Postmitotic neurons develop a p21-dependent senescence-like phenotype driven by a DNA damage response

In vivo 5-ethynyluridine (EU) labelling detects reduced transcription in Purkinje cell degeneration mouse mutants, but can itself induce neurodegeneration

Fluorescent staining of newly transcribed RNA via metabolic labelling with 5-ethynyluridine (EU) and click chemistry enables visualisation of changes in transcription, such as in conditions of cellular stress. Here, we tested whether EU labelling can be used to examine transcription in vivo in mouse models of nervous system disorders. We show that injection of EU directly into the cerebellum results in reproducible labelling of newly transcribed RNA in cerebellar neurons and glia, with cell type-specific differences in relative labelling intensities, such as Purkinje cells exhibiting the highest levels. We also observed EU-labelling accumulating into cytoplasmic inclusions, indicating that EU, like other modified uridines, may introduce non-physiological properties in labelled RNAs. Additionally, we found that EU induces Purkinje cell degeneration nine days after EU injection, suggesting that EU incorporation not only results in abnormal RNA transcripts, but also eventually becomes neurotoxic in highly transcriptionally-active neurons. However, short post-injection intervals of EU labelling in both a Purkinje cell-specific DNA repair-deficient mouse model and a mouse model of spinocerebellar ataxia 1 revealed reduced transcription in Purkinje cells compared to controls. We combined EU labelling with immunohistology to correlate altered EU staining with pathological markers, such as genotoxic signalling factors. These data indicate that the EU-labelling method provided here can be used to identify changes in transcription in vivo in nervous system disease models.

New insights into molecular changes in skeletal muscle aging and disease: Differential alternative splicing and senescence

Progressive loss of muscle mass and function due to muscle fiber atrophy and loss in the elderly and chronically ill is now defined as sarcopenia. It is a major contributor to loss of independence, disability, need of long-term care as well as overall mortality. Sarcopenia is a heterogenous disease and underlying mechanisms are not completely understood. Here, we newly identified and used Tmem158, alongside Cdkn1a, as relevant senescence and denervation markers (SDMs), associated with muscle fiber atrophy. Subsequent application of laser capture microdissection (LCM) and RNA analyses revealed age- and disease-associated differences in gene expression and alternative splicing patterns in a rodent sarcopenia model. Of note, genes exhibiting such differential alternative splicing (DAS) are mainly involved in the contractile function of the muscle. Many of these splicing events are also found in a mouse model for myotonic dystrophy type 1 (DM1), underscoring the premature aging phenotype of this disease. We propose to add differential alternative splicing to the hallmarks of aging.

Neuroimaging Spectrum at Pre-, Early, and Late Symptomatic Stages of SCA17 Mice

Spinocerebellar ataxia (SCA) is a hereditary neurodegenerative disease. We have generated SCA17 transgenic mice bearing human TBP with 109 CAG repeats under the Purkinje cell-specific L7/pcp2 promoter. These mice recapitulate the patients' phenotypes and are suitable for the study of the SCA17 pathomechanism. Magnetic resonance imaging (MRI) and immunostainings were performed to identify the neuroimaging spectrum during disease progression. The results indicate that despite an overall normal appearance at birth, postnatal brain damage takes place rapidly in SCA17. Cerebellar atrophy, fourth-ventricle enlargement, and reduced cerebellar N-acetylaspartate levels were detected at the presymptomatic stage, when the mice were juvenile. The aberrations, which included reductions in body weight; cerebral size; striatal size; and the mean, radial, and axial diffusivities of the cerebellum, became more salient as the disease progressed to the old, late-symptomatic stage. Phosphorylated H2A histone family, member X (γH2AX) immunostaining revealed that the cerebellum underwent severe cell senescence in the old stage while the striatum appeared relatively unaffected by aging. Morphometric analysis indicated that the cerebellar atrophy occurred in all subregions with aging. The data establish that the SCA17 mouse brain appears normal at birth but becomes aberrant at the presymptomatic/juvenile stage. More widespread deficits add to the pathological spectrum at the old stage. The study provides information for the expression and expansion of L7/pcp2 promoter and implies the disease progression of SCA17 patients.

Long-Term Administration of Triterpenoids From Ganoderma lucidum Mitigates Age-Associated Brain Physiological Decline via Regulating Sphingolipid Metabolism and Enhancing Autophagy in Mice

With the advent of the aging society, how to grow old healthily has become an important issue for the whole of society. Effective intervention strategies for healthy aging are most desired, due to the complexity and diversity of genetic information, it is a pressing concern to find a single drug or treatment to improve longevity. In this study, long-term administration of triterpenoids of Ganoderma lucidum (TGL) can mitigate brain physiological decline in normal aging mice. In addition, the age-associated pathological features, including cataract formation, hair loss, and skin relaxation, brown adipose tissue accumulation, the β-galactosidase staining degree of kidney, the iron death of spleen, and liver functions exhibit improvement. We used the APP/PS1 mice and 3 × Tg-AD mice model of Alzheimer's Disease (AD) to further verify the improvement of brain function by TGL and found that Ganoderic acid A might be the effective constituent of TGL for anti-aging of the brain in the 3 × Tg-AD mice. A potential mechanism of action may involve the regulation of sphingolipid metabolism, prolonging of telomere length, and enhance autophagy, which allows for the removal of pathological metabolites.

Hydrogen sulfide in longevity and pathologies: Inconsistency is malodorous

Hydrogen sulfide (H₂S) is one of the biologically active gases (gasotransmitters), which plays an important role in various physiological processes and aging. Its production in the course of methionine and cysteine catabolism and its degradation are finely balanced, and impairment of H₂S homeostasis is associated with various pathologies. Despite the strong geroprotective action of exogenous H₂S in C. elegans, there are controversial effects of hydrogen sulfide and its donors on longevity in other models, as well as on stress resistance, age-related pathologies and aging processes, including regulation of senescence-associated secretory phenotype (SASP) and senescent cell anti-apoptotic pathways (SCAPs). Here we discuss that the translation potential of H₂S as a geroprotective compound is influenced by a multiplicity of its molecular targets, pleiotropic biological effects, and the overlapping ranges of toxic and beneficial doses. We also consider the challenges of the targeted delivery of H₂S at the required dose. Along with this, the complexity of determining the natural levels of H₂S in animal and human organs and their ambiguous correlations with longevity are reviewed.

Telomerase in Brain: The New Kid on the Block and Its Role in Neurodegenerative Diseases

Telomerase is an enzyme that in its canonical function extends and maintains telomeres, the ends of chromosomes. This reverse transcriptase function is mainly important for dividing cells that shorten their telomeres continuously. However, there are a number of telomere-independent functions known for the telomerase protein TERT (Telomerase Reverse Transcriptase). This includes the shuttling of the TERT protein from the nucleus to mitochondria where it decreases oxidative stress, apoptosis sensitivity and DNA damage. Recently, evidence has accumulated on a protective role of TERT in brain and postmitotic neurons. This function might be able to ameliorate the effects of toxic proteins such as amyloid-β, pathological tau and α-synuclein involved in neurodegenerative diseases such as Alzheimer’s disease (AD) and Parkinson’s disease (PD). However, the protective mechanisms of TERT are not clear yet. Recently, an activation of autophagy as an important protein degradation process for toxic neuronal proteins by TERT has been described. This review summarises the current knowledge about the non-canonical role of the telomerase protein TERT in brain and shows its potential benefit for the amelioration of brain ageing and neurodegenerative diseases such as AD and PD. This might form the basis for the development of novel strategies and therapies against those diseases.

Up-regulation of APP endocytosis by neuronal aging drives amyloid dependent-synapse loss

Neuronal aging increases the risk of late-onset Alzheimer's disease. During normal aging, synapses decline, and β-amyloid (Aβ) accumulates intraneuronally. However, little is known about the underlying cell biological mechanisms. We studied normal neuronal aging using normal aged brain and aged mouse primary neurons that accumulate lysosomal lipofuscin and show synapse loss. We identify the up-regulation of amyloid precursor protein (APP) endocytosis as a neuronal aging mechanism that potentiates APP processing and Aβ production in vitro and in vivo. The increased APP endocytosis may contribute to the observed early endosomes enlargement in the aged brain. Mechanistically, we show that clathrin-dependent APP endocytosis requires F-actin and that clathrin and endocytic F-actin increase with neuronal aging. Finally, Aβ production inhibition reverts synaptic decline in aged neurons while Aβ accumulation, promoted by endocytosis up-regulation in younger neurons, recapitulates aging-related synapse decline. Overall, we identify APP endocytosis up-regulation as a potential mechanism of neuronal aging and, thus, a novel target to prevent late-onset Alzheimer's disease.

Vascular Senescence: A Potential Bridge Between Physiological Aging and Neurogenic Decline

The adult mammalian brain contains distinct neurogenic niches harboring populations of neural stem cells (NSCs) with the capacity to sustain the generation of specific subtypes of neurons during the lifetime. However, their ability to produce new progeny declines with age. The microenvironment of these specialized niches provides multiple cellular and molecular signals that condition NSC behavior and potential. Among the different niche components, vasculature has gained increasing interest over the years due to its undeniable role in NSC regulation and its therapeutic potential for neurogenesis enhancement. NSCs are uniquely positioned to receive both locally secreted factors and adhesion-mediated signals derived from vascular elements. Furthermore, studies of parabiosis indicate that NSCs are also exposed to blood-borne factors, sensing and responding to the systemic circulation. Both structural and functional alterations occur in vasculature with age at the cellular level that can affect the proper extrinsic regulation of NSCs. Additionally, blood exchange experiments in heterochronic parabionts have revealed that age-associated changes in blood composition also contribute to adult neurogenesis impairment in the elderly. Although the mechanisms of vascular- or blood-derived signaling in aging are still not fully understood, a general feature of organismal aging is the accumulation of senescent cells, which act as sources of inflammatory and other detrimental signals that can negatively impact on neighboring cells. This review focuses on the interactions between vascular senescence, circulating pro-senescence factors and the decrease in NSC potential during aging. Understanding the mechanisms of NSC dynamics in the aging brain could lead to new therapeutic approaches, potentially include senolysis, to target age-dependent brain decline.

Senescence and Inflammatory Markers for Predicting Clinical Progression in Parkinson's Disease: The ICICLE-PD Study

Background:

Cognitive decline is a frequent complication of Parkinson’s disease (PD) and the identification of predictive biomarkers for it would help in its management.

Objective:

Our aim was to analyse whether senescence markers (telomere length, p16 and p21) or their change over time could help to better predict cognitive and motor progression of newly diagnosed PD patients. We also compared these senescence markers to previously analysed markers of inflammation for the same purpose.

Methods:

This study examined the association of blood-derived markers of cell senescence and inflammation with motor and cognitive function over time in an incident PD cohort (the ICICLE-PD study). Participants (154 newly diagnosed PD patients and 99 controls) underwent physical and cognitive assessments over 36 months of follow up. Mean leukocyte telomere length and the expression of senescence markers p21 and p16 were measured at two time points (baseline and 18 months). Additionally, we selected five inflammatory markers from existing baseline data.

Results:

We found that PD patients had shorter telomeres at baseline and 18 months compared to age-matched healthy controls which also correlated to dementia at 36 months. Baseline p16 levels were associated with faster rates of motor and cognitive decline over 36 months in PD cases, while a simple inflammatory summary score at baseline best predicted cognitive score over this same time period in PD patients.

Conclusion:

Our study suggests that both inflammatory and senescence markers (p16) are valuable predictors of clinical progression in PD patients.

Cellular aging beyond cellular senescence: Markers of senescence prior to cell cycle arrest in vitro and in vivo

The field of research on cellular senescence experienced a rapid expansion from being primarily focused on in vitro aspects of aging to the vast territories of animal and clinical research. Cellular senescence is defined by a set of markers, many of which are present and accumulate in a gradual manner prior to senescence induction or are found outside of the context of cellular senescence. These markers are now used to measure the impact of cellular senescence on aging and disease as well as outcomes of anti-senescence interventions, many of which are at the stage of clinical trials. It is thus of primary importance to discuss their specificity as well as their role in the establishment of senescence. Here, the presence and role of senescence markers are described in cells prior to cell cycle arrest, especially in the context of replicative aging and in vivo conditions. Specifically, this review article seeks to describe the process of "cellular aging": the progression of internal changes occurring in primary cells leading to the induction of cellular senescence and culminating in cell death. Phenotypic changes associated with aging prior to senescence induction will be characterized, as well as their effect on the induction of cell senescence and the final fate of cells reviewed. Using published datasets on assessments of senescence markers in vivo, it will be described how disparities between quantifications can be explained by the concept of cellular aging. Finally, throughout the article the applicational value of broadening cellular senescence paradigm will be discussed.

Cell senescence in neuropathology: A focus on neurodegeneration and tumours

The study of cell senescence is a burgeoning field. Senescent cells can modify the cellular microenvironment through the secretion of a plethora of biologically active products referred to as the senescence-associated secretory phenotype (SASP). The consequences of these paracrine signals can be either beneficial for tissue homeostasis, if senescent cells are properly cleared and SASP activation is transient, or result in organ dysfunction, when senescent cells accumulate within the tissues and SASP activation is persistent. Several studies have provided evidence for the role of senescence and SASP in promoting age-related diseases or driving organismal ageing. The hype about senescence has been further amplified by the fact that a group of drugs, named senolytics, have been used to successfully ameliorate the burden of age-related diseases and increase health and life span in mice. Ablation of senescent cells in the brain prevents disease progression and improves cognition in murine models of neurodegenerative conditions. The role of senescence in cancer has been more thoroughly investigated, and it is now accepted that senescence is a double-edged sword that can paradoxically prevent or promote tumourigenesis in a context-dependent manner. In addition, senescence induction followed by senolytic treatment is starting to emerge as a novel therapeutic avenue that could improve current anti-cancer therapies and reduce tumour recurrence. In this review, we discuss recent findings supporting the role of cell senescence in the pathogenesis of neurodegenerative diseases and in brain tumours. A better understanding of senescence is likely to result in the development of novel and efficacious anti-senescence therapies against these brain pathologies.

Mechanisms of Cellular Senescence: Cell Cycle Arrest and Senescence Associated Secretory Phenotype

Cellular senescence is a stable cell cycle arrest that can be triggered in normal cells in response to various intrinsic and extrinsic stimuli, as well as developmental signals. Senescence is considered to be a highly dynamic, multi-step process, during which the properties of senescent cells continuously evolve and diversify in a context dependent manner. It is associated with multiple cellular and molecular changes and distinct phenotypic alterations, including a stable proliferation arrest unresponsive to mitogenic stimuli. Senescent cells remain viable, have alterations in metabolic activity and undergo dramatic changes in gene expression and develop a complex senescence-associated secretory phenotype. Cellular senescence can compromise tissue repair and regeneration, thereby contributing toward aging. Removal of senescent cells can attenuate age-related tissue dysfunction and extend health span. Senescence can also act as a potent anti-tumor mechanism, by preventing proliferation of potentially cancerous cells. It is a cellular program which acts as a double-edged sword, with both beneficial and detrimental effects on the health of the organism, and considered to be an example of evolutionary antagonistic pleiotropy. Activation of the p53/p21^WAF1/CIP1 and p16^INK4A/pRB tumor suppressor pathways play a central role in regulating senescence. Several other pathways have recently been implicated in mediating senescence and the senescent phenotype. Herein we review the molecular mechanisms that underlie cellular senescence and the senescence associated growth arrest with a particular focus on why cells stop dividing, the stability of the growth arrest, the hypersecretory phenotype and how the different pathways are all integrated.

Senescence-like phenotype in post-mitotic cells of mice entering middle age

Staining mice tissues for β-galactosidase activity is a fundamental tool to detect age- or disease-associated cellular senescence. However, reported analyses of positivity for senescence-associated β-galactosidase activity or for other markers of senescence in post-mitotic cells of healthy murine tissues have been fragmentary or inconclusive. Here, we attempted to independently deepen this knowledge using multiple senescence markers within the same cells of wild type mice entering middle age (9 months of age). A histochemistry protocol for the pH-dependent detection of β-galactosidase activity in several tissues was used. At pH 6, routinely utilized to detect senescence-associated β-galactosidase activity, only specific cellular populations in the mouse body (including Purkinje cells and choroid plexus in the central nervous system) were detected as strongly positive for β-galactosidase activity. These post-mitotic cells were also positive for other established markers of senescence (p16, p21 and DPP4), detected by immunofluorescence, confirming a potential senescent phenotype. These data might contribute to understanding the functional relation between the senescence-associated β-galactosidase activity and senescence markers in post-mitotic cells in absence of disease or advanced aging.

The Cellular Senescence Stress Response in Post-Mitotic Brain Cells: Cell Survival at the Expense of Tissue Degeneration

In 1960, Rita Levi-Montalcini and Barbara Booker made an observation that transformed neuroscience: as neurons mature, they become apoptosis resistant. The following year Leonard Hayflick and Paul Moorhead described a stable replicative arrest of cells in vitro, termed "senescence". For nearly 60 years, the cell biology fields of neuroscience and senescence ran in parallel, each separately defining phenotypes and uncovering molecular mediators to explain the 1960s observations of their founding mothers and fathers, respectively. During this time neuroscientists have consistently observed the remarkable ability of neurons to survive. Despite residing in environments of chronic inflammation and degeneration, as occurs in numerous neurodegenerative diseases, often times the neurons with highest levels of pathology resist death. Similarly, cellular senescence (hereon referred to simply as "senescence") now is recognized as a complex stress response that culminates with a change in cell fate. Instead of reacting to cellular/DNA damage by proliferation or apoptosis, senescent cells survive in a stable cell cycle arrest. Senescent cells simultaneously contribute to chronic tissue degeneration by secreting deleterious molecules that negatively impact surrounding cells. These fields have finally collided. Neuroscientists have begun applying concepts of senescence to the brain, including post-mitotic cells. This initially presented conceptual challenges to senescence cell biologists. Nonetheless, efforts to understand senescence in the context of brain aging and neurodegenerative disease and injury emerged and are advancing the field. The present review uses pre-defined criteria to evaluate evidence for post-mitotic brain cell senescence. A closer interaction between neuro and senescent cell biologists has potential to advance both disciplines and explain fundamental questions that have plagued their fields for decades.

The effect of aging and chronic microglia activation on the morphology and numbers of the cerebellar Purkinje cells

Reduced cerebellar volume and motor dysfunction have previously been observed in the GFAP-IL6 murine model of chronic neuroinflammation. This study aims to extend these findings by investigating the effect of microglial activation and ageing on the total number of Purkinje cells and the morphology of their dendritic arborization. Through comparison of transgenic GFAP-IL6 mice and their wild-type counterparts at the ages of 12 and 24-months, we were able to investigate the effects of ageing and chronic microglial activation on Purkinje cells. Unbiased stereology was used to estimate the number of microglia in Iba1⁺ stained tissue and Purkinje cells in calbindin stained tissue. Morphological analyses were made using 3D reconstructions of images acquired from the Golgi-stained cerebellar tissue. We found that the total number of microglia increased by approximately 5 times in the cerebellum of GFAP-IL6 mice compared to their WT littermates. The number of Purkinje cells decreased by as much as 50 % in aged wild type mice and 83 % in aged GFAP-IL6 mice. The remaining Purkinje cells in these cohorts were found to have significant reductions in their total dendritic length and number of branching points, indicating how the complexity of the Purkinje cell dendritic arbor reduces through age and inflammation. GFAP-IL6 mice, when compared to WT mice, had higher levels of microglial activation and more profound neurodegenerative changes in the cerebellum. The presence of constitutive IL6 production, driving chronic neuroinflammation, may account for these neurodegenerative changes in GFAP-IL6 mice.

Cellular Senescence in Brain Aging

Aging of the brain can manifest itself as a memory and cognitive decline, which has been shown to frequently coincide with changes in the structural plasticity of dendritic spines. Decreased number and maturity of spines in aged animals and humans, together with changes in synaptic transmission, may reflect aberrant neuronal plasticity directly associated with impaired brain functions. In extreme, a neurodegenerative disease, which completely devastates the basic functions of the brain, may develop. While cellular senescence in peripheral tissues has recently been linked to aging and a number of aging-related disorders, its involvement in brain aging is just beginning to be explored. However, accumulated evidence suggests that cell senescence may play a role in the aging of the brain, as it has been documented in other organs. Senescent cells stop dividing and shift their activity to strengthen the secretory function, which leads to the acquisition of the so called senescence-associated secretory phenotype (SASP). Senescent cells have also other characteristics, such as altered morphology and proteostasis, decreased propensity to undergo apoptosis, autophagy impairment, accumulation of lipid droplets, increased activity of senescence-associated-β-galactosidase (SA-β-gal), and epigenetic alterations, including DNA methylation, chromatin remodeling, and histone post-translational modifications that, in consequence, result in altered gene expression. Proliferation-competent glial cells can undergo senescence both in vitro and in vivo, and they likely participate in neuroinflammation, which is characteristic for the aging brain. However, apart from proliferation-competent glial cells, the brain consists of post-mitotic neurons. Interestingly, it has emerged recently, that non-proliferating neuronal cells present in the brain or cultivated in vitro can also have some hallmarks, including SASP, typical for senescent cells that ceased to divide. It has been documented that so called senolytics, which by definition, eliminate senescent cells, can improve cognitive ability in mice models. In this review, we ask questions about the role of senescent brain cells in brain plasticity and cognitive functions impairments and how senolytics can improve them. We will discuss whether neuronal plasticity, defined as morphological and functional changes at the level of neurons and dendritic spines, can be the hallmark of neuronal senescence susceptible to the effects of senolytics.

Cell cycle re-entry and arrest in G2/M phase induces senescence and fibrosis in Fuchs Endothelial Corneal Dystrophy

Fuchs endothelial corneal dystrophy (FECD) is an age-related disease whereby progressive loss of corneal endothelial cells (CEnCs) leads to loss of vision. There is currently a lack of therapeutic interventions as the etiology of the disease is complex, with both genetic and environmental factors. In this study, we have provided further insights into the pathogenesis of the disease, showing a causal relationship between senescence and endothelial-mesenchymal transition (EMT) using in vitro and in vivo models. Ultraviolet A (UVA) light induced EMT and senescence in CEnCs. Senescent cells were arrested in G2/M phase of the cell cycle and responsible for the resulting profibrotic phenotype. Inhibiting ATR signaling and subsequently preventing G2/M arrest attenuated EMT. In vivo, UVA irradiation induced cell cycle re-entry in post mitotic CEnCs, resulting in senescence and fibrosis at 1- and 2-weeks post-UVA. Selectively eliminating senescent cells using the senolytic cocktail of dasatinib and quercetin attenuated UVA-induced fibrosis, highlighting the potential for a new therapeutic intervention for FECD.

Evidence of the Cellular Senescence Stress Response in Mitotically Active Brain Cells-Implications for Cancer and Neurodegeneration

Cellular stress responses influence cell fate decisions. Apoptosis and proliferation represent opposing reactions to cellular stress or damage and may influence distinct health outcomes. Clinical and epidemiological studies consistently report inverse comorbidities between age-associated neurodegenerative diseases and cancer. This review discusses how one particular stress response, cellular senescence, may contribute to this inverse correlation. In mitotically competent cells, senescence is favorable over uncontrolled proliferation, i.e., cancer. However, senescent cells notoriously secrete deleterious molecules that drive disease, dysfunction and degeneration in surrounding tissue. In recent years, senescent cells have emerged as unexpected mediators of neurodegenerative diseases. The present review uses pre-defined criteria to evaluate evidence of cellular senescence in mitotically competent brain cells, highlights the discovery of novel molecular regulators and discusses how this single cell fate decision impacts cancer and degeneration in the brain. We also underscore methodological considerations required to appropriately evaluate the cellular senescence stress response in the brain.

Ageing, cellular senescence and the impact of diet: an overview

Ageing is a risk factor for chronic diseases including cancer, cardiovascular diseases, neurodegenerative disorders, and metabolic syndrome. Among others, senescence mechanisms have become a target of huge research on the topic of the ageing process. Cellular senescence is a state of an irreversible growth arrest that occurs in response to various forms of cellular stress and is characterized by a pro-inflammatory secretory phenotype. Multiple studies showed that cellular senescence occurs in both physiological and pathophysiological conditions. Senescent cells accumulate with ageing and can contribute to age-related decline in tissue function. Obesity is a metabolic condition that can accelerate the ageing process by promoting a premature induction of the senescent state of the cells. In contrast, caloric restriction without malnutrition is currently the most effective non-genetic intervention to delay ageing, and its potential in decreasing the cellular senescent burden is suggested. Here, it will be highlighted the cellular and molecular mechanisms involved in cellular senescence and discussed some of the research that is being done about how environmental conditions such as diet can affect the accumulation of senescent cells.

Evidence and perspectives of cell senescence in neurodegenerative diseases

Increased life expectancies have significantly increased the number of individuals suffering from geriatric neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). The financial cost for current and future patients with these diseases is overwhelming, resulting in substantial economic and societal costs. Unfortunately, most recent high-profile clinical trials for neurodegenerative diseases have failed to obtain efficacious results, indicating that novel approaches are desperately needed to treat these pathologies. Cell senescence, characterized by permanent cell cycle arrest, resistance to apoptosis, mitochondrial alterations, and secretion of senescence-associated secretory phenotype (SASP) components, has been extensively studied in mitotic cells such as fibroblasts, which is considered a hallmark of aging. Furthermore, multiple cell types in the senescent state in the brain, including neurons, microglia, astrocytes, and neural stem cells, have recently been observed in the context of neurodegenerative diseases, suggesting that these senescent cells may play an essential role in the pathological processes of neurodegenerative diseases. Therefore, this review begins by outlining key aspects of cell senescence constitution followed by examining the evidence implicating senescent cells in neurodegenerative diseases. In the final section, we review how cell senescence may be targeted as novel therapeutics to treat pathologies associated with neurodegenerative diseases.

Cellular senescence in ageing: from mechanisms to therapeutic opportunities

Cellular senescence, first described in vitro in 1961, has become a focus for biotech companies that target it to ameliorate a variety of human conditions. Eminently characterized by a permanent proliferation arrest, cellular senescence occurs in response to endogenous and exogenous stresses, including telomere dysfunction, oncogene activation and persistent DNA damage. Cellular senescence can also be a controlled programme occurring in diverse biological processes, including embryonic development. Senescent cell extrinsic activities, broadly related to the activation of a senescence-associated secretory phenotype, amplify the impact of cell-intrinsic proliferative arrest and contribute to impaired tissue regeneration, chronic age-associated diseases and organismal ageing. This Review discusses the mechanisms and modulators of cellular senescence establishment and induction of a senescence-associated secretory phenotype, and provides an overview of cellular senescence as an emerging opportunity to intervene through senolytic and senomorphic therapies in ageing and ageing-associated diseases.

Whole-body senescent cell clearance alleviates age-related brain inflammation and cognitive impairment in mice

Cellular senescence is characterized by an irreversible cell cycle arrest and a pro-inflammatory senescence-associated secretory phenotype (SASP), which is a major contributor to aging and age-related diseases. Clearance of senescent cells has been shown to improve brain function in mouse models of neurodegenerative diseases. However, it is still unknown whether senescent cell clearance alleviates cognitive dysfunction during the aging process. To investigate this, we first conducted single-nuclei and single-cell RNA-seq in the hippocampus from young and aged mice. We observed an age-dependent increase in p16Ink4a senescent cells, which was more pronounced in microglia and oligodendrocyte progenitor cells and characterized by a SASP. We then aged INK-ATTAC mice, in which p16Ink4a -positive senescent cells can be genetically eliminated upon treatment with the drug AP20187 and treated them either with AP20187 or with the senolytic cocktail Dasatinib and Quercetin. We observed that both strategies resulted in a decrease in p16Ink4a exclusively in the microglial population, resulting in reduced microglial activation and reduced expression of SASP factors. Importantly, both approaches significantly improved cognitive function in aged mice. Our data provide proof-of-concept for senolytic interventions' being a potential therapeutic avenue for alleviating age-associated cognitive impairment.

Senescence in Post-Mitotic Cells: A Driver of Aging?

Significance: Cell senescence was originally defined by an acute loss of replicative capacity and thus believed to be restricted to proliferation-competent cells. More recently, senescence has been recognized as a cellular stress and damage response encompassing multiple pathways or senescence domains, namely DNA damage response, cell cycle arrest, senescence-associated secretory phenotype, senescence-associated mitochondrial dysfunction, autophagy/mitophagy dysfunction, nutrient and stress signaling, and epigenetic reprogramming. Each of these domains is activated during senescence, and all appear to interact with each other. Cell senescence has been identified as an important driver of mammalian aging. Recent Advances: Activation of all these senescence domains has now also been observed in a wide range of post-mitotic cells, suggesting that senescence as a stress response can occur in nondividing cells temporally uncoupled from cell cycle arrest. Here, we review recent evidence for post-mitotic cell senescence and speculate about its possible relevance for mammalian aging. Critical Issues: Although a majority of senescence domains has been found to be activated in a range of post-mitotic cells during aging, independent confirmation of these results is still lacking for most of them. Future Directions: To define whether post-mitotic senescence plays a significant role as a driver of aging phenotypes in tissues such as brain, muscle, heart, and others. Antioxid. Redox Signal. 34, 308-323.

Cisplatin-induced peripheral neuropathy is associated with neuronal senescence-like response

BACKGROUND

Cisplatin-induced peripheral neuropathy (CIPN) is a frequent serious dose-dependent adverse event that can determine dosage limitations for cancer treatment. CIPN severity correlates with the amount of platinum detected in sensory neurons of the dorsal root ganglia (DRG). However, the exact pathophysiology of CIPN is poorly understood, so the chance of developing neuroprotective treatment is reduced. The aim of this study was to determine the exact mechanisms involved in CIPN development.

METHODS

By single-cell RNA-sequencing (scRNAseq), we have studied the transcriptomic profile of DRG sensory neurons from a well-characterized neurophysiological mouse model of CIPN.

RESULTS

Gene Ontology analysis of the scRNAseq data indicated that cisplatin treatment induces the upregulation of biological pathways related to DNA damage response (DDR) in the DRG neuronal population. Moreover, DRG neurons also upregulated the Cdkn1a gene, confirmed later by the measurement of its protein product p21. While apoptosis activation pathways were not observed in DRG sensory neurons of cisplatin-treated mice, these neurons did express several senescence hallmarks, including senescence-associated β-galactosidase, phospho-H2AX, and nuclear factor kappa B (Nfkb)-p65 proteins.

CONCLUSIONS

In this study, we determined that after cisplatin-induced DNA damage, p21 appears as the most relevant downstream factor of the DDR in DRG sensory neurons in vivo, which survive in a nonfunctional senescence-like state.

Principles of the Molecular and Cellular Mechanisms of Aging

Aging can be defined as a state of progressive functional decline accompanied by an increase in mortality. Time-dependent accumulation of cellular damage, namely lesions and mutations in the DNA and misfolded proteins, impair organellar and cellular function. Ensuing cell fate alterations lead to the accumulation of dysfunctional cells and hamper homeostatic processes, thus limiting regenerative potential; trigger low-grade inflammation; and alter intercellular and intertissue communication. The accumulation of molecular damage together with modifications in the epigenetic landscape, dysregulation of gene expression, and altered endocrine communication, drive the aging process and establish age as the main risk factor for age-associated diseases and multimorbidity.

Is Senescence-Associated β-Galactosidase a Reliable in vivo Marker of Cellular Senescence During Embryonic Development?

During vertebrate embryonic development, cellular senescence occurs at multiple locations. To date, it has been accepted that when there has been induction of senescence in an embryonic tissue, β-galactosidase activity is detectable at a pH as high as 6.0, and this has been extensively used as a marker of cellular senescence in vivo in both whole-mount and cryosections. Such senescence-associated β-galactosidase (SA-β-GAL) labeling appears enhanced in degenerating regions of the vertebrate embryo that are also affected by programmed cell death. In this sense, there is a strong SA-β-GAL signal which overlaps with the pattern of cell death in the interdigital tissue of the developing limbs, and indeed, many of the labeled cells detected go on to subsequently undergo apoptosis. However, it has been reported that β-GAL activity at pH 6.0 is also enhanced in healthy neurons, and some retinal neurons are strongly labeled with this histochemical technique when they begin to differentiate during early embryonic development. These labeled early post-mitotic neurons also express other senescence markers such as p21. Therefore, the reliability of this histochemical technique in studying senescence in cells such as neurons that undergo prolonged and irreversible cell-cycle arrest is questionable because it is also expressed in healthy post-mitotic cells. The identification of new biomarkers of cellular senescence would, in combination with established markers, increase the specificity and efficiency of detecting cellular senescence in embryonic and healthy mature tissues.

Emerging Technologies for Genome-Wide Profiling of DNA Breakage

Genome instability is associated with myriad human diseases and is a well-known feature of both cancer and neurodegenerative disease. Until recently, the ability to assess DNA damage-the principal driver of genome instability-was limited to relatively imprecise methods or restricted to studying predefined genomic regions. Recently, new techniques for detecting DNA double strand breaks (DSBs) and single strand breaks (SSBs) with next-generation sequencing on a genome-wide scale with single nucleotide resolution have emerged. With these new tools, efforts are underway to define the "breakome" in normal aging and disease. Here, we compare the relative strengths and weaknesses of these technologies and their potential application to studying neurodegenerative diseases.

Cellular senescence in the aging retina and developments of senotherapies for age-related macular degeneration

Age-related macular degeneration (AMD), a degenerative disease in the central macula area of the neuroretina and the supporting retinal pigment epithelium, is the most common cause of vision loss in the elderly. Although advances have been made, treatment to prevent the progressive degeneration is lacking. Besides the association of innate immune pathway genes with AMD susceptibility, environmental stress- and cellular senescence-induced alterations in pathways such as metabolic functions and inflammatory responses are also implicated in the pathophysiology of AMD. Cellular senescence is an adaptive cell process in response to noxious stimuli in both mitotic and postmitotic cells, activated by tumor suppressor proteins and prosecuted via an inflammatory secretome. In addition to physiological roles in embryogenesis and tissue regeneration, cellular senescence is augmented with age and contributes to a variety of age-related chronic conditions. Accumulation of senescent cells accompanied by an impairment in the immune-mediated elimination mechanisms results in increased frequency of senescent cells, termed “chronic” senescence. Age-associated senescent cells exhibit abnormal metabolism, increased generation of reactive oxygen species, and a heightened senescence-associated secretory phenotype that nurture a proinflammatory milieu detrimental to neighboring cells. Senescent changes in various retinal and choroidal tissue cells including the retinal pigment epithelium, microglia, neurons, and endothelial cells, contemporaneous with systemic immune aging in both innate and adaptive cells, have emerged as important contributors to the onset and development of AMD. The repertoire of senotherapeutic strategies such as senolytics, senomorphics, cell cycle regulation, and restoring cell homeostasis targeted both at tissue and systemic levels is expanding with the potential to treat a spectrum of age-related diseases, including AMD.

The aging endothelium

Cellular senescence is now recognized as one of the hallmarks of aging. Herein, we examine current findings on senescence of the vascular endothelium and its impacts on age-related vascular diseases. Endothelial senescence can result in systemic metabolic changes, implicating senescence in chronic diseases such as diabetes, obesity and atherosclerosis. Senolytics, drugs that eliminate senescent cells, afford new therapeutic strategies for control of these chronic diseases.

Mechanisms and therapeutic implications of cellular senescence in osteoarthritis

The development of osteoarthritis (OA) correlates with a rise in the number of senescent cells in joint tissues, and the senescence-associated secretory phenotype (SASP) has been implicated in cartilage degradation and OA. Age-related mitochondrial dysfunction and associated oxidative stress might induce senescence in joint tissue cells. However, senescence is not the only driver of OA, and the mechanisms by which senescent cells contribute to disease progression are not fully understood. Furthermore, it remains uncertain which joint cells and SASP-factors contribute to the OA phenotype. Research in the field has looked at developing therapeutics (namely senolytics and senomorphics) that eliminate or alter senescent cells to stop disease progression and pathogenesis. A better understanding of how senescence contributes to joint dysfunction may enhance the effectiveness of these approaches and provide relief for patients with OA.

Short Leukocyte Telomeres, But Not Telomere Attrition Rates, Predict Memory Decline in the 20-Year Longitudinal Betula Study

Leukocyte telomere length (LTL) is a proposed biomarker for aging-related disorders, including cognitive decline and dementia. Long-term longitudinal studies measuring intra-individual changes in both LTL and cognitive outcomes are scarce, precluding strong conclusions about a potential aging-related relationship between LTL shortening and cognitive decline. This study investigated associations between baseline levels and longitudinal changes in LTL and memory performance across an up to 20-year follow-up in 880 dementia-free participants from a population-based study (mean baseline age: 56.8 years, range: 40–80; 52% female). Shorter baseline LTL significantly predicted subsequent memory decline (r = .34, 95% confidence interval: 0.06, 0.82), controlling for age, sex, and other relevant covariates. No significant associations were however observed between intra-individual changes in LTL and memory, neither concurrently nor with a 5-year time-lag between LTL shortening and memory decline. These results support the notion of short LTL as a predictive factor for aging-related memory decline, but suggest that LTL dynamics in adulthood and older age may be less informative of cognitive outcomes in aging. Furthermore, the results highlight the importance of long-term longitudinal evaluation of outcomes in biomarker research.

Senescence research from historical theory to future clinical application

Historically, the findings from cellular lifespan studies have greatly affected aging research. The discovery of replicative senescence by Hayflick developed into research on telomeres and telomerase, while stress-induced senescence became known as a telomere-independent event. Senescence-inducing signals comprise several tumor suppressors or cell cycle inhibitors, e.g., p53, cyclin-dependent kinase inhibitor p16 Ink4a and others. Stress-induced senescence serves as a physiological barrier to oncogenesis in vivo, while it activates senescence-associated secretary phenotype, inducing chronic inflammation. Thus, beside telomere length, p16, p53 and inflammatory cytokines have been utilized as biomarkers for cellular senescence. Telomere lengths in human leukocytes correlate well with events of aging-related lifestyle diseases, indicating the importance of cellular senescence in organismal aging. As such, the development of senescence research will have significant future clinical applications, e.g., senolysis. Geriatr Gerontol Int 2021; 21: 125-130.

Senescent cells exacerbate chronic inflammation and contribute to periodontal disease progression in old mice

BACKGROUND

Coinciding with other chronic comorbidities, the prevalence of periodontal disease increases with aging. Mounting evidence has established that senescent cells accumulate at sites of age-related pathologies, where they promote "non-microbial" inflammation. We hypothesized that alveolar bone osteocytes develop senescence characteristics in old age.

METHODS

Alveolar bone samples were obtained from young (6 months) and old (20 to 22 months) mice to evaluate the expression of senescence biomarkers by immunofluorescent staining. Osteocyte-enriched fractions were used to characterize the age-related senescence-associated secretory phenotype (SASP) gene expression profile. Primary alveolar bone cells were exposed to the SASP via in vitro senescent conditioned media (SCM) administration. A multiplex assay confirmed protein levels of specific cytokines. Interactions with bacterial components were evaluated by stimulating cells with lipopolysaccharide (LPS).

RESULTS

Increased senescence-associated distension of satellites (SADS) and p16^Ink4a mRNA expression were identified in alveolar bone osteocytes with aging. These findings were associated with increased levels of DNA damage, and activated p38 MAPK, both inducers of senescence. Furthermore, interleukin-6 (IL6), IL17, IGFBP4, and MMP13 were significantly upregulated with aging in osteocyte-enriched samples. Interestingly, SCM potentiated the LPS-induced expression of IL1α, IL1β, and IL6. Cell migration and differentiation were also impeded by SCM. These in vitro effects were ameliorated by the p38 MAPK inhibitor SB202190.

CONCLUSIONS

Accumulation of senescent osteocytes contributes to deterioration of the periodontal environment by exacerbating chronic inflammation and reducing regeneration in old age. Cellular senescence is a cell-intrinsic response to DNA damage, and a host-related mechanism associated with aging that could potentiate inflammation induced by bacterial components.

Zika virus exposure affects neuron-glia communication in the hippocampal slices of adult rats

Zika virus (ZIKV) infection during pregnancy was associated with microcephaly in neonates, but clinical and experimental evidence indicate that ZIKV also causes neurological complications in adults. However, the changes in neuron-glial communication, which is essential for brain homeostasis, are still unknown. Here, we report that hippocampal slices from adult rats exposed acutely to ZIKV showed significant cellular alterations regarding to redox homeostasis, inflammatory process, neurotrophic functions and molecular signalling pathways associated with neurons and glial cells. Our findings support the hypothesis that ZIKV is highly neurotropic and its infection readily induces an inflammatory response, characterized by an increased expression and/or release of pro-inflammatory cytokines. We also observed changes in neural parameters, such as adenosine receptor A2a expression, as well as in the release of brain-derived neurotrophic factor and neuron-specific enolase, indicating plasticity synaptic impairment/neuronal damage. In addition, ZIKV induced a glial commitment, with alterations in specific and functional parameters such as aquaporin 4 expression, S100B secretion and glutathione synthesis. ZIKV also induced p21 senescence-associated gene expression, indicating that ZIKV may induce early senescence. Taken together, our results indicate that ZIKV-induced neuroinflammation, involving nuclear factor erythroid 2-related factor 2 (Nrf2) and nuclear factor κB (NFκB) pathways, affects important aspects of neuron-glia communication. Therefore, although ZIKV infection is transient, long-term consequences might be associated with neurological and/or neurodegenerative diseases.

Senescence in RASopathies, a possible novel contributor to a complex pathophenoype

Senescence is a biological process that induces a permanent cell cycle arrest and a specific gene expression program in response to various stressors. Following studies over the last few decades, the concept of senescence has evolved from an antiproliferative mechanism in cancer (oncogene-induced senescence) to a critical component of physiological processes associated with embryonic development, tissue regeneration, ageing and its associated diseases. In somatic cells, oncogenic mutations in RAS-MAPK pathway genes are associated with oncogene-induced senescence and cancer, while germline mutations in the same pathway are linked to a group of monogenic developmental disorders generally termed RASopathies. Here, we consider that in these disorders, senescence induction may result in opposing outcomes, a tumour protective effect and a possible contributor to a premature ageing phenotype identified in Costello syndrome, which belongs to the RASopathy group. In this review, we will highlight the role of senescence in organismal homeostasis and we will describe the current knowledge about senescence in RASopathies. Additionally, we provide a perspective on examples of experimentally characterised RASopathy mutations that, alone or in combination with various stressors, may also trigger an age-dependent chronic senescence, possibly contributing to the age-dependent worsening of RASopathy pathophenotype and the reduction of lifespan.

Targeting cellular senescence based on interorganelle communication, multilevel proteostasis, and metabolic control

Cellular senescence, a stable cell division arrest caused by severe damage and stress, is a hallmark of aging in vertebrates including humans. With progressing age, senescent cells accumulate in a variety of mammalian tissues, where they contribute to tissue aging, identifying cellular senescence as a major target to delay or prevent aging. There is an increasing demand for the discovery of new classes of small molecules that would either avoid or postpone cellular senescence by selectively eliminating senescent cells from the body (i.e., ‘senolytics’) or inactivating/switching damage‐inducing properties of senescent cells (i.e., ‘senostatics/senomorphics’), such as the senescence‐associated secretory phenotype. Whereas compounds with senolytic or senostatic activity have already been described, their efficacy and specificity has not been fully established for clinical use yet. Here, we review mechanisms of senescence that are related to mitochondria and their interorganelle communication, and the involvement of proteostasis networks and metabolic control in the senescent phenotype. These cellular functions are associated with cellular senescence in in vitro and in vivo models but have not been fully exploited for the search of new compounds to counteract senescence yet. Therefore, we explore possibilities to target these mechanisms as new opportunities to selectively eliminate and/or disable senescent cells with the aim of tissue rejuvenation. We assume that this research will provide new compounds from the chemical space which act as mimetics of caloric restriction, modulators of calcium signaling and mitochondrial physiology, or as proteostasis optimizers, bearing the potential to counteract cellular senescence, thereby allowing healthy aging.

Cellular senescence in age-related disorders

Much of the population is now faced with an enormous burden of age-associated chronic diseases. Recent discoveries in geroscience indicate that healthspan in model organisms such as mice can be manipulated by targeting cellular senescence, a hallmark mechanism of aging, defined as an irreversible proliferative arrest that occurs when cells experience oncogenic or other diverse forms of damage. Senescent cells and their proinflammatory secretome have emerged as contributors to age-related tissue dysfunction and morbidity. Cellular senescence has causal roles in mediating osteoporosis, frailty, cardiovascular diseases, osteoarthritis, pulmonary fibrosis, renal diseases, neurodegenerative diseases, hepatic steatosis, and metabolic dysfunction. Therapeutically targeting senescent cells in mice can prevent, delay, or alleviate each of these conditions. Therefore, senotherapeutic approaches, including senolytics and senomorphics, that either selectively eliminate senescent cells or interfere with their ability to promote tissue dysfunction, are gaining momentum as potential realistic strategies to abrogate human senescence to thereby compress morbidity and extend healthspan.

On the evolution of cellular senescence

The idea that senescent cells are causally involved in aging has gained strong support from findings that the removal of such cells alleviates many age‐related diseases and extends the life span of mice. While efforts proceed to make therapeutic use of such discoveries, it is important to ask what evolutionary forces might have been behind the emergence of cellular senescence, in order better to understand the biology that we might seek to alter. Cellular senescence is often regarded as an anti‐cancer mechanism, since it limits the division potential of cells. However, many studies have shown that senescent cells often also have carcinogenic properties. This is difficult to reconcile with the simple idea of an anti‐cancer mechanism. Furthermore, other studies have shown that cellular senescence is involved in wound healing and tissue repair. Here, we bring these findings and ideas together and discuss the possibility that these functions might be the main reason for the evolution of cellular senescence. Furthermore, we discuss the idea that senescent cells might accumulate with age because the immune system had to strike a balance between false negatives (overlooking some senescent cells) and false positives (destroying healthy body cells).

The Association Between Leukocyte Telomere Length and Cognitive Performance Among the American Elderly

Background

Age-related cognitive decline begins in middle age and persists with age. Leukocyte telomere length (LTL) decreases with age and is enhanced by inflammation and oxidative stress. However, whether shorter LTL correlates with cognitive decline remains controversial.

Aims

We aimed to investigate the relationship between LTL and cognitive decline in the American elderly.

Methods

We used data from the 1999 to 2002 U.S. National Health and Nutrition Examination Survey (NHANES). We included participants aged 65–80 with available data on LTL and cognitive assessments. The cognitive function assessment used the digit symbol substitution test (DSST). We applied multivariate modeling to estimate the association between LTL and cognitive performance. Additionally, to ensure robust data analysis, we converted LTL into categorical variables through quartile and then calculated the P for trend.

Results

After adjusting for age, cardiovascular disease (CAD) score, gender, race, body mass index (BMI), and educational level, LTL showed a positive correlation with DSST score (odds ratio [OR] 3.47 [0.14, 6.79], P = 0.04). Additionally, to further quantify the LTL–DSST interaction, we found a similar trend when LTL was regarded as a categorical variable (quartile) (P for trend = 0.03).

Conclusion

LTL was associated with cognitive capabilities among the elderly, implying that LTL might be a biomarker of cognitive aging.

Core cell cycle machinery is crucially involved in both life and death of post-mitotic neurons

A persistent dogma in neuroscience supported the idea that terminally differentiated neurons permanently withdraw from the cell cycle. However, since the late 1990s, several studies have shown that cell cycle proteins are expressed in post-mitotic neurons under physiological conditions, indicating that the cell cycle machinery is not restricted to proliferating cells. Moreover, many studies have highlighted a clear link between cell cycle-related proteins and neurological disorders, particularly relating to apoptosis-induced neuronal death. Indeed, cell cycle-related proteins can be upregulated or overactivated in post-mitotic neurons in case of acute or degenerative central nervous system disease. Given the considerable lack of effective treatments for age-related neurological disorders, new therapeutic approaches targeting the cell cycle machinery might thus be considered. This review aims at summarizing current knowledge about the role of the cell cycle machinery in post-mitotic neurons in healthy and pathological conditions.

Osteocyte Cellular Senescence

PURPOSE OF REVIEW

Senescent cells are now known to accumulate in multiple tissues with aging and through their inflammation (the senescence-associated secretory phenotype, SASP) contribute to aging and chronic diseases. Here, we review the roles of senescent osteocytes in the context of bone loss.

RECENT FINDINGS

Numerous studies have established that senescent osteocytes accumulate in the bone microenvironment with aging in mice and in humans. Moreover, at least in mice, elimination of senescent cells results in attenuation of age-related bone loss. Osteocyte senescence also occurs in response to other cellular stressors, including radiotherapy, chemotherapy, and metabolic dysfunction, where it appears to mediate skeletal deterioration. Osteocyte senescence is linked to bone loss associated with aging and other conditions. Senescent osteocytes are potential therapeutic targets to alleviate skeletal dysfunction. Additional studies better defining the underlying mechanisms as well as translating these exciting findings from mouse models to humans are needed.

Anti-inflammatory treatment rescues memory deficits during aging in nfkb1-/- mice

Chronic inflammation is a common feature of many age-related conditions including neurodegenerative diseases such as Alzheimer's disease. Cellular senescence is a state of irreversible cell-cycle arrest, thought to contribute to neurodegenerative diseases partially via induction of a chronic pro-inflammatory phenotype. In this study, we used a mouse model of genetically enhanced NF-κB activity (nfκb1-/- ), characterized by low-grade chronic inflammation and premature aging, to investigate the impact of inflammaging on cognitive decline. We found that during aging, nfkb1-/- mice show an early onset of memory loss, combined with enhanced neuroinflammation and increased frequency of senescent cells in the hippocampus and cerebellum. Electrophysiological measurements in the hippocampus of nfkb1-/- mice in vitro revealed deficits in gamma frequency oscillations, which could explain the decline in memory capacity. Importantly, treatment with the nonsteroidal anti-inflammatory drug (NASID) ibuprofen reduced neuroinflammation and senescent cell burden resulting in significant improvements in cognitive function and gamma frequency oscillations. These data support the hypothesis that chronic inflammation is a causal factor in the cognitive decline observed during aging.

Designing neuroreparative strategies using aged regenerating animal models

In our ever-aging world population, the risk of age-related neuropathies has been increasing, representing both a social and economic burden to society. Since the ability to regenerate in the adult mammalian central nervous system is very limited, brain trauma and neurodegeneration are often permanent. As a consequence, novel scientific challenges have emerged and many research efforts currently focus on triggering repair in the damaged or diseased brain. Nevertheless, stimulating neuroregeneration remains ambitious. Even though important discoveries have been made over the past decades, they did not translate into a therapy yet. Actually, this is not surprising; while these disorders mainly manifest in aged individuals, most of the research is being performed in young animal models. Aging of neurons and their environment, however, greatly affects the central nervous system and its capacity to repair. This review provides a detailed overview of the impact of aging on central nervous system functioning and regeneration potential, both in non-regenerating and spontaneously regenerating animal models. Additionally, we highlight the need for aging animal models with regenerative capacities in the search for neuroreparative strategies.

Depletion of senescent-like neuronal cells alleviates cisplatin-induced peripheral neuropathy in mice

Chemotherapy-induced peripheral neuropathy is among the most common dose-limiting adverse effects of cancer treatment, leading to dose reduction and discontinuation of life-saving chemotherapy and a permanently impaired quality of life for patients. Currently, no effective treatment or prevention is available. Senescence induced during cancer treatment has been shown to promote the adverse effects. Here, we show that cisplatin induces senescent-like neuronal cells in primary culture and in mouse dorsal root ganglia (DRG), as determined by the characteristic senescence markers including senescence-associated beta-galactosidase, accumulation of cytosolic p16^INK4A and HMGB1, as well as increased expression of p16Ink4a, p21, and MMP-9. The accumulation of senescent-like neuronal cells in DRG is associated with cisplatin-induced peripheral neuropathy (CIPN) in mice. To determine if depletion of senescent-like neuronal cells may effectively mitigate CIPN, we used a pharmacological ‘senolytic’ agent, ABT263, which inhibits the anti-apoptotic proteins BCL-2 and BCL-xL and selectively kills senescent cells. Our results demonstrated that clearance of DRG senescent neuronal cells reverses CIPN, suggesting that senescent-like neurons play a role in CIPN pathogenesis. This finding was further validated using transgenic p16-3MR mice, which permit ganciclovir (GCV) to selectively kill senescent cells expressing herpes simplex virus 1 thymidine kinase (HSV-TK). We showed that CIPN was alleviated upon GCV administration to p16-3MR mice. Together, the results suggest that clearance of senescent DRG neuronal cells following platinum-based cancer treatment might be an effective therapy for the debilitating side effect of CIPN.

Senescence in Wound Repair: Emerging Strategies to Target Chronic Healing Wounds

Cellular senescence is a fundamental stress response that restrains tumour formation. Yet, senescence cells are also present in non-cancerous states, accumulating exponentially with chronological age and contributing to age- and diabetes-related cellular dysfunction. The identification of hypersecretory and phagocytic behaviours in cells that were once believed to be non-functional has led to a recent explosion of senescence research. Here we discuss the profound, and often opposing, roles identified for short-lived vs. chronic tissue senescence. Transiently induced senescence is required for development, regeneration and acute wound repair, while chronic senescence is widely implicated in tissue pathology. We recently demonstrated that sustained senescence contributes to impaired diabetic healing via the CXCR2 receptor, which when blocked promotes repair. Further studies have highlighted the beneficial effects of targeting a range of senescence-linked processes to fight disease. Collectively, these findings hold promise for developing clinically viable strategies to tackle senescence in chronic wounds and other cutaneous pathologies.

Cellular senescence in aging and age-related diseases: Implications for neurodegenerative diseases

Aging is the major predictor for developing multiple neurodegenerative diseases, including Alzheimer's disease (AD) other dementias, and Parkinson's disease (PD). Senescent cells, which can drive aging phenotypes, accumulate at etiological sites of many age-related chronic diseases. These cells are resistant to apoptosis and can cause local and systemic dysfunction. Decreasing senescent cell abundance using senolytic drugs, agents that selectively target these cells, alleviates neurodegenerative diseases in preclinical models. In this review, we consider roles of senescent cells in neurodegenerative diseases and potential implications of senolytic agents as an innovative treatment.

Irradiation-Induced Upregulation of miR-711 Inhibits DNA Repair and Promotes Neurodegeneration Pathways

Radiotherapy for brain tumors induces neuronal DNA damage and may lead to neurodegeneration and cognitive deficits. We investigated the mechanisms of radiation-induced neuronal cell death and the role of miR-711 in the regulation of these pathways. We used in vitro and in vivo models of radiation-induced neuronal cell death. We showed that X-ray exposure in primary cortical neurons induced activation of p53-mediated mechanisms including intrinsic apoptotic pathways with sequential upregulation of BH3-only molecules, mitochondrial release of cytochrome c and AIF-1, as well as senescence pathways including upregulation of p21^WAF1/Cip1. These pathways of irradiation-induced neuronal apoptosis may involve miR-711-dependent downregulation of pro-survival genes Akt and Ang-1. Accordingly, we demonstrated that inhibition of miR-711 attenuated degradation of Akt and Ang-1 mRNAs and reduced intrinsic apoptosis after neuronal irradiation; likewise, administration of Ang-1 was neuroprotective. Importantly, irradiation also downregulated two novel miR-711 targets, DNA-repair genes Rad50 and Rad54l2, which may impair DNA damage responses, amplifying the stimulation of apoptotic and senescence pathways and contributing to neurodegeneration. Inhibition of miR-711 rescued Rad50 and Rad54l2 expression after neuronal irradiation, enhancing DNA repair and reducing p53-dependent apoptotic and senescence pathways. Significantly, we showed that brain irradiation in vivo persistently elevated miR-711, downregulated its targets, including pro-survival and DNA-repair molecules, and is associated with markers of neurodegeneration, not only across the cortex and hippocampus but also specifically in neurons isolated from the irradiated brain. Our data suggest that irradiation-induced miR-711 negatively modulates multiple pro-survival and DNA-repair mechanisms that converge to activate neuronal intrinsic apoptosis and senescence. Using miR-711 inhibitors to block the development of these regulated neurodegenerative pathways, thus increasing neuronal survival, may be an effective neuroprotective strategy.

Cellular senescence in vivo: From cells to tissues to pathologies

Senescent cells accumulate during aging in a variety of tissues. Although scarce, they could influence tissue function non-cell-autonomously via secretion of a range of factors in their neighborhood. Recent studies support a role of senescent cells in age-related morbidity, including neurodegenerative diseases, cardiovascular pathologies, cancers, aging-associated nephrological alterations, chronic pulmonary disease and osteoarthritis, indicating that senescent cells could represent an interesting target for therapeutic exploitation across a range of pathophysiological contexts. In this article, we review data available to indicate which cell types can undergo senescence within various mammalian tissue environments and how these processes may contribute to tissue-specific pathologies associated with old age. We also consider markers used to identify senescent cells in vitro and in vivo. The data discussed may serve as an important starting point for an extended definition of molecular and functional characteristics of senescent cells in different organs and may hence promote the development and refinement of targeting strategies aimed at removing senescent cells from aging tissues.