Cockayne syndrome proteins CSA and CSB maintain mitochondrial homeostasis through NAD+ signaling

Administration of nicotinamide mononucleotide suppresses the progression of age-related hearing loss in mice

Age-related hearing loss (ARHL) is a widespread problem in the elderly, significantly impairing their quality of life. Despite its high prevalence, no fundamental treatment for ARHL has been established. Nicotinamide adenine dinucleotide (NAD+) is required for various biological processes and tissue levels of the coenzyme NAD+ are known to decrease with age. A previous report suggested that declining NAD+ levels induce age-related diseases and NAD+ supplementation might be effective for treating or preventing age-related diseases. To clarify the effect of NAD+ supplementation on ARHL, C57BL/6J mice used as an animal model of ARHL were treated with nicotinamide mononucleotide (NMN), a precursor of NAD+. Oral administration of NMN at 500 mg/kg/day effectively suppressed the development of ARHL in C57BL/6J mice. To elucidate the mechanism by which NMN administration suppressed the development of ARHL, NAD+-related metabolites were assessed, and a comprehensive transcriptomic analysis of the inner ear tissue was performed. NMN administration resulted in increased NAD+ levels in inner ear tissues and induced changes in the transcriptome, specifically in genes related to metal ion metabolism. These findings suggest that NMN administration enhanced NAD+ levels in inner ear tissues, modulating metal ion metabolism to potentially protect against oxidative stress. This study provides a novel therapeutic approach to mitigating ARHL through NAD+ supplementation.

General loss of proteostasis links Huntington disease to Cockayne syndrome

Cockayne syndrome (CS) is an autosomal recessive disorder of developmental delay, multiple organ system degeneration and signs of premature ageing. We show here, using the RNA-seq data from two CS mutant cell lines, that the CS key transcriptional signature displays significant enrichment of neurodegeneration terms, including genes relevant in Huntington disease (HD). By using deep learning approaches and two published RNA-Seq datasets, the CS transcriptional signature highly significantly classified and predicted HD and control samples. Neurodegeneration is one hallmark of CS disease, and fibroblasts from CS patients with different causative mutations display disturbed ribosomal biogenesis and a consecutive loss of protein homeostasis - proteostasis. Encouraged by the transcriptomic data, we asked whether this pathomechanism is also active in HD. In different HD cell-culture models, we showed that mutant Huntingtin impacts ribosomal biogenesis and function. This led to an error-prone protein synthesis and, as shown in different mouse models and human tissue, whole proteome instability, and a general loss of proteostasis.

Cognitive Decline and Other Late-Stage Neurologic Complications in Cockayne Syndrome

Background and Objectives

Cockayne syndrome (CS) is an ultra-rare, autosomal recessive, premature aging disorder characterized by impaired growth, neurodevelopmental delays, neurodegeneration, polyneuropathy, and other multiorgan system complications. The anatomic aspects of CS neurodegeneration have long been known from postmortem examinations and MRI studies, but the clinical features of this neurodegeneration are not well characterized, especially at later stages of the disease.

Methods

This was a retrospective observational study in which individuals with CS who survived beyond 18 years were ascertained at 3 centers in the United States, France, and the United Kingdom. Medical records were examined to determine the frequencies and features of the following neurologic complications: neurocognitive/neuropsychiatric decline (8 symptoms), tremors, neuropathy, seizures, and strokes.

Results

Among 18 individuals who met inclusion criteria, all but one (94.4%) experienced at least one symptom of neurocognitive/neuropsychiatric decline, with most individuals experiencing at least half of those symptoms. Most participants experienced tremors and peripheral neuropathy, with a few experiencing seizures and strokes. For individuals with available data, 100.0% were reported to have gait ataxia and neuroimaging showed that 85.7% had generalized cerebral atrophy on MRI while 78.6% had white matter changes.

Discussion

Symptoms of neurocognitive/neuropsychiatric decline are nearly universal in our cohort of adults with CS, suggesting that these individuals are at risk of developing neurocognitive/neuropsychiatric decline, with symptoms related to but not specific to dementia. Considering the prominent role of DNA repair defects in CS disease mechanisms and emerging evidence for increased DNA damage in neurodegenerative disease, impaired genome maintenance may be a shared pathway underlying multiple forms of neurocognitive/neuropsychiatric decline. Components of the DNA damage response mechanism may bear further study as potential therapeutic targets that could alleviate neurocognitive/neuropsychiatric symptoms in CS and other neurodegenerative disorders.

Cockayne Syndrome Linked to Elevated R-Loops Induced by Stalled RNA Polymerase II during Transcription Elongation

Mutations in the Cockayne Syndrome group B (CSB) gene cause cancer in mice, but premature aging and severe neurodevelopmental defects in humans. CSB, a member of the SWI/SNF family of chromatin remodelers, plays diverse roles in regulating gene expression and transcription-coupled nucleotide excision repair (TC-NER); however, these functions do not explain the distinct phenotypic differences observed between CSB-deficient mice and humans. During investigating Cockayne Syndrome-associated genome instability, we uncover an intrinsic mechanism that involves elongating RNA polymerase II (RNAPII) undergoing transient pauses at internal T-runs where CSB is required to propel RNAPII forward. Consequently, CSB deficiency retards RNAPII elongation in these regions, and when coupled with G-rich sequences upstream, exacerbates genome instability by promoting R-loop formation. These R-loop prone motifs are notably abundant in relatively long genes related to neuronal functions in the human genome, but less prevalent in the mouse genome. These findings provide mechanistic insights into differential impacts of CSB deficiency on mice versus humans and suggest that the manifestation of the Cockayne Syndrome phenotype in humans results from the progressive evolution of mammalian genomes.

DNA repair deficiencies and neurodegeneration

Neurodegenerative diseases are the second most prevalent cause of death in industrialized countries. Alzheimer's Disease is the most widespread and also most acknowledged form of dementia today. Together with Parkinson's Disease they account for over 90 % cases of neurodegenerative disorders caused by proteopathies. Far less known are the neurodegenerative pathologies in DNA repair deficiency syndromes. Such diseases like Cockayne - or Werner Syndrome are described as progeroid syndromes - diseases that cause the premature ageing of the affected persons, and there are clear implications of such diseases in neurologic dysfunction and degeneration. In this review, we aim to draw the attention on commonalities between proteopathy-associated neurodegeneration and neurodegeneration caused by DNA repair defects and discuss how mitochondria are implicated in the development of both disorder classes. Furthermore, we highlight how nematodes are a valuable and indispensable model organism to study conserved neurodegenerative processes in a fast-forward manner.

Roles of NAD+ in Health and Aging

NAD+, the essential metabolite involved in multiple reactions such as the regulation of cellular metabolism, energy production, DNA repair, mitophagy and autophagy, inflammation, and neuronal function, has been the subject of intense research in the field of aging and disease over the last decade. NAD+ levels decline with aging and in some age-related diseases, and reduction in NAD+ affects all the hallmarks of aging. Here, we present an overview of the discovery of NAD+, the cellular pathways of producing and consuming NAD+, and discuss how imbalances in the production rate and cellular request of NAD+ likely contribute to aging and age-related diseases including neurodegeneration. Preclinical studies have revealed great potential for NAD+ precursors in promotion of healthy aging and improvement of neurodegeneration. This has led to the initiation of several clinical trials with NAD+ precursors to treat accelerated aging, age-associated dysfunctions, and diseases including Alzheimer's and Parkinson's. NAD supplementation has great future potential clinically, and these studies will also provide insight into the mechanisms of aging.

Multifaceted roles of mitochondria in wound healing and chronic wound pathogenesis

Mitochondria are intracellular organelles that play a critical role in numerous cellular processes including the regulation of metabolism, cellular stress response, and cell fate. Mitochondria themselves are subject to well-orchestrated regulation in order to maintain organelle and cellular homeostasis. Wound healing is a multifactorial process that involves the stringent regulation of several cell types and cellular processes. In the event of dysregulated wound healing, hard-to-heal chronic wounds form and can place a significant burden on healthcare systems. Importantly, treatment options remain limited owing to the multifactorial nature of chronic wound pathogenesis. One area that has received more attention in recent years is the role of mitochondria in wound healing. With regards to this, current literature has demonstrated an important role for mitochondria in several areas of wound healing and chronic wound pathogenesis including metabolism, apoptosis, and redox signalling. Additionally, the influence of mitochondrial dynamics and mitophagy has also been investigated. However, few studies have utilised patient tissue when studying mitochondria in wound healing, instead using various animal models. In this review we dissect the current knowledge of the role of mitochondria in wound healing and discuss how future research can potentially aid in the progression of wound healing research.

Long-term NAD+ supplementation prevents the progression of age-related hearing loss in mice

Age-related hearing loss (ARHL) is the most common sensory disability associated with human aging. Yet, there are no approved measures for preventing or treating this debilitating condition. With its slow progression, continuous and safe approaches are critical for ARHL treatment. Nicotinamide Riboside (NR), a NAD+ precursor, is well tolerated even for long-term use and is already shown effective in various disease models including Alzheimer's and Parkinson's disease. It has also been beneficial against noise-induced hearing loss and in hearing loss associated with premature aging. However, its beneficial impact on ARHL is not known. Using two different wild-type mouse strains, we show that long-term NR administration prevents the progression of ARHL. Through transcriptomic and biochemical analysis, we find that NR administration restores age-associated reduction in cochlear NAD+ levels, upregulates biological pathways associated with synaptic transmission and PPAR signaling, and reduces the number of orphan ribbon synapses between afferent auditory neurons and inner hair cells. We also find that NR targets a novel pathway of lipid droplets in the cochlea by inducing the expression of CIDEC and PLIN1 proteins that are downstream of PPAR signaling and are key for lipid droplet growth. Taken together, our results demonstrate the therapeutic potential of NR treatment for ARHL and provide novel insights into its mechanism of action.

The mitophagy pathway and its implications in human diseases

Mitochondria are dynamic organelles with multiple functions. They participate in necrotic cell death and programmed apoptotic, and are crucial for cell metabolism and survival. Mitophagy serves as a cytoprotective mechanism to remove superfluous or dysfunctional mitochondria and maintain mitochondrial fine-tuning numbers to balance intracellular homeostasis. Growing evidences show that mitophagy, as an acute tissue stress response, plays an important role in maintaining the health of the mitochondrial network. Since the timely removal of abnormal mitochondria is essential for cell survival, cells have evolved a variety of mitophagy pathways to ensure that mitophagy can be activated in time under various environments. A better understanding of the mechanism of mitophagy in various diseases is crucial for the treatment of diseases and therapeutic target design. In this review, we summarize the molecular mechanisms of mitophagy-mediated mitochondrial elimination, how mitophagy maintains mitochondrial homeostasis at the system levels and organ, and what alterations in mitophagy are related to the development of diseases, including neurological, cardiovascular, pulmonary, hepatic, renal disease, etc., in recent advances. Finally, we summarize the potential clinical applications and outline the conditions for mitophagy regulators to enter clinical trials. Research advances in signaling transduction of mitophagy will have an important role in developing new therapeutic strategies for precision medicine.

Cockayne syndrome group A protein localizes at centrosomes during mitosis and regulates Cyclin B1 ubiquitination

Mutations in CSA and CSB proteins cause Cockayne syndrome, a rare genetic neurodevelopment disorder. Alongside their demonstrated roles in DNA repair and transcription, these two proteins have recently been discovered to regulate cytokinesis, the final stage of the cell division. This last finding allowed, for the first time, to highlight an extranuclear localization of CS proteins, beyond the one already known at mitochondria. In this study, we demonstrated an additional role for CSA protein being recruited at centrosomes in a strictly determined step of mitosis, which ranges from pro-metaphase until metaphase exit. Centrosomal CSA exerts its function in specifically targeting the pool of centrosomal Cyclin B1 for ubiquitination and proteasomal degradation. Interestingly, a lack of CSA recruitment at centrosomes does not affect Cyclin B1 centrosomal localization but, instead, it causes its lasting centrosomal permanence, thus inducing Caspase 3 activation and apoptosis. The discovery of this unveiled before CSA recruitment at centrosomes opens a new and promising scenario for the understanding of some of the complex and different clinical aspects of Cockayne Syndrome.

High-Dosage NMN Promotes Ferroptosis to Suppress Lung Adenocarcinoma Growth through the NAM-Mediated SIRT1-AMPK-ACC Pathway

BACKGROUND

Nicotinamide mononucleotide (NMN) is the physiological circulating NAD precursor thought to elevate the cellular level of NAD⁺ and to ameliorate various age-related diseases. An inseparable link exists between aging and tumorigenesis, especially involving aberrant energetic metabolism and cell fate regulation in cancer cells. However, few studies have directly investigated the effects of NMN on another major ageing-related disease: tumors.

METHODS

We conducted a series of cell and mouse models to evaluate the anti-tumor effect of high-dose NMN. Transmission electron microscopy and a Mito-FerroGreen-labeled immunofluorescence assay (Fe²⁺) were utilized to demonstrate ferroptosis. The metabolites of NAM were detected via ELISA. The expression of the proteins involved in the SIRT1-AMPK-ACC signaling were detected using a Western blot assay.

RESULTS

The results showed that high-dose NMN inhibits lung adenocarcinoma growth in vitro and in vivo. Excess NAM is produced through the metabolism of high-dose NMN, whereas the overexpression of NAMPT significantly decreases intracellular NAM content, which, in turn, boosts cell proliferation. Mechanistically, high-dose NMN promotes ferroptosis through NAM-mediated SIRT1-AMPK-ACC signaling.

CONCLUSIONS

This study highlights the tumor influence of NMN at high doses in the manipulation of cancer cell metabolism, providing a new perspective on clinical therapy in patients with lung adenocarcinoma.

DNA Damage and Repair in Eye Diseases

Vision is vital for daily activities, and yet the most common eye diseases-cataracts, DR, ARMD, and glaucoma-lead to blindness in aging eyes. Cataract surgery is one of the most frequently performed surgeries, and the outcome is typically excellent if there is no concomitant pathology present in the visual pathway. In contrast, patients with DR, ARMD and glaucoma often develop significant visual impairment. These often-multifactorial eye problems can have genetic and hereditary components, with recent data supporting the role of DNA damage and repair as significant pathogenic factors. In this article, we discuss the role of DNA damage and the repair deficit in the development of DR, ARMD and glaucoma.

Approaches to Mitigate Mitochondrial Dysfunction in Sensorineural Hearing Loss

Mitochondria are highly dynamic multifaceted organelles with various functions including cellular energy metabolism, reactive oxygen species (ROS) generation, calcium homeostasis, and apoptosis. Because of these diverse functions, mitochondria are key regulators of cell survival and death, and their dysfunction is implicated in numerous diseases, particularly neurodegenerative disorders such as Alzheimer's Disease, Parkinson's Disease, and Huntington's Disease. One of the most common neurodegenerative disorders is sensorineural hearing loss (SNHL). SNHL primarily originates from the degenerative changes in the cochlea, which is the auditory portion of the inner ear. Many cochlear cells contain an abundance of mitochondria and are metabolically highly active, rendering them susceptible to mitochondrial dysfunction. Indeed, the causal role of mitochondrial dysfunction in SNHL progression is well established, and therefore, targeted for treatment. In this review, we aim to compile the emerging findings in the literature indicating the role of mitochondrial dysfunction in the progression of sensorineural hearing loss and highlight potential therapeutics targeting mitochondrial dysfunction for hearing loss treatment.

Reactive Species in Progeroid Syndromes and Aging-Related Processes

Significance: Reactive species have been classically considered causative of age-related degenerative processes, but the scenario appears considerably more complex and to some extent counterintuitive than originally anticipated. The impact of reactive species in precocious aging syndromes is revealing new clues to understand and perhaps challenge the resulting degenerative processes. Recent Advances: Our understanding of reactive species has considerably evolved, including their hormetic effect (beneficial at a certain level, harmful beyond this level), the occurrence of diverse hormetic peaks in different cell types and organisms, and the extended type of reactive species that are relevant in biological processes. Our understanding of the impact of reactive species has also expanded from the dichotomic damaging/signaling role to modulation of gene expression. Critical Issues: These new concepts are affecting the study of aging and diseases where aging is greatly accelerated. We discuss how notions arising from the study of the underlying mechanisms of a progeroid disease, Cockayne syndrome, represent a paradigm shift that may shed a new light in understanding the role of reactive species in age-related degenerative processes. Future Issues: Future investigations urge to explore established and emerging notions to elucidate the multiple contributions of reactive species in degenerative processes linked to pathophysiological aging and their possible amelioration. Antioxid. Redox Signal. 37, 208-228.

Effects of Oxygen Tension for Membrane Lipidome Remodeling of Cockayne Syndrome Cell Models

Oxygen is important for lipid metabolism, being involved in both enzymatic transformations and oxidative reactivity, and is particularly influent when genetic diseases impair the repair machinery of the cells, such as described for Cockayne syndrome (CS). We used two cellular models of transformed fibroblasts defective for CSA and CSB genes and their normal counterparts, grown for 24 h under various oxygen tensions (hyperoxic 21%, physioxic 5% and hypoxic 1%) to examine the fatty acid-based membrane remodeling by GC analysis of fatty acid methyl esters derived from membrane phospholipids. Overall, we first distinguished differences due to oxygen tensions: (a) hyperoxia induced a general boost of desaturase enzymatic activity in both normal and defective CSA and CSB cell lines, increasing monounsaturated fatty acids (MUFA), whereas polyunsaturated fatty acids (PUFA) did not undergo oxidative consumption; (b) hypoxia slowed down desaturase activities, mostly in CSA cell lines and defective CSB, causing saturated fatty acids (SFA) to increase, whereas PUFA levels diminished, suggesting their involvement in hypoxia-related signaling. CSB-deprived cells are the most sensitive to oxidation and CSA-deprived cells are the most sensitive to the radical-based formation of trans fatty acids (TFA). The results point to the need to finely differentiate biological targets connected to genetic impairments and, consequently, suggest the better definition of cell protection and treatments through accurate molecular profiling that includes membrane lipidomes.

Mitochondrial DNA damage as driver of cellular outcomes

Mitochondria are primarily involved in energy production through the process of oxidative phosphorylation (OXPHOS). Increasing evidence has shown that mitochondrial function impacts a plethora of different cellular activities, including metabolism, epigenetics, and innate immunity. Like the nucleus, mitochondria own their genetic material, but this organellar genome is circular, present in multiple copies, and maternally inherited. The mitochondrial DNA (mtDNA) encodes 37 genes that are solely involved in OXPHOS. Maintenance of mtDNA, through replication and repair, requires the import of nuclear DNA-encoded proteins. Thus, mitochondria completely rely on the nucleus to prevent mitochondrial genetic alterations. As most cells contain hundreds to thousands of mitochondria, it follows that the shear number of organelles allows for the buffering of dysfunction-at least to some extent-before tissue homeostasis becomes impaired. Only red blood cells lack mitochondria entirely. Impaired mitochondrial function is a hallmark of aging and is involved in a number of different disorders, including neurodegenerative diseases, diabetes, cancer, and autoimmunity. Although alterations in mitochondrial processes unrelated to OXPHOS, such as fusion and fission, contribute to aging and disease, maintenance of mtDNA integrity is critical for proper organellar function. Here, we focus on how mtDNA damage contributes to cellular dysfunction and health outcomes.

Cellular fractionation reveals transcriptome responses of human fibroblasts to UV-C irradiation

While cells activate a multifaceted DNA damage response to remove transcription-blocking DNA lesions, mechanisms to regulate genome-wide reduction of RNA synthesis and the paradoxical continuous loading of RNAP II at initiation sites are still poorly understood. Uncovering how dramatic changes to the transcriptional program contribute to TC-NER (transcription-coupled nucleotide excision repair) is important in DNA repair research. However, the functional significance of transcriptome dynamics and the mechanisms of chromatin attachment for thousands of unstudied human lncRNAs remain unclear. To address these questions, we examined UV-induced gene expression regulation in human fibroblasts by performing RNA-seq with fractionated chromatin-associated and cytoplasmic transcripts. This approach allowed us to separate the synthesis of nascent transcripts from the accumulation of mature RNAs. In addition to documenting the subcellular locations of coding transcripts, our results also provide a high-resolution view of the transcription activities of noncoding RNAs in response to cellular stress. At the same time, the data showed that vast majority of genes exhibit large changes in chromatin-associated nascent transcripts without corresponding changes in cytoplasmic mRNA levels. Distinct from protein-coding genes that transcripts with shorter length prefer to be recovered first, repression of lncRNA transcription after UV exposure is inactivated first on noncoding transcripts with longer length. This work provides an updated framework for cellular RNA organization in response to stress and may provide useful information in understanding how cells respond to transcription-blocking DNA damage.

Neurovascular dysfunction and neuroinflammation in a Cockayne syndrome mouse model

Cockayne syndrome (CS) is a rare, autosomal genetic disorder characterized by premature aging-like features, such as cachectic dwarfism, retinal atrophy, and progressive neurodegeneration. The molecular defect in CS lies in genes associated with the transcription-coupled branch of the nucleotide excision DNA repair (NER) pathway, though it is not yet clear how DNA repair deficiency leads to the multiorgan dysfunction symptoms of CS. In this work, we used a mouse model of severe CS with complete loss of NER (Csa-/-/Xpa-/-), which recapitulates several CS-related phenotypes, resulting in premature death of these mice at approximately 20 weeks of age. Although this CS model exhibits a severe progeroid phenotype, we found no evidence of in vitro endothelial cell dysfunction, as assessed by measuring population doubling time, migration capacity, and ICAM-1 expression. Furthermore, aortas from CX mice did not exhibit early senescence nor reduced angiogenesis capacity. Despite these observations, CX mice presented blood brain barrier disruption and increased senescence of brain endothelial cells. This was accompanied by an upregulation of inflammatory markers in the brains of CX mice, such as ICAM-1, TNFα, p-p65, and glial cell activation. Inhibition of neovascularization did not exacerbate neither astro- nor microgliosis, suggesting that the pro-inflammatory phenotype is independent of the neurovascular dysfunction present in CX mice. These findings have implications for the etiology of this disease and could contribute to the study of novel therapeutic targets for treating Cockayne syndrome patients.

Preclinical and clinical evidence of NAD+ precursors in health, disease, and ageing

NAD⁺ is a fundamental molecule in human life and health as it participates in energy metabolism, cell signalling, mitochondrial homeostasis, and in dictating cell survival or death. Emerging evidence from preclinical and human studies indicates an age-dependent reduction of cellular NAD⁺, possibly due to reduced synthesis and increased consumption. In preclinical models, NAD⁺ repletion extends healthspan and / or lifespan and mitigates several conditions, such as premature ageing diseases and neurodegenerative diseases. These findings suggest that NAD⁺ replenishment through NAD⁺ precursors has great potential as a therapeutic target for ageing and age-predisposed diseases, such as Alzheimer's disease. Here, we provide an updated review on the biological activity, safety, and possible side effects of NAD⁺ precursors in preclinical and clinical studies. Major NAD⁺ precursors focused on by this review are nicotinamide riboside (NR), nicotinamide mononucleotide (NMN), and the new discovered dihydronicotinamide riboside (NRH). In summary, NAD⁺ precursors have an exciting therapeutic potential for ageing, metabolic and neurodegenerative diseases.

DRP1 Inhibition Rescues Mitochondrial Integrity and Excessive Apoptosis in CS-A Disease Cell Models

Cockayne syndrome group A (CS-A) is a rare recessive progeroid disorder characterized by sun sensitivity and neurodevelopmental abnormalities. Cells derived from CS-A patients present as pathological hallmarks excessive oxidative stress, mitochondrial fragmentation and apoptosis associated with hyperactivation of the mitochondrial fission dynamin related protein 1 (DRP1). In this study, by using human cell models we further investigated the interplay between DRP1 and CSA and we determined whether pharmacological or genetic inhibition of DRP1 affects disease progression. Both reactive oxygen and nitrogen species are in excess in CS-A cells and when the mitochondrial translocation of DRP1 is inhibited a reduction of these species is observed together with a recovery of mitochondrial integrity and a significant decrease of apoptosis. This study indicates that the CSA-driven modulation of DRP1 pathway is key to control mitochondrial homeostasis and apoptosis and suggests DRP1 as a potential target in the treatment of CS patients.

The interplay between mitochondrial functionality and genome integrity in the prevention of human neurologic diseases

As mitochondria are vulnerable to oxidative damage and represent the main source of reactive oxygen species (ROS), they are considered key tuners of ROS metabolism and buffering, whose dysfunction can progressively impact neuronal networks and disease. Defects in DNA repair and DNA damage response (DDR) may also affect neuronal health and lead to neuropathology. A number of congenital DNA repair and DDR defective syndromes, indeed, show neurological phenotypes, and a growing body of evidence indicate that defects in the mechanisms that control genome stability in neurons acts as aging-related modifiers of common neurodegenerative diseases such as Alzheimer, Parkinson's, Huntington diseases and Amyotrophic Lateral Sclerosis. In this review we elaborate on the established principles and recent concepts supporting the hypothesis that deficiencies in either DNA repair or DDR might contribute to neurodegeneration via mechanisms involving mitochondrial dysfunction/deranged metabolism.

DNA Damage-Induced Neurodegeneration in Accelerated Ageing and Alzheimer's Disease

DNA repair ensures genomic stability to achieve healthy ageing, including cognitive maintenance. Mutations on genes encoding key DNA repair proteins can lead to diseases with accelerated ageing phenotypes. Some of these diseases are xeroderma pigmentosum group A (XPA, caused by mutation of XPA), Cockayne syndrome group A and group B (CSA, CSB, and are caused by mutations of CSA and CSB, respectively), ataxia-telangiectasia (A-T, caused by mutation of ATM), and Werner syndrome (WS, with most cases caused by mutations in WRN). Except for WS, a common trait of the aforementioned progerias is neurodegeneration. Evidence from studies using animal models and patient tissues suggests that the associated DNA repair deficiencies lead to depletion of cellular nicotinamide adenine dinucleotide (NAD⁺), resulting in impaired mitophagy, accumulation of damaged mitochondria, metabolic derailment, energy deprivation, and finally leading to neuronal dysfunction and loss. Intriguingly, these features are also observed in Alzheimer’s disease (AD), the most common type of dementia affecting more than 50 million individuals worldwide. Further studies on the mechanisms of the DNA repair deficient premature ageing diseases will help to unveil the mystery of ageing and may provide novel therapeutic strategies for AD.

Nucleotide Excision Repair: From Molecular Defects to Neurological Abnormalities

Nucleotide excision repair (NER) is the most versatile DNA repair pathway, which can remove diverse bulky DNA lesions destabilizing a DNA duplex. NER defects cause several autosomal recessive genetic disorders. Xeroderma pigmentosum (XP) is one of the NER-associated syndromes characterized by low efficiency of the removal of bulky DNA adducts generated by ultraviolet radiation. XP patients have extremely high ultraviolet-light sensitivity of sun-exposed tissues, often resulting in multiple skin and eye cancers. Some XP patients develop characteristic neurodegeneration that is believed to derive from their inability to repair neuronal DNA damaged by endogenous metabolites. A specific class of oxidatively induced DNA lesions, 8,5′-cyclopurine-2′-deoxynucleosides, is considered endogenous DNA lesions mainly responsible for neurological problems in XP. Growing evidence suggests that XP is accompanied by defective mitophagy, as in primary mitochondrial disorders. Moreover, NER pathway is absent in mitochondria, implying that the mitochondrial dysfunction is secondary to nuclear NER defects. In this review, we discuss the current understanding of the NER molecular mechanism and focuses on the NER linkage with the neurological degeneration in patients with XP. We also present recent research advances regarding NER involvement in oxidative DNA lesion repair. Finally, we highlight how mitochondrial dysfunction may be associated with XP.

The Mitochondrial Response to DNA Damage

Mitochondria are double membrane organelles in eukaryotic cells that provide energy by generating adenosine triphosphate (ATP) through oxidative phosphorylation. They are crucial to many aspects of cellular metabolism. Mitochondria contain their own DNA that encodes for essential proteins involved in the execution of normal mitochondrial functions. Compared with nuclear DNA, the mitochondrial DNA (mtDNA) is more prone to be affected by DNA damaging agents, and accumulated DNA damages may cause mitochondrial dysfunction and drive the pathogenesis of a variety of human diseases, including neurodegenerative disorders and cancer. Therefore, understanding better how mtDNA damages are repaired will facilitate developing therapeutic strategies. In this review, we focus on our current understanding of the mtDNA repair system. We also discuss other mitochondrial events promoted by excessive DNA damages and inefficient DNA repair, such as mitochondrial fusion, fission, and mitophagy, which serve as quality control events for clearing damaged mtDNA.

Current and emerging roles of Cockayne syndrome group B (CSB) protein

Cockayne syndrome (CS) is a segmental premature aging syndrome caused primarily by defects in the CSA or CSB genes. In addition to premature aging, CS patients typically exhibit microcephaly, progressive mental and sensorial retardation and cutaneous photosensitivity. Defects in the CSB gene were initially thought to primarily impair transcription-coupled nucleotide excision repair (TC-NER), predicting a relatively consistent phenotype among CS patients. In contrast, the phenotypes of CS patients are pleiotropic and variable. The latter is consistent with recent work that implicates CSB in multiple cellular systems and pathways, including DNA base excision repair, interstrand cross-link repair, transcription, chromatin remodeling, RNAPII processing, nucleolin regulation, rDNA transcription, redox homeostasis, and mitochondrial function. The discovery of additional functions for CSB could potentially explain the many clinical phenotypes of CSB patients. This review focuses on the diverse roles played by CSB in cellular pathways that enhance genome stability, providing insight into the molecular features of this complex premature aging disease.

Transcription-Coupled DNA Repair: From Mechanism to Human Disorder

DNA lesions pose a major obstacle during gene transcription by RNA polymerase II (RNAPII) enzymes. The transcription-coupled DNA repair (TCR) pathway eliminates such DNA lesions. Inherited defects in TCR cause severe clinical syndromes, including Cockayne syndrome (CS). The molecular mechanism of TCR and the molecular origin of CS have long remained enigmatic. Here we explore new advances in our understanding of how TCR complexes assemble through cooperative interactions between repair factors stimulated by RNAPII ubiquitylation. Mounting evidence suggests that RNAPII ubiquitylation activates TCR complex assembly during repair and, in parallel, promotes processing and degradation of RNAPII to prevent prolonged stalling. The fate of stalled RNAPII is therefore emerging as a crucial link between TCR and associated human diseases.

Nicotinamide Mononucleotide Alleviates Hyperosmolarity-Induced IL-17a Secretion and Macrophage Activation in Corneal Epithelial Cells/Macrophage Co-Culture System

Background

Hyperosmosis stress (HS) was a key pathological factor in the development of dry eye disease (DED). Nicotinamide mononucleotide (NMN) demonstrated protective effects in the corneal damage, however, its role in the HS-induced DED remained unclear.

Methods

A NaCl based HS in-vitro model (500 mOsm) was generated and used in a co-culture system including corneal epithelial cells (CEC) and macrophage cell line RAW264.7. The effect of NMN on NAD+ metabolism and the expression of HS biomarker, tonicity-responsive element binding protein (TonEBP), was studied in the CEC. The cellular activity, including cell viability, apoptosis status and lactate dehydrogenase (LDH) release through trypan blue staining, flow cytometry and LDH assay, respectively. The mitochondrial membrane potential (MMP) assay would be conducted using the JC1 kit. The expression of IL-17a were detected using RT-PCR, ELISA and Western blot. After co-culture with the CEC in different group for 24 h, the phagocytosis ability and macrophage polarization were assessed in RAW264.7 cells co-cultured with CEC with or without HS or NMN treatment. Besides, the involvement of Notch pathway in the RAW264.7 would be analyzed. The potential involvement of Sirtuin 1 (SIRT1) and IL-17a in the crosstalk between CEC and macrophage was studied with SIRT1 inhibitor EX 527 and anti-IL-17a monoclonal antibody, respectively.

Results

NMN treatment increased NAD+ concentration and thus improved cell viability, reduced apoptotic rate and decreased the LDH release in HS-treated CEC. Besides, NMN alleviated HS-induced MMP, intracellular ROS and LDH release. Besides, it was confirmed NMN improve SIRT1 function and decreased the HS related IL-17a expression in CEC and then alleviated macrophage phagocytosis ability and M1 polarization based on a CEC-macrophage co-culture system. Moreover, NMN treatment of CEC in the CEC could moderate the subsequent macrophage activation through Notch pathway. SIRT1 activation and IL-17a inhibition was regarded as key progress in the function of NMN based on the application of EX 527 and anti-IL-17a antibody in the CEC-macrophage co-culture system.

Conclusion

The findings demonstrated that NMN could alleviated HS-induced DED status through regulating the CEC/macrophage interaction. Our data pointed to the role of SIRT1, IL-17a and Notch pathway in the function of NMN and then provided updated knowledge of potential NMN application in the management of DED.