Regulation of protein phosphorylation by triiodothyronine (T3) in neural cell cultures. Part II: Neurons.

Cerebral hemisphere from 16- to 18-day-old rat fetuses were dissociated and cells were cultured in absence of thyroid hormones. Neuron-enriched cultures were obtained either by using cells after 6 days of culture (before extensive glial cell proliferation) or by adding cytosine arabinoside for 48 h after 4 days of culture and using cells on day 9. Cells were incubated with T3 (10(-8) M) for 0-72 h and [32P]phosphate was added for the last 4 h of incubation. HMG (high mobility group; 0.75 M perchloric acid-soluble proteins) were prepared and phosphorylated proteins were analyzed by polyacrylamide gel electrophoresis. T3 rapidly (4-7 h) increased the phosphorylation of histone H1 and of a protein with apparent molecular mass of 17000 Da identified as HMG 14. In addition, in cells not treated with cytosine arabinoside, histone H1 was resolved into 3 subfractions and each of these responded to the hormone with a different time course. These results indicate that thyroid hormones act on the phosphorylation of specific nuclear proteins and therefore may influence chromatin structure and gene expression in primary neuronal cell cultures.