SUMMARY Myelin ensheathes axons to allow rapid propagation of action potentials and proper nervous system function. In the peripheral nervous system, Schwann cells (SCs) radially sort axons into a 1:1 relationship before wrapping an axonal segment to form myelin. SC myelination requires the adhesion G protein-coupled receptor GPR126, which undergoes autoproteolytic cleavage into an N-terminal fragment (NTF) and a 7-transmembrane-containing C-terminal fragment (CTF). Here, we show that GPR126 has domain-specific functions in SC development whereby the NTF is necessary and sufficient for axon sorting while the CTF promotes wrapping through cAMP elevation. These biphasic roles of GPR126 are governed by interactions with Laminin-211, which we define as a novel ligand for GPR126 that modulates receptor signaling via a tethered agonist. Our work suggests a model in which Laminin-211 mediates GPR126-induced cAMP levels to control early and late stages of SC development.

Functionally and spatially distinct PI 3-K pathways act either early to promote myelination downstream of axonal Neuregulin1 or late to inhibit myelination downstream of α6β4 integrin and Sgk1.

SUMMARYRadial sorting allows the segregation of axons by a single Schwann cell (SC) and is a prerequisite for myelination during peripheral nerve development. Radial sorting is impaired in models of human diseases, congenital muscular dystrophy (MDC) 1A, MDC1D and Fukuyama, owing to loss-of-function mutations in the genes coding for laminin 2, Large or fukutin glycosyltransferases, respectively. It is not clear which receptor(s) are activated by laminin 211, or glycosylated by Large and fukutin during sorting. Candidates are 1 integrins, because their absence phenocopies laminin and glycosyltransferase deficiency, but the topography of the phenotypes is different and 1 integrins are not substrates for Large and fukutin. By contrast, deletion of the Large and fukutin substrate dystroglycan does not result in radial sorting defects. Here, we show that absence of dystroglycan in a specific genetic background causes sorting defects with topography identical to that of laminin 211 mutants, and recapitulating the MDC1A, MDC1D and Fukuyama phenotypes. By epistasis studies in mice lacking one or both receptors in SCs, we show that only absence of 1 integrins impairs proliferation and survival, and arrests radial sorting at early stages, that 1 integrins and dystroglycan activate different pathways, and that the absence of both molecules is synergistic. Thus, the function of dystroglycan and 1 integrins is not redundant, but is sequential. These data identify dystroglycan as a functional laminin 211 receptor during axonal sorting and the key substrate relevant to the pathogenesis of glycosyltransferase congenital muscular dystrophies.

Cell–cell interactions promote juxtacrine signals in specific subcellular domains, which are difficult to capture in the complexity of the nervous system. For example, contact between axons and Schwann cells triggers signals required for radial sorting and myelination. Failure in this interaction causes dysmyelination and axonal degeneration. Despite its importance, few molecules at the axo-glial surface are known. To identify novel molecules in axo-glial interactions, we modified the ‘pseudopodia' sub-fractionation system and isolated the projections that glia extend when they receive juxtacrine signals from axons. By proteomics we identified the signalling networks present at the glial-leading edge, and novel proteins, including members of the Prohibitin family. Glial-specific deletion of Prohibitin-2 in mice impairs axo-glial interactions and myelination. We thus validate a novel method to model morphogenesis and juxtacrine signalling, provide insights into the molecular organization of the axo-glial contact, and identify a novel class of molecules in myelination.

During development, Schwann cells extend lamellipodia-like processes to segregate large-and small-caliber axons during the process of radial sorting. Radial sorting is a prerequisite for myelination and is arrested in human neuropathies because of laminin deficiency. Experiments in mice using targeted mutagenesis have confirmed that laminins 211, 411, and receptors containing the ␤1 integrin subunit are required for radial sorting; however, which of the 11 ␣ integrins that can pair with ␤1 forms the functional receptor is unknown. Here we conditionally deleted all the ␣ subunits that form predominant laminin-binding ␤1 integrins in Schwann cells and show that only ␣6␤1 and ␣7␤1 integrins are required and that ␣7␤1 compensates for the absence of ␣6␤1 during development. The absence of either ␣7␤1 or ␣6␤1 integrin impairs the ability of Schwann cells to spread and to bind laminin 211 or 411, potentially explaining the failure to extend cytoplasmic processes around axons to sort them. However, double ␣6/␣7 integrin mutants show only a subset of the abnormalities found in mutants lacking all ␤1 integrins, and a milder phenotype. Double-mutant Schwann cells can properly activate all the major signaling pathways associated with radial sorting and show normal Schwann cell proliferation and survival. Thus, ␣6␤1 and ␣7␤1 are the laminin-binding integrins required for axonal sorting, but other Schwann cell ␤1 integrins, possibly those that do not bind laminins, may also contribute to radial sorting during peripheral nerve development.

Myelin is required for proper nervous system function. Schwann cells in developing nerves depend on extrinsic signals from the axon and from the extracellular matrix to first sort and ensheathe a single axon and then myelinate it. Neuregulin 1 type III (Nrg1III) and laminin α2β1γ1 (Lm211) are the key axonal and matrix signals, respectively, but how their signaling is integrated and if each molecule controls both axonal sorting and myelination is unclear. Here, we use a series of epistasis experiments to show that Lm211 modulates neuregulin signaling to ensure the correct timing and amount of myelination. Lm211 can inhibit Nrg1III by limiting protein kinase A (PKA) activation, which is required to initiate myelination. We provide evidence that excessive PKA activation amplifies promyelinating signals downstream of neuregulin, including direct activation of the neuregulin receptor ErbB2 and its effector Grb2-Associated Binder-1 (Gab1), thereby elevating the expression of the key transcription factors Oct6 and early growth response protein 2 (Egr2). The inhibitory effect of Lm211 is seen only in fibers of small caliber. These data may explain why hereditary neuropathies associated with decreased laminin function are characterized by focally thick and redundant myelin.

Myelination requires extensive plasma membrane rearrangements, implying that molecules controlling membrane dynamics play prominent roles. The large GTPase dynamin 2 (DNM2) is a well-known regulator of membrane remodeling, membrane fission, and vesicular trafficking. Here, we genetically ablated Dnm2 in Schwann cells (SCs) and in oligodendrocytes of mice. Dnm2 deletion in developing SCs resulted in severely impaired axonal sorting and myelination onset. Induced Dnm2 deletion in adult SCs caused a rapidly-developing peripheral neuropathy with abundant demyelination. In both experimental settings, mutant SCs underwent prominent cell death, at least partially due to cytokinesis failure. Strikingly, when Dnm2 was deleted in adult SCs, non-recombined SCs still expressing DNM2 were able to remyelinate fast and efficiently, accompanied by neuropathy remission. These findings reveal a remarkable self-healing capability of peripheral nerves that are affected by SC loss. In the central nervous system, however, we found no major defects upon Dnm2 deletion in oligodendrocytes.

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