Characterization of a testis specific protein localized to endoplasmic reticulum of spermatogenic cells

Nuclear Na+/K+-ATPase plays an active role in nucleoplasmic Ca2+ homeostasis

Na(+)/K(+)-ATPase, an integral membrane protein, has been studied for over a half century with respect to its transporter function in the plasma membrane, where it expels three Na(+) ions from the cell in exchange for two K(+) ions. In this study, we demonstrate a functioning Na(+)/K(+)-ATPase within HEK293 cell nuclei. This subcellular localization was confirmed by western blotting, ouabain-sensitive ATPase activity of the nuclear membrane fraction, immunocytochemistry and delivery of fluorescently tagged Na(+)/K(+)-ATPase α- and β-subunits. In addition, we observed an overlap between nuclear Na(+)/K(+)-ATPase and Na/Ca-exchanger (NCX) when nuclei were immunostained with commercially available Na(+)/K(+)-ATPase and NCX antibodies, suggesting a concerted physiological coupling between these transporters. In keeping with this, we observed an ATP-dependent, strophanthidin-sensitive Na(+) flux into the nuclear envelope (NE) lumen loaded with the Na-sensitive dye, CoroNa-Green. Analogous experiments using Fluo-5N, a low affinity Ca(2+) indicator, demonstrated a similar ATP-dependent and strophanthidin-sensitive Ca(2+) flux into the NE lumen. Our results reveal an intracellular physiological role for the coordinated efforts of the Na(+)/K(+)-ATPase and NCX to actively remove Ca(2+) from the nucleoplasm into the NE lumen (i.e. the nucleoplasmic reticulum).

Molecular chaperone calmegin localization to the endoplasmic reticulum of meiotic and post-meiotic germ cells in the mouse testis

Calmegin is a testis-specific Ca(2+)-binding protein that is homologous to calnexin. Recently, sperm from transgenic mice lacking calmegin have been shown to be infertile. To further characterize calmegin, we analyzed the precise stage of expression and the intracellular localization of this protein in germ cells during mouse spermatogenesis by an immunoperoxidase technique using the anti-calmegin monoclonal antibody TRA369. Light microscopic immunocytochemistry showed that calmegin appeared in early pachytene spermatocytes, with the highest expression in round and elongating spermatids, and disappeared in the maturation phase of spematids at step 15. Immunoelectron microscopy showed that selective localization was found at the endoplasmic reticulum membrane and the nuclear envelope of spermatogenic cells. During the maturation phase, a dramatic reduction in calmegin occurred in the endoplasmic reticulum of the spermatids, suggesting that the major function of calmegin has been completed by the time spermatids reach step 14. In addition, although the immunoreactivity was completely absent in the calmegin-deficient mutant mouse testis, ultrastructural analysis showed that mature sperm from the knockout mice were normal. This suggests that calmegin is not required for the morphogenesis of male germ cells. Thus, our results suggest that calmegin has a major role in mouse spermatogenesis, and also indicate that this protein would be useful as a maker molecule to study the functional role of the endoplasmic reticulum in the process of spermatid differentiation.

Characterization of domains in mice of calnexin-t, a putative molecular chaperone required in sperm fertility, with use of glutathione S-transferase-fusion proteins

Calnexin-t (calmegin) is a male germ cell-specific variant of calnexin, a membrane bound-molecular chaperone in the endoplasmic reticulum (ER). Although it is temporally expressed during spermatogenesis, it has recently been shown to be highly involved in sperm fertility. To investigate the biochemical states of calnexin-t during spermatogenesis, we produced a series of glutathione S-transferase-fusion proteins with several specific coding domains of calnexin-t. Immunostaining and 45Ca2+ overlay assays clearly showed that the internal proline-rich repeat region has Ca2+-binding ability and contains an epitope recognized by monoclonal antibody 1C9. Western blot analysis of protein extracts from the testes of 10-, 18-, 26-, and 60-day-old mice revealed only a single 101-kDa protein during testicular development by 1C9. Anti-C, a cytoplasmic domain-specific antibody generated by immunization with recombinant protein, produced the same results, indicating that the 101-kDa form of calnexin-t is prevalent at all stages of spermatogenesis expressing calnexin-t. In paraffin sections of mouse testis, Anti-C stained spermatocytes and spermatids intensely, whereas 1C9 stained spermatocytes only slightly but spermatids intensely, suggesting that the affinity of 1C9 for its epitope is lower in pachytene spermatocytes than in spermatids. Acid phosphatase treatment of the 101-kDa form generated a 93-kDa band that in turn could be recovered to the 101-kDa form by incubation with HeLa cell S100 fraction, indicating that the 101-kDa form is a phosphorylated type of calnexin-t. The sites of phosphorylation were shown to be restricted to the cytoplasmic domain. Our results suggest that the structure of the ER luminal domain of calnexin-t is likely to differ in middle pachytene versus haploid germ cell phases. In addition, the cytoplasmic domain of calnexin-t was shown to be highly phosphorylated immediately after protein synthesis and constitutively phosphorylated during spermatogenesis.

Characterization and expression of a stage specific antigen by monoclonal antibody TRA 54 in testicular germ cells

To study the mechanism of spermatogenesis, we have isolated many monoclonal antibodies (mAb) which recognize specific steps of mouse germ cell differentiation and then have evaluated the specific expression and characterization of antigenic molecules using immunohistochemistry and Western blotting. Monoclonal antibody TRA 54 recognized specific organelles in germ cell cytoplasm from spermatocytes to spermatids; that is, a large granule was stained in mid-late pachytene, diplotene and secondary spermatocytes and in round spermatids at stage I while the acrosome of spermatids at steps 2-3 to step 12 were also positive. Thereafter, the antigens disappeared from spermatids at more advanced stages of differentiation. Western blots using TRA 54 revealed broad bands with approximate molecular weights of >200, 190 and 85 kDa in the testis. The expression of these antigens during testicular germ cell development should be of interest in relation to the biogenesis of organelles such as the chromatoid body and acrosome and will be a useful stage-specific molecular marker for the study of spermatogenesis.

Apoptosis-like cell death in experimentally-induced cryptorchidism in adult mice

In order to elucidate the mechanism of germ cell degeneration in experimental cryptorchidism, we examined the testes of adult mice from a morphological standpoint. Adult ICR mice were made cryptorchid either unilaterally or bilaterally. In some mice, testes were surgically replaced back into the scrotum at 2 months after induction of cryptorchidism to observe the regenerative process. Morphological changes of cryptorchid and replaced testes have been studied by light and electron microscopy. Testes were also examined by the TUNEL (TdT-mediated dUTP-biotin nick end labelling) method to evaluate whether the degenerative cells, spermatocytes and spermatids, were dying by apoptosis or by any other process(es). At 8 weeks after the induction of cryptorchidism, the seminiferous epithelium consisted only of Sertoli cells, spermatogonia, and some spermatocytes of early meiotic stages. Soon after the replacement of testes to the scrotum, most of the seminiferous tubules resumed spermatogenic processes. Many degenerating cells, especially the spermatocytes, showed condensation of the nucleus and cytoplasm in cryptorchid testes. Although the cytoplasm was markedly eosinophilic under a light microscope to imply condensation of the cytoplasm, the extent of the condensation was not as pronounced under an electron microscope as reported in previous publications. The cytoplasm showed no expansion. By the TUNEL method, many of the degenerating cells, mainly the spermatocytes, have been shown as undergoing apoptosis. These data provide evidence that at least some of the cells die by apoptosis, or by a process similar to apoptosis.