Cell‐ and Region‐Specific Localization of Lysosomal and Secretory Proteins and Endocytic Receptors in Epithelial Cells of the Cauda Epididymitis and Vas Deferens of the Adult Rat

Abstract

ABSTRACT:The epithelial cells lining the cauda epididymidis and vas deferens are active in endocytosis and have an abundance of lysosomes and a well‐characterized secretory apparatus. However, little is known about the nature of lysosomal proteins contained within lysosomes, the types of receptors on the cell surface, and the types of proteins secreted by these cells. In the present study, cathepsins A, D, B, and sulfated glycoprotein (SGP)‐1, well‐characterized lysosomal proteins, as well as SGP‐2, a secretory protein and lowdensity lipoprotein receptor–related protein‐2 (LRP‐2), an endocytic receptor, were immunolocalized at the light‐microscopic level within epithelial cells of the cauda epididymidis and vas deferens. Principal cells showed numerous intensely reactive lysosomes for cathepsins A, D, and SGP‐1 in all regions of the cauda and vas deferens and for cathepsin B only in the cauda epididymidis. Basal cells were intensely reactive for cathepsin A, unreactive for cathepsins D and B, and weakly reactive for SGP‐1 in the cauda region. In the vas deferens, these cells were intensely reactive for cathepsin A and SGP‐1 and unreactive for cathepsin B; in the case of cathepsin D, basal cells were weakly reactive in the proximal vas deferens but intensely reactive in the middle and distal vas deferens. Clear cells, present in the cauda region and proximal vas deferens, were intensely reactive for cathepsin A, weakly reactive for SGP‐1, and unreactive for cathepsins D and B, while narrow cells found mainly in the proximal vas deferens were intensely reactive for cathepsins A, D, and SGP‐1 and unreactive for cathepsin B. Thus, the expression of different lysosomal enzymes in the cauda epididymidis and vas deferens is not only cell‐ but also region‐specific, suggesting differences in the type of substrates internalized by these cells. SGP‐2, a secretory protein, showed a checkerboardlike staining pattern in the cytoplasm of principal cells of the cauda epididymidis, while the cytoplasm of all principal cells were intensely reactive in the vas deferens. This type of reaction, as well as staining of sperm, suggests that SGP‐2 is secreted into the lumen, where it functions in relation to sperm. The endocytic receptor LRP‐2 was noted only on the apical surface of principal cells of the cauda and vas deferens and in spherical structures indicative of endosomes suggestive of their role in the uptake of various ligands, including SGP‐2, for which it has a high binding affinity. Thus SGP‐2 in the cauda and vas deferens is not only secreted but endocytosed by principal cells, suggestive of an active turnover in the lumen. In summary, the epithelial cells of the cauda and vas deferens show marked differences in expression of lysosomal proteins, SGP‐2, and LRP‐2 suggestive of differences in their functional activity while sperm are stored and protected in these regions.