Analysis on the function of a protein encoded by a thyroid hormone responsive gene, ZAKI-4, in order to elucidate the molecular mechanizm of thyroid hormone action on brain development

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It is well known that thyroid hormone (T_3) plays a pivotal role in brain development, however, its molecular mechanism remains to be elucidated. We previously identified a thyroid hormone-responsive gene, ZAKI-4. ZAKI-4 encodes a protein (ZAKI-4 protein) with a molecular mass of 26kd. Recently, it has been reported that ZAKI-4 belongs to a family protein that inhibits calcineurin activity in vivo. Sicne calcineurin is known to play important roles in brain development and function, we hypothesized, that ZAKI-4 protein is one of key factors to mediate T_3 action during brain development and designed the present research projects to elucidate the in vivo function of ZAKI-4 protein. For this purpose we tried to produce mice in which the expression of ZAKI-4 is completely eliminated. As the first step to knockout ZAKI-4 gene, we determined the genomic organization of mouse ZAKI-4 gene and a targeting vector was constructed. We are currently carrying out homologous recombination in embryon … More ic stem (ES) cells derived from a mouse train (129/Sv). In the present study, the spatial and ontogenic expression of ZAKI-4 mRNA and the effect of hypothyroidism on the expression were also examined in brain of rodents. In cerebral cortex, the expression of ZAKI-4 mRNA was observed as early as embryonic day 18 and the expression increased during the brain development. This increase was significantly blunted tn hypothyroid rats. On the other hand the expression was constant and was not affected by hypothyroidism in cerebellum. In situ hybridization revealed that ZAKI-4 mRNA was widely expressed in the entire brain. It was noted that ZAKI-4 was highly expressed in the area such as olfactory bulb, hipocampus, cerebral cortex and cerebellum, where calcineurin is abundt. These results demonstrated that the expression of ZAKI-4 mRNA was indeed regulated by thyroid status in vivo and would suggest that thyroid hormone excerts its biological function during brain development at least in part by regulating ZAKI-4 expression and thus by controling calcineruin activity. Less