Spermatogenic cells do internalize Sertoli androgen-binding protein: a transmission electron microscopy autoradiographic study in the rat.

Specific binding sites for androgen-binding protein (ABP) have been demonstrated recently to be present on germ cells from the rat and on membrane-enriched fractions from rat germ cells. The present study was undertaken to test if such receptors could lead to the activation of a specific internalization pathway as in other cells. Isolated rat germ cells and in situ rat germ cells, maintained within the seminiferous epithelium with either an intact or a bypassed blood testis barrier, were exposed to culture medium containing 12,000 cpm/ml [3H] delta 6-testosterone (2.5 pg) photoaffinity-labeled ABP, purified from rat testis. The follow-up of labeled ABP/germ cell interactions was based on qualitative and quantitative transmission electron microscopy autohistoradiography. Attention was focused on adluminal germ cells from pachytene spermatocytes to mature spermatids, which are normally present above the Sertoli cell tight junctions. Our observations revealed the presence in rat germ cells of structures related to specific endocytosis, namely coated pits and vesicles which stained positively with anticlathrin antibodies. When exposed to the [3H] delta 6-testosterone-ABP complex, adluminal germ cells showed marked labeling of these endocytic organelles. Preincubation either with excess unlabeled ABP or pretreatment by EDTA reduced the labeling significantly. Once internalized, ABP was found to be confined to the endocytic and nuclear compartments. The nuclear labeling was high in primary spermatocytes and round spermatids but was absent in elongated spermatids with condensed chromatin, in which transcriptional activity had almost stopped. In contrast, at later steps of spermiogenesis, the cytoplasm became heavily labeled, especially in the postnuclear part of elongated spermatids and in residual bodies about to be phagocytozed by Sertoli cells. Experiments using ligated seminiferous tubules, with an intact blood-testis barrier, clearly showed that ABP was captured at the basal part of Sertoli cells, transported up to the adluminal compartment and delivered to germ cells which finally internalized the protein. Experiments using largely opened seminiferous tubules allowing ABP to bypass the blood-testis barrier led to a delay in adluminal germ cell labeling compared with that in isolated germ cells. In all experiments in which germ cells were incubated in teh presence of Sertoli cells, the labeling observed at the surface of the germ cell line, especially over coated pits, was mostly found facing thin Sertoli cell processes, suggesting the possible existence of a specific mechanism for the presentation of ABP by the Sertoli cells to adluminal germ cells.(ABSTRACT TRUNCATED AT 400 WORDS)