Evolutionary Divergence of Platelet-Derived Growth Factor Alpha Receptor Signaling Mechanisms

Abstract

Receptor tyrosine kinases (RTKs) direct diverse cellular and developmental responses by stimulating a relatively small number of overlapping signaling pathways. Specificity may be determined by RTK expression patterns or by differential activation of individual signaling pathways. To address this issue we generated knock-in mice in which the extracellular domain of the mouse platelet-derived growth factor alpha receptor (PDGFαR) is fused to the cytosolic domain of Drosophila Torso (α^Tor) or the mouse fibroblast growth factor receptor 1 (α^FR). α^Tor homozygous embryos exhibit significant rescue of neural crest and angiogenesis defects normally found in PDGFαR-null embryos yet fail to rescue skeletal or extraembryonic defects. This phenotype was associated with the ability of α^Tor to stimulate the mitogen-activated protein (MAP) kinase pathway to near wild-type levels but failure to completely activate other pathways, such as phosphatidylinositol (PI) 3-kinase. The α^FR chimeric receptor fails to rescue any aspect of the PDGFαR-null phenotype. Instead, α^FR expression leads to a gain-of-function phenotype highlighted by ectopic bone development. The α^FR phenotype was associated with a failure to limit MAP kinase signaling and to engage significant PI3-kinase response. These results suggest that precise regulation of divergent downstream signaling pathways is critical for specification of RTK function.

ACKNOWLEDGMENTS

We thank Jason Frazier and Peter Mueting-Nelsen for excellent technical assistance; Alice Davy, Josée Aubin, and Weisheng Chen for continuing interest and many helpful discussions; Brad Olwin for providing the FGFR1 cDNA; Deborah Morrison for providing the Torso cDNA; I. Lax and J. Schlessinger for the anti-FRS2 antibody; and our laboratory colleagues for critical review of the manuscript.

During the course of this study T.G.H. was supported by National Institutes of Health (NIH) postdoctoral fellowship HD08741-01. This work was supported by NIH grants HD25326 and HD24875 to P.S.