Poly(ADP-ribose) accumulation and enhancement of postischemic brain damage in 110-kDa poly(ADP-ribose) glycohydrolase null mice

ADP-ribosylation, a multifaceted modification: Functions and mechanisms in aging and aging-related diseases

Aging, a complex biological process, plays key roles the development of multiple disorders referred as aging-related diseases involving cardiovascular diseases, stroke, neurodegenerative diseases, cancers, lipid metabolism-related diseases. ADP-ribosylation is a reversible modification onto proteins and nucleic acids to alter their structures and/or functions. Growing evidence support the importance of ADP-ribosylation and ADP-ribosylation-associated enzymes in aging and age-related diseases. In this review, we summarized ADP-ribosylation-associated proteins including ADP-ribosyl transferases, the ADP-ribosyl hydrolyses and ADP-ribose binding domains. Furthermore, we outlined the latest knowledge about regulation of ADP-ribosylation in the pathogenesis and progression of main aging-related diseases, organism aging and cellular senescence, and we also speculated the underlying mechanisms to better disclose this novel molecular network. Moreover, we discussed current issues and provided an outlook for future research, aiming to revealing the unknown bio-properties of ADP-ribosylation, and establishing a novel therapeutic perspective in aging-related diseases and health aging via targeting ADP-ribosylation.

Poly(ADP-ribose) mediates bioenergetic defects and redox imbalance in neurons following oxygen and glucose deprivation

PARP-1 over-activation results in cell death via excessive PAR generation in different cell types, including neurons following brain ischemia. Glycolysis, mitochondrial function, and redox balance are key cellular processes altered in brain ischemia. Studies show that PAR generated after PARP-1 over-activation can bind hexokinase-1 (HK-1) and result in glycolytic defects and subsequent mitochondrial dysfunction. HK-1 is the neuronal hexokinase and catalyzes the first reaction of glycolysis, converting glucose to glucose-6-phosphate (G6P), a common substrate for glycolysis, and the pentose phosphate pathway (PPP). PPP is critical in maintaining NADPH and GSH levels via G6P dehydrogenase activity. Therefore, defects in HK-1 will not only decrease cellular bioenergetics but will also cause redox imbalance due to the depletion of GSH. In brain ischemia, whether PAR-mediated inhibition of HK-1 results in bioenergetics defects and redox imbalance is not known. We used oxygen-glucose deprivation (OGD) in mouse cortical neurons to mimic brain ischemia in neuronal cultures and observed that PARP-1 activation via PAR formation alters glycolysis, mitochondrial function, and redox homeostasis in neurons. We used pharmacological inhibition of PARP-1 and adenoviral-mediated overexpression of wild-type HK-1 (wtHK-1) and PAR-binding mutant HK-1 (pbmHK-1). Our data show that PAR inhibition or overexpression of HK-1 significantly improves glycolysis, mitochondrial function, redox homeostasis, and cell survival in mouse cortical neurons exposed to OGD. These results suggest that PAR binding and inhibition of HK-1 during OGD drive bioenergetic defects in neurons due to inhibition of glycolysis and impairment of mitochondrial function.

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 - reflected by increased poly-ADP-ribosylation and accumulation of pADPr-modified proteins or free pADPr - contributes to depletion of NAD⁺ and mitochondrial dysfunction, potentially leading to cell death. Thus, PARP overactivation and increases in free pADPr have been identified as key contributors to the pathobiology of many diseases. In stark contrast, PARP inhibitors are in clinical use in cancer patients where they potentiate cell death induced by chemotherapeutic agents. Accordingly, monitoring PARP-1 activation - responsible for up to 80-90% of cellular pADPr synthesis - 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.

Therapeutic Potentials of Poly (ADP-Ribose) Polymerase 1 (PARP1) Inhibition in Multiple Sclerosis and Animal Models: Concept Revisiting

Poly (ADP-ribose) polymerase 1 (PARP1) plays a fundamental role in DNA repair and gene expression. Excessive PARP1 hyperactivation, however, has been associated with cell death. PARP1 and/or its activity are dysregulated in the immune and central nervous system of multiple sclerosis (MS) patients and animal models. Pharmacological PARP1 inhibition is shown to be protective against immune activation and disease severity in MS animal models while genetic PARP1 deficiency studies reported discrepant results. The inconsistency suggests that the function of PARP1 and PARP1-mediated PARylation may be complex and context-dependent. The article reviews PARP1 functions, discusses experimental findings and possible interpretations of PARP1 in inflammation, neuronal/axonal degeneration, and oligodendrogliopathy, three major pathological components cooperatively determining MS disease course and neurological progression, and points out future research directions. Cell type specific PARP1 manipulations are necessary for revisiting the role of PARP1 in the three pathological components prior to moving PARP1 inhibition into clinical trials for MS therapy.

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.

The function of KptA/Tpt1 gene - a minor review

Rapid response of uni- and multicellular organisms to environmental changes and their own growth is achieved through a series of molecular mechanisms, often involving modification of macromolecules, including nucleic acids, proteins and lipids. The ADP-ribosylation process has ability to modify these different macromolecules in cells, and is closely related to the biological processes, such as DNA replication, transcription, signal transduction, cell division, stress, microbial aging and pathogenesis. In addition, tRNA plays an essential role in the regulation of gene expression, as effector molecules, no-load tRNA affects the overall gene expression level of cells under some nutritional stress. KptA/Tpt1 is an essential phosphotransferase in the process of pre-tRNA splicing, releasing mature tRNA and participating in ADP-ribose. The objective of this review is concluding the gene structure, the evolution history and the function of KptA/Tpt1 from prokaryote to eukaryote organisms. At the same time, the results of promoter elements analysis were also shown in the present study. Moreover, the problems in the function of KptA/Tpt1 that have not been clarified at the present time are summarised, and some suggestions to solve those problems are given. This review presents no only a summary of clear function of KptA/Tpt1 in the process of tRNA splicing and ADP-ribosylation of organisms, but also gives some proposals to clarify unclear problems of it in the future.

(ADP-ribosyl)hydrolases: structure, function, and biology

ADP-ribosylation is an intricate and versatile posttranslational modification involved in the regulation of a vast variety of cellular processes in all kingdoms of life. Its complexity derives from the varied range of different chemical linkages, including to several amino acid side chains as well as nucleic acids termini and bases, it can adopt. In this review, we provide an overview of the different families of (ADP-ribosyl)hydrolases. We discuss their molecular functions, physiological roles, and influence on human health and disease. Together, the accumulated data support the increasingly compelling view that (ADP-ribosyl)hydrolases are a vital element within ADP-ribosyl signaling pathways and they hold the potential for novel therapeutic approaches as well as a deeper understanding of ADP-ribosylation as a whole.

Poly (ADP-ribose) (PAR)-dependent cell death in neurodegenerative diseases

Disruption of cellular functions with aging-induced accumulation of neuronal stressors causes cell death which is a common feature of neurodegenerative diseases. Studies in a variety of neurodegenerative disease models demonstrate that poly (ADP-ribose) (PAR)-dependent cell death, also named parthanatos, is responsible for neuronal loss in neurological diseases, such as Parkinson's disease (PD), Alzheimer's disease (AD), Huntington's disease (HD) and amyotrophic lateral sclerosis (ALS). Parthanatos has distinct features that differ from caspase-dependent apoptosis, necrosis or autophagic cell death. Parthanatos can be triggered by the accumulation of PAR due to overactivation of PAR polymerase-1 (PARP-1). Excess PAR, induces the mitochondrial release apoptosis-inducing factor (AIF), which binds to macrophage migration inhibitory factor (MIF) carrying MIF into the nucleus where it cleaves genomic DNA into large fragments. In this review, we will discuss the molecular mechanisms of parthanatos and their role in neurodegenerative diseases. Furthermore, we will discuss promising therapeutic interventions within the pathological PAR signaling cascade that could be designed to halt the progression of neurodegeneration.

NAD+ precursor modulates post-ischemic mitochondrial fragmentation and reactive oxygen species generation via SIRT3 dependent mechanisms

Global cerebral ischemia depletes brain tissue NAD⁺, an essential cofactor for mitochondrial and cellular metabolism, leading to bioenergetics failure and cell death. The post-ischemic NAD⁺ levels can be replenished by the administration of nicotinamide mononucleotide (NMN), which serves as a precursor for NAD⁺ synthesis. We have shown that NMN administration shows dramatic protection against ischemic brain damage and inhibits post-ischemic hippocampal mitochondrial fragmentation. To understand the mechanism of NMN-induced modulation of mitochondrial dynamics and neuroprotection we used our transgenic mouse models that express mitochondria targeted yellow fluorescent protein in neurons (mito-eYFP) and mice that carry knockout of mitochondrial NAD⁺-dependent deacetylase sirt3 gene (SIRT3KO). Following ischemic insult, the mitochondrial NAD⁺ levels were depleted leading to an increase in mitochondrial protein acetylation, high reactive oxygen species (ROS) production, and excessive mitochondrial fragmentation. Administration of a single dose of NMN normalized hippocampal mitochondria NAD⁺ pools, protein acetylation, and ROS levels. These changes were dependent on SIRT3 activity, which was confirmed using SIRT3KO mice. Ischemia induced increase in acetylation of the key mitochondrial antioxidant enzyme, superoxide dismutase 2 (SOD2) that resulted in inhibition of its activity. This was reversed after NMN treatment followed by reduction of ROS generation and suppression of mitochondrial fragmentation. Specifically, we found that the interaction of mitochondrial fission protein, pDrp1(S616), with neuronal mitochondria was inhibited in NMN treated ischemic mice. Our data thus provide a novel link between mitochondrial NAD⁺ metabolism, ROS production, and mitochondrial fragmentation. Using NMN to target these mechanisms could represent a new therapeutic approach for treatment of acute brain injury and neurodegenerative diseases.

Sex differences in Alzheimer's disease: Understanding the molecular impact

Alzheimer's disease (AD) is a common neurodegenerative disorder that presents with cognitive impairment and behavioral disturbance. Approximately 5.5 million people in the United States live with AD, most of whom are over the age of 65 with two-thirds being woman. There have been major advancements over the last decade or so in the understanding of AD neuropathological changes and genetic involvement. However, studies of sex impact in AD have not been adequately integrated into the investigation of disease development and progression. It becomes indispensable to acknowledge in both basic science and clinical research studies the importance of understanding sex-specific differences in AD pathophysiology and pathogenesis, which could guide future effort in the discovery of novel targets for AD. Here, we review the latest and most relevant literature on this topic, highlighting the importance of understanding sex dimorphism from a molecular perspective and its association to clinical trial design and development in AD research field.

ADP-ribosylation signalling and human disease

ADP-ribosylation (ADPr) is a reversible post-translational modification of proteins, which controls major cellular and biological processes, including DNA damage repair, cell proliferation and differentiation, metabolism, stress and immune responses. In order to maintain the cellular homeostasis, diverse ADP-ribosyl transferases and hydrolases are involved in the fine-tuning of ADPr systems. The control of ADPr network is vital, and dysregulation of enzymes involved in the regulation of ADPr signalling has been linked to a number of inherited and acquired human diseases, such as several neurological disorders and in cancer. Conversely, the therapeutic manipulation of ADPr has been shown to ameliorate several disorders in both human and animal models. These include cardiovascular, inflammatory, autoimmune and neurological disorders. Herein, we summarize the recent findings in the field of ADPr, which support the impact of this modification in human pathophysiology and highlight the curative potential of targeting ADPr for translational and molecular medicine.

Multi-targeted Effect of Nicotinamide Mononucleotide on Brain Bioenergetic Metabolism

Dysfunctions in NAD⁺ metabolism are associated with neurodegenerative diseases, acute brain injury, diabetes, and aging. Loss of NAD⁺ levels results in impairment of mitochondria function, which leads to failure of essential metabolic processes. Strategies to replenish depleted NAD⁺ pools can offer significant improvements of pathologic states. NAD⁺ levels are maintained by two opposing enzymatic reactions, one is the consumption of NAD⁺ while the other is the re-synthesis of NAD⁺. Inhibition of NAD⁺ degrading enzymes, poly-ADP-ribose polymerase 1 (PARP1) and ectoenzyme CD38, following brain ischemic insult can provide neuroprotection. Preservation of NAD⁺ pools by administration of NAD⁺ precursors, such as nicotinamide (Nam) or nicotinamide mononucleotide (NMN), also offers neuroprotection. However, NMN treatment demonstrates to be a promising candidate as a therapeutic approach due to its multi-targeted effect acting as PARP1 and CD38 inhibitor, sirtuins activator, mitochondrial fission inhibitor, and NAD⁺ supplement. Many neurodegenerative diseases or acute brain injury activate several cellular death pathways requiring a treatment strategy that will target these mechanisms. Since NMN demonstrated the ability to exert its effect on several cellular metabolic pathways involved in brain pathophysiology it seems to be one of the most promising candidates to be used for successful neuroprotection.

Cell Death Mechanisms of Neurodegeneration

There are common mechanisms shared by genetically or pathologically distinct neurodegenerative diseases, such as excitotoxicity, mitochondrial deficits and oxidative stress, protein misfolding and translational dysfunction, autophagy and microglia activation. This indicates that although the original cause may differ in individual diseases or even subtypes of certain disorders, these disrupted common cell functions and signaling, together with aging, may lead to final execution of cell death through similar pathways. The variable neurodegenerative disease symptoms are probably caused by the type, location, and connection of the cell populations that suffer from dysfunction and loss. Besides apoptosis, necroptosis, and autophagy, an important form of death termed parthanatos plays a prominent role in stroke and several neurodegenerative diseases, which is due to PARP-1 overactivation, PAR accumulation, nuclear translocation of the mitochondria protein AIF, and large-scale DNA cleavage. Understanding the mechanisms and interactions of cell death signaling will not only help to develop neuroprotective strategies to halt neurodegeneration, but also provide biomarkers for monitoring disease progression and recovery.

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.

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.

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.

CD38 Knockout Mice Show Significant Protection Against Ischemic Brain Damage Despite High Level Poly-ADP-Ribosylation

Several enzymes in cellular bioenergetics metabolism require NAD+ as an essential cofactor for their activity. NAD+ depletion following ischemic insult can result in cell death and has been associated with over-activation of poly-ADP-ribose polymerase PARP1 as well as an increase in NAD+ consuming enzyme CD38. CD38 is an NAD+ glycohydrolase that plays an important role in inflammatory responses. To determine the contribution of CD38 activity to the mechanisms of post-ischemic brain damage we subjected CD38 knockout (CD38KO) mice and wild-type (WT) mice to transient forebrain ischemia. The CD38KO mice showed a significant amelioration in both histological and neurologic outcome following ischemic insult. Decrease of hippocampal NAD+ levels detected during reperfusion in WT mice was only transient in CD38KO animals, suggesting that CD38 contributes to post-ischemic NAD+ catabolism. Surprisingly, pre-ischemic poly-ADP-ribose (PAR) levels were dramatically higher in CD38KO animals compared to WT animals and exhibited reduction post-ischemia in contrast to the increased levels in WT animals. The high PAR levels in CD38 mice were due to reduced expression levels of poly-ADP-ribose glycohydrolase (PARG). Thus, the absence of CD38 activity can not only directly affect inflammatory response, but also result in unpredicted alterations in the expression levels of enzymes participating in NAD+ metabolism. Although the CD38KO mice showed significant protection against ischemic brain injury, the changes in enzyme activity related to NAD+ metabolism makes the determination of the role of CD38 in mechanisms of ischemic brain damage more complex.

Nicotinamide mononucleotide inhibits post-ischemic NAD(+) degradation and dramatically ameliorates brain damage following global cerebral ischemia

Nicotinamide adenine dinucleotide (NAD(+)) is an essential cofactor for multiple cellular metabolic reactions and has a central role in energy production. Brain ischemia depletes NAD(+) pools leading to bioenergetics failure and cell death. Nicotinamide mononucleotide (NMN) is utilized by the NAD(+) salvage pathway enzyme, nicotinamide adenylyltransferase (Nmnat) to generate NAD(+). Therefore, we examined whether NMN could protect against ischemic brain damage. Mice were subjected to transient forebrain ischemia and treated with NMN or vehicle at the start of reperfusion or 30min after the ischemic insult. At 2, 4, and 24h of recovery, the proteins poly-ADP-ribosylation (PAR), hippocampal NAD(+) levels, and expression levels of NAD(+) salvage pathway enzymes were determined. Furthermore, animal's neurologic outcome and hippocampal CA1 neuronal death was assessed after six days of reperfusion. NMN (62.5mg/kg) dramatically ameliorated the hippocampal CA1 injury and significantly improved the neurological outcome. Additionally, the post-ischemic NMN treatment prevented the increase in PAR formation and NAD(+) catabolism. Since the NMN administration did not affect animal's temperature, blood gases or regional cerebral blood flow during recovery, the protective effect was not a result of altered reperfusion conditions. These data suggest that administration of NMN at a proper dosage has a strong protective effect against ischemic brain injury.

Readers of poly(ADP-ribose): designed to be fit for purpose

Post-translational modifications (PTMs) regulate many aspects of protein function and are indispensable for the spatio-temporal regulation of cellular processes. The proteome-wide identification of PTM targets has made significant progress in recent years, as has the characterization of their writers, readers, modifiers and erasers. One of the most elusive PTMs is poly(ADP-ribosyl)ation (PARylation), a nucleic acid-like PTM involved in chromatin dynamics, genome stability maintenance, transcription, cell metabolism and development. In this article, we provide an overview on our current understanding of the writers of this modification and their targets, as well as the enzymes that degrade and thereby modify and erase poly(ADP-ribose) (PAR). Since many cellular functions of PARylation are exerted through dynamic interactions of PAR-binding proteins with PAR, we discuss the readers of this modification and provide a synthesis of recent findings, which suggest that multiple structurally highly diverse reader modules, ranging from completely folded PAR-binding domains to intrinsically disordered sequence stretches, evolved as PAR effectors to carry out specific cellular functions.

Structures and Mechanisms of Enzymes Employed in the Synthesis and Degradation of PARP-Dependent Protein ADP-Ribosylation

The poly(ADP-ribose) polymerases (PARPs) are a major family of enzymes capable of modifying proteins by ADP-ribosylation. Due to the large size and diversity of this family, PARPs affect almost every aspect of cellular life and have fundamental roles in DNA repair, transcription, heat shock and cytoplasmic stress responses, cell division, protein degradation, and much more. In the past decade, our understanding of the PARP enzymatic mechanism and activation, as well as regulation of ADP-ribosylation signals by the readers and erasers of protein ADP-ribosylation, has been significantly advanced by the emergence of new structural data, reviewed herein, which allow for better understanding of the biological roles of this widespread post-translational modification.

ARTD1 (PARP1) activation and NAD(+) in DNA repair and cell death

Nicotinamide adenine dinucleotide, NAD(+), is a small metabolite coenzyme that is essential for the progress of crucial cellular pathways including glycolysis, the tricarboxylic acid cycle (TCA) and mitochondrial respiration. These processes consume and produce both oxidative and reduced forms of NAD (NAD(+) and NADH). NAD(+) is also important for ADP(ribosyl)ation reactions mediated by the ADP-ribosyltransferase enzymes (ARTDs) or deacetylation reactions catalyzed by the sirtuins (SIRTs) which use NAD(+) as a substrate. In this review, we highlight the significance of NAD(+) catabolism in DNA repair and cell death through its utilization by ARTDs and SIRTs. We summarize the current findings on the involvement of ARTD1 activity in DNA repair and most specifically its involvement in the trigger of cell death mediated by ARTD1 activation and energy depletion. By sharing the same substrate, the activities of ARTDs and SIRTs are tightly linked, are dependent on each other and are thereby involved in the same cellular processes that play an important role in cancer biology, inflammatory diseases and ischaemia/reperfusion.

PARP-1 activation causes neuronal death in the hippocampal CA1 region by increasing the expression of Ca(2+)-permeable AMPA receptors

An excessive activation of poly(ADP-ribose) polymerases (PARPs) may trigger a form of neuronal death similar to that occurring in neurodegenerative disorders. To investigate this process, we exposed organotypic hippocampal slices to N-methyl-N'-nitro-N'-nitrosoguanidine (MNNG, 100μM for 5min), an alkylating agent widely used to activate PARP-1. MNNG induced a pattern of degeneration of the CA1 pyramidal cells morphologically similar to that observed after a brief period of oxygen and glucose deprivation (OGD). MNNG exposure was also associated with a dramatic increase in PARP-activity and a robust decrease in NAD(+) and ATP content. These effects were prevented by PARP-1 but not PARP-2 inhibitors. In our experimental conditions, cell death was not mediated by AIF translocation (parthanatos) or caspase-dependent apoptotic processes. Furthermore, we found that PARP activation was followed by a significant deterioration of neuronal membrane properties. Using electrophysiological recordings we firstly investigated the suggested ability of ADP-ribose to open TRPM2 channels in MNNG-induced cells death, but the results we obtained showed that TRPM2 channels are not involved. We then studied the involvement of glutamate receptor-ion channel complex and we found that NBQX, a selective AMPA receptor antagonist, was able to effectively prevent CA1 neuronal loss while MK801, a NMDA antagonist, was not active. Moreover, we observed that MNNG treatment increased the ratio of GluA1/GluA2 AMPAR subunit expression, which was associated with an inward rectification of the IV relationship of AMPA sEPSCs in the CA1 but not in the CA3 subfield. Accordingly, 1-naphthyl acetyl spermine (NASPM), a selective blocker of Ca(2+)-permeable GluA2-lacking AMPA receptors, reduced MNNG-induced CA1 pyramidal cell death. In conclusion, our results show that activation of the nuclear enzyme PARP-1 may change the expression of membrane proteins and Ca(2+) permeability of AMPA channels, thus affecting the function and survival of CA1 pyramidal cells.

Parthanatos: mitochondrial-linked mechanisms and therapeutic opportunities

Cells die by a variety of mechanisms. Terminally differentiated cells such as neurones die in a variety of disorders, in part, via parthanatos, a process dependent on the activity of poly (ADP-ribose)-polymerase (PARP). Parthanatos does not require the mediation of caspases for its execution, but is clearly mechanistically dependent on the nuclear translocation of the mitochondrial-associated apoptosis-inducing factor (AIF). The nuclear translocation of this otherwise beneficial mitochondrial protein, occasioned by poly (ADP-ribose) (PAR) produced through PARP overactivation, causes large-scale DNA fragmentation and chromatin condensation, leading to cell death. This review describes the multistep course of parthanatos and its dependence on PAR signalling and nuclear AIF translocation. The review also discusses potential targets in the parthanatos cascade as promising avenues for the development of novel, disease-modifying, therapeutic agents.

The recognition and removal of cellular poly(ADP-ribose) signals

Poly(ADP-ribosyl)ation is involved in the regulation of a variety of cellular pathways, including, but not limited to, transcription, chromatin, DNA damage and other stress signalling. Similar to other tightly regulated post-translational modifications, poly(ADP-ribosyl)ation employs 'writers', 'readers' and 'erasers' to confer regulatory functions. The generation of poly(ADP-ribose) is catalyzed by poly(ADP-ribose) polymerase enzymes, which use NAD(+) as a cofactor to sequentially transfer ADP-ribose units generating long polymers, which, in turn, can affect protein function or serve as a recruitment platform for additional factors. Historically, research has focused on poly(ADP-ribose) generation pathways, with knowledge about PAR recognition and degradation lagging behind. Over recent years, several discoveries have significantly furthered our understanding of poly(ADP-ribose) recognition and, even more so, of poly(ADP-ribose) degradation. In this review, we summarize current knowledge about the protein modules recognizing poly(ADP-ribose) and discuss the newest developments on the complete reversibility of poly(ADP-ribosyl)ation.

Poly(ADP-ribose): PARadigms and PARadoxes

Poly(ADP-ribosyl)ation (PARylation) is a posttranslational protein modification (PTM) catalyzed by members of the poly(ADP-ribose) polymerase (PARP) enzyme family. PARPs use NAD(+) as substrate and upon cleaving off nicotinamide they transfer the ADP-ribosyl moiety covalently to suitable acceptor proteins and elongate the chain by adding further ADP-ribose units to create a branched polymer, termed poly(ADP-ribose) (PAR), which is rapidly degraded by poly(ADP-ribose) glycohydrolase (PARG) and ADP-ribosylhydrolase 3 (ARH3). In recent years several key discoveries changed the way we look at the biological roles and mode of operation of PARylation. These paradigm shifts include but are not limited to (1) a single PARP enzyme expanding to a PARP family; (2) DNA-break dependent activation extended to several other DNA dependent and independent PARP-activation mechanisms; (3) one molecular mechanism (covalent PARylation of target proteins) underlying the biological effect of PARPs is now complemented by several other mechanisms such as protein-protein interactions, PAR signaling, modulation of NAD(+) pools and (4) one principal biological role in DNA damage sensing expanded to numerous, diverse biological functions identifying PARP-1 as a real moonlighting protein. Here we review the most important paradigm shifts in PARylation research and also highlight some of the many controversial issues (or paradoxes) of the field such as (1) the mostly synergistic and not antagonistic biological effects of PARP-1 and PARG; (2) mitochondrial PARylation and PAR decomposition, (3) the cross-talk between PARylation and signaling pathways (protein kinases, phosphatases, calcium) and the (4) divergent roles of PARP/PARylation in longevity and in age-related diseases.

Long-lasting neuroprotection and neurological improvement in stroke models with new, potent and brain permeable inhibitors of poly(ADP-ribose) polymerase

BACKGROUND AND PURPOSES

Thienyl-isoquinolone (TIQ-A) is a relatively potent PARP inhibitor able to reduce post-ischaemic neuronal death in vitro. Here we have studied, in different stroke models in vivo, the neuroprotective properties of DAMTIQ and HYDAMTIQ, two TIQ-A derivatives able to reach the brain and to inhibit PARP-1 and PARP-2.

EXPERIMENTAL APPROACH

Studies were carried out in (i) transient (2 h) middle cerebral artery occlusion (tMCAO), (ii) permanent MCAO (pMCAO) and (iii) electrocoagulation of the distal portion of MCA in conjunction with transient (90 min) bilateral carotid occlusion (focal cortical ischaemia).

KEY RESULTS

In male rats with tMCAO, HYDAMTIQ (0.1-10 mg·kg(-1)) injected i.p. three times, starting 4 h after MCAO, reduced infarct volumes by up to 70%, reduced the loss of body weight by up to 60% and attenuated the neurological impairment by up to 40%. In age-matched female rats, HYDAMTIQ also reduced brain damage. Protection, however, was less pronounced than in the male rats. In animals with pMCAO, HYDAMTIQ administered 30 min after MCAO reduced infarct volumes by approximately 40%. In animals with focal cortical ischaemia, HYDAMTIQ treatment decreased post-ischaemic accumulation of PAR (the product of PARP activity) and the presence of OX42-positive inflammatory cells in the ischaemic cortex. It also reduced sensorimotor deficits for up to 90 days after MCAO.

CONCLUSION AND IMPLICATIONS

Our results show that HYDAMTIQ is a potent PARP inhibitor that conferred robust neuroprotection and long-lasting improvement of post-stroke neurological deficits.

Ischemic neuroprotection by TRPV1 receptor-induced hypothermia

Although treatment of stroke patients with mild hypothermia is a promising therapeutic approach, chemicals inducing prompt and safe reduction of body temperature are an unmet need. We measured the effects of the transient receptor potential vanilloid-1 (TRPV1) agonist rinvanil on thermoregulation and ischemic brain injury in mice. Intraperitoneal or intracerebroventricular injection of rinvanil induces mild hypothermia that is prevented by the receptor antagonist capsazepine. Both intraischemic and postischemic treatments provide permanent neuroprotection in animals subjected to transient middle cerebral artery occlusion (MCAo), an effect lost in mice artificially kept normothermic. Data indicate that TRPV1 receptor agonists are promising candidates for hypothermic treatment of stroke.

Neuroprotective effects of a novel water-soluble poly(ADP-ribose) polymerase-1 inhibitor, MP-124, in in vitro and in vivo models of cerebral ischemia

Cerebral ischemia induces excessive activation of poly(ADP-ribose) polymerase-1 (PARP-1), leading to neuronal cell death and the development of post-ischemic dysfunction. Blockade of PARP-related signals during cerebral ischemia has become a focus of interest as a new therapeutic approach for acute stroke treatment. The purpose of the present study was to examine the pharmacological profiles of MP-124, a novel water-soluble PARP-1 inhibitor, and its neuroprotective effects on ischemic injury in vitro and in vivo. MP-124 demonstrated competitive inhibition of the PARP-1 activity of human recombinant PARP-1 enzyme (Ki=16.5nmol/L). In P388D(1) cells, MP-124 inhibited the LDH leakage induced by H(2)O(2) in a concentration-dependent manner. (IC(50)=20.8nmol/L). In rat primary cortical neurons, MP-124 also inhibited the NAD depletion and polymerized ADP-ribose formation induced by H(2)O(2) exposure. Moreover, we investigated the neuroprotective effects of MP-124 in rat permanent and transient stroke models. In the rat permanent middle cerebral artery occlusion (MCAO) model, MP-124 was administered intravenously for 24h from 5min after the onset of MCAO. MP-124 (1, 3 and 10mg/kg/h) significantly inhibited the cerebral infarction in a dose-dependent manner (18, 42 and 48%). In rat transient MCAO model, MP-124 was administered intravenously from 30min after the onset of MCAO. MP-124 (3 and 10mg/kg/h) significantly reduced the infarct volume (53% and 50%). The present findings suggest that MP-124 acts as a potent neuroprotective agent in focal ischemia and its actions can be attributed to a reduction in NAD depletion and PAR formation.

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 the depletion of NAD(+) and mitochondrial dysfunction, potentially leading to cell death via apoptosis or necrosis. Since PARP over-activation has been identified as a key contributor 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.

Poly(ADP-ribose) metabolism in brain and its role in ischemia pathology

The biological roles of poly(ADP-ribose) polymers (PAR) and poly(ADP-ribosyl)ation of proteins in the central nervous system are diverse. The homeostasis of PAR orchestrated by poly(ADP-ribose) polymerase-1 (PARP-1) and poly(ADP-ribose) glycohydrolase (PARG) is crucial for cell physiology and pathology. Both enzymes are ubiquitously distributed in neurons and glia; however, they are segregated at the subcellular level. PARP-1 serves as a "nick sensor" for single- or double-stranded breaks in DNA and is involved in long and short patch base-excision repair, while PARG breaks down PAR. The stimulation of PARP-1 and PAR formation can activate proinflammatory transcription factors, including nuclear factor kappa B. However, hyperactivation of PARP-1 can result in depletion of NAD/ATP, and in PAR-dependent mitochondrial pore formation leading to release of apoptosis inducing factor and cell death. The role of PAR as a death signaling molecule in brain ischemia-reperfusion and inflammation as well as the effect of gender and aging is presented in this review. Modulating the PAR level through pharmacological or genetic intervention on PARP-1/PARG activity and gene expression should be a valuable way for neuroprotective strategy.

Novel neuroprotective strategies in ischemic retinal lesions

Retinal ischemia can be effectively modeled by permanent bilateral common carotid artery occlusion, which leads to chronic hypoperfusion-induced degeneration in the entire rat retina. The complex pathways leading to retinal cell death offer a complex approach of neuroprotective strategies. In the present review we summarize recent findings with different neuroprotective candidate molecules. We describe the protective effects of intravitreal treatment with: (i) urocortin 2; (ii) a mitochondrial ATP-sensitive K⁺ channel opener, diazoxide; (iii) a neurotrophic factor, pituitary adenylate cyclase activating polypeptide; and (iv) a novel poly(ADP-ribose) polymerase inhibitor (HO3089). The retinoprotective effects are demonstrated with morphological description and effects on apoptotic pathways using molecular biological techniques.

Mitochondrial calpain system: an overview

Calpain system is generally known to be comprised of three molecules: two Ca2+-dependent proteases: mu- and m-calpains, and their endogenous inhibitor, calpastatin. While calpains have previously been considered as the cytoplasmic enzymes, research in the recent past demonstrated that mu-calpain, m-calpain and calpain 10 are present in mitochondria, which play important roles in a variety of pathophysiological conditions including necrotic and apoptotic cell death phenomena. Although a number of original research articles on mitochondrial calpain system are available, yet to the best of our knowledge, a precise review article on mitochondrial calpain system has, however, not been available. This review outlines the key features of the mitochondrial calpain system, and its roles in several cellular and biochemical events under normal and some pathophysiological conditions.

Brain ischemic preconditioning does not require PARP-1

BACKGROUND AND PURPOSE

Poly(ADP-ribose) polymerase-1 (PARP-1) is involved in ischemic preconditioning of the heart and cultured neurons, but its role in brain ischemic preconditioning is unknown. Summary of Report- We report that 5-minute bilateral common carotid artery occlusion (BCCAO) in the mouse prompted reduction of infarct volumes triggered 24 hours later by 20-minute middle cerebral artery occlusion (MCAO). Pharmacological PARP-1 inhibition between BCCAO and MCAO did not impair preconditioning. The contents of the PARP-1 substrate NAD, those of its product poly(ADP-ribose), caspase-3 activation, and PARP-1 expression did not change after BCCAO within the preconditioned tissue. PARP-1 KO mice were similarly protected by the 5-minute BCCAO.

CONCLUSIONS

Data demonstrate that, at variance with the heart, PARP-1 is dispensable for brain ischemic preconditioning.

Poly(ADP-ribose) catabolism triggers AMP-dependent mitochondrial energy failure

Upon massive DNA damage, hyperactivation of the nuclear enzyme poly(ADP-ribose) polymerase (PARP)-1 causes severe depletion of intracellular NAD and ATP pools as well as mitochondrial dysfunction. Thus far, the molecular mechanisms contributing to PARP-1-dependent impairment of mitochondrial functioning have not been identified. We found that degradation of the PARP-1 product poly(ADP-ribose) through the concerted actions of poly(ADP-ribose) glycohydrolase and NUDIX (nucleoside diphosphate-X) hydrolases leads to accumulation of AMP. The latter, in turn, inhibits the ADP/ATP translocator, prompting mitochondrial energy failure. For the first time, our findings identify NUDIX hydrolases as key enzymes involved in energy derangement during PARP-1 hyperactivity. Also, these data disclose unanticipated AMP-dependent impairment of mitochondrial exchange of adenine nucleotides, which can be of relevance to organelle functioning and disease pathogenesis.

Calcium dysregulation induces apoptosis-inducing factor release: cross-talk between PARP-1- and calpain-signaling pathways

Recent discoveries show that caspase-independent cell death pathways are a pervasive mechanism in neurodegenerative diseases, and apoptosis-inducing factor (AIF) is an important effector of this mode of neuronal death. There are currently two known mechanisms underlying AIF release following excitotoxic stress, PARP-1 and calpain. To test whether there is an interaction between PARP-1 and calpain in triggering AIF release, we used the NMDA toxicity model in rat primary cortical neurons. Exposure to NMDA resulted in AIF truncation and nuclear translocation, and shRNA-mediated knockdown of AIF resulted in neuroprotection. Both calpain and PARP-1 are involved with AIF processing as AIF truncation, nuclear translocation and neuronal death were attenuated by calpain inhibition using adeno-associated virus-mediated overexpression of the endogenous calpain inhibitor, calpastatin, or treatment with the PARP-1 inhibitor 3-ABA. Activation of PARP-1 is necessary for calpain activation as PARP-1 inhibition blocked mitochondrial calpain activation. Finally, NMDA toxicity induces mitochondrial Ca(2+) dysregulation in a PARP-1 dependent manner. Thus, PARP-1 and mitochondrial calpain activation are linked via PARP-1-induced alterations in mitochondrial Ca(2+) homeostasis. Collectively, these findings link the two seemingly independent mechanisms triggering AIF-induced neuronal death.

Sex differences in the response to activation of the poly (ADP-ribose) polymerase pathway after experimental stroke

It is increasingly recognized that histological and functional outcomes after stroke are shaped by biologic sex. Emerging data suggests that ischemic cell death pathways are sexually dimorphic (Hurn, P., Vannucci, S., Hagberg, H. (2005) Adult or perinatal brain injury: does sex matter?. Stroke 36, 193-195 ; Lang, J.T., McCullough, L.D. (2008) Pathways to ischemic neuronal cell death: are sex differences relevant?. J. Transl. Med. 6). Reducing neuronal nitric oxide (NO) or poly-ADP-ribose polymerase (PARP1) activation protects only the male brain (Hagberg, H., et al. PARP-1 gene disruption in mice preferentially protects males from perinatal brain injury. J. Neurochem. 90, 1068-1075 (2004)), and paradoxically enhances ischemic injury in females (McCullough, L.D., et al. Ischemic nitric oxide and poly (ADP-ribose) polymerase-1 in cerebral ischemia: male toxicity, female protection. J. Cereb. Blood Flow Metab. 25, 502-512 (2005)). In this study, we examined downstream mediators of NO/PARP activation to investigate possible mediators of ischemic sexual dimorphism. Nuclear translocation of Apoptosis Inducing Factor (AIF) was equivalent in wild type males and females after stroke and was unaffected by estrogen exposure. Deletion of PARP1 led to a dramatic reduction in stroke-induced poly (ADP-ribose) polymerase (PAR) formation and AIF translocation in both sexes, yet ischemic damage was reduced only in males. Subsequent examination of AIF-deficient Harlequin mice demonstrated that male Harlequin mice had less PAR formation, reduced AIF translocation and less ischemic damage than male wild type mice. In contrast, female Harlequin mice had no neuroprotective effect of gene deletion despite robust reductions in PAR formation and AIF translocation. Although equivalent activation of this cell death pathway occurs in both sexes after ischemia, detrimental effects are only present in males. AIF translocation and PAR formation do not mediate ischemic injury in the female brain, therefore agents designed to reduce PARP1 activation are unlikely to benefit females.

Protection against chronic hypoperfusion-induced retinal neurodegeneration by PARP inhibition via activation of PI-3-kinase Akt pathway and suppression of JNK and p38 MAP kinases

Poly(ADP-ribose) polymerase (PARP) activation is considered as a major regulator of cell death in various pathophysiological conditions, however, no direct information is available about its role in chronic hypoperfusion-induced neuronal death. Here, we provide evidence for the protective effect of PARP inhibition on degenerative retinal damage induced by bilateral common carotid artery occlusion (BCCAO), an adequate chronic hypoperfusion murine model. We found that BCCAO in adult male Wistar rats led to severe degeneration of all retinal layers that was attenuated by a carboxaminobenzimidazol-derivative PARP inhibitor (HO3089) administered unilaterally into the vitreous body immediately following carotid occlusion and then 4 times in a 2-week-period. Normal morphological structure of the retina was preserved and the thickness of the retinal layers was increased in HO3089-treated eyes compared to the BCCAO eyes. For Western blot studies, HO3089 was administered immediately after BCCAO and retinas were removed 4 h later. According to Western blot analysis utilizing phosphorylation-specific primary antibodies, besides activating poly-ADP-ribose (PAR) synthesis, BCCAO induced phosphorylation of c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). HO3089 inhibited PAR synthesis, and decreased the phosphorylation of these proapoptotic MAPKs. In addition, HO3089 treatment induced phosphorylation, that is activation, of the protective Akt/glycogen synthase kinase (GSK)-3beta and extracellular signal-regulated kinase (ERK1/2) signaling pathways. These data indicate that PARP activation has a major role in mediating chronic hypoperfusion-induced neuronal death, and inhibition of the enzyme prevents the pathological changes both in the morphology and the kinase signaling cascades involved. These results identify PARP inhibition as a possible molecular target in the clinical management of chronic hypoperfusion-induced neurodegenerative diseases including ocular ischemic syndrome.

Selective down-regulation of nuclear poly(ADP-ribose) glycohydrolase

Background

The formation of ADP-ribose polymers on target proteins by poly(ADP-ribose) polymerases serves a variety of cell signaling functions. In addition, extensive activation of poly(ADP-ribose) polymerase-1 (PARP-1) is a dominant cause of cell death in ischemia-reperfusion, trauma, and other conditions. Poly(ADP-ribose) glycohydrolase (PARG) degrades the ADP-ribose polymers formed on acceptor proteins by PARP-1 and other PARP family members. PARG exists as multiple isoforms with differing subcellular localizations, but the functional significance of these isoforms is uncertain.

Methods / Principal Findings

Primary mouse astrocytes were treated with an antisense phosphorodiamidate morpholino oligonucleotide (PMO) targeted to exon 1 of full-length PARG to suppress expression of this nuclear-specific PARG isoform. The antisense-treated cells showed down-regulation of both nuclear PARG immunoreactivity and nuclear PARG enzymatic activity, without significant alteration in cytoplasmic PARG activity. When treated with the genotoxic agent MNNG to induced PARP-1 activation, the antisense-treated cells showed a delayed rate of nuclear PAR degradation, reduced nuclear condensation, and reduced cell death.

Conclusions/Significance

These results support a preferentially nuclear localization for full-length PARG, and suggest a key role for this isoform in the PARP-1 cell death pathway.

Mitochondrial and nuclear cross talk in cell death: parthanatos

Poly(ADP-ribose) polymerase-1 (PARP-1) is an abundant nuclear protein best known to facilitate DNA base excision repair. Recent work has expanded the physiologic functions of PARP-1, and it is clear that the full range of biologic actions of this important protein are not yet fully understood. Regulation of the product of PARP-1, poly(ADP-ribose) (PAR), is a dynamic process with PAR glycohydrolase playing the major role in the degradation of the polymer. Under pathophysiologic situations overactivation of PARP-1 results in unregulated PAR synthesis and widespread neuronal cell death. Once thought to be necrotic cell death resulting from energy failure, we have found that PARP-1-dependent cell death is dependent on the generation of PAR, which triggers the nuclear translocation of apoptosis-inducing factor resulting in caspase-independent cell death. This form of cell death is distinct from apoptosis, necrosis, or autophagy and is termed parthanatos. PARP-1-dependent cell death has been implicated in tissues throughout the body and in diseases afflicting hundreds of millions worldwide, including stroke, Parkinson's disease, heart attack, diabetes, and ischemia reperfusion injury in numerous tissues. The breadth of indications for PARP-1 injury make parthanatos a clinically important form of cell death to understand and control.

Parthanatos, a messenger of death

Poly-ADP-ribose polymerase-1 (PARP-1)'s roles in the cell span from maintaining life to inducing death. The processes PARP-1 is involved in include DNA repair, DNA transcription, mitosis, and cell death. Of PARP-1's different cellular functions, its role in cell death is of particular interest to designing therapies for diseases. Genetic deletion of PARP-1 revealed that PARP-1 overactivation underlies cell death in models of stroke, diabetes, inflammation and neurodegeneration. Since interfering with PARP-1 mediated cell death will be clinically beneficial, great effort has been invested into understanding mechanisms downstream of PARP-1 overactivation. Recent evidence shows that poly-ADP ribose (PAR) polymer itself can act as a cell death effector downstream of PARP-1. We coined the term parthanatos after Thanatos, the personification of death in Greek mythology, to refer to PAR-mediated cell death. In this review, we will present evidence and questions raised by these recent findings, and summarize the proposed mechanisms by which PARP-1 overactivation kills. It is evident that further understanding of parthanatos opens up new avenues for therapy in ameliorating diseases related to PARP-1 overactivation.

Mono-galloyl glucose derivatives are potent poly(ADP-ribose) glycohydrolase (PARG) inhibitors and partially reduce PARP-1-dependent cell death

BACKGROUND AND PURPOSE

Maintenance of poly(ADP-ribose) (PAR) polymers at homoeostatic levels by PAR glycohydrolase (PARG) is central in cell functioning and survival. Yet the pharmacological relevance of PARG inhibitors is still debated. Gallotannin, a complex mixture of hydrolysable tannins from oak gall, inhibits PARG but which of its constituents is responsible for the inhibition and whether the pharmacodynamic properties are due to its antioxidant properties, has not yet been established.

EXPERIMENTAL APPROACH

A structure-activity relationship study was conducted on different natural and synthetic tannins/galloyl derivatives as potential PARG inhibitors, using a novel in vitro enzymic assay. Cytotoxicity was assayed in cultured HeLa cells.

KEY RESULTS

Mono-galloyl glucose compounds were potent inhibitors of PARG, with activities similar to that of ADP-(hydroxymethyl) pyrrolidinediol, the most potent PARG inhibitor yet identified. When tested on HeLa cells exposed to the PAR polymerase (PARP)-1-activating compound 1-methyl-3-nitro-1-nitrosoguanidine (MNNG), 3-galloyl glucose weakly inhibited PAR degradation. Conversely, the more lipophilic, 3-galloyl-1,2-O-isopropylidene glucose, despite being inactive on the pure enzyme, efficiently prolonged the half-life of the polymers in intact HeLa cells. Also, PARG inhibitors, but not radical scavengers, reduced, in part, cell death caused by MNNG.

CONCLUSIONS AND IMPLICATIONS

Taken together, our findings identify mono-galloyl glucose derivatives as potent PARG inhibitors, and emphasize the active function of this enzyme in cell death.

Parp and cell death or protection in rat primary astroglial cell cultures under LPS/IFNgamma induced proinflammatory conditions

The enzyme poly(ADP-ribose)polymerase (PARP) has a leader role in the DNA damage survey mechanisms by its nick-sensor function, but it is also involved in the early events of the programmed cell death, particularly during inflammatory injury, as a coactivator of NF-kB. In the present study, we evaluated the PARP involvement in the mechanisms of protection and/or cell death in rat astroglial cell cultures during the early phase of proinflammatory commitment after lipopolysaccharide and interferon gamma treatment. According with the recent findings that PARP-1 phosphorylation by MAPK/ERK-2 pathway seems to modulate PARP activation, in time course experiments we demonstrated that a very early PARP activation and expression is able to trigger a cell death pathway, DNA damage independent, during strong proinflammatory insults, maintaining its role of guardian of the genome stability only during the normal cell cycling.

NAD+/NADH and NADP+/NADPH in cellular functions and cell death: regulation and biological consequences

Accumulating evidence has suggested that NAD (including NAD+ and NADH) and NADP (including NADP+ and NADPH) could belong to the fundamental common mediators of various biological processes, including energy metabolism, mitochondrial functions, calcium homeostasis, antioxidation/generation of oxidative stress, gene expression, immunological functions, aging, and cell death: First, it is established that NAD mediates energy metabolism and mitochondrial functions; second, NADPH is a key component in cellular antioxidation systems; and NADH-dependent reactive oxygen species (ROS) generation from mitochondria and NADPH oxidase-dependent ROS generation are two critical mechanisms of ROS generation; third, cyclic ADP-ribose and several other molecules that are generated from NAD and NADP could mediate calcium homeostasis; fourth, NAD and NADP modulate multiple key factors in cell death, such as mitochondrial permeability transition, energy state, poly(ADP-ribose) polymerase-1, and apoptosis-inducing factor; and fifth, NAD and NADP profoundly affect aging-influencing factors such as oxidative stress and mitochondrial activities, and NAD-dependent sirtuins also mediate the aging process. Moreover, many recent studies have suggested novel paradigms of NAD and NADP metabolism. Future investigation into the metabolism and biological functions of NAD and NADP may expose fundamental properties of life, and suggest new strategies for treating diseases and slowing the aging process.

Identification of poly-ADP-ribosylated mitochondrial proteins after traumatic brain injury

Poly-ADP-ribosylation is a post-translational modification performed by poly(ADP-ribose) polymerases (PARP), involved in many diverse cellular functions including DNA repair, transcription, and long-term potentiation. Paradoxically, PARP over-activation under pathologic conditions including traumatic brain injury (TBI) results in cell death. We previously demonstrated that intra-mitochondrial poly-ADP-ribosylation occurs following excitotoxic and oxidative injury in vitro. Here we sought to identify mitochondrial proteins modified by poly-ADP-ribosylation after TBI in vivo. Poly-ADP-ribosylation within mitochondria from injured brain after experimental TBI in rats was first verified using western blot and immuno-electron microscopy. Poly-ADP-ribosylated mitochondrial proteins identified using a targeted proteomic approach included voltage-dependent anion channel-1, mitofilin, mitochondrial stress proteins, and the electron transport chain components F1F0 ATPase, cytochrome c oxidase, and cytochrome c reductase. To examine the functional consequences of mitochondrial poly-ADP-ribosylation, isolated rat brain mitochondria were exposed to conditions of nitrosative stress known to activate PARP. PARP activation-induced reductions in State 3 respiration were prevented by the PARP-1 inhibitor 5-iodo-6-amino-1,2-benzopyrone or exogenous poly(ADP-ribose) glycohydrolase. As the effects of PARP activation on mitochondrial respiration appear regulated by poly(ADP-ribose) glycohydrolase, a direct effect of poly-ADP-ribosylation on electron transport chain function is suggested. These findings may be of relevance to TBI and other diseases where mitochondrial dysfunction occurs.

Two small enzyme isoforms mediate mammalian mitochondrial poly(ADP-ribose) glycohydrolase (PARG) activity

Poly(ADP-ribose)glycohydrolase (PARG) is the major enzyme capable of rapidly hydrolyzing poly(ADP-ribose) (PAR) formed by the diverse members of the PARP enzyme family. This study presents an alternative splice mechanism by which two novel PARG protein isoforms of 60 kDa and 55 kDa are expressed from the human PARG gene, termed hPARG60 and hPARG55, respectively. Homologous forms were found in the mouse (mPARG63 and mPARG58) supporting the hypothesis that expression of small PARG isoforms is conserved among mammals. A PARG protein of approximately 60 kDa has been described for decades but with its genetic basis unknown, it was hypothesized to be a product of posttranslational cleavage of larger PARG isoforms. While this is not excluded entirely, isolation and expression of cDNA clones from different sources of RNA indicate that alternative splicing leads to expression of a catalytically active hPARG60 in multiple cell compartments. A second enzyme, hPARG55, that can be expressed through alternative translation initiation from hPARG60 transcripts is strictly targeted to the mitochondria. Functional studies of a mitochondrial targeting signal (MTS) in PARG exon IV suggest that hPARG60 may be capable of shuttling between nucleus and mitochondria, which would be in line with a proposed function of PAR in genotoxic stress-dependent, nuclear-mitochondrial crosstalk.

Neither energy collapse nor transcription underlie in vitro neurotoxicity of poly(ADP-ribose) polymerase hyper-activation

Poly(ADP-ribose)polymerase-1 (PARP-1) overactivation is a key event in neurodegeneration but the underlying molecular mechanisms wait to be unequivocally identified. Energy failure, transcriptional derangement and deadly nucleus-mitochondria cross-talk have been proposed as mechanisms responsible for PARP-1 neurotoxicity. In this study, we sought to determine how these mechanisms contributes to PARP-1-dependent neuronal death. We report that the PARP-1 activating agent methyl-nitrosoguanidine (MNNG) caused poly(ADP-ribosyl)ation-dependent death of pure mouse cortical neurons in culture. Upon PARP-1 hyperactivation, NAD and ATP storages only partially decreased, neurons rapidly acquired apoptotic morphology, apoptosis inducing factor and cytochrome c were released from mitochondria and caspase activation occurred. No evidence for p53 activation was found, lactate dehydrogenase release occurred only 18h later, and JNK kinase was constitutively activated and not affected by PARP-1 activation. The PARP-1 inhibitors 6-(5)H-phenanthridinone and N-(6-oxo-5,6-dihydro-phenanthridin-2-yl)-N,N-dimethylacetamide (PJ-34) prevented nucleotide depletion and cell death, whereas the transcription inhibitor actinomycin D did not affect PARP-1-dependent neurotoxicity. Together, our findings provide the first evidence that neither energy collapse nor transcriptional changes are involved in PARP-1-dependent apoptotic neuronal death, and support the existence of a poly(ADP-ribose)-mediated death signaling targeting mitochondria.

Nuclear ADP-ribosylation reactions in mammalian cells: where are we today and where are we going?

Since poly-ADP ribose was discovered over 40 years ago, there has been significant progress in research into the biology of mono- and poly-ADP-ribosylation reactions. During the last decade, it became clear that ADP-ribosylation reactions play important roles in a wide range of physiological and pathophysiological processes, including inter- and intracellular signaling, transcriptional regulation, DNA repair pathways and maintenance of genomic stability, telomere dynamics, cell differentiation and proliferation, and necrosis and apoptosis. ADP-ribosylation reactions are phylogenetically ancient and can be classified into four major groups: mono-ADP-ribosylation, poly-ADP-ribosylation, ADP-ribose cyclization, and formation of O-acetyl-ADP-ribose. In the human genome, more than 30 different genes coding for enzymes associated with distinct ADP-ribosylation activities have been identified. This review highlights the recent advances in the rapidly growing field of nuclear mono-ADP-ribosylation and poly-ADP-ribosylation reactions and the distinct ADP-ribosylating enzyme families involved in these processes, including the proposed family of novel poly-ADP-ribose polymerase-like mono-ADP-ribose transferases and the potential mono-ADP-ribosylation activities of the sirtuin family of NAD(+)-dependent histone deacetylases. A special focus is placed on the known roles of distinct mono- and poly-ADP-ribosylation reactions in physiological processes, such as mitosis, cellular differentiation and proliferation, telomere dynamics, and aging, as well as "programmed necrosis" (i.e., high-mobility-group protein B1 release) and apoptosis (i.e., apoptosis-inducing factor shuttling). The proposed molecular mechanisms involved in these processes, such as signaling, chromatin modification (i.e., "histone code"), and remodeling of chromatin structure (i.e., DNA damage response, transcriptional regulation, and insulator function), are described. A potential cross talk between nuclear ADP-ribosylation processes and other NAD(+)-dependent pathways is discussed.