DNA-dependent DNA polymerase species in male germ cells of the mouse

Mouse models to explore the biological and organismic role of DNA polymerase beta

Gene knock-out (KO) mouse models for DNA polymerase beta (Polβ) revealed that loss of Polβ leads to neonatal lethality, highlighting the critical organismic role for this DNA polymerase. While biochemical analysis and gene KO cell lines have confirmed its biochemical role in base excision repair and in TET-mediated demethylation, more long-lived mouse models continue to be developed to further define its organismic role. The Polb-KO mouse was the first of the Cre-mediated tissue-specific KO mouse models. This technology was exploited to investigate roles for Polβ in V(D)J recombination (variable-diversity-joining rearrangement), DNA demethylation, gene complementation, SPO11-induced DNA double-strand break repair, germ cell genome stability, as well as neuronal differentiation, susceptibility to genotoxin-induced DNA damage, and cancer onset. The revolution in knock-in (KI) mouse models was made possible by CRISPR/cas9-mediated gene editing directly in C57BL/6 zygotes. This technology has helped identify phenotypes associated with germline or somatic mutants of Polβ. Such KI mouse models have helped uncover the importance of key Polβ active site residues or specific Polβ enzyme activities, such as the PolbY265C mouse that develops lupus symptoms. More recently, we have used this KI technology to mutate the Polb gene with two codon changes, yielding the PolbL301R/V303R mouse. In this KI mouse model, the expressed Polβ protein cannot bind to its obligate heterodimer partner, Xrcc1. Although the expressed mutant Polβ protein is proteolytically unstable and defective in recruitment to sites of DNA damage, the homozygous PolbL301R/V303R mouse is viable and fertile, yet small in stature. We expect that this and additional targeted mouse models under development are poised to reveal new biological and organismic roles for Polβ.

Isolation and analyses of enriched populations of male mouse germ cells by sedimentation velocity: the centrifugal elutriation

The studies of molecular events that occur in single cell types within a tissue often require the disaggregation of the tissue into a single cell suspension, followed by isolation of distinct cell populations. The germinal epithelium of mammals is composed of several cell types, which divide mitotically, before entering meiosis. In this chapter, we describe the isolation of five mouse germ-cell fractions by centrifugal elutriation, and characterize them by their DNA content (flow cytometry), cell morphology (DAPI staining of nuclei, Giemsa staining of squashed cells) and deposition of stage-specific meiotic markers (SYCP3, H1t, SAM68) on chromosome spreads and whole cells. Within 2 h it is possible to obtain enriched populations of elongated spermatids (up to approximately 50% of the fraction), round spermatids (up to approximately 80%), primary spermatocytes (up to approximately 89%), and secondary spermatocytes (up to approximately 17%). Furthermore, most of the collected spermatocytes of the primary spermatocyte fraction are in early-mid pachytene stage as judged by chromosome spreads, enriched up to approximately 89%. Elutriation and techniques used for characterization of germ cell fractions are described.

Down-regulation of DNA polymerase beta accompanies somatic hypermutation in human BL2 cell lines

Somatic hypermutation (SHM) is a fundamental process in immunoglobulin gene maturation that results in increased affinity of antibodies toward antigens. In one hypothesis explaining SHM in human B cells, the process is initiated by enzymatic deamination of cytosine to uracil in the immunoglobulin gene V-region and this in turn triggers mutation-prone forms of uracil-DNA base excision repair (BER). Yet, an uncertainty with this model is that BER of uracil-DNA in mammalian cells is generally error-free, wherein DNA polymerase beta (pol beta) conducts gap-filling synthesis by insertion of bases according to Watson-Crick rules. To evaluate this inconsistency, we examined pol beta expression in various SHM proficient human BL2 cell line subclones. We report that expression of pol beta in SHM proficient cell lines was strongly down-regulated. In contrast, in other BL2 subclones, we found that SHM was deficient and that pol beta expression was much higher than in the SHM proficient subclones. We also found that overexpression of recombinant human pol beta in a SHM proficient subclone abrogated its capacity for SHM. These results suggest that down-regulation of the normal BER gap-filling DNA polymerase, pol beta, accompanies induced SHM in BL2 cells. This is consistent with the hypothesis that normal error-free BER must be silenced to make way for an error-prone BER process that may be required during somatic hypermutation.

cDNA cloning of a stage-specific gene expressed during HCG-induced spermatogenesis in the Japanese eel

A single injection of human chorionic gonadotropin (HCG) can induce complete spermatogenesis in immature Japanese eel (Anguilla japonica) testes consisting of only premitotic spermatogonia. Proliferation of spermatogonia, meiosis and spermiogenesis begin on 3, 12 and 18 days after HCG injection, respectively. To isolate the genes responsible for regulating the initiation of meiosis, differential mRNA display using poly (A)+ RNA extracted from testes of eels at different times after HCG treatment was carried out. Five cDNA clones in which expression was initiated before the onset of meiosis were obtained. Northern blot analysis showed that one clone, which encoded activin betaB subunit, was expressed in the initial phase of spermatogenesis (1-6 days after HCG treatment), in agreement with the previous suggestion that activin B induces the initiation of spermatogenesis in the Japanese eel. The remaining four were expressed in the testes during the following time frames: 3-18 days (two clones), 6-18 days (one clone) and 9-18 days (one clone) after HCG treatment. One of the two clones expressed on day 3 exhibited strong expression on days 12 and 15, just at the initiation period of meiosis. This clone was selected as a candidate gene responsible for initiating meiosis, and its full-length cDNA isolated. The cDNA contained an open reading frame of 1571 nucleotides encoding a protein of 260 amino acid residues, which showed high homology with the proliferating cell nuclear antigen (PCNA) of human, mouse and Xenopus. Northern blot analysis using eel PCNA cDNA showed that a 1.6 kb transcript first appeared on day 3 and became abundant, reaching maximum levels on days 12-15. In situ hybridization analysis revealed that PCNA mRNA was expressed strongly in late type B spermatogonia before the sixth mitotic division. It has already been shown that spermatogonia have a regulatory point to enter meiosis between the fifth and sixth mitotic division. The coincidence of PCNA expression and this regulatory point suggests an involvement of PCNA in the progression of mitotic germ cells into meiosis during HCG-induced spermatogenesis in the eel.

Potential role of DNA polymerase beta in gene therapy against cancer: a case for colorectal cancer

Genetic instability characterized by the accumulation of mutations of tumor suppressor genes and oncogenes appears to be associated with carcinogenesis in colorectal and other cancers. Mutations of DNA polymerase beta (pol beta) and related chromosomal alterations appear to be consistent with the causal role of a "mutator phenotype' in carcinogenesis. However, homozygous knockout pol beta mutations appear to interfere with embryogenesis. Increased pol beta activity (i.e. relative to pol alpha activity) has been associated with cell cycle arrest. The related aphidicolin-resistant DNA replication has been observed primarily in differentiating cells, including the mammalian blastocyst, adrenal cortex, thyroid, anterior pituitary, and the mechanism of endoreduplication (amitotic over-replication of DNA) can be traced to lower eukaryotes. This increased activity in relation to terminal commitment is inconsistent with a simple "DNA repair' view of pol beta. It is therefore proposed that pol beta may play a more fundamental role in cellular differentiation through involvement in a putative subgenomic DNA replication-based model of terminal gene expression. Thus genetic instability, loss of differentiation, and carcinogenesis may result from aberration(s) or "derailment' of such replication-based mechanism of terminal gene expression. It is suggested to examine the relationship of DNA pol beta to genomic instability and carcinogenesis using genetic analyses and antisense technology with possible applications for gene therapy against colorectal cancer.

Changes in the DNA polymerase beta gene expression during development of lung, brain, and testis suggest an involvement of the enzyme in DNA recombination

Changes in the expression pattern of DNA polymerase beta gene during rat lung, brain, and testis development have been investigated. A decrease in the level of beta-pol mRNA was observed during postnatal development of lung and brain. By contrast, an almost 20-fold increase in the level of beta-pol mRNA was observed during spermatogenesis. For most adult rat tissues the abundance of beta-pol mRNA was low compared with that of beta-actin mRNA. Northern blot analysis revealed four distinct transcripts hybridizing to beta-pol probes. At least two of them, 1.4 kb and 4.0 kb, were products of a beta-polymerase gene. The changes in the expression pattern during lung and brain development, and during spermatogenesis, suggest involvement of DNA polymerase beta in gap-filling DNA synthesis during recombination.

Levels and size complexity of DNA polymerase beta mRNA in rat regenerating liver and other organs

A cDNA probe encoding DNA polymerase beta (beta-pol) was used to study the level and size complexity of beta-pol mRNA in regenerating rat liver and other rat tissues. An almost 2-fold increase in beta-pol mRNA was observed 18-24 h after partial hepatectomy. In most adult rat tissues (liver, heart, kidney, stomach, spleen, thymus, lung and brain) the abundance of beta-pol mRNA was low. In contrast, young brain and testes exhibited beta-pol mRNA levels 5- and 15-times higher, respectively. The observed changes in the level of beta-pol mRNA in regenerating rat liver and in developing brain are correlated with reported changes in DNA polymerase beta activity. Four different (4.0, 2.5, 2.2, 1.4 kb) transcripts hybridizing to beta-pol probe were found in all tissues examined. The 4.0 kb transcript was dominant for young and adult brain, whereas the 1.4 kb transcript was dominant for testes. The significance of these transcripts is discussed.

Replicating premeiotic germ cells of the mouse contain a novel DNA primase stimulatory factor

A protein factor that stimulates DNA primase activity associated with DNA polymerase alpha has been identified in mouse germ cell populations enriched in spermatogonia and preleptotene spermatocytes. The partially purified factor enhances DNA primase activity from homologous cell types as well as DNA primase activity from Xenopus laevis oocytes in a poly dT or M 13 directed reaction. The factor does not stimulate DNA polymerase alpha activity in a gapped salmon sperm or poly dT-rA directed reaction. The DNA primase stimulating factor is identified in a male mouse germ cell population enriched in premeiotic cells; it is not detectable in middle-late pachytene spermatocytes, spermatids, Sertoli cells or fibroblasts.

Biochemical changes in testicular varicocele

The precise mechanism of the hypospermatogenesis associated with varicocele has remained uncertain, although there have been a number of speculations on the etiology of the associated infertility. The altered spermatogenesis has been attributed to the reflux of toxic metabolites from either adrenal or renal origin, disturbed hormone status, spermatic venous hypertension, testicular hypoxia secondary to stasis, and abnormal temperature regulation. However, the biochemical changes of the testicular tissue with varicocele have been only partially explored. This overview includes the available information on the biochemical change in the testes associated with varicocele as well as the introduction of basic biochemical aspects on the testes, which may give new insights into the possible pathophysiological mechanism of male infertility.

Deoxyribonucleic acid polymerase activity in the testes of infertile men with varicocele

Activities of deoxyribonucleic acid (DNA) polymerase alpha, beta, and gamma were measured in extracts of testicular biopsy specimen obtained from 37 cases of male infertility with left varicocele and compared with those of 6 normal controls. It was observed that levels of DNA polymerase alpha, beta, and gamma were significantly lower in infertile men than normal controls on both sides of testes. Among three DNA polymerases, the level of DNA polymerase beta activity well correlated with the histological findings (Johnsen's score), i.e., the extent of differentiation of germinal cells. DNA polymerase beta activity appeared to be the lowest in the patients whose sperm density was less than 5 X 10(6)/ml. On the other hand, no correlation was apparent between levels of DNA polymerases and other clinical parameters, e.g., testicular volume, sperm motility, grade of varicocele, and serum hormone levels. These results suggest that the combined decrease in the DNA polymerase activities may be one of the factors that have deleterious effects on spermatogenesis in varicocele patients.

DNA repair synthesis in mouse spermatogenesis involves DNA polymerase beta activity

The role of DNA polymerase alpha-DNA primase complex and DNA polymerase beta in DNA replication and ultraviolet-induced DNA repair synthesis has been analyzed in mouse spermatogenesis. Autoradiographic experiments with germ cells in culture, indicating an involvement of DNA polymerase alpha and/or delta in DNA replication, and of DNA polymerase beta in DNA repair synthesis, have been confirmed by studying partially purified enzymes. These findings support the idea that, different from other biological systems, in meiotic and post meiotic male mouse germ cells DNA polymerase beta is the main DNA polymerase form needed for DNA repair.

DNA repair synthesis-related enzymes during spermatogenesis in the mouse

To assess whether uracil DNA glycosylase and dUTP nucleotidohydrolase (dUTPase) can be involved in repair-type DNA synthesis associated to crossing-over or induced by UV and X-ray treatments, we have studied these enzyme activities in male mouse germ cells at specific stages of differentiation. Although the highest uracil DNA glycosylase activity was observed in dividing germ cells (spermatogonia and preleptotene spermatocytes), some activity was also detected in meiotic (3.5%) and post-meiotic (1.0%) cells with a relative maximum of activity at pachytene stage (4.7%) when meiotic crossing-over takes place. These findings suggest that uracil DNA glycosylase is involved, in this biological system, in DNA replication and in repair-type DNA synthesis. dUTPase is present at all the stages of spermatogenesis studied but, unlike thymidylate synthetase which is mainly associated with replicating germ cells, dUTPase activity is maximal in spermatocytes at pachytene stages. The data reported suggest that, in this biological system, the main role of dUTPase is to degrade dUTP to prevent misincorporation of uracil into DNA during crossing-over, rather than to participate in the biosynthetic pathway of dTTP.

Changes in DNA polymerases alpha, beta and gamma in mouse liver as a function of age

The activities of DNA polymerases alpha, beta and gamma were determined in mouse liver as a function of age by a combination of glycerol density gradient centrifugation with polymerase specific assays. Although alpha polymerase was preserved throughout the life span, the activity dropped sharply from a high level at the fetal and neonatal stages to a level one order lower after maturation through adjustment of the amount of protein administered. beta polymerase showed similar but less drastic changes than alpha. DNA polymerase gamma activity increased about two-fold in going from newborn to adult stages and remained constant after maturation. According to the amount of DNA, DNA polymerase alpha decreased after birth, but the change was less drastic compared to that through adjustment of the amount of protein. DNA polymerase beta increased the activity 2-3-fold within a period of 3 months following birth. gamma polymerase underwent more than a 10-fold increase in activity through adjustment of the amount of DNA within the same period.

Haploid accumulation and translational control of phosphoglycerate kinase-2 messenger RNA during mouse spermatogenesis

The intracellular location of the mRNA for the testis-specific isozyme of phosphoglycerate kinase-2 (PGK-2) has been determined for two spermatogenic cell types. The mRNA activity for PGK-2 from the polysomal and nonpolysomal fractions of pachytene primary spermatocytes or round spermatids has been assayed by cell-free translation with the polypeptide products monitored by immunoprecipitation, followed by one-dimensional or two-dimensional electrophoresis and fluorography. The results reveal that the majority of PGK-2 mRNA activity of round spermatids was present in the polysomal fraction while the relatively less abundant PGK-2 mRNA of pachytene primary spermatocytes was present in the nonpolysomal fraction. No PGK-2 mRNA activity was observed in the cytoplasmic RNA from primitive type A spermatogonia or prepubertal Sertoli cells. These data indicate that mature PGK-2 mRNA first appears in the cytoplasm of spermatogenic cells during the prophase of meiosis and increases in amount after meiosis. Although mature PGK-2 mRNA is present in meiotic cells it is not actively translated until after meiosis has been completed. Thus, mRNA accumulation and translational mechanisms are involved in the control of phosphoglycerate kinase-2 synthesis during spermatogenesis.

Identification of living spermatogenic cells of the mouse by transillumination-phase contrast microscopic technique for 'in situ' analyses of DNA polymerase activities

The stages of spermatogenesis can be identified in freshly isolated, unstained adult mouse seminiferous tubules using a transillumination method. Late acrosome- and maturation phase spermatids, arranged in bundles at stages XII-VI give rise to a spotty transillumination pattern. Before spermiation, these cells form a continuous layer on the top of the seminiferous epithelium, recognized by a strong homogeneous central light absorption in the freshly isolated seminiferous tubules at stages VII and VIII. Other stages have a pale light absorption pattern. The accurate determination of the developmental stages of the germ cells was based on the morphology of the developing acrosomic system and of the nuclei of the spermatids, as revealed by phase contrast microscopy. Using this procedure, the activity levels of DNA polymerases alpha and beta have been studied by autoradiography of squash preparations. Using endogenous templates, assay conditions that differentiate between the solubilized DNA polymerases alpha and beta in vitro, were used to distinguish between these activities in situ in different stages of mouse spermatogenesis. Except in very late spermatids shortly before spermiation, DNA polymerases alpha and beta were detectable in all cell types examined. Coinciding with the nuclear protein transitions, elongating spermatids at steps 10-12 and maturation phase spermatids at steps 13-14 showed high DNA polymerase activities. As no replication occurs in these cells, the observations support the view that both DNA polymerases alpha and beta could be involved in repair DNA synthesis.

Study of dogfish (Scyliorhinus caniculus) deoxyribonucleic acid polymerase alpha and beta. Extraction, separation, characterization and changes during spermatogenesis

DNA polymerase activity was extracted from testis cells of the dogfish Scyliorhinus caniculus. On a sucrose gradient, two main peaks could be separated, corresponding to DNA polymerases beta (3.8 S) and alpha (7.5 S). DNA polymerase gamma could also be detected when poly(A) . (dT)12 was used as template. The properties of alpha and beta polymerases of this primitive vertebrate were similar to those generally described, especially in mammals. The beta enzyme was highly sensitive to N-ethylmaleimide, however, and could use poly(dT) . poly(A) as template. Polymerase alpha was present in spermatogonia, spermatocytes and spermatids. Activity was maximal in spermatocytes. DNA polymerase beta was present in all testis cells with similar activities in spermatogonia and spermatocytes. Decreased activities were observed during spermiogenesis. Some activity remained associated with the chromatin fraction of mature sperm cells.

Electrophoretic pattern of polypeptide synthesis in spermatocytes and spermatids of the mouse

The pattern of protein synthesis in different stages of spermatogenesis was examined using two-dimensional polyacrylamide gel electrophoresis followed fluorography. The [3H]leucine labelling was performed by incubating in culture either seminiferous tubules before cell separation or isolated germ cells after fractionation by velocity sedimentation at unit gravity in an albumin gradient. The patterns of soluble polypeptides synthesised in middle-late pachytene spermatocytes, round spermatides (steps 1--8 of spermiogenesis) and intermediate spermatids (steps 9--13) have been compared with each other. Approximately 250 fluorographic spots were detected in middle-late pachytene spermatocytes and round spermatids, but only 100 in intermediate spermatids. From the analysis of fluorograms in the three cell stages examined, 5 categories of labelled polypeptides can be identified: (1) those specific of each cell stage; (2) those present in pachytene spermatocytes and early spermatids but absent in intermediate spermatids; (3) polypeptides labelled during spermiogenesis but unlabelled in meiotic cells; (4) polypeptides showing quantitative differences among the three cell types; and finally (5) polypeptides common to the three cell stages. These results are discussed in relation to differential gene expression during spermatogenesis.