ADP-ribosylation of highly purified rat brain mitochondria

The Role of Poly(ADP-ribose) Polymerase 1 in Nuclear and Mitochondrial Base Excision Repair

Poly(ADP-ribose) (PAR) Polymerase 1 (PARP-1), also known as ADP-ribosyl transferase with diphtheria toxin homology 1 (ARTD-1), is a critical player in DNA damage repair, during which it catalyzes the ADP ribosylation of self and target enzymes. While the nuclear localization of PARP-1 has been well established, recent studies also suggest its mitochondrial localization. In this review, we summarize the differences between mitochondrial and nuclear Base Excision Repair (BER) pathways, the involvement of PARP-1 in mitochondrial and nuclear BER, and its functional interplay with other BER enzymes.

Neuralized-like protein 4 (NEURL4) mediates ADP-ribosylation of mitochondrial proteins

ADP-ribosylation is a reversible post-translational modification where an ADP-ribose moiety is covalently attached to target proteins by ADP-ribosyltransferases (ARTs). Although best known for its nuclear roles, ADP-ribosylation is increasingly recognized as a key regulatory strategy across cellular compartments. ADP-ribosylation of mitochondrial proteins has been widely reported, but the exact nature of mitochondrial ART enzymes is debated. We have identified neuralized-like protein 4 (NEURL4) as a mitochondrial ART enzyme and show that most ART activity associated with mitochondria is lost in the absence of NEURL4. The NEURL4-dependent ADP-ribosylome in mitochondrial extracts from HeLa cells includes numerous mitochondrial proteins previously shown to be ADP-ribosylated. In particular, we show that NEURL4 is required for the regulation of mtDNA integrity via poly-ADP-ribosylation of mtLIG3, the rate-limiting enzyme for base excision repair (BER). Collectively, our studies reveal that NEURL4 acts as the main mitochondrial ART enzyme under physiological conditions and provide novel insights in the regulation of mitochondria homeostasis through ADP-ribosylation.

Aiming for the target: Mitochondrial drug delivery in traumatic brain injury

Mitochondria are a keystone of neuronal function, serving a dual role as sustainer of life and harbinger of death. While mitochondria are indispensable for energy production, a dysregulated mitochondrial network can spell doom for both neurons and the functions they provide. Traumatic brain injury (TBI) is a complex and biphasic injury, often affecting children and young adults. The primary pathological mechanism of TBI is mechanical, too rapid to be mitigated by anything but prevention. However, the secondary injury of TBI evolves over hours and days after the initial insult providing a window of opportunity for intervention. As a nexus point of both survival and death during this second phase, targeting mitochondrial pathology in TBI has long been an attractive strategy. Often these attempts are mired by efficacy-limiting unintended off-target effects. Specific delivery to and enrichment of therapeutics at their submitochondrial site of action can reduce deleterious effects and increase potency. Mitochondrial drug localization is accomplished using (1) the mitochondrial membrane potential, (2) affinity of a carrier to mitochondria-specific components (e.g. lipids), (3) piggybacking on the cells own mitochondria trafficking systems, or (4) nanoparticle-based approaches. In this review, we briefly consider the mitochondrial delivery strategies and drug targets that illustrate the promise of these mitochondria-specific approaches in the design of TBI pharmacotherapy. This article is part of the Special Issue entitled "Novel Treatments for Traumatic Brain Injury".

Synthesis and Evaluation of a Mitochondria-Targeting Poly(ADP-ribose) Polymerase-1 Inhibitor

The poly(ADP-ribose) polymerase (PARP) family of enzymes plays a crucial role in cellular and molecular processes including DNA damage detection and repair and transcription; indeed, PARP inhibitors are under clinical evaluation as chemotherapeutic adjuncts given their capacity to impede genomic DNA repair in tumor cells. Conversely, overactivation of PARP can lead to NAD⁺ depletion, mitochondrial energy failure, and cell death. Since PARP activation facilitates genomic but impedes mitochondrial DNA repair, nonselective PARP inhibitors are likely to have opposing effects in these cellular compartments. Herein, we describe the synthesis and evaluation of the mitochondria-targeting PARP inhibitor, XJB-veliparib. Attachment of the hemigramicidin S pentapeptide isostere for mitochondrial targeting using a flexible linker at the primary amide site of veliparib did not disrupt PARP affinity or inhibition. XJB-veliparib was effective at low nanomolar concentrations (10-100 nM) and more potent than veliparib in protection from oxygen-glucose deprivation (OGD) in primary cortical neurons. Both XJB-veliparib and veliparib (10 nM) preserved mitochondrial NAD⁺ after OGD; however, only XJB-veliparib prevented release of NAD⁺ into cytosol. XJB-veliparib (10 nM) appeared to inhibit poly(ADP-ribose) polymer formation in mitochondria and preserve mitochondrial cytoarchitecture after OGD in primary cortical neurons. After 10 nM exposure, XJB-veliparib was detected by LC-MS in mitochondria but not nuclear-enriched fractions in neurons and was observed in mitoplasts stripped of the outer mitochondrial membrane obtained from HT22 cells. XJB-veliparib was also effective at preventing glutamate-induced HT22 cell death at micromolar concentrations. Importantly, in HT22 cells exposed to H₂O₂ to produce DNA damage, XJB-veliparib (10 μM) had no effect on nuclear DNA repair, in contrast to veliparib (10 μM) where DNA repair was retarded. XJB-veliparib and analogous mitochondria-targeting PARP inhibitors warrant further evaluation in vitro and in vivo, particularly in conditions where PARP overactivation leads to mitochondrial energy failure and maintenance of genomic DNA integrity is desirable, e.g., ischemia, oxidative stress, and radiation exposure.

Mitochondrial poly(ADP-ribose) polymerase: The Wizard of Oz at work

Among multiple members of the poly(ADP-ribose) polymerase (PARP) family, PARP1 accounts for the majority of PARP activity in mammalian cells. Although PARP1 is predominantly localized to the nucleus, and its nuclear regulatory roles are most commonly studied and are the best characterized, several lines of data demonstrate that PARP1 is also present in the mitochondria, and suggest that mitochondrial PARP (mtPARP) plays an important role in the regulation of various cellular functions in health and disease. The goal of the current article is to review the experimental evidence for the mitochondrial localization of PARP1 and its intra-mitochondrial functions, with focus on cellular bioenergetics, mitochondrial DNA repair and mitochondrial dysfunction. In addition, we also propose a working model for the interaction of mitochondrial and nuclear PARP during oxidant-induced cell death. MtPARP is similar to the Wizard of Oz in the sense that it is enigmatic, it has been elusive for a long time and it remains difficult to be interrogated. mtPARP - at least in some cell types - works incessantly "behind the curtains" as an orchestrator of many important cellular functions.

Sex differences in mitochondrial (dys)function: Implications for neuroprotection

Decades of research have revealed numerous differences in brain structure size, connectivity and metabolism between males and females. Sex differences in neurobehavioral and cognitive function after various forms of central nervous system (CNS) injury are observed in clinical practice and animal research studies. Sources of sex differences include early life exposure to gonadal hormones, chromosome compliment and adult hormonal modulation. It is becoming increasingly apparent that mitochondrial metabolism and cell death signaling are also sexually dimorphic. Mitochondrial metabolic dysfunction is a common feature of CNS injury. Evidence suggests males predominantly utilize proteins while females predominantly use lipids as a fuel source within mitochondria and that these differences may significantly affect cellular survival following injury. These fundamental biochemical differences have a profound impact on energy production and many cellular processes in health and disease. This review will focus on the accumulated evidence revealing sex differences in mitochondrial function and cellular signaling pathways in the context of CNS injury mechanisms and the potential implications for neuroprotective therapy development.

Regulation of mitochondrial poly(ADP-Ribose) polymerase activation by the β-adrenoceptor/cAMP/protein kinase A axis during oxidative stress

We investigated the regulation of mitochondrial poly(ADP-ribose) polymerase 1 (PARP1) by the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) system during oxidative stress in U937 monocytes. Oxidative stress induced an early (10 minutes) mitochondrial DNA damage, and concomitant activation of PARP1 in the mitochondria. These early events were followed by a progressive mitochondrial oxidant production and nuclear PARP1 activation (by 6 hours). These processes led to a functional impairment of mitochondria, culminating in cell death of mixed (necrotic/apoptotic) type. β-Adrenoceptor blockade with propranolol or inhibition of its downstream cAMP/PKA signaling attenuated, while β-adrenoceptor agonists and cAMP/PKA activators enhanced, the oxidant-mediated PARP1 activation. In the presence of cAMP, recombinant PKA directly phosphorylated recombinant PARP1 on serines 465 (in the automodification domain) and 782 and 785 (both in the catalytic domain). Inhibition of the β-adrenergic receptor/cAMP/PKA axis protected against the oxidant-mediated cell injury. Propranolol also suppressed PARP1 activation in peripheral blood leukocytes during bacterial lipopolysaccharide (LPS)-induced systemic inflammation in mice. We conclude that the activation of mitochondrial PARP1 is an early, active participant in oxidant-induced cell death, which is under the control of β-adrenoceptor/cAMP/PKA axis through the regulation of PARP1 activity by PARP1 phosphorylation.

Mitochondrial dysfunction and NAD(+) metabolism alterations in the pathophysiology of acute brain injury

Mitochondrial dysfunction is commonly believed to be one of the major players in mechanisms of brain injury. For several decades, pathologic mitochondrial calcium overload and associated opening of the mitochondrial permeability transition (MPT) pore were considered a detrimental factor causing mitochondrial damage and bioenergetics failure. Mitochondrial and cellular bioenergetic metabolism depends on the enzymatic reactions that require NAD(+) or its reduced form NADH as cofactors. Recently, it was shown that NAD(+) also has an important function as a substrate for several NAD(+) glycohydrolases whose overactivation can contribute to cell death mechanisms. Furthermore, downstream metabolites of NAD(+) catabolism can also adversely affect cell viability. In contrast to the negative effects of NAD(+)-catabolizing enzymes, enzymes that constitute the NAD(+) biosynthesis pathway possess neuroprotective properties. In the first part of this review, we discuss the role of MPT in acute brain injury and its role in mitochondrial NAD(+) metabolism. Next, we focus on individual NAD(+) glycohydrolases, both cytosolic and mitochondrial, and their role in NAD(+) catabolism and brain damage. Finally, we discuss the potential effects of downstream products of NAD(+) degradation and associated enzymes as well as the role of NAD(+) resynthesis enzymes as potential therapeutic targets.

NAD and ADP-ribose metabolism in mitochondria

Mitochondrial metabolism is intimately connected to the universal coenzyme NAD. In addition to its role in redox reactions of energy transduction, NAD serves as substrate in regulatory reactions that lead to its degradation. Importantly, all types of the known NAD-consuming signalling reactions have been reported to take place in mitochondria. These reactions include the generation of second messengers, as well as post-translational protein modifications such as ADP-ribosylation and protein deacetylation. Therefore, the availability and redox state of NAD emerged as important factors in the regulation of mitochondrial metabolism. Molecular mechanisms and targets of mitochondrial NAD-dependent protein deacetylation and mono-ADP-ribosylation have been established, whereas poly-ADP-ribosylation and NAD-derived messenger generation in the organelles await in-depth characterization. In this review, we highlight the major NAD-dependent reactions occurring within mitochondria and describe their metabolic and regulatory functions. We also discuss the metabolic fates of the NAD-degradation products, nicotinamide and ADP-ribose, and how the mitochondrial NAD pool is restored.

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.

Sirtuin-4 modulates sensitivity to induction of the mitochondrial permeability transition pore

The sustained opening of the mitochondrial permeability transition pore (PTP) is a decisive event in the onset of irreversible cell injury. The PTP is modulated by numerous exogenous and endogenous effectors, including mitochondrial membrane potential, ions and metabolites. Mitochondrial sirtuins have recently emerged as pivotal mediators of mitochondrial metabolism. In the present study, we demonstrate that sirt-4 modulates sensitivity to PTP onset induced by calcium and the oxidative cross linking reagent phenylarsine oxide, and PTP dependent cytotoxicity brought about by TNF or doxorubicin. Moreover, the ability of sirt-4 to modulate onset of the PTP is dependent on the expression of glutamate dehydrogenase-1.

Rutin prevents cognitive impairments by ameliorating oxidative stress and neuroinflammation in rat model of sporadic dementia of Alzheimer type

The objective of the present study was to assess the neuroprotective role of rutin (vitamin P) and delineate the mechanism of action. Recent evidence indicates that rutin exhibits antioxidant potential and protects the brain against various oxidative stressors. More precisely, the aim of the present study was to examine the modulating impacts of rutin against cognitive deficits and oxidative damage in intracerebroventricular-streptozotocin (ICV-STZ)-infused rats. Rats were injected bilaterally with ICV-STZ (3 mg/kg), whereas sham rats received the same volume of vehicle. After 2 weeks of streptozotocin (STZ) infusion, rats were tested for cognitive performance using Morris water maze tasks and thereafter euthanized for further biochemical, histopathological, and immunohistochemical studies. Rutin pretreatment (25 mg/kg, orally, once daily for 3 weeks) significantly attenuated thiobarbituric acid reactive substances (TBARS), activity of poly ADP-ribosyl polymerase, and nitrite level and decreased level of reduced glutathione (GSH) and activities of its dependent enzymes (glutathione peroxidase [GPx] and glutathione reductase [GR]) and catalase in the hippocampus of ICV-STZ rats. ICV-STZ rats showed significant cognitive deficits, which was improved significantly by rutin supplementation. The results indicate that rutin attenuates STZ-induced inflammation by reducing the expression of cyclooxygenase-2 (COX-2), glial fibrillary acidic protein (GFAP), interleukin-8 (IL-8), inducible nitric oxide synthase (iNOS), nuclear factor-kB, and preventing the morphological changes in hippocampus. The study thereby suggests the effectiveness of rutin in preventing cognitive deficits and might be beneficial for the treatment of sporadic dementia of Alzheimer type (SDAT).

Mitochondrial dysfunction and nicotinamide dinucleotide catabolism as mechanisms of cell death and promising targets for neuroprotection

Both acute and chronic neurodegenerative diseases are frequently associated with mitochondrial dysfunction as an essential component of mechanisms leading to brain damage. Although loss of mitochondrial functions resulting from prolonged activation of the mitochondrial permeability transition (MPT) pore has been shown to play a significant role in perturbation of cellular bioenergetics and in cell death, the detailed mechanisms are still elusive. Enzymatic reactions linked to glycolysis, the tricarboxylic acid cycle, and mitochondrial respiration are dependent on the reduced or oxidized form of nicotinamide dinucleotide [NAD(H)] as a cofactor. Loss of mitochondrial NAD(+) resulting from MPT pore opening, although transient, allows detrimental depletion of mitochondrial and cellular NAD(+) pools by activated NAD(+) glycohydrolases. Poly(ADP-ribose) polymerase (PARP) is considered to be a major NAD(+) degrading enzyme, particularly under conditions of extensive DNA damage. We propose that CD38, a main cellular NAD(+) level regulator, can significantly contribute to NAD(+) catabolism. We discuss NAD(+) catabolic and NAD(+) synthesis pathways and their role in different strategies to prevent cellular NAD(+) degradation in brain, particularly following an ischemic insult. These therapeutic approaches are based on utilizing endogenous intermediates of NAD(+) metabolism that feed into the NAD(+) salvage pathway and also inhibit CD38 activity.

Quercetin protects against oxidative stress associated damages in a rat model of transient focal cerebral ischemia and reperfusion

Experimental studies have demonstrated that oxidative stress and apoptosis play an important role in cerebral ischemic pathogenesis and may represent a target for treatment. The purpose of this study was to determine whether the quercetin dihydrate (Q) protects against cerebral ischemia neuronal damage. Male Wistar rats were subjected to transient middle cerebral artery occlusion (MCAO) for 2 h and reperfused for 72 h. Quercetin (30 mg/kg, i.p) was administrated 30 min before the onset of ischemia and after the ischemia at interval of 0, 24, 48, and 72 h. The administration of Q showed marked reduction in infarct size, reduced the neurological deficits in terms of behaviors, suppressed neuronal loss and diminished the p53 expression in MCAO rats. Q was found to be successful in upregulating the antioxidant status and lowering the TBARS level. Conversely, the elevated activity of poly (ADP-ribose) polymerase (PARP), and activity of caspase-3 in MCAO group was attenuated significantly in Q treated group when compared with MCAO group. Our study reveals that Q, as a powerful antioxidant, could prevent free radicals associated oxidative damage and morphological changes in the MCAO rats. Thus, it may have a therapeutic value for the treatment of stroke.

Mitochondrial localization of PARP-1 requires interaction with mitofilin and is involved in the maintenance of mitochondrial DNA integrity

Poly(ADP-ribose)polymerase-1 (PARP-1) is a predominantly nuclear enzyme that exerts numerous functions in cellular physiology and pathology, from maintenance of DNA stability to transcriptional regulation. Through a proteomic analysis of PARP-1 co-immunoprecipitation complexes, we identified Mitofilin, a mitochondrial protein, as a new PARP-1 interactor. This result prompted us to further investigate the presence and the role of the enzyme in mitochondria. Using laser confocal microscopy and Western blot analysis of purified mitochondria, we demonstrated the mitochondrial localization of a fraction of PARP-1. Further, the effects of overexpressing or down-regulating Mitofilin showed that this protein promotes and is required for PARP-1 mitochondrial localization. We also report several lines of evidence suggesting that intramitochondrial PARP-1 plays a role in mitochondrial DNA (mtDNA) damage signaling and/or repair. First, we show that PARP-1 binds to different regions throughout the mtDNA. Moreover, we demonstrated that the depletion of either PARP-1 or Mitofilin, which abrogates the mitochondrial localization of the enzyme, leads to the accumulation of mtDNA damage. Finally, we show that DNA ligase III, known to be required for mtDNA repair, participates in a PARP-1-containing complex bound to mtDNA. This work highlights a new environment for PARP-1, opening the possibility that at least some of the nuclear functions of the enzyme can be also extended to mtDNA metabolism.

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.

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.

Mitochondrial poly(ADP-ribosylation): from old data to new perspectives

Poly(ADP-ribosylation) is involved in DNA repair and replication, transcription, and cell death. For a long time, only one poly(ADP-ribosylating) enzyme was known, named ADPRT/PARP (EC 2.4.2.30). The recent discovery of a family of PARPs has provided a high degree of complexity in the field. Moreover, the finding that poly(ADP-ribosylation) is not confined to the nucleus but is also carried out by cytoplasmic enzymes supports the idea that it could regulate proteins localized in different cellular compartments. In this respect, a reappraisal of the literature on mitochondrial poly(ADP-ribosylation) could be useful, as well as a discussion of its relevance regarding the current "hot" view of poly(ADP-ribosylation) as a mediator of cell death.

Intra-mitochondrial poly(ADP-ribosylation) contributes to NAD+ depletion and cell death induced by oxidative stress

Poly(ADP-ribosylation), primarily via poly(ADP-ribose) polymerase-1 (PARP-1), is a pluripotent cellular process important for maintenance of genomic integrity and RNA transcription in cells. However, during conditions of oxidative stress and energy depletion, poly(ADP-ribosylation) paradoxically contributes to mitochondrial failure and cell death. Although it has been presumed that poly(ADP-ribosylation) within the nucleus mediates this pathologic process, PARP-1 and other poly(ADP-ribosyltransferases) are also localized within mitochondria. To this end, the presence of PARP-1 and poly(ADP-ribosylation) were verified within mitochondrial fractions from primary cortical neurons and fibroblasts. Inhibition of poly(ADP-ribosylation) within the mitochondrial compartment preserved transmembrane potential (DeltaPsi(m)), NAD(+) content, and cellular respiration, prevented release of apoptosis-inducing factor, and reduced neuronal cell death triggered by oxidative stress. Treatment with liposomal NAD(+) also preserved DeltaPsi(m) and cellular respiration during oxidative stress. Furthermore, inhibition of poly(ADP-ribosylation) prevented intranuclear localization of apoptosis-inducing factor and protected neurons from excitotoxic injury; and PARP-1 null fibroblasts were protected from oxidative stress-induced cell death. Collectively these data suggest that poly(ADP-ribosylation) compartmentalized to the mitochondria can be converted from a homeostatic process to a mechanism of cell death when oxidative stress is accompanied by energy depletion. These data implicate intra-mitochondrial poly(ADP-ribosylation) as an important therapeutic target for central nervous system and other diseases associated with oxidative stress and energy failure.

Effect of growth factors on nuclear and mitochondrial ADP-ribosylation processes during astroglial cell development and aging in culture

Epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), insulin-like growth factor-I (IGF-I) and insulin (INS) are powerful mitogens and may regulate gene expression in cultured astrocytes by ADP-ribosylation process. Nuclear poly-ADP ribose polymerase (PARP) and mitochondrial monoADP-ribosyltransferase (ADPRT) are the key enzymes involved in poly-ADP-ribosylation and mono ADP-ribosylation, respectively. In this investigation the effect of EGF, bFGF, IGF-I or INS on nuclear PARP and mitochondrial ADPRT activities were assessed in nuclei and mitochondria purified from developing (30 DIV) or aging (90 and 190 DIV) primary rat astrocyte cultures. A marked increase of PARP activity in bFGF or IGF-I treated astroglial cell cultures at 30 DIV was found. Nuclear PARP and mitochondrial ADPRT activities were greatly stimulated by treatment with EGF or INS alone or together in astrocyte cultures at 30 DIV. Nuclear PARP and mitochondrial ADPRT activities showed a more remarkable increase in control untreated astrocyte cultures at 190 DIV than at 90 DIV. These findings suggest that ADP-ribosylation process is involved in DNA damage and repair during cell differentiation and aging in culture. Twelve hours treatment with EGF, INS or bFGF significantly stimulated nuclear PARP and mitochondrial ADPRT activities in 190 DIV aging astrocyte cultures. The above results indicate that EGF, INS and bFGF may play a crucial role in the post-translational modification of chromosomal proteins including ADP-ribosylation process in in vitro models. This suggests that growth factors regulate genomic stability in glial cells during development and maturation, stimulating nuclear and mitochondrial ADP-ribosylation processes in developing or aging astrocyte cultures.

New functions of a long-known molecule. Emerging roles of NAD in cellular signaling

Over the past decades, the pyridine nucleotides have been established as important molecules in signaling pathways, besides their well known function in energy transduction. Similarly to another molecule carrying such dual functions, ATP, NAD(P)+ may serve as substrate for covalent protein modification or as precursor of biologically active compounds. Protein modification is catalyzed by ADP-ribosyl transferases that attach the ADP-ribose moiety of NAD+ to specific amino-acid residues of the acceptor proteins. For a number of ADP ribosylation reactions the specific transferases and their target proteins have been identified. As a result of the modification, the biological activity of the acceptor proteins may be severely changed. The cell nucleus contains enzymes catalyzing the transfer of ADP-ribose polymers (polyADP-ribose) onto the acceptor proteins. The best known enzyme of this type is poly(ADP-ribose) polymerase 1 (PARP1), which has been implicated in the regulation of several important processes including DNA repair, transcription, apoptosis, neoplastic transformation and others. The second group of reactions leads to the synthesis of an unusual cyclic nucleotide, cyclic ADP-ribose (cADPR). Moreover, the enzymes catalyzing this reaction may also replace the nicotinamide of NADP+ by nicotinic acid resulting in the synthesis of nicotinic acid adenine dinucleotide phosphate (NAADP+). Both cADPR and NAADP+ have been reported to be potent intracellular calcium-mobilizing agents. In concert with inositol 1,4,5-trisphosphate, they participate in cytosolic calcium regulation by releasing calcium from intracellular stores.

Poly(ADP-ribose) polymerase is found in both the nucleus and cytoplasm of human CNS neurons

There is evidence that inhibitors of poly(ADP-ribose) polymerase (PARP) may be therapeutically useful in neurodegenerative diseases. Using immunocytochemistry, we have investigated the distribution of PARP in the human CNS. Some neuronal groups showed cytoplasmic staining in addition to the expected staining of nuclei. Considerable variation between different neuronal groups was noted: motor neurons in the spinal cord showed greatest cytoplasmic staining, whereas staining was virtually absent in other neurons, notably in the hippocampus. These results indicate that PARP can be associated with sub-cellular components other than the nucleus, and may indicate additional roles for this enzyme.

Enzymic, cysteine-specific ADP-ribosylation in bovine liver mitochondria

NAD+ glycohydrolase (NADase) and non-enzymic ADP-ribosylation have been thought to be involved in the regulation of mitochondrial Ca2+ fluxes. In this study it was found that several conditions (5 mM nicotinamide, 5 mM 3-aminobenzamide, 2 mM EDTA, 1 mM ATP, 10 mM dithiothreitol) known to strongly inhibit the NADase decreased ADP-ribosylation in bovine liver mitochondrial membranes with [32P]NAD+ as substrate to only a limited extent, if at all. The reaction led to the specific modification of two proteins with apparent molecular masses of approx. 26 and 53 kDa. An excess of added free ADP-ribose diminished the incorporation of label from [32P]NAD+ only slightly. Dithiothreitol inactivated the NADase, whereas ADP-ribosylation was unaffected. At low concentrations (25 microM) ADP-ribosylation was efficient with NAD+, but not ADP-ribose, as substrate. Under these conditions mitochondrial ADP-ribosylation seems to occur as an enzymic reaction rather than a non-enzymic transfer of ADP-ribose previously liberated from NAD+ by NAD+ glycohydrolase. The chemical stability of the protein-ADP-ribose bonds in the mitochondrial membranes indicated that cysteine residues are the predominant acceptors. Moreover, yeast aldehyde dehydrogenase, known to be a substrate for thiol-associated ADP-ribosylation, was efficiently ADP-ribosylated by using the mitochondrial activity and NAD+ as substrate. The modification of a cysteine residue in the aldehyde dehydrogenase was verified by the observation that pretreatment of this acceptor protein with N-ethylmaleimide substantially decreased its modification. It is therefore concluded that bovine liver mitochondria contain a cysteine-specific ADP-ribosyltransferase.

Identification of bovine liver mitochondrial NAD+ glycohydrolase as ADP-ribosyl cyclase

The present investigation identifies bovine liver mitochondrial NADase (NAD+ glycohydrolase) as a member of the class of bifunctional ADP-ribosyl cyclases/cyclic ADP-ribose hydrolases, known to be potential second messenger enzymes. These enzymes catalyse the synthesis and degradation of cyclic ADP-ribose, a potent intracellular calcium-mobilizing agent. The mitochondrial enzyme utilized the NAD+ analogues nicotinamide guanine dinucleotide (NGD+) and nicotinamide hypoxanthine dinucleotide (NHD+) to form fluorescent cyclic purine nucleoside diphosphoriboses. ADP-ribosyl cyclase activity was also demonstrated using 32P-labelled NAD+ as substrate. The identity of NADase and ADP-ribosyl cyclase was supported by their co-migration in SDS/polyacrylamide gels. Cyclase activity was visualized directly within the gel by detecting the formation of fluorescent cyclic IDP-ribose from NHD+. The enzyme catalysed the hydrolysis of cyclic ADP-ribose to ADP-ribose. Moreover, in the presence of nicotinamide and cyclic ADP-ribose the enzyme synthesized NAD+. Both the ADP-ribosyl cyclase and NADase activities of the enzyme were strongly inhibited by reducing agents. Treatment of the NADase with dithiothreitol caused the apparent inactivation of the enzyme. Subsequent removal of the reducing agent and addition of oxidized glutathione led to a partial recovery of enzymic activity. The results support a model for pro-oxidant-induced calcium release from mitochondria involving cyclic ADP-ribose as a specific messenger, rather than the non-enzymic modification of proteins by ADP-ribose.

ADP-ribosylation of proteins in brain regions of rats during postnatal development

Post-translational modifications of chromatin-bound proteins play an important role in the regulation of eukaryotic gene expression. Processes such as acetylation, methylation, phosphorylation and ADP-ribosylation may alter the interaction of these proteins with DNA and consequently affect chromatin conformation and the binding of enzymes and other molecules involved in the regulation of gene expression. In the present study the process of ADP-ribosylation of chromosomal proteins (histone and non-histone proteins) in some rat brain regions during postnatal development was investigated; also the effect of epidermal growth factor (EGF) on this process in fetal brain slices was studied. It has been found that the process of ADP-ribosylation of total histones extracted from rat cerebral cortex and cerebellum at 1, 10 and 30 days of age, increases from 1 to 10 days of age (i.e. the period of maximal cell proliferation) and decreases thereafter, while the process of ADP-ribosylation of non-histone proteins (NHPs) sharply decreases during the same developmental period. The addition of EGF to fetal brain slices causes a significant increase of ADP-ribosylation of total histones (particularly of the histone H1 fraction) and also of NHPs and microsomal proteins. This result is in agreement with the effect of EGF as a mitogen factor, previously shown in astroglial cell cultures.

Unexpected stimulation of mitochondrial ADP-ribosylation by cyanide

Cyanide, the classical inhibitor of the mitochondrial respiratory chain at site III, stimulates ADP-ribosylation of a number of mitochondrial proteins, the major protein being the 50-55 kDa band. Sodium azide, sharing the same inhibitory site, does not have the same effect. Rotenone or antimycin A have no influence on mitochondrial ADP-ribosylation. Data suggest that no apparent correlation exists between oxidoreductase function and protein ADP-ribosylation. Purified nuclear poly(ADP-ribose) polymerase activity was not affected by cyanide. The cyanide effect on mitochondrial ADP-ribosylation seems intriguing and may be attributed to NAD+-CN complex formation, since NAD reacts with cyanide at pH greater than 8 with N-substituted nicotinamide which may prevent inhibition of ADP-ribosylation.