Endonuclease G: a mitochondrial protein released in apoptosis and involved in caspase-independent DNA degradation

Sodium-glucose cotransporter 2 inhibitor partially improves brain mitochondrial function, but does not mitigate cognitive impairment in rats with myocardial infarction

Sodium-glucose co-transporter 2 inhibitors (SGLT2i) are recommended to treat heart failure (HF) with the benefit of mitigating cognitive impairment in HF patients with type 2 diabetes. However, the underlying mechanisms are still unclear. This study aimed to investigate whether SGLT2i can improve cognitive function and synaptic plasticity in rats with myocardial infarction (MI) through attenuating inflammation, oxidative stress, impaired metabolism, and mitochondrial dysfunction in the brain. Male rats (n = 8/group) underwent either a sham operation or MI induced by permanent ligation of the left anterior descending coronary artery. MI rats with ejection fraction <50 % were divided into three groups to receive either a vehicle, SGLT2i (Dapagliflozin, 1 mg/kg), or angiotensin-converting enzyme inhibitor (Enalapril, 10 mg/kg, positive control) for four weeks. Cardiac function, cognitive function, synaptic plasticity, dendritic spine density, and brain biochemical changes were assessed at the end of the protocol. MI rats exhibited cardiac dysfunction, hippocampal-dependent cognitive impairment, impaired synaptic plasticity, and loss of dendritic spines. Brain oxidative stress, inflammation, and mitochondrial dysfunction were also observed in MI rats. Treatment with SGLT2i and ACEi improved cardiac function but failed to attenuate cognitive impairment, synaptic dysplasticity, and loss of dendritic spine density in MI rats. A decrease in brain glutamate level was found following MI, which can be restored by SGLT2i and ACEi. Only SGLT2i partially improved brain mitochondrial function. In summary, SGLT2i enhanced glutamate levels and partially improved mitochondrial function in the brain; however, these changes were insufficient to improve cognitive function in MI rats.

Mitochondrial DNA released by senescent tumor cells enhances PMN-MDSC-driven immunosuppression through the cGAS-STING pathway

Mitochondrial dysfunction is a hallmark of cellular senescence. Here, we investigated whether senescent cells release mitochondrial (mt)DNA into the extracellular space and its impact on innate immunity. We found that both primary senescent cells and tumor cells undergoing therapy-induced senescence actively released mtDNA into the extracellular environment. mtDNA released by senescent cells was packaged within extracellular vesicles and selectively transferred to polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) in the tumor microenvironment. Upon uptake, extracellular mtDNA enhanced the immunosuppressive activity of PMN-MDSCs via cGAS-STING-NF-κB signaling, thereby promoting tumor progression. While STING activation directly induced NF-κB signaling, it also activated PKR-like endoplasmic reticulum kinase (PERK), which further amplified NF-κB activity, in PMN-MDSCs. mtDNA release from senescent cells was mediated by voltage-dependent anion channels (VDACs), and pharmacological inhibition of VDAC reduced extracellular mtDNA levels, reversed PMN-MDSC-driven immunosuppression, and enhanced chemotherapy efficacy in prostate cancer mouse models. These findings suggest that targeting mtDNA release could reprogram the immunosuppressive tumor microenvironment, improving therapeutic outcomes for chemotherapy-treated patients.

The multifaceted role of mitochondria in autism spectrum disorder

Normal brain functioning relies on high aerobic energy production provided by mitochondria. Failure to supply a sufficient amount of energy, seen in different brain disorders, including autism spectrum disorder (ASD), may have a significant negative impact on brain development and support of different brain functions. Mitochondrial dysfunction, manifested in the abnormal activities of the electron transport chain and impaired energy metabolism, greatly contributes to ASD. The aberrant functioning of this organelle is of such high importance that ASD has been proposed as a mitochondrial disease. It should be noted that aerobic energy production is not the only function of the mitochondria. In particular, these organelles are involved in the regulation of Ca2+ homeostasis, different mechanisms of programmed cell death, autophagy, and reactive oxygen and nitrogen species (ROS and RNS) production. Several syndromes originated from mitochondria-related mutations display ASD phenotype. Abnormalities in Ca2+ handling and ATP production in the brain mitochondria affect synaptic transmission, plasticity, and synaptic development, contributing to ASD. ROS and Ca2+ regulate the activity of the mitochondrial permeability transition pore (mPTP). The prolonged opening of this pore affects the redox state of the mitochondria, impairs oxidative phosphorylation, and activates apoptosis, ultimately leading to cell death. A dysregulation between the enhanced mitochondria-related processes of apoptosis and the inhibited autophagy leads to the accumulation of toxic products in the brains of individuals with ASD. Although many mitochondria-related mechanisms still have to be investigated, and whether they are the cause or consequence of this disorder is still unknown, the accumulating data show that the breakdown of any of the mitochondrial functions may contribute to abnormal brain development leading to ASD. In this review, we discuss the multifaceted role of mitochondria in ASD from the various aspects of neuroscience.

Mechanistic study of leukopenia treatment by Qijiao shengbai Capsule via the Bcl2/Bax/CASAPSE3 pathway

Background

Leukopenia can be caused by chemotherapy, which suppresses bone marrow function and can impact the effectiveness of cancer treatment. Qijiao Shengbai Capsule (QJSB) is commonly used to treat leukopenia, but the specific bioactive components and mechanisms of action are not well understood.

Objectives and results

This study aimed to analyze the active ingredients of QJSB and its potential targets for treating leukopenia using network pharmacology and molecular docking. Through a combination of serum pharmacochemistry, multi-omics, network pharmacology, and validation experiments in a murine leukopenia model, the researchers sought to understand how QJSB improves leukopenia. The study identified 16 key components of QJSB that act in vivo to increase the number of white blood cells in leukopenic mice. Multi-omics analysis and network pharmacology revealed that the PI3K-Akt and MAPK signaling pathways are important in the treatment of leukopenia with QJSB. Five specific targets (JUN, FOS, BCl-2, CASPAS-3) were identified as key targets.

Conclusion

Validation experiments confirmed that QJSB regulates genes related to cell growth and inhibits apoptosis, suggesting that apoptosis may play a crucial role in leukopenia development and that QJSB may improve immune function by regulating apoptotic proteins and increasing CD4+ T cell count in leukopenic mice.

Impact of combined intermittent fasting and high-intensity interval training on apoptosis and atrophy signaling in rat fast- and slow-twitch muscles

This study aimed to evaluate the influence of combined intermittent fasting (IF) and high-intensity interval training (HIIT) on morphology, caspase-independent apoptosis signaling pathway, and myostatin expression in soleus and gastrocnemius (white portion) muscles from healthy rats. Sixty-day-old male Wistar rats (n = 60) were divided into four groups: control (C), IF, high-intensity-interval training (T), and high-intensity-interval training and intermittent fasting (T-IF). The C and T groups received ad libitum chow daily; IF and T-IF received the same standard chow every other day. Animals from T and T-IF underwent a HIIT protocol five times a week for 12 weeks. IF reduced gastrocnemius mass and increased pro-apoptotic proteins apoptosis-inducing factor (AIF) and endonuclease G (EndoG) in soleus and cleaved-to-non-cleaved PARP-1 ratio and myostatin expression in gastrocnemius white portion. HIIT increased AIF and apoptosis repressor with caspase recruitment domain expression in soleus and cleaved-to-total PARP-1 ratio in gastrocnemius muscle white portion. The combination of IF and HIIT reduced fiber cross-sectional area in both muscles, increased EndoG and AIF expression, and decreased cleaved-to-non-cleaved PARP-1 ratio in gastrocnemius muscle white portion. Muscle responses to IF and HIIT are directly impacted by the muscle fiber type composition and are modulated, at least in part, by myostatin and caspase-independent apoptosis signaling.

Tumor Cell-Derived Exosomal Hybrid Nanosystems Loaded with Rhubarbic Acid and Tanshinone IIA for Sepsis Treatment

Background

Sepsis continues to exert a significant impact on morbidity and mortality in clinical settings, with immunosuppression, multi-organ failure, and disruptions in gut microbiota being key features. Although rheinic acid and tanshinone IIA show promise in mitigating macrophage apoptosis in sepsis treatment, their precise targeting of macrophages remains limited. Additionally, the evaluation of intestinal flora changes following treatment, which plays a significant role in subsequent cytokine storms, has been overlooked. Leveraging the innate inflammation chemotaxis of tumor cell-derived exosomes allows for their rapid recognition and uptake by activated macrophages, facilitating phenotypic changes and harnessing anti-inflammatory effects.

Methods

We extracted exosomes from H1299 cells using a precipitation method. Then we developed a tumor cell-derived exosomal hybrid nanosystem loaded with rhubarbic acid and tanshinone IIA (R+T/Lipo/EXO) for sepsis treatment. In vitro studies, we verify the anti-inflammatory effect and the mechanism of inhibiting cell apoptosis of nano drug delivery system. The anti-inflammatory effects, safety, and modulation of intestinal microbiota by the nanoformulations were further validated in the in vivo study.

Results

Nanoformulation demonstrated enhanced macrophage internalization, reduced TNF-α expression, inhibited apoptosis, modulated intestinal flora, and alleviated immunosuppression.

Conclusion

R+T/Lipo/EXO presents a promising approach using exosomal hybrid nanosystems for treating sepsis.

Selected Functions and Disorders of Mitochondrial Metabolism under Lead Exposure

Mitochondria play a fundamental role in the energy metabolism of eukaryotic cells. Numerous studies indicate lead (Pb) as a widely occurring environmental factor capable of disrupting oxidative metabolism by modulating the mitochondrial processes. The multitude of known molecular targets of Pb and its strong affinity for biochemical pathways involving divalent metals suggest that it may pose a health threat at any given dose. Changes in the bioenergetics of cells exposed to Pb have been repeatedly demonstrated in research, primarily showing a reduced ability to synthesize ATP. In addition, lead interferes with mitochondrial-mediated processes essential for maintaining homeostasis, such as apoptosis, mitophagy, mitochondrial dynamics, and the inflammatory response. This article describes selected aspects of mitochondrial metabolism in relation to potential mechanisms of energy metabolism disorders induced by Pb.

CHCHD10 mutations induce tissue-specific mitochondrial DNA deletions with a distinct signature

Mutations affecting the mitochondrial intermembrane space protein CHCHD10 cause human disease, but it is not known why different amino acid substitutions cause markedly different clinical phenotypes, including amyotrophic lateral sclerosis-frontotemporal dementia, spinal muscular atrophy Jokela-type, isolated autosomal dominant mitochondrial myopathy and cardiomyopathy. CHCHD10 mutations have been associated with deletions of mitochondrial DNA (mtDNA deletions), raising the possibility that these explain the clinical variability. Here, we sequenced mtDNA obtained from hearts, skeletal muscle, livers and spinal cords of WT and Chchd10 G58R or S59L knockin mice to characterise the mtDNA deletion signatures of the two mutant lines. We found that the deletion levels were higher in G58R and S59L mice than in WT mice in some tissues depending on the Chchd10 genotype, and the deletion burden increased with age. Furthermore, we observed that the spinal cord was less prone to the development of mtDNA deletions than the other tissues examined. Finally, in addition to accelerating the rate of naturally occurring deletions, Chchd10 mutations also led to the accumulation of a novel set of deletions characterised by shorter direct repeats flanking the deletion breakpoints. Our results indicate that Chchd10 mutations in mice induce tissue-specific deletions which may also contribute to the clinical phenotype associated with these mutations in humans.

Vibrio parahaemolyticus thermostable direct haemolysin induces non-classical programmed cell death despite caspase activation

Thermostable direct haemolysin (TDH) is the key virulence factor secreted by the human gastroenteric bacterial pathogen Vibrio parahaemolyticus. TDH is a membrane-damaging pore-forming toxin. It evokes potent cytotoxicity, the mechanism of which still remains under-explored. Here, we have elucidated the mechanistic details of cell death response elicited by TDH. Employing Caco-2 intestinal epithelial cells and THP-1 monocytic cells, we show that TDH induces some of the hallmark features of apoptosis-like programmed cell death. TDH triggers caspase-3 and 7 activations in the THP-1 cells, while caspase-7 activation is observed in the Caco-2 cells. Interestingly, TDH appears to induce caspase-independent cell death. Higher XIAP level and lower Smac/Diablo level upon TDH intoxication provide plausible explanation for the functional inability of caspases in the THP-1 cells, in particular. Further exploration reveals that mitochondria play a central role in the TDH-induced cell death. TDH triggers mitochondrial damage, resulting in the release of AIF and endonuclease G, responsible for the execution of caspase-independent cell death. Among the other critical mediators of cell death, ROS is found to play an important role in the THP-1 cells, while PARP-1 appears to play a critical role in the Caco-2 cells. Altogether, our work provides critical new insights into the mechanism of cell death induction by TDH, showing a common central theme of non-classical programmed cell death. Our study also unravels the interplay of crucial molecules in the underlying signalling processes. Our findings add valuable insights into the role of TDH in the context of the host-pathogen interaction processes.

Molecular Mechanisms for the Regulation of Nuclear Membrane Integrity

The nuclear membrane serves a critical role in protecting the contents of the nucleus and facilitating material and signal exchange between the nucleus and cytoplasm. While extensive research has been dedicated to topics such as nuclear membrane assembly and disassembly during cell division, as well as interactions between nuclear transmembrane proteins and both nucleoskeletal and cytoskeletal components, there has been comparatively less emphasis on exploring the regulation of nuclear morphology through nuclear membrane integrity. In particular, the role of type II integral proteins, which also function as transcription factors, within the nuclear membrane remains an area of research that is yet to be fully explored. The integrity of the nuclear membrane is pivotal not only during cell division but also in the regulation of gene expression and the communication between the nucleus and cytoplasm. Importantly, it plays a significant role in the development of various diseases. This review paper seeks to illuminate the biomolecules responsible for maintaining the integrity of the nuclear membrane. It will delve into the mechanisms that influence nuclear membrane integrity and provide insights into the role of type II membrane protein transcription factors in this context. Understanding these aspects is of utmost importance, as it can offer valuable insights into the intricate processes governing nuclear membrane integrity. Such insights have broad-reaching implications for cellular function and our understanding of disease pathogenesis.

Mitochondrial Factors in the Cell Nucleus

The origin of eukaryotic organisms involved the integration of mitochondria into the ancestor cell, with a massive gene transfer from the original proteobacterium to the host nucleus. Thus, mitochondrial performance relies on a mosaic of nuclear gene products from a variety of genomes. The concerted regulation of their synthesis is necessary for metabolic housekeeping and stress response. This governance involves crosstalk between mitochondrial, cytoplasmic, and nuclear factors. While anterograde and retrograde regulation preserve mitochondrial homeostasis, the mitochondria can modulate a wide set of nuclear genes in response to an extensive variety of conditions, whose response mechanisms often merge. In this review, we summarise how mitochondrial metabolites and proteins-encoded either in the nucleus or in the organelle-target the cell nucleus and exert different actions modulating gene expression and the chromatin state, or even causing DNA fragmentation in response to common stress conditions, such as hypoxia, oxidative stress, unfolded protein stress, and DNA damage.

N-terminal proteoforms may engage in different protein complexes

Alternative translation initiation and alternative splicing may give rise to N-terminal proteoforms, proteins that differ at their N-terminus compared with their canonical counterparts. Such proteoforms can have altered localizations, stabilities, and functions. Although proteoforms generated from splice variants can be engaged in different protein complexes, it remained to be studied to what extent this applies to N-terminal proteoforms. To address this, we mapped the interactomes of several pairs of N-terminal proteoforms and their canonical counterparts. First, we generated a catalogue of N-terminal proteoforms found in the HEK293T cellular cytosol from which 22 pairs were selected for interactome profiling. In addition, we provide evidence for the expression of several N-terminal proteoforms, identified in our catalogue, across different human tissues, as well as tissue-specific expression, highlighting their biological relevance. Protein-protein interaction profiling revealed that the overlap of the interactomes for both proteoforms is generally high, showing their functional relation. We also showed that N-terminal proteoforms can be engaged in new interactions and/or lose several interactions compared with their canonical counterparts, thus further expanding the functional diversity of proteomes.

Ginger polysaccharide promotes myeloid-derived suppressor cell apoptosis by regulating lipid metabolism

Recently, targeting myeloid-derived suppressor cells (MDSCs) which mainly play an immunosuppressive role in tumor microenvironment has become a hot spot in tumor immunotherapy. This study focuses on biological effect of ginger polysaccharide extracted from natural plants on promoting apoptosis of MDSCs by regulating lipid metabolism. An MTT assay was used to detect the inhibitory effect of ginger polysaccharide on the growth of an MDSC-like cell line (MSC-2). The apoptosis-promoting effect of ginger polysaccharide on MSC-2 cells was detected by flow cytometry. Expression levels of apoptosis proteins (caspase 9 and Bcl-2) and lipid metabolism enzymes (fatty acid synthase (FASN) and diacylglycerol acyltransferase 2) in MSC-2 cells treated with different concentrations of ginger polysaccharide were detected by western blot assay. Nile red staining was used to quantitatively detect the effect of ginger polysaccharide on lipid droplet synthesis. Ginger polysaccharide inhibited proliferation of MSC-2 cells and promoted their apoptosis by upregulating pro-apoptotic caspase 9 protein, downregulating anti-apoptotic Bcl-2 protein, inhibiting expression of FASN and diacylglycerol acyltransferase 2 (key enzymes in fatty acid synthesis and lipid droplet formation, respectively). Ginger polysaccharide promoted apoptosis of MDSCs by regulating key lipid metabolism enzymes, inhibiting fatty acid synthesis and lipid droplet accumulation, and reducing the energy supply of cells.

Toxicological mechanism of ammonia-N on haematopoiesis and apoptosis of haemocytes in Litopenaeus vannamei

Ammonia, as an important pollutant, contributed to the reduction of immunity, disruption of physiology in animals. RNA interference (RNAi) was performed to understand the function of astakine (AST) in haematopoiesis and apoptosis in Litopenaeus vannamei under ammonia-N exposure. Shrimps were exposed to 20 mg/L ammonia-N from 0 to 48 h with injection of 20 μg AST dsRNA. Further, shrimps were exposed to 0, 2, 10 and 20 mg/L ammonia-N also from 0 to 48 h. The results showed that the total haemocytes count (THC) decreased under ammonia-N stress and the knockdown of AST resulted in a further decrease of THC, suggesting that 1) the proliferation was decreased through the reduction of AST and Hedgehog, the differentiation was interfered by Wnt4, Wnt5 and Notch, and the migration was inhibited by the decrease of VEGF; 2) oxidative stress was induced under ammonia-N stress, leading to the increase of DNA damage with the up-regulated gene expression of death receptor, mitochondrial and endoplasmic reticulum stress pathways; 3) the changes of THC resulted from the decrease of proliferation, differentiation and migration of haematopoiesis cells and the increase of apoptosis of haemocytes. This study helps to deepen our understanding of risk management in shrimp aquaculture.

The DNA damage potential of quantum dots: Toxicity, mechanism and challenge

Quantum dots (QDs) are semiconductor nanoparticles (1-10 nm) with excellent optical and electrical properties. As QDs show great promise for applications in fields such as biomedicine, their biosafety is widely emphasized. Therefore, studies on the potential 'nanotoxicity' of QDs in genetic material are warranted. This review summarizes and discusses recent reports derived from different cell lines or animal models concerning the effects of QDs on genetic material. QDs could induce many types of genetic material damage, which subsequently triggers a series of cellular adverse outcomes, including apoptosis, cell cycle arrest and senescence. However, the individual biological and ecological significance of the genotoxicity of QDs is not yet clear. In terms of mechanisms of genotoxicity, QDs can damage DNA either through their own nanomorphology or through the released metal ions. It also includes the reactive oxygen species generation, inflammation and failure of DNA damage repair. Notably, apoptosis may lead to false positive results in genotoxicity tests. Finally, given the different uses of QDs and the interference of the physicochemical properties of QDs on the test method, genotoxicity testing of QDs should be different from traditional toxic compounds, which requires further research.

Unleashing a novel function of Endonuclease G in mitochondrial genome instability

Having its genome makes the mitochondrion a unique and semiautonomous organelle within cells. Mammalian mitochondrial DNA (mtDNA) is a double-stranded closed circular molecule of about 16 kb coding for 37 genes. Mutations, including deletions in the mitochondrial genome, can culminate in different human diseases. Mapping the deletion junctions suggests that the breakpoints are generally seen at hotspots. '9 bp deletion' (8271-8281), seen in the intergenic region of cytochrome c oxidase II/tRNALys, is the most common mitochondrial deletion. While it is associated with several diseases like myopathy, dystonia, and hepatocellular carcinoma, it has also been used as an evolutionary marker. However, the mechanism responsible for its fragility is unclear. In the current study, we show that Endonuclease G, a mitochondrial nuclease responsible for nonspecific cleavage of nuclear DNA during apoptosis, can induce breaks at sequences associated with '9 bp deletion' when it is present on a plasmid or in the mitochondrial genome. Through a series of in vitro and intracellular studies, we show that Endonuclease G binds to G-quadruplex structures formed at the hotspot and induces DNA breaks. Therefore, we uncover a new role for Endonuclease G in generating mtDNA deletions, which depends on the formation of G4 DNA within the mitochondrial genome. In summary, we identify a novel property of Endonuclease G, besides its role in apoptosis and the recently described 'elimination of paternal mitochondria during fertilisation.

Shrimp miR-965 transfers tumoricidal mitochondria

Background

Micro RNA of Marsupenaeus japonicas has been known to promote apoptosis of tumor cells. However, the detailed mechanisms are not well understood.

Results

Using tomographic microscope, which can detect the internal structure of cells, we observed breast tumor cells following treatment of the miRNA. Intriguingly, we found that mitochondria migrate to an adjacent tumor cells through a tunneling nanotube. To recapitulate this process, we engineered a microfluidic device through which mitochondria were transferred. We show that this mitochondrial transfer process released endonuclease G (Endo G) into tumor cells, which we referred to herein as unsealed mitochondria. Importantly, Endo G depleted mitochondria alone did not have tumoricidal effects. Moreover, unsealed mitochondria had synergistic apoptotic effects with subtoxic dose of doxorubicin thereby mitigating cardiotoxicity.

Conclusions

Together, we show that the mitochondrial transfer through microfluidics can provide potential novel strategies towards tumor cell death.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12575-022-00178-8.

Limited evidence for protein products of non-coding transcripts in the HEK293T cellular cytosol

Ribosome profiling has revealed translation outside of canonical coding sequences (CDSs) including translation of short upstream ORFs, long non-coding RNAs, overlapping ORFs, ORFs in UTRs or ORFs in alternative reading frames. Studies combining mass spectrometry, ribosome profiling and CRISPR-based screens showed that hundreds of ORFs derived from non-coding transcripts produce (micro)proteins, while other studies failed to find evidence for such types of non-canonical translation products. Here, we attempted to discover translation products from non-coding regions by strongly reducing the complexity of the sample prior to mass spectrometric analysis. We used an extended database as the search space and applied stringent filtering of the identified peptides to find evidence for novel translation events. We show that, theoretically our strategy facilitates the detection of translation events of transcripts from non-coding regions, but experimentally only find 19 peptides that might originate from such translation events. Finally, Virotrap based interactome analysis of two N-terminal proteoforms originating from non-coding regions finally showed the functional potential of these novel proteins.

Lethal and Non-Lethal Functions of Caspases in the DNA Damage Response

Members of the caspase family are well known for their roles in the initiation and execution of cell death. Due to their function in the removal of damaged cells that could otherwise become malignant, caspases are important players in the DNA damage response (DDR), a network of pathways that prevent genomic instability. However, emerging evidence of caspases positively or negatively impacting the accumulation of DNA damage in the absence of cell death demonstrates that caspases play a role in the DDR that is independent of their role in apoptosis. This review highlights the apoptotic and non-apoptotic roles of caspases in the DDR and how they can impact genomic stability and cancer treatment.

Autophagy Is Involved in Mesenchymal Stem Cell Death in Coculture with Chondrocytes

OBJECTIVE

Cartilage formation is stimulated in mixtures of chondrocytes and human adipose-derived mesenchymal stromal cells (MSCs) both in vitro and in vivo. During coculture, human MSCs perish. The goal of this study is to elucidate the mechanism by which adipose tissue-derived MSC cell death occurs in the presence of chondrocytes.

METHODS

Human primary chondrocytes were cocultured with human MSCs derived from 3 donors. The cells were cultured in monoculture or coculture (20% chondrocytes and 80% MSCs) in pellets (200,000 cells/pellet) for 7 days in chondrocyte proliferation media in hypoxia (2% O₂). RNA sequencing was performed to assess for differences in gene expression between monocultures or coculture. Immune fluorescence assays were performed to determine the presence of caspase-3, LC3B, and P62.

RESULTS

RNA sequencing revealed significant upregulation of >90 genes in the 3 cocultures when compared with monocultures. STRING analysis showed interconnections between >50 of these genes. Remarkably, 75% of these genes play a role in cell death pathways such as apoptosis and autophagy. Immunofluorescence shows a clear upregulation of the autophagic machinery with no substantial activation of the apoptotic pathway.

CONCLUSION

In cocultures of human MSCs with primary chondrocytes, autophagy is involved in the disappearance of MSCs. We propose that this sacrificial cell death may contribute to the trophic effects of MSCs on cartilage formation.

Myostatin Deficiency Protects C2C12 Cells from Oxidative Stress by Inhibiting Intrinsic Activation of Apoptosis

Ischemia reperfusion (IR) injury remains an important topic in clinical medicine. While a multitude of prophylactic and therapeutic strategies have been proposed, recent studies have illuminated protective effects of myostatin inhibition. This study aims to elaborate on the intracellular pathways involved in myostatin signaling and to explore key proteins that convey protective effects in IR injury. We used CRISPR/Cas9 gene editing to introduce a myostatin (Mstn) deletion into a C2C12 cell line. In subsequent experiments, we evaluated overall cell death, activation of apoptotic pathways, ROS generation, lipid peroxidation, intracellular signaling via mitogen-activated protein kinases (MAPKs), cell migration, and cell proliferation under hypoxic conditions followed by reoxygenation to simulate an IR situation in vitro (hypoxia reoxygenation). It was found that mitogen-activated protein kinase kinase 3/6, also known as MAPK/ERK Kinase 3/6 (MEK3/6), and subsequent p38 MAPK activation were blunted in C2C12-Mstn^−/− cells in response to hypoxia reoxygenation (HR). Similarly, c-Jun N-terminal kinase (JNK) activation was negated. We also found the intrinsic activation of apoptosis to be more important in comparison with the extrinsic activation. Additionally, intercepting myostatin signaling mitigated apoptosis activation. Ultimately, this research validated protective effects of myostatin inhibition in HR and identified potential mediators worth further investigation. Intercepting myostatin signaling did not inhibit ROS generation overall but mitigated cellular injury. In particular, intrinsic activation of apoptosis origination from mitochondria was alleviated. This was presumably mediated by decreased activation of p38 caused by the diminished kinase activity increase of MEK3/6. Overall, this work provides important insights into HR signaling in C2C12-Mstn^−/− cells and could serve as basis for further research.

Induction of mitochondrial apoptosis pathway mediated through caspase-8 and c-Jun N-terminal kinase by cadmium-activated Fas in rat cortical neurons

Cadmium (Cd) is a toxic metal and an environmental pollutant and can cause neurotoxicity by inducing apoptosis. Fas (CD95/Apo-1) is a cell-surface receptor that triggers apoptosis upon ligand binding, mediated through the mitochondrial apoptotic pathway. However, the role and regulatory mechanism of Fas in Cd-induced neuronal apoptosis remain understudied. Here, we demonstrate that activation of caspase-8 and the c-Jun N-terminal kinase (JNK) pathway are mechanisms underlying Cd-induced Fas-mediated activation of the mitochondrial apoptotic pathway in rat cerebral cortical neurons. In vitro, Cd induced apoptosis in primary cortical neurons by activating caspase-8, JNK, and the mitochondrial apoptotic pathway. Fas knockdown enhanced cell viability in the presence of Cd and inhibited apoptosis by blocking Cd-activated Fas, caspase-8, and JNK. Fas knockdown also inhibited the decrease of mitochondrial membrane potential, cleavage of caspase-9/3 and poly (ADP-ribose) polymerase 1, and impaired nuclear translocation of apoptosis-inducing factor and endonuclease G. In vivo, Fas knockdown alleviated Cd-induced neuronal injury and inhibited apoptosis, activation of caspase-8, JNK, and mitochondrial apoptotic pathways in rat cerebral cortical neurons. In summary, our results demonstrate that Cd-activated Fas relays apoptotic signals from the cell surface to the mitochondria via caspase-8 and JNK activation in rat cerebral cortical neurons, leading to aggravation of the neuronal injury.

Garcinielliptone G from Garcinia subelliptica Induces Apoptosis in Acute Leukemia Cells

Cytotoxicity and apoptosis-inducing properties of compounds isolated from Garcinia subelliptica leaves were investigated. The hexane-soluble portion of MeOH extracts of G. subelliptica leaves that showed cytotoxic activity was separated to yield seven compounds 1–7. Chemical structure analysis using NMR spectroscopy and mass spectrometry confirmed that compound 1 was canophyllol, and compounds 2–7 were garcinielliptones N, O, J, G, F, and garcinielliptin oxide, respectively. Among them, garcinielliptone G (5) showed growth inhibition by causing apoptosis in THP-1 and Jurkat cells derived from human acute monocytic leukemia and T lymphocyte cells, respectively. Apoptosis induced by garcinielliptone G (5) was demonstrated by the detection of early apoptotic cells with fluorescein-labeled Annexin V and increases in cleaved caspase-3 and cleaved PARP protein levels. However, the addition of caspase inhibitor Z-VAD-FMK did not affect growth arrest or apoptosis induction. These results suggest that garcinielliptone G (5) can induce both caspase-3 activation and caspase-independent apoptosis. Therefore, garcinielliptone G (5) may be a potential candidate for acute leukemia treatment.

Autophagy restricts mitochondrial DNA damage-induced release of ENDOG (endonuclease G) to regulate genome stability

Genotoxic insult causes nuclear and mitochondrial DNA damages with macroautophagy/autophagy induction. The role of mitochondrial DNA (mtDNA) damage in the requirement of autophagy for nuclear DNA (nDNA) stability is unclear. Using site-specific DNA damage approaches, we show that specific nDNA damage alone does not require autophagy for repair unless in the presence of mtDNA damage. We provide evidence that after IR exposure-induced mtDNA and nDNA damages, autophagy suppression causes non-apoptotic mitochondrial permeability, by which mitochondrial ENDOG (endonuclease G) is released and translocated to nuclei to sustain nDNA damage in a TET (tet methylcytosine dioxygenase)-dependent manner. Furthermore, blocking lysosome function is sufficient to increase the amount of mtDNA leakage to the cytosol, accompanied by ENDOG-free mitochondrial puncta formation with concurrent ENDOG nuclear accumulation. We proposed that autophagy eliminates the mitochondria specified by mtDNA damage-driven mitochondrial permeability to prevent ENDOG-mediated genome instability. Finally, we showed that HBx, a hepatitis B viral protein capable of suppressing autophagy, also causes mitochondrial permeability-dependent ENDOG mis-localization in nuclei and is linked to hepatitis B virus (HBV)-mediated hepatocellular carcinoma development.Abbreviations: 3-MA: 3-methyladenine; 5-hmC: 5-hydroxymethylcytosine; ACTB: actin beta; ATG5: autophagy related 5; ATM: ATM serine/threonine kinase; DFFB/CAD: DNA fragmentation factor subunit beta; cmtDNA: cytosolic mitochondrial DNA; ConA: concanamycin A; CQ: chloroquine; CsA: cyclosporin A; Dox: doxycycline; DSB: double-strand break; ENDOG: endonuclease G; GFP: green fluorescent protein; Gy: gray; H2AX: H2A.X variant histone; HBV: hepatitis B virus; HBx: hepatitis B virus X protein; HCC: hepatocellular carcinoma; I-PpoI: intron-encoded endonuclease; IR: ionizing radiation; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MOMP: mitochondrial outer membrane permeability; mPTP: mitochondrial permeability transition pore; mtDNA: mitochondrial DNA; nDNA: nuclear DNA; 4-OHT: 4-hydroxytamoxifen; rDNA: ribosomal DNA; ROS: reactive oxygen species; SQSTM1/p62: sequestosome 1; TET: tet methylcytosine dioxygenase; TFAM: transcription factor A, mitochondrial; TOMM20: translocase of outer mitochondrial membrane 20; VDAC: voltage dependent anion channel.

Analysis of Apoptosis-Related Genes Reveals that Apoptosis Functions in Conidiation and Pathogenesis of Fusarium pseudograminearum

Apoptosis, a type of programmed cell death, plays crucial roles in various physiological processes, from development to adaptive responses. Key features of apoptosis have been verified in various fungal microbes but not yet in Fusarium species. Here, we identified 19 apoptosis-related genes in Fusarium pseudograminearum using a genome-wide survey. Expression profile analysis revealed that several apoptosis-related genes were significantly increased during conidiation and infection stages. Among these is FpBIR1, with two BIR (baculovirus inhibitor-of-apoptosis protein repeat) domains at the N-terminal end of the protein, a homolog of Saccharomyces cerevisiae BIR1, which is a unique apoptosis inhibitor. FpNUC1 is the ortholog of S. cerevisiae NUC1, which triggers AIF1- or YCA1-independent apoptosis. The functions of these two proteins were assessed by creating Δfpbir1 and Δfpnuc1 mutants via targeted gene deletion. The Δfpbir1 mutant had more cells with nuclear fragmentation and exhibited reduced conidiation, conidial formation, and infectivity. Correspondingly, the Δfpnuc1 mutant contained multiple nuclei, produced thicker and more branched hyphae, was reduced in conidiation, and exhibited faster conidial formation and higher infection rates. Taken together, our results indicate that the apoptosis-related genes FpBIR1 and FpNUC1 function in conidiation, conidial germination, and infection by F. pseudograminearumIMPORTANCE The plant-pathogenic fungus F. pseudograminearum is the causal agent of Fusarium crown rot (FCR) in wheat and barley, resulting in substantial yield losses worldwide. Particularly, in the Huanghuai wheat-growing region of China, F. pseudograminearum was reported as the dominant Fusarium species in FCR infections. Apoptosis is an evolutionarily conserved mechanism in eukaryotes, playing crucial roles in development and cell responses to biotic and abiotic stresses. However, few reports on apoptosis in plant fungal pathogens have been published. In this study, we identified 19 conserved apoptosis-related genes in F. pseudograminearum, several of which were significantly increased during conidiation and infection stages. Potential apoptosis functions were assessed by deletion of the putative apoptosis inhibitor gene FpBIR1 and apoptosis trigger gene FpNUC1 in F. pseudograminearum The FpBIR1 deletion mutant exhibited defects in conidial germination and pathogenicity, whereas the FpNUC1 deletion mutant experienced faster conidial formation and higher infection rates. Apoptosis appears to negatively regulate the conidial germination and pathogenicity of F. pseudograminearum To our knowledge, this study is the first report of apoptosis contributing to infection-related morphogenesis and pathogenesis in F. pseudograminearum.

Characterization of a cell death-inducing endonuclease-like venom protein from the parasitoid wasp Pteromalus puparum (Hymenoptera: Pteromalidae)

BACKGROUND

Parasitoid wasps are valuable natural enemies for controlling pests. To ensure successful parasitism, these wasps inject venoms along with their eggs that are deposited either into or on their hosts. Parasitoid venoms regulate host behaviors, development, metabolism and immune responses. Pteromalus puparum is a pupal endoparasitoid that parasitizes a number of butterflies, including the worldwide pest cabbage butterfly, Pieris rapae. Venom from P. puparum has a variety of effects on host hemocytes, including alteration of absolute and relative hemocyte counts, and inhibition of hemocyte spreading and encapsulation. In particular, P. puparum venom causes hemocyte cell death in vivo and in vitro.

RESULTS

Using assay-guided chromatography, a cell death-inducing venom fraction was identified and defined as P. puparum endonuclease-like venom protein (PpENVP). It belongs to the DNA/RNA nonspecific endonuclease family, which contains two conserved endonuclease activation sites. We analyzed its expression profiles and demonstrated that PpENVP inhibits gene expression in transfected cells relying on two activation sites. However, RNA interference of PpENVP did not significantly reduce P. puparum venom cytotoxicity, suggesting that PpENVP may not be the sole cytotoxic factor present.

CONCLUSION

Our results provide novel insight into the function of the P. puparum venom cocktail and identify a promising insecticide candidate endonuclease that targets insect hemocytes.

Making Connections: p53 and the Cathepsin Proteases as Co-Regulators of Cancer and Apoptosis

Simple Summary

This article describes an emerging area of significant interest in cancer and cell death and the relationships shared by these through the p53 and cathepsin proteins. While it has been demonstrated that the p53 protein can directly induce the leakage of cathepsin proteases from the lysosome, directly triggering cell death, little is known about what factors set the threshold at which the lysosome can become permeabilized. It appears that the expression levels of cathepsin proteases may be central to this process, with some of them being transcriptionally regulated by p53. The consequences of such a mechanism have serious implications for lysosomal-mediated apoptosis and have significant input into the design of therapeutics and their strategic use. In this review, we highlight the importance of extending such findings to other cathepsin family members and the need to assess the roles of p53 isoforms and mutants in furthering this mechanism.

Abstract

While viewed as the “guardian of the genome”, the importance of the tumor suppressor p53 protein has increasingly gained ever more recognition in modulating additional modes of action related to cell death. Slowly but surely, its importance has evolved from a mutated genetic locus heavily implicated in a wide array of cancer types to modulating lysosomal-mediated cell death either directly or indirectly through the transcriptional regulation of the key signal transduction pathway intermediates involved in this. As an important step in determining the fate of cells in response to cytotoxicity or during stress response, lysosomal-mediated cell death has also become strongly interwoven with the key components that give the lysosome functionality in the form of the cathepsin proteases. While a number of articles have been published highlighting the independent input of p53 or cathepsins to cellular homeostasis and disease progression, one key area that warrants further focus is the regulatory relationship that p53 and its isoforms share with such proteases in regulating lysosomal-mediated cell death. Herein, we review recent developments that have shaped this relationship and highlight key areas that need further exploration to aid novel therapeutic design and intervention strategies.

Ca2+-dependent nuclease is involved in DNA degradation during the formation of the secretory cavity by programmed cell death in fruit of Citrus grandis 'Tomentosa'

The secretory cavity is a typical structure in Citrus fruit and is formed by schizolysigeny. Previous reports have indicated that programmed cell death (PCD) is involved in the degradation of secretory cavity cells in the fruit, and that the spatio-temporal location of calcium is closely related to nuclear DNA degradation in this process; however, the molecular mechanisms underlying this Ca2+ regulation remain largely unknown. Here, we identified CgCaN that encodes a Ca2+-dependent DNase in the fruit of Citrus grandis 'Tomentosa', the function of which was studied using calcium ion localization, DNase activity assays, in situ hybridization, and protein immunolocalization. The results suggested that the full-length cDNA of CgCaN contains an ORF of 1011 bp that encodes a protein 336 amino acids in length with a SNase-like functional domain. CgCaN digests dsDNA at neutral pH in a Ca2+-dependent manner. In situ hybridization signals of CgCaN were particularly distributed in the secretory cavity cells. Ca2+ and Ca2+-dependent DNases were mainly observed in the condensed chromatin and in the nucleolus. In addition, spatio-temporal expression patterns of CgCaN and its protein coincided with the time-points that corresponded to chromatin degradation and nuclear rupture during the PCD in the development of the fruit secretory cavity. Taken together, our results suggest that Ca2+-dependent DNases play direct roles in nuclear DNA degradation during the PCD of secretory cavity cells during Citrus fruit development. Given the consistency of the expression patterns of genes regulated by calmodulin (CaM) and calcium-dependent protein kinases (CDPK) and the dynamics of calcium accumulation, we speculate that CaM and CDPK proteins might be involved in Ca2+ transport from the extracellular walls through the cytoplasm and into the nucleus to activate CgCaN for DNA degradation.

The role of caspase-dependent and caspase-independent pathways of apoptosis in the premature rupture of the membranes: A case-control study

Background

Premature rupture of membrane (PROM) remains a problem in obstetrics, the mechanisms of PROM have not been clearly defined. Apoptosis is thought to play a key role in the mechanism, via caspase-dependent and caspase-independent pathways. Caspase-3, Apoptosis-inducing factor (AIF), and anti-apoptosis B-cell lymphoma 2 (Bcl-2) are hypothesized to be involved in PROM.

Objective

To determine the role of caspase-dependent and caspase-independent pathways in the mechanism of PROM.

Materials and Methods

This was a case-control study involving 42 pregnant women with gestational age between 20-42 wk. Participants were divided into the case group (with PROM) and control group (without PROM). Amniotic membranes were collected immediately after the delivery, and samples were taken from the site of membrane rupture. Immunohistochemical examination was done to determine the expression of Caspase-3, AIF, and Bcl-2.

Results

The expressions of Caspase-3 (OR = 9.75; 95% CI = 2.16-43.95; p = 0.001) and AIF (OR = 6.60; 95% CI = 1.48-29.36; p = 0.009) were significantly increased, whereas, Bcl-2 expressions (OR = 8.00; 95% CI = 1.79-35.74; p = 0.004) were significantly decreased in the case group.

Conclusion

High Caspase-3, AIF, and low Bcl-2 expression were the risk factors for PROM. Thus, it is evident that caspase-dependent and caspase-independent pathways are involved in the mechanism of PROM.

Inhibition of nuclease activity by a splice-switching oligonucleotide targeting deoxyribonuclease 1 mRNA prevents apoptosis progression and prolong viability of normal human CD4+ T-lymphocytes

The nuclease activity of deoxyribonuclease 1 (DNase I) is regulated by alternative splicing (AS) of its mRNA. The aim of this study was to define the ability of a splice-switching oligonucleotide (SSO) that base-paired with DNase I pre-mRNA to induce AS and inhibit nuclease activity in human T, B and NK lymphocytes. The SSO for DNase I could significantly downregulate the expression of full-length active DNase I and upregulate a truncated splice variant with a deleted exon 4. Such an induction of AS resulted in inhibition of nuclease activity and slowed apoptosis progression in anti-CD95/FAS stimulated lymphocytes. These results should facilitate further investigations of apoptosis regulation in lymphocytes and demonstrate that SSOs for DNase I are promising cytoprotective agents.

Neuroprotective Effect of Rhodiola crenulata in D-Galactose-Induced Aging Model

The medicinal plant Rhodiola crenulata grows at high altitudes in the Arctic and mountainous regions and is commonly used in phytotherapy in Eastern European and Asian countries. In the present study, we investigated the anti-apoptotic effect of Rhodiola crenulata and its neuroprotective mechanism of action in a rat model of D-galactose-induced aging. Two groups of twelve-week-old male Wistar rats received a daily injection of D-galactose (150mg/kg/day, i.p.) and orally administered Rhodiola crenulata (0, 248mg/kg/day) for eight weeks, while a control group received a saline injection (1ml/kg/day, i.p.). We examined apoptosis in the cortex and hippocampus of three groups of rats based on a terminal deoxynucleotide transferase-mediated deoxy uridine triphosphate nick-end labeling (TUNEL) positive assay. The expression levels of apoptotic and anti-apoptotic proteins in excised brains were analyzed by Western blotting. Our findings indicated that D-galactose caused marked neuronal apoptosis via activation of both extrinsic-dependent and mitochondrial-dependent apoptotic pathways. When compared to the control group, the protein levels of Fas receptor, Fas ligand, Fas-associated death domain (FADD), and activated caspase-8 (Fas-dependent apoptotic pathways), as well as those of t-Bid, Bax, cytochrome c, activated caspase-9, and activated caspase-3 (mitochondrial-dependent apoptotic pathways), were significantly increased in the D-galactose treated group. In addition, D-galactose impaired the phosphorylation of PI3K/Akt, an important survival signaling event in neurons. Rhodiola crenulata, however, protected against all these neurotoxicities in aging brains. The present study suggests that neuronal survival promoted by Rhodiola crenulata may be a potentially effective method to enhance the resistance of neurons to age-related disorders.

Effect of caspase inhibitor Z-VAD-FMK on bovine sperm cryotolerance

The aim of this study was to evaluate the treatment of bovine semen with the pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (Z-VAD-FMK), before or after freezing on semen quality. After the initial assessment, sperm from 4 bulls were pooled (Experiment 1) and cryopreserved in BioXcell containing 0, 20 and 100 μM Z-VAD-FMK. After thawing semen viability, motility, membrane integrity, as well as DNA fragmentation and ΔΨm were evaluated. In Experiment 2, bovine frozen/thawed sperm were incubated for 1 hr with 0, 20 and 100 µM Z-VAD-FMK before assessing the semen quality. The treatment with Z -VAD-FMK before cryopreservation improved post-thawing sperm motility compared to the control group (p < .05), while no differences were recorded in sperm viability and membrane integrity among groups (on average 86.8 ± 1.5 and 69.1 ± 1.4, respectively). Interestingly, at the highest concentration, DNA fragmentation decreased (p < .05), while the percentage of spermatozoa with high ΔΨm increased (p < .05). The results of Experiment 2 showed that 1-hr treatment with Z-VAD-FMK did not affect sperm motility and viability (on average 63.4 ± 5.8 and 83.7.1 ± 1.2, respectively). However, Z-VAD-FMK improved sperm membrane integrity (p < .05) and at the highest concentration tested decreased the proportion of sperm showing DNA fragmentation (p < .05). No differences were recorded in the percentage of spermatozoa with high ΔΨm (on average 57.0 ± 11.4). In conclusion, the treatment with 100 µM of the caspase inhibitor Z-VAD-FMK before freezing increased bovine sperm mass motility and ΔΨm, while decreasing sperm DNA fragmentation. Treatment of semen after thawing with 100 µM Z-VAD-FMK improved sperm membrane integrity and reduced DNA fragmentation.

Cell death in Leishmania

Leishmaniases still represent a global scourge and new therapeutic tools are necessary to replace the current expensive, difficult to administer treatments that induce numerous adverse effects and for which resistance is increasingly worrying. In this context, the particularly original organization of the Leishmania parasite in comparison to higher eukaryotes is a great advantage. It allows for the development of new, very specific, and thus non-cytotoxic treatments. Among these originalities, Leishmania cell death can be cited. Despite a classic pattern of apoptosis, key mammalian apoptotic proteins are not present in Leishmania, such as caspases, cell death receptors, and anti-apoptotic molecules. Recent studies have helped to develop a better understanding of parasite cell death, identifying new proteins or even new apoptotic pathways. This review provides an overview of the current knowledge on Leishmania cell death, describing its physiological roles and its phenotype, and discusses the involvement of various proteins: endonuclease G, metacaspase, aquaporin Li-BH3AQP, calpains, cysteine proteinase C, LmjHYD36 and Lmj.22.0600. From these data, potential apoptotic pathways are suggested. This review also offers tools to identify new Leishmania cell death effectors. Lastly, different approaches to use this knowledge for the development of new therapeutic tools are suggested: either inhibition of Leishmania cell death or activation of cell death for instance by treating cells with proteins or peptides involved in parasite death fused to a cell permeant peptide or encapsulated into a lipidic vector to target intra-macrophagic Leishmania cells.

Combined addition of superoxide dismutase, catalase and glutathione peroxidase improves quality of cooled stored stallion semen

During cold storage stallion spermatozoa experience undergo oxidative stress, which can impair sperm function and fertilizing capacity. Superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX) are the main endogenous enzymatic antioxidants in stallion seminal plasma, and counteract reactive oxygen species. Semen dilution reduces the endogenous antioxidant concentrations. The aim of this study was to investigate whether addition of 15 IU/mL each of SOD, CAT, and GPX to diluted stallion semen would ameliorate a reactive oxygen-mediated decrease in semen quality during 72 h of storage at 5 °C. Ejaculates (n = 7) were divided in two aliquots and diluted in INRA 96 without (control) or with addition of antioxidants. Semen analysis was performed at the time of dilution and every 24 h during chilled storage. Antioxidant supplementation completely inhibited the storage-dependent increase in activated caspase 3 (P < 0.05). Concomitantly, the antioxidant-supplemented samples had a greater percentage of viable, motile and rapidly moving sperm than control samples after 72 h storage (P < 0.05). The DNA damage, as evaluated by TUNEL assay and SCSA, increased with storage time (P < 0.05). Antioxidant supplementation did not prevent, but did significantly reduce the increase in DNA strand breakage. The results indicate part of the intrinsic apoptotic pathway leading to effector caspase activation was inhibited, although an activation of molecules with endonuclease activity still occurred. In conclusion, adding equal concentrations of SOD, CAT and GPX to a semen extender suppressed caspase-3 activation and improved preservation of stallion sperm motility and viability during 72 h of storage at 5 °C.

Mitochondria-hubs for regulating cellular biochemistry: emerging concepts and networks

Mitochondria are iconic structures in biochemistry and cell biology, traditionally referred to as the powerhouse of the cell due to a central role in energy production. However, modern-day mitochondria are recognized as key players in eukaryotic cell biology and are known to regulate crucial cellular processes, including calcium signalling, cell metabolism and cell death, to name a few. In this review, we will discuss foundational knowledge in mitochondrial biology and provide snapshots of recent advances that showcase how mitochondrial function regulates other cellular responses.

Meiotic viral attenuation through an ancestral apoptotic pathway

The programmed release of apoptogenic proteins from mitochondria is a core event of apoptosis, although ancestral roles of this phenomenon are not known. In mammals, one such apoptogenic protein is Endonuclease G (EndoG), a conserved mitochondrial nuclease that fragments the DNA of dying cells. In this work, we show that budding yeast executes meiotically programmed mitochondrial release of an EndoG homolog, Nuc1, during sporulation. In contrast to EndoG's ostensible pro-death function during apoptosis, Nuc1 mitochondrial release is pro-survival, attenuating the cytosolic L-A and Killer double-stranded RNA mycoviruses and protecting meiotic progeny from the catastrophic consequences of their derepression. The protective viral attenuation role of this pathway illuminates a primordial role for mitochondrial release of EndoG, and perhaps of apoptosis itself.

Cell Death Mechanisms in a Mouse Model of Retinal Degeneration in Spinocerebellar Ataxia 7

Spino-cerebellar ataxia type 7 (SCA7) is a polyglutamine (polyQ) disorder characterized by neurodegeneration of the brain, cerebellum, and retina caused by a polyglutamine expansion in ataxin7. The presence of an expanded polyQ tract in a mutant protein is known to induce protein aggregation, cellular stress, toxicity, and finally cell death. However, the consequences of the presence of mutant ataxin7 in the retina and the mechanisms underlying photoreceptor degeneration remain poorly understood. In this study, we show that in a retinal SCA7 mouse model, polyQ ataxin7 induces stress within the retina and activates Muller cells. Moreover, unfolded protein response and autophagy are activated in SCA7 photoreceptors. We have also shown that the photoreceptor death does not involve a caspase-dependent apoptosis but instead involves apoptosis inducing factor (AIF) and Leukocyte Elastase Inhibitor (LEI/L-DNase II). When these two cell death effectors are downregulated by their siRNA, a significant reduction in photoreceptor death is observed. These results highlight the consequences of polyQ protein expression in the retina and the role of caspase-independent pathways involved in photoreceptor cell death.

Endonuclease G modulates the alternative splicing of deoxyribonuclease 1 mRNA in human CD4+ T lymphocytes and prevents the progression of apoptosis

Apoptotic endonucleases act cooperatively to fragment DNA and ensure the irreversibility of apoptosis. However, very little is known regarding the potential regulatory links between endonucleases. Deoxyribonuclease 1 (DNase I) inactivation is caused by alternative splicing (AS) of DNase I pre-mRNA skipping exon 4, which occurs in response to EndoG overexpression in cells. The current study aimed to determine the role of EndoG in the regulation of DNase I mRNA AS and the modulation of its enzymatic activity. A strong correlation was identified between the EndoG expression levels and DNase I splice variants in human lymphocytes. EndoG overexpression in CD4⁺ T cells down-regulated the mRNA levels of the active full-length DNase I variant and up-regulated the levels of the non-active spliced variant, which acts in a dominant-negative fashion. DNase I AS was induced by the translocation of EndoG from mitochondria into nuclei during the development of apoptosis. The DNase I spliced variant was induced by recombinant EndoG or by incubation with EndoG-digested cellular RNA in an in vitro system with isolated cell nuclei. Using antisense DNA oligonucleotides, we identified a 72-base segment that spans the adjacent segments of exon 4 and intron 4 and appears to be responsible for the AS. DNase I-positive CD4⁺ T cells overexpressing EndoG demonstrated decreased progression towards bleomycin-induced apoptosis. Therefore, EndoG is an endonuclease with the unique ability to inactivate another endonuclease, DNase I, and to modulate the development of apoptosis.

Graphene quantum dots in alveolar macrophage: uptake-exocytosis, accumulation in nuclei, nuclear responses and DNA cleavage

BACKGROUND

Given the tremendous potential for graphene quantum dots (QDs) in biomedical applications, a thorough understanding of the interaction of these materials with macrophages is essential because macrophages are one of the most important barriers against exogenous particles. Although the cytotoxicity and cellular uptake of graphene QDs were reported in previous studies, the interaction between nuclei and the internalized graphene QDs is not well understood. We thus systematically studied the nuclear uptake and related nuclear response associated with aminated graphene QDs (AG-QDs) exposure.

RESULTS

AG-QDs showed modest 24-h inhibition to rat alveolar macrophages (NR8383), with a minimum inhibitory concentration (MIC) of 200 μg/mL. Early apoptosis was significantly increased by AG-QDs (100 and 200 μg/mL) exposure and played a major role in cell death. The internalization of AG-QDs was mainly via energy-dependent endocytosis, phagocytosis and caveolae-mediated endocytosis. After a 48-h clearance period, more than half of the internalized AG-QDs remained in the cellular cytoplasm and nucleus. Moreover, AG-QDs were effectively accumulated in nucleus and were likely regulated by two nuclear pore complexes genes (Kapβ2 and Nup98). AG-QDs were shown to alter the morphology, area, viability and nuclear components of exposed cells. Significant cleavage and cross-linking of DNA chains after AG-QDs exposure were confirmed by atomic force microscopy investigation. Molecular docking simulations showed that H-bonding and π-π stacking were the dominant forces mediating the interactions between AG-QDs and DNA, and were the important mechanisms resulting in DNA chain cleavage. In addition, the generation of reactive oxygen species (ROS) (e.g., •OH), and the up-regulation of caspase genes also contributed to DNA cleavage.

CONCLUSIONS

AG-QDs were internalized by macrophages and accumulated in nuclei, which further resulted in nuclear damage and DNA cleavage. It is demonstrated that oxidative damage, direct contact via H-bonding and π-π stacking, and the up-regulation of caspase genes are the primary mechanisms for the observed DNA cleavage by AG-QDs.

The mitochondrial pathway is involved in sodium fluoride (NaF)-induced renal apoptosis in mice

The objective of the present study was to explore the molecular mechanism of apoptosis induced by sodium fluoride (NaF) in the mouse kidney by using the methods of flow cytometry, quantitative real-time polymerase chain reaction (qRT-PCR), western blotting, and experimental pathology. 240 four-week-old ICR mice were randomly divided into 4 groups and exposed to different concentrations of NaF (0 mg kg-1, 12 mg kg-1, 24 mg kg-1 and 48 mg kg-1) for a period of 42 days. The results demonstrated that NaF increased cell apoptosis and the depolarization of the mitochondrial membrane potential (MMP), and that the mitochondrial pathway was involved in NaF-induced apoptosis. Alteration of the mitochondrial pathway was characterized by significantly increasing mRNA and protein expression levels of cytosolic cytochrome c (Cyt c), the second mitochondrial activator of caspases/direct inhibitors of the apoptosis binding protein with low pI (Smac/Diablo), the serine protease high-temperature-requirement protein A2/Omi (HtrA2/Omi), the apoptosis inducing factor (AIF), endonuclease G (Endo G), cleaved-cysteine aspartate specific protease-9 (cleaved-caspase-9), cleaved-cysteine aspartate specific protease-3 (cleaved-caspase-3), Bcl-2 antagonist killer (Bak), Bcl-2 associated X protein (Bax), Bcl-2 interacting mediator of cell death (Bim), cleaved-poly-ADP-ribose polymerase (cleaved-PARP), p-p53, and decreasing mRNA and protein expression levels of B-cell lymphoma-2 (Bcl-2), Bcl-extra large (Bcl-xL), and X chromosome-linked inhibitors of apoptosis proteins (XIAPs). To our knowledge, the mitochondrial pathway is reported for the first time in NaF-induced apoptosis of the human or animal kidney. Also, this study provides novel insights for further studying fluoride-induced nephrotoxicity.

Metabolic Enhancer Piracetam Attenuates the Translocation of Mitochondrion-Specific Proteins of Caspase-Independent Pathway, Poly [ADP-Ribose] Polymerase 1 Up-regulation and Oxidative DNA Fragmentation

Piracetam, a nootropic drug, has been clinically used for decades; however, its mechanism of action still remains enigmatic. The present study was undertaken to evaluate the role of mitochondrion-specific factors of caspase-independent pathway like apoptotic-inducing factor (AIF) and endonuclease-G (endo-G) in piracetam-induced neuroprotection. N2A cells treated with lipopolysaccharide (LPS) exhibited significant cytotoxicity, impaired mitochondrial activity, and reactive oxygen species generation which was significantly attenuated with piracetam co-treatment. Cells co-treated with LPS and piracetam exhibited significant uptake of piracetam in comparison to only piracetam-treated cells as estimated by liquid chromatography-mass spectrometry (LC-MSMS). LPS treatment caused significant translocation of AIF and endonuclease-G in neuronal N2A cells which were significantly attenuated with piracetam co-treatment. Significant over-expression of proinflammatory cytokines was also observed after treatment of LPS to cells which was inhibited with piracetam co-treatment demonstrating its anti-inflammatory property. LPS-treated cells exhibited significant oxidative DNA fragmentation and poly [ADP-ribose] polymerase-1 (PARP-1) up-regulation in nucleus, both of which were attenuated with piracetam treatment. Antioxidant melatonin but not z-VAD offered the inhibited LPS-induced DNA fragmentation indicating the involvement of oxidative DNA fragmentation. Further, we did not observe the altered caspase-3 level after LPS treatment initially while at a later time point, significantly augmented level of caspase-3 was observed which was not inhibited with piracetam treatment. In total, our findings indicate the interference of piracetam in mitochondrion-mediated caspase-independent pathway, as well as its anti-inflammatory and antioxidative properties. Graphical Abstract Graphical abstract indicating the novel interference of metabolic enhancer piracetam (P) in neuronal death mechanisms.

Autophagy-dependent apoptosis is triggered by a semi-synthetic [6]-gingerol analogue in triple negative breast cancer cells

Triple negative breast cancer (TNBC) is very aggressive and lacks specific therapeutic targets, having limited treatment options and poor prognosis. [6]-gingerol is the most abundant and studied compound in ginger, presenting diverse biological properties such as antitumor activity against several types of cancer, including breast cancer. In this study, we show that the semi-synthetic analogue SSi6, generated after chemical modification of the [6]-gingerol molecule, using acetone-2,4-dinitrophenylhydrazone (2,4-DNPH) reagent, enhanced selective cytotoxic effects on MDA-MB-231 cells. Remarkably, unlike the original [6]-gingerol molecule, SSi6 enabled autophagy followed by caspase-independent apoptosis in tumor cells. We found a time-dependent association between SSi6-induced oxidative stress, autophagy and apoptosis. Initial SSi6-induced reactive oxygen species (ROS) accumulation (1h) led to autophagy activation (2-6h), which was followed by caspase-independent apoptosis (14h) in TNBC cells. Additionally, our data showed that SSi6 induction of ROS plays a key role in the promotion of autophagy and apoptosis. In order to investigate whether the observed cell death induction was dependent on preceding autophagy in MDA-MB-231 cells, we used siRNA to knock down LC3B prior to SSi6 treatment. Our data show that LC3B downregulation decreased the number of apoptotic cells after treatment with SSi6, indicating that autophagy is a key initial step on SSi6-induced caspase-independent apoptosis. Overall, the results of this study show that structural modifications of natural compounds can be an interesting strategy for developing antitumor drugs, with distinct mechanisms of actions, which could possibly be used against triple negative breast cancer cells that are resistant to canonical apoptosis-inducing drugs.

Endonuclease G takes part in AIF-mediated caspase-independent apoptosis in Mycobacterium bovis-infected bovine macrophages

Mycobacterium bovis, the causative agent of bovine tuberculosis encodes different virulence mechanisms to survive inside of host cells. One of the possible outcomes in this host-pathogen interaction is cell death. Previous results from our group showed that M. bovis induces a caspase-independent apoptosis in bovine macrophages with the possible participation of apoptosis inducing factor mitochondria associated 1 (AIFM1/AIF), a flavoprotein that functions as a cell-death regulator. However, contribution of other caspase-independent cell death mediators in M. bovis-infected macrophages is not known. In this study, we aimed to further characterize M. bovis-induced apoptosis, addressing Endonuclease G (Endo G) and Poly (ADP-ribose) polymerase 1 (PARP-1). In order to accomplish our objective, we infected bovine macrophages with M. bovis AN5 (MOI 10:1). Analysis of M. bovis-infected nuclear protein extracts by immunoblot, identified a 15- and 43-fold increase in concentration of mitochondrial proteins AIF and Endo G respectively. Interestingly, pretreatment of M. bovis-infected macrophages with cyclosporine A, a mitochondrial permeability transition pore inhibitor, abolished AIF and Endo G nuclear translocation. In addition, it also decreased macrophage DNA fragmentation to baseline and caused a 26.2% increase in bacterial viability. We also demonstrated that PARP-1 protein expression in macrophages did not change during M. bovis infection. Furthermore, pretreatment of M. bovis-infected bovine macrophages with 3-aminobenzamide, a PARP-1 inhibitor, did not change the proportion of macrophage DNA fragmentation. Our results suggest participation of Endo G, but not PARP-1, in M. bovis-induced macrophage apoptosis. To the best of our knowledge this is the first report associating Endo G with caspase-independent apoptosis induced by a member of the Mycobacterium tuberculosis complex.

Transcriptome and proteome profiling reveals stress-induced expression signatures of imiquimod-treated Tasmanian devil facial tumor disease (DFTD) cells

As a topical cancer immunotherapy, the toll-like receptor 7 ligand imiquimod activates tumor regression via stimulation of immune cell infiltration and cytotoxic responses. Imiquimod also exerts direct pro-apoptotic effects on tumor cells in vitro, but a role for these effects in imiquimod-induced tumor regression remains undefined. We previously demonstrated that cell lines derived from devil facial tumor disease (DFTD), a transmissible cancer threatening the survival of the Tasmanian devil (Sarcophilus harrisii), are sensitive to imiquimod-induced apoptosis. In this study, the pro-apoptotic effects of imiquimod in DFTD have been investigated using RNA-sequencing and label-free quantitative proteomics. This analysis revealed that changes to gene and protein expression in imiquimod treated DFTD cells are consistent with the onset of oxidative and endoplasmic reticulum stress responses, and subsequent activation of the unfolded protein response, autophagy, cell cycle arrest and apoptosis. Imiquimod also regulates the expression of oncogenic pathways, providing a direct mechanism by which this drug may increase tumor susceptibility to immune cytotoxicity in vivo. Our study has provided the first global analysis of imiquimod-induced effects in any tumor cell line. These findings have highlighted the potential of cell stress pathways as therapeutic targets in DFTD, and will allow for improved mechanistic use of imiquimod as a therapy in both the Tasmanian devil and human cancers.

Senoptosis: non-lethal DNA cleavage as a route to deep senescence

DNA-damage-induced apoptosis and cellular senescence are perceived as two distinct cell fates. We found that after ionizing radiation (IR)-induced DNA damage the majority (up to 70 %) of senescent human diploid fibroblasts (HDFs) were subjected to controlled cleavage of DNA, resulting in the establishment of a viable and stable sub-G1 population, i.e. deeply senescent cells. We show that in senescent HDFs this DNA cleavage is triggered by modest loss of the mitochondrial membrane potential, which is not sufficient to activate caspases, but strong enough to release mitochondrial endonuclease G (EndoG). We demonstrate that upon γ-irradiation in HDFs EndoG translocates into the nucleus playing an essential role in the non-lethal cleavage of damaged DNA. Notably, the established sub-G1 cell population does not contribute to the senescence-associated secretory phenotype (SASP), however, it exhibits increased senescence-associated β-galactosidase activity. We show that EndoG knockdown causes an increase in DNA damage, indicating a role of this enzyme in DNA repair. Thus, we conclude that IR-induced deep senescence of HDFs exhibits features of both senescence, such as cell cycle arrest and viability, and apoptosis like reduced DNA content and no SASP, and, resembles uncomplete or stalled apoptosis, a phenomenon we term senoptosis.

Apoptosis-induced lymphopenia in sepsis and other severe injuries

Sepsis and other acute injuries such as severe trauma, extensive burns, or major surgeries, are usually followed by a period of marked immunosuppression. In particular, while lymphocytes play a pivotal role in immune response, their functions and numbers are profoundly altered after severe injuries. Apoptosis plays a central role in this process by affecting immune response at various levels. Indeed, apoptosis-induced lymphopenia duration and depth have been associated with higher risk of infection and mortality in various clinical settings. Therapies modulating apoptosis represent an interesting approach to restore immune competence after acute injury, although their use in clinical practice still presents several limitations. After briefly describing the apoptosis process in physiology and during severe injuries, we will explore the immunological consequences of injury-induced lymphocyte apoptosis, and describe associations with clinically relevant outcomes in patients. Therapeutic perspectives targeting apoptosis will also be discussed.