Protective Effects and Network Analysis of Ginsenoside Rb1 Against Cerebral Ischemia Injury: A Pharmacological Review

Ischemic stroke is a leading cause of death and disability worldwide. Currently, only a limited number of drugs are available for treating ischemic stroke. Hence, studies aiming to explore and develop other potential strategies and agents for preventing and treating ischemic stroke are urgently needed. Ginseng Rb1 (GRb1), a saponin from natural active ingredients derived from traditional Chinese medicine (TCM), exerts neuroprotective effects on the central nervous system (CNS). We conducted this review to explore and summarize the protective effects and mechanisms of GRb1 on cerebral ischemic injury, providing a valuable reference and insights for developing new agents to treat ischemic stroke. Our summarized results indicate that GRb1 exerts significant neuroprotective effects on cerebral ischemic injury both in vivo and in vitro, and these network actions and underlying mechanisms are mediated by antioxidant, anti-inflammatory, and antiapoptotic activities and involve the inhibition of excitotoxicity and Ca2+ influx, preservation of blood–brain barrier (BBB) integrity, and maintenance of energy metabolism. These findings indicate the potential of GRb1 as a candidate drug for treating ischemic stroke. Further studies, in particular clinical trials, will be important to confirm its therapeutic value in a clinical setting.


INTRODUCTION
When the brain becomes blocked by a blood clot and blood is prevented from reaching the brain, an ischemic stroke occurs. Ischemic stroke, approximately accounting for 85% of all diagnosed strokes, has the characteristics of high morbidity, high mortality, high disability, and high recurrence rates and is mainly caused by cerebral ischemia and reperfusion (I/R) injury (CIRI) (Turley et al., 2005;Eltzschig and Eckle, 2011). I/R is a pathological condition characterized by an initial restriction of blood supply to an organ followed by the subsequent restoration of perfusion and concomitant reoxygenation (Turley et al., 2005;Eltzschig and Eckle, 2011), which is one of the leading causes of death worldwide (Turley et al., 2005;Woodruff et al., 2011;Feigin et al., 2014). I/R injury mainly includes ischemic stroke, acute kidney injury, and myocardial infarction. Due to the interrupted blood supply to the CNS, cerebral infarction causes ischemic stroke and other forms of CNS injury. As these mechanisms often involve complex combinations of necrosis, apoptosis, necroptosis, and autophagy, ischemic stroke-related pathogenesis is not totally clear (Turley et al., 2005;Eltzschig and Eckle, 2011). Based on accumulating evidence, once an ischemic stroke occurs, the insufficient blood supply directly stimulates neurons, causes the accumulation of glutamate, overactivates a plethora of downstream signaling pathways, and increases the intracellular calcium concentration, which triggers energy metabolism disorders (Bolaños et al., 2009), oxidative stress (Chamorro et al., 2016), Ca 2+ overload, excitatory neurotransmitter release, the immune-mediated inflammatory response following acute ischemic stroke, and its related apoptosis and necrosis processes (Kamel and Iadecola, 2012;Chamorro et al., 2016), finally resulting in ischemic infarction of the brain. Hence, the main aim of acute stroke treatment is to salvage the ischemic penumbra or volume of hypoperfused, nonfunctional, yet still viable tissue surrounding the infarcted core.
Currently, tissue plasminogen activator (TPA) is regarded as the main effective pharmacological therapy and drug for ischemic stroke (Turley et al., 2005;Woodruff et al., 2011;Chamorro et al., 2016). Scholars have conducted extensive studies and developed some neuroprotective drugs and other interventions for treating ischemic injury (Tuttolomondo et al., 2011;Woodruff et al., 2011), ranging from pharmacologically blocking neurotransmitter receptors to intercepting cell death pathways, as well as the induction of hypothermia or hyperoxygenation. However, most of these studies failed to show the efficacy of any of these promising strategies. The current existing neuroprotective drugs remain limited and insufficient for the clinical treatment needs for ischemic stroke (Turley et al., 2005;Woodruff et al., 2011;Chamorro et al., 2016). Therefore, the development of novel therapeutic strategies and agents for preventing and treating ischemic stroke is urgently needed.
Currently, accumulated experiments and data suggest that GRb1 exerts neuroprotective effects both in vivo and in vitro and has a great potential as a novel candidate agent for ischemic stroke. To date, researchers have not clearly determined whether GRb1 can be used to treat ischemic stroke and cerebral I/R injury. No systematic review or analysis has been conducted to assess the protective effects and mechanisms by which GRb1 combats ischemic stroke and I/R injury ( Figure 1). Hence, we conducted this analysis of preclinical studies of the effect of GRb1 on ischemic stroke. We searched the PubMed and China National Knowledge Infrastructure databases via using "Ginsenoside Rb1" and "Ischemia" as search terms. The PubMed database was comprehensively searched up to September 2020, and it showed 66 literatures; we excluded some irrelevant ones and then divided them into several aspects to further analyze the pharmacological effects and mechanisms of GRb1 in pre-treating and treating CIRI. Furthermore, we manually searched for other potential and relevant references, and there were no limitations in the language of all publications.
In this review, we found that GRb1 might alleviate cerebral/ neural ischemia injury via its antiapoptotic, antioxidant, and antiinflammatory activities, and effects on mitochondrial homeostasis, promoting neurogenesis, and improving brain functional connections and interactions, which provides more evidence for basic studies and further promotes the development of GRb1 as a candidate drug for the clinical treatment of ischemic stroke.

ANTIAPOPTOTIC EFFECTS AND NEURONS
Recent reports have suggested that GRb1, a natural saponin ingredient of TCM, exerts remarkable neuroprotective effects on the CNS (Ahmed et al., 2016), prevents and alleviates cerebral I/R injury via anti-inflammatory activity, antioxidant activity, enhanced neuroproliferation and neurodifferentiation, and improved energy metabolism Zhu et al., 2012;, confirming that Rb1 exerts antiapoptotic effects on neurons.
First, in vivo experiments showed that Rb1 significantly inhibited CA1 neuronal death caused by a 2-vessel occlusion (2-VO) model in rats  and delayed neuronal death in gerbils (Lim et al., 1997); noticeably reduced the infarct size and neuronal deficits, relieved pathological changes (Yuan et al., 2007), and decreased the number of neural apoptotic cells (Wen et al., 1996) in rats with middle cerebral artery occlusion (MCAO) (Lim et al., 1997;Yuan et al., 2007;Yang et al., 2008); and scavenged free radicals (Lim et al., 1997) and improved hippocampal blood flow at 5 min after transient forebrain ischemia (Lim et al., 1997). Rb1 significantly prolonged the response latency of ischemic gerbils and rescued a significant number of ischemic CA1 pyramidal neurons (Wen et al., 1996;Lim et al., 1997). Under abnormal ischemic microenvironmental conditions, Rb1 evidently decreased the concentrations of glutamic acid and Ca 2+ , noticeably alleviated memory deficits in rats, and reduced pyramidal cellular necrosis and apoptosis in the hippocampus induced by glutamate (Glu) and Ca 2+ Guo et al., 2018). Additionally, Rb1 suppressed the loss of BBB integrity by suppressing the induction of neuroinflammation in a model of ischemic stroke (Chen et al., 2015). Based on these results, Rb1, an effective neuroprotective drug, plays key roles in cerebral I/R injury.
Second, in vitro experiments (Table 1) suggested that Rb1 might increase cell viability, inhibit oxygen and glucose deprivation (OGD)-induced neuronal death, and reduce autophagic vacuoles in SH-SY5Y cells , changes that were blocked by the inhibitor LY294002. GRb1 significantly decreased the levels of free radicals, protected hippocampal neurons from lethal damage caused by the hydroxyl radical-promoting agent FeSO 4 in vitro (Lim et al., 1997), and markedly suppressed the uptake of Glu and overload of Ca 2+ in OGD-induced SH-SY5Y cells Guo et al., 2018). GRb1 also significantly reduced the levels of lactate dehydrogenase (LDH) (Park et al., 2005;Li et al., 2016), nitric oxide (NO), and superoxide (O-) (Zhu et al., 2012;Huang et al., 2014). Thus, Rb1 might be regarded as an antiapoptotic agent with neuroprotective effects.
Furthermore, GRb1 significantly inhibited the expression of the proapoptotic genes Bax, Bad, and caspase-3 Gao et al., 2010), upregulated Bcl-2 and the ratio of Bcl-2/Bax in vivo and in vitro (Yuan et al., 2007;Yang et al., 2008), increased the expression of glial-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) (Yuan et al., 2007;Gao et al., 2010), and thus prevented neuronal death induced by cerebral ischemia. GRb1 significantly increased the levels of Akt phosphorylated at Ser473 (P-Akt) and reduced the expression levels of LC3II and Beclin1, indicating that GRb1 might prevent ischemic neuronal death by modulating autophagy activation ; moreover, GRb1 increased the levels of P-Akt and P-mTOR, reduced P-PTEN levels in vivo and in vitro, and ameliorated the abnormal microenvironment by activating the P-AKT/P-mTOR pathway and inhibiting P-PTEN (Guo et al., 2018). In contrast, the LY294002 treatment reversed these changes induced by GRb1. Furthermore, GRb1 inhibited the expression of NMDAR, increased the expression of glial glutamate transporter 1 (GLT-1), and downregulated the levels of cytochrome C (Cyt-C) in response to neuronal mitochondrial stress, which reduced the excessive Glu and Ca 2+ levels Guo et al., 2018).
In general, evidence from recent studies suggests that GRb1 may exert its antiapoptotic effects by regulating the phosphoinositide 3-kinase (PI3K)-protein kinase B (AKT)-TOR signaling pathways, inhibiting autophagy, alleviating mitochondrial stress and apoptosis pathways, modulating the expression of N-methyl-D-aspartate-receptor (NMDAR) and GLT-1, and improving neurogenesis and BDNF levels. Nonetheless, currently, the mechanisms by which GRb1 regulates ischemic neuronal apoptosis are not completely elaborated and summarized and should be further explored in the future.

REGULATION OF NEUROINFLAMMATION AND MICROGLIA
Although CIRI is a complex pathology caused by the interaction of numerous pathophysiological factors (Allison and Ditor, 2014;Anderson et al., 2014), accumulating evidence indicates that acute inflammation and subsequent apoptosis and necrosis are involved in the progression of a cerebral ischemic insult (Jianhua et al., 2008;Eltzschig and Eckle, 2011;Jiang et al., 2014;Tao et al., 2015). Recently, GRb1 was reported to exert beneficial effects on cerebral ischemic stroke and to inhibit inflammatory cascades in the acute phases of cerebral ischemia Zhu et al., 2012;Jiang et al., 2015).
According to recent studies ( Table 2), GRb1 noticeably reduces the infarct volume, significantly alleviates neurological deficits, decreases neurological severity scores (Zhu et al., 2012;Liu et al., 2013;Liu et al., 2018;Chen et al., 2020), preserves the neuronal morphology and structure (Ke et al., 2014), improves pathological changes (Liu et al., 2018), and inhibits the activation of microglia in MCAO/R model rats (Zhu et al., 2012) and N9 microglia in vitro (Ke et al., 2014). These neuroprotective effects may be involved in microglia-mediated CNS inflammation and related neuronal damage in the acute phases of cerebral ischemia/ hypoxia.
Frontiers in Pharmacology | www.frontiersin.org July 2021 | Volume 12 | Article 604811 attenuated the activity of MMP-9 (Chen et al., 2015), upregulated the expression of γ-aminobutyric acid (GABA) receptors in I/R rats (Chen et al., 2020), protected BBB integrity in ischemic stroke by suppressing neuroinflammation, decreased the production of MMP-9 and NOX4-derived free radicals (Chen et al., 2015), and regulated the probiotic Lac.H and GABA receptor levels (Chen et al., 2020). Overall, GRb1 may exert a neuroprotective effect on I/R-induced injury by suppressing neuroinflammation and microglia-mediated inflammatory reactions.

ANTIOXIDANT EFFECTS AND MITOCHONDRIA
Oxidative stress plays an important role, and reactive oxygen species (ROS) is implicated in the tissue damage that occurs in cerebral ischemic pathogenesis, which results in the production of toxic molecules that alter cellular proteins, lipids, and ribonucleic acids, leading to cell dysfunction or death (Crack and Taylor, 2005;Chamorro et al., 2016). Excessive ROS severely impair mitochondria and their related energetic metabolism functions.  (Table 3) showed that GRb1 protected hippocampal CA1 neurons (Lim et al., 1997), reduced free radicals (Chen et al., 2015), and possessed potential for treating brain injuries ; meanwhile, GRb1 increased superoxide dismutase (SOD) activity, decreased malondialdehyde (MDA) contents in the serum and spinal cord tissue (Ye et al., 2019), and inhibited oxidative stress and extracellular signal-regulated kinase (ERK) activation in aged mice exposed to cerebral ischemia (Dong et al., 2017). All of the antioxidant effects of GRb1 are beneficial for reducing neuronal death, mitochondrial damage, and astrocyte injury induced by I/R, indicating that GRb1 may reduce pathological changes and decrease neural cell apoptosis Chen et al., 2015).
Second, in vitro experiments (Table 3) confirmed that GRb1 significantly improved cell viability, decreased intracellular ROS production, and increased catalase (CAT) activity and the mtDNA copy number in OGD/R-induced astrocytes, thus inhibiting oxidative phosphorylation (OXPHOS) (Xu et al., 2019). GRb1 exerted obvious protective effects on the mitochondria by distinctly attenuating MMP depolarization, improving the efficiency of mitochondrial OXPHOS, increasing the activities of complexes I, II, III, and V, and increasing the level of adenosine triphosphate (ATP) following OGD/R induction (Xu et al., 2019).
Further studies (Table 3) revealed the antioxidant effects of GRb1. On the one hand, GRb1 significantly downregulated NOX-4 expression and NOX activities in ischemic rat brain tissues (Chen et al., 2015;Dong et al., 2017), inhibited ischemiastimulated NADPH oxidase gene expression, including NOX-1, 2, and 4 (Dong et al., 2017), and prevented ERK activation in aged mice (Dong et al., 2017). On the other hand, GRb1 significantly suppressed mitochondrial damage, increased the mitochondrial membrane potential (MMP) in OGD/ R-induced astrocytes (Xu et al., 2019), and thus improved the antioxidant activity (CAT and SOD). GRb1 remarkably increased aquaporin (AQP) 4 levels , decreased Bax expression , and upregulated Bcl-2 , nerve growth factor (NGF), and BDNT expression  in vivo and in vitro. Thus, GRb1 is potentially useful for treating brain injury, due to its antioxidant effects and mitochondrial protection.

INHIBITION OF OTHER TYPES OF NEURAL ISCHEMIC INJURY
Related studies have shown that GRb1 significantly alleviates I/R injury of the kidney Sun et al., 2012;Sun et al., 2013;Chen et al., 2019) and brain Zhu et al., 2012;. However, few reports have assessed spinal cord ischemia-reperfusion injury (SCII). According to recent studies, GRb1 reduces cell apoptosis induced by SCII by inhibiting oxidative stress (Ye et al., 2019). GRb1 noticeably improves hindlimb locomotor dysfunction in rats , increases SOD activity, decreases the MDA content in serum and spinal cord tissue (Ye et al., 2019), and inhibits neuronal apoptosis. The potential mechanisms may be tightly associated with promoting the expression of survivin (Ye et al., 2019), downregulating the levels of caspase-3 and phosphorylated Ask-1 (p-Ask-1), and improving the Bcl-2/Bax ratio  in SCII rats. After the exposure of spiral ganglion cells (SGCs)to transient cochlear ischemia, GRb1 significantly reduced the percentage of the auditory brainstem response threshold shift and prevented hearing loss caused by ischemic injury to SGCs in Mongolian gerbils (Fujita et al., 2007), indicating that GRb1 might protect SGCs from ischemic injury.

ANALYSIS OF SAFETY AND THERAPEUTIC EFFECTIVENESS
Based on the above summarized findings (Tables 1-3), Rb1 has a potential therapeutic value for ischemic stroke via immunological, antioxidant, and neuroprotective effects. However, the efficacy and safety of Rb1 for ischemic stroke in human have not been tested, and its effective dose is not known. Nevertheless, there is evidence that red ginseng extract, which contains 20-30% of GRb1, at 23, 25, 50, and 100 mg with repeated oral administration for 1-2 weeks, is safe and well tolerant in healthy subjects (Kim, 2013;Jin et al., 2019;Choi et al., 2020). In addition, randomized controlled studies showed that 200-400 mg of ginsenosides or its preparations significantly ameliorated neurological deficit (He et al., 2011) and improved working memory and daily activities of patients with ischemic stroke (Reay et al., 2010;He et al., 2011), indicating that a daily dose of 40-120 mg of GRb1 may be safe and effective for treating ischemic stroke. The effective dose of Rb1 tested in animal studies with notably attenuated ischemia-induced cerebral I/R injury ranges from 10 to 40 mg/kg (Tables 1-3), which is equivalent nitric oxide; MMP-9, matrix metalloprotein 9; BBB, blood-brain barrier; NOX, NADPH oxidase; EB, Evans blue; OGD/R, oxygen-glucose deprivation/reperfusion; MCAO, middle cerebral artery occlusion; AQP, aquaporin; NGF, nerve growth factor; BNDF, brain-derived neurotrophic factor; ROS, reactive oxygen species; CAT, catalase; ATP, adenosine triphosphate; MMP, mitochondrial membrane potential; mtDNA, mitochondrial DNA; ERK, extracellular signal-regulated kinase; SOD, superoxide dismutase; MDA, malondialdehyde; BVA, bilateral vertebral arteries; CF, cited references.
Frontiers in Pharmacology | www.frontiersin.org July 2021 | Volume 12 | Article 604811 6 to 0.8-3.2 mg/kg in human (calculated from dose in mice) or 48-192 mg Rb1 for a 60 kg person. Thus, an effective concentration of Rb1 may be achievable in human.

CONCLUSION AND REMARKS
GRb1, a natural active ingredient, exerts neurotrophic and neuroprotective effects on the CNS. In the present review (Figure 2), we summarized the available reports on the therapeutic effects and the molecular mechanisms of GRb1 in cerebral I/R injury. The currently available data and our summarized results suggest that GRb1 may alleviate cerebral/neural ischemic injury via multiple pharmacological activities, such as its antiapoptotic, antioxidant, and anti-inflammatory properties, preservation of mitochondrial homeostasis, promotion of neurogenesis, and maintenance of the probiotic balance. Based on these results, GRb1, a special natural compound, has bright prospects for the prevention and treatment for ischemic stroke with a pharmacological network of multiple effects, targets, and molecular pathways.
Cerebral I/R injury is a complicated pathological process (Turley et al., 2005;Eltzschig and Eckle, 2011;Chamorro et al., 2016) that includes various types of I/R damage to the neurovascular unit (NVU), which is composed of nerve cells, BBB, microglia, and extracellular matrix (Lok et al., 2007;Guo and Lo, 2009;Lo, 2017). For many decades, neuronal injury has been considered the main cause of functional deficits after brain injury. Accordingly, almost all therapeutic strategies were FIGURE 2 | Summarized effects and molecular network analysis of GRb1 in cerebral ischemic injury. Rb1 exerts significant neuroprotective effects on the neurovascular unit (NVU) and other neural cells via the network actions of its antiapoptotic, antioxidant, and anti-inflammatory activities; mitochondrial homeostasis; neurogenesis promotion; and regulation of the probiotic balance. The molecular mechanisms involve multiple effects, multiple targets, and multiple pathways. NVU, neurovascular unit; CMVE, cerebral microvascular endothelium. GRb1, ginsenoside Rb1; I/R, indicates ischemia and reperfusion. CC, cleaved caspase.
Frontiers in Pharmacology | www.frontiersin.org July 2021 | Volume 12 | Article 604811 targeted at rescuing neurons and repairing neuronal damage. However, emerging data from both experimental models and patients now clearly show that, in patients with stroke, saving neurons alone may also be insufficient for treating brain infarcts (Lok et al., 2015) as brain functional connections and interactions among the different components in the NVU are also important (Cai et al., 2017;Jiang et al., 2018). Coincidentally, as shown in Figure 2, GRb1 exerts significant antiapoptotic effects on neurons exposed to cerebral ischemia stroke, suppresses neuroinflammation and microglia-mediated inflammatory stress in the acute phase of I/R, and protects ischemia-exposed astrocyte cells. Meanwhile, GRb1 inhibits oxidative stress and mitochondrial damage in cells exposed to I/R, preserves the BBB integrity, improves energy metabolism, and promotes neurogenesis. Moreover, GRb1 may alleviate spinal cord ischemia-reperfusion injury (SCII) and regulate the probiotic balance ( Figure 2). Hence, GRb1 obviously plays a vital role not only in neural protection but also via the inhibition of NVU damage caused by CIRI.
From the perspective of protecting the NVU, GRb1 prevents I/R-induced cell apoptosis and death, suppresses excessive Ca 2+ , glutamate, and RNS levels, alleviates mitochondrial stress, improves neurogenesis, and reduces the cerebral infarct volume in vitro and in vivo, indicating that GRb1 may exert antiapoptotic effects and protect the NVU from CIRI (Figure 2). Although the mechanisms have not been completely elaborated, our summarized results ( Table 1) further reveal the molecular mechanisms and vital biomarkers of the effects of GRb1 on CIRI, which may be related to the PI3K-AKT-mTOR, autophagy, and mitochondrial apoptosis signaling pathways, NMDA receptor and GLT-1 targets, and neurogenesis.
Neuroinflammation underlies the etiology of multiple neurodegenerative diseases and stroke (Eldahshan et al., 2019). In the acute phase of ischemic stroke, resident microglia and recruited macrophages assume an M2 phenotype, and the immune-mediated inflammatory response is activated, resulting in the secretion of a number of inflammatory cytokines (IL-6, IL-1β, and TNF-α and the proteolytic enzymes MMP 3 and 9) (Jiang et al., 2018). Proinflammatory cytokines further trigger severe inflammatory stress, aggravate BBB dysfunction (Jiang et al., 2018), and induce NVU damage. All these events lead to the exacerbation of ischemic injury. Based on the summarized results listed in Table 2, we found that GRb1 inhibited the NF-KB, HMGB-1/TLR4, and HMGB1 signaling pathways, regulated the MAPK signaling pathway, increased the expression of GABA receptors, and reduced the levels of proinflammatory cytokines (IL-6, MMP-9, IL-1β, and TNF-α). By regulating these pathways, GRb1 suppresses neuroinflammation and microglia-mediated inflammatory reactions, reduces the damage caused by inflammatory factors, improves the integrity and normal functions of the BBB, and thus inhibits necrosis and apoptosis associated with anti-inflammatory activities at the early stages after stroke, which plays a fundamental role in the maintenance of CNS homeostasis, BBB structural integrity, and normal functions of the NVU.
The neutralization of oxidative and nitrosative stresses is a potential therapeutic strategy because the ischemic brain is highly susceptible to oxidative damage (Crack and Taylor, 2005;Xie et al., 2020). After CI/RI, the production of ROS increases, leading to lipid peroxidation, mitochondrial and DNA damage, protein nitration, and mitochondrial injury that evokes the mitochondrial release of apoptosis inducers (Bolaños et al., 2009;Chamorro et al., 2016). According to recent studies (Table 3), GRb1 decreases the production of ROS, MDA, and NO, improves antioxidant defenses (CAT, SOD, and GSH-px), and inhibits I/R-induced mitochondrial injury in vivo and in vitro. Moreover, it remarkably increases AQP4 levels , decreases Bax expression , upregulates the expression of Bcl-2 , downregulates NOX-4 expression and NOX activities, and inhibits the cascade reactions of caspase-3 and caspase-9. Thus, GRb1 exerts significant neuroprotective effects due to its antioxidant activity and protects the mitochondria in the treatment of cerebral ischemic injury.
In summary, this review addresses the effects and efficacy of GRb1 by discussing research results obtained using cellular and animal models; the summarized results and analysis indicate that GRb1, a new candidate agent, has bright prospects for preventing and treating ischemic stroke.
However, unrecognized actions and limitations of GRb1 still exist. 1) Researchers have not clearly determined whether GRb1 alleviates ischemic injury of the cerebral microvascular endothelium (CMVE) of the NVU. 2) Because of the lack of clinical testing and validation of preclinical data, the use of GRb1 as a new drug for treating ischemic stroke remains a challenge. 3) The existing research and data only showed the effects of GRb1 on I/R in the early stages and acute phases. Therefore, currently available clinical trials and data collection are urgently needed, and future studies should focus on the effects and molecular mechanisms of GRb1 on the NVU system and the recovery phase of ischemic stroke.

AUTHOR CONTRIBUTIONS
WX and XW designed the review; WX, XW, and YW wrote the manuscript; YC, DY, and TX helped map the figures and revise the manuscript. SZ was mainly responsible for the supervision of the works and the management of related projects. All authors discussed, edited and approved the final version.

FUNDING
This work was funded by the Natural Science Foundation of China (No. 81801096). We greatly expressed our sincere gratitude to the foundations.