Heterochronic parabiosis for the study of the effects of aging on stem cells and their niches

Umbilical Cord Blood Plasma Enhances Cellular Repair and Senescence Suppression in Human Dermal Fibroblasts Under Oxidative Stress

Aging is associated with a gradual decline in cellular function, largely driven by oxidative stress, which leads to cellular senescence. These processes contribute to tissue degeneration and age-related dysfunction. Human dermal fibroblasts (HDFs), critical for maintaining skin structure, are highly vulnerable to oxidative damage, making them key contributors to skin aging. Umbilical cord blood plasma (UCBP), rich in growth factors and regenerative molecules, has shown potential in preventing cellular senescence and addressing key mechanisms of tissue aging. Based on findings from heterochronic parabiosis experiments that demonstrated the rejuvenating effect of young blood, we investigated the effects of UCBP on hydrogen peroxide (H2O2) induced oxidative stress in HDFs and compared its efficacy with adult blood plasma (ABP). Our results indicate that although both UCBP and ABP reduce reactive oxygen species (ROS), UCBP is more effective in suppressing cellular senescence and maintaining fibroblast proliferation. These findings suggest that UCBP's protective effects extend beyond ROS reduction, potentially by modulating the senescence-associated secretory phenotype and the enhancement of tissue repair mechanisms.

Metabolaging: a new geroscience perspective linking aging pathologies and metabolic dysfunction

With age, our metabolic systems undergo significant alterations, which can lead to a cascade of adverse effects that are implicated in both metabolic disorders, such as diabetes, and in the body's ability to respond to acute stress and trauma. To elucidate the metabolic imbalances arising from aging, we introduce the concept of "metabolaging." This framework encompasses the broad spectrum of metabolic disruptions associated with the hallmarks of aging, including the functional decline of key metabolically active organs, like the adipose tissue. By examining how these organs interact with essential nutrient-sensing pathways, "metabolaging" provides a more comprehensive view of the systemic metabolic imbalances that occur with age. This concept extends to understanding how age-related metabolic disturbances can influence the response to acute stressors, like burn injuries, highlighting the interplay between metabolic dysfunction and the ability to handle severe physiological challenges. Finally, we propose potential interventions that hold promise in mitigating the effects of metabolaging and its downstream consequences.

Replacement as an aging intervention

Substantial progress in aging research continues to deepen our understanding of the fundamental mechanisms of aging, yet there is a lack of interventions conclusively shown to attenuate the processes of aging in humans. By contrast, replacement interventions such as joint replacements, pacemaker devices and transplant therapies have a long history of restoring function in injury or disease contexts. Here, we consider biological and synthetic replacement-based strategies as aging interventions. We discuss innovations in tissue engineering, such as the use of scaffolds or bioprinting to generate functional tissues, methods for enhancing donor-recipient compatibility through genetic engineering and recent progress in both cell therapies and xenotransplantation strategies. We explore synthetic approaches including prostheses, external devices and brain-machine interfaces. Additionally, we evaluate the evidence from heterochronic parabiosis experiments in mice and donor-recipient age-mismatched transplants to consider whether systemic benefits could result from personalized replacement approaches. Finally, we outline key challenges and future directions required to advance replacement therapies as viable, scalable and ethical interventions for aging.

Systemic Rejuvenating Interventions: Perspectives on Neuroinflammation and Blood-Brain Barrier Integrity

The aging process results in structural, functional, and immunological changes in the brain, which contribute to cognitive decline and increase vulnerability to neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), and stroke-related complications. Aging leads to cognitive changes and also affect executive functions. Additionally, it causes neurogenic and neurochemical alterations, such as a decline in dopamine and acetylcholine levels, which also impact cognitive performance. The chronic inflammation caused by aging contributes to the impairment of the blood-brain barrier (BBB), contributing to the infiltration of immune cells and exacerbating neuronal damage. Therefore, rejuvenating therapies such as heterochronic parabiosis, cerebrospinal fluid (CSF) administration, plasma, platelet-rich plasma (PRP), and stem cell therapy have shown potential to reverse these changes, offering new perspectives in the treatment of age-related neurological diseases. This review focuses on highlighting the effects of rejuvenating interventions on neuroinflammation and the BBB.

The brain-body energy conservation model of aging

Aging involves seemingly paradoxical changes in energy metabolism. Molecular damage accumulation increases cellular energy expenditure, yet whole-body energy expenditure remains stable or decreases with age. We resolve this apparent contradiction by positioning the brain as the mediator and broker in the organismal energy economy. As somatic tissues accumulate damage over time, costly intracellular stress responses are activated, causing aging or senescent cells to secrete cytokines that convey increased cellular energy demand (hypermetabolism) to the brain. To conserve energy in the face of a shrinking energy budget, the brain deploys energy conservation responses, which suppress low-priority processes, producing fatigue, physical inactivity, blunted sensory capacities, immune alterations and endocrine 'deficits'. We term this cascade the brain-body energy conservation (BEC) model of aging. The BEC outlines (1) the energetic cost of cellular aging, (2) how brain perception of senescence-associated hypermetabolism may drive the phenotypic manifestations of aging and (3) energetic principles underlying the modifiability of aging trajectories by stressors and geroscience interventions.

Effect of Young Plasma Therapy on Cognition, Oxidative Stress, miRNA-134, BDNF, CREB, and SIRT-1 Expressions and Neuronal Survey in the Hippocampus of Aged Ovariectomized Rats with Alzheimer's

Menopause may increase the risk of Alzheimer's disease (AD) dementia. This study aimed to use young plasma therapy (YPT) to improve dementia caused by AD in aged ovariectomized rats. Female Wistar rats were used in the following groups: (a) young (CY) (180-200 g, 2-3 months, n = 10) and (b) old groups (250-350 g, 22-24 months, n = 60). The old rats were randomly assigned to six sub-groups: (1) control, (2) sham, (3) ovariectomized group (OVX), (4) OVX + Alzheimer disease (OVX + AD), (5) OVX + AD+ 17β-Estradiol (OVX + AD + E), and (6) OVX + AD + young plasma (OVX + AD + YP). Cognitive behaviors were evaluated using NOR, MWM, and PAL tests. MiR-134a, SIRT-1, CREB, and BDNF expressions were measured using real-time PCR and western blot, respectively. Oxidative stress in hippocampal tissue was assayed using ELISA kits. OVX and AD caused significant cognitive impairment (p < 0.001), up-regulated miR-134a (p < 0.001), down-regulated SIRT-1, CREB, and BDNF protein expression (p < 0.001), and decreased antioxidant marker levels (p < 0.001) compared to the sham group. YPT significantly restored miR-134a (p < 0.001), SIRT-1 (p < 0.001), CREB (p < 0.001), and BDNF (p < 0.001) protein expression in OVX + AD rats. YPT, as much as or more than estrogen therapy (ERT), significantly improved oxidative stress and down-regulated miR-134a expression and the up-regulation of SIRT-1, CREB, and BDNF proteins in OVX + AD rats (p < 0.001). YPT significantly improved histological alteration compared to the OVX + AD group (p < 0.001). As a non-pharmacological treatment, YPT can improve the expression of miR-134a and SIRT-1, CREB, and BDNF proteins as much as or more than estrogen therapy, ameliorating AD-induced dementia in aged OVX rats.

Sex-specific preservation of neuromuscular function and metabolism following systemic transplantation of multipotent adult stem cells in a murine model of progeria

Onset and rates of sarcopenia, a disease characterized by a loss of muscle mass and function with age, vary greatly between sexes. Currently, no clinical interventions successfully arrest age-related muscle impairments since the decline is frequently multifactorial. Previously, we found that systemic transplantation of our unique adult multipotent muscle-derived stem/progenitor cells (MDSPCs) isolated from young mice-but not old-extends the health-span in DNA damage mouse models of progeria, a disease of accelerated aging. Additionally, induced neovascularization in the muscles and brain-where no transplanted cells were detected-strongly suggests a systemic therapeutic mechanism, possibly activated through circulating secreted factors. Herein, we used ZMPSTE24-deficient mice, a lamin A defect progeria model, to investigate the ability of young MDSPCs to preserve neuromuscular tissue structure and function. We show that progeroid ZMPST24-deficient mice faithfully exhibit sarcopenia and age-related metabolic dysfunction. However, systemic transplantation of young MDSPCs into ZMPSTE24-deficient progeroid mice sustained healthy function and histopathology of muscular tissues throughout their 6-month life span in a sex-specific manner. Indeed, female-but not male-mice systemically transplanted with young MDSPCs demonstrated significant preservation of muscle endurance, muscle fiber size, mitochondrial respirometry, and neuromuscular junction morphometrics. These novel findings strongly suggest that young MDSPCs modulate the systemic environment of aged animals by secreted rejuvenating factors to maintain a healthy homeostasis in a sex-specific manner and that the female muscle microenvironment remains responsive to exogenous regenerative cues in older age. This work highlights the age- and sex-related differences in neuromuscular tissue degeneration and the future prospect of preserving health in older adults with systemic regenerative treatments.

Serum factors mediate changes in mitochondrial bioenergetics associated with diet and exercise interventions

Mitochondrial improvements resulting from behavioral interventions, such as diet and exercise, are systemic and apparent across multiple tissues. Here, we test the hypothesis that factors present in serum, and therefore circulating throughout the body, can mediate changes in mitochondrial function in response to intervention. To investigate this, we used stored serum from a clinical trial comparing resistance training (RT) and RT plus caloric restriction (RT + CR) to examine effects of blood borne circulating factors on myoblasts in vitro. We report that exposure to dilute serum is sufficient to mediate bioenergetic benefits of these interventions. Additionally, serum-mediated bioenergetic changes can differentiate between interventions, recapitulate sex differences in bioenergetic responses, and is linked to improvements in physical function and inflammation. Using metabolomics, we identified circulating factors associated with changes in mitochondrial bioenergetics and the effects of interventions. This study provides new evidence that circulating factors play a role in the beneficial effects of interventions that improve healthspan among older adults. Understanding the factors that drive improvements in mitochondrial function is a key step towards predicting intervention outcomes and developing strategies to countermand systemic age-related bioenergetic decline.

Anecdotal Evidence Elucidates the Aging Process

Our group at Yuvan Research has conducted several experiments in recent years that have demonstrated the reversibility of aging through the use of a young plasma fraction, following a historical line of research that began with heterochronic parabiosis. However, a one-of-a-kind discovery, in the form of anecdotal evidence, has recently clarified many doubts about the nature of aging and rejuvenation, and the conclusions that can be drawn from this discovery allow us to form a relatively clear picture of the mechanics of the aging and rejuvenation processes.

Insights into the Notch signaling pathway in degenerative musculoskeletal disorders: Mechanisms and perspectives

Degenerative musculoskeletal disorders are a group of age-related diseases of the locomotive system that severely affects the patient's ability to work and cause adverse sequalae such as fractures and even death. The incidence and prevalence of degenerative musculoskeletal disorders is rising owing to the aging of the world's population. The Notch signaling pathway, which is expressed in almost all organ systems, extensively regulates cell proliferation and differentiation as well as cellular fate. Notch signaling shows increased activity in degenerative musculoskeletal disorders and retards the progression of degeneration to some extent. The review focuses on four major degenerative musculoskeletal disorders (osteoarthritis, intervertebral disc degeneration, osteoporosis, and sarcopenia) and summarizes the pathophysiological functions of Notch signaling in these disorders, especially its role in stem/progenitor cells in each disorder. Finally, a conclusion will be presented to explore the research and application of the perspectives on Notch signaling in degenerative musculoskeletal disorders.

scDiffCom: a tool for differential analysis of cell-cell interactions provides a mouse atlas of aging changes in intercellular communication

Dysregulation of intercellular communication is a hallmark of aging. To better quantify and explore changes in intercellular communication, we present scDiffCom and scAgeCom. scDiffCom is an R package, relying on approximately 5,000 curated ligand-receptor interactions, that performs differential intercellular communication analysis between two conditions from single-cell transcriptomics data. Built upon scDiffCom, scAgeCom is an atlas of age-related cell-cell communication changes covering 23 mouse tissues from 58 single-cell RNA sequencing datasets from Tabula Muris Senis and the Calico murine aging cell atlas. It offers a comprehensive resource of tissue-specific and sex-specific aging dysregulations and highlights age-related intercellular communication changes widespread across the whole body, such as the upregulation of immune system processes and inflammation, the downregulation of developmental processes, angiogenesis and extracellular matrix organization and the deregulation of lipid metabolism. Our analysis emphasizes the relevance of the specific ligands, receptors and cell types regulating these processes. The atlas is available online ( https://scagecom.org ).

Disorganization of secondary lymphoid organs and dyscoordination of chemokine secretion as key contributors to immune aging

Aging is characterized by progressive loss of organ and tissue function, and the immune system is no exception to that inevitable principle. Of all the age-related changes in the body, reduction of the size of, and naïve T (Tn) cell output from, the thymus occurs earliest, being prominent already before or by the time of puberty. Therefore, to preserve immunity against new infections, over much of their lives, vertebrates dominantly rely on peripheral maintenance of the Tn cell pool in the secondary lymphoid organs (SLO). However, SLO structure and function subsequently also deteriorate with aging. Several recent studies have made a convincing case that this deterioration is of major importance to the erosion of protective immunity in the last third of life. Specifically, the SLO were found to accumulate multiple degenerative changes with aging. Importantly, the results from adoptive transfer and parabiosis studies teach us that the old microenvironment is the limiting factor for protective immunity in old mice. In this review, we discuss the extent, mechanisms, and potential role of stromal cell aging in the age-related alteration of T cell homeostatic maintenance and immune function decline. We use that discussion to frame the potential strategies to correct the SLO stromal aging defects - in the context of other immune rejuvenation approaches, - to improve functional immune responses and protective immunity in older adults.

Evaluation of quantitative biomarkers of aging in human PBMCs

Functional decline with age contributes significantly to the burden of disease in developed countries. There is growing interest in the development of therapeutic interventions which slow or even reverse aging. Time and cost constraints prohibit the testing of a large number of interventions for health and lifespan extension in model organisms. Cell-based models of aging could enable high throughput testing of potential interventions. Despite extensive reports in the literature of cell properties that correlate with donor age, few are robustly observed across different laboratories. This casts doubt on the extent that aging signatures are captured in cultured cells. We tested molecular changes previously reported to correlate with donor age in peripheral blood mononuclear cells (PBMCs) and evaluated their suitability for inclusion in a panel of functional aging measures. The tested measures spanned several pathways implicated in aging including epigenetic changes, apoptosis, proteostasis, and intracellular communication. Surprisingly, only two markers correlated with donor age. DNA methylation age accurately predicted donor age confirming this is a robust aging biomarker. Additionally, the apoptotic marker CD95 correlated with donor age but only within subsets of PBMCs. To demonstrate cellular rejuvenation in response to a treatment will require integration of multiple read-outs of cell function. However, building a panel of measures to detect aging in cells is challenging and further research is needed to identify robust predictors of age in humans.

Transcriptional Profiling of Skeletal Muscle Stem Cells After In Vivo Engraftment into a Heterologous Niche Environment

Adult stem cells play a critical role in the maintenance and repair of the organs in which they reside. However, their function is highly dependent on the crosstalk with their niche environment that changes during development and in disease states. The niche provides signals to stem cells to activate, proliferate, self-renew, or remain in quiescence. In skeletal muscle, the niche is perturbed in disease contexts such as aging, muscular dystrophies, and cachexia. Therefore, it is important to develop methods that permit the decoupling of niche-mediated from cell-intrinsic changes that occur in muscle stem cells (MuSCs) in development and disease contexts. With the purpose of determining the effect of the niche environment on the MuSC transcriptome, function, or health, we have coupled an allogeneic stem cell transplantation system, meaning the transplantation of MuSCs from a donor mouse into a recipient host mouse, with Switching Mechanism at 5' End of RNA Template (SMART-Seq) to quantify the effects of the niche on the MuSC transcriptome in vivo. Briefly, MuSCs are isolated from a GFP reporter donor mouse (Pax7-nGFP) and transplanted into the irradiated muscles of immunocompromised allogeneic hosts. The MuSCs are re-isolated by fluorescence-activated cell sorting (FACS) after three weeks of inhabiting the heterologous niche, defined as a niche that is different from their originating niche, and sequencing-ready libraries are created. This method allows for the direct comparison of the transcriptome of stem cells before and after transplantation into a host of a different age, disease status, or genetic background. This method can be used to accurately quantify the direct effect of the niche environment on the stem cell gene expression profile and to decouple cell-intrinsic versus niche-mediated alterations in the stem cell transcriptome. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol: Allogeneic muscle stem cell transplantation.

Multi-omic rejuvenation and life span extension on exposure to youthful circulation

Heterochronic parabiosis (HPB) is known for its functional rejuvenation effects across several mouse tissues. However, its impact on biological age and long-term health is unknown. Here we performed extended (3-month) HPB, followed by a 2-month detachment period of anastomosed pairs. Old detached mice exhibited improved physiological parameters and lived longer than control isochronic mice. HPB drastically reduced the epigenetic age of blood and liver based on several clock models using two independent platforms. Remarkably, this rejuvenation effect persisted even after 2 months of detachment. Transcriptomic and epigenomic profiles of anastomosed mice showed an intermediate phenotype between old and young, suggesting a global multi-omic rejuvenation effect. In addition, old HPB mice showed gene expression changes opposite to aging but akin to several life span-extending interventions. Altogether, we reveal that long-term HPB results in lasting epigenetic and transcriptome remodeling, culminating in the extension of life span and health span.

Aging Affects the Efficacy of Platelet-Rich Plasma Treatment for Osteoarthritis

OBJECTIVE

Despite the increased use of platelet-rich plasma in the treatment of osteoarthritis, whether and how age of the platelet-rich plasma donor affects therapeutic efficacy is unclear.

DESIGN

In vitro, male osteoarthritic human chondrocytes were treated with platelet-rich plasma from young (18-35 yrs) or old (≥65 yrs) donors, and the chondrogenic profile was evaluated using immunofluorescent staining for two markers of chondrogenicity, type II collagen and SOX-9. In vivo, we used a within-subjects design to compare Osteoarthritis Research Society International scores in aged mouse knee joints injected with platelet-rich plasma from young or old individuals.

RESULTS

In vitro experiments revealed that platelet-rich plasma from young donors induced a more youthful chondrocyte phenotype, as evidenced by increased type II collagen ( P = 0.033) and SOX-9 expression ( P = 0.022). This benefit, however, was significantly blunted when cells were cultured with platelet-rich plasma from aged donors. Accordingly, in vivo studies revealed that animals treated with platelet-rich plasma from young donors displayed a significantly improved cartilage integrity when compared with knees injected with platelet-rich plasma from aged donors ( P = 0.019).

CONCLUSIONS

Injection of platelet-rich plasma from a young individual induced a regenerative effect in aged cells and mice, whereas platelet-rich plasma from aged individuals showed no improvement in chondrocyte health or cartilage integrity.

Neuromuscular electrical stimulation enhances the ability of serum extracellular vesicles to regenerate aged skeletal muscle after injury

Exercise promotes healthy aging of skeletal muscle. This benefit may be mediated by youthful factors in the circulation released in response to an exercise protocol. While numerous studies to date have explored soluble proteins as systemic mediators of rejuvenating effect of exercise on tissue function, here we showed that the beneficial effect of skeletal muscle contractile activity on aged muscle function is mediated, at least in part, by regenerative properties of circulating extracellular vesicles (EVs). Muscle contractile activity elicited by neuromuscular electrical stimulation (NMES) decreased intensity of expression of the tetraspanin surface marker, CD63, on circulating EVs. Moreover, NMES shifted the biochemical Raman fingerprint of circulating EVs in aged animals with significant changes in lipid and sugar content in response to NMES when compared to controls. As a demonstration of the physiological relevance of these EV changes, we showed that intramuscular administration of EVs derived from aged animals subjected to NMES enhanced aged skeletal muscle healing after injury. These studies suggest that repetitive muscle contractile activity enhances the regenerative properties of circulating EVs in aged animals.

Muscle Injuries Induce a Prostacyclin-PPARγ/PGC1a-FAO Spike That Boosts Regeneration

It is well-known that muscle regeneration declines with aging, and aged muscles undergo degenerative atrophy or sarcopenia. While exercise and acute injury are both known to induce muscle regeneration, the molecular signals that help trigger muscle regeneration have remained unclear. Here, mass spectrometry imaging (MSI) is used to show that injured muscles induce a specific subset of prostanoids during regeneration, including PGG1, PGD2, and the prostacyclin PGI2. The spike in prostacyclin promotes skeletal muscle regeneration via myoblasts, and declines with aging. Mechanistically, the prostacyclin spike promotes a spike in PPARγ/PGC1a signaling, which induces a spike in fatty acid oxidation (FAO) to control myogenesis. LC-MS/MS and MSI further confirm that an early FAO spike is associated with normal regeneration, but muscle FAO became dysregulated during aging. Functional experiments demonstrate that the prostacyclin-PPARγ/PGC1a-FAO spike is necessary and sufficient to promote both young and aged muscle regeneration, and that prostacyclin can synergize with PPARγ/PGC1a-FAO signaling to restore aged muscles' regeneration and physical function. Given that the post-injury prostacyclin-PPARγ-FAO spike can be modulated pharmacologically and via post-exercise nutrition, this work has implications for how prostacyclin-PPARγ-FAO might be fine-tuned to promote regeneration and treat muscle diseases of aging.

Biological age is increased by stress and restored upon recovery

Aging is classically conceptualized as an ever-increasing trajectory of damage accumulation and loss of function, leading to increases in morbidity and mortality. However, recent in vitro studies have raised the possibility of age reversal. Here, we report that biological age is fluid and exhibits rapid changes in both directions. At epigenetic, transcriptomic, and metabolomic levels, we find that the biological age of young mice is increased by heterochronic parabiosis and restored following surgical detachment. We also identify transient changes in biological age during major surgery, pregnancy, and severe COVID-19 in humans and/or mice. Together, these data show that biological age undergoes a rapid increase in response to diverse forms of stress, which is reversed following recovery from stress. Our study uncovers a new layer of aging dynamics that should be considered in future studies. The elevation of biological age by stress may be a quantifiable and actionable target for future interventions.

Multiomics reveals glutathione metabolism as a driver of bimodality during stem cell aging

With age, skeletal muscle stem cells (MuSCs) activate out of quiescence more slowly and with increased death, leading to defective muscle repair. To explore the molecular underpinnings of these defects, we combined multiomics, single-cell measurements, and functional testing of MuSCs from young and old mice. The multiomics approach allowed us to assess which changes are causal, which are compensatory, and which are simply correlative. We identified glutathione (GSH) metabolism as perturbed in old MuSCs, with both causal and compensatory components. Contrary to young MuSCs, old MuSCs exhibit a population dichotomy composed of GSHhigh cells (comparable with young MuSCs) and GSHlow cells with impaired functionality. Mechanistically, we show that antagonism between NRF2 and NF-κB maintains this bimodality. Experimental manipulation of GSH levels altered the functional dichotomy of aged MuSCs. These findings identify a novel mechanism of stem cell aging and highlight glutathione metabolism as an accessible target for reversing MuSC aging.

Aging Hallmarks and the Role of Oxidative Stress

Aging is a complex biological process accompanied by a progressive decline in the physical function of the organism and an increased risk of age-related chronic diseases such as cardiovascular diseases, cancer, and neurodegenerative diseases. Studies have established that there exist nine hallmarks of the aging process, including (i) telomere shortening, (ii) genomic instability, (iii) epigenetic modifications, (iv) mitochondrial dysfunction, (v) loss of proteostasis, (vi) dysregulated nutrient sensing, (vii) stem cell exhaustion, (viii) cellular senescence, and (ix) altered cellular communication. All these alterations have been linked to sustained systemic inflammation, and these mechanisms contribute to the aging process in timing not clearly determined yet. Nevertheless, mitochondrial dysfunction is one of the most important mechanisms contributing to the aging process. Mitochondria is the primary endogenous source of reactive oxygen species (ROS). During the aging process, there is a decline in ATP production and elevated ROS production together with a decline in the antioxidant defense. Elevated ROS levels can cause oxidative stress and severe damage to the cell, organelle membranes, DNA, lipids, and proteins. This damage contributes to the aging phenotype. In this review, we summarize recent advances in the mechanisms of aging with an emphasis on mitochondrial dysfunction and ROS production.

Reduction, proliferation, and differentiation defects of stem cells over time: a consequence of selective accumulation of old centrioles in the stem cells?

BACKGROUND

All molecules, structures, cells in organisms are subjected to destruction during the process of vital activities. In the organisms of most multicellular animals and humans, the regeneration process always takes place: destruction of old cells and their replacement with the new. The replacement of cells happens even if the cells are in perfect condition. The sooner the organism destroys the cells that emerged a certain time ago and replaces them with the new (i.e., the higher is the regeneration tempo), the younger the organism is.

DISCUSSION

Stem cells are progenitor cells of the substituting young cells. Asymmetric division of a mother stem cell gives rise to one, analogous to the mother, daughter cell, and to a second daughter cell that takes the path of further differentiation. Despite such asymmetric divisions, the pool of stem cells diminishes in its quantity over time. Moreover, intervals between stem cell divisions increase. The combination of these two processes causes the decline of regeneration tempo and aging of the organism.

CONCLUSION

During asymmetric stem cell divisions daughter cells, with preserved potency of the stem cell, selectively conserve mother (old) centrioles. In contrast with molecules of nuclear DNA, reparations do not take place in centrioles. Hypothetically, old centrioles are more subjected to destruction than other structures of a cell-which makes centrioles potentially the main structure of aging.

Heterochronic parabiosis reprograms the mouse brain transcriptome by shifting aging signatures in multiple cell types

Aging is a complex process involving transcriptomic changes associated with deterioration across multiple tissues and organs, including the brain. Recent studies using heterochronic parabiosis have shown that various aspects of aging-associated decline are modifiable or even reversible. To better understand how this occurs, we performed single-cell transcriptomic profiling of young and old mouse brains after parabiosis. For each cell type, we cataloged alterations in gene expression, molecular pathways, transcriptional networks, ligand-receptor interactions and senescence status. Our analyses identified gene signatures, demonstrating that heterochronic parabiosis regulates several hallmarks of aging in a cell-type-specific manner. Brain endothelial cells were found to be especially malleable to this intervention, exhibiting dynamic transcriptional changes that affect vascular structure and function. These findings suggest new strategies for slowing deterioration and driving regeneration in the aging brain through approaches that do not rely on disease-specific mechanisms or actions of individual circulating factors.

Ageing at Molecular Level: Role of MicroRNAs

The progression of age triggers a vast number of diseases including cardiovascular, cancer, and neurodegenerative disorders. Regardless of our plentiful knowledge about age-related diseases, little is understood about molecular pathways that associate the ageing process with various diseases. Several cellular events like senescence, telomere dysfunction, alterations in protein processing, and regulation of gene expression are common between ageing and associated diseases. Accumulating information on the role of microRNAs (miRNAs) suggests targeting miRNAs can aid our understanding of the interplay between ageing and associated diseases. In the present chapter, we have attempted to explore the information available on the role of miRNAs in ageing of various tissues/organs and diseases and understand the molecular mechanism of ageing.

Dietary Flavonoids and Adult Neurogenesis: Potential Implications for Brain Aging

Adult neurogenesis deficiency has been proposed to be a common hallmark in different age-related neurodegenerative diseases. The administration of flavonoids is currently reported as a potentially beneficial strategy for preventing brain aging alterations, including adult neurogenesis decline. Flavonoids are a class of plant-derived dietary polyphenols that have drawn attention for their neuroprotective and pro-cognitive effects. Although they undergo extensive metabolism and localize in the brain at low concentrations, flavonoids are now believed to improve cerebral vasculature and interact with signal transduction cascades involved in the regulation of adult neurogenesis. Furthermore, many dietary flavonoids have been shown to reduce oxidative stress and neuroinflammation, improving the neuronal microenvironment where adult neurogenesis occurs. The overall goal of this review is to summarize the evidence supporting the role of flavonoids in modulating adult neurogenesis as well as to highlight how these dietary agents may be promising candidates in restoring healthy brain function during physiological and pathological aging.

Rejuvenation of Senescent Mesenchymal Stem Cells to Prevent Age-Related Changes in Synovial Joints

Mesenchymal/medicinal stem/signaling cells (MSCs), well known for regenerative potential, have been involved in hundreds of clinical trials. Even if equipped with reparative properties, aging significantly decreases their biological activity, representing a major challenge for MSC-based therapies. Age-related joint diseases, such as osteoarthritis, are associated with the accumulation of senescent cells, including synovial MSCs. An impaired ability of MSCs to self-renew and differentiate is one of the main contributors to the human aging process. Moreover, senescent MSCs (sMSCs) are characterized by the senescence-messaging secretome (SMS), which is typically manifested by the release of molecules with an adverse effect. Many factors, from genetic and metabolic pathways to environmental stressors, participate in the regulation of the senescent phenotype of MSCs. To better understand cellular senescence in MSCs, this review discusses the characteristics of sMSCs, their role in cartilage and synovial joint aging, and current rejuvenation approaches to delay/reverse age-related pathological changes, providing evidence from in vivo experiments as well.

Small-animal blood exchange is an emerging approach for systemic aging research

We describe a small-animal blood exchange approach developed for aging research as an alternative to heterochronic parabiosis or plasma injections. In parabiosis, animals are surgically coupled, which has several disadvantages, including difficulty controlling experimental procedure, the effects of shared organs, environmental enrichment from jointly exploring the housing enclosure, involuntary exercise and an imprecise onset of blood sharing. Likewise, in plasma injections, the added volumes need to be small, and there is little flexibility in changing the relative contributions of ectopic to endogenous blood components. These factors complicate the conclusions and interpretations, including the identification of key mechanisms and molecular or cellular determinants. Our approach, where blood is exchanged between animals without them being surgically coupled, is less invasive than parabiosis. The percentage of exchanged blood or other exchanged fluids is known and precise. The age of plasma and cells can be mixed and matched at all desired relative contributions to the endogenous systemic milieu, and the onset of the effects can be accurately delineated. In this protocol, we describe the preparatory and animal surgery steps required for small-animal blood exchange in mice and compare this process with parabiosis and plasma injections. We also provide the design, hardware and software for the blood exchange device and compare automated and manual exchange methods. Lastly, we report mathematical modeling of the dilution of blood factors. The fluid exchange takes ~30 min when performed by a well-trained biomedical scientist; the entire process takes ~2 h.

Therapeutic Antiaging Strategies

Aging constitutes progressive physiological changes in an organism. These changes alter the normal biological functions, such as the ability to manage metabolic stress, and eventually lead to cellular senescence. The process itself is characterized by nine hallmarks: genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. These hallmarks are risk factors for pathologies, such as cardiovascular diseases, neurodegenerative diseases, and cancer. Emerging evidence has been focused on examining the genetic pathways and biological processes in organisms surrounding these nine hallmarks. From here, the therapeutic approaches can be addressed in hopes of slowing the progression of aging. In this review, data have been collected on the hallmarks and their relative contributions to aging and supplemented with in vitro and in vivo antiaging research experiments. It is the intention of this article to highlight the most important antiaging strategies that researchers have proposed, including preventive measures, systemic therapeutic agents, and invasive procedures, that will promote healthy aging and increase human life expectancy with decreased side effects.

Breast Cancer-Stromal Interactions: Adipose-Derived Stromal/Stem Cell Age and Cancer Subtype Mediated Remodeling

Adipose tissue is characterized as an endocrine organ that acts as a source of hormones and paracrine factors. In diseases such as cancer, endocrine and paracrine signals from adipose tissue contribute to cancer progression. Young individuals with estrogen receptor-alpha positive (ER-α+) breast cancer (BC) have an increased resistance to endocrine therapies, suggesting that alternative estrogen signaling is activated within these cells. Despite this, the effects of stromal age on the endocrine response in BC are not well defined. To identify differences between young and aged ER-α+ breast tumors, RNA sequencing data were obtained from The Cancer Genome Atlas. Analysis revealed enrichment of matrix and paracrine factors in young (≤40 years old) patients compared to aged (≥65 years old) tumor samples. Adipose-derived stromal/stem cells (ASCs) from noncancerous lipoaspirate of young and aged donors were evaluated for alterations in matrix production and paracrine secreted factors to determine if the tumor stroma could alter estrogen signaling. Young and aged ASCs demonstrated comparable proliferation, differentiation, and matrix production, but exhibited differences in the expression levels of inflammatory cytokines (Interferon gamma, interleukin [IL]-8, IL-10, Tumor necrosis factor alpha, IL-2, and IL-6). Conditioned media (CM)-based experiments showed that young ASC donor age elevated endocrine response in ER-α+ BC cell lines. MCF-7 ER-α+ BC cell line treated with secreted factors from young ASCs had enhanced ER-α regulated genes (PGR and SDF-1) compared to MCF-7 cells treated with aged ASC CM. Western blot analysis demonstrated increased activation levels of p-ER ser-167 in the MCF-7 cell line treated with young ASC secreted factors. To determine if ER-α+ BC cells heightened the cytokine release in ASCs, ASCs were stimulated with MCF-7-derived CM. Results demonstrated no change in growth factors or cytokines when treated with the ER-α+ secretome. In contrast to ER-α+ CM, the ER-α negative MDA-MB-231 derived CM demonstrated increased stimulation of pro-inflammatory cytokines in ASCs. While there was no observed change in the release of selected paracrine factors, MCF-7 cells did induce matrix production and a pro-adipogenic lineage commitment. The adipogenesis was evident by increased collagen content through Sirius Red/Fast Green Collagen stain, lipid accumulation evident by Oil Red O stain, and significantly increased expression in PPARγ mRNA expression. The data from this study provide evidence suggesting more of a subtype-dependent than an age-dependent difference in stromal response to BC, suggesting that this signaling is not heightened by reciprocal signals from ER-α+ BC cell lines. These results are important in understanding the mechanisms of estrogen signaling and the dynamic and reciprocal nature of cancer cell-stromal cell crosstalk that can lead to tumor heterogeneity and variance in response to therapy.

Transplantation to study satellite cell heterogeneity in skeletal muscle

Skeletal muscle has a remarkable capacity to regenerate throughout life, which is mediated by its resident muscle stem cells, also called satellite cells. Satellite cells, located periphery to the muscle fibers and underneath the basal lamina, are an indispensable cellular source for muscle regeneration. Satellite cell transplantation into regenerating muscle contributes robustly to muscle repair, thereby indicating that satellite cells indeed function as adult muscle stem cells. Moreover, satellite cells are a heterogenous population in adult tissue, with subpopulations that can be distinguished based on gene expression, cell-cycle progression, ability to self-renew, and bi-potential ability. Transplantation assays provide a powerful tool to better understand satellite cell function in vivo enabling the separation of functionally distinct satellite cell subpopulations. In this review, we focus on transplantation strategies to explore satellite cells’ functional heterogeneity, approaches targeting the recipient tissue to improve transplantation efficiency, and common strategies to monitor the behaviour of the transplanted cells. Lastly, we discuss some recent approaches to overcome challenges to enhance the transplantation potential of muscle stem cells.

Aging clocks & mortality timers, methylation, glycomic, telomeric and more. A window to measuring biological age

As humans age multiple forms of biological decay ensue, and many aspects of human biology can be measured to determine how far biological machinery has drifted from homeostasis. Research has led to aging clocks being developed that claim to predict biological age as opposed to chronological age. Aging could be regarded as a measured loss of homeostatic biological equilibrium that augments biological decay in fully developed tissues. Measuring aspects of how far various elements of biology have drifted from a youthful state may allow us to make determinations on a subject's health but also make informed predictions on their biological age. As we see across human physiology, many facets that maintain human health taper off such as nicotinamide adenine dinucleotide, glutathione, catalase, super oxide dismutase, and more. Extracellular vesicle density also tapers off during age combined with epigenetic drift, telomere attrition, and stem cell exhaustion, whilst genomic instability and biological insults from environment and lifestyle factors increase. Measuring these types of biomarkers with aging clocks may allow subjects to understand their own health more accurately and enable subjects to better focus on their efforts in the pursuit of longevity and, in addition, allow healthcare practitioners to deliver better health advice.

Role of thymus in health and disease

The thymus is a primary lymphoid organ, essential for the development of T-cells that will protect from invading pathogens, immune disorders, and cancer. The thymus decreases in size and cellularity with age referred to as thymus involution or atrophy. This involution causes decreased T-cell development and decreased naive T-cell emigration to the periphery, increased proportion of memory T cells, and a restricted, altered T-cell receptor (TCR) repertoire. The changes in composition and function of the circulating T cell pool as a result of thymic involution led to increased susceptibility to infectious diseases including the recent COVID and a higher risk for autoimmune disorders and cancers. Thymic involution consisting of both structural and functional loss of the thymus has a deleterious effect on T cell development, T cell selection, and tolerance. The mechanisms which act on the structural (cortex and medulla) matrix of the thymus, the gradual accumulation of genetic mutations, and altered gene expressions may lead to immunosenescence as a result of thymus involution. Understanding the molecular mechanisms behind thymic involution is critical for identifying diagnostic biomarkers and targets for treatment help to develop strategies to mitigate thymic involution-associated complications. This review is focused on the consequences of thymic involution in infections, immune disorders, and diseases, identifying potential checkpoints and potential approaches to sustain or restore the function of the thymus particularly in elderly and immune-compromised individuals.

The Role of Systemic Filtrating Organs in Aging and Their Potential in Rejuvenation Strategies

Advances in aging studies brought about by heterochronic parabiosis suggest that aging might be a reversable process that is affected by changes in the systemic milieu of organs and cells. Given the broadness of such a systemic approach, research to date has mainly questioned the involvement of “shared organs” versus “circulating factors”. However, in the absence of a clear understanding of the chronological development of aging and a unified platform to evaluate the successes claimed by specific rejuvenation methods, current literature on this topic remains scattered. Herein, aging is assessed from an engineering standpoint to isolate possible aging potentiators via a juxtaposition between biological and mechanical systems. Such a simplification provides a general framework for future research in the field and examines the involvement of various factors in aging. Based on this simplified overview, the kidney as a filtration organ is clearly implicated, for the first time, with the aging phenomenon, necessitating a re-evaluation of current rejuvenation studies to untangle the extent of its involvement and its possible role as a potentiator in aging. Based on these findings, the review concludes with potential translatable and long-term therapeutics for aging while offering a critical view of rejuvenation methods proposed to date.

Immune Aging and How It Works for Inflammation and Fibrosis

Almost all mature cells that undergo apoptosis in an age-dependent or an accidental manner are completely recovered in tissue-specific microenvironments without any physiological changes. After peripheral blood leukocytes are released into the local region, fibroblast cells and new blood vessels commonly proliferate during wound healing. Inducible repair tools mainly supplied from blood vessels are cleared by peripheral blood phagocytic macrophages. Finally, hematopoietic stem cell (HSC)-derived precursor cells migrate from bone marrow (BM) to the microenvironment to rebuild damaged tissues (the mature immune system). In contrast to the mature immune system, the effects of aging on HSCs (long-term HSCs) and peripheral blood lymphocytes (long-term PBLs) are not clearly understood in the BM and thymus niches with tissue-specific microenvironments with some physiological changes (the aged BM niche) for incomplete rebuilding of damaged tissues (the aged immune system). In this review, the roles of the aged immune system in both a delay of acute inflammation and the development of chronic inflammation or fibrosis are discussed.

Multiple Sclerosis and Aging: The Dynamics of Demyelination and Remyelination

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) leading to demyelination and neurodegeneration. Life expectancy and age of onset in MS patients have been rising over the last decades, and previous studies have shown that age affects disease progression. Therefore, age appears as one of the most important factors in accumulating disability in MS patients. Indeed, the degeneration of oligodendrocytes (OGDs) and OGD precursors (OPCs) increases with age, in association with increased inflammatory activity of astrocytes and microglia. Similarly, age-related neuronal changes such as mitochondrial alterations, an increase in oxidative stress, and disrupted paranodal junctions can impact myelin integrity. Conversely, once myelination is complete, the long-term integrity of axons depends on OGD supply of energy. These alterations determine pathological myelin changes consisting of myelin outfolding, splitting, and accumulation of multilamellar fragments. Overall, these data demonstrate that old mature OGDs lose their ability to produce and maintain healthy myelin over time, to induce de novo myelination, and to remodel pre-existing myelinated axons that contribute to neural plasticity in the CNS. Furthermore, as observed in other tissues, aging induces a general decline in regenerative processes and, not surprisingly, progressively hinders remyelination in MS. In this context, this review will provide an overview of the current knowledge of age-related changes occurring in cells of the oligodendroglial lineage and how they impact myelin synthesis, axonal degeneration, and remyelination efficiency.

Stem cell aging in the skeletal muscle: The importance of communication

Adult stem cells sustain tissue homeostasis and regeneration; their functional decline is often linked to aging, which is characterized by the progressive loss of physiological functions across multiple tissues and organs. The resident stem cells in skeletal muscle, termed satellite cells, are normally quiescent but activate upon injury to reconstitute the damaged tissue. In this review, we discuss the current understanding of the molecular processes that contribute to the functional failure of satellite cells during aging. This failure is due not only to intrinsic changes but also to extrinsic factors, most of which are still undefined but originate from the muscle tissue microenvironment of the satellite cells (the niche), or from the systemic environment. We also highlight the emerging applications of the powerful single-cell sequencing technologies in the study of skeletal muscle aging, particularly in the heterogeneity of the satellite cell population and the molecular interaction of satellite cells and other cell types in the niche. An improved understanding of how satellite cells communicate with their environment, and how this communication is perturbed with aging, will be helpful for defining countermeasures against loss of muscle regenerative capacity in sarcopenia.

Ageing and rejuvenation models reveal changes in key microbial communities associated with healthy ageing

BACKGROUND

The gut microbiota is associated with diverse age-related disorders. Several rejuvenation methods, such as probiotic administration and faecal microbiota transplantation, have been applied to alter the gut microbiome and promote healthy ageing. Nevertheless, prolongation of the health span of aged mice by remodelling the gut microbiome remains challenging.

RESULTS

Here, we report the changes in gut microbial communities and their functions in mouse models during ageing and three rejuvenation procedures including co-housing, serum-injection and parabiosis. Our results showed that the compositional structure and gene abundance of the intestinal microbiota changed dynamically during the ageing process. Through the three rejuvenation procedures, we observed that the microbial community and intestinal immunity of aged mice were comparable to those of young mice. The results of metagenomic data analysis underscore the importance of the high abundance of Akkermansia and the butyrate biosynthesis pathway in the rejuvenated mouse group. Furthermore, oral administration of Akkermansia sufficiently ameliorated the senescence-related phenotype in the intestinal systems in aged mice and extended the health span, as evidenced by the frailty index and restoration of muscle atrophy.

CONCLUSIONS

In conclusion, the changes in key microbial communities and their functions during ageing and three rejuvenation procedures, and the increase in the healthy lifespan of aged mice by oral administration of Akkermansia. Our results provide a rationale for developing therapeutic strategies to achieve healthy active ageing. Video abstract.

Strategies for late phase preclinical and early clinical trials of senolytics

Cellular senescence and the hallmarks of aging contribute to age-related disease and dysfunction. The Unitary Theory of Fundamental Aging Mechanisms highlights the interdependence among the hallmarks of aging and suggests that by intervening in one fundamental aging process, most or all of the other processes could be impacted. Accumulation of senescent cells is associated with frailty, cardiovascular disease, obesity, diabetes, cognitive decline, and other age- and/or chronic disease-related disorders, suggesting that senescent cells are a target for intervention. Early preclinical data using senolytics, agents that target senescent cells, show promising results in several aging and disease models. The first in-human trials using the senolytic combination of Dasatinib and Quercetin indicated reduced senescent cell burden in adipose tissue of diabetic kidney disease patients and improved physical function in patients with idiopathic pulmonary fibrosis. Clinical trials with other senolytics, including the flavonoid Fisetin and BCL-xL inhibitors, are underway. These results from preclinical and early clinical trials illustrate the potential of senolytics to alleviate age-related dysfunction and diseases. However, multiple clinical trials across different aging and disease models are desperately needed. Parallel trials across institutions through the Translational Geroscience Network are facilitating testing to determine whether senolytics can be translated into clinical application.

Current Thoughts of Notch's Role in Myoblast Regulation and Muscle-Associated Disease

The evolutionarily conserved signaling pathway Notch is unequivocally essential for embryogenesis. Notch’s contribution to the muscle repair process in adult tissue is complex and obscure but necessary. Notch integrates with other signals in a functional antagonist manner to direct myoblast activity and ultimately complete muscle repair. There is profound recent evidence describing plausible mechanisms of Notch in muscle repair. However, the story is not definitive as evidence is slowly emerging that negates Notch’s importance in myoblast proliferation. The purpose of this review article is to examine the prominent evidence and associated mechanisms of Notch’s contribution to the myogenic repair phases. In addition, we discuss the emerging roles of Notch in diseases associated with muscle atrophy. Understanding the mechanisms of Notch’s orchestration is useful for developing therapeutic targets for disease.

Reproductive Suicide: Similar Mechanisms of Aging in C. elegans and Pacific Salmon

In some species of salmon, reproductive maturity triggers the development of massive pathology resulting from reproductive effort, leading to rapid post-reproductive death. Such reproductive death, which occurs in many semelparous organisms (with a single bout of reproduction), can be prevented by blocking reproductive maturation, and this can increase lifespan dramatically. Reproductive death is often viewed as distinct from senescence in iteroparous organisms (with multiple bouts of reproduction) such as humans. Here we review the evidence that reproductive death occurs in C. elegans and discuss what this means for its use as a model organism to study aging. Inhibiting insulin/IGF-1 signaling and germline removal suppresses reproductive death and greatly extends lifespan in C. elegans, but can also extend lifespan to a small extent in iteroparous organisms. We argue that mechanisms of senescence operative in reproductive death exist in a less catastrophic form in iteroparous organisms, particularly those that involve costly resource reallocation, and exhibit endocrine-regulated plasticity. Thus, mechanisms of senescence in semelparous organisms (including plants) and iteroparous ones form an etiological continuum. Therefore understanding mechanisms of reproductive death in C. elegans can teach us about some mechanisms of senescence that are operative in iteroparous organisms.

Enhancement of myogenic potential of muscle progenitor cells and muscle healing during pregnancy

The decline of muscle regenerative potential with age has been attributed to a diminished responsiveness of muscle progenitor cells (MPCs). Heterochronic parabiosis has been used as a model to study the effects of aging on stem cells and their niches. These studies have demonstrated that, by exposing old mice to a young systemic environment, aged progenitor cells can be rejuvenated. One interesting idea is that pregnancy represents a unique biological model of a naturally shared circulatory system between developing and mature organisms. To test this hypothesis, we evaluated the muscle regeneration potential of pregnant mice using a cardiotoxin (CTX) injury mouse model. Our results indicate that the pregnant mice demonstrate accelerated muscle healing compared to nonpregnant control mice following muscle injury based on improved muscle histology, superior muscle regeneration, and a reduction in inflammation and necrosis. Additionally, we found that MPCs isolated from pregnant mice display a significant improvement of myogenic differentiation capacity in vitro and muscle regeneration in vivo when compared to the MPCs from nonpregnant mice. Furthermore, MPCs from nonpregnant mice display enhanced myogenic capacity when cultured in the presence of serum obtained from pregnant mice. Our proteomics data from these studies provides potential therapeutic targets to enhance the myogenic potential of progenitor cells and muscle repair.

Separated parabiont reveals the fate and lifespan of peripheral-derived immune cells in normal and ischaemia-induced injured kidneys

Immune cell infiltration plays a key role in acute kidney injury (AKI) to chronic kidney disease (CKD) progression. T lymphocytes, neutrophils, monocytes/macrophages and other immune cells regulate inflammation, tissue remodelling and repair. To determine the kinetics of accumulation of various immune cell populations, we established an animal model combining parabiosis and separation surgery to explore the fate and lifespan of peripheral leucocytes that migrate to the kidney. We found that peripheral T lymphocytes could survive for a long time (more than 14 days), whereas peripheral neutrophils survived for a short time in both healthy and ischaemia-induced damaged kidneys. Nearly half of the peripheral-derived macrophages disappeared after 14 days in normal kidneys, while their existing time in the inflammatory kidneys was prolonged. A fraction of F4/80^high macrophages were renewed from the circulating monocyte pool. In addition, we found that after renal ischaemia reperfusion, neutrophils increased significantly in the early phase, and T lymphocytes mainly accumulated in the late stage, whereas macrophages infiltrated throughout AKI-CKD progression and were sustained longer in injured as opposed to normal kidneys. In conclusion, peripheral-derived macrophages, T lymphocytes and neutrophils exhibit different lifespans in the kidney, which may play different roles during AKI-CKD progression.

Terminal Schwann Cell Aging: Implications for Age-Associated Neuromuscular Dysfunction

Action potential is transmitted to muscle fibers through specialized synaptic interfaces called neuromuscular junctions (NMJs). These structures are capped by terminal Schwann cells (tSCs), which play essential roles during formation and maintenance of the NMJ. tSCs are implicated in the correct communication between nerves and muscles, and in reinnervation upon injury. During aging, loss of muscle mass and strength (sarcopenia and dynapenia) are due, at least in part, to the progressive loss of contacts between muscle fibers and nerves. Despite the important role of tSCs in NMJ function, very little is known on their implication in the NMJ-aging process and in age-associated denervation. This review summarizes the current knowledge about the implication of tSCs in the age-associated degeneration of NMJs. We also speculate on the possible mechanisms underlying the observed phenotypes.

Parabiosis: Assessing the Effects of Circulating Cells and Factors on the Skeleton

The circulatory system carries within it numerous types of cells, proteins, and other factors that are able to influence the local biology of tissues. Within this chapter, we present a protocol for parabiosis, a surgical model which results in shared circulation between two mice. Such chimeras have recently been used to probe the impact of age-associated changes in the circulation on skeletal, muscular, and neural biology. In conjunction with transgenic mouse models, parabiosis can be used as a tool to investigate the effects of specific factors on local tissues. Here we discuss our adaptation of this surgical procedure including technique details, pitfalls, and suggestions for optimization.

Heterochronic Parabiosis: Old Blood Induces Changes in Mitochondrial Structure and Function of Young Mice

Heterochronic parabiosis models have been utilized to demonstrate the role of blood-borne circulating factors in systemic effects of aging. In previous studies, heterochronic parabiosis has shown positive effects across multiple tissues in old mice. More recently, a study demonstrated old blood had a more profound negative effect on muscle performance and neurogenesis of young mice. In this study, we used heterochronic parabiosis to test the hypothesis that circulating factors mediate mitochondrial bioenergetic decline, a well-established biological hallmark of aging. We examined mitochondrial morphology, expression of mitochondrial complexes, and mitochondrial respiration from skeletal muscle of mice connected as heterochronic pairs, as well as young and old isochronic controls. Our results indicate that young heterochronic mice had significantly lower total mitochondrial content and on average had significantly smaller mitochondria compared to young isochronic controls. Expression of complex IV followed a similar pattern: young heterochronic mice had a trend for lower expression compared to young isochronic controls. Additionally, respirometric analyses indicate that young heterochronic mice had significantly lower complex I, complex I + II, and maximal mitochondrial respiration and a trend for lower complex II-driven respiration compared to young isochronic controls. Interestingly, we did not observe significant improvements in old heterochronic mice compared to old isochronic controls, demonstrating the profound deleterious effects of circulating factors from old mice on mitochondrial structure and function. We also found no significant differences between the young and old heterochronic mice, demonstrating that circulating factors can be a driver of age-related differences in mitochondrial structure and function.

Cellular senescence in ageing: from mechanisms to therapeutic opportunities

Cellular senescence, first described in vitro in 1961, has become a focus for biotech companies that target it to ameliorate a variety of human conditions. Eminently characterized by a permanent proliferation arrest, cellular senescence occurs in response to endogenous and exogenous stresses, including telomere dysfunction, oncogene activation and persistent DNA damage. Cellular senescence can also be a controlled programme occurring in diverse biological processes, including embryonic development. Senescent cell extrinsic activities, broadly related to the activation of a senescence-associated secretory phenotype, amplify the impact of cell-intrinsic proliferative arrest and contribute to impaired tissue regeneration, chronic age-associated diseases and organismal ageing. This Review discusses the mechanisms and modulators of cellular senescence establishment and induction of a senescence-associated secretory phenotype, and provides an overview of cellular senescence as an emerging opportunity to intervene through senolytic and senomorphic therapies in ageing and ageing-associated diseases.

Aging and age-related diseases: from mechanisms to therapeutic strategies

Aging is a physiological process mediated by numerous biological and genetic pathways, which are directly linked to lifespan and are a driving force for all age-related diseases. Human life expectancy has greatly increased in the past few decades, but this has not been accompanied by a similar increase in their healthspan. At present, research on aging biology has focused on elucidating the biochemical and genetic pathways that contribute to aging over time. Several aging mechanisms have been identified, primarily including genomic instability, telomere shortening, and cellular senescence. Aging is a driving factor of various age-related diseases, including neurodegenerative diseases, cardiovascular diseases, cancer, immune system disorders, and musculoskeletal disorders. Efforts to find drugs that improve the healthspan by targeting the pathogenesis of aging have now become a hot topic in this field. In the present review, the status of aging research and the development of potential drugs for aging-related diseases, such as metformin, rapamycin, resveratrol, senolytics, as well as caloric restriction, are summarized. The feasibility, side effects, and future potential of these treatments are also discussed, which will provide a basis to develop novel anti-aging therapeutics for improving the healthspan and preventing aging-related diseases.

Modulation of Ki67 and myogenic regulatory factor expression by tocotrienol-rich fraction ameliorates myogenic program of senescent human myoblasts

INTRODUCTION

Replicative senescence results in dysregulation of cell proliferation and differentiation, which plays a role in the regenerative defects observed during age-related muscle atrophy. Vitamin E is a well-known antioxidant, which potentially ameliorates a wide range of age-related manifestations. The aim of this study was to determine the effects of tocotrienol-rich fraction (TRF) in modulating the expression of proliferation- and differentiation-associated proteins in senescent human myoblasts during the differentiation phase.

MATERIAL AND METHODS

Human skeletal muscle myoblasts were cultured until senescence. Young and senescent cells were treated with TRF for 24 h before and after differentiation induction, followed by evaluation of cellular morphology and efficiency of differentiation. Expression of cell proliferation marker Ki67 protein and myogenic regulatory factors MyoD and myogenin were determined.

RESULTS

Our findings showed that treatment with TRF significantly improved the morphology of senescent myoblasts. Promotion of differentiation was observed in young and senescent myoblasts with TRF treatment as shown by the increased fusion index and larger size of myotubes. Increased Ki67 and myogenin expression with TRF treatment was also observed in senescent myoblasts, suggesting amelioration of the myogenic program by TRF during replicative senescence.

CONCLUSIONS

TRF modulates the expression of regulatory factors related to proliferation and differentiation in senescent human myoblasts and could be beneficial for ameliorating the regenerative defects during aging.