Evaluation of degenerating germ cells in normal juvenile mice.

Absence of spermiogenesis in mice with meiotic defects complicates the staging of meiotic arrest using light microscopy. Consequently, new methodologies are required to establish accurate relationships among germ cells. In this study, we utilized a novel approach to analyze germ cell degeneration in juvenile mice. We used terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) in combination with meiosis-specific antibodies. Germ cell degeneration is a normal component of early spermatogenesis in juvenile mice. The incidence of germ cell death was monitored at various postnatal ages of mice using the TUNEL assay to quantify the incidence of apoptosis. Cell death occurred predominantly at 15.5 days after birth. It was found that groups of apoptotic cells were apparent in tubules containing two generations of spermatocytes that form in two progressive cohorts. Electron microscopic observations further illustrated that the majority of cells in the first cohort are in late pachytene, while groups of cells in the second cohort can degenerate in early pachytene. The methodology utilized in this study is significant because it allows one to accurately determine the point at which germ cells arrest. Consequently, we believe that these methods can be applied to study animals with meiotic defects that prevent spermiogenesis.

Bat3 deficiency accelerates the degradation of Hsp70-2/HspA2 during spermatogenesis.

Meiosis is critical for sexual reproduction. During meiosis, the dynamics and integrity of homologous chromosomes are tightly regulated. The genetic and molecular mechanisms governing these processes in vivo, however, remain largely unknown. In this study, we demonstrate that Bat3/Scythe is essential for survival and maintenance of male germ cells (GCs). Targeted inactivation of Bat3/Scythe in mice results in widespread apoptosis of meiotic male GCs and complete male infertility. Pachytene spermatocytes exhibit abnormal assembly and disassembly of synaptonemal complexes as demonstrated by abnormal SYCP3 staining and sustained gamma-H2AX and Rad51/replication protein A foci. Further investigation revealed that a testis-specific protein, Hsp70-2/HspA2, is absent in Bat3-deficient male GCs at any stage of spermatogenesis; however, Hsp70-2 transcripts are expressed at normal levels. We found that Bat3 deficiency induces polyubiquitylation and subsequent degradation of Hsp70-2. Inhibition of proteasomal degradation restores Hsp70-2 protein levels. Our findings identify Bat3 as a critical regulator of Hsp70-2 in spermatogenesis, thereby providing a possible molecular target in idiopathic male infertility.

Initiation and resolution of interhomolog connections: crossover and non-crossover sites along mouse synaptonemal complexes.

Programmed double-strand breaks at prophase of meiosis acquire immunologically detectable RAD51-DMC1 foci or early nodules (ENs) that are associated with developing chromosome core segments; each focus is surrounded by a gammaH2AX-modified chromosome domain. The 250-300 ENs per nucleus decline in numbers during the development of full-length cores and the remaining foci are relatively evenly distributed along the mature cores (gamma distribution of nu=2.97). The ENs become transformed nodules (TNs) by the acquisition of RPA, BLM, MSH4 and topoisomerases that function in repair and Holliday junction resolution. At the leptotene-zygotene transition, TNs orient to positions between the aligned cores where they initiate structural interhomolog contacts prior to synaptonemal complex (SC) formation, possibly future crossover sites. Subsequently, TNs are associated with SC extension at the synaptic forks. Dephosphorylation of TN-associated histone gammaH2AX chromatin suggests annealing of single strands or repair of double-strand breaks DSBs at this time. Some 200 TNs per pachytene nucleus are distributed proportional to SC length and are evenly distributed along the SCs (nu= approximately 4). At this stage, gammaH2AX-modified chromatin domains are associated with transcriptionally silenced sex chromosomes and autosomal sites. Immunogold electron microscope evidence shows that one or two TNs of the 10-15 TNs per SC acquire MLH1 protein, the hallmark of reciprocal recombination, whereas the TNs that do not acquire MLH1 protein relocate from their positions along the midline of the SCs to the periphery of the SCs. Relocation of TNs may be associated with the conversion of potential crossovers into non-crossovers.

Mlh1 deficiency in zebrafish results in male sterility and aneuploid as well as triploid progeny in females.

In most eukaryotes, recombination of homologous chromosomes during meiosis is necessary for proper chromosome pairing and subsequent segregation. The molecular mechanisms of meiosis are still relatively unknown, but numerous genes are known to be involved, among which are many mismatch repair genes. One of them, mlh1, colocalizes with presumptive sites of crossing over, but its exact action remains unclear. We studied meiotic processes in a knockout line for mlh1 in zebrafish. Male mlh1 mutants are sterile and display an arrest in spermatogenesis at metaphase I, resulting in increased testis weight due to accumulation of prophase I spermatocytes. In contrast, females are fully fertile, but their progeny shows high rates of dysmorphology and mortality within the first days of development. SNP-based chromosome analysis shows that this is caused by aneuploidy, resulting from meiosis I chromosomal missegregation. Surprisingly, the small percentage of progeny that develops normally has a complete triploid genome, consisting of both sets of maternal and one set of paternal chromosomes. As adults, these triploid fish are infertile males with wild-type appearance. The frequency of triploid progeny of mlh1 mutant females is much higher than could be expected for random chromosome segregation. Together, these results show that multiple solutions exist for meiotic crossover/segregation problems.

Zebrafish: chiasmata and interference.

With immunofluorescence microscopy, the positions of centromeres and MLH1 (MutL homolog) foci representing the sites of presumptive chiasmata are shown for zebrafish (Danio rerio Hamilton 1822) synaptonemal complexes (SCs) in spermatocyte nuclei at meiotic prophase. Most SCs have a single focus and a few (7 of 140) have 2 chiasmata. MLH1 foci tend to be in the distal regions of SCs, with progressively fewer occurring towards the middle of the SCs. This non-random distribution suggests chiasma interference. Synaptic initiation, as well as replication protein A (RPA) foci at the chromosome ends, correlates with the distal localization of MLH1 foci. These observations may provide the physical basis for the reported limited genetic recombination in the centromeric region of androgenetic offspring of a male.

The evolution of meiosis: recruitment and modification of somatic DNA-repair proteins.

Several DNA-damage detection and repair mechanisms have evolved to repair double-strand breaks induced by mutagens. Later in evolutionary history, DNA single- and double-strand cuts made possible immune diversity by V(D)J recombination and recombination at meiosis. Such cuts are induced endogenously and are highly regulated and controlled. In meiosis, DNA cuts are essential for the initiation of homologous recombination, and for the formation of joint molecule and crossovers. Many proteins that function during somatic DNA-damage detection and repair are also active during homologous recombination. However, their meiotic functions may be altered from their somatic roles through localization, posttranslational modifications and/or interactions with meiosis-specific proteins. Presumably, somatic repair functions and meiotic recombination diverged during evolution, resulting in adaptations specific to sexual reproduction. (c) 2005 Wiley Periodicals, Inc.

Mutant meiotic chromosome core components in mice can cause apparent sexual dimorphic endpoints at prophase or X-Y defective male-specific sterility.

Genetic modifications causing germ cell death during meiotic prophase in the mouse frequently have sexually dimorphic phenotypes where oocytes reach more advanced stages than spermatocytes. To determine to what extent these dimorphisms are due to differences in male versus female meiotic prophase development, we compared meiotic chromosome events in the two sexes in both wild-type and mutant mice. We report the abundance and time course of appearance of structural and recombination-related proteins of fetal oocyte nuclei. Oocytes at successive days post coitus show rapid, synchronous meiotic prophase development compared with the continuous spermatocyte development in adult testis. Consequently, a genetic defect requiring 2-3 days from the onset of prophase to reach arrest registers pachytene as the developmental endpoint in oocytes. Pachytene spermatocytes, on the other hand, which normally accumulate during days 4-10 after the onset of prophase, will be rare, giving the appearance of an earlier endpoint than in oocytes. We conclude that these different logistics create apparent sexually dimorphic endpoints. For more pronounced sexual dimorphisms, we examined meiotic prophase of mice with genetic modifications of meiotic chromosome core components that cause male but not female sterility. The correlations between male sterility and alterations in the organization of the sex chromosome cores and X-Y chromatin may indicate that impaired signals from the XY domain (XY chromosome cores, chromatin, dense body and sex body) may interfere with the progression of the spermatocyte through prophase. Oocytes, in the absence of the X-Y pair, do not suffer such defects.

Association between MSH4 (MutS homologue 4) and the DNA strand-exchange RAD51 and DMC1 proteins during mammalian meiosis.

During meiotic prophase, chromosomes must undergo highly regulated recombination events, some of which lead to reciprocal exchanges. In yeast, MSH4, a meiosis-specific homologue of the bacterial MutS protein, is required for meiotic recombination. In mice, disruption of the Msh4 gene results in male and female infertility due to meiotic failure. To date, the implication of MSH4 mutations has not been established in human sterility. However, it is noteworthy that mutant mice exhibit a defect in the chromosome synapsis, strikingly similar to the clinical observations found in human infertility. As a step towards understanding the molecular mechanisms underlying the role of MSH4 in human gametogenesis, we decided to determine whether this protein interacts with recombination machinery enzymes. Our results provide biochemical evidence indicating that the human MSH4 protein physically interacts with both RAD51 and DMC1, two RecA homologues known to initiate DNA strand-exchange between homologous chromosomes. Immunolocalization analyses show that some MSH4 foci, located on mouse meiotic chromosomes, colocalize with DMC1/RAD51 complexes. Our data support the view that MSH4 is associated with the early meiotic recombination machinery in mammals. We consider the possibility that MSH4 is involved in the regulation of recombination events by exerting a function closely after DNA strand-exchange has been initiated.

TopBP1 localises to centrosomes in mitosis and to chromosome cores in meiosis.

Topoisomerase IIbeta binding protein 1 (TopBP1), previously shown to localise to sites of DNA damage and to stalled replication forks, has been implicated in DNA replication and in DNA damage response. In this work we showed that TopBP1 was localised in structures other than stalled replication forks. In late mitosis TopBP1 localises to centrosomes in a manner similar to other DNA damage response proteins such as BRCA1 and p53. Spindle checkpoint activation does not affect this centrosomal localisation. Moreover, in the testis, we detected high levels of TopBP1 associated with meiotic prophase chromosome cores and the X-Y pair. Together, these data suggest a direct role of TopBP1 during both mitosis and meiotic prophase I.

Irregular telomeres impair meiotic synapsis and recombination in mice.

Telomere shortening can lead to chromosome instability, replicative senescence, and apoptosis in both somatic and male germ cells. To study roles for mammalian telomeres in homologous pairing and recombination, we characterized effects of telomere shortening on spermatogenesis and oogenesis in late-generation telomerase-deficient mice. We show that shortened telomeres of late-generation telomerase-deficient mice impair meiotic synapsis and decrease recombination, in particular, in females. In response to telomere shortening, male germ cells mostly undergo apoptosis, whereas female germ cells preferentially arrest in early meiosis, suggesting sexually dimorphic surveillance mechanisms for telomere dysfunction during meiosis in mice. Further, meiocytes of late-generation telomerase-deficient females with shortened telomeres, bred with early-generation males harboring relatively long telomeres, exhibit severely impaired chromosome pairing and synapsis and reduced meiotic recombination. These findings imply that functional telomeres are important in mammalian meiotic synapsis and recombination.

Windows for sex-specific methylation marked by DNA methyltransferase expression profiles in mouse germ cells.

The acquisition of genomic methylation in the male germ line is initiated prenatally in diploid gonocytes, while DNA methylation in the female germ line is initiated postnatally in growing oocytes. We compared the temporal expression patterns of the DNA methyltransferases, DNMT1, DNMT3a, DNMT3b, and DNMT3l in the male and female germ lines. DNMT1 expression was examined by immunocytochemistry and Northerns with an emphasis on the prenatal period. In the female, there is a gradual down-regulation of DNMT1 protein in prenatal meiotic prophase I oocytes that is not associated with the production of an untranslated transcript, as it is in the male; these results suggest that the mechanism of meiotic down-regulation differs between the sexes. In the male, DNMT1 is unlikely to play a role in the prenatal acquisition of germ line methylation patterns since it is down-regulated in gonocytes between 14.5 and 18.5 days of gestation and is absent at the time of initiation of DNA methylation. To search for candidate DNMTs that could be involved in establishing methylation patterns in both germ lines, real-time RT-PCR was used to simultaneously study the expression profiles of the three DNMT3 enzymes in developing testes and ovaries; DNMT1 expression was included as a control. Expression profiles of DNMT3a and DNMT3l provide support for an interaction of the two enzymes during prenatal germ cell development and de novo methylation in the male. DNMT3l is the predominant DNMT3 enzyme expressed at high levels in the postnatal female germ line at the time of acquisition of DNA methylation patterns. DNMT1 and DNMT3b expression levels peak concomitantly, shortly after birth in the male, consistent with a role in the maintenance of methylation patterns in proliferating spermatogonia. Together, the results provide clues to specific roles for the different DNMT family members in de novo and maintenance methylation in the developing testis and ovary.

TopBP1 and ATR colocalization at meiotic chromosomes: role of TopBP1/Cut5 in the meiotic recombination checkpoint.

Mammalian TopBP1 is a BRCT domain-containing protein whose function in mitotic cells is linked to replication and DNA damage checkpoint. Here, we study its possible role during meiosis in mice. TopBP1 foci are abundant during early prophase I and localize mainly to histone gamma-H2AX-positive domains, where DNA double-strand breaks (required to initiate recombination) occur. Strikingly, TopBP1 showed a pattern almost identical to that of ATR, a PI3K-like kinase involved in mitotic DNA damage checkpoint. In the synapsis-defective Fkbp6(-/-) mouse, TopBP1 heavily stains unsynapsed regions of chromosomes. We also tested whether Schizosaccharomyces pombe Cut5 (the TopBP1 homologue) plays a role in the meiotic recombination checkpoint, like spRad3, the ATR homologue. Indeed, we found that a cut5 mutation suppresses the checkpoint-dependent meiotic delay of a meiotic recombination defective mutant, indicating a direct role of the Cut5 protein in the meiotic checkpoint. Our findings suggest that ATR and TopBP1 monitor meiotic recombination and are required for activation of the meiotic recombination checkpoint.

The double-stranded DNA helix in recombination at meiosis.

The nature of meiotic genetic recombination was resolved at the DNA level by the 1953 Watson and Crick model. What remains to be determined are the roles of the various recombination proteins and the distribution and localization of recombination events in the meiotic prophase nucleus.

Essential role of Fkbp6 in male fertility and homologous chromosome pairing in meiosis.

Meiosis is a critical stage of gametogenesis in which alignment and synapsis of chromosomal pairs occur, allowing for the recombination of maternal and paternal genomes. Here we show that FK506 binding protein (Fkbp6) localizes to meiotic chromosome cores and regions of homologous chromosome synapsis. Targeted inactivation of Fkbp6 in mice results in aspermic males and the absence of normal pachytene spermatocytes. Moreover, we identified the deletion of Fkbp6 exon 8 as the causative mutation in spontaneously male sterile as/as mutant rats. Loss of Fkbp6 results in abnormal pairing and misalignments between homologous chromosomes, nonhomologous partner switches, and autosynapsis of X chromosome cores in meiotic spermatocytes. Fertility and meiosis are normal in Fkbp6 mutant females. Thus, Fkbp6 is a component of the synaptonemal complex essential for sex-specific fertility and for the fidelity of homologous chromosome pairing in meiosis.

Infertility and aneuploidy in mice lacking a type IA DNA topoisomerase III beta.

We report that disruption of the mouse TOP3 beta gene encoding DNA topoisomerase III beta, one of the two mammalian type IA DNA topoisomerases, leads to a progressive reduction in fecundity. The litter size in crosses of top3 beta(-/-) mice decreases over time and through successive generations, and this decrease seems to reflect embryonic death rather than impaired fertilization. These observations are suggestive of a gradual accumulation of chromosomal defects in germ cells lacking DNA topoisomerase III beta, and this interpretation is supported by the observation of a high incidence of aneuploidy in the spermatocytes of infertile top3 beta(-/-) males. Cytogenetic examination of spermatocytes of wild-type mice also indicates that DNA topoisomerase III beta becomes prominently associated with the asynaptic regions of the XY bivalents during pachytene, and that there is a time lag between the appearance of chromosome-bound DNA topoisomerase III beta and Rad51, a protein known to be involved in an early step of homologous recombination. We interpret these findings, together with the known mechanistic characteristics of different subfamilies of DNA topoisomerases, in terms of a specific role of a type IA DNA topoisomerase in the resolution of meiotic double-Holliday junctions without crossing over. This interpretation is most likely applicable to mitotic cells as well and can explain the universal presence of at least one type IA DNA topoisomerase in all organisms.

Meiotic arrest and aneuploidy in MLH3-deficient mice.

MutL homolog 3 (Mlh3) is a member of a family of proteins conserved during evolution and having dual roles in DNA mismatch repair and meiosis. The pathway in eukaryotes consists of the DNA-binding components, which are the homologs of the bacterial MutS protein (MSH 2 6), and the MutL homologs, which bind to the MutS homologs and are essential for the repair process. Three of the six homologs of MutS that function in these processes, Msh2, Msh3 and Msh6, are involved in the mismatch repair of mutations, frameshifts and replication errors, and two others, Msh4 and Msh5, have specific roles in meiosis. Of the four MutL homologs, Mlh1, Mlh3, Pms1 and Pms2, three are involved in mismatch repair and at least two, Pms2 and Mlh1, are essential for meiotic progression in both yeast and mice. To assess the role of Mlh3 in mammalian meiosis, we have generated and characterized Mlh3(-/-) mice. Here we show that Mlh3(-/-) mice are viable but sterile. Mlh3 is required for Mlh1 binding to meiotic chromosomes and localizes to meiotic chromosomes from the mid pachynema stage of prophase I. Mlh3(-/-) spermatocytes reach metaphase before succumbing to apoptosis, but oocytes fail to complete meiosis I after fertilization. Our results show that Mlh3 has an essential and distinct role in mammalian meiosis.

The time course and chromosomal localization of recombination-related proteins at meiosis in the mouse are compatible with models that can resolve the early DNA-DNA interactions without reciprocal recombination.

During mouse meiosis, the early prophase RAD51/DMC1 recombination protein sites, which are associated with the chromosome cores and which serve as markers for ongoing DNA-DNA interactions, are in ten-fold excess of the eventual reciprocal recombinant events. Most, if not all, of these early interactions are eliminated as prophase progresses. The manner in which these sites are eliminated is the focus of this investigation. We report that these sites acquire replication protein A, RPA and the Escherichia coli MUTS homologue, MSH4p, and somewhat later the Bloom helicase, BLM, while simultaneously losing the RAD51/DMC1 component. Eventually the RPA component is also lost and BLM sites remain. At that time, the MUTL homologue, MLH1p, which is essential for reciprocal recombination in the mouse, appears in numbers and locations that correspond to the distribution of reciprocal recombination events. However, the MLH1 foci do not appear to coincide with the remaining BLM sites. The MLH1p is specifically localized to electron-microscope-defined recombination nodules. We consider the possibility that the homology-search RAD51/DMC1 complexes are involved in homologous chromosome synapsis but that most of these early DNA-DNA interactions are later resolved by the anti-recombination RPA/MSH4/BLM-topoisomerase complex, thereby preventing the formation of superfluous reciprocal recombinant events.