Structural and functional properties of CiNTH, an endonuclease III homologue of the ascidian Ciona intestinalis: critical role of N-terminal region

CiApex1 has AP endonuclease activity and abrogated AP site repair disrupts early embryonic development in Ciona intestinalis

Apurinic/apyrimidinic (AP) sites are the most common form of cytotoxic DNA damage. Since AP sites inhibit DNA replication and transcription, repairing them is critical for cell growth. However, the significance of repairing AP sites during early embryonic development has not yet been clearly determined. Here, we focused on APEX1 from the ascidian Ciona intestinalis (CiApex1), a homolog of human AP endonuclease 1 (APEX1), and examined its role in early embryonic development. Recombinant CiApex1 protein complemented the drug sensitivities of an AP endonuclease-deficient Escherichia coli mutant, and exhibited Mg²⁺-dependent AP endonuclease activity, like human APEX1, in vitro. Next, the effects of abnormal AP site repair on embryonic development were investigated. Treatment with methyl methanesulfonate, which alkylates DNA bases and generates AP sites, induced abnormal embryonic development. This abnormal phenotype was also caused by treatment with methoxyamine, which inhibits AP endonuclease activity. Furthermore, we constructed dominant-negative CiApex1, which inhibits CiApex1 action, and found that its expression impaired embryonic growth. These results suggested that AP site repair is essential for embryonic development and CiApex1 plays an important role in AP site repair during early embryonic development in C. intestinalis.

CiAPEX2 and CiP0, candidates of AP endonucleases in Ciona intestinalis, have 3'-5' exonuclease activity and contribute to protection against oxidative stress

Apurinic/apyrimidinic (AP) sites are one of the most frequent DNA lesions. AP sites inhibit transcription and DNA replication, and induce cell death. AP endonucleases are key enzymes in AP site repair. Several types of AP endonucleases have been reported, such as AP endonuclease 2 (APEX2) and ribosomal protein P0 (P0). However, it is not known how the functions and roles differ among AP endonucleases. To clarify the difference of roles among AP endonucleases, we conducted biochemical analysis focused on APEX2 and P0 homologues in Ciona intestinalis. Amino acid sequence analysis suggested that CiAPEX2 and CiP0 are AP endonuclease homologues. Although we could not detect AP endonuclease or 3'-phosphodiesterase activity, these two purified proteins exhibited 3'-5' exonuclease activity. This 3'-5' exonuclease activity was sensitive to ethylenediaminetetraacetic acid (EDTA), and the efficiency of this activity was influenced by the 3'-terminus of substrate DNA. Both CiAPEX2 and CiP0 degraded not only a 5'-protruding DNA end, but also nicked DNA, which is generated through AP endonuclease 1 (APEX1) cleavage. These two genes partially complemented the growth rate of AP endonuclease-deficient Escherichia coli treated with hydrogen peroxide. These results indicate that 3'-5' exonuclease activity is an evolutionarily conserved enzymatic activity of APEX2 and P0 homologues and this enzymatic activity may be important for AP endonucleases.

A novel DNA damage response mediated by DNA mismatch repair in Caenorhabditis elegans: induction of programmed autophagic cell death in non-dividing cells

DNA mismatch repair (MMR) contributes to genome integrity by correcting errors of DNA polymerase and inducing cell death in response to DNA damage. Dysfunction of MMR results in increased mutation frequency and cancer risk. Clinical researches revealed that MMR abnormalities induce cancers of non-dividing tissues, such as kidney and liver. However, how MMR suppresses cancer in non-dividing tissues is not understood.

To address that mechanism, we analyzed the roles of MMR in non-dividing cells using Caenorhabditis elegans (C. elegans), in which all somatic cells are non-dividing in the adult stage. In this study, we used stable MMR-mutant lines with a balancer chromosome. First, we confirmed that deficiency of MMR leads to resistance to various mutagens in C. elegans dividing cells. Next, we performed drug resistance assays, and found that MMR-deficient adult worms were resistant to SN1-type alkylating and oxidizing agents. In addition, dead cell staining and reporter assays of an autophagy-related gene demonstrated that the cell death was autophagic cell death. Interestingly, this autophagic cell death was not suppressed by caffeine, implying that MMR induces death of non-dividing cells in an atl-1-independent manner. Hence, we propose the hypothesis that MMR prevents cancers in non-dividing tissues by directly inducing cell death.

Low-intensity red and infrared laser effects at high fluences on Escherichia coli cultures

Semiconductor laser devices are readily available and practical radiation sources providing wavelength tenability and high monochromaticity. Low-intensity red and near-infrared lasers are considered safe for use in clinical applications. However, adverse effects can occur via free radical generation, and the biological effects of these lasers from unusually high fluences or high doses have not yet been evaluated. Here, we evaluated the survival, filamentation induction and morphology of Escherichia coli cells deficient in repair of oxidative DNA lesions when exposed to low-intensity red and infrared lasers at unusually high fluences. Cultures of wild-type (AB1157), endonuclease III-deficient (JW1625-1), and endonuclease IV-deficient (JW2146-1) E. coli, in exponential and stationary growth phases, were exposed to red and infrared lasers (0, 250, 500, and 1000 J/cm2) to evaluate their survival rates, filamentation phenotype induction and cell morphologies. The results showed that low-intensity red and infrared lasers at high fluences are lethal, induce a filamentation phenotype, and alter the morphology of the E. coli cells. Low-intensity red and infrared lasers have potential to induce adverse effects on cells, whether used at unusually high fluences, or at high doses. Hence, there is a need to reinforce the importance of accurate dosimetry in therapeutic protocols.