Ca2+ influx-linked protein kinase C activity regulates the β-catenin localization, micromere induction signalling and the oral–aboral axis formation in early sea urchin embryos

Sea urchinembryos initiate cell specifications at the 16-cellstage by forming the mesomeres, macromeres and micromeres according to the relative position of the cells in the animal–vegetal axis. The most vegetal cells, micromeres, autonomously differentiate into skeletons and induce the neighbouring macromere cells to become mesoendoderm in the β- catenin-dependent Wnt8 signalling pathway. Although the underlying molecular mechanism for this progression is largely unknown, we have previously reported that the initial events might be triggered by the Ca2+ influxes through the egg-originated L-type Ca2+ channels distributed asymmetrically along the animal–vegetal axis and through the stretch-dependent Ca2+channels expressed specifically in the micromere at the 4th cleavage. In this communication, we have examined whether one of the earliest Ca2+ targets, protein kinase C (PKC), plays a role in cell specification upstream of β- catenin. To this end, we surveyed the expression pattern of β- cateninin early embryos in the presence or absence of the specific peptide inhibitor of Hemicentrotuspulcherrimus PKC (HpPKC-I). Unlike previous knowledge, we have found that the initial nuclear entrance of β- catenindoes not take place in the micromeres, but in the macromeres at the 16-cellstage. Using the HpPKC-I, we have demonstrated further that PKC not only determines cell-specific nucleation of β- catenin, but also regulates a variety of cell specification events in the early sea urchinembryos by modulating the cell adhesion structures, actin dynamics, intracellular Ca2+ signalling, and the expression of key transcription factors.