Aging, which is a complex and progressive degenerative process, can be recognized by degenerative decline in the tissue integrity and physiologic functions of the body, accompanied by cellular senescence, matrix changes, tissue inflammation, and metabolic dysfunction [
1]. It is an important contributing factor for the development of multiple age-related diseases, such as neurodegenerative diseases, cardiovascular diseases, cancer, and inflammation-related diseases. Furthermore, aging presents a substantial risk to the global healthcare system and the well-being of the elderly, and slowing down the progression of aging is essential for promoting healthy aging. Currently, many studies have identified a range of interventions aimed at delaying aging or age-related diseases, such as caloric restriction, increased physical exercise, improved sleep quality, and exogenous molecular interventions targeting specific molecular targets [
2]. However, modifying diet and lifestyle alone is insufficient to extend healthy lifespan or prevent age-related diseases in older adults. Hence, many studies mainly focus on the mechanisms of the aging process and explore methods to target the hallmarks of aging. Indeed, numerous critical factors trigger aging, such as telomere dysregulation, DNA damage, mitochondrial dysregulation, stem cell exhaustion, and autophagy disorders (Fig.1). Thus, targeting these potential triggers of aging may be a promising therapeutic approach for delaying aging.
Telomere dysfunction is a significant manifestation of aging, leading to molecular and cellular damage. It functions as a driver or amplifier of the molecular circuits that contribute to the aging process and the development of associated diseases. Telomeres are repetitive DNA sequences (TTAGGG) at the ends of chromosomes. Telomerase, which is the reverse transcriptase and the ribonucleoprotein complex, can add new DNA to telomeres for telomere elongation [
3]. Telomerase is primarily comprised of telomerase RNA component (TERC) and telomerase reverse transcriptase (TERT) protein [
4]. Telomerase has a vital role in maintaining both genomic integrity and cell proliferation [
5]. In humans, telomerase is expressed in stem cells, germline cells, and somatic cells with high replicative demands, such as lymphocytes [
6]. Nevertheless, the expression of telomerase is tightly regulated in most adult human somatic cells and telomere length decreases with age in most human and mouse tissues [
7,
8]. Furthermore, evidence shows a positive correlation between aging and decreased telomerase activity during cell division [
9]. Conversely, increased expression of human telomerase in lymphocytes can promote telomere elongation and extend lifespan [
10]. Moreover, humans and mice display significant differences in telomere length and telomerase regulation. In adult humans, the telomere length ranges from 5 to 15 kilobases (kb), whereas in mice, telomere length can start around 50 kb [
11]. Unlike in humans, telomerase activity in mice can be detected in most tissues. Intriguingly, this difference may contribute to more abundant telomere reserves in mice, thereby providing a valuable model for studying the regulatory complexity of human telomerase. Furthermore, mouse models play a vital role in biomedical research in understanding human development and age-related diseases. Specifically, telomerase-deficient mouse models reveal a critical role for telomerase in organismal lifespan and tissue renewal. In a pure C57BL6 genetic background, these telomerase-deficient (TERC
−/−) mice show shortened telomere length as well as elevated genomic instability starting from the first generation (G1) to the third (G3) or fourth (G4) generation [
12]. Notably, studies have shown that telomerase reactivation in aged telomerase-deficient mice can partially reverse tissue degeneration and rescue telomere dysfunction [
13]. Therefore, these accumulated studies clarified the significant role of telomerase in maintaining telomere function, suggesting that targeting telomerase may have beneficial therapeutic effects in alleviating aging phenotypes and aging-related diseases.
As an indispensable regulator of telomerase activity, TERT contributes to the reverse transcription of hexameric repeat sequences onto the ends of chromosomes. Specifically, increased
TERT expression in aged mice is able to elevate telomerase activity, which delays aging and extends longevity without tumorigenic activity [
14]. Recent studies have shed light on the multifaceted roles of TERT in postmitotic cells, such as neurons and cardiomyocytes, which extend beyond the traditional telomere synthesis function of TERT in aging and age-related diseases [
5]. To date, numerous synthetic and natural small molecule compounds have been reported to possess the potential to genetically protect or modulate aging in one or more model organisms. The natural compound TA-65 (cyclastragenol) in mice can increase TERT expression and rescue short telomeres [
12]. The synthetic compound AGS-499 promotes TERT expression in mice and human cells [
15]. Although these exogenous compounds that induce telomerase activation have broad prospects in human aging and age-related diseases, their downstream mechanisms of action remain inadequately defined. Therefore, elucidating the therapeutic mechanisms of anti-aging interventions and developing novel drugs to delay the aging process hold significant clinical importance.
Recent research has shown that a novel TERT activator compound (TAC) can induce endogenous TERT activation [
5]. The group further explored the specific downstream mechanisms that delay natural aging after TERT activation, beyond its role in telomere elongation (Fig.1).
Specifically, Shim et al. used fibroblasts from transgenic mice with human TERT (hTERT)-Renilla luciferase (Rluc) to screen 653 000 compounds. Ultimately, a novel small-molecule compound called TAC (< 400 Da) was identified that can significantly increase TERT expression (Fig.1). TAC further induced TERT gene expression in proliferating and postmitotic cells (like cardiomyocytes, neurons, and microglia) and tissues (such as the brain, heart, and skeletal muscle) in mice. Additionally, TAC could extend telomere length and attenuate foci of DNA damage triggered by telomere dysfunction in primary Werner syndrome fibroblasts. Thus, these results suggest that TAC induces physiologic TERT expression in various tissues and cells. In addition, TAC possesses lipophilicity, which facilitates drug uptake across tissues, including the central nervous system (CNS). Intraperitoneal administration of TAC resulted in favorable plasma exposure for TAC (T1/2, 0.568 h; AUC, 285 h·ng/mL) and showed CNS exposure in mice, suggesting that TAC can penetrate the blood-brain barrier and enter the central nervous system.
To further clarify the detailed mechanisms involved in TAC-mediated TERT expression, the authors observed that TAC in primary human fibroblasts MRC-5 cells increases the phosphorylation of extracellular signal-regulated kinase (ERK), which is translocated into the nucleus and phosphorylates multiple transcription factors (including FOS) to control the expression of target genes. This suggests that TAC facilitates TERT expression through the MEK/ERK axis. They further explored the underlying transcription factors of TERT and their binding elements that link ERK activation to TERT transcriptional control. In human fibroblasts and neurons, TAC significantly upregulated the transcription factor FOS, which is an essential component of the transcription complex encoding activator protein 1 (AP-1). The authors further identified that mutations of AP-1 binding sites (−1672, −3725) in MRC-5 cells could reduce TAC-induced TERT promoter activity. Next, it will be determined whether the AP-1 complex is specifically recruited to TERT promoters during TAC induction of TERT expression. ChIP-qPCR analysis suggested that TAC results in the recruitment of FOS to two AP-1 binding motifs in the TERT promoter. The selective AP-1 inhibitor T-5224 also attenuated TAC-induced TERT expression by blocking FOS/AP-1 binding to DNA. Taken together, these results indicate that TAC specifically mediates the transcriptional activation of TERT through the MEK/ERK/FOS/AP-1 pathway (Fig.1).
Based on the significant role of TERT expression in the aging organism, the authors further explored whether TAC-induced TERT expression can mitigate organismal aging in naturally aged mice. Surprisingly, in middle-aged mice (12-month-old mice), they revealed for the first time that TAC-induced TERT upregulation represses the expression of cyclin-dependent kinase inhibitor p16Ink4a, a key driver and biomarker of aging. Furthermore, they elucidated the molecular mechanism of TAC-mediated p16Ink4a inhibition. It was reported that de novo DNA methylase DNMT3b is involved in the hypermethylation of the p16Ink4a promoter and is linked to the transcriptional repression of p16Ink4a. They found that TERT level is correlated with DNMT3b expression in vivo and TERT can bind to the DNMT3b promoter in human iPSC-derived neurons, suggesting that TERT acts as a transcriptional modulator in terminally differentiated cells such as neurons. Further, TAC could increase methylated CpG sites in the p16Ink4a promoter region in middle-aged mouse tissues. These results indicate that TAC can facilitate hypermethylation of the p16Ink4a promoter by inducing TERT upregulation, which leads to repression of p16Ink4a transcription. Additionally, TAC attenuated the expression of senescence-associated secretory phenotype (SASP) components, including interleukin (IL)-1α, IL-1β, matrix metalloproteinase-3 (Mmp-3), and vascular endothelial growth factor (Vegf) in mice. They also clarified that long-term administration of TAC for 6 months can delay cellular senescence in multiple tissues of aged mice (about 26–27 months) and decrease the pro-inflammatory cytokines IL-1β and IL-6. Therefore, these results show that TAC-induced TERT expression can mitigate age-related tissue senescence (such as the brain) and modulate the expression of p16Ink4a and SASP components (like IL-1β) (Fig.1).
In addition, this study further evaluated the specific impacts of TAC in naturally aged mice. Shim et al. revealed that short-term treatment with TAC for 1 week in the hippocampus of middle-aged mice strikingly promotes the expression of TERT and mature brain-derived neurotrophic factor (BDNF). Astonishingly, TAC-induced activation of TERT enhanced hippocampal neurogenesis-associated genes such as doublecortin (Dcx) in adult mice after 3 weeks and elevated the number of DCX-expressing newly developed neurons after treatment for 4 weeks. Moreover, TAC treatment for 4 weeks in the hippocampus of middle-aged mice decreased IL-1β and IL-6 as well as IBA1-positive activated microglia, a hallmark of neuroinflammation in brain aging. Long-term TAC treatment for 6 months improved hippocampal-dependent cognitive function in naturally aged mice (26–27 months old) without marked adverse consequences. Besides, TAC-treated aged mice improved motor coordination and muscle strength, suggesting that TAC may ameliorate age-related neuromuscular decline. In summary, these experiments unravel that TAC-induced TERT expression not only promotes neurogenesis but also effectively improves neuroinflammation, hippocampal-dependent cognitive function, and neuromuscular function in naturally aged mice (Fig.1).
The interaction and mutual promotion between aging phenotypic characteristics, aging triggers, and inherited or acquired age-related diseases have become key hotspots of interest for scientists to explore anti-aging strategies. Encouragingly, the authors further confirmed that TAC promotes high
TERT expression and mitigates natural aging (Fig.1). Increasing evidence supports that small molecule drug therapies in the treatment of chronic and age-related diseases are considered to have lower immunogenicity and lower treatment costs compared with biologicals [
16]. Thus, TAC may have extensive application prospects in delaying aging, which deserves further validation. In another study, Shim
et al. indicated that the physiologic elevation of TERT can downregulate the hallmark of aging in neurons and attenuate Aβ levels in an Alzheimer’s disease mouse model and cultured human iPSC-derived neurons containing genomic amyloid precursor protein (APP) duplication [
17]. These results suggest that the physiologic activation of TERT in somatic cells may have a potential role in improving Alzheimer’s disease. Therefore, whether TAC-induced physiologic activation of TERT serves as a novel strategy to relieve age-related diseases such as Alzheimer’s disease warrants further investigation. In addition, TAC induces TERT expression in cardiomyocytes, and cardiomyocyte senescence is associated with the progression of cardiovascular diseases. Hence, the relationship between TAC and cardiovascular diseases deserves further elucidation.
Additionally, TAC has shown great potential in delaying natural aging in mice, whereas there are significant differences between humans and mice in telomerase biology, growth and development, lifespan, and experimental cycle. As mentioned above, Shim
et al. found that TAC-induced telomerase activation can delay tissue senescence in naturally aged mice during their relatively short life cycle. However, it is crucial to recognize that the translational gap between preclinical research and human clinical trials remains a challenge. On the one hand, the human lifespan is relatively long compared to that of mice; on the other hand, laboratory mouse models also have limitations in mimicking human physiologic mechanisms and disease progression [
11]. Thus, by using advanced gene editing to optimize existing mouse models, it may be possible to better mimic human telomere homeostasis, thereby improving the success rate of drug clinical trials. Zhang
et al. designed a mouse model harboring a humanized
mTERT gene, which can mimic the expression of human telomerase and telomere length (10–12 kb). It is worthwhile to utilize this valuable model to verify the potential of TAC in delaying aging. Given the multifaceted differences between the two species, the specific role of TAC in delaying human natural aging is still unclear. Therefore, further studies are needed.
Notably, the timing of TAC administration may be important for these potential outcomes in delaying aging. Hence, further clinical trials are needed to explore the age at which optimal efficacy of TAC can be achieved in humans. In addition, the authors showed that TAC-induced
TERT expression does not lead to
in vivo cancer effects. However, given the role of telomere dysfunction in cancer initiation, there is a growing body of disease-based and epidemiological evidence that telomerase is expressed in the majority of tumors from all cancer types [
18]. These studies indicate that TAC may be protective in telomerase-positive tumors. Thus, long-term TAC administration in future clinical studies requires further toxicity studies to evaluate safety.
Together, positive outcomes from this study utilizing the TAC may facilitate larger-scale exploration, and if proven, the novel drug will provide new therapeutic opportunities for telomerase activation to delay aging in the future.
PDF
(790KB)
