Fight Aging! Newsletter, November 2nd 2020

All major United States institutional advocates for research on the biology of aging and for the leading causes of death assert that aging is the greatest risk factor for these deaths. Nevertheless, all fail to support research on the etiology of aging despite having mechanisms to do so. Bordering on scandal, research on the cause of aging in life forms is not a major priority for any organization in this country with "Age" or "Aging" in its title. This neglect is inexplicable because the mantra believed by most physicians, geriatricians and biogerontologists is that "Aging is the greatest risk factor for the leading causes of death." It does not require a great leap of intellect to ask: "Then, why is research on the etiology of the greatest risk factor that increases vulnerability to cancer, cardiovascular disease, stroke, and Alzheimer's disease (AD), ignored?"

The field of aging is the only area of biomedical research where causation is ignored. This inexplicable omission is compounded by the fact that aging is a universal human phenomenon for all who live long enough to experience it. Even for those in good health, that condition is merely the slowest rate of aging and dying. Today, like the former rich and powerful, their modern counterparts have the same goals in the form of funding hundreds of biotech startup companies. Plus ça change, plus c'est la meme chose. In these modern efforts there seems to be little understanding that there is an enormous difference between the molecular biology of what determines the longevity of life forms and what causes their aging. Longevity is determined by anabolic processes and addresses the question, "Why do life forms live as long as they do?" Aging is a catabolic process that addresses the question, "Why do longevity systems eventually fail?"

There is a general failure to understand that manipulating the genome or anabolic processes in living forms which may increase longevity, or cure a disease, tells us nothing about the dysfunctional or missing molecules that characterize the catabolic process of aging. Research on the biogerontology of aging is unique because of the common belief that the goal is to interfere or manipulate the process. The availability of funds for age-associated disease research is several orders of magnitude greater than what is available for research on the fundamental biology of their greatest risk factor. The resolution of any age-associated disease has not in the past, nor will it in the future, improve our understanding of the etiology of aging. A century ago, the leading cause of death in old age was pneumonia, often called "the old man's friend" (with its sexist overtones). Pneumonia is no longer one of the leading causes of death in old age and its resolution did not advance our knowledge of the cause of aging. Nor will the resolution of any other age-associated pathology.

Studies indicate that the Wnt signaling plays multiple roles in adult hippocampal neurogenesis including neural progenitor cell (NPC) proliferation, fate-commitment, development and maturation of newborn neurons. Evidences suggest a stage-specific expression of particular receptors that might activate different Wnt signaling cascades to control the progression of neurogenesis. Although the role of the canonical Wnt co-receptor LRP6 support this notion, the role of other co-receptors that control the activation of non-canonical Wnt signaling remains to be elucidated. The identification of Wnt co-receptors involved in adult neurogenesis is a critical issue that should be addressed to gain a more comprehensive understanding of how canonical and non-canonical Wnt signaling are regulated during adult neurogenesis. In addition, it will be interesting to further study the downstream signaling components and effectors involved in the regulation of adult hippocampal neurogenesis by non-canonical Wnt signaling.

Several studies indicate that Wnt proteins released by hippocampal astrocytes and progenitor cells are crucial components of the subgranular zone (SGZ) neural stem cell niche. In addition, endogenous Wnt inhibitors are also components of the neurogenic microenvironment that dynamically regulate Wnt-mediated neurogenesis under physiological conditions. Considering the increasing number of Wnt regulators identified to date, it will be interesting to further investigate the contribution of these molecules to the dynamic control of neurogenesis.

In agreement with the critical roles of Wnt signaling in adult neurogenesis, evidence indicates that Wnt signaling is associated with the age-dependent decline in neurogenesis. Concomitantly with the decrease in the generation of new neurons, in normal aging there is a reduction in the expression of Wnt proteins, an increase in the expression of Wnt inhibitors, and a decrease in canonical Wnt signaling activity in the dentate gyrus. Wnt dysfunction might also underlie the impairment of neurogenesis observed in Alzheimer's disease (AD).

Interestingly, genetic and pharmacological activation of Wnt signaling was shown to restore adult hippocampal neurogenesis, and also to improve cognitive performance in animal models of AD. Although it is not yet known how neurogenesis contribute to hippocampal function in humans, compelling evidence in animal models suggest that adult-born neurons are important for learning and memory, cognitive flexibility and mood regulation. In addition, recent findings support that neurogenesis impairment contributes to cognitive decline in aging and AD. Therefore, a better understanding on the molecular mechanisms involved in the regulation of neurogenesis may have important therapeutic implications.

Frailty is connected to cellular aging, which in turn is connected to cellular senescence. Senescent cells are permanently withdrawn from the cell cycle and generally develop a persistent pro-inflammatory phenotype called the senescence-associated secretory phenotype (SASP) which is comprised of proinflammatory cytokines and chemokines. Selective killing of senescent cells with therapeutics (i.e., senolytics) have gained attention as a new therapeutic approach for age-related diseases. Targeting of pro-survival Senescent Cell Anti-apoptotic Pathways (SCAPs) has emerged as the primary strategy for senescent cell killing.

The translational value of many senolytic drugs in vivo is limited due to their chronic toxicity. The identification of agents that selectively kill senescent cells while sparing other cell populations represents a scientific challenge. Current senolytic drugs target molecular pathways shared between senescent and proliferating cells, thus achieving cell killing but not specificity. As a matter of fact, many known senolytic agents were initially developed as cytotoxic anti-cancer agents and subsequently repurposed for 'selective' removal of senescent cell populations.

Senescent cells are characterized by a notable change in biological properties such as an increase in the levels of mitochondria, reactive oxygen species, lysosomal content, and upregulation of many lysosomal proteins, including the lysosomal enzyme senescence-associated β-galactosidase (SA-βGal). Recently, a promising strategy has been proposed based on galactose-derivative prodrugs. These prodrugs are selectively activated in senescent cells upon conversion into the parent active drug by the hydrolase activity of SA-βGal. In particular, specific senotoxic compounds such as duocarmycin, gemcitabine, and navitoclax have been modified into galacto-derivative prodrugs showing increased selectivity in targeting senescence cells and efficacy in treating cancer and aged mouse models.

Here, we report a novel prodrug design to target senescent cells, allowing systemic removal of senescent cells in geriatric mice without noticeable side effects. We took advantage of the senescence-specific activity of SA-βGal in the design of a non-toxic senolytic prodrug derivative of the compound 5-Fluorouridine, a metabolic precursor of the clinically approved anti-cancer medication 5-Fluorouracil. We first tested the specificity of this prodrug on senescent cells in vitro. We then confirmed safety and efficacy of the prodrug in young (5 month-old), aged (22 month old) and in geriatric (30 month old) mice. Importantly, we showed that geriatric mice that received the prodrug treatment for four weeks altogether improved significantly: 1) frailty profile; 2) skeletal muscle function; 3) muscle stem cell function; 4) cognitive function; and 5) survival.

In Alzheimer's disease (AD), mitochondrial dysfunction and the bioenergetic deficit contribute to the amyloid-β (Aβ) and phosphorylated Tau (p-Tau) pathologies; in turn, these two pathologies promote mitochondrial defects. As a consequence, a fundamental characteristic of AD is the impairment of mitochondria. Pharmacological agents, fasting, physical exercise, and caloric restriction can reverse this impairment. The main target of these methods is to enhance autophagy and mitophagy. Mitophagy plays a fundamental role in mitochondrial quality control and homeostasis, and the pathological consequences of its misregulation demonstrate its importance. However, the exact positions of mitophagy in AD etiology are still unclear as multiple steps are affected. Cells regulate mitochondrial degradation not only through control of the mitophagy machinery but also through delicate tuning of mitochondrial fusion and fission. It remains to see whether other cellular processes linked with mitochondria also have a role to play in mitophagy regulation.

Accumulating studies suggest that dysfunctional mitochondria are mainly due to impaired mitophagy in neurons in AD. The 'vicious cycle' hypothesis proposed that loss-of-function mitophagy and Aβ and p-Tau, the biomarkers in AD pathophysiology, strongly influence each other. Moreover, the 'vicious cycle' experiments state that Aβ-dependent neuronal hyperactivity supports circuit dysfunction in the early stages of AD. Recently, researchers successfully stimulated mitophagy and reversed memory impairment using NAD+ supplementation, urolithin A, and action in both Aβ and tau Caenorhabditis elegans models. In human neurons derived from the hippocampus of AD patients and in AD animal models, enhanced mitophagy can even diminish insoluble Aβ and prevent cognitive impairment in AD mouse model through the suppression of neuroinflammation and microglial phagocytosis of Aβ plaques. These findings predict that enhancing mitophagy could be a novel approach to delay or even treat AD. To this end, plentiful pharmacological agents have been examined in preclinical studies.

In the past 20 years, most of the drugs tested in the clinic for AD have targeted the Aβ accumulation; however, none of these anti-Aβ therapies overcome the central problem. Today, a promising alternative option for AD therapeutics is to maintain mitochondrial homeostasis by enhancing autophagy and stimulating mitophagy. Dysfunctional mitophagy can increase Aβ and Tau pathologies, while aggregating Aβ can impair neuronal mitophagy in reverse. These outcomes indicate pivotal roles for mitophagy dysfunction, both upstream and downstream of Aβ and Tau pathways.

Ageing is associated with broad decline in organ function and increased risk for chronic disease. The immune system undergoes dramatic changes associated with age, including decreased immune response, loss of immune memory, and increased chronic inflammation. Ageing broadly impacts humoral immunity, as antibody affinity and the adaptive immune processes that lead to their production suffer with age. For instance, plasma cells produce less antibody, germinal center B cell selection results in lower affinity antibodies in mouse, and the CD4+ T cell receptor diversity decreases. Additionally, hematopoiesis broadly declines, professional antigen presenting cells reduce expression of peptide-MHC-II complex, and antibody effector cells show decreased functional clearance of IgG-bound pathogens. These age-dependent declines in humoral immunity can be manifested in less effective antibody binding.

To better understand and quantify the impact of ageing on the immune response, we identified age-associated patterns in serum antibody binding profiles. We profiled IgG antibody binding using peptide microarrays in a cohort of 1675 donors. We created a machine learning model that estimates an "immune age" from a donor's antibody binding profile that is highly correlated with chronological age.

The immune age is highly robust with respect to technical parameters, such as reagents, peptide microarray design, and serum handling. The machine learning regression model was validated on an independent donor cohort and longitudinal profiling revealed that a donor's immune age is typically consistent over multiple years suggesting that this could be a robust long-term biomarker of age-associated humoral immune decline. We show that accelerated immune ageing, when a donor has an older immune age than chronological age, is associated with autoimmunity, autoinflammatory disease, and acute disease flares.

In conclusion, the circulating antibody repertoire has increased binding to thousands of peptides in older donors, which can be represented as an immune age. Increased immune age is associated with autoimmune disease, acute inflammatory disease severity, and may be a broadly relevant biomarker of immune function in health, disease, and therapeutic intervention. The immune age has the potential for wide-spread use in clinical and consumer settings.

As organisms age, some epigenetic markers are modified. It has been proposed that the removal of these aging-dependent epigenetic modifications may reverse some features of aging. Temporal expression of Oct4, Sox2, Klf4, and c-Myc (also known as the Yamanaka factors, YFs), used for pluripotency cell reprogramming, can cause this removal of epigenetic marks and subsequent reversal of aging features. Indeed, this approach has been successfully used to improve age-associated hallmarks in peripheral tissues of mice. However, little attention has been given to the therapeutic use of YFs in the central nervous system. Importantly, YF expression must be tightly regulated, since it can lead to aberrant mitogenic stimulation or apoptosis.

In this study, we addressed age-dependent changes in brain structures susceptible to premature degeneration. It has been postulated that age-related brain decline mirrors developmental maturation and, accordingly, brain structures with a late development may be the first to degenerate. This notion was first described as Ribot's law. The dentate gyrus (DG) exemplifies a brain structure that matures after birth and whose functions decline early with age. For example, the DG of 10-month-old mice shows a clear decrease in adult neurogenesis, the process through which functional neurons are generated from adult neural precursors and integrated into existing circuits. In the adult mouse brain, adult neurogenesis occurs at the interface between the DG and hilus, in a region known as the subgranular zone. This type of neurogenesis is involved in learning and memory.

Here, we examined several markers for adult neurogenesis in mice. We found impaired adult hippocampal neurogenesis as the animals aged, thereby supporting previous observations. Our aim here has been to rejuvenate old hippocampal neurons by expressing YFs. However, an extended expression of YFs (continuous protocol) can cause aberrant transcription and cell death. Indeed, around 50% of YF-expressing mice died after 10 days of this protocol. We then tested cyclic induction of YFs. In this protocol, mouse death was prevented. Our results indicate that in mature mice, the expression of YFs results in a partial prevention of those aging-associated changes found in the newborn neurons of adult mice. In addition, YFs show an effect on DG mature neurons that could increase synaptic plasticity in old mice. This increase could explain why mice expressing YFs outperformed same-age wild-type counterparts in a memory test.

Aging doesn't kill people - diseases kill people. Right? In today's world, and in a country like the United States, most people die of diseases such as heart attack and stroke, cancer, and Alzheimer's. These diseases tend to be complex, challenging, difficult, and extremely ugly to experience. And they are by nature chronic, caused by multifactorial triggers and predispositions and lifestyle choices. What we are only now beginning to understand is that the diseases that ultimately kill us are inseparable from the aging process itself. Aging is the root cause. This means that studying these diseases without taking aging into account could be dangerously misleading ... and worst of all, impede real progress.

Take Alzheimer's disease. To truly treat a disease like Alzheimer's, we would need to identify and understand the biological targets and mechanisms that trigger the beginning of the disease, allowing us to intervene early - ideally, long before the onset of disease, to prevent any symptoms from happening. But in the case of diseases like Alzheimer's, the huge problem is that we actually understand very little about those early targets and mechanisms. The biology underlying such diseases is incredibly complex. We aren't sure what the cause is, we know for sure there isn't only one target to hit, and all prior attempts to hit any targets at all have failed. When you start to think about how much of what we think we know about Alzheimer's comes from very broken models - for example, mice, which don't get Alzheimer's naturally - it becomes totally obvious why we're at a scientific stalemate in developing treatments for the disease, and that we've likely been coming at this from the wrong direction entirely.

The biggest risk factor for Alzheimer's isn't your APOE status; it's your age. People in their twenties don't get Alzheimer's. But after you hit the age of 65, your risk of Alzheimer's doubles every five years, with your risk reaching nearly one out of three by the time you're 85. What if going after this one biggest risk factor is the best vector of attack? Maybe even the only way to truly address it? This isn't about the vanity of staying younger, about holding on to your good looks or your ability to run an 8 minute mile. It's about the only concrete possibility we have to cure these diseases. Instead of choosing targets for a single specific disease, i.e. a specific condition that arises in conjunction with aging, we can get out in front of disease by choosing targets that promote health. And we can identify these by looking at disease through the lens of the biology of aging.

Using a magnetic field and hydrogels, a team of researchers have demonstrated a new possible way to rebuild complex body tissues. "We found that we were able to arrange objects, such as cells, in ways that could generate new, complex tissues without having to alter the cells themselves. Others have had to add magnetic particles to the cells so that they respond to a magnetic field, but that approach can have unwanted long-term effects on cell health. Instead, we manipulated the magnetic character of the environment surrounding the cells, allowing us to arrange the objects with magnets."

In humans, tissues like cartilage can often break down, causing joint instability or pain. Often, the breakdown isn't in total, but covers an area, forming a hole. Current fixes are to fill those holes in with synthetic or biologic materials, which can work but often wear away because they are not the same exact material as what was there before. What complicates fixing cartilage or other similar tissues is that their make-up is complex. "There is a natural gradient from the top of cartilage to the bottom, where it contacts the bone. Superficially, or at the surface, cartilage has a high cellularity, meaning there is a higher number of cells. But where cartilage attaches to the bone, deeper inside, its cellularity is low."

With that in mind, the research team found that if they added a magnetic liquid to a three-dimensional hydrogel solution, cells, and other non-magnetic objects including drug delivery microcapsules, could be arranged into specific patterns that mimicked natural tissue through the use of an external magnetic field. After brief contact with the magnetic field, the hydrogel solution (and the objects in it) was exposed to ultraviolet light in a process called "photo crosslinking" to lock everything in place, and the magnetic solution subsequently was diffused out. After this, the engineered tissues maintained the necessary cellular gradient. With this magneto-patterning technique, the team was able to recreate articular cartilage, the tissue that covers the ends of bones.

"These magneto-patterned engineered tissues better resemble the native tissue, in terms of their cell disposition and mechanical properties, compared to standard uniform synthetic materials or biologics that have been produced. By locking cells and other drug delivering agents in place via magneto-patterning, we are able to start tissues on the appropriate trajectory to produce better implants for cartilage repair." While the technique was restricted to in vitro studies, it's the first step toward potential longer-lasting, more efficient fixes in living subjects.

Although more than 300 theories have emerged over the years to explain the intrinsic molecular and evolutionary drivers behind organismal aging, the onset of cellular senescence seems to act as a foundational pillar for organ and organismal aging. Diabetes mellitus (DM) is a heterogeneous metabolic disease characterized by chronic hyperglycemia resulting from defects in insulin secretion, insulin action, or both. An intense debate has existed so far addressing whether senescence precedes or follows the onset of chronic inflammation and insulin resistance (IR). Irrespective to "who-precedes-who," diabetic patients experience an obvious accelerated aging process that increases their susceptibility to morbidity and earlier mortality. Hence, diabetes-affected patients have a significantly shorter life expectancy than non-diabetic individuals, while this life expectancy reduction is largely dependent on diabetes duration.

The major clinical challenge of diabetes is the progressive and expansive morbidity and mortality resulting from the long-term secondary complications. Within the constellation of diabetic complications, the delayed and poorness in triggering and progressing along a physiological repair response following wounding is of major clinical significance. Diabetes undermines skin cells physiology and progressively intoxicates the dermal layer by the accumulation of advanced glycation end products (AGEs) and free radicals derivatives. Accordingly, most if not all of the events encompassed within the cutaneous healing process including hemostasis, inflammation, matrix deposition, angiogenesis, contraction, remodeling, and re-epithelialization are somewhat buffeted by diabetes.

Chronic low-grade inflammation and an increased burden of senescent cells are hallmarks of aging in diabetic subjects. Hyperglycemia per se is known to act as a senescence-promoting factor for cultured cells, and steadily precipitates organs complications and functional demise by different mechanistic pathways. The notion that cellular senescence is an imperceptible underlying force in the pathogenesis of wound chronicity and ulcer recurrence has accrued for years. Consequently, we suggest that diabetes-associated wound healing failure and reduced tissue resilience are clinical translations of an "entrenched" wound senescent cell population, with self-perpetuating and propagating abilities.

This population may be fostered by a diabetic archetypal secretome that induces replicative senescence in dermal fibroblasts, endothelial cells, and keratinocytes. Mesenchymal stem cells are also susceptible to major diabetic senescence drivers, which accounts for the inability of these cells to appropriately assist in diabetic wound healing. The senescent cell population and its adjunctive secretome could be an ideal local target to manipulate diabetic ulcers and resolve non-healing wounds.

Scientists are in the initial stages of development of a method to detect the biomarkers for Alzheimer's disease that is 10 times more sensitive than current blood testing technology. For Alzheimer's disease, doctors most often diagnose patients based on their symptoms. By that time, the patients often already have severe brain damage. Imaging technology such as magnetic resonance imaging and CT scans can also be used to help confirm the disease, but they are not suitable for early stage diagnosis. Occasionally, doctors may test spinal fluid to look for beta-amyloid proteins, markers of the disease, but the process is more invasive than a simple blood test would be.

One common way of testing blood is the ELISA, or enzyme-linked immunosorbent assay, which is used to test for a variety of diseases. The ELISA uses a natural enzyme found in the roots of horseradish that can change color to indicate the presence of disease biomarkers. But, using the technique to detect the beta-amyloid proteins of Alzheimer's is difficult because their levels in the blood are too small.

Last year, researchers created an artificial enzyme using a single-atom architecture that was able to work as efficiently as natural enzymes. Their artificial enzyme, called a nanozyme, is made of single iron atoms embedded in nitrogen-doped carbon nanotubes. For this work, the researchers were able to use their single-atom nanozyme to mimic the active site of a natural enzyme and to detect the Alzheimer's disease proteins at levels 10 times lower than commercially available ELISA tests. The nanozyme was also more robust than natural enzymes, which can degrade in acidic environments or in high temperatures. It is also less expensive and could be stored for long periods of time.

Resting metabolic rate (RMR) changes over the life span and has been related to changes in health status. RMR reflects the energy expended by the human body in a prolonged resting state in the absence of food digestion, physical, or cognitive activities. As such, RMR can be understood as the "cost of living", i.e., the energetic cost of maintaining all physiological processes that preserve homeostatic equilibrium and cognitive alertness and sets the stage for all activities of life. RMR is affected by changes in body size, with greater RMR associated with larger body size, especially large lean body mass. RMR is widely determined by the most metabolically active tissues, such as muscle, heart, brain, and liver, and, as the function and metabolic activity of these organs and tissues decline with aging, RMR also declines with aging.

In an analysis of data from the Baltimore Longitudinal Study of Aging (BLSA), subjects in good health and functional status had lower RMR than those affected by chronic diseases and functional limitations, independent of age, sex and body composition. Also, independent of relevant confounders, higher RMR was cross-sectionally associated with both a higher number of chronic diseases and with significantly higher risk of developing multimorbidity, defined as two or more out of 15 chronic conditions. Similarly, in community-dwelling women 60 years and older, increasing multimorbidity was associated with an increase in RMR independent of body composition and age. Moreover, two studies evaluating the longitudinal association between energy metabolism and mortality found higher RMR and 24 hour energy expenditure (24EE), which are predictive of future negative health outcomes and early mortality.

Overall, these data suggest that while healthy aging is associated with a progressive decline of RMR, independent of changes in body composition, superimposed adverse changes in health and functional status tend to attenuate such decline. This attenuation has been attributed to the potential extra-energetic cost of maintaining homeostasis in response to disease-related processes. However, a comprehensive analysis of how various diseases that ensue with aging affect age-associated changes in RMR is still lacking.

A hypothesis that could explain the increased basal metabolism observed in these conditions is the accumulation of senescent cells. The cell stops replicating, withdrawing from the cell cycle, and develops specific features such as resistance to apoptosis, increased energy metabolism, and production of bioactive molecules globally defined as "Senescence Associated Secretory Phenotype" (SASP). SASP includes several pro-inflammatory proteins that determine damage to tissues and produce a chronic inflammatory environment. We argue that the enhanced metabolic activity we observe in this analysis for some diseases could be attributable to the presence of increasing numbers of metabolically active senescent cells.

The biological bases of longevity are not well understood, and there are limited biomarkers for the prediction of long life. We used a high-throughput, discovery-based proteomics approach to identify serum peptides and proteins that were associated with the attainment of longevity in a longitudinal study of community-dwelling men age ≥65 years. Baseline serum in 1196 men were analyzed using liquid chromatography - ion mobility - mass spectrometry, and lifespan was determined during ~12 years of follow-up. Men who achieved longevity (≥90% expected survival) were compared to those who died earlier.

Rigorous statistical methods that controlled for false positivity were utilized to identify 25 proteins that were associated with longevity. All these proteins were in lower abundance in long-lived men and included a variety involved in inflammation or complement activation. Lower levels of longevity-associated proteins were also associated with better health status, but as time to death shortened, levels of these proteins increased. Pathway analyses implicated a number of compounds as important upstream regulators of the proteins and implicated shared networks that underlie the observed associations with longevity.

Overall, these results suggest that complex pathways, prominently including inflammation, are linked to the likelihood of attaining longevity. This work may serve to identify novel biomarkers for longevity and to understand the biology underlying lifespan.

The coronavirus disease 2019 (COVID-19) pandemic does not affect everyone equally. While anyone can contract COVID-19, accumulating data suggest that older people or those with pre-existing comorbidities are far more likely to have severe complications or die from the disease. While researchers scramble to unravel the mechanisms of action underlying the disease's wide-ranging effects, news that the disease hits older people hardest has been received without demur: it is widely accepted that to be old is to be fragile. Indeed, even in so-called normal times, everyone expects more things break as people age: bones, hearts, brains. In the context of the pandemic, being old is seen as just one more comorbidity. It should not be.

We accept growing old and losing our vitality as an inevitability of life. To do so is to overlook the fact that ageing is, fundamentally, a plastic trait-influenced both by our genetic predispositions and many (controllable) environmental factors. Anecdotally we know this to be true: for some, being in their eighties means being confined to a wheelchair whereas for others, like Eileen Noble, who at 84 years old was the oldest runner in 2019's London Marathon, it decidedly does not. The burgeoning field of biogerontology is now beginning to amass data in support of such observations. Single genetic mutations in evolutionarily conserved pathways across model organisms - ranging from fruit flies to mice - increase lifespan by up to 80%. Crucially, not only do these animals live longer, they also have a longer youthspan - the proportion of their lives in which they retain the trappings of youth such as peak mobility, immunity, and stress resilience. These data show something amazing: the rate of ageing is not fixed. Fragility, vulnerability, and poor health need not necessarily follow advancing age.

This is an unprecedented crossroads in global society, raising fundamental questions about how we live as individuals, and collectively. Will an ageing population mean people experience longer periods of good health, a sustained sense of wellbeing, and extended periods of social engagement and productivity - or will it be associated with a higher burden of illness, disability, and dependence on others? The science suggests that we have a choice.

Glial cells carry out a variety of support and maintenance functions, and one type in particular - the astrocytic glial cell - has the unique ability to form scar tissue around damaged neurons. The presence of scar tissue is associated with inhibitory effects on the regrowth of mature neurons that are damaged by spinal cord injury. Recent evidence suggests, however, that these inhibitory effects are reversible, and in new work, scientists show that astrocytic glial cells can in fact play a major role in facilitating neuron repair.

The research is the first to establish a link between glucose metabolism in glial cells and functional regeneration of damaged neurons in the central nervous system. Scientists set out to investigate how scar tissue formation induced by glial cells impacts axon regeneration, using both fly and mouse models of axon injury. In initial experiments, they confirmed that the negative effects of glial cell activity on axon regeneration are indeed reversible. But the researchers also found that the switch between positive and negative effects on axon regrowth is directly related to the glial cells' metabolic status.

In follow-up experiments in flies, the researchers focused specifically on glycolysis - the metabolic pathway responsible for the breakdown of glucose - and discovered that upregulating this pathway alone in glial cells was sufficient to promote axon regeneration. This same result was observed in mice. Further investigation in fly and mouse models led to the identification of two glucose metabolites, lactate and hydroxyglutarate, that act as key mediators of the glial switch from an inhibitory reaction to a stimulatory response. "In the fly model, we observed axon regeneration and dramatic improvements in functional recovery when we applied lactate to damaged neuronal tissue. We also found that in injured mice, treatment with lactate significantly improved locomotor ability, restoring some walking capability, relative to untreated animals."

Experiments revealed that when glial cells are activated, they release glucose metabolites, which subsequently attach to molecules known as GABAB receptors on the neuron surface and thereby activate pathways in neurons that stimulate axon growth. "Our findings indicate that GABAB receptor activation induced by lactate can have a critical role in neuronal recovery after spinal cord injury. Moreover, this process is driven by a metabolic switch to aerobic glycolysis, which leads specifically to the production of lactate and other glucose metabolites."

A major assumption of the free radical theory of aging is that random de novo or somatic mitochondrial DNA (mtDNA) mutations gradually accumulate over time, eventually reaching pathological levels. However, data supports the hypothesis that, rather than gradually accumulating over time, mtDNA turnover can lead to the clonal expansion of pre-existing age-related mutations. Once amplified, these higher frequency mtDNA mutations, that are potentially pathogenic, are referred to as heteroplasmy.

To further understand the potential link between mtDNA mutations and the free radical theory of aging, our group examined aging in the context of tobacco smoking and human immunodeficiency virus (HIV) infection, both believed to accelerate aging. Data suggests that smoking and HIV may distinctly contribute to the accumulation of mtDNA mutations. Indeed, smoking showed an association with increased mtDNA heteroplasmy but not somatic mutations, while the reverse was observed with HIV participants, but only in those with a history of high viremia, reflecting poor control of HIV. These results suggest that the chronic immune activation and subsequent oxidative stress induced by HIV may lead to de novo mtDNA mutations, while oxidative damage associated with exposure to tobacco smoking may promote the clonal amplification of pre-existing mtDNA mutations.

Such a pattern is not consistent with the gradual build-up of random mtDNA mutations. Taken together, our findings do not support the slow accumulation of mtDNA transversion mutations as proposed by the free radical theory of aging. Rather, they suggest that randomly mutated molecules of mtDNA are being clonally amplified to generate unique patterns of heteroplasmy in our participants.

Although the accumulation of mtDNA mutations has been linked to older age and age-associated conditions, several studies have provided new insight that challenge the connection between oxidative damage and mtDNA mutations. For example, the most studied oxidative lesion, 8-oxodG, is one of the 37 major oxidative lesions, and is known to induce transversion mutations (A ↔ C, A ↔ T, C ↔ G, G ↔ T). However, recent studies showing the accumulation of mtDNA mutations with aging did not observe increases in mtDNA transversion mutations, but rather increases in mtDNA transition mutations (A ↔ G, C ↔ T), believed to be the hallmark of mitochondrial polymerase γ errors rather than oxidative damage. Additionally, in our study, although both somatic transition and transversion mutations increased with older age, transition mutations were over 30 times more abundant than transversion mutations, once again suggesting that mtDNA replication errors are the major contributors to mtDNA mutation burden.

In conclusion, recent research support the theory that mtDNA replication errors are the major drivers of cellular mtDNA mutation burden. Nonetheless they do not exclude a comparatively minor role for 8-oxodG-induced transversion mutations, or the many other DNA oxidative lesions that can induce transition mutations. Based on recent findings, an updated understanding regarding the role of free radicals in contemporary theories of mtDNA aging is needed. It seems likely that rather than directly contributing to mtDNA mutations via oxidative lesions, free radicals may affect the mitochondrial polymerase and decrease its fidelity, indirectly increasing somatic transition mutations. Free radicals may also act as a signaling molecule and influence mitochondrial biogenesis and/or mitochondrial turnover via mitophagy, which could in turn promote the clonal expansion of pre-existing mtDNA mutations.