Towards a genetic basis for kidney development

Expression patterns of Id1, Id2, and Id3 are highly related but distinct from that of Id4 during mouse embryogenesis

The murine dominant negative helix-loop-helix (dnHLH) proteins inhibit the activities of bHLH transcription factors in diverse cell lineages (Benezra et al. [1990] Cell 61:49-59; Christy et al [1991] Proc. Natl. Acad. Sci. U.S.A. 88:1815-1819; Sun et al [1991] Mol. Cell Biol. 11: 5603-5611; Riechmann et al. [1994] Nucleic Acids Res. 22:749-755). Currently, there are four members in the dnHLH family, Id1, Id2, Id3, and Id4. In this report, we have performed a detailed comparative in situ hybridization analysis to examine their expression pattern during post-gastrulational mouse development. Id1, 2, and 3 are expressed in multiple tissues, whereas Id4 expression can only be detected in neuronal tissues and in the ventral portion of the epithelium of the developing stomach. The regions where Id1-3 genes are expressed, such as gut, lung, kidney, tooth, whisker, and several glandular structures, are undergoing active morphogenetic activities. The expression patterns of Id1, 2, and 3 overlap in many organs, except in the tissue derived from primitive gut. In the latter, Id1 and Id3 signals are detected in the mesenchyme surrounding the epithelium, whereas Id2 is expressed within the epithelium. The difference in the patterns of expressions of Id2-3 and Id4 suggest that the dominant negative transcriptional activity of these two subclasses of the Id family may have different physiological consequences.

Genetic control of epithelial tube fusion during Drosophila tracheal development

During development of tubular networks such as the mammalian vascular system, the kidney and the Drosophila tracheal system, epithelial tubes must fuse to each other to form a continuous network. Little is known of the cellular mechanisms or molecular control of epithelial tube fusion. We describe the cellular dynamics of a tracheal fusion event in Drosophila and identify a gene regulatory hierarchy that controls this extraordinary process. A tracheal cell located at the developing fusion point expresses a sequence of specific markers as it grows out and contacts a similar cell from another tube; the two cells adhere and form an intercellular junction, and they become doughnut-shaped cells with the lumen passing through them. The early fusion marker Fusion-1 is identified as the escargot gene. It lies near the top of the regulatory hierarchy, activating the expression of later fusion markers and repressing genes that promote branching. Ectopic expression of escargot activates the fusion process and suppresses branching throughout the tracheal system, leading to ectopic tracheal connections that resemble certain arteriovenous malformations in humans. This establishes a simple genetic system to study fusion of epithelial tubes.

Induction of nephrogenic mesenchyme by osteogenic protein 1 (bone morphogenetic protein 7)

The definitive mammalian kidney forms as the result of reciprocal interactions between the ureteric bud epithelium and metanephric mesenchyme. As osteogenic protein 1 (OP-1/bone morphogenetic protein 7), a member of the TGF-beta superfamily of proteins, is expressed predominantly in the kidney, we examined its involvement during metanephric induction and kidney differentiation. We found that OP-1 mRNA is expressed in the ureteric bud epithelium before mesenchymal condensation and is subsequently seen in the condensing mesenchyme and during glomerulogenesis. Mouse kidney metanephric rudiments cultured without ureteric bud epithelium failed to undergo mesenchymal condensation and further epithelialization, while exogenously added recombinant OP-1 was able to substitute for ureteric bud epithelium in restoring the induction of metanephric mesenchyme. This OP-1-induced nephrogenic mesenchyme differentiation follows a developmental pattern similar to that observed in the presence of the spinal cord, a metanephric inducer. Blocking OP-1 activity using either neutralizing antibodies or antisense oligonucleotides in mouse embryonic day 11.5 mesenchyme, cultured in the presence of metanephric inducers or in intact embryonic day 11.5 kidney rudiment, greatly reduced metanephric differentiation. These results demonstrate that OP-1 is required for metanephric mesenchyme differentiation and plays a functional role during kidney development.

Selection for genes encoding secreted proteins and receptors

Extracellular proteins play an essential role in the formation, differentiation, and maintenance of multicellular organisms. Despite that, the systematic identification of genes encoding these proteins has not been possible. We describe here a highly efficient method to isolate genes encoding secreted and membrane-bound proteins by using a single-step selection in yeast. Application of this method, termed signal peptide selection, to various tissues yielded 559 clones that appear to encode known or novel extracellular proteins. These include members of the transforming growth factor and epidermal growth factor protein families, endocrine hormones, tyrosine kinase receptors, serine/threonine kinase receptors, seven transmembrane receptors, cell adhesion molecules, extracellular matrix proteins, plasma proteins, and ion channels. The eventual identification of most, or all, extracellular signaling molecules will advance our understanding of fundamental biological processes and our ability to intervene in disease states.

Essential role of stromal mesenchyme in kidney morphogenesis revealed by targeted disruption of Winged Helix transcription factor BF-2

Metanephric mesenchyme gives rise to both the epithelial cells of the nephron and the stromal cells of the mature kidney. The function of the stroma. in kidney morphogenesis is poorly understood. We have generated mice with a null mutation in the Winged Helix (WH) transcription factor BF-2 to examine its function during development. BF-2 expression within the developing kidney is restricted to the stromal cell lineage. Homozygotes die within the first 24 hr after birth with abnormal kidneys. Mutant kidneys are small, fused longitudinally, and rotated 90 degrees ventrally. Histological examination reveals a smaller collecting system, numerous large condensations of mesenchyme, and a decrease in the number of nephrons. Using molecular markers we show that induction and condensation of the nephrogenic mesenchyme occurs normally in mutant. The disruption of BF-2 reduces the rate of differentiation of the condensed mesenchyme into tubular epithelium, as well as the rate of growth and branching of the ureter and collecting system. Our findings demonstrate that BF-2 and stromal cells have essential functions during kidney morphogenesis. Furthermore, they suggest that BF-2 controls the production, by the stroma, of signals or factors that are required for the normal transition of induced mesenchyme into tubular epithelium and full growth and branching of the collecting system.

Metanephros organogenesis is highly stimulated by vitamin A derivatives in organ culture

Vitamin A and its metabolic derivatives are known to be key signalling molecules in regulating morphogenetic events in vertebrate development. Here we investigated their possible involvement during mammalian kidney development using paired rat metanephros organ culture. Metanephroi were explanted from 14-day-old embryos and cultured for six days in a chemically defined medium containing a retinoid at a dose of 10(-11) to 10(-4) M. Retinol, all-trans and 9-cis retinoic acid were able to promote metanephros growth and differentiation in vitro. A significant increase in the number of nephrons was observed from 10(-8) M of retinol and 10(-10) M of all-trans retinoic acid, before any change in growth parameters. A threefold increase in the number of nephrons was obtained at a dose of 10(-6) M. At low retinoid concentrations, there was a modulating effect of triiodothyronine on retinoid-stimulated nephrogenesis since the absence of triiodothyronine in the medium enhanced the nephrogenic stimulation. Exposure of metanephroi from 13-day-old embryos to all-trans retinoic acid (10(-7) M) led to a sixfold increase of nephron formation. Finally, we analyzed the branching pattern of the ureteric bud and showed that within 48 hours of culture, it was significantly more developed upon retinoid exposure. In conclusion, this study demonstrates that retinoic acid is a key regulator of renal organogenesis in controlling nephrogenic induction processes and ureteric bud patterning, and that the younger the metanephros, the greater the effect.

Pexicrine effects of basement membrane components on paracrine signaling by renal tubular cells

Paracrine interactions between tubular epithelium and interstitial cells have been assumed to be mediated largely by soluble cytokines. While the role of extracellular matrix (ECM) and matrix metalloproteinases (MMPs) in modifying cell function is widely appreciated, the role of the renal tubular basement membrane in modulation of tubulointerstitial function has not been studied. To establish whether those components of the ECM which support tubular epithelial cells also influence cell function (that is, a pexicrine effect), we studied their effects on paracrine signaling between epithelium and fibroblasts. Primary cultures of rat renal proximal tubular epithelial cells (PTE) were cultured on laminin (LN), collagen types-IV and -I (COL-IV, COL-I) and fibronectin (FN). PTE attained confluence more rapidly when grown on LN = COL-IV > COL-I = FN = plastic. On all substrates PTE produced the MMPS, gelatinase-A and -B and collagenase with an apparent increase in gelatinase-A and -B production when cultured on LN. MMPs were found to be secreted both apically and basally with basal secretion predominating, except on LN where secretion was primarily from the apical surface. Cultures of rat renal cortical interstitial fibroblasts were established and characterized. Cortical fibroblasts (CF) were found to secrete gelatinase-A and collagenase. Conditioned medium (CM) from PTE cultured on COL-IV stimulated proliferation of CF but proliferation was unaltered by CM from PTE grown on other substrates. By contrast, co-culture of PTE on LN with CF suppressed collagenase and gelatinase activity in both cell types, indicating a bi-directional, paracrine modulation of MMP production. Thus in the tubulointerstitium, the BM components LN and COL-IV not only fulfill a structural role but act as signaling molecules with differential effects which modify the function of the tubular epithelium and its paracrine interaction with adjacent fibroblasts. The initiation of interstitial fibrosis induced by injury to the tubular basement membrane may reside in the perturbation of this interaction.

The GATA-3 gene is expressed during human kidney embryogenesis

GATA-3 is a transcription factor involved in the differentiation of T lymphocytes and additionally expressed in several chicken and mouse embryonic tissues. Using in situ hybridization, we found that the human GATA-3 gene is selectively expressed in the developing kidney. GATA-3 mRNA is first detected in the Wolffian duct from the time of its emergence in the embryonic intermediate mesoderm and further expressed in the collecting ducts of the mesonephros until its involution. In the metanephros, GATA-3 is expressed in the ureteric bud where it is constitutively transcribed, throughout development, along the branching process that gives rise to the whole collecting system of the definitive kidney. Besides the Wolffian duct and derivatives, we also report the expression of GATA-3 gene in the glomerular mesangium and adjacent endocapillary cells, in both meso- and metanephros. This early and specific expression of the GATA-3 gene suggests a role for this transcription factor in the differentiation of the human kidney.

Hoxa 11 structure, extensive antisense transcription, and function in male and female fertility

Hoxa 11 is a murine Abdominal-B-type homeobox gene. The structure of this gene is presented, including genomic and cDNA sequence. The cDNA includes the complete open reading frame and based on primer extension results is near full length. Surprisingly, the antisense strand of Hoxa 11 was found to be transcribed. Moreover, these antisense transcripts were processed and polyadenylated. The developmental expression patterns for both sense and antisense transcripts were examined using serial section and whole-mount in situ hybridizations. Hoxa 11 transcription patterns were defined in the limbs, kidney and stromal cells surrounding the Mullerian and Wolffian ducts. Of particular interest, in the developing limbs, the sense and antisense transcripts showed complementary expression patterns, with antisense RNAs increasing in abundance in regions where sense RNAs were diminishing in abundance. Furthermore, targeted mutation of Hoxa 11 is shown to result in both male and female sterility. The female mutants produce normal ova, which develop properly post-fertilization when transferred to wild-type surrogate mothers. The Hoxa 11 homozygous mutants are shown to provide a defective uterine environment. The mutant males exhibited a malformation of the vas deferens that resembles a partial homeotic transformation to an epididymis. In addition, the mutant testes fail to descend properly into the scrotum and, likely as a result, spermatogenesis is perturbed.

Sulphated proteoglycan is required for collecting duct growth and branching but not nephron formation during kidney development

Kidney epithelia have separate origins; collecting ducts develop by ureteric bud growth and arborisation, nephrons by induced mesenchyme-epithelium transition. Both express sulphated glycosaminoglycans (GAGs) which are strikingly upregulated during nephron differentiation. However, sodium chlorate, an inhibitor of GAG sulphation, and the GAG-degrading enzymes heparitinase plus chondroitinase, did not prevent nephron development. In contrast, ureteric bud growth and branching were reversibly inhibited by the above reagents, the inhibition correlating quantitatively with sulphated GAG deprivation caused by a range of chlorate concentrations. Growth and branching could be independently restored during GAG deprivation by hepatocyte growth factor and phorbol-12-myristate acetate (PMA) respectively. Together these signalling effectors stimulated both branch initiation and growth. Thus growth and morphogenesis of ureteric bud involve distinct signalling pathways both regulated by GAGs.