Q. Behfar1, A. Ramirez Zuniga1,2,3,4,5, P.V. Martino-Adami1
1. Division of Neurogenetics and Molecular Psychiatry, Department of Psychiatry and Psychotherapy, Faculty of Medicine and University Hospital Cologne, University of Cologne, Cologne, Germany; 2. Department of Neurodegenerative Diseases and Geriatric Psychiatry, University Hospital Bonn, Medical Faculty, Bonn, Germany;
3. German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany; 4. Department of Psychiatry and Glenn Biggs Institute for Alzheimer’s and Neurodegenerative Diseases, San Antonio, Texas, USA; 5. Cluster of Excellence Cellular Stress Responses in Aging-associated Diseases (CECAD), University of Cologne. Cologne, Germany.
Corresponding Author: Pamela V. Martino-Adami, PhD., Division of Neurogenetics and Molecular Psychiatry, Department of Psychiatry and Psychotherapy, Faculty of Medicine and University Hospital Cologne, University of Cologne. Kerpener Str. 62, 50937 Cologne, Germany, E-mail: firstname.lastname@example.org, Phone: +49-(0)221- 478-51653, Fax: +49-(0)221-478 478-98042
J Prev Alz Dis 2022;
Published online April 28, 2022, http://dx.doi.org/10.14283/jpad.2022.42
The underlying processes occurring in aging are complex, involving numerous biological changes that result in chronic cellular stress and sterile inflammation. One of the main hallmarks of aging is senescence. While originally the term senescence was defined in the field of oncology, further research has established that also microglia, astrocytes and neurons become senescent. Since age is the main risk factor for neurodegenerative diseases, it is reasonable to argue that cellular senescence might play a major role in Alzheimer’s disease. Specific cellular changes seen during Alzheimer’s disease are similar to those observed during senescence across all resident brain cell types. Furthermore, increased levels of senescence-associated secretory phenotype proteins such as IL-6, IGFBP, TGF-β and MMP-10 have been found in both CSF and plasma samples from Alzheimer’s disease patients. In addition, genome-wide association studies have identified that individuals with Alzheimer’s disease carry a high burden of genetic risk variants in genes known to be involved in senescence, including ADAM10, ADAMTS4, and BIN1. Thus, cellular senescence is emerging as a potential underlying disease process operating in Alzheimer’s disease. This has also attracted more attention to exploiting cellular senescence as a therapeutic target. Several senolytic compounds with the capability to eliminate senescent cells have been examined in vivo and in vitro with notable results, suggesting they may provide a novel therapeutic avenue. Here, we reviewed the current knowledge of cellular senescence and discussed the evidence of senescence in various brain cell types and its putative role in inflammaging and neurodegenerative processes.
Key words: Aging, senescence, senescence-associated secretory phenotype, neuroinflammation, Alzheimer’s disease.
Life expectancy in developed nations is at record highs (1); however healthy aging is not increasing as fast as lifespan (2), mainly due to an increase in the incidence of age-related non-communicable chronic diseases (NCDs), including neurodegenerative disorders.
Aging is a biological process that leads to a progressive structural and functional decline that finally results in an inability to adapt to environmental stressors (3). Understanding the mechanisms driving this process -one of the main goals of geroscience- is, therefore, of critical importance to mitigate the burden of age-related NCDs, alleviate functional decline, and promote healthy aging (4–6).
Aging has negative impacts on cognition, and even in physically healthy elderly individuals, a mild decline in brain functions is observable, which heralds the predisposition to subsequent dementia (7). Large studies have shown a rapid increase in the incidence of dementia after 65 years of age that doubles every five years (8), with Alzheimer’s disease (AD) accounting for 60 to 70% of all cases (9). Nevertheless, not every person will develop dementia during aging, indicating that age-associated processes may not inevitably lead to dementia, and despite their synchronicity, normal aging and AD might be representatives of different pathways. Hence, elucidation of the fundamental processes occurring in normal aging might pave the way to prevent or postpone the development of dementia. One of these key processes in aging is cellular senescence, which results in cell cycle arrest and the release of a senescence-associated secretory phenotype (SASP).
In this review, we present an overview of cellular senescence, SASP, and neuroinflammation. We concentrated on studies reporting the role of cellular senescence and neuroinflammation as a biological manifestation of aging and their association with dementia. Finally, we discuss specific trajectories and implications for future research in brain aging, senescence, and dementia.
Senescence is a cellular homeostatic response to inhibit propagation of damaged cells and neoplastic transformation (10). However, it might also promote phenotypes and pathologies associated with aging (11). Senescent cells undergo both morphological and functional alterations. Morphological changes include cytoskeletal rearrangements (12, 13) and changes in cell membrane composition (14) leading to enlargement and flattening, which result in an irregular shape. In addition, cellular senescence may lead to a reorganization of the nuclear lamina and downregulation of lamin B1, causing alterations in nuclear morphology and gene expression (15). The senescence process is also accompanied by certain chromatic changes, such as senescence-associated heterochromatin formation (SAHF) with the deactivation of proliferation-associated genes (16–19).
From a functional perspective, senescent cells show increased activation of pathways related to cellular stress, such as unfolded protein response (UPR) (20), SASP release (21), upregulated lysosomal senescence-associated-β-galactosidase (SA-β-gal) expression (22), (22), a shift from oxidative phosphorylation to glycolysis (23, 24), and cytoplasmic accumulation of lipofuscin (25–27).
The cellular program activated during senescence consists of two main components – the intrinsic and the extrinsic arms. The intrinsic arm regulates cell cycle arrest and is mediated by regulatory proteins including the p53, p16, and p21 tumor suppressors. Some senescence inducers cause DNA damage that triggers the DNA damage response (DDR) (28), which in turn activates p53 and can result in different outcomes depending on the damage level.
Mild DNA damage normally induces cell cycle arrest, while severe injury can activate senescence or death programs. During the initiation of senescence, also called “primary senescence”, the stressed cells may be still able to repair the damage and then can escape from cell cycle arrest. However, persistent exposure to a damaging environment leads to “developing senescence” and activation of the extrinsic arm of the program, consisting of SASP release (29). In a physiological setting, the immune cells contribute to the clearance of senescent cells and cellular debris, thus eliminating the initial source of the damage and SASP. However, in a pathological setting, the senescent cells are not sufficiently cleared. This leads to an increased number of senescent cells and increased SASP from these cells (Figure 1). Both p53 and DDR proteins seem to be involved in the earlier stages of senescence (30). p53 activity decreases with time, making it a crucial factor for senescence induction and a gatekeeper to an early phase and still reversible senescence (31).
Persistent exposure to a damaging environment leads to cellular senescence, characterized by cell cycle arrest, morphological alterations, upregulation of tumor suppressor proteins and lysosomal SA-β-gal, and the release of the senescence-associated secretory phenotype. SASP, senescence-associated secretory phenotype; ROS, reactive oxygen species; SAHF, senescence-associated heterochromatin formation.
SASP factors can be classified into the following major categories: soluble signaling factors (interleukins, chemokines, and growth factors), secreted proteases, and secreted insoluble proteins/extracellular matrix (ECM) components. Soluble signaling factors include IL-6, IL-1, IL-8, GROα, IGF, GM-CSF. The group of secreted proteases contains MMP-1, MMP-3, MMP-10, uPA. The ECM components comprise proteins such as fibronectin (Table 1). SASP proteases have three major functions: (a) shedding of membrane-associated proteins, (b) cleavage/degradation of signaling molecules, and (c) ECM degradation or processing, thus altering local and systemic tissue milieus (21). Even though there is a significant increase of factor secretion upon senescence, the SASP is neither a general nor unspecific up-regulator of cellular secretion. This claim is supported by the observation that the expression levels of many secreted factors do not change when cells senesce. Although a core of SASP factors is a common feature of all senescent cells, there are variations depending on the cell type and senescence inducer.
SASP, senescence-associated secretory phenotype; ECM, extracellular matrix; MMP, metalloproteinase.
Even though the majority of SASP factors are up-regulated at the mRNA level, partially due to increased nuclear factor kappa B (NF-κB) (32, 33), there is also evidence of epigenetic regulation. For instance, Sirtuin-1, a class III histone deacetylase that can deacetylate both histone and non-histone targets, was found to dissociate from the promoter regions of IL-6 and IL-8 during senescence (34). Both mRNA and protein levels of IL-6 and IL-8 increase quickly in Sirtuin-1 depleted cells, suggesting Sirtuin-1 represses the expression of SASP factors through the deacetylation of histones in their promoter regions. The dynamic epigenetic changes over the life span have recently attracted attention. DNA methylation changes have been employed to develop “epigenetic clocks” to identify the discrepancies between chronological and “biological” age of different tissues (35, 36).
However, SASP secretion does not always mean “bad news” for the cells. For instance, chemokines or cytokines can recruit natural killer cells, thus facilitating the removal of neighboring tumor cells. Other SASP factors can communicate cellular damage to the surrounding tissue and stimulate repair or limit damage-induced fibrosis (37). However, data from many studies strongly support the idea that SASP drive multiple age-related phenotypes and pathologies, including atherosclerosis (38), osteoarthritis (39), and overall decrements in health span (40). SASP factors, therefore, hold potential as biomarkers for aging and age-related diseases.
Brain cellular senescence
The brain requires multiple cell types working together to ensure proper functioning, including neurons, astrocytes, microglia, and endothelial cells. Like all aging tissues in the body, the aging brain is characterized by a low-level chronic inflammation that results in cellular and tissue changes, termed “inflammaging” (41). While acute inflammatory response causes robust short-acting and self-limiting oxidative and nitrosative damages, chronic inflammation is a weaker but long-standing self-perpetuating process (42). Since this inflammation is not caused by a pathogen or a foreign body, it is considered “sterile”. A potential contributor to brain inflammaging is cellular senescence, most likely occurring in replication-competent glial cells. An age-related build-up of senescent cells could set an ideal proinflammatory environment for the onset of neurodegenerative diseases. Thus, characterization of cellular senescence in the brain could uncover novel therapeutic targets for the prevention and treatment of chronic age-related brain diseases.
Astrocytes are the most abundant glial cells in the brain and have a prominent role in brain physiology and neuronal function. They support neurons, balance the content of the synaptic cleft, and maintain the ion-homeostasis and blood-brain-barrier integrity. Dysfunctional astrocytes are implicated in neuropathology associated with both normal brain aging and various age-related neurodegenerative diseases (43). Different external stressors have been shown to induce astrocyte senescence in vitro, including oxidative stress, proteasome inhibitors, and irradiation. These stressors led cultured human astrocytes to increase the expression levels of well-established markers of senescence, such as the proteins p53, p21, and p16, as well as production of a SASP (44–46). Importantly, senescent astrocytes can affect the vulnerability of neurons to glutamate-induced toxicity in vitro (47). A recent study has shown a severe reduction of lamin B1 and nuclear deformations in astrocytes from the granular cell layer of the hippocampus of post-mortem tissue from non-demented aged individuals (48). Differences in lamin B1 levels and astrocyte nuclear morphology were also found between the granular cell and polymorphic layers in the elderly human hippocampus, suggesting an intra-regional-dependent aging response of human astrocytes.
Microglia are of mesenchymal origin and function as resident macrophages in the central nervous system (49). They continuously sample the extracellular space by extending their numerous ramified processes, which makes them quite dynamic cells even in an apparently healthy brain (50). The execution of microglial responses to injury or disease has been termed “microglial activation”, and even though it may protect the brain through much of the lifespan, the efficiency of these functions seems to deteriorate with age. Thus, the effects of age on microglia may be characterized as dysfunctional or even hyper-reactive responses. Mosher and Wyss-Coray (51) have grouped the major physiological effector functions of microglia into six broad categories: 1) proliferation, 2) morphological transformation, 3) motility and migration, 4) intercellular communication, 5) phagocytosis, and 6) proteostasis; and considered aging phenotypes within each of these functions: 1) senescence, 2) dystrophy, 3) impaired movement, 4) altered signaling, 5) impaired phagocytosis, and 6) impaired proteostasis. These aging phenotypes were termed as “hallmarks of microglia aging”. Senescent microglia have been detected in different brain regions from aged individuals using immunohistochemistry and positron emission tomography (PET) (52, 53), suggesting that it is a process that affects the vast majority of microglia and not a single population. Thus, the brain would become less capable of a defensive immune response when confronted with injury or disease.
Although it is well known that neuronal functions decline with aging (54), the concept of neuronal senescence is relatively new and data regarding this subject are still scarce. Since neurons are post-mitotic cells, senescence must rely on processes different from proliferation arrest. Even if most of the studies were derived from rodent models, increased levels of p16 and GATA4 (a SASP initiator) were reported in pyramidal neurons from the prefrontal cortex of old humans (55). Interestingly, a study using single nucleus RNA sequencing reported the presence of senescent excitatory neurons containing neurofibrillary tangles (NFT) in postmortem human brains with various levels of AD pathology, with p19 as the most significant senescence contributor (56).
Cerebrovascular epithelial cells
Cerebrovascular endothelial cells (CECs) are integral components of both the blood-brain barrier (BBB) and the neurovascular unit. CECs are the primary cell type of the BBB, which makes them responsible for the tight regulation of molecular transport between the brain parenchyma and the periphery. CEC senescence could explain many of the endothelial dysfunction phenotypes seen in aging and cerebrovascular diseases, such as BBB leakage and neurovascular uncoupling. A senescent-associated phenotype in endothelial cells has been described in age-related pathologies such as atherosclerosis (57) and in the brain of a mouse model for premature aging (58). However, human CEC senescence has yet to be well-defined.
Although it is currently unclear whether inflammaging is the cause or consequence of neurodegenerative processes, it seems to orchestrate and accelerate the cellular dysfunction observed during aging. Natural damage accumulated throughout life probably also leads to brain injury. The damage occurring in the brain is sensed by microglia, the first line of defense of the central nervous system, leading to microglia activation and fast recruitment to the damage sites to phagocytose dead cells and debris (59). Secondary to microglial reaction is astrocyte activation, which releases inflammatory mediators that signal back to microglia and can recruit peripheral hematogenous cells that infiltrate the brain. This microglia-astrocyte crosstalk following brain damage has probably a beneficial effect by removing injured brain tissue and defective synapses. However, during aging, chronic stress and accumulating pathology may lead to hyperactivation of glial cells. Hyperactivated glia in the aging brain will increase the release of SASP proteins such as IL-6, TNF-β, TGFβ family ligands, MIF, and YKL-40, which are all associated with cognitive impairment (60).
Beyond microglia and astrocytes, BBB maintenance is crucial for normal neuronal function. As the brain ages, the tight control of the neuronal milieu is affected by the normal age-driven alteration of BBB permeability, a critical process involved in the initiation or worsening of cerebrovascular diseases. Enhanced levels of pro-inflammatory factors in the bloodstream result in the disruption of BBB (61, 62), which consequently leads to an increased infiltration of peripheral immune cells (63, 64) and cytokines, further stimulating microglia and astrocytes (Figure 2).
Brain senescence in Alzheimer’s disease
The most common cause of dementia is AD, characterized by the accumulation of amyloid β (Aβ), phosphorylated tau, and neuroinflammation. Age is the main risk factor for neurodegenerative diseases, so it is reasonable to argue that cellular senescence might play a major role in AD. Although there is no clear causal relationship between brain senescence and AD pathology, it is tempting to speculate that the age-related build-up of senescent cells in the brain could set an ideal proinflammatory environment for the onset of AD. In this regard, the pathology seen in AD may then increase the burden of senescent cells and inflammation in the aged brain, establishing then a positive feedback loop that might, in turn, exacerbate the disease. Thus, understanding the key events that enable the shift from healthy brain aging to pathological aging and neurodegeneration is essential (Figures 2-3).
Resident brain cells like astrocytes, microglia, and neurons can become senescent during aging (A) and contribute to brain inflammaging, setting up an ideal proinflammatory environment for the onset of neurodegenerative diseases (B). NSC, neural stem cells; ROS, reactive oxygen species; SASP, senescence-associated secretory phenotype; SAHF, senescence-associated heterochromatin formation; BBB, blood-brain barrier.
Amyloid pathology in the aged brain may increase the burden of hyperactivated senescent glia and lead to the upregulation of neuroinflammatory molecules associated with cognitive impairment, establishing a positive feedback loop that might, in turn, exacerbate Alzheimer’s disease. ROS, reactive oxygen species; NO, nitric oxide.
Specific cellular changes seen during AD are similar to those observed during senescence across all resident brain cell types. For example, it has been reported that p16INK4a is upregulated in pyramidal neurons in the hippocampus from AD patients (65). In addition, transcriptomic analyses of microdissected NFT-containing neurons from post-mortem AD brains revealed an expression profile consistent with cellular senescence (66). Senescent astrocytes have also been identified in cultures from post-mortem AD brain tissue (45, 67). Interestingly, these senescent astrocytes showed upregulated p16INK4a expression and β-galactosidase activity, and SASP (68–74). Likewise, senescent microglia has been linked to AD, including identification of senescent disease-associated microglia phenotype (DAM) with intracellular/phagocytic Aβ particles in human brain slices (75). In addition, research has shown that Aβ oligomers can induce senescence in aged microglia cultures from AD brains (76). Further supporting the contribution of senescence to AD, increased levels of SASP-associated proteins such as IL-6, IGFBP, TGF-β, and matrix metalloproteinases (MMPs) have been found in both CSF and plasma samples from AD patients (77–83). In addition, genome-wide association studies (GWAS) have identified that individuals with AD carry a high burden of genetic risk variants in genes known to be involved in senescence (84), including metalloproteinase domain 10 (ADAM10) (85), ADAM metallopeptidase with thrombospondin type 1 motif 4 (ADAMTS4) (86), and bridging integrator 1 (BIN1) (87).
Senolytic therapies in Alzheimer’s disease
The emergence of cellular senescence as an important player in AD pathophysiology attracted the attention of researchers to senotherapies as an interesting therapeutic alternative. The first senolytic drugs, i.e. drugs capable of removing senescent cells, were developed using a bioinformatic hypothesis-driven approach. Since senescent cells can resist apoptotic stimuli, it was speculated that senescent cells might prevent their self-destruction through senescent cell anti-apoptotic pathway (SCAP) networks. This hypothesis was confirmed when SCAP networks were interrogated using RNA interference and it was demonstrated that pro-apoptotic senescent cells with SASP secretion depended on SCAP to evade their removal (88).
Next, using bioinformatics, 46 compounds already in use for other indications in humans were selected as candidates for transiently disabling the SCAP network. These compounds included dasatinib, a tyrosine kinase inhibitor approved for clinical use in the United States since 2006, and quercetin, a naturally occurring flavonoid. Since their target is multiple senescent cell types and not a single protein, senolytics reduce off-target effects on non-senescent cells. Given that senescent cells do not divide and accumulate slowly, senolytic drugs do not need to be administered continuously to be effective. Besides, intermittent dosing reduces the risk of associated medication side effects.
The first clinical trial with senolytic treatment showed that a combination of dasatinib and quercetin (D+Q) was generally well tolerated and suggested improvements in the physical function of individuals with idiopathic pulmonary fibrosis (89). Likewise, a short-term treatment with D+Q in diabetic patients with kidney disease reduced the senescent cell burden in adipose tissue and attenuated plasma levels of key SASP factors (90). Interestingly, it has been demonstrated that both dasatinib and quercetin penetrate the BBB in rodent models and individuals undergoing cancer treatment (91, 92).
Senolytic therapies have shown promising results in a mouse model of tau-dependent neurodegeneration. Bussian et al. (93) bred (MAPT*P301S)PS19 mice, which accumulate p16INK4A-positive senescent astrocytes and microglia, with INK-ATTAC mice that eliminates p16INK4a-positive cells upon administration of the synthetic drug AP201 (87). The authors observed that the double transgenic (MAPT*P301S)PS19-INK-ATTAC mice showed a clearance of senescent cells as they arose. Interestingly, this clearance of senescent cells prevented gliosis, tau hyperphosphorylation, and cortical and hippocampal neurodegeneration, thereby preserving cognitive function. These observations suggested that targeting senescent cells may provide a therapeutic avenue in tau-dependent neurodegeneration, and they led to the design of clinical trials with the hope to translate these results into clinical practice. To test this hypothesis, two clinical trials (SToMP-AD and ALSENLITE) are assessing the therapeutic potential of D+Q in AD.
SToMP-AD (NCT04063124) is an open-label pilot study to evaluate whether D+Q penetrate the brain and to establish the feasibility and safety in older adults with early-stage AD, as an initial proof-of-concept for a larger phase II multisite clinical trial (NCT04685590) (94). Secondary aims include measurement of changes in CSF and blood SASP, cognition, functional status, physical performance, and MRI-derived neuroimaging markers of brain structure and function. Adults aged 65 years and older with diagnosed early-stage AD (MoCA 10-20 and Clinical Dementia Rating Scale/CDR = 1) on a stable dose of cholinesterase inhibitors for at least three months are considered for inclusion. A total of five eligible participants will be enrolled and will complete 10 scheduled visits across a study period not to exceed 24 weeks. Participants will be administered D+Q daily for two consecutive days followed by a 14-day (+/- 2 day) no-drug period (intermittent administration) to complete a single cycle. Six cycles will be completed across 12 consecutive weeks. D+Q will be taken orally. The results of this study will be used to inform the development of a randomized, double-blind, placebo-controlled multicenter phase II trial to further explore of the safety, feasibility, and efficacy of senolytics for modulating AD progression.
ALSENLITE (NCT04785300) is an open-label pilot study to evaluate the safety and feasibility of using D+Q in subjects with mild cognitive impairment (MCI) or AD. Adults aged 55 or older with a clinical diagnosis of symptomatic probable AD (MMSE 26 to 15 or Short Test of Mental Status 31 to 15 inclusive and/or Clinical Dementia Rating Scale/CDR = 0.5 to 2, inclusive) and tau positivity by brain PET imaging will be considered for inclusion. Twenty eligible participants will be enrolled and administered D+Q by mouth at the same times for 2 days out of every 15 days for 6 cycles lasting for a total of 77 days (12 concurrent doses of each agent).
Despite these promising translational initiatives, it is still early days to adventure any conclusion from the potential benefit of this kind of therapeutic approach. Our knowledge of the potential role of cellular senescence in AD, either detrimental or protective, remains sparse. Consequently, the potential use of senolytics should be cautiously approached until a clearer picture of the functions of senescence during brain aging and AD is drawn.
Research has established neuroinflammation as a driver for processes related to aging, probably through induction of brain senescence. Although a large body of evidence supports the involvement of senescence in the pathogenic processes leading to dementia, there is still a lack of a clear definition for “cellular senescence” in the brain, which hampers the development of specific targets related to senescence for drug discovery and development. Furthermore, understanding how senescence may distinguish healthy aging brain from pathological aging brain disease remains a major challenge, especially because not all senescent cell populations have detrimental effects. Hypothetically, senolytic therapies would prevent neuronal loss and tissue destruction leading to a reduction in risk of disease progression. Unfortunately, there is still a gap in this strategy because biomarkers for senescent cells in the brain are needed for both defining inclusion criteria for asymptomatic individuals at risk of progressing to dementia and assessing target engagement. Although several ongoing efforts are trying to identify them, it is still early days to consider their translation to clinical trials or primary care. Brain senescence biomarkers are particularly challenging. For example, proposed senescence biomarkers like SA β-gal have shown inconsistencies since some neuronal cell types are always positive for SA β-gal staining, while it is reversible in others. Likewise, inconclusive data have been found for the cyclin-dependent kinase inhibitor p21. Increased levels of these markers have been reported in the brains of AD patients when compared to controls, while other studies have found no significant differences. Despite these conflicting results, SASP components like IL-6, IL-1β, TGF-β, TNF-α, MMP-1, MMP-3 and MMP-10 are promising candidates to monitor senescence occurring in AD brain. Nevertheless, additional studies are required to delineate the biology modulating SASP and its changes during healthy and pathological aging. This might provide us with a powerful biomarker toolbox to evaluate how senescence and brain aging evolve together, and help improve our definition for population at risk of progressing to dementia based on “biological” rather than chronological age. However, we are still at the dawn of a new research perspective targeting biological aging as a potentially modifiable risk factor.
Funding: This review has been funded by JPco-fuND-2 “Multinational research projects on Personalised Medicine for Neurodegenerative Diseases” PREADAPT project (BMBF grant: 01ED2007A). PVMA was supported by a Georg Foster Humboldt Fellowship. The sponsors had no role in the design, preparation, or review of the manuscript.
Conflict of interest: The authors report no conflicts of interest.
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