Early development of the zebrafish pronephros and analysis of mutations affecting pronephric function

The zebrafish pronephric kidney provides a simplified model of nephron deve lopment and epithelial cell differentiation which is amenable to genetic an alysis, The pronephros consists of two nephrons with fused glomeruli and pa ired pronephric tubules and ducts. Nephron formation occurs after the diffe rentiation of the pronephric duct with both the glomeruli and tubules being derived from a nephron primordium. Fluorescent dextran injection experimen ts demonstrate that vascularization of the zebrafish pronephros and the ons et of glomerular filtration occurs between 40 and 48 hpf. We isolated fifte en recessive mutations that affect development of the pronephros. All have visible cysts in place of the pronephric tubule at 2-2.5 days of developmen t. Mutants were grouped in three classes: (1) a group of twelve mutants wit h defects in body axis curvature and manifesting the most rapid and severe cyst formation involving the glomerulus, tubule and duct, (2) the fleer mut ation with distended glomerular capillary loops and cystic tubules, and (3) the mutation pao pao tang with a normal glomerulus and pronephric tubules, double bubble was analyzed as a representative of mutations that perturb t he entire length of the pronephros and body axis curvature. Cyst formation begins in the glomerulus at 40 hpf at the time when glomerular filtration i s established suggesting a defect associated with the onset of pronephric f unction. Basolateral membrane protein targeting in the pronephric duct epit helial cells is also severely affected, suggesting a failure in terminal ep ithelial cell differentiation and alterations in electrolyte transport. The se studies reveal the similarity of normal pronephric development to kidney organogenesis in all vertebrates and allow for a genetic dissection of gen es needed to establish the earliest renal function.