Mesenchymal stem cells mediate the temporal modulation of aging-related changes

Abstract

Currently, mesenchymal stem cells (MSCs) and MSC-based transplantation are used to improve health conditions and increase life expectancy. These therapies are based on the principle that the decline of cell populations is the main cause of reduced regenerative potential and aging. However, metabolic cycle disorders could result from several mechanisms of intercellular communication and cell senescence transmission, independent of special phenotype development. MSCs are sensitive to external influences; therefore, the accumulation of changes in the surrounding tissues, rather than in the cells themselves, contributes to accelerated aging through several biophysical mechanisms. These systemic physiological mechanisms could partially explain such effects, such as the systemic role of MSCs as drivers of inflammaging, an effect that increases with age. These findings have prompted a reevaluation of the potential of MSC subpopulations as novel, unconventional therapeutic targets.

Key Words: Adverse events; Aging; Cell therapy; Immunosenescence; Mesenchymal stem cells; Rejuvenation; Senescence-associated secretory phenotype; Senescence

Core Tip: Mesenchymal stem cells (MSCs) show promise in regenerative medicine, but their use in elderly patients requires careful consideration. Aging presents unique challenges, such as immunosenescence, an increased risk of cancer, cellular senescence, and vascular complications. These challenges highlight the importance of performing thorough preclinical and clinical studies to assess the safety and effectiveness of MSC therapy. Furthermore, they could lead to a reevaluation of the role of MSC subpopulations as novel, unconventional therapeutic targets.

INTRODUCTION

Currently, mesenchymal stem cells (MSCs) and MSC-based transplantation are used to improve health conditions and increase life expectancy. The limited clinical approvals of MSC-based therapies highlight the complexity of cellular product formulation and quality assurance. This complexity is potentially related to the extreme heterogeneity in cell properties and their related effects[1-5], which is highly affected by in vitro culturing conditions[2,3,6,7]. Addressing these challenges requires precise prediction of clinical responses. This, in turn, requires well-defined populations of MSCs and harmonised assessment of their specific functions[2,8].

Cell therapies based on MSCs derived from various sources show promise in regenerative medicine, but their use in elderly patients requires careful consideration due to the risks of adverse events, side effects, and complications[9,10]. These risks are related not only to the specific systemic responses, but also the features of cell preparing and culturing. However, while MSC-based cell therapy can extend lifespan[11,12], this effect is observed only under limited conditions and in specific cases associated with the reduction of inflammation[13,14]. Aging affects the quantity, quality, and responsiveness of stem cells to their microenvironment, thereby compromising therapeutic outcomes[10,15]. Indeed, investigators have examined the development of pathological conditions, primarily those associated with the senescent phenotype[16,17]. It is not surprising that induced pluripotent stem cells became the relevant model for investigating the wide range of MSC alterations[18]. For example, immune-mediated responses are involved in the development of age-related complications[19,20]. It could be hypothesized that the responses to MCS-based therapy could be explained by the previously investigated systemic physiological responses rather than a definitive mechanism from cellular biology.

The series of studies by Ababneh et al[21] comparing functional outcomes of differentiation protocols for induced pluripotent stem cell-derived MSCs (iMSCs) highlights the optimization and standardization of differentiation methods to produce reproducible, high-quality iMSCs for research and therapeutic applications. Previously, the same authors investigated the tissue-specific senescence-associated secretion phenotype of iMSC cells, and identified their specific secretomic properties with sustained anti-proliferative effects[22,23]. Many recently published studies have disclosed the issue of age-dependent loss of MSCs, including the compensation of cell deficit by the transplantation of cultured MSCs[24,25]. While Ababneh et al[21] optimized differentiation protocols, the long-term senescence risks of iMSCs remain unaddressed. Senescent cells are well known to exhibit decreased functionality and specific phenotypes, making them primary targets of senolytic therapy[26,27]. In MSCs, increased secretion activity is associated with cell stage factors. Previously published studies have provided insight into a previously rarely investigated phenomenon of tissue physiology related to the basic principle of ‘accumulation disease’ and the systemic switching effects of MSCs[28-30]. Interestingly, this phenomenon has only recently been investigated and remains not fully elucidated. Our aim is to elucidate the fundamental mechanisms of the MSC-derived factors that promote aging.

PHYSIOLOGICAL CAUSALITY OF TEMPORAL MODULATION OF AGING AND DISEASES

The well-known effects of MSC-associated therapeutic and health improvements contrast with the less obvious, yet previously documented, effects of reversed system responses. Physiological feedback and therapeutic outcomes may be affected by three biological effects that are grounded in firm biophysical basics and can be calculated directly using quantitative approaches[31,32]. Although often overlooked, these factors appear to be important and thus warrant illustration through examples from different fields. Rather than being considered separately, these effects should be examined through tissue-specific interactions, which vary greatly across the body and result in heterogeneous responses—particularly due to the involvement of tissue-specific macrophages[33,34].

The first biophysical issue relates to resource depletion and structural damage. Metabolic and structural injury to the cell leads to an excessive intensification of the response that consumes nutrients, mediators, and energy reserves[35,36]. This alteration not only affects the cells, but also regulates the key cellular microenvironment, including the extracellular matrix and cellular glycocalyx[37]. Any biological process in which a protective or adaptive response exceeds an organism’s capacity to supply energy and substrates will eventually result in resource depletion and structural damage[38]. For example, overtraining and muscle cell damage can lead to a continuous metabolic demand in skeletal muscle, as repeated excessive exertion without recovery can create this condition[39]. Glycogen stores and intracellular ATP are depleted, and reactive oxygen species accumulate. Muscle fibers experience structural disruption, such as Z-line streaming and membrane rupture[40]. The over-intensifying normal adaptive pathways can result in metabolic exhaustion and physical injury to cells, which could explain the outcome of cells affected by the secretory activity of transplanted or over-activated MSCs. This physiological response could explain why MSC-based cell therapy alone leads to primarily short-term outcomes[41,42]. Often, there are no prolonged or long-term robust physiological effects.

The second relates to the disruption of the balance between positive and negative physiological feedback mechanisms. Under normal conditions, negative feedback limits the progression of a process. However, when negative feedback is insufficient or impaired, the system experiences uncontrolled amplification or “runaway”[43,44]. This mechanism is well known in quantitative physiology, particularly in fever regulation and hyperthermia[45]. The hypothalamus maintains core body temperature through negative feedback, and when body temperature rises, sweating and vasodilation promote heat loss. Conversely, impaired negative feedback (e.g., in heat stroke) causes thermoregulation to fail[46,47]. This creates a positive feedback loop that can rapidly become fatal unless interrupted externally, as the rising temperature causes further metabolic acceleration, producing more heat[48]. Another example is the cytokine storm that occurs during inflammation. This storm happens when inflammatory responses are tempered by anti-inflammatory cytokines (e.g., interleukin-10) and regulatory immune cells. However, when these mechanisms fail due to severe viral infections or sepsis, proinflammatory cytokines (e.g., tumor necrosis factor-α and interleukin-1) amplify each other’s release[49]. This leads to a systemic inflammatory response, which is a “runaway” positive feedback loop that causes shock and tissue injury. A “runaway” physiological process occurs when stabilizing (negative feedback) controls fail to oppose a self-reinforcing (positive feedback) process. To prevent runaway activation, intrinsic inhibitory mechanisms are triggered to counterbalance the surge[50]. An imbalance of intercellular signaling and the pleiotropic properties of signaling proteins can lead to a “runaway” effect during MSC-based therapies.

The third is the formation of closed positive feedback loops, also known as vicious cycles. The activation of one factor triggers subsequent factors that reinforce the initial stimulus, creating a self-sustaining process. For example, a closed positive feedback loop means that activation of one component amplifies the activity of the same or related components[51]. This reinforces the process until an external limit is reached, the system collapses, or pathology ensues. For example, action potential depolarization in neurons occurs during slight membrane depolarization (e.g., in response to a stimulus). This opens voltage-gated sodium channels, allowing sodium to enter and further depolarize the membrane, which in turn opens more sodium channels and leads to the formation of a positive feedback loop. Another example is calcium-induced calcium release in muscle and synaptic cells in response to a small rise in cytosolic Ca²⁺ due to extracellular influx or a signal[52,53]. Calcium binds to ryanodine receptors on the sarcoplasmic/endoplasmic reticulum, releasing more calcium and amplifying the signal, and results in a swift, large release of Ca²⁺ for contraction or secretion. Interestingly, the observed therapeutic effects of MSCs could be also based on positive biological feedback loops because they accelerate and reinforce processes that are useful for “all-or-none” events, such as birth, firing, and clotting, or that require natural “off” mechanisms to restore balance afterward.

UNDERLYING MECHANISMS OF MSC-DERIVED FACTORS THAT PROMOTE AGING

The proposed mechanism of MSCs in the treatment of injuries and chronic diseases is based on their secretory activity of vesicles and soluble factors. This secretion phenotype forms the unique tissue-specific niche and related cellular microenvironment. Subpopulations of MSCs differ as the secretion of growth factors and vesicles depend on the source derivation[54,55]. Age-related involution of the thymus due to MSC migration and displacement of naive immune cells is hypothetically associated with aging acceleration[56]; however, recent investigations could shift this paradigm. Up to now, it has been hypothesized that intercellular communication and its weak-scale alterations are the primary mechanism driving systemic pathological effects[34]. The key mechanisms underlying the long-term effects of MSC-based therapy on aging acceleration are summarized in Table 1.

Table 1 Key mechanisms underlying the long-term effects of mesenchymal stem cell-based therapy on aging acceleration.

Physiological process	Cell-mediated mechanism(s)	Short-term and long-term effects
Resource depletion and structural damage	Primarily cellular microenvironment-mediated mechanisms (extracellular matrix and cellular glycocalyx)	Short-term: Visible
		Long-term: Altered non-functional tissue maturation
Disrupting the balance between positive and negative physiological feedback mechanisms	Extremely specific stimulation through the secretion of targeted molecular complexes and extracellular vesicles	Short-term: Primarily not visible
		Long-term: Reducing reparation abilities
Formation of closed positive feedback loops (also known as vicious cycles)	Induction of a chain reaction of metabolic secretion, such as inflammaging	Short-term: Not visible, but rather anti-inflammation effects
		Long-term: Systemic physiological effects related to accelerated aging

Despite the promising potential of young or rejuvenated MSC transplantation, their effects have two-sided unconventional effects. A decrease in the number of MSCs may not be the cause of regeneration loss and increased inflammation; rather, it may reflect age-dependent defense responses. MSCs are able to acquire an immunosuppressive state, termed the ‘MSC-2’ phenotype, in a manner similar to other innate immune cells such as macrophages[57]. This ‘MSC-2’ state is crucial for the therapeutic potential of resolving inflammation in autoimmune diseases and tissue repair[9]. MSCs are capable of undergoing alterations that contribute to inflammaging, and their secretory activity can drive senescent drift in affected tissues. Indeed, the deterioration of stem cell function may contribute to human aging itself[10,58]. Transplantation of senescent MSCs can induce physical dysfunction[59], as can the transplantation of MSCs in senescence-induced conditions. The secretome of senescent MSCs has been shown to act as a ‘key-driver’ of inflammaging[60,61], and aged MSCs could cause toxicity and induce senescent drift in affected tissues[62]. Unlike the senescence-associated secretion phenotype, which is a known type of cell secretion, senescent drift in tissues is associated with the secretion of an affinity-targeted secretome and may play a significant role in driving aging (Figure 1).

Figure 1 An example of the robust mechanism associated with the altered paracrine activity of mesenchymal stem cells, which leads to senescent drift and promotes inflammaging. Created with Biorender.com (Supplementary material).

The factors associated with chronic inflammation could lead to adverse events and side effects of MSC-based therapies[9]. MSC subpopulations could also lead to long-term effects, some of which could be part of accelerated aging, especially in elderly populations. There are many lysosomal storage diseases associated with the accumulation of metabolites and gene products. Regarding aging, an increase in senescent cells with age and the effects of various senolytics have been demonstrated[63]. However, the underlying mechanism of this effect may be much deeper as promising therapies may not aim to combat senescent cells, but rather specific populations of secretory-active cells, such as MSCs from adipose tissue.

CLINICAL IMPLICATIONS

The potential risks associated with MSC-based therapy in elderly populations necessitate careful ethical consideration. Informed consent is paramount, as elderly patients must fully understand the potential benefits and risks of MSC treatment. Additionally, the principle of “do-no-harm” must guide clinical decision-making, ensuring that the potential for harm does not outweigh the anticipated benefits. Recently published studies have investigated ways to optimize and standardize differentiation methods to produce high-quality, reproducible MSCs for research and therapeutic applications[21,64,65]. We believe that without dependence on MSC phenotype, an increase in the number of these cells in the elderly could lead to rapid aging.

CONCLUSIONS

The role of MSCs in inflammation and aging is not well understood. Transplantation of these cells has been associated with adverse events and side effects attributable to unresolved underlying mechanisms. The systemic physiological mechanisms investigated to date can only explain isolated components of these effects, such as the systemic role of MSCs as drivers of inflammaging, a phenomenon that may exacerbate with age. This has prompted a re-evaluation of MSC subpopulations as potential novel and unconventional therapeutic targets.