LOW-DENSITY-LIPOPROTEIN RECEPTOR-RELATED PROTEIN-2 EXPRESSION IN EFFERENT DUCT AND EPIDIDYMAL EPITHELIA - EVIDENCE IN RATS FOR ITS IN-VIVO ROLE IN ENDOCYTOSIS OF APOLIPOPROTEIN J CLUSTERIN/

Apolipoprotein J/clusterin/sulfated glycoprotein-2 (ape J) disassociat es from spermatozoa and is endocytosed by epithelial cells lining the efferent ducts and epididymis. The low density lipoprotein receptor-re lated protein-2/megalin (LRP-2) has been shown to bind to apo J and me diates its endocytosis and lysosomal degradation In cultured cells. In this study, immunocytological techniques were used to localize LRP-2 in rat efferent ducts and epididymis and to determine whether its expr ession correlated with those epithelial cells involved in ape endocyto sis. Pronounced LRP-2 immunochemical staining was observed on the apic al surfaces of epithelial cells lining the efferent ducts and in the i ntermediate zone, proximal caput, and corpus and cauda regions of the epididymis. Single immunogold labeling at the electron microscopic lev el showed LRP-2 to be present within coated pits, endocytic vesicles, and early endosomes of the nonciliated cells of the efferent ducts and the principal cells of the epididymis. In efferent ducts, double immu nogold labeling showed both LRP-2 and apo J to be present in endocytic compartments including coated pits, endocytic vesicles, and early end osomes of nonciliated cells. However, while ape J was detected in late endosomes and lysosomes of nonciliated cells, LRP-2 was not. Apical t ubules, possibly emerging from late endosomes, contained labeling for LRP-2 but not for ape J. Ciliated cells lying adjacent to nonciliated cells displayed no labeling for either LRP-2 or apo J. These results a re consistent with the possibility that LRP-2 serves as an endocytic r eceptor for apo I In vivo and that after endocytosis the LRP-2 Is recy cled back to the cell surface while ape J is delivered to the lysosome s for degradation. To provide additional evidence Implicating LRP-2 In apo I endocytosis,a receptor-associated protein (RAP), an antagonist of ape J binding to LRP-2, was Injected Into the efferent duct lumen. Subsequent immunocytological analysis of the efferent duct showed that the RAP treatment abolished the endocytosis of ape I by the nonciliat ed cells. Taken together, these data indicate that LRP-2 is a likely m ediator of ape I endocytosis by the nonciliated efferent duct cells.