Meiotic prophase arrest with failure of chromosome synapsis in mice deficient for Dmc1, a germline-specific RecA homolog

Human Dmc1 protein binds DNA as an octameric ring

The bacterial RecA protein has been the most intensively studied enzyme in homologous genetic recombination. The core of RecA is structurally homologous to that of the F1-ATPase and helicases. Like the F1-ATPase and ring helicases, RecA forms a hexameric ring. The human Dmc1 (hDmc1) protein, a meiosis-specific recombinase, is homologous to RecA. We show that hDmc1 forms octameric rings. Unlike RecA and Rad51, however, hDmc1 protein does not form helical filaments. The hDmc1 ring binds DNA in the central channel, as do the ring helicases, which is likely to represent the active form of the protein. These observations indicate that the conservation of the RecA-like ring structure extends from bacteria to humans, and that some RecA homologs may form both rings and filaments, whereas others may function only as rings.

Completion of meiosis is not always required for acrosome formation in HSP70-2 null mice

Hsp70-2 is a unique member of the mouse 70-kDa heat shock protein family that is synthesized during meiosis in spermatogenic cells. Germ cells in male mice homozygous for a targeted mutation in the Hsp70-2 gene (Hsp70-2(-/-)) arrest in development and undergo apoptosis at the end of the pachytene spermatocyte stage of meiotic prophase. However, cells with a putative acrosome were present occasionally in histological sections of the testes of juvenile and adult Hsp70-2(-/-) mice. This study verified that acrosomes were present and investigated the relationship between acrosome formation and the process of meiosis. Histochemistry with the periodic acid-Schiff procedure and immunostaining with monoclonal antibody MN7 verified that acrosomes were present in Hsp70-2(-/-) mice, and electron microscopy showed that some of these cells had condensing nuclei characteristic of step 8-9 spermatids. The frequency of acrosome-containing cells in Hsp70-2(-/-) mice was less than 0.01% of that in wild-type mice. Propidium iodide staining and cytophotometry indicated that the average DNA content of nuclei in MN7-positive cells in Hsp70-2(-/-) mice was usually about twice, or occasionally the same as, that of nuclei in round spermatids of wild-type mice. Meiotic metaphase I and II chromosome spreads were observed in spermatogenic cells from Hsp70-2(-/-) mice but at a much lower frequency than in wild-type mice. These results indicate that not all pachytene spermatocytes in Hsp70-2(-/-) mice arrest in meiosis, but they may divide once or sometimes twice and begin acrosome formation and nuclear condensation. This demonstrates that some aspects of spermatid development can occur without the completion of meiosis in mice, as has been reported recently for Drosophila.

Atm inactivation results in aberrant telomere clustering during meiotic prophase

A-T (ataxia telangiectasia) individuals frequently display gonadal atrophy, and Atm-/- mice show spermatogenic failure due to arrest at prophase of meiosis I. Chromosomal movements take place during meiotic prophase, with telomeres congregating on the nuclear envelope to transiently form a cluster during the leptotene/zygotene transition (bouquet arrangement). Since the ATM protein has been implicated in telomere metabolism of somatic cells, we have set out to investigate the effects of Atm inactivation on meiotic telomere behavior. Fluorescent in situ hybridization and synaptonemal complex (SC) immunostaining of structurally preserved spermatocytes I revealed that telomere clustering occurs aberrantly in Atm-/- mice. Numerous spermatocytes of Atm-/- mice displayed locally accumulated telomeres with stretches of SC near the clustered chromosome ends. This contrasted with spermatogenesis of normal mice, where only a few leptotene/zygotene spermatocytes I with clustered telomeres were detected. Pachytene nuclei, which were much more abundant in normal mice, displayed telomeres scattered over the nuclear periphery. It appears that the timing and occurrence of chromosome polarization is altered in Atm-/- mice. When we examined telomere-nuclear matrix interactions in spermatocytes I, a significant difference was observed in the ratio of soluble versus matrix-associated telomeric DNA sequences between meiocytes of Atm-/- and control mice. We propose that the severe disruption of spermatogenesis during early prophase I in the absence of functional Atm may be partly due to altered interactions of telomeres with the nuclear matrix and distorted meiotic telomere clustering.

Multiple pathways of recombination induced by double-strand breaks in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has been the principal organism used in experiments to examine genetic recombination in eukaryotes. Studies over the past decade have shown that meiotic recombination and probably most mitotic recombination arise from the repair of double-strand breaks (DSBs). There are multiple pathways by which such DSBs can be repaired, including several homologous recombination pathways and still other nonhomologous mechanisms. Our understanding has also been greatly enriched by the characterization of many proteins involved in recombination and by insights that link aspects of DNA repair to chromosome replication. New molecular models of DSB-induced gene conversion are presented. This review encompasses these different aspects of DSB-induced recombination in Saccharomyces and attempts to relate genetic, molecular biological, and biochemical studies of the processes of DNA repair and recombination.

The ubiquitin system in gametogenesis

Ubiquitin is a ubiquitous and highly conserved protein of 76 amino acid residues, that can be covalently attached to cellular acceptor proteins. The attachment of ubiquitin to target proteins is achieved through a multi-step enzymatic pathway, which involves activities of ubiquitin-activating E1 enzymes, ubiquitin-conjugating E2 enzymes, and ligating E3 enzymes. Mono- or poly-ubiquitination of proteins can lead to protein degradation or modification of protein activity. Many components of the complex ubiquitin system show remarkable evolutionary conservation, from yeast to mammalian species. The ubiquitin system is essential to all eukaryotic cells. Among others, several signal transduction cascades show involvement of the ubiquitin system, but there are currently little data supporting a specific role of the ubiquitin system in hormonal control of reproduction. Interestingly, during gametogenesis, many specialized and important aspects of the ubiquitin system become apparent. Components of the ubiquitin system appear to be involved in different steps and processes during gametogenesis, including control of meiosis, and reorganization of chromatin structure.

Pch2 links chromatin silencing to meiotic checkpoint control

The PCH2 gene of Saccharomyces cerevisiae is required for the meiotic checkpoint that prevents chromosome segregation when recombination and chromosome synapsis are defective. Mutation of PCH2 relieves the checkpoint-induced pachytene arrest of the zip1, zip2, and dmc1 mutants, resulting in chromosome missegregation and low spore viability. Most of the Pch2 protein localizes to the nucleolus, where it represses meiotic interhomolog recombination in the ribosomal DNA, apparently by excluding the meiosis-specific Hop1 protein. Nucleolar localization of Pch2 depends on the silencing factor Sir2, and mutation of SIR2 also bypasses the zip1 pachytene arrest. Under certain circumstances, Sir3-dependent localization of Pch2 to telomeres also provides checkpoint function. These unexpected findings link the nucleolus, chromatin silencing, and the pachytene checkpoint.

Mouse MutS-like protein Msh5 is required for proper chromosome synapsis in male and female meiosis

Members of the mammalian mismatch repair protein family of MutS and MutL homologs have been implicated in postreplicative mismatch correction and chromosome interactions during meiotic recombination. Here we demonstrate that mice carrying a disruption in MutS homolog Msh5 show a meiotic defect, leading to male and female sterility. Histological and cytological examination of prophase I stages in both sexes revealed an extended zygotene stage, characterized by impaired and aberrant chromosome synapsis, that was followed by apoptotic cell death. Thus, murine Msh5 promotes synapsis of homologous chromosomes in meiotic prophase I.

The leptotene-zygotene transition of meiosis

The leptotene/zygotene transition of meiosis, as defined by classical cytological studies, is the period when homologous chromosomes, already being discernible individualized entities, begin to be close together or touching over portions of their lengths. This period also includes the bouquet stage: Chromosome ends, which have already become integral components of the inner nuclear membrane, move into a polarized configuration, along with other nuclear envelope components. Chromosome movements, active or passive, also occur. The detailed nature of interhomologue interactions during this period, with special emphasis on the involvement of chromosome ends, and the overall role for meiosis and recombination of chromosome movement and, especially, the bouquet stage are discussed.

Acquisition of competence to condense metaphase I chromosomes during spermatogenesis

Little is known about the timing of meiotic prophase events during spermatogenesis in the mouse or how these events are related to cell-cycle progression. This work was designed to test hypotheses about the timing and biochemical correlates of developmental acquisition of competence to condense bivalent pairs of homologous chromosomes held together by chiasmata. The experimental approach takes advantage of the fact that okadaic acid (OA) treatment of pachytene spermatocytes causes precocious entry into metaphase I (MI) of meiosis. Leptotene and zygotene (L/Z) spermatocytes are not competent to respond to OA with condensation of chiasmate bivalent chromosomes. Competence for MI condensation of chiasmate bivalents is acquired by the middle of the pachytene stage of meiotic prophase, several days after homologous chromosomes become fully synapsed. The acquisition of MI competence is paralleled by the accumulation of histone H1t in the nuclei of mid-pachytene spermatocytes. Biochemical differences also exist between the incompetent L/Z spermatocytes and the competent pachytene spermatocytes. Both have the molecular components of metaphase promoting factor, CDC2 and CYCLIN B1; however, the histone H1 kinase activity of metaphase promoting factor of incompetent L/Z spermatocytes is not activated by OA, as it is in competent pachytene spermatocytes. Additionally, the CDC25C protein phosphatase is present in competent pachytene spermatocytes, but not in incompetent L/Z or early pachytene spermatocytes. Both incompetent and competent spermatocytes accumulate MPM-2 phosphoepitopes and phosphorylated histone H3 in response to OA treatment, indicating that presence of these antigens is not sufficient to promote condensation of meiotic chromosomes. These data demonstrate that meiotic competence of spermatocytes is acquired after homologous chromosome pairing is established and is coincident with first appearance of histone H1t and CDC25C protein phosphatase in spermatocytes.

Mammalian MutS homologue 5 is required for chromosome pairing in meiosis

MSH5 (MutS homologue 5) is a member of a family of proteins known to be involved in DNA mismatch repair. Germline mutations in MSH2, MLH1 and GTBP (also known as MSH6) cause hereditary non-polyposis colon cancer (HNPCC) or Lynch syndrome. Inactivation of Msh2, Mlh1, Gtmbp (also known as Msh6) or Pms2 in mice leads to hereditary predisposition to intestinal and other cancers. Early studies in yeast revealed a role for some of these proteins, including Msh5, in meiosis. Gene targeting studies in mice confirmed roles for Mlh1 and Pms2 in mammalian meiosis. To assess the role of Msh5 in mammals, we generated and characterized mice with a null mutation in Msh5. Msh5-/- mice are viable but sterile. Meiosis in these mice is affected due to the disruption of chromosome pairing in prophase I. We found that this meiotic failure leads to a diminution in testicular size and a complete loss of ovarian structures. Our results show that normal Msh5 function is essential for meiotic progression and, in females, gonadal maintenance.

The role of homologous recombination processes in the repair of severe forms of DNA damage in mammalian cells

The role of homologous recombination processes in the repair of severe forms of DNA damage is reviewed, with particular attention to the functions of members of the recA/RAD51 family of genes. In the yeast Saccharomyces cerevisiae, several of the gene products involved in homologous recombination repair (HRR) have been studied in detail, and a picture is beginning to emerge of the repair mechanism for DNA double-strand breaks. Knowledge is fragmentary for other eukaryotic organisms and for other types of DNA damage. In mammalian cells, while it has been known for some years that HRR occurs, the relative importance of the process in repairing DNA damage is unknown and very few of the gene products involved have been identified. Very recently, a number of RAD51-like genes have been identified in mammals, either through cloning genes complementing cell lines sensitive to DNA-damaging agents (XRCC2, XRCC3), or through homology searches (RAD51L1, RAD51L2, RAD51L3). As yet the role of these genes and their possible functions are speculative, although the combination of sequence conservation and gene expression patterns suggest that they function in HRR pathways.

Meiosis: Avoiding inappropriate relationships

Meiosis is distinguished from mitosis by the way double-strand breaks are made and by the synapsis and segregation of homologous chromosomes. Recent studies with the yeast Saccharomyces cerevisiae have identified some of the key players that link homologous recombination to synaptonemal complex formation.

Atm deficiency results in severe meiotic disruption as early as leptonema of prophase I

Infertility is a common feature of the human disorder ataxia-telangiectasia and Atm-deficient mice are completely infertile. To gain further insight into the role of ATM in meiosis, we examined meiotic cells in Atm-deficient mice during development. Spermatocyte degeneration begins between postnatal days 8 and 16.5, soon after entry into prophase I of meiosis, while oocytes degenerate late in embryogenesis prior to dictyate arrest. Using electron microscopy and immunolocalization of meiotic proteins in mutant adult spermatocytes, we found that male and female gametogenesis is severely disrupted in Atm-deficient mice as early as leptonema of prophase I, resulting in apoptotic degeneration. A small number of mutant cells progress into later stages of meiosis, but no cells proceed beyond prophase I. ATR, a protein related to ATM, DMC1, a RAD51 family member, and RAD51 are mislocalized to chromatin and have reduced localization to developing synaptonemal complexes in spermatocytes from Atm-deficient mice, suggesting dysregulation of the orderly progression of meiotic events. ATM protein is normally present at high levels primarily in ova cytoplasm of developing ovarian follicles, and in the nucleus of spermatogonia and to a lesser extent in spermatoctyes, but without localization to the synaptonemal complex. We propose a model in which ATM acts to monitor meiosis by participation in the regulation or surveillance of meiotic progression, similar to its role as a monitor of mitotic cell cycle progression.