A role for Timeless in epithelial morphogenesis during kidney development

Central to the process of epithelial organogenesis is branching morphogenes is into tubules and ducts. In the kidney, this can be modeled by a very sim ple system consisting of isolated ureteric bud (UB) cells, which undergo br anching morphogenesis in response to soluble factors present in the conditi oned medium of a metanephric mesenchyme cell line. By employing a targeted screen to identify transcription factors involved early in the morphogeneti c program leading to UB branching, we identified the mammalian ortholog of Timeless (mTim) as a potential immediate early gene (IEG) important in this process. In the embryo, mTim was found to be expressed in patterns very su ggestive of a role in epithelial organogenesis with high levels of expressi on in the developing lung, liver, and kidney, as well as neuroepithelium. I n the embryonic kidney, the expression of mTim was maximal in regions of ac tive UB branching, and a shift from the large isoform of mTim to a smaller isoform occurred as the kidney developed. Selective down-regulation of mTim resulted in profound inhibition of embryonic kidney growth and UB morphoge nesis in organ culture. A direct effect on the branching UB was supported b y the observation that down-regulation of mTim in the isolated UB (cultured in the absence of mesenchyme) resulted in marked inhibition of morphogenes is, suggesting a key role for Tim in the epithelial cell morphogenetic path way leading to the formation of branching tubules.