Skeletal Muscle Mitochondrial Dysfunction and Oxidative Stress in Peripheral Arterial Disease: A Unifying Mechanism and Therapeutic Target

Abstract

Peripheral artery disease (PAD) is caused by atherosclerosis in the lower extremities, which leads to a spectrum of life-altering symptomatology, including claudication, ischemic rest pain, and gangrene requiring limb amputation. Current treatments for PAD are focused primarily on re-establishing blood flow to the ischemic tissue, implying that blood flow is the decisive factor that determines whether or not the tissue survives. Unfortunately, failure rates of endovascular and revascularization procedures remain unacceptably high and numerous cell- and gene-based vascular therapies have failed to demonstrate efficacy in clinical trials. The low success of vascular-focused therapies implies that non-vascular tissues, such as skeletal muscle and oxidative stress, may substantially contribute to PAD pathobiology. Clues toward the importance of skeletal muscle in PAD pathobiology stem from clinical observations that muscle function is a strong predictor of mortality. Mitochondrial impairments in muscle have been documented in PAD patients, although its potential role in clinical pathology is incompletely understood. In this review, we discuss the underlying mechanisms causing mitochondrial dysfunction in ischemic skeletal muscle, including causal evidence in rodent studies, and highlight emerging mitochondrial-targeted therapies that have potential to improve PAD outcomes. Particularly, we will analyze literature data on reactive oxygen species production and potential counteracting endogenous and exogenous antioxidants.

Keywords: bioenergetics; ischemia; myopathy; peripheral vascular disease; reactive oxygen species.

Figure 1

Pathogenesis of ischemic myopathy in peripheral artery disease (PAD). A graphical depiction of the pathogenesis of ischemic myopathy in PAD. This figure was created with Biorender.com.

Figure 2

Overview of mitochondrial energetics. A graphical overview of the mitochondrial energy transduction pathway. Electrons are transferred from complex carbon changes (sugars, proteins, fats) to carrier molecules (NAD⁺ and FAD⁺), which feed the electrons in the electron transport system (ETS) located in the inner mitochondrial membrane. The ETS transfers electrons across several protein complexes down a redox potential to oxygen. Consequently, the fall in redox potential powers the proton pumping capabilities of complexes I, III, and IV, which establish the proton motive force (mitochondrial membrane potential). The proton motive force is then used by the ATP synthase to drive phosphorylation of ADP to form ATP and establish the cellular energy charge (ATP/ADP), which powers all cellular work. Abbreviations: CoQ = ubiquinone/co-enzyme Q; TCA = tricarboxylic acid; ETFQOR = electron transferring flavoprotein-ubiquinone oxidoreducatase; Cyt C = cytochrome c. This figure was created with Biorender.com.

Figure 3

Summary of muscle mitochondrial assessments in PAD patients. A summary of studies assessing mitochondrial function via respirometry in muscle biopsy specimens of in vivo using ³¹P magnetic resonance spectroscopy (MRS). Data are presented as a percentage of the control group and patients were classified as intermittent claudicants (IC—mild PAD) or chronic limb-threatening ischemia (CLTI—severe PAD) based on the clinical characteristics provided by the authors of each paper. Please refer to the following references for these original studies: [53,74,75,76,77,78,79,80,81,82,83,84,85,86,87].

Figure 4

Ischemia-reperfusion injury as a source of oxidative stress in PAD skeletal muscle. A graphical depiction of the mitochondrial electron transport system (ETS) during normoxic conditions, ischemia conditions, and reperfusion conditions. Under normoxia, the functions to transfer potential energy from electrons on NADH/succinate to a proton motive force, which drives ATP synthesis to establish a high energy charge. During periods of ischemia, the low oxygen levels halt electron flow in the ETS due to the lack of oxygen availability, and subsequently, succinate accumulates and the cellular energy charge declines. With reperfusion of ischemic muscle, high levels of succinate oxidation result in reverse electron flow through complex I to drive high levels of ROS production. This figure was created with Biorender.com.

Figure 5

Redox-sensitive transcriptional signaling for muscle atrophy in PAD. Muscle biopsy specimens were collected from non-PAD control (Age = 72.8 ± 7.8 years, ABI = 1.2 ± 0.2) and severe PAD patients undergoing limb amputation (Age = 69.5 ± 6.2 years, ABI = 0.5 ± 0.3) following obtainment of informed consent (study approved by the IRB at the University of Florida). Other clinical characteristics of patients are described in [119]. RNA was isolated using a Qiagen RNeasy minikit and reverse transcribed into cDNA using Superscript IV (Thermofisher). Quantitative RT-PCR was performed using pre-validated Taqman probes (Thermofisher) to assess the levels of redox-sensitive atrogenes (A), autophagy genes (B), and caspases (C) known to be involved in muscle atrophy. Violin plots are shown with dashed lines representing the median and dotted lines the quartiles. n = 15/group. * p < 0.05, ** p < 0.01, **** p < 0.001 using two-tailed, unpaired Student’s t-test. Full Gene names are as follows: FBXO30 = F-box only protein 30; FBXO31 = F-box only protein 31; FBXO32 = F-box only protein 32; TRIM63 = tripartite motif containing 63 or muscle-specific RING finger; FoxO1 = forkhead box O1; FoxO3 = forkhead box O3; LC3B = microtubule associated protein 1 light chain 3 beta; GABARAPAL1 = GABA type A receptor associated protein like 2; Gadd45a = growth arrest and DNA damage inducible alpha; CASP1 = caspase 1; CASP3 = caspase 3; CASP9 = caspase 9; CASP12 = caspase 12.