Endogenous Stem Cells in Homeostasis and Aging

Abstract

In almost all human tissues and organs, adult stem cells or tissue stem cells are present in a unique location, the so-called stem cell niche or its equivalent, continuously replenishing functional differentiated cells. Those endogenous stem cells can be expanded for cell therapeutics using ex vivo cell culture or recalled for tissue repair in situ through cell trafficking and homing. In the aging process, inefficiency in the endogenous stem cell–mediated healing mechanism can emerge from a variety of impairments that accumulate in the processes of stem cell self-renewal, function, differentiation capacity, and trafficking through cell autonomous intrinsic pathways (such as epigenetic alterations) or systemic extrinsic pathways. This review examines the homeostasis of endogenous stem cells, particularly bone marrow stem cells, and their dysregulation in disease and aging and discusses possible intervention strategies. Several systemic pro-aging and rejuvenating factors, recognized in heterochronic parabiosis or premature aging progeroid animal models, are reviewed as possible anti-aging pharmaceutical targets from the perspective of a healthy environment for endogenous stem cells. A variety of epigenetic modifications and chromosome architectures are reviewed as an intrinsic cellular pathway for aging and senescence. A gradual increase in inflammatory burden during aging is also reviewed. Finally, the tissue repair and anti-aging effects of Substance-P, a peptide stimulating stem cell trafficking from the bone marrow and modifying the inflammatory response, are discussed as a future anti-aging target.

Introduction

Aging is a natural phenomenon marked by a progressive decrease in a body’s homeostatic and regenerative functions, including age-related deterioration of stem cell functions [1]. Recent studies between young and old animals and humans have revealed aged-related alterations in stem cell traits, stem cell pool size, and differentiation diversity [2–4]. In the aged, cell-autonomous epigenetic alterations in stem cells accumulate, which can cause senescence of stem/progenitor cells and reduce both the stem cell pool and stem cell function. Heterochronic parabiosis experiments between young and old animals have clearly demonstrated the existence of age-related alterations in systemic and local environments, including systemic rejuvenating and pro-aging factors [5–7], that could control the function and lifespan of endogenous stem cells and stem cell niches. More prominently, experiments have revealed aged-related elevation of inflammatory cytokines and chronic inflammation [8–10], which could also affect stem cell function, stem cell niche, stem cell trafficking, and differentiation diversity. Those reports about aging and degenerative diseases such as Parkinson’s disease, Alzheimer’s disease, diabetes, chronic vascular disease, and osteoarthritis have pointed out possible links between stem cell status and disease etiology and between stem cell status and life expectancy.

In 2013, Carlos Lopez-Otin et al. [2] identified and categorized nine cellular and molecular hallmarks that contribute to the aging process: genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. Most age-related damage and deterioration accumulates gradually over time, which has been extensively studied in the fields of cancer, metabolic diseases, and degenerative diseases. In particular, the concept of stem cell exhaustion, through cell-autonomous intrinsic pathways or extrinsic pathways, could explain the integrative consequences of multiple types of aging-associated damage and the decline in regenerative potential [11, 12]. Progeroid mice and recent parabiosis experiments using young and old mice have demonstrated that systemic factors such as hepatocyte growth factor activator (HGFA) that are abundant in young mice can reverse the decline in neural and muscle stem cell function found in old or progeroid mice [13–15]. Those seminal works propose that systemic extrinsic factors, acting as a pharmacological intervention, could stimulate stem cell rejuvenation and reverse the aging phenotype.

This review discusses endogenous stem cells in vivo, especially bone marrow stem cells such as hematopoietic stem cells (HSCs), bone marrow stromal (or mesenchymal) stem cells (BMSCs), and endothelial precursor cells (EPCs), based on their ontogeny, normal functions, and age-related functional alterations. Second, age-related alterations in systemic factors, candidates for anti-aging or pro-aging factors, are considered for their possible links to stem cell homeostasis and aging and for their potential as pharmacological targets to retard or control the aging process. Third, age-related alterations in the bone marrow stem cell niche are reviewed. Fourth, epigenetic changes to stem cells during aging, so-called cell autonomous stem cell aging, are reviewed and discussed in terms of histone modification, DNA methylation, and non-coding RNA. Fifth, the effects of enhanced inflammatory responses during aging are discussed, particularly in age-related degenerative diseases and defective tissue repair. Conclusions and perspectives for future research include a consideration of whether age-related deterioration in endogenous stem cells and their environment can be controlled and new pharmacological targets for human health.

Endogenous stem cells in bone marrow and aging

Homeostasis, tissue repair, and regeneration are continuously maintained and stimulated by endogenous tissue stem cells in the body. Bone marrow is a stem cell reservoir for HSCs, BMSCs, EPCs, and other stem cells yet to be identified. The stem cells in bone marrow are vitally involved in tissue regeneration, and senescence or loss of the function of those stem cells is a main cause of failure in tissue homeostasis and repair (Fig. 1) [4].

Fig. 1.

Age-related alterations in endogenous stem cells in the bone marrow. In the aging process, stem/precursor cells such as HSCs, BMSCs, and EPCs in the bone marrow decline in stem cell pool, function, and differentiation potential. HSCs tend to shift toward myeloid-biased differentiation. BMSCs preferentially differentiate to adipocytes, which in turn increase the inflammatory cytokine-enriched environment and affect HSCs by reducing the function of their niche. EPCs also reduce their vasculogenic function. All of those age-related alterations in bone marrow stem cells could result from cell autonomous changes, such as epigenetic modification, inflammation burden, and mitochondrial dysfunction, which could be targets for rejuvenation strategies

HSCs continuously supply blood cells, including immune cells, red blood cells, and white blood cells, throughout life. Hematopoietic dysfunction emerges in the aged, resulting in decreased adaptive immune response and increased myeloid responses and anemia. A prominent aging-associated alteration, so called inflammaging, a smoldering pro-inflammatory phenotype [9, 16], could also result from HSC dysfunction or loss of differentiation diversity. BMSCs are stromal cells in the bone marrow that might support hematopoiesis as stem cell niche cells as well as osteogenesis for bone formation. In the aged, BMSCs also decline, and their differentiation potential skews toward adipogenesis, resulting in yellow marrow and osteoporosis. EPCs, precursors to endothelial cells, also decline during aging, causing impairment of vascular repair and impaired vascular niche function for HSCs.

HSC definition/ontogeny/aging

HSCs are the only cells that can produce the whole blood cell lineage. They generate ~ 1 × 10¹⁰ red blood cells and ~ 1 × 10⁸ white blood cells every hour throughout life [11]. This continuous process of mature blood cell production, called hematopoiesis, involves the proliferation, self-renewal, and differentiation of HSCs and the egress of mature progenitor cells into the circulating blood [12]. Hematopoiesis occurs in ontogenetic flows. Primitive phases and definitive phases produce true HSCs. The emergence of primitive HSCs starts in the yolk sac at 30 days post-conception in humans and E7.5 in mice and then moves to the allantois and placenta. At 4 weeks post-conception (wpc) in humans and E10.5 in mice, HSCs lie in the aorta-gonad mesonephros, a region of the embryonic mesoderm that gives rise to definitive HSCs. HSCs are subsequently seeded into the placenta, thymus, and liver at 5 wpc in humans and E11 in mice, into the spleen at 8 wpc in humans and E14 in mice, and into the bone marrow at 12 wpc in humans and E18 in mice. After birth, HSCs exist only in the bone marrow and thymus, and the bone marrow supports the majority of hematopoiesis [17]. HSCs give rise to both common myeloid progenitor cells (CMPs) and common lymphoid progenitors (CLPs) to generate all blood cells. CMPs differentiate into monocytes, macrophages, dendritic cells, neutrophils, basophils, eosinophils, erythrocytes, megakaryocytes and platelets. CLPs differentiate into T cells, B cells, and NK cells (Fig. 2).

Fig. 2.

HSC hierarchy and markers for HSCs and other supporting niche cells in bone marrow. A HSC ontogeny. B Surface markers of bone marrow stem cells and niche cells. HSC hematopoietic stem cell, LT-HSC long term-HSC, ST-HSC short term-HSC, MMPs multipotent progenitors, SLAM signaling lymphocyte activation molecule, CLP common lymphoid progenitor, CMP common myeloid progenitor, GMP granulocyte macrophage progenitor, MEP megakaryocyte erythroid progenitor, DC dendritic cell, Mφ macrophage, EC endothelial cell, BMSC bone marrow stromal cell, OB osteoblast

An HSC hierarchy can be defined based on bone marrow–repopulation capacity, differentiation potential, and multiple surface marker expression. In 1994, Spangrude et al. [18, 19] divided the population of HSCs into three multipotent types: long-term (LT)-HSCs, short-term (ST)-HSCs, and multipotent progenitors (MPPs). They identified those cells using a diverse cluster of differentiation markers, such as CD34, CD38, CD90, CD133, CD105, CD45, and the stem cell factor c-kit. In 2005, to identify the functionally distinct subpopulation of HSCs from non-self-renewing MPPs, Morrison et al. [20, 21] reported that the signaling lymphocyte activation molecule (SLAM) family of receptors including CD150 (slamf1), CD48 (slamf2), CD229 (slamf3), and CD224 (slamf4) was differentially expressed among functionally distinct stem cells and progenitor cells in mice. In addition, weak staining with vital dyes such as Hoechst 33342 (side population) or rhodamine 123 can be used to isolate HSCs with small cell size. The ability of hematopoietic progenitors to proliferate and differentiate into colonies is examined with the colony forming unit (CFU) assay using methylcellulose-based semi-solid media in response to cytokine stimulation for lymphocytes (CFU-L), pre-B cells (CFU-Pre-B), granulocytes, erythrocytes, macrophages, and megakaryocytes (CFU-GEMM). To identify the cell cycle status, Ki-67 labeling, BrdU incorporation analysis, and the fluorescence ubiquitination-based cell cycle indicator (Fucci) system are used. Using the fusion of fluorescent proteins to the cell cycle–specific proteins geminin, cdt1, and p27, the Fucci system can visualize cells by staining cell nuclei in the G1, S/G2/M, and G0 phases of the cell cycle in cyan, green, and red, respectively [22].

A variety of age-related cellular and molecular alterations in HSCs and hematopoiesis have been reported. In HSC function, aging leads to the over-proliferation of HSCs, whereas aging decreases stem cell numbers and cell-cycle activity in skeletal muscle stem cells, germline stem cells, and neural stem cells [1, 23]. How those changes are controlled and regulated remains unclear. However, it was reported that aged HSCs showed decreased self-renewal, loss of cell polarity, egress into the bloodstream, impaired homing ability, and myeloid- and platelet-biased differentiation [12, 24].

Age-related changes in systemic factors, intracellular signaling pathways, and cell cycle controlling molecules also seem to be implicated in HSC self-renewal and differentiation diversity. TGF-β1 enhances myeloid differentiation rather than lymphoid differentiation [25], and the chromatin regulator Satb1, which is induced during lymphoid differentiation, decrease in aged HSCs [26]. Aged HSCs activate the non-canonical Wnt signaling pathway instead of canonical Wnt pathway, which increases Wnt5a expression and activates cell division control protein 42 (Cdc42) polarity. The regulation of Cdc42 polarity and distribution is important for regenerative capacity and bone marrow homing [27]. Also, in the aged, cell cycle checkpoints such as p16INK4a, BCL-2, BATF, and p53 are activated. Abolishing p16INK4a increases the regenerative potential of stem cells from bone marrow and brain, and a low level of p53 promotes stem cell maintenance, while a high level induces cell death and senescence [28].

Reactive oxygen species (ROS) and nitric oxide (NO) could also be important regulators in HSC aging. The accumulation of ROS in aged cells induces FOXO depletion, NF-κB activation, p38-mTOR activation, telomere shortening, DNA damage, and mitochondrial dysfunction [12]. Insulin and IGF activate the PI3K-Akt signaling pathway and phosphorylate FOXO, followed by inhibition of the expression of the anti-oxidant N-acetyl-l-cysteine [28]. After oxidative stress, HSCs increase NO levels, which results in loss of self-renewal, abnormal proliferation, and malignancy [12, 29].

BMSCs in young and old bone marrow

BMSCs, also generally called mesenchymal stem cells (BMSCs), do not express the HSC and EPC marker CD34, but they reveal plastic adherent clonogenic properties and differentiate into a variety of cell types, such as osteoblasts, chondrocytes, adipocytes, and myocytes, upon in vitro cell culture [30–32]. BMSCs express CD73, CD90, CD105, CD29, CD44, CD71, CD106, CD120a, CD124, CD56, and CD271 on their surfaces, but they lack CD11b, CD14, CD117, CD19, CD34, CD45, CD79a, and HLA-DR surface markers [33]. While in vivo BMSCs express CD146, SCA1, PDGFRα, CXCL12, and nestin, ex vivo aged BMSCs express CD106 and CD295 [34]. Enhanced expression of CD295, a leptin receptor, marks apoptotic cells and non–self renewal cells.

BMSCs have self-renewal capacity, mobilize to injury sites, and participate in immune modulation, wound healing, and repair of almost all tissues [34–37]. The main physiological capacity of BMSCs is an immune modulatory function by releasing cytokines, which seems to be independent of traditional stem cell activity. In aging, the proliferation capacity, differentiation potential, and genomic stability of BMSCs decline. Whereas extensive HSC aging research has been carried out, research into BMSCs in the aging process remains insufficient. In aged bone marrow, aberrations within the BMSC microenvironment, such as chronic inflammation, result in fat deposits that coincide with a decrease in mesenchymal progenitors, bone loss, and fibrosis [38]. This age-dependent decline in BMSC function weakens its immune modulation capacity. BMSCs from the bone marrow of aged mice showed decreased colony forming capacity. Moreover, BMSCs from aged bone marrow showed prominently reduced mobilization, possibly through downregulation of the phosphorylation of JNK signaling [39]. Therefore, maintaining a sufficient BMSC pool and competent BMSC trafficking from the bone marrow are essential to a healthy marrow environment for inflammation modulation and also to facilitate tissue regeneration following several types of peripheral tissue damage. In the aged, decreased regeneration potential, enhanced autoimmune response, and enhanced inflammatory response could all be strongly interconnected with age-related BMSC decay and dysfunction [38].

EPCs in the bone marrow and in aging

EPCs in the bone marrow and circulating EPCs in peripheral blood can differentiate into endothelial cells and form the endothelial lining of the vasculature. Both EPCs and HSCs in the bone marrow are derived from hemangioblast [40]. EPCs in the bone marrow express CD34, CD133, and VEGFR2 (KDR/Flk1). After moving from the bone marrow to the blood, circulating EPCs lose their progenitor capacity and start to endothelial differentiation, expressing von Willebrand factor, CD31, CD144, VE-cadherin, and eNOS [41, 42].

EPCs can be mobilized from the bone marrow and play a pivotal role in tissue repair with mechanisms to regenerate and maintain the endothelium by regulating coagulation, arterial tone, permeability, vessel growth, and inflammation. As aging progresses, the number of EPCs and their function decrease with increasing oxidative stress, inflammation, senescent phenotype oxidized low density lipoprotein (ox-LDL), and telomere shortening, which eventually increases the risk for vascular diseases such as atherosclerosis and cardiovascular disease [43]. EPCs from aged humans are sensitive to oxidative stress, probably due to reduced levels and activity of the antioxidant enzyme glutathione peroxidase-1 (GPX1), which then lowers cell survival [44]. Ox-LDL, a risk factor of cardiovascular disease, accumulates with age and reduces the survival and function of EPCs by inhibiting eNOS expression and activity [45]. Aged EPCs are identified by their cell survival ability and the colony forming unit assay, even though mechanisms of EPC aging have not been sufficiently studied. Therefore, EPC dysfunction with aging could also be interconnected with delayed repair of ischemic vascular damage and a higher risk of cardiovascular disease.

Systemic rejuvenation factors and pro-aging factors

Systemic change produce differences between young and old people and between healthy and diseased individuals. Active factors in stem cells can systemically and directly influence their fitness and guide their destinies.

Heterochronic parabiosis experiments have elucidated the presence of systemic rejuvenating and pro-aging factors and their alterations with age (Fig. 3). In 1864, the French physiologist Paul Bert carried out heterochronic parabiosis experiments on albino rats to study the effects of aging. Parabiosis (from the Greek words, para “besides” and bios “life”) uses a surgical technique to physically connect the blood vessels and create a shared circulating system for two living organisms of different ages [5]. In the 1950s, researchers found that old mice experienced rejuvenating effects, and the young mice had shorter than average lifespans in between surgically connecting two animals. In the 1970s, parabiosis experiments were banned by animal research regulations. However, they became active again in the 2000s for research on aging. Recent parabiosis studies have shown that rejuvenation factors in the blood can turn back the stem cell clock: the functions of old stem cells were rejuvenated, and younger stem cell functions were weakened when they were exposed to young or old serum, respectively. Systemic rejuvenating and pro-aging factors are summarized in Table 1. Growth differentiation factor (GDF) 11, oxytocin, bursicon, and HGFA are all candidate systemic rejuvenating factors, and C–C motif chemokine (CCL) 11 and β2-microglobulin (B2M) are pro-aging factors.

Fig. 3.

Heterochronic parabiosis experimental model. A Basic principle of shared-circulation in the parabiosis experimental model. Effects of systemic factors on different pairs of individuals, such as young and old, lean and obese, non-irradiated and irradiated, normal and mutant, can be explored in parabiosis animal models. B Exploration of candidate systemic pro-aging and anti-aging factors using young and old heterochronic parabiosis experiments and confirmation of their efficacy using old mice. Y young, O old

Table 1.

Systemic rejuvenation factors and pro-aging systemic factors

	Factor	Original function	Function in aging and disease	References
Systemic rejuvenation factor	GDF11	Bone morphogenetic protein 11	Regression of Cardiac Hypertrophy in C57BL/6 mice	[6]
			Vascular and neurogenic rejuvenation in C57BL/6 mice brain	[13]
	Oxytocin	Circulating hormone and neuropeptide	Muscle maintenance and regeneration in C57BL/6 mice	[14]
	Bursicon	Neuroendocrine hormone	Intestinal stem cell homeostasis in Drosophila midgut	[52]
	HGFA	Hepatocyte growth factor activator	Stimulation to GAlert in skeletal muscle stem cells in C57BL/6 mice	[15]
Pro-aging systemic factor	CCL11	Eosinophil chemotactic protein, eotaxin-1	Aging of neurogenesis and cognitive function in C57BL/6 mice	[7]
	B2M	A component of MHC class I molecules	Aging of neurogenesis and cognitive function in C57BL/6 mice	[57]

GDF11 growth differentiation factor, HGFA hepatocyte growth factor activator, CCL11 C–C motif chemokine 11, B2M β2-microglobulin

GDF11

The rejuvenating effect of GDF11 was first identified in parabiosis experiments. Bone morphogenetic protein 11, another name for GDF11, was reported as a circulating factor that reversed age-related cardiac hypertrophy in mice [6]. GDF11, a TGF-β superfamily member, decreases during the aging process. Restoring youthful GDF11 through parabiosis or treating old mice with recombinant GDF11 reversed age-related hypertrophy. Several studies showed that GDF11 promotes vascular and neurogenic rejuvenation in aged mouse brains and restores the genomic integrity of old skeletal muscle satellite cells through increased stem cell activity [13, 46]. In humans, older males who were consistently active throughout their lives showed higher concentrations of GDF11 than inactive older men, and the concentration of circulating GDF11 correlated with leg power output when cycling [47]. However, in 2016–2017, conflicting reviews were published suggesting that GDF11 induced wasting of skeletal and cardiac muscle [48–51] and that GDF11 does not decline in rats or humans during aging [48]. Aging is the lifelong, complex phenomena that accumulates over time. Therefore, more intensive cause-and-effect studies should be conducted to decisively elaborate the role of GDF11 as a rejuvenating factor.

Oxytocin

Oxytocin is a circulating hormone and neuropeptide whose plasma level declines with aging. Muscle regeneration in young animals decreased when oxytocin was inhibited, and systemic administration of oxytocin enhanced aged muscle stem cell activation and proliferation by activating the MAPK/ERK signaling pathway, which increased muscle regeneration [14]. As an FDA-approved drug, oxytocin is considered a potentially therapeutic and safe way to treat muscle aging. Moreover, oxytocin is released when hugging or bonding, which could also have anti-aging benefits.

Bursicon

Bursicon is an insect hormone and neuroendocrine hormone. Neurotransmitters released by nerve cells stimulate neuroendocrine cells to release bursicon into the bloodstream. Enteroendocrine cells in the gastro-intestine secrete bursicon, which binds to its receptor DLGR2 and inhibits epidermal growth factor (EGF) expression. The interaction of bursicon and DLGR2 in the Drosophila intestine maintains the quiescence of intestinal stem cells [52]. Bursicon is considered a local controller for maintenance of intestinal stem cell homeostasis.

HGFA

HGF, a mesenchyme-derived heparin-binding glycoprotein, binds to the c-Met receptor, and that interaction regulates cell proliferation, cell survival, cell motility, and tissue regeneration through a tyrosine kinase pathway [53, 54]. Many previous studies showed that HGF activated the entry of quiescent satellite cells into the cell cycle. In 2014, Dr. Thomas Rando and his group reported that satellite cells and extracellular matrix of damaged muscles secrete HGF, which promotes the entry of quiescent satellite cell from the G0 state to the G_Alert stage through the mTORC1 downstream signaling pathway. G_Alert cells are primed for proliferation and activation. Injury-induced secretion of a low level of HGF stimulates quiescent satellite cells to enter the G_Alert stage, whereas prolonged injury-induced HGF signals quickly promote proliferation and activation for repair [55]. In 2017, the Rondo group showed that the systemic injection of HGFA induces proteolytic processing for HGF activation and stimulates the G_Alert state in skeletal muscle stem cells. The administration of HGFA sufficiently achieved stem cell activation and tissue repair; thus, HGFA is proposed as a potential candidate for therapeutic applications in regenerative medicine [15].

CCL11

In 2011, Tony Wyss-Coray and his group reported that CCL11 level in blood plasma was increased in aged mice and humans. CCL11 inhibits neurogenesis and impairs learning and memory, so it was proposed as a circulating pro-aging factor using heterochronic parabiosis experiments [7]. The same group has struggled to determine the systemic factors associated with aging and tissue degeneration using a proteomic approach. Sixty-six cytokines, chemokines, and secreted signaling proteins were measured in the plasma of normal aging mice using standardized, multiplex sandwich enzyme-linked immunosorbent assays (ELISAs; Luminex). Among those 66 proteins, CCL2, CCL11, CCL12, CCL19, haptoglobin, and β2-microglobulin (B2M) increased in aged mice and young heterochronic parabiosis mice. CCL11 is a chemokine involved in allergic responses that has not been reported as an aging factor. However, a high level of CCL11 is detected in cannabis users and in people suffering from schizophrenia triggered by cannabis [56]. Although CCL11 does seem to be an aging-associated systemic molecule, exactly how it promotes the aging process needs further study.

B2M

The Wyss-Coray and Saul A Villeda group also reported B2M as a systemic pro-aging factor that impairs cognitive function and neurogenesis in mice [57]. B2M is a component of major histocompatibility complex class I (MHC I) molecules, which are elevated in the blood and hippocampus of aging humans and mice. Exogenous injection of B2M systemically or locally to the hippocampus impairs hippocampal-dependent cognitive function and neurogenesis in young mice, and the absence of endogenous B2M in B2m ^−/− mice eliminates age-related cognitive dysfunction and enhances neurogenesis in aged mice [57]. Furthermore, an increased systemic level of soluble B2M has been detected in the cerebrospinal fluid of patients with HIV-dementia [58, 59] or Alzheimer’s disease [60]. The strong association between systemic B2M level and cognitive function decline suggests B2M as a strong potential pro-aging systemic factor. Although the Wyss-Coray and Villeda group has not yet isolated any anti-aging factors from young plasma, they did find that young blood reversed age-related impairments in cognitive function and synaptic plasticity of old mice [61]. In addition, they have conducted a clinical trial in humans that used a series of plasma transfusions from young donors (< 30) to older Alzheimer’s patients [62]. There is therapeutic promise for rejuvenating factors that could turn back the clock for aging people, but such factors are only a starting point to understand aging.

Stem cell niche as a positive or negative regulator

Within the bone marrow, HSCs, BMSCs, and EPCs exist in a tightly controlled microenvironment, a niche, that contributes to the control of quiescence, proliferation, self-renewal, and differentiation. This stem cell niche is composed of a specialized population of cells that includes soluble factors such as cytokines, chemokines, and growth factors and produces chemical, physical, and mechanical signals for oxygen tension, extracellular matrices, shear forces, temperature, surrounding pH, and monoatomic ions, all of which regulate stem cell behavior. This section considers how many age-related changes in these cellular or microenvironmental components of the niche affect the function of stem cells or the stem cell niche itself.

HSC niche in young and old bone marrow

The stem cell niche for HSCs, which contains many cellular components and their secretions, regulates HSC homeostasis and trafficking using distinct mechanisms. BMSCs, endothelial cells of arterioles/capillaries/sinusoids, osteoblasts, the sympathetic nervous system, T cells, and macrophages are all recognized as positive regulators, whereas adipocytes and osteoclasts are generally known to be negative regulators (Fig. 4). Nestin^mid PDGFα⁺ CD51⁺ perivascular cells reside near HSCs and the adrenergic nerve and maintain the quiescence of HSCs by secreting cytokine chemokine (C–X–C motif) ligand 12 (CXCL12) and stem cell factor (SCF) [63]. CXCL12-abundant reticular (CAR) cells, which are Nestin⁺ Mx-1⁺ Lepr⁺Prx-1⁺ MSC-like cells near the endosteum, regulate HSC self-renewal, proliferation, and mobilization [64]. Endothelial cells on the arteriole and sinusoid support HSCs by secreting FGF, EGF, DLL1, IGFBP2, ANGPT1, DHH, pleiotrophin, and Jagged-2 [12, 65, 66].

Fig. 4.

Identification of HSC niche and supporting cells as immune-privileged sites and regulating factors of HSC functions. A Location of HSC niche cells within the bone marrow substructure and characteristics. (1) LT-HSCs localized closely to the endosteal surface with SNO cells and non-myelinating Schwann cells surrounded by Treg cells and an IL-10/TGF-β-enriched immune privileged area that can be protected from inflammatory insults. More actively cycling HSCs are present near the arteriole (2) or sinusoidal endothelia (3). A newly identified quiescent LT-HSC niche is surrounded by unique bone marrow macrophages expressing α-SMA, CD169, or DARC and enriched by megakaryocyte-releasing factors such as CXCL4 and TGF-β (4). In the aged, yellow marrow with ample fat tissue forms. HSCs become surrounded and entrapped by adipocytes and fat tissue releasing inflammatory cytokines (5). B Factors regulating HSC hierarchy and differentiation. Mφ macrophage, SCF stem cell factor, CXCL12 C-X-C motif chemokine ligand 12, TPO thrombopoietin, BMP bone morphogenic protein, Ang angiogenin, ANGPT angiopoietin, TGF transforming growth factor, IL interleukin, EPO erythropoietin, FGF fibroblast growth factor

The endosteal stem cell niche contains a specialized hypoxic immune privilege site to protect HSCs from inflammatory insult. Approximately 80% of Lin⁻Sca1⁺KIT⁺ CD41⁻CD150⁺CD48⁻ HSCs are found close to the endosteal niche on the bone surface, and only 20% of HSCs reside near the central vein of the bone center [12]. In the endosteal niche, specialized osteoblastic cells (SNO cells) provide quiescent signals for HSC maintenance. SNO cells and HSCs form a cell–cell junction with N-cadherin and induce β-catenin-mediated PTEN activation that blocks PI3K/Akt/mTOR-induced cell cycle entry. Both SNO cells and HSCs form an adhesive junction between VCAM and VLA4 and between fibronectin and VLA5. SNO cells inhibit HSC activation, differentiation, and apoptosis by providing BMP, Jagged, and Ang-1 [67]. Treg cells, which accumulate on the endosteal surface, provide immune suppression privilege to HSCs by secreting the anti-inflammatory cytokine IL-10. HSCs seem to reside within 20 µm of CD4⁺CD25⁺FoxP3⁺ Treg [68]. Sympathetic nerve systems in the bone marrow also protect HSCs by secreting TGF-β activator from non-myelinating Schwann cells, which interacts with the TGF-β receptor on the HSC surface to activate the phosphorylation of SMAD2/3 signaling and cause the maintenance and quiescence of HSCs [69]. Adrenergic nerve and Nestin⁺ BMSCs highly express HSC maintenance genes, including CXCL12, ANGPT1, Kit ligand, and VCAM-1, whereas activation of the β-adrenergic receptor downregulates that gene expression [70].

Recently, researchers have found that macrophages play an important role in the maintenance of LT-HSCs, whose depletion by granulocyte–colony stimulating factor (G-CSF) activates a sympathetic nerve to secrete norepinephrine, which causes HSC egress into the blood [71]. CD82/KAI1, which is highly expressed on the surface of LT-HSCs, promotes LT-HSC quiescence by interacting with DARC (CD234) on the macrophages following the TGF-β1/Smad3 signaling pathway [72]. Low CD82 expression is associated with tumor progression. Endothelial DARC is known to induce the senescence of CD82⁺ tumor cells [72]. α-SMA⁺COX-2⁺ monocytes and macrophages maintain hematopoietic stem progenitor cells (HSPCs) through the production of prostaglandin E₂, which inhibits the production of ROS in HSPCs by limiting stromal-cell expression of the chemokine CXCL12 [73]. CD169⁺ macrophages in bone marrow are distinct from the M1/M2 macrophage subtypes and express CD206, VCAM-1, and CCL22 [74]. CD169 (Sialo-adhesion) is a cell adhesion molecule present on the surface macrophages. CD169⁺ macrophages constitute approximately 2.6% of all bone marrow cells, support the HSC niche, and promote HSC retention with Nestin⁺ BMSCs and β-adrenergic neurons in the bone marrow [75].

What negative regulators affect the HSC niche? After irradiation and in advanced aging, BMSCs tend to differentiate into adipocytes and fill the marrow space, which is understood to inhibit HSC function. Inhibiting adipogenesis enhances bone marrow recovery [76]. Also, in 2017, Ambrosi et al. [77] used a competitive repopulation assay following lethal irradiation and reported that adipocyte accumulation in the bone marrow during obesity and aging impairs stem cell–based hematopoiesis and bone regeneration. As part of that mechanism, they proposed that the bone-resident adipocytic lineage produced an excessive amount of dipeptidyl peptidase-4 (DPP-4), a target for diabetes therapies that could deteriorate the HSC niche. Another dispensable type of cell for a healthy HSC niche is the bone-degrading osteoclast. In an osteoprotegerin-deficient model mouse, osteoclasts degrade the endosteal niche and induce HSC mobilization [78, 79]. In aging bodies, the exhaustion of niche-supporting cells and the upregulation of adiposity in bone marrow can occur at the same time, which deteriorates normal HSC homeostasis.

BMSC/EPC niche

For decades, BMSC research has emphasized the pivotal role of BMSCs in wound healing and tissue regeneration. However, current understanding of the cells and signals that actually constitute the BMSC niche is very limited. BMSCs are generally mixtures of progenitor cells in the bone marrow. CD31⁺ EPCs are known to be closely neighbored with BMSCs and enhance the self-renewal capacity of BMSCs in vivo and in vitro [80]. Specifically, the fibronectin in EPCs, a component of the extracellular matrix for the HSC niche, is known to activate the integrin in BMSCs and promote self-renewal [81, 82]. EPCs are a major biological component of the BMSC niche and affect the biological processes of BMSCs. IGF-1/IGF-2 bind to IGF-1R/IGF-2R on BMSCs and increase proliferation and self-renewal [83]. CXCL12 enhances BMSC colony forming capacity, which is promoted by activation of the β3-adrenergic neuron and followed by promotion of EPC colony forming potency [84].

Epigenetic changes in stem cells during aging

During the past decade, accumulating evidence has supported changes in epigenetic information during aging in both stem cells and somatic cells. Epigenetic refers to all heritable changes in gene expression that are independent of DNA sequencing. Epigenetic modifications in the genome and DNA-associated proteins are required for the maintenance of biological states such as cell identity, with particularly extensive implications for stem cells. Different types of epigenetic changes are encoded within the epigenome, including reduced core histones, histone posttranslational modifications, structural and functional variants of histones, DNA methylation, chromatin remodeling, and altered noncoding RNA expression during both aging and senescence. Those epigenetic changes cause aberrant gene expression, telomere shortening, and genomic instability [85]. Histone modifications such as methylation, phosphorylation, ubiquitination, and acetylation can act separately or synergistically to regulate gene expression through changes in chromatin structure. Histone methylation is a complex epigenetic event that leads to different transcriptional outputs depending on the lysine (K) residue, such as K4, K9, K20, K27, K36, and K79. For instance, histone H3 methylated on K4 or K36 is generally associated with active genes, whereas methylation at K9 and K27 is mostly linked to gene repression.

DNA methylation is a relatively stable epigenetic modification restricted to cytosine residues in the CpG dinucleotides by DNA methyl transferase enzymes (DNMTs). Chromatin remodeling is a protein-mediated event that leads to chromatin rearrangements and helps access the nucleosomal DNA using SWItch/sucrose non-fermentation (SWI/SNF), polycomb group (PcG), and Trithorax group (TrxG) proteins. Non-coding RNAs, including microRNAs (miRNAs, ~ 20 nucleotides) and long-non-coding RNAs (lncRNAs, ~ 200 nucleotides), which are not translated into proteins, participate in the posttranscriptional regulation of gene expression.

Epigenetic research in diverse species and progeria model

Invertebrates are popular models for aging studies because they have short lifespans and provide easy genetic and environmental manipulation. In addition, vertebrate models such as mice, rat and zebrafish are usually used. In yeast, removing the chromatin remodeler SWI/SNF (ISW2) demonstrated extension of the replicative lifespan and mimicked caloric restrictions through changes in the nucleosome positioning critical for regulating gene expression [86]. In the past 10 years, histone methylation regulators such as H3K4me3, H3K27me3, and H3K36me3 were found to be involved in regulating the lifespan of C. elegans [87–89]. Additionally, increased levels of key metabolites, such as acetyl CoA, and increased global histone acetylation, including acetylated lysine 12 on histone H4 (H4K12ac), were confirmed during the aging process. H4K12ac mutation extended the lifespan of Drosophila [90]. Mainly in metabolism and aging, the mammalian sirtuin protein family (SIRT1–SIRT7) regulates mitochondrial function and enhances stem cell survival. Sirtuins deacetylate histones and several transcriptional regulators in the nucleus in cellular compartments [91]. In mice, studies on the causes of aging have been conducted by examining premature aging (progeria), such as in Werner’s syndrome and Hutchinson–Gilford progeria syndrome (HGPS), genetic disorders that lead to a shortened lifespan and occur by recessive mutation of the Lamin A gene, which leads to chromatin modification, metabolic defects, stem cell exhaustion, cell cycle deregulation, and inflammation [92, 93].

Epigenetic changes in aged stem cells

Modifications to DNA/histones and non-coding RNA-mediated mechanisms are epigenetic events that play important roles in the regulation of stem/progenitor cell functions by changing the chromatin structure. Epigenetic changes in adult stem cells are critical during aging because they alter the function, clonal composition, and lineage fate of stem cells which have been regulated by intrinsic and extrinsic epigenetic modifiers. The activating H3K4me3 mark and the repressive H3K27me3 mark are bivalent domains thought to affect developmental gene expression. These bivalent domains also allow timely activation and repression in the absence of differentiation signals [94]. The H3K27me3 mark is essential for maintaining the repressed form of these genes, whereas the H3K4me3/1 mark could lead to activation upon induction of differentiation through external signals.

Epigenetic changes of HSCs in the aging and stem cell hierarchy

Interdependency between histone modification and DNA methylation could be related to HSC aging. In stem cells, the repressive H3K27me3 mark in young and aged HSCs, together with two chromatin marks related to active transcription, H3K4me3 and H3K36me3, has been epigenetically profiled by ChIP-seq. The presence of H3K4me3 during HSC aging could increase self-renewal in old HSCs [95]. In contrast, in adult skeletal muscle stem cells or satellite cells, the presence of H3K4me3 marks showed little or no difference in young and old mice [96]. This conflicting result could suggest that repressive H3K4me3 marks are the cause of other mechanisms, not transcription suppression, among different stem cells. Additionally, HSCs are connected to loss of the H4K16ac activation mark during aging [27]. Moreover, Kdm3a and Kdm5b have been reported to regulate stem cell aging because their levels decrease during the aging process [97, 98]. Knockdown of the lysine demethylase Kdm5b (Jarid1b) enhanced the in vitro expansion of HSCs and their in vivo lymphomyeloid differentiation potential [99]. In addition, histone methylation of H3K27me3 is a key regulator of hematopoiesis. Recent studies have reported that young HSCs showed a low level of methylation in the genomic region associated with blood cell production, whereas aged HSCs showed DNA hypermethylation in the genomic regions associated with the lymphoid/erythroid lineages [100].

Epigenetic regulators, such as the ten-eleven translocation (Tet) enzymes that regulate demethylation and the DNMTs that cause the methylation of CpG motifs, appear within the HSC compartment during aging. Different expression levels have been revealed for both Tet2 and DNMTs in young and old HSCs [95]. A recent mouse study demonstrated that Tet2 loss causes myeloid transformation and malignancies [101]. DNMT1 plays a role in maintaining methylation, and DNMT1 deficiency in HSCs demonstrated myeloid skewing and a defective self-renewal process [102, 103]. Moreover, lost function of DNMT3A and DNMT3B caused a lack of HSC differentiation [104].

LT-HSCs express an evolutionarily conserved miRNA cluster that includes miR-99b, let-7e, and miR-125a. miR-125a is involved in increasing the number of HSCs in vivo by eightfold and enriches lymphoid-balanced HSCs [105, 106]. The lncRNA Xist is essential for HSC survival; Xist-deficient HSCs cause abnormal hematopoiesis and age-dependent loss [107].

Epigenetic changes of BMSCs during ex vivo cell expansion and differentiation

The epigenetic mechanisms of BMSCs are relatively well investigated because of their therapeutic applications in regenerative medicine. Epigenetic changes help regulate the expression of several genes associated with the stemness of BMSCs and the differentiation of diverse cell types containing osteocytes, chondrocytes, or adipocytes.

Li et al. [108] compared the epigenetic modifications of histone H3 acetylation in early- and late-passage BMSCs. Histone H3 acetylation is an essential event in regulating BMSC aging and differentiation. For instance, H3 acetylation level coincides with gene expression levels such that K9 and K14 histone H3 acetylation occurs in various genes, including Oct4, Sox2, TERT, ALP, and Runx2, in the late passage. Histone deacetylase inhibitors promote apoptosis and senescence in human BMSCs [109]. DNA methylation levels decline during BMSC senescence, and inhibition of DNMT1 and DNMT3b promotes cellular senescence in umbilical cord blood BMSCs through increased expression of p16INK4A and p21^CIP1/WAF1 [110].

Recently, research has examined the epigenetic mechanisms that regulate BMSC differentiation. In the osteogenesis of BMSCs, chromatin hyperacetylation, histone methylation of H3K4me3 at the promotor of the HOXA10 gene, and demethylation of H3K27me3 contribute to osteogenic determination of BMSCs by inducing activation of the osteogenic transcription factor Runx2 [111, 112]. Histone methylation of H3K4 and H3K36 at the AP-2α gene increases the osteogenic and odontogenic potential of BMSCs [113]. The acetylation of H3 and H4 at the osteocalcin gene or the acetylation of H3K9 induces osteogenic differentiation of BMSCs [114, 115]. DNA methylation at the promoter of osteopontin decreases during induction of osteogenic differentiation in BMSCs [116]. In addition, miR-27a, miR-489, miR-204, and miR-138 decrease osteogenic differentiation by reducing the expression of alkaline phosphatase, Runx2, or osterix [117, 118], and miRNA-20A, miR-148b, miRNA-2861, and miR-335 induce osteogenic differentiation by activating BMP/Runx2 [108, 119, 120].

During BMSC chondrogenesis, histone modifications and non-coding RNAs could be involved, rather than DNA methylation. Genes transcriptionally upregulated during chondrogenesis are marked by H3K36me3 of the gene body, H3K4me3 and H3K9ac of the 5′ end of genes and promoters, and H3K4me1 and H3K27ac [121]. The expression of miR-130b, miR152, miR28, and miR26b increases in chondrogenesis [122], and miR-145 expression decreases, which causes increased expression of SOX9 [123]. Moreover, miR-29 induces chondrogenesis by regulating FOXO3A [124], and miR-574-3p inhibits chondrogenesis [125].

Only a few studies have been conducted on BMSC adipogenesis. Histone methylation of H3K4me2 occurs at the promotor of adipogenic genes such as adiponectin, glut4, and leptin [126]. In addition, the methyltransferase enhancer of zeste 2 (EZH2) induces adipogenesis by trimethylation of H3K27, but the demethylase KDM6A causes osteogenesis by removing the methylation of H3K27me3. Knockdown of EZH2 increased osteogenesis, whereas knockdown of KDM6A increased adipogenesis [127].

Epigenetic changes of EPCs in angiogenesis and aging

Interest in EPCs has risen because of their potential in stem cell therapies for ischemic injuries to facilitate revascularization. Epigenetic regulators that increase the vascular repair function of endothelial progenitor cells offer potential breakthroughs for clinical application strategies. The use of epigenetic drugs to reverse those epigenetic marks and enhance revascularization is being studied.

In histone modification of EPCs, the full extent of bivalent genes between the activating H3K4me3 mark and the repressive H3K27me3 mark is unclear. H3K4 methylation is essential for angiogenesis because lysine-specific demethylase 1 (LSD1) suppresses metastasis and angiogenesis by inducing H3K4 demethylation [128]. EZH2 is a negative regulator of endothelial cell differentiation that is overexpressed in cancer cells, represses differentiation genes, and maintains stemness through the deposition of repressive H3K27me3 marks [129]. H3K36me3 methyltransferase is required for vascular development, endothelial cell differentiation, and function [130]. Epigenetic changes in EPCs with increased H3K4m3 and reduced H3K9me3 induce the secretion of pro-inflammatory cytokines such as MCP-1 and IL-6 and impair the angiopoietic phenotype, which leads to increased risk for cardiovascular disease [131].

The contribution of DNA methylation/demethylation to endothelial gene regulation remains poorly understood. Recent studies have reported repressive H3K27me3 and DNA methylation marks in eNOS promoters in early EPC, which can be reversed in hypoxic conditions to increase eNOS expression and thus increase endothelial recruitment and differentiation [132]. The histone deacetylase enzyme HDAC1 is also recognized to have an essential part in the inhibition of endothelial proliferation and differentiation [133].

Research about the non-coding RNAs in EPCs has progressed steadily. miR-21, miR-27a, miR-27b, miR-126, and miR-130a are expressed in EPCs but are decreased in circulating EPCs. In diabetic patients, EPCs express low levels of miR-126 and miR-130. Inhibiting miR-126 decreases proliferation and migration, and inhibition of miR-130 represses EPC differentiation [134, 135]. miR-10A* and miR-21 progress EPC aging, and blocking those miRNAs induces an anti-aging process, enhancing angiogenic capacity [136].

Inflammation and aging

Inflammation is a pivotal pathophysiological process that protects bodies from infection and repairs injuries. As people age, they experience many infections and diseases and build an antigenic burden, which can induce unbalanced inflammatory circumstances. During aging, increased adipose tissue caused by a high glucose/fat diet, decreased sex hormones, smoking, and stress combine to cause chronic, low-grade, systemic inflammation. The so-called inflammaging [8, 9] gives rise to age-related diseases such as cardiovascular disease, cancer, diabetes, and osteoporosis [137]. Many studies about relationships among aging, inflammation, and disease have been conducted. The levels of pro-inflammatory cytokines and chemokines increase in aged people. IL-6, TNF-α, IL-1β, and C-reactive protein (CRP), which is released in the liver in response to IL-6, all increase in the serum of older humans [137]. Pro-inflammatory markers are considered to be predictors of age-related diseases such as cardiovascular disease, cancer, diabetes, osteoporosis [137] and neurodegenerative disorders such as Alzheimer’s disease [138]. In addition, HIV-infected patients can experience premature aging through chronic inflammation and immune-senescence, which causes T cell dysfunction and progenitor cell exhaustion [139]. Chronic inflammation causes oxidative stress, mitochondrial dysfunction, age-related diseases, myeloid-biased differentiation, telomere shortening, epigenetic modifications, etc. [140]. It is still a mystery whether inflammation is a cause or outcome of aging.

Inflammation and stem cell aging

Aged stem cells show downregulation of colony forming units and growth factor/cytokine/chemokine release. Quiescence and retention of HSCs are both supported by vascular endothelial growth factor (VEGF), TGF-β1, and IL-10, which all function as anti-inflammatory signals [12]. However, in aged bone marrow, those factors decrease, and inflammatory signals increase, which induces immunogenicity. Inflammatory signals act as emergency calls for HSCs. Within 1 h of an infection induced by E. coli-derived LPS injection, LPS reaches the bone marrow, interacts with TLR4 on HSCs, and causes proliferation [10, 141]. IFNs such as IFN-α and IFN-γ act directly on HSCs to move quiescent HSCs into the proliferation and differentiation stages. Under stressed conditions, such as a bacterial or viral infection, cytotoxic CD8⁺ T cells and HSCs secrete IFN-γ, which causes the expression of the myeloid transcription factors C/EBPα and Runx1 in HSCs [142], and CD4⁺ T cells produce IFN-γ through the TLR/MyD88 pathway [143]. Activated BMSCs and HSCs produce IL-1, IL-6, TNF-α, G-CSF, and granulocyte–macrophage colony-stimulating factor (GM-CSF) [141], which lead to myeloid-biased differentiation, mobilization, and proliferation [141–144]. These results show that anti-inflammatory therapies that control the inflammatory signals in the bone marrow microenvironment could modulate stem cell fate.

Anti-inflammatory control of stem cell aging

Ten years ago, Bente K Pedesen published a paper titled “Anti-inflammation—just another word for anti-ageing?” [145]. He reported that, in age-related and chronic inflammatory diseases such as atherosclerosis, diabetes, cardiovascular disease, Alzheimer’s disease, and cancer, the administration of non-steroidal anti-inflammatory drugs at systemic concentrations would reduce occurrence rates and counteract the inflammation-induced inhibition of protein synthesis in old people [145]. For instance, salsalate, an anti-inflammatory drug that inhibits NF-κB and a dimer of salicylic acid, reduces systemic inflammation and prevents type 2 diabetes in obese adults [146]. Dr. Thomas von Zglinicki and his group studied low-grade inflammatory nfkb1^−/− knockout mice, which age prematurely and have decreased regeneration capacity in the liver and intestine [147]. The nfkb^−/− fibroblasts increased NF-kB, COX-2, and ROS through a feedback loop, causing DNA damage such as telomere dysfunction and senescence. However, anti-inflammatory or antioxidant administration recovered the tissue regeneration [147]. In addition, his team measured the blood cell numbers, metabolism, liver and kidney function, telomere length, and level of inflammation of Japanese centenarians [148]. They reported that centenarians showed longer telomere length than the general population and also that inflammation is an important cause of aging in very old humans. Immune-modulation and safer anti-inflammatory medicines have the foremost potential and are good candidate therapies to promote a healthy lifespan [148].

Stem cell self renewal and trafficking: expectations for tissue repair and anti-aging

Stem cells with differential potentials, such as adult stem cells, embryonic stem cells, and induced pluripotent stem cells, are expected to be used to repair tissue loss and injury for functional recovery from a variety of degenerative diseases and trauma. Several critical issues, such as well-controlled lineage-specific differentiation, mass production of stem cells, high cost of ex vivo cell culture, low engraftment at transplantation site, and short half-life of the therapeutic cells, have to be resolved for clinical applications to advance. On the other hand, endogenous adult or tissue stem cells reserved in the tissue and organs of the body could be used for tissue repair without elaborate ex vivo cell culture if small molecules or growth factors that regulate stem cell self-renewal and trafficking can be identified.

Stem cell trafficking for tissue repair

Several growth factors and peptides, such as stromal cell-derived factor 1 alpha (SDF-1α), VEGF, G-CSF, and Substance-P, are known to mobilize bone marrow stem/progenitors cells such as HSCs, EPCs, and BMSCs and facilitate tissue repair in well-defined animal models. The sequential events of the healing mechanisms have not been fully elucidated, but several studies strongly support the participation of mobilized EPCs and BMSCs, directly or indirectly, in tissue regeneration in situ [149–153]. Such small molecules or biologics could be efficiently and immediately used to treat patients in emergency situations, such as following stroke or acute myocardial infarction (AMI), by using patients’ own stem cells in a stem cell mobilization and homing strategy.

Among the recombinant growth factors currently being produced at clinical grade, G-CSF, GM-CSF, VEGF, erythropoietin (EPO), and SDF-1α and their combinations are actively being studied based on their EPC and HSC mobilizing capacities to find a therapeutic modality for ischemic vascular disease, such as AMI, ischemic limb disease, diabetic ulcer, and stroke. Other new candidate molecules, such as Substance-P (SP), which retains BMSC mobilization capacity, are also actively being studied in specific injury and disease animal models, such as AMI, stroke, diabetic ulcer, ischemic limb disease, rheumatoid arthritis, spinal cord injury (SCI), radiation-induced gastrointestinal damage, and corneal injury [153–161].

Some clinical studies have shown variable outcomes and more limited effects than expected. Their differential results could point to a fundamental issue of tissue regeneration in adults: tissue regeneration in situ requires recapitulation of developmental organogenesis. Thus, strategies need to be designed to fine tune and regulate complex factors and cells, spatially as well as temporally, based on organogenesis and the traits of specific tissue injuries and patients. Specifically, the critical threshold concentrations of factors needed, locally and systemically, to initiate the trafficking of progenitor and stem cells to the target organ are largely unknown. In addition, accessory cellular components, such as myeloid cells and lymphocytes, might secrete auxiliary factors to control the inflammatory environment, remove dead cells to prepare a receptive tissue environment for incoming reparative stem cells, and facilitate specific local recruitment and positioning of circulating cells to reconstruct a well-defined tissue architecture. Those roles have not been considered in depth for tissue regeneration in situ.

Because a variety of age-related degenerative diseases could be considered to result from impaired tissue repair and recovery from the daily low grade tissue damage that occurs from a variety of insults and infections throughout life, several endogenous factors, previously identified in stem cell self-renewal and trafficking and facilitation of tissue repair, could be candidates for anti-aging therapeutics. Some of them are highly correlated with the appearance of chronic disease and the aging phenotype, but proof of their roles in aging is a future topic for this field.

SP as a BMSC mobilizer and anti-inflammatory modulator: expectations for anti-aging

SP is an 11 amino acid neuropeptide secreted from the peripheral terminals of sensory nerve fibers, where it acts as a neurotransmitter or hormone. Subsets of neurons in the central and peripheral nervous systems [162], non-neuronal cells including macrophages and T lymphocytes, immune cells, and bone marrow stroma [163, 164] express SP and other structurally related peptides [165], all of which are encoded by the same gene, preprotachykinin-1 (PPT-1). Moreover, the SP receptor neurokinin 1 receptor is expressed on a variety of non-neuronal cells, such as BMSCs, chondrocytes, osteocytes, osteoblasts, osteoclasts, and mast cells [166–168]. SP mediates pain perception, neuro-immune modulation, cell proliferation, and enhanced proliferation and differentiation of endothelial cells, all of which are expected from its local action: direct nerve innervation and direct cellular contacts [169, 170]. In addition to its local action, intravenously injected SP works systemically to mobilize CD29⁺ stromal-like cells (namely BMSCs) from the bone marrow to the periphery blood, resulting in accelerated wound healing [153–161]. This new function of SP was initially identified as an injury-inducible messenger to trigger an endogenous wound healing mechanism, which recalls BMSCs mobilizing and homing to injured tissue.

In addition to its BMSC mobilizer function, SP enhances BMSC-mediated immune modulation at the late passage of BMSC by secreting TGF-β1. SP-induced BMSCs inhibit the activation of CD4⁺ jurkat T cells and decrease the secretion of IL-2 and IFN-γ from T cells even in the presence of an activation factor such as LPS or CD3/CD28 antibodies [171]. Recently, SP’s novel function as a cytokine was identified; SP can directly polarize monocyte and macrophage phenotypes [172]. SP stimulates bone marrow-derived monocytes and macrophages to become tissue-repairing M2 macrophages through NK-1R signaling that expresses arginase-1 and secretes anti-inflammatory cytokine IL-10 [172]. Furthermore, SP stimulated the emigration of monocytes from the bone marrow and their infiltration to the injured tissue of a rat with SCI. In consequence, adoptively transferred SP-induced M2 macrophages reached the SCI lesion site and enhanced SCI functional recovery. Collectively, SP could have an integral role in tissue repair by recruiting reparative stem cells from the bone marrow, along with immune modulation systemically, locally, and in the bone marrow stem cell niche. It is a potential systemic factor regulating the proliferation, maintenance, and function of HSCs, BMSCs, and EPCs. Because SP level in the blood is low in diabetic patients and those with chronic cardiovascular disease, its role in the pursuit of successful tissue repair, especially in the case of acute tissue injury, might not be executed properly in the aged and people with those diseases. Thus, SP or its equivalent medication might be developed to recover a homeostatic basal level of SP and its injury-mediated induction mechanisms.

Conclusion and perspectives

This study has provided a comprehensive overview of the physiological homeostatic role of endogenous stem/precursor cells in the bone marrow (HSCs, BMSCs, and EPCs), along with their dysfunctions in a variety of chronic degenerative diseases and aging. Candidate systemic factors or small molecules that promote aging or rejuvenation, inflammation, stem cell trafficking, and tissue repair were reviewed from the perspective of age- or disease-related alterations, and possible pharmacological targets were elucidated for anti-aging therapeutics, retardation of senescence in ex vivo cell culture, and disease curing agents. Clinical studies of stem cell therapies have revealed many limitations of the current state of ex vivo cultured stem cell therapy. However, the factors stimulating stem cell self-renewal and retarding senescence mentioned in this review could offer a new pathway for stem cell therapy. Future studies using aging and age-related degenerative disease models might confirm those promising expectations.

Conflict of interest

The authors have no financial conflicts of interest.

Ethical statement

There are no animal experiments carried out for the article.