Insights from In Vivo Studies of Cellular Senescence

Biomarkers of Cellular Senescence and Aging: Current State-of-the-Art, Challenges and Future Perspectives

Population aging has increased the global prevalence of aging-related diseases, including cancer, sarcopenia, neurological disease, arthritis, and heart disease. Understanding aging, a fundamental biological process, has led to breakthroughs in several fields. Cellular senescence, evinced by flattened cell bodies, vacuole formation, and cytoplasmic granules, ubiquitously plays crucial roles in tissue remodeling, embryogenesis, and wound repair as well as in cancer therapy and aging. The lack of universal biomarkers for detecting and quantifying senescent cells, in vitro and in vivo, constitutes a major limitation. The applications and limitations of major senescence biomarkers, including senescence-associated β-galactosidase staining, telomere shortening, cell-cycle arrest, DNA methylation, and senescence-associated secreted phenotypes are discussed. Furthermore, explore senotherapeutic approaches for aging-associated diseases and cancer. In addition to the conventional biomarkers, this review highlighted the in vitro, in vivo, and disease models used for aging studies. Further, technologies from the current decade including multi-omics and computational methods used in the fields of senescence and aging are also discussed in this review. Understanding aging-associated biological processes by using cellular senescence biomarkers can enable therapeutic innovation and interventions to improve the quality of life of older adults.

Senotherapeutics to Counteract Senescent Cells Are Prominent Topics in the Context of Anti-Ageing Strategies

Cellular senescence is implicated in ageing and associated with a broad spectrum of age-related diseases. Importantly, a cell can initiate the senescence program irrespective of the organism's age. Various stress signals, including those defined as ageing hallmarks and alterations leading to cancer development, oncogene activation, or loss of cancer-suppressive functions, can trigger cellular senescence. The primary outcome of these alterations is the activation of nuclear factor (NF)-κB, thereby inducing the senescence-associated secretory phenotype (SASP). Proinflammatory cytokines and chemokines, components of this phenotype, contribute to chronic systemic sterile inflammation, commonly referred to as inflamm-ageing. This inflammation is linked to age-related diseases (ARDs), frailty, and increased mortality in older individuals. Additionally, senescent cells (SCs) accumulate in multiple tissues with age and are believed to underlie the organism functional decline, as demonstrated by models. An escalating effort has been dedicated to identify senotherapeutics that selectively target SCs by inducing apoptosis; these drugs are termed senolytics. Concurrently, small molecules that suppress senescent phenotypes without causing cell death are known as senomorphics. Both natural and synthetic senotherapeutics, along with immunotherapies employing immune cell-mediated clearance of SCs, currently represent the most promising strategies to combat ageing and ARDs. Indeed, it is fascinating to observe that information regarding the immune reaction to SCs indicates that regulation by specific lymphocyte subsets, elevated in the oldest centenarians, plays a role in attaining extreme longevity. Regardless, the application of methods already utilized in cancer treatment, such as CAR cells and monoclonal antibodies, broadens the spectrum of potential approaches to be utilized.

Calcium's Role and Signaling in Aging Muscle, Cellular Senescence, and Mineral Interactions

Calcium research, since its pivotal discovery in the early 1800s through the heating of limestone, has led to the identification of its multi-functional roles. These include its functions as a reducing agent in chemical processes, structural properties in shells and bones, and significant role in cells relating to this review: cellular signaling. Calcium signaling involves the movement of calcium ions within or between cells, which can affect the electrochemical gradients between intra- and extracellular membranes, ligand binding, enzyme activity, and other mechanisms that determine cell fate. Calcium signaling in muscle, as elucidated by the sliding filament model, plays a significant role in muscle contraction. However, as organisms age, alterations occur within muscle tissue. These changes include sarcopenia, loss of neuromuscular junctions, and changes in mineral concentration, all of which have implications for calcium's role. Additionally, a field of study that has gained recent attention, cellular senescence, is associated with aging and disturbed calcium homeostasis, and is thought to affect sarcopenia progression. Changes seen in calcium upon aging may also be influenced by its crosstalk with other minerals such as iron and zinc. This review investigates the role of calcium signaling in aging muscle and cellular senescence. We also aim to elucidate the interactions among calcium, iron, and zinc across various cells and conditions, ultimately deepening our understanding of calcium signaling in muscle aging.

Therapy-induced senescence as a component of tumor biology: Evidence from clinical cancer

Therapy-Induced Senescence (TIS) is an established response to anticancer therapy in a variety of cancer models. Ample evidence has characterized the triggers, hallmarks, and functional outcomes of TIS in preclinical studies; however, limited evidence delineates TIS in clinical cancer (human tumor samples). We examined the literature that investigated the induction of TIS in samples derived from human cancers and highlighted the major findings that suggested that TIS represents a main constituent of tumor biology. The most frequently utilized approach to identify TIS in human cancers was to investigate the protein expression of senescence-associated markers (such as cyclins, cyclin-dependent kinase inhibitors, Ki67, DNA damage repair response markers, DEC1, and DcR1) via immunohistochemical techniques using formalin-fixed paraffin-embedded (FFPE) tissue samples and/or testing the upregulation of Senescence-Associated β-galactosidase (SA-β-gal) in frozen sections of unfixed tumor samples. Collectively, and in studies where the extent of TIS was determined, TIS was detected in 31-66% of tumors exposed to various forms of chemotherapy. Moreover, TIS was not only limited to both malignant and non-malignant components of tumoral tissue but was also identified in samples of normal (non-transformed) tissue upon chemo- or radiotherapy exposure. Nevertheless, the available evidence continues to be limited and requires a more rigorous assessment of in vivo senescence based on novel approaches and more reliable molecular signatures. The accurate assessment of TIS will be beneficial for determining its relevant contribution to the overall outcome of cancer therapy and the potential translatability of senotherapeutics.

Senescent alveolar macrophages promote early-stage lung tumorigenesis

Senescent cells play relevant but context-dependent roles during tumorigenesis. Here, in an oncogenic Kras-driven lung cancer mouse model, we found that senescent cells, specifically alveolar macrophages, accumulate early in neoplasia. These macrophages have upregulated expression of p16INK4a and Cxcr1, are distinct from previously defined subsets and are sensitive to senolytic interventions, and suppress cytotoxic T cell responses. Their removal attenuates adenoma development and progression in mice, indicating their tumorigenesis-promoting role. Importantly, we found that alveolar macrophages with these properties increase with normal aging in mouse lung and in human lung adenocarcinoma in situ. Collectively, our study indicates that a subset of tissue-resident macrophages can support neoplastic transformation through altering their local microenvironment, suggesting that therapeutic interventions targeting senescent macrophages may attenuate lung cancer progression during early stages of disease.

Aging microenvironment and antitumor immunity for geriatric oncology: the landscape and future implications

The tumor microenvironment (TME) has been extensively investigated; however, it is complex and remains unclear, especially in elderly patients. Senescence is a cellular response to a variety of stress signals, which is characterized by stable arrest of the cell cycle and major changes in cell morphology and physiology. To the best of our knowledge, senescence leads to consistent arrest of tumor cells and remodeling of the tumor-immune microenvironment (TIME) by activating a set of pleiotropic cytokines, chemokines, growth factors, and proteinases, which constitute the senescence-associated secretory phenotype (SASP). On the one hand, the SASP promotes antitumor immunity, which enhances treatment efficacy; on the other hand, the SASP increases immunosuppressive cell infiltration, including myeloid-derived suppressor cells (MDSCs), regulatory T cells (Tregs), M2 macrophages, and N2 neutrophils, contributing to TIME suppression. Therefore, a deeper understanding of the regulation of the SASP and components contributing to robust antitumor immunity in elderly individuals with different cancer types and the available therapies is necessary to control tumor cell senescence and provide greater clinical benefits to patients. In this review, we summarize the key biological functions mediated by cytokines and intercellular interactions and significant components of the TME landscape, which influence the immunotherapy response in geriatric oncology. Furthermore, we summarize recent advances in clinical practices targeting TME components and discuss potential senescent TME targets.

Delayed effects of radiation in adipose tissue reflect progenitor damage and not cellular senescence

The pathogenesis of many age-related diseases is linked to cellular senescence, a state of inflammation-inducing, irreversible cell cycle arrest. The consequences and mechanisms of age-associated cellular senescence are often studied using in vivo models of radiation exposure. However, it is unknown whether radiation induces persistent senescence, like that observed in ageing. We performed analogous studies in mice and monkeys, where young mice and rhesus macaques received sub-lethal doses of ionizing radiation and were observed for ~ 15% of their expected lifespan. Assessments of 8-hydroxy-2' -deoxyguanosine (8-OHdG), senescence-associated beta-galactosidase (SAβ-gal), and p16Ink4a and p21 were performed on mitotic and post-mitotic tissues - liver and adipose tissue - 6 months and 3 years post-exposure for the mice and monkeys, respectively. No elevations in 8-OHdG, SA-βgal staining, or p16 Ink4a or p21 gene or protein expression were found in mouse and monkey liver or adipose tissue compared to control animals. Despite no evidence of senescence, progenitor cell dysfunction persisted after radiation exposure, as indicated by lower in situ CD34+ adipose cells (p = 0.03), and deficient adipose stromal vascular cell proliferation (p < 0.05) and differentiation (p = 0.04) ex vivo. Our investigation cautions that employing radiation to study senescence-related processes should be limited to the acute post-exposure period and that stem cell damage likely underpins the dysfunction associated with delayed effects of radiation.

Senescence atlas reveals an aged-like inflamed niche that blunts muscle regeneration

Tissue regeneration requires coordination between resident stem cells and local niche cells^1,2. Here we identify that senescent cells are integral components of the skeletal muscle regenerative niche that repress regeneration at all stages of life. The technical limitation of senescent-cell scarcity³ was overcome by combining single-cell transcriptomics and a senescent-cell enrichment sorting protocol. We identified and isolated different senescent cell types from damaged muscles of young and old mice. Deeper transcriptome, chromatin and pathway analyses revealed conservation of cell identity traits as well as two universal senescence hallmarks (inflammation and fibrosis) across cell type, regeneration time and ageing. Senescent cells create an aged-like inflamed niche that mirrors inflammation associated with ageing (inflammageing⁴) and arrests stem cell proliferation and regeneration. Reducing the burden of senescent cells, or reducing their inflammatory secretome through CD36 neutralization, accelerates regeneration in young and old mice. By contrast, transplantation of senescent cells delays regeneration. Our results provide a technique for isolating in vivo senescent cells, define a senescence blueprint for muscle, and uncover unproductive functional interactions between senescent cells and stem cells in regenerative niches that can be overcome. As senescent cells also accumulate in human muscles, our findings open potential paths for improving muscle repair throughout life.

Molecular Mechanisms to Target Cellular Senescence in Aging and Disease

Cellular senescence is a state of irreversible cell cycle arrest in response to several stressors, including DNA damage, increased cellular oxidative stress, telomere shortening, oncogene activation, and a deep epigenetic remodeling [...].

Senescence and the tumor-immune landscape: Implications for cancer immunotherapy

Cancer therapies, including conventional chemotherapy, radiation, and molecularly targeted agents, can lead to tumor eradication through a variety of mechanisms. In addition to their effects on tumor cell growth and survival, these regimens can also influence the surrounding tumor-immune microenvironment in ways that ultimately impact therapy responses. A unique biological outcome of cancer therapy is induction of cellular senescence. Senescence is a damage-induced stress program that leads to both the durable arrest of tumor cells and remodeling the tumor-immune microenvironment through activation of a collection pleiotropic cytokines, chemokines, growth factors, and proteinases known as the senescence-associated secretory phenotype (SASP). Depending on the cancer context and the mechanism of action of the therapy, the SASP produced following therapy-induced senescence (TIS) can promote anti-tumor immunity that enhances therapeutic efficacy, or alternatively chronic inflammation that leads to therapy failure and tumor relapse. Thus, a deeper understanding of the mechanisms regulating the SASP and components necessary for robust anti-tumor immune surveillance in different cancer and therapy contexts are key to harnessing senescence for tumor control. Here we draw a roadmap to modulate TIS and its immune-stimulating features for cancer immunotherapy.

Ross J, Scott Herron G. Genetic and Epigenetic Influences on Cutaneous Cellular Senescence

Skin is the largest human organ system, and its protective function is critical to survival. The epithelial, dermal, and subcutaneous compartments are heterogeneous mixtures of cell types, yet they all display age-related skin dysfunction through the accumulation of an altered phenotypic cellular state called senescence. Cellular senescence is triggered by complex and dynamic genetic and epigenetic processes. A senescence steady state is achieved in different cell types under various and overlapping conditions of chronological age, toxic injury, oxidative stress, replicative exhaustion, DNA damage, metabolic dysfunction, and chromosomal structural changes. These inputs lead to outputs of cell-cycle withdrawal and the appearance of a senescence-associated secretory phenotype, both of which accumulate as tissue pathology observed clinically in aged skin. This review details the influence of genetic and epigenetic factors that converge on normal cutaneous cellular processes to create the senescent state, thereby dictating the response of the skin to the forces of both intrinsic and extrinsic aging. From this work, it is clear that no single biomarker or process leads to senescence, but that it is a convergence of factors resulting in an overt aging phenotype.

Brain cellular senescence in mouse models of Alzheimer's disease

The accumulation of senescent cells contributes to aging pathologies, including neurodegenerative diseases, and its selective removal improves physiological and cognitive function in wild-type mice as well as in Alzheimer's disease (AD) models. AD models recapitulate some, but not all components of disease and do so at different rates. Whether brain cellular senescence is recapitulated in some or all AD models and whether the emergence of cellular senescence in AD mouse models occurs before or after the expected onset of AD-like cognitive deficits in these models are not yet known. The goal of this study was to identify mouse models of AD and AD-related dementias that develop measurable markers of cellular senescence in brain and thus may be useful to study the role of cellular senescence in these conditions. We measured the levels of cellular senescence markers in the brains of P301S(PS19), P301L, hTau, and 3xTg-AD mice that model amyloidopathy and/or tauopathy in AD and related dementias and in wild-type, age-matched control mice for each strain. Expression of cellular senescence markers in brains of transgenic P301L and 3xTg-AD mice was largely indistinguishable from that in WT control age-matched mice. In contrast, markers of cellular senescence were differentially increased in brains of transgenic hTau and P301S(PS19) mice as compared to WT control mice before the onset of AD-like cognitive deficits. Taken together, our data suggest that P301S(PS19) and hTau mice may be useful models for the study of brain cellular senescence in tauopathies including, but not limited to, AD.

The Fate of Sialic Acid and PEG Modified Epirubicin Liposomes in Aged versus Young Cells and Tumor Mice Models

In preclinical studies of young mice, nanoparticles showed excellent anti-tumor therapeutic effects by harnessing Peripheral Blood Monocytes (PBMs) and evading the immune system. However, the changes of age will inevitably affect PBMs and the immune system, and there is a serious lack of relevant research. Sialic acid (SA)-octadecylamine (ODA) was synthesized, and SA- or polyethylene glycol (PEG)-modified epirubicin (EPI) liposomes (EPI-SL and EPI-PL, respectively) were prepared to explore differences in antitumor treatment using 8-month-old and 8-week-old Kunming mice. Based on presented data, 8-month-old mice had more PBMs in peripheral blood than 8-week-old mice, and age differences resulted in different anti-tumor treatment effects following EPI-SL and EPI-PL treatment. Following EPI-PL administration, the tumor volume was significantly smaller in 8-week-old mice than in 8-month-old mice (* p < 0.05). Eight-month-old mice treated with EPI-SL (8M-SL) presented no damage to healthy tissue, with a 100% survival rate, and 50% mice in 8M-SL showed ‘shedding’ of tumor tissues from the growth site. Accordingly, 8-month-old mice treated with EPI-SL achieved the best therapeutic effect at different ages and with different liposomes. EPI-SL could improve the antitumor effect of 8-week-old and 8-month-old mice.

Cellular Senescence in Sarcopenia: Possible Mechanisms and Therapeutic Potential

Aging promotes most degenerative pathologies in mammals, which are characterized by progressive decline of function at molecular, cellular, tissue, and organismal levels and account for a host of health care expenditures in both developing and developed nations. Sarcopenia is a prominent age-related disorder in musculoskeletal system. Defined as gradual and generalized chronic skeletal muscle disorder, sarcopenia involves accelerated loss of muscle mass, strength and function, which is associated with increased adverse functional outcomes and evolutionally refers to muscle wasting accompanied by other geriatric syndromes. More efforts have been made to clarify mechanisms underlying sarcopenia and new findings suggest that it may be feasible to delay age-related sarcopenia by modulating fundamental mechanisms such as cellular senescence. Cellular senescence refers to the essentially irreversible growth arrest mainly regulated by p53/p21^CIP1 and p16^INK4a/pRB pathways as organism ages, possibly detrimentally contributing to sarcopenia via muscle stem cells (MuSCs) dysfunction and the senescence-associated secretory phenotype (SASP) while cellular senescence may have beneficial functions in counteracting cancer progression, tissue regeneration and wound healing. By now diverse studies in mice and humans have established that targeting cellular senescence is a powerful strategy to alleviating sarcopenia. However, the mechanisms through which senescent cells contribute to sarcopenia progression need to be further researched. We review the possible mechanisms involved in muscle stem cells (MuSCs) dysfunction and the SASP resulting from cellular senescence, their associations with sarcopenia, current emerging therapeutic opportunities based on targeting cellular senescence relevant to sarcopenia, and potential paths to developing clinical interventions genetically or pharmacologically.

Aging, Cell Senescence, the Pathogenesis and Targeted Therapies of Osteoarthritis

Osteoarthritis (OA) is a chronic, debilitating joint disease characterized by progressive destruction of articular cartilage. For a long time, OA has been considered as a degenerative disease, while recent observations indicate the mechanisms responsible for the pathogenesis of OA are multifaceted. Aging is a key factor in its development. Current treatments are palliative and no disease modifying anti-osteoarthritis drugs (DMOADs) are available. In addition to articular cartilage degradation, cellular senescence, synovial inflammation, and epigenetic alterations may all have a role in its formation. Accumulating data demonstrate a clear relationship between the senescence of articular chondrocytes and OA formation and progression. Inhibition of cell senescence may help identify new agents with the properties of DMOADs. Several anti-cellular senescence strategies have been proposed and these include sirtuin-activating compounds (STACs), senolytics, and senomorphics drugs. These agents may selectively remove senescent cells or ameliorate their harmful effects. The results from preclinical experiments and clinical trials are inspiring. However, more studies are warranted to confirm their efficacy, safety profiles and adverse effects of these agents.

The interplay between apoptosis and cellular senescence: Bcl-2 family proteins as targets for cancer therapy

Cell death by apoptosis and permanent cell cycle arrest by senescence serve as barriers to the development of cancer. Chemotherapeutic agents not only induce apoptosis, they can also induce senescence known as therapy-induced senescence (TIS). There are, however, controversies whether TIS improves or worsens therapeutic outcome. Unlike apoptosis, which permanently removes cancer cells, senescent cells are metabolically active, and can contribute to tumor progression and relapse. If senescent cells are not cleared by the immune system or if cancer cells escape senescence, they may acquire resistance to apoptotic stimuli and become highly aggressive. Thus, there have been significant efforts in developing senolytics, drugs that target these pro-survival molecules to eliminate senescent cells. The anti-apoptotic Bcl-2 family proteins not only protect against cell death by apoptosis, but they also allow senescent cells to survive. While combining senolytics with chemotherapeutic drugs is an attractive approach, there are also limitations. Moreover, members of the Bcl-2 family have distinct effects on apoptosis and senescence. The purpose of this review article is to discuss recent literatures on how members of the Bcl-2 family orchestrate the interplay between apoptosis and senescence, and the challenges and progress in targeting these Bcl-2 family proteins for cancer therapy.

Perlingeiro R, Cisneros B. Loss of Dystroglycan Drives Cellular Senescence via Defective Mitosis-Mediated Genomic Instability

Nuclear β-dystroglycan (β-DG) is involved in the maintenance of nuclear architecture and function. Nonetheless, its relevance in defined nuclear processes remains to be determined. In this study we generated a C2C12 cell-based DG-null model using CRISPR-Cas9 technology to provide insights into the role of β-DG on nuclear processes. Since DG-null cells exhibited decreased levels of lamin B1, we aimed to elucidate the contribution of DG to senescence, owing to the central role of lamin B1 in this pathway. Remarkably, the lack of DG enables C2C12 cells to acquire senescent features, including cell-cycle arrest, increased senescence-associated-β-galactosidase activity, heterochromatin loss, aberrant nuclear morphology and nucleolar disruption. We demonstrated that genomic instability is one driving cause of the senescent phenotype in DG-null cells via the activation of a DNA-damage response associated with mitotic failure, as shown by the presence of multipolar mitotic spindles, which in turn induced the formation of micronuclei and γH2AX foci (DNA-damage marker), telomere shortening and p53/p21 upregulation. Altogether, these events might ultimately lead to premature senescence, impeding the replication of the damaged genome. In summary, we present evidence supporting a role for DG in protecting against senescence, through the maintenance of proper lamin B1 expression/localization and proper mitotic spindle organization.

Cellular Senescence Is Associated with Faster Progression of Focal Segmental Glomerulosclerosis

BACKGROUND

A current, albeit unproven, hypothesis is that an acceleration of cellular senescence is involved in impaired renal repair and progression of glomerular diseases. Focal segmental glomerulosclerosis (FSGS) is a glomerular disease with a substantial risk for progression to ESRD. However, if and to what extent cell senescence predicts a negative outcome in FSGS is still unknown.

METHODS

The hypothesis that cell senescence represents a proximate mechanism by which the kidney is damaged in FSGS (NOS phenotype) was investigated in 26 consecutive kidney biopsies from adult FSGS cases (eGFR 72 ± 4 mL/min, proteinuria 2.3 ± 0.6 g/day) who were incident for 2 years in a Northern Italian nephrology center and had a 6-year clinical follow-up.

RESULTS

Cell senescence (p16INK4A, SA-β-galactosidase [SA-β-Gal]) was upregulated by ∼3- to 4-fold in both glomerular and tubular cells in kidney biopsies of FSGS as compared to age-matched controls (p < 0.05-0.01). Tubular SA-β-Gal correlated with proteinuria and glomerulosclerosis, while only as a trend, tubular p16INK4A was directly associated with interstitial fibrosis. At univariate analysis, basal eGFR, proteinuria, and tubular expression of SA-β-Gal and p16INK4A were significantly directly related to the annual loss of eGFR. No correlation was observed between glomerular p16INK4A and eGFR loss. However, at multivariate analysis, eGFR, proteinuria, and tubular p16INK4A, but not SA-β-Gal, contributed significantly to the prediction of eGFR loss.

CONCLUSIONS

The results indicate that an elevated cell senescence rate, expressed by an upregulation of p16INK4A in tubules at the time of initial biopsy, represents an independent predictor of progression to ESRD in adult patients with FSGS.