Introduction to special issue: Challenges and opportunities for regeneration in the peripheral nervous system

Therapeutic effect of exosomes derived from Schwann cells in the repair of peripheral nerve injury

Peripheral nerve injury (PNI) can cause nerve demyelination, neuronal apoptosis, axonal atrophy, inflammatory infiltration, glial scar formation, and other pathologies that can lead to sensory and motor dysfunction and seriously affect the psychosomatic health of patients. There is currently no effective treatment method, so exploring a promising treatment method is of great significance. Several studies have revealed the therapeutic roles of Schwann cells (SCs) and their exosomes in nerve injury repair. Exosomes are extracellular nanovesicles secreted by cells that act as key molecules in intercellular communication. Progress has been made in understanding the role of exosomes derived from SCs (SC-EXOs) in peripheral nerve regeneration, including the promotion of axonal regeneration and myelin formation, anti-inflammation, vascular regeneration, neuroprotection, and neuroregulation. Therefore, in this paper, we summarize the functional characteristics of SC-EXOs and discuss their potential therapeutic effects on PNI repair as well as some existing problems and future challenges.

Target specification and therapeutic potential of extracellular vesicles for regulating corneal angiogenesis, lymphangiogenesis, and nerve repair

Extracellular vesicles, including exosomes, are small extracellular vesicles that range in size from 30 nm to 10 μm in diameter and have specific membrane markers. They are naturally secreted and are present in various bodily fluids, including blood, urine, and saliva, and through the variety of their internal cargo, they contribute to both normal physiological and pathological processes. These processes include immune modulation, neuronal synapse formation, cell differentiation, cancer metastasis, angiogenesis, lymphangiogenesis, progression of infectious disease, and neurodegenerative disorders like Alzheimer's and Parkinson's disease. In recent years, interest has grown in the use of exosomes as a potential drug delivery system for various diseases and injuries. Importantly, exosomes originating from a patient's own cells exhibit minimal immunogenicity and possess remarkable stability along with inherent and adjustable targeting capabilities. This review explores the roles of exosomes in angiogenesis, lymphangiogenesis, and nerve repair with a specific emphasis on these processes within the cornea. Furthermore, it examines exosomes derived from specific cell types, discusses the advantages of exosome-based therapies in modulating these processes, and presents some of the most established methods for exosome isolation. Exosome-based treatments are emerging as potential minimally invasive and non-immunogenic therapies that modulate corneal angiogenesis and lymphangiogenesis, as well as enhance and accelerate endogenous corneal nerve repair.

Bioelectronics for electrical stimulation: materials, devices and biomedical applications

Bioelectronics is a hot research topic, yet an important tool, as it facilitates the creation of advanced medical devices that interact with biological systems to effectively diagnose, monitor and treat a broad spectrum of health conditions. Electrical stimulation (ES) is a pivotal technique in bioelectronics, offering a precise, non-pharmacological means to modulate and control biological processes across molecular, cellular, tissue, and organ levels. This method holds the potential to restore or enhance physiological functions compromised by diseases or injuries by integrating sophisticated electrical signals, device interfaces, and designs tailored to specific biological mechanisms. This review explains the mechanisms by which ES influences cellular behaviors, introduces the essential stimulation principles, discusses the performance requirements for optimal ES systems, and highlights the representative applications. From this review, we can realize the potential of ES based bioelectronics in therapy, regenerative medicine and rehabilitation engineering technologies, ranging from tissue engineering to neurological technologies, and the modulation of cardiovascular and cognitive functions. This review underscores the versatility of ES in various biomedical contexts and emphasizes the need to adapt to complex biological and clinical landscapes it addresses.

Beyond boundaries: The therapeutic potential of exosomes in neural microenvironments in neurological disorders

Neurological disorders are a diverse group of conditions that can significantly impact individuals' quality of life. The maintenance of neural microenvironment homeostasis is essential for optimal physiological cellular processes. Perturbations in this delicate balance underlie various pathological manifestations observed across various neurological disorders. Current treatments for neurological disorders face substantial challenges, primarily due to the formidable blood-brain barrier and the intricate nature of neural tissue structures. These obstacles have resulted in a paucity of effective therapies and inefficiencies in patient care. Exosomes, nanoscale vesicles that contain a complex repertoire of biomolecules, are identifiable in various bodily fluids. They hold substantial promise in numerous therapeutic interventions due to their unique attributes, including targeted drug delivery mechanisms and the ability to cross the BBB, thereby enhancing their therapeutic potential. In this review, we investigate the therapeutic potential of exosomes across a range of neurological disorders, including neurodegenerative disorders, traumatic brain injury, peripheral nerve injury, brain tumors, and stroke. Through both in vitro and in vivo studies, our findings underscore the beneficial influence of exosomes in enhancing the neural microenvironment following neurological diseases, offering promise for improved neural recovery and management in these conditions.

Efficacy of using adipose-derived stem cells and PRP on regeneration of 40 -mm long sciatic nerve defect bridged by polyglycolic-polypropylene mesh in canine model

BACKGROUND

Sciatic nerve repair becomes a focus of research in neurological aspect to restore the normal physical ability of the animal to stand and walk. Tissue engineered nerve grafts (TENGs) provide a promising alternative therapy for regeneration of large gap defects. The present study investigates the regenerative capacity of PRP, ADSCs, and PRP mixed ADSCs on a long sciatic nerve defect (40-mm) bridged by a polyglycolic polypropylene (PGA-PRL) mesh which acts as a neural scaffold.

MATERIALS AND METHODS

The study was conducted on 12 adult male mongrel dogs that were randomly divided into 4 groups: Group I (scaffold group); where the sciatic defect was bridged by a (PGA-PRL) mesh only while the mesh was injected with ADSCs in Group II (ADSCs group), PRP in Group III (PRP group). Mixture of PRP and ADSCs was allocated in Group IV (PRP + ADSCs group). Monthly, all animals were monitored for improvement in their gait and a numerical lameness score was recorded for all groups. 6 months-post surgery, the structural and functional recovery of sciatic nerve was evaluated electrophysiologically, and on the level of gene expression, and both sciatic nerve and the gastrocnemius muscle were evaluated morphometrically, histopathologically.

RESULTS

Numerical lameness score showed improvement in the motor activities of both Group II and Group III followed by Group IV and the scaffold group showed mild improvement even after 6 months. Histopathologically, all treated groups showed axonal sprouting and numerous regenerated fascicles with obvious angiogenesis in proximal cut, and distal portion where Group IV exhibited a significant remyelination with the MCOOL technique. The regenerative ratio of gastrocnemius muscle was 23.81%, 56.68%, 52.06% and 40.69% for Group I, II, III and IV; respectively. The expression of NGF showed significant up regulation in the proximal portion for both Group III and Group IV (P ≤ 0.0001) while Group II showed no significant difference. PDGF-A, and VEGF expressions were up-regulated in Group II, III, and IV whereas Group I showed significant down-regulation for NGF, PDGF-A, and VEGF (P ≤ 0.0001).

CONCLUSION

ADSCs have a great role in restoring the damaged nerve fibers by secreting several types of growth factors like NGF that have a proliferative effect on Schwann cells and their migration. In addition, PRP therapy potentiates the effect of ADSCs by synthesis another growth factors such as PDGF-A, VEGF, NGF for better healing of large sciatic gap defects.

Schwann Cells as Orchestrators of Nerve Repair: Implications for Tissue Regeneration and Pathologies

Peripheral nerves exist in a stable state in adulthood providing a rapid bidirectional signaling system to control tissue structure and function. However, following injury, peripheral nerves can regenerate much more effectively than those of the central nervous system (CNS). This multicellular process is coordinated by peripheral glia, in particular Schwann cells, which have multiple roles in stimulating and nurturing the regrowth of damaged axons back to their targets. Aside from the repair of damaged nerves themselves, nerve regenerative processes have been linked to the repair of other tissues and de novo innervation appears important in establishing an environment conducive for the development and spread of tumors. In contrast, defects in these processes are linked to neuropathies, aging, and pain. In this review, we focus on the role of peripheral glia, especially Schwann cells, in multiple aspects of nerve regeneration and discuss how these findings may be relevant for pathologies associated with these processes.

Application of Electrical Stimulation to Enhance Axon Regeneration Following Peripheral Nerve Injury

Enhancing axon regeneration is a major focus of peripheral nerve injury research. Although peripheral axons possess a limited ability to regenerate, their functional recovery is very poor. Various activity-based therapies like exercise, optical stimulation, and electrical stimulation as well as pharmacologic treatments can enhance spontaneous axon regeneration. In this protocol, we use a custom-built cuff to electrically stimulate the whole sciatic nerve for an hour prior to transection and repair. We used a Thy-1-YFP-H mouse to visualize regenerating axon profiles. We compared the regeneration of axons from nerves that were electrically stimulated to nerves that were not stimulated (untreated). Electrically stimulated nerves had longer axon growth than the untreated nerves. We detail how variations of this method can be used to measure acute axon growth.

Nerve trunk healing and neuroma formation after nerve transection injury

The nerve trunk healing process of a transected peripheral nerve trunk is composed of angiogenesis, nerve fiber regeneration, and scarring. Nerve trunk healing and neuroma formation probably share identical molecular mediators and similar regulations. At the nerve transection site, angiogenesis is sufficient and necessary for nerve fiber regeneration. Angiogenesis and nerve fiber regeneration reveal a positive correlation in the early time. Scarring and nerve fiber regeneration show a negative correlation in the late phase. We hypothesize that anti-angiogenesis suppresses neuromas. Subsequently, we provide potential protocols to test our hypothesis. Finally, we recommend employing anti-angiogenic small-molecule protein kinase inhibitors to investigate nerve transection injuries.

Collagen for neural tissue engineering: Materials, strategies, and challenges

Neural tissue engineering (NTE) has made remarkable strides in recent years and holds great promise for treating several devastating neurological disorders. Selecting optimal scaffolding material is crucial for NET design strategies that enable neural and non-neural cell differentiation and axonal growth. Collagen is extensively employed in NTE applications due to the inherent resistance of the nervous system against regeneration, functionalized with neurotrophic factors, antagonists of neural growth inhibitors, and other neural growth-promoting agents. Recent advancements in integrating collagen with manufacturing strategies, such as scaffolding, electrospinning, and 3D bioprinting, provide localized trophic support, guide cell alignment, and protect neural cells from immune activity. This review categorises and analyses collagen-based processing techniques investigated for neural-specific applications, highlighting their strengths and weaknesses in repair, regeneration, and recovery. We also evaluate the potential prospects and challenges of using collagen-based biomaterials in NTE. Overall, this review offers a comprehensive and systematic framework for the rational evaluation and applications of collagen in NTE.

Effect of combined intervention of exercise and autologous bone marrow stromal cell transplantation on neurotrophic factors and pain-related cascades over time after sciatic nerve injury

The purpose of this study was to determine whether combined inter-vention of treadmill exercise and bone marrow stromal cell (BMSC) transplantation would affect the expression of neurotrophic factors in the sciatic nerve injury (SNI) and neuropathic pain-related cascades in ipsilateral lumbar 4-5 dorsal root ganglion (DRG) during the early or late stage of sciatic nerve regeneration. The rats were randomly divided into the normal control group (CONT, n=6), sedentary group (SS, n=24), exercise group (SE, n=24), BMSC transplantation group (SB, n=24), BMSC transplantation+exercise group (SBE, n=24) 1, 2, 3, and 5 weeks after SNI. Single dose of 5×10⁶ harvested BMSC was injected into the injury area sing by a 30 gauge needle. Treadmill exercise was performed at a speed of 8 m/min for 30 min once a day. Tropomyosin-receptor kinase B, brain-derived neurotrophic factor and ciliary neurotrophic fac-tor were significantly upregulated in the SE and SBE groups at 1- and 2-week postinjury than those in the CONT and SS groups, and SB and SBE groups continuously kept up proinflammatory cytokines until the late stage of regeneration. Nuclear factor kappa-light-chain-enhancer of activated B cells, interleukin and tumor necrosis factor alpha in ipsi-lateral DRG were progressively decreased by exercise alone application and/or BMSC transplantation at early and late stage of regeneration. Present results provide reliable information that combined intervention of treadmill exercise and BMSC transplantation might be one of the effective treatment strategies for recovering sciatic nerve injury-induced neuropathic pain over time.

Schwann cell functions in peripheral nerve development and repair

The glial cell of the peripheral nervous system (PNS), the Schwann cell (SC), counts among the most multifaceted cells of the body. During development, SCs secure neuronal survival and participate in axonal path finding. Simultaneously, they orchestrate the architectural set up of the developing nerves, including the blood vessels and the endo-, peri- and epineurial layers. Perinatally, in rodents, SCs radially sort and subsequently myelinate individual axons larger than 1 μm in diameter, while small calibre axons become organised in non-myelinating Remak bundles. SCs have a vital role in maintaining axonal health throughout life and several specialized SC types perform essential functions at specific locations, such as terminal SC at the neuromuscular junction (NMJ) or SC within cutaneous sensory end organs. In addition, neural crest derived satellite glia maintain a tight communication with the soma of sensory, sympathetic, and parasympathetic neurons and neural crest derivatives are furthermore an indispensable part of the enteric nervous system. The remarkable plasticity of SCs becomes evident in the context of a nerve injury, where SC transdifferentiate into intriguing repair cells, which orchestrate a regenerative response that promotes nerve repair. Indeed, the multiple adaptations of SCs are captivating, but remain often ill-resolved on the molecular level. Here, we summarize and discuss the knowns and unknowns of the vast array of functions that this single cell type can cover in peripheral nervous system development, maintenance, and repair.

Exploring the Nerve Regenerative Capacity of Compounds with Differing Affinity for PPARγ In Vitro and In Vivo

Damage to peripheral nerves can cause debilitating consequences for patients such as lifelong pain and disability. At present, no drug treatments are routinely given in the clinic following a peripheral nerve injury (PNI) to improve regeneration and remyelination of damaged nerves. Appropriately targeted therapeutic agents have the potential to be used at different stages following nerve damage, e.g., to maintain Schwann cell viability, induce and sustain a repair phenotype to support axonal growth, or promote remyelination. The development of therapies to promote nerve regeneration is currently of high interest to researchers, however, translation to the clinic of drug therapies for PNI is still lacking. Studying the effect of PPARγ agonists for treatment of peripheral nerve injures has demonstrated significant benefits. Ibuprofen, a non-steroidal anti-inflammatory drug (NSAID), has reproducibly demonstrated benefits in vitro and in vivo, suggested to be due to its agonist action on PPARγ. Other NSAIDs have demonstrated differing levels of PPARγ activation based upon their affinity. Therefore, it was of interest to determine whether affinity for PPARγ of selected drugs corresponded to an increase in regeneration. A 3D co-culture in vitro model identified some correlation between these two properties. However, when the drug treatments were screened in vivo, in a crush injury model in a rat sciatic nerve, the same correlation was not apparent. Further differences were observed between capacity to increase axon number and improvement in functional recovery. Despite there not being a clear correlation between affinity and size of effect on regeneration, all selected PPARγ agonists improved regeneration, providing a panel of compounds that could be explored for use in the treatment of PNI.

The Role of Microglia in Neuroinflammation of the Spinal Cord after Peripheral Nerve Injury

Peripheral nerve injuries induce a pronounced immune reaction within the spinal cord, largely governed by microglia activation in both the dorsal and ventral horns. The mechanisms of activation and response of microglia are diverse depending on the location within the spinal cord, type, severity, and proximity of injury, as well as the age and species of the organism. Thanks to recent advancements in neuro-immune research techniques, such as single-cell transcriptomics, novel genetic mouse models, and live imaging, a vast amount of literature has come to light regarding the mechanisms of microglial activation and alluding to the function of microgliosis around injured motoneurons and sensory afferents. Herein, we provide a comparative analysis of the dorsal and ventral horns in relation to mechanisms of microglia activation (CSF1, DAP12, CCR2, Fractalkine signaling, Toll-like receptors, and purinergic signaling), and functionality in neuroprotection, degeneration, regeneration, synaptic plasticity, and spinal circuit reorganization following peripheral nerve injury. This review aims to shed new light on unsettled controversies regarding the diversity of spinal microglial-neuronal interactions following injury.

Identification of Potential miRNA-mRNA Regulatory Network in Denervated Muscular Atrophy by Bioinformatic Analysis

Muscle atrophy caused by long-term denervation leads to the loss of skeletal muscle mass and strength, resulting in a poor recovery of functional muscles and decreasing quality of life. Increasing differentially expressed microRNAs (DEMs) have been reported to be involved in the pathogenesis of denervated muscle atrophy. However, there is still insufficient evidence to explain the role of miRNAs and their target genes in skeletal muscle atrophy. Therefore, an integrative exploration of the miRNA-mRNA regulatory network in denervated muscle atrophy is necessary. A total of 21 (16 upregulated and 5 downregulated) DEMs were screened out in the GSE81914 dataset. Med1, Myod1, Nfkb1, Rela, and Camta1 were predicted and verified to be significantly upregulated in denervated muscle atrophy, from which 6 key TF-miRNA relationship pairs, including Med1-mir-1949, Med1-mir-146b, Myod1-mir-29b, Nfkb1-mir-21, Rela-mir-21, and Camta1-mir-132, were obtained. 60 target genes were then predicted by submitting candidate DEMs to the miRNet database. GO and KEGG pathway enrichment analysis showed that target genes of DEMs were mainly enriched in the apoptotic process and PI3K/Akt signaling pathway. Through the PPI network construction, key modules and hub genes were obtained and potentially modulated by mir-29b, mir-132, and mir-133a. According to the qRT-PCR results, the expression of COL1A1 and Ctgf is opposite to their related miRNAs in denervated muscle atrophy. In the study, a potential miRNA-mRNA regulatory network was firstly constructed in denervated muscle atrophy, in which the mir-29b-COL1A1 and mir-133a-Ctgf pathways may provide new insights into the pathogenesis and treatment.

Novel Tissue-Engineered Multimodular Hyaluronic Acid-Polylactic Acid Conduits for the Regeneration of Sciatic Nerve Defect

The gold standard for the treatment of peripheral nerve injuries, the autograft, presents several drawbacks, and engineered constructs are currently suitable only for short gaps or small diameter nerves. Here, we study a novel tissue-engineered multimodular nerve guidance conduit for the treatment of large nerve damages based in a polylactic acid (PLA) microfibrillar structure inserted inside several co-linear hyaluronic acid (HA) conduits. The highly aligned PLA microfibers provide a topographical cue that guides axonal growth, and the HA conduits play the role of an epineurium and retain the pre-seeded auxiliary cells. The multimodular design increases the flexibility of the device. Its performance for the regeneration of a critical-size (15 mm) rabbit sciatic nerve defect was studied and, after six months, very good nerve regeneration was observed. The multimodular approach contributed to a better vascularization through the micrometrical gaps between HA conduits, and the pre-seeded Schwann cells increased axonal growth. Six months after surgery, a cross-sectional available area occupied by myelinated nerve fibers above 65% at the central and distal portions was obtained when the multimodular device with pre-seeded Schwann cells was employed. The results validate the multi-module approach for the regeneration of large nerve defects and open new possibilities for surgical solutions in this field.

The Therapeutic Potential of Exosomes in Soft Tissue Repair and Regeneration

Soft tissue defects are common following trauma and tumor extirpation. These injuries can result in poor functional recovery and lead to a diminished quality of life. The healing of skin and muscle is a complex process that, at present, leads to incomplete recovery and scarring. Regenerative medicine may offer the opportunity to improve the healing process and functional outcomes. Barriers to regenerative strategies have included cost, regulatory hurdles, and the need for cell-based therapies. In recent years, exosomes, or extracellular vesicles, have gained tremendous attention in the field of soft tissue repair and regeneration. These nanosized extracellular particles (30-140 nm) can break the cellular boundaries, as well as facilitate intracellular signal delivery in various regenerative physiologic and pathologic processes. Existing studies have established the potential of exosomes in regenerating tendons, skeletal muscles, and peripheral nerves through different mechanisms, including promoting myogenesis, increasing tenocyte differentiation and enhancing neurite outgrowth, and the proliferation of Schwann cells. These exosomes can be stored for immediate use in the operating room, and can be produced cost efficiently. In this article, we critically review the current advances of exosomes in soft tissue (tendons, skeletal muscles, and peripheral nerves) healing. Additionally, new directions for clinical applications in the future will be discussed.

A Dual-Modal Magnetic Resonance/Photoacoustic Imaging Tracer for Long-Term High-Precision Tracking and Facilitating Repair of Peripheral Nerve Injuries

Neuroanatomical tracing is considered a crucial technique to assess the axonal regeneration level after injury, but traditional tracers do not meet the needs of in vivo neural tracing in deep tissues. Magnetic resonance (MR) and photoacoustic (PA) imaging have high spatial resolution, great penetration depth, and rich contrast. Fe₃ O₄ nanoparticles may work well as a dual-modal diagnosis probe for neural tracers, with the potential to improve nerve regeneration. The present study combines antegrade neural tracing imaging therapy for the peripheral nervous system. Fe₃ O₄ @COOH nanoparticles are successfully conjugated with biotinylated dextran amine (BDA) to produce antegrade nano-neural tracers, which are encapsulated by microfluidic droplets to control leakage and allow sustained, slow release. They have many notable advantages over traditional tracers, including dual-modal real-time MR/PA imaging in vivo, long-duration release effect, and limitation of uncontrolled leakage. These multifunctional anterograde neural tracers have potential neurotherapeutic function, are reliable and may be used as a new platform for peripheral nerve injury imaging and treatment integration.

Studies on the Manner of Collateral Regeneration From Nerve Stem to Motor Endplate

Despite recent evidence suggesting that nerve transfer techniques help improve clinical outcomes, the underlying manner by which collateral-regenerated nerve enters skeletal muscles to restore an organized pattern of the neuromuscular junction (NMJ) is unclear. To construct the animal models of collateral regeneration, the proximal peroneal nerve was fixed to the distal tibial nerve stump. Three months after surgery, the spatial distribution of motor endplates (MEPs) and corresponding in-muscle nerve branches in long flexor digitorum muscles were observed with tissue optical clearing combined with light-sheet microscopy in transgenic fluorescent mice. The results showed that the number of fibers in the proximal donor peroneal nerve was 415 ± 11, while regenerated nerve fibers in the distal tibial stump were 781 ± 43, which indicates a collateral regeneration ratio of 1.88. The spatial distribution of MEPs was restored to an organized pattern of the lamella, and the corresponding in-muscle nerve branches reverted to the normal manner such as after collateral regeneration. Beyond this, the numbers of MEPs dominated by the single distal nerve fiber were 25.58 ± 0.50 and 26.42 ± 0.94, respectively (n = 6, p > 0.05, collateral regeneration group vs. normal group). However, the numbers of distal-regenerated nerve fibers were less than those in normal control groups (781 ± 43 vs. 914 ± 55, n = 6, p < 0.05), and the number and perforations of MEPs were lower than those in normal control groups as such. In summary, this is the first study to show the manner of collateral regeneration of the peripheral nerve that the smaller proximal donor nerve can sprout more axonal buds to connect distal larger nerves and finally restore to an organized pattern of lamella dominated by corresponding in-muscle nerve branches.

The Role of c-Jun and Autocrine Signaling Loops in the Control of Repair Schwann Cells and Regeneration

After nerve injury, both Schwann cells and neurons switch to pro-regenerative states. For Schwann cells, this involves reprogramming of myelin and Remak cells to repair Schwann cells that provide the signals and mechanisms needed for the survival of injured neurons, myelin clearance, axonal regeneration and target reinnervation. Because functional repair cells are essential for regeneration, it is unfortunate that their phenotype is not robust. Repair cell activation falters as animals get older and the repair phenotype fades during chronic denervation. These malfunctions are important reasons for the poor outcomes after nerve damage in humans. This review will discuss injury-induced Schwann cell reprogramming and the concept of the repair Schwann cell, and consider the molecular control of these cells with emphasis on c-Jun. This transcription factor is required for the generation of functional repair cells, and failure of c-Jun expression is implicated in repair cell failures in older animals and during chronic denervation. Elevating c-Jun expression in repair cells promotes regeneration, showing in principle that targeting repair cells is an effective way of improving nerve repair. In this context, we will outline the emerging evidence that repair cells are sustained by autocrine signaling loops, attractive targets for interventions aimed at promoting regeneration.

Impact of Heat Shock Proteins in Neurodegeneration: Possible Therapeutical Targets

Human neurodegenerative diseases occur as a result of various factors. Regardless of the variety in the etiology of development, many of these diseases are characterized by the accumulation of pathological, misfolded proteins; hence, such diseases are considered as proteinopathies. While plenty of research study has been conducted in order to identify the pathophysiology of these proteinopathies, there is still a lack of understanding in terms of potential therapeutic targets. Molecular chaperones present the main workforce for cellular protection and stress response. Therefore, considering these functions, molecular chaperones present a promising target for research within the field of conformational diseases that arise from proteinopathies. Since the association between neurodegenerative disorders and their long-term consequences is well documented, the need for the development of new therapeutic strategies becomes even more critical. In this review, we summarized the molecular function of heat shock proteins and recent progress on their role, involvement, and other mechanisms related to neurodegeneration caused by different etiological factors. Based on the relevant scientific data, we will highlight the functional classification of heat shock proteins, regulatin, and their therapeutic potential for neurodegenerative disorders.

The role of exercise on peripheral nerve regeneration: from animal model to clinical application

Peripheral nerve injury is a complex condition with a variety of signs and symptoms depending on the severity and nerves involved. Peripheral nerve damage may lead to sensory and motor functions deficits and even lifelong disability, causing important socioeconomic costs worldwide. Despite the increase in knowledge of the mechanisms of injury and regeneration, a full functional recovery is still unsatisfying in the majority of patients. It is well known that exercise promotes physical and psychological well-being, by ameliorating general health. In the last years, there has been a growing interest in evaluating the effects of exercise on the peripheral nervous system. Experimental works with rodent models showed the potential utility of exercise following peripheral nerve injuries, as evinced by increasing axon regeneration, muscle reinnervation, better recovery of strength, muscle mass and higher expression of neurotrophic factors. Moreover, clinical evidence showed positive trends in favour of physical therapy following peripheral nerve damage based on the improvement of range of motion (ROM), muscle power grade and pain. After a brief overview of peripheral nerve anatomy and the different types of nerve injury, the present review aims to summarize the impact of exercise on peripheral nerve regeneration. Some clinical evidence regarding the effect of exercise after peripheral nerve injury will also be discussed.

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

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

Schwann cell-derived exosomes: Janus-faced mediators of regeneration and disease

The phenotypic plasticity of Schwann cells (SCs) has contributed to the regenerative potential of the peripheral nervous system (PNS), but also pathological processes. This double-sided effect has led to an increasing attention to the role of extracellular vesicles (EVs) or exosomes in SCs to examine the intercellular communication between SCs and their surroundings. Here, we first describe the current knowledge of SC and EV biology, which forms the basis for the updates on advances in SC-derived exosomes research. We seek to explore in-depth the exosome-mediated molecular mechanisms involved in the regulation of SCs and their microenvironment. This review concludes with potential applications of SC-derived exosomes as delivery vehicles for therapeutics and biomarkers. The goal of this review is to emphasize the crucial role of SC-derived exosomes in the functional integration of the PNS, highlighting an emerging area in which there is much to explore and re-explore.

Repurposing Small Molecules to Target PPAR-γ as New Therapies for Peripheral Nerve Injuries

The slow rate of neuronal regeneration that follows peripheral nerve repair results in poor recovery, particularly where reinnervation of muscles is delayed, leading to atrophy and permanent loss of function. There is a clear clinical need to develop drug treatments that can accelerate nerve regeneration safely, restoring connections before the target tissues deteriorate irreversibly. The identification that the Rho/Rho-associated kinase (ROCK) pathway acts to limit neuronal growth rate is a promising advancement towards the development of drugs. Targeting Rho or ROCK directly can act to suppress the activity of this pathway; however, the pathway can also be modulated through the activation of upstream receptors; one of particular interest being peroxisome proliferator-activated receptor gamma (PPAR-γ). The connection between the PPAR-γ receptor and the Rho/ROCK pathway is the suppression of the conversion of inactive guanosine diphosphate (GDP)-Rho to active guanosine triphosphate GTP-Rho, resulting in the suppression of Rho/ROCK activity. PPAR-γ is known for its role in cellular metabolism that leads to cell growth and differentiation. However, more recently there has been a growing interest in targeting PPAR-γ in peripheral nerve injury (PNI). The localisation and expression of PPAR-γ in neural cells following a PNI has been reported and further in vitro and in vivo studies have shown that delivering PPAR-γ agonists following injury promotes nerve regeneration, leading to improvements in functional recovery. This review explores the potential of repurposing PPAR-γ agonists to treat PNI and their prospective translation to the clinic.

Defining the regenerative effects of native spider silk fibers on primary Schwann cells, sensory neurons, and nerve-associated fibroblasts

The search for a suitable material to promote regeneration after long-distance peripheral nerve defects turned the spotlight on spider silk. Nerve conduits enriched with native spider silk fibers as internal guiding structures previously demonstrated a regenerative outcome similar to autologous nerve grafts in animal studies. Nevertheless, spider silk is a natural material with associated limitations for clinical use. A promising alternative is the production of recombinant silk fibers that should mimic the outstanding properties of their native counterpart. However, in vitro data on the regenerative features that native silk fibers provide for cells involved in nerve regeneration are scarce. Thus, there is a lack of reference parameters to evaluate whether recombinant silk fiber candidates will be eligible for nerve repair in vivo. To gain insight into the regenerative effect of native spider silk, our study aims to define the behavioral response of primary Schwann cells (SCs), nerve-associated fibroblasts (FBs), and dorsal root ganglion (DRG) neurons cultured on native dragline silk from the genus Nephila and on laminin coated dishes. The established multi-color immunostaining panels together with confocal microscopy and live cell imaging enabled the analysis of cell identity, morphology, proliferation, and migration on both substrates in detail. Our findings demonstrated that native spider silk rivals laminin coating as it allowed attachment and proliferation and supported the characteristic behavior of all tested cell types. Axonal out-growth of DRG neurons occurred along longitudinally aligned SCs that formed sustained bundled structures resembling Bungner bands present in regenerating nerves. The migration of SCs along the silk fibers achieved the reported distance of regenerating axons of about 1 mm per day, but lacked directionality. Furthermore, rFBs significantly reduced the velocity of rSCs in co-cultures on silk fibers. In summary, this study (a) reveals features recombinant silk must possess and what modifications or combinations could be useful for enhanced nerve repair and (b) provides assays to evaluate the regenerative performance of silk fibers in vitro before being applied as internal guiding structure in nerve conduits in vivo.

Asparagine Endopeptidase (δ Secretase), an Enzyme Implicated in Alzheimer's Disease Pathology, Is an Inhibitor of Axon Regeneration in Peripheral Nerves

Asparagine endopeptidase (AEP) is a lysosomal protease implicated in the pathology of Alzheimer’s disease (AD). It is known to cleave the axonal microtubule associated protein, Tau, and amyloid precursor protein (APP), both of which might impede axon regeneration following peripheral nerve injury (PNI). Active AEP, AEP-cleaved fragments of Tau (Tau N368), and APP (APP N585) were found in injured peripheral nerves. In AEP null mice, elongation of regenerating axons after sciatic nerve transection and repair was increased relative to wild-type (WT) controls. Compound muscle action potentials (M responses) were restored in reinnervated muscles twice as fast after injury in AEP knock-out (KO) mice as WT controls. Neurite elongation in cultures of adult dorsal root ganglion (DRG) neurons derived from AEP KO mice was increased significantly relative to cultures from WT controls. In AEP KO mice exposed to 1 h of 20-Hz electrical stimulation (ES) at the time of nerve injury, no further enhancement of axon regeneration was observed. These findings support inhibition of AEP as a therapeutic target to enhance axon regeneration after PNI.

Cathepsins in neuronal plasticity

Proteases comprise a variety of enzymes defined by their ability to catalytically hydrolyze the peptide bonds of other proteins, resulting in protein lysis. Cathepsins, specifically, encompass a class of at least twenty proteases with potent endopeptidase activity. They are located subcellularly in lysosomes, organelles responsible for the cell’s degradative and autophagic processes, and are vital for normal lysosomal function. Although cathepsins are involved in a multitude of cell signaling activities, this chapter will focus on the role of cathepsins (with a special emphasis on Cathepsin B) in neuronal plasticity. We will broadly define what is known about regulation of cathepsins in the central nervous system and compare this with their dysregulation after injury or disease. Importantly, we will delineate what is currently known about the role of cathepsins in axon regeneration and plasticity after spinal cord injury. It is well established that normal cathepsin activity is integral to the function of lysosomes. Without normal lysosomal function, autophagy and other homeostatic cellular processes become dysregulated resulting in axon dystrophy. Furthermore, controlled activation of cathepsins at specialized neuronal structures such as axonal growth cones and dendritic spines have been positively implicated in their plasticity. This chapter will end with a perspective on the consequences of cathepsin dysregulation versus controlled, localized regulation to clarify how cathepsins can contribute to both neuronal plasticity and neurodegeneration.

Controlled local release of PPARγ agonists from biomaterials to treat peripheral nerve injury

OBJECTIVE

Poor clinical outcomes following peripheral nerve injury (PNI) are partly attributable to the limited rate of neuronal regeneration. Despite numerous potential drug candidates demonstrating positive effects on nerve regeneration rate in preclinical models, no drugs are routinely used to improve restoration of function in clinical practice. A key challenge associated with clinical adoption of drug treatments in nerve injured patients is the requirement for sustained administration of doses associated with undesirable systemic sideeffects. Local controlled-release drug delivery systems could potentially address this challenge, particularly through the use of biomaterials that can be implanted at the repair site during the microsurgical repair procedure.

APPROACH

In order to test this concept, this study used various biomaterials to deliver ibuprofen sodium or sulindac sulfide locally in a controlled manner in a rat sciatic nerve injury model. Following characterisation of release parameters in vitro, ethylene vinyl acetate tubes or polylactic-co-glycolic acid wraps, loaded with ibuprofen sodium or sulindac sulfide, were placed around directly-repaired nerve transection or nerve crush injuries in rats.

MAIN RESULTS

Ibuprofen sodium, but not sulindac sulfide caused an increase in neurites in distal nerve segments and improvements in functional recovery in comparison to controls with no drug treatment.

SIGNIFICANCE

This study showed for the first time that local delivery of ibuprofen sodium using biomaterials improves neurite growth and functional recovery following PNI and provides the basis for future development of drug-loaded biomaterials suitable for clinical translation.

Axo-glial interaction in the injured PNS

Axons share a close relationship with Schwann cells, their glial partners in peripheral nerves. An intricate axo-glia network of signals and bioactive molecules regulates the major aspects of nerve development and normal functioning of the peripheral nervous system. Disruptions to these complex axo-glial interactions can have serious neurological consequences, as typically seen in injured nerves. Recent studies in inherited neuropathies have demonstrated that damage to one of the partners in this symbiotic unit ultimately leads to impairment of the other partner, emphasizing the bidirectional influence of axon to glia and glia to axon signaling in these diseases. After physical trauma to nerves, dramatic alterations in the architecture and signaling environment of peripheral nerves take place. Here, axons and Schwann cells respond adaptively to these perturbations and change the nature of their reciprocal interactions, thereby driving the remodeling and regeneration of peripheral nerves. In this review, we focus on the nature and importance of axon-glia interactions in injured nerves, both for the reshaping and repair of nerves after trauma, and in driving pathology in inherited peripheral neuropathies.

Adipose Stem Cell-Derived Extracellular Vesicles Induce Proliferation of Schwann Cells via Internalization

Recent studies showed a beneficial effect of adipose stem cell-derived extracellular vesicles (ADSC-EVs) on sciatic nerve repair, presumably through Schwann cell (SC) modulation. However, it has not yet been elucidated whether ADSC-EVs exert this supportive effect on SCs by extracellular receptor binding, fusion to the SC membrane, or endocytosis mediated internalization. ADSCs, ADSC-EVs, and SCs were isolated from rats and characterized according to associated marker expression and properties. The proliferation rate of SCs in response to ADSC-EVs was determined using a multicolor immunofluorescence staining panel followed by automated image analysis. SCs treated with ADSC-EVs and silica beads were further investigated by 3-D high resolution confocal microscopy and live cell imaging. Our findings demonstrated that ADSC-EVs significantly enhanced the proliferation of SCs in a time- and dose-dependent manner. 3-D image analysis revealed a perinuclear location of ADSC-EVs and their accumulation in vesicular-like structures within the SC cytoplasm. Upon comparing intracellular localization patterns of silica beads and ADSC-EVs in SCs, we found striking resemblance in size and distribution. Live cell imaging visualized that the uptake of ADSC-EVs preferentially took place at the SC processes from which the EVs were transported towards the nucleus. This study provided first evidence for an endocytosis mediated internalization of ADSC-EVs by SCs and underlines the therapeutic potential of ADSC-EVs in future approaches for nerve regeneration.

Removal of the Potassium Chloride Co-Transporter from the Somatodendritic Membrane of Axotomized Motoneurons Is Independent of BDNF/TrkB Signaling But Is Controlled by Neuromuscular Innervation

The potassium-chloride cotransporter (KCC2) maintains the low intracellular chloride found in mature central neurons and controls the strength and direction of GABA/glycine synapses. We found that following axotomy as a consequence of peripheral nerve injuries (PNIs), KCC2 protein is lost throughout the somatodendritic membrane of axotomized spinal cord motoneurons after downregulation of kcc2 mRNA expression. This large loss likely depolarizes the reversal potential of GABA/glycine synapses, resulting in GABAergic-driven spontaneous activity in spinal motoneurons similar to previous reports in brainstem motoneurons. We hypothesized that the mechanism inducing KCC2 downregulation in spinal motoneurons following peripheral axotomy might be mediated by microglia or motoneuron release of BDNF and TrkB activation as has been reported on spinal cord dorsal horn neurons after nerve injury, motoneurons after spinal cord injury (SCI), and in many other central neurons throughout development or a variety of pathologies. To test this hypothesis, we used genetic approaches to interfere with microglia activation or delete bdnf from specifically microglia or motoneurons, as well as pharmacology (ANA-12) and pharmacogenetics (F616A mice) to block TrkB activation. We show that KCC2 dysregulation in axotomized motoneurons is independent of microglia, BDNF, and TrkB. KCC2 is instead dependent on neuromuscular innervation; KCC2 levels are restored only when motoneurons reinnervate muscle. Thus, downregulation of KCC2 occurs specifically while injured motoneurons are regenerating and might be controlled by target-derived signals. GABAergic and glycinergic synapses might therefore depolarize motoneurons disconnected from their targets and contribute to augment motoneuron activity known to promote motor axon regeneration.

Sciatic Nerve Cut and Repair Using Fibrin Glue in Adult Mice

Peripheral nerve injury (PNI) is an excellent model for studying neural responses to injury and elucidating the mechanisms that can facilitate axon regeneration. As such, several animal models have been employed to study regenerative mechanisms after PNI, including Aplysia, zebrafish, rabbits, cats and rodents. This protocol describes how to perform a sciatic nerve injury and repair in mice, one of the most frequently used models to study mechanisms that facilitate recovery after PNI, and that takes advantage of the availability of many genetic models. In this protocol, we describe a method for using fibrin glue to secure the proximal and distal stumps of an injured nerve in close alignment. This method facilitates the alignment of nerve stumps, which aids in regeneration of both sensory and motor axons and allows successful reconnection with peripheral targets.

Growth Hormone Improves Nerve Regeneration, Muscle Re-innervation, and Functional Outcomes After Chronic Denervation Injury

This study investigates the efficacy of systemic growth hormone (GH) therapy in ameliorating the deleterious effects of chronic denervation (CD) injury on nerve regeneration and resulting motor function. Using a forelimb CD model, 4 groups of Lewis rats were examined (n = 8 per group): Group-1 (negative control) 8 weeks of median nerve CD followed by ulnar-to-median nerve transfer; Group-2 (experimental) 8 weeks of median nerve CD followed by ulnar-to-median nerve transfer and highly purified lyophilized pituitary porcine GH treatment (0.6 mg/day); Group-3 (positive control) immediate ulnar-to-median nerve transfer without CD; Group-4 (baseline) naïve controls. All animals underwent weekly grip strength testing and were sacrificed 14 weeks following nerve transfer for histomorphometric analysis of median nerve regeneration, flexor digitorum superficialis atrophy, and neuromuscular junction reinnervation. In comparison to untreated controls, GH-treated animals demonstrated enhanced median nerve regeneration as measured by axon density (p < 0.005), axon diameter (p < 0.0001), and myelin thickness (p < 0.0001); improved muscle re-innervation (27.9% vs 38.0% NMJs re-innervated; p < 0.02); reduced muscle atrophy (1146 ± 93.19 µm2 vs 865.2 ± 48.33 µm2; p < 0.02); and greater recovery of motor function (grip strength: p < 0.001). These findings support the hypothesis that GH-therapy enhances axonal regeneration and maintains chronically-denervated muscle to thereby promote motor re-innervation and functional recovery.

The Success and Failure of the Schwann Cell Response to Nerve Injury

The remarkable plasticity of Schwann cells allows them to adopt the Remak (non-myelin) and myelin phenotypes, which are specialized to meet the needs of small and large diameter axons, and differ markedly from each other. It also enables Schwann cells initially to mount a strikingly adaptive response to nerve injury and to promote regeneration by converting to a repair-promoting phenotype. These repair cells activate a sequence of supportive functions that engineer myelin clearance, prevent neuronal death, and help axon growth and guidance. Eventually, this response runs out of steam, however, because in the long run the phenotype of repair cells is unstable and their survival is compromised. The re-programming of Remak and myelin cells to repair cells, together with the injury-induced switch of peripheral neurons to a growth mode, gives peripheral nerves their strong regenerative potential. But it remains a challenge to harness this potential and devise effective treatments that maintain the initial repair capacity of peripheral nerves for the extended periods typically required for nerve repair in humans.

Developing an In Vitro Model to Screen Drugs for Nerve Regeneration

Peripheral nerve injuries (PNI) have a high prevalence and can be debilitating, resulting in life‐long loss or disturbance in end‐organ function, which compromises quality of life for patients. Current therapies use microsurgical approaches but there is the potential for enhancing recovery through other therapeutic modalities such as; cell‐based conduits, gene therapy and small molecules. A number of molecular targets and drugs which have the potential to improve nerve regeneration have been identified, however, there are challenges associated with moving therapies toward clinical translation. Due to the lack of detailed knowledge about the pro‐regenerative effect of potential drug treatments, there is a need for effective in vitro models to screen compounds to inform future pre‐clinical and clinical studies. The interaction between regenerating neurites and supporting Schwann cells is a key feature of the nerve environment, therefore, in vitro models that mimic this cellular association are useful tools. In this study, we have investigated various cell culture models, including simple monolayer systems and more complex 3D‐engineered co‐cultures, as models for use in PNI drug development. Anat Rec, 301:1628–1637, 2018. © 2018 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Glial-derived growth factor and pleiotrophin synergistically promote axonal regeneration in critical nerve injuries

The repair of nerve gap injuries longer than 3 cm is limited by the need to sacrifice donor tissue and the morbidity associated with the autograft gold standard, while decellularized grafts and biodegradable conduits are effective only in short nerve defects. The advantage of isogenic nerve implants seems to be the release of various growth factors by the denervated Schwann cells. We evaluated the effect of vascular endothelial growth factor, neurotrophins, and pleiotrophin (PTN) supplementation of multi-luminal conduits, in the repair of 3 and 4 cm nerve gaps in the rabbit peroneal nerve. In vitro screening revealed a synergistic regenerative effect of PTN with glial-derived neurotrophic factor (GDNF) in promoting sensory axon density, and motor axonal growth from spinal cord explants. In vivo, pleiotrophins were able to support nerve regrowth across a 3 cm gap. In the 4 cm lesions, PTN-GDNF had a modest effect in the number of axons distal to the implant, while increasing the mean axon diameter (1 ± 0.4; p ≤ 0.001) over PTN or GDNF alone (0.80 ± 0.2, 0.84 ± 0.5; respectively). Some regenerated axons reinnervated muscle targets as indicated by neuromuscular junction staining. However, many were wrapped in Remak bundles, suggesting a delay in axonal sorting, explaining the limited electrophysiological function of the reinnervated muscle, and the modest recovery in toe spreading in the PTN-GDNF repaired animals. These results support the use of synergistic neurotrophic/pleiotrophic growth factors in long gap repair and underscore the need for re-myelination strategies distal to the injury site.

STATEMENT OF SIGNIFICANCE

Nerve injuries due to trauma or tumor resection often result in long gaps that are challenging to repair. The best clinical option demands the use of autologous grafts that are associated with serious side effects. Bioengineered nerves are considered a good alternative, particularly if supplemented with growth factors, but current options do not match the regenerative capacity of autografts. This study revealed the synergistic effect of neurotrophins and pleiotrophins designed to achieve a broad cellular regenerative effect, and that GDNF-PTN are able to mediated axonal growth and partial functional recovery in a 4 cm nerve gap injury, albeit delays in remyelination. This report underscores the need for defining an optimal growth factor support for biosynthetic nerve implants.

The Whole Transcriptome Involved in Denervated Muscle Atrophy Following Peripheral Nerve Injury

Peripheral nerve injury (PNI) usually leads to progressive muscle atrophy and poor functional recovery. Previous studies have demonstrated that non-coding ribonucleic acid (ncRNA) is a key regulator of muscle atrophy and beneficial for the treatment of PNI. We aimed to analyze the whole transcriptome involved in denervated muscle atrophy after PNI. Animal models of sciatic nerve injury were assessed at 0 (control group), 1, 2, 4, and 8 weeks after injury. The expression patterns in the whole transcriptome in the gastrocnemius muscle were profiled using RNA sequencing at each time point and compared to that obtained in the control group. Six-hundred and sixty-four long non-coding RNAs, 671 microRNAs, 236 circular RNAs, and 12,768 messenger RNAs (mRNAs) were differentially expressed (DE) after injury. Changes in some of the DE ncRNAs and mRNAs were validated using quantitative polymerase chain reaction. Gene Ontology and Kyoko Encyclopedia of Genes and Genomes analysis revealed the potential functions of and relationships among the DE ncRNAs and mRNAs. To our knowledge, this is the first study to expound the whole transcriptome involved in denervated muscle atrophy, and provides a theoretical basis for further research targeting ncRNAs.

GSK3β inhibitor promotes myelination and mitigates muscle atrophy after peripheral nerve injury

Delay of axon regeneration after peripheral nerve injury usually leads to progressive muscle atrophy and poor functional recovery. The Wnt/β-catenin signaling pathway is considered to be one of the main molecular mechanisms that lead to skeletal muscle atrophy in the elderly. We hold the hypothesis that the innervation of target muscle can be promoted by accelerating axon regeneration and decelerating muscle cell degeneration so as to improve functional recovery of skeletal muscle following peripheral nerve injury. This process may be associated with the Wnt/β-catenin signaling pathway. Our study designed in vitro cell models to simulate myelin regeneration and muscle atrophy. We investigated the effects of SB216763, a glycogen synthase kinase 3 beta inhibitor, on the two major murine cell lines RSC96 and C2C12 derived from Schwann cells and muscle satellite cells. The results showed that SB216763 stimulated the Schwann cell migration and myotube contraction. Quantitative polymerase chain reaction results demonstrated that myelin related genes, myelin associated glycoprotein and cyclin-D1, muscle related gene myogenin and endplate-associated gene nicotinic acetylcholine receptors levels were stimulated by SB216763. Immunocytochemical staining revealed that the expressions of β-catenin in the RSC96 and C2C12 cytosolic and nuclear compartments were increased in the SB216763-treated cells. These findings confirm that the glycogen synthase kinase 3 beta inhibitor, SB216763, promoted the myelination and myotube differentiation through the Wnt/β-catenin signaling pathway and contributed to nerve remyelination and reduced denervated muscle atrophy after peripheral nerve injury.

Noncovalent Bonding of RGD and YIGSR to an Electrospun Poly(ε-Caprolactone) Conduit through Peptide Self-Assembly to Synergistically Promote Sciatic Nerve Regeneration in Rats

The nerve conduit with biofunctionalities can regulate neurite outgrowth, as well as the migration, proliferation, and myelination activity of Schwann cells. In the present study, polycaprolactone (PCL) conduits are coated with Naphthalene-phenylalanine-phenylalanine-glycine-arginine-glycine-aspartic (Nap-FFGRGD) and Naphthalene-phenylalanine-phenylalanine-glycine-cysteine-aspartic-proline-glycine-tyrosine-isoleucine-glycine-serine-arginine (Nap-FFGCDPGYIGSR) by self-assembly. In vitro studies demonstrate that arginine-glycine-aspartic (RGD) and tyrosine-isoleucine-glycine-serine-arginine (YIGSR) are capable of synergistically enhancing the ability of PCL to support the adhesion and proliferation of Schwann cells, as well as increasing neurite outgrowth from dorsal root ganglions explants. This synergistic effect may occur via the activation of both the phosphoinositide 3-kinase/protein kinase B and mitogen-activated protein kinase/extracellular signal-regulated protein kinase pathways. RGD/YIGSR modifications demonstrate beneficial effects across a 15 mm sciatic nerve gap in axonal regeneration and functional recovery. In addition, increased vascularization is observed in the RGD/YIGSR-PCL group, which might contribute to their beneficial effects on nerve regeneration. These findings indicate the potential of the RGD/YIGSR-PCL conduit to promote axonal regeneration and functional recovery, making the RGD/YIGSR-PCL conduit an attractive candidate for the treatment of a critical nerve defect.

Upslope treadmill exercise enhances motor axon regeneration but not functional recovery following peripheral nerve injury

Following peripheral nerve injury, moderate daily exercise conducted on a level treadmill results in enhanced axon regeneration and modest improvements in functional recovery. If the exercise is conducted on an upwardly inclined treadmill, even more motor axons regenerate successfully and reinnervate muscle targets. Whether this increased motor axon regeneration also results in greater improvement in functional recovery from sciatic nerve injury was studied. Axon regeneration and muscle reinnervation were studied in Lewis rats over an 11 wk postinjury period using stimulus evoked electromyographic (EMG) responses in the soleus muscle of awake animals. Motor axon regeneration and muscle reinnervation were enhanced in slope-trained rats. Direct muscle (M) responses reappeared faster in slope-trained animals than in other groups and ultimately were larger than untreated animals. The amplitude of monosynaptic H reflexes recorded from slope-trained rats remained significantly smaller than all other groups of animals for the duration of the study. The restoration of the amplitude and pattern of locomotor EMG activity in soleus and tibialis anterior and of hindblimb kinematics was studied during treadmill walking on different slopes. Slope-trained rats did not recover the ability to modulate the intensity of locomotor EMG activity with slope. Patterned EMG activity in flexor and extensor muscles was not noted in slope-trained rats. Neither hindblimb length nor limb orientation during level, upslope, or downslope walking was restored in slope-trained rats. Slope training enhanced motor axon regeneration but did not improve functional recovery following sciatic nerve transection and repair.

Therapeutic augmentation of the growth hormone axis to improve outcomes following peripheral nerve injury

INTRODUCTION

Peripheral nerve injuries often result in debilitating motor and sensory deficits. There are currently no therapeutic agents that are clinically available to enhance the regenerative process. Following surgical repair, axons often must regenerate long distances to reach and reinnervate distal targets. Progressive atrophy of denervated muscle and Schwann cells (SCs) prior to reinnervation contributes to poor outcomes. Growth hormone (GH)-based therapies have the potential to accelerate axonal regeneration while at the same time limiting atrophy of muscle and the distal regenerative pathway prior to reinnervation.

AREAS COVERED

In this review, we discuss the potential mechanisms by which GH-based therapies act on the multiple tissue types involved in peripheral nerve regeneration to ultimately enhance outcomes, and review the pertinent mechanistic and translational studies that have been performed. We also address potential secondary benefits of GH-based therapies pertaining to improved bone, tendon and wound healing in the setting of peripheral nerve injury.

EXPERT OPINION

GH-based therapies carry great promise for the treatment of peripheral nerve injuries, given the multi-modal mechanism of action not seen with other experimental therapies. A number of FDA-approved drugs that augment the GH axis are currently available, which may facilitate clinical translation.

Shocking therapy: Brief electrical stimulation for delayed nerve repair

This commentary provides perspective on a recent paper published in Experimental Neurology by Elzinga et al. where the authors investigated the effect of brief electrical stimulation (ES) on nerve regeneration after delayed nerve repair in a rodent model. Their results from a well controlled series of experiments indicated that brief ES promoted axonal outgrowth after chronic axotomy as well after chronic Schwann cell and muscle denervation. ES also increased chronically axotomized neurons regenerating into chronically denervated stumps, which represent a true delayed repair. The authors conclude that brief ES promotion of nerve regeneration after delayed nerve repair is as effective as after immediate repair. Given the prior experimental evidence, and the prior clinical data from patients with carpal tunnel syndrome and digital nerve repair, the implication of this new work is to consider a well designed clinical trial for use of brief ES in nerve graft and nerve transfer repairs.

Normalized activation in the somatosensory cortex 30 years following nerve repair in children: an fMRI study

The clinical outcome following a peripheral nerve injury in the upper extremity is generally better in young children than in teenagers and in adults, but the mechanism behind this difference is unknown. In 28 patients with a complete median nerve injury sustained at the ages of 1-13 years (n = 13) and 14-20 years (n = 15), the cortical activation during tactile finger stimulation of the injured and healthy hands was monitored at a median time since injury of 28 years using functional magnetic resonance imaging (fMRI) at 3 Tesla. The results from the fMRI were compared with the clinical outcome and electroneurography. The cortical activation pattern following sensory stimulation of the median nerve-innervated fingers was dependent on the patient's age at injury. Those injured at a young age (1-13 years) had an activation pattern similar to that of healthy controls. Furthermore, they showed a clinical outcome significantly superior (P = 0.001) to the outcome in subjects injured at a later age; however, electroneurographical parameters did not differ between the groups. In subjects injured at age 14-20 years, a more extended activation of the contralateral hemisphere was seen in general. Interestingly, these patients also displayed changes in the ipsilateral hemisphere where a reduced inhibition of somatosensory areas was seen. This loss of ipsilateral inhibition correlated to increasing age at injury as well as to poor recovery of sensory functions in the hand. In conclusion, cerebral changes in both brain hemispheres may explain differences in clinical outcome following a median nerve injury in childhood or adolescence.

Delaying the onset of treadmill exercise following peripheral nerve injury has different effects on axon regeneration and motoneuron synaptic plasticity

Transection of a peripheral nerve results in withdrawal of synapses from motoneurons. Some of the withdrawn synapses are restored spontaneously, but those containing the vesicular glutamate transporter 1 (VGLUT1), and arising mainly from primary afferent neurons, are withdrawn permanently. If animals are exercised immediately after nerve injury, regeneration of the damaged axons is enhanced and no withdrawal of synapses from injured motoneurons can be detected. We investigated whether delaying the onset of exercise until after synapse withdrawal had occurred would yield similar results. In Lewis rats, the right sciatic nerve was cut and repaired. Reinnervation of the soleus muscle was monitored until a direct muscle (M) response was observed to stimulation of the tibial nerve. At that time, rats began 2 wk of daily treadmill exercise using an interval training protocol. Both M responses and electrically-evoked H reflexes were monitored weekly for an additional seven wk. Contacts made by structures containing VGLUT1 or glutamic acid decarboxylase (GAD67) with motoneurons were studied from confocal images of retrogradely labeled cells. Timing of full muscle reinnervation was similar in both delayed and immediately exercised rats. H reflex amplitude in delayed exercised rats was only half that found in immediately exercised animals. Unlike immediately exercised animals, motoneuron contacts containing VGLUT1 in delayed exercised rats were reduced significantly, relative to intact rats. The therapeutic window for application of exercise as a treatment to promote restoration of synaptic inputs onto motoneurons following peripheral nerve injury is different from that for promoting axon regeneration in the periphery.

Morphological Proof of nerve regeneration after long-term defects of rat sciatic nerves

Unsatisfactory efficacy of clinical cure for long-term delayed injuries and other disadvantages such as the low regeneration rate and speed of axotomized neurons and the questionable reinnervation ability of atrophic target organ lead to inaction to the long-term delayed injuries. Here we attempted to use autologous nerve to bridge a long-term delayed 10-mm defect in SD rats based on some previous positive messages of basic and clinical research. In this study, for experimental groups, the rat sciatic nerve had been transected leaving a 10-mm defect, which was maintained for 3 or 6 months before implantation with the autologous graft. The non-grafted animals served as negative control. Measuring with electrophysiological and histological techniques, we find: (1) A number of long-term axotomized neurons survived and sustained certain degree of axonal regenerative capacity; (2) A few denervated Schwann cells survived and retained their ability to provide trophic support and myelinate axons in at least 6 months; (3) the chronically denervated muscle can partially be reinnervated by regenerated axons. But the quantity and the quality of the regenerated nerve fibers and the reinnervated muscle fibers were all poor. Thus these observations provide new positive morphological proof of nerve regeneration after long-term defects and further studies will be needed to increase the survival rate and the regenerative speed of long-term chronic axotomized neurons, enhance the support provided by denervated distal stumps and protect the target muscle.