Failure to degrade poly(ADP-ribose) causes increased sensitivity to cytotoxicity and early embryonic lethality

The PARP side of the nucleus: molecular actions, physiological outcomes, and clinical targets

The abundant nuclear enzyme PARP-1, a multifunctional regulator of chromatin structure, transcription, and genomic integrity, plays key roles in a wide variety of processes in the nucleus. Recent studies have begun to connect the molecular functions of PARP-1 to specific physiological and pathological outcomes, many of which can be altered by an expanding array of chemical inhibitors of PARP enzymatic activity.

Investigation of PARP-1, PARP-2, and PARG interactomes by affinity-purification mass spectrometry

Background

Poly(ADP-ribose) polymerases (PARPs) catalyze the formation of poly(ADP-ribose) (pADPr), a post-translational modification involved in several important biological processes, namely surveillance of genome integrity, cell cycle progression, initiation of the DNA damage response, apoptosis, and regulation of transcription. Poly(ADP-ribose) glycohydrolase (PARG), on the other hand, catabolizes pADPr and thereby accounts for the transient nature of poly(ADP-ribosyl)ation. Our investigation of the interactomes of PARP-1, PARP-2, and PARG by affinity-purification mass spectrometry (AP-MS) aimed, on the one hand, to confirm current knowledge on these interactomes and, on the other hand, to discover new protein partners which could offer insights into PARPs and PARG functions.

Results

PARP-1, PARP-2, and PARG were immunoprecipitated from human cells, and pulled-down proteins were separated by gel electrophoresis prior to in-gel trypsin digestion. Peptides were identified by tandem mass spectrometry. Our AP-MS experiments resulted in the identifications of 179 interactions, 139 of which are novel interactions. Gene Ontology analysis of the identified protein interactors points to five biological processes in which PARP-1, PARP-2 and PARG may be involved: RNA metabolism for PARP-1, PARP-2 and PARG; DNA repair and apoptosis for PARP-1 and PARP-2; and glycolysis and cell cycle for PARP-1.

Conclusions

This study reveals several novel protein partners for PARP-1, PARP-2 and PARG. It provides a global view of the interactomes of these proteins as well as a roadmap to establish the systems biology of poly(ADP-ribose) metabolism.

PARP and PARG inhibitors--new therapeutic targets in cancer treatment

Today, the number of cancer patients throughout the world is increasing alarmingly and as per the World Health Organisation (WHO) data and statistics the prediction for the year 2020 will be 15 million new cases as compared to only 10 million cases in year 2000 leaving us dumbfounded. A lot of effort has been put in by researchers and scientists over decades to find drugs helpful in the treatment of cancers for the benefit of patients--the latest being the Poly ADP-ribose polymerase (PARP) and the Poly ADP-ribose glycohydrolase (PARG) inhibitors. This review highlights their mechanism of action under the rationale of their use and current development in the field of cancer.

Oocyte numbers in the mouse increase after treatment with 5-aminoisoquinolinone: a potent inhibitor of poly(ADP-ribosyl)ation

Poly(ADP-ribosyl)ation is a posttranslational protein modification carried out by a family of enzymes referred to as poly(ADP-ribose) polymerases (PARPs). It has been proposed that the broad nuclear distribution of PARPs may allow them to modulate gene expression in addition to their more accepted role as DNA repair mediators. The role of poly(ADP-ribosyl)ation during oogenesis and folliculogenesis is unknown. Here we found that when 3- to 4-wk-old mice were injected with 5-amninoisoquinolinone, a water soluble inhibitor of poly(ADP-ribosyl)ation, it leads to considerably increased oocyte numbers and a dramatic increase in primordial follicle numbers. Furthermore, we show that inhibition of poly(ADP-ribosyl)ation leads to an increased expression of specific genes and pathways in mouse ovaries, in particular, transforming growth factor superfamily members. Our results demonstrate that poly(ADP-ribosyl)ation, is important in oogenesis and folliculogenesis, and it may have a differential role in regulating gene expression, DNA repair, and apoptosis. The novel function of poly(ADP-ribosyl)ation in oogenesis and folliculogenesis sheds light on the alternative role that DNA repair mediators may play in cellular development and differentiation.

Disruption of poly(ADP-ribosyl)ation mechanisms alters responses of Arabidopsis to biotic stress

Poly(ADP-ribosyl)ation is a posttranslational protein modification in which ADP-ribose (ADP-Rib) units derived from NAD(+) are attached to proteins by poly(ADP-Rib) polymerase (PARP) enzymes. ADP-Rib groups are removed from these polymer chains by the enzyme poly(ADP-Rib) glycohydrolase (PARG). In animals, poly(ADP-ribosyl)ation is associated with DNA damage responses and programmed cell death. Previously, we hypothesized a role for poly(ADP-ribosyl)ation in plant defense responses when we detected defense-associated expression of the poly(ADP-ribosyl)ation-related genes PARG2 and NUDT7 and observed altered callose deposition in the presence of a chemical PARP inhibitor. The role of poly(ADP-ribosyl)ation in plant defenses was more extensively investigated in this study, using Arabidopsis (Arabidopsis thaliana). Pharmacological inhibition of PARP using 3-aminobenzamide perturbs certain innate immune responses to microbe-associated molecular patterns (flg22 and elf18), including callose deposition, lignin deposition, pigment accumulation, and phenylalanine ammonia lyase activity, but does not disrupt other responses, such as the initial oxidative burst and expression of some early defense-associated genes. Mutant parg1 seedlings exhibit exaggerated seedling growth inhibition and pigment accumulation in response to elf18 and are hypersensitive to the DNA-damaging agent mitomycin C. Both parg1 and parg2 knockout plants show accelerated onset of disease symptoms when infected with Botrytis cinerea. Cellular levels of ADP-Rib polymer increase after infection with avirulent Pseudomonas syringae pv tomato DC3000 avrRpt2(+), and pathogen-dependent changes in the poly(ADP-ribosyl)ation of discrete proteins were also observed. We conclude that poly(ADP-ribosyl)ation is a functional component in plant responses to biotic stress.

Potential biological role of poly (ADP-ribose) polymerase (PARP) in male gametes

Maintaining the integrity of sperm DNA is vital to reproduction and male fertility. Sperm contain a number of molecules and pathways for the repair of base excision, base mismatches and DNA strand breaks. The presence of Poly (ADP-ribose) polymerase (PARP), a DNA repair enzyme, and its homologues has recently been shown in male germ cells, specifically during stage VII of spermatogenesis. High PARP expression has been reported in mature spermatozoa and in proven fertile men. Whenever there are strand breaks in sperm DNA due to oxidative stress, chromatin remodeling or cell death, PARP is activated. However, the cleavage of PARP by caspase-3 inactivates it and inhibits PARP's DNA-repairing abilities. Therefore, cleaved PARP (cPARP) may be considered a marker of apoptosis. The presence of higher levels of cPARP in sperm of infertile men adds a new proof for the correlation between apoptosis and male infertility. This review describes the possible biological significance of PARP in mammalian cells with the focus on male reproduction. The review elaborates on the role played by PARP during spermatogenesis, sperm maturation in ejaculated spermatozoa and the potential role of PARP as new marker of sperm damage. PARP could provide new strategies to preserve fertility in cancer patients subjected to genotoxic stresses and may be a key to better male reproductive health.

Global analysis of transcriptional regulation by poly(ADP-ribose) polymerase-1 and poly(ADP-ribose) glycohydrolase in MCF-7 human breast cancer cells

Poly(ADP-ribose) polymerase-1 (PARP-1) and poly(ADP-ribose) glycohydrolase (PARG) are enzymes that modify target proteins by the addition and removal, respectively, of ADP-ribose polymers. Although a role for PARP-1 in gene regulation has been well established, the role of PARG is less clear. To investigate how PARP-1 and PARG coordinately regulate global patterns of gene expression, we used short hairpin RNAs to stably knock down PARP-1 or PARG in MCF-7 cells followed by expression microarray analyses. Correlation analyses showed that the majority of genes affected by the knockdown of one factor were similarly affected by the knockdown of the other factor. The most robustly regulated common genes were enriched for stress-response and metabolic functions. In chromatin immunoprecipitation assays, PARP-1 and PARG localized to the promoters of positively and negatively regulated target genes. The levels of chromatin-bound PARG at a given promoter generally correlated with the levels of PARP-1 across the subset of promoters tested. For about half of the genes tested, the binding of PARP-1 at the promoter was dependent on the binding of PARG. Experiments using stable re-expression of short hairpin RNA-resistant catalytic mutants showed that PARP-1 and PARG enzymatic activities are required for some, but not all, target genes. Collectively, our results indicate that PARP-1 and PARG, nuclear enzymes with opposing enzymatic activities, localize to target promoters and act in a similar, rather than antagonistic, manner to regulate gene expression.

Structure/function analysis of PARP-1 in oxidative and nitrosative stress-induced monomeric ADPR formation

Poly adenosine diphosphate-ribose polymerase-1 (PARP-1) is a multifunctional enzyme that is involved in two major cellular responses to oxidative and nitrosative (O/N) stress: detection and response to DNA damage via formation of protein-bound poly adenosine diphosphate-ribose (PAR), and formation of the soluble 2(nd) messenger monomeric adenosine diphosphate-ribose (mADPR). Previous studies have delineated specific roles for several of PARP-1's structural domains in the context of its involvement in a DNA damage response. However, little is known about the relationship between the mechanisms through which PARP-1 participates in DNA damage detection/response and those involved in the generation of monomeric ADPR. To better understand the relationship between these events, we undertook a structure/function analysis of PARP-1 via reconstitution of PARP-1 deficient DT40 cells with PARP-1 variants deficient in catalysis, DNA binding, auto-PARylation, and PARP-1's BRCT protein interaction domain. Analysis of responses of the respective reconstituted cells to a model O/N stressor indicated that PARP-1 catalytic activity, DNA binding, and auto-PARylation are required for PARP-dependent mADPR formation, but that BRCT-mediated interactions are dispensable. As the BRCT domain is required for PARP-dependent recruitment of XRCC1 to sites of DNA damage, these results suggest that DNA repair and monomeric ADPR 2(nd) messenger generation are parallel mechanisms through which PARP-1 modulates cellular responses to O/N stress.

Dual role of poly(ADP-ribose) glycohydrolase in the regulation of cell death in oxidatively stressed A549 cells

Activation of poly(ADP-ribose) polymerase-1 (PARP1) has been shown to mediate cell death induced by genotoxic stimuli. The role of poly(ADP-ribose) glycohydrolase (PARG), the enzyme responsible for polymer degradation, has been largely unexplored in the regulation of cell death. Using lentiviral gene silencing we generated A549 lung adenocarcinoma cell lines with stably suppressed PARG and PARP1 expression (shPARG and shPARP1 cell lines, respectively) and determined parameters of apoptotic and necrotic cell death following hydrogen peroxide exposure. shPARG cells accumulated large amounts of poly(ADP-ribosyl)ated proteins and exhibited reduced PARP activation. Hydrogen peroxide-induced cell death is regulated by PARG in a dual fashion. Whereas the shPARG cell line (similarly to shPARP1 cells) was resistant to the necrotic effect of high concentrations of hydrogen peroxide, these cells exhibited stronger apoptotic response. Both shPARP1 and especially shPARG cells displayed a delayed repair of DNA breaks and exhibited reduced clonogenic survival following hydrogen peroxide treatment. Translocation of apoptosis-inducing factor could not be observed, but cells could be saved by methyl pyruvate and alpha-ketoglutarate, indicating that energy failure may mediate cytotoxicity in our model. These data indicate that PARG is a survival factor at mild oxidative damage but contributes to the apoptosis-necrosis switch in severely damaged cells.

Dual role of poly(ADP-ribose) glycohydrolase in the regulation of cell death in oxidatively stressed A549 cells

Activation of poly(ADP-ribose) polymerase-1 (PARP1) has been shown to mediate cell death induced by genotoxic stimuli. The role of poly(ADP-ribose) glycohydrolase (PARG), the enzyme responsible for polymer degradation, has been largely unexplored in the regulation of cell death. Using lentiviral gene silencing we generated A549 lung adenocarcinoma cell lines with stably suppressed PARG and PARP1 expression (shPARG and shPARP1 cell lines, respectively) and determined parameters of apoptotic and necrotic cell death following hydrogen peroxide exposure. shPARG cells accumulated large amounts of poly(ADP-ribosyl)ated proteins and exhibited reduced PARP activation. Hydrogen peroxide-induced cell death is regulated by PARG in a dual fashion. Whereas the shPARG cell line (similarly to shPARP1 cells) was resistant to the necrotic effect of high concentrations of hydrogen peroxide, these cells exhibited stronger apoptotic response. Both shPARP1 and especially shPARG cells displayed a delayed repair of DNA breaks and exhibited reduced clonogenic survival following hydrogen peroxide treatment. Translocation of apoptosis-inducing factor could not be observed, but cells could be saved by methyl pyruvate and alpha-ketoglutarate, indicating that energy failure may mediate cytotoxicity in our model. These data indicate that PARG is a survival factor at mild oxidative damage but contributes to the apoptosis-necrosis switch in severely damaged cells.

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.

Radiation-induced mitotic catastrophe in PARG-deficient cells

Poly(ADP-ribosyl)ation is a post-translational modification of proteins involved in the regulation of chromatin structure, DNA metabolism, cell division and cell death. Through the hydrolysis of poly(ADP-ribose) (PAR), Poly(ADP-ribose) glycohydrolase (PARG) has a crucial role in the control of life-and-death balance following DNA insult. Comprehension of PARG function has been hindered by the existence of many PARG isoforms encoded by a single gene and displaying various subcellular localizations. To gain insight into the function of PARG in response to irradiation, we constitutively and stably knocked down expression of PARG isoforms in HeLa cells. PARG depletion leading to PAR accumulation was not deleterious to undamaged cells and was in fact rather beneficial, because it protected cells from spontaneous single-strand breaks and telomeric abnormalities. By contrast, PARG-deficient cells showed increased radiosensitivity, caused by defects in the repair of single- and double-strand breaks and in mitotic spindle checkpoint, leading to alteration of progression of mitosis. Irradiated PARG-deficient cells displayed centrosome amplification leading to mitotic supernumerary spindle poles, and accumulated aberrant mitotic figures, which induced either polyploidy or cell death by mitotic catastrophe. Our results suggest that PARG could be a novel potential therapeutic target for radiotherapy.

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.

Poly(ADP-ribosyl)ation of heterogeneous nuclear ribonucleoproteins modulates splicing

The biological functions of poly(ADP-ribosyl)ation of heterogeneous nuclear ribonucleoproteins (hnRNPs) are not well understood. However, it is known that hnRNPs are involved in the regulation of alternative splicing for many genes, including the Ddc gene in Drosophila. Therefore, we first confirmed that poly(ADP-ribose) (pADPr) interacts with two Drosophila hnRNPs, Squid/hrp40 and Hrb98DE/hrp38, and that this function is regulated by Poly(ADP-ribose) Polymerase 1 (PARP1) and Poly(ADP-ribose) Glycohydrolase (PARG) in vivo. These findings then provided a basis for analyzing the role of pADPr binding to these two hnRNPs in terms of alternative splicing regulation. Our results showed that Parg null mutation does cause poly(ADP-ribosyl)ation of Squid and hrp38 protein, as well as their dissociation from active chromatin. Our data also indicated that pADPr binding to hnRNPs inhibits the RNA-binding ability of hnRNPs. Following that, we demonstrated that poly(ADP-ribosyl)ation of Squid and hrp38 proteins inhibits splicing of the intron in the Hsrω-RC transcript, but enhances splicing of the intron in the Ddc pre-mRNA. Taken together, these findings suggest that poly(ADP-ribosyl)ation regulates the interaction between hnRNPs and RNA and thus modulates the splicing pathways.

Poly(ADP-ribose) signals to mitochondrial AIF: a key event in parthanatos

Poly(ADP-ribose) polymerase-1 (PARP-1) plays a pivotal role in multiple neurologic diseases by mediating caspase-independent cell death, which has recently been designated parthanatos to distinguish it from other forms of cell death such as apoptosis, necrosis and autophagy. Mitochondrial apoptosis-inducing factor (AIF) release and translocation to the nucleus is the commitment point for parthanatos. This process involves a pathogenic role of poly(ADP-ribose) (PAR) polymer. It generates in the nucleus and translocates to the mitochondria to mediate AIF release following lethal PARP-1 activation. PAR polymer itself is toxic to cells. Thus, PAR polymer signaling to mitochondrial AIF is the key event initiating the deadly crosstalk between the nucleus and the mitochondria in parthanatos. Targeting PAR-mediated AIF release could be a potential approach for the therapy of neurologic disorders.

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.

Disruption of poly(ADP-ribose) homeostasis affects spermiogenesis and sperm chromatin integrity in mice

The major function of sperm is the delivery of the paternal genome to the metaphase II oocyte, ensuring transmission of the genetic information to the next generation. For successful fertilization and healthy offspring, sperm DNA must be protected from exogenous insults. This is achieved by packaging the sperm DNA into a condensed protamine-bound form, preceded by the precisely orchestrated removal of histones and intermittent insertion and removal of transition proteins. This remodeling process requires relaxation of supercoiled DNA by transient formation of physiological strand breaks that spermatids, being haploid, cannot repair by homologous recombination. In somatic cells, the presence of DNA strand breaks rapidly induces the formation of poly(ADP-ribose) by nuclear poly(ADP-ribose) polymerases, which in turn facilitates DNA strand break signaling and assembly of DNA repair complexes. We reported earlier that chromatin remodeling steps during spermiogenesis trigger poly(ADP-ribose) (PAR) formation. Here, we show that knockout mice deficient in PARP1, PARG (110-kDa isoform), or both display morphological and functional sperm abnormalities that are dependent on the individual genotypes, including residual DNA strand breaks associated with varying degrees of subfertility. The data presented highlight the importance of PAR metabolism, particularly PARG function, as a prerequisite of proper sperm chromatin quality.

Poly(ADP-ribose) polymerase suppression protects rheumatoid synovial fibroblasts from Fas-induced apoptosis

OBJECTIVES

To investigate the effect of poly(ADP-ribose) polymerase 1 (PARP-1) suppression on CD95/Apo-1 (Fas)-induced apoptosis in fibroblast-like synoviocytes (FLS) from RA patients.

METHODS

Apoptosis, determined by Hoechst staining and quantification of nucleosomal release by ELISA, was induced by stimulation with anti-Fas antibody with or without pre-treatment with cycloheximide (CHX). PARP-1 and poly(ADP-ribose) glycohydrolase (PARG) were suppressed in RA FLS by small interfering RNA (siRNA) transfection. Fas-associated via death domain (FADD), pro-caspase-8, Fas, c-Fas-associated death domain-like IL-1b-converting enzyme-inhibitory protein (FLIP) expression, and AKT and GSK phosphorylation were analysed by western blot.

RESULTS

PARP-1-deficient FLS showed significantly lower apoptosis than non-transfected and control siRNA-transfected FLS. The expression of death-inducing signaling complex (DISC) components such as Fas, FADD and pro-caspase-8 was not modified by PARP-1 suppression; however, FLS lacking PARP-1 showed high activation of the Akt-GSK survival pathway and up-regulation of the c-FLIP-S isoform after Fas triggering. Inhibition of PI3K/Akt pathway did not modify the difference between PARP-1-competent or -deficient FLS in Fas-mediated apoptosis or c-FLIP-S expression. Poly(ADP-ribose) accumulation induced by PARG supression did not modify the apoptotic response.

CONCLUSION

PARP-1 deficiency increases the resistance of RA FLS to Fas-induced apoptosis through activation of the Akt-GSK survival pathway and up-regulation of c-FLIP-S isoform.

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.

Effector caspase activation, in the absence of a conspicuous apoptosis induction, in mononuclear cells treated with azidothymidine

In the present study we focused our attention on the effect of AZT, at pharmacological and suprapharmacological concentrations, on some apoptosis-related key events and, particularly, on caspase activation in fresh human peripheral blood mononuclear cells (PBMCs). The main results can be summarized as follows: (i) AZT induced a strong, dose-dependent antiproliferative effect in mitogen-stimulated PBMCs, but low levels of cytotoxicity. in comparison with 5FU; (ii) low levels of cytotoxicity were coupled with a poor increase of apoptosis after AZT treatment in PBMCs; (iii) despite low levels of apoptosis, remarkable signs of both initiator and effector caspase enhanced expression with respect to control were detected by immunoblot analysis in AZT-treated PBMCs; (iv) enhanced caspase expression was associated with an increased expression of both anti-apoptotic Bcl-2 and pro-apoptotic Fas and p53 proteins, as detected by flow cytometry analysis; (v) combination treatment in vitro with AZT and anti-Fas significantly increased apoptosis in PBMCs with respect to single treatments. Overall, these results suggest that AZT treatment activates a complex, and apparently contrasting apoptosis-related signaling activity in PBMCs and that additional events are necessary to disrupt the balance induced by AZT towards apoptosis, on these cells.

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.

CCCTC-binding factor activates PARP-1 affecting DNA methylation machinery

Our previous data have shown that in L929 mouse fibroblasts the control of methylation pattern depends in part on poly(ADP-ribosyl)ation and that ADP-ribose polymers (PARs), both present on poly(ADP-ribosyl)ated PARP-1 and/or protein-free, have an inhibitory effect on Dnmt1 activity. Here we show that transient ectopic overexpression of CCCTC-binding factor (CTCF) induces PAR accumulation, PARP-1, and CTCF poly(ADP-ribosyl)ation in the same mouse fibroblasts. The persistence in time of a high PAR level affects the DNA methylation machinery; the DNA methyltransferase activity is inhibited with consequences for the methylation state of genome, which becomes diffusely hypomethylated affecting centromeric minor satellite and B1 DNA repeats. In vitro data show that CTCF is able to activate PARP-1 automodification even in the absence of nicked DNA. Our new finding that CTCF is able per se to activate PARP-1 automodification in vitro is of great interest as so far a burst of poly(ADP-ribosyl)ated PARP-1 has generally been found following introduction of DNA strand breaks. CTCF is unable to inhibit DNMT1 activity, whereas poly(ADP-ribosyl)ated PARP-1 plays this inhibitory role. These data suggest that CTCF is involved in the cross-talk between poly(ADP-ribosyl)ation and DNA methylation and underscore the importance of a rapid reversal of PARP activity, as DNA methylation pattern is responsible for an important epigenetic code.

Poly ADP-ribose polymerase-1: an international molecule of mystery

Poly(ADP-ribose) polymerase-1 (PARP-1) is one of the most abundant proteins within mammalian cells. First described more than 45 years ago, PARP-1 has been the subject of many studies and was shown to be involved in multiple aspects of cellular metabolism. Despite many interesting studies that implicate PARP-1 in transcription, chromatin remodelling, apoptosis, DNA repair and several neurological disorders, its precise role is still unclear. This review will discuss the role of PARP-1 in DNA repair and propose a model whereby PARP-1 operates as a modulator of base excision repair capacity.

Discovery of ADP-ribosylation and other plant defense pathway elements through expression profiling of four different Arabidopsis-Pseudomonas R-avr interactions

A dissection of plant defense pathways was initiated through gene expression profiling of the responses of a single Arabidopsis thaliana genotype to isogenic Pseudomonas syringae strains expressing one of four different cloned avirulence (avr) genes. Differences in the expression profiles elicited by different resistance (R)-avr interactions were observed. A role for poly(ADP-ribosyl)ation in plant defense responses was suggested initially by the upregulated expression of genes encoding NUDT7 and poly(ADP-ribose) glycohydrolase in multiple R-avr interactions. Gene knockout plant lines were tested for 20 candidate genes identified by the expression profiling, and Arabidopsis NUDT7 mutants allowed less growth of virulent P. syringae (as previously reported) but also exhibited a reduced hypersensitive-response phenotype. Inhibitors of poly(ADP-ribose) polymerase (PARP) disrupted FLS2-mediated basal defense responses such as callose deposition. EIN2 (ethylene response) and IXR1 and IXR2 (cellulose synthase) mutants impacted the FLS2-mediated responses that occur during PARP inhibition, whereas no impacts were observed for NPR1, PAD4, or NDR1 mutants. In the expression profiling work, false-positive selection and grouping of genes was reduced by requiring simultaneous satisfaction of statistical significance criteria for each of three separate analysis methods, and by clustering genes based on statistical confidence values for each gene rather than on average fold-change of transcript abundance.

Functional localization of two poly(ADP-ribose)-degrading enzymes to the mitochondrial matrix

Recent discoveries of NAD-mediated regulatory processes in mitochondria have documented important roles of this compartmentalized nucleotide pool in addition to energy transduction. Moreover, mitochondria respond to excessive nuclear NAD consumption arising from DNA damage-induced poly-ADP-ribosylation because poly(ADP-ribose) (PAR) can trigger the release of apoptosis-inducing factor from the organelles. To functionally assess mitochondrial NAD metabolism, we overexpressed the catalytic domain of nuclear PAR polymerase 1 (PARP1) and targeted it to the matrix, which resulted in the constitutive presence of PAR within the organelles. As a result, stably transfected HEK293 cells exhibited a decrease in NAD content and typical features of respiratory deficiency. Remarkably, inhibiting PARP activity revealed PAR degradation within mitochondria. Two enzymes, PAR glycohydrolase (PARG) and ADP-ribosylhydrolase 3 (ARH3), are known to cleave PAR. Both full-length ARH3 and a PARG isoform, which arises from alternative splicing, localized to the mitochondrial matrix. This conclusion was based on the direct demonstration of their PAR-degrading activity within mitochondria of living cells. The visualization of catalytic activity establishes a new approach to identify submitochondrial localization of proteins involved in the metabolism of NAD derivatives. In addition, targeted PARP expression may serve as a compartment-specific "knock-down" of the NAD content which is readily detectable by PAR formation.

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.

Spatial and functional relationship between poly(ADP-ribose) polymerase-1 and poly(ADP-ribose) glycohydrolase in the brain

Poly(ADP-ribose) polymerases (PARPs) are members of a family of enzymes that utilize nicotinamide adenine dinucleotide (NAD(+)) as substrate to form large ADP-ribose polymers (PAR) in the nucleus. PAR has a very short half-life due to its rapid degradation by poly(ADP-ribose) glycohydrolase (PARG). PARP-1 mediates acute neuronal cell death induced by a variety of insults including cerebral ischemia, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinsonism, and CNS trauma. While PARP-1 is localized to the nucleus, PARG resides in both the nucleus and cytoplasm. Surprisingly, there appears to be only one gene encoding PARG activity, which has been characterized in vitro to generate different splice variants, in contrast to the growing family of PARPs. Little is known regarding the spatial and functional relationships of PARG and PARP-1. Here we evaluate PARG expression in the brain and its cellular and subcellular distribution in relation to PARP-1. Anti-PARG (alpha-PARG) antibodies raised in rabbits using a purified 30 kDa C-terminal fragment of murine PARG recognize a single band at 111 kDa in the brain. Western blot analysis also shows that PARG and PARP-1 are evenly distributed throughout the brain. Immunohistochemical studies using alpha-PARG antibodies reveal punctate cytosolic staining, whereas anti-PARP-1 (alpha-PARP-1) antibodies demonstrate nuclear staining. PARG is enriched in the mitochondrial fraction together with manganese superoxide dismutase (MnSOD) and cytochrome C (Cyt C) following whole brain subcellular fractionation and Western blot analysis. Confocal microscopy confirms the co-localization of PARG and Cyt C. Finally, PARG translocation to the nucleus is triggered by NMDA-induced PARP-1 activation. Therefore, the subcellular segregation of PARG in the mitochondria and PARP-1 in the nucleus suggests that PARG translocation is necessary for their functional interaction. This translocation is PARP-1 dependent, further demonstrating a functional interaction of PARP-1 and PARG in the brain.

Poly(ADP-ribose) polymerase 1 accelerates single-strand break repair in concert with poly(ADP-ribose) glycohydrolase

Single-strand breaks are the commonest lesions arising in cells, and defects in their repair are implicated in neurodegenerative disease. One of the earliest events during single-strand break repair (SSBR) is the rapid synthesis of poly(ADP-ribose) (PAR) by poly(ADP-ribose) polymerase (PARP), followed by its rapid degradation by poly(ADP-ribose) glycohydrolase (PARG). While the synthesis of poly(ADP-ribose) is important for rapid rates of chromosomal SSBR, the relative importance of poly(ADP-ribose) polymerase 1 (PARP-1) and PARP-2 and of the subsequent degradation of PAR by PARG is unclear. Here we have quantified SSBR rates in human A549 cells depleted of PARP-1, PARP-2, and PARG, both separately and in combination. We report that whereas PARP-1 is critical for rapid global rates of SSBR in human A549 cells, depletion of PARP-2 has only a minor impact, even in the presence of depleted levels of PARP-1. Moreover, we identify PARG as a novel and critical component of SSBR that accelerates this process in concert with PARP-1.

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.

Poly(etheno ADP-ribose) blocks poly(ADP-ribose) glycohydrolase activity

Poly(ADP-ribose) is a biopolymer synthesized by poly(ADP-ribose) polymerases. Recent findings suggest the possibility for modulation of cellular functions including cell death and mitosis by poly(ADP-ribose). Derivatization of poly(ADP-ribose) may be useful for investigating the effects of poly(ADP-ribose) on various cellular processes. We prepared poly(etheno ADP-ribose) (poly(epsilonADP-ribose)) by converting the adenine moiety of poly(ADP-ribose) to 1-N(6)-etheno adenine residues. Poly(epsilonADP-ribose) is shown to be highly resistant to digestion by poly(ADP-ribose) glycohydrolase (Parg). On the other hand, poly(epsilonADP-ribose) could be readily digested by phosphodiesterase. Furthermore, poly(epsilonADP-ribose) inhibited Parg activity to hydrolyse ribose-ribose bonds of poly(ADP-ribose). This study suggests the possibility that poly(epsilonADP-ribose) might be a useful tool for studying the poly(ADP-ribose) dynamics and function of Parg. This study also implies that modification of the adenine moiety of poly(ADP-ribose) abrogates the susceptibility to digestion by Parg.

Poly(ADP-ribose) (PAR) polymer is a death signal

Excessive activation of the nuclear enzyme, poly(ADP-ribose) polymerase-1 (PARP-1) plays a prominent role in various of models of cellular injury. Here, we identify poly(ADP-ribose) (PAR) polymer, a product of PARP-1 activity, as a previously uncharacterized cell death signal. PAR polymer is directly toxic to neurons, and degradation of PAR polymer by poly(ADP-ribose) glycohydrolase (PARG) or phosphodiesterase 1 prevents PAR polymer-induced cell death. PARP-1-dependent, NMDA excitotoxicity of cortical neurons is reduced by neutralizing antibodies to PAR and by overexpression of PARG. Neuronal cultures with reduced levels of PARG are more sensitive to NMDA excitotoxicity than WT cultures. Transgenic mice overexpressing PARG have significantly reduced infarct volumes after focal ischemia. Conversely, mice with reduced levels of PARG have significantly increased infarct volumes after focal ischemia compared with WT littermate controls. These results reveal PAR polymer as a signaling molecule that induces cell death and suggests that interference with PAR polymer signaling may offer innovative therapeutic approaches for the treatment of cellular injury.

MNNG-induced cell death is controlled by interactions between PARP-1, poly(ADP-ribose) glycohydrolase, and XRCC1

PARP-1 (poly(ADP-ribose) polymerases) modifies proteins with poly(ADP-ribose), which is an important signal for genomic stability. ADP-ribose polymers also mediate cell death and are degraded by poly(ADP-ribose) glycohydrolase (PARG). Here we show that the catalytic domain of PARG interacts with the automodification domain of PARP-1. Furthermore, PARG can directly down-regulate PARP-1 activity. PARG also interacts with XRCC1, a DNA repair factor that is recruited by DNA damage-activated PARP-1. We investigated the role of XRCC1 in cell death after treatment with supralethal doses of the alkylating agent MNNG. Only in XRCC1-proficient cells MNNG induced a considerable accumulation of poly(ADP-ribose). Similarly, extracts of XRCC1-deficient cells produced large ADP-ribose polymers if supplemented with XRCC1. Consequently, MNNG triggered in XRCC1-proficient cells the translocation of the apoptosis inducing factor from mitochondria to the nucleus followed by caspase-independent cell death. In XRCC1-deficient cells, the same MNNG treatment caused non-apoptotic cell death without accumulation of poly(ADP-ribose). Thus, XRCC1 seems to be involved in regulating a poly(ADP-ribose)-mediated apoptotic cell death.

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.

Poly(ADP-ribose): novel functions for an old molecule

The addition to proteins of the negatively charged polymer of ADP-ribose (PAR), which is synthesized by PAR polymerases (PARPs) from NAD(+), is a unique post-translational modification. It regulates not only cell survival and cell-death programmes, but also an increasing number of other biological functions with which novel members of the PARP family have been associated. These functions include transcriptional regulation, telomere cohesion and mitotic spindle formation during cell division, intracellular trafficking and energy metabolism.

Poly(ADP-ribose) glycohydrolase silencing protects against H2O2-induced cell death

PAR [poly(ADP-ribose)] is a structural and regulatory component of multiprotein complexes in eukaryotic cells. PAR catabolism is accelerated under genotoxic stress conditions and this is largely attributable to the activity of a PARG (PAR glycohydrolase). To overcome the early embryonic lethality of parg-knockout mice and gain more insights into the biological functions of PARG, we used an RNA interference approach. We found that as little as 10% of PARG protein is sufficient to ensure basic cellular functions: PARG-silenced murine and human cells proliferated normally through several subculturing rounds and they were able to repair DNA damage induced by sublethal doses of H2O2. However, cell survival following treatment with higher concentrations of H2O2 (0.05-1 mM) was increased. In fact, PARG-silenced cells were more resistant than their wild-type counterparts to oxidant-induced apoptosis while exhibiting delayed PAR degradation and transient accumulation of ADP-ribose polymers longer than 15-mers at early stages of drug treatment. No difference was observed in response to the DNA alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine, suggesting a specific involvement of PARG in the cellular response to oxidative DNA damage.

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

Poly(ADP-ribose) (PAR) is a polymer synthesized by poly(ADP-ribose) polymerases (PARPs) and metabolized into free adenosine diphosphate (ADP)-ribose units by poly(ADP-ribose) glycohydrolase (PARG). Perturbations in PAR synthesis have been shown to play a key role in brain disorders including postischemic brain damage. A single parg gene but two PARG isoforms (110 and 60 kDa) have been detected in mouse cells. Complete suppression of parg gene causes early embryonic lethality, whereas mice selectively lacking the 110 kDa PARG isoform (PARG(110)(-/-)) develop normally. We used PARG(110)(-/-) mice to evaluate the importance of PAR catabolism to postischemic brain damage. Poly(ADP-ribose) contents were higher in the brain tissue of PARG(110)(-/-) than PARG(110)(+/+) mice, both under basal conditions and after PARP activation. Distal middle cerebral artery occlusion caused higher increase of brain PAR levels and larger infarct volumes in PARG(110)(-/-) mice than in wild-type counterparts. Of note, the brain of PARG(110)(-/-) mice showed reduced heat-shock protein (HSP)-70 and increased cyclooxygenase-2 expression under both control and ischemic conditions. No differences were detected in brain expression/activation of procaspase-3, PARP-1, Akt, HSP-25 and interleukin-1beta. Our findings show that PAR accumulation worsens ischemic brain injury, and highlight the therapeutic potential of strategies capable of maintaining PAR homeostasis.

Poly(ADP-ribose) glycohydrolase is a component of the FMRP-associated messenger ribonucleoparticles

PARG [poly(ADP-ribose) glycohydrolase] is the only known enzyme that catalyses the hydrolysis of poly(ADP-ribose), a branched polymer that is synthesized by the poly(ADP-ribose) polymerase family of enzymes. Poly(ADP-ribosyl)ation is a transient post-translational modification that alters the functions of the acceptor proteins. It has mostly been studied in the context of DNA-damage signalling or DNA transaction events, such as replication and transcription reactions. Growing evidence now suggests that poly(ADP-ribosyl)ation could have a much broader impact on cellular functions. To elucidate the roles that could be played by PARG, we performed a proteomic identification of PARG-interacting proteins by mass spectrometric analysis of PARG pulled-down proteins. In the present paper, we report that PARG is resident in FMRP (Fragile-X mental retardation protein)-associated messenger ribonucleoparticles complexes. The localization of PARG in these complexes, which are components of the translation machinery, was confirmed by sedimentation and microscopy analysis. A functional link between poly(ADP-ribosyl)ation modulation and FMRP-associated ribonucleoparticle complexes are discussed in a context of translational regulation.

The expanding role of poly(ADP-ribose) metabolism: current challenges and new perspectives

Recent discoveries have resulted in significant breakthroughs in the understanding of PARPs and PARG functions within a broad range of cellular processes. The novel and sometimes unexpected pathways that are regulated by poly(ADP-ribosylation) bring new questions and hypotheses, some of them being contentious. In this review, we highlight current areas of investigation such as the clinical potential of PARP and PARG inhibitors and the important mitotic regulatory functions of poly(ADP-ribose) in cell-cycle progression, a recent discovery that has broadened our knowledge regarding poly(ADP-ribose) functions. A special emphasis is placed on recent advances in relation to PARG that are stimulating new directions in future research. Noticeably, the existence of various PARG isoforms characterized by distinct cellular localizations and nucleocytoplasmic shuttling properties challenges our current comprehension of pADPr metabolism. Observations and suppositions towards functionally important regulatory elements in the N-terminal portion of PARG are also discussed.

Dynamic relocation of poly(ADP-ribose) glycohydrolase isoforms during radiation-induced DNA damage

Poly(ADP-ribosyl)ation is a very early cellular response to DNA damage. Poly(ADP-ribose) (PAR) accumulation is transient since PAR is rapidly hydrolyzed by poly(ADP-ribose) glycohydrolase (PARG). PARG may play a prominent role in DNA damage response and repair by removing PAR from modified proteins including PARP-1. Using living cells, we provide evidence that in response to DNA damage induced by gamma-irradiation the cytoplasmic 103 kDa PARG isoform translocates into the nucleus. We further observed that the nuclear GFP-hPARG110 enzyme relocalizes to the cytoplasm in response to DNA damage. Using different GFP-PARG fusion proteins specific for the nuclear and cytoplasmic forms, we demonstrate their dynamic distribution between cytoplasm and nucleoplasm and a high mobility of major PARG isoforms by fluorescence recovery after photobleaching (FRAP). The dynamic relocation of all PARG isoforms presented in this report reveals a novel biological mechanism by which PARG could be involved in DNA damage response.

Poly(ADP-ribose) glycohydrolase inhibitor as chemosensitiser of malignant melanoma for temozolomide

Disruption of poly(ADP-ribose) polymerase (PARP) pathways by inhibitors of PARP catalytic domain has been shown to increase the anti-tumour activity of temozolomide (TMZ). Since PARP is inhibited by poly(ADP)ribosylation, herein we tested whether inhibition of poly(ADP-ribose) glycohydrolase (PARG) might enhance TMZ efficacy. The PARG inhibitor N-bis-(3-phenyl-propyl)9-oxo-fluorene-2,7-diamide (GPI 16552) was administered in combination with TMZ to mice injected subcutaneously or intracranially with B16 melanoma cells. The ability of treatment to reduce melanoma metastatic spreading and invasion of the extracellular matrix was also tested. The results indicated that combined treatment with GPI 16552 and TMZ significantly reduced melanoma growth, increased life-span of mice bearing tumour at the CNS site, and decreased the ability of melanoma cells to form lung metastases and to invade the extracellular matrix. In conclusion, PARG inhibition represents an alternative strategy to enhance TMZ efficacy against melanoma in peripheral as well as at CNS site.

Molecular mechanisms of trophoblast survival: From implantation to birth

Fetal development depends upon a coordinated series of events in both the embryo and in the supporting placenta. The initial event in placentation is appropriate lineage allocation of stem cells followed by the formation of a spheroidal trophoblastic shell surrounding the embryo, facilitating implantation into the uterine stroma and exclusion of oxygenated maternal blood. In mammals, cellular proliferation, differentiation, and death accompany early placental development. Programmed cell death is a critical driving force behind organ sculpturing and eliminating abnormal, misplaced, nonfunctional, or harmful cells in the embryo proper, although very little is known about its physiological function during placental development. This review summarizes current knowledge of the cell death patterns and molecular pathways governing the survival of cells within the blastocyst, with a focus on the trophoblast lineage prior to and after implantation. Particular emphasis is given to human placental development in the context of normal and pathological conditions. As molecular pathways in humans are poorly elucidated, we have also included an overview of pertinent genetic animal models displaying defects in trophoblast survival.

Identification and characterization of a mammalian 39-kDa poly(ADP-ribose) glycohydrolase

ADP-ribosylation is a post-translational modification resulting from transfer of the ADP-ribose moiety of NAD to protein. Mammalian cells contain mono-ADP-ribosyltransferases that catalyze the formation of ADP-ribose-(arginine) protein, which can be cleaved by a 39-kDa ADP-ribose-(arginine) protein hydrolase (ARH1), resulting in release of free ADP-ribose and regeneration of unmodified protein. Enzymes involved in poly(ADP-ribosylation) participate in several critical physiological processes, including DNA repair, cellular differentiation, and carcinogenesis. Multiple poly(ADP-ribose) polymerases have been identified in the human genome, but there is only one known poly(ADP-ribose) glycohydrolase (PARG), a 111-kDa protein that degrades the (ADP-ribose) polymer to ADP-ribose. We report here the identification of an ARH1-like protein, termed poly(ADP-ribose) hydrolase or ARH3, which exhibited PARG activity, generating ADP-ribose from poly-(ADP-ribose), but did not hydrolyze ADP-ribose-arginine, -cysteine, -diphthamide, or -asparagine bonds. The 39-kDa ARH3 shares amino acid sequence identity with both ARH1 and the catalytic domain of PARG. ARH3 activity, like that of ARH1, was enhanced by Mg(2+). Critical vicinal acidic amino acids in ARH3, identified by mutagenesis (Asp(77) and Asp(78)), are located in a region similar to that required for activity in ARH1 but different from the location of the critical vicinal glutamates in the PARG catalytic site. All findings are consistent with the conclusion that ARH3 has PARG activity but is structurally unrelated to PARG.

Poly(ADP-ribose). The most elaborate metabolite of NAD+

One of the most drastic post-translational modification of proteins in eukaryotic cells is poly(ADP-ribosyl)ation, catalysed by a family enzymes termed poly(ADP-ribose) polymerases (PARPs). In the human genome, 18 different genes have been identified that all encode PARP family members. Poly(ADP-ribose) metabolism plays a role in a wide range of biological structures and processes, including DNA repair and maintenance of genomic stability, transcriptional regulation, centromere function and mitotic spindle formation, centrosomal function, structure and function of vault particles, telomere dynamics, trafficking of endosomal vesicles, apoptosis and necrosis. In this article, the most recent advances in this rapidly growing field are summarized.

Identification of three critical acidic residues of poly(ADP-ribose) glycohydrolase involved in catalysis: determining the PARG catalytic domain

PARG [poly(ADP-ribose) glycohydrolase] catalyses the hydrolysis of alpha(1''-->2') or alpha(1'''-->2'') O-glycosidic linkages of ADP-ribose polymers to produce free ADP-ribose. We investigated possible mechanistic similarities between PARG and glycosidases, which also cleave O-glycosidic linkages. Glycosidases typically utilize two acidic residues for catalysis, thus we targeted acidic residues within a conserved region of bovine PARG that has been shown to contain an inhibitor-binding site. The targeted glutamate and aspartate residues were changed to asparagine in order to minimize structural alterations. Mutants were purified and assayed for catalytic activity, as well as binding, to an immobilized PARG inhibitor to determine ability to recognize substrate. Our investigation revealed residues essential for PARG catalytic activity. Two adjacent glutamic acid residues are found in the conserved sequence Gln755-Glu-Glu757, and a third residue found in the conserved sequence Val737-Asp-Phe-Ala-Asn741. Our functional characterization of PARG residues, along with recent identification of an inhibitor-binding residue Tyr796 and a glycine-rich region Gly745-Gly-Gly747 important for PARG function, allowed us to define a PARG 'signature sequence' [vDFA-X3-GGg-X6-8-vQEEIRF-X3-PE-X14-E-X12-YTGYa], which we used to identify putative PARG sequences across a range of organisms. Sequence alignments, along with our mapping of PARG functional residues, suggest the presence of a conserved catalytic domain of approx. 185 residues which spans residues 610-795 in bovine PARG.

Poly(ADP-ribosyl)ation by PARP-1: 'PAR-laying' NAD+ into a nuclear signal

Poly(ADP-ribose) (PAR) and the PAR polymerases (PARPs) that catalyze its synthesis from donor nicotinamide adenine dinucleotide (NAD+) molecules have received considerable attention in the recent literature. Poly(ADP-ribosyl)ation (PARylation) plays diverse roles in many molecular and cellular processes, including DNA damage detection and repair, chromatin modification, transcription, cell death pathways, insulator function, and mitotic apparatus function. These processes are critical for many physiological and pathophysiological outcomes, including genome maintenance, carcinogenesis, aging, inflammation, and neuronal function. This review highlights recent work on the biochemistry, molecular biology, physiology, and pathophysiology of PARylation, focusing on the activity of PARP-1, the most abundantly expressed member of a family of PARP proteins. In addition, connections between nuclear NAD+ metabolism and nuclear signaling through PARP-1 are discussed.