Microtubules and brain development: The effects of thyroid hormones

The data accumulated during the past twenty years suggest that thyroid hormones have a direct effect on the differentiation of both the neurons and the glial cell during the critical period of brain development. A fast survey of the available data (which is presented in the introduction of this article) on the mechanism of action of thyroid hormones and on their different effects during brain development suggests that the most dramatic effect of hypothyroidism is a hypoplastic neuropile. Both in vivo, during the critical period of nerve cell differentiation and in vitro, when added to primary cultures of embryonic nerve cells thyroid hormones stimulate neurite outgrowth. Since neurite outgrowth requires massive microtubule assembly the assumption was made that thyroid hormones stimulate nerve cell differentiation by changing the concentration and/or activity of the different proteins (tubulin and "microtubule associated proteins", MAPs) which co-polymerize to form microtubules. Preliminary information was obtained by following the kinetics of microtubule assembly in crude brain supernatants. The data showed that: (1) the rate of in vitro microtubule assembly increases with age during brain development; (2) hypothyroidism, when produced in the rat at late pregnancy, slows this evolution; (3) early replacement therapy with thyroid hormones restores normal rates of assembly; (4) the addition of purified MAPs to normal young or 15-day-old hypothyroid brain preparations restores normal rates of polymerization. These and other data suggested that thyroid hormones regulate microtubule assembly by changing the concentration and/or activity of one or more of the MAPs. Further analysis revealed that striking qualitative changes in MAPs composition occur during brain development. For instance, the TAU fraction, a group of 4-5 proteins with a molecular weight of ? 60-68 K which is present in adult brain, is absent at early stages of postnatal development: two other entities are present, TAU slow and TAU fast, with different molecular weights, lower activity and different peptide mapping. This latter observation suggests that different TAU genes are expressed during brain development; a conclusion which has been confirmed by cell-free translation of the mRNas coding for these proteins. Analysis of the TAU fraction prepared from hypothyroid rat brains also revealed that a group of TAU proteins. "TAU(3)", is almost missing, whereas thyroid hormone administration markedly increases its concentration. Two-dimensional gel electrophoresis showed that the TAU fraction is composed with more than 15 entities, with at least five of them being under thyroid hormone control. The precise physiological significance of the heterogeneity of MAPs and of the changes in MAPs composition seen during development and in hypothyroid rat brain remains to be determined. The assumption is made that these changes might be of utmost importance to regulate the number and length of the microtubules, and therefore the number and length of the neurites which are formed during the differentiation process of the different neurons. Thyroid hormones would be in these respects one of the epigenic factors required to synchronize sequentially the expression of the genes coding for these proteins in the different nerve cells.