Decellularized peripheral nerve supports Schwann cell transplants and axon growth following spinal cord injury

The phenotypic changes of Schwann cells promote the functional repair of nerve injury

Functional recovery after nerve injury is a significant challenge due to the complex nature of nerve injury repair and the non-regeneration of neurons. Schwann cells (SCs), play a crucial role in the nerve injury repair process because of their high plasticity, secretion, and migration abilities. Upon nerve injury, SCs undergo a phenotypic change and redifferentiate into a repair phenotype, which helps in healing by recruiting phagocytes, removing myelin fragments, promoting axon regeneration, and facilitating myelin formation. However, the repair phenotype can be unstable, limiting the effectiveness of the repair. Recent research has found that transplantation of SCs can be an effective treatment option, therefore, it is essential to comprehend the phenotypic changes of SCs and clarify the related mechanisms to develop the transplantation therapy further.

Biomaterials targeting the microenvironment for spinal cord injury repair: progression and perspectives

Spinal cord injury (SCI) disrupts nerve pathways and affects sensory, motor, and autonomic function. There is currently no effective treatment for SCI. SCI occurs within three temporal periods: acute, subacute, and chronic. In each period there are different alterations in the cells, inflammatory factors, and signaling pathways within the spinal cord. Many biomaterials have been investigated in the treatment of SCI, including hydrogels and fiber scaffolds, and some progress has been made in the treatment of SCI using multiple materials. However, there are limitations when using individual biomaterials in SCI treatment, and these limitations can be significantly improved by combining treatments with stem cells. In order to better understand SCI and to investigate new strategies for its treatment, several combination therapies that include materials combined with cells, drugs, cytokines, etc. are summarized in the current review.

Exploring the properties and potential of the neural extracellular matrix for next-generation regenerative therapies

The extracellular matrix (ECM) is a dynamic and complex network of proteins and molecules that surrounds cells and tissues in the nervous system and orchestrates a myriad of biological functions. This review carefully examines the diverse interactions between cells and the ECM, as well as the transformative chemical and physical changes that the ECM undergoes during neural development, aging, and disease. These transformations play a pivotal role in shaping tissue morphogenesis and neural activity, thereby influencing the functionality of the central nervous system (CNS). In our comprehensive review, we describe the diverse behaviors of the CNS ECM in different physiological and pathological scenarios and explore the unique properties that make ECM-based strategies attractive for CNS repair and regeneration. Addressing the challenges of scalability, variability, and integration with host tissues, we review how advanced natural, synthetic, and combinatorial matrix approaches enhance biocompatibility, mechanical properties, and functional recovery. Overall, this review highlights the potential of decellularized ECM as a powerful tool for CNS modeling and regenerative purposes and sets the stage for future research in this exciting field. This article is categorized under: Implantable Materials and Surgical Technologies > Nanotechnology in Tissue Repair and Replacement Therapeutic Approaches and Drug Discovery > Nanomedicine for Neurological Disease Implantable Materials and Surgical Technologies > Nanomaterials and Implants.

PEG-SH-GNPs-SAPNS@miR-29a delivery system promotes neural regeneration and recovery of motor function after spinal cord injury

Spinal cord injury (SCI) is a serious disease characterized by hemorrhage, edema, local ischemia and hypoxia, inflammatory reaction, and degeneration of the injured spinal cord, which lacks effective clinical treatments. We design a PEG-SH-GNPs-SAPNS@miR-29a delivery system to repair impaired spinal cord by building a regenerative microenvironment for the recruitment of endogenous neural stem cells. The miR-29a, as an axonal regeneration-related miRNA that overexpression of miR-29a significantly inhibits the expression of PTEN and promotes axonal regeneration of the injured spinal cord. The gold nanoparticles and self-assembling peptide hydrogel composite scaffold (PEG-SH-GNPs-SAPNS@miR-29a delivery system) applied to deliver miR-29a, which recruit endogenous neural stem cells simultaneously. Sustained release of miR-29a and recruitment of endogenous neural stem cells give rise to favorable axonal regeneration and recovery of motor function after spinal cord injury. These findings suggest that the PEG-SH-GNPs-SAPNS@miR-29a delivery system may be an alternative strategy for the treatment of SCI.

Advances in Large Gap Peripheral Nerve Injury Repair and Regeneration with Bridging Nerve Guidance Conduits

Peripheral nerve injury is a common complication of accidents and diseases. The traditional autologous nerve graft approach remains the gold standard for the treatment of nerve injuries. While sources of autologous nerve grafts are very limited and difficult to obtain. Nerve guidance conduits are widely used in the treatment of peripheral nerve injuries as an alternative to nerve autografts and allografts. However, the development of nerve conduits does not meet the needs of large gap peripheral nerve injury. Functional nerve conduits can provide a good microenvironment for axon elongation and myelin regeneration. Herein, the manufacturing methods and different design types of functional bridging nerve conduits for nerve conduits combined with electrical or magnetic stimulation and loaded with Schwann cells, etc., are summarized. It summarizes the literature and finds that the technical solutions of functional nerve conduits with electrical stimulation, magnetic stimulation and nerve conduits combined with Schwann cells can be used as effective strategies for bridging large gap nerve injury and provide an effective way for the study of large gap nerve injury repair. In addition, functional nerve conduits provide a new way to construct delivery systems for drugs and growth factors in vivo.

Decellularized extracellular matrix in the treatment of spinal cord injury

Functional limitation caused by spinal cord injury (SCI) has the problem of significant clinical and economic burden. Damaged spinal axonal connections and an inhibitory environment severely hamper neuronal function. Regenerative biomaterials can fill the cavity and produce an optimal microenvironment at the site of SCI, inhibiting apoptosis, inflammation, and glial scar formation while promoting neurogenesis, axonal development, and angiogenesis. Decellularization aims to eliminate cells from the ultrastructure of tissues while keeping tissue-specific components that are similar to the structure of real tissues, making decellularized extracellular matrix (dECM) a suitable scaffold for tissue engineering. dECM has good biocompatibility, it can be widely obtained from natural organs of different species, and can be co-cultured with cells for 3D printing to obtain the target scaffold. In this paper, we reviewed the pathophysiology of SCI, the characteristics of dECM and its preparation method, and the application of dECM in the treatment of SCI. Although dECM has shown its therapeutic effect at present, there are still many indicators that need to be taken into account, such as the difficulty in obtaining materials and standardized production mode for large-scale use, the effect of decellularization on the physical and chemical properties of dECM, and the study on the synergistic effect of dECM and cells.

Injectable neural hydrogel as in vivo therapeutic delivery vehicle

Purpose

This study demonstrated in vivo delivery of a decellularized, injectable peripheral nerve (iPN) hydrogel and explored options for using iPN in combination with regenerative biomolecular therapies like stem cell secretome.

Methods

Rat-derived iPN hydrogel solutions were combined with a dextran-dye before subcutaneous injection into adult Sprague Dawley rats. After injection, an in vivo imaging system (IVIS) was used to visualize hydrogels and quantify dextran-dye release over time. Poly(lactic-co-glycolic) acid (PLGA) was used to encapsulate the dextran-dye to prolong molecular release from the hydrogel scaffolds. Lastly, we investigated use of adipose-derived stem cell (ASC) secretome as a potential future combination strategy with iPN. ASC secretome was assessed for growth factor levels in response to media stimulation and was encapsulated in PLGA to determine loading efficiency.

Results

Gelation of iPN hydrogels was successful upon subcutaneous injection. When combined with iPN, a 10 kDa dextran-dye was reduced to 54% its initial signal at 24 hours, while PLGA-encapsulated dextran-dye in iPN was only reduced to 78% by 24 hours. Modified media stimulation resulted in changes in ASC phenotype and dramatic upregulation of VEGF secretion. The PLGA encapsulation protocol was adapted for use with temperature sensitive biomolecules, however, considerations must be made with loading efficiency for cell secretome as the maximum efficiency was 28%.

Conclusion

The results of this study demonstrated successful injection and subsequent gelation of our iPN hydrogel formulation in vivo. Biomolecular payloads can be encapsulated in PLGA to help prolong their release from the soft iPN hydrogels in future combination therapies.

Lay Summary

We developed an injectable decellularized tissue scaffold from rat peripheral nerve tissue (called iPN), a potential minimally invasive therapeutic meant to fill lesion spaces after injury. This study was the first demonstration of iPN delivery to a living animal. The iPN solution was injected subcutaneously in a rat and properly formed a gelled material upon entering the body. Our results showed that encapsulating biomolecules in an FDA-approved polymer (PLGA) slowed the release of biomolecules from the iPN, which could allow therapeutics more time around the scaffold to help repair native tissue. Lastly, we investigated one potential avenue for combining iPN with other regenerative cues obtained from adipose-derived stem cells.

Description of Future Works

Future work must focus on optimal loading conditions and release profiles from the iPN hydrogels. Next steps will be applying iPN in various combination therapies for spinal cord injury. We will focus efforts on developing a pro-regenerative secretome that directly promotes neurite extension and neural cell infiltration into iPN scaffolds upon transplantation in spinal cord.

Cooperative assembly of a designer peptide and silk fibroin into hybrid nanofiber gels for neural regeneration after spinal cord injury

Local reconstruction of a permissive environment with biomaterials is a promising strategy to treat spinal cord injury (SCI). We reported a hybrid hydrogel fabricated from a small functional self-assembling peptide (F-SAP) and large silk fibroin (SF). The diffusion of SF micelles into F-SAP solution was driven by the dynamic synergy between osmotic pressure and F-SAP/SF electrostatic interactions, resulting in the rearrangement of SF micelles and the formation of rod-like filaments with axes nearly perpendicular to F-SAP nanofibers. Spectroscopy analysis, including circular dichroism, Raman and fluorescence, indicated conformation changes of SF from random coil to β sheet, which contributed to enhanced mechanical properties of the resultant hybrid hydrogel. Furthermore, the F-SAP/SF hybrid hydrogel coupled with controlled release of NT-3 provided a permissive environment for neural regeneration by providing nanofibrous substrates for regenerating axons, inflammatory modulation and remyelination, consequently resulting in improved locomotion and electrophysiological properties. This hydrogel could be used as a long-term stent in vivo for the treatment of SCI.

Chitosan Nerve Grafts Incorporated with SKP-SC-EVs Induce Peripheral Nerve Regeneration

BACKGROUND

Repair of long-distance peripheral nerve defects remains an important clinical problem. Nerve grafts incorporated with extracellular vesicles (EVs) from various cell types have been developed to bridge peripheral nerve defects. In our previous research, EVs obtained from skin-derived precursor Schwann cells (SKP-SC-EVs) were demonstrated to promote neurite outgrowth in cultured cells and facilitate nerve regeneration in animal studies.

METHODS

To further assess the functions of SKP-SC-EVs in nerve repair, we incorporated SKP-SC-EVs and Matrigel into chitosan nerve conduits (EV-NG) for repairing a 15-mm long-distance sciatic nerve defect in a rat model. Behavioral analysis, electrophysiological recording, histological investigation, molecular analysis, and morphometric assessment were carried out.

RESULTS

The results revealed EV-NG significantly improved motor and sensory function recovery compared with nerve conduits (NG) without EVs incorporation. The outgrowth and myelination of regenerated axons were improved, while the atrophy of target muscles induced by denervation was alleviated after EVs addition.

CONCLUSION

Our data indicated SKP-SC-EVs incorporation into nerve grafts represents a promising method for extended peripheral nerve damage repair.

Biodegradable polyurethane-incorporating decellularized spinal cord matrix scaffolds enhance Schwann cell reprogramming to promote peripheral nerve repair

Decellularized extracellular matrix (dECM) nerve guide conduits (NGCs) are a promising strategy to replace autogenous nerve grafting for the treatment of peripheral nerve system (PNS) injury. However, dECM conduits with mechanical properties that match those of peripheral nerves are yet to be well developed. Herein, we developed polyurethane-based NGCs incorporating decellularized spinal cord (BWPU-DSC NGCs) to repair peripheral nerves. BWPU-DSC NGCs have an inner three-dimensional micro-nanostructure. The mechanical properties of BWPU-DSC NGCs were similar to those of polyurethane NGCs, which were proven to promote peripheral nerve regeneration. An in vitro study indicated that BWPU-DSC NGCs could boost the proliferation and growth of cell processes in Schwann and neuron-like cells. In a rat sciatic nerve transected injury model, BWPU-DSC NGCs exhibited a dramatic increase in nerve repair, similar to that obtained by the current gold standard autograft implantation at only 6 weeks post-implantation, whereas polyurethane NGCs still displayed incomplete nerve repair. Histological analysis revealed that BWPU-DSC NGCs could induce the reprogramming of Schwann cells to promote axon regeneration and remyelination. Moreover, reprogrammed Schwann cells together with BWPU-DSC NGCs had anti-inflammatory effects and altered the activation state of macrophages to M2 phenotypes to enhance PNS regeneration. In this study, we provided a strategy to prepare polyurethane-based dECM NGCs enriched with bioactive molecules to promote PNS regeneration.

Transplantation of A2 type astrocytes promotes neural repair and remyelination after spinal cord injury

BACKGROUND

Limited progress in terms of an effective treatment for spinal cord injury (SCI) emphasizes the urgent need for novel therapies. As a vital central nervous system component, the resident astrocytes play crucial roles in regulating recovery after SCI. In this study, recovery after SCI was compared following the transplantation of either A1 or A2 astrocytes. A1 astrocytes are harmful as they upregulate the neurotoxic classical complement cascade genes. Conversely, A2 astrocytes are characterized as neuroprotective as they upregulate the production of many neurotrophic factors.

METHODS

We used different supernatant obtained from microglia stimulated with lipopolysaccharide or interleukin-4 to generate A1 and A2 astrocytes. We detected the influence of astrocytes on neurons by co-culturing A1 and A2 astrocytes with neurons. We transplanted astrocytes into the lesion site of the spinal cord and assessed lesion progression, neural restoration, glia formation and locomotor recovery.

RESULTS

Astrocytes were polarized into A1 and A2 phenotypes following culture in the supernatant obtained from microglia stimulated with lipopolysaccharide or interleukin-4, respectively. Furthermore, co-culturing A2 astrocytes with neurons significantly suppressed glutamate-induced neuronal apoptosis and promoted the degree of neuron arborization. Transplantation of these A2 astrocytes into the lesion site of the spinal cord of mice significantly improved motor function recovery, preserved spared supraspinal pathways, decreased glia scar deposition, and increased neurofilament formation at the site of injury compared to the transplantation of A1 astrocytes. Additionally, enhanced A2 astrocytes with potentially beneficial A2-like genes were also detected in the A2 group. Moreover, luxol fast blue staining and electron microscopy indicated increased preservation of myelin with organized structure after transplantation of A2 astrocytes than of A1 astrocytes.

CONCLUSIONS

A2 astrocyte transplantation could be a promising potential therapy for SCI. Video abstract.

A Dual-network Nerve Adhesive with Enhanced Adhesion Strength Promotes Transected Peripheral Nerve Repair

Peripheral nerve transection has a high prevalence and results in functional loss of affected limbs. The current clinical treatment using suture anastomosis significantly limits nerve recovery due to severe inflammation, secondary damage, and fibrosis. Fibrin glue, a commercial nerve adhesive as an alternative, avoids secondary damage but suffers from poor adhesion strength. To address their limitations, a highly efficacious nerve adhesive based on dual-crosslinking of dopamine-isothiocyanate modified hyaluronic acid and decellularized nerve matrix is reported in this paper. This dual-network nerve adhesive (DNNA) shows controllable gelation behaviors feasible for surgical applications, robust adhesion strength, and promoted axonal outgrowth in vitro. The in vivo therapeutic efficacy is tested using a rat-based sciatic nerve transection model. The DNNA decreases fibrosis and accelerates axon/myelin debris clearance at 10 days post-surgery, compared to suture and commercial fibrin glue treatments. At 10 weeks post-surgery, the strong adhesion and bioactivity allow DNNA to significantly decrease intraneural inflammation and fibrosis, enhance axon connection and remyelination, aid motor and sensory function recovery, as well as improve muscle contraction, compared to suture and fibrin treatments. Overall, this dual-network hydrogel with robust adhesion provides a rapid and highly efficacious nerve transection treatment to facilitate nerve repair and neuromuscular function recovery.

The Application of Biomaterials in Spinal Cord Injury

The spinal cord and the brain form the central nervous system (CNS), which is the most important part of the body. However, spinal cord injury (SCI) caused by external forces is one of the most difficult types of neurological injury to treat, resulting in reduced or even absent motor, sensory and autonomic functions. It leads to the reduction or even disappearance of motor, sensory and self-organizing nerve functions. Currently, its incidence is increasing each year worldwide. Therefore, the development of treatments for SCI is urgently needed in the clinic. To date, surgery, drug therapy, stem cell transplantation, regenerative medicine, and rehabilitation therapy have been developed for the treatment of SCI. Among them, regenerative biomaterials that use tissue engineering and bioscaffolds to transport cells or drugs to the injured site are considered the most promising option. In this review, we briefly introduce SCI and its molecular mechanism and summarize the application of biomaterials in the repair and regeneration of tissue in various models of SCI. However, there is still limited evidence about the treatment of SCI with biomaterials in the clinic. Finally, this review will provide inspiration and direction for the future study and application of biomaterials in the treatment of SCI.

Pathophysiology of Spinal Cord Injury and Tissue Engineering Approach for Its Neuronal Regeneration: Current Status and Future Prospects

A spinal cord injury (SCI) is a very debilitating condition causing loss of sensory and motor function as well as multiple organ failures. Current therapeutic options like surgery and pharmacotherapy show positive results but are incapable of providing a complete cure for chronic SCI symptoms. Tissue engineering, including neuroprotective or growth factors, stem cells, and biomaterial scaffolds, grabs attention because of their potential for regeneration and ability to bridge the gap in the injured spinal cord (SC). Preclinical studies with tissue engineering showed functional recovery and neurorestorative effects. Few clinical trials show the safety and efficacy of the tissue engineering approach. However, more studies should be carried out for potential treatment modalities. In this review, we summarize the pathophysiology of SCI and its current treatment modalities, including surgical, pharmacological, and tissue engineering approaches following SCI in preclinical and clinical phases.

Worldwide productivity and research trend of publications concerning electroactive materials and spinal cord injury: A bibliometric study

Purpose: We investigated the current state and trends in the area during the previous 10 years using bibliometric approaches to evaluate the global scientific output of research on electroactive materials and spinal cord injury. Methods: Studies on spinal cord injury in electroactive materials that were published between 2012 and 2022 were located using the Web of science (WOS) datebase. The software programs bibliometrix R-package and CiteSpace were used to do quantitative analyses of annual publications, nation, author, institution, journal source, co-cited references, and keywords. The studies were categorized by the research's main points using a qualitative analysis, and publications having more than 10 citations each year. Results: In the final analysis, 1,330 relevant papers or reviews were included. There is an increased tendency in both the average annual citation rate and the number of publications in the discipline. The United States and the University of Toronto are the countries and institutions that have contributed the most to this discipline, respectively. The majority of authors are from the China and United States. Zhang Y is the author with the most published articles and holds the top position in the cited author h-index species. The journal with the highest number of published articles is "Disability and rehabilitation"; the journal is divided into four main areas including physics, materials, chemistry, molecular, and biology. The keyword analysis revealed a shift in research hotspots from schwann cell, fracture, and urinary disorders to carbon-based materials, functional recovery, and surgery. Analysis of qualitative data revealed that the role and mechanism of injectable conductive hydrogels in spinal cord healing after damage is a hot topic of current study, with the mechanism primarily focusing on the inhibition of oxidative stress (Nrf2) and apoptosis (Casepase 3). Conclusion: Our bibliometric analysis indicates that research on electroactive materials for spinal cord injury remains an active field of study. Moreover, contemporary research is concentrated on carbon-based materials, functional rehabilitation, and surgery.

Neural grafts containing exosomes derived from Schwann cell-like cells promote peripheral nerve regeneration in rats

Background

Schwann cell-like cells (SCLCs), differentiated from mesenchymal stem cells, have shown promising outcomes in the treatment of peripheral nerve injuries in preclinical studies. However, certain clinical obstacles limit their application. Hence, the primary aim of this study was to investigate the role of exosomes derived from SCLCs (SCLCs-exo) in peripheral nerve regeneration.

Methods

SCLCs were differentiated from human amniotic mesenchymal stem cells (hAMSCs) in vitro and validated by immunofluorescence, real-time quantitative PCR and western blot analysis. Exosomes derived from hAMSCs (hAMSCs-exo) and SCLCs were isolated by ultracentrifugation and validated by nanoparticle tracking analysis, WB analysis and electron microscopy. A prefabricated nerve graft was used to deliver hAMSCs-exo or SCLCs-exo in an injured sciatic nerve rat model. The effects of hAMSCs-exo or SCLCs-exo on rat peripheral nerve injury (PNI) regeneration were determined based on the recovery of neurological function and histomorphometric variation. The effects of hAMSCs-exo or SCLCs-exo on Schwann cells were also determined via cell proliferation and migration assessment.

Results

SCLCs significantly expressed the Schwann cell markers glial fibrillary acidic protein and S100. Compared to hAMSCs-exo, SCLCs-exo significantly enhanced motor function recovery, attenuated gastrocnemius muscle atrophy and facilitated axonal regrowth, myelin formation and angiogenesis in the rat model. Furthermore, hAMSCs-exo and SCLCs-exo were efficiently absorbed by Schwann cells. However, compared to hAMSCs-exo, SCLCs-exo significantly promoted the proliferation and migration of Schwann cells. SCLCs-exo also significantly upregulated the expression of a glial cell-derived neurotrophic factor, myelin positive regulators (SRY-box transcription factor 10, early growth response protein 2 and organic cation/carnitine transporter 6) and myelin proteins (myelin basic protein and myelin protein zero) in Schwann cells.

Conclusions

These findings suggest that SCLCs-exo can more efficiently promote PNI regeneration than hAMSCs-exo and are a potentially novel therapeutic approach for treating PNI.

Evaluation and Comparison of the Effects of Mature Silkworm (Bombyx mori) and Silkworm Pupae Extracts on Schwann Cell Proliferation and Axon Growth: An In Vitro Study

Background

Silkworm products were first used by physicians more than 8500 years ago, in the early Neolithic period. In Persian medicine, silkworm extract has several uses for treating and preventing neurological, cardiac, and liver diseases. Mature silkworms (Bombyx mori) and their pupae contain a variety of growth factors and proteins that can be used in many repair processes, including nerve regeneration.

Objectives

The study aimed to evaluate the effects of mature silkworm (Bombyx mori), and silkworm pupae extract on Schwann cell proliferation and axon growth.

Methods

Silkworm (Bombyx mori) and silkworm pupae extracts were prepared. Then, the concentration and type of amino acids and proteins in the extracts were evaluated by Bradford assay, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and liquid chromatograph-mass spectrometer (LC-MS/MS). Also, the regenerative potential of extracts for improving Schwann cell proliferation and axon growth was examined by 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide (MTT) assay, electron microscopy, and NeuroFilament-200 (NF-200) immunostaining.

Results

According to the results of the Bradford test, the total protein content of pupae extract was almost twice that of mature worm extract. Also, SDS-PAGE analysis revealed numerous proteins and growth factors, such as bombyrin and laminin, in extracts that are involved in the repair of the nervous system. In accordance with Bradford's results, the evaluation of extracts using LC-MS/MS revealed that the number of amino acids in pupae extract was higher than in mature silkworm extract. It was found that the proliferation of Schwann cells at a concentration of 0.25 mg/mL in both extracts was higher than the concentrations of 0.01 and 0.05 mg/mL. When using both extracts on dorsal root ganglion (DRGs), an increase in length and number was observed in axons.

Conclusions

The findings of this study demonstrated that extracts obtained from silkworms, especially pupae, can play an effective role in Schwann cell proliferation and axonal growth, which can be strong evidence for nerve regeneration, and, consequently, repairing peripheral nerve damage.

Thermosensitive hydrogel carrying extracellular vesicles from adipose-derived stem cells promotes peripheral nerve regeneration after microsurgical repair

Peripheral nerve injuries are commonly occurring traumas of the extremities; functional recovery is hindered by slow nerve regeneration (<1 mm/day) following microsurgical repair and subsequent muscle atrophy. Functional recovery after peripheral nerve repair is highly dependent on local Schwann cell activity and axon regeneration speed. Herein, to promote nerve regeneration, paracrine signals of adipose-derived stem cells were applied in the form of extracellular vesicles (EVs) loaded in a thermosensitive hydrogel (PALDE) that could solidify rapidly and sustain high EV concentration around a repaired nerve during surgery. Cell experiments revealed that PALDE hydrogel markedly promotes Schwann-cell migration and proliferation and axon outgrowth. In a rat sciatic nerve repair model, the PALDE hydrogel increased repaired-nerve conduction efficacy; contraction force of leg muscles innervated by the repaired nerve also recovered. Electromicroscopic examination of downstream nerves indicated that fascicle diameter and myeline thickness in the PALDE group (1.91 ± 0.61 and 1.06 ± 0.40 μm, respectively) were significantly higher than those in PALD and control groups. Thus, this EV-loaded thermosensitive hydrogel is a potential cell-free therapeutic modality to improve peripheral-nerve regeneration, offering sustained and focused EV release around the nerve-injury site to overcome rapid clearance and maintain EV bioactivity in vivo.

Acellular nerve xenografts based on supercritical extraction technology for repairing long-distance sciatic nerve defects in rats

Compared to conventional artificial nerve guide conduits (NGCs) prepared using natural polymers or synthetic polymers, acellular nerve grafts (ACNGs) derived from natural nerves with eliminated immune components have natural bionic advantages in composition and structure that polymer materials do not have. To further optimize the repair effect of ACNGs, in this study, we used a composite technology based on supercritical carbon dioxide (scCO₂) extraction to process the peripheral nerve of a large mammal, the Yorkshire pig, and obtained an innovative Acellular nerve xenografts (ANXs, namely, CD + scCO₂ NG). After scCO₂ extraction, the fat and DNA content in CD + scCO₂ NG has been removed to the greatest extent, which can better supported cell adhesion and proliferation, inducing an extremely weak inflammatory response. Interestingly, the protein in the CD + scCO₂ NG was primarily involved in signaling pathways related to axon guidance. Moreover, compared with the pure chemical decellularized nerve graft (CD NG), the DRG axons grew naturally on the CD + scCO₂ NG membrane and extended long distances. In vivo studies further revealed that the regenerated nerve axons had basically crossed the CD + scCO₂ NG 3 weeks after surgery. 12 weeks after surgery, CD + scCO2 NG was similar to autologous nerves in improving the quality of nerve regeneration, target muscle morphology and motor function recovery and was significantly better than hollow NGCs and CD NG. Therefore, we believe that the fully decellularized and fat-free porcine ACNGs may be the most promising “bridge” for repairing human nerve defects at this stage and for some time to come.

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The native adipose tissue inside acellular nerve xenografts hinders regenerated nerve fibers.

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Environmentally friendly scCO2 extraction has natural advantages in reducing fat content.

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Natural three-dimensional nerve basement membrane tube structure guides regenerating axons.

In vitro study of decellularized rat tissues for nerve regeneration

Peripheral nerve injuries cause an absence or destruction of nerves. Decellularized nerves, acting as a replacement for autografts, have been investigated in the promotion of nerve repair and regeneration, always being incorporated with stem cells or growth factors. However, such a strategy is limited by size availability. The potential application in heterotopic transplantation of other decellularized tissues needs to be further explored. In this study, rat decellularized kidney (dK) was selected to be compared with decellularized peripheral nerve (dN), since dK has aboundant ECM components and growth factors. The PC-12 cells were cultured on dK and dN scaffolds, as shown in the similar behaviors of cell metabolism and viability, but have a more regular arrangement on dN compared to dK, indicating that the natural structure plays an important role in guiding cell extension. However, we found significant upregulation of axon–growth–associated genes and proteins of PC-12 cells in the dK group compared to the dN group by qRT-PCR, immunofluorescence, and western blotting. Furthermore, various neurotrophic factors and growth factors of acellular kidney and nerve were evaluated by ELISA assay. The lower expression of neurotrophic factors but higher expression of growth factors such as VEGF and HGF from dK suggests that axon growth and extension for PC-12 cells may be partially mediated by VEGF and HGF expression from decellularized kidney, which further points to a potential application in nerve repair and regeneration.

Decellularization alters the unfavorable regenerative adverse microenvironment of the injured spinal cord to support neurite outgrowth

Background

Acellular tissue has been transplanted into the injury site as an external microenvironment to intervene with imbalance microenvironment that occurs after spinal cord injury (SCI) and stimulating axonal regeneration, although the mechanism is unclear. Given decellularization is the key means to obtain acellular tissues, we speculated changes in the internal components of tissue caused by decellularization may be the key reason why acellular tissues affect remodeling of the microenvironment.

Methods

Complete spinal cord crush in a mouse model was established, and the dynamic of extracellular matrix (ECM) expression and distribution during SCI was studied with immunohistochemistry (IHC). Normal spinal cord (NSC) and 14-day injury spinal cord (ISC) were obtained to prepare the decellularized NSC (DNSC) and decellularized ISC (DISC) through a well-designed decellularization method, and the decellularization effects were evaluated by residual DNA content determination, hematoxylin and eosin staining (H&E), and IHC. Rat dorsal root ganglia (DRG) were co-cultured with NSC, ISC, DNSC, and DISC to evaluate their effect on neurite outgrowth. Furthermore, the mechanisms by which decellularized tissue promotes axonal growth were explored with proteomics analysis of the protein components and function of 14-day ISC and DISC.

Results

We found the expression of the four main ECM components (collagen type I and IV, fibronectin, and laminin) gradually increased with the progression of SCI compared to NSC, peaking at 14 days of injury then slightly decreasing at 21 days, and the distribution of the four ECM proteins in the ISC also changed dynamically. H&E staining, residual DNA content determination, and IHC showed decellularization removed cellular components and preserved an intact ECM. The results of co-cultured DRG with NSCs, ISCs, DNSCs, and DISCs showed DNSCs and DISCs had a stronger ability in supporting neurite outgrowth than NSC and ISC. We found through proteomics that decellularization could remove proteins associated with inflammatory responses, scarring, and other pathological factors, while completely retaining the ECM proteins.

Conclusions

Taken together, our findings demonstrate decellularization can optimize the imbalanced microenvironment after SCI by removing components that inhibit spinal cord regeneration, providing a theoretical basis for clinical application of acellular tissue transplantation to repair SCI.

Atractylenolide III ameliorates spinal cord injury in rats by modulating microglial/macrophage polarization

Background

Inflammatory reactions induced by spinal cord injury (SCI) are essential for recovery after SCI. Atractylenolide III (ATL‐III) is a natural monomeric herbal bioactive compound that is mainly derived in Atractylodes macrocephala Koidz and has anti‐inflammatory and neuroprotective effects.

Objective

Here, we speculated that ATL‐III may ameliorate SCI by modulating microglial/macrophage polarization. In the present research, we focused on investigating the role of ATL‐III on SCI in rats and explored the potential mechanism.

Methods

The protective and anti‐inflammatory effects of ATL‐III on neuronal cells were examined in a rat SCI model and lipopolysaccharide (LPS)‐stimulated BV2 microglial line. The spinal cord lesion area, myelin integrity, and surviving neurons were assessed by specific staining. Locomotor function was evaluated by the Basso, Beattie, and Bresnahan (BBB) scale, grid walk test, and footprint test. The activation and polarization of microglia/macrophages were assessed by immunohistofluorescence and flow cytometry. The expression of corresponding inflammatory factors from M1/M2 and the activation of relevant signaling pathways were assessed by Western blotting.

Results

ATL‐III effectively improved histological and functional recovery in SCI rats. Furthermore, ATL‐III promoted the transformation of M1 into M2 and attenuated the activation of microglia/macrophages, further suppressing the expression of corresponding inflammatory mediators. This effect may be partly mediated by inhibition of neuroinflammation through the NF‐κB, JNK MAPK, p38 MAPK, and Akt pathways.

Conclusion

This study reveals a novel effect of ATL‐III in the regulation of microglial/macrophage polarization and provides initial evidence that ATL‐III has potential therapeutic benefits in SCI rats.

Biomaterial-Based Schwann Cell Transplantation and Schwann Cell-Derived Biomaterials for Nerve Regeneration

Schwann cells (SCs) dominate the regenerative behaviors after peripheral nerve injury by supporting axonal regrowth and remyelination. Previous reports also demonstrated that the existence of SCs is beneficial for nerve regeneration after traumatic injuries in central nervous system. Therefore, the transplantation of SCs/SC-like cells serves as a feasible cell therapy to reconstruct the microenvironment and promote nerve functional recovery for both peripheral and central nerve injury repair. However, direct cell transplantation often leads to low efficacy, due to injection induced cell damage and rapid loss in the circulatory system. In recent years, biomaterials have received great attention as functional carriers for effective cell transplantation. To better mimic the extracellular matrix (ECM), many biodegradable materials have been engineered with compositional and/or topological cues to maintain the biological properties of the SCs/SCs-like cells. In addition, ECM components or factors secreted by SCs also actively contribute to nerve regeneration. Such cell-free transplantation approaches may provide great promise in clinical translation. In this review, we first present the current bio-scaffolds engineered for SC transplantation and their achievement in animal models and clinical applications. To this end, we focus on the physical and biological properties of different biomaterials and highlight how these properties affect the biological behaviors of the SCs/SC-like cells. Second, the SC-derived biomaterials are also reviewed and discussed. Finally, the relationship between SCs and functional biomaterials is summarized, and the trends of their future development are predicted toward clinical applications.

Intramuscular delivery of neural crest stem cell spheroids enhances neuromuscular regeneration after denervation injury

BACKGROUND

Muscle denervation from trauma and motor neuron disease causes disabling morbidities. A limiting step in functional recovery is the regeneration of neuromuscular junctions (NMJs) for reinnervation. Stem cells have the potential to promote these regenerative processes, but current approaches have limited success, and the optimal types of stem cells remain to be determined. Neural crest stem cells (NCSCs), as the developmental precursors of the peripheral nervous system, are uniquely advantageous, but the role of NCSCs in neuromuscular regeneration is not clear. Furthermore, a cell delivery approach that can maintain NCSC survival upon transplantation is critical.

METHODS

We established a streamlined protocol to derive, isolate, and characterize functional p75+ NCSCs from human iPSCs without genome integration of reprogramming factors. To enhance survival rate upon delivery in vivo, NCSCs were centrifuged in microwell plates to form spheroids of desirable size by controlling suspension cell density. Human bone marrow mesenchymal stem cells (MSCs) were also studied for comparison. NCSC or MSC spheroids were injected into the gastrocnemius muscle with denervation injury, and the effects on NMJ formation and functional recovery were investigated. The spheroids were also co-cultured with engineered neuromuscular tissue to assess effects on NMJ formation in vitro.

RESULTS

NCSCs cultured in spheroids displayed enhanced secretion of soluble factors involved in neuromuscular regeneration. Intramuscular transplantation of spheroids enabled long-term survival and retention of NCSCs, in contrast to the transplantation of single-cell suspensions. Furthermore, NCSC spheroids significantly improved functional recovery after four weeks as shown by gait analysis, electrophysiology, and the rate of NMJ innervation. MSC spheroids, on the other hand, had insignificant effect. In vitro co-culture of NCSC or MSC spheroids with engineered myotubes and motor neurons further evidenced improved innervated NMJ formation with NCSC spheroids.

CONCLUSIONS

We demonstrate that stem cell type is critical for neuromuscular regeneration and that NCSCs have a distinct advantage and therapeutic potential to promote reinnervation following peripheral nerve injury. Biophysical effects of spheroidal culture, in particular, enable long-term NCSC survival following in vivo delivery. Furthermore, synthetic neuromuscular tissue, or "tissues-on-a-chip," may offer a platform to evaluate stem cells for neuromuscular regeneration.

Decellularized peripheral nerve as an injectable delivery vehicle for neural applications

Damage to the nervous system can result in loss of sensory and motor function, paralysis, or even death. To facilitate neural regeneration and functional recovery, researchers have employed biomaterials strategies to address both peripheral and central nervous system injuries. Injectable hydrogels that recapitulate native nerve extracellular matrix are especially promising for neural tissue engineering because they offer more flexibility for minimally invasive applications and provide a growth-permissive substrate for neural cell types. Here, we explore the development of injectable hydrogels derived from decellularized rat peripheral nerves (referred to as "injectable peripheral nerve [iPN] hydrogels"), which are processed using a newly developed sodium deoxycholate and DNase (SDD) decellularization method. We assess the gelation kinetics, mechanical properties, cell bioactivity, and drug release kinetics of the iPN hydrogels. The iPN hydrogels thermally gel when exposed to 37°C in under 20 min and have mechanical properties similar to neural tissue. The hydrogels demonstrate in vitro biocompatibility through support of Schwann cell viability and metabolic activity. Additionally, iPN hydrogels promote greater astrocyte spreading compared to collagen I hydrogels. Finally, the iPN is a promising delivery vehicle of drug-loaded microparticles for a combinatorial approach to neural injury therapies.

Repair of the Injured Spinal Cord by Schwann Cell Transplantation

Spinal cord injury (SCI) can result in sensorimotor impairments or disability. Studies of the cellular response to SCI have increased our understanding of nerve regenerative failure following spinal cord trauma. Biological, engineering and rehabilitation strategies for repairing the injured spinal cord have shown impressive results in SCI models of both rodents and non-human primates. Cell transplantation, in particular, is becoming a highly promising approach due to the cells’ capacity to provide multiple benefits at the molecular, cellular, and circuit levels. While various cell types have been investigated, we focus on the use of Schwann cells (SCs) to promote SCI repair in this review. Transplantation of SCs promotes functional recovery in animal models and is safe for use in humans with subacute SCI. The rationales for the therapeutic use of SCs for SCI include enhancement of axon regeneration, remyelination of newborn or sparing axons, regulation of the inflammatory response, and maintenance of the survival of damaged tissue. However, little is known about the molecular mechanisms by which transplanted SCs exert a reparative effect on SCI. Moreover, SC-based therapeutic strategies face considerable challenges in preclinical studies. These issues must be clarified to make SC transplantation a feasible clinical option. In this review, we summarize the recent advances in SC transplantation for SCI, and highlight proposed mechanisms and challenges of SC-mediated therapy. The sparse information available on SC clinical application in patients with SCI is also discussed.

Implantation of nanofibrous silk scaffolds seeded with bone marrow stromal cells promotes spinal cord regeneration (6686 words)

Spinal cord injury (SCI) is a common pathology often resulting in permanent loss of sensory, motor, and autonomic function. Numerous studies in which stem cells have been transplanted in biomaterial scaffolds into animals have demonstrated their considerable potential for recovery from SCI. In the present study, a three-dimensional porous silk fibroin (SF) scaffold with a mean pore size of approximately 383 μm and nanofibrous structure was fabricated, the silk scaffold enabling the enhanced attachment and proliferation of bone marrow stromal cells (BMSCs). Investigation of its therapeutic potential was conducted by implantation of the nanofibrous SF scaffold seeded with BMSCs into a transected spinal cord model. Recovery of the damaged spinal cord was significantly improved after 2 months, compared with a non-nanofibrous scaffold, in combination with decreased glial fibrillary acidic protein (GFAP) expression and improved axonal regeneration at the site of injury. Furthermore, elevated Basso-Beattie-Bresnahan (BBB) scores indicated greatly improved hindlimb movement. Together, these results demonstrate that transplantation of neural scaffolds consisting of nanofibrous SF and BMSCs is an attractive strategy for the promotion of functional recovery following SCI.

Repair of peripheral nerve defects by nerve grafts incorporated with extracellular vesicles from skin-derived precursor Schwann cells

Our previous studies have shown that extracellular vesicles from skin-derived precursor Schwann cells (SKP-SC-EVs) promote neurite outgrowth of sensory and motor neurons in vitro. This study was aimed at generating an artificial nerve graft incorporated with SKP-SC-EVs to examine in vivo effects of SKP-SC-EVs on peripheral nerve regeneration. Here SKP-SC-EVs were isolated and then identified by morphological observation and phenotypic marker expression. Following co-culture with SCs or motoneurons, SKP-SC-EVs were internalized, showing the capability to enhance SC viability or motoneuron neurite outgrowth. In vitro, SKP-SC-EVs released from Matrigel could maintain cellular uptake property and neural activity. Nerve grafts were developed by incorporating Matrigel-encapsulated SKP-SC-EVs into silicone conduits. Functional evaluation, histological investigation, and morphometric analysis were performed to compare the nerve regenerative outcome after bridging the 10-mm long sciatic nerve defect in rats with our developed nerve grafts, silicone conduits (filled with vehicle), and autografts respectively. Our developed nerve grafts significantly accelerated the recovery of motor, sensory, and electrophysiological functions of rats, facilitated outgrowth and myelination of regenerated axons, and alleviated denervation-induced atrophy of target muscles. Collectively, our findings suggested that incorporation of SKP-SC-EVs into nerve grafts might represent a promising paradigm for peripheral nerve injury repair. STATEMENT OF SIGNIFICANCE: Nerve grafts have been progressively developed to meet the increasing requirements for peripheral nerve injury repair. Here we reported a design of nerve grafts featured by incorporation of Matrigel-encapsulated extracellular vesicles from skin-derived precursor Schwann cells (SKP-SC-EVs), because SKP-SC-EVs were found to possess in vitro neural activity, thus raising the possibility of cell-free therapy. Our developed nerve grafts yielded the satisfactory outcome of nerve grafting in rats with a 10-mm long sciatic nerve defect, as evaluated by functional and morphological assessments. The promoting effects of SKP-SC-EVs-incorporating nerve grafts on peripheral nerve regeneration might benefit from in vivo biological cues afforded by SKP-SC-EVs, which had been released from Matrigel and then internalized by residual neural cells in sciatic nerve stumps.

Optimizing Olfactory Ensheathing Cell Transplantation for Spinal Cord Injury Repair

Cell transplantation constitutes an important avenue for development of new treatments for spinal cord injury (SCI). These therapies are aimed at supporting neural repair and/or replacing lost cells at the injury site. To date, various cell types have been trialed, with most studies focusing on different types of stem cells or glial cells. Here, we review commonly used cell transplantation approaches for spinal cord injury (SCI) repair, with focus on transplantation of olfactory ensheathing cells (OECs), the glial cells of the primary olfactory nervous system. OECs are promising candidates for promotion of neural repair given that they support continuous regeneration of the olfactory nerve that occurs throughout life. Further, OECs can be accessed from the nasal mucosa (olfactory neuroepithelium) at the roof of the nasal cavity and can be autologously transplanted. OEC transplantation has been trialed in many animal models of SCI, as well as in human clinical trials. While several studies have been promising, outcomes are variable and the method needs improvement to enhance aspects such as cell survival, integration, and migration. As a case study, we include the approaches used by our team (the Clem Jones Centre for Neurobiology and Stem Cell Research, Griffith University, Nathan, QLD, Australia) to address the current problems with OEC transplantation and discuss how the therapeutic potential of OEC transplantation can be improved. Our approach includes discovery research to improve our knowledge of OEC biology, identifying natural and synthetic compounds to stimulate the neural repair properties of OECs, and designing three-dimensional cell constructs to create stable and transplantable cell structures.

Properties Regulation and Biological Applications of Decellularized Peripheral Nerve Matrix Hydrogel

Decellularized peripheral nerve matrix hydrogel (DNM-G) has drawn increasing attention in the field of neural tissue engineering, owing to its high tissue-specific bioactivity, drug/cell delivery capability, and multifunctional processability. However, the mechanisms and influencing factors of DNM-G formation have been rarely reported. To enable potential biological applications, the relationship between gelation conditions (including digestion time and gel concentration) and mechanical properties/stability (sol-gel transition temperature, gelation time, nanotopology, and storage modulus) of the DNM-G were systematically investigated in this study. The adequate-digested decellularized nerve matrix solution exhibited higher mechanical property, shorter gelation time, and a lower gelation temperature. A noteworthy increase of β-sheet proportion was identified through Fourier-transform infrared spectroscopy (FTIR) and circular dichroism (CD) characterizations, which suggested the possible major secondary structure formation during the phase transition. Besides, the DNM-G degraded fast that over 70% mass loss was noted after 4 weeks when immersing in PBS. A natural cross-linking agent, genipin, was gently introduced into DNM-G to enhance its mechanical properties and stability without changing its microstructure and biological performance. As a prefabricated scaffold, DNM-G remarkably increased the length and penetration depth of dorsal root ganglion (DRG) neurites compared to collagen gel. Furthermore, the DNM-G promoted the myelination and facilitated the formation of the morphological neural network. Finally, we demonstrated the feasibility of applying DNM-G in support-free extrusion-based 3D printing. Overall, the mechanical and biological performance of DNM-G can be manipulated by tuning the processing parameters, which is key to the versatile applications of DNM-G in regenerative medicine.

Increasing toll-like receptor 2 on astrocytes induced by Schwann cell-derived exosomes promotes recovery by inhibiting CSPGs deposition after spinal cord injury

Background

Traumatic spinal cord injury (SCI) is a severely disabling disease that leads to loss of sensation, motor, and autonomic function. As exosomes have great potential in diagnosis, prognosis, and treatment of SCI because of their ability to easily cross the blood–brain barrier, the function of Schwann cell-derived exosomes (SCDEs) is still largely unknown.

Methods

A T10 spinal cord contusion was established in adult female mice. SCDEs were injected into the tail veins of mice three times a week for 4 weeks after the induction of SCI, and the control group was injected with PBS. High-resolution transmission electron microscope and western blot were used to characterize the SCDEs. Toll-like receptor 2 (TLR2) expression on astrocytes, chondroitin sulfate proteoglycans (CSPGs) deposition and neurological function recovery were measured in the spinal cord tissues of each group by immunofluorescence staining of TLR2, GFAP, CS56, 5-HT, and β-III-tublin, respectively. TLR2^f/f mice were crossed to the GFAP-Cre strain to generate astrocyte specific TLR2 knockout mice (TLR2^−/−). Finally, western blot analysis was used to determine the expression of signaling proteins and IKKβ inhibitor SC-514 was used to validate the involved signaling pathway.

Results

Here, we found that TLR2 increased significantly on astrocytes post-SCI. SCDEs treatment can promote functional recovery and induce the expression of TLR2 on astrocytes accompanied with decreased CSPGs deposition. The specific knockout of TLR2 on astrocytes abolished the decreasing CSPGs deposition and neurological functional recovery post-SCI. In addition, the signaling pathway of NF-κB/PI3K involved in the TLR2 activation was validated by western blot. Furthermore, IKKβ inhibitor SC-514 was also used to validate this signaling pathway.

Conclusion

Thus, our results uncovered that SCDEs can promote functional recovery of mice post-SCI by decreasing the CSPGs deposition via increasing the TLR2 expression on astrocytes through NF-κB/PI3K signaling pathway.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02215-x.

Emerging Exosomes and Exosomal MiRNAs in Spinal Cord Injury

Acute spinal cord injury (SCI) is a serious traumatic event to the spinal cord with considerable morbidity and mortality. This injury leads to short- and long-term variations in the spinal cord, and can have a serious effect on the patient's sensory, motor, or autonomic functions. Due to the complicated pathological process of SCI, there is currently no successful clinical treatment strategy. Exosomes, extracellular vesicles (EVs) with a double-layer membrane structure of 30-150 nm diameter, have recently been considered as critical mediators for communication between cells and tissues by transferring proteins, lipids, and nucleic acids. Further studies verified that exosomes participate in the pathophysiological process of several diseases, including cancer, neurodegenerative diseases, and cardiovascular diseases, and could have a significant impact in their treatment. As natural carriers of biologically active cargos, exosomes have emerged as pathological mediators of SCI. In this review article, we critically discuss the functions of exosomes as intracellular mediators and potential treatments in SCI and provide an outlook on future research.

3D printing of functional nerve guide conduits

Nerve guide conduits (NGCs), as alternatives to nerve autografts and allografts, have been widely explored as an advanced tool for the treatment of peripheral nerve injury. However, the repairing efficiency of NGCs still needs significant improvements. Functional NGCs that provide a more favorable microenvironment for promoting axonal elongation and myelination are of great importance. In recent years, 3D printing technologies have been widely applied in the fabrication of customized and complex constructs, exhibiting great potential for tissue engineering applications, especially for the construction of functional NGCs. In this review, we introduce the 3D printing technologies for manufacturing functional NGCs, including inkjet printing, extrusion printing, stereolithography-based printing and indirect printing. Further, we summarize the current methods and strategies for constructing functional NGCs, such as designing special conduit architectures, using appropriate materials and co-printing with different biological cues. Finally, the challenges and prospects for construction of functional NGCs are also presented.

Importance of brain alterations in spinal cord injury

Spinal cord injury (SCI) destroys the sensorimotor pathway and blocks the information flow between the peripheral nerve and the brain, resulting in autonomic function loss. Numerous studies have explored the effects of obstructed information flow on brain structure and function and proved the extensive plasticity of the brain after SCI. Great progress has also been achieved in therapeutic strategies for SCI to restore the "re-innervation" of the cerebral cortex to the limbs to some extent. Although no thorough research has been conducted, the changes of brain structure and function caused by "re-domination" have been reported. This article is a review of the recent research progress on local structure, functional changes, and circuit reorganization of the cerebral cortex after SCI. Alterations of structure and electrical activity characteristics of brain neurons, features of brain functional reorganization, and regulation of brain functions by reconfigured information flow were also explored. The integration of brain function is the basis for the human body to exercise complex/fine movements and is intricately and widely regulated by information flow. Hence, its changes after SCI and treatments should be considered.

Biomaterial Strategies to Bolster Neural Stem Cell-Mediated Repair of the Central Nervous System

Stem cell therapies have the potential to not only repair, but to regenerate tissue of the central nervous system (CNS). Recent studies demonstrate that transplanted stem cells can differentiate into neurons and integrate with the intact circuitry after traumatic injury. Unfortunately, the positive findings described in rodent models have not been replicated in clinical trials, where the burden to maintain the cell viability necessary for tissue repair becomes more challenging. Low transplant survival remains the greatest barrier to stem cell-mediated repair of the CNS, often with fewer than 1-2% of the transplanted cells remaining after 1 week. Strategic transplantation parameters, such as injection location, cell concentration, and transplant timing achieve only modest improvements in stem cell transplant survival and appear inconsistent across studies. Biomaterials provide researchers with a means to significantly improve stem cell transplant survival through two mechanisms: (1) a vehicle to deliver and protect the stem cells and (2) a substrate to control the cytotoxic injury environment. These biomaterial strategies can alleviate cell death associated with delivery to the injury and can be used to limit cell death after transplantation by limiting cell exposure to cytotoxic signals. Moreover, it is likely that control of the injury environment with biomaterials will lead to a more reliable support for transplanted cell populations. This review will highlight the challenges associated with cell delivery in the CNS and the advances in biomaterial development and deployment for stem cell therapies necessary to bolster stem cell-mediated repair.

Development of novel apoptosis-assisted lung tissue decellularization methods

Decellularized tissues hold great potential for both regenerative medicine and disease modeling applications. The acellular extracellular matrix (ECM)-enriched scaffolds can be recellularized with patient-derived cells prior to transplantation, or digested to create thermally-gelling ECM hydrogels for 3D cell culture. Current methods of decellularization clear cellular components using detergents, which can result in loss of ECM proteins and tissue architectural integrity. Recently, an alternative approach utilizing apoptosis to decellularize excised murine sciatic nerves resulted in superior ECM preservation, cell removal, and immune tolerance in vivo. However, this apoptosis-assisted decellularization approach has not been optimized for other tissues with a more complex geometry, such as lungs. To this end, we developed an apoptosis-assisted lung tissue decellularization method using a combination of camptothecin and sulfobetaine-10 (SB-10) to induce apoptosis and facilitate gentle and effective removal of cell debris, respectively. Importantly, combination of the two agents resulted in superior cell removal and ECM preservation compared to either of the treatments alone, presumably because of pulmonary surfactants. In addition, our method was superior in cell removal compared to a previously established detergent-based decellularization protocol. Furthermore, thermally-gelling lung ECM hydrogels supported high viability of rat lung epithelial cells for up to 2 weeks in culture. This work demonstrates that apoptosis-based lung tissue decellularization is a superior technique that warrants further utilization for both regenerative medicine and disease modeling purposes.

Biomaterials for Neural Tissue Engineering

The therapy of neural nerve injuries that involve the disruption of axonal pathways or axonal tracts has taken a new dimension with the development of tissue engineering techniques. When peripheral nerve injury (PNI), spinal cord injury (SCI), traumatic brain injury (TBI), or neurodegenerative disease occur, the intricate architecture undergoes alterations leading to growth inhibition and loss of guidance through large distance. To improve the limitations of purely cell-based therapies, the neural tissue engineering philosophy has emerged. Efforts are being made to produce an ideal scaffold based on synthetic and natural polymers that match the exact biological and mechanical properties of the tissue. Furthermore, through combining several components (biomaterials, cells, molecules), axonal regrowth is facilitated to obtain a functional recovery of the neural nerve diseases. The main objective of this review is to investigate the recent approaches and applications of neural tissue engineering approaches.

IL-10 lentivirus-laden hydrogel tubes increase spinal progenitor survival and neuronal differentiation after spinal cord injury

A complex cellular cascade characterizes the pathophysiological response following spinal cord injury (SCI) limiting regeneration. Biomaterial and stem cell combination therapies together have shown synergistic effects, compared to the independent benefits of each intervention, and represent a promising approach towards regaining function after injury. In this study, we combine our polyethylene glycol (PEG) cell delivery platform with lentiviral-mediated overexpression of the anti-inflammatory cytokine interleukin (IL)-10 to improve mouse embryonic Day 14 (E14) spinal progenitor transplant survival. Immediately following injury in a mouse SCI hemisection model, five PEG tubes were implanted followed by direct injection into the tubes of lentivirus encoding for IL-10. Two weeks after tube implantation, mouse E14 spinal progenitors were injected directly into the integrated tubes, which served as a soft substrate for cell transplantation. Together, the tubes with the IL-10 encoding lentivirus improved E14 spinal progenitor survival, assessed at 2 weeks posttransplantation (4 weeks postinjury). On average, 8.1% of E14 spinal progenitors survived in mice receiving IL-10 lentivirus-laden tubes compared with 0.7% in mice receiving transplants without tubes, an 11.5-fold difference. Surviving E14 spinal progenitors gave rise to neurons when injected into tubes. Axon elongation and remyelination were observed, in addition to a significant increase in functional recovery in mice receiving IL-10 lentivirus-laden tubes with E14 spinal progenitor delivery compared to the injury only control by 4 weeks postinjury. All other conditions did not exhibit increased stepping until 8 or 12 weeks postinjury. This system affords increased control over the transplantation microenvironment, offering the potential to improve stem cell-mediated tissue regeneration.

Decellularized optic nerve functional scaffold transplant facilitates directional axon regeneration and remyelination in the injured white matter of the rat spinal cord

Axon regeneration and remyelination of the damaged region is the most common repair strategy for spinal cord injury. However, achieving good outcome remains difficult. Our previous study showed that porcine decellularized optic nerve better mimics the extracellular matrix of the embryonic porcine optic nerve and promotes the directional growth of dorsal root ganglion neurites. However, it has not been reported whether this material promotes axonal regeneration in vivo. In the present study, a porcine decellularized optic nerve was seeded with neurotrophin-3-overexpressing Schwann cells. This functional scaffold promoted the directional growth and remyelination of regenerating axons. In vitro, the porcine decellularized optic nerve contained many straight, longitudinal channels with a uniform distribution, and microscopic pores were present in the channel wall. The spatial micro topological structure and extracellular matrix were conducive to the adhesion, survival and migration of neural stem cells. The scaffold promoted the directional growth of dorsal root ganglion neurites, and showed strong potential for myelin regeneration. Furthermore, we transplanted the porcine decellularized optic nerve containing neurotrophin-3-overexpressing Schwann cells in a rat model of T10 spinal cord defect in vivo. Four weeks later, the regenerating axons grew straight, the myelin sheath in the injured/transplanted area recovered its structure, and simultaneously, the number of inflammatory cells and the expression of chondroitin sulfate proteoglycans were reduced. Together, these findings suggest that porcine decellularized optic nerve loaded with Schwann cells overexpressing neurotrophin-3 promotes the directional growth of regenerating spinal cord axons as well as myelin regeneration. All procedures involving animals were conducted in accordance with the ethical standards of the Institutional Animal Care and Use Committee of Sun Yat-sen University (approval No. SYSU-IACUC-2019-B034) on February 28, 2019.

Progress in clinical trials of cell transplantation for the treatment of spinal cord injury: how many questions remain unanswered?

Spinal cord injury can lead to severe motor, sensory and autonomic nervous dysfunctions. However, there is currently no effective treatment for spinal cord injury. Neural stem cells and progenitor cells, bone marrow mesenchymal stem cells, olfactory ensheathing cells, umbilical cord blood stem cells, adipose stem cells, hematopoietic stem cells, oligodendrocyte precursor cells, macrophages and Schwann cells have been studied as potential treatments for spinal cord injury. These treatments were mainly performed in animals. However, subtle changes in sensory function, nerve root movement and pain cannot be fully investigated with animal studies. Although these cell types have shown excellent safety and effectiveness in various animal models, sufficient evidence of efficacy for clinical translation is still lacking. Cell transplantation should be combined with tissue engineering scaffolds, local drug delivery systems, postoperative adjuvant therapy and physical rehabilitation training as part of a comprehensive treatment plan to provide the possibility for patients with SCI to return to normal life. This review summarizes and analyzes the clinical trials of cell transplantation therapy in spinal cord injury, with the aim of providing a rational foundation for the development of clinical treatments for spinal cord injury.

Decellularized nerve matrix hydrogel scaffolds with longitudinally oriented and size-tunable microchannels for peripheral nerve regeneration

The scaffolding biomaterials and their internal structures are crucial in constructing growth-permissive microenvironment for tissue regeneration. A functional bioscaffold not only requires sufficient extracellular matrix components, but also provides topological guidance by mimicry of the ultrastructure of the native tissue. In our laboratory, a decellularized nerve matrix hydrogel derived from porcine sciatic nerve (pDNM-G) is successfully prepared, which shows great promise for peripheral nerve regeneration. Herein, longitudinally oriented microchannel structures were introduced into pDNM-G bioscaffolds (A-pDNM-G) through controlled unidirectional freeze-drying. The axially aligned microchannels effectively directed and significantly promoted neurite extension and Schwann cell migration, assessed by culturing dorsal root ganglion explants on the longitudinal sections of A-pDNM-G scaffolds. Such regenerative cellular responses can be further optimized by tuning the channel sizes. In vivo studies confirmed that the implanted nerve guidance conduits containing A-pDNM-G scaffolds significantly facilitated axonal extension, myelination, and reached considerable functional recovery in 15-mm rat sciatic nerve defects. The incorporation of nerve growth factor further improved the overall performance in the grafted nerve. The bioactive pDNM-G enables controlled release of neurotrophic factor and easy integration of topological cue provided by the axially aligned microchannels into implantable bioscaffolds, which may serve in future clinical treatments of peripheral nerve injury.

Bioreactivity of decellularized animal, plant, and fungal scaffolds: perspectives for medical applications

Numerous biomedical applications imply supportive materials to improve protective, antibacterial, and regenerative abilities upon surgical interventions, oncotherapy, regenerative medicine, and others. With the increasing variability of the possible sources, the materials of natural origin are among the safest and most accessible biomedical tools. Animal, plant, and fungal tissues can further undergo decellularization to improve their biocompatibility. Decellularized scaffolds lack the most reactive cellular material, nuclear and cytoplasmic components, that predominantly trigger immune responses. At the same time, the outstanding initial three-dimensional microarchitecture, biomechanical properties, and general composition of the scaffolds are preserved. These unique features make the scaffolds perfect ready-to-use platforms for various biomedical applications, implying cell growth and functionalization. Decellularized materials can be repopulated with various cells upon request, including epithelial, endothelial, muscle and neuronal cells, and applied for structural and functional biorepair within diverse biological sites, including the skin and musculoskeletal, cardiovascular, and central nervous systems. However, the molecular and cellular mechanisms behind scaffold and host tissue interactions remain not fully understood, which significantly restricts their integration into clinical practice. In this review, we address the essential aspects of decellularization, scaffold preparation techniques, and its biochemical composition and properties, which determine the biocompatibility and immunogenicity of the materials. With the integrated evaluation of the scaffold profile in living systems, decellularized animal, plant, and fungal scaffolds have the potential to become essential instruments for safe and controllable biomedical applications.

MSC spheroids-loaded collagen hydrogels simultaneously promote neuronal differentiation and suppress inflammatory reaction through PI3K-Akt signaling pathway

It is critical for the clinical success to take the anti-inflammatory function into consideration when integrating the neurogenesis into the nerve repair materials. To this aim, we prepared mesenchymal stem cell (MSC) spheroids-loaded collagen (Col) hydrogels with combined superior anti-inflammatory efficacy and neurogenic activity. The size of the MSC spheroids showed a strong modulation effect on both functions, and the MSC spheroids-100 sample exhibited the best neuronal and anti-inflammatory potentials. The observed dual functions were likely based on the elevated intrinsic cell-cell contacts and cell-extracellular matrix interactions from the MSC spheroids. MSC self-assembly as spheroids expedited the secretions of endogenous trophic factors and extracellular matrix (ECM), which was beneficial to drive neural stem cell differentiation into the neuronal lineage. In addition, the formation of the MSC spheroids secreted more amounts and types of cytokines as well as immunomodulatory paracrine factors to suppress LPS-induced inflammatory reaction. LC-MS/MS analysis further demonstrated that MSC spheroids contributed to the activation of neuroactive ligand-receptor interaction, thereby triggering downstream PI3K-Akt signal pathway, which was likely due to the acceleration of ECM-receptor interaction, gap junction and tight junction. Importantly, inhibiting Akt pathway significantly suppressed the neuronal differentiation, indicating that PI3K-Akt signal pathway was critically involved in the Col-MSC spheroid hydrogel mediated neuroprotection and neurogenesis. Such findings not only provided a simple approach for improving MSC-based therapies for neuron-related diseases, but also shed insight on understanding the underlying mechanisms of MSC-mediated neuronal differentiation.

Silencing Celsr2 inhibits the proliferation and migration of Schwann cells through suppressing the Wnt/β-catenin signaling pathway

After a peripheral nerve injury, the remaining Schwann cells undergo proliferation and adopt a migratory phenotype to prepare for the regeneration of nerves. Celsr2 has been reported to play an important role in the development and maintenance of the function of the nervous system. However, the role and mechanism of Celsr2 during peripheral nerve regeneration remain unknown. Here, we showed that after sciatic nerve injury, Celsr2 mRNA and protein were significantly increased in nerve tissues. In addition, silencing Celsr2 decreased the ki67-positve portion and the migration distance of Schwann cells in vivo. In vitro, the results of MTT and EdU staining, transwell and wound healing assays indicated that Celsr2 siRNA-transfected primary Schwann cells showed significant decrease in proliferation and migration compared to that seen in negative control (NC)-transfected cells. Furthermore, we found that Wnt/β-catenin luciferase activity was reduced, as were the expression of β-catenin in the nucleus and the mRNA levels of its downstream genes Cyclin D1 and MMP-7 in Celsr2 siRNA-transfected primary Schwann cells. Further investigations showed that silencing Celsr2 inhibited the phosphorylation of GSK3β. Moreover, specific activators of the Wnt/β-catenin pathway, LiCl or mutant β-catenin (S33Y), partially reversed the inhibitory effect of Celsr2 siRNA. Taken together, our data indicated that silencing Celsr2 inhibited Schwann cells migration and proliferation through the suppressing Wnt/β-catenin pathway, providing a potential target for peripheral nerve regeneration.

Novel Approach for Efficient Recovery for Spinal Cord Injury Repair via Biofabricated Nano-Cerium Oxide Loaded PCL With Resveratrol to Improve in Vitro Biocompatibility and Autorecovery Abilities

It is more difficult to develop the low-cost spinal cord injury repair materials with high stability and biocompatibility for the biomedical applications. Herein, for the first time, we demonstrated the functional restoration of an injured spinal cord by the nano CeO₂ particles assembled onto poly (∊-caprolactone) (PCL)/resveratrol (RVL) were synthesized using the biocompatible ionic liquid. The as-prepared biocompatible nanomaterials were characterized and confirmed by using different instruments such as Fourier transform infra-red spectroscopy for functional groups identification, X-ray diffraction for crystalline nature, Scanning electron microscopy, transmission electron microscopy for morphological structure, Dynamic light scattering for size distribution of the nanoparticles and thermogravimetric analysis for thermal properties. The synergetic effect between the uniform distributions of nano-sized CeO₂ particles onto the PCL polymer with RVL can remarkably enhance the catalytic performance. Biofabricated nano-cerium oxide loaded PCL with RVL revealed that treatment significantly preserved hydrogen peroxide and also good catalytic performance. This study presents a nano-sized cerium oxide particles loaded PCL with RVL biocompatible materials have been providing highly efficient regenerative activity and biocompatibility in spinal card regeneration.

The biology and engineered modeling strategies of cancer-nerve crosstalk

A recent finding critical to cancer aggravation is the interaction between cancer cells and nerves. There exist two main modes of cancer-nerve interaction: perineural invasion (PNI) and tumor innervation. PNI occurs when cancer cells infiltrate the adjacent nerves, and its relative opposite, tumor innervation, occurs when axons extend into tumor bodies. Like most cancer studies, these crosstalk interactions have mostly been observed in patient samples and animal models at this point, making it difficult to understand the mechanisms in a controlled manner. As such, in recent years in vitro studies have emerged that have helped identify various microenvironmental factors responsible for cancer-nerve crosstalk, including but not limited to neurotrophic factors, neurotransmitters, chemokines, cancer-derived exosomes, and Schwann cells. The versatility of in vitro systems warrants continuous development to increase physiological relevance to study PNI and tumor innervation, for example by utilizing biomimetic three-dimensional (3D) culture systems. Despite the wealth of 3D in vitro cancer models, comparatively there exists a lack of 3D in vitro models of nerve, PNI, and tumor innervation. Native-like 3D in vitro models of cancer-nerve interactions may further help develop therapeutic strategies to curb nerve-mediated cancer aggravation. As such, we provide an overview of the key players of cancer-nerve crosstalk and current in vitro models of the crosstalk, as well as cancer and nerve models. We also discuss a few future directions in cancer-nerve crosstalk research.

Strategies and prospects of effective neural circuits reconstruction after spinal cord injury

Due to the disconnection of surviving neural elements after spinal cord injury (SCI), such patients had to suffer irreversible loss of motor or sensory function, and thereafter enormous economic and emotional burdens were brought to society and family. Despite many strategies being dealing with SCI, there is still no effective regenerative therapy. To date, significant progress has been made in studies of SCI repair strategies, including gene regulation of neural regeneration, cell or cell-derived exosomes and growth factors transplantation, repair of biomaterials, and neural signal stimulation. The pathophysiology of SCI is complex and multifaceted, and its mechanisms and processes are incompletely understood. Thus, combinatorial therapies have been demonstrated to be more effective, and lead to better neural circuits reconstruction and functional recovery. Combinations of biomaterials, stem cells, growth factors, drugs, and exosomes have been widely developed. However, simply achieving axon regeneration will not spontaneously lead to meaningful functional recovery. Therefore, the formation and remodeling of functional neural circuits also depend on rehabilitation exercises, such as exercise training, electrical stimulation (ES) and Brain-Computer Interfaces (BCIs). In this review, we summarize the recent progress in biological and engineering strategies for reconstructing neural circuits and promoting functional recovery after SCI, and emphasize current challenges and future directions.