Characterization of nuclear pore distribution in freeze-fracture replicas of seminiferous tubules isolated by transillumination

Involvement of classical bipartite/karyopherin nuclear import pathway components in acrosomal trafficking and assembly during bovine and murid spermiogenesis

This study arose from our finding that SubH2Bv, a histone H2B variant residing in the subacrosomal compartment of mammalian spermatozoa, contains a bipartite nuclear localization signal (bNLS) but in spite of this did not enter the spermatid nucleus. Instead, it associated with proacrosomic and acrosomic vesicles, which were targeted to the nuclear surface to form the acrosome. On this basis we proposed that SubH2Bv targets proacrosomic/acrosomic vesicles from the Golgi apparatus to the nuclear envelope by utilizing the classical bipartite/karyopherin alpha (KPNA) nuclear import pathway. To test the protein's nuclear targeting ability, SubH2Bv, with and without targeted mutations of the basic residues of bNLS, as well as bNLS alone, were transfected into mammalian cells as GFP-fusion proteins. Only the intact bNLS conferred nuclear entry. Subsequently, we showed that a KPNA, most likely KPNA6, occupies the same sperm head compartment and follows the same pattern of acrosomal association during spermiogenesis as SubH2Bv. Sperm head fractionation combined with Western blotting located this KPNA to the subacrosomal layer of the perinuclear theca, while immunocytochemistry of testicular sections showed that it associates with the surface of proacrosomic/acrosomic vesicles during acrosomal biogenesis. The identical sperm-localization and testicular-expression patterns between KPNA and SubH2Bv suggested a potential binding interaction between these proteins. This was supported by recombinant SubH2Bv affinity pull-down assays on germ cell extracts. The results of this study provide a compelling argument that these two nuclear homing proteins work in concert to direct the acrosomic vesicle to the nucleus. Their final residence in the subacrosomal layer of the perinuclear theca of spermatozoa indicates a role for SubH2Bv and KPNA in acrosomal-nuclear docking.

Regulated nucleocytoplasmic transport during gametogenesis

Gametogenesis is the process by which sperm or ova are produced in the gonads. It is governed by a tightly controlled series of gene expression events, with some common and others distinct for males and females. Nucleocytoplasmic transport is of central importance to the fidelity of gene regulation that is required to achieve the precisely regulated germ cell differentiation essential for fertility. In this review we discuss the physiological importance for gamete formation of the molecules involved in classical nucleocytoplasmic protein transport, including importins/karyopherins, Ran and nucleoporins. To address what functions/factors are conserved or specialized for these developmental processes between species, we compare knowledge from mice, flies and worms. The present analysis provides evidence of the necessity for and specificity of each nuclear transport factor and for nucleoporins during germ cell differentiation. This article is part of a Special Issue entitled: Nuclear Transport and RNA Processing.

P granules in the germ cells of Caenorhabditis elegans adults are associated with clusters of nuclear pores and contain RNA

The germ cells, and germ cell precursors, in the nematode Caenorhabditis elegans contain distinctive granules called P granules. During early embryogenesis, P granules are segregated asymmetrically into those blastomeres that eventually produce the germ line. Because of the correlation between P granule distribution and the development of the germ line, P granules are widely thought to function in some aspect of germ line specification or differentiation. Most of the analysis of P granule structure and localization has focused on the early embryo, when P granules are located in the cytoplasm. However, during most of development P granules are associated with germ cell nuclei. We report here an ultrastructural analysis of the nuclear-associated P granules in the germ cells of the adult hermaphrodite gonad. We show that P granules are tightly associated with nuclear pores and that the positions of certain structures within the P granules correspond to the positions of pores on the nuclear envelope. We present immunocytochemical and ultrastructural data suggesting that P granules can associate, or remain associated, with pore-like structures even after they detach from the nuclear envelope during oogenesis. Finally, we show that nuclear-associated P granules in the gonad contain RNA, complementing previous studies showing that cytoplasmic P granules in embryos contain RNA.

Sertoli cell nuclear pore number changes in some stages of the spermatogenic cycle of the rat seminiferous epithelium

In an earlier study we described changes in the number and distribution of nuclear pores during maturation of germ cells at given stages of the spermatogenic cycle; these changes were related to the activity of nucleus-cytoplasm transport. Similarly, the present work was performed by combining freeze-fracture techniques with Sertoli nuclei identification criteria, using fragments of tubules isolated by transillumination under stereomicroscopy. We studied the density of nuclear pores in freeze-fracture replicas of the Sertoli nuclear envelope at stages XIII-XIV-I compared with stages IX-XII. Pore counts were carried out on photographs of the platinum replicas using a digitalized morphometric board. The results were statistically analyzed using Student's t test. The difference in density (pore number/micron2 +/- SEM) was significant between stages IX-XII (8.25 +/- 0.63) and XIII-XIV-I (10.80 +/- 0.60). We postulate that this density appears to be increased at the time of increased metabolic requirements of the Sertoli cell.

cDNA cloning and characterization of Npap60: a novel rat nuclear pore-associated protein with an unusual subcellular localization during male germ cell differentiation

We have cloned and characterized a cDNA, Npap60, encoding a rat nuclear pore-associated protein. The 3-kb cDNA was obtained by antibody screening of a rat testis expression library. The predicted NPAP60 contains 381 amino acids with a composition of 25.6% charged residues and is highly hydrophilic. The Npap60 gene appears to be conserved in mouse, rat, and human. Immunofluorescence studies with anti-NPAP60 fusion protein antibody show that the NPAP60 protein colocalizes with nuclear pore complexes in RAT1A cells. The expression of Npap60 is about 10-20 times higher in rat testis than in somatic tissues. The subcellular localization of NPAP60 protein changes dramatically during male germ cell differentiation, from nuclear pore complex-like staining in spermatocytes to whole nucleus staining in spermatids and finally to a nuclear surface staining in mature spermatozoa. These changes are temporally and spatially related to nuclear reorganization during male germ cell differentiation.

Extractions reveal specific argentophilic proteins in rat and bull sperm heads

BACKGROUND

Silver-stainability (argentophilia) of cytoplasmic structures occurring in spermatids have been localized into the organizing perinuclear theca, but the biochemical nature and structural associations of these proteins with the cytoskeletal and membranous elements are unresolved and, therefore, were the aim of the present study.

METHODS

Light and electron microscopic analysis of the silver-stainability in the rat spermatids and spermatozoa was carried out in the intact testis tissue and epididymal spermatozoa and after their chemical and mechanical extraction. Correlation of argentophilia with specific proteins of rat and bovine spermatids and spermatozoa was investigated using a recently developed technique for silver nitrate staining of proteins on nitrocellulose.

RESULTS

Sequential formation of the silver-stainable domains seemed to proceed from the argentophilic acrosomal ring. Various extractions indicated that argentophilia in the spermatids and spermatozoa was mainly associated with the perinuclear theca and to some extent to the plasma membrane. Hyamine-soluble extract from spermatozoa of rat and bull revealed only a single argentophilic protein of 130 kDa. Hyamine and SDS-soluble extracts of rat testis tissue contained an additional group of argentophilic polypeptides of lower molecular weight (115, 94, 36, 23, and 21 kDa).

CONCLUSIONS

Reduction in the number of argentophilic proteins appears to be involved in a series of changes in the cyto-architecture of developing spermatids. Tentative cytoskeletal nature of argentophilic proteins remains to be identified. Nevertheless, they may have important physical relations with the higher-order organization of the sperm head cytoskeleton and overlying membranes.