REVIEW article

Front. Mater., 22 July 2022

Sec. Biomaterials

Volume 9 - 2022 | https://doi.org/10.3389/fmats.2022.932157

Applications of Nonviral Biomaterials for microRNA Transfection in Bone Tissue Engineering

  • Department of Orthodontics, School of Stomatology, Capital Medical University, Beijing, China

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Abstract

Bone tissue engineering, which involves scaffolds, growth factors, and cells, has been of great interest to treat bone defects in recent years. MicroRNAs (miRNAs or miRs) are small, single-stranded, noncoding RNAs that closely monitor and regulate the signaling pathway of osteoblast differentiation. Thus, the role of miRNAs in bone tissue engineering has attracted much attention. However, there are some problems when miRNAs are directly applied in the human body, including negative charge rejection of the cell membrane, nuclease degradation, immunotoxicity, and neurotoxicity. Therefore, it is necessary to use a suitable carrier to transfect miRNAs into cells. In contrast to viral vectors, nonviral vectors are advantageous because they are less immunogenic and toxic; they can deliver miRNAs with a higher molecular weight; and they are easier to construct and modify. This article reviews the application of different miRNAs or anti-miRNAs in bone tissue engineering and the related signaling pathways when they promote osteogenic gene expression and osteogenic differentiation of target cells. An overview of the properties of different types of nonviral miRNA-transfected biomaterials, including calcium phosphates, nanosystems, liposomes, nucleic acids, silk-based biomaterials, cell-penetrating peptides, bioactive glass, PEI, and exosomes, is also provided. In addition, the evaluations in load efficiency, release efficiency, cell uptake rate, biocompatibility, stability, and biological immunity of nonviral miRNA-transfected biomaterials are given. This article also confirms that these biomaterials stably deliver miRNA to promote osteogenic gene expression, osteogenic differentiation of target cells, and mineralization of the extracellular matrix. Because there are differences in the properties of various nonviral materials, future work will focus on identifying suitable transfection materials and improving the transfection efficiency and biocompatibility of materials.

1 Introduction

1.1 Bone Tissue Engineering

Bone defects caused by disease, trauma, or surgery are common clinical problems encountered by plastic surgeons. Severe bone defects can result in delayed bone union, leading to a high disability rate. The self-healing ability of bone defects is strong, but the self-healing of large-scale bone defects is often delayed or impossible, thereby requiring external intervention. Traditional bone defect repair methods include autogenous and allogeneic bone transplantation, but these methods have limitations. Although the former is the “gold standard” of bone transplantation, it has the disadvantages of limited materials, complications at the donor site, and the need for secondary surgery. Furthermore, the latter may lead to disease transmission, injection reactions, and poor prognosis due to its reduced osteoinduction capacity (Wang and Yeung, 2017; Han et al., 2020). Therefore, bone tissue engineering has emerged and has been developed in the past 2 decades. Bone tissue engineering involves culturing and expanding stem cells in vitro. The cells are then seeded onto a scaffold material with good biological activity and degradability, and the cells are cultivated for a period of time. Later, the scaffold with cells is transplanted to the corresponding defect site. Cells continue to grow and reproduce in vivo and secrete extracellular matrix. With the gradual degradation of the materials, the new bone tissue finally replaces the scaffold materials to repair the structure and restore the function (Qu et al., 2019). (Figure 1) As an alternative to bone defect repair, bone tissue engineering reduces the defects of autologous bone transplantation and allogeneic bone transplantation. The goal of bone tissue engineering is to employ the biochemical signaling pathway during the natural bone healing process to promote self-healing and bone regeneration to restore and maintain bone morphology and function (Arriaga et al., 2019). This technology involves the use of scaffold biomaterials and the introduction of appropriate growth factors and multipotent cells (Marew and Birhanu, 2021). Currently, through bone tissue engineering technology, many new bioactive materials and technologies have been developed, such as three-dimensional (3D)-printed scaffolds (Zhang et al., 2019) and gene delivery technology (Gantenbein et al., 2020), to reduce the shortcomings of traditional transplantation methods and improve the biocompatibility and the osteogenic, osteoconductive, and osteoinductive properties of grafts. The study of the molecular mechanism of osteogenesis induced by these biomaterials is also a hot field.

FIGURE 1

FIGURE 1

The procession of the microRNA-involved bone tissue engineering. (A) Nonviral vectors and microRNA are combined in vitro.(B) Stem cells are cultured, expanded, and transfected in vitro. (C) The cells are then seeded onto a scaffold material. (D) The scaffold with cells is transplanted to the corresponding defect site of the animal model. (E) Cells continue to grow and reproduce in vivo and secrete extracellular matrix. With the gradual degradation of the materials, the new bone tissue finally replaces the scaffold materials to repair the structure and restore the function.

1.2 miRNAs in Bone Tissue Engineering

1.2.1 Mechanism of miRNAs in Bone Tissue Engineering

Bioactive factors used in bone tissue engineering include the following three categories: 1) growth factors, 2) genetic substances, and 3) drugs (Dasari et al., 2022). The main effect of drug delivery is to resist local inflammation, to reduce the immune response, and to provide bone nutrition in bone tissue engineering. Drug delivery has a small effect on promoting osteogenesis. Growth factors have been widely used in bone tissue engineering; they bind to cell surface receptors to induce cell migration, differentiation, and proliferation, among which bone morphogenic protein (BMP) is the most commonly used (Fuerkaiti et al., 2022). However, the cost of synthesizing enough growth factors for clinical use is high, and proteolytic degradation easily occurs, resulting in a short half-life of the biological activity. Thus, the delivery of genetic material, which includes DNA and RNA, is a good alternative to growth factor delivery (Samorezov and Alsberg, 2015). This review mainly discusses the delivery of microRNAs (miRNAs or miRs).

miRNAs are small, single-stranded, noncoding RNAs that mainly exist in cells, and a small amount exist in extracellular environments, such as serum, plasma, and tears (Lanzillotti et al., 2021). MiRNAs are a class of regulators of gene expression (Reda El Sayed et al., 2021). By affecting the translation of target messenger RNAs (mRNAs), miRNAs can negatively regulate gene expression at the posttranscriptional level. miRNAs are able to bind the 3′ untranslated region (3′-UTR) of target mRNAs, which are subsequently degraded or translationally silenced, to regulate cell proliferation, differentiation and apoptosis (Figure 2) (Mazziotta et al., 2021).

FIGURE 2

FIGURE 2

miRNAs negatively regulate gene expression by affecting the translation of target mRNAs. (RISC, RNA-induced silencing complex).

miRNAs play an important role in bone formation involving mesenchymal stem cells (MSCs) derived from blood, pericytes, and bone marrow (Iaquinta et al., 2019; Liu J. et al., 2019). miRNAs promote or inhibit osteogenic differentiation by targeting transcription factor gene expression or targeting either positive or negative regulatory genes associated with osteogenesis (Lanzillotti et al., 2021). During the process of MSC differentiation into osteoblasts, miRNAs closely monitor and regulate the following two key signaling cascades: the transforming growth factor-beta (TGF-β)/BMP pathway and the Wingless/Int-1(Wnt)/β-catenin signaling pathway (Mazziotta et al., 2021). For example, miR-93-5p suppresses osteogenic differentiation in a rabbit model of traumatic femoral head necrosis by binding the 3′-UTR of Smad5 and reducing BMP-2 and RUNX2 (Zhang Y. et al., 2021). Xu et al. confirmed that miR-889 binds the 3′UTR of WNT7A and negatively regulates the osteogenic differentiation of bone mesenchymal stem cells (BMSCs) through the Wnt/β-catenin signaling pathway (Xu et al., 2019). In addition, miR-486-3p activates the Wnt/β-catenin signaling pathway by targeting catenin beta interacting protein 1 (CTNNBIP1) to promote the osteogenesis of BMSCs, which has been confirmed in bone marrow samples from patients with osteoporosis and in mice undergoing ovariectomy (Zhang Z. et al., 2021). In addition, in other cells, such as fibroblasts, miRNAs regulate osteogenesis through similar signaling pathways. Ding et al. obtained fibroblasts from the capsular ligament of patients with ankylosing spondylitis and studied its osteogenic differentiation mechanism, and they reported that miR-214-3p targets the BMP2 gene and blocks the BMP-TGFβ axis, thereby preventing fibroblast osteogenesis (Ding et al., 2020). Apart from these two pathways, some miRNAs regulate bone differentiation through other signaling pathways, such as the Notch signaling pathway and Nrf2 pathway (Wang et al., 2013; Liu H. et al., 2019). In conclusion, the miRNAs that have been applied to bone tissue engineering and tested in vivo are listed in Table 1, and their known mechanisms of regulating osteogenic differentiation are listed in Table 2.

TABLE 1

miRNAOther bioactive factorTransfection agentBiomaterial scaffoldCell typeAnimal modelTimepointResultsReferences
miR let-7dLayered double hydroxide (LDH) nanoparticlesFibrin GelBMSCssubcutaneous pockets on the backs of the athymic nude mice (Subcutaneous Ectopic Osteogenesis Model)2 weeksSignificant improvement of bone volume fraction (bone volume/total volume, BV/TV)Yang L et al. (2021)
miR-10aIL-2, TGF-βPLLA/PEG co-functionalized MSN, PLGA MSPLLA nanofibrous spongy microspheres (NF-SMS)--Mouse periodontal disease model10 daysSubstantially rescue the alveolar bone lossLiu et al. (2018b)
antimiR-138SDF-1αChitosan/tripolyphosphate/hyaluronic acid/antimiRNA-138 nanoparticles (CTH/antimiR-138 NPs)Thermosensitive chitosan/β-glycerol phosphate (CS/GP) hydrogel--8 mm calvarial defect in rats8 weeksHigher regenerated bone (32.74 ± 4.89%), higher BMD of newly formed bone, greater Tb.N (0.90 ± 0.05)Wu et al. (2018)
miR-19b-3pLentivirusPLLA/POSSBMSCs8 mm calvarial defect in rats3 monthsAlmost complete repair of bone defects and higher bone mineral densityXiong et al. (2020)
miR-20aPoly(ethylene glycol) (PEG) hydrogelshMSCs5 mm calvarial defect in rats12 weeksSignificantly higher average bone volume fraction (24.51%), significantly higher trabecular number and lower trabecular separationNguyen et al. (2018)
miR-21NanocapsulesO-carboxymethyl chitosan (CMCS)--2 mm diameter bilateral bone defects of the proximal tibia in rats with bilateral ovaries removal4 weeksAt 4 weeks: significantly higher BV/TV and Tb.Th, higher calcium nodule formation; at 8 weeks: less new cancellous bone, lower BV/TV, lower Tb.Th, the new cancellous bone had been absorbed and the marrow cavity had been dredgedSun et al. (2020)
miR-21Nanocapsules and O-carboxymethyl chitosan (CMCS) powder mixed gelTitanium cylinders--New Zealand White rabbits3 monthsIncreased content of mineral (Ca and P), plenty of nodules, a dense structure combined with collagenous fiber and apatite, similar to mature boneGeng et al. (2020)
miR-21Lentivirusβ-TCPrBMSCs5 mm calvarial defect in rats60 dayshigher BMD and Tb.ThYang et al. (2019)
Lentivirusβ-TCPBMSCs20 mm × 10 mm osteoperiosteal segmental defect in canine mandibular6 monthsHigher BV/TV, BMD, bone mineral content (BMC), improved percentage of new bone area (52.21 ± 3.87%), lower percentage of the β-TCP residual area (6.82 ± 1.43%)
miR-21N-(3-aminopropyl) methacrylamide, acrylamide, and ethylene glycol dimethacrylate nanocapsulesTitanium (Ti)-based SrHA/miR-21 composite coatingOsteoblast-like MG63 cells4 mm defect at distal femur and tibia in New Zealand white rabbits3 monthsSignificantly higher bone-implant contact, higher biomechanical strength (287 ± 25 N), highest υ1PO43−/amide I values (13.1 ± 1.4)Geng et al. (2018)
miR-21NanocapsulesO-carboxymethyl chitosan (CMCS) network3 mm tibial plateau bone defect in rats8 weeksSignificantly higher BV/TV, 2.4-fold bone formationMeng et al. (2016b)
miR-26aInjectable poly(ethylene glycol) (PEG) hydrogel--hMSCs7 mm calvarial defect in rats8 weeksStatistically increase in BV/TV and bone surface density (bone surface/bone volume, BS/BV)Gan et al. (2021)
miR-26aA comb-shaped polycation (HA-SS-PGEA) consisting of hyaluronic acid (HA), disulfide groups, and ethanolamine (EA)-functionalized poly(glycidyl methacrylate) (PGMA)Three-dimensional (3D) hybrid nanofiber aerogelsBMSCs8 mm diameter cranial defect in rats4 weeksMuch larger defect healing area (new bone volumes: 21.8 mm3, corresponding closure percentages: 62.2%, coverage: 56.4%)Li et al. (2020)
antimiR-26a-5pLentivirusBiphasic calcium phosphate (BCP)Adipose-derived mesenchymal stem cells (ADSCs)4 mm-long ×2 mm-deep femoral defect in rats2\4\8 weeksSignificantly higher BV/TV (8 weeks), higher BMD (2, 4 weeks), higher Tb.N. (2, 4 weeks), higher Tb.Th (2, 4 weeks), lower residual bcp/TVYuan et al. (2019)
miR-26alentivirusβ-TCPmBMSCs5 mm calvarial defect in mice2 monthsA marked increase in the volume of newly formed bones, which almost filled the whole defect areaLiu et al. (2018a)
miR-26asiPORT NeoFX transfection agentHyStem-HP™ hydrogelmBMMSCsectopic bone formation model of immunocompromised mice8 weekssignificantly more bone formation and high density of blood vesselsLi et al. (2013)
siPORT NeoFX transfection agentHyStem-HP™ hydrogelmBMMSCs5 mm calvarial defect in mice12 weeksincreasing vascular volume showed by immunofluorescence staining for CD31
miR-29b-3ppTRE2-Tet-on plasmidmicrobubble-ultrasound system----femoral fracture in mice6 weekssignificant reduction in callus area, higher BV/TV (including BVh/TV, BVl/TV, BV1/TV) and BMD, enhanced stiffness and relative stiffnessLee et al. (2016)
miR-29bO-carboxymethyl chitosan (CMCS) coating nanocapsulestitanium Alloy--3 mm tibial defect in rats8 weekssignificantly higher rate of calcification (2.80-fold), 24% increase in BIC, more new bone (∼2.01-fold at 2 weeks)Meng et al. (2016a)
as-miR-31lentivirusβ-tricalcium phosphate (β-TCP)rat ASCs5 mm calvarial defect in rats8 weekshigher BMD (0.553 ± 0.081 g/cm3), BV/TV (35.42 ± 6.12%), new bone formation at 2 weeks: 4.58 ± 0.51%, 4 weeks: 7.62 ± 1.18%, 6 weeks: 8.11 ± 0.89%, 8 weeks: 36.81 ± 3.54%)Deng et al. (2013)
antimiR-31lentiviruspoly (glycerol sebacate) (PGS)rat BMSCs8 mm calvarial defect in rats8 weekshigher BV/TV (41.82 ± 6.54%), BMD (0.492 ± 0.062 g/cm3), and percentage of new bone area (60.92 ± 7.34%)Deng et al. (2014)
miR-33a-5pLipofectamine 3000collagen-based hydrogelshASCsectopic bone formation model of nude mice8 weeksmore newly constructed bone, more collagen fiber bundles arranged compactlyShen et al. (2020)
miR-34aLipofectamine 2000rBMSCsectopic bone formation model of nude mice8 weekssignificantly higher percentage of bone area to total area (BA/TA)Liu J et al. (2019)
Lipofectamine 2000collagen-based hydrogel--3 mm tibial defect in rats8 weekssignificantly higher BV/TV
miR-92blentivirusMSCectopic bone formation model of nude mice8 weeksmiR-92b was superior to GFP in ectopic bone formation by HE stainingHou et al. (2021)
open femur fracture model of rats3 weekshigher newly formed bone, higher volume of low-density bone/total tissue volume, higher percentage of bone in callus
miR-93-5p inhibitorLipofectamine 2000--rabbit BMSCsTrauma-induced osteonecrosis of the femoral head (TIONFH) rabbit model8 weekssignificantly fewer empty lacunae and more osteoblastsZhang Y et al. (2021)
miR-106a InhibitorLiposome 2000autologous oxygen release nano-bionic scaffoldrBMSCsrat tibia fracture model6 weeksmarkedly higher BMD, significantly promoted collagen II productionSun et al. (2018)
miR-129-5plentivirusmatrigelBMSCs3 mm diameter defect on each side of the calvaria in mice8 weeksmuch higher BV/TV (0.702 ± 0.027), significantly higher BMD (1,296 ± 53 g/cm3), more bone-like structures and collagen depositsZhao et al. (2021)
miR-133aaspartate, serine, serine (AspSerSer)6-liposome--osteoblasthindlimb unloading (HU)-challenged mice3 weeksless bone loss and osteoclast numbers, enhanced BMD, BV/TV, Tb.Th and Tb.N, lower Tb.Sp, trabecular bone pattern factor (TbPF) and BS/BVZhou et al. (2021)
antagomiR-133acollagen-hydroxyapatite (coll-HA)coll-nHA--7 mm calvarial defect in rats4 weeksstatistically more calcified tissue (8.71 ± 7.48%), statistically more and thicker new trabeculae, statistically more de novo bone (≥70% increase)Castaño et al. (2020)
antagomiR-133a/bCTH nanoparticle (chitosan solution (CS), Sodium tripolyphosphate (TPP), hyaluronic acid (HA))--5 mm calvarial defect in mice12 weekssignificantly increased new bone areaJiang et al. (2020)
miR-135lentivirusPSeDrat ADSCs8 mm calvarial defect in rats8 weekssignificantly higher BV/TV (50.53 ± 4.45%), BMD (0.0165 ± 0.0012 g/cc) and Tb.N (0.3352 ± 0.0529), larger newly formed bone (820.4 ± 77.3 mm2), higher percentage of newly formed bone in the total area of bone tissue (40.13 ± 1.94%), larger area of fluorochrome stained boneXie et al. (2016)
miR-142-5pperiosteal injection at the fracture site----femoral fracture in mice4 weekssignificantly higher BMDTu et al. (2017)
miR-146a inhibitorlentiviruspoly (sebacoyl diglyceride) (PSeD) porous scaffoldrat ADSCs8 mm calvarial defect in rats8 weekssignificantly higher BV/TV, Tb.N and BMD (49.8 ± 5.49%, 0.4094 ± 0.0687, 0.01581 ± 0.00299 g/cc), larger new bone areas in weeks 2–4: 92.38 ± 16.69 mm2, weeks 4–6: 115.32 ± 11.87 mm2, and weeks 6–8: 90.93 ± 9.95 mm2Xie et al. (2017)
miR-148a-3plentiviruses--BMSCsovariectomy (OVX)-induced osteoporosis model in mice6 weekshigher BMD, BV/TV ratio, Tb.N and Tb.Th, lower trabecular spacing (Tb.Sp)Liu and Sun, (2021)
miR-148b(hydroxypropyl) cellulose (HPC)-modified silver nanoparticles (SNPs)collagen-infilled 3D printed hybridrBMSCs5 mm in diameter and 1 mm in thickness calvarial defect in rats8 weekssignificantly higher BV/TV (%), higher normalized BMD (34.7 ± 8.9%), larger bone coverage area (78.1 ± 20.8%), higher connectivity density (2.86 ± 1.23)Moncal et al. (2019)
miR-148bBMP-2baculoviruspoly (L-lactide-co-glycolide) (PLGA)hASCs4 mm calvarial defect in nude mice12 weeksthe new bone nearly filled the entire defect after 12W, higher bone area (94.7 ± 0.8%), volume (89.4 ± 11.1%) and density (95.7 ± 3.9%)Liao et al. (2014)
miR-187lentivirushMSCsosteoporosis (OP) mouse model28 dayssignificantly reverse the decreased bone healing rateZhang J et al. (2021)
agomiR-199a-5pchitosan nanoparticleshydroxyapatite/collagen (HA/collagen)hMSCectopic bone formation model of NOD/SCID mice6 weekshigher density, darker MSCs and more collagen deposition in Masson trichrome staining for collagenChen X et al. (2015)
chitosan nanoparticlesfibrin gel--3 mm tibial defect in rats8 weekshigher density, more regenerated bone in the center of the repaired area
miR-200cpDNA3D-printed β-tricalcium phosphate (β-TCP) with collagen coatingsBMSCs9 mm diameter parietal defect in rats4 weeksstatistically increase in bone formationRemy et al. (2021)
miR-205lentivirusendothelial colony-forming cells (ECFCs)mandibular distraction osteogenesis (MDO) canine model4 weeksthe distraction gap was fully bridgedJiang et al. (2021)
miR-210simvastatin (Siv)dual-sized pore structure calcium-silicon nanospheres (DPNPs)β-TCP--5 mm calvarial defect in mice8 weeksthe highest bone density and bone volume, strong positive expression of CD31 (the platelet endothelial cell adhesion molecule-1)Liu et al. (2021)
miR-222aspirinmesoporous silica nanoparticles (MSN)injectable colloidal hydrogel--5 mm mandibular defect in rats10 weekshigher BV/TV% (21.97% ± 3.99%), significant increased neurogenic proteins expressionLei et al. (2019)
antimiR-222Lipofectamine 2000HA/tricalcium phosphate (HA/TCP)hBMSCsectopic bone formation model of non-obese diabetic/severe combined immunodeficiency (NOD/SCID) mice8 weeksstatistically higher quantified bone volume (% bone/total area)Chang et al. (2018)
miR-335-5plipidoid nanoparticlesempty silk scaffold (SS)mBMSCs4 mm calvarial defect in mice5 weekssignificantly higher BV/TVSui et al. (2018)
miR-335-5ptetrahedral DNA nanostructures (TDNs)Heparin lithium hydrogel (Li-hep-gel)BMSCs3 mm femoral defect in rabbits12 weeks70% new bone area, new blood vessels 12n, empty lacunae 22% ± 6%Li et al. (2022)
antimiR-467gLipofectamine RNAi MAX--mouse calvarial osteoblast (MCO) cells0.8 mm femoral defect in mice21 dayssignificantly higher BV/TV, Tb.Th and Tb.NKureel et al. (2017)
miR-672lentivirusCPCADSCs5 mm critical size skull defect in rats8 weekshighest blood vessel volume and number (3.56 ± 6.46 mm3, 15.43 ± 7.67 mm−2), enhanced BMD(0.78 g ± 4.28cm−3), BV/TV (17.83 ± 8.42%), significantly higher new bone area and new bone area/total areaChen et al. (2021)
miR-2861sticky-end tetrahedral framework nucleic acids (stFNAs)BMSCs1 mm spherical femoral defect in mice2 weeksThe surface defect had almost completely healed after 2 weeksLi X et al. (2021)
miR-5106novel monodispersed bioactive glass nanoclusters (BGNCs) with PEIhydrogelBMSCs5 mm calvarial defect in rats4 weekssignificantly high new bone volume and trabecular thicknessXue et al. (2017)

The application of miRNAs in bone tissue engineering in vivo.

TABLE 2

miRNAsPathwayRelated signaling pathwaysCell typesReferences
miR-1-3pSOX9 pathwaymiR-1-3p targets and decreases Sox9 transcription factor activity. Sox9 negatively regulates Runx2 and type X collagen expression to modulate endochondral ossification-related disorders. Ding et al. (2021)BMSCsDing et al. (2021)
miR-10aTGF-β pathwayInterleukin 2 (IL-2) and transforming growth factor beta (TGF-β) are cytokines known to enhance Treg recruitment, proliferation, and differentiation. miR-10a can facilitate naïve T cells to differentiate to Tregs. The higher number and possibly more mature Tregs substantially suppressed the destructive osteoclastogenesis and enhanced the osteoblastic activity, synergistically rescuing periodontal bone lossmice T cellsLiu et al. (2018b)
miR-let-7dBMP pathwaymiRNA-let-7d targets the 3′-UTR of HMGA2, resulting in the suppression of the expression of GSK3β protein, positively regulating osteogenic differentiation and negatively regulates adipogenic differentiation of hADSCshuman adipose-derived mesenchymal stem cells (hADSCs)Wei et al. (2014)
BMSCsYang L et al. (2021)
miR-19b-3pBMP pathwaymiR-19b-3p could bind to the 3′UTR of Smurf1, suppressing the expression of Smurf1 which is a negative regulator of osteogenesis. Smurf1 could mediate Runx2 degradation to inhibit osteoblast differentiation and bone formation. Smurf1 can also mediate the degradation of Smad1/5 which is the down-stream factor of BMP signal channel, resulting in the suppression of the osteoblast differentiationBMSCsXiong et al. (2020)
miR-20aBMP pathwaymiRNA-20a has a positive effect on hMSC osteogenic differentiation by inhibiting the expression of PPAR-γ, a down regulator of BMP signaling in osteogenesishMSCsNguyen et al. (2018)
miR-21PI3K-AKT signaling pathwaymiR-21 directly targets and inhibits PTEN by binding its 3′-UTR, thus leading to the activation of AKT and HIF-1α. The PI3K-AKT signaling pathway activity has an increasing tendency responding to miR-21 up-regulation. This enhancement promotes the phosphorylation of GSK-3β, leading to the stabilization and high concentration accumulation of β-catenin in cytoplasm to activate the transcription of RUNX-2, and finally increases the osteogenesis of hUMSCshUMSCsMeng et al. (2015); Yang et al. (2019)
miR-26aBMP pathwaymiR-26a interacts with the 3′-UTR of the Smad1 mRNA, diminishing the availability of the active SMAD1 transcription factor to participate in the differentiation process of hADSCs and elevating the mRNA and protein expression levels of Runx2. SMAD1 is the downstream effector of BMP signaling, and it is phosphorylated by BMP type I receptorshADSCsGan et al. (2021); Liu et al. (2018a)
Wnt/β-catenin pathwaymiRNA-26a targets the 3′-UTR of GSK3 β to activate Wnt signaling for promoting osteogenic differentiation of BMSCs by inhibiting the expression of GSK3β and increasing the level of active β-cateninBMSCsSu et al. (2015)
miR-26a-5p inhibits the translation of Wnt5a by directly binding to the 3′-UTR of Wnt5a. WNT5A is a noncanonical Wnt ligand and activates two noncanonical Wnt pathways, one of which is the Wnt/Ca2+ signaling pathway. Yuan et al. (2019)ADSCsYuan et al. (2019)
miR-29bM-CSF and RANK-L signaling pathwaysmiR-29b targets C-FOS and MMP2 within osteoclasts (OCLs). In OCL precursors, M-CSF promotes RANK expression through C-FOS and sustains survival and cytoskeletal reorganization. RANK controls NFKB activation, which in turn leads to upregulated expression of NFATc-1, the master transcription factor for OCL generation and function. MMP2 belongs to the gelatinase protein family and participates to bone matrix degradationosteoclast (OCL)Rossi et al. (2013)
miR-31BMP pathwaymiR-31 typically binds to the mRNA and targets and inhibits the translation of the master transcription factor special AT-rich sequence-binding protein 2 (Satb2). SATB2 interacts with and enhances the transcriptional activity of Runx2 and activating transcription factor 4 (ATF4) Yan et al. (2011). As-miR-31 promotes bone regeneration and bone defect repairBMSCsDeng et al. (2014)
miR-33a-5pcircFOXP1/miR-33a-5p/FOXP1 pathwaymiR-33a-5p inhibits osteogenesis by targeting forkhead box P1 (FOXP1) 3′-UTR and down-regulating FOXP1 expression (Shen et al., 2020). FOXP1 regulates cell-fate choice of MSCs through interactions with the CEBPβ/δ complex and recombination signal binding protein for immunoglobulin κ J region (RBPjκ), key modulators of adipogenesis and osteogenesis, respectively. Li X et al. (2017)hASCsLi X et al. (2017); Shen et al. (2020)
miR-34aNotch signaling pathwaymiR-34a directly targets Notch1, improving the osteogenic differentiation of irradiated BMSCs by suppressing NOTCH1, since downregulation of NOTCH1 enhanced the mRNA and protein expression of RUNX2 and OCN.BMSCsLiu J et al. (2019)
mir-92bERK and JNK signaling pathwaysEzh2 is a potential target of mir-92b and down-regulated by it. Ezh2 is the catalytic subunit of the Polycomb Repressive Complex 2 (PRC2) and catalyzes tri-methylation of histone H3 at lysine 27 (H3K27me3) to silence target genes. And extracellular signal-regulated kinases (ERK) and c-Jun N-terminal protein kinase (JNK) signaling pathways were activated by mir-92b, which could finally lead to the enhanced osteogenesis of MSCsMSCsHou et al. (2021)
miR-93-5pBMP-2/Smad5 pathwaymiR-93-5p suppresses osteogenic differentiation of BMSCs by binding the 3′-UTR of Smad5 and reducing BMP-2 and RUNX2BMSCsZhang Y et al. (2021)
miR-106aBMP-2/Smad5 pathwaymiR-106b-5p regulates Smad5 expression negatively, and they functioned as an inhibitory factor in the physiological process of bone formation and osteoblast differentiation. Smad5 is a downstream transcription factor phosphorylated and activated by of BMP-2 receptors which is a key signaling component in osteoblast differentiation, a member of TGF-β superfamily. The phosphorylated Smad5 forms a complex with Smad4 (Co-Smad), then translocates into the nucleus to activate transcription factor Cbfa1/Runx2BMSCsFang et al. (2016)
miR-129-5pWnt/β-catenin pathwaymiR-129-5p targets the 3′-UTR of Dickkopf3 (Dkk3) and repress it to enhance osteoblast differentiation. Dkk3 could bind to β-catenin, mediating Wnt signaling pathwayBMSCsZhao et al. (2021)
miR-133BMP pathwayMiR-133 directly regulates the 3′UTR of distal-less homeobox 3 (Dlx3), a member of the Dlx family of homeobox proteins. It is a transcriptional activator of runt-related transcription factor 2 (Runx2) during osteogenic differentiation. Mir-133a inhibits Dlx3 expression via direct targeting of the Dlx3 3′-UTR.MSCsQadir et al. (2018)
miR-133 inhibits the bone formation by targeting the 3′-UTR of RUNX2 and decreasing the expression level of RUNX2Peng et al. (2018); Jiang et al. (2020)osteoblastsZhou et al. (2021)
miR-135Hoxa2/Runx2 pathwaymiR-135 negatively regulates Hoxa2 expression by targeting the 3′-UTR of Hoxa2. And Hoxa2 negatively regulates Runx2 expression in ADSCs. The overexpression of miR-135 enhances the expression of bone markers and extracellular matrix calcium depositionADSCsXie et al. (2016)
antimiR-138ERK1/2 pathwayThe antimiR-138 delivery down-regulates the endogenous miR-138 levels in BMSC sheets, activates the extracellular signal regulated kinases 1/2 (ERK1/2) pathway and enhances the expression of RUNX2 finally leading to enhanced osteogenesisBMSCsYan et al. (2014)
miR-142-5pUbiquitination pathwaymiR-142-5p promotes osteoblast activity and matrix mineralization by targeting the gene encoding WW-domain-containing E3 ubiquitin protein ligase 1. And miR-142-5p stimulates osteocalcin and Runx2 expression by targeting Wwp1. Agomir-142-5p in the fracture areas stimulates osteoblast activitypreosteoblast cellsTu et al. (2017)
miR-146aBMP pathwaymiR-146a exerts its repressive effect on Drosophila mothers against decapentaplegic protein 4 (SMAD4) through interacting with 3′-untranslated region (3′-UTR) of SMAD4 mRNA which is an important co-activator in the BMP signaling pathwayADSCsXie et al. (2017)
miR-148a-3pNrf2 pathwaymiR-148a-3p negatively regulates p300 expression in osteoblasts by binding to the 3-UTR of p300 mRNA, which could inactivate the Nrf2 pathway, consequently down regulating RUNX2\ALP activity, and blunting osteoblast differentiation and subsequent bone reconstruction, ultimately leading to osteoporosisosteoblasticsWang et al. (2013); Liu and Sun, (2021)
miR-148bBMP pathwaymiR-148b directly targets NOG, whose gene product (noggin) is an antagonist to BMPs and negatively regulates BMP-induced osteogenic differentiation and bone formationrBMSCsLi K.-C et al. (2017)
miR-187BMP pathwaymiR-187 downregulates human BarH-like homeobox 2 (BARX2) through targeted regulation, inducing osteogenic differentiation of hMSCs. (33550149) Barx2 regulates the expression of several genes encoding cell-adhesion molecules and extracellular matrix proteins, including NCAM and collagen II (Col2a1) in the limb bud. Two members of the BMP family that are crucial for chondrogenesis, GDF5 and BMP4, regulate the pattern of Barx2 expression in developing limbs. Barx2 acts downstream of BMP signaling and in concert with Sox proteins to regulate chondrogenesis. Meech et al. (2005)hMSCsZhang J et al. (2021)
miR-199a-5pHIF1α-Twist1 pathwayAt early stage of differentiation, hypoxia induces HIF1a-Twist1 pathway to enhance osteogenesis by up-regulating miR-199a-5p, while at late stage of differentiation, miR-199a-5p enhances osteogenesis maturation by inhibiting HIF1α-Twist1 pathway. And Runx2 might be negatively regulated by HIF1α, which is the direct target of miR-199a-5phMSCsChen X et al. (2015)
miR-200cWnt/β-catenin pathwaymiR-200c overexpression is shown to downregulate SRY (sex detg. region Y)-box 2 (Sox2) and Kruppel-like factor 4 by directly targeting 3′-untranslated regions and upregulate the activity of Wnt signaling inhibited by Sox2hBMSCsAkkouch et al. (2019)
BMP pathwaymiR-200c effectively inhibits Noggin, an antagonist of BMP signals, by directly targeting the 3′UTR of Nogginhuman embryonic palatal mesenchyme (HEPM)Hong et al. (2016)
miR-205Notch signaling pathwaymiR-205 targets the 3′-untranslated region (UTR) of cfa-NOTCH2, which is a unique transcription regulator in bone angiogenesis. Inhibit miR-205 increases NOTCH2 expression, resulting in the elevated secretion of VEGF proteins and thereby stimulating angiogenesis and osteogenesis within the skeletal systemendothelial colony-forming cells (ECFCs)Jiang et al. (2021)
miR-214BMP pathwaymiR-214 targets the 3′-UTR of the transcription factor ATF4 to inhibit bone formation. There is a runt-related transcription factor 2 (Runx2) binding site in Atf4 promoter Wang et al. (2013)preosteoblast cells (MC3T3-E1 cells)Wang Y et al. (2021)
Pten/PI3k/Akt pathwaymiR-214 targets the 3′-UTR of phosphatase and tensin homolog (Pten). It has been demonstrated that Pten regulates RANKL-induced osteoclast differentiation from RAW 264.7 osteoclast precursors through PI3K/Akt pathwayOsteoclastsZhao et al. (2015)
miR-222Wnt/β-catenin pathwaymiR-222 promotes neural differentiation of hBMSCs in vitro by targeting Nemo-like kinase (NLK) and decreasing NLK protein level. NLK is an inhibitor of Wnt/β-catenin signaling, which plays as a vital role in neuronal differentiationhBMSCsLei et al. (2019)
TGFβ pathwayAnti-miR-222 enhances in vivo ectopic bone formation through targeting to the 3′UTR of cyclin-dependent kinase inhibitor 1B (CDKN1B), a cell-cycle inhibitor Chang et al. (2018). CDKN1B regulate osteoblast differentiation through cell-cycle arrest, and cell-cycle arrest is a prerequisite for differentiationhMSCsChang et al. (2018)
STAT5A signaling pathwayMiR-222 was found to negatively modulate angiogenesis by targeting the c-Kit receptor Mazziotta et al. (2021) together with the signal transducer and activator of transcription 5A (STAT5A) Zhang Y et al. (2021). The c-Kit receptor is the receptor for the angiogenic activity of stem cell factor (SFC), and is expressed on the surface of ECs Mazziotta et al. (2021). STAT5A activates bFGF and IL-3, which in turn trigger vascular EC morphogenesis in the STAT5A signaling pathway Zhang J et al. (2021). The present study demonstrated that although the target genes of miR-222 related to angiogenesis were not validated, down-regulation of the c-kit receptor and STAT5A by the miR-222 inhibitor might contribute to the enhanced neovascularization at the fracture sitehMSCsYoshizuka et al. (2016)
miR-335-5pWnt/β-catenin pathwayMiR335-5p may inhibit Wnt antagonist Dickkopf-1 (DKK1) expression and upregulate the Wnt pathway, promoting osteogenesis and angiogenesis as well as enhancing bone regeneration in steroid-associated osteonecrosis (SAON)BMSCsLi et al. (2022); Sui et al. (2018)
miR-467gIhh/Runx-2 signaling pathwaymiR-467g targets the 3′-UTR of Runx-2, and down regulates Runx-2, inhibiting osteoblast differentiationosteoblastKureel et al. (2017)
miR-590-5pBMP pathwayThe 3′-untranslated region of Smad7 was directly targeted by miR-590-5p. Smad7 inhibits osteoblast differentiation via Smurf2-mediated Runx2 degradation.miR-590-5p promotes osteoblast differentiation by indirectly protecting and stabilizing the Runx2 protein by targeting Smad7 gene expression Vishal et al. (2017)mMSCsBrenner et al. (2021)
Wnt/β-catenin pathwaymiR-590-3p binds to 3′UTR of APC mRNA. miR-590-3p can promote osteogenic differentiation via suppressing APC expression and stabilizing β-cateninhMSCsWu S et al. (2016)
miR-672TGFβ pathwaymiR-672 negatively regulated the expression of TIMP2 by interacting with 3′-UTR of TIMP2 mRNA, regulating ADSCs angiogenesis in vitro. TIMP2, a member of the TIMP family, regulates the proteolytic activity of matrix metalloproteinases (MMPs), a group of proteolytic enzymes, and maintain the balance between extracellular matrix (ECM) breakdown and synthesisADSCsChen et al. (2021)
miR-2861BMP pathwayMiR-2861 bound to the amino acid coding sequences (CDSs) of histone deacetylase 5 (HDAC5) mRNA with complementarity to the miR-2861 seed region, inhibiting the expression of HDAC5 protein at the translational level, thereby upregulating the expression of the runt-related transcription factor 2 (Runx2) protein and ultimately promoting the osteogenic differentiation of BMSCsBMSCsLi Z et al. (2021)
miR-5106Wnt/β-catenin pathwaymiR-5106 targets and increases Sox9 transcription factor activity Xue et al. (2017). Sox9 negatively regulates Runx2 and type X collagen expression to modulate endochondral ossification-related disorders Ding et al. (2021)BMSCsXue et al. (2017)

The mechanism of miRNAs tested in vivo regulating osteogenic differentiation.

1.2.2 Evaluation of Osteogenic Effect of miRNA

The existing in vivo and in vitro studies have confirmed that miRNA can effectively promote bone regeneration. The main observation indexes of in vitro experiments are target mRNA and protein, Runx2, ALP, and the osteoblast-specific microRNA (Raj Preeth et al., 2021). The main indexes of in vivo experiments are as follows: significant improvement of bone volume fraction (bone volume/total volume, BV/TV), thickness of trabecularized spicules (Tb.Th), and trabecular number (Tb.N) (Kureel et al., 2017), bone mineral density (BMD) (Wu et al., 2018), bone mineral content (BMC) (Yang et al., 2019), bone surface density (bone surface/bone volume, BS/BV) (Gan et al., 2021), trabecular spacing (Tb.Sp), trabecular bone pattern factor (TbPF) (Zhou et al., 2021), and higher percentage of bone area to total area (BA/TA) (Liu H. et al., 2019).

1.3 Biomaterials for miRNA Transfection in Bone Tissue Engineering

Based on the understanding of the RNA expression profile in tissues and diseases, miRNA delivery strategies have been developed to enhance osteogenesis, such as using miRNA replacement therapy to administer double-stranded oligonucleotide miRNA mimics to treat conditions in which the target genes are overexpressed due to miRNA downregulation (Sriram et al., 2015). However, there are some difficulties when RNAs are directly applied in the human body. First, miRNA is a negatively charged molecule, which itself has difficulty penetrating the negatively charged cell membrane (Samorezov and Alsberg, 2015). Second, unmodified miRNA antagonists and miRNA mimics are rapidly degraded and eliminated in the blood circulation by nucleases that are rich in the patient bloodstream (Grixti et al., 2021). Third, miRNAs can lead to immunotoxicity by activating interferons or Toll-like receptors (Md et al., 2021) and neurotoxicity by triggering neurodegeneration through Toll-like receptors (Chen Y. et al., 2015). Therefore, it is necessary to use a suitable carrier to deliver miRNAs to protect them from inactivation in the process of matrix formation, storage, and release. The carrier should also deliver miRNA to specific tissues or organs continuously, stably and efficiently as well as ensure efficient cell uptake (Meng et al., 2016b). For most miRNA therapies developed thus far, cells are transfected or transduced with miRNAs or anti-miRNAs and loaded into scaffolds that are implanted into target sites rather than locally releasing the miRNAs/anti-miRNAs directly from scaffolds. Therefore, future research may focus on developing miR carriers to deliver miRNA/anti-miRNAs to cells in vivo (Arriaga et al., 2019).

RNA carriers are divided into viral vectors and nonviral vectors. Viral gene delivery methods present some intrinsic drawbacks, including difficult production processes (Tarach and Janaszewska, 2021), triggering acute inflammation, delayed humeral or cellular immune reactions (Levingstone et al., 2020), foreign DNA insertional mutagenesis (Dasgupta and Chatterjee, 2021), and limitation of insert molecule size (Nayerossadat et al., 2012). In contrast, nonviral vectors are advantageous due to their following properties: They are less immunogenic and toxic, they can deliver miRNAs with a higher molecular weight, and they are easier to construct and modify (Levingstone et al., 2020). The binding modes of miRNA and transfection agents (as illustrated in Table 3) mainly include electrostatic interactions (Liu et al., 2018b; Yang L. et al., 2021; Hosseinpour et al., 2021), hydrogen bonding (Meng et al., 2016b; Geng et al., 2018), polymer network wrapping (Geng et al., 2018), chemical crosslinking (Moncal et al., 2019), physical adsorption (Yu et al., 2017), and photosensitive linking (Qureshi et al., 2013; Gan et al., 2021). In addition, nonviral vectors can also transport synthetic siRNA and miRNA mimics, thus avoiding the need for nuclear localization performed according to plasmid DNA (pDNA) constructs containing RNA interference (RNAi) expression cassettes (Gantenbein et al., 2020). This effect allows siRNA/miRNA mimics to interact with the RNAi machinery directly in the cytosol, reducing the degree of intracellular trafficking required for RNAi-mediated gene repression and silencing (Levingstone et al., 2020).

TABLE 3

Types of vectorTransfection agentLoading methods and loading efficiencyUnloadingStabilityCellsCellular uptakeCytotoxicitymiRNAsReferences
Nucleic acidssticky-end tetrahedral framework nucleic acids (stFNAs)complementary base pairingRNase H cutsdegradation in >0.8 U ml−1 RNase A, still existed at 35% FBSBMSCsthe BMSCs had adsorbed a large amount of stFNA after 12 hdecreased cell viability when concentrationmiR-2861Li S et al. (2021)
>300 nm, altered cell viability only when carrying 2000 nm miR
tetrahedral DNA nanostructures (TDNs)complementary base pairing----BMSCs≈37.6% of BMSCs absorbed MiR@TDNs--miR-335-5pLi et al. (2022)
Calcium phosphatescollagen-hydroxyapatite (coll-HA)extending its half-liferMSCs55.4 ± 9.76% and 10.8 ± 6.37% at 3 and 7 days respectivelyantagomiR-133aCastaño et al. (2020); Mencía Castaño et al. (2016)
nanohydroxyapatite (nHA) particleselectrostatic interaction Bose and Tarafder, (2012)CaPs dissoluted in the acidic environment of the endocytic vesicle Bose and Tarafder, (2012)hMSCsprecipitates at the cell surface and undergo endocytosis, 33.5 ± 1.5% and 39.6 ± 4.7% respectively for the 10 and 20 nM doses Dy547 nanoantagomiR by day 3 Bose and Tarafder, (2012); Mencía Castaño et al. (2015)no cytotoxic effectsantagomiR-16Mencía Castaño et al. (2019)
Nano systemsLayered double hydroxide (LDH) nanoparticleselectrostatic interactions with mild orbital agitationproton sponge effectprotective effect of LDH against serum degradationBMSCsclathrin-mediated endocytosis, the buffering capacity facilitates endosomal escape, internalization was increased in 24 hdid not remarkably affect cell proliferation at any concentration testedmiRNA let-7dYang K et al. (2021)
mesoporous silica nanoparticles (MSN)miR222 filled the pores of MSNs, optimal loading capacity: around 6.6 wt% (MSN: miR222 = 15 : 1)disulfide bonds will break up by glutathione (GSH), 15% released on the first day, and 80% at the end of 35 daysdisulfide bonds and amino groups to stabilize miR222, release <3% after 3 days in phosphate-buffered saline (PBS)hBMSCsendocytosnon-toxicmiR-222Lei et al. (2019)
CTH nanoparticle (chitosan solution (CS), Sodium tripolyphosphate (TPP), hyaluronic acid (HA))The loading efficiency was over 90% when the N/P ratio was 15:140–50% at 21 daysmurine BMSCsreached to the greatest transfection efficiency with 2 mM CTH-antagomiR-133a/bdid not impair BMSCs proliferation and exhibited no cytotoxicity in BMSCsantagomiR-133a/bJiang et al. (2020)
(hydroxypropyl) cellulose (HPC)-modified silver nanoparticles (SNPs)a nitrobenzyl photocleavable linker between the 3′ terminal and miR-148b sequence, NP ratio: 5000:1405 nm irradiationrBMSCs56.1 ± 2.2% of cells contained SNP-miR148b-TAMRA 12 h post-transfection, just before illumination. After photo-activation, the percentage decreasedno any major inhibition on the proliferation of rBMSCsmiR-148bMoncal et al. (2019)
dual-sized pore structure calcium-silicon nanospheres (DPNPs)coordination bond formatio between calcium ions in the DPNPs and phosphate in miR-210, miR-210 could be adsorbed on the surface of the mesoporous structure of DPNPsreaches maximum releasing amount after 4 daysthe electrophoretic bands can be detected even after 10 hmBMSCs79.20%no any mass cell deathmiR-210Liu et al. (2021)
PLLA/polyethylene glycol (PEG) co-functionalized mesoporous MSN, poly (lactic acid- co-glycolic acid) microspheres (PLGA MS)electrostatic interactions between an amino-functionalized multi-armed cationic polymer and miR90% by day 50 at 260 nmmice T cellsthe cationic polymer can bind and transfer the miRNA into T cellsno inhibitory effect on T cellsmiR-10aLiu et al. (2018b)
chitosan/tripolyphosphate/hyaluronic acid/antimiRNA-138 nanoparticles (CTH/antimiR-138 NPs)ionic gelation and encapsulation of miR26.8% in the first 2 d, and 52.4% by 21 drMSCsantimiR-138Wu et al. (2018)
chitosan nanoparticleselectrostatic interactions, 82%About 30%, 55% and 65% within 7, 14 and 21 days, respectivelynanoparticle/agomir complexes showed constant expression of miRNA in long-term culturehMSCsChitosan binding to negatively charged cellular membranes can enhance cellular uptakeno significant cytotoxicityagomiR-199a-5pChen X et al. (2015)
electrospun polycaprolactone (PCL) nanofibers>50% in the first 72 h, release sustained up to 14 daysinduced pluripotent stem cells (iPSCs)iPSCs showed an increasing proliferation trendmiR-22 and miR-126Tahmasebi et al. (2020)
bioactive glass nanoparticles (BGNs) silica nanoparticles (SNs) polyethylenimineSNs: physical adsorption, BGNs: a strong surface interaction, BGNs showed a higher miRNA binding amount (∼200 μg/mg) at 40–320 μg/ml nanoparticle concentrations compared to that of SNs (∼20 μg/mg)miRNA was released from BGNs after degradation>82% of the intact miRNA left after 3 h of incubation in 25% FBS, in BGN/miRNA and PEI/LIPO groups, while almost completely degraded in SN/miRNA groupBMSCsBGN group: ∼45%, PEI: 25%, LIPO: 35%live cell attachment: BGNs group: good at 30–240 μg/ml, PEI 25K and LIPO groups: significantly low; cell viability: significantly higher with 30 and 60 μg/ml BGNs after 72 hmiR-5106Yu et al. (2017)
monodispersed BGNs with polyethyleneimine (PEI)a strong binding affinity bwteen the Ca2+ in BGs framework and the phosphates groups in miRNA>80% of the intact miRNA after 24 h nuclease incubation, but only 50% and 35% in BGN and Lipo groupBMSCsBGNCs group: 45.3%, BGNs group: 40.1%, Lipo group: 35.1% (after 48 h)good cells attachment morphology, no dead cells after 1 dmiR-5106Xue et al. (2017)
chitosan/hyaluronic acid nanoparticles (CS/HA NPs)electrostatic interactions, N/P ratio was 20:1release sustained up to 14 dayshBMMSCsa moderate and long-lasting transfection process up to 14 daysno any obvious cytotoxicity after 24 hoursmiR-21Wang et al. (2016)
chitosan (CS)/tripolyphosphate (TPP)/Hyaluronic Acid (HA) nanoparticles (CTH NPs)electrostatic interactions, N/P ratio >20:1losing the tight binding between the gene and the carrier, and prolong circulation time of the delivery system by reducing their non-specific interactions with serum proteinsWith N/P ratio increasing, the antmiR-138 in complexes was subject to less RNase degradationrMSCs40% to nearly 70% with the antmiR-138 concentration from 50 to 150 nMno toxicityantimiR-138Wu G et al. (2016)
polyethylenimine (PEI) bound to magnetic nanoparticles (MNPs)a salt-induced aggregation called “magnetofection"Released the DNA in the perinuclear region due to strong biotin-streptavidin connections of MNPs. Delyagina et al. (2011)Appropriate condensation of miR protects miR from early enzymatic degradationhMSCsmiR/PEI/MNP group (N/P ratio of 2.5): 79%, miR/PEI/CombiMag: 56%, miR/Magnetofectamine: 75%no significant cell mortalitymiRSchade et al. (2014)
photocleavable (PC) silver nanoparticleThe light activated technology links a truncated single stranded miRNA to SNP surface via a PC linkerPC linker release miRNA from the particle by a discrete photo-triggerThe HPC was displaced with negatively charged PC-miR-148b, increasing colloidal stability in aqueous solutionhASCsminimal cytotoxicity (90.08 ± 2.12% viable hASCsmiR-148bQureshi et al. (2013)
gold nanoparticlesmiR can be attached to the gold nanoparticlehMSCsAt 1 h GNPs were mainly at the cell periphery, whilst at 48 h the NPs were within the cell, mainly packaged into endosomesno adverse effectsantagomiR-31McCully et al. (2018)
PEI-capped gold nanoparticles (AuNPs)electrostatic interactionsstable in serum for 6 hhMSCs and MC3T3-E1 cellsAuNPs/Cy3-miR-29b: 54 ± 0.71% and 88 ± 1.42% for hMSCs and MC3T3-E1 cells, lipo/miR-29b: 65.12 ± 1.85% and 80.57 ± 1.77%no significant cytotoxicity, lower toxicity than lipomiR-29bPan et al. (2016)
GNP SNPA stable covalently bound linker, amide bond, conjugated molecular cargo to surfacesphotothermal release at temperatures ≥60°C or at ≈400 nm irradiationthe 2′-O-methyl modified RNA mimics prolonged the lifetime of RNA in serumhASCstoxic cellular response to the 405 nm LED light sourcemiR-148b and miR-21Abu-Laban et al. (2019)
lyophilized mesoporous silica nanoparticles with core-cone structure and coated with polyethylenimine (MSN-CC-PEI)electrostatic attraction, the loading efficiency achieved 60% with 40 μg/ml nanoparticlesproton sponge (proton buffering)rBMSCsconsiderably more cytotoxicity at 40 μg ml−1 PEI coated particles, no significant cytotoxicity in uncoated particles, the optimal exposure conditions for the PEI coated NPs would be less than 24 hrno-miRNA-26a-5pHosseinpour et al. (2021)
MSNs-PEI-KALA peptidemiR-26a is bonded to the MSN surfaceKALA’s membrane disrupting activityno release of miRNA in RNase ArBMSCs∼23% fluorescence intensity at 12 h with 20 μg/ml complexesno significant cytotoxicitymiR-26aYan et al. (2020)
lipidoid nanoparticleselectrostatic interactionsmBMSCsa similar or higher transfection efficiency than Lipofectamine 2000no significant cytotoxicitymiR-335-5pSui et al. (2018)
R9-LK15 nanocomplexeselectrostatic interactionsstable in serum for up to 24 hrBMSCsR9-LK15/miR-29b nanocomplexes: 78.33% ± 5.90, Lipo/miR-29b nanocomplexes: 36.43% ± 1.75no significant cytotoxicity, much less cytotoxic than LipomiR-29bLiu Q et al. (2019)
nanocapsules and O-carboxymethyl chitosan (CMCS) powder mixed gela fast release within the first 20 h (∼50%), ∼10% remained in the coating after 100 hMSCsgood biocompatibilitymiR-21Geng et al. (2020)
N-(3-aminopropyl) methacrylamide, acrylamide and ethylene glycol dimethacrylate nanocapsuleselectrostatic interactions, hydrogen bonding, free-radical polymerization wraps the miRNA molecules with thin shells of network polymer Liu et al. (2015)The crosslinker molecules are degradable in acidic environment (pH 5.4) Liu et al. (2015)no extraction of As-miR-21 by heparin from the nanocapsules, better stability against RNase and serum than lipo/AS-miR-21 Liu et al. (2015)Osteoblast-like MG63 cellstwo and fivefold higher than that of lipo/AS-miR-21 Liu et al. (2015)no any obvious rejection phenomenon 1 month after surgery Geng et al. (2018), low cytotoxicity Liu et al. (2015)miR-21Geng et al. (2018)
CMCS nanocapsuleselectrostatic interactions, hydrogen bonding, free-radical polymerization wraps the miRNA molecules with thin shells of network polymerno extraction from nanocapsules by heparin, compared to the lipo/miR-21 complexhUMSCs61.6% after 48 h (nearly 3.6-fold that of the CMCS/lipo/miR-21 group), 1.6-fold greater at 3 days97.6 ± 9.3% cell viability at a miR-21 concentration of 50 nMmiR-21Meng et al. (2016b)
CMCS coating nanocapsules77.14% cells presented green fluorescence after 4 h of incubation, while 67.56% in lipo2000 group78.28% hUMSCs viability at the nanocapsule concentration of 500 nM, and 93.05% at 50 nMmiR-29bMeng et al. (2016a)
nanocapsulesElectrostatic interaction and hydrogen bonding interaction formed a polymer shell around the miR-21rMSCshigh efficiencyincreased cytotoxicity when concentration >100 nm, metabolic activity was 98.06% at 50 nmmiR-21Sun et al. (2020)
Liposomesaspartate, serine, serine (AspSerSer)6-liposomeosteoblastsmir-133aZhou et al. (2021)
Lipofectamine 3000hASCsmiR-33a-5pShen et al. (2020)
Lipofectamine 2000rBMSCs, hBMSCs, rabbit BMSCsmiR-34a, miR-106a Inhibitor, antimiR-222, antimiR-138, miR-93-5p inhibitorLiu H et al. (2019); Sun et al. (2018); Chang et al. (2018); Yan et al. (2014); Zhang Y et al. (2021)
Lipofectamine RNAi MAX Carthew et al. (2020)Complexes gradually diffused, with full release taking ∼7 days Carthew et al. (2020)MCO cellstransfection efficiency: 97%Carthew et al. (2020)antimiR-467gKureel et al. (2017)
siPORT NeoFX transfection agentproton spongehBMMSCs, mBMMSCs, ADSCsmiR-26a, anti-hsa-miR-221Li et al. (2013); Hoseinzadeh et al. (2016)
X-tremeGENE transfection reagentmBMSCs, mMSCsmiR-590-5p, miR-590–5, miR-15bBalagangadharan et al. (2018); Vishal et al. (2017); Vimalraj et al. (2016)
Other typesinjectable poly (ethylene glycol) (PEG) hydrogelcovalently connected by an ultraviolet (UV) light-cleavable linkerThe release rate was ∼70% upon 365 nm UV irradiation for 5 minno miR-26a release in the absence of UV irradiationhMSCsThe internalization efficiency was about 60% in 1 dayThe biocompatibility of the gel are suitable for the surface-cultured cellsmiR-26aGan et al. (2021)
poly (ethylene glycol) (PEG) hydrogelshMSCsmiR-20aNguyen et al. (2018)
a comb-shaped polycation (HA-SS-PGEA) consisting of Hyaluronic acid (HA), disulfide groups and ethanolamine (EA)-functionalized poly (glycidyl methacrylate) (PGMA)electrostatic interactionsDL-dithiothreitol (DTT) induced disulfide bond cleavage; Burst release in first 3 days, and sustained release for 1 monthThe HA-SS-PGEA completely retarded miRNA migration at the N/P ratio of 1 and enabled NPs stable to resist anionic macromolecules to prevent premature failure of miRNA.BMSCsThe ability of HA-SS-PGEA to transport miRNA through cellular membrane is stronger than PEI and PGEA.The cell viability reached above 90%miR-26aLi et al. (2020)
Bio-inspired bioactive glassesThe miR-7–FAM labelling efficiency >90% when the PBGs–NH2/microRNA ratio was 40 or 80human Hela cell line>90% after 4 h incubationThe cells were not in a good state and many of them became irregular when ≥100 μg ml−1, although the positive expression rate was still >95%miR-7Li X et al. (2017)
zinc(II) quercetin complexes (Zn + Q (PHt)) (BMC)MG-63 cellsno declined cytocompatibility till >60 μMpre-miR-15bRaj Preeth et al. (2021)
Ascorbic Acid-PEI Carbon Dots (CD)electrostatic interactions, miR-2861 fully complexed with the CD at a 4:1 weight ratio of the CD and miRBMSCsCellular uptake had started before 5 min and exponentially boosted with time before 4 h and then became slow and plateau phases/saturation; transfection efficiency: 47.44%With the increase of PEI concentration, the viability of BMSCs reduced remarkably, only 35.00% at 50 μg/ml; no significant cytotoxicity of CDmiR-2861Bu et al. (2020)
PEIhMSCsendocytosis, transfection efficiency: 79% (4 kDa PEI) and 77% (40 kDa PEI)no significant cytotoxicitymiR-100-5p, miR-143-3p, miR-20aCarthew et al. (2020); Huynh et al. (2016); Nguyen et al. (2014)
PEI-functionalized graphene oxide (GO) complexelectrostatic interactions; At the N/P ratio of 30 (Figure 3D), GPM complex would wrap miR-inhibitor inside and prevent it being degradedRNase A have no influence on the GFP RNA delivered into cellsmouse osteoblastic cells (MC3T3-E1)cell uptake and transfection efficacy for 24 h greater than naked miR-inhibitor or Lipofectamine 2000 groupsno significant toxicitymiR-214 inhibitorOu et al. (2019)
silk-based orthopedic devices, Xfect RNA Transfection Reagentelectrostatic interactionsinitial release of ∼20% in the first 24 h, the continued release of an additional 42% in the next 48 h, an additional 36% after 168 hhMSCsCells are typically transfected within 24 h and have bioactivity up to 2 or 3 weekssignificantly higher cell viability after 72 h and 7 days, no clear difference in morphology, no apoptoticantimiR-214James et al. (2019)
Low molecular weight protamine (LMWP)electrostatic interactionsno degradation in serum for up to 24 hhMSCs6.5-fold transfection efficacy than the cationic lipids in 5 hno significant change in the viability of hMSCs, mild toxicity (around 14%) in liposomal groupmiR-29bSuh et al. (2013)
Electrospinning of gelatinThe miRNA was loaded uniformly throughout the fibersan initial burst followed by sustained release for up to 72 hMC3T3-E1 osteoblast-like cellsno significant differences in cell viability after 24 hmiR-29a inhibitorJames et al. (2014)

The information of miRNA transfection vectors applied to bone tissue engineering.

To our knowledge, this article is the first review to summary and compare the characteristics of different types of nonviral miRNA-loaded biomaterials to provide examples of their application in in vitro or in vivo experiments in the future. In addition, the review confirms that nonviral miRNA vector materials can stably transmit miRNA. We also discuss the application prospects of miRNAs in bone tissue engineering.

2 Application of Different Nonviral Vectors

2.1 Calcium Phosphates as Nonviral Vectors

Calcium phosphate (CAP) has long been used as a nonviral gene delivery vector. Calcium phosphate precipitates oligonucleotides on cells, and the precipitate is adsorbed on the cell membrane. Cells take up oligonucleotides along with the natural calcium uptake (Ruedel and Bosserhoff, 2012). Among all CAP materials, hydroxyapatite (HA) remains the most frequently used CAP to date. In addition, amorphous calcium phosphate, beta tricalcium phosphate, and dicalcium phosphate dihydrate are promising (Levingstone et al., 2020). Calcium phosphate nanoparticles present good binding affinity for RNA molecules; CAP nanoparticles have good osteoinduction, osteoconduction (Levingstone et al., 2020), biocompatibility, and biodegradability, and they are nontoxic and nonimmunogenic (Bakan, 2018). Compared to cationic lipids, CAP nanoparticles show improved cytocompatibility, and spherical CAP nanoparticles increase osteoblast proliferation and osteogenic gene expression. However, the disadvantage of CAP nanoparticles is that the transfection efficiency of CAP is lower than that of viral vectors. Currently, surface functionalization, such as functionalization with cationic polymers, natural polymers, cell-penetrating peptides, biodegradable lipids, and polyethyleneimine/poly(ethylene)glycol, has been shown to improve cellular uptake and increase transfection efficiency (Levingstone et al., 2020). In addition, calcium phosphate material itself also promotes osteogenesis. Ca2+ and PO43- play an important role in regulating bone resorption and bone deposition. Ca2+ induces the chemotaxis of monocytes, osteoblasts, and hematopoietic stem cells to the injury site, and it induces osteoblast proliferation, osteoblast differentiation, and osteogenic gene expression. PO43- participates in the proliferation and differentiation of osteoblasts by entering the mitochondria and stimulating the production of adenosine triphosphate (ATP), which is converted to adenosine and promotes osteogenesis (Levingstone et al., 2020).

Irene et al. combined nanohydroxyapatite (nHA) particles with collagen-nHA scaffolds to deliver antagomiR-16 to human bone mesenchymal stem cells (hMSCs). The levels of Runx2 (the key transcription factor for osteogenesis) and osteocalcin as well as the mineral calcium deposition of hMSCs were significantly increased, indicating the bone repair potential of the combination (Mencía Castaño et al., 2019) (Figure 3). Castaño et al. implanted collagen-nanohydroxyapatite (coll-nHA) scaffolds without cells into calvarial defects of rats to deliver antagomir-133a; 1 week after implantation, antagomir-133a began to be released at the implantation site, and the bone repair volume was ten times that in the negative control group after 4 weeks, indicating that the platform accelerates bone repair in vivo without the participation of exogenous cells (Castaño et al., 2020). This system did not inoculate cells before implantation because recent studies have emphasized that adding cells to scaffolds is a limiting factor in the field of tissue engineering (Zhang et al., 2016). Coll-nHA has been demonstrated to be both a nonviral vector and a scaffold (Curtin et al., 2015).

FIGURE 3

FIGURE 3

The mechanism of calcium phosphates vectors transfecting antagomiR-16. (A) AntagomiR-16 binds to nHA particles through electrostatic interaction between Ca2+ in CaP vector and phosphate groups in miRNA structure. The complex are found in multiparticulate formations. (B) nHA-antagomiR-16 particles pass through lipid bilayer cell membranes along endocytosis. (C) AntagomiR-16 undergoes endosomal escape before the fusion of endosome with lysosome. (D) AntagomiR specifically complement to their mature target miRNA, inducing the repression of miRNA, preventing translation repression or Smad5 and AcvR2a mRNA degradation via RISC.

2.2 Nanosystem as a Nonviral Vector

Nanotransfection materials mainly include nanoparticles and nanocapsules. Nanoparticles are structures with a size (1–100 nm) similar to that of biomolecules (protein, DNA, and RNA) (Rahim et al., 2018; Chiang et al., 2021). Nanoparticles covalently bind biomaterials, and the physicochemical properties of a variety of biomedical applications are met through surface modification (Kashapov et al., 2021). The types of nanoparticles commonly used for miRNA delivery applications include organic nanoparticles, such as lipid nanoparticles, as well as inorganic nanoparticles, such as metal nanoparticles and silica nanoparticles. Despite this classification, many inorganic and organic composite systems have been developed to achieve synergy (Pan et al., 2016; Kashapov et al., 2021). Because miRNAs are negatively charged, the nanoparticles are adjusted to be neutral or slightly negatively charged (Chiang et al., 2021; Kashapov et al., 2021).

Gold nanoparticles (GNPs) and silver nanoparticles (SNPs) are commonly used metal nanoparticles. In a previous study, researchers encapsulated antagomiR-31 with GNPs and delivered them to preosteoblastic and primary human mesenchymal stem cells (hMSCs) in vitro, which resulted in increased osterix protein and osteocalcin in the 2 cell types, indicating cell osteogenesis (McCully et al., 2018). Pan et al. combined polyethyleneimine (PEI)-capped gold nanoparticles with miR-29b, which effectively entered hMSCs and mouse embryonic osteoblasts (MC3T3-E1 cells), promoting the expression of alkaline phosphatase (ALP), the early-stage osteogenic gene, and medium-stage marker Runx2 as well as the expression of OCN and OPN, the late-stage osteogenic differentiation markers. Moreover, the combination showed negligible cytotoxicity (Pan et al., 2016). Moncal et al. synthesized (hydroxypropyl) cellulose (HPC)-modified SNPs and functionalized them with photolytic miR-148b. miR-148b mimics were photoactivated at wavelengths ranging from 350 to 450 nm, causing them to be released from the surface of SNPs. The proliferation rate of rat bone marrow-derived mesenchymal stem cells (rBMSCs) transfected with miR-148b was significantly higher than that of the control group. Calvarial defects in rats were almost completely repaired (Moncal et al., 2019) (Figure 4). Abu-Laban et al. cotransfected human adipose stem cells (hASCs) with SNP-miR-21 and GNP-miR-148b and activated the constructs at wavelengths of 405 and 503 nm; they reported that the degree of cell mineralization in the cotransfection group was higher than that in the group treated with one particle alone (Abu-Laban et al., 2019).

FIGURE 4

FIGURE 4

The procession of silver nanoparticles (SNPs) with a nitrobenzyl photocleavable group transfecting miR-148b into rBMSCs.

There are many other inorganic nanoparticles as well as the calcium phosphate nanoparticles mentioned above. Through appropriate synthesis and functionalization technology, inorganic nanoparticles show unique optical, magnetic, and electrical properties as well as strong loading capacity, mechanical stability, controllable size, and controllable porosity (Yang K. et al., 2021; Kashapov et al., 2021). Mesoporous silica nanoparticles (MSNs), for instance, have many favorable properties, such as low toxicity, ideal degradability, flexible design, tunable size, and high porosity. Hosseinpour et al. and Yan et al. loaded the miR-26a simulant into MSNs and delivered the complex to rBMSCs. miRNA stably bonded to the surface of nanoparticles via electrostatic attractions, and the carrier protected the miRNA from degradation by RNase A. The complex significantly enhanced osteogenic differentiation and extracellular matrix deposition and mineralization (Yan et al., 2020; Hosseinpour et al., 2021). Liu et al. constructed silica nanoparticles containing poly(lactic acid-coglycolic acid) (PLGA) microspheres (MSs). PLGA MSs release miR-10a, locally recruit T cells, and stimulate them to differentiate into Treg cells, mediating immunotherapy against bone loss in a mouse periodontitis model (Liu et al., 2018b). In addition, nanoparticles made of pinecone-like bioactive glasses have shown excellent apatite mineralization properties. After chemical modification of miR-7 to produce a miR-7-FAM complex, both the miRNA-loading efficiency and cell transfection efficiency of the complex are greater than 90% (Li X. et al., 2017).

The common organic material is chitosan, which has good aqueous solubility, good biocompatibility, controllable biodegradability, and strong bioactivity. Jiang et al. used CTH nanoparticles (chitosan solution, CS; sodium tripolyphosphate, TPP; and hyaluronic acid, HA) to transfect antagomiR-133a/b into murine BMSCs. The loading efficiency was over 90% when the N/P ratio was 15:1 and exhibited no cytotoxicity in BMSCs (Jiang et al., 2020). Chen et al. reported that approximately 30%, 55%, and 65% of agomiR-199a-5p was released within 7, 14, and 21 days, respectively, from chitosan nanoparticles, indicating that this transfection agent could continuously release miRNA in long-term culture (Chen X. et al., 2015). Another common organic material is polyethyleneimine (PEI), which is often combined with bioactive glass nanoparticles (BGNs) or MSNs. Xue et al. used monodispersed BGNs with PEI to transfect miR-5106 into BMSCs. The complex effectively protected miRNA from degradation, and more than 80% of the intact miRNA existed after 24 h of nuclease incubation compared to 35% in the Lipo group (Xue et al., 2017).

The formation of nanocapsules begins with the enrichment of monomers and crosslinkers around miRNA molecules. Monomers and crosslinkers form polymer shells around miRNA molecules through in situ polymerization, thereby forming nanocapsules (Meng et al., 2016b). Meng et al. used O-carboxymethyl chitosan (CMCS) to encapsulate miRNA-21 mimics. CMCS provides protection for the miRNA from heparin and improves transfection efficiency to 61.6% after 48 h and by 1.6-fold at 3 days, significantly promoting the osteogenic differentiation of human umbilical cord mesenchymal stem cells (HUMSCs) and bone formation (Meng et al., 2016b). Sun et al. first designed a metalloproteinase-sensitive nanocapsule, which was bound to the surface of miRNA-21 through in situ free radical polymerization; they mixed the miR-21/nanocapsule with CMCS until a gel material with good fluidity and injection was formed, and they applied the material to a rat fracture model to promote bone repair (Sun et al., 2020). Geng et al. used N-(3-aminopropyl) methylacrylamide, acrylamide, and ethylene glycol dimethacrylate nanocapsules to encapsulate miR-21 and delivered it to osteoblast-like MG63 cells, promoting the formation and mineralization of new bone (Geng et al., 2018). The nanocapsules showed more than twice the transfection capacity of commercial Lipofectamine transfectants (Liu et al., 2015).

2.3 Liposomes as Nonviral Vectors

Lipofection is an effective method to transfect miRNA into cells (Ruedel and Bosserhoff, 2012). In lipid transfection, microvesicular liposomes are formed by cationic lipids. The hydrophilic head of cationic lipids can condense with nucleic acids, and the hydrophobic tail of cationic lipids can form micelles or lipid bilayer structures arranged in spherical shell shapes to wrap the cargo in the middle (Goodwin and Huang, 2014; Carter and Shieh, 2015). When the liposome collides or attaches with the cell membrane, it fuses with the cell membrane or undergoes endocytosis to release the cargo (Carter and Shieh, 2015; Hori, 2019). A large number of commercial liposome transfection agents have been used in bone tissue engineering. It is convenient to use liposomes to study the effect of miRNA or scaffolds on osteogenesis because liposomes have good biocompatibility, and researchers can easily functionalize the surface of liposomes to carry certain targeted ligands for cell recruitment or to anchor to scaffolds (Kang et al., 2021; Scheideler et al., 2020). In addition, because of the clear transfection effect of liposomes, they are now the control group in many studies of nonviral vectors. Lipofectamine RNAi Max transfection agent has become the gold standard of miRNA nonviral vectors, and its cell transfection efficiency has reached 97% (Carthew et al., 2020). Meng et al. used the CMCS/Lipofectamine 2000 complex as the positive control of CMCS powder to deliver miR-21 to hUMSCs. The results showed that the delivery efficiency of CMCS/n (miR-21) (61.6%) was approximately 3.6 times that of the positive control group (17.2%) (Meng et al., 2016b). Other scholars have used Lipofectamine 2000 as a control to verify that there is no significant cytotoxicity of R9-LK15 nanocomplexes to cells, which is lower than lipo (Liu Q. et al., 2019). In addition, lipid vectors also include stable nuclear acid lipid particles (SNALPs), solid lipid nanoparticles (SLNs), and pH-responsive lipids. SLNs are defined as colloidal drug carriers that are 50 nm to 1 μm in diameter (Mishra and Singh, 2020). SLNs with cations condense with anionic miRNA through electrostatic interactions to form SLN/miRNA complexes (Liu et al., 2016). The other two are mainly used in delivering siRNA and drugs. At present, there is no case of delivering miRNA in bone tissue engineering using these three vectors.

2.4 Nucleic Acids as Nonviral Vectors

Transporters based on nucleic acid structures have natural advantages, including high precision brought by Watson–Crick base pairing, structural predictability, good biocompatibility, simple manufacture, and high yield (Tian et al., 2020). Li et al. made sticky-end tetrahedral framework nucleic acids (stFNAs) that carry double-stranded miRNA, including a guide chain and a passenger chain with sticky ends. miRNA and stFNAs are combined by base complementary pairing. When the complex enters the BMSCs through the cell membrane, RNase H cuts the complex to unload miRNA (Li S. et al., 2021) (Figure 5). Li et al. used tetrahedral DNA nanostructures (TDNs) to carry miR-335-5p (Li et al., 2022). TDNs have been demonstrated to have high mechanical stiffness, high stability and rich functional modification sites as well as to be able to carry siRNA, CGP, and miRNA (Zhang et al., 2018; Li et al., 2022).

FIGURE 5

FIGURE 5

StFNA-miRs promote the healing of mice femoral defects. (A) StFNA-miRs are synthesized at room temperature. (B) The drugs are injected locally around the operation area of 1 mm diameter spherical femoral defect in mice. (C) StFNA-miRs pass through cell membranes along a caveolin-mediated pathway. (D) Double-stranded miR and stFNA form stFNA–miRs through sticky ends complementary pairing. The structures contain the binding site of RNase (H) (E) RNase H degrades the miR strands of heterozygous RNA/DNA molecules. (F) The strands that is less thermodynamically stable at the 5′ end are actively chose as the guide strands and the passenger strands undergo degradation. (G) RNA-induced silencing complex binds to target histone deacetylase 5 (HDAC5)mRNA and the translation is inhibited. (H) The expression of the runt-related transcription factor 2 (Runx2) protein is upregulated and the osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) is ultimately promoted. (I) There is obvious bone regeneration in the bone defects of mice.

2.5 Other Types of Materials as Nonviral Vectors

Other biomaterials that can be applied for miRNA delivery include silk-based biomaterials, cell-penetrating peptides, and PEI. Silk-based bioactive materials have attracted much attention due to their good orthopedic repair ability (James et al., 2019). James et al. delivered antisense miRNA-214 (AS-miR-214), which inhibited the endogenous expression of osteoinductive antagonists via a silk-based orthopedic device. This device released miRNA continuously for 7 days, promoting osteogenic gene expression and increasing ALP levels and calcium deposition of hMSCs (James et al., 2019).

Suh et al. used a cell-penetrating peptide rich in arginine, called low molecular weight protamine (LMWP), whose transfection efficacy increased 6.5-fold that of the cationic lipids in 5 h. The LMW1/miR-29b complex enhanced the expression of ALP, OCN, OPN, and Runx2 as well as induced the differentiation of hMSCs into osteoblasts (Suh et al., 2013). Additionally, there is a new cell-penetrating peptide called R9-LK15. R9-LK15/miRNA-29b nanocomplexes maintained the stability of miR-29b in serum for up to 24 h. Moreover, the efficiency of R9-LK15 in delivering miR-29b to BMSCs was 10 times higher than that of Lipofectamine 2000. The complex promoted osteogenic differentiation and extracellular matrix mineralization of BMSCs by upregulating the expression of ALP and downregulating the expression of histone deacetylase-4 (Liu Q. et al., 2019).

PEI is also a common transfectant. PEI is a positively charged cationic polymer that combines with negatively charged miRNA through electrostatic interactions, and it promotes miRNA escape from lysosomes through the “proton sponge” effect to avoid degradation (Carthew et al., 2020; Hosseinpour et al., 2021). However, the cytotoxicity of PEI is its disadvantage. Therefore, constructing low toxicity PEI-based transport materials is a research direction (Bu et al., 2020). Lim et al. constructed ascorbic acid-PEI carbon dots (CDs) by taking advantage of the characteristics of low toxicity, high biocompatibility and chemical inertia of carbon dots, and they reported that the transfection efficiency of miR-2861 into BMSCs was 47.44% (Lim et al., 2015; Bu et al., 2020) (Figure 6). Ou et al. constructed a PEI-functionalized graphene oxide (GO) complex to transfect miR-214 inhibitor into mouse osteoblastic cells (MC3T3-E1), which did not show significant cytotoxicity (Ou et al., 2019).

FIGURE 6

FIGURE 6

The procession of ascorbic acid-PEI carbon dots (CD) transfecting miR-2861 into BMSCs.

Information on the transfection vectors, including loading mode, and efficiency as well as release mode, miRNA stability, biocompatibility, and cytotoxicity (Arriaga et al., 2019), is summarized in Table 3.

In addition, some materials that have not yet been used in bone tissue engineering can deliver miRNA, such as carbon nanotubes. Carbon nanotubes (CNTs) are nanomaterials whose advantages include high surface-to-volume ratios, needle-like structures, high strength, high stability, good biocompatibility, flexible interactions with carrying materials, high drug-loading capacity and ability to release drugs at specific targets. However, the disadvantages include a lack of biodegradability and toxicity (Zare et al., 2021). Andrea et al. used coated low toxicity carbon nanotubes to deliver miR-503 to mouse endothelial cells, which not only improved the stability of miR-503 but also promoted angiogenesis in vivo (Masotti et al., 2016). These studies provide ideas for the formation of vascularized bone in bone defects.

3 Other Types of Methods to Transfect miRNA Into Cells

Exosomes are lipid structural vesicles with a diameter of 50–100 nm formed by stem cells. Exosomes deliver bioactive proteins, lipids, and RNA to target cells for intercellular communication, and they have characteristics of high compatibility, low toxicity, and low immune stimulation (Chiang et al., 2021; Nan et al., 2021). Exosomes secreted by osteoblasts inhibit the differentiation of osteoclast progenitor cells (Wang Q. et al., 2021). Exosomes derived from M2 macrophages (M2D-Exos) inhibit adipogenesis and promote osteogenesis of BMSCs (Li Z. et al., 2021). Therefore, Nan et al. constructed exosomes from miRNA-378-transfected adipose-derived stem cells (ASCs) and cocultured the extracted exosomes with BMSCs and human umbilical vein endothelial cells (HUVECs). The results showed that miR-378-ASCs-Exos promoted the osteogenic differentiation of BMSCs and improved the angiogenesis of HUVECs in vitro, and they enhanced neovascularization and osteogenesis in vivo in a glucocorticoid (GC)-induced osteonecrosis of the femoral head (ONFH) rat model (Nan et al., 2021). Peng et al. extracted exosomes produced by BMSCs transfected with a miR-196a simulant or inhibitor and cocultured them with HFOB1.19 osteoblasts. The results showed that the BMSC-Exos entered HFOB1.19 cells and that exosomes overexpressing miR-196a promoted osteogenic differentiation and inhibited HFOB1.19 cell apoptosis (Peng et al., 2021). However, at present, the loading and excretion mechanism of exosomes is still unclear, and the cargo is often retained in exosomes (Chiang et al., 2021).

In addition, there are some carrier-free delivery methods. Lee et al. used a microbubble-ultrasound system. These researchers first constructed the femoral fracture in mice, and miR-29b-3p was injected through the tail vein. The probe was then immediately placed on the skin of the fracture site, and the site was irradiated with ultrasound for 4 min, which allowed the miRNA to be delivered to highly vascularized callus (Lee et al., 2016). Tu et al. injected agomiR-142-5p directly at the periosteal of the fracture site once a week for 4 weeks (Tu et al., 2017). Qin et al. isolated an extracellular vesicle (EV) delivery system from BMSCs, and the resulting EVs were rich in miR-196a, miR-27a, and miR-206. The results showed that after the EVs entered osteoblasts through endocytosis, they existed in the endoplasmic reticulum, Golgi apparatus, and lysosomes, releasing cargo along the way (Qin et al., 2016). However, the transfection efficiency and the tissue toxicity of the methods were not shown in the above studies.

4 Evaluation of Nonviral Vectors

4.1 The Physicochemical and Biological Properties of Nonviral miRNA-Transfected Biomaterials

Various physicochemical and biological properties of nonviral miRNA-transfected biomaterials deserve attention, which are necessary to promote osteogenic gene expression and osteogenic differentiation of target cells.

Most studies have reported the zeta potential of carrier materials. Zeta potential is the potential difference between the mobile dispersion medium and the fluid stationary layer attached to the dispersed particles, which can be directly measured by electromotive phenomena (Lu and Gao, 2010). Zeta potential is generally used to evaluate or predict the physical stability of particle dispersion systems (Ding et al., 2018). Generally, the higher the absolute value of zeta potential, the greater the electrostatic repulsion between particles, that is, dissolution or dispersion can resist aggregation, and the better the physical stability (Lu and Gao, 2010). It is generally believed that the zeta potential value of an electrostatically stable suspension should reach at least ±30 mV. On the other hand, low values, less than 5 mV, can cause agglomeration (Gumustas et al., 2017). In the experiment of Professor Liu (Liu et al., 2018b), a multibiological delivery vector was created to encapsulate miR-10a, which contained poly(l-lactic acid) (PLLA)/polyethylene glycol (PEG) co-functionalized mesoporous silica nanoparticles (MSN), and poly(lactic acid-co-glycolic acid) microspheres (PLGA MS). However, the zeta potential of the complex was 12 mV. Pan et al. created a PEI-capped gold nanoparticle to deliver miR-29b, and the zeta potential was +9.34 mV when the optimal w/w ratio between nanoparticles and miR-29b was selected as 3 (Pan et al., 2016). The stability of the above system deserves attention.

In terms of loading modes, nucleic acid materials combine with miRNA mainly through complementary base pairing (Li S. et al., 2021). The combination mode of calcium phosphates is electrostatic interaction (Mencía Castaño et al., 2019). Nanoparticles load miRNA through chemical bond, including coordination bond (Liu et al., 2021) and covalent bond (Abu-Laban et al., 2019), or intermolecular force, including electrostatic interaction (Yang L. et al., 2021), physical adsorption (Yu et al., 2017), or light-activated connection (Qureshi et al., 2013; Moncal et al., 2019). Nanocapsules encapsulate miRNA through electrostatic interaction, hydrogen bonding, or a network polymer formed by free radical polymerization (Liu et al., 2015; Meng et al., 2016b; Geng et al., 2018). Other types of nonviral vehicles bind miRNA mainly by electrostatic interaction, such as ascorbic Acid-PEI Carbon Dots (CD) (Bu et al., 2020), PEI-functionalized graphene oxide (GO) complex (Ou et al., 2019), silk-based orthopedic devices (James et al., 2019), low molecular weight protamine (LMWP) (Suh et al., 2013), and comb-shaped polycation (HA-SS-PGEA) consisting of hyaluronic acid (HA), disulfide groups, and ethanolamine (EA)-functionalized poly(glycidyl methacrylate) (PGMA) (Li et al., 2020).

The way for virus vectors to enter cells is gathering on the cell surface through adhesion factors, and then endocytosis is started by the real signal protein, invasion receptor (Mercer et al., 2020). Similarly, nonviral vectors carrying miRNAs enter cells mainly through endocytosis. Receptor-mediated endocytosis is currently recognized as a main way for organisms to ingest biological macromolecules. Li et al. created a unique tetrahedral DNA framework structure so that it can smoothly pass through cell membranes along caveolin-mediated endocytosis (Li S. et al., 2021). Another common endocytosis is mediated by clathrin (Bu et al., 2020). It has been reported that the pits of clathrin coating cover 2% of the plasma membrane. Because their life span is about 1 min on average, about 2% of the cell surface membrane is internalized every minute (Donaldson, 2013). It is reported that positively charged nanoparticles are generally considered to be able to electrostatically combine with anionic cell membranes to produce positive endocytosis and improve transfection efficiency (Lei et al., 2019; Yang L. et al., 2021).

For the unloading mode, nucleic acid vectors separated from miRNA by the cut of RNase H (Li S. et al., 2021). Calcium phosphates dissolve in the acidic environment of endocytic vesicles to release miRNA (Mencía Castaño et al., 2019). For nanoparticles, there are many ways to release miRNA. For some cationic nanoparticles, such as PEI nanoparticles, they first efficiently escape from the endosomes mediated by the proton sponge effect into the cytoplasm. PEI contain different types of amino groups, and their pKa values span the physiological pH range, resulting in a buffer capacity. When miR-PEI complexes are encapsulated in the membrane invagination to form endosomes, the environmental pH is in normal physiological range, so the PEI nanoparticles are inactive. However, when the endosomes are combined with lysosomes, the pH value decreases. The unsaturated amino groups on the particles chelate the protons captured by a vacuolar-type H+-ATPase (V-ATPase) proton pump, which cause lysosomes capturing a large number of protons, and Cl and water molecules influx. Cl and water molecules cause retention in lysosomes, causing lysosomes swelling and rupture, and the release of particles (Omote and Moriyama, 2013; Bu et al., 2020; Yang L. et al., 2021; Wang et al., 2022). When the complex is released into the cytoplasm, glutathione (GSH) can break down disulfide bonds (Lei et al., 2019), irradiation or discrete photo-trigger can release miRNA from the light-activated link (Qureshi et al., 2013; Moncal et al., 2019), photothermal release at temperature no less than 60°C or the irradiation that is about 400 nm causes the decomposition of covalent bonds (Abu-Laban et al., 2019). For nanocapsules, the acid environment decomposes the electrostatic interactions, hydrogen bonds, and the wrapping of free radical polymerization nets between the carrier material and miRNA (Liu et al., 2015).

In terms of stability, all types of carriers performed well. PBS (Lei et al., 2019), FBS (Yu et al., 2017), serum (Yang L. et al., 2021), heparin (Liu et al., 2015; Meng et al., 2016b), and nuclease (Xue et al., 2017) including RNase A (Li S. et al., 2021)are commonly used materials to detect the ability of the vectors protecting miRNA. Generally, the carriers and miRNA complex is cultured with one of the above materials for 24 h, and then the integrity of miRNA is tested to verify the ability of the vectors to protect miRNA from degradation.

4.2 Comparison of Various Nonviral miRNA-Transfected Biomaterials

For the selection of different types of miRNA vectors, many studies have utilized commercial lipid products because they have clear transfection effects; therefore, commercial lipid products are often the control group in the research of other miRNA vectors (Carthew et al., 2020). Moreover, commercial lipid products still have low toxicity to cells (Liu Q. et al., 2019).

In contrast, nucleic acid transporters have better biocompatibility. Lipofectamine 2000 was obviously toxic to BMSCs when carrying 500 nm miR, while stFNA changed cell viability only when carrying 4 times the amount of miR carried by Lipofectamine 2000 (Li S. et al., 2021). However, the transfection efficiency of nucleic acid transporters may be lower than that of other types of vectors (Li et al., 2022).

Similarly, The disadvantage of calcium phosphates is that the delivery efficiency is lower than that of Lipofectamine 2000, PEG or PEI, but calcium phosphates are low-toxicity, biodegradable and easy to use (Mencía Castaño et al., 2015).

In addition, nanoparticles and nanocapsules are currently the most studied carriers. Nanomaterials are mostly connected to miRNA through electrostatic interactions. Nanoparticles include inorganic nanoparticles and organic nanoparticles. Inorganic nanoparticles are relatively smaller. For example, the volumes of silica nanoparticles (SNs) and bioactive glass nanoparticles (BGNs) are less than one hundredth of the volume of nanohydroxyapatite (nHA) particles or Lipofectamine RNAi max (Kureel et al., 2017; Yu et al., 2017; Mencía Castaño et al., 2019). However, the release time of organic particles is relatively long, ranging from 100 h to 50 days (Chen X. et al., 2015; Wang et al., 2016; Liu et al., 2018b; Wu et al., 2018; Geng et al., 2020; Jiang et al., 2020). At the same time, nanoparticles well protect miRNA from the degradation of nuclease, serum, and heparin (Meng et al., 2016b; Yan et al., 2020; Yang L. et al., 2021). However, the transfection efficiencies of various types of nanoparticles are quite different, but the efficiencies may also be related to the different types of transfected cells and the different types of miRNA (Liu et al., 2015; Wu G. et al., 2016; Pan et al., 2016; Xue et al., 2017; Yu et al., 2017; Abu-Laban et al., 2019; Liu Q. et al., 2019).

At present, the transfection efficiencies of bioactive glass and Lipofectamine RNAi Max are the highest, more than 90% (Carthew et al., 2020). However, studies have shown that bioactive glass has cytotoxicity. When the concentration of bioactive glass was greater than 100 μg/ml, the cell morphology became irregular, and the live cell attachment was not good with concentrations greater than 240 μg/ml (Li H et al., 2017; Yu et al., 2017).

In addition, the “indirect” transfection method of extracting exosomes from miRNA-transfected cells and coculturing them with target cells seems to reduce cytotoxicity, which is an interesting new idea.

4.3 The Superiority and Insufficiency of Nonviral miRNA-Transfected Biomaterials

The performance of a carrier can be comprehensively evaluated from the aspects of load efficiency, release efficiency, cell uptake rate, biocompatibility, stability, biological immunity, manufacturing difficulty, cost, osteogenesis time, and in vivo experimental osteogenic effect. It is certain that the materials with high loading and unloading efficiency, high transfection efficiency, favorable stability, and low toxicity are the most ideal. These properties will allow miRNA to fully promote bone regeneration.

Compared to viral materials, nonviral materials have unique advantages, including low toxicity, low immunogenicity, good stability, high loading capacity, flexible design, controllable biodegradability, and relatively simple production and construction processes, and they lack the insertion mutation risk brought by viral vectors. In addition, nonviral materials are less likely to cause local acute reactions, thus allowing repeated administrations (Al-Dosari and Gao, 2009). Most importantly, various studies have shown that the application of nonviral miRNA delivery materials stably and efficiently deliver miRNAs, significantly enhancing the expression levels of osteogenic genes in target cells, the activity of osteogenic-related enzymes, the differentiation of stem cells into osteoblasts, and the deposition of calcifications, ultimately promoting osteogenesis. Some miR delivery material complexes also enhance nerve and vascular regeneration to assist bone regeneration.

However, there are still some insufficiency of nonviral miRNA-transfected biomaterials. To date, the transfection efficiency of nonviral biological delivery materials is still generally lower than that of viral vectors (Jiao et al., 2020). The reason may be that the viral vectors have adhesion factors and specific invasion receptors. The former can make the vectors gather on the cell surface, and the latter can promote endocytosis and improve the efficiency of the vectors entering the cell. Hence, it is needy to enhance the surface specificity of nonviral vectors through physical and chemical methods to increase the efficiency of cellular uptake. Second, some materials have slight toxicity to cells, tissues, and organs. It is important to reduce or even eliminate their toxicity and further improve histocompatibility. Third, the types of carriers, the types of miRNA, the types of stem cells, material concentrations, and the animal models used in different experiments are quite different, and the observation time reported in different literatures is also different. Quantitative comparison in the loading efficiency and the cell transfection efficiency between different materials is absent. Only a few experiments use commercial lipofectamine transfectants as the control group for comparison. More experiments that control the variables of miRNA, cell types, and animal models are needed to detect the transfection ability of different transfection materials. Fourth, few literatures report the reason why they chose the certain kind of vector. Different kinds of miRNA and stem cells may have their own suitable transfection materials to meet the best their unique properties, but unfortunately, there is no related discussion in the literatures. Finally, there are still some new and promising biological materials that can deliver miRNA but have not been sufficiently applied to bone tissue engineering, such as carbon nanotubes and exosomes. These new materials can be tested more for their miRNA transfection efficiency and bone regeneration effects in vivo and in vitro to develop new or compound carriers to improve the safety, efficiency, and targeting of miRNA delivery materials in bone tissue engineering.

5 Conclusion

With the development of bone tissue regeneration engineering, researchers have gradually realized that miRNAs play an important role in bone regeneration. Research on miRNA-loaded biomaterials is of great significance in bone tissue regeneration engineering because miRNA delivery materials protect miRNAs from degradation in the release process, extending the release time and making the process more controllable, stable, and efficient. Among existing delivery biomaterials, nonviral miRNA delivery materials are increasingly used in bone tissue regeneration engineering because they overcome the shortcomings of viral materials.

In short, we reviewed the properties of miRNA-transfected materials used in different bone regeneration engineering studies, including calcium phosphates, nanosystems, liposomes, nucleic acids, silk-based biomaterials, cell-penetrating peptides, bioactive glass, and PEI (Figure 7).

FIGURE 7

FIGURE 7

Nonviral biomaterials for microRNA transfection in bone tissue engineering.

For the existing defects of nonviral vectors, such as relatively low transfection efficiency and the lack of quantitative comparison, future research should focus on overcoming these problems, developing new or compound carriers, improving the safety, specificity, and the transfection efficiency of the materials through physical or chemical methods, so that miRNA transfection vectors can be better used in bone tissue engineering.

Statements

Author contributions

Conceptualization, NZ, XX, and YB; writing—original draft preparation, MZ, and YG; writing—review and editing, YG and CB; supervision, NZ; funding acquisition, NZ.

Funding

This study was supported by Beijing Municipal Education Commission (KM202010025012), Innovation Research Team Project of Beijing Stomatological Hospital, Capital Medical University (CXTD202203), Beijing Hospitals Authority’ Ascent Plan (DFL20191501), and Young Scientist Program of Beijing Stomatological Hospital Capital Medical University (YSP202001).

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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Summary

Keywords

bone tissue engineering, microRNA, calcium phosphates, nanoparticles, transfection, nonviral vectors, osteogenesis

Citation

Zhu M, Gu Y, Bian C, Xie X, Bai Y and Zhang N (2022) Applications of Nonviral Biomaterials for microRNA Transfection in Bone Tissue Engineering. Front. Mater. 9:932157. doi: 10.3389/fmats.2022.932157

Received

29 April 2022

Accepted

09 June 2022

Published

22 July 2022

Volume

9 - 2022

Edited by

Anuj Kumar, Yeungnam University, South Korea

Reviewed by

Xiaoyu Du, ETH Zürich, Switzerland

Sangram Keshari Samal, Regional Medical Research Center (ICMR), India

Ganesh Ingavle, Symbiosis International University, India

Updates

Copyright

*Correspondence: Ning Zhang,

†These authors have contributed equally to this work

This article was submitted to Biomaterials, a section of the journal Frontiers in Materials

Disclaimer

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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