Calpain I induces cleavage and release of apoptosis-inducing factor from isolated mitochondria

A genetically encoded fluorescent protein sensor for mitochondrial membrane damage detection

Mitochondria are essential cellular organelles; detecting mitochondrial damage is crucial in cellular biology and toxicology. Compared with existing chemical probe detection methods, genetically encoded fluorescent protein sensors can directly indicate cellular and molecular events without involving exogenous reagents. In this study, we introduced a molecular sensor system, MMD-Sensor, for monitoring mitochondrial membrane damage. The sensor consists of two molecular modules. Module I is a fusion structure of the mitochondrial localization sequence (MLS), AIF cleavage site sequence (CSS), nuclear localization sequence (NLS), N-terminus of mNeonGreen and mCherry. Module II is a fusion structure of the C-terminus of mNeonGreen, NLS sequence, and mtagBFP2. Under normal condition, Module I is constrained in the inner mitochondrial membrane anchored by MLS, while Module II is restricted to the nucleus by its NLS fusion component. If the mitochondrial membrane is damaged, CSS is cut from the inner membrane, causing Module I to shift into the nucleus guided by the NLS fusion component. After Module I enters the nucleus, the N- and C-terminus of mNeonGreen meet each other and rebuild its intact 3D structure through fragment complementation and thus generates green fluorescence in the nucleus. Dynamic migration of red fluorescence from mitochondria to the nucleus and generation of green fluorescence in the nucleus indicate mitochondrial membrane damage. Using the MMD-Sensor, mitochondrial membrane damage induced by various reagents, such as uncoupling agents, ATP synthase inhibitors, monovalent cationic carriers, and ROS, in HeLa and 293T cells are directly observed and evaluated.

Research progress on morphology and mechanism of programmed cell death

Programmed cell death (PCD) is a basic process of life that is closely related to the growth, development, aging and disease of organisms and is one of the hotspots of life science research today. PCD is a kind of genetic control, autonomous and orderly important cell death that involves the activation, expression, and regulation of a series of genes. In recent years, with the deepening of research in this field, new mechanisms of multiple PCD pathways have been revealed. This article reviews and summarizes the multiple PCD pathways that have been discovered, analyses and compares the morphological characteristics and biomarkers of different types of PCD, and briefly discusses the role of various types of PCD in the diagnosis and treatment of different diseases, especially malignant tumors.

Exploring the role of parthanatos in CNS injury: Molecular insights and therapeutic approaches

BACKGROUND

Central nervous system (CNS) injury causes severe organ damage due to both damage resulting from the injury and subsequent cell death. However, there are currently no effective treatments for countering the irreversible loss of cell function. Parthanatos is a poly (ADP-ribose) polymerase 1 (PARP-1)-dependent form of programmed cell death that is partly responsible for neural cell death. Consequently, the mechanism by which parthanatos promotes CNS injury has attracted significant scientific interest.

AIM OF REVIEW

Our review aims to summarize the potential role of parthanatos in CNS injury and its molecular and pathophysiological mechanisms. Understanding the role of parthanatos and related molecules in CNS injury is crucial for developing effective treatment strategies and identifying important directions for future in-depth research.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Parthanatos (from Thanatos, the personification of death according to Greek mythology) is a type of programmed cell death that is initiated by the overactivation of PARP-1. This process triggers a cascade of reactions, including the accumulation of poly(ADP-ribose) (PAR), the nuclear translocation of apoptosis-inducing factor (AIF) after its release from mitochondria, and subsequent massive DNA fragmentation caused by migration inhibitory factor (MIF) forming a complex with AIF. Secondary molecular mechanisms, such as excitotoxicity and oxidative stress-induced overactivation of PARP-1, significantly exacerbate neuronal damage following initial mechanical injury to the CNS. Furthermore, parthanatos is not only associated with neuronal damage but also interacts with various other types of cell death. This review focuses on the latest research concerning the parthanatos cell death pathway, particularly considering its regulatory mechanisms and functions in CNS damage. We highlight the associations between parthanatos and different cell types involved in CNS damage and discuss potential therapeutic agents targeting the parthanatos pathway.

Protease-independent control of parthanatos by HtrA2/Omi

HtrA2/Omi is a mitochondrial serine protease with ascribed pro-apoptotic as well as pro-necroptotic functions. Here, we establish that HtrA2/Omi also controls parthanatos, a third modality of regulated cell death. Deletion of HtrA2/Omi protects cells from parthanatos while reconstitution with the protease restores the parthanatic death response. The effects of HtrA2/Omi on parthanatos are specific and cannot be recapitulated by manipulating other mitochondrial proteases such as PARL, LONP1 or PMPCA. HtrA2/Omi controls parthanatos in a manner mechanistically distinct from its action in apoptosis or necroptosis, i.e., not by cleaving cytosolic IAP proteins but rather exerting its effects without exiting mitochondria, and downstream of PARP-1, the first component of the parthanatic signaling cascade. Also, previously identified or candidate substrates of HtrA2/Omi such as PDXDC1, VPS4B or moesin are not cleaved and dispensable for parthanatos, whereas DBC-1 and stathmin are cleaved, and thus represent potential parthanatic downstream mediators of HtrA2/Omi. Moreover, mass-spectrometric screening for novel parthanatic substrates of HtrA2/Omi revealed that the induction of parthanatos does not cause a substantial proteolytic cleavage or major alterations in the abundance of mitochondrial proteins. Resolving these findings, reconstitution of HtrA2/Omi-deficient cells with a catalytically inactive HtrA2/Omi mutant restored their sensitivity against parthanatos to the same level as the protease-active HtrA2/Omi protein. Additionally, an inhibitor of HtrA2/Omi's protease activity did not confer protection against parthanatic cell death. Our results demonstrate that HtrA2/Omi controls parthanatos in a protease-independent manner, likely via novel, unanticipated functions as a scaffolding protein and an interaction with so far unknown mitochondrial proteins.

Newly synthesized AIFM1 determines the hypersensitivity of T lymphocytes to STING activation-induced cell apoptosis

STING is a well-known signaling adaptor essential for sensing cytosolic dsDNA to produce type I interferon. Although the detailed underlying mechanisms remain enigmatic, recent studies show that STING activation can lead to T lymphocyte apoptosis. Here, we report that AIFM1 facilitates STING activation-induced cell apoptosis in T lymphocytes. Mechanistically, AIFM1 is upregulated after STING activation in T cells but not in HEK293T-STING and THP-1 cells, rendering T cells more sensitive to apoptosis. In contrast to the canonical role of AIFM1 in the caspase-independent parthanatos, the function of AIFM1 is operated by the formation of an AIFM1/IRF3/BAX complex and mitochondrial outer membrane permeabilization, which cause cytochrome c release and caspase activation. Furthermore, supplementation with newly synthesized AIFM1 can reconstitute STING activation-induced cell apoptosis in HEK293T-STING and THP-1 cells. Our study identifies AIFM1 as a key regulating factor determining the hypersensitivity of T lymphocytes to STING activation-induced cell apoptosis.

The mitochondrial permeability transition pore in Ca2+ homeostasis

The mitochondrial Permeability Transition Pore (PTP) can be defined as a Ca²⁺ activated mega-channel involved in mitochondrial damage and cell death, making its inhibition a hallmark for therapeutic purposes in many PTP-related paradigms. Although long-lasting PTP openings have been widely studied, the physiological implications of transient openings (also called "flickering" behavior) are still poorly understood. The flickering activity was suggested to play a role in the regulation of Ca²⁺ and ROS homeostasis, and yet this hypothesis did not reach general consensus. This state of affairs might arise from the lack of unquestionable experimental evidence, due to limitations of the available techniques for capturing transient PTP activity and to a still partial understanding of its molecular identity. In this review we will focus on possible implications of the PTP in physiology, in particular its role as a Ca²⁺ release pathway, discussing the consequences of its forced inhibition. We will also consider the recent hypothesis of the existence of more permeability pathways and their potential involvement in mitochondrial physiology.

Anti-apoptosis effect of traditional Chinese medicine in the treatment of cerebral ischemia-reperfusion injury

Cerebral ischemia, one of the leading causes of neurological dysfunction of brain cells, muscle dysfunction, and death, brings great harm and challenges to individual health, families, and society. Blood flow disruption causes decreased glucose and oxygen, insufficient to maintain normal brain tissue metabolism, resulting in intracellular calcium overload, oxidative stress, neurotoxicity of excitatory amino acids, and inflammation, ultimately leading to neuronal cell necrosis, apoptosis, or neurological abnormalities. This paper summarizes the specific mechanism of cell injury that apoptosis triggered by reperfusion after cerebral ischemia, the related proteins involved in apoptosis, and the experimental progress of herbal medicine treatment through searching, analyzing, and summarizing the PubMed and Web Of Science databases, which includes active ingredients of herbal medicine, prescriptions, Chinese patent medicines, and herbal extracts, providing a new target or new strategy for drug treatment, and providing a reference for future experimental directions and using them to develop suitable small molecule drugs for clinical application. With the research of anti-apoptosis as the core, it is important to find highly effective, low toxicity, safe and cheap compounds from natural plants and animals with abundant resources to prevent and treat Cerebral ischemia/reperfusion (I/R) injury (CIR) and solve human suffering. In addition, understanding and summarizing the apoptotic mechanism of cerebral ischemia-reperfusion injury, the microscopic mechanism of CIR treatment, and the cellular pathways involved will help to develop new drugs.

Cathepsins Trigger Cell Death and Regulate Radioresistance in Glioblastoma

Treatment of glioblastoma (GBM) remains very challenging, and it is particularly important to find sensitive and specific molecular targets. In this work, we reveal the relationship between the expression of cathepsins and radioresistance in GBM. We analyzed cathepsins (cathepsin B, cathepsin D, cathepsin L, and cathepsin Z/X), which are highly associated with the radioresistance of GBM by regulating different types of cell death. Cathepsins could be potential targets for GBM treatment.

Germinated brown rice protects against glutamate toxicity in HT22 hippocampal neurons through the jnk-mediated apoptotic pathway via the GABAA receptor

The anti-apoptosis effect of germinated brown rice (GBR) focusing on differentiated HT22 cells results in improved nutritional values after the germination process of GBR which contains total phenolic compounds and γ-aminobutyric acid (GABA). Cell death induced by 5 mM glutamate was investigated for 24 h to determine whether GBR mediates cell death through GABA receptors by using antagonists. The results showed that GBR (100 µg/ml) suppressed glutamate-induced cytotoxicity and caused arrest at the G1/S phase of the cell cycle in differentiated HT22 cells. Furthermore, GBR significantly decreased the expression level of c-Jun, while its active form, p-c-Jun, is the downstream product of the JNK-mediated apoptotic pathway and causes subsequent cell death. In addition, bicuculline (12.5 nM), a GABA_A antagonist, could eliminate GBR effects, but phaclofen (1 mM), a GABA_B antagonist, could not. Surprisingly, GBR exhibited a better neuroprotective effect than a pure commercial GABA compound (0.115 µM). These results indicated that GBR possessed high anti-apoptotic activity and inhibited cell death in differentiated HT22 cells by perturbing re-entry of the cell cycle and apoptosis via the GABA_A receptor. Hence, GBR could be further used as a valuable nutritional compound to prevent apoptosis-induced neurodegenerative diseases.

Upregulation of Ca2+-binding proteins contributes to VTA dopamine neuron survival in the early phases of Alzheimer's disease in Tg2576 mice

BACKGROUND

Recent clinical and experimental studies have highlighted the involvement of Ventral Tegmental Area (VTA) dopamine (DA) neurons for the early pathogenesis of Alzheimer's Disease (AD). We have previously described a progressive and selective degeneration of these neurons in the Tg2576 mouse model of AD, long before amyloid-beta plaque formation. The degenerative process in DA neurons is associated with an autophagy flux impairment, whose rescue can prevent neuronal loss. Impairments in autophagy can be the basis for accumulation of damaged mitochondria, leading to disturbance in calcium (Ca²⁺) homeostasis, and to functional and structural deterioration of DA neurons.

METHODS

In Tg2576 mice, we performed amperometric recordings of DA levels and analysis of dopaminergic fibers in the Nucleus Accumbens - a major component of the ventral striatum precociously affected in AD patients - together with retrograde tracing, to identify the most vulnerable DA neuron subpopulations in the VTA. Then, we focused on these neurons to analyze mitochondrial integrity and Apoptosis-inducing factor (AIF) localization by electron and confocal microscopy, respectively. Stereological cell count was also used to evaluate degeneration of DA neuron subpopulations containing the Ca²⁺-binding proteins Calbindin-D28K and Calretinin. The expression levels for these proteins were analyzed by western blot and confocal microscopy. Lastly, using electrophysiology and microfluorometry we analyzed VTA DA neuron intrinsic properties and cytosolic free Ca²⁺ levels.

RESULTS

We found a progressive degeneration of mesolimbic DA neurons projecting to the ventral striatum, located in the paranigral nucleus and parabrachial pigmented subnucleus of the VTA. At the onset of degeneration (3 months of age), the vulnerable DA neurons in the Tg2576 accumulate damaged mitochondria, while AIF translocates from the mitochondria to the nucleus. Although we describe an age-dependent loss of the DA neurons expressing Calbindin-D28K or Calretinin, we observed that the remaining cells upregulate the levels of Ca²⁺-binding proteins, and the free cytosolic levels of Ca²⁺ in these neurons are significantly decreased. Coherently, TUNEL-stained Tg2576 DA neurons express lower levels of Calbindin-D28K when compared with non-apoptotic cells.

CONCLUSION

Overall, our results suggest that the overexpression of Ca²⁺-binding proteins in VTA DA neurons might be an attempt of cells to survive by increasing their ability to buffer free Ca²⁺. Exploring strategies to overexpress Ca²⁺-binding proteins could be fundamental to reduce neuronal suffering and improve cognitive and non-cognitive functions in AD.

Covalent conjugation of ubiquitin-like ISG15 to apoptosis inducing factor exacerbates toxic stimuli-induced apoptotic cell death

Apoptosis-inducing factor (AIF) is a mitochondrion-localized flavoprotein with NADH oxidase activity. AIF normally acts as an oxidoreductase to catalyze the transfer of electrons between molecules, but it can also kill cells when exposed to certain stimuli. For example, intact AIF is cleaved upon exposure to DNA-damaging agents such as etoposide, and truncated AIF (tAIF) is released from the mitochondria to the cytoplasm and translocated to the nucleus where it induces apoptosis. Although the serial events during tAIF-mediated apoptosis and the transition of AIF function have been widely studied from various perspectives, their underlying regulatory mechanisms and the factors involved are not fully understood. Here, we demonstrated that tAIF is a target of the covalent conjugation of the ubiquitin-like moiety ISG15 (referred to as ISGylation), which is mediated by the ISG15 E3 ligase HERC5. In addition, ISGylation increases the stability of tAIF protein as well as its K6-linked polyubiquitination. Moreover, we found that ISGylation increases the nuclear translocation of tAIF upon cytotoxic etoposide treatment, subsequently causing apoptotic cell death in human lung A549 carcinoma cells. Collectively, these results suggest that HERC5-mediated ISG15 conjugation is a key factor in the positive regulation of tAIF-mediated apoptosis, highlighting a novel role of posttranslational ISG15 modification as a switch that allows cells to live or die under the stress that triggers tAIF release.

AIFM1 beyond cell death: An overview of this OXPHOS-inducing factor in mitochondrial diseases

Apoptosis-inducing factor (AIF) is a mitochondrial intermembrane space flavoprotein with diverse functions in cellular physiology. In this regard, a large number of studies have elucidated AIF's participation to chromatin condensation during cell death in development, cancer, cardiovascular and brain disorders. However, the discovery of rare AIFM1 mutations in patients has shifted the interest of biomedical researchers towards AIF's contribution to pathogenic mechanisms underlying inherited AIFM1-linked metabolic diseases. The functional characterization of AIF binding partners has rapidly advanced our understanding of AIF biology within the mitochondria and beyond its widely reported role in cell death. At the present time, it is reasonable to assume that AIF contributes to cell survival by promoting biogenesis and maintenance of the mitochondrial oxidative phosphorylation (OXPHOS) system. With this review, we aim to outline the current knowledge around the vital role of AIF by primarily focusing on currently reported human diseases that have been linked to AIFM1 deficiency.

Inflammation: Roles in Skeletal Muscle Atrophy

Various diseases can cause skeletal muscle atrophy, usually accompanied by inflammation, mitochondrial dysfunction, apoptosis, decreased protein synthesis, and enhanced proteolysis. The underlying mechanism of inflammation in skeletal muscle atrophy is extremely complex and has not been fully elucidated, thus hindering the development of effective therapeutic drugs and preventive measures for skeletal muscle atrophy. In this review, we elaborate on protein degradation pathways, including the ubiquitin-proteasome system (UPS), the autophagy-lysosome pathway (ALP), the calpain and caspase pathways, the insulin growth factor 1/Akt protein synthesis pathway, myostatin, and muscle satellite cells, in the process of muscle atrophy. Under an inflammatory environment, various pro-inflammatory cytokines directly act on nuclear factor-κB, p38MAPK, and JAK/STAT pathways through the corresponding receptors, and then are involved in muscle atrophy. Inflammation can also indirectly trigger skeletal muscle atrophy by changing the metabolic state of other tissues or cells. This paper explores the changes in the hypothalamic-pituitary-adrenal axis and fat metabolism under inflammatory conditions as well as their effects on skeletal muscle. Moreover, this paper also reviews various signaling pathways related to muscle atrophy under inflammatory conditions, such as cachexia, sepsis, type 2 diabetes mellitus, obesity, chronic obstructive pulmonary disease, chronic kidney disease, and nerve injury. Finally, this paper summarizes anti-amyotrophic drugs and their therapeutic targets for inflammation in recent years. Overall, inflammation is a key factor causing skeletal muscle atrophy, and anti-inflammation might be an effective strategy for the treatment of skeletal muscle atrophy. Various inflammatory factors and their downstream pathways are considered promising targets for the treatment and prevention of skeletal muscle atrophy.

Calpain-Independent Intracellular Protease Activity Is Elevated in Excitotoxic Cortical Neurons Prior to Delayed Calcium Deregulation and Mitochondrial Dysfunction

Glutamate excitotoxicity contributes to many neurodegenerative diseases. Excessive glutamate receptor-mediated calcium entry causes delayed calcium deregulation (DCD) that coincides with abrupt mitochondrial depolarization. We developed cA-TAT, a live-cell protease activity reporter based on a vimentin calpain cleavage site, to test whether glutamate increases protease activity in neuronal cell bodies prior to DCD. Treatment of rat cortical neurons with excitotoxic (100 µM) glutamate increased the low baseline rate of intracellular cA-TAT proteolysis by approximately three-fold prior to DCD and by approximately seven-fold upon calcium deregulation. The glutamate-induced rate enhancement prior to DCD was suppressed by glutamate receptor antagonists, but not by calpain or proteasome inhibitors, whereas DCD-stimulated proteolysis was partly attenuated by the proteasome inhibitor MG132. Further suggesting that cA-TAT cleavage is calpain-independent, cA-TAT fluorescence was observed in immortalized Capn4 knockout fibroblasts lacking the regulatory calpain subunit. About half of the neurons lost calcium homeostasis within two hours of a transient, 20 min glutamate receptor stimulation. These neurons had a significantly (49%) higher mean baseline cA-TAT proteolysis rate than those maintaining calcium homeostasis, suggesting that the unknown protease(s) cleaving cA-TAT may influence DCD susceptibility. Overall, the results indicate that excitotoxic glutamate triggers the activation of calpain-independent neuronal protease activity prior to the simultaneous loss of calcium homeostasis and mitochondrial bioenergetic function.

Retinitis Pigmentosa: Progress in Molecular Pathology and Biotherapeutical Strategies

Retinitis pigmentosa (RP) is genetically heterogeneous retinopathy caused by photoreceptor cell death and retinal pigment epithelial atrophy that eventually results in blindness in bilateral eyes. Various photoreceptor cell death types and pathological phenotypic changes that have been disclosed in RP demand in-depth research of its pathogenic mechanism that may account for inter-patient heterogeneous responses to mainstream drug treatment. As the primary method for studying the genetic characteristics of RP, molecular biology has been widely used in disease diagnosis and clinical trials. Current technology iterations, such as gene therapy, stem cell therapy, and optogenetics, are advancing towards precise diagnosis and clinical applications. Specifically, technologies, such as effective delivery vectors, CRISPR/Cas9 technology, and iPSC-based cell transplantation, hasten the pace of personalized precision medicine in RP. The combination of conventional therapy and state-of-the-art medication is promising in revolutionizing RP treatment strategies. This article provides an overview of the latest research on the pathogenesis, diagnosis, and treatment of retinitis pigmentosa, aiming for a convenient reference of what has been achieved so far.

A novel peptide encoded by N6-methyladenosine modified circMAP3K4 prevents apoptosis in hepatocellular carcinoma

Background

Circular RNAs (circRNAs) regulate various biological activities and have been shown to play crucial roles in hepatocellular carcinoma (HCC) progression. However, only a few coding circRNAs have been identified in cancers, and their roles in HCC remain elusive. This study aimed to identify coding circRNAs and explore their function in HCC.

Methods

CircMAP3K4 was selected from the CIRCpedia database. We performed a series of experiments to determine the characteristics and coding capacity of circMAP3K4. We then used in vivo and in vitro assays to investigate the biological function and mechanism of circMAP3K4 and its protein product, circMAP3K4-455aa, in HCC.

Results

We found circMAP3K4 to be an upregulated circRNA with coding potential in HCC. IGF2BP1 recognized the circMAP3K4 N6-methyladenosine modification and promoted its translation into circMAP3K4-455aa. Functionally, circMAP3K4-455aa prevented cisplatin-induced apoptosis in HCC cells by interacting with AIF, thus protecting AIF from cleavage and decreasing its nuclear distribution. Moreover, circMAP3K4-455aa was degraded through the ubiquitin–proteasome E3 ligase MIB1 pathway. Clinically, a high level of circMAP3K4 is an independent prognostic factor for adverse overall survival and adverse disease-free survival of HCC patients.

Conclusions

CircMAP3K4 is a highly expressed circRNA in HCC. Driven by m6A modification, circMAP3K4 encoded circMAP3K4-455aa, protected HCC cells from cisplatin exposure, and predicted worse prognosis of HCC patients. Targeting circMAP3K4-455aa may provide a new therapeutic strategy for HCC patients, especially for those with chemoresistance.

Graphical Abstract

CircMAP3K4 is a highly expressed circRNA in HCC. Driven by m6A modification, IGF2BP1 facilitates circMAP3K4 peptide translation, then the circMAP3K4 peptide inhibits AIF cleavage and nuclear distribution, preventing HCC cells from cell death under stress and promoting HCC progression.

Ars moriendi: Proteases as sculptors of cellular suicide

The Ars moriendi, which translates to "The Art of Dying," encompasses two Latin texts that gave advice on how to die well and without fear according to the Christian precepts of the late Middle Ages. Given that ten to hundred billion cells die in our bodies every day, it is obvious that the concept of a well and orderly ("regulated") death is also paramount at the cellular level. In apoptosis, as the most well-studied form of regulated cell death, proteases of the caspase family are the central mediators. However, caspases are not the only proteases that act as sculptors of cellular suicide, and therefore, we here provide an overview of the impact of proteases in apoptosis and other forms of regulated cell death.

Reversing mitochondrial defects in aged hearts: role of mitochondrial calpain activation

Aging chronically increases endoplasmic reticulum (ER) stress that contributes to mitochondrial dysfunction. Activation of calpain 1 (CPN1) impairs mitochondrial function during acute ER stress. We proposed that aging-induced ER stress led to mitochondrial dysfunction by activating CPN1. We posit that attenuation of the ER stress or direct inhibition of CPN1 in aged hearts can decrease cardiac injury during ischemia-reperfusion by improving mitochondrial function. Male young (3 mo) and aged mice (24 mo) were used in the present study, and 4-phenylbutyrate (4-PBA) was used to decrease the ER stress in aged mice. Subsarcolemmal (SSM) and interfibrillar mitochondria (IFM) were isolated. Chronic 4-PBA treatment for 2 wk decreased CPN1 activation as shown by the decreased cleavage of spectrin in cytosol and apoptosis inducing factor (AIF) and the α1 subunit of pyruvate dehydrogenase (PDH) in mitochondria. Treatment improved oxidative phosphorylation in 24-mo-old SSM and IFM at baseline compared with vehicle. When 4-PBA-treated 24-mo-old hearts were subjected to ischemia-reperfusion, infarct size was decreased. These results support that attenuation of the ER stress decreased cardiac injury in aged hearts by improving mitochondrial function before ischemia. To challenge the role of CPN1 as an effector of the ER stress, aged mice were treated with MDL-28170 (MDL, an inhibitor of calpain 1). MDL treatment improved mitochondrial function in aged SSM and IFM. MDL-treated 24-mo-old hearts sustained less cardiac injury following ischemia-reperfusion. These results support that age-induced ER stress augments cardiac injury during ischemia-reperfusion by impairing mitochondrial function through activation of CPN1.

The key players of parthanatos: opportunities for targeting multiple levels in the therapy of parthanatos-based pathogenesis

Parthanatos is a form of regulated cell death involved in the pathogenesis of many diseases, particularly neurodegenerative disorders, such as Parkinson's disease, Alzheimer's disease, Huntington's disease, and amyotrophic lateral sclerosis. Parthanatos is a multistep cell death pathway cascade that involves poly (ADP-ribose) polymerase 1 (PARP-1) overactivation, PAR accumulation, PAR binding to apoptosis-inducing factor (AIF), AIF release from the mitochondria, nuclear translocation of the AIF/macrophage migration inhibitory factor (MIF) complex, and MIF-mediated large-scale DNA fragmentation. All the key players in the parthanatos pathway are pleiotropic proteins with diverse functions. An in-depth understanding of the structure-based activity of the key factors, and the biochemical mechanisms of parthanatos, is crucial for the development of drugs and therapeutic strategies. In this review, we delve into the key players of the parthanatos pathway and reveal the multiple levels of therapeutic opportunities for treating parthanatos-based pathogenesis.

Baicalein ameliorated cerebral ischemia-reperfusion injury dependent on calpain 1/AIF pathway

Cerebral ischemia-reperfusion (CIR) has become the leading cause of death and disability. Baicalein is a natural bioactive ingredient extracted from Scutellaria baicalensis Georgi and has neuroprotective activity. In our work, baicalein was found to reduce neurological deficits, brain water content, infarct area and neuronal death of rats induced by middle cerebral artery occlusion/reperfusion. In vitro, oxygen-glucose deprivation/reperfusion induced inordinate ROS production and apoptosis that could be reversed by baicalein. Our study revealed for the first time that baicalein has the potential to binds and inhibits the activity of calpain 1, thereby inhibiting AIF nuclear translocation. These findings demonstrated that baicalein protected against CIR injury via inhibiting AIF nuclear translocation by inhibiting calpain 1 activity.

RNA Biological Characteristics at the Peak of Cell Death in Different Hereditary Retinal Degeneration Mutants

Purpose: The present work investigated changes in the gene expression, molecular mechanisms, and pathogenesis of inherited retinal degeneration (RD) in three different disease models, to identify predictive biomarkers for their varied phenotypes and to provide a better scientific basis for their diagnosis, treatment, and prevention. Methods: Differentially expressed genes (DEGs) between retinal tissue from RD mouse models obtained during the photoreceptor cell death peak period (Pde6b ^rd1 at post-natal (PN) day 13, Pde6b ^rd10 at PN23, Prph ^rd2 at PN29) and retinal tissue from C3H wild-type mice were identified using Illumina high-throughput RNA-sequencing. Co-expression gene modules were identified using a combination of GO and KEGG enrichment analyses and gene co-expression network analysis. CircRNA-miRNA-mRNA network interactions were studied by genome-wide circRNA screening. Results: Pde6b ^rd1 , Pde6b ^rd10 , and Prph ^rd2 mice had 1,926, 3,096, and 375 DEGs, respectively. Genes related to ion channels, stress, inflammatory processes, tumor necrosis factor (TNF) production, and microglial cell activation were up-regulated, while genes related to endoplasmic reticulum regulation, metabolism, and homeostasis were down-regulated. Differential expression of transcription factors and non-coding RNAs generally implicated in other human diseases was detected (e.g., glaucoma, diabetic retinopathy, and inherited retinal degeneration). CircRNA-miRNA-mRNA network analysis indicated that these factors may be involved in photoreceptor cell death. Moreover, excessive cGMP accumulation causes photoreceptor cell death, and cGMP-related genes were generally affected by different pathogenic gene mutations. Conclusion: We screened genes and pathways related to photoreceptor cell death. Additionally, up-stream regulatory factors, such as transcription factors and non-coding RNA and their interaction networks were analyzed. Furthermore, RNAs involved in RD were functionally annotated. Overall, this study lays a foundation for future studies on photoreceptor cell death mechanisms.

Mitochondrial Carrier Homolog 2 Functionally Co-operates With BH3 Interacting-Domain Death Agonist in Promoting Ca2+-Induced Neuronal Injury

The BH3 interacting-domain death agonist (BID) is a pro-apoptotic member of the Bcl-2 protein family. While proteolytic processing of BID links death receptor-induced apoptosis to the mitochondrial apoptosis pathway, we previously showed that full length BID also translocates to mitochondria during Ca²⁺-induced neuronal cell death. Moreover, mitochondrial carrier homolog 2 (MTCH2) was identified as a mitochondrial protein that interacts with BID during cell death. We started our studies by investigating the effect of Mtch2 silencing in a well-established model of Ca²⁺-induced mitochondrial permeability transition pore opening in non-neuronal HCT116 cells. We found that silencing of Mtch2 inhibited mitochondrial swelling and the associated decrease in mitochondrial energetics, suggesting a pro-death function for MTCH2 during Ca²⁺-induced injury. Next, we explored the role of BID and MTCH2 in mediating Ca²⁺-induced injury in primary cortical neurons triggered by prolonged activation of NMDA glutamate receptors. Analysis of intracellular Ca²⁺ transients, using time-lapse confocal microscopy, revealed that neurons lacking Bid showed markedly reduced Ca²⁺ levels during the NMDA excitation period. These Ca²⁺ transients were further decreased when Mtch2 was also silenced. Collectively, our data suggest that BID and MTCH2 functionally interact to promote Ca²⁺-induced neuronal injury.

Catecholamine-induced cardiotoxicity: A critical element in the pathophysiology of stroke-induced heart injury

Cerebrovascular diseases such as ischemic stroke, brain hemorrhage, and subarachnoid hemorrhage provoke cardiac complications such as heart failure, neurogenic stress-related cardiomyopathy and Takotsubo cardiomyopathy. With regards to the pathophysiology of stroke-induced heart injury, several mechanisms have been postulated to contribute to this complex interaction between brain and heart, including damage from gut dysbiosis, immune and systematic inflammatory responses, microvesicle- and microRNA-mediated vascular injury and damage from a surge of catecholamines. All these cerebrovascular diseases may trigger pronounced catecholamine surges through diverse ways, including stimulation of hypothalamic-pituitary adrenal axis, dysregulation of autonomic system, and secretion of adrenocorticotropic hormone. Primary catecholamines involved in this pathophysiological response include norepinephrine (NE) and epinephrine. Both are important neurotransmitters that connect the nervous system with the heart, leading to cardiac damage via myocardial ischemia, calcium (Ca²⁺) overload, oxidative stress, and mitochondrial dysfunction. In this review, we will aim to summarize the molecular mechanisms behind catecholamine-induced cardiotoxicity including Ca²⁺ overload, oxidative stress, apoptosis, cardiac hypertrophy, interstitial fibrosis, and inflammation. In addition, we will focus on how synchronization among these pathways evokes cardiotoxicity.

Cyclophilin D Regulates the Nuclear Translocation of AIF, Cardiac Endothelial Cell Necroptosis and Murine Cardiac Transplant Injury

Ischemia-reperfusion injury (IRI) is an inevitable consequence of organ transplant procedure and associated with acute and chronic organ rejection in transplantation. IRI leads to various forms of programmed cell death, which worsens tissue damage and accelerates transplant rejection. We recently demonstrated that necroptosis participates in murine cardiac microvascular endothelial cell (MVEC) death and murine cardiac transplant rejection. However, MVEC death under a more complex IRI model has not been studied. In this study, we found that simulating IRI conditions in vitro by hypoxia, reoxygenation and treatment with inflammatory cytokines induced necroptosis in MVECs. Interestingly, the apoptosis-inducing factor (AIF) translocated to the nucleus during MVEC necroptosis, which is regulated by the mitochondrial permeability molecule cyclophilin D (CypD). Furthermore, CypD deficiency in donor cardiac grafts inhibited AIF translocation and mitigated graft IRI and rejection (n = 7; p = 0.002). Our studies indicate that CypD and AIF play significant roles in MVEC necroptosis and cardiac transplant rejection following IRI. Targeting CypD and its downstream AIF may be a plausible approach to inhibit IRI-caused cardiac damage and improve transplant survival.

Cell Death and Survival Pathways Involving ATM Protein Kinase

Cell death is the ultimate form of cellular dysfunction, and is induced by a wide range of stresses including genotoxic stresses. During genotoxic stress, two opposite cellular reactions, cellular protection through DNA repair and elimination of damaged cells by the induction of cell death, can occur in both separate and simultaneous manners. ATM (ataxia telangiectasia mutated) kinase (hereafter referred to as ATM) is a protein kinase that plays central roles in the induction of cell death during genotoxic stresses. It has long been considered that ATM mediates DNA damage-induced cell death through inducing apoptosis. However, recent research progress in cell death modality is now revealing ATM-dependent cell death pathways that consist of not only apoptosis but also necroptosis, ferroptosis, and dysfunction of autophagy, a cellular survival mechanism. In this short review, we intend to provide a brief outline of cell death mechanisms in which ATM is involved, with emphasis on pathways other than apoptosis.

Staurosporine-induced cleavage of apoptosis-inducing factor in human fibrosarcoma cells is independent of matrix metalloproteinase-2

Apoptosis-inducing factor (AIF) is a mitochondrial flavoprotein which mediates staurosporine (STS)-induced cell death. AIF cleavage and translocation to the cytosol is thought to be calpain-1-dependent as calpain inhibitors reduced AIF proteolysis. However, many calpain inhibitors also inhibit matrix metalloproteinase-2 (MMP-2) activity, an intracellular and extracellular protease implicated in apoptosis. Here we investigated whether MMP-2 activity is affected in response to STS and if contributes to AIF cleavage. Human fibrosarcoma HT1080 cells were treated with STS (0.1 µM, 0.25-24 hr). A significant increase in cellular MMP-2 activity was seen by gelatin zymography after 6 hr STS treatment, prior to induction of cell necrosis. Western blot showed the time-dependent appearance of two forms of AIF (~60 and 45 kDa) in the cytosol which were significantly increased at 6 hr. Surprisingly, knocking down MMP-2 or inhibiting its activity with MMP-2 preferring inhibitors ARP-100 or ONO-4817, or inhibiting calpain activity with ALLM or PD150606, did not prevent the STS-induced increase in cytosolic AIF. These results show that although STS rapidly increases MMP-2 activity, the cytosolic release of AIF may be independent of the proteolytic activities of MMP-2 or calpain.

Identification of Phosphorylated Calpain 3 in Rat Brain Mitochondria under mPTP Opening

The protein phosphorylation of the membrane-bound mitochondrial proteins has become of interest from the point of view of its regulatory role of the function of the respiratory chain, opening of the mitochondrial permeability transition pore (mPTP), and initiation of apoptosis. Earlier, we noticed that upon phosphorylation of proteins in some proteins, the degree of their phosphorylation increases with the opening of mPTP. Two isoforms of myelin basic protein and cyclic nucleotide phosphodiesterase were identified in rat brain non-synaptic mitochondria and it was concluded that they are involved in mPTP regulation. In the present study, using the mass spectrometry method, the phosphorylated protein was identified as Calpain 3 in rat brain non-synaptic mitochondria. In the present study, the phosphoprotein Calpain-3 (p94) (CAPN3) was identified in the rat brain mitochondria as a phosphorylated truncated form of p60–62 kDa by two-dimensional electrophoresis and mass spectrometry. We showed that the calpain inhibitor, calpeptin, was able to suppress the Ca²⁺ efflux from mitochondria, preventing the opening of mPTP. It was found that phosphorylated truncated CALP3 with a molecular weight of 60–62 contains p-Tyr, which indicates the possible involvement of protein tyrosine phosphatase in this process.

Calpain-1 C2L domain peptide protects mouse hippocampus-derived neuronal HT22 cells against glutamate-induced oxytosis

Calpains are Ca²⁺-dependent cysteine proteases; their aberrant activation is associated with several neurodegenerative diseases. The μ-calpain catalytic subunit, calpain-1, is located in the cytoplasm as well as in the mitochondria. Mitochondrial calpain-1 cleaves apoptosis-inducing factor (AIF), leading to apoptotic cell death. We have previously reported that short peptides of calpain-1 C2-like domain conjugated with cell penetrating peptide HIV-Tat (Tat-μCL) selectively inhibit mitochondrial calpain-1 and effectively prevent neurodegenerative diseases of the eye. In this study, we determined whether mitochondrial calpain-1 mediates oxytosis (oxidative glutamate toxicity) in hippocampal HT22 cells using Tat-μCL and newly generated polyhistidine-conjugated μCL peptide and compared their efficacies in preventing oxytosis. TUNEL assay and single strand DNA staining revealed that both μCL peptides inhibited glutamate-induced oxytosis. Additionally, both the peptides suppressed the mitochondrial AIF translocation into the nucleus. All polyhistidine-μCL peptides (containing 4–16 histidine residues) showed higher cell permeability than Tat-μCL. Notably, tetrahistidine (H4)-μCL exerted the highest cytoprotective activity. Thus, H4-μCL may be a potential peptide drug for calpain-1-mediated neurodegenerative diseases such as Alzheimer's disease.

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Mitochondrial calpain-1 mediates glutamate-induced oxytosis in HT22 cells.

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CPP-μCL inhibits AIF translocation and DNA fragmentation in oxytosis.

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Polyhistidine conjugation enhances intracellular μCL peptide uptake efficiency.

Exercise triggers CAPN1-mediated AIF truncation, inducing myocyte cell death in arrhythmogenic cardiomyopathy

Myocyte death occurs in many inherited and acquired cardiomyopathies, including arrhythmogenic cardiomyopathy (ACM), a genetic heart disease plagued by the prevalence of sudden cardiac death. Individuals with ACM and harboring pathogenic desmosomal variants, such as desmoglein-2 (DSG2), often show myocyte necrosis with progression to exercise-associated heart failure. Here, we showed that homozygous Dsg2 mutant mice (Dsg2 ^mut/mut), a model of ACM, die prematurely during swimming and display myocardial dysfunction and necrosis. We detected calcium (Ca²⁺) overload in Dsg2 ^mut/mut hearts, which induced calpain-1 (CAPN1) activation, association of CAPN1 with mitochondria, and CAPN1-induced cleavage of mitochondrial-bound apoptosis-inducing factor (AIF). Cleaved AIF translocated to the myocyte nucleus triggering large-scale DNA fragmentation and cell death, an effect potentiated by mitochondrial-driven AIF oxidation. Posttranslational oxidation of AIF cysteine residues was due, in part, to a depleted mitochondrial thioredoxin-2 redox system. Hearts from exercised Dsg2 ^mut/mut mice were depleted of calpastatin (CAST), an endogenous CAPN1 inhibitor, and overexpressing CAST in myocytes protected against Ca²⁺ overload-induced necrosis. When cardiomyocytes differentiated from Dsg2 ^mut/mut embryonic stem cells (ES-CMs) were challenged with β-adrenergic stimulation, CAPN1 inhibition attenuated CAPN1-induced AIF truncation. In addition, pretreatment of Dsg2 ^mut/mut ES-CMs with an AIF-mimetic peptide, mirroring the cyclophilin-A (PPIA) binding site of AIF, blocked PPIA-mediated AIF-nuclear translocation, and reduced both apoptosis and necrosis. Thus, preventing CAPN1-induced AIF-truncation or barring binding of AIF to the nuclear chaperone, PPIA, may avert myocyte death and, ultimately, disease progression to heart failure in ACM and likely other forms of cardiomyopathies.

Phosphorylation-Dependent Interactome of Ryanodine Receptor Type 2 in the Heart

Hyperphosphorylation of the calcium release channel/ryanodine receptor type 2 (RyR2) at serine 2814 (S2814) is associated with multiple cardiac diseases including atrial fibrillation and heart failure. Despite recent advances, the molecular mechanisms driving pathological changes associated with RyR2 S2814 phosphorylation are still not well understood. Methods: Using affinity-purification coupled to mass spectrometry (AP-MS), we investigated the RyR2 interactome in ventricles from wild-type (WT) mice and two S2814 knock-in mutants: the unphosphorylated alanine mutant (S2814A) and hyperphosphorylated mimic aspartic acid mutant (S2814D). Western blots were used for validation. Results: In WT mouse ventricular lysates, we identified 22 proteins which were enriched with RyR2 pull-down relative to both IgG control and no antibody (beads-only) pull-downs. Parallel AP-MS using WT, S2814A, and S2814D mouse ventricles identified 72 proteins, with 20 being high confidence RyR2 interactors. Of these, 14 had an increase in their binding to RyR2 S2814A but a decrease in their binding to RyR2 S2814D. We independently validated three protein hits, Idh3b, Aifm1, and Cpt1b, as RyR2 interactors by western blots and showed that Aifm1 and Idh3b had significantly decreased binding to RyR2 S2814D compared to WT and S2814A, consistent with MS findings. Conclusion: By applying state-of-the-art proteomic approaches, we discovered a number of novel RyR2 interactors in the mouse heart. In addition, we found and defined specific alterations in the RyR2 interactome that were dependent on the phosphorylation status of RyR2 at S2814. These findings yield mechanistic insights into RyR2 regulation which may guide future drug designs.

Association between Cav3 channel upregulation in spiral ganglion neurons and age-dependent hearing loss

Ca_v3 channels play a critical role in maintaining calcium homeostasis, and its dysregulation is related to age-related diseases, such as age-related hearing loss (AHL). However, the underlying mechanism of the Ca_v3 channels involved in AHL remains unknown. Previous studies have shown that the degeneration of spiral ganglion neurons (SGNs) plays a critical role in AHL. Here, we explored the involvement of Ca_v3 channels in the dysregulation of SGNs in AHL. We used C57BL/6 mice as the AHL mouse model and found that the expression of Ca_v3 channels was increased in SGNs associated with age. The three subtypes of Ca_v3 channels were present in the apical, middle, and basal SGNs from young and older (AHL) mice. The immunostaining data suggest that Ca_v3.1 and Ca_v3.2 may contribute to Ca_v3 upregulation in SGNs of AHL mice. Additionally, we found that calpain-2 and apoptosis-inducing factor (AIF) were activated in SGNs from AHL mice. The inhibition of Ca_v3 channels or calpain-2 reduced AIF-activation in SGNs may affect neuronal survival. In conclusion, the findings suggest that Ca_v3 channels are upregulated in SGNs from AHL mice that may contribute to the degeneration of SGNs through the calpain-2-AIF apoptosis pathway in AHL mice.

Molecular characterization and functional analysis of apoptosis-inducing factor (AIF) in palmitic acid-induced apoptosis in Ctenopharyngodon idellus kidney (CIK) cells

Palmitic acid (PA), the most common saturated free fatty acid, may cause apoptosis when overloaded in non-fat cells. Apoptosis-inducing factor (AIF) is known to translocate from the mitochondria into the nucleus to induce apoptosis. However, it remains to be investigated whether AIF involved in palmitic acid-induced lipoapoptosis in fish. In the present study, we cloned a coding sequence of grass carp (Ctenopharyngodon idella) AIF (CiAIF) gene, and determined its function in Ctenopharyngodon idellus kidney (CIK) cells. The open reading frame (ORF) of CiAIF gene is 1863 bp, encoding a precursor protein of 620 amino acids (aa). Sequence analysis indicated that CiAIF contains a mitochondrial localization sequence, a conserved Pyr_redox and a C-terminal domain. Phylogenetic analyses showed that the CiAIF gene tended to cluster with sequences from Danio rerio. CiAIF gene was ubiquitously expressed in all tested tissues, including heart, liver, spleen, muscle, brain, eye, kidney, intestine, and fat. Moreover, we demonstrated that PA treatment induced the expression level of CiAIF and increases in markers of endoplasmic reticulum (ER) stress and apoptosis. Meanwhile, ER stress-inducing agent thapsigargin (TG) induced CiAIF translocated into the nucleus in CIK cells, whereas the suppression of ER stress inhibited PA-induced CiAIF expression and apoptosis. In addition, overexpression of CiAIF caused apoptosis by upregulating capase9, capase8, and capase3b, and affects protein translation via directly interacting with CieIF3g. Taken together, our data indicate that in Ctenopharyngodon idellus, PA is key elements that affect not only ER stress and mitochondrial apoptosis but also different physiological functions, such as protein translation, and CiAIF might play a key role in this progress.

OTUD1 Activates Caspase-Independent and Caspase-Dependent Apoptosis by Promoting AIF Nuclear Translocation and MCL1 Degradation

Apoptosis-inducing factor (AIF) plays a dual role in regulating cell survival and apoptosis, acting as a prosurvival factor in mitochondria via its NADH oxidoreductase activity and activating the caspase-independent apoptotic pathway (i.e., parthanatos) after nuclear translocation. However, whether one factor conjunctively controls the separated functions of AIF is not clear. Here, it is shown that OTU deubiquitinase 1 (OTUD1) acts as a link between the two functions of AIF via deubiquitination events. Deubiquitination of AIF at K244 disrupts the normal mitochondrial structure and compromises oxidative phosphorylation, and deubiquitination of AIF at K255 enhances its DNA-binding ability to promote parthanatos. Moreover, OTUD1 stabilizes DDB1 and CUL4 associated factor 10 (DCAF10) and recruits the cullin 4A (CUL4A)-damage specific DNA binding protein 1 (DDB1) complex to promote myeloid cell leukemia sequence 1 (MCL1) degradation, thereby activating caspase-dependent apoptotic signaling. Collectively, these results reveal the central role of OTUD1 in activating both caspase-independent and caspase-dependent apoptotic signaling and propose decreased OTUD1 expression as a key event promoting chemoresistance in esophageal squamous cell carcinoma.

PAK5-mediated AIF phosphorylation inhibits its nuclear translocation and promotes breast cancer tumorigenesis

Although p21 activated kinase 5 (PAK5) is related to the progression of multiple cancers, its biological function in breast cancer remains unclear. Apoptosis-inducing factor (AIF) is a vital apoptosis factor in mitochondria, which can be released from mitochondria and enter the nucleus, causing caspase-independent apoptosis. In this study, we reveal that PAK5 inhibits apoptosis by preventing the nuclear translocation of AIF. PAK5 inhibits the release of AIF from mitochondria in breast cancer cells by decreasing the mitochondria membrane permeability and increasing the membrane potential. Furthermore, PAK5 phosphorylates AIF at Thr281 site to inhibit the formation of AIF/importin α3 complex, leading to decrease AIF nuclear translocation. Functionally, we demonstrate that PAK5-mediated AIF phosphorylation promotes the proliferation of breast cancer cells and accelerates the growth of breast cancer in vivo. Significantly, PAK5 and AIF expression in breast cancer are positively correlated with poor patient prognosis. PAK5 expression is negatively correlated with AIF nuclear translocation. These results suggest that PAK5-AIF signaling pathway may play an essential role in mammary tumorigenesis, providing a new therapeutic target for the treatment of breast cancer.

Mitochondrial Permeability Transition: A Pore Intertwines Brain Aging and Alzheimer's Disease

Advanced age is the greatest risk factor for aging-related brain disorders including Alzheimer's disease (AD). However, the detailed mechanisms that mechanistically link aging and AD remain elusive. In recent years, a mitochondrial hypothesis of brain aging and AD has been accentuated. Mitochondrial permeability transition pore (mPTP) is a mitochondrial response to intramitochondrial and intracellular stresses. mPTP overactivation has been implicated in mitochondrial dysfunction in aging and AD brains. This review summarizes the up-to-date progress in the study of mPTP in aging and AD and attempts to establish a link between brain aging and AD from a perspective of mPTP-mediated mitochondrial dysfunction.

Role of apoptosis-inducing factor in perinatal hypoxic-ischemic brain injury

Perinatal complications, such as asphyxia, can cause brain injuries that are often associated with subsequent neurological deficits, such as cerebral palsy or mental retardation. The mechanisms of perinatal brain injury are not fully understood, but mitochondria play a prominent role not only due to their central function in metabolism but also because many proteins with apoptosis-related functions are located in the mitochondrion. Among these proteins, apoptosis-inducing factor has already been shown to be an important factor involved in neuronal cell death upon hypoxia-ischemia, but a better understanding of the mechanisms behind these processes is required for the development of more effective treatments during the early stages of perinatal brain injury. In this review, we focus on the molecular mechanisms of hypoxic-ischemic encephalopathy, specifically on the importance of apoptosis-inducing factor. The relevance of apoptosis-inducing factor is based not only because it participates in the caspase-independent apoptotic pathway but also because it plays a crucial role in mitochondrial energetic functionality, especially with regard to the maintenance of electron transport during oxidative phosphorylation and in oxidative stress, acting as a free radical scavenger. We also discuss all the different apoptosis-inducing factor isoforms discovered, focusing especially on apoptosis-inducing factor 2, which is only expressed in the brain and the functions of which are starting now to be clarified. Finally, we summarized the interaction of apoptosis-inducing factor with several proteins that are crucial for both apoptosis-inducing factor functions (pro-survival and pro-apoptotic) and that are highly important in order to develop promising therapeutic targets for improving outcomes after perinatal brain injury.

The 89-kDa PARP1 cleavage fragment serves as a cytoplasmic PAR carrier to induce AIF-mediated apoptosis

Poly(ADP-ribose) polymerase 1 (PARP1) is a nuclear protein that is activated by binding to DNA lesions and catalyzes poly(ADP-ribosyl)ation of nuclear acceptor proteins, including PARP1 itself, to recruit DNA repair machinery to DNA lesions. When excessive DNA damage occurs, poly(ADP-ribose) (PAR) produced by PARP1 is translocated to the cytoplasm, changing the activity and localization of cytoplasmic proteins, e.g., apoptosis-inducing factor (AIF), hexokinase, and resulting in cell death. This cascade, termed parthanatos, is a caspase-independent programmed cell death distinct from necrosis and apoptosis. In contrast, PARP1 is a substrate of activated caspases 3 and 7 in caspase-dependent apoptosis. Once cleaved, PARP1 loses its activity, thereby suppressing DNA repair. Caspase cleavage of PARP1 occurs within a nuclear localization signal near the DNA-binding domain, resulting in the formation of 24-kDa and 89-kDa fragments. In the present study, we found that caspase activation by staurosporine- and actinomycin D-induced PARP1 autopoly(ADP-ribosyl)ation and fragmentation, generating poly(ADP-ribosyl)ated 89-kDa and 24-kDa PARP1 fragments. The 89-kDa PARP1 fragments with covalently attached PAR polymers were translocated to the cytoplasm, whereas 24-kDa fragments remained associated with DNA lesions. In the cytoplasm, AIF binding to PAR attached to the 89-kDa PARP1 fragment facilitated its translocation to the nucleus. Thus, the 89-kDa PARP1 fragment is a PAR carrier to the cytoplasm, inducing AIF release from mitochondria. Elucidation of the caspase-mediated interaction between apoptosis and parthanatos pathways extend the current knowledge on mechanisms underlying programmed cell death and may lead to new therapeutic targets.

Inhibition of mitochondrial and cytosolic calpain attenuates atrophy in myotubes co-cultured with colon carcinoma cells

Cancer cachexia is a life-threatening syndrome characterized by muscle atrophy. Cancer cachectic muscle atrophy (CCMA) is associated with mitochondrial injury. Mitochondrial calpains have been reported to induce mitochondrial injury in mouse cardiomyocytes and pulmonary smooth muscle. In the present study, the presence of calpain in the mitochondria of skeletal muscle and its potential role in CCMA were investigated. Transwell plates were used to develop a myotube-carcinoma cell co-culture model to simulate the cancer cachexia environment in vitro. The calpain inhibitors, calpastatin (CAST) and calpeptin (CAPT), were used to inhibit calpain activity in myotubes during co-culture. Calpain-1, calpain-2 and CAST were found to be present in mouse myotube mitochondria. Co-culture activated calpain in both cytoplasm and mitochondria, which caused myotube atrophy. CAST and CAPT treatment prevented calpain activation in both cytoplasm and mitochondria, which inhibited myotube atrophy during co-culture. Additionally, CAST and CAPT treatment increased mitochondrial complex I activity, decreased mitochondrial permeability transition pore opening and improved mitochondrial membrane potential in myotubes during co-culture. In addition, CAST and CAPT treatment increased AKT/mTOR activity, inhibited FoxO3a activity and decreased atrogin-1 content in myotubes during co-culture. The present findings provide new insights to understand the mechanism of CCMA and further help the development of focused approaches to treat CCMA by manipulating the mitochondrial and cytosolic calpain activity.

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.

Severe congenital neutropenia-associated JAGN1 mutations unleash a calpain-dependent cell death programme in myeloid cells

Severe congenital neutropenia (SCN) of autosomal recessive inheritance, also known as Kostmann disease, is characterised by a lack of neutrophils and a propensity for life-threatening infections. Using whole-exome sequencing, we identified homozygous JAGN1 mutations (p.Gly14Ser and p.Glu21Asp) in three patients with Kostmann-like SCN, thus confirming the recent attribution of JAGN1 mutations to SCN. Using the human promyelocytic cell line HL-60 as a model, we found that overexpression of patient-derived JAGN1 mutants, but not silencing of JAGN1, augmented cell death in response to the pro-apoptotic stimuli, etoposide, staurosporine, and thapsigargin. Furthermore, cells expressing mutant JAGN1 were remarkably susceptible to agonists that normally trigger degranulation and succumbed to a calcium-dependent cell death programme. This mode of cell death was completely prevented by pharmacological inhibition of calpain but unaffected by caspase inhibition. In conclusion, our results confirmed the association between JAGN1 mutations and SCN and showed that SCN-associated JAGN1 mutations unleash a calcium- and calpain-dependent cell death in myeloid cells.

Revisiting Traumatic Brain Injury: From Molecular Mechanisms to Therapeutic Interventions

Studying the complex molecular mechanisms involved in traumatic brain injury (TBI) is crucial for developing new therapies for TBI. Current treatments for TBI are primarily focused on patient stabilization and symptom mitigation. However, the field lacks defined therapies to prevent cell death, oxidative stress, and inflammatory cascades which lead to chronic pathology. Little can be done to treat the mechanical damage that occurs during the primary insult of a TBI; however, secondary injury mechanisms, such as inflammation, blood-brain barrier (BBB) breakdown, edema formation, excitotoxicity, oxidative stress, and cell death, can be targeted by therapeutic interventions. Elucidating the many mechanisms underlying secondary injury and studying targets of neuroprotective therapeutic agents is critical for developing new treatments. Therefore, we present a review on the molecular events following TBI from inflammation to programmed cell death and discuss current research and the latest therapeutic strategies to help understand TBI-mediated secondary injury.

Apoptosis inducing factor and mitochondrial NADH dehydrogenases: redox-controlled gear boxes to switch between mitochondrial biogenesis and cell death

The mitochondrial complex I serves as entry point for NADH into the electron transport chain. In animals, fungi and plants, additional NADH dehydrogenases carry out the same electron transfer reaction, however they do not pump protons. The apoptosis inducing factor (AIF, AIFM1 in humans) is a famous member of this group as it was the first pro-apoptotic protein identified that can induce caspase-independent cell death. Recent studies on AIFM1 and the NADH dehydrogenase Nde1 of baker's yeast revealed two independent and experimentally separable activities of this class of enzymes: On the one hand, these proteins promote the functionality of mitochondrial respiration in different ways: They channel electrons into the respiratory chain and, at least in animals, promote the import of Mia40 (named MIA40 or CHCHD4 in humans) and the assembly of complex I. On the other hand, they can give rise to pro-apoptotic fragments that are released from the mitochondria to trigger cell death. Here we propose that AIFM1 and Nde1 serve as conserved redox switches which measure metabolic conditions on the mitochondrial surface and translate it into a binary life/death decision. This function is conserved among eukaryotic cells and apparently used to purge metabolically compromised cells from populations.

Taurine attenuates isoproterenol-induced H9c2 cardiomyocytes hypertrophy by improving antioxidative ability and inhibiting calpain-1-mediated apoptosis

Pathological cardiac hypertrophy is ultimately accompanied by cardiomyocyte apoptosis. Apoptosis mainly related to calpain-1-mediated apoptotic pathways. Studies had proved that taurine can maintain heart health through antioxidation and antiapoptotic functions, but the effect of taurine on cardiac hypertrophy is still unclear. This study aimed to determine whether taurine could inhibit calpain-1-mediated mitochondria-dependent apoptotic pathways in isoproterenol (ISO)-induced hypertrophic cardiomyocytes. We found that taurine could inhibit the increase in cell surface area and reduce the protein expression levels of the hypertrophic markers atrial natriuretic peptide, brain natriuretic polypeptide, and β-myosin heavy chain. Taurine also reduced ROS, intracellular Ca²⁺ overload and mitochondrial membrane potential. Moreover, taurine inhibited cardiomyocyte apoptosis by decreasing the protein expression of calpain-1, Bax, t-Bid, cytosolic cytochrome c, Apaf-1, cleaved caspase-9 and cleaved caspase-3 and by enhancing calpastatin and Bcl-2 protein expression. Calpain-1 small interfering RNA transfection results showed similar antiapoptotic effects as the taurine prevention group. However, compared with the two treatments, taurine inhibited the expression of cleaved caspase-9 more significantly. Therefore, we believe that taurine prevents ISO-induced H9c2 cardiomyocyte hypertrophy by inhibiting oxidative stress, intracellular Ca²⁺ overload, the calpain-1-mediated mitochondria-dependent apoptotic pathway and cleaved caspase-9 levels.

Taurine prevents cardiomyocyte apoptosis by inhibiting the calpain-1/cytochrome c pathway during RVH in broilers

The calpain-1-activated apoptotic pathway plays a key role in right ventricular hypertrophy (RVH). Taurine has been shown to attenuate apoptosis by inhibiting calpain activity. This experiment aimed to determine whether taurine could prevent RVH by inhibiting the calpain-1/cytochrome c apoptotic pathway. The broilers were given 1% taurine dissolved in drinking water and were raised at 10 °C ~ 12 °C from day 21 to day 42. At 21 d, 28 d, 35 d and 42 d, the right ventricular (RV) tissues were collected. Increased RVH index, angiotensin II, norepinephrine and atrial natriuretic peptide mRNA expression were reduced by taurine in the broiler RVs. Taurine obviously inhibited cardiomyocyte apoptosis via maintaining the mitochondrial membrane potential and decreased the activation of caspase-9 and caspase-3 in the broiler RVs. The antioxidant assay demonstrated that taurine enhanced the activities of superoxide dismutase, total antioxidant capacity and glutathione peroxidase and the glutathione/glutathione disulfide ratio. Western blot results revealed that taurine also downregulated the expression of calpain-1 and cytosolic cytochrome c while upregulating the expression of Bcl-2/Bax and mitochondrial cytochrome c in broiler cardiomyocytes during RVH. In summary, we found that taurine could enhance cardiomyocyte antioxidant ability and further prevented cardiomyocyte apoptosis by inhibiting the calpain-1/cytochrome c pathway during RVH in broilers.

mTOR may interact with PARP-1 to regulate visible light-induced parthanatos in photoreceptors

BACKGROUND

Excessive light exposure is a detrimental environmental factor that plays a critical role in the pathogenesis of retinal degeneration. However, the mechanism of light-induced death of retina/photoreceptor cells remains unclear. The mammalian/mechanistic target of rapamycin (mTOR) and Poly (ADP-ribose) polymerase-1 (PARP-1) have become the primary targets for treating many neurodegenerative disorders. The aim of this study was to elucidate the mechanisms underlying light-induced photoreceptor cell death and whether the neuroprotective effects of mTOR and PARP-1 inhibition against death are mediated through apoptosis-inducing factor (AIF).

METHODS

Propidium iodide (PI)/Hoechst staining, lentiviral-mediated short hairpin RNA (shRNA), Western blot analysis, cellular fraction separation, plasmid transient transfection, laser confocal microscopy, a mice model, electroretinography (ERG), and hematoxylin-eosin (H & E) staining were employed to explore the mechanisms by which rapamycin/3-Aminobenzamide (3AB) exert neuroprotective effects of mTOR/PARP-1 inhibition in light-injured retinas.

RESULTS

A parthanatos-like death mechanism was evaluated in light-injured 661 W cells that are an immortalized photoreceptor-like cell line that exhibit cellular and biochemical feature characteristics of cone photoreceptor cells. The death process featured over-activation of PARP-1 and AIF nuclear translocation. Either PARP-1 or AIF knockdown played a significantly protective role for light-damaged photoreceptors. More importantly, crosstalk was observed between mTOR and PARP-1 signaling and mTOR could have regulated parthanatos via the intermediate factor sirtuin 1 (SIRT1). The parthanatos-like injury was also verified in vivo, wherein either PARP-1 or mTOR inhibition provided significant neuroprotection against light-induced injury, which is evinced by both structural and functional retinal analysis. Overall, these results elucidate the mTOR-regulated parthanatos death mechanism in light-injured photoreceptors/retinas and may facilitate the development of novel neuroprotective therapies for retinal degeneration diseases.

CONCLUSIONS

Our results demonstrate that inhibition of the mTOR/PARP-1 axis exerts protective effects on photoreceptors against visible-light-induced parthanatos. These protective effects are conducted by regulating the downstream factors of AIF, while mTOR possibly interacts with PARP-1 via SIRT1 to regulate parthanatos. Video Abstract Schematic diagram of mTOR interacting with PARP-1 to regulate visible light-induced parthanatos. Increased ROS caused by light exposure penetrates the nuclear membrane and causes nuclear DNA strand breaks. PARP-1 detects DNA breaks and synthesizes PAR polymers to initiate the DNA repair system that consumes a large amount of cellular NAD+. Over-production of PAR polymers prompts the release of AIF from the mitochondria and translocation to the nucleus, which leads to parthanatos. Activated mTOR may interact with PARP-1 via SIRT1 to regulate visible light-induced parthanatos.

Targeting molecular pathways for the treatment of inherited retinal degeneration

Inherited retinal degeneration is a major cause of incurable blindness characterized by loss of retinal photoreceptor cells. Inherited retinal degeneration is characterized by high genetic and phenotypic heterogeneity with several genes mutated in patients affected by these genetic diseases. The high genetic heterogeneity of these diseases hampers the development of effective therapeutic interventions for the cure of a large cohort of patients. Common cell demise mechanisms can be envisioned as targets to treat patients regardless the specific mutation. One of these targets is the increase of intracellular calcium ions, that has been detected in several murine models of inherited retinal degeneration. Recently, neurotrophic factors that favor the efflux of calcium ions to concentrations below toxic levels have been identified as promising molecules that should be evaluated as new treatments for retinal degeneration. Here, we discuss therapeutic options for inherited retinal degeneration and we will focus on neuroprotective approaches, such as the neuroprotective activity of the Pigment epithelium-derived factor. The characterization of specific targets for neuroprotection opens new perspectives together with many questions that require deep analyses to take advantage of this knowledge and develop new therapeutic approaches. We believe that minimizing cell demise by neuroprotection may represent a promising treatment strategy for retinal degeneration.

Novel Positive Allosteric Modulators of Glutamate Transport Have Neuroprotective Properties in an in Vitro Excitotoxic Model

Dysfunction of excitatory amino acid transporters (EAATs) has been implicated in the pathogenesis of various neurological disorders, such as stroke, brain trauma, epilepsy, and several neurodegenerative disorders. EAAT2 is the main transporter subtype responsible for glutamate clearance in the brain, and plays a key role in regulating neurotransmission and preventing excitotoxicity. Therefore, compounds that increase the activity of EAAT2 have therapeutic potential for neuroprotection. In previous studies, we used virtual screening approaches to identify novel positive allosteric modulators (PAMs) of EAAT2. These compounds were shown to selectively increase the activity of EAAT2 and increase V_max of transport, without changing substrate affinity. In this work, our major effort was to investigate whether increasing the activity of EAAT2 by allosteric modulation would translate to neuroprotection in in vitro primary culture models of excitotoxicity. To investigate potential neuroprotective effects of one EAAT2 PAM, GT949, we subjected cultures to acute and prolonged excitotoxic insults by exogenous application of glutamate, or oxidative stress by application of hydrogen peroxide. GT949 administration did not result in neuroprotection in the oxidative stress model, likely due to damage of the glutamate transporters. However, GT949 displayed neuroprotective properties after acute and prolonged glutamate-mediated excitotoxicity. We propose that this compound prevents excess glutamate signaling by increasing the rate of glutamate clearance by EAAT2, thereby preventing excitotoxic damage and cell death. This novel class of compounds is therefore an innovative approach for neuroprotection with potential for translation in in vivo animal models of excitotoxicity.