Genetic dissection of kidney development: Distinct pathways for glomerulus and tubule

The vertebrate kidney is essential for osmoregulation and the clearing of metabolic wastes. The development of the kidney proceeds through a series of transient structures that form along the nephric duct: the pronephros, mesonephros and metanephros. The lineage relationships between the various renal cell types have not been well explored. I have used cell lineage tracing methods and a genetic screen to define both the embryological origin of renal cell precursors and essential genes required for kidney formation.; I employed direct cell marking to define the pronephric organ field in the zebrafish embryo. The field is patterned along the antero-posterior axis with glomerular, tubular and duct progenitors spatially segregated. Cell fate is dictated by the combinatorial expression of three transcription factors: wt1, pax2.1 and sim1.; I performed a genetic screen to identify mutations affecting kidney development. I isolated two types of mutations. The first group perturb the glomerulus. Based on the mutations, I proposed a link between vascular function and glomerular morphogenesis. Eliminating vascular flow at the site of glomerulogenesis by pharmacological means and by focal occlusion of the vessel support the evidence that blood flow is required for normal glomerular morphogenesis. I find that the stretch-sensitive signal acts via matrix metalloproteinase-2. Inhibition of MMP-2 activity does not affect vascular flow but prevents normal formation of the glomerulus. This provides the first evidence for a biomechanical signal operating during vertebrate development.; The second group of mutations have abnormal tubules. I have positionally cloned one such mutation, fusilli. Embryos mutant at the fusilli locus develop tubular cysts and display defects in renal cell polarity. The sodium potassium ATPase normally localizes to the basolateral membrane of tubular epithelium, while in the fusilli mutant, apical localization is also observed. fusilli encodes a novel leucine-rich repeat protein with a LRRcap motif that has been implicated in RNA binding. The identification of fusilli may define a new genetic pathway involved in tubular cell differentiation.