Histone ubiquitination and chromatin remodeling in mouse spermatogenesis

A putative, ubiquitin-dependent mechanism for the recognition and elimination of defective spermatozoa in the mammalian epididymis

The normal structure and function of sperm are prerequisites for successful fertilization and embryonic development, but little is known about how defective sperm are eliminated during mammalian spermatogenesis. Here, we describe a ubiquitin-dependent, sperm quality control mechanism that resides in the mammalian epididymis, the site of sperm maturation and storage. We used immunofluorescence, electron microscopy, western blotting and pulse-chase experiments to show that ubiquitin is secreted by the epididymal epithelium and binds to the surface of defective sperm. Most of the ubiquitinated sperm are subsequently phagocytosed by the epididymal epithelial cells. A portion of defective sperm escapes phagocytosis and can be found in the ejaculate. Cultured epididymal cells maintain their ability to produce ubiquitin and phagocytose the defective sperm, as well as the ubiquitin-coated microspheres, in vitro. The surprising phenomenon of cell-surface ubiquitination in defective sperm provides a possible mechanism for sperm quality control in mammals and a new marker of semen abnormalities in men and animals.

Male fertility defects in mice lacking the serine protease inhibitor protease nexin-1

Understanding infertility and sterility requires knowledge of the molecular mechanisms underlying sexual reproduction. We have found that male mice deficient for the gene encoding the protease inhibitor protease nexin-1 (PN-1) show a marked impairment in fertility from the onset of sexual maturity. Absence of PN-1 results in altered semen protein composition, which leads to inadequate semen coagulation and deficient vaginal plug formation upon copulation. Progressive morphological changes of the seminal vesicles also are observed. Consistent with these findings, abnormal PN-1 expression was found in the semen of men displaying seminal dysfunction. The data demonstrate that the level of extracellular proteolytic activity is a critical element in controlling male fertility.

FOG-2, a novel F-box containing protein, associates with the GLD-1 RNA binding protein and directs male sex determination in the C. elegans hermaphrodite germline

Male sex determination in the Caenorhabditis elegans hermaphrodite germline requires translational repression of tra-2 mRNA by the GLD-1 RNA binding protein. We cloned fog-2 by finding that its gene product physically interacts with GLD-1, forming a FOG-2/GLD-1/tra-2 3′untranslated region ternary complex. FOG-2 has an N-terminal F-box and a novel C-terminal domain called FTH. Canonical F-box proteins act as bridging components of the SCF ubiquitin ligase complex; the N-terminal F-box binds a Skp1 homolog, recruiting ubiquination machinery, while a C-terminal protein-protein interaction domain binds a specific substrate for degradation. However, since both fog-2 and gld-1 are necessary for spermatogenesis, FOG-2 cannot target GLD-1 for ubiquitin-mediated degradation. We propose that FOG-2 also acts as a bridge, bringing GLD-1 bound to tra-2 mRNA into a multiprotein translational repression complex, thus representing a novel function for an F-box protein. fog-2 is a member of a large, apparently rapidly evolving, C. elegans gene family that has expanded, in part, by local duplications; fog-2 related genes have not been found outside nematodes. fog-2 may have arisen during evolution of self-fertile hermaphroditism from an ancestral female/male species.

Structural features of the 26S proteasome complex isolated from rat testis and sperm tail

We have previously cloned a cDNA encoding TBP-1, a protein present in the rat spermatid manchette and outer dense fibers of the developing sperm. TBP-1 contains a heptad repeat of six-leucine zipper fingers at the amino terminus and highly conserved ATPase and DNA/RNA helicase motifs toward the carboxyl terminus. TBP-1 is one of the 20 subunits forming the 19S regulatory complex of the 26S proteasome, an ATP-dependent multisubunit protease found in most eukaryotic cells. We now report the isolation of the 26S proteasome from rat testis and sperm tail and its visualization by whole-mount electron microscopy using negative staining. The 26S proteasome from rat testis was fractionated by Sephacryl S-400/Mono-Q chromatography using homogenates suspended in a 10% glycerol-supplemented buffer. Chromatographic fractions were analyzed by immunoblotting using a specific anti-TBP-1 serum. During the purification of Sak57, a keratin filament present in outer dense fibers from epididymal sperm, we detected a substantial amount of 26S proteasomes. Intact 26S proteasomes from rat testis display a rod-shaped particles about 45 nm in length and 11-17 nm in diameter. Each particle consists of a 20S barrel-shaped component formed by four rings (alphabetabetaalpha), capped by two polar 19S regulatory complexes, each identified by an element known as the "Chinese dragon head motif". TBP-1 is an ATPase-containing subunit of the 19S regulatory cap. Rat sperm preparations displayed both dissociated 26S proteasomes and Sak57 filaments. We hypothesize that 26S proteasomes in the perinuclear-arranged manchette are in a suitable location for recognition, sequestration, and degradation of accumulating ubiquitin-conjugated somatic and transient testis-specific histones during spermiogenesis. In the sperm tail, the 26S proteasome may have a role in the remodeling of the outer dense fibers and other tail components during epididymal transit.

Specific aspects of the ubiquitin system in spermatogenesis

The ubiquitin system is involved in numerous cellular processes, regulating the amounts and/or activities of specific proteins through posttranslational coupling with ubiquitin or ubiquitin-like proteins. In spermatogenesis, there appears to be a special requirement for certain components of the ubiquitin system, as exemplified in human and mouse by mutation of USP9Y and HR6B, respectively. Both genes encode proteins which take part in the ubiquitin system and are ubiquitously expressed, but their mutation generates no apparent phenotype other than male infertility. Different phases of mammalian spermatogenesis probably require different specialized activities of the ubiquitin system. It is anticipated that ubiquitination activities similar to those required during mitotic cell cycle regulation will play some role in control of the meiotic divisions. In spermatocytes, there is an intricate link among DNA repair, the ubiquitin system, and regulation of meiotic chromatin structure, as indicated by the co-localization of proteins involved in these processes on meiotic recombination complexes. HR6B and its nearly identical homolog HR6A are multiple function proteins, with ubiquitin-conjugating activity and essential roles in post-replication DNA repair. HR6B, possibly together with the ubiquitin-ligating enzyme mRAD1 8Sc, is most likely involved in chromatin re-organization during the meiotic and post-meiotic phases of spermatogenesis. Biochemical data indicate that, in particular during spermiogenesis, the general activity of the ubiquitin system is high, which most likely is related to the high requirement for massive breakdown of cytoplasmatic and nuclear proteins during this last phase of spermatogenesis.

Molecular and cellular mechanisms in spermatogenesis

Mammalian spermatogenesis shows a strict control of many specific molecular and cellular events. This control involves Sertoli cell-germ cell interaction, as well as a programmed performance of changes in chromatin structure and gene expression in the developing germ cells. In recent years, much knowledge about the functions of defined genes in spermatogenesis has been gained by making use of mouse transgenic and gene knockout models. Several of these models are discussed in this brief overview, with an emphasis on genes encoding proteins involved in the control of gene transcription, mRNA translation, DNA repair and protein ubiquitination. A better understanding of the molecular and cellular biology of spermatogenesis in the mouse may provide concepts that can improve our understanding of human male infertility and may also lead to the identification of novel targets for contraceptive intervention.

Histone macroH2A1.2 is concentrated in the XY-body by the early pachytene stage of spermatogenesis

The pairing of sex chromosomes during meiosis in male mammals is associated with ongoing heterochromatinization and X inactivation. This process occurs in a specific area of the nucleus that can be discerned morphologically: the sex vesicle or XY-body. In contrast to X inactivation in the somatic cells of female mammals the reasons for X inactivation in the male germline remain obscure. We have recently demonstrated that the inactive X chromosome in somatic cells of female mammals is marked by a high concentration of histone macroH2A. Here we investigate X inactivation in the meiotic cells of the male germline. We demonstrate here that macroH2A1.2 is present in the nuclei of germ cells starting first with localization that is largely, if not exclusively, to the developing XY-body in early pachytene spermatocytes. Our results suggest that inactivation of sex chromosomes in the male germ cell includes a major alteration of the nucleosomal structure.

Ubiquitinated sperm mitochondria, selective proteolysis, and the regulation of mitochondrial inheritance in mammalian embryos

The strictly maternal inheritance of mitochondria and mitochondrial DNA (mtDNA) in mammals is a developmental paradox promoted by an unknown mechanism responsible for the destruction of the sperm mitochondria shortly after fertilization. We have recently reported that the sperm mitochondria are ubiquitinated inside the oocyte cytoplasm and later subjected to proteolysis during preimplantation development (P. Sutovsky et al., Nature 1999; 402:371-372). Here, we provide further evidence for this process by showing that the proteolytic destruction of bull sperm mitochondria inside cow egg cytoplasm depends upon the activity of the universal proteolytic marker, ubiquitin, and the lysosomal apparatus of the egg. Binding of ubiquitin to sperm mitochondria was visualized by monospecific antibodies throughout pronuclear development and during the first embryonic divisions. The recognition and disposal of the ubiquitinated sperm mitochondria was prevented by the microinjection of anti-ubiquitin antibodies and by the treatment of the fertilized zygotes with lysosomotropic agent ammonium chloride. The postfecundal ubiquitination of sperm mitochondria and their destruction was not seen in the hybrid embryos created using cow eggs and sperm of wild cattle, gaur, thus supporting the hypothesis that sperm mitochondrion destruction is species specific. The initial ligation of ubiquitin molecules to sperm mitochondrial membrane proteins, one of which could be prohibitin, occurs during spermatogenesis. Even though the ubiquitin cross-reactivity was transiently lost from the sperm mitochondria during epididymal passage, likely as a result of disulfide bond cross-linking, it was restored and amplified after fertilization. Ubiquitination therefore may represent a mechanism for the elimination of paternal mitochondria during fertilization. Our data have important implications for anthropology, treatment of mitochondrial disorders, and for the new methods of assisted procreation, such as cloning, oocyte cytoplasm donation, and intracytoplasmic sperm injection.

Proteasomes in human spermatozoa

In the present study we describe the localization of proteasomes in human spermatozoa by means of immunolabelling with different monoclonal and polyclonal antibodies detected by confocal microscopy. Western blotting confirmed the specificity of the antibodies and has shown that proteasomes are present in spermatozoa and in seminal fluid. In spermatozoa proteasomes are concentrated in the neck region where the centrioles are located. Some labelling was also detected at the periphery of the head, but no proteasomal antigens were detected in either the nucleus or associated with the flagellum. Proteasome inhibitors did not affect the motility of the spermatozoa, acrosome reaction nor zona binding. It is hypothesized that paternal proteasomes enter the oocyte during fertilization in tight association with the centrioles and may serve a special function during further development which can be associated with the function of a hypothetical proteolysis centre.

Meiotic chromosomes: integrating structure and function

Meiotic chromosomes have been studied for many years, in part because of the fundamental life processes they represent, but also because meiosis involves the formation of homolog pairs, a feature which greatly facilitates the study of chromosome behavior. The complex events involved in homolog juxtaposition necessitate prolongation of prophase, thus permitting resolution of events that are temporally compressed in the mitotic cycle. Furthermore, once homologs are paired, the chromosomes are connected by a specific structure: the synaptonemal complex. Finally, interaction of homologs includes recombination at the DNA level, which is intimately linked to structural features of the chromosomes. In consequence, recombination-related events report on diverse aspects of chromosome morphogenesis, notably relationships between sisters, development of axial structure, and variations in chromatin status. The current article reviews recent information on these topics in an historical context. This juxtaposition has suggested new relationships between structure and function. Additional issues were addressed in a previous chapter (551).

Rad6-dependent ubiquitination of histone H2B in yeast

Although ubiquitinated histones are present in substantial levels in vertebrate cells, the roles they play in specific biological processes and the cellular factors that regulate this modification are not well characterized. Ubiquitinated H2B (uH2B) has been identified in the yeast Saccharomyces cerevisiae, and mutation of the conserved ubiquitination site is shown to confer defects in mitotic cell growth and meiosis. uH2B was not detected in rad6 mutants, which are defective for the ubiquitin-conjugating enzyme Ubc2, thus identifying Rad6 as the major cellular activity that ubiquitinates H2B in yeast.

Novel human and mouse homologs of Saccharomyces cerevisiae DNA polymerase eta

The Saccharomyces cerevisiae RAD30 gene encodes a novel eukaryotic DNA polymerase, pol eta that is able to replicate across cis-syn cyclobutane pyrimidine dimers both accurately and efficiently. Very recently, a human homolog of RAD30 was identified, mutations in which result in the sunlight-sensitive, cancer-prone, Xeroderma pigmentosum variant group phenotype. We report here the cloning and localization of a second human homolog of RAD30. Interestingly, RAD30B is localized on chromosome 18q21.1 in a region that is often implicated in the etiology of many human cancers. The mouse homolog (Rad30b) is located on chromosome 18E2. The human RAD30B and mouse Rad30b mRNA transcripts, like many repair proteins, are highly expressed in the testis. In situ hybridization analysis indicates that expression of mouse Rad30b occurs predominantly in postmeiotic round spermatids. Database searches revealed genomic and EST sequences from other eukaryotes such as Aspergillus nidulans, Schizosaccharomyces pombe, Brugia malayi, Caenorhabditis elegans, Trypanosoma cruzi, Arabidopsis thaliana, and Drosophila melanogaster that also encode putative homologs of RAD30, thereby suggesting that Rad30-dependent translesion DNA synthesis is conserved within the eukaryotic kingdom.

The ubiquitin system in gametogenesis

Ubiquitin is a ubiquitous and highly conserved protein of 76 amino acid residues, that can be covalently attached to cellular acceptor proteins. The attachment of ubiquitin to target proteins is achieved through a multi-step enzymatic pathway, which involves activities of ubiquitin-activating E1 enzymes, ubiquitin-conjugating E2 enzymes, and ligating E3 enzymes. Mono- or poly-ubiquitination of proteins can lead to protein degradation or modification of protein activity. Many components of the complex ubiquitin system show remarkable evolutionary conservation, from yeast to mammalian species. The ubiquitin system is essential to all eukaryotic cells. Among others, several signal transduction cascades show involvement of the ubiquitin system, but there are currently little data supporting a specific role of the ubiquitin system in hormonal control of reproduction. Interestingly, during gametogenesis, many specialized and important aspects of the ubiquitin system become apparent. Components of the ubiquitin system appear to be involved in different steps and processes during gametogenesis, including control of meiosis, and reorganization of chromatin structure.