The discovery that single gene manipulations can significantly modulate longevity is arguably the major breakthrough in biogerontology thus far. Genetic manipulations of aging in mice are crucial to gather insights into the underlying mechanisms of aging, to discover pathways modulating longevity and to identify candidate genes for drug discovery. Moreover, the manipulation of the aging process in mammalian models (particularly mice) via genetic manipulation (gene knockouts, overexpression, etc.) is crucial to test mechanistic hypotheses of aging. However, determining if such genetic interventions actually affect the aging process and not some others factor of health is not always straightforward.

For example, should a genetic intervention reduce an organism's resistance to disease, this could conceivably reduce the lifespan of the organism, although the rate of aging would not have been affected. Differentiating between genetic interventions that affect the lifespan of an organism through altered health as opposed to changes in the rate of aging is therefore essential to gain insights on aging, and determine interventions with wide ranging effects.

There are two fundamental methods to determine if a life-extending genetic intervention has altered the rate of aging rather than general health. One can track the onset and progression of age-related ailments and physiological degeneration to determine if there is a shift in the onset and on progression of the ailment. In addition, efforts have been made to quantify aging rates with mathematical models such as the Gompertz law of mortality. From the Gompertz parameters, the mortality rate doubling time (MRDT) can be calculated. The MRDT is the amount of time it takes for the mortality rate to double for a given cohort.

A change in MRDT indicates a change in the demographic rate of aging, which is not a perfect reflection of biological aging but a metric that correlates with physiological deterioration and health. Although some studies have investigated MRDT, many authors still often assume that changes in the lifespan of mice following a genetic intervention directly equates to changes in the rate of aging, leading to the misrepresentation of certain genes as having a causal role in aging, when in reality they do not.

Many studies have reported altered median and/or maximum lifespan as a result of an intervention but lifespan alterations may have a number of causes, including altered age at onset of senescence and age-independent mortality. To address this lack of distinction, we previously used linear regression to fit the Gompertz model to longevity data from published mouse studies, and statistically compared the rates of aging in these cohorts. For example, we showed that caloric restriction increases the MRDT and thus retards the demographic rate of aging. Here, the same methodology was employed to reassess mouse longevity data published since 2005 and to identify which genes are more important in determining the demographic rate of aging.

Overall, only 7 of 54 genes were found to have a statistically significant effect on the demographic rate of aging as expected from longevity manipulations. These results suggest that only a relatively small proportion of interventions reported to affect longevity in mice do so through directly influencing the demographic rate of aging. Surprisingly, 20 of 54 genes had a statistically significant impact on the demographic rate of aging in the opposite direction than would be expected for the published longevity effects. One possible explanation is that many mutations impacted on various parameters affecting longevity in non-linear ways, and indeed we observed that increases in aging independent mortality correlated with a slower demographic aging rate. For instance, Sirt1 deficiency extended lifespan but increased the demographic rate of aging; its effect appeared to be exerted instead by delaying the age of onset of mortality rate escalation. This highlights the complex relationship between lifespan and the demographic rate of aging.

Another caveat of our approach concerns the number of mice used in some of the original studies, which ranged from 10 to 146 animals per cohort. Whilst research reported here has attempted to compensate for this by using the Gompertz equation which allows for small sample sizes, one cannot escape the low statistical power that accompanies such small sample sizes. Interestingly, caloric restriction has been shown to significantly retard the demographic rate of aging, but this was a large study with over 200 animals in total. Therefore, caution must be taken when interpreting some of the results detailed here from studies with small sample sizes. Indeed, we observed that in smaller experimental cohorts subjective decisions in estimating Gompertz parameters can significantly affect the results.

Our main conclusions are: 1) most genetic manipulations of longevity in mice do so by modulating aging-independent mortality; 2) there is substantial variation in the lifespan of controls of the same strain across experiments; 3) studies in which the lifespan of the controls is short have a greater lifespan increase, emphasizing the importance of having adequate control groups; 4) mouse lifespan studies employing small cohorts can yield unreliable results; 5) lifespan-reducing experiments tend to be noisier and more difficult to analyze for demographic parameters than life-extending experiments; 6) a greater aging-independent mortality is usually accompanied by a slower demographic aging rate.

Cellular senescence is a cell fate program that entails essentially irreversible proliferative arrest in response to damage signals. Tumor necrosis factor-alpha (TNFα), an important pro-inflammatory cytokine secreted by some types of senescent cells, can induce senescence in mouse and human cells. However, downstream signaling pathways linking TNFα-related inflammation to senescence are not fully characterized. Using human umbilical vein endothelial cells (HUVECs) as a model, we show that TNFα induces permanent growth arrest and increases p21CIP1, p16INK4A, and SA-β-gal, accompanied by persistent DNA damage and ROS production. By gene expression profiling, we identified the crucial involvement of inflammatory and JAK/STAT pathways in TNFα-mediated senescence. We found that TNFα activates a STAT-dependent autocrine loop that sustains cytokine secretion and an interferon signature to lock cells into senescence.

Cellular senescence has been viewed as a tumor suppression mechanism and also as a contributor to individual aging. Widespread shortening of 3′ untranslated regions (3′ UTRs) in messenger RNAs (mRNAs) by alternative polyadenylation (APA) has recently been discovered in cancer cells. However, the role of APA in the process of cellular senescence remains elusive. Here, we found that hundreds of genes in senescent cells tended to use distal poly(A) (pA) sites, leading to a global lengthening of 3′ UTRs and reduced gene expression. Genes that harbor longer 3′ UTRs in senescent cells were enriched in senescence-related pathways. Rras2, a member of the Ras superfamily that participates in multiple signal transduction pathways, preferred longer 3′ UTR usage and exhibited decreased expression in senescent cells. Depletion of Rras2 promoted senescence, while rescue of Rras2 reversed senescence-associated phenotypes.

Oncogenic signals lead to premature senescence in normal human cells causing a proliferation arrest and the elimination of these defective cells by immune cells. Oncogene-induced senescence (OIS) prevents aberrant cell division and tumor initiation. In order to identify new regulators of OIS, we performed a loss-of-function genetic screen and identified that the loss of SCN9A allowed cells to escape from OIS. The expression of this sodium channel increased in senescent cells during OIS. This upregulation was mediated by NF-κB transcription factors, which are well-known regulators of senescence. Importantly, the induction of SCN9A by an oncogenic signal or by p53 activation led to plasma membrane depolarization, which in turn, was able to induce premature senescence. Computational and experimental analyses revealed that SCN9A and plasma membrane depolarization mediated the repression of mitotic genes through a calcium/Rb/E2F pathway to promote senescence.

A relevant feature of aging is chronic low-grade inflammation, termed inflammaging, a key process promoting the development of all major age-related diseases. Senescent cells can acquire the senescence-associated (SA) secretory phenotype (SASP), characterized by the secretion of proinflammatory factors fuelling inflammaging. Cellular senescence is also accompanied by a deep reshaping of microRNA expression and by the modulation of mitochondrial activity, both master regulators of the SASP. Here, we synthesize novel findings regarding the role of mitochondria in the SASP and in the inflammaging process and propose a network linking nuclear-encoded SA-miRNAs to mitochondrial gene regulation and function in aging cells. In this conceptual structure, SA-miRNAs can translocate to mitochondria (SA-mitomiRs) and may affect the energetic, oxidative, and inflammatory status of senescent cells.

During aging, the number of neural stem/progenitor cells (NPCs), and subsequently neurogenesis, precipitously declines in the subgranular zone of the dentate gyrus (DG) in the hippocampus. Mounting evidence in animal models indicates the potential for rejuvenation of regenerative and cognitive functions in the aging brain through interventions, such as heterochronic parabiosis (which exposes aged animals to young blood). However, the ability to utilize this neurogenic potential is predicated on identifying molecular targets that reverse the effects of aging in the brain.

Recent studies have begun to link changes in the functions of epigenetic mediators to age-related regenerative decline. Interestingly ten eleven translocation methylcytosine dioxygenase 2 (Tet2) is emerging as a potential epigenetic regulator of aging. Human genetic studies identified an increased frequency of somatic TET2 mutations with age that are associated with elevated risk for aging-associated disorders, such as cancer, cardiovascular disease, and stroke. Notwithstanding, the involvement of Tet2 in mediating the aging process in the adult brain has yet to be investigated.

Here we demonstrate that Tet2 offsets age-related neurogenic decline and enhances cognition in the hippocampus of adult mice. We detect an age-dependent decrease in the levels of Tet2 in the aging hippocampus coincident with decreased adult neurogenesis. Mimicking an age-related loss of Tet2 in the adult neurogenic niche of the hippocampus, or adult NPCs, impairs regenerative capacity and associated hippocampal-dependent learning and memory processes. Conversely, increasing Tet2 in the hippocampus of mature animals increases restores adult neurogenesis to youthful levels and enhances cognitive function.

Recently, it has been demonstrated that constitutive whole-body loss of Tet2 yields opposing effects on neurogenic processes, resulting in increased adult NPC proliferation but decreased neuronal differentiation. In contrast, our data indicate that decreasing Tet2 expression acutely in the adult neurogenic niche impairs all stages of hippocampal neurogenesis, while loss of Tet2 in adult NPCs impairs neuronal differentiation processes. These data point to differential regulation of distinct stages of neurogenesis by Tet2 that arise from the loss of Tet2 at the level of the whole organism, neurogenic niche, and adult NSC during development versus adult ages. In the context of aging, our data implicate decreased Tet2 in the aging hippocampus with age-related regenerative decline.

It has already been established that natural ageing impairs the function of our skeletal muscles. We also know that the number and the activity of the muscles' stem cells decline with age. However, the reasons for this has not been fully understood. In a new study, researchers have investigated the number of mutations that accumulate in the muscle's stem cells (satellite cells). "What is most surprising is the high number of mutations. We have seen how a healthy 70-year-old has accumulated more than 1,000 mutations in each stem cell in the muscle, and that these mutations are not random but there are certain regions that are better protected."

The researchers have benefited from new methods to complete the study. The study was performed using single stem cells cultivated to provide sufficient DNA for whole genome sequencing. The mutations occur during natural cell division, and the regions that are protected are those that are important for the function or survival of the cells. Nonetheless, the researchers were able to identify that this protection declines with age. "We can demonstrate that this protection diminishes the older you become, indicating an impairment in the cell's capacity to repair their DNA. And this is something we should be able to influence with new drugs."

"We achieved this in the skeletal muscle tissue, which is absolutely unique. We have also found that there is very little overlap of mutations, despite the cells being located close to each other, representing an extremely complex mutational burden." The researchers will now continue their work to investigate whether physical exercise can affect the number of accumulated mutations. Is it true that physical exercise from a young age clears out cells with many mutations, or does it result in the generation of a higher number of such cells?

Satellite cells (SCs) are a heterogeneous population of stem and progenitor cells that have been demonstrated to play a pivotal role in skeletal muscle (SkM) regeneration. The SCs are normally kept in a quiescent state and activated upon exposure to stimuli, such as exercise or SkM injury. When committed to myogenic differentiation, SCs proliferate further, fuse to existing SkM fibers, and contribute new nuclei to the growing and regenerating fibers. Aged human SkMs show a decline in the number and proliferative potential of the SCs. As a consequence, a dysfunctional SC compartment is envisaged as a major contributor to age-related defects, including reduced capacity to respond to hypertrophic stimuli such as exercise and impaired recovery from muscle disuse and injury.

A well-known factor in the decline of stem cell function is the loss of genome integrity, for example, caused by the appearance of somatic mutations. These modifications of the genome range from single-base changes (single-nucleotide variants) to insertions or deletions of a few bases (indels) to chromosomal rearrangements and occur during the whole life, starting from the first division of the embryo. In contrast to germline variants, somatic variants are not propagated to the whole individual but to a subpopulation of cells in the body, with the final consequence that adult human tissues are a mosaic of genetically different cells. Moreover, somatic mutation burden increases during a lifetime as a result of accumulating errors occurring either during cell division or because of environment-induced DNA damage. At present, nothing is known about somatic mutation burden in human SCs or SkM.

Here, we investigate the genetic changes that occur with aging in the genome of human adult SCs and use the results to elucidate mutational processes and SC replication rate occurring in vivo in adult human muscles. We assess the functional effects of somatic mutations on SC proliferation and differentiation and predict the global consequence on muscle aging and sarcopenia. Our analyses reveal an accumulation of 13 mutations per genome per year that results in a 2-3-fold higher mutation load in active genes and promoters in aged SCs. High mutation burden correlates with defective SC function. Overall, our work points to the accumulation of somatic mutations as an intrinsic factor contributing to impaired muscle function with aging.

Researchers have successfully treated a cancerous tumor using a "nanofactory" - a synthetic cell that produces anti-cancer proteins within the tumor tissue. The research combines synthetic biology, to artificially produce proteins, and targeted drug delivery, to direct the synthetic cell to abnormal tissues. The synthetic cells are artificial systems with capacities similar to, and, at times, superior to those of natural cells. Just as human cells can generate a variety of biological molecules, the synthetic cell can produce a wide range of proteins. Such systems bear vast potential in the tissue engineering discipline, in production of artificial organs and in studying the origins of life. Design of artificial cells is a considerably complex engineering challenge being pursued by many research groups across the globe.

The researchers integrated molecular machines within lipid-based particles resembling the natural membrane of biological cells. They engineered the particles such that when they "sense" the biological tissue, they are activated and produce therapeutic proteins, dictated by an integrated synthetic DNA template. The particles recruit the energy sources and building blocks necessary for their continued activity, from the external microenvironment (e.g., the tumor tissue).

After experiments in cell cultures in the lab, the novel technology was also tested in mice. When the engineered particles reached the tumor, they produced a protein that eradicated the cancer cells. The particles and their activity were monitored using a green fluorescent protein (GFP), generated by the particles. This protein can be viewed in real-time, using a fluorescence microscope. "By coding the integrated DNA template, the particles we developed can produce a variety of protein medicines. They are modular, meaning they allow for activation of protein production in accordance with the environmental conditions. Therefore, the artificial cells we've developed may take an important part in the personalized medicine trend - adjustment of treatment to the genetic and medical profile of a specific patient."

Induced pluripotent stem cells (iPSCs) avoid many of the restrictions that hamper the application of human embryonic stem cells, and the donor's clinical phenotype is often known when working with iPSCs. Therefore, iPSCs seem ideal to tackle the two biggest tasks of regenerative medicine: degenerative diseases with genetic cause (e.g., Duchenne's muscular dystrophy) and organ replacement in age-related diseases (e.g., end-stage heart or renal failure), especially in combination with recently developed gene-editing tools.

In the setting of autologous transplantation in elderly patients, donor age becomes a potentially relevant factor that needs to be assessed. Here, we review and critically discuss available data pertinent to the questions: How does donor age influence the reprogramming process and iPSC functionality? Would it even be possible to reprogram senescent somatic cells? How does donor age affect iPSC differentiation into specialised cells and their functionality? We also identify research needs, which might help resolve current unknowns.

Until recently, most hallmarks of ageing were attributed to an accumulation of DNA damage over time, and it was thus expected that DNA damage from a somatic cell would accumulate in iPSCs and the cells derived from them. In line with this, a decreased lifespan of cloned organisms compared with the donor was also observed in early cloning experiments. Therefore, it was questioned for a time whether iPSC derived from an old individual's somatic cells would suffer from early senescence and, thus, may not be a viable option either for disease modelling nor future clinical applications. Instead, typical signs of cellular ageing are reverted in the process of iPSC reprogramming, and iPSCs from older donors do not show diminished differentiation potential nor do iPSC-derived cells from older donors suffer early senescence or show functional impairments when compared with those from younger donors.

Thus, the data would suggest that donor age does not limit iPSC application for modelling genetic diseases nor regenerative therapies. However, open questions remain, e.g., regarding the potential tumourigenicity of iPSC-derived cells and the impact of epigenetic pattern retention.

The decline of cognitive function occurs with aging, but the mechanisms responsible are unknown. Astrocytes instruct the formation, maturation, and elimination of synapses, and impairment of these functions has been implicated in many diseases. These findings raise the question of whether astrocyte dysfunction could contribute to cognitive decline in aging. We performed RNA sequencing of astrocytes from different brain regions across the lifespan of the mouse. We found that astrocytes have region-specific transcriptional identities that change with age in a region-dependent manner.

Detailed analysis of the differentially expressed genes in aging revealed that aged astrocytes take on a reactive phenotype of neuroinflammatory A1-like reactive astrocytes. Hippocampal and striatal astrocytes up-regulated a greater number of reactive astrocyte genes compared with cortical astrocytes. Moreover, aged brains formed many more A1 reactive astrocytes in response to the neuroinflammation inducer lipopolysaccharide.

We found that the aging-induced up-regulation of reactive astrocyte genes was significantly reduced in mice lacking the microglial-secreted cytokines (IL-1α, TNF, and C1q) known to induce A1 reactive astrocyte formation, indicating that microglia promote astrocyte activation in aging. Since A1 reactive astrocytes lose the ability to carry out their normal functions, produce complement components, and release a toxic factor which kills neurons and oligodendrocytes, the aging-induced up-regulation of reactive genes by astrocytes could contribute to the cognitive decline in vulnerable brain regions in normal aging and contribute to the greater vulnerability of the aged brain to injury.

Biomedical engineers are growing tracheas by coaxing cells to form three distinct tissue types after assembling them into a tube structure - without relying on scaffolding strategies currently being investigated by other groups. "The unique approach we are taking to this problem of trachea damage or loss is forming tissue modules using a patient's cells and assembling them into a more complex tissue." Recent tissue engineering approaches using synthetic or natural materials as scaffolding for cells have met with challenges. Difficulties have included uniformly seeding cells on the scaffolding, recreating the multiple different tissue types found in the native trachea, tailoring the scaffolding degradation rate to equal the rate of new tissue formation, and recreating important contacts between cells because of the intervening scaffold.

The trachea engineering strategy now being pursued, however, wouldn't have those problems because it doesn't rely on a separate scaffold structure. A new trachea replacement must do three critical things to function properly: (1) maintain rigidity to prevent airway collapse when the patient breathes; (2) contain immunoprotective respiratory epithelium, the tissue lining the respiratory tract, which moistens and protects the airway and functions as a barrier to potential pathogens and foreign particles; and (3) integrate with the host vasculature, or system of blood vessels, to support epithelium viability.

The self-assembling rings developed by researchers meet all three of those requirements because they can fuse together to form tubes of both cartilage and "prevascular" tissue types. Prevascular refers to tissues potentially ready to participate in the formation of blood vessels, though not yet functional in that way. The cartilage rings are formed by aggregating marrow-derived-stem cells in ring-shaped wells. Polymer microspheres containing a protein that induces the stem cells to become "chondrocytes," or cells that form cartilage, are also incorporated into the cell aggregates. These prevascular rings are comprised of both these marrow-derived stem cells and endothelial cells, the thin layer of cells that line the interior of blood vessels.

The researchers then coat the tubes with epithelial cells to form multi-tissue constructs that satisfy all of those requirements: cartilage provides rigidity, epithelium serves the role of immunoprotection and the vascular network would ultimately permit blood flow to feed and integrate the new trachea tissue. Using this method, the team has been able to engineer highly elastic "neo-tracheas" of various sizes, including tissues similar to human trachea. When these tracheas were implanted under the skin in mice, there was evidence the prevascular structures could join up with the host vascular supply.

The hallmarks of Alzheimer's disease (AD) pathology are marked by accumulation of extracellular amyloid-β (Aβ) plaques in the brain followed by intracellular neurofibrillary tangle (NFT) growth. Aβ upregulates the generation of NFTs by increasing glycogen synthase kinase-3 (GSK-3) activity, leading to the phosphorylation of tau. Phosphorylated tau (pTau) begins to self-assemble to form NFTs. Aβ plaques, soluble Aβ oligomers, and NFTs interfere with normal neuronal cell function by disrupting synaptic signaling. Each protein's accumulation leads to neuron damage, eliciting diminished brain mass and cognitive function.

The removal of Aβ plaques does not influence elimination of NFTs after NFTs have been established in the brain, but early intervention can prevent pTau development. Therefore, targeted late stage treatments may specifically eliminate Aβ without impacting pTau levels that have already accumulated, which enables NFTs to continue amplifying cognitive deficits. Comparison of differences in pTau and Aβ levels in treated mice illuminate differences between the proteins' impact on cognitive function. For example, pTau levels were reduced by chemical treatment as Aβ levels continued to increase, yet cognitive function improved. This result implies that there is a quantitative difference between how the two proteins effect cognitive deterioration, and moreover, that decreasing pTau may ultimately be more important than reducing Aβ in the quest to successfully treat AD.

The Amyloid Cascade Hypothesis states that Aβ is the center piece in AD pathology leading to hyperphosphorylation of tau and numerous neurotoxic pathways causing cell death. Treatments targeting Aβ and Aβ precursors have failed to pass clinical trials to improve patient outcomes. The presence of Aβ is associated with a decrease in cognitive performance; however, the quantitative level of Aβ inconsistently predicts the amount of cognitive decline. Instead, it is suggested that other contributors, such as the hyperphosphorylation of tau, are the functional cause of degeneration after the initial onset of AD.

The present study compares the effects of Aβ and pTau levels on cognitive performance in the Morris water maze (MWM) and Novel Object Recognition (NOR) through a large-scale meta-analysis of 3xTg-AD mouse model experiments. The triple-transgenic mouse model (3xTg-AD) of AD expresses tangle and plaque pathology as well as synaptic dysfunction. Multiple linear regression confirmed pTau is a stronger predictor of MWM performance than Aβ. Despite pTau's lower physical concentration than Aβ, pTau levels more directly and quantitatively correlate with 3xTg-AD cognitive decline.

Physicists have devised a new method of investigating brain function. This new non-invasive technique could potentially be used for any diagnosis based on cardiovascular and metabolic-related diseases of the brain. The researchers deciphered oscillations in the cerebrospinal fluid which lies between the scalp and skull; a device for non-invasive recordings of this translucent fluid was developed and recordings on healthy subjects were made.

It has been shown that the circulation throughout the brain of this fluid is highly fluctuating, and that these fluctuations are slow but interconnected by the rhythms of breathing and the heart rate. Researchers found that some of these oscillations are linked with blood pressure, but are generally slower, occurring at lower frequencies, which have been shown in previous studies to be related to oscillations in vascular motion and blood oxygenation.

Preliminary results showed evidence of a decline in the coherence between these oscillations in participants over the age of 25, indicating that brain ageing may begin earlier than expected. "Combining the technique to noninvasively record the fluctuation corresponding to cerebrospinal fluid and our sophisticated methods to analyse oscillations which are not clock-like but rather vary in time around their natural values, we have come to an interesting and non-invasive method that can be used to study ageing."

A mainstay of preventive medicine innovators and medical futurists has been the concept of longevity escape velocity (LEV). LEV represents the time at which someone is gaining greater than 1 year of predicted healthy life expectancy per year, essentially making his or her healthy life expectancy unlimited. But practically, what are the likely requirements to reach such a "longevity escape velocity"? Around 95% of medical service budgets today are spent on acute medicine, with only around 5% on preventive care. How can today's medicine adapt to bring around a care system that provides LEV on a population scale?

To help patients achieve and maintain LEV, medical knowledge is required from diverse medical specialties and from outside specialties typically practiced by doctors. General practice (also known as primary care or family medicine) is the current specialty with the most similarity to LEV medicine; however it lacks in knowledge in many key areas, as well as in availability of time. An LEV medical specialty could be a subspecialty training of general practice, internal medicine, geriatrics, or clinical research. Core elements of an LEV medical specialty training would include education in prioritization of clinical problems according to magnitude and probability of clinical outcome or surrogate marker impact to a specific person's budget; understanding clinical biogerontology frameworks, including pathology based frameworks (such as the Strategies for Engineered Negligible Senescence [SENS] framework) and process based frameworks (such as the Hallmarks of Aging framework) and the associated markers and current state of therapies and clinical or research access to these.

How can LEV be measured accurately? Initial models are needed that take into account a minimum number of quality measurements across broad clinical outcomes and frameworks of aging (such as SENS and Hallmarks of Aging). Optimal ranges for clinical outcomes can be established for diverse markers and used to create an effective "biological age" for individual organs or systemic aging pathologies. Combined with current best risk prediction calculators for broad sets of diseases as well as a current annual "coefficient of baseline gain in life expectancy" due to current innovation rates, and taking into account a qualitative measure of a person's financial budget, motivation, and "LEV-related education," a client's LEV might be be determined to fall within a certain range.

Research is paramount to accelerate the generation of evidence of efficacy and safety of new measurements, therapies, and clinical pathways that are relevant to LEV. A core element of LEV medicine should be that any novel practice across any aspect of LEV medicine, be it a new annual screening panel, an off label pharmaceutical, an experimental stem cell or gene therapy, should be part of a formal registry, with all data captured and published open access, and ideally collated to a central LEV society or organization for analysis, methodological and ethical critique, and distribution to parties that may benefit. For example, what proportion of potentially useful surrogate marker or clinical outcomes data is captured, collated, and distributed from the proportion of people globally experimenting with novel therapies? It is likely under 0.01%; global standards to capture such data usefully - such as via guidelines for basic experimental protocol that doctors and patients may follow for each novel intervention, as well as systems to capture, collate, analyze, and disseminate such data - could have ensured perhaps 1000 or 10,000 times more data on all novel practices to date, providing benefits for everyone.

Due to the relevance of mitochondria to cell physiology and whole-body metabolism, a comprehensive set of adaptive quality control mechanisms is in place to ensure the preservation of mitochondrial structural and functional integrity. Mitochondrial quality control (MQC) mechanisms also allow for the dynamic modulation of organelle function and number to meet the heterogeneous energy demands of the various tissues. MQC is accomplished through a set of interrelated processes (i.e., protein folding and degradation, mitochondrial autophagy, mitochondrial fission and fusion, and mitochondrial biogenesis).

The regulation of mitochondrial content is achieved through the dynamic balance between mitochondrial biogenesis and degradation. Mitochondrial biogenesis is a multistage process finalized to producing new mitochondria upon the coordinated expression of nuclear and mtDNA-encoded genes. Mitochondrial fusion allows for mtDNA mixing within the network, thereby preventing focal accumulation of mutant mtDNA and preserving mtDNA integrity. Mitochondrial fission, instead, segregates defective or unnecessary organelles for their subsequent removal through mitophagy. The integration of mitochondrial dynamics with the selective removal of dysfunctional mitochondria, referred to as mitophagy, ensures an efficient MQC process and preserves metabolic cellular "fitness."

Derangements of the MQC axis have been described during aging and in the context of a number of disease conditions, including cancer, cardiovascular disease, diabetes, and neurodegenerative disorders. Along with mitochondrial dysfunction, chronic inflammation is a hallmark of both aging and degenerative diseases. The two phenomena may be linked to one another. Indeed, emerging evidence indicates that circulating cell-free mtDNA, one of the damage-associated molecular patterns (DAMPs), may establish a functional relationship between mitochondrial damage and systemic inflammation. mtDNA can be released into the circulation in response to cell insults. Here, it is able to induce an inflammatory response through hypomethylated CpG motifs resembling those of bacterial DNA. These regions, indeed, bind and activate membrane or cytoplasmic pattern recognition receptors (PRRs), such as the Toll-like receptor (TLR), the nucleotide-binding oligomerization domain (NOD)-like receptor (NLR), and cytosolic cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) DNA sensing system-mediated pathways.

The possible contribution of mitochondrial DAMPs to the inflammatory milieu that characterizes muscle wasting disorders has not yet been explored. However, this hypothesis is worth being pursued as it could help identify novel biological targets for the management of muscle loss. Here, we summarize the current evidence on circulating mtDNA as a trigger for age-related systemic inflammation. We first describe two candidate mechanisms generating and releasing cell-free mtDNA: (1) dysregulation of TFAM binding to mtDNA, and (2) impairment of mitophagy. Subsequently, we illustrate the pathways linking mitochondrial dysfunction with systemic inflammation during aging. Finally, we propose a role for the triad "MQC failure/cell-free mtDNA/inflammation" in two major muscle wasting disorders, sarcopenia and cachexia.

Years of research have demonstrated that existing on a calorie restricted diet can boost healthy lifespan, reducing the risk of heart attack, diabetes, and other age-related conditions. Other, more recent work has shown that calorie-restricted animals regenerate tissue more effectively following injury. "The beneficial effects of calorie restriction are at this point not really up for debate; it's quite clear. But there are all sorts of questions about the cellular and molecular basis to these benefits."

One theory has been that calorie restriction slows age-related degeneration and enables more efficient tissue function by influencing the integrity and activity of adult stem cells, the precursor cells that dwell within specific tissues and give rise to the diversity of cell types that compose that tissue. Recent studies focused on the effects of calorie restriction on the active intestinal stem cells. While these active stem cells bear the burden of daily tissue turnover and act as the workhorses of intestinal function, they are also known to be highly susceptible to DNA damage, such as that induced by radiation exposure, and thus are unlikely to be the cells mediating the enhanced regeneration seen under calorie restriction. Instead of looking at these active stem cells, researchers examined a second population of intestinal stem cells known as reserve stem cells. The team had previously shown that these reserve stem cells normally reside in a dormant state and are protected from chemotherapy and radiation. Upon a strong injury that kills the active cells, these reserve stem cells "wake up" to regenerate the tissue.

To investigate this hypothesis, the scientists focused on how a subpopulation of mouse intestinal stem cells responded under calorie restriction and then when the animals were exposed to radiation. When mice were fed a diet reduced in calories by 40 percent from normal, the researchers observed that reserve intestinal stem cells expanded five-fold. Paradoxically, these cells also seemed to divide less frequently, a mystery the researchers hope to follow up on in later work. When the research team selectively deleted the reserve stem cells in calorie-restricted mice, their intestinal tissue's regeneration capabilities were cut in half, implicating these cells as having an important role in carrying out the benefits of calorie restriction.

"These reserve stem cells are rare cells. In a normal animal they may make up less than half a percentage of the intestinal epithelium and in calorie restricted animals maybe slightly more. Normally, in the absence of injury, the tissue can tolerate the loss, due to the presence of the active stem cells, but, when you injure the animal, the regeneration is compromised and the enhanced regeneration after calorie restriction was compromised in the absence of the reserve stem cell pool. These reserve stem cells that we had shown were important for the beneficial effects of calorie restriction, were repressing many pathways that are all known to be regulated by the protein complex mTOR, which is most well known as being a nutrient-sensing complex. Curiously, we see that, when they're injured, the calorie-restricted mice were actually better able to activate mTOR than their counterparts. So somehow, even though mTOR is being suppressed initially, it's also better poised to become activated after injury. That's something we don't fully understand.

The researchers conducted experiments using leucine, an amino acid that activates mTOR, and rapamycin, a drug which inhibits mTOR, to confirm that mTOR acted within these reserve stem cells to regulate their activity. Reserve stem cells exposed to leucine proliferated, while those exposed to rapamycin were blocked. Pretreating the animals with leucine make the reserve stem cells more sensitive to radiation and less able to regenerate tissue following radiation injury, while rapamycin protected the reserve stem cells as they were more likely to remain dormant. The researchers caution, however, that rapamycin cannot be used as a stand-in for calorie restriction, as it would linger and continue to block mTOR activation even following injury, hindering the ability of the reserve stem cells to spring into action and regenerate intestinal tissue.

Of the seven strands of the SENS platform, the ablation of senescent cells (ApoptoSENS) has thus far made the most progress towards the clinic; drugs that selectively eliminate these toxic and superfluous cells are referred to as senolytics, and several are now undergoing or are soon to enter clinical trials. Recent evidence from preclinical work has indicated that the role of senescent cells in the aging process is remarkably significant, such that resolving this single form of damage yields dramatic benefits across the spectrum of age-related decline - simultaneously extending both lifespan and healthspan in mouse models.

Although the existence of a true senescent phenotype in postmitotic cells such as neurons is still unproven, its existence in their crucial support cells, the astrocytes, has been recognized since the beginning of this decade. A recent dissertation makes vital progress towards proving the clinical relevance of the phenomenon - laying the groundwork for the translational application of senolytics to major neurodegenerative diseases. Glutamate (together with aspartate) is the major excitatory neurotransmitter in the human brain, and dysfunctions of its handling are clearly associated with both acute and chronic neurological conditions. That such dysfunction is here shown to be an intrinsic consequence of physiologically realistic levels of astrocyte senescence leaves little doubt that a mechanistic connection must exist. In Alzheimer's disease specifically, it is notable that the loss of glutamate receptors in postmortem samples tracks both the brain's major excitatory pathways and also the very well-established progressive staging of the disease. These results are good news indeed!

Replacing intrinsically aged neurons without disrupting synaptic connectivity has always been accepted to be a daunting task, but astrocyte turnover - while low in healthy tissue - is a routine process following injury (albeit one that has side effects of its own when driven to excess in the context of chronic inflammation, although these appear somewhat treatable). Thus, depleting senescent astrocytes and so neutralizing their inflammatory effects may well automatically induce their replacement by healthy new cells; and even if not, stimulating that process is not an insurmountable challenge. At the very least, such a therapy should prevent further degeneration - and perhaps even create the conditions for the repair of pre-existing neuronal decline as well, especially since a subset of those astrocytes may be able to function as neural stem cells.