Benefits of Maintaining Mitochondrial Health in Multiple Sclerosis

Vitória Carvalho Troitiño^1,3,a, Natália Pressuto Pennachioni^1,3,a, Natália Gabriele Hösch¹, Gisele Picolo¹, Vanessa Olzon Zambelli^1,2*

¹Laboratory of Pain and Signalling, Butantan Institute, Sao Paulo - SP, Brazil

²Centre of Excellence in New Target Discovery, Butantan Institute, Sao Paulo – SP, Brazil

³Institute of Biomedical Sciences, University of Sao Paulo, Sao Paulo – SP, Brazil

^aThese authors have contributed equally to this work, and share first authorship

Abstract

Impaired mitochondrial functions are implicated in the pathogenesis of the neurodegenerative process of Multiple Sclerosis (MS). MS is an autoimmune disease that leads to neurodegeneration, axonal loss, and demyelination in the brain and spinal cord white and grey matter areas. The existing treatments for this autoimmune disease primarily target acute neuroinflammation episodes. While this strategy effectively reduces relapse occurrences, it has limited effectiveness in protecting against neurodegeneration, especially in the progressive stages of MS. In this mini review we highlight studies showing how misfunctioned mitochondria may contribute to MS pathogenesis. We also bring evidence that targeting mitochondria may open new perspectives for future prevention of neuroinflammation and neuronal loss in MS.

Introduction

Strong evidence has emerged to implicate disturbed mitochondrial functions as central pathological components of neurodegenerative disorders, such as Multiple Sclerosis (MS). MS is a chronic autoimmune and inflammatory disease of the central nervous system (CNS) with no known cure. Although its exact onset remains unclear, MS is triggered by an autoimmune response that targets myelin content and oligodendrocytes, leading to neurodegeneration, axonal loss, and demyelination in the brain and spinal cord white and grey matter areas¹. As a consequence, patients experience varying degrees of motor deficits, coordination disorders, and sensory alterations. Symptoms differ among individuals but often include physical, psychological, and social challenges such as fatigue, cognitive impairments, bladder dysfunction, limb weakness, ataxia, and pain^2,3. Three distinct clinical forms of the disease can be observed: primary progressive (PPMS), secondary progressive (SPMS), and relapsing-remitting (RRMS). Each form presents varying degrees of acute and chronic inflammation as well as neurodegeneration, with the latter occurring even in the early stages of the disease⁴.

Current biological knowledge suggests that autoreactive T-lymphocytes primarily drive this autoimmune condition, targeting self-antigens in the CNS. A disruption of the blood-brain barrier (BBB), caused by pro-inflammatory cytokines and chemokines, increases its permeability, allowing the influx of immune cells into the CNS and triggering the activation and proliferation of resident glial cells^5,6. T helper 17 (Th17) cells, which release pro-inflammatory cytokines such as interleukin (IL)-17, and Th1 cells, which produce lymphokines like interferon-γ (IFN-γ) and interleukin-2 (IL-2), play crucial roles in MS development. B cells have also been recognized as relevant players in this process, contributing to antibody-dependent and antibody-independent mechanisms in the inflammatory process³.

Mitochondria are highly complex organelles originating from the endosymbiosis of a prokaryote that was once a free-living organism. They are composed of a DNA (mtDNA), outer and inner membranes, an intermembrane space, and a matrix. The outer membrane plays a critical role in maintaining the structural integrity of the organelle, while the inner membrane is extensively folded into structures known as cristae. These folds significantly increase the surface area, enhancing its capacity for adenosine triphosphate (ATP) production⁷. Although mitochondria possess their own genome, only a limited number of genes encoded by mtDNA are essential for the respiratory machinery; several mitochondrial proteins are encoded by nuclear DNA⁸.

Mitochondria are essential organelles responsible for generating energy in eukaryotic cells through a process known as oxidative phosphorylation (OxPhos). Briefly, the mitochondria inner membrane comprises five respiratory chain complexes, which integrate the electron transport chain (mETC). These complexes transfer electrons to oxygen, the terminal electron acceptor, a process that establishes an electrochemical gradient by moving protons across the membrane. This gradient drives protons back through ATP synthase in Complex V, a process that generates ATP⁹.

Beyond ATP production, mitochondria play central roles in a wide range of physiological processes, including intracellular calcium regulation, biological aging, apoptosis, β-oxidation of fatty acids, heme biosynthesis, neurotransmitter synthesis and inactivation, as well as the production and removal of reactive oxygen species (ROS)^8,10. This organelle is the main source of ROS, which are byproducts of mitochondrial metabolism generated by the complexes of the respiratory chain. Under physiological conditions, these reactive molecules are neutralized by antioxidant enzymes, such as superoxide dismutase, maintaining cellular homeostasis and preventing oxidative damage¹¹. However, under pathological conditions, the redox balance can be disrupted, leading to excessive ROS production and oxidative stress. This state is characterized by damage to cellular components, including DNA, proteins, and lipids¹². Furthermore, the imbalance between ROS production and clearance adversely affects mitochondrial function, resulting in impaired ATP production and exacerbating cellular dysfunction^12,13.

Although mitochondria may appear as isolated organelles, they form a dynamic and interconnected network undergoing fusion and fission. These processes are crucial for maintaining the shape, number, and size of mitochondria. The balance between fusion and fission enables the organelle to respond effectively to metabolic and environmental changes⁸. Fusion refers to the process where two mitochondria merge their outer membranes to form a single, larger mitochondrion. This process is mediated by GTPases, such as mitofusins (MFN1 and MFN2) and optic atrophy protein 1 (OPA1). Conversely, fission involves the division of one mitochondrion into two smaller mitochondria, a process regulated by dynamin-related protein 1 (DRP1) and mitochondrial fission protein 1 (Fis1). Fission plays a key role in mitophagy, facilitating the removal of damaged mitochondria, as well as in mitochondrial transport to specific cellular regions. Fusion, on the other hand, supports the exchange of mitochondrial DNA and metabolites, contributing to mitochondrial complementation—a compensatory mechanism that mitigates dysfunction in mitochondria with mutated DNA—and the maintenance of oxidative phosphorylation^14,15.

Mitochondrial Dysfunction and Multiple Sclerosis

Impaired mitochondrial functions are implicated in the pathogenesis of the neurodegenerative process of MS. For example, pro-inflammatory cytokines can induce oxidative stress, and conversely, oxidative stress can further amplify inflammation, creating a self-perpetuating cycle, leading to neuronal oxidative damage, which is observed in both RRMS and progressive forms of MS. Oxidative stress leads to mitochondrial dysfunction, primarily by interfering with several components of the respiratory chain, resulting in cell membrane disruption and, ultimately, neuronal cell death¹⁶. Similarly, impaired mitochondrial function leads to increased ROS production, resulting in oxidative damage⁵. Then, oxidative stress and mitochondrial dysfunction are extremely interconnected processes. Finally, current research has identified several mitochondrial abnormalities implicated in the development and progression of multiple sclerosis, including (1) mitochondrial DNA defects, (2) abnormal mitochondrial gene expression, (3) impaired mitochondrial enzymatic activity, (4) deficient mitochondrial DNA repair mechanisms, and (5) overall mitochondrial dysfunction¹⁶.

Post-mortem analysis of chronically active demyelinating lesions in brain and spinal cord sections, common in long-standing MS, revealed a reduction in mitochondrial mass and defects in mitochondrial respiratory chain complex IV (cytochrome c oxidase), which may represent a decreased cellular energy demand⁷. Conversely, chronic inactive demyelinating lesions, i.e., areas of past myelin damage where inflammation is no longer active, exhibit increased mitochondrial density and complex IV activity, suggesting that a compensatory metabolic adaptation may occur in this stage of the disease. Importantly, defects in mitochondrial respiratory chain complexes and reduced mitochondrial density may induce cytosolic deleterious effects through impaired mitochondrial calcium homeostasis leading to neuronal death¹⁷.

Neuroinflammation is a hallmark of MS, triggered by leukocyte infiltration and glial cell activation. Increased macrophage/microglial density in chronic lesions are important sources of ROS, contributing to the oxidative stress scenario. Histopathological studies of active cerebral white matter lesions from MS post-mortem patients suggest that neuronal mitochondrial dysfunction may be a consequence of increased amounts of reactive oxygen and nitrogen species produced by infiltrated leukocytes and activated microglia¹⁸. These reactive species can damage mitochondrial proteins and contribute to mitochondrial respiratory chain impairment, impacting neuronal mitochondrial function.

Excessive ROS can cause oxidative modification of lipids and originate a process called lipid peroxidation. Neurons, with their high lipid content, is highly susceptible to lipid peroxidation. Toxic aldehydes, such as 4-hydroxynonenal (4-HNE), are the main end-products of lipid peroxidation, which are metabolized by the mitochondrial aldehyde dehydrogenase-2 (ALDH2). Recent findings highlight the key role of ALDH2 in metabolizing these toxic aldehydes during chronic inflammation¹⁹ and neurodegeneration, as occurs in MS. In this context, studies using an EAE mouse model demonstrate that sustained activation of ALDH2 is essential to reduce the central aldehydic load and improve clinical signs and neuroinflammation. As a proof of concept, knock-in mice carrying the inactivating point mutation in ALDH2, identical to the mutation found in Han Chinese, exhibit heightened motor disabilities and hypernociception in MS mice. The deleterious clinical signs are followed by increased systemic and SNC aldehyde levels^20-22.

In summary, mitochondrial dysfunction, leading to oxidative stress and energy insufficiency, is a key factor in the pathogenesis of MS. This indicates that mitochondria are a potential molecular target for MS control. (Figure 1)

Figure 1: Multiple sclerosis (MS) is an inflammatory neurodegenerative disease resulting in demyelination of the Central Nerve System (CNS). Impairment in mitochondrial functions, such as, aldehydic load clearance, increased oxidative stress, energy imbalance and impaired calcium handling, play a key role in the MS development.

Targeting mitochondria to prevent MS

The existing treatments for MS primarily target acute neuroinflammation episodes. While this strategy effectively reduces relapse occurrences, it has limited effectiveness in protecting against neurodegeneration, especially in the progressive stages of the disease. Given that mitochondrial dysfunction is involved in the pathophysiology of MS, the development of new therapeutic strategies targeting mitochondria may become attractive²³. Indeed, lipoic acid, an endogenously produced antioxidant displayed a clinical benefit in patients with in secondary progressive MS. On the other hand, a Phase I/II clinical trial of idebenone (NCT01854359), showed that CoQ10 supplementation failed in preventing disability progression in PPMS²⁴. CoQ10 is a lipophilic constituent of the mitochondrial ETC, whose supplementation could indirectly affect mitochondrial oxidative stress. The authors speculate that idebenone can limit mitochondrial dysfunction in astrocytes, but not in neurons. While mitochondrial dysfunction is a significant factor for MS, it is mainly considered a consequence of demyelination, and not a direct cause. Therefore, preventing astrocytic mitochondrial dysfunction was not sufficient to prevent MS.

An attractive compound to treat MS approved by FDA is teriflunomide, an immunomodulatory agent that works by inhibiting dihydro-orotate dehydrogenase (DHODH), a mitochondrial enzyme involved in the pyrimidine synthesis pathway, which is expressed at high levels in proliferating lymphocytes. Therefore, teriflunomide exerts a cytostatic effect on proliferating T and B cells, limiting their involvement in the inflammatory processes of MS pathogenesis²⁵.

Another shared characteristic of neurodegenerative and neurological disorders, such as MS, is the impairment of the Nrf2 (nuclear factor erythroid 2-related factor 2) signalling pathway. This pathway is essential for regulating antioxidants and protecting from mitochondrial dysfunction, oxidative stress, and neuroinflammation. Dimethyl fumarate, diroximel fumarate, and monomethyl fumarate - oral immunomodulatory drugs to treat MS - appear to activate the Nrf2-dependent and independent antioxidant pathway in the CNS. As mentioned before, aldehydes play an important role in the pathophysiology of MS²². Therefore, pharmacological interventions improving aldehyde removal might become a useful strategy to ameliorate MS. Studies showed that the improvement of the aldehyde clearance by Alda-1, an ALDH2 activator, is sufficient to recover hypernociception and motor impairment in a rodent model of MS. The beneficial effects of Alda-1 have also been reported in inflammatory¹⁹ and neuropathic pain models²⁶. Overall, these preclinical studies suggest ALDH2 as a new molecular target for the control of neurodegenerative disease.

The recent advancements in the understanding of mitochondrial dysfunction in neurodegenerative diseases, and in particular MS, bring new perspectives for future prevention of neuroinflammation and neuronal loss. These emerging therapies hold promise for providing new avenues for treatment and, potentially improving outcomes for individuals affected by MS.

Acknowledgements

VOZ and GP research were funded by National Council for Scientific and Technological Development (CNPq, process number 467211/2014-0), Grant FAPESP 2020/13139-0, 2015/50040-4 Sao Paulo Research Foundation and GlaxoSmithKline, VOZ; São Paulo Research Foundation (FAPESP, grants 2024/04023-0, VOZ; 2023/13449-8, VCT; 2023/05626-7, NGH; 2023/14271-8, NPP; 2013/07467-1, GP). GP is CNPq researcher PQ-2. Schematic images created with BioRender.com.

Conflict of Interest

The authors declare no potential conflict of interest. 

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