Restoring aged stem cell functionality: Current progress and future directions

1.1 |. Aging phenotypes of stem cells

Stem cell aging phenotypes can be either tissue-specific or generalized to most adult stem cells, and age-associated alterations in function are driven by contributions from both intrinsic and extrinsic factors. Extrinsic contributions include elevated inflammation, also referred to as inflammaging, in which the increase in pro-inflammatory cytokines compromises the function of stem cells, ultimately reducing their ability to maintain homeostasis within the body.¹ Reactive oxygen species (ROS) also contribute to genetic and organelle-level damage leading to reduced stem cell function as damage can accumulate over an organism’s lifetime.^2,3 Additionally, dysregulation of autophagy in stem cells can result in the buildup of critically misfolded proteins affecting stem cell potential.⁴ There are also unique epigenetic landscapes in aged stem cells, which may further contribute to their altered function.^5–7 Interestingly, with the exception of DNA damage, many of these features are not permanent, suggesting opportunities for manipulation to mitigate their effects on tissue-specific stem cells, described in this section (Figure 1).

1.2 |. Neural stem/progenitor cells (NSPCs)

Neurogenesis is maintained in specific regions of the adult mammalian brain to facilitate memory formation by NSPCs. Recent studies using clonal tracing assays suggest the adult NPCs can be bi-potent, generating oligodendrocytes and neurons.⁸ With age, mouse NSPCs in the ventricular-subventricular zone and the hippocampal region decline in number and have increased p16^INK4a expression (perhaps indicative of senescence).^9–12 However, unlike many other stem cell types, the function of each non-senescent NSC is not changed significantly with age when measured in vitro.¹¹

1.3 |. Muscle stem cells/satellite cells (MuSCs)

MuSCs, also called satellite cells, are found in skeletal muscle tissue below the lamina surrounding muscle fibers and are responsible for skeletal muscle repair and growth.^13,14 With age, muscle repair slows, resulting in increased repair time, reduced muscle mass, and increased muscle fibrosis.^15–17 MuSCs not only decline in number over time,¹⁸ but the functional potential of individual MuSCs decline while the number of senescent MuSCs increase.^15,18–22 The functional decline of MuSCs has been associated with increased DNA damage in both mice and humans, reduced autophagy leading to increased ROS, and changes in the TGF-β1 and Wnt pathways.^3,22–24 Recent single cell data has shown aged MuSCs have increased DNA methylation in repeat elements and more heterogeneity in promotors regions compared to young MuSCs, and these epigenetic changes are associated with increased transcriptional variability in the aged cells.⁶

1.4 |. Hematopoietic stem cells (HSCs)

HSCs, predominantly located within the bone marrow, are responsible for the maintenance of all blood cells, many of which have high turnover rates. Unlike MuSCs and NSCs, HSCs increase in number with age. However, aging is still associated with reduced competitive fitness and myeloid-biased differentiation potential of these stem cells.^25,26 Age-associated changes in HSCs have been linked to epigenetic alterations,^5,7 metabolic changes,²⁷ and DNA damage accumulation.^28,29

1.5 |. Intestinal stem cells (ISCs)

The gut epithelial layer is constantly replenished by a reservoir of ISCs. In Drosophila, ISCs increase in number but decrease in function with age, similar to HSCs, as both cell types are responsible for maintaining high turnover rates.³⁰ However in ISCs, the increase in number is partially attributed to environmental factors, as suggested by studies measuring ISC changes using bacterial infection.³¹ Aged ISCs from mice show an inhibited DNA repair response to radiation compared to young,³² and mutations in these cells can drive adenomas.³³

1.6 |. Skin stem cells (SSCs)

SSCs can be classified into epidermal- (EpSCs), hair follicle- (HFSCs), and melanocyte- stem cells (MeSCs). In young, healthy individuals, the integrity of the skin is maintained by stem cells in the epidermal layer. Both EpSCs and HFSCs preserve their numbers with age but exhibit functional loss due to lengthened dormancy periods,^34,35 leading to phenotypes such as balding and wrinkles. MeSC number declines with age, which results in loss of pigmentation underlying phenotypes such as greying hair.³⁶ Many of these skin stem cell changes are attributed to extrinsic stressors due to constant environmental exposure.^37–39

1.7 |. Mesenchymal stromal cells (MSCs)

MSCs are progenitor cells, which differentiate into connective tissue such as bone, cartilage, and muscle tissue.⁴⁰ As MSCs age, gradual genetic and epigenetic changes accumulate and contribute to drastically impaired functionality.⁴ Bone-marrow derived MSCs (bmMSCs) collected from aged donors exhibit reduced colony forming efficiency and osteogenic potential when compared to those from young individuals.⁴¹ Aged MSCs also have increased expression of p53, p21, and BAX and have significantly higher levels of ROS corresponding with decreased levels of superoxide dismutase activity.⁴² MSCs are known to modulate their tissue micro-environment through their highly diverse secretomic profiles,^43,44 which are drastically altered with age.^45–47 As a result, the secretomes from aged bone-marrow derived MSCs have decreased immune-modulatory properties and can impede HSC differentiation and proliferation.⁴⁶

1.8 |. Other adult stem cells

In addition to these stem cell types, there are other adult stem cells including skeletal,⁴⁸ lung,⁴⁹ germline,⁵⁰ and possibly others. Many of these stem cell types are still being characterized and/or little has been described about their aging phenotypes; thus, they will not be discussed in this review.

2.1 |. Reprogramming

Since the discovery that both mouse and human somatic cells could generate induced pluripotent stem cells (iPSCs) by overexpression of the transcription factors Oct4, Sox2, Klf4, and c-Myc (OSKM),^51,52 researchers have been studying ways to harness the potential of cellular reprogramming for regenerative medicine purposes.

Several studies have specifically focused on reprogramming aged stem cells into iPSCs and then re-differentiating them back into the original adult stem cell to restore function. This technique was initially used to reprogram aged murine HSCs. The HSC-derived iPSCs were implanted into mouse blastocysts and differentiated back to HSCs in vivo.^53,54 The bone marrow from these chimeric mice was transplanted into lethally irradiated mice to examine the potential of the iPSC-derived HSCs, originally from old mice. There were no measurable differences in the functional potential between these HSCs and HSCs isolated from young mice. More recently, a similar technique was used with human MSCs isolated from 60–70-year-old individuals.⁵⁵ These rederived MSCs had functional potential similar to that of young MSCs and presented a rejuvenated gene signature. To the best of our knowledge, no study has yet used this paradigm to examine restoration of functional potential to old NSCs, MuSCs, ISCs, or SSCs. However, high-functioning iPSC-derived NSCs, SSCs, ISCs, and MuSCs have been successfully made and used in non-aging-focused studies, suggesting this may be a potential avenue of restoration for multiple types of stem cells.^56–59

Rather than isolating and implanting rejuvenated stem cells, reprogramming at the organismal level aimed to directly produce the same benefits in vivo. However, while pulsing of reprogramming factors has been shown to generate short-term beneficial effects, long-term expression of these factors causes teratoma formation.⁶⁰ To obtain the benefits of reprogramming while minimizing the risk of cancer, the idea of partial reprogramming was introduced. Transient expression of OSKM in a transgenic model of progeria (LAKI-4F mice) was shown to alleviate hallmarks of aging, including cellular senescence, as measured by a decrease in p16^INK4a, MMP13, IL-6, and senescence-associated β-galactosidase activity.⁶¹ Moreover, an increase in the number of MuSCs and HFSCs was seen in partially-reprogrammed mice. Using surrogate analyses including rescued lymphoid output of the white pulp in the spleen, increased epidermal and dermal thickness, and rescued loss of parietal cells and gastric epithelium thinning, suggested improvements in HSCs, SSCs, and ISCs function, respectively. Short-term pulsing of the OSKM factors in these mice was also associated with significant improvement in overall health and lifespan. This study, along with others, shows that it is possible to reset some age-associated phenotypes without significantly affecting the epigenetic landscape required for defining cell identity.^62,63

To demonstrate the benefits of partial reprogramming in a non-transgenic model, modified mRNA for OKSM, LIN2, and NANOG were expressed for 2 days in MuSCs isolated from 20 to 24-month-old lead to enhanced myogenic potential. Similar results were also seen in treated human MuSCs isolated from patients between 60 and 80 years of age. Functionally, transplants of the aged, treated MuSCs generated myofibers with a greater cross-sectional area than those generated by young MuSCs.^64,65

Another avenue for reprogramming focuses not on generating adult stem cells by reprogramming back to iPSCs, but rather on transcription-mediated reprogramming of differentiated cells to adult tissue-specific stem cells. These reprogramming assays have been done in HSCs,^66,67 MSCs,^68,69 NSCs,⁷⁰ intestinal progenitor cells,⁷¹ and skeletal muscle progenitors.⁷² Though many of these studies were performed in young animals, they suggest the potential for manipulation of adult, and even aged, cells to generate tissue-specific stem cells. In an aged paradigm, reprogrammed stem cells may be able to compensate for stem cell populations that have become depleted or functionally impaired. It is yet unclear, though, if transcription-mediated reprogramming to tissue specific stem cells, and not iPSCs, will fully reset age-associated alterations.

2.2 |. Diet-based interventions

Another approach to functional restoration is through diet. Various diet-based interventions have been shown to improve life and health span across a number of model organisms.^73–75 While there are many nuances between these interventions, they can be generally categorized into three groups: caloric restriction (CR), fasting, and fasting-mimicking (FM).

3.1.1 |. Heterochronic blood exchange & young plasma injections

In a parabiosis-inspired experiment, the blood of young and old mice was exchanged, leading to improved aged MuSC function as measured by muscle wound healing and improved hepatogenesis, indicating improved aged liver progenitor cell function.¹⁰² These improvements were, in part, attributed to a decrease in beta-2 microglobulin levels in aged tissue. Similarly, in another parabiosis-inspired experiment, young plasma injected into old mice increased K-Creb levels and improved neurogenesis and synaptic plasticity, which may indicate improved NSC function.¹⁰³ These data suggest that restoring circulating factors found in young blood have wide-reaching positive effects, though the mechanisms by which they affect different stem cells do not seem to be universal.

Analysis of one such circulating factor, α-Klotho, was examined in muscle repair aging studies. α-Klotho is decreased with aging and associated with decreased tissue repair.¹⁰⁴ This factor is typically upregulated upon acute injury in young muscle, but this response is mitigated in old tissue. By supplementing this single circulating factor, important for mitochondrial potential, to aged mice, there were significant improvements in muscle regeneration.¹⁰⁴

Another circulating factor, GDF11, was administered to aged mice and showed reversal of age-related cardiac hypertrophy,¹⁰⁵ restored genomic integrity and functional potential of MuSCs²⁴ and improved neurogenesis and NSC numbers in the SVZ and hippocampus in aged mice.^101,106 However, other studies have demonstrated that GDF11 may have some negative effects as well, and others were not able to reproduce the positive benefits in MuSCs.^107–109 Another study showed that many assays cannot distinguish GDF11 and myostatin. Using a method that can discriminate them, they demonstrated GDF11 levels do not change with age in humans, and increases in GDF11 are associated with comorbidity, frailty, and worse surgical outcomes in individuals with cardiovascular disease.¹¹⁰ These differing results regarding GDF11 may thus be due to detection of myostatin, differences in GDF11 sources, dosages, in vitro vs in vivo studies, and possibly differences in animal age.

3.1.2 |. Secretome-based Rejuvenation

Studies from circulating factors have demonstrated that rejuvenation of aged stem cells can be mediated by factors derived from young tissue.^20,99,102 Thus, it should follow that the secretome harvested from MSCs may also potentially have rejuvenating effects on aged tissue. Indeed, the secretome of young MSCs has been shown to promote cardiac repair,¹¹¹ differentiation of NSCs into functioning glial cells and astrocytes,^112,113 and differentiation of MuSCs into functional muscle tissue.¹¹⁴ More recently, it was discovered that the secretome from umbilical cord-derived MSCs was able to induce differentiation and self-renewal of old MSCs.¹¹⁵

3.1.3. |. Stem cell niche rejuvenation

Although blood-based circulating factors have been most commonly explored for stem cell rejuvenation, mechanical and biological cues from the extracellular matrix (ECM) can influence stem cell phenotype and behavior. Of particular interest, each stem cell niche has its own unique microenvironment, differing in stiffness, porosity, and topography.¹¹⁶ Therefore, it logically follows that age-related changes of the ECM can negatively affect the local stem cell niche, whereas targeted ECM modifications can improve specific stem cell function.¹¹⁷

The proliferation and differentiation capacity of passage 3 adult human synovium-derived MSCs (sMSCs) in different microenvironments were compared in vitro.¹¹⁸ MSCs seeded onto a decellularized ECM secreted by fetal sMSCs greatly increased proliferation and differentiation capacity, chondrogenic potential, and expression of pluripotency markers compared to MSCs seeded on plastic or decellularized ECM secreted by adult sMSCs.¹¹⁸

Other strategies utilized graphene and graphene oxide-based scaffolds to improve osteogenic and chondrogenic potential of MSCs in vitro.¹¹⁹ Similarly, graphene nanoparticles have been used to increase long-term proliferation of human NSCs in vitro.¹²⁰ Other studies have shown increased osteogenesis of MSCs on the surface of polycaprolactone nanofibers and increased NSC proliferation on the surface of a graphene oxide-coated polycaprolactone nanofibers in vitro.¹¹⁹ These results suggest that ECM modification may be capable of promoting stemness. Thus, delivery of these synthetic material in vivo may recapitulate similar effects to promote proper tissue restoration and phenotype recovery.^119–121

ECM modifications can also be performed by directly injecting ECM molecules into the aged stem cell niche. MuSCs from 20–24-month-old C57BL/6Rj mice displayed improved muscle regeneration and repair in muscle healing, increased proliferation, and greater myogenic commitment to levels seen in young MuSCs when injected with fibronectin.¹²² These improvements were measured by expression of Pax7+, MyoD1, and Ki67. Another study found that loss of Collagen XVII (COL17A1) expression resulted in skin stem cell aging.¹²³ Through an in silico screen, they determined that small molecules Y-27632 and apocynin could induce COL17A1 expression. Young C57BL/6 mice injected with either Y-27632 or apocynin had improved wound healing in full-thickness skin wounds indicating improvement in EpSC function.¹²³ Furthermore, transplants of endothelial cells from young mice into old mice have been used to improve HSPC function, as well as lung and liver regeneration.¹²⁴ Markedly, aged HSPCs transplanted with young endothelial cells showed significantly improved engraftment ability and increased output of B and T cells suggesting these young cells provide HSCs provide a hospitable niche and secrete factors that are important for lymphoid cell development.

3.2.1 |. NAD⁺ precursors

NAD⁺ is an energy carrier molecule within the body that has been shown to decline with age. Many studies have shown that increasing NAD⁺ levels by supplementing various precursors improves many aging phenotypes.¹²⁵ Looking specifically at MuSCs in C57BL/6JRj mice, 6 weeks of nicotinamide riboside (NR) treatment increased numbers of MuSC in young and old mice, as well as NAD⁺ concentrations within those MuSCs. In the aged mice, there was enhanced muscle function, regeneration, and stemness of the MuSCs, as measured by a reduction in differentiating factors and an increase in self-renewal factors.¹²⁶ The improvement in old, NR-treated MuSCs was associated with decreased DNA damage and improved mitochondrial function.

Ex vivo NR treatment of ISCs derived from old mice showed complete rescue of colony formation efficiency and number per organoid of differentiated buds, while young mice-derived ISCs showed no change.¹²⁷ Interestingly, they also found that rapamycin treatment blocks this functional improvement. In old mice treated for 6 weeks with NR, ISCs showed equivalent colony formation efficiency, regenerative function, and number compared to young and young NR-treated ISCs. These improvements were associated with a significant increase in SIRT1 protein in the old NR-treated ISCs.¹²⁷

In addition, NAD⁺ precursors have shown improved neurogenesis in aged mice associated with the prevention of age-associated NSC loss,^126,128 and NR treatment rescued the decline in the number of MeSCs.¹²⁶ However, to the best of our knowledge, the effect of NR and nicotinamide mononucleotide (NMN) on aged HSCs has not yet been reported.

These studies suggest that NAD⁺ boosters could have a positive effect on aged stem cells in humans. Indeed, human studies have shown promising results as NR supplementation is highly bioavailable, well tolerated in adults, and was shown to contribute to a decrease in systolic blood pressure, aortic stiffness, and levels of inflammatory cytokines.^129–132

3.2.2 |. Rapamycin

While NAD+ precursors focuses on replenishing a metabolite that declines with age to elucidate benefits, rapamycin functions by inhibition of an upregulated pathway. The mammalian target of rapamycin (mTOR) is a serine-threonine kinase which belongs to the phosphatidylinositol-3 kinases (PI3K) family that senses and integrates environmental and intracellular signals to direct cellular response.^133,134 Activation of mTOR can drive cellular aging, and inhibition of mTOR by rapamycin has been shown to delay aging and induce autophagy in experimental models and has the potential for translation for age-associated diseases such as cancer.¹³⁴

In the murine model, mTOR activity is increased in aged HSCs compared to young.¹³⁵ Administration of rapamycin improved the competitive reconstitution ability of aged HSCs and enabled effective vaccination against influenza in the aged mice.¹³⁵ Rapamycin treatment in young mice also led to a significant increase in the frequency of ISCs as well as Paneth cells, and treated ISC exhibited more robust formation of primary and secondary organoid bodies.⁸¹ mTOR repression via rapamycin also had positive effects on aged murine basal cells (which differentiate to maintain the airway epithelia), leading to a slight increase in the frequency of these cells and a balanced output between differentiated cells and basal cells.¹³⁶ Inhibition of mTOR1 rescued age-related changes in muscle-derived progenitors cells from a progeroid (Zmpste24^−/−) mouse model.¹³⁷ Furthermore, treatment with rapamycin increased autophagy, reduced senescence, and improved engraftment of 28-month-old MuSCs transplanted into injured young mice.²²

Rapamycin also partially restored aged bmMSC functionality by elevating autophagy and reducing ROS, p53 levels, and damaged cell levels.¹³⁸ Additionally, ex vivo experiments have attributed the ability of rapamycin to delay both replicative and H₂O₂-induced senescence (measured by p16) of human epithelial stem/progenitor cells by decreasing levels of oxidative stress and DNA damage.¹³⁹

3.2.3 |. Resveratrol

Another promising small molecule is resveratrol, a naturally occurring polyphenol derived from plants that has been shown to protect against age-associated issues.¹⁴⁰ Young C57BL/6 mice injected with resveratrol for 3 weeks showed an increased frequency and total number of bone marrow HSCs and progenitor cells, though no improvement was seen in functional potential.¹⁴¹ Resveratrol treatment before an induced damage (total body irradiation) protected the reconstitution potential of HSCs, suggesting a protective benefit conferred to HSCs,¹⁴² but it is unclear if resveratrol can restore lost potential to already compromised HSC compartments (as seen in aging).

Aged (15-month-old) rats treated with resveratrol for 45 days showed significantly increased neurogenesis in the hippocampus compared to aged matched control rats, suggesting a potential improvement in NPCs.¹⁴³ Rat MSCs also showed improved peri-odontal regeneration after resveratrol treatment even in an inflammatory microenvironment.¹⁴⁴ Resveratrol also improved in vitro cell aggregate formation potential and osteogenesis of third passage MSCs derived from human periodontitis patients between the ages of 25 to 41.¹⁴⁴

Resveratrol treatment of passage 3 MSCs isolated from one-year-old mice showed a significant reduction in markers of cellular senescence (SA-B-gal, p21, p16, and p53 expression). After 2 days of ex vivo resveratrol treatment, MSCs also demonstrated reduced levels of ROS and improved osteoblast differentiation.¹⁴⁵ Additionally, long-term resveratrol treatment maintained osteogenic potential and delayed senescence in adult human MSCs over 10 passages.¹⁴⁶

Resveratrol treatment in Nothobranchius guentheri fish reduced SA-β-gal-associated senescence and SASP factors in the gut with age as measured in 6-, 9-, and 12-month-old fish.¹⁴⁷ In addition, the treatment reversed the decline in the number and proliferation ability of ISCs at both the 9 and 12 month age groups. Resveratrol has also been shown to restore SIRT1 activity in MuSCs exposed to high levels of oxidative stress,¹⁴⁸ similar to the improved MSC response associated with increased SIRT1.¹⁴⁶ Additional studies of resveratrol treatment on MSCs¹⁴⁹ with seemingly inconsistent results highlight the importance of dose, cell age, and experimental conditions on the interpretations of the data. Furthermore, studies on other aged tissue-specific stem cells are needed to more robustly examine the cell type-specific effects of resveratrol.

3.2.4 |. Metformin

Differing in origin from the other small molecules mentioned thus far, metformin, a small molecule largely prescribed for type 2 diabetes, also has been shown to have anti-aging effects, and several studies have examined its effect on aged tissue-specific stem cells.

Aged OPCs isolated from rats typically have delayed differentiation into oligodendrocytes; however, upon metformin treatment, these cells showed enhanced differentiation capacity ex vivo.⁹⁴ In addition, 18-month-old rats given metformin showed improved remyelination, attributed to the improved OPC function. This improved function, similar to results from alternate-day fasting,⁹⁴ was associated with decreased DNA damage and Cdkn2a expression⁹⁴ and was further attributed to upregulation of the AMPK pathway. The AMPK pathway was also implicated in MSC functional restoration with resveratrol treatment.¹⁴⁵ Furthermore, NPC proliferation, differentiation, and self-renewal have also been improved by metformin treatment in vitro and in vivo in adult mice.¹⁵⁰

In a Fanconi anemia (FA) mouse model, 6 months of metformin in young mice improved hematopoiesis, increased HSPC quiescence, reduced DNA damage, and increased the number of HSCs suggesting that metformin improved HSC function in the FA model.¹⁵¹ This suggests potential beneficial effects for aged HSCs given the similar HSC features of FA and aging, including accumulated DNA damage and reduced hematopoietic repopulation capacity.

Both Drosophila ISCs and murine HFSCs showed mitigated age-associated effects (reduction of DNA damage, hyperproliferation, and centrosome amplification of aged ISCs)^152,153 and overall improved function of HFSCs¹⁵⁴ after metformin or IM176OUT05, a molecule derived from metformin, treatment. However, in mouse MuSCs, 21 days of metformin treatment led to undesirable responses including delayed activation, slowed muscle regeneration, and reduced MuSC size.¹⁵⁵

Metformin is a promising intervention for improving some aged stem cells function, and several studies have been reported on its effects on young stem cells. Interestingly, this intervention did not have universally beneficial effects on all stem cell compartments. However, it is unclear if aged stem cells will have the same response to metformin treatment, and these aged stem cell studies will be necessary to determine the efficacy of metformin as a therapeutic for stem cell aging and aging-related diseases.

3.2.5 |. Oxytocin and Alk5 inhibitor

Oxytocin is a hormone commonly known for its role in lactation and social bonding whose plasma levels are known to decline with age.¹⁵⁶ Administration of oxytocin has been shown to improve the muscle regenerative function of MuSCs in mice through activation of the MAPK/ERK signaling pathway.¹⁵⁶ Alk5 inhibition has been shown to improve myogenesis and neurogenesis in 24-month-old mice by attenuating TGFβ signaling.¹⁵⁷ In combination, co-administration of oxytocin and Alk5 inhibitor (Alk5i) for 7 days in 22- to 24-month-old C57BL/6N male mice results in further improvements in muscle regenerative function of MuSCs beyond that of either treatment alone.¹⁵⁸ In addition, hippocampal neurogenesis was improved and the number of Ki67⁺ NSCs were increased compared to control mice, resulting in improved cognitive abilities and decreased neuro-inflammation. Finally, oxytocin and Alk5i administration resulted in decreased liver adiposity and fibrosis.¹⁵⁸

Since both molecules are currently FDA-approved drugs, they represent a very appealing intervention for clinical use to improve the function of many stem cell compartments. Further in vivo studies are needed to determine the effects on ISCs, SSCs, HSCs, and MSCs and clinical trials in aged individuals are needed to evaluate the effectiveness and safety of these treatments.

3.2.6 |. Curcumin & Noggin

Turning back to plant-derived compounds, curcumin is produced by the Curcuma longa plants which has been well-noted for its anti-inflammatory and anti-oxidative properties.¹⁵⁹ This has made it a popular compound in anti-aging research.¹⁶⁰ Curcumin treatment in vitro protects human adipose-derived MSCs (aMSCs) from death when exposed to H₂O₂ and enhances osteogenic potential through attenuation of oxidative stress and Wnt/β-catenin signaling inhibition.¹⁶¹ However, this effect may be cell type-dependent. Rat-derived bmMSCs showed increased proliferation and survival in a dose-dependent manner, but no effect was observed in NPCs (with the exception of toxicity at 10 μM).¹⁶² Rat aMSCs isolated from 6- to 8-week-old animals treated with 1 to 5 μM concentrations of curcumin exhibited improved proliferation after 48 hours and a decreased number of senescent cells.¹⁶³ This effect was attributed to curcumin-induced TERT expression.

Noggin, a BMP6 antagonist, was injected into SAMP8 mice, a senescence-associated strain used to model aspects of aging, and restored neurogenesis and NSC count, and improved performance on cognitive-decline behavioral tests compared to control SAMP8 mice.¹⁶⁴

3.2.7 |. Senolytics

Long-term senescent cells and senescence-associated secretory phenotypes (SASPs) cause deleterious effects in aged organisms by both not performing normal cellular functions, but by also inhibiting proper function of other cells. Clearance of these senescent cells has been shown to be beneficial in transgenic mouse models.¹⁶⁵ Inspired by these results, researchers discovered and developed senolytics, molecules designed to clear senescent cells and reduce SASPs.¹⁶⁶ ABT263 administered to both total body irradiated mice and 20-month-old mice effectively cleared senescent cells which led to improved HSC function, as measured by balanced multilineage differentiation and improved clonogenicity and long-term engraftment ability in primary and secondary transplants.¹⁶⁷ ABT263 treatment in aged mice also showed improved clonogenicity and function of MuSCs compared to control aged mice.¹⁶⁷ In human bmMSCs, ABT263 removed both senescent and non-senescent MSCs while quercetin and dasatinib (two senolytics shown to increase lifespan)¹⁶⁸ did not have effects on either the senescent and non-senescent MSCs.¹⁶⁹ This contrasts with the results found in mouse bmMSCs, which showed that quercetin and quercetin + dasatinib significantly removed senescent MSCs.¹⁷⁰ A newer senolytic, 17-DMAG, has been shown to greatly reduce senescent bmMSCs in a progeroid mouse model (Errc1).¹⁷¹ However, these senolytics have not yet shown any functional rejuvenation of stem cells. Returning to dasatinib and quercetin, administration of this combination to mice on a high fat diet led to increased neural precursor cells and CD133⁺ ependymal cells, cells with potential NSC properties.^172,173 Furthermore, treatment of K5-rtTA/tet-p14 mice, a model which generates senescent epidermal cells, with another senolytic, ABT737, eliminated senescent epidermal cells which lead to increased proliferation and number of HFSCs.¹⁷⁴

Senolytics present a promising avenue for stem cell rejuvenation. However, more research is needed to find the most effective senolytics and to fully characterize the effects of senolytics in the various stem cell compartments.