FGF-MEDIATED MESODERM INDUCTION INVOLVES THE SRC-FAMILY KINASE LALOO

During embryogenesis, inductive interactions underlie the development of much of the body plan. In Xenopus laevis factors secreted from the vegetal pole induce mesoderm in the adjacent marginal zone; members of both the transforming growth factor-beta (TGF-beta) and fibroblast gr owth factor (FGF) ligand families seem to have critical roles in this process(1). Here we report the identification and characterization of laloo a novel participant in the signal transduction cascade linking e xtracellular, mesoderm-inducing signals to the nucleus, where alterati on of cell fate is driven by changes in gene expression. Overexpressio n of laloo, a member of the Src-related gene family, in Xenopus embryo s gives rise to ectopic posterior structures that frequently contain a xial tissue. Laloo induces mesoderm in Xenopus ectodermal explants; th is induction is blocked by reagents that disrupt the FGF signalling pa thway. Conversely, expression of a dominant-inhibitory Laloo mutant bl ocks mesoderm induction by FGF and causes severe posterior truncations in vivo. This work provides the first evidence that a Src-related kin ase is involved in vertebrate mesoderm induction.