Among the most basic questions remaining in biology are what are the inductive molecules that commit stem cells to a particular pathway of differentiation and what prevents this commitment in tumorigenesis? While in vitro systems for studying tissue-specific induction and development have been available in some cases for more than 40 years, the molecules that direct these processes remain elusive. For kidney, it has long been possible in culture to induce tubules in undifferentiated metanephric mesenchyme by recombining isolated mesenchyme with an inductive tissue but not by using soluble factors. We have devised tissue culture models that permit differentiation in the absence of an inductive tissue and are using these models to define the functional secreted patterning molecules critical to normal renal development. Previously, we demonstrated that basic fibroblast growth factor (FGF2) induces the early events in renal tubulogenesis, including upregulation of the critical molecules, Wt1, wnt- 4, and pax-2 and compaction of the mesenchyme. We find now that it probably also mediates the divergent differentiation of metanephric mesenchyme to form nephrogenic blastemal cells and stromal mesenchyme. This is based on the reciprocal expression patterns of hepatocyte growth factor and its receptor c-met in cultured mesenchymes following bFGF treatment. More importantly, we have also established that a cell line (RUB-1) derived in our lab from the ureteric bud, the natural inductive tissue for metanephric mesenchyme, secretes an inductive molecule(s) for renal tubulogenesis and glomerulogenesis. In characterizing the RUB-1 line, we have found that it expresses the secreted patterning molecules, bone morphogenetic protein-7 and wnt7B, as well as several members of the fgf family, i.e., fgf1, fgf2, and fgf9. We are currently assessing the activities of BMP-7 and WNT-7B in combination with an FGF in morphogenesis as we continue to purify the inductive activity secreted by RUB-1 cells. These studies suggest that soluble factors similar to those implicated in the development of other tissues are also involved in renal morphogenesis, and this system provides a powerful approach to examining the relevance of various BMPs, FGFs, and WNTs in the induction process. Furthermore, identification of the factors critical to development should permit insight into possible targets of carcinogens which when altered might block stem cell commitment and cause tumor formation.
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