Molecular signaling mechanisms of axon-glia communication in the peripheral nervous system

Peripheral myelin: From development to maintenance

Peripheral myelin is synthesized by glial cells called Schwann cells (SCs). SC development and differentiation must be tightly regulated to avoid any pathological consequence affecting peripheral nerve function. Neuropathic symptoms can arise from developmental issues in SCs, as well as in adult life through processes affecting mature SCs. In this review we focus on SC differentiation from the immature towards the myelinating and non-myelinating SC stages, defining molecular mechanisms outlining radial sorting, a multi-stepped event essential for immature SC differentiation and myelination. We also describe mechanisms regulating myelin sheath maintenance and SC homeostasis during aging. Finally, we will conclude with some remaining questions in the field of SC biology.

Single-cell transcriptome analysis reveals distinct cell populations in dorsal root ganglia and their potential roles in diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is a common complication associated with diabetes, and can affect quality of life considerably. Dorsal root ganglion (DRG) plays an important role in the development of DPN. However, the relationship between DRG and the pathogenesis of DPN still lacks a thorough exploration. Besides, a more in-depth understanding of the cell type composition of DRG, and the roles of different cell types in mediating DPN are needed. Here we conducted single-cell RNA-seq (scRNA-seq) for DRG tissues isolated from healthy control and DPN rats. Our results demonstrated DRG includes eight cell-type populations (e.g., neurons, satellite glial cells (SGCs), Schwann cells (SCs), endothelial cells, fibroblasts). In the heterogeneity analyses of cells, six neuron sub-types, three SGC sub-types and three SC sub-types were identified, additionally, biological functions related to cell sub-types were further revealed. Cell communication analysis showed dynamic interactions between neurons, SGCs and SCs. We also found that the aberrantly expressed transcripts in sub-types of neurons, SGCs and SCs with DPN were associated with diabetic neuropathic pain, cell apoptosis, oxidative stress, etc. In conclusion, this study provides a systematic perspective of the cellular composition and interactions of DRG tissues, and suggests that neurons, SGCs and SCs play vital roles in the progression of DPN. Our data may provide a valuable resource for future studies regarding the pathophysiological effect of particular cell type in DPN.

Novel CTNNB1 variant leading to neurodevelopmental disorder with spastic diplegia and visual defects plus peripheral neuropathy: A case report

Pathogenic variants in the β1-catenin (CTNNB1) gene have been identified in patients with various diseases, including syndromic intellectual disability, autism spectrum disorder, familial exudative vitreoretinopathy, and neurodevelopmental disorder with spastic diplegia and visual defects (NEDSDV). We report on the clinical, genetic, neuroimaging, and neurophysiological data of a 15-year-old patient with complex hereditary spastic paraplegias with exotropia, dyskinesia, and cerebellar signs and a so-far unreported demyelinating (mainly sensory) polyneuropathy in her lower limbs. She carries the novel, de novo, likely pathogenic heterozygous c.603_605delinsAATA, p.(Met202Ilefs*6) frameshift variant in the CTNNB1 gene. Although peripheral neuropathy was not previously associated with NEDSDV, in light of the role of β1-catenin as a junction protein in the peripheral as well as in the central nervous system documented in experimental studies, it might represent a causally linked and under-reported finding to be further explored.

Mature but not developing Schwann cells promote axon regeneration after peripheral nerve injury

Since Schwann cells (SCs) support axonal growth at development as well as after peripheral nerve injury (PNI), developing SCs might be able to promote axon regeneration after PNI. The purpose of the current study was to elucidate the capability of developing SCs to induce axon regeneration after PNI. SC precursors (SCPs), immature SCs (ISCs), repair SCs (RSCs) from injured nerves, and non-RSCs from intact nerves were tested by grafting into acellular region of rat sciatic nerve with crush injury. Both of developing SCs completely failed to support axon regeneration, whereas both of mature SCs, especially RSCs, induced axon regeneration. Further, RSCs but not SCPs promoted neurite outgrowth of adult dorsal root ganglion neurons. Transcriptome analysis revealed that the gene expression profiles were distinctly different between RSCs and SCPs. These findings indicate that developing SCs are markedly different from mature SCs in terms of functional and molecular aspects and that RSC is a viable candidate for regenerative cell therapy for PNI.

The role of exosomes in inter-cellular and inter-organ communication of the peripheral nervous system

Exosomes, nano-sized extracellular vesicles, are produced via the endosomal pathway and released in the extracellular space upon fusion of multivesicular bodies with the plasma membrane. Recent evidence shows that these extracellular vesicles play a key role in cell-to-cell communication. Exosomes transport bioactive proteins, messenger RNA (mRNAs) and microRNA (miRNAs) in an active form to adjacent cells or to distant organs. In this review, we focus on the role of exosomes in peripheral nerve maintenance and repair, as well as peripheral nerve/organ crosstalk, and discuss the potential benefits of exploiting exosomes for treating PNS injuries. In addition, we will highlight the emerging role of exosomes as new important vehicles for physiological systemic crosstalk failures, which could lead to organ dysfunction during neuroinflammation or aging.

Peripheral Nerve Development and the Pathogenesis of Peripheral Neuropathy: the Sorting Point

Nerve development requires a coordinated sequence of events and steps to be accomplished for the generation of functional peripheral nerves to convey sensory and motor signals. Any abnormality during development may result in pathological structure and function of the nerve, which evolves in peripheral neuropathy. In this review, we will briefly describe different steps of nerve development while we will mostly focus on the molecular mechanisms involved in radial sorting of axons, one of these nerve developmental steps. We will summarize current knowledge of molecular pathways so far reported in radial sorting and their possible interactions. Finally, we will describe how disruption of these pathways may result in human neuropathies.

MicroRNAs as Biomarkers of Charcot-Marie-Tooth Disease Type 1A

OBJECTIVE

To determine whether microRNAs (miRs) are elevated in the plasma of individuals with the inherited peripheral neuropathy Charcot-Marie-Tooth disease type 1A (CMT1A), miR profiling was employed to compare control and CMT1A plasma.

METHODS

We performed a screen of CMT1A and control plasma samples to identify miRs that are elevated in CMT1A using next-generation sequencing, followed by validation of selected miRs by quantitative PCR, and correlation with protein biomarkers and clinical data: Rasch-modified CMT Examination and Neuropathy Scores, ulnar compound muscle action potentials, and motor nerve conduction velocities.

RESULTS

After an initial pilot screen, a broader screen confirmed elevated levels of several muscle-associated miRNAs (miR1, -133a, -133b, and -206, known as myomiRs) along with a set of miRs that are highly expressed in Schwann cells of peripheral nerve. Comparison to other candidate biomarkers for CMT1A (e.g., neurofilament light) measured on the same sample set shows a comparable elevation of several miRs (e.g., miR133a, -206, -223) and ability to discriminate cases from controls. Neurofilament light levels were most highly correlated with miR133a. In addition, the putative Schwann cell miRs (e.g., miR223, -199a, -328, -409, -431) correlate with the recently described transmembrane protease serine 5 (TMPRSS5) protein biomarker that is most highly expressed in Schwann cells and also elevated in CMT1A plasma.

CONCLUSIONS

These studies identify a set of miRs that are candidate biomarkers for clinical trials in CMT1A. Some of the miRs may reflect Schwann cell processes that underlie the pathogenesis of the disease.

CLASSIFICATION OF EVIDENCE

This study provides Class III evidence that a set of plasma miRs are elevated in patients with CMT1A.

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.

The significance of the neuregulin-1/ErbB signaling pathway and its effect on Sox10 expression in the development of terminally differentiated Schwann cells in vitro

PURPOSE

The purpose of this study was to explore the significance of the neuregulin-1/ErbB signaling pathway and its effect on Sox10 expression in the course of the differentiation of mouse bone marrow mesenchymal stem cells into Schwann-like cells in vitro.

MATERIALS AND METHODS

The experiment was conducted with three groups-control, TAK 165, and HRG-off. In the control group, we used the classical induction method of adding β-ME, RA, FSK, b-FGF, PDGF, and neuregulin (HRG); the cells were collected on the 7^th day. Using the same basic protocol as the control group, the specific ErbB2 inhibitor mubritinib (TAK 165) was added to block the neuregulin-1/ErbB pathway in the TAK 165 group, while HRG was not added in the HRG-off group. We detected the degree of differentiation of stem cells into Schwann-like cells by using RT-PCR to examine the expression of Sox10, NRG-1, ErbB2, ErbB3, and ErbB4 and by using immunofluorescence staining to examine the Schwann cell marker S100B, Glial Fibrillary Acidic Protein (GFAP) and P75.

RESULTS

Our results showed that the proliferation of Schwann cells was reduced and apoptosis was increased in the TAK 165 group and the HRG-off group. Sox10 was stably expressed and NRG-1, ErbB2, and ErbB3 increased in the control group. However, the expression of Sox10 in the TAK 165 group was obviously decreased at the end of induced differentiation; meanwhile, the degree of stem cell differentiation also decreased.

CONCLUSIONS

the neuregulin-1/ErbB signaling pathway plays an important role in the differentiation of bone marrow mesenchymal stem cells into Schwann-like cells and can promote the maintenance of Sox10 。.

Adult tissue-derived neural crest-like stem cells: Sources, regulatory networks, and translational potential

Neural crest (NC) cells are a multipotent stem cell population that give rise to a diverse array of cell types in the body, including peripheral neurons, Schwann cells (SC), craniofacial cartilage and bone, smooth muscle cells, and melanocytes. NC formation and differentiation into specific lineages takes place in response to a set of highly regulated signaling and transcriptional events within the neural plate border. Premigratory NC cells initially are contained within the dorsal neural tube from which they subsequently emigrate, migrating to often distant sites in the periphery. Following their migration and differentiation, some NC‐like cells persist in adult tissues in a nascent multipotent state, making them potential candidates for autologous cell therapy. This review discusses the gene regulatory network responsible for NC development and maintenance of multipotency. We summarize the genes and signaling pathways that have been implicated in the differentiation of a postmigratory NC into mature myelinating SC. We elaborate on the signals and transcription factors involved in the acquisition of immature SC fate, axonal sorting of unmyelinated neuronal axons, and finally the path toward mature myelinating SC, which envelope axons within myelin sheaths, facilitating electrical signal propagation. The gene regulatory events guiding development of SC in vivo provides insights into means for differentiating NC‐like cells from adult human tissues into functional SC, which have the potential to provide autologous cell sources for the treatment of demyelinating and neurodegenerative disorders.

Axo-glial interdependence in peripheral nerve development

During the development of the peripheral nervous system, axons and myelinating Schwann cells form a unique symbiotic unit, which is realized by a finely tuned network of molecular signals and reciprocal interactions. The importance of this complex interplay becomes evident after injury or in diseases in which aspects of axo-glial interaction are perturbed. This Review focuses on the specific interdependence of axons and Schwann cells in peripheral nerve development that enables axonal outgrowth, Schwann cell lineage progression, radial sorting and, finally, formation and maintenance of the myelin sheath.

Neuron-Glia Signaling in Synapse Elimination

Maturation of neuronal circuits requires selective elimination of synaptic connections. Although neuron-intrinsic mechanisms are important in this process, it is increasingly recognized that glial cells also play a critical role. Without proper functioning of these cells, the number, morphology, and function of synaptic contacts are profoundly altered, resulting in abnormal connectivity and behavioral abnormalities. In addition to their role in synaptic refinement, glial cells have also been implicated in pathological synapse loss and dysfunction following injury or nervous system degeneration in adults. Although mechanisms regulating glia-mediated synaptic elimination are still being uncovered, it is clear this complex process involves many cues that promote and inhibit the removal of specific synaptic connections. Gaining a greater understanding of these signals and the contribution of different cell types will not only provide insight into this critical biological event but also be instrumental in advancing knowledge of brain development and neural disease.

Direct conversion of adult human skin fibroblasts into functional Schwann cells that achieve robust recovery of the severed peripheral nerve in rats

Direct conversion is considered a promising approach to obtain tissue-specific cells for cell therapies; however, this strategy depends on exogenous gene expression that may cause undesired adverse effects such as tumorigenesis. By optimizing the Schwann cell induction system, which was originally developed for trans-differentiation of bone marrow mesenchymal stem cells into Schwann cells, we established a system to directly convert adult human skin fibroblasts into cells comparable to authentic human Schwann cells without gene introduction. Serial treatments with beta-mercaptoethanol, retinoic acid, and finally a cocktail of basic fibroblast growth factor, forskolin, platelet-derived growth factor-AA, and heregulin-β1 (EGF domain) converted fibroblasts into cells expressing authentic Schwann cell markers at an efficiency of approximately 75%. Genome-wide gene expression analysis suggested the conversion of fibroblasts into the Schwann cell-lineage. Transplantation of induced Schwann cells into severed peripheral nerve of rats facilitated axonal regeneration and robust functional recovery in sciatic function index comparable to those of authentic human Schwann cells. The contributions of induced Schwann cells to myelination of regenerated axons and re-formation of neuromuscular junctions were also demonstrated. Our data clearly demonstrated that cells comparable to functional Schwann cells feasible for the treatment of neural disease can be induced from adult human skin fibroblasts without gene introduction. This direct conversion system will be beneficial for clinical applications to peripheral and central nervous system injuries and demyelinating diseases.

The LKB1-AMPK and mTORC1 Metabolic Signaling Networks in Schwann Cells Control Axon Integrity and Myelination: Assembling and upholding nerves by metabolic signaling in Schwann cells

The Liver kinase B1 with its downstream target AMP activated protein kinase (LKB1-AMPK), and the key nutrient sensor mammalian target of rapamycin complex 1 (mTORC1) form two signaling systems that coordinate metabolic and cellular activity with changes in the environment in order to preserve homeostasis. For example, nutritional fluctuations rapidly feed back on these signaling systems and thereby affect cell-specific functions. Recent studies have started to reveal important roles of these strategic metabolic regulators in Schwann cells for the trophic support and myelination of axons. Because aberrant intermediate metabolism along with mitochondrial dysfunction in Schwann cells is mechanistically linked to nerve abnormalities found in acquired and inherited peripheral neuropathies, manipulation of the LKB1-AMPK, and mTORC1 signaling hubs may be a worthwhile therapeutic target to mitigate nerve damage in disease. Here, recent advances in our understanding of LKB1-AMPK and mTORC1 functions in Schwann cells are covered, and future research areas for this key metabolic signaling network are proposed.

Tonic ATP-mediated growth suppression in peripheral nerve glia requires arrestin-PP2 and is evaded in NF1

Normal Schwann cells (SCs) are quiescent in adult nerves, when ATP is released from the nerve in an activity dependent manner. We find that suppressing nerve activity in adult nerves causes SC to enter the cell cycle. In vitro, ATP activates the SC G-protein coupled receptor (GPCR) P2Y2. Downstream of P2Y2, β-arrestin-mediated signaling results in PP2-mediated de-phosphorylation of AKT, and PP2 activity is required for SC growth suppression. NF1 deficient SC show reduced growth suppression by ATP, and are resistant to the effects of β-arrestin-mediated signaling, including PP2-mediated de-phosphorylation of AKT. In patients with the disorder Neurofibromatosis type 1, NF1 mutant SCs proliferate and form SC tumors called neurofibromas. Elevating ATP levels in vivo reduced neurofibroma cell proliferation. Thus, the low proliferation characteristic of differentiated adult peripheral nerve may require ongoing, nerve activity-dependent, ATP. Additionally, we identify a mechanism through which NF1 SCs may evade growth suppression in nerve tumors.

Glycans and glycosaminoglycans in neurobiology: key regulators of neuronal cell function and fate

The aim of the present study was to examine the roles of l-fucose and the glycosaminoglycans (GAGs) keratan sulfate (KS) and chondroitin sulfate/dermatan sulfate (CS/DS) with selected functional molecules in neural tissues. Cell surface glycans and GAGs have evolved over millions of years to become cellular mediators which regulate fundamental aspects of cellular survival. The glycocalyx, which surrounds all cells, actuates responses to growth factors, cytokines and morphogens at the cellular boundary, silencing or activating downstream signaling pathways and gene expression. In this review, we have focused on interactions mediated by l-fucose, KS and CS/DS in the central and peripheral nervous systems. Fucose makes critical contributions in the area of molecular recognition and information transfer in the blood group substances, cytotoxic immunoglobulins, cell fate-mediated Notch-1 interactions, regulation of selectin-mediated neutrophil extravasation in innate immunity and CD-34-mediated new blood vessel development, and the targeting of neuroprogenitor cells to damaged neural tissue. Fucosylated glycoproteins regulate delivery of synaptic neurotransmitters and neural function. Neural KS proteoglycans (PGs) were examined in terms of cellular regulation and their interactive properties with neuroregulatory molecules. The paradoxical properties of CS/DS isomers decorating matrix and transmembrane PGs and the positive and negative regulatory cues they provide to neurons are also discussed.

Advances in Controlling Differentiation of Adult Stem Cells for Peripheral Nerve Regeneration

Adult stems cells, possessing the ability to grow, migrate, proliferate, and transdifferentiate into various specific phenotypes, constitute a great asset for peripheral nerve regeneration. Adult stem cells' ability to undergo transdifferentiation is sensitive to various cell-to-cell interactions and external stimuli involving interactions with physical, mechanical, and chemical cues within their microenvironment. Various studies have employed different techniques for transdifferentiating adult stem cells from distinct sources into specific lineages (e.g., glial cells and neurons). These techniques include chemical and/or electrical induction as well as cell-to-cell interactions via co-culture along with the use of various 3D conduit/scaffold designs. Such scaffolds consist of unique materials that possess controllable physical/mechanical properties mimicking cells' natural extracellular matrix. However, current limitations regarding non-scalable transdifferentiation protocols, fate commitment of transdifferentiated stem cells, and conduit/scaffold design have required new strategies for effective stem cells transdifferentiation and implantation. In this progress report, a comprehensive review of recent advances in the transdifferentiation of adult stem cells via different approaches along with multifunctional conduit/scaffolds designs is presented for peripheral nerve regeneration. Potential cellular mechanisms and signaling pathways associated with differentiation are also included. The discussion with current challenges in the field and an outlook toward future research directions is concluded.

Myelinating cocultures of rodent stem cell line-derived neurons and immortalized Schwann cells

Myelination is one of the most remarkable biological events in the neuron-glia interactions for the development of the mammalian nervous system. To elucidate molecular mechanisms of cell-to-cell interactions in myelin synthesis in vitro, establishment of the myelinating system in cocultures of continuous neuronal and glial cell lines are desirable. In the present study, we performed co-culture experiments using rat neural stem cell-derived neurons or mouse embryonic stem (ES) cell-derived motoneurons with immortalized rat IFRS1 Schwann cells to establish myelinating cultures between these cell lines. Differentiated neurons derived from an adult rat neural stem cell line 1464R or motoneurons derived from a mouse ES cell line NCH4.3, were mixed with IFRS1 Schwann cells, plated, and maintained in serum-free F12 medium with B27 supplement, ascorbic acid, and glial cell line-derived neurotrophic factor. Myelin formation was demonstrated by electron microscopy at 4 weeks in cocultures of 1464R-derived neurons or NCH4.3-derived motoneurons with IFRS1 Schwann cells. These in vitro coculture systems utilizing the rodent stable stem and Schwann cell lines can be useful in studies of peripheral nerve development and regeneration.

Molecular Mechanisms Involved in Schwann Cell Plasticity

Schwann cell incredible plasticity is a hallmark of the utmost importance following nerve damage or in demyelinating neuropathies. After injury, Schwann cells undergo dedifferentiation before redifferentiating to promote nerve regeneration and complete functional recovery. This review updates and discusses the molecular mechanisms involved in the negative regulation of myelination as well as in the reprogramming of Schwann cells taking place early following nerve lesion to support repair. Significant advance has been made on signaling pathways and molecular components that regulate SC regenerative properties. These include for instance transcriptional regulators such as c-Jun or Notch, the MAPK and the Nrg1/ErbB2/3 pathways. This comprehensive overview ends with some therapeutical applications targeting factors that control Schwann cell plasticity and highlights the need to carefully modulate and balance this capacity to drive nerve repair.

Dual specificity phosphatase 15 regulates Erk activation in Schwann cells

Schwann cells and oligodendrocytes are the myelinating cells of the peripheral and central nervous system, respectively. Despite having different myelin components and different transcription factors driving their terminal differentiation there are shared molecular mechanisms between the two. Sox10 is one common transcription factor required for several steps in development of myelinating glia. However, other factors are divergent as Schwann cells need the transcription factor early growth response 2/Krox20 and oligodendrocytes require Myrf. Likewise, some signaling pathways, like the Erk1/2 kinases, are necessary in both cell types for proper myelination. Nonetheless, the molecular mechanisms that control this shared signaling pathway in myelinating cells remain only partially characterized. The hypothesis of this study is that signaling pathways that are similarly regulated in both Schwann cells and oligodendrocytes play central roles in coordinating the differentiation of myelinating glia. To address this hypothesis, we have used genome-wide binding data to identify a relatively small set of genes that are similarly regulated by Sox10 in myelinating glia. We chose one such gene encoding Dual specificity phosphatase 15 (Dusp15) for further analysis in Schwann cell signaling. RNA interference and gene deletion by genome editing in cultured RT4 and primary Schwann cells showed Dusp15 is necessary for full activation of Erk1/2 phosphorylation. In addition, we show that Dusp15 represses expression of several myelin genes, including myelin basic protein. The data shown here support a mechanism by which early growth response 2 activates myelin genes, but also induces a negative feedback loop through Dusp15 to limit over-expression of myelin genes.

Concomitant differentiation of a population of mouse embryonic stem cells into neuron-like cells and schwann cell-like cells in a slow-flow microfluidic device

BACKGROUND

To send meaningful information to the brain, an inner ear cochlear implant (CI) must become closely coupled to as large and healthy a population of remaining spiral ganglion neurons (SGN) as possible. Inner ear gangliogenesis depends on macrophage migration inhibitory factor (MIF), a directionally attractant neurotrophic cytokine made by both Schwann and supporting cells (Bank et al., 2012). MIF-induced mouse embryonic stem cell (mESC)-derived "neurons" could potentially substitute for lost or damaged SGN. mESC-derived "Schwann cells" produce MIF, as do all Schwann cells (Huang et al., a; Roth et al., 2007; Roth et al., 2008) and could attract SGN to a "cell-coated" implant.

RESULTS

Neuron- and Schwann cell-like cells were produced from a common population of mESCs in an ultra-slow-flow microfluidic device. As the populations interacted, "neurons" grew over the "Schwann cell" lawn, and early events in myelination were documented. Blocking MIF on the Schwann cell side greatly reduced directional neurite outgrowth. MIF-expressing "Schwann cells" were used to coat a CI: Mouse SGN and MIF-induced "neurons" grew directionally to the CI and to a wild-type but not MIF-knockout organ of Corti explant.

CONCLUSIONS

Two novel stem cell-based approaches for treating the problem of sensorineural hearing loss are described. Developmental Dynamics 246:7-27, 2017. © 2016 Wiley Periodicals, Inc.

Rab8a/Rab11a regulate intercellular communications between neural cells via tunneling nanotubes

Tunneling nanotubes (TNTs) are F-actin-based membrane tubes, and can form between cultured cells and within vital tissues. TNTs mediate intercellular communications that range from electrical signaling to the transfer of organelles. Following peripheral nerve injury, the orchestrated intercellular communications among neural and non-neural cells are required for effective nerve regeneration. It remains unknown whether TNTs exist between neural cells in the peripheral nerve system and how TNTs affect neural regeneration. To address these interesting questions, we investigated the transfer of neurotropic factors, membrane protein, cytoplasmic protein, mitochondria and RNA in functional TNTs formed between cultured Schwann cells (SCs). TNT-like structures were increased not only in cultured SCs after exposure to serum depletion but also in longitudinal sections of proximal sciatic nerve stump harvested after rat peripheral nerve transection. Meanwhile, downregulation of Rab8a or Rab11a in cultured SCs inhibited the formation of functional TNTs and vesicle transfer and led to decrease in cell migration, increase in SCs apoptosis. Likewise, knockdown of Rab8a or Rab11a in primary SCs also suppressed axonal outgrowth from co-cultured dorsal root ganglion (DRG) neurons. Overall, our results suggested that the gene of Rab8a or Rab11a might be involved in the formation of TNTs structures in the peripheral nerve system, while TNTs structures were likely to affect peripheral nerve regeneration through the regulation of neural cell communications.

Specific inhibition of secreted NRG1 types I-II by heparin enhances Schwann Cell myelination

Primary cultures of mixed neuron and Schwann cells prepared from dorsal root ganglia (DRG) are extensively used as a model to study myelination. These dissociated DRG cultures have the particular advantage of bypassing the difficulty in purifying mouse Schwann cells, which is often required when using mutant mice. However, the drawback of this experimental system is that it yields low amounts of myelin. Here we report a simple and efficient method to enhance myelination in vitro. We show that the addition of heparin or low molecular weight heparin to mixed DRG cultures markedly increases Schwann cells myelination. The myelin promoting activity of heparin results from specific inhibition of the soluble immunoglobulin (Ig)-containing isoforms of neuregulin 1 (i.e., NRG1 types I and II) that negatively regulates myelination. Heparin supplement provides a robust and reproducible method to increase myelination in a simple and commonly used culture system. GLIA 2016;64:1227-1234.

Tuning the Mechanical Properties of Poly(Ethylene Glycol) Microgel-Based Scaffolds to Increase 3D Schwann Cell Proliferation

2D in vitro studies have demonstrated that Schwann cells prefer scaffolds with mechanical modulus approximately 10× higher than the modulus preferred by nerves, limiting the ability of many scaffolds to promote both neuron extension and Schwann cell proliferation. Therefore, the goals of this work are to develop and characterize microgel-based scaffolds that are tuned over the stiffness range relevant to neural tissue engineering and investigate Schwann cell morphology, viability, and proliferation within 3D scaffolds. Using thiol-ene reaction, microgels with surface thiols are produced and crosslinked into hydrogels using a multiarm vinylsulfone (VS). By varying the concentration of VS, scaffold stiffness ranges from 0.13 to 0.76 kPa. Cell morphology in all groups demonstrates that cells are able to spread and interact with the scaffold through day 5. Although the viability in all groups is high, proliferation of Schwann cells within the scaffold of G* = 0.53 kPa is significantly higher than other groups. This result is ≈ 5× lower than previously reported optimal stiffnesses on 2D surfaces, demonstrating the need for correlation of 3D cell response to mechanical modulus. As proliferation is the first step in Schwann cell integration into peripheral nerve conduits, these scaffolds demonstrate that the stiffness is a critical parameter to optimizing the regenerative process.

Glial Cells and Integrity of the Nervous System

Today, there is enormous progress in understanding the function of glial cells, including astroglia, oligodendroglia, Schwann cells, and microglia. Around 150 years ago, glia were viewed as a glue among neurons. During the course of the twentieth century, microglia were discovered and neuroscientists' views evolved toward considering glia only as auxiliary cells of neurons. However, over the last two to three decades, glial cells' importance has been reconsidered because of the evidence on their involvement in defining central nervous system architecture, brain metabolism, the survival of neurons, development and modulation of synaptic transmission, propagation of nerve impulses, and many other physiological functions. Furthermore, increasing evidence shows that glia are involved in the mechanisms of a broad spectrum of pathologies of the nervous system, including some psychiatric diseases, epilepsy, and neurodegenerative diseases to mention a few. It appears safe to say that no neurological disease can be understood without considering neuron-glia crosstalk. Thus, this book aims to show different roles played by glia in the healthy and diseased nervous system, highlighting some of their properties while considering that the various glial cell types are essential components not only for cell function and integration among neurons, but also for the emergence of important brain homeostasis.

What We Talk About When We Talk About Evolution

Currently, the biologic sciences are a Tower of Babel, having become so highly specialized that one discipline cannot effectively communicate with another. A mechanism for evolution that integrates development and physiologic homeostasis phylogenetically has been identified—cell-cell interactions. By reducing this process to ligand-receptor interactions and their intermediate down-stream signaling partners, it is possible, for example, to envision the functional homologies between such seemingly disparate structures and functions as the lung alveolus and kidney glomerulus, the skin and brain, or the skin and lung. For example, by showing the continuum of the lung phenotype for gas exchange at the cell-molecular level, being selected for increased surface area by augmenting lung surfactant production and function in lowering surface tension, we have determined an unprecedented structural-functional continuum from proximate to ultimate causation in evolution. It is maintained that tracing the changes in structure and function that have occurred over both the short-term history of the organism (as ontogeny), and the long-term history of the organism (as phylogeny), and how the mechanisms shared in common can account for both biologic stability and novelty, will provide the key to understanding the mechanisms of evolution. We need to better understand evolution from its unicellular origins as the Big Bang of biology.

SncRNA715 Inhibits Schwann Cell Myelin Basic Protein Synthesis

Myelin basic proteins (MBP) are major constituents of the myelin sheath in the central nervous system (CNS) and the peripheral nervous system (PNS). In the CNS Mbp translation occurs locally at the axon-glial contact site in a neuronal activity-dependent manner. Recently we identified the small non-coding RNA 715 (sncRNA715) as a key inhibitor of Mbp translation during transport in oligodendrocytes. Mbp mRNA localization in Schwann cells has been observed, but has not been investigated in much detail. Here we could confirm translational repression of Mbp mRNA in Schwann cells. We show that sncRNA715 is expressed and its levels correlate inversely with MBP in cultured Schwann cells and in the sciatic nerve in vivo. Furthermore we could reduce MBP protein levels in cultured Schwann cells by increasing the levels of the inhibitory sncRNA715. Our findings suggest similarities in sncRNA715-mediated translational repression of Mbp mRNA in oligodendrocytes and Schwann cells.