A @entity8496 / XXXX / @entity3963 signaling pathway is required for development of the sea urchin embryo micromere lineage through phosphorylation of the transcription factor Ets.

@entity8047 @entity12136 @entity5732 @entity25820 @entity3963 @entity3546 @entity1889 @entity493 @entity8496
In the sea urchin embryo, the skeleton of the larva is built from a population of mesenchymal cells known as the primary mesenchyme cells (PMCs). These derive from the large micromeres that originate from the vegetal pole at fourth cleavage. At the blastula stage, the 32 cells of this lineage detach from the epithelium and ingress into the blastocoel by a process of epithelial-mesenchymal transition. We report that shortly before ingression, there is a transient and highly localized activation of the MAP-kinase @entity3963 in the micromere lineage. We show that ingression of the PMCs requires the activity of @entity3963 , @entity8047 and @entity8496 , and depends on the maternal Wnt/ @entity3546 pathway. Dissociation experiments and injection of mRNA encoding a dominant-negative form of Ras indicated that this activation is probably cell autonomous. We identified the transcription factors @entity12136 and @entity25820 as putative targets of the phosphorylation by @entity3963 . Both proteins contain a single consensus site for phosphorylation by the MAP kinase @entity3963 . In addition, the @entity12136 protein sequence contains a putative @entity3963 docking site. Overexpression of @entity12136 by injection of synthetic mRNA in the egg caused a dramatic increase in the number of cells becoming mesenchymal at the blastula stage. This effect could be largely inhibited by treating embryos with the @entity8047 inhibitor @entity5732 . Moreover, mutations in the consensus phosphorylation motif substituting @entity1889 107 by an aspartic or an @entity493 residue resulted respectively in a constitutively active form of @entity12136 that could not be inhibited by @entity5732 or in an inactive form of @entity12136 . These results show that the MAP kinase pathway, working through phosphorylation of @entity12136 , is required for full specification of the PMCs and their subsequent transition from epithelial to mesenchymal state.
