Microglial phagocytosis in aging and Alzheimer's disease

Oxygen Sensing and Signaling in Alzheimer's Disease: A Breathtaking Story!

Oxygen sensing and homeostasis is indispensable for the maintenance of brain structural and functional integrity. Under low-oxygen tension, the non-diseased brain has the ability to cope with hypoxia by triggering a homeostatic response governed by the highly conserved hypoxia-inducible family (HIF) of transcription factors. With the advent of advanced neuroimaging tools, it is now recognized that cerebral hypoperfusion, and consequently hypoxia, is a consistent feature along the Alzheimer's disease (AD) continuum. Of note, the reduction in cerebral blood flow and tissue oxygenation detected during the prodromal phases of AD, drastically aggravates as disease progresses. Within this scenario a fundamental question arises: How HIF-driven homeostatic brain response to hypoxia "behaves" during the AD continuum? In this sense, the present review is aimed to critically discuss and summarize the current knowledge regarding the involvement of hypoxia and HIF signaling in the onset and progression of AD pathology. Importantly, the promises and challenges of non-pharmacological and pharmacological strategies aimed to target hypoxia will be discussed as a new "hope" to prevent and/or postpone the neurodegenerative events that occur in the AD brain.

PIP2 depletion and altered endocytosis caused by expression of Alzheimer's disease-protective variant PLCγ2 R522

Variants identified in genome-wide association studies have implicated immune pathways in the development of Alzheimer's disease (AD). Here, we investigated the mechanistic basis for protection from AD associated with PLCγ2 R522, a rare coding variant of the PLCG2 gene. We studied the variant's role in macrophages and microglia of newly generated PLCG2-R522-expressing human induced pluripotent cell lines (hiPSC) and knockin mice, which exhibit normal endogenous PLCG2 expression. In all models, cells expressing the R522 mutation show a consistent non-redundant hyperfunctionality in the context of normal expression of other PLC isoforms. This manifests as enhanced release of cellular calcium ion stores in response to physiologically relevant stimuli like Fc-receptor ligation or exposure to Aβ oligomers. Expression of the PLCγ2-R522 variant resulted in increased stimulus-dependent PIP₂ depletion and reduced basal PIP₂ levels in vivo. Furthermore, it was associated with impaired phagocytosis and enhanced endocytosis. PLCγ2 acts downstream of other AD-related factors, such as TREM2 and CSF1R, and alterations in its activity directly impact cell function. The inherent druggability of enzymes such as PLCγ2 raises the prospect of PLCγ2 manipulation as a future therapeutic approach in AD.

Acute TBK1/IKK-ε Inhibition Enhances the Generation of Disease-Associated Microglia-Like Phenotype Upon Cortical Stab-Wound Injury

Traumatic brain injury has a poorer prognosis in elderly patients, possibly because of the enhanced inflammatory response characteristic of advanced age, known as “inflammaging.” Recently, reduced activation of the TANK-Binding-Kinase 1 (Tbk1) pathway has been linked to age-associated neurodegeneration and neuroinflammation. Here we investigated how the blockade of Tbk1 and of the closely related IKK-ε by the small molecule Amlexanox could modify the microglial and immune response to cortical stab-wound injury in mice. We demonstrated that Tbk1/IKK-ε inhibition resulted in a massive expansion of microglial cells characterized by the TMEM119⁺/CD11c⁺ phenotype, expressing high levels of CD68 and CD317, and with the upregulation of Cst7a, Prgn and Ccl4 and the decrease in the expression levels of Tmem119 itself and P2yr12, thus a profile close to Disease-Associated Microglia (DAM, a subset of reactive microglia abundant in Alzheimer’s Disease and other neurodegenerative conditions). Furthermore, Tbk1/IKK-ε inhibition increased the infiltration of CD3⁺ lymphocytes, CD169⁺ macrophages and CD11c⁺/CD169⁺ cells. The enhanced immune response was associated with increased expression of Il-33, Ifn-g, Il-17, and Il-19. This upsurge in the response to the stab wound was associated with the expanded astroglial scars and increased deposition of chondroitin-sulfate proteoglycans at 7 days post injury. Thus, Tbk1/IKK-ε blockade results in a massive expansion of microglial cells with a phenotype resembling DAM and with the substantial enhancement of neuroinflammatory responses. In this context, the induction of DAM is associated with a detrimental outcome such as larger injury-related glial scars. Thus, the Tbk1/IKK-ε pathway is critical to repress neuroinflammation upon stab-wound injury and Tbk1/IKK-ε inhibitors may provide an innovative approach to investigate the consequences of DAM induction.

Scaffolding protein Gab2 is involved in postnatal development and lipopolysaccharide-induced activation of microglia in the mouse brain

Grb2-associated-binding protein-2 (Gab2) is a member of the Gab/DOS family and functions as an adapter protein downstream of several growth factor signaling pathways. Gab2 is considered an Alzheimer's disease susceptibility gene. However, the role of Gab2 in the brain is still largely unknown. Herein, we report that Gab2 is involved in the postnatal development of microglia in mice. The Gab2 expression in the brain was detected at postnatal day 1 (P1) and increased until P14 but decreased thereafter. The tyrosine phosphorylation of Gab2 (pGab2) was also detected at P1 and increased until P14. Next, we focused on microglial development in Gab2 knockout and heterozygous mice. Although differences were not detected in the cytoplasmic area of Iba1-labeled microglia between Gab2(±) and Gab2(-/-) mice, the analysis of CD68 and cathepsin D (indicators of microglial lysosomal activation) immunolabeling within Iba1+ cells revealed significant underdevelopment of microglial lysosomes in Gab2(-/-) mice at P60. In addition to the developmental abnormality of microglia in Gab2(-/-) mice, lipopolysaccharide-induced lysosomal activation was selectively suppressed in Gab2(-/-) mice compared to that in Gab2(±) mice. Our findings suggest that Gab2 is involved not only in postnatal development but also in lysosomal activation of microglia, therefore Gab2 dysfunction in microglia might potentially contribute to the development of neurodegenerative diseases.

Impact of Alcohol Abuse on Susceptibility to Rare Neurodegenerative Diseases

Despite the prevalence and well-recognized adverse effects of prenatal alcohol exposure and alcohol use disorder in the causation of numerous diseases, their potential roles in the etiology of neurodegenerative diseases remain poorly characterized. This is especially true of the rare neurodegenerative diseases, for which small population sizes make it difficult to conduct broad studies of specific etiological factors. Nonetheless, alcohol has potent and long-lasting effects on neurodegenerative substrates, at both the cellular and systems levels. This review highlights the general effects of alcohol in the brain that contribute to neurodegeneration across diseases, and then focuses on specific diseases in which alcohol exposure is likely to play a major role. These specific diseases include dementias (alcohol-induced, frontotemporal, and Korsakoff syndrome), ataxias (cerebellar and frontal), and Niemann-Pick disease (primarily a Type B variant and Type C). We conclude that there is ample evidence to support a role of alcohol abuse in the etiology of these diseases, but more work is needed to identify the primary mechanisms of alcohol's effects.

Safety and Tolerability of GRF6019 Infusions in Severe Alzheimer's Disease: A Phase II Double-Blind Placebo-Controlled Trial

BACKGROUND

The plasma fraction GRF6019 shows multiple benefits on brain aging in mice, including enhanced cognition, neurogenesis, and synaptic density, as well as reduced neuroinflammation.

OBJECTIVE

To evaluate the safety, tolerability, and preliminary efficacy of GRF6019 in patients with severe Alzheimer's disease (AD).

METHODS

A phase II, double-blind, placebo-controlled study in patients with severe AD (Mini-Mental State Examination score 0-10). Patients were randomized 2 : 1 to GRF6019 (N = 18) or placebo (N = 8) and received daily 250 mL intravenous infusions over 5 days. The primary endpoints were the rates of adverse events (AEs) and the tolerability of GRF6019 as assessed by the number of patients completing the study. Change from baseline in cognitive and functional assessments was also evaluated.

RESULTS

All patients completed 100%of study visits and infusions. The rate of AEs was similar in the GRF6019 (8/18 patients [44.4%]) and placebo (3/8 patients [37.5%]) groups, and there were no deaths or serious AEs. The most common AEs considered related to treatment were mild, transient changes in blood pressure in the GRF6019 group (hypotension: 2 patients [11.1%]; hypertension: 1 patient [5.6%]); there were no related AEs in the placebo group. The trial was not powered to detect statistically significant differences between treatment groups. At the end of the study, patients in both treatment groups remained stable or improved on all cognitive and functional endpoints.

CONCLUSION

GRF6019 demonstrated excellent safety, feasibility, and tolerability. Future trials designed to characterize the potential functional benefits of GRF6019 and related plasma fractions in severe AD are warranted.

Phosphoinositides: Roles in the Development of Microglial-Mediated Neuroinflammation and Neurodegeneration

Microglia are increasingly recognized as vital players in the pathology of a variety of neurodegenerative conditions including Alzheimer’s (AD) and Parkinson’s (PD) disease. While microglia have a protective role in the brain, their dysfunction can lead to neuroinflammation and contributes to disease progression. Also, a growing body of literature highlights the seven phosphoinositides, or PIPs, as key players in the regulation of microglial-mediated neuroinflammation. These small signaling lipids are phosphorylated derivates of phosphatidylinositol, are enriched in the brain, and have well-established roles in both homeostasis and disease.Disrupted PIP levels and signaling has been detected in a variety of dementias. Moreover, many known AD disease modifiers identified via genetic studies are expressed in microglia and are involved in phospholipid metabolism. One of these, the enzyme PLCγ2 that hydrolyzes the PIP species PI(4,5)P₂, displays altered expression in AD and PD and is currently being investigated as a potential therapeutic target.Perhaps unsurprisingly, neurodegenerative conditions exhibiting PIP dyshomeostasis also tend to show alterations in aspects of microglial function regulated by these lipids. In particular, phosphoinositides regulate the activities of proteins and enzymes required for endocytosis, toll-like receptor signaling, purinergic signaling, chemotaxis, and migration, all of which are affected in a variety of neurodegenerative conditions. These functions are crucial to allow microglia to adequately survey the brain and respond appropriately to invading pathogens and other abnormalities, including misfolded proteins. AD and PD therapies are being developed to target many of the above pathways, and although not yet investigated, simultaneous PIP manipulation might enhance the beneficial effects observed. Currently, only limited therapeutics are available for dementia, and although these show some benefits for symptom severity and progression, they are far from curative. Given the importance of microglia and PIPs in dementia development, this review summarizes current research and asks whether we can exploit this information to design more targeted, or perhaps combined, dementia therapeutics. More work is needed to fully characterize the pathways discussed in this review, but given the strength of the current literature, insights in this area could be invaluable for the future of neurodegenerative disease research.

Inhibition of Na+/H+exchanger modulates microglial activation and scar formation following microelectrode implantation

Objective.Intracortical microelectrodes are an important tool for neuroscience research and have great potential for clinical use. However, the use of microelectrode arrays to treat neurological disorders and control prosthetics is limited by biological challenges such as glial scarring, which can impair chronic recording performance. Microglia activation is an early and prominent contributor to glial scarring. After insertion of an intracortical microelectrode, nearby microglia transition into a state of activation, migrate, and encapsulate the device. Na⁺/H⁺exchanger isoform-1 (NHE-1) is involved in various microglial functions, including their polarity and motility, and has been implicated in pro-inflammatory responses to tissue injury. HOE-642 (cariporide) is an inhibitor of NHE-1 and has been shown to depress microglial activation and inflammatory response in brain injury models.Approach.In this study, the effects of HOE-642 treatment on microglial interactions to intracortical microelectrodes was evaluated using two-photon microscopyin vivo.Main results.The rate at which microglia processes and soma migrate in response to electrode implantation was unaffected by HOE-642 administration. However, HOE-642 administration effectively reduced the radius of microglia activation at 72 h post-implantation from 222.2µm to 177.9µm. Furthermore, treatment with HOE-642 significantly reduced microglial encapsulation of implanted devices at 5 h post-insertion from 50.7 ± 6.0% to 8.9 ± 6.1%, which suggests an NHE-1-specific mechanism mediating microglia reactivity and gliosis during implantation injury.Significance.This study implicates NHE-1 as a potential target of interest in microglial reactivity and HOE-642 as a potential treatment to attenuate the glial response and scar formation around implanted intracortical microelectrodes.

Regulation of neuroimmune processes by damage- and resolution-associated molecular patterns

Sterile inflammatory processes are essential for the maintenance of central nervous system homeostasis, but they also contribute to various neurological disorders, including neurotrauma, stroke, and demyelinating or neurodegenerative diseases. Immune mechanisms in the central nervous system and periphery are regulated by a diverse group of endogenous proteins, which can be broadly divided into the pro-inflammatory damage-associated molecular patterns (DAMPs) and anti-inflammatory resolution-associated molecular patterns (RAMPs), even though there is notable overlap between the DAMP- and RAMP-like activities for some of these molecules. Both groups of molecular patterns were initially described in peripheral immune processes and pathologies; however, it is now evident that at least some, if not all, of these immunomodulators also regulate neuroimmune processes and contribute to neuroinflammation in diverse central nervous system disorders. The review of recent literature demonstrates that studies on DAMPs and RAMPs of the central nervous system still lag behind the much broader research effort focused on their peripheral counterparts. Nevertheless, this review also reveals that over the last five years, significant advances have been made in our understanding of the neuroimmune functions of several well-established DAMPs, including high-mobility group box 1 protein and interleukin 33. Novel neuroimmune functions have been demonstrated for other DAMPs that previously were considered almost exclusively as peripheral immune regulators; they include mitochondrial transcription factor A and cytochrome C. RAMPs of the central nervous system are an emerging area of neuroimmunology with very high translational potential since some of these molecules have already been used in preclinical and clinical studies as candidate therapeutic agents for inflammatory conditions, such as multiple sclerosis and rheumatoid arthritis. The therapeutic potential of DAMP antagonists and neutralizing antibodies in central nervous system neuroinflammatory diseases is also supported by several of the identified studies. It can be concluded that further studies of DAMPs and RAMPs of the central nervous system will continue to be an important and productive field of neuroimmunology.

Innate Immunity in Age-Related Macular Degeneration

Multiple lines of investigation have demonstrated that inflammation plays significant roles in etiology of age-related macular degeneration (AMD). Although interventional trials in AMD therapy targeting inflammatory pathways have been conducted, they have not yet been successful and a detailed understanding as to why some have failed is still elusive. One limitation is the relative dearth of information on how immune cells interact with retinal cells to generate AMD phenotypes at each disease stage. Here, we summarize current research evidence and hypotheses regarding potential pathogenic roles of innate immune cells in the eye, which include resident retinal microglia, macrophages derived from infiltrating systemic monocytes, and macrophages resident in the choroid. We relate recent findings regarding the physiology, function, and cellular interactions involving innate immune cells in the retina and choroid to AMD-related processes, including: (1) drusen formation and regression, (2) the onset and spread of degeneration in late atrophic AMD, and (3) the initiation, growth, and exudation of neovascular vessels in late "wet" AMD. Understanding how innate immune cells contribute to specific AMD phenotypes can assist in generating a comprehensive view on the inflammatory etiology of AMD and aid in identifying anti-inflammatory therapeutic strategies and selecting appropriate clinical outcomes for the planned interventions.

PET Imaging of Neutrophils Infiltration in Alzheimer's Disease Transgenic Mice

Neutrophils are important components in the innate immune system. Neutrophil hyperactivation is regarded as a characteristic of Alzheimer's disease (AD). But in vivo imaging tools observing neutrophil activity in AD dynamically is lacking. This study aimed to identify neutrophil infiltration in AD transgenic mice. We used the AD triple-mutant transgenic mouse model and identified the genotype with RT-PCR. Behavioral experiments including an open-field test, a Morris water maze, and a Y-maze test were performed to evaluate the status of this AD model. ¹⁸F-AV45, ¹⁸F-PM-PBB3, ⁶⁸Ga-PEG-cFLFLFK, and ¹⁸F-DPA714 were synthesized according to previous reports. We employed microPET to detect tracer uptake in the AD model and the control mice at different stages. Western blotting was used to observe the expression of functional proteins. We proved the successful establishment of AD models by RT-PCR, behavioral tests, and ¹⁸F-AV45 and ¹⁸F-PM-PBB3 PET imaging. We found an increased neutrophil accumulation in the brains of the AD mice through ⁶⁸Ga-PEG-cFLFLFK PET imaging and Western blot assay. Our studies also demonstrated an elevated level of CAP37, which is produced by neutrophils, in the AD brain, and treatment with CAP37 promoted the expression of Iba1, iNOS, and COX-2 in BV2 cultures. Furthermore, our ¹⁸F-DPA714 PET imaging studies verified the raised activation of microglia in the brain of transgenic AD mice. Collectively, our findings indicate the increased activity of neutrophils in the brain and heart of AD model mice, ⁶⁸Ga-PEG-cFLFLFK PET imaging represents a sensitive method to observe the status of neutrophils in AD, and infiltrated neutrophils can induce the activation of microglia by releasing CAP37 and blocking the activity of neutrophils may be beneficial for the control of AD progression.

Safety and tolerability of GRF6019 in mild-to-moderate Alzheimer's disease dementia

INTRODUCTION

This phase 2 trial evaluated the safety, tolerability, and feasibility of repeated infusions of the plasma fraction GRF6019 in mild-to-moderate Alzheimer's disease.

METHODS

In this randomized, double-blind, dose-comparison trial, 47 patients were randomized 1:1 to receive daily infusions of 100 mL (n = 24) or 250 mL (n = 23) of GRF6019 for 5 consecutive days over two dosing periods separated by a treatment-free interval of 3 months.

RESULTS

The mean (standard deviation [SD]) age of the enrolled patients was 74.3 (6.9), and 62% were women. Most adverse events (55%) were mild, with no clinically significant differences in safety or tolerability between the two dose levels. The mean (SD) baseline Mini-Mental State Examination score was 20.6 (3.7) in the 100 mL group and 19.6 (3.7) in the 250 mL group; at 24 weeks, the within-patient mean change from baseline was -1.0 points (95% confidence interval [CI], -3.1 to 1.1) in the 100 mL group and +1.5 points (95% CI, -0.4 to 3.3) in the 250 mL group. The within-patient mean change from baseline on the Alzheimer's Disease Assessment Scale-Cognitive subscale was -0.4 points (95% CI, -2.9 to 2.2) in the 100 mL group, while in the 250 mL group it was -0.9 points (95% CI, -3.0 to 1.2). The within-patient mean change from baseline on the Alzheimer's Disease Cooperative Study-Activities of Daily Living was -0.7 points in the 100 mL group (95% CI, -4.3 to 3.0) and -1.3 points (95% CI, -3.4 to 0.7) in the 250 mL group. The mean change from baseline on the Category Fluency Test, Clinical Dementia Rating Scale-Sum of Boxes, Alzheimer's Disease Cooperative Study-Clinical Global Impression of Change, and Neuropsychiatric Inventory Questionnaire was similar for both treatment groups and did not show any worsening.

DISCUSSION

GRF6019 was safe and well tolerated, and patients experienced no cognitive decline and minimal functional decline. These results support further development of GRF6019.

Tissue-specific features of microglial innate immune responses

As tissue-resident macrophages of the brain, microglia are increasingly considered as cellular targets for therapeutical intervention. Innate immune responses in particular have been implicated in central nervous system (CNS) infections, neuro-oncology, neuroinflammatory and neurodegenerative diseases. We here review the impact of 'nature and nurture' on microglial innate immune responses and summarize documented tissue-specific adaptations. Overall, such adaptations are associated with regulatory processes rather than with overt differences in the expressed repertoire of activating receptors of different tissue-resident macrophages. Microglial responses are characterized by slower kinetics, by a more persistent nature and by a differential usage of downstream enzymes and accessory receptors. We further consider factors like aging, previous exposure to inflammatory stimuli, and differences in the microenvironment that can modulate innate immune responses. The long-life span of microglia in the metabolically active CNS renders them susceptible to the phenomenon of 'inflammaging', and major challenges lie in the unraveling of the factors that underlie age-related alterations in microglial behavior.

Astrocytic phagocytosis is a compensatory mechanism for microglial dysfunction

Microglia are the principal phagocytes that clear cell debris in the central nervous system (CNS). This raises the question, which cells remove cell debris when microglial phagocytic activity is impaired. We addressed this question using Siglech^dtr mice, which enable highly specific ablation of microglia. Non‐microglial mononuclear phagocytes, such as CNS‐associated macrophages and circulating inflammatory monocytes, did not clear microglial debris. Instead, astrocytes were activated, exhibited a pro‐inflammatory gene expression profile, and extended their processes to engulf microglial debris. This astrocytic phagocytosis was also observed in Irf8‐deficient mice, in which microglia were present but dysfunctional. RNA‐seq demonstrated that even in a healthy CNS, astrocytes express TAM phagocytic receptors, which were the main astrocytic phagocytic receptors for cell debris in the above experiments, indicating that astrocytes stand by in case of microglial impairment. This compensatory mechanism may be important for the maintenance or prolongation of a healthy CNS.

Ketotherapeutics for neurodegenerative diseases

Alzheimer's disease (AD) and Parkinson's disease (PD) are, respectively, the most prevalent and fastest growing neurodegenerative diseases worldwide. The former is primarily characterized by memory loss and the latter by the motor symptoms of tremor and bradykinesia. Both AD and PD are progressive diseases that share several key underlying mitochondrial, inflammatory, and other metabolic pathologies. This review will detail how these pathologies intersect with ketone body metabolism and signaling, and how ketone bodies, particularly d-β-hydroxybutyrate (βHB), may serve as a potential adjunctive nutritional therapy for two of the world's most devastating conditions.

Glycolysis - a key player in the inflammatory response

The inflammatory response involves the activation of several cell types to fight insults caused by a plethora of agents, and to maintain the tissue homoeostasis. On the one hand, cells involved in the pro‐inflammatory response, such as inflammatory M1 macrophages, Th1 and Th17 lymphocytes or activated microglia, must rapidly provide energy to fuel inflammation, which is essentially accomplished by glycolysis and high lactate production. On the other hand, regulatory T cells or M2 macrophages, which are involved in immune regulation and resolution of inflammation, preferentially use fatty acid oxidation through the TCA cycle as a main source for energy production. Here, we discuss the impact of glycolytic metabolism at the different steps of the inflammatory response. Finally, we review a wide variety of molecular mechanisms which could explain the relationship between glycolytic metabolites and the pro‐inflammatory phenotype, including signalling events, epigenetic remodelling, post‐transcriptional regulation and post‐translational modifications. Inflammatory processes are a common feature of many age‐associated diseases, such as cardiovascular and neurodegenerative disorders. The finding that immunometabolism could be a master regulator of inflammation broadens the avenue for treating inflammation‐related pathologies through the manipulation of the vascular and immune cell metabolism.

Alzheimer's disease: microglia targets and their modulation to promote amyloid phagocytosis and mitigate neuroinflammation

Introduction: Despite the revolutionary progress in neurodegenerative disease research, there is no cure for Alzheimer's disease (AD). This is a chronic progressive neurodegenerative disease affecting aged people and is associated with chronic neuroinflammation and amyloid-beta (Aβ) deposition in the brain parenchyma. Microglia, the resident myeloid cells in the central nervous system, are critically involved in the pathogenesis of AD and have emerged as a potential therapeutic target for treating or preventing AD. The failure of microglia to keep up with persistent amyloid-beta development along with secretion of inflammatory cytokines is detrimental to neurons and favors Aβ accumulation.Areas covered: This review illuminates the latest research that is focused on molecules and their intracellular targets that promote microglial phagocytosis and /or its polarization to an anti-inflammatory state.Expert opinion: A robust inflammatory response of microglia is not necessary to improve their efficiency of Aβ clearance. The challenge is to master inflammatory/anti-inflammatory phenotypes depending on the stage of AD and to maintain efficient responses to remove Aβ. Therefore, promoting microglia phagocytosis without a persistent excessive inflammatory response could be a potential therapeutic strategy.

Control of Inflammation by Calorie Restriction Mimetics: On the Crossroad of Autophagy and Mitochondria

Mitochondrial metabolism and autophagy are two of the most metabolically active cellular processes, playing a crucial role in regulating organism longevity. In fact, both mitochondrial dysfunction or autophagy decline compromise cellular homeostasis and induce inflammation. Calorie restriction (CR) is the oldest strategy known to promote healthspan, and a plethora of CR mimetics have been used to emulate its beneficial effects. Herein, we discuss how CR and CR mimetics, by modulating mitochondrial metabolism or autophagic flux, prevent inflammatory processes, protect the intestinal barrier function, and dampen both inflammaging and neuroinflammation. We outline the effects of some compounds classically known as modulators of autophagy and mitochondrial function, such as NAD⁺ precursors, metformin, spermidine, rapamycin, and resveratrol, on the control of the inflammatory cascade and how these anti-inflammatory properties could be involved in their ability to increase resilience to age-associated diseases.