The role of poly(ADP-ribose) polymerase-1 in CNS disease

Poly (ADP-ribose) polymerase 1 and neurodegenerative diseases: Past, present, and future

Poly (ADP-ribose) polymerase 1 (PARP1) is a first responder that recognizes DNA damage and facilitates its repair. Neurodegenerative diseases, characterized by progressive neuron loss driven by various risk factors, including DNA damage, have increasingly shed light on the pivotal involvement of PARP1. During the early phases of neurodegenerative diseases, PARP1 experiences controlled activation to swiftly address mild DNA damage, thereby contributing to maintain brain homeostasis. However, in late stages, exacerbated PARP1 activation precipitated by severe DNA damage exacerbates the disease condition. Consequently, inhibition of PARP1 overactivation emerges as a promising therapeutic approach for neurodegenerative diseases. In this review, we comprehensively synthesize and explore the multifaceted role of PARP1 in neurodegenerative diseases, with a particular emphasis on its over-activation in the aggregation of misfolded proteins, dysfunction of the autophagy-lysosome pathway, mitochondrial dysfunction, neuroinflammation, and blood-brain barrier (BBB) injury. Additionally, we encapsulate the therapeutic applications and limitations intrinsic of PARP1 inhibitors, mainly including limited specificity, intricate pathway dynamics, constrained clinical translation, and the heterogeneity of patient cohorts. We also explore and discuss the potential synergistic implementation of these inhibitors alongside other agents targeting DNA damage cascades within neurodegenerative diseases. Simultaneously, we propose several recommendations for the utilization of PARP1 inhibitors within the realm of neurodegenerative disorders, encompassing factors like the disease-specific roles of PARP1, combinatorial therapeutic strategies, and personalized medical interventions. Lastly, the encompassing review presents a forward-looking perspective along with strategic recommendations that could guide future research endeavors in this field.

Preclinical evaluation of a brain penetrant PARP PET imaging probe in rat glioblastoma and nonhuman primates

PURPOSE

Currently, there are multiple active clinical trials involving poly(ADP-ribose) polymerase (PARP) inhibitors in the treatment of glioblastoma. The noninvasive quantification of baseline PARP expression using positron emission tomography (PET) may provide prognostic information and lead to more precise treatment. Due to the lack of brain-penetrant PARP imaging agents, the reliable and accurate in vivo quantification of PARP in the brain remains elusive. Herein, we report the synthesis of a brain-penetrant PARP PET tracer, (R)-2-(2-methyl-1-(methyl-11C)pyrrolidin-2-yl)-1H-benzo[d]imidazole-4-carboxamide ([11C]PyBic), and its preclinical evaluations in a syngeneic RG2 rat glioblastoma model and healthy nonhuman primates.

METHODS

We synthesized [11C]PyBic using veliparib as the labeling precursor, performed dynamic PET scans on RG2 tumor-bearing rats and calculated the distribution volume ratio (DVR) using simplified reference region method 2 (SRTM2) with the contralateral nontumor brain region as the reference region. We performed biodistribution studies, western blot, and immunostaining studies to validate the in vivo PET quantification results. We characterized the brain kinetics and binding specificity of [11C]PyBic in nonhuman primates on FOCUS220 scanner and calculated the volume of distribution (VT), nondisplaceable volume of distribution (VND), and nondisplaceable binding potential (BPND) in selected brain regions.

RESULTS

[11C]PyBic was synthesized efficiently in one step, with greater than 97% radiochemical and chemical purity and molar activity of 148 ± 85 MBq/nmol (n = 6). [11C]PyBic demonstrated PARP-specific binding in RG2 tumors, with 74% of tracer binding in tumors blocked by preinjected veliparib (i.v., 5 mg/kg). The in vivo PET imaging results were corroborated by ex vivo biodistribution, PARP1 immunohistochemistry and immunoblotting data. Furthermore, brain penetration of [11C]PyBic was confirmed by quantitative monkey brain PET, which showed high specific uptake (BPND > 3) and low nonspecific uptake (VND < 3 mL/cm3) in the monkey brain.

CONCLUSION

[11C]PyBic is the first brain-penetrant PARP PET tracer validated in a rat glioblastoma model and healthy nonhuman primates. The brain kinetics of [11C]PyBic are suitable for noninvasive quantification of available PARP binding in the brain, which posits [11C]PyBic to have broad applications in oncology and neuroimaging.

Cellular and Mitochondrial NAD Homeostasis in Health and Disease

The mitochondrion has a unique position among other cellular organelles due to its dynamic properties and symbiotic nature, which is reflected in an active exchange of metabolites and cofactors between the rest of the intracellular compartments. The mitochondrial energy metabolism is greatly dependent on nicotinamide adenine dinucleotide (NAD) as a cofactor that is essential for both the activity of respiratory and TCA cycle enzymes. The NAD level is determined by the rate of NAD synthesis, the activity of NAD-consuming enzymes, and the exchange rate between the individual subcellular compartments. In this review, we discuss the NAD synthesis pathways, the NAD degradation enzymes, and NAD subcellular localization, as well as NAD transport mechanisms with a focus on mitochondria. Finally, the effect of the pathologic depletion of mitochondrial NAD pools on mitochondrial proteins' post-translational modifications and its role in neurodegeneration will be reviewed. Understanding the physiological constraints and mechanisms of NAD maintenance and the exchange between subcellular compartments is critical given NAD's broad effects and roles in health and disease.

Poly (ADP-ribose) polymerases as PET imaging targets for central nervous system diseases

Poly (ADP-ribose) polymerases (PARPs) constitute of 17 members that are associated with divergent cellular processes and play a crucial role in DNA repair, chromatin organization, genome integrity, apoptosis, and inflammation. Multiple lines of evidence have shown that activated PARP1 is associated with intense DNA damage and irritating inflammatory responses, which are in turn related to etiologies of various neurological disorders. PARP1/2 as plausible therapeutic targets have attracted considerable interests, and multitudes of PARP1/2 inhibitors have emerged for treating cancer, metabolic, inflammatory, and neurological disorders. Furthermore, PARP1/2 as imaging targets have been shown to detect, delineate, and predict therapeutic responses in many diseases by locating and quantifying the expression levels of PARP1/2. PARP1/2-directed noninvasive positron emission tomography (PET) has potential in diagnosing and prognosing neurological diseases. However, quantitative PARP PET imaging in the central nervous system (CNS) has evaded us due to the challenges of developing blood-brain barrier (BBB) penetrable PARP radioligands. Here, we review PARP1/2's relevance in CNS diseases, summarize the recent progress on PARP PET and discuss the possibilities of developing novel PARP radiotracers for CNS diseases.

Neuroprotective Effects of PARP Inhibitors in Drosophila Models of Alzheimer’s Disease

Alzheimer’s disease (AD) is an irreversible age-related neurodegenerative disorder clinically characterized by severe memory impairment, language deficits and cognitive decline. The major neuropathological hallmarks of AD include extracellular deposits of the β-amyloid (Aβ) peptides and cytoplasmic neurofibrillary tangles (NFTs) of hyperphosphorylated tau protein. The accumulation of plaques and tangles in the brain triggers a cascade of molecular events that culminate in neuronal damage and cell death. Despite extensive research, our understanding of the molecular basis of AD pathogenesis remains incomplete and a cure for this devastating disease is still not available. A growing body of evidence in different experimental models suggests that poly(ADP-ribose) polymerase-1 (PARP-1) overactivation might be a crucial component of the molecular network of interactions responsible for AD pathogenesis. In this work, we combined genetic, molecular and biochemical approaches to investigate the effects of two different PARP-1 inhibitors (olaparib and MC2050) in Drosophila models of Alzheimer’s disease by exploring their neuroprotective and therapeutic potential in vivo. We found that both pharmacological inhibition and genetic inactivation of PARP-1 significantly extend lifespan and improve the climbing ability of transgenic AD flies. Consistently, PARP-1 inhibitors lead to a significant decrease of Aβ42 aggregates and partially rescue the epigenetic alterations associated with AD in the brain. Interestingly, olaparib and MC2050 also suppress the AD-associated aberrant activation of transposable elements in neuronal tissues of AD flies.

Role of melatonin in Alzheimer's disease: From preclinical studies to novel melatonin-based therapies

Melatonin and novel melatonin-based therapies such as melatonin-containing hybrid molecules, melatonin analogues, and melatonin derivatives have been investigated as potential therapeutics against Alzheimer's disease (AD) pathogenesis. In this review, we examine the developmental trends of melatonin therapies for AD from 1997 to 2021. We then highlight the neuroprotective mechanisms of melatonin therapy derived from preclinical studies. These mechanisms include the alleviation of amyloid-related burden, neurofibrillary tangle accumulation, oxidative stress, neuroinflammation, apoptosis, mitochondrial dysfunction, and impaired neuroplasticity and neurotransmission. We further illustrate the beneficial effects of melatonin on behavior in animal models of AD. Next, we discuss the clinical effects of melatonin on sleep, cognition, behavior, psychiatric symptoms, electroencephalography findings, and molecular biomarkers in patients with mild cognitive impairment and AD. We then explore the effectiveness of novel melatonin-based therapies. Lastly, we discuss the limitations of current melatonin therapies for AD and suggest two emerging research themes for future study.

Discovery of an NAD+ analogue with enhanced specificity for PARP1

Among various protein posttranslational modifiers, poly-ADP-ribose polymerase 1 (PARP1) is a key player for regulating numerous cellular processes and events through enzymatic attachments of target proteins with ADP-ribose units donated by nicotinamide adenine dinucleotide (NAD⁺). Human PARP1 is involved in the pathogenesis and progression of many diseases. PARP1 inhibitors have received approvals for cancer treatment. Despite these successes, our understanding about PARP1 remains limited, partially due to the presence of various ADP-ribosylation reactions catalyzed by other PARPs and their overlapped cellular functions. Here we report a synthetic NAD⁺ featuring an adenosyl 3′-azido substitution. Acting as an ADP-ribose donor with high activity and specificity for human PARP1, this compound enables labelling and profiling of possible protein substrates of endogenous PARP1. It provides a unique and valuable tool for studying PARP1 in biology and pathology and may shed light on the development of PARP isoform-specific modulators.

An analogue of nicotinamide adenine dinucleotide (NAD⁺) featuring an azido group at 3′-OH of adenosine moiety is found to possess high specificity for human PARP1-catalyzed protein poly-ADP-ribosylation.

Trimetazidine Modulates Mitochondrial Redox Status and Disrupted Glutamate Homeostasis in a Rat Model of Epilepsy

Mitochondrial oxidative status exerts an important role in modulating glia–neuron interplay during epileptogenesis. Trimetazidine (TMZ), a well-known anti-ischemic drug, has shown promising potential against a wide range of neurodegenerative disorders including epilepsy. Nevertheless, the exact mechanistic rationale behind its anti-seizure potential has not been fully elucidated yet. Herein, the impact of TMZ against mitochondrial oxidative damage as well as glutamate homeostasis disruption in the hippocampus has been investigated in rats with lithium/pilocarpine (Li/PIL) seizures. Animals received 3 mEq/kg i.p. LiCl₃ followed by PIL (single i.p.; 150 mg/kg) 20 h later for induction of seizures with or without TMZ pretreatment (25 mg/kg; i.p.) for five consecutive days. Seizure score and seizure latency were observed. Mitochondrial redox status as well as ATP and uncoupling protein 2 was recorded. Moreover, glutamate homeostasis was unveiled. The present findings demonstrate the TMZ-attenuated Li/PIL seizure score and latency. It improved mitochondrial redox status, preserved energy production mechanisms, and decreased reactive astrocytes evidenced as decreased glial fibrillary acidic protein immune-stained areas in hippocampal tissue. In addition, it modulated phosphorylated extracellular signal-regulated kinases (p-ERK1/2) and p-AMP–activated protein kinase (p-AMPK) signaling pathways to reflect a verified anti-apoptotic effect. Consequently, it upregulated mRNA expression of astroglial glutamate transporters and reduced the elevated glutamate level. The current study demonstrates that TMZ exhibits robust anti-seizure and neuroprotective potentials. These effects are associated with its ability to modulate mitochondrial redox status, boost p-ERK1/2 and p-AMPK signaling pathways, and restore glutamate homeostasis in hippocampus.

PARP overactivation in neurological disorders

Poly (ADP-ribose) polymerases (PARPs) constitute a family of enzymes associated with divergent cellular processes that are not limited to DNA repair, chromatin organization, genome integrity, and apoptosis but also found to play a crucial role in inflammation. PARPs mediate poly (ADP-ribosylation) of DNA binding proteins that is often responsible for chromatin remodeling thereby ensure effective repairing of DNA stand breaks although during the conditions of severe genotoxic stress PARPs direct the cell fate towards apoptotic events. Recent discoveries have pushed PARPs into the spotlight as targets for treating cancer, metabolic, inflammatory and neurological disorders. Of note, PARP-1 is the most abundant isoform of PARPs (18 member super family) which executes more than 90% of PARPs functions. Since oxidative/nitrosative stress actuated PARP-1 is linked to vigorous DNA damage and wide spread provocative inflammatory response that underlie the aetiopathogenesis of different neurological disorders, possibility of developing PARP-1 inhibitors as plausible neurotherapeutic agents attracts considerable research interest. This review outlines the recent advances in PARP-1 biology and examines the capability of PARP-1 inhibitors as treatment modalities in intense and interminable diseases of neuronal origin.

PARP-DNA trapping ability of PARP inhibitors jeopardizes astrocyte viability: Implications for CNS disease therapeutics

There is emerging interest in the role of poly(ADP-ribose) polymerase-1 (PARP-1) in neurodegeneration and potential of its therapeutic targeting in neurodegenerative disorders. New generations of PARP inhibitors exhibit polypharmacological properties; they do not only block enzymatic activity with lower doses, but also alter how PARP-1 interacts with DNA. While these new inhibitors have proven useful in cancer therapy due to their ability to kill cancer cell, their use in neurodegenerative disorders has an opposite goal: cell protection. We hypothesize that newer generation PARP-1 inhibitors jeopardize the viability of dividing CNS cells by promoting DNA damage upon the PARP-DNA interaction. Using enriched murine astrocyte cultures, our study evaluates the effects of a variety of drugs known to inhibit PARP; talazoparib, olaparib, PJ34 and minocycline. Despite similar PARP enzymatic inhibiting activities, we show here that these drugs result in varied cell viability. Talazoparib and olaparib reduce astrocyte growth in a dose-dependent manner, while astrocytes remain unaffected by PJ34 and minocycline. Similarly, PJ34 and minocycline do not jeopardize DNA integrity, while treatment with talazoparib and olaparib promote DNA damage. These two drugs impact astrocytes similarly in basal conditions and upon nitrosative stress, a pathological condition typical for neurodegeneration. Mechanistic assessment revealed that talazoparib and olaparib promote PARP trapping onto DNA in a dose-dependent manner, while PJ34 and minocycline do not induce PARP-DNA trapping. This study provides unique insight into the selective use of PARP inhibitors to treat neurodegenerative disorders whereby inhibition of PARP enzymatic activity must occur without deleteriously trapping PARP onto DNA.

Inhibition of PARP activity improves therapeutic effect of ARPE-19 transplantation in RCS rats through decreasing photoreceptor death

Photoreceptor (PR) dysfunction or death is the key pathological change in retinal degeneration (RD). The death of PRs might be due to a primary change in PRs themselves or secondary to the dysfunction of the retinal pigment epithelium (RPE). Poly(ADP-ribose) polymerase (PARP) was reported to be involved in primary PR death, but whether it plays a role in PR death secondary to RPE dysfunction has not been determined. To clarify this question and develop a new therapeutic approach, we studied the changes in PAR/PARP in the RCS rat, a RD model, and tested the effect of PARP intervention when given alone or in combination with RPE cell transplantation. The results showed that poly(ADP-ribosyl)ation of proteins was increased in PRs undergoing secondary death in RCS rats, and this result was confirmed by the observation of similar changes in sodium iodate (SI)-induced secondary RD in SD rats. The increase in PAR/PARP was highly associated with increased apoptotic PRs and decreased visual function, as represented by lowered b-wave amplitudes on electroretinogram (ERG). Then, as we expected, when the RCS rats were treated with subretinal injection of the PARP inhibitor PJ34, the RD process was delayed. Furthermore, when PJ34 was given simultaneously with subretinal ARPE-19 cell transplantation, the therapeutic effects were significantly improved and lasted longer than those of ARPE-19 or PJ34 treatment alone. These results provide a potential new approach for treating RD.

The Modified Phenanthridine PJ34 Unveils an Exclusive Cell-Death Mechanism in Human Cancer Cells

This overview summarizes recent data disclosing the efficacy of the PARP inhibitor PJ34 in exclusive eradication of a variety of human cancer cells without impairing healthy proliferating cells. Its cytotoxic activity in cancer cells is attributed to the insertion of specific un-repairable anomalies in the structure of their mitotic spindle, leading to mitotic catastrophe cell death. This mechanism paves the way to a new concept of cancer therapy.

Resilience to Injury: A New Approach to Neuroprotection?

Despite thousands of neuroprotectants demonstrating promise in preclinical trials, a neuroprotective therapeutic has yet to be approved for the treatment of acute brain injuries such as stroke or traumatic brain injury. Developing a more detailed understanding of models and populations demonstrating "neurological resilience" in spite of brain injury can give us important insights into new translational therapies. Resilience is the process of active adaptation to a stressor. In the context of neuroprotection, models of preconditioning and unique animal models of extreme physiology (such as hibernating species) reliably demonstrate resilience in the laboratory setting. In the clinical setting, resilience is observed in young patients and can be found in those with specific genetic polymorphisms. These important examples of resilience can help transform and extend the current neuroprotective framework from simply countering the injurious cascade into one that anticipates, monitors, and optimizes patients' physiological responses from the time of injury throughout the process of recovery. This review summarizes the underpinnings of key adaptations common to models of resilience and how this understanding can be applied to new neuroprotective approaches.

Ischemia-Triggered Glutamate Excitotoxicity From the Perspective of Glial Cells

A plethora of neurological disorders shares a final common deadly pathway known as excitotoxicity. Among these disorders, ischemic injury is a prominent cause of death and disability worldwide. Brain ischemia stems from cardiac arrest or stroke, both responsible for insufficient blood supply to the brain parenchyma. Glucose and oxygen deficiency disrupts oxidative phosphorylation, which results in energy depletion and ionic imbalance, followed by cell membrane depolarization, calcium (Ca²⁺) overload, and extracellular accumulation of excitatory amino acid glutamate. If tight physiological regulation fails to clear the surplus of this neurotransmitter, subsequent prolonged activation of glutamate receptors forms a vicious circle between elevated concentrations of intracellular Ca²⁺ ions and aberrant glutamate release, aggravating the effect of this ischemic pathway. The activation of downstream Ca²⁺-dependent enzymes has a catastrophic impact on nervous tissue leading to cell death, accompanied by the formation of free radicals, edema, and inflammation. After decades of “neuron-centric” approaches, recent research has also finally shed some light on the role of glial cells in neurological diseases. It is becoming more and more evident that neurons and glia depend on each other. Neuronal cells, astrocytes, microglia, NG2 glia, and oligodendrocytes all have their roles in what is known as glutamate excitotoxicity. However, who is the main contributor to the ischemic pathway, and who is the unsuspecting victim? In this review article, we summarize the so-far-revealed roles of cells in the central nervous system, with particular attention to glial cells in ischemia-induced glutamate excitotoxicity, its origins, and consequences.

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

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

PARP Inhibitor Protects Against Chronic Hypoxia/Reoxygenation-Induced Retinal Injury by Regulation of MAPKs, HIF1α, Nrf2, and NFκB

Purpose

In the eye, chronic hypoxia/reoxygenation (H/R) contributes to the development of a number of ocular disorders. H/R induces the production of reactive oxygen species (ROS), leading to poly(ADP-ribose) polymerase-1 (PARP1) activation that promotes inflammation, cell death, and disease progression. Here, we analyzed the protective effects of the PARP1 inhibitor olaparib in H/R-induced retina injury and investigated the signaling mechanisms involved.

Methods

A rat retinal H/R model was used to detect histologic and biochemical changes in the retina.

Results

H/R induced reductions in the thickness of most retinal layers, which were prevented by olaparib. Furthermore, H/R caused increased levels of Akt and glycogen synthase kinase-3β phosphorylation, which were further increased by olaparib, contributing to retina protection. By contrast, H/R-induced c-Jun N-terminal kinase and p38 mitogen-activated protein kinases (MAPK) phosphorylation and activation were reduced by olaparib, via mitogen-activated protein kinase phosphatase 1 (MKP-1) expression. In addition, H/R-induced hypoxia-inducible factor 1α (HIF1α) levels were decreased by olaparib, which possibly contributed to reduced VEGF expression. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) expression was slightly increased by H/R and was further activated by olaparib. Nuclear factor-κB (NFκB) was also activated by H/R through phosphorylation (Ser536) and acetylation (Lys310) of the p65 subunit, although this was significantly reduced by olaparib.

Conclusions

Olaparib reduced H/R-induced degenerative changes in retinal morphology. The protective mechanisms of olaparib most probably involved Nrf2 activation and ROS reduction, as well as normalization of HIF1α and related VEGF expression. In addition, olaparib reduced inflammation by NFκB dephosphorylation/inactivation, possibly via the PARP1 inhibition-MKP-1 activation-p38 MAPK inhibition pathway. PARP inhibitors represent potential therapeutics in H/R-induced retinal disease.

Nicotinamide mononucleotide alters mitochondrial dynamics by SIRT3-dependent mechanism in male mice

Nicotinamide adenine dinucleotide (NAD+ ) is a central signaling molecule and enzyme cofactor that is involved in a variety of fundamental biological processes. NAD+ levels decline with age, neurodegenerative conditions, acute brain injury, and in obesity or diabetes. Loss of NAD+ results in impaired mitochondrial and cellular functions. Administration of NAD+ precursor, nicotinamide mononucleotide (NMN), has shown to improve mitochondrial bioenergetics, reverse age-associated physiological decline, and inhibit postischemic NAD+ degradation and cellular death. In this study, we identified a novel link between NAD+ metabolism and mitochondrial dynamics. A single dose (62.5 mg/kg) of NMN, administered to male mice, increases hippocampal mitochondria NAD+ pools for up to 24 hr posttreatment and drives a sirtuin 3 (SIRT3)-mediated global decrease in mitochondrial protein acetylation. This results in a reduction of hippocampal reactive oxygen species levels via SIRT3-driven deacetylation of mitochondrial manganese superoxide dismutase. Consequently, mitochondria in neurons become less fragmented due to lower interaction of phosphorylated fission protein, dynamin-related protein 1 (pDrp1 [S616]), with mitochondria. In conclusion, manipulation of mitochondrial NAD+ levels by NMN results in metabolic changes that protect mitochondria against reactive oxygen species and excessive fragmentation, offering therapeutic approaches for pathophysiologic stress conditions.

Multi-targeted Effect of Nicotinamide Mononucleotide on Brain Bioenergetic Metabolism

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

Nicotinamide reverses behavioral impairments and provides neuroprotection in 3-nitropropionic acid induced animal model ofHuntington's disease: implication of oxidative stress- poly(ADP- ribose) polymerase pathway

Huntington's disease (HD) is characterized by cognitive and psychiatric impairment caused by neuronal degeneration in the brain. Several studies have supported the hypothesis that oxidative stress is the main pathogenic factor in HD. The current study aims to determine the possible neuroprotective effects of nicotinamide on 3-nitropropionic acid (3-NP) induced HD. Male Wistar albino rats were divided into six groups. Group I was the vehicle-treated control, group II received 3-NP (20 mg/kg, intraperitoneally (i.p.) for 4 days, group III received nicotinamide (500 mg/kg, i.p.). The remaining groups received a combination of 3-NP plus nicotinamide 100, 300 or 500 mg/kg, i.p. respectively for 8 days. Afterward, the motor function and hind paw activity in the limb withdrawal were tested; rats were then euthanized for biochemical and histopathological analyses. Treatment of rats with 3-NP altered the motor function, elevated oxidative stress and caused significant histopathological changes in the brain. The treatment of rats with nicotinamide (100, 300 and 500 mg/kg) improved the motor function tested by locomotor activity test, movement analysis, and limb withdrawal test, which was associated with decreased oxidative stress markers (malondialdehyde, nitrites) and increased antioxidant enzyme (glutathione) levels. In addition, nicotinamide treatment decreased lactate dehydrogenase and prevented neuronal death in the striatal region. Our study, therefore, concludes that antioxidant drugs like nicotinamide might slow progression of clinical HD and may improve the motor functions in HD patients. To the best of our knowledge, this study is the first to explore the neuroprotective effects of nicotinamide on 3-NP-induced HD.

Interplay between NAD+ and acetyl‑CoA metabolism in ischemia-induced mitochondrial pathophysiology

Brain injury caused by ischemic insult due to significant reduction or interruption in cerebral blood flow leads to disruption of practically all cellular metabolic pathways. This triggers a complex stress response followed by overstimulation of downstream enzymatic pathways due to massive activation of post-translational modifications (PTM). Mitochondria are one of the most sensitive organelle to ischemic conditions. They become dysfunctional due to extensive fragmentation, inhibition of acetyl‑CoA production, and increased activity of NAD⁺ consuming enzymes. These pathologic conditions ultimately lead to inhibition of oxidative phosphorylation and mitochondrial ATP production. Both acetyl‑CoA and NAD⁺ are essential intermediates in cellular bioenergetics metabolism and also serve as substrates for post-translational modifications such as acetylation and ADP‑ribosylation. In this review we discuss ischemia/reperfusion-induced changes in NAD⁺ and acetyl‑CoA metabolism, how these affect relevant PTMs, and therapeutic approaches that restore the physiological levels of these metabolites leading to promising neuroprotection.

ADP sensitizes ZL55 cells to the activity of cisplatin

Malignant pleural mesothelioma (MPM) is an aggressive malignant tumor in which cisplatin therapy is commonly used, although its effectiveness is limited. It follows that research efforts dedicated to identify promising combinations that can synergistically kill cancer cells are needed. Because we recently demonstrated that ADP inhibits the proliferation of ZL55 cells, an MPM-derived cell line obtained from bioptic samples of asbestos-exposed patients. Our objective in this study was to investigate the hypothesis that ADP also potentiates the cytotoxic activity of cisplatin. Results show that in ZL55 cells ADP enhanced (a) the cytotoxicity of cisplatin by 12-fold, (b) the restraint of cell clonogenic potential cisplatin-mediated, and (c) the number of apoptotic cells. Cisplatin, but not ADP, caused caspases activation; nevertheless, poly(ADP-ribose) polymerase-1 was not only cleaved in cisplatin-treated cells but also in cells treated with ADP alone. Furthermore, ADP, but not cisplatin, decreased mTOR and 6SK phosphorylations. Both ADP and cisplatin increased p53 protein, but ADP was also able to enhance p53 messenger RNA. P53 silencing resulted in a very large decrement of cell death induced by ADP or by cisplatin and reverted ADP effects on mTOR/S6K phosphorylation, suggesting that activated p53 may act as a negative regulator of mTOR. Consistently, the inhibition of mTOR by rapamycin also sensitized cells to cisplatin, and the effects of cisplatin plus rapamycin were identical to those obtained with cisplatin plus ADP. These findings suggest that the combination of ADP and cisplatin may be a promising strategy for the clinical treatment of cisplatin-resistant MPM.

Adenosine diphosphate regulates MMP2 and MMP9 activity in malignant mesothelioma cells

Although an association between cancer progression and matrix metalloproteinase (MMP) 2 and MPP9 expression has been known, the expression, nuclear localization, and physiologically controlled activation of these two MMPs have not been investigated in malignant mesothelioma cells. We examined the expression and intracellular localization of MMP2/9 in ZL55 malignant mesothelioma cells, as well as their regulation by ADP. Using real-time PCR, we showed that activation of the P2Y1 receptor by ADP increased the expression of MMP2/9 mRNAs; MMP2/9 collected from conditioned media also showed an increase in activity; and ADP induced the nuclear localization of MMP2/9. The effects of ADP on transcription of the MMPs were due to activation of c-Src, Akt, and NF-κB, while ERK1/2 phosphorylation was needed for the increase in enzymatic activity and the regulation of nuclear import. We also showed that the nuclear localization of MMP2/9 induced by ADP causes the cleavage and inactivation of poly-ADP-ribose polymerase-1. These findings may help to elucidate the mechanisms regulating MMP2/9 activation in ZL55 human epithelioid mesothelioma cells, and perhaps other cells. Therapeutic approaches that promote ADP accumulation in a tumor environment may constitute an effective means to induce anticancer activity.

Mitochondrial Zn2+ Accumulation: A Potential Trigger of Hippocampal Ischemic Injury

Ischemic stroke is a major cause of death and disabilities worldwide, and it has been long hoped that improved understanding of relevant injury mechanisms would yield targeted neuroprotective therapies. While Ca²⁺ overload during ischemia-induced glutamate excitotoxicity has been identified as a major contributor, failures of glutamate targeted therapies to achieve desired clinical efficacy have dampened early hopes for the development of new treatments. However, additional studies examining possible contributions of Zn²⁺, a highly prevalent cation in the brain, have provided new insights that may help to rekindle the enthusiasm. In this review, we discuss both old and new findings yielding clues as to sources of the Zn²⁺ that accumulates in many forebrain neurons after ischemia, and mechanisms through which it mediates injury. Specifically, we highlight the growing evidence of important Zn²⁺ effects on mitochondria in promoting neuronal injury. A key focus has been to examine Zn²⁺ contributions to the degeneration of highly susceptible hippocampal pyramidal neurons. Recent studies provide evidence of differences in sources of Zn²⁺ and its interactions with mitochondria in CA1 versus CA3 neurons that may pertain to their differential vulnerabilities in disease. We propose that Zn²⁺-induced mitochondrial dysfunction is a critical and potentially targetable early event in the ischemic neuronal injury cascade, providing opportunities for the development of novel neuroprotective strategies to be delivered after transient ischemia.

The Polyphenol Oleuropein Aglycone Modulates the PARP1-SIRT1 Interplay: An In Vitro and In Vivo Study

Poly(ADP-ribose) polymerase-1 (PARP1) activation contributes to the cascade of events initiated by amyloid-β (Aβ) peptide eventually leading to cell death in Alzheimer's disease brain. A significant accumulation of PAR polymers and increase of PARP1 expression were detected in the cortex at the early (3.5 months) and intermediate (6 months) stage of Aβ deposition in the TgCRND8 mouse model. Our previous data highlighted the beneficial effects of oleuropein aglycone (OLE), the main polyphenol found in the olive oil, against neurodegeneration both in cultured cells and in model organisms. Here we found that 8-week OLE treatment (50 mg/kg of diet) to 6-month-old TgCRND8 mice rescued to control values PARP1 activation and the levels of its product, PAR. In N2a neuroblastoma cells, PARP1 activation and PAR formation upon exposure to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were abolished by pretreatment for 24 h with either OLE (100μM) or PARP inhibitors. A significant reduction of the NAD+ content, compared to controls, was found in N2a cells exposed to MNNG (100μM) for 90 min; the latter was slightly attenuated by cell treatment for 24 h with PJ-34 or with OLE. In vitro and in vivo, the OLE-induced reduction of PARP1 activation was paralleled by the overexpression of Sirtuin1 (SIRT1), and, in vivo, by a decrease of NF-κB and the pro-apoptotic marker p53. In N2a cells, we also found that OLE potentiates the MNNG-induced increase of Beclin1 levels. In conclusion, our data show that OLE treatment counteracts neuronal damage through modulation of the PARP1-SIRT1 interplay.

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

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

PARP inhibitors protect against sex- and AAG-dependent alkylation-induced neural degeneration

Alkylating agents are commonly used to treat cancer. Although base excision repair (BER) is a major pathway for repairing DNA alkylation damage, under certain conditions, the initiation of BER produces toxic repair intermediates that damage healthy tissues. The initiation of BER by the alkyladenine DNA glycosylase (AAG, a.k.a. MPG) can mediate alkylation-induced cytotoxicity in specific cells in the retina and cerebellum of male mice. Cytotoxicity in both wild-type and Aag-transgenic (AagTg) mice is abrogated in the absence of Poly(ADP-ribose) polymerase-1 (PARP1). Here, we tested whether PARP inhibitors can also prevent alkylation-induced retinal and cerebellar degeneration in male and female WT and AagTg mice. Importantly, we found that WT mice display sex-dependent alkylation-induced retinal damage (but not cerebellar damage), with WT males being more sensitive than females. Accordingly, estradiol treatment protects males against alkylation-induced retinal degeneration. In AagTg male and female mice, the alkylation-induced tissue damage in both the retina and cerebellum is exacerbated and the sex difference in the retina is abolished. PARP inhibitors, much like Parp1 gene deletion, protect against alkylation-induced AAG-dependent neuronal degeneration in WT and AagTg mice, regardless of the gender, but their efficacy in preventing alkylation-induced neuronal degeneration depends on PARP inhibitor characteristics and doses. The recent surge in the use of PARP inhibitors in combination with cancer chemotherapeutic alkylating agents might represent a powerful tool for obtaining increased therapeutic efficacy while avoiding the collateral effects of alkylating agents in healthy tissues.

Molecular evolutionary patterns of NAD+/Sirtuin aging signaling pathway across taxa

A deeper understanding of the conserved molecular mechanisms in different taxa have been made possible only because of the evolutionary conservation of crucial signaling pathways. In the present study, we explored the molecular evolutionary pattern of selection signatures in 51 species for 10 genes which are important components of NAD+/Sirtuin pathway and have already been directly linked to lifespan extension in worms and mice. Selection pressure analysis using PAML program revealed that MRPS5 and PPARGC1A were under significant constraints because of their functional significance. FOXO3a also displayed strong purifying selection. All three sirtuins, which were SIRT1, SIRT2 and SIRT6, displayed a great degree of conservation between taxa, which is consistent with the previous report. A significant evolutionary constraint is seen on the anti-oxidant gene, SOD3. As expected, TP53 gene was under significant selection pressure in mammals, owing to its major role in tumor progression. Poly-ADP-ribose polymerase (PARP) genes displayed the most sites under positive selection. Further 3D structural analysis of PARP1 and PARP2 protein revealed that some of these positively selected sites caused a change in the electrostatic potential of the protein structure, which may allow a change in its interaction with other proteins and molecules ultimately leading to difference in the function. Although the functional significance of the positively selected sites could not be established in the variants databases, yet it will be interesting to see if these sites actually affect the function of PARP1 and PARP2.

Sex differences in ischaemic stroke: potential cellular mechanisms

Stroke remains a leading cause of mortality and disability worldwide. More women than men have strokes each year, in part because women live longer. Women have poorer functional outcomes, are more likely to need nursing home care and have higher rates of recurrent stroke compared with men. Despite continued advancements in primary prevention, innovative acute therapies and ongoing developments in neurorehabilitation, stroke incidence and mortality continue to increase due to the aging of the U.S.

POPULATION

Sex chromosomes (XX compared with XY), sex hormones (oestrogen and androgen), epigenetic regulation and environmental factors all contribute to sex differences. Ischaemic sensitivity varies over the lifespan, with females having an "ischaemia resistant" phenotype that wanes after menopause, which has recently been modelled in the laboratory. Pharmacological therapies for acute ischaemic stroke are limited. The only pharmacological treatment for stroke approved by the Food and Drug Administration (FDA) is tissue plasminogen activator (tPA), which must be used within hours of stroke onset and has a number of contraindications. Pre-clinical studies have identified a number of potentially efficacious neuroprotective agents; however, nothing has been effectively translated into therapy in clinical practice. This may be due, in part, to the overwhelming use of young male rodents in pre-clinical research, as well as lack of sex-specific design and analysis in clinical trials. The review will summarize the current clinical evidence for sex differences in ischaemic stroke, and will discuss sex differences in the cellular mechanisms of acute ischaemic injury, highlighting cell death and immune/inflammatory pathways that may contribute to these clinical differences.

Vulnerability to a Metabolic Challenge Following Perinatal Asphyxia Evaluated by Organotypic Cultures: Neonatal Nicotinamide Treatment

The hypothesis of enhanced vulnerability following perinatal asphyxia was investigated with a protocol combining in vivo and in vitro experiments. Asphyxia-exposed (AS) (by 21 min water immersion of foetuses containing uterine horns) and caesarean-delivered control (CS) rat neonates were used at P2-3 for preparing triple organotypic cultures (substantia nigra, neostriatum and neocortex). At DIV 18, cultures were exposed to different concentrations of H₂O₂ (0.25-45 mM), added to the culture medium for 18 h. After a 48-h recovery period, the cultures were either assessed for cell viability or for neurochemical phenotype by confocal microscopy. Energy metabolism (ADP/ATP ratio), oxidative stress (GSH/GSSG) and a modified ferric reducing/antioxidant power assay were applied to homogenates of parallel culture series. In CS cultures, the number of dying cells was similar in substantia nigra, neostriatum and neocortex, but it was several times increased in AS cultures evaluated under the same conditions. A H₂O₂ challenge led to a concentration-dependent increase in cell death (>fourfold after 0.25 mM of H₂O₂) in CS cultures. In AS cultures, a significant increase in cell death was only observed after 0.5 mM of H₂O₂. At higher than 1 mM of H₂O₂ (up to 45 mM), cell death increased several times in all cultures, but the effect was still more prominent in CS than in AS cultures. The cell phenotype of dying/alive cells was investigated in formalin-fixed cultures exposed to 0 or 1 mM of H₂O₂, co-labelling for TUNEL (apoptosis), MAP-2 (neuronal phenotype), GFAP (astroglial phenotype) and TH (tyrosine hydroxylase; for dopamine phenotype), counterstaining for DAPI (nuclear staining), also evaluating the effect of a single dose of nicotinamide (0.8 nmol/kg, i.p. injected in 100 μL, 60 min after delivery). Perinatal asphyxia produced a significant increase in the number of DAPI/TUNEL cells/mm³, in substantia nigra and neostriatum. One millimolar of H₂0₂ increased the number of DAPI/TUNEL cells/mm³ by ≈twofold in all regions of CS and AS cultures, an effect that was prevented by neonatal nicotinamide treatment. In substantia nigra, the number of MAP-2/TH-positive cells/mm³ was decreased in AS compared to CS cultures, also by 1 mM of H₂0₂, both in CS and AS cultures, prevented by nicotinamide. In agreement, the number of MAP-2/TUNEL-positive cells/mm³ was increased by 1 mM H₂O₂, both in CS (twofold) and AS (threefold) cultures, prevented by nicotinamide. The number of MAP-2/TH/TUNEL-positive cells/mm³ was only increased in CS (>threefold), but not in AS (1.3-fold) cultures. No TH labelling was observed in neostriatum, but 1 mM of H₂O₂ produced a strong increase in the number of MAP-2/TUNEL-positive cells/mm³, both in CS (>2.9-fold) and AS (>fourfold), decreased by nicotinamide. In neocortex, H₂O₂ increased the number of MAP-2/TUNEL-positive cells/mm³, both in CS and AS cultures (≈threefold), decreased by nicotinamide. The ADP/ATP ratio was increased in AS culture homogenates (>sixfold), compared to CS homogenates, increased by 1 mM of H₂0₂, both in CS and AS homogenates. The GSH/GSSG ratio was significantly decreased in AS, compared to CS cultures. One millimolar of H₂O₂ decreased that ratio in CS and AS homogenates. The present results demonstrate that perinatal asphyxia induces long-term changes in metabolic pathways related to energy and oxidative stress, priming cell vulnerability with both neuronal and glial phenotype. The observed effects were region dependent, being the substantia nigra particularly prone to cell death. Nicotinamide administration in vivo prevented the deleterious effects observed after perinatal asphyxia in vitro, a suitable pharmacological strategy against the deleterious consequences of perinatal asphyxia.

Opportunities for the repurposing of PARP inhibitors for the therapy of non-oncological diseases

The recent clinical availability of the PARP inhibitor olaparib (Lynparza) opens the door for potential therapeutic repurposing for non-oncological indications. Considering (a) the preclinical efficacy data with PARP inhibitors in non-oncological diseases and (b) the risk-benefit ratio of treating patients with a compound that inhibits an enzyme that has physiological roles in the regulation of DNA repair, we have selected indications, where (a) the severity of the disease is high, (b) the available therapeutic options are limited, and (c) the duration of PARP inhibitor administration could be short, to provide first-line options for therapeutic repurposing. These indications are as follows: acute ischaemic stroke; traumatic brain injury; septic shock; acute pancreatitis; and severe asthma and severe acute lung injury. In addition, chronic, devastating diseases, where alternative therapeutic options cannot halt disease development (e.g. Parkinson's disease, progressive multiple sclerosis or severe fibrotic diseases), should also be considered. We present a preclinical and clinical action plan for the repurposing of PARP inhibitors.

LINKED ARTICLES

This article is part of a themed section on Inventing New Therapies Without Reinventing the Wheel: The Power of Drug Repurposing. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.2/issuetoc.

Base excision repair of oxidative DNA damage: from mechanism to disease

Reactive oxygen species continuously assault the structure of DNA resulting in oxidation and fragmentation of the nucleobases. Both oxidative DNA damage itself and its repair mediate the progression of many prevalent human maladies. The major pathway tasked with removal of oxidative DNA damage, and hence maintaining genomic integrity, is base excision repair (BER). The aphorism that structure often dictates function has proven true, as numerous recent structural biology studies have aided in clarifying the molecular mechanisms used by key BER enzymes during the repair of damaged DNA. This review focuses on the mechanistic details of the individual BER enzymes and the association of these enzymes during the development and progression of human diseases, including cancer and neurological diseases. Expanding on these structural and biochemical studies to further clarify still elusive BER mechanisms, and focusing our efforts toward gaining an improved appreciation of how these enzymes form co-complexes to facilitate DNA repair is a crucial next step toward understanding how BER contributes to human maladies and how it can be manipulated to alter patient outcomes.

Protective effect of nicotinamide adenine dinucleotide (NAD+) against spinal cord ischemia-reperfusion injury via reducing oxidative stress-induced neuronal apoptosis

As previous studies demonstrate that oxidative stress and apoptosis play crucial roles in ischemic pathogenesis and nicotinamide adenine dinucleotide (NAD⁺) treatment attenuates oxidative stress-induced cell death among primary neurons and astrocytes as well as significantly reduce cerebral ischemic injury in rats. We used a spinal cord ischemia injury (SCII) model in rats to verify our hypothesis that NAD⁺ could ameliorate oxidative stress-induced neuronal apoptosis. Adult male rats were subjected to transient spinal cord ischemia for 60min, and different doses of NAD⁺ were administered intraperitoneally immediately after the start of reperfusion. Neurological function was determined by Basso, Beattie, Bresnahan (BBB) scores. The oxidative stress level was assessed by superoxide dismutase (SOD) activity and malondialdehyde (MDA) content. The degree of apoptosis was analyzed by deoxyuridinetriphosphate nick-end labeling (TUNEL) staining and protein levels of cleaved caspase-3 and AIF (apoptosis inducing factor). The results showed that NAD⁺ at 50 or 100mg/kg significantly decreased the oxidative stress level and neuronal apoptosis in the spinal cord of ischemia-reperfusion rats compared with saline, as accompanied with the decreased oxidative stress, NAD⁺ administration significantly restrained the neuronal apoptosis after ischemia injury while improved the neurological and motor function. These findings suggested that NAD⁺ might protect against spinal cord ischemia-reperfusion via reducing oxidative stress-induced neuronal apoptosis.

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

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

MUTYH promotes oxidative microglial activation and inherited retinal degeneration

Oxidative stress is implicated in various neurodegenerative disorders, including retinitis pigmentosa (RP), an inherited disease that causes blindness. The biological and cellular mechanisms by which oxidative stress mediates neuronal cell death are largely unknown. In a mouse model of RP (rd10 mice), we show that oxidative DNA damage activates microglia through MutY homolog-mediated (MUYTH-mediated) base excision repair (BER), thereby exacerbating retinal inflammation and degeneration. In the early stage of retinal degeneration, oxidative DNA damage accumulated in the microglia and caused single-strand breaks (SSBs) and poly(ADP-ribose) polymerase activation. In contrast, Mutyh deficiency in rd10 mice prevented SSB formation in microglia, which in turn suppressed microglial activation and photoreceptor cell death. Moreover, Mutyh-deficient primary microglial cells attenuated the polarization to the inflammatory and cytotoxic phenotype under oxidative stress. Thus, MUTYH-mediated BER in oxidative microglial activation may be a novel target to dampen the disease progression in RP and other neurodegenerative disorders that are associated with oxidative stress.

Pathophysiological links between traumatic brain injury and post-traumatic headaches

This article reviews possible ways that traumatic brain injury (TBI) can induce migraine-type post-traumatic headaches (PTHs) in children, adults, civilians, and military personnel. Several cerebral alterations resulting from TBI can foster the development of PTH, including neuroinflammation that can activate neural systems associated with migraine. TBI can also compromise the intrinsic pain modulation system and this would increase the level of perceived pain associated with PTH. Depression and anxiety disorders, especially post-traumatic stress disorder (PTSD), are associated with TBI and these psychological conditions can directly intensify PTH. Additionally, depression and PTSD alter sleep and this will increase headache severity and foster the genesis of PTH. This article also reviews the anatomic loci of injury associated with TBI and notes the overlap between areas of injury associated with TBI and PTSD.

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

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

Ouabain Enhances ADPKD Cell Apoptosis via the Intrinsic Pathway

Progression of autosomal dominant polycystic kidney disease (ADPKD) is highly influenced by factors circulating in blood. We have shown that the hormone ouabain enhances several characteristics of the ADPKD cystic phenotype, including the rate of cell proliferation, fluid secretion and the capacity of the cells to form cysts. In this work, we found that physiological levels of ouabain (3 nM) also promote programmed cell death of renal epithelial cells obtained from kidney cysts of patients with ADPKD (ADPKD cells). This was determined by Alexa Fluor 488 labeled-Annexin-V staining and TUNEL assay, both biochemical markers of apoptosis. Ouabain-induced apoptosis also takes place when ADPKD cell growth is blocked; suggesting that the effect is not secondary to the stimulatory actions of ouabain on cell proliferation. Ouabain alters the expression of BCL family of proteins, reducing BCL-2 and increasing BAX expression levels, anti- and pro-apoptotic mediators respectively. In addition, ouabain caused the release of cytochrome c from mitochondria. Moreover, ouabain activates caspase-3, a key “executioner” caspase in the cell apoptotic pathway, but did not affect caspase-8. This suggests that ouabain triggers ADPKD cell apoptosis by stimulating the intrinsic, but not the extrinsic pathway of programmed cell death. The apoptotic effects of ouabain are specific for ADPKD cells and do not occur in normal human kidney cells (NHK cells). Taken together with our previous observations, these results show that ouabain causes an imbalance in cell growth/death, to favor growth of the cystic cells. This event, characteristic of ADPKD, further suggests the importance of ouabain as a circulating factor that promotes ADPKD progression.

Poly(ADP-ribosylation) and neurodegenerative disorders

Impaired mitochondrial structure and function are common features of neurodegenerative disorders, ultimately characterized by the death of neural cells promoted by still unknown signals. Among the possible modulators of neurodegeneration, the activation of poly(ADP-ribosylation), a post-translational modification of proteins, has been considered, being the product of the reaction, poly(ADP-ribose), a signaling molecule for different cell death paradigms. The basic properties of poly(ADP-ribosylation) are here described, focusing on the mitochondrial events; cell death paradigms such as apoptosis, parthanatos, necroptosis and mitophagy are illustrated. Finally, the promising use of poly(ADP-ribosylation) inhibitors to rescue neurodegeneration is addressed.

Experimental subarachnoid hemorrhage in rats: comparison of two endovascular perforation techniques with respect to success rate, confounding pathologies and early hippocampal tissue lesion pattern

Recently aside from the “classic” endovascular monofilament perforation technique to induce experimental subarachnoid hemorrhage (SAH) a modification using a tungsten wire advanced through a guide tube has been described. We aim to assess both techniques for their success rate (induction of SAH without confounding pathologies) as primary endpoint. Further, the early tissue lesion pattern as evidence for early brain injury will be analyzed as secondary endpoint. Sprague Dawley rats (n=39) were randomly assigned to receive either Sham surgery (n=4), SAH using the “classic” technique (n=18) or using a modified technique (n=17). Course of intracranial pressure (ICP) and regional cerebral blood flow (rCBF) was analyzed; subsequent pathologies were documented either 6 or 24 h after SAH. Hippocampal tissue samples were analyzed via immunohistochemistry and western blotting. SAH-induction, regardless of confounding pathologies, was independent from type of technique (p=0.679). There was no significant difference concerning case fatality rate (classic: 40%; modified: 20%; p=0.213). Successful induction of SAH without collateral ICH or SDH was possible in 40% with the classic and in 86.7% with the modified technique (p=0.008). Peak ICP levels differed significantly between the two groups (classic: 94 +/- 23 mmHg; modified: 68 +/- 19 mmHg; p=0.003). Evidence of early cellular stress response and activation of apoptotic pathways 6 h after SAH was demonstrated. The extent of stress response is not dependent on type of technique. Both tested techniques successfully produce SAH including activation of an early stress response and apoptotic pathways in the hippocampal tissue. However, the induction of SAH with less confounding pathologies was more frequently achieved with the modified tungsten wire technique.

PARP-1 involvement in neurodegeneration: A focus on Alzheimer's and Parkinson's diseases

DNA damage is the prime activator of the enzyme poly(ADP-ribose)polymerase1 (PARP-1) whose overactivation has been proven to be associated with the pathogenesis of numerous central nervous system disorders, such as ischemia, neuroinflammation, and neurodegenerative diseases. Under oxidative stress conditions PARP-1 activity increases, leading to an accumulation of ADP-ribose polymers and NAD(+) depletion, that induces energy crisis and finally cell death. This review aims to explain the contribution of PARP-1 in neurodegenerative diseases, focusing on Alzheimer's and Parkinson's disease, to stimulate further studies on this issue and thereby engage a new perspective regarding the design of possible therapeutic agents or the identification of biomarkers.

Neuronal death induced by misfolded prion protein is due to NAD+ depletion and can be relieved in vitro and in vivo by NAD+ replenishment

The mechanisms of neuronal death in protein misfolding neurodegenerative diseases such as Alzheimer's, Parkinson's and prion diseases are poorly understood. We used a highly toxic misfolded prion protein (TPrP) model to understand neurotoxicity induced by prion protein misfolding. We show that abnormal autophagy activation and neuronal demise is due to severe, neuron-specific, nicotinamide adenine dinucleotide (NAD(+)) depletion. Toxic prion protein-exposed neuronal cells exhibit dramatic reductions of intracellular NAD(+) followed by decreased ATP production, and are completely rescued by treatment with NAD(+) or its precursor nicotinamide because of restoration of physiological NAD(+) levels. Toxic prion protein-induced NAD(+) depletion results from PARP1-independent excessive protein ADP-ribosylations. In vivo, toxic prion protein-induced degeneration of hippocampal neurons is prevented dose-dependently by intracerebral injection of NAD(+). Intranasal NAD(+) treatment of prion-infected sick mice significantly improves activity and delays motor impairment. Our study reveals NAD(+) starvation as a novel mechanism of autophagy activation and neurodegeneration induced by a misfolded amyloidogenic protein. We propose the development of NAD(+) replenishment strategies for neuroprotection in prion diseases and possibly other protein misfolding neurodegenerative diseases.

Perinatal asphyxia leads to PARP-1 overactivity, p65 translocation, IL-1β and TNF-α overexpression, and apoptotic-like cell death in mesencephalon of neonatal rats: prevention by systemic neonatal nicotinamide administration

Perinatal asphyxia (PA) is a leading cause of neuronal damage in newborns, resulting in long-term neurological and cognitive deficits, in part due to impairment of mesostriatal and mesolimbic neurocircuitries. The insult can be as severe as to menace the integrity of the genome, triggering the overactivation of sentinel proteins, including poly (ADP-ribose) polymerase-1 (PARP-1). PARP-1 overactivation implies increased energy demands, worsening the metabolic failure and depleting further NAD⁺ availability. Using a global PA rat model, we report here evidence that hypoxia increases PARP-1 activity, triggering a signalling cascade leading to nuclear translocation of the NF-κB subunit p65, modulating the expression of IL-1β and TNF-α, pro-inflammatory molecules, increasing apoptotic-like cell death in mesencephalon of neonate rats, monitored with Western blots, qPCR, TUNEL and ELISA. PARP-1 activity increased immediately after PA, reaching a maximum 1–8 h after the insult, while activation of the NF-κB signalling pathway was observed 8 h after the insult, with a >twofold increase of p65 nuclear translocation. IL-1β and TNF-α mRNA levels were increased 24 h after the insult, together with a >twofold increase in apoptotic-like cell death. A single dose of the PARP-1 inhibitor nicotinamide (0.8 mmol/kg, i.p.), 1 h post delivery, prevented the effect of PA on PARP-1 activity, p65 translocation, pro-inflammatory cytokine expression and apoptotic-like cell death. The present study demonstrates that PA leads to PARP-1 overactivation, increasing the expression of pro-inflammatory cytokines and cell death in mesencephalon, effects prevented by systemic neonatal nicotinamide administration, supporting the idea that PARP-1 inhibition represents a therapeutic target against the effects of PA.