β-catenin causes renal dysplasia via upregulation of Tgfβ2 and Dkk1

Renal dysplasia, defined by defective ureteric branching morphogenesis and nephrogenesis, is the major cause of renal failure in infants and children. Here, we define a pathogenic role for a β-catenin-activated genetic pathway in murine renal dysplasia. Stabilization of β-catenin in the ureteric cell lineage before the onset of kidney development increased β-catenin levels and caused renal aplasia or severe hypodysplasia. Analysis of gene expression in the dysplastic tissue identified downregulation of genes required for ureteric branching and upregulation of Tgfβ2 and Dkk1. Treatment of wild-type kidney explants with TGFβ2 or DKK1 generated morphogenetic phenotypes strikingly similar to those observed in mutant kidney tissue. Stabilization of β-catenin after the onset of kidney development also caused dysplasia and upregulation of Tgfβ2 and Dkk1 in the epithelium. Together, these results demonstrate that elevation of β-catenin levels during kidney development causes dysplasia.

TGF-β superfamily members modulate growth, branching, shaping, and patterning of the ureteric bud

Protein-rich fractions inhibitory for isolated ureteric bud (UB) growth were separated from a conditioned medium secreted by cells derived from the metanephric mesenchyme (MM). Elution profiles and immunoblotting indicated the presence of members of the transforming growth factor-beta (TGF-beta) superfamily. Treatment of cultured whole embryonic kidney with BMP2, BMP4, activin, or TGF-beta1 leads to statistically significant differences in the overall size of the kidney, the number of UB branches, the length and angle of the branches, as well as in the thickness of the UB stalks. Thus, the pattern of the ureteric tree is altered. LIF, however, appeared to have only minimal effect on growth and development of the whole embryonic kidney in organ culture. The factors all directly inhibited, in a concentration-dependent fashion, the growth and branching of the isolated UB, albeit to different extents. Antagonists of some of these factors reduced their inhibitory effect. Detailed examination of TGF-beta1-treated UBs revealed only a slight increase in the amount of apoptosis in tips by TUNEL staining, but diminished proliferation throughout by Ki67 staining. These data suggest an important direct modulatory role for BMP2, BMP4, LIF, TGF-beta1, and activin (as well as their antagonists) on growth and branching of the UB, possibly in shaping the growing UB by playing a role in determining the number of branches, as well as where and how the branches occur. In support of this notion, UBs cultured in the presence of fibroblast growth factor 7 (FGF7), which induces the formation of globular structures with little distinction between the stalk and ampullae [Mech. Dev. 109 (2001) 123], and TGF-beta superfamily members lead to the formation of UBs with clear stalks and ampullae. This indicates that positive (i.e., growth and branch promoting) and negative (i.e., growth and branch inhibiting) modulators of UB morphogenesis can cooperate in the formation of slender arborized UB structures similar to those observed in the intact developing kidney or in whole embryonic kidney organ culture. Finally, purification data also indicate the presence of an as yet unidentified soluble non-heparin-binding activity modulating UB growth and branching. The data suggest how contributions of positive and negative growth factors can together (perhaps as local bipolar morphogenetic gradients existing within the mesenchyme) modulate the vectoral arborization pattern of the UB and shape branches as they develop, thereby regulating both nephron number and tubule/duct caliber. We suggest that TGF-beta-like molecules and other non-heparin-binding inhibitory factors can, in the appropriate matrix context, facilitate "braking" of the branching program as the UB shifts from a rapid branching stage (governed by a feed-forward mechanism) to a stage where branching slows down (negative feedback) and eventually stops.