The role of Schwann cell-axon interaction in peripheral nerve regeneration

Study on the Role and Mechanism of Exosomes Derived from Dental Pulp Stem Cells in Promoting Regeneration of Myelin Sheath in Rats with Sciatic Nerve Injury

The prognosis of peripheral nerve injury (PNI) is usually poor, and currently, there is no effective treatment for PNI. Studies have shown that exosomes derived from mesenchymal stem cells could promote nerve regeneration by optimizing the function of endogenous Schwann cells (SCs), while the mechanism is unclear. Autophagy, a highly conserved intracellular catabolic process responsible for maintaining cellular homeostasis, has been proved to be involved in the regulation of nerve repair after injury. We explored the effect of exosomes derived from dental pulp stem cells (DPSC-Exos) on the regeneration of myelin sheath in rats with sciatic nerve injury (SNI). In vitro and in vivo experiments were performed to clarify whether the effect of DPSC-Exos is associated with autophagy of SCs and to reveal the mechanism at the molecular level. Our results showed that the SCs of SNI rats exhibited the obvious autophagic characteristics, and the increase of P53 expression was an internal factor of autophagy. Our mechanism research indicated that DPSC-Exos could deliver miR-122-5p from DPSCs into SCs and suppressed the rapamycin (RAPA)-induced autophagy in SCs by inhibiting P53 expression. Rescue experiments showed that both the use of GW4869 and overexpression of exogenous P53 in SCs could reverse the inhibitory effect of DPSCs on the autophagy in SCs from co-culture system. In short, our study indicated that DPSC-Exos could promote the regeneration of the myelin sheath through suppressing the autophagy in SCs caused by PNI via miR-122-5p/P53 pathway; this provides researchers with another option for precise repair of PNI.

Immobilized NRG1 Accelerates Neural Crest like Cell Differentiation Toward Functional Schwann Cells Through Sustained Erk1/2 Activation and YAP/TAZ Nuclear Translocation

Neural Crest cells (NC) are a multipotent cell population that give rise to a multitude of cell types including Schwann cells (SC) in the peripheral nervous system (PNS). Immature SC interact with neuronal axons via the neuregulin 1 (NRG1) ligand present on the neuronal surface and ultimately form the myelin sheath. Multiple attempts to derive functional SC from pluripotent stem cells have met challenges with respect to expression of mature markers and axonal sorting. Here, they hypothesized that sustained signaling from immobilized NRG1 (iNRG1) might enhance the differentiation of NC derived from glabrous neonatal epidermis towards a SC phenotype. Using this strategy, NC derived SC expressed mature markers to similar levels as compared to explanted rat sciatic SC. Signaling studies revealed that sustained NRG1 signaling led to yes-associated protein 1 (YAP) activation and nuclear translocation. Furthermore, NC derived SC on iNRG1 exhibited mature SC function as they aligned with rat dorsal root ganglia (DRG) neurons in an in vitro coculture model; and most notably, aligned on neuronal axons upon implantation in a chick embryo model in vivo. Taken together their work demonstrated the importance of signaling dynamics in SC differentiation, aiming towards development of drug testing platforms for de-myelinating disorders.

Epothilone B loaded in acellular nerve allograft enhanced sciatic nerve regeneration in rats

BACKGROUND

Epothilone B (EpoB) is a microtubule-stabilizing agent with neuroprotective properties.

OBJECTIVES

This study examines the regenerative properties of ANA supplemented with EpoB on a sciatic nerve deficit in male Wistar rats.

METHODS

For this purpose, the 10 mm nerve gap was filled with acellular nerve allografts (ANAs) containing EpoB at 0.1, 1, and 10 nM concentrations. The sensorimotor recovery was evaluated up to 16 weeks after the operation. Real-time PCR, histomorphometry analysis, and electrophysiological evaluation were also used to evaluate the process of nerve regeneration.

RESULTS

ANA/EpoB (0.1 nM) significantly improved sensorimotor recovery in rats compared to ANA, ANA/EpoB (1 nM), and ANA/EpoB (10 nM) groups. This led to reduced muscle atrophy, improved sciatic functional index, and thermal paw withdrawal reflex latency, indicating nerve regeneration and target organ reinnervation. The electrophysiological and histomorphometry findings also confirmed the ANA/EpoB regenerative properties (0.1 nM). EpoB only enhanced ANA regenerative properties at 0.1 nM, with no therapeutic effects at higher concentrations.

CONCLUSION

Totally, we concluded that ANA loaded with 0.1 nM EpoB can effectively reconstruct the transected sciatic nerve in rats, likely by enhancing axonal sprouting and extension.

Dental Pulp Stem Cell-Derived Exosomes Promote Sciatic Nerve Regeneration via Optimizing Schwann Cell Function

Repair strategies for injured peripheral nerve have achieved great progresses in recent years. However, the clinical outcomes remain unsatisfactory. Recent studies have found that exosomes secreted by dental pulp stem cells (DPSC-exos) have great potential for applications in nerve repair. In this study, we evaluated the effects of human DPSC-exos on improving peripheral nerve regeneration. Initially, we established a coculture system between DPSCs and Schwann cells (SCs) in vitro to assess the effect of DPSC-exos on the activity of embryonic dorsal root ganglion neurons (DRGs) growth in SCs. We extracted and labeled human DPSC-exos, which were subsequently utilized in uptake experiments in DRGs and SCs. Subsequently, we established a rat sciatic nerve injury model to evaluate the therapeutic potential of DPSC-exos in repairing sciatic nerve damage. Our findings revealed that DPSC-exos significantly promoted neurite elongation by enhancing the proliferation, migration, and secretion of neurotrophic factors by SCs. In vivo, DPSC-exos administration significantly improved the walking behavior, axon regeneration, and myelination in rats with sciatic nerve injuries. Our study underscores the vast potential of DPSC-exos as a therapeutic tool for tissue-engineered nerve construction.

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.

PHA is not just a bioplastic!

Polyhydroxyalkanoates (PHA) have evolved into versatile biopolymers, transcending their origins as mere bioplastics. This extensive review delves into the multifaceted landscape of PHA applications, shedding light on the diverse industries that have harnessed their potential. PHA has proven to be an invaluable eco-conscious option for packaging materials, finding use in films foams, paper coatings and even straws. In the textile industry, PHA offers a sustainable alternative, while its application as a carbon source for denitrification in wastewater treatment showcases its versatility in environmental remediation. In addition, PHA has made notable contributions to the medical and consumer sectors, with various roles ranging from 3D printing, tissue engineering implants, and cell growth matrices to drug delivery carriers, and cosmetic products. Through metabolic engineering efforts, PHA can be fine-tuned to align with the specific requirements of each industry, enabling the customization of material properties such as ductility, elasticity, thermal conductivity, and transparency. To unleash PHA's full potential, bridging the gap between research and commercial viability is paramount. Successful PHA production scale-up hinges on establishing direct supply chains to specific application domains, including packaging, food and beverage materials, medical devices, and agriculture. This review underscores that PHA's future rests on ongoing exploration across these industries and more, paving the way for PHA to supplant conventional plastics and foster a circular economy.

Progress and mechanism of graphene oxide-composited materials in application of peripheral nerve repair

Peripheral nerve injuries (PNI) are one of the most common nerve injuries, and graphene oxide (GO) has demonstrated significant potential in the treatment of PNI. GO could enhance the proliferation, adhesion, migration, and differentiation of neuronal cells by upregulating the expression of relevant proteins, and regulate the angiogenesis process and immune response. Therefore, GO is a suitable additional component for fabricating artificial nerve scaffolds (ANS), in which the slight addition of GO could improve the physicochemical performance of the matrix materials, through hydrogen bonds and electrostatic attraction. GO-composited ANS can increase the expression of nerve regeneration-associated genes and factors, promoting angiogenesis by activating the RAS/MAPK and AKT-eNOS-VEGF signaling pathway, respectively. Moreover, GO could be metabolized and excreted from the body through the pathway of peroxidase degradation in vivo. Consequently, the application of GO in PNI regeneration exhibits significant potential for transitioning from laboratory research to clinical use.

Brief Electrical Stimulation Promotes Recovery after Surgical Repair of Injured Peripheral Nerves

Injured peripheral nerves regenerate their axons in contrast to those in the central nervous system. Yet, functional recovery after surgical repair is often disappointing. The basis for poor recovery is progressive deterioration with time and distance of the growth capacity of the neurons that lose their contact with targets (chronic axotomy) and the growth support of the chronically denervated Schwann cells (SC) in the distal nerve stumps. Nonetheless, chronically denervated atrophic muscle retains the capacity for reinnervation. Declining electrical activity of motoneurons accompanies the progressive fall in axotomized neuronal and denervated SC expression of regeneration-associated-genes and declining regenerative success. Reduced motoneuronal activity is due to the withdrawal of synaptic contacts from the soma. Exogenous neurotrophic factors that promote nerve regeneration can replace the endogenous factors whose expression declines with time. But the profuse axonal outgrowth they provoke and the difficulties in their delivery hinder their efficacy. Brief (1 h) low-frequency (20 Hz) electrical stimulation (ES) proximal to the injury site promotes the expression of endogenous growth factors and, in turn, dramatically accelerates axon outgrowth and target reinnervation. The latter ES effect has been demonstrated in both rats and humans. A conditioning ES of intact nerve days prior to nerve injury increases axonal outgrowth and regeneration rate. Thereby, this form of ES is amenable for nerve transfer surgeries and end-to-side neurorrhaphies. However, additional surgery for applying the required electrodes may be a hurdle. ES is applicable in all surgeries with excellent outcomes.

Voluntary wheel running prevents formation of membrane attack complexes and myelin degradation after peripheral nerve injury

Regular aerobic activity is associated with a reduced risk of chronic pain in humans and rodents. Our previous studies in rodents have shown that prior voluntary wheel running can normalize redox signaling at the site of peripheral nerve injury, attenuating subsequent neuropathic pain. However, the full extent of neuroprotection offered by voluntary wheel running after peripheral nerve injury is unknown. Here, we show that six weeks of voluntary wheel running prior to chronic constriction injury (CCI) reduced the terminal complement membrane attack complex (MAC) at the sciatic nerve injury site. This was associated with increased expression of the MAC inhibitor CD59. The levels of upstream complement components (C3) and their inhibitors (CD55, CR1 and CFH) were altered by CCI, but not increased by voluntary wheel running. Since MAC can degrade myelin, which in turn contributes to neuropathic pain, we evaluated myelin integrity at the sciatic nerve injury site. We found that the loss of myelinated fibers and decreased myelin protein which occurs in sedentary rats following CCI was not observed in rats with prior running. Substitution of prior voluntary wheel running with exogenous CD59 also attenuated mechanical allodynia and reduced MAC deposition at the nerve injury site, pointing to CD59 as a critical effector of the neuroprotective and antinociceptive actions of prior voluntary wheel running. This study links attenuation of neuropathic pain by prior voluntary wheel running with inhibition of MAC and preservation of myelin integrity at the sciatic nerve injury site.

LIPUS-SCs-Exo promotes peripheral nerve regeneration in cavernous nerve crush injury-induced ED rats via PI3K/Akt/FoxO signaling pathway

OBJECTIVE

Clinical treatment of erectile dysfunction (ED) caused by cavernous nerve (CN) injury during pelvic surgery is difficult. Low-intensity pulsed ultrasound (LIPUS) can be a potential strategy for neurogenic ED (NED). However, whether Schwann cells (SCs) can respond to LIPUS stimulation signals is unclear. This study aims to elucidate the signal transmission between SCs paracrine exosome (Exo) and neurons stimulated by LIPUS, as well as to analyze the role and potential mechanisms of exosomes in CN repair after injury.

METHODS

The major pelvic ganglion (MPG) neurons and MPG/CN explants were stimulated with LIPUS of different energy intensities to explore the appropriate LIPUS energy intensity. The exosomes were isolated and purified from LIPUS-stimulated SCs (LIPUS-SCs-Exo) and non-stimulated SCs (SCs-Exo). The effects of LIPUS-SCs-Exo on neurite outgrowth, erectile function, and cavernous penis histology were identified in bilateral cavernous nerve crush injury (BCNI)-induced ED rats.

RESULTS

LIPUS-SCs-Exo group can enhance the axon elongation of MPG/CN and MPG neurons compared to SCs-Exo group in vitro. Then, the LIPUS-SCs-Exo group showed a stronger ability to promote the injured CN regeneration and SCs proliferation compared to the SCs-Exo group in vivo. Furthermore, the LIPUS-SCs-Exo group increased the Max intracavernous pressure (ICP)/mean arterial pressure (MAP), lumen to parenchyma and smooth muscle to collagen ratios compared to the SCs-Exo group in vivo. Additionally, high-throughput sequencing combined with bioinformatics analysis revealed the differential expression of 1689 miRNAs between the SCs-Exo group and the LIPUS-SCs-Exo group. After LIPUS-SCs-Exo treatment, the phosphorylated levels of Phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt) and forkhead box O (FoxO) in MPG neurons increased significantly compared to negative control (NC) and SCs-Exo groups.

CONCLUSION

Our study revealed that LIPUS stimulation could regulate the gene of MPG neurons by changing miRNAs derived from SCs-Exo, then activating the PI3K-Akt-FoxO signal pathway to enhance nerve regeneration and restore erectile function. This study had important theoretical and practical significance for improving the NED treatment.

PGC-1α Inhibits Schwann Cell Dedifferentiation and Delays Peripheral Nerve Degeneration by Targeting PON1

PPARγ coactivator-1 alpha (PGC-1α) is an essential transcription factor co-activator that regulates gene transcription and neural regeneration. Schwann cells, which are unique glial cells in peripheral nerves that dedifferentiate after peripheral nerve injury (PNI) and are released from degenerative nerves. Wallerian degeneration is a series of stereotypical events that occurs in response to nerve fibers after PNI. The role of PGC-1α in Schwann cell dedifferentiation and Wallerian degeneration is not yet clear. As Wallerian degeneration plays a crucial role in PNI, we conducted a study to determine whether PGC-1α has an effect on peripheral nerve degeneration after injury. We examined the expression of PGC-1α after sciatic nerve crush or transection using Western blotting and found that PGC-1α expression increased after PNI. Then we utilized ex vivo and in vitro models to investigate the effects of PGC-1α inhibition and activation on Schwann cell dedifferentiation and nerve degeneration. Our findings indicate that PGC-1α negatively regulates Schwann cell dedifferentiation and nerve degeneration. Through the use of RNA-seq, siRNA/plasmid transfection and reversal experiments, we identified that PGC-1α targets inhibit the expression of paraoxonase 1 (PON1) during Schwann cell dedifferentiation in degenerated nerves. In summary, PGC-1α plays a crucial role in preventing Schwann cell dedifferentiation and its activation can reduce peripheral nerve degeneration by targeting PON1. PGC-1α inhibits Schwann cell dedifferentiation and peripheral nerve degeneration. PGC-1α negatively regulates Schwann cell dedifferentiation and peripheral nerve degeneration after injury by targeting PON1.

Role of autophagy in the pathogenesis and regulation of pain

Pain is a ubiquitous and highly concerned clinical symptom, usually caused by peripheral or central nervous injury, tissue damage, or other diseases. The long-term existence of pain can seriously affect daily physical function and quality of life and produce great torture on the physiological and psychological levels. However, the complex pathogenesis of pain involving molecular mechanisms and signaling pathways has not been fully elucidated, and managing pain remains highly challenging. As a result, finding new targets to pursue effective and long-term pain treatment strategies is required and urgent. Autophagy is an intracellular degradation and recycling process that maintains tissue homeostasis and energy supply, which can be cytoprotective and is vital in maintaining neural plasticity and proper nervous system function. Much evidence has shown that autophagy dysregulation is linked to the emergence of neuropathic pain, such as postherpetic neuralgia and cancer-related pain. Autophagy has also been connected to pain caused by osteoarthritis and lumbar disc degeneration. It is worth noting that in recent years, studies on traditional Chinese medicine have also proved that several traditional Chinese medicine monomers involve autophagy in the mechanism of pain relief. Therefore, autophagy can serve as a potential regulatory target to provide new ideas and inspiration for pain management.

α7 Nicotinic Acetylcholine Receptors May Improve Schwann Cell Regenerating Potential via Metabotropic Signaling Pathways

BACKGROUND

Schwann cells (SCs) are glial cells involved in peripheral axon myelination. SCs also play a strategic role after peripheral nerve injury, regulating local inflammation and axon regeneration. Our previous studies demonstrated the presence of cholinergic receptors in SCs. In particular, the α7 nicotinic acetylcholine receptors (nAChRs) are expressed in SCs after peripheral axotomy, suggesting their involvement in the regulation of SC-regenerating properties. To clarify the role that α7 nAChRs may play after peripheral axon damage, in this study we investigated the signal transduction pathways triggered by receptor activation and the effects produced by their activation.

METHODS

Both ionotropic and metabotropic cholinergic signaling were analyzed by calcium imaging and Western blot analysis, respectively, following α7 nAChR activation. In addition, the expression of c-Jun and α7 nAChRs was evaluated by immunocytochemistry and Western blot analysis. Finally, the cell migration was studied by a wound healing assay.

RESULTS

Activation of α7 nAChRs, activated by the selective partial agonist ICH3, did not induce calcium mobilization but positively modulated the PI3K/AKT/mTORC1 axis. Activation of the mTORC1 complex was also supported by the up-regulated expression of its specific p-p70 S6KThr389 target. Moreover, up-regulation of p-AMPKThr172, a negative regulator of myelination, was also observed concomitantly to an increased nuclear accumulation of the transcription factor c-Jun. Cell migration and morphology analyses proved that α7 nAChR activation also promotes SC migration.

CONCLUSIONS

Our data demonstrate that α7 nAChRs, expressed by SCs only after peripheral axon damage and/or in an inflammatory microenvironment, contribute to improve the SCs regenerating properties. Indeed, α7 nAChR stimulation leads to an upregulation of c-Jun expression and promotes Schwann cell migration by non-canonical pathways involving the mTORC1 activity.

Nanosuspension of Foeniculum Vulgare Promotes Accelerated Sensory and Motor Function Recovery after Sciatic Nerve Injury

The seed extract of Foeniculum vulgare (FV) was used for the preparation of a nanosuspension (NS) with an enhanced bioavailability of phytoconstituents. Subsequently, it was employed as a potent source of polyphenols, such as quercetin and kaempferol, to accelerate the regeneration and recovery of motor and sensory function in injured nerves. The NS was administered through daily gauging as NS1 (0.5 mg/mL) and NS2 (15 mg/mL), at a dose rate of 2 g/kg body weight until the end of the study. The NS-mediated retrieval of motor functions was studied by evaluating muscle grip strength and the sciatic functional index. The recovery of sensory functions was assessed by the hotplate test. Several well-integrated biochemical pathways mediate the recovery of function and the regeneration of nerves under controlled blood glucose and oxidative stress. Consequently, the NS-treated groups were screened for blood glucose, total antioxidant capacity (TAC), and total oxidant status (TOS) compared to the control. The NS administration showed a significant potential to enhance the recuperation of motor and sensory functions. Moreover, the oxidative stress was kept under check as a result of NS treatments to facilitate neuronal generation. Thus, the nanoformulation of FV with polyphenolic contents accelerated the reclamation of motor and sensory function after nerve lesion.

Neuroregeneration of injured peripheral nerve by fraction B of catfish epidermal secretions through the reversal of the apoptotic pathway and DNA damage

Introduction: Crush injuries occur from acute traumatic nerve compression resulting in different degrees of neural damage leading to permanent functional deficits. Recently, we have shown that administration of Fraction B (FB) derived from catfish epidermal secretions accelerates healing of damaged nerve in a sciatic nerve crush injury, as it ameliorates the neurobehavioral deficits and enhances axonal regeneration, as well as protects spinal neurons and increases astrocytic activity and decreasing GAP-43 expression. The present study aimed to investigate the role of FB treatment on the apoptotic pathway in the neuroregeneration of the sciatic nerve crush injury. Methods: Male Wistar rats were randomly assigned into five groups: (I) SHAM, (II) CRUSH, (III) CRUSH + (1.5 mg/kg) FB, (IV) CRUSH + (3 mg/kg) FB, and (V) CRUSH + (4.5 mg/kg) FB. Rats underwent sciatic nerve crush surgery, followed by treatment with FB administered intraperitoneally (IP) daily for two weeks and then sacrificed at the end of the fourth week. Results: FB improved the recovery of neurobehavioral functions with a concomitant increase in axonal regeneration and neuroprotective effects on spinal cord neurons following crush injury. Further, FB enhanced Schwann cells (SCs) proliferation with a significant increase in myelin basic protein expression. FB-treated animals demonstrated higher numbers of neurons in the spinal cord, possibly through ameliorating oxidative DNA damage and alleviating the mitochondrial-dependent apoptotic pathway by inhibiting the release of cytochrome c and the activation of caspase-3 in the spinal cord neurons. Conclusion: FB alleviates the neurodegenerative changes in the lumbar spinal cord neurons and recovers the decrease in the neuronal count through its anti-apoptotic and DNA antioxidative properties.

Mechanism of Schwann cells in diabetic peripheral neuropathy: A review

Diabetic peripheral neuropathy (DPN) is the most common neuropathy in the world, mainly manifested as bilateral symmetry numbness, pain or paresthesia, with a high rate of disability and mortality. Schwann cells (SCs), derived from neural ridge cells, are the largest number of glial cells in the peripheral nervous system, and play an important role in DPN. Studies have found that SCs are closely related to the pathogenesis of DPN, such as oxidative stress, endoplasmic reticulum stress, inflammation, impaired neurotrophic support and dyslipidemia. This article reviews the mechanism of SCs in DPN.

Effects of ECM proteins (laminin, fibronectin, and type IV collagen) on the biological behavior of Schwann cells and their roles in the process of remyelination after peripheral nerve injury

Introduction: It is important to note that complete myelination and formation of myelinated fibers are essential for functional nerve regeneration after peripheral nerve injury (PNI). However, suboptimal myelin regeneration is common and can hinder ideal nerve regeneration. Therefore, it is important to closely monitor and support myelin regeneration in patients with PNI to achieve optimal outcomes. Methods: This study analyzed the effects of three extracellular matrix (ECM) proteins on Schwann cells (SCs) in the nerve regeneration environment, including their adhesion, proliferation, and migration. The study also explored the use of composite sodium alginate hydrogel neural scaffolds with ECM components and investigated the effects of ECM proteins on remyelination following peripheral nerve injury. Results: The results showed that laminin (LN), fibronectin (FN), and collagen Ⅳ (type IV Col) promoted the early adhesion of SCs in 2-dimensional culture but the ratios of early cell adhesion were quite different and the maintenance of cells' morphology by different ECM proteins were significantly different. In transwell experiment, the ability of LN and FN to induce the migration of SCs was obviously higher than that of type IV Col. An vitro co-culture model of SCs and dorsal root ganglia (DRG) neurons showed that LN promoted the transition of SCs to a myelinated state and the maturation of the myelin sheath, and increased the thickness of neurofilaments. Animal experiments showed that LN had superior effects in promoting myelin sheath formation, axon repair, and reaching an ideal G-ratio after injury compared to FN and Col IV. The situation of gastrocnemius atrophy was significantly better in the LN group. Notably, the thickness of the regenerated myelin sheaths in the type IV Col group was the thickest. Conclusion: In this experiment, we analyzed and compared the effects of LN, FN, and type IV Col on the biological behavior of SCs and their effects on remyelination after PNI and further clarified their unique roles in the process of remyelination. Further research is necessary to explore the underlying mechanisms.

Review: Myelin clearance is critical for regeneration after peripheral nerve injury

Traumatic peripheral nerve injury occurs frequently and is a major clinical and public health problem that can lead to functional impairment and permanent disability. Despite the availability of modern diagnostic procedures and advanced microsurgical techniques, active recovery after peripheral nerve repair is often unsatisfactory. Peripheral nerve regeneration involves several critical events, including the recreation of the microenvironment and remyelination. Results from previous studies suggest that the peripheral nervous system (PNS) has a greater capacity for repair than the central nervous system. Thus, it will be important to understand myelin and myelination specifically in the PNS. This review provides an update on myelin biology and myelination in the PNS and discusses the mechanisms that promote myelin clearance after injury. The roles of Schwann cells and macrophages are considered at length, together with the possibility of exogenous intervention.

The Effect of Electrical Stimulation on Nerve Regeneration Following Peripheral Nerve Injury

Peripheral nerve injuries (PNI) are common and often result in lifelong disability. The peripheral nervous system has an inherent ability to regenerate following injury, yet complete functional recovery is rare. Despite advances in the diagnosis and repair of PNIs, many patients suffer from chronic pain, and sensory and motor dysfunction. One promising surgical adjunct is the application of intraoperative electrical stimulation (ES) to peripheral nerves. ES acts through second messenger cyclic AMP to augment the intrinsic molecular pathways of regeneration. Decades of animal studies have demonstrated that 20 Hz ES delivered post-surgically accelerates axonal outgrowth and end organ reinnervation. This work has been translated clinically in a series of randomized clinical trials, which suggest that ES can be used as an efficacious therapy to improve patient outcomes following PNIs. The aim of this review is to discuss the cellular physiology and the limitations of regeneration after peripheral nerve injuries. The proposed mechanisms of ES protocols and how they facilitate nerve regeneration depending on timing of administration are outlined. Finally, future directions of research that may provide new perspectives on the optimal delivery of ES following PNI are discussed.

An overview of substrate stiffness guided cellular response and its applications in tissue regeneration

Cell-matrix interactions play a critical role in tissue repair and regeneration. With gradual uncovering of substrate mechanical characteristics that can affect cell-matrix interactions, much progress has been made to unravel substrate stiffness-mediated cellular response as well as its underlying mechanisms. Yet, as a part of cell-matrix interaction biology, this field remains in its infancy, and the detailed molecular mechanisms are still elusive regarding scaffold-modulated tissue regeneration. This review provides an overview of recent progress in the area of the substrate stiffness-mediated cellular responses, including 1) the physical determination of substrate stiffness on cell fate and tissue development; 2) the current exploited approaches to manipulate the stiffness of scaffolds; 3) the progress of recent researches to reveal the role of substrate stiffness in cellular responses in some representative tissue-engineered regeneration varying from stiff tissue to soft tissue. This article aims to provide an up-to-date overview of cell mechanobiology research in substrate stiffness mediated cellular response and tissue regeneration with insightful information to facilitate interdisciplinary knowledge transfer and enable the establishment of prognostic markers for the design of suitable biomaterials.

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Substrate stiffness physically determines cell fate and tissue development.

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Rational design of scaffolds requires the understanding of cell-matrix interactions.

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Substrate stiffness depends on scaffold molecular-constituent-structure interaction.

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Substrate stiffness-mediated cellular responses vary in different tissues.

Proteomics reveals the key molecules involved in curcumin-induced protection against sciatic nerve injury in rats

We generated a rat model of sciatic nerve crush injury and characterized the effects of curcumin on sciatic nerve recovery by using behavioral experiments, hematoxylin-eosin staining, toluidine blue staining, and immunohistochemical. Proteomic analysis using tandem mass tagging was performed to determine differentially expressed proteins (DEPs), and GO and KEGG pathway analyses of overlapping DEPs was conducted, following which, qPCR, western blotting, and immunofluorescence were further performed to validate the proteins of interest. Finally, a Schwann cell injury model was used to verify the effect of curcumin on potential targets. The rat model was successful established and curcumin improved the sciatic nerve function index of rats with sciatic nerve injury (SNI) and increased the number and diameter of myelinated axons in the sciatic nerve. In the Sham group versus the Injured group and in the Injured group versus the Curcumin group, we identified a total of 4,175 proteins, of which 953 were DEPs, and 218 were known overlapping DEPs. Ten associated pathways, such as calcium signaling pathway, biosynthesis of antibiotics, and long-term potentiation, were identified. The 218 overlapping DEPs were primarily involved in negative regulation of apoptotic process, biological processes, cytoplasm cellular component, and protein binding molecular function based on GO annotation. Curcumin promoted increased expression of ApoD and inhibited the expression of Cyba in vivo and in vitro. These results indicated that curcumin promoted sciatic nerve repair through regulation of various proteins, targets, and pathways. Cyba and ApoD may be potential targets of curcumin in the treatment of SNI.

Surgical Approaches for Prevention of Neuroma at Time of Peripheral Nerve Injury

Painful neuroma is a frequent sequela of peripheral nerve injury which can result in pain and decreased quality of life for the patient, often necessitating surgical intervention. End neuromas are benign neural tumors that commonly form after nerve transection, when axons from the proximal nerve stump regenerate in a disorganized manner in an attempt to recreate nerve continuity. Inflammation and collagen remodeling leads to a bulbous end neuroma which can become symptomatic and result in decreased quality of life. This review covers surgical prophylaxis of end neuroma formation at time of injury, rather than treatment of existing neuroma and prevention of recurrence. The current accepted methods to prevent end neuroma formation at time of injury include different mechanisms to inhibit the regenerative response or provide a conduit for organized regrowth, with mixed results. Approaches include proximal nerve stump capping, nerve implantation into bone, muscle and vein, various pharmacologic methods to inhibit axonal growth, and mechanisms to guide axonal growth after injury. This article reviews historical treatments that aimed to prevent end neuroma formation as well as current and experimental treatments, and seeks to provide a concise, comprehensive resource for current and future therapies aimed at preventing neuroma formation.

Role of Transforming Growth Factor Beta in Peripheral Nerve Regeneration: Cellular and Molecular Mechanisms

Peripheral nerve injury (PNI) is one of the most common concerns in trauma patients. Despite significant advances in repair surgeries, the outcome can still be unsatisfactory, resulting in morbidities such as loss of sensory or motor function and reduced quality of life. This highlights the need for more supportive strategies for nerve regrowth and adequate recovery. Multifunctional cytokine transforming growth factor-β (TGF-β) is essential for the development of the nervous system and is known for its neuroprotective functions. Accumulating evidence indicates its involvement in multiple cellular and molecular responses that are critical to peripheral nerve repair. Following PNI, TGF-β is released at the site of injury where it can initiate a series of phenotypic changes in Schwann cells (SCs), modulate immune cells, activate neuronal intrinsic growth capacity, and regulate blood nerve barrier (BNB) permeability, thus enhancing the regeneration of the nerves. Notably, TGF-β has already been applied experimentally in the treatment of PNI. These treatments with encouraging outcomes further demonstrate its regeneration-promoting capacity. Herein, we review the possible roles of TGF-β in peripheral nerve regeneration and discuss the underlying mechanisms, thus providing new cues for better treatment of PNI.

Modulation of the Crosstalk between Schwann Cells and Macrophages for Nerve Regeneration: A Therapeutic Strategy Based on a Multifunctional Tetrahedral Framework Nucleic Acids System

Peripheral nerve injury (PNI) is currently recognized as one of the most significant public health issues and affects the general well-being of millions of individuals worldwide. Despite advances in nerve tissue engineering, nerve repair still cannot guarantee complete functional recovery. In the present study, an innovative approach is adopted to establish a multifunctional tetrahedral framework nucleic acids (tFNAs) system, denoted as MiDs, which can integrate the powerful programmability, permeability, and structural stability of tFNAs, with the nerve regeneration potential of microRNA-22 to enhance the communication between Schwann cells (SCs) and macrophages for more effective functional rehabilitation of peripheral nerves. Relevant results demonstrate that MiDs can amplify the ability of SCs to recruit macrophages and facilitate their polarization into the pro-healing M2 phenotype to reconstruct the post-injury microenvironment. Furthermore, MiDs can initiate the adaptive intracellular reprogramming of SCs within a short period to further promote axon regeneration and remyelination. MiDs represent a new possibility for enhancing nerve repair and may have critical clinical applications in the future.

Implantable Biomaterials for Peripheral Nerve Regeneration-Technology Trends and Translational Tribulations

The use of autografted nerve in surgical repair of peripheral nerve injuries (PNI) is severely limited due to donor site morbidity and restricted tissue availability. As an alternative, synthetic nerve guidance channels (NGCs) are available on the market for surgical nerve repair, but they fail to promote nerve regeneration across larger critical gap nerve injuries. Therefore, such injuries remain unaddressed, result in poor healing outcomes and are a limiting factor in limb reconstruction and transplantation. On the other hand, a myriad of advanced biomaterial strategies to address critical nerve injuries are proposed in preclinical literature but only few of those have found their way into clinical practice. The design of synthetic nerve grafts should follow rational criteria and make use of a combination of bioinstructive cues to actively promote nerve regeneration. To identify the most promising NGC designs for translation into applicable products, thorough mode of action studies, standardized readouts and validation in large animals are needed. We identify design criteria for NGC fabrication according to the current state of research, give a broad overview of bioactive and functionalized biomaterials and highlight emerging composite implant strategies using therapeutic cells, soluble factors, structural features and intrinsically conductive substrates. Finally, we discuss translational progress in bioartificial conduits for nerve repair from the surgeon's perspective and give an outlook toward future challenges in the field.

Peripheral Nerve Management in Extremity Amputations

The effective management of peripheral nerves in amputation surgery is critical to optimizing patient outcomes. Nerve-related pain after amputation is common, maybe a source of dissatisfaction and functional impairment, and should be considered in all amputees presenting with pain and dysfunction. While traction neurectomy or transposition has long been the standard of care, both regenerative peripheral nerve interface (RPNI) and targeted muscle reinnervation (TMR) have emerged as promising techniques to improve neuroma-related and phantom pain. A multi-disciplinary and multi-modal approach is essential for the optimal management of amputees both acutely and in the delayed or chronic setting.

Insulin enhances neurite extension and myelination of diabetic neuropathy neurons

Background

The authors established an in vitro model of diabetic neuropathy based on the culture system of primary neurons and Schwann cells (SCs) to mimic similar symptoms observed in in vivo models of this complication, such as impaired neurite extension and impaired myelination. The model was then utilized to investigate the effects of insulin on enhancing neurite extension and myelination of diabetic neurons.

Methods

SCs and primary neurons were cultured under conditions mimicking hyperglycemia prepared by adding glucose to the basal culture medium. In a single culture, the proliferation and maturation of SCs and the neurite extension of neurons were evaluated. In a co-culture, the percentage of myelination of diabetic neurons was investigated. Insulin at different concentrations was supplemented to culture media to examine its effects on neurite extension and myelination.

Results

The cells showed similar symptoms observed in in vivo models of this complication. In a single culture, hyperglycemia attenuated the proliferation and maturation of SCs, induced apoptosis, and impaired neurite extension of both sensory and motor neurons. In a co-culture of SCs and neurons, the percentage of myelinated neurites in the hyperglycemia-treated group was significantly lower than that in the control group. This impaired neurite extension and myelination was reversed by the introduction of insulin to the hyperglycemic culture media.

Conclusions

Insulin may be a potential candidate for improving diabetic neuropathy. Insulin can function as a neurotrophic factor to support both neurons and SCs. Further research is needed to discover the potential of insulin in improving diabetic neuropathy.

Axon Regeneration: A Subcellular Extension in Multiple Dimensions

Axons are a unique cellular structure that allows for the communication between neurons. Axon damage compromises neuronal communications and often leads to functional deficits. Thus, developing strategies that promote effective axon regeneration for functional restoration is highly desirable. One fruitful approach is to dissect the regenerative mechanisms used by some types of neurons in both mammalian and nonmammalian systems that exhibit spontaneous regenerative capacity. Additionally, numerous efforts have been devoted to deciphering the barriers that prevent successful axon regeneration in the most regeneration-refractory system-the adult mammalian central nervous system. As a result, several regeneration-promoting strategies have been developed, but significant limitations remain. This review is aimed to summarize historic progression and current understanding of this exciting yet incomplete endeavor.

Autophagic Schwann cells promote perineural invasion mediated by the NGF/ATG7 paracrine pathway in pancreatic cancer

Background

Perineural invasion (PNI) and autophagy are two common features in the tumor microenvironment of pancreatic cancer (PanCa) and have a negative effect on prognosis. Potential mediator cells and the molecular mechanism underlying their relationships need to be fully elucidated.

Methods

To investigate the autophagy of Schwann cells (SCs) in PNI, we reproduced the microenvironment of PNI by collecting clinical PNI tissue, performing sciatic nerve injection of nude mice with cancer cells and establishing a Dorsal root ganglion (DRG) coculture system with cancer cell lines. Autophagy was detected by IHC, IF, transmission electron microscopy (TEM) and western blotting assays. Apoptosis was detected by IF, TEM and western blotting. NGF targeting molecular RO 08–2750(RO) and the autophagy inhibitor Chloroquine (CQ) were utilized to evaluate the effect on autophagy and apoptosis in SCs and PanCa cells in PNI samples.

Results

SC autophagy is activated in PNI by paracrine NGF from PanCa cells. Autophagy-activated Schwann cells promote PNI through a) enhanced migration and axon guidance toward PanCa cells and b) increased chemoattraction to PanCa cells. The NGF-targeting reagent RO and autophagy inhibitor CQ inhibited Schwann cell autophagic flux and induced Schwann cell apoptosis. Moreover, RO and CQ could induce PanCa cell apoptosis and showed good therapeutic effects in the PNI model.

Conclusions

PanCa cells can induce autophagy in SCs through paracrine pathways such as the NGF/ATG7 pathway. Autophagic SCs exert a “nerve-repair like effect”, induce a high level of autophagy of cancer cells, provide a “beacon” for the invasion of cancer cells to nerve fibers, and induce directional growth of cancer cells. Targeting NGF and autophagy for PNI treatment can block nerve infiltration and is expected to provide new directions and an experimental basis for the research and treatment of nerve infiltration in pancreatic cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-021-02198-w.

Curcumin enhances the proliferation and myelinization of Schwann cells through Runx2 to repair sciatic nerve injury

BACKGROUND

RUNX Family Transcription Factor 2 (Runx2) promotes neurite outgrowth after sciatic nerve injury, and Curcumin can promote the expression of Runx2. It is worthwhile to explore whether curcumin's repair effect on sciatic nerve injury is related to Runx2.

OBJECTIVE

To investigate the repair effect of curcumin on sciatic nerve injury and its possible mechanism.

RESULTS

Curcumin improved the sciatic functional index (SFI) and toe spread index (TSI) of rats with sciatic nerve injury and increased the number and diameter of myelinated axons in the sciatic nerve. Curcumin promoted the myelination of SCs (Schwann cells) by increasing the expression of peripheral myelin protein 22 (PMP22), fibrin, S100, and proliferating cell nuclear antige (PCNA). Curcumin treatment increased the proliferation of SCs and the expression of Runx2. Cell experiments further confirmed that curcumin promoted Schwann cell proliferation and myelination through Runx2.

CONCLUSION

Curcumin promotes SCs proliferation and myelination through Runx2 and improves sciatic nerve repair.

Endocytosis-associated patterns in nerve regeneration after peripheral nerve injury

Background

Clearance of myelin debris and remyelination of myelin are necessary steps for peripheral nerve remodeling and regeneration. It has yet to be clarified which genes or proteins are involved in endocytosis or exocytosis in the removal of myelin debris during peripheral nerve repair.

Methods

For this project, a rat model of subacute stage of sciatic nerve injury was established first. Subsequently, normal Schwann cells (NSCs) and activated Schwann cells (ASCs) were harvest before and after peripheral nerve injury (PNI). Following methylated DNA immunoprecipitation sequencing (MeDIP-seq) and tandem mass tags (TMT) labeling analysis of NSCs and ASCs, what common biomarkers changes in peripheral nervous systems remain to be elucidated.

Results

A total of 14,770 different expression genes (DEGs) and 3249 different expression proteins (DEPs) were screened between ASCs and NSCs. For the exosomes, the diameter and particles concentration of exosomes were 141.7 ​nm and 2.97 ​× ​10⁷ particles/mL, respectively. The size distribution of exosomes was 50–200 ​nm. ASCs showed higher cellular uptake ability than the NSCs by cellular uptake test. Moreover, RAB7A, ARF6, ARF1, VPS45, RAB11A, DNM3, and NEDD4 were the core markers and may control the molecular mechanism of the Endocytosis pathway.

Conclusion

These biomarkers may play significant roles in the initiation phase of demyelination and axon regeneration.

The translational potential of this article

This study explores that the endocytosis-associated patterns of Schwann cells may be new therapeutic strategy for nerve tissue engineering and nerve regeneration.

Artemisinin Facilitates Motor Function Recovery by Enhancing Motoneuronal Survival and Axonal Remyelination in Rats Following Brachial Plexus Root Avulsion

Artemisinin (ART), a well-known antimalarial medicine originally isolated from the plant Artemisia annua, exerts neuroprotective effects in the nervous system owing to an antioxidant effect. Here, we determined whether ART is capable of inhibiting the oxidative stress to enhance motoneuronal (MN) survival to promote motor function recovery of rats following brachial plexus root avulsion (BPRA) with reimplantation surgery. Rats following BPRA and reimplantation were subcutaneously injected with 500 μL of PBS or 16 mg/mL ART once daily for 7 days after surgery. Terzis grooming test (TGT), histochemical staining, real-time polymerase chain reaction, and Western blot were conducted to determine the recovery of motor function of the upper limb, the survival rate of MNs, the oxidative stress levels in the ventral horn of the spinal cord, the morphology of abnormal musculocutaneous nerve fibers, the remyelination of axons in musculocutaneous nerves, and the degree of bicep atrophy. ART significantly increased TGT score, improved the survival of MNs, inhibited the oxidative stress, ameliorated the abnormal morphology of fibers in the musculocutaneous nerve, promoted the remyelination of axons, and alleviated muscle atrophy. Take together, ART can improve the survival of MNs and axonal remyelination to promote the motor function recovery via inhibiting oxidative stress, suggesting that ART may represent a new approach to the therapy of spinal root avulsion.

Schwann cells and trigeminal neuralgia

Schwann cells are components of the peripheral nerve myelin sheath, which supports and nourishes axons. Upon injury of the trigeminal nerve, Schwann cells are activated and cause trigeminal neuralgia by engulfing the myelin sheath and secreting various neurotrophic factors. Further, Schwann cells can repair the damaged nerve and thus alleviate trigeminal neuralgia. Here, we briefly describe the development and activation of Schwann cells after nerve injury. Moreover, we expound on the occurrence, regulation, and treatment of trigeminal neuralgia; further, we point out the current research deficiencies and future research directions.

Peripheral nerves preferentially regenerate in intramuscular endoneurial tubes to reinnervate denervated skeletal muscles

Schwann cells are essential for peripheral nerve regeneration but, over short distances in acellular nerve grafts, extracellular matrix (ECM) molecules can support growth. The ECM molecules are present also on denervated muscle surfaces where they can support nerve growth. In this study, we addressed the efficacy of the ECM molecules of denervated muscle to support nerve fiber regeneration and muscle reinnervation. In the hindlimb of Sprague-Dawley rats, the proximal stump of the transected posterior tibial nerve, was cross-sutured to the distal nerve stump (NN) of each of three denervated muscles, tibialis anterior, extensor digitorum longus, and soleus, or implanted onto the denervated muscles' surfaces (N-M), proximal or distal to the endplate zone. Recordings of muscle and motor unit (MU) isometric forces and silver/cholinesterase histochemical staining of longitudinal muscle cryosections were used to determine the numbers of reinnervated MUs and the spatial course of regenerating nerve fibers, respectively. MU numbers declined significantly after N-M (>50%) as compared to those after NN. Muscle forces were reduced despite each nerve reinnervating up to three times the normal MU muscle fiber number. Regenerating nerves 'streamed' from the N-M site either proximal or distal to endplate zones toward the denervated intramuscular endoneurial tubes, with reduced numbers reinnervating endplates. We conclude that there is preferential reinnervation through the endoneurial tube and that it is important to drive implanted nerve fibers to enter endoneurial tubes for optimal muscle reinnervation. Schwann cells play the essential role in guiding regenerating nerve fibers to reinnervate denervated muscle fibers.

Engineering polysialic acid on Schwann cells using polysialyltransferase gene transfer or purified enzyme exposure for spinal cord injury transplantation

Polysialic acid (PolySia) is a critical post-translational modification on the neural cell adhesion molecule (NCAM, a.k.a., CD56), important for cell migration and axon growth during nervous system development, plasticity and repair. PolySia induction on Schwann cells (SCs) enhances their migration, axon growth support and ability to improve functional recovery after spinal cord injury (SCI) transplantation. In the current investigation two methods of PolySia induction on SCs, lentiviral vector transduction of the mouse polysialytransferase gene ST8SIA4 (LV-PST) or enzymatic engineering with a recombinant bacterial PST (PSTNm), were examined comparatively for their effects on PolySia induction, SC migration, the innate immune response and axon growth after acute SCI. PSTNm produced significant PolySia induction and a greater diversity of surface molecule polysialylation on SCs as evidenced by immunoblot. In the scratch wound assay, PSTNm was superior to LV-PST in the promotion of SC migration and gap closure. At 24 h after SCI transplantation, PolySia induction on SCs was most pronounced with LV-PST. Co-delivery of PSTNm with SCs, but not transient cell exposure, led to broader induction of PolySia within the injured spinal cord due to polysialylation upon both host cells and transplanted SCs. The innate immune response after SCI, measured by CD68 immunoreactivity, was similar among PolySia induction methods. LV-PST or PSTNm co-delivery with SCs provided a similar enhancement of SC migration and axon growth support above that of unmodified SCs. These studies demonstrate that LV-PST and PSTNm provide comparable acute effects on SC polysialation, the immune response and neurorepair after SCI.

Effective decellularization of human nerve matrix for regenerative medicine with a novel protocol

Injuries to the peripheral nerves represent a frequent cause of permanent disability in adults. The repair of large nerve lesions involves the use of autografts, but they have several inherent limitations. Overcoming these limitations, the use of decellularized nerve matrix has emerged as a promising treatment in tissue regenerative medicine. Here, we generate longer human decellularized nerve segments with a novel decellularization method, using nonionic, zwitterionic, and enzymatic incubations. Efficiency of decellularization was measured by DNA quantification and cell remnant analysis (myelin, S100, neurofilament). The evaluation of the extracellular matrix (collagen, laminin, and glycosaminoglycans) preservation was carried out by enzyme-linked immunosorbent assay (ELISA) or biochemical methods, along with histological and immunofluorescence analysis. Moreover, biomechanical properties and cytocompatibility were tested. Results showed that the decellularized nerves generated with this protocol have a concentration of DNA below the threshold of 50 ng/mg of dry tissue. Furthermore, myelin, S100, and MHCII proteins were absent, although some neurofilament remnants could be observed. Moreover, extracellular matrix proteins were well maintained, as well as the biomechanical properties, and the decellularized nerve matrix did not generate cytotoxicity. These results show that our method is effective for the generation of decellularized human nerve grafts. The generation of longer decellularized nerve segments would allow the understanding of the regenerative neurobiology after nerve injuries in both clinical assays and bigger animal models. Effective decellularization of human nerve matrix for regenerative medicine with a novel protocol. Combination of zwitterionic, non-ionic detergents, hyperosmotic solution and nuclease enzyme treatment remove cell remnants, maintain collagen, laminin and biomechanics without generating cytotoxic leachables.

Ascorbic acid accelerates Wallerian degeneration after peripheral nerve injury

Wallerian degeneration occurs after peripheral nerve injury and provides a beneficial microenvironment for nerve regeneration. Our previous study demonstrated that ascorbic acid promotes peripheral nerve regeneration, possibly through promoting Schwann cell proliferation and phagocytosis and enhancing macrophage proliferation, migration, and phagocytosis. Because Schwann cells and macrophages are the main cells involved in Wallerian degeneration, we speculated that ascorbic acid may accelerate this degenerative process. To test this hypothesis, 400 mg/kg ascorbic acid was administered intragastrically immediately after sciatic nerve transection, and 200 mg/kg ascorbic acid was then administered intragastrically every day. In addition, rat sciatic nerve explants were treated with 200 μM ascorbic acid. Ascorbic acid significantly accelerated the degradation of myelin basic protein-positive myelin and neurofilament 200-positive axons in both the transected nerves and nerve explants. Furthermore, ascorbic acid inhibited myelin-associated glycoprotein expression, increased c-Jun expression in Schwann cells, and increased both the number of macrophages and the amount of myelin fragments in the macrophages. These findings suggest that ascorbic acid accelerates Wallerian degeneration by accelerating the degeneration of axons and myelin in the injured nerve, promoting the dedifferentiation of Schwann cells, and enhancing macrophage recruitment and phagocytosis. The study was approved by the Southern Medical University Animal Care and Use Committee (approval No. SMU-L2015081) on October 15, 2015.

Interaction between Schwann cells and other cells during repair of peripheral nerve injury

Peripheral nerve injury (PNI) is common and, unlike damage to the central nervous system injured nerves can effectively regenerate depending on the location and severity of injury. Peripheral myelinating glia, Schwann cells (SCs), interact with various cells in and around the injury site and are important for debris elimination, repair, and nerve regeneration. Following PNI, Wallerian degeneration of the distal stump is rapidly initiated by degeneration of damaged axons followed by morphologic changes in SCs and the recruitment of circulating macrophages. Interaction with fibroblasts from the injured nerve microenvironment also plays a role in nerve repair. The replication and migration of injury-induced dedifferentiated SCs are also important in repairing the nerve. In particular, SC migration stimulates axonal regeneration and subsequent myelination of regenerated nerve fibers. This mobility increases SC interactions with other cells in the nerve and the exogenous environment, which influence SC behavior post-injury. Following PNI, SCs directly and indirectly interact with other SCs, fibroblasts, and macrophages. In addition, the inter- and intracellular mechanisms that underlie morphological and functional changes in SCs following PNI still require further research to explain known phenomena and less understood cell-specific roles in the repair of the injured peripheral nerve. This review provides a basic assessment of SC function post-PNI, as well as a more comprehensive evaluation of the literature concerning the SC interactions with macrophages and fibroblasts that can influence SC behavior and, ultimately, repair of the injured nerve.

Microtopographical patterns promote different responses in fibroblasts and Schwann cells: A possible feature for neural implants

The chronic reliability of bioelectronic neural interfaces has been challenged by foreign body reactions (FBRs) resulting in fibrotic encapsulation and poor integration with neural tissue. Engineered microtopographies could alleviate these challenges by manipulating cellular responses to the implanted device. Parallel microchannels have been shown to modulate neuronal cell alignment and axonal growth, and Sharklet™ microtopographies of targeted feature sizes can modulate bio-adhesion of an array of bacteria, marine organisms, and epithelial cells due to their unique geometry. We hypothesized that a Sharklet™ micropattern could be identified that inhibited fibroblasts partially responsible for FBR while promoting Schwann cell proliferation and alignment. in vitro cell assays were used to screen the effect of Sharklet™ and channel micropatterns of varying dimensions from 2 to 20 μm on fibroblast and Schwann cell metrics (e.g., morphology/alignment, nuclei count, metabolic activity), and a hierarchical analysis of variance was used to compare treatments. In general, Schwann cells were found to be more metabolically active and aligned than fibroblasts when compared between the same pattern. 20 μm wide channels spaced 2 μm apart were found to promote Schwann cell attachment and alignment while simultaneously inhibiting fibroblasts and warrant further in vivo study on neural interface devices. No statistically significant trends between cellular responses and geometrical parameters were identified because mammalian cells can change their morphology dependent on their environment in a manner dissimilar to bacteria. Our results showed although surface patterning is a strong physical tool for modulating cell behavior, responses to micropatterns are highly dependent on the cell type.

Fundamentals and Current Strategies for Peripheral Nerve Repair and Regeneration

A body of evidence indicates that peripheral nerves have an extraordinary yet limited capacity to regenerate after an injury. Peripheral nerve injuries have confounded professionals in this field, from neuroscientists to neurologists, plastic surgeons, and the scientific community. Despite all the efforts, full functional recovery is still seldom. The inadequate results attained with the "gold standard" autograft procedure still encourage a dynamic and energetic research around the world for establishing good performing tissue-engineered alternative grafts. Resourcing to nerve guidance conduits, a variety of methods have been experimentally used to bridge peripheral nerve gaps of limited size, up to 30-40 mm in length, in humans. Herein, we aim to summarize the fundamentals related to peripheral nerve anatomy and overview the challenges and scientific evidences related to peripheral nerve injury and repair mechanisms. The most relevant reports dealing with the use of both synthetic and natural-based biomaterials used in tissue engineering strategies when treatment of nerve injuries is envisioned are also discussed in depth, along with the state-of-the-art approaches in this field.

Peripheral Nerve Regeneration and Muscle Reinnervation

Injured peripheral nerves but not central nerves have the capacity to regenerate and reinnervate their target organs. After the two most severe peripheral nerve injuries of six types, crush and transection injuries, nerve fibers distal to the injury site undergo Wallerian degeneration. The denervated Schwann cells (SCs) proliferate, elongate and line the endoneurial tubes to guide and support regenerating axons. The axons emerge from the stump of the viable nerve attached to the neuronal soma. The SCs downregulate myelin-associated genes and concurrently, upregulate growth-associated genes that include neurotrophic factors as do the injured neurons. However, the gene expression is transient and progressively fails to support axon regeneration within the SC-containing endoneurial tubes. Moreover, despite some preference of regenerating motor and sensory axons to “find” their appropriate pathways, the axons fail to enter their original endoneurial tubes and to reinnervate original target organs, obstacles to functional recovery that confront nerve surgeons. Several surgical manipulations in clinical use, including nerve and tendon transfers, the potential for brief low-frequency electrical stimulation proximal to nerve repair, and local FK506 application to accelerate axon outgrowth, are encouraging as is the continuing research to elucidate the molecular basis of nerve regeneration.

Phosphorylation of STAT3 by axonal Cdk5 promotes axonal regeneration by modulating mitochondrial activity

Cyclin-dependent kinase 5 (Cdk5) is involved in neural organization and synaptic functions in developing and adult brains, yet its role in axonal regeneration is not known well. Here, we characterize Cdk5 function for axonal regeneration after peripheral nerve injury. Levels of Cdk5 and p25 were elevated in sciatic nerve axons after injury. Cdk5 activity was concomitantly induced from injured nerve and increased the phosphorylation of signal transducer and activator of transcription 3 (STAT3) on the serine 727 residue. Pharmacological and genetic blockades of Cdk5 activity phosphorylating STAT3 resulted in the inhibition of axonal regeneration as evidenced by reduction of retrograde labeling of dorsal root ganglion (DRG) sensory neurons and spinal motor neurons and also of neurite outgrowth of preconditioned DRG neurons in culture. Cdk5 and STAT3 were found in mitochondrial membranes of the injured sciatic nerve. Cdk5-GFP, which was translocated into the mitochondria by the mitochondrial target sequence (MTS), induced STAT3 phosphorylation in transfected DRG neurons and was sufficient to induce neurite outgrowth. In the mitochondria, Cdk5 activity was positively correlated with increased mitochondrial membrane potential as measured by fluorescence intensity of JC-1 aggregates. Our data suggest that Cdk5 may play a role in modulating mitochondrial activity through STAT3 phosphorylation, thereby promoting axonal regeneration.

Effects of 660-nm and 780-nm Laser Therapy on ST88-14 Schwann Cells

The aim of the present study was to evaluate the comparative effects of red (660-nm) and near-infrared (780-nm) low-level laser therapy (LLLT) on viability, mitochondrial activity, morphology and gene expression of growth factors on Schwann cells (SC). ST88-14 cells were grown in RPMI 1640 with 10 mM of HEPES, 2 mM of glutamine, 10% fetal bovine serum and 1% antibiotic-antimycotic solution at 37°C in humidified atmosphere of 5% CO₂ . Cells were detached with trypsin and centrifugated at 231 g for 5 min at 10°C, and the pellet (8 × 10⁴  cells/tube) was irradiated at the bottom of 50 ml polypropylene tube with a Twin-Laser system (660 and 780 nm, 40 mW, 1 mW cm^-2 , 3.2 and 6.4 J, 80 and 160 J cm^-2 with 80 and 160 s). After 1, 3 and 7 days, the analysis was performed. After irradiation, the SC increase mitochondrial activity, gene expression of the neural growth factors NGF and BDNF, and cell migration and increase the G2/M cells. SC showed neuronal morphology, normal F-actin cytoskeleton organization and positive labeling for S100. PBM increased metabolic activity, mitosis and gene expression when irradiated with red and infrared LLLT. An increase in cell migration was obtained when irradiated with infrared LLLT.

Augmented Buyang Huanwu Decoction facilitates axonal regeneration after peripheral nerve transection through the regulation of inflammatory cytokine production

ETHNOPHARMACOLOGICAL RELEVANCE

Herbal formulation Buyang Huanwu Decoction (BYHWD) has been used to treat cardiovascular disorders including cerebral ischemia. Recent studies showed its effects on promoting axonal regeneration after nerve injury. However, compositional reformulation supplemented with herbal components that regulates inflammation may increase its efficacy for nerve repair.

AIM OF THE STUDY

We prepared a new herbal decoction by adding selected herbal components to BYHWD (augmented BYHWD; ABHD) and investigated the effect of ABHD on the production of inflammatory cytokines and axonal regeneration using an animal model of nerve transection and coaptation (NTC).

MATERIALS AND METHODS

A rat model of NTC was performed on the sciatic nerve. The sciatic nerve and dorsal root ganglion (DRG) were isolated and used for immunofluorescence staining and western blot analysis. DRG tissue was also used to prepare primary neuron culture and the length of neurites was analyzed. Sensorimotor nerve activities were assessed by rotarod and von Frey tests.

RESULTS

Three herbal components that facilitated neurite outgrowth were chosen to formulate ABHD. ABHD administration into the sciatic nerve 1 week or 3 months after NTC facilitated axonal regeneration. Cell division cycle 2 (Cdc2) and brain-derived neurotrophic factor (BDNF) proteins were induced from the reconnected distal portion of the sciatic nerve and the levels were further elevated by in vivo administration of ABHD. Phospho-Erk1/2 level was increased by ABHD treatment as well, implying its role in mediating retrograde transport of BDNF signals into the neuronal cell body. Production of inflammatory cytokines IL-1β and TNF-α was induced in the reconnected nerve but attenuated by ABHD treatment. Behavioral tests revealed that ABHD treatment improved functional recovery of sensorimotor activities.

CONCLUSIONS

A newly formulated ABHD is effective at regulating the production of inflammatory cytokines and promoting axonal regeneration after nerve transection and may be considered to develop therapeutic strategies for peripheral nerve injury disorders.

Berberine enhances L1 expression and axonal remyelination in rats after brachial plexus root avulsion

BACKGROUND AND PURPOSE

Enhanced remyelination of the regenerated axons results in functional re-innervation and improved functional motor recovery after brachial plexus root avulsion (BPRA). The neural cell adhesion molecule L1 (L1CAM, L1) regulates myelination and promotes regeneration after acute injury in the nervous system. Berberine (BBR) can exert neuroprotective roles against the lesion. Herein, we investigated whether berberine (BBR) can affect the expression of L1 and enhance the axonal remyelination in rats following BPRA.

METHODS

The surgical procedures were performed to build the rat brachial plexus avulsion and re-implantation model, and then, the rats were treated with BBR. After the rehabilitation for 12 weeks, the musculocutaneous nerves were collected for quantitative real-time PCR, Western blot analysis, and histochemical and immunofluorescence staining.

RESULTS

We observed that, BBR treatment ameliorated the abnormal musculocutaneous nerve fibers morphology, up-regulated the L1 expression, increased the myelination-related genes, decreased the differentiated-associated genes, and up-regulated the phosphorylation of ERK.

CONCLUSION

These results suggest that BBR may enhance L1 expression and promote axonal remyelination after spinal root avulsion.

Lithium promotes proliferation and suppresses migration of Schwann cells

Schwann cell proliferation, migration and remyelination of regenerating axons contribute to regeneration after peripheral nervous system injury. Lithium promotes remyelination by Schwann cells and improves peripheral nerve regeneration. However, whether lithium modulates other phenotypes of Schwann cells, especially their proliferation and migration remains elusive. In the current study, primary Schwann cells from rat sciatic nerve stumps were cultured and exposed to 0, 5, 10, 15, or 30 mM lithium chloride (LiCl) for 24 hours. The effects of LiCl on Schwann cell proliferation and migration were examined using the Cell Counting Kit-8, 5-ethynyl-2'-deoxyuridine, Transwell and wound healing assays. Cell Counting Kit-8 and 5-ethynyl-2'-deoxyuridine assays showed that 5, 10, 15, and 30 mM LiCl significantly increased the viability and proliferation rate of Schwann cells. Transwell-based migration assays and wound healing assays showed that 10, 15, and 30 mM LiCl suppressed the migratory ability of Schwann cells. Furthermore, the effects of LiCl on the proliferation and migration phenotypes of Schwann cells were mostly dose-dependent. These data indicate that lithium treatment significantly promotes the proliferation and inhibits the migratory ability of Schwann cells. This conclusion will inform strategies to promote the repair and regeneration of peripheral nerves. All of the animal experiments in this study were ethically approved by the Administration Committee of Experimental Animal Center of Nantong University, China (approval No. 20170320-017) on March 2, 2017.

Growth factors-based therapeutic strategies and their underlying signaling mechanisms for peripheral nerve regeneration

Peripheral nerve injury (PNI), one of the most common concerns following trauma, can result in a significant loss of sensory or motor function. Restoration of the injured nerves requires a complex cellular and molecular response to rebuild the functional axons so that they can accurately connect with their original targets. However, there is no optimized therapy for complete recovery after PNI. Supplementation with exogenous growth factors (GFs) is an emerging and versatile therapeutic strategy for promoting nerve regeneration and functional recovery. GFs activate the downstream targets of various signaling cascades through binding with their corresponding receptors to exert their multiple effects on neurorestoration and tissue regeneration. However, the simple administration of GFs is insufficient for reconstructing PNI due to their short half‑life and rapid deactivation in body fluids. To overcome these shortcomings, several nerve conduits derived from biological tissue or synthetic materials have been developed. Their good biocompatibility and biofunctionality made them a suitable vehicle for the delivery of multiple GFs to support peripheral nerve regeneration. After repairing nerve defects, the controlled release of GFs from the conduit structures is able to continuously improve axonal regeneration and functional outcome. Thus, therapies with growth factor (GF) delivery systems have received increasing attention in recent years. Here, we mainly review the therapeutic capacity of GFs and their incorporation into nerve guides for repairing PNI. In addition, the possible receptors and signaling mechanisms of the GF family exerting their biological effects are also emphasized.

Perspectives on 3D Bioprinting of Peripheral Nerve Conduits

The peripheral nervous system controls the functions of sensation, movement and motor coordination of the body. Peripheral nerves can get damaged easily by trauma or neurodegenerative diseases. The injury can cause a devastating effect on the affected individual and his aides. Treatment modalities include anti-inflammatory medications, physiotherapy, surgery, nerve grafting and rehabilitation. 3D bioprinted peripheral nerve conduits serve as nerve grafts to fill the gaps of severed nerve bodies. The application of induced pluripotent stem cells, its derivatives and bioprinting are important techniques that come in handy while making living peripheral nerve conduits. The design of nerve conduits and bioprinting require comprehensive information on neural architecture, type of injury, neural supporting cells, scaffold materials to use, neural growth factors to add and to streamline the mechanical properties of the conduit. This paper gives a perspective on the factors to consider while bioprinting the peripheral nerve conduits.

Biological characteristics of dynamic expression of nerve regeneration related growth factors in dorsal root ganglia after peripheral nerve injury

The regenerative capacity of peripheral nerves is limited after nerve injury. A number of growth factors modulate many cellular behaviors, such as proliferation and migration, and may contribute to nerve repair and regeneration. Our previous study observed the dynamic changes of genes in L4-6 dorsal root ganglion after rat sciatic nerve crush using transcriptome sequencing. Our current study focused on upstream growth factors and found that a total of 19 upstream growth factors were dysregulated in dorsal root ganglions at 3, 9 hours, 1, 4, or 7 days after nerve crush, compared with the 0 hour control. Thirty-six rat models of sciatic nerve crush injury were prepared as described previously. Then, they were divided into six groups to measure the expression changes of representative genes at 0, 3, 9 hours, 1, 4 or 7 days post crush. Our current study measured the expression levels of representative upstream growth factors, including nerve growth factor, brain-derived neurotrophic factor, fibroblast growth factor 2 and amphiregulin genes, and explored critical signaling pathways and biological process through bioinformatic analysis. Our data revealed that many of these dysregulated upstream growth factors, including nerve growth factor, brain-derived neurotrophic factor, fibroblast growth factor 2 and amphiregulin, participated in tissue remodeling and axon growth-related biological processes Therefore, the experiment described the expression pattern of upstream growth factors in the dorsal root ganglia after peripheral nerve injury. Bioinformatic analysis revealed growth factors that may promote repair and regeneration of damaged peripheral nerves. All animal surgery procedures were performed in accordance with Institutional Animal Care Guidelines of Nantong University and ethically approved by the Administration Committee of Experimental Animals, China (approval No. 20170302-017) on March 2, 2017.

3D Printing and Bioprinting Nerve Conduits for Neural Tissue Engineering

Fabrication of nerve conduits for perfectly repairing or replacing damaged peripheral nerve is an urgent demand worldwide, but it is also a formidable clinical challenge. In the last decade, with the rapid development of manufacture technologies, 3D printing and bioprinting have been becoming remarkable stars in the field of neural engineering. In this review, we explore that the biomaterial inks (hydrogels, thermoplastic, and thermoset polyesters and composite) and bioinks have been selected for 3D printing and bioprinting of peripheral nerve conduits. This review covers 3D manufacturing technologies, including extrusion printing, inkjet printing, stereolithography, and bioprinting with inclusion of cells, bioactive molecules, and drugs. Finally, an outlook on the future directions of 3D printing and 4D printing in customizable nerve therapies is presented.