How Changes in Cell Energy Networks with Aging Affect Cell Communication and Tissue Flexibility

Mitochondrial fusion is mediated by membrane-bound GTPases, with mitofusins (MFN1 and MFN2) regulating outer membrane fusion and optic atrophy 1 (OPA1) controlling inner membrane fusion [5]. Fusion promotes the mixing of mitochondrial content mainly to compensate for deficient units and to increase functional efficiency [6,7]. Fusion is typically adopted during high cellular energetic demand or mild stress, when cells require coordinated distribution of metabolites, mitochondrial DNA (mtDNA), and membrane potential to sustain optimal oxidative phosphorylation (OXPHOS) processes. This state is characteristic of the proliferating, differentiated, or metabolically active cells 2that maintain bioenergetic stability. Reduced protein synthesis or induction of autophagy, particularly via inhibition of the mammalian target of rapamycin (mTOR) or nutrient deprivation, promotes mitochondrial fusion and coordinates mitochondrial activity to support cell viability [8,9]. Beyond protein expression levels, mitochondrial fusion is an energetically costly process. Both outer membrane fusion (mediated by MFN1 and MFN2) and inner membrane fusion (mediated by OPA1) require GTP hydrolysis, linking network maintenance directly to cellular energetic state [10,11]. During aging, declining ATP production and impaired nucleotide regeneration reduce the availability of GTP, thereby limiting the cell’s capacity to sustain fusion even when fusion machinery remains present. Under such conditions, fission becomes energetically favored not only as a quality-control mechanism but also as a passive consequence of energetic insufficiency, during which sustained network fusion becomes metabolically unaffordable.

Fission is governed primarily by dynamin-related protein 1 (DRP1) and its adaptors (Mitochondrial fission protein 1-Fis1, Mitochondrial fission factor-Mff, Mitochondrial Dynamics proteins of 49 and 51 kDa-MiD49/51). Fission allows segregation of dysfunctional segments and facilitates organelle redistribution during mitosis or stress [7]. By fragmenting the network, fission enables the isolation of damaged or dysfunctional mitochondria to further undergo PTEN-induced putative kinase 1 (PINK1/Parkin-mediated mitophagy [12]. The PINK1/Parkin pathway selectively labels mitochondria that have lost membrane potential for autophagic removal. When ΔΨm collapses, PINK1 accumulates on the outer membrane and activates Parkin, which ubiquitylates outer-membrane proteins to recruit autophagy adaptors and initiate engulfment. This pathway prevents damaged, reactive oxygen species (ROS)-producing mitochondria from remaining as “faulty nodes” within the network, thereby protecting energetic coupling and limiting inflammatory signaling. During cell division and differentiation, fission enables equal mitochondrial partitioning between the two daughter cells. Fission is employed as well in response to nutrient deprivation, oxidative stress, or calcium overload [13,14] and mediates apoptotic cell clearance by macrophages [15]. A shift toward excessive fission is observed in apoptotic, senescent, or metabolically altered states, contributing to mitochondrial depolarization and impaired bioenergetic efficiency [16].

The interplay between fusion and fission ensures metabolic flexibility and quality control, linking mitochondrial morphology to cellular energetic and redox states. With aging and disease, disruption of this balance undermines network integration and can lead to functional decoupling of the mitochondrial reticulum.

Mitochondrial network deterioration during aging is more than a morphological disruption. While increased fission and fragmentation are frequently observed, aged tissues also display hyper-fused, swollen, or toroidal (“donut-shaped”) mitochondria, as well as networks that remain morphologically continuous yet functionally compromised. These configurations likely represent stress-adaptive or pre-degenerative states rather than preserved network health. Loss of mitochondrial connectivity during aging is often functional rather than purely topological, reflecting impaired energy conduction, quality-control responsiveness, and signal integration despite apparent structural continuity.

Mitochondrial connectivity must be considered at two distinct architectural levels. Outer mitochondrial membrane fusion governs reticular continuity and physical networking, whereas inner membrane fusion and cristae junction integrity determine electrical conduction, proton-motive force distribution, and respiratory efficiency [17]. Emerging evidence indicates that in aged tissues, mitochondria may remain connected at the outer membrane level while exhibiting profound inner membrane disorganization, including cristae shortening, vesiculation, or junction collapse. Such alterations uncouple mitochondrial inner membrane potential (ΔΨm) propagation and ATP distribution, even if morphological continuity may be preserved [18]. Functional decoupling is not only a matter of impaired conduction within an otherwise continuous reticulum but is also a progressive shift in both the number and functional quality of mitochondrial nodes.

Both the number and the functional quality of mitochondria—which can be viewed as nodes within a network—are key determinants of collective mitochondrial behavior. Node density and competence are maintained by the coupled processes of mitophagy (selective removal) and mitochondrial biogenesis (renewal), together preserving coherence within the dynamic reticulum. Damaged or depolarized mitochondria are removed through the PINK1–Parkin pathway, whereas renewal depends on PGC-1α–driven biogenesis. The balance between these processes determines the pool of functional nodes, network resilience, and bioenergetic stability [19,20].

Energetic and nutrient states influence mitochondrial turnover through the AMPK–mTORC1 axis. Under energy deficit, AMPK activates ULK1 to promote autophagy/mitophagy and supports biogenesis via the SIRT1–PGC-1α pathway [21,22]. In contrast, mTORC1 suppresses ULK1-dependent autophagy under nutrient sufficiency and adjusts mitochondrial gene expression in anabolic states [23]. At the transcriptional level, PGC-1α co-activates NRF1/NRF2 and induces TFAM, supporting mtDNA replication/transcription and replenishment of the mitochondrial pool [24,25]. Receptor-mediated mitophagy (e.g., FUNDC1) is transcriptionally coupled to PGC-1α/NRF1 signaling, linking degradation and renewal within a quality-control loop [26]. PINK1/Parkin-mediated mitophagy, PGC-1α biogenesis, and AMPK/mTOR-regulated signaling operate as an integrated homeostatic system aligning the mitochondria network with the cell state [27,28]. Mitophagy is activated during energetic stress, hypoxia, oxidative injury, mtDNA damage, or cell-cycle arrest, conditions in which ROS-producing organelles accumulate. PINK1/Parkin and receptor-mediated pathways (BNIP3, NIX, FUNDC1) remove dysfunctional mitochondria, therefore limiting ROS [29,30].

In stem or progenitor cells, mitophagy supports quiescence or early differentiation by maintaining a glycolytic, low-ROS profile [31,32]. Conversely, biogenesis predominates during stress recovery, proliferation, or differentiation, when increased OXPHOS capacity is required. PGC-1α–NRF1/2–TFAM signaling is induced downstream of AMPK/SIRT1 to restore mitochondrial content after mitophagic clearance and to match increased energetic demand [33,34,35]. Mitophagy and biogenesis form a physiological continuum in which stress promotes selective removal, whereas recovery and anabolism trigger PGC-1α–mediated renewal, restoring network quality and density [36]. Disruption of this balance is a hallmark of aging and contributes to mtDNA instability, cristae alterations, and impaired energy metabolism. Accordingly, healthy cellular homeostasis depends on an intracellular mitochondrial network capable of adaptive remodeling through fusion, fission, biogenesis, and autophagy/mitophagy (Figure 1).

Tunneling nanotubes (TNTs) are actin-rich membranous protrusions connecting donor and recipient cells, along which entire mitochondria or mitochondrial fragments can be mobilized [37,38]. TNTs vary in diameter (from ~50 nm up to ~1 µm) and mediate either direct cytoplasmic continuity or close apposition between donor and recipient cells [39].

TNT-based mitochondrial transfer mediates the antimicrobial effect of human MSC administration in an in vivo model of acute bacterial pneumonia, improving alveolar macrophage bioenergetics and enhancing their phagocytic ability [40,41]. TNT-transferred mitochondria influence multiple physiological and pathological processes, including immune and reparative responses, neural plasticity, and malignant progression with acquired resistance to therapy [42,43]. TNT transfer is typically initiated by donor cells in response to mitochondrial dysfunction or oxidative stress in recipient cells. Donor cells activate RalA–M-Sec–exocyst signaling together with actin polymerization to form TNTs. Mitochondria are subsequently recruited through the MIRO1–TRAK–myosin/kinesin transport machinery and transported along actin- or microtubule-based tracks within the TNT toward the recipient cell. There, integration with the endogenous mitochondrial network can restore membrane potential, oxidative phosphorylation, and ATP production [44], being increasingly discussed as a potential therapeutic target [45].

To date, there is no evidence of TNT mitochondrial transfer to rescue senescent cells in vivo; rather, senescent phenotype appears to employ this mechanism to propagate to surrounding cells. Life imaging tracking was able to document that senescent cells act rather as mitochondrial donors in a mammalian target of rapamycin (mTOR) and of small GTP-ase CDC42-dependent manner, contributing to spreading senescence-associated phenotype (SASP) to surrounding cells and, potentially, tissues [46].

Mitochondria can circulate as well via EVs containing the entire organelle or components (mtDNA or proteins). Mitochondria-containing EVs can be released under physiological stress and subsequently internalized by recipient cells through endocytosis. After uptake, mitochondrial cargo contributes to restoring or reshaping recipient-cell bioenergetics, for example, by supplementing respiratory capacity or supporting recovery from mitochondrial dysfunction. In support of this concept, mitochondria-containing EVs derived from autologous stem cell–derived cardiomyocytes were reported to improve ischemic myocardial energetics, at least in part through functional integration and fusion with the endogenous mitochondrial network of recipient cells [47]. Conversely, stressed MSCs can also export mitochondria via EVs; under oxidative stress conditions, MSCs have been shown to package and release partially depolarized mitochondria in EVs, pointing towards a context-dependent role for EV-mediated mitochondrial trafficking in intercellular adaptation and mitochondrial quality control.

EV-mediated mitochondrial transfer involving MSCs is increasingly recognized as bidirectional. MSCs can donate functional mitochondria to metabolically compromised cells, restoring OXPHOS and supporting tissue repair [48,49], but stressed or damaged niche cells may also transfer dysfunctional mitochondria to MSCs. Such reverse transfer induces mitochondrial overload, metabolic stress, and premature senescence in recipient stem cells, potentially limiting long-term regenerative capacity [50,51]. Mitochondrial exchange between stem cells and their niche therefore appears to be a double-edged process, supporting repair or propagating aging signals depending on organelle quality.

EV-based export may also serve as a mechanism for clearance of dysfunctional mitochondria. Under oxidative stress, MSCs export depolarized mitochondria within EVs, which are subsequently recycled by macrophages, restoring intracellular bioenergetics [52,53]. Declining plasma levels of mtDNA-containing EVs correlate with advancing age and have been proposed as a biomarker of health status in middle-aged and older individuals [54]. However, standardized methods for sampling, isolation, characterization, assessment of mitochondrial origin, and biomarker validation are still lacking [55].

A third mechanism of intercellular exchange is cell–cell or heterotypic fusion, during which cytoplasmic organelles, including mitochondria and mitochondrial DNA, are shared [56]. Human multipotent adipose-derived stem cells (hMADS) and human bone marrow-derived stem cells (BMSCs), but not human fibroblasts, were shown to reprogram fully differentiated mouse cardiomyocytes in vitro through cell fusion and mitochondrial transfer [57], suggesting a regenerative mechanism that may be conserved across species. Cell-fusion-mediated mitochondrial transfer from donor BMSCs also improved Müller glial cells in vitro and in a rat model of degenerative retinopathy, reducing oxidative stress and improving function and vision [58]. Cellular fusion events have also been reported to induce senescence through nuclear/genomic stress in viral oncogenesis and tumor metastasis [59,60]. At present, however, no evidence shows that cell fusion-based mitochondrial trafficking either promotes or reverses senescence.

Less common routes of intercellular exchange include direct release of free mitochondria into the extracellular space, followed by uptake by recipient cells through phagocytosis or endocytosis. Intact mitochondria have been detected in conditioned media and subsequently internalized by neighboring cells, mainly through fluid-phase or nonspecific endocytosis; some escaped the endosomal compartment and may have integrated into the host mitochondrial network [61]. Release of dysfunctional mitochondria has also been described in some cell types, such as neurons and adipocytes, as part of the quality control process, followed by macrophage clearance [62]. Intercellular mitochondrial exchange has been described as a mode of communication that can rescue stressed neighboring cells by restoring bioenergetic function and may extend beyond local interactions to inter-tissue and inter-organ coordination, with possible whole-body implications (Figure 2).

Effective intracellular integration and intercellular coupling of cardiomyocytes are required for proper cardiac function and rhythmic contraction. In adult cardiomyocytes, mitochondria form an electrically coupled reticulum segmented into inter-mitochondrial junction (IMJ)-linked sub-networks that conduct ΔΨm and redistribute ATP supply on a beat-to-beat basis while disconnecting damaged regions as a “circuit-breaker” [64,65]. With aging, this network shifts toward fragmentation due to altered dynamics (reduced MFN2/OPA1-mediated fusion relative to DRP1-driven fission) and cristae destabilization, impairing energy conduction and favoring ROS propagation and loss of contractility [66,67]. OPA1/MICOS (Mitochondrial Contact site and Cristae Organizing System)-dependent inner-membrane and crista junction integrity is particularly important for maintaining networked respiration. Different degrees of perturbations of these systems are linked to cardiac dysfunction in both experimental and clinical settings. OPA1 is involved in the occurrence and progress of dilatative cardiomyopathy, governing fusion–fission imbalance by disrupting mitochondrial inner membrane fusion, as well as the modified cristae structure [68]. Cristae structure and dynamics have been recently identified as a modality to fine-tune oxidative phosphorylation. Cristae vesicles could trap protons as well as ATP, influencing their availability to cytosol, while the fusion of cristae to IBM increases OXPHOS exchanges [69].

With advanced age, cardiomyocytes exhibit downregulation of the MFN2/OPA1 axis as well as relative upregulation/activation of DRP1 involved in fission and impaired cristae architecture. This leads to the build-up of a cardiac muscle that is more prone to impaired excitation–contraction energetics, diastolic/systolic dysfunction, and vulnerability to stress [70]. Excess DRP1-dependent fission promotes apoptotic signaling and pathological remodeling, contributing to heart-failure phenotypes [71]. Mitochondrial function and positive fusion/fission balance can be increased by targeting the sirtuin1 (SIRT1)/AMPK/Drp1 pathway to restore cardiac function [72].

Mitochondrial networks populated with dysfunctional elements contribute to decreased connectivity. In normal adult hearts, the PINK1/Parkin mitophagy cascade is triggered by loss of mitochondrial membrane potential: depolarization blocks PINK1 import and PARL-mediated cleavage, causing PINK1 to accumulate on the outer mitochondrial membrane, where it recruits and activates the E3 ligase Parkin by phosphorylating both Parkin and ubiquitin [73]. The resulting ubiquitin chains are read by autophagy receptors such as Optineurin, which bind ubiquitinated outer mitochondrial membrane (OMM) proteins and simultaneously engage microtubule-associated protein 1A/1B-light chain 3 (LC3) via LC3-interacting region (LIR) motifs, thereby coupling damaged mitochondria to nascent autophagosomes [74]. Aging reduces effective mitophagy signaling, such as the PINK1–Parkin axis, allowing dysfunctional mitochondria to persist within the network [75]. An experimental increase in mitophagy has been demonstrated to improve cardiac function. Urolithin A (UA), a mitophagy activator, enhanced cardiac mitophagy and improved systolic/diastolic function in naturally aged and heart-failure mouse models; electron microscopy and molecular markers indicated restored mitochondrial quality control [76]. In a clinical observational study, a 4-month duration of oral urolithin supplementation in healthy older adults was associated with reduced validated biomarkers of cardiovascular risk (plasma ceramides) [77,78,79,80,81,82].

Mitochondrial autophagy declines in the aging heart. DNA alterations, a hallmark of aging [83], impair mitochondrial network connectivity. Aging hearts accumulate mtDNA lesions and heteroplasmy, which depress respiratory chain function, reduce ΔΨm, and destabilize cristae, thereby compromising reticular coupling across cardiomyocyte mitochondrial subnetworks. The result is regional conduction disturbance, increased intracellular ROS, and reduced contractile reserve. In Polg mutant mice, age-related mtDNA mutations are associated with myocardial hypertrophy, systolic and diastolic dysfunction, and increasing fibrosis, supporting a role for mtDNA damage in cardiomyopathy [84].

Since inter-mitochondrial junctions enable rapid ΔΨm conduction, mtDNA-driven loss of membrane potential or crista integrity can decouple subnetworks and impair myocardial contractility. Nutritional, pharmacological, and genetic interventions that reduce mtDNA heteroplasmy restore mitophagy, dynamics, and biogenesis, delaying age-related cardiovascular degeneration [85].

Adult human cardiomyocyte renewal is very slow—about 1% per year at age 25, falling to about 0.45% per year by age 75—so damaged cells persist for decades, and defects in mitochondrial networking and quality control accumulate rather than being diluted by cell replacement. This favors functional decoupling of the mitochondrial network through altered dynamics (MFN2/OPA1, ↑DRP1), mtDNA lesion and heteroplasmy build-up, and declining mitophagy, allowing dysfunctional organelles to persist [86]. Together, these disturbances reduce ATP reserve, increase intracellular ROS, and impair beat-to-beat contractile performance, predisposing the aging heart to systolic/diastolic dysfunction and stress intolerance (Figure 3).

With advancing age, the decline in mitochondrial biogenesis, mitophagy efficiency, and respiratory capacity contributes to age-related sarcopenia and decreased endurance. Mitochondrial DNA mutations and the accumulation of dysfunctional organelles disrupt energy homeostasis and redox signaling, amplifying anabolic resistance and inflammation and perturbing mitochondrial intracellular and intercellular networking.

In the young adult skeletal muscle, mitochondria form a reticular network extending along myofibrils and across sarcomeres, ensuring efficient distribution of ATP, metabolites, and calcium [87]. This interconnected network, known as the mitochondrial reticulum, enables rapid energy distribution by providing a conductive pathway for the proton-motive force. Coordinated mitochondrial depolarization and conduction via the mitochondrial reticulum have been proposed as the main modality of energy distribution within skeletal muscle [88]. The proximity of the mitochondrial reticulum to calcium stores in the sarcoplasmic reticulum enables bidirectional signaling: calcium stimulates ATP production, while ATP is used during calcium removal. This bidirectional communication ensures that energy supply is closely matched to the muscle’s contraction and relaxation needs, optimizing muscle performance [89].

With advancing age, both structural continuity and functional coupling within the mitochondrial reticulum deteriorate. Electron microscopy and high-resolution 3D imaging have shown fragmented, swollen, and spatially isolated mitochondria in aged myofibers, in contrast with the highly interconnected networks seen in young muscle [90]. These morphological changes parallel functional decline: aged mitochondria produce ATP less efficiently and are more susceptible to oxidative stress, contributing to muscle atrophy and sarcopenia [91]. Mitochondrial organization depends not only on morphology but also on interconnectivity. Reduced inter-mitochondrial contacts and altered cytoskeletal anchoring limit calcium and metabolite diffusion, further decoupling bioenergetic domains. Studies in human and animal muscle further show that aging disrupts the microarchitecture linking excitation–contraction (EC) coupling to mitochondrial function. Muscle biopsies from sedentary older adults show a 40–50% reduction in calcium-release units (CRUs) compared with young individuals, a loss associated with impaired specific force generation [92]. Sarcopenic mice and human muscle biopsies show parallel declines in total mitochondrial number and in CRU–mitochondrial pairs, indicating that mitochondria progressively lose their strategic positioning near SR Ca²⁺ release sites with age [93,94]. This uncoupling is accompanied by reduced SR Ca²⁺ release, impaired mitochondrial Ca²⁺ uptake, and increased oxidative stress, underscoring the importance of CRU–mitochondrial alignment for coupling Ca²⁺ signaling to ATP synthesis and buffering ROS/RNS [95,96]. Disorganization of the mitochondrial network lowers the respiratory control ratio, slows ATP turnover, and delays recovery, all features of sarcopenic decline [97,98]

ATP turnover declines with age, correlating with slower phosphocreatine (PCr) recovery kinetics (kPCr) after exercise, lower oxidative capacity reduced cardiorespiratory fitness, and reduced donor muscle strength [99,100].

Aging skeletal muscle decreases intracellular connectivity between the mitochondria nodes of the network. Mitochondrial fusion mediators MFN2 and OPA1were found to be downregulated in aging muscle, with relative upregulation or hyperactivation of DRP1, contributing to excessive fission, inducing network fragmentation and loss of cristae integrity [101]. In adult mice, specific deletion of Drp-1 induces ER stress, which has as a consequence catabolic muscle loss and systemic senescence mediated by the unfolded protein response (UPR) and FoxOs signaling. In mice with double deletion of Opa1 and Fgf21 genes, the aging phenotype was completely reversed. Sedentary but not physically active subjects display age-correlated decline of OPA-1 in skeletal muscle biopsy. [102]. The reduced capacity for complementation between partially damaged mitochondria amplifies local oxidative stress and mtDNA instability. In this “negative feedback loop,” mitochondria are a major source of ROS, further impairing mtDNA [103]. Not only interconnectivity but also mitochondrial number and quality decline in aging muscle. Transcriptomic analyses reveal reduced expression of PGC-1α, TFAM, and NRF1/2, the main regulators of mitochondrial biogenesis, thereby limiting renewal of network components [104]. Reduced biogenesis is associated with impaired skeletal muscle performance, decreased respiratory capacity, and accumulation of dysfunctional mitochondrial elements. However, targeted overexpression of biogenesis regulators does not appear to have a straightforward restorative effect. In aging mice, skeletal muscle-specific overexpression of PGC-1α enhanced mitochondrial biogenesis and supported anabolic signaling but also induced oxidative stress and inflammatory responses. In addition to fragmentation and swelling, aging muscle fibers may show focal mitochondrial clustering, reflecting disruption of the normally ordered interfibrillar distribution of mitochondria. Such aggregates likely result from cumulative alterations in fusion–fission balance, impaired mitochondrial transport, and declining quality-control mechanisms, ultimately contributing to local heterogeneity in membrane potential and reduced bioenergetic coupling within the mitochondrial network [105] (Figure 4).

NF-κB is a stress- and cytokine-responsive transcription factor that coordinates inflammatory and pro-survival gene programs. In the canonical pathway, NF-κB dimers are retained in the cytosol by IκB proteins; inflammatory cues activate the IKK complex, leading to IκB phosphorylation and degradation, NF-κB nuclear translocation, and induction of targets such as TNF-α and other inflammatory mediators. In aging skeletal muscle, persistent NF-κB activation is commonly associated with oxidative stress and mitochondrial damage signaling and correlates with impaired mitochondrial quality control and anabolic resistance.

Consistent with this profile, aged muscle shows increased protein carbonylation and altered expression of stress-response and repair markers, including increased IκB-α, NF-κB, TNF-α, SOD2, and NRF2, together with reduced expression of the DNA repair enzyme OGG1. Notably, a comparable inflammatory/oxidative signature was reported in the hippocampus despite PGC-1α overexpression being restricted to skeletal muscle, supporting a context-dependent and potentially systemic impact of muscle mitochondrial remodeling during aging [106]. In line with this, muscle PGC-1α overexpression enhances the biogenic response, whereas its absence attenuates exercise-induced mitochondrial remodeling [107,108,109,110,111].

Defects in mitochondrial quality control further aggravate this inflammatory state. Reduced mitophagy can lead to accumulation of fragmented organelles due to impaired PINK1–Parkin signaling and defective lysosomal flux, contributing to decreased myofibrillar area and muscle atrophy [112,113,114,115,116]. Persisting dysfunctional mitochondria release ROS and mitochondrial DAMPs, reinforcing NF-κB–driven inflammatory cascades and further promoting anabolic resistance in aging skeletal muscle [117,118,119]. Taken together, aging muscle progressively loses mitochondrial network connectivity—both morphologically and functionally. Energetic inefficiency, redox imbalance and chronic low-grade inflammation lead to loss of function and motor impairment.

With advancing age, the tightly regulated processes that maintain mitochondrial networking in the nervous system gradually deteriorate. Mitochondria become less efficient, more fragmented, and increasingly unable to meet the bioenergetic demands of neuronal circuits. As a result, neurons accumulate oxidative damage, display impaired proteostasis, and lose synaptic integrity [120,121]. Although these alterations are well described in neurodegenerative disorders, they are not exclusive to them but represent broader age-related vulnerabilities affecting neuronal resilience and function.

Aging neurons shift from elongated, interconnected mitochondrial networks toward shorter, rounded, and fragmented organelles. Longitudinal imaging in C. elegans showed that neuronal aging is accompanied by progressive mitochondrial fragmentation associated with functional decline, whereas maintenance of fused networks delays behavioral deterioration [122]. Similar changes are seen in mammals: ultrastructural studies in aged mouse hippocampus revealed shortened, spherical mitochondria with disrupted cristae compared with the elongated profiles found in young animals [123,124,125]. These changes reflect an altered fission–fusion balance rather than morphology alone. In the aged hippocampus, increased Drp1-Ser616 phosphorylation together with reduced Mfn2 expression is associated with higher fragmentation indices, impaired mitochondrial respiration, and memory decline. Senescent cortical astrocytes and astrocytes from aged mouse brain likewise show increased numbers of small, fragmented mitochondria, reduced membrane potential, and marked DRP1 upregulation, indicating that aging shifts mitochondrial dynamics toward fragmentation in both neurons and glia, with likely consequences for metabolic support of neural circuits [126].

In postmortem samples from patients with Alzheimer’s disease, electron microscopy revealed swollen and fragmented mitochondria with damaged cristae in both pre- and postsynaptic compartments, together with reduced synaptic vesicle content in neurons and glia across several brain regions [127]. Because direct neural tissue access is limited, studies have also examined more accessible cells. Fibroblasts from patients with Alzheimer’s disease showed reduced average mitochondrial length, a higher proportion of short mitochondria, and altered OPA1/MFN1 processing, consistent with a fragmented network compared with age-matched controls [128]. Increased proportions of morphologically abnormal, ROS-producing mitochondria, together with lysosomal/autophagic defects, were also reported in these cells [129], further supporting the view that fragmentation and accumulation of damaged mitochondria are common features of age-related neurodegenerative disease.

In Caenorhabditis elegans mechanosensory neurons, mitochondrial trafficking in distal processes was found to decrease progressively from early adulthood, in parallel with late-life reductions in axonal mitochondrial density and resistance to oxidative stress [130]. In vivo imaging studies (multiphoton imaging) of retinal ganglion cell axons in mice showed that between adulthood (4 months) and old age (23–25 months), the duration and distance of mitochondrial runs and the duty cycle of movement progressively decline, and mitochondria-free gaps along the axon and the proportion of shorter mitochondria increase, indicating less efficient delivery of organelles to distal axonal segments even though the number of transported mitochondria per axon is conserved. The decline is worsened by the co-existence of specific diseases (such as glaucoma) [131]. In sciatic nerve and central nervous (optical nerve, hippocampal region) explants from aging mice, quantitative live imaging demonstrated that the fraction of motile axonal mitochondria and other cargoes declines in two age-dependent phases, one in early adulthood and another one occurring in old subjects, supporting the view that reduced transport is not a continuous, gradual process. Furthermore, at least in the peripheral nervous system, triggers of regeneration were able to increase axonal transport, suggesting that this might be an environment-responsive process and that axonal transport is not obligatorily diminished by age but rather by loss of functional demand. The interference and impact of neurodegenerative diseases on this process remain to be further evaluated [132].

Indirect data from human postmortem samples regarding motor protein levels, localization, and mitochondrial localization also support the fact that in the absence of neurodegenerative disease, adaptive changes in retrograde transport machinery occur rather than clear failure. Dynein adaptor dynactin-P50, βAPP, synaptophysin, and phospho-tau were quantified in hippocampal tissue from cognitively intact individuals of different ages and from AD patients. In neuropathology-free aged cases, advanced age was associated with coordinated increases in dynactin-P50 and βAPP in pyramidal neurons, interpreted as an adaptive up-regulation of retrograde transport capacity during normal aging. In contrast, AD brains showed reduced overall dynactin-P50 and βAPP levels and misplacement to dystrophic neurites and plaques, suggestive of the breakdown of retrograde axonal transport and cargo recycling in disease [133]. Post-mortem frontal cortex from AD patients and age-matched cognitive unaffected controls revealed that kinesin light chain (KLC) and dynein intermediate chain are significantly reduced in AD tissue, with concomitant changes in βAPP and phospho-tau distribution. These can be regarded as evidence for impaired axoplasmic (axonal) transport in human AD cortex rather than an epiphenomenon [134]. In another study, perikarya and neurites from AD patients have depleted or mislocalized mitochondria despite preserved individual mitochondrial volume. These findings are similar to those seen in a tauopathy mouse model and are reversible when soluble tau expression is suppressed, implying the tau-linked disruption of mitochondrial trafficking/axonal transport in AD brain [135]. Synaptic mitochondrial dynamics disruption in AD patients might be cortical-region-dependent, as revealed by EM studies also reporting aberrant mitochondrial morphology, multivesicular bodies within synapses, and diminished synapse apposition length proximal to AD plaques [136].

Currently, direct axonal trafficking studies in animal models, as well as indirect reports from animal and human samples, suggest age-related adaptive changes in axonal transport. Neurodegenerative diseases (particularly reflecting AD-related changes, which appear to be the most well studied) are accompanied by failure of mitochondrial distribution around the synaptic region, and reduced levels and anomalies in phosphorylation of transport proteins are related to structural changes induced by tau plaque accumulation.

Deficient mitophagy, a common mechanism of network impairment during aging, appears to differently impact various CNS regions while acquiring a different profile in neurodegenerative diseases. In animal models, mitophagy reporter mice have provided the clearest evidence that neuronal mitophagy declines in specific brain regions during aging. In mt-Keima reporter mice, lysosome-engulfed mitochondria were detected in several tissues; aging was associated with a marked reduction in basal mitophagy, in hippocampal dentate gyrus neurons of 21-month-old versus 3-month-old subjects, supporting the fact that “normal” aging does not imply overspread defective mitophagy but rather a dynamic adaptive process. Longitudinal mapping with dual autophagy/mitophagy reporters showed that neuronal mitophagy in the mouse brain follows cell- and region-specific trajectories: mitophagy can increase through adulthood and mid-life in hippocampal and cortical circuits and then decline in old age (e.g., CA1 pyramidal neurons, dentate gyrus parvalbumin interneurons), whereas macro autophagy shows an earlier and more global age-related decrease [137].

In aged mice lacking the phospholipase iPLA2β, age-dependent cognitive decline in the prefrontal cortex is accompanied by reduced PINK1/Parkin-dependent mitophagy, decreased LC3-II and increased p62, and accumulation of damaged mitochondria in cortical neurons, partially rescued by pharmacological mitophagy activation [138].

These data support a model in which physiological aging produces a nuanced remodeling of neuronal mitophagy—declining in some neuronal populations and brain regions but not globally extinguished—a possible adaptation of mitochondrial network functioning for preserving more frequently utilized neural groups’ functioning.

In AD, mitophagy defects in neurons have a mechanistically distinct profile. Hippocampal and cortical neurons from AD mouse models and human AD brain tissue exhibit reduced mitophagy flux, with altered levels of PINK1/Parkin-pathway proteins, accumulation of mitochondrial fragments and autophagosomes, and impaired clearance of damaged mitochondria. Pharmacological enhancement of mitophagy (e.g., urolithin A, NAD⁺ boosters) restores mitochondrial turnover and reverses cognitive deficits in AD models [139]. Post-mortem human AD brains and AD-derived cellular models show that amyloid-β and phosphorylated tau interact with Drp1, PINK1 and parkin, with excessive mitochondrial fission and simultaneously disturbing autophagosome–lysosome fusion and lysosomal acidity. This results in accumulation of mitochondria-containing autophagic vacuoles and increased p62 in affected neurons [140].

Human pathological studies used phospho-ubiquitin (p-S65-Ub)—a PINK1-dependent “mitophagy tag” to distinguish between normal aging and neurodegeneration. In large cohorts of post-mortem human brains, p-S65-Ub-positive structures were detected and quantified in substantia nigra, hippocampus, amygdala, putamen and nucleus basalis. In neurologically normal individuals, p-S65-Ub density increased with age in the amygdala and nucleus basalis, consistent with an age-related rise or compensation in mitophagy signaling in these regions. In Lewy body disease, p-S65-Ub levels were significantly higher in the substantia nigra and hippocampus than in age-matched controls, correlating with Lewy body and neurofibrillary tangle burden. PRKN or PINK1 mutation carriers showed reduced or absent p-S65-Ub [141].

Basal mitophagy signaling increases with age in several human brain regions, but in Parkinsonian disorders, it becomes dysregulated, acquiring both region- and mutation-dependent failure of mitochondrial quality control.

Patient-derived organoid neuronal models have introduced an advanced modality to gain mechanistic insights into pathological aging. Human iPSC-derived dopaminergic neurons with PINK1 loss-of-function display normal mitochondrial morphology and preserved Parkin recruitment but show inhibited ionophore-induced mitophagy and reduced mitochondrial membrane potential, indicating a specific defect in the execution of mitophagy rather than in biogenesis or gross morphology [142]. Age-related neurodegenerative diseases such as AD and PD superimpose additional blocks at multiple levels of the mitophagy pathway (PINK1/Parkin signaling, receptor recruitment, autophagosome–lysosome fusion), disrupting mitochondrial network adaptability. If such disturbances are a cause or consequence of the disease, they need to be further explored.

In aging mice, senescent astrocytes accumulate morphologically damaged, fragmented mitochondria due to impaired mitophagy (PINK1–Parkin axis downregulation and increased fission), with reduced respiratory capacity and increased susceptibility to further mitochondrial stress [143]. Such changes can compromise metabolic and redox support to adjacent neurons and induce age-associated neuroinflammation. In human brain samples, 3D Electron microscopy (EM) and morphometry show mitochondrial malfunction and atrophy specifically in astrocytes—shorter, swollen mitochondria, reduced mitochondrial content, and astrocytic process atrophy—while neuronal mitochondria are comparatively preserved. This can be interpreted as an age-related shift where astrocyte mitochondrial failure undermines synaptic and metabolic support to neurons in the aged human cortex [144]. In human induced pluripotent stem cells (IPSC) models, astrocytes carrying POLG gene mutations (a gene that encodes the catalytic subunit of polymerase γ associated with mitochondrial inherited diseases) display loss of mitochondrial membrane potential I/IV deficiency, ATP failure and mtDNA depletion. Such astrocytes have an A1-like, reactive signature with neurotoxic effects on co-cultured neurons [145]. Limiting microglial mitochondrial fragmentation with the Drp1–Fis1 inhibitory peptide P110 stops A1 astrocyte conversion and protects neurons [146,147].

Metabolic coupling between oligodendrocytes and axons depends on intact oligodendrocyte mitochondria and their glycolytic/oxidative capacity. NMDA receptor activation in oligodendrocytes mobilizes GLUT1 and enhances glucose uptake and lactate delivery to axons, supporting axonal energy metabolism.

In demyelinating/neurodegenerative diseases, mtDNA double-strand breaks induced selectively in oligodendrocytes in mice lead to oligodendrocyte mitochondrial dysfunction, followed by axonal degeneration, even in the absence of oligodendrocyte loss. In patients with multiple sclerosis (MS), oligodendrocyte mitochondrial failure contributes to “dying-back” axonopathy and chronic neurodegeneration [148].

While this evidence is disease-focused (MS and white-matter degeneration), it exemplifies how disrupted mitochondrial networks in myelinating glia can compromise glia–axon metabolic crosstalk, promoting neurodegeneration (Figure 5).

Mitochondrial connectivity operates in a tissue-dependent manner (), contributing to adaptive responses that can propagate at the organismal level. Figure S1