cAMP and bFGF negatively regulate tropomyosin expression in rat cultured astroblasts.

We have examined the expression of tropomyosin (TM) messenger RNAs (mRNAs) and protein isoforms in primary cultures of rat astroblasts during morphological changes. Three messenger RNA bands of 2.5, 1.8 and 1.2 kilobase pairs (kb) were detected by Northern blot. Using an antibody cross-reacting with all tropomyosin isoforms, we found that rat cerebellar neonatal astroblasts expressed three tropomyosin protein isoforms termed TM-As1, TM-As2 and TM-As3 (As for Astroblast) with respective molecular masses of 38,000, 33,000 and 31,000. Treatment of cells with agents which promote or mimick the action of cyclic AMP, or with growth factors, is known to induce astroblast morphological alteration from flat, polygonal epitheloid cells into star-shaped, process-bearing cells. In the presence of dibutyryl cAMP (dBcAMP), forskolin or basic fibroblast growth factor (bFGF), these morphological changes were found to be associated with dramatic decreases of the three mRNA transcripts and also of the three protein isoforms. This decrease was reversed upon removal of the drugs. The pattern of the tropomyosin protein isoforms in cultured astroblasts showed that TM-Asl, the most immunoreactive isoform recovered in the cytoskeletal insoluble cell fraction, had a developmental profile similar to that of F-actin. Therefore this isoform, which belongs to the high-molecular-mass family of proteins known to interact strongly with F-actin, could specifically be involved in the regulation/control of F-actin stability and thus be associated with the plasticity of astroblasts.