Schwann cells and mesenchymal stem cells in laminin- or fibronectin-aligned matrices and regeneration across a critical size defect of 15 mm in the rat sciatic nerve

Research advancements on nerve guide conduits for nerve injury repair

Peripheral nerve injury (PNI) is one of the most serious causes of disability and loss of work capacity of younger individuals. Although PNS has a certain degree of regeneration, there are still challenges like disordered growth, neuroma formation, and incomplete regeneration. Regarding the management of PNI, conventional methods such as surgery, pharmacotherapy, and rehabilitative therapy. Treatment strategies vary depending on the severity of the injury. While for the long nerve defect, autologous nerve grafting is commonly recognized as the preferred surgical approach. Nevertheless, due to lack of donor sources, neurological deficits and the low regeneration efficiency of grafted nerves, nerve guide conduits (NGCs) are recognized as a future promising technology in recent years. This review provides a comprehensive overview of current treatments for PNI, and discusses NGCs from different perspectives, such as material, design, fabrication process, and composite function.

Advances of Schwann cells in peripheral nerve regeneration: From mechanism to cell therapy

Peripheral nerve injuries (PNIs) frequently occur due to various factors, including mechanical trauma such as accidents or tool-related incidents, as well as complications arising from diseases like tumor resection. These injuries frequently result in persistent numbness, impaired motor and sensory functions, neuropathic pain, or even paralysis, which can impose a significant financial burden on patients due to outcomes that often fall short of expectations. The most frequently employed clinical treatment for PNIs involves either direct sutures of the severed ends or bridging the proximal and distal stumps using autologous nerve grafts. However, autologous nerve transplantation may result in sensory and motor functional loss at the donor site, as well as neuroma formation and scarring. Transplantation of Schwann cells/Schwann cell-like cells has emerged as a promising cellular therapy to reconstruct the microenvironment and facilitate peripheral nerve regeneration. In this review, we summarize the role of Schwann cells and recent advances in Schwann cell therapy in peripheral nerve regeneration. We summarize current techniques used in cell therapy, including cell injection, 3D-printed scaffolds for cell delivery, cell encapsulation techniques, as well as the cell types employed in experiments, experimental models, and research findings. At the end of the paper, we summarize the challenges and advantages of various cells (including ESCs, iPSCs, and BMSCs) in clinical cell therapy. Our goal is to provide the theoretical and experimental basis for future treatments targeting peripheral nerves, highlighting the potential of cell therapy and tissue engineering as invaluable resources for promoting nerve regeneration.

Preparation and evaluation of decellularized epineurium as an anti-adhesive biofilm in peripheral nerve repair

Following peripheral nerve anastomosis, the anastomotic site is prone to adhesions with surrounding tissues, consequently impacting the effectiveness of nerve repair. This study explores the development and efficacy of a decellularized epineurium as an anti-adhesive biofilm in peripheral nerve repair. Firstly, the entire epineurium was extracted from fresh porcine sciatic nerves, followed by a decellularization process. The decellularization efficiency was then thoroughly assessed. Subsequently, the decellularized epineurium underwent proteomic analysis to determine the remaining bioactive components. To ensure biosafety, the decellularized epineurium underwent cytotoxicity assays, hemolysis tests, cell affinity assays, and assessments of the immune response following subcutaneous implantation. Finally, the functionality of the biofilm was determined using a sciatic nerve transection and anastomosis model in rats. The result indicated that the decellularization process effectively removed cellular components from the epineurium while preserving a number of bioactive molecules, and this decellularized epineurium was effective in preventing adhesion while promoting nerve repairment and functional recovery. In conclusion, the decellularized epineurium represents a novel and promising anti-adhesion biofilm for enhancing surgical outcomes of peripheral nerve repair.

Enhancing neuroinduction activity of PLCL-based nerve conduits through native epineurium integration

Autologous nerve grafts have been considered the gold standard for peripheral nerve grafts. However, due to drawbacks such as functional loss in the donor area and a shortage of donor sources, nerve conduits are increasingly being considered as an alternative approach. Polymer materials have been widely studied as nerve repair materials due to their excellent processing performance. However, their limited biocompatibility has restricted further clinical applications. The epineurium is a natural extra-neural wrapping structure. After undergoing decellularization, the epineurium not only reduces immune rejection but also retains certain bioactive components. In this study, decellularized epineurium (DEP) derived from the sciatic nerve of mammals was prepared, and a bilayer nerve conduit was created by electrospinning a poly (l-lactide-co-ε-caprolactone) (PLCL) membrane layer onto the outer surface of the DEP. Components of the DEP were examined; the physical properties and biosafety of the bilayer nerve conduit were evaluated; and the functionality of the nerve conduit was evaluated in rats. The results demonstrate that the developed bilayer nerve conduit exhibits excellent biocompatibility and mechanical properties. Furthermore, this bilayer nerve conduit shows significantly superior therapeutic effects for sciatic nerve defects in rats compared to the pure PLCL nerve conduit. In conclusion, this research provides a novel strategy for the design of nerve regeneration materials and holds promising potential for further clinical translation.

Interactions between Schwann cell and extracellular matrix in peripheral nerve regeneration

Peripheral nerve injuries, caused by various reasons, often lead to severe sensory, motor, and autonomic dysfunction or permanent disability, posing a challenging problem in regenerative medicine. Autologous nerve transplantation has been the gold standard in traditional treatments but faces numerous limitations and risk factors, such as donor area denervation, increased surgical complications, and diameter or nerve bundle mismatches. The extracellular matrix (ECM) is a complex molecular network synthesized and released into the extracellular space by cells residing in tissues or organs. Its main components include collagen, proteoglycans/glycosaminoglycans, elastin, laminin, fibronectin, etc., providing structural and biochemical support to surrounding cells, crucial for cell survival and growth. Schwann cells, as the primary glial cells in the peripheral nervous system, play various important roles. Schwann cell transplantation is considered the gold standard in cell therapy for peripheral nerve injuries, making ECM derived from Schwann cells one of the most suitable biomaterials for peripheral nerve repair. To better understand the mechanisms of Schwann cells and the ECM in peripheral nerve regeneration and their optimal application, this review provides an overview of their roles in peripheral nerve regeneration.

Identification of Anoikis-Related Genes in Spinal Cord Injury: Bioinformatics and Experimental Validation

Spinal cord injury (SCI) is a serious disease without effective therapeutic strategies. To identify the potential treatments for SCI, it is extremely important to explore the underlying mechanism. Current studies demonstrate that anoikis might play an important role in SCI. In this study, we aimed to identify the key anoikis-related genes (ARGs) providing therapeutic targets for SCI. The mRNA expression matrix of GSE45006 was downloaded from the Gene Expression Omnibus (GEO) database, and the ARGs were downloaded from the Molecular Signatures Database (MSigDB database). Then, the potential differentially expressed ARGs were identified. Next, correlation analysis, gene ontology (GO) enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis, and protein-protein interaction (PPI) analysis were employed for the differentially expressed ARGs. Moreover, miRNA-gene networks were constructed by the hub ARGs. Finally, RNA expression of the top ten hub ARGs was validated in the SCI cell model and rat SCI model. A total of 27 common differentially expressed ARGs were identified at different time points (1, 3, 7, and 14 days) following SCI. The GO and KEGG enrichment analysis of these ARGs indicated several enriched terms related to proliferation, cell cycle, and apoptotic process. The PPI results revealed that most of the ARGs interacted with each other. Ten hub ARGs were further screened, and all the 10 genes were validated in the SCI cell model. In the rat model, only seven genes were validated eventually. We identified 27 differentially expressed ARGs of the SCI through bioinformatic analysis. Seven real hub ARGs (CCND1, FN1, IGF1, MYC, STAT3, TGFB1, and TP53) were identified eventually. These results may expand our understanding of SCI and contribute to the exploration of potential SCI targets.

Role of transforming growth factor-β in peripheral nerve regeneration

Injuries caused by trauma and neurodegenerative diseases can damage the peripheral nervous system and cause functional deficits. Unlike in the central nervous system, damaged axons in peripheral nerves can be induced to regenerate in response to intrinsic cues after reprogramming or in a growth-promoting microenvironment created by Schwann cells. However, axon regeneration and repair do not automatically result in the restoration of function, which is the ultimate therapeutic goal but also a major clinical challenge. Transforming growth factor (TGF) is a multifunctional cytokine that regulates various biological processes including tissue repair, embryo development, and cell growth and differentiation. There is accumulating evidence that TGF-β family proteins participate in peripheral nerve repair through various factors and signaling pathways by regulating the growth and transformation of Schwann cells; recruiting specific immune cells; controlling the permeability of the blood-nerve barrier, thereby stimulating axon growth; and inhibiting remyelination of regenerated axons. TGF-β has been applied to the treatment of peripheral nerve injury in animal models. In this context, we review the functions of TGF-β in peripheral nerve regeneration and potential clinical applications.

Biomaterials and Cell Therapy Combination in Central Nervous System Treatments

Current pharmacological and surgical therapies for the central nervous system (CNS) show a limited capacity to reduce the damage progression; that together with the intrinsic limited capability of the CNS to regenerate greatly reduces the hopes of recovery. Among all the therapies proposed, the tissue engineering strategies supplemented with therapeutic stem cells remain the most promising. Neural tissue engineering strategies are based on the development of devices presenting optimal physical, chemical, and mechanical properties which, once inserted in the injured site, can support therapeutic cells, limiting the effect of a hostile environment and supporting regenerative processes. Thus, this review focuses on the employment of hydrogel and nanofibrous scaffolds supplemented with stem cells as promising therapeutic tools for the central and peripheral nervous systems in preclinical and clinical applications.

Strain-induced bands of Büngner formation promotes axon growth in 3D tissue-engineered constructs

Treatment of peripheral nerve lesions remains a major challenge due to poor functional recovery; hence, ongoing research efforts strive to enhance peripheral nerve repair. In this study, we aimed to establish three-dimensional tissue-engineered bands of Büngner constructs by subjecting Schwann cells (SCs) embedded in fibrin hydrogels to mechanical stimulation. We show for the first time that the application of strain induces (i) longitudinal alignment of SCs resembling bands of Büngner, and (ii) the expression of a pronounced repair SC phenotype as evidenced by upregulation of BDNF, NGF, and p75NTR. Furthermore, we show that mechanically aligned SCs provide physical guidance for migrating axons over several millimeters in vitro in a co-culture model with rat dorsal root ganglion explants. Consequently, these constructs hold great therapeutic potential for transplantation into patients and might also provide a physiologically relevant in vitro peripheral nerve model for drug screening or investigation of pathologic or regenerative processes.

Potential role of Schwann cells in neuropathic pain

Neuropathic pain (NPP) is a common syndrome associated with most forms of disease, which poses a serious threat to human health. NPP may persist even after the nociceptive stimulation is eliminated, and treatment is extremely challenging in such cases. Schwann cells (SCs) form the myelin sheaths around neuronal axons and play a crucial role in neural information transmission. SCs can secrete trophic factors to nourish and protect axons, and can further secrete pain-related factors to induce pain. SCs may be activated by peripheral nerve injury, triggering the transformation of myelinated and non-myelinated SCs into cell phenotypes that specifically promote repair. These differentiated SCs provide necessary signals and spatial clues for survival, axonal regeneration, and nerve regeneration of damaged neurons. They can further change the microenvironment around the regions of nerve injury, and relieve the pain by repairing the injured nerve. Herein, we provide a comprehensive overview of the biological characteristics of SCs, discuss the relationship between SCs and nerve injury, and explore the potential mechanism of SCs and the occurrence of NPP. Moreover, we summarize the feasible strategies of SCs in the treatment of NPP, and attempt to elucidate the deficiencies and defects of SCs in the treatment of NPP.

Benefit of Adjuvant Mesenchymal Stem Cell Transplantation to Critical-Sized Peripheral Nerve Defect Repair: A Systematic Review and Meta-Analysis of Preclinical Studies

Critically sized nerve defects cause devastating life-long disabilities and require interposition for reconstruction. Additional local application of mesenchymal stem cells (MSCs) is considered promising to enhance peripheral nerve regeneration. To better understand the role of MSCs in peripheral nerve reconstruction, we performed a systematic review and meta-analysis of the effects of MSCs on critically sized segment nerve defects in preclinical studies. 5146 articles were screened following PRISMA guidelines using PubMed and Web of Science. A total of 27 preclinical studies (n = 722 rats) were included in the meta-analysis. The mean difference or the standardized mean difference with 95% confidence intervals for motor function, conduction velocity, and histomorphological parameters of nerve regeneration, as well as the degree of muscle atrophy, was compared in rats with critically sized defects and autologous nerve reconstruction treated with or without MSCs. The co-transplantation of MSCs increased the sciatic functional index (3.93, 95% CI 2.62 to 5.24, p < 0.00001) and nerve conduction velocity recovery (1.49, 95% CI 1.13 to 1.84, p = 0.009), decreased the atrophy of targeted muscles (gastrocnemius: 0.63, 95% CI 0.29 to 0.97 p = 0.004; triceps surae: 0.08, 95% CI 0.06 to 0.10 p = 0.71), and promoted the regeneration of injured axons (axon number: 1.10, 95% CI 0.78 to 1.42, p < 0.00001; myelin sheath thickness: 0.15, 95% CI 0.12 to 0.17, p = 0.28). Reconstruction of critically sized peripheral nerve defects is often hindered by impaired postoperative regeneration, especially in defects that require an autologous nerve graft. This meta-analysis indicates that additional application of MSC can enhance postoperative peripheral nerve regeneration in rats. Based on the promising results in vivo experiments, further studies are needed to demonstrate potential clinical benefits.

Development and In Vitro Differentiation of Schwann Cells

Schwann cells are glial cells of the peripheral nervous system. They exist in several subtypes and perform a variety of functions in nerves. Their derivation and culture in vitro are interesting for applications ranging from disease modeling to tissue engineering. Since primary human Schwann cells are challenging to obtain in large quantities, in vitro differentiation from other cell types presents an alternative. Here, we first review the current knowledge on the developmental signaling mechanisms that determine neural crest and Schwann cell differentiation in vivo. Next, an overview of studies on the in vitro differentiation of Schwann cells from multipotent stem cell sources is provided. The molecules frequently used in those protocols and their involvement in the relevant signaling pathways are put into context and discussed. Focusing on hiPSC- and hESC-based studies, different protocols are described and compared, regarding cell sources, differentiation methods, characterization of cells, and protocol efficiency. A brief insight into developments regarding the culture and differentiation of Schwann cells in 3D is given. In summary, this contribution provides an overview of the current resources and methods for the differentiation of Schwann cells, it supports the comparison and refinement of protocols and aids the choice of suitable methods for specific applications.

Peripheral Nerve Regeneration–Adipose-Tissue-Derived Stem Cells Differentiated by a Three-Step Protocol Promote Neurite Elongation via NGF Secretion

The lack of supportive Schwann cells in segmental nerve lesions seems to be one cornerstone for the problem of insufficient nerve regeneration. Lately, adipose-tissue-derived stem cells (ASCs) differentiated towards SC (Schwann cell)-like cells seem to fulfill some of the needs for ameliorated nerve recovery. In this study, three differentiation protocols were investigated for their ability to differentiate ASCs from rats into specialized SC phenotypes. The differentiated ASCs (dASCs) were compared for their expressions of neurotrophins (NGF, GDNF, BDNF), myelin markers (MBP, P0), as well as glial-marker proteins (S100, GFAP) by RT-PCR, ELISA, and Western blot. Additionally, the influence of the medium conditioned by dASCs on a neuron-like cell line was evaluated. The dASCs were highly diverse in their expression profiles. One protocol yielded relatively high expression rates of neurotrophins, whereas another protocol induced myelin-marker expression. These results were reproducible when the ASCs were differentiated on surfaces potentially used for nerve guidance conduits. The NGF secretion affected the neurite outgrowth significantly. It remains uncertain what features of these SC-like cells contribute the most to adequate functional recovery during the different phases of nerve recovery. Nevertheless, therapeutic applications should consider these diverse phenotypes as a potential approach for stem-cell-based nerve-injury treatment.

Systematic review of the therapeutic use of Schwann cells in the repair of peripheral nerve injuries: Advancements from animal studies to clinical trials

Objective

To systematically evaluate the literature on the therapeutic use of Schwann cells (SC) in the repair of peripheral nerve injuries.

Methods

The Cochrane Library and PubMed databases were searched using terms [(“peripheral nerve injury” AND “Schwann cell” AND “regeneration”) OR (“peripheral nerve injuries”)]. Studies published from 2008 to 2022 were eligible for inclusion in the present study. Only studies presenting data from in-vivo investigations utilizing SCs in the repair of peripheral nerve injuries qualified for review. Studies attempting repair of a gap of ≥10 mm were included. Lastly, studies needed to have some measure of quantifiable regenerative outcome data such as histomorphometry, immunohistochemical, electrophysiology, or other functional outcomes.

Results

A search of the PubMed and Cochrane databases revealed 328 studies. After screening using the abstracts and methods, 17 studies were found to meet our inclusion criteria. Good SC adherence and survival in conduit tubes across various studies was observed. Improvement in morphological and functional outcomes with the use of SCs in long gap peripheral nerve injuries was observed in nearly all studies.

Conclusion

Based on contemporary literature, SCs have demonstrated clear potential in the repair of peripheral nerve injury in animal studies. It has yet to be determined which nerve conduit or graft will prove superior for delivery and retention of SCs for nerve regeneration. Recent developments in isolation and culturing techniques will enable further translational utilization of SCs in future clinical trials.

Therapeutic effect of paeoniflorin on chronic constriction injury of the sciatic nerve via the inhibition of Schwann cell apoptosis

Therapeutic drugs of chronic neuralgia have a high risk of addiction, making it crucial to identify novel drugs for chronic neuralgia. This study aimed to explore the therapeutic effect of paeoniflorin on chronic sciatica via inhibiting Schwann cell apoptosis. 28 SD rats were randomly divided into four groups, including the sham operation group, chronic constriction injury (CCI) group, mecobalamin group, and paeoniflorin group. The therapeutic effect and mechanism of paeoniflorin were evaluated via rat and cell experiments. Mechanical, hot, or cold hyperalgesia was induced in the rats after CCI operation, while paeoniflorin relieved chronic neuralgia. Besides, paeoniflorin decreased the levels of IL1, IL6, TNF‐α, CRP, and LPS and increased the level of IL10 in serum. As for the sciatic nerve, the number of inflammatory cells was decreased, and Schwann cells were present after paeoniflorin treatment, and paeoniflorin promoted the recovery of nerve structure. In cell experiments, LPS induced Schwann cell apoptosis via the TLR4/NF‐kB pathway. And paeoniflorin attenuated LPS‐induced Schwann cell apoptosis by decreasing the levels of TLR4, p‐NF‐kB, caspase3, cleaved‐caspase3, and cleaved‐caspase7. Overall, these results suggest that paeoniflorin alleviates chronic sciatica by decreasing inflammatory factor levels and promotes the repair of damaged nerves by reducing Schwann cell apoptosis.

Regulation of Schwann Cell and DRG Neurite Behaviors within Decellularized Peripheral Nerve Matrix

Decellularized nerve hydrogels (dNHs) containing bioactive molecules are promising biomaterials for peripheral nerve injury (PNI) treatment and have been extensively applied in clinical and preclinical practice. However, most previous research projects studied their influences on nerve-related cellular behaviors in two dimensions (2D) without taking hydrogel biomechanics into consideration. The molecular mechanisms underlying the beneficial microenvironment provided by dNHs also remain unclear. In this study, dNHs from rat sciatic nerves were prepared, and their effects on Schwann cell (SC) and dorsal root ganglion (DRG) neurite behaviors were evaluated and compared to commercial rat tail type I collagen (Col) hydrogels in three-dimensional (3D) environments. We found that dNHs could promote SC proliferation and neurite outgrowth, and both the hydrogel mechanics and components contributed to the dNH functionalization. Through proteomics analysis, we found that laminin (LAM) and type V collagen (COLV) exclusively and abundantly existed in dNHs. By adding exogenous LAM and COLV into Col hydrogels, we demonstrated that they regulated SC gene expression and that LAM could promote SC spreading and neurite outgrowth, while COLV improved SC proliferation. Lastly, dNHs were fabricated into paper-like, aligned nerve scaffolds through unidirectional freezing to expand the dNH applications in PNI treatment.

Fibronectin-Enriched Biomaterials, Biofunctionalization, and Proactivity: A Review

Modern innovation in reconstructive medicine implies the proposition of material-based strategies suitable for tissue repair and regeneration. The development of such systems necessitates the design of advanced materials and the control of their interactions with their surrounding cellular and molecular microenvironments. Biomaterials must actively engage cellular matter to direct and modulate biological responses at implant sites and beyond. Indeed, it is essential that a true dialogue exists between the implanted device and the cells. Biomaterial engineering implies the knowledge and control of cell fate considering the globality of the adhesion process, from initial cell attachment to differentiation. The extracellular matrix (ECM) represents a complex microenvironment able to meet these essential needs to establish a relationship between the material and the contacting cells. The ECM exhibits specific physical, chemical, and biochemical characteristics. Considering the complexity, heterogeneity, and versatility of ECM actors, fibronectin (Fn) has emerged among the ECM protagonists as the most pertinent representative key actor. The following review focuses on and synthesizes the research supporting the potential to use Fn in biomaterial functionalization to mimic the ECM and enhance cell–material interactions.

Tissue Engineering Strategies for Peripheral Nerve Regeneration

A peripheral nerve injury (PNI) has severe and profound effects on the life of a patient. The therapeutic approach remains one of the most challenging clinical problems. In recent years, many constructive nerve regeneration schemes are proposed at home and abroad. Nerve tissue engineering plays an important role. It develops an ideal nerve substitute called artificial nerve. Given the complexity of nerve regeneration, this review summarizes the pathophysiology and tissue-engineered repairing strategies of the PNI. Moreover, we discussed the scaffolds and seed cells for neural tissue engineering. Furthermore, we have emphasized the role of 3D printing in tissue engineering.

Engineered neural tissue made using clinical-grade human neural stem cells supports regeneration in a long gap peripheral nerve injury model

A surgical autograft remains the clinical gold-standard therapy for gap repair following peripheral nerve injury, however, challenges remain with achieving full recovery and reducing donor-site morbidity. Engineered Neural Tissue (EngNT) manufactured using differentiated CTX0E03 human stem cells (EngNT-CTX) has been developed as a potential 'off the shelf' allogeneic autograft replacement. Ensheathed within a collagen membrane developed to facilitate biomechanical integration, EngNT-CTX was used to bridge a critical-length (15 mm) sciatic nerve gap injury in athymic nude rats. The effectiveness of EngNT-CTX was compared to an autograft using outcome measures that assessed neuronal regeneration and functional recovery at 8 and 16 weeks. At both time points EngNT-CTX restored electrophysiological nerve conduction and functional reinnervation of downstream muscles to the same extent as the autograft. Histological analysis confirmed that more motor neurons had successfully regenerated through the repair in EngNT-CTX in comparison to the autograft at 8 weeks, which was consistent with the electrophysiology, with the number of motor neurons similar in both groups by 16 weeks. The total number of neurons (motor + sensory) was greater in autografts than EngNT-CTX at 8 weeks, indicating that more sensory fibres may have sprouted in those animals at this time point. In conclusion, this study provides evidence to support the effectiveness of EngNT-CTX as a replacement for the nerve autograft, as the functional regeneration assessed through histological and electrophysiological outcome measures demonstrated equivalent performance. STATEMENT OF SIGNIFICANCE: Following injury a peripheral nerve has the capacity to regenerate naturally, however, in the case of severe damage where there is a gap the current gold-standard microsurgical intervention is an autograft. This is associated with serious limitations including tissue availability and donor-site morbidity. Tissue engineering aims to overcome these limitations by building a construct from therapeutic cells and biomaterials as a means to mimic and replace the autograft. In this study engineered neural tissue (EngNT) was manufactured using human stem cells (CTX) to bridge a critical-length gap injury. When compared to the autograft in an animal model the EngNT-CTX construct restored function to an equivalent or greater extent.

'EngNT' - Engineering live neural tissue for nerve replacement

Peripheral nerve injury can result in severe long-term disability and current clinical approaches for repairing large gaps rely on the nerve autograft. Engineered Neural Tissue (EngNT) has been developed to provide living aligned therapeutic cells in a stabilised collagen hydrogel, mimicking the key features of the autograft. This Perspective article will introduce the field and discuss the current stage of translation, highlighting the key opportunities for commercial and clinical development.

Natural Biomaterials as Instructive Engineered Microenvironments That Direct Cellular Function in Peripheral Nerve Tissue Engineering

Nerve tissue function and regeneration depend on precise and well-synchronised spatial and temporal control of biological, physical, and chemotactic cues, which are provided by cellular components and the surrounding extracellular matrix. Therefore, natural biomaterials currently used in peripheral nerve tissue engineering are selected on the basis that they can act as instructive extracellular microenvironments. Despite emerging knowledge regarding cell-matrix interactions, the exact mechanisms through which these biomaterials alter the behaviour of the host and implanted cells, including neurons, Schwann cells and immune cells, remain largely unclear. Here, we review some of the physical processes by which natural biomaterials mimic the function of the extracellular matrix and regulate cellular behaviour. We also highlight some representative cases of controllable cell microenvironments developed by combining cell biology and tissue engineering principles.

Comprehensive strategy of conduit guidance combined with VEGF producing Schwann cells accelerates peripheral nerve repair

Peripheral nerve regeneration requires stepwise and well-organized establishment of microenvironment. Since local delivery of VEGF-A in peripheral nerve repair is expected to promote angiogenesis in the microenvironment and Schwann cells (SCs) play critical role in nerve repair, combination of VEGF and Schwann cells may lead to efficient peripheral nerve regeneration. VEGF-A overexpressing Schwann cells were established and loaded into the inner wall of hydroxyethyl cellulose/soy protein isolate/polyaniline sponge (HSPS) conduits. When HSPS is mechanically distorted, it still has high durability of strain strength, thus, can accommodate unexpected strain of nerve tissues in motion. A 10 mm nerve defect rat model was used to test the repair performance of the HSPS-SC (VEGF) conduits, meanwhile the HSPS, HSPS-SC, HSPS-VEGF conduits and autografts were worked as controls. The immunofluorescent co-staining of GFP/VEGF-A, Ki67 and MBP showed that the VEGF-A overexpressing Schwann cells could promote the proliferation, migration and differentiation of Schwann cells as the VEGF-A was secreted from the VEGF-A overexpressing Schwann cells. The nerve repair performance of the multifunctional and flexible conduits was examined though rat behavioristics, electrophysiology, nerve innervation to gastrocnemius muscle (GM), toluidine blue (TB) staining, transmission electron microscopy (TEM) and NF200/S100 double staining in the regenerated nerve. The results displayed that the effects on the repair of peripheral nerves in HSPS-SC (VEGF) group was the best among the conduits groups and closed to autografts. HSPS-SC (VEGF) group exhibited notably increased CD31⁺ endothelial cells and activation of VEGFR2/ERK signaling pathway in the regenerated nerve tissues, which probably contributed to the improved nerve regeneration. Altogether, the comprehensive strategy including VEGF overexpressing Schwann cells-mediated and HSPS conduit-guided peripheral nerve repair provides a new avenue for nerve tissue engineering.

Human Platelet Lysate Acts Synergistically With Laminin to Improve the Neurotrophic Effect of Human Adipose-Derived Stem Cells on Primary Neurons in vitro

Background

Despite the advancements in microsurgical techniques and noteworthy research in the last decade, peripheral nerve lesions have still weak functional outcomes in current clinical practice. However, cell transplantation of human adipose-derived stem cells (hADSC) in a bioengineered conduit has shown promising results in animal studies. Human platelet lysate (hPL) has been adopted to avoid fetal bovine serum (FBS) in consideration of the biosafety concerns inherent with the use of animal-derived products in tissue processing and cell culture steps for translational purposes. In this work, we investigate how the interplay between hPL-expanded hADSC (hADSC^hPL) and extracellular matrix (ECM) proteins influences key elements of nerve regeneration.

Methods

hADSC were seeded on different ECM coatings (laminin, LN; fibronectin, FN) in hPL (or FBS)-supplemented medium and co-cultured with primary dorsal root ganglion (DRG) to establish the intrinsic effects of cell–ECM contact on neural outgrowth. Co-cultures were performed “direct,” where neural cells were seeded in contact with hADSC expanded on ECM-coated substrates (contact effect), or “indirect,” where DRG was treated with their conditioned medium (secretome effect). Brain-derived nerve factor (BDNF) levels were quantified. Tissue culture plastic (TCPS) was used as the control substrate in all the experiments.

Results

hPL as supplement alone did not promote higher neurite elongation than FBS when combined with DRG on ECM substrates. However, in the presence of hADSC, hPL could dramatically enhance the stem cell effect with increased DRG neurite outgrowth when compared with FBS conditions, regardless of the ECM coating (in both indirect and direct co-cultures). The role of ECM substrates in influencing neurite outgrowth was less evident in the FBS conditions, while it was significantly amplified in the presence of hPL, showing better neural elongation in LN conditions when compared with FN and TCPS. Concerning hADSC growth factor secretion, ELISA showed significantly higher concentrations of BDNF when cells were expanded in hPL compared with FBS-added medium, without significant differences between cells cultured on the different ECM substrates.

Conclusion

The data suggest how hADSC grown on LN and supplemented with hPL could be active and prone to support neuron–matrix interactions. hPL enhanced hADSC effects by increasing both proliferation and neurotrophic properties, including BDNF release.

Engineered Tissues Made from Human iPSC-Derived Schwann Cells for Investigating Peripheral Nerve Regeneration In Vitro

Peripheral nerves have a limited ability to regenerate and current clinical approaches involving microsurgery give suboptimal recovery. Engineered tissues using aligned cellular collagen hydrogels can be used as in vitro models through the incorporation of human Schwann cells. However, primary human Schwann cells are difficult to obtain and can be challenging to culture. The ability to generate Schwann cells from human-induced pluripotent stem cells (hiPSCs) provides a more reliable cell source for modeling peripheral nerve tissue. Here, we describe protocols for generating hiPSC-derived Schwann cells and incorporating them into 3D engineered tissue culture models for peripheral nerve research.

Advances and clinical challenges for translating nerve conduit technology from bench to bed side for peripheral nerve repair

Injuries to the peripheral nervous system remain a large-scale clinical problem. These injuries often lead to loss of motor and/or sensory function that significantly affects patients' quality of life. The current neurosurgical approach for peripheral nerve repair involves autologous nerve transplantation, which often leads to clinical complications. The most pressing need is to increase the regenerative capacity of existing tubular constructs in the repair of large nerve gaps through development of tissue-engineered approaches that can surpass the performance of autografts. To fully realize the clinical potential of nerve conduit technology, there is a need to reconsider design strategies, biomaterial selection, fabrication techniques and the various potential modifications to optimize a conduit microenvironment that can best mimic the natural process of regeneration. In recent years, a significant progress has been made in the designing and functionality of bioengineered nerve conduits to bridge long peripheral nerve gaps in various animal models. However, translation of this work from lab to commercial scale has not been achieve. The current review summarizes recent advances in the development of tissue engineered nerve guidance conduits (NGCs) with regard to choice of material, novel fabrication methods, surface modifications and regenerative cues such as stem cells and growth factors to improve regeneration performance. Also, the current clinical potential and future perspectives to achieve therapeutic benefits of NGCs will be discussed in context of peripheral nerve regeneration.

Biomimetic Approaches for Separated Regeneration of Sensory and Motor Fibers in Amputee People: Necessary Conditions for Functional Integration of Sensory-Motor Prostheses With the Peripheral Nerves

The regenerative capacity of the peripheral nervous system after an injury is limited, and a complete function is not recovered, mainly due to the loss of nerve tissue after the injury that causes a separation between the nerve ends and to the disorganized and intermingled growth of sensory and motor nerve fibers that cause erroneous reinnervations. Even though the development of biomaterials is a very promising field, today no significant results have been achieved. In this work, we study not only the characteristics that should have the support that will allow the growth of nerve fibers, but also the molecular profile necessary for a specific guidance. To do this, we carried out an exhaustive study of the molecular profile present during the regeneration of the sensory and motor fibers separately, as well as of the effect obtained by the administration and inhibition of different factors involved in the regeneration. In addition, we offer a complete design of the ideal characteristics of a biomaterial, which allows the growth of the sensory and motor neurons in a differentiated way, indicating (1) size and characteristics of the material; (2) necessity to act at the microlevel, on small groups of neurons; (3) combination of molecules and specific substrates; and (4) temporal profile of those molecules expression throughout the regeneration process. The importance of the design we offer is that it respects the complexity and characteristics of the regeneration process; it indicates the appropriate temporal conditions of molecular expression, in order to obtain a synergistic effect; it takes into account the importance of considering the process at the group of neuron level; and it gives an answer to the main limitations in the current studies.

Tyrosine-derived polycarbonate nerve guidance tubes elicit proregenerative extracellular matrix deposition when used to bridge segmental nerve defects in swine

Promising biomaterials should be tested in appropriate large animal models that recapitulate human inflammatory and regenerative responses. Previous studies have shown tyrosine-derived polycarbonates (TyrPC) are versatile biomaterials with a wide range of applications across multiple disciplines. The library of TyrPC has been well studied and consists of thousands of polymer compositions with tunable mechanical characteristics and degradation and resorption rates that are useful for nerve guidance tubes (NGTs). NGTs made of different TyrPCs have been used in segmental nerve defect models in small animals. The current study is an extension of this work and evaluates NGTs made using two different TyrPC compositions in a 1 cm porcine peripheral nerve repair model. We first evaluated a nondegradable TyrPC formulation, demonstrating proof-of-concept chronic regenerative efficacy up to 6 months with similar nerve/muscle electrophysiology and morphometry to the autograft repair control. Next, we characterized the acute regenerative response using a degradable TyrPC formulation. After 2 weeks in vivo, TyrPC NGT promoted greater deposition of pro-regenerative extracellular matrix (ECM) constituents (in particular collagen I, collagen III, collagen IV, laminin, and fibronectin) compared to commercially available collagen-based NGTs. This corresponded with dense Schwann cell infiltration and axon extension across the lumen. These findings confirmed results reported previously in a mouse model and reveal that TyrPC NGTs were well tolerated in swine and facilitated host axon regeneration and Schwann cell infiltration in the acute phase across segmental defects - likely by eliciting a favorable neurotrophic ECM milieu. This regenerative response ultimately can contribute to functional recovery.

Application of bioactive hydrogels combined with dental pulp stem cells for the repair of large gap peripheral nerve injuries

Due to the limitations in autogenous nerve grafting or Schwann cell transplantation, large gap peripheral nerve injuries require a bridging strategy supported by nerve conduit. Cell based therapies provide a novel treatment for peripheral nerve injuries. In this study, we first experimented an optimal scaffold material synthesis protocol, from where we selected the 10% GFD formula (10% GelMA hydrogel, recombinant human basic fibroblast growth factor and dental pulp stem cells (DPSCs)) to fill a cellulose/soy protein isolate composite membrane (CSM) tube to construct a third generation of nerve regeneration conduit, CSM-GFD. Then this CSM-GFD conduit was applied to repair a 15-mm long defect of sciatic nerve in a rat model. After 12 week post implant surgery, at histologic level, we found CSM-GFD conduit could regenerate nerve tissue like neuron and Schwann like nerve cells and myelinated nerve fibers. At physical level, CSM-GFD achieved functional recovery assessed by a sciatic functional index study. In both levels, CSM-GFD performed like what gold standard, the nerve autograft, could do. Further, we unveiled that almost all newly formed nerve tissue at defect site was originated from the direct differentiation of exogeneous DPSCs in CSM-GFD. In conclusion, we claimed that this third-generation nerve regeneration conduit, CSM-GFD, could be a promising tissue engineering approach to replace the conventional nerve autograft to treat the large gap defect in peripheral nerve injuries.

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A novel 3^rd generation nerve conduit was successfully constructed and applied for repairing peripheral nerve injuries (PNI).

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Dental pulp stem cells (DPSCs) was optimized as an ideal seeding cells for nerve regeneration.

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A bioactive system combining GelMA with human bFGF and DPSCs could reconstruct the long gap PNI within 12 weeks in vivo.

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Our system could achieve the same outcome in nerve repair as that of autografting, a routine treatment for PNI.

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The proposed bioactive system may trigger an evolutional change into the current clinical practice in managing PNI.

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The majority of the regenerated nerve tissue was originated from the donor’s dental pulp stem cells.

Modification of tubular chitosan-based peripheral nerve implants: applications for simple or more complex approaches

Surgical treatment of peripheral nerve injuries is still a major challenge in human clinic. Up to now, none of the well-developed microsurgical treatment options is able to guarantee a complete restoration of nerve function. This restriction is also effective for novel clinically approved artificial nerve guides. In this review, we compare surgical repair techniques primarily for digital nerve injuries reported with relatively high prevalence to be valuable attempts in clinical digital nerve repair and point out their advantages and shortcomings. We furthermore discuss the use of artificial nerve grafts with a focus on chitosan-based nerve guides, for which our own studies contributed to their approval for clinical use. In the second part of this review, very recent future perspectives for the enhancement of tubular (commonly hollow) nerve guides are discussed in terms of their clinical translatability and ability to form three-dimensional constructs that biomimick the natural nerve structure. This includes materials that have already shown their beneficial potential in in vivo studies like fibrous intraluminal guidance structures, hydrogels, growth factors, and approaches of cell transplantation. Additionally, we highlight upcoming future perspectives comprising co-application of stem cell secretome. From our overview, we conclude that already simple attempts are highly effective to increase the regeneration supporting properties of nerve guides in experimental studies. But for bringing nerve repair with bioartificial nerve grafts to the next level, e.g. repair of defects > 3 cm in human patients, more complex intraluminal guidance structures such as innovatively manufactured hydrogels and likely supplementation of stem cells or their secretome for therapeutic purposes may represent promising future perspectives.

A Systematic Review of the Effectiveness of Cell-Based Therapy in Repairing Peripheral Nerve Gap Defects

Nerve prostheses are widely utilized to reconstruct segmental (gap) defects in peripheral nerves as an alternative to nerve grafting. However, with increasing gap length, the effectiveness of a nerve prosthesis becomes sub-optimal, which subsequently has made repairing larger gaps in peripheral nerves a significant challenge in the field of regenerative medicine. Recently, the structure of nerve prostheses has been significantly revised, which interestingly, has provided a promising avenue for the housing and proliferation of supportive cells. In this systematic review, cell implantation in synthetic nerve prostheses to enhance the regenerative capability of an injured nerve with a focus on identifying the cell type and mode of cell delivery is discussed. Of interest are the studies employing supportive cells to bridge gaps greater than 10 mm without the aid of nerve growth factors. The results have shown that cell therapy in conjunction with nerve prostheses becomes inevitable and has dramatically boosted the ability of these prostheses to maintain sustainable nerve regeneration across larger gaps and helped to attain functional recovery, which is the ultimate goal. The statistical analysis supports the use of differentiated bone-marrow-derived mesenchymal stem cells suspended in oxygen-carrying hydrogels in chitosan prostheses for bridging gaps of up to 40 mm; however, based on the imperfect repair outcomes, nerve grafting should not yet be replaced altogether.

Regeneration of sciatic nerves by transplanted microvesicles of human neural stem cells derived from embryonic stem cells

Injured nerves cannot regenerate on their own, and a lack of engraftable human nerves has been a major obstacle in cell-based therapies for regenerating damaged nerves. A monolayer culture approach to obtain adherent neural stem cells from human embryonic stem cells (hESC-NSCs) was established, and the greatest number of stemness characteristics were achieved by the eighth generation of hESC-NSCs (P8 hESC-NSCs). To overcome deficits in cell therapy, we used microvesicles secreted from P8 hESC-NSCs (hESC-NSC-MVs) instead of entire hESC-NSCs. To investigate the therapeutic efficacy of hESC-NSC-MVs in vitro, hESC-NSC-MVs were cocultured with dorsal root ganglia to determine the length of axons. In vivo, we transected the sciatic nerve in SD rats and created a 5-mm gap. A sciatic nerve defect was bridged using a silicone tube filled with hESC-NSC-MVs (45 μg) in the MVs group, P8 hESC-NSCs (1 × 106 single cells) in the cell group and PBS in the control group. The hESC-NSC-MVs group showed better morphological recovery and a significantly greater number of regenerated axons than the hESC-NSCs group 12 weeks after nerve injury. These results indicated that the hESC-NSC-MVs group had the greatest ability to repair and reconstruct nerve structure and function. As a result, hESC-NSC-MVs may have potential for applications in the field of nerve regenerative repair.

Strategies for Peripheral Nerve Repair

PURPOSE OF REVIEW

This review focuses on biomechanical and cellular considerations required for development of biomaterials and engineered tissues suitable for implantation following PNI, as well as translational requirements relating to outcome measurements for testing success in patients.

RECENT FINDINGS

Therapies that incorporate multiple aspects of the regenerative environment are likely to be key to improving therapies for nerve regeneration. This represents a complex challenge when considering the diversity of biological, chemical and mechanical factors involved. In addition, clinical outcome measures following peripheral nerve repair which are sensitive and responsive to changes in the tissue microenvironment following neural injury and regeneration are required.

SUMMARY

Effective new therapies for the treatment of PNI are likely to include engineered tissues and biomaterials able to evoke a tissue microenvironment that incorporates both biochemical and mechanical features supportive to regeneration. Translational development of these technologies towards clinical use in humans drives a concomitant need for improved clinical measures to quantify nerve regeneration.

Restoration of Neurological Function Following Peripheral Nerve Trauma

Following peripheral nerve trauma that damages a length of the nerve, recovery of function is generally limited. This is because no material tested for bridging nerve gaps promotes good axon regeneration across the gap under conditions associated with common nerve traumas. While many materials have been tested, sensory nerve grafts remain the clinical “gold standard” technique. This is despite the significant limitations in the conditions under which they restore function. Thus, they induce reliable and good recovery only for patients < 25 years old, when gaps are <2 cm in length, and when repairs are performed <2–3 months post trauma. Repairs performed when these values are larger result in a precipitous decrease in neurological recovery. Further, when patients have more than one parameter larger than these values, there is normally no functional recovery. Clinically, there has been little progress in developing new techniques that increase the level of functional recovery following peripheral nerve injury. This paper examines the efficacies and limitations of sensory nerve grafts and various other techniques used to induce functional neurological recovery, and how these might be improved to induce more extensive functional recovery. It also discusses preliminary data from the clinical application of a novel technique that restores neurological function across long nerve gaps, when repairs are performed at long times post-trauma, and in older patients, even under all three of these conditions. Thus, it appears that function can be restored under conditions where sensory nerve grafts are not effective.

Regenerative Effects and Development Patterns of Solid Neural Tissue Grafts Located in Gelatin Hydrogel Conduit for Treatment of Peripheral Nerve Injury

Supplemental Digital Content is available in the text.

Background:

The regeneration of the peripheral nerves after injuries is still a challenging fundamental and clinical problem. The cell therapy and nerve guide conduit construction are promising modern approaches. Nowadays, different sources of cells for transplantation are available. But it is little known about the interaction between fetal central nervous system cells and peripheral nerve tissue. In this study, we analyzed the development of the fetal neocortex and spinal cord solid grafts injected into the gelatin hydrogel conduits and their effects on sciatic nerve regeneration after cut injury.

Methods:

Frontal neocortex tissue was obtained from E19.5 and spinal cord tissue was obtained from E14.5 fetuses harvested from transgenic EGFP mice. The grafts were injected into the hydrogel conduits which were connected to the nerve stumps after cut injury. The recovery of motor function was estimated with walking track analysis at 2, 5, and 8 weeks after surgery. Then immunohistochemical study was performed.

Results:

The histological examination showed that only fetal neocortex solid graft cells had survived after implantation. Immunostaining revealed that some of the transplanted cells expressed neural markers such as neurofilament protein and NeuN. But the cells mostly differentiated in glial lineage, which was confirmed with immunostaining for GFAP and S100β. The walking-track analysis has shown that 8 weeks after surgery bioengineered conduit differed significantly from the control.

Conclusions:

We revealed that the hydrogel conduit is suitable for nerve re-growth and that the fetal neocortex grafted cells can survive and differentiate. Bioengineered conduit can stimulate functional recovery after the nerve injury.

In Vivo and In Vitro Evaluation of a Novel Hyaluronic Acid-Laminin Hydrogel as Luminal Filler and Carrier System for Genetically Engineered Schwann Cells in Critical Gap Length Tubular Peripheral Nerve Graft in Rats

In the current study we investigated the suitability of a novel hyaluronic acid-laminin hydrogel (HAL) as luminal filler and carrier system for co-transplanted cells within a composite chitosan-based nerve graft (CNG) in a rat critical nerve defect model. The HAL was meant to improve the performance of our artificial nerve guides by giving additional structural and molecular support to regrowing axons. We filled hollow CNGs or two-chambered nerve guides with an inserted longitudinal chitosan film (CNG[F]s), with cell-free HAL or cell-free HA or additionally suspended either naïve Schwann cells (SCs) or fibroblast growth factor 2-overexpressing Schwann cells (FGF2-SCs) within the gels. We subjected female Lewis rats to immediate 15 mm sciatic nerve gap reconstruction and comprehensively compared axonal and functional regeneration parameters with the gold standard autologous nerve graft (ANG) repair. Motor recovery was surveyed by means of electrodiagnostic measurements at 60, 90, and 120 days post-reconstruction. Upon explantation after 120 days, lower limb target muscles were harvested for calculation of muscle-weight ratios. Semi-thin cross-sections of nerve segments distal to the grafts were evaluated histomorphometrically. After 120 days of recovery, only ANG treatment led to full motor recovery. Surprisingly, regeneration outcomes revealed no regeneration-supportive effect of HAL alone and even an impairment of peripheral nerve regeneration when combined with SCs and FGF2-SCs. Furthermore, complementary in vitro studies, conducted to elucidate the reason for this unexpected negative result, revealed that SCs and FGF2-SCs suspended within the hydrogel relatively downregulated gene expression of regeneration-supporting neurotrophic factors. In conclusion, cell-free HAL in its current formulation did not qualify for optimizing regeneration outcome through CNG[F]s. In addition, we demonstrate that our HAL, when used as a carrier system for co-transplanted SCs, changed their gene expression profile and deteriorated the pro-regenerative milieu within the nerve guides.

Stem-cell-based therapies to enhance peripheral nerve regeneration

Peripheral nerve injury remains a major cause of morbidity in trauma patients. Despite advances in microsurgical techniques and improved understanding of nerve regeneration, obtaining satisfactory outcomes after peripheral nerve injury remains a difficult clinical problem. There is a growing body of evidence in preclinical animal studies demonstrating the supportive role of stem cells in peripheral nerve regeneration after injury. The characteristics of both mesoderm-derived and ectoderm-derived stem cell types and their role in peripheral nerve regeneration are discussed, specifically focusing on the presentation of both foundational laboratory studies and translational applications. The current state of clinical translation is presented, with an emphasis on both ethical considerations of using stems cells in humans and current governmental regulatory policies. Current advancements in cell-based therapies represent a promising future with regard to supporting nerve regeneration and achieving significant functional recovery after debilitating nerve injuries.

The advances in nerve tissue engineering: From fabrication of nerve conduit to in vivo nerve regeneration assays

Peripheral nerve damage is a common clinical complication of traumatic injury occurring after accident, tumorous outgrowth, or surgical side effects. Although the new methods and biomaterials have been improved recently, regeneration of peripheral nerve gaps is still a challenge. These injuries affect the quality of life of the patients negatively. In the recent years, many efforts have been made to develop innovative nerve tissue engineering approaches aiming to improve peripheral nerve treatment following nerve injuries. Herein, we will not only outline what we know about the peripheral nerve regeneration but also offer our insight regarding the types of nerve conduits, their fabrication process, and factors associated with conduits as well as types of animal and nerve models for evaluating conduit function. Finally, nerve regeneration in a rat sciatic nerve injury model by nerve conduits has been considered, and the main aspects that may affect the preclinical outcome have been discussed.

Adipose Stem Cells Enhance Nerve Regeneration and Muscle Function in a Peroneal Nerve Ablation Model

Severe peripheral nerve injuries have devastating consequences on the quality of life in affected patients, and they represent a significant unmet medical need. Destruction of nerve fibers results in denervation of targeted muscles, which, subsequently, undergo progressive atrophy and loss of function. Timely restoration of neural innervation to muscle fibers is crucial to the preservation of muscle homeostasis and function. The goal of this study was to evaluate the impact of addition of adipose stem cells (ASCs) to polycaprolactone (PCL) nerve conduit guides on peripheral nerve repair and functional muscle recovery in the setting of a critical size nerve defect. To this end, peripheral nerve injury was created by surgically ablating 6 mm of the common peroneal nerve in a rat model. A PCL nerve guide, filled with ASCs and/or poloxamer hydrogel, was sutured to the nerve ends. Negative and positive controls included nerve ablation only (no repair), and reversed polarity autograft nerve implant, respectively. Tibialis anterior (TA) muscle function was assessed at 4, 8, and 12 weeks postinjury, and nerve and muscle tissue was retrieved at the 12-week terminal time point. Inclusion of ASCs in the PCL nerve guide elicited statistically significant time-dependent increases in functional recovery (contraction) after denervation; ∼25% higher than observed in acellular (poloxamer-filled) implants and indistinguishable from autograft implants, respectively, at 12 weeks postinjury (p < 0.05, n = 7-8 in each group). Analysis of single muscle fiber cross-sectional area (CSA) revealed that ASC-based treatment of nerve injury provided a better recapitulation of the overall distribution of muscle fiber CSAs observed in the contralateral TA muscle of uninjured limbs. In addition, the presence of ASCs was associated with improved features of re-innervation distal to the defect, with respect to neurofilament and S100 (Schwann cell marker) expression. In conclusion, these initial studies indicate significant benefits of inclusion of ASCs to the rate and magnitude of both peripheral nerve regeneration and functional recovery of muscle contraction, to levels equivalent to autograft implantation. These findings have important implications to improved nerve repair, and they provide input for future work directed to restoration of nerve and muscle function after polytraumatic injury. Impact Statement This works explores the application of adipose stem cells (ASCs) for peripheral nerve regeneration in a rat model. Herein, we demonstrate that the addition of ASCs in poloxamer-filled PCL nerve guide conduits impacts nerve regeneration and recovery of muscle function, to levels equivalent to autograft implantation, which is considered to be the current gold standard treatment. This study builds on the importance of a timely restoration of innervation to muscle fibers for preservation of muscle homeostasis, and it will provide input for future work aiming at restoring nerve and muscle function after polytraumatic injury.

Cell Encapsulation in Soft, Anisometric Poly(ethylene) Glycol Microgels Using a Novel Radical-Free Microfluidic System

Complex 3D artificial tissue constructs are extensively investigated for tissue regeneration. Frequently, materials and cells are delivered separately without benefitting from the synergistic effect of combined administration. Cell delivery inside a material construct provides the cells with a supportive environment by presenting biochemical, mechanical, and structural signals to direct cell behavior. Conversely, the cell/material interaction is poorly understood at the micron scale and new systems are required to investigate the effect of micron-scale features on cell functionality. Consequently, cells are encapsulated in microgels to avoid diffusion limitations of nutrients and waste and facilitate analysis techniques of single or collective cells. However, up to now, the production of soft cell-loaded microgels by microfluidics is limited to spherical microgels. Here, a novel method is presented to produce monodisperse, anisometric poly(ethylene) glycol microgels to study cells inside an anisometric architecture. These microgels can potentially direct cell growth and can be injected as rod-shaped mini-tissues that further assemble into organized macroscopic and macroporous structures post-injection. Their aspect ratios are adjusted with flow parameters, while mechanical and biochemical properties are altered by modifying the precursors. Encapsulated primary fibroblasts are viable and spread and migrate across the 3D microgel structure.

Novel microRNA, miR-sc6, modulates Schwann cell phenotype via targeting ErbB4

MicroRNAs (miRNAs) are non-coding RNAs that regulate various tissues and organs, including the nervous system. Peripheral nerve injury is a common pathology of the nervous system and leads to differential expressions of a variety of miRNAs. Previously, a group of novel miRNAs have been identified in rat proximal nerve segments after sciatic nerve transection. However, the biological functions of these novel miRNAs remain undetermined. The aim of the current study was therefore to identify the function of a novel miRNA, miR-sc6, following nerve injury. Its target genes and effects on phenotypic modulation of Schwann cells were determined using a miR-sc6 mimic transfection. These observations contribute to the understanding of miRNA involvement in peripheral nerve injury and the cognition of regulatory mechanisms in peripheral nerve regeneration.

Novel approaches using mesenchymal stem cells for curing peripheral nerve injuries

Peripheral nerve injury (PNI) is a common life-changing disability of peripheral nervous system with significant socioeconomic consequences. Conventional therapeutic approaches for PNI have several drawbacks such as need to autologous nerve scarifying, surplus surgery, and difficult accessibility to donor nerve; therefore, other therapeutic strategies such as mesenchymal stem cells (MSCs) therapy are getting more interesting. MSCs have been proved to be safe and efficient in numerous degenerative diseases of central and peripheral nervous systems. In this paper, we review novel biotechnological advancements in treating PNI using MSCs.

Increased levels of miR-3099 induced by peripheral nerve injury promote Schwann cell proliferation and migration

MicroRNAs (miRNAs) can regulate the modulation of the phenotype of Schwann cells. Numerous novel miRNAs have been discovered and identified in rat sciatic nerve segments, including miR-3099. In the current study, miR-3099 expression levels following peripheral nerve injury were measured in the proximal stumps of rat sciatic nerves after surgical crush. Real-time reverse transcription-polymerase chain reaction was used to determine miR-3099 expression in the crushed nerve segment at 0, 1, 4, 7, and 14 days post sciatic nerve injury, which was consistent with Solexa sequencing outcomes. Expression of miR-3099 was up-regulated following peripheral nerve injury. EdU and transwell chamber assays were used to observe the effect of miR-3099 on Schwann cell proliferation and migration. The results showed that increased miR-3099 expression promoted the proliferation and migration of Schwann cells. However, reduced miR-3099 expression suppressed the proliferation and migration of Schwann cells. The potential target genes of miR-3099 were also investigated by bioinformatic tools and high-throughput outcomes. miR-3099 targets genes Aqp4, St8sia2, Tnfsf15, and Zbtb16 and affects the proliferation and migration of Schwann cells. This study examined the levels of miR-3099 at different time points following peripheral nerve injury. Our results confirmed that increased miR-3099 level induced by peripheral nerve injury can promote the proliferation and migration of Schwann cells.

In vivo Evaluation of Nanostructured Fibrin-Agarose Hydrogels With Mesenchymal Stem Cells for Peripheral Nerve Repair

The regenerative capability of peripheral nerves is very limited, and several strategies have been proposed to increase nerve regeneration. In the present work, we have analyzed the in vivo usefulness of a novel nanostructured fibrin-agarose bio-artificial nerve substitute (Nano) used alone or in combination with NeuraGen^® collagen type I conduits (Coll-Nano) in laboratory rats with a 10-mm sciatic nerve defect. Control animals were subjected to the gold-standard autograft technique (Auto). Results first demonstrated that the percentage of self-amputations was lower in Nano and Coll-Nano groups as compared to the Auto group. Neurotrophic ulcers were more abundant in the Auto group (60%, with 66.6% of them being >2-mm) than Nano and Coll-Nano groups (0%) at 4 weeks, although Nano showed more ulcers after 12 weeks. Foot length was significantly altered in Auto animals due to neurogenic retraction, but not in Nano and Coll-Nano groups after 12 weeks. At the functional level, all animals showed a partial sensory recovery as determined by the pinch test, especially in Nano and Auto groups, but did not reach the levels of native animals. Toe-spread test revealed a partial motor function recovery only in Nano animals at 4 weeks and Auto and Nano at 12 weeks. Electromyography showed clear denervation signs in all experimental groups, with few differences between Auto and Nano animals. After 12 weeks, an important denervation decrease and an increase of the reinnervation process was found in Auto and Nano groups, with no differences between these groups. Histological analyses demonstrated an active peripheral nerve regeneration process with newly formed peripheral nerve fascicles showing S-100, GAP-43 and myelin in all experimental groups. The peripheral nerve regeneration process was more abundant in Auto group, followed by Nano group, and both were better than Coll-Nano group. Muscle histology confirmed the electromyography results and showed some atrophy and fibrosis signs and an important weight and volume loss in all groups, especially in the Coll-Nano group (56.8% weight and 60.4% volume loss). All these results suggest that the novel Nano substitutes used in in vivo were able to contribute to bridge a 10-mm peripheral nerve defect in rats.

Matrices, scaffolds & carriers for cell delivery in nerve regeneration

Nerve injuries can be life-long debilitating traumas that severely impact patients' quality of life. While many acellular neural scaffolds have been developed to aid the process of nerve regeneration, complete functional recovery is still very difficult to achieve, especially for long-gap peripheral nerve injury and most cases of spinal cord injury. Cell-based therapies have shown many promising results for improving nerve regeneration. With recent advances in neural tissue engineering, the integration of biomaterial scaffolds and cell transplantation are emerging as a more promising approach to enhance nerve regeneration. This review provides an overview of important considerations for designing cell-carrier biomaterial scaffolds. It also discusses current biomaterials used for scaffolds that provide permissive and instructive microenvironments for improved cell transplantation.

An update-tissue engineered nerve grafts for the repair of peripheral nerve injuries

Peripheral nerve injuries (PNI) are caused by a range of etiologies and result in a broad spectrum of disability. While nerve autografts are the current gold standard for the reconstruction of extensive nerve damage, the limited supply of autologous nerve and complications associated with harvesting nerve from a second surgical site has driven groups from multiple disciplines, including biomedical engineering, neurosurgery, plastic surgery, and orthopedic surgery, to develop a suitable or superior alternative to autografting. Over the last couple of decades, various types of scaffolds, such as acellular nerve grafts (ANGs), nerve guidance conduits, and non-nervous tissues, have been filled with Schwann cells, stem cells, and/or neurotrophic factors to develop tissue engineered nerve grafts (TENGs). Although these have shown promising effects on peripheral nerve regeneration in experimental models, the autograft has remained the gold standard for large nerve gaps. This review provides a discussion of recent advances in the development of TENGs and their efficacy in experimental models. Specifically, TENGs have been enhanced via incorporation of genetically engineered cells, methods to improve stem cell survival and differentiation, optimized delivery of neurotrophic factors via drug delivery systems (DDS), co-administration of platelet-rich plasma (PRP), and pretreatment with chondroitinase ABC (Ch-ABC). Other notable advancements include conduits that have been bioengineered to mimic native nerve structure via cell-derived extracellular matrix (ECM) deposition, and the development of transplantable living nervous tissue constructs from rat and human dorsal root ganglia (DRG) neurons. Grafts composed of non-nervous tissues, such as vein, artery, and muscle, will be briefly discussed.