Acetylation of rat testis histones H2B and TH2B

Roles of Histone H2B, H3 and H4 Variants in Cancer Development and Prognosis

Histone variants are the paralogs of core histones (H2A, H2B, H3 and H4). They are stably expressed throughout the cell cycle in a replication-independent fashion and are capable of replacing canonical counterparts under different fundamental biological processes. Variants have been shown to take part in multiple processes, including DNA damage repair, transcriptional regulation and X chromosome inactivation, with some of them even specializing in lineage-specific roles like spermatogenesis. Several reports have recently identified some unprecedented variants from different histone families and exploited their prognostic value in distinct types of cancer. Among the four classes of canonical histones, the H2A family has the greatest number of variants known to date, followed by H2B, H3 and H4. In our prior review, we focused on summarizing all 19 mammalian histone H2A variants. Here in this review, we aim to complete the full summary of the roles of mammalian histone variants from the remaining histone H2B, H3, and H4 families, along with an overview of their roles in cancer biology and their prognostic value in a clinical context.

Reduce, Retain, Recycle: Mechanisms for Promoting Histone Protein Degradation versus Stability and Retention

The eukaryotic genome is packaged into chromatin. The nucleosome, the basic unit of chromatin, is composed of DNA coiled around a histone octamer. Histones are among the longest-lived protein species in mammalian cells due to their thermodynamic stability and their associations with DNA and histone chaperones. Histone metabolism plays an integral role in homeostasis. While histones are largely stable, the degradation of histone proteins is necessary under specific conditions. Here, we review the physiological and cellular contexts that promote histone degradation. We describe specific known mechanisms that drive histone proteolysis. Finally, we discuss the importance of histone degradation and regulation of histone supply for organismal and cellular fitness.

Lead-mediated inhibition of lysine acetylation and succinylation causes reproductive injury of the mouse testis during development

Lead (Pb), a widespread heavy metal, may induce serious diseases, particularly male reproductive injury. However, the mechanisms by which Pb induces testicular injury remain unclear. In this paper, we established a mouse model of Pb-induced testicular injury via an intraperitoneal injection of lead chloride at a concentration of 1.5 mg/kg body weight. We confirmed that Pb could induce a series of injuries, including a low litter size, smaller testes, more weak offspring, direct injury, and aberrant spermiogenesis. Our study demonstrated that Pb could inhibit lysine acetylation (Kac) and succinylation (Ksuc) via western blot (WB) and immunofluorescence (IF) analyses. We subsequently separated different germ cells that contained Pre-meiotic spermatogonia (SPG), meiotic spermatocyte (SPC), and round spermatid (RS) into the Pb-treated and control groups and verified that Pb inhibited Kac in SPC, RS, and particularly, during meiosis. Furthermore, our results regarding the inhibition of pyruvate kinase and mitochondrial electron transport chain complex I and II in the Pb-treated groups suggested that Pb may restrain key enzymes to block the TCA cycle and that the low TCA cycle activity could reduce the contents of two important metabolites, acetyl-CoA and succinyl-CoA, to inhibit Kac and Ksuc. Moreover, we examined the influences of the inhibition of Kac and Ksuc on spermiogenesis, which indicated that decreased Kac and Ksuc could impede the replacement of transition proteins in elongating sperm and disorder the distribution of germ cells in the seminiferous tubule. Our research provides novel insights into the mechanisms of Pb reproductive toxicity with respect to lysine acetylation and succinylation.

Essential Role of Histone Replacement and Modifications in Male Fertility

Spermiogenesis is a complex cellular differentiation process that the germ cells undergo a distinct morphological change, and the protamines replace the core histones to facilitate chromatin compaction in the sperm head. Recent studies show the essential roles of epigenetic events during the histone-to-protamine transition. Defects in either the replacement or the modification of histones might cause male infertility with azoospermia, oligospermia or teratozoospermia. Here, we summarize recent advances in our knowledge of how epigenetic regulators, such as histone variants, histone modification and their related chromatin remodelers, facilitate the histone-to-protamine transition during spermiogenesis. Understanding the molecular mechanism underlying the modification and replacement of histones during spermiogenesis will enable the identification of epigenetic biomarkers of male infertility, and shed light on potential therapies for these patients in the future.

Cadmium inhibits lysine acetylation and succinylation inducing testicular injury of mouse during development

The toxic effects of cadmium (Cd) in the reproductive system have been confirmed, and lysine acetylation and succinylation play important roles in spermatogenesis. However, little attention determined whether Cd could affect lysine acylation and how it might have an impact on the reproductive system. Therefore, with the goal of contributing to this subject, we have examined the effects of Cd on lysine acetylation and succinylation of proteins in the germ cells of male mice testes during different developmental stages. We adopted intraperitoneal injection of cadmium chloride (1.2 mg/kg body weight) in mice once every 5 days from postnatal day 5-60. The results showed that Cd could restrict GAPDH activity, ATP and cAMP levels of germ cells to inhibit lysine acetylation and succinylation in the testes, inducing reproductive injuries. Cd also restricts acetylation of histone H4K5 and H4K12, which could result in failure of spermiogenesis. Remarkably, polarized acetylation occurs in meiosis, and high-level acetylation occurs earlier than high-level succinylation during spermatogenesis. Moreover, Cd has a limited effect on body weight but reduces the weight of the testis and litter size. Our research may provide a new way to reveal the mechanisms of Cd reproductive toxicity related to lysine acetylation and succinylation.

SIRT6 in mouse spermatogenesis is modulated by diet-induced obesity

Male obesity is associated with reduced sperm function and increased incidence of sperm DNA damage; however, the underlying molecular mechanisms have not yet been identified. Mammalian SIRT6 protein is involved in caloric-dependant DNA damage repair in other tissue types, yet a possible role for SIRT6 in male obesity and subfertility has not been investigated previously. To assess SIRT6 levels and activity in the testes, male mice (n=12 per diet) were fed either a control diet (CD; 6% fat) or a high-fat diet (HFD; 21% fat) for 16 weeks before the collection of testes and spermatozoa. SIRT6 protein was localised to the nucleus of transitional spermatids and the acrosome of mature spermatozoa, with levels significantly decreased in HFD-fed male mice (P<0.05). This decrease in SIRT6 protein was associated with transitional spermatids having increased levels of acetylated H3K9 in the nucleus (P<0.01) and increased DNA damage (P<0.001). We propose a role for SIRT6 in spermiogenesis and potentially protamination processes, which are known to be compromised by male obesity.

Mapping of lysine methylation and acetylation in core histones of Neurospora crassa

Core histones are susceptible to a variety of post-translational modifications (PTMs), among which methylation and acetylation play critical roles in various chromatin-dependent processes. The nature and biological functions of these PTMs have been extensively studied in plants, animals, and yeasts. In contrast, the histone modifications in Neurospora crassa, a convenient model organism for multicellular eukaryotes, remained largely undefined. In this study, we used several mass spectrometric techniques, coupled with HPLC separation and multiple-protease digestion, to identify the methylation and acetylation sites in core histones isolated from Neurospora. Electron transfer dissociation (ETD) was employed to fragment the heavily modified long N-terminal peptides. In addition, accurate mass measurement of fragment ions allowed for unambiguous differentiation of acetylation from trimethylation. Many modification sites conserved in other organisms were identified in Neurospora. In addition, some unique modification sites in histone H2B, including N-terminal alpha methylation, methylation at K3, and acetylation at K19, K28, and K29, were observed. Our analysis provides a potentially comprehensive picture of methylation and acetylation of core histones in Neurospora, which should serve as a foundation for future studies of the function of histone PTMs in this model organism.

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.

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

Here we report a detailed analysis of waves of histone acetylation that occurs throughout spermatogenesis in mouse. Our data showed that spermatogonia and preleptotene spermatocytes contained acetylated core histones H2A, H2B and H4, whereas no acetylated histones were observed throughout meiosis in leptotene or pachytene spermatocytes. Histones remained unacetylated in most round spermatids. Acetylated forms of H2A and H2B, H3 and H4 reappeared in step 9 to 11 elongating spermatids, and disappeared later in condensing spermatids. The spatial distribution pattern of acetylated H4 within the spermatids nuclei, analyzed in 3D by immunofluorescence combined with confocal microscopy, showed a spatial sequence of events tightly associated with chromatin condensation. In order to gain an insight into mechanisms controlling histone hyperacetylation during spermiogenesis, we treated spermatogenic cells with a histone deacetylase inhibitor, trichostatin A (TSA), which showed a spectacular increase of histone acetylation in round spermatids. This observation suggests that deacetylases are responsible for maintaining a deacetylated state of histones in these cells. TSA treatment could not induce histone acetylation in condensing spermatids, suggesting that acetylated core histones are replaced by transition proteins without being previously deacetylated. Moreover, our data showed a dramatic decrease in histone deacetylases in condensing spermatids. Therefore, the regulation of histone deacetylase activity/concentration appears to play a major role in controling histone hyperacetylation and probably histone replacement during spermiogenesis.

The promoter of the Poly(A) binding protein 2 (Pabp2) retroposon is derived from the 5'-untranslated region of the Pabp1 progenitor gene

The mouse Pabp2 retroposon encodes an isoform of poly(A) binding protein that is expressed in meiotic and early haploid spermatogenic cells. In the present study, we have determined the transcription start site of the Pabp2 gene to clarify the source of its promoter, a prerequisite for expression of retroposons and preservation of their function by natural selection. The 5' end of the mouse Pabp2 retroposon exhibits extensive similarity to the entire 5' UTR of the human PABP1 mRNA, but there is no similarity upstream of the transcription start site of the human PABP1 mRNA, indicating that the Pabp2 gene lacks 5' flanking sequences of the parental PABP1 gene. Oligonucleotide-directed RNase H cleavage and 5' rapid amplification of cDNA ends both indicate that the transcription start site of the mouse Pabp2 gene is located approximately 330 bases downstream of the capsite of the PABP1 mRNA, indicating that the Pabp2 promoter is derived from the PABP1 5' UTR.

Influence of core histone acetylation on SV40 minichromosome replication in vitro

We have used the SV40 in vitro replication system to analyze the replication efficiencies of SV40 minichromosomes associated with normal or hyperacetylated histones. We found that elongation of replication occurs with higher efficiency in hyperacetylated minichromosomes in comparison with normal minichromosomes. Our results indicate that the movement of the replication machinery through nucleosomal DNA is facilitated by charge neutralization due to acetylation of the histone tails.

Increased accessibility of the N-terminus of testis-specific histone TH2B to antibodies in elongating spermatids

Changes in chromatin structure during spermatogenesis were investigated using a monoclonal antibody that immunoreacts with the N-terminus of the testis-specific histone TH2B. This monoclonal antibody, which had been raised against rat tyrosine hydroxylase (TH), cross-reacted with TH2B because of sequence homology at the N-termini of TH and TH2B. The epitope was localized to the N-terminus of TH2B as trypsin-digested chromatin which lacked the N-terminal tail did not react with anti-TH and preincubating anti-TH with a synthetic peptide made from the homologous sequence between TH2B and TH inhibited its binding to TH and TH2B. In histological sections of rat testis, the primary spermatocytes and round spermatids immunoreacted weakly, whereas elongating spermatids at steps 10-12 immunoreacted intensely with anti-TH. Increased staining of elongating spermatids was also observed in mouse and hamster by immunohistochemistry. However, immunoblotting proteins extracted from separated rat testis cells showed no increase in the TH2B content of these late steps of spermatids. The apparent increase in the immunohistochemical staining corresponds to increased accessibility of the epitope in the elongating spermatids. This indicated that the N-terminus of TH2B is less tightly bound to DNA or to other proteins at this time in preparation for the removal of TH2B and other histones.

Histone hyperacetylation can induce unfolding of the nucleosome core particle

A direct correlation exists between the level of histone H4 hyperacetylation induced by sodium butyrate and the extent to which nucleosomes lose their compact shape and become elongated (62.0% of the particles have a length/width ratio over 1.6; overall mean in the length/width ratio = 1.83 +/- 0.48) when bound to electron microscope specimen grids at low ionic strength (1mM EDTA, 10mM Tris, pH 8.0). A marked proportion of elongated core particles is also observed in the naturally occurring hyperacetylated chicken testis chromatin undergoing spermatogenesis when analyzed at low ionic strength (36.8% of the particles have a length/width ratio over 1.6). Core particles of elongated shape (length/width ratio over 1.6) generated under low ionic strength conditions are absent in the hypoacetylated chicken erythrocyte chromatin and represent only 2.3% of the untreated Hela S3 cell core particles containing a low proportion of hyperacetylated histones. The marked differences between control and hyperacetylated core particles are absent if the particles are bound to the carbon support film in the presence of 0.2 M NaCl, 6mM MgCl2 and 10mM Tris pH 8.0, conditions known to stabilize nucleosomes. A survey of the published work on histone hyperacetylation together with the present results indicate that histone hyperacetylation does not produce any marked disruption of the core particle 'per se', but that it decreases intranucleosomal stabilizing forces as judged by the lowered stability of the hyperacetylated core particle under conditions of shearing stress such as cationic competition by the carbon support film of the EM grid for DNA binding.

On the biological role of histone acetylation

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Marked differences in the ability of distinct protamines to disassemble nucleosomal core particles in vitro

In accordance with the results of classical experiments performed in vitro with calf thymus chromatin and the fish protamine salmine, we have observed that this highly basic, small molecular weight protamine cannot cause major displacement of histones from nucleosomal core particles at concentrations several times higher than physiological (arginine/nucleotide ratios 1-8) and that hyperacetylation of histones facilitates nucleosome disassembly. However, the avian protamine galline, with molecular weight and number of arginine residues almost twice those of common fish protamines, is able to displace the nucleosomal core histones from DNA in vitro at concentrations (arginine/nucleotide ratios 0.6-1.2) within the physiological range (0.8). Our results suggest that the binding of the avian protamine galline to chromatin could be directly involved in the rapid disassembly of nucleosomes that takes place during the nucleohistone nucleoprotamine transition in chicken spermiogenesis.

Histones and their modifications

Histones constitute the protein core around which DNA is coiled to form the basic structural unit of the chromosome known as the nucleosome. Because of the large amount of new histone needed during chromosome replication, the synthesis of histone and DNA is regulated in a complex manner. During RNA transcription and DNA replication, the basic nucleosomal structure as well as interactions between nucleosomes must be greatly altered to allow access to the appropriate enzymes and factors. The presence of extensive and varied post-translational modifications to the otherwise highly conserved histone primary sequences provides obvious opportunities for such structural alterations, but despite concentrated and sustained effort, causal connections between histone modifications and nucleosomal functions are not yet elucidated.

Changes in the structural organization of chromatin during spermatogenesis in the rat

In this study, histone H4 was shown to be extensively hyperacetylated in mid-spermatids of the rat during the time period when the entire complement of histones is replaced by basic spermatidal transition proteins. The degree of hyperacetylation of histone H4 was minimal in pachytene spermatocytes. Therefore, the hyperacetylation appears to be directly involved in the histone replacement process late in spermatogenesis in mid-spermatids. In order to investigate further the possible effects of histone H4 hyperacetylation and the other dramatic changes in the nuclear proteins on the structure of chromatin in germinal cells, we examined the thermal denaturation profiles of chromatin from various purified germinal cell types. Our analyses revealed that chromatins from pachytene spermatocytes and early spermatids have similar thermal denaturation profiles, with their major thermal transitions slightly lower than those for rat liver. However, the major thermal transitions for chromatin from mid-spermatids are much lower than those from pachytene spermatocytes and early-spermatids. We propose that the greatly lowered thermal stability of mid-spermatid chromatin represents a dramatic relaxation or decondensation of the chromatin in this cell type in preparation for the replacement of histone by the basic spermatidal transition proteins and that the decondensation is due in large part to the extensive histones hyperacetylation which occurs in these cells.

Biochemical and immunological characterization of an H2A variant from the mouse testis

A histone H2A variant, protein 'A', has been isolated and purified from the mouse testis. Amino acid composition analysis and electrophoretic properties indicate it to be apparently similar to H2A X X variant present in the mouse L1210 cells. Specific antibodies against protein 'A' have been generated in rabbits and used to study tissue and species distribution in mammals. Protein transfer experiments indicate the presence of antigenically similar proteins in somatic tissues of the mouse. Immunologically similar proteins were also detected in other mammalian testes. The data further indicate that protein A is antigenically distinct from the other members of the H2A family, H2A X 1, H2A X 2 and H2A X Z.

Protein O-carboxylmethylation in relation to male gamete production and function

Protein O-carboxylmethyltransferase (PCM) activity of differentiating male germ cells in the testis and of spermatozoa is strikingly high. PCM catalyzes the methylesterification by S-adenosylmethionine of dicarboxylic amino acid residues in proteins. PCM appears to be the only type of protein methyltransferase present in mature spermatozoa. Mammalian sperms contain considerable amounts of S-adenosylmethionine and can apparently synthesize this nucleoside from L-methionine and ATP. Spermatozoa are rich in S-adenosylhomocysteine hydrolase. The characteristics of this enzyme in testicular germ cells and in sperms are very similar to those in other mammalian tissues; the very sub-stoichiometric extent of methylation of various pure protein substrates, and the rapid spontaneous hydrolysis of the protein methyl ester products at physiological and especially higher pH values, are particularly remarkable. From studies on processes related to protein O-carboxylmethylation in rat spermatozoa from different regions of the epididymis, and in ejaculated spermatozoa from normal and infertile men, unequivocal evidence could not be obtained for hypotheses of other investigators that PCM-catalyzed reactions are of regulatory importance for the acquisition of a potentiality for motility in sperms during their transit and maturation in the epididymis, or for the locomotion of ejaculated sperms. The findings are discussed in the light of the recent hypothesis of S. Clarke that PCM catalyzes methylesterification of D-aspartyl residues that accumulate in certain proteins as a result of slow spontaneous racemization of L-aspartyl residues, and that the methyl esterification of D-aspartyl residues may be related to disposal or repair of proteins damaged in this fashion.

Release of ribonucleoprotein during digestion of rat testis chromatin with deoxyribonuclease II (3.1.4.6)

The composition of rat testis chromatin proteins in fractions produced by limited DNase II digestion followed by differential precipitation with MgCl2 has been studied. Over 50% of the acid-soluble proteins in the soluble chromatin fraction appeared to be quite similar to proteins which are associated with ribonucleoprotein (RNP) particles in HeLa cells. Although the ratios of the testis RNP protein components differed from those of HeLa RNP particles, the three major polypeptides were most similar to the HeLa components designated A2, B2, and C1. The soluble chromatin fraction was also enriched in the high mobility group proteins HMG1 and HMG2.