Thyroid hormone-induced maturation of astrocytes is associated with the expression of new variants of vimentin and their phosphorylation.

The role of thyroid hormone (TH)-induced vimentin expression in promoting the maturation of astrocytes has been investigated. Comparative immunocytochemical staining with vimentin antibody demonstrated distinctly different patterns of expression of vimentin during the progressive maturation of astrocytes of normal and TH-deficient astrocyte cultures. [35S]Methionine labeling of cells showed a significant decline in the level of vimentin in the hypothyroid cultures at all ages from day 5 to 20. Western blot analysis suggested that the maturation of normal astrocytes was associated with the appearance of several acidic and basic variants of vimentin, whose expression was significantly delayed or reduced in the TH-deficient astrocytes. In normal astrocyte cultures, two acidic variants of vimentin, one of similar molecular weight and the other of lower molecular weight, were found to be phosphorylated during the final transformation of the epithelioid form into the mature stellate form. None of these phosphorylated forms was evident in the TH-deficient astrocytes. This was further confirmed by negative immunocytochemical staining of 5- to 18-day-old cultures of TH-deficient astrocytes with antibody TM71, specific to phosphorylated vimentin, compared with age-matched normal astrocytes, which displayed consistent positive staining. The overall results indicate that TH-induced expression of phosphorylated forms of vimentin may be essential for the intracellular organization of the cytoskeleton in a way conducive to the morphological maturation of astrocytes.