Po mRNA expression in cultures of Schwann cells and neurons that lack basal lamina and myelin

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Abstract
Schwann cells of the peripheral nervous system depend on the presence of both axons and basal lamina to achieve a myelinating phenotype. Furthermore, removal of axonal influence results in the cessation of myelination and down‐regulation of myelin protein expression by Schwann cells. Here we examine whether both axons and basal lamina are required by Schwann cells for the expression of mRNA encoding the major myelin glycoprotein, Po. Cultures of Schwann cells and neurons obtained from dorsal root ganglia of 15 day embryonic rat pups were grown for up to 20 days in vitro under conditions that either allowed or prohibited basal lamina and myelin formation. These cultures were assayed for the expression of Po. mRNA by using an S1 nuclease‐protection assay. After 20 days in vitro, the cultures that did not assemble basal lamina and that were incapable of myelin formation expressed Po. mRNA at a level which was comparable to that seen in identically aged, myelinating cultures. Both the myelinating and nonmyelinating cultures demonstrated an appreciable increase in Po mRNA when compared to the starting embryonic dorsal root ganglia Schwann cells. The latter had a low, but detectable, level of mRNA for this myelin glycoprotein. The cultures that were devoid of basal lamina and myelin showed a clear increase in Po mRNA by 11–15 days in culture. This increase in expression depended on the presence of neurons/neurites, since Schwann cells which were grown in neuron‐depleted cultures expressed little, if any, Po mRNA. In contrast to the levels of Po. mRNA, the nonmyelinating cultures had a significantly lower amount of Po. glycoprotein than did the cultures which assemble myelin. This suggests that the nonmyelinating Schwann cells regulate the level of this glycoprotein at the translational and/or the posttranslational level. The data presented here suggest that myelin protein mRNA expression and myelin assembly by Schwann cells are separable events, with the former depending on one or more neuronal/axonal factors.