Therapeutic Approaches in Mitochondrial Dysfunction, Proteolysis, and Structural Alterations of Diaphragm and Gastrocnemius in Rats With Chronic Heart Failure

Histological changes in skeletal muscle induced by heart failure in human patients and animal models: A scoping review

OBJECTIVE

This scoping review aimed to characterize the histological changes in skeletal muscle after heart failure (HF) and to identify gaps in knowledge.

METHODS

On April 03, 2024, systematic searches were performed for papers in which histological analyses were conducted on skeletal muscle sampled from patients with HF or animal models of HF. Screening and data extraction were conducted by two independent authors.

RESULTS AND CONCLUSION

A total of 118 papers were selected, including 33 human and 85 animal studies. Despite some disagreements among studies, some trends were observed. These trends included a slow-to-fast transition, a decrease in muscle fiber size, capillary to muscle fiber ratio, and mitochondrial activity and content, and an increase in apoptosis. These changes may contribute to the fatigability and decrease in muscle strength observed after HF. Although there were some disagreements between the results of human and animal studies, the results were generally similar. Animal models of HF will therefore be useful in elucidating the histological changes in skeletal muscle that occur in human patients with HF. Because the muscles subjected to histological analysis were mostly thigh muscles in humans and mostly lower leg muscles in animals, it remains uncertain whether changes similar to those seen in lower limb (hindlimb) muscles after HF also occur in upper limb (forelimb) muscles. The results of this review will consolidate the current knowledge on HF-induced histological changes in skeletal muscle and consequently aid in the rehabilitation of patients with HF and future studies.

Identification of three mechanistic pathways for iron-deficient heart failure

Current understanding of iron-deficient heart failure is based on blood tests that are thought to reflect systemic iron stores, but the available evidence suggests greater complexity. The entry and egress of circulating iron is controlled by erythroblasts, which (in severe iron deficiency) will sacrifice erythropoiesis to supply iron to other organs, e.g. the heart. Marked hypoferraemia (typically with anaemia) can drive the depletion of cardiomyocyte iron, impairing contractile performance and explaining why a transferrin saturation < ≈15%-16% predicts the ability of intravenous iron to reduce the risk of major heart failure events in long-term trials (Type 1 iron-deficient heart failure). However, heart failure may be accompanied by intracellular iron depletion within skeletal muscle and cardiomyocytes, which is disproportionate to the findings of systemic iron biomarkers. Inflammation- and deconditioning-mediated skeletal muscle dysfunction-a primary cause of dyspnoea and exercise intolerance in patients with heart failure-is accompanied by intracellular skeletal myocyte iron depletion, which can be exacerbated by even mild hypoferraemia, explaining why symptoms and functional capacity improve following intravenous iron, regardless of baseline haemoglobin or changes in haemoglobin (Type 2 iron-deficient heart failure). Additionally, patients with advanced heart failure show myocardial iron depletion due to both diminished entry into and enhanced egress of iron from the myocardium; the changes in iron proteins in the cardiomyocytes of these patients are opposite to those expected from systemic iron deficiency. Nevertheless, iron supplementation can prevent ventricular remodelling and cardiomyopathy produced by experimental injury in the absence of systemic iron deficiency (Type 3 iron-deficient heart failure). These observations, taken collectively, support the possibility of three different mechanistic pathways for the development of iron-deficient heart failure: one that is driven through systemic iron depletion and impaired erythropoiesis and two that are characterized by disproportionate depletion of intracellular iron in skeletal and cardiac muscle. These mechanisms are not mutually exclusive, and all pathways may be operative at the same time or may occur sequentially in the same patients.

Research progress on the pathogenesis and treatment of ventilator-induced diaphragm dysfunction

Prolonged controlled mechanical ventilation (CMV) can cause diaphragm fiber atrophy and inspiratory muscle weakness, resulting in diaphragmatic contractile dysfunction, called ventilator-induced diaphragm dysfunction (VIDD). VIDD is associated with higher rates of in-hospital deaths, nosocomial pneumonia, difficulty weaning from ventilators, and increased costs. Currently, appropriate clinical strategies to prevent and treat VIDD are unavailable, necessitating the importance of exploring the mechanisms of VIDD and suitable treatment options to reduce the healthcare burden. Numerous animal studies have demonstrated that ventilator-induced diaphragm dysfunction is associated with oxidative stress, increased protein hydrolysis, disuse atrophy, and calcium ion disorders. Therefore, this article summarizes the molecular pathogenesis and treatment of ventilator-induced diaphragm dysfunction in recent years so that it can be better served clinically and is essential to reduce the duration of mechanical ventilation use, intensive care unit (ICU) length of stay, and the medical burden.

Locomotor and respiratory muscle abnormalities in HFrEF and HFpEF

Heart failure (HF) is a chronic and progressive syndrome affecting worldwide billions of patients. Exercise intolerance and early fatigue are hallmarks of HF patients either with a reduced (HFrEF) or a preserved (HFpEF) ejection fraction. Alterations of the skeletal muscle contribute to exercise intolerance in HF. This review will provide a contemporary summary of the clinical and molecular alterations currently known to occur in the skeletal muscles of both HFrEF and HFpEF, and thereby differentiate the effects on locomotor and respiratory muscles, in particular the diaphragm. Moreover, current and future therapeutic options to address skeletal muscle weakness will be discussed focusing mainly on the effects of exercise training.

Beneficial Effects of Resveratrol in Mouse Gastrocnemius: A Hint to Muscle Phenotype and Proteolysis

We hypothesized that the phenolic compound resveratrol mitigates muscle protein degradation and loss and improves muscle fiber cross-sectional area (CSA) in gastrocnemius of mice exposed to unloading (7dI). In gastrocnemius of mice (female C57BL/6J, 10 weeks) exposed to a seven-day period of hindlimb immobilization with/without resveratrol treatment, markers of muscle proteolysis (tyrosine release, systemic troponin-I), atrophy signaling pathways, and muscle phenotypic features and function were analyzed. In gastrocnemius of unloaded mice treated with resveratrol, body and muscle weight and function were attenuated, whereas muscle proteolysis (tyrosine release), proteolytic and apoptotic markers, atrophy signaling pathways, and myofiber CSA significantly improved. Resveratrol treatment of mice exposed to a seven-day period of unloading prevented body and muscle weight and limb strength loss, while an improvement in muscle proteolysis, proteolytic markers, atrophy signaling pathways, apoptosis, and muscle fiber CSA was observed in the gastrocnemius muscle. These findings may have potential therapeutic implications in the management of disuse muscle atrophy in clinical settings.

Airway Redox Homeostasis and Inflammation Gone Awry: From Molecular Pathogenesis to Emerging Therapeutics in Respiratory Pathology

As aerobic organisms, we are continuously and throughout our lifetime subjected to an oxidizing atmosphere and, most often, to environmental threats. The lung is the internal organ most highly exposed to this milieu. Therefore, it has evolved to confront both oxidative stress induced by reactive oxygen species (ROS) and a variety of pollutants, pathogens, and allergens that promote inflammation and can harm the airways to different degrees. Indeed, an excess of ROS, generated intrinsically or from external sources, can imprint direct damage to key structural cell components (nucleic acids, sugars, lipids, and proteins) and indirectly perturb ROS-mediated signaling in lung epithelia, impairing its homeostasis. These early events complemented with efficient recognition of pathogen- or damage-associated recognition patterns by the airway resident cells alert the immune system, which mounts an inflammatory response to remove the hazards, including collateral dead cells and cellular debris, in an attempt to return to homeostatic conditions. Thus, any major or chronic dysregulation of the redox balance, the air-liquid interface, or defects in epithelial proteins impairing mucociliary clearance or other defense systems may lead to airway damage. Here, we review our understanding of the key role of oxidative stress and inflammation in respiratory pathology, and extensively report current and future trends in antioxidant and anti-inflammatory treatments focusing on the following major acute and chronic lung diseases: acute lung injury/respiratory distress syndrome, asthma, chronic obstructive pulmonary disease, pulmonary fibrosis, and cystic fibrosis.

Prolonged Immobilization Exacerbates the Loss of Muscle Mass and Function Induced by Cancer-Associated Cachexia through Enhanced Proteolysis in Mice

We hypothesized that in mice with lung cancer (LC)-induced cachexia, periods of immobilization of the hindlimb (7 and 15 days) may further aggravate the process of muscle mass loss and function. Mice were divided into seven groups (n = 10/group): (1) non-immobilized control mice, (2) 7-day unloaded mice (7-day I), (3) 15-day unloaded mice (15-day I), (4) 21-day LC-cachexia group (LC 21-days), (5) 30-day LC-cachexia group (LC 30-days), (6) 21-day LC-cachexia group besides 7 days of unloading (LC 21-days + 7-day I), (7) 30-day LC-cachexia group besides 15 days of unloading (LC 30-days + 15-day I). Physiological parameters, body weight, muscle and tumor weights, phenotype and morphometry, muscle damage (including troponin I), proteolytic and autophagy markers, and muscle regeneration markers were identified in gastrocnemius muscle. In LC-induced cachexia mice exposed to hindlimb unloading, gastrocnemius weight, limb strength, fast-twitch myofiber cross-sectional area, and muscle regeneration markers significantly decreased, while tumor weight and area, muscle damage (troponin), and proteolytic and autophagy markers increased. In gastrocnemius of cancer-cachectic mice exposed to unloading, severe muscle atrophy and impaired function was observed along with increased muscle proteolysis and autophagy, muscle damage, and impaired muscle regeneration.

Satellite Cells and Markers of Muscle Regeneration during Unloading and Reloading: Effects of Treatment with Resveratrol and Curcumin

We hypothesized that treatment with pharmacological agents known to increase sirtuin-1 activity (resveratrol and curcumin) may enhance muscle regeneration. In limb muscles of mice (C57BL/6J, 10 weeks) exposed to reloading for seven days following a seven-day period of hindlimb immobilization with/without curcumin or resveratrol treatment, progenitor muscle cell numbers (FACS), satellite cell subtypes (histology), early and late muscle regeneration markers, phenotype and morphometry, sirtuin-1 activity and content, and muscle function were assessed. Treatment with either resveratrol or curcumin in immobilized muscles elicited a significant improvement in numbers of progenitor, activated, quiescent, and total counts of muscle satellite cells, compared to non-treated animals. Treatment with either resveratrol or curcumin in reloaded muscles compared to non-treated mice induced a significant improvement in the CSA of both hybrid (curcumin) and fast-twitch fibers (resveratrol), sirtuin-1 activity (curcumin), sirtuin-1 content (resveratrol), and counts of progenitor muscle cells (resveratrol). Treatment with the pharmacological agents resveratrol and curcumin enhanced the numbers of satellite cells (muscle progenitor, quiescent, activated, and total satellite cells) in the unloaded limb muscles but not in the reloaded muscles. These findings have potential clinical implications as treatment with these phenolic compounds would predominantly be indicated during disuse muscle atrophy to enhance the muscle regeneration process.

The hypoglycemic mechanism of catalpol involves increased AMPK-mediated mitochondrial biogenesis

Mitochondria serve as sensors of energy regulation and glucose levels, which are impaired by diabetes progression. Catalpol is an iridoid glycoside that exerts a hypoglycemic effect by improving mitochondrial function, but the underlying mechanism has not been fully elucidated. In the current study we explored the effects of catalpol on mitochondrial function in db/db mice and C2C12 myotubes in vitro. After oral administration of catalpol (200 mg·kg⁻¹·d⁻¹) for 8 weeks, db/db mice exhibited a decreased fasting blood glucose level and restored mitochondrial function in skeletal muscle. Catalpol increased mitochondrial biogenesis, evidenced by significant elevations in the number of mitochondria, mitochondrial DNA levels, and the expression of three genes associated with mitochondrial biogenesis: peroxisome proliferator-activated receptor gammaco-activator 1 (PGC-1α), mitochondrial transcription factor A (TFAM) and nuclear respiratory factor 1 (NRF1). In C2C12 myotubes, catalpol significantly increased glucose uptake and ATP production. These effects depended on activation of AMP-activated protein kinase (AMPK)-mediated mitochondrial biogenesis. Thus, catalpol improves skeletal muscle mitochondrial function by activating AMPK-mediated mitochondrial biogenesis. These findings may guide the development of a new therapeutic approach for type 2 diabetes.

Novel role of Tieg1 in muscle metabolism and mitochondrial oxidative capacities

AIM

Tieg1 is involved in multiple signalling pathways, human diseases, and is highly expressed in muscle where its functions are poorly understood.

METHODS

We have utilized Tieg1 knockout (KO) mice to identify novel and important roles for this transcription factor in regulating muscle ultrastructure, metabolism and mitochondrial functions in the soleus and extensor digitorum longus (EDL) muscles. RNA sequencing, immunoblotting, transmission electron microscopy, MRI, NMR, histochemical and mitochondrial function assays were performed.

RESULTS

Loss of Tieg1 expression resulted in altered sarcomere organization and a significant decrease in mitochondrial number. Histochemical analyses demonstrated an absence of succinate dehydrogenase staining and a decrease in cytochrome c oxidase (COX) enzyme activity in KO soleus with similar, but diminished, effects in the EDL. Decreased complex I, COX and citrate synthase (CS) activities were detected in the soleus muscle of KO mice indicating altered mitochondrial function. Complex I activity was also diminished in KO EDL. Significant decreases in CS and respiratory chain complex activities were identified in KO soleus. ¹ H-NMR spectra revealed no significant metabolic difference between wild-type and KO muscles. However, ³¹ P spectra revealed a significant decrease in phosphocreatine and ATPγ. Altered expression of 279 genes, many of which play roles in mitochondrial and muscle function, were identified in KO soleus muscle. Ultimately, all of these changes resulted in an exercise intolerance phenotype in Tieg1 KO mice.

CONCLUSION

Our findings have implicated novel roles for Tieg1 in muscle including regulation of gene expression, metabolic activity and organization of tissue ultrastructure. This muscle phenotype resembles diseases associated with exercise intolerance and myopathies of unknown consequence.

Muscle Phenotype, Proteolysis, and Atrophy Signaling During Reloading in Mice: Effects of Curcumin on the Gastrocnemius

We hypothesized that curcumin may mitigate muscle protein degradation and loss through attenuation of proteolytic activity in limb muscles of mice exposed to reloading (7dR) following immobilization (7dI). In gastrocnemius of mice (female C57BL/6J, 10 weeks) exposed to recovery following a seven-day period of hindlimb immobilization with/without curcumin treatment, markers of muscle proteolysis (systemic troponin-I), atrophy signaling pathways and histone deacetylases, protein synthesis, and muscle phenotypic characteristics and function were analyzed. In gastrocnemius of reloading mice compared to unloaded, muscle function, structure, sirtuin-1, and protein synthesis improved, while proteolytic and signaling markers (FoxO1/3) declined. In gastrocnemius of unloaded and reloaded mice treated with curcumin, proteolytic and signaling markers (NF-kB p50) decreased and sirtuin-1 activity and hybrid fibers size increased (reloaded muscle), while no significant improvement was seen in muscle function. Treatment with curcumin elicited a rise in sirtuin-1 activity, while attenuating proteolysis in gastrocnemius of mice during reloading following a period of unloading. Curcumin attenuated muscle proteolysis probably via activation of histone deacetylase sirtuin-1, which also led to decreased levels of atrophy signaling pathways. These findings offer an avenue of research in the design of therapeutic strategies in clinical settings of patients exposed to periods of disuse muscle atrophy.

Mitochondrial respiration and H2O2 emission in saponin-permeabilized murine diaphragm fibers: optimization of fiber separation and comparison to limb muscle

Diaphragm abnormalities in aging or chronic diseases include impaired mitochondrial respiration and H₂O₂ emission, which can be measured using saponin-permeabilized muscle fibers. Mouse diaphragm presents a challenge for isolation of fibers due to relatively high abundance of connective tissue in healthy muscle that is exacerbated in disease states. We tested a new approach to process mouse diaphragm for assessment of intact mitochondria respiration and ROS emission in saponin-permeabilized fibers. We used the red gastrocnemius (RG) as "standard" limb muscle. Markers of mitochondrial content were two- to fourfold higher in diaphragm (Dia) than in RG (P < 0.05). Maximal O₂ consumption (JO₂: pmol·s^-1·mg^-1) in Dia was higher with glutamate, malate, and succinate (Dia 399 ± 127, RG 148 ± 60; P < 0.05) and palmitoyl-CoA + carnitine (Dia 15 ± 5, RG 7 ± 1; P < 0.05) than in RG, but not different between muscles when JO₂ was normalized to citrate synthase activity. Absolute JO₂ for Dia was two- to fourfold higher than reported in previous studies. Mitochondrial JH₂O₂ was higher in Dia than in RG (P < 0.05), but lower in Dia than in RG when JH₂O₂ was normalized to citrate synthase activity. Our findings are consistent with an optimized diaphragm preparation for assessment of intact mitochondria in permeabilized fiber bundles. The data also suggest that higher mitochondrial content potentially makes the diaphragm more susceptible to "mitochondrial onset" myopathy. Overall, the new approach will facilitate testing and understanding of diaphragm mitochondrial function in mouse models that are used to advance biomedical research and human health.

Reduced lung cancer burden by selective immunomodulators elicits improvements in muscle proteolysis and strength in cachectic mice

Identification of to what extent tumor burden influences muscle mass independently of specific treatments for cancer-cachexia remains to be elucidated. We hypothesized that reduced tumor burden by selective treatment of tumor with immunomodulators may exert beneficial effects on muscle wasting and function in mice. Body and muscle weight, grip strength, physical activity, muscle morphometry, apoptotic nuclei, troponin-I systemic levels, interleukin-6, proteolytic markers, and tyrosine release, and apoptosis markers were determined in diaphragm and gastrocnemius muscles of lung cancer (LP07 adenocarcinoma cells) mice (BALB/c) treated with monoclonal antibodies (mAbs), against immune check-points and pathways (CD-137, cytotoxic T-lymphocyte associated protein-4, programed cell death-1, and CD-19; N = 10/group). Nontreated lung cancer cachectic mice were the controls. T and B cell numbers and macrophages were counted in tumors of both mouse groups. Compared to nontreated cachectic mice, in the mAbs-treated animals, T cells increased, no differences in B cells or macrophages, the variables final body weight, body weight and grip strength gains significantly improved. In diaphragm and gastrocnemius of mAbs-treated cachectic mice, number of apoptotic nuclei, tyrosine release, proteolysis, and apoptosis markers significantly decreased compared to nontreated cachectic mice. Systemic levels of troponin-I significantly decreased in treated cachectic mice compared to nontreated animals. We conclude that reduced tumor burden as a result of selective treatment of the lung cancer cells with immunomodulators elicits per se beneficial effects on muscle mass loss through attenuation of several biological mechanisms that lead to increased protein breakdown and apoptosis, which translated into significant improvements in limb muscle strength but not in physical activity parameters.

Diaphragm plasticity in aging and disease: therapies for muscle weakness go from strength to strength

The diaphragm is the main inspiratory muscle and is required to be highly active throughout the life span. The diaphragm muscle must be able to produce and sustain various behaviors that range from ventilatory to nonventilatory such as those required for airway maintenance and clearance. Throughout the life span various circumstances and conditions may affect the ability of the diaphragm muscle to generate requisite forces, and in turn the diaphragm muscle may undergo significant weakness and dysfunction. For example, hypoxic stress, critical illness, cancer cachexia, chronic obstructive pulmonary disorder, and age-related sarcopenia all represent conditions in which significant diaphragm muscle dysfunction exits. This perspective review article presents several interesting topics involving diaphragm plasticity in aging and disease that were presented at the International Union of Physiological Sciences Conference in 2017. This review seeks to maximize the broad and collective research impact on diaphragm muscle dysfunction in the search for transformative treatment approaches to improve the diaphragm muscle health during aging and disease.

Effects of the beta2 agonist formoterol on atrophy signaling, autophagy, and muscle phenotype in respiratory and limb muscles of rats with cancer-induced cachexia

Muscle mass loss and wasting are characteristic features of patients with chronic conditions including cancer. Beta-adrenoceptors attenuate muscle wasting. We hypothesized that specific muscle atrophy signaling pathways and altered metabolism may be attenuated in cancer cachectic animals receiving treatment with the beta₂ agonist formoterol. In diaphragm and gastrocnemius of tumor-bearing rats (intraperitoneal inoculum, 10⁸ AH-130 Yoshida ascites hepatoma cells, 7-day study period) with and without treatment with formoterol (0.3 mg/kg body weight/day/7days, subcutaneous), atrophy signaling pathways (NF-κB, MAPK, FoxO), proteolytic markers (ligases, proteasome, ubiquitination), autophagy markers (p62, beclin-1, LC3), myostatin, apoptosis, muscle metabolism markers, and muscle structure features were analyzed (immunoblotting, immunohistochemistry). In diaphragm and gastrocnemius of cancer cachectic rats, fiber sizes were reduced, levels of structural alterations, atrophy signaling pathways, proteasome content, protein ubiquitination, autophagy, and myostatin were increased, while those of regenerative and metabolic markers (myoD, mTOR, AKT, and PGC-1alpha) were decreased. Formoterol treatment attenuated such alterations in both muscles. Muscle wasting in this rat model of cancer-induced cachexia was characterized by induction of significant structural alterations, atrophy signaling pathways, proteasome activity, apoptotic and autophagy markers, and myostatin, along with a significant decline in the expression of muscle regenerative and metabolic markers. Treatment of the cachectic rats with formoterol partly attenuated the structural alterations and atrophy signaling, while improving other molecular perturbations similarly in both respiratory and limb muscles. The results reported in this study have relevant therapeutic implications as they showed beneficial effects of the beta₂ agonist formoterol in the cachectic muscles through several key biological pathways.

The phosphodiesterase-4 inhibitor roflumilast reverts proteolysis in skeletal muscle cells of patients with COPD cachexia

Peripheral muscle weakness and mass loss are characteristic features in severe chronic obstructive pulmonary disease (COPD). We hypothesized that the phosphodiesterase (PDE)-4 inhibitor roflumilast-induced cAMP may ameliorate proteolysis and metabolism in skeletal muscles of COPD patients with severe muscle wasting. In myogenic precursor cells (isolated from muscle biopsies and cultured up to obtain differentiated myotubes) from 10 severe COPD patients and 10 healthy controls, which were treated with 1 μM roflumilast N-oxide (RNO) for three time cohorts (1, 6, and 24 h), genes of antioxidant defense and oxidative stress marker, myogenesis and muscle metabolism, proteolysis (tyrosine release assay) and ubiquitin-proteasome system markers, autophagy, and myosin isoforms were analyzed using RT-PCR and immunoblotting. In COPD patients at 6 h RNO treatment, myotube tyrosine release, total protein ubiquitination, and tripartite motif-containing protein 32 levels were significantly lower than healthy controls, whereas at 24 h RNO treatment, myotube myosin heavy chain ( MyHC) -I and MyHC-IIx expression levels were upregulated in both patients and controls. In the 6-h RNO cohort, in patients and controls, myotube expression of nuclear factor (erythroid-derived 2)-like 2 ( NRF2) and its downstream antioxidants sirtuin-1, FGF-inducible 14, and insulin-like growth factor-1 was upregulated, whereas that of myocyte-specific enhancer factor 2C, myogenic differentiation, myogenin, myostatin, atrogin-1, and muscle RING-finger protein-1 was downregulated. In myotubes of severe COPD patients with cachexia, roflumilast-induced cAMP signaling exerts beneficial effects by targeting muscle protein breakdown (tyrosine release), along with reduced expression of proteolytic markers of the ubiquitin-proteasome system and that of myostatin. In both patients and controls, roflumilast also favored antioxidant defense through upregulation of the NRF2 pathway and that of the histone deacetylase sirtuin-1, whereas it improved the expression of slow- and fast-twitch myosin isoforms. These findings show that muscle dysfunction and wasting may be targeted by roflumilast-induced cAMP signaling in COPD. These results have potential therapeutic implications, as this PDE-4 inhibitor is currently available for the treatment of systemic inflammation and exacerbations in patients with severe COPD. NEW & NOTEWORTHY In myotubes of cachectic chronic obstructive pulmonary disease (COPD) patients, cAMP signaling exerted beneficial effects by targeting muscle proteolysis and reducing gene expression of proteolytic markers of the ubiquitin-proteasome system and that of myostatin. In myotubes of patients and controls, roflumilast also favored antioxidant defense through upregulation of the nuclear factor (erythroid-derived 2)-like 2 pathway, of sirtuin-1, and of gene expression of slow- and fast-twitch isoforms. These findings have potential clinical implications for the treatment of muscle wasting in patients with COPD and cachexia.

Skeletal muscle alterations in HFrEF vs. HFpEF

PURPOSE OF REVIEW

Severe exercise intolerance and early fatigue are hallmarks of heart failure patients either with a reduced (HFrEF) or a still preserved (HFpEF) ejection fraction. This review, therefore, will provide a contemporary summary of the alterations currently known to occur in the skeletal muscles of both HFrEF and HFpEF, and provide some further directions that will be required if we want to improve our current understanding of this area.

RECENT FINDINGS

Skeletal muscle alterations are well documented for over 20 years in HFrEF, and during the recent years also data are presented that in HFpEF muscular alterations are present. Alterations are ranging from a shift in fiber type and capillarization to an induction of atrophy and modulation of mitochondrial energy supply. In general, the molecular alterations are more severe in the skeletal muscle of HFrEF when compared to HFpEF. The alterations occurring in the skeletal muscle at the molecular level may contribute to exercise intolerance in HFrEF and HFpEF. Nevertheless, the knowledge of changes in the skeletal muscle of HFpEF is still sparsely available and more studies in this HF cohort are clearly warranted.

Molecular effects of exercise training in patients with cardiovascular disease: focus on skeletal muscle, endothelium, and myocardium

For decades, we have known that exercise training exerts beneficial effects on the human body, and clear evidence is available that a higher fitness level is associated with a lower incidence of suffering premature cardiovascular death. Despite this knowledge, it took some time to also incorporate physical exercise training into the treatment plan for patients with cardiovascular disease (CVD). In recent years, in addition to continuous exercise training, further training modalities such as high-intensity interval training and pyramid training have been introduced for coronary artery disease patients. The beneficial effect for patients with CVD is clearly documented, and during the last years, we have also started to understand the molecular mechanisms occurring in the skeletal muscle (limb muscle and diaphragm) and endothelium, two systems contributing to exercise intolerance in these patients. In the present review, we describe the effects of the different training modalities in CVD and summarize the molecular effects mainly in the skeletal muscle and cardiovascular system.

Muscle-specific and age-related changes in protein synthesis and protein degradation in response to hindlimb unloading in rats

Disuse is a potent inducer of muscle atrophy, but the molecular mechanisms driving this loss of muscle mass are highly debated. In particular, the extent to which disuse triggers decreases in protein synthesis or increases in protein degradation, and whether these changes are uniform across muscles or influenced by age, is unclear. We aimed to determine the impact of disuse on protein synthesis and protein degradation in lower limb muscles of varied function and fiber type in adult and old rats. Alterations in protein synthesis and degradation were measured in the soleus, medial gastrocnemius, and tibialis anterior (TA) muscles of adult and old rats subjected to hindlimb unloading (HU) for 3, 7, or 14 days. Loss of muscle mass was progressive during the unloading period, but highly variable (-9 to -38%) across muscle types and between ages. Protein synthesis decreased significantly in all muscles, except for the old TA. Atrophy-associated gene expression was only loosely associated with protein degradation as muscle RING finger-1, muscle atrophy F-box (MAFbx), and Forkhead box O1 expression significantly increased in all muscles, but an increase in proteasome activity was only observed in the adult soleus. MAFbx protein levels were significantly higher in the old muscles compared with adult muscles, despite the old having higher expression of microRNA-23a. These results indicate that adult and old muscles respond similarly to HU, and the greatest loss in muscle mass occurs in predominantly slow-twitch extensor muscles due to a concomitant decrease in protein synthesis and increase in protein degradation.NEW & NOTEWORTHY In this study, we showed that age did not intensify the atrophy response to unloading in rats, but rather that the degree of atrophy was highly variable across muscles, indicating that changes in protein synthesis and protein degradation occur in a muscle-specific manner. Our data emphasize the importance of studying muscles of varying fiber-type and physiological function at multiple time points to fully understand the molecular mechanisms responsible for disuse atrophy.

Time-Course of Muscle Mass Loss, Damage, and Proteolysis in Gastrocnemius following Unloading and Reloading: Implications in Chronic Diseases

BACKGROUND

Disuse muscle atrophy is a major comorbidity in patients with chronic diseases including cancer. We sought to explore the kinetics of molecular mechanisms shown to be involved in muscle mass loss throughout time in a mouse model of disuse muscle atrophy and recovery following immobilization.

METHODS

Body and muscle weights, grip strength, muscle phenotype (fiber type composition and morphometry and muscle structural alterations), proteolysis, contractile proteins, systemic troponin I, and mitochondrial content were assessed in gastrocnemius of mice exposed to periods (1, 2, 3, 7, 15 and 30 days) of non-invasive hindlimb immobilization (plastic splint, I cohorts) and in those exposed to reloading for different time-points (1, 3, 7, 15, and 30 days, R cohorts) following a seven-day period of immobilization. Groups of control animals were also used.

RESULTS

Compared to non-exposed controls, muscle weight, limb strength, slow- and fast-twitch cross-sectional areas, mtDNA/nDNA, and myosin content were decreased in mice of I cohorts, whereas tyrosine release, ubiquitin-proteasome activity, muscle injury and systemic troponin I levels were increased. Gastrocnemius reloading following splint removal improved muscle mass loss, strength, fiber atrophy, injury, myosin content, and mtDNA/nDNA, while reducing ubiquitin-proteasome activity and proteolysis.

CONCLUSIONS

A consistent program of molecular and cellular events leading to reduced gastrocnemius muscle mass and mitochondrial content and reduced strength, enhanced proteolysis, and injury, was seen in this non-invasive mouse model of disuse muscle atrophy. Unloading of the muscle following removal of the splint significantly improved the alterations seen during unloading, characterized by a specific kinetic profile of molecular events involved in muscle regeneration. These findings have implications in patients with chronic diseases including cancer in whom physical activity may be severely compromised.

Systemic and Tumor Th1 and Th2 Inflammatory Profile and Macrophages in Lung Cancer: Influence of Underlying Chronic Respiratory Disease

INTRODUCTION

Chronic respiratory conditions, especially chronic obstructive pulmonary disease (COPD), and inflammatory events underlie lung cancer (LC). We hypothesized that profiles of T helper 1 and T helper 2 cytokines and type 1 and type 2 macrophages (M1 and M2) are differentially expressed in lung tumors and blood of patients with NSCLC with and without COPD and that the ratio M1/M2 specifically may influence their survival.

METHODS

In blood, inflammatory cytokines (determined by enzyme-linked immunosorbent assay) were quantified in 80 patients with LC (60 with LC and COPD [the LC-COPD group] and 20 with LC only [the LC-only group]) and lung specimens (tumor and nontumor) from those undergoing thoracotomy (20 in the LC-COPD group and 20 in the LC-only group).

RESULTS

In the LC-COPD group compared with in the LC-only group, systemic levels of tumor necrosis factor-α, interleukin-2 (IL-2), transforming growth factor-β, and IL-10 were increased, whereas vascular endothelial growth factor and IL-4 levels were decreased. In lung tumors, levels of tumor necrosis factor-α, transforming growth factor-β, and IL-10 were higher than in nontumor parenchyma in the LC-COPD group, whereas IL-2 and vascular endothelial growth factor levels were higher in tumors of both the LC-only and LC-COPD groups. Compared with in nontumor lung, M1 macrophage counts were reduced whereas M2 counts were increased in tumors of both patient groups, and the M1/M2 ratio was higher in the LC-COPD group than the LC-only group. M1 and M2 counts did not influence patients' survival.

CONCLUSIONS

The relative predominance of T helper 1 cytokines and M1 macrophages in the blood and tumors of patients with underlying COPD imply that a stronger proinflammatory pattern exists in these patients. Inflammation should not be targeted systematically in all patients with LC. Screening for the presence of underlying respiratory diseases and identification of the specific inflammatory pattern should be carried out in patients with LC, at least in early stages of their disease.

Phenotypic and metabolic features of mouse diaphragm and gastrocnemius muscles in chronic lung carcinogenesis: influence of underlying emphysema

BACKGROUND

Muscle wasting negatively impacts the progress of chronic diseases such as lung cancer (LC) and emphysema, which are in turn interrelated.

OBJECTIVES

We hypothesized that muscle atrophy and body weight loss may develop in an experimental mouse model of lung carcinogenesis, that the profile of alterations in muscle fiber phenotype (fiber type composition and morphometry, muscle structural alterations, and nuclear apoptosis), and in muscle metabolism are similar in both respiratory and limb muscles of the tumor-bearing mice, and that the presence of underlying emphysema may influence those events.

METHODS

Diaphragm and gastrocnemius muscles of mice with urethane-induced lung cancer (LC-U) with and without elastase-induced emphysema (E-U) and non-exposed controls (N = 8/group) were studied: fiber type composition, morphometry, muscle abnormalities, apoptotic nuclei (immunohistochemistry), and proteolytic and autophagy markers (immunoblotting) at 20- and 35-week exposure times. In the latter cohort, structural contractile proteins, creatine kinase (CK), peroxisome proliferator-activated receptor (PPAR) expression, oxidative stress, and inflammation were also measured. Body and muscle weights were quantified (baseline, during follow-up, and sacrifice).

RESULTS

Compared to controls, in U and E-U mice, whole body, diaphragm and gastrocnemius weights were reduced. Additionally, both in diaphragm and gastrocnemius, muscle fiber cross-sectional areas were smaller, structural abnormalities, autophagy and apoptotic nuclei were increased, while levels of actin, myosin, CK, PPARs, and antioxidants were decreased, and muscle proteolytic markers did not vary among groups.

CONCLUSIONS

In this model of lung carcinogenesis with and without emphysema, reduced body weight gain and muscle atrophy were observed in respiratory and limb muscles of mice after 20- and 35-week exposure times most likely through increased nuclear apoptosis and autophagy. Underlying emphysema induced a larger reduction in the size of slow- and fast-twitch fibers in the diaphragm of U and E-U mice probably as a result of the greater inspiratory burden imposed onto this muscle.