Role of extracellular matrix, growth factors and proto-oncogenes in metanephric development

Isolation of rat fibrillin-1 cDNA and its relevance in metanephric development

The role of fibrillin-1 in metanephrogenesis was investigated. Fibrillin-1 cDNA was isolated from the rat kidney cDNA library and sequenced, and its spatiotemporal expression was studied. It had approximately 88% homology with human fibrillin-1 and had Ca2+ binding epidermal growth factor-like domains, transforming growth factor-beta binding protein motifs, and an RGD binding site. Northern blot analysis revealed an approximately 10-kb transcript, and fibrillin-1 expression was developmentally regulated. In situ hybridization and immunofluorescence studies indicated that at day 15 of gestation, fibrillin-1 is expressed in the metanephric mesenchyme. At day 18, its expression was confined to nascent blood vessels and glomeruli, and it increased in the newborn and neonatal kidneys. Immunoprecipitation revealed an approximately 300-kDa band by SDS-PAGE. Treatment with fibrillin-1 antisense oligodeoxynucleotide induced marked dysmorphogenesis of the embryonic metanephroi. Concomitantly, the fibrillin-1 mRNA, antibody reactivity in the metanephroi, and fibrillin-1-specific radioincorporation were reduced. These data indicate that, like alphavbeta3 integrin, a known morphogen and a putative receptor of fibrillin-1, the fibrillin-1 modulates events related to early organogenesis and possibly also the vascularization of the rat kidney.

Relevance of extracellular matrix, its receptors, and cell adhesion molecules in mammalian nephrogenesis

Mammalian nephrogenesis begins by the reciprocal interaction of the ureteric bud with the undifferentiated mesenchyme. The mesenchyme differentiates into an epithelial phenotype with the development of the glomerulus and proximal and distal tubules. At the same time, the mesenchyme stimulates the branching morphogenesis of the ureteric bud that differentiates into the collecting ducts. These inductive interactions and differentiation events are modulated by a number of macromolecules, including the extracellular matrix (ECM), integrin receptors, and cell adhesion molecules. Many of these macromolecules exhibit spatiotemporal developmental regulation in the metanephros. Some are expressed in the mesenchyme, whereas others appear in the ureteric bud epithelia. The molecules expressed in the mesenchyme or at the epithelial:mesenchymal interface may serve as ligands while those in the epithelia serve as the receptors. In such a scenario the ligand and the receptor would be ideally suited for epithelial:mesenchymal paracrine/juxtacrine interactions that are also influenced by RGD sequences and Ca2+ binding domains of the ECM proteins and their receptors. This review addresses the role of such interactions in metanephric development.

Origins and formation of microvasculature in the developing kidney

Regulation of microvessel assembly in the developing kidney is not known and may occur through vasculogenic, angiogenic, or both processes. To examine this question, we grafted rat and mice embryonic (E) day 12 (E12) kidneys, which have only a rudimentary vasculature, into anterior eye chambers of mouse and rat hosts. Species-specific, monoclonal anti-basement membrane antibodies showed that glomerular basement membranes, mesangial matrices, and microvessel basement membranes were always derived from the graft. When wild-type E12 mouse kidneys were grafted into anterior chambers of ROSA26 mice, in which the beta-galactosidase transgene is expressed ubiquitously, glomerular and microvascular endothelial cells stemmed from the graft, even after maintenance of kidneys in organ culture for 6 days before grafting. Immunolabeling with antibodies against the vascular endothelial growth factor (VEGF) receptor, Flk1, the EphB1 receptor, and its ligand, ephrin-B1, labeled discrete mesenchymal cells in embryonic and newborn kidney cortex, as well as developing microvessel and glomerular endothelium. In adult kidneys, Flk1 labeled glomeruli weakly, other vascular structures were unlabeled. When wild-type E12 kidneys were grafted under renal capsules of adult ROSA26 hosts, endothelium developing within the graft again came from the implanted kidney. In contrast, when E12 kidneys were grafted into renal cortices of newborns, glomeruli within grafts now contained host-derived endothelium. Similarly, when ROSA26 E12 kidneys were implanted into newborn wild-type hosts, chimeric vessels containing graft- and host-derived endothelium were seen in nearby host tissue. Our results indicate that cells capable of forming the entire microvascular tree of grafted metanephroi are already present in the E12 kidney. We hypothesize that Flk1/VEGF and EphB1/ephrin-B1 mediate renal endothelial mitosis-motility and cell guidance-aggregation behavior, respectively.

Direct visualization of renal vascular morphogenesis in Flk1 heterozygous mutant mice

Flk1, a receptor tyrosine kinase for vascular endothelial growth factor (VEGF), is the earliest known marker for endothelial precursors (angioblasts). We examined heterozygous mice in which the Flk1 gene was partially replaced by a promoter-less LacZ insert and used beta-galactosidase histochemistry to view cells transcribing Flk1. In day 10 (E10) embryos, a Flk1-positive network surrounded the metanephric blastema, and, at E11, a vessel entered the metanephros from its ventral aspect alongside the ingrowing ureteric bud. However, aortic branches did not engage embryonic kidneys at these time points. In newborns, beta-galactosidase was localized exclusively and intensely to endothelial cells of all vessels and glomeruli. In contrast, when E12 kidneys grown in organ culture for 6 days were examined, only scattered Flk1-positive cells were seen, glomeruli were unlabeled, and vessels were absent. When organ-cultured kidneys were then grafted into wild-type anterior eye chambers, numerous Flk1-positive endothelial cells in vessels and glomeruli were found, all stemming from the graft. Image analysis showed that grafts with the most abundant glomerulo- and tubulogenesis were also those with the richest expression of Flk1. We conclude that 1) kidney microvessels precede renal artery development, 2) angioblast differentiation is arrested in organ culture but released on grafting when vasculogenesis resumes, and 3) nephrogenesis and microvessel assembly are tightly coupled in vivo.

Capsulin: a novel bHLH transcription factor expressed in epicardial progenitors and mesenchyme of visceral organs

Members of the basic helix-loop-helix (bHLH) family of transcription factors have been shown to control development and differentiation of a variety of cell types. We describe a novel bHLH protein, called capsulin, which is expressed specifically in mesodermally-derived cells that surround the epithelium of the developing gastrointestinal, genitourinary and respiratory systems during mouse embryogenesis. Capsulin transcripts also mark the spiral septum of the heart and progenitor cells that give rise to the pericardium and coronary arteries. Capsulin shares high homology with a recently identified bHLH protein from Drosophila, called bHLH102C, which is expressed in visceral muscle cells that surround the midgut. Capsulin binds a specific E-box consensus sequence (CANNTG) as a heterodimer with the widely-expressed bHLH protein E12, but it does not activate transcription through that sequence on its own. Its restricted expression pattern and DNA binding activity suggest that capsulin regulates gene expression in specific subtypes of visceral mesodermal cells involved in organogenesis and in precursor cells that contribute to the pericardium, coronary arteries and regions of the heart.