Gpr126 is essential for peripheral nerve development and myelination in mammals

In peripheral nerves, Schwann cells form the myelin sheath that insulates axons and allows rapid propagation of action potentials. Although a number of regulators of Schwann cell development are known, the signaling pathways that control myelination are incompletely understood. In this study, we show that Gpr126 is essential for myelination and other aspects of peripheral nerve development in mammals. A mutation in Gpr126 causes a severe congenital hypomyelinating peripheral neuropathy in mice, and expression of differentiated Schwann cell markers, including Pou3f1, Egr2, myelin protein zero and myelin basic protein, is reduced. Ultrastructural studies of Gpr126-/- mice showed that axonal sorting by Schwann cells is delayed, Remak bundles (non-myelinating Schwann cells associated with small caliber axons) are not observed, and Schwann cells are ultimately arrested at the promyelinating stage. Additionally, ectopic perineurial fibroblasts form aberrant fascicles throughout the endoneurium of the mutant sciatic nerve. This analysis shows that Gpr126 is required for Schwann cell myelination in mammals, and defines new roles for Gpr126 in axonal sorting, formation of mature non-myelinating Schwann cells and organization of the perineurium.     

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

Neuregulin-1 provides an important axonally derived signal for the survival and growth of developing Schwann cells, which is transmitted by the ErbB2/ErbB3 receptor tyrosine kinases. Null mutations of the neuregulin-1, erbB2, or erbB3 mouse genes cause severe deficits in early Schwann cell development. Here, we employ Cre-loxP technology to introduce erbB2 mutations late in Schwann cell development, using a Krox20-cre allele. Cre-mediated erbB2 ablation occurs perinatally in peripheral nerves, but already at E11 within spinal roots. The mutant mice exhibit a widespread peripheral neuropathy characterized by abnormally thin myelin sheaths, containing fewer myelin wraps. In addition, in spinal roots the Schwann cell precursor pool is not correctly established. Thus, the Neuregulin signaling system functions during multiple stages of Schwann cell development and is essential for correct myelination. The thickness of the myelin sheath is determined by the axon diameter, and we suggest that trophic signals provided by the nerve determine the number of times a Schwann cell wraps an axon.     

A genetic screen identifies genes essential for development of myelinated axons in zebrafish

The myelin sheath insulates axons in the vertebrate nervous system, allowing rapid propagation of action potentials via saltatory conduction. Specialized glial cells, termed Schwann cells in the PNS and oligodendrocytes in the CNS, wrap axons to form myelin, a compacted, multilayered sheath comprising specific proteins and lipids. Disruption of myelinated axons causes human diseases, including multiple sclerosis and Charcot-Marie-Tooth peripheral neuropathies. Despite the progress in identifying human disease genes and other mutations disrupting glial development and myelination, many important unanswered questions remain about the mechanisms that coordinate the development of myelinated axons. To address these questions, we began a genetic dissection of myelination in zebrafish. Here we report a genetic screen that identified 13 mutations, which define 10 genes, disrupting the development of myelinated axons. We present the initial characterization of seven of these mutations, defining six different genes, along with additional characterization of mutations that we have described previously. The different mutations affect the PNS, the CNS, or both, and phenotypic analyses indicate that the genes affect a wide range of steps in glial development, from fate specification through terminal differentiation. The analysis of these mutations will advance our understanding of myelination, and the mutants will serve as models of human diseases of myelin.     

Neurotrophin-4 induces myelin protein zero expression in cultured Schwann cells via the TrkB/PI3K/Akt/mTORC1 pathway

Myelin formation during peripheral nervous system development, as well as myelin repair after injury and in disease, requires multiple intrinsic and extrinsic signals. Neurotrophin-4 (NT-4) is a member of the neurotrophin family, which regulates the development of neuronal networks by participating in the growth of neuronal processes, synaptic development and plasticity, neuronal survival, and differentiation. However, the intracellular signaling pathways by which NT-4 participates in myelination by Schwann cells remain elusive. In this study, we examined the effects of NT-4 on the expression of compact myelin proteins in cultured Schwann cells. Using real-time quantitative RT-PCR and western blotting, we found that NT-4 could significantly enhance the expression of myelin protein zero (MPZ) but not the expression of myelin basic protein or peripheral myelin protein 22. Further, knockdown of truncated TrkB with small interfering RNA could eliminate the effect of NT-4 on MPZ expression. Moreover, we demonstrated that the NT-4-enhanced MPZ expression depended on Akt and mTORC1 signaling. Taken together, these results suggest that NT-4 binds TrkB to enhance the expression of MPZ in Schwann cells, probably through the PI3K/Akt/mTORC1 signaling pathway, thus contributing to myelination.     

Erythropoietin (EPO) Increases Myelin Gene Expression in CG4 Oligodendrocyte Cells through the Classical EPO Receptor

Erythropoietin (EPO) has protective effects in neurodegenerative and neuroinflammatory diseases, including in animal models of multiple sclerosis, where EPO decreases disease severity. EPO also promotes neurogenesis and is protective in models of toxic demyelination. In this study, we asked whether EPO could promote neurorepair by also inducing remyelination. In addition, we investigated whether the effect of EPO could be mediated by the classical erythropoietic EPO receptor (EPOR), since it is still questioned if EPOR is functional in nonhematopoietic cells. Using CG4 cells, a line of rat oligodendrocyte precursor cells, we found that EPO increases the expression of myelin genes (myelin oligodendrocyte glycoprotein [MOG] and myelin basic protein [MBP]). EPO had no effect in wild-type CG4 cells, which do not express EPOR, whereas it increased MOG and MBP expression in cells engineered to overexpress EPOR (CG4-EPOR). This was reflected in a marked increase in MOG protein levels, as detected by Western blot. In these cells, EPO induced by 10-fold the early growth response gene 2 (Egr2), which is required for peripheral myelination. However, Egr2 silencing with a siRNA did not reverse the effect of EPO, indicating that EPO acts through other pathways. In conclusion, EPO induces the expression of myelin genes in oligodendrocytes and this effect requires the presence of EPOR. This study demonstrates that EPOR can mediate neuroreparative effects.     

Transcriptional profile of the human peripheral nervous system by serial analysis of gene expression

The peripheral nerve contains both nonmyelinating and myelinating Schwann cells. The interactions between axons, surrounding myelin, and Schwann cells are thought to be important for the correct functioning of the nervous system. To get insight into the genes involved in human myelination and maintenance of the myelin sheath and nerve, we performed a serial analysis of gene expression of human sciatic nerve and cultured Schwann cells. In the sciatic nerve library, we found high expression of genes encoding proteins related to lipid metabolism, the complement system, and the cell cycle, while cultured Schwann cells showed mainly high expression of genes encoding extracellular matrix proteins. The results of our study will assist in the identification of genes involved in maintenance of myelin and peripheral nerve and of genes involved in inherited peripheral neuropathies.