Fight Aging! Newsletter, May 28th 2018

Researchers have discovered that the flow of cerebrospinal fluid is a key signal for neural stem cell renewal. Neural stem cells in the brain can divide and mature into neurons and this process plays important roles in various regions of the brain - including olfactory sense and memory. These cells are located in what is known as the neurogenic stem cell niche one of which is located at the walls of the lateral ventricles, where they are in contact with circulating cerebrospinal fluid.

The cerebrospinal fluid fills the brain and its roles are still poorly understood. This work highlights the role of this fluid as a key signal - but this time not a chemical but a physical signal. The mechanism is controlled by the ENaC molecule. This abbreviation stands for epithelial sodium (Na) channel and describes a channel protein on the cell surface through which sodium ions stream into the cell's interior. "We were able to show in an experimental model that brain stem cells are no longer able to divide in the absence of ENaC. Conversely, a stronger ENaC function promotes cell proliferation."

Further tests showed that the function of ENaC is augmented by shear forces exerted on the cells by the cerebrospinal fluid. The physical stimulation causes the channel protein to open for longer time and allow sodium ions to flow into the cell, thus stimulating division. "The results came as a big surprise, since ENaC had previously only been known for its functions in the kidneys and lungs." Pharmacological ENaC blockers are already used clinically to relieve certain types of hypertension. Now it is known that they can also influence stem cells in the brain and thus brain function.

One hallmark of adult neurogenesis is its adaptability to environmental influences. Here, we uncovered the epithelial sodium channel (ENaC) as a key regulator of adult neurogenesis as its deletion in neural stem cells (NSCs) and their progeny in the murine subependymal zone (SEZ) strongly impairs their proliferation and neurogenic output in the olfactory bulb.

Importantly, alteration of fluid flow promotes proliferation of SEZ cells in an ENaC-dependent manner, eliciting sodium and calcium signals that regulate proliferation via calcium-release-activated channels and phosphorylation of ERK. Flow-induced calcium signals are restricted to NSCs in contact with the ventricular fluid, thereby providing a highly specific mechanism to regulate NSC behavior at this special interface with the cerebrospinal fluid. Thus, ENaC plays a central role in regulating adult neurogenesis, and among multiple modes of ENaC function, flow-induced changes in sodium signals are critical for NSC biology.

Central arterial stiffness increases with sedentary aging. While near-daily, vigorous lifelong (more than 25 years) endurance exercise training prevents arterial stiffening with aging, this rigorous routine of exercise training over a lifetime is impractical for most individuals. The aim was to examine whether a less frequent 'dose' of lifelong exercise training (4-5 sessions per week for more than 30 minutes) that is consistent with current physical activity recommendations elicits similar benefits on central arterial stiffening with aging.

A cross-sectional examination of 102 seniors (60 years and older), who had a consistent lifelong exercise history was performed. Subjects were stratified into 4 groups based on exercise frequency as an index of exercise 'dose': sedentary: fewer than 2 sessions per week; casual exercisers: 2-3 sessions per week; committed exercisers: 4-5 sessions per week; Masters athletes: 6-7 sessions per week plus regular competitions. Detailed measures of arterial stiffness and left ventricular afterload were collected.

Biological aortic age and central pulse wave velocity were younger in committed exercisers and athletes compared to sedentary seniors. TACi (total arterial compliance) was lower, while carotid β-stiffness index and Eai (effective arterial elastance) were higher in sedentary seniors compared to the other groups. There appeared to be a dose-response threshold for carotid β-stiffness index and TACi. Peripheral arterial stiffness was not significantly different among the groups. This suggest that 4-5 weekly exercise sessions over a lifetime is associated with reduced central arterial stiffness in the elderly. A less frequent dose of lifelong exercise (2-3 sessions/wk) is associated with decreased ventricular afterload and peripheral resistance, while peripheral arterial stiffness is unaffected by any dose of exercise.

Regular exercise improves the physical capacity and reduces the risk of developing chronic and age-related diseases by improving the metabolic state, antioxidant protection, and redox regulation. Lifelong training was reported to slow down aging-associated skeletal muscle fiber atrophy and prevent the reduction in muscular strength. Notably, acute intensive exercise induces the production of reactive oxygen species (ROS) that can evoke macromolecular damage, oxidative stress, endoplasmic reticulum (ER) stress, and activation of the unfolded protein response (UPR).

On the other hand, regular exercise training results in adaptations in antioxidant defense and improves redox signaling to protect cells against stress-related diseases, thus delaying the aging processes. In addition, the UPR, which is activated by exercise in skeletal muscles, may exert protective effects against ER stress and can promote metabolic adaptation to physical activity. Long-term exercise was reported to upregulate heat shock protein (HSP) production in skeletal muscle, which would be beneficial in coping with oxidative stress, ER stress, and ER stress-related apoptosis. Nevertheless, the ability to induce HSPs in aged skeletal muscle is compromised, which may impair the exercise-mediated adaptation processes.

There is only limited information available on the association of aging and exercise training concerning oxidative stress, ER (SR) stress, UPR, and/or ER stress-related apoptosis in skeletal muscle. Our hypothesis is based on the fact that there is an age-induced disruption of redox regulation, increased redox ER stress, and ER stress-related apoptosis, and that long-term exercise can exert protective effects against these processes. We investigated the key molecular markers associated with redox state, ER stress, and apoptosis in skeletal muscle of old animals in a life-long running model and compared them to young animals. Our data demonstrated that aging induced oxidative stress and activated ER stress-related apoptosis signaling in skeletal muscle, whereas long-term wheel-running improved redox regulation, ER stress adaptation and attenuated ER stress-related apoptosis signaling. These findings suggest that life-long exercise can protect against age-related cellular stress.

Intermittent fasting (IF), an umbrella term for diets that cycle between a period of fasting and non-fasting, has become increasingly popular as a weight loss regime (e.g., "every-other-day fasting" and the "5:2" diet). Advocates of IF argue that it shows many of the benefits seen with traditional daily energy restriction diets but with a simplified nutritional regime and increased compliance. Recent pilots and clinical trials used a fasting mimicking diet (FMD) (consisting of monthly cycles of a 5-day fast during which daily food intake was reduced to ∼50% normal caloric intake), which reduced multiple health risk factors during the post-fast recovery period, including lowered blood pressure, and reduced blood glucose and insulin-like growth factor-1 (IGF-1) levels. IF can extend lifespan in a variety of organisms, including bacteria, yeast, nematode worms, and mice. In animal models, IF has been shown to reduce the risk of developing a variety of age-related pathologies. IF is effective in preventing neurodegeneration in rodents and can attenuate cancer and cardiometabolic diseases, such as type II diabetes.

Reduced activity of nutrient-sensing pathways, with corresponding decrease in global protein translation, is implicated as an important mechanism underlying the pro-longevity effects of dietary interventions, such as dietary restriction (DR). Reduced TOR signaling is a hallmark of pro-longevity interventions, including DR, and treatment with the TOR inhibitor rapamycin extends healthy lifespan in a range of organisms. Although DR may exert some of its pro-longevity effects through reduced fecundity, DR can still extend lifespan in sterile Drosophila, implying that fecundity and lifespan can be uncoupled and that other mechanisms are also important.

Previous studies examining potential pro-longevity effects of IF in flies have produced mainly negative results. The first studies, almost 90 years ago, found that 6 hr of starvation in every 24 hr was beneficial and could extend lifespan. However, the effects of this IF regime may be strain or food medium specific, because a similar, more rigorous experiment ∼80 years later found that daily bouts of either 3 hr or 6 hr starvation throughout the adult life of the fly had neither a positive nor a negative effect on lifespan.

Here, we investigated a variety of IF regimes in flies and their effects on a range of health outcomes, including feeding behavior, gut and metabolic health, survival after stress, and lifespan. Importantly, short-term IF (the "2:5" diet) confined to early life robustly increased subsequent lifespan, particularly in females, independent of TOR signaling. Short-term IF also led to long-lasting health improvements, including increased stress resistance and a lower incidence of gut pathology that was associated with reduced bacterial abundance. Guts of flies 40 days post-IF showed a significant reduction in age-related pathologies and improved gut barrier function. We conclude that short-term IF during early life can induce long-lasting beneficial effects, with robust increase in lifespan in a TOR-independent manner, probably at least in part by preserving gut health.

Alzheimer's disease and cerebrovascular disease are probably the two most common causes of cognitive impairment in the elderly, but even though they often co-occur in individual patients, they are typically viewed as independent contributors. While the presence of amyloid plaques in the brain is considered a hallmark of Alzheimer's disease, some individuals with elevated amyloid levels never develop cognitive impairment. This has led to a search for additional markers beyond brain amyloid to help identify those at increased risk for cognitive decline.

The current study was designed to investigate whether the effects of increased brain amyloid and of vascular risk on cognitive decline are merely additive, reflecting a simple combination of the risks independently contributed by each factor, or synergistic, in which interaction of the two produces an even higher level of risk. The study analyzed data from 223 participants in the Harvard Aging Brain Study, an ongoing study of cognitively normal individuals ages 50 to 90 designed to improve understanding of brain changes affecting memory and cognition that occur with aging.

Upon enrollment in the study, participants receive standard imaging biomarker studies, including PET scans with a compound that reveals amyloid deposits in the brain. Assessment of vascular risk is determined by the Framingham cardiovascular risk score, which is based on factors such as hypertension, body mass index, and histories of diabetes or smoking. Participants also receive standard tests of memory, attention and language, which are repeated at annual follow-up visits.

The results showed that both elevated brain amyloid levels and higher vascular risk, as measured upon study enrollment, were associated with more rapid cognitive decline, with the most rapid changes seen in participants with elevations in both factors. The extent of the interaction between the two measures suggested a synergistic, rather than simply an additive effect.

Identifying asymptomatic individuals at high risk of impending cognitive decline because of Alzheimer disease (AD) is crucial to the success of clinical trials aimed at preventing dementia. The advent of in vivo measures of β-amyloid (Aβ) burden highlighted a preclinical phase of AD allowing for the identification of clinically normal individuals with objective evidence of AD pathology. However, a substantial portion of individuals who are amyloid positive do not show clear evidence of cognitive decline in available longitudinal follow-up data. This is consistent with autopsy data indicating that approximately 30% of clinically normal elderly individuals have signs of elevated Aβ burden on pathological examination. These findings have prompted the search for additional biomarkers that can be used with Aβ burden to identify individuals at maximal risk of cognitive decline.

Multiple studies have demonstrated that cardiovascular risk factors, such as hypertension and hyperlipidemia (which often occur together), are also risk factors for cognitive decline and AD. Consistent with this, recent epidemiological data suggest that declining dementia incidence may be partially because of advances in the treatment of cardiovascular disease. Neuropathological studies indicate that vascular brain changes frequently co-occur with AD pathology in late-onset dementia and that vascular pathology may lower the threshold for cognitive impairment.

The goal of the present study was to examine whether a well-validated, multivariable measure of vascular risk is associated with prospective cognitive decline in a large cohort of clinically normal elderly individuals, either additively or synergistically with Aβ burden.

The LMNA gene encodes lamin and lamin C, which are major components of the nuclear lamina. Mutations in the LMNA gene have been implicated in premature aging disorders, including Hutchinson-Gilford progeria syndrome (HGPS). HGPS is caused by splicing defect and consequent generation of progerin, mutant-truncated lamin protein. Cells of HGPS patients exhibit an abnormal nuclear structure, increased DNA damage and premature senescence. In addition to the effects of progerin, accumulation of prelamin A, precursor of lamin A, induces defects in nuclear structures. ZMPSTE24 is an enzyme that produces mature lamin by cleavage of amino acids in prelamin A.

Zmpste24 knock-out mice have been widely used to study the mechanisms of aging and progeria. Depletion of Zmpste24 causes premature senescence in mice, including decreases in life span and bone density. Increased prelamin expression caused by ZMPSTE24 deficiency causes defective DNA repair. Zmpste24 knock-out mice have been extensively studied because of their impaired DNA damage response (DDR). Lamin also functions as a structural barrier to DDR. Altogether, these findings indicate that defects in the nuclear structure induced by progerin or prelamin lead to the accumulation of DNA damage, which results in accelerated aging.

It has been reported that exogenous expression of progerin in human mesenchymal stem cells (hMSCs) can impair their differentiation potential. Furthermore, production of induced pluripotent stem cells (iPSCs) from HGPS patients has revealed that the progerin expression levels are the highest in MSCs, vascular smooth muscle cells, and fibroblasts. These results indicate that MSCs are a specific target cell type of progerin-induced senescence. Like progerin, excessive accumulation of prelamin induces premature senescence in MSCs, including wrinkled nuclei. Downregulation of ZMPSTE24 in hMSCs also induces the senescence phenotype. These investigations imply that both progerin and prelamin can induce senescence in hMSCs with change in nuclear morphology.

Senescent cells secrete a group of factors that induce senescence in neighboring cells, a phenomenon termed senescence-associated secretory phenotype (SASP). The secreted inflammatory factors propagate senescence and recruit immune cells to senescent tissues by the generation of a pro-inflammatory environment. Among the factors reported to regulate the SASP, GATA4 has been recently identified as a regulator of senescence and inflammation. GATA4 is expressed during oncogene- and irradiation-induced senescence in fibroblasts in response to DNA damage. During the process of cellular senescence, GATA4 has a regulatory role in the SASP of fibroblasts. Because GATA4-dependent cellular senescence is closely associated with DDR, the role of GATA4 in other senescence models and other cell types may reveal a new mechanism.

Senescent hMSCs also induce senescence in neighboring cells. Monocyte chemoattractant protein-1 (MCP-1) secreted from senescent human umbilical cord blood-derived mesenchymal stem cells (hUCB-MSCs) induces premature senescence in neighboring cells. Insulin-like growth factor binding proteins are also produced by senescent hMSCs, and they trigger senescence in adjacent normal cells. These studies investigated the mechanisms of the SASP by inducing senescence in hMSCs through prolonged passaging. However, cellular senescence of MSCs can be regulated by various factors other than passaging. In our previous report, we have demonstrated that depletion of ZMPSTE24 and introduction of progerin induce premature senescence in hUCB-MSCs. It remains to be determined whether defective lamin triggers paracrine senescence via inflammatory factors in hMSCs.

In this study, we identified that paracrine senescence is triggered in senescent hMSCs with abnormal nuclear structures by increasing the expression of MCP-1 and that inhibition of MCP-1 decreases the SASP. Furthermore, we found that GATA4 mediates the senescence of hMSCs induced by defective lamin A. We assessed whether down-regulation of GATA4 disturbs the progerin- or prelamin A-dependent senescence phenotype. Elucidating how GATA4 regulates senescence in hMSCs with nuclear defects may aid in understanding the etiology of complex aging disorders. We show that inhibition of GATA4 expression protects hMSCs from cellular senescence, implying unique therapeutic opportunity against progeroid syndromes and physiological aging.

Cellular senescence occurs when irreversible cell cycle arrest is triggered by telomere shortening or exposure to stress. Senescent cells (SCs) accumulate if they cannot be removed rapidly by the immune system due to immune dysfunction and/or a sustained, overwhelming increase in SC production. This occurs during aging or under certain pathological conditions. Under these circumstances, SCs can be detrimental and play a causal role in aging, age-related diseases, and chemotherapy- and radiotherapy-induced side effects, in part through the expression of the senescence-associated secretory phenotype.

This hypothesis is supported by recent studies demonstrating that the genetic clearance of SCs prolongs the lifespan of mice and delays the onset of several age-related diseases and disorders in both progeroid and naturally aged mice. Therefore, the pharmacological clearance of SCs with a small molecule, a senolytic agent that can selectively kill SCs, is potentially a novel anti-aging strategy and a new treatment for chemotherapy- and radiotherapy-induced side effects.

However, a major challenge facing the discovery and development of effective senolytic agents is to identify and validate more senolytic targets. Since the first senolytic was published, twelve molecular targets have been identified. These findings led to the discovery of a few senolytic agents, but the clinical application of these senolytic agents for age-related diseases and cytotoxic cancer therapy-induced side effects may be limited by agent toxicity and manufacturing challenges.

Piperlongumine (PL) is one of a few natural products identified to have the ability to selectively kill SCs. Compared to other known senolytic agents, PL has the advantage of low toxicity, an excellent PK/PD profile, and oral bioavailability. However, its molecular targets and mechanisms of action are unknown. To facilitate the development of PL and its analogues as senolytic drug candidates, it is critical to identify PL molecular targets, which can form a molecular basis for the rational design of new PL analogues.

Herein, we report the identification and validation of oxidation resistance 1 (OXR1) as a molecular target of PL in SCs. OXR1 is a cellular oxidative stress sensor that regulates the expression of a variety of antioxidant enzymes and modulates the cell cycle and apoptosis. We found that OXR1 was upregulated in SCs induced by ionizing radiation or extensive replication. PL bound to OXR1 directly and induced its degradation through the ubiquitin-proteasome system in an SC-specific manner. Knocking down OXR1 selectively induced apoptosis in SCs and sensitized the cells to oxidative stress caused by hydrogen peroxide (H2O2). These findings suggest that OXR1 is a potential senolytic target that can be exploited for the development of selective senolytic agents with improved potency and selectivity. In addition, these findings also provide new insight into the mechanism by which SCs are highly resistant to oxidative stress.

Unknown 15 years ago, PCSK9 (proprotein convertase subtilisin/kexin type 9) is now common parlance among scientists and clinicians interested in prevention and treatment of atherosclerotic cardiovascular disease. What makes this story so special is not its recent discovery nor the fact that it uncovered previously unknown biology but rather that these important scientific insights have been translated into an effective medical therapy in record time. Indeed, the translation of this discovery to novel therapeutic serves as one of the best examples of how genetic insights can be leveraged into intelligent target drug discovery.

Initial clues were provided by a French family with familial hypercholesterolemia (FH) in 2003. Gain-of-function mutations in PCSK9 were linked with hypercholesterolemia and ultimately uncovered a key new player in lipid metabolism. This seminal discovery led to a series of investigations that demonstrated that loss-of-function (LOF) mutations in PCSK9 associate with lifelong low cholesterol levels and marked reductions in the risk of atherosclerotic cardiovascular disease (ASCVD). The rare individuals with homozygous LOF mutations in PCSK9 (and no circulating protein) demonstrated extremely LDL cholesterol (LDL-C; ≈15 mg/dL), normal health and reproductive capacity, and no evidence of neurological or cognitive dysfunction.

This complementary set of observations has been leveraged into the most important therapy for the treatment of hypercholesterolemia and ASCVD since the introduction of statins. Indeed, the so-called PCSK9 inhibitors, fully human monoclonal antibodies that bind PCSK9, reduce LDL-C by ≈60% and risk of myocardial infarction and stroke by ≈20% after more than 2 years of treatment. Remarkably, these agents antagonizing PCSK9 action were approved by regulatory agencies spanning the globe only a decade after its discovery - although the scientific and medical communities have swiftly uncovered many facets of PCSK9 biology, there is still much to learn.

New research has shown that older people with very low heart disease risks also have very little frailty, raising the possibility that frailty could be prevented. The largest study of its kind found that even small reductions in risk factors helped to reduce frailty, as well as dementia, chronic pain, and other disabling conditions of old age. Many perceive frailty to be an inevitable consequence of ageing - but the study found that severe frailty was 85% less likely in those with near ideal cardiovascular risk factors.

"This study indicates that frailty and other age-related diseases could be prevented and significantly reduced in older adults. Getting our heart risk factors under control could lead to much healthier old ages. Unfortunately, the current obesity epidemic is moving the older population in the wrong direction, however our study underlines how even small reductions in risk are worthwhile." The study analysed data from more than 421,000 people aged 60-69 in both GP medical records and in the UK Biobank research study. Participants were followed up over ten years.

The researchers analysed six factors that could impact on heart health. They looked at uncontrolled high blood pressure, cholesterol and glucose levels, plus being overweight, doing little physical activity and being a current smoker. "Individuals with untreated cardiovascular disease or other common chronic diseases appear to age faster and with more frailty. In the past, we viewed ageing and these common chronic diseases as being both inevitable and unrelated to each other. Now our growing body of scientific evidence on ageing shows what we have previously considered as inevitable might be prevented or delayed through earlier and better recognition and treatment of cardiac disease."

Osteoarthritis (OA) is a prevalent debilitating joint disorder characterized by erosion of articular cartilage. The degradation of network of collagen and proteoglycan in OA cartilage leads to a loss in tensile strength and shear properties of cartilage. Interestingly, though OA manifests as loss of the articular cartilage, it also includes all tissues of the joint, particularly the subchondral bone. Besides aging, the increase in level of accumulation of advanced glycation end products (AGEs), oxidative stress, and senescence-related secretory phenotypes are a few reported factors associated with pathogenesis of OA.

The potential of stem cells to differentiate into osteoblasts, chondroblasts, and adipocytes, if stimulated properly, can regenerate cartilage both in vivo and in vitro. Recent progress in tissue engineering has highlighted the regenerative potential of stem cells for therapeutic purposes. The multilineage potential of stem cells, suitable scaffolds, and appropriate chondrogenic agent (chemical and mechanical stimuli) have been implicated to regenerate damaged cartilage. Mesenchymal stem cell (MSC) based therapy is also emerging as alternative to joint replacement with prostheses, due to its long-lasting effect.

MSCs derived from bone marrow (BMSCs) are capable enough to differentiate into tissues such as bone and cartilage and mobilize at an injured cartilage site in knee joints thereby assisting in cartilage regeneration in OA. In a study, the intra-articularly transplanted BMSC successfully regenerated injured cartilage in an animal model of OA and also improved osteoarthritic symptoms in humans without any major side effect even in the long-term. This study demonstrated the possibility of intra-articular injection of MSCs for the treatment of injured articular tissue including anterior cruciate ligament, meniscus, or cartilage. Therefore, if this treatment option is well-established, it may be minimally invasive procedure compared to conventional surgeries.

In a first-of-its-kind finding, a new stroke-healing gel helped regrow neurons and blood vessels in mice with stroke-damaged brains. The results suggest that such an approach may someday be a new therapy for stroke in people. The brain has a limited capacity for recovery after stroke and other diseases. Unlike some other organs in the body, such as the liver or skin, the brain does not regenerate new connections, blood vessels, or new tissue structures. Tissue that dies in the brain from stroke is absorbed, leaving a cavity, devoid of blood vessels, neurons, or axons, the thin nerve fibers that project from neurons.

To see if healthy tissue surrounding the cavity could be coaxed into healing the stroke injury, researchers engineered a gel to inject into the stroke cavity that thickens to mimic the properties of brain tissue, creating a scaffolding for new growth. The gel is infused with molecules that stimulate blood vessel growth and suppress inflammation, since inflammation results in scars and impedes regrowth of functional tissue.

After 16 weeks, stroke cavities in treated mice contained regenerated brain tissue, including new neural networks - a result that had not been seen before. The mice with new neurons showed improved motor behavior, though the exact mechanism wasn't clear. "The new axons could actually be working. Or the new tissue could be improving the performance of the surrounding, unharmed brain tissue." The gel was eventually absorbed by the body, leaving behind only new tissue.

The nervous system is a key player in maintaining homeostasis and the structural and functional integrity of living beings and, hence, in controlling aging and longevity. Given the role of the nervous system in life span control, a reasonable question would be whether genes defining the cellular specificity of neurons are also involved, in some way, in the regulation of longevity.

We have already demonstrated that several genes that encode RNA polymerase II transcription factors and that are involved in neural development affect life span in Drosophila melanogaster. Among other genes, escargot (esg) was identified as a candidate gene affecting life span in a screen of more than 1,500 insertion mutations and the insertion located downstream of esg was further confirmed to be causally associated with life span control.

The gene esg belongs to the Snail family of genes that are involved in the development of the nervous system in arthropods and chordates. In Drosophila melanogaster, Esg and other Snail proteins act to control asymmetric neuroblast division during embryogenesis; however, Esg functions are not exclusively neuronal, and it also participates in the maintenance of intestinal and male germ cells, regulates tracheal morphogenesis and development of the genital disk, and determines wing cell fate.

Here, we present new data on the role of esg in life span control. Analysis of the esg-BG01042 mutation allowed us to show that esg is involved in the regulation of life span, to varying degrees, in unmated and mated males and females. The esg-BG01042 mutation also increased locomotion, specifically during old age, indicating that the mutation slowed down aging. The increase in longevity was caused by decreased esg transcription associated with structural changes in the DNA sequences downstream of the gene.

Targets of esg encoded enzymes involved in the biosynthesis of neurotransmitters, neuropeptides, cationic transporters, and other proteins. Among others, genes involved in the defense/immune response were both up- and down-regulated. Of the genes known to be involved in life span control, at least two genes associated with increased life span, heat shock protein 26 (hsp26) and NAD-dependent methylenetetrahydrofolate dehydrogenase (Nmdmc), increased transcription.

Exercise benefits young and old organisms, including increased skeletal mass, improvement in the cardiovascular system, and metabolic regulation, as well as in brain functions associated with cognition, memory, and mood. In particular, exercise promotes adult hippocampal neurogenesis and neuronal plasticity, and is associated with increased memory performance and cognition, and is considered to counter cognitive decline caused by aging and by neurodegenerative diseases.

Skeletal muscle is the most abundant tissue in the human body and the most highly activated organ in response to physical activity. Aerobic exercise affects skeletal muscle by inducing a substantial switch in composition from fast-twitching, glycolytic type IIb fibers to the more oxidative, slow-twitching type I fibers. Endurance training results in an increase in mitochondrial biogenesis and activity, vascularization, oxygen consumption and an overall improvement of aerobic capacity. Furthermore, the resulting activation of signaling pathways relevant to energy metabolism, such as the AMPK-SIRT1-PGC-1α pathway in muscle may contribute to the benefits of exercise for brain function.

The vast beneficial consequences of exercise might not be within reach of debilitated, diseased, and elderly patients. The development of compounds capable of activating cellular targets of exercise may be a new therapeutic approach. Indeed, recent research indicates that factors secreted by skeletal muscle during exercise may exert beneficial effects on brain function. This review will focus on the identified targets relevant to energy metabolism in muscle and the molecules affecting it.

An active lifestyle, despite the promising of compounds currently under study, remains the preferred choice for improving body and brain function. Indeed, the mechanisms of action of exercise mimetics still require further investigation, and the possibility of a treatment capable of replacing exercise in its entirety is remote. In order to achieve an artificial exercise regimen, potential adverse effects of prolonged treatment with exercise mimetics have to be overcome. Nonetheless, a possible use of this class of compounds could be envisioned in parallel with light training paradigms, helping to achieve a more complete exercise-induced benefit, both on brain and on peripheral functions. This is especially poignant for conditions, such as morbid obesity or neurodegenerative diseases, which may preclude exercise.

Using a targeted metabolomics approach, participants with low muscle quality presented significantly higher plasma concentrations of isoleucine and leucine, suggesting that low muscle quality is characterized by impaired transport of amino acids, especially branched chain amino acids (BCAAs), across the muscle cell membrane. The exact reasons for why amino acid uptake is reduced in older persons with low muscle quality are unknown, and further work is required to identify putative intervention/therapeutic targets.

Physiologically, amino acid uptake in muscle cells is regulated by three fundamental mechanisms: insulin signalling, BCAA (primarily leucine) blood concentration, and physical activity. Previous studies have also suggested that these 'anabolic' signals cause increased amino acid entry by dynamically enhancing muscle perfusion, and all three signals exhibit a dose-response relationship that is steeper in younger than in older persons. In other words, older persons tend to develop an 'anabolic resistance' to the three stimuli. Since muscle perfusion adaptation is mediated by endothelial reactivity, which is hampered by a pro-inflammatory state, this hypothesis can also explain why inflammation is such a strong correlate and predictor of age-related sarcopenia.

During ageing, mitochondria lose the ability to produce energy during maximal efforts but not when the energetic demand is lower. This impaired mitochondrial function could be due to inadequate perfusion or reduced muscle blood flow, resulting in lower oxygen delivery in skeletal muscle and diminished aerobic capacity. This hypothesis is interesting because it connects both energetic and anabolic deficits to the same mechanism. These results indicate that oxidative phosphorylation is progressively impaired with ageing; it is unclear whether this is because the number of mitochondria per muscle volume is diminished, the intrinsic capacity of mitochondria to generate ATP is impaired, or the availability of oxygen and nutrients at different levels of effort is compromised.

Oxidative stress and defective mitophagy (mitochondrial autophagy) are potentially involved in the decline of muscle quality with ageing and need to be considered. Dysfunctional mitochondria are characterized by reduced oxidative phosphorylation efficiency and excessive production of reactive oxygen species, which oxidize and damage macromolecules. The hypothesis that oxidative stress causes degenerative changes in tissues that are highly metabolically active, such as the brain and the muscle, has been proposed for many years. Oxidative stress may also affect satellite cells or muscle stem cell pools in skeletal muscle.

Defective mitochondrial function has been studied in regard to the neuromuscular junction (NMJ) remodelling that occurs with ageing, producing cycles of denervation-innervation that lead to motor unit loss, specifically in type II fibres, as well as muscle fibre atrophy. However, it is not clear whether these changes in the NMJ precede or follow the observed decline in muscle mass and strength that is observed with ageing. Some studies have reported altered mitochondria morphology in the NMJ that produce increased levels of oxidative stress, decreased enzymatic activity and ATP production, and impaired calcium buffering. The combination of these biological changes may have a strong negative impact on excitation-contraction coupling and eventually lead to the loss of motor units.

Overall, low muscle quality seems to be associated with (i) metabolic impairments that lead to reduced incorporation of the three major BCAAs, which are used by muscle as energy sources and are associated with muscle strength and endurance; (ii) fat accumulation in muscle tissue that ultimately leads to architectural disruption and loss of function; and (iii) high concentration of lipid species that are associated with impaired mitochondrial function and unrecycled mitochondrial proteins, potentially due to defective mitophagy or proteostasis. The extent and complexity to which these mechanisms are interconnected is unknown and should be examined in future studies. In addition, other factors that impact ageing muscle could also modulate mitochondrial function, such as (i) defects in the NMJ that leads to myofiber denervation-due to reduced capacity in motor neurons to reinnervate muscle fibres-consequently causing fibres to become atrophied; (ii) the age-associated decline in the satellite cell pool, reducing muscle regeneration after injury; and (iii) 'inflammaging', the chronic low-grade inflammation observed in older persons.

Immune aging (immunosenescence) is characterized by the reduced competence of acquired immunity, leading to increased susceptibility to infection as well as decreased vaccination efficiency. Recent accumulating evidence indicates that immunosenescence underlies an increased proinflammatory trait with age, including various chronic inflammatory and metabolic disorders, such as atherosclerosis and diabetes mellitus, as well as an increased risk for autoimmunity. Cellular senescence is characterized by irreversible arrest of proliferation, grossly altered gene expression, and relative resistance to apoptosis. Notably, senescent cells are often metabolically active and may become foci of host reactions in tissues by secreting various inflammatory factors. The features and consequences of cellular senescence in T cells in the immune system, however, remain elusive.

One of the most prominent changes occurring in the immune organs with age is an early involution of the thymus. The thymus is a central immune organ to support T cell development and establish T cell self-tolerance. T cell generation in the thymus sharply declines after the juvenile stage, eventually replaced almost entirely by fat tissues at later stages of life. In concordance with the decrease of T cell genesis, the peripheral naïve T cells are gradually reduced with age. Although the peripheral T cell pool is well maintained in aged individuals, the population shows a steady increase in the proportions of memory phenotype (MP) T cells.

We reported that a unique PD-1+ MP CD4+ T cell population is increased with age, now termed senescence-associated (SA-) T cells. The SA-T cells show characteristic signs and features of cellular senescence and emerge as follicular T cells in spontaneous germinal centers (GCs) that occur in aged mice. Spontaneous development of GCs is a hallmark of systemic autoimmune diseases, and among a number of changes in immune function with age is an increasing risk for autoimmunity.

Ischemic heart disease affects a majority of people, especially elderly patients. Recent studies have utilized autologous adult stem cells and progenitor cells as a treatment option to heal cardiac tissue after myocardial infarction. However, donor cells from aging patients are more likely to be in a senescent stage. Rejuvenation is required to reverse the damage levied by aging and promote a youthful phenotype. This review aims to discuss current strategies that are effective in rejuvenating aging cardiac stem cells and represent novel therapeutic methods to treat the aging heart.

Recent literature mainly focuses on three approaches that aim to reverse cardiac aging: genetic modification, pharmaceutical administration, and optimization of extracellular factors. In vitro genetic modification can be used to overexpress or knock down certain genes and allow for reversal of the aging phenotype. Pharmaceutical administration is another approach that allows for manipulation of signaling pathways related to cell proliferation and cell senescence. Since the stem cell niche can contribute to the age-related decline in stem cell function, rejuvenation strategies also include optimization of extracellular factors.

Overall, improving the intrinsic properties of aging stem cells as well as the surrounding environment allows these cells to adopt a phenotype similar to their younger counterparts. Recent studies show promising results of the ability of these techniques to rejuvenate the aging heart. However, more understanding of the combinatorial effects of these interventions and fine-tuning of these techniques is required to evaluate the translational potential of these methods. Each strategy has its own advantages and disadvantages. The success of myocardial regenerative treatment will require teamwork across various disciplines to make stem cell therapy a reliable method for cardiac repair.