Podocyte differentiation in the absence of endothelial cells as revealed in the zebrafish avascular mutant, cloche.

Abstract

The physiological functions of the zebrafish pronephros are blood plasma filtration and osmoregulation. The pronephric glomerulus is vascularized through a capillary network sprouting from the dorsal aorta. Vascularization of the glomerulus, visualized by flk-1 expression and alkaline phosphatase reactivity, involves the intimate association between podocytes and endothelial cells and the formation of an intervening glomerular basement membrane (GBM). Cell-cell interactions between podocytes and endothelial cells are thought to play an important role in glomerular angiogenesis. In order to determine whether endothelial cell-derived signals were required for podocyte differentiation, we employed in situ hybridization and electron microscopy to investigate glomerulogenesis in the zebrafish mutant cloche (clo), where endothelial cell development is blocked at an early stage. In clo mutants, glomerular epithelial cells expressing the podocyte specific marker wt1 display well-formed foot processes and are able to form a GBM, suggesting podocytes are able to morphologically differentiate in the absence of endothelia or endothelial-derived signals. The presence of irregular aggregates in the clo GBM as well as the apparent effacement of podocyte foot processes implies a role for endothelial cells in the maintenance of the mature glomerular filtration barrier.