SULFATED GLYCOPROTEIN-2 SYNTHESIZED BY NONCILIATED CELLS OF THE EFFERENT DUCTS IS TARGETED TO THE LYSOSOMAL COMPARTMENT

The epithelial nonciliated cells of the efferent ducts are specialized in internalizing many luminal substances. The nonciliated cells activ ely endocytose sulfated glycoprotein-a (SGP-2), a major secretory prot ein of Sertoli cells and a homologue of human apolipoprotein J. This s tudy was undertaken to investigate the internalization of Sertoli-deri ved SGP-2 and synthesis of an endogenous efferent duct form of SGP-2 b y nonciliated cells targeted to their secondary lysosomes on animals w hose efferent ducts were ligated and/or received injections of tunicam ycin. The regulation of synthesis of the endogenous form of SGP-2 with in nonciliated cells by hormones in general and testosterone in partic ular was also examined using hypophysectomized and castrated animals w ith or without subsequent testosterone replacement. Quantitative elect ron microscope immunocytochemistry was performed on groups of animals fixed with 4% paraformaldehyde and 0.5% glutaraldehyde in phosphate bu ffer for each experimental condition and their controls. In each case, the labeling density (number of gold particles/mu m(2)) within the en dosomal (endosomes) and lysosomal (dense multivesicular bodies and sec ondary lysosomes) compartments was calculated. The results revealed th at ligation of the efferent ducts resulted in a significant decrease i n the labeling density of the endosomal and lysosomal compartments. Ho wever, a baseline of about 18% of controls was still observed in the l ysosomal compartment 24 h after ligation. In this compartment similar values were noted 24 h after tunicamycin treatment in conjunction with or without ligation. These results suggest that an endogenous form of SGP-2 is synthesized by nonciliated cells and presumably targeted via small vesicles from the Golgi apparatus to the lysosomal compartment, but that the major portion of SGP-2 within this compartment is derive d via endocytosis of testicular SGP-2 Hypophysectomy and castration al so showed significant decreases in the labeling densities of these two compartments, but again a baseline level of labeling was noted in the lysosomal compartment. Subsequent testosterone administration to 7-da y hypophysectomized or castrated animals had no effect on the labeling density of the lysosomal compartment, as values comparable to the eff ect of hypophysectomy or castration alone were noted. Taken together t hese results suggest that the nonciliated cells of the efferent ducts synthesize an endogenous form of SGP-2 that is targeted to the lysosom al compartment and which is not regulated by pituitary factors or test osterone. (C) 1994 Wiley-Liss, Inc.