The development of the mammalian kidney requires the inductive interactions of two progenitor tissues, the ureteric bud epithelium and the metanephric mesenchyme. Signals emanating from the ureteric bud induce the mesenchyme to proliferate and differentiate, thus generating most of the epithelial cell types in the nephron. Reciprocally, mesenchyme derived signals induce the ureteric bud epithelium to proliferate and undergo branching morphogenesis to generate the collecting duct system. Although these signals and their receptors have remained elusive, their mechanisms of action ar fundamental to understanding the molecular basis of renal cell growth and differentiation. The receptor tyrosine kinase RET is expressed at the tips of the ureteric bud and regulates proliferation and branching morphogenesis. Recent work in the PI's lab has demonstrated that the glial cell derived growth factor (GDNF) interacts with RET and stimulates RET phosphorylation. GDNF and RET are part of a common signaling pathway that promotes ureteric bud proliferation and branching. This pathway may not only be critical for ealry kidney development and growth, but may also function in aberrant proliferation of ureteric bud derived cells, such as in cystic diseases of the collecting duct. We have established a biological assay for RET activity in transformed MDCK renal epithelial cells. These cells show increased scattering, cell motility, and morphological changes in response to RET activation. It is now well established that the GDNF/RET pathway promotes migration of ureteric bud epthelium towards the metanephric mesenchyme, a potent source of GDNF. Our preliminary data shows that RET expressing epithelial cells can migrate towards a localized source of GDNF. Thus, ureteric bud outgrowth is due to chemoattraction towards a target derived guidance cue. How the GDNF signal is received and interpreted by the epithelial cell is the major focus of this proposal.
The specific aims will utilize the yeast two hybrid system to identify signaling molecules that interact wih the cytoplasmic domain of RET. We will also develop a differential screeen using our model MDCK cell system to identify genes that are activated by the GDNF/RET pathway. This proposal will address the underlying molecular mechanisms of GDNF/RET mediated cell migration and chemoattaction. In addition to advancing our understanding of basic developmental processes, the mechnisms of migration and invasion by oncogenic RET derived tumor cells will also be illuminated.
