Intracellular calcium signal at the leading edge regulates mesodermal sheet migration during Xenopus gastrulation

During the gastrulation stage in animal embryogenesis, the cells leading the axial mesoderm migrate toward the anterior side of the embryo, vigorously extending cell protrusions such as lamellipodia. It is thought that the leading cells sense gradients of chemoattractants emanating from the ectodermal cells and translate them to initiate and maintain the cell movements necessary for gastrulation. However, it is unclear how the extracellular information is converted to the intracellular chemical reactions that lead to motion. Here we demonstrated that intracellular Ca²⁺ levels in the protrusion-forming leading cells are markedly higher than those of the following cells and the axial mesoderm cells. We also showed that inhibiting the intracellular Ca²⁺ significantly retarded the gastrulation cell movements, while increasing the intracellular Ca²⁺ with an ionophore enhanced the migration. We further found that the ionophore treatment increased the active form of the small GTPase Rac1 in these cells. Our results suggest that transient intracellular Ca²⁺ signals play an essential role in the active cell migration during gastrulation.