Fight Aging! Newsletter, March 29th 2021

Discussing the age-related dynamics of mortality, we should consider the following relevant question in the study of aging: how is it possible for different diseases and causes of death to "negotiate" with each other in order to produce a simple exponential function for mortality from all causes of death combined (given that contribution of the different causes of death to total mortality varies greatly with age)? Linked to this question is the traditional approach to life extension, based on combating individual causes of death.

Indeed, it is well known which causes of death were reduced in order produce the mortality decrease that took place in the first half of the 20th century. These are primarily pneumonia, influenza, tuberculosis, enteritis and other infectious diseases. It is also known that mortality from each of these causes changes with age. Therefore, their elimination should inevitably change the age dynamics of total mortality and the size of its age-related component. However, mortality increases with age according to the fairly simple Gompertz formula (the Makeham term is close to zero in recent decades and has little effect on mortality dynamics). The only way to resolve this contradiction is to admit that the causes of death are not independent of each other, but are coordinated so that the age-related component of mortality increases exponentially with age, despite a dramatic change in the structure of causes of death. However, then the following question arises: how do the causes of death "agree" with each other so that the age-related component of mortality grows with age in accordance with a fairly simple Gompertz law?

The above facts can be explained by using the hypothesis of limited organism's reliability. According to this hypothesis, an organism is a multi-redundant system with high, but not infinitely high reliability. Therefore, there is always some probability that the interference in the work of individual elements of the organism will coincide randomly in time and the organism will move into a state of non-specific vulnerability. Such failure causes a whole cascade of dependent failures of other systems in the organism, so there are many observed causes of death.

In the simplest illustration of the idea of this hypothesis, an organism in a normal state can die only in extreme situations, certainly lethal for any organism (corresponding to the background component of mortality, which in the developed countries is already close to zero). In addition, as a result of the failure of one of the bodily systems, it may also pass into a state of non-specific vulnerability, which is called "non-survivor". It should be noted that this state has a quite clear biological meaning. For example, failures of immune system, the frequency of which sharply increases with age, create a nonspecific vulnerability to the widest range of diseases, both endogenous and exogenous.

Having fallen into a state of nonspecific vulnerability, an organism quickly dies from any of the first causes it has caught. This concept to a certain extent echoes the new concept of phenoptosis, when an organism is eliminated from the population as a result of multiple systems failure. The age-related component of mortality is determined by the rate of the first limiting stage of the organism's transition from a normal state to a state of non-specific vulnerability ("non-survivor"). This means that the age-related component of mortality is not summed-up of individual causes of death but, on the contrary, is being distributed between them.

In other words, the rate of the first limiting stage determines the value of "death quota", which is then distributed among its various particular manifestations, called "causes" of death. This explains why elimination of the separate age-dependent causes of death is not always capable of changing the size of the age component of mortality. In fact, any reduction of the death rate of organisms, being in a state of non-specific vulnerability, inevitably leads to the increase of share of the organisms being in this state, and to restoration of the former mortality rate due to increase of mortality from other causes.

The hypothesis of limited organism's reliability explains the phenomenon of historical stability of the age-related component of mortality before the early 1950s, as well as the facts of "independent" behavior of the total mortality in relation to its components. Moreover, this hypothesis makes it possible to justify the Gompertz-Makehan formula using such simple notions of the nature of aging as reduction of reserves of organism systems with age. Therefore, the idea of limited reliability of the organism is sufficiently well-founded and natural to be used as a working hypothesis in determining the ways and prospects for extension of human life.

This hypothesis argues that the problem of human lifespan extension is not reduced to fighting individual causes of death. Moreover, the hypothesis of limited reliability predicts that reduction of mortality from individual causes of death will only lead to a significant reduction of total mortality when the initial stage of organism's destruction (transition to a state of non-specific vulnerability) ceases to be a limiting stage of the whole process.

Apparently, the future belongs to another strategy based on explaining the mechanisms of organism's reliability providing nonspecific resistance to a wide range of damaging factors. If successful in this direction, we can expect simultaneous reduction in mortality from a wide variety of diseases. These ideas are conceptually close to the currently developing direction in gerontology, which is called geroscience. This direction is based on the well-known idea that in order to increase lifespan and healthy longevity in particular, it is necessary to move from combating specific diseases of old age to slowing down the pathological processes leading to aging (e.g., reduction of systemic sterile inflammation). Apparently, further success in gerontology should be expected in the development of this particular direction of research.

"Everyone is familiar with the brain's grey matter, but very few know about the white matter, which comprises of the insulated electrical wires that connect all the different parts of our brains. A key feature of the ageing brain is the progressive loss of white matter and myelin, but the reasons behind these processes are largely unknown. The brain cells that produce myelin - called oligodendrocytes - need to be replaced throughout life by stem cells called oligodendrocyte precursors. If this fails, then there is a loss of myelin and white matter, resulting in devastating effects on brain function and cognitive decline. An exciting new finding of our study is that we have uncovered one of the reasons that this process is slowed down in the ageing brain."

"By comparing the genome of a young mouse brain to that of a senile mouse, we identified which processes are affected by ageing. These very sophisticated analysis allowed us to unravel the reasons why the replenishment of oligodendrocytes and the myelin they produce is reduced in the ageing brain. We identified GPR17, the gene associated to these specific precursors, as the most affected gene in the ageing brain and that the loss of GPR17 is associated to a reduced ability of these precursors to actively work to replace the lost myelin."

Brain ageing is characterised by a decline in neuronal function and associated cognitive deficits. There is increasing evidence that myelin disruption is an important factor that contributes to the age-related loss of brain plasticity and repair responses. In the brain, myelin is produced by oligodendrocytes, which are generated throughout life by oligodendrocyte progenitor cells (OPCs). Currently, a leading hypothesis points to ageing as a major reason for the ultimate breakdown of remyelination in Multiple Sclerosis (MS). However, an incomplete understanding of the cellular and molecular processes underlying brain ageing hinders the development of regenerative strategies.

Here, our combined systems biology and neurobiological approach demonstrate that oligodendroglial and myelin genes are amongst the most altered in the ageing mouse cerebrum. This was underscored by the identification of causal links between signalling pathways and their downstream transcriptional networks that define oligodendroglial disruption in ageing. The results highlighted that the G-protein coupled receptor Gpr17 is central to the disruption of OPCs in ageing and this was confirmed by genetic fate-mapping and cellular analyses. Finally, we used systems biology strategies to identify therapeutic agents that rejuvenate OPCs and restore myelination in age-related neuropathological contexts.

Chronic neuroinflammation is well accepted as the most relevant pathological features of AD, regulating other pathological hallmarks of Alzheimer's disease (AD), such as the accumulation of amyloid-β (Aβ) and hyperphosphorylation of Tau, both of which are involved in the neuronal dysfunction in AD. However, there is a great deal of evidence suggesting the important role of systemic inflammation in the pathogenesis of AD, especially in neuroinflammation.

There is increasing evidence suggesting that chronic neuroinflammation, and indeed inflammation in general, is the most relevant pathological feature of Alzheimer's disease (AD), regulating other pathological features, such as the accumulation of amyloid-β (Aβ) and hyperphosphorylation of Tau. Therapies aimed at reducing systemic inflammation in individuals with mild cognitive impairment (MCI) and AD have proven beneficial by delaying the cognitive decline in these individuals, suggesting that recognition of the cross-talk between systemic inflammation and neuroinflammation has important implications for AD therapeutic strategies.

It is well accepted that the pro-inflammatory mediators, including interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α, are co-related factors involved in both systemic inflammation and neuroinflammation and affecting their sustainment and convergence. In contrast, intracellular enzymes, such as lysosomal cathepsins, mediate the production of pro-inflammatory mediators from both the periphery and brain.

Systemic inflammatory signals caused by systemic disorders are known to strongly influence neuroinflammation as a consequence of microglial activation, inflammatory mediator production, and the recruitment of peripheral immune cells to the brain, resulting in neuronal dysfunction. However, the neuroinflammation-accelerated neuronal dysfunction in AD also influences the functions of peripheral organs.

In the present review, we highlight the link between systemic inflammatory disorders and AD, with inflammation serving as the common explosion. We discuss the molecular mechanisms that govern the crosstalk between systemic inflammation and neuroinflammation. In our view, inflammation spreading indicates a negative spiral between systemic diseases and AD. Therefore, "dampening inflammation" may be a novel therapeutic approach for delaying the onset of and enacting early intervention for AD.

Chronic administration of D-galactose induces brain aging and accelerates artificial senescence which is used for different anti-aging pharmacological research. D-galactose is a monosaccharide, which exists throughout the body. At higher concentrations, in the presence of galactose oxidase, it converts to hydrogen peroxide and aldose, causing disposition of a superoxide anion, oxygen-derived free radicals, and cellular damage. Chronic administration of d-galactose for 2 months induces cognitive and memory impairment through the accumulated reactive oxygen species (ROS), mitochondrial deficits, neuroinflammation, and neurodegeneration.

Recently, the use of phytonutrients and medicinal herbs have gained a special interest to treat neurological disorders such as Alzheimer's disease. Among the phytonutrients, Fisetin, a natural flavonoid is found in different fruits. Fisetin has shown strong anticarcinogenic, anti-inflammatory, antioxidant, neurotrophic, and neuroprotective effects against different neurodegenerative diseases.

Here, we explore the underlying neuroprotective mechanism of fisetin against d-galactose-induced aging in mice. Normal mice were injected with d-galactose (100 mg/kg/day for 60 days) and fisetin (20 mg/kg/day for 30 days). To elucidate the protective effects of fisetin against d-galactose induced oxidative stress-mediated neuroinflammation, we conducted western blotting, biochemical, behavioral, and immunofluorescence analyses.

According to our findings, d-galactose induced oxidative stress, neuroinflammation, synaptic dysfunctions, and cognitive impairment. Conversely, fisetin prevented the d-galactose-mediated ROS accumulation, by regulating the endogenous anti-oxidant mechanisms, such as Sirt1/Nrf2 signaling, suppressed the activated phosphorylated-JNK/NF-kB pathway, and its downstream targets, such as inflammatory cytokines. Hence, our results together with the previous reports suggest that fisetin may be beneficial in age-related neurological disorders.

Dysregulation of tissue-specific gene expression programs is a hallmark of cancer. Such dysregulation leads to cancer-promoting gene expression programs and, in various types of cancer, the consequent reprogramming of cells into those with stem or progenitor (stem/progenitor) properties. A study of cancer initiation revealed that normal differentiated cells with oncogenic mutations remain in a nonmalignant state until they undergo cellular reprogramming into a stem/progenitor state. This suggests that differentiated cells have an inherent resistance mechanism against malignant transformation and indicates that cellular reprogramming is indispensable for malignancy. Thus, we speculated that malignant properties might be eradicated if the tissue-specific gene expression program is reinstated.

In colorectal cancer, cellular differentiation is impeded through processes involving both oncogenic mutations and microenvironmental alterations. This cancer provides a model for exploring whether the malignant cells could be converted to normal-like cells through restoration of the tissue-specific gene expression program. To address this challenge in a systematic way, we employed a computational framework to identify the core factors to revert cancer cells back to their normal state. A recent computational framework for inferring gene regulatory networks (GRN) has effectively applied to the cell fate conversion study through identification of master regulators of tissue-specific gene expression programs.

Here, we reconstructed normal colon-specific GRNs and colorectal cancer-specific GRNs, and identified core transcription factors (TF) for differentiation of colorectal cancer cells. We further identified SET Domain Bifurcated 1 (SETDB1) as a key factor that hinders the function of core TFs. We demonstrated that SETDB1 depletion effectively reestablishes the normal colon-specific gene expression profile and induces a postmitotic differentiated state in three stem-like colorectal cancer cell lines and patient-derived colon cancer organoids by recapitulating the transcriptional activities of the core TFs.

Longevity medicine is advanced personalised preventive medicine powered by deep biomarkers of aging and longevity, and is a fast-emerging field. The field encompasses the likewise rapidly evolving areas of biogerontology, geroscience, and precision, preventive, and functional medicine. With modern advances in artificial intelligence and machine learning, biomarker research and drug development have produced numerous tools for early diagnostics and prevention of communicable and non-communicable diseases, which remain largely unknown to the global medical community.

The notion of longevity and healthy aging as a major priority for healthcare will undoubtedly substantially impact primary, secondary, and tertiary prevention. Therefore, it is essential that practicing doctors have access to the appropriate education through a credible curriculum in longevity medicine.

The development of longevity-focused medical practices greatly depends on bridging the gap between health-care providers and interdisciplinary experts, such as academic biogerontologists, artificial intelligence experts, computer scientists, and informaticians. Health-care providers require customised courses on the most recent advances in longevity medicine and on how this knowledge can be implemented in the practice. Patients have insufficient access to the health-care providers who have been adequately trained in longevity medicine and can manage a patient from a longevity medicine standpoint. Viable longevity education with practical translation will thus ultimately improve health-care systems worldwide and decrease disease occurrence by training health-care providers to tackle the most common and strongest contributor of disease - unhealthy aging.

Aging is the greatest risk factor for most acute and chronic diseases. Previous decades have shown that we are now on the cusp of being able to intervene in the aging process, probably allowing us to decrease overall mortality and morbidity rates in elderly individuals. Although this progress has mostly occurred at the academic level, there is now a great need for expanding this knowledge into the realm of clinical practice. With this Comment, we hope to encourage this necessary step towards implementation of longevity education for health-care providers worldwide.

Hormones produced by the thyroid gland are essential regulators of organ function. The absence of these hormones either through thyroid dysfunction due to, for example, irradiation, thyroid cancer, autoimmune disease, or thyroidectomy leads to symptoms like fatigue, feeling cold, constipation, and weight gain. Although hypothyroidism can be treated by hormone replacement therapy, some patients have persistent symptoms and/or experience side effects. To investigate potential alternatives to present treatment strategies for these patients, researchers have now for the first time succeeded in generating thyroid mini-organs in the lab.

In a new study researchers used healthy thyroid tissue from patients undergoing surgical removal of the thyroid to grow mini-thyroid organs in a lab which resembled thyroid glands in their structure and protein content. The thyroid mini-organs contained stem cells which re-grew and formed new mini-organs when the structures were dissociated, providing a potentially unlimited source of lab-grown thyroid tissue. Importantly, the thyroid mini-organs could be matured and produced thyroid hormones in the cultures.

Preliminary proof that these structures could potentially replace thyroid tissue came from experiments in mice with hypothyroidism, where transplantation of the mini-organs increased serum levels of thyroid hormones and extended the lifespan of the animals compared to un-transplanted mice. Further studies are required, however the study lays the foundation for generating thyroid mini-organs from surgically removed tissue and may potentially lead to a new therapy for hypothyroidism in the future.

The blood transcriptome is expected to provide a detailed picture of an organism's physiological state with potential outcomes for applications in medical diagnostics and molecular and epidemiological research. We here present the analysis of blood specimens of 3,388 adult individuals, together with phenotype characteristics such as disease history, medication status, lifestyle factors, and body mass index (BMI). The size and heterogeneity of this data challenges analytics in terms of dimension reduction, knowledge mining, feature extraction, and data integration.

Self-organizing maps (SOM)-machine learning was applied to study transcriptional states on a population-wide scale. This method permits a detailed description and visualization of the molecular heterogeneity of transcriptomes and of their association with different phenotypic features.

The diversity of transcriptomes is described by personalized SOM-portraits, which specify the samples in terms of modules of co-expressed genes of different functional context. We identified two major blood transcriptome types where type 1 was found more in men, the elderly, and overweight people and it upregulated genes associated with inflammation and increased heme metabolism, while type 2 was predominantly found in women, younger, and normal weight participants and it was associated with activated immune responses, transcriptional, ribosomal, mitochondrial, and telomere-maintenance cell-functions. We find a striking overlap of signatures shared by multiple diseases, aging, and obesity driven by an underlying common pattern, which was associated with the immune response and the increase of inflammatory processes.

As we age, the risk that we will experience chronic diseases (for example, heart disease, diabetes, and cancer) and declining capacities (for example, reduced strength, impaired hearing, and poorer memory) increases. All individuals age chronologically at the same rate, but there is marked variation in their rate of biological aging; this may help explain why some adults experience age-related decline faster than others.

Biological aging can be defined as decline that (1) simultaneously involves multiple organ systems and (2) is gradual and progressive5. Across the lifespan, the consequences of individual differences in genetic endowment, cellular biology, and life experiences accumulate, driving the divergence of biological age from chronological age for some people. Among older adults of the same chronological age, those with accelerated biological aging (as measured by blood and DNA methylation biomarkers) are more likely to develop heart disease, diabetes, and cancer and have a higher rate of cognitive decline, disability, and mortality.

Current disease-management strategies usually treat and manage each age-related chronic disease independently. In contrast, the geroscience hypothesis proposes that many age-related chronic diseases could be prevented by slowing biological aging itself. The geroscience hypothesis states that biological aging drives cellular-level deterioration across all organ systems, thereby causing the exponential rise in multimorbidity across the second half of the lifespan. The implication is that by slowing biological aging directly, instead of managing each disease separately, the risk for all chronic age-related diseases could be simultaneously ameliorated.

We measured biological aging in a population-representative 1972-1973 birth cohort of 1,037 individuals followed from birth to age 45 years in 2019 with 94% retention: the Dunedin Study. Over 20 years - at ages 26, 32, 38 and 45 - we repeatedly collected 19 biomarkers to assess changes in the function of cardiovascular, metabolic, renal, immune, dental, and pulmonary systems, and quantified age-related decline shared among these systems. We call this index of biological aging in the Dunedin Study the 'Pace of Aging'.

At age 45 in 2019, participants with faster Pace of Aging had more cognitive difficulties, signs of advanced brain aging, diminished sensory-motor functions, older appearances, and more pessimistic perceptions of aging. People who are aging more rapidly than same-age peers in midlife may prematurely need supports to sustain independence that are usually reserved for older adults. Chronological age does not adequately identify need for such supports.

Great strides have been made in advancing extracellular vesicles (EVs) to clinical testing. By late 2020, approximately 250 trials that utilize EVs in some way had been registered. Diagnostic, prognostic, and monitoring uses of EVs are evident in these registrations as well as applications of EVs in therapeutics. Interest in EVs stems in part from their biology. They are involved in natural processes of communication in the body and have a perceived safety profile that features low immunogenicity.

Additionally, EVs are 'targetable'. Display of specific proteins, and possibly other biomolecules, allows EVs to be sorted to certain cell types and tissues or away from undesired recipients. EV engineering, by manipulating the EV source or by altering EVs post-production, can be used to enhance such targeting. Modified EVs have been used for some time as delivery vehicles for small molecule drugs and natural products, short hairpin RNA (shRNA), short interfering RNA (siRNA), plasmid DNA, and microRNAs. However, a key factor in the success of this and other EV therapies is whether and how EVs can be targeted to, or away from, specific cells.

Targeting EVs to specific cell types could indeed be considered a holy grail of EV therapeutics, since cell specificity reduces the necessary dose and minimizes off-target effects. However, we should be clear that the word 'targeting' is used colloquially. The typical EV cannot move towards a destination as a result of interpreting signals, for example, by crawling along a chemical gradient, so the EV cannot truly 'home' to a specific cell. Instead, the word 'targeting' refers more accurately to 'selective retention' or 'capture' by the target cell.

To the extent possible, administering EVs at the site of intended action will enhance selective retention and help to avoid clearance. Many studies use intravenous delivery of EVs, but this results in delivery predominantly to just a few organs, especially lung and liver, as well as bone marrow, spleen, and kidney. Introducing EVs by different routes, or even directly to the target site by application (e.g. skin wound healing, eye) or tissue injection avoids rapid clearance and maximizes dosage.

The first and the most studied manipulation shown to increase lifespan in mammals is caloric restriction (CR). Numerous laboratories have shown that reducing food consumption by 30% to 50% (without malnutrition) consistently increases both the mean and maximum lifespans of both laboratory rats and mice. The effect of CR on longevity is not limited to rodents as CR has been shown to increase the lifespan of a large number of diverse animal models ranging from invertebrates (yeast, C. elegans, and Drosophila) to dogs and non-human primates.

Because the gastrointestinal (GI) system is the first organ/tissue that encounters the impact of reduced food consumption, there have been several studies on the effect of CR on the GI-system. With the advent of metagenomics, it is now possible to interrogate the colon microbiome and study the effect of CR. Two groups have reported that long-term CR had a significant impact on the microbiome of old mice. These studies were conducted with aging colonies of mice specific to those particular laboratories and for which there were no lifespan data. Because the institutional animal husbandry environment and the health status of the host can have a major impact on the microbiome, we felt it was important to establish the effect of CR on the microbiome of well characterized mice from the aging colony maintained by National Institute on Aging (NIA).

Life-long CR increased microbial diversity and the Bacteroidetes/Firmicutes ratio and prevented the age-related changes in the microbiota, shifting it to a younger microbial and fecal metabolite profile in both C57BL/6JN and B6D2F1 mice. Old mice fed CR were enriched in the Rikenellaceae, S24-7, and Bacteroides families. The changes in the microbiome that occur with age and CR were initiated in the cecum and further modified in the colon. Short-term CR in adult mice had a minor effect on the microbiome but a major effect on the transcriptome of the colon mucosa. These data suggest that CR has a major impact on the physiological status of the gastrointestinal system, maintaining it in a more youthful state, which in turn could result in a more diverse and youthful microbiome.

The most common aging-associated diseases are cardiovascular diseases which affect 40% of elderly people. It is well accepted that the origin of aging-associated cardiovascular diseases is mitochondrial dysfunction. Mitochondria have their own genome (mtDNA) that is circular and double-stranded. There are between 500 to 6000 mtDNA copies per cell, depending on tissue type. As a by-product of ATP production, reactive oxygen species (ROS) are generated which damage proteins, lipids, and mtDNA.

ROS-mutated mtDNA co-existing with wild type mtDNA is called mtDNA heteroplasmy. The progressive increase in mtDNA heteroplasmy causes progressive mitochondrial dysfunction leading to a loss in their bioenergetic capacity, disruption in the balance of mitochondrial fusion and fission events (mitochondrial dynamics, MtDy) and decreased mitophagy. This failure in mitochondrial physiology leads to the accumulation of depolarized and ROS-generating mitochondria. Thus, besides attenuated ATP production, dysfunctional mitochondria interfere with proper cellular metabolism and signaling pathways in cardiac cells, contributing to the development of aging-associated cardiovascular diseases.

In this context, there is a growing interest to enhance mitochondrial function by decreasing mtDNA heteroplasmy. Reduction in mtDNA heteroplasmy is associated with increased mitophagy, proper MtDy balance and mitochondrial biogenesis; and those processes can delay the onset or progression of cardiovascular diseases. This has led to the development of mitochondrial therapies based on the application of nutritional, pharmacological, and genetic treatments, seeking to have a positive impact on mtDNA integrity, mitochondrial biogenesis, dynamics, and mitophagy in old and sick hearts.

Head injuries did not appear to contribute to brain damage characteristic of Alzheimer's disease, but might make people more vulnerable to dementia symptoms, according to new findings. "Here we found compelling evidence that head injuries in early or mid-life can have a small but significant impact on brain health and thinking skills in the long term. It might be that a head injury makes the brain more vulnerable to, or accelerates, the normal brain ageing process."

The study involved 502 participants of the UK's longest-running cohort study, the MRC National Survey of Health and Development Cohort, which has been following participants since their birth in the same week in 1946. At age 53, they were asked 'Have you ever been knocked unconscious?' to assess whether they had ever suffered a substantial head injury; 21% of their sample had answered yes to this question. And then around age 70 (69-71), the study participants underwent brain scans (PET/MRI), and they took a suite of cognitive tests.

The participants had all completed standardised cognitive tests at age eight, so the researchers were able to compare their results at age 70 with expected results based on their childhood cognition and other factors such as educational attainment and socioeconomic status. The researchers found that 70-year-olds who had experienced a serious head injury more than 15 years earlier performed slightly worse than expected on cognitive tests for attention and quick thinking (a difference of two points, scoring 46 versus 48 on a 93-point scale). They also had smaller brain volumes (by 1%) and differences in brain microstructural integrity, in line with evidence from previous studies, which may explain the subtle cognitive differences.

The researchers did not find any differences in levels of the amyloid protein, implicated in Alzheimer's disease, or other signs of Alzheimer's-related damage. "It looks like head injuries can make our brains more vulnerable to the normal effects of ageing. We have not found evidence that a head injury would cause dementia, but it could exacerbate or accelerate some dementia symptoms."

Let us imagine that Gustav Embden (1874-1933), one of the ingenious discoverers of glycolysis, would have had modern transgenic techniques at hand and intended to use them to investigate the role of phosphoglycerate kinase (PGK) in the biochemical degradation of glucose to pyruvate. He would have probably overexpressed the enzyme 10x first, and he would not have seen any relevant change in the rate of pyruvate formation in the perfused working heart, whereas the addition of insulin would have shown a clear effect. He then would have generated 90% knockdown animals and again would not have seen any decrease in the rate of glycolysis. Hence, he would have confidently concluded that PGK was not involved in glycolysis. Thus, he would have arrived at an overtly wrong conclusion (merely hypothetical; sorry, Gustav!).

In essence, this is what we do today when we conclude from unsuccessful overexpression or knockdown studies of antioxidant enzymes that free radicals were not involved in aging. We arrive at a wrong conclusion.

What is the mistake here, and what did Embden and his successors do better? First, they looked at the intrinsic chemical logic of the overall system. This should also be done in the study of aging. In particular, they recognized that steps can be involved and essential in a causal chain of (chemical) events even without ever being rate-limiting (or "flux-controlling") for the overall passage through the chain of events. This principle applies to linear chains, branched chains, branching-converging chains and even cyclic chains. Because aging certainly represents an arrangement of causally chained elementary steps (of whatever type and complexity), the decisive point will be to identify the flux-controlling steps of aging as narrowly as possible and then determine their control coefficients for the overall process.

Thus, the only thing we can learn from the fact that superoxide dismutase (SOD) modulation does not influence lifespan is that superoxide degradation is not flux-controlling for aging (in mice). This is still a valuable conclusion, even if it may not be particularly surprising: flux control is usually exerted by low-level, low-efficiency, or highly regulated enzymes, none of which applies to SOD. Moreover, if simple overexpression of SOD indeed would have had a measurable effect on lifespan, one might wonder why evolution has not yielded such a parsimonious solution before. Hence, it is quite unlikely that any isolated enzyme overexpression approach will ever substantially extend life in a species in which longevity is under positive selective pressure (like, arguably, in mammals). Extensive data support this generalization. We have to grab for higher-hanging fruit.

Researchers introduced the "Tripartite Phenotype of Aging" as a new conceptual model that addresses why lifespan varies so much, even among human identical twins who share the same genes. Only about 10 to 35 percent of longevity can be traced to genes inherited from our parents. Researchers propose that the limited heritability of aging patterns and longevity in humans is an outcome of gene-environment interactions, together with stochastic, or chance, variations in the body's cells. These random changes can include cellular changes that happen during development, molecular damage that occurs later in life, and more.

The new model is a natural extension of the idea of the exposome, which was first proposed in 2005 to draw attention to the need for more data on lifetime exposure to environmental carcinogens. The exposome concept illustrates how external factors, ranging from air pollution and socioeconomic status to individual diet and exercise patterns, interact with endogenous, or internal, factors such as the body's microbiome and fat deposits.

The new model illustrates that cell-by-cell variations in gene expression, variations arising during development, random mutations, and epigenetic changes - turning genes "off" or "on" - should be explicitly considered apart from traditional genetic or environmental research regarding aging. More detailed study into these chance processes has been enabled by cutting-edge research techniques, including the study of gene transcription within single cells as well as ChIP-sequencing, which can illustrate how individual proteins interact with DNA.

The researchers offer several examples of how risks of age-related disease are poorly predicted by DNA alone but are heavily influenced by environmental exposures as well as the time and duration of the exposure, including during development or over the course of decades. One well-known example of a gene that is associated with increased Alzheimer's risk is ApoE-4; however, having the ApoE-4 gene doesn't definitively mean someone will get Alzheimer's. Studies in both mice and humans revealed that ApoE-4 and clusters of related genes interact with exposures such as air pollution or cigarette smoke to influence risk, and Alzheimer's patients also show differences in their epigenetics as compared to individuals without the disease.