Accumulation of senescent cells in mitotic tissue of aging primates

The many types of heterogeneity in replicative senescence

Replicative senescence, which is induced by telomere shortening, underlies the loss of regeneration capacity of organs and is ultimately detrimental to the organism. At the same time, it is required to protect organisms from unlimited cell proliferation that may arise from numerous stimuli or deregulations. One important feature of replicative senescence is its high level of heterogeneity and asynchrony, which promote genome instability and senescence escape. Characterizing this heterogeneity and investigating its sources are thus critical to understanding the robustness of replicative senescence. Here we review the different aspects of senescence driven by telomere attrition that are subject to variation in Saccharomyces cerevisiae, the current understanding of the molecular processes at play, and the consequences of heterogeneity in replicative senescence.

Genomic-Destabilization-Associated Mutagenesis and Clonal Evolution of Cells with Mutations in Tumor-Suppressor Genes

The development of cancer is driven by genomic instability and mutations. In general, cancer develops via multiple steps. Each step involves the clonal evolution of cells with abrogated defense systems, such as cells with mutations in cancer-suppressor genes. However, it remains unclear how cellular defense systems are abrogated and the associated clonal evolution is triggered and propagated. In this manuscript, we review current knowledge regarding mutagenesis associated with genomic destabilization and its relationship with the clonal evolution of cells over the course of cancer development, focusing especially on mechanistic aspects.

Whole Blood Transcriptome Analysis for Lifelong Monitoring in Elite Sniffer Dogs Produced by Somatic Cell Nuclear Transfer

Reproductive cloning by somatic cell nuclear transfer (SCNT) is a valuable method to propagate service dogs with desirable traits because of higher selection rates in cloned dogs. However, incomplete reprogramming is a major barrier to SCNT, and the assessment of reprogramming is limited to preimplantation embryos and tissues from dead and/or adult tissue. Thus, lifelong monitoring in SCNT dogs can be useful to evaluate the SCNT service dogs for propagation. We applied microarray and qRT-PCR to profile of mRNA and miRNA in whole blood samples collected from four cloned dogs (S), three age-matched control dogs (A), and a donor dog (D). In the analysis of differentially expressed genes in S-A, A-D, and S-D pairs, most genomes were completely reprogrammed and rejuvenated in the cloned offspring. However, several RNAs were differentially expressed. Interestingly, the altered genes are associated with aging and senescence. Furthermore, we identified potential biomarkers such as mirR-223 (NFIB; CLIC4), miRN-494 (ARHGEF12), miR-106b (PPP1R3B; CC2D1A), miR-20a (CC2D1A; PPP1R3B), miR-30e (IGJ; HIRA), and miR-19a (TNRC6A) by miRNA-target mRNA pairing for monitoring rejuvenation, aging/senescence, and reprogramming in cloned dogs. The novel comparative transcriptomic information about SCNT and age-matched dogs can be used to assess the lifelong health of cloned dogs and to facilitate the selection of training animals with minimal invasive procedures.

Benefits of curcumin in brain disorders

Curcumin is widely consumed in Asia either as turmeric directly or as one of the culinary ingredients in food recipes. The benefits of curcumin in different organ systems have been reported extensively in several neurological diseases and cancer. Curcumin has got its global recognition because of its strong antioxidant, anti-inflammatory, anti-cancer, and antimicrobial activities. Additionally, it is used in diabetes and arthritis as well as in hepatic, renal, and cardiovascular diseases. Recently, there is growing attention on usage of curcumin to prevent or delay the onset of neurodegenerative diseases. This review summarizes available data from several recent studies on curcumin in various neurological diseases such as Alzheimer's disease, Parkinson's disease, Multiple Sclerosis, Huntington's disease, Prions disease, stroke, Down's syndrome, autism, Amyotrophic lateral sclerosis, anxiety, depression, and aging. Recent advancements toward increasing the therapeutic efficacy of curcuma/curcumin formulation and the novel delivery strategies employed to overcome its minimal bioavailability and toxicity studies have also been discussed. This review also summarizes the ongoing clinical trials on curcumin for different neurodegenerative diseases and patent details of curcuma/curcumin in India.

A regulatory loop connecting WNT signaling and telomere capping: possible therapeutic implications for dyskeratosis congenita

The consequences of telomere dysfunction are most apparent in rare inherited syndromes caused by genetic deficiencies in factors that normally maintain telomeres. The principal disease is known as dyskeratosis congenita (DC), but other syndromes with similar underlying genetic defects share some clinical aspects with this disease. Currently, there are no curative therapies for these diseases of telomere dysfunction. Here, we review recent findings demonstrating that dysfunctional (i.e., uncapped) telomeres can downregulate the WNT pathway, and that restoration of WNT signaling helps to recap telomeres by increasing expression of shelterins, proteins that naturally bind and protect telomeres. We discuss how these findings are different from previous observations connecting WNT and telomere biology, and discuss potential links between WNT and clinical manifestations of the DC spectrum of diseases. Finally, we argue for exploring the use of WNT agonists, specifically lithium, as a possible therapeutic approach for patients with DC.

Age-associated bimodal transcriptional drift reduces intergenic disparities in transcription

This study addressed the question of how well the quantitative transcriptome structure established in early life is maintained and how consistently it appears with increasing age, and if there is age-associated alteration of gene expression (A₃GE), how much influence the Huntington’s disease (HD) genotype exerts on it. We examined 285 exonic sequences of 175 genes using targeted PCR sequencing in skeletal muscle, brain, and splenic CD4+ T cells of wild-type and HD mice. In contrast to the muscle and brain, T cells exhibited large A₃GE, suggesting a strong contribution to functional decline of the organism. This A₃GE was markedly intensified in age-matched HD T cells, which exhibited accelerated aging as determined by reduced telomere length. Regression analysis suggested that gene expression levels change at a rate of approximately 3% per month with age. We found a bimodal relationship in A₃GE in T cells in that weakly expressed genes in young mice were increasingly transcribed in older animals whereas highly expressed genes in the young were decreasingly expressed with age. This bimodal transcriptional drift in the T cell transcriptome data causes the differences in transcription rate between genes to progressively reduce with age.

Rejuvenating Strategies of Tissue-specific Stem Cells for Healthy Aging

Although aging is a physiological process, it has raised interest in the science of aging and rejuvenation because of the increasing burden on the rapidly aging global population. With advanced age, there is a decline in homeostatic maintenance and regenerative responsiveness to the injury of various tissues, thereby contributing to the incidence of age-related diseases. The primary cause of the functional declines that occur along with aging is considered to be the exhaustion of stem cell functions in their corresponding tissues. Age-related changes in the systemic environment, the niche, and stem cells contribute to this loss. Thus, the reversal of stem cell aging at the cellular level might lead to the rejuvenation of the animal at an organismic level and the prevention of aging, which would be critical for developing new therapies for age-related dysfunction and diseases. Here, we will explore the effects of aging on stem cells in different tissues. The focus of this discussion is on pro-youth interventions that target intrinsic stem cell properties, environmental niche component, systemic factors, and senescent cellular clearance, which are promising for developing strategies related to the reversal of aged stem cell function and optimizing tissue repair processes.

Quantitative assessment of changes in cell growth, size and morphology during telomere-initiated cellular senescence in Saccharomyces cerevisiae

Telomerase-deficient cells of the budding yeast S. cerevisiae experience progressive telomere shortening and undergo senescence in a manner similar to that seen in cultured human fibroblasts. The cells exhibit a DNA damage checkpoint-like stress response, undergo changes in size and morphology, and eventually stop dividing. In this study, a new assay is described that allowed quantitation of senescence in telomerase-deficient est2 cells with applied statistics. Use of the new technique revealed that senescence was strongly accelerated in est2 mutants that had homologous recombination genes RAD51, RAD52 or RAD54 co-inactivated, but was only modestly affected when RAD55, RAD57 or RAD59 were knocked out. Additionally, a new approach for calculating population doublings indicated that loss of growth capacity occurred after approximately 64 generations in est2 cells but only 42 generations in est2 rad52 cells. Phase contrast microscopy experiments demonstrated that senescing est2 cells became enlarged in a time-dependent manner, ultimately exhibiting a 60% increase in cell size. Progressive alterations in physical properties were also observed, including striking changes in light scattering characteristics and cellular sedimentation rates. The results described herein will facilitate future studies of genetic and environmental factors that affect telomere shortening-associated cell senescence rates using the yeast model system.

Stress-induced and ATF7-dependent epigenetic change influences cellular senescence

Cellular senescence plays an important role in aging and is induced by cyclin-dependent kinase (Cdk) inhibitors that accumulate following stresses during aging. However, the underlying mechanism remains elusive. Herein, we demonstrate that activating transcription factor 7 (ATF7), the stress-responsive recruiter of histone H3K9 di- and trimethyltransferases, functions in the accumulation of Cdk inhibitors. Atf7-deficient (Atf7^-/- ) mice have a shorter lifespan than wild-type (WT) mice. Levels of p16^Ink4a Cdk inhibitor mRNA increased with age more rapidly in Atf7^-/- mice than in WT animals. ATF7 binds to the p16^Ink4a gene promoter and was released with age. Consistently, histone H3K9me2 levels on the p16^Ink4a gene promoter were lower in Atf7^-/- mice than in WT animals. Similar results were obtained when Atf7^-/- and WT mouse embryonic fibroblasts (MEFs) were cultured under 20% oxygen conditions, which induces cellular senescence via oxidative stress. Phosphorylation of ATF7 by p38 was also increased with the passage of MEFs, consistent with previous observations that ATF7 phosphorylation by p38 induces its release from chromatin. These results indicate that stress-induced and ATF7-dependent epigenetic changes on p16^Ink4a genes play an important role in cellular senescence.

Senescence-associated β-galactosidase activity in the developing avian retina

BACKGROUND

Senescence-associated β-galactosidase (SA-β-GAL) histochemistry is the most commonly used biomarker of cellular senescence. These SA-β-GAL-positive cells are senescent embryonic cells that are usually removed by apoptosis from the embryo, followed by macrophage-mediated clearance.

RESULTS

Some authors have proposed that SA-β-GAL activity in differentiated neurons from young and adult mammals cannot be uniquely attributed to cell senescence, whether in vivo or in vitro. Using the developing visual system of the chicken as a model, the present study found that SA-β-GAL detected in the developing retina corresponded to lysosomal β-galactosidase activity, and that SA-β-GAL activity did not correlate with the chronotopographical distribution of apoptotic cells. However, SA-β-GAL staining in the undifferentiated retina coincided with the appearance of early differentiating neurons. In the laminated retina, SA-β-GAL staining was concentrated in the ganglion, amacrine, and horizontal cell layers. The photoreceptors and pigment epithelial cells also exhibited SA-β-GAL activity throughout retinal development. We have also found that SA-β-GAL staining strongly correlated p21 immunoreactivity.

CONCLUSION

In conclusion, the results clearly show that SA-β-GAL activity cannot be regarded as a specific marker of senescence during retinal development, and that it is mainly expressed in subpopulations of postmitotic neurons, which are nonproliferative cells, even at early stages of cell differentiation.

Mitochondrial dysfunction and cell senescence: deciphering a complex relationship

Cellular senescence and mitochondrial dysfunction have both been defined as classical hallmarks of the ageing process. Here, we review the intricate relationship between the two. In the context of ageing, it is now well regarded that cellular senescence is a key driver in both ageing and the onset of a number of age-related pathologies. Emerging evidence has pinpointed mitochondria as one of the key modulators in the development of the senescence phenotype, particularly the pro-inflammatory senescence associated secretory phenotype (SASP). This review focuses on the contribution of homeostatic mechanisms, as well as of reactive oxygen species and mitochondrial metabolites in the senescence programme. Furthermore, we discuss emerging pathways and mitochondrial-mediated mechanisms that may be influencing the SASP and, subsequently, explore how these may be exploited to open up new therapeutic avenues.

Lipotoxicity, aging, and muscle contractility: does fiber type matter?

Sarcopenia is a universal characteristic of the aging process and is often accompanied by increases in whole-body adiposity. These changes in body composition have important clinical implications, given that loss of muscle and gain of fat mass are both significantly and independently associated with declining physical performance as well as an increased risk for disability, hospitalizations, and mortality in older individuals. This increased fat mass is not exclusively stored in adipose depots but may become deposited in non-adipose tissues, such as skeletal muscle, when the oxidative capacity of the adipose tissue itself is exceeded. The redistributed adipose tissue is thought to exert detrimental local effects on the muscle environment given the close proximity. Thus, sarcopenia observed with aging may be better defined in the context of loss of muscle quality rather than loss of muscle quantity per se. In this perspective, we briefly review the age-related physiological changes in cellularity, secretory profiles, and inflammatory status of adipose tissue which drive lipotoxicity (spillover) of skeletal muscle and then provide evidence of how this may affect specific fiber type contractility. We focus on biological contributors (cellular machinery) to contractility for which there is some evidence of vulnerability to lipid stress distinguishing between fiber types.

Nuclear Genomic Instability and Aging

The nuclear genome decays as organisms age. Numerous studies demonstrate that the burden of several classes of DNA lesions is greater in older mammals than in young mammals. More challenging is proving this is a cause rather than a consequence of aging. The DNA damage theory of aging, which argues that genomic instability plays a causal role in aging, has recently gained momentum. Support for this theory stems partly from progeroid syndromes in which inherited defects in DNA repair increase the burden of DNA damage leading to accelerated aging of one or more organs. Additionally, growing evidence shows that DNA damage accrual triggers cellular senescence and metabolic changes that promote a decline in tissue function and increased susceptibility to age-related diseases. Here, we examine multiple lines of evidence correlating nuclear DNA damage with aging. We then consider how, mechanistically, nuclear genotoxic stress could promote aging. We conclude that the evidence, in toto, supports a role for DNA damage as a nidus of aging.

Senescent cells in the development of cardiometabolic disease

PURPOSE OF REVIEW

Senescent cells have recently been identified as key players in the development of metabolic dysfunction. In this review, we will highlight recent developments in this field and discuss the concept of targeting these cells to prevent or treat cardiometabolic diseases.

RECENT FINDINGS

Evidence is accumulating that cellular senescence contributes to adipose tissue dysfunction, presumably through induction of low-grade inflammation and inhibition of adipogenic differentiation leading to insulin resistance and dyslipidaemia. Senescent cells modulate their surroundings through their bioactive secretome and only a relatively small number of senescent cells is sufficient to cause persistent physical dysfunction even in young mice. Proof-of-principle studies showed that selective elimination of senescent cells can prevent or delay the development of cardiometabolic diseases in mice.

SUMMARY

The metabolic consequences of senescent cell accumulation in various tissues are now unravelling and point to new therapeutic opportunities for the treatment of cardiometabolic diseases.

LncRNAs Regulatory Networks in Cellular Senescence

Long noncoding RNAs (lncRNAs) are a class of transcripts longer than 200 nucleotides with no open reading frame. They play a key role in the regulation of cellular processes such as genome integrity, chromatin organization, gene expression, translation regulation, and signal transduction. Recent studies indicated that lncRNAs are not only dysregulated in different types of diseases but also function as direct effectors or mediators for many pathological symptoms. This review focuses on the current findings of the lncRNAs and their dysregulated signaling pathways in senescence. Different functional mechanisms of lncRNAs and their downstream signaling pathways are integrated to provide a bird’s-eye view of lncRNA networks in senescence. This review not only highlights the role of lncRNAs in cell fate decision but also discusses how several feedback loops are interconnected to execute persistent senescence response. Finally, the significance of lncRNAs in senescence-associated diseases and their therapeutic and diagnostic potentials are highlighted.

Inhibition of nucleotide synthesis promotes replicative senescence of human mammary epithelial cells

Cellular senescence is a mechanism by which cells permanently withdraw from the cell cycle in response to stresses including telomere shortening, DNA damage, or oncogenic signaling. Senescent cells contribute to both age-related degeneration and hyperplastic pathologies, including cancer. In culture, normal human epithelial cells enter senescence after a limited number of cell divisions, known as replicative senescence. Here, to investigate how metabolic pathways regulate replicative senescence, we used LC-MS-based metabolomics to analyze senescent primary human mammary epithelial cells (HMECs). We did not observe significant changes in glucose uptake or lactate secretion in senescent HMECs. However, analysis of intracellular metabolite pool sizes indicated that senescent cells exhibit depletion of metabolites from nucleotide synthesis pathways. Furthermore, stable isotope tracing with ¹³C-labeled glucose or glutamine revealed a dramatic blockage of flux of these two metabolites into nucleotide synthesis pathways in senescent HMECs. To test whether cellular immortalization would reverse these observations, we expressed telomerase in HMECs. In addition to preventing senescence, telomerase expression maintained metabolic flux from glucose into nucleotide synthesis pathways. Finally, we investigated whether inhibition of nucleotide synthesis in proliferating HMECs is sufficient to induce senescence. In proliferating HMECs, both pharmacological and genetic inhibition of ribonucleotide reductase regulatory subunit M2 (RRM2), a rate-limiting enzyme in dNTP synthesis, induced premature senescence with concomitantly decreased metabolic flux from glucose into nucleotide synthesis. Taken together, our results suggest that nucleotide synthesis inhibition plays a causative role in the establishment of replicative senescence in HMECs.

Uncoupling the Senescence-Associated Secretory Phenotype from Cell Cycle Exit via Interleukin-1 Inactivation Unveils Its Protumorigenic Role

Cellular senescence has emerged as a potent tumor suppressor mechanism in numerous human neoplasias. Senescent cells secrete a distinct set of factors, collectively termed the senescence-associated secretory phenotype (SASP), which has been postulated to carry both pro- and antitumorigenic properties depending on tissue context. However, the in vivo effect of the SASP is poorly understood due to the difficulty of studying the SASP independently of other senescence-associated phenotypes. Here, we report that disruption of the interleukin-1 (IL-1) pathway completely uncouples the SASP from other senescence-associated phenotypes such as cell cycle exit. Transcriptome profiling of IL-1 receptor (IL-1R)-depleted senescent cells indicates that IL-1 controls the late arm of the senescence secretome, which consists of proinflammatory cytokines induced by NF-κB. Our data suggest that both IL-1α and IL-1β signal through IL-1R to upregulate the SASP in a cooperative manner. Finally, we show that IL-1α inactivation impairs tumor progression and immune cell infiltration without affecting cell cycle arrest in a mouse model of pancreatic cancer, highlighting the protumorigenic property of the IL-1-dependent SASP in this context. These findings provide novel insight into the therapeutic potential of targeting the IL-1 pathway in inflammatory cancers.

Aging and Senescence in Canine Testes

Senescent cells accumulate with age but tissue-based studies of senescent cells are limited to selected organs from humans, mice, and primates. Cell culture and xenograft studies have indicated that senescent cells in the microenvironment may play a role in tumor proliferation via paracrine activities. Dogs develop age-related conditions, including in the testis, but cellular senescence has not been confirmed. We hypothesized that senescent cells accumulate with age in canine testes and in the microenvironment of testicular tumors. We tested the expression of the established senescence markers γH2AX and p21 on normal formalin-fixed, paraffin-embedded testes from 15 young dogs (<18 months of age) and 15 old dogs (7-15 years of age) and correlated the findings with age-dependent morphological changes. A statistically significant age-dependent increase in the percentage of p21-expressing cells was observed for testicular fibroblasts (4-fold) and Leydig cells (8-fold). However, p21-expressing cells were still a rare event. In contrast, the percentage of γH2AX-positive cells did not increase with age. P21- and γH2AX-expressing cells were rare in the microenvironments of tumors. Age-dependent morphological changes included an increased mean number of Leydig cells per intertubular triangle (2.95-fold) and a decreased spermatogenesis score. To our surprise, no age-related changes were recorded for interstitial collagen content, mean tubular diameter, and epithelial area. Opposed to our expectations based on previous in vitro data, we did not identify evidence of a correlation between age-associated accumulation of senescent cells and testicular tumor development. Understanding the role of the microenvironment in senescence obviously remains a challenging task.

Cellular senescence in the lung across the age spectrum

Cellular senescence results in cell cycle arrest with secretion of cytokines, chemokines, growth factors, and remodeling proteins (senescence-associated secretory phenotype; SASP) that have autocrine and paracrine effects on the tissue microenvironment. SASP can promote remodeling, inflammation, infectious susceptibility, angiogenesis, and proliferation, while hindering tissue repair and regeneration. While the role of senescence and the contributions of senescent cells are increasingly recognized in the context of aging and a variety of disease states, relatively less is known regarding the portfolio and influences of senescent cells in normal lung growth and aging per se or in the induction or progression of lung diseases across the age spectrum such as bronchopulmonary dysplasia, asthma, chronic obstructive pulmonary disease, or pulmonary fibrosis. In this review, we introduce concepts of cellular senescence, the mechanisms involved in the induction of senescence, and the SASP portfolio that are relevant to lung cells, presenting the potential contribution of senescent cells and SASP to inflammation, hypercontractility, and remodeling/fibrosis: aspects critical to a range of lung diseases. The potential to blunt lung disease by targeting senescent cells using a novel class of drugs (senolytics) is discussed. Potential areas for future research on cellular senescence in the lung are identified.

The Role of Curcumin in the Modulation of Ageing

It is believed that postponing ageing is more effective and less expensive than the treatment of particular age-related diseases. Compounds which could delay symptoms of ageing, especially natural products present in a daily diet, are intensively studied. One of them is curcumin. It causes the elongation of the lifespan of model organisms, alleviates ageing symptoms and postpones the progression of age-related diseases in which cellular senescence is directly involved. It has been demonstrated that the elimination of senescent cells significantly improves the quality of life of mice. There is a continuous search for compounds, named senolytic drugs, that selectively eliminate senescent cells from organisms. In this paper, we endeavor to review the current knowledge about the anti-ageing role of curcumin and discuss its senolytic potential.

VEGF receptor-1 modulates amyloid β 1-42 oligomer-induced senescence in brain endothelial cells

Aggregated amyloid β (Aβ) peptides in the Alzheimer's disease (AD) brain are hypothesized to trigger several downstream pathologies, including cerebrovascular dysfunction. Previous studies have shown that Aβ peptides can have antiangiogenic properties, which may contribute to vascular dysfunction in the early stages of the disease process. We have generated data showing that brain endothelial cells (ECs) exposed to toxic Aβ1-42 oligomers can readily enter a senescence phenotype. To determine the effect of Aβ oligomers on brain ECs, we treated early passaged human brain microvascular ECs and HUVECs with high MW Aβ1-42 oligomers (5 µM, for 72 h). For controls, we used no peptide treatment, 5 µM Aβ1-42 monomers, and 5 µM Aβ1-42 fibrils, respectively. Brain ECs treated with Aβ1-42 oligomers showed increased senescence-associated β-galactosidase staining and increased senescence-associated p21/p53 expression. Treatment with either Aβ1-42 monomer or Aβ1-42 fibrils did not induce senescence in this assay. We then measured vascular endothelial growth factor receptor (VEGFR) expression in the Aβ1-42 oligomer-treated ECs, and these cells showed significantly increased VEGFR-1 expression and decreased VEGFR-2 levels. Overexpression of VEGFR-1 in brain ECs readily induced senescence, suggesting a direct role of VEGFR-1 signaling events in this paradigm. More importantly, small interfering RNA-mediated knockdown of VEGFR-1 expression in brain ECs was able to prevent up-regulation of p21 protein expression and significantly reduced induction of senescence following Aβ1-42 oligomer treatment. Our studies show that exposure to Aβ1-42 oligomers may impair vascular functions by altering VEGFR-1 expression and causing ECs to enter a senescent phenotype. Altered VEGFR expression has been documented in brains of AD patients and suggests that this pathway may play a role in AD disease pathogenesis. These studies suggest that modulating VEGFR-1 expression and signaling events could potentially prevent senescence and rejuvenate EC functions, and provides us with a novel target to pursue for prevention and treatment of cerebrovascular dysfunction in AD.-Angom, R. S., Wang, Y., Wang, E., Pal, K., Bhattacharya, S., Watzlawik, J. O., Rosenberry, T. L., Das, P., Mukhopadhyay, D. VEGF receptor-1 modulates amyloid β 1-42 oligomer-induced senescence in brain endothelial cells.

Mitochondria and cellular senescence: Implications for musculoskeletal ageing

Musculoskeletal ageing and its associated diseases are major contributors to the loss of independence and reduced quality of life in older people. Several recent studies indicate that cellular senescence is a contributor to age-related loss of function in various organs including muscle, bones and joints. Importantly, these studies indicate that therapies targeting specifically senescent cells have great therapeutic potential in improving musculoskeletal health during ageing. Senescent cells are characterised by dramatic changes in mitochondrial function, metabolism and homeostasis. Mitochondrial dysfunction has been shown to contribute to senescence and the SASP. Here we review the role of cellular senescence in musculoskeletal ageing as well as the potential mechanisms by which mitochondrial dysfunction may impact on the induction and development of the senescent phenotype.

New Insights into the Role of Epithelial⁻Mesenchymal Transition during Aging

Epithelial–mesenchymal transition (EMT) is a cellular process by which differentiated epithelial cells undergo a phenotypic conversion to a mesenchymal nature. The EMT has been increasingly recognized as an essential process for tissue fibrogenesis during disease and normal aging. Higher levels of EMT proteins in aged tissues support the involvement of EMT as a possible cause and/or consequence of the aging process. Here, we will highlight the existing understanding of EMT supporting the phenotypical alterations that occur during normal aging or pathogenesis, covering the impact of EMT deregulation in tissue homeostasis and stem cell function.

The bystander effect contributes to the accumulation of senescent cells in vivo

Senescent cells accumulate with age in multiple tissues and may cause age-associated disease and functional decline. In vitro, senescent cells induce senescence in bystander cells. To see how important this bystander effect may be for accumulation of senescent cells in vivo, we xenotransplanted senescent cells into skeletal muscle and skin of immunocompromised NSG mice. 3 weeks after the last transplantation, mouse dermal fibroblasts and myofibres displayed multiple senescence markers in the vicinity of transplanted senescent cells, but not where non-senescent or no cells were injected. Adjacent to injected senescent cells, the magnitude of the bystander effect was similar to the increase in senescence markers in myofibres between 8 and 32 months of age. The age-associated increase of senescence markers in muscle correlated with fibre thinning, a widely used marker of muscle aging and sarcopenia. Senescent cell transplantation resulted in borderline induction of centrally nucleated fibres and no significant thinning, suggesting that myofibre aging might be a delayed consequence of senescence-like signalling. To assess the relative importance of the bystander effect versus cell-autonomous senescence, we compared senescent hepatocyte frequencies in livers of wild-type and NSG mice under ad libitum and dietary restricted feeding. This enabled us to approximate cell-autonomous and bystander-driven senescent cell accumulation as well as the impact of immunosurveillance separately. The results suggest a significant impact of the bystander effect for accumulation of senescent hepatocytes in liver and indicate that senostatic interventions like dietary restriction may act as senolytics in immunocompetent animals.

Cellular Senescence: Aging, Cancer, and Injury

Cellular senescence is a permanent state of cell cycle arrest that occurs in proliferating cells subjected to different stresses. Senescence is, therefore, a cellular defense mechanism that prevents the cells to acquire an unnecessary damage. The senescent state is accompanied by a failure to re-enter the cell cycle in response to mitogenic stimuli, an enhanced secretory phenotype and resistance to cell death. Senescence takes place in several tissues during different physiological and pathological processes such as tissue remodeling, injury, cancer, and aging. Although senescence is one of the causative processes of aging and it is responsible of aging-related disorders, senescent cells can also play a positive role. In embryogenesis and tissue remodeling, senescent cells are required for the proper development of the embryo and tissue repair. In cancer, senescence works as a potent barrier to prevent tumorigenesis. Therefore, the identification and characterization of key features of senescence, the induction of senescence in cancer cells, or the elimination of senescent cells by pharmacological interventions in aging tissues is gaining consideration in several fields of research. Here, we describe the known key features of senescence, the cell-autonomous, and noncell-autonomous regulators of senescence, and we attempt to discuss the functional role of this fundamental process in different contexts in light of the development of novel therapeutic targets.

Turning back time with emerging rejuvenation strategies

Ageing is associated with the functional decline of all tissues and a striking increase in many diseases. Although ageing has long been considered a one-way street, strategies to delay and potentially even reverse the ageing process have recently been developed. Here, we review four emerging rejuvenation strategies-systemic factors, metabolic manipulations, senescent cell ablation and cellular reprogramming-and discuss their mechanisms of action, cellular targets, potential trade-offs and application to human ageing.

Obesity May Accelerate the Aging Process

Lines of evidence from several studies have shown that increases in life expectancy are now accompanied by increased disability rate. The expanded lifespan of the aging population imposes a challenge on the continuous increase of chronic disease. The prevalence of overweight and obesity is increasing at an alarming rate in many parts of the world. Further to increasing the onset of metabolic imbalances, obesity leads to reduced life span and affects cellular and molecular processes in a fashion resembling aging. Nine key hallmarks of the aging process have been proposed. In this review, we will review these hallmarks and discuss pathophysiological changes that occur with obesity, that are similar to or contribute to those that occur during aging. We present and discuss the idea that obesity, in addition to having disease-specific effects, may accelerate the rate of aging affecting all aspects of physiology and thus shortening life span and health span.

Senescent cell clearance by the immune system: Emerging therapeutic opportunities

Senescent cells (SCs) arise from normal cells in multiple organs due to inflammatory, metabolic, DNA damage, or tissue damage signals. SCs are non-proliferating but metabolically active cells that can secrete a range of pro-inflammatory and proteolytic factors as part of the senescence-associated secretory phenotype (SASP). Senescent cell anti-apoptotic pathways (SCAPs) protect SCs from their own pro-apoptotic SASP. SCs can chemo-attract immune cells and are usually cleared by these immune cells. During aging and in multiple chronic diseases, SCs can accumulate in dysfunctional tissues. SCs can impede innate and adaptive immune responses. Whether immune system loss of capacity to clear SCs promotes immune system dysfunction, or conversely whether immune dysfunction permits SC accumulation, are important issues that are not yet fully resolved. SCs may be able to assume distinct states that interact differentially with immune cells, thereby promoting or inhibiting SC clearance, establishing a chronically pro-senescent and pro-inflammatory environment, leading to modulation of the SASP by the immune cells recruited and activated by the SASP. Therapies that enhance immune cell-mediated clearance of SCs could provide a lever for reducing SC burden. Such therapies could include vaccines, small molecule immunomodulators, or other approaches. Senolytics, drugs that selectively eliminate SCs by transiently disabling their SCAPs, may prove to alleviate immune dysfunction in older individuals and thereby accelerate immune-mediated clearance of SCs. The more that can be understood about the interplay between SCs and the immune system, the faster new interventions may be developed to delay, prevent, or treat age-related dysfunction and the multiple senescence-associated chronic diseases and disorders.

The role of myeloid-derived suppressor cells (MDSC) in the inflammaging process

A chronic low-grade inflammation is one of the hallmarks of the aging process. This gradually augmenting inflammatory state has been termed inflammaging. Inflammaging is associated with increased myelopoiesis in the bone marrow. This myelopoiesis-biased process increases the generation not only of mature myeloid cells, e.g. monocytes, macrophages, and neutrophils, but also immature myeloid progenitors and myeloid-derived suppressor cells (MDSCs). It is known that the aging process is associated with a significant increase in the presence of MDSCs in the bone marrow, blood, spleen, and peripheral lymph nodes. Consequently, MDSCs will become recruited into inflamed tissues where they suppress acute inflammatory responses and trigger the resolution of inflammation. However, if the perpetrator cannot be eliminated, the long-term presence of MDSCs suppresses the host's immune defence and increases the susceptibility to infections and tumorigenesis. Chronic immunosuppression also impairs the clearance of waste products and dead cells, impairs energy metabolism, and disturbs tissue proteostasis. This immunosuppressive state is reminiscent of the immunosenescence observed in inflammaging. It seems that proinflammatory changes in tissues with aging stimulate the myelopoietic production of MDSCs which subsequently induces immunosenescence and maintains the chronic inflammaging process. We will briefly describe the functions of MDSCs and then examine in detail how inflammaging enhances the generation MDSCs and how MDSCs are involved in the control of immunosenescence occurring in inflammaging.

Adaptation to DNA damage checkpoint in senescent telomerase-negative cells promotes genome instability

Here, Coutelier et al. used a microfluidic-based approach and live-cell imaging in yeast to capture early mutation events during replicative senescence and observed that prolonged checkpoint arrests occurred frequently in telomerase-negative lineages. Their results demonstrate that the adaptation pathway is a major contributor to the genome instability induced during replicative senescence.

In cells lacking telomerase, telomeres gradually shorten during each cell division to reach a critically short length, permanently activate the DNA damage checkpoint, and trigger replicative senescence. The increase in genome instability that occurs as a consequence may contribute to the early steps of tumorigenesis. However, because of the low frequency of mutations and the heterogeneity of telomere-induced senescence, the timing and mechanisms of genome instability increase remain elusive. Here, to capture early mutation events during replicative senescence, we used a combined microfluidic-based approach and live-cell imaging in yeast. We analyzed DNA damage checkpoint activation in consecutive cell divisions of individual cell lineages in telomerase-negative yeast cells and observed that prolonged checkpoint arrests occurred frequently in telomerase-negative lineages. Cells relied on the adaptation to the DNA damage pathway to bypass the prolonged checkpoint arrests, allowing further cell divisions despite the presence of unrepaired DNA damage. We demonstrate that the adaptation pathway is a major contributor to the genome instability induced during replicative senescence. Therefore, adaptation plays a critical role in shaping the dynamics of genome instability during replicative senescence.

Hyperphosphatemia Promotes Senescence of Myoblasts by Impairing Autophagy Through Ilk Overexpression, A Possible Mechanism Involved in Sarcopenia

In mammalians, advancing age is associated with sarcopenia, the progressive and involuntary loss of muscle mass and strength. Hyperphosphatemia is an aging-related condition involved in several pathologies. The aim of this work was to assess whether hyperphosphatemia plays a role in the age-related loss of mass muscle and strength by inducing cellular senescence in murine myoblasts and to explore the intracellular mechanism involved in this effect. Cultured mouse C2C12 cells were treated with 10 mM beta-glycerophosphate (BGP] at different periods of time to induce hyperphosphatemia. BGP promoted cellular senescence after 24 h of treatment, assessed by the increased expression of p53, acetylated-p53 and p21 and senescence associated β-galactosidase activity. In parallel, BGP increased ILK expression and activity, followed by mTOR activation and autophagy reduction. Knocking-down ILK expression increased autophagy and protected cells from senescence induced by hyperphosphatemia. BGP also reduced the proliferative capacity of cultured myoblasts. Old mice (24-months-old] presented higher serum phosphate concentration, lower forelimb strength, higher expression of p53 and ILK and less autophagy in vastus muscle than young mice (5-months-old]. In conclusion, we propose that hyperphosphatemia induces senescence in cultured myoblasts through ILK overexpression, reducing their proliferative capacity, which could be a mechanism involved in the development of sarcopenia, since old mice showed loss of muscular strength correlated with high serum phosphate concentration and increased levels of ILK and p53.

Cellular senescence: a view throughout organismal life

Cellular senescence is the final fate of most cells in response to specific stimuli, but is not the end. Indeed, it is the beginning of a singular life, with multiple side roads leading to diverse effects on the organism. Many studies have been done in the last few years to elucidate the intriguing role of senescent cells in the organism, demonstrating them as the cause of several age-related diseases. However, these cells are also positively implicated in other important pathways, such as embryogenesis and wound healing. It appears that the multiple effects are time-dependent: long-term senescence is mostly implicated in chronic inflammation and disease, whereas in the short term, senescent cells seem to be beneficial, being rapidly targeted by the innate immune system. The influence of senescent cells on their neighbors by paracrine factors, differential activity depending on developmental stage, and duration of the effects make the cellular senescent program a unique spatial-temporal mechanism. During pathological conditions such as progeroid syndromes, this mechanism is deregulated, leading to accelerated onset of some aging-related diseases and a shorter lifespan, among other physiological defects. Here, we review the three primary cell senescence programs described so far (replicative, stress-induced, and developmentally programmed senescence), their onset during development, and their potential roles in diseases with premature aging. Finally, we discuss the role of immune cells in keeping senescence burden below the threshold of disease.

Biomarkers of Cellular Senescence and Skin Aging

Cellular senescence is an irreversible growth arrest that occurs as a result of different damaging stimuli, including DNA damage, telomere shortening and dysfunction or oncogenic stress. Senescent cells exert a pleotropic effect on development, tissue aging and regeneration, inflammation, wound healing and tumor suppression. Strategies to remove senescent cells from aging tissues or preneoplastic lesions can delay tissue dysfunction and lead to increased healthspan. However, a significant hurdle in the aging field has been the identification of a universal biomarker that facilitates the unequivocal detection and quantification of senescent cell types in vitro and in vivo. Mammalian skin is the largest organ of the human body and consists of different cell types and compartments. Skin provides a physical barrier against harmful microbes, toxins, and protects us from ultraviolet radiation. Increasing evidence suggests that senescent cells accumulate in chronologically aged and photoaged skin; and may contribute to age-related skin changes and pathologies. Here, we highlight current biomarkers to detect senescent cells and review their utility in the context of skin aging. In particular, we discuss the efficacy of biomarkers to detect senescence within different skin compartments and cell types, and how they may contribute to myriad manifestations of skin aging and age-related skin pathologies.

Myeloid-derived suppressor cells (MDSC): an important partner in cellular/tissue senescence

The aging process is associated with a low-grade chronic inflammation and the accumulation of senescent cells into tissues. Diverse stresses can trigger cellular senescence, a cell fate characterized by cell-cycle arrest and flat morphology. Oncogenic signaling can also induce cellular senescence which has been termed oncogene-induced senescence (OIS). Senescent cells display a pro-inflammatory phenotype which has been called the senescence-associated secretory phenotype (SASP). The secretomes associated with SASP contain colony-stimulating factors and chemokines which stimulate the generation of myeloid-derived suppressor cells (MDSC) by enhancing myelopoiesis in bone marrow and spleen. Enhanced myelopoiesis and increased level of MDSCs have been observed in bone marrow, spleen, and blood in both tumor-bearing and aged mice. Immunosuppressive MDSCs are recruited via chemotaxis into inflamed tissues where they proliferate and consequently suppress acute inflammatory reactions by inhibiting the functions of distinct components of innate and adaptive immunity. For instance, MDSCs stimulate the activity of immunosuppressive regulatory T-cells (Tregs). They also increase the expression of amino acid catabolizing enzymes and the secretion of anti-inflammatory cytokines, e.g. IL-10 and TGF-β, and reactive oxygen species (ROS). On the other hand, the accumulation of MDSCs into tissues exerts harmful effects in chronic pathological disorders, e.g. tumors and many age-related diseases, since the immunosuppression induced by MDSCs impairs the clearance of senescent and cancer cells and also disturbs the maintenance of energy metabolism and tissue proteostasis. The co-operation between senescent cells and immunosuppressive MDSCs regulates not only tumorigenesis and chronic inflammatory disorders but it also might promote inflammaging during the aging process.

Epigenetic Mechanisms Impacting Aging: A Focus on Histone Levels and Telomeres

Aging and age-related diseases pose some of the most significant and difficult challenges to modern society as well as to the scientific and medical communities. Biological aging is a complex, and, under normal circumstances, seemingly irreversible collection of processes that involves numerous underlying mechanisms. Among these, chromatin-based processes have emerged as major regulators of cellular and organismal aging. These include DNA methylation, histone modifications, nucleosome positioning, and telomere regulation, including how these are influenced by environmental factors such as diet. Here we focus on two interconnected categories of chromatin-based mechanisms impacting aging: those involving changes in the levels of histones or in the functions of telomeres.

Is senescence-associated β-galactosidase a marker of neuronal senescence?

One of the features of cellular senescence is the activity of senescence-associated- β-galactosidase (SA-β-gal). The main purpose of this study was to evaluate this marker of senescence in aging neurons. We found that cortical neurons exhibited noticeable SA-β-gal activity quite early in culture. Many SA-β-gal-positive neurons were negative for another canonical marker of senescence, namely, double-strand DNA breaks (DSBs). Moreover, DDR signalling triggered by low doses of doxorubicin did not accelerate the appearance of neuronal SA-β-gal. In vivo, we observed pronounced induction of SA-β-gal activity in the hippocampus of 24-month-old mice, which is consistent with previous findings and supports the view that at this advanced age neurons developed a senescence-like phenotype. Surprisingly however, relatively high SA-β-gal activity, probably unrelated to the senescence process, was also observed in much younger, 3-month-old mice. In conclusion, we propose that SA-β-gal activity in neurons cannot be attributed uniquely to cell senescence either in vitro or in vivo. Additionally, we showed induction of REST protein in aging neurons in long-term culture and we propose that REST could be a marker of neuronal senescence in vitro.

NOX4 downregulation leads to senescence of human vascular smooth muscle cells

Senescence is a stress response characterized by an irreversible growth arrest and alterations in certain cell functions. It is believed that both double-strand DNA breaks (DSB) and increased ROS level are the main culprit of senescence. Excessive ROS production is also particularly important in the development of a number of cardiovascular disorders. In this context the involvement of professional ROS-producing enzymes, NADPH oxidases (NOX), was postulated. In contrary to the common knowledge, we have shown that not only increased ROS production but also diminished ROS level could be involved in the induction of senescence.

Accordingly, our studies revealed that stress-induced premature senescence (SIPS) of vascular smooth muscle cells (VSMCs) induced by doxorubicin or H₂O₂, correlates with increased level of DSB and ROS. On the other hand, both SIPS and replicative senescence were accompanied by diminished expression of NOX4. Moreover, inhibition of NOX activity or decrease of NOX4 expression led to permanent growth arrest of VSMCs and secretion of interleukins and VEGF. Interestingly, cells undergoing senescence due to NOX4 depletion neither acquired DSB nor activated DNA damage response. Instead, transient induction of the p27, upregulation of HIF-1alpha, decreased expression of cyclin D1 and hypophosphorylated Rb was observed. Our results showed that lowering the level of ROS-producing enzyme - NOX4 oxidase below physiological level leads to cellular senescence of VSMCs which is correlated with secretion of pro-inflammatory cytokines. Thus the use of specific NOX4 inhibitors for pharmacotherapy of vascular diseases should be carefully considered.

Small-molecule MDM2 antagonists attenuate the senescence-associated secretory phenotype

Processes that have been linked to aging and cancer include an inflammatory milieu driven by senescent cells. Senescent cells lose the ability to divide, essentially irreversibly, and secrete numerous proteases, cytokines and growth factors, termed the senescence-associated secretory phenotype (SASP). Senescent cells that lack p53 tumor suppressor function show an exaggerated SASP, suggesting the SASP is negatively controlled by p53. Here, we show that increased p53 activity caused by small molecule inhibitors of MDM2, which promotes p53 degradation, reduces inflammatory cytokine production by senescent cells. Upon treatment with the MDM2 inhibitors nutlin-3a or MI-63, human cells acquired a senescence-like growth arrest, but the arrest was reversible. Importantly, the inhibitors reduced expression of the signature SASP factors IL-6 and IL-1α by cells made senescent by genotoxic stimuli, and suppressed the ability of senescent fibroblasts to stimulate breast cancer cell aggressiveness. Our findings suggest that MDM2 inhibitors could reduce cancer progression in part by reducing the pro-inflammatory environment created by senescent cells.

Transcriptomic alterations during ageing reflect the shift from cancer to degenerative diseases in the elderly

Disease epidemiology during ageing shows a transition from cancer to degenerative chronic disorders as dominant contributors to mortality in the old. Nevertheless, it has remained unclear to what extent molecular signatures of ageing reflect this phenomenon. Here we report on the identification of a conserved transcriptomic signature of ageing based on gene expression data from four vertebrate species across four tissues. We find that ageing-associated transcriptomic changes follow trajectories similar to the transcriptional alterations observed in degenerative ageing diseases but are in opposite direction to the transcriptomic alterations observed in cancer. We confirm the existence of a similar antagonism on the genomic level, where a majority of shared risk alleles which increase the risk of cancer decrease the risk of chronic degenerative disorders and vice versa. These results reveal a fundamental trade-off between cancer and degenerative ageing diseases that sheds light on the pronounced shift in their epidemiology during ageing.

Ageing is associated with a pronounced shift in mortality from cancer to degenerative diseases. Here, the authors show that in concordance with this shift, conserved transcriptional alterations during ageing across four vertebrates align with degenerative diseases but are opposite to those in cancer.

Organ reserve, excess metabolic capacity, and aging

"Organ reserve" refers to the ability of an organ to successfully return to its original physiological state following repeated episodes of stress. Clinical evidence shows that organ reserve correlates with the ability of older adults to cope with an added workload or stress, suggesting a role in the process of aging. Although organ reserve is well documented clinically, it is not clearly defined at the molecular level. Interestingly, several metabolic pathways exhibit excess metabolic capacities (e.g., bioenergetics pathway, antioxidants system, plasticity). These pathways comprise molecular components that have an excess of quantity and/or activity than that required for basic physiological demand in vivo (e.g., mitochondrial complex IV or glycolytic enzymes). We propose that the excess in mtDNA copy number and tandem DNA repeats of telomeres are additional examples of intrinsically embedded structural components that could comprise excess capacity. These excess capacities may grant intermediary metabolism the ability to instantly cope with, or manage, added workload or stress. Therefore, excess metabolic capacities could be viewed as an innate mechanism of adaptability that substantiates organ reserve and contributes to the cellular defense systems. If metabolic excess capacities or organ reserves are impaired or exhausted, the ability of the cell to cope with stress is reduced. Under these circumstances cell senescence, transformation, or death occurs. In this review, we discuss excess metabolic and structural capacities as integrated metabolic pathways in relation to organ reserve and cellular aging.

"Social Life" of Senescent Cells: What Is SASP and Why Study It?

Cellular senescence was first described as a failure of normal human cells to divide indefinitely in culture. Until recently, the emphasis in the study of cell senescence has been focused on the accompanying intracellular processes. The focus of the attention has been on the irreversible growth arrest and two important physiological functions that rely on it: suppression of carcinogenesis due to the proliferation loss of damaged cells, and the acceleration of organism aging due to the deterioration of the tissue repair mechanism with age. However, the advances of the past years have revealed that senescent cells can impact the surrounding tissue microenvironment, and, thus, that the main consequences of senescence are not solely mediated by intracellular alterations. Recent studies have provided evidence that a pool of molecules secreted by senescent cells, including cytokines, chemokines, proteases and growth factors, termed the senescence-associated secretory phenotype (SASP), via autocrine/paracrine pathways can affect neighboring cells. Today it is clear that SASP functionally links cell senescence to various biological processes, such as tissue regeneration and remodeling, embryonic development, inflammation, and tumorigenesis. The present article aims to describe the "social" life of senescent cells: basically, SASP constitution, molecular mechanisms of its regulation, and its functional role.

The role of cellular senescence in aging through the prism of Koch-like criteria

Since Hayflick's discovery of cellular senescence (CS), a great volume of knowledge in the field has been accumulated and intensively discussed. Here, we attempted to organize the evidence "for" and "against" the hypothesized causal role of CS in aging. For that purpose, we utilized robust Koch-like logical criteria, based on the assumption that some quantitative relationships between the accumulation of senescent cells and aging rate should exist. If so, it could be expected that (i) the "CS load" would be greater in the premature aging phenotype and lesser in longevity phenotype; (ii) CS would promote age-related diseases, and (iii) the interventions that modulate the levels of senescent cells should also modulate health/lifespan. The analysis shows that CS can be considered a causal factor of aging and an important player in various age-related diseases, though its contribution may greatly vary across species. While the relative impact of senescent cells to aging could overall be rather limited and their elimination is hardly expected to be the "fountain of youth", the potential benefits of the senolytic strategy seems a promising option in combating age-related diseases and extending healthspan.