Glial cell line-derived neurotrophic factor is a chemoattractant for Ret-activated kidney epithelial cells

Expression of glial cell line-derived neurotrophic factor (GDNF) and its receptor tyrosine kinase protooncogene c-ret is critical for the development of kidney, nervous and gastrointestinal systems. Functional depletion of GDNF or Ret results in renal agenesis or failure of kidney organogenesis. Activation of Ret through GDNF requires the presence of GDNF receptor (GDNFR) and serves as survival signals for neuronal cells. However, effects of GDNF-GDNFR-Ret interactions in epithelial cells has not been elucidated. Using MDCK cells stably transfected with Ret and its kinase mutant (KM), we demonstrate that both lowering serum concentration and addition of GDNF and GDNFR confer activation of Ret tyrosine phosphorylation. Activation of Ret induces chemokinesis and chemotaxis, as assessed by morphology of cells in culture or in type-I collagen gel, and Boyden chamber assays, respectively. However, addition of GDNF and soluble GDNFR does not augment proliferation of Ret expressing cells. Furthermore, we developed a novel method to examine GDNF-induced responses on Ret expressing cells. GDNF bead inserted in collagen gel activates the Ret-expressing cells and attracts them to migrate toward the bead, which coud not be observed in KM. In embyonic kidney cultures, GDNF bead attracts ureteric buds to tightly adhere around them and thus interferes the normal branching morphology. Our data indicate GDNF serves as a chemoattractant for Ret activated kidney cells, which is essential for the invasion and also branching morphogenesis of ureteric buds during kidney organogenesis.