A dual role of erbB2 in myelination and in expansion of the schwann cell precursor pool

Pleiotrophin-Neuregulin1 promote axon regeneration and sorting in conduit repair of critical nerve gap injuries

Significant challenges remain in the treatment of critical nerve gap injuries using artificial nerve conduits. We previously reported successful axon regeneration across a 40 mm nerve gap using a biosynthetic nerve implant (BNI) with multi-luminal synergistic growth factor release. However, axon sorting, remyelination, and functional recovery were limited. Neuregulin1 (NRG1) plays a significant role in regulating the proliferation and differentiation of Schwann cells (SCs) during development and after injury. We hypothesize that the release of NRG1 type III combined with pleiotrophin (PTN) in the BNI will enhance axon growth, remyelination, and function of regenerated nerves across a critical gap. A rabbit 40 mm peroneal gap injury model was used to investigate the therapeutic efficacy of BNIs containing either NRG1, PTN, or PTN+NRG1 growth factor release. We found that NRG1 treatment doubled the number of regenerated axons (1276±895) compared to empty controls (633±666) and PTN tripled this number (2270±989). NRG1 also significantly increased the number of SOX10+ Schwann cells in mid-conduit (20.42%±11.78%) and reduced the number of abnormal Remak axon bundles. The combination of PTN+NRG1 increased axon diameter (1.70±1.06) vs control (1.21±0.77) (p<0.01), with 15.35% of axons above 3 μm, comparable to autograft. However, the total number of remyelinated axons was not increased by the added NRG1 release, which correlated with absence of axonal NRG1 type III expression in the regenerated axons. Electrophysiological evaluation showed higher muscle force recruitment (23.8±16.0 mN vs 17.4±1.4 mN) and maximum evoked compound motor action potential (353 μV vs 37 μV) in PTN-NRG1 group versus control, which correlated with the improvement in the toe spread recovery observed in PTN-NRG1 treated animals (0.64±0.02) vs control (0.50±0.01). These results revealed the need of a combination of pro-regenerative and remyelinating growth factor combination therapy for the repair of critical nerve gaps.

The interplay of common genetic variants NRG1 rs2439302 and RET rs2435357 increases the risk of developing Hirschsprung's disease

Introduction: As a congenital and genetically related disease, many single nucleotide polymorphisms (SNPs) have been reported to be associated with the risk of HSCR. Our previous research showed that SNP rs2439302 (NRG1) interacted with rs2435357 (RET) to increase the risk of HSCR development. However, the underlying molecular mechanism is still not well understood. Methods: SNP rs2439302 (NRG1) and rs2435357 (RET) were genotyped in 470 HSCR cases. The expression of NRG1 and RET was investigated in the colon of HSCR patients. Knockdown of the NRG1 and RET homologs was performed in zebrafish to investigate their synergistic effect on ENS development. The effect of SNP rs2439302 and rs2435357 polymorphism on neuron proliferation, migration, and differentiation were investigated in SHSY-5Y cells and IPSCs. Results: Significant downregulation of NRG1 and RET expression was noticed in the aganglionic segment of HSCR patients and SHSY-5Y cells with rs2439302 GG/rs2435357 TT genotype. NRG1 and RET double mutants caused the most severe reduction in enteric neuron numbers than NRG1 single mutant or RET single mutant in the hindgut of zebrafish. SHSY-5Y cells and IPSCs with rs2439302 GG/rs2435357 TT genotype exhibited a decreased proliferative, migration, and differentiative capacity. CTCF showed a considerably higher binding ability to SNP rs2439302 CC than GG. NRG1 reduction caused a further decrease in SOX10 expression via the PI3K/Akt pathway, which regulates RET expression by directly binding to rs2435357. Discussion: SNP rs2439302 (NRG1) GG increases the risk of developing HSCR by affecting the binding of transcription factor CTCF and interacting with rs2435357 (RET) to regulate RET expression via the PI3K/Akt/SOX10 pathway.

Development and In Vitro Differentiation of Schwann Cells

Schwann cells are glial cells of the peripheral nervous system. They exist in several subtypes and perform a variety of functions in nerves. Their derivation and culture in vitro are interesting for applications ranging from disease modeling to tissue engineering. Since primary human Schwann cells are challenging to obtain in large quantities, in vitro differentiation from other cell types presents an alternative. Here, we first review the current knowledge on the developmental signaling mechanisms that determine neural crest and Schwann cell differentiation in vivo. Next, an overview of studies on the in vitro differentiation of Schwann cells from multipotent stem cell sources is provided. The molecules frequently used in those protocols and their involvement in the relevant signaling pathways are put into context and discussed. Focusing on hiPSC- and hESC-based studies, different protocols are described and compared, regarding cell sources, differentiation methods, characterization of cells, and protocol efficiency. A brief insight into developments regarding the culture and differentiation of Schwann cells in 3D is given. In summary, this contribution provides an overview of the current resources and methods for the differentiation of Schwann cells, it supports the comparison and refinement of protocols and aids the choice of suitable methods for specific applications.

Peripheral glia diversity

Recent years have seen an evolving appreciation for the role of glial cells in the nervous system. As we move away from the typical neurocentric view of neuroscience, the complexity and variability of central nervous system glia is emerging, far beyond the three main subtypes: astrocytes, oligodendrocytes, and microglia. Yet the diversity of the glia found in the peripheral nervous system remains rarely discussed. In this review, we discuss the developmental origin, morphology, and function of the different populations of glia found in the peripheral nervous system, including: myelinating Schwann cells, Remak Schwann cells, repair Schwann cells, satellite glia, boundary cap-derived glia, perineurial glia, terminal Schwann cells, glia found in the skin, olfactory ensheathing cells, and enteric glia. The morphological and functional heterogeneity of glia found in the periphery reflects the diverse roles the nervous system performs throughout the body. Further, it highlights a complexity that should be appreciated and considered when it comes to a complete understanding of the peripheral nervous system in health and disease.

Partial loss-of-function variant in neuregulin 1 identified in family with heritable peripheral neuropathy

Neuregulin 1 signals are essential for the development and function of Schwann cells, which form the myelin sheath on peripheral axons. Disruption of myelin in the peripheral nervous system can lead to peripheral neuropathy, which is characterized by reduced axonal conduction velocity and sensorimotor deficits. Charcot-Marie-Tooth disease is a group of heritable peripheral neuropathies that may be caused by variants in nearly 100 genes. Despite the evidence that Neuregulin 1 is essential for many aspects of Schwann cell development, previous studies have not reported variants in the neuregulin 1 gene (NRG1) in patients with peripheral neuropathy. We have identified a rare missense variant in NRG1 that is homozygous in a patient with sensory and motor deficits consistent with mixed axonal and de-myelinating peripheral neuropathy. Our in vivo functional studies in zebrafish indicate that the patient variant partially reduces NRG1 function. This study tentatively suggests that variants at the NRG1 locus may cause peripheral neuropathy and that NRG1 should be investigated in families with peripheral neuropathy of unknown cause.

Disruption of Endosomal Sorting in Schwann Cells Leads to Defective Myelination and Endosomal Abnormalities Observed in Charcot-Marie-Tooth Disease

Endosomal sorting plays a fundamental role in directing neural development. By altering the temporal and spatial distribution of membrane receptors, endosomes regulate signaling pathways that control the differentiation and function of neural cells. Several genes linked to inherited demyelinating peripheral neuropathies, known as Charcot-Marie-Tooth (CMT) disease, encode proteins that directly interact with components of the endosomal sorting complex required for transport (ESCRT). Our previous studies demonstrated that a point mutation in the ESCRT component hepatocyte growth-factor-regulated tyrosine kinase substrate (HGS), an endosomal scaffolding protein that identifies internalized cargo to be sorted by the endosome, causes a peripheral neuropathy in the neurodevelopmentally impaired teetering mice. Here, we constructed a Schwann cell-specific deletion of Hgs to determine the role of endosomal sorting during myelination. Inactivation of HGS in Schwann cells resulted in motor and sensory deficits, slowed nerve conduction velocities, delayed myelination and hypomyelinated axons, all of which occur in demyelinating forms of CMT. Consistent with a delay in Schwann cell maturation, HGS-deficient sciatic nerves displayed increased mRNA levels for several promyelinating genes and decreased mRNA levels for genes that serve as markers of myelinating Schwann cells. Loss of HGS also altered the abundance and activation of the ERBB2/3 receptors, which are essential for Schwann cell development. We therefore hypothesize that HGS plays a critical role in endosomal sorting of the ERBB2/3 receptors during Schwann cell maturation, which further implicates endosomal dysfunction in inherited peripheral neuropathies.SIGNIFICANCE STATEMENT Schwann cells myelinate peripheral axons, and defects in Schwann cell function cause inherited demyelinating peripheral neuropathies known as CMT. Although many CMT-linked mutations are in genes that encode putative endosomal proteins, little is known about the requirements of endosomal sorting during myelination. In this study, we demonstrate that loss of HGS disrupts the endosomal sorting pathway in Schwann cells, resulting in hypomyelination, aberrant myelin sheaths, and impairment of the ERBB2/3 receptor pathway. These findings suggest that defective endosomal trafficking of internalized cell surface receptors may be a common mechanism contributing to demyelinating CMT.

Wnt5a Promotes Lysosomal Cholesterol Egress and Protects Against Atherosclerosis

BACKGROUND

Impairment of cellular cholesterol trafficking is at the heart of atherosclerotic lesions formation. This involves egress of cholesterol from the lysosomes and 2 lysosomal proteins, the NPC1 (Niemann-Pick C1) and NPC2 that promotes cholesterol trafficking. However, movement of cholesterol out the lysosome and how disrupted cholesterol trafficking leads to atherosclerosis is unclear. As the Wnt ligand, Wnt5a inhibits the intracellular accumulation of cholesterol in multiple cell types, we tested whether Wnt5a interacts with the lysosomal cholesterol export machinery and studied its role in atherosclerotic lesions formation.

METHODS

We generated mice deleted for the Wnt5a gene in vascular smooth muscle cells. To establish whether Wnt5a also protects against cholesterol accumulation in human vascular smooth muscle cells, we used a CRISPR/Cas9 guided nuclease approach to generate human vascular smooth muscle cells knockout for Wnt5a.

RESULTS

We show that Wnt5a is a crucial component of the lysosomal cholesterol export machinery. By increasing lysosomal acid lipase expression, decreasing metabolic signaling by the mTORC1 (mechanistic target of rapamycin complex 1) kinase, and through binding to NPC1 and NPC2, Wnt5a senses changes in dietary cholesterol supply and promotes lysosomal cholesterol egress to the endoplasmic reticulum. Consequently, loss of Wnt5a decoupled mTORC1 from variations in lysosomal sterol levels, disrupted lysosomal function, decreased cholesterol content in the endoplasmic reticulum, and promoted atherosclerosis.

CONCLUSIONS

These results reveal an unexpected function of the Wnt5a pathway as essential for maintaining cholesterol homeostasis in vivo.

Effective in vitro differentiation of adipose-derived stem cells into Schwann-like cells with folic acid supplementation

Peripheral nerve injury (PNI) after pelvic surgery is a common issue with a significant impact on patients. Autologous nerve grafting is the gold standard treatment for PNI, but this technique cannot be applied to fine nerve fibers in the pelvis. Schwann-like cell (SLC) differentiation is a novel therapeutic strategy for this clinical condition. However, the efficiency of SLC differentiation remains unsatisfactory. We modified an SLC differentiation protocol using adipose-derived stem cells (ADSCs) and folic acid. Morphology, gene expression and secretion of neurotrophic factors were examined to assess the differentiation quality and phenotypic characteristics. Our new modified protocol effectively induced a Schwann cell (SC) phenotype in ADSCs as assessed by morphology and expression of SC markers [S100 calcium-binding protein B (S100B), P < 0.01 ; p75 neurotrophic receptor (p75NTR), P < 0.05]. SLCs produced by the new protocol displayed a repair phenotype with decreased expression of ERBB2 and early growth response protein 2 (EGR2) / KROX20 (P < 0.01). Furthermore, our new protocol enhanced both mRNA expression and secretion of nerve growth factors by SLCs (P < 0.01). This protocol enhanced the SC characteristics and functions of ADSC-derived SLCs. This promising protocol requires further research and may contribute to SC-based nerve regeneration. J. Med. Invest. 68 : 347-353, August, 2021.

Distinct roles for the Charcot-Marie-tooth disease-causing endosomal regulators Mtmr5 and Mtmr13 in axon radial sorting and Schwann cell myelination

The form of Charcot-Marie-Tooth type 4B (CMT4B) disease caused by mutations in myotubularin-related 5 (MTMR5; also called SET Binding Factor 1; SBF1) shows a spectrum of axonal and demyelinating nerve phenotypes. This contrasts with the CMT4B subtypes caused by MTMR2 or MTMR13 (SBF2) mutations, which are characterized by myelin outfoldings and classic demyelination. Thus, it is unclear whether MTMR5 plays an analogous or distinct role from that of its homolog, MTMR13, in the peripheral nervous system (PNS). MTMR5 and MTMR13 are pseudophosphatases predicted to regulate endosomal trafficking by activating Rab GTPases and binding to the phosphoinositide 3-phosphatase MTMR2. In the mouse PNS, Mtmr2 was required to maintain wild type levels of Mtmr5 and Mtmr13, suggesting that these factors function in discrete protein complexes. Genetic elimination of both Mtmr5 and Mtmr13 in mice led to perinatal lethality, indicating that the two proteins have partially redundant functions during embryogenesis. Loss of Mtmr5 in mice did not cause CMT4B-like myelin outfoldings. However, adult Mtmr5-/- mouse nerves contained fewer myelinated axons than control nerves, likely as a result of axon radial sorting defects. Consistently, Mtmr5 levels were highest during axon radial sorting and fell sharply after postnatal day seven. Our findings suggest that Mtmr5 and Mtmr13 ensure proper axon radial sorting and Schwann cell myelination, respectively, perhaps through their direct interactions with Mtmr2. This study enhances our understanding of the non-redundant roles of the endosomal regulators MTMR5 and MTMR13 during normal peripheral nerve development and disease.

Primary Cilia in Glial Cells: An Oasis in the Journey to Overcoming Neurodegenerative Diseases

Many neurodegenerative diseases have been associated with defects in primary cilia, which are cellular organelles involved in diverse cellular processes and homeostasis. Several types of glial cells in both the central and peripheral nervous systems not only support the development and function of neurons but also play significant roles in the mechanisms of neurological disease. Nevertheless, most studies have focused on investigating the role of primary cilia in neurons. Accordingly, the interest of recent studies has expanded to elucidate the role of primary cilia in glial cells. Correspondingly, several reports have added to the growing evidence that most glial cells have primary cilia and that impairment of cilia leads to neurodegenerative diseases. In this review, we aimed to understand the regulatory mechanisms of cilia formation and the disease-related functions of cilia, which are common or specific to each glial cell. Moreover, we have paid close attention to the signal transduction and pathological mechanisms mediated by glia cilia in representative neurodegenerative diseases. Finally, we expect that this field of research will clarify the mechanisms involved in the formation and function of glial cilia to provide novel insights and ideas for the treatment of neurodegenerative diseases in the future.

Myelin Biology

Myelin is a key evolutionary specialization and adaptation of vertebrates formed by the plasma membrane of glial cells, which insulate axons in the nervous system. Myelination not only allows rapid and efficient transmission of electric impulses in the axon by decreasing capacitance and increasing resistance but also influences axonal metabolism and the plasticity of neural circuits. In this review, we will focus on Schwann cells, the glial cells which form myelin in the peripheral nervous system. Here, we will describe the main extrinsic and intrinsic signals inducing Schwann cell differentiation and myelination and how myelin biogenesis is achieved. Finally, we will also discuss how the study of human disorders in which molecules and pathways relevant for myelination are altered has enormously contributed to the current knowledge on myelin biology.

The Role of Epidermal Growth Factor Receptor Family of Receptor Tyrosine Kinases in Mediating Diabetes-Induced Cardiovascular Complications

Diabetes mellitus is a major debilitating disease whose global incidence is progressively increasing with currently over 463 million adult sufferers and this figure will likely reach over 700 million by the year 2045. It is the complications of diabetes such as cardiovascular, renal, neuronal and ocular dysfunction that lead to increased patient morbidity and mortality. Of these, cardiovascular complications that can result in stroke and cardiomyopathies are 2- to 5-fold more likely in diabetes but the underlying mechanisms involved in their development are not fully understood. Emerging research suggests that members of the Epidermal Growth Factor Receptor (EGFR/ErbB/HER) family of tyrosine kinases can have a dual role in that they are beneficially required for normal development and physiological functioning of the cardiovascular system (CVS) as well as in salvage pathways following acute cardiac ischemia/reperfusion injury but their chronic dysregulation may also be intricately involved in mediating diabetes-induced cardiovascular pathologies. Here we review the evidence for EGFR/ErbB/HER receptors in mediating these dual roles in the CVS and also discuss their potential interplay with the Renin-Angiotensin-Aldosterone System heptapeptide, Angiotensin-(1-7), as well the arachidonic acid metabolite, 20-HETE (20-hydroxy-5, 8, 11, 14-eicosatetraenoic acid). A greater understanding of the multi-faceted roles of EGFR/ErbB/HER family of tyrosine kinases and their interplay with other key modulators of cardiovascular function could facilitate the development of novel therapeutic strategies for treating diabetes-induced cardiovascular complications.

Microvesicles from Schwann-Like Cells as a New Biomaterial Promote Axonal Growth

Schwann cells promote axonal regeneration following peripheral nerve injury. However, in terms of clinical treatment, the therapeutic effects of Schwann cells are limited by their source. The transmission of microvesicles from neuroglia cells to axons is a novel communication mechanism in axon regeneration.To evaluate the effect of microvesicles released from Schwann-like cells on axonal regeneration, neural stem cells derived from human embryonic stem cells differentiated into Schwann-like cells, which presented a typical morphology and characteristics similar to those of schwann cells. The glial markers like MBP, P0, P75^NTR, PMP-22, GFAP, HNK-1 and S100 were upregulated, whereas the neural stem markers like NESTIN, SOX1 and SOX2 were significantly downregulated in schwann-like cells. Microvesicles enhanced axonal growth in dorsal root ganglia neurons and regulated GAP43 expression in neuron-like cells (N2A and PC12) through the PTEN/PI3 K/Akt signaling pathway. A 5 mm section of sciatic nerve was transected in Sprague-Dawley rats. With microvesicles transplantation, regenerative nerves were evaluated after 6 weeks. Microvesicles increased sciatic function index scores, delayed gastrocnemius muscle atrophy and elevated βIII-tubulin-labeled axons in vivo. Schwann-like cells serve as a convenient source and promote axonal growth by secreting microvesicles, which may potentially be used as bioengineering materials for nerve tissue repair.

Mesenchymal plasticity of devil facial tumour cells during in vivo vaccine and immunotherapy trials

Immune evasion is critical to the growth and survival of cancer cells. This is especially pertinent to transmissible cancers, which evade immune detection across genetically diverse hosts. The Tasmanian devil (Sarcophilus harrisii) is threatened by the emergence of Devil Facial Tumour Disease (DFTD), comprising two transmissible cancers (DFT1 and DFT2). The development of effective prophylactic vaccines and therapies against DFTD has been restricted by an incomplete understanding of how allogeneic DFT1 and DFT2 cells maintain immune evasion upon activation of tumour-specific immune responses. In this study, we used RNA sequencing to examine tumours from three experimental DFT1 cases. Two devils received a vaccine prior to inoculation with live DFT1 cells, providing an opportunity to explore changes to DFT1 cancers under immune pressure. Analysis of DFT1 in the non-immunised devil revealed a 'myelinating Schwann cell' phenotype, reflecting both natural DFT1 cancers and the DFT1 cell line used for the experimental challenge. Comparatively, immunised devils exhibited a 'dedifferentiated mesenchymal' DFT1 phenotype. A third 'immune-enriched' phenotype, characterised by increased PDL1 and CTLA-4 expression, was detected in a DFT1 tumour that arose after immunotherapy. In response to immune pressure, mesenchymal plasticity and upregulation of immune checkpoint molecules are used by human cancers to evade immune responses. Similar mechanisms are associated with immune evasion by DFTD cancers, providing novel insights that will inform modification of DFTD vaccines. As DFT1 and DFT2 are clonal cancers transmitted across genetically distinct hosts, the Tasmanian devil provides a 'natural' disease model for more broadly exploring these immune evasion mechanisms in cancer.

Axon-glia interactions in the ascending auditory system

The auditory system detects and encodes sound information with high precision to provide a high-fidelity representation of the environment and communication. In mammals, detection occurs in the peripheral sensory organ (the cochlea) containing specialized mechanosensory cells (hair cells) that initiate the conversion of sound-generated vibrations into action potentials in the auditory nerve. Neural activity in the auditory nerve encodes information regarding the intensity and frequency of sound stimuli, which is transmitted to the auditory cortex through the ascending neural pathways. Glial cells are critical for precise control of neural conduction and synaptic transmission throughout the pathway, allowing for the precise detection of the timing, frequency, and intensity of sound signals, including the sub-millisecond temporal fidelity is necessary for tasks such as sound localization, and in humans, for processing complex sounds including speech and music. In this review, we focus on glia and glia-like cells that interact with hair cells and neurons in the ascending auditory pathway and contribute to the development, maintenance, and modulation of neural circuits and transmission in the auditory system. We also discuss the molecular mechanisms of these interactions, their impact on hearing and on auditory dysfunction associated with pathologies of each cell type.

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.

Motoneuron expression profiling identifies an association between an axonal splice variant of HDGF-related protein 3 and peripheral myelination

Disorders that disrupt myelin formation during development or in adulthood, such as multiple sclerosis and peripheral neuropathies, lead to severe pathologies, illustrating myelin's crucial role in normal neural functioning. However, although our understanding of glial biology is increasing, the signals that emanate from axons and regulate myelination remain largely unknown. To identify the core components of the myelination process, here we adopted a microarray analysis approach combined with laser-capture microdissection of spinal motoneurons during the myelinogenic phase of development. We identified neuronal genes whose expression was enriched during myelination and further investigated hepatoma-derived growth factor-related protein 3 (HRP3 or HDGFRP3). HRP3 was strongly expressed in the white matter fiber tracts of the peripheral (PNS) and central (CNS) nervous systems during myelination and remyelination in a cuprizone-induced demyelination model. The dynamic localization of HPR3 between axons and nuclei during myelination was consistent with its axonal localization during neuritogenesis. To study this phenomenon, we identified two splice variants encoded by the HRP3 gene: the canonical isoform HRP3-I and a newly recognized isoform, HRP3-II. HRP3-I remained solely in the nucleus, whereas HRP3-II displayed distinct axonal localization both before and during myelination. Interestingly, HRP3-II remained in the nuclei of unmyelinated neurons and glial cells, suggesting the existence of a molecular machinery that transfers it to and retains it in the axons of neurons fated for myelination. Overexpression of HRP3-II, but not of HRP3-I, increased Schwann cell numbers and myelination in PNS neuron-glia co-cultures. However, HRP3-II overexpression in CNS co-cultures did not alter myelination.

Role of EGFR in the Nervous System

Epidermal growth factor receptor (EGFR) is the first discovered member of the receptor tyrosine kinase superfamily and plays a fundamental role during embryogenesis and in adult tissues, being involved in growth, differentiation, maintenance and repair of various tissues and organs. The role of EGFR in the regulation of tissue development and homeostasis has been thoroughly investigated and it has also been demonstrated that EGFR is a driver of tumorigenesis. In the nervous system, other growth factors, and thus other receptors, are important for growth, differentiation and repair of the tissue, namely neurotrophins and neurotrophins receptors. For this reason, for a long time, the role of EGFR in the nervous system has been underestimated and poorly investigated. However, EGFR is expressed both in the central and peripheral nervous systems and it has been demonstrated to have specific important neurotrophic functions, in particular in the central nervous system. This review discusses the role of EGFR in regulating differentiation and functions of neurons and neuroglia. Furthermore, its involvement in regeneration after injury and in the onset of neurodegenerative diseases is examined.

Neurogenesis From Neural Crest Cells: Molecular Mechanisms in the Formation of Cranial Nerves and Ganglia

The neural crest (NC) is a transient multipotent cell population that originates in the dorsal neural tube. Cells of the NC are highly migratory, as they travel considerable distances through the body to reach their final sites. Derivatives of the NC are neurons and glia of the peripheral nervous system (PNS) and the enteric nervous system as well as non-neural cells. Different signaling pathways triggered by Bone Morphogenetic Proteins (BMPs), Fibroblast Growth Factors (FGFs), Wnt proteins, Notch ligands, retinoic acid (RA), and Receptor Tyrosine Kinases (RTKs) participate in the processes of induction, specification, cell migration and neural differentiation of the NC. A specific set of signaling pathways and transcription factors are initially expressed in the neural plate border and then in the NC cell precursors to the formation of cranial nerves. The molecular mechanisms of control during embryonic development have been gradually elucidated, pointing to an important role of transcriptional regulators when neural differentiation occurs. However, some of these proteins have an important participation in malformations of the cranial portion and their mutation results in aberrant neurogenesis. This review aims to give an overview of the role of cell signaling and of the function of transcription factors involved in the specification of ganglia precursors and neurogenesis to form the NC-derived cranial nerves during organogenesis.

Unraveling ERBB network dynamics upon betacellulin signaling in pancreatic ductal adenocarcinoma in mice

Pancreatic ductal adenocarcinoma (PDAC) will soon belong to the top three cancer killers. The only approved specific PDAC therapy targets the epidermal growth factor receptor (EGFR). Although EGFR is a crucial player in PDAC development, EGFR-based therapy is disappointing. In this study, we evaluated the role of the EGFR ligand betacellulin (BTC) in PDAC. The expression of BTC was investigated in human pancreatic cancer specimen. Then, we generated a BTC knockout mouse model by CRISPR/Cas9 technology and a BTC overexpression model. Both models were crossed with the Ptf1aCre/+ ;KRASG12D/+ (KC) mouse model (B-/- KC or BKC, respectively). In addition, EGFR, ERBB2, and ERBB4 were investigated by the pancreas-specific deletion of each receptor using the Cre-loxP system. Tumor initiation and progression were analyzed in all mouse lines, and the underlying molecular biology of PDAC was investigated at different time points. BTC is expressed in human and murine PDAC. B-/- KC mice showed a decelerated PDAC progression, associated with decreased EGFR activation. BKC mice developed severe PDAC with a poor survival rate. The dramatically increased BTC-mediated tumor burden was EGFR-dependent, but also ERBB4 and ERBB2 were involved in PDAC development or progression, as depletion of EGFR, ERBB2, or ERBB4 significantly improved the survival rate of BTC-mediated PDAC. BTC increases PDAC tumor burden dramatically by enhanced RAS activation. EGFR signaling, ERBB2 signaling, and ERBB4 signaling are involved in accelerated PDAC development mediated by BTC indicating that targeting the whole ERBB family, instead of a single receptor, is a promising strategy for the development of future PDAC therapies.

The impact of EGFR gene polymorphisms on the response and toxicity derived from neoadjuvant chemotherapy for breast cancer

Background

Neoadjuvant chemotherapy is usually used for treating locally advanced breast cancer. However, not all patients achieve pathologic complete response (pCR). In this study, we selected two epidermal growth factor receptor (EGFR) single nucleotide polymorphism (SNP) sites, rs1468727 and rs845552, to investigate the association between the genotypes and the response and toxicity derived from neoadjuvant chemotherapy for breast cancer.

Methods

All participants took part in clinical trial SHPD001 and SHPD002. For univariate analyses, the association between SNP and pCR or toxicity was analyzed by Chi-square or Fisher's exact test. For multivariate analyses, logistic regression was used instead.

Results

In all, one hundred and eighteen patients were enrolled. We found that the frequency of AA genotype in rs845552 was higher than that of other genotypes in HER2-positive breast cancer (AA vs. AG, P=0.039; AA vs. GG, P=0.005; AA vs. AG+GG, P=0.009). Multivariate logistic regression analyses showed that pCR was more difficult to be achieved in patients with a CT genotype in rs1468727 compared to those with a CC+TT genotype (OR =0.288, 95% CI: 0.109-0.762, P=0.012) or a CC genotype (OR =0.254, 95% CI: 0.076-0.849, P=0.026). Moreover, we demonstrated that both rs1468727 and rs845552 were associated with toxicity that results in complications such as increased total bilirubin, skin rash, peripheral neuropathy, and alopecia (P<0.05).

Conclusions

Our study reported for the first time, that in treating breast cancer with neoadjuvant chemotherapy, EGFR SNP rs1468727 is associated with treatment response, and that both rs1468727 and rs845552 are related to treatment-derived toxicity. In addition, we also found that rs845552 may be related to the status of HER2 in breast cancer.

Differential impact of the ERBB receptors EGFR and ERBB2 on the initiation of precursor lesions of pancreatic ductal adenocarcinoma

Earlier diagnosis of pancreatic ductal adenocarcinoma (PDAC) requires better understanding of the mechanisms driving tumorigenesis. In this context, depletion of Epidermal Growth Factor Receptor (EGFR) is known to impair development of PDAC-initiating lesions called acinar-to-ductal metaplasia (ADM) and Pancreatic Intraepithelial Neoplasia (PanIN). In contrast, the role of v-erb-b2 erythroblastic leukemia viral oncogene homolog 2 (ERBB2), the preferred dimerization partner of EGFR, remains poorly understood. Here, using a mouse model with inactivation of Erbb2 in pancreatic acinar cells, we found that Erbb2 is dispensable for inflammation- and KRas^G12D-induced development of ADM and PanIN. A mathematical model of EGFR/ERBB2-KRAS signaling, which was calibrated on mouse and human data, supported the observed roles of EGFR and ERBB2. However, this model also predicted that overexpression of ERBB2 stimulates ERBB/KRAS signaling; this prediction was validated experimentally. We conclude that EGFR and ERBB2 differentially impact ERBB signaling during PDAC tumorigenesis, and that the oncogenic potential of ERBB2 is only manifested when it is overexpressed. Therefore, the level of ERBB2, not only its mere presence, needs to be considered when designing therapies targeting ERBB signaling.

Defective transcription of ATF3 responsive genes, a marker for Cockayne Syndrome

Cockayne syndrome (CS) is a rare genetic disorder caused by mutations (dysfunction) in CSA and CSB. CS patients exhibit mild photosensitivity and severe neurological problems. Currently, CS diagnosis is based on the inefficiency of CS cells to recover RNA synthesis upon genotoxic (UV) stress. Indeed, upon genotoxic stress, ATF3, an immediate early gene is activated to repress up to 5000 genes encompassing its responsive element for a short period of time. On the contrary in CS cells, CSA and CSB dysfunction impairs the degradation of the chromatin-bound ATF3, leading to a permanent transcriptional arrest as observed by immunofluorescence and ChIP followed by RT-PCR. We analysed ChIP-seq of Pol II and ATF3 promoter occupation analysis and RNA sequencing-based gene expression profiling in CS cells, as well as performed immunofluorescence study of ATF3 protein stability and quantitative RT-PCR screening in 64 patient cell lines. We show that the analysis of few amount (as for example CDK5RAP2, NIPBL and NRG1) of ATF3 dependent genes, could serve as prominent molecular markers to discriminate between CS and non-CS patient’s cells. Such assay can significantly simplify the timing and the complexity of the CS diagnostic procedure in comparison to the currently available methods.

The role of teratogens in neural crest development

The neural crest (NC), discovered by Wilhelm His 150 years ago, gives rise to a multipotent migratory embryonic cell population that generates a remarkably diverse and important array of cell types during the development of the vertebrate embryo. These cells originate in the neural plate border (NPB), which is the ectoderm between the neural plate and the epidermis. They give rise to the neurons and glia of the peripheral nervous system, melanocytes, chondrocytes, smooth muscle cells, odontoblasts and neuroendocrine cells, among others. Neurocristopathies are a class of congenital diseases resulting from the abnormal induction, specification, migration, differentiation or death of NC cells (NCCs) during embryonic development and have an important medical and societal impact. In general, congenital defects affect an appreciable percentage of newborns worldwide. Some of these defects are caused by teratogens, which are agents that negatively impact the formation of tissues and organs during development. In this review, we will discuss the teratogens linked to the development of many birth defects, with a strong focus on those that specifically affect the development of the NC, thereby producing neurocristopathies. Although increasing attention is being paid to the effect of teratogens on embryonic development in general, there is a strong need to critically evaluate the specific role of these agents in NC development. Therefore, increased understanding of the role of these factors in NC development will contribute to the planning of strategies aimed at the prevention and treatment of human neurocristopathies, whose etiology was previously not considered.

Elmo1 function, linked to Rac1 activity, regulates peripheral neuronal numbers and myelination in zebrafish

Peripheral nervous system development involves a tight coordination of neuronal birth and death and a substantial remodelling of the myelinating glia cytoskeleton to achieve myelin wrapping of its projecting axons. However, how these processes are coordinated through time is still not understood. We have identified engulfment and cell motility 1, Elmo1, as a novel component that regulates (i) neuronal numbers within the Posterior Lateral Line ganglion and (ii) radial sorting of axons by Schwann cells (SC) and myelination in the PLL system in zebrafish. Our results show that neuronal and myelination defects observed in elmo1 mutant are rescued through small GTPase Rac1 activation. Inhibiting macrophage development leads to a decrease in neuronal numbers, while peripheral myelination is intact. However, elmo1 mutants do not show defective macrophage activity, suggesting a role for Elmo1 in PLLg neuronal development and SC myelination independent of macrophages. Forcing early Elmo1 and Rac1 expression specifically within SCs rescues elmo1-/- myelination defects, highlighting an autonomous role for Elmo1 and Rac1 in radial sorting of axons by SCs and myelination. This uncovers a previously unknown function of Elmo1 that regulates fundamental aspects of PNS development.

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.

Schwann cell plasticity-roles in tissue homeostasis, regeneration, and disease

How tissues are maintained over a lifetime and repaired following injury are fundamental questions in biology with a disruption to these processes underlying pathologies such as cancer and degenerative disorders. It is becoming increasingly clear that each tissue has a distinct mechanism to maintain homeostasis and respond to injury utilizing different types of stem/progenitor cell populations depending on the insult and/or with a contribution from more differentiated cells that are able to dedifferentiate to aid tissue regeneration. Peripheral nerves are highly quiescent yet show remarkable regenerative capabilities. Remarkably, there is no evidence for a classical stem cell population, rather all cell-types within the nerve are able to proliferate to produce new nerve tissue. Co-ordinating the regeneration of this tissue are Schwann cells (SCs), the main glial cells of the peripheral nervous system. SCs exist in architecturally stable structures that can persist for the lifetime of an animal, however, they are not postmitotic, in that following injury they are reprogrammed at high efficiency to a progenitor-like state, with these cells acting to orchestrate the nerve regeneration process. During nerve regeneration, SCs show little plasticity, maintaining their identity in the repaired tissue. However, once free of the nerve environment they appear to exhibit increased plasticity with reported roles in the repair of other tissues. In this review, we will discuss the mechanisms underlying the homeostasis and regeneration of peripheral nerves and how reprogrammed progenitor-like SCs have broader roles in the repair of other tissues with implications for pathologies such as cancer.

Spatiotemporal Role of Transforming Growth Factor Beta 2 in Developing and Mature Mouse Hindbrain Serotonergic Neurons

Transforming growth factor betas are integral molecular components of the signalling cascades defining development and survival of several neuronal groups. Among TGF-β ligands, TGF-β2 has been considered as relatively more important during development. We have generated a conditional knockout mouse of the Tgf-β2 gene with knock-in of an EGFP reporter and subsequently a mouse line with cell-type specific deletion of TGF-β2 ligand from Krox20 expressing cells (i.e., in cells from rhombomeres r3 and r5). We performed a phenotypic analysis of the hindbrain serotonergic system during development and in adulthood, determined the neurochemical profile in hindbrain and forebrain, and assessed behavioural performance of wild type and mutant mice. Mutant mice revealed significantly decreased number of caudal 5-HT neurons at embryonic day (E) 14, and impaired development of caudal dorsal raphe, median raphe, raphe magnus, and raphe obscurus neurons at E18, a phenotype that was largely restored and even overshot in dorsal raphe of mutant adult mice. Serotonin levels were decreased in hindbrain but significantly increased in cortex of adult mutant mice, though without any behavioural consequences. These results highlight differential and temporal dependency of developing and adult neurons on TGF-β2. The results also indicate TGF-β2 being directly or indirectly potent to modulate neurotransmitter synthesis and metabolism. The novel floxed TGF-β2 mouse model is a suitable tool for analysing the in vivo functions of TGF-β2 during development and in adulthood in many organs.

Nrg1 Intracellular Signaling Is Neuroprotective upon Stroke

The schizophrenia risk gene NRG1 controls the formation of excitatory and inhibitory synapses in cortical circuits. While the expression of different NRG1 isoforms occurs during development, adult neurons primarily express the CRD-NRG1 isoform characterized by a highly conserved intracellular domain (NRG1-ICD). We and others have demonstrated that Nrg1 intracellular signaling promotes dendrite elongation and excitatory connections during neuronal development. However, the role of Nrg1 intracellular signaling in adult neurons and pathological conditions remains largely unaddressed. Here, we investigated the role of Nrg1 intracellular signaling in neuroprotection and stroke. Our bioinformatic analysis revealed the evolutionary conservation of the NRG1-ICD and a decrease in NRG1 expression with age in the human frontal cortex. Hence, we first evaluated whether Nrg1 signaling may affect pathological hallmarks in an in vitro model of neuronal senescence; however, our data failed to reveal a role for Nrg1 in the activation of the stress-related pathway p38 MAPK and DNA damage. Previous studies demonstrated that the soluble EGF domain of Nrg1 alleviated brain ischemia, a pathological process involving the generation of free radicals, reactive oxygen species (ROS), and excitotoxicity. Hence, we tested the hypothesis that Nrg1 intracellular signaling could be neuroprotective in stroke. We discovered that Nrg1 expression significantly increased neuronal survival upon oxygen-glucose deprivation (OGD), an established in vitro model for stroke. Notably, the specific activation of Nrg1 intracellular signaling by expression of the Nrg1-ICD protected neurons from OGD. Additionally, time-lapse experiments confirmed that Nrg1 intracellular signaling increased the survival of neurons exposed to OGD. Finally, we investigated the relevance of Nrg1 intracellular signaling in stroke in vivo. Using viral vectors, we expressed the Nrg1-ICD in cortical neurons and subsequently challenged them by a focal hemorrhagic stroke; our data indicated that Nrg1 intracellular signaling improved neuronal survival in the infarcted area. Altogether, these data highlight Nrg1 intracellular signaling as neuroprotective upon ischemic lesion both in vitro and in vivo. Given the complexity of the neurotoxic effects of stroke and the involvement of various mechanisms, such as the generation of ROS, excitotoxicity, and inflammation, further studies are required to determine the molecular bases of the neuroprotective effect of Nrg1 intracellular signaling. In conclusion, our research highlights the stimulation of Nrg1 intracellular signaling as a promising target for cortical stroke treatment.

Human hair keratins promote the regeneration of peripheral nerves in a rat sciatic nerve crush model

Axon regeneration and functional recovery after peripheral nerve injury remains a clinical challenge. Injury leads to axonal disintegration after which Schwann cells (SCs) and macrophages re-engage in the process of regeneration. At present, biomaterials are regarded as the most promising way to repair peripheral nerve damage. As a natural material, keratin has a wide range of sources and has good biocompatibility and biodegradability. Here, a keratin was extracted from human hair by reducing method and a keratin sponge with porous structure was obtained by further processing. The results suggested that keratin can promote cell adhesion, proliferation, migration as well as the secretion of neurotrophic factors by SCs and the regulation of the expression of macrophage inflammatory cytokines in vitro. We report for the first time that human hair keratin can promote the extension of axon in DRG neurons. The motor deficits caused by a sciatic nerve crush injury were alleviated by keratin sponge dressing in vivo. Thus, keratin has been identified as a valuable biomaterial that can enhance peripheral nerve regeneration.

Neuregulin-1/ErbB network: An emerging modulator of nervous system injury and repair

Neuregulin-1 (Nrg-1) is a member of the Neuregulin family of growth factors with essential roles in the developing and adult nervous system. Six different types of Nrg-1 (Nrg-1 type I-VI) and over 30 isoforms have been discovered; however, their specific roles are not fully determined. Nrg-1 signals through a complex network of protein-tyrosine kinase receptors, ErbB2, ErbB3, ErbB4 and multiple intracellular pathways. Genetic and pharmacological studies of Nrg-1 and ErbB receptors have identified a critical role for Nrg-1/ErbB network in neurodevelopment including neuronal migration, neural differentiation, myelination as well as formation of synapses and neuromuscular junctions. Nrg-1 signaling is best known for its characterized role in development and repair of the peripheral nervous system (PNS) due to its essential role in Schwann cell development, survival and myelination. However, our knowledge of the impact of Nrg-1/ErbB on the central nervous system (CNS) has emerged in recent years. Ongoing efforts have uncovered a multi-faceted role for Nrg-1 in regulating CNS injury and repair processes. In this review, we provide a timely overview of the most recent updates on Nrg-1 signaling and its role in nervous system injury and diseases. We will specifically highlight the emerging role of Nrg-1 in modulating the glial and immune responses and its capacity to foster neuroprotection and remyelination in CNS injury. Nrg-1/ErbB network is a key regulatory pathway in the developing nervous system; therefore, unraveling its role in neuropathology and repair can aid in development of new therapeutic approaches for nervous system injuries and associated disorders.

Toxic Peripheral Neuropathies: Agents and Mechanisms

Toxic peripheral neuropathies are an important form of acquired polyneuropathy produced by a variety of xenobiotics and different exposure scenarios. Delineating the mechanisms of neurotoxicants and determining the degenerative biological pathways triggered by peripheral neurotoxicants will facilitate the development of sensitive and specific biochemical-based methods for identifying neurotoxicants, designing therapeutic interventions, and developing structure-activity relationships for predicting potential neurotoxicants. This review presents an overview of the general concepts of toxic peripheral neuropathies with the goal of providing insight into why certain agents target the peripheral nervous system and produce their associated lesions. Experimental data and the main hypotheses for the mechanisms of selected agents that produce neuronopathies, axonopathies, or myelinopathies including covalent or noncovalent modifications, compromised energy or protein biosynthesis, and oxidative injury and disruption of ionic gradients across membranes are presented. The relevance of signaling between the main components of peripheral nerve, that is, glia, neuronal perikaryon, and axon, as a target for neurotoxicants and the contribution of active programmed degenerative pathways to the lesions observed in toxic peripheral neuropathies is also discussed.

Neuregulin 1 type III improves peripheral nerve myelination in a mouse model of congenital hypomyelinating neuropathy

Myelin sheath thickness is precisely regulated and essential for rapid propagation of action potentials along myelinated axons. In the peripheral nervous system, extrinsic signals from the axonal protein neuregulin 1 (NRG1) type III regulate Schwann cell fate and myelination. Here we ask if modulating NRG1 type III levels in neurons would restore myelination in a model of congenital hypomyelinating neuropathy (CHN). Using a mouse model of CHN, we improved the myelination defects by early overexpression of NRG1 type III. Surprisingly, the improvement was independent from the upregulation of Egr2 or essential myelin genes. Rather, we observed the activation of MAPK/ERK and other myelin genes such as peripheral myelin protein 2 and oligodendrocyte myelin glycoprotein. We also confirmed that the permanent activation of MAPK/ERK in Schwann cells has detrimental effects on myelination. Our findings demonstrate that the modulation of axon-to-glial NRG1 type III signaling has beneficial effects and improves myelination defects during development in a model of CHN.

Schwann Cell Precursors; Multipotent Glial Cells in Embryonic Nerves

The cells of the neural crest, often referred to as neural crest stem cells, give rise to a number of sub-lineages, one of which is Schwann cells, the glial cells of peripheral nerves. Crest cells transform to adult Schwann cells through the generation of two well defined intermediate stages, the Schwann cell precursors (SCP) in early embryonic nerves, and immature Schwann cells (iSch) in late embryonic and perinatal nerves. SCP are formed when neural crest cells enter nascent nerves and form intimate relationships with axons, a diagnostic feature of glial cells. This involves large-scale changes in gene expression, including the activation of established glial cell markers. Like early glia in the CNS, radial glia, SCP retain developmental multipotency and contribute to other crest-derived lineages during embryonic development. SCP, as well as closely related cells termed boundary cap cells, and later stages of the Schwann cell lineage have all been implicated as the tumor initiating cell in NF1 associated neurofibromas. iSch are formed from SCP in a process that involves the appearance of additional differentiation markers, autocrine survival circuits, cellular elongation, a formation of endoneurial connective tissue and basal lamina. Finally, in peri- and post-natal nerves, iSch are reversibly induced by axon-associated signals to form the myelin and non-myelin Schwann cells of adult nerves. This review article discusses early Schwann cell development in detail and describes a large number of molecular signaling systems that control glial development in embryonic nerves.

NRG1 signalling regulates the establishment of Sertoli cell stock in the mouse testis

Testis differentiation requires high levels of proliferation of progenitor cells that give rise to two cell lineages forming the testis, the Sertoli and the Leydig cells. Hence defective cell cycling leads to testicular dysgenesis that has profound effects on androgen production and fertility. The growth factor NRG1 has been implicated in adult Leydig cell proliferation, but a potential function in the fetal testis has not been analysed to date. Here we show that Nrg1 and its receptors ErbB2/3 are already expressed in early gonadal development. Using tissue-specific deletion, we further demonstrate that Nrg1 is required in a dose-dependent manner to induce proliferation of Sertoli progenitor cells and then differentiated Sertoli cells. As a result of reduced numbers of Sertoli cells, Nrg1 knockout mice display a delay in testis differentiation and defects in sex cord partitioning. Taken together Nrg1 signalling is essential for the establishment of the stock of Sertoli cells and thus required to prevent testicular hypoplasia.

Maf links Neuregulin1 signaling to cholesterol synthesis in myelinating Schwann cells

Kim et al. define a crucial role of the transcription factor Maf in myelination and cholesterol biosynthesis and show that Maf acts downstream from Neuregulin1 in myelinating Schwann cells.

Cholesterol is a major constituent of myelin membranes, which insulate axons and allow saltatory conduction. Therefore, Schwann cells, the myelinating glia of the peripheral nervous system, need to produce large amounts of cholesterol. Here, we define a crucial role of the transcription factor Maf in myelination and cholesterol biosynthesis and show that Maf acts downstream from Neuregulin1 (Nrg1). Maf expression is induced when Schwann cells begin myelination. Genetic ablation of Maf resulted in hypomyelination that resembled mice with defective Nrg1 signaling. Importantly, loss of Maf or Nrg1 signaling resulted in a down-regulation of the cholesterol synthesis program, and Maf directly binds to enhancers of cholesterol synthesis genes. Furthermore, we identified the molecular mechanisms by which Nrg1 signaling regulates Maf levels. Transcription of Maf depends on calmodulin-dependent kinases downstream from Nrg1, whereas Nrg1–MAPK signaling stabilizes Maf protein. Our results delineate a novel signaling cascade regulating cholesterol synthesis in myelinating Schwann cells.

Neuregulin-1 is a chemoattractant and chemokinetic molecule for trunk neural crest cells

BACKGROUND

Trunk neural crest cells migrate rapidly along characteristic pathways within the developing vertebrate embryo. Proper trunk neural crest cell migration is necessary for the morphogenesis of much of the peripheral nervous system, melanocytes, and the adrenal medulla. Numerous molecules help guide trunk neural crest cell migration throughout the early embryo.

RESULTS

The trophic factor NRG1 is a chemoattractant through in vitro chemotaxis assays and in vivo silencing via a DN-erbB receptor. Interestingly, we also observed changes in migratory responses consistent with a chemokinetic effect of NRG1 in trunk neural crest velocity.

CONCLUSIONS

NRG1 is a trunk neural crest cell chemoattractant and chemokinetic molecule. Developmental Dynamics 247:888-902, 2018.. © 2018 Wiley Periodicals, Inc.

Neurotropin® Accelerates the Differentiation of Schwann Cells and Remyelination in a Rat Lysophosphatidylcholine-Induced Demyelination Model

Neurotropin^® (NTP), a non-protein extract of inflamed rabbit skin inoculated with vaccinia virus, is clinically used for the treatment of neuropathic pain in Japan and China, although its effect on peripheral nerve regeneration remains to be elucidated. The purpose of this study was to investigate the effects of NTP on Schwann cells (SCs) in vitro and in vivo, which play an important role in peripheral nerve regeneration. In SCs, NTP upregulated protein kinase B (AKT) activity and Krox20 and downregulated extracellular signal-regulated kinase1/2 activity under both growth and differentiation conditions, enhanced the expression of myelin basic protein and protein zero under the differentiation condition. In a co-culture of dorsal root ganglion neurons and SCs, NTP accelerated myelination of SCs. To further investigate the influence of NTP on SCs in vivo, lysophosphatidylcholine was injected into the rat sciatic nerve, leading to the focal demyelination. After demyelination, NTP was administered systemically with an osmotic pump for one week. NTP improved the ratio of myelinated axons and motor, sensory, and electrophysiological function. These findings reveal novel effects of NTP on SCs differentiation in vitro and in vivo, and indicate NTP as a promising treatment option for peripheral nerve injuries and demyelinating diseases.

Sustained MAPK/ERK Activation in Adult Schwann Cells Impairs Nerve Repair

The MAPK/ERK pathway has a critical role in PNS development. It is required for Schwann cell (SC) differentiation and myelination; sustained embryonic MAPK/ERK activation in SCs enhances myelin growth overcoming signals that normally end myelination. Excess activation of this pathway can be maladaptive as in adulthood acute strong activation of MAPK/ERK has been shown to cause SC dedifferentiation and demyelination. We used a mouse model (including male and female animals) in which the gain-of-function MEK1DD allele produces sustained MAPK/ERK activation in adult SCs, and we determined the impact of such activation on nerve repair. In the uninjured nerve, MAPK/ERK activation neither impaired myelin nor reactivated myelination. However, in the injured nerve it was detrimental and resulted in delayed repair and functional recovery. In the early phase of injury, the rate of myelin clearance was faster. Four weeks following injury, when nerve repair is normally advanced, myelinated axons of MEK1DD mutants demonstrated higher rates of myelin decompaction, a reduced number of Cajal bands. and decreased internodal length. We noted the presence of abnormal Remak bundles with long SCs processes and reduced numbers of C-fibers/Remak bundle. Both the total number of regenerating axons and the intraepidermal nerve fiber density in the skin were reduced. Sustained activation of MAPK/ERK in adult SCs is therefore deleterious to successful nerve repair, emphasizing the differences in the signaling processes coordinating nerve development and repair. Our results also underline the key role of SCs in axon regeneration and successful target reinnervation.SIGNIFICANCE STATEMENT The MAPK/ERK pathway promotes developmental myelination and its sustained activation in SCs induced continuous myelin growth, compensating for the absence of essential myelination signals. However, the strength of activation is fundamental because acute strong induction of MAPK/ERK in adulthood induces demyelination. What has been unknown is the effect of a mild but sustained MAPK/ERK activation in SCs on nerve repair in adulthood. This promoted myelin clearance but led to abnormalities in nonmyelinating and myelinating SCs in the later phases of nerve repair, resulting in slowed axon regeneration, cutaneous reinnervation, and functional recovery. Our results emphasize the distinct role of the MAPK/ERK pathway in developmental myelination versus remyelination and the importance of signaling between SCs and axons for successful axon regeneration.

Cockayne's Syndrome A and B Proteins Regulate Transcription Arrest after Genotoxic Stress by Promoting ATF3 Degradation

Cockayne syndrome (CS) is caused by mutations in CSA and CSB. The CSA and CSB proteins have been linked to both promoting transcription-coupled repair and restoring transcription following DNA damage. We show that UV stress arrests transcription of approximately 70% of genes in CSA- or CSB-deficient cells due to the constitutive presence of ATF3 at CRE/ATF sites. We found that CSB, CSA/DDB1/CUL4A, and MDM2 were essential for ATF3 ubiquitination and degradation by the proteasome. ATF3 removal was concomitant with the recruitment of RNA polymerase II and the restart of transcription. Preventing ATF3 ubiquitination by mutating target lysines prevented recovery of transcription and increased cell death following UV treatment. Our data suggest that the coordinate action of CSA and CSB, as part of the ubiquitin/proteasome machinery, regulates the recruitment timing of DNA-binding factors and provide explanations about the mechanism of transcription arrest following genotoxic stress.

Perineurial Glial Plasticity and the Role of TGF-β in the Development of the Blood-Nerve Barrier

Precisely orchestrated interactions between spinal motor axons and their ensheathing glia are vital for forming and maintaining functional spinal motor nerves. Following perturbations to peripheral myelinating glial cells, centrally derived oligodendrocyte progenitor cells (OPCs) ectopically exit the spinal cord and myelinate peripheral nerves in myelin with CNS characteristics. However, whether remaining peripheral ensheathing glia, such as perineurial glia, properly encase the motor nerve despite this change in glial cell and myelin composition, remains unknown. Using zebrafish mutants in which OPCs migrate out of the spinal cord and myelinate peripheral motor axons, we assayed perineurial glial development, maturation, and response to injury. Surprisingly, in the presence of OPCs, perineurial glia exited the CNS normally. However, aspects of their development, response to injury, and function were altered compared with wildtype larvae. In an effort to better understand the plasticity of perineurial glia in response to myelin perturbations, we identified transforming growth factor-β1 as a partial mediator of perineurial glial development. Together, these results demonstrate the incredible plasticity of perineurial glia in the presence of myelin perturbations.SIGNIFICANCE STATEMENT Peripheral neuropathies can result from damage or dysregulation of the insulating myelin sheath surrounding spinal motor axons, causing pain, inefficient nerve conduction, and the ectopic migration of oligodendrocyte progenitor cells (OPCs), the resident myelinating glial cell of the CNS, into the periphery. How perineurial glia, the ensheathing cells that form the protective blood-nerve barrier, are impacted by this myelin composition change is unknown. Here, we report that certain aspects of perineurial glial development and injury responses are mostly unaffected in the presence of ectopic OPCs. However, perineurial glial function is disrupted along nerves containing centrally derived myelin, demonstrating that, although perineurial glial cells display plasticity despite myelin perturbations, the blood-nerve barrier is compromised in the presence of ectopic OPCs.

Co-cultures with stem cell-derived human sensory neurons reveal regulators of peripheral myelination

See Saporta and Shy (doi:10.1093/awx048) for a scientific commentary on this article.Effective bidirectional signalling between axons and Schwann cells is essential for both the development and maintenance of peripheral nerve function. We have established conditions by which human induced pluripotent stem cell-derived sensory neurons can be cultured with rat Schwann cells, and have produced for the first time long-term and stable myelinating co-cultures with human neurons. These cultures contain the specialized domains formed by axonal interaction with myelinating Schwann cells, such as clustered voltage-gated sodium channels at the node of Ranvier and Shaker-type potassium channel (Kv1.2) at the juxtaparanode. Expression of type III neuregulin-1 (TIIINRG1) in induced pluripotent stem cell-derived sensory neurons strongly enhances myelination, while conversely pharmacological blockade of the NRG1-ErbB pathway prevents myelination, providing direct evidence for the ability of this pathway to promote the myelination of human sensory axons. The β-secretase, BACE1 is a protease needed to generate active NRG1 from the full-length form. Due to the fact that it also cleaves amyloid precursor protein, BACE1 is a therapeutic target in Alzheimer's disease, however, consistent with its role in NRG1 processing we find that BACE1 inhibition significantly impairs myelination in our co-culture system. In order to exploit co-cultures to address other clinically relevant problems, they were exposed to anti-disialosyl ganglioside antibodies, including those derived from a patient with a sensory predominant, inflammatory neuropathy with mixed axonal and demyelinating electrophysiology. The co-cultures reveal that both mouse and human disialosyl antibodies target the nodal axolemma, induce acute axonal degeneration in the presence of complement, and impair myelination. The human, neuropathy-associated IgM antibody is also shown to induce complement-independent demyelination. Myelinating co-cultures using human induced pluripotent stem cell-derived sensory neurons thus provide insights into the cellular and molecular specialization of axoglial signalling, how pharmacological agents may promote or impede such signalling and the pathogenic effects of ganglioside antibodies.awx012media15372351982001.

ERBB2 Is Essential for the Growth of Chemically Induced Skin Tumors in Mice

Although the epidermal growth factor receptor has established roles in skin carcinogenesis, inflammation, and wound healing, the functions of the structurally related receptor ERBB2 in this tissue remain poorly explored. To assess the functions of ERBB2 in skin homeostasis, tumorigenesis, and wound healing, we employed keratin 5-directed, cre recombinase-mediated targeting of Erbb2 alleles in mice. Erbb2^del mice, lacking ERBB2 specifically in keratinocytes, showed no noticeable spontaneous skin abnormalities. During early wound healing, the thickness and the number and proliferation rate of keratinocytes in the wound epithelium of Erbb2^del mice were significantly reduced. Compared with control littermates, Erbb2^del mice remained free of papillomas for a longer time and had significantly reduced tumor burden after application of the 7,12-dimethylbenz[a]anthracene/12-O-tetradecanoylphorbol-13-acetate multistage chemical carcinogenesis protocol. Furthermore, tumor cell proliferation was substantially reduced in Erbb2^del mice, and loss of ERBB2 also decreased keratinocyte proliferation after 12-O-tetradecanoylphorbol-13-acetate application. Thus, ERBB2 is dispensable for the development and homeostasis of the epidermis and its appendages. However, reflecting its pro-proliferative role, ERBB2 is required for the normal healing of skin wounds and for the progression of tumors during skin chemical carcinogenesis in mice. Thus, ERBB2 may be a promising target for inhibiting human nonmelanoma skin cancer progression.

Effect of Laminin on Neurotrophic Factors Expression in Schwann-Like Cells Induced from Human Adipose-Derived Stem Cells In Vitro

The Schwann-like cells can be considered as promising in stem cell therapies, at least in experimental models. Human adipose-derived stem cells (ADSCs) are induced into Schwann-like cells (SC-like cells) and are cultured on either a plastic surface or laminin-coated plates. The findings here reveal that laminin is a critical component in extracellular matrix (ECM) of SC-like cells at in vitro. The survival rate of SC-like cells on a laminin matrix are measured through MTT assay and it is found that this rate is significantly higher than that of the cells grown on a plastic surface (P < 0.05). Schwann cell markers and the myelinogenic ability of SC-like cells at the presence versus absence of laminin are assessed through immunocytochemistry. The analysis of GFAP/S100β and S100β/MBP markers indicate that laminin can increase the differentiated rate and myelinogenic potential of SC-like cells. The expression levels of SCs markers, myelin basic proteins (MBP), and neurotrophic factors in two conditions are analyzed by real-time reverse transcription polymerase chain reaction (RT-PCR). The findings here demonstrated that gene expression of SCs markers, MBP, and brain-derived neurotrophic factors (BDNF) increase significantly on laminin compared to plastic surface (P < 0.01). In contrast, the nerve growth factor (NGF) expression is downregulated significantly on laminin-coated plates (P < 0.05). The obtained data suggest that production of neurotrophic factors in SC-like cell in presence of laminin can induce appropriate microenvironment for nerve repair in neurodegenerative diseases.

Regulating Axonal Responses to Injury: The Intersection between Signaling Pathways Involved in Axon Myelination and The Inhibition of Axon Regeneration

Following spinal cord injury (SCI), a multitude of intrinsic and extrinsic factors adversely affect the gene programs that govern the expression of regeneration-associated genes (RAGs) and the production of a diversity of extracellular matrix molecules (ECM). Insufficient RAG expression in the injured neuron and the presence of inhibitory ECM at the lesion, leads to structural alterations in the axon that perturb the growth machinery, or form an extraneous barrier to axonal regeneration, respectively. Here, the role of myelin, both intact and debris, in antagonizing axon regeneration has been the focus of numerous investigations. These studies have employed antagonizing antibodies and knockout animals to examine how the growth cone of the re-growing axon responds to the presence of myelin and myelin-associated inhibitors (MAIs) within the lesion environment and caudal spinal cord. However, less attention has been placed on how the myelination of the axon after SCI, whether by endogenous glia or exogenously implanted glia, may alter axon regeneration. Here, we examine the intersection between intracellular signaling pathways in neurons and glia that are involved in axon myelination and axon growth, to provide greater insight into how interrogating this complex network of molecular interactions may lead to new therapeutics targeting SCI.

YAP and TAZ control peripheral myelination and the expression of laminin receptors in Schwann cells

Myelination is essential for nervous system function. Schwann cells interact with neurons and the basal lamina to myelinate axons, using known receptors, signals and transcription factors. In contrast, the transcriptional control of axonal sorting and the role of mechanotransduction in myelination are largely unknown. Yap and Taz are effectors of the Hippo pathway that integrate chemical and mechanical signals in cells. We describe a previously unknown role for the Hippo pathway in myelination. Using conditional mutagenesis in mice we show that Taz is required in Schwann cells for radial sorting and myelination, and that Yap is redundant with Taz. Yap/Taz are activated in Schwann cells by mechanical stimuli, and regulate Schwann cell proliferation and transcription of basal lamina receptor genes, both necessary for proper radial sorting of axons and subsequent myelination. These data link transcriptional effectors of the Hippo pathway and of mechanotransduction to myelin formation in Schwann cells.

Development of the intrinsic and extrinsic innervation of the gut

The gastrointestinal (GI) tract is innervated by intrinsic enteric neurons and by extrinsic efferent and afferent nerves. The enteric (intrinsic) nervous system (ENS) in most regions of the gut consists of two main ganglionated layers; myenteric and submucosal ganglia, containing numerous types of enteric neurons and glial cells. Axons arising from the ENS and from extrinsic neurons innervate most layers of the gut wall and regulate many gut functions. The majority of ENS cells are derived from vagal neural crest cells (NCCs), which proliferate, colonize the entire gut, and first populate the myenteric region. After gut colonization by vagal NCCs, the extrinsic nerve fibers reach the GI tract, and Schwann cell precursors (SCPs) enter the gut along the extrinsic nerves. Furthermore, a subpopulation of cells in myenteric ganglia undergoes a radial (inward) migration to form the submucosal plexus, and the intrinsic and extrinsic innervation to the mucosal region develops. Here, we focus on recent progress in understanding the developmental processes that occur after the gut is colonized by vagal ENS precursors, and provide an up-to-date overview of molecular mechanisms regulating the development of the intrinsic and extrinsic innervation of the GI tract.