Thyroidal induction of tubulin in brain development--identification of the target cell

Exposure of organ cultures of newborn rat brains to tri-iodothyronine (T3) followed by cell fractionation as well as direct exposure of prefractionated neuronal (N) and glial (G) cells to the hormone results in an almost selective induction of tubulin in the glial cells. This is established from two independent assays of tubulin, viz. colchicine binding and vinblastin precipitation. In the newborn rat brain, the tubulin content of the G cells is almost 3-fold higher than that of the N cells. Treatment with T3 elicits 40-50% stimulation of tubulin in the G cells within 2 hr without any significant increase in the N cells. Brains from 8- or 50-day-old rats are irresponsive to induction to tubulin by T3. The rate of incorporation of [3H]leucine into total protein is very similar in both N and G cells of newborn rat brain but that into tubulin of G cells is about 3-fold higher than that of N cells. T3 promotes this incorporation by over 30% in the G cells with only a marginal 5% increase in the N cells. The overall results suggest that the glial cells represent the target cells for the T3-induced synthesis of tubulin, the major structural protein of the developing brain.