The activation of MAP kinase during vertical palatal shelf development in hamster

Mitogen-activated protein (MAP) kinase has been implicated in signal transduction pathways that regulate cell cycle progression during the proliferation of eukaryotic cells. Previous studies have shown that a rapid burst of cell proliferation is a major event of the development of mammalian palatal shelves in a vertical direction. The present study analyzed the involvement of MAP kinase during the vertical development of the secondary palate in hamster. Palates were dissected at various times between days 10:00 and 12:00 of gestation, homogenized, centrifuged and fractionated on a Mono Q column by fast protein liquid chromatography. The fractions were assayed for phosphotransferase activity toward myelin basic protein, and also toward a synthetic peptide APRTPGGRR (S5), which was more specifically utilized by MAP kinase. The data showed that MAP kinase activity increased during the initial phase, i.e., between days 10:00 and 11:12, and then decreased during the latter half of vertical palate development, i.e., between days 11:12, and 12:00 of gestation. Western blotting studies, using antibodies raised against the subdomain I ATP binding sequence (GEGA), subdomain III (ERK1-III), and the C-terminus (ERK1-CT) of MAP kinases, demonstrated the presence of both the 42-kDa and 44-kDa MAP kinase isoforms between days 10:12 and 12:12 of gestation. A monoclonal antibody (4G10), which detects phosphotyrosine, demonstrated phosphorylation of both the 42-kDa and 44-kDa isoforms. The amount of protein remained constant during vertical palatal shelf development indicating that the differential activity of MAP kinase was most likely due to post-translational modification (i.e., phosphorylation). There was a good correlation between the temporal expression of MAP kinase activity and the rates of cell proliferation in the developing vertical palate suggesting a possible involvement of MAP kinase in regulation of cell proliferation during secondary palate development.