The multicellular complexity of peripheral nerve regeneration

Hydrogel system with growth cone-targeted hydroxyapatite nanorods: Regulating calcium signals for peripheral nerve injury repair

Calcium (Ca²⁺) regulation assumes a critical role in the repair course of peripheral nerve injury (PNI). However, effective calcium sources capable of providing sustained Ca²⁺ signals to promote growth cone extension remain limited. Herein, the coupling of biotinylated dextran amine (BDA) to amine-functionalized hydroxyapatite nanorods (nHAP-NH2) remarkably promoted and maintained the extension of growth cones throughout nerve regeneration. Therefore, a newly developed therapeutic system for PNI was constructed based on a hydrogel (Gel) loaded with BDA-nHAP (nHAP-NH2 with surface modification of BDA) and vascular endothelial growth factor (VEGF). The sustained-release BDA-nHAP has the potential to continuously and targetedly increase the Ca2+ levels within the growth cone, and further boost neurite outgrowth by modulating the PI3K-PAK and MAPK signalling pathways. Moreover, VEGF can significantly promote angiogenesis in the early stage of nerve repair, which is critical for optimizing the functional efficacy of BDA-nHAP in enhancing neurogenesis. Thus, this innovative integrated therapeutic system with neurogenesis and angiogenesis capabilities may offer a new solution for achieving high-quality functional recovery from PNI. STATEMENT OF SIGNIFICANCE: For PNI, there remains a scarcity of effective calcium sources capable of providing sustained Ca²⁺ signals within the growth cone to enhance its extension. Herein, a newly developed therapeutic system for PNI was constructed based on a Gel loaded with BDA-nHAP and VEGF. The sustained-release BDA-nHAP has the potential to continuously and targetedly increase the Ca2+ levels within the growth cone, and further boost neurite outgrowth throughout the whole process of nerve regeneration. Moreover, VEGF can significantly promote angiogenesis in the early stage of nerve repair, which is critical for optimizing the functional efficacy of BDA-nHAP in enhancing neurogenesis. Thus, this innovative therapeutic system may offer a new solution for achieving high-quality functional recovery from PNI.

Neuroprotective effects ofGarcinia kolaand curcumin on diabetic transected sciatic nerve

The growing interest in peripheral nerve regeneration and developing post-traumatic repair methods under diabetes was the impetus for this study, which aims to investigate the effect of curcumin andGarcinia kola(GK) on the transected and diabetic sciatic nerves. Thirty-five male Wistar albino rats were used. The animals were divided into five groups; each consisted of seven rats. The sciatic nerve was transected in all groups of rats except the control (Cont) group, which underwent no treatment. In the transected animals, a 10 mm nerve stump was removed from the 2 cm distal to the sciatic notch. The external jugular vein was used as a conduit to repair the gap between the two ends of the sciatic nerve. Diabetes was induced in the transected + diabetes mellitus (T + DM), the transected + diabetes mellitus + GK (T + DM + GK), and the transected + diabetes mellitus + Curcumin (T + DM + Cur) groups except for the sham group. A dose of 300 mg kg-1d-1of curcumin dissolved in olive oil was administered to the T + DM + Cur group (via oral gavage every day for 28 d) and 200 mg kg-1d-1of GK to the T + DM + GK group (via oral gavage every day for 7 d). All animals were sacrificed after three months. Stereological analysis and functional and microscopic evaluations were done to evaluate the sciatic nerve regeneration and function. In the T + DM + GK and the sham groups, the number of axons increased. A slight improvement in the axonal area in the T + DM + Cur and the sham groups was also observed, and an increase in the myelin sheath thickness was found in the T + DM + GK and the sham group. When the SFI test results were evaluated, it was seen that GK had a stronger effect than curcumin in terms of functional regeneration. Additionally, no significant difference was observed between T + DM and Cont groups when the electrophysiological results were examined. The study showed GK's efficiency in treating diabetic peripheral nerve regeneration.

Dock1 functions in Schwann cells to regulate development, maintenance, and repair

Schwann cells, the myelinating glia of the peripheral nervous system (PNS), are critical for myelin development, maintenance, and repair. Rac1 is a known regulator of radial sorting, a key step in developmental myelination. Previously, in zebrafish, we showed that the loss of Dock1, a Rac1-specific guanine nucleotide exchange factor, resulted in delayed peripheral myelination during development. Here, we demonstrate that Dock1 is necessary for myelin maintenance and remyelination after injury in adult zebrafish. Furthermore, Dock1 performs an evolutionarily conserved role in mice, functioning cell autonomously in Schwann cells to regulate the development, maintenance, and repair of peripheral myelin. Pharmacological and genetic manipulation of Rac1 in larval zebrafish, along with the analysis of active Rac1 levels in developing Dock1 mutant mouse nerves, revealed an interaction between these two proteins. We propose that the interplay between Dock1 and Rac1 signaling in Schwann cells is required to establish, maintain, and facilitate repair and remyelination within the PNS.

Hydrogen Peroxide Modulates the Timely Activation of Jun and Erk in Schwann Cells at the Injury Site and Is Required for Motor Axon Regeneration

Peripheral nervous system (PNS) neurons, including motor neurons (MNs), possess a remarkable ability to regenerate and reinnervate target muscles following nerve injury. This process is orchestrated by a combination of intrinsic neuronal properties and extrinsic factors, with Schwann cells (SCs) playing a central role. Upon injury, SCs transition into a repair phenotype that allows axonal regeneration through molecular signaling and structural guidance. However, the identity of the SCs' reprogramming factors is only partially known. We previously identified hydrogen peroxide (H2O2) as an early and key driver of nerve repair, inducing gene expression rewiring in SCs to support nerve re-growth. In this study, we quantitatively assessed the role of H2O2 in the activation of key pro-regenerative signaling pathways in SCs following sciatic nerve compression, specifically the extracellular signal-regulated kinase 1/2 (ERK1/2) and c-Jun, which are essential for functional nerve recovery. Notably, we found that H2O2 neutralization does not impact degeneration, but it significantly affects the regenerative response. Collectively, our findings establish H2O2 as a promising regulator of the Schwann cell injury response at the injury site, linking oxidative signaling to the molecular mechanisms governing nerve regeneration.

Neuroplasticity and Nervous System Recovery: Cellular Mechanisms, Therapeutic Advances, and Future Prospects

Neuroplasticity, the ability of the nervous system to adapt structurally and functionally in response to environmental interactions and injuries, is a cornerstone of recovery in the central (CNS) and peripheral nervous systems (PNS). This review explores the mechanisms underlying neuroplasticity, focusing on the dynamic roles of cellular and molecular processes in recovery from nervous system injuries. Key cellular players, including Schwann cells, oligodendrocytes, and neural stem cells, are highlighted for their contributions to nerve repair, myelination, and regeneration. Advances in therapeutic interventions, such as electrical stimulation, bioluminescent optogenetics, and innovative nerve grafting techniques, are discussed alongside their potential to enhance recovery and functional outcomes. The molecular underpinnings of plasticity, involving synaptic remodeling, homeostatic mechanisms, and activity-dependent regulation of gene expression, are elucidated to illustrate their role in learning, memory, and injury repair. Integrating emerging technologies and therapeutic approaches with a foundational understanding of neuroplasticity offers a pathway toward more effective strategies for restoring nervous system functionality after injury.

Lactoferrin-derived peptide PXL01 impacts nerve regeneration after sciatic nerve reconstruction in healthy and diabetic rats

Introduction

Although advanced surgical techniques are available, satisfactory functional outcomes after peripheral nerve injuries are uncommon. Hence, immune-modulating factors such as PXL01, a lactoferrin-derived peptide that improves axonal outgrowth in injured human digital nerves, have gained attention. We previously reported a short-term immunosuppressive effect of PXL01 after the repair of transected rat sciatic nerves, but it had no effect on nerve regeneration. Here, we investigated the potential of PXL01 to improve nerve regeneration in healthy rats and in a rat model of type 2 diabetes (Goto-Kakizaki [GK] rats).

Methods

A 10-mm sciatic nerve defect was created in healthy (n = 14) and diabetic GK rats (n = 14) and reconstructed using nerve autografts. Immediately after surgery, PXL01 or sodium chloride (control, placebo) (n = 7 for each treatment) was administered around the autograft. On day 8, immunohistochemical staining of the sciatic nerve and dorsal root ganglia (DRGs) was performed to analyze axonal outgrowth (neurofilament staining); inflammation (CD68 and CD206 macrophage staining in nerve); Schwann cell and sensory neuron activation (transcription factor ATF3 staining in nerve and DRGs) and apoptosis (cleaved caspase 3 staining in nerve); and neuroprotection (heat shock protein [HSP27] staining in nerve and DRGs).

Results

PXL01 had no impact on the macrophage response in the autografts but increased axonal outgrowth and HSP27 expression in the DRGs of healthy and diabetic rats, despite a lower number of activated Schwann cells in the autograft. Diabetes affected axonal outgrowth, Schwann cell and macrophage responses, and HSP27 expression. These effects were observed in the sciatic nerve as well as the DRG.

Discussion

Application of PXL01, despite having no impact on macrophages, may improve axonal outgrowth and affects Schwann cell activation in autograft-reconstructed sciatic nerves, as well as conveys neuroprotection (HSP27 expression) in the DRGs of healthy and diabetic GK rats. Diabetes influenced nerve regeneration in such autografts. Therefore, PXL01 is a promising candidate to improve nerve regeneration.

Engineering the Future of Restorative Clinical Peripheral Nerve Surgery

Peripheral nerve injury is a significant clinical challenge, often leading to permanent functional deficits. Standard interventions, such as autologous nerve grafts or distal nerve transfers, require sacrificing healthy nerve tissue and typically result in limited motor or sensory recovery. Nerve regeneration is complex and influenced by several factors: 1) the regenerative capacity of proximal neurons, 2) the ability of axons and support cells to bridge the injury, 3) the capacity of Schwann cells to maintain a supportive environment, and 4) the readiness of target muscles or sensory organs for reinnervation. Emerging bioengineering solutions, including biomaterials, drug delivery systems, fusogens, electrical stimulation devices, and tissue-engineered products, aim to address these challenges. Effective translation of these therapies requires a deep understanding of the physiology and pathology of nerve injury. This article proposes a comprehensive framework for developing restorative strategies that address all four major physiological responses in nerve repair. By implementing this framework, we envision a paradigm shift that could potentially enable full functional recovery for patients, where current approaches offer minimal hope.

Potentially commercializable nerve guidance conduits for peripheral nerve injury: Past, present, and future

Peripheral nerve injuries are a prevalent global issue that has garnered great concern. Although autografts remain the preferred clinical approach to repair, their efficacy is hampered by factors like donor scarcity. The emergence of nerve guidance conduits as novel tissue engineering tools offers a promising alternative strategy. This review aims to interpret nerve guidance conduits and their commercialization from both clinical and laboratory perspectives. To enhance comprehension of clinical situations, this article provides a comprehensive analysis of the clinical efficacy of nerve conduits approved by the United States Food and Drug Administration. It proposes that the initial six months post-transplantation is a critical window period for evaluating their efficacy. Additionally, this study conducts a systematic discussion on the research progress of laboratory conduits, focusing on biomaterials and add-on strategies as pivotal factors for nerve regeneration, as supported by the literature analysis. The clinical conduit materials and prospective optimal materials are thoroughly discussed. The add-on strategies, together with their distinct obstacles and potentials are deeply analyzed. Based on the above evaluations, the development path and manufacturing strategy for the commercialization of nerve guidance conduits are envisioned. The critical conclusion promoting commercialization is summarized as follows: 1) The optimization of biomaterials is the fundamental means; 2) The phased application of additional strategies is the emphasized direction; 3) The additive manufacturing techniques are the necessary tools. As a result, the findings of this research provide academic and clinical practitioners with valuable insights that may facilitate future commercialization endeavors of nerve guidance conduits.

Schwann cells in regeneration and cancer: an epithelial-mesenchymal transition perspective

In the peripheral nervous system, glial cells, known as Schwann cells (SCs), are responsible for supporting and maintaining nerves. One of the most important characteristics of SCs is their remarkable plasticity. In various injury contexts, SCs undergo a reprogramming process that generates specialized cells to promote tissue regeneration and repair. However, in pathological conditions, this same plasticity and regenerative potential can be hijacked. Different studies highlight the activation of the epithelial-mesenchymal transition (EMT) as a driver of SC phenotypic plasticity. Although SCs are not epithelial, their neural crest origin makes EMT activation crucial for their ability to adopt repair phenotypes, mirroring the plasticity observed during development. These adaptive processes are essential for regeneration. However, EMT activation in SCs-derived tumours enhances cancer progression and aggressiveness. Furthermore, in the tumour microenvironment (TME), SCs also acquire activated phenotypes that contribute to tumour migration and invasion by activating EMT in cancer cells. In this review, we will discuss how EMT impacts SC plasticity and function from development and tissue regeneration to pathological conditions, such as cancer.

Identification of CCL3 as a Schwann cell chemotactic factor essential for nerve regeneration

Peripheral nerves regenerate following injury, in contrast to those of the central nervous system. This involves the collective migration of Schwann cell (SC) cords, which transport regrowing axons across the wound site. The SC cords migrate along a newly formed vasculature, which bridges the wound site in response to vascular endothelial growth factor, secreted by hypoxic macrophages. However, the directional signals by which SC cords navigate the long distances across the wound, in the absence of those that guide axons during development, remain unknown. Here, we identify CCL3 as the SC chemotactic factor, secreted by hypoxic macrophages, responsible for this process. We show that CCL3 promotes collective SC migration and axonal regrowth in vivo and, using genetic mouse models and widely used CCL3 inhibitors, that CCL3 is essential for effective nerve regeneration. These findings have therapeutic implications for both promoting nerve repair and inhibiting the aberrant nerve growth associated with trauma and disease.

Neuroregulation during Bone Formation and Regeneration: Mechanisms and Strategies

The skeleton is highly innervated by numerous nerve fibers. These nerve fibers, in addition to transmitting information within the bone and mediating bone sensations, play a crucial role in regulating bone tissue formation and regeneration. Traditional bone tissue engineering (BTE) often fails to achieve satisfactory outcomes when dealing with large-scale bone defects, which is frequently related to the lack of effective reconstruction of the neurovascular network. In recent years, increasing research has revealed the critical role of nerves in bone metabolism. Nerve fibers regulate bone cells through neurotransmitters, neuropeptides, and peripheral glial cells. Furthermore, nerves also coordinate with the vascular and immune systems to jointly construct a microenvironment favorable for bone regeneration. As a signaling driver of bone formation, neuroregulation spans the entire process of bone physiological activities from the embryonic formation to postmaturity remodeling and repair. However, there is currently a lack of comprehensive summaries of these regulatory mechanisms. Therefore, this review sketches out the function of nerves during bone formation and regeneration. Then, we elaborate on the mechanisms of neurovascular coupling and neuromodulation of bone immunity. Finally, we discuss several novel strategies for neuro-bone tissue engineering (NBTE) based on neuroregulation of bone, focusing on the coordinated regeneration of nerve and bone tissue.

Leptin deficiency leads to nerve degeneration and impairs axon remyelination by inducing Schwann cell apoptosis and demyelination in type 2 diabetic peripheral neuropathy in rats

Diabetic peripheral neuropathy, characterized by symptoms such as paresthesia, neuropathic pain, and potential lower limb amputation, poses significant clinical management challenges. Recent studies suggest that chronic hyperglycemia-induced Schwann cells (SCs) apoptosis contributes to neurodegeneration and impaired nerve regeneration, but the detailed mechanisms are still unknown. Our study investigated a mixed-sex type 2 diabetes mellitus (T2DM) rat model using leptin knockout (KO) to simulate obesity and diabetes-related conditions. Through extensive assessments, including mechanical allodynia, electrophysiology, and microcirculation analyses, along with myelin degradation studies in KO versus wild-type rats, we focused on apoptosis, autophagy, and SCs dedifferentiation in the sciatic nerve and examined nerve regeneration in KO rats. KO rats exhibited notable reductions in mechanical withdrawal force, prolonged latency, decreased compound muscle action potential (CMAP) amplitude, reduced microcirculation, myelin sheath damage, and increases in apoptosis, autophagy, and SCs dedifferentiation. Moreover, leptin KO was found to impair peripheral nerve regeneration postinjury, as indicated by reduced muscle weight, lower CMAP amplitude, extended latency, and decreased remyelination and SCs density. These findings underscore the effectiveness of the T2DM rat model in clarifying the impact of leptin KO on SCs apoptosis, dedifferentiation, and demyelination, providing valuable insights into new therapeutic avenues for treating T2DM-induced peripheral neuropathy.

Innervated Coculture Device to Model Peripheral Nerve-Mediated Fibroblast Activation

Cutaneous wound healing is a complex process involving various cellular and molecular interactions, resulting in the formation of a collagen-rich scar with imperfect function and morphology. Dermal fibroblasts are crucial to successful wound healing, migrating to the wound site where they are activated to provide extracellular matrix remodeling and wound closure. Peripheral nerves have been shown to play an important role in wound healing, with loss or damage to these nerves often leading to impaired healing and the formation of chronic nonhealing wounds. Previous research has suggested that sensory nerves secrete trophic factors that can regulate wound healing, including fibroblast activation; however, the direct cell-cell interaction between nerves and fibroblasts has not been extensively studied. To address this knowledge gap, we developed an in vitro co-culture model using a device called the IFlowPlate. This model supports the long-term viability of multiple cell types while allowing for direct contact between sensory nerve cells and dermal fibroblasts. Using the IFlowPlate, we demonstrate that co-culture of dorsal root ganglia with dermal fibroblasts increases fibroblast proliferation, collagen and α-smooth muscle actin expression, and secretion of pro-wound healing factors, suggesting that nerves can promote wound healing by modulating fibroblast activation. The IFlowPlate offers a user-friendly and high-throughput platform to study the in vitro interactions between nerves and a variety of cell types that can be applied to wound healing and other important biological processes.

A method for quantitative spatial analysis of immunolabeled fibers at regenerative electrode interfaces

BACKGROUND

Regenerative electrodes are being explored as robust peripheral nerve interfaces for neuro-prosthetic control and sensory feedback. Current designs differ in electrode number, spatial arrangement, and porosity which impacts the regeneration, activation, and spatial distribution of fibers at the device interface. Knowledge of sensory and motor fiber distributions are important in optimizing selective fiber activation and recording.

NEW METHOD

We use confocal microscopy and immunofluorescence methods to conduct spatial analysis of immunolabeled fibers across whole nerve cross sections.

RESULTS

This protocol was implemented to characterize motor fiber distribution within 3 macro-sieve electrode regenerated (MSE), 3 silicone-conduit regenerated, and 3 unmanipulated control rodent sciatic nerves. Total motor fiber counts were 1485 [SD: +/- 50.11], 1899 [SD: +/- 359], and 5732 [SD: +/- 1410] for control, MSE, and conduit nerves respectively. MSE motor fiber distributions exhibited evidence of deviation from complete spatial randomness and evidence of dispersion and clustering tendencies at varying scales. Notably, MSE motor fibers exhibited clustering within the central portion of the cross section, whereas conduit regenerated motor fibers exhibited clustering along the periphery.

COMPARISON WITH EXISTING METHODS

Prior exploration of fiber distributions at regenerative interfaces was limited to either quadrant-based density analysis of randomly sampled subregions or qualitative description. This method extends existing sample preparation and microscopy techniques to quantitatively assess immunolabeled fiber distributions within whole nerve cross-sections.

CONCLUSIONS

This approach is an effective way to examine the spatial organization of fiber subsets at regenerative electrode interfaces, enabling robust assessment of fiber distributions relative to electrode arrangement.

Wnt 3a-Modified Scaffolds Improve Nerve Regeneration by Boosting Schwann Cell Function

A pivotal approach in engineering artificial peripheral nerve sheaths encompasses the augmentation of the regenerative microenvironment via the manipulation of Schwann cells (SCs). Our investigation employed single-cell sequencing analysis to elucidate the potential functions of Schwann cells and the Wnt pathway in facilitating peripheral nerve regeneration. In vitro studies showed that activating the Wnt signaling pathway promotes the transition to repair SCs, boosting their growth, movement, and immune functions. To better understand the peripheral nerve regeneration environment, we created a polymer scaffold using ammonization and electrospinning. The Wnt3a protein was incorporated into the polycaprolactone (PCL) electrospun fiber surface. In a rat sciatic nerve defect model, the Wnt3a-modified scaffold showed better nerve repair outcomes than traditional electrospun scaffolds. After a week, the test group showed better immune regulation and angiogenesis, with a significant increase in axon growth rate observed after 3 weeks. Three-month-long animal experiments revealed notable improvements in neuroelectrophysiology, reduced organ atrophy, and enhanced sciatic nerve recovery. In this nerve defect model, Wnt3a-modified neural scaffolds achieved repair effects.

Tau Isoform-Regulated Schwann Cell Proliferation and Migration Improve Peripheral Nerve Regeneration After Injury

Tau is a microtubule-associated protein that plays a vital role in the mammalian nervous system. Alternative splicing of the MAPT gene leads to the formation of tau isoforms with varying N-terminal inserts and microtubule-binding repeats. Dysregulation of tau alternative splicing has been linked to diseases in the central nervous system, but the roles of tau isoforms in the peripheral nervous system remain unclear. Here, we investigated the alternative splicing of tau exons 4A and 10 in the sciatic nerve and Schwann cells during development and following injury. We discovered that low-molecular-weight (LMW) tau, resulting from the exclusion of exon 4A, and 3R tau, generated by the exclusion of exon 10, diminishes with aging in rat sciatic nerve and Schwann cells. High-molecular-weight (HMW) tau and 3R tau increase in the adult sciatic nerve post-injury. We constructed viruses that expressed HMW-4R, LMW-4R, HMW-3R, and LMW-3R and introduced them into cultured cells or the distal part of the injured sciatic nerve to assess their effects on Schwann cell migration and proliferation. We also examined the effects of the four isoforms on axon growth and debris clearance after sciatic nerve injury. Our results demonstrated that tau isoforms inhibit Schwann cell proliferation while promoting Schwann cell migration and sciatic nerve regeneration. Specifically, the 3R-tau isoforms were more effective than the 4R-tau isoforms in promoting nerve regeneration. In conclusion, our study reveals the roles of tau isoforms in the peripheral nervous system and provides insights into the development of new therapeutic strategies for peripheral nerve injuries.

Inducible deletion of endothelial cell Efnb2 delays capillary regeneration and attenuates myofibre reinnervation following myotoxin injury in mice

Acute injury of skeletal muscle disrupts myofibres, microvessels and motor innervation. Myofibre regeneration is well characterized, however its relationship with the regeneration of microvessels and motor nerves is undefined. Endothelial cell (EC) ephrin-B2 (Efnb2) is required for angiogenesis during embryonic development and promotes neurovascular regeneration in the adult. We hypothesized that, following acute injury to skeletal muscle, loss of EC Efnb2 would impair microvascular regeneration and the recovery of neuromuscular junction (NMJ) integrity. Mice (aged 3-6 months) were bred for EC-specific conditional knockout (CKO) of Efnb2 following tamoxifen injection with non-injected CKO mice as controls (CON). The gluteus maximus, tibialis anterior or extensor digitorum longus muscle was then injured with local injection of BaCl2. Intravascular staining with wheat germ agglutinin revealed diminished capillary area in the gluteus maximus of CKO vs. CON at 5 days post-injury (dpi); both recovered to uninjured (0 dpi) level by 10 dpi. At 0 dpi, tibialis anterior isometric force of CKO was less than CON. At 10 dpi, isometric force was reduced by half in both groups. During intermittent contractions (75 Hz, 330 ms s-1, 120 s), isometric force fell during indirect (sciatic nerve) stimulation whereas force was maintained during direct (electrical field) stimulation of myofibres. Neuromuscular transmission failure correlated with perturbed presynaptic (terminal Schwann cells) and postsynaptic (nicotinic acetylcholine receptors) NMJ morphology in CKO. Resident satellite cell number on extensor digitorum longus myofibres did not differ between groups. Following acute injury of skeletal muscle, loss of Efnb2 in ECs delays capillary regeneration and attenuates recovery of NMJ structure and function. KEY POINTS: The relationship between microvascular regeneration and motor nerve regeneration following skeletal muscle injury is undefined. Expression of Efnb2 in endothelial cells (ECs) is essential to vascular development and promotes neurovascular regeneration in the adult. To test the hypothesis that EfnB2 in ECs is required for microvascular regeneration and myofibre reinnervation, we induced conditional knockout of Efnb2 in ECs of mice. Acute injury was then induced by BaCl2 injection into gluteus maximus, tibialis anterior or extensor digitorum longus (EDL) muscle. Capillary regeneration was reduced at 5 days post-injury (dpi) in gluteus maximus of conditional knockout vs. controls; at 10 dpi, neither differed from uninjured. Nerve stimulation revealed neuromuscular transmission failure in tibialis anterior with perturbed neuromuscular junction structure. Resident satellite cell number on EDL myofibres did not differ between groups. Conditional knockout of EC Efnb2 delays capillary regeneration and attenuates recovery of neuromuscular junction structure and function.

Advancements in stimulation therapies for peripheral nerve regeneration

Soft-tissue injuries affecting muscles, nerves, vasculature, tendons, and ligaments often diminish the quality of life due to pain, loss of function, and financial burdens. Both natural healing and surgical interventions can result in scarring, which potentially may impede functional recovery and lead to persistent pain. Scar tissue, characterized by a highly disorganized fibrotic extracellular matrix, may serve as a physical barrier to regeneration and drug delivery. While approaches such as drugs, biomaterials, cells, external stimulation, and other physical forces show promise in mitigating scarring and promoting regenerative healing, their implementation remains limited and challenging. Ultrasound, laser, electrical, and magnetic forms of external stimulation have been utilized to promote soft tissue as well as neural tissue regeneration. After stimulation, neural tissues experience increased proliferation of Schwann cells, secretion of neurotropic factors, production of myelin, and growth of vasculature, all aimed at supporting axon regeneration and innervation. Yet, the outcomes of healing vary depending on the pathophysiology of the damaged nerve, the timing of stimulation following injury, and the specific parameters of stimulation employed. Increased treatment intensity and duration have been noted to hinder the healing process by inducing tissue damage. These stimulation modalities, either alone or in combination with nerve guidance conduits and scaffolds, have been demonstrated to promote healing. However, the literature currently lacks a detailed understanding of the stimulation parameters used for nerve healing applications. In this article, we aim to address this gap by summarizing existing reports and providing an overview of stimulation parameters alongside their associated healing outcomes.

Towards the development of sensation-enabled skin substitutes

Recent advances in cell and biofabrication technologies have contributed to the development of complex human organs. In particular, several skin substitutes are being generated using tissue engineering and regenerative medicine (TERM) technologies. However, recent studies mainly focus on the restoration of the dermis and epidermis layers rather than the regeneration of a fully functional innervated skin organ. Innervation is a critical step in functional tissue repair which has been overlooked in the current TERM studies. In the current study, we highlight the importance of sensation in the skin as the largest sensory organ in the human body. In large non-healing skin wounds, the skin sensation is severely diminished or completely lost and ultimately lead to chronic pain and wound healing process interruption. Current therapeutics for restoring skin sensation after trauma are limited. Recent regenerative medicine-based studies could successfully induce neural networks in skin substitutes, but the effectiveness of these technologies in enhancing sensory capability needs further investigation.

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.

PXL01 alters macrophage response with no effect on axonal outgrowth or Schwann cell response after nerve repair in rats

Background: Adjunctive pharmacological treatment may improve nerve regeneration. We investigated nerve regeneration processes of PXL01 - a lactoferrin-derived peptide - after repair of the sciatic nerve in healthy Wistar rats.Materials & methods: PXL01, sodium hyaluronate (carrier) or sodium chloride was administered around the repair. After 6 days axonal outgrowth, Schwann cell response, pan- (CD68) and pro-healing (CD206) macrophages in sciatic nerve, sensory neuronal response in dorsal root ganglia (DRG) and expression of heat shock protein 27 (HSP27) in sciatic nerves and DRGs were analyzed.Results: Despite a lower number of pan-macrophages, other investigated variables in sciatic nerves or DRGs did not differ between the treatment groups.Conclusion: PLX01 applied locally inhibits inflammation through pan-macrophages in repaired sciatic nerves without any impact on nerve regeneration or pro-healing macrophages.

N-cadherin directs the collective Schwann cell migration required for nerve regeneration through Slit2/3-mediated contact inhibition of locomotion

Collective cell migration is fundamental for the development of organisms and in the adult for tissue regeneration and in pathological conditions such as cancer. Migration as a coherent group requires the maintenance of cell-cell interactions, while contact inhibition of locomotion (CIL), a local repulsive force, can propel the group forward. Here we show that the cell-cell interaction molecule, N-cadherin, regulates both adhesion and repulsion processes during Schwann cell (SC) collective migration, which is required for peripheral nerve regeneration. However, distinct from its role in cell-cell adhesion, the repulsion process is independent of N-cadherin trans-homodimerisation and the associated adherens junction complex. Rather, the extracellular domain of N-cadherin is required to present the repulsive Slit2/Slit3 signal at the cell surface. Inhibiting Slit2/Slit3 signalling inhibits CIL and subsequently collective SC migration, resulting in adherent, nonmigratory cell clusters. Moreover, analysis of ex vivo explants from mice following sciatic nerve injury showed that inhibition of Slit2 decreased SC collective migration and increased clustering of SCs within the nerve bridge. These findings provide insight into how opposing signals can mediate collective cell migration and how CIL pathways are promising targets for inhibiting pathological cell migration.

An Adaptable Drug Delivery System Facilitates Peripheral Nerve Repair by Remodeling the Microenvironment

The multifaceted process of nerve regeneration following damage remains a significant clinical issue, due to the lack of a favorable regenerative microenvironment and insufficient endogenous biochemical signaling. However, the current nerve grafts have limitations in functionality, as they require a greater capacity to effectively regulate the intricate microenvironment associated with nerve regeneration. In this regard, we proposed the construction of a functional artificial scaffold based on a "two-pronged" approach. The whole system was developed by encapsulating Tazarotene within nanomicelles formed through self-assembly of reactive oxygen species (ROS)-responsive amphiphilic triblock copolymer, all of which were further loaded into a thermosensitive injectable hydrogel. Notably, the hydrogel exhibits obvious temperature sensitivity at a concentration of 6 wt %, and the nanoparticles possess concentration-dependent H2O2-response capability with a controlled release profile in 48 h. The combined strategy promoted the repair of injured peripheral nerves, attributed to the dual role of the materials, which mainly involved providing structural support, modulating the immune microenvironment, and enhancing angiogenesis. Overall, this study opens up intriguing prospects in tissue engineering.

Skin derived precursors induced Schwann cells mediated tissue engineering-aided neuroregeneration across sciatic nerve defect

A central question in neural tissue engineering is how the tissue-engineered nerve (TEN) translates detailed transcriptional signals associated with peripheral nerve regeneration into meaningful biological processes. Here, we report a skin-derived precursor-induced Schwann cell (SKP-SC)-mediated chitosan/silk fibroin-fabricated tissue-engineered nerve graft (SKP-SCs-TEN) that can promote sciatic nerve regeneration and functional restoration nearly to the levels achieved by autologous nerve grafts according to behavioral, histological, and electrophysiological evidence. For achieving better effect of neuroregeneration, this is the first time to jointly apply a dynamic perfusion bioreactor and the ascorbic acid to stimulate the SKP-SCs secretion of extracellular matrix (ECM). To overcome the limitation of traditional tissue-engineered nerve grafts, jointly utilizing SKP-SCs and their ECM components were motivated by the thought of prolongating the effect of support cells and their bioactive cues that promote peripheral nerve regeneration. To further explore the regulatory model of gene expression and the related molecular mechanisms involved in tissue engineering-aided peripheral nerve regeneration, we performed a cDNA microarray analysis of gene expression profiling, a comprehensive bioinformatics analysis and a validation study on the grafted segments and dorsal root ganglia tissues. A wealth of transcriptomic and bioinformatics data has revealed complex molecular networks and orchestrated functional regulation that may be responsible for the effects of SKP-SCs-TEN on promoting peripheral nerve regeneration. Our work provides new insights into transcriptomic features and patterns of molecular regulation in nerve functional recovery aided by SKP-SCs-TEN that sheds light on the broader possibilities for novel repair strategies of peripheral nerve injury.

Cultivation of Schwann cells from fresh and non-fresh adult equine peripheral nerves

BACKGROUND

Over the past 25 years, acquired equine polyneuropathy (AEP) has emerged as a neurological disease in Scandinavian horses. This condition is characterized by histopathological features including the presence of Schwann cell (SC) inclusions. Cultivated equine SCs would serve as a valuable resource for investigations of factors triggering this Schwannopathy. Ideally, cells should be sampled for cultivation from fresh nerves immediately after death of the animal, however the availability of fresh material is limited, due to the inconsistent case load and the inherent technical and practical challenges to collection of samples in the field. This study aimed to cultivate SCs from adult equine peripheral nerves and assess their ability to survive in sampled nerve material over time to simulate harvesting of SCs in field situations.

NEW METHODS

Peripheral nerves from five non-neurological horses were used. After euthanasia, both fresh and non-fresh nerve samples were harvested from each horse. Flow cytometry was employed to confirm the cellular identity and to determine the SC purity.

RESULTS

The results revealed successful establishment of SC cultures from adult equine peripheral nerves, with the potential to achieve high SC purity from both fresh and non-fresh nerve samples.

COMPARISON WITH EXISTING METHOD

While most SC isolation methods focus on harvest of cells from fresh nerve materials from laboratory animals, our approach highlights the possibility of utilizing SC cultures from field-harvested and transported nerve samples from horses.

CONCLUSIONS

We describe a method for isolating SCs with high purity from both fresh and non-fresh peripheral nerves of adult horses.

Immune-cell-mediated tissue engineering strategies for peripheral nerve injury and regeneration

During the process of peripheral nerve repair, there are many complex pathological and physiological changes, including multi-cellular responses and various signaling molecules, and all these events establish a dynamic microenvironment for axon repair, regeneration, and target tissue/organ reinnervation. The immune system plays an indispensable role in the process of nerve repair and function recovery. An effective immune response not only involves innate-immune and adaptive-immune cells but also consists of chemokines and cytokines released by these immune cells. The elucidation of the orchestrated interplay of immune cells with nerve regeneration and functional restoration is meaningful for the exploration of therapeutic strategies. This review mainly enumerates the general immune cell response to peripheral nerve injury and focuses on their contributions to functional recovery. The tissue engineering-mediated strategies to regulate macrophages and T cells through physical and biochemical factors combined with scaffolds are discussed. The dynamic immune responses during peripheral nerve repair and immune-cell-mediated tissue engineering methods are presented, which provide a new insight and inspiration for immunomodulatory therapies in peripheral nerve regeneration.

Structured wound angiogenesis instructs mesenchymal barrier compartments in the regenerating nerve

Organ injury stimulates the formation of new capillaries to restore blood supply raising questions about the potential contribution of neoangiogenic vessel architecture to the healing process. Using single-cell mapping, we resolved the properties of endothelial cells that organize a polarized scaffold at the repair site of lesioned peripheral nerves. Transient reactivation of an embryonic guidance program is required to orient neovessels across the wound. Manipulation of this structured angiogenic response through genetic and pharmacological targeting of Plexin-D1/VEGF pathways within an early window of repair has long-term impact on configuration of the nerve stroma. Neovessels direct nerve-resident mesenchymal cells to mold a provisionary fibrotic scar by assembling an orderly system of stable barrier compartments that channel regenerating nerve fibers and shield them from the persistently leaky vasculature. Thus, guided and balanced repair angiogenesis enables the construction of a "bridge" microenvironment conducive for axon regrowth and homeostasis of the regenerated tissue.

An Updated Review of Magnetic Resonance Neurography for Plexus Imaging

Magnetic resonance neurography (MRN) is increasingly used to visualize peripheral nerves in vivo. However, the implementation and interpretation of MRN in the brachial and lumbosacral plexi are challenging because of the anatomical complexity and technical limitations. The purpose of this article was to review the clinical context of MRN, describe advanced magnetic resonance (MR) techniques for plexus imaging, and list the general categories of utility of MRN with pertinent imaging examples. The selection and optimization of MR sequences are centered on the homogeneous suppression of fat and blood vessels while enhancing the visibility of the plexus and its branches. Standard 2D fast spin-echo sequences are essential to assess morphology and signal intensity of nerves. Moreover, nerve-selective 3D isotropic images allow improved visualization of nerves and multiplanar reconstruction along their course. Diffusion-weighted and diffusion-tensor images offer microscopic and functional insights into peripheral nerves. The interpretation of MRN in the brachial and lumbosacral plexi should be based on a thorough understanding of their anatomy and pathophysiology. Anatomical landmarks assist in identifying brachial and lumbosacral plexus components of interest. Thus, understanding the varying patterns of nerve abnormalities facilitates the interpretation of aberrant findings.

A feasibility study transplanting macrophages to a segmental nerve injury

INTRODUCTION/AIMS

Promoting regeneration after segmental nerve injury repair is a challenge, but improving angiogenesis could be beneficial. Macrophages facilitate regeneration after injury by promoting angiogenesis. Our aim in this study was to evaluate the feasibility and effects of transplanting exogenous macrophages to a segmental nerve injury.

METHODS

Bone marrow-derived cells were harvested from donor mice and differentiated to macrophages (BMDM), then suspended within fibrin hydrogels to facilitate BMDM transplantation. BMDM survival was characterized in vitro. The effect of this BMDM fibrin hydrogel construct at a nerve injury site was assessed using a mouse sciatic nerve gap injury. Mice were equally distributed to "fibrin+Mφ" (fibrin hydrogels containing culture medium and BMDM) or "fibrin" hydrogel control (fibrin hydrogels containing culture medium alone) groups. Flow cytometry (n = 3/group/endpoint) and immunohistochemical analysis (n = 5/group/endpoint) of the nerve gap region were performed at days 3, 5, and 7 after repair.

RESULTS

Incorporating macrophage colony-stimulating factor (M-CSF) improved BMDM survival and expansion. Transplanted BMDM survived for at least 7 days in a nerve gap (~40% retained at day 3 and ~15% retained at day 7). From transplantation, macrophage quantities within the nerve gap were elevated when comparing fibrin+Mφ with fibrin control (~25% vs. 3% at day 3 and ~14% vs. 6% at day 7). Endothelial cells increased by about fivefold within the nerve gap, and axonal extension into the nerve gap increased almost twofold for fibrin+Mφ compared with fibrin control.

DISCUSSION

BMDM suspended within fibrin hydrogels at a nerve gap do not impair regeneration.

Dock1 acts cell-autonomously in Schwann cells to regulate the development, maintenance, and repair of peripheral myelin

Schwann cells, the myelinating glia of the peripheral nervous system (PNS), are critical for myelin development, maintenance, and repair. Rac1 is a known regulator of radial sorting, a key step in developmental myelination, and we previously showed in zebrafish that loss of Dock1, a Rac1-specific guanine nucleotide exchange factor, results in delayed peripheral myelination in development. We demonstrate here that Dock1 is necessary for myelin maintenance and remyelination after injury in adult zebrafish. Furthermore, it performs an evolutionary conserved role in mice, acting cell-autonomously in Schwann cells to regulate peripheral myelin development, maintenance, and repair. Additionally, manipulating Rac1 levels in larval zebrafish reveals that dock1 mutants are sensitized to inhibition of Rac1, suggesting an interaction between the two proteins during PNS development. We propose that the interplay between Dock1 and Rac1 signaling in Schwann cells is required to establish, maintain, and facilitate repair and remyelination within the peripheral nervous system.

The Dynamics of Nerve Degeneration and Regeneration in a Healthy Milieu and in Diabetes

Appropriate animal models, mimicking conditions of both health and disease, are needed to understand not only the biology and the physiology of neurons and other cells under normal conditions but also under stress conditions, like nerve injuries and neuropathy. In such conditions, understanding how genes and different factors are activated through the well-orchestrated programs in neurons and other related cells is crucial. Knowledge about key players associated with nerve regeneration intended for axonal outgrowth, migration of Schwann cells with respect to suitable substrates, invasion of macrophages, appropriate conditioning of extracellular matrix, activation of fibroblasts, formation of endothelial cells and blood vessels, and activation of other players in healthy and diabetic conditions is relevant. Appropriate physical and chemical attractions and repulsions are needed for an optimal and directed regeneration and are investigated in various nerve injury and repair/reconstruction models using healthy and diabetic rat models with relevant blood glucose levels. Understanding dynamic processes constantly occurring in neuropathies, like diabetic neuropathy, with concomitant degeneration and regeneration, requires advanced technology and bioinformatics for an integrated view of the behavior of different cell types based on genomics, transcriptomics, proteomics, and imaging at different visualization levels. Single-cell-transcriptional profile analysis of different cells may reveal any heterogeneity among key players in peripheral nerves in health and disease.

Fibroblast growth factor signaling in axons: from development to disease

The fibroblast growth factor (FGF) family regulates various and important aspects of nervous system development, ranging from the well-established roles in neuronal patterning to more recent and exciting functions in axonal growth and synaptogenesis. In addition, FGFs play a critical role in axonal regeneration, particularly after spinal cord injury, confirming their versatile nature in the nervous system. Due to their widespread involvement in neural development, the FGF system also underlies several human neurological disorders. While particular attention has been given to FGFs in a whole-cell context, their effects at the axonal level are in most cases undervalued. Here we discuss the endeavor of the FGF system in axons, we delve into this neuronal subcompartment to provide an original view of this multipurpose family of growth factors in nervous system (dys)function. Video Abstract.

Effects of 4-Aminopyridine on Combined Nerve and Muscle Injury and Bone Loss

PURPOSE

Musculoskeletal injuries are common, and peripheral nerve injury (PNI) causes significant muscle and bone loss within weeks. After PNI, 4-aminopyridine (4-AP) improves functional recovery and muscle atrophy. However, it is unknown whether 4-AP has any effect on isolated traumatic muscle injury and PNI-induced bone loss.

METHODS

A standardized crush injury was performed on the sciatic nerve and muscles in mice, and the mice were assigned to receive normal saline or 4-AP treatment daily for 21 days. The postinjury motor and sensory function recovery was assessed, injured muscles were processed for histomorphometry, and the tibial bone was scanned for bone density.

RESULTS

4-Aminopyridine significantly accelerated the postinjury motor and sensory function recovery, improved muscle histomorphometry, increased muscle satellite cell numbers, and shifted muscle fiber types after combined nerve and muscle injury. Importantly, the 4-AP treatment significantly reduced PNI-induced bone loss. In contrast, in the case of isolated muscle injury, 4-AP had no effect on functional recovery and bone density, but it improved muscle-specific histomorphometry to a limited extent.

CONCLUSIONS

These findings demonstrate the potential beneficial effects of 4-AP on the recovery of muscle morphology and bone density after combined muscle and nerve injury.

CLINICAL RELEVANCE

Nerve injuries frequently involve muscle and result in rapid muscle and bone atrophy. In this scenario, 4-AP, in addition to accelerating nerve functional recovery, might work as an adjunctive agent to improve the recovery of injured muscle and attenuate PNI-induced bone loss.

Medium chain length polyhydroxyalkanoates as potential matrix materials for peripheral nerve regeneration

Polyhydroxyalkanoates are natural, biodegradable, thermoplastic and sustainable polymers with a huge potential in fabrication of bioresorbable implantable devices for tissue engineering. We describe a comparative evaluation of three medium chain length polyhydroxyalkanoates (mcl-PHAs), namely poly(3-hydroxyoctanoate), poly(3-hydroxyoctanoate-co-3-hydoxydecanoate) and poly(3-hydroxyoctanoate-co-3-hydroxydecanoate-co-3-hydroxydodecanoate), one short chain length polyhydroxyalkanoate, poly(3-hydroxybutyrate), P(3HB) and synthetic aliphatic polyesters (polycaprolactone and polylactide) with a specific focus on nerve regeneration, due to mechanical properties of mcl-PHAs closely matching nerve tissues. In vitro biological studies with NG108-15 neuronal cell and primary Schwann cells did not show a cytotoxic effect of the materials on both cell types. All mcl-PHAs supported cell adhesion and viability. Among the three mcl-PHAs, P(3HO-co-3HD) exhibited superior properties with regards to numbers of cells adhered and viable cells for both cell types, number of neurite extensions from NG108-15 cells, average length of neurite extensions and Schwann cells. Although, similar characteristics were observed for flat P(3HB) surfaces, high rigidity of this biomaterial, and FDA-approved polymers such as PLLA, limits their applications in peripheral nerve regeneration. Therefore, we have designed, synthesized and evaluated these materials for nerve tissue engineering and regenerative medicine, the interaction of mcl-PHAs with neuronal and Schwann cells, identifying mcl-PHAs as excellent materials to enhance nerve regeneration and potentially their clinical application in peripheral nerve repair.

Deferoxamine promotes peripheral nerve regeneration by enhancing Schwann cell function and promoting axon regeneration of dorsal root ganglion

Deferoxamine (DFO) is a potent iron chelator for clinical treatment of various diseases. Recent studies have also shown its potential to promote vascular regeneration during peripheral nerve regeneration. However, the effect of DFO on the Schwann cell function and axon regeneration remains unclear. In this study, we investigated the effects of different concentrations of DFO on Schwann cell viability, proliferation, migration, expression of key functional genes, and axon regeneration of dorsal root ganglia (DRG) through a series of in vitro experiments. We found that DFO improves Schwann cell viability, proliferation, and migration in the early stages, with an optimal concentration of 25 μM. DFO also upregulates the expression of myelin-related genes and nerve growth-promoting factors in Schwann cells, while inhibiting the expression of Schwann cell dedifferentiation genes. Moreover, the appropriate concentration of DFO promotes axon regeneration in DRG. Our findings demonstrate that DFO, with suitable concentration and duration of action, can positively affect multiple stages of peripheral nerve regeneration, thereby improving the effectiveness of nerve injury repair. This study also enriches the theory of DFO promoting peripheral nerve regeneration and provides a basis for the design of sustained-release DFO nerve grafts.

Loss of Gata1 decreased eosinophils, macrophages, and type 2 cytokines in regenerating nerve and delayed axon regeneration after a segmental nerve injury

The immune system has garnered attention for its role in peripheral nerve regeneration, particularly as it pertains to regeneration across segmental injuries. Previous work demonstrated that eosinophils are recruited to regenerating nerve and express interleukin-4, amongst potential cytokines. These results suggest a direct role for eosinophils in promoting nerve regeneration. Therefore, we further considered eosinophils roles in nerve regeneration using a segmental nerve injury and Gata1 knockout (KO) mice, which are severely eosinophil deficient, compared to wild-type BALB/c mice (WT). Mice receiving a sciatic nerve gap injury demonstrated distinct cytokine expression and leukocytes within regenerating nerve. Compared to controls, Gata1 KO regenerated nerves contained decreased expression of type 2 cytokines, including Il-5 and Il-13, and decreased recruitment of eosinophils and macrophages. At this early time point during ongoing regeneration, the macrophages within Gata1 KO nerves also demonstrated significantly less M2 polarization compared to controls. Subsequently, motor and sensory axon regeneration across the gap injury was decreased in Gata1 KO compared to WT during ongoing nerve regeneration. Over longer observation to allow for more complete nerve regeneration, behavioral recovery measured by grid-walk assessment was not different comparing groups but modestly delayed in Gata1 KO compared to WT. The extent of final axon regeneration was not different amongst groups. Our data provide additional evidence suggesting eosinophils contribute to nerve regeneration across a nerve gap injury, but are not essential to regeneration in this context. Our evidence also suggests eosinophils may regulate cytokines that promote distinct macrophage phenotypes and axon regeneration.

Culture Conditions for Human Induced Pluripotent Stem Cell-Derived Schwann Cells: A Two-Centre Study

Adult human Schwann cells represent a relevant tool for studying peripheral neuropathies and developing regenerative therapies to treat nerve damage. Primary adult human Schwann cells are, however, difficult to obtain and challenging to propagate in culture. One potential solution is to generate Schwann cells from human induced pluripotent stem cells (hiPSCs). Previously published protocols, however, in our hands did not deliver sufficient viable cell numbers of hiPSC-derived Schwann cells (hiPSC-SCs). We present here, two modified protocols from two collaborating laboratories that overcome these challenges. With this, we also identified the relevant parameters to be specifically considered in any proposed differentiation protocol. Furthermore, we are, to our knowledge, the first to directly compare hiPSC-SCs to primary adult human Schwann cells using immunocytochemistry and RT-qPCR. We conclude the type of coating to be important during the differentiation process from Schwann cell precursor cells or immature Schwann cells to definitive Schwann cells, as well as the amounts of glucose in the specific differentiation medium to be crucial for increasing its efficiency and the final yield of viable hiPSC-SCs. Our hiPSC-SCs further displayed high similarity to primary adult human Schwann cells.

Sensory and motor fibroblasts have different protein expression patterns and exert different growth promoting effects on sensory and motor neurons

Functional reconstruction after peripheral nerve injury depends on the ability of the regenerated sensory and motor axons to re-innervate the suitable target organs. Therefore, it is essential to explore the cellular mechanisms of peripheral nerve-specific regeneration. In a previous study, we found that sensory and motor fibroblasts can guide Schwann cells to migrate towards the same phenotype. In the present paper, we analyzed the different effects of sensory and motor fibroblasts on sensory or motor neurons. The fibroblasts and neurons co-culture assay showed that compared with motor fibroblasts, sensory fibroblasts promote the neurite outgrowth of sensory neurons on a larger scale, and vice versa. Furthermore, a higher proportion of sensory or motor fibroblasts migrated towards their respective (sensory or motor) neurons. Meanwhile, a comparative proteomic approach was applied to obtain the protein expression profiles of sensory and motor fibroblasts. Among a total of 2597 overlapping proteins identified, we counted 148 differentially expressed items, of those 116 had a significantly higher expression in sensory fibroblasts, and 32 had a significantly greater expression in motor fibroblasts. Functional categorization revealed that differentially expressed proteins were involved in regeneration, axon guidance and cytoskeleton organization, all of which might play a critical role in peripheral nerve-specific regeneration. After nerve crush injury, ITB1 protein expression decreased significantly in motor nerves and increased in sensory nerves. In vitro, ITB1 significantly promoted axonal regeneration of sensory neurons, but had no significant effect on motor neurons. Overall, sensory and motor fibroblasts express different proteins and exert different growth promoting effects on sensory and motor neurons. This comparative proteomic database of sensory and motor fibroblasts could provide future directions for in-depth research on peripheral nerve-specific regeneration. Data are available via ProteomeXchange with identifier PXD034827.

Characterization of the structure and control of the blood-nerve barrier identifies avenues for therapeutic delivery

The blood barriers of the nervous system protect neural environments but can hinder therapeutic accessibility. The blood-brain barrier (BBB) is well characterized, consisting of endothelial cells with specialized tight junctions and low levels of transcytosis, properties conferred by contacting pericytes and astrocytes. In contrast, the blood-nerve barrier (BNB) of the peripheral nervous system is poorly defined. Here, we characterize the structure of the mammalian BNB, identify the processes that confer barrier function, and demonstrate how the barrier can be opened in response to injury. The homeostatic BNB is leakier than the BBB, which we show is due to higher levels of transcytosis. However, the barrier is reinforced by macrophages that specifically engulf leaked materials, identifying a role for resident macrophages as an important component of the BNB. Finally, we demonstrate the exploitation of these processes to effectively deliver RNA-targeting therapeutics to peripheral nerves, indicating new treatment approaches for nervous system pathologies.

Driving axon regeneration by orchestrating neuronal and non-neuronal innate immune responses via the IFNγ-cGAS-STING axis

The coordination mechanism of neural innate immune responses for axon regeneration is not well understood. Here, we showed that neuronal deletion of protein tyrosine phosphatase non-receptor type 2 sustains the IFNγ-STAT1 activity in retinal ganglion cells (RGCs) to promote axon regeneration after injury, independent of mTOR or STAT3. DNA-damage-induced cGAMP synthase (cGAS)-stimulator of interferon genes (STINGs) activation is the functional downstream signaling. Directly activating neuronal STING by cGAMP promotes axon regeneration. In contrast to the central axons, IFNγ is locally translated in the injured peripheral axons and upregulates cGAS expression in Schwann cells and infiltrating blood cells to produce cGAMP, which promotes spontaneous axon regeneration as an immunotransmitter. Our study demonstrates that injured peripheral nervous system (PNS) axons can direct the environmental innate immune response for self-repair and that the neural antiviral mechanism can be harnessed to promote axon regeneration in the central nervous system (CNS).

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.

Enhancing the Effect of Placental Extract on the Regeneration of Crush Injured Facial Nerve

There is a scarcity of experimental studies on peripheral nerve regeneration using placental extract (PE). This study aimed to investigate the effects of topical PE application on recovery after crush injury to the rat facial nerve using functional, electrophysiological, and morphological evaluations. The viability of the RSC96 Schwann cells treated with PE (0.5~4 mg/ml) increased significantly. Immunoblot test revealed that PE application enhanced the migration of RSC96 cells. Quantitative polymerase chain reaction demonstrated that PE increased the expression of neurotropic genes. The recovery from vibrissa fibrillation in the PE-treated group was superior to that in the control group. The threshold of action potential was also significantly lower in the PE group. Histopathological examination showed that crushed facial nerves treated with PE exhibited larger axons. The surrounding myelin sheaths were more distinct and thicker in the PE-treated group. Hence, PE may be considered a topical therapeutic agent for treating traumatic facial nerve paralysis.

Promoting Nerve Regeneration: Electrical Stimulation, Gene Therapy, and Beyond

Peripheral nerve injuries often result in life-altering functional deficits even with optimal management. Unlike the central nervous system, peripheral nerves have the ability to regenerate lost axons after injury; however, axonal regeneration does not equate to full restoration of function. To overcome this physiological shortcoming, advances in nerve regeneration and repair are paramount, including electrical stimulation, gene therapy, and surgical technique advancements.

Models and methods to study Schwann cells

Schwann cells (SCs) are fundamental components of the peripheral nervous system (PNS) of all vertebrates and play essential roles in development, maintenance, function, and regeneration of peripheral nerves. There are distinct populations of SCs including: (1) myelinating SCs that ensheath axons by a specialized plasma membrane, called myelin, which enhances the conduction of electric impulses; (2) non-myelinating SCs, including Remak SCs, which wrap bundles of multiple axons of small caliber, and perysinaptic SCs (PSCs), associated with motor axon terminals at the neuromuscular junction (NMJ). All types of SCs contribute to PNS regeneration through striking morphological and functional changes in response to nerve injury, are affected in peripheral neuropathies and show abnormalities and a diminished plasticity during aging. Therefore, methodological approaches to study and manipulate SCs in physiological and pathophysiological conditions are crucial to expand the present knowledge on SC biology and to devise new therapeutic strategies to counteract neurodegenerative conditions and age-derived denervation. We present here an updated overview of traditional and emerging methodologies for the study of SCs for scientists approaching this research field.

Role of osteopontin in the process of pulpal healing following tooth replantation in mice

Introduction

The role of osteopontin (OPN) following severe injury remains to be elucidated, especially its relationship with type I collagen (encoded by the Col1a1 gene) secretion by newly-differentiated odontoblast-like cells (OBLCs). In this study, we examined the role of OPN in the process of reparative dentin formation with a focus on reinnervation and revascularization after tooth replantation in Opn knockout (KO) and wild-type (WT) mice.

Methods

Maxillary first molars of 2- and 3-week-old-Opn KO and WT mice (Opn KO 2W, Opn KO 3W, WT 2W, and WT 3W groups) were replanted, followed by fixation 3–56 days after operation. Following micro-computed tomography analysis, the decalcified samples were processed for immunohistochemistry for Ki67, Nestin, PGP 9.5, and CD31 and in situ hybridization for Col1a1.

Results

An intense inflammatory reaction occurred to disrupt pulpal healing in the replanted teeth of the Opn KO 3W group, whereas dental pulp achieved healing in the Opn KO 2W and WT groups. The tertiary dentin in the Opn KO 3W group was significantly decreased in area compared with the Opn KO 2W and WT groups, with a significantly low percentage of Nestin-positive, newly-differentiated OBLCs during postoperative days 7–14. In the Opn KO 3W group, the blood vessels were significantly decreased in area and pulp healing was disturbed with a failure of pulpal revascularization and reinnervation.

Conclusions

OPN is necessary for proper reinnervation and revascularization to deposit reparative dentin following severe injury within the dental pulp of erupted teeth with advanced root development.

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Osteopontin deficiency inhibits hard tissue formation in advanced erupted teeth.

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Odontoblast-like cells may be different origins between mild and severe injuries.

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Osteopontin has an important role for proper reinnervation and revascularization.

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Osteopontin is necessary to deposit reparative dentin in advanced erupted teeth.

Artificial nerve graft constructed by coculture of activated Schwann cells and human hair keratin for repair of peripheral nerve defects

Studies have shown that human hair keratin (HHK) has no antigenicity and excellent mechanical properties. Schwann cells, as unique glial cells in the peripheral nervous system, can be induced by interleukin-1β to secrete nerve growth factor, which promotes neural regeneration. Therefore, HHK with Schwann cells may be a more effective approach to repair nerve defects than HHK without Schwann cells. In this study, we established an artificial nerve graft by loading an HHK skeleton with activated Schwann cells. We found that the longitudinal HHK microfilament structure provided adhesion medium, space and direction for Schwann cells, and promoted Schwann cell growth and nerve fiber regeneration. In addition, interleukin-1β not only activates Schwann cells, but also strengthens their activity and increases the expression of nerve growth factors. Activated Schwann cells activate macrophages, and activated macrophages secrete interleukin-1β, which maintains the activity of Schwann cells. Thus, a beneficial cycle forms and promotes nerve repair. Furthermore, our studies have found that the newly constructed artificial nerve graft promotes the improvements in nerve conduction function and motor function in rats with sciatic nerve injury, and increases the expression of nerve injury repair factors fibroblast growth factor 2 and human transforming growth factor B receptor 2. These findings suggest that this artificial nerve graft effectively repairs peripheral nerve injury.

Basic mechanisms of peripheral nerve injury and treatment via electrical stimulation

Previous studies on the mechanisms of peripheral nerve injury (PNI) have mainly focused on the pathophysiological changes within a single injury site. However, recent studies have indicated that within the central nervous system, PNI can lead to changes in both injury sites and target organs at the cellular and molecular levels. Therefore, the basic mechanisms of PNI have not been comprehensively understood. Although electrical stimulation was found to promote axonal regeneration and functional rehabilitation after PNI, as well as to alleviate neuropathic pain, the specific mechanisms of successful PNI treatment are unclear. We summarize and discuss the basic mechanisms of PNI and of treatment via electrical stimulation. After PNI, activity in the central nervous system (spinal cord) is altered, which can limit regeneration of the damaged nerve. For example, cell apoptosis and synaptic stripping in the anterior horn of the spinal cord can reduce the speed of nerve regeneration. The pathological changes in the posterior horn of the spinal cord can modulate sensory abnormalities after PNI. This can be observed in cases of ectopic discharge of the dorsal root ganglion leading to increased pain signal transmission. The injured site of the peripheral nerve is also an important factor affecting post-PNI repair. After PNI, the proximal end of the injured site sends out axial buds to innervate both the skin and muscle at the injury site. A slow speed of axon regeneration leads to low nerve regeneration. Therefore, it can take a long time for the proximal nerve to reinnervate the skin and muscle at the injured site. From the perspective of target organs, long-term denervation can cause atrophy of the corresponding skeletal muscle, which leads to abnormal sensory perception and hyperalgesia, and finally, the loss of target organ function. The mechanisms underlying the use of electrical stimulation to treat PNI include the inhibition of synaptic stripping, addressing the excessive excitability of the dorsal root ganglion, alleviating neuropathic pain, improving neurological function, and accelerating nerve regeneration. Electrical stimulation of target organs can reduce the atrophy of denervated skeletal muscle and promote the recovery of sensory function. Findings from the included studies confirm that after PNI, a series of physiological and pathological changes occur in the spinal cord, injury site, and target organs, leading to dysfunction. Electrical stimulation may address the pathophysiological changes mentioned above, thus promoting nerve regeneration and ameliorating dysfunction.

Transforming growth factor-beta signaling modulates perineurial glial bridging following peripheral spinal motor nerve injury in zebrafish

Spinal motor nerves are necessary for organismal locomotion and survival. In zebrafish and most vertebrates, these peripheral nervous system structures are composed of bundles of axons that naturally regenerate following injury. However, the cellular and molecular mechanisms that mediate this process are still only partially understood. Perineurial glia, which form a component of the blood-nerve barrier, are necessary for the earliest regenerative steps by establishing a glial bridge across the injury site as well as phagocytosing debris. Without perineurial glial bridging, regeneration is impaired. In addition to perineurial glia, Schwann cells, the cells that ensheath and myelinate axons within the nerve, are essential for debris clearance and axon guidance. In the absence of Schwann cells, perineurial glia exhibit perturbed bridging, demonstrating that these two cell types communicate during the injury response. While the presence and importance of perineurial glial bridging is known, the molecular mechanisms that underlie this process remain a mystery. Understanding the cellular and molecular interactions that drive perineurial glial bridging is crucial to unlocking the mechanisms underlying successful motor nerve regeneration. Using laser axotomy and in vivo imaging in zebrafish, we show that transforming growth factor-beta (TGFβ) signaling modulates perineurial glial bridging. Further, we identify connective tissue growth factor-a (ctgfa) as a downstream effector of TGF-β signaling that works in a positive feedback loop to mediate perineurial glial bridging. Together, these studies present a new signaling pathway involved in the perineurial glial injury response and further characterize the dynamics of the perineurial glial bridge.

Platelet-rich plasma loaded nerve guidance conduit as implantable biocompatible materials for recurrent laryngeal nerve regeneration

Vocal cord paralysis caused by recurrent laryngeal nerve (RLN) injury during thyroidectomy results in hoarseness, aspiration, and dyspnea. We evaluated the usefulness of nerve guidance conduits (NGCs) constructed from an asymmetric polycaprolactone (PCL)/Pluronic F127 porous membrane and filled with platelet-rich plasma (PRP) for functional RLN regeneration. We evaluated the proliferation and migration of Schwann cells (SCs) after PRP treatment in vitro. For the in vivo study, rabbits were divided into a non-loaded NGC group and a PRP-loaded NGC group. The left RLNs were resected and interposed with the NGCs. Functional and histological examinations of the vocal cords were performed. SC proliferation and migration increased in a PRP dose-dependent manner, with the PRP increasing the levels of neurotrophic factors, myelin-associated glycoprotein, and ERK. In vivo, the PRP group showed significantly better vocal cord mobility and less vocalis muscle atrophy than the non-loaded NGC group. Histologically, the ingrowth of nerve endings occurred more rapidly in the PRP group, and acetylcholinesterase, neurofilament, and S-100 expression in neural endings were significantly higher in the PRP group. Furthermore, transmission electron microscopy showed that myelinated axons were more tightly packed in the PRP group. This study shows that PRP-loaded NGCs provide a favorable environment for neural regeneration and suggests that this technique has therapeutic potential for promoting RLN recovery.