NGF controls axonal receptivity to myelination by Schwann cells or oligodendrocytes

Axons dictate whether or not they will become myelinated in both the central and peripheral nervous systems by providing signals that direct the development of myelinating glia. Here we identify the neurotrophin nerve growth factor (NGF) as a potent regulator of the axonal signals that control myelination of TrkA-expressing dorsal root ganglion neurons (DRGs). Unexpectedly, these NGF-regulated axonal signals have opposite effects on peripheral and central myelination, promoting myelination by Schwann cells but reducing myelination by oligodendrocytes. These findings indicate a novel role for growth factors in regulating the receptivity of axons to myelination and reveal that different axonal signals control central and peripheral myelination.     

Epineurial adipocytes are dispensable for Schwann cell myelination

Previous clinical observations and data from mouse models with defects in lipid metabolism suggested that epineurial adipocytes may play a role in peripheral nervous system myelination. We have used adipocyte‐specific Lpin1 knockout mice to characterize the consequences of the presence of impaired epineurial adipocytes on the myelinating peripheral nerve. Our data revealed that the capacity of Schwann cells to establish myelin, and the functional properties of peripheral nerves, were not affected by compromised epineurial adipocytes in adipocyte‐specific Lpin1 knockout mice. To evaluate the possibility that Lpin1‐negative adipocytes are still able to support endoneurial Schwann cells, we also characterized sciatic nerves from mice carrying epiblast‐specific deletion of peroxisome proliferator‐activated receptor gamma, which develop general lipoatrophy. Interestingly, even the complete loss of adipocytes in the epineurium of peroxisome proliferator‐activated receptor gamma knockout mice did not lead to detectable defects in Schwann cell myelination. However, probably as a consequence of their hyperglycemia, these mice have reduced nerve conduction velocity, thus mimicking the phenotype observed under diabetic condition. Together, our data indicate that while adipocytes, as regulators of lipid and glucose homeostasis, play a role in nerve function, their presence in epineurium is not essential for establishment or maintenance of proper myelin.     

Nrg1/ErbB signaling networks in Schwann cell development and myelination

Neuregulin-1 (Nrg1) provides a key axonal signal that regulates Schwann cell proliferation, migration and myelination through binding to ErbB2/3 receptors. The analysis of a number of genetic models has unmasked fundamental mechanisms underlying the specificity of the Nrg1/ErbB signaling axis. Differential expression of Nrg1 isoforms, Nrg1 processing, and ErbB receptor localization and trafficking represent important regulatory themes in the control of Nrg1/ErbB function. Nrg1 binding to ErbB2/3 receptors results in the activation of intracellular signal transduction pathways that initiate changes in Schwann cell behavior. Here, we review data that has defined the role of key Nrg1/ErbB signaling components like Shp2, ERK1/2, FAK, Rac1/Cdc42 and calcineurin in development of the Schwann cell lineage in vivo. Many of these regulators receive converging signals from other cues that are provided by Notch, integrin or G-protein coupled receptors. Signaling by multiple extracellular factors may act as key modifiers and allow Schwann cells at different developmental stages to respond in distinct manners to the Nrg1/ErbB signal.     

Schwann cell myelination: induction by exogenous basement membrane-like extracellular matrix

Exposing rat Schwann cells co-cultured with nerve cells to a reconstituted basement membrane induced the formation of myelin segments by Schwann cells. This occurred in a serum-free culture medium in which, in the absence of this matrix, Schwann cells proliferate but fail to differentiate. This reconstituted basement membrane was prepared from solubilized extracellular matrix proteins synthesized by a basement membrane-producing murine tumor. The major constituents of this reconstituted matrix are collagen type IV, laminin, heparan sulfate proteoglycan, entactin, and nidogen. The matrix also elicited striking morphological changes in Schwann cells, inducing them to spread longitudinally along the nerve fibers (a necessary early step in the process of ensheathment of nerve fibers). Several observations indicated that the effect of the matrix was exerted directly on Schwann cells and not indirectly through an effect on nerve cells. First, the matrix-induced cell spreading occurred only in areas in which Schwann cells directly contacted the matrix; Schwann cells that were associated with the same nerve fibers but that did not themselves directly contact the matrix did not exhibit spreading. Second, the matrix-induced alteration in Schwann cell morphology was observed in cultures in which the nerve cells were removed. These results provide direct evidence that basement membrane contact induces normal Schwann cell differentiation, and support the idea that Schwann cell differentiation in vivo may be regulated by the appearance of the basement membrane, which normally envelops terminally differentiating Schwann cells.     

Movements of the Schwann cell nucleus implicate progression of the inner (axon-related) Schwann cell process during myelination

Although it has been known for several decades that peripheral myelin is formed from an extended, spiraled, and compacted sheet of Schwann cell (SC) plasma membrane, the mechanism by which this unique spiraling is accomplished remains unknown. We have studied the movements of SC nuclei before, during, and subsequent to myelin formation (over periods of 24-72 h) to determine if this nuclear motion (noted in earlier reports) would provide useful insights into the mechanism of myelinogenesis. We used rodent sensory neuron and SC cultures in which initiation of myelinogenesis is relatively synchronized and bright field conditions that allowed resolution of the axon, compact myelin, and position of the SC nucleus. Observed areas were subsequently examined by electron microscopy (EM); eight myelinating SCs with known nuclear movement history were subjected to detailed EM analysis. We observed that, prefatory to myelination, SCs extended along the length of larger axons, apparently competing with adjacent SCs for axonal surface contact. This lengthening preceded the deposition of compact myelin. SC nuclear circumnavigation of the axon was found to attend early myelin sheath formation. This movement was rarely greater than 0.25 turns per 3 h; on the average, more nuclear motion was seen in relation to internodes that formed during observation (0.8 +/- 0.1 turns/24 h) than in relation to those that had begun to form before observation (0.3 +/- 0.1 turns/24 h). Nuclear circumnavigation generally proceeded in one direction, could be in similar or opposite direction in neighboring myelinating SCs on the same axon, and was not proportional to the number of major dense lines within the myelin sheath. A critical finding was that, in all eight cases examined, the overall direction of nuclear movement was the same as that of the inner end of the spiraling SC process, and thus opposite the direction of the outer end of the spiral. We conclude that the correspondence of the direction of nuclear rotation and inner end of the spiraling cytoplasmic lip implicates active progression of the inner lip over the axonal surface to form the membranous spiral of myelin, the nuclear motion resulting from towing by the advancing adaxonal lip. This interpretation fits with finding basal lamina and macular adhering junctions associated with the external lip of SC cytoplasm; these attributes would imply anchorage rather than movement of this region of the SC.     

Calcitonin gene-related peptide promotes Schwann cell proliferation

Schwann cells in culture divide in response to defined mitogens such as PDGF and glial growth factor (GGF), but proliferation is greatly enhanced if agents such as forskolin, which increases Schwann cell intracellular cAMP, are added at the same time as PDGF or GGF (Davis, J. B., and P. Stroobant. 1990. J. Cell Biol. 110:1353-1360). The effect of forskolin is probably due to an increase in numbers of PDGF receptors (Weinmaster, G., and G. Lemke. 1990. EMBO (Eur. Mol. Biol. Organ.) J. 9:915-920. Neuropeptides and beta-adrenergic agonists have been reported to have no effect on potentiating the mitogenic response of either PDGF or GGF. We show that the neuropeptide calcitonin gene- related peptide (CGRP) increases Schwann cell cAMP levels, but the cells rapidly desensitize. We therefore stimulated the cells in pulsatile fashion to partly overcome the effects of desensitization and show that CGRP can synergize with PDGF to stimulate Schwann cell proliferation, and that CGRP is as effective as forskolin in the pulsatile regime. CGRP is a good substrate for the neutral endopeptidase 24.11. Schwann cells in vivo have this protease on their surface, so the action of CGRP could be terminated by this enzyme and desensitization prevented. We therefore suggest that CGRP may play an important role in stimulating Schwann cell proliferation by regulating the response of mitogenic factors such as PDGF.     

Schwann cell myelination requires timely and precise targeting of P(0) protein

This report investigated mechanisms responsible for failed Schwann cell myelination in mice that overexpress P(0) (P(0)(tg)), the major structural protein of PNS myelin. Quantitative ultrastructural immunocytochemistry established that P(0) protein was mistargeted to abaxonal, periaxonal, and mesaxon membranes in P(0)(tg) Schwann cells with arrested myelination. The extracellular leaflets of P(0)-containing mesaxon membranes were closely apposed with periodicities of compact myelin. The myelin-associated glycoprotein was appropriately sorted in the Golgi apparatus and targeted to periaxonal membranes. In adult mice, occasional Schwann cells myelinated axons possibly with the aid of endocytic removal of mistargeted P(0). These results indicate that P(0) gene multiplication causes P(0) mistargeting to mesaxon membranes, and through obligate P(0) homophilic adhesion, renders these dynamic membranes inert and halts myelination.     

Nectin-like proteins mediate axon Schwann cell interactions along the internode and are essential for myelination

Axon-glial interactions are critical for the induction of myelination and the domain organization of myelinated fibers. Although molecular complexes that mediate these interactions in the nodal region are known, their counterparts along the internode are poorly defined. We report that neurons and Schwann cells express distinct sets of nectin-like (Necl) proteins: axons highly express Necl-1 and -2, whereas Schwann cells express Necl-4 and lower amounts of Necl-2. These proteins are strikingly localized to the internode, where Necl-1 and -2 on the axon are directly apposed by Necl-4 on the Schwann cell; all three proteins are also enriched at Schmidt-Lanterman incisures. Binding experiments demonstrate that the Necl proteins preferentially mediate heterophilic rather than homophilic interactions. In particular, Necl-1 on axons binds specifically to Necl-4 on Schwann cells. Knockdown of Necl-4 by short hairpin RNA inhibits Schwann cell differentiation and subsequent myelination in cocultures. These results demonstrate a key role for Necl-4 in initiating peripheral nervous system myelination and implicate the Necl proteins as mediators of axo-glial interactions along the internode.     

Association of glucocerebroside homolog biosynthesis with Schwann cell proliferation

The biosynthesis of myelin-associated glycolipids was studied in quiescent secondary cultures of Schwann cells and in a rapidly proliferating population of transfected Schwann cells (TSC) by in vitro incorporation of [³H]galactose. The TSC demonstrated a marked increase (>10-fold) in [³H]galactose incorporation when compared to quiescent Schwann cells. The level (or amount) of [³H]galactose incorporation into lipids is dependent upon the number of TSC in culture. The majority of³H-labeled lipids were oligohexosylceramides (GL-2, GL-3, and GL-4). Substrates that inhibit TSC proliferation, collagen type I and Matrigel, an artificial basement membrane, decrease the [³H]galactose incorporation by 25% and 80%, respectively. Our results indicate that the synthesis of glucocerebroside and its homologs is associated with Schwann cell proliferation.Abbreviations HPTLC high-performance thin-layer chromatography - TL total lipids - NL non-polar lipids - GL glycolipids - PL phospholipids - MGDG monogalactosyl diacylglycerol - GalCe galactocerebroside - GalCe-OH galacto hydroxycerebroside - GlcCe glucocerebroside - Su sulfatide - Su-OH hydroxysulfatide - GL-2 lactosylceramide - GL-3 trihexosylceramide - GL-4 tetrahexosylceramide - PE phosphatidylethanolamine - PC phosphatidylcholine - PS phosphatidylserine - PI phosphatidylinositol - TSC transfected Schwann cells A preliminary report of this work was presented at the 22nd Annual Meeting of the American Society for Neurochemistry, Charleston, South Carolina, March 13, 1991.     

Effects of ethanol on rat schwann cell proliferation and myelination in culture

Summary It is possible to treat dissociated embryonic rat dorsal root ganglia in culture to inhibit proliferation of all nonneuronal cells except Schwann cells. Neurons have been shown to produce a mitogenic stimulus for Schwann cells under these conditions. Additionally, myelin-competent neurons induce Schwann cells to elaborate myelin sheaths. Groups of sibling cultures were exposed to various nonlethal concentrations of ethanol (0, 43, 86, or 172 mM) for 4 wk. Culture were assessed weekly by light microscopy in a blind fashion for evidence of Schwann cell proliferation and myelin formation. Ethanol adversely affected both Schwann cell proliferation and myelin formation in culture. No obvious differences in neuronal morphology were observed among the various groups of cultures by light or electron microscopy. These observations suggest that ethanol might interfere with Schwann cell proliferation and myelin formation in culture by one or both of the following means: a) inhibit neuronal production of signals for Schwann cell proliferation and myelination or b) impede Schwann cell responses to neuronal signals. Investigation of these possibilities in culture may provide insight into neuropathologic mechanisms operative in the fetal alcohol syndrome or alcohol-associated peripheral neuropathy in humans. This work was supported by the Department of Veterans Affairs, Washington, D.C.     

Social controls on cell proliferation in plants

Plant cell division occurs mainly in developing tissues and appears to be highly regulated in both space and time. Recently, genetic and molecular analyses have been able to dissect the function of cell proliferation in the processes of growth and development. Mutant studies have shown that plants have a compensatory mechanism whereby increased cell expansion can partially cover for defects in proliferation. Ectopic expression of developmental and cell-cycle regulators has indicated how growth rate is controlled at the molecular level in meristems and lateral organs.     

N-WASp is required for Schwann cell cytoskeletal dynamics, normal myelin gene expression and peripheral nerve myelination

Schwann cells elaborate myelin sheaths around axons by spirally wrapping and compacting their plasma membranes. Although actin remodeling plays a crucial role in this process, the effectors that modulate the Schwann cell cytoskeleton are poorly defined. Here, we show that the actin cytoskeletal regulator, neural Wiskott-Aldrich syndrome protein (N-WASp), is upregulated in myelinating Schwann cells coincident with myelin elaboration. When N-WASp is conditionally deleted in Schwann cells at the onset of myelination, the cells continue to ensheath axons but fail to extend processes circumferentially to elaborate myelin. Myelin-related gene expression is also severely reduced in the N-WASp-deficient cells and in vitro process and lamellipodia formation are disrupted. Although affected mice demonstrate obvious motor deficits these do not appear to progress, the mutant animals achieving normal body weights and living to advanced age. Our observations demonstrate that N-WASp plays an essential role in Schwann cell maturation and myelin formation.