EXPRESSION OF THE PROTEIN ZERO MYELIN GENE IN AXON-RELATED SCHWANN-CELLS IS LINKED TO BASAL LAMINA FORMATION

A Schwann cell has the potential to differentiate into either a myelin ating or ensheathing cell depending upon signals received from the axo n that it contacts. Studies focusing on the pathway leading to myelina tion demonstrated that Schwann cells must form a basal lamina in order to myelinate an axon. In this report, we describe studies that indica te that initiation of basal lamina synthesis is required for Schwann c ells to distinguish between myelination-inducing axons and axons that do not induce myelination, and to respond by undergoing the appropriat e genetic and cellular changes. We have used high resolution in situ h ybridization, immunocytochemistry and electron microscopy to examine c hanges in gene expression and morphology of Schwann cells differentiat ing into myelin-forming cells in vitro. These experiments were carried out in dorsal root ganglion neuron/Schwann cell co-cultures maintaine d in either serum-free, serum-only or serum-plus-ascorbate-containing medium. We have made four novel observations that contribute significa ntly to our understanding of how basal lamina and myelination are link ed. (1) The addition of ascorbate (in the presence of serum), which pr omotes basal lamina production, appears to induce expression of the pr otein zero gene encoding the major structural protein of myelin. Moreo ver, expression of protein zero mRNA and protein, and its insertion in to myelin membranes, occurs only in the subset of Schwann cells contac ting myelination-inducing axons. Schwann cells in contact with axons t hat do not induce myelination, or Schwann cells that have not establis hed a unitary relationship with an axon, do not express protein zero m RNA although they produce basal lamina components. (2) In serum-free c onditions, a majority of Schwann cells express protein zero mRNA and p rotein, but this change in gene expression is not associated with basa l lamina formation or with elongation of the Schwann cell along the ax on and elaboration of myelin. (3) In the presence of serum (and the ab sence of ascorbate), Schwann cells again fail to form basal lamina or elongate but no longer express protein zero mRNA or protein. (4) Myeli n-associated glycoprotein and galactocerebroside, two additional myeli n-specific components, can be expressed by Schwann cells under any of the three culture conditions. Therefore, we have demonstrated that axo nal induction of protein zero gene expression in Schwann cells is subj ect to regulation by both serum- and ascorbate-dependent pathways and that not all myelin-specific proteins are regulated in the same manner . Only when Schwann cells contact axons and initiate basal lamina synt hesis is expression of myelin-specific genes restricted to the subset of Schwann cells contacting myelination-inducing axons and coupled to cellular differentiation. In the absence of basal lamina formation, Sc hwann cells in contact with axons seem to express myelin-specific prot eins spuriously without undergoing further differentiation, In sum, th ese findings suggest that basal lamina serves to simultaneously induce myelin gene expression and cell shape changes in those Schwann cells associated with axons destined for myelination. Basal lamina also supp resses the expression of myelin genes in these SCs in contact with axo ns that do not induce myelination.