The origin and development of glial cells in peripheral nerves

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

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

Implications of FBXW7 in Neurodevelopment and Neurodegeneration: Molecular Mechanisms and Therapeutic Potential

The ubiquitin-proteasome system (UPS) mediated protein degradation is crucial to maintain quantitive and functional homeostasis of diverse proteins. Balanced cellular protein homeostasis controlled by UPS is fundamental to normal neurological functions while impairment of UPS can also lead to some neurodevelopmental and neurodegenerative disorders. Functioning as the substrate recognition component of the SCF-type E3 ubiquitin ligase, FBXW7 is essential to multiple aspects of cellular processes via targeting a wide range of substrates for proteasome-mediated degradation. Accumulated evidence shows that FBXW7 is fundamental to neurological functions and especially implicated in neurodevelopment and the nosogenesis of neurodegeneration. In this review, we describe general features of FBXW7 gene and proteins, and mainly present recent findings that highlight the vital roles and molecular mechanisms of FBXW7 in neurodevelopment such as neurogenesis, myelination and cerebral vasculogenesis and in the pathogenesis of some typical neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease and Huntington’s disease. Additionally, we also provide a prospect on focusing FBXW7 as a potential therapeutic target to rescue neurodevelopmental and neurodegenerative impairment.

Diversity of developing peripheral glia revealed by single-cell RNA sequencing

The peripheral nervous system responds to a wide variety of sensory stimuli, a process that requires great neuronal diversity. These diverse neurons are closely associated with glial cells originating from the neural crest. However, the molecular nature and diversity among peripheral glia are not understood. Here, we used single-cell RNA sequencing to profile developing and mature glia from somatosensory dorsal root ganglia and auditory spiral ganglia. We found that glial precursors (GPs) in these two systems differ in their transcriptional profiles. Despite their unique features, somatosensory and auditory GPs undergo convergent differentiation to generate molecularly uniform myelinating and non-myelinating Schwann cells. By contrast, somatosensory and auditory satellite glial cells retain system-specific features. Lastly, we identified a glial signature gene set, providing new insights into commonalities among glia across the nervous system. This survey of gene expression in peripheral glia constitutes a resource for understanding functions of glia across different sensory modalities.

Humanized neurofibroma model from induced pluripotent stem cells delineates tumor pathogenesis and developmental origins

Neurofibromatosis type 1 (NF1) is a common tumor predisposition syndrome caused by NF1 gene mutation, in which affected patients develop Schwann cell lineage peripheral nerve sheath tumors (neurofibromas). To investigate human neurofibroma pathogenesis, we differentiated a series of isogenic, patient-specific NF1-mutant human induced pluripotent stem cells (hiPSCs) into Schwannian lineage cells (SLCs). We found that, although WT and heterozygous NF1-mutant hiPSCs-SLCs did not form tumors following mouse sciatic nerve implantation, NF1-null SLCs formed bona fide neurofibromas with high levels of SOX10 expression. To confirm that SOX10+ SLCs contained the cells of origin for neurofibromas, both Nf1 alleles were inactivated in mouse Sox10+ cells, leading to classic nodular cutaneous and plexiform neurofibroma formation that completely recapitulated their human counterparts. Moreover, we discovered that NF1 loss impaired Schwann cell differentiation by inducing a persistent stem-like state to expand the pool of progenitors required to initiate tumor formation, indicating that, in addition to regulating MAPK-mediated cell growth, NF1 loss also altered Schwann cell differentiation to promote neurofibroma development. Taken together, we established a complementary humanized neurofibroma explant and, to our knowledge, first-in-kind genetically engineered nodular cutaneous neurofibroma mouse models that delineate neurofibroma pathogenesis amenable to future therapeutic target discovery and evaluation.

GDNF to the rescue: GDNF delivery effects on motor neurons and nerves, and muscle re-innervation after peripheral nerve injuries

Peripheral nerve injuries commonly occur due to trauma, like a traffic accident. Peripheral nerves get severed, causing motor neuron death and potential muscle atrophy. The current golden standard to treat peripheral nerve lesions, especially lesions with large (≥ 3 cm) nerve gaps, is the use of a nerve autograft or reimplantation in cases where nerve root avulsions occur. If not tended early, degeneration of motor neurons and loss of axon regeneration can occur, leading to loss of function. Although surgical procedures exist, patients often do not fully recover, and quality of life deteriorates. Peripheral nerves have limited regeneration, and it is usually mediated by Schwann cells and neurotrophic factors, like glial cell line-derived neurotrophic factor, as seen in Wallerian degeneration. Glial cell line-derived neurotrophic factor is a neurotrophic factor known to promote motor neuron survival and neurite outgrowth. Glial cell line-derived neurotrophic factor is upregulated in different forms of nerve injuries like axotomy, sciatic nerve crush, and compression, thus creating great interest to explore this protein as a potential treatment for peripheral nerve injuries. Exogenous glial cell line-derived neurotrophic factor has shown positive effects in regeneration and functional recovery when applied in experimental models of peripheral nerve injuries. In this review, we discuss the mechanism of repair provided by Schwann cells and upregulation of glial cell line-derived neurotrophic factor, the latest findings on the effects of glial cell line-derived neurotrophic factor in different types of peripheral nerve injuries, delivery systems, and complementary treatments (electrical muscle stimulation and exercise). Understanding and overcoming the challenges of proper timing and glial cell line-derived neurotrophic factor delivery is paramount to creating novel treatments to tend to peripheral nerve injuries to improve patients’ quality of life.

Reprint of: Schwann cell precursors: Where they come from and where they go

Schwann cell precursors (SCPs) are a transient population in the embryo, closely associated with nerves along which they migrate into the periphery of the body. Long considered to be progenitors that only form Schwann cells-the myelinating cells of nerves, current evidence suggests that SCPs have much broader developmental potential. Indeed, different cell marking techniques employed over the past 20 years have identified multiple novel SCP derivatives throughout the body. It is now clear that SCPs represent a multipotent progenitor population, which also display a level of plasticity in response to injury. Moreover, they originate from multiple origins in the embryo and may reflect several distinct subpopulations in terms of molecular identity and fate. Here we review SCP origins, derivatives and plasticity in development, growth and repair.

Let-7a-5p regulated by lncRNA-MEG3 promotes functional differentiation to Schwann cells from adipose derived stem cells via directly inhibiting RBPJ-mediating Notch pathway

Schwann cells (SCs) have important roles in supporting and repairing peripheral neurons, and thus have great potential for nerve injury treatment. Adipose tissue-derived stem cells (ADSCs) can be reliably induced to differentiate into SCs. However, the underlying molecular mechanisms are unclear. We explored the roles of MEG3/let-7a-5p/RBPJ axis in the differentiation into SCs from ADSCs. Primary ADSCs were induced to differentiate into SCs by appropriate reagents. ELISA, immunostaining, Western blotting, and qRT-PCR were employed to examine levels of SC-markers such as S100, GFAP, SOX10, p75NTR, GAP43, MPZ, β-NGF, BDNF, and NCAM and let-7 family, MEG3, RBPJ, and Notch signaling related proteins. Dual luciferase assay and RNA immunoprecipitation were performed to validate interactions of let-7a-5p/RBPJ mRNA and MEG3/let-7a-5p. Cultured ADSCs could be induced to differentiate into functional SCs. Let-7a-5p and let-7d-5p were elevated during the differentiation while MEG3 and RBPJ/Notch-signaling were suppressed. Let-7a-5p mimics promoted ADSC differentiation into SCs and up-regulated the levels of SC-related markers including S100, GFAP, SOX10, p75NTR, GAP43, MPZ, β-NGF, and NCAM, while RBPJ or MEG3 overexpression retarded the differentiation and reduced those levels. Let-7a-5p directly targeted RBPJ and MEG3 disinhibited Notch-RBPJ signaling via sponging let-7a-5p. RBPJ overexpression reversed the acceleration of let-7a-5p mimics on SC differentiation while let-7a-5p mimics blocked MEG3-mediated suppression on SC differentiation. Let-7a-5p sponged by MEG3 promotes differentiation of ADSCs into SCs via suppressing Notch signaling by targeting RBPJ. These findings shed light on mechanisms underlying the differentiation of ADSCs to SCs and provide avenues to accelerate the process.

Optimizing Olfactory Ensheathing Cell Transplantation for Spinal Cord Injury Repair

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

Toward Understanding the Mechanisms of Malignant Peripheral Nerve Sheath Tumor Development

Malignant peripheral nerve sheath tumors (MPNSTs) originate from the neural crest lineage and are associated with the neurofibromatosis type I syndrome. MPNST is an unmet clinical need. In this review article, we summarize the knowledge and discuss research perspectives related to (1) the natural history of MPNST development; (2) the mouse models recapitulating the progression from precursor lesions to MPNST; (3) the role of the tumor microenvironment in MPNST development, and (4) the signaling pathways linked to MPNST development.

Insulin Promotes Schwann-Like Cell Differentiation of Rat Epidermal Neural Crest Stem Cells

Schwann cells (SCs) are considered potentially attractive candidates for transplantation therapies in neurodegenerative diseases. However, problems arising from the isolation and expansion of the SCs restrict their clinical applications. Establishing an alternative Schwann-like cell type is a prerequisite. Epidermal neural crest stem cells (EPI-NCSCs) are well studied for their autologous accessibility, along with the ability to produce major neural crest derivatives and neurotrophic factors. In the current study, we explored insulin influence, a well-known growth factor, on directing EPI-NCSCs into the Schwann cell (SC) lineage. EPI-NCSCs were isolated from rat hair bulge explants. The viability of cells treated with a range of insulin concentrations (0.05-100 μg/ml) was defined by MTT assay at 24, 48, and 72 h. The gene expression profiles of neurotrophic factors (BDNF, FGF-2, and IL-6), key regulators involved in the development of SC (EGR-1, SOX-10, c-JUN, GFAP, OCT-6, EGR-2, and MBP), and oligodendrocyte (PDGFR-α and NG-2) were quantified 1 and 9 days post-treatment with 0.05 and 5 μg/ml insulin. Furthermore, the protein expression of nestin (stemness marker), SOX-10, PDGFR-α, and MBP was analyzed following the long-term insulin treatment. Insulin downregulated the early-stage SC differentiation marker (EGR-1) and increased neurotrophins (BDNF and IL-6) and pro-myelinating genes, including OCT-6, SOX-10, EGR-2, and MBP, as well as oligodendrocyte differentiation markers, upon exposure for 9 days. Insulin can promote EPI-NCSC differentiation toward SC lineage and possibly oligodendrocytes. Thus, employing insulin might enhance the EPI-NCSCs efficiency in cell transplantation strategies.

Dynamic early clusters of nodal proteins contribute to node of Ranvier assembly during myelination of peripheral neurons

Voltage-gated sodium channels cluster in macromolecular complexes at nodes of Ranvier to promote rapid nerve impulse conduction in vertebrate nerves. Node assembly in peripheral nerves is thought to be initiated at heminodes at the extremities of myelinating Schwann cells, and fusion of heminodes results in the establishment of nodes. Here we show that assembly of 'early clusters' of nodal proteins in the murine axonal membrane precedes heminode formation. The neurofascin (Nfasc) proteins are essential for node assembly, and the formation of early clusters also requires neuronal Nfasc. Early clusters are mobile and their proteins are dynamically recruited by lateral diffusion. They can undergo fusion not only with each other but also with heminodes, thus contributing to the development of nodes in peripheral axons. The formation of early clusters constitutes the earliest stage in peripheral node assembly and expands the repertoire of strategies that have evolved to establish these essential structures.

A tuned gelatin methacryloyl (GelMA) hydrogel facilitates myelination of dorsal root ganglia neurons in vitro

Investigating axonal myelination by Schwann cells (SCs) is crucial for understanding mechanisms underlying demyelination and remyelination, which may help gain insights into incurable disorders like neurodegenerative diseases. In this study, a gelatin-based hydrogel, gelatin methacryloyl (GelMA), was optimized to achieve the biocompatibility, porosity, mechanical stability, and degradability needed to provide high cell viability for dorsal root ganglia (DRG) neurons and SCs, and to enable their long-term coculture needed for myelination studies. The results of cell viability, neurite elongation, SC function and maturation, SC-axon interaction, and myelination were compared with two other commonly used substrates, namely collagen and Poly-d Lysine (PDL). The tuned GelMA constructs (Young's modulus of 32.6 ± 1.9 kPa and the median value of pore size of 10.3 μm) enhanced single axon generation (unlike collagen) and promoted the interaction of DRG neurons and SCs (unlike PDL). While DRG cells exhibited relatively higher viability on PDL after 48 h, i.e., 83.8%, the cells had similar survival rate on GelMA and collagen substrates, 66.7% and 61.5%, respectively. Further adjusting the hydrogel properties to achieve two distinct ranges of relatively small and large pores supported SCs to extend their processes freely and enabled physical contact with and wrapping around their corresponding axons. Staining the cells with myelin basic protein (MBA) and myelin-associated glycoprotein (MAG) revealed enhanced myelination on GelMA hydrogel compared to PDL and collagen. Moreover, the engineered porosity enhanced DRGs and SCs attachments and flexibility of movement across the substrate. This engineered hydrogel structure can now be further explored to model demyelination in neurodegenerative diseases, as well as to study the effects of various compounds on myelin regeneration.

Intracellular potassium depletion enhances apoptosis induced by staurosporine in cultured trigeminal satellite glial cells

PURPOSE

Satellite glial cells (SGC) surrounding neurons in sensory ganglia can buffer extracellular potassium, regulating the excitability of injured neurons and possibly influencing a shift from acute to neuropathic pain. SGC apoptosis may be a key component in this process. This work evaluated induction or enhancement of apoptosis in cultured trigeminal SGC following changes in intracellular potassium [K]ic.

MATERIALS AND METHODS

We developed SGC primary cultures from rat trigeminal ganglia (TG). Purity of our cultures was confirmed using immunofluorescence and western blot analysis for the presence of the specific marker of SGC, glutamine synthetase (GS). SGC [K]ic was depleted using hypo-osmotic shock and 4 mM bumetanide plus 10 mM ouabain. [K]ic was measured using the K⁺ fluorescent indicator potassium benzofuran isophthalate (PBFI-AM).

RESULTS

SGC tested positive for GS and hypo-osmotic shock induced a significant decrease in [K]ic at every evaluated time. Cells were then incubated for 5 h with either 2 mM staurosporine (STS) or 20 ng/ml of TNF-α and evaluated for early apoptosis and late apoptosis/necrosis by flow cytometry using annexin V and propidium iodide. A significant increase in early apoptosis, from 16 to 38%, was detected in SGC with depleted [K]ic after incubation with STS. In contrast, TNF-α did not increase early apoptosis in normal or [K]ic depleted SGC.

CONCLUSION

Hypo-osmotic shock induced a decrease in intracellular potassium in cultured trigeminal SGC and this enhanced apoptosis induced by STS that is associated with the mitochondrial pathway. These results suggest that K⁺ dysregulation may underlie apoptosis in trigeminal SGC.

Schwann cell precursors generate sympathoadrenal system during zebrafish development

The autonomic portion of the peripheral nervous system orchestrates tissue homeostasis through direct innervation of internal organs, and via release of adrenalin and noradrenalin into the blood flow. The developmental mechanisms behind the formation of autonomic neurons and chromaffin cells are not fully understood. Using genetic tracing, we discovered that a significant proportion of sympathetic neurons in zebrafish originates from Schwann cell precursors (SCPs) during a defined period of embryonic development. Moreover, SCPs give rise to the main portion of the chromaffin cells, as well as to a significant proportion of enteric and other autonomic neurons associated with internal organs. The conversion of SCPs into neuronal and chromaffin cells is ErbB receptor dependent, as the pharmacological inhibition of the ErbB pathway effectively perturbed this transition. Finally, using genetic ablations, we revealed that SCPs producing neurons and chromaffin cells migrate along spinal motor axons to reach appropriate target locations. This study reveals the evolutionary conservation of SCP-to-neuron and SCP-to-chromaffin cell transitions over significant growth periods in fish and highlights relevant cellular-genetic mechanisms. Based on this, we anticipate that multipotent SCPs might be present in postnatal vertebrate tissues, retaining the capacity to regenerate autonomic neurons and chromaffin cells.

Mek/ERK1/2-MAPK and PI3K/Akt/mTOR signaling plays both independent and cooperative roles in Schwann cell differentiation, myelination and dysmyelination

Multiple signals are involved in the regulation of developmental myelination by Schwann cells and in the maintenance of a normal myelin homeostasis throughout adult life, preserving the integrity of the axons in the PNS. Recent studies suggest that Mek/ERK1/2-MAPK and PI3K/Akt/mTOR intracellular signaling pathways play important, often overlapping roles in the regulation of myelination in the PNS. In addition, hyperactivation of these signaling pathways in Schwann cells leads to a late onset of various pathological changes in the sciatic nerves. However, it remains poorly understood whether these pathways function independently or sequentially or converge using a common mechanism to facilitate Schwann cell differentiation and myelin growth during development and in causing pathological changes in the adult animals. To address these questions, we analyzed multiple genetically modified mice using simultaneous loss- and constitutive gain-of-function approaches. We found that during development, the Mek/ERK1/2-MAPK pathway plays a primary role in Schwann cell differentiation, distinct from mTOR. However, during active myelination, ERK1/2 is dependent on mTOR signaling to drive the growth of the myelin sheath and regulate its thickness. Finally, our data suggest that peripheral nerve pathology during adulthood caused by hyperactivation of Mek/ERK1/2-MAPK or PI3K is likely to be independent or dependent on mTOR-signaling in different contexts. Thus, this study highlights the complexities in the roles played by two major intracellular signaling pathways in Schwann cells that affect their differentiation, myelination, and later PNS pathology and predicts that potential therapeutic modulation of these pathways in PNS neuropathies could be a complex process.

Phenotypical peculiarities and species-specific differences of canine and murine satellite glial cells of spinal ganglia

Satellite glial cells (SGCs) are located in the spinal ganglia (SG) of the peripheral nervous system and tightly envelop each neuron. They preserve tissue homeostasis, protect neurons and react in response to injury. This study comparatively characterizes the phenotype of murine (mSGCs) and canine SGCs (cSGCs). Immunohistochemistry and immunofluorescence as well as 2D and 3D imaging techniques were performed to describe a SGC-specific marker panel, identify potential functional subsets and other phenotypical, species-specific peculiarities. Glutamine synthetase (GS) and the potassium channel Kir 4.1 are SGC-specific markers in murine and canine SG. Furthermore, a subset of mSGCs showed CD45 immunoreactivity and the majority of mSGCs were immunopositive for neural/glial antigen 2 (NG2), indicating an immune and a progenitor cell character. The majority of cSGCs were immunopositive for glial fibrillary acidic protein (GFAP), 2',3'-cyclic-nucleotide 3'-phosphodiesterase (CNPase) and Sox2. Therefore, cSGCs resemble central nervous system glial cells and progenitor cells. SGCs lacked expression of macrophage markers CD107b, Iba1 and CD204. Double labelling with GS/Kir 4.1 highlights the unique anatomy of SGC-neuron units and emphasizes the indispensability of further staining and imaging techniques for closer insights into the specific distribution of markers and potential colocalizations.

Vitamin C regulates Schwann cell myelination by promoting DNA demethylation of pro-myelinating genes

Ascorbic acid (vitamin C) is critical for Schwann cells to myelinate peripheral nerve axons during development and remyelination after injury. However, its exact mechanism remains elusive. Vitamin C is a dietary nutrient that was recently discovered to promote active DNA demethylation. Schwann cell myelination is characterized by global DNA demethylation in vivo and may therefore be regulated by vitamin C. We found that vitamin C induces a massive transcriptomic shift (n = 3,848 genes) in primary cultured Schwann cells while simultaneously producing a global increase in genomic 5-hydroxymethylcytosine (5hmC), a DNA demethylation intermediate which regulates transcription. Vitamin C up-regulates 10 pro-myelinating genes which exhibit elevated 5hmC content in both the promoter and gene body regions of these loci following treatment. Using a mouse model of human vitamin C metabolism, we found that maternal dietary vitamin C deficiency causes peripheral nerve hypomyelination throughout early development in resulting offspring. Additionally, dietary vitamin C intake regulates the expression of myelin-related proteins such as periaxin (PRX) and myelin basic protein (MBP) during development and remyelination after injury in mice. Taken together, these results suggest that vitamin C cooperatively promotes myelination through 1) increased DNA demethylation and transcription of pro-myelinating genes, and 2) its known role in stabilizing collagen helices to form the basal lamina that is necessary for myelination.

The Effects of Insulin on Immortalized Rat Schwann Cells, IFRS1

Schwann cells play an important role in peripheral nerve function, and their dysfunction has been implicated in the pathogenesis of diabetic neuropathy and other demyelinating diseases. The physiological functions of insulin in Schwann cells remain unclear and therefore define the aim of this study. By using immortalized adult Fischer rat Schwann cells (IFRS1), we investigated the mechanism of the stimulating effects of insulin on the cell proliferation and expression of myelin proteins (myelin protein zero (MPZ) and myelin basic protein (MBP). The application of insulin to IFRS1 cells increased the proliferative activity and induced phosphorylation of Akt and ERK, but not P38-MAPK. The proliferative potential of insulin-stimulated IFRS1 was significantly suppressed by the addition of LY294002, a PI3 kinase inhibitor. The insulin-stimulated increase in MPZ expression was significantly suppressed by the addition of PD98059, a MEK inhibitor. Furthermore, insulin-increased MBP expression was significantly suppressed by the addition of LY294002. These findings suggest that both PI3-K/Akt and ERK/MEK pathways are involved in insulin-induced cell growth and upregulation of MPZ and MBP in IFRS1 Schwann cells.

Retracing Schwann Cell Developmental Transitions in Embryonic Dissociated DRG/Schwann Cell Cocultures in Mice

Embryonic Dissociated Dorsal Root Ganglia (DRG) cultures are often used to investigate the role of novel molecular pathways or drugs in Schwann cell development and myelination. These cultures largely recapitulate the order of cellular and molecular events that occur in Schwann cells of embryonic nerves. However, the timing of Schwann cell developmental transitions, notably the transition from Schwann Cell Precursors (SCP) to immature Schwann cells (iSC) and then to myelinating Schwann cells, has not been estimated so far in this culture system. In this study, we determined the expression profiles of Schwann cell developmental genes during the first week of culture and then compared our data to the expression profiles of these genes in developing spinal nerves. This helped in identifying that SCP transition into iSC between the 5th and 7th day in vitro. Furthermore, we also investigated the transition of immature cells into pro-myelinating and myelinating Schwann cells upon the induction of myelination in vitro. Our results suggest that Schwann cell differentiation beyond the immature stage can be observed as early as 4 days post the induction of myelination in cocultures. Finally, we compared the myelinating potential of coculture-derived Schwann cell monocultures to cultures established from neonatal sciatic nerves and found that both these culture systems exhibit similar myelinating phenotypes. In effect, our results allow for a better understanding and interpretation of coculture experiments especially in studies that aim to elucidate the role of a novel actor in Schwann cell development and myelination.

The Expression of Chemokines Is Downregulated in a Pre-Clinical Model of TTR V30M Amyloidosis

Inflammation is a hallmark of several neurodegenerative disorders including hereditary amyloidogenic transthyretin amyloidosis (ATTRv). ATTRv is an autosomal dominant neurodegenerative disorder with extracellular deposition of mutant transthyretin (TTR) aggregates and fibrils, particularly in nerves and ganglia of the peripheral nervous system. Nerve biopsies from ATTRv patients show increased cytokine production, but interestingly no immune inflammatory cellular infiltrate is observed around TTR aggregates. Here we show that as compared to Wild Type (WT) animals, the expression of several chemokines is highly downregulated in the peripheral nervous system of a mouse model of the disease. Interestingly, we found that stimulation of mouse Schwann cells (SCs) with WT TTR results in the secretion of several chemokines, a process that is mediated by toll-like receptor 4 (TLR4). In contrast, the secretion of all tested chemokines is compromised upon stimulation of SCs with mutant TTR (V30M), suggesting that V30M TTR fails to activate TLR4 signaling. Altogether, our data shed light into a previously unappreciated mechanism linking TTR activation of SCs and possibly underlying the lack of inflammatory response observed in the peripheral nervous system of ATTRv patients.

Schwann cell precursors: Where they come from and where they go

Schwann cell precursors (SCPs) are a transient population in the embryo, closely associated with nerves along which they migrate into the periphery of the body. Long considered to be progenitors that only form Schwann cells-the myelinating cells of nerves, current evidence suggests that SCPs have much broader developmental potential. Indeed, different cell marking techniques employed over the past 20 years have identified multiple novel SCP derivatives throughout the body. It is now clear that SCPs represent a multipotent progenitor population, which also display a level of plasticity in response to injury. Moreover, they originate from multiple origins in the embryo and may reflect several distinct subpopulations in terms of molecular identity and fate. Here we review SCP origins, derivatives and plasticity in development, growth and repair.

Transcriptional profiling of mouse peripheral nerves to the single-cell level to build a sciatic nerve ATlas (SNAT)

Peripheral nerves are organ-like structures containing diverse cell types to optimize function. This interactive assembly includes mostly axon-associated Schwann cells, but also endothelial cells of supporting blood vessels, immune system-associated cells, barrier-forming cells of the perineurium surrounding and protecting nerve fascicles, and connective tissue-resident cells within the intra-fascicular endoneurium and inter-fascicular epineurium. We have established transcriptional profiles of mouse sciatic nerve-inhabitant cells to foster the fundamental understanding of peripheral nerves. To achieve this goal, we have combined bulk RNA sequencing of developing sciatic nerves up to the adult with focused bulk and single-cell RNA sequencing of Schwann cells throughout postnatal development, extended by single-cell transcriptome analysis of the full sciatic nerve both perinatally and in the adult. The results were merged in the transcriptome resource Sciatic Nerve ATlas (SNAT: https://www.snat.ethz.ch). We anticipate that insights gained from our multi-layered analysis will serve as valuable interactive reference point to guide future studies.

Genetic Events and Signaling Mechanisms Underlying Schwann Cell Fate in Development and Cancer

In this review, we describe Schwann cell development from embryonic neural crest cells to terminally differentiated myelinated and nonmyelinated mature Schwann cells. We focus on the genetic drivers and signaling mechanisms mediating decisions to proliferate versus differentiate during Schwann cell development, highlighting pathways that overlap with Schwann cell development and are dysregulated in tumorigenesis. We conclude by considering how our knowledge of the events underlying Schwann cell development and mouse models of schwannoma, neurofibroma, and malignant peripheral nerve sheath tumor can inform novel therapeutic strategies for patients with cancers derived from Schwann cell lineages.

Activation of skeletal muscle-resident glial cells upon nerve injury

Here, we report on the identification of Itga7-expressing muscle-resident glial cells activated by loss of neuromuscular junction (NMJ) integrity. Gene expression analysis at the bulk and single-cell level revealed that these cells are distinct from Itga7-expressing muscle satellite cells. We show that a selective activation and expansion of Itga7+ glial cells occur in response to muscle nerve lesion. Upon activation, muscle glial-derived progenies expressed neurotrophic genes, including nerve growth factor receptor, which enables their isolation by FACS. We show that activated muscle glial cells also expressed genes potentially implicated in extracellular matrix remodeling at NMJs. We found that tenascin C, which was highly expressed by muscle glial cells, activated upon nerve injury and preferentially localized to NMJ. Interestingly, we observed that the activation of muscle glial cells by acute nerve injury was reversible upon NMJ repair. By contrast, in a mouse model of ALS, in which NMJ degeneration is progressive, muscle glial cells steadily increased over the course of the disease. However, they exhibited an impaired neurotrophic activity, suggesting that pathogenic activation of glial cells may be implicated in ALS progression.

Completion of neuronal remodeling prompts myelination along developing motor axon branches

Neuronal remodeling and myelination are two fundamental processes during neurodevelopment. How they influence each other remains largely unknown, even though their coordinated execution is critical for circuit function and often disrupted in neuropsychiatric disorders. It is unclear whether myelination stabilizes axon branches during remodeling or whether ongoing remodeling delays myelination. By modulating synaptic transmission, cytoskeletal dynamics, and axonal transport in mouse motor axons, we show that local axon remodeling delays myelination onset and node formation. Conversely, glial differentiation does not determine the outcome of axon remodeling. Delayed myelination is not due to a limited supply of structural components of the axon-glial unit but rather is triggered by increased transport of signaling factors that initiate myelination, such as neuregulin. Further, transport of promyelinating signals is regulated via local cytoskeletal maturation related to activity-dependent competition. Our study reveals an axon branch-specific fine-tuning mechanism that locally coordinates axon remodeling and myelination.

Terminal Schwann Cell Aging: Implications for Age-Associated Neuromuscular Dysfunction

Action potential is transmitted to muscle fibers through specialized synaptic interfaces called neuromuscular junctions (NMJs). These structures are capped by terminal Schwann cells (tSCs), which play essential roles during formation and maintenance of the NMJ. tSCs are implicated in the correct communication between nerves and muscles, and in reinnervation upon injury. During aging, loss of muscle mass and strength (sarcopenia and dynapenia) are due, at least in part, to the progressive loss of contacts between muscle fibers and nerves. Despite the important role of tSCs in NMJ function, very little is known on their implication in the NMJ-aging process and in age-associated denervation. This review summarizes the current knowledge about the implication of tSCs in the age-associated degeneration of NMJs. We also speculate on the possible mechanisms underlying the observed phenotypes.

Cannabidiol and Other Cannabinoids in Demyelinating Diseases

A growing body of preclinical evidence indicates that certain cannabinoids, including cannabidiol (CBD) and synthetic derivatives, may play a role in the myelinating processes and are promising small molecules to be developed as drug candidates for management of demyelinating diseases such as multiple sclerosis (MS), stroke and traumatic brain injury (TBI), which are three of the most prevalent demyelinating disorders. Thanks to the properties described for CBD and its interesting profile in humans, both the phytocannabinoid and derivatives could be considered as potential candidates for clinical use. In this review we will summarize current advances in the use of CBD and other cannabinoids as future potential treatments. While new research is accelerating the process for the generation of novel drug candidates and identification of druggable targets, the collaboration of key players such as basic researchers, clinicians and pharmaceutical companies is required to bring novel therapies to the patients.

Pronociceptive Roles of Schwann Cell-Derived Galectin-3 in Taxane-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a severe dose-limiting side effect of taxanes such as paclitaxel and docetaxel. Despite the high medical needs, insufficient understanding of the complex mechanism underlying CIPN pathogenesis precludes any endorsed causal therapy to prevent or relieve CIPN. In this study, we report that elevation of plasma galectin-3 level is a pathologic change common to both patients with taxane-treated breast cancer with CIPN and a mouse model of taxane-related CIPN. Following multiple intraperitoneal injections of paclitaxel in mice, galectin-3 levels were elevated in Schwann cells within the sciatic nerve but not in other peripheral organs or cells expressing galectin-3. Consistent with this, paclitaxel treatment of primary cultures of rat Schwann cells induced upregulation and secretion of galectin-3. In vitro migration assays revealed that recombinant galectin-3 induced a chemotactic response of the murine macrophage cell line RAW 264.7. In addition, perineural administration of galectin-3 to the sciatic nerve of naive mice mimicked paclitaxel-induced macrophage infiltration and mechanical hypersensitivity. By contrast, chemical depletion of macrophages by clodronate liposomes suppressed paclitaxel-induced mechanical hypersensitivity despite the higher level of plasma galectin-3. Deficiency (Galectin-3 ^-/- mice) or pharmacologic inhibition of galectin-3 inhibited paclitaxel-induced macrophage infiltration and mechanical hypersensitivity. In conclusion, we propose that Schwann cell-derived galectin-3 plays a pronociceptive role via macrophage infiltration in the pathogenesis of taxane-induced peripheral neuropathy. Therapies targeting this phenomenon, which is common to patients with CIPN and mouse models, represent a novel approach to suppress taxane-related CIPN. SIGNIFICANCE: These findings demonstrate that the elevation of plasma galectin-3 is a CIPN-related pathologic change common to humans and mice, and that targeting galectin-3 is a therapeutic option to delay CIPN progression.

Challenges and solutions for fabrication of three-dimensional cocultures of neural cell-loaded biomimetic constructs

Fabrication of three-dimensional (3D) constructs to model body tissues and organs can contribute to research into tissue development and models for studying disease, as well as supporting preclinical drug screening in vitro. Furthermore, 3D constructs can also be used for diagnosis and therapy of disease conditions via lab on a chip and microarrays for diagnosis and engineered products for tissue repair, replacement, and regeneration. While cell culture approaches for studying tissue development and disease in two dimensions are long-established, the translation of this knowledge into 3D environments remains a fertile field of research. In this Tutorial, we specifically focus on the application of biosynthetic hydrogels for neural cell encapsulation. The Tutorial briefly covers background on using biosynthetic hydrogels for cell encapsulation, as well as common fabrication techniques. The Methods section focuses on the hydrogel design and characterization, highlighting key elements and tips for more effective approaches. Coencapsulation of different cell types, and the challenges associated with different growth and maintenance requirements, is the main focus of this Tutorial. Much care is needed to blend different cell types, and this Tutorial provides tips and insights that have proven successful for 3D coculture in biosynthetic hydrogels.

Spinal cord precursors utilize neural crest cell mechanisms to generate hybrid peripheral myelinating glia

During development, oligodendrocytes and Schwann cells myelinate central and peripheral nervous system axons, respectively, while motor exit point (MEP) glia are neural tube-derived, peripheral glia that myelinate axonal territory between these populations at MEP transition zones. From which specific neural tube precursors MEP glia are specified, and how they exit the neural tube to migrate onto peripheral motor axons, remain largely unknown. Here, using zebrafish, we found that MEP glia arise from lateral floor plate precursors and require foxd3 to delaminate and exit the spinal cord. Additionally, we show that similar to Schwann cells, MEP glial development depends on axonally derived neuregulin1. Finally, our data demonstrate that overexpressing axonal cues is sufficient to generate additional MEP glia in the spinal cord. Overall, these studies provide new insight into how a novel population of hybrid, peripheral myelinating glia are generated from neural tube precursors and migrate into the periphery.

Satellite Glial Cells and Astrocytes, a Comparative Review

Astroglia are neural cells, heterogeneous in form and function, which act as supportive elements of the central nervous system; astrocytes contribute to all aspects of neural functions in health and disease. Through their highly ramified processes, astrocytes form close physical contacts with synapses and blood vessels, and are integrated into functional syncytia by gap junctions. Astrocytes interact among themselves and with other cells types (e.g., neurons, microglia, blood vessel cells) by an elaborate repertoire of chemical messengers and receptors; astrocytes also influence neural plasticity and synaptic transmission through maintaining homeostasis of neurotransmitters, K⁺ buffering, synaptic isolation and control over synaptogenesis and synaptic elimination. Satellite glial cells (SGCs) are the most abundant glial cells in sensory ganglia, and are believed to play major roles in sensory functions, but so far research into SGCs attracted relatively little attention. In this review we compare SGCs to astrocytes with the purpose of using the vast knowledge on astrocytes to explore new aspects of SGCs. We survey the main properties of these two cells types and highlight similarities and differences between them. We conclude that despite the much greater diversity in morphology and signaling mechanisms of astrocytes, there are some parallels between them and SGCs. Both types serve as boundary cells, separating different compartments in the nervous system, but much more needs to be learned on this aspect of SGCs. Astrocytes and SGCs employ chemical messengers and calcium waves for intercellular signaling, but their significance is still poorly understood for both cell types. Both types undergo major changes under pathological conditions, which have a protective function, but an also contribute to disease, and chronic pain in particular. The knowledge obtained on astrocytes is likely to benefit future research on SGCs.

Mesenchymal Stem Cells Derived from Wharton's Jelly Can Differentiate into Schwann Cell-Like Cells and Promote Peripheral Nerve Regeneration in Acellular Nerve Grafts

BACKGROUND

Schwann cells (SCs) secrete neurotrophic factors and provide structural support and guidance during axonal regeneration. However, nearby nerves may be damaged to obtain primary SCs, and there is a lack of nervous tissue donors. We investigated the potential of Wharton's Jelly-derived mesenchymal stem cells (WJ-MSCs) in differentiating into Schwann cell-like cells (WJ-SCLCs) as an alternative to SCs. We also examined whether implantation of WJ-SCLCs-laden acellular nerve grafts (ANGs) are effective in inducing functional recovery and nerve regeneration in an animal model of peripheral nerve injury.

METHODS

The differentiation of WJ-MSCs into WJ-SCLCs was determined by analyzing SC-specific markers. The secretion of neurotrophic factors was assessed by the Neuro Discovery antibody array. Neurite outgrowth and myelination of axons were found in a co-culture system involving motor neuron cell lines. The effects of ANGs on repairing sciatic nerves were evaluated using video gait angle test, isometric tetanic force analysis, and toluidine blue staining.

RESULTS

Compared with undifferentiated WJ-MSCs, WJ-SCLCs showed higher expression levels of SC-specific markers such as S100β, GFAP, KROX20, and NGFR. WJ-SCLCs also showed higher secreted amounts of brain-derived neurotrophic factor, glial cell-derived neurotrophic factor, and granulocyte-colony stimulating factor than did WJ-MSCs. WJ-SCLCs effectively promoted the outgrowth and myelination of neurites in motor neuron cells, and WJ-SCLCs laden ANGs significantly facilitated peripheral nerve regeneration in an animal model of sciatic nerve injury.

CONCLUSION

WJ-MSCs were readily differentiated into WJ-SCLCs, which effectively promoted the regeneration of peripheral nerves. Transplantation of WJ-SCLCs with ANGs might be useful for assisting peripheral nerve regeneration.

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.

Acidic fibroblast growth factor attenuates type 2 diabetes-induced demyelination via suppressing oxidative stress damage

Prolonged type 2 diabetes mellitus (T2DM) produces a common complication, peripheral neuropathy, which is accompanied by nerve fiber disorder, axon atrophy, and demyelination. Growing evidence has characterized the beneficial effects of acidic fibroblast growth factor (aFGF) and shown that it relieves hyperglycemia, increases insulin sensitivity, and ameliorates neuropathic impairment. However, there is scarce evidence on the role of aFGF on remodeling of aberrant myelin under hyperglycemia condition. Presently, we observed that the expression of aFGF was rapidly decreased in a db/db T2DM mouse model. Administration of exogenous aFGF was sufficient to block acute demyelination and nerve fiber disorganization. Furthermore, this strong anti-demyelinating effect was most likely dominated by an aFGF-mediated increase of Schwann cell (SC) proliferation and migration as well as suppression of its apoptosis. Mechanistically, the beneficial biological effects of aFGF on SC behavior and abnormal myelin morphology were likely due to the inhibition of hyperglycemia-induced oxidative stress activation, which was most likely activated by kelch-like ECH-associated protein 1 (Keap1)/nuclear factor erythroid-derived-like 2 (Nrf2) signaling. Thus, this evidence indicates that aFGF is a promising protective agent for relieving myelin pathology through countering oxidative stress signaling cascades under diabetic conditions.

Androgen action in cell fate and communication during prostate development at single-cell resolution

Androgens/androgen receptor (AR)-mediated signaling pathways are essential for prostate development, morphogenesis and regeneration. Specifically, stromal AR signaling has been shown to be essential for prostatic initiation. However, the molecular mechanisms underlying AR-initiated mesenchymal-epithelial interactions in prostate development remain unclear. Here, using a newly generated mouse model, we have directly addressed the fate and role of genetically marked AR-expressing cells during embryonic prostate development. Androgen signaling-initiated signaling pathways were identified in mesenchymal niche populations at single-cell transcriptomic resolution. The dynamic cell-signaling networks regulated by stromal AR were additionally characterized in relation to prostatic epithelial bud formation. Pseudotime analyses further revealed the differentiation trajectory and fate of AR-expressing cells in both prostatic mesenchymal and epithelial cell populations. Specifically, the cellular properties of Zeb1-expressing progenitors were assessed. Selective deletion of AR signaling in a subpopulation of mesenchymal rather than epithelial cells dysregulated the expression of the master regulators and significantly impaired prostatic bud formation. These data provide novel, high-resolution evidence demonstrating the important role of mesenchymal androgen signaling in the cellular niche controlling prostate early development by initiating dynamic mesenchyme-epithelia cell interactions.

Neuronal lineages derived from the nerve-associated Schwann cell precursors

For a long time, neurogenic placodes and migratory neural crest cells were considered the immediate sources building neurons of peripheral nervous system. Recently, a number of discoveries revealed the existence of another progenitor type-a nerve-associated multipotent Schwann cell precursors (SCPs) building enteric and parasympathetic neurons as well as neuroendocrine chromaffin cells. SCPs are neural crest-derived and are similar to the crest cells by their markers and differentiation potential. Such similarities, but also considerable differences, raise many questions pertaining to the medical side, fundamental developmental biology and evolution. Here, we discuss the genesis of Schwann cell precursors, their role in building peripheral neural structures and ponder on their role in the origin in congenial diseases associated with peripheral nervous systems.

Schwann cell remyelination of the central nervous system: why does it happen and what are the benefits?

Myelin sheaths, by supporting axonal integrity and allowing rapid saltatory impulse conduction, are of fundamental importance for neuronal function. In response to demyelinating injuries in the central nervous system (CNS), oligodendrocyte progenitor cells (OPCs) migrate to the lesion area, proliferate and differentiate into new oligodendrocytes that make new myelin sheaths. This process is termed remyelination. Under specific conditions, demyelinated axons in the CNS can also be remyelinated by Schwann cells (SCs), the myelinating cell of the peripheral nervous system. OPCs can be a major source of these CNS-resident SCs-a surprising finding given the distinct embryonic origins, and physiological compartmentalization of the peripheral and central nervous system. Although the mechanisms and cues governing OPC-to-SC differentiation remain largely undiscovered, it might nevertheless be an attractive target for promoting endogenous remyelination. This article will (i) review current knowledge on the origins of SCs in the CNS, with a particular focus on OPC to SC differentiation, (ii) discuss the necessary criteria for SC myelination in the CNS and (iii) highlight the potential of using SCs for myelin regeneration in the CNS.

Xenogeneic Decellularized Extracellular Matrix-based Biomaterials For Peripheral Nerve Repair and Regeneration

Peripheral nerve injury could lead to either impairment or a complete loss of function for affected patients, and a variety of nerve repair materials have been developed for surgical approaches to repair it. Although autologous or autologous tissue-derived biomaterials remain preferred treatment for peripheral nerve injury, the lack of donor sources has led biomedical researchers to explore more other biomaterials. As a reliable alternative, xenogeneic decellularized extracellular matrix (dECM)-based biomaterials have been widely employed for surgical nerve repair. The dECM derived from animal donors is an attractive and unlimited source for xenotransplantation. Meanwhile, as an increasingly popular technique, decellularization could retain a variety of bioactive components in native ECM, such as polysaccharides, proteins, and growth factors. The resulting dECM-based biomaterials preserve a tissue's native microenvironment, promote Schwann cells proliferation and differentiation, and provide cues for nerve regeneration. Although the potential of dECM-based biomaterials as a therapeutic agent is rising, there are many limitations of this material restricting its use. Herein, this review discusses the decellularization techniques that have been applied to create dECM-based biomaterials, the main components of nerve ECM, and the recent progress in the utilization of xenogeneic dECM-based biomaterials through applications as a hydrogel, wrap, and guidance conduit in nerve tissue engineering. In the end, the existing bottlenecks of xenogeneic dECM-based biomaterials and developing technologies that could be eliminated to be helpful for utilization in the future have been elaborated.

αV integrins in Schwann cells promote attachment to axons, but are dispensable in vivo

In the developing peripheral nervous system, Schwann cells (SCs) extend their processes to contact, sort, and myelinate axons. The mechanisms that contribute to the interaction between SCs and axons are just beginning to be elucidated. Using a SC-neuron coculture system, we demonstrate that Arg-Gly-Asp (RGD) peptides that inhibit αV -containing integrins delay the extension of SCs elongating on axons. αV integrins in SC localize to sites of contact with axons and are expressed early in development during radial sorting and myelination. Short interfering RNA-mediated knockdown of the αV integrin subunit also delays SC extension along axons in vitro, suggesting that αV -containing integrins participate in axo-glial interactions. However, mice lacking the αV subunit in SCs, alone or in combination with the potentially compensating α5 subunit, or the αV partners β3 or β8 , myelinate normally during development and remyelinate normally after nerve crush, indicating that overlapping or compensatory mechanisms may hide the in vivo role of RGD-binding integrins.

Single-cell RNA sequencing in vision research: Insights into human retinal health and disease

Gene expression provides valuable insight into cell function. As such, vision researchers have frequently employed gene expression studies to better understand retinal physiology and disease. With the advent of single-cell RNA sequencing, expression experiments provide an unparalleled resolution of information. Instead of studying aggregated gene expression across all cells in a heterogenous tissue, single-cell technology maps RNA to an individual cell, which facilitates grouping of retinal and choroidal cell types for further study. Single-cell RNA sequencing has been quickly adopted by both basic and translational vision researchers, and single-cell level gene expression has been studied in the visual systems of animal models, retinal organoids, and primary human retina, RPE, and choroid. These experiments have generated detailed atlases of gene expression and identified new retinal cell types. Likewise, single-cell RNA sequencing investigations have characterized how gene expression changes in the setting of many retinal diseases, including how choroidal endothelial cells are altered in age-related macular degeneration. In addition, this technology has allowed vision researchers to discover drivers of retinal development and model rare retinal diseases with induced pluripotent stem cells. In this review, we will overview the growing number of single-cell RNA sequencing studies in the field of vision research. We will summarize experimental considerations for designing single-cell RNA sequencing experiments and highlight important advancements in retinal, RPE, choroidal, and retinal organoid biology driven by this technology. Finally, we generalize these findings to genes involved in retinal degeneration and outline the future of single-cell expression experiments in studying retinal disease.

Motor function recovery: deciphering a regenerative niche at the neuromuscular synapse

The coordinated movement of many organisms relies on efficient nerve–muscle communication at the neuromuscular junction (NMJ), a peripheral synapse composed of a presynaptic motor axon terminal, a postsynaptic muscle specialization, and non‐myelinating terminal Schwann cells. NMJ dysfunctions are caused by traumatic spinal cord or peripheral nerve injuries as well as by severe motor pathologies. Compared to the central nervous system, the peripheral nervous system displays remarkable regenerating abilities; however, this capacity is limited by the denervation time frame and depends on the establishment of permissive regenerative niches. At the injury site, detailed information is available regarding the cells, molecules, and mechanisms involved in nerve regeneration and repair. However, a regenerative niche at the final functional step of peripheral motor innervation, i.e. at the mature neuromuscular synapse, has not been deciphered. In this review, we integrate classic and recent evidence describing the cells and molecules that could orchestrate a dynamic ecosystem to accomplish successful NMJ regeneration. We propose that such a regenerative niche must ensure at least two fundamental steps for successful NMJ regeneration: the proper arrival of incoming regenerating axons to denervated postsynaptic muscle domains, and the resilience of those postsynaptic domains, in morphological and functional terms. We here describe and combine the main cellular and molecular responses involved in each of these steps as potential targets to help successful NMJ regeneration.

3D Spheroids of Umbilical Cord Blood MSC-Derived Schwann Cells Promote Peripheral Nerve Regeneration

Schwann cells (SCs) are promising candidates for cell therapy due to their ability to promote peripheral nerve regeneration. However, SC-based therapies are hindered by the lack of a clinically renewable source of SCs. In this study, using a well-defined non-genetic approach, umbilical cord blood mesenchymal stem cells (cbMSCs), a clinically applicable cell type, were phenotypically, epigenetically, and functionally converted into SC-like cells (SCLCs) that stimulated effective sprouting of neuritic processes from neuronal cells. To further enhance their therapeutic capability, the cbMSC-derived SCLCs were assembled into three-dimensional (3D) cell spheroids by using a methylcellulose hydrogel system. The cell-cell and cell-extracellular matrix interactions were well-preserved within the formed 3D SCLC spheroids, and marked increases in neurotrophic, proangiogenic and anti-apoptotic factors were detected compared with cells that were harvested using conventional trypsin-based methods, demonstrating the superior advantage of SCLCs assembled into 3D spheroids. Transplantation of 3D SCLC spheroids into crush-injured rat sciatic nerves effectively promoted the recovery of motor function and enhanced nerve structure regeneration. In summary, by simply assembling cells into a 3D-spheroid conformation, the therapeutic potential of SCLCs derived from clinically available cbMSCs for promoting nerve regeneration was enhanced significantly. Thus, these cells hold great potential for translation to clinical applications for treating peripheral nerve injury.

Comparison of the Efficacy of Optogenetic Stimulation of Glia versus Neurons in Myelination

Increasing evidence demonstrates that optogenetics contributes to the regulation of brain behavior, cognition, and physiology, particularly during myelination, potentially allowing for the bidirectional modulation of specific cell lines with spatiotemporal accuracy. However, the type of cell to be targeted, namely, glia vs neurons, and the degree to which optogenetically induced cell activity can regulate myelination during the development of the peripheral nervous system (PNS) are still underexplored. Herein, we report the comparison of optogenetic stimulation (OS) of Schwann cells (SCs) and motor neurons (MNs) for activation of myelination in the PNS. Capitalizing on these optogenetic tools, we confirmed that the formation of the myelin sheath was initially promoted more by OS of calcium translocating channelrhodopsin (CatCh)-transfected SCs than by OS of transfected MNs at 7 days in vitro (DIV). Additionally, the level of myelination was substantially enhanced even until 14 DIV. Surprisingly, after OS of SCs, > 91.1% ± 5.9% of cells expressed myelin basic protein, while that of MNs was 67.8% ± 6.1%. The potent effect of OS of SCs was revealed by the increased thickness of the myelin sheath at 14 DIV. Thus, the OS of SCs could highly accelerate myelination, while the OS of MNs only somewhat promoted myelination, indicating a clear direction for the optogenetic application of unique cell types for initiating and promoting myelination. Together, our findings support the importance of precise cell type selection for use in optogenetics, which in turn can be broadly applied to overcome the limitations of optogenetics after injury.

Mesenteric Neural Crest Cells Are the Embryological Basis of Skip Segment Hirschsprung's Disease

BACKGROUND & AIMS

Defective rostrocaudal colonization of the gut by vagal neural crest cells (vNCCs) results in Hirschsprung's disease (HSCR), which is characterized by aganglionosis in variable lengths of the distal bowel. Skip segment Hirschsprung's disease (SSHD), referring to a ganglionated segment within an otherwise aganglionic intestine, contradicts HSCR pathogenesis and underscores a significant gap in our understanding of the development of the enteric nervous system. Here, we aimed to identify the embryonic origin of the ganglionic segments in SSHD.

METHODS

Intestinal biopsy specimens from HSCR patients were prepared via the Swiss-roll technique to search for SSHD cases. NCC migration from the neural tube to the gut was spatiotemporally traced using targeted cell lineages and gene manipulation in mice.

RESULTS

After invading the mesentery surrounding the foregut, vNCCs separated into 2 populations: mesenteric NCCs (mNCCs) proceeded to migrate along the mesentery, whereas enteric NCCs invaded the foregut to migrate along the gut. mNCCs not only produced neurons and glia within the gut mesentery, but also continuously complemented the enteric NCC pool. Two new cases of SSHD were identified from 183 HSCR patients, and Ednrb-mutant mice, but not Ret^-/- mice, showed a high incidence rate of SSHD-like phenotypes.

CONCLUSIONS

mNCCs, a subset of vNCCs that migrate into the gut via the gut mesentery to give rise to enteric neurons, could provide an embryologic explanation for SSHD. These findings lead to novel insights into the development of the enteric nervous system and the etiology of HSCR.

Why Does the Face Predict the Brain? Neural Crest Induction, Craniofacial Morphogenesis, and Neural Circuit Development

Mesenchephalic and rhombencephalic neural crest cells generate the craniofacial skeleton, special sensory organs, and subsets of cranial sensory receptor neurons. They do so while preserving the anterior-posterior (A-P) identity of their neural tube origins. This organizational principle is paralleled by central nervous system circuits that receive and process information from facial structures whose A-P identity is in register with that in the brain. Prior to morphogenesis of the face and its circuits, however, neural crest cells act as "inductive ambassadors" from distinct regions of the neural tube to induce differentiation of target craniofacial domains and establish an initial interface between the brain and face. At every site of bilateral, non-axial secondary induction, neural crest constitutes all or some of the mesenchymal compartment for non-axial mesenchymal/epithelial (M/E) interactions. Thus, for epithelial domains in the craniofacial primordia, aortic arches, limbs, the spinal cord, and the forebrain (Fb), neural crest-derived mesenchymal cells establish local sources of inductive signaling molecules that drive morphogenesis and cellular differentiation. This common mechanism for building brains, faces, limbs, and hearts, A-P axis specified, neural crest-mediated M/E induction, coordinates differentiation of distal structures, peripheral neurons that provide their sensory or autonomic innervation in some cases, and central neural circuits that regulate their behavioral functions. The essential role of this neural crest-mediated mechanism identifies it as a prime target for pathogenesis in a broad range of neurodevelopmental disorders. Thus, the face and the brain "predict" one another, and this mutual developmental relationship provides a key target for disruption by developmental pathology.

Stem cells, evolutionary aspects and pathology of the adrenal medulla: A new developmental paradigm

The mammalian adrenal gland is composed of two main components; the catecholaminergic neural crest-derived medulla, found in the center of the gland, and the mesoderm-derived cortex producing steroidogenic hormones. The medulla is composed of neuroendocrine chromaffin cells with oxygen-sensing properties and is dependent on tissue interactions with the overlying cortex, both during development and in adulthood. Other relevant organs include the Zuckerkandl organ containing extra-adrenal chromaffin cells, and carotid oxygen-sensing bodies containing glomus cells. Chromaffin and glomus cells reveal a number of important similarities and are derived from the multipotent nerve-associated descendants of the neural crest, or Schwann cell precursors. Abnormalities in complex developmental processes during differentiation of nerve-associated and other progenitors into chromaffin and oxygen-sensing populations may result in different subtypes of paraganglioma, neuroblastoma and pheochromocytoma. Here, we summarize recent findings explaining the development of chromaffin and oxygen-sensing cells, as well as the potential mechanisms driving neuroendocrine tumor initiation.

Hedging against Neuropathic Pain: Role of Hedgehog Signaling in Pathological Nerve Healing

The peripheral nervous system has important regenerative capacities that regulate and restore peripheral nerve homeostasis. Following peripheral nerve injury, the nerve undergoes a highly regulated degeneration and regeneration process called Wallerian degeneration, where numerous cell populations interact to allow proper nerve healing. Recent studies have evidenced the prominent role of morphogenetic Hedgehog signaling pathway and its main effectors, Sonic Hedgehog (SHH) and Desert Hedgehog (DHH) in the regenerative drive following nerve injury. Furthermore, dysfunctional regeneration and/or dysfunctional Hedgehog signaling participate in the development of chronic neuropathic pain that sometimes accompanies nerve healing in the clinical context. Understanding the implications of this key signaling pathway could provide exciting new perspectives for future research on peripheral nerve healing.

Engineering an Injectable Electroactive Nanohybrid Hydrogel for Boosting Peripheral Nerve Growth and Myelination in Combination with Electrical Stimulation

Electrical stimulation (ES) can be used to manipulate recovery after peripheral nerve injuries. Although biomaterial-based strategies have already been implemented to gain momentum for ES and engineer permissive microenvironments for neural regeneration, the development of biomaterials for specific stimuli-responsive modulation of neural cell properties remains a challenge. Herein, we homogeneously incorporate pristine carbon nanotubes into a functional self-assembling peptide to prepare a hybrid hydrogel with good injectability and conductivity. Two-dimensional (on the surface) and three-dimensional (within the hybrid hydrogel) culturing experiments demonstrate that ES promotes axon outgrowth and Schwann cell (SC) migration away from dorsal root ganglia spheres, further revealing that ES-enhanced interactions between SCs and axons result in improved myelination. Thus, our study not only advances the development of tailor-made materials but also provides useful insights into comprehensive approaches for promoting nerve growth and presents a practical strategy of repairing peripheral nerve injuries.

Deletion of the Gene Encoding the NMDA Receptor GluN1 Subunit in Schwann Cells Causes Ultrastructural Changes in Remak Bundles and Hypersensitivity in Pain Processing

Abnormalities in interactions between sensory neurons and Schwann cells (SCs) may result in heightened pain processing and chronic pain states. We previously reported that SCs express the NMDA receptor (NMDA-R), which activates cell signaling in response to glutamate and specific protein ligands, such as tissue-type plasminogen activator. Herein, we genetically targeted grin1 encoding the essential GluN1 NMDA-R subunit, conditionally in SCs, to create a novel mouse model in which SCs are NMDA-R-deficient (GluN1- mice). These mice demonstrated increased sensitivity to light touch, pinprick, and thermal hyperalgesia in the absence of injury, without associated changes in motor function. Ultrastructural analysis of adult sciatic nerve in GluN1- mice revealed increases in the density of Aδ fibers and Remak bundles and a decrease in the density of Aβ fibers, without altered g-ratios. Abnormalities in adult Remak bundle ultrastructure were also present including aberrant C-fiber ensheathment, distances between axons, and increased poly-axonal pockets. Developmental and post radial sorting defects contributed to altered nerve fiber densities in adult. Uninjured sciatic nerves in GluN1- mice did not demonstrate an increase in neuroinflammatory infiltrates. Transcriptome profiling of dorsal root ganglia (DRGs) revealed 138 differentially regulated genes in GluN1- mice. One third of the regulated genes are known to be involved in pain processing, including sprr1a, npy, fgf3, atf3, and cckbr, which were significantly increased. The intraepidermal nerve fiber density (IENFD) was significantly decreased in the skin of GluN1- mice. Collectively, these findings demonstrate that SC NMDA-R is essential for normal PNS development and for preventing development of pain states.SIGNIFICANCE STATEMENT Chronic unremitting pain is a prevalent medical condition; however, the molecular mechanisms that underlie heightened pain processing remain incompletely understood. Emerging data suggest that abnormalities in Schwann cells (SCs) may cause neuropathic pain. We established a novel mouse model for small fiber neuropathy (SFN) in which grin1, the gene that encodes the NMDA receptor (NMDA-R) GluN1 subunit, is deleted in SCs. These mice demonstrate hypersensitivity in pain processing in the absence of nerve injury. Changes in the density of intraepidermal small fibers, the ultrastructure of Remak bundles, and the transcriptome of dorsal root ganglia (DRGs) provide possible explanations for the increase in pain processing. Our results support the hypothesis that abnormalities in communication between sensory nerve fibers and SCs may result in pain states.

Perspective on Schwann Cells Derived from Induced Pluripotent Stem Cells in Peripheral Nerve Tissue Engineering

Schwann cells play a crucial role in successful peripheral nerve repair and regeneration by supporting both axonal growth and myelination. Schwann cells are therefore a feasible option for cell therapy treatment of peripheral nerve injury. However, sourcing human Schwann cells at quantities required for development beyond research is challenging. Due to their availability, rapid in vitro expansion, survival, and integration within the host tissue, stem cells have attracted considerable attention as candidate cell therapies. Among them, induced pluripotent stem cells (iPSCs) with the associated prospects for personalized treatment are a promising therapy to take the leap from bench to bedside. In this critical review, we firstly focus on the current knowledge of the Schwann cell phenotype in regard to peripheral nerve injury, including crosstalk with the immune system during peripheral nerve regeneration. Then, we review iPSC to Schwann cell derivation protocols and the results from recent in vitro and in vivo studies. We finally conclude with some prospects for the use of iPSCs in clinical settings.