Mitochondrial dysfunction and mitophagy activation in blood mononuclear cells of fibromyalgia patients: implications in the pathogenesis of the disease

Fibromyalgia (FM) is a common chronic pain syndrome accompanied by other symptoms such as fatigue, headache, sleep disturbances, and depression. It is diagnosed according to the classification criteria established by the American College of Rheumatology (ACR) [1]. Despite being a common disorder that affects at least 5 million individuals in the United States [2], its pathogenic mechanism remains elusive. Recently oxidative stress markers were proposed as a relevant event in the pathogenesis of this disorder [3, 4].

Previously, we detected decreased coenzyme Q₁₀ (CoQ₁₀) levels and increased reactive oxygen species (ROS) production in blood mononuclear cells of FM patients, providing direct evidence of increased oxidative stress at the cellular level [5]. CoQ₁₀ plays a crucial role in cellular metabolism, acting as an electron carrier between complexes I and II and complex III of the mitochondrial respiratory chain. CoQ₁₀ also has been reported to play an important role in the regulation of uncoupling proteins, mitochondrial permeability transition pore, β-oxidation of fatty acids, and the nucleotide-biosynthesis pathway [6]. Moreover, CoQ₁₀ levels have been suggested to be useful as a mitochondrial-dysfunction marker [7]. CoQ₁₀ deficiency induces decreased activities of complex II + III, complex III and complex IV, reduced expression of mitochondrial proteins involved in oxidative phosphorylation, decreased mitochondrial membrane potential, increased production of reactive oxygen species (ROS), activation of mitochondrial permeability transition (MPT), mitophagy of dysfunctional mitochondria, and reduced growth rates [8, 9].

The purpose of the present work was to assess the mitochondrial dysfunction in blood mononuclear cells of FM patients and to elucidate whether mitochondrial disturbance was involved in the pathophysiology of oxidative stress present in FM.

Mitochondria generate energy primarily in the form of the electrochemical proton gradient, which fuels ATP production, ion transport, and metabolism. Mitochondria are also the major source of ROS. Both complexes I and III, along with CoQ₁₀, leak electrons to oxygen [14‐17]. CoQ₁₀deficiency has been associated with a variety of human disorders, some of them caused by a direct defect of CoQ₁₀ biosynthesis genes or as a secondary consequence of other diseases [18, 19]. Recent findings show that CoQ₁₀ deficiency alters mitochondrial function and mitochondrial respiratory complex organization, leading to increased ROS generation, activation of MPT, and increased autophagy of dysfunctional mitochondria by mitophagy [8, 9]. In the present study, we found that CoQ₁₀-deficient BMCs in FM patients showed high levels of ROS production in mitochondria and increased levels of lipid peroxidation in both cells and plasma. In this respect, high levels of lipid peroxidation and protein carbonyls [20, 21] and disturbances in the homeostasis of platelet ATP have been observed in FM patients [22]. The fact that CoQ₁₀ and α-toc, two lipophilic antioxidants, significantly reduced mitochondrial ROS production, also suggests that ROS are produced in the lipophilic environment of mitochondrial membranes and that CoQ₁₀ deficiency may be involved in oxidative stress in FM.

If oxidative damage plays a role in FM through the activation of MPT and mitophagy, then therapeutic strategies that reduce ROS may ameliorate the pathologic process. What is the relation between oxidative stress and FM symptoms? Recent studies showed that oxidative stress can cause peripheral and central sensitization and alter nociception [23], resulting in hyperalgesia mediated by both local and spinal oxidant mechanisms. Furthermore, oxidative stress is increased in patients with chronic-fatigue syndrome [24, 25]. Superoxide plays a major role in the development of pain through direct peripheral sensitization, the release of various cytokines (for example, TNF-α, IL-1β, and IL-6), the formation of peroxynitrite (ONOO^-), and PARP activation [23]. In addition, studies on depression, a typical symptom in FM patients, have elucidated the possible link between depression and lipid peroxidation [26]. Lipid peroxidation may play an important role in depression, and the peroxidation-reducing effect of different selective serotonin reuptake inhibitors in major depression was demonstrated by Bilici and associates [27].

In addition, and supporting the role of mitochondrial dysfunction, BMCs of FM patients showed a decrease of 36% of mitochondrial membrane potential (ΔΨm), possibly reflecting a reduced electron flow and proton pumping caused by CoQ deficiency. Interestingly, a positive correlation between the content of CoQ₁₀ in BMCs and skeletal muscle [28, 29] was demonstrated; therefore, CoQ₁₀ deficiency and mitochondrial dysfunction can also be present in other cells and tissues in FM patients. Furthermore, changes in the morphology and number of mitochondria have been shown in skeletal muscle from FM patients [30‐32], suggesting the role of mitochondrial dysfunction in this disorder. CoQ deficiency, mitochondrial dysfunction, and cell bioenergetics alteration could also explain the low muscular and aerobic capacity observed in some groups of FM patients [33, 34].

It has been proposed that ROS damage can induce MPT by opening of permeability transition pores in the mitochondrial inner membrane [35‐37]. This, in turn, leads to a simultaneous collapse of mitochondrial membrane potential and the elimination of dysfunctional mitochondria by mitophagy. Our results support this hypothesis, showing the presence of mitophagy in BMCs of FM patients. Autophagy is a regulated lysosomal pathway involved in the degradation and recycling of cytoplasmic materials [38‐42]. During autophagy, cytoplasmic materials are sequestered into double-membraned vesicles, 'autophagosomes', which then fuse with lysosomes to form autolysosomes, in which degradation of cellular structures occurs. Many cellular stresses can cause induction of autophagy, such as endoplasmic reticulum stress, mitochondrial dysfunction, or oxidative stress [42‐44]. 'Mitophagy' was coined to describe the selective removal of mitochondria by autophagy during development and under pathologic conditions [36, 45]. In our work, biochemical analysis of citrate synthase indicated a depletion of mitochondrial mass, suggesting selective mitochondrial degradation by mitophagy in BMCs from FM patients. These results were confirmed with electron microscopy that clearly shows autophagosomes where mitochondria are being degraded. Autophagy can be beneficial for the cells by eliminating dysfunctional mitochondria, but massive autophagy can promote cell injury [41] and may contribute to the pathophysiology of FM.

Our study supports the hypothesis that CoQ₁₀ deficiency, oxidative stress, and extensive mitophagy can contribute to cell-bioenergetics imbalance, compromising cell functionality. Abnormal BMC performance can promote oxidative stress and may contribute to altered nociception in FM.