Acceleration of trophoblast differentiation by heparin-binding EGF-like growth factor is dependent on the stage-specific activation of calcium influxby ErbB receptors in developing mouse blastocysts

Heparin-binding EGF-like growth factor (HB-EGF) is expressed in the mouse e ndometrial epithelium during implantation exclusively at sites apposed to e mbryos and accelerates the development of cultured blastocysts, suggesting that it may regulate peri-implantation development in utero, We have examin ed the influence of HB-EGF on mouse trophoblast differentiation in vitro an d the associated intracellular signaling pathways, HB-EGF both induced intr acellular Ca2+ signaling and accelerated trophoblast development to an adhe sion-competent stage, but only late on gestation day 4 after ErbB4, a recep tor for HB-EGF, translocated from the cytoplasm to the apical surface of tr ophoblast cells. The acceleration of blastocyst differentiation by HB-EGF w as attenuated after inhibition of protein tyrosine kinase activity or remov al of surface heparan sulfate, as expected. Chelation of intracellular Ca2 blocked the ability of HB-EGF to accelerate development, as did inhibitors of protein kinase C or calmodulin, The absence of any effect by a phosphol ipase C inhibitor and the requirement for extracellular Ca2+ suggested that the accrued free cytoplasmic Ca2+ did not originate from inositol phosphat e-sensitive intracellular stores, but through Ca2+ influx. Indeed, N-type C a2+ channel blockers specifically inhibited the ability of HB-EGF to both i nduce Ca2+ signaling and accelerate trophoblast development. We conclude th at HB-EGF accelerates the differentiation of trophoblast cells to an adhesi on-competent stage by inducing Ca2+ influx, which activates calmodulin and protein kinase C. An upstream role for ErbB4 in this pathway is implicated by the timing of its translocation to the trophoblast surface.