Mitochondrial dysfunction in the pathogenesis of Parkinson disease: current concepts and potential therapeutic strategies

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Introduction

Parkinson disease (PD) is one of the most common neurodegenerative disorders. PD manifests clinically through typical motor and non-motor symptoms. Motor symptoms include resting tremor, bradykinesia, muscular rigidity, hypomimia, and postural instability. Non-motor symptoms, which usually precede motor symptoms during the prodromal period, include cognitive impairment, hyposmia, insomnia, constipation, and depression. The prodromal period lasts for 5–15 years [1]. The clinical symptoms of PD are associated with the death of dopaminergic neurons in the compact region of the substantia nigra in the midbrain. Motor symptoms appear only after 50–80% of these neurons have died. PD is pathologically characterized by the accumulation of Lewy bodies, which are round, eosinophilic intracellular inclusions primarily composed of aberrant alpha-synuclein [2]. The growing elderly population and improved medical care for patients with PD are expected to increase the PD prevalence in the next 20–30 years. It is estimated that the total number of PD patients worldwide will rise from 4.1 million in 2005 to 8.7 million in 2030. This increase will put intense strain on healthcare systems in many countries [2]. The epidemiological parameters of PD prevalence and incidence vary considerably by Russian region. Tomsk has the highest prevalence (238 cases per 100,000 people), whereas Moscow has the lowest prevalence (27 cases per 100,000 people). The incidence rates also vary widely. The lowest incidence rate is reported in Karelia (1.88 cases per 100,000 people per year). The highest rate is reported in the Solnechnogorsky district in the Moscow Oblast (16.3 cases per 100,000 people per year) [3]. In addition, symptomatic therapy becomes less effective as patients’ conditions worsen. Currently, there are no treatments that can prevent the onset or progression of PD. Understanding the pathogenesis of PD is critical for the development and clinical implementation of novel, highly effective therapeutic strategies in the near future.

PD is considered a multisystem and multifactorial disease that can be initiated by various etiological factors, including genetic, biological, and environmental factors [3]. From a pathophysiological perspective, familial forms of PD are classified as genetic diseases with Mendelian inheritance patterns. Sporadic forms of PD, which account for 85–90% of cases, belong to multifactorial diseases, meaning that they are genetically predisposed [4]. Sporadic forms have a specific genetic predisposition, but its penetrance depends on environmental factors that induce and potentiate PD development. In recent years, tremendous progress has been made in understanding the molecular basis of PD pathogenesis, leading to various theories. Pathogenetic factors include disturbances in both apoptotic and non-apoptotic programmed neuronal cell death, as well as aberrant autophagy regulation, endoplasmic reticulum dysfunction, and elevated intracellular calcium levels. However, their exact role in neuronal degeneration is still being investigated [5].

Recently, the pathogenetic role of mitochondria in PD has been actively studied. This is because neurons have a complex mitochondrial network extending from the soma to the synaptic terminals. These terminals relay information from one neuron to another. Mitochondria perform many functions, such as adenosine triphosphate (ATP) generation, calcium buffering, and epigenetic neuronal signaling. Unlike many other cell types, neurons have higher bioenergetic needs. For example, ionic homeostasis requires ATP, which is constantly consumed to generate transmembrane ionic fluxes, sequester neurotransmitters into vesicles, facilitate vesicle fusion during synaptic activity, and mediate reuptake during vesicular recycling. ATP is also necessary to maintain and restore a large pool of neurotransmitters. ATP is synthesized for these processes within the mitochondria. Therefore, mitochondrial dysfunction is considered an essential part of PD pathogenesis [6]. This review focuses on the latest advances in understanding how mitochondrial dysfunction contributes to the development of sporadic and familial forms of PD.

Mitochondrial Dysfunction in the Pathogenesis of Sporadic Parkinson Disease

An electron transport chain (ETC) within mitochondria is the primary source of reactive oxygen species (ROS) in eukaryotic cells. The ETC sequentially reduces molecular oxygen to water. Complexes I and III produce a small amount of superoxide (O₂^–) during this process. Superoxide is formed in the mitochondria and can be converted into hydrogen peroxide by an manganese superoxide dismutase. However, under certain conditions ROS production may exceed the cellular antioxidant capacity. This state, called oxidative stress, can cause irreversible damage to cellular macromolecules and lead to cell death. Markers of oxidative stress, such as oxidized lipids, proteins, and DNA, are found at high concentrations in patients with PD [6]. In addition, patients at risk for PD, including those with frequent constipation, impaired olfactory function, anxiety and depressive thoughts, and sleep behavior disorders, have higher levels of oxidative stress than those at no risk. Increased oxidative stress is one of the effects of complex I deficiency observed in sporadic PD. This is supported by data from a study by Esteves et al., which showed increased oxidative stress and reduced complex I activity in neuronal cells of patients with PD compared to healthy individuals [7].

A major breakthrough in understanding the PD pathogenesis was achieved through the analysis of specific cases of induced parkinsonism in California during the 1980s. Langston et al. (1983) identified individuals with histories of intravenous substance abuse who had inadvertently injected 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a synthetic analog of heroin [6]. They developed parkinsonism within a few days. An autopsy showed significant damage to dopaminergic neurons in the substantia nigra, as well as typical alpha-synuclein inclusions. MPTP easily crosses the blood-brain barrier and is taken up by astrocytes, where it is metabolized into 1-methyl-4-phenylpyridinium (MPP⁺) and released into the extracellular space. MPP⁺ is a substrate of the dopamine transporter that is selectively taken up by dopaminergic neurons, where it inhibits mitochondrial complex I. After inhibiting complex I, the excess superoxide suppresses the antioxidant capacity of dopaminergic neurons, leading to their death [6]. MPP⁺ is toxic to dopaminergic neurons in humans, primates, and rodents. Therefore, MPTP is recommended for use in animal models of parkinsonism by Guidelines for Preclinical Studies of Medicinal Products by the Scientific Center for Expert Evaluation of Medicinal Products.¹ However, MPTP models of PD have some limitations. For example, MPTP experiments rarely result in the formation of Lewy bodies. MPTP induces acute or subacute neurodegeneration, which is differs from the chronic neurodegenerative process in PD. In addition, MPTP-induced parkinsonism models often fail to demonstrate the motor impairments characteristic of PD [8–10]. In experimental settings, other inhibitors of mitochondrial complex I, such as rotenone and annonacin, as well as other pesticides affecting mitochondria (paraquat, maneb, dieldrin, heptachlor, and atrazine), induce pathological, biochemical, and behavioral changes typical of PD [11, 12].

Another molecular hypothesis suggests that the mitochondrial defects in PD result from the accumulation of point mutations in mitochondrial DNA (mtDNA). In eukaryotic cells, mtDNA is organized into nucleoids, which consist of proteins and nucleic acids. Each nucleoid contains an average of 1.4 million copies of mtDNA, and cells can contain up to 2,000 nucleoids [6]. mtDNA is circular and encodes 13 proteins, as well as mitochondrial transfer RNA (tRNA) and ribosomal RNA (rRNA) [6]. The proteins encoded by mtDNA include subunits from all parts of the ETC. Six of these genes encode subunits of complex I [6]. Therefore, point mutations in any of six genes can affect the activity of complex I. Therefore, mitochondria are involved in the pathogenesis of Parkinson-like syndromes. Mitochondrial dysfunction has been reported in the neurons of the substantia nigra, as well as in the myocytes, platelets, lymphocytes, and fibroblasts of patients with PD. These findings support the idea that mitochondrial dysfunction is not limited to neurons alone and is an important feature of PD’s multisystem nature.

Alpha-synuclein, a characteristic marker of PD, binds to voltage-dependent anion-selective channel 1 (VDAC1), translocase of the outer membrane (TOM) 40, and TOM20, thereby mediating mitochondrial dysfunction [13, 14]. In sporadic PD, reduced levels of VDAC1 are found in neurons of the substantia nigra due to alpha-synuclein aggregation, which contributes to mitochondrial dysfunction [15]. In addition, alpha-synuclein activates a channel that depolarizes the mitochondrial membrane, leading to mitochondrial fragmentation and degradation. The aggregation of alpha-synuclein disrupts proteostasis, impairing function and transport within the endoplasmic reticulum, the Golgi apparatus, and the autophagolysosomal system. This process destabilizes organelle connections and contributes to mitochondrial dysfunction. Oxidative stress is closely related to mitochondrial dysfunction. Mitochondria produce up to 90% of cellular ROS [16]. Synucleinopathy, oxidative stress, and mitochondrial dysfunction appear to form a vicious cycle in the pathogenesis of sporadic PD [17]. The accumulation of iron in the substantia nigra of patients with sporadic PD can also cause increased ROS production and enhanced alpha-synuclein aggregation [15, 18]. Mitochondria actively exchange iron with the cytoplasm. This exchange is necessary for the synthesis of various enzyme systems that are integral components of mitochondrial complexes I and III [18]. The inhibition of complex I by rotenone, MPTP, and paraquat leads to iron accumulation and induces the development of PD [19]. Inhibition of the ubiquitin-proteasome system also causes an imbalance of iron in cells, further enhancing ROS generation and alpha-synuclein aggregation [20].

Mitochondrial Dysfunction in the Pathogenesis of Autosomal Dominant Parkinson Disease

Alpha-synuclein was first associated with PD due to its presence in Lewy bodies. Subsequently, the SNCA gene, which encodes alpha-synuclein, was identified as the first gene responsible for autosomal dominant PD [21]. Alpha-synuclein is a small polypeptide consisting of 140 amino acids. It mediates neurotransmitter release in presynaptic terminals and interacts with the membranes of various organelles, including mitochondria. Mullin et al. discovered that alpha-synuclein is present in mitochondrial membranes, where it directly influences the structure and function of organelles [22]. Both in vitro and in vivo models have demonstrated that SNCA gene mutations (A53T, E46K and H50Q) lead to the production of a defective protein, resulting in mitochondrial fragmentation and excessive ROS production [23]. Alpha-synuclein is normally located in a specialized structure called the mitochondria-associated endoplasmic reticulum membrane (MAM). The MAM acts as a barrier between the endoplasmic reticulum and the mitochondria and plays a critical role in regulating calcium signaling and apoptosis. Negative mutations in the SNCA gene decrease alpha-synuclein binding to MAM and increase mitochondrial fragmentation. These findings suggest that alpha-synuclein is involved in the regulation of mitochondrial morphology [14, 24]. Overexpression of mutant alpha-synuclein causes the dissociation of mitochondria from MAM, disrupting calcium exchange and reducing mitochondrial energy production [25]. Ryan et al. discovered that alpha-synuclein directly influences mitochondrial morphology and biogenesis by regulating the PGC1α receptor [26].

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene, which encodes a protein called dardarin, are a frequent cause of autosomal dominant and familial forms of PD [27]. LRRK2 is a multifunctional protein kinase, where PD-associated mutations lead to increased kinase activity. Increased mitochondrial sensitivity to toxins, defects in mitochondrial homeostasis, and increased ROS production have been demonstrated in experimental animal models with mutant LRRK2 associated with PD [23]. Studies have shown that the G2019S mutation in the LRRK2 gene is associated with mitochondrial abnormalities in dopaminergic neurons of the substantia nigra in patients with PD [26], as well as in PD mouse models [27].

Several proteins interact with LRRK2, mediating pathological effects on mitochondria. For example, the mitochondrial fission protein, dynamin-related protein 1 (DRP1), acts as an effector of mitochondrial fragmentation through LRRK2-mediated phosphorylation [28]. In addition, LRRK2 appears to interact with other mitochondrial fission proteins, including mitofusin and the dynamin-like protein [29]. Excessive activity of uncoupling proteins 2 and 4 may cause increased proton leakage and LRRK2-mediated loss of mitochondrial membrane potential [30]. The G2019S mutation in the LRRK2 gene impairs the proteasomal degradation of an outer mitochondrial membrane protein, which connects mitochondria to microtubule motor proteins, contributing to defective mitophagy [31].

Recently, a link was discovered between PD and the vacuolar protein sorting 35 (VPS35) gene in European patient cohorts with a family history of PD, suggesting autosomal dominant transmission [27, 32]. VPS35 is an essential component of a complex that mediates the retrograde delivery of substances from endosomes to the Golgi apparatus and the recycling of substances from endosomes to the cell surface (33). Early studies showed that PD-associated mutations in the VPS35 gene made cells more vulnerable to the mitochondrial toxin MPP⁺ in vitro [34]. The primary function of VPS35 in mitochondria appears to be the regulation of mitochondrial dynamics through interaction with fission and fusion proteins. Recent studies have demonstrated that the mutant VPS35 protein may cause mitochondrial fragmentation and lead to neurodegeneration [14]. This may occur through decreased degradation of the E3 ubiquitin ligase-1 protein, which increases mitofusin degradation [35], or by enhancing DRP1 complex turnover through vesicle-mediated transport from mitochondria to lysosomes [36]. In addition, the D620N mutation in the VPS35 gene is shown to increase mitochondrial fragmentation, disrupting the assembly and activity of complex I [37].

Another gene, CHCHD2 (coiled-coil-helix-coiled-coil-helix domain-containing 2), has mutations that have been identified as a potential cause of late-onset, autosomal dominant PD in three Japanese families [38]. This gene produces a protein containing a coiled-coil-helix-coiled-coil-helix domain, which is found in the intermembrane space of mitochondria and the cell nucleus. CHCHD2 is usually found in mitochondria and associated with complex IV. The under-expression of CHCHD2 suppresses the activity of complex IV, which leads to increased ROS production and mitochondrial fragmentation [39]. It should be noted that CHCHD2 translocates into the nucleus and acts as a transcription factor under stress conditions, thereby regulating the expression of subunit 4 isoforms of mitochondrial complex IV [40]. In addition, Drosophila with low CHCHD2 [41] expression or PD-linked CHCHD gene mutations exhibit mitochondrial structural and biochemical abnormalities resulting in dopaminergic neurodegeneration in the substantia nigra and motor dysfunction. These findings strongly suggest that the CHCHD2 gene mutation leads to nigrostriatal neurodegeneration and the development of PD specifically due to mitochondrial dysfunction.

Mitochondrial Dysfunction in the Pathogenesis of Autosomal Recessive Parkinson Disease

The most common cause of autosomal recessive PD is mutations (>120) in the Parkingene, which encodes the Parkin protein. The Parkin protein is a cytosolic E3 ubiquitin ligase that attaches ubiquitin to target proteins for signaling or proteasomal degradation. Parkin primarily functions in association with mitochondria. Parkin-deficient models show severe defects in mitochondrial morphology and function [43]. The E3 ubiquitin ligase plays different functions in maintaining mitochondrial homeostasis and regulating mitochondrial biogenesis and mitophagy. Mitophagy is the process by which dysfunctional mitochondria are removed from a healthy mitochondrial pool and degraded using the autophagolysosomal pathway [44]. During the early stages of mitochondrial degradation, Parkin is recruited to damaged or dysfunctional mitochondria and activated by kinase 1, leading to ubiquitylation of proteins and subsequent proteasomal degradation [14]. Pickrell et al. demonstrated a defect in Parkin-mediated mitophagy in distal neuronal axons in a rodent model of age-related dopaminergic neurodegeneration accompanied by PD symptoms [45]. These findings further highlight the pathophysiological importance of Parkin-mediated mitophagy in PD, as opposed to data from in vitro studies. Apart from its role in mitophagy, Parkin maintains a functional mitochondrial pool by regulating biogenesis [43]. Parkin normally mediates the degradation of peroxisome proliferator-activated receptor gamma coactivator (PGC1α), which leads to its translocation to the nucleus and the activation of transcription of mitochondrial-related genes [46]. Therefore, Parkin dysfunction suppresses mitochondrial biogenesis, resulting in a decrease in the number and function of mitochondria [47]. These results also highlight a key role of Parkin in maintaining the balance between mitochondrial biosynthesis and biodegradation.

Mutations in the PINK1 gene are the second most common cause of early-onset, autosomal recessive PD [48]. PINK1 (PTEN-induced putative kinase 1) is a serine-threonine kinase associated with mitochondria and plays a critical role in maintaining mitochondrial homeostasis. PINK1 is shown to enhance mitochondrial fission by increasing the activation of protein kinase A [49] and to modulate mitochondrial biogenesis by regulating Parkin-dependent degradation [50]. A defective PINK1 gene impairs mitochondrial function, resulting in the destruction of mitochondria. Mitophagy is the most widely studied function of PINK1 [45, 51]. PINK1 activates Parkin through two mechanisms: direct phosphorylation [52] and transactivation via the phosphorylation of ubiquitin, followed by Parkin binding [51, 53, 54]. In addition, PINK1 can mediate mitophagy independently of Parkin by recruiting nuclear dot protein and optineurin [55]. Like LRRK2, PINK1 promotes mitophagy by arresting mitochondrial transport through phosphorylation, followed by proteasomal degradation [56]. Experiments on Drosophila and mice have shown that partially inhibiting PINK1 results in various mitochondrial dysfunctions. This is primarily due to the loss of PINK1/Parkin-mediated mitophagy. However, PINK1 regulates mitochondrial homeostasis through another mechanism [43]: a PINK1 deficiency leads to mitochondrial calcium overload [57] and a decrease in mitochondrial complexes I and III [58].

A rare form of autosomal recessive juvenile PD, Kufor–Rakeb syndrome, is caused by mutations in the ATP13A2 gene [59]. This gene encodes a P5B-type ATPase that mainly localizes to the endolysosomal compartment. Although ATP13A2 is believed to transport cations across organelle membranes [59], its transport activity is not fully defined. However, a loss of ATP13A2 in cells in patients with PD reveals increased sensitivity to Zn²⁺ and Mn⁺, suggesting a critical role for ATP13A2 in maintaining the balance of these trace elements [14, 59]. The association between the ATP13A2 gene and mitochondrial dysfunction was first identified in skin fibroblasts from patients with mutations in this gene [14, 60]. Grünewald et al. [60] and Ramonet et al. [61] demonstrated mitochondrial dysfunction, characterized by reduced ATP production, increased mitochondrial fragmentation, and elevated ROS production, in a model of ATP13A2-deficient cells [60, 61]. Park et al. proposed that ATP13A2 has a broader impact on cellular bioenergetics. They found that loss of ATP13A2 worsens glycolysis and causes more severe mitochondrial dysfunction [62]. In addition, the literature describes ATP13A2 mutations that cause Zn²⁺ homeostasis disruption due to an imbalance in vesicular sequestration, resulting in mitochondrial dysfunction [61]. Disruption in Zn²⁺ metabolism can also cause lysosomal dysfunction [63] and contribute to defective mitophagy. This highlights the complex relationship between interconnected intracellular processes in PD pathogenesis.

Potential Therapeutic Strategies

Since mitochondrial dysfunction plays a significant role in the development of PD, novel pathogenetic approaches are needed to treat PD. Various strategies are being developed to improve mitochondrial function in both familial and sporadic forms of PD. An effective approach to treating PD appears to involve targeting the mitophagy process in defective mitochondria. Increasing the activity of the cytosolic ubiquitin ligase E3 (Parkin) by nilotinib, which inhibits phosphorylation, is shown to provide a neuroprotective effect [64]. Inhibition of the deubiquitinating enzymes increases Parkin-mediated mitophagy because a ubiquitin-specific peptidase counteracts Parkin’s effects, whereas inhibition of the ubiquitin-specific peptidase increases mitochondrial degradation [14, 65]. In addition, activation of mitophagy in PD may create alternative conditions that restore mitochondrial function. Hamacher-Brady et al. demonstrated that the proteins FUNDC1 (FUN14 domain containing 1) and Ambra1 (autophagy and Beclin 1 regulator 1) are able to modulate mitophagy independently of PINK1 or Parkin activity [66]. However, it has been discovered that Nip3-like protein-mediated mitophagy [14, 67] restores mitochondrial function and prevents neurodegeneration in case of Parkin or PINK1 deficiency, which justifies this mechanism as a new potential target for PD treatment [14].

Another neuroprotection strategy involves increasing mitochondrial biogenesis. Hayashi et al. demonstrated in animal experiments and human subjects that dimethyl fumarate (BG-12) increases mitochondrial biogenesis through the transcription factor NRF2 (nuclear factor erythroid 2-related factor 2) [68]. Phase III clinical trials demonstrated the efficacy of BG-12 in treating relapsing multiple sclerosis [69], and the agent was approved for patient use. These findings highlight the potential use of BG-12 in PD treatment. Other activators of the NRF2-mediated pathway include synthetic triterpenoids, which have demonstrated protective effects on dopaminergic neurons against MPTP [70]. Johri et al. reported that PGC-1α, a powerful inducer of mitochondrial biogenesis, might be another candidate for a target in PD treatment [71]. Other studies on animal models of neurodegeneration have demonstrated that bezafibrate [71] and quercetin [72] increase mitochondrial numbers. These findings open up opportunities for developing new PD treatment strategies.

Conclusion

A literature review has revealed the significant role of mitochondrial dysfunction in PD development. Both exogenous environmental factors and endogenous factors, such as genetic aberrations characteristic of familial forms of PD, can lead to mitochondrial dysfunction. Mitochondria are affected by these etiological factors directly and indirectly, through the activation or inhibition of secondary messenger systems. Pathogenetically, mitochondrial dysfunction can arise from either defective mitophagy or disrupted mitochondrial biogenesis. Therefore, new treatment strategies for PD should aim to enhance mitophagy of defective mitochondria or increase the biogenesis of new mitochondria.

About the authors

Natalia G. Zhukova

Siberian State Medical University

Email: sayfutdinxodjaev2002@gmail.com
ORCID iD: 0000-0001-6547-6622

Dr. Sci. (Med.), Professor, Professor, Department of neurology and neurosurgery

Russian Federation, 2, Moskovsky tract, Tomsk, 634050

Julia V. Kolobovnikova

Siberian State Medical University

Email: sayfutdinxodjaev2002@gmail.com
ORCID iD: 0000-0001-7156-2471

Dr. Sci. (Med.), Associate Professor, Dean, Faculty of medicine and biology, Head, Department of normal physiology, Professor, Department of pathophysiology

Russian Federation, 2, Moskovsky tract, Tomsk, 634050

Zaynutdinkhuzha F. Sayfitdinkhuzhaev

Siberian State Medical University

Author for correspondence.
Email: sayfutdinxodjaev2002@gmail.com
ORCID iD: 0009-0007-2184-2708
SPIN-code: 9065-3080

research assistant, Department of scientific and educational laboratory of cognitive neurophysiology of psychosomatic relationships

Russian Federation, 2, Moskovsky tract, Tomsk, 634050

References

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