Actin plays a role in both changes in cell shape and gene-expression associated with Schwann cell myelination

Schwann cell (SC) differentiation into a myelinating cell requires concurrent interactions with basal lamina and an axon destined for myelination. As SCs differentiate, they undergo progressive morphological changes and initiate myelin-specific gene expression. We find that disrupting actin polymerization with cytochalasin D (CD) inhibits myelination of SC/neuron cocultures. Basal lamina is present, neurons are healthy, and the inhibition is reversible. Electron microscopic analysis reveals that actin plays a role at two stages of SC differentiation. At 0.75-1.0 μg/ml CD, SCs do not differentiate and appear as 'rounded' cells in contact with axons. This morphology is consistent with disruption of actin filaments and cell shape changes. However, at 0.25 μg/ml CD, SCs partially differentiate; they elongate and segregate axons but generally fail to form one-to-one relationships and spiral around the axon. In situ hybridizations reveal that SCs in CD-treated cultures do not express mRNAs encoding the myelin-specific proteins 2',3'-cyclic nucleotide phosphodiesterase (CNP), myelin-associated glycoprotein (MAG), and PO. Our results suggest that at the lower CD dose, SCs commence differentiation as evidenced by changes in cell shape but are unable to elaborate myelin lamellae because of a lack of myelin-specific mRNAs. We propose that F-actin influences myelin-specific gene expression in SCs.