The age-associated decline in neuroplasticity and its implications for post-stroke recovery in animal models of cerebral ischemia: The therapeutic role of extracellular vesicles

Abstract

Older individuals are typically more susceptible to stroke, and age-related differences in brain plasticity significantly affect recovery and treatment responses following cerebral ischemia and traumatic brain injury. Extracellular vesicles (EVs) have emerged as promising diagnostic and therapeutic tools due to their role in intercellular communication and ability to cross the blood–brain barrier. While EVs hold potential in promoting brain repair, their efficacy is influenced by donor age—those derived from young stem cells exhibit more regenerative profiles, whereas aged donor EVs may carry senescence-related signals that impede recovery. Emerging therapies, including senolytics, exosome-based approaches, and immune modulation, aim to enhance post-stroke repair, yet a substantial translational gap persists, especially in adapting these strategies to the aged brain. Differences in immune responses, neurovascular integrity, and repair mechanisms between young and aged individuals further complicate therapeutic development. Incorporating aged animal models in preclinical research is thus essential for ensuring the relevance and safety of interventions in elderly patients. These findings underscore the need for age-tailored strategies that reflect the unique biological landscape of aging, paving the way for more effective treatments for stroke and related neurological conditions in older adults.

Introduction

Aging is the most significant non-modifiable risk factor for ischemic stroke—the leading cause of adult disability and a major contributor to mortality worldwide. ¹ Both the incidence and severity of brain ischemia increase markedly with age, with individuals over 65 accounting for the majority of stroke cases. ² This age-related vulnerability results from a complex interplay of vascular, metabolic, inflammatory, and cellular mechanisms that impair brain plasticity and reduce the brain’s resilience to ischemic injury. Understanding how aging predisposes the brain to ischemic damage is therefore essential—not only for identifying individuals at higher risk but also for developing age-specific preventive and therapeutic strategies. This review examines the multifaceted impact of aging on brain plasticity and the pathophysiology of ischemic stroke, with a particular focus on the therapeutic potential of mesenchymal cell-derived extracellular vesicles in aged rodent models of cerebral ischemia.

In preparation for this review, we conducted a literature search in PubMed using combinations of the following keywords: (“aging”) and (“brain” or “brain plasticity”) and (“macroscopic alterations” or “microscopic alterations” or “neuronal plasticity”, or “axonal plasticity”). In a separate search, we combined the keywords: (“extracellular vesicle” or “exosome”) with (“aging”, “ischemic stroke”, “focal cerebral ischemia”, “hypoxia-ischemia”, “traumatic brain injury”, “brain trauma”, “brain injury”, “inflammation”, “inflammatory”, “neurodegeneration”, “neurodegenerative”, “behavioural recuperation”, “Alzheimer”, or “oxidative stress”).

The aging brain: Macroscopic and microscopic alterations

Brain volume is dynamic and undergoes continuous changes throughout the lifespan. MRI studies have shown that brain volume increases during childhood and early adolescence, peaking around the age of 13. Following this peak, a gradual decline begins, which accelerates significantly after the age of 35 as age-related neuronal loss becomes more pronounced. ³ Structural atrophy is particularly evident in the prefrontal cortex, hippocampus, and basal ganglia—regions critical for executive function, memory processing, and motor coordination. ⁴ Importantly, atrophy in these areas is not uniform across individuals and may be influenced by factors such as lifestyle, genetics, and overall systemic health.^5,6

As the brain ages, it undergoes a complex interplay of macroscopic and microscopic alterations that impact neural integrity, cognitive function, and overall physiological resilience. These changes manifest across multiple domains, including cortical and subcortical atrophy, ⁷ white matter degeneration, ⁸ synaptic and neurotransmitter alterations, ⁹ and mitochondrial dysfunction.^10,11 Emerging evidence further suggests that these neural modifications may contribute to—or occur in parallel with—the development of frailty in older adults.

Contrary to the long-standing belief that brain development and degeneration are opposing processes, growing evidence indicates that they are deeply interwoven. ¹² With age, the brain undergoes a gradual reduction in volume accompanied by subtle neurodegenerative changes, including the progressive loss of neuronal structure, function, and number.^13,14 While neurodegeneration is a natural part of aging, it does not eliminate the brain’s capacity for neuroplasticity.^15,16 In fact, studies have shown that certain developmental mechanisms—such as neurogenesis—persist into adulthood and even old age. ¹⁷ A notable example is the dentate gyrus of the hippocampus, a key region involved in memory encoding, where new neurons continue to be generated throughout life.^18,19 However, the extent of such neuroplastic changes is modulated by factors including lifestyle, systemic health, and exposure to neurological insults.

As the brain ages, mitochondrial efficiency progressively declines, leading to increased production of reactive oxygen species (ROS), reduced ATP generation, and heightened susceptibility to metabolic stress. ²⁰ This deterioration in mitochondrial function contributes to cellular energy deficits, rendering neurons more vulnerable to degeneration and impairing their capacity to recover from stress and injury. ²¹ At the same time, significant vascular changes occur in the aging brain, compromising neurovascular function and reducing overall resilience to insults. ²² One of the most critical consequences of age-related vascular decline is the breakdown of the blood-brain barrier (BBB). ²³ As the BBB weakens, its permeability increases, allowing neurotoxic substances, inflammatory mediators, and immune cells to infiltrate the central nervous system, thereby exacerbating neuroinflammation and neuronal damage.^23,24

Cell-cell communication in the brain via extracellular vesicles

Extracellular vesicles (EVs) are secreted by nearly all cell types in the central nervous system (CNS), including neurons, astrocytes, microglia, and oligodendrocytes.^25–27 These vesicles—comprising exosomes, microvesicles, and apoptotic bodies—carry a diverse cargo of proteins, lipids, nucleic acids (e.g., miRNAs, mRNAs, and DNA), and metabolites, which they deliver to recipient cells to modulate their function, play essential roles in maintaining homeostasis, support neuronal development, and influence both physiological and pathological processes.

For instance, neuronal exosomes can transfer miRNAs to astrocytes, modulating their metabolic support functions.^28,29 Conversely, astrocyte-derived exosomes can deliver neuroglobin to neurons, offering protection against cellular stress. ³⁰ Microglial exosomes contribute to neuronal survival and function by transporting neurotrophic factors such as nerve growth/differentiation factor, and may also carry N-arachidonoylethanolamine, which targets GABAergic neurons to modulate synaptic transmission. ³¹ Additionally, microglia can internalize oligodendrocyte-derived exosomes to facilitate the clearance of myelin debris. ³²

Mitochondria—traditionally known for their roles in energy production and apoptosis regulation—also interact with EVs, suggesting a synergistic relationship that supports CNS homeostasis and contributes to disease pathology. ³³

EVs mediate intercellular communication through several mechanisms. They can directly fuse with the plasma membrane of recipient cells, releasing their cargo into the cytoplasm. Alternatively, EVs may be internalized via endocytosis, phagocytosis, or receptor-mediated uptake.^34–37 Once inside the recipient cell, EV cargo can influence gene expression, protein synthesis, and signaling pathways. For example, neuronal-derived EVs have been shown to transfer miRNAs to astrocytes, regulating their metabolic functions. ²⁸ Similarly, astrocyte-derived EVs can modulate synaptic activity by delivering signaling molecules to neurons. ³⁸ (Figure 1).

Figure 1.

Hypothetical modulation of motor neuron activity via astrocyte-derived extracellular vesicles (EVs).

EVs play a role in neurodevelopment and plasticity

EVs, including exosomes and microvesicles, are secreted by virtually all neural cell types—neurons, astrocytes, microglia, and oligodendrocytes. They have emerged as essential mediators of intercellular communication in the nervous system, facilitating both local and long-distance signaling. EVs contribute to the regulation of neurodevelopment, synaptic plasticity, immune responses, and neural repair.^39–42

During early brain development, EVs mediate the transfer of proteins, lipids, and non-coding RNAs which play critical roles in neuronal differentiation, axonal growth, and synaptogenesis. EVs derived from neural stem cells have been shown to influence both neurogenesis and gliogenesis by modulating key signaling pathways, including Notch, Wnt, and PI3K/Akt, which are essential for the proper formation and wiring of neural circuits.^43–47

Oligodendrocyte-derived EVs support neuronal survival and promote axonal integrity by delivering bioactive molecules such as proteins, lipids, and RNAs. ⁴⁸ EVs also contribute to synaptic plasticity by regulating synaptic protein turnover and modulating local translation at synaptic terminals. ⁴⁹

In the mature brain, EVs continue to play a role in activity-dependent synaptic plasticity by transferring proteins and RNAs essential for long-term potentiation (LTP) and memory formation. ⁵⁰ Astrocyte-derived EVs also modulate synaptic activity by delivering signaling molecules to neurons. ³⁸ Furthermore, EVs influence myelination by supplying enzymes and lipids necessary for the formation and maintenance of the myelin sheath, thus supporting efficient nerve conduction.

Notably, EVs contribute to long-term potentiation and synaptic scaling through the delivery of synaptic modulators such as miR-132, miR-124 and miR-324, which are known to affect dendritic spine morphology and synaptic strength.^51–54

Age-related decline in neuroplasticity and functional recovery after cerebral ischemia

The brain’s capacity for adaptation is essential not only for cognitive function but also for responding to injury. With age, the efficiency of neural repair mechanisms declines, rendering older individuals more vulnerable to prolonged recovery and cognitive deterioration after events such as stroke or traumatic brain injury (TBI).^55,56 Although certain neuroplastic processes can compensate for damage, the balance between repair and degeneration becomes increasingly fragile with age. When this balance is disrupted, maladaptive plasticity may emerge, further contributing to functional impairments.^57–60

Cerebral ischemia triggers a cascade of pathophysiological events—including excitotoxicity, oxidative stress, neuroinflammation, and cell death—that lead to significant neurological impairments and cognitive decline.^61,62 Although acute-phase interventions such as intravenous thrombolysis with tissue plasminogen activator (tPA) and mechanical thrombectomy have been developed, their therapeutic window remains narrow, and many patients are left with long-term deficits. ⁶³ As a result, there is an urgent need for effective therapies that can restore brain function during the subacute and chronic phases of recovery. ⁶⁴

Post-ischemic recovery depends on a complex interplay of endogenous repair mechanisms, including neurogenesis, synaptic plasticity, angiogenesis, and neural network remodeling. However, spontaneous recovery is often incomplete. In recent years, a broad range of therapeutic strategies has been investigated to enhance these repair processes. These include pharmacological agents targeting excitotoxicity, oxidative stress, and inflammation; stem cell-based therapies designed to replace lost cells and modulate the immune environment; and neurorehabilitative approaches such as neuromodulation and activity-based training to promote functional reorganization of the brain.^65–67

Neuroplasticity refers to the brain’s remarkable capacity to adapt and reorganize in response to both intrinsic and extrinsic stimuli. ⁶⁸ Aging is accompanied by profound structural and functional changes that impact cognitive abilities, ⁶⁹ neuroplasticity, ⁷⁰ and the brain’s resilience to injuries such as stroke and traumatic brain injury (TBI). ⁷¹ These changes occur at multiple levels including cellular, molecular, and systemic, shaping the delicate balance between neuroprotection and degeneration. Although often discussed in the context of recovery from injury or neurological disease, neuroplasticity is equally relevant in the aging brain.

Non-invasive neuromodulatory techniques, such as transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), have attracted growing attention as adjunctive therapies to enhance neuroplasticity and support functional recovery when combined with conventional rehabilitation. These techniques modulate cortical excitability and synaptic plasticity, thereby creating a more favorable neural environment for learning and recovery. TMS uses focal magnetic pulses to induce long-lasting changes in cortical activity, whereas tDCS applies low-intensity electrical currents to alter neuronal membrane potentials. When integrated with task-specific rehabilitation, both approaches have shown the ability to improve motor and cognitive outcomes, particularly in individuals recovering from stroke or traumatic brain injury. Recent studies highlight their potential to increase the brain’s responsiveness to therapy and accelerate recovery trajectories.^72,73

Acute phase: Brain reactivity

During the acute phase, the brain’s response to cerebral ischemia involves a combination of reactive and plastic mechanisms that unfold across different phases of injury and recovery.

Immediately after an ischemic event, the brain activates a series of reactive processes aimed at limiting tissue damage and initiating early repair. These include:

1. Inflammatory Response: Activation of microglia and astrocytes triggers the release of cytokines and other inflammatory mediators. While this response is essential for clearing dead cells and debris, excessive inflammation can worsen neuronal injury in the post-stroke aged rodent brain.^74–76

2. Excitotoxicity: Ischemia often causes an excessive release of glutamate, which overstimulates NMDA receptors and leads to neuronal injury or death. ⁷⁷

3. Oxidative Stress: An imbalance between the production of free radicals and the availability of antioxidants contributes to cellular damage and apoptosis during ischemic episodes. ⁷⁸

4. Reactive gliosis—where astrocytes become hyperactivated in response to injury—contributes to maladaptive repair mechanisms, leading to increased scarring and neuronal dysfunction rather than facilitating recovery. ⁷⁹ These deficits collectively contribute to delayed or incomplete functional recovery following neurological injuries.

Rehabilitation phase: Neuroplasticity and functional recovery

As the brain enters the recovery phase, neuroplastic mechanisms become more prominent, supporting rehabilitation and functional restoration.^59,64

Key processes include: (i) Synaptic Remodeling and Neuronal Sprouting : Neuroplasticity is crucial for recovery after stroke. A 2023 review highlighted that enriched environments can enhance synaptic remodeling and axonal sprouting, both of which are essential for functional restoration. ⁸⁰ (ii) Cortical Reorganization : Neuroimaging studies have shown that the brain reorganizes its functional networks after stroke, offering valuable insights for the development of targeted rehabilitation strategies. ⁸¹

However, the brain’s capacity for recovery varies across the lifespan. In younger individuals, plasticity mechanisms—such as synaptogenesis, dendritic remodeling, and neurogenesis—are typically more robust, enabling better functional outcomes. In contrast, aged brains often show reduced plastic potential due to cumulative cellular senescence, diminished trophic support, chronic inflammation, and impaired neural stem cell function.^82,83

The importance of neuroplasticity in post-stroke recovery is well illustrated in preclinical models. In a study using a rat model of focal ischemia targeting the right middle cerebral artery, researchers induced comparable lesion volumes in rats aged 10, 63, and 180 days. Behavioral performance was assessed on post-injury days 3, 7, 14, 28, and 56 using a battery of tests including the rotarod, foot fault, open field, inclined screen, tape-removal, and postural reflex assessments. Results showed that younger rats exhibited faster and more complete functional recovery compared to older groups. These findings align with previous studies comparing young adult and aged rodents, collectively demonstrating that neuroplasticity is more pronounced in young brains.^75,84–86

Age-related brain gene expression in response to cerebral ischemia

Ischemic stroke triggers a cascade of gene expression changes in the brain, which vary across the different phases of injury and recovery. These changes are further modulated by age, significantly influencing the brain’s response and reparative capacity. ⁸⁷ In young brains, the acute phase is characterized by a strong upregulation of stress response and inflammatory genes. As the injury progresses, these responses gradually subside, giving way to the expression of reparative genes during the recovery phase. In contrast, aged brains exhibit a prolonged and heightened inflammatory gene expression profile, along with a diminished induction of genes critical for repair and plasticity.

Aging profoundly affects post-ischemic gene expression dynamics. Compared to younger individuals, aged brains demonstrate an exaggerated and sustained inflammatory response, contributing to increased neuroinflammation and impaired recovery. Additionally, the expression of genes associated with neurogenesis and synaptic plasticity is significantly reduced in older individuals, correlating with limited neuronal regeneration and poorer functional outcomes. ⁸⁸

Immediately after ischemia, there is a rapid induction of immediate early genes such as c-fos, c-jun, and egr-1, which are involved in neuronal stress responses and apoptosis. Simultaneously, genes related to inflammation including PTGS2 (encoding cyclooxygenase-2), TLR4 (Toll-like receptor 4), and CCR2 (C-C motif chemokine receptor 2), ⁸⁹ along with oxidative stress markers (NFE2L2, NQO1, GCLC) and excitotoxicity-related genes (e.g., NMDA receptors), are upregulated, contributing to neuronal damage. ⁹⁰

Recent research has explored whether the ischemic brain in young adults can reactivate genes typically expressed during early postnatal brain development. Findings suggest that while there is partial re-expression of juvenile gene programs during the acute phase, many key plasticity-related genes are not sufficiently upregulated, indicating inherent limitations in the adult brain’s capacity to engage developmental plasticity pathways following stroke. ⁹¹ Moreover, immune-related genes that facilitate synaptic remodeling during early development through processes such as pruning and sprouting in response to environmental stimuli, may also contribute to post-stroke damage, supporting the theory of antagonistic pleiotropy. ⁹²

As the brain enters the recovery phase, typically days to weeks after stroke, there is a shift towards the upregulation of genes involved in neurogenesis, synaptic plasticity, and angiogenesis. For example, increased expression of brain-derived neurotrophic factor (BDNF) promotes neuronal survival and synaptic remodeling. However, inflammatory gene expression often remains elevated, indicating a persistent immune response. ⁹³ This imbalance suggests that while young brains are capable of transitioning from injury to repair, aged brains are often trapped in a state of chronic inflammation, with impaired regenerative capacity.

These age-associated changes not only hinder repair mechanisms but may also promote maladaptive plasticity such as aberrant neural connectivity, glial scarring, and ultimately, impaired behavioral recovery.

EVs and neurological disorders: Friend or foe?

EVs facilitate intercellular communication by transporting proteins, lipids, and nucleic acids, thereby influencing the behavior of recipient cells. Through these mechanisms, EVs shape neurophysiological responses and can contribute to the progression of neurological diseases.^94,95

Under physiological conditions, EVs support synaptic function and neurovascular coupling by transferring microRNAs and other regulatory molecules. For example, astrocyte-derived EVs have been shown to modulate synaptic activity by delivering specific proteins and RNAs to neurons, thereby influencing synaptic strength and plasticity. ⁹⁶

Glial cells, particularly microglia and astrocytes, release EVs containing inflammatory mediators such as cytokines and miRNAs, which can propagate neuroinflammatory signals. ⁹⁷ In pathological states, EVs released by activated microglia can amplify inflammatory responses, contributing to the progression of neurodegenerative diseases. ⁹⁸ At the same time, EVs may carry anti-inflammatory factors that help resolve inflammation, underscoring their dual regulatory role. ⁹⁹

EVs are implicated in both neuroprotective and neurotoxic processes. On the neuroprotective side, EVs promote cell survival, modulate inflammation, and support neuronal repair mechanisms.^100,101 Conversely, they can also propagate pathological protein aggregates, such as amyloid-beta and tau in Alzheimer’s disease, or alpha-synuclein in Parkinson’s disease, thereby accelerating disease progression.^102–104 For instance, EVs containing aggregated tau have been shown to induce tau pathology in recipient neurons. ¹⁰⁵

Beyond inflammation and aggregation, EVs contribute to maintaining white matter integrity by supporting oligodendrocyte function and myelin production. ¹⁰⁶ This dual capacity to both support and impair neuronal function highlights the importance of tightly regulated EV activity in central nervous system (CNS) health.

In neuroinflammatory conditions, EVs often carry pro-inflammatory cytokines and microRNAs that intensify immune responses, potentially exacerbating diseases such as multiple sclerosis and other neurodegenerative disorders. ¹⁰⁷ Similarly, following traumatic brain injury (TBI), EVs released from damaged cells contain damage-associated molecular patterns (DAMPs), which trigger immune activation and worsen secondary injury. ¹⁰⁸ However, not all EVs released after injury are detrimental. EVs derived from neural stem cells have been shown to promote brain repair by enhancing progenitor cell proliferation and differentiation. ¹⁰⁹

Exosome-based therapies for cerebral ischemia

As the brain ages, its ability to recover from injuries such as stroke, TBI, or neurodegenerative conditions becomes increasingly impaired. ¹¹⁰ A major contributor to this decline in recovery potential is the exaggerated and prolonged neuroinflammatory response commonly observed in the aged brain.^111,112 Microglial activation that is essential for clearing cellular debris and modulating immune responses, becomes dysregulated with age, resulting in chronic inflammation that hinders tissue repair and exacerbates secondary neurodegeneration. ¹¹³

In addition to this maladaptive inflammatory state, the brain’s capacity for neuronal regeneration and adaptive plasticity is significantly diminished. The hippocampal neurogenic niche, which plays a key role in generating new neurons, deteriorates with age, leading to a marked reduction in neurogenesis following injury and thereby limiting functional recovery. ¹¹⁴ Moreover, glial support systems are also compromised in the aging brain. Astrocytes, which provide critical metabolic and trophic support to neurons, show reduced functional efficiency in older individuals.

In the context of stroke, EVs have been shown to transport molecules that can either worsen injury by promoting apoptosis and inflammation, or support recovery by delivering neuroprotective agents, depending on their cellular origin and molecular cargo. A meta-analysis of preclinical studies evaluated the effects of stem cell-derived EVs on outcomes following ischemic stroke. The analysis revealed that these EVs significantly improved neurological function, reduced infarct volume, and decreased apoptosis and inflammation. These therapeutic effects are largely attributed to the EVs’ ability to modulate neuroinflammatory responses and promote neuronal survival. ¹¹⁵

Emerging evidence also indicates that astrocyte-derived exosomes influence neuronal activity and survival. Specifically, exosomes carrying miRNAs such as miR-138-5p have been shown to support neuronal signaling and protect against apoptosis, thereby contributing to neuroprotection. ¹¹⁶ However, injury-associated reactive astrocytes may also contribute to neuronal damage. For instance, astrocyte-derived EVs carrying miR-382-5p have been shown to mediate astrocyte–neuron communication that promotes mitochondrial dysfunction in neurons, potentially driving neuronal death. ¹¹⁷

A recent study explored the role of microglia-derived EVs in ischemic stroke, showing that they can promote neuroprotection and repair by reducing neuronal apoptosis, inhibiting autophagy, and alleviating inflammatory responses. ¹¹⁸ Additionally, microglia have been shown to internalize exosomes derived from oligodendrocytes, facilitating the clearance of myelin debris, a critical step in resolving neuroinflammation. Efficient clearance of such debris supports remyelination and enhances neural repair mechanisms, thereby influencing disease progression. ¹¹⁹

Therapeutic potential of EVs in cerebral ischemia

EVs have emerged as a promising therapeutic strategy for ischemic stroke due to their ability to cross the BBB, deliver bioactive molecules to specific cell types, and promote angiogenesis, neuroprotection, neurorepair, and BBB stabilization (Figure 2).

Figure 2.

Extracellular vesicle (EV)-based stroke therapy: Intravenously administered extracellular vesicles derived from mesenchymal stem cells promote angiogenesis, neurogenesis, and behavioral recovery, likely through a bystander effect.

Engineered EVs are currently being explored as delivery vehicles for drugs, gene therapies, and regenerative agents within the central nervous system (CNS). ¹²⁰ In particular, EVs derived from stem cells have shown encouraging results in preclinical models of brain injury and neurodegeneration, demonstrating the ability to enhance neuroprotection and facilitate repair processes. ¹²¹

Recent advances in stem cell biology have highlighted the therapeutic potential of neural and mesenchymal stem cells in promoting post-ischemic neuroregeneration. Rather than relying solely on direct cell replacement, these therapies exert their effects primarily through paracrine signaling, immunomodulation, and trophic support. ¹²²

A recent meta-analysis of preclinical studies investigated the impact of stem cell-derived EVs on outcomes following ischemic stroke. The analysis revealed that these EVs significantly improved neurological function, reduced infarct volume, and decreased both apoptosis and inflammation. These therapeutic effects are largely attributed to the EVs’ ability to modulate neuroinflammatory responses and support neuronal survival. ¹¹⁵

Stem cell-derived EVs for stroke treatment in young versus aged animal models

Stem cell-derived EVs are increasingly recognized as potent acellular therapeutics for ischemic stroke, capable of modulating inflammation, promoting neuroregeneration, and supporting brain repair. These nano-sized vesicles carrying a complex cargo of molecules that can influence recipient cells and potentially overcome some limitations of direct stem cell transplantation, such as immune rejection and tumorigenesis. ¹²³

In young animal models, numerous studies have demonstrated that MSC-EVs, neural stem cells (NSCs), and induced pluripotent stem cells (iPSCs) can reduce infarct size, promote neurogenesis and angiogenesis, and enhance behavioral recovery following stroke.^124–127 The therapeutic potential of these EVs is largely attributed to their cargo, particularly miRNAs such as miR-124, the miR-17–92 cluster, ¹²⁸ and others known to support neural plasticity and modulate immune responses. Notably, exosome treatments enriched with the miR-17–92 cluster have demonstrated significantly enhanced effects on neurological recovery—specifically, increased oligodendrogenesis, neurogenesis, and neurite remodeling/dendritic plasticity in the ischemic boundary zone—compared to standard MSC-derived exosome treatments. ¹²⁹ Similarly, EVs isolated from IL-4-treated microglia (M2-EVs) and administered intravenously to post-stroke young mice were shown to reduce brain atrophy, promote functional recovery, and enhance oligodendrogenesis and white matter repair via miR-23a-5p. ¹³⁰

However, translating these findings into aged animal models, which more closely resemble the human condition, has proven more challenging. Aging is accompanied by chronic low-grade inflammation (inflammaging), diminished endogenous repair capacity, altered EV biodistribution, and a reduced responsiveness to regenerative signals.^131,132 As a result, the therapeutic efficacy of stem cell-derived EVs appears to be attenuated in aged subjects.

For example, studies in aged rats have shown that while mesenchymal stem cell (MSC)-derived EVs still promote functional recovery, the extent of improvement is significantly less than that observed in younger counterparts.^133,134 Similarly, human bone marrow–derived MSCs (hBM-MSCs) have been found to alleviate sensorimotor deficits in aged mice after stroke. However, hBM-MSC transplantation did not reduce brain atrophy or enhance neuronal survival 56 days after distal middle cerebral artery occlusion. Instead, their beneficial effects were associated with preserved white matter integrity and increased angiogenesis and oligodendrogenesis, without a corresponding increase in neurogenesis in the post-stroke brain. ¹³⁵

Moreover, EVs derived from young and aged donor stem cells exhibit distinct therapeutic capacities. EVs from young stem cells carry a more regenerative and anti-inflammatory cargo profile, whereas those from aged cells often contain pro-inflammatory signals and senescence-associated molecules. To investigate whether these age-related differences influence other cells, a study examined the effects of plasma-derived EVs from young (18–25 years) and elderly (45–60 years) human donors on human umbilical cord blood (hUCB)-derived hematopoietic stem cells (hUCB-HSCs) in vitro. The results demonstrated that EVs modulate the growth, functionality, and differentiation potential of HSCs in an age-dependent manner. Specifically, EVs from young donors enhanced HSC proliferation and self-renewal, while EVs from older donors promoted senescence-associated differentiation, particularly toward the myeloid lineage. These findings underscore the importance of considering both the age of the host and the donor cells when developing EV-based therapies for stroke. ¹³⁶

Exosomes derived from aged MSCs can impair the regenerative potential of young cells

Earlier studies reported that mice injected with young bone marrow-derived mesenchymal stem cells (BMSCs) had a longer lifespan compared with mice injected with senescent BMSCs. ¹³⁷ Similarly, recent research has demonstrated that BMSCs from naturally aged and progeroid Ercc1⁻/⁻ mice exhibit reduced proliferative capacity and increased expression of senescence markers, such as SA-β-gal, in culture. Notably, intraperitoneal injection of young BMSCs extended the lifespan of Ercc1⁻/⁻ mice. Importantly, the progeroid Ercc1⁻/Δ mouse model, characterized by a rapid accumulation of senescent cells across multiple tissues and accelerated development of age-related pathologies similar to those observed in aged wild-type mice, provides an excellent system for evaluating health span interventions. ¹³⁸

The therapeutic efficacy of EVs derived from mesenchymal stem cells (MSCs) is highly dependent on the age of the MSCs. During in vitro expansion and in vivo transplantation, MSCs are susceptible to senescence—an irreversible growth arrest accompanied by morphological alterations, gene expression changes, and functional impairments in genomic regulation. The senescence phenotype of MSCs is critically modulated by their microenvironment, with factors such as hypoxia, inflammation, and organismal aging playing key roles. ¹³⁹

Senescent MSCs adopt a Senescence-Associated Secretory Phenotype (SASP), characterized by the secretion of pro-inflammatory cytokines and altered exosomal content. These exosomes can negatively influence the tissue microenvironment, potentially impairing the function and regenerative capacity of neighboring young cells. ¹⁴⁰ A recent study investigated the effects of plasma-derived exosomes from older and younger individuals on human HSCs. The findings revealed that exosomes from older donors decreased the expression of hypoxia-inducible factor 1-alpha (HIF-1α), that is the principal mediator in response to hypoxia, and increased the expression of the senescence marker P21 in HSCs. In contrast, exosomes from younger donors enhanced HIF-1α expression and reduced P21 levels, suggesting that aged exosomes promote cellular aging in young HSCs. ¹⁴¹

Molecular investigations have established that aging profoundly reshapes the miRNA profile of mesenchymal stem cell (MSC)-derived exosomes, impairing intercellular communication and contributing to the decline in regenerative capacity characteristic of aged tissues. For instance, exosomes secreted by bone marrow-derived MSCs (BM-MSCs) from aged mice are efficiently internalized by adipocytes, myocytes, and hepatocytes, where they induce insulin resistance both in vivo and in vitro. Comprehensive miRNA array profiling further revealed a significant upregulation of miR-29b-3p in exosomes derived from aged BM-MSCs. ¹⁴²

Conversely, exosomes derived from young MSCs have been shown to rejuvenate aged MSCs. For example, exosomes secreted by young MSCs enhance bone regeneration during distraction osteogenesis—a clinically effective procedure for repairing large bone defects—by promoting the proliferation and osteogenic differentiation of aged BM-MSCs in older rats. ¹⁴³ Likewise, exosomes derived from human umbilical cord MSCs have been shown to rejuvenate aged MSCs and enhance their functional capacity for myocardial repair. Specifically, exosomal miR-136 from young MSCs promotes the viability and function of aged MSCs by downregulating apoptotic factors, such as Apaf1, thereby improving their therapeutic potential for myocardial regeneration. ¹⁴⁴

Extending these findings beyond peripheral tissues, the loss of hypothalamic NSCs has been identified as a central driver of systemic aging. Remarkably, transplantation of young hypothalamic NSCs into middle-aged mice delayed aging and extended lifespan, an effect partially mediated by the exosomal miRNAs secreted by the transplanted NSCs. ¹⁴⁵ These results underscore the critical role of stem cell-derived exosomal miRNAs in modulating both local and systemic aging processes.

These studies collectively suggest that exosomes from aged MSCs carry altered molecular signals that can induce senescence and impair the regenerative functions of young cells. Understanding the mechanisms behind these effects is crucial for developing therapeutic strategies aimed at mitigating age-related decline in tissue regeneration. ¹⁴⁶

Conclusions

In conditions such as stroke and TBI, elderly individuals typically experience worse outcomes due to impaired compensatory plasticity, diminished angiogenesis, and increased vulnerability to secondary damage. These age-related changes significantly hinder neural repair and plasticity, exacerbating the long-term consequences of brain injuries and limiting the effectiveness of therapeutic interventions.

EVs, owing to their role in mediating cellular responses and their ability to cross the blood–brain barrier, are increasingly being investigated both as biomarkers and as therapeutic vectors in neurological diseases.^147–149

Although several studies have begun to address the translational gap by employing aged stroke models, further research is needed to refine EV formulations and delivery methods specifically for the aged brain, assess long-term safety, and explore combination therapies—such as EVs combined with rehabilitation or pharmacological agents.

Importantly, the age of donor stem cells has a profound impact on the therapeutic potential of derived EVs. EVs from aged stem cells demonstrate reduced reparative capacity and may carry senescence-associated signals that compromise recovery. In contrast, EVs from young donor cells display a more regenerative profile, indicating that both donor age and preconditioning are critical factors for optimizing clinical outcomes.

These insights underscore the need for age-specific therapeutic strategies that take into account the unique molecular and immune characteristics of the aging brain. Collectively, the evidence suggests that stroke mechanisms and therapeutic responses vary significantly with age. Differences in immune responses, neurovascular integrity, and cellular repair processes between aged and young individuals highlight the importance of incorporating aged animal models into preclinical stroke research. Without such models, the translation of experimental therapies into effective clinical treatments for elderly patients remains limited.

Finally, exosomes derived from aged MSCs carry altered molecular cargo capable of inducing senescence and impairing the regenerative functions of young recipient cells.

Data availability statement

The data supporting the findings of this study are openly available at PubMed.