Expression of calcitonin receptors in mouse preimplantation embryos and their function in the regulation of blastocyst differentiation by calcitonin

Calcitonin secretion in the pregnant uterus is tightly regulated by the ova rian hormones, estrogen and progesterone, which limit its expression to a b rief period preceding blastocyst implantation. The binding of calcitonin to a G protein-coupled receptor activates adenylate cyclase and elevates cyto solic Ca2+ levels. The acceleration of preimplantation embryonic developmen t that is known to occur upon elevation of intracellular Ca2+ prompted an i nvestigation into calcitonin regulation of blastocyst differentiation. Usin g reverse transcription and the polymerase chain reaction to estimate the r elative abundance of calcitonin receptor mRNA, a 25-fold accumulation of th e splice variant, CR-1a, was observed in embryos between the 1-cell and 8-c ell stages. Cytosolic free Ca2+ levels were rapidly elevated in embryos at the 4-cell to blastocyst stages after exposure to 10 nM calcitonin, Blastoc ysts treated for 30 minutes with 10 nM calcitonin differentiated in vitro a t an accelerated rate, as assessed by the translocation of alpha(5)beta(1) integrin to the apical surface of trophoblast cells, the corresponding elev ation of fibronectin-binding activity and the timing of trophoblast cell mi gration, Chelation of cytosolic free Ca2+ with BAPTA-AM, but not inhibition of protein kinase A activity by H-89, attenuated the effects of calcitonin on blastocyst development, These findings support the concept that calcito nin secretion within the progesterone-primed uterus and the coordinate expr ession of CR-1a by preimplantation embryos regulates blastocyst differentia tion through receptor-mediated Ca2+ signaling.