Differentiation of axon-related Schwann cells in vitro: II. Control of myelin formation by basal lamina

Abstract

Several recent observations suggest that Schwann cell (SC) differentiation, including myelin formation, is dependent upon the development of basal lamina which characteristically surrounds each axon-SC unit in peripheral nerve. This dependence can be tested in a neuron-SC culture system developed in our laboratory in which SC differentiation, including basal lamina formation and myelination, is faithfully reproduced. The use of serum-free, defined medium (DM) with this culture system allows axon-driven SC proliferation but not basal lamina formation or myelination. We previously demonstrated that ascorbic acid, in the presence of a nondialyzable serum factor(s), stimulates basal lamina assembly and myelin formation with similar dose-response relationships (Eldridge et al., 1987). We hypothesized that ascorbic acid acts to promote SC myelination indirectly, by enabling the assembly of basal lamina. We now provide support for this hypothesis by demonstrating the following. (1) Pepsin-resistant triple-helical collagen molecules were produced only by SCs grown in the presence of ascorbic acid, suggesting that triple-helical type IV collagen may mediate the effect of ascorbic acid on basal lamina formation. (2) The formation of myelin by oligodendrocytes, which myelinate axons in the CNS without the concomitant deposition of basal lamina, was little affected by ascorbic acid, suggesting that the biosynthesis and assembly of myelin per se does not require ascorbic acid. (3) The provision of exogenous basal lamina matrix to SCs grown with neurons in DM without ascorbic acid promoted control levels of myelination (and basal lamina formation); the provision of exogenous fibrillar collagen matrix did not. (4) Purified laminin promoted control levels of myelination in the absence of ascorbic acid, but purified type IV collagen and heparan sulfate proteoglycan (HSPG) did not. Laminin caused SCs to assemble basal lamina-like structures that contained not only laminin but also HSPG and non-triple-helical type IV collagen. Thus, several types of experiments demonstrate that SC myelin formation can be controlled by regulating the ability of the SC to assemble basal lamina, illustrating that acquisition of basal lamina is a crucial prefatory step for further SC differentiation.

α6β1 and α7β1 Integrins Are Required in Schwann Cells to Sort Axons

“…One possibility is that a high density of laminin and integrin molecules is required for basal lamina assembly and function. This hypothesis is consistent with data showing that PNS myelination in vivo is particularly sensitive to laminin dosage (McKee et al, 2012) and in vitro high concentrations of laminins are necessary to polymerize and bind integrins required for myelination, whereas lower concentrations induce Schwann cell proliferation (Eldridge et al, 1989;McKee et al, 2012). Still, the fact that different laminins and integrin receptors are used suggests that some specificity exists in their function during radial sorting.…”

Glial Cell Line-Derived Neurotrophic Factor Alters Axon Schwann Cell Units and Promotes Myelination in Unmyelinated Nerve Fibers

“…Purified neuronal and Schwann cell cultures were prepared as described by Eldridge et al (1989). The purified DRG neuronal cultures were established from Sprague Dawley rats at embryonic day 14.…”

Peripheral Myelin Protein 22 Is in Complex with α6β4 Integrin, and Its Absence Alters the Schwann Cell Basal Lamina

“…This linkage with the extracellular matrix (ECM) stabilizes axon-SC contacts and permits the initiation of myelination (Eldridge et al, 1987(Eldridge et al, , 1989. Integrins, specifically ␣6␤1 and ␣6␤4, play key roles in mediating the interaction between SCs and basal lamina constituents, such as laminin (Previtali et al, 2001).…”