Biology subjects

Masai, H.

Publications and source records attributed to Masai, H..

9 recordsLinked to original sources

bioRxiv2025

dif-XerCD is required for chromosome segregation during cSDR-dependent growth in E. coli

Escherichia coli cells can grow in a DnaA- and oriC-independent manner in rnhA-deficient cells that lack the major RNaseH activity. This mode of replication is named cSDR (constitutive Stable DNA Replication) and is believed to initiate from the sites of RNA-DNA hybrids, although the detailed mechanisms of cSDR are unclear. In this study, we discovered that dif sequence and XerC/D are essential for cSDR-dependent growth. cSDR was observed in either dif or xerC/D mutant cells as efficiently as in the control cells, as measured by the incorporation of [3H]-thymine. However, we found that these mutants accumulate cells with extra DNA contents in the presence of rifampicin and cephalexin, suggesting accumulation of unresolved dimeric or multimeric chromosomes or that of catenated chromosomes due to potential problems in resolution of the replicated chromosomes or in decatenation process. These results indicate that site-specific recombination mediated by dif-XerC/D is essential for cSDR-dependent growth.

molecular biology↗

bioRxiv2025

Phosphorylation of Claspin by elF2α kinase protects cells from heat stress

Various biological stresses can induce replication stress, potentially leading to cancer development. Replication checkpoint protects cells against the threat of DNA damages by suspending cell cycle, allowing DNA repair. Chk1 kinase is activated by various stresses and is required for replication checkpoint. An adaptor protein, Claspin, mediates Chk1 activation, but specific mechanisms involved are still unknown. We demonstrate that heat stress induces hyperphosphorylation of Claspin, resulting in Chk1 activation. Claspin phosphorylation is impaired in cells lacking two eIF2 kinases, GCN2 and HRI, which are parts of the integrated stress response (ISR) pathway. Mass spectrometry identified five heat-inducible phosphorylation sites in the C-terminal region of Claspin, and Claspin lacking these sites failed to activate checkpoint. Our data support a model that the unphosphorylated C-terminal tail of Claspin masks its CKBD (Chk1 Binding Domain)-AP (Acidic Patch) domain, preventing it from interacting with Cdc7. Phosphorylation of the tail causes it to dissociate from CKBD-AP and facilitates Cdc7 interaction, leading to increased Chk1 activation. Our findings demonstrate crosstalk between replication checkpoint and ISR pathways, and suggest potential new strategies for cancer therapy.

molecular biology↗

bioRxiv2024

Phosphorylation inhibits intramolecular interactions, DNA-binding and protein interactions of Claspin through disordered/ structured conformation transition

Claspin, known to be highly disordered, plays important roles in replication fork progression, initiation and cellular responses to replication stress. However, regulation of its structure and molecular interactions is not completely understood. We show here, through Proximity-Ligation-Assays, the evidence for intramolecular interaction between the N- and C-terminal segments of Claspin, which depends on the Acidic-Patch [AP] segment near its C-terminus. Interaction of Claspin with DNA and replication factors is highly stimulated in {Delta}AP mutant and by prior dephosphorylation. The wild-type Claspin inhibits the helicase activity of MCM in an AP-dependent manner. {Delta}AP and dephosphorylated Claspin exhibit resistance to trypsin digestion compared to wild-type, suggesting the presence of structural domains in the formers. We propose that Claspin is converted from disordered (closed) to structured (open) conformation at initiation, which stimulates its DNA binding and interaction with replication factors and counteracts its helicase inhibitory activity to trigger initiation of DNA replication.

molecular biology↗

bioRxiv2023

Homologous recombination contributes to the repair of acetaldehyde-induced DNA damages.

Acetaldehyde, a chemical that can cause DNA damage and contribute to cancer, is prevalently present in our environment, e.g., in alcohol, tobacco, and food. Although aldehyde potentially promotes crosslinking reaction among biological substances including DNA, RNA, and proteins, it remains unclear what types of DNA damage are caused by acetaldehyde and how they are repaired. In this study, we examined acetaldehyde sensitivity of DNA damage-deficient cells established from human TK6 cell line. Among the mutants, mismatch repair mutants did not show a hypersensitivity to acetaldehyde, while cells deficient in base and nucleotide excision repair pathways increased its sensitivity. We found a delayed repair and hypersensitivity in homologous recombination (HR)-deficient cells but not in non-homologous end joining-deficient cells after acetaldehyde treatment. By analyzing the formation of acetaldehyde-induced RAD51 foci, which represent HR intermediates, HR-deficient cells, but not NHEJ, exhibits delayed repair of acetaldehyde-induced DNA damages, compared with wild-type. These results suggest that acetaldehyde causes complex DNA damages that requires various types of repair pathways. Interestingly, mutants deficient in TDP1 and TDP2, which are involved in the removal of protein adducts from DNA ends, exhibited hypersensitivity to acetaldehyde. the acetaldehyde sensitivity of the TDP1-/-/RAD54-/- double mutant was similar to that of each single mutant. This epistatic relationship between TDP1 and RAD54 suggests that that the removal of protein-DNA adducts generated by acetaldehyde needs to be removed for efficient repair by HR. Our study would help understand the molecular mechanism of genotoxic and mutagenic effects of acetaldehyde.

molecular biology↗

bioRxiv2022

Claspin-dependent and -independent Chk1 activation by a panel of biological stresses

Replication stress has been suggested to be an ultimate trigger of carcinogenesis. Oncogenic signal, such as overexpression of CyclinE, has been shown to induce replication stress. Here, we show that various biological stresses, including heat, oxidative stress, osmotic stress, LPS, hypoxia, and arsenate induce activation of Chk1, a key effector kinase for replication checkpoint. Some of these stresses indeed reduce the fork rate, inhibiting DNA replication. Analyses of Chk1 activation in the cell population with western analyses showed that Chk1 activation by these stresses is largely dependent on Claspin. On the other hand, single cell analyses with Fucci cells indicated that while Chk1 activation during S phase is dependent on Claspin, that in G1 is mostly independent of Claspin. We propose that various biological stresses activate Chk1 either directly by stalling DNA replication fork or by some other mechanism that does not involve replication inhibition. The former pathway predominantly occurs in S phase and depends on Claspin, while the latter pathway, which may occur throughout the cell cycle, is largely independent of Claspin. Our findings provide evidence for novel links between replication stress checkpoint and other biological stresses and points to the presence of unknown mechanisms of Chk1 activation in mammalian cells.

molecular biology↗

bioRxiv2022

Cell fusion upregulates PD-L1 expression and promotes tumor formation

MSCs (mesenchymal stem cells), responsible for tissue repair, rarely undergo cell fusion with somatic cells. Here, we show that approximately 5% of bladder cancer cells (UMUC-3) fuses with bone marrow-derived MSC (BM-MSC) in co-culture and exhibits increased tumorigenicity. Eleven fusion cell clones are established, and 116 genes are identified whose expression is specifically altered in the fusion cells. Many of them are interferon-stimulated genes (ISG), but are activated in a manner independent of interferon. Among them, we show that PD-L1 is induced in fusion cells, and its knockout decreases tumorigenesis in a xenograft model. PD-L1 is induced in a manner independent of STAT1 known to regulate PD-L1 expression, but is regulated by histone modification, and is likely to inhibit phagocytosis by PD1-expressing macrophages, thus protecting cancer cells from immunological attacks. The fusion cells overexpress multiple cytokines including CCL2 that causes tumor progression by converting infiltrating macrophages to tumor-associated-macrophage (TAM). The results present mechanisms of how cell fusion promotes tumorigenesis, revealing a novel link between cell fusion and PD-L1, and underscores the efficacy of cancer immunotherapy.

cancer biology↗

bioRxiv2022

Aberrant association of chromatin with nuclear periphery induced by Rif1 leads to mitotic defect and cell death.

Chromatin is compartmentalized in nuclei and its architecture and nuclear location may have impacts on chromatin events. Rif1, identified as a potent suppressor of hsk1-null mutation (defective in initiation of DNA replication) in fission yeast, recognizes G-quadruplex structures and inhibits origin firing in their 50[~]100-kb vicinity, leading us to postulate that Rif1 may generate chromatin higher-order structures inhibitory for initiation. However, effects of Rif1 on chromatin localization in nuclei have not been known. We show here that overexpression of Rif1 causes growth inhibition and eventually cell death in fission yeast. Chromatin binding activities of Rif1, but not recruitment of phosphatase PP1, are required for growth inhibitory effect. Overexpression of a PP1 binding site mutant of Rif1 does not delay S-phase, but still causes cell death, indicating that cell death is caused not by S-phase problems but by issues in other phases of cell cycle, most likely M-phase. Indeed, Rif1 overexpression generates cells with unequally segregated chromosomes. Rif1 overexpression relocates chromatin near nuclear periphery in a manner dependent on its chromatin-binding ability, and this correlates with growth inhibition and cell death induction. Thus, regulated Rif1-mediated chromatin association with nuclear periphery is important for coordinated progression of S- and M-phases.

molecular biology↗

bioRxiv2022

Claspin is required for growth recovery from serum starvation through regulating the PI3K-PDK1-mTOR pathway

Growth recovery from serum starvation requires the activation of PI3 kinase (PI3K)-PDK1-Akt-mTOR pathways. Claspin plays multiple important roles in regulation of DNA replication as a mediator for the cellular response to replication stress, an integral replication fork factor that facilitates replication fork progression and a factor that promotes initiation by recruiting Cdc7 kinase. Here, we report a novel role of Claspin in growth recovery from serum starvation. In the absence of Claspin, cells do not proceed into S phase and eventually die. Claspin interacts with PI3K and mTOR, and is required for activation of PI3K-PDK1-mTOR and for that of mTOR downstream factors, p70S6K and 4E-BP1, but not for p38 MAPK cascade during the recovery from serum starvation. PDK1 interacts with Claspin, notably with CKBD, in a manner dependent on phosphorylation of the latter protein, and is required for interaction of mTOR with Claspin. p53 and ROS (Reactive Oxygen Species) inhibitors increased survival of Claspin-deficient cells released from serum starvation. Thus, Claspin plays a novel role as a mediator/protein platform for nutrition-induced proliferation/survival signaling by activating the mTOR pathway.

molecular biology↗

bioRxiv2020

Extracellular DJ-1 induces sterile inflammation in the ischemic brain

Inflammation is implicated in the onset and progression of various diseases, including cerebral pathologies. Here we report that DJ-1, which plays a role within cells as an antioxidant protein, functions as a damage-associated molecular pattern (DAMP), and triggers inflammation if released from dead cells into the extracellular space. We first found that recombinant DJ-1 protein induces the production of various inflammatory cytokines in bone marrow-derived macrophages (BMMs). We further identified a unique peptide sequence in the G and H helices of DJ-1 that activates Toll-like receptor 2 (TLR2) and TLR4. In the ischemic brain, DJ-1 is released into the extracellular space from necrotic neurons within 24 hours after stroke onset and makes direct contact with the surfaces of infiltrating myeloid cells. Administration of an antibody against DJ-1 suppresses the expression of inflammatory cytokines in infiltrating immune cells and attenuates ischemic neuronal damage. Our results demonstrate a previously unknown function of DJ-1 as a DAMP and suggest that extracellular DJ-1 could be a therapeutic target to prevent inflammation in tissue injuries and neurodegenerative diseases. Significance statementDJ-1 has been thoroughly investigated as a cytoprotective antioxidant protein in neurons. However, here we demonstrate that extracellularly released DJ-1 triggers neurotoxic inflammation after ischemic stroke. Intracellular DJ-1 increases in response to oxidative stress in ischemic neurons, but if ischemic stresses result in necrotic cell death, DJ-1 is released extracellularly. Released DJ-1 interacts with TLR2 and TLR4 on the surface of infiltrating myeloid cells and triggers post-ischemic inflammation, leading to the exacerbated pathologies of ischemic stroke. Thus, extracellular DJ-1 is a previously unknown inflammatogenic DAMP, and may be a putative target for therapeutic intervention to prevent progression of inflammatory and neurodegenerative diseases.

immunology↗