DNA synthesis at selective sites during pachytene in mouse spermatocytes

Key mediators of somatic ATR signaling localize to unpaired chromosomes in spermatocytes

Meiotic silencing of unpaired chromatin (MSUC) occurs during the first meiotic prophase, as chromosomes that fail to pair are sequestered into a transcriptionally repressive nuclear domain. This phenomenon is exemplified by the heterologous sex chromosomes of male mammals, where the ATR DNA damage response kinase is crucial for this silencing event. However, the mechanisms underlying the initiation of MSUC remain unknown. Here, we show that essential components of ATR signaling in murine somatic cells are spatially confined to unpaired chromosomes in spermatocytes, including the ATR-dependent phosphorylation of the single-stranded DNA (ssDNA)-binding complex replication protein A (RPA) and the checkpoint kinase CHK1. These observations support a model in which ssDNA plays a central role in the recruitment of ATR during MSUC, and provide a link to meiotic progression through activation of CHK1.

The evolutionary tangle of aging, sex, and reproduction and an experimental approach to its molecular dissection

Exchange of genetic materials by two individual members of the same species is considered to be the origin of primitive sex. During evolution, this primitive form of molecular sex has been transformed into a complex biological function involving specialized sexual structures and multiple hormonal interactions. Development and maintenance of these reproductive structures are also dependent on hormones and hormone receptors. Furthermore, reproductive specialization in higher forms of life has led to customized species-specific rates of aging and life-span potentials that are commensurate with the reproductive needs of the particular type of organism. Because of this reproductive imposition on aging of the organism, temporal regulation of the hormone response is a significant component of the genetics of aging. We have observed a marked age-dependent alteration in the hepatic expression of the rat androgen receptor (rAR) gene. Among the large number of transcription factors that control the rAR gene, at least three appear to participate in its age-dependent regulation. Two of these are positively acting and yet/to be characterized transcription factors, while the third is a negative regulator the nuclear factor kappa B (NF-kappa B). NF-kappa B is the major trans-regulator for genes involved in the immune response, inflammation, and oxidative stress. Involvement of NF-kB in the modulation of both oxidative stress and sex function provides the first example of a common molecular link between sex and aging.

Mammalian DNA ligase III: molecular cloning, chromosomal localization, and expression in spermatocytes undergoing meiotic recombination

Three biochemically distinct DNA ligase activities have been identified in mammalian cell extracts. We have recently purified DNA ligase II and DNA ligase III to near homogeneity from bovine liver and testis tissue, respectively. Amino acid sequencing studies indicated that these enzymes are encoded by the same gene. In the present study, human and murine cDNA clones encoding DNA ligase III were isolated with probes based on the peptide sequences. The human DNA ligase III cDNA encodes a polypeptide of 862 amino acids, whose sequence is more closely related to those of the DNA ligases encoded by poxviruses than to replicative DNA ligases, such as human DNA ligase I. In vitro transcription and translation of the cDNA produced a catalytically active DNA ligase similar in size and substrate specificity to the purified bovine enzyme. The DNA ligase III gene was localized to human chromosome 17, which eliminated this gene as a candidate for the cancer-prone disease Bloom syndrome that is associated with DNA joining abnormalities. DNA ligase III is ubiquitously expressed at low levels, except in the testes, in which the steady-state levels of DNA ligase III mRNA are at least 10-fold higher than those detected in other tissues and cells. Since DNA ligase I mRNA is also present at high levels in the testes, we examined the expression of the DNA ligase genes during spermatogenesis. DNA ligase I mRNA expression correlated with the contribution of proliferating spermatogonia cells to the testes, in agreement with the previously defined role of this enzyme in DNA replication. In contrast, elevated levels of DNA ligase III mRNA were observed in primary spermatocytes undergoing recombination prior to the first meiotic division. Therefore, we suggest that DNA ligase III seals DNA strand breaks that arise during the process of meiotic recombination in germ cells and as a consequence of DNA damage in somatic cells.

Culture of pachytene spermatocytes for analysis of meiosis

An impediment to the investigation of mammalian spermatogenic meiosis has been the lack of an appropriate system for experimental manipulation of meiotic prophase cells. We report here the use of a simple system for the short-term culture of pachytene spermatocytes. We have assayed parameters of cell function pertinent to meiotic prophase, namely chromosome pairing and synapsis. During the culture period of 24-48 hr, cells maintained typical pachytene morphology, chromatin condensation patterns, and chromosome pairing, as assessed by light and electron microscopy. Uridine incorporation, monitored by autoradiography, reflected the chromosomal distribution found in vivo in that the autosomal chromosomes were transcriptionally active, while the sex chromosomes were not. Thus features of chromosome pairing and sex chromatin inactivation are maintained in these cultures. We have conducted experiments to demonstrate that cultured pachytene spermatocytes can be useful for the analysis of agents, some of which may be suspected mutagens, that might affect chromosome structure and function during meiosis. Treatment of cells with actinomycin D revealed a differential effect on chromatin condensation in the autosomes versus the sex chromosomes. Camptothecin, a topoisomerase inhibitor, induced desynapsis of paired chromosomes. Okadaic acid, a phosphatase inhibitor, induced premature metaphase-I condensation of pachytene chromosomes. This last experiment suggests that these cultured cells may be useful for analysis of meiotic cell cycle controls. Taken together, these results demonstrate a culture system that can be useful for analysis of meiotic events as well as in screening for potential mutagenic agents that might affect meiotic chromosome structure and function.

Identification and sequence characterization of a 1.3 Kb EcoRI repeat fragment that harbors a DNA repair site of rat pachytene spermatocytes

Introduction of well-programmed nicks and gaps and the associated DNA repair activity in the genome at the pachytene interval is a characteristic feature of the meiotic prophase in organisms as varied as lilium and mouse. In the present study we have shown that the DNA synthetic activity in rat pachytene spermatocytes is insensitive to aphidicolin, a specific inhibitor of DNA polymerase alpha, delta and epsilon, suggesting DNA beta-polymerase-mediated repair synthesis in these cells. We have developed a novel approach for the isolation of the DNA repair sites by combining two independent techniques. Following incorporation of BrdUrd into pachytene spermatocytes in the presence of aphidicolin, the repair sites were released as ssDNA fragments by treatment of nuclei with 30 mM NaOH. Subsequently, the BrdUrd containing ssDNA fragments were specifically isolated using polyclonal anti-BrdUrd antibodies. The DNA fragments released were of two size classes, namely 4-7S (major) and 9-12S (minor) and constituted approximately 1.75% of the pachytene genomic DNA. These DNA repair fragments were distinct from Okazaki fragments and other replicative intermediates isolated from rat bone marrow cells as evidenced by (a) their different size distribution and (b) little cross-hybridization. Southern hybridization of restriction enzyme digests of rat genomic DNA with probes made against BrdUrd-ssDNA fragments revealed that although the repair sites were distributed throughout the genome, strong hybridization signals were observed in EcoRI. (1.3 kb and 2.4 kb), BamH1 (9 kb) and HindIII (5 kb) repetetive DNA fragments. The EcoRI 1.3 kb family were cloned into M13 mp19, and a repair positive (1.3 A) and a repair negative (1.3 B) were identified and sequenced. The repair positive clone contained (a) (CA)22 repeat, (b) a (CAGA)6 repeat and (c) 4 sequences sharing high homology with various hypervariable minisatellite (HVMS) sequences. One of the HVMS sequence contained a GGCAGG motif known to be responsible for germline instability. The repair negative clone had (a) (CA)6 repeat and (b) a HVMS like sequence without GGCAGG. The significance of these motifs and their relevance to the events of DNA metabolism at pachytene interval have been discussed.

cDNA cloning and functional characterization of a meiosis-specific protein (MNS1) with apparent nuclear association

It is well known that cytoskeleton and karyoskeleton proteins are associated with changes in cell shape and with the rearrangement of the dynamic structures involved in cell division and motility. In higher vertebrates, there are three major skeletal protein groups: microfilaments, microtubules and intermediate filaments, each representing a multigene family. Some of these skeletal proteins are expressed in a temporally- and spatially-specific fashion, and they establish cell-specific cytoplasmic and nucleoplasmic organization during development. Here we report the cDNA cloning of a novel 60 kDa skeletal protein from mouse spermatocytes, termed MNS 1 (meiosis-specific nuclear structural protein), whose computer-predicted protein configuration indicates long alpha-helical coiled-coil domains flanked by non-helical terminal domains. Functional characterization of MNS1 by ectopic expression in culture cells indicated that it is a detergent- and high salt-resistant skeletal protein which is involved in organization of the nuclear or perinuclear architecture. The MNS1 protein is specifically expressed at the pachytene stage during spermatogenesis, so that its function may involve the determination and maintenance of the appropriate nuclear morphology during meiotic prophase.

Mice with reduced levels of p53 protein exhibit the testicular giant-cell degenerative syndrome

Transgenic mice which carry hybrid p53 promoter-chloramphenicol acetyltransferase (CAT) transgenes were found to express CAT enzymatic activity predominantly in the testes. Endogenous levels of p53 mRNA and protein were lower than in the nontransgenic control mice. The various p53 promoter-CAT transgenic mice exhibited in their testes multinucleated giant cells, a degenerative syndrome resulting presumably from the inability of the tetraploid primary spermatocytes to complete meiotic division. The giant-cell degenerative syndrome was also observed in some genetic strains of homozygous p53 null mice. In view of the hypothesis that p53 plays a role in DNA repair mechanisms, it is tempting to speculate that the physiological function of p53 that is specifically expressed in the meiotic pachytene phase of spermatogenesis is to allow adequate time for the DNA reshuffling and repair events which occur at this phase to be properly completed. Primary spermatocytes which have reduced p53 levels are probably impaired with respect to DNA repair, thus leading to the development of genetically defective giant cells that do not mature.

Differential expression of poly(ADP-ribose) polymerase and DNA polymerase beta in rat tissues

The activities of two DNA repair-related enzymes, poly(ADP-ribose) polymerase and DNA polymerase beta, and their mRNA levels were measured in 17 tissues of Wistar rats. A large variety in enzyme activity values could be detected in the tissues examined; the highest levels of activity for both enzymes were found in the testis. A good correlation between poly(ADP-ribose) polymerase activity and the level of the transcript of the gene coding for the enzyme was observed in many tissues. A less satisfactory correlation could be evidenced for DNA polymerase beta. The almost parallel amounts of the mRNAs for poly(ADP-ribose) polymerase and DNA polymerase beta in the tissues examined suggest a possible coexpression of the genes coding for these enzymes. Additional studies have been carried out in testis and liver by immunohistochemical techniques and by in situ hybridization analyses. While in the testis the spermatocytes were shown to contain both enzymes and their transcripts, in other types of cells this could not be observed. In the liver mRNAs and enzymes were only found in 20% of the hepatocytes. This may in part explain both the low levels of the mRNAs and the modest activities of the two enzymes in that tissue.

Characterization of poly(ADP-ribosyl)ated domains of rat pachytene chromatin

Poly(ADP-ribosyl)ation of nuclear proteins was several-fold higher in the pachytene spermatocytes than in the premeiotic germ cells of the rat. Among the histones of the pachytene nucleus, histone subtypes H2A, H1 and H3 were poly(ADP-ribosyl)ated. Based on the immunoaffinity fractionation procedure of Malik, Miwa, Sugimara & Smulson [(1983) Proc. Natl. Acad. Sci. U.S.A. 80, 2554-2558] we have fractionated DNAase-II-solubilized chromatin into poly(ADP-ribosyl)ated chromatin (PAC) and non-poly(ADP-ribosyl)ated chromatin (non-PAC) domains on an anti-[poly(ADP-ribose)] IgG affinity matrix. Approx. 2.5% of the pachytene chromatin represented the PAC domains. A significant amount of [alpha-32P]dATP-labelled pachytene chromatin (labelled in vitro) was bound to the affinity matrix. The DNA of pachytene PAC domains had internal strand breaks, significant length of gaps and ligatable ends, namely 5'-phosphoryl and 3'-hydroxyl termini. On the other hand, the PAC domains from 18 h regenerating liver had very few gaps, if any. The presence of gaps in the pachytene PAC DNA was also evident from thermal denaturation studies. Although many of the polypeptides were common to the PAC domains of both pachytene and regenerating liver, the DNA sequences associated with these domains were quite different. A 20 kDa protein and the testis-specific histone H1t were selectively enriched in the pachytene PAC domains. The pachytene PAC domains also contained approx. 10% of the messenger coding sequences present in the DNAase-II-solubilized chromatin. The pachytene PAC domains, therefore, may represent highly enriched DNA-repair domains of the pachytene nucleus.

Difference in the expression level of DNA polymerase beta among mouse tissues: high expression in the pachytene spermatocyte

The expression level of DNA polymerase beta was determined in various mouse tissues. Northern blot hybridization analysis using rat cDNA as a probe revealed that the mRNA of about 1.5 kb for this enzyme is present in all kinds of tissues examined, but its content widely varies among tissues; the most abundant DNA polymerase beta mRNA was present in the testis, which was followed by brain, thymus, and spleen. The mRNA content was low in heart, kidney, and liver. In testis and brain, two minor species of transcripts of 3.3 and 6.2 kb were detected in addition to that of 1.5 kb. DNA polymerase beta activities in these tissues were closely correlated with the mRNA content, indicating that the expression of this enzyme is mainly regulated by the level of the mRNA. A survey of DNA polymerase beta mRNA levels in the testes at successive postnatal developmental stages and in isolated spermatogenic cells indicated that DNA polymerase beta mRNA was most abundant in spermatocytes at early pachytene. Since meiotic recombination occurs in this period, DNA polymerase beta may be involved in the repair-type DNA synthesis associated with the recombination process.

UV-induced DNA repair is not detectable in pre-dictyate oocytes of the mouse

Oocytes from fetal and neonatal mice were UV-irradiated, cultured in medium containing tritiated thymidine and analysed following autoradiography. As previously reported irradiated dictyate cells clearly displayed an increased grain count. However we detected no UV-induced response in oocytes at the leptotene, zygotene or pachytene stages of meiosis. This contrasts with the situation in spermatogenesis where high levels of repair can be induced in equivalent early prophase stages.