Phosphatidylinositol-3 kinase acts in parallel to the ERK MAP kinase in the FGF pathway during Xenopus mesoderm induction

Phosphoinositide 3-kinases (PI3Ks) are lipid kinases that can phosphorylate phosphaditylinositides leading to the cell type-specific regulation of int racellular protein kinases. PI3Ks are involved in a wide variety of cellula r events including mitogenic signalling, regulation of growth and survival, vesicular trafficking, and control of the cytoskeleton. Some of these enzy mes also act downstream of receptor tyrosine kinases or G-protein-coupled r eceptors. Using two strategies to inhibit PI3K signalling in embryos, we have analyse d the role of PI3Ks during early Xenopus development. We find that a class 1A PI3K catalytic activity is required for the definition of trunk mesoderm during the blastula stages, but is less important for endoderm and prechor dal plate mesoderm induction or for organiser formation, It is required in the FGF signalling pathway downstream of Pas and in parallel to the extrace llular signal-regulated kinase (ERK) MAP kinases. In addition, our results show that ERKs and PI3Ks can synergise to convert ectoderm into mesoderm. T hese data provide the first evidence that class 1 PI3Ks are required for a specific set of patterning events in vertebrate embryos. Furthermore, they bring new insight into the FGF signalling cascade in Xenopus.