Restoration of hippocampal neural precursor function by ablation of senescent cells in the aging stem cell niche

Senescent brain cell types in Alzheimer's disease: Pathological mechanisms and therapeutic opportunities

Cellular senescence is a cell state triggered by programmed physiological processes or cellular stress responses. Stress-induced senescent cells often acquire pathogenic traits, including a toxic secretome and resistance to apoptosis. When pathogenic senescent cells form faster than they are cleared by the immune system, they accumulate in tissues throughout the body and contribute to age-related diseases, including neurodegeneration. This review highlights evidence of pathogenic senescent cells in the brain and their role in Alzheimer's disease (AD), the leading cause of dementia in older adults. We also discuss the progress and challenges of senotherapies, pharmacological strategies to clear senescent cells or mitigate their toxic effects, which hold promise as interventions for AD and related dementias (ADRD).

hESC-derived extracellular vesicles enriched with MFGE-8 and the GSH redox system act as senotherapeutics for neural stem cells in ischemic stroke

Human embryonic stem cells (hESCs) and their extracellular vesicles (EVs) hold significant potential for tissue repair and regeneration. Neural stem cells (NSCs) in the adult brain often acquire senescent phenotypes after ischemic injuries, releasing neurodegenerative senescence-associated secretory phenotype factors. In this study, we investigated the senotherapeutic effects of hESC-EVs on NSCs and confirmed their neuroprotective effects in neurons via rejuvenation of NSC secretions. Proteomic profiling of hESC-EVs identified MFGE-8 as a critical bridging molecule to NSCs. We also found that the glutathione (GSH) redox system is a key contributor to the therapeutic antioxidant activity of hESC-EVs. Additionally, EVs produced by the hypoxic preconditioning of hESCs (hESC-HypoxEVs) exhibited reinforced GSH redox capacity and further enhanced the senotherapeutic effects on NSCs compared to naïve hESC-EVs. We also demonstrated that administration of hESC-HypoxEVs, precoated with MFGE-8, significantly increased the populations of NSCs and newborn neurons in the subventricular zone of the brain and improved sensorimotor functions in a rat model of ischemic stroke. Our study suggests that combining hESC-HypoxEVs with MFGE-8 may serve as an effective therapeutic modality for reversing senescence and enhancing the neurogenic potential of NSCs to treat neurodegenerative diseases.

The senolytic ABT-263 improves cognitive functions in middle-aged male, but not female, atherosclerotic LDLr-/-;hApoB100+/+ mice

Accumulation of cerebral senescent cells may compromise the continuum between vascular and neuronal function, leading to damage and cognitive decline. Elimination of senescent cells might therefore preserve vascular and neuronal functions. To test this hypothesis, we used male and female atherosclerotic LDLr-/-;hApoB100+/+ mice (ATX-mice), a model of vascular cognitive impairment (VCI), treated with the senolytic ABT-263 for 3 months (3- to 6-month or 9- to 12-month old). In young male ATX mice, prevention with ABT-263 improved spatial retention memory, in association with a higher endothelial sensitivity to shear stress and a higher hippocampal CD31+ endothelial cell density, lower activation of both astrocytes and glial cells. In young females, ABT-263 tended to improve delayed memory; however, atherosclerotic plaque was magnified by ABT-263, endothelial function was unaffected, hippocampal astrocyte activation increased and expression of CD31+ cells decreased. Hence, unlike in males, ABT-263 appears deleterious in young ATX females. In middle-aged males, the curative treatment improved the learning process and memory. Although no change in endothelial function was observed, the benefits of ABT-263 were associated with a decreased expression of several inflammaging markers, a higher density of CD31+ cells and a lower activation of glial cells. In middle-aged females, ABT-263 induced a surge of inflammaging markers, associated with a slower learning process. Altogether, our data demonstrate that ABT-263 differentially affects VCI, improving cognition in male while being deleterious in female ATX mice. More studies are needed to understand the mechanisms at the basis of the sexual dimorphic effects of the senolytic ABT-263.

The mechanisms, hallmarks, and therapies for brain aging and age-related dementia

Age-related cognitive decline and dementia are significant manifestations of brain aging. As the elderly population grows rapidly, the health and socio-economic impacts of cognitive dysfunction have become increasingly significant. Although clinical treatment of dementia has faced considerable challenges over the past few decades, with limited breakthroughs in slowing its progression, there has been substantial progress in understanding the molecular mechanisms and hallmarks of age-related dementia (ARD). This progress brings new hope for the intervention and treatment of this disease. In this review, we categorize the latest findings in ARD biomarkers into four stages based on disease progression: Healthy brain, pre-clinical, mild cognitive impairment, and dementia. We then systematically summarize the most promising therapeutic approaches to prevent or slow ARD at four levels: Genome and epigenome, organelle, cell, and organ and organism. We emphasize the importance of early prevention and detection, along with the implementation of combined treatments as multimodal intervention strategies, to address brain aging and ARD in the future.

Podocyte senescence: from molecular mechanisms to therapeutics

As an important component of the glomerular filtration membrane, the state of the podocytes is closely related to kidney function, they are also key cells involved in aging and play a central role in the damage caused by renal aging. Therefore, understanding the aging process of podocytes will allow us to understand their susceptibility to injury and identify targeted protective mechanisms. In fact, the process of physiological aging itself can induce podocyte senescence. Pathological stresses, such as oxidative stress, mitochondrial damage, secretion of senescence-associated secretory phenotype, reduced autophagy, oncogene activation, altered transcription factors, DNA damage response, and other factors, play a crucial role in inducing premature senescence and accelerating aging. Senescence-associated-β-galactosidase (SA-β-gal) is a marker of aging, and β-hydroxybutyric acid treatment can reduce SA-β-gal activity to alleviate cellular senescence and damage. In addition, CCAAT/enhancer-binding protein-α, transforming growth factor-β signaling, glycogen synthase kinase-3β, cycle-dependent kinase, programmed cell death protein 1, and plasminogen activator inhibitor-1 are closely related to aging. The absence or elevation of these factors can affect aging through different mechanisms. Podocyte injury is not an independent process, and injured podocytes interact with the surrounding epithelial cells or other kidney cells to mediate the injury or loss of podocytes. In this review, we discuss the manifestations, molecular mechanisms, biomarkers, and therapeutic drugs for podocyte senescence. We included elamipretide, lithium, calorie restriction, rapamycin; and emerging treatment strategies, such as gene and immune therapies. More importantly, we summarize how podocyte interact with other kidney cells.

SenNet recommendations for detecting senescent cells in different tissues

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

Emerging insights in senescence: pathways from preclinical models to therapeutic innovations

Senescence is a crucial hallmark of ageing and a significant contributor to the pathology of age-related disorders. As committee members of the young International Cell Senescence Association (yICSA), we aim to synthesise recent advancements in the identification, characterisation, and therapeutic targeting of senescence for clinical translation. We explore novel molecular techniques that have enhanced our understanding of senescent cell heterogeneity and their roles in tissue regeneration and pathology. Additionally, we delve into in vivo models of senescence, both non-mammalian and mammalian, to highlight tools available for advancing the contextual understanding of in vivo senescence. Furthermore, we discuss innovative diagnostic tools and senotherapeutic approaches, emphasising their potential for clinical application. Future directions of senescence research are explored, underscoring the need for precise, context-specific senescence classification and the integration of advanced technologies such as machine learning, long-read sequencing, and multifunctional senoprobes and senolytics. The dual role of senescence in promoting tissue homoeostasis and contributing to chronic diseases highlights the complexity of targeting these cells for improved clinical outcomes.

Molecular Targeting of Ischemic Stroke: The Promise of Naïve and Engineered Extracellular Vesicles

Ischemic stroke (IS) remains a leading cause of mortality and long-term disability worldwide, with limited therapeutic options available. Despite the success of early interventions, such as tissue-type plasminogen activator administration and mechanical thrombectomy, many patients continue to experience persistent neurological deficits. The pathophysiology of IS is multifaceted, encompassing excitotoxicity, oxidative and nitrosative stress, inflammation, and blood-brain barrier disruption, all of which contribute to neural cell death, further complicating the treatment of IS. Recently, extracellular vesicles (EVs) secreted naturally by various cell types have emerged as promising therapeutic agents because of their ability to facilitate selective cell-to-cell communication, neuroprotection, and tissue regeneration. Furthermore, engineered EVs, designed to enhance targeted delivery and therapeutic cargo, hold the potential to improve their therapeutic benefits by mitigating neuronal damage and promoting neurogenesis and angiogenesis. This review summarizes the characteristics of EVs, the molecular mechanisms underlying IS pathophysiology, and the emerging role of EVs in IS treatment at the molecular level. This review also explores the recent advancements in EV engineering, including the incorporation of specific proteins, RNAs, or pharmacological agents into EVs to enhance their therapeutic efficacy.

Therapeutic targeting of senescent cells in the CNS

Senescent cells accumulate throughout the body with advanced age, diseases and chronic conditions. They negatively impact health and function of multiple systems, including the central nervous system (CNS). Therapies that target senescent cells, broadly referred to as senotherapeutics, recently emerged as potentially important treatment strategies for the CNS. Promising therapeutic approaches involve clearing senescent cells by disarming their pro-survival pathways with 'senolytics'; or dampening their toxic senescence-associated secretory phenotype (SASP) using 'senomorphics'. Following the pioneering discovery of first-generation senolytics dasatinib and quercetin, dozens of additional therapies have been identified, and several promising targets are under investigation. Although potentially transformative, senotherapies are still in early stages and require thorough testing to ensure reliable target engagement, specificity, safety and efficacy. The limited brain penetrance and potential toxic side effects of CNS-acting senotherapeutics pose challenges for drug development and translation to the clinic. This Review assesses the potential impact of senotherapeutics for neurological conditions by summarizing preclinical evidence, innovative methods for target and biomarker identification, academic and industry drug development pipelines and progress in clinical trials.

Senolytic treatment diminishes microglia and decreases severity of experimental autoimmune encephalomyelitis

BACKGROUND

The role of senescence in disease contexts is complex, however there is considerable evidence that depletion of senescent cells improves outcomes in a variety of contexts particularly related to aging, cognition, and neurodegeneration. Much research has shown previously that inflammation can promote cellular senescence. Microglia are a central nervous system innate immune cell that undergo senescence with aging and during neurodegeneration. The contribution of senescent microglia to multiple sclerosis, an inflammatory neurodegenerative disease, is not clear, but microglia are strongly implicated in chronic active lesion pathology, tissue injury, and disease progression. Drugs that could specifically eliminate dysregulated microglia in multiple sclerosis are therefore of great interest to the field.

RESULTS

A single-cell analysis of brain tissue from mice subjected to experimental autoimmune encephalomyelitis (EAE), a mouse model of CNS inflammation that models aspects of multiple sclerosis (MS), identified microglia with a strong transcriptional signature of senescence including the presence of BCL2-family gene transcripts. Microglia expressing Bcl2l1 had higher expression of pro-inflammatory and senescence associated genes than their Bcl2l1 negative counterparts in EAE, suggesting they may exacerbate inflammation. Notably, in human single-nucleus sequencing from MS, BCL2L1 positive microglia were enriched in lesions with active inflammatory pathology, and likewise demonstrated increased expression of immune genes suggesting they may be proinflammatory and contribute to disease processes in chronic active lesions. Employing a small molecule BCL2-family inhibitor, Navitoclax (ABT-263), significantly reduced the presence of microglia and macrophages in the EAE spinal cord, suggesting that these cells can be targeted by senolytic treatment. ABT-263 treatment had a profound effect on EAE mice: decreasing motor symptom severity, improving visual acuity, promoting neuronal survival, and decreasing white matter inflammation.

CONCLUSION

These results support the hypothesis that microglia and macrophages exhibit transcriptional features of cellular senescence in EAE and MS, and that microglia expressing Bcl2l1 demonstrate a proinflammatory signature that may exacerbate inflammation resulting in negative outcomes in neuroinflammatory disease. Depleting microglia and macrophages using a senolytic results in robust improvement in EAE disease severity, including across measures of neurodegeneration, inflammation, and demyelination, and may therefore represent a novel strategy to address disease progression in multiple sclerosis.

Prophylactic senolytic treatment in aged mice reduces seizure severity and improves survival from Status Epilepticus

Increased vulnerability to seizures in aging has been well documented both clinically and in various models of aging in epilepsy. Seizures can exacerbate cognitive decline that is already prominent in aging. Senescent cells are thought to contribute to cognitive impairment in aging and clearing senescent cells with senolytic drugs improves cognitive function in animal models. It remains unclear whether senescent cells render the aged brain vulnerable to seizures. Here, we demonstrate that prophylactic senolytic treatment with Dasatinib and Quercetin (D&Q) reduced both seizure severity and mortality in aged C57BL/6J mice. We subjected the D&Q and VEH-treated aged mice to spatial memory testing before and after an acute seizure insult, Status Epilepticus [SE], which leads to epilepsy development. We found that senolytic therapy improved spatial memory before injury, however, spatial memory was not rescued after SE. Senescence-related proteins p16 and senescence-associated β-galactosidase were reduced in D&Q-treated aged mice. Our findings indicate that senescent cells increase seizure susceptibility in aging. Thus, prophylactically targeting senescent cells may prevent age-related seizure vulnerability.

Navitoclax safety, tolerability, and effect on biomarkers of senescence and neurodegeneration in aged nonhuman primates

Alzheimer's disease (AD) is the most common global dementia and is universally fatal. Most late-stage AD disease-modifying therapies are intravenous and target amyloid beta (Aβ), with only modest effects on disease progression: there remains a high unmet need for convenient, safe, and effective therapeutics. Senescent cells (SC) and the senescence-associated secretory phenotype (SASP) drive AD pathology and increase with AD severity. Preclinical senolytic studies have shown improvements in neuroinflammation, tau, Aβ, and CNS damage; most were conducted in transgenic rodent models with uncertain human translational relevance. In this study, aged cynomolgus monkeys had significant elevation of biomarkers of senescence, SASP, and neurological damage. Intermittent treatment with the senolytic navitoclax induced modest reversible thrombocytopenia; no serious drug-related toxicity was noted. Navitoclax reduced several senescence and SASP biomarkers, with CSF concentrations sufficient for senolysis. Finally, navitoclax reduced TSPO-PET frontal cortex binding and showed trends of improvement in CSF biomarkers of neuroinflammation, neuronal damage, and synaptic dysfunction. Overall, navitoclax administration was safe and well tolerated in aged monkeys, inducing trends of biomarker changes relevant to human neurodegenerative disease.

Senescent cell clearance ameliorates temporal lobe epilepsy and associated spatial memory deficits in mice

Current therapies for the epilepsies only treat the symptoms, but do not prevent epileptogenesis (the process in which epilepsy develops). Many cellular responses during epileptogenesis are also common hallmarks of cellular senescence , which halts proliferation of damaged cells. Clearing senescent cells (SCs) restores function in several age-associated and neurodegenerative disease models. It is unknown whether SC accumulation contributes to epileptogenesis and associated cognitive impairments. To address this question, we used a mouse model of temporal lobe epilepsy (TLE) and characterized the senescence phenotype throughout epileptogenesis. SCs accumulated 2 weeks after SE and were predominantly microglia. We ablated SCs and reduced (and in some cases prevented) the emergence of spontaneous seizures and normalized cognitive function in mice. Suggesting that this is a translationally-relevant target we also found SC accumulation in resected hippocampi from patients with TLE. These findings indicate that SC ablation after an epileptogenic insult is a potential anti-epileptogenic therapy.

Aging Modulates the Ability of Quiescent Radial Glia-Like Stem Cells in the Hippocampal Dentate Gyrus to be Recruited into Division by Pro-neurogenic Stimuli

A delicate balance between quiescence and division of the radial glia-like stem cells (RGLs) ensures continuation of adult hippocampal neurogenesis (AHN) over the lifespan. Transient or persistent perturbations of this balance due to a brain pathology, drug administration, or therapy can lead to unfavorable long-term outcomes such as premature depletion of the RGLs, decreased AHN, and cognitive deficit. Memantine, a drug used for alleviating the symptoms of Alzheimer's disease, and electroconvulsive seizure (ECS), a procedure used for treating drug-resistant major depression or bipolar disorder, are known strong AHN inducers; they were earlier demonstrated to increase numbers of dividing RGLs. Here, we demonstrated that 1-month stimulation of quiescent RGLs by either memantine or ECS leads to premature exhaustion of their pool and altered AHN at later stages of life and that aging of the brain modulates the ability of the quiescent RGLs to be recruited into the cell cycle by these AHN inducers. Our findings support the aging-related divergence of functional features of quiescent RGLs and have a number of implications for the practical assessment of drugs and treatments with respect to their action on quiescent RGLs at different stages of life in animal preclinical studies.

Cellular Aging and Senescence in Cancer: A Holistic Review of Cellular Fate Determinants

This comprehensive review navigates the complex relationship between cellular aging, senescence, and cancer, unraveling the determinants of cellular fate. Beginning with an overview of cellular aging's significance in cancer, the review explores processes, changes, and molecular pathways influencing senescence. The review explores senescence as a dual mechanism in cancer, acting as a suppressor and contributor, focusing on its impact on therapy response. This review highlights opportunities for cancer therapies that target cellular senescence. The review further examines the senescence-associated secretory phenotype and strategies to modulate cellular aging to influence tumor behavior. Additionally, the review highlights the mechanisms of senescence escape in aging and cancer cells, emphasizing their impact on cancer prognosis and resistance to therapy. The article addresses current advances, unexplored aspects, and future perspectives in understanding cellular aging and senescence in cancer.

The potential for senotherapy as a novel approach to extend life quality in veterinary medicine

Cellular senescence, a condition where cells undergo arrest and can assume an inflammatory phenotype, has been associated with initiation and perpetuation of inflammation driving multiple disease processes in rodent models and humans. Senescent cells secrete inflammatory cytokines, proteins, and matrix metalloproteinases, termed the senescence associated secretory phenotype (SASP), which accelerates the aging processes. In preclinical models, drug interventions termed "senotherapeutics" selectively clear senescent cells and represent a promising strategy to prevent or treat multiple age-related conditions in humans and veterinary species. In this review, we summarize the current available literature describing in vitro evidence for senotheraputic activity, preclinical models of disease, ongoing human clinical trials, and potential clinical applications in veterinary medicine. These promising data to date provide further justification for future studies identifying the most active senotherapeutic combinations, dosages, and routes of administration for use in veterinary medicine.

Senolytic-facilitated Reversal of End-Organ Dysfunction in a Murine Model of Obstructive Sleep Apnea

Rationale: Obstructive sleep apnea (OSA) is a highly prevalent condition that is associated with accelerated biological aging and multiple end-organ morbidities. Current treatments, such as continuous positive airway pressure (CPAP), have shown limited cognitive, metabolic, and cardiovascular beneficial outcomes despite adherence. Thus, adjunct therapies aiming to reduce OSA burden, such as senolytics, could improve OSA outcomes.Objectives: To assess if targeting senescence in addition to partial normoxia mimicking "good" CPAP adherence can improve physiological outcomes in mice exposed to chronic intermittent hypoxia.Methods: We compared the effects of 6 weeks of therapy with either partial normoxic recovery alone or combined with the senolytic navitoclax after 16 weeks of intermittent hypoxia exposures, a hallmark of OSA, on multiphenotypic cardiometabolic and neurocognitive parameters.Measurements and Main Results: Our findings indicate that only when combined with navitoclax, partial normoxic recovery significantly improved sleepiness (sleep in the dark phase: 34% ± 4% vs. 26% ± 3%; P < 0.01), cognition (preference score: 51% ± 19% vs. 70% ± 11%; P = 0.048), coronary artery function (response to acetylcholine [vasodilation]: 56% ± 13% vs. 72% ± 10%; P < 0.001), glucose, and lipid metabolism and reduced intestinal permeability and senescence in multiple organs.Conclusions: These findings indicate that the reversibility of end-organ morbidities induced by OSA is not only contingent on restoration of normal oxygenation patterns but can be further enhanced by targeting other OSA-mediated detrimental cellular processes, such as accelerated senescence.

The role of DNA damage in neural stem cells ageing

Neural stem cells (NSCs) are pluripotent stem cells with the potential to differentiate into a variety of nerve cells. NSCs are susceptible to both intracellular and extracellular insults, thus causing DNA damage. Extracellular insults include ultraviolet, ionizing radiation, base analogs, modifiers, alkyl agents and others, while intracellular factors include Reactive oxygen species (ROS) radicals produced by mitochondria, mismatches that occur during DNA replication, deamination of bases, loss of bases, and more. When encountered with DNA damage, cells typically employ three coping strategies: DNA repair, damage tolerance, and apoptosis. NSCs, like many other stem cells, have the ability to divide, differentiate, and repair DNA damage to prevent mutations from being passed down to the next generation. However, when DNA damage accumulates over time, it will lead to a series of alterations in the metabolism of cells, which will cause cellular ageing. The ageing and exhaustion of neural stem cell will have serious effects on the body, such as neurodegenerative diseases. The purpose of this review is to examine the processes by which DNA damage leads to NSCs ageing and the mechanisms of DNA repair in NSCs.

Potential Clinical Implications of Senotherapies for Cardiovascular Disease

Aging is a major risk factor for cardiovascular diseases (CVDs) and accumulating evidence indicates that biological aging has a significant effect on the onset and progression of CVDs. In recent years, therapies targeting senescent cells (senotherapies), particularly senolytics that selectively eliminate senescent cells, have been developed and show promise for treating geriatric syndromes and age-associated diseases, including CVDs. In 2 pilot studies published in 2019 the senolytic combination, dasatinib plus quercetin, improved physical function in patients with idiopathic pulmonary fibrosis and eliminated senescent cells from adipose tissue in patients with diabetic kidney disease. More than 30 clinical trials using senolytics are currently underway or planned. In preclinical CVD models, senolytics appear to improve heart failure, ischemic heart disease, valvular heart disease, atherosclerosis, aortic aneurysm, vascular dysfunction, dialysis arteriovenous fistula patency, and pre-eclampsia. Because senotherapies are completely different strategies from existing treatment paradigms, they might alleviate diseases for which there are no current effective treatments or they could be used in addition to current therapies to enhance efficacy. Moreover, senotherapies might delay, prevent, alleviate or treat multiple diseases in the elderly and reduce polypharmacy, because senotherapies target fundamental aging mechanisms. We comprehensively summarize the preclinical evidence about senotherapies for CVDs and discuss future prospects for their clinical application.

Alzheimer's Amyloid-β Accelerates Cell Senescence and Suppresses SIRT1 in Human Neural Stem Cells

As a lifelong source of neurons, neural stem cells (NSCs) serve multiple crucial functions in the brain. The senescence of NSCs may be associated with the onset and progression of Alzheimer's disease (AD). Our study reveals a noteworthy finding, indicating that the AD-associated pathogenic protein amyloid-β (Aβ) substantially enhances senescence-related characteristics of human NSCs. These characteristics encompass the enhanced expression of p16 and p21, the upregulation of genes associated with the senescence-associated secretory phenotype (SASP), increased SA-β-gal activity, and the activation of the DNA damage response. Further studies revealed that Aβ treatment significantly downregulates the SIRT1 protein which plays a crucial role in regulating the aging process and decreases downstream PGC-1α and FOXO3. Subsequently, we found that SIRT1 overexpression significantly alleviates a range of Aβ-induced senescent markers in human NSCs. Taken together, our results uncover that Aβ accelerates cellular senescence in human NSCs, making SIRT1 a highly promising therapeutic target for senescent NSCs which may contribute to age-related neurodegenerative diseases, including AD.

Pharmacological Targeting of Senescence with Senolytics as a New Therapeutic Strategy for Neurodegeneration

Cellular senescence is a state of permanent cell-cycle arrest. Early in life, senescence has a physiologic role in tumor suppression and wound healing. However, gradually, as these senescent cells accumulate over the lifespan of an organism, they contribute to inflammation and the progression of age-related diseases, including neurodegeneration. Targeting senescent cells using a class of drugs known as "senolytics" holds great promise for the management of Alzheimer's and Parkinson's disease. Already, several senolytic compounds have been shown to ameliorate cognitive deficits across several preclinical models of neurodegeneration. Most of these senolytics (e.g., dasatinib) are repurposed clinical or experimental anticancer drugs, which trigger apoptosis of senescent cells by interfering with pro-survival pathways. However, outside of their senolytic function, many first-generation senolytics also have other less appreciated neuroprotective effects, such as potent antioxidant and anti-inflammatory activity. In addition, some senolytic drugs may also have negative dose-limiting toxicities, including thrombocytopenia. In this review, we discuss the various biologic pathways targeted by the leading senolytic drugs, namely dasatinib, quercetin, fisetin, and navitoclax. We further evaluate the clinical transability of these compounds for neurodegeneration, assessing their adverse effects, pharmacokinetic properties, and chemical structure. SIGNIFICANCE STATEMENT: Currently, there are no effective disease-modifying treatments for the most prevalent neurodegenerative disorders, including Alzheimer's and Parkinson's disease. Some of the drugs currently available for treating these diseases are associated with unwanted side-effects and/or become less efficacious with time. Therefore, researchers have begun to explore new innovative treatments for these belligerent diseases, including senolytic drugs. These agents lead to the apoptosis of senescent cells thereby preventing their deleterious role in neurodegeneration.

Cellular senescence in brain aging and neurodegeneration

Cellular senescence is a state of terminal cell cycle arrest associated with various macromolecular changes and a hypersecretory phenotype. In the brain, senescent cells naturally accumulate during aging and at sites of age-related pathologies. Here, we discuss the recent advances in understanding the accumulation of senescent cells in brain aging and disorders. Here we highlight the phenotypical heterogeneity of different senescent brain cell types, highlighting the potential importance of subtype-specific features for physiology and pathology. We provide a comprehensive overview of various senescent cell types in naturally occurring aging and the most common neurodegenerative disorders. Finally, we critically discuss the potential of adapting senotherapeutics to improve brain health and reduce pathological progression, addressing limitations and future directions for application and development.

From Youthful Vigor to Aging Decline: Unravelling the Intrinsic and Extrinsic Determinants of Hippocampal Neural Stem Cell Aging

Since Joseph Altman published his pioneering work demonstrating neurogenesis in the hippocampus of adult rats, the number of publications in this field increased exponentially. Today, we know that the adult hippocampus harbors a pool of adult neural stem cells (NSCs) that are the source of life-long neurogenesis and plasticity. The functions of these NSCs are regulated by extrinsic cues arising from neighboring cells and the systemic environment. However, this tight regulation is subject to imbalance with age, resulting in a decline in adult NSCs and neurogenesis, which contributes to the progressive deterioration of hippocampus-related cognitive functions. Despite extensive investigation, the mechanisms underlying this age-related decline in neurogenesis are only incompletely understood, but appear to include an increase in NSC quiescence, changes in differentiation patterns, and NSC exhaustion. In this review, we summarize recent work that has improved our knowledge of hippocampal NSC aging, focusing on NSC-intrinsic mechanisms as well as cellular and molecular changes in the niche and systemic environment that might be involved in the age-related decline in NSC functions. Additionally, we identify future directions that may advance our understanding of NSC aging and the concomitant loss of hippocampal neurogenesis and plasticity.

Cellular senescence and neurodegeneration

Advancing age is a major risk factor of Alzheimer's disease (AD). The worldwide prevalence of AD is approximately 50 million people, and this number is projected to increase substantially. The molecular mechanisms underlying the aging-associated susceptibility to cognitive impairment in AD are largely unknown. As a hallmark of aging, cellular senescence is a significant contributor to aging and age-related diseases including AD. Senescent neurons and glial cells have been detected to accumulate in the brains of AD patients and mouse models. Importantly, selective elimination of senescent cells ameliorates amyloid beta and tau pathologies and improves cognition in AD mouse models, indicating a critical role of cellular senescence in AD pathogenesis. Nonetheless, the mechanisms underlying when and how cellular senescence contributes to AD pathogenesis remain unclear. This review provides an overview of cellular senescence and discusses recent advances in the understanding of the impact of cellular senescence on AD pathogenesis, with brief discussions of the possible role of cellular senescence in other neurodegenerative diseases including Down syndrome, Parkinson's disease, multiple sclerosis, and amyotrophic lateral sclerosis.

Ageing in the brain: mechanisms and rejuvenating strategies

Ageing is characterized by the progressive loss of cellular homeostasis, leading to an overall decline of the organism's fitness. In the brain, ageing is highly associated with cognitive decline and neurodegenerative diseases. With the rise in life expectancy, characterizing the brain ageing process becomes fundamental for developing therapeutic interventions against the increased incidence of age-related neurodegenerative diseases and to aim for an increase in human life span and, more importantly, health span. In this review, we start by introducing the molecular/cellular hallmarks associated with brain ageing and their impact on brain cell populations. Subsequently, we assess emerging evidence on how systemic ageing translates into brain ageing. Finally, we revisit the mainstream and the novel rejuvenating strategies, discussing the most successful ones in delaying brain ageing and related diseases.

A short dasatinib and quercetin treatment is sufficient to reinstate potent adult neuroregenesis in the aged killifish

The young African turquoise killifish has a high regenerative capacity, but loses it with advancing age, adopting several aspects of the limited form of mammalian regeneration. We deployed a proteomic strategy to identify pathways that underpin the loss of regenerative power caused by aging. Cellular senescence stood out as a potential brake on successful neurorepair. We applied the senolytic cocktail Dasatinib and Quercetin (D + Q) to test clearance of chronic senescent cells from the aged killifish central nervous system (CNS) as well as rebooting the neurogenic output. Our results show that the entire aged killifish telencephalon holds a very high senescent cell burden, including the parenchyma and the neurogenic niches, which could be diminished by a short-term, late-onset D + Q treatment. Reactive proliferation of non-glial progenitors increased substantially and lead to restorative neurogenesis after traumatic brain injury. Our results provide a cellular mechanism for age-related regeneration resilience and a proof-of-concept of a potential therapy to revive the neurogenic potential in an already aged or diseased CNS.

Senolytics: opening avenues in drug discovery to find novel therapeutics for Parkinson's disease

Aging is one of the major risk factors for most neurodegenerative disorders including Parkinson's disease (PD). More than 10 million people are affected with PD worldwide. One of the predominant factors accountable for progression of PD pathology could be enhanced accumulation of senescent cells in the brain with the progress of age. Recent investigations have highlighted that senescent cells can ignite PD pathology via increased oxidative stress and neuroinflammation. Senolytics are agents that kill senescent cells. This review mainly focuses on understanding the pathological connection between senescence and PD, with emphasis on some of the recent advances made in the area of senolytics and their evolution to potential clinical candidates for future pharmaceuticals against PD.

Epigenetic aging in adult neurogenesis

Neural stem cells (NSCs) in the hippocampus generate new neurons throughout life, which functionally contribute to cognitive flexibility and mood regulation. Yet adult hippocampal neurogenesis substantially declines with age and age-related impairments in NSC activity underlie this reduction. Particularly, increased NSC quiescence and consequently reduced NSC proliferation are considered to be major drivers of the low neurogenesis levels in the aged brain. Epigenetic regulators control the gene expression programs underlying NSC quiescence, proliferation and differentiation and are hence critical to the regulation of adult neurogenesis. Epigenetic alterations have also emerged as central hallmarks of aging, and recent studies suggest the deterioration of the NSC-specific epigenetic landscape as a driver of the age-dependent decline in adult neurogenesis. In this review, we summarize the recently accumulating evidence for a role of epigenetic dysregulation in NSC aging and propose perspectives for future research directions.

Anti-Oxidative and Anti-Aging Effects of Probiotic Fermented Ginseng by Modulating Gut Microbiota and Metabolites in Caenorhabditis elegans

Antioxidative and antiaging abilities of probiotic fermented ginseng (PG) were evaluated in Caenorhabditis elegans (C. elegans). Lifespan and effect on heat stress and acute oxidative stress in C. elegans were significantly enhanced by PG. Antioxidative enzymes such as T-SOD, GSH-PX, CAT were significantly up-regulated, and MDA, ROS and apoptosis levels were significantly down-regulated. At the same time, PG exerted antioxidant and anti-aging activities by reducing the expression of DAF-2 mRNA and increasing the expression of SKN-1 and SOD-3 mRNA in C. elegans. In addition, the mechanism of antioxidative and antiaging activities of PG was explored through gut microbiota sequencing and untargeted metabolomics. The results of gut microbiota indicated that PG could significantly improve the composition and structure of microbes in the gut of C. elegans, and the relative abundance of beneficial bacteria was up-regulated. Untargeted metabolomic results elucidated that PG modulated antioxidant and antiaging activities through neuroactive ligand-receptor interaction, Citrate cycle (TCA cycle), pyruvate metabolism, ascorbate and aldarate metabolism and D-Arginine and D-ornithine metabolism of C. elegans. These results indicated that PG had excellent antioxidant and anti-aging activities, providing research value for the development of functional foods and improvement of aging-related diseases.

Exercise suppresses neuroinflammation for alleviating Alzheimer's disease

Alzheimer's disease (AD) is a chronic neurodegenerative disease, with the characteristics of neurofibrillary tangle (NFT) and senile plaque (SP) formation. Although great progresses have been made in clinical trials based on relevant hypotheses, these studies are also accompanied by the emergence of toxic and side effects, and it is an urgent task to explore the underlying mechanisms for the benefits to prevent and treat AD. Herein, based on animal experiments and a few clinical trials, neuroinflammation in AD is characterized by long-term activation of pro-inflammatory microglia and the NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasomes. Damaged signals from the periphery and within the brain continuously activate microglia, thus resulting in a constant source of inflammatory responses. The long-term chronic inflammatory response also exacerbates endoplasmic reticulum oxidative stress in microglia, which triggers microglia-dependent immune responses, ultimately leading to the occurrence and deterioration of AD. In this review, we systematically summarized and sorted out that exercise ameliorates AD by directly and indirectly regulating immune response of the central nervous system and promoting hippocampal neurogenesis to provide a new direction for exploring the neuroinflammation activity in AD.

Senescent cells in the brain and where to find them

Cellular senescence is a process in which cells change their characteristic phenotype in response to stress and enter a state of prolonged cell cycle arrest accompanied by a distinct secretory phenotype. Cellular senescence has both beneficial and detrimental outcomes. With age, senescent cells progressively accumulate in tissues and might be the bridge connecting ageing to many age-related pathologies. In recent years, evidence emerged supporting the accumulation of brain senescent cells during neurological disorders and ageing. Here, we will discuss the different brain cell populations that exhibit a senescent phenotype. Subsequently, we will explore several senolytic strategies which have been developed to eliminate senescent cells. Finally, we will examine their potential to directly eliminate these senescent brain cells.

Adult hippocampal neurogenesis in Alzheimer's disease: A roadmap to clinical relevance

Adult hippocampal neurogenesis (AHN) drops sharply during early stages of Alzheimer's disease (AD), via unknown mechanisms, and correlates with cognitive status in AD patients. Understanding AHN regulation in AD could provide a framework for innovative pharmacological interventions. We here combine molecular, behavioral, and clinical data and critically discuss the multicellular complexity of the AHN niche in relation to AD pathophysiology. We further present a roadmap toward a better understanding of the role of AHN in AD by probing the promises and caveats of the latest technological advancements in the field and addressing the conceptual and methodological challenges ahead.

Identification and validation of biomarkers based on cellular senescence in mild cognitive impairment

Background

Mild cognitive impairment (MCI), a syndrome defined as decline of cognitive function greater than expected for an individual's age and education level, occurs in up to 22.7% of elderly patients in United States, causing the heavy psychological and economic burdens to families and society. Cellular senescence (CS) is a stress response that accompanies permanent cell-cycle arrest, which has been reported to be a fundamental pathological mechanism of many age-related diseases. This study aims to explore biomarkers and potential therapeutic targets in MCI based on CS.

Methods

The mRNA expression profiles of peripheral blood samples from patients in MCI and non-MCI group were download from gene expression omnibus (GEO) database (GSE63060 for training and GSE18309 for external validation), CS-related genes were obtained from CellAge database. Weighted gene co-expression network analysis (WGCNA) was conducted to discover the key relationships behind the co-expression modules. The differentially expressed CS-related genes would be obtained through overlapping among the above datasets. Then, pathway and GO enrichment analyses were performed to further elucidate the mechanism of MCI. The protein-protein interaction network was used to extract hub genes and the logistic regression was performed to distinguish the MCI patients from controls. The hub gene-drug network, hub gene-miRNA network as well as transcription factor-gene regulatory network were used to analyze potential therapeutic targets for MCI.

Results

Eight CS-related genes were identified as key gene signatures in MCI group, which were mainly enriched in the regulation of response to DNA damage stimulus, Sin3 complex and transcription corepressor activity. The receiver operating characteristic curves of logistic regression diagnostic model were constructed and presented great diagnostic value in both training and validation set.

Conclusion

Eight CS-related hub genes - SMARCA4, GAPDH, SMARCB1, RUNX1, SRC, TRIM28, TXN, and PRPF19 - serve as candidate biomarkers for MCI and display the excellent diagnostic value. Furthermore, we also provide a theoretical basis for targeted therapy against MCI through the above hub genes.

Neurons and glial cells acquire a senescent signature after repeated mild traumatic brain injury in a sex-dependent manner

Mild traumatic brain injury (mTBI) is an important public health issue, as it can lead to long-term neurological symptoms and risk of neurodegenerative disease. The pathophysiological mechanisms driving this remain unclear, and currently there are no effective therapies for mTBI. In this study on repeated mTBI (rmTBI), we have induced three mild closed-skull injuries or sham procedures, separated by 24 h, in C57BL/6 mice. We show that rmTBI mice have prolonged righting reflexes and astrogliosis, with neurological impairment in the Morris water maze (MWM) and the light dark test. Cortical and hippocampal tissue analysis revealed DNA damage in the form of double-strand breaks, oxidative damage, and R-loops, markers of cellular senescence including p16 and p21, and signaling mediated by the cGAS-STING pathway. This study identified novel sex differences after rmTBI in mice. Although these markers were all increased by rmTBI in both sexes, females had higher levels of DNA damage, lower levels of the senescence protein p16, and lower levels of cGAS-STING signaling proteins compared to their male counterparts. Single-cell RNA sequencing of the male rmTBI mouse brain revealed activation of the DNA damage response, evidence of cellular senescence, and pro-inflammatory markers reminiscent of the senescence-associated secretory phenotype (SASP) in neurons and glial cells. Cell-type specific changes were also present with evidence of brain immune activation, neurotransmission alterations in both excitatory and inhibitory neurons, and vascular dysfunction. Treatment of injured mice with the senolytic drug ABT263 significantly reduced markers of senescence only in males, but was not therapeutic in females. The reduction of senescence by ABT263 in male mice was accompanied by significantly improved performance in the MWM. This study provides compelling evidence that senescence contributes to brain dysfunction after rmTBI, but may do so in a sex-dependent manner.

Cellular senescence and senolytics: the path to the clinic

Interlinked and fundamental aging processes appear to be a root-cause contributor to many disorders and diseases. One such process is cellular senescence, which entails a state of cell cycle arrest in response to damaging stimuli. Senescent cells can arise throughout the lifespan and, if persistent, can have deleterious effects on tissue function due to the many proteins they secrete. In preclinical models, interventions targeting those senescent cells that are persistent and cause tissue damage have been shown to delay, prevent or alleviate multiple disorders. In line with this, the discovery of small-molecule senolytic drugs that selectively clear senescent cells has led to promising strategies for preventing or treating multiple diseases and age-related conditions in humans. In this Review, we outline the rationale for senescent cells as a therapeutic target for disorders across the lifespan and discuss the most promising strategies-including recent and ongoing clinical trials-for translating small-molecule senolytics and other senescence-targeting interventions into clinical use.