DEVELOPMENTAL EXPRESSION AND VITAMIN-D REGULATION OF CALBINDIN-D-28K IN CHICK EMBRYONIC YOLK-SAC ENDODERM

The yolk is an important calcium source for the developing chick embry o. The epithelial yolk sac endodermal cells lie in direct contact with the yolk and are the principal nutrient-transporting cell type. We pr eviously reported that vitamin D treatment stimulated yolk calcium mob ilization and that the vitamin D-dependent Ca2+-binding protein, calbi ndin-D-25K, is present in the yolk sac. We report here the development al expression and regulation of calbindin-D-28K in the yolk sac. Calbi ndin-D-28K is expressed as early as incubation d 3 and is found exclus ively within the cytoplasm of endodermal cells. Comparative protein an d mRNA analyses of yolk sac and dissociated yolk sac endodermal cells as a function of development and treatment with calcitriol (1,25-dihyd roxyvitamin D-3) in vitro and in vivo showed a development-specific an d vitamin D-inducible expression of calbindin-D-28K. Northern analysis revealed the expression of vitamin D receptor mRNA in the yolk sac, b eginning as early as d 3, strongly indicating that the extraembryonic yolk sac is an early vitamin D target tissue. Cultured yolk sac endode rmal cells should serve as a useful in vitro cell model for analyzing the cellular and molecular mechanisms of vitamin D action.