P120 catenin is required for thickening of Schwann cell myelin

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.

Identification of potential regulatory long non-coding RNA-associated competing endogenous RNA axes in periplaque regions in multiple sclerosis

Slow-burning inflammation at the lesion rim is connected to the expansion of chronic multiple sclerosis (MS) lesions. However, the underlying processes causing expansion are not clearly realized. In this context, the current study used a bioinformatics approach to identify the expression profiles and related lncRNA-associated ceRNA regulatory axes in the periplaque region in MS patients. Expression data (GSE52139) from periplaque regions in the secondary progressive MS spinal cord and controls were downloaded from the Gene Expression Omnibus database (GEO), which has details on mRNAs and lncRNAs. Using the R software's limma package, the differentially expressed lncRNAs (DElncRNAs) and mRNAs (DEmRNAs) were found. The RNA interactions were also found using the DIANA-LncBase, miRTarBase, and HMDD databases. The Pearson correlation coefficient was used to determine whether there were any positive correlations between DEmRNAs and DElncRNAs in the ceRNA network. Finally, lncRNA-associated ceRNA axes were created based on co-expression and connections between DElncRNA, miRNA, and DEmRNA. We used the Enrichr tool to enrich the biological process, molecular function, and pathways for DEmRNAs and DElncRNAs. A network of DEmRNAs' protein-protein interactions was developed, and the top five hub genes were found using Cytoscape and STRING. The current study indicates that 15 DEmRNAs, including FOS, GJA1, NTRK2, CTNND1, and SP3, are connected to the MS ceRNA network. Additionally, four DElncRNAs (such as TUG1, ASB16-AS1, and LINC01094) that regulated the aforementioned mRNAs by sponging 14 MS-related miRNAs (e.g., hsa-miR-145-5p, hsa-miR-200a-3p, hsa-miR-20a-5p, hsa-miR-22-3p, hsa-miR-23a-3p, hsa-miR-27a-3p, hsa-miR-29b-3p, hsa-miR-29c-3p, hsa-miR-34a-5p) were found. In addition, the analysis of pathway enrichment revealed that DEmRNAs were enriched in the pathways for the "MAPK signaling pathway", "Kaposi sarcoma-associated herpesvirus infection", "Human immunodeficiency virus one infection", "Lipid and atherosclerosis", and "Amphetamine addiction". Even though the function of these ceRNA axes needs to be investigated further, this study provides research targets for studying ceRNA-mediated molecular mechanisms related to periplaque demyelination in MS.

Comprehensive Atlas of the Myelin Basic Protein Interaction Landscape

Intrinsically disordered myelin basic protein (MBP) is one of the key autoantigens in autoimmune neurodegeneration and multiple sclerosis particularly. MBP is highly positively charged and lacks distinct structure in solution and therefore its intracellular partners are still mostly enigmatic. Here we used combination of formaldehyde-induced cross-linking followed by immunoprecipitation and liquid chromatography-tandem mass spectrometry (LC-MS/MS) to elucidate the interaction network of MBP in mammalian cells and provide the list of potential MBP interacting proteins. Our data suggest that the largest group of MBP-interacting proteins belongs to cellular proteins involved in the protein translation machinery, as well as in the spatial and temporal regulation of translation. MBP interacts with core ribosomal proteins, RNA helicase Ddx28 and RNA-binding proteins STAU1, TDP-43, ADAR-1 and hnRNP A0, which are involved in various stages of RNA biogenesis and processing, including specific maintaining MBP-coding mRNA. Among MBP partners we identified CTNND1, which has previously been shown to be necessary for myelinating Schwann cells for cell-cell interactions and the formation of a normal myelin sheath. MBP binds proteins MAGEB2/D2 associated with neurotrophin receptor p75NTR, involved in pathways that promote neuronal survival and neuronal death. Finally, we observed that MBP interacts with RNF40-a component of heterotetrameric Rnf40/Rnf20 E3 ligase complex, recruited by Egr2, which is the central transcriptional regulator of peripheral myelination. Concluding, our data suggest that MBP may be more actively involved in myelination not only as a main building block but also as a self-regulating element.

Dysregulation of myelin synthesis and actomyosin function underlies aberrant myelin in CMT4B1 neuropathy

Charcot-Marie-Tooth type 4B1 (CMT4B1) is a severe autosomal recessive demyelinating neuropathy with childhood onset, caused by loss-of-function mutations in the myotubularin-related 2 (MTMR2) gene. MTMR2 is a ubiquitously expressed catalytically active 3-phosphatase, which in vitro dephosphorylates the 3-phosphoinositides PtdIns3P and PtdIns(3,5)P ₂, with a preference for PtdIns(3,5)P ₂ A hallmark of CMT4B1 neuropathy are redundant loops of myelin in the nerve termed myelin outfoldings, which can be considered the consequence of altered growth of myelinated fibers during postnatal development. How MTMR2 loss and the resulting imbalance of 3'-phosphoinositides cause CMT4B1 is unknown. Here we show that MTMR2 by regulating PtdIns(3,5)P ₂ levels coordinates mTORC1-dependent myelin synthesis and RhoA/myosin II-dependent cytoskeletal dynamics to promote myelin membrane expansion and longitudinal myelin growth. Consistent with this, pharmacological inhibition of PtdIns(3,5)P ₂ synthesis or mTORC1/RhoA signaling ameliorates CMT4B1 phenotypes. Our data reveal a crucial role for MTMR2-regulated lipid turnover to titrate mTORC1 and RhoA signaling thereby controlling myelin growth.

Structures and Molecular Composition of Schmidt-Lanterman Incisures

Schmidt-Lanterman incisure (SLI) is a circular-truncated cone shape in the myelin internode that is a specific feature of myelinated nerve fibers formed in Schwann cells in the peripheral nervous system (PNS). The SLI circular-truncated cones elongate like spring at the narrow sites of beaded appearance nerve fibers under the stretched condition. In this chapter, we demonstrate various molecular complexes in SLI, and especially focus on membrane skeleton, protein 4.1G-membrane protein palmitoylated 6 (MPP6)-cell adhesion molecule 4 (CADM4). 4.1G was essential for the molecular targeting of MPP6 and CADM4 in SLI. Motor activity and myelin ultrastructures were abnormal in 4.1G-deficient mice, indicating the 4.1G function as a signal for proper formation of myelin in PNS. Thus, SLI probably has potential roles in the regulation of adhesion and signal transduction as well as in structural stability in Schwann cell myelin formation.

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.

In Vivo Introduction of Transgenes into Mouse Sciatic Nerve Cells Using Viral Vectors

Myelinated fibers are essential for the rapid and efficient propagation of nerve information throughout the body. These fibers result from an intimate crosstalk between myelinating glia and the myelinated axons and, because it is difficult to fully reproduce these interactions in vitro, the basic molecular mechanisms that regulate myelination, demyelination, and remyelination remain unclear. Schwann cells produce myelin in the peripheral nervous system (PNS) and remain associated with the axons of peripheral neurons throughout axonal migration to the target. In order to investigate more closely the biology of myelinated fibers, we developed a local transgenesis approach based on the injection of engineered viral vectors in the sciatic nerve of mice to locally transduce peripheral nerve cells. This approach represents an alternative to germline modifications as it facilitates and speed up the investigation of peripheral nerve biology in vivo. Indeed the protocol we describe here requires just 3 weeks to complete. The injection of engineered viral vectors in the sciatic nerve of mice is a reproducible and straightforward method for introducing exogenous factors into myelinating Schwann cells and myelinated axons in vivo in order to investigate specific molecular mechanisms.

Transcriptome analysis of adherens junction pathway-related genes after peripheral nerve injury

The neural regeneration process is driven by a wide range of molecules and pathways. Adherens junctions are critical cellular junctions for the integrity of peripheral nerves. However, few studies have systematically characterized the transcript changes in the adherens junction pathway following injury. In this study, a rat model of sciatic nerve crush injury was established by forceps. Deep sequencing data were analyzed using comprehensive transcriptome analysis at 0, 1, 4, 7, and 14 days after injury. Results showed that most individual molecules in the adherens junctions were either upregulated or downregulated after nerve injury. The mRNA expression of ARPC1B, ARPC3, TUBA8, TUBA1C, CTNNA2, ACTN3, MET, HGF, NME1 and ARF6, which are involved in the adherens junction pathway and in remodeling of adherens junctions, was analyzed using quantitative real-time polymerase chain reaction. Most of these genes were upregulated in the sciatic nerve stump following peripheral nerve injury, except for CTNNA2, which was downregulated. Our findings reveal the dynamic changes of key molecules in adherens junctions and in remodeling of adherens junctions. These key genes provide a reference for the selection of clinical therapeutic targets for peripheral nerve injury.

Poly(ADP-ribosylation) is present in murine sciatic nerve fibers and is altered in a Charcot-Marie-Tooth-1E neurodegenerative model

Background

Poly-ADP-ribose (PAR) is a polymer synthesized by poly-ADP-ribose polymerases (PARPs) as a postranslational protein modification and catabolized mainly by poly-ADP-ribose glycohydrolase (PARG). In spite of the existence of cytoplasmic PARPs and PARG, research has been focused on nuclear PARPs and PAR, demonstrating roles in the maintenance of chromatin architecture and the participation in DNA damage responses and transcriptional regulation. We have recently detected non-nuclear PAR structurally and functionally associated to the E-cadherin rich zonula adherens and the actin cytoskeleton of VERO epithelial cells. Myelinating Schwann cells (SC) are stabilized by E-cadherin rich autotypic adherens junctions (AJ). We wondered whether PAR would map to these regions. Besides, we have demonstrated an altered microfilament pattern in peripheral nerves of Trembler-J (Tr-J) model of CMT1-E. We hypothesized that cytoplasmic PAR would accompany such modified F-actin pattern.

Methods

Wild-type (WT) and Tr-J mice sciatic nerves cryosections were subjected to immunohistofluorescence with anti-PAR antibodies (including antibody validation), F-actin detection with a phalloidin probe and DAPI/DNA counterstaining. Confocal image stacks were subjected to a colocalization highlighter and to semi-quantitative image analysis.

Results

We have shown for the first time the presence of PAR in sciatic nerves. Cytoplasmic PAR colocalized with F-actin at non-compact myelin regions in WT nerves. Moreover, in Tr-J, cytoplasmic PAR was augmented in close correlation with actin. In addition, nuclear PAR was detected in WT SC and was moderately increased in Tr-J SC.

Discussion

The presence of PAR associated to non-compact myelin regions (which constitute E-cadherin rich autotypic AJ/actin anchorage regions) and the co-alterations experienced by PAR and the actin cytoskeleton in epithelium and nerves, suggest that PAR may be a constitutive component of AJ/actin anchorage regions. Is PAR stabilizing the AJ-actin complexes? This question has strong implications in structural cell biology and cell signaling networks. Moreover, if PAR played a stabilizing role, such stabilization could participate in the physiological control of axonal branching. PARP and PAR alterations exist in several neurodegenerative pathologies including Alzheimer’s, Parkinson’s and Hungtington’s diseases. Conversely, PARP inhibition decreases PAR and promotes neurite outgrowth in cortical neurons in vitro. Coherently, the PARP inhibitor XAV939 improves myelination in vitro, ex vivo and in vivo. Until now such results have been interpreted in terms of nuclear PARP activity. Our results indicate for the first time the presence of PARylation in peripheral nerve fibers, in a healthy environment. Besides, we have evidenced a PARylation increase in Tr-J, suggesting that the involvement of cytoplasmic PARPs and PARylation in normal and neurodegenerative conditions should be re-evaluated.

Optimal myelin elongation relies on YAP activation by axonal growth and inhibition by Crb3/Hippo pathway

Fast nerve conduction relies on successive myelin segments that electrically isolate axons. Segment geometry—diameter and length—is critical for the optimization of nerve conduction and the molecular mechanisms allowing this optimized geometry are partially known. We show here that peripheral myelin elongation is dynamically regulated by stimulation of YAP (Yes-associated protein) transcription cofactor activity during axonal elongation and limited by inhibition of YAP activity via the Hippo pathway. YAP promotes myelin and non-myelin genes transcription while the polarity protein Crb3, localized at the tips of the myelin sheath, activates the Hippo pathway to temper YAP activity, therefore allowing for optimal myelin growth. Dystrophic Dy^2j/2j mice mimicking human peripheral neuropathy with reduced internodal lengths have decreased nuclear YAP which, when corrected, leads to longer internodes. These data show a novel mechanism controlling myelin growth and nerve conduction, and provide a molecular ground for disease with short myelin segments.

Molecular mechanisms regulating optimal myelin geometry are only partially understood. Here authors show that peripheral myelin growth is orchestrated by the Crb3/Hippo/YAP pathway, and that defects in YAP activation may underlie peripheral neuropathies caused by shorter myelin.

Rac1 controls Schwann cell myelination through cAMP and NF2/merlin

During peripheral nervous system development, Schwann cells (SCs) surrounding single large axons differentiate into myelinating SCs. Previous studies implicate RhoGTPases in SC myelination, but the mechanisms involved in RhoGTPase regulation of SC myelination are unknown. Here, we show that SC myelination is arrested in Rac1 conditional knock-out (Rac1-CKO) mice. Rac1 knock-out abrogated phosphorylation of the effector p21-activated kinase and decreased NF2/merlin phosphorylation. Mutation of NF2/merlin rescued the myelin deficit in Rac1-CKO mice in vivo and the shortened processes in cultured Rac1-CKO SCs in vitro. Mechanistically, cAMP levels and E-cadherin expression were decreased in the absence of Rac1, and both were restored by mutation of NF2/merlin. Reduced cAMP is a cause of the myelin deficiency in Rac1-CKO mice, because elevation of cAMP by rolipram in Rac1-CKO mice in vivo allowed myelin formation. Thus, NF2/merlin and cAMP function downstream of Rac1 signaling in SC myelination, and cAMP levels control Rac1-regulated SC myelination.

Identification of genes expressed by zebrafish oligodendrocytes using a differential microarray screen

Myelination of central nervous system axons requires that oligodendrocytes extend multiple membrane processes that specifically recognize and wrap axons, which is followed by expression of proteins necessary for formation of myelin sheaths. To identify new genes that might be important for myelination, we used microarrays to analyze the expression profiles of cells sorted from transgenic zebrafish embryos and larvae under conditions that permitted or blocked oligodendrocyte development. Here, we describe eight genes that have not been previously implicated in oligodendrocyte development. Among the predicted functions of proteins encoded by these genes are lipid sensing, cell-cell junction formation, cytoskeleton regulation, and intracellular signaling. The predicted functions raise the possibility that these genes are involved in multiple cellular events during oligodendrocyte differentiation and myelin formation.

p120ctn and P-cadherin but not E-cadherin regulate cell motility and invasion of DU145 prostate cancer cells

Background

Adherens junctions consist of transmembrane cadherins, which interact intracellularly with p120ctn, ß-catenin and α-catenin. p120ctn is known to regulate cell-cell adhesion by increasing cadherin stability, but the effects of other adherens junction components on cell-cell adhesion have not been compared with that of p120ctn.

Methodology/Principal Findings

We show that depletion of p120ctn by small interfering RNA (siRNA) in DU145 prostate cancer and MCF10A breast epithelial cells reduces the expression levels of the adherens junction proteins, E-cadherin, P-cadherin, ß-catenin and α-catenin, and induces loss of cell-cell adhesion. p120ctn-depleted cells also have increased migration speed and invasion, which correlates with increased Rap1 but not Rac1 or RhoA activity. Downregulation of P-cadherin, β-catenin and α-catenin but not E-cadherin induces a loss of cell-cell adhesion, increased migration and enhanced invasion similar to p120ctn depletion. However, only p120ctn depletion leads to a decrease in the levels of other adherens junction proteins.

Conclusions/Significance

Our data indicate that P-cadherin but not E-cadherin is important for maintaining adherens junctions in DU145 and MCF10A cells, and that depletion of any of the cadherin-associated proteins, p120ctn, ß-catenin or α-catenin, is sufficient to disrupt adherens junctions in DU145 cells and increase migration and cancer cell invasion.

Pals1 is a major regulator of the epithelial-like polarization and the extension of the myelin sheath in peripheral nerves

Diameter, organization, and length of the myelin sheath are important determinants of the nerve conduction velocity, but the basic molecular mechanisms that control these parameters are only partially understood. Cell polarization is an essential feature of differentiated cells, and relies on a set of evolutionarily conserved cell polarity proteins. We investigated the molecular nature of myelin sheath polarization in connection with the functional role of the cell polarity protein pals1 (Protein Associated with Lin Seven 1) during peripheral nerve myelin sheath extension. We found that, in regard to epithelial polarity, the Schwann cell outer abaxonal domain represents a basolateral-like domain, while the inner adaxonal domain and Schmidt-Lanterman incisures form an apical-like domain. Silencing of pals1 in myelinating Schwann cells in vivo resulted in a severe reduction of myelin sheath thickness and length. Except for some infoldings, the structure of compact myelin was not fundamentally affected, but cells produced less myelin turns. In addition, pals1 is required for the normal polarized localization of the vesicular markers sec8 and syntaxin4, and for the distribution of E-cadherin and myelin proteins PMP22 and MAG at the plasma membrane. Our data show that the polarity protein pals1 plays an essential role in the radial and longitudinal extension of the myelin sheath, likely involving a functional role in membrane protein trafficking. We conclude that regulation of epithelial-like polarization is a critical determinant of myelin sheath structure and function.

Erbin regulates NRG1 signaling and myelination

Neuregulin 1 (NRG1) plays a critical role in myelination. However, little is known about regulatory mechanisms of NRG1 signaling. We show here that Erbin, a protein that contains leucine-rich repeats (LRR) and a PSD95-Dlg-Zol (PDZ) domain and that interacts specifically with ErbB2, is necessary for NRG1 signaling and myelination of peripheral nervous system (PNS). In Erbin null mice, myelinated axons were hypomyelinated with reduced expression of P0, a marker of mature myelinating Schwann cells (SCs), whereas unmyelinated axons were aberrantly ensheathed in Remak bundles, with increased numbers of axons in the bundles and in pockets. The morphological deficits were associated with decreased nerve conduction velocity and increased sensory threshold to mechanistic stimulation. These phenotypes were duplicated in erbin(DeltaC/DeltaC) mice, in which Erbin lost the PDZ domain to interact with ErbB2. Moreover, ErbB2 was reduced at protein levels in both Erbin mutant sciatic nerves, and ErbB2 became unstable and NRG1 signaling compromised when Erbin expression was suppressed. These observations indicate a critical role of Erbin in myelination and identify a regulatory mechanism of NRG1 signaling. Our results suggest that Erbin, via the PDZ domain, binds to and stabilizes ErbB2, which is necessary for NRG1 signaling that has been implicated in tumorigenesis, heart development, and neural function.

E-cadherin expression in postnatal Schwann cells is regulated by the cAMP-dependent protein kinase a pathway

Expression of E-cadherin in the peripheral nervous system is a highly regulated process that appears postnatally in concert with the development of myelinating Schwann cell lineage. As a major component of autotypic junctions, E-cadherin plays an important role in maintaining the structural integrity of noncompact myelin regions. In vivo, the appearance of E-cadherin in postnatal Schwann cell is accompanied by the disappearance of N-cadherin, suggesting reciprocal regulation of the two cadherins during Schwann cell development. The molecular signal that regulates the cadherin switch in Schwann cell is unclear. Using a neuron-Schwann cell co-culture system, here we show that E-cadherin expression is induced by components on the axonal membrane. We also show that the axonal effect is mediated through cAMP-dependent protein kinase A (cAMP-PKA) activation in the Schwann cell: (1) inhibition of cAMP-PKA blocks axon-induced E-cadherin expression and (2) cAMP elevation in the Schwann cell is sufficient to induce E-cadherin expression. In addition, cAMP-dependent E-cadherin expression is promoted by contact between adjacent Schwann cell membranes, suggesting its role in autotypic junction formation during myelination. Furthermore, cAMP-induced E-cadherin expression is accompanied by suppression of N-cadherin expression. Therefore, we propose that axon-dependent activation of cAMP-PKA serves as a signal that promotes cadherin switch during postnatal development of Schwann cells.

Tumor suppressor schwannomin/merlin is critical for the organization of Schwann cell contacts in peripheral nerves

Schwannomin/merlin is the product of a tumor suppressor gene mutated in neurofibromatosis type 2 (NF2). Although the consequences of NF2 mutations on Schwann cell proliferation are well established, the physiological role of schwannomin in differentiated cells is not known. To unravel this role, we studied peripheral nerves in mice overexpressing in Schwann cells schwannomin with a deletion occurring in NF2 patients (P0-SCH-Delta39-121) or a C-terminal deletion. The myelin sheath and nodes of Ranvier were essentially preserved in both lines. In contrast, the ultrastructural and molecular organization of contacts between Schwann cells and axons in paranodal and juxtaparanodal regions were altered, with irregular juxtaposition of normal and abnormal areas of contact. Similar but more severe alterations were observed in mice with conditional deletion of the Nf2 gene in Schwann cells. The number of Schmidt-Lanterman incisures, which are cytoplasmic channels interrupting the compact myelin and characterized by distinct autotypic contacts, was increased in the three mutant lines. P0-SCH-Delta39-121 and conditionally deleted mice displayed exuberant wrapping of nonmyelinated fibers and short internodes, an abnormality possibly related to altered control of Schwann cell proliferation. In support of this hypothesis, Schwann cell number was increased along fibers before myelination in P0-SCH-Delta39-121 mice but not in those with C-terminal deletion. Schwann cell numbers were also more numerous in mice with conditional deletion. Thus, schwannomin plays an important role in the control of Schwann cell number and is necessary for the correct organization and regulation of axoglial heterotypic and glio-glial autotypic contacts.