Mitochondrial Aging in the Nervous System and Its Possible Impact on Brain Health and Disease

In mammals, the clearance of damaged mitochondria, both under physiological conditions and in response to cellular stressors, is primarily achieved through selective autophagy, known as mitophagy. Mitophagy, first identified in 2005 by Lemasters et al. [3], is a core cellular quality-control process that safeguards the functionality of the mitochondrial network and allows its adaptation to different stimuli [4,5].

Mitophagy can be elicited through two different pathways: the PINK1/Parkin-dependent pathway, and the receptor-mediated, PINK1/Parkin-independent mechanism [6]. The best-characterized of these is the ubiquitin-mediated pathway, orchestrated by the mitochondrial kinase PINK1 (PTEN-induced putative kinase 1) and the cytosolic E3 ubiquitin ligase Parkin (PARK2). Upon mitochondrial damage and loss of membrane potential, PINK1 accumulates on the outer mitochondrial membrane (OMM) and becomes activated, triggering a phosphorylation cascade. A key step involves the PINK1-mediated phosphorylation of Parkin [6], which, in turn, activates Parkin and promotes its translocation to the OMM. Once there, Parkin ubiquitinates multiple outer membrane proteins, such as mitofusin-1/2 (Mfn1/2) and dynamin-related protein 1 (Drp1) [7,8], targeting the mitochondria for degradation. Ubiquitinated mitochondria are subsequently recognized by autophagy adaptor proteins, including p62/SQSTM1, optineurin (OPTN), NDP52, NBR1, and TAX1BP1 [9,10], which facilitate mitochondrial engulfment by autophagosomes.

The involvement of PINK1/Parkin signaling is particularly noted in the context of neurodegenerative diseases. Their levels are downregulated in Alzheimer's, Parkinson's, and amyotrophic lateral sclerosis (ALS), and are accompanied by aberrant mitophagy and/or impaired mitophagy flux in human brains [11,12,13]. However, in mice tissues or cells of high metabolic demand, the loss of PINK1 is not sufficient to alter basal mitophagy including microglia [14], suggesting that other pathways may contribute to basal mitophagy process in these contexts. Moreover, neither PINK1 or Parkin knockout mice exhibit clear neurodegenerative phenotypes under normal physiological conditions [15].

Notably, despite their cooperation in stress responses, PINK1 and Parkin have distinct subcellular localizations, and may perform independent functions under basal conditions [16]. For example, PINK1 is believed to phosphorylate a broader set of substrates beyond Parkin, while Parkin activation can also be influenced by aging-related factors and PINK1-independent signaling pathways [17].

Beyond the ubiquitin-dependent mechanism, PINK1/Parkin-independent mitophagy plays a crucial role in maintaining mitochondrial health under normal conditions. Studies in PINK1-deficient mice [14] and Drosophila lacking either PINK1 or Parkin [18] have shown that basal mitophagy can occur independently of mitochondrial depolarization and ubiquitin signaling, underscoring the existence of alternative receptor-mediated pathways [18,19,20]. These pathways are mediated by a range of mitophagy receptors that localize predominantly to the OMM and contain an LC3-interacting region (LIR) motif, enabling direct interaction with autophagosomal proteins LC3 and GABARAP. Key examples include FUN14 domain-containing 1 (FUNDC1), Autophagy and Beclin 1 Regulator 1 (AMBRA1), B-cell lymphoma 2 (BCL2)/adenovirus E1B 19-kDa-interacting protein 3-like (BNIP3L/NIX), and its homolog BCL2/adenovirus E1B 19-kDa-interacting protein 3 (BNIP3) [6]. Interestingly, Schmid and colleagues recently found that activation of BNIP3 in Drosophila brain tissue could reverse the accumulation of dysfunctional mitochondria across aging by increasing mitophagy, resulting in improved organism lifespan [21]. However, while BNIP3 activation in Drosophila shows promising effects on mitophagy and lifespan, caution is needed when extrapolating these findings to humans, given the significant physiological differences between invertebrates and mammals. Further studies in vertebrate models are needed to assess the translational potential.

As previously mentioned, an established consequence of altered mitophagy is cellular senescence. Cellular senescence is a critical feature of aging and has been recognized as the primary component of age-associated neurodegenerative disorders [22]. Indeed, excessive senescence can have detrimental effects and cause or contribute to different aging-associated phenotypes and diseases [23]. It is noteworthy that prevalent neurodegenerative diseases in humans, such as Alzheimer's, Parkinson's, and ALS, are accompanied by aberrant mitophagy and/or impaired mitophagy flux [12] and this is paralleled by a reduced presence of specific markers, such as PINK1, Parkin, phosphoglycerate mutase 5 (PGAM5), and other already-mentioned mitophagy-associated proteins (MAPs), in the plasma from patients with several types of dementia, cognitive impairment or neurodegenerative diseases, reflecting a significant downregulation of this pathway in these pathological conditions [13,17] (Figure 1). Moreover, the maintenance of mitophagy in order to blunt the dramatic decline observed with aging can be exploited pharmacologically to modulate this biological process with the aim to counteract the progression of neurodegeneration and preserve a healthy lifespan. Indeed, supplementation with chemical inducers of mitophagy has been shown to mitigate cognitive deficits in different models of Alzheimer's disease [24,25]. Examples of pharmacological agents and phytochemicals that have been proposed as mitophagy promoters acting on different molecular targets include resveratrol, curcumin, spermidine, taurine, catechins, and melatonin [26,27,28,29]. On the other hand, there are also reported cases where uncontrolled mitophagy proves detrimental and aggravates neuronal cell death [30,31,32,33].

The complexity of mitophagy regulation is further underscored by the prominent differences that have been characterized in basal mitophagy activity in neurons of different brain regions in vivo [34]. Moreover, although less extensively studied, mitophagy in glial cells is an emerging area of interest, with growing recognition of its potential physiological and pathological relevance [7]. For example, dysfunction in mitochondrial metabolism in astrocytes, which are responsible for nearly 20% of the brain's total oxygen consumption through oxidative phosphorylation within their mitochondria [35], can lead to decreased neuronal activity and contribute to the development of neurodegenerative diseases [36]. Moreover, most astrocytes become senescent during aging [37,38]; therefore, it is likely that astrocytic senescence participates in the initiation and progression of neurodegenerative disease, including Parkinson's disease [39,40]. Importantly, senescent astrocytes exhibit increased mitochondrial fragmentation and impaired mitophagy coupled with upregulated mitochondrial biogenesis, leading to the accumulation of damaged mitochondria that are not efficiently cleared [41]. Recently, Chen et al. [42] have proposed a small aminopropyl carbazole compound, P7C3, as a new neuroprotective compound, capable of suppressing the increased production and secretion of senescence-associated secretory phenotype (SASP) factors exhibited by senescent astrocytes in vitro. This occurs through concomitant promotion of mitophagy and inhibition of mitochondrial ROS generation. This is particularly relevant since the massive release of chemokines and proinflammatory cytokines from senescent astrocytes can lead to the death of surrounding neurons through inflammatory damage, loss of neuroprotective functions, impaired synaptic plasticity, and induced glutamate cytotoxicity [42,43].

Collectively, these studies strongly suggest that mitophagy must be tightly regulated to preserve neuronal function along with aging, since its dysregulation, whether increased or decreased, can affect brain homeostasis promoting neuropathology and neurodegeneration.

Mitochondria form an interconnected network within the cell, and homeostasis of this network greatly affects cellular function and survival. Moreover, finely tuned mitochondrial dynamics allow effective adaptation of these organelles in response to various cellular needs and environmental cues [44,45]. Indeed, it can be assumed that maintaining a balance between the two opposing processes, mitochondrial fission and fusion, is an emerging essential factor of longevity [46]. Mitochondrial fission is essential for mediating stress response, metabolic regulation, and mitophagy, but excessive levels can result in fragmented and defective mitochondria. On the other hand, fusion allows a constant mixing and exchange of proteins, metabolites, and mtDNA between mitochondria, which can be beneficial, leading to restoration of mitochondrial function [1,47]. Indeed, mitochondrial fusion promotes oxygen consumption, oxidative phosphorylation, and, consequently, increased ATP production [48,49]. As organisms age, mitochondrial fission and fusion become dysregulated and mitochondrial networks become fragmented [44,50,51,52]. Furthermore, dysregulated expression of fission and fusion proteins occurs in both aging and age-related diseases [53,54,55] (Figure 1). Coherently, modulation of mitochondrial dynamics has been shown to affect longevity in different species. Notably, increased mitochondrial fusion has been associated with increased longevity [56]. Healthy human centenarians have highly connected mitochondrial networks compared to younger individuals [57]. Moreover, it has long been demonstrated that mitochondrial fusion impairment resulting from decreased mitofusin expression is associated with impaired cell growth and respiration, an heterogeneous mitochondrial population, and decreased membrane potential [58]. These effects are characteristic of brain aging and, in particular, may play a detrimental role by damaging neighbouring neuronal precursors [46]. Indeed, some studies propose a link between aging and stem cell loss in the CNS [59,60], although the causes of this phenomenon have not yet been elucidated.

On the other hand, activated mitochondrial fission can be beneficial for the elimination of defective mitochondria fragments accumulated during aging. However, it has been suggested that increased expression of mitochondrial fission proteins may disrupt the mitochondrial membrane potential and inhibit the respiratory chain, leading to slower cell growth and exacerbated neuronal aging [46]. On the contrary, mitochondrial fission inhibition is sufficient to increase lifespan in yeast and fungal models [61,62]. In particular, deletion of dynamin-related protein 1 (Drp-1) increases lifespan and restores motility in a neuronal model of polyglutamine toxicity [63]. Moreover, studies in C. elegans have shown that promoting mitochondrial fusion by deleting Drp-1 amplifies the longevity-promoting effects of dietary restriction model daf-2, but not in wild-type [64,65]. Moreover, it has been observed that the amount of the activated (phosphorylated) form of Drp1 increases with aging [66]. In particular, phosphorylation of Drp1 at Serine 616 (S616) activates Drp1 and results in mitochondrial fragmentation, and is possibly involved in neuronal death [67], suggesting that the increased level of phospho-Drp1 correlates with the mitochondrial morphological changes in the hippocampus and the cognitive decline observed with aging [46]. Consistently, Li Y. et al. demonstrated that murine hippocampus and cerebral cortex aging processes are accompanied by significant increase in fission proteins Fis1 and Drp1 [68], which also contributes to the mitochondrial fragmentation activation (Figure 1).

Importantly, dysregulated mitochondrial fission was observed in inflamed microglia [69,70], whereas mitochondrial division inhibitor 1 (mdivi-1) and Drp1 knockdown inhibited mitochondrial fission, mitochondrial ROS generation, and proinflammatory mediators in LPS-stimulated microglia [71]. Indeed, mitochondrial homeostasis disruption contributes to microglia-mediated inflammation and vice versa. Moreover, the natural compound atractylenolide III was reported to suppress neuroinflammation and protect against brain ischemia by inhibiting JAK2/STAT3-dependent mitochondrial fission in microglia [72].

Finally, it has recently been shown that the simultaneous overexpression of proteins regulating both mitochondrial fission and fusion despite (or perhaps due to) an increased level of mitochondrial fragmentation in C. elegans was positively correlated with stress tolerance and longevity [73].

All these evidences, together emphasize the relevance of developing therapeutic approaches that would maintain mitochondrial dynamics balance in order to actively improve health and lifespan, as well as to counteract CNS aging and neurodegenerative diseases development.

Mitochondria–nuclear communication represents a fundamental inter-organellar process essential for maintaining cellular homeostasis and balancing survival–death signaling under cellular stress [133].

To date, studies on mitochondria–nucleus communication have primarily focused on signals sent from stressed mitochondria to the nucleus and the subsequent cellular responses, as well as on nuclear-to-mitochondrial communication in the context of DNA damage and repair. However, particularly during aging, this interaction can be seen as a dynamic, bidirectional dialogue with feedback mechanisms extending beyond these two organelles and influenced by various physiological cues [134].

In neuronal cells, the relationship between mitochondria and the nucleus is especially intricate. Neurons rely heavily on oxidative phosphorylation (OXPHOS), making them particularly susceptible to oxidative stress caused by ROS production. Given that OXPHOS components are encoded by both mitochondrial and nuclear genomes, while the "ROS defense system" is exclusively nuclear-encoded, a well-coordinated supply of these factors is essential for sustaining ATP production and maintaining neuronal integrity. Additionally, the unique and highly specialized structure of neurons necessitates a continuous supply of nuclear-encoded mitochondrial proteins to support their extensive mitochondrial network [135].

Given the central role of mitochondria in cellular metabolism, it is unsurprising that metabolites function as mitochondrial messengers capable of communicating with the nucleus, thereby rewiring key bioenergetic and stress-response signaling pathways [136]. For instance, a decline in ATP production increases the AMP/ATP ratio, activating the AMPK pathway. This pathway regulates crucial metabolic and cellular processes, including lipid and glucose metabolism, mitochondrial dynamics, autophagy, and protein synthesis, while also inducing transcriptional changes to restore energy balance [137]. Similarly, the NAD+/NADH ratio reflects mitochondrial metabolic status and modulates various enzymatic reactions, influencing deacetylation processes mediated by sirtuins across multiple cellular compartments [138]. Oxygen availability is another critical factor, as mitochondria require it for ATP generation. Under hypoxic conditions, cells activate hypoxia-inducible factors (HIFs) via prolyl hydroxylase domain proteins, enabling adaptation of mitochondrial function to oxygen scarcity [139]. Furthermore, mitochondria-derived reactive oxygen species (ROS) serve not only as toxic by-products but also as essential signaling molecules, regulating hypoxic responses, immunity, and stem cell function. They may also contribute to longevity through HIF stabilization [140,141]. Additionally, metabolites produced within mitochondria, such as acetyl-CoA and tricarboxylic acid (TCA) cycle intermediates such as α-ketoglutarate, succinate, and fumarate act as signaling molecules that link energy production with gene expression and epigenetic modifications, orchestrating a comprehensive cellular response to metabolic status [142].

As previously mentioned, mitochondria become dysfunctional during aging. This dysfunction can trigger increased ROS production, mtDNA mutations, and impaired energy production, all of which might disrupt communication with the nucleus and further contribute to the aging process.

Mitochondria-nucleus contact sites, also referred to as nucleus-associated mitochondria (NAMs), represent a recently described form of inter-organellar communication, in which mitochondria are physically tethered to the nuclear envelope. These structures facilitate the local exchange of signals, proteins, and possibly metabolites, supporting the maintenance of cellular homeostasis, particularly under stress conditions.

These contacts are not randomly distributed but are mediated by conserved protein complexes that orchestrate their formation. In mammalian cells, components such as the translocator protein (TSPO) located on the mitochondrial outer membrane, and ACBD3 and PKA on the nuclear side are essential for establishing these tethers, suggesting a regulated and structured architecture. Additionally, ancient membrane pores like the voltage-dependent anion channel (VDAC) and nuclear pore complex-associated proteins have been implicated in bridging these organelles [143,144].

Functionally, mitochondria–nucleus contact sites play a crucial role in coordinating the mitochondrial retrograde response (MRR), a signaling cascade activated upon mitochondrial dysfunction. MRR promotes the transcription of nuclear genes involved in stress adaptation, mitochondrial biogenesis, and metabolic reprogramming, ultimately helping to restore cellular homeostasis [144].

In the CNS, where mitochondrial function is vital due to high energy demands and limited regenerative capacity, the role of NAMs may be particularly significant. These contacts could be essential in coordinating stress responses, regulating gene expression, and promoting neuronal survival and plasticity. Conversely, their disruption may compromise stress adaptation and contribute to the development or progression of neurodegenerative diseases. Although direct studies on NAMs within the CNS remain limited, altered mitochondrial dynamics and defective retrograde signaling, both potentially linked to NAM dysfunction, are well-established features of conditions such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis.

In summary, in the CNS, mitochondrial–nuclear communication is emerging as a key player in maintaining neuronal homeostasis by coordinating energy metabolism, redox balance, and stress responses. This bidirectional dialogue ensures the proper expression, transport, and function of mitochondrial proteins, thereby preserving neuronal integrity.

Recent research has highlighted the importance of specialized contact sites between mitochondria and other organelles, including the endoplasmic reticulum (ER) [145,146], lysosomes [147], and peroxisomes [148] in maintaining cellular homeostasis and regulating metabolic processes.

Mitochondria-Associated Membranes (MAMs), the contact points between mitochondria and the ER, are essential for calcium homeostasis, lipid transfer, and mitochondrial dynamics [145,149]. With aging, MAM function becomes compromised, leading to disrupted calcium signaling, increased ER stress, and mitochondrial dysfunction [149]. These alterations contribute significantly to the pathogenesis of age-related neurodegenerative diseases, such as Alzheimer's and Parkinson's [149].

Mitochondria–lysosome contacts (MLCs) have also emerged as critical regulators of mitochondrial quality control and cellular metabolism [147,150]. During neuronal aging, MLC efficiency declines, resulting in impaired mitophagy and the accumulation of dysfunctional mitochondria. This accumulation exacerbates oxidative stress and inflammation—hallmarks of aging in the CNS [147].

The interaction between mitochondria and peroxisomes is also affected by the aging process. These organelles cooperate in fatty acid oxidation and reactive oxygen species (ROS) metabolism [148,151]. Age-related alterations in mitochondria–peroxisome interactions lead to metabolic imbalances and increased oxidative stress in neurons, contributing to the progression of neurodegenerative disorders [151].

Mitochondria–organelle contact sites not only facilitate the exchange of lipids, calcium, and signaling molecules but also provide structural and spatial cues that influence mitochondria-derived vesicles (MDVs) biogenesis [152]. MDVs selectively remove damaged mitochondrial components for lysosomal degradation [153]. With age, MDV formation and trafficking become less efficient, leading to the accumulation of oxidized proteins and lipids within mitochondria [154]. This impairment is particularly relevant in Parkinson's disease, where mutations in PINK1 and PARKIN affect both mitophagy and MDV formation [153,154].

The disruption of these inter-organelle contacts during aging contributes to a cascade of cellular dysfunctions, including impaired calcium homeostasis, increased oxidative stress, accumulation of misfolded proteins, and compromised energy metabolism [145,147,148,149]. These changes collectively accelerate neuronal aging and increase susceptibility to neurodegenerative diseases.

Understanding the intricate relationships between mitochondria and other cellular organelles presents promising avenues for therapeutic interventions. Potential strategies include enhancing MAM formation, improving MLC efficiency, stimulating MDV production, and restoring proper mitochondria–peroxisome interactions [145,147,154]. Such approaches could help preserve mitochondrial function, reduce oxidative stress, and ultimately slow down the neuronal aging process.

In conclusion, mitochondrial inter-organelle contacts play a pivotal role in neuronal aging. Further research into these complex interactions will not only deepen our understanding of age-related neurodegeneration but also pave the way for novel therapeutic strategies to promote healthy brain aging.

Alzheimer's disease (AD) is the most common progressive neurodegenerative disorder worldwide, mainly affecting individuals over 65 years old [155].

The neuropathological hallmarks of AD include the abnormal accumulation and aggregation of amyloid-β (Aβ) peptides in the extracellular compartment of the CNS forming so-called senile plaques (SPs), alongside the intracellular deposition of neurofibrillary tangles (NFTs), containing hyperphosphorylated tau (p-tau) protein [156]. Importantly, both Aβ plaques and p-tau NFTs are neurotoxic, synergistically triggering progressive neuronal death and brain tissue loss in the CNS. This leads to profound cortical degeneration, with clinical symptoms including severe memory loss, cognitive decline, and behavioral abnormalities in patients [157].

Key contributors to the neurodegenerative process in AD include neuroinflammation, mitochondrial dysfunction, excitotoxicity, and oxidative stress. Among these, mitochondrial impairment has been proposed as an early metabolic alteration and a major feature of the disease, playing a pivotal role as both a driver and predisposing factor in the onset and/or progression of AD (Figure 2) [158].

More specifically, the aberrant accumulation of neuroinflammatory signals and p-tau NFTs in close proximity to mitochondria disrupts the mitochondrial membrane potential, triggering excessive production of reactive oxygen species (ROS). The resulting oxidative stress further compromises mitochondrial function [159], and triggers oxidation and denaturation of proteins, lipids, and nucleic acids within CNS cells [160]. This cascade exacerbates the accumulation of Aβ and p-tau, which, in turn, further induce mitochondrial dysfunction and oxidative stress, thus creating a vicious cycle (Figure 2).

To date, numerous studies have demonstrated significant alterations affecting the content of various proteins involved in mitophagy, mitochondrial dynamics, and mitochondrial biogenesis in AD [157]. In this section, we summarize the principal mitochondria-targeted therapeutic strategies for the treatment of AD (Figure 2):

Parkinson's disease (PD) is the most common neurodegenerative disorder of aging and the second most common brain aging disorder [182]. This syndrome is mainly characterized by the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc), thus leading to a dopamine deficiency in the basal ganglia. Lewy bodies are the, primary protein aggregates in PD that are mainly composed of toxic α-synuclein (α-syn) (or its phosphorylated form), ubiquitin, neurofilaments, and molecular chaperones, which are detrimental to neurons [183,184]. PD presents with a range of symptoms, including key motor impairments, such as bradykinesia, hypokinesia, resting tremor, muscle rigidity, and postural instability, as well as non-motor issues like autonomic dysfunction, sleep disturbances, depression, and cognitive decline. About 10% of cases are familial, indicating a genetic component, while the remaining 90% are sporadic, likely resulting from environmental factors or unknown genetic influences. Mitochondrial deficits and specifically reduced mitophagy are evident in both sporadic and familial PD.

In four sporadic cases of Parkinsonism, exposure to MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) led to approximately 60% and 65% deficiency in Complex I and Complex II, respectively, in dopaminergic neurons of the substantia nigra [185]. Several mitochondrial-related genes have been identified as being associated with PD in different meta-analyses published, such as RAB7L1, MCCC1, ACMSD, and LRRK2 [186].

On the other hand, several genes have been identified as being involved in causing familial PD, that are linked with mitochondrial dysfunction. Among them, the PRKN (Parkin) gene and PINK1 genes are the most common cause of autosomal recessive PD [187]. Loss of function of PRKN contributes to mitochondrial swelling, cytochrome c release, and caspase-3 activation, which triggers cellular death [188], while mutations in PINK1 have been correlated with reduced mitochondrial membrane potential, abnormal morphology, decreased Complex I and IV activity, impaired mitochondrial transport, reduced ATP, and increased ROS production, triggering apoptosis and deficient synaptic transmission (Figure 2) [189,190].

In this section, we summarize the principal mitochondrial-targeted therapeutic strategies for treatment, depending on the specific gene mutation of PD (Figure 2):

HD is a progressive, autosomal dominant, hereditary neurodegenerative disorder, globally affecting 5–10 individuals per 100,000 people, and is therefore the most common hereditary neurodegenerative disease [204]. HD is caused by a mutation in the huntingtin (HTT) gene on chromosome 4 [205] encoding the HTT protein, which plays a crucial role in the CNS contributing to several cellular functions, including neurogenesis, embryonic development, vesicle trafficking, autophagy regulation, autophagosome transport in neurons, gene expression, and cell signaling [206]. Importantly, HTT is also essential for mitochondrial functions, including mitochondrial transport, localization, fusion, fission, and maintenance of the mitochondrial membrane potential [207].

In HD, mutation in the HTT gene involves the amplification of a cytosine-adenine-guanine (CAG) trinucleotide repeat encoding the amino acid glutamine (Gln), in the first exon of the gene, with 36 or more repetitions. This expansion leads to the production of a mutant protein (mHTT9) with a long poly-glutamine (polyQ) stretch, resulting in protein misfolding and progressive accumulation and aggregation in the cytoplasm and nucleus of cells [208]. In particular, in the brain, the accumulation of mHTT interferes with mitochondrial functions, leading to bioenergetic deficits and an increase in oxidative stress, causing neuronal damage, impaired calcium homeostasis, and altered mitochondrial dynamics [209]. Furthermore, mHTT disrupts synaptic morphology and function, as well as neuronal proteostasis, ultimately contributing to neuronal loss and nerve damage [210].

Given that HD affects different regions of the brain, its typical clinical manifestations include motor and cognitive disorders, as well as psychiatric disturbances. Additionally, motor abnormalities resulting from striatal dysfunction, which is characterized by progressive, involuntary dance-like movements, are also observed [211].

In recent years, an increasing body of evidence has highlighted the critical role of mitochondrial dysfunction in the etiology of HD, making it a significant study area for possible therapeutic approaches. However, the precise pathogenic pathways involved have not yet been thoroughly elucidated, even though the neurotoxic effects of mHTT are mainly mediated by the induction of aberrant mitochondrial energy metabolism and HD-related neuronal dysfunction.

In this section, we describe the principal mitochondrial-targeted therapeutic strategies for the treatment of Huntington's disease (Figure 2):A.Enhancing mitochondrial biogenesis by increasing PGC-1α: Research evidence indicates that mutant huntingtin (mHTT) directly interacts with the promoter region of PGC-1α and interferes with the CREB/TAF-dependent transcriptional pathway, leading to reduced transcriptional activity. This downregulation impairs the expression of key downstream targets, including mitochondrial transcription factor A (TFAM), thereby decreasing biogenesis and contributing to mitochondrial dysfunction, increased oxidative stress susceptibility, and progressive neuronal degeneration [212]. Additionally, mHTT-mediated suppression of PGC-1α affects genes critical for the maintenance of neuromuscular junction stability [213]. Oral administration of bezafibrate restored levels of PGC-1α and downstream targets such as NRF-1 and TFAM, showing improvements in an HD mouse model by increasing mitochondrial biogenesis and reducing lipid accumulation [209,214].B.Enhancing mitophagy through PINK activation: Research studies have demonstrated that mHTT is also involved in the process of mitophagy. mHTT inhibits the commitment of mitophagy receptors, such as p62 and OPTN, to damaged mitochondria and impairs their interaction with LC3, thereby blocking autophagosome formation and disrupting mitophagy initiation [207]. mHTT also impairs Parkin recruitment to mitochondria and inhibits PINK1 accumulation on the outer mitochondrial membrane [207,215]. This results in defects in mitophagy, where a large number of dysfunctional mitochondria are not removed in time and excessively accumulate in neuronal cells, inducing a vicious cycle [216]. It was observed that the overexpression of PINK1 in a Drosophila model of HD resulted in the amelioration of Parkin-mediated mitophagy defects and the attenuation of mHTT-induced neurotoxicity, promoting neuronal activity and protecting neuronal integrity [215]. Consequently, mitophagy serves to protect neurons from damage in HD, and a deficiency in this process contributes to further malignant deterioration of HD pathology [157].C.Decreasing mitochondria fragmentation through Drp1 inhibition: The accumulation of mHTT in neuronal cells in the brains of HD patients has been shown to induce mitochondrial hyper-fragmentation and to affect the mitochondrial respiratory chain complex by disrupting the balance of mitochondrial dynamics, which, in turn, induces impairment of mitochondrial function [217]. Moreover, in the brains of HD patients, the expression levels of the mitochondrial fission/fusion proteins Drp1, Fis1, and Mfn are significantly altered [218]. Sawant et al. demonstrated that mHTT is able to bind to the proteins Mfn and Drp1. This binding enhances the activity of Drp1 and, together with a downregulation of fusion proteins such as Mfn1/2 and OPA1, results in mitochondrial fusion and fission imbalances. Fragmented mitochondria are less efficient at ATP production and produce more ROS. Moreover, this fragmentation triggers mitochondrial distribution abnormalities and mitochondrial axonal transport defects, which ultimately lead to impaired synaptic plasticity [219]. Coherently, inhibition of Drp1 has been demonstrated to restore mitochondrial and neuronal dysfunction in mHTT-induced HD animal models. In particular, treatment with CHIR99021 stabilizes calpastatin (CAST), thereby inhibiting calpain activation and preventing Drp1-mediated mitochondrial fragmentation, reducing HD-associated neuropathology and behavioral defects [157,220].

Amyotrophic lateral sclerosis (ALS) represents the most prevalent subtype of motor neuron diseases (MNDs), a group of neurodegenerative disorders characterized by the involvement of both upper and lower motor neurons. ALS is a relentlessly progressive condition marked by the degeneration and eventual loss of motor neurons, leading to increasing muscle weakness and, ultimately, respiratory compromise [221].

Among the key pathological features of ALS, mitochondrial dysfunction plays a central role in disease onset and progression. Mitochondrial impairments in ALS trigger altered dynamics, defective mitophagy, impaired bioenergetics, increased oxidative stress, and defective biogenesis (Figure 2). Several ALS-linked genes are directly involved in these mitochondrial processes [222,223]. The most frequently implicated include the hexanucleotide repeat expansion in the chromosome 9 open reading frame 72 (C9ORF72), Tank-binding kinase 1 (TBK1), superoxide dismutase 1 (SOD1), TAR DNA-binding protein 43 (TARDBP, encoding TDP-43), fused in sarcoma (FUS), and CHCHD10. Different strategies are known to ameliorate disease progression targeting mitochondria-related genes (Figure 2):A.Promoting mitophagy: In ALS, ninety distinct mutations in TBK1 are linked to ALS and frontotemporal dementia, including missense mutations that disrupt TBK1's ability to dimerize and associate with the mitophagy receptor optineurin (OPTN), thereby impairing autophagy/mitophagy initiation and mitochondrial clearance. Additionally, mutations in C9ORF72 disrupt endosomal and autophagic trafficking, leading to defective clearance of damaged mitochondria and accumulation of dysfunctional mitochondria [224]. Moreover, TDP-43 and FUS aggregates sequester essential mitochondrial proteins and impair mitochondrial transport along axons, further compounding mitophagy defects [225,226]. While urolithin A has shown promise as treatment for improving motor dysfunction by activating mitophagy [227], rapamycin, which also increases mitophagy by inhibiting mTOR, only showed positive effects in decreasing neuroinflammation in clinical studies, even though preclinical studies have shown reduced TDP-43 aggregation and improved motor neuron function in ALS [228]. The same has been observed with trehalose, another autophagy activator, for which preclinical studies showed promising results, but in large-scale trials did not demonstrate efficacy in patients [229]. However, trehalose ameliorates the pathogenesis of some subgroups, suggesting that more personalized research is needed.B.Reducing fragmentation though downregulation of Drp-1: Mutant SOD1, TDP-43, and FUS mislocalize to mitochondria and promote excessive fission via Drp1 upregulation and OPA1/Mfn2 downregulation [230,231]. On the other hand, CHCHD10 mutations affect cristae structure and fusion [232]. Excessive Drp1-mediated mitochondrial fission contributes to mitochondrial fragmentation, which impairs ATP synthesis and facilitates cytochrome c release, triggering apoptotic pathways and motor neuron death. Suppression of the Drp-1 cascade prevents ALS-related symptoms [233].C.Reducing oxidative stress and ROS production: Mitochondria in ALS show increased ROS production, partly due to SOD1 mutations, which impair detoxification of superoxide radicals, and TDP-43, which inhibits mitochondrial antioxidant responses. Increased mitochondrial ROS also trigger activation of the mitochondrial permeability transition pore (mPTP), promoting mitochondrial swelling and release of pro-apoptotic factors [234,235]. Targeting ROS with antioxidants such as edaravone, which is FDA-approved for ALS [236], helps scavenge ROS, thereby reducing oxidative stress and protecting mtDNA and mitochondrial function.D.PGC-1α activation: ALS models show reduced PGC-1α levels and downstream regulators such as NRF1 and TFAM, limiting mitochondrial renewal and repair [213], which further exacerbates energy failure and oxidative stress. PGC-1α also regulates the expression of key antioxidant enzymes and mitochondrial fusion/fission proteins; thus, its reduction amplifies mitochondrial vulnerability in ALS [237]. Activation of PGC-1α by resveratrol ameliorates mitochondrial function in ALS animal models [238].

Nevertheless, the majority of sporadic ALS cases do not involve mutations in these known ALS-associated genes. Emerging evidence suggests that genetic predispositions, environmental exposures, for example, heavy metals or organic chemicals such as pesticides and solvents, traumatic brain injury, as well as lifestyle and age-related factors, may all contribute to the etiology of ALS [223] but more research is needed to unravel the molecular pathogenesis underlying these factors.