Defective imprint resetting in carriers of Robertsonian translocation Rb (8.12)

Meiotic silencing of unsynapsed chromatin (MSUC) occurs in the germ cells of translocation carriers and may cause meiotic arrest and infertility. We hypothesized that if bypassing meiotic checkpoints MSUC may cause epigenetic defects in sperm. We investigated the meiotic behavior of the Robertsonian translocation Rb (8.12) in mice. The unsynapsed 8 and 12 trivalent was associated with the XY body during early and mid-pachynema in heterozygous Rb (8.12) carriers, suggesting possible silencing of pericentromeric genes, such as the Dnmt3a gene. In wild-type mice, DNMT3A protein showed a dramatic accumulation in the nucleus during the mid-pachytene stage and distinct association with the XY body. In translocation carriers, DNMT3A was less abundant in a proportion of pachytene spermatocytes that also had unsynapsed pericentromeric regions of chromosomes 8 and 12. The same mice had incomplete methylation of the imprinted H19 differentially methylated region (DMR) in sperm. We propose that impaired H19 imprint establishment results from lack of synapsis in chromosomes 8 and 12 probably through transient silencing of a chromosome 8 or 12 gene during pachynema. Furthermore, our findings support the notion that imprint establishment at the H19 locus extends into pachynema.     

CYP26B1 promotes male germ cell differentiation by suppressing STRA8-dependent meiotic and STRA8-independent mitotic pathways

Germ cell sex is defined by factors derived from somatic cells. CYP26B1 is known to be a male sex-promoting factor that inactivates retinoic acid (RA) in somatic cells. In CYP26B1-null XY gonads, germ cells are exposed to a higher level of RA than in normal XY gonads and this activates Stra8 to induce meiosis while male-specific gene expression is suppressed. However, it is unknown whether meiotic entry by an elevated level of RA is responsible for the suppression of male-type gene expression. To address this question, we have generated Cyp26b1/Stra8 double knockout (dKO) embryos. We successfully suppressed the induction of meiosis in CYP26B1-null XY germ cells by removing the Stra8 gene. Concomitantly, we found that the male genetic program represented by the expression of NANOS2 and DNMT3L was totally rescued in about half of dKO germ cells, indicating that meiotic entry causes the suppression of male differentiation. However, half of the germ cells still failed to enter the appropriate male pathway in the dKO condition. Using microarray analyses together with immunohistochemistry, we found that KIT expression was accompanied by mitotic activation, but was canceled by inhibition of the RA signaling pathway. Taken together, we conclude that inhibition of RA is one of the essential factors to promote male germ cell differentiation, and that CYP26B1 suppresses two distinct genetic programs induced by RA: a Stra8-dependent meiotic pathway, and a Stra8-independent mitotic pathway.     

Genomic imprinting and epigenetic reprogramming: unearthing the garden of forking paths

Genomic imprinting, an epigenetic form of gene regulation, determines the parent-dependent gene expression of marked or imprinted genes during gametogenesis and embryonic development. Imprinting involves differential allele DNA methylation in one sex cell lineage but not in the other. Egg and sperm each contributes the same DNA sequences to the zygote but epigenetic imprinting of a subset of genes determines that only one of the parent alleles are expressed relative to the parental origin. Primordial germ cells inherit biallelically imprinted genes from maternal and paternal origin and erase their imprints to start de novo monoallelic imprinting during gametogenesis. Epigenetic paternalization is an ongoing process in the mitotically-dividing spermatogonial stem cell and derived meiotically-dividing spermatocyte progeny to endow sperm with imprinted alleles. Epigenetic maternalization is restricted to the oocyte growth phase of folliculogenesis and is unrelated to DNA replication since it takes place while the oocyte remains in the diplotene stage of meiotic prophase I. Sperm and oocyte genomic methylation patterns depend on the activity of DNA methyltransferases (Dnmt). A variant of Dnmt1, designated Dnmt1o, accumulates in oocyte nuclei during the follicular growth phase. Dnmt3L, an isoform of Dnmt3a and Dnmt3b, but lacking enzymatic activity, interacts with Dnmt2a and Dnmt3b and is required for spermatogenesis. In the mouse early zygote, the male pronucleus is demethylated within 4 h of fertilization. Global demethylation takes place gradually up to the morula stage. In the blastocyst, de novo methylation is reestablished in the inner cell mass but not in the trophectoderm. Both the significance of genomic imprinting and the severe developmental defects caused by disrupted Dnmt activity, point to a need for a better understanding of the causes of low cloning efficiency by somatic nuclear transfer to enucleated ovulated oocyte.     

Increased recombination frequency showing evidence of loss of interference is associated with abnormal testicular histopathology

Nondisjunction leading to aneuploid gametes has been linked genetically to both increases and decreases in recombination frequency on the aneuploid chromosome. In the present study, we present physical evidence of increased frequency of recombination nodules as measured by Mut-S-like homologue-1 (MLH1) foci on pachytene chromosomes from sterile male mice homozygous for a mutation in the protein phosphatase 1cgamma (PP1cgamma) gene. The pattern of elevated recombination frequency in PP1cgamma mutant spermatocytes is consistent with a loss of interference. Previous studies demonstrated: (1) spermiogenesis is impaired starting at step 8 with a severe reduction in elongating and condensed spermatids; (2) spermatids and sperm exhibit elevated rates of DNA fragmentation; and (3) haploid gametes exhibit elevated levels of aneuploidy. Morphometric analysis of developing testes revealed that the first wave of meiosis proceeds at a normal rate in mutant testes, a surprising result given that the PP1 inhibitor okadaic acid has been shown to accelerate progression of spermatocytes from pachytene to the first meiotic division (MI). Evidence of abnormal testicular histopathology is apparent at 3 weeks, before the appearance of haploid gametes, eliminating the possibility that the mutant phenotype is caused by the presence of abnormal spermatids, but coincident with the appearance of the first set of mid to late pachytene spermatocytes. These observations lead us to conclude that the PP1cgamma mutation causes a complex phenotype, including subtle adverse effects on meiosis, possibly mediated by defective signaling between germ cells and Sertoli cells.     

Meiotic and epigenetic aberrations in Dnmt3L-deficient male germ cells

The DNA methyltransferase-like protein Dnmt3L is necessary for the establishment of genomic imprints in oogenesis and for normal spermatogenesis (Bourc'his et al., 2001; Hata et al., 2002). Also, a paternally imprinted gene, H19, loses DNA methylation in Dnmt3L-/- spermatogonia (Bourc'his and Bestor, 2004; Kaneda et al., 2004). To determine the reason for the impaired spermatogenesis in the Dnmt3L-/- testes, we have carried out a series of histological and molecular studies. We show here that Dnmt3L-/- germ cells were arrested and died around the early meiotic stage. A microarray-based gene expression-profiling analysis revealed that various gonad-specific and/or sex-chromosome-linked genes were downregulated in the Dnmt3L-/- testes. In contrast, expression of retrovirus-like intracisternal A-particle (IAP) sequences was upregulated; consistent with this observation, a specific IAP copy showed complete loss of DNA methylation. These findings indicate that Dnmt3L regulates germ cell-specific gene expression and IAP suppression, which are critical for male germ cell proliferation and meiosis.     

RNA interference during spermatogenesis in mice

Spermatogenesis consists of complex cellular and developmental processes, such as the mitotic proliferation of spermatogonial stem cells, meiotic division of spermatocytes, and morphogenesis of haploid spermatids. In this study, we show that RNA interference (RNAi) functions throughout spermatogenesis in mice. We first carried out in vivo DNA electroporation of the testis during the first wave of spermatogenesis to enable foreign gene expression in spermatogenic cells at different stages of differentiation. Using prepubertal testes at different ages and differentiation stage-specific promoters, reporter gene expression was predominantly observed in spermatogonia, spermatocytes, and round spermatids. This method was next applied to introduce DNA vectors that express small hairpin RNAs, and the sequence-specific reduction in the reporter gene products was confirmed at each stage of spermatogenesis. RNAi against endogenous Dmc1, which encodes a DNA recombinase that is expressed and functionally required in spermatocytes, led to the same phenotypes observed in null mutant mice. Thus, RNAi is effective in male germ cells during mitosis and meiosis as well as in haploid cells. This experimental system provides a novel tool for the rapid, first-pass assessment of the physiological functions of spermatogenic genes in vivo.     

Bromodomain‐dependent stage‐specific male genome programming by Brdt

Male germ cell differentiation is a highly regulated multistep process initiated by the commitment of progenitor cells into meiosis and characterized by major chromatin reorganizations in haploid spermatids. We report here that a single member of the double bromodomain BET factors, Brdt, is a master regulator of both meiotic divisions and post‐meiotic genome repackaging. Upon its activation at the onset of meiosis, Brdt drives and determines the developmental timing of a testis‐specific gene expression program. In meiotic and post‐meiotic cells, Brdt initiates a genuine histone acetylation‐guided programming of the genome by activating essential genes and repressing a ‘progenitor cells’ gene expression program. At post‐meiotic stages, a global chromatin hyperacetylation gives the signal for Brdt's first bromodomain to direct the genome‐wide replacement of histones by transition proteins. Brdt is therefore a unique and essential regulator of male germ cell differentiation, which, by using various domains in a developmentally controlled manner, first drives a specific spermatogenic gene expression program, and later controls the tight packaging of the male genome.     

Timing of entry of meiosis depends on a mark generated by DNA methyltransferase 3a in testis

Reprogramming of DNA methylation is an essential part of gametogenesis, and a role of two members of the DNA methyltransferase (Dnmt) family, Dnmt3a and Dnmt3L, has been recognized. In an attempt to elucidate the role of Dnmt3a, we analyzed the progression of spermatogenesis in Dnmt3a (-/-) homozygotes during the first 3 weeks of post-natal development. The emerging picture was markedly different from that recently reported for the Dnmt3L protein. In the Dnmt3a (-/-) testis, at the expected time of entry into meiosis (11-13 dpp), the number of spermatocytes was greatly reduced. They progressively accumulated during the following days, but at a slower rate than in the wild type. Once started, however, the pachytene stage was apparently completed with normal chromosome pairing and formation of the sex vesicle, and spermatogenesis further progressed with the appearance and the expression of round spermatid specific markers. Interestingly and unlike Dnmt3L (-/-) spermatocytes, Dnmt3a (-/-) germ cells showed only a minor reduction in the methylation of interspersed repetitive elements and retroposons. The Dnmt3a might thus generate a mark important for the initiation of male meiosis that is distinct from that created by Dnmt3L.     

Evidence for Paternal Age-Related Alterations in Meiotic Chromosome Dynamics in the Mouse

Increasing age in a woman is a well-documented risk factor for meiotic errors, but the effect of paternal age is less clear. Although it is generally agreed that spermatogenesis declines with age, the mechanisms that account for this remain unclear. Because meiosis involves a complex and tightly regulated series of processes that include DNA replication, DNA repair, and cell cycle regulation, we postulated that the effects of age might be evident as an increase in the frequency of meiotic errors. Accordingly, we analyzed spermatogenesis in male mice of different ages, examining meiotic chromosome dynamics in spermatocytes at prophase, at metaphase I, and at metaphase II. Our analyses demonstrate that recombination levels are reduced in the first wave of spermatogenesis in juvenile mice but increase in older males. We also observed age-dependent increases in XY chromosome pairing failure at pachytene and in the frequency of prematurely separated autosomal homologs at metaphase I. However, we found no evidence of an age-related increase in aneuploidy at metaphase II, indicating that cells harboring meiotic errors are eliminated by cycle checkpoint mechanisms, regardless of paternal age. Taken together, our data suggest that advancing paternal age affects pairing, synapsis, and recombination between homologous chromosomes—and likely results in reduced sperm counts due to germ cell loss—but is not an important contributor to aneuploidy.     

Male differentiation of germ cells induced by embryonic age-specific Sertoli cells in mice

Retinoic acid (RA) is a meiosis-inducing factor. Primordial germ cells (PGCs) in the developing ovary are exposed to RA, resulting in entry into meiosis. In contrast, PGCs in the developing testis enter mitotic arrest to differentiate into prospermatogonia. Sertoli cells express CYP26B1, an RA-metabolizing enzyme, providing a simple explanation for why XY PGCs do not initiate meios/is. However, regulation of entry into mitotic arrest is likely more complex. To investigate the mechanisms that regulate male germ cell differentiation, we cultured XX and XY germ cells at 11.5 and 12.5 days postcoitus (dpc) with an RA receptor inhibitor. Expression of Stra8, a meiosis initiation gene, was suppressed in all groups. However, expression of Dnmt3l, a male-specific gene, during embryogenesis was elevated but only in 12.5-dpc XY germ cells. This suggests that inhibiting RA signaling is not sufficient for male germ cell differentiation but that the male gonadal environment also contributes to this pathway. To define the influence of Sertoli cells on male germ cell differentiation, Sertoli cells at 12.5, 15.5, and 18.5 dpc were aggregated with 11.5 dpc PGCs, respectively. After culture, PGCs aggregated with 12.5 dpc Sertoli cells increased Nanos2 and Dnmt3l expression. Furthermore, these PGCs established male-specific methylation imprints of the H19 differentially methylated domains. In contrast, PGCs aggregated with Sertoli cells at late embryonic ages did not commit to the male pathway. These findings suggest that male germ cell differentiation is induced both by inhibition of RA signaling and by molecule(s) production by embryonic age-specific Sertoli cells.     

Increased incidence of hyperhaploid 24,XY spermatozoa detected by three-colour FISH in a 46,XY/47,XXY male

Meiotic segregation of gonosomes from a 46,XY/47,XXY male was analysed by a three-colour fluorescence in situ hybridisation (FISH) procedure. This method allows the identification of hyperhaploid spermatozoa (with 24 chromosomes), diploid spermatozoa (with 46 chromosomes) and their meiotic origin (meiosis I or 11). Alpha satellite DNA probes specific for chromosomes X, Y and 1 were observed on 27,097 sperm nuclei. The proportions of X-and Y -bearing sperm were estimated to 52.78% and 43.88%, respectively. Disomy (24,XX, 24,YY, 24,X or Y,+1) and diploidy (46,XX, 46,YY, 46,XY) frequencies were close to those obtained from control sperm, whereas the frequency of hyperhaploid 24,XY spermatozoa (2.09%) was significantly increased compared with controls (0.36%). These results support the hypothesis that a few 47,XXY germ cells would be able to complete meiosis and to produce mature spermatozoa.     

Acrosome biogenesis begins during meiosis: evidence from the synthesis and distribution of an acrosomal glycoprotein, acrogranin, during guinea pig spermatogenesis.

The biogenesis of the sperm-specific organelle, the acrosome, was investigated using an acrosomal glycoprotein as a marker of development. This component, which we have named acrogranin, was purified from an acid extract of guinea pig testes by standard chromatographic procedures. The molecular weight of reduced acrogranin was determined to be 67,000 by analytical sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Immunization of female rabbits with purified acrogranin produced an antiserum that recognized a single protein with Mr = 67,000 in an acid extract of guinea pig testes. By indirect immunofluorescence, acrogranin was found only in the acrosome of mature sperm. In haploid spermatids, acrogranin was localized in the developing acrosome and, weakly, in the cytoplasm. Acrogranin was also detected in the cytoplasm and juxtanuclear region in putative proacrosomal granules of meiotic cells (pachytene spermatocytes). Detergent extracts from different purified germ cell populations contained only the Mr = 67,000 form of acrogranin, but sperm extracts had four lower Mr immunoreactive forms not present in the testicular extracts. By two-dimensional gel electrophoresis, acrogranin was found to be an acidic glycoprotein. Analysis of glycosylated and trifluoromethanesulfonic acid-deglycosylated acrogranin indicated that the antibody recognized polypeptide determinants. After highly enriched germ cell populations were labeled overnight with [35S]methionine and extracted with detergent, anti-acrogranin immunoprecipitated a single protein of Mr = 67,000. The synthesis of acrogranin by pachytene spermatocytes and round spermatids was similar, but the synthesis of the glycoprotein by condensing spermatids was markedly reduced. These studies demonstrate that acrosome biogenesis, as determined by the synthesis of a specific acrosomal component, begins during meiosis and continues through the early stages of spermiogenesis.     

DNMT3L stimulates the DNA methylation activity of Dnmt3a and Dnmt3b through a direct interaction

In mammals, the resetting of DNA methylation patterns in early embryos and germ cells is crucial for development. Two DNA methyltransferases, Dnmt3a and Dnmt3b, are responsible for the creation of DNA methylation patterns. Dnmt3L, a member of the Dnmt3 family, has been reported to be necessary for maternal methylation imprinting, possibly by interacting with Dnmt3a and/or Dnmt3b (Hata, K., Okano, M., Lei, H., and Li, E. (2002) Development 129, 1983-1993). In the present study, the effect of DNMT3L, a human homologue of Dnmt3L, on the DNA methylation activity of mouse Dnmt3a and Dnmt3b was examined in vitro. DNMT3L enhanced the DNA methylation activity of Dnmt3a and Dnmt3b about 1.5-3-fold in a dose-dependent manner but did not enhance the DNA methylation activity of Dnmt1. Although the extents of stimulation were different, a stimulatory effect on the DNA methylation activity was observed for all of the substrate DNA sequences examined, such as those of the maternally methylated SNRPN and Lit-1 imprinting genes, the paternally methylated H19 imprinting gene, the CpG island of the myoD gene, the 5 S ribosomal RNA gene, an artificial 28-bp DNA, poly(dG-dC)-poly(dG-dC), and poly(dI-dC)-poly(dI-dC). DNMT3L could not bind to DNA but could bind to Dnmt3a and Dnmt3b, indicating that the stimulatory effect of DNMT3L on the DNA methylation activity may not be due to the guiding of Dnmt3a and Dnmt3b to the targeting DNA sequence but may comprise a direct effect on their catalytic activity. The carboxyl-terminal half of DNMT3L was found to be responsible for the enhancement of the enzyme activity.