THYROIDAL STIMULATION OF TUBULIN AND ACTIN IN RAT-BRAIN CYTOSKELETON

In cultures of neonatal rat brain cells, labeled with S-35-methionine in the presence or absence of triiodothyronine (T-3), the hormone prom oted a significant enhancement of labeled tubulin and actin in the ins oluble fraction (30,000 g pellet) of cell homogenate. To identify the specific sub-cellular fraction associated with this induction, organ c ultures of 1 day rat cerebra were labelled with S-35-methionine in the presence and absence of T-3 and the insoluble fraction (30,000 g pell et) was subfractionated into mitochondria, plasma membrane and cytoske leton. Analysis of the labeled proteins by SDS-PAGE, autoradiography a nd densitometry revealed a T-3-induced increase of 50-80% for both tub ulin and actin, only in the cytoskeleton fraction without any signific ant effect on the other fractions. Similar results were obtained when plasma membrane or cytoskeleton were isolated directly from labeled ce rebrum by conventional methods instead of fractionating from the 30,00 0 g pellet. Analysis of relative stimulation of labeled tubulin and ac tin by T-3 in cytoskeleton fraction derived from primary cultures of n euronal (N) and glial (G) cells labeled with (35)-methionine show that the stimulatory effect is predominantly on the N cells. Studies on th e kinetics of induction of labeled tubulin and actin by T-3 in the cyt oskeleton fraction prepared from cerebra labeled with S-35-methionine for 2, 8 and 18 hrs revealed no significant difference at 2 hrs; at 8 hrs, an increased incorporation into both tubulin and actin was reprod ucibly seen in the controls relative to T-3-treated samples. However b y 18 hrs, this pattern reversed and an enhanced accumulation of both l abeled tubulin and actin was observed under the influence of T-3. The mechanism of this apparently intriguing effect of T-3 On the kinetics of association of tubulin and actin with the cytoskeleton has been dis cussed in the light of the dual effect of the hormone on tubulin viz. enhancing its stability as well as rate of synthesis. The overall resu lts indicate that the thyroid hormones play a major role in the cytosk eletal transport of tubulin and actin from their site of synthesis to that of assembly thus facilitating axodendritic outgrowth and morpholo gical differentiation.