A Schwann cell has the potential to differentiate into either a myelinating or ensheathing cell depending upon signals received from the axon that it contacts. Studies focusing on the pathway leading to myelination demonstrated that Schwann cells must form a basal lamina in order to myelinate an axon. In this report, we describe studies that indicate that initiation of basal lamina synthesis is required for Schwann cells to distinguish between myelination-inducing axons and axons that do not induce myelination, and to respond by undergoing the appropriate genetic and cellular changes. We have used high resolution in situ hybridization, immunocytochemistry and electron microscopy to examine changes in gene expression and morphology of Schwann cells differentiating into myelin-forming cells in vitro. These experiments were carried out in dorsal root ganglion neuron/Schwann cell co-cultures maintained in either serum-free, serum-only or serum-plus-ascorbate-containing medium. We have made four novel observations that contribute significantly to our understanding of how basal lamina and myelination are linked. (1) The addition of ascorbate (in the presence of serum), which promotes basal lamina production, appears to induce expression of the protein zero gene encoding the major structural protein of myelin. Moreover, expression of protein zero mRNA and protein, and its insertion into myelin membranes, occurs only in the subset of Schwann cells contacting myelination-inducing axons. Schwann cells in contact with axons that do not induce myelination, or Schwann cells that have not established a unitary relationship with an axon, do not express protein zero mRNA although they produce basal lamina components. (2) In serum-free conditions, a majority of Schwann cells express protein zero mRNA and protein, but this change in gene expression is not associated with basal lamina formation or with elongation of the Schwann cell along the axon and elaboration of myelin. (3) In the presence of serum (and the absence of ascorbate), Schwann cells again fail to form basal lamina or elongate but no longer express protein zero mRNA or protein. (4) Myelin-associated glycoprotein and galactocerebroside, two additional myelin-specific components, can be expressed by Schwann cells under any of the three culture conditions. Therefore, we have demonstrated that axonal induction of protein zero gene expression in Schwann cells is subject to regulation by both serum- and ascorbate-dependent pathways and that not all myelin-specific proteins are regulated in the same manner
