Nuclear protein accumulation in cellular senescence and organismal aging revealed with a novel single-cell resolution fluorescence microscopy assay

Molecular signatures of neurodegenerative diseases identified by proteomic and phosphoproteomic analyses in aging mouse brain

A central hallmark of neurodegenerative diseases is the irreversible accumulation of misfolded proteins in the brain by aberrant phosphorylation. Understanding the mechanisms underlying protein phosphorylation and its role in pathological protein aggregation within the context of aging is crucial for developing therapeutic strategies aimed at preventing or reversing such diseases. Here, we applied multi-protease digestion and quantitative mass spectrometry to compare and characterize dysregulated proteins and phosphosites in the mouse brain proteome using three different age groups: young-adult (3-4 months), middle-age (10 months), and old mice (19-21 months). Proteins associated with senescence, neurodegeneration, inflammation, cell cycle regulation, the p53 hallmark pathway, and cytokine signaling showed significant age-dependent changes in abundances and level of phosphorylation. Several proteins implicated in Alzheimer's and Parkinson's diseases including tau (Mapt), Nefh, and Dpysl2 (also known as Crmp2) were hyperphosphorylated in old mice brain suggesting their susceptibility to the diseases. Cdk5 and Gsk3b, which are known to phosphorylate Dpysl2 at multiple specific sites, had also increased phosphorylation levels in old mice suggesting a potential crosstalk between them to contribute to AD. Hapln2, which promotes α-synuclein aggregation in patients with PD, was one of the proteins with highest abundance in old mice. CD9, which regulates senescence through the PI3K-AKT-mTOR-p53 signaling was upregulated in old mice and its regulation was correlated with the activation of phosphorylated AKT1. Overall, the findings identify a significant association between aging and the dysregulation of proteins involved in various pathways linked to neurodegenerative diseases with potential therapeutic implications.

Cellular senescence in lung cancer: Molecular mechanisms and therapeutic interventions

Lung cancer stands as the primary contributor to cancer-related fatalities worldwide, affecting both genders. Two primary types exist where non-small cell lung cancer (NSCLC), accounts for 80-85% and SCLC accounts for 10-15% of cases. NSCLC subtypes include adenocarcinoma, squamous cell carcinoma, and large cell carcinoma. Smoking, second-hand smoke, radon gas, asbestos, and other pollutants, genetic predisposition, and COPD are lung cancer risk factors. On the other hand, stresses such as DNA damage, telomere shortening, and oncogene activation cause a prolonged cell cycle halt, known as senescence. Despite its initial role as a tumor-suppressing mechanism that slows cell growth, excessive or improper control of this process can cause age-related diseases, including cancer. Cellular senescence has two purposes in lung cancer. Researchers report that senescence slows tumor growth by constraining multiplication of impaired cells. However, senescent cells also demonstrate the pro-inflammatory senescence-associated secretory phenotype (SASP), which is widely reported to promote cancer. This review will look at the role of cellular senescence in lung cancer, describe its diagnostic markers, ask about current treatments to control it, look at case studies and clinical trials that show how senescence-targeting therapies can be used in lung cancer, and talk about problems currently being faced, and possible solutions for the same in the future.

Accumulation of senescent cells in the adrenal gland induces hypersecretion of corticosterone via IL1β secretion

Aging progresses through the interaction of metabolic processes, including changes in the immune and endocrine systems. Glucocorticoids (GCs), which are regulated by the hypothalamic-pituitary-adrenal (HPA) axis, play an important role in regulating metabolism and immune responses. However, the age-related changes in the secretion mechanisms of GCs remain elusive. Here, we found that corticosterone (CORT) secretion follows a circadian rhythm in young mice, whereas it oversecreted throughout the day in aged mice >18 months old, resulting in the disappearance of diurnal variation. Furthermore, senescent cells progressively accumulated in the zF of the adrenal gland as mice aged beyond 18 months. This accumulation was accompanied by an increase in the number of Ad4BP/SF1 (SF1), a key transcription factor, strongly expressing cells (SF1-high positive: HP). Removal of senescent cells with senolytics, dasatinib, and quercetin resulted in the reduction of the number of SF1-HP cells and recovery of CORT diurnal oscillation in 24-month-old mice. Similarly, administration of a neutralizing antibody against IL1β, which was found to be strongly expressed in the adrenocortical cells of the zF, resulted in a marked decrease in SF1-HP cells and restoration of the CORT circadian rhythm. Our findings suggest that the disappearance of CORT diurnal oscillation is a characteristic of aging individuals and is caused by the secretion of IL1β, one of the SASPs, from senescent cells that accumulate in the zF of the adrenal cortex. These findings provide a novel insight into aging. Age-related hypersecretory GCs could be a potential therapeutic target for aging-related diseases.

Cell-cycle and Age-Related Modulations in Mouse Chromosome Stiffness

The intricate structure of chromosomes is complex, and many aspects of chromosome configuration/organization remain to be fully understood. Measuring chromosome stiffness can provide valuable insights into their structure. However, the nature of chromosome stiffness, whether static or dynamic, remains elusive. In this study, we analyzed chromosome stiffness in MI and MII oocytes. We revealed that MI oocytes had a ten-fold increase in stiffness compared to mitotic chromosomes, whereas chromosome stiffness in MII oocytes was relatively low chromosome. We then investigated the contribution of meiosis-specific cohesin complexes to chromosome stiffness in MI and MII oocytes. Surprisingly, the Young's modulus of chromosomes from the three meiosis-specific cohesin mutants did not exhibit significant differences compared to the wild type, indicating that these proteins may not play a substantial role in determining chromosome stiffness. Additionally, our findings revealed an age-related increase in chromosome stiffness in MI oocytes. Age correlates with elevated DNA damage levels, so we investigated the impact of etoposide-induced DNA damage on chromosome stiffness, discovering a reduction in stiffness in response to such damage in MI oocytes. Overall, our study underscores the dynamic nature of chromosome stiffness, subject to changes influenced by the cell cycle and age.

Nuclear Phosphoproteome Reveals Prolyl Isomerase PIN1 as a Modulator of Oncogene-Induced Senescence

Mammalian cells possess intrinsic mechanisms to prevent tumorigenesis upon deleterious mutations, including oncogene-induced senescence (OIS). The molecular mechanisms underlying OIS are, however, complex and remain to be fully characterized. In this study, we analyzed the changes in the nuclear proteome and phosphoproteome of human lung fibroblast IMR90 cells during the progression of OIS induced by oncogenic RASG12V activation. We found that most of the differentially regulated phosphosites during OIS contained prolyl isomerase PIN1 target motifs, suggesting PIN1 is a key regulator of several promyelocytic leukemia nuclear body proteins, specifically regulating several proteins upon oncogenic Ras activation. We showed that PIN1 knockdown promotes cell proliferation, while diminishing the senescence phenotype and hallmarks of senescence, including p21, p16, and p53 with concomitant accumulation of the protein PML and the dysregulation of promyelocytic leukemia nuclear body formation. Collectively, our data demonstrate that PIN1 plays an important role as a tumor suppressor in response to oncogenic ER:RasG12V activation.

Androgen Receptor Activation Induces Senescence in Thyroid Cancer Cells

Thyroid cancer (TC) is the most common endocrine malignancy, with an approximately three-fold higher incidence in women. TCGA data indicate that androgen receptor (AR) RNA is significantly downregulated in PTC. In this study, AR-expressing 8505C (anaplastic TC) (84E7) and K1 (papillary TC) cells experienced an 80% decrease in proliferation over 6 days of exposure to physiological levels of 5α-dihydrotestosterone (DHT). In 84E7, continuous AR activation resulted in G1 growth arrest, accompanied by a flattened, vacuolized cell morphology, with enlargement of the cell and the nuclear area, which is indicative of senescence; this was substantiated by an increase in senescence-associated β-galactosidase activity, total RNA and protein content, and reactive oxygen species. Additionally, the expression of tumor suppressor proteins p16, p21, and p27 was significantly increased. A non-inflammatory senescence-associated secretory profile was induced, significantly decreasing inflammatory cytokines and chemokines such as IL-6, IL-8, TNF, RANTES, and MCP-1; this is consistent with the lower incidence of thyroid inflammation and cancer in men. Migration increased six-fold, which is consistent with the clinical observation of increased lymph node metastasis in men. Proteolytic invasion potential was not significantly altered, which is consistent with unchanged MMP/TIMP expression. Our studies provide evidence that the induction of senescence is a novel function of AR activation in thyroid cancer cells, and may underlie the protective role of AR activation in the decreased incidence of TC in men.

Minoxidil Regulates Aging-Like Phenotypes in Rat Cortical Astrocytes In Vitro

Mainly due to the slanted focus on the mechanism and regulation of neuronal aging, research on astrocyte aging and its modulation during brain aging is scarce. In this study, we established aged astrocyte culture model by long-term culturing. Cellular senescence was confirmed through SA-β-gal staining as well as through the examination of morphological, molecular, and functional markers. RNA sequencing and functional analysis of astrocytes were performed to further investigate the detailed characteristics of the aged astrocyte model. Along with aged phenotypes, decreased astrocytic proliferation, migration, mitochondrial energetic function and support for neuronal survival and differentiation has been observed in aged astrocytes. In addition, increased expression of cytokines and chemokine-related factors including plasminogen activator inhibitor -1 (PAI-1) was observed in aged astrocytes. Using the RNA sequencing results, we searched potential drugs that can normalize the dysregulated gene expression pattern observed in long-term cultured aged astrocytes. Among several candidates, minoxidil, a pyrimidine-derived anti-hypertensive and anti-pattern hair loss drug, normalized the increased number of SA-β-gal positive cells and nuclear size in aged astrocytes. In addition, minoxidil restored up-regulated activity of PAI-1 and increased mitochondrial superoxide production in aged astrocytes. We concluded that long term culture of astrocytes can be used as a reliable model for the study of astrocyte senescence and minoxidil can be a plausible candidate for the regulation of brain aging.

Associations between exercise capacity, p16INK4a expression and inflammation among adult survivors of childhood cancer

BACKGROUND

Over 50% of childhood cancer survivors are exercise intolerant, with maximal aerobic capacities comparable to individuals decades older, suggesting early physiologic ageing. In addition, 36% of survivors are obese. Optimal exercise capacity provides a foundation to support daily function and healthy body habitus and is associated with benefits to cognition, cardiovascular health, and longevity. Cellular senescence and inflammation are key mechanisms that drive age-related disease, quantifiable as biomarkers in peripheral blood.

AIMS

This study aimed to evaluate associations between p16INKa, a biomarker of cellular senescence, and inflammation and exercise capacity among adult survivors of childhood cancer.

MATERIALS AND METHODS

Eligible survivors were recruited from the St. Jude Lifetime (SJLIFE) Cohort Study. Exercise capacity was assessed by maximal oxygen uptake (VO2, ml/kg/min) obtained via cardiopulmonary exercise testing using a modified Bruce protocol. Body fat (%) was determined from dual energy x-ray absorptiometry (DEXA). Peripheral blood samples were used to evaluate log2 p16INK4a mRNA expression, a biomarker of cellular senescence, and inflammation with high sensitivity C-reactive protein (hs-CRP) levels. Multivariable regression evaluated associations between p16INK4a, hs-CRP, body fat, and exercise capacity.

RESULTS

Participants included 185 five-year childhood cancer survivors (mean age 36.6 [range 20.1 - 55.7] years, 44% male, 77% non-Hispanic white, 53% leukemia/lymphoma). Compared to males, females had lower peak VO2 (mean ± SD, 22.5 ± 8.2 vs. 28.8 ± 7.7 ml/kg/min, p<0.01), higher p16INK4a expression (9.6 ± 1.2 vs. 9.2 ± 1.2 fold, p=0.02), and hs-CRP concentration (5.9 ± 8.4 vs. 3.3 ± 3.9 mg/L, p=0.01). Among females (n=103), hs-CRP concentration (β -0.2, 95% CI -0.34 to -0.05, p=0.01) and p16INK4a expression (β-5.32, 95% CI 10.42 to -0.22, p=0.04) were inversely associated and statistically significant with peak exercise capacity, with a significant interaction between p16INK4a expression and body fat (β 0.15, 95% CI 0.02 to 0.28, p=0.03). Among males (n=82), p16INK4a expression (β -1.01, 95% CI -2.14 to 0.12, p=0.08), and body fat (β -0.54, 95% CI -0.70 to -0.38, p<0.01) were inversely associated with peak exercise capacity.

CONCLUSION

Inflammation and p16INK4a expression, a biomarker of cellular senescence, are associated with lower exercise capacity in childhood cancer survivors, suggesting potential targets or outcome measures for interventions designed to prevent or remediate accelerated physiologic ageing in this population.

Late Passage Cultivation Induces Aged Astrocyte Phenotypes in Rat Primary Cultured Cells

Astrocytes play various important roles such as maintaining brain homeostasis, supporting neurons, and secreting inflammatory mediators to protect the brain cells. In aged subjects, astrocytes show diversely changed phenotypes and dysfunctions. But, the study of aged astrocytes or astrocytes from aged subjects is not yet sufficient to provide a comprehensive understanding of their important processes in the regulation of brain function. In this study, we induced an in vitro aged astrocyte model through late passage cultivation of rat primary cultured astrocytes. Astrocytes were cultured until passage 7 (P7) as late passage astrocytes and compared with passage 1 (P1) astrocytes as early passage astrocytes to confirm the differences in phenotypes and the effects of serial passage. In this study, we confirmed the morphological, molecular, and functional changes of late passage astrocytes showing aging phenotypes through SA-β-gal staining and measurement of nuclear size. We also observed a reduced expression of inflammatory mediators including IL-1β, IL-6, TNFα, iNOS, and COX2, as well as dysregulation of wound-healing, phagocytosis, and mitochondrial functions such as mitochondrial membrane potential and mitochondrial oxygen consumption rate. Culture-conditioned media obtained from P1 astrocytes promoted neurite outgrowth in immature primary cultures of rat cortices, which is significantly reduced when we treated the immature neurons with the culture media obtained from P7 astrocytes. These results suggest that late passage astrocytes show senescent astrocyte phenotypes with functional defects, which makes it a suitable model for the study of the role of astrocyte senescence on the modulation of normal and pathological brain aging.

Nanomechanical insights: Amyloid beta oligomer-induced senescent brain endothelial cells

Senescent cells accumulate in various peripheral tissues during aging and have been shown to exacerbate age-related inflammatory responses. We recently showed that exposure to neurotoxic amyloid β (Aβ1-42) oligomers can readily induce a senescence phenotype in human brain microvascular endothelial cells (HBMECs). In the present work, we used atomic force microscopy (AFM) to further characterize the morphological properties such as cell membrane roughness and cell height and nanomechanical properties such as Young's modulus of the membrane (membrane stiffness) and adhesion resulting from the interaction between AFM tip and cell membrane in Aβ1-42 oligomer-induced senescent human brain microvascular endothelial cells. Morphological imaging studies showed a flatter and spread-out nucleus in the senescent HBMECs, both characteristic features of a senescent phenotype. Furthermore, the mean cell body roughness and mean cell height were lower in senescent HBMECs compared to untreated normal HBMECs. We also observed increased stiffness and alterations in the adhesion properties in Aβ1-42 oligomer-induced senescent endothelial cells compared to the untreated normal HBMECs suggesting dynamic reorganization of cell membrane. We then show that vascular endothelial growth factor receptor 1 (VEGFR-1) knockdown or overexpression of Rho GTPase Rac 1 in the endothelial cells inhibited senescence and reversed these nanomechanical alterations, confirming a direct role of these pathways in the senescent brain endothelial cells. These results illustrate that nanoindentation and topographic analysis of live senescent brain endothelial cells can provide insights into cerebrovascular dysfunction in neurodegenerative diseases such as Alzheimer's disease.

Etoposide Induces Mitochondrial Dysfunction and Cellular Senescence in Primary Cultured Rat Astrocytes

Brain aging is an inevitable process characterized by structural and functional changes and is a major risk factor for neurodegenerative diseases. Most brain aging studies are focused on neurons and less on astrocytes which are the most abundant cells in the brain known to be in charge of various functions including the maintenance of brain physical formation, ion homeostasis, and secretion of various extracellular matrix proteins. Altered mitochondrial dynamics, defective mitophagy or mitochondrial damages are causative factors of mitochondrial dysfunction, which is linked to age-related disorders. Etoposide is an anti-cancer reagent which can induce DNA stress and cellular senescence of cancer cell lines. In this study, we investigated whether etoposide induces senescence and functional alterations in cultured rat astrocytes. Senescence-associated β-galactosidase (SA-β-gal) activity was used as a cellular senescence marker. The results indicated that etoposide-treated astrocytes showed cellular senescence phenotypes including increased SA-β-gal-positive cells number, increased nuclear size and increased senescence-associated secretory phenotypes (SASP) such as IL-6. We also observed a decreased expression of cell cycle markers, including Phospho- Histone H3/Histone H3 and CDK2, and dysregulation of cellular functions based on wound-healing, neuronal protection, and phagocytosis assays. Finally, mitochondrial dysfunction was noted through the determination of mitochondrial membrane potential using tetramethylrhodamine methyl ester (TMRM) and the measurement of mitochondrial oxygen consumption rate (OCR). These data suggest that etoposide can induce cellular senescence and mitochondrial dysfunction in astrocytes which may have implications in brain aging and neurodegenerative conditions.

Golgi Outpost Synthesis Impaired by Toxic Polyglutamine Proteins Contributes to Dendritic Pathology in Neurons

Dendrite aberration is a common feature of neurodegenerative diseases caused by protein toxicity, but the underlying mechanisms remain largely elusive. Here, we show that nuclear polyglutamine (polyQ) toxicity resulted in defective terminal dendrite elongation accompanied by a loss of Golgi outposts (GOPs) and a decreased supply of plasma membrane (PM) in Drosophila class IV dendritic arborization (da) (C4 da) neurons. mRNA sequencing revealed that genes downregulated by polyQ proteins included many secretory pathway-related genes, including COPII genes regulating GOP synthesis. Transcription factor enrichment analysis identified CREB3L1/CrebA, which regulates COPII gene expression. CrebA overexpression in C4 da neurons restores the dysregulation of COPII genes, GOP synthesis, and PM supply. Chromatin immunoprecipitation (ChIP)-PCR revealed that CrebA expression is regulated by CREB-binding protein (CBP), which is sequestered by polyQ proteins. Furthermore, co-overexpression of CrebA and Rac1 synergistically restores the polyQ-induced dendrite pathology. Collectively, our results suggest that GOPs impaired by polyQ proteins contribute to dendrite pathology through the CBP-CrebA-COPII pathway.

Caveolin-1, cancer and therapy resistance

Resistance of solid tumors to chemo- and radiotherapy remains a major obstacle in anti-cancer treatment. Herein, the membrane protein caveolin-1 (CAV1) came into focus as it is highly expressed in many tumors and high CAV1 levels were correlated with tumor progression, invasion and metastasis, and thus a worse clinical outcome. Increasing evidence further indicates that the heterogeneous tumor microenvironment, also known as the tumor stroma, contributes to therapy resistance resulting in poor clinical outcome. Again, CAV1 seems to play an important role in modulating tumor host interactions by promoting tumor growth, metastasis, therapy resistance and cell survival. However, the mechanisms driving stroma-mediated tumor growth and radiation resistance remain to be clarified. Understanding these interactions and thus, targeting CAV1 may offer a novel strategy for preventing cancer therapy resistance and improving clinical outcomes. In this review, we will summarize the resistance-promoting effects of CAV1 in tumors, and emphasize its role in the tumor-stroma communication as well as the resulting malignant phenotype of epithelial tumors.

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.

Biomarkers to identify and isolate senescent cells

Aging is the main risk factor for many degenerative diseases and declining health. Senescent cells are part of the underlying mechanism for time-dependent tissue dysfunction. These cells can negatively affect neighbouring cells through an altered secretory phenotype: the senescence-associated secretory phenotype (SASP). The SASP induces senescence in healthy cells, promotes tumour formation and progression, and contributes to other age-related diseases such as atherosclerosis, immune-senescence and neurodegeneration. Removal of senescent cells was recently demonstrated to delay age-related degeneration and extend lifespan. To better understand cell aging and to reap the benefits of senescent cell removal, it is necessary to have a reliable biomarker to identify these cells. Following an introduction to cellular senescence, we discuss several classes of biomarkers in the context of their utility in identifying and/or removing senescent cells from tissues. Although senescence can be induced by a variety of stimuli, senescent cells share some characteristics that enable their identification both in vitro and in vivo. Nevertheless, it may prove difficult to identify a single biomarker capable of distinguishing senescence in all cell types. Therefore, this will not be a comprehensive review of all senescence biomarkers but rather an outlook on technologies and markers that are most suitable to identify and isolate senescent cells.

Cellular Senescence as the Causal Nexus of Aging

In this paper we present cellular senescence as the ultimate driver of the aging process, as a "causal nexus" that bridges microscopic subcellular damage with the phenotypic, macroscopic effect of aging. It is important to understand how the various types of subcellular damage correlated with the aging process lead to the larger, visible effects of anatomical aging. While it has always been assumed that subcellular damage (cause) results in macroscopic aging (effect), the bridging link between the two has been hard to define. Here, we propose that this bridge, which we term the "causal nexus", is in fact cellular senescence. The subcellular damage itself does not directly cause the visible signs of aging, but rather, as the damage accumulates and reaches a critical mass, cells cease to proliferate and acquire the deleterious "senescence-associated secretory phenotype" (SASP) which then leads to the macroscopic consequences of tissue breakdown to create the physiologically aged phenotype. Thus senescence is a precondition for anatomical aging, and this explains why aging is a gradual process that remains largely invisible during most of its progression. The subcellular damage includes shortening of telomeres, damage to mitochondria, aneuploidy, and DNA double-strand breaks triggered by various genetic, epigenetic, and environmental factors. Damage pathways acting in isolation or in concert converge at the causal nexus of cellular senescence. In each species some types of damage can be more causative than in others and operate at a variable pace; for example, telomere erosion appears to be a primary cause in human cells, whereas activation of tumor suppressor genes is more causative in rodents. Such species-specific mechanisms indicate that despite different initial causes, most of aging is traced to a single convergent causal nexus: senescence. The exception is in some invertebrate species that escape senescence, and in non-dividing cells such as neurons, where senescence still occurs, but results in the SASP rather than loss of proliferation plus SASP. Aging currently remains an inevitable endpoint for most biological organisms, but the field of cellular senescence is primed for a renaissance and as our understanding of aging is refined, strategies capable of decelerating the aging process will emerge.

Reorganization of chromosome architecture in replicative cellular senescence

Replicative cellular senescence is a fundamental biological process characterized by an irreversible arrest of proliferation. Senescent cells accumulate a variety of epigenetic changes, but the three-dimensional (3D) organization of their chromatin is not known. We applied a combination of whole-genome chromosome conformation capture (Hi-C), fluorescence in situ hybridization, and in silico modeling methods to characterize the 3D architecture of interphase chromosomes in proliferating, quiescent, and senescent cells. Although the overall organization of the chromatin into active (A) and repressive (B) compartments and topologically associated domains (TADs) is conserved between the three conditions, a subset of TADs switches between compartments. On a global level, the Hi-C interaction matrices of senescent cells are characterized by a relative loss of long-range and gain of short-range interactions within chromosomes. Direct measurements of distances between genetic loci, chromosome volumes, and chromatin accessibility suggest that the Hi-C interaction changes are caused by a significant reduction of the volumes occupied by individual chromosome arms. In contrast, centromeres oppose this overall compaction trend and increase in volume. The structural model arising from our study provides a unique high-resolution view of the complex chromosomal architecture in senescent cells.

Autophagy through 4EBP1 and AMPK regulates oxidative stress-induced premature senescence in auditory cells

The aim of this study was to determine whether autophagy and AMPK contribute to premature senescence in auditory cells. Incubating HEI-OC1 auditory cells with 5 mM H2O2 for 1 h induced senescence, as demonstrated by senescence-associated β-galactosidase (SA-β-gal) staining. H2O2 treatment significantly delayed population-doubling time, leaving cell viability unchanged. Furthermore, the proportion of SA-β-gal-positive cells significantly increased. Autophagy-related protein expression increased, with Atg7 and LC3-II peaking 6 h and Lamp2 peaking 24 h after H2O2 treatment. The expression of these proteins decreased 48 h after treatment. Transmission electron microscopy revealed lipofuscin and aggregates within autolysosomes, which accumulated markedly in the cytoplasm of HEI-OC1 cells 48 h after treatment. Akt and P70S6 phosphorylation markedly decreased after H2O2 treatment, but 4EBP1 phosphorylation significantly increased 48 h after treatment. After RNAi-mediated knockdown (KD) of Atg7 and AMPK, H2O2-treated cells displayed dense SA-β-gal staining. Also, premature senescence was significantly induced. These suggest that a negative feedback loop may exist between autophagy and AMPK signaling pathways in HEI-OC1 cells. In our model, oxidative stress-induced premature senescence occurred due to impaired autophagy function through 4EBP1 phosphorylation. Our results also indicate that AMPK may regulate premature senescence in auditory cells in an autophagy-dependent and independent manner.

Cellular Senescence and the Biology of Aging, Disease, and Frailty

Population aging simultaneously highlights the remarkable advances in science, medicine, and public policy, and the formidable challenges facing society. Indeed, aging is the primary risk factor for many of the most common chronic diseases and frailty, which result in profound social and economic costs. Population aging also reveals an opportunity, i.e. interventions to disrupt the fundamental biology of aging could significantly delay the onset of age-related conditions as a group, and, as a result, extend the healthy life span, or health span. There is now considerable evidence that cellular senescence is an underlying mechanism of aging and age-related conditions. Cellular senescence is a process in which cells lose the ability to divide and damage neighboring cells by the factors they secrete, collectively referred to as the senescence-associated secretory phenotype (SASP). Herein, we discuss the concept of cellular senescence, review the evidence that implicates cellular senescence and SASP in age-related deterioration, hyperproliferation, and inflammation, and propose that this underlying mechanism of aging may play a fundamental role in the biology of frailty.

Geroconversion: irreversible step to cellular senescence

Cellular senescence happens in 2 steps: cell cycle arrest followed, or sometimes preceded, by gerogenic conversion (geroconversion). Geroconvesrion is a form of growth, a futile growth during cell cycle arrest. It converts reversible arrest to irreversible senescence. Geroconversion is driven by growth-promoting, mitogen-/nutrient-sensing pathways such as mTOR. Geroconversion leads to hyper-secretory, hypertrophic and pro-inflammatory cellular phenotypes, hyperfunctions and malfunctions. On organismal level, geroconversion leads to age-related diseases and death. Rapamycin, a gerosuppressant, extends life span in diverse species from yeast to mammals. Stress-and oncogene-induced accelerated senescence, replicative senescence in vitro and life-long cellular aging in vivo all can be described by 2-step model.

Active Degradation Explains the Distribution of Nuclear Proteins during Cellular Senescence

The amount of cellular proteins is a crucial parameter that is known to vary between cells as a function of the replicative passages, and can be important during physiological aging. The process of protein degradation is known to be performed by a series of enzymatic reactions, ranging from an initial step of protein ubiquitination to their final fragmentation by the proteasome. In this paper we propose a stochastic dynamical model of nuclear proteins concentration resulting from a balance between a constant production of proteins and their degradation by a cooperative enzymatic reaction. The predictions of this model are compared with experimental data obtained by fluorescence measurements of the amount of nuclear proteins in murine tail fibroblast (MTF) undergoing cellular senescence. Our model provides a three-parameter stationary distribution that is in good agreement with the experimental data even during the transition to the senescent state, where the nuclear protein concentration changes abruptly. The estimation of three parameters (cooperativity, saturation threshold, and maximal velocity of the reaction), and their evolution during replicative passages shows that only the maximal velocity varies significantly. Based on our modeling we speculate the reduction of functionality of the protein degradation mechanism as a possible competitive inhibition of the proteasome.

The emerging role of senescent cells in tissue homeostasis and pathophysiology

Cellular senescence is a state of permanent growth arrest and is thought to play a pivotal role in tumor suppression. Cellular senescence may play an important role in tumor suppression, wound healing, and protection against tissue fibrosis in physiological conditions in vivo. However, accumulating evidence that senescent cells may have harmful effects in vivo and may contribute to tissue remodeling, organismal aging, and many age-related diseases also exists. Cellular senescence can be induced by various intrinsic and extrinsic factors. Both p53/p21 and p16/RB pathways are important for irreversible growth arrest in senescent cells. Senescent cells secret numerous biologically active factors. This specific secretion phenotype by senescent cells may largely contribute to physiological and pathological consequences in organisms. Here I review the molecular basis of cell cycle arrest and the specific secretion phenotype in cellular senescence. I also summarize the current knowledge of the role of cellular senescence in vivo in physiological and pathological settings.

Mast cells exert pro-inflammatory effects of relevance to the pathophyisology of tendinopathy

Introduction

We have previously found an increased mast cell density in tendon biopsies from patients with patellar tendinopathy compared to controls. This study examined the influence of mast cells on basic tenocyte functions, including production of the inflammatory mediator prostaglandin E2 (PGE₂), extracellular matrix remodeling and matrix metalloproteinase (MMP) gene transcription, and collagen synthesis.

Methods

Primary human tenocytes were stimulated with an established human mast cell line (HMC-1). Extracellular matrix remodeling was studied by culturing tenocytes in a three-dimensional collagen lattice. Survival/proliferation was assessed with the 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium salt (MTS) assay. Levels of mRNA for COX-2, COL1A1, MMP1, and MMP7 were determined by quantitative real-time polymerase chain reaction (qPCR). Cox-2 protein level was assessed by Western blot analysis and type I procollagen was detected by immunofluorescent staining. PGE₂ levels were determined using an enzyme-linked immunosorbent assay (ELISA).

Results

Mast cells stimulated tenocytes to produce increased levels of COX-2 and the pro-inflammatory mediator PGE₂, which in turn decreased COL1A1 mRNA expression. Additionally, mast cells reduced the type I procollagen protein levels produced by tenocytes. Transforming growth factor beta 1 (TGF-β1) was responsible for the induction of Cox-2 and PGE₂ by tenocytes. Mast cells increased MMP1 and MMP7 transcription and increased the contraction of a three-dimensional collagen lattice by tenocytes, a phenomenon which was blocked by a pan-MMP inhibitor (Batimastat).

Conclusion

Our data demonstrate that mast cell-derived PGE₂ reduces collagen synthesis and enhances expression and activities of MMPs in human tenocytes.

Metabolic alterations accompanying oncogene-induced senescence

Senescence is defined as a stable cell growth arrest. Oncogene-induced senescence (OIS) occurs in normal primary human cells after activation of an oncogene in the absence of other cooperating oncogenic stimuli. OIS is therefore considered a bona fide tumor suppression mechanism in vivo. Indeed, overcoming OIS-associated stable cell growth arrest can lead to tumorigenesis. Although cells that have undergone OIS do not replicate their DNA, they remain metabolically active. A number of recent studies report significant changes in cellular metabolism during OIS, including alterations in nucleotide, glucose, and mitochondrial metabolism and autophagy. These alterations may be necessary for stable senescence-associated cell growth arrest, and overcoming these shifts in metabolism may lead to tumorigenesis. This review highlights what is currently known about alterations in cellular metabolism during OIS and the implication of OIS-associated metabolic changes in cellular transformation and the development of cancer therapeutic strategies.

Myogenic potential of canine craniofacial satellite cells

The skeletal fibers have different embryological origin; the extraocular and jaw-closer muscles develop from prechordal mesoderm while the limb and trunk muscles from somites. These different origins characterize also the adult muscle stem cells, known as satellite cells (SCs) and responsible for the fiber growth and regeneration. The physiological properties of presomitic SCs and their epigenetics are poorly studied despite their peculiar characteristics to preserve muscle integrity during chronic muscle degeneration. Here, we isolated SCs from canine somitic [somite-derived muscle (SDM): vastus lateralis, rectus abdominis, gluteus superficialis, biceps femoris, psoas] and presomitic [pre-somite-derived muscle (PSDM): lateral rectus, temporalis, and retractor bulbi] muscles as myogenic progenitor cells from young and old animals. In addition, SDM and PSDM-SCs were obtained also from golden retrievers affected by muscular dystrophy (GRMD). We characterized the lifespan, the myogenic potential and functions, and oxidative stress of both somitic and presomitic SCs with the aim to reveal differences with aging and between healthy and dystrophic animals. The different proliferation rate was consistent with higher telomerase activity in PSDM-SCs compared to SDM-SCs, although restricted at early passages. SDM-SCs express early (Pax7, MyoD) and late (myosin heavy chain, myogenin) myogenic markers differently from PSDM-SCs resulting in a more efficient and faster cell differentiation. Taken together, our results showed that PSDM-SCs elicit a stronger stem cell phenotype compared to SDM ones. Finally, myomiR expression profile reveals a unique epigenetic signature in GRMD SCs and miR-206, highly expressed in dystrophic SCs, seems to play a critical role in muscle degeneration. Thus, miR-206 could represent a potential target for novel therapeutic approaches.

Targets of immune regeneration in rheumatoid arthritis

Many of the aging-related morbidities, including cancer, cardiovascular disease, neurodegenerative disease, and infectious susceptibility, are linked to a decline in immune competence with a concomitant rise in proinflammatory immunity, placing the process of immune aging at the center of aging biology. Immune aging affects individuals older than 50 years and is accelerated in patients with the autoimmune disease rheumatoid arthritis. Immune aging results in a marked decline in protective immune responses and a parallel increase in tissue inflammatory responses. By studying immune cells in patients with rheumatoid arthritis, several of the molecular underpinnings of the immune aging process have been delineated, such as the loss of telomeres and inefficiencies in the repair of damaged DNA. Aging T cells display a series of abnormalities, including the unopposed up-regulation of cytoplasmic phosphatases and the loss of glycolytic competence, that alter their response to stimulating signals and undermine their longevity. Understanding the connection between accelerated immune aging and autoimmunity remains an area of active research. With increasing knowledge of the molecular pathways that cause immunosenescence, therapeutic interventions can be designed to slow or halt the seemingly inevitable deterioration of protective immunity with aging.

Transposable elements become active and mobile in the genomes of aging mammalian somatic tissues

Transposable elements (TEs) were discovered by Barbara McClintock in maize and have since been found to be ubiquitous in all living organisms. Transposition is mutagenic and organisms have evolved mechanisms to repress the activity of their endogenous TEs. Transposition in somatic cells is very low, but recent evidence suggests that it may be derepressed in some cases, such as cancer development. We have found that during normal aging several families of retrotransposable elements (RTEs) start being transcribed in mouse tissues. In advanced age the expression culminates in active transposition. These processes are counteracted by calorie restriction (CR), an intervention that slows down aging. Retrotransposition is also activated in age-associated, naturally occurring cancers in the mouse. We suggest that somatic retrotransposition is a hitherto unappreciated aging process. Mobilization of RTEs is likely to be an important contributor to the progressive dysfunction of aging cells.

Dynamics of histone H3.3 deposition in proliferating and senescent cells reveals a DAXX-dependent targeting to PML-NBs important for pericentromeric heterochromatin organization

Oncogene-induced senescence is a permanent cell cycle arrest characterized by extensive chromatin reorganization. Here, we investigated the specific targeting and dynamics of histone H3 variants in human primary senescent cells. We show that newly synthesized epitope-tagged H3.3 is incorporated in senescent cells but does not accumulate in senescence-associated heterochromatin foci (SAHF). Instead, we observe that new H3.3 colocalizes with its specific histone chaperones within the promyelocytic leukemia nuclear bodies (PML-NBs) and is targeted to PML-NBs in a DAXX-dependent manner both in proliferating and senescent cells. We further show that overexpression of DAXX enhances targeting of H3.3 in large PML-NBs devoid of transcriptional activity and promotes the accumulation of HP1, independently of H3K9me3. Loss of H3.3 from pericentromeric heterochromatin upon DAXX or PML depletion suggests that the targeting of H3.3 to PML-NBs is implicated in pericentromeric heterochromatin organization. Together, our results underline the importance of the replication-independent chromatin assembly pathway for histone replacement in non-dividing senescent cells and establish PML-NBs as important regulatory sites for the incorporation of new H3.3 into chromatin.

Lamins are rapamycin targets that impact human longevity: a study in centenarians

The dynamic organisation of the cell nucleus is profoundly modified during growth, development and senescence as a result of changes in chromatin arrangement and gene transcription. A plethora of data suggests that the nuclear lamina is a key player in chromatin dynamics and argues in favour of a major involvement of prelamin A in fundamental mechanisms regulating cellular senescence and organism ageing. As the best model to analyse the role of prelamin A in normal ageing, we used cells from centenarian subjects. We show that prelamin A is accumulated in fibroblasts from centenarians owing to downregulation of its specific endoprotease ZMPSTE24, whereas other nuclear envelope constituents are mostly unaffected and cells do not enter senescence. Accumulation of prelamin A in nuclei of cells from centenarians elicits loss of heterochromatin, as well as recruitment of the inactive form of 53BP1, associated with rapid response to oxidative stress. These effects, including the prelamin-A-mediated increase of nuclear 53BP1, can be reproduced by rapamycin treatment of cells from younger individuals. These data identify prelamin A and 53BP1 as new targets of rapamycin that are associated with human longevity. We propose that the reported mechanisms safeguard healthy ageing in humans through adaptation of the nuclear environment to stress stimuli.

Comparative effect of Piper betle, Chlorella vulgaris and tocotrienol-rich fraction on antioxidant enzymes activity in cellular ageing of human diploid fibroblasts

Background

Human diploid fibroblasts (HDFs) undergo a limited number of cellular divisions in culture and progressively reach a state of irreversible growth arrest, a process termed cellular ageing. Even though beneficial effects of Piper betle, Chlorella vulgaris and tocotrienol-rich fraction (TRF) have been reported, ongoing studies in relation to ageing is of interest to determine possible protective effects that may reverse the effect of ageing. The aim of this study was to evaluate the effect of P. betle, C. vulgaris and TRF in preventing cellular ageing of HDFs by determining the activity of antioxidant enzymes viz.; catalase, superoxide dismutase (SOD) and glutathione peroxidase.

Methods

Different passages of HDFs were treated with P. betle, C. vulgaris and TRF for 24 h prior to enzymes activity determination. Senescence-associated beta-galactosidase (SA β-gal) expression was assayed to validate cellular ageing.

Results

In cellular ageing of HDFs, catalase and glutathione peroxidase activities were reduced, but SOD activity was heightened during pre-senescence. P. betle exhibited the strongest antioxidant activity by reducing SA β-gal expression, catalase activities in all age groups, and SOD activity. TRF exhibited a strong antioxidant activity by reducing SA β-gal expression, and SOD activity in senescent HDFs. C. vulgaris extract managed to reduce SOD activity in senescent HDFs.

Conclusion

P. betle, C. vulgaris, and TRF have the potential as anti-ageing entities which compensated the role of antioxidant enzymes in cellular ageing of HDFs.

Lysosome-mediated processing of chromatin in senescence

Senescent cells extrude fragments of chromatin from the nucleus into the cytoplasm, where they are processed by an autophagic/lysosomal pathway.

Cellular senescence is a stable proliferation arrest, a potent tumor suppressor mechanism, and a likely contributor to tissue aging. Cellular senescence involves extensive cellular remodeling, including of chromatin structure. Autophagy and lysosomes are important for recycling of cellular constituents and cell remodeling. Here we show that an autophagy/lysosomal pathway processes chromatin in senescent cells. In senescent cells, lamin A/C–negative, but strongly γ-H2AX–positive and H3K27me3-positive, cytoplasmic chromatin fragments (CCFs) budded off nuclei, and this was associated with lamin B1 down-regulation and the loss of nuclear envelope integrity. In the cytoplasm, CCFs were targeted by the autophagy machinery. Senescent cells exhibited markers of lysosomal-mediated proteolytic processing of histones and were progressively depleted of total histone content in a lysosome-dependent manner. In vivo, depletion of histones correlated with nevus maturation, an established histopathologic parameter associated with proliferation arrest and clinical benignancy. We conclude that senescent cells process their chromatin via an autophagy/lysosomal pathway and that this might contribute to stability of senescence and tumor suppression.

Probing the depths of cellular senescence

Cellular senescence is a state of irreversible cell cycle arrest that has been documented to both suppress cancer and promote aging. Although not well understood, extensive nuclear changes, including the remodeling of chromatin, take place as cells become senescent. In this issue, Ivanov et al. (2013. J. Cell Biol.http://dx.doi.org/jcb.201212110) report that chromatin fragments are released from the nuclei of senescent cells and are subsequently targeted for processing through the autophagy/lysosomal pathway.

Cellular senescence and the senescent secretory phenotype: therapeutic opportunities

Aging is the largest risk factor for most chronic diseases, which account for the majority of morbidity and health care expenditures in developed nations. New findings suggest that aging is a modifiable risk factor, and it may be feasible to delay age-related diseases as a group by modulating fundamental aging mechanisms. One such mechanism is cellular senescence, which can cause chronic inflammation through the senescence-associated secretory phenotype (SASP). We review the mechanisms that induce senescence and the SASP, their associations with chronic disease and frailty, therapeutic opportunities based on targeting senescent cells and the SASP, and potential paths to developing clinical interventions.