Inactivation of the HR6B ubiquitin-conjugating DNA repair enzyme in mice causes male sterility associated with chromatin modification

Cadmium-induced activation of stress signaling pathways, disruption of ubiquitin-dependent protein degradation and apoptosis in primary rat Sertoli cell-gonocyte cocultures

Cadmium (Cd) is a ubiquitous environmental pollutant that has been associated with male reproductive toxicity in both humans and animal models. The underlying mechanism of this response, however, is still uncharacterized. To address this issue, we employed a recently developed and optimized three-dimensional primary Sertoli cell-gonocyte coculture system and examined the time- and dose-dependent effects of Cd on morphological alterations, cell viability, activation of stress signaling pathway proteins, and the disruption of the ubiquitin proteasome system (UPS). Our results demonstrated that Cd exposure lead to time- and dose-dependent morphological changes that are associated with the induction of apoptosis. In response to Cd, we also saw a disruption of the UPS as evaluated through the accumulation of high-molecular weight polyubiquitinated proteins (HMW-polyUb) as well as alterations in proteasome activity. Robust activation of cellular stress response, measured through the increased phosphorylation of stress-activated protein kinase/c-jun N-terminal kinase and p38, paralleled the accumulation of HMW-polyUb. In addition, p53, a key regulatory protein, was upregulated and underwent increased ubiquitination in response to Cd. To further characterize the role of the UPS in Cd cellular response, we compared the above changes with two classic proteasomal inhibitors, lactacystin, and MG132. The stress response and the accumulation of HWM-polyUb induced by Cd were consistent with the response seen with MG132 but not with lactacystin. In addition, Cd treatment resulted in a dose- and time-dependent effect on proteasome activity, but the overall Cd-induced proteasomal inhibition was unique as compared to MG132 and lactacystin. Taken together, our studies further characterize Cd-induced in vitro testicular toxicity and highlight the potential role of the UPS in this response.

Histone ubiquitination: triggering gene activity

Recently, many of the enzymes responsible for the addition and removal of ubiquitin from the histones H2A and H2B have been identified and characterized. From these studies, it has become clear that H2A and H2B ubiquitination play critical roles in regulating many processes within the nucleus, including transcription initiation and elongation, silencing, and DNA repair. In this review, we present the enzymes involved in H2A and H2B ubiquitination and discuss new evidence that links histone ubiquitination to other chromatin modifications, which has provided a model for the role of H2B ubiquitination, in particular, in transcription initiation and elongation.

Regulated expression of the ubiquitin protein ligase, E3(Histone)/LASU1/Mule/ARF-BP1/HUWE1, during spermatogenesis

A ubiquitin protein ligase (E3), E3(Histone)/LASU1 (Mule/ARF-BP1/HUWE1), was recently identified that mediates ubiquitination of core histones, the Mcl-1 anti-apoptotic protein, and the p53 tumor suppressor protein. However, the expression of E3(Histone)/LASU1 remains poorly studied. Because we identified E3(Histone)/LASU1 from the testis, we explored its regulation during spermatogenesis. In the first wave of rat spermatogenesis, E3(Histone)/LASU1 mRNA and protein had peak expression at days 10 and 20, respectively, and decreased with age. Consistent with these findings, immunohistochemistry revealed that E3(Histone)/LASU1 was highly expressed in nuclei from spermatogonia to mid-pachytene spermatocytes. There was no obvious staining in spermatids, when histones are ubiquitinated and degraded. E3(Histone)/LASU1 was also expressed in other tissues. However, except in neuronal cells of the brain, expression was cytoplasmic. Thus, E3(Histone)/LASU1 may play a role in chromatin modification in early germ cells of the testis, but also has functions in other tissues.

Roles and potential therapeutic targets of the ubiquitin proteasome system in muscle wasting

Muscle wasting, characterized by the loss of protein mass in myofibers, is in most cases largely due to the activation of intracellular protein degradation by the ubiquitin proteasome system (UPS). During the last decade, mechanisms contributing to this activation have been unraveled and key mediators of this process identified. Even though much remains to be understood, the available information already suggests screens for new compounds inhibiting these mechanisms and highlights the potential for pharmaceutical drugs able to treat muscle wasting when it becomes deleterious. This review presents an overview of the main pathways contributing to UPS activation in muscle and describes the present state of efforts made to develop new strategies aimed at blocking or slowing muscle wasting.

Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; ).

The mammalian N-end rule pathway: new insights into its components and physiological roles

The N-end rule pathway is a ubiquitin-dependent proteolytic system, in which destabilizing N-terminal residues of short-lived proteins function as an essential determinant of an N-terminal degradation signal (N-degron). An N-degron can be created from a pre-N-degron through specific N-terminal modifications, providing a means conditionally to destabilize otherwise stable polypeptides. The pathway has been found in all organisms examined, from prokaryotes to eukaryotes. Recent biochemical and proteomic studies identified many components of the mammalian N-end rule pathway, including a family of substrate recognition ubiquitin ligases and their substrates. The genetic dissection in animals and humans revealed its essential role in various vital physiological processes, ranging from cardiovascular development and meiosis to the pathogenesis of human genetic diseases. These discoveries have provided new insights into the components, functions and mechanics of this unique proteolytic system.

UBPy/MSJ-1 system during male germ cell progression in the frog, Rana esculenta

mUBPy (mouse ubiquitin specific processing protease) is a de-ubiquitinating enzyme expressed in mouse testis and brain. In testis, it interacts with the DnaJ protein MSJ-1 (mouse sperm cell specific DnaJ first homologue), a molecular chaperone expressed in spermatids and spermatozoa. Since MSJ-1 is conserved among vertebrates, to demonstrate an evolutionarily conserved function of UBPy/MSJ-1 system, we assayed mUBPy presence in the anuran amphibian, the frog, Rana esculenta, during the annual sexual cycle. By Western blot we have detected a specific signal of 126kDa in testis and isolated spermatozoa. During the annual sexual cycle, the signal gradually increases as soon as spermatogenesis resumes after the winter stasis. Using immunocytochemistry, we have localized the protein in spermatids and spermatozoa. In conclusion, UBPy/MSJ-1 system is available in R. esculenta testis suggesting a conserved fundamental function in spermatogenesis and sperm formation.

Increased phosphorylation and dimethylation of XY body histones in the Hr6b-knockout mouse is associated with derepression of the X chromosome

Mono-ubiquitylated H2A marks the transcriptionally silenced XY body during male meiotic prophase. Concomitant with H2AK119ub1, the ubiquitin-conjugating enzyme HR6B is also enriched on the XY body. We analyzed H2A and H2B ubiquitylation in Hr6b-knockout mouse spermatocytes, but no global changes were detected. Next, we analyzed phosphorylation of the threonine residues T120 and T119 that are adjacent to the K119 and K120 target sites for ubiquitylation in H2A and H2B, respectively. In wild-type cells, H2AT120ph and H2BT119ph mark meiotically unpaired and silenced chromatin, including the XY body. In Hr6b-knockout spermatocytes, the H2BT119ph signal was unchanged, but H2AT120ph was enhanced from late pachytene until metaphase I. Furthermore, we found increased H3K4 dimethylation on the X and Y chromosomes of diplotene Hr6b-knockout spermatocytes, persisting into postmeiotic round spermatids. In these cells, the X and Y chromosomes maintained an unchanged H3K9m2 level, even when this modification was lost from centromeric heterochromatin. Analysis of gene expression showed derepression of X chromosome genes in postmeiotic Hr6b-knockout spermatids. We conclude that HR6B exerts control over different histone modifications in spermatocytes and spermatids, and that this function contributes to the postmeiotic maintenance of X chromosome silencing.

Transition from a nucleosome-based to a protamine-based chromatin configuration during spermiogenesis inDrosophila

In higher organisms, the chromatin of sperm is organised in a highly condensed protamine-based structure. In pre-meiotic stages and shortly after meiosis, histones carry multiple modifications. Here, we focus on post-meiotic stages and show that also after meiosis, histone H3 shows a high overall methylation of K9 and K27 and we hypothesise that these modifications ensure maintenance of transcriptional silencing in the haploid genome. Furthermore, we show that histones are lost during the early canoe stage and that just before this stage, hyper-acetylation of histone H4 and mono-ubiquitylation of histone H2A occurs. We believe that these histone modifications within the histone-based chromatin architecture may lead to better access of enzymes and chromatin remodellers. This notion is supported by the presence of the architectural protein CTCF, numerous DNA breaks, SUMO, UbcD6 and high content of ubiquitin, as well as testes-specific nuclear proteasomes at this time. Moreover, we report the first transition protein-like chromosomal protein, Tpl94D, to be found in Drosophila. We propose that Tpl94D – an HMG box protein – and the numerous DNA breaks facilitate chromatin unwinding as a prelude to protamine and Mst77F deposition. Finally, we show that histone modifications and removal are independent of protamine synthesis.

Essential role of T-cell factor/beta-catenin in regulation of Rad6B: a potential mechanism for Rad6B overexpression in breast cancer cells

We have previously shown that the postreplication DNA repair gene Rad6B plays a critical role in the maintenance of genomic integrity of human breast cells. Whereas normal breast cells express low levels of Rad6B, increases in Rad6B expression occur in hyperplasia with overexpression in breast carcinomas. Here, we show that the human Rad6B gene is a transcriptional target of T-cell factor (TCF)-4/beta-catenin/p300. Rad6B promoter activity is subject to negative regulation in normal human MCF10A breast cells whereas it is constitutively active in metastatic MDA-MB-231 breast cancer cells. Derepression and activation of Rad6B promoter in MCF10A cells require coexpression of beta-catenin and p300. Using electrophoresis mobility shift assay, Western blot analysis of electrophoresis mobility shift assay, UV cross-linking, and chromatin immunoprecipitation assay, we show that Rad6B transcriptional repression in MCF10A cells is due to paucity of transcriptionally active beta-catenin assembled on the TCF binding sequence in the Rad6B promoter rather than to a deficit/decreased affinity of TCF-4 for the TCF binding element in Rad6B promoter. Three-dimensional epithelial acini generated in vitro from MCF10A cells cotransfected with beta-catenin and p300 showed beta-catenin expression on the membrane, cytoplasm, and/or nuclei with concomitant Rad6 overexpression, whereas control acini showed beta-catenin on the membranes and negligible Rad6 expression. Immunohistochemical analysis of 12 breast carcinomas showed an approximately 80% correlation between Rad6 and beta-catenin expression, and combined nuclear and cytoplasmic staining of beta-catenin and Rad6 was detected in 25% of the breast carcinomas. In vivo implantation of MCF10A-Rad6B cells produced hyperplastic lesions. These data reveal a potentially important role for transcriptionally active beta-catenin in the regulation of Rad6B gene expression, and link aberrant beta-catenin signaling with transcriptional deregulation of Rad6B and breast cancer development.

DNA repair in mammalian embryos

Mammalian cells have developed complex mechanisms to identify DNA damage and activate the required response to maintain genome integrity. Those mechanisms include DNA damage detection, DNA repair, cell cycle arrest and apoptosis which operate together to protect the conceptus from DNA damage originating either in parental gametes or in the embryo's somatic cells. DNA repair in the newly fertilized preimplantation embryo is believed to rely entirely on the oocyte's machinery (mRNAs and proteins deposited and stored prior to ovulation). DNA repair genes have been shown to be expressed in the early stages of mammalian development. The survival of the embryo necessitates that the oocyte be sufficiently equipped with maternal stored products and that embryonic gene expression commences at the correct time. A Medline based literature search was performed using the keywords 'DNA repair' and 'embryo development' or 'gametogenesis' (publication dates between 1995 and 2006). Mammalian studies which investigated gene expression were selected. Further articles were acquired from the citations in the articles obtained from the preliminary Medline search. This paper reviews mammalian DNA repair from gametogenesis to preimplantation embryos to late gestational stages.

Differential Expression of Genes Encoding Constitutive and Inducible 20S Proteasomal Core Subunits in the Testis and Epididymis of Theophylline- or 1,3-Dinitrobenzene-Exposed Rats1

Theophylline (THP) and 1,3-dinitrobenzene (DNB) are thought to induce infertility by incapacitating the nurturing Sertoli cells and causing germ cell apoptosis in the testicular seminiferous epithelium, respectively. We hypothesized that THP and DNB exposure would alter the expression of the genes within the ubiquitin-proteasome pathway (UPP), implicated in spermatogenesis and epididymal sperm quality control. Rats were fed 0 or 8000 ppm of THP and necropsied on Days 18, 30, and 42 or administered 0, 2, or 6 mg/kg DNB via oral gavage and necropsied on Day 7. Tissues were collected from the testis and the caput, corpus, and cauda regions of the epididymis for transcriptional profiling by semiquantitative RT-PCR, real-time RT-PCR, and histopathology. Target UPP genes included those encoding for constitutive the 20S proteasomal core subunits Psmb1 (beta1), Psmb2 (beta2), and Psmb5 (beta5); the inducible 20S core subunits Psmb9 (LMP2), Psmb8 (LMP7), and Psmb10 (LMP10); and Ube1 (ubiquitin-activating enzyme E1), Ube2d3 (ubiquitin-conjugating enzyme E2), and Uchl1 (ubiquitin C-terminal hydrolase PGP9.5). Spermatozoa were collected from the cauda epididymis for analysis by light microscopy and flow cytometric evaluation of sperm surface ubiquitin. These data show that reprotoxic exposure alters the tissue-specific expression of UPP genes in the testis and epididymis, which may contribute to the aberrant spermatogenesis and epididymal processing of both normal and defective spermatozoa. Transcriptional profiling and flow cytometric analysis of the UPP thus captures the prodromal effects of reproductive toxicity not captured by conventional histology and functional cytology. Complementing seminal analysis with these measures may be useful in screening drug-induced toxicity or environmental infertility.

Poly(ADP-ribose) polymerase-2 contributes to the fidelity of male meiosis I and spermiogenesis

Besides the established central role of poly(ADP-ribose) polymerase-1 (Parp-1) and Parp-2 in the maintenance of genomic integrity, accumulating evidence indicates that poly(ADP-ribosyl)ation may modulate epigenetic modifications under physiological conditions. Here, we provide in vivo evidence for the pleiotropic involvement of Parp-2 in both meiotic and postmeiotic processes. We show that Parp-2-deficient mice exhibit severely impaired spermatogenesis, with a defect in prophase of meiosis I characterized by massive apoptosis at pachytene and metaphase I stages. Although Parp-2(-/-) spermatocytes exhibit normal telomere dynamics and normal chromosome synapsis, they display defective meiotic sex chromosome inactivation associated with derailed regulation of histone acetylation and methylation and up-regulated X- and Y-linked gene expression. Furthermore, a drastically reduced number of crossover-associated Mlh1 foci are associated with chromosome missegregation at metaphase I. Moreover, Parp-2(-/-) spermatids are severely compromised in differentiation and exhibit a marked delay in nuclear elongation. Altogether, our findings indicate that, in addition to its well known role in DNA repair, Parp-2 exerts essential functions during meiosis I and haploid gamete differentiation.

Replication-dependent and -independent responses of RAD18 to DNA damage in human cells

Postreplication repair facilitates tolerance of DNA damage during replication, overcoming termination of replication at sites of DNA damage. A major post-replication repair pathway in mammalian cells is translesion synthesis, which is carried out by specialized polymerase(s), such as polymerase eta, and is identified by focus formation by the polymerase after irradiation with UVC light. The formation of these foci depends on RAD18, which ubiquitinates PCNA for the exchange of polymerases. To understand the initial processes in translesion synthesis, we have here analyzed the response to damage of RAD18 in human cells. We find that human RAD18 accumulates very rapidly and remains for a long period of time at sites of different types of DNA damage, including UVC light-induced lesions, and x-ray microbeam- and laser-induced single-strand breaks, in a cell cycle-independent manner. The accumulation of RAD18 at DNA damage is observed even when DNA replication is inhibited, and a small region containing a zinc finger motif located in the middle of RAD18 is essential and sufficient for the replication-independent damage accumulation. The zinc finger motif of RAD18 is not necessary for UV-induced polymerase eta focus formation, but another SAP (SAF-A/B, Acinus and PIAS) motif near the zinc finger is required. These data indicate that RAD18 responds to DNA damage in two distinct ways, one replication-dependent and one replication-independent, involving the SAP and zinc finger motifs, respectively.

Spermatozoal RNA: why is it there and what does it do?

The mammalian male gamete is transcriptionally silent as a consequence of the highly condensed architecture of its chromatin and there is also little or no cytoplasm capable of supporting translation; however, we now understand that these cells carry a full complement of mRNAs and that under certain conditions, spermatozoa can translate their mRNAs de novo. Moreover, there is now good evidence that spermatozoal RNA can potentially affect phenotypic traits in offspring. This epigenetic phenomenon may involve the transmission of extra-chromosomal episomal elements. These data indicate that the spermatozoon is well matched to its role of delivering the paternal genome, but that additional male-benefits may also be accrued by the delivery of spermatozoal RNA.

UBE2A, which encodes a ubiquitin-conjugating enzyme, is mutated in a novel X-linked mental retardation syndrome

We report a mutation of UBE2A/HR6A, which encodes a ubiquitin-conjugating enzyme (E2), a member of the ubiquitin proteasome pathway, as the cause of a novel X-linked mental retardation (XLMR) syndrome that affects three males in a two-generation family. A single-nucleotide substitution, c.382C-->T in UBE2A, led to a premature UAG stop codon (Q128X). As a consequence, the predicted polypeptide lacks the 25 C-terminal amino acid residues. The importance of this terminal sequence for UBE2 function is inferred by its conservation in vertebrates and in Drosophila. UBE2A mutations do not appear to significantly contribute to XLMR, since no UBE2A mutations were identified in 15 families with nonsyndromic and 4 families with syndromic idiopathic XLMR previously mapped to intervals encompassing this gene. This is the first description of a mutation in a ubiquitin-conjugating enzyme gene as the cause of a human disease.

Mutations that affect meiosis in male mice influence the dynamics of the mid-preleptotene and bouquet stages

Meiosis pairs and segregates homologous chromosomes and thereby forms haploid germ cells to compensate the genome doubling at fertilization. Homologue pairing in many eukaryotic species depends on formation of DNA double strand breaks (DSBs) during early prophase I when telomeres begin to cluster at the nuclear periphery (bouquet stage). By fluorescence in situ hybridization criteria, we observe that mid-preleptotene and bouquet stage frequencies are altered in male mice deficient for proteins required for recombination, ubiquitin conjugation and telomere length control. The generally low frequencies of mid-preleptotene spermatocytes were significantly increased in male mice lacking recombination proteins SPO11, MEI1, MLH1, KU80, ubiquitin conjugating enzyme HR6B, and in mice with only one copy of the telomere length regulator Terf1. The bouquet stage was significantly enriched in Atm(-/-), Spo11(-/-), Mei1(m1Jcs/m1Jcs), Mlh1(-/-), Terf1(+/-) and Hr6b(-/-) spermatogenesis, but not in mice lacking recombination proteins DMC1 and HOP2, the non-homologous end-joining DNA repair factor KU80 and the ATM downstream effector GADD45a. Mice defective in spermiogenesis (Tnp1(-/-), Gmcl1(-/-), Asm(-/-)) showed wild-type mid-preleptotene and bouquet frequencies. A low frequency of bouquet spermatocytes in Spo11(-/-)Atm(-/-) spermatogenesis suggests that DSBs contribute to the Atm(-/-)-correlated bouquet stage exit defect. Insignificant changes of bouquet frequencies in mice with defects in early stages of DSB repair (Dmc1(-/-), Hop2(-/-)) suggest that there is an ATM-specific influence on bouquet stage duration. Altogether, it appears that several pathways influence telomere dynamics in mammalian meiosis.

Towards a better understanding of RNA carriage by ejaculate spermatozoa

Research on spermatozoal RNA has made considerable progress since the original reports on its presence appeared in the late 1950s and early 1960s. Through the use of stringent procedures aimed at eliminating contamination artefacts, we now appreciate that a complex cohort of mRNAs persists in the ejaculate cell but that 80S (cytoplasmic) ribosomal complexes are not present in sufficient quantities to support cytoplasmic mRNA translation. Despite this, under certain conditions, at least some cytoplasmic mRNAs can apparently be translated de novo, possibly on mitochondrial polysomes. The detection of mRNA translation by mature spermatozoa essentially supports the earliest research reports on spermatozoal gene expression although the suggested relationship with protein turnover and capacitation is wholly unexpected. We also examine some alternative explanations and roles for RNA carriage, including the RNAs passive retention as a consequence of nuclear shutdown and a more active role in chromatin repackaging, genomic imprinting, gene silencing and post-fertilization requirements of essential paternal RNAs. The recent report of an RNA-mediated epigenetic alteration to phenotype that is likely to be sperm derived is of particular interest in this regard. We finally show that regardless of the biological role(s) of spermatozoal RNA, its utility in infertility studies, particularly when coupled with modern techniques in gene-expression analysis (e.g. microarrays), is obvious. As a wholly non-invasive proxy for the testis, this RNA offers considerable potential as a marker for fertility status and the genetic and environmental influences that could make all the difference between a fertile and an infertile phenotype.

Proteasome activator PA200 is required for normal spermatogenesis

The PA200 proteasome activator is a broadly expressed nuclear protein. Although how PA200 normally functions is not fully understood, it has been suggested to be involved in the repair of DNA double-strand breaks (DSBs). The PA200 gene (Psme4) is composed of 45 coding exons spanning 108 kb on mouse chromosome 11. We generated a PA200 null allele (PA200(Delta)) through Cre-loxP-mediated interchromosomal recombination after targeting loxP sites at either end of the locus. PA200(Delta/Delta) mice are viable and have no obvious developmental abnormalities. Both lymphocyte development and immunoglobulin class switching, which rely on the generation and repair of DNA DSBs, are unperturbed in PA200(Delta/Delta) mice. Additionally, PA200(Delta/Delta) embryonic stem cells do not exhibit increased sensitivity to either ionizing radiation or bleomycin. Thus, PA200 is not essential for the repair of DNA DSBs generated in these settings. Notably, loss of PA200 led to a marked reduction in male, but not female, fertility. This was due to defects in spermatogenesis observed in meiotic spermatocytes and during the maturation of postmeiotic haploid spermatids. Thus, PA200 serves an important nonredundant function during spermatogenesis, suggesting that the efficient generation of male gametes has distinct protein metabolic requirements.

Knockout mouse models of sperm flagellum anomalies

To date, 21 knockout mouse models are known to bear specific anomalies of the sperm flagellum structures leading to motility disorders. In addition, genes responsible for flagellar defects of two well-known spontaneous mutant mice have recently been identified. These models reveal genetic factors, which are required for the proper assembly of the axoneme, the annulus, the mitochondrial sheath and the fibrous sheath. Many of these genetic factors follow unexpected cellular pathways to act on sperm flagellum morphogenesis. These mouse models may bear anomalies which are restricted to the spermatozoa or display more complex phenotypes that often include neuropathies and/or cilia-related diseases. In human, several structural disorders of the sperm flagellum found in brothers or consanguineous men probably have a genetic origin, but the genes involved have not yet been identified. The mutant mice we present in this review are invaluable models, which can be used to identify potential candidate genes for infertile men with specific sperm flagellum anomalies.

Histone Ubiquitylation and the Regulation of Transcription

The small (76 amino acids) and highly conserved ubiquitin protein plays key roles in the physiology of eukaryotic cells. Protein ubiquitylation has emerged as one of the most important intracellular signaling mechanisms, and in 2004 the Nobel Prize was awarded to Aaron Ciechanower, Avram Hersko, and Irwin Rose for their pioneering studies of the enzymology of ubiquitin attachment. One of the most common features of protein ubiquitylation is the attachment of polyubiquitin chains (four or more ubiquitin moieties attached to each other), which is a widely used mechanism to target proteins for degradation via the 26S proteosome. However, it is noteworthy that the first ubiquitylated protein to be identified was histone H2A, to which a single ubiquitin moiety is most commonly attached. Following this discovery, other histones (H2B, H3, H1, H2A.Z, macroH2A), as well as many nonhistone proteins, have been found to be monoubiquitylated. The role of monoubiquitylation is still elusive because a single ubiquitin moiety is not sufficient to target proteins for turnover, and has been hypothesized to control the assembly or disassembly of multiprotein complexes by providing a protein-binding site. Indeed, a number of ubiquitin-binding domains have now been identified in both polyubiquitylated and monoubiquitylated proteins. Despite the early discovery of ubiquitylated histones, it has only been in the last five or so years that we have begun to understand how histone ubiquitylation is regulated and what roles it plays in the cell. This review will discuss current research on the factors that regulate the attachment and removal of ubiquitin from histones, describe the relationship of histone ubiquitylation to histone methylation, and focus on the roles of ubiquitylated histones in gene expression.

Analysis of the Human Sperm Proteome

As part of our effort to identify putative protein targets for the development of male contraceptives, we performed an in-depth proteomic analysis of human sperm by liquid chromatography and tandem mass spectrometry. Motile sperm were collected from a single fertile individual and fractionated into detergent-soluble and detergent-insoluble fractions. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis separation of these fractions, followed by manual cutting of the gel, yielded 35 gel sections for each fraction to include proteins across the full range of electrophoretic mobility. Proteomic analysis of these gel sections identified more than 1,760 proteins with high confidence, with 1,350 proteins identified in the soluble fraction, 719 identified in the insoluble fraction, and 309 identified in both fractions. This characterization of the human sperm proteome provides a high-resolution, physiologically relevant index of the proteins that comprise human sperm.

Expression profiling of purified male germ cells: stage-specific expression patterns related to meiosis and postmeiotic development

Gene expression profiling was performed using the National Institute on Aging 15,000-cDNA microarray to reveal the differential expression pattern of 160 genes between meiotic pachytene spermatocytes and postmeiotic round spermatids of the mouse. Our results indicate that more genes are expressed in spermatids than in spermatocytes. Genes participating in cell cycle regulation and chromatin structure and dynamics are preferentially expressed in spermatocytes, while genes for protein turnover, signal transduction, energy metabolism, and intracellular transport are prevalent in spermatids. This suggests that a switch of functional requirement occurs when meiotic germ cells differentiate into haploid spermatids. Concordant expression patterns were obtained when quantitative real-time polymerase chain reaction was performed to verify the microarray data. Interestingly, the majority of the differentially expressed genes were underrepresented in mitotic type A spermatogonia, and they were preferentially expressed in the testis. Our results suggest that an even higher proportion of the mouse genome is devoted to male gamete development from meiosis than was previously estimated. We also provide evidence that underscores the advantage of using purified germ cells over whole testes in profiling spermatogenic gene expression to identify transcripts that demonstrate stage-specific expression patterns.

Database of mouse strains carrying targeted mutations in genes affecting biological responses to DNA damage Version 7

We present Version 7 of a database of mouse mutant strains that affect biological responses to DNA damage. This database is also electronically available at http://pathcuricl.swmed.edu/research/research.htm.

The controversy, potential and roles of spermatozoal RNA

The majority of cellular and molecular andrologists endorse the view that the sperm is a vessel for transporting the paternal genome to the waiting egg and nothing more. Any requirement for additional spermatozoal components that enter the ooplasm apart from the paternal centriole and the soluble egg-activating factor is generally dismissed. Many studies, however, have reported RNAs in ejaculate spermatozoa and we now know that mRNAs are delivered to the egg on fertilisation. The function and utility of sperm mRNA remains essentially unexplored. Here, we examine the controversy surrounding spermatozoal mRNA carriage, the evidence refuting its presence as an artefact and how spermatozoal mRNA is leading us to suspect that, quite apart from its undoubted diagnostic potential, it might have an important role in the establishment and maintenance of a viable paternal genome.

Regulation of nuclear proteasome by Rhp6/Ubc2 through ubiquitination and destruction of the sensor and anchor Cut8

While proteasome is central to the degradation of cellular ubiquitinated proteins, the control of its nuclear function is barely understood. Here we show that the fission yeast ubiquitin-conjugating Rhp6/Ubc2/Rad6 and ligating enzymes Ubr1 are responsible for nuclear enrichment of proteasome through the function of Cut8, a nuclear envelope protein. Cut8 is an Rhp6 substrate that physically interacts with and tethers proteasome. Nonubiquitinatable K-all-R Cut8 weakly interacts with proteasome and fails to enrich nuclear proteasome. Consistently, the nuclear enrichment of proteasome also fails in rhp6 and ubr1 null mutants. Further, cut8 null and cut8 K-all-R mutants are hypersensitive to DNA damage, probably due to the paucity of nuclear proteasome. Thus, Rhp6 enhances the retention of nuclear proteasome through regulating Cut8. The short-lived nature of Cut8 is crucial for feedback enrichment of the proteasome within the nucleus. This is likely to be a conserved mechanism as we describe a Cut8 homolog in flies.

Mice lacking the UBC4-testis gene have a delay in postnatal testis development but normal spermatogenesis and fertility

Activation of ubiquitination occurs during spermatogenesis and is dependent on the induction of isoforms of the UBC4 family of ubiquitin-conjugating enzymes. The UBC4-testis isoform is testis specific, is induced in round spermatids, and demonstrates biochemical functions distinct from a ubiquitously expressed isoform UBC4-1. To explore further the function of UBC4-testis, mice bearing inactivation of this gene were produced. Homozygous (-/-) mice showed normal body growth and fertility. Although testis weight and morphology were normal in testes from adult mice, examination of young mice during the first wave of spermatogenesis revealed that testes were approximately 10% smaller in weight at 40 and 45 days of age but had become normal at 65 days of age. Overall protein content, levels of ubiquitinated proteins, and ubiquitin-conjugating activity did not differ between wild-type and homozygous (-/-) mice. Spermatid number, as well as the motility of spermatozoa isolated from the epididymis, was also normal in homozygous (-/-) mice. To determine whether the germ cells lacking UBC4-testis might be more sensitive to stress, testes from wild-type and knockout mice were exposed to heat stress by implantation in the abdominal cavity. Testes from both strains of mice showed similar rates of degeneration in response to heat. The lack of an obvious phenotype did not appear to be due to induction of other UBC4 isoforms, as shown by two-dimensional gel immunoblotting. Our data indicate that UBC4-testis plays a role in early maturation of the testis and suggest that the many UBC4 isoforms have mixed redundant and specific functions.

Expression and possible functions of DNA lesion bypass proteins in spermatogenesis

In mammalian cells, there is a complex interplay of different DNA damage response and repair mechanisms. Several observations suggest that, in particular in gametogenesis, proteins involved in DNA repair play an intricate role in and outside the context of DNA repair. Here, we discuss the possible roles of proteins that take part in replicative damage bypass (RDB) mechanisms, also known as post-replication DNA repair (PRR), in germ line development. In yeast, and probably also in mammalian somatic cells, RDB [two subpathways: damage avoidance and translesion synthesis (TLS)] prevents cessation of replication forks during the S phase of the cell cycle, in situations when the replication machinery encounters a lesion present in the template DNA. Many genes encoding proteins involved in RDB show an increased expression in testis, in particular in meiotic and post-meiotic spermatogenic cells. Several RDB proteins take part in protein ubiquitination, and we address relevant aspects of the ubiquitin system in spermatogenesis. RDB proteins might be required for damage avoidance and TLS of spontaneous DNA damage during gametogenesis. In addition, we consider the possible functional relation between TLS and the induction of mutations in spermatogenesis. TLS requires the activity of highly specialized polymerases, and is an error-prone process that may induce mutations. In evolutionary terms, controlled generation of a limited number of mutations in gametogenesis might provide a mechanism for evolvability.

Impairment of spermatogenesis leading to infertility

Highly differentiated spermatozoa are generated through multiple cellular and molecular processes maintained by Sertoli cells. The cellular events associated with germ cells include proliferation, protein folding and transportation, as well as sequential changes in chromatin and cell organelles. These processes are strictly controlled by the expression of specific genes, including transcription and DNA replication/repair. This complex spermatogenesis is impaired by a mutation such as gene knockout, which leads to a variety of morphological and functional abnormalities found in mature spermatozoa. An overview of spermatogenesis impairment induced by gene knockout is provided in the present review.

Tissue distribution of the "N-end rule" ubiquitin-conjugating enzyme, HR6, in the rat

The conjugation of multiple ubiquitin molecules is required for recognition and degradation of a protein by the proteasome. The ubiquitination pathway responsible for the bulk of constitutive protein degradation targets proteins carrying basic or large hydrophobic amino acids at the N-terminus. In mammalian cells, this "N-end rule" pathway requires the ubiquitin-conjugating enzyme HR6. Until now, it has not been known which mammalian tissues and cell types predominantly utilize this pathway for degradation. Therefore, the distribution and intracellular localization of HR6 was determined by indirect immunofluorescence techniques and protein blotting of adult rat tissues. Intense immunoreactivity against HR6 was detected in various epithelia, muscle, testis, peripheral neurons, chromaffin cells and macrophages, whereas lower HR6 protein levels were found in the gut or in the kidney. Autonomic and sensory neurons, glandular cells and spermatocytes revealed prominent nuclear HR6 immunoreactivity. Plasma membrane labeling was observed in peripheral neurons, spermatocytes and skeletal muscle cells. Smooth muscle cells, macrophages, endothelial and epithelial cells exhibited primarily cytoplasmic staining. The clear differences in the regional and intracellular distribution of HR6 are suggestive for the involvement of N-end rule protein degradation in various physiological processes dependent on cell type and subcellular structure.

Poly(ADP-ribosyl)ation during chromatin remodeling steps in rat spermiogenesis

In spermiogenesis, spermatid differentiation is marked by dramatic changes in chromatin density and composition. The extreme condensation of the spermatid nucleus is characterized by an exchange of histones to transition proteins and then to protamines as the major nuclear proteins. Alterations in DNA topology that occur in this process have been shown to require the controlled formation of DNA strand breaks. Poly(ADP-ribosyl)ation is a posttranslational modification of proteins mediated by a family of poly(ADP-ribose) polymerase (PARP) proteins, and two family members, PARP-1 and PARP-2, are activated by DNA strand breaks that are directly detected by the DNA-binding domains of these enzymes. Here, we show for the first time that poly(ADP-ribose) formation, mediated by poly(ADP-ribose) polymerases (PARP-1 and presumably PARP-2), occurs in spermatids of steps 11-14, steps that immediately precede the most pronounced phase of chromatin condensation in spermiogenesis. High levels of ADP-ribose polymer were observed in spermatid steps 12-13 in which the highest rates of chromatin nucleoprotein exchanges take place. We also detected gamma-H2AX, indicating the presence of DNA double-strand breaks during the same steps. Thus, we hypothesize that transient ADP-ribose polymer formation may facilitate DNA strand break management during the chromatin remodeling steps of sperm cell maturation.

Chromatin remodelling and epigenetic features of germ cells

Germ cells have the unique capacity to start a new life upon fertilization. They are generated during a sex-specific differentiation programme called gametogenesis. Maturation of germ cells is characterized by an impressive degree of cellular restructuring and gene regulation that involves remarkable genomic reorganization. These events are finely tuned, but are also susceptible to the introduction of various types of error. Because stable genetic transmission to future generations is essential for life, understanding the control of these processes has far-reaching implications for human health and reproduction.

Database of mouse strains carrying targeted mutations in genes affecting biological responses to DNA damage (Version 6)

We present Version 6 of a database of mouse mutant strains that affect biological responses to DNA damage. This database is also electronically available at http://pathcuric1.swmed.edu/research/research.htm.

Silencing of unpaired chromatin and histone H2A ubiquitination in mammalian meiosis

During meiotic prophase in male mammals, the X and Y chromosomes are incorporated in the XY body. This heterochromatic body is transcriptionally silenced and marked by increased ubiquitination of histone H2A. This led us to investigate the relationship between histone H2A ubiquitination and chromatin silencing in more detail. First, we found that ubiquitinated H2A also marks the silenced X chromosome of the Barr body in female somatic cells. Next, we studied a possible relationship between H2A ubiquitination, chromatin silencing, and unpaired chromatin in meiotic prophase. The mouse models used carry an unpaired autosomal region in male meiosis or unpaired X and Y chromosomes in female meiosis. We show that ubiquitinated histone H2A is associated with transcriptional silencing of large chromatin regions. This silencing in mammalian meiotic prophase cells concerns unpaired chromatin regions and resembles a phenomenon described for the fungus Neurospora crassa and named meiotic silencing by unpaired DNA.

The ubiquitin system: pathogenesis of human diseases and drug targeting

With the many processes and substrates targeted by the ubiquitin pathway, it is not surprising to find that aberrations in the system underlie, directly or indirectly, the pathogenesis of many diseases. While inactivation of a major enzyme such as E1 is obviously lethal, mutations in enzymes or in recognition motifs in substrates that do not affect vital pathways or that affect the involved process only partially may result in a broad array of phenotypes. Likewise, acquired changes in the activity of the system can also evolve into certain pathologies. The pathological states associated with the ubiquitin system can be classified into two groups: (a) those that result from loss of function-mutation in a ubiquitin system enzyme or in the recognition motif in the target substrate that lead to stabilization of certain proteins, and (b) those that result from gain of function-abnormal or accelerated degradation of the protein target. Studies that employ targeted inactivation of genes coding for specific ubiquitin system enzymes and substrates in animals can provide a more systematic view into the broad spectrum of pathologies that may result from aberrations in ubiquitin-mediated proteolysis. Better understanding of the processes and identification of the components involved in the degradation of key regulatory proteins will lead to the development of mechanism-based drugs that will target specifically only the involved proteins.

Ubiquitin C-terminal hydrolase L-1 is essential for the early apoptotic wave of germinal cells and for sperm quality control during spermatogenesis

Ubiquitination is required throughout all developmental stages of mammalian spermatogenesis. Ubiquitin C-terminal hydrolase (UCH) L1 is thought to associate with monoubiquitin to control ubiquitin levels. Previously, we found that UCHL1-deficient testes of gad mice have reduced ubiquitin levels and are resistant to cryptorchid stress-related injury. Here, we analyzed the function of UCHL1 during the first round of spermatogenesis and during sperm maturation, both of which are known to require ubiquitin-mediated proteolysis. Testicular germ cells in the immature testes of gad mice were resistant to the early apoptotic wave that occurs during the first round of spermatogenesis. TUNEL staining and cell quantitation demonstrated decreased germ cell apoptosis and increased numbers of premeiotic germ cells in gad mice between Postnatal Days 7 and 14. Expression of the apoptotic proteins TRP53, Bax, and caspase-3 was also significantly lower in the immature testes of gad mice. In adult gad mice, cauda epididymidis weight, sperm number in the epididymis, and sperm motility were reduced. Moreover, the number of defective spermatozoa was significantly increased; however, complete infertility was not detected. These data indicate that UCHL1 is required for normal spermatogenesis and sperm quality control and demonstrate the importance of UCHL1-dependent apoptosis in spermatogonial cell and sperm maturation.

The RING domain of Mdm2 mediates histone ubiquitylation and transcriptional repression

Histone modifications play a pivotal role in regulating transcription and other chromatin-associated processes. In yeast, histone H2B monoubiquitylation affects gene silencing. However, mammalian histone ubiquitylation remains poorly understood. We report that the Mdm2 oncoprotein, a RING domain E3 ubiquitin ligase known to ubiquitylate the p53 tumor suppressor protein, can interact directly with histones and promote in vitro monoubiquitylation of histones H2A and H2B. Moreover, Mdm2 induces H2B monoubiquitylation in vivo. Endogenous Mdm2 is tethered in vivo, presumably via p53, to chromatin comprising the p53-responsive p21(waf1) promoter, and Mdm2 overexpression enhances protein ubiquitylation in the vicinity of a p53 binding site within that promoter. Moreover, when recruited to a promoter in the absence of p53, Mdm2 can repress transcription dependently on its RING domain, suggesting that its E3 activity contributes to repression. Histone ubiquitylation may thus constitute a novel mechanism of transcriptional repression by Mdm2, possibly underlying some of its oncogenic activities.

The Deubiquitinating Enzyme mUBPy Interacts with the Sperm-Specific Molecular Chaperone MSJ-1: The Relation with the Proteasome, Acrosome, and Centrosome in Mouse Male Germ Cells1

The mouse USP8/mUBPy gene codifies a deubiquitinating enzyme expressed preferentially in testis and brain. While the ubiquitin-specific processing proteases (UBPs) are known to be important for the early development in invertebrate organisms, their specific functions remain still unclear in mammals. Using specific antibodies, raised against a recombinant mUBPy protein, we studied mUBPy in mouse testis. The mUBPy is expressed exclusively by the germ cell component and is maintained in epididymal spermatozoa. The enzyme is functionally active, being able to detach ubiquitin moieties from endogenous protein substrates. Protein interaction assays showed that sperm UBPy interacts with MSJ-1, the sperm-specific DnaJ protein evolutionarily conserved for spermiogenesis. Immunocytochemistry revealed that mUBPy shares with MSJ-1 the intracellular localization during spermatid cell differentiation; intriguingly, we show here that the proteasomes also locate in mUBPy/MSJ-1-positive sites, such as the cytoplasmic surface of the developing acrosome and the centrosomal region. These colocalization sites are maintained in epididymal spermatozoa. The demonstration of a protein interaction between a deubiquitinating enzyme and a molecular chaperone and the documentation on the proteasomes in both differentiating and mature mouse male germ cells suggest that members of the chaperone and ubiquitin/proteasome systems could cooperate in the fine control of protein quality to yield functional spermatozoa.

How to pack the genome for a safe trip

The transformation of the somatic chromatin into a unique and highly compact structure occurring during the post-meiotic phase of spermatogenesis is one of the most dramatic known processes of chromatin remodeling. Paradoxically, no information is available on the mechanisms controlling this specific reorganization of the haploid cell genome. The only existing hints suggest a role for histone variants, as well as for stage-specific post-translational histone modifications,before and during the incorporation of testis-specific basic nuclear proteins. Moreover, the exact functions of the latter remain obscure. This chapter summarizes the major chromatin-associated events taking place during the post-meiotic differentiation of male haploid cells in mammals and discusses some of the basic issues that remain to be solved to finally understand chromatin remodeling during spermatogenesis.

Ubiquitin-protein ligases in muscle wasting

Muscle wasting occurs when rates of protein degradation outstrip rates of protein synthesis. Accelerated rates of protein degradation develop in atrophying muscle largely through activation of the ubiquitin-proteasome pathway. The complexity of the ubiquitination process, however, has hampered our understanding of how this pathway is activated in atrophying muscles and which enzymes of the ubiquitin conjugation system are responsible. Recent studies demonstrate that two ubiquitin-protein ligases (E3s), atrogin-1/MAFbx and MuRF1 are critical in the development of muscle atrophy. Other experiments implicate E2(14k) and E3alpha, of the N-end rule pathway, as important players in the process. It seems likely that multiple pathways of ubiquitin conjugation are activated in parallel in atrophying muscle, perhaps to target for degradation specific classes of muscle proteins. The emerging challenge will be to define the protein targets for, as well as to develop inhibitors of, these E3s.

Effects of endocrine-disrupting chemicals on expression of ubiquitin C-terminal hydrolase mRNA in testis and brain of the Japanese common goby

We investigated the effects of endocrine-disrupting chemicals (EDCs) on the expression of ubiquitin C-terminal hydrolase (UCH) mRNA in the testis and brain of the Japanese common goby, Acanthogobius flavimanus. The cDNA sequence of goby UCH contained an open reading frame encoding 220 amino acid residues (M(r)=24,223) with 51.3% overall sequence identity with human and mouse UCHL1. A competitive PCR assay was used to quantify the levels of UCH mRNA in the testis and brain of male gobies after exposure to bisphenol A, nonylphenol, or estradiol-17beta for 3 weeks. Exposure to estradiol-17beta at a nominal concentration of 100 ng/L induced significant increase in UCH mRNA levels in both testis and brain (P<0.05), whereas exposure to nonylphenol induced a significant decrease in UCH mRNA levels in the testis (P<0.01). These results suggest that EDCs can either positively or negatively regulate UCH mRNA levels.

The role of histones in chromatin remodelling during mammalian spermiogenesis

One of the most dramatic chromatin remodelling processes takes place during mammalian spermatogenesis. Indeed, during the postmeiotic maturation of male haploid germ cells, or spermiogenesis, histones are replaced by small basic proteins, which in mammals are transition proteins and protamines. However, nothing is known of the mechanisms controlling the process of histone replacement. Two hints from the literature could help to shed light on the underlying molecular events: one is the massive synthesis of histone variants, including testis-specific members, and the second is a stage specific post-translational modification of histones. A new testis-specific 'histone code' can therefore be generated combining both histone variants and histone post-translational modifications. This review will detail these two phenomena and discuss possible functional significance of the global chromatin alterations occurring prior to histone replacement during spermiogenesis.

Ubiquitin ligase Rad18Sc localizes to the XY body and to other chromosomal regions that are unpaired and transcriptionally silenced during male meiotic prophase

In replicative damage bypass (RDB) in yeast, the ubiquitinconjugating enzyme RAD6 interacts with the ubiquitin ligase RAD18. In the mouse, these enzymes are represented by two homologs of RAD6, HR6a and HR6b, and one homolog of RAD18, Rad18Sc. Expression of these genes and the encoded proteins is ubiquitous, but there is relatively high expression in the testis. We have studied the subcellular localization by immunostaining Rad18Sc and other RDB proteins in mouse primary spermatocytes passing through meiotic prophase in spermatogenesis. The highest Rad18Sc protein level is found at pachytene and diplotene, and the protein localizes mainly to the XY body, a subnuclear region that contains the transcriptionally inactivated X and Y chromosomes. In spermatocytes that carry translocations for chromosomes 1 and 13, Rad18Sc protein concentrates on translocation bivalents that are not fully synapsed. The partly synapsed bivalents are often localized in the vicinity of the XY body, and show a very low level of RNA polymerase II, indicating that the chromatin is in a silent configuration similar to transcriptional silencing of the XY body. Thus, Rad18Sc localizes to unsynapsed and silenced chromosome segments during the male meiotic prophase. All known functions of RAD18 in yeast are related to RDB. However, in contrast to Rad18Sc, expression of UBC13 and polη, known to be involved in subsequent steps of RDB, appears to be diminished in the XY body and regions containing the unpaired translocation bivalents. Taken together, these observations suggest that the observed subnuclear localization of Rad18Sc may involve a function outside the context of RDB. This function is probably related to a mechanism that signals the presence of unsynapsed chromosomal regions and subsequently leads to transcriptional silencing of these regions during male meiotic prophase.

Targeted disruption of Drosophila Roc1b reveals functional differences in the Roc subunit of Cullin-dependent E3 ubiquitin ligases

Cullin-dependent ubiquitin ligases regulate a variety of cellular and developmental processes by recruiting specific proteins for ubiquitin-mediated degradation. Cullin proteins form a scaffold for two functional modules: a catalytic module comprised of a small RING domain protein Roc1/Rbx1 and a ubiquitin-conjugating enzyme (E2), and a substrate recruitment module containing one or more proteins that bind to and bring the substrate in proximity to the catalytic module. Here, we present evidence that the three Drosophila Roc proteins are not functionally equivalent. Mutation of Roc1a causes lethality that cannot be rescued by expression of Roc1b or Roc2 by using the Roc1a promoter. Roc1a mutant cells hyperaccumulate Cubitus interruptus, a transcription factor that mediates Hedgehog signaling. This phenotype is not rescued by expression of Roc2 and only partially by expression of Roc1b. Targeted disruption of Roc1b causes male sterility that is partially rescued by expression of Roc1a by using the Roc1b promoter, but not by similar expression of Roc2. These data indicate that Roc proteins play nonredundant roles during development. Coimmunoprecipitation followed by Western or mass spectrometric analysis indicate that the three Roc proteins preferentially bind certain Cullins, providing a possible explanation for the distinct biological activities of each Drosophila Roc/Rbx.

Developmental Regulation of Ubiquitin C-Terminal Hydrolase Isozyme Expression During Spermatogenesis in Mice

The ubiquitin pathway functions in the process of protein turnover in eukaryotic cells. This pathway comprises the enzymes that ubiquitinate/deubiquitinate target proteins and the proteasome that degrades ubiquitin-conjugated proteins. Ubiquitin C-terminal hydrolases (UCHs) are thought to be essential for maintaining ubiquitination activity by releasing ubiquitin (Ub) from its substrates. Mammalian UCH-L1 and UCH-L3 are small proteins that share considerable homology at the amino acid level. Both of these UCHs are highly expressed in the testis/ ovary and neuronal cells. Our previous work demonstrated that UCH-L1-deficient gracile axonal dystrophy (gad) mice exhibit progressively decreasing spermatogonial stem cell proliferation, suggesting that UCH isozymes in the testis function during spermatogenesis. To analyze the expression patterns of UCH isozymes during spermatogenesis, we isolated nearly homogeneous populations of spermatogonia, spermatocytes, spermatids, and Sertoli cells from mouse testes. Western blot analysis detected UCH-L1 in spermatogonia and Sertoli cells, whereas UCH-L3 was detected in spermatocytes and spermatids. Moreover, reverse transcription-polymerase chain reaction analysis of UCH isozymes showed that UCH-L1 and UCH-L4 mRNAs are expressed in spermatogonia, whereas UCH-L3 and UCH-L5 mRNAs are expressed mainly in spermatocytes and spermatids. These results suggest that UCH-L1 and UCH-L3 have distinct functions during spermatogenesis, namely, that UCH-L1 may act during mitotic proliferation of spermatogonial stem cells whereas UCH-L3 may function in the meiotic differentiation of spermatocytes into spermatids.

RAD6B overexpression confers chemoresistance: RAD6 expression during cell cycle and its redistribution to chromatin during DNA damage-induced response

The HR6A and HR6B genes, homologs of the yeast RAD6 gene, encode ubiquitin conjugating enzymes that are required for postreplication repair (PRR) of DNA and damage-induced mutagenesis. We show here that consistent with its role as a PRR protein, HR6 protein (referred as RAD6) expression is cell cycle regulated, with maximal levels expressed in late S/G2 phases of the cell cycle. Exposure of MCF10A cells to adriamycin (ADR) causes enhancement in the levels of RAD6B mRNA and protein. Inclusion of actinomycin D abolishes both basal and ADR-induced RAD6B transcription indicating that ADR-induced effects on RAD6B transcription result from an increase in transcriptional activity rather than from regulation of RAD6B mRNA stability. The increase in RAD6 protein expression observed in ADR-treated cells is dependent upon transcription and de novo protein synthesis, as addition of actinomycin D and cycloheximide eliminated the induction effects. Using in vivo crosslinking experiments, we demonstrate that only a small proportion of RAD6 is associated with chromatin in untreated MCF10A cells. However, treatment with ADR or cisplatin is accompanied by a significant increase and redistribution of RAD6 to DNA, and RAD6, RAD18, PCNA, phosphohistone H3, as well as p53 proteins are all found in the DNA fractions. These findings suggest that although RAD6 protein is present in the nucleus, its recruitment to the chromatin appears to be modulated by DNA damage. Whereas MCF10A cells engineered to overexpress ectopic RAD6B are significantly more resistant to ADR and cisplatin as compared to empty vector-transfected cells, MCF10A cells stably transfected with antisense RAD6B display hypersensitivity to these damage-inducing drugs. Analysis of PRR capacities in cisplatin-treated MCF10A cells stably transfected with empty vector, RAD6B or antisense RAD6B showed that whereas RAD6B-overexpressing and vector control MCF10A cells possessed the ability to convert newly synthesized DNA to higher molecular weight species, MCF10A cells depleted of RAD6B are PRR-compromised. Although no human diseases have been linked to mutations in the PRR pathway genes, these data suggest that RAD6 may play an essential role in DNA damage tolerance and recovery via modulation of PRR, and that imbalances in the levels of RAD6 could lead to changes in drug sensitivity and damage-induced mutagenesis.

The ubiquitin-conjugating DNA repair enzyme HR6A is a maternal factor essential for early embryonic development in mice

The Saccharomyces cerevisiae RAD6 protein is required for a surprising diversity of cellular processes, including sporulation and replicational damage bypass of DNA lesions. In mammals, two RAD6-related genes, HR6A and HR6B, encode highly homologous proteins. Here, we describe the phenotype of cells and mice deficient for the mHR6A gene. Just like mHR6B knockout mouse embryonic fibroblasts, mHR6A-deficient cells appear to have normal DNA damage resistance properties, but mHR6A knockout male and female mice display a small decrease in body weight. The necessity for at least one functional mHR6A (X-chromosomal) or mHR6B (autosomal) allele in all somatic cell types is supported by the fact that neither animals lacking both proteins nor females with only one intact mHR6A allele are viable. In striking contrast to mHR6B knockout males, which show a severe spermatogenic defect, mHR6A knockout males are normally fertile. However, mHR6A knockout females fail to produce offspring despite a normal ovarian histology and ovulation. The absence of mHR6A in oocytes prevents development beyond the embryonic two-cell stage but does not result in an aberrant methylation pattern of histone H3 at this early stage of mouse embryonic development. These observations support redundant but dose-dependent roles for HR6A and HR6B in somatic cell types and germ line cells in mammals.

Specific arrests of spermatogenesis in genetically modified and mutant mice

In naturally occurring mutant mice but also in mice genetically modified for the study of other organs, relatively often a spermatogenic arrest is seen. In a number of cases the arrests appear to be very specific causing apoptosis of germ cells at a particular step in their development, while before this step cells progress normally. These steps include: proliferation/migration of primordial germ cells, the production of differentiating spermatogonia by gonocytes, the regulation of stem cell renewal/differentiation, the differentiation of A(al) into A1 spermatogonia, proliferation of A1-A4 spermatogonia, germ cell density regulation, start of meiosis, epithelial stage IV checkpoint of pachytene spermatocytes, the first meiotic division, the formation of the acrosomic vesicle in spermatids and several other steps in spermatid development. In addition, there are many mice that have not been studied in enough detail for a proper categorization. In this review an overview is given of the various mutations and genetically modified mice showing a direct effect on specific spermatogenic cell types. In addition, the relevance of these models to our understanding of the spermatogenic process is discussed.

Molecular aspects of XY body formation

More than a century ago, a densely stained area inside the nucleus of male meiotic cells was described. It was later shown to harbor the sex chromosomes which undergo transcriptional inactivation in conjunction with heterochromatinisation and synapsis to form the XY body. Formation of the XY body is conserved throughout the mammalian phylogenetic tree and is thought to be essential for successful spermatogenesis. However, its biological role as well as the molecular mechanisms underlying XY body formation are still far from being understood. A lot of effort has already been undertaken to characterize components of the XY body and to investigate their functional implications in sex chromatin heterochromatinisation and meiotic sex chromosome inactivation (MSCI). This review gives an overview of those components and their possible implications in XY body formation and function.