SenNet recommendations for detecting senescent cells in different tissues.

Once considered a tissue culture-specific phenomenon, cellular senescence has now been linked to various biological processes with both beneficial and detrimental roles in humans, rodents and other species. Much of our understanding of senescent cell biology still originates from tissue culture studies, where each cell in the culture is driven to an irreversible cell cycle arrest. By contrast, in tissues, these cells are relatively rare and difficult to characterize, and it is now established that fully differentiated, postmitotic cells can also acquire a senescence phenotype. The SenNet Biomarkers Working Group was formed to provide recommendations for the use of cellular senescence markers to identify and characterize senescent cells in tissues. Here, we provide recommendations for detecting senescent cells in different tissues based on a comprehensive analysis of existing literature reporting senescence markers in 14 tissues in mice and humans. We discuss some of the recent advances in detecting and characterizing cellular senescence, including molecular senescence signatures and morphological features, and the use of circulating markers. We aim for this work to be a valuable resource for both seasoned investigators in senescence-related studies and newcomers to the field.

Cellular Senescence: Defining a Path Forward.

Cellular senescence is a cell state implicated in various physiological processes and a wide spectrum of age-related diseases. Recently, interest in therapeutically targeting senescence to improve healthy aging and age-related disease, otherwise known as senotherapy, has been growing rapidly. Thus, the accurate detection of senescent cells, especially in vivo, is essential. Here, we present a consensus from the International Cell Senescence Association (ICSA), defining and discussing key cellular and molecular features of senescence and offering recommendations on how to use them as biomarkers. We also present a resource tool to facilitate the identification of genes linked with senescence, SeneQuest (available at http://Senequest.net). Lastly, we propose an algorithm to accurately assess and quantify senescence, both in cultured cells and in vivo.

Apoptotic stress causes mtDNA release during senescence and drives the SASP.

Senescent cells drive age-related tissue dysfunction partially through the induction of a chronic senescence-associated secretory phenotype (SASP)1. Mitochondria are major regulators of the SASP; however, the underlying mechanisms have not been elucidated2. Mitochondria are often essential for apoptosis, a cell fate distinct from cellular senescence. During apoptosis, widespread mitochondrial outer membrane permeabilization (MOMP) commits a cell to die3. Here we find that MOMP occurring in a subset of mitochondria is a feature of cellular senescence. This process, called minority MOMP (miMOMP), requires BAX and BAK macropores enabling the release of mitochondrial DNA (mtDNA) into the cytosol. Cytosolic mtDNA in turn activates the cGAS-STING pathway, a major regulator of the SASP. We find that inhibition of MOMP in vivo decreases inflammatory markers and improves healthspan in aged mice. Our results reveal that apoptosis and senescence are regulated by similar mitochondria-dependent mechanisms and that sublethal mitochondrial apoptotic stress is a major driver of the SASP. We provide proof-of-concept that inhibition of miMOMP-induced inflammation may be a therapeutic route to improve healthspan.

Mitochondria-to-nucleus retrograde signaling drives formation of cytoplasmic chromatin and inflammation in senescence.

Cellular senescence is a potent tumor suppressor mechanism but also contributes to aging and aging-related diseases. Senescence is characterized by a stable cell cycle arrest and a complex proinflammatory secretome, termed the senescence-associated secretory phenotype (SASP). We recently discovered that cytoplasmic chromatin fragments (CCFs), extruded from the nucleus of senescent cells, trigger the SASP through activation of the innate immunity cytosolic DNA sensing cGAS-STING pathway. However, the upstream signaling events that instigate CCF formation remain unknown. Here, we show that dysfunctional mitochondria, linked to down-regulation of nuclear-encoded mitochondrial oxidative phosphorylation genes, trigger a ROS-JNK retrograde signaling pathway that drives CCF formation and hence the SASP. JNK links to 53BP1, a nuclear protein that negatively regulates DNA double-strand break (DSB) end resection and CCF formation. Importantly, we show that low-dose HDAC inhibitors restore expression of most nuclear-encoded mitochondrial oxidative phosphorylation genes, improve mitochondrial function, and suppress CCFs and the SASP in senescent cells. In mouse models, HDAC inhibitors also suppress oxidative stress, CCF, inflammation, and tissue damage caused by senescence-inducing irradiation and/or acetaminophen-induced mitochondria dysfunction. Overall, our findings outline an extended mitochondria-to-nucleus retrograde signaling pathway that initiates formation of CCF during senescence and is a potential target for drug-based interventions to inhibit the proaging SASP.

Neutrophils induce paracrine telomere dysfunction and senescence in ROS-dependent manner.

Cellular senescence is characterized by an irreversible cell cycle arrest as well as a pro-inflammatory phenotype, thought to contribute to aging and age-related diseases. Neutrophils have essential roles in inflammatory responses; however, in certain contexts their abundance is associated with a number of age-related diseases, including liver disease. The relationship between neutrophils and cellular senescence is not well understood. Here, we show that telomeres in non-immune cells are highly susceptible to oxidative damage caused by neighboring neutrophils. Neutrophils cause telomere dysfunction both in vitro and ex vivo in a ROS-dependent manner. In a mouse model of acute liver injury, depletion of neutrophils reduces telomere dysfunction and senescence. Finally, we show that senescent cells mediate the recruitment of neutrophils to the aged liver and propose that this may be a mechanism by which senescence spreads to surrounding cells. Our results suggest that interventions that counteract neutrophil-induced senescence may be beneficial during aging and age-related disease.

Senolytic Drugs: Reducing Senescent Cell Viability to Extend Health Span.

Senescence is the consequence of a signaling mechanism activated in stressed cells to prevent proliferation of cells with damage. Senescent cells (Sncs) often develop a senescence-associated secretory phenotype to prompt immune clearance, which drives chronic sterile inflammation and plays a causal role in aging and age-related diseases. Sncs accumulate with age and at anatomical sites of disease. Thus, they are regarded as a logical therapeutic target. Senotherapeutics are a new class of drugs that selectively kill Sncs (senolytics) or suppress their disease-causing phenotypes (senomorphics/senostatics). Since 2015, several senolytics went from identification to clinical trial. Preclinical data indicate that senolytics alleviate disease in numerous organs, improve physical function and resilience, and suppress all causes of mortality, even if administered to the aged. Here, we review the evidence that Sncs drive aging and disease, the approaches to identify and optimize senotherapeutics, and the current status of preclinical and clinical testing of senolytics.

Anti-Cholestatic Therapy with Obeticholic Acid Improves Short-Term Memory in Bile Duct-Ligated Mice.

Patients with cholestatic liver disease, including those with primary biliary cholangitis, can experience symptoms of impaired cognition or brain fog. This phenomenon remains unexplained and is currently untreatable. Bile duct ligation (BDL) is an established rodent model of cholestasis. In addition to liver changes, BDL animals develop cognitive symptoms early in the disease process (before development of cirrhosis and/or liver failure). The cellular mechanisms underpinning these cognitive symptoms are poorly understood. Herein, the study explored the neurocognitive symptom manifestations, and tested potential therapies, in BDL mice, and used human neuronal cell cultures to explore translatability to humans. BDL animals exhibited short-term memory loss and showed reduced astrocyte coverage of the blood-brain barrier, destabilized hippocampal network activity, and neuronal senescence. Ursodeoxycholic acid (first-line therapy for most human cholestatic diseases) did not reverse symptomatic or mechanistic aspects. In contrast, obeticholic acid (OCA), a farnesoid X receptor agonist and second-line anti-cholestatic agent, normalized memory function, suppressed blood-brain barrier changes, prevented hippocampal network deficits, and reversed neuronal senescence. Co-culture of human neuronal cells with either BDL or human cholestatic patient serum induced cellular senescence and increased mitochondrial respiration, changes that were limited again by OCA. These findings provide new insights into the mechanism of cognitive symptoms in BDL animals, suggesting that OCA therapy or farnesoid X receptor agonism could be used to limit cholestasis-induced neuronal senescence.

Expansion and Cell-Cycle Arrest: Common Denominators of Cellular Senescence.

Cellular senescence is a major driver of age-related diseases, and senotherapies are being tested in clinical trials. Despite its popularity, cellular senescence is weakly defined and is frequently referred to as irreversible cell-cycle arrest. In this article we hypothesize that cellular senescence is a phenotype that results from the coordination of two processes: cell expansion and cell-cycle arrest. We provide evidence for the compatibility of the proposed model with recent findings showing senescence in postmitotic tissues, wound healing, obesity, and development. We believe our model also explains why some characteristics of senescence can be found in non-senescent cells. Finally, we propose new avenues for research from our model.

Senescent human melanocytes drive skin ageing via paracrine telomere dysfunction.

Cellular senescence has been shown to contribute to skin ageing. However, the role of melanocytes in the process is understudied. Our data show that melanocytes are the only epidermal cell type to express the senescence marker p16INK4A during human skin ageing. Aged melanocytes also display additional markers of senescence such as reduced HMGB1 and dysfunctional telomeres, without detectable telomere shortening. Additionally, senescent melanocyte SASP induces telomere dysfunction in paracrine manner and limits proliferation of surrounding cells via activation of CXCR3-dependent mitochondrial ROS. Finally, senescent melanocytes impair basal keratinocyte proliferation and contribute to epidermal atrophy in vitro using 3D human epidermal equivalents. Crucially, clearance of senescent melanocytes using the senolytic drug ABT737 or treatment with mitochondria-targeted antioxidant MitoQ suppressed this effect. In conclusion, our study provides proof-of-concept evidence that senescent melanocytes affect keratinocyte function and act as drivers of human skin ageing.

Length-independent telomere damage drives post-mitotic cardiomyocyte senescence.

Ageing is the biggest risk factor for cardiovascular disease. Cellular senescence, a process driven in part by telomere shortening, has been implicated in age-related tissue dysfunction. Here, we address the question of how senescence is induced in rarely dividing/post-mitotic cardiomyocytes and investigate whether clearance of senescent cells attenuates age-related cardiac dysfunction. During ageing, human and murine cardiomyocytes acquire a senescent-like phenotype characterised by persistent DNA damage at telomere regions that can be driven by mitochondrial dysfunction and crucially can occur independently of cell division and telomere length. Length-independent telomere damage in cardiomyocytes activates the classical senescence-inducing pathways, p21CIP and p16INK4a, and results in a non-canonical senescence-associated secretory phenotype, which is pro-fibrotic and pro-hypertrophic. Pharmacological or genetic clearance of senescent cells in mice alleviates detrimental features of cardiac ageing, including myocardial hypertrophy and fibrosis. Our data describe a mechanism by which senescence can occur and contribute to age-related myocardial dysfunction and in the wider setting to ageing in post-mitotic tissues.

Moderate Exercise Inhibits Age-Related Inflammation, Liver Steatosis, Senescence, and Tumorigenesis.

Age-related chronic inflammation promotes cellular senescence, chronic disease, cancer, and reduced lifespan. In this study, we wanted to explore the effects of a moderate exercise regimen on inflammatory liver disease and tumorigenesis. We used an established model of spontaneous inflammaging, steatosis, and cancer (nfkb1-/- mouse) to demonstrate whether 3 mo of moderate aerobic exercise was sufficient to suppress liver disease and cancer development. Interventional exercise when applied at a relatively late disease stage was effective at reducing tissue inflammation (liver, lung, and stomach), oxidative damage, and cellular senescence, and it reversed hepatic steatosis and prevented tumor development. Underlying these benefits were transcriptional changes in enzymes driving the conversion of tryptophan to NAD+, this leading to increased hepatic NAD+ and elevated activity of the NAD+-dependent deacetylase sirtuin. Increased SIRT activity was correlated with enhanced deacetylation of key transcriptional regulators of inflammation and metabolism, NF-κB (p65), and PGC-1α. We propose that moderate exercise can effectively reprogram pre-established inflammatory and metabolic pathologies in aging with the benefit of prevention of disease.

Mitochondria are required for pro-ageing features of the senescent phenotype.

Cell senescence is an important tumour suppressor mechanism and driver of ageing. Both functions are dependent on the development of the senescent phenotype, which involves an overproduction of pro-inflammatory and pro-oxidant signals. However, the exact mechanisms regulating these phenotypes remain poorly understood. Here, we show the critical role of mitochondria in cellular senescence. In multiple models of senescence, absence of mitochondria reduced a spectrum of senescence effectors and phenotypes while preserving ATP production via enhanced glycolysis. Global transcriptomic analysis by RNA sequencing revealed that a vast number of senescent-associated changes are dependent on mitochondria, particularly the pro-inflammatory phenotype. Mechanistically, we show that the ATM, Akt and mTORC1 phosphorylation cascade integrates signals from the DNA damage response (DDR) towards PGC-1ß-dependent mitochondrial biogenesis, contributing to aROS-mediated activation of the DDR and cell cycle arrest. Finally, we demonstrate that the reduction in mitochondrial content in vivo, by either mTORC1 inhibition or PGC-1ß deletion, prevents senescence in the ageing mouse liver. Our results suggest that mitochondria are a candidate target for interventions to reduce the deleterious impact of senescence in ageing tissues.

DNA damage response at telomeres contributes to lung aging and chronic obstructive pulmonary disease.

Cellular senescence has been associated with the structural and functional decline observed during physiological lung aging and in chronic obstructive pulmonary disease (COPD). Airway epithelial cells are the first line of defense in the lungs and are important to COPD pathogenesis. However, the mechanisms underlying airway epithelial cell senescence, and particularly the role of telomere dysfunction in this process, are poorly understood. We aimed to investigate telomere dysfunction in airway epithelial cells from patients with COPD, in the aging murine lung and following cigarette smoke exposure. We evaluated colocalization of γ-histone protein 2A.X and telomeres and telomere length in small airway epithelial cells from patients with COPD, during murine lung aging, and following cigarette smoke exposure in vivo and in vitro. We found that telomere-associated DNA damage foci increase in small airway epithelial cells from patients with COPD, without significant telomere shortening detected. With age, telomere-associated foci increase in small airway epithelial cells of the murine lung, which is accelerated by cigarette smoke exposure. Moreover, telomere-associated foci predict age-dependent emphysema, and late-generation Terc null mice, which harbor dysfunctional telomeres, show early-onset emphysema. We found that cigarette smoke accelerates telomere dysfunction via reactive oxygen species in vitro and may be associated with ataxia telangiectasia mutated-dependent secretion of inflammatory cytokines interleukin-6 and -8. We propose that telomeres are highly sensitive to cigarette smoke-induced damage, and telomere dysfunction may underlie decline of lung function observed during aging and in COPD.

Determining the feasibility of characterising cellular senescence in human skeletal muscle and exploring associations with muscle morphology and physical function at different ages: findings from the MASS_Lifecourse Study.

Cellular senescence may be associated with morphological changes in skeletal muscle and changes in physical function with age although there have been few human studies. We aimed to determine the feasibility of characterising cellular senescence in skeletal muscle and explored sex-specific associations between markers of cellular senescence, muscle morphology, and physical function in participants from the MASS_Lifecourse Study. Senescence markers (p16, TAF (Telomere-Associated DNA Damage Foci), HMGB1 (High Mobility Group Box 1), and Lamin B1) and morphological characteristics (fibre size, number, fibrosis, and centrally nucleated fibres) were assessed in muscle biopsies from 40 men and women (age range 47-84) using spatially-resolved methods (immunohistochemistry, immunofluorescence, and RNA and fluorescence in situ hybridisation). The associations between senescence, morphology, and physical function (muscle strength, mass, and physical performance) at different ages were explored. We found that most senescence markers and morphological characteristics were weakly associated with age in men but more strongly, although non-significantly, associated with age in women. Associations between senescence markers, morphology, and physical function were also stronger in women for HMGB1 and grip strength (r = 0.52); TAF, BMI, and muscle mass (r > 0.4); Lamin B1 and fibrosis (r = - 0.5); fibre size and muscle mass (r ≥ 0.4); and gait speed (r = - 0.5). However, these associations were non-significant. In conclusion, we have demonstrated that it is feasible to characterise cellular senescence in human skeletal muscle and to explore associations with morphology and physical function in women and men of different ages. The findings require replication in larger studies.

Sustained telomere length in hepatocytes and cholangiocytes with increasing age in normal liver.

UNLABELLED: Telomeres, a validated biomarker of aging, comprise multiple nucleotide repeats capping chromosomes that shorten with each cell cycle until a critical length is achieved, precipitating cell senescence. Only two previous studies focused on the effect of aging in "normal" liver tissue, but these studies were compromised by small sample size, limited age range, tissue derived from individuals with an increased risk of senescence, and the use of liver homogenates. We developed a robust large-volume, four-color quantitative fluorescent in situ hybridization technique to measure telomere length in large numbers of hepatocytes, Kupffer cells, hepatic stellate cells, CD4-positive and CD8-positive lymphocytes, and cholangiocytes. Following validation against the gold standard (Southern blotting), the technique was applied to normal archived paraffin-embedded liver tissue obtained following reperfusion of implanted donor liver. We studied 73 highly selected donors aged 5-79 years with a short medical illness preceding death and no history of liver disease, reperfusion injury, or steatosis and normal graft function 1-year posttransplantation. Cholangiocytes had significantly longer telomeres compared with all other intrahepatic lineages over a wide age range (P < 0.05). Age-related telomere attrition was restricted to sinusoidal cells (i.e., Kupffer cells [P = 0.0054] and stellate cells [P = 0.0001]). Cholangiocytes and hepatocytes showed no age-related telomere shortening. CONCLUSION: In normal liver and over a broad age range, cholangiocytes have longer telomeres than all other intrahepatic lineages. Age-related telomere length decline is restricted to Kupffer cells and stellate cells.

A new face of old cells: An overview about the role of senescence and telomeres in inflammatory bowel diseases.

Cellular senescence is a pivotal factor contributing to aging and the pathophysiology of age-related diseases. Despite the presence of inflammation and abnormal immune system function in both inflammatory bowel diseases (IBD) and senescence, the relationship between the two remains largely unexplored. Therefore, our study aimed to investigate the intricate connection between cellular senescence, telomeres, and IBD. The review highlights the presence of senescence markers, particularly p16 and p21, in IBD patients, suggesting their potential association with disease progression and mucosal inflammation. We emphasize the critical role of macrophages in eliminating senescent cells and how disturbance in effective clearance may contribute to persistent senescence and inflammation in IBD. Additionally, we shed light on the involvement of telomeres in IBD, as their dysfunction impairs enterocyte function and disrupts colonic barrier integrity, potentially exacerbating the pathogenesis of the disease. Targeting senescence and telomere dysfunctions holds promise for the development of innovative therapeutic approaches to mitigate intestinal inflammation and alleviate symptoms in IBD patients. By unraveling the precise role of senescence in IBD, we can pave the way for the discovery of novel therapeutic interventions that effectively address the underlying mechanisms of intestinal inflammation, offering hope for improved management and treatment of IBD patients.

Telomere dysfunction promotes cholangiocyte senescence and biliary fibrosis in primary sclerosing cholangitis.

Cellular senescence and biliary fibrosis are prototypical features of obliterative cholangiopathies, such as primary sclerosing cholangitis (PSC). Telomere dysfunction can lead to senescence either through telomere erosion or damaged telomeres. Our goal was to investigate a mechanistic relationship between telomere damage and biliary fibrosis in PSC. Telomere attrition was observed in the bile ducts of patients with PSC along with a reduction in telomerase reverse transcriptase (TERT) expression, compared with that in normal livers. Similarly, liver tissue from mouse models of biliary fibrosis showed telomere attrition with increased damage at telomeres measured as telomere-associated foci (TAF). Cellular models of senescence induction increased the TAF in cholangiocytes. This coincided with decreased TERT expression and increased senescence, which was rescued by modulating TERT levels. Epigenetic analysis revealed increased acquisition of repressive histone methylation at the TERT promoter, which correlated with decreased TERT transcription. Cholangiocyte-selective deletion of TERT in mice exacerbated fibrosis, whereas androgen therapy toward telomerase rescued liver fibrosis and liver function in a genetic mouse model of PSC. Our results demonstrate a mechanistic role for telomere dysfunction in cellular senescence and fibrosis that characterize PSC. This suggests that PSC may be, in part, a telomere biology disorder, and identifies TERT as a potential therapeutic target.

A chronic wound model to investigate skin cellular senescence.

Wound healing is an essential physiological process for restoring normal skin structure and function post-injury. The role of cellular senescence, an essentially irreversible cell cycle state in response to damaging stimuli, has emerged as a critical mechanism in wound remodeling. Transiently-induced senescence during tissue remodeling has been shown to be beneficial in the acute wound healing phase. In contrast, persistent senescence, as observed in chronic wounds, contributes to delayed closure. Herein we describe a chronic wound murine model and its cellular senescence profile, including the senescence-associated secretory phenotype.