Poly(ADP-ribose) polymerase-dependent energy depletion occurs through inhibition of glycolysis

Reduced oxygen tension culturing conditionally alters toxicogenic response of differentiated H9c2 cardiomyoblasts to acrolein

Acrolein is a reactive electrophilic aldehyde known to cause mitochondrial dysfunction, oxidative stress, and dysregulation of signaling transduction in vitro. Most in vitro systems employ standard cell culture maintenance conditions of 95% air/5% CO₂, translating to a culture oxygen tension of approximately 20%, far above most physiological tissues. The purpose of this investigation was to examine whether low-serum, retinoic acid differentiated H9c2 cells were less sensitive to acrolein insult when cultured under reduced oxygen tension. H9c2 cells were maintained separately in 20% and 5% oxygen, differentiated for 5 d, and then exposed to acrolein for 30 min in media containing varying concentrations of tricarboxylic acid and glycolytic substrates, followed by fresh medium replacement. Cells were then assessed for MTT reduction at 2 h and 24 h after acrolein insult. We showed that pyruvate supplementation in combination with lowered oxygen culturing significantly attenuated acrolein-induced viability loss at 24 h. Poly(ADP-ribose) polymerase inhibition and EGTA preferentially provided partial rescue to low oxygen cultures, but not for standard cultures. Collectively, these results offer evidence supporting altered toxicogenic response of H9c2 during physiologically relevant oxygen tension culturing.

Apoptosis-Inducing Factor (AIF) in Physiology and Disease: The Tale of a Repented Natural Born Killer

Apoptosis-inducing factor (AIF) is a mitochondrial oxidoreductase that contributes to cell death programmes and participates in the assembly of the respiratory chain. Importantly, AIF deficiency leads to severe mitochondrial dysfunction, causing muscle atrophy and neurodegeneration in model organisms as well as in humans. The purpose of this review is to describe functions of AIF and AIF-interacting proteins as regulators of cell death and mitochondrial bioenergetics. We describe how AIF deficiency induces pathogenic processes that alter metabolism and ultimately compromise cellular homeostasis. We report the currently known AIFM1 mutations identified in humans and discuss the variability of AIFM1-related disorders in terms of onset, organ involvement and symptoms. Finally, we summarize how the study of AIFM1-linked pathologies may help to further expand our understanding of rare inherited forms of mitochondrial diseases.

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AIF is a mitochondrial NADH-dependent oxidoreductase.

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Nuclear translocation of AIF occurs during cell death and has been associated with human disorders.

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Under physiological settings, AIF participates to the biogenesis of the respiratory complexes.

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AIFM1 mutations have been identified in patients with impaired mitochondrial bioenergetics.

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Inherited AIFM1 mutations lead to a variety of clinical manifestations, including severe childhood-onset mitochondrial diseases.

α-Lactalbumin-oleic acid complex kills tumor cells by inducing excess energy metabolism but inhibiting mRNA expression of the related enzymes

Previous studies have demonstrated that the anti-tumor α-lactalbumin-oleic acid complex (α-LA-OA) may target the glycolysis of tumor cells. However, few data are available regarding the effects of α-LA-OA on energy metabolism. In this study, we measured glycolysis and mitochondrial functions in HeLa cells in response to α-LA-OA using the XF flux analyzer (Seahorse Bioscience, North Billerica, MA). The gene expression of enzymes involved in glycolysis, tricarboxylic acid cycle, electron transfer chain, and ATP synthesis were also evaluated. Our results show that α-LA-OA significantly enhanced the basal glycolysis and glycolytic capacity. Mitochondrial oxidative phosphorylation, including the basal respiration, maximal respiration, spare respiratory capacity and ATP production were also improved in response to α-LA-OA. The enhanced mitochondrial functions maybe partly due to the increased capacity of utilizing fatty acids and glutamine as the substrate. However, the gene expressions of pyruvate kinase M2, lactate dehydrogenase A, aconitate hydratase, and isocitrate dehydrogenase 1 were inhibited, suggesting an insufficient ability for the glycolysis process and the tricarboxylic acid cycle. The increased expression of acetyl-coenzyme A acyltransferase 2, a central enzyme involved in the β-oxidation of fatty acids, would enhance the unbalance due to the decreased expression of electron transfer flavoprotein β subunit, which acts as the electron acceptor. These results indicated that α-LA-OA may induce oxidative stress due to conditions in which the ATP production is exceeding the energy demand. Our results may help clarify the mechanism of apoptosis induced by reactive oxygen species and mitochondrial destruction.

Molecular mechanisms of cell death: recommendations of the Nomenclature Committee on Cell Death 2018

Over the past decade, the Nomenclature Committee on Cell Death (NCCD) has formulated guidelines for the definition and interpretation of cell death from morphological, biochemical, and functional perspectives. Since the field continues to expand and novel mechanisms that orchestrate multiple cell death pathways are unveiled, we propose an updated classification of cell death subroutines focusing on mechanistic and essential (as opposed to correlative and dispensable) aspects of the process. As we provide molecularly oriented definitions of terms including intrinsic apoptosis, extrinsic apoptosis, mitochondrial permeability transition (MPT)-driven necrosis, necroptosis, ferroptosis, pyroptosis, parthanatos, entotic cell death, NETotic cell death, lysosome-dependent cell death, autophagy-dependent cell death, immunogenic cell death, cellular senescence, and mitotic catastrophe, we discuss the utility of neologisms that refer to highly specialized instances of these processes. The mission of the NCCD is to provide a widely accepted nomenclature on cell death in support of the continued development of the field.

Ferroptosis: bug or feature?

Ferroptosis is an iron-dependent, oxidative form of non-apoptotic cell death. This form of cell death does not share morphological, biochemical, or genetic similarities with classic necrosis, necroptosis, parthanatos, or other forms of non-apoptotic cell death. Ferroptosis can be triggered by depleting the cell of the amino acid cysteine, or by inhibiting the phospholipid hydroperoxidase glutathione peroxidase 4 (GPX4). Why certain stimuli trigger ferroptosis instead of another form of cell death, and whether this process could be adaptive in vivo, are two major unanswered questions concerning this process. Emerging evidence and consideration of related non-apoptotic pathways suggest that ferroptosis could be an adaptive process, albeit one regulated and executed in a manner very different from apoptosis and other forms of cell death.

Reciprocal Regulation of Glycolysis-Driven Th17 Pathogenicity and Regulatory T Cell Stability by Cdc42

A balance between Th17 cells and regulatory T cells (Tregs) is important for host immunity and immune tolerance. The underlying molecular mechanisms remain poorly understood. Here we have identified Cdc42 as a central regulator of Th17/Treg balance. Deletion of Cdc42 in T cells enhanced Th17 differentiation but diminished induced Treg differentiation and suppressive function. Treg-specific deletion of Cdc42 decreased natural Tregs but increased effector T cells including Th17 cells. Notably, Cdc42-deficient Th17 cells became pathogenic associated with enhanced glycolysis and Cdc42-deficient Tregs became unstable associated with weakened glycolytic signaling. Inhibition of glycolysis in Cdc42-deficient Th17 cells diminished their pathogenicity and restoration of glycolysis in Cdc42-deficient Tregs rescued their instability. Intriguingly, Cdc42 deficiency in T cells led to exacerbated wasting disease in mouse models of colitis and Treg-specific deletion of Cdc42 caused early, fatal lymphoproliferative diseases. In summary, we show that Cdc42 is a bona fide regulator of peripheral tolerance through suppression of Th17 aberrant differentiation/pathogenicity and promotion of Treg differentiation/stability/function involving metabolic signaling and thus Cdc42 pathway might be harnessed in autoimmune disease therapy.

Specificity of reversible ADP-ribosylation and regulation of cellular processes

Proper and timely regulation of cellular processes is fundamental to the overall health and viability of organisms across all kingdoms of life. Thus, organisms have evolved multiple highly dynamic and complex biochemical signaling cascades in order to adapt and survive diverse challenges. One such method of conferring rapid adaptation is the addition or removal of reversible modifications of different chemical groups onto macromolecules which in turn induce the appropriate downstream outcome. ADP-ribosylation, the addition of ADP-ribose (ADPr) groups, represents one of these highly conserved signaling chemicals. Herein we outline the writers, erasers and readers of ADP-ribosylation and dip into the multitude of cellular processes they have been implicated in. We also review what we currently know on how specificity of activity is ensured for this important modification.

Bioenergetic Impairment in Animal and Cellular Models of Alzheimer's Disease: PARP-1 Inhibition Rescues Metabolic Dysfunctions

Amyloid-beta peptide accumulation in the brain is one of the main hallmarks of Alzheimer's disease. The amyloid aggregation process is associated with the generation of free radical species responsible for mitochondrial impairment and DNA damage that in turn activates poly(ADP-ribose)polymerase 1 (PARP-1). PARP-1 catalyzes the poly(ADP-ribosylation), a post-translational modification of proteins, cleaving the substrate NAD+ and transferring the ADP-ribose moieties to the enzyme itself or to an acceptor protein to form branched polymers of ADP-ribose. In this paper, we demonstrate that a mitochondrial dysfunction occurs in Alzheimer's transgenic mice TgCRND8, in SH-SY5Y treated with amyloid-beta and in 7PA2 cells. Moreover, PARP-1 activation contributes to the functional energetic decline affecting cytochrome oxidase IV protein levels, oxygen consumption rates, and membrane potential, resulting in cellular bioenergetic deficit. We also observed, for the first time, an increase of pyruvate kinase 2 expression, suggesting a modulation of the glycolytic pathway by PARP-1. PARP-1 inhibitors are able to restore both mitochondrial impairment and pyruvate kinase 2 expression. The overall data here presented indicate a pivotal role for this enzyme in the bioenergetic network of neuronal cells and open new perspectives for investigating molecular mechanisms underlying energy charge decline in Alzheimer's disease. In this scenario, PARP-1 inhibitors might represent a novel therapeutic intervention to rescue cellular energetic metabolism.

PARP Inhibition Prevents Ethanol-Induced Neuroinflammatory Signaling and Neurodegeneration in Rat Adult-Age Brain Slice Cultures

Using rat adult-age hippocampal-entorhinal cortical (HEC) slice cultures, we examined the role of poly [ADP-ribose] polymerase (PARP) in binge ethanol's brain inflammatory and neurodegenerative mechanisms. Activated by DNA strand breaks, PARP (principally PARP1 in the brain) promotes DNA repair via poly [ADP-ribose] (PAR) products, but PARP overactivation triggers regulated neuronal necrosis (e.g., parthanatos). Previously, we found that brain PARP1 levels were upregulated by neurotoxic ethanol binges in adult rats and HEC slices, and PARP inhibitor PJ34 abrogated slice neurodegeneration. Binged HEC slices also exhibited increased Ca⁺²-dependent phospholipase A2 (PLA2) isoenzymes (cPLA2 IVA and sPLA2 IIA) that mobilize proinflammatory ω6 arachidonic acid (ARA). We now find in 4-day-binged HEC slice cultures (100 mM ethanol) that PARP1 elevations after two overnight binges precede PAR, cPLA2, and sPLA2 enhancements by 1 day and high-mobility group box-1 (HMGB1), an ethanol-responsive alarmin that augments proinflammatory cytokines via toll-like receptor-4 (TLR4), by 2 days. After verifying that PJ34 effectively blocks PARP activity (↑PAR), we demonstrated that, like PJ34, three other PARP inhibitors-olaparib, veliparib, and 4-aminobenzamide-provided neuroprotection from ethanol. Importantly, PJ34 and olaparib also prevented ethanol's amplification of the PLA2 isoenzymes, and two PLA2 inhibitors were neuroprotective-thus coupling PARP to PLA2, with PLA2 activity promoting neurodegeneration. Also, PJ34 and olaparib blocked ethanol-induced HMGB1 elevations, linking brain PARP induction to TLR4 activation. The results provide evidence in adult brains that induction of PARP1 may mediate dual neuroinflammatory pathways (PLA2→phospholipid→ARA and HMGB1→TLR4→proinflammatory cytokines) that are complicit in binge ethanol-induced neurodegeneration.

An Overview of Pathways of Regulated Necrosis in Acute Kidney Injury

Necrosis is the predominant form of regulated cell death in acute kidney injury (AKI) and represents results in the formation of casts that appear in the urine sedimentation, referred to as muddy brown casts, which are part of the diagnosis of AKI. Pathologists referred to this typical feature as acute tubular necrosis. We are only beginning to understand the dynamics and the molecular pathways that underlie such typical necrotic morphology. In this review, we provide an overview of candidate pathways and summarize the emerging evidence for the relative contribution of these pathways of regulated necrosis, such as necroptosis, ferroptosis, mitochondrial permeability transition-mediated regulated necrosis, parthanatos, and pyroptosis. Inhibitors of each of these pathways are available, and clinical trials may be started after the detection of the most promising drug targets, which will be discussed here. With the global burden of AKI in mind, inhibitiors of regulated necrosis represent promising means to prevent this disease.

Absence of REV3L promotes p53-regulated cancer cell metabolism in cisplatin-treated lung carcinoma cells

Lung cancer is one of the deadliest cancers in the world because of chemo-resistance to the commonly used cisplatin-based treatments. The use of low fidelity DNA polymerases in the translesional synthesis (TLS) DNA damage response pathway that repairs lesions caused by cisplatin also presents a mutational carcinogenic burden on cells that needs to be regulated by the tumor suppressor protein p53. However, there is much debate over the roles of the reversionless 3-like (REV3L) protein responsible for TLS and p53 in regulating cancer cell metabolism. In this study, the fluorescence lifetime of the metabolic coenzyme NADH reveals that the absence of REV3L can promote the p53-mediated upregulation of oxidative phosphorylation in cisplatin-treated H1299 lung carcinoma cells and increases cancer cell sensitivity to this platinum-based chemotherapy. These results demonstrate a previously unrecognized relationship between p53 and REV3L in cancer cell metabolism and may lead to improvements in chemotherapy treatment plans that reduce cisplatin resistance in lung cancer.

Enhanced Repair of UV-Induced DNA Damage by 1,25-Dihydroxyvitamin D3 in Skin Is Linked to Pathways that Control Cellular Energy

Inadequately repaired post-UV DNA damage results in skin cancers. DNA repair requires energy but skin cells have limited capacity to produce energy after UV insult. We examined whether energy supply is important for DNA repair after UV exposure, in the presence of 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), which reduces UV-induced DNA damage and photocarcinogenesis in a variety of models. After UV exposure of primary human keratinocytes, the addition of 1,25(OH)₂D₃ increased unscheduled DNA synthesis, a measure of DNA repair. Oxidative phosphorylation was depleted in UV-irradiated keratinocytes to undetectable levels within an hour of UV irradiation. Treatment with 1,25(OH)₂D₃ but not vehicle increased glycolysis after UV. 2-Deoxyglucose-dependent inhibition of glycolysis abolished the reduction in cyclobutane pyrimidine dimers by 1,25(OH)₂D₃, whereas inhibition of oxidative phosphorylation had no effect. 1,25(OH)₂D₃ increased autophagy and modulated PINK1/Parkin consistent with enhanced mitophagy. These data confirm that energy availability is limited in keratinocytes after exposure to UV. In the presence of 1,25(OH)₂D₃, glycolysis is enhanced along with energy-conserving processes such as autophagy and mitophagy, resulting in increased repair of cyclobutane pyrimidine dimers and decreased oxidative DNA damage. Increased energy availability in the presence of 1,25(OH)₂D₃ is an important contributor to DNA repair in skin after UV exposure.

ADP-Ribosylation, a Multifaceted Posttranslational Modification Involved in the Control of Cell Physiology in Health and Disease

Posttranslational modifications (PTMs) regulate protein functions and interactions. ADP-ribosylation is a PTM, in which ADP-ribosyltransferases use nicotinamide adenine dinucleotide (NAD⁺) to modify target proteins with ADP-ribose. This modification can occur as mono- or poly-ADP-ribosylation. The latter involves the synthesis of long ADP-ribose chains that have specific properties due to the nature of the polymer. ADP-Ribosylation is reversed by hydrolases that cleave the glycosidic bonds either between ADP-ribose units or between the protein proximal ADP-ribose and a given amino acid side chain. Here we discuss the properties of the different enzymes associated with ADP-ribosylation and the consequences of this PTM on substrates. Furthermore, the different domains that interpret either mono- or poly-ADP-ribosylation and the implications for cellular processes are described.

Nitric Oxide Signaling in Neurodegeneration and Cell Death

In this tribute to Solomon H. Snyder (Sol) we discuss the mechanisms by which nitric oxide (NO) kills neurons. We provide a historical perspective regarding the discovery that glutamate excitotoxicity is mediated by NO. It also contains a discussion of the discovery that neuronal nitric oxide synthase (nNOS) catalytic activity accounts for NADPH diaphorase activity and its localization in the central nervous system. NADPH diaphorase/nNOS neurons are unique in that they are resistant to toxic effects of excess glutamate and that they are resistant to neurodegeneration in a variety of neurodegenerative diseases. NADPH diaphorase/nNOS neurons are resistant to neurotoxicity and neurodegeneration through the overexpression of manganese superoxide dismutase. The review also delves into the mechanisms by which NO kills neurons including NO's activation of the glyceraldehyde-3-phosphate dehydrogenase-dependent cell pathway. In addition, there is a review of parthanatos in which NO combines with the superoxide anion ( [Formula: see text] ) to form peroxynitrite (ONOO^-) that damages DNA and activates poly (ADP-ribose) (PAR) polymerase (PARP). This ultimately leads to activation of the PARP-dependent apoptosis-inducing factor-associated nuclease, the final executioner in NO-dependent cell death. Finally, there is a discussion of potential targets that are under development that target the mechanisms by which NO kills neurons.

Endoplasmic reticulum stress regulates oxygen-glucose deprivation-induced parthanatos in human SH-SY5Y cells via improvement of intracellular ROS

AIMS

Endoplasmic reticulum (ER) stress has been demonstrated to regulate neuronal death caused by ischemic insults via activation of apoptosis, but it still remains unclear whether ER stress participates in regulation of parthanatos, a new type of programmed cell death characterized by PARP-1 overactivation and intracellular accumulation of PAR polymer.

METHODS

we used oxygen-glucose deprivation (OGD) and human SH-SY5Y cells to simulate neuronal damage caused by ischemia.

RESULTS

Oxygen-glucose deprivation induced time-dependent death in SH-SY5Y cells, which was accompanied with upregulation of PARP-1, accumulation of PAR polymer, decline of mitochondrial membrane potentials and nuclear translocation of AIF. Pharmacological inhibition of PARP-1 with its specific inhibitor 3AB rescued OGD-induced cell death, as well as prevented PAR polymer accumulation, mitochondrial depolarization, and AIF translocation into nucleus. Similar results could be found when PARP-1 was genetically knocked down with SiRNA. These indicated that OGD triggered parthanatos in SH-SY5Y cells. Then, we found inhibition of overproduction of ROS with antioxidant NAC attenuated obviously OGD-induced parthanatos in SH-SY5Y cells, suggesting ROS regulated OGD-induced parthanatos. Additionally, OGD also induced upregulation of ER stress-related proteins. Mitigation of ER stress with chemical chaperone 4-PBA or trehalose suppressed significantly OGD-induced overproduction of ROS, PARP-1 upregulation, PAR polymer accumulation, and nuclear accumulation of AIF, and cell death in SH-SY5Y cells.

CONCLUSION

Endoplasmic reticulum stress regulates OGD-induced parthanatos in human SH-SY5Y cells via improvement of intracellular ROS.

Mitochondrial NUDIX hydrolases: A metabolic link between NAD catabolism, GTP and mitochondrial dynamics

NAD+ catabolism and mitochondrial dynamics are important parts of normal mitochondrial function and are both reported to be disrupted in aging, neurodegenerative diseases, and acute brain injury. While both processes have been extensively studied there has been little reported on how the mechanisms of these two processes are linked. This review focuses on how downstream NAD+ catabolism via NUDIX hydrolases affects mitochondrial dynamics under pathologic conditions. Additionally, several potential targets in mitochondrial dysfunction and fragmentation are discussed, including the roles of mitochondrial poly(ADP-ribose) polymerase 1(mtPARP1), AMPK, AMP, and intra-mitochondrial GTP metabolism. Mitochondrial and cytosolic NUDIX hydrolases (NUDT9α and NUDT9β) can affect mitochondrial and cellular AMP levels by hydrolyzing ADP- ribose (ADPr) and subsequently altering the levels of GTP and ATP. Poly (ADP-ribose) polymerase 1 (PARP1) is activated after DNA damage, which depletes NAD+ pools and results in the PARylation of nuclear and mitochondrial proteins. In the mitochondria, ADP-ribosyl hydrolase-3 (ARH3) hydrolyzes PAR to ADPr, while NUDT9α metabolizes ADPr to AMP. Elevated AMP levels have been reported to reduce mitochondrial ATP production by inhibiting the adenine nucleotide translocase (ANT), allosterically activating AMPK by altering the cellular AMP: ATP ratio, and by depleting mitochondrial GTP pools by being phosphorylated by adenylate kinase 3 (AK3), which uses GTP as a phosphate donor. Recently, activated AMPK was reported to phosphorylate mitochondria fission factor (MFF), which increases Drp1 localization to the mitochondria and promotes mitochondrial fission. Moreover, the increased AK3 activity could deplete mitochondrial GTP pools and possibly inhibit normal activity of GTP-dependent fusion enzymes, thus altering mitochondrial dynamics.

Alkylation induced cerebellar degeneration dependent on Aag and Parp1 does not occur via previously established cell death mechanisms

Alkylating agents are ubiquitous in our internal and external environments, causing DNA damage that contributes to mutations and cell death that can result in aging, tissue degeneration and cancer. Repair of methylated DNA bases occurs primarily through the base excision repair (BER) pathway, a multi-enzyme pathway initiated by the alkyladenine DNA glycosylase (Aag, also known as Mpg). Previous work demonstrated that mice treated with the alkylating agent methyl methanesulfonate (MMS) undergo cerebellar degeneration in an Aag-dependent manner, whereby increased BER initiation by Aag causes increased tissue damage that is dependent on activation of poly (ADP-ribose) polymerase 1 (Parp1). Here, we dissect the molecular mechanism of cerebellar granule neuron (CGN) sensitivity to MMS using primary ex vivo neuronal cultures. We first established a high-throughput fluorescent imaging method to assess primary neuron sensitivity to treatment with DNA damaging agents. Next, we verified that the alkylation sensitivity of CGNs is an intrinsic phenotype that accurately recapitulates the in vivo dependency of alkylation-induced CGN cell death on Aag and Parp1 activity. Finally, we show that MMS-induced CGN toxicity is independent of all the cellular events that have previously been associated with Parp-mediated toxicity, including mitochondrial depolarization, AIF translocation, calcium fluxes, and NAD+ consumption. We therefore believe that further investigation is needed to adequately describe all varieties of Parp-mediated cell death.

Poly(ADP-ribosylated) proteins in mononuclear cells from patients with type 2 diabetes identified by proteomic studies

AIMS

In diabetes, hyperglycemia increases reactive oxygen species that induce DNA damage and poly(ADP-ribose)polymerase activation. The aim of this study is to characterize the proteomic profile and the role of poly(ADP-ribosylation) in patients with type 2 diabetes.

METHODS

A proteomic platform based on 2DE and MALDI-ToF spectrometry was applied to peripheral blood mononuclear cells obtained from two different cohorts in which diabetic (n = 14) and normoglycemic patients (n = 11) were enrolled.

RESULTS

Proteomic maps identified WD repeat protein, 78-kDa glucose-regulated protein precursor and myosin regulatory light chain 2, as unique proteins in diabetic patients; vimentin, elongation factor 2, annexin A1, glutathione S-transferase P, moesin and cofilin-1 as unique in the normoglycemic; and calreticulin, rho GDP-dissociation inhibitor 2, protein disulfide isomerase and tropomyosin alpha-4-chain as differentially expressed between the two cohorts. An enrichment in PARylation in diabetic patients was observed in particular, affecting GAPDH and α-Enolase leading to a decrease in their enzymatic activity.

CONCLUSIONS

As the GAPDH and α-Enolase are involved in energy metabolism, protein synthesis and DNA repair, loss of their function or change in their activity can significantly contribute to the molecular mechanisms responsible for the development of type 2 diabetes. These data along with the proteomic profile associated with the disease may provide new insight into the pathophysiology of type 2 diabetes.

ALKBH7 drives a tissue and sex-specific necrotic cell death response following alkylation-induced damage

Regulated necrosis has emerged as a major cell death mechanism in response to different forms of physiological and pharmacological stress. The AlkB homolog 7 (ALKBH7) protein is required for regulated cellular necrosis in response to chemotherapeutic alkylating agents but its role within a whole organism is unknown. Here, we show that ALKBH7 modulates alkylation-induced cellular death through a tissue and sex-specific mechanism. At the whole-animal level, we find that ALKBH7 deficiency confers increased resistance to MMS-induced toxicity in male but not female mice. Moreover, ALKBH7-deficient mice exhibit protection against alkylation-mediated cytotoxicity in retinal photoreceptor and cerebellar granule cells, two cell types that undergo necrotic death through the initiation of the base excision repair pathway and hyperactivation of the PARP1/ARTD1 enzyme. Notably, the protection against alkylation-induced cerebellar degeneration is specific to ALKBH7-deficient male but not female mice. Our results uncover an in vivo role for ALKBH7 in mediating a sexually dimorphic tissue response to alkylation damage that could influence individual responses to chemotherapies based upon alkylating agents.

Mitochondrial Dysfunction Mediated by Poly(ADP-Ribose) Polymerase-1 Activation Contributes to Hippocampal Neuronal Damage Following Status Epilepticus

Mitochondrial dysfunction plays a central role in the neuropathology associated with status epilepticus (SE) and is implicated in the development of epilepsy. While excitotoxic mechanisms are well-known mediators affecting mitochondrial health following SE, whether hyperactivation of poly(ADP-ribose) polymerase-1 (PARP-1) also contributes to SE-induced mitochondrial dysfunction remains to be examined. Here we first evaluated the temporal evolution of poly-ADP-ribosylated protein levels in hippocampus following kainic acid-induced SE as a marker for PARP-1 activity, and found that PARP-1 was hyperactive at 24 h following SE. We evaluated oxidative metabolism and found decreased NAD⁺ levels by enzymatic cycling, and impaired NAD⁺-dependent mitochondrial respiration as measured by polarography at 24 h following SE. Stereological estimation showed significant cell loss in the hippocampal CA₁ and CA₃ subregions 72 h following SE. PARP-1 inhibition using N-(6-Oxo-5,6-dihydro-phenanthridin-2-yl)- N,N-dimethylacetamide (PJ-34) in vivo administration was associated with preserved NAD⁺ levels and NAD⁺-dependent mitochondrial respiration, and improved CA₁ neuronal survival. These findings suggest that PARP-1 hyperactivation contributes to SE-associated mitochondrial dysfunction and CA₁ hippocampal damage. The deleterious effects of PARP-1 hyperactivation on mitochondrial respiration are in part mediated through intracellular NAD⁺ depletion. Therefore, modulating PARP-1 activity may represent a potential therapeutic target to preserve intracellular energetics and mitochondrial function following SE.

Metabolic correction by pyruvate halts acquired epilepsy in multiple rodent models

Metabolic intervention strategy of epilepsy treatment has been gaining broader attention due to accumulated evidence that hypometabolism, manifested in humans as reduced brain glucose consumption, is a principal factor in acquired epilepsy. Therefore, targeting deficient energy metabolism may be an effective approach for treating epilepsy. To confront this pathology we utilized pyruvate, which besides being an anaplerotic mitochondrial fuel possesses a unique set of neuroprotective properties as it: (i) is a potent reactive oxygen species scavenger; (ii) abates overactivation of Poly [ADP-ribose] polymerase 1 (PARP-1); (iii) facilitates glutamate efflux from the brain; (iv) augments brain glycogen stores; (v) is anti-inflammatory; (vi) prevents neuronal hyperexcitability; and (vii) normalizes the cytosolic redox state. In vivo, chronic oral pyruvate administration completely abolished established epileptic phenotypes in three accepted and fundamentally different rodent acquired epilepsy models. Our study reports metabolic correction by pyruvate as a potentially highly effective treatment of acquired epilepsies.

Mitochondrial form, function and signalling in aging

Aging is often accompanied by a decline in mitochondrial mass and function in different tissues. Additionally, cell resistance to stress is frequently found to be prevented by higher mitochondrial respiratory capacity. These correlations strongly suggest mitochondria are key players in aging and senescence, acting by regulating energy homeostasis, redox balance and signalling pathways central in these processes. However, mitochondria display a wide array of functions and signalling properties, and the roles of these different characteristics are still widely unexplored. Furthermore, differences in mitochondrial properties and responses between tissues and cell types, and how these affect whole body metabolism are also still poorly understood. This review uncovers aspects of mitochondrial biology that have an impact upon aging in model organisms and selected mammalian cells and tissues.

S100B impairs glycolysis via enhanced poly(ADP-ribosyl)ation of glyceraldehyde-3-phosphate dehydrogenase in rodent muscle cells

S100 calcium-binding protein B (S100B), a multifunctional macromolecule mainly expressed in nerve tissues and adipocytes, has been suggested to contribute to the pathogenesis of obesity. To clarify the role of S100B in insulin action and glucose metabolism in peripheral tissues, we investigated the effect of S100B on glycolysis in myoblast and myotube cells. Rat myoblast L6 cells were treated with recombinant mouse S100B to examine glucose consumption, lactate production, glycogen accumulation, glycolytic metabolites and enzyme activity, insulin signaling, and poly(ADP-ribosyl)ation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Glycolytic metabolites were investigated by enzyme assays or metabolome analysis, and insulin signaling was assessed by Western blot analysis. Enzyme activity and poly(ADP-ribosyl)ation of GAPDH was evaluated by an enzyme assay and immunoprecipitation followed by dot blot with an anti-poly(ADP-ribose) antibody, respectively. S100B significantly decreased glucose consumption, glucose analog uptake, and lactate production in L6 cells, in either the presence or absence of insulin. In contrast, S100B had no effect on glycogen accumulation and insulin signaling. Metabolome analysis revealed that S100B increased the concentration of glycolytic intermediates upstream of GAPDH. S100B impaired GAPDH activity and increased poly(ADP-ribosyl)ated GAPDH proteins. The effects of S100B on glucose metabolism were mostly canceled by a poly(ADP-ribose) polymerase inhibitor. Similar results were obtained in C₂C₁₂ myotube cells. We conclude that S100B as a humoral factor may impair glycolysis in muscle cells independent of insulin action, and the effect may be attributed to the inhibition of GAPDH activity from enhanced poly(ADP-ribosyl)ation of the enzyme.

The vicious circle of hypometabolism in neurodegenerative diseases: Ways and mechanisms of metabolic correction

Hypometabolism, characterized by decreased brain glucose consumption, is a common feature of many neurodegenerative diseases. Initial hypometabolic brain state, created by characteristic risk factors, may predispose the brain to acquired epilepsy and sporadic Alzheimer's and Parkinson's diseases, which are the focus of this review. Analysis of available data suggests that deficient glucose metabolism is likely a primary initiating factor for these diseases, and that resulting neuronal dysfunction further promotes the metabolic imbalance, establishing an effective positive feedback loop and a downward spiral of disease progression. Therefore, metabolic correction leading to the normalization of abnormalities in glucose metabolism may be an efficient tool to treat the neurological disorders by counteracting their primary pathological mechanisms. Published and preliminary experimental results on this approach for treating Alzheimer's disease and epilepsy models support the efficacy of metabolic correction, confirming the highly promising nature of the strategy. © 2017 Wiley Periodicals, Inc.

Chemical signaling between gut microbiota and host chromatin: What is your gut really saying?

Mammals and their gut microbial communities share extensive and tightly coordinated co-metabolism of dietary substrates. A large number of microbial metabolites have been detected in host circulation and tissues and, in many cases, are linked to host metabolic, developmental, and immunological states. The presence of these metabolites in host tissues intersects with regulation of the host's epigenetic machinery. Although it is established that the host's epigenetic machinery is sensitive to levels of endogenous metabolites, the roles for microbial metabolites in epigenetic regulation are just beginning to be elucidated. This review focuses on eukaryotic chromatin regulation by endogenous and gut microbial metabolites and how these regulatory events may impact host developmental and metabolic phenotypes.

At the Interface of Three Nucleic Acids: The Role of RNA-Binding Proteins and Poly(ADP-ribose) in DNA Repair

RNA-binding proteins (RBPs) regulate RNA metabolism, from synthesis to decay. When bound to RNA, RBPs act as guardians of the genome integrity at different levels, from DNA damage prevention to the post-transcriptional regulation of gene expression. Recently, RBPs have been shown to participate in DNA repair. This fact is of special interest as DNA repair pathways do not generally involve RNA. DNA damage in higher organisms triggers the formation of the RNA-like polymer - poly(ADP-ribose) (PAR). Nucleic acid-like properties allow PAR to recruit DNA- and RNA-binding proteins to the site of DNA damage. It is suggested that poly(ADP-ribose) and RBPs not only modulate the activities of DNA repair factors, but that they also play an important role in the formation of transient repairosome complexes in the nucleus. Cytoplasmic biomolecules are subjected to similar sorting during the formation of RNA assemblages by functionally related mRNAs and promiscuous RBPs. The Y-box-binding protein 1 (YB-1) is the major component of cytoplasmic RNA granules. Although YB-1 is a classic RNA-binding protein, it is now regarded as a non-canonical factor of DNA repair.

Retrograde signaling from autophagy modulates stress responses

Macroautophagy is a process in which cytoplasmic components, including whole organelles, are degraded within lysosomes. Basally, this process is essential for homeostasis and is constitutively functional in most cells, but it can also be implemented as part of stress responses. We discuss findings showing that autophagy proteins can modulate and amplify the activities of transcription factors involved in stress responses, such as those in the p53, FOXO, MiT/TFE, Nrf2, and NFκB/Rel families. Thus, transcription factors not only amplify stress responses and autophagy but are also subject to retrograde regulation by autophagy-related proteins. Physical interactions with autophagy-related proteins, competition for activating intermediates, and "signalphagy," which is the role autophagy plays in the degradation of specific signaling proteins, together provide powerful tools for implementing negative feedback or positive feed-forward loops on the transcription factors that regulate autophagy. We present examples illustrating how this network interacts to regulate metabolic and physiologic responses.

Chemosensitivity of IDH1-Mutated Gliomas Due to an Impairment in PARP1-Mediated DNA Repair

Mutations in isocitrate dehydrogenase (IDH) are the most prevalent genetic abnormalities in lower grade gliomas. The presence of these mutations in glioma is prognostic for better clinical outcomes with longer patient survival. In the present study, we found that defects in oxidative metabolism and 2-HG production confer chemosensitization in IDH1-mutated glioma cells. In addition, temozolomide (TMZ) treatment induced greater DNA damage and apoptotic changes in mutant glioma cells. The PARP1-associated DNA repair pathway was extensively compromised in mutant cells due to decreased NAD⁺ availability. Targeting the PARP DNA repair pathway extensively sensitized IDH1-mutated glioma cells to TMZ. Our findings demonstrate a novel molecular mechanism that defines chemosensitivity in IDH-mutated gliomas. Targeting PARP-associated DNA repair may represent a novel therapeutic strategy for gliomas. Cancer Res; 77(7); 1709-18. ©2017 AACR.

Functional Role of ADP-Ribosyl-Acceptor Hydrolase 3 in poly(ADP-Ribose) Polymerase-1 Response to Oxidative Stress

Poly-ADP-ribosylation has been proposed to be a reversible protein modification, participating in diverse cellular functions including DNA repair, chromatin remodeling, genetic stability, mitosis, and cell death. Poly-ADP-ribosylation is initiated by the transfer of the ADP-ribose moiety of NAD+ primarily to the carboxyl groups of glutamate and aspartate and amino group of lysine residues in target proteins, followed by elongation of poly(ADP-ribose) (PAR) chains via α-O-glycosidic (C- 1"-C-2') ribose-ribose bonds. PAR consists of polymers of ADP-ribose (up to 200 units) with branching via α-O-glycosidic (C-1"'-C-2") ribose-ribose bonds. Further, the pyrophosphate group of each ADP-ribose has two negative charges. Therefore, in proteins modified by PAR, a complex structure with negative charges may lead to dynamic changes of functions. PAR formation is catalyzed by poly(ADP-ribose) polymerases (PARPs) and terminated by several types of enzymes with PAR-degrading activities; poly(ADP-ribose) glycohydrolase (PARG), ADP-ribosylacceptor hydrolase (ARH) 3, ARH1, and macrodomain-containing proteins. PARG has been thought to be primarily responsible for PAR degradation. In 2006, ARH3 was cloned and identified as another type of PAR-degrading protein. Although PAR-degrading activity of ARH3 is less than that of PARG, different mechanisms of PAR recognition and the cellular localization of ARH3 appear to be responsible for unique cellular roles of ARH3 involving PAR. In the present review, we focused on our findings regarding structure, biological properties, and cellular functions of ARH3. In addition, we describe the current knowledge of poly-ADP-ribosylation and cell death pathways regulated PARP1, PARG, and ARH3.

Role of NAD+ and mitochondrial sirtuins in cardiac and renal diseases

The coenzyme nicotinamide adenine dinucleotide (NAD⁺) has key roles in the regulation of redox status and energy metabolism. NAD⁺ depletion is emerging as a major contributor to the pathogenesis of cardiac and renal diseases and NAD⁺ repletion strategies have shown therapeutic potential as a means to restore healthy metabolism and physiological function. The pleotropic roles of NAD⁺ enable several possible avenues by which repletion of this coenzyme could have therapeutic efficacy. In particular, NAD⁺ functions as a co-substrate in deacylation reactions carried out by the sirtuin family of enzymes. These NAD⁺-dependent deacylases control several aspects of metabolism and a wealth of data suggests that boosting sirtuin activity via NAD⁺ supplementation might be a promising therapy for cardiac and renal pathologies. This Review summarizes the role of NAD⁺ metabolism in the heart and kidney, and highlights the mitochondrial sirtuins as mediators of some of the beneficial effects of NAD⁺-boosting therapies in preclinical animal models. We surmise that modulating the NAD⁺-sirtuin axis is a clinically relevant approach to develop new therapies for cardiac and renal diseases.

Base excision repair imbalance in colorectal cancer has prognostic value and modulates response to chemotherapy

Colorectal cancer (CRC) is prevalent worldwide, and treatment often involves surgery and genotoxic chemotherapy. DNA repair mechanisms, such as base excision repair (BER) and mismatch repair (MMR), may not only influence tumour characteristics and prognosis but also dictate chemotherapy response. Defective MMR contributes to chemoresistance in colorectal cancer. Moreover, BER affects cellular survival by repairing genotoxic base damage in a process that itself can disrupt metabolism. In this study, we characterized BER and MMR gene expression in colorectal tumours and the association between this repair profile with patients’ clinical and pathological features. In addition, we exploited the possible mechanisms underlying the association between altered DNA repair, metabolism and response to chemotherapy. Seventy pairs of sporadic colorectal tumour samples and adjacent non-tumour mucosal specimens were assessed for BER and MMR gene and protein expression and their association with pathological and clinical features. MMR-deficient colon cancer cells (HCT116) transiently overexpressing MPG or XRCC1 were treated with 5-FU or TMZ and evaluated for viability and metabolic intermediate levels. Increase in BER gene and protein expression is associated with more aggressive tumour features and poor pathological outcomes in CRC. However, tumours with reduced MMR gene expression also displayed low MPG, OGG1 and PARP1 expression. Imbalancing BER by overexpression of MPG, but not XRCC1, sensitises MMR-deficient colon cancer cells to 5-FU and TMZ and leads to ATP depletion and lactate accumulation. MPG overexpression alters DNA repair and metabolism and is a potential strategy to overcome 5-FU chemotherapeutic resistance in MMR-deficient CRC.

Natural Compounds As Modulators of Non-apoptotic Cell Death in Cancer Cells

Cell death is an innate capability of cells to be removed from microenvironment, if and when they are damaged by multiple stresses. Cell death is often regulated by multiple molecular pathways and mechanism, including apoptosis, autophagy, and necroptosis. The molecular network underlying these processes is often intertwined and one pathway can dynamically shift to another one acquiring certain protein components, in particular upon treatment with various drugs. The strategy to treat human cancer ultimately relies on the ability of anticancer therapeutics to induce tumor-specific cell death, while leaving normal adjacent cells undamaged. However, tumor cells often develop the resistance to the drug-induced cell death, thus representing a great challenge for the anticancer approaches. Numerous compounds originated from the natural sources and biopharmaceutical industries are applied today in clinics showing advantageous results. However, some exhibit serious toxic side effects. Thus, novel effective therapeutic approaches in treating cancers are continued to be developed. Natural compounds with anticancer activity have gained a great interest among researchers and clinicians alike since they have shown more favorable safety and efficacy then the synthetic marketed drugs. Numerous studies in vitro and in vivo have found that several natural compounds display promising anticancer potentials. This review underlines certain information regarding the role of natural compounds from plants, microorganisms and sea life forms, which are able to induce non-apoptotic cell death in tumor cells, namely autophagy and necroptosis.

Nonapoptotic cell death in acute kidney injury and transplantation

Acute tubular necrosis causes a loss of renal function, which clinically presents as acute kidney failure (AKI). The biochemical signaling pathways that trigger necrosis have been investigated in detail over the past 5 years. It is now clear that necrosis (regulated necrosis, RN) represents a genetically driven process that contributes to the pathophysiology of AKI. RN pathways such as necroptosis, ferroptosis, parthanatos, and mitochondrial permeability transition-induced regulated necrosis (MPT-RN) may be mechanistically distinct, and the relative contributions to overall organ damage during AKI in living organisms largely remain elusive. In a synchronized manner, some necrotic programs induce the breakdown of tubular segments and multicellular functional units, whereas others are limited to killing single cells in the tubular compartment. Importantly, the means by which a renal cell dies may have implications for the subsequent inflammatory response. In this review, the recent advances in the field of renal cell death in AKI and key enzymes that might serve as novel therapeutic targets will be discussed. As a consequence of the interference with RN, the immunogenicity of dying cells in AKI in renal transplants will be diminished, rendering inhibitors of RN indirect immunosuppressive agents.

Mitochondrial Mechanisms of Neuronal Cell Death: Potential Therapeutics

Mitochondria lie at the crossroads of neuronal survival and cell death. They play important roles in cellular bioenergetics, control intracellular Ca2+ homeostasis, and participate in key metabolic pathways. Mutations in genes involved in mitochondrial quality control cause a myriad of neurodegenerative diseases. Mitochondria have evolved strategies to kill cells when they are not able to continue their vital functions. This review provides an overview of the role of mitochondria in neurologic disease and the cell death pathways that are mediated through mitochondria, including their role in accidental cell death, the regulated cell death pathways of apoptosis and parthanatos, and programmed cell death. It details the current state of parthanatic cell death and discusses potential therapeutic strategies targeting initiators and effectors of mitochondrial-mediated cell death in neurologic disorders.

Detecting and Quantifying pADPr In Vivo

Poly(ADP-ribose) polymerases (PARP) participate in diverse biological processes contributing to cellular homeostasis or exacerbating injury. PARP catalyzes the addition of ADP-ribose molecules (pADPr) to the target proteins, a process termed poly-ADP-ribosylation. Overactivation of PARP, as reflected by increased poly-ADP-ribosylation, accumulation of pADPr-modified proteins or free pADPr, contributes to depletion of NAD⁺ and mitochondrial dysfunction, potentially leading to cell death. Since PARP overactivation and increases in free pADPr have been identified as key contributors to the pathobiology of many diseases, monitoring PARP-1 activation by detecting and quantifying pADPr may provide valuable mechanistic insights as well as facilitating therapeutic drug monitoring for PARP inhibitors.Several non-isotopic immunodetection methods for quantifying pADPr are discussed: western blotting of poly-ADP-ribosylated proteins, cellular localization of pADPr by immunohistochemistry, quantification of pADPr by enzyme-linked immunoassay and small scale two-dimensional gel electrophoresis.

Proteome-wide Profiling of Clinical PARP Inhibitors Reveals Compound-Specific Secondary Targets

Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) are a promising class of targeted cancer drugs, but their individual target profiles beyond the PARP family, which could result in differential clinical use or toxicity, are unknown. Using an unbiased, mass spectrometry-based chemical proteomics approach, we generated a comparative proteome-wide target map of the four clinical PARPi, olaparib, veliparib, niraparib, and rucaparib. PARPi as a class displayed high target selectivity. However, in addition to the canonical targets PARP1, PARP2, and several of their binding partners, we also identified hexose-6-phosphate dehydrogenase (H6PD) and deoxycytidine kinase (DCK) as previously unrecognized targets of rucaparib and niraparib, respectively. Subsequent functional validation suggested that inhibition of DCK by niraparib could have detrimental effects when combined with nucleoside analog pro-drugs. H6PD silencing can cause apoptosis and further sensitize cells to PARPi, suggesting that H6PD may be, in addition to its established role in metabolic disorders, a new anticancer target.

Metabolic roles of poly(ADP-ribose) polymerases

Poly(ADP-ribosyl)ation (PARylation) is an evolutionarily conserved reaction that had been associated with numerous cellular processes such as DNA repair, protein turnover, inflammatory regulation, aging or metabolic regulation. The metabolic regulatory tasks of poly(ADP-ribose) polymerases (PARPs) are complex, it is based on the regulation of metabolic transcription factors (e.g. SIRT1, nuclear receptors, SREBPs) and certain cellular energy sensors. PARP over-activation can cause damage to mitochondrial terminal oxidation, while the inhibition of PARP-1 or PARP-2 can induce mitochondrial oxidation by enhancing the mitotropic tone of gene transcription and signal transduction. These PARP-mediated processes impact on higher order metabolic regulation that modulates lipid metabolism, circadian oscillations and insulin secretion and signaling. PARP-1, PARP-2 and PARP-7 are related to metabolic diseases such as diabetes, alcoholic and non-alcoholic fatty liver disease (AFLD, NAFLD), or on a broader perspective to Warburg metabolism in cancer or the metabolic diseases accompanying aging.

Metabolic fate of glucose in rats with traumatic brain injury and pyruvate or glucose treatments: A NMR spectroscopy study

Administration of sodium pyruvate (SP; 9.08 μmol/kg, i.p.), ethyl pyruvate (EP; 0.34 μmol/kg, i.p.) or glucose (GLC; 11.1 μmol/kg, i.p.) to rats after unilateral controlled cortical impact (CCI) injury has been reported to reduce neuronal loss and improve cerebral metabolism. In the present study these doses of each fuel or 8% saline (SAL; 5.47 nmoles/kg) were administered immediately and at 1, 3, 6 and 23 h post-CCI. At 24 h all CCI groups and non-treated Sham injury controls were infused with [1,2 ¹³C] glucose for 68 min ¹³C nuclear magnetic resonance (NMR) spectra were obtained from cortex + hippocampus tissues from left (injured) and right (contralateral) hemispheres. All three fuels increased lactate labeling to a similar degree in the injured hemisphere. The amount of lactate labeled via the pentose phosphate and pyruvate recycling (PPP + PR) pathway increased in CCI-SAL and was not improved by SP, EP, and GLC treatments. Oxidative metabolism, as assessed by glutamate labeling, was reduced in CCI-SAL animals. The greatest improvement in oxidative metabolism was observed in animals treated with SP and fewer improvements after EP or GLC treatments. Compared to SAL, all three fuels restored glutamate and glutamine labeling via pyruvate carboxylase (PC), suggesting improved astrocyte metabolism following fuel treatment. Only SP treatments restored the amount of [4 ¹³C] glutamate labeled by the PPP + PR pathway to sham levels. Milder injury effects in the contralateral hemisphere appear normalized by either SP or EP treatments, as increases in the total pool of ¹³C lactate and labeling of lactate in glycolysis, or decreases in the ratio of PC/PDH labeling of glutamine, were found only for CCI-SAL and CCI-GLC groups compared to Sham. The doses of SP, EP and GLC examined in this study all enhanced lactate labeling and restored astrocyte-specific PC activity but differentially affected neuronal metabolism after CCI injury. The restoration of astrocyte metabolism by all three fuel treatments may partially underlie their abilities to improve cerebral glucose utilization and to reduce neuronal loss following CCI injury.

Neuroprotective Strategies during Cardiac Surgery with Cardiopulmonary Bypass

Aortocoronary bypass or valve surgery usually require cardiac arrest using cardioplegic solutions. Although, in principle, in a number of cases beating heart surgery (so-called off-pump technique) is possible, aortic or valve surgery or correction of congenital heart diseases mostly require cardiopulmonary arrest. During this condition, the heart-lung machine also named cardiopulmonary bypass (CPB) has to take over the circulation. It is noteworthy that the invention of a machine bypassing the heart and lungs enabled complex cardiac operations, but possible negative effects of the CPB on other organs, especially the brain, cannot be neglected. Thus, neuroprotection during CPB is still a matter of great interest. In this review, we will describe the impact of CPB on the brain and focus on pharmacological and non-pharmacological strategies to protect the brain.

Poly(ADP-ribose)polymerase-1 hyperactivation in neurodegenerative diseases: The death knell tolls for neurons

Neurodegeneration is a salient feature of chronic refractory brain disorders like Alzheimer's, Parkinson's, Huntington's, amyotropic lateral sclerosis and acute conditions like cerebral ischemia/reperfusion etc. The pathological protein aggregates, mitochondrial mutations or ischemic insults typifying these disease conditions collude with and intensify existing oxidative stress and attendant mitochondrial dysfunction. Interlocking these mechanisms is poly(ADP-ribose) polymerase (PARP-1) hyperactivation that invokes a distinct form of neuronal cell death viz., 'parthanatos'. PARP-1, a typical 'moonlighting protein' by virtue of its ability to poly(ADP-ribosyl)ate a plethora of cellular proteins exerts diverse functions that impinge significantly on cellular processes. In addition, its interactions with various nuclear proteins like transcription factors and chromatin modifiers elicit varied transcriptional outcomes that wield pathological cellular responses. Further, emerging leitmotifs like mitochondrial and nucleolar PARPs and the novel aspects of gene expression regulation by PARP-1 and poly(ADP-ribosyl)ation can provide a holistic view of PARP-1's influence on cell vitality. In this review, we discuss the pathological underpinnings of PARP-1, with a special emphasis on mitochondrial dysfunction and cell death subroutines, in the realm of neurodegeneration. This would provide a deeper insight into the functions of PARP-1 in neurodegenerative conditions that would enable the development of more effective therapeutic strategies.

Post-translational modifications in mitochondria: protein signaling in the powerhouse

There is an intimate interplay between cellular metabolism and the pathophysiology of disease. Mitochondria are essential to maintaining and regulating metabolic function of cells and organs. Mitochondrial dysfunction is implicated in diverse diseases, such as cardiovascular disease, diabetes and metabolic syndrome, neurodegeneration, cancer, and aging. Multiple reversible post-translational protein modifications are located in the mitochondria that are responsive to nutrient availability and redox conditions, and which can act in protein-protein interactions to modify diverse mitochondrial functions. Included in this are physiologic redox signaling via reactive oxygen and nitrogen species, phosphorylation, O-GlcNAcylation, acetylation, and succinylation, among others. With the advent of mass proteomic screening techniques, there has been a vast increase in the array of known mitochondrial post-translational modifications and their protein targets. The functional significance of these processes in disease etiology, and the pathologic response to their disruption, are still under investigation. However, many of these reversible modifications act as regulatory mechanisms in mitochondria and show promise for mitochondrial-targeted therapeutic strategies. This review addresses the current knowledge of post-translational processing and signaling mechanisms in mitochondria, and their implications in health and disease.

FAF1 mediates regulated necrosis through PARP1 activation upon oxidative stress leading to dopaminergic neurodegeneration

Cumulative damage caused by oxidative stress results in diverse pathological conditions. Therefore, elucidating the molecular mechanisms underlying cell death following oxidative stress is important. Here, we describe a novel role for Fas-associated factor 1 (FAF1) as a crucial regulator of necrotic cell death elicited by hydrogen peroxide. Upon oxidative insult, FAF1 translocated from the cytoplasm to the nucleus and promoted the catalytic activation of poly(ADP-ribose) polymerase 1 (PARP1) through physical interaction. Moreover, FAF1 depletion prevented PARP1-linked downstream events involved in the triggering of cell death, including energetic collapse, mitochondrial depolarization and nuclear translocation of apoptosis-inducing factor (AIF), implying that FAF1 has a key role in PARP1-dependent necrosis in response to oxidative stress. We further investigated whether FAF1 might contribute to the pathogenesis of Parkinson's disease through excessive PARP1 activation. Indeed, the overexpression of FAF1 using a recombinant adeno-associated virus system in the mouse ventral midbrain promoted PARP1 activation and dopaminergic neurodegeneration in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease. Collectively, our data demonstrate the presence of an FAF1-PARP1 axis that is involved in oxidative stress-induced necrosis and in the pathology of Parkinson's disease.

Deficiency of Parkinson's disease-related gene Fbxo7 is associated with impaired mitochondrial metabolism by PARP activation

The Parkinson's disease (PD)-related protein F-box only protein 7 (Fbxo7) is the substrate-recognition component of the Skp1-Cullin-F-box protein E3 ubiquitin ligase complex. We have recently shown that PD-associated mutations in Fbxo7 disrupt mitochondrial autophagy (mitophagy), suggesting a role for Fbxo7 in modulating mitochondrial homeostasis. Here we report that Fbxo7 deficiency is associated with reduced cellular NAD⁺ levels, which results in increased mitochondrial NADH redox index and impaired activity of complex I in the electron transport chain. Under these conditions of compromised respiration, mitochondrial membrane potential and ATP contents are reduced, and cytosolic reactive oxygen species (ROS) production is increased. ROS activates poly (ADP-ribose) polymerase (PARP) activity in Fbxo7-deficient cells. PARP inhibitor restores cellular NAD⁺ content and redox index and ATP pool, suggesting that PARP overactivation is cause of decreased complex I-driven respiration. These findings bring new insight into the mechanism of Fbxo7 deficiency, emphasising the importance of mitochondrial dysfunction in PD.

Analyzing structure-function relationships of artificial and cancer-associated PARP1 variants by reconstituting TALEN-generated HeLa PARP1 knock-out cells

Genotoxic stress activates PARP1, resulting in the post-translational modification of proteins with poly(ADP-ribose) (PAR). We genetically deleted PARP1 in one of the most widely used human cell systems, i.e. HeLa cells, via TALEN-mediated gene targeting. After comprehensive characterization of these cells during genotoxic stress, we analyzed structure–function relationships of PARP1 by reconstituting PARP1 KO cells with a series of PARP1 variants. Firstly, we verified that the PARP1\E988K mutant exhibits mono-ADP-ribosylation activity and we demonstrate that the PARP1\L713F mutant is constitutively active in cells. Secondly, both mutants exhibit distinct recruitment kinetics to sites of laser-induced DNA damage, which can potentially be attributed to non-covalent PARP1–PAR interaction via several PAR binding motifs. Thirdly, both mutants had distinct functional consequences in cellular patho-physiology, i.e. PARP1\L713F expression triggered apoptosis, whereas PARP1\E988K reconstitution caused a DNA-damage-induced G2 arrest. Importantly, both effects could be rescued by PARP inhibitor treatment, indicating distinct cellular consequences of constitutive PARylation and mono(ADP-ribosyl)ation. Finally, we demonstrate that the cancer-associated PARP1 SNP variant (V762A) as well as a newly identified inherited PARP1 mutation (F304L\V762A) present in a patient with pediatric colorectal carcinoma exhibit altered biochemical and cellular properties, thereby potentially supporting human carcinogenesis. Together, we establish a novel cellular model for PARylation research, by revealing strong structure–function relationships of natural and artificial PARP1 variants.

L-Lactate-Mediated Neuroprotection against Glutamate-Induced Excitotoxicity Requires ARALAR/AGC1

ARALAR/AGC1/Slc25a12, the aspartate-glutamate carrier from brain mitochondria, is the regulatory step in the malate-aspartate NADH shuttle, MAS. MAS is used to oxidize cytosolic NADH in mitochondria, a process required to maintain oxidative glucose utilization. The role of ARALAR was analyzed in two paradigms of glutamate-induced excitotoxicity in cortical neurons: glucose deprivation and acute glutamate stimulation. ARALAR deficiency did not aggravate glutamate-induced neuronal death in vitro, although glutamate-stimulated respiration was impaired. In contrast, the presence of L-lactate as an additional source protected against glutamate-induced neuronal death in control, but not ARALAR-deficient neurons.l-Lactate supplementation increased glutamate-stimulated respiration partially prevented the decrease in the cytosolic ATP/ADP ratio induced by glutamate and substantially diminished mitochondrial accumulation of 8-oxoguanosine, a marker of reactive oxygen species production, only in the presence, but not the absence, of ARALAR. In addition,l-lactate potentiated glutamate-induced increase in cytosolic Ca(2+), in a way independent of the presence of ARALAR. Interestingly,in vivo, the loss of half-a-dose of ARALAR in aralar(+/-)mice enhanced kainic acid-induced seizures and neuronal damage with respect to control animals, in a model of excitotoxicity in which increased L-lactate levels and L-lactate consumption have been previously proven. These results suggest that,in vivo, an inefficient operation of the shuttle in the aralar hemizygous mice prevents the protective role of L-lactate on glutamate excitotoxiciy and that the entry and oxidation of L-lactate through ARALAR-MAS pathway is required for its neuroprotective function.

SIGNIFICANCE STATEMENT

Lactate now stands as a metabolite necessary for multiple functions in the brain and is an alternative energy source during excitotoxic brain injury. Here we find that the absence of a functional malate-aspartate NADH shuttle caused by aralar/AGC1 disruption causes a block in lactate utilization by neurons, which prevents the protective role of lactate on excitotoxicity, but not glutamate excitotoxicity itself. Thus, failure to use lactate is detrimental and is possibly responsible for the exacerbated in vivo excitotoxicity in aralar(+/-)mice.

Poly(ADP-ribose) polymerase regulates glycolytic activity in kidney proximal tubule epithelial cells

After renal injury, selective damage occurs in the proximal tubules as a result of inhibition of glycolysis. The molecular mechanism of damage is not known. Poly(ADP-ribose) polymerase (PARP) activation plays a critical role of proximal tubular cell death in several renal disorders. Here, we studied the role of PARP on glycolytic flux in pig kidney proximal tubule epithelial LLC-PK1 cells using XFp extracellular flux analysis. Poly(ADP-ribosyl)ation by PARP activation was increased approximately 2-fold by incubation of the cells in 10 mM glucose for 30 minutes, but treatment with the PARP inhibitor 3-aminobenzamide (3-AB) does-dependently prevented the glucose-induced PARP activation (approximately 14.4% decrease in 0.1 mM 3-AB–treated group and 36.7% decrease in 1 mM 3-AB–treated group). Treatment with 1 mM 3-AB significantly enhanced the glucose-mediated increase in the extracellular acidification rate (61.1±4.3 mpH/min vs. 126.8±6.2 mpH/min or approximately 2-fold) compared with treatment with vehicle, indicating that PARP inhibition increases only glycolytic activity during glycolytic flux including basal glycolysis, glycolytic activity, and glycolytic capacity in kidney proximal tubule epithelial cells. Glucose increased the activities of glycolytic enzymes including hexokinase, phosphoglucose isomerase, phosphofructokinase-1, glyceraldehyde-3-phosphate dehydrogenase, enolase, and pyruvate kinase in LLC-PK1 cells. Furthermore, PARP inhibition selectively augmented the activities of hexokinase (approximately 1.4-fold over vehicle group), phosphofructokinase-1 (approximately 1.6-fold over vehicle group), and glyceraldehyde-3-phosphate dehydrogenase (approximately 2.2-fold over vehicle group). In conclusion, these data suggest that PARP activation may regulate glycolytic activity via poly(ADP-ribosyl)ation of hexokinase, phosphofructokinase-1, and glyceraldehyde-3-phosphate dehydrogenase in kidney proximal tubule epithelial cells.

The PARP family: insights into functional aspects of poly (ADP-ribose) polymerase-1 in cell growth and survival

PARP family members can be found spread across all domains and continue to be essential molecules from lower to higher eukaryotes. Poly (ADP-ribose) polymerase 1 (PARP-1), newly termed ADP-ribosyltransferase D-type 1 (ARTD1), is a ubiquitously expressed ADP-ribosyltransferase (ART) enzyme involved in key cellular processes such as DNA repair and cell death. This review assesses current developments in PARP-1 biology and activation signals for PARP-1, other than conventional DNA damage activation. Moreover, many essential functions of PARP-1 still remain elusive. PARP-1 is found to be involved in a myriad of cellular events via conservation of genomic integrity, chromatin dynamics and transcriptional regulation. This article briefly focuses on its other equally important overlooked functions during growth, metabolic regulation, spermatogenesis, embryogenesis, epigenetics and differentiation. Understanding the role of PARP-1, its multidimensional regulatory mechanisms in the cell and its dysregulation resulting in diseased states, will help in harnessing its true therapeutic potential.