Abstract
The constant release of human bone morphogenetic protein 2 (rhBMP-2) in the picomolar range (Pico-Stat) from PDLLA-biohybrids led to the detection of intrinsic novel pro- and anti-angiogenic functions of this cytokine. As integrant part in this perspective of previous work, first evidence for the binding of rhBMP-2, as an inverse agonist, to allosteric angiogenic receptors in cocultures of human endothelial cells is reported.
1 Introduction
BMP-2 initiates osteogenesis requiring angiogenesis, with the ingrowth of mesenchymal stem cells and osteoblasts. The global market size () of the recombinant form (rhBMP-2) [INFUSE® Bone Graft ()] reached USD 498.1 million in 2021 (). This protein, rhBMP-2CHO, is produced by a genetically engineered Chinese hamster ovary cell line, as a disulfide-linked heterodimeric protein mixture of two different polypeptide chains of 114 and 131 amino acids. Each chain is glycosylated at one site with high-mannose-type glycans (; ; ). As applied, BMP-2CHO comprises a microheterogenous mixture of three different BMP-2 isoforms and up to 15 additional different glycoforms ().
In contrast, the non-glycosylated rhBMP-2COL second species () from E. coli (Figure 1) is homodimeric and displays no microheterogeneity (; ). The molecular masses of the commercial glycosylated (rhBMP-2CHO) and the non-glycosylated rhBMP-2 species (rhBMP-2COL) correspond to 33–36 kDa for the former and 26 kDa for the latter, respectively (). In humans, 12 mg of rhBMP-2CHO must be applied for osteoinduction in a concentration of 1.5 mg/mL (∼ 44 µM) in an absorbable collagen sponge (ACS). After intravenous injection, rhBMP-2 is eliminated with a half-life of t1/2 = 6.7 min in nonhuman primates (). Physiological serum levels of BMP-2 in healthy human controls are reported to be 17.1 ± 0.6 pg/mL (= 4.8 × 10−13 M) ().
FIGURE 1
In receptor-binding studies, the binding function (θ) and the state function (r) are distinguished (). The binding function (θ) of rhBMP-2 is a direct measure of receptor occupation by the ligand with KθD in the range of ∼0.45 nM (). The state function (r) (; ) of rhBMP-2 correlates with the receptor activity state by monitoring downstream products (e.g., alkaline phosphatase) in activity tests in MC3T3-E1 cell cultures, with both species of rhBMP-2CHO and rhBMP-2COL having indistinguishable biological activities of nanomolar affinities (Kr′D ∼ 2–10 nM ()). BMP-2 also specifically binds to its prodomain for ECM targeting, presumably with a similar affinity constant (K′D ∼ 4–8 nM) as BMP-7 (). RhBMP-2 is known to indirectly initiate angiogenesis in the nanomolar range (0.38–3.8 nM) () by activating paracrine VEGF-mediated osteoblast‐endothelial cell cross‐talk (). The combination of biomolecular materials, such as rhBMP-2, with the other biomaterial classes such as metals, polymers, or ceramics, forms a biohybrid material [see ()]. In protein immobilization, we distinguish adsorbates (adsorption), covalates (covalent binding), and inclusates (encapsulated biohybrids) (). A biohybrid is classed as bioactive if it has been “designed to induce a specific biological activity” (). A paracrine biohybrid is, e.g., a growth factor releasing biomaterial. Recently, we reported the first evidence of a direct influence of rhBMP-2COL on angiogenesis in co-cultures at picomolar concentrations (). Applications of rhBMP-2 in solute and PDLLA solid-phase forms are described.
2 Methods
RhBMP-2COL from E. coli (Morphoplant GmbH Bochum, D) and rhBMP-2CHO from CHO-cells (InductOs®, Medtronic BioPharma B.V., Heerlen, NL) were employed (). The dose–response derived biological equivalent activity equals K'D ∼ 5–15 nM for both species of rhBMP-2 (). The for many weeks stable PDLLA nanofiber fleece preparation is described in (). Labeled 125I-rhBMP-2 () was for the analytical preparation of adsorbate PDLLA biohybrids. Non-isotope biohybrids were made in parallel for the other experiments. In a 24-well cell culture plate, the single well volume corresponds to 0.5–1.0 mL. Human outgrowth endothelial cells (OECs) and human primary osteoblasts (pOBs) were prepared as described (). Cocultures consisted of endothelial cells (130.000/well) and primary osteoblasts (20.000/well). Relative gene expressions (RQ) of mRNA for various proteins are described in (). In dose-response experiments, rhBMP-2COL was added in defined concentrations to the OEC/pOB co-culture system and harvested after 7 days of incubation for fluorescence microcopy and quantitative Nikon NIS image processing as described (). The pixel values were converted to length according to 1 pixel = 0.46 µm. Cell viability was tested by the LDH cytotoxicity assay CyQUANT™ as described (). In desorption experiments, 1 cm2 (1 mg) fleeces with an rhBMP-2COL loads of ∼ 6.98 mg/g PDLLA were placed in 1.5 mL flow-through chambers (). They were perfused with sterile phosphate-buffered saline (PBS) pH 7.4 at a flow rate of 10 mL/h in a 16-channel Watson-Marlow peristaltic pump for 22 days at ca. 22°C-26°C. Statistical calculations were done with the PC program GraphPad Prism 4/5 (La Jolla, CA). Prism software fails in exponential decays with r2-values of <0.2 when decays parallel the abscissa at t1/2 > 100–200 days (slope ∼ zero). Kinetic and thermodynamic constants under non-standard conditions are termed “apparent” (K′, k′). Equilibrium dissociation constants (KD, K0.5) represent affinity constants ().
3 Results
3.1 Pico-paracrine biohybrids
In preparing solid-state paracrine adsorbate biohybrids, rhBMP-2COL was adsorbed to the surface of a non-woven fleece composed of PDLLA nanofibers (∅ = ∼ 100 nm), prepared by electrospinning (; ) (see Table 1).
TABLE 1
 | ||
|---|---|---|
| BMP-2 [µg/mL] concentration | Adsorbed amount [mg/g] | |
| rhBMP-2COL | rhBMP-2CHO | |
| 10 (low) | 0.33 ± 0.06 | 0.58 ± 0.10 |
| 30 (high) | 2.61 ± 0.81 | 3.40 ± 0.61 |
Preparation of rhBMP-2-PDLLA-adsorbate biohybrids*.
The adsorbed load (high and low) to the nanofiber fleece from the adsorption solution (10 and 30 μg/mL) was determined at room temperature after 10–20 h of adsorption by tracer detection with1251-rhBMP-2 (). Mean ± SD, n = 3.
Decisive is the 22-day rhBMP-2COL adsorbate release kinetics from biohybrids loaded with ∼8.9 mg/g Table 2. The two-phase exponential decay function displays a half-life of 3–4 h for the burst phase (K′D = 8.2 nM) and a half-life of 208 days for sustained high-affinity release (K′D = k−2/k+1 = 0.59 × 10−12 M; = pico paracrine biohybrid). Such rhBMP-2COL adsorbate PDLLA biohybrids (Table 1) were incubated in OEC/pOB co-cultures for 14 days and led to large numbers of capillary–like microvessels with no VEGF-A gene activation () (Figure 2A; 0.33 mg/g). Fewer microvessels, but more than in controls, form at an 8-fold higher rhBMP-2COL adsorbate load (Figure 2B).
TABLE 2
Initial  | Burst | Sustained release | ||||
|---|---|---|---|---|---|---|
| k′-1 | t1/2 | K′D | k′-2 | t1/2 | K′D | |
| mg/g | 10−4 [s−1] | [d] | nM | 10−8 [s−1] | [d] | pM |
| 8.97 | 5.34 | 0.015 | 8.2 | 3.86 | 208 | 0.59 |
| ± | ± | ± | ||||
| 1.53 | 0.16 | 2.91 | ||||
125I-rhBMP-2COL release kinetics from biohybrids (Table 1).
Data are derived from desorption kinetics of a 22-day duration [for details see ()] SEM; for k+1 = 6.5 ± 0.4 × 104 M−1 s−1 see ().
FIGURE 2
In contrast to the pro-angiogenic activities of rhBMP-2COL, the glycosylated species rhBMP-2CHO was anti-angiogenic, fully inhibiting control angiogenesis () and forming instead a cobblestone layer of endothelial cells [not shown, see ()].
These results were confirmed in Table 3. At a low rhBMP-2COL, adsorbate loads of 0.33 mg/g and a total length of microvessels of 23.9 ± 5.1 mm formed without an increased expression of VEGF-A (). At high adsorbate loads of 2.6 mg/g, the total length decreased 5-fold to 4.7 ± 0.44 mm, and in controls lacking adsorbate down 10-fold to intrinsic 2.4 ± 0.046 mm. The knots in the microvessel mesh paralleled the length changes. At low concentrations of rhBMP-2COL, they totaled a high of 51.9 ± 11.5 knots, decreasing 4.3-fold to 12.2 ± 0.6 at higher concentrations and 12-fold less in controls of 4.3 ± 1.1 knots. These changes were statistically significant (Table 3).
TABLE 3
| rhBMP-2COL Biohybrid | Adsorbed amount of rhBMP-2COL | ||
|---|---|---|---|
| Ø mg/g Controls | 0.33 mg/g | 2.6 mg/g | |
| (After 14 days) | (intrinsic activity) | (pro-angiogenic activity) | |
| Length Analysis (mm) | 2.4 ± 0.046 | 23.9 ± 5.1 | 4.7 ± 0.44 |
| Knot Analysis (dimensionless) | 4.3 ± 1.1 | 51.9 ± 11.5 | 12.2 ± 0.6 |
Length and knot data of capillary-like structures induced by rhBMP-2COL adsorbate biohybrids after 14 days in co-culture*.
3.2 Solution studies
In the following, concentration-response experiments in solution are shown. Instead of correlating with rising concentrations, the pro-angiogenic microvessel response increased as a function of factor-10 graded dilutions of rhBMP-2COL (Table 4). Microvessel formation thus followed an inverse concentration gradient, exposing a hitherto unseen angiogenic activity by rhBMP-2COL in the picomolar range, confirming results in Figure 2 and Table 3. Anti-angiogenic concentrations of rhBMP-2COL >10 pM fully abolished pro-angiogenic responses.
TABLE 4
| [rhBMP-2COL] | Microvessel total length | ||
|---|---|---|---|
| Gross, pixel | Net, pixel | Net, mm | |
| Control, 0 | 13,070 ± 1,456 | - | - |
| 1 × 10−7 M | 8 ± 0.6 | 0 | 0 |
| 1 × 10−8 M | 12 ± 3 | 0 | 0 |
| 1 × 10−9 M | 22 ± 1 | 0 | 0 |
| 1 × 10−10 M | 1,217 ± 436 | 0 | 0 |
| 1 × 10−11 M | 16,360 ± 377 | 3,290 ± 75 | 1.5 ± 0.015 |
| 1 × 10−12 M | 42,103 ± 4,276 | 29,034 ± 2,961 | 13.4 ± 1.4 |
| 1 × 10−13 M | 46,245 ± 3,978 | 33,175 ± 2,853 | 15.3 ± 1.3 |
Dose-response angioinduction in OEC/pOB co-cultures after 7 days by decreasing concentrations of rhBMP-2COL in solution*.
Microvessels (capillary-like structures) were determined by quantitative histologic staining (CD31; 1 pixel ∼0.46 µm). For further details see Figure 2, (mean ± SD, n = 3). [data from ()].
The data of Table 4 are plotted in Figure 3 down to concentrations of 10−16 M for a classical receptor type. Inhibition increases with the concentration of rhBMP-2COL. The apparent half-saturation constant is not a classical affinity constant but an “autoinhibition” constant (K′I0.5) with a value of 3.2 pM. The hypothetical (see dotted line) dissociation constant (K′D) for the presumed association of rhBMP-2COL to endothelial cell receptors lies in the subpicomolar range ∼10−14 M. The anti-angiogenic regulation by rhBMP-2COL prevents too many blood vessels from being formed as a “high concentration cut-off regulation”, in which an apparent agonist turns into an antagonist at high concentrations.
FIGURE 3
Figure 4 shows a compound extended dose-response plot comprising the data of Table 4 on inverse related angioinduction () together with data on direct related osteoinduction [derived from activity measurements with MC3T3-E1 cells in culture ()] and their respective affinity constants. The inverse angiogenic branch saturates at ca. 15 mm and the direct osteogenic branch at 1.2 OD units. The apparent half-saturation autoinhibition constant (KI0.5) can now be termed as an inverse-related affinity constant (K′DI-R) for angiogenesis with a value of 3.2 pM which contrasts with the direct-related affinity constant (K′D) for osteoinduction at 7.3 nM. Since the constants of the two activities are separated by three and more orders of magnitude, identical receptors, shared, cross-reactive receptors and homologous desensitization () of receptors are improbable, strongly indicating an angiogenic receptor with constitutive activity () (see saturation in Figure 4) and rhBMP-2COL as a full inverse agonist in a typical dose response curve in Figure 4.
FIGURE 4
Control of local picomolar concentrations of rhBMP-2COL in solute form (Table 4) are short-term and difficult, but via solid-state technology, e.g., as rhBMP-2COL-PDLLA biohybrids (Figure 2; Table 3), are simple and long-term by sustained release (Table 2), comparable to a pH-Stat. In this study, not the pH but the growth factor concentration in the pico- or subpicomolar range is maintained as constant and guaranteed by high-affinity dissociation constants K′D (= Pico-Stat) ().
3.3 rhBMP-2 an angiogenesis inhibitor
As shown, the microvessel control values in the absence of rhBMP-2 (experiments of Tables 3, 4) are not zero but exhibit a significant spontaneous, endogenous pro-angiogenic control activity in the OEC/pOB cultures by VEGF-A (). This endogenous activity is inhibited by both species of rhBMP-2 (), for rhBMP-2COL by concentrations above 10–20 pM (see Figures 3, 4). Thus it could be argued that the observed proangiogenic activity of rhBMP-2COL (Figures 2, 3) is not of a stimulatory but only of a deinhibitory nature on dilution. However such a “deinhibition” neither accounts for a 9-fold net higher angiogenic biohybrid activity in vitro (Table 3) nor for a severalfold net higher vessel density in vivo (21 days, rats) in low versus high load comparison () (see Tables 1, 3). The above paradox angiogenic activity agrees with the evidence of rhBMP-2COL being a full inverse agonist of a novel allosteric receptor complex together with an as yet unknown orthosteric agonist (see: ; ).
4 Conclusion
Adsorbate load-dependent pro- and anti-angiogenic functions of the considered inverse agonist rh-BMP-2COL on endothelial cells, has been determined by the novel method of solid-state biohybrids as nano- or pico-stats.
Statements
Data availability statement
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.
Author contributions
The author confirms being the sole contributor of this work and has approved it for publication.
Funding
This study was funding by Deutsche Forschungsgemeinschaft Grant JE 84/15-3.
Conflict of interest
The author declares 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
angioinduction, osteoinduction, pico-technology, constitutive receptor activity, allosteric inverse agonist, orthosteric agonist, inverse concentration dependence, proangiogenic and antiangiogenic factors
Citation
Jennissen HP (2023) Camouflaged angiogenic BMP-2 functions exposed by pico-paracrine biohybrids. Front. Bioeng. Biotechnol. 11:1226649. doi: 10.3389/fbioe.2023.1226649
Received
22 May 2023
Accepted
22 June 2023
Published
07 September 2023
Volume
11 - 2023
Edited by
Babatunde Okesola, University of Nottingham, United Kingdom
Reviewed by
Cosimo Ligorio, University of Nottingham, United Kingdom
Updates
Copyright
© 2023 Jennissen.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Herbert P. Jennissen, hp.jennissen@uni-due.de
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.