Zebrafish Kidney Development

Abstract

The zebrafish pronephric kidney provides a useful and relevant model of kidney development and function. It is composed of cell types common to all vertebrate kidneys and pronephric organogenesis is regulated by transcription factors that have highly conserved functions in mammalian kidney development. Pronephric nephrons are a good model of tubule segmentation and differentiation of epithelial cell types. The pronephric glomerulus provides a simple model to assay gene function in regulating cell structure and cell interactions that form the blood filtration apparatus. The relative simplicity of the pronephric kidney combined with the ease of genetic manipulation in zebrafish makes it well suited for mutation analysis and gene discovery, in vivo imaging, functional screens of candidate genes from other species, and cell isolation by FACS . In addition, the larval and adult zebrafish kidneys have emerged as systems to study kidney regeneration after injury. This chapter provides a review of pronephric structure and development as well as current methods to study the pronephros.

Introduction

The kidney has two principal functions: to remove waste from the blood and to balance ion and metabolite concentrations in the blood within physiological ranges that support proper functioning of all other cells (Vize et al., 2002). Kidney function is achieved largely by first filtering the blood and then recovering useful ions and small molecules by directed epithelial transport. This work is performed by nephrons, the functional units of the kidney (Fig. 1). The nephron is comprised of a blood filter, called the glomerulus, attached to a tubular epithelium (Fig. 1C and D). The glomerulus contains specialized epithelial cells called podocytes that form a basket-like extension of cellular processes around a capillary tuft. The basement membrane between podocytes and capillary endothelial cells together with the specialized junctions between the podocyte cell processes (slit diaphragms) function as a blood filtration barrier, allowing passage of small molecules, ions, and blood fluid into the urinary space, while retaining high molecular weight proteins in the vascular system (Fig. 1; see also Fig. 12D). The blood filtrate travels down the lumen of the kidney tubule, encountering distinct proximal and distal tubule segments that modify the composition of the urine via specific solute transport activities. The urine is drained by the collecting ducts, which further modify its salt and water composition, until eventually being voided outside the body (Fig. 1; Vize et al., 2002).

In the course of vertebrate evolution, three distinct forms of kidneys of increasing complexity have been generated: the pronephros, mesonephros, and metanephros (Saxén, 1987). The pronephros is the first kidney to form during embryogenesis. In vertebrates with free-swimming larvae, including amphibians and teleost fish, the pronephros is the functional kidney of early larval life (Howland, 1921, Tytler, 1988, Tytler et al., 1996, Vize et al., 1997) and is required for proper osmoregulation (Howland, 1921). Later, in juvenile stages of fish and frog development, a mesonephros forms around and along the length of the pronephros and later serves as the final adult kidney. The metanephric kidney forms exclusively in the amniotes (mammals, birds, and reptiles) and, in the case of mammals, is adapted for water retention and producing concentrated urine. Despite some differences in organ morphology between the various kidney forms, many common elements exist at the cellular and molecular level that can be exploited to further our general understanding of renal development and biology. In particular, the zebrafish pronephros has provided a useful model of nephrogenic mesoderm differentiation, kidney cell type differentiation, nephron patterning, kidney, vasculature interactions, glomerular function, and diseases affecting glomerular filtration and tubule lumen size, i.e., cystic kidney disease. While much remains to be done, the basic features of zebrafish pronephric development and patterning have emerged from studies using simple histology, cell lineage tracing, gene expression patterns, and analysis of zebrafish mutants affecting this process.