Early in development, the human zygote undergoes repeated cell divisions to form a blastocyst. Trophoectoderm cells, which form the blastocyst shell, mediate blastocyst attachment to uterine epithelial cells, triggering trophoblast proliferation and uterine invasion. Sugihara et al., a group that previously identified trophinin as a homophilic cell adhesion molecule expressed by trophoblasts and endometrial epithelium, found that adhesion to trophinin-expressing cells stimulated tyrosine phosphorylation in trophinin-expressing trophoblastic embryonal carcinoma HT-H cells. Peptides containing a glycine-tryptophan-arginine-glutamine motif (GWRQ, which the authors implicated in trophinin binding) stimulated tyrosine phosphorylation in HT-H cells, as well as actin polymerization and cell proliferation and motility. Western analysis indicated that the major protein phosphorylated in response to GWRQ was of a size consistent with that of ErbB-family epidermal growth factor (EGF) receptors, an identification supported by immunoprecipitation analysis. In vitro binding assays revealed that bystin, a protein known to interact with trophinin, also bound ErbB4. GWRQ reduced coimmunoprecipitation of trophinin with ErbB4 from HT-H cells and reduced their colocalization as assessed with immunofluorescence. In the absence of fetal calf serum in the culture medium, GWRQ failed to stimulate tyrosine phosphorylation in HT-H cells. GWRQ-dependent phosphorylation was restored by EGF or heparin-binding EGF-like growth factor (HB-EGF), neither of which stimulated tyrosine phosphorylation in the absence of GWRQ. When trophinin and bystin were knocked down with small interfering RNA, however, HB-EGF stimulated tyrosine phosphorylation whether or not GWRQ was present. Thus, the authors propose that trophinin inhibits ErbB4, an inhibition that is released by trophinin-mediated cell adhesion (mimicked by GWRQ binding), enabling ErbB4 activation and stimulating trophoblast proliferation and motility.
K. Sugihara, D. Sugiyama, J. Byrne, D. P. Wolf, K. P. Lowitz, Y. Kobayashi, M. Kabir-Salmani, D. Nadano, D. Aoki, S. Nozawa, J. Nakayama, T. Mustelin, E. Ruoslahti, N. Yamaguchi, M. N. Fukuda, Trophoblast cell activation by trophinin ligation is implicated in human embryo implantation. Proc. Natl. Acad. Sci. U.S.A. 104, 3799-3804 (2007). [Abstract] [Full Text]