Expression of mammalian spermatozoal nucleoproteins

The testes-specific bZip type transcription factor Tisp40 plays a role in ER stress responses and chromatin packaging during spermiogenesis

We previously reported that the spermatid-specific transcription factor Tisp40 functions through UPRE and CRE. To investigate Tisp40 function in vivo, we generated TISP40(-/-) mice. TISP40(-/-) mice were born at expected ratios, were healthy, and mutant males bred normally. However, the ER stress-response protein Grp78/BiP accumulated in the TISP40(-/-) testis and RAMP4 (Ribosome-associated membrane protein 4) mRNA level was up-regulated. Disruption of TISP40 caused ER stress and activation of caspase 12 but not caspase 9, leading to apoptosis of meiotic/postmeiotic germ cells. On the other hand, DAPI staining and electron microscopy revealed that epididymal sperm nuclei were abnormally relaxed in the TISP40(-/-) testis, a phenotype that was independent of the expression and maturation of transition proteins and protamines but due to abnormally retained histones. Histones localized to the cytoplasm as well as to the nucleus and were also retained in epididymal sperm. Histones H2A and H4 were dramatically up-regulated and the acetylation of H2A, H2B and H4 was also enhanced in the TISP40(-/-) testis. Taken together, we conclude that Tisp40 plays an important role in the unfolded protein response of the testis and in regulating the maturation of sperm head nuclei.

Globozoospermia revisited

Globozoospermia is a rare (incidence <0.1%) but severe disorder in male infertility. Total globozoospermia is diagnosed by the presence of 100% round-headed spermatozoa lacking an acrosome. It is still unclear whether patients whose ejaculate contains both normal and globozoospermic cells (partial globozoospermia) suffer from a variation of the same syndrome. Apart from the fact that affected males suffer from reduced fertility or even infertility, no other physical characteristics can be associated with the syndrome. ICSI is a treatment option for these patients, although low fertilization rates after ICSI show a reduced ability to activate the oocyte. In globozoospermic cells, the use of acrosome markers has demonstrated an absent or severely malformed acrosome. Chromatin compaction appears to be disturbed but is not consistently over- or undercondensed. In some cases, an increased number of cells with DNA fragmentation have been observed. The analysis of the cytogenetic composition revealed an increased aneuploidy rate in some cases. Nonetheless, no increased number of spontaneous abortions or congenital defects has been reported in pregnancies conceived after ICSI. The pathogenesis of globozoospermia most probably originates in spermiogenesis, more specifically in acrosome formation and sperm head elongation. In several knockout mouse models, a phenotype similar to that in humans was found. Together with the occurrence of affected siblings, these findings indicate a genetic origin, which makes globozoospermia a good candidate for genetic analysis. More research is needed to elucidate the pathogenesis of human globozoospermia to further understand globozoospermia as well as (abnormalities in) spermiogenesis and spermatogenesis in general.

Effects of ageing on spermatozoal chromatin and its sensitivity to in vivo and in vitro oxidative challenge in the Brown Norway rat

BACKGROUND

The goals of our study were to examine chromatin packaging and integrity in spermatozoa taken from the caput and cauda epididymides of young (4-month-old) and old (21-month-old) Brown Norway rats and to assess whether spermatozoal sensitivity to oxidative treatments is altered with age.

METHODS

Oxidative treatments consisted of (i) in vivo oxidative challenge by systemic administration of the glutathione-depleting drug l-buthionine-[S,R]-sulphoximine (BSO) and (ii) in vitro oxidative challenge by incubating collected spermatozoa with hydrogen peroxide (H(2)O(2)). Chromatin parameters assessed included quantification of thiols, nuclear chromomycin A3 (CMA3) penetration, DNA breaks by TdT (terminal deoxynucleotidyl transferase)-mediated dUDP nick-end labelling (TUNEL) and ease of DNA dissociation by acridine orange (AO) staining.

RESULTS

In spermatozoa from older rats, we found decreases in thiols, CMA3 penetration and the percentage of cells that undergo DNA dissociation. Administration of BSO had oxidizing effects on the thiol groups. It also decreased CMA3 penetration and DNA dissociation and increased TUNEL staining. Furthermore, BSO treatment sensitized cauda epididymidis spermatozoa, from older animals, to H(2)O(2).

CONCLUSIONS

Overall, we show that spermatozoa from older rats have altered chromatin packaging and integrity and that spermatozoa from the cauda epididymidis are more responsive to combined in vivo and in vitro oxidative challenge than spermatozoa from young rats.

Protamines and male infertility

Protamines are the major nuclear sperm proteins. The human sperm nucleus contains two types of protamine: protamine 1 (P1) encoded by a single-copy gene and the family of protamine 2 (P2) proteins (P2, P3 and P4), all also encoded by a single gene that is transcribed and translated into a precursor protein. The protamines were discovered more than a century ago, but their function is not yet fully understood. In fact, different hypotheses have been proposed: condensation of the sperm nucleus into a compact hydrodynamic shape, protection of the genetic message delivered by the spermatozoa, involvement in the processes maintaining the integrity and repair of DNA during or after the nucleohistone-nucleoprotamine transition and involvement in the epigenetic imprinting of the spermatozoa. Protamines are also one of the most variable proteins found in nature, with data supporting a positive Darwinian selection. Changes in the expression of P1 and P2 protamines have been found to be associated with infertility in man. Mutations in the protamine genes have also been found in some infertile patients. Transgenic mice defective in the expression of protamines also present several structural defects in the sperm nucleus and have variable degrees of infertility. There is also evidence that altered levels of protamines may result in an increased susceptibility to injury in the spermatozoan DNA causing infertility or poor outcomes in assisted reproduction. The present work reviews the articles published to date on the relationship between protamines and infertility.

Detection of damage in mammalian sperm cells

Ejaculated semen is washed for in vitro fertilization or diluted and processed to allow optimal and long-term low temperature liquid- and cryo-preservation. However, sperm are vulnerable to the washing, dilution, temperature and osmotic changes involved in sperm storage. In this review, a number of techniques are considered for detecting damaged spermatozoa. Staining protocols have been developed to detect the membrane and organelle integrity of mammalian sperm cells. Plasma membrane integrity is usually assessed after staining cells with membrane-impermeable dyes or alternatively with acetylated membrane (AM) permeable probes that are selectively de-esterified and become membrane impermeable and thus entrapped into viable cells only (AM ester loading). Organelle-specific dyes are commonly used to detect functionality of mitochondria or the acrosome. A distortion in the lateral and bilayer organization of lipids as well as the peroxidation of fatty acid moieties can be quantified and localized in living sperm. The relation of a disordering in the sperm membrane's lipid architecture and sperm deterioration versus capacitation is discussed. Finally, the integrity of sperm DNA can be measured at three different levels by assessing the degree of DNA-protamine condensation, the incidence of breaks and nicks in the DNA and the frequency of fragmentation of the nuclei into sub-haploid apoptotic bodies. The relevance of detecting DNA aberrations and especially the putative link to the incidence of apoptosis is critically considered.

Sperm DNA fragmentation: threshold value in male fertility

BACKGROUND

The extent of sperm DNA fragmentation, which can be measured by the TUNEL assay, is one of the determinants of male fertility. However, the clinical application of this test to in-vivo situations is difficult owing to the absence of a statistically validated threshold value.

METHODS

The aim of this study was to compare the results of TUNEL assay applied to semen samples from men of proven fertility (n = 47) and patients from an infertile population (n = 66), in order to establish a discriminating threshold value.

RESULTS

Infertile patients had a higher mean level of DNA fragmentation than men of proven fertility (40.9 +/- 14.3% versus 13.1 +/- 7.3%, respectively; P < 0.001). The area under the receiver operating characteristics curve was 0.93 for 20% sperm DNA fragmentation. The calculated threshold value for TUNEL assay to distinguish between fertile controls and infertile men was 20%. At this threshold, specificity was 89.4 [95% confidence interval (CI) 83.7-95.1] and sensitivity was 96.9% (95% CI 93.8-100). The positive and negative predictive values of the 20% sperm DNA fragmentation threshold were high: 92.8% (95% CI 87.9-97.5) and 95.5% (95% CI 91.6-99.3), respectively.

CONCLUSION

This study demonstrates that sperm DNA fragmentation, as measured by TUNEL assay, is a highly valuable indicator of male fertility.

Replacement by Drosophila melanogaster protamines and Mst77F of histones during chromatin condensation in late spermatids and role of sesame in the removal of these proteins from the male pronucleus

Chromatin condensation is a typical feature of sperm cells. During mammalian spermiogenesis, histones are first replaced by transition proteins and then by protamines, while little is known for Drosophila melanogaster. Here we characterize three genes in the fly genome, Mst35Ba, Mst35Bb, and Mst77F. The results indicate that Mst35Ba and Mst35Bb encode dProtA and dProtB, respectively. These are considerably larger than mammalian protamines, but, as in mammals, both protamines contain typical cysteine/arginine clusters. Mst77F encodes a linker histone-like protein showing significant similarity to mammalian HILS1 protein. ProtamineA-enhanced green fluorescent protein (eGFP), ProtamineB-eGFP, and Mst77F-eGFP carrying Drosophila lines show that these proteins become the important chromosomal protein components of elongating spermatids, and His2AvDGFP vanishes. Mst77F mutants [ms(3)nc3] are characterized by small round nuclei and are sterile as males. These data suggest the major features of chromatin condensation in Drosophila spermatogenesis correspond to those in mammals. During early fertilization steps, the paternal pronucleus still contains protamines and Mst77F but regains a nucleosomal conformation before zygote formation. In eggs laid by sesame-deficient females, the paternal pronucleus remains in a protamine-based chromatin status but Mst77F-eGFP is removed, suggesting that the sesame gene product is essential for removal of protamines while Mst77F removal is independent of Sesame.

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

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

Intégrité de l'ADN des spermatozoïdes comme élément diagnostique et pronostique de la fertilité masculine

Recent progress in reproductive biology has improved comprehension physiology of the spermatozoa and on the fertilization mechanisms. This new knowledge has carried out the elaboration of tests on male fertility based on sperm genomic integrity. This review presents some of these techniques and brings a reflexion element on the application and use of sperm DNA integrity in the investigation of male fertility. The single cell gel electrophoresis (COMET assay), Sperm Chromatin Structure Assay (SCSA), In Situ Nick Translation (NT: Nick Translation) and Terminal Uridine Nick-End Labelling (TUNEL assay) are actually the most currently used techniques for the measure of sperm DNA integrity in research clinic. From a certain point of view, TUNEL assay, SCSA, COMET assay and NT assay are complementary. The TUNEL and COMET can measure single and double strand breaks of DNA, the SCSA can detect the abnormalities in the chromatin compaction and the NT assay can detect the single strand breaks of DNA. The exact origin of sperm DNA fragmentation is not established yet. However, several mechanisms have been proposed: defect in the chromatin compaction during spermiogenesis; reactive oxygen species production by immature spermatozoa; apoptosis during spermatogenesis. It becomes important to consider the possible consequences of the oocyte fertilization by a spermatozoon having a high degree of DNA fragmentation. The use in routine of some of these tests must however pass by a standardization of the inter laboratory protocols and obviously, by the establishment of both in vivo and in vitro discriminating threshold values in order for these tests to present a good predictive value for pregnancy outcome.

Identification of transcripts by macroarrays, RT–PCR and in situ hybridization in human ejaculate spermatozoa

Round spermatids contain high levels of extremely varied mRNAs that are synthesized either throughout early spermatogenesis or during spermiogenesis from the haploid genome. Concomitantly, with major changes in the chromatin organization, arrest of transcription occurs at midspermiogenesis. However, previous investigations using RT-PCR have revealed the persistence of numerous and different transcripts in ejaculated spermatozoa. In the present study, a step-by-step analysis by means of macroarray hybridization, RT-PCR and in situ hybridization was performed to identify more accurately the different mRNA species found in the human ejaculated spermatozoa. The data showed an extended pattern of various transcripts encoding a diverse range of proteins involved in signal transduction and cell proliferation. For the first time, they demonstrated that mRNAs coding for the transcription factors NFkappaB, HOX2A, ICSBP, protein kinase JNK2, growth factor HBEGF and receptors RXRbeta and ErbB3 accumulate within the sperm nucleus. The origin and fate of the sperm transcripts remain subject to discussion.

Novel human testis-specific histone H2B encoded by the interrupted gene on the X chromosome

Testis-specific histones are synthesized and accumulated at specific stages of mammalian spermatogenesis. Their proposed functions range from facilitation of the replacement of somatic histones by protamines to epigenetic control of gene transcription. Several testis histone variants were characterized in mouse and rat; however, few are known in humans. Here we report the identification and characterization of a novel human histone 2B gene (TH2B-175) located at Xq22.2, which encodes a highly divergent H2B variant. The TH2B-175 gene contains two introns and is transcribed exclusively in testis, where the spliced polyadenylated mRNA was detected. Genomic PCR, Southern blot analysis, and BLAST-based searches indicate that TH2B-175 evolved in the primate lineage or has been lost in rodents. In transfected Chinese hamster cells, GFP-tagged TH2B-175 targeted to large fluorescent bodies that partially colocalize with the interstitial telomeric blocks. Therefore, TH2B-175 may have telomere-associated functions and participate in the telomere-binding complex in the human sperm [1].

The nuclear envelope lamina network has elasticity and a compressibility limit suggestive of a molecular shock absorber

Mechanical properties of the nuclear envelope have implications for cell and nuclear architecture as well as gene regulation. Using isolated Xenopus oocyte nuclei, we have established swelling conditions that separate the intact nuclear envelope (membranes, pore complexes and underlying lamin filament network) from nucleoplasm and the majority of chromatin. Swelling proves reversible with addition of high molecular mass dextrans. Micropipette aspiration of swollen and unswollen nuclear envelopes is also reversible and yields a network elastic modulus, unaffected by nucleoplasm, that averages 25 mN/m. Compared to plasma membranes of cells, the nuclear envelope is much stiffer and more resilient. Our results suggest that the nuclear lamina forms a compressed network shell of interconnected rods that is extensible but limited in compressibility from the native state, thus acting as a 'molecular shock absorber'. In light of the conservation of B-type lamins in metazoan evolution, the mechanical properties determined in this investigation suggest physical mechanisms by which mutated lamins can either destabilize nuclear architecture or influence nuclear responses to mechanical signals in Emery-Dreifuss muscular dystrophy, cardiomyopathy, progeria syndromes (premature 'aging') and other laminopathies.

Male germline-specific histones in mouse and man

In mice and humans, the production of male gametes is a result of a complex multistep process of stem cell differentiation. The final product, the mature spermatozoon, is designed for the safe delivery of a haploid copy of the paternal genetic information to the oocyte in a structural state suitable for zygote formation and embryogenesis. A remarkable structural reorganization of chromosomes in germline cells during mammalian spermatogenesis has been characterized. The most important steps are connected with the recombination events during meiosis and the final packaging of the haploid genome in the genetically inert, compacted nucleus of the sperm. Underlying the changes in chromatin organization is the appearance of testis-specific histones. Although the existence of such histones has been known for decades, their exact functions still are not established. Deciphering of the mouse and human genomes has allowed a more detailed description of the organization and regulation of the testis-specific histone genes. In addition, it has facilitated the discovery of previously unknown proteins. This review summarizes contemporary information on these germline-specific/enriched histones in both the mouse and human and outlines early achievements in the identification of their functions.

Novel aspect of perinuclear theca assembly revealed by immunolocalization of non-nuclear somatic histones during bovine spermiogenesis

The perinuclear theca (PT) is an important accessory structure of the sperm head, yet its biogenesis is not well defined. To understand the developmental origins of PT-derived somatic histones during spermiogenesis, we used affinity-purified antibodies against somatic-type histones H3, H2B, H2A, and H4 to probe bovine testicular tissue using three different immunolocalization techniques. While undetectable in elongating spermatid nuclei, immunoperoxidase light microscopy showed all four somatic histones remained associated to the caudal head region of spermatids from steps 11 to 14 of the 14 steps in bovine spermiogenesis. Immunogold electron microscopy confirmed the localization of somatic histones on two nonnuclear structures, namely transient manchette microtubules of step-9 to step-11 spermatids and the developing postacrosomal sheath of step-13 and -14 spermatids. Immunofluorescence demonstrated somatic histone immunoreactivity in the developing postacrosomal sheath, and on anti-beta-tubulin decorated manchette microtubules of step-12 spermatids. Focal antinuclear pore complex labeling on the base of round spermatid nuclei was detected by electron microscopy and immunofluorescence, occurring before the nucleoprotein transition period during spermatid elongation. This indicated that, if nuclear histone export precedes their degradation, this process could only occur in this region, thereby questioning the proposed role of the manchette in nucleocytoplasmic trafficking. Somatic histone immunodetection on the manchette during postacrosomal sheath formation supports a role for the manchette in PT assembly, signifying that some PT components have origins in the distal spermatid cytoplasm. Furthermore, these findings suggest that somatic histones are de novo synthesized in late spermiogenesis for PT assembly.

Transient expression analysis of the mouse ornithine decarboxylase antizyme haploid-specific promoter using in vivo electroporation

The testicular isoform of the ornithine decarboxylase antizyme (OAZt) gene is expressed exclusively in the haploid spermatids of mice. The 357-bp region, which includes a TATA-less promoter and an untranslated region, is sufficient for OAZt gene expression in the spermatids of transgenic mice. In this study, in vivo transient transfection to living mouse testes was used to define the transcriptional regulatory elements of the OAZt gene promoter. We found that the 10-bp element that contains an initiator (Inr) plays a central role as the core promoter, in combination with a downstream element, while two cyclic adenosine monophosphate-responsive element (CRE)-like sites in the upstream region also contribute to promoter activity. The electrophoretic mobility shift assay showed binding of the testis-specific factors to these elements. Our results show that the in vivo DNA transfer technique enables detailed analysis of haploid germ cell-specific gene regulation in mice.

Transcription in haploid male germ cells

Major modifications in chromatin organization occur in spermatid nuclei, resulting in a high degree of DNA packaging within the spermatozoon head. However, before arrest of transcription during midspermiogenesis, high levels of mRNA are found in round spermatids. Some transcripts are the product of genes expressed ubiquitously, whereas some are generated from male germ cell-specific gene homologs of somatic cell genes. Others are transcript variants derived from genes with expression regulated in a testis-specific fashion. The haploid genome of spermatids also initiates the transcription of testis-specific genes. Various general transcription factors, distinct promoter elements, and specific transcription factors are involved in transcriptional regulation. After meiosis, spermatids are genetically but not phenotypically different, because of transcript and protein sharing through cytoplasmic bridges connecting spermatids of the same generation. Interestingly, different types of mRNAs accumulate in the sperm cell nucleus, raising the question of their origin and of a possible role after fertilization.

Control of mouse hils1 gene expression during spermatogenesis: identification of regulatory element by transgenic mouse

Histone H1-like protein in spermatids 1 (Hils1) is a testis- specific histone H1-like protein exclusively expressed in haploid spermatids and should be involved in chromatin remodeling during mouse spermatogenesis. Spatial and temporal regulation of the hils1 gene expression would be critical for the formation of functional sperm, controlled at both transcriptional and translational levels. Previously, we reported that transcripts of the hils1 gene are exclusively expressed in mouse testis from 23 days of age whereas the Hils1 protein is not detected until 28 days of age, suggesting that hils1 is a member of a class of translationally regulated genes. By analyzing transgenic mice, we could demonstrate that 318-base pair (bp) 5'-proximal region corresponding to the first 70-bp proximal TATA-less promoter, and 248 bp of 5'-untranslated region is sufficient to confer testis- and spermatid-specific transcription as well as posttranscriptional control of the mouse hils1 gene in vivo.

Female lethality and apoptosis of spermatocytes in mice lacking the UBR2 ubiquitin ligase of the N-end rule pathway

Substrates of the ubiquitin-dependent N-end rule pathway include proteins with destabilizing N-terminal residues. UBR1(-/-) mice, which lacked the pathway's ubiquitin ligase E3alpha, were viable and retained the N-end rule pathway. The present work describes the identification and analysis of mouse UBR2, a homolog of UBR1. We demonstrate that the substrate-binding properties of UBR2 are highly similar to those of UBR1, identifying UBR2 as the second E3 of the mammalian N-end rule pathway. UBR2(-/-) mouse strains were constructed, and their viability was found to be dependent on both gender and genetic background. In the strain 129 (inbred) background, the UBR2(-/-) genotype was lethal to most embryos of either gender. In the 129/B6 (mixed) background, most UBR2(-/-) females died as embryos, whereas UBR2(-/-) males were viable but infertile, owing to the postnatal degeneration of the testes. The gross architecture of UBR2(-/-) testes was normal and spermatogonia were intact as well, but UBR2(-/-) spermatocytes were arrested between leptotene/zygotene and pachytene and died through apoptosis. A conspicuous defect of UBR2(-/-) spermatocytes was the absence of intact synaptonemal complexes. We conclude that the UBR2 ubiquitin ligase and, hence, the N-end rule pathway are required for male meiosis and spermatogenesis and for an essential aspect of female embryonic development.

Isolation and characterization of a novel cDNA encoding a DNA-binding protein (Hils1) specifically expressed in testicular haploid germ cells

A cDNA encoding a protein homologous with histone H1 has been cloned from a haploid germ cell specific cDNA library. Deduced amino acid sequence (170 amino acids) showed 40% identity with histone H1 globular domain. Messenger RNA of the gene was observed exclusively in the testis, and was accumulated after post-natal day 23. Western blotting analysis showed that the protein encoded by this gene is about 19 kDa in molecular weight, and it was exclusively recovered from the nuclei of testicular germ cells. Immunohistochemical analysis showed that the protein was localized to the nuclei of round and elongating spermatids, consistent with the results of immunoblot analysis. Thus, the gene product was named Hils1 (histone H1 like protein in spermatids 1). In vitro DNA-binding experiments using DNA-cellulose mini-columns showed that Hils1 was able to bind to both double and single stranded-DNAs in a non-sequence-specific manner. These findings suggest that Hils1 may play an important role in the structural changes of spermatid nuclei, such as nuclear condensation, and gene regulation of haploid germ cell differentiation.