Creating a Signaling Gradient

Abstract

During development of the vertebrate embryo, anterior tissues develop and differentiate earlier than do posterior tissues, as the rapidly proliferating cells of the tail bud, the caudal-most region of the embryo, move progressively rearward. The undifferentiated state of the cells in the posterior-most tissues is believed to depend on high activity of fibroblast growth factor (FGF) signaling in this region. Dubrulle and Pourquié studied the relation of a gradient in fgf8 transcripts (which they recently described in chick embryos) to the spatial activation of FGF signaling, as well as investigated the basis for this gradient. Using in situ hybridization and real-time reverse transcription polymerase chain reaction to measure fgf8 RNA and immunohistochemistry and Western analysis to measure FGF8 protein, the authors found a gradient in fgf8 expression in the posterior parts of mouse embryos and a shallower gradient of FGF8 protein across the same region. Further, they found a gradient of phosphorylation of Akt, a kinase activated by FGF through the phosphoinositide 3-kinase pathway. Explants of the posterior half of chick embryos showed similar rates of fgf8 gradient regression regardless of the presence of the tail bud, which suggested that a gradient in fgf8 transcription did not depend on a signal from the tail bud. Rather, active transcription of fgf8 (determined using probes directed against fgf8 introns to detect nascent transcripts) was restricted to cells of the tail bud. Pharmacological inhibition of transcription indicated that the gradient depended on progressive degradation of fgf8 transcripts as cells became more remote from the tail bud during embryo elongation. J. Dubrulle, O. Pourquié, Fgf8 mRNA decay establishes a gradient that couples axial elongation to patterning in the vertebrate embryo. Nature 427 , 419-422 (2004). [Online Journal]