Caenorhabditis elegans dna-2 is involved in DNA repair and is essential for germ-line development

Continuous double-strand break induction and their differential processing sustain chiasma formation during Caenorhabditis elegans meiosis

Faithful chromosome segregation into gametes depends on Spo11-induced DNA double-strand breaks (DSBs). These yield single-stranded 3' tails upon resection to promote crossovers (COs). While early Mre11-dependent end resection is the predominant pathway in most organisms, Exo1 or Dna2/BLM can also contribute to the efficient processing of meiotic DSBs. Although its enzymatic activity has been thoroughly dissected, the temporal dynamics underlying Spo11 activity have remained mostly elusive. We show that, in Caenorhabditis elegans, SPO-11-mediated DSB induction takes place throughout early meiotic prophase I until mid-late pachynema. We find that late DSBs are essential for CO formation and are preferentially processed by EXO-1 and DNA-2 in a redundant fashion. Further, EXO-1-DNA-2-mediated resection ensures completion of conservative DSB repair and discourages activation of KU-dependent end joining. Taken together, our data unveil important temporal aspects of DSB induction and identify previously unknown functional implications for EXO-1-DNA-2-mediated resection activity in C. elegans.

VIG-1 is required for maintenance of genome stability in Caenorhabditis elegans

To explore the function of VIG-1 in Caenorhabditis elegans, we analyzed the phenotypes of two vig-1 deletion mutants: vig-1(tm3383) and vig-1(ok2536). Both vig-1 mutants exhibited phenotypes associated with genome instability, such as a high incidence of males (Him) and increased embryonic lethality. These phenotypes became more evident in succeeding generations, implying that the germline of vig-1 accumulates DNA damage over generations. To examine whether vig-1 causes a defect in the DNA damage response, we treated worms with UV or camptothecin, a specific topoisomerase I inhibitor. We observed that the embryonic survival of the vig-1 mutants was reduced compared with that of the wild-type worms. Our results thus suggest that VIG-1 is required for maintaining genome stability in response to endogenous and exogenous genotoxic stresses.

Differential subcellular localization of DNA topoisomerase-1 isoforms and their roles during Caenorhabditis elegans development

DNA topoisomerase-1 (TOP-1) resolves the topological problems associated with DNA replication, transcription and recombination by introducing temporary single-strand breaks in the DNA. Caenorhabditis elegans TOP-1 has two isoforms, TOP-1α and TOP-1β. TOP-1β is broadly localized to the nuclei of many cells at all developmental stages and concentrated in nucleoli in embryo gut and oogenic cells. However, TOP-1α is specifically localized to centrosomes, neuronal cells, excretory cells and chromosomes of germ cells in embryonic and larval stages. Reporter gene analysis also shows that top-1 transcription is highly activated in several sensory neurons, speculating the possible role of TOP-1α in neuronal development. From RNA interference (RNAi) experiments, we demonstrated that C. elegans TOP-1 is required for chromosomal segregation, germline proliferation and gonadal migration, which are all correlated with the expression and activity of TOP-1. Therefore, our findings may provide an insight into a new role of TOP-1 in development of multicellular organisms.

Caenorhabditis elegans DNA-2 helicase/endonuclease plays a vital role in maintaining genome stability, morphogenesis, and life span

In eukaryotes, highly conserved Dna2 helicase/endonuclease proteins are involved in DNA replication, DNA double-strand break repair, telomere regulation, and mitochondrial function. The Dna2 protein assists Fen1 (Flap structure-specific endonuclease 1) protein in the maturation of Okazaki fragments. In yeast, Dna2 is absolutely essential for viability, whereas Fen1 is not. In Caenorhabditis elegans, however, CRN-1 (a Fen1 homolog) is essential, but Dna2 is not. Here we explored the biological function of C. elegans Dna2 (Cedna-2) in multiple developmental processes. We find that Cedna-2 contributes to embryonic viability, the morphogenesis of both late-stage embryos and male sensory rays, and normal life span. Our results support a model whereby CeDNA-2 minimizes genetic defects and maintains genome integrity during cell division and DNA replication. These finding may provide insight into the role of Dna2 in other multi-cellular organisms, including humans, and could have important implications for development and treatment of human conditions linked to the accumulation of genetic defects, such as cancer or aging.

DNA double-strand break repair in Caenorhabditis elegans

Faithful repair of DNA double-strand breaks (DSBs) is vital for animal development, as inappropriate repair can cause gross chromosomal alterations that result in cellular dysfunction, ultimately leading to cancer, or cell death. Correct processing of DSBs is not only essential for maintaining genomic integrity, but is also required in developmental programs, such as gametogenesis, in which DSBs are deliberately generated. Accordingly, DSB repair deficiencies are associated with various developmental disorders including cancer predisposition and infertility. To avoid this threat, cells are equipped with an elaborate and evolutionarily well-conserved network of DSB repair pathways. In recent years, Caenorhabditis elegans has become a successful model system in which to study DSB repair, leading to important insights in this process during animal development. This review will discuss the major contributions and recent progress in the C. elegans field to elucidate the complex networks involved in DSB repair, the impact of which extends well beyond the nematode phylum.

Dicer-related drh-3 gene functions in germ-line development by maintenance of chromosomal integrity in Caenorhabditis elegans

In the course of systematic RNA interference (RNAi)-based screens with helicase-like genes in Caenorhabditis elegans, we have identified the drh-3(D2005.5) gene as a candidate gene for protection against X-ray irradiation. This gene encodes a novel RNA helicase-like protein that is similar to two nematode Dicer-related helicases (DRH). Here, we have showed the increased expression of drh-3 transcripts during maturation of larvae to adults, and characterized the phenotype of drh-3-interferred nematodes using feeding RNAi method. RNAi-mediated depletion of the drh-3 transcripts caused embryonic lethality of F1 progeny and temperature-sensitive reproductive capacity but did not affect the nematode life span. F1 progeny from drh-3(RNAi) animals exhibited increased lethality after X-ray irradiation or exposure to camptothecin. In drh-3(RNAi) worms, aggregated chromosomes were observed in diakinesis oocyte nuclei. In developing early zygotic embryos from drh-3(RNAi) worms, abnormally segregated chromosomes were observed and embryonic development was largely arrested at the mid-stages of embryogenesis. Finally, examination of checkpoint responses in mitotic germ cells with regards to replication arrest by hydroxyurea and X-ray-induced DNA damage suggested that both checkpoints function normally under these genotoxic stress conditions. Taken together, these results indicate that the drh-3 gene is essential for the development of germ-lines by maintaining chromosomal integrity in C. elegans.

URI-1 is required for DNA stability in C. elegans

Unconventional prefoldin RPB5 interactor (URI), an evolutionary conserved member of the prefoldin family of molecular chaperones, plays a central role in the regulation of nutrient-sensitive, TOR (target-of-rapamycin)-dependent gene expression programs in yeast. Mammalian URI has been shown to associate with key components of the transcriptional machinery, including RPB5, a shared subunit of all three RNA polymerases, the ATPases TIP48 and TIP49, which are present in various chromatin remodeling complexes, and human PAF1 and parafibromin, which are components of a transcription elongation complex. Here, we provide the first functional characterization of a URI-1 homolog in a multicellular organism and show that the C. elegans gene uri-1 is essential for germ cell proliferation. URI-1-deficient cells exhibit cell cycle arrest and display DNA breaks as evidenced by TUNEL staining and the appearance of HUS-1::GFP foci formation. In addition, uri-1(lf) mutants and uri-1(RNAi) worms show a p53-dependent increase in germline apoptosis. Our findings indicate that URI-1 has an important function in the mitotic and meiotic cell cycles. Furthermore, they imply that URI-1 participates in a pathway(s) that is associated with the suppression of endogenous genotoxic DNA damage and highlight a role for URI-1 in the control of genome integrity.

Identification of two apurinic/apyrimidinic endonucleases from Caenorhabditis elegans by cross-species complementation

The Saccharomyces cerevisiae mutant strain YW778, which lacks apurinic/apyrimidinic (AP) endonuclease and 3'-diesterase DNA repair activities, displays high levels of spontaneous mutations and hypersensitivities to several DNA damaging agents. We searched a cDNA library derived from the nematode Caenorhabditis elegans for gene products that would rescue the DNA repair defects of this yeast mutant. We isolated two genes, apn-1 and exo-3, encoding proteins that have not been previously characterized. Both APN-1 and EXO-3 share significant identity with the functionally established Escherichia coli AP endonucleases, endonuclease IV and exonuclease III, respectively. Strain YW778 expressing either apn-1 or exo-3 shows parental levels of spontaneous mutations, as well as resistance to DNA damaging agents that produce AP sites and DNA single strand breaks with blocked 3'-ends. Using an in vitro assay, we show that the apn-1 and exo-3 genes independently express AP endonuclease activity in the yeast mutant. We further characterize the EXO-3 protein and three of its mutated variants E68A, D190A, and H279A. The E68A variant retains both AP endonuclease and 3'-diesterase repair activities in vitro, yet severely lacks the ability to protect strain YW778 from spontaneous and drug-induced DNA lesions, suggesting that this variant E68A may possess a defect that interferes with the repair process in vivo. In contrast, D190A and H279A are completely devoid of DNA repair activities and fail to rescue the genetic instability of strain YW778. Our data strongly suggest that EXO-3 and APN-1 are enzymes possessing intrinsic AP endonuclease and 3'-diesterase activities.

A small molecular activator of cardiac hypertrophy uncovered in a chemical screen for modifiers of the calcineurin signaling pathway

The calcium, calmodulin-dependent phosphatase calcineurin, regulates growth and gene expression of striated muscles. The activity of calcineurin is modulated by a family of cofactors, referred to as modulatory calcineurin-interacting proteins (MCIPs). In the heart, the MCIP1 gene is activated by calcineurin and has been proposed to fulfill a negative feedback loop that restrains potentially pathological calcineurin signaling, which would otherwise lead to abnormal cardiac growth. In a high-throughput screen for small molecules capable of regulating MCIP1 expression in muscle cells, we identified a unique 4-aminopyridine derivative exhibiting an embedded partial structural motif of serotonin (5-hydroxytryptamine, 5-HT). This molecule, referred to as pyridine activator of myocyte hypertrophy, acts as a selective agonist for 5-HT(2A/2B) receptors and induces hypertrophy of cardiac muscle cells through a signaling pathway involving calcineurin and a kinase-dependent mechanism that inactivates class II histone deacetylases, which act as repressors of cardiac growth. These findings identify MCIP1 as a downstream target of 5-HT(2A/2B) receptor signaling in cardiac muscle cells and suggest possible uses for 5-HT(2A/2B) agonists and antagonists as modulators of cardiac growth and gene expression.