Matrix metalloproteinases and their inhibitors regulate in vitro ureteric bud branching morphogenesis

Mammalian kidney development is initiated by the mutual interaction between embryonic metanephric mesenchyme (MM) and the ureteric bud (UB), leading t o tightly controlled UB branching morphogenesis. In a three-dimensional cel l culture model, which employs MM cell-derived conditioned medium (BSN-CM) to induce UB cell branching morphogenesis in extracellular matrix (ECM) gel s (Sakurai H, Barros EJ, Tsukamoto T, Barasch J, and Nigam SK. Proc Natl Ac ad Sci USA 94: 6279-6284, 1997), branching morphogenesis was inhibited by b oth chemical agents (ilomastat and 1,10-orthophenanthroline) and a physiolo gical protein factor [tissue inhibitor of metalloproteinases (TIMP)-2], kno wn to act as matrix metalloproteinase (MMP) inhibitors. In addition, UB bra nching was inhibited in isolated UB culture (Qiao J, Sakurai H, and Nigam S K. Proc Natl Acad Sci USA 96: 7330-7335, 1999) by TIMP-2 and ilomastat, sug gesting a direct role for MMPs in UB branching. Gelatin zymography and enzy matic measurement of MMP activity revealed that MMPs could originate from a t least three different sources: the conditioned medium, the ECM, and the U B cells themselves. In the UB cells, transcription of several MMPs [gelatin ase A (MMP2) and B (MMP9), stromelysin (MMP3), MT1-MMP] and TIMPs was alter ed by BSN-CM and changed as more complex branching structures formed. The E CM appeared to serve as both a reservoir for MMPs and modulated their expre ssion because different ECM compositions altered the total MMP activity as well as specific subsets of MMPs expressed by the UB cells (as determined b y zymography and Northern analysis). In the context of UB branching morphog enesis during kidney development, our data suggest a complex model in which soluble factors produced by the MM, in the context of specific ECM compone nts, modulate the expression of specific subsets of MMPs and TIMPs in the U B, which alter as structures develop and the matrix environment changes. Th is suggests distinct roles for different subsets of MMPs and their inhibito rs during different phases of branching morphogenesis.