ROLE OF SULFATED GLYCOPROTEIN-1 (SGP-1) IN THE DISPOSAL OF RESIDUAL BODIES BY SERTOLI CELLS OF THE RAT

Sulfated glycoprotein-1 (SGP-1) is a polypeptide secreted by Sertoli c ells in the rat. Sequence analysis revealed a 76% sequence similarity with human prosaposin produced by various cell types. Human prosaposin is a 70 kDa protein which is cleaved in the lysosomes into four 10-15 kDa polypeptides termed saposins A, B, C, and D. The function of lyso somal saposins is to either solubilize certain membrane glycolipids or to form complexes with lysosomal enzymes and/or their glycolipid subs trate to facilitate their hydrolysis. The present investigation dealt with the delivery of SGP-1 into the phagosomes of Sertoli cells; these phagosomes contain the residual bodies which detach from the late spe rmatids at the time of spermiation. Immunogold labeling with anti-SGP- 1 antibody was found over Sertoli cell lysosomes, but was absent from phagosomes formed after phagocytosis of spermatid residual bodies in t he apical Sertoli cell cytoplasm in stages VIII and early IX of the cy cle of the seminiferous epithelium. The phagosomes found later in the basal Sertoli cell cytoplasm in stages IX and X of the cycle became la beled with the antibody as the components of the residual bodies rapid ly underwent lysis and disappeared from the Sertoli cells. Sertoli cel l lysosomes isolated by cell fractionation (estimated purity of 80%) w ere found to contain a 65 kDa form of SGP-1 or prosaposin, as well as the 15 kDa polypeptides or saposins. Thus, it appears that this unique lysosomal form of SGP-1 reached the Sertoli cell phagosomes and that their derived polypeptides, the saposins, must play a role in the hydr olysis of membrane glycolipids found in phagocytosed residual bodies. (C) 1995 Wiley-Liss, Inc.