Apical Targeting and Endocytosis of the Sialomucin Endolyn are Essential for Establishment of Zebrafish Pronephric Kidney Function

Abstract

Kidney function requires the appropriate distribution of membrane proteins between the apical and basolateral surfaces along the kidney tubule. Further, the absolute amount of a protein at the cell surface vs. intracellular compartments must be attuned to specific physiological needs. Endolyn (CD164) is a transmembrane protein that is expressed at the brush border and in apical endosomes of the proximal convoluted tubule and in lysosomes of more distal segments. Endolyn has been shown to regulate CXCR4 signaling in hematopoietic precursor cells and myoblasts; however, little is known about endolyn function in adult or developing kidney. Here we identify endolyn as a novel gene important for zebrafish pronephric kidney function. Zebrafish endolyn lacks the amino terminal mucin-like domain of the mammalian protein, but is otherwise highly conserved. Using in situ hybridization we show that endolyn is expressed early during development in zebrafish brain, eye, gut, and pronephric kidney. Embryos injected with a translation inhibiting morpholino targeted against endolyn developed pericardial edema, hydrocephaly, and body curvature. The pronephric kidney appeared normal morphologically, but clearance of fluorescent dextran injected into the common cardinal vein was delayed, consistent with a defect in the regulation of water balance in morphant embryos. Heterologous expression of rat endolyn rescued the morphant phenotypes. Interestingly, rescue experiments using mutant rat endolyn constructs revealed that both apical sorting and endocytic/lysosomal targeting motifs are required for normal pronephric kidney function. This suggests that both polarized targeting and postendocytic trafficking of endolyn are essential for the protein's proper function in mammalian kidney.