The polarity protein Par-3 directly interacts with p75NTR to regulate myelination

The role of neurotrophin polymorphisms and susceptibility to neural damage in leprosy

OBJECTIVES

Mycobacterium leprae is able to infect Schwann cells leading to neural damage. Neurotrophins are involved in nervous system plasticity and impact neural integrity during diseases. Investigate the association between single nucleotide polymorphisms in neurotrophin genes and leprosy phenotypes, especially neural damage.

DESIGN

We selected single nucleotide polymorphisms in neurotrophins or their receptors genes associated with neural disorders: rs6265 and rs11030099 of brain-derived neurotrophic factor (BDNF), rs6330 of BDNF, rs6332 in NT3 and rs2072446 of P75NTR. The association of genetic frequencies with leprosy phenotypes was investigated in a case-control study.

RESULTS

An association of the BDNF single nucleotide polymorphism rs11030099 with the number of affected nerves was demonstrated. The "AA+AC" genotypes were demonstrated to be protective against nerve impairment. However, this variation does not affect BDNF serum levels. BDNF is an important factor for myelination of Schwann cells and polymorphisms in this gene can be associated with leprosy outcome. Moreover, rs11030099 is located in the binding region for micro-RNA (miRNA) 26a that could be involved in control of BDNF expression. We demonstrated different expression levels of this miRNA in polar forms of leprosy.

CONCLUSION

Our findings demonstrate for the first time an association between the polymorphism rs11030099 in the BDNF gene and neural commitment in leprosy and may indicate a possible role of miRNA-26a acting synergistically to these genetic variants in neural damage development.

Schwann cells are axo-protective after injury irrespective of myelination status in mouse Schwann cell-neuron cocultures

Myelinating Schwann cell (SC)-dorsal root ganglion (DRG) neuron cocultures are an important technique for understanding cell-cell signalling and interactions during peripheral nervous system (PNS) myelination, injury, and regeneration. Although methods using rat SCs and neurons or mouse DRG explants are commonplace, there are no established protocols for compartmentalised myelinating cocultures with dissociated mouse cells. There consequently is a need for a coculture protocol that allows separate genetic manipulation of mouse SCs or neurons, or use of cells from different transgenic animals to complement in vivo mouse experiments. However, inducing myelination of dissociated mouse SCs in culture is challenging. Here, we describe a new method to coculture dissociated mouse SCs and DRG neurons in microfluidic chambers and induce robust myelination. Cocultures can be axotomised to study injury and used for drug treatments, and cells can be lentivirally transduced for live imaging. We used this model to investigate axon degeneration after traumatic axotomy and find that SCs, irrespective of myelination status, are axo-protective. At later timepoints after injury, live imaging of cocultures shows that SCs break up, ingest and clear axonal debris.

Protein kinase C epsilon activation regulates proliferation, migration, and epithelial to mesenchymal-like transition in rat Schwann cells

Introduction

Protein kinase type C-ε (PKCε) plays an important role in the sensitization of primary afferent nociceptors, promoting mechanical hyperalgesia. In accordance, we showed that PKCε is present in sensory neurons of the peripheral nervous system (PNS), participating in the control of pain onset and chronification. Recently, it was found that PKCε is also implicated in the control of cell proliferation, promoting mitogenesis and metastatic invasion in some types of cancer. However, its role in the main glial cell of the PNS, the Schwann cells (SCs), was still not investigated.

Methods

Rat primary SCs culture were treated with different pharmacologic approaches, including the PKCε agonist dicyclopropyl-linoleic acid (DCP-LA) 500 nM, the human recombinant brain derived neurotrophic factor (BDNF) 1 nM and the TrkB receptor antagonist cyclotraxin B 10 nM. The proliferation (by cell count), the migration (by scratch test and Boyden assay) as well as some markers of SCs differentiation and epithelial-mesenchymal transition (EMT) process (by qRT-PCR and western blot) were analyzed.

Results

Overall, we found that PKCε is constitutively expressed in SCs, where it is likely involved in the switch from the proliferative toward the differentiated state. Indeed, we demonstrated that PKCε activation regulates SCs proliferation, increases their migration, and the expression of some markers (e.g., glycoprotein P0 and the transcription factor Krox20) of SCs differentiation. Through an autocrine mechanism, BDNF activates TrkB receptor, and controls SCs proliferation via PKCε. Importantly, PKCε activation likely promoted a partial EMT process in SCs.

Discussion

PKCε mediates relevant actions in the neuronal and glial compartment of the PNS. In particular, we posit a novel function for PKCε in the transformation of SCs, assuming a role in the mechanisms controlling SCs' fate and plasticity.

Cellular mechanisms of heterogeneity in NF2-mutant schwannoma

Schwannomas are common sporadic tumors and hallmarks of familial neurofibromatosis type 2 (NF2) that develop predominantly on cranial and spinal nerves. Virtually all schwannomas result from inactivation of the NF2 tumor suppressor gene with few, if any, cooperating mutations. Despite their genetic uniformity schwannomas exhibit remarkable clinical and therapeutic heterogeneity, which has impeded successful treatment. How heterogeneity develops in NF2-mutant schwannomas is unknown. We have found that loss of the membrane:cytoskeleton-associated NF2 tumor suppressor, merlin, yields unstable intrinsic polarity and enables Nf2^-/- Schwann cells to adopt distinct programs of ErbB ligand production and polarized signaling, suggesting a self-generated model of schwannoma heterogeneity. We validated the heterogeneous distribution of biomarkers of these programs in human schwannoma and exploited the synchronous development of lesions in a mouse model to establish a quantitative pipeline for studying how schwannoma heterogeneity evolves. Our studies highlight the importance of intrinsic mechanisms of heterogeneity across human cancers.

Pals1a and aPKCλ are not essential for Schwann cell migration, division or myelination in zebrafish

BACKGROUND

Schwann cells (SCs) are specialized glial cells of the peripheral nervous system that produce myelin and promote fast action potential propagation. In order to myelinate, SCs engage in a series of events that include migration and division along axons, followed by extensive cytoskeletal rearrangements that ensure axonal ensheathment and myelination. SCs are polarized and extend their processes along an abaxonal-adaxonal axis. Here, we investigate the role of the apical polarity proteins, Pals1a, and aPKCλ, in SC behavior during zebrafish development.

RESULTS

We analyzed zebrafish nok and has mutants deficient for pals1a and aPKCλ function respectively. Using live imaging, transmission electron microscopy and whole mount immunostaining, we show that SCs can migrate and divide appropriately, exhibit normal radial sorting, express myelin markers and ensheath axons on time in has and nok mutants.

CONCLUSIONS

Pals1a and aPKCλ are not essential for SC migration, division or myelination in zebrafish.

A potential role of p75NTR in the regulation of circadian rhythm and incremental growth lines during tooth development

Objective: Tooth morphogenesis and the formation of hard tissues have been reported to be closely related to circadian rhythms. This study investigates the spatiotemporal expression and relationship of p75NTR with core clock genes, mineralization-related or odontogenesis-related genes, and aims to derive the potential role of p75NTR in regulating circadian rhythm and incrementality growth line formation during tooth development.

Materials and methods: The dynamic morphology of the rat dental germ was observed at seven stages (E14.5 d, E16.5 d, E18.5 d, P.N. 4 d, P.N. 7 d, P.N. 10 d, and P.N. 15 d). Next, the expressions of p75NTR and other target factors were traced. The ectomesenchymal stem cells (EMSCs) were isolated from the E18.5d rat dental germs and synchronized using 50% of fetal bovine serum. Then, they were cultured in light/light (L.L.), dark/dark (D.D.), and light/dark (L.D.) conditions for 48 h. The total RNA was collected every 4 h, and the circadian rhythm dynamics of target factors were observed. To reveal the mechanism further, p75NTR was down-regulated in p75NTR^ExIII−/− mice and up-regulated in immortalized mouse dental apical papilla progenitor cells. The change tendencies of other target factors were also detected.

Results: The clock genes Bmal1, Clock, Per1, and Per2 were all expressed in tooth germs before the formation of dental hard tissues and demonstrated a regular oscillating expression pattern in EMSCs from dental germs. Their expression was affected by the L.D. stimulus, and most of them were promoted by D.D. conditions. p75NTR presented a similar expression pattern and a positive or negative relationship with most clock genes, mineralization-related and odontogenesis-related factors, such as brain and muscle ARNT-like protein-1 (Bmal1), runt-related transcription factor 2 (Runx2), alkaline phosphatase (ALP), MSH-like 1 (MSX1), dentin matrix acidic phosphoprotein 1 (Dmp1), and dentin sialophosphoprotein (Dspp). Moreover, the arrangement, morphology, and even boundary in pre-odontoblast/pre-ameloblast layers were disordered in the p75NTR^ExIII−/− mice.

Conclusion: Circadian rhythm was found to affect tooth development. p75NTR might play a crucial role in regulating clock genes in the mineralization and formation of the dental hard tissues. p75NTR is actively involved in the odontoblast-ameloblast junction and cell polarity establishment during tooth morphogenesis.

The Roles of Par3, Par6, and aPKC Polarity Proteins in Normal Neurodevelopment and in Neurodegenerative and Neuropsychiatric Disorders

Normal neural circuits and functions depend on proper neuronal differentiation, migration, synaptic plasticity, and maintenance. Abnormalities in these processes underlie various neurodevelopmental, neuropsychiatric, and neurodegenerative disorders. Neural development and maintenance are regulated by many proteins. Among them are Par3, Par6 (partitioning defective 3 and 6), and aPKC (atypical protein kinase C) families of evolutionarily conserved polarity proteins. These proteins perform versatile functions by forming tripartite or other combinations of protein complexes, which hereafter are collectively referred to as "Par complexes." In this review, we summarize the major findings on their biophysical and biochemical properties in cell polarization and signaling pathways. We next summarize their expression and localization in the nervous system as well as their versatile functions in various aspects of neurodevelopment, including neuroepithelial polarity, neurogenesis, neuronal migration, neurite differentiation, synaptic plasticity, and memory. These versatile functions rely on the fundamental roles of Par complexes in cell polarity in distinct cellular contexts. We also discuss how cell polarization may correlate with subcellular polarization in neurons. Finally, we review the involvement of Par complexes in neuropsychiatric and neurodegenerative disorders, such as schizophrenia and Alzheimer's disease. While emerging evidence indicates that Par complexes are essential for proper neural development and maintenance, many questions on their in vivo functions have yet to be answered. Thus, Par3, Par6, and aPKC continue to be important research topics to advance neuroscience.

Structural Organization of Human Full-Length PAR3 and the aPKC-PAR6 Complex

The tripartite partition defect (PAR) polarity complex, which includes the proteins PAR3, atypical protein kinase C (aPKC), and PAR6, is a major regulator of cellular polarity. It is highly conserved and expressed in various tissues. Its largest component, PAR3, controls protein–protein interactions of the PAR complex with a variety of interaction partners, and PAR3 self-association is critical for the formation of filament-like structures. However, little is known about the structure of the PAR complex. Here, we purified non-filamentous PAR3 and the aPKC–PAR6 complex and characterized them by single-particle electron microscopy (EM). We expressed and purified an oligomerization-deficient form of PAR3, PAR3_V13D,D70K, and the active aPKC–PAR6 dimer. For PAR3, engineering at two positions is sufficient to form stable single particles with a maximum dimension of 20 nm. aPKC–PAR6 forms a complex with a maximum dimension of 13.5 nm that contains single copies of aPKC. Thus, the data present a basis for further high-resolution studies of PAR proteins and PAR complex formation.

The adaptor SH2B1 and the phosphatase PTP4A1 regulate the phosphorylation of cytohesin-2 in myelinating Schwann cells in mice

Mature myelin sheaths insulate axons to increase nerve conduction velocity and protect nerve fibers from stress and physical injury. In the peripheral nervous system, the myelin sheath is produced by Schwann cells. The guanine-nucleotide exchange factor cytohesin-2 activates the protein Arf6 to promote Schwann cell myelination. Here, we investigated the regulation of cytohesin-2 and found that the phosphorylation status of Tyr³⁸¹ in cytohesin-2 is central to Schwann cell myelination. Knockin mice with a nonphosphorylatable Y381F mutation in cytohesin-2 exhibited reduced myelin thickness and decreased Arf6 activity in sciatic nerve tissue. In HEK293T cells, cytohesin-2 was dephosphorylated at Tyr³⁸¹ by the protein tyrosine phosphatase PTP4A1, whereas phosphorylation at this site was maintained by interaction with the adaptor protein SH2B1. Schwann cell-specific knockdown of PTP4A1 in mice increased cytohesin-2 phosphorylation and myelin thickness. Conversely, Schwann cell-specific loss of SH2B1 resulted in reduced myelin thickness and decreased cytohesin-2 phosphorylation. Thus, a signaling unit centered on cytohesin-2-with SH2B1 as a positive regulator and PTP4A1 as a negative regulator-controls Schwann cell myelination in the peripheral nervous system.

Metabolic Control of Sensory Neuron Survival by the p75 Neurotrophin Receptor in Schwann Cells

We report that the neurotrophin receptor p75 contributes to sensory neuron survival through the regulation of cholesterol metabolism in Schwann cells. Selective deletion of p75 in mouse Schwann cells of either sex resulted in a 30% loss of dorsal root ganglia (DRG) neurons and diminished thermal sensitivity. P75 regulates Schwann cell cholesterol biosynthesis in response to BDNF, forming a co-receptor complex with ErbB2 and activating ErbB2-mediated stimulation of sterol regulatory element binding protein 2 (SREBP2), a master regulator of cholesterol synthesis. Schwann cells lacking p75 exhibited decreased activation of SREBP2 and a reduction in 7-dehydrocholesterol (7-DHC) reductase (DHCR7) expression, resulting in accumulation of the neurotoxic intermediate, 7-dehyrocholesterol in the sciatic nerve. Restoration of DHCR7 in p75 null Schwann cells in mice significantly attenuated DRG neuron loss. Together, these results reveal a mechanism by which the disruption of lipid metabolism in glial cells negatively influences sensory neuron survival, which has implications for a wide range of peripheral neuropathies.SIGNIFICANCE STATEMENT Although expressed in Schwann cells, the role of p75 in myelination has remained unresolved in part because of its dual expression in sensory neurons that Schwann cells myelinate. When p75 was deleted selectively among Schwann cells, myelination was minimally affected, while sensory neuron survival was reduced by 30%. The phenotype is mainly due to dysregulation of cholesterol biosynthesis in p75-deficient Schwann cells, leading to an accumulation of neurotoxic cholesterol precursor, 7-dehydrocholesterol (7-DHC). Mechanism-wise, we discovered that in response to BDNF, p75 recruits and activates ErbB2 independently of ErbB3, thereby stimulating the master regulator, sterol regulatory element binding protein 2 (SREBP2). These results together highlight a novel role of p75 in Schwann cells in regulating DRG neuron survival by orchestrating proper cholesterol metabolism.

Lessons from Injury: How Nerve Injury Studies Reveal Basic Biological Mechanisms and Therapeutic Opportunities for Peripheral Nerve Diseases

Since Waller and Cajal in the nineteenth and early twentieth centuries, laboratory traumatic peripheral nerve injury studies have provided great insight into cellular and molecular mechanisms governing axon degeneration and the responses of Schwann cells, the major glial cell type of peripheral nerves. It is now evident that pathways underlying injury-induced axon degeneration and the Schwann cell injury-specific state, the repair Schwann cell, are relevant to many inherited and acquired disorders of peripheral nerves. This review provides a timely update on the molecular understanding of axon degeneration and formation of the repair Schwann cell. We discuss how nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) and sterile alpha TIR motif containing protein 1 (SARM1) are required for axon survival and degeneration, respectively, how transcription factor c-JUN is essential for the Schwann cell response to nerve injury and what each tells us about disease mechanisms and potential therapies. Human genetic association with NMNAT2 and SARM1 strongly suggests aberrant activation of programmed axon death in polyneuropathies and motor neuron disorders, respectively, and animal studies suggest wider involvement including in chemotherapy-induced and diabetic neuropathies. In repair Schwann cells, cJUN is aberrantly expressed in a wide variety of human acquired and inherited neuropathies. Animal models suggest it limits axon loss in both genetic and traumatic neuropathies, whereas in contrast, Schwann cell secreted Neuregulin-1 type 1 drives onion bulb pathology in CMT1A. Finally, we discuss opportunities for drug-based and gene therapies to prevent axon loss or manipulate the repair Schwann cell state to treat acquired and inherited neuropathies and neuronopathies.

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.

Schwann cell development: From neural crest to myelin sheath

Vertebrate nervous system function requires glial cells, including myelinating glia that insulate axons and provide trophic support that allows for efficient signal propagation by neurons. In vertebrate peripheral nervous systems, neural crest-derived glial cells known as Schwann cells (SCs) generate myelin by encompassing and iteratively wrapping membrane around single axon segments. SC gliogenesis and neurogenesis are intimately linked and governed by a complex molecular environment that shapes their developmental trajectory. Changes in this external milieu drive developing SCs through a series of distinct morphological and transcriptional stages from the neural crest to a variety of glial derivatives, including the myelinating sublineage. Cues originate from the extracellular matrix, adjacent axons, and the developing SC basal lamina to trigger intracellular signaling cascades and gene expression changes that specify stages and transitions in SC development. Here, we integrate the findings from in vitro neuron-glia co-culture experiments with in vivo studies investigating SC development, particularly in zebrafish and mouse, to highlight critical factors that specify SC fate. Ultimately, we connect classic biochemical and mutant studies with modern genetic and visualization tools that have elucidated the dynamics of SC development. This article is categorized under: Signaling Pathways > Cell Fate Signaling Nervous System Development > Vertebrates: Regional Development.

Partial deletion of p75NTR in large-diameter DRG neurons exerts no influence upon the survival of peripheral sensory neurons in vivo

The p75 neurotrophin receptor (p75NTR ) is required for maintaining peripheral sensory neuron survival and function; however, the underlying cellular mechanism remains unclear. The general view is that expression of p75NTR by the neuron itself is required for maintaining sensory neuron survival and myelination in the peripheral nervous system (PNS). Adopting a neuronal-specific conditional knockout strategy, we demonstrate the partial depletion of p75NTR in neurons exerts little influence upon maintaining sensory neuron survival and peripheral nerve myelination in health and after demyelinating neuropathy. Our data show that the density and total number of dorsal root ganglion (DRG) neurons in 2-month-old mice is not affected following the deletion of p75NTR in large-diameter myelinating neurons, as assessed by stereology. Adopting experimental autoimmune neuritis induced in adult male mice, an animal model of demyelinating peripheral neuropathy, we identify that deleting p75NTR in myelinating neurons exerts no influence upon the disease progression, the total number of DRG neurons, and the extent of myelin damage in the sciatic nerve, indicating that the expression of neuronal p75NTR is not essential for maintaining peripheral neuron survival and myelination after a demyelinating insult in vivo. Together, results of this study suggest that the survival and myelination of peripheral sensory neurons is independent of p75NTR expressed by a subtype of neurons in vivo. Thus, our findings provide new insights into the mechanism underpinning p75NTR -mediated neuronal survival in the PNS.

Conductive Hydroxyethyl Cellulose/Soy Protein Isolate/Polyaniline Conduits for Enhancing Peripheral Nerve Regeneration via Electrical Stimulation

Nerve regeneration remains a challenge to the treatment of peripheral nerve injury. Electrical stimulation (ES) is an assistant treatment to enhance recovery from peripheral nerve injury. A conductive nerve guide conduit was prepared from hydroxyethyl cellulose (HEC)/soy protein isolate (SPI)/PANI sponge (HSPS) and then the HSPS conduits were used to repair 10 mm sciatic nerve injury in rat model with or without ES, using HSPS+brain-derived neurotrophic factor (BDNF) and autografts as controls. The nerve repairing capacities were evaluated by animal experiments of behavioristics, electrophysiology, toluidine blue staining, and transmission electron microscopy (TEM) in the regenerated nerves. The results revealed that the nerve regeneration efficiency of HSPS conduits with ES (HSPS+ES) group was the best among the conduit groups but slightly lower than that of autografts group. HSPS+ES group even exhibited notably increased in the BDNF expression of regenerated nerve tissues, which was also confirmed through in vitro experiments that exogenous BDNF could promote Schwann cells proliferation and MBP protein expression. As a result, this work provided a strategy to repair nerve defect using conductive HSPS as nerve guide conduit and using ES as an extrinsic physical cue to promote the expression of endogenous BDNF.

Schwann cell interactions during the development of the peripheral nervous system

Schwann cells play a critical role in the development of the peripheral nervous system (PNS), establishing important relationships both with the extracellular milieu and other cell types, particularly neurons. In this review, we discuss various Schwann cell interactions integral to the proper establishment, spatial arrangement, and function of the PNS. We include signals that cascade onto Schwann cells from axons and from the extracellular matrix, bidirectional signals that help to establish the axo-glial relationship and how Schwann cells in turn support the axon. Further, we speculate on how Schwann cell interactions with other components of the developing PNS ultimately promote the complete construction of the peripheral nerve.

Loss-of-function of EBP50 is a new cause of hereditary peripheral neuropathy: EBP50 functions in peripheral nerve system

Finding causative genetic mutations is important in the diagnosis and treatment of hereditary peripheral neuropathies. This study was conducted to find new genes involved in the pathophysiology of hereditary peripheral neuropathy. We identified a new mutation in the EBP50 gene, which is co-segregated with neuropathic phenotypes, including motor and sensory deficit in a family with Charcot-Marie-Tooth disease. EBP50 is known to be important for the formation of microvilli in epithelial cells, and the discovery of this gene mutation allowed us to study the function of EBP50 in the nervous system. EBP50 was strongly expressed in the nodal and paranodal regions of sciatic nerve fibers, where Schwann cell microvilli contact the axolemma, and at the growth tips of primary Schwann cells. In addition, EBP50 expression was decreased in mouse models of peripheral neuropathy. Knockout mice were used to study EBP50 function in the peripheral nervous system. Interestingly motor function deficit and abnormal histology of nerve fibers were observed in EBP50^+/- heterozygous mice at 12 months of age, but not 3 months. in vitro studies using Schwann cells showed that NRG1-induced AKT activation and migration were significantly reduced in cells overexpressing the I325V mutant of EBP50 or cells with knocked-down EBP50 expression. In conclusion, we show for the first time that loss of function due to EBP50 gene deficiency or mutation can cause peripheral neuropathy.

Pou2f1 and Pou2f2 cooperate to control the timing of cone photoreceptor production in the developing mouse retina

Multipotent retinal progenitor cells (RPCs) generate various cell types in a precise chronological order, but how exactly cone photoreceptor production is restricted to early stages remains unclear. Here, we show that the POU-homeodomain factors Pou2f1/Pou2f2, the homologs of Drosophila temporal identity factors nub/pdm2, regulate the timely production of cones in mice. Forcing sustained expression of Pou2f1 or Pou2f2 in RPCs expands the period of cone production, whereas misexpression in late-stage RPCs triggers ectopic cone production at the expense of late-born fates. Mechanistically, we report that Pou2f1 induces Pou2f2 expression, which binds to a POU motif in the promoter of the rod-inducing factor Nrl to repress its expression. Conversely, conditional inactivation of Pou2f2 in RPCs increases Nrl expression and reduces cone production. Finally, we provide evidence that Pou2f1 is part of a cross-regulatory cascade with the other temporal identity factors Ikzf1 and Casz1. These results uncover Pou2f1/2 as regulators of the temporal window for cone genesis and, given their widespread expression in the nervous system, raise the possibility of a general role in temporal patterning.

Cellular Signal-Regulated Schwann Cell Myelination and Remyelination

Increasing studies have demonstrated multiple signaling molecules responsible for oligodendrocytes and Schwann cells development such as migration, differentiation, myelination, and axo-glial interaction. However, complicated roles in these events are still poorly understood. This chapter focuses on well established intracellular signaling transduction and recent topics that control myelination and are elucidated from accumulating evidences. The underlying molecular mechanisms, which involved in membrane trafficking through small GTPase Arf6 and its activator cytohesins, demonstrate the crosstalk between well established intracellular signaling transduction and a new finding signaling pathway in glial cells links to physiological phenotype and essential role in peripheral nerve system (PNS). Since Arf family proteins affect the expression levels of myelin protein zero (MPZ) and Krox20, which is a transcription factor regulatory factor in early developmental stages of Schwann cells, Arf proteins likely to be key regulator for Schwann cells development. Herein, we discuss how intracellular signaling transductions in Schwann cells associate with myelination in CNS and PNS.

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.

aPKC in neuronal differentiation, maturation and function

The atypical Protein Kinase Cs (aPKCs)—PRKCI, PRKCZ and PKMζ—form a subfamily within the Protein Kinase C (PKC) family. These kinases are expressed in the nervous system, including during its development and in adulthood. One of the aPKCs, PKMζ, appears to be restricted to the nervous system. aPKCs are known to play a role in a variety of cellular responses such as proliferation, differentiation, polarity, migration, survival and key metabolic functions such as glucose uptake, that are critical for nervous system development and function. Therefore, these kinases have garnered a lot of interest in terms of their functional role in the nervous system. Here we review the expression and function of aPKCs in neural development and in neuronal maturation and function. Despite seemingly paradoxical findings with genetic deletion versus gene silencing approaches, we posit that aPKCs are likely candidates for regulating many important neurodevelopmental and neuronal functions, and may be associated with a number of human neuropsychiatric diseases.

Myelinating Schwann cells ensheath multiple axons in the absence of E3 ligase component Fbxw7

In the central nervous system (CNS), oligodendrocytes myelinate multiple axons; in the peripheral nervous system (PNS), Schwann cells (SCs) myelinate a single axon. Why are the myelinating potentials of these glia so fundamentally different? Here, we find that loss of Fbxw7, an E3 ubiquitin ligase component, enhances the myelinating potential of SCs. Fbxw7 mutant SCs make thicker myelin sheaths and sometimes appear to myelinate multiple axons in a fashion reminiscent of oligodendrocytes. Several Fbxw7 mutant phenotypes are due to dysregulation of mTOR; however, the remarkable ability of mutant SCs to ensheathe multiple axons is independent of mTOR signaling. This indicates distinct roles for Fbxw7 in SC biology including modes of axon interactions previously thought to fundamentally distinguish myelinating SCs from oligodendrocytes. Our data reveal unexpected plasticity in the myelinating potential of SCs, which may have important implications for our understanding of both PNS and CNS myelination and myelin repair.

The authors find that deletion from Schwann cells of an E3 ubiquitin ligase component called Fbxw7 leads to a phenotype reminiscent of myelination in the central nervous system where a single oligodendrocyte ensheaths multiple axons.

Necl-4/Cadm4 recruits Par-3 to the Schwann cell adaxonal membrane

Interactions between axons and Schwann cells are essential for the acquisition of Schwann cell radial and longitudinal polarity and myelin sheath assembly. In the internode, the largest of these longitudinal domains, axon-Schwann cell interactions are mediated by the Nectin-like (Necl) cell adhesion proteins, also known as SynCAMs or Cadms. In particular, Necl-1/Cadm3 expressed on the axon surface binds to Necl-4/Cadm4 expressed along the adaxonal membrane of myelinating Schwann cells. Necl-4 promotes myelination in vitro and is required for the timely onset of myelination and the fidelity of the organization of the myelin sheath and the internode in vivo. A key question is the identity of the downstream effectors of Necl-4 that mediate its effects. The cytoplasmic terminal region (CTR) of Necl-4 contains a PDZ-domain binding motif. Accordingly, we used the CTR of Necl-4 in an unbiased proteomic screen of PDZ-domain proteins. We identify Par-3, a multi-PDZ domain containing protein of the Par-aPKC polarity complex previously implicated in myelination, as an interacting protein. Necl-4 and Par-3 are colocalized along the inner Schwann cell membrane and coprecipitate from Schwann cell lysates. The CTR of Necl-4 binds to the first PDZ domain of Par-3 thereby recruiting Par-3 to sites of Necl-4/Necl-1 interaction. Knockdown of Necl-4 perturbs Par-3 localization to the inner membrane of Schwann cells in myelinating co-cultures. These findings implicate interactions of Necl-1/Necl-4 in the recruitment of Par-3 to the Schwann cell adaxonal membrane and the establishment of Schwann cell radial polarity.

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.

BIG1/Arfgef1 and Arf1 regulate the initiation of myelination by Schwann cells in mice

During development of the peripheral nervous system in mammals, Schwann cells wrap their plasma membranes around neuronal axons, forming multiple myelin sheaths. A mature myelin sheath insulates axons and increases nerve conduction velocity while protecting nerve fibers from various stresses such as physical ones. Despite this functional importance, the molecular units that underlie dynamic morphological changes in formation of myelin sheaths are not sufficiently understood. Arf1 is a small guanosine triphosphate-binding protein that plays multiple roles in intracellular trafficking and related signaling, both of which are processes involved in cell morphogenesis. We demonstrate that the Arf1 guanine nucleotide exchange factor, brefeldin A-inhibited guanine nucleotide-exchange protein 1 (BIG1)/Arfgef1, and the effector Arf1 regulate the initiation of myelination of axons by Schwann cells. Schwann cell-specific BIG1 conditional knockout mice, which have been generated here, exhibit reduced myelin thickness and decreased localization of myelin protein zero in the myelin membrane, compared with their littermate controls. BIG1 knockout mouse nerves specifically decrease the amounts of Arf1 in the AP1 clathrin adaptor protein subunits but not the Arf1 binding to GGA1 (Golgi-localized, gamma-adaptin ear-containing, Arf-binding protein 1) transporting proteins. The amounts of Arf1 in the COPI coatomer protein subunits were comparable in the knockout mice and controls. Similar results in myelin thickness are observed in Arf1 conditional knockout mice, which have also been generated here. Thus, the BIG1 and Arf1 unit plays a key role in Schwann cell myelination, newly adding it to the list of molecular units controlling myelination.

Myelinating Schwann Cell Polarity and Mechanically-Driven Myelin Sheath Elongation

Myelin sheath geometry, encompassing myelin sheath thickness relative to internodal length, is critical to optimize nerve conduction velocity and these parameters are carefully adjusted by the myelinating cells in mammals. In the central nervous system these adjustments could regulate neuronal activities while in the peripheral nervous system they lead to the optimization and the reliability of the nerve conduction velocity. However, the physiological and cellular mechanisms that underlie myelin sheath geometry regulation are not yet fully elucidated. In peripheral nerves the myelinating Schwann cell uses several molecular mechanisms to reach and maintain the correct myelin sheath geometry, such that myelin sheath thickness and internodal length are regulated independently. One of these mechanisms is the epithelial-like cell polarization process that occurs during the early phases of the myelin biogenesis. Epithelial cell polarization factors are known to control cell size and morphology in invertebrates and mammals making these processes critical in the organogenesis. Correlative data indicate that internodal length is regulated by postnatal body growth that elongates peripheral nerves in mammals. In addition, the mechanical stretching of peripheral nerves in adult animals shows that myelin sheath length can be increased by mechanical cues. Recent results describe the important role of YAP/TAZ co-transcription factors during Schwann cell myelination and their functions have linked to the mechanotransduction through the HIPPO pathway and the epithelial polarity factor Crb3. In this review the molecular mechanisms that govern mechanically-driven myelin sheath elongation and how a Schwann cell can modulate internodal myelin sheath length, independent of internodal thickness, will be discussed regarding these recent data. In addition, the potential relevance of these mechanosensitive mechanisms in peripheral pathologies will be highlighted.

Junctional Adhesion Molecules (JAMs): Cell Adhesion Receptors With Pleiotropic Functions in Cell Physiology and Development

Junctional adhesion molecules (JAM)-A, -B and -C are cell-cell adhesion molecules of the immunoglobulin superfamily which are expressed by a variety of tissues, both during development and in the adult organism. Through their extracellular domains, they interact with other adhesion receptors on opposing cells. Through their cytoplasmic domains, they interact with PDZ domain-containing scaffolding and signaling proteins. In combination, these two properties regulate the assembly of signaling complexes at specific sites of cell-cell adhesion. The multitude of molecular interactions has enabled JAMs to adopt distinct cellular functions such as the regulation of cell-cell contact formation, cell migration, or mitotic spindle orientation. Not surprisingly, JAMs regulate diverse processes such as epithelial and endothelial barrier formation, hemostasis, angiogenesis, hematopoiesis, germ cell development, and the development of the central and peripheral nervous system. This review summarizes the recent progress in the understanding of JAMs, including their characteristic structural features, their molecular interactions, their cellular functions, and their contribution to a multitude of processes during vertebrate development and homeostasis.

Changes in the Coding and Non-coding Transcriptome and DNA Methylome that Define the Schwann Cell Repair Phenotype after Nerve Injury

Repair Schwann cells play a critical role in orchestrating nerve repair after injury, but the cellular and molecular processes that generate them are poorly understood. Here, we perform a combined whole-genome, coding and non-coding RNA and CpG methylation study following nerve injury. We show that genes involved in the epithelial-mesenchymal transition are enriched in repair cells, and we identify several long non-coding RNAs in Schwann cells. We demonstrate that the AP-1 transcription factor C-JUN regulates the expression of certain micro RNAs in repair Schwann cells, in particular miR-21 and miR-34. Surprisingly, unlike during development, changes in CpG methylation are limited in injury, restricted to specific locations, such as enhancer regions of Schwann cell-specific genes (e.g., Nedd4l), and close to local enrichment of AP-1 motifs. These genetic and epigenomic changes broaden our mechanistic understanding of the formation of repair Schwann cell during peripheral nervous system tissue repair.

A Brain-Derived Neurotrophic Factor-Based p75NTR Peptide Mimetic Ameliorates Experimental Autoimmune Neuritis Induced Axonal Pathology and Demyelination

Axonal damage and demyelination are major determinants of disability in patients with peripheral demyelinating neuropathies. The neurotrophin family of growth factors are essential for the normal development and myelination of the peripheral nervous system (PNS), and as such are potential therapeutic candidates for ameliorating axonal and myelin damage. In particular, BDNF promotes peripheral nerve myelination via p75 neurotrophin receptor (p75NTR) receptors. Here, we investigated the therapeutic efficacy of a small structural mimetic of the region of BDNF that binds to p75NTR (cyclo-dPAKKR) in experimental autoimmune neuritis (EAN), an established animal model of peripheral demyelinating neuropathy. Examination of rodents induced with EAN revealed that p75NTR is abundantly expressed in affected peripheral nerves. We found that systemic administration of cyclo-dPAKKR ameliorates EAN disease severity and accelerates recovery. Animals treated with cyclo-dPAKKR displayed significantly better motor performance compared to control animals. Histological assessment revealed that cyclo-dPAKKR administration limits the extent of inflammatory demyelination and axonal damage, and protects against the disruption of nodal architecture in affected peripheral nerves. In contrast, a structural control peptide of cyclo-dPAKKR exerted no influence. Moreover, all the beneficial effects of cyclo-dPAKKR in EAN are abrogated in p75NTR heterozygous mice, strongly suggesting a p75NTR-dependent effect. Taken together, our data demonstrate that cyclo-dPAKKR ameliorates functional and pathological defects of EAN in a p75NTR-dependant manner, suggesting that p75NTR is a therapeutic target to consider for future treatment of peripheral demyelinating diseases and targeting of p75NTR is a strategy worthy of further investigation.

Schwann cell mitochondria as key regulators in the development and maintenance of peripheral nerve axons

Formation of myelin sheaths by Schwann cells (SCs) enables rapid and efficient transmission of action potentials in peripheral axons, and disruption of myelination results in disorders that involve decreased sensory and motor functions. Given that construction of SC myelin requires high levels of lipid and protein synthesis, mitochondria, which are pivotal in cellular metabolism, may be potential regulators of the formation and maintenance of SC myelin. Supporting this notion, abnormal mitochondria are found in SCs of neuropathic peripheral nerves in both human patients and the relevant animal models. However, evidence for the importance of SC mitochondria in myelination has been limited, until recently. Several studies have recently used genetic approaches that allow SC-specific ablation of mitochondrial metabolic activity in living animals to show the critical roles of SC mitochondria in the development and maintenance of peripheral nerve axons. Here, we review current knowledge about the involvement of SC mitochondria in the formation and dysfunction of myelinated axons in the peripheral nervous system.

The ins and outs of polarized axonal domains

Myelinated axons are divided into polarized subdomains including axon initial segments and nodes of Ranvier. These domains initiate and propagate action potentials and regulate the trafficking and localization of somatodendritic and axonal proteins. Formation of axon initial segments and nodes of Ranvier depends on intrinsic (neuronal) and extrinsic (glial) interactions. Several levels of redundancy in both mechanisms and molecules also exist to ensure efficient node formation. Furthermore, the establishment of polarized domains at and near nodes of Ranvier reflects the intrinsic polarity of the myelinating glia responsible for node assembly. Here, we discuss the various polarized domains of myelinated axons, how they are established by both intrinsic and extrinsic interactions, and the polarity of myelinating glia.

Dual function of partitioning-defective 3 in the regulation of YAP phosphorylation and activation

Partitioning-defective 3 (Par3), a key component of the evolutionarily conserved polarity PAR complex (Par3/Par6/aPKC), controls cell polarity and contributes to cell migration, proliferation and tumor development. Emerging evidence indicates that cell polarity proteins function as upstream modulators that regulate the Hippo pathway. However, little is known about Par3’s involvement in the Hippo pathway. Here, we find Par3 and YAP dynamically co-localize in different subcellular compartments; that is, the membrane, cytoplasm and nucleus, in a cell-density-dependent manner. Interestingly, Par3 knockdown promotes YAP phosphorylation, leading to a significant impairment of YAP nuclear translocation at low cell density, but not at high density, in MDCK cells. Furthermore, via its third PDZ domain, Par3 directly binds to the PDZ-binding motif of YAP. The interaction is required for regulating YAP phosphorylation and nuclear localization. Mechanistically, Par3, as a scaffold protein, associates with LATS1 and protein phosphatase 1, α subunit (PP1A) in the cytoplasm and nucleus. Par3 promotes the dephosphorylation of LATS1 and YAP, thus enhancing YAP activation and cell proliferation. Strikingly, we also find that under the condition of PP1A knockdown, Par3 expression promotes YAP hyperphosphorylation, leading to the suppression of YAP activity and its downstream targets. Par3 expression results in differential effects on YAP phosphorylation and activation in different tumor cell lines. These findings indicate that Par3 may have a dual role in regulating the activation of the Hippo pathway, in a manner possibly dependent on cellular context or cell type in response to cell–cell contact and cell polarity signals.

Lithium accelerates functional motor recovery by improving remyelination of regenerating axons following ventral root avulsion and reimplantation

Brachial plexus injury (BPI) often involves the complete or partial avulsion of one or more of the cervical nerve roots, which leads to permanent paralysis of the innervated muscles. Reimplantation surgery has been attempted as a clinical treatment for brachial plexus root avulsion but has failed to achieve complete functional recovery. Lithium is a mood stabilizer drug that is used to treat bipolar disorder; however, its effects on spinal cord or peripheral nerve injuries have also been reported. The purpose of this study was to investigate whether lithium can improve functional motor recovery after ventral root avulsion and reimplantation in a rat model of BPI. The results showed that systemic treatment with a clinical dose of lithium promoted motor neuron outgrowth and increased the efficiency of motor unit regeneration through enhanced remyelination. An analysis of myelin-associated genes showed that the effects of lithium started during the early phase of remyelination and persisted through the late stage of the process. Efficient remyelination of the regenerated axons in the lithium-treated rats led to an earlier functional recovery. Therefore, we demonstrated that lithium might be a potential clinical treatment for BPI in combination with reimplantation surgery.

The Polarity Protein Pals1 Regulates Radial Sorting of Axons

Myelin is essential for rapid and efficient action potential propagation in vertebrates. However, the molecular mechanisms regulating myelination remain incompletely characterized. For example, even before myelination begins in the PNS, Schwann cells must radially sort axons to form 1:1 associations. Schwann cells then ensheathe and wrap axons, and establish polarized, subcellular domains, including apical and basolateral domains, paranodes, and Schmidt-Lanterman incisures. Intriguingly, polarity proteins, such as Pals1/Mpp5, are highly enriched in some of these domains, suggesting that they may regulate the polarity of Schwann cells and myelination. To test this, we generated mice with Schwann cells and oligodendrocytes that lack Pals1. During early development of the PNS, Pals1-deficient mice had impaired radial sorting of axons, delayed myelination, and reduced nerve conduction velocities. Although myelination and conduction velocities eventually recovered, polyaxonal myelination remained a prominent feature of adult Pals1-deficient nerves. Despite the enrichment of Pals1 at paranodes and incisures of control mice, nodes of Ranvier and paranodes were unaffected in Pals1-deficient mice, although we measured a significant increase in the number of incisures. As in other polarized cells, we found that Pals1 interacts with Par3 and loss of Pals1 reduced levels of Par3 in Schwann cells. In the CNS, loss of Pals1 affected neither myelination nor the establishment of polarized membrane domains. These results demonstrate that Schwann cells and oligodendrocytes use distinct mechanisms to control their polarity, and that radial sorting in the PNS is a key polarization event that requires Pals1. Significance statement: This paper reveals the role of the canonical polarity protein Pals1 in radial sorting of axons by Schwann cells. Radial sorting is essential for efficient and proper myelination and is disrupted in some types of congenital muscular dystrophy.

New insights on Schwann cell development

In the peripheral nervous system, Schwann cells are glial cells that are in intimate contact with axons throughout development. Schwann cells generate the insulating myelin sheath and provide vital trophic support to the neurons that they ensheathe. Schwann cell precursors arise from neural crest progenitor cells, and a highly ordered developmental sequence controls the progression of these cells to become mature myelinating or nonmyelinating Schwann cells. Here, we discuss both seminal discoveries and recent advances in our understanding of the molecular mechanisms that drive Schwann cell development and myelination with a focus on cell-cell and cell-matrix signaling events.

Glioma invasion mediated by the p75 neurotrophin receptor (p75(NTR)/CD271) requires regulated interaction with PDLIM1

The invasive nature of glioblastoma renders them incurable by current therapeutic interventions. Using a novel invasive human glioma model, we previously identified the neurotrophin receptor p75^NTR (aka CD271) as a mediator of glioma invasion. Herein, we provide evidence that preventing phosphorylation of p75^NTR on S303 by pharmacological inhibition of PKA, or by a mutational strategy (S303G), cripples p75^NTR-mediated glioma invasion resulting in serine phosphorylation within the C-terminal PDZ-binding motif (SPV) of p75^NTR. Consistent with this, deletion (ΔSPV) or mutation (SPM) of the PDZ motif results in abrogation of p75^NTR-mediated invasion. Using a peptide-based strategy, we identified PDLIM1 as a novel signaling adaptor for p75^NTR and provide the first evidence for a regulated interaction via S425 phosphorylation. Importantly, PDLIM1 was shown to interact with p75^NTR in highly invasive patient-derived glioma stem cells/tumor-initiating cells and shRNA knockdown of PDLIM1 in vitro and in vivo results in complete ablation of p75^NTR-mediated invasion. Collectively, these data demonstrate a requirement for a regulated interaction of p75^NTR with PDLIM1 and suggest that targeting either the PDZ domain interactions and/or the phosphorylation of p75^NTR by PKA could provide therapeutic strategies for patients with glioblastoma.

Myelination of the nervous system: mechanisms and functions

Myelination of axons in the nervous system of vertebrates enables fast, saltatory impulse propagation, one of the best-understood concepts in neurophysiology. However, it took a long while to recognize the mechanistic complexity both of myelination by oligodendrocytes and Schwann cells and of their cellular interactions. In this review, we highlight recent advances in our understanding of myelin biogenesis, its lifelong plasticity, and the reciprocal interactions of myelinating glia with the axons they ensheath. In the central nervous system, myelination is also stimulated by axonal activity and astrocytes, whereas myelin clearance involves microglia/macrophages. Once myelinated, the long-term integrity of axons depends on glial supply of metabolites and neurotrophic factors. The relevance of this axoglial symbiosis is illustrated in normal brain aging and human myelin diseases, which can be studied in corresponding mouse models. Thus, myelinating cells serve a key role in preserving the connectivity and functions of a healthy nervous system.

Schwann cell myelination

Myelinated nerve fibers are essential for the rapid propagation of action potentials by saltatory conduction. They form as the result of reciprocal interactions between axons and Schwann cells. Extrinsic signals from the axon, and the extracellular matrix, drive Schwann cells to adopt a myelinating fate, whereas myelination reorganizes the axon for its role in conduction and is essential for its integrity. Here, we review our current understanding of the development, molecular organization, and function of myelinating Schwann cells. Recent findings into the extrinsic signals that drive Schwann cell myelination, their cognate receptors, and the downstream intracellular signaling pathways they activate will be described. Together, these studies provide important new insights into how these pathways converge to activate the transcriptional cascade of myelination and remodel the actin cytoskeleton that is critical for morphogenesis of the myelin sheath.

The Role of PB1 Domain Proteins in Endothelial Cell Dysfunction and Disease

SIGNIFICANCE

There are a limited number of proteins containing the Phox-Bem1 (PB1) protein interaction domain, and almost all of them play some role in endothelial cell (EC) function, health, and homeostasis.

RECENT ADVANCES

Most of these proteins have been shown to physically interact through PB1-PB1 binding and, as such, are linked together to form complexes that are responsive to hemodynamic force. These complexes range from redox regulation to inflammation to autophagy and back, and they employ multiple feedback mechanisms that are reliant on PB1 domain proteins.

CRITICAL ISSUES

Pathologic roles for PB1 domain-containing proteins have been demonstrated in multiple diseases, including vascular disease, cancer, liver disease, and myriad other concerns. Findings cited in this review show that dimerization of PB1 proteins exerts novel effects on EC function that may be important in multiple cardiovascular diseases, including atherosclerosis, thrombosis, inflammation, and hypertension.

FUTURE DIRECTIONS

As mechanistic understanding of the component pathways (redox regulation, cell polarity, inflammation, atheroprotection, and autophagy) is continually increasing, the larger picture of how these pathways interact with one another is evolving rapidly. We can now evaluate the PB1 domain proteins as a family in the context of multiple phenotypic readouts in EC function as well as evaluate them as drug targets against disease.

The polarity protein Scribble regulates myelination and remyelination in the central nervous system

The development and regeneration of myelin by oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), requires profound changes in cell shape that lead to myelin sheath initiation and formation. Here, we demonstrate a requirement for the basal polarity complex protein Scribble in CNS myelination and remyelination. Scribble is expressed throughout oligodendroglial development and is up-regulated in mature oligodendrocytes where it is localised to both developing and mature CNS myelin sheaths. Knockdown of Scribble expression in cultured oligodendroglia results in disrupted morphology and myelination initiation. When Scribble expression is conditionally eliminated in the myelinating glia of transgenic mice, myelin initiation in CNS is disrupted, both during development and following focal demyelination, and longitudinal extension of the myelin sheath is disrupted. At later stages of myelination, Scribble acts to negatively regulate myelin thickness whilst suppressing the extracellular signal-related kinase (ERK)/mitogen-activated protein kinase (MAP) kinase pathway, and localises to non-compact myelin flanking the node of Ranvier where it is required for paranodal axo-glial adhesion. These findings demonstrate an essential role for the evolutionarily-conserved regulators of intracellular polarity in myelination and remyelination.

The polarity protein Scribble regulates the formation and properties of myelin sheaths in the central nervous system during development and after demyelinating injury.

The formation of myelin, a fatty, multilayered structure that surrounds certain neuronal axons in the nervous system, is essential for the proper communication of electrical signals by neurons, acting both as an insulator and to promote metabolic support to the axon. Loss of myelin can have severe functional consequences and trigger serious diseases, such as multiple sclerosis. Bidirectional communication between the oligodendrocytes, the myelinating cells of the central nervous system, and the axon is essential for the proper formation and function of myelin membranes; however, the signals that control myelination by oligodendrocytes in the central nervous system are poorly understood. In this paper, we use a combination of cell culture and animal studies to demonstrate that the protein Scribble, which is known to be a highly evolutionarily conserved regulator of cell polarity, plays a role in controlling whether oligodendrocytes myelinate axons. We show that Scribble regulates the length and thickness of myelin sheaths formed, as well as the tight adhesion of oligodendroglial membranes to the axonal surface, which is required for the organization of the axon into specialized domains at the nodes of Ranvier (gaps formed between the myelin sheaths generated by different cells). In addition, we show that Scribble plays a key role in the repair of myelin sheaths in a mouse model of demyelinating disease. The discovery of novel regulators of myelination in the central nervous system may allow for the identification of novel therapeutic targets for the promotion of myelin repair in patients suffering from demyelinating diseases.

PACAP interacts with PAC1 receptors to induce tissue plasminogen activator (tPA) expression and activity in schwann cell-like cultures

Regeneration of peripheral nerves depends on the abilities of rejuvenating axons to migrate at the injury site through cellular debris and altered extracellular matrix, and then grow along the residual distal nerve sheath conduit and reinnervate synaptic targets. Considerable evidence suggest that glial cells participate in this process, although the mechanisms remain to be clarified. In cell culture, regenerating neurites secrete PACAP, a peptide shown to induce the expression of the protease tissue plasminogen activator (tPA) in neural cell types. In the present studies, we tested the hypothesis that PACAP can stimulate peripheral glial cells to produce tPA. More specifically, we addressed whether or not PACAP promoted the expression and activity of tPA in the Schwann cell line RT4-D6P2T, which shares biochemical and physical properties with Schwann cells. We found that PACAP dose- and time-dependently stimulated tPA expression both at the mRNA and protein level. Such effect was mimicked by maxadilan, a potent PAC1 receptor agonist, but not by the PACAP-related homolog VIP, suggesting a PAC1-mediated function. These actions appeared to be mediated at least in part by the Akt/CREB signaling cascade because wortmannin, a PI3K inhibitor, prevented peptide-driven CREB phosphorylation and tPA increase. Interestingly, treatment with BDNF mimicked PACAP actions on tPA, but acted through both the Akt and MAPK signaling pathways, while causing a robust increase in PACAP and PAC1 expression. PACAP6-38 totally blocked PACAP-driven tPA expression and in part hampered BDNF-mediated effects. We conclude that PACAP, acting through PAC1 receptors, stimulates tPA expression and activity in a Akt/CREB-dependent manner to promote proteolytic activity in Schwann-cell like cultures.

Matrix RGD ligand density and L1CAM-mediated Schwann cell interactions synergistically enhance neurite outgrowth

The innate biological response to peripheral nerve injury involves a complex interplay of multiple molecular cues to guide neurites across the injury gap. Many current strategies to stimulate regeneration take inspiration from this biological response. However, little is known about the balance of cell-matrix and Schwann cell-neurite dynamics required for regeneration of neural architectures. We present an engineered extracellular matrix (eECM) microenvironment with tailored cell-matrix and cell-cell interactions to study their individual and combined effects on neurite outgrowth. This eECM regulates cell-matrix interactions by presenting integrin-binding RGD (Arg-Gly-Asp) ligands at specified densities. Simultaneously, the addition or exclusion of nerve growth factor (NGF) is used to modulate L1CAM-mediated Schwann cell-neurite interactions. Individually, increasing the RGD ligand density from 0.16 to 3.2mM resulted in increasing neurite lengths. In matrices presenting higher RGD ligand densities, neurite outgrowth was synergistically enhanced in the presence of soluble NGF. Analysis of Schwann cell migration and co-localization with neurites revealed that NGF enhanced cooperative outgrowth between the two cell types. Interestingly, neurites in NGF-supplemented conditions were unable to extend on the surrounding eECM without the assistance of Schwann cells. Blocking studies revealed that L1CAM is primarily responsible for these Schwann cell-neurite interactions. Without NGF supplementation, neurite outgrowth was unaffected by L1CAM blocking or the depletion of Schwann cells. These results underscore the synergistic interplay between cell-matrix and cell-cell interactions in enhancing neurite outgrowth for peripheral nerve regeneration.

Delayed nerve stimulation promotes axon-protective neurofilament phosphorylation, accelerates immune cell clearance and enhances remyelination in vivo in focally demyelinated nerves

Rapid and efficient axon remyelination aids in restoring strong electrochemical communication with end organs and in preventing axonal degeneration often observed in demyelinating neuropathies. The signals from axons that can trigger more effective remyelination in vivo are still being elucidated. Here we report the remarkable effect of delayed brief electrical nerve stimulation (ES; 1 hour @ 20 Hz 5 days post-demyelination) on ensuing reparative events in a focally demyelinated adult rat peripheral nerve. ES impacted many parameters underlying successful remyelination. It effected increased neurofilament expression and phosphorylation, both implicated in axon protection. ES increased expression of myelin basic protein (MBP) and promoted node of Ranvier re-organization, both of which coincided with the early reappearance of remyelinated axons, effects not observed at the same time points in non-stimulated demyelinated nerves. The improved ES-associated remyelination was accompanied by enhanced clearance of ED-1 positive macrophages and attenuation of glial fibrillary acidic protein expression in accompanying Schwann cells, suggesting a more rapid clearance of myelin debris and return of Schwann cells to a nonreactive myelinating state. These benefits of ES correlated with increased levels of brain derived neurotrophic factor (BDNF) in the acute demyelination zone, a key molecule in the initiation of the myelination program. In conclusion, the tremendous impact of delayed brief nerve stimulation on enhancement of the innate capacity of a focally demyelinated nerve to successfully remyelinate identifies manipulation of this axis as a novel therapeutic target for demyelinating pathologies.

Pard3 regulates contact between neural crest cells and the timing of Schwann cell differentiation but is not essential for neural crest migration or myelination

BACKGROUND

Schwann cells, which arise from the neural crest, are the myelinating glia of the peripheral nervous system. During development neural crest and their Schwann cell derivatives engage in a sequence of events that comprise delamination from the neuroepithelium, directed migration, axon ensheathment, and myelin membrane synthesis. At each step neural crest and Schwann cells are polarized, suggesting important roles for molecules that create cellular asymmetries. In this work we investigated the possibility that one polarity protein, Pard3, contributes to the polarized features of neural crest and Schwann cells that are associated with directed migration and myelination.

RESULTS

We analyzed mutant zebrafish embryos deficient for maternal and zygotic pard3 function. Time-lapse imaging revealed that neural crest delamination was normal but that migrating cells were disorganized with substantial amounts of overlapping membrane. Nevertheless, neural crest cells migrated to appropriate peripheral targets. Schwann cells wrapped motor axons and, although myelin gene expression was delayed, myelination proceeded to completion.

CONCLUSIONS

Pard3 mediates contact inhibition between neural crest cells and promotes timely myelin gene expression but is not essential for neural crest migration or myelination.

Phosphorylation of LKB1/Par-4 establishes Schwann cell polarity to initiate and control myelin extent

The Schwann cell (SC)-axon interface represents a membrane specialization that integrates axonal signals to coordinate cytoskeletal dynamics resulting in myelination. Here we show that LKB1/Par-4 is asymmetrically localized to the SC-axon interface and colocalizes with the polarity protein Par-3. Using purified SCs and myelinating cocultures, we demonstrate that localization is dependent on the phosphorylation of LKB1 at serine-431. SC-specific deletion of LKB1 significantly attenuates developmental myelination, delaying the initiation and altering the myelin extent into adulthood, resulting in a 30% reduction in the conduction velocity along adult sciatic nerves. Phosphorylation of LKB1 by protein kinase A is essential to establish the asymmetric localization of LKB1 and Par-3 and rescues the delay in myelination observed in the SC-specific knockout of LKB1. Our findings suggest that SC polarity may coordinate multiple signaling complexes that couple SC-axon contact to the redistribution of specific membrane components necessary to initiate and control myelin extent.

Neuregulin-ERBB signaling in the nervous system and neuropsychiatric diseases

Neuregulins (NRGs) comprise a large family of growth factors that stimulate ERBB receptor tyrosine kinases. NRGs and their receptors, ERBBs, have been identified as susceptibility genes for diseases such as schizophrenia (SZ) and bipolar disorder. Recent studies have revealed complex Nrg/Erbb signaling networks that regulate the assembly of neural circuitry, myelination, neurotransmission, and synaptic plasticity. Evidence indicates there is an optimal level of NRG/ERBB signaling in the brain and deviation from it impairs brain functions. NRGs/ERBBs and downstream signaling pathways may provide therapeutic targets for specific neuropsychiatric symptoms.

Role of p75 neurotrophin receptor in stem cell biology: more than just a marker

p75(NTR), the common receptor for both neurotrophins and proneurotrophins, has been widely studied because of its role in many tissues, including the nervous system. More recently, a close relationship between p75(NTR) expression and pluripotency has been described. p75(NTR) was shown to be expressed in various types of stem cells and has been used to prospectively isolate stem cells with different degrees of potency. Here, we give an overview of the current knowledge on p75(NTR) in stem cells, ranging from embryonic to adult stem cells, and cancer stem cells. In an attempt to address its potential role in the control of stem cell biology, the molecular mechanisms underlying p75(NTR) signaling in different models are also highlighted. p75(NTR)-mediated functions include survival, apoptosis, migration, and differentiation, and depend on cell type, (pro)neurotrophin binding, interacting transmembrane co-receptors expression, intracellular adaptor molecule availability, and post-translational modifications, such as regulated proteolytic processing. It is therefore conceivable that p75(NTR) can modulate cell-fate decisions through its highly ramified signaling pathways. Thus, elucidating the potential implications of p75(NTR) activity as well as the underlying molecular mechanisms of p75(NTR) will shed new light on the biology of both normal and cancer stem cells.

Brain patterning perturbations following PTEN loss

This review will consider the impact of compromised PTEN signaling in brain patterning. We approach understanding the contribution of PTEN to nervous system development by surveying the findings from the numerous genetic loss-of-function models that have been generated as well as other forms of PTEN inactivation. By exploring the developmental programs influenced by this central transduction molecule, we can begin to understand the molecular mechanisms that shape the developing brain. A wealth of data indicates that PTEN plays critical roles in a variety of stages during brain development. Many of them are considered here including: stem cell proliferation, fate determination, polarity, migration, process outgrowth, myelination and somatic hypertrophy. In many of these contexts, it is clear that PTEN phosphatase activity contributes to the observed effects of genetic deletion or depletion, however recent studies have also ascribed non-catalytic functions to PTEN in regulating cell function. We also explore the potential impact this alternative pool of PTEN may have on the developing brain. Together, these elements begin to form a clearer picture of how PTEN contributes to the emergence of brain structure and binds form and function in the nervous system.

The BDNF effects on dendritic spines of mature hippocampal neurons depend on neuronal activity

The fine tuning of neural networks during development and learning relies upon both functional and structural plastic processes. Changes in the number as well as in the size and shape of dendritic spines are associated to long-term activity-dependent synaptic plasticity. However, the molecular mechanisms translating functional into structural changes are still largely unknown. In this context, neurotrophins, like Brain-Derived Neurotrophic Factor (BDNF), are among promising candidates. Specifically BDNF-TrkB receptor signaling is crucial for activity-dependent strengthening of synapses in different brain regions. BDNF application has been shown to positively modulate dendritic and spine architecture in cortical and hippocampal neurons as well as structural plasticity in vitro. However, a global BDNF deprivation throughout the central nervous system (CNS) resulted in very mild structural alterations of dendritic spines, questioning the relevance of the endogenous BDNF signaling in modulating the development and the mature structure of neurons in vivo. Here we show that a loss-of-function approach, blocking BDNF results in a significant reduction in dendritic spine density, associated with an increase in spine length and a decrease in head width. These changes are associated with a decrease in F-actin levels within spine heads. On the other hand, a gain-of-function approach, applying exogenous BDNF, could not reproduce the increase in spine density or the changes in spine morphology previously described. Taken together, we show here that the effects exerted by BDNF on the dendritic architecture of hippocampal neurons are dependent on the neuron's maturation stage. Indeed, in mature hippocampal neurons in vitro as shown in vivo BDNF is specifically required for the activity-dependent maintenance of the mature spine phenotype.