Skeletal muscle mitochondrial DNA injury in patients with unilateral peripheral arterial disease

A 6-minute Limb Function Assessment for Therapeutic Testing in Experimental Peripheral Artery Disease Models

Background

The translation of promising therapies from pre-clinical models of hindlimb ischemia (HLI) to patients with peripheral artery disease (PAD) has been inadequate. While this failure is multifactorial, primary outcome measures in preclinical HLI models and clinical trials involving patients with PAD are not aligned well. For example, laser Doppler perfusion recovery measured under resting conditions is the most used outcome in HLI studies, whereas clinical trials involving patients with PAD primarily assess walking performance. Here, we sought to develop a 6-min limb function test for preclinical HLI models that assess muscular performance and hemodynamics congruently.

Methods

We developed an in situ 6-min limb function test that involves repeated isotonic (shortening) contractions performed against a submaximal load. Continuous measurement of muscle blood flow was performed using laser Doppler flowmetry. Quantification of muscle power, work, and perfusion are obtained across the test. To assess the efficacy of this test, we performed HLI via femoral artery ligation on several mouse strains: C57BL6J, BALBc/J, and MCK-PGC1α (muscle-specific overexpression of PGC1α). Additional experiments were performed using an exercise intervention (voluntary wheel running) following HLI.

Results

The 6-min limb function test was successful at detecting differences in limb function of C57BL6/J and BALBc/J mice subjected to HLI with effect sizes superior to laser Doppler perfusion recovery. C57BL6/J mice randomized to exercise therapy following HLI had smaller decline in muscle power, greater hyperemia, and performed more work across the 6-min limb function test compared to non-exercise controls with HLI. Mice with muscle-specific overexpression of PGC1α had no differences in perfusion recovery in resting conditions, but exhibited greater capillary density, increased muscle mass and absolute force levels, and performed more work across the 6-min limb function test compared to their wildtype littermates without the transgene.

Conclusion

These results demonstrate the efficacy of the 6-min limb function test to detect differences in the response to HLI across several interventions including where traditional perfusion recovery, capillary density, and muscle strength measures were unable to detect therapeutic differences.

Nuclear receptors as potential therapeutic targets in peripheral arterial disease and related myopathy

Peripheral arterial disease (PAD) is a prevalent cardiovascular complication of limb vascular insufficiency, causing ischemic injury, mitochondrial metabolic damage and functional impairment in the skeletal muscle, and ultimately leading to immobility and mortality. While potential therapies have been mostly focussed on revascularization, none of the currently available pharmacological treatments are fully effective in PAD, often leading to amputations, particularly in chronic metabolic diseases. One major limitation of focussed angiogenesis and revascularization as a therapeutic strategy is a limited effect on metabolic restoration and muscle regeneration in the affected limb. Therefore, additional preclinical investigations are needed to discover novel treatment options for PAD preferably targeting multiple aspects of muscle recovery. In this review, we propose nuclear receptors expressed in the skeletal muscle as potential candidates for ischemic muscle repair in PAD. We review classic steroid and orphan receptors that have been reported to be involved in the regulation of paracrine muscle angiogenesis, oxidative metabolism, mitochondrial biogenesis and muscle regeneration, and discuss how these receptors could be critical for recovery from ischemic muscle damage. Furthermore, we identify existing gaps in our understanding of nuclear receptor signalling in the skeletal muscle and propose future areas of research that could be instrumental in exploring nuclear receptors as therapeutic candidates for treating PAD.

Diet-induced obesity augments ischemic myopathy and functional decline in a murine model of peripheral artery disease

Peripheral artery disease (PAD) causes an ischemic myopathy contributing to patient disability and mortality. Most preclinical models to date use young, healthy rodents with limited translatability to human disease. Although PAD incidence increases with age, and obesity is a common comorbidity, the pathophysiologic association between these risk factors and PAD myopathy is unknown. Using our murine model of PAD, we sought to elucidate the combined effect of age, diet-induced obesity and chronic hindlimb ischemia (HLI) on (1) mobility, (2) muscle contractility, and markers of muscle (3) mitochondrial content and function, (4) oxidative stress and inflammation, (5) proteolysis, and (6) cytoskeletal damage and fibrosis. Following 16-weeks of high-fat, high-sucrose, or low-fat, low-sucrose feeding, HLI was induced in 18-month-old C57BL/6J mice via the surgical ligation of the left femoral artery at 2 locations. Animals were euthanized 4-weeks post-ligation. Results indicate mice with and without obesity shared certain myopathic changes in response to chronic HLI, including impaired muscle contractility, altered mitochondrial electron transport chain complex content and function, and compromised antioxidant defense mechanisms. However, the extent of mitochondrial dysfunction and oxidative stress was significantly greater in obese ischemic muscle compared to non-obese ischemic muscle. Moreover, functional impediments, such as delayed post-surgical recovery of limb function and reduced 6-minute walking distance, as well as accelerated intramuscular protein breakdown, inflammation, cytoskeletal damage, and fibrosis were only evident in mice with obesity. As these features are consistent with human PAD myopathy, our model could be a valuable tool to test new therapeutics.

Activation of the Aryl Hydrocarbon Receptor in Muscle Exacerbates Ischemic Pathology in Chronic Kidney Disease

BACKGROUND

Chronic kidney disease (CKD) accelerates the development of atherosclerosis, decreases muscle function, and increases the risk of amputation or death in patients with peripheral artery disease (PAD). However, the mechanisms underlying this pathobiology are ill-defined. Recent work has indicated that tryptophan-derived uremic solutes, which are ligands for AHR (aryl hydrocarbon receptor), are associated with limb amputation in PAD. Herein, we examined the role of AHR activation in the myopathy of PAD and CKD.

METHODS

AHR-related gene expression was evaluated in skeletal muscle obtained from mice and human PAD patients with and without CKD. AHRmKO (skeletal muscle-specific AHR knockout) mice with and without CKD were subjected to femoral artery ligation, and a battery of assessments were performed to evaluate vascular, muscle, and mitochondrial health. Single-nuclei RNA sequencing was performed to explore intercellular communication. Expression of the constitutively active AHR was used to isolate the role of AHR in mice without CKD.

RESULTS

PAD patients and mice with CKD displayed significantly higher mRNA expression of classical AHR-dependent genes (Cyp1a1, Cyp1b1, and Aldh3a1) when compared with either muscle from the PAD condition with normal renal function (P<0.05 for all 3 genes) or nonischemic controls. AHRmKO significantly improved limb perfusion recovery and arteriogenesis, preserved vasculogenic paracrine signaling from myofibers, increased muscle mass and strength, as well as enhanced mitochondrial function in an experimental model of PAD/CKD. Moreover, viral-mediated skeletal muscle-specific expression of a constitutively active AHR in mice with normal kidney function exacerbated the ischemic myopathy evidenced by smaller muscle masses, reduced contractile function, histopathology, altered vasculogenic signaling, and lower mitochondrial respiratory function.

CONCLUSIONS

These findings establish AHR activation in muscle as a pivotal regulator of the ischemic limb pathology in CKD. Further, the totality of the results provides support for testing of clinical interventions that diminish AHR signaling in these conditions.

Chronic activation of the aryl hydrocarbon receptor in muscle exacerbates ischemic pathology in chronic kidney disease

Chronic kidney disease (CKD) accelerates the development of atherosclerosis, decreases muscle function, and increases the risk of amputation or death in patients with peripheral artery disease (PAD). However, the cellular and physiological mechanisms underlying this pathobiology are ill-defined. Recent work has indicated that tryptophan-derived uremic toxins, many of which are ligands for the aryl hydrocarbon receptor (AHR), are associated with adverse limb outcomes in PAD. We hypothesized that chronic AHR activation, driven by the accumulation of tryptophan-derived uremic metabolites, may mediate the myopathic condition in the presence of CKD and PAD. Both PAD patients with CKD and mice with CKD subjected to femoral artery ligation (FAL) displayed significantly higher mRNA expression of classical AHR-dependent genes ( Cyp1a1 , Cyp1b1 , and Aldh3a1 ) when compared to either muscle from the PAD condition with normal renal function ( P <0.05 for all three genes) or non-ischemic controls. Skeletal-muscle-specific AHR deletion in mice (AHR mKO ) significantly improved limb muscle perfusion recovery and arteriogenesis, preserved vasculogenic paracrine signaling from myofibers, increased muscle mass and contractile function, as well as enhanced mitochondrial oxidative phosphorylation and respiratory capacity in an experimental model of PAD/CKD. Moreover, viral-mediated skeletal muscle-specific expression of a constitutively active AHR in mice with normal kidney function exacerbated the ischemic myopathy evidenced by smaller muscle masses, reduced contractile function, histopathology, altered vasculogenic signaling, and lower mitochondrial respiratory function. These findings establish chronic AHR activation in muscle as a pivotal regulator of the ischemic limb pathology in PAD. Further, the totality of the results provide support for testing of clinical interventions that diminish AHR signaling in these conditions.

BAG3 Attenuates Ischemia-Induced Skeletal Muscle Necroptosis in Diabetic Experimental Peripheral Artery Disease

Peripheral artery disease (PAD) is characterized by impaired blood flow to the lower extremities, resulting in ischemic limb injuries. Individuals with diabetes and PAD typically have more severe ischemic limb injuries and limb amputations, but the mechanisms involved are poorly understood. Previously, we identified BAG3 as a gene within a mouse genetic locus termed limb salvage QTL1 on mouse chromosome 7 that determined the extent of limb necrosis following ischemic injury in C57Bl/6 mice. Whether BAG3 deficiency plays a role in the severe ischemic injury observed in diabetic PAD is not known. In vitro, we found simulated ischemia enhanced BAG3 expression in primary human skeletal muscle cells, whereas BAG3 knockdown increased necroptosis markers and decreased cell viability. In vivo, ischemic skeletal muscles from hind limbs of high-fat diet (HFD)-fed mice showed poor BAG3 expression compared to normal chow diet (NCD)-fed mice, and this was associated with increased limb amputations. BAG3 overexpression in ischemic skeletal muscles from hind limbs of HFD mice rescued limb amputation and improved autophagy, necroptosis, skeletal muscle function and regeneration. Therefore, BAG3 deficiency in ischemic skeletal muscles contributes to the severity of ischemic limb injury in diabetic PAD, likely through autophagy and necroptosis pathways.

Impaired microcirculatory function, mitochondrial respiration, and oxygen utilization in skeletal muscle of claudicating patients with peripheral artery disease

Peripheral artery disease (PAD) is an atherosclerotic disease that impairs blood flow and muscle function in the lower limbs. A skeletal muscle myopathy characterized by mitochondrial dysfunction and oxidative damage is present in PAD; however, the underlying mechanisms are not well established. We investigated the impact of chronic ischemia on skeletal muscle microcirculatory function and its association with leg skeletal muscle mitochondrial function and oxygen delivery and utilization capacity in PAD. Gastrocnemius samples and arterioles were harvested from patients with PAD (n = 10) and age-matched controls (Con, n = 11). Endothelium-dependent and independent vasodilation was assessed in response to flow (30 μL·min-1), acetylcholine, and sodium nitroprusside (SNP). Skeletal muscle mitochondrial respiration was quantified by high-resolution respirometry, microvascular oxygen delivery, and utilization capacity (tissue oxygenation index, TOI) were assessed by near-infrared spectroscopy. Vasodilation was attenuated in PAD (P < 0.05) in response to acetylcholine (Con: 71.1 ± 11.1%, PAD: 45.7 ± 18.1%) and flow (Con: 46.6 ± 20.1%, PAD: 29.3 ± 10.5%) but not SNP (P = 0.30). Complex I + II state 3 respiration (P < 0.01) and TOI recovery rate were impaired in PAD (P < 0.05). Both flow and acetylcholine-mediated vasodilation were positively associated with complex I + II state 3 respiration (r = 0.5 and r = 0.5, respectively, P < 0.05). Flow-mediated vasodilation and complex I + II state 3 respiration were positively associated with TOI recovery rate (r = 0.8 and r = 0.7, respectively, P < 0.05). These findings suggest that chronic ischemia attenuates skeletal muscle arteriole endothelial function, which may be a key mediator for mitochondrial and microcirculatory dysfunction in the PAD leg skeletal muscle. Targeting microvascular dysfunction may be an effective strategy to prevent and/or reverse disease progression in PAD.NEW & NOTEWORTHY Ex vivo skeletal muscle arteriole endothelial function is impaired in claudicating patients with PAD, and this is associated with attenuated skeletal muscle mitochondrial respiration. In vivo skeletal muscle oxygen delivery and utilization capacity is compromised in PAD, and this may be due to microcirculatory and mitochondrial dysfunction. These results suggest that targeting skeletal muscle arteriole function may lead to improvements in skeletal muscle mitochondrial respiration and oxygen delivery and utilization capacity in claudicating patients with PAD.

Circulating Biomarkers in Lower Extremity Artery Disease

Lower extremity artery disease (LEAD), a chronic condition with disturbed lower extremity circulation due to narrowing of the arteries, is predominantly caused by atherosclerosis and is associated with the presence of cardiovascular risk factors and an increased risk of cardiovascular events. LEAD is prevalent among older individuals and predicted to rise with the ageing population. In progressive disease, the patient experiences symptoms of ischaemia when walking and, in advanced critical limb-threatening ischaemia, even at rest. However, LEAD is asymptomatic in most patients, delaying diagnosis and treatment. In this setting, circulating biomarkers may facilitate earlier diagnosis in selected individuals. This review provides a broad overview of the circulating biomarkers investigated to date in relation to LEAD and discusses their usefulness in clinical practice.

Macrophage Paired Immunoglobulin-Like Receptor B Deficiency Promotes Peripheral Atherosclerosis in Apolipoprotein E–Deficient Mice

Background: Peripheral atherosclerotic disease (PAD) is the narrowing or blockage of arteries that supply blood to the lower limbs. Given its complex nature, bioinformatics can help identify crucial genes involved in the progression of peripheral atherosclerosis.

Materials and Methods: Raw human gene expression data for 462 PAD arterial plaque and 23 normal arterial samples were obtained from the GEO database. The data was analyzed using an integrated, multi-layer approach involving differentially-expressed gene analysis, KEGG pathway analysis, GO term enrichment analysis, weighted gene correlation network analysis, and protein-protein interaction analysis. The monocyte/macrophage-expressed leukocyte immunoglobulin-like receptor B2 (LILRB2) was strongly associated with the human PAD phenotype. To explore the role of the murine LILRB2 homologue PirB in vivo, we created a myeloid-specific PirB-knockout Apoe^−/− murine model of PAD (PirB^MΦKO) to analyze femoral atherosclerotic burden, plaque features of vulnerability, and monocyte recruitment to femoral atherosclerotic lesions. The phenotypes of PirB^MΦKO macrophages under various stimuli were also investigated in vitro.

Results:PirB^MΦKO mice displayed increased femoral atherogenesis, a more vulnerable plaque phenotype, and enhanced monocyte recruitment into lesions. PirB^MΦKO macrophages showed enhanced pro-inflammatory responses and a shift toward M1 over M2 polarization under interferon-γ and oxidized LDL exposure. PirB^MΦKO macrophages also displayed enhanced efferocytosis and reduced lipid efflux under lipid exposure.

Conclusion: Macrophage PirB reduces peripheral atherosclerotic burden, stabilizes peripheral plaque composition, and suppresses macrophage accumulation in peripheral lesions. Macrophage PirB inhibits pro-inflammatory activation, inhibits efferocytosis, and promotes lipid efflux, characteristics critical to suppressing peripheral atherogenesis.

Endothelial cell-derived pro-fibrotic factors increase TGF-β1 expression by smooth muscle cells in response to cycles of hypoxia-hyperoxia

BACKGROUND

The vascular pathology of peripheral artery disease (PAD) encompasses abnormal microvascular architecture and fibrosis in response to ischemia-reperfusion (I/R) cycles. We aimed to investigate the mechanisms by which pathological changes in the microvasculature direct fibrosis in the context of I/R.

METHODS

Primary human aortic endothelial cells (ECs) were cultured under cycles of normoxia-hypoxia (NH) or normoxia-hypoxia-hyperoxia (NHH) to mimic I/R. Primary human aortic smooth muscle cells (SMCs) were cultured and treated with media from the ECs.

FINDINGS

The mRNA and protein expression of the pro-fibrotic factors platelet derived growth factor (PDGF)-BB and connective tissue growth factor (CTGF) were significantly upregulated in ECs undergoing NH or NHH cycles. Treatment of SMCs with media from ECs undergoing NH or NHH cycles led to significant increases in TGF-β1, TGF-β pathway signaling intermediates, and collagen expression. Addition of neutralizing antibodies against PDGF-BB and CTGF to the media blunted the increases in TGF-β1 and collagen expression. Treatment of SMCs with PAD patient-derived serum also led to increased TGF-β1 levels.

INTERPRETATION

In an in-vitro model of I/R, which recapitulates the pathophysiology of PAD, increased secretion of PDGF-BB and CTGF by ECs was shown to be predominantly driving TGF-β1-mediated expression by SMCs. These cell culture experiments help elucidate the mechanism and interaction between ECs and SMCs in microvascular fibrosis associated with I/R. Thus, targeting these pro-fibrotic factors may be an effective strategy to combat fibrosis in response to cycles of I/R.

FUNDING

National Institute on Aging at the National Institutes of Health grant number R01AG064420.

RESEARCH IN CONTEXT

Evidence before this study: Previous studies in gastrocnemius biopsies from peripheral artery disease (PAD) patients showed that transforming growth factor beta 1 (TGF-β1), the most potent inducer of pathological fibrosis, is increased in the vasculature of PAD patients and correlated with collagen deposition. However, the exact cellular source of TGF-β1 remained unclear. Added value of this study: Exposing cells to cycles of normoxia-hypoxia-hyperoxia (NHH) resulted in pathological changes that are consistent with human PAD. This supports the idea that the use of NHH may be a reliable, novel in vitro model of PAD useful for studying associated pathophysiological mechanisms. Furthermore, pro-fibrotic factors (PDGF-BB and CTGF) released from endothelial cells were shown to induce a fibrotic phenotype in smooth muscle cells. This suggests a potential interaction between these cell types in the microvasculature that drives increased TGF-β1 expression and collagen deposition. Thus, targeting these pro-fibrotic factors may be an effective strategy to combat fibrosis in response to cycles of ischemia-reperfusion.

Sarcopenia as a comorbidity of cardiovascular disease

Sarcopenia, the lowered skeletal muscle mass, weakened skeletal muscle strength, and reduced physical performance with aging, is a component of frailty and high-risk factor for falls, resulting in an increase in mortality. In cardiovascular disease (CVD) patients, systemic inflammation, oxidative stress, overactivation of ubiquitin-proteasome system, endothelial dysfunction, lowering muscle blood flow, impaired glucose tolerance, hormonal changes, and physical inactivity possibly contribute to CVD-related sarcopenia. Prevalence of sarcopenia and osteosarcopenia, which is osteopenia and sarcopenia coexisting together, seems to be higher in CVD patients than in community-dwelling adults, suggesting the necessity of early diagnosis and prevention of CVD-related sarcopenia. Atrial stiffness, coronary artery calcification score, and serum vitamin D levels may be of help as the biomarkers to suspect sarcopenia, and renin-angiotensin-aldosterone system inhibitors may play a role in the medical prevention and treatment of CVD-related sarcopenia. There are few reports to convince the efficacies of dietary and antioxidant supplementation on sarcopenia at present, whereas aerobic and resistance training exercises have been recognized as an effective strategy to prevent and treat sarcopenia.

Diffusion tensor imaging indices of acute muscle damage are augmented after exercise in peripheral arterial disease

PURPOSE

Although it is known that peripheral arterial disease (PAD) is associated with chronic myopathies, the acute muscular responses to exercise in this population are less clear. This study used diffusion tensor imaging (DTI) to compare acute exercise-related muscle damage between PAD patients and healthy controls.

METHODS

Eight PAD patients and seven healthy controls performed graded plantar flexion in the bore of a 3T MRI scanner. Exercise began at 2 kg and increased by 2 kg every 2 min until failure, or completion of 10 min of exercise. DTI images were acquired from the lower leg pre- and post-exercise, and were analyzed for mean diffusivity, fractional anisotropy (FA), and eigenvalues 1-3 (λ1-3) of the medial gastrocnemius (MG) and tibialis anterior (TA).

RESULTS

Results indicated a significant leg by time interaction for mean diffusivity, explained by a significantly greater increase in diffusivity of the MG in the most affected legs of PAD patients (11.1 × 10-4 ± 0.5 × 10-4 mm2/s vs. 12.7 × 10-4 ± 1.2 × 10-4 mm2/s at pre and post, respectively, P = 0.02) compared to healthy control subjects (10.8 × 10-4 ± 0.3 × 10-4 mm2/s vs. 11.2 × 10-4 ± 0.5 × 10-4 mm2/s at pre and post, respectively, P = 1.0). No significant differences were observed for the TA, or λ1-3 (all P ≥ 0.06). Moreover, no reciprocal changes were observed for FA in either group (all P ≥ 0.29).

CONCLUSION

These data suggest that calf muscle diffusivity increases more in PAD patients compared to controls after exercise. These findings are consistent with the notion that acute exercise results in increased muscle damage in PAD.

Unique Metabolomic Profile of Skeletal Muscle in Chronic Limb Threatening Ischemia

Chronic limb threatening ischemia (CLTI) is the most severe manifestation of peripheral atherosclerosis. Patients with CLTI have poor muscle quality and function and are at high risk for limb amputation and death. The objective of this study was to interrogate the metabolome of limb muscle from CLTI patients. To accomplish this, a prospective cohort of CLTI patients undergoing either a surgical intervention (CLTI Pre-surgery) or limb amputation (CLTI Amputation), as well as non-peripheral arterial disease (non-PAD) controls were enrolled. Gastrocnemius muscle biopsy specimens were obtained and processed for nuclear magnetic resonance (NMR)-based metabolomics analyses using solution state NMR on extracted aqueous and organic phases and ¹H high-resolution magic angle spinning (HR-MAS) on intact muscle specimens. CLTI Amputation specimens displayed classical features of ischemic/hypoxic metabolism including accumulation of succinate, fumarate, lactate, alanine, and a significant decrease in the pyruvate/lactate ratio. CLTI Amputation muscle also featured aberrant amino acid metabolism marked by elevated branched chain amino acids. Finally, both Pre-surgery and Amputation CLTI muscles exhibited pronounced accumulation of lipids, suggesting the presence of myosteatosis, including cholesterol, triglycerides, and saturated fatty acids. Taken together, these metabolite differences add to a growing body of literature that have characterized profound metabolic disturbance’s in the failing ischemic limb of CLTI patients.

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

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.

Mitochondrial DNA damage in calf skeletal muscle and walking performance in people with peripheral artery disease

BACKGROUND

Peripheral artery disease (PAD) is associated with mitochondrial dysfunction in calf skeletal muscle and a greater abundance of mitochondrial DNA (mtDNA) heteroplasmy. However, it is unknown whether calf skeletal muscle mtDNA of PAD participants harbors a greater abundance of mitochondrial DNA 4977-bp common deletion (mtDNA⁴⁹⁷⁷), strand breaks and oxidative damage (i.e., oxidized purines) compared to non-PAD participants and whether these mtDNA abnormalities are associated with poor walking performance in participants with PAD.

METHODS

Calf muscle biopsies were obtained from 50 PAD participants (ankle-brachial index (ABI) < 0.95) and 25 non-PAD participants (ABI = 0.99-1.40) matched by age, sex, and race. The abundance of mtDNA copy number, mtDNA⁴⁹⁷⁷ deletion, strand breaks, and oxidized purines in selected mtDNA regions coding for electron transport chain (ETC) constituents and the non-coding D-Loop region was determined in calf muscle. All participants completed measurement of 6-min walk and usual and fast-paced 4-m walking velocity test.

RESULTS

Participants with PAD (mean age = 65.4 years, SD = 6.9; 14 (28%) women, 38 (76%) black) and without PAD (mean age = 65.2 years, SD = 6.7; 7 (28%) women, 16 (64%) black) did not differ in the abundance of calf muscle mtDNA⁴⁹⁷⁷ deletion, mtDNA strand breaks, and oxidized purines. Though, a greater abundance of mtDNA strand breaks within ND4/5 genes was significantly associated with poorer 6-min walk distance, lower usual-paced 4-m walking velocity, and lower fast-paced 4-m walking velocity in non-PAD participants. Significant associations were also found in the density of strand break damage (i.e., damage per mtDNA copy) within ND1/2, ND4/5 and COII/ATPase 6/8 region with 6-min walk distance, usual-paced 4-m walking velocity and fast-paced 4-m walking velocity in non-PAD participants. Significant interactions were found between PAD presence vs. absence and density of strand break damage within ND1/2, ND4/5, COII/ATPase 6/8 regions for the associations with 6-min walk distance, usual-paced 4-m walking velocity, fast-paced 4-m walking velocity. Conversely, of the three walking performance measures only the usual-paced 4-m walking velocity showed a significant, although modest, negative association with the abundance of oxidized purines in the D-Loop (P = 0.031) and ND4/5 (P = 0.033) regions in the calf skeletal muscle of people with PAD.

CONCLUSION

Overall, these data suggest that the abundance of calf muscle mtDNA strand breaks and mtDNA⁴⁹⁷⁷ common deletion are not associated with walking performance in people with PAD and may not be directly involved in the pathophysiology of PAD. Conversely, strand breaks in specific mtDNA regions may contribute to poor walking performance in people without PAD. Further study is needed to confirm whether usual-paced 4-m walking velocity is associated significantly with a greater abundance of oxidized purines in the D-loop, a "mutational hotspot" for oxidative damage, and why this association may differ from the association with 6-min walk distance and fast-paced 4-m walking velocity.

Sarcopenia and peripheral arterial disease: a systematic review

BACKGROUND

Patients with lower extremity peripheral arterial disease (PAD) and sarcopenia are a population at risk requiring specific and targeted care. The aim of this review is to gather all relevant studies associating sarcopenia and PAD and to identify the underlying pathophysiological mechanisms as well as potential therapeutic strategies to improve skeletal muscle function.

METHODS

A systematic review was carried out following the recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA).

RESULTS

Data extraction allowed the evaluation of 140 publications; 87 met the inclusion criteria; of which 79 were included in the final review, reporting sufficient data for epidemiological and diagnostic criteria, mechanical analysis, and therapeutic approaches. Epidemiological analysis and diagnostic criteria were based on 18 studies following 2362 PAD patients [31.39% (SD 7.61) women], aged 72.42 (SD 2.84); sarcopenia was present in 34.63% (SD 12.86) of the patients. Mechanical and pathway analysis were based on five animal studies and 29 clinical reports, showing significantly altered muscle strength and function in 1352 PAD patients [26.49% (SD 17.32) women], aged 67.67 (SD 5.14) years; impaired muscle histology in 192 PAD patients (9.2% (SD 11.22) women), aged 64.3 (SD 0.99) years; +58.63% (SD 25.48) of oxidative stress in 69 PAD patients [16.96% (SD 8.10) women], aged 63.17 (SD 1.43) years; mitochondriopathy in 153 PAD patients [29.39% (SD 28.27) women], aged 63.50 (SD 1.83) years; +15.58% (SD 7.41) of inflammation in 900 PAD patients [40.77% (SD 3.71) women], aged 74.88 (SD 2.76) years; and altered signalling pathways in 51 PAD patients [34.45% (SD 32.23) women], aged 72.25 (SD 5.25) years. Therapeutic approaches analysis was based on seven animal studies and 21 clinical reports. In total, 884 patients followed an exercise therapy, and 18 received an angiogenesis treatment; 30.84% (SD 17.74) were women. Mean ages of patients studied were 66.85 (SD 3.96).

CONCLUSIONS

Sarcopenia and lower extremity PAD have musculoskeletal consequences that directly impair patients' quality of life and prognosis. Although PAD is primarily a vascular disease, all etiological factors of sarcopenia identified so far are present in PAD. Indeed, both sarcopenia and PAD are accompanied by oxidative stress, skeletal muscle mitochondrial impairments, inflammation, inhibition of specific pathways regulating muscle synthesis or protection (i.e. IGF-1, RISK, and SAFE), and activation of molecules associated with muscle degradation. To date, besides revascularization, the best therapeutic strategy includes exercise, but approaches targeting the underlying mechanisms still deserve further studies.

Associations of Peripheral Artery Disease With Calf Skeletal Muscle Mitochondrial DNA Heteroplasmy

Background Patients with peripheral artery disease (PAD) undergo frequent episodes of ischemia-reperfusion in lower extremity muscles that may negatively affect mitochondrial health and are associated with impaired mobility. We hypothesized that skeletal muscle from PAD patients will show high mitochondrial DNA heteroplasmy, especially in regions more susceptible to oxidative damage, such as the displacement loop, and that the degree of heteroplasmy will be correlated with the severity of ischemia and mobility impairment. Methods and Results Mitochondrial mutations and deletions and their relative abundance were identified by targeted mitochondrial DNA sequencing in biopsy specimens of gastrocnemius muscle from 33 PAD (ankle brachial index <0.9) and 9 non-PAD (ankle brachial index >0.9) subjects aged ≥60 years. The probability of heteroplasmy per DNA base was significantly higher for PAD subjects than non-PAD within each region. In adjusted models, PAD was associated with higher heteroplasmy than non-PAD (P=0.003), but the association was limited to microheteroplasmy, that is heteroplasmy found in 1% to 5% of all mitochondrial genomes (P=0.004). Heteroplasmy in the displacement loop and coding regions were significantly higher for PAD than non-PAD subjects after adjustment for age, sex, race, and diabetes mellitus (P=0.037 and 0.004, respectively). Low mitochondrial damage, defined by both low mitochondrial DNA copy number and low microheteroplasmy, was associated with better walking performance. Conclusions People with PAD have higher "low frequency" heteroplasmy in gastrocnemius muscle compared with people without PAD. Among people with PAD, those who had evidence of least mitochondrial damage, had better walking performance than those with more mitochondrial damage. Registration URL: http://www.clinicaltrials.gov. Unique identifier: NCT02246660.

The Rise of Mitochondria in Peripheral Arterial Disease Physiopathology: Experimental and Clinical Data

Peripheral arterial disease (PAD) is a frequent and serious condition, potentially life-threatening and leading to lower-limb amputation. Its pathophysiology is generally related to ischemia-reperfusion cycles, secondary to reduction or interruption of the arterial blood flow followed by reperfusion episodes that are necessary but also—per se—deleterious. Skeletal muscles alterations significantly participate in PAD injuries, and interestingly, muscle mitochondrial dysfunctions have been demonstrated to be key events and to have a prognosis value. Decreased oxidative capacity due to mitochondrial respiratory chain impairment is associated with increased release of reactive oxygen species and reduction of calcium retention capacity leading thus to enhanced apoptosis. Therefore, targeting mitochondria might be a promising therapeutic approach in PAD.

Chronic kidney disease exacerbates ischemic limb myopathy in mice via altered mitochondrial energetics

Chronic kidney disease (CKD) substantially increases the severity of peripheral arterial disease (PAD) symptomology, however, the biological mechanisms remain unclear. The objective herein was to determine the impact of CKD on PAD pathology in mice. C57BL6/J mice were subjected to a diet-induced model of CKD by delivery of adenine for six weeks. CKD was confirmed by measurements of glomerular filtration rate, blood urea nitrogen, and kidney histopathology. Mice with CKD displayed lower muscle force production and greater ischemic lesions in the tibialis anterior muscle (78.1 ± 14.5% vs. 2.5 ± 0.5% in control mice, P < 0.0001, N = 5-10/group) and decreased myofiber size (1661 ± 134 μm2 vs. 2221 ± 100 μm2 in control mice, P < 0.01, N = 5-10/group). This skeletal myopathy occurred despite normal capillary density (516 ± 59 vs. 466 ± 45 capillaries/20x field of view) and limb perfusion. CKD mice displayed a ~50-65% reduction in muscle mitochondrial respiratory capacity in ischemic muscle, whereas control mice had normal mitochondrial function. Hydrogen peroxide emission was modestly higher in the ischemic muscle of CKD mice, which coincided with decreased oxidant buffering. Exposure of cultured myotubes to CKD serum resulted in myotube atrophy and elevated oxidative stress, which were attenuated by mitochondrial-targeted therapies. Taken together, these findings suggest that mitochondrial impairments caused by CKD contribute to the exacerbation of ischemic pathology.

High prevalence of exercise-induced ischemia in the asymptomatic limb of patients with apparently strictly unilateral symptoms and unilateral peripheral artery disease

BACKGROUND:

The prevalence of exercise-induced ischemia in the asymptomatic limb of patients with unilateral claudication based on history and treadmill evaluation, and with unilateral ipsilateral peripheral artery disease (i.e ankle-to-brachial systolic pressure index <0.90) is unknown.

METHODS:

We detected exercise-induced ischemia in the asymptomatic limb of patients with apparently unilateral claudication. Among 6059 exercise-oximetry tests performed in 3407 nondiabetic and 961 diabetic patients. We estimated the intensity of ischemia in the both limb (buttocks and calves) using the lowest minimum value of the decrease from rest of oxygen pressure (DROP; limb changes minus chest changes from rest), with significant ischemia defined as DROP lower than -15 mmHg.

RESULTS:

We found 152 tests performed in 142 nondiabetic patients and 40 tests performed in 38 diabetic patients. The asymptomatic limb showed significant ischemia in 46.7% and 37.5% of the tests. Strictly unilateral exercise-induced claudication with apparently unilateral peripheral artery disease was rare (<4% of all tests). However, among these highly selected tests, significant ischemia was found in the asymptomatic limb in more than one-third of cases.

CONCLUSION:

The asymptomatic limb of patients with peripheral artery disease should not be considered a normal limb.

Metabolic Response to Claudication in Peripheral Arterial Disease: A Microdialysis Pilot Study

OBJECTIVES

This is a pilot study, and the objective of the study was to investigate the possible uses of microdialysis in the calf muscle to assess the metabolic response to intermittent claudication (IC) and in addition evaluate the simultaneous systemic inflammatory reaction.

METHODS

Dialysate and venous blood sampling was performed before, during and after walking on a treadmill to maximal tolerable claudication (controls 10 min) using 1 microdialysis catheter inserted in the gastrocnemius muscle, 1 subcutaneously in the pectoral region (as a reference), and a peripheral venous catheter.

RESULTS

A total of 9 participants were recruited, 6 patients with IC and 3 healthy control subjects. At baseline, patients with IC and control subjects did not differ in metabolic findings (glucose, lactate, pyruvate, and glycerol) in the gastrocnemius muscle. Subcutaneous glucose concentration was higher in control subjects. After physical exertion, gastrocnemius and subcutaneous glycerol, lactate, and pyruvate concentrations increased in patients with IC. Plasma concentrations of tumor necrosis factor α (TNF-α), interleukin-6 (IL-6), interleukin-1ß (IL-1ß), hepatocyte growth factor, and vascular endothelial growth factor were higher in IC subjects at baseline, and TNF-α, IL-6, and IL-18 increased after walking as did IL-6 and IL-1ß in control subjects. The muscle catheters did not show any signs of harm.

CONCLUSIONS

Microdialysis can be used to study the ongoing metabolic response during walking and claudication. Our results suggest both an acute local and a systemic inflammatory reaction during development of claudication.

Peripheral artery disease, calf skeletal muscle mitochondrial DNA copy number, and functional performance

In people without lower extremity peripheral artery disease (PAD), mitochondrial DNA copy number declines with aging, and this decline is associated with declines in mitochondrial activity and functional performance. However, whether lower extremity ischemia is associated with lower mitochondrial DNA copy number and whether mitochondrial DNA copy number is associated with the degree of functional impairment in people with PAD is unknown. In people with and without PAD, age 65 years and older, we studied associations of the ankle-brachial index (ABI) with mitochondrial DNA copy number and associations of mitochondrial DNA copy number with functional impairment. Calf muscle biopsies were obtained from 34 participants with PAD (mean age: 73.5 years (SD 6.4), mean ABI: 0.67 (SD 0.15), mean 6-minute walk distance: 1191 feet (SD 223)) and 10 controls without PAD (mean age: 73.1 years (SD 4.7), mean ABI: 1.14 (SD 0.07), mean 6-minute walk distance: 1387 feet (SD 488)). Adjusting for age and sex, lower ABI values were associated with higher mitochondrial DNA copy number, measured in relative copy number (ABI<0.60: 914, ABI 0.60-0.90: 731, ABI 0.90-1.50: 593; p trend=0.016). The association of mitochondrial DNA copy number with the 6-minute walk distance and 4-meter walking velocity differed significantly between participants with versus without PAD ( p-value for interaction=0.001 and p=0.015, respectively). The correlation coefficient between mitochondrial DNA copy number and the 6-minute walk distance was 0.653 ( p=0.056) among people without PAD and -0.254 ( p=0.154) among people with PAD and ABI < 0.90. In conclusion, lower ABI values are associated with increased mitochondrial DNA copy number. Associations of mitochondrial DNA copy number with the 6-minute walk distance and 4-meter walking velocity significantly differed between people with versus without PAD, with stronger positive associations observed in people without PAD than in people with PAD. The cross-sectional and exploratory nature of the analyses precludes conclusions regarding causal inferences. ClinicalTrials.gov Identifier: NCT02246660.

Gait pattern in patients with peripheral artery disease

Background

The aim of the present paper is to assess the gait pattern of patients with Peripheral Artery Disease (PAD). A more specific aim is to compare the gait pattern of PAD patients before and after the appearance of intermittent claudication symptoms.

Methods

The study involved 34 PAD patients with a claudication distance ≥200 m and 20 participants without PAD, who formed the control group. The gait pattern of PAD patients was assessed twice: before the appearance of intermittent claudication symptoms (pain-free conditions) and after the appearance of intermittent claudication symptoms (pain conditions).

Results

Compared to the control group, PAD patients presented a statistically significant decrease in step length both during pain-free conditions (52.6 ± 12.5 vs. 72.8 ± 18.5 cm, p = 0.008) and in pain conditions (53.3 ± 13.3 vs. 72.8 ± 18.5 cm, p = 0.006). As for the remaining spatiotemporal parameters, there were no differences observed between the patient group and the controls. Intermittent claudication symptom induced by the walking test on the treadmill did not bring about any new abnormalities in the gait pattern or intensify the existing abnormalities of the gait.

Conclusions

PAD patients have a tendency to shorten their step length regardless of the presence of intermittent claudication.

Growth Hormone (GH) and Cardiovascular System

This review describes the positive effects of growth hormone (GH) on the cardiovascular system. We analyze why the vascular endothelium is a real internal secretion gland, whose inflammation is the first step for developing atherosclerosis, as well as the mechanisms by which GH acts on vessels improving oxidative stress imbalance and endothelial dysfunction. We also report how GH acts on coronary arterial disease and heart failure, and on peripheral arterial disease, inducing a neovascularization process that finally increases flow in ischemic tissues. We include some preliminary data from a trial in which GH or placebo is given to elderly people suffering from critical limb ischemia, showing some of the benefits of the hormone on plasma markers of inflammation, and the safety of GH administration during short periods of time, even in diabetic patients. We also analyze how Klotho is strongly related to GH, inducing, after being released from the damaged vascular endothelium, the pituitary secretion of GH, most likely to repair the injury in the ischemic tissues. We also show how GH can help during wound healing by increasing the blood flow and some neurotrophic and growth factors. In summary, we postulate that short-term GH administration could be useful to treat cardiovascular diseases.

Optical probing of gastrocnemius in patients with peripheral artery disease characterizes myopathic biochemical alterations and correlates with stage of disease

Peripheral artery disease (PAD) is a condition caused by atherosclerotic blockages in the arteries supplying the lower limbs and is characterized by ischemia of the leg, progressive myopathy, and increased risk of limb loss. The affected leg muscles undergo significant changes of their biochemistry and metabolism including variations in the levels of many key proteins, lipids, and nucleotides. The mechanisms behind these changes are poorly understood. The objective of this study was to correlate the severity of the PAD disease stage and associated hemodynamic limitation (determined by the ankle brachial index, ABI) in the legs of the patients with alterations in the biochemistry of chronically ischemic leg muscle as determined by ATR‐Fourier transform infrared micro‐spectroscopy. Muscle (gastrocnemius) biopsies were collected from 13 subjects including four control patients (ABI≥0.9), five claudicating patients (0.4 ≤ ABI<0.9), and four critical limb ischemia (CLI) patients (ABI<0.4). Slide mounted specimens were analyzed by ATR‐Fourier transform infrared micro‐spectroscopy. An analysis of variance and a partial least squares regression model were used to identify significant differences in spectral peaks and correlate them with the ABI. The spectra revealed significant differences (P < 0.05) across control, claudicating, and CLI patients in the fingerprint and functional group regions. Infrared microspectroscopic probing of ischemic muscle biopsies demonstrates that PAD produces significant and unique changes to muscle biochemistry in comparison to control specimens. These distinctive biochemical profiles correlate with disease progression and may provide insight and direction for new targets in the diagnosis and therapy of muscle degeneration in PAD.

Exercise in claudicants increase or decrease walking ability and the response relates to mitochondrial function

Background

Exercise of patients with intermittent claudication improves walking performance. Exercise does not usually increase blood flow, but seems to increase muscle mitochondrial enzyme activities. Although exercise is beneficial in most patients, it might be harmful in some. The mitochondrial response to exercise might therefore differ between patients. Our hypothesis was that changes in walking performance relate to changes in mitochondrial function after 8 weeks of exercise. At a subgroup level, negative responders decrease and positive responders increase mitochondrial capacity.

Methods

Two types of exercise were studied, calf raising and walking (n = 28). We wanted to see whether there were negative and positive responders, independent of type of exercise. Measurements of walking performance, peripheral hemodynamics, mitochondrial respiration and content (citrate synthase activity) were obtained on each patient before and after the intervention period. Multiple linear regression was used to test whether changes in peak walking time relate to mitochondrial function. Subgroups of negative (n = 8) and positive responders (n = 8) were defined as those that either decreased or increased peak walking time following exercise. Paired t test and analysis of covariance was used to test changes within and between subgroups.

Results

Changes in peak walking time were related to changes in mitochondrial respiration supported by electron transferring flavoprotein (ETF + CI)_P (p = 0.004), complex I (CI + ETF)_P (p = 0.003), complex I + complex II (CI + CII + ETF)_P (p = 0.037) and OXPHOS coupling efficiency (p = 0.046) in the whole group. Negative responders had more advanced peripheral arterial disease. Mitochondrial respiration supported by electron transferring flavoprotein (ETF + CI)_P (p = 0.0013), complex I (CI + ETF)_P (p = 0.0005), complex I + complex II (CI + CII + ETF)_P (p = 0.011) and electron transfer system capacity (CI + CII + ETF)_E (p = 0.021) and OXPHOS coupling efficiency decreased in negative responders (p = 0.0007) after exercise. Positive responders increased citrate synthase activity (p = 0.010).

Conclusions

Changes in walking performance seem to relate to changes in mitochondrial function after exercise. Negative responders have more advanced peripheral arterial disease and decrease, while positive responders increase mitochondrial capacity.

Trial registration ClinicalTrials.gov ID: NCT023110256

Mitochondrial Bioenergetics in the Metabolic Myopathy Accompanying Peripheral Artery Disease

Peripheral artery disease (PAD) is a serious but relatively underdiagnosed and undertreated clinical condition associated with a marked reduction in functional capacity and a heightened risk of morbidity and mortality. The pathophysiology of lower extremity PAD is complex, and extends beyond the atherosclerotic arterial occlusion and subsequent mismatch between oxygen demand and delivery to skeletal muscle mitochondria. In this review, we evaluate and summarize the available evidence implicating mitochondria in the metabolic myopathy that accompanies PAD. Following a short discussion of the available in vivo and in vitro methodologies to quantitate indices of muscle mitochondrial function, we review the current evidence implicating skeletal muscle mitochondrial dysfunction in the pathophysiology of PAD myopathy, while attempting to highlight questions that remain unanswered. Given the rising prevalence of PAD, the detriment in quality of life for patients, and the associated significant healthcare resource utilization, new alternate therapies that ameliorate lower limb symptoms and the functional impairment associated with PAD are needed. A clear understanding of the role of mitochondria in the pathophysiology of PAD may contribute to the development of novel therapeutic interventions.

Disruption of mitochondrial quality control in peripheral artery disease: New therapeutic opportunities

Peripheral artery disease (PAD) is a multifactorial disease initially triggered by reduced blood supply to the lower extremities due to atherosclerotic obstructions. It is considered a major public health problem worldwide, affecting over 200 million people. Management of PAD includes smoking cessation, exercise, statin therapy, antiplatelet therapy, antihypertensive therapy and surgical intervention. Although these pharmacological and non-pharmacological interventions usually increases blood flow to the ischemic limb, morbidity and mortality associated with PAD continue to increase. This scenario raises new fundamental questions regarding the contribution of intrinsic metabolic changes in the distal affected skeletal muscle to the progression of PAD. Recent evidence suggests that disruption of skeletal muscle mitochondrial quality control triggered by intermittent ischemia-reperfusion injury is associated with increased morbidity in individuals with PAD. The mitochondrial quality control machinery relies on surveillance systems that help maintaining mitochondrial homeostasis upon stress. In this review, we describe some of the most critical mechanisms responsible for the impaired skeletal muscle mitochondrial quality control in PAD. We also discuss recent findings on the central role of mitochondrial bioenergetics and quality control mechanisms including mitochondrial fusion-fission balance, turnover, oxidative stress and aldehyde metabolism in the pathophysiology of PAD, and highlight their potential as therapeutic targets.

Diminished force production and mitochondrial respiratory deficits are strain-dependent myopathies of subacute limb ischemia

OBJECTIVE

Reduced skeletal muscle mitochondrial function might be a contributing mechanism to the myopathy and activity based limitations that typically plague patients with peripheral arterial disease (PAD). We hypothesized that mitochondrial dysfunction, myofiber atrophy, and muscle contractile deficits are inherently determined by the genetic background of regenerating ischemic mouse skeletal muscle, similar to how patient genetics affect the distribution of disease severity with clinical PAD.

METHODS

Genetically ischemia protected (C57BL/6) and susceptible (BALB/c) mice underwent either unilateral subacute hind limb ischemia (SLI) or myotoxic injury (cardiotoxin) for 28 days. Limbs were monitored for blood flow and tissue oxygen saturation and tissue was collected for the assessment of histology, muscle contractile force, gene expression, mitochondrial content, and respiratory function.

RESULTS

Despite similar tissue O₂ saturation and mitochondrial content between strains, BALB/c mice suffered persistent ischemic myofiber atrophy (55.3% of C57BL/6) and muscle contractile deficits (approximately 25% of C57BL/6 across multiple stimulation frequencies). SLI also reduced BALB/c mitochondrial respiratory capacity, assessed in either isolated mitochondria (58.3% of C57BL/6 at SLI on day (d)7, 59.1% of C57BL/6 at SLI d28 across multiple conditions) or permeabilized myofibers (38.9% of C57BL/6 at SLI d7; 76.2% of C57BL/6 at SLI d28 across multiple conditions). SLI also resulted in decreased calcium retention capacity (56.0% of C57BL/6) in BALB/c mitochondria. Nonischemic cardiotoxin injury revealed similar recovery of myofiber area, contractile force, mitochondrial respiratory capacity, and calcium retention between strains.

CONCLUSIONS

Ischemia-susceptible BALB/c mice suffered persistent muscle atrophy, impaired muscle function, and mitochondrial respiratory deficits during SLI. Interestingly, parental strain susceptibility to myopathy appears specific to regenerative insults including an ischemic component. Our findings indicate that the functional deficits that plague PAD patients could include mitochondrial respiratory deficits genetically inherent to the regenerating muscle myofibers.

Mitochondrial Respiration after One Session of Calf Raise Exercise in Patients with Peripheral Vascular Disease and Healthy Older Adults

PURPOSE

Mitochondria are essential for energy production in the muscle cell and for this they are dependent upon a sufficient supply of oxygen by the circulation. Exercise training has shown to be a potent stimulus for physiological adaptations and mitochondria play a central role. Whether changes in mitochondrial respiration are seen after exercise in patients with a reduced circulation is unknown. The aim of the study was to evaluate the time course and whether one session of calf raise exercise stimulates mitochondrial respiration in the calf muscle of patients with peripheral vascular disease.

METHODS

One group of patients with peripheral vascular disease (n = 11) and one group of healthy older adults (n = 11) were included. Patients performed one session of continuous calf raises followed by 5 extra repetitions after initiation of pain. Healthy older adults performed 100 continuous calf raises. Gastrocnemius muscle biopsies were collected at baseline and 15 minutes, one hour, three hours and 24 hours after one session of calf raise exercise. A multi substrate (octanoylcarnitine, malate, adp, glutamate, succinate, FCCP, rotenone) approach was used to analyze mitochondrial respiration in permeabilized fibers. Mixed-linear model for repeated measures was used for statistical analyses.

RESULTS

Patients with peripheral vascular disease have a lower baseline respiration supported by complex I and they increase respiration supported by complex II at one hour post-exercise. Healthy older adults increase respiration supported by electron transfer flavoprotein and complex I at one hour and 24 hours post-exercise.

CONCLUSION

Our results indicate a shift towards mitochondrial respiration supported by complex II as being a pathophysiological component of peripheral vascular disease. Furthermore exercise stimulates mitochondrial respiration already after one session of calf raise exercise in patients with peripheral vascular disease and healthy older adults.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01842412.

Multiple skeletal muscle mitochondrial DNA deletions in patients with unilateral peripheral arterial disease

Peripheral arterial disease (PAD) is associated with metabolic derangements and accumulation of the common 4977 bp mitochondrial DNA (mtDNA) deletion mutation. The current study was undertaken to test the hypothesis that PAD is associated with multiple mtDNA deletions. Gastrocnemius biopsies were obtained from nine patients with unilateral PAD. DNA extracted from the biopsies was analyzed for mtDNA deletions using a primer- shift PCR strategy. Multiple primers and strict, prospective criteria were used to identify deletions. PAD was associated with multiple mtDNA deletions (average of 8.2 distinct deletions in muscle from the hemodynamically affected limb). mtDNA injury was present in both the worse- and less-affected limbs of the unilateral PAD patients, and the estimated degree of mtDNA injury was strongly correlated in the two limbs on an intra-subject basis. The 4977 bp deletion was frequently identified, but was not always the deletion of highest frequency in individual samples. The estimated relative frequency of the 4977 bp deletion was correlated with the overall mtDNA injury in the biopsies. In summary, PAD is associated with mtDNA injury as reflected by multiple deletion mutations. As the mutations are not limited to the ischemic limb in unilateral patients, they are unlikely to contribute to the pathophysiology of claudication.

Skeletal muscle metabolic changes in peripheral arterial disease contribute to exercise intolerance: a point-counterpoint discussion

Patients with claudication have a marked impairment in exercise performance. Several factors contribute to this limitation, including reductions in large vessel blood flow and oxygen delivery as well as metabolic abnormalities in skeletal muscle. The relative contribution of these factors and their role in the pathophysiology of the exercise limitation is discussed using a point-counterpoint approach. Future directions for research conclude the discussion.

Histopathological changes in skeletal muscle associated with chronic ischaemia

Muscle biopsy is an essential part in the diagnostic workup in patients with suspected neuromuscular disorders. It is therefore important to be aware of morphological alterations that can be caused by systemic factors or natural ageing. Chronic limb ischaemia is frequent in elderly individuals. This study was performed to examine histopathological and mitochondrial changes in muscle in patients with chronic critical limb ischaemia. Muscle biopsy of skeletal muscle of the lower limb of patients with chronic ischaemia leading to amputation was performed and compared with muscle biopsies of healthy, age-matched controls. The histopathological abnormalities included fibrosis, necrosis, atrophy, glycogen depletion, internal nuclei, rimmed vacuoles, fibre type grouping, cytochrome c oxidase deficient fibres, MHC-I upregulation, and signs of microangiopathy. The only alteration found in age-matched controls was a few cytochrome c oxidase deficient fibres. There were also increased levels of multiple mitochondrial DNA deletions in ischaemic muscles compared with controls. Critical limb ischaemia is associated with significant histopathological changes in muscle tissue and also increased levels of mitochondrial DNA deletions. Since the alterations mimic different primary myopathic changes, chronic ischaemia is important to consider as a differential diagnosis in elderly individuals, investigated with muscle biopsy for muscle disease.

Mitochondrial therapy improves limb perfusion and myopathy following hindlimb ischemia

Critical limb ischemia is a devastating manifestation of peripheral arterial disease with no effective strategies for improving morbidity and mortality outcomes. We tested the hypothesis that cellular mitochondrial function is a key component of limb pathology and that improving mitochondrial function represents a novel paradigm for therapy. BALB/c mice were treated with a therapeutic mitochondrial-targeting peptide (MTP-131) and subjected to limb ischemia (HLI). Compared to vehicle control, MTP-131 rescued limb muscle capillary density and blood flow (64.7±11% of contralateral vs. 39.9±4%), and improved muscle regeneration. MTP-131 also increased electron transport system flux across all conditions at HLI day-7. In vitro, primary muscle cells exposed to experimental ischemia demonstrated markedly reduced (~75%) cellular respiration, which was rescued by MTP-131 during a recovery period. Compared to muscle cells, endothelial cell (HUVEC) respiration was inherently protected from ischemia (~30% reduction), but was also enhanced by MTP-131. These findings demonstrate an important link between ischemic tissue bioenergetics and limb blood flow and indicate that the mitochondria may be a pharmaceutical target for therapeutic intervention during critical limb ischemia.

The impact of peripheral arterial disease: A proposal for a new classification

Clasically, intermittent claudication, an intermediate stage in peripheral arterial disease, has been considered as a benign condition when considering only the muscular pain on walking. In this paper our aim is to attract attention about the effects linked to ischemic pain and the oxidative injury resulting from episodes of ischemia/reperfusion. Throughout this process alterations in calcium homeostasis as well as uncontrolled generation of reactive oxygen species, in association with the mitochondrial dysfunction and inflammatory phenomena, could lead to accelerate atherosclerosis, with an increased cardiovascular risk stated by means of a reduced ankle-brachial index. Taking this idea into account we propose a possible new classification for the management of the peripheral arterial disease, combining the Fontaine and Rutherford classifications and thinking about the described systemic effects in order to change the traditional management of peripheral arterial disease.

Mitochondrial Regulation of the Muscle Microenvironment in Critical Limb Ischemia

Critical limb ischemia (CLI) is the most severe clinical presentation of peripheral arterial disease and manifests as chronic limb pain at rest and/or tissue necrosis. Current clinical interventions are largely ineffective and therapeutic angiogenesis based trials have shown little efficacy, highlighting the dire need for new ideas and novel therapeutic approaches. Despite a decade of research related to skeletal muscle as a determinant of morbidity and mortality outcomes in CLI, very little progress has been made toward an effective therapy aimed directly at the muscle myopathies of this disease. Within the muscle cell, mitochondria are well positioned to modulate the ischemic cellular response, as they are the principal sites of cellular energy production and the major regulators of cellular redox charge and cell death. In this mini review, we update the crucial importance of skeletal muscle to CLI pathology and examine the evolving influence of muscle and endothelial cell mitochondria in the complex ischemic microenvironment. Finally, we discuss the novelty of muscle mitochondria as a therapeutic target for ischemic pathology in the context of the complex co-morbidities often associated with CLI.

Subacute limb ischemia induces skeletal muscle injury in genetically susceptible mice independent of vascular density

OBJECTIVE

The primary preclinical model of peripheral artery disease, which involves acute limb ischemia (ALI), can result in appreciable muscle injury that is attributed to the acuity of the ischemic injury. A less acute model of murine limb ischemia using ameroid constrictors (ACs) has been developed in an attempt to mimic the chronic nature of human disease. However, there is currently little understanding of how genetics influence muscle injury following subacute arterial occlusion in the mouse.

METHODS

We investigated the influence of mouse genetics on skeletal muscle tissue survival, blood flow, and vascular density by subjecting two different mouse strains, C57BL/6 (BL6) and BALB/c, to ALI or subacute limb ischemia using single (1AC) or double (2AC) AC placement on the femoral artery.

RESULTS

Similar to ALI, the 2AC model resulted in significant tissue necrosis and limb perfusion deficits in genetically susceptible BALB/c but not BL6 mice. In the 1AC model, no outward evidence of tissue necrosis was observed, and there were no differences in limb blood flow between BL6 and BALB/c. However, BALB/c mice displayed significantly greater muscle injury, as evidenced by increased inflammation and myofiber atrophy, despite having no differences in CD31(+) and SMA(+) vascular density and area. BALB/c mice also displayed significantly greater centralized myonuclei, indicating increased muscle regeneration.

CONCLUSIONS

The susceptibility of skeletal muscle to ischemia-induced injury is at least partly independent of muscle blood flow and vascular density, consistent with a muscle cell autonomous response that is genetically determined. Further development of preclinical models of peripheral artery disease that more accurately reflect the nature of the human disease may allow more accurate identification of genetic targets for therapeutic intervention.

A review of the pathophysiology and potential biomarkers for peripheral artery disease

Peripheral artery disease (PAD) is due to the blockage of the arteries supplying blood to the lower limbs usually secondary to atherosclerosis. The most severe clinical manifestation of PAD is critical limb ischemia (CLI), which is associated with a risk of limb loss and mortality due to cardiovascular events. Currently CLI is mainly treated by surgical or endovascular revascularization, with few other treatments in routine clinical practice. There are a number of problems with current PAD management strategies, such as the difficulty in selecting the appropriate treatments for individual patients. Many patients undergo repeated attempts at revascularization surgery, but ultimately require an amputation. There is great interest in developing new methods to identify patients who are unlikely to benefit from revascularization and to improve management of patients unsuitable for surgery. Circulating biomarkers that predict the progression of PAD and the response to therapies could assist in the management of patients. This review provides an overview of the pathophysiology of PAD and examines the association between circulating biomarkers and PAD presence, severity and prognosis. While some currently identified circulating markers show promise, further larger studies focused on the clinical value of the biomarkers over existing risk predictors are needed.

Association of mutations in the mitochondrial genome with the subclinical carotid atherosclerosis in women

The importance of the study of an association of mitochondrial DNA mutations with asymptomatic atherosclerosis in women is undeniable. In the present study, a series of PCR with primers for mutation region and further amplificate pyrosequencing were carried out to identify point substitutions or microdeletions of the mitochondrial genome. The results obtained were processed using the original method of estimating the level of heteroplasmy. Five mutations in the mitochondrial genome, namely C3256T, G14709A, G12315A, G13513A and G14846A, in which the heteroplasmy level was associated with the degree of preclinical atherosclerosis in women, were identified. The data obtained in the study showed that C3256T, G14709A and G12315A mutations have a positive correlation with atherosclerosis while G13513A and G14846A mutations have a negative correlation with atherosclerotic lesions. Total mutational load of the mitochondrial genome for C3256T, G14709A, G12315A, G13513A and G14846A mutations explains 68% of the variability of thickness of the carotid intima-medial layer, while the complex of traditional risk factors for cardiovascular diseases explains only 8% of the IMT variability. Data on the correlation between heteroplasmy levels of C3256T, G14709A, G12315A, G13513A and G14846A mutations prompt a suggestion that these mutations may be present on the same haplotypes of mitochondrial genome, associated with atherosclerosis.

Protein concentration and mitochondrial content in the gastrocnemius predicts mortality rates in patients with peripheral arterial disease

OBJECTIVE

This study evaluated the hypothesis that protein concentration and mitochondrial content in gastrocnemius biopsies from patients with peripheral arterial disease (PAD) predict mortality rates.

BACKGROUND

PAD patients experience advancing myopathy characterized by mitochondrial dysfunction, myofiber degradation, and fibrosis in their ischemic legs, along with increased mortality rates.

METHODS

Samples from the gastrocnemius of PAD patients were used for all analyses. Protein concentration was normalized to muscle wet weight, and citrate synthase activity (standard measure of mitochondrial content in cells) was normalized to muscle wet weight and protein concentration. Protein and citrate synthase data were grouped into tertiles and 5-year, all-cause mortality for each tertile was determined with Kaplan-Meier curves and compared by the modified Peto-Peto test. A Cox-regression model for each variable controlled for the effects of clinical characteristics.

RESULTS

Of the 187 study participants, 46 died during a mean follow-up of 23.0 months. Five-year mortality rate was highest for patients in the lowest tertile of protein concentration. Mortality was lowest for patients in the middle tertile of citrate synthase activity when normalized to either muscle wet weight or protein concentration. The mortality hazard ratios (HRs) from the Cox analysis were statistically significant for protein concentration normalized to muscle wet weight (lowest vs middle tertile; HR = 2.93; P = 0.008) and citrate synthase normalized to protein concentration (lowest vs middle tertile; HR = 4.68; P = 0.003; and lowest vs highest tertile; HR = 2.36; P = 0.027).

CONCLUSIONS

Survival analysis of a contemporaneous population of PAD patients identifies protein and mitochondrial content of their gastrocnemius as predictors of mortality rate.

Role of stereotactic radiosurgery in gynecologic cancer

PURPOSE OF REVIEW

This current work offers an opinion on the evidence accumulated in humans regarding stereotactic ablative radiation (SABR) therapy, a favorable option for the treatment of persistent or recurrent metastatic gynecologic cancers.

RECENT FINDINGS

SABR precisely and accurately kills cancer cells, making durable disease control possible. This article evaluates SABR's clinical performance and the variations in the main components of SABR technology, such as target localization and radiation beam collimation coupled with respiratory motion tracking, in the context of their contribution to the banked clinical successes in gynecologic radiation oncology.

SUMMARY

In an era of patient requests for short radiotherapy courses, SABR now provides well tolerated and efficacious treatment for women with gynecologic cancers.

Exercise performance and peripheral vascular insufficiency improve with AMPK activation in high-fat diet-fed mice

Intermittent claudication is a form of exercise intolerance characterized by muscle pain during walking in patients with peripheral artery disease (PAD). Endothelial cell and muscle dysfunction are thought to be important contributors to the etiology of this disease, but a lack of preclinical models that incorporate these elements and measure exercise performance as a primary end point has slowed progress in finding new treatment options for these patients. We sought to develop an animal model of peripheral vascular insufficiency in which microvascular dysfunction and exercise intolerance were defining features. We further set out to determine if pharmacological activation of 5'-AMP-activated protein kinase (AMPK) might counteract any of these functional deficits. Mice aged on a high-fat diet demonstrate many functional and molecular characteristics of PAD, including the sequential development of peripheral vascular insufficiency, increased muscle fatigability, and progressive exercise intolerance. These changes occur gradually and are associated with alterations in nitric oxide bioavailability. Treatment of animals with an AMPK activator, R118, increased voluntary wheel running activity, decreased muscle fatigability, and prevented the progressive decrease in treadmill exercise capacity. These functional performance benefits were accompanied by improved mitochondrial function, the normalization of perfusion in exercising muscle, increased nitric oxide bioavailability, and decreased circulating levels of the endogenous endothelial nitric oxide synthase inhibitor asymmetric dimethylarginine. These data suggest that aged, obese mice represent a novel model for studying exercise intolerance associated with peripheral vascular insufficiency, and pharmacological activation of AMPK may be a suitable treatment for intermittent claudication associated with PAD.

Unacylated ghrelin promotes skeletal muscle regeneration following hindlimb ischemia via SOD-2-mediated miR-221/222 expression

Background

Surgical treatment of peripheral artery disease, even if successful, does not prevent reoccurrence. Under these conditions, increased oxidative stress is a crucial determinant of tissue damage. Given its reported antioxidant effects, we investigated the potential of unacylated‐ghrelin (UnAG) to reduce ischemia‐induced tissue damage in a mouse model of peripheral artery disease.

Methods and Results

We show that UnAG but not acylated ghrelin (AG) induces skeletal muscle regeneration in response to ischemia via canonical p38/mitogen‐actived protein kinase signaling UnAG protected against reactive oxygen species–induced cell injuries by inducing the expression of superoxide dismutase‐2 (SOD‐2) in satellite cells. This led to a reduced number of infiltrating CD68⁺ cells and was followed by induction of the myogenic process and a reduction in functional impairment. Moreover, we found that miR‐221/222, previously linked to muscle regeneration processes, was up‐regulated and negatively correlated with p57^Kip2 expression in UnAG‐treated mice. UnAG, unlike AG, promoted cell‐cycle entry in satellite cells of mice lacking the genes for ghrelin and its receptor (GHSR1a). UnAG‐induced p38/mitogen‐actived protein kinase phosphorylation, leading to activation of the myogenic process, was prevented in SOD‐2–depleted SCs. By siRNA technology, we also demonstrated that SOD‐2 is the antioxidant enzyme involved in the control of miR‐221/222–driven posttranscriptional p57^Kip2 regulation. Loss‐of‐function experiments targeting miR‐221/222 and local pre–miR‐221/222 injection in vivo confirmed a role for miR‐221/222 in driving skeletal muscle regeneration after ischemia.

Conclusions

These results indicate that UnAG‐induced skeletal muscle regeneration after ischemia depends on SOD‐2–induced miR‐221/222 expression and highlight its clinical potential for the treatment of reactive oxygen species–mediated skeletal muscle damage.