Receptor-mediated endocytosis of testicular sulfated glycoprotein-1 (SGP-1) by the nonciliated cells of the rat ductuli efferentes

The present study examines the endocytosis of testicular sulfated glycoprotein-1 (SGP-1) by the nonciliated cells of the efferent ducts. SGP-1 is a 70 KDa protein secreted by the Sertoli cells. Once secreted in the seminiferous lumen, the protein binds to the tail of spermatozoa. In the efferent ducts, it is endocytosed by the nonciliated cells, presumably via a receptor-mediated process. Because the initial steps of receptor-mediated endocytosis result from the binding of a ligand's terminal oligosaccharide to a receptor on the ceil surface, several monosaccharides were injected into the lumen of the rete testis to study their effect on the endocytosis of SGP-1 in the efferent duct. The labeling density of various endocytic compartments was estimated and compared in untreated and treated animals with various sugars. The following sugars were tested: glucose, galactose, mannose, mannose 6-phosphate, N-acetylglucosamine, N-acetylgalactosamine, and sialic acid. The findings suggest that, via its sialic acid binding domain, SGP-1 may bind to glycolipids on the tail of spermatozoa, remove them from the membrane forming a lipo-protein complex. The complex may then be endocytosed by the nonciliated cells, via a receptor that would recognize SGP-1's terminal sialic acid residues, and be delivered to the lysosomes to be degraded. (Abstract shortened by UMI.