Background: We have suggested that bone morphogenetic protein 4 (BMP4), acting on the Wolffian duct and ureter epithelium, determines the budding site of the ureter by locally antagonizing ubiquitous inductive signal(s) from the metanephric mesenchyme. In the present study, we examine the effect of BMP4 on the development of metanephric and periureteral mesenchymal cells, which express the BMP type I receptor gene, Bmpr1a (Alk3).

Methods: Urogenital tissues obtained from Bmp4 heterozygous null mutant (Bmp4+/-) embryos at different stages, and metanephric and ureteral tissue explants cultured in the presence of recombinant BMP4 were subjected to morphologic, immunohistochemical and in situ hybridization analyses. To examine the chemotactic activity of BMP4 for periureteral mesenchymal cells, a modified Boyden chamber assay was performed.

Results: Many of the kidneys of newborn Bmp4+/- mice contained multicystic dysplastic regions. This morphology was preceded by abnormally high apoptotic activity in the metanephric mesenchyme of mutant embryos at E14.5. In whole metanephric explants, BMP4 uniformly promoted the expansion of the Pax2-negative and weakly Foxd1 (previously Bf2) -positive peripheral stromal compartment of metanephric mesenchyme in the presence of fibroblast growth factor 2 (FGF2). In addition, in isolated metanephric mesenchyme, BMP4-loaded beads prevented apoptosis locally. Thus, BMP4 prevents cell death and promotes the growth of the metanephric mesenchyme. The effect of BMP4 on periureteral mesenchyme is different from its effect on metanephric mesenchyme. In utero, periureteral mesenchymal cells condense around the ureter epithelium, followed by differentiation into smooth muscle cells at a site where Bmp4 is intensely expressed. Analysis of Bmp4+/- ureters at E15.5 reveals that the alpha-smooth muscle actin (alpha-SMA)-positive cells are low in number. In vitro, BMP4-loaded beads promote the accumulation of periureteral mesenchymal cells to form several cell layers surrounding the beads. In addition, in a Boyden chamber assay, BMP4 increases the migration of periureteral mesenchymal cells through the filter. Thus, BMP4 can serve as a chemoattractant for periureteral mesenchymal cells and induce locally the smooth muscle layer of the ureter at Bmp4-expressing sites.

Conclusion: Depending on local context, BMP4 has several biological actions on the morphogenesis of different portions of the excretory system, namely, the development of the ureterovesical junction, the ureter, and the kidney.