Aging and “rejuvenation” of resident stem cells — a new way to active longevity?

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BACKGROUND

Body aging is characterized by a gradual decline in many physiological functions, such as a decrease in metabolic activity, increase in adipose tissue mass, decrease in muscle mass [1], disruption of circadian wake–sleep cycles, weakening of the immune system [2], cognitive impairment, retinal dysfunction, etc. [3]. The achievements of modern healthcare have increased life expectancy in developed countries; thus, maintaining a high quality of life in the aging process is becoming one of the most urgent problems.

Like any biological process, aging is determined by the complex effect of genetic programs and environmental factors. Both endogenous and exogenous effects on aging are implemented through biological mechanisms, such as chromatin modification, based on which the so-called epigenetic clock has been developed [4]. The concept of the epigenetic clock was proposed in 2013 by Hannum et al. [5] and Horvath [6], who demonstrated that the DNA methylation (DNAm) profile is one of the most accurate markers of cellular aging. Further studies have revealed a multilevel genetically determined program that is implemented by modifying chromatin by methylation of the CG dinucleotides or CpG islands of the DNA [7]. This program determines the epigenetic aging of cells and the organism as a whole.

The methylation of CpG islands is currently considered the most canonical epigenetic sign of aging [8, 9]. The total level of DNAm may decrease with age; however, CpG islands, especially in the target genes of polycomb proteins that can remodel chromatin and genes that regulate transcription, proliferation, and cell differentiation, are hypermethylated [10]. Changes in CpG methylation reflecting epigenetic age can be analyzed using various approaches, including whole-genome sequencing of single cells, which makes it possible to analyze methylome in each cell in various tissues and study their epigenetic characteristics [6, 11–14].

The epigenetic clock is a regression model that links changes in the methylation profile of CpG islands with biological age [5, 15]. In the past decade, these predictive models based on DNAm levels have been constructed based on machine learning, and this technology has truly revolutionized aging research. DNAm-based epigenetic clock is currently better at estimating actual chronological age than any other transcriptomic and proteomic data, including telomere length [16]. The epigenetic clock, originally designed to determine chronological age, can now integrate and predict various indicators of biological aging and disease risk, which determines their clinical significance [16].

The first epigenetic clock was developed by Hannum et al. [5] based on the assessment of 71 CpG from the DNA of peripheral blood mononuclear cells. This was followed by the pan-tissue epigenetic clock, which was more accurate in many aspects; it was created by Horvath [6] based on the assessment of 353 CpG methylations in DNA obtained from several normal and tumor tissues. Interestingly, several mammalian pan-tissue clocks that can determine epigenetic age with notable accuracy in virtually any mammalian tissue have recently been developed [17].

The true biological age is determined under the influence of many external and internal factors; thus, the most modern epigenetic clocks are either specialized in body tissues or are based on the analysis of not only the DNAm profile but also various other indicators to determine more accurately the biological age of cells and the rate of aging (e.g., PhenoAge and GrimAge) [8, 18]. Specifically, the GrimAge clock, in addition to the methylation profile analysis, determines the serum levels of plasminogen activator inhibitor-1 and growth and differentiation factor-15, accounting for smoking and comorbidity, which contribute to a more accurate prediction of the duration and quality life [18]. In light of the development of regenerative medicine, epigenetic clocks are of particular interest because these models can detect even small changes in biological age after various interventions aimed at increasing the lifespan or reprogramming of cells [19].

Recently, a new method for analyzing DNAm scAge profiles has been developed, which can be used to determine the epigenetic age of each cell [20]. This method reproduces the chronological age of the tissue on average and reveals the intrinsic epigenetic heterogeneity between cells. As a result, a combination of several individual cell predictions yielded an accurate average of the age of a specific tissue. These results suggest a high heterogeneity of the aging process even within the same tissue, depending on many epigenetic factors, cell type, and biological characteristics (stem, proliferating, terminally differentiated, etc.). The authors also suggested that some cells undergo accelerated or delayed epigenetic aging, which was previously impossible to establish [20]. The data obtained led to the conclusion that during the aging process of cells and tissues, the epigenetic clock in each cell or group of cells probably ticks independently.

LIFE IS IMPOSSIBLE TO TURN BACK, AND TIME CANNOT BE STOPPED EVEN FOR A MOMENT

The preservation of youth and prolongation of active life have been important for mankind. The development of the epigenetic clock concept has enabled the study of the aging processes at a new level, both for individual cells and tissues and for the whole organism. Certain therapeutic interventions can change the readings of the epigenetic clock toward a decrease in biological age (with the same chronological age) and an increase in the predicted lifespan. Specifically, the Thymus Regeneration, Immunorestoration, and Insulin Mitigation study showed that therapy for several months with recombinant human growth hormone together with dehydroepiandrosterone and metformin in apparently healthy men aged 51–65 years contributes to slowing down aging, which is eventually accompanied by a significant decrease in epigenetic age according to the GrimAge predictor [21].

In a recent study on rats [22], which used six different epigenetic clocks developed based on DNAm data, the infusion of plasma components from young rats into old rats for 5 months led to normalization of the biochemical parameters of old rats, bringing them closer to those of young animals, whereas the epigenetic age of the blood and liver and heart tissues decreased by nearly three times (from the age of 25 months to the age of 7 months). Other indicators of cellular aging, not related to the epigenetic clock, also decreased significantly after the transfusion of plasma components, which demonstrated a rejuvenation effect.

Despite the very impressive results, the extent to which such approaches can affect the epigenetic age of the whole organism and whether such effects are reversible upon withdrawal of pharmacological support remain disputable questions. This issue is relevant given the pronounced heterogeneity of aging processes in various tissues, which is largely due to the peculiarities of the functioning of resident stem cells (SCs). The presence of SC niches determines the regenerative potential of organs and tissues, and their functional state undoubtedly affects the epigenetic age. In this regard, the aging of SCs and approaches to their rejuvenation should be considered separately.

RESIDENT SCs AND AGING

The regeneration of nearly all organs and tissues occurs with the participation of resident SCs which can self-renew and differentiate into various cell types due to asymmetric division. Initial beliefs stated that SCs were not subject to replicative senescence; however, sufficient evidence has revealed that they, like other cells, accumulate metabolic and genetic damage with age and are exposed to age-related epigenetic factors [23]. In addition, the criterion for organ and tissue aging is a decrease in the proportion of SCs in the corresponding niches [23].

Let us consider in more detail the aging of the main types of SCs.

Hematopoietic stem cells (HSCs) are located in the bone marrow of adult mammals and are responsible for hematopoiesis. In HSC aging, their clonal diversity decreases because of a decrease in the intensity of the proliferation of individual clones, which results in the depletion of the clonal composition of all subsequent cell generations [24]. Although the total proportion of HSCs does not change or even increase, the depletion of the clonal composition indicates a decrease in the number of functionally active HSC clones [25, 26]. Another defining characteristic of HSC aging is a shift in the profile of their differentiation toward the myeloid series due to a decrease in the lymphoid series [27]. These data are consistent well with the concept of immunological aging, accompanied by a decrease in adaptive cellular immunity caused by the depletion of the repertoire of TCR lymphocytes [21]. Age-related changes that affect all parts of the lymphoid lineage and a shift in differentiation toward myelogenesis determine a higher incidence of myeloid leukemia in older people [28, 29]; these same changes are probably the cause of the age-related growth of oncological pathology in general.

Multipotent mesenchymal stem cells (MSCs) are located in the bone marrow, adipose tissue, and several other organs and tissues of adult humans, differentiate into many cell types, such as fibroblasts, bone, cartilage, fat, muscle, and other stromal cells, and play an important role in the regeneration of these tissues [30, 31]. The aging of MSCs is functionally characterized by a decrease in their proliferative activity and consequently a decrease in their share in the bone marrow and other tissues and a decrease in their ability to differentiate [32]. Signs of MSC aging include the appearance of granular morphology in the presence of decreased secretion of various molecules, which is called the senescence-associated secretory phenotype [33]. In addition, MSC aging is accompanied by changes in nuclear morphology and the formation of a distinct chromatin structure called senescence-associated heterochromatic foci [34]. Currently, MSC aging is assessed by measuring β-galactosidase activity, telomere length, gene expression markers, CpG methylation, and other epigenetic markers [35].

Intestinal stem cells (ISCs) support the regular renewal of the gastrointestinal tract epithelium. Most of our knowledge of ISC aging is obtained from studies in Drosophila, where ISCs are easily identified by the expression of the snail transcription factor (Esg) and the Notch delta ligand (D1). During aging, the count of cells with the ISC immunophenotype increases several times, which is accompanied by a decrease in their function [36, 37]. This increase is associated with an impaired self-renewal process, accompanied by the partial differentiation of ISCs with the preservation of some markers of SCs [36]. Mammals have two interconvertible ISC populations, namely, proliferatively active Lgr5-expressing cells located at the base of the crypt and resting SCs located above the base of the crypt [38]. Experiments with gamma irradiation have shown that although the intestine becomes more susceptible to damage with age, the total number of clonogenic units increases [39]. Aging of human ISCs, accompanied by impaired self-renewal and differentiation ability, was assumed to be one of the etiological factors of hyperplasia of the intestinal mucosa, which, in turn, leads to an increase in the incidence of colorectal cancer with age [40].

Satellite SCs are involved in the regeneration of damaged skeletal muscles [41, 42]. Unlike HSCs and ISCs, the satellite cell count markedly decreases with age [43, 44]. The proliferation rate in vitro and the potential for engraftment and regeneration of satellite cells after in vivo transplantation also decrease with age [45–47]. Moreover, like HSCs, old satellite cells demonstrate a distorted differentiation potential, whereby they differentiate toward fibroblasts rather than myoblasts, mainly due to changes in Wnt and transforming growth factor beta (TGF-β) signaling [48, 49]. Heterochronic transplantation of satellite cells from old to young mice indicates that the mechanisms underlying changes in satellite cell regeneration potential include changes in the availability of Wnt superfamily ligands, Notch, fibroblast growth factors, and TGF-β [48, 50, 51] and changes in cytokine signaling via the janus kinase/signal transducer and activator of transcription protein pathway [52]. In addition to microenvironmental changes, self-renewal defects and increased stress-induced p38-MAPK signaling are associated with satellite cell aging [45, 46], and these changes are not reversed even after transplantation into a young environment [46, 47]. Thus, irreversible changes in satellite cells underlie senile sarcopenia, which is present to varying degrees in all older people.

Neural stem cells (NSCs). Neurogenesis in the adult mammalian brain occurs throughout life in the subventricular (SVZ) and subgranular (SGZ) zones of the brain [53]. In the adult brain, neurogenesis is mainly involved in the regeneration of olfactory cells, formation of new neurons and gliocytes in structures that implement memory and other cognitive processes, and maintenance of neuronal plasticity.

Aging is a long and progredient process, and increasing homeostasis disorders affect all niches of resident SCs, including those in the brain [4]. The NSC count, like many other resident SCs in an adult organism, decreases with age, which in turn is accompanied by a decrease in the neurogenesis level, up to its complete termination in anthropoid primates and humans and consequently deterioration in cognitive functions. This is primarily due to a decrease in the pool of proliferating NSCs [54].

Recent studies have shown that in aging in the SVZ and SGZ, there is a decrease in the total count of proliferating NSCs with the Nestin + Mcm2+ phenotype and an increase in the proportion of resting NSCs with the Nestin + MCM2- phenotype and in a state of cell cycle arrest [55], i.e., a decrease in neurogenesis with age may be due not only to an absolute decrease in NSCs but also to the transition of NSCs to a resting state due to an age-related decrease in activating signals. The resting state for NSCs does not imply a transition to some kind of anabiosis, and from the point of view of the synthesis of extracellular matrix proteins and signaling molecules, a resting cell can be even more active than a proliferating one [56]. In addition, the resting state is biologically important for maintaining the SC pool throughout life.

Experimental studies have shown that NSCs in the SVZ of old mice become less sensitive to activation signals; however, once activated, old NSCs do not differ from young ones in terms of proliferation and differentiation [57]. Age-related changes in the transcriptome of activated old NSCs were also not detected, which suggests that the aging of SC niches affects NSCs that are dormant throughout life to a lesser extent than their environment. Microenvironmental aging of the SC niche is manifested by a decrease in the serum level of important signaling molecules, such as insulin and insulin-like growth factor (IGF), a decrease in the secretory activity of the choroid plexus, local changes in the activity of signaling pathways, specifically the inactivation of the Wnt pathway [57], and changes in the IGF-1/BMP5 ratio, which leads to an increase in the proportion of resting NSCs [56–57].

From the point of view of active longevity, functions of old NSCs can be quickly normalized to the state of young ones when stimulated in vitro. Thus, the conditioned medium from the choroid plexus cells of young mice stimulates the proliferation of old NSCs, and vice versa, the medium from the choroid plexus cells of old mice inactivates the proliferation of young NSCs due to a change in the BMP5/IGF1 ratio [58]. Experiments with heterochronous parabiosis (combining the circulatory systems of young and old mice) have shown that perfusing the blood of young mice to the brain of old mice restores the levels of IGF-1, GH, Wnt3, TGF-β, or GDF11 in old mice to the “young” level, which significantly activates neurogenesis and cognitive functions [59–61]. Interestingly, several age-associated factors are somehow associated with immune response and inflammation. In addition to TGF-β, which, along with its effect on neurogenesis, is an inducer of T cells, the role of β2-microglobulin in the aging of NSC niches and age-related deterioration of cognitive functions is registered [58].

Important data were obtained by NGS sequencing of individual cell transcriptomes in SC niches of young and old mice [62]. This precision study detected age-related changes in the transcriptome of endothelial cells and microglia and, interestingly, an increase in T-cell infiltration in senescent SC niches. The T cells found in the brain differed in the repertoire of T-cell receptors from the T cells in the peripheral blood of the same old mice; therefore, they might be activated by some cerebral antigens [62]. T cells in the old brain expressed γ-interferon, whereas NSC subpopulations responding to interferon were characterized by a reduced level of in vivo proliferation. In vitro experiments have also shown that T cells can inhibit NSC proliferation during co-culture, including through interferon secretion. Thus, immune mechanisms may play a special role in the aging of SC niches and age-related cognitive decline [62].

Although transcriptomic studies did not reveal age-related changes in the genetic expression in old NSCs, aging also affects directly NSCs, which is manifested by biochemical disorders, particularly lysosome dysfunction, manifested by impaired protein degradation and intracellular accumulation of protein aggregates (specifically β-amyloid in Alzheimer’s disease) [57]. The lysosomal activity of NSC decreases with age, which leads to the accumulation of protein aggregates and loss of the ability to activate. The stimulation of lysosome function prevents this aging-associated decline in activity [63].

Ibrayeva et al. [55] revealed that activated NSCs in the dentate gyrus of the hippocampus decrease significantly with age and detected certain molecular signs of aging, including an increase in the expression of Abl1 tyrosine kinase which is a protooncogene. Notably, the inhibitor of Bcr-Abl-tyrosine kinase, the antitumor drug imatinib, can partially restore the function of aging NSCs and slow down their aging process.

Changes in NSC niches in human aging are much less studied than similar processes in laboratory animals; however, most researchers believe that adult neurogenesis decreases considerably with age, starting from the first year of postnatal life [64–66].

Skin SCs. The skin contains several types of SCs, including basal layer SCs that ensure epithelium regeneration, hair follicle SCs (HFSC) that support hair growth, and melanocyte SCs that generate pigment-producing melanocytes. Hair follicles have phases of growth, regression, and rest. The most pronounced change during aging is an increase in the dormant period and, in some cases, a complete cessation of hair growth and loss of hair follicles (alopecia) [67]. Despite the natural loss of hair with aging, the HFSC count does not decrease with age [68]. Instead, their loss of function underlies the lengthening of periods of rest. Unlike HFSCs, melanocyte SC count in the skin decreases dramatically with age. This decrease is associated not with apoptosis but with ectopic differentiation and impaired self-renewal of SCs [69]. Ionizing radiation and genotoxic stress enhance melanocyte differentiation, which is the main cause of age-related graying of hair [70].

Germinal SCs. In mammals, the male germ line is maintained by spermatogonial SCs (SSCs), and its count decreases progressively with aging [71, 72]. Despite the decline in SSCs, in the vast majority of mammalian species, including anthropoid primates, males remain fertile throughout their lives. In contrast to males, female oogenesis from oogonia in the developing ovary ceases before birth in most mammals, except for a few species of bats, bush dogs, and chinchillas [73, 74]. Some researchers have suggested that these species, and perhaps even all other mammals, have oogonial SCs (OSCs) capable of generating oocytes postnatally. OSCs have been isolated from mice and rhesus monkeys that generate oocytes in vitro and, in the case of mice, have been used to generate transgenic offspring [75, 76]. OSCs have also been described in adult ovaries that can form oocyte-like cells when transplanted into ovarian tissue [77]. However, other groups found no evidence of postnatal oogenesis [78] or tried unsuccessfully to isolate OSCs [79].

SC REPROGRAMMING WITH REDUCED EPIGENETIC AGE

The expression of four transcription factors from the Yamanaka cocktail (OCT4, SOX2, KLF4, and c-MYC; OSKM) converts somatic cells into induced pluripotent SCs (IPSCs) [80]. Reprogramming occurs through global chromatin remodeling, which ultimately returns the cell to a pluripotent state corresponding to embryonic SCs, including the DNAm pattern [81]. However, this opens up great prospects for cell therapy, since having obtained autologous IPSCs, they can differentiate into the desired cell type and thus rejuvenate cells, tissues, and organs.

Transcriptomic single-cell analysis during obtaining IPSCs revealed some interesting patterns [82–84]. Thus, two phases were identified in cellular reprogramming. The stochastic phase is characterized by differential expression of genes involved in the cell cycle (e.g., Ccnb1 and Cdkn2b), mesenchymal–epithelial transition (e.g., Snai1 and Cdh1), and silencing of genes involved in cell adhesion and differentiation (e.g., Col1a1, Fbln5, and Mmp14) [83]. These initial changes are followed by phase 2, a deterministic or hierarchical one, characterized by progressive activation of pluripotent master genes (Nanog, Oct4, Sox2, c-Mic, Dnmt3L, etc.) [83]. Epigenetic remodeling starts as early as phase 1 of reprogramming and is characterized by the modification of histones, such as H3K4 me3 and H3K27 me3, followed by changes in the DNAm profile, including those in Nanog, Oct4, and Rex1.

Olova et al. [85] showed that even with partial reprogramming, in which the cell does not reach pluripotency, age-related epigenetic signatures are nullified in the first 10 days of reprogramming, i.e., during the period of increased activity of pluripotency genes, early expressions of NANOG, SALL4, ZFP42, TRA-1-60, UTF1, DPPA4, and LEFTY2. Similar results were demonstrated by Ocampo et al. [86], who performed incomplete reprogramming of cells using OKSM cyclic induction. Consequently, they showed nullification of the epigenetic age to an early postnatal state without loss of cell specialization. Transient expression of only two transforming factors from the Yamanaka cocktail, SOX2 and c-MYC, was sufficient to make the resulting cell population comparable in terms of age signatures to the population differentiated from embryonic SCs [87]. Rejuvenation of reprogrammed cells has been demonstrated at the level of telomere measurement [88], rejuvenation of mitochondria, etc. [89, 90]. Thus, partial reprogramming without reaching pluripotency may be a method of epigenetic rejuvenation of cells and tissues.

A study described reprogramming methods by obtaining IPSCs in vivo in experiments on mice [91], which theoretically makes partial reprogramming with a rejuvenating effect in vivo possible. Thus, cyclic expression of Yamanaka factors can increase the lifespan of progeria mice and improve cellular function in wild-type mice [86]. An alternative approach to in vivo reprogramming has demonstrated the reversibility of age-related changes in retinal ganglion cells and the possibility of restoring vision in a glaucoma mouse model [19]. More recently, transient expression of transcription factors in vitro was reported to reverse human fibroblast and chondrocyte aging, including the reversal of epigenetic clocks, decreased expression of pro-inflammatory genes, and rejuvenation of regenerative potential [92].

The bioRxiv service provides a very recent publication by Gill et al. [93] who claimed to have developed an unprecedented technology of rejuvenation using transient expression of transcription factors, which enables the rejuvenation of human fibroblasts by 30 years. The technology is called maturation phase transient reprogramming (MPTR), which involves the transfection of a polycistronic cassette with Oct4, Sox2, Klf4, c-Myc, and GFP genes under the control of a doxycycline promoter. The genes were expressed ectopically until reaching the maturation phase, after which the expression was deactivated. MPTR rejuvenates significantly all measured molecular signs of aging, including the methylation profile, without loss of the original cellular phenotype.

The aforementioned studies have suggested that in the future, the “rejuvenation” technology of SCs ex vivo with their subsequent transplantation into resident SC niches will become possible, which will probably result in a radical rejuvenation of organs and tissues.

GENETIC AND EPIGENETIC APPROACHES TO SC REJUVENATION

Let us consider approaches to the rejuvenation of SCs using MSCs as one of the most studied types of SCs in regenerative medicine. Interest in MSCs is associated with the fact that, along with the availability and relative ease of propagation [94], they have a significant paracrine therapeutic potential, which generally led to their wide clinical application [95, 96]. The efficiency of MSCs has been demonstrated in the treatment of various diseases, including graft-versus-host disease [97], Crohn’s disease [98], diabetes mellitus [99], multiple sclerosis [100], and myocardial infarction [101]. As we have already noted, the functional activity of MSCs decreases significantly with age. In an attempt to rejuvenate MSCs, various genetic modifications, treatment with miRNAs and noncoding RNAs [102], preconditioning under hypoxic conditions or in the presence of various cytokines, etc., have been undertaken [102, 104]. Many researchers use MSCs as an object of partial reprogramming by pluripotency factors, as described in the previous section [104–107].

Liang et al. [108] revealed that CLOCK gene deficiency accelerates hMSC aging, whereas CLOCK overexpression, even in a transcriptionally inactive form, rejuvenates old hMSCs. Based on the idea that CLOCKs form complexes with nuclear membrane proteins and KRAB-associated protein 1 (KAP1) and thus stabilize heterochromatin, the authors used CRISPR/Cas9 genome editing to increase the level of CLOCK expression in senescent MSCs, which resulted in their rejuvenation and promoted cartilage regeneration in mice.

Recently, Jiao et al. [109] performed transcriptomic analysis of conventional MSCs and rejuvenated MSCs passed through the pluripotency stage. In the latter, the level of expression of GATA-binding protein 6 (GATA6), whose expression is associated with the development of some types of chemoresistant pancreatic carcinomas, was reduced [110]. The GATA6 expression level is inversely correlated with the activity of the sonic hedgehog signaling pathway and the expression level of forkhead box P1, which is known to slow down MSC aging [110]. Thus, GATA6 knockout could be used for MSC rejuvenation ex vivo.

Recently, L. Deng et al. [112] obtained an experimental model of DiGeorge syndrome on MSCs with a deficiency of critical region 8 (DiGeorge syndrome critical region 8, DGCR8) with an accelerated aging phenotype. DGCR8 maintains heterochromatin organization by interacting with the nuclear envelope protein Lamin B1, heterochromatin-associated KAP1, and heterochromatin protein 1 (HP1), thus regulating MSC aging. Similarly, yes-associated protein (YAP) was also found to significantly slow down aging. Fu et al. [110] generated YAP-deficient MSCs with a premature cellular senescence phenotype and found that YAP cooperates with the TEA domain transcription factor to activate the FOXD1 anti-aging factor.

Ren et al. [113] obtained CBX4-deficient MSCs and showed that CBX4 counteracts MSC aging by maintaining nucleolar homeostasis by recruiting nuclear protein fibrillarin and heterochromatin component KAP1 to nucleolar rDNA and stabilize it, thereby limiting rRNA overexpression and slowing down cell aging. Gene therapy using lentiviral transduction of the DGCR8 gene attenuates effectively MSC aging, as evidenced by the suppression of cellular senescence markers and inflammation factors [112]. By analogy, lentivirus-mediated transfer of YAP, FOXD1, or CBX4 also rejuvenates old MSCs [110, 113]. Thus, the correction of the expression of certain genes in resident SCs can induce their rejuvenation, which in turn can contribute to the f repair and rejuvenation of the corresponding body tissue.

Several studies have focused on the epigenetic modulation of senescent MSCs. As noted above, aging is accompanied by selective methylation of GPG DNA islands. Indeed, the expressions of several DNA methyltransferases increase during replicative senescence and correlate with CpG hypermethylation in old MSCs [114]. The deactivation of DNAm with the DNA methyltransferase inhibitor 5-azacytidine reverses the aging phenotype of MSCs, decreases the accumulation of reactive oxygen species (ROS), improves the activity of superoxide dismutase, and increases the BCL-2/BAX ratio [114].

Several studies have revealed that MSC aging can be reversed by modulating ROS generation and activating antioxidant systems. Ascorbic acid inhibits ROS production by activating AKT/mTOR signaling in MSCs [116]. Another group showed that the iron-containing protein lactoferrin inhibits ROS production induced by hydrogen peroxide and suppresses caspase-3 and AKT activation [117]. MSCs pretreated with a natural plant-derived antioxidant derived from Cirsium setidens can inhibit ROS production and reduce the expression of phosphorylated mitogen-activated protein kinase p38, N-terminal c-Jun kinase, and p53 [118].

Mitochondrial dysfunction is often regarded as a typical sign of senescent cells. Melatonin can slow down MSC aging by activating mitochondrial function through the involvement of 70-kDa heat shock protein 1L (HSPA1L) [119]. HSPA1L binds to COX4IA, a protein of the mitochondrial complex IV, which leads to an increase in the potential of the mitochondrial membrane and activity of antioxidant enzymes [119]. A decrease in carnitine palmitoyltransferase 1A (CPT1A) also activates mitochondrial respiration and reverses MSC aging [120]. Similarly, an increase in FGF21 level improves mitochondrial function, rejuvenates aging MSCs, and increases mitochondrial dynamics [121].

Various studies in regenerative medicine have focused on the use of exosomes containing various biologically active substances, both from MSCs and other SC types. In some studies, exosomes have been used for cell rejuvenation. Thus, extracellular vesicles derived from embryonic SCs (ESC-Exos) were used as a rejuvenation factor for MSCs and showed that the rejuvenating effect is mediated through the activation of the IGF1 signaling pathway [122]. The use of ESC-Exos rejuvenates endothelial cells, enhances angiogenesis, reduces ROS levels, and promotes pressure ulcer regeneration in aging mice [123, 124]. Compared with reprogrammed cells, the use of exosomes is not accompanied by any oncological risks and therefore appears to be a very promising and interesting direction in anti-aging.

CONCLUSION

Aging is a complex genetically determined process, which implementation has its characteristics both at the cellular level in resting, proliferating, and differentiating SCs and at the organ and organism levels. Such a large “orchestra,” which is the mammalian organism, cannot but have a common “conductor” for synchronizing the aging processes and adequate inclusion of genetic programs in response to external factors of aging in cells, organs, and tissues. The role of the main “conductor” can be claimed by the genetically regulated DNAm system, which forms the basis of the epigenetic clock that determines the biological age of cells, tissues, and organs. Organ and tissue aging is largely due to age-related changes in resident SCs. The latter represents an attractive target for cellular rejuvenation because they can be selected, cultured ex vivo, modified, and re-introduced into resident niches. Thus, the rejuvenation of SCs with a decrease in their epigenetic age may contribute to the juvenilization of the corresponding organs and tissues.

The close relationship of aging, regeneration, and oncogenesis with each other and with the functioning of resident SC niches requires further precision research, which will undoubtedly result in the creation of an effective anti-aging strategy and prolongation of human life actively.

ADDITIONAL INFORMATION

Author contribution. V.P. Baklaushev — review concept, manuscript writing and editing; E.M. Samoylova — collection of material, manuscript writing and editing; V.A. Kalsin, G.M. Yusubalieva — collection of material, manuscript editing. The authors made a substantial contribution to the conception of the work, acquisition, analysis, interpretation of data for the work, drafting and revising the work, final approval of the version to be published and agree to be accountable for all aspects of the work.

Funding source. The work was carried out within the framework of the state task of the FMBA of Russia (code “Reprogramming”).

Competing interests. The authors declare that they have no competing interests.

About the authors

Vladimir P. Baklaushev

Federal Scientific and Clinical Center for Specialized Medical Assistance and Medical Technologies of the Federal Medical Biological Agency; Engelhardt Institute of Molecular Biology; Pulmonology Scientific Research Institute under Federal Medical and Biological Agency of Russian Federation

Email: baklaushev.vp@fnkc-fmba.ru
ORCID iD: 0000-0003-1039-4245
SPIN-code: 3968-2971
https://fnkc-fmba.ru/about/komanda-upravleniya/

MD, PhD, Chief Scientific Officer

Russian Federation, 28, Orekhovy blvd, Moscow, 115682; Moscow; Moscow

Ekaterina M. Samoilova

Federal Scientific and Clinical Center for Specialized Medical Assistance and Medical Technologies of the Federal Medical Biological Agency; Engelhardt Institute of Molecular Biology

Email: samoyket@gmail.com
ORCID iD: 0000-0002-0485-6581
SPIN-code: 3014-6243

MD

Russian Federation, 28, Orekhovy blvd, Moscow, 115682; Moscow

Vladimir A. Kalsin

Federal Scientific and Clinical Center for Specialized Medical Assistance and Medical Technologies of the Federal Medical Biological Agency; Engelhardt Institute of Molecular Biology

Author for correspondence.
Email: vkalsin@mail.ru
ORCID iD: 0000-0003-2705-3578
SPIN-code: 1046-8801

научный сотрудник лаборатории клеточных технологий

Russian Federation, 28, Orekhovy blvd, Moscow, 115682; Moscow

Gaukhar M. Yusubalieva

Federal Scientific and Clinical Center for Specialized Medical Assistance and Medical Technologies of the Federal Medical Biological Agency; Engelhardt Institute of Molecular Biology

Email: gaukhar@gaukhar.org
ORCID iD: 0000-0003-3056-4889
SPIN-code: 1559-5866

MD, PhD

Russian Federation, 28, Orekhovy blvd, Moscow, 115682; Moscow

References

Supplementary files