The Role of GDNF/Ret Signaling in Ureteric Bud Cell Fate and Branching Morphogenesis

Abstract

While GDNF signaling through the Ret receptor is critical for kidney development, its specific role in branching morphogenesis of the epithelial ureteric bud (UB) is unclear. Ret expression defines a population of UB "tip cells" distinct from cells of the tubular "trunks," but how these cells contribute to UB growth is unknown. We have used time-lapse mosaic analysis to investigate normal cell fates within the growing UB and the developmental potential of cells lacking Ret. We found that normal tip cells are bipotential, contributing to both tips and trunks. Cells lacking Ret are specifically excluded from the tips, although they contribute to the trunks, revealing that the tips form and expand by GDNF-driven cell proliferation. Surprisingly, the mutant cells assumed an asymmetric distribution in the UB trunks, suggesting a model of branching in which the epithelium of the tip and the adjacent trunk is remodeled to form new branches.

GDNF/Ret signaling and the development of the kidney

“…However, the ability of the mutant cells to contribute to the trunks of multiple branch generations, without having ever occupied the tips, required a new model for UB branching, in which not only the epithelium of the ampulla, but also the adjacent trunk epithelium, is remodeled to form the new branches. (74) These studies established that Ret is required in a cell autonomous manner in UB cells in order for these cells to contribute to the tip domain. Why is Ret signaling specifically required for tip formation?…”

“…(59,71) These are not two entirely separate lineages, as shown by studies using chimeric kidneys, in which some cells carried a Hoxb7/GFP marker gene, which is expressed only in ureteric bud cells (73) and allows their fates to be followed by time lapse imaging of organ cultures. (74) Cells that were initially located in the tip of an elongating branch gave rise to daughter cells, some of which remain at the tip, and others of which were left behind in the forming trunk (Fig. 3c).…”

“…Not all the growth occurs at the tips, as UB trunks can elongate without incorporation of cells from the adjacent tips (Fig. 3c,d); (74) furthermore, much of the growth of the UB at later stages of kidney development apparently occurs by interstitial growth of the trunks. (8,56) However, proliferation of UB tip cells is clearly an important mechanism of UB growth and possibly of branching.…”

“…(77) New insight into the role of GDNF/Ret signaling in the formation and behavior of UB tip and trunk cells has recently been obtained from the analysis of chimeric mouse kidneys in which only a subset of cells lacked the Ret gene (and carried a Hoxb7/GFP transgene that allowed the fates of mutant UB cells to be traced ) while the remaining cells were wild type. (74) As long as the percentage of mutant cells was low, the chimeric kidneys developed normally. Ret-mutant (GFPþ) cells contributed without restriction to the Wolffian duct, consistent with the normal formation of the WD in Ret-mutant embryos.…”

“…(78) However, analysis of cell movements during culture of wild-type kidneys provided no indication that cells normally migrate from the trunk into the UB tip. (74) Perhaps the simplest explanation is that, during the initial outgrowth of the UB from the Wolffian duct, the UB tip is formed entirely by Ret-expressing cells that proliferate rapidly in response to GDNF; therefore, in the Ret Figure 4. Inability of Ret À/À cells to contribute to ureteric bud tips in the kidneys of chimeric mice.…”