Mitochondrial Dysfunction in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome

Abstract

ME/CFS is a debilitating multisystem disorder of unclear etiology that affects many individuals worldwide. One of its hallmark symptoms is prolonged fatigue following exertion, a feature also observed in long COVID, suggesting an underlying dysfunction in energy production in both conditions. Here, mitochondrial dysfunction and its potential pathogenetic role in these disorders are reviewed.

Summary:

Mitochondrial dysfunction, its underlying mechanisms, and possible targeted treatment approaches in ME/CFS are reviewed.

Introduction

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a chronic multisystem disorder that causes significant disability worldwide. The characteristic symptoms of ME/CFS include debilitating fatigue, post-exertional malaise (PEM), unrefreshing sleep, and cognitive impairment (7). It has been estimated to affect ~1% of the general population in the United States according to the Center for Disease Control and Prevention (CDC), and women are affected at a rate 3 times higher than men (42, 109). This disorder takes a tremendous toll on patients, family members and caretakers and on society through healthcare costs and loss of productivity. Although significant advances have been made in characterizing the clinical and pathogenetic aspects of ME/CFS, the lack of specific diagnostic tests and targeted therapies have limited its effective management in the clinics. Furthermore, the suffering and impact of “chronic fatigue” symptoms have significantly increased worldwide due to the growing number of individuals affected by long COVID, a condition closely resembling ME/CFS, highlighting the urgency of advancing research into these disorders (2, 47, 81).

Reports of conditions resembling ME/CFS began emerging in the early 20th century, describing sporadic cases of encephalomyelitis (likened to “abortive poliomyelitis”) with unclear origins following illnesses (79). Notably, one of the earliest diagnostic criteria for “myalgic encephalomyelitis” emphasized exercise intolerance and the more recent Institute of Medicine (U.S.) renaming proposal has continued to highlight this key bioenergetic feature by calling it “systemic exertion intolerance disorder (SEID)” (42, 85). Despite over many years of research, ME/CFS has remained a formidable challenge in medicine, significantly affecting both the physical and mental capacities of the patients (7). ME/CFS could be a complex manifestation of multiple systems being affected by the derangement of a critically important cellular process that has wide-ranging effects depending on the characteristics of each tissue or organ. Furthermore, the unique genetic makeup, medical history and lifestyle of each patient may also shape how the disease manifests and progresses. Therefore, this complex interplay of cellular dysfunction, genetic predisposition, and environmental factors could explain why ME/CFS is such a diverse and often unpredictable condition, affecting patients in different ways (42, 84).

As expected, most of the major case definitions used to diagnose ME/CFS list fatigue, usually lasting greater than 6 months, as a cardinal symptom (55). Focusing on the aspect of fatigue that refers to the diminution of muscular force versus its “sensation” (84), the role of mitochondrial dysfunction as a contributing factor in ME/CFS has gained considerable attention due to its expected negative impact on cellular energy production after depletion by exercise (25, 40, 106). This potentially diminished ability to regenerate energy levels is reflective of the delayed or even lack of recovery from fatigue, one of the multiple symptoms of post-exertional malaise (PEM), exacerbated by physical or mental exertion in ME/CFS (7, 35).

Understanding the mechanism and role of mitochondrial dysfunction in ME/CFS may offer insights into the underlying cause of this complex disorder and potentially identify novel therapeutic targets. This review aims to provide an overview of what is known about the mitochondrion in ME/CFS, and how its dysfunction may interplay with other systems, such as the immune and central nervous system. We also provide a review of recent work identifying the Wiskott-Aldrich Syndrome Protein Family Member 3 (WASF3) and its regulation by the endoplasmic reticulum (ER) stress as mediating mitochondrial dysfunction in ME/CFS. Such mechanistic finding may facilitate the development of targeted interventions aimed at alleviating the heavy burden of this debilitating condition (115).

Mitochondria in ME/CFS

The mitochondria are best known for oxidizing metabolic substrates and using oxidative phosphorylation for generating the high energy molecule adenosine triphosphate (ATP), but it also performs many other activities critical for maintaining cellular homeostasis (114). The activities of mitochondria are important for maintaining health, supported by the observation that >40% of the mitochondrial proteome is linked to human diseases (71), while its dysfunction can promote diseases, particularly those associated with aging such as neurodegeneration and cancer (114). Given the essential roles of mitochondria in cellular activities, their dysfunction would likely impair muscle, brain, and immune cell function, contributing to the hallmark symptoms of ME/CFS which include fatigue, cognitive impairment, and immune system abnormalities (7, 100). Whether mitochondrial dysfunction in ME/CFS, reported by numerous groups, is a cause or simply an effect of the underlying pathogenesis is currently unclear. Nonetheless, studying the molecular mechanism of the mitochondrial dysfunction could illuminate interacting networks that in turn may provide clues to the etiology of ME/CFS.

Evidence of Mitochondrial Abnormalities in ME/CFS

Given the important role of the mitochondrion in generating cellular energy and potentially alleviating fatigue, its function in ME/CFS has been highly scrutinized over the years (see reviews) (25, 40, 80, 107). Earlier studies using in vivo phosphorus-31 magnetic resonance spectroscopy (³¹P-MRS) to measure ME/CFS patient skeletal muscles during exercise recovery reported lower mitochondrial metabolism and ATP synthesis rate and increased intracellular acidosis consistent with more glycolytic activity (5, 17, 49, 54, 120). Decreased oxygen extraction during exercise also suggested deficiencies in mitochondrial function in these patients that did not appear to be related to deconditioning (112).

Subsequent metabolic profiling of ME/CFS patient compared with healthy subject blood samples under basal or stress states have revealed specific differences informing on potential metabolic perturbations and augmenting earlier in vivo observations using techniques such as ³¹P-MRS (4, 26, 29, 38, 77, 123). MRS analysis of the brain has also revealed regional differences in specific metabolites in ME/CFS compared with healthy control subjects (74). In this study, the correlation between elevated lactate levels and tissue temperature was interpreted as being consistent with neuroinflammation, which could be promoted in part by mitochondrial dysfunction and release of mtDNA (108). Other brain MRS studies have also showed elevated lactate levels in the ventricular cerebrospinal fluid of CFS patients, postulated to be caused by oxidative stress and/or conditions favoring anaerobic metabolism (76, 91).

Experimental studies have been performed on various tissue/cell types using different mitochondrial techniques with sometimes contrasting or negative findings (6, 93), but generally, there appears to be a detectable pattern of mitochondrial dysfunction in ME/CFS. Impaired mitochondrial function in subsets or whole peripheral blood mononuclear cells (PBMCs) have been reported in ME/CFS patients (62, 68, 106). Another study consisting of 138 ME/CFS patients correlated an index of mitochondrial function with the severity of the illness although this work was done using isolated neutrophils (11). As comprehensively reviewed (40), some reports have included in vitro demonstrations of impaired mitochondrial ATP synthesis, increased oxidative stress, and changes in mitochondrial morphology and dynamics.

Despite all these abnormalities, ME/CFS is not classified as a mitochondrial disease largely because no specific genetic changes have been identified to explain the mechanism of dysfunction (31). A comprehensive next-generation sequencing study of the mitochondrial genome (mtDNA) in a large cohort of ME/CFS patients compared with matched controls did not reveal any significant associations by mtDNA single nucleotide polymorphisms or degree of heteroplasmy (mtDNA variants) (9). However, the mtDNA haplogroups J, U and H and eight single nucleotide polymorphisms (SNPs) were significantly linked to individual symptoms and their characteristics in ME/CFS patients. These findings were reflective of another study that had associated a mtDNA SNP with ME/CFS symptoms but no conclusions could be made about the mechanism or the role of nuclear genome encoded mitochondrial proteins (10).

Increased Oxidative Stress and Redox Imbalance in ME/CFS

Under normal conditions, the body maintains a balance between the production and detoxification of reactive oxygen species (ROS), a state known as redox homeostasis, while an imbalance can cause oxidative stress (92). Although low levels of ROS can play physiologic roles as signaling molecules (51), excessive amounts of these oxygen free radicals can oxidize virtually every component within the cell including lipids, proteins and DNA, resulting in deleterious outcomes. Various studies have reported evidence of increased oxidative stress in ME/CFS patients as manifested by elevated levels of lipid peroxidation products, protein carbonyls and oxidized DNA bases and lower levels of antioxidants such as coenzyme Q10 (CoQ10) (80, 121). CoQ10, also known as ubiquinone, is a naturally occurring antioxidant that protects cells from oxidative stress. It is also involved in the electron transport chain thereby playing an essential role in the production of ATP. The increased oxidative stress in ME/CFS patients could be speculated to damage cellular function and the CoQ10 deficiency may impair respiration and antioxidant responses, contributing to the wide array of abnormalities observed in this disorder (60).

In mitochondrial dysfunction, the aberrant transfer of electrons along the respiratory complex chain is thought to result in the generation of ROS by the spurious reduction of molecular oxygen (37, 75). Generally, the mitochondria are thought to be the major source of ROS in the cell, but this is still debatable in the field given the number of contrary views (12, 33, 39, 125). In fact, the mitochondrion can serve as a powerful antioxidant mechanism for survival in our oxygen-rich oxidizing environment, originally hypothesized by the symbiotic theory of the mitochondrion (63). Therefore, mitochondrial dysfunction would be expected to increase the reducing potential of a cell along with greater availability of free oxygen, which could result in the generation of ROS and oxidative stress (43, 72, 73, 99). Whatever the mechanism may be, it is widely accepted that normal mitochondrial respiratory activity and oxidative metabolic capacity are important for modulating oxidative stress. Other subcellular sources of ROS are also possible as plasma metabolomic profiling has suggested peroxisomal dysfunction in ME/CFS patients (18). In addition to regulating fatty acid metabolism in cross-talk with the mitochondria, peroxisomes are crucial cellular sites for both the generation and scavenging of ROS so its dysfunction may further contribute to oxidative stress and cellular dysfunction in ME/CFS (12).

WASF3 as a Mediator of Mitochondrial Dysfunction in Response to ER Stress in ME/CFS

In the course of examining mechanisms of mitochondrial regulation, we identified overexpression of the protein encoded by the Wiskott-Aldrich Syndrome Protein Family Member 3 (WASF3) as mediating decreased respiration in the fibroblasts of a patient with a life-long history of fatigue and exercise intolerance (115). Compared to cells from her healthy sibling, the patient’s fibroblasts exhibited higher levels of WASF3 protein and ER stress, along with reduced respiratory complex IV (CIV) subunit levels. Notably, pharmacologically alleviating ER stress in the patient’s cells decreased WASF3 levels and restored mitochondrial function.

WASF3 is best known as a regulator of actin polymerization and cell motility, but it also serves as a scaffold for protein complexes (48, 58). In addition to its role in the actin cytoskeleton, the overexpression of WASF3 was found to disrupt mitochondrial respiratory supercomplexes (SC) which have been associated with endurance exercise training (32, 115). Mechanistically, WASF3 appears to interact with specific subunits of CIII, the deficiency of which has been associated with exercise intolerance (83), and destabilizes CIV subunits by interfering with SC III₂+IV formation (52). In vivo, transgenic mice overexpressing human WASF3 displayed significantly reduced exercise capacity and higher blood lactate levels compared to wild-type littermates, consistent with decreased oxidative metabolic capacity (115).

In support of the involvement of WASF3 in ME/CFS, skeletal muscle biopsy samples obtained from a well-characterized cohort of ME/CFS patients demonstrated elevated levels of WASF3 protein and ER stress markers, along with decreased mitochondrial CIV subunits, when compared with that of healthy volunteers (113, 115). This was consistent with a previous report of decreased CIV levels in the skeletal muscle of elderly individuals with idiopathic chronic fatigue (117). The association between ER stress and mitochondrial dysfunction in individuals with ME/CFS was intriguing given the link between this disorder and viral infections which are well-known to disrupt ER homeostasis (34, 36, 86).

The endoplasmic reticulum is a membranous organelle whose major function is the synthesis of luminal/membrane proteins and their maturation, sorting and delivery. However, the ER apparatus can be usurped by viruses for utilization at different stages of their life cycle such as during viral entry, protein synthesis, replication, assembly and exit from the cell (87). As expected with such extraneous activities, viral infections can disrupt ER homeostasis and stress the system. Interestingly, ER stress has been associated with oxidative stress and the two often coexist in various pathological conditions but the underlying mechanism has remained elusive (13). Our observation that ER stress induces WASF3 through a post-transcriptional mechanism, disrupting mitochondrial function and thereby causing oxidative stress, could provide an explanation for this phenomenon (72, 115). It also suggests that the increased WASF3 in ME/CFS could be part of an immune response to a perceived stimulus.

Pathophysiological Effects of Mitochondrial Dysfunction in ME/CFS

Exercise intolerance and slow recovery from fatigue after physical exertion are hallmark symptoms of ME/CFS. From a bioenergetic perspective, these exertional symptoms could be explained by the rapid depletion and slow regeneration of ATP, slow oxidative metabolism of fuel substrates, and increased oxidative stress in skeletal muscle, all secondary to impaired mitochondrial function. However, the consequences of dysfunctional mitochondria can extend beyond energy insufficiency in skeletal muscle and limit the function of virtually every system in the body. Given the high energy demands of neurons for action potential generation and cell signaling, decreased mitochondrial respiration could affect brain function in unpredictable ways given the complexity of neuronal circuitries. The neurocognitive symptoms of diminished memory, concentration and coordination, which feature prominently in ME/CFS, have been explained by various mechanisms including impaired cerebral blood flow (119). However, it is possible that these symptoms as well as others such as autonomic dysfunction could be largely due to the impairment of mitochondria, which are required for neuronal activities such as Ca²⁺ homeostasis for cell signaling at the junction between the ER and the mitochondria (64).

Alterations in the profiles of immunoglobulins, cytokines and cellular components as well as evidence of chronic low grade inflammation, autoimmunity, neuroinflammation and gut microbiome changes have implicated a dysregulated immune system in ME/CFS pathogenesis (21, 47, 96, 122). It is notable that mitochondria also play an important role in immune cells, especially T cells which undergo metabolic reprogramming to mount an effective response against pathogenic stimuli and have been reported to be abnormal in ME/CFS (20, 47, 62, 66). The mitochondria can also act as signaling hubs that dictate the modulation of specific immune responses such as by the release of mtDNA into the cytosol with initiation of innate immune signaling, a topic that has been highly reviewed (88, 118). In this regard, it could be revealing to investigate whether PEM, a key symptom of ME/CFS with neuroimmune features, is mediated in part by the mitochondrial innate immune signaling mechanism involved in skeletal muscle adaptation to exercise (59) (35).

Potential Role of WASF3 in Regulating Immune Metabolism and Function

The actin cytoskeleton is well established to regulate T cell and B cell receptor signaling, but it also plays other roles including the motility of T cells, monocytes and neutrophils essential for their normal function (8, 24, 53) (45). WASF3 belongs to a family of genes with homology to the Wiskott–Aldrich Syndrome (WAS) gene which when mutated can cause the rare X-linked immunodeficiency disorder associated with B cell dysfunction (65). It should be noted here that the more ubiquitously expressed WASF2 gene has been reported to be important for actin regulation, Ca²⁺ entry, and mTOR suppression in T cells, but the immune role of its family member WASF3 remains unexplored (56, 78).

WASF3 protein has a N-terminal WASF homology domain (WHD) and a C-terminal verprolin-cofilin-acidic (VCA) region, which binds actin and actin-related proteins (Arp2/3) for regulating actin polymerization (Figure 1) (48). The C-terminal truncated form of WASF3 missing the VCA region can still interact with mitochondria and disrupt respiration as effectively as full-length WASF3 (105, 115). This suggests that WASF3 inhibits mitochondrial function independently of its interaction with the actin cytoskeleton. On the other hand, cycles of actin polymerization and depolymerization has been reported to regulate mitochondrial dynamics (70). Inhibiting actin polymerization enhances brain mitochondrial function, while inducing actin polymerization activates glycolysis (16, 101). Thus, WASF3 could reprogram cell metabolism by suppressing mitochondrial oxidative phosphorylation while simultaneously enhancing glycolysis through more than one mechanism (Figure 1).

Figure 1. A model of how WASF3 may play a central role in regulating metabolism and immunity in response to ER stress and other signals.

Upon ER stress, the level of WASF3 protein may rise at the contact sites between the endoplasmic reticulum (ER) and mitochondria. WASF3 disrupts the assembly of respiratory complexes, inhibiting mitochondrial respiration and oxidative phosphorylation. In parallel, WASF3 overexpression promotes actin polymerization, which drives glycolysis and further suppresses mitochondrial respiration. This metabolic shift may support immune system activation for host defense. However, prolonged activation of this pathway can lead to chronic inflammation and energy deficiency, contributing to the symptoms of ME/CFS. Created in BioRender. Hwang, P. (2024) https://BioRender.com/g19a699

Potential clues that WASF3 may be involved in the immune system come from early observations that JAK2/STAT3 pathway can regulate WASF3 through both transcriptional and protein phosphorylation mechanisms, whereas overexpression of WASF3 alone activates the NFKB pathway (22, 104). In our recent study, we found that ER stress increases WASF3 levels, and the subcellular fraction of WASF3 localized to mitochondria appears to be post-translationally modified by phosphorylation in mouse myoblasts (115). Because WASF3 is highly expressed in brain tissue (95, 98), it is also tempting to speculate that disruption of WASF3 homeostasis could be involved in neuroinflammation as observed in ME/CFS (Figure 1) (50, 110).

Elevated levels of WASF3 activates the immune and key cellular stress mediator p38 MAPK through a non-canonical pathway, possibly as a feedback signal to promote mitochondrial biogenesis due to impaired respiratory activity (94, 115). Interestingly, the meta-analysis study that initially associated WASF3 to ME/CFS suggested that the central mechanism of fatigue may be mediated by p38 MAPK which is known to enhance the expression of interferon-α (IFN-α) in the brain (46, 57, 82). In the presence of pathogens, it is also conceivable that the metabolic switch toward glycolysis for activating T cells and macrophages is coordinated with the induction of a mitochondrial inhibitory factor such as WASF3 (20). Considering these disparate yet potentially interconnected observations, it could be fruitful to investigate the role of WASF3 in immune cells and explore whether some of the chronic inflammatory characteristics seen in ME/CFS patients can be replicated in WASF3 mouse models.

Impaired Mitochondria Function in Long COVID

The COVID-19 pandemic and subsequent reports of the long term complications from the post-acute sequelae of SARS-CoV-2 infection (PASC), also known as long COVID, has highlighted the rapidly growing public health impact of fatigue syndromes. Like long COVID, ME/CFS often has an antecedent history of infection, especially with the Epstein-Barr Virus (EBV) as one of the more commonly associated triggers of this disorder. Following SARS-CoV-2 infection, up to 20% of individuals who were infected have reported experiencing lingering fatigue, cognitive impairment and other disabling symptoms resembling ME/CFS (47). The seminal finding of persistence of viral particles in different organs of patients months after SARS-CoV-2 infection suggests that there could be ongoing immune activation, such as the ER stress response, resulting in long COVID symptoms (97).

Emerging research has revealed various pathophysiologic findings in long COVID that suggest mitochondrial dysfunction (2, 81). The disruption of redox homeostasis and oxidative stress, potentially caused by alterations in mitochondrial respiration (72, 80), may lead to subsequent changes in cellular metabolism in both ME/CFS and long COVID (30, 61, 90). A recent study has reported that individuals experiencing long COVID symptoms showed an increased fraction of fatigue prone glycolytic fibers in their skeletal muscle and impaired mitochondrial function that worsened with PEM (3). These factors would be expected to diminish exercise endurance, cause easy fatigability and slow energy regeneration after exertion. It could also be speculated that the observed increase in immune cell infiltrates in the skeletal muscle of long COVID patients with PEM may be mediated in part by innate immune signaling activated in muscle cells by exercise stimulation (3, 59). Overall, this study underscores the potential direct contribution of mitochondrial dysfunction and muscle tissue changes to fatigue and exercise intolerance in long COVID and ME/CFS.

Therapeutic Targeting of Mitochondria in ME/CFS

If mitochondrial dysfunction underlies the varied symptoms of ME/CFS, therapeutic approaches to correct this dysfunction could provide symptomatic relief although it may not address the upstream etiologic cause(s). While not including all the many treatment studies on ME/CFS, we attempt to provide a brief overview of the ones focused on mitochondria. Multiple approaches, such as the use of mitochondrial biogenesis enhancers, metabolic modulators and antioxidant supplements, have been explored to enhance mitochondrial function and improve symptoms in ME/CFS patients (Table 1). Changes in mitochondrial metabolism and oxidative stress in ME/CFS may cause deficiencies in essential factors such as CoQ10 and NADH which are required for oxidative phosphorylation and ATP production. Therefore, these cofactors in the form of nutritional supplements have been the focus of multiple ME/CFS treatment studies over the years (23).

Table 1.

A representative list of ME/CFS treatment studies targeting mitochondria

Treatment	Targets	Dose, duration, study size/design	Outcomes	Ref.
CoQ10; selenium	Mitochondrial coenzyme; oxidative stress; inflammation	400mg and 200μg daily for 8 wks; n=27/open label	Improved fatigue severity (FIS-40), quality of life (SF-36) and oxidative stress/cytokine levels	(14)
CoQ10 (reduced form Ubiquinol-10)	Mitochondrial coenzyme	150mg daily for 12 wks; n=20/open label; n=43/RCT	Improved fatigue symptoms (CFS), arithmetic task (UKP Test), autonomic function (APG system), cognition and sleep	(28)
CoQ10; NADH	Mitochondrial coenzyme	200mg or 20mg daily for 12 wks; n=207/RCT	Reduced cognitive fatigue (FIS-40); improved quality of life (HRQoL) and sleep (PSQI)	(15)
D-ribose	Mitochondrial homeostasis	5g 3x daily for 3 wks; n=41/open label	Improved energy, sleep, mental clarity, pain intensity and well-being (VAS)	(103)
Dichloroacetate, sodium (DCA)	Inhibit pyruvate dehydrogenase kinase	1 capsule daily for 30 d; n=22/open label	Reduced fatigue severity (FSS)	(19)
NADH (ENADA)	Mitochondrial coenzyme	10mg daily for 4 wks; 26 subjects/RCT	31% of patients had improved fatigue (50-item questionnaire) in response to NADH	(27)
Acetyl-L-carnitine (ALC); propionyl-L-carnitine (PLC)	Mitochondrial β-oxidation	2g daily ALC, PLC or ALC+PLC combination for 24 wks; n=30/RCT	Both supplements individually improved fatigue (MFI-20) and concentration (Stroop test) but combination was not effective	(111)
KPAX002 (methylphenidate HCl, mitochondrial nutrients)	Improve hypometabolic state; mitochondrial nutrients	Methylphenidate 10–20mg daily; mito nutrients for 12 wks; n=15/open label, n=135/RCT	Significantly improved fatigue (CIS) and concentration disturbance (VAS) symptoms in >50% of patients	(44) (69)
Nutraceutical combination	Mitochondrial nutrients	Twice daily for 16 wks; n=10/open label	Significant improvement in fatigue symptoms (CFS); no benefit was observed in mood (MADRS) or other functions	(67)

Abbreviations (Outcomes): APG, Acceleration Plethysmography system; CFS, Chalder’s Fatigue Scale questionnaire; CIS, Checklist Individual Strength; FIS-40, 40 item Fatigue Impact Scale questionnaire; FSS, Fatigue Severity Scale, Dutch version; HRQoL, Health-Related Quality of Life; MPQ, McGill Pain Questionnaire; MFI 20, Multidimensional Fatigue Inventory; MADRS, Montgomery Asberg Depression Rating Scale; PSQI, Pittsburgh Sleep Quality Index questionnaire; PFS, Piper Fatigue Scale; RCT, randomized controlled trial; SF-36, Short-Form 36-item health survey; UKP, Uchida Kraepelin Psychodiagnostic test; VAS, Visual Analog Scale questions.

Two randomized, double-blind, placebo-controlled crossover studies have shown that NADH can improve ME/CFS patients’ symptoms based on a questionnaire or reduce anxiety and heart rate during exercise stress testing (1, 27). Coenzyme Q10 (CoQ10, ubiquinone) is an antioxidant and essential cofactor for the transport of electrons through mitochondrial respiratory complexes I, II and III. Because of its critical role in the mitochondria along with the various reports of decreased CoQ10 levels in ME/CFS patients (121), it is a supplement that has been tested more extensively in both randomized and open-label treatment studies (Table 1). Randomized controlled trials (RCT) and open-label studies testing CoQ10 alone or in addition to NADH or other supplements (selenium, magnesium, alpha-lipoic acid, N-acetyl cysteine, acetyl-L-carnitine, vitamins) have been reported to improve various combinations of subjective symptoms, functional tasks, sleep-wake cycle, and/or specific biochemical parameters in ME/CFS patients (14, 15, 28, 67).

Other factors directed at improving mitochondrial function have also been explored in ME/CFS treatment studies. D-Ribose, a key structural component in nucleic acids and redox enzymes, essential for the mitochondria to maintain cellular ATP and redox homeostasis, was tested on ME/CFS patients (103). The results showed improvements in several ME/CFS-associated symptoms, including energy levels, sleep, mental clarity, pain, and overall well-being. KPAX002, a combination of methylphenidate hydrochloride and mitochondrial-supporting supplements such as acetyl-L-carnitine and selenium, was also tested with symptomatic improvement in a small group of ME/CFS patients but with negative results in a subsequent larger clinical study (44, 69). Treatment with sodium dichloroacetate (DCA), proposed to increase mitochondrial ATP production by inhibiting pyruvate dehydrogenase kinase (PDK) and thereby activating pyruvate dehydrogenase, was observed to improve fatigue scale scores of ME/CFS patients (19). Other supplements that promote mitochondrial function or serve as antioxidants such as red ginseng (HRG80) and quercetin have been tested in open-label and RCT studies, respectively, with various reported functional improvements (41, 89, 102, 111, 124).

Despite their reported beneficial effects, these treatment trials have not significantly impacted the management of ME/CFS patients largely due to their modest benefits and variable efficacy among individuals. Elucidation of the molecular mediators of the complex symptoms of ME/CFS could assist in the development of targeted treatment strategies. For example, the identification of ER stress as inducing WASF3 and causing mitochondrial dysfunction in ME/CFS has revealed a new treatment approach (115). Salubrinal, a potent inhibitor of protein phosphatase 1 (PP1) that selectively targets the eukaryotic translation initiation factor 2α (EIF2A), has been shown to effectively reduce ER stress and restore mitochondrial function in ME/CFS patient cells. A recent review has highlighted the therapeutic potential of salubrinal for both ME/CFS and long-COVID, but there are challenges to be overcome in considering a pharmacologic agent that inhibits general protein translation and is not approved for human use (116).

Conclusions

Various lines of evidence suggesting mitochondrial dysfunction appear to be a recurring theme in the complex pathophysiology of ME/CFS. It is conceivable that the impairment of ATP production and the many other vital functions that mitochondria play, such as redox and electrochemical homeostasis, could contribute to the multi-system symptoms of this disorder. The heterogeneity in the clinical characteristics of ME/CFS patients has presented significant challenges for both clinicians and researchers. For instance, while some individuals primarily suffer from cognitive impairments and post-exertional malaise, others may experience predominantly autonomic, hemodynamic or immune abnormalities. It has also led to inconsistent observations and difficulties in developing consensus treatment protocols, in part due to the lack of specific molecular mediators as treatment targets and/or diagnostic biomarkers of ME/CFS. Despite significant progress in characterizing the biology of individuals with ME/CFS, further collaborations amongst the different basic science disciplines and clinical specialties may aid in advancing our knowledge of this disorder.

Insights into the molecular mechanism of the mitochondrial anomalies holds promise for alleviating symptoms and improving the quality of life for those suffering from this debilitating condition. Our recent study identifying WASF3 as a potential mediator of mitochondrial dysfunction highlights the involvement of cellular energy production, ER stress and immunity in ME/CFS (115). Oxidative stress and redox imbalance can also trigger ER stress and this remains to be explored in the context of WASF3 and ME/CFS. Regardless of the underlying etiology, targeting WASF3 through ER stress modulation may yield more insights into the specific molecular pathways associated with the pathogenesis of ME/CFS as well as provide a potential biomarker of treatment efficacy.