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 versus 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 of the kidney.
Endolyn has been shown to regulate CXCR4 signaling in hematopoietic precursor cells and myoblasts; however, little is known about
endolyn function in the adult or developing kidney. Here we identify
endolyn as a gene important for zebrafish pronephric kidney function. Zebrafish
endolyn lacks the N-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 oligonucleotide 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.