Ca(2+) 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 urchin embryos initiate cell specifications at the 16-cell stage 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 Ca(2+) influxes through the egg-originated L-type Ca(2+) channels distributed asymmetrically along the animal–vegetal axis and through the stretch-dependent Ca(2+)channels expressed specifically in the micromere at the 4th cleavage. In this communication, we have examined whether one of the earliest Ca(2+) 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-catenin in early embryos in the presence or absence of the specific peptide inhibitor of Hemicentrotus pulcherrimus PKC (HpPKC-I). Unlike previous knowledge, we have found that the initial nuclear entrance of β-catenin does not take place in the micromeres, but in the macromeres at the 16-cell stage. 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 urchin embryos by modulating the cell adhesion structures, actin dynamics, intracellular Ca(2+) signalling, and the expression of key transcription factors.