The taming of PARP1 and its impact on NAD+ metabolism

Poly (ADP-ribose): A double-edged sword governing cancer cell survival and death

Poly (ADP-ribose) (PAR), a polymer of ADP-ribose, is synthesized by PAR polymerase and is crucial for the survival of cancer cells due to its vital functions in DNA repair and post-translational modifications. Beyond its supportive role, PAR also triggers cancer cell death by excessive accumulation of PAR leading to an energy crisis and parthanatos. This phenomenon underscores the potential of targeting PAR regulation as a novel anticancer strategy, and the rationale would present an engaging topic in the field of anticancer research. Therefore, this editorial provides an overview of the mechanisms determining cancer cell fate, emphasizing the central role of PAR. It further introduces promising methods for modulating PAR concentrations that may pave the way for innovative anticancer therapies.

Nicotinamide Mononucleotide Supplementation: Understanding Metabolic Variability and Clinical Implications

Recent years have seen a surge in research focused on NAD+ decline and potential interventions, and despite significant progress, new discoveries continue to highlight the complexity of NAD+ biology. Nicotinamide mononucleotide (NMN), a well-established NAD+ precursor, has garnered considerable interest due to its capacity to elevate NAD+ levels and induce promising health benefits in preclinical models. Clinical trials investigating NMN supplementation have yielded variable outcomes while shedding light on the intricacies of NMN metabolism and revealing the critical roles played by gut microbiota and specific cellular uptake pathways. Individual variability in factors such as lifestyle, health conditions, genetics, and gut microbiome composition likely contributes to the observed discrepancies in clinical trial results. Preliminary evidence suggests that NMN's effects may be context-dependent, varying based on a person's physiological state. Understanding these nuances is critical for definitively assessing the impact of manipulating NAD+ levels through NMN supplementation. Here, we review NMN metabolism, focusing on current knowledge, pinpointing key areas where further research is needed, and outlining future directions to advance our understanding of its potential clinical significance.

CHK1 inhibitor induced PARylation by targeting PARG causes excessive replication and metabolic stress and overcomes chemoresistance in ovarian cancer

Chemoresistance contributes to the majority of deaths in women with ovarian cancer (OC). Altered DNA repair and metabolic signaling is implicated in mediating therapeutic resistance. DNA damage checkpoint kinase 1 (CHK1) integrates cell cycle and DNA repair in replicating cells, and its inhibition causes replication stress, repair deficiency and cell cycle dysregulation. We observed elevated Poly-ADP-ribosylation (PAR) of proteins (PARylation) and subsequent decrease in cellular NAD+ levels in OC cells treated with the CHK1 inhibitor prexasertib, indicating activation of NAD+ dependent DNA repair enzymes poly-ADP-ribose polymerases (PARP1/2). While multiple PARP inhibitors are in clinical use in treating OC, tumor resistance to these drugs is highly imminent. We reasoned that inhibition of dePARylation by targeting Poly (ADP-ribose) glycohydrolase (PARG) would disrupt metabolic and DNA repair crosstalk to overcome chemoresistance. Although PARG inhibition (PARGi) trapped PARylation of the proteins and activated CHK1, it did not cause any significant OC cell death. However, OC cells deficient in CHK1 were hypersensitive to PARGi, suggesting a role for metabolic and DNA repair crosstalk in protection of OC cells. Correspondingly, OC cells treated with a combination of CHK1 and PARG inhibitors exhibited excessive replication stress-mediated DNA lesions, cell cycle dysregulation, and mitotic catastrophe compared to individual drugs. Interestingly, increased PARylation observed in combination treatment resulted in depletion of NAD+ levels. These decreased NAD+ levels were also paralleled with reduced aldehyde dehydrogenase (ALDH) activity, which requires NAD+ to maintain cancer stem cells. Furthermore, prexasertib and PARGi combinations exhibited synergistic cell death in OC cells, including an isogenic chemoresistant cell line and 3D organoid models of primary patient-derived OC cell lines. Collectively, our data highlight a novel crosstalk between metabolism and DNA repair involving replication stress and NAD+-dependent PARylation, and suggest a novel combination therapy of CHK1 and PARG inhibitors to overcome chemoresistance in OC.

Respiratory Virus-Induced PARP1 Alters DC Metabolism and Antiviral Immunity Inducing Pulmonary Immunopathology

Previous studies from our laboratory and others have established the dendritic cell (DC) as a key target of RSV that drives infection-induced pathology. Analysis of RSV-induced transcriptomic changes in RSV-infected DC revealed metabolic gene signatures suggestive of altered cellular metabolism. Reverse phase protein array (RPPA) data showed significantly increased PARP1 phosphorylation in RSV-infected DC. Real-time cell metabolic analysis demonstrated increased glycolysis in PARP1-/- DC after RSV infection, confirming a role for PARP1 in regulating DC metabolism. Our data show that enzymatic inhibition or genomic ablation of PARP1 resulted in increased ifnb1, il12, and il27 in RSV-infected DC which, together, promote a more appropriate anti-viral environment. PARP1-/- mice and PARP1-inhibitor-treated mice were protected against RSV-induced immunopathology including airway inflammation, Th2 cytokine production, and mucus hypersecretion. However, delayed treatment with PARP1 inhibitor in RSV-infected mice provided only partial protection, suggesting that PARP1 is most important during the earlier innate immune stage of RSV infection.

Luteolin Alleviates Cadmium-Induced Kidney Injury by Inhibiting Oxidative DNA Damage and Repairing Autophagic Flux Blockade in Chickens

Chickens are a major source of meat and eggs in human food and have significant economic value. Cadmium (Cd) is a common environmental pollutant that can contaminate feed and drinking water, leading to kidney injury in livestock and poultry, primarily by inducing the generation of free radicals. It is necessary to develop potential medicines to prevent and treat Cd-induced nephrotoxicity in poultry. Luteolin (Lut) is a natural flavonoid compound mainly extracted from peanut shells and has a variety of biological functions to defend against oxidative damage. In this study, we aimed to demonstrate whether Lut can alleviate kidney injury under Cd exposure and elucidate the underlying molecular mechanisms. Renal histopathology and cell morphology were observed. The indicators of renal function, oxidative stress, DNA damage and repair, NAD+ content, SIRT1 activity, and autophagy were analyzed. In vitro data showed that Cd exposure increased ROS levels and induced oxidative DNA damage and repair, as indicated by increased 8-OHdG content, increased γ-H2AX protein expression, and the over-activation of the DNA repair enzyme PARP-1. Cd exposure decreased NAD+ content and SIRT1 activity and increased LC3 II, ATG5, and particularly p62 protein expression. In addition, Cd-induced oxidative DNA damage resulted in PARP-1 over-activation, reduced SIRT1 activity, and autophagic flux blockade, as evidenced by reactive oxygen species scavenger NAC application. The inhibition of PARP-1 activation with the pharmacological inhibitor PJ34 restored NAD+ content and SIRT1 activity. The activation of SIRT1 with the pharmacological activator RSV reversed Cd-induced autophagic flux blockade and cell injury. In vivo data demonstrated that Cd treatment caused the microstructural disruption of renal tissues, reduced creatinine, and urea nitrogen clearance, raised MDA content, and decreased the activities or contents of antioxidants (GSH, T-SOD, CAT, and T-AOC). Cd treatment caused oxidative DNA damage and PARP-1 activation, decreased NAD+ content, decreased SIRT1 activity, and impaired autophagic flux. Notably, the dietary Lut supplement observably alleviated these alterations in chicken kidney tissues induced by Cd. In conclusion, the dietary Lut supplement alleviated Cd-induced chicken kidney injury through its potent antioxidant properties by relieving the oxidative DNA damage-activated PARP-1-mediated reduction in SIRT1 activity and repairing autophagic flux blockade.

Antioxidant Glutathione Analogues UPF1 and UPF17 Modulate the Expression of Enzymes Involved in the Pathophysiology of Chronic Obstructive Pulmonary Disease

Increased oxidative stress (OS) and systemic inflammation are key players in the pathophysiology of chronic obstructive pulmonary disease (COPD). We aimed to clarify the effects of synthetic glutathione (GSH) analogue peptides UPF1 and UPF17 on the mRNA levels of enzymes involved in systemic inflammation and GSH metabolism in peripheral blood mononuclear cells (PBMCs) from patients with acute exacerbation of COPD (AE-COPD) and stable COPD along with non-obstructive smokers and non-smokers. UPF1 and UPF17 increased the expression of enzymes involved in the formation of the antioxidant capacity: superoxide dismutase 1 (SOD1) and the catalytic subunit of glutamyl-cysteine ligase (GCLC) in patients with AE-COPD and stable COPD, but also in non-obstructive smokers and non-smokers. Similarly, both UPF1 and UPF17 increased the expression of inflammatory enzymes poly(ADP-ribose) polymerase-1 (PARP-1), dipeptidyl peptidase 4 (DPP4), and cyclooxygenase-2 (COX-2). Both UPF analogues acted in a gender-dependent manner by increasing the expression of certain anti-inflammatory (histone deacetylase 2 (HDAC2)) and GSH metabolism pathway (SOD1 and GSH reductase (GSR))-related enzymes in females and decreasing them in males. UPF1 and UPF17 are able to increase the expression of the enzymes involved in GSH metabolism and could serve as a lead for designing potential COPD therapies against excessive OS.

Anemonin ameliorates human diploid fibroblasts 2BS and IMR90 cell senescence by PARP1-NAD+-SIRT1 signaling pathway

OBJECTIVE

Aging becomes the most predominant risk factor for all age-associated pathological conditions with the increase of life expectancy and the aggravation of social aging. Slowing down the speed of aging is considered an effective way to improve health, but so far, effective anti-aging methods are relatively lacking.

METHODS

Anemonin (ANE) was screened from eight existing small-molecule compounds by cell viability assay. The function of ANE was determined by the analysis of cell proliferation, β -galactosidase (SA-β -Gal) activity, cell cycle, SASP secretion, NAD+/NADH ratio, and other aging-related indicators. The targets of ANE were predicted by Drug Target Prediction System (DTPS) and Swiss Targe Prediction System. The effect of ANE on PARP-1-NAD+-SIRT1 signaling pathway was assessed by quantitative reverse-transcription polymerase chain reaction (RT-PCR), Western blot, PARP1, NAD+ and SIRT1 activity detection.

RESULTS

ANE can delay cell senescence; PARP1 is one of the targets of ANE and plays a crucial role in ANE anti-aging; ANE release more NAD+ by inhibiting PARP1 activity, thereby conversely promoting the function of SIRT1 and delay cell senescence.

CONCLUSIONS

Our study indicates that ANE can delay cellular senescence through the PARP1-NAD+-SIRT1 signaling pathway, which may be considered as an effective anti-aging strategy.

NAD metabolism: Role in senescence regulation and aging

The geroscience hypothesis proposes that addressing the biology of aging could directly prevent the onset or mitigate the severity of multiple chronic diseases. Understanding the interplay between key aspects of the biological hallmarks of aging is essential in delivering the promises of the geroscience hypothesis. Notably, the nucleotide nicotinamide adenine dinucleotide (NAD) interfaces with several biological hallmarks of aging, including cellular senescence, and changes in NAD metabolism have been shown to be involved in the aging process. The relationship between NAD metabolism and cellular senescence appears to be complex. On the one hand, the accumulation of DNA damage and mitochondrial dysfunction induced by low NAD+ can promote the development of senescence. On the other hand, the low NAD+ state that occurs during aging may inhibit SASP development as this secretory phenotype and the development of cellular senescence are both highly metabolically demanding. However, to date, the impact of NAD+ metabolism on the progression of the cellular senescence phenotype has not been fully characterized. Therefore, to explore the implications of NAD metabolism and NAD replacement therapies, it is essential to consider their interactions with other hallmarks of aging, including cellular senescence. We propose that a comprehensive understanding of the interplay between NAD boosting strategies and senolytic agents is necessary to advance the field.

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.

Epithelial NAD+ depletion drives mitochondrial dysfunction and contributes to intestinal inflammation

Introduction

We have previously demonstrated that a pathologic downregulation of peroxisome proliferator-activated receptor-gamma coactivator 1-alpha (PGC1α) within the intestinal epithelium contributes to the pathogenesis of inflammatory bowel disease (IBD). However, the mechanism underlying downregulation of PGC1α expression and activity during IBD is not yet clear.

Methods

Mice (male; C57Bl/6, Villincre/+;Pgc1afl/fl mice, and Pgc1afl/fl) were subjected to experimental colitis and treated with nicotinamide riboside. Western blot, high-resolution respirometry, nicotinamide adenine dinucleotide (NAD+) quantification, and immunoprecipitation were used to in this study.

Results

We demonstrate a significant depletion in the NAD+ levels within the intestinal epithelium of mice undergoing experimental colitis, as well as humans with ulcerative colitis. While we found no decrease in the levels of NAD+-synthesizing enzymes within the intestinal epithelium of mice undergoing experimental colitis, we did find an increase in the mRNA level, as well as the enzymatic activity, of the NAD+-consuming enzyme poly(ADP-ribose) polymerase-1 (PARP1). Treatment of mice undergoing experimental colitis with an NAD+ precursor reduced the severity of colitis, restored mitochondrial function, and increased active PGC1α levels; however, NAD+ repletion did not benefit transgenic mice that lack PGC1α within the intestinal epithelium, suggesting that the therapeutic effects require an intact PGC1α axis.

Discussion

Our results emphasize the importance of PGC1α expression to both mitochondrial health and homeostasis within the intestinal epithelium and suggest a novel therapeutic approach for disease management. These findings also provide a mechanistic basis for clinical trials of nicotinamide riboside in IBD patients.

Regulation of Biomolecular Condensates by Poly(ADP-ribose)

Biomolecular condensates are reversible compartments that form through a process called phase separation. Post-translational modifications like ADP-ribosylation can nucleate the formation of these condensates by accelerating the self-association of proteins. Poly(ADP-ribose) (PAR) chains are remarkably transient modifications with turnover rates on the order of minutes, yet they can be required for the formation of granules in response to oxidative stress, DNA damage, and other stimuli. Moreover, accumulation of PAR is linked with adverse phase transitions in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis. In this review, we provide a primer on how PAR is synthesized and regulated, the diverse structures and chemistries of ADP-ribosylation modifications, and protein-PAR interactions. We review substantial progress in recent efforts to determine the molecular mechanism of PAR-mediated phase separation, and we further delineate how inhibitors of PAR polymerases may be effective treatments for neurodegenerative pathologies. Finally, we highlight the need for rigorous biochemical interrogation of ADP-ribosylation in vivo and in vitro to clarify the exact pathway from PARylation to condensate formation.

Early loss of endogenous NAD+ following rotenone treatment leads to mitochondrial dysfunction and Sarm1 induction that is ameliorated by PARP inhibition

Sarm1 is an evolutionary conserved innate immune adaptor protein that has emerged as a primary regulator of programmed axonal degeneration over the past decade. In vitro structural insights have revealed that although Sarm1 induces energy depletion by breaking down nicotinamide adenine dinucleotide⁺ (NAD⁺ ), it is also allosterically inhibited by NAD⁺ . However, how NAD⁺ levels modulate the activation of intracellular Sarm1 has not been elucidated so far. This study focuses on understanding the events leading to Sarm1 activation in both neuronal and non-neuronal cells using the mitochondrial complex I inhibitor rotenone. Here, we report the regulation of rotenone-induced cell death by loss of NAD⁺ that may act as a 'biological trigger' of Sarm1 activation. Our study revealed that early loss of endogenous NAD⁺ levels arising due to PARP1 hyperactivation preceded Sarm1 induction following rotenone treatment. Interestingly, replenishing NAD⁺ levels by the PARP inhibitor, PJ34 restored mitochondrial complex I activity and also prevented subsequent Sarm1 activation in rotenone-treated cells. These cellular data were further validated in Drosophila melanogaster where a significant reduction in rotenone-mediated loss of locomotor abilities, and reduced dSarm expression was observed in the flies following PARP inhibition. Taken together, these observations not only uncover a novel regulation of Sarm1 induction by endogenous NAD⁺ levels but also point towards an important understanding on how PARP inhibitors could be repurposed in the treatment of mitochondrial complex I deficiency disorders.

Druggable Metabolic Vulnerabilities Are Exposed and Masked during Progression to Castration Resistant Prostate Cancer

There is an urgent need for exploring new actionable targets other than androgen receptor to improve outcome from lethal castration-resistant prostate cancer. Tumor metabolism has reemerged as a hallmark of cancer that drives and supports oncogenesis. In this regard, it is important to understand the relationship between distinctive metabolic features, androgen receptor signaling, genetic drivers in prostate cancer, and the tumor microenvironment (symbiotic and competitive metabolic interactions) to identify metabolic vulnerabilities. We explore the links between metabolism and gene regulation, and thus the unique metabolic signatures that define the malignant phenotypes at given stages of prostate tumor progression. We also provide an overview of current metabolism-based pharmacological strategies to be developed or repurposed for metabolism-based therapeutics for castration-resistant prostate cancer.

The Role of NAD+ in Regenerative Medicine

The understanding of the molecular and cellular basis of aging has grown exponentially over recent years, and it is now accepted within the scientific community that aging is a malleable process; just as it can be accelerated, it can also be slowed and even reversed. This has far-reaching implications for our attitude and approach toward aging, presenting the opportunity to enter a new era of cellular regenerative medicine to not only manage the external signs of aging but also to develop therapies that support the body to repair and restore itself back to a state of internal well-being. A wealth of evidence now demonstrates that a decline in cellular nicotinamide adenine dinucleotide (NAD+) is a feature of aging and may play a role in the process. NAD+ plays a pivotal role in cellular metabolism and is a co-substrate for enzymes that play key roles in pathways that modify aging. Thus, interventions that increase NAD+ may slow aspects of the aging trajectory, and there is great interest in methods for cellular NAD+ restoration. Given these recent advancements in understanding the cellular aging process, it is important that there is an integration between the basic scientists who are investigating the underlying mechanisms of cellular aging and the surgeons and aesthetic practitioners who are providing antiaging therapies. This will allow the effective translation of this vastly complex area of biology into clinical practice so that people can continue to not only stay looking younger for longer but also experience improved health and wellness.

Augmentation of NAD+ by Dunnione Ameliorates Imiquimod-Induced Psoriasis-Like Dermatitis in Mice

Background

Dunnione has anti-inflammatory properties arising from its ability to alter the ratio of NAD⁺/NADH through NAD(P)H quinone oxidoreductase 1 (NQO1) enzymatic action, followed by subsequent inhibition of NF-κB and inflammatory cytokines. Psoriasis is a chronic, inflammatory skin disorder in which the IL-23/Th17 axis plays an important role in inflammation. However, it is unclear whether modulation of NAD⁺ levels affects psoriasis, such as skin inflammation. Therefore, in this study, we investigated the effect of NAD⁺/NADH ratio modulation on imiquimod (IMQ)-induced, psoriasis-like skin inflammation in mice.

Methods

Psoriasis-like skin inflammation was generated by daily topical application of IMQ cream. The severity of dermatitis was assessed using the Psoriasis Area Severity Index (PASI) and histochemistry. Expression of inflammatory cytokines was detected by enzyme-linked immunosorbent assay and quantitative PCR. Acetylation of NF-κB p65 and STAT3 was determined by Western blotting.

Results

Dunnione improved IMQ-induced epidermal hyperplasia and inflammation, consistent with decreased levels of inflammatory cytokines (IL-17, IL-22, and IL-23) in skin lesions. Moreover, we found that an increase in the NAD⁺/NADH ratio by dunnione restored SIRT1 activity, thereby reduced imiquimod-induced STAT3 acetylation, which modulates the expression of psoriasis-promoting inflammatory cytokines, such as IL-17, IL-22, and IL-23.

Conclusion

Pharmacological modulation of cellular NAD⁺ levels could be a promising therapeutic approach for psoriasis-like skin disease.

Analysis of changes in the proteomic profile of porcine corpus luteum during different stages of the oestrous cycle: effects of PPAR gamma ligands

CONTEXT

The corpus luteum (CL) is an endocrine gland in the ovary of mature females during the oestrous cycle and pregnancy. There is evidence of a relationship between the secretory function of the CL and PPARs.

AIMS

In this study, we investigated the changes in the proteome of the CL in relation to the phase of the oestrous cycle and the impact of PPARγ ligands on the proteomic profile of the CL during the mid- and late-luteal phase of the oestrous cycle.

METHODS

The porcine CL explants were incubated in vitro for 6h in the presence of PPARγ ligands (agonist pioglitazone, antagonist T0070907) or without ligands. Global proteomic analysis was performed using the TMT-based LC-MS/MS method.

KEY RESULTS

The obtained results showed the disparity in proteomic profile of the untreated CL - different abundance of 23 and 28 proteins for the mid- and late-luteal phase, respectively. Moreover, seven proteins were differentially regulated in the CL tissue treated with PPARγ ligands. In the mid-luteal phase, one protein, CAND1, was downregulated after treatment with T0070907. In the late-luteal phase, the proteins SPTAN1, GOLGB1, TP53BP1, MATR3, RRBP1 and SRRT were upregulated by pioglitazone.

CONCLUSIONS

Comparative proteomic analysis revealed that certain proteins constitute a specific proteomic signature for each examined phase. Moreover, the study showed that the effect of PPARγ ligands on the CL proteome was rather limited.

IMPLICATIONS

The results provide a broader insight into the processes that may be responsible for the structural luteolysis of the porcine CL, in addition to apoptosis and autophagy.

Deciphering the functional mechanism of zinc ions of PARP1 binding with single strand breaks and double strand breaks

Poly(ADP-ribose)polymerase 1 (PARP1) is a key target for the treatment of cancer-related diseases, and plays an important role in biological processes such as DNA repair, regulating a variety of metabolic and signal transduction processes. Understanding the dynamic binding mechanisms between each domain of PARP1 and DNA is of great significance to deepen the understanding on the function of PARP1 and to facilitate the design of inhibitors. Herein, strategies such as classical molecular dynamics simulation, conformational analysis, binding free energy calculation and energy decomposition were used to shed light on the binding mechanisms of different DNA binding domains (DBDs, including ZnF1, ZnF2 and ZnF3) in PARP1 with DNA and on the influences of zinc ions on the binding process. On one hand, during binding with DNA, ZnF2 tends to expand its space to identify the DNA damage sites and ZnF1/ZnF2 recognizes the interfaces on both sides of DNA damage rather than one side during the process of DNA repair. More importantly, the stable secondary structure of L₂ of ZnF2 (PRO146 to MET153) is the key conformational change for ZnF1 and ZnF2 to recognize DNA damage. Meanwhile, ZnF3 has little effect on the binding mechanisms of PARP1. On the other hand, for the structural differences of DBD domains, zinc ions in ZnF1 and ZnF2 (Zn1 and Zn2) have an impact not only on the conformational changes of PARP1, but also on the conformational changes brought by the interaction of double strand breaks (DSB) and single strand breaks (SSB). And meanwhile, Zn3 also has little effect on ZnF3 for the system of ZnF3/DSB. The findings presented in this work deepen the understanding on the functional mechanism of PARP1 and provide a theoretical basis for further study on the interaction between different inhibitors and DBD domains to design more potential inhibitors.

Poly(ADP-ribose)polymerase 1 (PARP1) is a key target for treatment of cancer-related diseases. Detailed structural changes DBD in PARP1 during the binding process with DNA were investigated and the dynamic conformational differences of DBD caused by zinc ions were revealed.

Fueling genome maintenance: On the versatile roles of NAD+ in preserving DNA integrity

NAD⁺ is a versatile biomolecule acting as a master regulator and substrate in various cellular processes, including redox regulation, metabolism, and various signaling pathways. In this article, we concisely and critically review the role of NAD⁺ in mechanisms promoting genome maintenance. Numerous NAD⁺-dependent reactions are involved in the preservation of genome stability, the cellular DNA damage response, and other pathways regulating nucleic acid metabolism, such as gene expression and cell proliferation pathways. Of note, NAD⁺ serves as a substrate to ADP-ribosyltransferases, sirtuins, and potentially also eukaryotic DNA ligases, all of which regulate various aspects of DNA integrity, damage repair, and gene expression. Finally, we critically analyze recent developments in the field as well as discuss challenges associated with therapeutic actions intended to raise NAD⁺ levels.

Joining the PARty: PARP Regulation of KDM5A during DNA Repair (and Transcription?)

The lysine demethylase KDM5A collaborates with PARP1 and the histone variant macroH2A1.2 to modulate chromatin to promote DNA repair. Indeed, KDM5A engages poly(ADP-ribose) (PAR) chains at damage sites through a previously uncharacterized coiled-coil domain, a novel binding mode for PAR interactions. While KDM5A is a well-known transcriptional regulator, its function in DNA repair is only now emerging. Here we review the molecular mechanisms that regulate this PARP1-macroH2A1.2-KDM5A axis in DNA damage and consider the potential involvement of this pathway in transcription regulation and cancer. Using KDM5A as an example, we discuss how multifunctional chromatin proteins transition between several DNA-based processes, which must be coordinated to protect the integrity of the genome and epigenome. The dysregulation of chromatin and loss of genome integrity that is prevalent in human diseases including cancer may be related and could provide opportunities to target multitasking proteins with these pathways as therapeutic strategies.

Oncology Drug Repurposing for Sepsis Treatment

Sepsis involves life-threatening organ dysfunction caused by a dysregulated host response to infection. Despite three decades of efforts and multiple clinical trials, no treatment, except antibiotics and supportive care, has been approved for this devastating syndrome. Simultaneously, numerous preclinical studies have shown the effectiveness of oncology-indicated drugs in ameliorating sepsis. Here we focus on cataloging these efforts with both oncology-approved and under-development drugs that have been repositioned to treat bacterial-induced sepsis models. In this context, we also envision the exciting prospect for further standard and oncology drug combination testing that could ultimately improve clinical outcomes in sepsis.

Evolution, structure and function of divergent macroH2A1 splice isoforms

The replacement of replication-coupled histones with non-canonical histone variants provides chromatin with additional properties and contributes to the plasticity of the epigenome. MacroH2A histone variants are counterparts of the replication-coupled histone H2A. They are characterized by a unique tripartite structure, consisting of a histone fold, an unstructured linker, and a globular macrodomain. MacroH2A1.1 and macroH2A1.2 are the result of alternative splicing of the MACROH2A1 gene and can have opposing biological functions. Here, we discuss the structural differences between the macrodomains of the two isoforms, resulting in differential ligand binding. We further discuss how this modulates gene regulation by the two isoforms, in cases resulting in opposing role of macroH2A1.1 and macroH2A1.2 in development and differentiation. Finally, we share recent insight in the evolution of macroH2As. Taken together, in this review, we aim to discuss in unprecedented detail distinct properties and functions of the fascinating macroH2A1 splice isoforms.

Mammalian AKT, the Emerging Roles on Mitochondrial Function in Diseases

Mitochondrial dysfunction may play a crucial role in various diseases due to its roles in the regulation of energy production and cellular metabolism. Serine/threonine kinase (AKT) is a highly recognized antioxidant, immunomodulatory, anti-proliferation, and endocrine modulatory molecule. Interestingly, increasing studies have revealed that AKT can modulate mitochondria-mediated apoptosis, redox states, dynamic balance, autophagy, and metabolism. AKT thus plays multifaceted roles in mitochondrial function and is involved in the modulation of mitochondria-related diseases. This paper reviews the protective effects of AKT and its potential mechanisms of action in relation to mitochondrial function in various diseases.

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

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

Regulation and role of glycophagy in skeletal muscle energy metabolism

Glycophagy is the autophagic degradation of glycogen via the lysosomal enzyme GAA/alpha-acid glucosidase. Glycophagy is considered a housekeeping process to degrade poorly branched glycogen particles, but the regulation and role of glycophagy in skeletal muscle metabolism remains enigmatic. Herein, prior muscle contraction promoted glycogen supercompensation 24 and 48 h post contraction, an effect associated with reduced glycophagy. Moreover, NOTCH or cAMP signaling promoted glycophagy, whereas acute glycophagy deficiency rewired cell metabolism by reducing glycolysis and enhancing AMPK and PPAR signaling and fatty acid and glutamine metabolism. These metabolic adaptations were associated with reduced inflammation and triglyceride content but enhanced phosphoinositide 3-kinase (PI3K)-AKT/protein kinase B signaling and insulin action, the latter of which was abolished by exogenous oxidative stress. Collectively, these data suggest glycophagy is dynamically regulated, while the function of glycophagy can be extended beyond a housekeeping process to having an additional role in regulating energy metabolism and insulin action.Abbreviations: AMPK, AMP-activated protein kinase; ASM, acid soluble metabolites; cAMP, cyclic adenosine monophosphate; EPS, electrical pulse stimulation; FCCP, carbonyl cyanide-p-trifluoromethoxyphenylhydrazone; GAA, glucosidase, alpha, acid; mTOR, mechanistic target of rapamycin kinase; NAD, nicotinamide adenine dinucleotide; PARP, poly (ADP-ribose) polymerase family; PI3K, phosphoinositide 3-kinase; PPAR, peroxisome proliferator activated receptor ; PYGM, muscle glycogen phosphorylase; STBD1, starch binding domain 1; TFEB, transcription factor EB.

Why structure and chain length matter: on the biological significance underlying the structural heterogeneity of poly(ADP-ribose)

Poly(ADP-ribosyl)ation (PARylation) is a multifaceted post-translational modification, carried out by poly(ADP-ribosyl)transferases (poly-ARTs, PARPs), which play essential roles in (patho-) physiology, as well as cancer therapy. Using NAD⁺ as a substrate, acceptors, such as proteins and nucleic acids, can be modified with either single ADP-ribose units or polymers, varying considerably in length and branching. Recently, the importance of PAR structural heterogeneity with regards to chain length and branching came into focus. Here, we provide a concise overview on the current knowledge of the biochemical and physiological significance of such differently structured PAR. There is increasing evidence revealing that PAR’s structural diversity influences the binding characteristics of its readers, PAR catabolism, and the dynamics of biomolecular condensates. Thereby, it shapes various cellular processes, such as DNA damage response and cell cycle regulation. Contrary to the knowledge on the consequences of PAR’s structural diversity, insight into its determinants is just emerging, pointing to specific roles of different PARP members and accessory factors. In the future, it will be interesting to study the interplay with other post-translational modifications, the contribution of natural PARP variants, and the regulatory role of accessory molecules. This has the exciting potential for new therapeutic approaches, with the targeted modulation and tuning of PARPs’ enzymatic functions, rather than their complete inhibition, as a central premise.

Cohesin Mutations in Cancer: Emerging Therapeutic Targets

The cohesin complex is crucial for mediating sister chromatid cohesion and for hierarchal three-dimensional organization of the genome. Mutations in cohesin genes are present in a range of cancers. Extensive research over the last few years has shown that cohesin mutations are key events that contribute to neoplastic transformation. Cohesin is involved in a range of cellular processes; therefore, the impact of cohesin mutations in cancer is complex and can be cell context dependent. Candidate targets with therapeutic potential in cohesin mutant cells are emerging from functional studies. Here, we review emerging targets and pharmacological agents that have therapeutic potential in cohesin mutant cells.

Genotoxic therapy and resistance mechanism in gliomas

Glioma is one of the most common and lethal brain tumors. Surgical resection followed by radiotherapy plus chemotherapy is the current standard of care for patients with glioma. The existence of resistance to genotoxic therapy, as well as the nature of tumor heterogeneity greatly limits the efficacy of glioma therapy. DNA damage repair pathways play essential roles in many aspects of glioma biology such as cancer progression, therapy resistance, and tumor relapse. O6-methylguanine-DNA methyltransferase (MGMT) repairs the cytotoxic DNA lesion generated by temozolomide (TMZ), considered as the main mechanism of drug resistance. In addition, mismatch repair, base excision repair, and homologous recombination DNA repair also play pivotal roles in treatment resistance as well. Furthermore, cellular mechanisms, such as cancer stem cells, evasion from apoptosis, and metabolic reprogramming, also contribute to TMZ resistance in gliomas. Investigations over the past two decades have revealed comprehensive mechanisms of glioma therapy resistance, which has led to the development of novel therapeutic strategies and targeting molecules.

The Role of MacroH2A Histone Variants in Cancer

Simple Summary

The structural unit of chromatin is the nucleosome that is composed of DNA wrapped around a core of eight histone proteins. Histone variants can replace ‘standard’ histones at specific sites of the genome. Thus, histone variants modulate all functions in the context of chromatin, such as gene expression. Here, we provide a concise review on a group of histone variants termed macroH2A. They contain two additional domains that contribute to their increased size. We discuss how these domains mediate molecular functions in normal cells and the role of macroH2As in gene expression and cancer.

Abstract

The epigenome regulates gene expression and provides a molecular memory of cellular events. A growing body of evidence has highlighted the importance of epigenetic regulation in physiological tissue homeostasis and malignant transformation. Among epigenetic mechanisms, the replacement of replication-coupled histones with histone variants is the least understood. Due to differences in protein sequence and genomic distribution, histone variants contribute to the plasticity of the epigenome. Here, we focus on the family of macroH2A histone variants that are particular in having a tripartite structure consisting of a histone fold, an intrinsically disordered linker and a globular macrodomain. We discuss how these domains mediate different molecular functions related to chromatin architecture, transcription and DNA repair. Dysregulated expression of macroH2A histone variants has been observed in different subtypes of cancer and has variable prognostic impact, depending on cellular context and molecular background. We aim to provide a concise review regarding the context- and isoform-dependent contributions of macroH2A histone variants to cancer development and progression.

MacroH2A1.2 deficiency leads to neural stem cell differentiation defects and autism-like behaviors

The development of the nervous system requires precise regulation. Any disturbance in the regulation process can lead to neurological developmental diseases, such as autism and schizophrenia. Histone variants are important components of epigenetic regulation. The function and mechanisms of the macroH2A (mH2A) histone variant during brain development are unknown. Here, we show that deletion of the mH2A isoform mH2A1.2 interferes with neural stem cell differentiation in mice. Deletion of mH2A1.2 affects neurodevelopment, enhances neural progenitor cell (NPC) proliferation, and reduces NPC differentiation in the developing mouse brain. mH2A1.2-deficient mice exhibit autism-like behaviors, such as deficits in social behavior and exploratory abilities. We identify NKX2.2 as an important downstream effector gene and show that NKX2.2 expression is reduced after mH2A1.2 deletion and that overexpression of NKX2.2 rescues neuronal abnormalities caused by mH2A1.2 loss. Our study reveals that mH2A1.2 reduces the proliferation of neural progenitors and enhances neuronal differentiation during embryonic neurogenesis and that these effects are at least in part mediated by NKX2.2. These findings provide a basis for studying the relationship between mH2A1.2 and neurological disorders.

HPF1 and nucleosomes mediate a dramatic switch in activity of PARP1 from polymerase to hydrolase

Poly(ADP-ribose) polymerase 1 (PARP1) is an important player in the response to DNA damage. Recently, Histone PARylation Factor (HPF1) was shown to be a critical modulator of the activity of PARP1 by facilitating PARylation of histones and redirecting the target amino acid specificity from acidic to serine residues. Here, we investigate the mechanism and specific consequences of HPF1-mediated PARylation using nucleosomes as both activators and substrates for PARP1. HPF1 provides that catalytic base Glu284 to substantially redirect PARylation by PARP1 such that the histones in nucleosomes become the primary recipients of PAR chains. Surprisingly, HPF1 partitions most of the reaction product to free ADP-ribose (ADPR), resulting in much shorter PAR chains compared to reactions in the absence of HPF1. This HPF1-mediated switch from polymerase to hydrolase has important implications for the PARP1-mediated response to DNA damage and raises interesting new questions about the role of intracellular ADPR and depletion of NAD⁺.

Role of NAD+ in regulating cellular and metabolic signaling pathways

Background

Nicotinamide adenine dinucleotide (NAD⁺), a critical coenzyme present in every living cell, is involved in a myriad of metabolic processes associated with cellular bioenergetics. For this reason, NAD⁺ is often studied in the context of aging, cancer, and neurodegenerative and metabolic disorders.

Scope of review

Cellular NAD⁺ depletion is associated with compromised adaptive cellular stress responses, impaired neuronal plasticity, impaired DNA repair, and cellular senescence. Increasing evidence has shown the efficacy of boosting NAD⁺ levels using NAD⁺ precursors in various diseases. This review provides a comprehensive understanding into the role of NAD⁺ in aging and other pathologies and discusses potential therapeutic targets.

Major conclusions

An alteration in the NAD⁺/NADH ratio or the NAD⁺ pool size can lead to derailment of the biological system and contribute to various neurodegenerative disorders, aging, and tumorigenesis. Due to the varied distribution of NAD⁺/NADH in different locations within cells, the direct role of impaired NAD⁺-dependent processes in humans remains unestablished. In this regard, longitudinal studies are needed to quantify NAD⁺ and its related metabolites. Future research should focus on measuring the fluxes through pathways associated with NAD⁺ synthesis and degradation.

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NAD⁺ regulates energy metabolism, DNA damage repair, gene expression, and stress response.

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NAD⁺ deterioration contributes to the progression of multiple metabolic disorders, cancers, and neurodegenerative diseases.

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Nicotinamide mononucleotide and nicotinamide riboside raise NAD⁺ levels in different tissues in preclinical models.

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Imaging studies on genetic models can illustrate the pathways of NAD⁺metabolism and their downstream functional effects.

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Human clinical trials to determine benefits of restoration of NAD⁺ by using NAD precursors are in progress.

Connecting vascular aging and frailty in Alzheimer's disease

Aging plays an important role in the etiology of the most common age-related diseases (ARDs), including Alzheimer's disease (AD). The increasing number of AD patients and the lack of disease-modifying drugs warranted intensive research to tackle the pathophysiological mechanisms underpinning AD development. Vascular aging/dysfunction is a common feature of almost all ARDs, including cardiovascular (CV) diseases, diabetes and AD. To this regard, interventions aimed at modifying CV outcomes are under extensive investigation for their pleiotropic role in ameliorating and slowing down cognitive impairment in middle-life and elderly individuals. Evidence from observational and clinical studies confirm the notion that the earlier the interventions are conducted, the most favorable are the effects on cognitive function. Therefore, epidemiological research should focus on the early detection of deviations from a healthy cognitive aging trajectory, through the stratification of adult individuals according to the rate of aging. Here, we review the interplay between vascular and cognitive dysfunctions associated with aging, to disentangle the complex mechanisms underpinning the development and progression of neurodegenerative disorders, with a specific focus on AD.

Poly (ADP-ribose) polymerase-1 as a promising drug target for neurodegenerative diseases

AIMS

Poly (ADP-ribose) polymerase- (PARP)-1 is predominantly triggered by DNA damage. Overexpression of PARP-1 is known for its association with the pathogenesis of several CNS disorders, such as Stroke, Parkinson's disease (PD), Alzheimer's disease (AD), Huntington (HD) and Amyotrophic lateral sclerosis (ALS). NAD+ depletion resulted PARP related cell death only happened when the trial used extreme high oxidization treatment. Inhibition of PARP1/2 may induce replication related cell death due to un-repaired DNA damage. This review has discussed PARP-1 modulated downstream pathways in neurodegeneration and various FDA approved PARP-1 inhibitors.

MATERIALS AND METHODS

A systematic literature review of PubMed, Medline, Bentham, Scopus and EMBASE (Elsevier) databases was carried out to understand the nature of the extensive work done on mechanistic role of Poly (ADP-ribose) polymerase and its inhibition in Neurodegenerative diseases.

KEY FINDINGS

Several researchers have put forward number of potential treatments, of which PARP-1 enzyme has been regarded as a potent target intended for the handling of neurodegenerative ailments. Targeting PARP using its chemical inhibitors in various neurodegenerative may have therapeutic outcomes by reducing neuronal death mediated by PARPi. Numerous PARP-1 inhibitors have been studied in neurodegenerative diseases but they haven't been clinically evaluated.

SIGNIFICANCE

In this review, the pathological role of PARP-1 in various neurodegenerative diseases has been discussed along with the therapeutic role of PARP-1 inhibitors in various neurodegenerative diseases.

Immunotherapy of COVID-19 with poly (ADP-ribose) polymerase inhibitors: starting with nicotinamide

COVID-19 induces a proinflammatory environment that is stronger in patients requiring intensive care. The cytokine components of this environment may determine efficacy or otherwise of glucocorticoid therapy. The immunity modulators, the aryl hydrocarbon receptor (AhR) and the nuclear NAD+-consuming enzyme poly (ADP-ribose) polymerase 1 (PARP 1) may play a critical role in COVID-19 pathophysiology. The AhR is overexpressed in coronaviruses, including COVID-19 and, as it regulates PARP gene expression, the latter is likely to be activated in COVID-19. PARP 1 activation leads to cell death mainly by depletion of NAD+ and adenosine triphosphate (ATP), especially when availability of these energy mediators is compromised. PARP expression is enhanced in other lung conditions: the pneumovirus respiratory syncytial virus (RSV) and chronic obstructive pulmonary disease (COPD). I propose that PARP 1 activation is the terminal point in a sequence of events culminating in patient mortality and should be the focus of COVID-19 immunotherapy. Potent PARP 1 inhibitors are undergoing trials in cancer, but a readily available inhibitor, nicotinamide (NAM), which possesses a highly desirable biochemical and activity profile, merits exploration. It conserves NAD+ and prevents ATP depletion by PARP 1 and Sirtuin 1 (silent mating type information regulation 2 homologue 1) inhibition, enhances NAD+ synthesis, and hence that of NADP+ which is a stronger PARP inhibitor, reverses lung injury caused by ischaemia/reperfusion, inhibits proinflammatory cytokines and is effective against HIV infection. These properties qualify NAM for therapeutic use initially in conjunction with standard clinical care or combined with other agents, and subsequently as an adjunct to stronger PARP 1 inhibitors or other drugs.

NAD+ metabolism: pathophysiologic mechanisms and therapeutic potential

Nicotinamide adenine dinucleotide (NAD+) and its metabolites function as critical regulators to maintain physiologic processes, enabling the plastic cells to adapt to environmental changes including nutrient perturbation, genotoxic factors, circadian disorder, infection, inflammation and xenobiotics. These effects are mainly achieved by the driving effect of NAD+ on metabolic pathways as enzyme cofactors transferring hydrogen in oxidation-reduction reactions. Besides, multiple NAD+-dependent enzymes are involved in physiology either by post-synthesis chemical modification of DNA, RNA and proteins, or releasing second messenger cyclic ADP-ribose (cADPR) and NAADP+. Prolonged disequilibrium of NAD+ metabolism disturbs the physiological functions, resulting in diseases including metabolic diseases, cancer, aging and neurodegeneration disorder. In this review, we summarize recent advances in our understanding of the molecular mechanisms of NAD+-regulated physiological responses to stresses, the contribution of NAD+ deficiency to various diseases via manipulating cellular communication networks and the potential new avenues for therapeutic intervention.

A unifying mechanism of ketogenic diet action: The multiple roles of nicotinamide adenine dinucleotide

The ability of a ketogenic diet to treat seizures and render a neuronal network more resistant to strong electrical activity has been observed for a century in clinics and for decades in research laboratories. Alongside ongoing efforts to understand how this therapy works to stop seizures, metabolic health is increasingly appreciated as critical buffer to resisting and recovering from acute and chronic disease. Accordingly, links between metabolism and health, and the broader emerging impact of the ketogenic diet in improving diverse metabolic, immunological and neurological conditions, have served to intensify the search for its key and/or common mechanisms. Here we review diverse evidence for increased levels of NAD⁺, and thus an altered ratio of NAD⁺/NADH, during metabolic therapy with a ketogenic diet. We propose this as a potential unifying mechanism, and highlight some of the evidence linking altered NAD⁺/NADH with reduced seizures and with a range of short and long-term changes associated with the beneficial effects of a ketogenic diet. An increase in NAD⁺/NADH is consistent with multiple lines of evidence and hypotheses, and therefore we suggest that increased NAD⁺ may be a common mechanism underlying beneficial effects of ketogenic diet therapy.

CD38: T Cell Immuno-Metabolic Modulator

Activation and subsequent differentiation of T cells following antigenic stimulation are triggered by highly coordinated signaling events that lead to instilling cells with a discrete metabolic and transcriptional feature. Compelling studies indicate that intracellular nicotinamide adenine dinucleotide (NAD⁺) levels have profound influence on diverse signaling and metabolic pathways of T cells, and hence dictate their functional fate. CD38, a major mammalian NAD⁺ glycohydrolase (NADase), expresses on T cells following activation and appears to be an essential modulator of intracellular NAD⁺ levels. The enzymatic activity of CD38 in the process of generating the second messenger cADPR utilizes intracellular NAD^+, and thus limits its availability to different NAD⁺ consuming enzymes (PARP, ART, and sirtuins) inside the cells. The present review discusses how the CD38-NAD⁺ axis affects T cell activation and differentiation through interfering with their signaling and metabolic processes. We also describe the pivotal role of the CD38-NAD⁺ axis in influencing the chromatin remodeling and rewiring T cell response. Overall, this review emphasizes the crucial contribution of the CD38^-NAD⁺ axis in altering T cell response in various pathophysiological conditions.

Macrophage Immunometabolism and Inflammaging: Roles of Mitochondrial Dysfunction, Cellular Senescence, CD38, and NAD

Aging is a complex process that involves dysfunction on multiple levels, all of which seem to converge on inflammation. Macrophages are intimately involved in initiating and resolving inflammation, and their dysregulation with age is a primary contributor to inflammaging—a state of chronic, low-grade inflammation that develops during aging. Among the age-related changes that occur to macrophages are a heightened state of basal inflammation and diminished or hyperactive inflammatory responses, which seem to be driven by metabolic-dependent epigenetic changes. In this review article we provide a brief overview of mitochondrial functions and age-related changes that occur to macrophages, with an emphasis on how the inflammaging environment, senescence, and NAD decline can affect their metabolism, promote dysregulation, and contribute to inflammaging and age-related pathologies.

The Histone Variant MacroH2A1 Regulates Key Genes for Myogenic Cell Fusion in a Splice-Isoform Dependent Manner

MacroH2A histone variants have functions in differentiation, somatic cell reprogramming and cancer. However, at present, it is not clear how macroH2As affect gene regulation to exert these functions. We have parted from the initial observation that loss of total macroH2A1 led to a change in the morphology of murine myotubes differentiated ex vivo. The fusion of myoblasts to myotubes is a key process in embryonic myogenesis and highly relevant for muscle regeneration after acute or chronic injury. We have focused on this physiological process, to investigate the functions of the two splice isoforms of macroH2A1. Individual perturbation of the two isoforms in myotubes forming in vitro from myogenic C2C12 cells showed an opposing phenotype, with macroH2A1.1 enhancing, and macroH2A1.2 reducing, fusion. Differential regulation of a subset of fusion-related genes encoding components of the extracellular matrix and cell surface receptors for adhesion correlated with these phenotypes. We describe, for the first time, splice isoform-specific phenotypes for the histone variant macroH2A1 in a physiologic process and provide evidence for a novel underlying molecular mechanism of gene regulation.