Sulfated glycoprotein-2 synthesized by nonciliated cells of the efferent ducts is targeted to the lysosomal compartment
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The epithelial nonciliated cells of the efferent ducts are specialized in internalizing many luminal substances. The nonciliated cells actively endocytose sulfated glycoprotein-2 (SGP-2), a major secretory protein of Sertoli cells and a homologue of human apolipoprotein J. This study was undertaken to investigate the internalization of Sertoli-derived SGP-2 and synthesis of an endogenous efferent duct form of SGP-2 by nonciliated cells targeted to their secondary lysosomes on animals whose efferent ducts were ligated and/or received injections of tunicamycin. The regulation of synthesis of the endogenous form of SGP-2 within nonciliated cells by hormones in general and testosterone in particular was also examined using hypophysectomized and castrated animals with or without subsequent testosterone replacement. Quantitative electron microscope immunocytochemistry was performed on groups of animals fixed with 4% paraformaldehyde and 0.5% glutaraldehyde in phosphate buffer for each experimental condition and their controls. In each case, the labeling density (number of gold particles/micron 2) within the endosomal (endosomes) and lysosomal (dense multivesicular bodies and secondary lysosomes) compartments was calculated. The results revealed that ligation of the efferent ducts resulted in a significant decrease in the labeling density of the endosomal and lysosomal compartments. However, a baseline of about 18% of controls was still observed in the lysosomal 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 derived via endocytosis of testicular SGP-2. Hypophysectomy and castration also 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-day hypophysectomized or castrated animals had no effect on the labeling density of the lysosomal compartment, as values comparable to the effect of hypophysectomy or castration alone were noted. Taken together these results suggest that the nonciliated cells of the efferent ducts synthesize an endogenous form of SGP-2 that is targeted to the lysosomal compartment and which is not regulated by pituitary factors or testosterone.
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