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

Isoform-specific antibodies to TGF beta 1, TGF beta 2, and TGF beta 3 proteins were generated and have been used to examine the expression of these factors in the developing mouse embryo from 12.5-18.5 d post coitum (d.p.c.). These studies demonstrate the initial characterization of both TGF beta 2 and beta 3 in mammalian embryogenesis and are compared with TGF beta 1. Expression of one or all three TGF beta proteins was observed in many tissues, e.g., cartilage, bone, teeth, muscle, heart, blood vessels, lung, kidney, gut, liver, eye, ear, skin, and nervous tissue. Furthermore, all three TGF beta proteins demonstrated discrete cell-specific patterns of expression at various stages of development and the wide variety of tissues expressing TGF beta proteins represent all three primary embryonic germ layers. For example, specific localization of TGF beta 1 was observed in the lens fibers of the eye (ectoderm), TGF beta 2 in the cortex of the adrenal gland (mesoderm), and TGF beta 3 in the cochlear epithelium of the inner ear (endoderm). Compared to the expression of TGF beta mRNA transcripts in a given embryonic tissue, TGF beta proteins were frequently colocalized within the same cell type as the mRNA, but in some cases were observed to localize to different cells than the mRNA, thereby indicating that a complex pattern of transcription, translation, and secretion for TGF beta s 1-3 exists in the mouse embryo. This also indicates that TGF beta 1, beta 2, and beta 3 act through both paracrine and autocrine mechanisms during mammalian embryogenesis.

In the normal mouse embryo, Bmp4 is expressed in mesenchymal cells surrounding the Wolffian duct (WD) and ureter stalk, whereas bone morphogenetic protein (BMP) type I receptor genes are transcribed either ubiquitously (Alk3) or exclusively in the WD and ureter epithelium (Alk6). Bmp4 heterozygous null mutant mice display, with high penetrance, abnormalities that mimic human congenital anomalies of the kidney and urinary tract (CAKUT), including hypo/dysplastic kidneys, hydroureter, ectopic ureterovesical (UV) junction, and double collecting system. Analysis of mutant embryos suggests that the kidney hypo/dysplasia results from reduced branching of the ureter, whereas the ectopic UV junction and double collecting system are due to ectopic ureteral budding from the WD and accessory budding from the main ureter, respectively. In the cultured metanephros deprived of sulfated glycosaminoglycans (S-GAGs), BMP4-loaded beads partially rescue growth and elongation of the ureter. By contrast, when S-GAGs synthesis is not inhibited, BMP4 beads inhibit ureter branching and expression of Wnt 11, a target of glial cell-derived neurotrophic factor signaling. Thus, Bmp4 has 2 functions in the early morphogenesis of the kidney and urinary tract. One is to inhibit ectopic budding from the WD or the ureter stalk by antagonizing inductive signals from the metanephric mesenchyme to the illegitimate sites on the WD. The other is to promote the elongation of the branching ureter within the metanephros, thereby promoting kidney morphogenesis.

BMP7 is expressed at diverse sites in the developing mouse embryo, including visceral endoderm, notochord, heart, eye, kidney, and bone. A null mutation in BMP7 results in defects largely confined to the developing kidney and eye. To examine whether other bone morphogenetic protein (BMP) family members potentially substitute for BMP7 in mutant embryos, thereby restricting the observed defects, we analyzed the expression patterns of BMP2 through BMP7 in wild-type and mutant tissues. In the central nervous system and heart, which develop normally in the absence of BMP7 signaling, expression domains of other BMP family members completely overlap with that of BMP7. The variable expressivity of the eye defect correlates with partially overlapping BMP4 and BMP7 expression domains during early eye induction. The loss of BMP7 signaling in the kidney results in apoptosis in the metanephric mesenchyme, a cell population that exclusively expresses BMP7. Thus, tissue defects observed in BMP7 deficient embryos are restricted to cell populations exclusively expressing BMP7. These data suggest that BMP family members can functionally substitute for BMP7 at sites where they colocalize in vivo.