Regulated hyperacetylation of core histones during mouse spermatogenesis: involvement of histone deacetylases

Nuclear regulator Pygo2 controls spermiogenesis and histone H3 acetylation

Mammalian spermiogenesis, a process where haploid male germ cells differentiate to become mature spermatozoa, entails dramatic morphological and biochemical changes including remodeling of the germ cell chromatin. Proteins that contain one or more plant homeodomain (PHD) fingers have been implicated in the regulation of chromatin structure and function. Pygopus 2 (Pygo2) belongs to a family of evolutionarily conserved PHD finger proteins thought to act as co-activators of Wnt signaling effector complexes composed of beta-catenin and LEF/TCF transcription factor. Here we analyze mice containing hypomorphic alleles of pygopus 2 (Pygo2 or mpygo2) and uncover a beta-catenin-independent involvement of the Pygo2 protein in spermiogenesis. Pygo2 is expressed in elongating spermatids at stages when chromatin remodeling occurs, and block of Pygo2 function leads to spermiogenesis arrest and consequent infertility. Analysis of spermiogenesis in Pygo2 mutants reveals reduced expression of select post-meiotic genes including protamines, transition protein 2, and H1fnt, all of which are required for germ cell chromatin condensation, and drastically altered pattern of histone H3 hyperacetylation. These findings suggest that Pygo2 is involved in the chromatin remodeling events that lead to nuclear compaction of male germ cells.

Acetylation of histone H4 in complex structural transitions of spermiogenic chromatin

In spermiogenic nuclei of the cephalopod mollusc Sepia officinalis histones are replaced by a precursor-protamine molecule, which is later converted into protamine. Simultaneously, spermiogenic chromatin undergoes a complex structural change. Somatic-like chromatin belonging to the earliest spermatid is progressively reorganized into: (a) granules of 20 nm diameter, (b) fibres of 30-35 nm, and (c) fibres of 40-50 nm. In the final phases of spermiogenesis these fibres of 40-50 nm join to form larger structures of condensed chromatin, and lastly, the uniformly packed chromatin in the sperm nucleus. Using specific antibodies for mono- and hyperacetylated forms of histone H4, in this work we show that the first structural remodelling of chromatin (from somatic-like organization into 20 nm granules) is given concomitantly with a massive mono-acetylation of H4 (acetylation in lysine 12), whereas the structural remodelling from 30-35 to 40-50 nm fibres is produced simultaneously with hyperacetylation of H4 and the nuclear removal of histones.

The dual bromodomain and WD repeat-containing mouse protein BRWD1 is required for normal spermiogenesis and the oocyte-embryo transition

A novel mutation, repro5, was isolated in a forward genetic screen for infertility mutations induced by ENU mutagenesis. Homozygous mutant mice were phenotypically normal but were infertile. Oocytes from mutant females appeared normal, but were severely maturation-defective in that they had reduced ability to progress to metaphase II (MII), and those reaching MII were unable to progress beyond the two pronuclei stage following in vitro fertilization (IVF). Mutant males exhibited defective spermiogenesis, resulting in oligoasthenoteratospermia. Genetic mapping, positional cloning, and complementation studies with a disruption allele led to the identification of a mutation in Brwd1 (Bromodomain and WD repeat domain containing 1) as the causative lesion. Bromodomain-containing proteins typically interact with regions of chromatin containing histones hyperacetylated at lysine residues, a characteristic of chromatin in early spermiogenesis before eventual replacement of histones by the protamines. Previous data indicated that Brwd1 is broadly expressed, encoding a putative transcriptional regulator that is believed to act on chromatin through interactions with the Brg1-dependent SWI/SNF chromatin-remodeling pathway. Brwd1 represents one of a small number of genes whose elimination disrupts gametogenesis in both sexes after the major events of meiotic prophase I have been completed.

Mice lacking histone deacetylase 6 have hyperacetylated tubulin but are viable and develop normally

Posttranslational modifications play important roles in regulating protein structure and function. Histone deacetylase 6 (HDAC6) is a mostly cytoplasmic class II HDAC, which has a unique structure with two catalytic domains and a domain binding ubiquitin with high affinity. This enzyme was recently identified as a multisubstrate protein deacetylase that can act on acetylated histone tails, alpha-tubulin and Hsp90. To investigate the in vivo functions of HDAC6 and the relevance of tubulin acetylation/deacetylation, we targeted the HDAC6 gene by homologous recombination in embryonic stem cells and generated knockout mice. HDAC6-deficient mice are viable and fertile and show hyperacetylated tubulin in most tissues. The highest level of expression of HDAC6 is seen in the testis, yet development and function of this organ are normal in the absence of HDAC6. Likewise, lymphoid development is normal, but the immune response is moderately affected. Furthermore, the lack of HDAC6 results in a small increase in cancellous bone mineral density, indicating that this deacetylase plays a minor role in bone biology. HDAC6-deficient mouse embryonic fibroblasts show apparently normal microtubule organization and stability and also show increased Hsp90 acetylation correlating with impaired Hsp90 function. Collectively, these data demonstrate that mice survive well without HDAC6 and that tubulin hyperacetylation is not detrimental to normal mammalian development.

Histone H4 acetylation and AZFc involvement in germ cells of specimens of impaired spermatogenesis

OBJECTIVE

To measure histone-H4 acetylation and involvement of the AZFc region in testicular mixed atrophy.

DESIGN

Prospective study.

SETTING

University-affiliated medical center.

PATIENT(S)

Azoospermic men (n = 23) who underwent testicular sperm extraction and preparation for intracytoplasmic sperm injection (ICSI) divided into obstructive azoospermia with complete spermatogenesis (group A), testicular mixed atrophy (group B), and testicular mixed atrophy associated with AZFc deletion (group C).

INTERVENTION(S)

Testicular biopsy evaluation by Western blotting and quantitative immunohistochemistry of histone-H4 hyperacetylation (Hypac-H4) and lysine-12 acetylation (Lys12ac-H4).

MAIN OUTCOME MEASURE(S)

Percentage of spermatogonia and spermatids stained by Hypac-H4 and Lys12ac-H4 antibodies in retrieved specimens.

RESULT(S)

The percentage of spermatogonia stained for Hypac-H4 and Lys12ac-H4 in groups B and C was statistically significantly reduced. The percentage of elongated spermatids showing positive staining to Hypac-H4 was statistically significantly lower in group B than group A. The percentage of Lys12ac-H4-labeled spermatids was similar for all groups. Hypac-H4 and Lys12ac-H4 processes were highly correlated in spermatogonia but not in spermatids.

CONCLUSION(S)

The reduced percentage of spermatogonia with Hypac-H4 and Lys12ac-H4 in groups B and C may contribute to lower sperm production in mixed atrophy. Spermatids Hypac-H4 impairment in mixed atrophy did not deteriorate further by AZFc region deletion.

Investigation of MYST4 histone acetyltransferase and its involvement in mammalian gametogenesis

Background

Various histone acetylases (HATs) play a critical role in the regulation of gene expression, but the precise functions of many of those HATs are still unknown. Here we provide evidence that MYST4, a known HAT, may be involved in early mammalian gametogenesis.

Results

Although MYST4 mRNA transcripts are ubiquitous, protein expression was restricted to select extracts (including ovary and testis). Immunohistochemistry experiments performed on ovary sections revealed that the MYST4 protein is confined to oocytes, granulosa and theca cells, as well as to cells composing the blood vessels. The transcripts for MYST4 and all-MYST4-isoforms were present in oocytes and in in vitro produced embryos. In oocytes and embryos the MYST4 protein was localized in both the cytoplasm and nucleus. Within testis sections, the MYST4 protein was specific to only one cell type, the elongating spermatids, where it was exclusively nuclear.

Conclusion

We established that MYST4 is localized into specialized cells of the ovary and testis. Because the majority of these cells are involved in male and female gametogenesis, MYST4 may contribute to important and specific acetylation events occurring during gametes and embryo development.

Pericentric heterochromatin reprogramming by new histone variants during mouse spermiogenesis

During male germ cell postmeiotic maturation, dramatic chromatin reorganization occurs, which is driven by completely unknown mechanisms. For the first time, we describe a specific reprogramming of mouse pericentric heterochromatin. Initiated when histones undergo global acetylation in early elongating spermatids, this process leads to the establishment of new DNA packaging structures organizing the pericentric regions in condensing spermatids. Five new histone variants were discovered, which are expressed in late spermiogenic cells. Two of them, which we named H2AL1 and H2AL2, specifically mark the pericentric regions in condensing spermatids and participate in the formation of new nucleoprotein structures. Moreover, our investigations also suggest that TH2B, an already identified testis-specific H2B variant of unknown function, could provide a platform for the structural transitions accompanying the incorporation of these new histone variants.

Immunolocalization assessment of metastasis-associated protein 1 in human and mouse mature testes and its association with spermatogenesis

AIM

To investigate the stage-specific localization of metastasis-associated protein 1 (MTA1) during spermatogenesis in adult human and mouse testis.

METHODS

The immunolocalization of MTA1 was studied by immunohistochemistry and Western blot analysis. The distribution pattern of MTA1 in mouse testis was confirmed by using quantitative analysis of purified spermatogenic cells.

RESULTS

The specificity of polyclonal antibody was confirmed by Western blot analysis. MTA1 was found expressed in the nucleus of germ cells, except elongate spermatids, and in the cytoplasm of Sertoli cells; Leydig cells did not show any specific reactivity. MTA1 possessed different distribution patterns in the two species: in humans, the most intensive staining was found in the nucleus of round spermatids and of primary spermatocytes while in mice, the most intense MTA1 staining was in the nucleus of leptotene, zygotene and pachytene spermatocytes. In both species the staining exhibited a cyclic pattern.

CONCLUSION

The present communication initially provides new evidence for the potential role of MTA1 in mature testis. In addition, its distinctive expression in germ cells suggests a regulatory role of the peptide during spermatogenesis.

Transition of nuclear proteins and chromatin structure in spermiogenesis of Sepia officinalis

During spermiogenesis of Sepia officinalis histones are directly substituted by a molecule of precursor protamine, which is later transformed into the protamine through a deletion of the amino terminal end. In the present work, it is shown that the pattern of spermiogenic chromatin condensation consists of a phase of "patterning" and a phase of "condensation." In the phase of patterning, three structural remodelings are produced in the chromatin structure: [somatic-like chromatin --> 18 nm granules --> 25 nm fibers --> 44 nm fibers]. The first remodeling of the chromatin into granules of 18 nm takes place without the entrance of specific proteins in the spermiogenic nuclei. The second remodeling [granules of 18 nm --> fibers of 25 nm] is due to the entrance of the precursor protamine and its interaction with the DNA-histone complex. The third remodeling [fibers of 25 nm --> fibers of 44 nm] occurs simultaneously with the disappearance of histones from the chromatin. In the phase of condensation, the fibers of 44 nm coalesce among themselves to form progressively larger aggregates of chromatin. In this phase there are no substantial variations in the nuclear proteins, so that the condensation of the chromatin must respond to posttranscriptional changes of the precursor protamine (dephosphorylation, deletion of the amino-terminal end).

Differential histone modifications mark mouse imprinting control regions during spermatogenesis

Only some imprinting control regions (ICRs) acquire their DNA methylation in the male germ line. These imprints are protected against the global demethylation of the sperm genome following fertilisation, and are maintained throughout development. We find that in somatic cells and tissues, DNA methylation at these ICRs is associated with histone H4-lysine-20 and H3-lysine-9 trimethylation. The unmethylated allele, in contrast, has H3-lysine-4 dimethylation and H3 acetylation. These differential modifications are also detected at maternally methylated ICRs, and could be involved in the somatic maintenance of imprints. To explore whether the post-fertilisation protection of imprints relates to events during spermatogenesis, we assayed chromatin at stages preceding the global histone-to-protamine exchange. At these stages, H3-lysine-4 methylation and H3 acetylation are enriched at maternally methylated ICRs, but are absent at paternally methylated ICRs. H4 acetylation is enriched at all regions analysed. Thus, paternally and maternally methylated ICRs carry different histone modifications during the stages preceding the global histone-to-protamine exchange. These differences could influence the way ICRs are assembled into specific structures in late spermatogenesis, and may thus influence events after fertilisation.

Immunohistochemical distribution of hyperacetylated histone H4 in testis of paddlefish Polyodon spathula: ultrastructural correlation with chromatin condensation

The acetylation of core histones has been correlated with the deposition of free histones onto newly replicated DNA, transcriptional activity and the displacement of histones by protamines during spermiogenesis. The aim of the present study was to investigate the immunohistochemical distribution of hyperacetylated H4 during spermatogenesis in Polyodon spathula and to correlate these findings with the pattern of chromatin condensation in spermatids. In immature testis, the Sertoli cells showed more intense immunoreactivity for highly acetylated H4 than that of most primary spermatogonia. The testis of paddlefish at the beginning of spermatogenesis possessed early secondary spermatogonia and late secondary spermatogonia/preleptotene primary spermatocyte with intense nuclear immunoreactivity for highly acetylated H4. In seminiferous tubules in which secondary spermatogonia nuclei were intensely immunostained, Sertoli cell nuclei were unstained. Testes in late spermatogenesis contained tubules in which the immunohistochemical reaction for highly acetylated H4 was positive in the nuclei of preleptotene primary spermatocytes and secondary spermatocytes and in spermatids at the beginning of the elongation process. No immunostaining was found in round spermatids and spermatozoa. During the resting stage, immunostaining was confined to the nuclei of spermatogonia and the cells from the interstitial tissue. Transmission electron microscopy revealed that early spermatids had a round nucleus with filaments of fine chromatin that were dispersed or condensed in clumps. In later stages of maturation, the spermatids had slightly oval nuclei and homogeneous granular chromatin. The chromatin of advanced spermatids was organized into thick fibres. At the end of spermiogenesis, spermatozoan nuclei consisted of homogeneous highly compacted chromatin.

Post-meiotic shifts in HSPA2/HSP70.2 chaperone activity during mouse spermatogenesis

HSPA2 (formerly HSP70.2) is a testis-specific member of the HSP70 family known to play a critical role in the completion of meiosis during male germ cell differentiation. Although abundantly present in post-meiotic cells, its function during spermiogenesis remained obscure. Here, using a global proteomic approach to identify genome-organizing proteins in condensing spermatids, we discovered an unexpected role for HSPA2, which acquires new functions and becomes tightly associated with major spermatid DNA-packaging proteins, transition proteins 1 and 2. Hence, HSPA2 is identified here as the first transition protein chaperone, and these data shed a new light on the yet totally unknown process of genome-condensing structure assembly in spermatids.

Post-meiotic shifts in HSPA2/HSP70.2 chaperone activity during mouse spermatogenesis

HSPA2 (formerly HSP70.2) is a testis-specific member of the HSP70 family known to play a critical role in the completion of meiosis during male germ cell differentiation. Although abundantly present in post-meiotic cells, its function during spermiogenesis remained obscure. Here, using a global proteomic approach to identify genome-organizing proteins in condensing spermatids, we discovered an unexpected role for HSPA2, which acquires new functions and becomes tightly associated with major spermatid DNA-packaging proteins, transition proteins 1 and 2. Hence, HSPA2 is identified here as the first transition protein chaperone, and these data shed a new light on the yet totally unknown process of genome-condensing structure assembly in spermatids.

DNMT1 and HDAC1 gene expression in impaired spermatogenesis and testicular cancer

DNA methylation catalyzed by DNA methyltransferases (DNMTs) and histone deacetylation catalyzed by histone deacetylases (HDACs) play an important role for the regulation of gene expression during carcinogenesis and spermatogenesis. We therefore studied the cell-specific expression of DNMT1 and HDAC1 for the first time in human testicular cancer and impaired human spermatogenesis. During normal spermatogenesis, DNMT1 and HDAC1 were colocalized in nuclei of spermatogonia. While HDAC1 was additionally present in nuclei of Sertoli cells, DNMT1 was restricted to germ cells exhibiting a different expression pattern of mRNA (in pachytene spermatocytes and round spermatids) and protein (in round spermatids). Interestingly, in infertile patients revealing round spermatid maturation arrest, round spermatids lack DNMT1 protein, while pachytene spermatocytes became immunopositive for DNMT1. In contrast, no changes in the expression pattern could be observed for HDAC1. This holds true also in testicular tumors, where HDAC1 has been demonstrated in embryonal carcinoma, seminoma and teratoma. Interestingly, DNMT1 was not expressed in seminoma, but upregulated in embryonal carcinoma.

[Epigenetics of the sperm cell]

In addition to genetic information, the spermatozoon carries another type of information, named epigenetic, which is not associated with variations of the DNA sequence. In somatic cells, it is now generally admitted that epigenetic information is not only regulated by DNA methylation but also involves modifications of the genome structure, or epigenome. During male germ cell maturation, the epigenome is globally re-organized, since most histones, which are associated to DNA in somatic cells, are removed and replaced by sperm specific nuclear proteins, the protamines, responsible for the tight compaction of the sperm DNA. However, a small proportion of histones, and probably other proteins, are retained within the sperm nucleus, and the structure of the sperm genome is actually heterogeneous. This heterogeneity of the sperm epigenome could support an epigenetic information, transmitted to the embryo, which could be crucial for its development. Although it is nowadays possible to appreciate the global structure of the sperm genome, the precise constitution of the sperm epigenome remains unknown. In particular, very recent data suggest that specific regions of the genome could be associated with particular proteins and define specific structures. This structural partitioning of the sperm genome could convey important epigenetic information, crucial for the embryo development.

Histone acetylation-mediated chromatin compaction during mouse spermatogenesis

One of the most dramatic chromatin remodelling events takes place during mammalian spermatogenesis involving massive incorporation of somatic and testis-specific histone variants, as well as generalized histone modifications before their replacement by new DNA packaging proteins. Our data suggest that the induced histone acetylation occurring after meiosis may direct the first steps of genome compaction. Indeed, a double bromodomain-containing protein expressed in postmeiotic cells, Brdt, shows the extraordinary capacity to specifically condense acetylated chromatin in vivo and in vitro. In elongating spermatids, Brdt widely co-localizes with acetylated histones before accumulating in condensed chromatin domains. These domains preferentially maintain their acetylation status until late spermatogenesis. Based on these data, we propose that Brdt mediates a general histone acetylation-induced chromatin compaction and also maintains differential acetylation of specific regions, and is therefore involved in organizing the spermatozoon's genome.

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.

Dynamic expression of DNMT3a and DNMT3b isoforms during male germ cell development in the mouse

In the male germ line, sequence-specific methylation patterns are initially acquired prenatally in diploid gonocytes and are further consolidated after birth during spermatogenesis. It is still unclear how DNA methyltransferases are involved in establishing and/or maintaining these patterns in germ cells, or how their activity is regulated. We compared the temporal expression patterns of the postulated de novo DNA methyltransferases DNMT3a and DNMT3b in murine male germ cells. Mitotic, meiotic and post-meiotic male germ cells were isolated, and expression of various transcript variants and isoforms of Dnmt3a and Dnmt3b was examined using Quantitative RT-PCR and Western blotting. We found that proliferating and differentiating male germ cells were marked by distinctive expression profiles. Dnmt3a2 and Dnmt3b transcripts were at their highest levels in type A spermatogonia, decreased dramatically in type B spermatogonia and preleptotene spermatocytes and rose again in leptotene/zygotene spermatocytes, while Dnmt3a expression was mostly constant, except in type B spermatogonia where it increased. In all cases, expression declined as pachynema progressed. At the protein level, DNMT3a was the predominant isoform in type B spermatogonia, while DNMT3a2, DNMT3b2, and DNMT3b3 were expressed throughout most of spermatogenesis, except in pachytene spermatocytes. We also detected DNMT3a2 and DNMT3b2 in round spermatids. Taken together, these data highlight the tightly regulated expression of these genes during spermatogenesis and provide evidence that DNMTs may be contributing differentially to the establishment and/or maintenance of methylation patterns in male germ cells.

Transmission of modified nucleosomes from the mouse male germline to the zygote and subsequent remodeling of paternal chromatin

Rapidly after gamete fusion, the sperm nucleus loses its specific chromatin conformation and the DNA is repopulated with maternally derived nucleosomes. We evaluated the nature of paternally derived nucleosomes and the dynamics of sperm chromatin remodeling in the zygote directly after gamete fusion. We observed histone H4 acetylated at K8 or K12 already prior to full decondensation of the sperm nucleus, suggesting that these marks are transmitted by the spermatozoon. Tracking down the origin of H4K8ac and H4K12ac during spermiogenesis revealed the retention of nucleosomes with these modifications in the chromocenter of elongating spermatids. We show that sperm constitutive heterochromatin is enriched for nucleosomes carrying specific histone modifications which are transmitted to the zygote. Our results suggest an epigenetic mechanism for inheritance of chromosomal architecture. Furthermore, up to pronucleus formation, histone acetylation and phosphorylation build up in a cascade-like fashion in the paternal chromatin. After formation of the pronucleus, a subset of these marks is removed from the heterochromatin, which suggests a reestablishment of the euchromatin-heterochromatin partition.

Basic mechanisms for the control of germ cell gene expression

The patterns of gene expression in spermatocytes and oocytes are quite different from those in somatic cells. The messenger RNAs produced by these cells are not only required to support germ cell development but, in the case of oocytes, they are also used for maturation, fertilization, and early embryogenesis. Recent studies have begun to provide an explanation for how germ-cell-specific programs of gene expression are generated. Part of the answer comes from the observation that germ cells express core promoter-associated regulatory factors that are different from those expressed in somatic cells. These factors supplement or replace their somatic counterparts to direct expression during meiosis and gametogenesis. In addition, germ cell transcription involves the recognition and use of specialized core promoter sequences. Finally, transcription must occur on chromosomal DNA templates that are reorganized into new chromatin-packaging configurations using alternate histone subunits. This article will review recent advances in our understanding of the factors and mechanisms that control transcription in ovary and testis and will discuss models for germ cell gene expression.

Mechanisms and consequences of paternally-transmitted chromosomal abnormalities

Paternally-transmitted chromosomal damage has been associated with pregnancy loss, developmental and morphological defects, infant mortality, infertility, and genetic diseases in the offspring, including cancer. There is epidemiological evidence linking paternal exposure to occupational or environmental agents with an increased risk of abnormal reproductive outcomes. There is also a large body of literature on germ cell mutagenesis in rodents showing that treatment of male germ cells with mutagens has dramatic consequences on reproduction, producing effects such as those observed in human epidemiological studies. However, we know very little about the etiology, transmission, and early embryonic consequences of paternally-derived chromosomal abnormalities. The available evidence suggests that: 1) there are distinct patterns of germ cell-stage differences in the sensitivity of induction of transmissible genetic damage, with male postmeiotic cells being the most sensitive; 2) cytogenetic abnormalities at first metaphase after fertilization are critical intermediates between paternal exposure and abnormal reproductive outcomes; and 3) there are maternal susceptibility factors that may have profound effects on the amount of sperm DNA damage that is converted into chromosomal aberrations in the zygote and that directly affect the risk for abnormal reproductive outcomes.

On the Nature and Origin of DNA Strand Breaks in Elongating Spermatids1

Transient DNA strand breaks are generated in the whole population of elongating spermatids and are perfectly coincident with histone H4 hyperacetylation at chromatin-remodeling steps. Given the limited DNA repair capacity of elongating spermatids, chromatin remodeling may present a threat to genetic integrity of the male gamete. The nature of the DNA strand breakage, the enzymes involved, and the role of H4 hyperacetylation in the process must be determined to further investigate the potential mutagenic consequences of this important transition. We used the metachromatic dye acridine orange in combination with fluorescence-activated cell sorting to achieve separation of spermatids according to their condensation state. Using single-cell electrophoresis (comet assay), in both alkaline and neutral conditions, we demonstrated that double-stranded breaks account for most of the DNA fragmentation observed in purified elongating spermatids. DNA strand breaks were generated in round spermatids as a result of de novo histone hyperacetylation induced by trichostatin A, whereas an increase in endogenous DNA strand breaks was observed in elongating spermatids. Using a short-term culture of testicular cells, we demonstrated that DNA strand breaks in spermatids were abolished on incubation with two functionally different topoisomerase II inhibitors. Hence, topoisomerase II appears as the unique enzyme responsible for the transient double-stranded breaks in elongating spermatids but depends on histone hyperacetylation for its activity.

Epigenetic programming in the preimplantation rat embryo is disrupted by chronic paternal cyclophosphamide exposure

Preconceptional paternal exposure to cyclophosphamide, a widely used anticancer agent, leads to increases in embryo loss, malformations, and behavioral deficits in offspring; these abnormalities are transmissible to subsequent generations [Auroux, M., Dulioust, E., Selva, J. & Rince, P. (1990) Mutat. Res. 229, 189-200]. Little information exists on the mechanisms underlying this male-mediated developmental toxicity. We assessed the impact of paternal cyclophosphamide exposure on the dynamic regulation of histone H4 acetylation at lysine 5 and DNA methylation in preimplantation rat embryos. Zygotes sired by drug-treated males displayed advanced developmental progression, increased pronuclear areas, and disruption of the epigenetic programming of both parental genomes. Early postfertilization zygotic pronuclei were hyperacetylated; by mid-zygotic development, male pronuclei were dramatically hypomethylated, whereas female pronuclei were hypermethylated. Micronuclei were substantially elevated, and histone H4 acetylation at lysine 5 localization to the nuclear periphery was disrupted in two-cell embryos fertilized by cyclophosphamide-exposed spermatozoa. This finding demonstrates that paternal exposure to this drug induces aberrant epigenetic programming in early embryos. We hypothesize that disturbances in epigenetic programming contribute to heritable instabilities later in development, emphasizing the importance of epigenetic risk assessment after chemotherapy.

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.

Meiotic and epigenetic defects in Dnmt3L-knockout mouse spermatogenesis

The production of mature germ cells capable of generating totipotent zygotes is a highly specialized and sexually dimorphic process. The transition from diploid primordial germ cell to haploid spermatozoa requires genome-wide reprogramming of DNA methylation, stage- and testis-specific gene expression, mitotic and meiotic division, and the histone-protamine transition, all requiring unique epigenetic control. Dnmt3L, a DNA methyltransferase regulator, is expressed during gametogenesis, and its deletion results in sterility. We found that during spermatogenesis, Dnmt3L contributes to the acquisition of DNA methylation at paternally imprinted regions, unique nonpericentric heterochromatic sequences, and interspersed repeats, including autonomous transposable elements. We observed retrotransposition of an LTR-ERV1 element in the DNA from Dnmt3L-/- germ cells, presumably as a result of hypomethylation. Later in development, in Dnmt3L-/- meiotic spermatocytes, we detected abnormalities in the status of biochemical markers of heterochromatin, implying aberrant chromatin packaging. Coincidentally, homologous chromosomes fail to align and form synaptonemal complexes, spermatogenesis arrests, and spermatocytes are lost by apoptosis and sloughing. Because Dnmt3L expression is restricted to gonocytes, the presence of defects in later stages reveals a mechanism whereby early genome reprogramming is linked inextricably to changes in chromatin structure required for completion of spermatogenesis.

In vivo effects of histone-deacetylase inhibitor trichostatin-A on murine spermatogenesis

The acetylation state of core histones is controlled by two classes of enzymes, histone acetyl transferases (HATs) and histone deacetylases (HDACs). HDAC inhibitors, such as trichostatin-A (TSA), are able to induce cell cycle arrest by stimulating transcription of genes that negatively regulate cell growth and survival. However, little is known about the effect of HDAC inhibitors on spermatogenesis. TSA treatment of cultured murine germ cells from whole testes resulted in an increase of histone H4 acetylation in round spermatids, suggesting that a hypoacetylated state of these cells is important for their normal differentiation. In the present study, the in vivo effects of TSA on murine spermatogenesis were investigated. Subcutaneously applied TSA resulted in a dose-dependent decrease in relative testis weight due to impaired spermatogenesis. No obvious toxic effects of TSA treatment could be found. A second animal experiment confirmed that male mice receiving TSA under the same conditions as in the first experiment became infertile. This phenomenon was completely reversible. No evidence of histone H4 hyperacetylation in round spermatids could be found; however, the number of spermatids significantly decreased with increasing TSA concentrations. Additionally, a dramatic loss of pachytene-diplotene spermatocytes due to increased apoptosis was observed. This suggests that TSA was mainly effective at the level of meiosis. The other male reproductive organs showed no morphological changes compared to controls, suggesting that TSA action on the testis was not mediated by sex hormones.

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.

Major chromatin remodeling in the germinal vesicle (GV) of mammalian oocytes is dispensable for global transcriptional silencing but required for centromeric heterochromatin function

Global silencing of transcriptional activity in the oocyte genome occurs just before the resumption of meiosis and is a crucial developmental transition at the culmination of oogenesis. Transcriptionally quiescent oocytes rely on stored maternal transcripts to sustain the completion of meiosis, fertilization, and early embryonic cleavage stages. Thus, the timing of silencing is key for successful embryo development. Yet, the cellular and molecular pathways coordinating dynamic changes in large-scale chromatin structure with the onset of transcriptional repression are poorly understood. Here, oocytes obtained from nucleoplasmin 2 knockout (Npm2-/-) mice were used to investigate the relationship between transcriptional repression and chromatin remodeling in the germinal vesicle (GV) of mammalian oocytes. Although temporally linked, global silencing of transcription and chromatin remodeling in the oocyte genome can be experimentally dissociated and therefore must be regulated through distinct pathways. Detection of centromeric heterochromatin DNA sequences with a mouse pan-centromeric chromosome paint revealed that most centromeres are found in close apposition with the nucleolus in transcriptionally quiescent oocytes and therefore constitute an important component of the perinucleolar heterochromatin rim or karyosphere. Pharmacological inhibition of histone deacetylases (HDACs) with trichostatin A (TSA) revealed that HDACs are essential for large-scale chromatin remodeling in the GV. Importantly, the specialized nuclear architecture acquired upon transcriptional repression is essential for meiotic progression as interference with global deacetylation and partial disruption of the karyosphere resulted in a dramatic increase in the proportion of oocytes exhibiting abnormal meiotic chromosome and spindle configuration. These results indicate that the unique chromatin remodeling mechanism in oocytes may be specifically related to meiotic cell division in female mammals.

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.

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.

Trichostatin A-induced histone acetylation causes decondensation of interphase chromatin

The effect of trichostatin A (TSA)-induced histone acetylation on the interphase chromatin structure was visualized in vivo with a HeLa cell line stably expressing histone H2A, which was fused to enhanced yellow fluorescent protein. The globally increased histone acetylation caused a reversible decondensation of dense chromatin regions and led to a more homogeneous distribution. These structural changes were quantified by image correlation spectroscopy and by spatially resolved scaling analysis. The image analysis revealed that a chromatin reorganization on a length scale from 200 nm to >1 microm was induced consistent with the opening of condensed chromatin domains containing several Mb of DNA. The observed conformation changes could be assigned to the folding of chromatin during G1 phase by characterizing the effect of TSA on cell cycle progression and developing a protocol that allowed the identification of G1 phase cells on microscope coverslips. An analysis by flow cytometry showed that the addition of TSA led to a significant arrest of cells in S phase and induced apoptosis. The concentration dependence of both processes was studied.

Transient DNA Strand Breaks During Mouse and Human Spermiogenesis:New Insights in Stage Specificity and Link to Chromatin Remodeling1

In the course of mammalian spermiogenesis, a unique chromatin remodeling process takes place within elongating and condensing spermatid nuclei. The histone-to-protamine exchange results in efficient packaging and increased stability of the paternal genome. Although not fully understood, this change in chromatin architecture must require a global but transient appearance of endogenous DNA strand breaks because most of the DNA supercoiling is eliminated in the mature sperm. To establish the extent of DNA strand breakage and the stage specificity at which these breaks are created and repaired, we performed a sensitive terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end labeling (TUNEL) assay to detect in situ DNA strand breaks on both mice and human testis cross sections. In the mouse, we established that DNA strand breaks are indeed detected in the whole population of elongating spermatids between stages IX and XI of the seminiferous epithelium cycle perfectly coincident with the chromatin remodeling as revealed by histone H4 hyperacetylation. Similarly, TUNEL analyses performed on human testis sections revealed an elevated and global increase in the levels of DNA strand breaks present in nuclei of round-shaped spermatids also coincident with chromatin remodeling. The demonstration of the global character of the transient DNA strand breaks in mammalian spermiogenesis suggests that deleterious consequences on genetic integrity of the male gamete may arise from any disturbance in the process. In addition, this investigation may shed some light on the origin of the low success rate that has been encountered so far with intracytoplasmic injection procedures making use of round spermatids in humans.

Members of the CDY family have different expression patterns: CDY1 transcripts have the best correlation with complete spermatogenesis

The CDY family of genes is of special interest because some of them are included in chromosome-Y microdeletions detected among infertile men and are apparently involved in the spermiogenetic process. In this study, we employed the reverse transcriptase/polymerase chain reaction technique to test the RNA expression of the various transcripts of these genes in testicular biopsies of 84 azoospermic men who had been classified by comprehensive histology and cytology analyses. We also evaluated the feasibility of detecting CDY expression in biopsies taken by testicular sperm extraction versus acquisition by aspiration. There was a significant association between the type of testicular impairment and the expression of CDY1 and CDY2 transcripts. CDY2 was expressed whenever germ cells were present, but CDY1 major and especially CDY1 minor and short transcripts were identified almost exclusively when mature spermatids/spermatozoa were detected. The expression of CDY1 minor and short transcripts detected in aspirated specimens was less efficient than that in testicular tissue acquired by extraction. It is suggested that CDY2 is apparently required in the early stages of spermatogenesis, whereas CDY1 transcripts are required later on in the process. The findings of this study imply different functional roles for CDY isoforms during spermatogenesis. However, in consideration of the high levels of identity between CDY1 and CDY2 (98% at the protein level), the delayed up-regulation of CDY1 transcripts could be attributable to temporal changes in dosage requirements.

Somatic histones are components of the perinuclear theca in bovine spermatozoa

The perinuclear theca is a non-ionic detergent-resistant, electron-dense layer surrounding the condensed nucleus of mammalian sperm. The known proteins originating from the perinuclear theca have implicated the structure in a variety of important cellular processes during spermiogenesis and fertilization. Nonetheless, the composition of the perinuclear theca remains largely unexplored. We have isolated a group of low molecular mass (14-19 kDa) perinuclear theca-derived proteins from acrosome-depleted bovine sperm heads by salt (1 M KCl) extraction and have identified them as core somatic histones. N-terminal sequencing and immunoblotting with anti-histone antibodies confirmed the presence of both intact and proteolytically cleaved somatic histones H3, H2B, H2A, and H4. Identical proteins were isolated using 2% SDS or 1 N HCl extractions. Subsequent acid and SDS extractions of intact bovine sperm revealed the presence of all four intact histone subtypes, with minimal proteolysis. Two-dimensional acid/urea/Triton-SDS-PAGE, coupled with immunoblotting analysis, confirmed the somatic nature of these perinuclear theca-derived histones. Estimates of the abundance of perinuclear theca-derived histones showed that up to 0.2 pg per sperm of each histone subtype was present. Immunogold labeling at the ultrastructural level localized all four core somatic histones to the post-acrosomal sheath region of bovine epididymal sperm, when probed with affinity-purified anti-histone antibodies. Little immunoreactivity was detected in residual perinuclear theca structures following the extractions. Taken together, these findings indicate the unprecedented and stable localization of non-nuclear somatic histones in bovine sperm perinuclear theca.

Acetylation-dependent chromatin reorganization by BRDT, a testis-specific bromodomain-containing protein

The association between histone acetylation and replacement observed during spermatogenesis prompted us to consider the testis as a source for potential factors capable of remodelling acetylated chromatin. A systematic search of data banks for open reading frames encoding testis-specific bromodomain-containing proteins focused our attention on BRDT, a testis-specific protein of unknown function containing two bromodomains. BRDT specifically binds hyperacetylated histone H4 tail depending on the integrity of both bromodomains. Moreover, in somatic cells, the ectopic expression of BRDT triggered a dramatic reorganization of the chromatin only after induction of histone hyperacetylation by trichostatin A (TSA). We then defined critical domains of BRDT involved in its activity. Both bromodomains of BRDT, as well as flanking regions, were found indispensable for its histone acetylation-dependent remodelling activity. Interestingly, we also observed that recombinant BRDT was capable of inducing reorganization of the chromatin of isolated nuclei in vitro only when the nuclei were from TSA-treated cells. This assay also allowed us to show that the action of BRDT was ATP independent, suggesting a structural role for the protein in the remodelling of acetylated chromatin. This is the first demonstration of a large-scale reorganization of acetylated chromatin induced by a specific factor.

A haploid affair: core histone transitions during spermatogenesis

The process of meiosis reduces a diploid cell to four haploid gametes and is accompanied by extensive recombination. Thus, the dynamics of chromatin during meiosis are significantly different than in mitotic cells. As spermatogenesis progresses, there is a widespread reorganization of the haploid genome followed by extensive DNA compaction. It has become increasingly clear that the dynamic composition of chromatin plays a critical role in the activities of enzymes and processes that act upon it. Therefore, an analysis of the role of histone variants and modifications in these processes may shed light upon the mechanisms involved and the control of chromatin structure in general. Histone variants such as histone H3.3, H2AX, and macroH2A appear to play key roles in the various stages of spermiogenesis, in addition to the specifically modulated acetylation of histone H4 (acH4), ubiquitination of histones H2A and H2B (uH2A, uH2B), and phosphorylation of histone H3 (H3p). This review will examine recent discoveries concerning the role of histone modifications and variants during meiosis and spermatogenesis.

Chromatin remodeling by nuclear receptors

The eukaryotic genome is structurally organized into nucleosomes to form chromatin, which regulates gene expression, in part, by controlling the accessibility of regulatory factors. When packaged as chromatin, many promoters are transcriptionally repressed, thus reducing the access of transcription factors to their binding sites. However, nuclear receptors (NRs) are a group of transcription factors that have the ability to access their binding sites in this repressive chromatin structure. Nuclear receptors are able to bind to their sites and recruit chromatin-remodeling proteins such as ATP-dependent chromatin-remodeling complexes and histone-modifying enzymes, resulting in transcriptional activation. In this review, we present the role of NRs in recruiting these chromatin-modifying enzymes by means of an extensively studied model system, the glucocorticoid receptor-mediated transactivation of the mouse mammary tumor virus (MMTV) promoter. We use these findings as a template to begin to understand the effect of chromatin changes on gene expression during spermatogenesis.

Dynamics of histone acetylation in vivo. A function for acetylation turnover?

Histone acetylation, discovered more than 40 years ago, is a reversible modification of lysines within the amino-terminal domain of core histones. Amino-terminal histone domains contribute to the compaction of genes into repressed chromatin fibers. It is thought that their acetylation causes localized relaxation of chromatin as a necessary but not sufficient condition for processes that repackage DNA such as transcription, replication, repair, recombination, and sperm formation. While increased histone acetylation enhances gene transcription and loss of acetylation represses and silences genes, the function of the rapid continuous or repetitive acetylation and deacetylation reactions with half-lives of just a few minutes remains unknown. Thirty years of in vivo measurements of acetylation turnover and rates of change in histone modification levels have been reviewed to identify common chromatin characteristics measured by distinct protocols. It has now become possible to look across a wider spectrum of organisms than ever before and identify common features. The rapid turnover rates in transcriptionally active and competent chromatin are one such feature. While ubiquitously observed, we still do not know whether turnover itself is linked to chromatin transcription beyond its contribution to rapid changes towards hyper- or hypoacetylation of nucleosomes. However, recent experiments suggest that turnover may be linked directly to steps in gene transcription, interacting with nucleosome remodeling complexes.

Expression of hyperacetylated histone H4 during normal and impaired human spermatogenesis

Histone-to-protamine exchange in haploid spermatids is preceded by hyperacetylation of core histones resulting in decreased DNA-histone interaction. During normal spermatogenesis, immunohistochemistry with a polyclonal antihyperacetylated histone H4 antibody displayed a strong signal in nuclei of elongating spermatids and, in addition, spermatogonia. Quantitative analysis revealed 98.2 +/- 1.1% of immunopositive spermatids. The percentage of positive spermatids was significantly reduced in infertile men exhibiting at least qualitatively normal spermatogenesis (scores 10-8, 93.1 +/- 6.6%) and impaired spermatogenesis (scores 7-1, 74.9 +/- 23.4%). In seminiferous tubules showing spermatogenic arrest at the level of round spermatids, only 59.5 +/- 16.5% of spermatids were immunopositive for hyperacetylated histone H4. These data demonstrate that the decrease of histone acetylation in spermatids associated with impaired spermatogenesis corresponds with the well known reduction of protamine expression in these cells and confirms the essential role of histone hyperacetylation for correct histone-to-protamine exchange. In seminiferous tubules exhibiting round spermatid maturation arrest, there was an additional signal in nuclei of spermatocytes, suggesting that premature hyperacetylation of histone H4 may result in precocious histone-to-protamine exchange followed by infertility. This is in accordance with data from transgenic mice, where it has been demonstrated that premature expression of protamine-1 results in precocious chromatin condensation followed by sterility.

Chromatin modification and epigenetic reprogramming in mammalian development

The developmental programme of embryogenesis is controlled by both genetic and epigenetic mechanisms. An emerging theme from recent studies is that the regulation of higher-order chromatin structures by DNA methylation and histone modification is crucial for genome reprogramming during early embryogenesis and gametogenesis, and for tissue-specific gene expression and global gene silencing. Disruptions to chromatin modification can lead to the dysregulation of developmental processes, such as X-chromosome inactivation and genomic imprinting, and to various diseases. Understanding the process of epigenetic reprogramming in development is important for studies of cloning and the clinical application of stem-cell therapy.