Clustering of pericentromeres initiates in step 9 of spermiogenesis of the rat (Rattus norvegicus) and contributes to a well defined genome architecture in the sperm nucleus

ADAD1 is required for normal translation of nuclear pore and transport protein transcripts in spermatids of Mus musculus†

ADAD1 is a testis-specific RNA-binding protein expressed in post-meiotic spermatids whose loss leads to defective sperm and male infertility. However, the drivers of the Adad1 phenotype remain unclear. Morphological and functional analysis of Adad1 mutant sperm showed defective DNA compaction, abnormal head shaping, and reduced motility. Mutant testes demonstrated minimal transcriptome changes; however, ribosome association of many transcripts was reduced, suggesting ADAD1 may be required for their translational activation. Further, immunofluorescence of proteins encoded by select transcripts showed delayed protein accumulation. Additional analyses demonstrated impaired subcellular localization of multiple proteins, suggesting protein transport is also abnormal in Adad1 mutants. To clarify the mechanism giving rise to this, the manchette, a protein transport microtubule network, and the LINC (linker of nucleoskeleton and cytoskeleton) complex, which connects the manchette to the nuclear lamin, were assessed across spermatid development. Proteins of both displayed delayed translation and/or localization in mutant spermatids implicating ADAD1 in their regulation, even in the absence of altered ribosome association. Finally, ADAD1's impact on the NPC (nuclear pore complex), a regulator of both the manchette and the LINC complex, was examined. Reduced ribosome association of NPC encoding transcripts and reduced NPC protein abundance along with abnormal localization in Adad1 mutants confirmed ADAD1's impact on translation is required for a NPC in post-meiotic germ cells. Together, these studies lead to a model whereby ADAD1's influence on nuclear transport leads to deregulation of the LINC complex and the manchette, ultimately generating the range of physiological defects observed in the Adad1 phenotype.

DOT1L promotes spermatid differentiation by regulating expression of genes required for histone-to-protamine replacement

Unique chromatin remodeling factors orchestrate dramatic changes in nuclear morphology during differentiation of the mature sperm head. A crucial step in this process is histone-to-protamine exchange, which must be executed correctly to avoid sperm DNA damage, embryonic lethality and male sterility. Here, we define an essential role for the histone methyltransferase DOT1L in the histone-to-protamine transition. We show that DOT1L is abundantly expressed in mouse meiotic and postmeiotic germ cells, and that methylation of histone H3 lysine 79 (H3K79), the modification catalyzed by DOT1L, is enriched in developing spermatids in the initial stages of histone replacement. Elongating spermatids lacking DOT1L fail to fully replace histones and exhibit aberrant protamine recruitment, resulting in deformed sperm heads and male sterility. Loss of DOT1L results in transcriptional dysregulation coinciding with the onset of histone replacement and affecting genes required for histone-to-protamine exchange. DOT1L also deposits H3K79me2 and promotes accumulation of elongating RNA Polymerase II at the testis-specific bromodomain gene Brdt. Together, our results indicate that DOT1L is an important mediator of transcription during spermatid differentiation and an indispensable regulator of male fertility.

Chromatin remodelers HELLS, WDHD1 and BAZ1A are dynamically expressed during mouse spermatogenesis

In brief

Proper regulation of heterochromatin is critical for spermatogenesis. This study reveals the dynamic localization patterns of distinct chromatin regulators during spermatogenesis and disrupted sex chromatin status in spermatocytes in the absence of DICER.

Abstract

Heterochromatin is dynamically formed and organized in differentiating male germ cells, and its proper regulation is a prerequisite for normal spermatogenesis. While heterochromatin is generally transcriptionally silent, we have previously shown that major satellite repeat (MSR) DNA in the pericentric heterochromatin (PCH) is transcribed during spermatogenesis. We have also shown that DICER associates with PCH and is involved in the regulation of MSR-derived transcripts. To shed light on the heterochromatin regulation in the male germline, we studied the expression, localization and heterochromatin association of selected testis-enriched chromatin regulators in the mouse testis. Our results show that HELLS, WDHD1 and BAZ1A are dynamically expressed during spermatogenesis. They display limited overlap in expression, suggesting involvement in distinct heterochromatin-associated processes at different steps of differentiation. We also show that HELLS and BAZ1A interact with DICER and MSR chromatin. Interestingly, deletion of Dicer1 affects the sex chromosome heterochromatin status in late pachytene spermatocytes, as demonstrated by mislocalization of Polycomb protein family member SCML1 to the sex body. These data substantiate the importance of dynamic heterochromatin regulation during spermatogenesis and emphasize the key role of DICER in the maintenance of chromatin status in meiotic male germ cells.

ADAD2 regulates heterochromatin in meiotic and post-meiotic male germ cells via translation of MDC1

Male germ cells establish a unique heterochromatin domain, the XY-body, early in meiosis. How this domain is maintained through the end of meiosis and into post-meiotic germ cell differentiation is poorly understood. ADAD2 is a late meiotic male germ cell-specific RNA-binding protein, loss of which leads to post-meiotic germ cell defects. Analysis of ribosome association in Adad2 mouse mutants revealed defective translation of Mdc1, a key regulator of XY-body formation, late in meiosis. As a result, Adad2 mutants show normal establishment but failed maintenance of the XY-body. Observed XY-body defects are concurrent with abnormal autosomal heterochromatin and ultimately lead to severely perturbed post-meiotic germ cell heterochromatin and cell death. These findings highlight the requirement of ADAD2 for Mdc1 translation, the role of MDC1 in maintaining meiotic male germ cell heterochromatin and the importance of late meiotic heterochromatin for normal post-meiotic germ cell differentiation.

Detailed cell-level analysis of sperm nuclear quality among the different hypo-osmotic swelling test (HOST) classes

PURPOSE

We studied the quality differences between the different hypo-osmotic swelling test (HOST) classes, as measured by criteria of DNA fragmentation, DNA decondensation, and nuclear architecture. The aim was to find particular HOST classes associated with good-quality metrics, which may be potentially used in ICSI (intra-cytoplasmic sperm injection).

METHODS

Ten patients from the Department of Reproductive Medicine at Tenon Hospital (Paris, France) were included. Their semen samples were collected and divided into two fractions: one was incubated in a hypo-osmotic solution as per HOST protocol and sorted by sperm morphology, and a second was incubated without undergoing the HOST protocol to serve as an unsorted baseline. Three parameters were assessed: DNA fragmentation (TUNEL assay), DNA decondensation (chromomycin A3 assay), and nuclear architecture (FISH, with telomeric and whole chromosome painting probes). The different HOST classes were evaluated for these three parameters, and statistical analysis was performed for each class versus the unsorted non-HOST-treated sperm. Results with p<0.05 were considered statistically significant.

RESULTS

For each of the parameters evaluated, we found significant differences between HOST-selected spermatozoa and non-selected spermatozoa. Overall, spermatozoa of HOST classes B and B+ exhibited the highest quality based on four metrics (low DNA fragmentation, low DNA decondensation, short inter-telomeric distance, and small chromosome 1 territory area), while spermatozoa of HOST classes A and G exhibited the poorest quality by these metrics.

CONCLUSION

In addition to their pathophysiological interest, our results open possibilities of sperm selection prior to ICSI, which may allow for optimization of reproductive outcomes in heretofore unstudied patient populations.

LINCking the Nuclear Envelope to Sperm Architecture

Nuclear architecture undergoes an extensive remodeling during spermatogenesis, especially at levels of spermatocytes (SPC) and spermatids (SPT). Interestingly, typical events of spermiogenesis, such as nuclear elongation, acrosome biogenesis, and flagellum formation, need a functional cooperation between proteins of the nuclear envelope and acroplaxome/manchette structures. In addition, nuclear envelope plays a key role in chromosome distribution. In this scenario, special attention has been focused on the LINC (linker of nucleoskeleton and cytoskeleton) complex, a nuclear envelope-bridge structure involved in the connection of the nucleoskeleton to the cytoskeleton, governing mechanotransduction. It includes two integral proteins: KASH- and SUN-domain proteins, on the outer (ONM) and inner (INM) nuclear membrane, respectively. The LINC complex is involved in several functions fundamental to the correct development of sperm cells such as head formation and head to tail connection, and, therefore, it seems to be important in determining male fertility. This review provides a global overview of the main LINC complex components, with a special attention to their subcellular localization in sperm cells, their roles in the regulation of sperm morphological maturation, and, lastly, LINC complex alterations associated to male infertility.

Automated Nuclear Cartography Reveals Conserved Sperm Chromosome Territory Localization across 2 Million Years of Mouse Evolution

Measurements of nuclear organization in asymmetric nuclei in 2D images have traditionally been manual. This is exemplified by attempts to measure chromosome position in sperm samples, typically by dividing the nucleus into zones, and manually scoring which zone a fluorescence in-situ hybridisation (FISH) signal lies in. This is time consuming, limiting the number of nuclei that can be analyzed, and prone to subjectivity. We have developed a new approach for automated mapping of FISH signals in asymmetric nuclei, integrated into an existing image analysis tool for nuclear morphology. Automatic landmark detection defines equivalent structural regions in each nucleus, then dynamic warping of the FISH images to a common shape allows us to generate a composite of the signal within the entire cell population. Using this approach, we mapped the positions of the sex chromosomes and two autosomes in three mouse lineages (Mus musculus domesticus, Mus musculus musculus and Mus spretus). We found that in all three, chromosomes 11 and 19 tend to interact with each other, but are shielded from interactions with the sex chromosomes. This organization is conserved across 2 million years of mouse evolution.

The telomere bouquet is a hub where meiotic double-strand breaks, synapsis, and stable homolog juxtaposition are coordinated in the zebrafish, Danio rerio

Meiosis is a cellular program that generates haploid gametes for sexual reproduction. While chromosome events that contribute to reducing ploidy (homologous chromosome pairing, synapsis, and recombination) are well conserved, their execution varies across species and even between sexes of the same species. The telomere bouquet is a conserved feature of meiosis that was first described nearly a century ago, yet its role is still debated. Here we took advantage of the prominent telomere bouquet in zebrafish, Danio rerio, and super-resolution microscopy to show that axis morphogenesis, synapsis, and the formation of double-strand breaks (DSBs) all take place within the immediate vicinity of telomeres. We established a coherent timeline of events and tested the dependence of each event on the formation of Spo11-induced DSBs. First, we found that the axis protein Sycp3 loads adjacent to telomeres and extends inward, suggesting a specific feature common to all telomeres seeds the development of the axis. Second, we found that newly formed axes near telomeres engage in presynaptic co-alignment by a mechanism that depends on DSBs, even when stable juxtaposition of homologous chromosomes at interstitial regions is not yet evident. Third, we were surprised to discover that ~30% of telomeres in early prophase I engage in associations between two or more chromosome ends and these interactions decrease in later stages. Finally, while pairing and synapsis were disrupted in both spo11 males and females, their reproductive phenotypes were starkly different; spo11 mutant males failed to produce sperm while females produced offspring with severe developmental defects. Our results support zebrafish as an important vertebrate model for meiosis with implications for differences in fertility and genetically derived birth defects in males and females.

Inherent to reproduction is the transmission of genetic information from one generation to the next. In sexually reproducing organisms, each parent contributes an equal amount of genetic information, packaged in chromosomes, to the offspring. Diploid organisms, like humans, have two copies of every chromosome, while their haploid gametes (e.g. eggs and sperm) have only one. This reduction in ploidy depends on the segregation of chromosomes during meiosis, resulting in gametes with one copy of each chromosome. Missegregation of the chromosomes in the parents leads to abnormal chromosome numbers in the offspring, which is usually lethal or has detrimental developmental effects. While it has been known for over a century that homologous chromosomes pair and recombine to facilitate proper segregation, how homologs find their partners has remained elusive. A structure that has been central to the discussion of homolog pairing is the bouquet, or the dynamic clustering of telomeres during early stages of meiosis. Here we use zebrafish to show that the telomere bouquet is the site where key events leading to homologous chromosome pairing are coordinated. Furthermore, we show that deletion of spo11, a gene required for proper recombination in most studied organisms, resulted in very different effects in males and females where males were sterile while females produced deformed progeny.

Nuclear Integrity but Not Topology of Mouse Sperm Chromosome is Affected by Oxidative DNA Damage

Recent studies have revealed a well-defined higher order of chromosome architecture, named chromosome territories, in the human sperm nuclei. The purpose of this work was, first, to investigate the topology of a selected number of chromosomes in murine sperm; second, to evaluate whether sperm DNA damage has any consequence on chromosome architecture. Using fluorescence in situ hybridization, confocal microscopy, and 3D-reconstruction approaches we demonstrate that chromosome positioning in the mouse sperm nucleus is not random. Some chromosomes tend to occupy preferentially discrete positions, while others, such as chromosome 2 in the mouse sperm nucleus are less defined. Using a mouse transgenic model (Gpx5^−/−) of sperm nuclear oxidation, we show that oxidative DNA damage does not disrupt chromosome organization. However, when looking at specific nuclear 3D-parameters, we observed that they were significantly affected in the transgenic sperm, compared to the wild-type. Mild reductive DNA challenge confirmed the fragility of the organization of the oxidized sperm nucleus, which may have unforeseen consequences during post-fertilization events. These data suggest that in addition to the sperm DNA fragmentation, which is already known to modify sperm nucleus organization, the more frequent and, to date, the less highly-regarded phenomenon of sperm DNA oxidation also affects sperm chromatin packaging.

SCML2 promotes heterochromatin organization in late spermatogenesis

Spermatogenesis involves the progressive reorganization of heterochromatin. However, the mechanisms that underlie the dynamic remodeling of heterochromatin remain unknown. Here, we identify SCML2, a germline-specific Polycomb protein, as a critical regulator of heterochromatin organization in spermatogenesis. We show that SCML2 accumulates on pericentromeric heterochromatin (PCH) in male germ cells, where it suppresses PRC1-mediated monoubiquitylation of histone H2A at Lysine 119 (H2AK119ub) and promotes deposition of PRC2-mediated H3K27me3 during meiosis. In postmeiotic spermatids, SCML2 is required for heterochromatin organization, and the loss of SCML2 leads to the formation of ectopic patches of facultative heterochromatin. Our data suggest that, in the absence of SCML2, the ectopic expression of somatic lamins drives this process. Furthermore, the centromere protein CENP-V is a specific marker of PCH in postmeiotic spermatids, and SCML2 is required for CENP-V localization on PCH. Given the essential functions of PRC1 and PRC2 for genome-wide gene expression in spermatogenesis, our data suggest that heterochromatin organization and spermatogenesis-specific gene expression are functionally linked. We propose that SCML2 coordinates the organization of heterochromatin and gene expression through the regulation of Polycomb complexes.

Preferable location of chromosomes 1, 29, and X in bovine spermatozoa

Chromosome positioning in sperm nucleus may have a functional significance by influencing the sequence of post-fertilization events. In this study we present data on preferential locations of chromosomes 1, 29 and X in Bos taurus spermatozoa. Here we demonstrate that the position of X chromosome in the sperm nucleus is more restricted as compared to the position of chromosome 1, which is about of the same size. Our data support the concept of the functional significance of genome architecture in male germline cells.

Topology of chromosome centromeres in human sperm nuclei with high levels of DNA damage

Several studies have shown that the 'poor' sperm DNA quality appears to be an important factor affecting male reproductive ability. In the case of sperm cells from males with the correct somatic karyotype but with deficient spermatogenesis, resulting in a high degree of sperm DNA fragmentation, we observed changes in the preferential topology of the chromosome 7, 9, 15, 18, X and Y centromeres. The changes occurred in radial localization and may have been directly linked to the sperm chromatin damage. This conclusion is mainly based on a comparison of FISH signals that were observed simultaneously in the TUNEL-positive and TUNEL-negative sperm cells. The analyzed cells originated from the same ejaculated sample and FISH was performed on the same slides, after in situ TUNEL reaction. Based on the observed changes and previous data, it appears that the sperm nucleus architecture can be disrupted by a variety of factors and has a negative influence on spermatogenesis at the same time. Often, these factors coexist (e.g. chromosomal translocations, aneuploidies, a higher DNA fragmentation, abnormal seminology), but no direct correlations between the factors were observed.

Impact of sperm DNA chromatin in the clinic

The paternal contribution to fertilization and embryogenesis is frequently overlooked as the spermatozoon is often considered to be a silent vessel whose only function is to safely deliver the paternal genome to the maternal oocyte. In this article, we hope to demonstrate that this perception is far from the truth. Typically, infertile men have been unable to conceive naturally (or through regular IVF), and therefore, a perturbation of the genetic integrity of sperm heads in infertile males has been under-considered. The advent of intracytoplasmic sperm injection (ICSI) however has led to very successful treatment of male factor infertility and subsequent widespread use in IVF clinics worldwide. Until recently, little concern has been raised about the genetic quality of sperm in ICSI patients or the impact genetic aberrations could have on fertility and embryogenesis. This review highlights the importance of chromatin packaging in the sperm nucleus as essential for the establishment and maintenance of a viable pregnancy.

Telomere distribution pattern and synapsis initiation during spermatogenesis in zebrafish

BACKGROUND

Telomeres are located at ends of eukaryotic chromosomes and can affect proper chromosomal positioning. During spermatogenesis, the appropriate dynamics and behavior of chromosomes is crucial to generate haploid cells through meiosis. Here, we describe telomere distribution patterns during spermatogenesis in zebrafish, especially during meiotic prophase I, using fluorescence in situ hybridization. This was combined with synaptonemal complex protein 3 immunostaining, which allows the staging of spermatocytes.

RESULTS

During spermatogonial proliferation and the preleptotene stage, telomeres were dispersed throughout the nucleus. During the leptotene stage, telomeres temporarily moved to one pole of the nucleus at which γ-tubulin was located, forming the telomere bouquet. The cluster lasted until the onset of zygotene where it coincided with terminal synapsis initiation. They then spread around the periphery of the nucleus during the zygotene to pachytene stages. During postmeiotic stages, telomeres in spermatids and sperm were again dispersed throughout the nuclei. Application of this procedure in meiotic mutants confirmed that meiotic telomere clustering is independent of axial element formation of the synaptonemal complex.

CONCLUSIONS

These data clearly showed the clustering and distributions of telomeres throughout spermatogenesis in zebrafish. This procedure could be used to screen for mutants that have primary defects in telomere clustering.

Sperm nuclear architecture is locally modified in presence of a Robertsonian translocation t(13;17)

In mammals, the non-random organization of the sperm nucleus supports an early function during embryonic development. Altering this organization may interfere with the zygote development and reduce fertility or prolificity. Thus, rare studies on sperm cells from infertile patients described an altered nuclear organization that may be a cause or a consequence of their respective pathologies. Thereby, chromosomal rearrangements and aneuploidy can be studied not only for their adverse effects on production of normal/balanced gametes at meiosis but also for their possible impact on sperm nuclear architecture and the epigenetic consequences of altered chromosome positioning. We decided to compare the global architecture of sperm nuclei from boars, either with a normal chromosome composition or with a Robertsonian translocation involving chromosomes 13 and 17. We hypothesized that the fusion between these chromosomes may change their spatial organization and we examined to what extend it could also modify the global sperm nuclear architecture. Analysis of telomeres, centromeres and gonosomes repartition does not support a global nuclear disorganization. But specific analysis of chromosomes 13 and 17 territories highlights an influence of chromosome 17 for the positioning of the fused chromosomes within the nucleus. We also observed a specific clustering of centromeres depending of the chromosome subtypes. Altogether our results showed that chromosome fusion does not significantly alter sperm nucleus architecture but suggest that centromere remodelling after chromosome fusion locally impacts chromosome positioning.

Alteration of poly(ADP-ribose) metabolism affects murine sperm nuclear architecture by impairing pericentric heterochromatin condensation

The mammalian sperm nucleus is characterized by unique properties that are important for fertilization. Sperm DNA retains only small numbers of histones in distinct positions, and the majority of the genome is protamine associated, which allows for extreme condensation and protection of the genetic material. Furthermore, sperm nuclei display a highly ordered architecture that is characterized by a centrally located chromocenter comprising the pericentromeric chromosome regions and peripherally positioned telomeres. Establishment of this unique and well-conserved nuclear organization during spermiogenesis is not well understood. Utilizing fluorescence in situ hybridization (FISH), we show that a large fraction of the histone-associated sperm genome is repetitive in nature, while a smaller fraction is associated with unique DNA sequences. Coordinated activity of poly(ADP-ribose) (PAR) polymerase and topoisomerase II beta has been shown to facilitate DNA relaxation and histone to protamine transition during spermatid condensation, and altered PAR metabolism is associated with an increase in sperm histone content. Combining FISH with three-dimensional laser scanning microscopy technology, we further show that altered PAR metabolism by genetic or pharmacological intervention leads to a disturbance of the overall sperm nuclear architecture with a lower degree of organization and condensation of the chromocenters formed by chromosomal pericentromeric heterochromatin.

Remodeling of three-dimensional organization of the nucleus during terminal keratinocyte differentiation in the epidermis

The nucleus of epidermal keratinocytes (KCs) is a complex and highly compartmentalized organelle, whose structure is markedly changed during terminal differentiation and transition of the genome from a transcriptionally active state seen in the basal and spinous epidermal cells to a fully inactive state in the keratinized cells of the cornified layer. Here, using multicolor confocal microscopy, followed by computational image analysis and mathematical modeling, we demonstrate that in normal mouse footpad epidermis, transition of KCs from basal epidermal layer to the granular layer is accompanied by marked differences in nuclear architecture and microenvironment including the following: (i) decrease in the nuclear volume; (ii) decrease in expression of the markers of transcriptionally active chromatin; (iii) internalization and decrease in the number of nucleoli; (iv) increase in the number of pericentromeric heterochromatic clusters; and (v) increase in the frequency of associations between the pericentromeric clusters, chromosomal territory 3, and nucleoli. These data suggest a role for nucleoli and pericentromeric heterochromatin clusters as organizers of nuclear microenvironment required for proper execution of gene expression programs in differentiating KCs, and provide important background information for further analyses of alterations in the topological genome organization seen in pathological skin conditions, including disorders of epidermal differentiation and epidermal tumors.

The role of the double bromodomain-containing BET genes during mammalian spermatogenesis

The double bromodomain-containing BET (bromodomain and extra terminal) family of proteins is highly conserved from yeast to humans and consists not just of transcriptional regulators but also histone-interacting chromatin remodelers. The four mammalian BET genes are each expressed at unique times during spermatogenesis, and the testis-specific gene Brdt is essential for spermatogenesis. Loss of the first bromodomain of BRDT results in improper/incomplete spermatid elongation and severely morphologically defective sperm. The elongation defects observed in mutant spermatids can be directly tied to altered postmeiotic chromatin architecture. BRDT is required for creation/maintenance of the chromocenter of round spermatids, a structure that forms just after completion of meiosis. The chromocenter creates a defined topology in spermatids, and the presence of multiple chromocenters rather than a single intact chromocenter correlates with loss of spermatid polarity, loss of heterochromatin foci at the nuclear envelope, and loss of proper spermatid elongation. BRDT is not only essential for proper chromatin organization but also involved in regulation of transcription and in cotranscriptional processing. That is, transcription and alternative splicing are altered in spermatocytes and spermatids that lack full-length BRDT. Additionally, the transcription of mRNAs with short 3' UTRs, which is characteristic of round spermatids, is also altered. Examination of the genes regulated by BRDT yields many possible targets that could in part explain the morphologically abnormal sperm produced by the BRDT mutant testes. Thus, BRDT and possibly the other BET genes are required for proper spermatogenesis, which opens up the possibility that the recently discovered small molecule inhibitors of the BET family could be useful as reversible male contraceptives.

The first bromodomain of the testis-specific double bromodomain protein Brdt is required for chromocenter organization that is modulated by genetic background

Mice homozygous for a mutation (Brdt(∆BD1/∆BD1)) lacking the first bromodomain of Brdt, a testis-specific member of the BET family of double-bromodomain containing proteins, are sterile and exhibit profound defects in chromatin remodeling during spermiogenesis. We have now observed that a prominent feature of the aberrant spermatid nuclei is a fragmented chromocenter, a structure comprised of peri-centromeric heterochromatin. There was a concomitant increase in the levels of heterochromatin protein 1 alpha (Hp1α), suggesting that the presence of multiple chromocenters was correlated with a spread of heterochromatin beyond the normal centromeric region. Brdt protein was normally present throughout the nucleus but was excluded from the chromocenter. A more densely staining region of Brdt protein appeared to separate sirtuin 1 (Sirt1) protein from contact with the chromocenter. Although still nuclear, this unique localization of Brdt protein was lost in Brdt(∆BD1/∆BD1) mutant spermatids and Brdt and Sirt1 overlapped around the chromocenters. There was also ectopic localization of the H1 histone family, member N, testis-specific (H1fnt) protein in Brdt(∆BD1/∆BD1) round spermatids, which may be linked to the previously reported loss of polarized localization of peri-nuclear heterochromatin foci. The extent of chromocenter fragmentation was more severe and penetrant in mutant testes on a pure 129Sv/Ev as compared to a pure C57Bl/6 background. Indeed, all aspects of the mutant phenotype were more severe on the 129Sv/Ev background. Contrary to previous studies in genetic models where fragmented chromocenters were observed in spermatids, the Brdt(∆BD1/∆BD1) mutant spermatids do not undergo apoptosis (on either background). These observations suggest that the first bromodomain of Brdt is critical in the formation and/or maintenance of an intact chromocenter and implicate this structure in proper remodeling of the chromatin architecture of the sperm head.

Nuclear organisation of sperm remains remarkably unaffected in the presence of defective spermatogenesis

Organisation of chromosome territories in interphase nuclei has been studied in many systems and positional alterations have been associated with disease phenotypes (e.g. laminopathies, cancer) in somatic cells. Altered nuclear organisation is also reported in developmental processes such as mammalian spermatogenesis where a "chromocentre" model is proposed with the centromeres and sex chromosomes repositioning to the nuclear centre. The purpose of this study was to test the hypothesis that alterations in nuclear organisation of human spermatozoa are associated with defects upstream in spermatogenesis (as manifest in certain infertility phenotypes). The nuclear address of (peri-) centromeric loci for 18 chromosomes (1-4, 6-12, 15-18, 20, X and Y) was assayed in 20 males using established algorithms for 3D extrapolations of 2D data. The control group comprised 10 fertile sperm donors while the test group was 10 patients with severely compromised semen parameters including high sperm aneuploidy. All loci examined in the control group adopted defined, interior positions thus providing supporting evidence for the presence of a chromocentre and interior sex chromosome territories. In the test group however there were subtle alterations in the nuclear address for certain centromeres in individual patients and, when all patient results were pooled, some different nuclear addresses were observed for chromosomes 3, 6, 12 and 18. Considering the extensive impairment of spermatogenesis in the test group (evidenced by compromised semen parameters and increased chromosome abnormalities), the observed differences in nuclear organisation for centromeric loci compared to the controls were modest. A defined pattern of nuclear reorganisation of centromeric loci in sperm heads therefore appears to be a remarkably robust process, even if spermatogenesis is severely compromised.

Constitutive heterochromatin reorganization during somatic cell reprogramming

Induced pluripotent stem (iPS) cell reprogramming is a gradual epigenetic process that reactivates the pluripotent transcriptional network by erasing and establishing repressive epigenetic marks. In contrast to loci-specific epigenetic changes, heterochromatin domains undergo epigenetic resetting during the reprogramming process, but the effect on the heterochromatin ultrastructure is not known. Here, we characterize the physical structure of heterochromatin domains in full and partial mouse iPS cells by correlative electron spectroscopic imaging. In somatic and partial iPS cells, constitutive heterochromatin marked by H3K9me3 is highly compartmentalized into chromocentre structures of densely packed chromatin fibres. In contrast, chromocentre boundaries are poorly defined in pluripotent embryonic stem and full iPS cells, and are characterized by unusually dispersed 10 nm heterochromatin fibres in high Nanog-expressing cells, including pluripotent cells of the mouse blastocyst before differentiation. This heterochromatin reorganization accompanies retroviral silencing during conversion of partial iPS cells by MEK/GSK3 2i inhibitor treatment. Thus, constitutive heterochromatin is compacted in partial iPS cells but reorganizes into dispersed 10 nm chromatin fibres as the fully reprogrammed iPS cell state is acquired.

Specialization of a Drosophila capping protein essential for the protection of sperm telomeres

BACKGROUND

A critical function of telomeres is to prevent fusion of chromosome ends by the DNA repair machinery. In Drosophila somatic cells, assembly of the protecting capping complex at telomeres notably involves the recruitment of HOAP, HP1, and their recently identified partner, HipHop. We previously showed that the hiphop gene was duplicated before the radiation of the melanogaster subgroup of species, giving birth to K81, a unique paternal effect gene specifically expressed in the male germline.

RESULTS

Here we show that K81 specifically associates with telomeres during spermiogenesis, along with HOAP and HP1, and is retained on paternal chromosomes until zygote formation. In K81 mutant testes, capping proteins are not maintained at telomeres in differentiating spermatids, resulting in the transmission of uncapped paternal chromosomes that fail to properly divide during the first zygotic mitosis. Despite the apparent similar capping roles of K81 and HipHop in their respective domain of expression, we demonstrate by in vivo reciprocal complementation analyses that they are not interchangeable. Strikingly, HipHop appeared to be unable to maintain capping proteins at telomeres during the global chromatin remodeling of spermatid nuclei.

CONCLUSIONS

Our data demonstrate that K81 is essential for the maintenance of capping proteins at telomeres in postmeiotic male germ cells. In species of the melanogaster subgroup, HipHop and K81 have not only acquired complementary expression domains, they have also functionally diverged following the gene duplication event. We propose that K81 specialized in the maintenance of telomere protection in the highly peculiar chromatin environment of differentiating male gametes.

Nuclear architecture of resting and LPS-stimulated porcine neutrophils by 3D FISH

Neutrophils are essential components of the innate immune system due to their ability to kill and phagocytose invading microbes. They possess a lobulated nucleus and are capable of extensive and complex changes in response to bacterial stimulation. The aim of our study was to investigate whether the 3D nuclear organization of porcine neutrophils was modified upon stimulation. The organization of centromeres, telomeres, and chromosome territories (chromosomes 2, 3, 7, 8, 12, 13, and 17) was studied on structurally preserved nuclei using 3D fluorescence in situ hybridization, confocal microscopy, and image analysis. By differential labeling of centromeres of acrocentric and metacentric/submetacentric chromosomes, we showed that centromeres associated to form chromocenters but did so preferentially between chromosomes with the same morphology. Upon activation, some of these chromocenters dispersed. Telomeres were also found to form clusters, but their number remained unchanged in lipopolysaccharide-stimulated neutrophils. The analysis of the position of chromocenters and telomere clusters showed a more internal location of the latter compared to the former. The analysis of chromosome territories revealed that homologs were distributed randomly among lobes whatever the cell's status. The volume of these territories was not proportional to chromosome length, and some chromosomes (chr 3, 12, 13, and 17) were more prone to decondensation when neutrophils were stimulated. Thus, our study demonstrated that activation of neutrophils resulted in several modifications of their nuclear architecture: a decrease in the number of non-acrocentric chromocenters and the decondensation of several chromosomes.

Analysis of telomere dynamics in mouse spermatogenesis

A complex meiotic differentiation program generates genetically diverse haploid cells (gametes or spores) to compensate for the genome doubling that occurs at fertilization. To this end, homologous chromosomes must undergo pairing and recombination before they become partitioned in haploid sets by two consecutive meiotic divisions. Chromosome ends (telomeres) contain a protective complex that is crucial for genomic stability. In meiosis, telomeres become key players in the chromosome pairing process during prophase to the first meiotic division. At the onset of prophase I, telomeres attach to the nuclear envelope, about which they move and transiently cluster in a limited sector of the nuclear periphery. The dynamic clustering of telomeres (bouquet formation) occurs at the onset of the zygotene substage and supports homologue recognition, pairing and telomere DNA metabolism. The following chapter outlines the protocols that have been useful in studies on telomere dynamics and the frequency of earliest prophase I stages in testis suspensions of the mouse, and may be useful to address similar questions in particular mouse mutants that become increasingly available.

Higher-order genome organization in platypus and chicken sperm and repositioning of sex chromosomes during mammalian evolution

In mammals, chromosomes occupy defined positions in sperm, whereas previous work in chicken showed random chromosome distribution. Monotremes (platypus and echidnas) are the most basal group of living mammals. They have elongated sperm like chicken and a complex sex chromosome system with homology to chicken sex chromosomes. We used platypus and chicken genomic clones to investigate genome organization in sperm. In chicken sperm, about half of the chromosomes investigated are organized non-randomly, whereas in platypus chromosome organization in sperm is almost entirely non-random. The use of genomic clones allowed us to determine chromosome orientation and chromatin compaction in sperm. We found that in both species chromosomes maintain orientation of chromosomes in sperm independent of random or non-random positioning along the sperm nucleus. The distance of loci correlated with the total length of sperm nuclei, suggesting that chromatin extension depends on sperm elongation. In platypus, most sex chromosomes cluster in the posterior region of the sperm nucleus, presumably the result of postmeiotic association of sex chromosomes. Chicken and platypus autosomes sharing homology with the human X chromosome located centrally in both species suggesting that this is the ancestral position. This suggests that in some therian mammals a more anterior position of the X chromosome has evolved independently.

Epigenetic reprogramming of the male genome during gametogenesis and in the zygote

During post-meiotic maturation, male germ cells undergo a formidable reorganization and condensation of their genome. During this phase most histones are globally acetylated and then replaced by sperm-specific basic proteins, named protamines, which compact the genome into a very specific structure within the sperm nucleus. Several studies suggest that this sperm-specific genome packaging structure conveys an important epigenetic message to the embryo. This paper reviews what is known about this fundamental, yet poorly understood, process, which involves not only global changes of the structure of the haploid genome, but also localized specific modifications of particular genomic regions, including pericentric heterochromatin and sex chromosomes. After fertilization, the male genome undergoes a drastic decondensation, and rapidly incorporates new histones. However, it remains different from the maternal genome, bearing specific epigenetic marks, especially in the pericentric heterochromatin region. The functional implications of male post-meiotic and post-fertilization genome reprogramming are not well known, but there is experimental evidence to show that it affects early embryonic development.

Interindividual differences and alterations in the topology of chromosomes in human sperm nuclei of fertile donors and carriers of reciprocal translocations

Recently it has been shown that the nucleus of the human spermatozoon appears to possess a specific architecture. The current prevailing view is that spatial organization of the male genome contains information critical for the spermatozoon's function as well as for early embryonic development. The purpose of this study was to determine whether there are alterations in intranuclear localization of centromeres in spermatozoa of chromosomes associated with particular reciprocal chromosome translocations (RCT). We analyzed the longitudinal and spatial localization of centromeres of selected chromosomes in sperm nuclei of four control males with normal karyotypes as well as in six carriers of reciprocal chromosome translocations: t(1;7), t(7;2), t(7;13), t(7;9), t(9;14), and t(4;13). Our study revealed that chromosomes with translocations may have shifted their intranuclear localization and that these translocations may influence the localization of other chromosomes in sperm nuclei. The chromocenter in sperm nuclei of translocation carriers was widened toward the apical side in comparison with chromocenter sites visible in control males. Our study also revealed interindividual differences in the localization of the Y chromosome centromere in the chromocenter area of sperm from fertile individuals.

Organization of chromosomes in spermatozoa: an additional layer of epigenetic information?

Elaborate non-random organization of human sperm chromosomes at different structural levels, starting from the DNA packing by protamines up to the higher-order chromosome configuration and nuclear positioning of chromosome territories, has been discovered. Here, we put forward a hypothesis that the unique genome architecture in sperm provides a mechanism for orchestrated unpacking and ordered activation of the male genome during fertilization, thus offering an additional level of epigenetic information that will be deciphered in the descendant cells.

The X and Y chromosomes assemble into H2A.Z-containing [corrected] facultative heterochromatin [corrected] following meiosis

Spermatogenesis is a complex sequential process that converts mitotically dividing spermatogonia stem cells into differentiated haploid spermatozoa. Not surprisingly, this process involves dramatic nuclear and chromatin restructuring events, but the nature of these changes are poorly understood. Here, we linked the appearance and nuclear localization of the essential histone variant H2A.Z with key steps during mouse spermatogenesis. H2A.Z cannot be detected during the early stages of spermatogenesis, when the bulk of X-linked genes are transcribed, but its expression begins to increase at pachytene, when meiotic sex chromosome inactivation (MSCI) occurs, peaking at the round spermatid stage. Strikingly, when H2A.Z is present, there is a dynamic nuclear relocalization of heterochromatic marks (HP1beta and H3 di- and tri-methyl K9), which become concentrated at chromocenters and the inactive XY body, implying that H2A.Z may substitute for the function of these marks in euchromatin. We also show that the X and the Y chromosome are assembled into facultative heterochromatic structures postmeiotically that are enriched with H2A.Z, thereby replacing macroH2A. This indicates that XY silencing continues following MSCI. These results provide new insights into the large-scale changes in the composition and organization of chromatin associated with spermatogenesis and argue that H2A.Z has a unique role in maintaining sex chromosomes in a repressed state.

Chromosome architecture in the decondensing human sperm nucleus

Whereas recent studies demonstrated a well-defined nuclear architecture in human sperm nuclei, little is known about the mode of DNA compaction above the elementary structural unit of nucleoprotamine toroids. Here, using fluorescence in-situ hybridization (FISH) with arm-specific DNA probes of chromosomes 1, 2 and 5, we visualized arm domains and established hierarchical levels of sperm chromatin structures. The compact chromosome territories, which in sperm have a preferred intranuclear localization, have an extended conformation represented by a 2000 nm chromatin fiber. This fiber is composed of a 1000 nm chromatin thread bent at 180 degrees near centromere. Two threads of 1000 nm, representing p-arm and q-arm chromatin, run in antiparallel fashion and join at the telomeres. Each 1000 nm thread, in turn, resolves into two rows of chromatin globules 500 nm in diameter interconnected with thinner chromatin strands. We propose a unified comprehensive model of chromosomal and nuclear architecture in human sperm that, as we suggest, is important for successful fertilization and early development.

Nature of telomere dimers and chromosome looping in human spermatozoa

Specific and well-organized chromosome architecture in human sperm cells is supported by the prominent interactions between centromeres and between telomeres. The telomere-telomere interactions result in telomere dimers that are positioned at the nuclear periphery. It is unknown whether composition of sperm telomere dimers is random or specific. We now report that telomere dimers result from specific interactions between the two ends of each chromosome. FISH using pairs of subtelomeric DNA probes that correspond to the small and long arms of seven human chromosomes demonstrates that subtelomeres of one chromosome are brought together. Statistical analysis confirmed that telomere associations could not result from the random proximity of DNA sequences. Therefore, chromosomes in human sperm nuclei adopt a looped conformation. This higher-order chromosome structure is most likely required for chromosome withdrawal/decondensation during the early fertilization events leading to zygote formation.

Dynamic rearrangement of telomeres during spermatogenesis in mice

Chromosomal structure within the nucleus influences various biological processes such as transcription and replication. Telomeres are located at the end of eukaryotic chromosomes and they can be a decisive factor for correct chromosomal positioning. To gain new insight into telomere dynamics, we examined telomere length and positional changes during spermatogenesis using improved fluorescence in situ hybridization (FISH) and in situ telomeric repeat amplification protocols (TRAP) on histological sections. FISH revealed telomere length and chromosome position within nuclei change dynamically. Telomere extension occurred during spermiogenesis. In situ TRAP analysis verified elevated telomerase activity in elongating spermatids. Together, these data show that elongated spermatids have longer telomeres than precursor spermatogenic cells. This observation indicates that telomere elongation in haploid cells occurs after meiosis and in the absence of genomic replication. Analyses of testes from telomerase null mice further support the significance of telomere dynamics during spermatogenesis and the existence of an alternative telomere extension pathway.

Non-random chromosome positioning in mammalian sperm nuclei, with migration of the sex chromosomes during late spermatogenesis

Chromosomes are highly organized and compartmentalized in cell nuclei. The analysis of their position is a powerful way to monitor genome organization in different cell types and states. Evidence suggests that the organization of the genome could be functionally important for influencing different cellular and developmental processes, particularly at early stages of development (i.e. fertilization and the consequent entry of the sperm nucleus into the egg). The position of chromosomes in the sperm nucleus might be crucial, because their location could determine the time at which particular chromatin domains are decondensed and remodelled, allowing some epigenetic level of control or influence over subsequent paternal gene expression in the embryo. Here, we analyse genome organization by chromosome position in mammalian sperm nuclei from three breeds of pig, as a model species. We have mapped the preferential position of all chromosomes (bar one) in sperm nuclei in two dimensions and have established that the sex chromosomes are the most internally localized chromosomes in mature sperm. The distribution of two autosomes and chromosomes X and Y in sperm heads was compared in primary and secondary spermatocytes and spermatids in porcine testes. The sex chromosomes were found at the nuclear edge in primary spermatocytes, which correlates with the known position of the XY body and their position in somatic cells, whereas, in spermatids, the sex chromosomes were much more centrally located, mirroring the position of these chromosomes in ejaculated spermatozoa. This study reveals the temporal repositioning of chromosome territories in spermatogenesis.

Polar nuclear localization of H1T2, a histone H1 variant, required for spermatid elongation and DNA condensation during spermiogenesis

Spermiogenesis entails a major biochemical and morphological restructuring of the germ cell involving replacement of the somatic histones by protamines packing the DNA into the condensed spermatid nucleus and elimination of the cytoplasm during the elongation phase. We describe H1T2, an histone H1 variant selectively and transiently expressed in male haploid germ cells during spermiogenesis. In round and elongating spermatids, H1T2 specifically localizes to a chromatin domain at the apical pole, revealing a polarity in the spermatid nucleus. Inactivation by homologous recombination shows that H1T2 is critical for spermiogenesis as male H1t2(-/-) mice have greatly reduced fertility. Analysis of spermiogenesis in H1t2 mutant mice shows delayed nuclear condensation and aberrant elongation. As a result, mutant spermatids are characterized by the presence of residual cytoplasm, acrosome detachment, and fragmented DNA. Hence, H1T2 is a protein required for proper cell restructuring and DNA condensation during the elongation phase of spermiogenesis.

Establishment of male-specific epigenetic information

The setting of male-specific epigenetic information is a complex process, which involves a major global re-organisation, as well as localized changes of the nucleus structure during the pre-meiotic, meiotic and post-meiotic stages of the male germ cell differentiation. Although it has long been known that DNA methylation in targeted regions of the genome is associated with male-specific genomic imprinting, or that most core histones are hyperacetylated and then replaced by sperm-specific proteins during the post-meiotic condensation of the nucleus, many questions remain unanswered. How these changes interact, how they affect the epigenetic information and how the paternal epigenetic marks contribute to the future genome are indeed major issues remaining to be explored.

The monotreme genome: a patchwork of reptile, mammal and unique features?

The first specimen of platypus (Ornithorhynchus anatinus) that reached Britain in the late 18th century was regarded a scientific hoax. Over decades the anatomical characteristics of these unique mammals, such as egg laying and the existence of mammary glands, were hotly debated before they were accepted. Within the last 40 years, more and more details of monotreme physiology, histology, reproduction and genetics have been revealed. Some show similarities with birds or reptiles, some with therian mammals, but many are very specific to monotremes. The genome is no exception to monotreme uniqueness. An early opinion was that the karyotype, composed of a few large chromosomes and many small ones, resembled bird and reptile macro- and micro-chromosomes. However, the platypus genome also features characteristics that are not present in other mammals, such as a complex translocation system. The sex chromosome system is still not resolved. Nothing is known about dosage compensation and, unlike in therian mammals, there seems to be no genomic imprinting. In this article we will recount the mysteries of the monotreme genome and describe how we are using recently developed technology to identify chromosomes in mitosis, meiosis and sperm, to map genes to chromosomes, to unravel the sex chromosome system and the translocation chain and investigate X inactivation and genomic imprinting in monotremes.

Telomere-Binding TRF2/MTBP Localization during Mouse Spermatogenesis and Cell Cycle of the Mouse Cells L929

Observations of the organization and distribution of telomeres (Tel) in somatic tissues still remain controversial. The Tel topography revealed by modern microscopy shows them to be associated with the nuclear envelope (NE) in a wide variety of eukaryotic cells, although not at the Rabl orientation (peripheral position at one pole of the nucleus at prophase). We used two cell types that have different nuclear architectures. The cell line L929 shows lack of any rigid Tel architecture in the nucleus. In contrast, spermatozoa have a precise architecture established during spermiogenesis. We observed Tel and membrane Tel binding protein (MTBP/TRF2) position by immunoFISH in L929 cells and by immunofluorescence and immunogold electron microscopy, using antibodies against Membrane Tel Binding Protein (MTBP/TRF2), during different stages of spermiogenesis. At all stages of the L929 cell cycle, MTBP/TRF2 is co-localized with Tel. The only Tel order found in this cell type is similar to the Rabl-orientation, probably due to fast divisions. In the mouse pachytene spermatocytes, the membrane structures abut on the synaptonemal complex (SC) attachment sites contain MTBP/TRF2. In fully formed spermatozoa and during spermiogenesis, apart from the expected MTBP/TRF2 position at the nuclear periphery, MTBP/TRF2 unexpectedly localized at the acrosomal membrane that is adjacent to the nucleus. The difference in the MTBP/TRF2 distribution in the oocyte and spermatozoa leads to the suggestion that the MTBP/TRF2 location might reflect preparation for fertilization events. The Tel distribution is not static in cultured cells throughout the cell cycle or during spermatogenesis. When the Tel are attached to the NE, as during SC formation, MTBP/TRF2 is the member of the protein complex, which appears to be responsible for this attachment.

Telomeres in mammalian male germline cells

Telomeres are terminal chromosomal domains that protect chromosome ends from degradation and fusion and promote complete replication of DNA. Telomeres are involved in the regulation of cellular replicative lifespan and tumorigenesis. These important functions of the telomeres have evoked high interest: numerous studies have resulted in a detailed description of telomere composition and structure in somatic cells. Much less is known about telomeres in germline cells. Emerging novel features and unique behavior of telomeres in the process of gamete differentiation suggest that they may have additional germline-specific function(s). This review describes recent studies revealing changes in the telomere organization in the course of differentiation from the germline stem cells to mature sperm in mammals. Similarities and differences between somatic and spermatogenic cells in telomere nuclear localization, protein composition, DNA length, telomerase activity, and chromatin structure are discussed. The exceptional features of the germline telomeres may be important for regulation of telomerase activity during spermatogenesis, homologous chromosome pairing during recombination, as well as for male pronucleus development and ordered chromosome withdrawal post-fertilization.

Distinct functions of TBP and TLF/TRF2 during spermatogenesis: requirement of TLF for heterochromatic chromocenter formation in haploid round spermatids

TLF (TBP-like factor) is a protein commonly thought to belong to the general transcription initiation complex. TLF is evolutionarily conserved and has been shown to be essential for early development in C. elegans, zebrafish and Xenopus. In mammals however, TLF has a specialised function, as revealed by targeted mutation of the gene in the mouse germline. The TLF mutation elicits a complete arrest of late spermiogenesis and increased haploid cell apoptosis. We explored in more detail the molecular function that TLF plays in the differentiation program of male germ cells. A comparison of TBP and TLF reveals drastic differences, both in their temporal expression pattern and in their intracellular location. While TBP is ubiquitously expressed, TLF expression is strictly developmentally regulated, being very high in late pachytene spermatocytes, suggesting a function prior to the apoptosis of the haploid cells. A refined study of TLF-deficient mice reveals defective acrosome formation in early stage spermatids. Most importantly, our results uncover an unsuspected function of TLF in chromatin organisation. Indeed, early spermatids in TLF-deficient mice display a fragmentation of the chromocenter, a condensed structure formed by the association of centromeric heterochromatin and containing the HP1 proteins. This defect is likely to be the primary cause of spermatogenic failure in the TLF mutant mice.

A Caenorhabditis elegans cohesion protein with functions in meiotic chromosome pairing and disjunction

We have studied four Caenorhabditis elegans homologs of the Rad21/Scc1/Rec8 sister-chromatid cohesion protein family. Based on the RNAi phenotype and protein localization, it is concluded that one of them, W02A2.6p, is the likely worm ortholog of yeast Rec8p. The depletion of C. elegans W02A2.6p (called REC-8) by RNAi, induced univalent formation and splitting of chromosomes into sister chromatids at diakinesis. Chromosome synapsis at pachytene was defective, but primary homology recognition seemed unaffected, as a closer-than-random association of homologous fluorescence in situ hybridization (FISH) signals at leptotene/zygotene was observed. Depletion of REC-8 also induced chromosome fragmentation at diakinesis. We interpret these fragments as products of unrepaired meiotic double-stranded DNA breaks (DSBs), because fragmentation was suppressed in a spo-11 background. Thus, REC-8 seems to be required for successful repair of DSBs. The occurrence of DSBs in REC-8-depleted meiocytes suggests that DSB formation does not depend on homologous synapsis. Anti-REC-8 immunostaining decorated synaptonemal complexes (SCs) at pachytene and chromosomal axes in bivalents and univalents at diakinesis. Between metaphase I and metaphase II, REC-8 is partially lost from the chromosomes. The partial loss of REC-8 from chromosomes between metaphase I and metaphase II suggests that worm REC-8 might function similarly to yeast Rec8p. The loss of yeast Rec8p from chromosome arms at meiosis I and centromeres at meiosis II coordinates the disjunction of homologs and sister chromatids at the two meiotic divisions.

Human sperm telomere-binding complex involves histone H2B and secures telomere membrane attachment

Telomeres are unique chromatin domains located at the ends of eukaryotic chromosomes. Telomere functions in somatic cells involve complexes between telomere proteins and TTAGGG DNA repeats. During the differentiation of germ-line cells, telomeres undergo significant reorganization most likely required for additional specific functions in meiosis and fertilization. A telomere-binding protein complex from human sperm (hSTBP) has been isolated by detergent treatment and was partially purified. hSTBP specifically binds double-stranded telomeric DNA and does not contain known somatic telomere proteins TRF1, TRF2, and Ku. Surprisingly, the essential component of this complex has been identified as a specific variant of histone H2B. Indirect immunofluorescence shows punctate localization of H2B in sperm nuclei, which in part coincides with telomeric DNA localization established by fluorescent in situ hybridization. Anti-H2B antibodies block interactions of hSTBP with telomere DNA, and spH2B forms specific complex with this DNA in vitro, indicating that this protein plays a role in telomere DNA recognition. We propose that hSTBP participates in the membrane attachment of telomeres that may be important for ordered chromosome withdrawal after fertilization.

Chromatin structure of telomere domain in human sperm

Telomeres in human sperm nucleus are clustered at the nuclear periphery. Chromosomes in the sperm are highly condensed with protamines, however, a small portion of DNA remains associated with histones; the role of the nucleohistone is unknown. To examine structure of the telomeric chromatin, the sperm nuclei were treated with micrococcal nuclease. Chromatin released by the digestion was free from protamines, but contained histones and revealed nucleosomal organization. It was enriched with telomeric DNA organized into closely spaced nucleosomes with a periodicity of 148 +/- bp. Thus, while the most of the sperm genome is packed into extremely dense nucleoprotamine structure, at least a part of the telomeric DNA is arranged into nucleosomes and can be released by the nuclease. We suggest that telomeres might be among the first structures in the sperm nucleus that respond to oocyte signals for male pronucleus development at fertilization.

Mammalian meiotic telomeres: protein composition and redistribution in relation to nuclear pores

Mammalian telomeres consist of TTAGGG repeats, telomeric repeat binding factor (TRF), and other proteins, resulting in a protective structure at chromosome ends. Although structure and function of the somatic telomeric complex has been elucidated in some detail, the protein composition of mammalian meiotic telomeres is undetermined. Here we show, by indirect immunofluorescence (IF), that the meiotic telomere complex is similar to its somatic counterpart and contains significant amounts of TRF1, TRF2, and hRap1, while tankyrase, a poly-(ADP-ribose)polymerase at somatic telomeres and nuclear pores, forms small signals at ends of human meiotic chromosome cores. Analysis of rodent spermatocytes reveals Trf1 at mouse, TRF2 at rat, and mammalian Rap1 at meiotic telomeres of both rodents. Moreover, we demonstrate that telomere repositioning during meiotic prophase occurs in sectors of the nuclear envelope that are distinct from nuclear pore-dense areas. The latter form during preleptotene/leptotene and are present during entire prophase I.

Isolation and characterization of Suv39h2, a second histone H3 methyltransferase gene that displays testis-specific expression

Higher-order chromatin has been implicated in epigenetic gene control and in the functional organization of chromosomes. We have recently discovered mouse (Suv39h1) and human (SUV39H1) histone H3 lysine 9-selective methyltransferases (Suv39h HMTases) and shown that they modulate chromatin dynamics in somatic cells. We describe here the isolation, chromosomal assignment, and characterization of a second murine gene, Suv39h2. Like Suv39h1, Suv39h2 encodes an H3 HMTase that shares 59% identity with Suv39h1 but which differs by the presence of a highly basic N terminus. Using fluorescent in situ hybridization and haplotype analysis, the Suv39h2 locus was mapped to the subcentromeric region of mouse chromosome 2, whereas the Suv39h1 locus resides at the tip of the mouse X chromosome. Notably, although both Suv39h loci display overlapping expression profiles during mouse embryogenesis, Suv39h2 transcripts remain specifically expressed in adult testes. Immunolocalization of Suv39h2 protein during spermatogenesis indicates enriched distribution at the heterochromatin from the leptotene to the round spermatid stage. Moreover, Suv39h2 specifically accumulates with chromatin of the sex chromosomes (XY body) which undergo transcriptional silencing during the first meiotic prophase. These data are consistent with redundant enzymatic roles for Suv39h1 and Suv39h2 during mouse development and suggest an additional function of the Suv39h2 HMTase in organizing meiotic heterochromatin that may even impart an epigenetic imprint to the male germ line.

Genome organization in the human sperm nucleus studied by FISH and confocal microscopy

The sperm nucleus has a unique chromatin structure where the DNA is highly condensed and associated with specific proteins, the protamines. It is a nondividing cell which is also transcriptionally inactive. After fusion with an oocyte, the sperm nucleus undergoes decondensation and, in the same time, starts replication and transcription. It has been suggested that somatic chromosomes during interphase are organized in territories which display a cell type and cell cycle specific distribution. The purpose of this work was to investigate whether chromosomes would also have a specific distribution in the sperm nucleus, which could be related to its inactive state, and have implications on the early stages of fertilization. In the present study, centromeric and telomeric sequences were detected by fluorescent techniques performed on human decondensed spermatozoa. Chromosome painting probes were used to detect the chromosome X and chromosome 13 on interphase sperm nuclei. The fluorescent signals were captured in 3D with a confocal microscope. For each of these chromatin structures, the volume, position, and distribution of the signals were analyzed in samples of 30 nuclei with the help of image analysis software. The centromeres appeared grouped in several foci that were randomly distributed within the sperm nucleus. The telomeres gave an approximately haploid number of small signals, evenly distributed throughout the nucleus. The chromosomes X and 13 occupied 4.7% and 3. 7% of the total nuclear volume, respectively. Interestingly, the X chromosome territory showed a preferential position in the anterior half of the volume of the nucleus, whereas chromosome 13 had a random position. This work shows a particular distribution of chromosome territories in the human sperm nucleus that could be related to mechanisms implicated in its specific functions. The analysis of more chromosomes and chromosomal structures, including the Y chromosome, would help to understand the structure of the human sperm chromatin, and its fundamental and clinical implications.

Identification and characterization of a bovine sperm protein that binds specifically to single-stranded telomeric deoxyribonucleic acid

Telomere DNA at the physical termini of chromosomes forms a single-stranded 3' overhang. In lower eukaryotes, e.g., ciliated protozoa, this DNA extension is capped by specific proteins that have been structurally and functionally characterized. Much less is known about single-stranded telomere DNA-binding proteins in vertebrates. Here we describe a new protein from bovine sperm designated bsSSTBP that specifically interacts with single-stranded (TTAGGG)(N) DNA. The bsSSTBP was extracted from nuclei by 0.6 M KCl. The native size of this protein, estimated by gel filtration, was 20-40 kDa. SDS-PAGE of the UV cross-linked complex between bsSSTBP and telomere DNA indicated that several polypeptides are involved in complex formation. Bovine sSSTB had high specificity toward nucleotide sequence, since single nucleotide substitutions in the (TTAGGG)(4) substrate suppressed binding. The minimal number of (TTAGGG) repeats required for binding of bsSSTBP was 3, and the protein recognized linear but not folded DNA structures. We propose that the bsSSTBP participates in telomere-telomere interactions and the telomere membrane localization observed in mature sperm. In mammals, somatic telomere-binding proteins are apparently substituted by sperm-specific ones that may lead to a structural reorganization of telomere domains to fulfill functions important during meiosis and fertilization.

The murine heterochromatin protein M31 is associated with the chromocenter in round spermatids and Is a component of mature spermatozoa

In mature sperm the normal nucleosomal packaging of DNA found in somatic and meiotic cells is transformed into a highly condensed form of chromatin which consists mostly of nucleoprotamines. Although sperm DNA is highly condensed it is nevertheless packaged into a highly defined nuclear architecture which may be organized by the heterochromatic chromocenter. One major component of heterochromatin is the heterochromatin protein 1 which is involved in epigenetic gene silencing. In order to investigate the possible involvement of heterochromatin protein in higher order organization of sperm DNA we studied the localization of the murine homologue of heterochromatin protein 1, M31, during chromatin reorganization in male germ cell differentiation. Each cell type in the testis showed a unique distribution pattern of M31. Colocalization to the heterochromatic regions were found in Sertoli cells, in midstage pachytene spermatocytes, and in round spermatids in which M31 localizes to the centromeric chromocenter. M31 cannot be detected in elongated spermatids or mature spermatozoa immunocytologically, but could be detected in mature spermatozoa by Western blotting. We suggest that M31, a nuclear protein involved in the organization of chromatin architecture, is involved in higher order organization of sperm DNA.