The function of signaling gradients in zebrafish somitogenesis

Somitogenesis is the process by which the vertebrate body axis is divided into repeated segments called somites. Somites form in an anterior to posterior wave, with new somites deriving from tissue generated by the growing tail bud. Segment boundary formation is presaged by "waves" of oscillating gene expression that begin in the tail bud and move anteriorly through presomitic mesoderm (PSM) until they encounter a "determination front". The opposing gradients of FGF/Wnt signaling and Retinoic Acid (RA) signaling for positioning the determination front. However, the molecular interactions at the determination front that transition cells from being immature to being competent to form somites is unknown. To better understand these molecular events, I investigated the expression and function of gadd45beta and gadd45betal, two highly related genes expressed in the zebrafish anterior PSM. I find that gadd45beta and gadd45betal are expressed at the determination front and are sensitive to perturbation of Fgf and RA signaling gradients. Fgf negatively regulates, and RA positively regulates, gadd45beta expression. By depleting Gadd45beta and Gadd45betal, I show that they are not necessary for identified a downstream gadd45betal/gadd45betal target, dusp1, which encodes a phosphatase that acts as a negative regulator of Fgf signaling. I showed that dusp1, a gene encoding a phosphatase that negatively regulates Fgf signaling, is a gadd45beta/gadd45betal target that requires Gadd45beta/Gadd45betal for expression and is sensitive to modulation by addition of exogenous RA. My work sets the stage for future investigations addressing a potential role for Gadd45beta and Gadd45betal in creating a permissive environment for initiation of gene transcription as cells respond to the changes in Fgf and RA signaling gradients and prepare to differentiate as muscle cells.