Immunocytochemical localization of sulfated glycoprotein-1 (SGP-1) and identification of its transcripts in epithelial cells of the extratesticular duct system of the rat.

The localization of sulfated glycoprotein-1 (SGP-1) in the extratesticular duct system was analyzed using an affinity purified antibody raised against the protein in conjunction with light (LM) and electron (EM) microscope immunocytochemistry. In the LM an intense immunoperoxidase reaction product was observed over the cytoplasm of Sertoli cells as well as over the tails of late spermatids. The rete epithelial cells and nonciliated cells of the efferent ducts also showed an intense uniform reaction over their entire cytoplasm. In the EM, immunogold labeling was noted over the entire endocytic apparatus of these cells including coated pits, endosomes, multivesicular bodies, and secondary lysosomes. Since there was no labeling of the luminal contents including sperm along the epididymis, it was concluded that the Sertoli-derived SGP-1 must dissociate from the sperm and be taken up by epithelial cells at the level of the rete testis and efferent ducts. In all regions of the epididymis, except the cauda, the principal cells showed, in the LM, an intense reaction over bodies of various shapes and sizes in their supranuclear region; this corresponded in the EM to a strong immunogold labeling of secondary lysosomes. No labeling was noted, however, over coated pits, endosomes, or pale multivesicular bodies, suggesting that SGP-1 was not being endocytosed from the lumen. Similar observations were noted for the epithelial clear cells along the entire epididymis. In the cauda epididymidis, principal cells presented a weak immunolabeling of their secondary lysosomes. Northern blot analysis revealed a strong 2.6 Kb band corresponding to the mRNA of SGP-1 in the efferent ducts and all regions of the epididymis with the exception of the cauda. Coincident with the mRNA expression of SGP-1 it was found that small clusters of gold particles representing anti SGP-1, presumably membrane bound, were associated with the Golgi apparatus as well as in close proximity to secondary lysosomes. There was, however, no evidence for the secretion of SGP-1 into the lumen. These results suggest that SGP-1 is synthesized by the epithelial cells of the male duct system and ferried by small vesicles derived from the Golgi apparatus to secondary lysosomes. Because SGP-1 has recently been shown to have substantial sequence similarity to prosaposin, it may be speculated that SGP-1 is instrumental in the degradation of membrane glycolipids present within secondary lysosomes of epithelial cells of the extratesticular duct system.