Human Endonuclease ANKLE1 Localizes at the Midbody and Processes Chromatin Bridges to Prevent DNA Damage and cGAS-STING Activation

Engineering Chromosome Bridges Through CRISPR/Cas9 to Decipher the Impact of Intercentromeric Distance on Resolution Dynamics

Resolution of chromosome bridges during mitosis is a critical yet incompletely understood process with implications for genomic stability and cancer development. In this study, we investigated the impact of the bridging chromatin length on the timing and mechanism of chromosome bridge resolution. Using CRISPR/Cas9 technology, we engineered chromosome bridges with precisely defined intercentromeric distances in human RPE-1 cells. Our study revealed a decline in the frequency of chromosome bridges as cells progressed from early anaphase to late telophase, indicating resolution during mitosis. Moreover, the longer the bridging chromatin length, the higher the frequency of chromosome bridges observed at the mitotic exit, demonstrating that the size of the bridge influences its resolution during mitosis. Additionally, the separation between the bridge kinetochores needed for bridge breakage was strongly dependent on the megabase length of the bridging chromatin, with longer chromosome bridges requiring greater separation for their resolution. Given that chromosome bridge resolution occurs in a concerted manner with spindle elongation and is influenced by the length of the bridging chromatin, we posit that the traction forces generated by microtubules attaching to dicentric chromosomes play a significant role in resolving chromosome bridges during mitosis. Our study underscores the intricate interplay between chromosome bridge geometry and mechanical forces in mitotic chromosome bridge resolution. Our model offers a valuable framework for future investigations into the molecular mechanisms underlying chromosome bridge resolution, with potential implications for cancer biology and genomic stability maintenance.

Mechanistic insight into anaphase bridge signaling to the abscission checkpoint

During cytokinesis in human cells, a failure to resolve persistent DNA bridges that span the cell-division plane maintains the Aurora B-dependent abscission checkpoint in an active state. However, the molecular mechanism by which unresolved sister-chromatid bridging signals to this checkpoint is poorly defined. Here, we define an essential role for the Bloom's syndrome helicase, BLM, in signaling to the abscission-checkpoint machinery in response to replication stress through the conversion of dsDNA bridges into RPA-coated ssDNA. RPA then promotes ATR-CHK1 signaling to Aurora B, utilizing a kinase cascade shared with the S-phase checkpoint. BLM-deficient cells ultimately abandon cytokinesis in response to replication stress, which promotes binucleation and hence aneuploidy. Considering that aneuploidy is a hallmark of cancer, we propose that this role for BLM in cytokinesis is a plausible reason for cancer predisposition in Bloom's syndrome individuals. Consistent with this, BLM deficiency promotes anchorage-independent growth of non-cancer cells.

Verteporfin-Mediated In Situ Nanovaccine Based on Local Conventional-Dose Hypofractionated Radiotherapy Enhances Antitumor and Immunomodulatory Effect

In situ radiotherapy is the most successful cytotoxic therapy available for the treatment of solid tumors, while high-dose radiotherapy per fraction is not yet widely and reliably used. To some extent, the major considerations of the disappointing results are on the risk of high-dose irradiation-induced damage to the surrounding normal tissues and the difficulty in distant metastasis control. To break these restraints, a gelatinase-responsive amphiphilic methoxypolyethyleneglycol-PVGLIG-polycaprolactone (mPEG-PVGLIG-PCL) nanoparticles' loading verteporfin (N@VP), a special photosensitizer that can also be excited by X-rays to produce cytotoxic singlet oxygen and greatly enhance radiotherapy efficacy, is prepared in this study. Herein, it is shown that the formed N@VP combined with conventional-dose radiation therapy (RT, 2 Gy (gray, a radiation dose unit)) can realize an antitumor effect no less than high-dose RT (8 Gy) and minimize radiation dose necessary to achieve local tumor control. Moreover, this radiosensitive nanosystem can exert excellent systemic antitumor immunity and abscopal effect, providing a preferable "in situ vaccine" strategy based on conventional-dose RT to achieve efficient systemic management of distant tumor metastasis. When combined with immunotherapy, this novel strategy for radiosensitization results in better immunotherapy sensitivity by stimulating significant immunogenic tumor cell death and synergistic antitumor immune responses.

Functional dissection of the conserved C. elegans LEM-3/ANKLE1 nuclease reveals a crucial requirement for the LEM-like and GIY-YIG domains for DNA bridge processing

Faithful chromosome segregation requires the removal of all DNA bridges physically linking chromatids before the completion of cell division. While several redundant safeguard mechanisms to process these DNA bridges exist from S-phase to late anaphase, the conserved LEM-3/ANKLE1 nuclease has been proposed to be part of a 'last chance' mechanism that acts at the midbody to eliminate DNA bridges that persist until late cytokinesis. We show that LEM-3 can cleave a wide range of branched DNA substrates, including flaps, forks, nicked, and intact Holliday junctions. AlphaFold modelling data suggest that the catalytic mechanism of LEM-3/ANKLE1 is conserved, mirroring the mechanism observed in bacterial GIY-YIG nucleases. We present evidence that LEM-3 may form a homodimeric complex on the Holliday junction DNA. LEM-3 LEM-like and GIY-YIG nuclease domains are essential for LEM-3 recruitment to the midbody and its nuclease activity, while its LEM-like domain is sufficient for DNA binding. Finally, we show that preventing LEM-3 nuclear access is important to avoid toxicity, likely caused by branched DNAs cleavage during normal DNA metabolism. Our data suggest that Caenorhabditis elegans LEM-3 acts as a 'last chance catch-all' enzyme that processes DNA bridges caused by various perturbations of DNA metabolism just before cells divide.

Deficiency of ATF3 facilitates both angiotensin II-induced and spontaneously formed aortic aneurysm and dissection development by activating cGAS-STING pathway

BACKGROUND

Sporadic aortic aneurysm and dissection (AAD) is a critical condition characterised by the progressive loss of vascular smooth muscle cells (VSMCs) and the breakdown of the extracellular matrix. However, the molecular mechanisms responsible for the phenotypic switch and loss of VSMCs in AAD are not fully understood.

METHODS AND RESULTS

In this study, we employed a discovery-driven, unbiased approach. This approach encourages us to explore the unknown functions of activating transcription factor 3 (ATF3) rather than merely confirming existing hypotheses, while no assumptions were made about ATF3 prior to the experiments. We ensured the unbiased nature of our assessment by conducting morphological evaluations with two independent observers in a blinded manner. We identified elevated expression of ATF3 in both human sporadic AAD tissues and mouse AAD models. VSMC-specific ATF3 conditional knockout (Atf3 cKO) mice showed notable enlargement, dissection and rupture in both thoracic and abdominal aortic regions after exposure to Ang II. Interestingly, older Atf3 cKO mice exhibited spontaneous aortic dissections and senescence of the aortic wall. Mechanistically, ATF3 deficiency led to the degradation of P21 through ubiquitination. Impaired DNA repair in VSMCs resulted in micronuclei formation in the cytoplasm, activating the  cyclicGMP-AMP synthase- stimulator of interferon genes (cGAS-STING) pathway and inducing VSMC phenotypic switching and apoptosis. Finally, both pharmacological complementation of P21 function and knockdown of STING expression alleviated ATF3 deficiency-induced AAD.

CONCLUSIONS

Our study indicates that ATF3 is essential for genomic DNA stability in VSMCs through the P21-cGAS-STING pathway, suggesting that enhancing ATF3 expression in VSMCs could help prevent sporadic AAD.

KEY POINTS

ATF3 deficiency led to degradation of P21 through ubiquitination, which abolished the G1 phase arrest. VSMCs had no time window to repair the damaged DNA, leading to generation of micronuclei in cytoplasm. Cytoplasmic micronuclei facilitating the activation of cGAS-STING pathway, thus inducing the phenotypic switch and apoptosis of VSMCs.

Emerin mislocalization during chromatin bridge resolution can drive prostate cancer cell invasiveness in a collagen-rich microenvironment

Micronuclei (MN) can form through many mechanisms, including the breakage of aberrant cytokinetic chromatin bridges. The frequent observation of MN in tumors suggests that they might not merely be passive elements but could instead play active roles in tumor progression. Here, we propose a mechanism through which the presence of micronuclei could induce specific phenotypic and functional changes in cells and increase the invasive potential of cancer cells. Through the integration of diverse in vitro imaging and molecular techniques supported by clinical samples from patients with prostate cancer (PCa) defined as high-risk by the D'Amico classification, we demonstrate that the resolution of chromosome bridges can result in the accumulation of Emerin and the formation of Emerin-rich MN. These structures are negative for Lamin A/C and positive for the Lamin-B receptor and Sec61β. MN can act as a protein sinks and result in the pauperization of Emerin from the nuclear envelope. The Emerin mislocalization phenotype is associated with a molecular signature that is correlated with a poor prognosis in PCa patients and is enriched in metastatic samples. Emerin mislocalization corresponds with increases in the migratory and invasive potential of tumor cells, especially in a collagen-rich microenvironment. Our study demonstrates that the mislocalization of Emerin to MN results in increased cell invasiveness, thereby worsening patient prognosis.

CRISPR/Pepper-tDeg: A Live Imaging System Enables Non-Repetitive Genomic Locus Analysis with One Single-Guide RNA

CRISPR-based genomic-imaging systems have been utilized for spatiotemporal imaging of the repetitive genomic loci in living cells, but they are still challenged by limited signal-to-noise ratio (SNR) at a non-repetitive genomic locus. Here, an efficient genomic-imaging system is proposed, termed CRISPR/Pepper-tDeg, by engineering the CRISPR sgRNA scaffolds with the degron-binding Pepper aptamers for binding fluorogenic proteins fused with Tat peptide derived degron domain (tDeg). The target-dependent stability switches of both sgRNA and fluorogenic protein allow this system to image repetitive telomeres sensitively with a 5-fold higher SNR than conventional CRISPR/MS2-MCP system using "always-on" fluorescent protein tag. Subsequently, CRISPR/Pepper-tDeg is applied to simultaneously label and track two different genomic loci, telomeres and centromeres, in living cells by combining two systems. Given a further improved SNR by the split fluorescent protein design, CRISPR/Pepper-tDeg system is extended to non-repetitive sequence imaging using only one sgRNA with two aptamer insertions. Neither complex sgRNA design nor difficult plasmid construction is required, greatly reducing the technical barriers to define spatiotemporal organization and dynamics of both repetitive and non-repetitive genomic loci in living cells, and thus demonstrating the large application potential of this genomic-imaging system in biological research, clinical diagnosis and therapy.

cGAS: action in the nucleus

As a canonical cytoplasmic DNA sensor, cyclic GMP-AMP synthase (cGAS) plays a key role in innate immunity. In recent years, a growing number of studies have shown that cGAS can also be located in the nucleus and plays new functions such as regulating DNA damage repair, nuclear membrane repair, chromosome fusion, DNA replication, angiogenesis and other non-canonical functions. Meanwhile, the mechanisms underlying the nucleo-cytoplasmic transport and the regulation of cGAS activation have been revealed in recent years. Based on the current understanding of the structure, subcellular localization and canonical functions of cGAS, this review focuses on summarizing the mechanisms underlying nucleo-cytoplasmic transport, activity regulation and non-canonical functions of cGAS in the nucleus. We aim to provide insights into exploring the new functions of cGAS in the nucleus and advance its clinical translation.

The cytosolic DNA-sensing cGAS-STING pathway in neurodegenerative diseases

BACKGROUND

With the widespread prevalence of neurodegenerative diseases (NDs) and high rates of mortality and disability, it is imminent to find accurate targets for intervention. There is growing evidence that neuroimmunity is pivotal in the pathology of NDs and that interventions targeting neuroimmunity hold great promise. Exogenous or dislocated nucleic acids activate the cytosolic DNA sensor cyclic GMP-AMP synthase (cGAS), activating the stimulator of interferon genes (STING). The activated STING triggers innate immune responses and then the cGAS-STING signaling pathway links abnormal nucleic acid sensing to the immune response. Recently, numerous studies have shown that neuroinflammation regulated by cGAS-STING signaling plays an essential role in NDs.

AIMS

In this review, we summarized the mechanism of cGAS-STING signaling in NDs and focused on inhibitors targeting cGAS-STING.

CONCLUSION

The cGAS-STING signaling plays an important role in the pathogenesis of NDs. Inhibiting the cGAS-STING signaling may provide new measures in the treatment of NDs.

Scrambling the genome in cancer: causes and consequences of complex chromosome rearrangements

Complex chromosome rearrangements, known as chromoanagenesis, are widespread in cancer. Based on large-scale DNA sequencing of human tumours, the most frequent type of complex chromosome rearrangement is chromothripsis, a massive, localized and clustered rearrangement of one (or a few) chromosomes seemingly acquired in a single event. Chromothripsis can be initiated by mitotic errors that produce a micronucleus encapsulating a single chromosome or chromosomal fragment. Rupture of the unstable micronuclear envelope exposes its chromatin to cytosolic nucleases and induces chromothriptic shattering. Found in up to half of tumours included in pan-cancer genomic analyses, chromothriptic rearrangements can contribute to tumorigenesis through inactivation of tumour suppressor genes, activation of proto-oncogenes, or gene amplification through the production of self-propagating extrachromosomal circular DNAs encoding oncogenes or genes conferring anticancer drug resistance. Here, we discuss what has been learned about the mechanisms that enable these complex genomic rearrangements and their consequences in cancer.

A benzochalcone derivative synchronously induces apoptosis and ferroptosis in pancreatic cancer cells

Background

Pancreatic cancer is a highly aggressive and lethal disease with limited treatment options. In this study, we investigated the potential therapeutic effects of compound KL-6 on pancreatic cancer cells.

Methods

The study involved assessing the inhibitory effects of KL-6 on cell proliferation, clonogenic potential, cell cycle progression, apoptosis, migration, and invasion. Additionally, we examined the action mechanism of KL-6 by RNA-seq and bioinformatic analysis and validated by qRT-PCR and western blot in pancreatic cancer cells.

Results

Our results demonstrated that KL-6 effectively inhibited the growth of pancreatic cancer cells in a dose-dependent manner. It induced G2/M phase cell cycle arrest and apoptosis, disrupting the cell cycle progression and promoting cell death. KL-6 also exhibited inhibitory effects on cell migration and invasion, suggesting its potential to suppress the metastatic properties of pancreatic cancer cells. Furthermore, KL-6 modulated the expression of genes involved in various cancer-related pathways including apoptosis and ferroptosis.

Conclusion

These findings collectively support the potential of KL-6 as a promising therapeutic option for pancreatic cancer treatment. Further research is needed to fully understand the underlying mechanisms and evaluate the clinical efficacy of KL-6 in pancreatic cancer patients.

The Calpain-7 protease functions together with the ESCRT-III protein IST1 within the midbody to regulate the timing and completion of abscission

The Endosomal Sorting Complexes Required for Transport (ESCRT) machinery mediates the membrane fission step that completes cytokinetic abscission and separates dividing cells. Filaments composed of ESCRT-III subunits constrict membranes of the intercellular bridge midbody to the abscission point. These filaments also bind and recruit cofactors whose activities help execute abscission and/or delay abscission timing in response to mitotic errors via the NoCut/Abscission checkpoint. We previously showed that the ESCRT-III subunit IST1 binds the cysteine protease Calpain-7 (CAPN7) and that CAPN7 is required for both efficient abscission and NoCut checkpoint maintenance (Wenzel et al., 2022). Here, we report biochemical and crystallographic studies showing that the tandem microtubule-interacting and trafficking (MIT) domains of CAPN7 bind simultaneously to two distinct IST1 MIT interaction motifs. Structure-guided point mutations in either CAPN7 MIT domain disrupted IST1 binding in vitro and in cells, and depletion/rescue experiments showed that the CAPN7-IST1 interaction is required for (1) CAPN7 recruitment to midbodies, (2) efficient abscission, and (3) NoCut checkpoint arrest. CAPN7 proteolytic activity is also required for abscission and checkpoint maintenance. Hence, IST1 recruits CAPN7 to midbodies, where its proteolytic activity is required to regulate and complete abscission.