Abstract
Skin is the largest organ in human body, harboring a plethora of cell types and serving as the organismal barrier. Skin aging such as wrinkling and hair graying is graphically pronounced, and the molecular mechanisms behind these phenotypic manifestations are beginning to unfold. As in many other organs and tissues, epigenetic and metabolic deregulations have emerged as key aging drivers. Particularly in the context of the skin epithelium, the epigenome and metabolome coordinately shape lineage plasticity and orchestrate stem cell function during aging. Our review discusses recent studies that proposed molecular mechanisms that drive the degeneration of hair follicles, a major appendage of the skin. By focusing on skin while comparing it to model organisms and adult stem cells of other tissues, we summarize literature on genotoxic stress, nutritional sensing, metabolic rewiring, mitochondrial activity, and epigenetic regulations of stem cell plasticity. Finally, we speculate about the rejuvenation potential of rate-limiting upstream signals during aging and the dominant role of the tissue microenvironment in dictating aged epithelial stem cell function.
Introduction
As the body’s largest organ, skin harbors a cadre of cell types. First and foremost, epithelial cells serve as the fundamental units of our barrier; they reside in the interfollicular epidermis and pilosebaceous unit, the latter including the sebaceous gland and the hair follicle (; ; ; ; ). Fibroblasts in the dermis secrete bulk extracellular matrix in the tissue for not only structural support but also mediate signaling (; ; ; ; ). Subcutaneous adipocytes and dermal pre-adipocytes exhibit remarkable lineage plasticity to mediate metabolic and signaling regulations (). Immune cells including those of the innate and adaptive systems provide tissue surveillance (; ; ), support repair and regeneration (; ; ), and communicate with the microbiome (; ; ). Oxygen and nutrients are exchanged via the endothelial and lymphatic vasculature, sensations are conducted through the intertwining neuronal network, ultraviolet radiation protection is afforded by melanocytes, and hair follicles are erected by arrector pili muscles; all of these cell types exert their respective functions while maintaining crosstalk with juxtaposed epithelial stem cells (; ; ; ; ; ; ).
The skin epithelium is maintained by its resident stem cells, harboring the capacity for long-term self-renewal and multi-lineage differentiation. The hair follicle as a major skin appendage is maintained by hair follicle stem cells (HFSCs) residing in the bulge, an anatomic location beneath the sebaceous gland and isthmus of the pilosebaceous unit () (Figure 1A). HFSCs fuel the cyclic regeneration of hair follicles during homeostatic hair growth (; ; ; ). They contribute to both follicular and epidermal regeneration during transplantation and wound repair (; ; ; ; ), and they initiate skin squamous cell carcinomas upon oncogenic transformation (). Likewise, exhibiting remarkable plasticity, several HFSC populations adjacent to the bulge contribute to wound repair and tumorigenesis (; ; ; ; ; ; ; ) (Figure 1B).
FIGURE 1
Aging in the skin is graphically pronounced and includes a wrinkly surface due to dermal extracellular matrix atrophy and gray hair owing to a loss of melanocytes. Accompanying these overtly notable signs, the old skin manifests many hallmarks of aging () such as stem cell exhaustion, genotoxic stress, metabolic deregulation, and epigenetic erosion. In our current essay, we focus on skin aging, especially in the context of stem cell function and the origin and consequence of lineage deregulation. Whenever applicable, we compare mammalian skin to model organisms and adult stem cell of other tissues and organs. Overall, we entertain the idea that skin is an ideal system to understand and tackle organismal aging.
Molecular Mechanisms Underlying Hair Follicle Miniaturization
Stem cells decline over time in number and/or in activity across organs and tissues. Curiously, this functional decline is often accompanied by skewed lineage output. For example, aging in the hematopoietic system is signified by pronounced myeloid lineage expansion at the expense of lymphoid cells () (; ). Aged skeletal stem cells have decreased bone- and cartilage-forming potential but produce more stromal lineages, leading to not only bone fragility but also hematopoietic skewing (). Secretory cell types dominate the aged intestinal stem cell output (). The question of the fate of HFSCs during aging is a particularly intriguing one. Consistent with the lineage-skewing phenomenon at the organ level, hair follicles undergo miniaturization as HFSCs are diminished during aging, resulting in macroscopically sparse hair. In contrast, the epidermis and sebaceous gland undergo hyperplasia. Several cellular and molecular mechanisms have been recently proposed to explain the fate of aging HFSCs in murine models (Figures 1C–E).
In the skin, stem cell lineage infidelity where HFSCs expand their fate to regenerate the epidermis is observed during wound repair () (Figure 1B) and could be recapitulated upon the ablation of several key HFSC fate transcription factors in adult skin, including SOX9 () and NFIB/NFIX (), suggesting a default HFSC response of epidermal differentiation under injury. In a more extreme scenario, epidermal cysts form in Notch signaling–deficient skin, suggesting blocked hair follicle differentiation (; ; ; ; ). During aging, stress signals including DNA damage and a high-fat diet have been shown to drive epidermal conversion of HFSCs by compromising basement membrane integrity (), altering stem cell symmetric divisions (), and inducing oxidative stress (), all of which contribute to the epidermal conversion of HFSCs and hair follicle miniaturization (Figure 1C).
On the other hand, deficiency of several HFSC quiescence regulators, such as LHX2 () or beta-catenin (), appears to push HFSCs into the sebaceous gland lineage. In this regard, loss of BMP (; ; ) or aberrant activation of LEF1 (; ; ), NOTCH1 (), and GLI2 (; ) also manifests as ectopic sebaceous glands, although in some of these scenarios, it remains unclear whether sebaceous glands were mis-specified from HFSCs at the expense of hair follicles. Since sebaceous gland hyperplasia, like that of the epidermis, is a prominent signature of skin aging, it would be tempting to speculate that aged HFSCs also skew toward sebaceous gland differentiation, resulting in hair follicle miniaturization (Figures 1B,C).
An alternative model for the loss of aged HFSCs was recently proposed (Figure 1D): mechanical compression induces Piezo channel activation, calcium influx, and subsequently HFSC apoptosis (). While the loss of inhibitory K6+ niche cells is known to cause HFSC activation (), in this context, mechanical cues play a dominant function, since the reinsertion of the hair shaft alone without the K6+ cells returned bulge HFSCs to quiescence (). Corroborating this model, repetitive hair depilation induces hair follicle aging (; ), likely evoking mechanical compressions and the Piezo1-calcium-TNFα axis (). Therefore, mechanical compressions induced HFSC apoptosis is likely a major contribution of HFSC exhaustion and hair follicle miniaturization during aging.
Remarkably, via intravital imaging, aged HFSCs have been shown to escape into the dermis () (Figure 1E). This escape could be recapitulated by depletion of HFSC quiescence regulators FOXC1 (; ) and NFATC1 () in the hair follicles and is caused by the aging-associated loss of extracellular matrix proteins (; ). On the other hand, deregulation of the aging extracellular matrix due to niche stiffening underlies nuclear cytoskeletal remodeling and subsequently epigenome remodeling in the aging HFSCs (), providing an important molecular link as to how the extracellular matrix niche determines aged HFSC fate (; ).
Impact of Genotoxic Stress on Skin Aging
Telomeres shorten each time the genome duplicates, serving as a major driver for replicative aging. In mammals, telomerase expression is present and often abundant in adult stem cells (), including those of the skin (Figure 2). Among the defects in many highly proliferative organs, a signature phenotype of telomerase-deficient mice is alopecia and hair graying (; ; ; ). Likewise, in humans, mutations in the gene encoding for dyskerin, critical for telomere stability, lead to dyskeratosis congenita, a progressive bone-marrow failure syndrome characterized by abnormal skin pigmentation (; ).
FIGURE 2
Similar to telomere attrition, DNA damage–induced genotoxic stress due to replication error, mutagens, and reactive oxygen species (ROS) is well documented to accelerate aging (Figure 2). Among DNA repair pathways, it has been shown that non-homologous end joining (NHEJ) is specifically enhanced in quiescent hematopoietic stem cells (HSCs), whereas committed progenitors preferentially use homologous recombination as their repair mechanism (). Similarly, quiescent bulge HFSCs are relatively protected against ionizing radiation, likely because of their elevated anti-apoptotic programs and enhanced NHEJ capacity (). Deficiencies in various NHEJ components (; ) (; ), spindle assembly checkpoint proteins (), and DNA damage response pathways (; ) collectively contribute to premature aging of the hair follicles.
Disruption of the circadian clock results in pre-mature aging (; ; ), and its associated molecular mechanisms are only beginning to emerge (Figure 2). It has been suggested the transit-amplifying cells of the hair follicles exhibit elevated sensitivity toward DNA damage during the day when their mitotic activities peak (), whereas the epidermis preferentially proliferates at night to avoid the high level of oxidative phosphorylation (with its byproduct ROS) during the day, presumably as a protective mechanism against genotoxicity (). In aged epidermal stem cells, arrhythmic and prolonged DNA replication combined with otherwise-normal oscillatory oxidative phosphorylation programs may therefore underlie the increased oxidative damage (). Nevertheless, complete disruption of the clock did not recapitulate selective rhythmic deregulation in a physiological aging setting, nor did circadian rewiring induced by a high-fat diet, suggesting the direct cause of aging-associated circadian reprogramming remains unclear ().
Furthermore, chronic inflammatory signals also contribute to DNA damage and genotoxicity. For example, IFNα stimulate HSCs to exit quiescence via IFNAR and STAT1 signaling, elevate mitochondrial ROS levels, and lead to DNA damage (; ; ) (Figure 2). ROS may act upstream of DNA damage, evidenced by the observation that antioxidant treatment is able to suppress DNA damage and thus delay HSC aging (; ). Genotoxic stress could further exacerbate ROS deregulation and subsequently activate p38MAPK and upregulate the expression of P16, resulting in HSC exhaustion () (). In the case of HFSCs, DNA damage induces proteolysis of collagen XVII, a critical cell junction collagen, and induces stem cell differentiation (). Likewise, ionizing radiation–induced DNA damage leads to melanocyte stem cell exhaustion and hair graying (). Strikingly, hair graying is completely suppressed in mice that are deficient in the double-stranded DNA-sensing cGAS/STING pathway (), suggesting that the innate immunity pathway serves as a key mediator of radiation-induced aging.
Nutritional and Metabolic Regulation of Skin Aging
Pioneering work has established several nutrient-sensing and energy-sensing pathways to be critically involved in lifespan extension across model organisms (Figure 3A), including insulin/insulin-like growth factor (IIS) (), AMP-activated protein kinase (AMPK) (), mammalian or mechanistic target of rapamycin (mTOR) (; ), and sirtuin (; ; ) pathways. While mechanistic studies of these pathways in skin aging are rapidly emerging, in this section, we will review the historical aspects of these molecular mechanisms in various model organisms and adult stem cells in order to provide a broader context for skin aging.
FIGURE 3
In the IIS pathway, the selective ablation of signaling cascade components led to activation of downstream transcription factors FOXO (; ) and NRF (; ), eliciting cellular protective programs against oxidative stress and promoting longevity (; ). AMPK senses cellular energy decline and activates a plethora of catabolic pathways while suppressing anabolic processes, promoting lifespan extension (; ). The antidiabetic antineoplastic drug metformin exerts both AMPK-dependent () and AMPK-independent activities () and, when used at a high dose, inhibits mitochondrial electron transport chain complex I (described further below), all of which may contribute to its utility in anti-aging (). Both IIS and AMPK pathways converge on mTOR. Nutrient and amino acid starvation inhibits mTOR activity, resulting in suppressed protein translation and enhanced autophagy via key mTOR targets S6K1 (ribosome protein S6 kinase B1), 4EBP1 (eukaryotic translation initiation factor 4E-binding protein 1), and ULK1 (unc-51 like autophagy activating kinase 1) (; ). Sirtuins are NAD+ (nicotinamide adenine dinucleotide)–dependent deacetylases that act on both histone substrates to silence heterochromatin (; ) and on non-histone substrates such as P53 (; ), HSF1 (heat shock factor 1), FOXO (), and PGC1 (), among others, to regulate aging.
Among these regulators, mTOR function has been genetically tackled in several adult stem cell, aging, and malignancy contexts. Hyperactivation of mTOR by deletion of Pten or TSC1 exhausted neural stem cells () and HSCs (; ; ; ), and the HSCs were transformed to become precursors of myeloproliferative disorder. On the other hand, loss of mTOR function via ablation of the mTORC1 component Raptor (regulatory-associated protein of mTOR) in mouse HSCs leads to non-lethal pancytopenia, splenomegaly, and accumulation of monocytoid cells. Raptor conditional knockout also compromised HSC regeneration and inhibited leukemogenesis evoked by Pten deficiency (). p53 and p16 mediate HSC exhaustion and serve as a roadblock to leukemic transformation upon Pten deletion (). In adult but not childhood leukemia, deletion of the mTORC2 component Rictor blocked leukemogenesis and HSC depletion (). Mechanistically, it has been shown that in Drosophila larvae, chronic stimulation of TOR (via constitutively active insulin receptor expression) induces ROS and activates JNK and FOXO, resulting in accumulation of Sestrin (a family of stress-sensing proteins) and activation of AMPK, facilitating autophagic clearance of damaged mitochondria, protein aggregates, or lipids (). Loss of Sestrin resulted in age-associated pathologies including triglyceride accumulation, mitochondrial dysfunction, muscle degeneration, and cardiac malfunction, which could be reversed by AMPK activation or TOR inhibition. In the skin, deletion of either Mtor itself or its complex components Raptor of mTORC1 or Rictor of mTORC2 led to skin barrier defects during development (). Treatment with rapamycin (preferentially targeting mTORC1) reversed HFSC exhaustion induced by WNT1 overactivation (), whereas mTORC2 has been suggested to mediate glutaminase suppression in returning HFSCs to quiescence ().
Calorie restriction or dietary restriction is widely known to ameliorate aging-associated decline of stem cells, such as HSCs (; ; ), germline stem cells (), and stem cells of the intestine (; ; ), skeletal muscle (), and skin (; ). Mechanistically, in the context of intestinal stem cells, calorie restriction enhances stem cell activity through a niche-dependent paracrine signal (). In HSCs, calorie restriction cell-autonomously preserves the functional autophagy-high and oxidative phosphorylation–low populations during aging (; ). Similar to basal autophagy, chaperon mediated autophagy is also required for maintaining HSC function during aging (). In aged neural stem cells, calorie restriction induced lysosome activation clears aggregates and restores stem cell activation (), and likewise, chaperon mediated autophagy is essential for preventing proteome collapse and neurodegenerations (). Calorie restriction also restores the transcriptional circadian rhythm of aged epidermal and muscle stem cells to their youthful level, such as replication and autophagy, respectively ().
By sensing energy demand and nutrient supply, stem cells adaptively tune the level of glycolysis (breakdown of glucose into pyruvate under aerobic conditions or lactate under anaerobic conditions) and oxidative phosphorylation pathways (including oxidation of pyruvate, glutamine, or fatty acid in mitochondria, TCA cycle, electron transport chain activity) (Figure 3B). It is commonly believed that long-term self-renewing stem cells reside in a hypoxic niche and downregulate mitochondrial activity (; ; ). Blocking the influx of glycolytic metabolites into mitochondria by overexpression of pyruvate dehydrogenase kinase (which inhibits pyruvate dehydrogenase and hence pyruvate oxidation) increases the long-term self-renewal capacity of HSCs (; ), while deletion of mitochondrial pyruvate carrier (which transports pyruvate into mitochondria for oxidation) enhances the organoid-forming potential of intestinal stem cells (). Likewise, in the skin, depletion of mitochondrial pyruvate carrier induced HFSC activation, whereas ablation of lactate dehydrogenase (which converts pyruvate to lactate during anaerobic glycolysis) blocked HFSC activation (). Interestingly, glutamine oxidation is highly upregulated in HFSC progenies compared to HFSCs in organoid culture, and it has been proposed to promote HFSCs returning to quiescence at the end of the hair cycle (). While oxidative phosphorylation and mitochondrial activity are unequivocally induced upon stem cell activation, whether glucose or glutamine influx contributes to the increased oxidation is challenging to directly determine in vivo. Tissue-specific depletion of glutaminase (which converts glutamine to glutamate), similar to the experiments performed on mitochondrial pyruvate carrier and lactate dehydrogenase, will likely provide further insights into this interesting question.
Mitochondrial biology has long been intertwined with aging biology (). Mice with mutated mitochondrial DNA polymerase Polg age prematurely (; ), inducing a differentiation block in tissues with high turnover rate including skin (; ). Mice with epidermis-specific loss of TFAM (mitochondrial transcription factor A), required for the transcription of mitochondrial genes encoding electron transport chain subunits, showed impaired epidermal differentiation and hair follicle growth (; ). Indeed, electron transport chain activity declines in aging, and enhancing mitochondrial function by overexpression of PGC1 (; ) or supply of NAD+ () delays aging. Consistently, during aging, telomere dysfunction compromises mitochondrial function via p53-mediated inhibition of PGC1 levels (). In an interesting twist, it has been shown that mild mitochondrial stress prolongs lifespan in worms () and yeast (). The specific context under which these pathways operate and the cell types responding to these various manipulations likely play a major role in explaining these seemingly complex observations. It would be of future interest to examine how restoring ETC activity or inducing mild mitochondrial stress would affect skin ageing.
Deciphering Transcriptional and Epigenetic Noise in Aging Stem Cells
A hallmark of aging-associated stem cell degeneration is transcriptional noise and epigenetic erosion, in which heterochromatin silencing and transcriptional fidelity appear crucial. Pioneering genetic and biochemical experiments have mapped out the central pathway for heterochromatin regulation (; ; ; ; ; ; ; ; ; ; ), a critical vulnerability during organismal aging. Epigenetic deregulations in aging have been extensively characterized in model organisms, where several parallels could be drawn into the mammalian systems including skin (Figure 4).
FIGURE 4
A key gene-silencing mechanism in mammals, DNA methylation is not conserved or has a very limited role in yeast, flies, and worms. Rather, histone modifications are heavily involved in transcriptional silencing in the latter. For example, during yeast replicative aging, decline of the deacetylase Sir2 results in acetylated histone 4 lysine 16 (H4K16ac) and increased transcription at subtelomeric regions (), as well as enhanced ribosomal DNA (rDNA) transcription by RNA polymerase I and formation of extrachromosomal rDNA circles () (H4K16ac and RNA polymerase I transcription factors are Sir2 substrates). Interestingly, aged HSCs exhibit global DNA hypomethylation that includes the rDNA regions (), and they accumulate non-canonical gH2AX and ATR at nucleoli where rDNA transcription occurs, disrupting ribosome biogenesis (). Notably, SIRT1 (the mammalian homolog of yeast Sir2) appears to contribute to repeat silencing and is involved in the DNA damage response in mammalian cells (), although its role in mammalian rDNA regulation is unknown. Since, unlike yeast, mammals use both DNA and histone methylation to mediate transcriptional repression, it will be interesting to see which of these pathways are involved in rDNA silencing during skin aging. Such effort will now be greatly facilitated by the recently completed draft of gapless human genome covering highly repetitive regions including rDNAs ().
Besides repetitive rDNA, another type of highly abundant repeats in the mammalian genome is transposons (Figure 4). The aging epigenome is shaped by global DNA hypomethylation, typically at gene-poor and late-replicating regions, accompanied by focal hypermethylation at genic regions, bearing strong resemblance to the cancer genome (; ). In yeast, decline of histone chaperones and global loss of histones during aging lead to transposon derepression and genome instability (; ; ). Deficiencies in histone deacetylase SIRT6 in mice (), exonuclease TREX1 in human cells (), and endonuclease AGO2 in flies () have been shown to induce transposons in aging and inflammatory diseases. In a unique group of premature aging diseases, progeria and laminopathies, lamin-associated domains become disintegrated (), accompanied by global loss of H3K9me3 and structural changes in the heterochromatin (; ) and subsequently nuclear autophagy and cytosolic chromatin fragment–induced inflammation (). Indeed, in keratinocytes, it has been shown that mechanical strain leads to H3K9me3 heterochromatin loss at the nuclear lamina and disruption of lineage gene expression during aging (; ), suggesting a viable hypothesis that deregulation of heterochromatin drives skin aging. Among H3K9me3 methyltransferases, G9a has been functionally examined in the mammalian epithelium. While G9a is minimally involved in homeostasis potentially because of functional redundancy, it is critical for tumorigenesis (; ).
Another source of spurious transcription in aging could be emanated from gene bodies (Figure 4). H3K36me3 methyltransferases are among the first discovered epigenetic regulators of longevity: Set2 in yeast (; ) and Met-1 in worms (; ). In these model systems, it has been shown that RNA polymerase II–associated methyltransferase deposits H3K36me3 along the gene body, recruiting histone deacetylase to suppress spurious transcription initiated from the gene body, and this transcriptional fidelity becomes compromised in aging. A similar mechanism has recently been described in mammalian cells (; ), in which DNA methyltransferase is likely recruited by H3K36me3 along the gene body to mediate the suppression of cryptic transcription (). The mammalian H3K36me3 is maintained by multiple methyltransferases, Setd2 and Nsd1/2/3, which exhibit both overlapping and distinct functions ().
Both H3K9me3 and H3K36me3 are closely associated with DNA methylation and frequently exhibit interdependency. Epidermis-specific deletion of the de novo DNA methyltransferase Dnmt1 led to hair follicle miniaturization along with epidermal and sebaceous gland hyperplasia, a signature of premature aging in the skin (). Consistently, in the epidermis, Dnmt1 along with its foreshadowing E3 ubiquitin ligase Uhrf1 are required to maintain stem cell self-renewal (; ), loss of which results in autoinflammatory conditions in the skin (). A related enzyme, Tet2, that catalyzes 5-hydroxymethylcytosine (5hmC) and promotes DNA demethylation, is essential to maintain epidermal differentiation (). Furthermore, deletion of the maintenance DNA methyltransferases Dnmt3a/b from epithelial cells largely spared skin homeostasis but contributed to malignant transformation (; ). Interesting, deletion of mediator complex component Med1 () or the SWI/SNF-like BAF complex catalytic subunit Brg1 () also elicits HFSC exhaustion and alopecia phenotype, although in the case of BAF complex, its function is context dependent and regulates either stem cell expansion or differentiation based on which lineage is examined (; ).
Another repressive histone post-translational modification proposed to regulate aging is the inactive or poised chromatin marker H3K27me3, which declines due to increased demethylase UTX-1 during aging (; ). In this case, the impact of H3K27me3 loss on aging has been attributed to its specific regulation of the IIS (insulin/insulin-like growth factor) genes. In contrast, the excessive levels of the active chromatin marker H3K4me3 and its responsible methyltransferase, trithorax, has been shown to be detrimental to longevity (; ). On the other hand, H3K4me3 demethylase promotes longevity in worms, with a remarkable transgenerational memory effect observed in this context (). Mild mitochondrial stress-induced lifespan extension is associated with epigenetic remodeling via histone methyltransferases () and demethylases (). Remarkably, genome-wide mapping of 5mC and 5hmC in HSCs revealed extended regions of low-methylation canyons that are distinct from CpG islands and shores (). These canyons harbor either H3K27me3 or poised H3K27me3/H3K4me3 histone markers, whose borders are demarcated by 5hmC, and are deregulated upon Dnmt3a/Tet1 deletion or in aging (; ; ).
In skin, histone methyltransferases of H3K27me3 (polycomb group) and H3K4me3 (trithorax group) regulate epidermal stem cell self-renewal and differentiation, respectively, during development and regeneration (; ; ; ). For example, in PRC2 Ezh2 conditional knockout, P16 is derepressed, resulting in cell cycle arrest and epithelium hypoplasia (), similar to PRC1 deficiency (; ). In contrast, the H3K27me3 demethylase promotes epidermal differentiation (). Likewise, histone deacetylases HDAC1/2 () () and HDAC3 () have opposing functions in stem cell proliferation versus differentiation. On the other hand, the histone methyltransferase MLL4 (KMT2D) that catalyzes H3K4me1 is required to maintain epidermal homeostasis, whose loss resulted in the disruption of skin stratification and lipid metabolism (; ). Interestingly, lack of Bmi1, a component of PRC1, leads to impaired mitochondrial function and to increased ROS and DNA damage, which could be reversed by antioxidant treatment or genetic disruption of Chk2 (). These findings suggest that global chromatin disruption may override defects from specific gene deregulations, that it is not an individual gene or a few genes per se that elicit the phenotype. These molecular and cellular mechanisms are potentially extendable to aging.
Compared to global hypomethylation, equally noteworthy is the accompanying focal hypermethylation in aging chromatin (Figure 4). A prominent feature in cellular senescence is the formation of the senescence-associated secretory phenotype (SASP) on one hand () and senescence-associated heterochromatin foci (SAHF) on the other (; ; ; ; ). Remarkably, a sparse hair coat in aged mice is recovered upon FOXO4 peptide treatment, which blocks p53 nuclear localization and induces the apoptosis of senescent cells (), supporting the functional significance of senescence in skin aging. SAHF has been suggested as a survival mechanism to “plug the hole” that assembles heterochromatin onto euchromatin regions, given the compromised nuclear lamina that otherwise is necessary to maintain the physiological heterochromatin. Alternatively, one could speculate these foci may titrate away rate-limiting factors (H3.3, methyltransferase, or metabolites required for transcriptional repression; see Discussion) from transposon repeats and heterochromatic regions, exacerbating the already-weakened epigenetic regulation during aging. Although this model has not been formally tested in aging, a similar titration- or competition-based inhibitory mechanism from H3K27M and H3K36M oncohistones has been shown in cancer (; ), leaving the field with the intriguing question of how the lineage specificity and gene selectivity is achieved in the context of global epigenetic deregulations.
Discussion
While the identity of upstream signals that lead to defective epigenetic machinery in aging remains unclear, it appears safe to assume these factors would be rate-limiting. As mentioned, nutrients and metabolites, in addition to their impact on cell signaling transductions and mitochondrial biology, are appealing candidates that appear to dictate the aging transcriptome and epigenome. Many epigenetic enzymes that regulate methylation or acetylation exhibit KM in the range of observed substrate concentrations in cells, suggesting rates of these reactions are highly sensitive to the cellular fluctuations of the corresponding metabolites, such as S-adenosyl methionine, acetyl-CoA, alpha-ketoglutarate, NAD+, and beta-hydroxybutyrate (), raising the intriguing question of the extent to which cellular metabolite deregulation shapes the aging epigenome. Indeed, it has been shown that in autophagy-deficient HSCs, DNA methylation and lineage gene expression are regulated by S-adenosyl methionine and alpha-ketoglutarate levels (). Ascorbate and its downstream target TET2 dictate HSCs () as well as () epidermal function and tumorigenesis. NAD+ being the rate-limiting factor, competition between PARP-1 (also NAD+ dependent) and SIRT1 dictates cellular response in DNA damage and epigenome regulation () (). RNA sequencing (RNA-seq) and assay for transposase-accessible chromatin with sequencing (ATAC-seq) at single cell level, and together with spatial transcriptomics provided unprecedented throughput and resolution of cell type, lineage trajectory, and tissue-level crosstalk in many biological contexts, and are likely to be powerful technologies in aging research. The current challenge lies in the metabolomics at a sub-cellular level and within the intact organism in vivo, both of which are critical to understand molecular determinants of aging.
Local tissue microenvironment plays a dominant role in dictating stem cell function, including aging. Aged dermal fibroblasts maintain their positional identity () while manifesting adipogenic and inflammatory traits (; ). Secreted factors of BMP and WNT pathways enriched in adipocytes (; ) and pre-adipocytes () known to dictate hair follicle regeneration are disrupted in aged dermis (; ). Skin-resident immune cells are drastically remodeled during aging (; ), including regulatory T cells that are known to support hair follicle regeneration () and decline significantly in aged dermis (). So are the arrector pili muscles and nerves (; ; ) that provide niche input to HFSCs, both of which become dislodged in aged skin (). Of significance, aged HFSCs can be rejuvenated by resident cell types and niche components of the young skin (; ; ), suggesting that the tissue microenvironment drives stem cell function during aging. Future work will need to exploit the molecular identity, regulatory signals, and therapeutic potential of such rejuvenation factors.
Statements
Author contributions
YL and YG conceived the concept and wrote the manuscript.
Funding
YG is supported by grants from the NIH (1K01AR072132), CPRIT (FP00006955), UT Rising STARs program, Cancer Center Support Grant new faculty award, Andrew Sabin Family Award and MD Anderson Cancer Center startup funding. This manuscript was edited by Sarah Bronson, ELS, of the Research Medical Library at The University of Texas MD Anderson Cancer Center.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s note
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Summary
Keywords
skin aging, stem cell lineage plasticity, inflammaging, epigenetics, metabolism
Citation
Lyu Y and Ge Y (2022) Toward Elucidating Epigenetic and Metabolic Regulation of Stem Cell Lineage Plasticity in Skin Aging. Front. Cell Dev. Biol. 10:903904. doi: 10.3389/fcell.2022.903904
Received
24 March 2022
Accepted
21 April 2022
Published
19 May 2022
Volume
10 - 2022
Edited by
Wen-Hui Lien, Catholic University of Louvain, Belgium
Reviewed by
Brian C. Capell, University of Pennsylvania, United States
Ramiro Iglesias-Bartolome, Center for Cancer Research (NIH), United States
Updates
Copyright
© 2022 Lyu and Ge.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Yejing Ge, YGe1@mdanderson.org
This article was submitted to Stem Cell Research, a section of the journal Frontiers in Cell and Developmental Biology
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