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 (PDGF␣R) is fused to the cytosolic domain of Drosophila Torso (␣ Tor ) or the mouse fibroblast growth factor receptor 1 (␣ FR ). ␣ Tor homozygous embryos exhibit significant rescue of neural crest and angiogenesis defects normally found in PDGF␣R-null embryos yet fail to rescue skeletal or extraembryonic defects. This phenotype was associated with the ability of ␣ Tor to stimulate the mitogen-activated protein (MAP) kinase pathway to near wildtype levels but failure to completely activate other pathways, such as phosphatidylinositol (PI) 3-kinase. The ␣ FR chimeric receptor fails to rescue any aspect of the PDGF␣R-null phenotype. Instead, ␣ FR expression leads to a gain-of-function phenotype highlighted by ectopic bone development. The ␣ 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.Evolutionary conservation of a gene requires that it contributes specific functions that enhance the fitness of the organism in which it is expressed. Receptor tyrosine kinases (RTKs) represent a large family of genes that have been conserved due to their ability to control multiple fundamental cellular processes. Upon binding to specific extracellular ligands, activated RTKs regulate cell proliferation, survival, migration, differentiation, and metabolism. The conservation of over 50 RTKs in mammals suggests that each executes critical specific cellular functions. How functional specificity is generated remains a matter of controversy. A vast amount of research has focused on understanding the molecular mechanisms that underlie the ability of these receptors to mediate diverse cellular functions. Surprisingly, studies have shown that even divergent RTKs activate highly redundant array of downstream signaling proteins, such as mitogen-activated protein (MAP) kinases, Src family kinases, phospholipase C (PLC)-␥, and phosphatidylinositol (PI) 3-kinase. Several models have been proposed to explain how RTKs achieve functional specificity at the cellular level while at the molecular level appearing to activate redundant signaling pathways (12,27).One model proposes that specific responses to RTKs are a result of differential activation of downstream pathways or biochemical differences in RTK signaling. A great deal of evidence based on experiments performed in cultured cell lines to assess individual contributions of downstream signaling pathways supports this idea (14,26). Differences in the strength and/or duration ...

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