Loss of NAMPT in aging retinal pigment epithelium reduces NAD+ availability and promotes cellular senescence

Integrating Multi-omics to Identify Age-Related Macular Degeneration Subtypes and Biomarkers

Age-related macular degeneration (AMD) is one of the most common causes of irreversible vision loss in the elderly. Its pathogenesis is likely multifactorial, involving a complex interaction of metabolic and environmental factors, and remains poorly understood. Previous studies have shown that mitochondrial dysfunction and oxidative stress play a crucial role in the development of AMD. Oxidative damage to the retinal pigment epithelium (RPE) has been identified as one of the major mediators in the pathogenesis of age-related macular degeneration (AMD). Therefore, this article combines transcriptome sequencing (RNA-seq) and single-cell sequencing (scRNA-seq) data to explore the role of mitochondria-related genes (MRGs) in AMD. Firstly, differential expression analysis was performed on the raw RNA-seq data. The intersection of differentially expressed genes (DEGs) and MRGs was performed. This paper proposes a deep subspace nonnegative matrix factorization (DS-NMF) algorithm to perform a multi-layer nonlinear transformation on the intersection of gene expression profiles corresponding to AMD samples. The age of AMD patients is used as prior information at the network's top level to change the data distribution. The classification is based on reconstructed data with altered distribution. The types obtained significantly differ in scores of multiple immune-related pathways and immune cell infiltration abundance. Secondly, an optimal AMD diagnosis model was constructed using multiple machine learning algorithms for external and qRT-PCR verification. Finally, ten potential therapeutic drugs for AMD were identified based on cMAP analysis. The AMD subtypes identified in this article and the diagnostic model constructed can provide a reference for treating AMD and discovering new drug targets.

Aging of the eye: Lessons from cataracts and age-related macular degeneration

Aging is the greatest risk factor for chronic human diseases, including many eye diseases. Geroscience aims to understand the effects of the aging process on these diseases, including the genetic, molecular, and cellular mechanisms that underlie the increased risk of disease over the lifetime. Understanding of the aging eye increases general knowledge of the cellular physiology impacted by aging processes at various biological extremes. Two major diseases, age-related cataract and age-related macular degeneration (AMD) are caused by dysfunction of the lens and retina, respectively. Lens transparency and light refraction are mediated by lens fiber cells lacking nuclei and other organelles, which provides a unique opportunity to study a single aging hallmark, i.e., loss of proteostasis, within an environment of limited metabolism. In AMD, local dysfunction of the photoreceptors/retinal pigmented epithelium/Bruch's membrane/choriocapillaris complex in the macula leads to the loss of photoreceptors and eventually loss of central vision, and is driven by nearly all the hallmarks of aging and shares features with Alzheimer's disease, Parkinson's disease, cardiovascular disease, and diabetes. The aging eye can function as a model for studying basic mechanisms of aging and, vice versa, well-defined hallmarks of aging can be used as tools to understand age-related eye disease.

KLF4 inhibited the senescence-associated secretory phenotype in ox-LDL-treated endothelial cells via PDGFRA/NAMPT/mitochondrial ROS

BACKGROUND

Inflammation is one of the significant consequences of ox-LDL-induced endothelial cell (EC) dysfunction. The senescence-associated secretory phenotype (SASP) is a critical source of inflammation factors. However, the molecular mechanism by which the SASP is regulated in ECs under ox-LDL conditions remains unknown.

RESULTS

The level of SASP was increased in ox-LDL-treated ECs, which could be augmented by KLF4 knockdown whereas restored by KLF4 knock-in. Furthermore, we found that KLF4 directly promoted PDGFRA transcription and confirmed the central role of the NAPMT/mitochondrial ROS pathway in KLF4/PDGFRA-mediated inhibition of SASP. Animal experiments showed a higher SASP HFD-fed mice, compared with normal feed (ND)-fed mice, and the endothelium of EC-specific KLF4-/- mice exhibited a higher proportion of SA-β-gal-positive cells and lower PDGFRA/NAMPT expression.

CONCLUSIONS

Our results revealed that KLF4 inhibits the SASP of endothelial cells under ox-LDL conditions through the PDGFRA/NAMPT/mitochondrial ROS.

METHODS

Ox-LDL-treated ECs and HFD-fed mice were used as endothelial senescence models in vitro and in vivo. SA-β-gal stain, detection of SAHF and the expression of inflammatory factors determined SASP and senescence of ECs. The direct interaction of KLF4 and PDGFRA promotor was analyzed by EMSA and fluorescent dual luciferase reporting analysis.

Kaempferol: Paving the path for advanced treatments in aging-related diseases

Aging-related diseases (ARDs) are a major global health concern, and the development of effective therapies is urgently needed. Kaempferol, a flavonoid found in several plants, has emerged as a promising candidate for ameliorating ARDs. This comprehensive review examines Kaempferol's chemical properties, safety profile, and pharmacokinetics, and highlights its potential therapeutic utility against ARDs. Kaempferol's therapeutic potential is underpinned by its distinctive chemical structure, which confers antioxidative and anti-inflammatory properties. Kaempferol counteracts reactive oxygen species (ROS) and modulates crucial cellular pathways, thereby combating oxidative stress and inflammation, hallmarks of ARDs. Kaempferol's low toxicity and wide safety margins, as demonstrated by preclinical and clinical studies, further substantiate its therapeutic potential. Compelling evidence supports Kaempferol's substantial potential in addressing ARDs through several mechanisms, notably anti-inflammatory, antioxidant, and anti-apoptotic actions. Kaempferol exhibits a versatile neuroprotective effect by modulating various proinflammatory signaling pathways, including NF-kB, p38MAPK, AKT, and the β-catenin cascade. Additionally, it hinders the formation and aggregation of beta-amyloid protein and regulates brain-derived neurotrophic factors. In terms of its anticancer potential, kaempferol acts through diverse pathways, inducing apoptosis, arresting the cell cycle at the G2/M phase, suppressing epithelial-mesenchymal transition (EMT)-related markers, and affecting the phosphoinositide 3-kinase/protein kinase B signaling pathways. Subsequent studies should focus on refining dosage regimens, exploring innovative delivery systems, and conducting comprehensive clinical trials to translate these findings into effective therapeutic applications.

The use of a systems approach to increase NAD+ in human participants

Reversal or mitigation against an age-related decline in NAD+ has likely benefits, and this premise has driven academic and commercial endeavour to develop dietary supplements that achieve this outcome. We used a systems-based approach to improve on current supplements by targeting multiple points in the NAD+ salvage pathway. In a double-blind, randomised, crossover trial, the supplement - Nuchido TIME+® (NT) - increased NAD+ concentration in whole blood. This was associated with an increase in SIRT1 and an increase in nicotinamide phosphoribosyltransferase (NAMPT) in peripheral blood mononucleocytes, lower concentrations of pro-inflammatory cytokines in plasma, including a reduction in interleukin 2 (IL2), a reduction in glycated serum protein and a shift in the glycosylation profile of immunoglobulin G (IgG) toward a younger biological age, all of which are likely to promote a healthier ageing trajectory.

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.

Dynamic human retinal pigment epithelium (RPE) and choroid architecture based on single-cell transcriptomic landscape analysis

The retinal pigment epithelium (RPE) and choroid are located behind the human retina and have multiple functions in the human visual system. Knowledge of the RPE and choroid cells and their gene expression profiles are fundamental for understanding retinal disease mechanisms and therapeutic strategies. Here, we sequenced the RNA of about 0.3 million single cells from human RPE and choroids across two regions and seven ages, revealing regional and age differences within the human RPE and choroid. Cell-cell interactions highlight the broad connectivity networks between the RPE and different choroid cell types. Moreover, the transcription factors and their target genes change during aging. The coding of somatic variations increases during aging in the human RPE and choroid at the single-cell level. Moreover, we identified ELN as a candidate for improving RPE degeneration and choroidal structure during aging. The mapping of the molecular architecture of the human RPE and choroid improves our understanding of the human vision support system and offers potential insights into the intervention targets for retinal diseases.

NAD+ metabolism and eye diseases: current status and future directions

Currently, there are no truly effective treatments for a variety of eye diseases, such as glaucoma, age-related macular degeneration (AMD), and inherited retinal degenerations (IRDs). These conditions have a significant impact on patients' quality of life and can be a burden on society. However, these diseases share a common pathological process of NAD+ metabolism disorders. They are either associated with genetically induced primary NAD+ synthase deficiency, decreased NAD+ levels due to aging, or enhanced NAD+ consuming enzyme activity during disease pathology. In this discussion, we explore the role of NAD+ metabolic disorders in the development of associated ocular diseases and the potential advantages and disadvantages of various methods to increase NAD+ levels. It is essential to carefully evaluate the possible adverse effects of these methods and conduct a more comprehensive and objective assessment of their function before considering their use.

Mesenchymal Stem Cell Senescence during Aging:From Mechanisms to Rejuvenation Strategies

In cell transplantation therapy, mesenchymal stem cells(MSCs)are ideal seed cells due to their easy acquisition and cultivation, strong regenerative capacity, multi-directional differentiation abilities, and immunomodulatory effects. Autologous MSCs are better applicable compared with allogeneic MSCs in clinical practice. The elderly are the main population for cell transplantation therapy, but as donor aging, MSCs in the tissue show aging-related changes. When the number of generations of in vitro expansion is increased, MSCs will also exhibit replicative senescence. The quantity and quality of MSCs decline during aging, which limits the efficacy of autologous MSCs transplantation therapy. In this review, we examine the changes in MSC senescence as a result of aging, discuss the progress of research on mechanisms and signalling pathways of MSC senescence, and discuss possible rejuvenation strategies of aged MSCs to combat senescence and enhance the health and therapeutic potential of MSCs.

IKKβ Inhibition Attenuates Epithelial Mesenchymal Transition of Human Stem Cell-Derived Retinal Pigment Epithelium

Epithelial-mesenchymal transition (EMT), which is well known for its role in embryonic development, malignant transformation, and tumor progression, has also been implicated in a variety of retinal diseases, including proliferative vitreoretinopathy (PVR), age-related macular degeneration (AMD), and diabetic retinopathy. EMT of the retinal pigment epithelium (RPE), although important in the pathogenesis of these retinal conditions, is not well understood at the molecular level. We and others have shown that a variety of molecules, including the co-treatment of human stem cell-derived RPE monolayer cultures with transforming growth factor beta (TGF-β) and the inflammatory cytokine tumor necrosis factor alpha (TNF-α), can induce RPE-EMT; however, small molecule inhibitors of RPE-EMT have been less well studied. Here, we demonstrate that BAY651942, a small molecule inhibitor of nuclear factor kapa-B kinase subunit beta (IKKβ) that selectively targets NF-κB signaling, can modulate TGF-β/TNF-α-induced RPE-EMT. Next, we performed RNA-seq studies on BAY651942 treated hRPE monolayers to dissect altered biological pathways and signaling events. Further, we validated the effect of IKKβ inhibition on RPE-EMT-associated factors using a second IKKβ inhibitor, BMS345541, with RPE monolayers derived from an independent stem cell line. Our data highlights the fact that pharmacological inhibition of RPE-EMT restores RPE identity and may provide a promising approach for treating retinal diseases that involve RPE dedifferentiation and EMT.

Interactions between Intestinal Homeostasis and NAD+ Biology in Regulating Incretin Production and Postprandial Glucose Metabolism

The intestine has garnered attention as a target organ for developing new therapies for impaired glucose tolerance. The intestine, which produces incretin hormones, is the central regulator of glucose metabolism. Glucagon-like peptide-1 (GLP-1) production, which determines postprandial glucose levels, is regulated by intestinal homeostasis. Nicotinamide phosphoribosyltransferase (NAMPT)-mediated nicotinamide adenine dinucleotide (NAD⁺) biosynthesis in major metabolic organs such as the liver, adipose tissue, and skeletal muscle plays a crucial role in obesity- and aging-associated organ derangements. Furthermore, NAMPT-mediated NAD⁺ biosynthesis in the intestines and its upstream and downstream mediators, adenosine monophosphate-activated protein kinase (AMPK) and NAD⁺-dependent deacetylase sirtuins (SIRTs), respectively, are critical for intestinal homeostasis, including gut microbiota composition and bile acid metabolism, and GLP-1 production. Thus, boosting the intestinal AMPK-NAMPT-NAD⁺-SIRT pathway to improve intestinal homeostasis, GLP-1 production, and postprandial glucose metabolism has gained significant attention as a novel strategy to improve impaired glucose tolerance. Herein, we aimed to review in detail the regulatory mechanisms and importance of intestinal NAMPT-mediated NAD⁺ biosynthesis in regulating intestinal homeostasis and GLP-1 secretion in obesity and aging. Furthermore, dietary and molecular factors regulating intestinal NAMPT-mediated NAD⁺ biosynthesis were critically explored to facilitate the development of new therapeutic strategies for postprandial glucose dysregulation.

Role of Nuclear NAD+ in Retinal Homeostasis

The retina is one of the most metabolically active tissues and maintenance of metabolic homeostasis is critical for retinal function. Nicotinamide adenine dinucleotide (NAD+) is a cofactor that is required for key processes, including the electron transport chain, glycolysis, fatty acid oxidation, and redox reactions. NAD+ also acts as a co-substrate for enzymes involved in maintaining genomic DNA integrity and cellular homeostasis, including poly-ADP ribose polymerases (PARPs) and Sirtuins. This review highlights the importance of NAD+ in the retina, including the role of enzymes involved in NAD+ production in the retina and how NAD+-consuming enzymes may play a role in disease pathology. We also suggest a cell death pathway that may be common in multiple models of photoreceptor degeneration and highlight the role that NAD+ likely plays in this process. Finally, we explore future experimental approaches to enhance our understanding of the role of NAD+ in the retina.

Aging and aging-related diseases: from molecular mechanisms to interventions and treatments

Aging is a gradual and irreversible pathophysiological process. It presents with declines in tissue and cell functions and significant increases in the risks of various aging-related diseases, including neurodegenerative diseases, cardiovascular diseases, metabolic diseases, musculoskeletal diseases, and immune system diseases. Although the development of modern medicine has promoted human health and greatly extended life expectancy, with the aging of society, a variety of chronic diseases have gradually become the most important causes of disability and death in elderly individuals. Current research on aging focuses on elucidating how various endogenous and exogenous stresses (such as genomic instability, telomere dysfunction, epigenetic alterations, loss of proteostasis, compromise of autophagy, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, altered intercellular communication, deregulated nutrient sensing) participate in the regulation of aging. Furthermore, thorough research on the pathogenesis of aging to identify interventions that promote health and longevity (such as caloric restriction, microbiota transplantation, and nutritional intervention) and clinical treatment methods for aging-related diseases (depletion of senescent cells, stem cell therapy, antioxidative and anti-inflammatory treatments, and hormone replacement therapy) could decrease the incidence and development of aging-related diseases and in turn promote healthy aging and longevity.

An Altered Neurovascular System in Aging-Related Eye Diseases

The eye has a complex and metabolically active neurovascular system. Repeated light injuries induce aging and trigger age-dependent eye diseases. Damage to blood vessels is related to the disruption of the blood-retinal barrier (BRB), altered cellular communication, disrupted mitochondrial functions, and exacerbated aggregated protein accumulation. Vascular complications, such as insufficient blood supply and BRB disruption, have been suggested to play a role in glaucoma, age-related macular degeneration (AMD), and Alzheimer's disease (AD), resulting in neuronal cell death. Neuronal loss can induce vision loss. In this review, we discuss the importance of the neurovascular system in the eye, especially in aging-related diseases such as glaucoma, AMD, and AD. Beneficial molecular pathways to prevent or slow down retinal pathologic processes will also be discussed.

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.

Dysfunctional peroxisomal lipid metabolisms and their ocular manifestations

Retina is rich in lipids and dyslipidemia causes retinal dysfunction and eye diseases. In retina, lipids are not only important membrane component in cells and organelles but also fuel substrates for energy production. However, our current knowledge of lipid processing in the retina are very limited. Peroxisomes play a critical role in lipid homeostasis and genetic disorders with peroxisomal dysfunction have different types of ocular complications. In this review, we focus on the role of peroxisomes in lipid metabolism, including degradation and detoxification of very-long-chain fatty acids, branched-chain fatty acids, dicarboxylic acids, reactive oxygen/nitrogen species, glyoxylate, and amino acids, as well as biosynthesis of docosahexaenoic acid, plasmalogen and bile acids. We also discuss the potential contributions of peroxisomal pathways to eye health and summarize the reported cases of ocular symptoms in patients with peroxisomal disorders, corresponding to each disrupted peroxisomal pathway. We also review the cross-talk between peroxisomes and other organelles such as lysosomes, endoplasmic reticulum and mitochondria.

Current Uncertainties and Future Challenges Regarding NAD+ Boosting Strategies

Precursors of nicotinamide adenine dinucleotide (NAD+), modulators of enzymes of the NAD+ biosynthesis pathways and inhibitors of NAD+ consuming enzymes, are the main boosters of NAD+. Increasing public awareness and interest in anti-ageing strategies and health-promoting lifestyles have grown the interest in the use of NAD+ boosters as dietary supplements, both in scientific circles and among the general population. Here, we discuss the current trends in NAD+ precursor usage as well as the uncertainties in dosage, timing, safety, and side effects. There are many unknowns regarding pharmacokinetics and pharmacodynamics, particularly bioavailability, metabolism, and tissue specificity of NAD+ boosters. Given the lack of long-term safety studies, there is a need for more clinical trials to determine the proper dose of NAD+ boosters and treatment duration for aging prevention and as disease therapy. Further research will also need to address the long-term consequences of increased NAD+ and the best approaches and combinations to increase NAD+ levels. The answers to the above questions will contribute to the more efficient and safer use of NAD+ boosters.

Nicotinamide Mononucleotide Ameliorates Cellular Senescence and Inflammation Caused by Sodium Iodate in RPE

Senescent cells have been demonstrated to have lower cellular NAD⁺ levels and are involved in the development of various age-related diseases, including age-related macular degeneration (AMD). Sodium iodate (NaIO₃) has been primarily used as an oxidant to establish a model of dry AMD. Results of previous studies have showed that NaIO₃ induced retinal tissue senescence in vivo. However, the role of NaIO₃ and the mechanism by which it induces retinal pigment epithelium (RPE) senescence remains unknown. In this study, RPE cell senescence was confirmed to be potentially induced by NaIO₃. The results showed that the number of senescence-associated-β-galactosidase (SA-β-gal-)-positive cells and the protein levels of p16 and p21 increased after NaIO₃ treatment. Additionally, the senescent RPE cells underwent oxidative stress and NAD⁺ depletion. Furthermore, significant DNA damage and mitochondrial dysfunction were also detected in senescent RPE cells. The antioxidant N-acetylcysteine (NAC) could alleviate cellular senescence only by a minimal degree, whereas supplementation with nicotinamide mononucleotide (NMN) strongly ameliorated RPE senescence through the alleviation of DNA damage and the maintenance of mitochondrial function. The protective effects of NMN were demonstrated to rely on undisturbed Sirt1 signaling. Moreover, both the expression of senescence markers of RPE and subretinal inflammatory cell infiltration were decreased by NMN treatment in vivo. Our results indicate that RPE senescence induced by NaIO₃ acquired several key features of AMD. More importantly, NMN may potentially be used to treat RPE senescence and senescence-associated pre-AMD changes by restoring the NAD⁺ levels in cells and tissues.

NAMPT mitigates colitis severity by supporting redox-sensitive activation of phagocytosis in inflammatory macrophages

Nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme in the nicotinamide adenine dinucleotide (NAD+) salvage pathway and plays a crucial role in the maintenance of the NAD+ pool during inflammation. Considering that macrophages are essential for tissue homeostasis and inflammation, we sought to examine the functional impact of NAMPT in inflammatory macrophages, particularly in the context of inflammatory bowel disease (IBD). In this study, we show that mice with NAMPT deletion within the myeloid compartment (Namptf/fLysMCre+/-, Nampt mKO) have more pronounced colitis with lower survival rates, as well as numerous uncleared apoptotic corpses within the mucosal layer. Nampt-deficient macrophages exhibit reduced phagocytic activity due to insufficient NAD+ abundance, which is required to produce NADPH for the oxidative burst. Nicotinamide mononucleotide (NMN) treatment rescues NADPH levels in Nampt mKO macrophages and sustains superoxide generation via NADPH oxidase. Consequently, Nampt mKO mice fail to clear dead cells during tissue repair, leading to substantially prolonged chronic colitis. Moreover, systemic administration of NMN, to supply NAD+, effectively suppresses the disease severity of DSS-induced colitis. Collectively, our findings suggest that activation of the NAMPT-dependent NAD+ biosynthetic pathway, via NMN administration, is a potential therapeutic strategy for managing inflammatory diseases.

The CD38 glycohydrolase and the NAD sink: implications for pathological conditions

Nicotinamide adenine dinucleotide (NAD) acts as a cofactor in several oxidation-reduction (redox) reactions and is a substrate for a number of nonredox enzymes. NAD is fundamental to a variety of cellular processes including energy metabolism, cell signaling, and epigenetics. NAD homeostasis appears to be of paramount importance to health span and longevity, and its dysregulation is associated with multiple diseases. NAD metabolism is dynamic and maintained by synthesis and degradation. The enzyme CD38, one of the main NAD-consuming enzymes, is a key component of NAD homeostasis. The majority of CD38 is localized in the plasma membrane with its catalytic domain facing the extracellular environment, likely for the purpose of controlling systemic levels of NAD. Several cell types express CD38, but its expression predominates on endothelial cells and immune cells capable of infiltrating organs and tissues. Here we review potential roles of CD38 in health and disease and postulate ways in which CD38 dysregulation causes changes in NAD homeostasis and contributes to the pathophysiology of multiple conditions. Indeed, in animal models the development of infectious diseases, autoimmune disorders, fibrosis, metabolic diseases, and age-associated diseases including cancer, heart disease, and neurodegeneration are associated with altered CD38 enzymatic activity. Many of these conditions are modified in CD38-deficient mice or by blocking CD38 NADase activity. In diseases in which CD38 appears to play a role, CD38-dependent NAD decline is often a common denominator of pathophysiology. Thus, understanding dysregulation of NAD homeostasis by CD38 may open new avenues for the treatment of human diseases.

Mechanisms of RPE senescence and potential role of αB crystallin peptide as a senolytic agent in experimental AMD

Oxidative stress in the retinal pigment epithelium (RPE) can cause mitochondrial dysfunction and is likely a causative factor in the pathogenesis of age-related macular degeneration (AMD). Under oxidative stress conditions, some of the RPE cells become senescent and a contributory role for RPE senescence in AMD pathology has been proposed. The purpose of this study is to 1) characterize senescence in human RPE; 2) investigate the effect of an αB Crystallin chaperone peptide (mini Cry) in controlling senescence, in particular by regulating mitochondrial function and senescence-associated secretory phenotype (SASP) production and 3) develop mouse models for studying the role of RPE senescence in dry and nAMD. Senescence was induced in human RPE cells in two ways. First, subconfluent cells were treated with 0.2 μg/ml doxorubicin (DOX); second, subconfluent cells were treated with 500 μM H₂O₂. Senescence biomarkers (senescence-associated beta-galactosidase (SA-βgal), p21, p16) and mitochondrial proteins (Fis1, DRP1, MFN2, PGC1-α, mtTFA) were analyzed in control and experimental groups. The effect of mini Cry on mitochondrial bioenergetics, glycolysis and SASP was determined. In vivo, retinal degeneration was induced by intravenous injection of NaIO3 (20 mg/kg) and subretinal fibrosis by laser-induced choroidal neovascularization. Increased SA-βgal staining and p16 and p21 expression was observed after DOX- or H₂O₂-induced senescence and mini Cry significantly decreased senescence-positive cells. The expression of mitochondrial biogenesis proteins PGC-1 and mTFA increased with senescence, and mini Cry reduced expression significantly. Senescent RPE cells were metabolically active, as evidenced by significantly enhanced oxidative phosphorylation and anaerobic glycolysis, mini Cry markedly reduced rates of respiration and glycolysis. Senescent RPE cells maintain a proinflammatory phenotype characterized by significantly increased production of cytokines (IFN-ˠ, TNF-α, IL1-α IL1-β, IL-6, IL-8, IL-10), and VEGF-A; mini Cry significantly inhibited their secretion. We identified and localized senescent RPE cells for the first time in NaIO3-induced retinal degeneration and laser-induced subretinal fibrosis mouse models. We conclude that mini Cry significantly impairs stress-induced senescence by modulating mitochondrial biogenesis and fission proteins in RPE cells. Characterization of senescence could provide further understanding of the metabolic changes that accompany the senescent phenotype in ocular disease. Future studies in vivo may better define the role of senescence in AMD and the therapeutic potential of mini Cry as a senotherapeutic.

Hsp90-associated DNA replication checkpoint protein and proteasome-subunit components are involved in the age-related macular degeneration

Background:

Age-related macular degeneration (AMD) is the leading cause of vision loss worldwide. However, the mechanisms involved in the development and progression of AMD are poorly delineated. We aimed to explore the critical genes involved in the progression of AMD.

Methods:

The differentially expressed genes (DEGs) in AMD retinal pigment epithelial (RPE)/choroid tissues were identified using the microarray datasets GSE99248 and GSE125564, which were downloaded from the gene expression omnibus database. The overlapping DEGs from the two datasets were screened to identify DEG-related biological pathways using gene ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses. The hub genes were identified from these DEGs through protein-protein interaction network analyses. The expression levels of hub genes were evaluated by quantitative real-time polymerase chain reaction following the induction of senescence in ARPE-19 with FK866. Following the identification of AMD-related key genes, the potential small molecule compounds targeting the key genes were predicted by PharmacoDB. Finally, a microRNA-gene interaction network was constructed.

Results:

Microarray analyses identified 174 DEGs in the AMD RPE compared to the healthy RPE samples. These DEGs were primarily enriched in the pathways involved in the regulation of DNA replication, cell cycle, and proteasome-mediated protein polyubiquitination. Among the top ten hub genes, HSP90AA1, CHEK1, PSMA4, PSMD4, and PSMD8 were upregulated in the senescent ARPE-19 cells. Additionally, the drugs targeting HSP90AA1, CHEK1, and PSMA4 were identified. We hypothesize that Hsa-miR-16-5p might target four out of the five key DEGs in the AMD RPE.

Conclusions:

Based on our findings, HSP90AA1 is likely to be a central gene controlling the DNA replication and proteasome-mediated polyubiquitination during the RPE senescence observed in the progression of AMD. Targeting HSP90AA1, CHEK1, PSMA4, PSMD4, and/or PSMD8 genes through specific miRNAs or small molecules might potentially alleviate the progression of AMD through attenuating RPE senescence.

Nicotinamide Adenine Nucleotide-The Fountain of Youth to Prevent Oocyte Aging?

According to the U.S. Special Operations Command (SOCOM), new clinical trials of an anti-aging oral treatment using nicotinamide adenine nucleotide are planned for 2022. All over the globe, the discovery of the fountain of youth is still a great goal to reach, not only among aging researchers, since people desire to stay longer healthy and feel young when reaching old age. Since the 1960s, women delaying pregnancy to pursue higher educational levels and a career path has contributed to drastically diminished overall female fertility rates (e.g., number of born offspring/woman). Consequently, a growing number of advanced-aged women depend on assisted reproductive technologies (ART) to become pregnant. In 2019, the Society for Assisted Reproductive Technology reported 293,672 cycles for oocyte retrieval. This change of demographics influenced women's age of having their first child, which has increased significantly. However, their reproductive tract shows hallmarks of aging very early in life without an effective preventive treatment. Therefore, we will present whether NAD+ could help to prevent oocyte aging.

Dynamic Aging: Channeled Through Microenvironment

Aging process is a complicated process that involves deteriorated performance at multiple levels from cellular dysfunction to organ degeneration. For many years research has been focused on how aging changes things within cell. However, new findings suggest that microenvironments, circulating factors or inter-tissue communications could also play important roles in the dynamic progression of aging. These out-of-cell mechanisms pass on the signals from the damaged aging cells to other healthy cells or tissues to promote systematic aging phenotypes. This review discusses the mechanisms of how senescence and their secretome, NAD⁺ metabolism or circulating factors change microenvironments to regulate systematic aging, as well as the potential therapeutic strategies based on these findings for anti-aging interventions.

Grape Seed Proanthocyanidin Extract Moderated Retinal Pigment Epithelium Cellular Senescence Through NAMPT/SIRT1/NLRP3 Pathway

Background

Retinal pigment epithelium (RPE) cellular senescence is an important process in degenerative retinal disorders. Grape seed proanthocyanidin extract (GSPE) alleviates senescence-related degenerative disorders; however, the potential effects of GSPE intake on RPE cellular senescence through regulating NAMPT/SIRT1/NLRP3 pathway remain unclear.

Methods

The effects of GSPE on NAMPT expression and NAD+ contents were detected with Western blot and assay kit in both in-vivo and in-vitro AMD models. Senescence-related biomarkers, including p16, p21 expressions and β-gal staining, were conducted in different groups. The protective effects of GSPE treatment on the mitochondrial homeostasis and barrier function of RPE cells were detected using mtDNA lesions analyses, JC-1 staining, ZO1 staining and trans-epithelial cell resistance (TEER) detection. The expression of senescence-associated secretory phenotype (SASP) in different groups would be conducted with qPCR. To demonstrate the potential effects of NAMPT/SIRT1/NLRP3 pathway after GSPE treatment, the protein levels of relevant key regulators after applications of NAMPT inhibitor, Fk866, and SIRT1 inhibitor, EX-527.

Results

GSPE significantly improves the NAMPT expression and NAD+ content in aging mice, and thus alleviates the RPE cellular senescence. In advanced in-vitro studies, GSPE significantly up-regulated NAMPT content and thus relieved H₂O₂ induced NAD+ depression through analyzing the NAD+ contents in different groups. In advanced analyses, it was reported that GSPE could alleviate mitochondrial permeability, mtDNA damage, ZO1 expression and SASP levels in aging RPE cells. Thus, GSPE treatment significantly decreased senescence-related protein p16 and p21, as well as SASP levels in in-vitro aging model, and it was demonstrated that GSPE could illustrate a significant anti-aging effect. The Western blot data in GSPE treatment of aging RPE cells demonstrated that GSPE could significantly improve NAMPT and SIRT1 levels, and thus depressed NLRP3 expression.

Conclusion

This study indicated that GSPE alleviated RPE cellular senescence through NAMPT/SIRT1/NLRP3 pathway. This study highlighted the potential effects of GSPE on degenerative retinopathy through the crosstalk of NAD+ metabolism, SIRT1 function and NLRP3 activation.

NAD+ metabolism and retinal degeneration (Review)

The recent years has revealed an intense interest in the study of nicotinamide adenine dinucleotide (NAD+), particularly in regards to its intermediates, such as nicotinamide and nicotinic acid known as niacin, and also nicotinamide riboside. Besides its participation as a coenzyme in the redox transformations of nutrients during catabolism, NAD+ is also involved in DNA repair and epigenetic modification of gene expression and also plays an essential role in calcium homeostasis. Clinical and experimental data emphasize the age-dependent decline in NAD+ levels and its relation with the onset and progression of various age-related diseases. Maintaining optimal levels of NAD+ has aroused a therapeutic interest in such pathological conditions; NAD+ being currently regarded as an important target to extend health and lifespan. Based on a systematic exploration of the experimental data and literature surrounding the topic, this paper reviews some of the recent research studies related to the roles of the pyridine nucleotide family focusing on biosynthesis, NAD+ deficiency-associated diseases, pathobiochemistry related to retinal degeneration and potential therapeutic effects on human vision as well.

Inflammation, epigenetics, and metabolism converge to cell senescence and ageing: the regulation and intervention

Remarkable progress in ageing research has been achieved over the past decades. General perceptions and experimental evidence pinpoint that the decline of physical function often initiates by cell senescence and organ ageing. Epigenetic dynamics and immunometabolic reprogramming link to the alterations of cellular response to intrinsic and extrinsic stimuli, representing current hotspots as they not only (re-)shape the individual cell identity, but also involve in cell fate decision. This review focuses on the present findings and emerging concepts in epigenetic, inflammatory, and metabolic regulations and the consequences of the ageing process. Potential therapeutic interventions targeting cell senescence and regulatory mechanisms, using state-of-the-art techniques are also discussed.

Restoring NAD+ by NAMPT is essential for the SIRT1/p53-mediated survival of UVA- and UVB-irradiated epidermal keratinocytes

Nicotinamide adenine dinucleotide (NAD⁺) is a crucial coenzyme in energy production. The imbalance of NAD⁺ synthesis has been found to trigger age-related diseases, such as metabolic disorders, cancer, and neurodegenerative diseases. Also, UV irradiation induces NAD⁺ depletion in the skin. In mammals, nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme in the NAD⁺ salvage pathway and essential for NAD⁺ homeostasis. However, but few studies have focused on the role of NAMPT in response to UV irradiation. Here, we show that NAMPT prevents NAD⁺ depletion in epidermal keratinocytes to protect against the mild-dose UVA and UVB (UVA/B)-induced proliferation defects. We showed that poly(ADP-ribose) polymerase (PARP) inhibitor rescued the NAD⁺ depletion in UVA/B-irradiated human keratinocytes, confirming that PAPR transiently exhausts cellular NAD⁺ to repair DNA damage. Notably, the treatment with a NAMPT inhibitor exacerbated the UVA/B-induced loss of energy production and cell viability. Moreover, the NAMPT inhibitor abrogated the sirtuin-1 (SIRT1)-mediated deacetylation of p53 and significantly inhibited the proliferation of UVA/B-irradiated cells, suggesting that the NAMPT-NAD⁺-SIRT1 axis regulates p53 functions upon UVA/B stress. The supplementation with NAD⁺ intermediates, nicotinamide mononucleotide and nicotinamide riboside, rescued the UVA/B-induced phenotypes in the absence of NAMPT activity. Therefore, NAD⁺ homeostasis is likely essential for the protection of keratinocytes from UV stress in mild doses. Since the skin is continuously exposed to UVA/B irradiation, understanding the protective role of NAMPT in UV stress will help prevent and treat skin photoaging.

Aging of the Retina: Molecular and Metabolic Turbulences and Potential Interventions

Multifaceted and divergent manifestations across tissues and cell types have curtailed advances in deciphering the cellular events that accompany advanced age and contribute to morbidities and mortalities. Increase in human lifespan during the past century has heightened awareness of the need to prevent age-associated frailty of neuronal and sensory systems to allow a healthy and productive life. In this review, we discuss molecular and physiological attributes of aging of the retina, with a goal of understanding age-related impairment of visual function. We highlight the epigenome-metabolism nexus and proteostasis as key contributors to retinal aging and discuss lifestyle changes as potential modulators of retinal function. Finally, we deliberate promising intervention strategies for promoting healthy aging of the retina for improved vision. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Citrus Alkaline Extracts Inhibit Senescence of A549 Cells to Alleviate Pulmonary Fibrosis via the β-Catenin/P53 Pathway

BACKGROUND Idiopathic pulmonary fibrosis (IPF) is a disease related to aging, which has become increasingly prevalent as the population has aged. However, there remains no effective treatment for the disease. Alveolar epithelial type II cell (AEC II) senescence plays an important role in the occurrence and development of IPF. Therefore, enhancing our understanding of aging AEC IIs might facilitate the development of a new therapeutic strategy for the prevention and treatment of IPF. The aim of this study was to investigate the effect of citrus alkaline extracts (CAE) on senescence in A549 cells and elucidate the mechanism by which CAE function. MATERIAL AND METHODS Adriamycin RD (ARD) induces the senescence of A549 cells. Relevant indicators were identified following administration of 3 concentrations of CAE (50 μg/mL, 100 μg/mL, and 200 μg/mL) to A549 cells. RESULTS CAE inhibited senescence in ARD-induced A549 cells. It inhibited p16, p21, p53, and a senescence-associated secretory phenotype, and reduced expression of the senescence-related positive cells of ß-galactosidase. Further study revealed that activation of the ß-catenin signaling pathway is closely associated with p53. CAE inhibited senescence in A549 cells via the ß-catenin/p53 pathway. Further, inhibition of b-catenin was associated with reduced expression levels of p53 and p21, and the anti-aging effects of CAE were enhanced. When expression of p53 was inhibited, expression levels of ß-catenin also tended to decrease. CONCLUSIONS In summary, our study showed that CAE can inhibit aging in A549 cells to alleviate pulmonary fibrosis, and thus limit the secretion of the extracellular matrix and collagen in lung fibroblasts.

Nicotinamide adenine dinucleotide (NAD+): essential redox metabolite, co-substrate and an anti-cancer and anti-ageing therapeutic target

Nicotinamide adenine dinucleotide (NAD+) and its reduced form NADH are essential coupled redox metabolites that primarily promote cellular oxidative (catabolic) metabolic reactions. This enables energy generation through glycolysis and mitochondrial respiration to support cell growth and survival. In addition, many key enzymes that regulate diverse cell functions ranging from gene expression to proteostasis require NAD+ as a co-substrate for their catalytic activity. This includes the NAD+-dependent sirtuin family of protein deacetylases and the PARP family of DNA repair enzymes. Whilst their vital activity consumes NAD+ which is cleaved to nicotinamide, several pathways exist for re-generating NAD+ and sustaining NAD+ homeostasis. However, there is growing evidence of perturbed NAD+ homeostasis and NAD+-regulated processes contributing to multiple disease states. NAD+ levels decline in the human brain and other organs with age and this is associated with neurodegeneration and other age-related diseases. Dietary supplementation with NAD+ precursors is being investigated to counteract this. Paradoxically, many cancers have increased dependency on NAD+. Clinical efforts to exploit this have so far shown limited success. Emerging new opportunities to exploit dysregulation of NAD+ metabolism in cancers are critically discussed. An update is also provided on other key NAD+ research including perturbation of the NAD+ salvage enzyme NAMPT in the context of the tumour microenvironment (TME), methodology to study subcellular NAD+ dynamics in real-time and the regulation of differentiation by competing NAD+ pools.

Selenomethionine (Se-Met) Induces the Cystine/Glutamate Exchanger SLC7A11 in Cultured Human Retinal Pigment Epithelial (RPE) Cells: Implications for Antioxidant Therapy in Aging Retina

Oxidative damage has been identified as a major causative factor in degenerative diseases of the retina; retinal pigment epithelial (RPE) cells are at high risk. Hence, identifying novel strategies for increasing the antioxidant capacity of RPE cells, the purpose of this study, is important. Specifically, we evaluated the influence of selenium in the form of selenomethionine (Se-Met) in cultured RPE cells on system xc- expression and functional activity and on cellular levels of glutathione, a major cellular antioxidant. ARPE-19 and mouse RPE cells were cultured with and without selenomethionine (Se-Met), the principal form of selenium in the diet. Promoter activity assay, uptake assay, RT-PCR, northern and western blots, and immunofluorescence were used to analyze the expression of xc-, Nrf2, and its target genes. Se-Met activated Nrf2 and induced the expression and function of xc- in RPE. Other target genes of Nrf2 were also induced. System xc- consists of two subunits, and Se-Met induced the subunit responsible for transport activity (SLC7A11). Selenocysteine also induced xc- but with less potency. The effect of Se-met on xc- was associated with an increase in maximal velocity and an increase in substrate affinity. Se-Met increased the cellular levels of glutathione in the control, an oxidatively stressed RPE. The Se-Met effect was selective; under identical conditions, taurine transport was not affected and Na+-coupled glutamate transport was inhibited. This study demonstrates that Se-Met enhances the antioxidant capacity of RPE by inducing the transporter xc- with a consequent increase in glutathione.

Systemic Treatment With Nicotinamide Riboside Is Protective in a Mouse Model of Light-Induced Retinal Degeneration

Purpose

Maintaining levels of nicotinamide adenine dinucleotide (NAD+), a coenzyme critical for cellular energetics and biosynthetic pathways, may be therapeutic in retinal disease because retinal NAD+ levels decline during retinal damage and degeneration. The purpose of this study was to investigate whether systemic treatment with nicotinamide riboside (NR), a NAD+ precursor that is orally deliverable and well-tolerated by humans, is protective in a mouse model of light-induced retinal degeneration.

Methods

Mice were injected intraperitoneally with vehicle or NR the day before and the morning of exposure to degeneration-inducing levels of light. Retinal function was assessed by electroretinography and in vivo retinal morphology and inflammation was assessed by optical coherence tomography. Post mortem retina sections were assessed for morphology, TUNEL, and inflammatory markers Iba1 and GFAP. Retinal NAD+ levels were enzymatically assayed.

Results

Exposure to degeneration-inducing levels of light suppressed retinal NAD+ levels. Mice undergoing light-induced retinal degeneration exhibited significantly suppressed retinal function, severely disrupted photoreceptor cell layers, and increased apoptosis and inflammation in the outer retina. Treatment with NR increased levels of NAD+ in retina and prevented these deleterious outcomes.

Conclusions

This study is the first to report the protective effects of NR treatment in a mouse model of retinal degeneration. The positive outcomes, coupled with human tolerance to NR dosing, suggest that maintaining retinal NAD+ via systemic NR treatment should be further explored for clinical relevance.

Nicotinamide phosphoribosyltransferase postpones rat bone marrow mesenchymal stem cell senescence by mediating NAD+-Sirt1 signaling

In vitro replicative senescence affects MSC characteristics and functionality, thus severely restricting their application in regenerative medicine and MSC-based therapies. Previously, we found that MSC natural senescence is accompanied by altered intracellular nicotinamide adenine dinucleotide (NAD⁺) metabolism, in which Nampt plays a key role. However, whether Nampt influences MSC replicative senescence is still unclear. Our study showed that Nampt expression is down-regulated during MSC replicative senescence. Nampt depletion via a specific Nampt inhibitor FK866 or Nampt knockdown in early passage MSCs led to enhanced senescence as indicated by senescence-like morphology, reduced proliferation, and adipogenic and osteogenic differentiation, and increased senescence-associated-β-galactosidase activity and the expression of the senescence-associated factor p16^INK4a. Conversely, Nampt overexpression ameliorated senescence-associated phenotypic features in late passage MSCs. Further, Nampt inhibition resulted in reduced intracellular NAD⁺ content, NAD⁺/NADH ratio, and Sirt1 activity, whereas overexpression had the opposite effects. Exogenous intermediates involved in NAD⁺ biosynthesis not only rescued replicative senescent MSCs but also alleviated FK866-induced MSC senescence. Thus, Nampt suppresses MSC senescence via mediating NAD⁺-Sirt1 signaling. This study provides novel mechanistic insights into MSC replicative senescence and a promising strategy for the severe shortage of cells for MSC-based therapies.

Lamivudine Inhibits Alu RNA-induced Retinal Pigment Epithelium Degeneration via Anti-inflammatory and Anti-senescence Activities

Purpose

Accumulation of the long noncoding Alu element RNA activates the NLRP3 inflammasome and leads to retinal pigment epithelium (RPE) cell death, a key event in the pathogenesis of geographic atrophy during late-stage age-related macular degeneration. Lamivudine (3TC) is a nucleoside analog reverse transcriptase inhibitor known to inhibit the NLRP3 inflammasome. Currently, the intracellular response of the senescence marker p16^Ink4a to the long noncoding RNA is being actively studied. The present study aimed to assess the efficacy of 3TC against Alu RNA-induced RPE inflammation and senescence by evaluating changes in expression of the proinflammatory cytokines IL-18 and IL-1β and of p16^INK4a in RPE cells.

Methods

Cultured human RPE cells and in vivo mouse RPE cells were transfected with an in vitro-transcribed Alu RNA, and changes in IL-18, IL-1β, and p16^Ink4a expression measured in the presences of 3TC or 3,4-(M)CA as a negative control.

Results

Treatment with 3TC markedly reduced Alu RNA-induced expression of IL-18 and IL-1β in human and mouse RPE cells compared with the negative control. Further, Alu RNA-induced p16^INK4a expression was suppressed by 3TC in human RPE cells.

Conclusions

Our data suggest that Alu RNA accumulation contributes to RPE cell senescence in age-related macular degeneration and that this pathogenic process can be suppressed by 3TC.

Translational Relevance

Further verifying this study leads to potential targets for age-related macular degeneration therapy.

Improving retinal mitochondrial function as a treatment for age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of blindness among the elderly. Currently, there are no treatments for dry AMD, which is characterized by the death of retinal pigment epithelium (RPE) and photoreceptors. Reports from human donors with AMD suggest that RPE mitochondrial defects are a key event in AMD pathology. Thus, the most effective strategy for treating dry AMD is to identify compounds that enhance mitochondrial function and subsequently, preserve the RPE. In this study, primary cultures of RPE from human donors with (n = 20) or without (n = 8) AMD were used to evaluate compounds that are designed to protect mitochondria from oxidative damage (N-acetyl-l-cysteine; NAC), remove damaged mitochondria (Rapamycin), increase mitochondrial biogenesis (Pyrroloquinoline quinone; PQQ), and improve oxidative phosphorylation (Nicotinamide mononucleotide, NMN). Mitochondrial function measured after drug treatments showed an AMD-dependent response; only RPE from donors with AMD showed improvements. All four drugs caused a significant increase in maximal respiration (p < 0.05) compared to untreated controls. Treatment with Rapamycin, PQQ, or NMN significantly increased ATP production (p < 0.05). Only Rapamycin increased basal respiration (p < 0.05). Notably, robust responses were observed in only about 50% of AMD donors, with attenuated responses observed in the remaining AMD donors. Further, within the responders, individual donors exhibited a distinct reaction to each drug. Our results suggest drugs targeting pathways involved in maintaining healthy mitochondria can improve mitochondrial function in a select population of RPE from AMD donors. The unique response of individual donors to specific drugs supports the need for personalized medicine when treating AMD.

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Human primary RPE cultures were used to test the efficacy of drugs on mitochondrial function.

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Drugs targeting mitochondrial homeostasis pathways improved mitochondrial function in AMD RPE.

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The donor-specific response to drugs suggests personalized medicine is needed to treat AMD.

Implications of NAD+ Metabolism in the Aging Retina and Retinal Degeneration

Nicotinamide adenine dinucleotide (NAD⁺) plays an important role in various key biological processes including energy metabolism, DNA repair, and gene expression. Accumulating clinical and experimental evidence highlights an age-dependent decline in NAD⁺ levels and its association with the development and progression of several age-related diseases. This supports the establishment of NAD⁺ as a critical regulator of aging and longevity and, relatedly, a promising therapeutic target to counter adverse events associated with the normal process of aging and/or the development and progression of age-related disease. Relative to the above, the metabolism of NAD⁺ has been the subject of numerous investigations in various cells, tissues, and organ systems; however, interestingly, studies of NAD⁺ metabolism in the retina and its relevance to the regulation of visual health and function are comparatively few. This is surprising given the critical causative impact of mitochondrial oxidative damage and bioenergetic crises on the development and progression of degenerative disease of the retina. Hence, the role of NAD⁺ in this tissue, normally and aging and/or disease, should not be ignored. Herein, we discuss important findings in the field of NAD⁺ metabolism, with particular emphasis on the importance of the NAD⁺ biosynthesizing enzyme NAMPT, the related metabolism of NAD⁺ in the retina, and the consequences of NAMPT and NAD⁺ deficiency or depletion in this tissue in aging and disease. We discuss also the implications of potential therapeutic strategies that augment NAD⁺ levels on the preservation of retinal health and function in the above conditions. The overarching goal of this review is to emphasize the importance of NAD⁺ metabolism in normal, aging, and/or diseased retina and, by so doing, highlight the necessity of additional clinical studies dedicated to evaluating the therapeutic utility of strategies that enhance NAD⁺ levels in improving vision.

The Emerging Role of Senescence in Ocular Disease

Cellular senescence is a state of irreversible cell cycle arrest in response to an array of cellular stresses. An important role for senescence has been shown for a number of pathophysiological conditions that include cardiovascular disease, pulmonary fibrosis, and diseases of the skin. However, whether senescence contributes to the progression of age-related macular degeneration (AMD) has not been studied in detail so far and the present review describes the recent research on this topic. We present an overview of the types of senescence, pathways of senescence, senescence-associated secretory phenotype (SASP), the role of mitochondria, and their functional implications along with antisenescent therapies. As a central mechanism, senescent cells can impact the surrounding tissue microenvironment via the secretion of a pool of bioactive molecules, termed the SASP. An updated summary of a number of new members of the ever-growing SASP family is presented. Further, we introduce the significance of mechanisms by which mitochondria may participate in the development of cellular senescence. Emerging evidence shows that extracellular vesicles (EVs) are important mediators of the effects of senescent cells on their microenvironment. Based on recent studies, there is reasonable evidence that senescence could be a modifiable factor, and hence, it may be possible to delay age-related diseases by modulating basic aging mechanisms using SASP inhibitors/senolytic drugs. Thus, antisenescent therapies in aging and age-related diseases appear to have a promising potential.

Senescence in the pathogenesis of age-related macular degeneration

Age-related macular degeneration (AMD) is a complex eye disease underlined by the death of photoreceptors and degeneration of retinal pigment epithelium (RPE) and choriocapillaris (CC). The mechanism(s) responsible for massive and progressive retinal degeneration is not completely known. Senescence, a state of permanent inhibition of cell growth, may be induced by many factors important for AMD pathogenesis and results in senescence-associated secretory phenotype (SASP) that releases growth factors, cytokines, chemokines, proteases and other molecules inducing inflammation and other AMD-related effects. These effects can be induced in the affected cell and neighboring cells, leading to progression of AMD phenotype. Senescent cells also release reactive oxygen species that increase SASP propagation. Many other pathways of senescence-related AMD pathogenesis, including autophagy, the cGAS-STING signaling, degeneration of CC by membrane attack complex, can be considered. A2E, a fluorophore present in lipofuscin, amyloid-beta peptide and humanin, a mitochondria-derived peptide, may link AMD with senescence. Further studies on senescence in AMD pathogenesis to check the possibility of opening a perspective of the use of drugs killing senescent cells (senolytics) and terminating SASP bystander effects (senostatics) might be beneficial for AMD that at present is an incurable disease.

Age-related NAD+ decline

Nicotinamide adenine dinucleotide (NAD+) is an essential metabolite that is reported to decline in concentration in tissues of aged animals. Strategies to increase NAD+ availability have shown promise in treating many conditions in rodents, including age-related degeneration, which has in turn driven intense interest in the effects of supplements on human health. However, many aspects of NAD+ metabolism remain poorly understood, and human data are limited. Here, we discuss the state of the evidence for an age-related decline in NAD+, along with potential mechanistic explanations, including increased consumption or decreased synthesis of NAD+ and changes in the composition of cells or tissues with age. Key challenges for the field involve the development of better tools to resolve information on the NAD+ content of specific cells and subcellular compartments as well as determining the threshold levels at which NAD+ depletion triggers physiological consequences in different tissues. Understanding how NAD+ metabolism changes with age in humans may ultimately allow the design of more targeted strategies to maintain its availability, such as inhibition of key consumers in specific tissues or direct delivery of precursors to sites of deficiency. In the meantime, human clinical trials with oral supplements are poised to provide some of the first direct evidence as to whether increasing NAD+ availability can impact human physiology. Thus, it is an exciting time for NAD+ research, with much remaining to be learned in terms of both basic biology and potential therapeutic applications.

mTOR may interact with PARP-1 to regulate visible light-induced parthanatos in photoreceptors

BACKGROUND

Excessive light exposure is a detrimental environmental factor that plays a critical role in the pathogenesis of retinal degeneration. However, the mechanism of light-induced death of retina/photoreceptor cells remains unclear. The mammalian/mechanistic target of rapamycin (mTOR) and Poly (ADP-ribose) polymerase-1 (PARP-1) have become the primary targets for treating many neurodegenerative disorders. The aim of this study was to elucidate the mechanisms underlying light-induced photoreceptor cell death and whether the neuroprotective effects of mTOR and PARP-1 inhibition against death are mediated through apoptosis-inducing factor (AIF).

METHODS

Propidium iodide (PI)/Hoechst staining, lentiviral-mediated short hairpin RNA (shRNA), Western blot analysis, cellular fraction separation, plasmid transient transfection, laser confocal microscopy, a mice model, electroretinography (ERG), and hematoxylin-eosin (H & E) staining were employed to explore the mechanisms by which rapamycin/3-Aminobenzamide (3AB) exert neuroprotective effects of mTOR/PARP-1 inhibition in light-injured retinas.

RESULTS

A parthanatos-like death mechanism was evaluated in light-injured 661 W cells that are an immortalized photoreceptor-like cell line that exhibit cellular and biochemical feature characteristics of cone photoreceptor cells. The death process featured over-activation of PARP-1 and AIF nuclear translocation. Either PARP-1 or AIF knockdown played a significantly protective role for light-damaged photoreceptors. More importantly, crosstalk was observed between mTOR and PARP-1 signaling and mTOR could have regulated parthanatos via the intermediate factor sirtuin 1 (SIRT1). The parthanatos-like injury was also verified in vivo, wherein either PARP-1 or mTOR inhibition provided significant neuroprotection against light-induced injury, which is evinced by both structural and functional retinal analysis. Overall, these results elucidate the mTOR-regulated parthanatos death mechanism in light-injured photoreceptors/retinas and may facilitate the development of novel neuroprotective therapies for retinal degeneration diseases.

CONCLUSIONS

Our results demonstrate that inhibition of the mTOR/PARP-1 axis exerts protective effects on photoreceptors against visible-light-induced parthanatos. These protective effects are conducted by regulating the downstream factors of AIF, while mTOR possibly interacts with PARP-1 via SIRT1 to regulate parthanatos. Video Abstract Schematic diagram of mTOR interacting with PARP-1 to regulate visible light-induced parthanatos. Increased ROS caused by light exposure penetrates the nuclear membrane and causes nuclear DNA strand breaks. PARP-1 detects DNA breaks and synthesizes PAR polymers to initiate the DNA repair system that consumes a large amount of cellular NAD+. Over-production of PAR polymers prompts the release of AIF from the mitochondria and translocation to the nucleus, which leads to parthanatos. Activated mTOR may interact with PARP-1 via SIRT1 to regulate visible light-induced parthanatos.

Visfatin Induces Senescence of Human Dental Pulp Cells

Dental pulp plays an important role in the health of teeth. The aging of teeth is strongly related to the senescence of dental pulp cells. A novel adipokine, visfatin, is closely associated with cellular senescence. However, little is known about the effect of visfatin on the senescence of human dental pulp cells (hDPCs). Here, it was found that in vivo visfatin levels in human dental pulp tissues increase with age and are upregulated in vitro in hDPCs during premature senescence activated by H₂O₂, suggesting a correlation between visfatin and senescence. In addition, visfatin knockdown by small interfering RNA led to the reduction in hDPC senescence; however, treatment with exogenous visfatin protein induced the senescence of hDPCs along with increased NADPH consumption, which was reversed by FK866, a chemical inhibitor of visfatin. Furthermore, visfatin-induced senescence was associated with both the induction of telomere damage and the upregulation of senescence-associated secretory phenotype (SASP) factors as well as NF-κB activation, which were all inhibited by FK866. Taken together, these results demonstrate, for the first time, that visfatin plays a pivotal role in hDPC senescence in association with telomere dysfunction and the induction of SASP factors.

The NADase CD38 is induced by factors secreted from senescent cells providing a potential link between senescence and age-related cellular NAD+ decline

Tissue nicotinamide adenine dinucleotide (NAD⁺) decline has been implicated in aging. We have recently identified CD38 as a central regulator involved in tissue NAD⁺ decline during the aging process. CD38 is an ecto-enzyme highly expressed in endothelial and inflammatory cells. To date, the mechanisms that regulate CD38 expression in aging tissues characterized by the presence of senescent cells is not completely understood. Cellular senescence has been described as a hallmark of the aging process and these cells are known to secrete several factors including cytokines and chemokines through their senescent associated secretory phenotype (SASP). Here we investigated if the cellular senescence phenotype is involved in the regulation of CD38 expression and its NADase activity. We observed that senescent cells do not have high expression of CD38. However, the SASP factors secreted by senescent cells induced CD38 mRNA and protein expression and increased CD38-NADase activity in non-senescent cells such as endothelial cells or bone marrow derived macrophages. Our data suggest a link between cellular senescence and NAD⁺ decline in which SASP-mediated upregulation of CD38 can disrupt cellular NAD⁺ homeostasis.