Fight Aging! Newsletter, February 17th 2020

Hematopoietic stem cells (HSCs) consist of a small cell-population in the bone-marrow (BM) that are responsible for lifelong production of all mature blood cells in an organism. A delicate balance of different HSC fates, namely, quiescence, self-renewal, and differentiation is decisive in maintaining the HSC pool and blood cell homeostasis. Cellular metabolism has emerged as one of the fundamental regulators of HSC fate decision process. HSCs rely primarily on anaerobic glycolysis while downstream progenitors use mitochondrial metabolism to fulfil their energy requirements.

In a recent study we have tested the mitochondrial modulator and NAD+ boosting agent, Nicotinamide Riboside (NR), in the context of regenerative hematopoiesis. One week of NR dietary supplementation to wild type mice resulted in increased BM cellularity and expansion of hematopoietic progenitor cells, this reflected in a significant increase in terminally differentiated circulating blood and immune cells. Importantly, mitochondrial profiling of HSCs derived from mice supplemented with NR revealed significant reduction of mitochondrial membrane potential (an indirect readout on mitochondrial activity), indicating that NR has a direct effect on HSC metabolism when administered systemically.

To understand the molecular mechanisms driving the effect of NR, we performed transcriptome analysis (by RNA sequencing) on ex vivo cultured HSCs. We found upregulation of autophagy (and mitophagy) and of NAD salvage pathway genes upon NR treatment, and a concomitant downregulation of mitochondrial metabolism pathway genes (TCA cycle and Oxidative Phosphorylation). NR-induced mitophagy was confirmed by image analysis of in vitro cultured HSCs and by bone marrow analysis of mitophagy reporter mice (mito-QC). We discovered that mitophagy induction was coupled with activation of mitochondrial unfolded protein response (UPRmt), that has been recently proposed as a conservation mechanism of the HSCs pool during aging. We hypothesize that NR-induced mitochondrial stress leads to the clearance of damaged mitochondria unable to coop with the metabolic stress. This complex mechanism initiates an instruction process where cells are primed toward asymmetric self-renewing cell division via differential distribution of active mitochondria in daughter cells.

Given that previous studies have implicated the importance of mitophagy and autophagy in HSC function and aging, we believe that NAD boosting strategies could be used to improve functionality of the hematopoietic stem cell pool in the elderly, where HSCs lose their capacity to produce a balanced immune system, being strongly primed toward a myeloid fate.

Although there could be asynchrony of telomere length among different tissues, peripheral leukocyte DNA has been most commonly used in clinical studies to measure leukocyte telomere length (LTL). Traditional risk factors for cardiovascular diseases (CVD), such as smoking, diabetes mellitus, dyslipidemia, hypertension, obesity, and shift work, have been associated with short LTL. In the prospective WOSCOPS (West of Scotland Primary Prevention Study) trial, subjects in the lowest tertile of LTL had a 44% increased risk of 5-year major cardiovascular events compared with subjects in the highest tertile of LTL. In a prospective WHI (Women's Health Initiative) study, hen patients developed chronic heart failure, they were also observed to have shorter LTL. Moreover, short LTL was also associated with congestive heart failure severity and clinical outcome.

Robust epidemiological and genetic evidence linking telomere length and CVD risk support the therapeutic hypothesis that genetic manipulations of the telomere system can be a potential treatment target for CVDs. Telomerase gene therapy was first achieved by delivering mouse TERT with an adeno-associated virus (AAV) into young and old mice. This nonintegrative gene therapy resulted in elongated telomeres, extended lifespans, and delayed age-associated pathologies. Importantly, telomerase-treated mice did not develop cancer at a higher rate than the corresponding control group. With the nonintegrative and replication incompetent properties of AAVs, this strategy restricted TERT expression to a few cell divisions and provided a relatively genome-safe TERT activation.

A report for age-associated diseases, such as CVDs, demonstrated improved ventricular function and limited infarct scars after acute myocardial infarction with TERT gene therapy in a preclinical mouse model. TERT gene therapy is a promising candidate that deserves further research efforts for clinical implementation for the treatment of age-associated diseases. Apart from direct TERT delivery by nonintegrative AAV vectors, new gene therapy methods using modified mRNA for in vitro encoding of TERT in human fibroblasts can transiently increase telomerase activity, rapidly extend telomeres, and increase proliferative capacity without the risks of insertional mutagenesis and off-target effects. In addition to proof-of-concept experimental data in mice, the development of safe strategies for transient and controllable telomerase activation in humans can be a subject of future studies.

What I'm going to do today is try to explain why I believe it makes sense for effective altruists (EAs) to prioritize the issue of aging. To make that case, there are a number of questions that need to be answered in the affirmative. First, is aging a really big problem? I believe that it is, by a good distance, the world's biggest problem. But I understand that this group thinks very carefully about such statements, so I need to justify my opinion. The second argument I need to make is that we have a sufficient understanding today of what aging is, and generally how we might go about addressing it. Therefore, if we throw money at this problem, there's a good chance of having a significant impact. This is not trivial. Other times that I've spoken at EA events, I've received a lot of pushback. Many EAs believe we understand so little about aging that what we do at the SENS Research Foundation is essentially random, and therefore spending money on it is unjustified. The third argument I need to make is very new. It's really only arisen over the past few years, and it is this: philanthropy is still critical, even though private investment in the study of aging has exploded.

To address the first point - why aging is important - I'm just going to tell you why I think that is clearly true. To me, it's just a fact that aging causes far more suffering than anything else in the world today or in the foreseeable future. It's not just the death part. We're talking about effective altruism here, and altruism means worrying about other people. People dying makes other people unhappy. That's not arguable. But what might be much more important is that when people die of aging, they do it slowly. They do it as a consequence of a chronic, progressive accumulation of damage in the body, a decline in mental and physical function. So, to me, it's self-evident that aging is, by far, the source of the largest amount of suffering in the world today. You could even argue that it was true a long time ago.

Now I'm going to address the second question. In order to do that, I'm going to fill in a lot of background information. Aging is the combination of two processes, metabolism and damage, which together result in pathology. A network of processes keeps us alive - that's what metabolism means - and, over time, generates damage. Currently, the overwhelming majority of money and effort spent to prevent the pathologies of late life is spent wrongly. It is spent on trying to break the link between damage and pathology. Damage, by definition, is accumulating, which means that efforts to stop it from causing pathology are bound to become progressively less effective as someone gets older. It's obvious. What we might be able to do is separate the component processes of metabolism and damage from each other. That's the maintenance approach - it's damage repair. We might be able to periodically repair some of the damage that metabolism generates, so that even though it continues to generate it, the damage does not reach a level of abundance that causes pathology. I think it is reasonable to call this the common sense alternative.

Seventeen years ago, I described the damage of aging in only seven words, as seven types of damage: cell loss or atrophy, division-obsessed cells, death-resistant cells, mitochondrial mutations, intracellular waste products, extracellular waste products, extracellular matrix stiffening. What's most important is the fact that for each of these types of damage, we can describe a generic therapy that could potentially represent the maintenance approach - the way to repair this type of damage. However, one thing I want to emphasize is that I'm not the only one saying this anymore. Five or 10 years ago, this was an argument that still needed to be made. But now it has been made. As an illustration, the Hallmarks of Aging paper came out only six years ago, and will be by far the most highly-cited paper this decade in the whole of the biology of aging. It's simply a restatement of what I said more than a decade earlier. The important point is that a divide-and-conquer, damage-repair approach is now a completely mainstream, orthodox idea.

On the the third point, that philanthropy still matters despite the growth of a longevity industry, SENS Research Foundation views itself these days as the engine room of that industry. We work on early-stage projects for as long as it takes to establish sufficient proof of concept to spin them out into startup companies. We're not the only ones. The Longevity Research Institute (LRI) is a new organization that's working in a more narrowly defined space. But they may grow, and, of course, there are going to be other organizations like this. Philanthropy matters enormously in this. We believe that we can get all - or at least almost all - of the key technologies in rejuvenation biotechnology into clinical trials within a few years. But the key point here is that the things being funded effectively by the private sector are the low-hanging fruit. And damage repair is an inherently divide-and-conquer concept. You can't just focus on the low-hanging fruit. You've got to address all of the components. It's more important than ever to make progress on the most difficult areas, and that is still a goal for philanthropy.

Why do we get old? How much of aging is genetic? And in what genes? There is clearly a genetic basis of aging, as demonstrated from yeast to worms to humans. As one example, different mouse strains have different potential lifespans. Much effort has been invested in understanding the genetic underpinnings of lifespan differences between the long-lived C57Bl/6 strain and the short-lived DBA/2 strain, with 50% mortality in captivity by 914 and 687 days, respectively. Quantitative trait loci mapping in C57Bl/6 X DBA/2 (BXD) recombinant inbred strains identified a locus on chromosome 11 that is linked to lifespan. Subsequent analyses revealed that this locus confers differential expression of the pituitary tumor-transforming gene-1 (Pttg1)/Securin gene. PTTG1 level-dependent impacts on chromosomal segregation during mitosis could influence longevity.

Natural selection only acts to promote longevity to the extent that it benefits the passage of genetic material to subsequent generations. Different animals have evolved different strategies for somatic maintenance that maximize reproductive success, and the extension of youth through additional investment in tissue maintenance would be disfavored if the costs (often manifested through reduced investment in reproduction) outweigh benefits. For a small vulnerable animal like a field mouse that faces high extrinsic hazards (such as predation), natural selection has favored a "fast" life history - a breed early, breed often strategy with little investment in longevity. For larger animals like humans, elephants, and whales, or for animals like tortoises, moles, bats and birds that have evolved other strategies to greatly reduce extrinsic hazards, natural selection has favored a "slow" life history, with greater and/or prolonged tissue maintenance leading to longer potential lifespans.

While we understand how natural selection has shaped the pathways that control longevity, we know less about what these pathways actually are. Studies from model organisms have clearly demonstrated that modulation of the insulin-like growth factor-1 (IGF-1) pathway, which positively regulates the mTOR pathway and negatively regulates autophagy, can significantly impact longevity. Decreases in IGF-1 and mTOR, or increases in autophagy, have been shown to prolong lifespans in organisms ranging from yeast to mammals. Additional studies have shown how inflammation can contribute to aging-associated phenotypes, and polymorphisms in genes controlling the IGF-1 pathway and inflammation are enriched in human centenarians, but the extent to which these polymorphisms and their impact on inflammation are contributing to differences in longevity has not been established.

While genetic screens in model organisms have revealed key pathways that regulate lifespan, the mechanisms employed by natural selection in the evolution of lifespans largely remain a mystery. Although one could argue that the selective breeding to generate different mouse strains over the last couple of centuries may not qualify as "natural" selection, the studies focused on PTTG1 reveal at least one potential (and novel) mediator of lifespan control. Key questions remain: Do variations in PTTG1 expression or activity contribute to lifespan differences across species, and perhaps within a species (including variability in the human population)? Would modulation of PTTG1 expression or activity promote the extension of healthspan or lifespan? How do activities known to modulate lifespan, such as dietary restriction and exercise, influence PTTG1 activity? Are there links between known aging pathways such as via IGF-1 and PTTG1? Good science generates good questions, leading to new insights (and sometimes even solutions). As a senior colleague once told me after I had told him that I worked on aging - "Hurry up".

The aged immune system has various characteristics. One of which is immunosenescence, which describes the vast and varied changes in the structure and function of the immune system as a result of age. Many of the early observations, such as reduced ability to fight new infections, diminished vaccine immunity, and reduced tumor clearance are generally categorized as immune insufficiencies. Immunosenescence is not due to the lack of immune cells, but due to reduced immune repertoire diversity, attributed to insufficient production of naïve immune cells and amplified oligo-clonal expansion of memory immune cells. Immunosenescence is therefore linked to the thymus. Natural aging causes the thymus to progressively atrophy, a process called thymic involution. This phenomenon is readily observed in most vertebrates and results in structural alterations, as well as functional decline, ultimately resulting in significantly decreased thymic output of naïve T cells that reduces the diversity of the T cell antigen receptor (TCR) repertoire, culminating in disrupted T cell homeostasis.

The second characteristic of aged immunity is termed inflammaging. Inflammaging describes the elevated self-reactivity in the elderly, resulting in the typical chronic, low-grade, but above baseline, systemic inflammatory phenotype observed in the absence of acute infection. Although immunosenescence and inflammaging appear to be opposing phenotypes, they comprise two sides of the same coin when attempting to holistically understand age-related immune dysfunction. It has been proposed that the basal inflammatory state in the elderly, defined by inflammaging, greatly contributes to many age-related degenerative diseases.

T lymphocyte (T cell) development and selection occurs in the thymus. Included in this process is central tolerance establishment. First is thymocyte negative selection, during which the majority of self (auto)-reactive T cells are depleted from the repertoire via apoptosis. Second is the generation of CD4 single positive FoxP3+ regulatory T (Treg) cells, whose primary function is to suppress T cell-mediated self-reactivity and preserve immune homeostasis in the periphery. These arms of central T cell tolerance work in tandem, and Treg cells most likely compensate for imperfections of negative selection, as some self-reactive T cells escape negative selection. With age, however, the atrophied thymus declines in its capacity to establish central tolerance, thereby, causing increased self-reactive T cells to escape to the periphery and participate in the process of inflammaging.

Sarcopenia is one of the hallmarks of the aging process. Human muscle undergoes constant changes with the most alterations taking place with age. As shown by different studies, on average, the prevalence of sarcopenia in older adults aged 60-70 years lies at 5-13%, increasing to 11-50% in people aged 80 or older. Sarcopenia is closely related to negative outcomes in older adults, such as an increased risk of falls and fractures and impaired cognitive function and physical performance. Researchers estimated that the economic costs for sarcopenia in the USA were about 18.5bn in 2000. Strikingly, if the prevalence of sarcopenia would be reduced by only 10%, it would save 1.1bn in medical costs per year in the US health care expenditures. Therefore, it is urgent to develop more effective research strategies and therapeutic approaches for preventing sarcopenia based on a better understanding of the potential mechanisms of this disease.

Over the years, many researches have investigated physiological and pathological conditions related to poor muscle regeneration in sarcopenia. However, the underlying molecular mechanisms associated with sarcopenia remain not completely understood. Some evidence suggests that such factors as anabolic resistance and endothelial dysfunction may contribute to the development of sarcopenia. Another one of the recent studies reported that exosomes released by muscles into the bloodstream may also play an essential role in muscle regeneration. This opens a novel field of research in preventing muscle loss. It is hypothesized that exosomes as carrier of a cargo of proteins, mRNA, miRNA, and other non-coding RNAs play a crucial role in myogenesis and muscle development. It has been shown in a mouse model of muscle injury that human skeletal myoblasts-derived exosomes containing all sorts of signal molecules can promote muscle regeneration.

Although the treatment of sarcopenia remains challenging, it is widely accepted that such strategies as nutritional supplementation and physical training (both aerobic exercise and resistance exercise) are the key interventions that can help maintain skeletal muscle mass. However, the molecular mechanisms behind the prevention from the age-related muscle loss by nutrition and exercise are still poorly understood. More recent data indicate that exercise attenuates sarcopenia mainly through increasing peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC1α) signalling, which can be activated by heat shock protein 60-bearing exosomes released after physical training. This connection between sports and release of exosomes supports the assumption about the key role of exosomes in muscle regeneration.

Other recent data also indicates that exosomes, secreted by skeletal muscle cells and carrying miRNAs and other factors, may act as vital modulators of skeletal muscle function, and may have the potential in the research strategies of sarcopenia. Owing to their diverse pathological and therapeutic effects, exosomes has attracted more attention in the scientific community in recent years. It has been shown that exosomes are related to organ crosstalk and can be beneficial in research of many diseases, including kidney injury, myocardial infarction, Parkinson's disease, and cancer. Importantly, the present data suggest that exosomes may mediate and enhance the beneficial effects of exercise. Emerging evidence has presented that exosomes as carriers can increase muscle regeneration after skeletal muscle injury and improve muscle protein synthesis and hypertrophy, which could support exosomes as vectors for future research strategies of sarcopenia. All these mechanisms are interconnected, but the underlying pathways are still not understood completely and should be examined more thoroughly in future studies.

The DNA methylation (DNAm) based estimator of biological age, DNAm-Age, has become a well-known molecular measure of human aging. DNAm-Age has been associated with cancers, cardiovascular diseases, neurological diseases, and chronic inflammation diseases. Subsequently, another DNAm based marker, DNAm-PhenoAge, was developed to be an improved predictor of mortality and health span using phenotypic age estimated from a range of aging-related clinical measures. Most recently, another metric, DNAm-GrimAge, has been developed to predict all cause mortality and health span.

Unfortunately, the underlying biological and molecular processes that drive these epigenetic age biomarkers are still unknown. Despite the observation that the DNAm-Age is associated with metabolic processes, the relationship between mitochondrial health and DNAm-Age remains understudied. Mitochondria are vital for metabolic processes as they are responsible for ATP production and are known to be involved in the aging process, become larger and less numerous with age. In addition, mitochondrial function may be related to DNAm aging. Activity of DNA methyltransferases (DNMT), as with any cellular enzyme, depend on ATP levels and impaired energy production as a result of mitochondrial dysfunction may influence normal function of DNMTs.

Mitochondrial DNA copy number (mtDNAcn), a measure of mitochondrial genome abundance, is commonly used as a reflection of the mitochondria's response to oxidative stress as well as general dysfunction. Mitochondria DNA (mtDNA) is sensitive to oxidative stress because it lacks a robust DNA repair system to restore oxidative stress induced damage and mtDNA damage persists longer compared to genomic DNA. Typically, mtDNA will increase when the endogenous antioxidant response is no longer able to recover its redox balance, possibly as a compensatory response. Previous studies have shown that mtDNAcn decreases with age and is positively associated with telomere length.

Recently, our group has shown that mtDNAcn is negatively correlated with DNAm-Age and hypothesized mtDNAcn may be a proxy of mitochondrial buffer capacity. Reduced mtDNAcn may be a consequence of exhausted mitochondrial buffering capacity, leading to adverse outcomes such as aging. In a population of 812 aging male veterans, we found contrasting results between cross-sectional and prospective analyses of mtDNAcn with aging biomarkers DNAm-Age, DNAm-PhenoAge, DNAm-GrimAge and leukocyte telomere length. We observed that mtDNAcn is negatively associated with cross-sectional measures of DNAm-Age and DNAm-PhenoAge. We found suggestive evidence that mtDNAcn is positively associated with prospective measures of DNAm-PhenoAge and negatively associated with prospective measures of leukocyte telomere length. These results suggest that while the negative cross-sectional associations reflect the opposing time-trends of mtDNAcn and aging biomarkers, it may be driven by unmeasured confounders such as underlying biological processes that drives both the decrease of mtDNAcn over time and the increase of DNAm-Age and DNAm-PhenoAge over time.

Several reports indicate that caloric restriction and intermittent periods of fasting may reduce the risk of complications associated with aging. Cells use nutrient sensing to identify and respond to differences in nutrient levels; the sensing mechanisms are dysregulated during the aging process. AMP-activated protein kinase (AMPK) and the mammalian target of rapamycin (mTOR) pathways are nutrient sensors that have been involved in lifespan. Additionally, PASK (a serine/threonine kinase that contains PAS domains) can sense intracellular oxygen, redox state, and various metabolites.

We have previously described how PASK is a critical regulator of AMPK and mTOR pathways in the hypothalamus and liver, as well as a key regulator of oxidative stress and glucose and lipid liver metabolism. PASK-deficient mice are protected against the development of obesity and insulin resistance induced by a high-fat diet (HFD). PASK has recently been described as a target of mTORC1 during regenerative myogenesis in muscle stem cells.

To investigate PASK's role in hepatic oxidative stress during aging, we analyzed the mitochondrial function, glucose tolerance, insulin resistance, and lipid-related parameters in aged PASK-deficient mice. Hepatic Pask mRNA decreased in step with aging, being undetectable in aged wild-type (WT) mice. Aged PASK-deficient mice recorded lower levels of reactive oxygen species and reactive nitrogen species compared to aged WT. The regulators of mitochondrial biogenesis, PGC1a, SIRT1, and NRF2, decreased in aged WT, while aged PASK-deficient mice recorded a higher expression of NRF2, GCLm, and HO1 proteins and CS activity under fasted conditions. Additionally, aged PASK-deficient mice recorded an overexpression of the longevity gene FoxO3a, and maintained elevated PCNA protein, suggesting that hepatic cell repair mechanisms might be functional. PASK-deficient mice have better insulin sensitivity and no glucose intolerance. PASK may be a good target for reducing damage during aging.

Age-related retinal diseases, such as late-stage macular degeneration have an impact on cortical morphology. For example, researchers found that the regions of the striate cortex that usually sample the visual input from the injured area of the retina were thinner in participants with macular disease when compared to controls. On the contrary, regions corresponding to non-damaged areas demonstrated a significant increase in cortical thickness. Overall, these findings suggest a strong retinocortical coupling of structural and functional changes in retinal disorders.

Healthy aging is characterized by diverse structural changes in both brain and retina, which association remains to be studied. Concerning the former, there is substantial evidence suggesting that widespread cortical shrinkage takes place with increasing aging. The nature of such effects in early visual areas remains controversial. Whereas some studies have shown volume loss or cortical thinning of visual cortices, others have reported a certain sparing of these areas during aging. Specifically to the primary visual cortex such discrepancy in the literature could be explained by methodological issues, since they demonstrated that different sub-regions of primary visual cortex were unequally affected by aging, depending on their retinotopic eccentricity. Nonetheless, studies directly examining the structure and function of primary visual areas are scarce in healthy aging, in spite of the fact that the organization of early visual areas is well documented in young adults.

The retina also undergoes substantial modifications throughout the lifespan. Histological studies have reported a reduction in the density of photoreceptors, ganglion cells, and pigment epithelial cells with age. Overall, retinal thickness studies using optical coherence tomography (OCT) imaging have revealed regional age-dependent differences in global macular integrity. The improvement of OCT image processing techniques has now allowed for the automatic segmentation of retinal individual layers, which provides a more detailed and specific perspective on such alterations.

we aimed to investigate the association of retinal layer and cortical integrity, in a healthy cohort aged between 20-80 years old. To that end, we performed magnetic resonance structural data imaging measurements of cortical thickness in the primary visual cortex - BA17, the cortical area that receives direct retinal input - and OCT to measure the thickness of the macula and their individual layers, in the same set of participants. We found an age-related decay of primary visual cortical thickness that was significantly correlated with a decrease in global and multiple layer retinal thicknesses. The atrophy of both structures might jointly account for the decline of various visual capacities that accompany the aging process. Furthermore, associations with other cortical regions suggest that retinal status may index cortical integrity in general.

People with chronic kidney disease have a higher risk for heart disease and heart-disease death. Coronary microvascular dysfunction, or CMD, is decreased blood flow in the small blood vessels inside the heart muscle that provide oxygen and fuel to feed the pumping heart. A new study links kidney disease to progressive heart disease via this mechanism. In healthy hearts, visualized postmortem, these blood vessels look like a tight filigree network that fills the heart muscle tissue. A diseased postmortem heart has lost much of this network. In living patients, however, those small blood vessels inside the heart muscle cannot be visualized; blood flow scans of living patients visualize only the larger, exterior coronary arteries.

Thus researchers needed an indirect way to gauge CMD. That measure is coronary flow reserve, or CFR, assessed via positron emission tomography. CFR is the maximum increase in blood flow through the coronary arteries above the normal resting volume. In a longitudinal study of 352 patients with chronic kidney disease, all with healthy heart function as measured by ejection fraction and none with signs of overt coronary artery disease, the researchers measured CFR and also measured signs of subclinical heart dysfunction. The patients were then followed a median of 4.4 years for major adverse cardiac events. A total of 108 patients had such major events, including death and hospitalization for non-fatal heart attack or heart failure.

The researchers found that CMD was a significant predictor of abnormal mechanics of the left ventricle - the heart's major pumping chamber - and a significant predictor of clinical risk of adverse cardiovascular outcomes. A statistical model called mediation analysis examined the relationship between impaired kidney function and heart disease. It showed that CMD accounted for 19 to 24 percent of left ventricle diastolic dysfunction, 19 to 42 percent of left ventricle systolic dysfunction and 32 percent of major adverse cardiovascular events. This evidence suggests that the development of severe microvascular dysfunction likely signals the transition from physiological to pathological left ventricle remodeling that increases the risk of heart failure and death in patients with chronic kidney disease.

Human peripheral blood mononuclear cells (PBMCs) undergo both cell-intrinsic and cell-compositional changes (i.e., cell frequencies) with age, where certain immune functions are impaired and others are remodeled1. Analyses of human blood samples uncovered significant aging-related changes in gene expression and DNA methylation levels. Recent studies revealed that chromatin accessibility of purified immune cells, especially CD8+ T cells, change significantly with aging, impacting the activity of important receptor molecules, signaling pathways, and transcription factors. Together, these changes likely contribute to aging-related immunodeficiency and ultimately to increased frequency of morbidity and mortality among older adults. However, it is unclear to what extent these aging-associated changes are shared between men and women.

Immune systems of men and women function and respond to infections and vaccination differently. For example, 80% of autoimmune diseases occur in women, who typically show stronger immune responses than males. Stronger responses in women produce faster pathogen clearance and better vaccine responsiveness, but also contribute to increased susceptibility to inflammatory and autoimmune diseases. Although not systematically described, these differences likely stem from differences in both cell frequencies and cell-intrinsic programs. For example, a study in young individuals showed that women have more B cells (percentage and absolute cell counts) in their blood than men. Moreover, hundreds of genes are differentially expressed between young men and young women in sorted B cells.

Despite the importance of sex and age in shaping immune cell functions and responses, it is not known whether men's and women's immune systems go through similar changes throughout their lifespan, and whether these changes occur at the same time and at the same rate. To study this, we profiled PBMCs of healthy adults by carefully matching the ages of male and female donors. These data reveal a shared epigenomic signature of aging including declining naïve T cell and increasing monocyte and cytotoxic cell functions. These changes are greater in magnitude in men and accompanied by a male-specific decline in B-cell specific loci. Age-related epigenomic changes first spike around late-thirties with similar timing and magnitude between sexes, whereas the second spike is earlier and stronger in men. Unexpectedly, genomic differences between sexes increase after age 65, with men having higher innate and pro-inflammatory activity and lower adaptive activity.

Chronic inflammation, which results when old age, stress, or environmental toxins keep the body's immune system in overdrive, can contribute to a variety of devastating diseases, from Alzheimer's and Parkinson's to diabetes and cancer. Researchers now show that a bulky collection of immune proteins called the NLRP3 inflammasome - responsible for sensing potential threats to the body and launching an inflammation response - can be essentially switched off by removing a small bit of molecular matter in a process called deacetylation. Overactivation of the NLRP3 inflammasome has been linked to a variety of chronic conditions, including multiple sclerosis, cancer, diabetes, and dementia. The results suggest that drugs targeted toward deacetylating, or switching off, this NLRP3 inflammasome might help prevent or treat these conditions and possibly age-related degeneration in general.

By studying mice and immune cells called macrophages, the team found that a protein called SIRT2 is responsible for deacetylating the NLRP3 inflammasome. Mice that were bred with a genetic mutation that prevented them from producing SIRT2 showed more signs of inflammation at the ripe old age of two than their normal counterparts. These mice also exhibited higher insulin resistance, a condition associated with type 2 diabetes and metabolic syndrome. The team also studied older mice whose immune systems had been destroyed with radiation and then reconstituted with blood stem cells that produced either the deacetylated or the acetylated version of the NLRP3 inflammasome. Those who were given the deacetylated, or "off," version of the inflammasome had improved insulin resistance after six weeks, indicating that switching off this immune machinery might actually reverse the course of metabolic disease.

"We are asking to what extent can aging be reversed. And we are doing that by looking at physiological conditions, like inflammation and insulin resistance, that have been associated with aging-related degeneration and diseases. I think this finding has very important implications in treating major human chronic diseases. It's also a timely question to ask, because in the past year, many promising Alzheimer's disease trials ended in failure. One possible explanation is that treatment starts too late, and it has gone to the point of no return. So, I think it's more urgent than ever to understand the reversibility of aging-related conditions and use that knowledge to aid a drug development for aging-related diseases."

Adipose-derived mesenchymal stem cells (ADSCs) or "preadipocytes" have been increasingly suggested for use in regenerative medicine as a treatment for a wide range of diseases due to their multipotency and accessibility. Older adults represent most likely recipients of ADSC therapies given the high burden of diseases in this population. Since autologous ADSCs are preferred in the clinic, it is essential to understand age-related changes influencing these cells. Emerging evidence suggests that ADSCs from aged donors have reduced regenerative potential, leading to diminished therapeutic efficacy. However, it is unknown whether transplanting ADSCs from aged donors might cause unexpected or even harmful effects in recipients. This is especially important for older adults, since they tend to be more vulnerable and less resilient to such stresses.

To examine this, we isolated ADSCs from 12 young (6-7 months, referred to as young ADSCs) and 12 old (28-31 months, referred to as old ADSCs) C57BL/6 male mice. We transplanted ADSCs from young or old donors into syngeneic 20-month-old C57BL/6 male mice. Four to six weeks after transplantation, we tested maximal walking speed, grip strength, physical endurance, daily activity, food intake, and body weight change to assess overall physical function in recipients, based on criteria used in clinical practice. ADSCs from old donors significantly impaired walking speed, grip strength, endurance, and daily activity of older recipient mice after transplantation, compared with mice transplanted with the same number of ADSCs from young donors.

Using single-cell transcriptomic analysis, we identified a naturally occurring senescent cell-like population in ADSCs primarily from old donors that resembles in vitro-generated senescent cells with regard to a number of key pathways. Overall, these findings suggest that ADSCs from old donors can induce physical frailty, which is highly associated with morbidity and loss of independence. Our study potentially begins new avenues of research to discover whether pharmacological interventions, such as senolytic drugs or anti-inflammatory drugs, can prevent or reverse dysfunction caused by transplanting ADSCs or even organs from old donors and improve clinical outcomes of transplantation for older patients.

The bone marrow is an important reservoir of stem cells and progenitor cells which cross-talk with peripheral organs to help maintain tissue function. Hematopoietic stem/progenitor cells (HSCs) are responsible for producing blood cells throughout life and these downstream cells play an active role in maintaining tissue homeostasis. With aging reduced function of bone marrow cells correlates with dysfunction of peripheral organs. For example, the decline in immune function with age, referred to as immunosenescence, contributes to the accumulation of senescent cells, persistent low grade inflammation, and reduced responses to injury. The bone marrow is also an important source of endothelial progenitor cells (EPCs) which participate in the generation and repair of vasculature endothelium; aging leads to a decline in circulating EPC number and function.

Different strategies have been proposed to rejuvenate the aged bone marrow such as pharmacological treatments, gene therapy, and dietary interventions. However, most approaches have focused on the effect of rejuvenation on HSC differentiation and EPC colony formation rather than effects on peripheral tissues. Therefore, we hypothesized that reconstituting aged mice with young bone marrow leads to stable engraftment of young cells in aged mice and rejuvenates tissue repair responses.

We recently utilized this bone marrow rejuvenation approach to study the effect aging has on the repair processes initiated post-myocardial infarction. Aged mice were reconstituted with young Sca-1+ bone marrow stem cells and examined 4 months later to allow cross talk between the bone marrow and heart. Young bone marrow reconstitution rejuvenated cardiac endothelial cells which contributed to improved repair and better outcome following myocardial infarction. In addition to improved angiogenesis, our lab has shown that rejuvenation using reconstitution of young cells improves multiple repair processes. Young bone marrow cell transplantation increases the proliferation of resident cardiac cells, increases epicardial derived cell migration/activation, and enhances the acute inflammatory response that follows myocardial infarction in aged mice.

Beyond cardiac repair, we have shown that bone marrow cells interact with other tissues and that bone marrow rejuvenation can benefit multiple organ systems. Reconstituting aged mice with young cells leads to the repopulation of the retina with young bone marrow derived microglia. Within the retina these cells secrete cytoprotective factors such as fibroblast growth factor-2 and insulin-like growth factor-1 which limit cell death following ischemia/reperfusion injury. More recently, we also demonstrated that bone marrow rejuvenation leads to the introduction of young bone marrow-derived microglia in the brain and that these cells act to improve learning and memory responses compared with mice receiving old bone marrow. Mechanistically, young bone marrow-derived microglia adopt a neutral or anti-inflammatory phenotype while old bone marrow-derived microglia adopt a pro-inflammatory phenotype. These results are consistent with studies which have linked increased neuroinflammation to a decline in cognitive function.