Risk factors for cefmetazole-non-susceptible bacteremia in acute cholangitis

INTRODUCTION

Cefmetazole (CMZ), an antibiotic with limited international distribution, is recommended by the Tokyo Guidelines 2018 (TG18) for non-severe cases of acute cholangitis (AC). However, the risk factors for CMZ-non-susceptible (CMZ-NS) bacteremia in AC remain unclear. Here, we aimed to investigate the risk factors for CMZ-NS bacteremia and evaluate mortality in patients with AC.

METHODS

This single-center, retrospective, observational study included all patients diagnosed with definite bacteremic AC, based on TG18, from April 2019 to March 2023. Risk factors for CMZ-NS bacteremia were analyzed by univariate, and age- and sex-adjusted, logistic regression analyses. Mortality was compared by cause of obstruction, CMZ-susceptible/CMZ-NS bacteremia, and initial treatment.

RESULTS

In total, 165 patients were enrolled. CMZ-NS bacteremia was diagnosed in 46 (27.9 %) patients. Histories of diabetes mellitus, hepato-biliary-pancreatic cancer, malignant biliary obstruction, and endoscopic sphincterotomy were identified as significant factors associated with the risk of CMZ-NS bacteremia. Thirteen patients died within 30 days of hospital admission. The mortality of patients with AC and malignant biliary obstruction was statistically higher than that of patients with bile duct stones. No patients with AC and bile duct stones died in the group with CMZ-NS bacteremia and inappropriate initial antibiotics.

CONCLUSIONS

In AC, a history of diabetes mellitus, hepato-biliary-pancreatic cancer, malignant biliary obstruction, and endoscopic sphincterotomy are associated with an increased risk of CMZ-NS bacteremia. Therefore, the choice of empiric therapy for AC should be based on the etiology and patient background, rather than on the severity.

APE2 Promotes AID-Dependent Somatic Hypermutation in Primary B Cell Cultures That Is Suppressed by APE1

Somatic hypermutation (SHM) is necessary for Ab diversification and involves error-prone DNA repair of activation-induced cytidine deaminase-induced lesions in germinal center (GC) B cells but can also cause genomic instability. GC B cells express low levels of the DNA repair protein apurinic/apyrimidinic (AP) endonuclease (APE)1 and high levels of its homolog APE2. Reduced SHM in APE2-deficient mice suggests that APE2 promotes SHM, but these GC B cells also exhibit reduced proliferation that could impact mutation frequency. In this study, we test the hypothesis that APE2 promotes and APE1 suppresses SHM. We show how APE1/APE2 expression changes in primary murine spleen B cells during activation, impacting both SHM and class-switch recombination (CSR). High levels of both APE1 and APE2 early after activation promote CSR. However, after 2 d, APE1 levels decrease steadily with each cell division, even with repeated stimulation, whereas APE2 levels increase with each stimulation. When GC-level APE1/APE2 expression was engineered by reducing APE1 genetically (apex1+/-) and overexpressing APE2, bona fide activation-induced cytidine deaminase-dependent VDJH4 intron SHM became detectable in primary B cell cultures. The C terminus of APE2 that interacts with proliferating cell nuclear Ag promotes SHM and CSR, although its ATR-Chk1-interacting Zf-GRF domain is not required. However, APE2 does not increase mutations unless APE1 is reduced. Although APE1 promotes CSR, it suppresses SHM, suggesting that downregulation of APE1 in the GC is required for SHM. Genome-wide expression data compare GC and cultured B cells and new models depict how APE1 and APE2 expression and protein interactions change during B cell activation and affect the balance between accurate and error-prone repair during CSR and SHM.

APE2 Is a General Regulator of the ATR-Chk1 DNA Damage Response Pathway to Maintain Genome Integrity in Pancreatic Cancer Cells

The maintenance of genome integrity and fidelity is vital for the proper function and survival of all organisms. Recent studies have revealed that APE2 is required to activate an ATR-Chk1 DNA damage response (DDR) pathway in response to oxidative stress and a defined DNA single-strand break (SSB) in Xenopus laevis egg extracts. However, it remains unclear whether APE2 is a general regulator of the DDR pathway in mammalian cells. Here, we provide evidence using human pancreatic cancer cells that APE2 is essential for ATR DDR pathway activation in response to different stressful conditions including oxidative stress, DNA replication stress, and DNA double-strand breaks. Fluorescence microscopy analysis shows that APE2-knockdown (KD) leads to enhanced γH2AX foci and increased micronuclei formation. In addition, we identified a small molecule compound Celastrol as an APE2 inhibitor that specifically compromises the binding of APE2 but not RPA to ssDNA and 3′-5′ exonuclease activity of APE2 but not APE1. The impairment of ATR-Chk1 DDR pathway by Celastrol in Xenopus egg extracts and human pancreatic cancer cells highlights the physiological significance of Celastrol in the regulation of APE2 functionalities in genome integrity. Notably, cell viability assays demonstrate that APE2-KD or Celastrol sensitizes pancreatic cancer cells to chemotherapy drugs. Overall, we propose APE2 as a general regulator for the DDR pathway in genome integrity maintenance.

Serum Anti-oligodendrocyte Autoantibodies in Patients With Multiple Sclerosis Detected by a Tissue-Based Immunofluorescence Assay

Multiple sclerosis (MS), the most prevalent inflammatory disease of the central nervous system (CNS), is characterized by damaged to myelin sheaths and oligodendrocytes. Because MS patients have variable clinical courses and disease severities, it is important to identify biomarkers that predict disease activity and severity. In this study, we assessed the frequencies of serum autoantibodies against mature oligodendrocytes in MS patients using a tissue-based immunofluorescence assay (IFA) to determine whether anti-oligodendrocyte antibodies are associated with the clinical features of MS patients and whether they might be a biomarker to assess CNS tissue damage in MS patients. We assessed the binding of serum autoantibodies to mouse oligodendrocytes expressing Nogo-A, a reliable mature oligodendrocyte marker, by IFA with mouse brain and sera from 147 MS patients, comprising 103 relapsing–remitting MS (RRMS), 22 secondary progressive MS (SPMS), and 22 primary progressive MS (PPMS) patients, 38 neuromyelitis optica spectrum disorder (NMOSD) patients, 23 other inflammatory neurological disorder (OIND) patients, and 39 healthy controls (HCs). Western blotting (WB) was performed using extracted mouse cerebellum proteins and IgG from anti-oligodendrocyte antibody-positive MS patients. Tissue-based IFA showed that anti-oligodendrocyte antibodies were positive in 3/22 (13.6%) PPMS and 1/22 (4.5%) SPMS patients but not in RRMS, NMOSD, and OIND patients or HCs. WB demonstrated the target CNS proteins recognized by serum anti-oligodendrocyte antibodies were approximately 110 kDa and/or 150 kDa. Compared with anti-oligodendrocyte antibody-negative MS patients, MS patients with anti-oligodendrocyte antibodies were significantly older at the time of serum sampling, scored significantly higher on the Expanded Disability Status Scale and the Multiple Sclerosis Severity Score, and had a higher frequency of mental disturbance. Although the clinical significance of anti-oligodendrocyte antibodies is still unclear because of their low frequency, anti-oligodendrocyte autoantibodies are potential biomarkers for monitoring the disease pathology and progression in MS.

MTH1 and OGG1 maintain a low level of 8-oxoguanine in Alzheimer's brain, and prevent the progression of Alzheimer's pathogenesis

8-Oxoguanine (8-oxoG), a major oxidative base lesion, is highly accumulated in Alzheimer’s disease (AD) brains during the pathogenic process. MTH1 hydrolyzes 8-oxo-dGTP to 8-oxo-dGMP, thereby avoiding 8-oxo-dG incorporation into DNA. 8-OxoG DNA glycosylase-1 (OGG1) excises 8-oxoG paired with cytosine in DNA, thereby minimizing 8-oxoG accumulation in DNA. Levels of MTH1 and OGG1 are significantly reduced in the brains of sporadic AD cases. To understand how 8-oxoG accumulation in the genome is involved in AD pathogenesis, we established an AD mouse model with knockout of Mth1 and Ogg1 genes in a 3xTg-AD background. MTH1 and OGG1 deficiency increased 8-oxoG accumulation in nuclear and, to a lesser extent, mitochondrial genomes, causing microglial activation and neuronal loss with impaired cognitive function at 4–5 months of age. Furthermore, minocycline, which inhibits microglial activation and reduces neuroinflammation, markedly decreased the nuclear accumulation of 8-oxoG in microglia, and inhibited microgliosis and neuronal loss. Gene expression profiling revealed that MTH1 and OGG1 efficiently suppress progression of AD by inducing various protective genes against AD pathogenesis initiated by Aß/Tau accumulation in 3xTg-AD brain. Our findings indicate that efficient suppression of 8-oxoG accumulation in brain genomes is a new approach for prevention and treatment of AD.

Cisplatin-Mediated Upregulation of APE2 Binding to MYH9 Provokes Mitochondrial Fragmentation and Acute Kidney Injury

Cisplatin chemotherapy is standard care for many cancers but is toxic to the kidneys. How this toxicity occurs is uncertain. In this study, we identified apurinic/apyrimidinic endonuclease 2 (APE2) as a critical molecule upregulated in the proximal tubule cells (PTC) following cisplatin-induced nuclear DNA and mitochondrial DNA damage in cisplatin-treated C57B6J mice. The APE2 transgenic mouse phenotype recapitulated the pathophysiological features of C-AKI (acute kidney injury, AKI) in the absence of cisplatin treatment. APE2 pulldown-MS analysis revealed that APE2 binds myosin heavy-Chain 9 (MYH9) protein in mitochondria after cisplatin treatment. Human MYH9-related disorder is caused by mutations in MYH9 that eventually lead to nephritis, macrothrombocytopenia, and deafness, a constellation of symptoms similar to the toxicity profile of cisplatin. Moreover, cisplatin-induced C-AKI was attenuated in APE2-knockout mice. Taken together, these findings suggest that cisplatin promotes AKI development by upregulating APE2, which leads to subsequent MYH9 dysfunction in PTC mitochondria due to an unrelated role of APE2 in DNA damage repair. This postulated mechanism and the availability of an engineered transgenic mouse model based on the mechanism of C-AKI provides an opportunity to identify novel targets for prophylactic treatment of this serious disease. SIGNIFICANCE: These results reveal and highlight an unexpected role of APE2 via its interaction with MYH9 and suggest that APE2 has the potential to prevent acute kidney injury in patients with cisplatin-treated cancer. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/3/713/F1.large.jpg.

Neural stem cell-specific ITPA deficiency causes neural depolarization and epilepsy

Inosine triphosphate pyrophosphatase (ITPA) hydrolyzes inosine triphosphate (ITP) and other deaminated purine nucleotides to the corresponding nucleoside monophosphates. In humans, ITPA deficiency causes severe encephalopathy with epileptic seizure, microcephaly, and developmental retardation. In this study, we established neural stem cell-specific Itpa-conditional KO mice (Itpa-cKO mice) to clarify the effects of ITPA deficiency on the neural system. The Itpa-cKO mice showed growth retardation and died within 3 weeks of birth. We did not observe any microcephaly in the Itpa-cKO mice, although the female Itpa-cKO mice did show adrenal hypoplasia. The Itpa-cKO mice showed limb-clasping upon tail suspension and spontaneous and/or audiogenic seizure. Whole-cell patch-clamp recordings from entorhinal cortex neurons in brain slices revealed a depolarized resting membrane potential, increased firing, and frequent spontaneous miniature excitatory postsynaptic current and miniature inhibitory postsynaptic current in the Itpa-cKO mice compared with ITPA-proficient controls. Accumulated ITP or its metabolites, such as cyclic inosine monophosphates, or RNA containing inosines may cause membrane depolarization and hyperexcitability in neurons and induce the phenotype of ITPA-deficient mice, including seizure.

8-Oxoguanine accumulation in aged female brain impairs neurogenesis in the dentate gyrus and major island of Calleja, causing sexually dimorphic phenotypes

In mammals, including humans, MTH1 with 8-oxo-dGTPase and OGG1 with 8-oxoguanine DNA glycosylase minimize 8-oxoguanine accumulation in genomic DNA. We investigated age-related alterations in behavior, 8-oxoguanine levels, and neurogenesis in the brains of Mth1/Ogg1-double knockout (TO-DKO), Ogg1-knockout, and human MTH1-transgenic (hMTH1-Tg) mice. Spontaneous locomotor activity was significantly decreased in wild-type mice with age, and females consistently exhibited higher locomotor activity than males. This decrease was significantly suppressed in female but not male TO-DKO mice and markedly enhanced in female hMTH1-Tg mice. Long-term memory retrieval was impaired in middle-aged female TO-DKO mice. 8-Oxoguanine accumulation significantly increased in nuclear DNA, particularly in the dentate gyrus (DG), subventricular zone (SVZ) and major island of Calleja (ICjM) in middle-aged female TO-DKO mice. In middle-aged female TO-DKO mice, neurogenesis was severely impaired in SVZ and DG, accompanied by ICjM and DG atrophy. Conversely, expression of hMTH1 efficiently suppressed 8-oxoguanine accumulation in both SVZ and DG with hypertrophy of ICjM. These findings indicate that newborn neurons from SVZ maintain ICjM in the adult brain, and increased accumulation of 8-oxoguanine in nuclear DNA of neural progenitors in females is caused by 8-oxo-dGTP incorporation during proliferation, causing depletion of neural progenitors, altered behavior, and cognitive function changes with age.

Deficiency of base excision repair enzyme NEIL3 drives increased predisposition to autoimmunity

Alterations in the apoptosis of immune cells have been associated with autoimmunity. Here, we have identified a homozygous missense mutation in the gene encoding the base excision repair enzyme Nei endonuclease VIII-like 3 (NEIL3) that abolished enzymatic activity in 3 siblings from a consanguineous family. The NEIL3 mutation was associated with fatal recurrent infections, severe autoimmunity, hypogammaglobulinemia, and impaired B cell function in these individuals. The same homozygous NEIL3 mutation was also identified in an asymptomatic individual who exhibited elevated levels of serum autoantibodies and defective peripheral B cell tolerance, but normal B cell function. Further analysis of the patients revealed an absence of LPS-responsive beige-like anchor (LRBA) protein expression, a known cause of immunodeficiency. We next examined the contribution of NEIL3 to the maintenance of self-tolerance in Neil3-/- mice. Although Neil3-/- mice displayed normal B cell function, they exhibited elevated serum levels of autoantibodies and developed nephritis following treatment with poly(I:C) to mimic microbial stimulation. In Neil3-/- mice, splenic T and B cells as well as germinal center B cells from Peyer's patches showed marked increases in apoptosis and cell death, indicating the potential release of self-antigens that favor autoimmunity. These findings demonstrate that deficiency in NEIL3 is associated with increased lymphocyte apoptosis, autoantibodies, and predisposition to autoimmunity.

Deoxyinosine triphosphate induces MLH1/PMS2- and p53-dependent cell growth arrest and DNA instability in mammalian cells

Deoxyinosine (dI) occurs in DNA either by oxidative deamination of a previously incorporated deoxyadenosine residue or by misincorporation of deoxyinosine triphosphate (dITP) from the nucleotide pool during replication. To exclude dITP from the pool, mammals possess specific hydrolysing enzymes, such as inosine triphosphatase (ITPA). Previous studies have shown that deficiency in ITPA results in cell growth suppression and DNA instability. To explore the mechanisms of these phenotypes, we analysed ITPA-deficient human and mouse cells. We found that both growth suppression and accumulation of single-strand breaks in nuclear DNA of ITPA-deficient cells depended on MLH1/PMS2. The cell growth suppression of ITPA-deficient cells also depended on p53, but not on MPG, ENDOV or MSH2. ITPA deficiency significantly increased the levels of p53 protein and p21 mRNA/protein, a well-known target of p53, in an MLH1-dependent manner. Furthermore, MLH1 may also contribute to cell growth arrest by increasing the basal level of p53 activity.

PKCη deficiency improves lipid metabolism and atherosclerosis in apolipoprotein E-deficient mice

Genomewide association studies have shown that a nonsynonymous single nucleotide polymorphism in PRKCH is associated with cerebral infarction and atherosclerosis-related complications. We examined the role of PKCη in lipid metabolism and atherosclerosis using apolipoprotein E-deficient (Apoe^-/- ) mice. PKCη expression was augmented in the aortas of mice with atherosclerosis and exclusively detected in MOMA2-positive macrophages within atherosclerotic lesions. Prkch^+/+ Apoe^-/- and Prkch^-/- Apoe^-/- mice were fed a high-fat diet (HFD), and the dyslipidemia observed in Prkch^+/+ Apoe^-/- mice was improved in Prkch^-/- Apoe^-/- mice, with a particular reduction in serum LDL cholesterol and phospholipids. Liver steatosis, which developed in Prkch^+/+ Apoe^-/- mice, was improved in Prkch^-/- Apoe^-/- mice, but glucose tolerance, adipose tissue and body weight, and blood pressure were unchanged. Consistent with improvements in LDL cholesterol, atherosclerotic lesions were decreased in HFD-fed Prkch^-/- Apoe^-/- mice. Immunoreactivity against 3-nitrotyrosine in atherosclerotic lesions was dramatically decreased in Prkch^-/- Apoe^-/- mice, accompanied by decreased necrosis and apoptosis in the lesions. ARG2 mRNA and protein levels were significantly increased in Prkch^-/- Apoe^-/- macrophages. These data show that PKCη deficiency improves dyslipidemia and reduces susceptibility to atherosclerosis in Apoe^-/- mice, showing that PKCη plays a role in atherosclerosis development.

MUTYH, an adenine DNA glycosylase, mediates p53 tumor suppression via PARP-dependent cell death

p53-regulated caspase-independent cell death has been implicated in suppression of tumorigenesis, however, the regulating mechanisms are poorly understood. We previously reported that 8-oxoguanine (8-oxoG) accumulation in nuclear DNA (nDNA) and mitochondrial DNA triggers two distinct caspase-independent cell death through buildup of single-strand DNA breaks by MutY homolog (MUTYH), an adenine DNA glycosylase. One pathway depends on poly-ADP-ribose polymerase (PARP) and the other depends on calpains. Deficiency of MUTYH causes MUTYH-associated familial adenomatous polyposis. MUTYH thereby suppresses tumorigenesis not only by avoiding mutagenesis, but also by inducing cell death. Here, we identified the functional p53-binding site in the human MUTYH gene and demonstrated that MUTYH is transcriptionally regulated by p53, especially in the p53/DNA mismatch repair enzyme, MLH1-proficient colorectal cancer-derived HCT116+Chr3 cells. MUTYH-small interfering RNA, an inhibitor for p53 or PARP suppressed cell death without an additive effect, thus revealing that MUTYH is a potential mediator of p53 tumor suppression, which is known to be upregulated by MLH1. Moreover, we found that the p53-proficient, mismatch repair protein, MLH1-proficient colorectal cancer cell line express substantial levels of MUTYH in nuclei but not in mitochondria, suggesting that 8-oxoG accumulation in nDNA triggers MLH1/PARP-dependent cell death. These results provide new insights on the molecular mechanism of tumorigenesis and potential new strategies for cancer therapies.

Apurinic/apyrimidinic endonuclease 2 regulates the expansion of germinal centers by protecting against activation-induced cytidine deaminase-independent DNA damage in B cells

Activation-induced cytidine deaminase (AID) initiates a process generating DNA mutations and breaks in germinal center (GC) B cells that are necessary for somatic hypermutation and class-switch recombination. GC B cells can "tolerate" DNA damage while rapidly proliferating because of partial suppression of the DNA damage response by BCL6. In this study, we develop a model to study the response of mouse GC B cells to endogenous DNA damage. We show that the base excision repair protein apurinic/apyrimidinic endonuclease (APE) 2 protects activated B cells from oxidative damage in vitro. APE2-deficient mice have smaller GCs and reduced Ab responses compared with wild-type mice. DNA double-strand breaks are increased in the rapidly dividing GC centroblasts of APE2-deficient mice, which activate a p53-independent cell cycle checkpoint and a p53-dependent apoptotic response. Proliferative and/or oxidative damage and AID-dependent damage are additive stresses that correlate inversely with GC size in wild-type, AID-, and APE2-deficient mice. Excessive double-strand breaks lead to decreased expression of BCL6, which would enable DNA repair pathways but limit GC cell numbers. These results describe a nonredundant role for APE2 in the protection of GC cells from AID-independent damage, and although GC cells uniquely tolerate DNA damage, we find that the DNA damage response can still regulate GC size through pathways that involve p53 and BCL6.

Fosb gene products contribute to excitotoxic microglial activation by regulating the expression of complement C5a receptors in microglia

The Fosb gene encodes subunits of the activator protein-1 transcription factor complex. Two mature mRNAs, Fosb and ΔFosb, encoding full-length FOSB and ΔFOSB proteins respectively, are formed by alternative splicing of Fosb mRNA. Fosb products are expressed in several brain regions. Moreover, Fosb-null mice exhibit depressive-like behaviors and adult-onset spontaneous epilepsy, demonstrating important roles in neurological and psychiatric disorders. Study of Fosb products has focused almost exclusively on neurons; their function in glial cells remains to be explored. In this study, we found that microglia express equivalent levels of Fosb and ΔFosb mRNAs to hippocampal neurons and, using microarray analysis, we identified six microglial genes whose expression is dependent on Fosb products. Of these genes, we focused on C5ar1 and C5ar2, which encode receptors for complement C5a. In isolated Fosb-null microglia, chemotactic responsiveness toward the truncated form of C5a was significantly lower than that in wild-type cells. Fosb-null mice were significantly resistant to kainate-induced seizures compared with wild-type mice. C5ar1 mRNA levels and C5aR1 immunoreactivity were increased in wild-type hippocampus 24 hours after kainate administration; however, such induction was significantly reduced in Fosb-null hippocampus. Furthermore, microglial activation after kainate administration was significantly diminished in Fosb-null hippocampus, as shown by significant reductions in CD68 immunoreactivity, morphological change and reduced levels of Il6 and Tnf mRNAs, although no change in the number of Iba-1-positive cells was observed. These findings demonstrate that, under excitotoxicity, Fosb products contribute to a neuroinflammatory response in the hippocampus through regulation of microglial C5ar1 and C5ar2 expression.

8-Oxoguanine causes neurodegeneration during MUTYH-mediated DNA base excision repair

8-Oxoguanine (8-oxoG), a common DNA lesion caused by reactive oxygen species, is associated with carcinogenesis and neurodegeneration. Although the mechanism by which 8-oxoG causes carcinogenesis is well understood, the mechanism by which it causes neurodegeneration is unknown. Here, we report that neurodegeneration is triggered by MUTYH-mediated excision repair of 8-oxoG-paired adenine. Mutant mice lacking 8-oxo-2'-deoxyguanosine triphosphate-depleting (8-oxo-dGTP-depleting) MTH1 and/or 8-oxoG-excising OGG1 exhibited severe striatal neurodegeneration, whereas mutant mice lacking MUTYH or OGG1/MUTYH were resistant to neurodegeneration under conditions of oxidative stress. These results indicate that OGG1 and MTH1 are protective, while MUTYH promotes neurodegeneration. We observed that 8-oxoG accumulated in the mitochondrial DNA of neurons and caused calpain-dependent neuronal loss, while delayed nuclear accumulation of 8-oxoG in microglia resulted in PARP-dependent activation of apoptosis-inducing factor and exacerbated microgliosis. These results revealed that neurodegeneration is a complex process caused by 8-oxoG accumulation in the genomes of neurons and microglia. Different signaling pathways were triggered by the accumulation of single-strand breaks in each type of DNA generated during base excision repair initiated by MUTYH, suggesting that suppression of MUTYH may protect the brain under conditions of oxidative stress.

Cancer-related PRUNE2 protein is associated with nucleotides and is highly expressed in mature nerve tissues

Human PRUNE is thought to enhance the metastasis of tumor cells. We found that a hypothetical paralog of PRUNE, PRUNE2, binds to 8-oxo-GTP, an oxidized form of GTP. Hypothetical PRUNE2 gene consists of C9orf65 and BMCC1/BNIPXL, both of which are malignant tumor-associated genes. We isolated PRUNE2 complementary DNA and revealed that the protein is composed of 3,062 residues. C9orf65 and BMCC1/BNIPXL encode the N-terminal part (259 residues) and C-terminal part (2,729 residues) of PRUNE2, respectively. We demonstrated the endogenous full-length PRUNE2 protein (338 kDa) by Western blot and mass spectrometry. PRUNE2 bound to 8-oxo-GTP as well as GTP. The expression levels of human PRUNE2 and mouse Prune2 messenger RNA (mRNA) were highest in the dorsal root ganglia (DRG) and, to a lesser extent, in other nerve tissues. DRG neurons express higher levels of PRUNE2 in their soma compared with adjacent cells. In addition, their expression levels in the adult nerve tissues were higher than those in fetal or neonatal nerve tissues. The present study indicates that C9orf65 and BMCC1/BNIPXL are transcribed as PRUNE2 mRNA, which is translated to a large PRUNE2 protein. The nerve tissue-specific and post-development expression of PRUNE2/Prune2 suggests that PRUNE2 may contribute to the maintenance of mature nervous systems.

Apurinic/apyrimidinic endonuclease 2 is necessary for normal B cell development and recovery of lymphoid progenitors after chemotherapeutic challenge

B cell development involves rapid cellular proliferation, gene rearrangements, selection, and differentiation, and it provides a powerful model to study DNA repair processes in vivo. Analysis of the contribution of the base excision repair pathway in lymphocyte development has been lacking primarily owing to the essential nature of this repair pathway. However, mice deficient for the base excision repair enzyme, apurinic/apyrimidinic endonuclease 2 (APE2) protein develop relatively normally, but they display defects in lymphopoiesis. In this study, we present an extensive analysis of bone marrow hematopoiesis in mice nullizygous for APE2 and find an inhibition of the pro-B to pre-B cell transition. We find that APE2 is not required for V(D)J recombination and that the turnover rate of APE2-deficient progenitor B cells is nearly normal. However, the production rate of pro- and pre-B cells is reduced due to a p53-dependent DNA damage response. FACS-purified progenitors from APE2-deficient mice differentiate normally in response to IL-7 in in vitro stromal cell cocultures, but pro-B cells show defective expansion. Interestingly, APE2-deficient mice show a delay in recovery of B lymphocyte progenitors following bone marrow depletion by 5-fluorouracil, with the pro-B and pre-B cell pools still markedly decreased 2 wk after a single treatment. Our data demonstrate that APE2 has an important role in providing protection from DNA damage during lymphoid development, which is independent from its ubiquitous and essential homolog APE1.

A comprehensive screening system for damaged nucleotide-binding proteins

To identify novel nucleotide pool sanitizing enzymes, we have established a comprehensive screening system for damaged nucleotide-binding proteins based on proteomics technology. In the screening system, affinity chromatography with resins carrying various damaged nucleotides is used for the purification of binding proteins, and the purified proteins are identified by mass-spectrometry. Inosine triphosphate (ITP) is a deleterious damaged nucleotide, and can be generated by nitrosative deamination of ATP or phosphorylation of inosine monophosphate (IMP). Using the above system, we performed screens for ITP-binding proteins from mouse and human cell extracts, and identified several ITP-binding enzymes. We identified both mouse inosine triphosphatase (ITPA) and human ITPA, well-known ITP hydrolyzing enzymes, as ITP-binding proteins. These results support the validity of this screening system. In addition to ITPA, we identified human nucleoside diphosphate linked moiety X-type motif 16 (NUDT16) protein as an ITP-binding protein. Biochemical analysis revealed that NUDT16 selectively hydrolyzes deoxyinosine diphosphate (dIDP) and IDP to deoxyinosine monophosphate (dIMP) and IMP, respectively. dITP and ITP are also hydrolyzed by NUDT16 to a lesser extent. The knockdown of NUDT16 in HeLa MR cells suppressed cell proliferation, and was accompanied by a significantly increased accumulation of strand breaks in nuclear DNA, suggesting that NUDT16 has an essential role in the maintenance of genome stability. RS21-C6, another ITP-binding protein identified in our screen, binds not only to ITP, but also to ATP. RS21-C6 hydrolyzes dCTP and 5-halo-dCTP, but does not hydrolyze ITP or ATP. It is likely that RS21-C6 may control dCTP levels or eliminate 5-halo-dCTP in the nucleotide pools. In conclusion, the results of these studies show that our screening system is applicable in studying the health effects of damaged nucleotides and cellular sanitizing systems for nucleotide pools.

ITPA protein, an enzyme that eliminates deaminated purine nucleoside triphosphates in cells

Inosine triphosphate pyrophosphatase (ITPA protein) (EC 3.6.1.19) hydrolyzes deaminated purine nucleoside triphosphates, such as ITP and dITP, to their corresponding purine nucleoside monophosphate and pyrophosphate. In mammals, this enzyme is encoded by the Itpa gene. Using the Itpa gene-disrupted mouse as a model, we have elucidated the biological significance of the ITPA protein and its substrates, ITP and dITP. Itpa(-/-) mice exhibited peri- or post-natal lethality dependent on the genetic background. The heart of the Itpa(-/-) mouse was found to be structurally and functionally abnormal. Significantly higher levels of deoxyinosine and inosine were detected in nuclear DNA and RNA prepared from Itpa(-/-) embryos compared to wild type embryos. In addition, an accumulation of ITP was observed in the erythrocytes of Itpa(-/-) mice. We found that Itpa(-/-) primary mouse embryonic fibroblasts (MEFs), which have no detectable ability to generate IMP from ITP in whole cell extracts, exhibited a prolonged population-doubling time, increased chromosome abnormalities and accumulation of single-strand breaks in their nuclear DNA, in comparison to primary MEFs prepared from wild type embryos. These results revealed that (1) ITP and dITP are spontaneously produced in vivo and (2) accumulation of ITP and dITP is responsible for the harmful effects observed in the Itpa(-/-) mouse. In addition to its effect as the precursor nucleotide for RNA transcription, ITP has the potential to influence the activity of ATP/GTP-binding proteins. The biological significance of ITP and dITP in the nucleotide pool remains to be elucidated.

Programmed cell death triggered by nucleotide pool damage and its prevention by MutT homolog-1 (MTH1) with oxidized purine nucleoside triphosphatase

Accumulation of oxidized bases such as 8-oxoguanine in either nuclear or mitochondrial DNA triggers various cellular dysfunctions including mutagenesis, and programmed cell death or senescence. Recent studies have revealed that oxidized nucleoside triphosphates such as 8-oxo-dGTP in the nucleotide pool are the main source of oxidized bases accumulating in the DNA of cells under oxidative stress. To counteract such deleterious effects of nucleotide pool damage, mammalian cells possess MutT homolog-1 (MTH1) with oxidized purine nucleoside triphosphatase and related enzymes, thus minimizing the accumulation of oxidized bases in cellular DNA. Depletion or increased expression of the MTH1 protein have revealed its significant roles in avoiding programmed cell death or senescence as well as mutagenesis, and accumulating evidences indicate that MTH1 is involved in suppression of degenerative disorders such as neurodegeneration.

NUDT16 and ITPA play a dual protective role in maintaining chromosome stability and cell growth by eliminating dIDP/IDP and dITP/ITP from nucleotide pools in mammals

Mammalian inosine triphosphatase encoded by ITPA gene hydrolyzes ITP and dITP to monophosphates, avoiding their deleterious effects. Itpa(-) mice exhibited perinatal lethality, and significantly higher levels of inosine in cellular RNA and deoxyinosine in nuclear DNA were detected in Itpa(-) embryos than in wild-type embryos. Therefore, we examined the effects of ITPA deficiency on mouse embryonic fibroblasts (MEFs). Itpa(-) primary MEFs lacking ITP-hydrolyzing activity exhibited a prolonged doubling time, increased chromosome abnormalities and accumulation of single-strand breaks in nuclear DNA, compared with primary MEFs prepared from wild-type embryos. However, immortalized Itpa(-) MEFs had neither of these phenotypes and had a significantly higher ITP/IDP-hydrolyzing activity than Itpa(-) embryos or primary MEFs. Mammalian NUDT16 proteins exhibit strong dIDP/IDP-hydrolyzing activity and similarly low levels of Nudt16 mRNA and protein were detected in primary MEFs derived from both wild-type and Itpa(-) embryos. However, immortalized Itpa(-) MEFs expressed significantly higher levels of Nudt16 than the wild type. Moreover, introduction of silencing RNAs against Nudt16 into immortalized Itpa(-) MEFs reproduced ITPA-deficient phenotypes. We thus conclude that NUDT16 and ITPA play a dual protective role for eliminating dIDP/IDP and dITP/ITP from nucleotide pools in mammals.

NUDT16 is a (deoxy)inosine diphosphatase, and its deficiency induces accumulation of single-strand breaks in nuclear DNA and growth arrest

Nucleotides function in a variety of biological reactions; however, they can undergo various chemical modifications. Such modified nucleotides may be toxic to cells if not eliminated from the nucleotide pools. We performed a screen for modified-nucleotide binding proteins and identified human nucleoside diphosphate linked moiety X-type motif 16 (NUDT16) protein as an inosine triphosphate (ITP)/xanthosine triphosphate (XTP)/GTP-binding protein. Recombinant NUDT16 hydrolyzes purine nucleoside diphosphates to the corresponding nucleoside monophosphates. Among 29 nucleotides examined, the highest k_cat/K_m values were for inosine diphosphate (IDP) and deoxyinosine diphosphate (dIDP). Moreover, NUDT16 moderately hydrolyzes (deoxy)inosine triphosphate ([d]ITP). NUDT16 is mostly localized in the nucleus, and especially in the nucleolus. Knockdown of NUDT16 in HeLa MR cells caused cell cycle arrest in S-phase, reduced cell proliferation, increased accumulation of single-strand breaks in nuclear DNA as well as increased levels of inosine in RNA. We thus concluded that NUDT16 is a (deoxy)inosine diphosphatase that may function mainly in the nucleus to protect cells from deleterious effects of (d)ITP.

ITPase-deficient mice show growth retardation and die before weaning

Inosine triphosphate pyrophosphatase (ITPase), the enzyme that hydrolyzes ITP and other deaminated purine nucleoside triphosphates to the corresponding purine nucleoside monophosphate and pyrophosphate, is encoded by the Itpa gene. In this study, we established Itpa knockout (KO) mice and used them to show that ITPase is required for the normal organization of sarcomeres in the heart. Itpa(-/-) mice died about 2 weeks after birth with features of growth retardation and cardiac myofiber disarray, similar to the phenotype of the cardiac alpha-actin KO mouse. Inosine nucleotides were found to accumulate in both the nucleotide pool and RNA of Itpa(-/-) mice. These data suggest that the role of ITPase in mice is to exclude ITP from the ATP pool, and the main target substrate of this enzyme is rITP. Our data also suggest that cardiomyopathy, which is mainly caused by mutations in sarcomeric protein-encoding genes, is also caused by a defect in maintaining the quality of the ATP pool, which is an essential requirement for sarcomere function.

Apex2 is required for efficient somatic hypermutation but not for class switch recombination of immunoglobulin genes

The DNA cleavage step in both the class switch recombination (CSR) and somatic hypermutation (SHM) of Ig genes is initiated by activation-induced cytidine deaminase (AID). However, the detailed mechanisms of the DNA strand cleavage in SHM and CSR are still largely unknown. Recently, the apurinic/apyrimidinic endonucleases, Apex1 and Apex2, were reported to be involved in the DNA cleavage step of CSR. Here, we examined the role of Apex2 in SHM using Apex2-deficient mice and found that the Apex2 deficiency caused a drastic reduction in the frequency of SHM and the number of mutations per mutated clone without affecting the pattern of base substitution. These results suggest that Apex2 may play a critical role in SHM through its 3'-5' exonuclease activity. Unexpectedly, the efficiency of CSR was not reduced in Apex2-deficient B cells. In addition, Apex1 knockdown in CH12F3-2 B lymphoma cells did not affect the CSR frequency, suggesting that neither Apex1 nor Apex2 plays a major role in CSR.

Hematopoietic tissue-specific expression of mouse Neil3 for endonuclease VIII-like protein

We cloned cDNA and genomic DNA containing exon 1 of mouse Neil3. Neil3 spans 52.4 kb and consists of 10 exons. Northern blot analysis revealed that Neil3 mRNA was selectively expressed in thymus, spleen and bone marrow. High levels of Neil3 mRNA were also detected in various mouse B cell lines by RT-PCR. Immunofluorescence microscopy using anti-NEIL3 revealed that recombinant mouse NEIL3 is localized in the nuclei. In mouse splenocytes, the level of Neil3 mRNA significantly increased after mitogen stimulation in vitro. We established NEIL3-null mice, which are viable and fertile. We found candidate sequences for NEIL3 orthologues in a DNA database from dog and zebrafish in addition to human and mouse, but not invertebrates. NEIL3 may function exclusively in vertebrates, such as mammals.

Mouse RS21-C6 is a mammalian 2'-deoxycytidine 5'-triphosphate pyrophosphohydrolase that prefers 5-iodocytosine

Free nucleotides in living cells play important roles in a variety of biological reactions, and often undergo chemical modifications of their base moieties. As modified nucleotides may have deleterious effects on cells, they must be eliminated from intracellular nucleotide pools. We have performed a screen for ITP-binding proteins because ITP is a deaminated product of ATP, the most abundant nucleotide, and identified RS21-C6 protein, which bound not only ITP but also ATP. Purified, recombinant RS21-C6 hydrolyzed several canonical nucleoside triphosphates to the corresponding nucleoside monophosphates. The pyrophosphohydrolase activity of RS21-C6 showed a preference for deoxynucleoside triphosphates and cytosine bases. The k(cat)/K(m) (s(-1) m(-1)) values were 3.11 x 10(4), 4.49 x 10(3) and 1.87 x 10(3) for dCTP, dATP and dTTP, respectively, and RS21-C6 did not hydrolyze dGTP. Of the base-modified nucleotides analyzed, 5-I-dCTP showed an eightfold higher k(cat)/K(m) value compared with that of its corresponding unmodified nucleotide, dCTP. RS21-C6 is expressed in both proliferating and non-proliferating cells, and is localized to the cytoplasm. These results show that RS21-C6 produces dCMP, an upstream precursor for the de novo synthesis of dTTP, by hydrolyzing canonical dCTP. Moreover, RS21-C6 may also prevent inappropriate DNA methylation, DNA replication blocking or mutagenesis by hydrolyzing modified dCTP.

Altered gene expression profiles and higher frequency of spontaneous DNA strand breaks in APEX2-null thymus

A second class II AP endonuclease, APEX2, possesses strong 3'-5' exonuclease and 3'-phosphodiesterase activities but only very weak AP-endonuclease activity. APEX2 associates with proliferating cell nuclear antigen (PCNA), and the progression of S phase of the cell cycle is accompanied by its expression. APEX2-null mice exhibit severe dyslymphopoiesis in thymus as well as moderate dyshematopoiesis and growth retardation. Comparative gene expression profiling of wild-type and APEX2-null mice using an oligonucleotide microarray revealed that APEX2-null thymus has significantly altered gene expression profiles, reflecting its altered populations of thymocytes. Beyond these altered populations, APEX2-null thymus exhibits significant alterations in expression of genes involved in DNA replication, recombination and repair, including Apex1, Exo1 and Fen1 as well as master genes for the DNA damage response, such as E2f1, Chek1, and proapoptotic genes. We therefore examined the extent of DNA strand breakage, and found that both of single-strand breaks detected as comets and double-strand breaks detected as gammaH2AX foci were significantly higher in frequency in most APEX2-null thymocytes compared to wild-type thymocytes. This higher frequency of DNA breaks was accompanied by increased expression of PCNA and increased phosphorylation of p53 at Ser23 and to a lesser extent, at Ser18. The present study clearly demonstrates that APEX2-null lymphocytes have a higher frequency of DNA breaks, indicating that APEX2 may play an important role(s) during their generation and/or repair.

APE1- and APE2-dependent DNA breaks in immunoglobulin class switch recombination

Antibody class switch recombination (CSR) occurs by an intrachromosomal deletion requiring generation of double-stranded breaks (DSBs) in switch-region DNA. The initial steps in DSB formation have been elucidated, involving cytosine deamination by activation-induced cytidine deaminase and generation of abasic sites by uracil DNA glycosylase. However, it is not known how abasic sites are converted into single-stranded breaks and, subsequently, DSBs. Apurinic/apyrimidinic endonuclease (APE) efficiently nicks DNA at abasic sites, but it is unknown whether APE participates in CSR. We address the roles of the two major mammalian APEs, APE1 and APE2, in CSR. APE1 deficiency causes embryonic lethality in mice; we therefore examined CSR and DSBs in mice deficient in APE2 and haploinsufficient for APE1. We show that both APE1 and APE2 function in CSR, resulting in the DSBs necessary for CSR and thereby describing a novel in vivo function for APE2.

Oxidative damage in nucleic acids and Parkinson's disease

Oxidative DNA lesions, such as 8-oxoguanine (8-oxoG), accumulate in nuclear and mitochondrial genomes during aging, and such accumulation can increase dramatically in patients with Parkinson's disease (PD). To counteract oxidative damage to nucleic acids, human and rodents are equipped with three distinct enzymes. One of these, MTH1, hydrolyzes oxidized purine nucleoside triphosphates, such as 8-oxo-2'-deoxyguanosine triphosphate and 2-hydroxy-2'-deoxyadenosine triphosphate, to their monophosphate forms. The other two enzymes are 8-oxoG DNA glycosylase encoded by the OGG1 gene and adenine/2-hydroxyadenine DNA glycosylase encoded by the MUTYH gene. We have shown a significant increase in 8-oxoG in mitochondrial DNA as well as an elevated expression of MTH1, OGG1, and MUTYH in nigrostriatal dopaminergic neurons of PD patients, suggesting that the buildup of these lesions may cause dopamine neuron loss. We established MTH1-null mice and found that MTH1-null fibroblasts were highly susceptible to cell death caused by H(2)O(2) characterized by pyknosis and electron-dense deposits in the mitochondria, and that this was accompanied by an ongoing accumulation of 8-oxoG in nuclear and mitochondrial DNA. We also showed that MTH1-null mice exhibited an increased accumulation of 8-oxoG in striatal mitochondrial DNA, followed by more extreme neuronal dysfunction after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine administration than that of wild-type mice. In conclusion, oxidative damage in nucleic acids is likely to be a major risk factor for Parkinson's disease, indicating that a solid understanding of the defense mechanisms involved will enable us to develop new strategies for protecting the brain against oxidative stress.

Mutagenesis and carcinogenesis caused by the oxidation of nucleic acids

Genomes and their precursor nucleotides are highly exposed to reactive oxygen species, which are generated both as byproducts of oxygen respiration or molecular executors in the host defense, and by environmental exposure to ionizing radiation and chemicals. To counteract such oxidative damage in nucleic acids, mammalian cells are equipped with three distinct enzymes. MTH1 protein hydrolyzes oxidized purine nucleoside triphosphates, such as 8-oxo-2'-deoxyguanosine triphosphate and 2-hydroxy-2'-deoxyadenosine triphosphate (2-OH-dATP), to the corresponding monophosphates. We observed increased susceptibility to spontaneous carcinogenesis in MTH1-null mice, which exhibit an increased occurrence of A:T-->C:G and G:C-->T:A transversion mutations. 8-Oxoguanine (8-oxoG) DNA glycosylase, encoded by the OGG1 gene, and adenine DNA glycosylase, encoded by the MUTYH gene, are responsible for the suppression of G:C to T:A transversions caused by the accumulation of 8-oxoG in the genome. Deficiency of these enzymes leads to increased tumorigenesis in the lung and intestinal tract in mice, respectively. MUTYH deficiency may also increase G:C to T:A transversions through the misincorporation of 2-OH-dATP, especially in the intestinal tract, since MUTYH can excise 2-hydroxyadenine opposite guanine in genomic DNA and the repair activity is selectively impaired by a mutation found in patients with autosomal recessive colorectal adenomatous polyposis.

A genome-wide distribution of 8-oxoguanine correlates with the preferred regions for recombination and single nucleotide polymorphism in the human genome

8-Oxoguanine (8-oxoG), a major spontaneous form of oxidative DNA damage, is considered to be a natural cause of genomic diversity in organisms because of its mutagenic potential. The steady-state level of 8-oxoG in the nuclear genome of a human cell has been estimated to be several residues per 10(6) guanines. In the present study, to clarify the genome-wide distribution of 8-oxoG in the steady state, we performed fluorescence in situ detection of 8-oxoG on human metaphase chromosomes using a monoclonal antibody. Multiple dot-like signals were observed on each metaphase chromosome. We then mapped the position of the signal at megabase resolution referring to the cytogenetically identified chromosomal band, and demonstrated that 8-oxoG is unevenly distributed in the normal human genome and that the distribution pattern is conserved among different individuals. Moreover, we found that regions with a high frequency of recombination and single nucleotide polymorphisms (SNPs) are preferentially located within chromosomal regions with a high density of 8-oxoG. Our findings suggest that 8-oxoG is one of the main causes of frequent recombinations and SNPs in the human genome, which largely contribute to the genomic diversity in human beings.

MTH1, an oxidized purine nucleoside triphosphatase, prevents the cytotoxicity and neurotoxicity of oxidized purine nucleotides

In human and rodent cells, MTH1, an oxidized purine nucleoside triphosphatase, efficiently hydrolyzes oxidized dGTP, GTP, dATP and ATP such as 2'-deoxy-8-oxoguanosine triphosphate (8-oxo-dGTP) and 2'-deoxy-2-hydroxyadenosine triphosphate (2-OH-dATP) in nucleotide pools, thus avoiding their incorporation into DNA or RNA. MTH1 is expressed in postmitotic neurons as well as in proliferative tissues, and it is localized both in the mitochondria and nucleus, thus suggesting that MTH1 plays an important role in the prevention of the mutagenicity and cytotoxicity of such oxidized purines as 8-oxoG which are known to accumulate in the cellular genome. Our recent studies with MTH1-deficient mice or cells revealed that MTH1 efficiently minimizes accumulation of 8-oxoG in both nuclear and mitochondrial DNA in the mouse brain as well as in cultured cells, thus contributing to the protection of the brain from oxidative stress.

A mechanism of transcriptional regulation of the CSF-1 gene by interferon-gamma

Interferon (IFN)-gamma exerts multiple functions including antiviral, anti-proliferative and immunomodulatory activities, which are mediated through the JAK-STAT pathway. We observed that IFN-gamma significantly increases the production of colony stimulating factor-1 (CSF-1) by a human lung carcinoma cell line, A549. To gain insight into this mechanism, we determined the unknown nucleotide sequences of 5'-flanking region of human CSF-1 gene. About 60 bases upstream of the transcription start site of the CSF-1 gene contains a possible gamma interferon activated site (GAS), TTCCCATAA. The promoter assay and the electrophoretic mobility gel shift assay revealed that IFN-gamma stimulates transcription of the CSF-1 gene through this sequence, which binds STAT1.

Characterization of the structure and expression of mouse Itpa gene and its related sequences in the mouse genome

In the mouse genome, we found one processed Itpa gene-like sequence and two processed Itpa pseudogenes as well as the Itpa gene itself with introns, located on chromosome 2F3, which was isolated by a retro-recombination method. We also identified three types (A, B, C) of Itpa transcripts in mouse tissues. The processed Itpa gene-like sequence located on chromosome 2E1 has a complete open reading frame for exactly the same polypeptide as ITPA encoded by the type A transcript, with a polyadenylation signal. However, no transcribed sequence derived from the Itpa gene-like sequence was detectable in any of the mouse tissues examined, thus naming the sequence as Itpa processed pseudogene alpha. The type A Itpa mRNA, which was expressed in all mouse tissues examined, only encodes mouse ITPA polypeptide consisting of 198 amino acid residues with a capacity to hydrolyze dITP into dIMP. Itpa mRNA was detected in all tissues examined, and its expression is especially high in the testis, brain, and thymus. ITPA protein was mostly detected in the cytoplasm, to a lesser extent in the nuclei of neurons in the brain, and also those of hepatocytes, epithelial cells lining the bile duct, and endothelial cells lining the portal vein in the liver.

A functional analysis of the DNA glycosylase activity of mouse MUTYH protein excising 2-hydroxyadenine opposite guanine in DNA

2-Hydroxy-2-deoxyadenosine triphosphate (2-OH-dATP), generated by the oxidation of dATP, can be misincorporated by DNA polymerases opposite guanine in template DNA during DNA replication, thus causing spontaneous mutagenesis. We demonstrated that mouse MUTYH (mMUTYH) has a DNA glycosylase activity excising not only adenine opposite 8-oxoguanine (8-oxoG) but also 2-hydroxyadenine (2-OH-A) opposite guanine, using purified recombinant thioredoxin-mMUTYH fusion protein. mMUTYH formed a stable complex with duplex oligonucleotides containing an adenine:8-oxoG pair, but the binding of mMUTYH to oligonucleotides containing a 2-OH-A:guanine pair was barely detectable, thus suggesting that mMUTYH recognizes and interacts with these two substrates in a different manner which may reflect the difference in the base excision repair process for each substrate. Mutant mMUTYH with G365D amino acid substitution, corresponding to a G382D germline mutation of human MUTYH found in familial adenomatous polyposis patients, almost completely retained its DNA glycosylase activity excising adenine opposite 8-oxoG; however, it possessed 1.5% of the wild-type activity excising 2-OH-A opposite guanine. Our results imply that the reduced repair capacity of the mutant hMUTYH(G382D), which inefficiently excises 2-OH-A opposite guanine, results in an increased occurrence of somatic G:C to T:A transversion mutations in the APC gene as well as tumorigenesis in the colon.

The defense mechanisms in mammalian cells against oxidative damage in nucleic acids and their involvement in the suppression of mutagenesis and cell death

To counteract oxidative damage in nucleic acids, mammalian cells are equipped with several defense mechanisms. We herein review that MTH1, MUTYH and OGG1 play important roles in mammalian cells avoiding an accumulation of oxidative DNA damage, both in the nuclear and mitochondrial genomes, thereby suppressing carcinogenesis and cell death. MTH1 efficiently hydrolyzes oxidized purine nucleoside triphosphates, such as 8-oxo-dGTP, 8-oxo-dATP and 2-hydroxy (OH)-dATP, to the monophosphates, thus avoiding the incorporation of such oxidized nucleotides into the nuclear and mitochondrial genomes. OGG1 excises 8-oxoG in DNA as a DNA glycosylase and thus minimizes the accumulation of 8-oxoG in the cellular genomes. MUTYH excises adenine opposite 8-oxoG, and thus suppresses 8-oxoG-induced mutagenesis. MUTYH also possesses a 2-OH-A DNA glycosylase activity for excising 2-OH-A incorporated into the cellular genomes. Increased susceptibilities to spontaneous carcinogenesis of the liver, lung or intestine were observed in MTH1-, OGG1- and MUTYH-null mice, respectively. The increased occurrence of lung tumors in OGG1-null mice was abolished by the concomitant disruption of the Mth1 gene, indicating that an increased accumulation of 8-oxoG and/or 2-OH-A might cause cell death. Furthermore, these defense mechanisms also likely play an important role in neuroprotection.

Growth retardation and dyslymphopoiesis accompanied by G2/M arrest in APEX2-null mice

APEX2/APE2 is a secondary mammalian apurinic/apyrimidinic endonuclease that associates with proliferating cell nuclear antigen (PCNA), and the progression of S phase of the cell cycle is accompanied by its expression. To determine the biologic significance of APEX2, we established APEX2-null mice. These mice were about 80% the size of their wild-type littermates and exhibited a moderate dyshematopoiesis and a relatively severe defect in lymphopoiesis. A significant accumulation of both thymocytes and mitogen-stimulated splenocytes in G(2)/M phase was seen in APEX2-null mice compared with the wild type, indicating that APEX2 is required for proper cell cycle progression of proliferating lymphocytes. Although APEX2-null mice exhibited an attenuated immune response against ovalbumin in comparison with wild-type mice, they produced both antiovalbumin immunoglobulin M (IgM) and IgG, indicating that class switch recombination can occur even in the absence of APEX2.

MUTYH prevents OGG1 or APEX1 from inappropriately processing its substrate or reaction product with its C-terminal domain

MutY homolog (MUTYH) excises adenine opposite 8-oxoguanine (8-oxoG) in DNA, thus preventing occurrence of G:C to T:A transversion. In cell-free extract prepared from the thymocytes of wild type but not MUTYH-null mice, adenine opposite 8-oxoG in DNA was excised by MUTYH, however, the generated apurinic (AP) site opposite 8-oxoG mostly remained unincised. Recombinant mouse MUTYH (mMUTYH) efficiently excised adenine opposite 8-oxoG and prevented mouse AP endonuclease (mAPEX1) from incising the generated AP site. In contrast, an AP site opposite 8-oxoG created by uracil DNA glycosylase or tetrahydrofuran opposite 8-oxoG was efficiently incised by mAPEX1 in the presence of an excess amount of mMUTYH. Mutant mMUTYH with R361A or G365D substitution, excised adenine opposite 8-oxoG as efficiently as did wild-type mMUTYH, but failed to prevent mAPEX1 from incising the generated AP site. Wild-type mMUTYH bound duplex oligonucleotides containing A:8-oxoG pair with a lower apparent K(d) than that of the mutants, and prevented OGG1 from excising 8-oxoG opposite adenine or the generated AP site. The G365D mutant failed to prevent OGG1 from excising 8-oxoG opposite the generated AP site, thus indicating that the protection of its own product by mMUTYH is an intrinsic function which depends on the C-terminal domain of mMUTYH.

Galectin-1β, a natural monomeric form of galectin-1 lacking its six amino-terminal residues promotes axonal regeneration but not cell death

We previously identified a novel N-terminally processed form of galectin-1, galectin-1beta (Gal-1beta) whose expression was induced by DeltaFosB. In the present study, the biochemical properties and biological functions of Gal-1beta were compared with the full-length form of galectin-1 (Gal-1alpha). We first purified recombinant mouse Gal-1alpha and beta (rmGal-1alpha, beta) to near homogeneity. The rmGal-1alpha exists as a monomer under oxidized conditions and forms a dimer under reduced conditions, while the rmGal-1beta exists as a monomer regardless of redox conditions. The affinity of rmGal-1beta to beta-lactose was approximately two-fold lower than that of rmGal-1alpha under reduced conditions. The viability of Jurkat cells efficiently decreased when they were exposed to rmGal-1alpha, however, rmGal-1beta barely induced such a reduction. In contrast, both rmGal-1alpha and rmGal-1beta exhibited an equivalent capacity to promote axonal regeneration from the dorsal root ganglion explants. Our results suggest that the biochemical properties of rmGal-1beta determine its biological functions.

Biological significance of the defense mechanisms against oxidative damage in nucleic acids caused by reactive oxygen species: from mitochondria to nuclei

In mammalian cells, more than one genome in a single cell has to be maintained throughout the entire life of the cell, namely, one in the nucleus and the other in the mitochondria. The genomes and their precursor nucleotides are highly exposed to reactive oxygen species, which are inevitably generated as a result of the respiratory function in mitochondria. To counteract such oxidative damage in nucleic acids, cells are equipped with several defense mechanisms. Modified nucleotides in the nucleotide pools are hydrolyzed, thus avoiding their incorporation into DNA or RNA. Damaged bases in DNA with relatively small chemical alterations are mainly repaired by the base excision repair (BER) system, which is initiated by the excision of damaged bases by specific DNA glycosylases. MTH1 protein hydrolyzes oxidized purine nucleoside triphosphates, such as 8-oxo-dGTP, 8-oxo-dATP, and 2-hydroxy (OH)-dATP to the monophosphates, and MTH1 are located in the cytoplasm, mitochondria, and nucleus. We observed an increased susceptibility to spontaneous carcinogenesis in Mth1-deficient mice and an alteration of MTH1 expression along with the accumulation of 8-oxo-dG in patients with various neurodegenerative diseases. Enzymes for the BER pathway, namely, 8-oxoG DNA glycosylase (OGG1), 2-OH-A/adenine DNA glycosylase (MUTYH), and AP endonuclease (APEX2) are also located both in the mitochondria and in the nuclei, and the expression of mitochondrial OGG1 is altered in patients with various neurodegenerative diseases. We also observed increased susceptibilities to spontaneous carcinogenesis in OGG1 and MUTYH-deficient mice. The increased occurrence of lung tumor in OGG1-deficient mice was completely abolished by the concomitant disruption of the Mth1 gene.

Identification and characterization of two forms of mouse MUTYH proteins encoded by alternatively spliced transcripts

There are three types of mouse Mutyh mRNAs (type a, b and c) generated by alternative splicing, and type b mRNA is a major form among the three in most of the tissues examined. The level of type c mRNA is relatively high in brain. Type a and b mRNAs were expected to encode 57.7 kDa protein (MUTYHalpha), while type c mRNA had a partly different open reading frame encoding a 50.2 kDa protein (MUTYHbeta). An in vitro translation of type b and c mRNAs produced a 50 kDa MUTYHalpha and 47 kDa MUTYHbeta, respectively. MUTYHalpha and MUTYHbeta were detected in wild-type embryonic stem (ES) cells or thymocytes prepared from wild-type mice, but neither MUTYH-null ES cells nor thymocytes prepared from MUTYH-null mice. Both MUTYHalpha and MUTYHbeta were mainly localized in the nuclei and some in mitochondria in wild-type ES cells. Recombinant MUTYHalpha and beta were expressed as fusion proteins with thioredoxin in Escherichia coli, but only MUTYHalpha was partly soluble and thus could be purified. Recombinant MUTYHalpha possessed DNA glycosylase activities to excise adenine opposite 8-oxoguanine and guanine but not AP lyase activity.

Mutator phenotype of MUTYH-null mouse embryonic stem cells

To evaluate the antimutagenic role of a mammalian mutY homolog, namely the Mutyh gene, which encodes adenine DNA glycosylase excising adenine misincorporated opposite 8-oxoguanine in the template DNA, we generated MUTYH-null mouse embryonic stem (ES) cells. In the MUTYH-null cells carrying no adenine DNA glycosylase activity, the spontaneous mutation rate increased 2-fold in comparison with wild type cells. The expression of wild type mMUTYH or mutant mMUTYH protein with amino acid substitutions at the proliferating cell nuclear antigen binding motif restored the increased spontaneous mutation rates of the MUTYH-null ES cells to the wild type level. The expression of a mutant mMUTYH protein with an amino acid substitution (G365D) that corresponds to a germ-line mutation (G382D) found in patients with multiple colorectal adenomas could not suppress the elevated spontaneous mutation rate of the MUTYH-null ES cells. Although the recombinant mMUTYH(G365D) purified from Escherichia coli cells had a substantial level of adenine DNA glycosylase activity as did wild type MUTYH, no adenine DNA glycosylase activity was detected in the MUTYH-null ES cells expressing the mMUTYH(G365D) mutant protein. The germ-line mutation (G382D) of the human MUTYH gene is therefore likely to be responsible for the occurrence of a mutator phenotype in these patients.

DeltaFosB, but not FosB, induces delayed apoptosis independent of cell proliferation in the Rat1a embryo cell line

The fates of Rat1a cells expressing FosB and DeltaFosB as fusion proteins (ER-FosB, ER-DeltaFosB) with the ligand binding domain of human estrogen receptor were examined. The binding of estrogen to the fusion proteins resulted in their nuclear translocation and triggered cell proliferation, and thereafter delayed cell death was observed only in cells expressing ER-DeltaFosB. The proliferation of Rat1a cells, but not cell death triggered by ER-DeltaFosB, was completely abolished by butyrolactone I, an inhibitor of cycline-dependent kinases, and was partly suppressed by antisense oligonucleotides against galectin-1, whose expression is induced after estrogen administration. The cell death was accompanied by the activation of caspase-3 and -9, the fragmentation of the nuclear genome and cytochrome c release from the mitochondria, and was suppressed by zDEVD-fmk and zLEHD-fmk but not zIETD-fmk. The cell death was not suppressed by exogenous His-PTD-Bcl-x(L) at all, suggesting involvement of a Bcl-x(L)-resistant pathway for cytochrome c release.

Characterization of the genomic structure and expression of the mouse Apex2 gene

We isolated a mouse cDNA encoding APEX2 protein and demonstrated that APEX2 binds to PCNA. The level of Apex2 mRNA was high in the thymus, bone marrow, spleen, and kidney in adult mice. Apex2 consists of six exons and is flanked on the 3' end by Alas2 on X chromosome 63.0. Furthermore, Apex2 is flanked on the 5' end by a novel gene with a 106-bp intergenic sequence. We disrupted Apex2 in embryonic stem cells derived from a male mouse, and a 55-kDa APEX2 protein was detected in the nuclei of Apex2(+) but not Apex2-disrupted cells. Immunoelectron microscopy revealed that APEX2 is also localized in the mitochondria of Apex2(+) cells. In serum-stimulated BALB/c 3T3 cells, the level of Apex2 mRNA was transiently increased and the level of APEX2 reached a maximum in the late S phase, thus indicating that APEX2 may participate in postreplicative base excision repair.