Schwann cells transduced with a lentiviral vector encoding Fgf-2 promote motor neuron regeneration following sciatic nerve injury

Growth Factors to Enhance Nerve Regeneration: Approaching Clinical Translation

Following nerve injury, growth factors (GFs) are transiently upregulated in injured neurons, proliferating Schwann cells, and denervated muscle and skin. They act on these same cells and tissues to promote nerve regeneration and end-organ reinnervation. Consequently, much attention has been focused on developing GF-based therapeutics. A major barrier to clinical translation of GFs is their short half-life. To provide sustained GF treatment to the affected nerve, muscle, and skin in a safe and practical manner, engineered drug delivery systems are needed. This review highlights recent advancements in GF-based therapeutics and discusses the remaining hurdles for clinical translation.

Runx2 regulates peripheral nerve regeneration to promote Schwann cell migration and re-myelination

JOURNAL/nrgr/04.03/01300535-202407000-00038/figure1/v/2023-11-20T171125Z/r/image-tiff

Runx2 is a major regulator of osteoblast differentiation and function; however, the role of Runx2 in peripheral nerve repair is unclear. Here, we analyzed Runx2 expression following injury and found that it was specifically up-regulated in Schwann cells. Furthermore, using Schwann cell-specific Runx2 knockout mice, we studied peripheral nerve development and regeneration and found that multiple steps in the regeneration process following sciatic nerve injury were Runx2-dependent. Changes observed in Runx2 knockout mice include increased proliferation of Schwann cells, impaired Schwann cell migration and axonal regrowth, reduced re-myelination of axons, and a block in macrophage clearance in the late stage of regeneration. Taken together, our findings indicate that Runx2 is a key regulator of Schwann cell plasticity, and therefore peripheral nerve repair. Thus, our study shows that Runx2 plays a major role in Schwann cell migration, re-myelination, and peripheral nerve functional recovery following injury.

Fabrication of Artificial Nerve Conduits Used in a Long Nerve Gap: Current Reviews and Future Studies

There are many commercially available artificial nerve conduits, used mostly to repair short gaps in sensory nerves. The stages of nerve regeneration in a nerve conduit are fibrin matrix formation between the nerve stumps joined to the conduit, capillary extension and Schwann cell migration from both nerve stumps, and, finally, axon extension from the proximal nerve stump. Artificial nerves connecting transected nerve stumps with a long interstump gap should be biodegradable, soft and pliable; have the ability to maintain an intrachamber fibrin matrix structure that allows capillary invasion of the tubular lumen, inhibition of scar tissue invasion and leakage of intratubular neurochemical factors from the chamber; and be able to accommodate cells that produce neurochemical factors that promote nerve regeneration. Here, we describe current progress in the development of artificial nerve conduits and the future studies needed to create nerve conduits, the nerve regeneration of which is compatible with that of an autologous nerve graft transplanted over a long nerve gap.

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.

Stem cell therapy combined with controlled release of growth factors for the treatment of sphincter dysfunction

Sphincter dysfunction often occurs at the end of tubule organs such as the urethra, anus, or gastroesophageal sphincters. It is the primary consequence of neuromuscular impairment caused by trauma, inflammation, and aging. Despite intensive efforts to recover sphincter function, pharmacological treatments have not achieved significant improvement. Cell- or growth factor-based therapy is a promising approach for neuromuscular regeneration and the recovery of sphincter function. However, a decrease in cell retention and viability, or the short half-life and rapid degradation of growth factors after implantation, remain obstacles to the translation of these therapies to the clinic. Natural biomaterials provide unique tools for controlled growth factor delivery, which leads to better outcomes for sphincter function recovery in vivo when stem cells and growth factors are co-administrated, in comparison to the delivery of single therapies. In this review, we discuss the role of stem cells combined with the controlled release of growth factors, the methods used for delivery, their potential therapeutic role in neuromuscular repair, and the outcomes of preclinical studies using combination therapy, with the hope of providing new therapeutic strategies to treat incontinence or sphincter dysfunction of the urethra, anus, or gastroesophageal tissues, respectively.

Growth factors and molecular-driven plasticity in neurological systems

It has been almost 70 years since the discovery of nerve growth factor (NGF), a period of a dramatic evolution in our understanding of dynamic growth, regeneration, and rewiring of the nervous system. In 1953, the extraordinary finding that a protein found in mouse submandibular glands generated a halo of outgrowing axons has now redefined our concept of the nervous system connectome. Central and peripheral neurons and their axons or dendrites are no longer considered fixed or static "wiring." Exploiting this molecular-driven plasticity as a therapeutic approach has arrived in the clinic with a slate of new trials and ideas. Neural growth factors (GFs), soluble proteins that alter the behavior of neurons, have expanded in numbers and our understanding of the complexity of their signaling and interactions with other proteins has intensified. However, beyond these "extrinsic" determinants of neuron growth and function are the downstream pathways that impact neurons, ripe for translational development and potentially more important than individual growth factors that may trigger them. Persistent and ongoing nuances in clinical trial design in some of the most intractable and irreversible neurological conditions give hope for connecting new biological ideas with clinical benefits. This review is a targeted update on neural GFs, their signals, and new therapeutic ideas, selected from an expansive literature.

A mouse model of peripheral nerve injury induced by Japanese encephalitis virus

Japanese encephalitis virus (JEV) is the most important cause of acute encephalitis in Eastern/Southern Asia. Infection with this virus also induces peripheral nerve injury. However, the disease pathogenesis is still not completely understood. Reliable animal models are needed to investigate the molecular pathogenesis of this condition. We studied the effect of Japanese encephalitis virus infection in C57BL/6 mice after a subcutaneous challenge. Limb paralysis was determined in mice using behavioral tests, including a viral paralysis scale and the hanging wire test, as well as by changes in body weight. Nerve conduction velocity and electromyography testing indicated the presence of demyelinating neuropathy of the sciatic nerve. Pathological changes in neural tissues were examined by immunofluorescence and transmission electron microscopy, which confirmed that the predominant pathologic change was demyelination. Although Western blots confirmed the presence of the virus in neural tissue, additional studies demonstrated that an immune-induced inflammatory response resulted in severe never injury. Immunofluorescence confirmed the presence of Japanese encephalitis virus in the brains of infected mice, and an inflammatory reaction was observed with hematoxylin-eosin staining as well. However, these observations were inconsistent at the time of paralysis onset. In summary, our results demonstrated that Japanese encephalitis virus infection could cause inflammatory demyelination of the peripheral nervous system in C57BL/6 mice.

Unleashing Intrinsic Growth Pathways in Regenerating Peripheral Neurons

Common mechanisms of peripheral axon regeneration are recruited following diverse forms of damage to peripheral nerve axons. Whether the injury is traumatic or disease related neuropathy, reconnection of axons to their targets is required to restore function. Supporting peripheral axon regrowth, while not yet available in clinics, might be accomplished from several directions focusing on one or more of the complex stages of regrowth. Direct axon support, with follow on participation of supporting Schwann cells is one approach, emphasized in this review. However alternative approaches might include direct support of Schwann cells that instruct axons to regrow, manipulation of the inflammatory milieu to prevent ongoing bystander axon damage, or use of inflammatory cytokines as growth factors. Axons may be supported by a growing list of growth factors, extending well beyond the classical neurotrophin family. The understanding of growth factor roles continues to expand but their impact experimentally and in humans has faced serious limitations. The downstream signaling pathways that impact neuron growth have been exploited less frequently in regeneration models and rarely in human work, despite their promise and potency. Here we review the major regenerative signaling cascades that are known to influence adult peripheral axon regeneration. Within these pathways there are major checkpoints or roadblocks that normally check unwanted growth, but are an impediment to robust growth after injury. Several molecular roadblocks, overlapping with tumour suppressor systems in oncology, operate at the level of the perikarya. They have impacts on overall neuron plasticity and growth. A second approach targets proteins that largely operate at growth cones. Addressing both sites might offer synergistic benefits to regrowing neurons. This review emphasizes intrinsic aspects of adult peripheral axon regeneration, emphasizing several molecular barriers to regrowth that have been studied in our laboratory.

New developments in the biology of fibroblast growth factors

The fibroblast growth factor (FGF) family is composed of 18 secreted signaling proteins consisting of canonical FGFs and endocrine FGFs that activate four receptor tyrosine kinases (FGFRs 1-4) and four intracellular proteins (intracellular FGFs or iFGFs) that primarily function to regulate the activity of voltage-gated sodium channels and other molecules. The canonical FGFs, endocrine FGFs, and iFGFs have been reviewed extensively by us and others. In this review, we briefly summarize past reviews and then focus on new developments in the FGF field since our last review in 2015. Some of the highlights in the past 6 years include the use of optogenetic tools, viral vectors, and inducible transgenes to experimentally modulate FGF signaling, the clinical use of small molecule FGFR inhibitors, an expanded understanding of endocrine FGF signaling, functions for FGF signaling in stem cell pluripotency and differentiation, roles for FGF signaling in tissue homeostasis and regeneration, a continuing elaboration of mechanisms of FGF signaling in development, and an expanding appreciation of roles for FGF signaling in neuropsychiatric diseases. This article is categorized under: Cardiovascular Diseases > Molecular and Cellular Physiology Neurological Diseases > Molecular and Cellular Physiology Congenital Diseases > Stem Cells and Development Cancer > Stem Cells and Development.

Application of neurotrophic and proangiogenic factors as therapy after peripheral nervous system injury

The intrinsic ability of peripheral nerves to regenerate after injury is extremely limited, especially in case of severe injury. This often leads to poor motor function and permanent disability. Existing approaches for the treatment of injured nerves do not provide appropriate conditions to support survival and growth of nerve cells. This drawback can be compensated by the use of gene therapy and cell therapy-based drugs that locally provide an increase in the key regulators of nerve growth, including neurotrophic factors and extracellular matrix proteins. Each growth factor plays its own specific angiotrophic or neurotrophic role. Currently, growth factors are widely studied as accelerators of nerve regeneration. Particularly noteworthy is synergy between various growth factors, that is essential for both angiogenesis and neurogenesis. Fibroblast growth factor 2 and vascular endothelial growth factor are widely known for their proangiogenic effects. At the same time, fibroblast growth factor 2 and vascular endothelial growth factor stimulate neural cell growth and play an important role in neurodegenerative diseases of the peripheral nervous system. Taken together, their neurotrophic and angiogenic properties have positive effect on the regeneration process. In this review we provide an in-depth overview of the role of fibroblast growth factor 2 and vascular endothelial growth factor in the regeneration of peripheral nerves, thus demonstrating their neurotherapeutic efficacy in improving neuron survival in the peripheral nervous system.

Engineered Schwann Cell-Based Therapies for Injury Peripheral Nerve Reconstruction

Compared with the central nervous system, the adult peripheral nervous system possesses a remarkable regenerative capacity, which is due to the strong plasticity of Schwann cells (SCs) in peripheral nerves. After peripheral nervous injury, SCs de-differentiate and transform into repair phenotypes, and play a critical role in axonal regeneration, myelin formation, and clearance of axonal and myelin debris. In view of the limited self-repair capability of SCs for long segment defects of peripheral nerve defects, it is of great clinical value to supplement SCs in necrotic areas through gene modification or stem cell transplantation or to construct tissue-engineered nerve combined with bioactive scaffolds to repair such tissue defects. Based on the developmental lineage of SCs and the gene regulation network after peripheral nerve injury (PNI), this review summarizes the possibility of using SCs constructed by the latest gene modification technology to repair PNI. The therapeutic effects of tissue-engineered nerve constructed by materials combined with Schwann cells resembles autologous transplantation, which is the gold standard for PNI repair. Therefore, this review generalizes the research progress of biomaterials combined with Schwann cells for PNI repair. Based on the difficulty of donor sources, this review also discusses the potential of “unlimited” provision of pluripotent stem cells capable of directing differentiation or transforming existing somatic cells into induced SCs. The summary of these concepts and therapeutic strategies makes it possible for SCs to be used more effectively in the repair of PNI.

Delivery of Basic Fibroblast Growth Factor Through an In Situ Forming Smart Hydrogel Activates Autophagy in Schwann Cells and Improves Facial Nerves Generation via the PAK-1 Signaling Pathway

Although studies have shown that basic fibroblast growth factor (bFGF) can activate autophagy and promote peripheral nerve repair, the role and the molecular mechanism of action of bFGF in the facial nerve are not clear. In this study, a thermosensitive in situ forming poloxamer hydrogel was used as a vehicle to deliver bFGF for treating facial nerve injury (FNI) in the rat model. Using H&E and Masson’s staining, we found that bFGF hydrogel can promote the functional recovery and regeneration of the facial nerve. Furthermore, studies on the mechanism showed that bFGF can promote FNI recovery by promoting autophagy and inhibiting apoptosis. Additionally, this study demonstrated that the role of hydrogel binding bFGF in nerve repair was mediated through the activation of the PAK1 signaling pathway in Schwann cells (SCs). These results indicated that poloxamer thermosensitive hydrogel loaded with bFGF can significantly restore the morphology and function of the injured facial nerve by promoting autophagy and inhibiting apoptosis by activating the PAK1 pathway, which can provide a promising strategy for FNI recovery.

FGF binding protein 3 is required for spinal cord motor neuron development and regeneration in zebrafish

Fibroblast growth factor binding protein 3 (Fgfbp3) have been known to be crucial for the process of neural proliferation, differentiation, migration, and adhesion. However, the specific role and the molecular mechanisms of fgfbp3 in regulating the development of motor neurons remain unclear. In this study, we have investigated the function of fgfbp3 in morphogenesis and regeneration of motor neuron in zebrafish. Firstly, we found that fgfbp3 was localized in the motor neurons and loss of fgfbp3 caused the significant decrease of the length and branching number of the motor neuron axons, which could be partially rescued by fgfbp3 mRNA injection. Moreover, the fgfbp3 knockdown (KD) embryos demonstrated similar defects of motor neurons as identified in fgfbp3 knockout (KO) embryos. Furthermore, we revealed that the locomotion and startle response of fgfbp3 KO embryos were significantly restricted, which were partially rescued by the fgfbp3 overexpression. In addition, fgfbp3 KO remarkably compromised axonal regeneration of motor neurons after injury. Lastly, the malformation of motor neurons in fgfbp3 KO embryos was rescued by overexpressing drd1b or neurod6a, respectively, which were screened by transcriptome sequencing. Taken together, our results provide strong cellular and molecular evidence that fgfbp3 is crucial for the axonal morphogenesis and regeneration of motor neurons in zebrafish.

Fibroblast Growth Factor Signalling in the Diseased Nervous System

Fibroblast growth factors (FGFs) act as key signalling molecules in brain development, maintenance, and repair. They influence the intricate relationship between myelinating cells and axons as well as the association of astrocytic and microglial processes with neuronal perikarya and synapses. Advances in molecular genetics and imaging techniques have allowed novel insights into FGF signalling in recent years. Conditional mouse mutants have revealed the functional significance of neuronal and glial FGF receptors, not only in tissue protection, axon regeneration, and glial proliferation but also in instant behavioural changes. This review provides a summary of recent findings regarding the role of FGFs and their receptors in the nervous system and in the pathogenesis of major neurological and psychiatric disorders.

All-Polymer Printed Low-Cost Regenerative Nerve Cuff Electrodes

Neural regeneration after lesions is still limited by several factors and new technologies are developed to address this issue. Here, we present and test in animal models a new regenerative nerve cuff electrode (RnCE). It is based on a novel low-cost fabrication strategy, called "Print and Shrink", which combines the inkjet printing of a conducting polymer with a heat-shrinkable polymer substrate for the development of a bioelectronic interface. This method allows to produce miniaturized regenerative cuff electrodes without the use of cleanroom facilities and vacuum based deposition methods, thus highly reducing the production costs. To fully proof the electrodes performance in vivo we assessed functional recovery and adequacy to support axonal regeneration after section of rat sciatic nerves and repair with RnCE. We investigated the possibility to stimulate the nerve to activate different muscles, both in acute and chronic scenarios. Three months after implantation, RnCEs were able to stimulate regenerated motor axons and induce a muscular response. The capability to produce fully-transparent nerve interfaces provided with polymeric microelectrodes through a cost-effective manufacturing process is an unexplored approach in neuroprosthesis field. Our findings pave the way to the development of new and more usable technologies for nerve regeneration and neuromodulation.

Basic fibroblast growth factor accelerates myelin debris clearance through activating autophagy to facilitate early peripheral nerve regeneration

The successful removal of damaged myelin sheaths during Wallerian degeneration (WD) is essential for ensuring structural remodelling and functional recovery following traumatic peripheral nerve injury (PNI). Recent studies have established that autophagy involves myelin phagocytosis and cellular homoeostasis, and its disorder impairs myelin clearance. Based on the role of basic fibroblast growth factor (bFGF) on exerting neuroprotection and angiogenesis during nerve tissue regeneration, we now explicitly focus on the issue about whether the therapeutic effect of bFGF on supporting nerve regeneration is closely related to accelerate the autophagic clearance of myelin debris during WD. Using sciatic nerve crushed model, we found that bFGF remarkedly improved axonal outgrowth and nerve reconstruction at the early phase of PNI (14 days after PNI). More importantly, we further observed that bFGF could enhance phagocytic capacity of Schwann cells (SCs) to engulf myelin debris. Additionally, this enhancing effect is accomplished by autophagy activation and the increase of autophagy flux by immunoblotting and immune‐histochemical analyses. Taken together, our data suggest that the action of bFGF on modulating early peripheral nerve regeneration is closely associated with myelin debris removal by SCs, which might result in SC‐mediated autophagy activation, highlighting its insight molecular mechanism as a neuroprotective agent for repairing PNI.

Insights Into the Role and Potential of Schwann Cells for Peripheral Nerve Repair From Studies of Development and Injury

Peripheral nerve injuries arising from trauma or disease can lead to sensory and motor deficits and neuropathic pain. Despite the purported ability of the peripheral nerve to self-repair, lifelong disability is common. New molecular and cellular insights have begun to reveal why the peripheral nerve has limited repair capacity. The peripheral nerve is primarily comprised of axons and Schwann cells, the supporting glial cells that produce myelin to facilitate the rapid conduction of electrical impulses. Schwann cells are required for successful nerve regeneration; they partially “de-differentiate” in response to injury, re-initiating the expression of developmental genes that support nerve repair. However, Schwann cell dysfunction, which occurs in chronic nerve injury, disease, and aging, limits their capacity to support endogenous repair, worsening patient outcomes. Cell replacement-based therapeutic approaches using exogenous Schwann cells could be curative, but not all Schwann cells have a “repair” phenotype, defined as the ability to promote axonal growth, maintain a proliferative phenotype, and remyelinate axons. Two cell replacement strategies are being championed for peripheral nerve repair: prospective isolation of “repair” Schwann cells for autologous cell transplants, which is hampered by supply challenges, and directed differentiation of pluripotent stem cells or lineage conversion of accessible somatic cells to induced Schwann cells, with the potential of “unlimited” supply. All approaches require a solid understanding of the molecular mechanisms guiding Schwann cell development and the repair phenotype, which we review herein. Together these studies provide essential context for current efforts to design glial cell-based therapies for peripheral nerve regeneration.

AAV Targeting of Glial Cell Types in the Central and Peripheral Nervous System and Relevance to Human Gene Therapy

Different glial cell types are found throughout the central (CNS) and peripheral nervous system (PNS), where they have important functions. These cell types are also involved in nervous system pathology, playing roles in neurodegenerative disease and following trauma in the brain and spinal cord (astrocytes, microglia, oligodendrocytes), nerve degeneration and development of pain in peripheral nerves (Schwann cells, satellite cells), retinal diseases (Müller glia) and gut dysbiosis (enteric glia). These cell type have all been proposed as potential targets for treating these conditions. One approach to target these cell types is the use of gene therapy to modify gene expression. Adeno-associated virus (AAV) vectors have been shown to be safe and effective in targeting cells in the nervous system and have been used in a number of clinical trials. To date, a number of studies have tested the use of different AAV serotypes and cell-specific promoters to increase glial cell tropism and expression. However, true glial-cell specific targeting for a particular glial cell type remains elusive. This review provides an overview of research into developing glial specific gene therapy and discusses some of the issues that still need to be addressed to make glial cell gene therapy a clinical reality.

Magnetic nanocomplexes for gene delivery applications

Gene delivery is an indispensable technique for various biomedical applications such as gene therapy, stem cell engineering and gene editing. Recently, magnetic nanoparticles (MNPs) have received increasing attention for their use in promoting gene delivery efficiency. Under magnetic attraction, gene delivery efficiency using viral or nonviral gene carriers could be universally enhanced. Besides, magnetic nanoparticles could be utilized in magnetic resonance imaging or magnetic hyperthermia therapy, providing extra theranostic opportunities. In this review, recent research integrating MNPs with a viral or nonviral gene vector is summarized from both technical and application perspectives. Applications of MNPs in cutting-edge research technologies, such as biomimetic cell membrane nano-gene carriers, exosome-based gene delivery, cell-based drug delivery systems or CRISPR/Cas9 gene editing, are also discussed.

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

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

Schwann Cell Role in Selectivity of Nerve Regeneration

Peripheral nerve injuries result in the loss of the motor, sensory and autonomic functions of the denervated segments of the body. Neurons can regenerate after peripheral axotomy, but inaccuracy in reinnervation causes a permanent loss of function that impairs complete recovery. Thus, understanding how regenerating axons respond to their environment and direct their growth is essential to improve the functional outcome of patients with nerve lesions. Schwann cells (SCs) play a crucial role in the regeneration process, but little is known about their contribution to specific reinnervation. Here, we review the mechanisms by which SCs can differentially influence the regeneration of motor and sensory axons. Mature SCs express modality-specific phenotypes that have been associated with the promotion of selective regeneration. These include molecular markers, such as L2/HNK-1 carbohydrate, which is differentially expressed in motor and sensory SCs, or the neurotrophic profile after denervation, which differs remarkably between SC modalities. Other important factors include several molecules implicated in axon-SC interaction. This cell–cell communication through adhesion (e.g., polysialic acid) and inhibitory molecules (e.g., MAG) contributes to guiding growing axons to their targets. As many of these factors can be modulated, further research will allow the design of new strategies to improve functional recovery after peripheral nerve injuries.

FGF5 Regulates Schwann Cell Migration and Adhesion

The fibroblast growth factor (FGF) family polypeptides play key roles in promoting tissue regeneration and repair. FGF5 is strongly up-regulated in Schwann cells of the peripheral nervous system following injury; however, a role for FGF5 in peripheral nerve regeneration has not been shown up to now. In this report, we examined the expression of FGF5 and its receptors FGFR1-4 in Schwann cells of the mouse sciatic nerve following injury, and then measured the effects of FGF5 treatment upon cultured primary rat Schwann cells. By microarray and mRNA sequencing data analysis, RT-PCR, qPCR, western blotting and immunostaining, we show that FGF5 is highly up-regulated in Schwann cells of the mouse distal sciatic nerve following injury, and FGFR1 and FGFR2 are highly expressed in Schwann cells of the peripheral nerve both before and following injury. Using cultured primary rat Schwann cells, we show that FGF5 inhibits ERK1/2 MAP kinase activity but promotes rapid Schwann cell migration and adhesion via the upregulation of N-cadherin. Thus, FGF5 is an autocrine regulator of Schwann cells to regulate Schwann cell migration and adhesion.

SIRT2 Inhibition Improves Functional Motor Recovery After Peripheral Nerve Injury

Sirtuin-2 (Sirt2) is a member of the NAD (+)-dependent protein deacetylase family involved in neuroprotection, cellular metabolism, homeostasis, and stress responses after injury of the nervous system. So far, no data have been published describing the role of SIRT2 in motor functional recovery after damage. We found that SIRT2 expression and deacetylase activity were increased within motoneurons after axotomy. To shed light onto the biological relevance of this change, we combined in vitro and in vivo models with pharmacological and genetic ablation approaches. We found that SIRT2 KO (knockout) mice exhibited improved functional recovery after sciatic nerve crush. SIRT2 activity blockage, using AK7, increased neurite outgrowth and length in organotypic spinal cord cultures and human cell line models. SIRT2 blockage enhanced the acetyltransferase activity of p300, which in turn increased the levels of an acetylated form of p53 (Ac-p53 k373), histone 3 (Ac-H3K9), and expression of GAP43, a downstream marker of regeneration. Lastly, we verified that p300 acetyltransferase activity is essential for these effects. Our results suggest that bolstering an epigenetic shift that promotes SIRT2 inhibition can be an effective therapy to increase functional recovery after peripheral nerve injury.

Ginsenoside Rg1 ameliorates the cognitive deficits in D-galactose and AlCl3-induced aging mice by restoring FGF2-Akt and BDNF-TrkB signaling axis to inhibit apoptosis

Ginsenoside Rg1 is the main active ingredient of Panax ginseng with the activity of neuroprotective, antioxidant and strengthening the immune system. Therefore, we hypothesized that Rg1 may afford anti-aging effects although the mechanism remains to be elucidated. In this study, chemically induced aging mice were established by consecutive administration of D-galactose and AlCl₃. We found that Rg1 effectively ameliorates spatial learning and memory deficits in aging mice demonstrated by their improved performance in step down avoidance tests and Morris water maze experiments. Rg1 restored aging-induced decline of FGF2 and BDNF, reactivated TrkB/Akt signaling pathways in the hippocampus and prefrontal cortex to inhibit apoptosis, for the expression of anti-apoptotic protein Bcl-2 and apoptosis promoting enzyme cleaved-Caspase3 were antagonistically restored. Therefore, these results established the anti-aging effects of Rg1, and FGF2, BDNF and associated signaling pathways might be promising targets. Our data may provide a new avenue to the pharmacological research and diet therapeutic role of ethnic products such as Rg1 in anti-aging and aging associated diseases.

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

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

A Schwann cell-enriched circular RNA circ-Ankib1 regulates Schwann cell proliferation following peripheral nerve injury

Schwann cells (SCs) play an essential role in nerve injury repair. A striking feature of the cellular response to peripheral nerve injury is the proliferation of SCs. Circular (circ)RNAs are enriched in the nervous system and are involved in physiologic and pathologic processes. However, the potential role of circRNAs in SC proliferation post nerve injury remains largely unknown. Using a sciatic nerve crush model, we obtained an expression profiling of circRNAs in injured sciatic nerves in rats by RNA sequencing and bioinformatics analysis, and we further identified a circRNA [circ-ankyrin repeat and in-between Ring finger (IBR) domain containing 1 (Ankib1)] involved in SC proliferation by the transfection of specific small interfering RNAs. Overexpression of circ-Ankib1, which was specifically and highly enriched in SCs, impaired SC proliferation and axon regeneration following sciatic nerve injury. Mechanistically, increased expression of DEx/H-box helicase 9 (DHX9) postinjury might contribute to the down-regulation of circ-Ankib1, which further suppressed cytochrome P450, family 26, subfamily B, polypeptide 1 expression by sponging miR-423-5p, miR-485-5p, and miR-666-3p, leading to the induction of SC proliferation and nerve regeneration. Taken together, our results reveal a crucial role for circRNAs in regulating proliferation of SCs involved in sciatic nerve regeneration; as such, circRNAs may serve as a potential therapeutic avenue for nerve injury repair.-Mao, S., Zhang, S., Zhou, S., Huang, T., Feng, W., Gu, X., Yu, B. A Schwann cell-enriched circular RNA circ-Ankib1 regulates Schwann cell proliferation following peripheral nerve injury.

The IS2 Element Improves Transcription Efficiency of Integration-Deficient Lentiviral Vector Episomes

Integration-defective lentiviral vectors (IDLVs) have become an important alternative tool for gene therapy applications and basic research. Unfortunately, IDLVs show lower transgene expression as compared to their integrating counterparts. In this study, we aimed to improve the expression levels of IDLVs by inserting the IS2 element, which harbors SARs and HS4 sequences, into their LTRs (SE-IS2-IDLVs). Contrary to our expectations, the presence of the IS2 element did not abrogate epigenetic silencing by histone deacetylases. In addition, the IS2 element reduced episome levels in IDLV-transduced cells. Interestingly, despite these negative effects, SE-IS2-IDLVs outperformed SE-IDLVs in terms of percentage and expression levels of the transgene in several cell lines, including neurons, neuronal progenitor cells, and induced pluripotent stem cells. We estimated that the IS2 element enhances the transcriptional activity of IDLV LTR circles 6- to 7-fold. The final effect the IS2 element in IDLVs will greatly depend on the target cell and the balance between the negative versus the positive effects of the IS2 element in each cell type. The better performance of SE-IS2-IDLVs was not due to improved stability or differences in the proportions of 1-LTR versus 2-LTR circles but probably to a re-positioning of IS2-episomes into transcriptionally active regions.

Fibroblast growth factor 21 facilitates peripheral nerve regeneration through suppressing oxidative damage and autophagic cell death

Seeking for effective drugs which are beneficial to facilitating axonal regrowth and elongation after peripheral nerve injury (PNI) has gained extensive attention. Fibroblast growth factor 21 (FGF21) is a metabolic factor that regulates blood glucose and lipid homeostasis. However, there is little concern for the potential protective effect of FGF21 on nerve regeneration after PNI and revealing related molecular mechanisms. Here, we firstly found that exogenous FGF21 administration remarkably promoted functional and morphologic recovery in a rat model of sciatic crush injury, manifesting as persistently improved motor and sensory function, enhanced axonal remyelination and regrowth and accelerated Schwann cells (SCs) proliferation. Furthermore, local FGF21 application attenuated the excessive activation of oxidative stress, which was accompanied with the activation of nuclear factor erythroid‐2‐related factor 2 (Nrf‐2) transcription and extracellular regulated protein kinases (ERK) phosphorylation. We detected FGF21 also suppressed autophagic cell death in SCs. Additionally, treatment with the ERK inhibitor U0126 or autophagy inhibitor 3‐MA partially abolishes anti‐oxidant effect and reduces SCs death. Taken together, these results indicated that the role of FGF21 in remyelination and nerve regeneration after PNI was probably related to inhibit the excessive activation of ERK/Nrf‐2 signalling‐regulated oxidative stress and autophagy‐induced cell death. Overall, our work suggests that FGF21 administration may provide a new therapy for PNI.

FGF binding proteins (FGFBPs): Modulators of FGF signaling in the developing, adult, and stressed nervous system

Members of the fibroblast growth factor (FGF) family are involved in a variety of cellular processes. In the nervous system, they affect the differentiation and migration of neurons, the formation and maturation of synapses, and the repair of neuronal circuits following insults. Because of the varied yet critical functions of FGF ligands, their availability and activity must be tightly regulated for the nervous system, as well as other tissues, to properly develop and function in adulthood. In this regard, FGF binding proteins (FGFBPs) have emerged as strong candidates for modulating the actions of secreted FGFs in neural and non-neural tissues. Here, we will review the roles of FGFBPs in the peripheral and central nervous systems.

The PI3K/Akt/FOXO3a pathway regulates regeneration following spinal cord injury in adult rats through TNF-α and p27kip1 expression

The aim of the present study was to elucidate the expression and role of the phosphatidylinositol 3‑kinase (PI3K)/Akt/forkhead box O3 (FOXO3a) pathway in the regeneration of the spinal cord following spinal cord injury (SCI), and its regulatory effect on tumor necrosis factor (TNF)-α and cyclin-dependent kinase inhibitor 1B (p27kip1) expression. Firstly, in a Sprague-Dawley rat model of SCI, western blot analysis revealed that the protein levels of PI3K, phosphorylated Akt and FOXO3a were markedly inhibited compared with those in the sham control group. In vitro experiments were also conducted, in which primary dissociated cultures of rat dorsal spinal cord cells were induced with lipopolysaccharide (LPS; 4 µg/ml). The downregulation of PI3K using LY294002 markedly suppressed cell viability, reduced the protein levels of FOXO3a and p27kip1, and increased TNF-α protein production in the LPS-induced spinal cord cells. In addition, when the LPS-induced spinal cord cells were infected with FOXO3a adenoviral vectors, the overexpression of FOXO3 markedly promoted cell proliferation, activated p27kip1 protein levels and inhibited TNF-α protein production in the spinal cord cells. These results suggest that the PI3K/Akt/FOXO3a pathway regulates regeneration following SCI in adult rats via its modulatory effects on TNF-α and p27kip1 expression.

Purification and Characterization of Schwann Cells from Adult Human Skin and Nerve

Despite its modest capacity for regeneration, peripheral nervous system injury often results in significant long-term disability. Supplementing peripheral nervous system injury with autologous Schwann cells (SCs) may serve to rejuvenate the postinjury environment to enhance regeneration and ultimately improve functional outcomes. However, human nerve-derived SC (hN-SC) collection procedures require invasive surgical resection. Here, we describe the characterization of SCs from adult human skin (hSk-SCs) of four male donors ranging between 27 and 46 years old. Within five weeks of isolating and culturing adherent mixed skin cells, we were able to obtain 3–5 million purified SCs. We found that hSk-SCs appeared transcriptionally indistinguishable from hN-SCs with both populations exhibiting expression of SC genes including: SOX10, SOX9, AP2A1, CDH19, EGR1, ETV5, PAX3, SOX2, CX32, DHH, NECL4, NFATC4, POU3F1, S100B, and YY1. Phenotypic analysis of hSk-SCs and hN-SCs cultures revealed highly enriched populations of SCs indicated by the high percentage of NES^+ve, SOX10^+ve, s100^+ve and p75^+ve cells, as well as the expression of a battery of other SC-associated proteins (PAX3, CDH19, ETV5, SOX2, POU3F1, S100B, EGR2, and YY1). We further show that both hSk-SCs and hN-SCs are capable of promoting axonal growth to similar degrees and that a subset of both associate with regenerating axons and form myelin following transplantation into the injured mouse sciatic nerve. Interestingly, although the majority of both hSk-SCs and hN-SCs maintained SOX10 immunoreactivity following transplant, only a subset of each activated the promyelinating factor, POU3F1, and were able to myelinate. Taken together, we demonstrate that adult hSk-SCs are genetically and phenotypically indistinguishable to hN-SCs.

Time-dependent differential expression of long non-coding RNAs following peripheral nerve injury

Long non-coding RNAs (lncRNAs) are widely accepted as key players in various biological processes. However, the roles of lncRNA in peripheral nerve regeneration remain completely unknown. Thus, in this study, we performed microarray analysis to measure lncRNA expression in the distal segment of the sciatic nerve at 0, 3, 7 and 14 days following injury. We identified 5,354 lncRNAs that were differentially expressed: 3,788 lncRNAs were differentially expressed between days 0 and 3; 3,314 lncRNAs were differentially expressed between days 0 and 7; and 2,400 lncRNAs were differentially expressed between days 0 and 14. The results of RT-qPCR of two dysregulated lncRNAs were consistent with those of microarray analysis. Bioinformatics approaches, including lncRNA classification, gene ontology (GO) analysis and target prediction, were utilized to investigate the functions of these dysregulated lncRNAs in peripheral nerve damage. Importantly, we predicted that several lncRNA-mRNA pairs may participate in biological processes related to peripheral nerve injury. RT-qPCR was performed for the preliminary verification of three lncRNA-mRNA pairs. The overexpression of NONMMUG014387 promoted the proliferation of mouse Schwann cells. Thus, the findings of our study may enhance our knowledge of the role of lncRNAs in nerve injury.

Cell-free artificial implants of electrospun fibres in a three-dimensional gelatin matrix support sciatic nerve regeneration in vivo

Surgical repair of larger peripheral nerve lesions requires the use of autologous nerve grafts. At present, clinical alternatives to avoid nerve transplantation consist of empty tubes, which are only suitable for the repair over short distances and have limited success. We developed a cell-free, three-dimensional scaffold for axonal guidance in long-distance nerve repair. Sub-micron scale fibres of biodegradable poly-ε-caprolactone (PCL) and collagen/PCL (c/PCL) blends were incorporated in a gelatin matrix and inserted in collagen tubes. The conduits were tested by replacing 15-mm-long segments of rat sciatic nerves in vivo. Biocompatibility of the implants and nerve regeneration were assessed histologically, with electromyography and with behavioural tests for motor functions. Functional repair was achieved in all animals with autologous transplants, in 12 of 13 rats that received artificial implants with an internal structure and in half of the animals with empty nerve conduits. In rats with implants containing c/PCL fibres, the extent of recovery (compound muscle action potentials, motor functions of the hind limbs) was superior to animals that had received empty implants, but not as good as with autologous nerve transplantation. Schwann cell migration and axonal regeneration were observed in all artificial implants, and muscular atrophy was reduced in comparison with animals that had received no implants. The present design represents a significant step towards cell-free, artificial nerve bridges that can replace autologous nerve transplants in the clinic. Copyright © 2017 John Wiley & Sons, Ltd.

Combining Gene and Stem Cell Therapy for Peripheral Nerve Tissue Engineering

Despite a substantially increased understanding of neuropathophysiology, insufficient functional recovery after peripheral nerve injury remains a significant clinical challenge. Nerve regeneration following injury is dependent on Schwann cells, the supporting cells in the peripheral nervous system. Following nerve injury, Schwann cells adopt a proregenerative phenotype, which supports and guides regenerating nerves. However, this phenotype may not persist long enough to ensure functional recovery. Tissue-engineered nerve repair devices containing therapeutic cells that maintain the appropriate phenotype may help enhance nerve regeneration. The combination of gene and cell therapy is an emerging experimental strategy that seeks to provide the optimal environment for axonal regeneration and reestablishment of functional circuits. This review aims to summarize current preclinical evidence with potential for future translation from bench to bedside.

Peripheral Nerve Regeneration through Hydrogel-Enriched Chitosan Conduits Containing Engineered Schwann Cells for Drug Delivery

Critical length nerve defects in the rat sciatic nerve model were reconstructed with chitosan nerve guides filled with Schwann cells (SCs) containing hydrogel. The transplanted SCs were naive or had been genetically modified to overexpress neurotrophic factors, thus providing a cellular neurotrophic factor delivery system. Prior to the assessment in vivo, in vitro studies evaluating the properties of engineered SCs overexpressing glial cell line-derived neurotrophic factor (GDNF) or fibroblast growth factor 2 (FGF-218kDa) demonstrated their neurite outgrowth inductive bioactivity for sympathetic PC-12 cells as well as for dissociated dorsal root ganglion cell drop cultures. SCs within NVR-hydrogel, which is mainly composed of hyaluronic acid and laminin, were delivered into the lumen of chitosan hollow conduits with a 5% degree of acetylation. The viability and neurotrophic factor production by engineered SCs within NVR-Gel inside the chitosan nerve guides was further demonstrated in vitro. In vivo we studied the outcome of peripheral nerve regeneration after reconstruction of 15-mm nerve gaps with either chitosan/NVR-Gel/SCs composite nerve guides or autologous nerve grafts (ANGs). While ANGs did guarantee for functional sensory and motor regeneration in 100% of the animals, delivery of NVR-Gel into the chitosan nerve guides obviously impaired sufficient axonal outgrowth. This obstacle was overcome to a remarkable extent when the NVR-Gel was enriched with FGF-218kDa overexpressing SCs.

Transplantation of human telomerase reverse transcriptase gene-transfected Schwann cells for repairing spinal cord injury

Transfection of the human telomerase reverse transcriptase (hTERT) gene has been shown to increase cell proliferation and enhance tissue repair. In the present study, hTERT was transfected into rat Schwann cells. A rat model of acute spinal cord injury was established by the modified free-falling method. Retrovirus PLXSN was injected at the site of spinal cord injury as a vector to mediate hTERT gene-transfected Schwann cells (1 × 10(10)/L; 10 μL) or Schwann cells (1 × 10(10)/L; 10 μL) without hTERT gene transfection. Between 1 and 4 weeks after model establishment, motor function of the lower limb improved in the hTERT-transfected group compared with the group with non-transfected Schwann cells. Terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling and reverse transcription-polymerase chain reaction results revealed that the number of apoptotic cells, and gene expression of aquaporin 4/9 and matrix metalloproteinase 9/2 decreased at the site of injury in both groups; however, the effect improved in the hTERT-transfected group compared with the Schwann cells without hTERT transfection group. Hematoxylin and eosin staining, PKH26 fluorescent labeling, and electrophysiological testing demonstrated that compared with the non-transfected group, spinal cord cavity and motor and sensory evoked potential latencies were reduced, while the number of PKH26-positive cells and the motor and sensory evoked potential amplitude increased at the site of injury in the hTERT-transfected group. These findings suggest that transplantation of hTERT gene-transfected Schwann cells repairs the structure and function of the injured spinal cord.

Gene therapy and peripheral nerve repair: a perspective

Clinical phase I/II studies have demonstrated the safety of gene therapy for a variety of central nervous system disorders, including Canavan's, Parkinson's (PD) and Alzheimer's disease (AD), retinal diseases and pain. The majority of gene therapy studies in the CNS have used adeno-associated viral vectors (AAV) and the first AAV-based therapeutic, a vector encoding lipoprotein lipase, is now marketed in Europe under the name Glybera. These remarkable advances may become relevant to translational research on gene therapy to promote peripheral nervous system (PNS) repair. This short review first summarizes the results of gene therapy in animal models for peripheral nerve repair. Secondly, we identify key areas of future research in the domain of PNS-gene therapy. Finally, a perspective is provided on the path to clinical translation of PNS-gene therapy for traumatic nerve injuries. In the latter section we discuss the route and mode of delivery of the vector to human patients, the efficacy and safety of the vector, and the choice of the patient population for a first possible proof-of-concept clinical study.

Nur77 Was Essential for Neurite Outgrowth and Involved in Schwann Cell Differentiation After Sciatic Nerve Injury

Nur77, together with Nurr1 and NOR-1, constitutes the NR4A subgroup of orphan nuclear receptors and plays critical roles in cell proliferation, differentiation, migration, and apoptosis. Among them, Nur77 is universally well known to contribute to neurite outgrowth. However, information regarding its regulation and possible function in the peripheral nervous system is still limited. In this study, we performed a sciatic nerve injury model in adult rats and detected an increased expression of Nur77 in the sciatic nerve, which was similar to the expression of Oct-6. Immunofluorescence indicated that Nur77 was located in both axons and Schwann cells. In vitro, we observed enhanced expression of Nur77 during the process of both basic fibroblast growth factor (bFGF)-induced Schwann cells differentiation and nerve growth factor (NGF)-induced PC12 cell neurite outgrowth. In vitro and in vivo experiments indicated that inhibiting the function of Nur77 by specific short hairpin RNA could depress Schwann cells myelinization and axons regeneration. Collectively, all these results suggested that upregulation of Nur77 might be involved in Schwann cells differentiation and neurite elongation following sciatic nerve crush.

Calcium alginate gels as stem cell matrix-making paracrine stem cell activity available for enhanced healing after surgery

Regeneration after surgery can be improved by the administration of anabolic growth factors. However, to locally maintain these factors at the site of regeneration is problematic. The aim of this study was to develop a matrix system containing human mesenchymal stem cells (MSCs) which can be applied to the surgical site and allows the secretion of endogenous healing factors from the cells. Calcium alginate gels were prepared by a combination of internal and external gelation. The gelling behaviour, mechanical stability, surface adhesive properties and injectability of the gels were investigated. The permeability of the gels for growth factors was analysed using bovine serum albumin and lysozyme as model proteins. Human MSCs were isolated, cultivated and seeded into the alginate gels. Cell viability was determined by AlamarBlue assay and fluorescence microscopy. The release of human VEGF and bFGF from the cells was determined using an enzyme-linked immunoassay. Gels with sufficient mechanical properties were prepared which remained injectable through a syringe and solidified in a sufficient time frame after application. Surface adhesion was improved by the addition of polyethylene glycol 300,000 and hyaluronic acid. Humans MSCs remained viable for the duration of 6 weeks within the gels. Human VEGF and bFGF was found in quantifiable concentrations in cell culture supernatants of gels loaded with MSCs and incubated for a period of 6 weeks. This work shows that calcium alginate gels can function as immobilization matrices for human MSCs.

Recent proteomic advances in developmental, regeneration, and cancer governing signaling pathways

Embryonic development, adult tissue repair, and cancer share a number of common regulating pathways. The basic processes that govern the events that induce mesenchymal properties in epithelial cells-a process known as epithelial-mesenchymal transition-are central for embryonic development, and can be resumed in adults either during wound healing or tissue regeneration. A misregulation of these pathways is involved in pathological situations, such as tissue fibrosis and cancer. Proteomic approaches have emerged as promising tools to better understand the signaling pathways that govern these complex biological processes. This review focuses on the recent proteomic-based contributions to better understand the modulation of transforming growth factor-beta (TGF-β), wingless-type MMTV integration site family (Wnt), Notch and Receptor tyrosine kinase (RTK) signaling pathways. New advances include the description of new protein interactions, the formation of new protein complexes or the description on how some PTMs are regulating these pathways. Understanding protein interactions and the tempo-spatial modulation of these pathways might lead us to interesting research quests in cancer, embryonic development or even on improving adult tissue regeneration capabilities.