β2-Adrenergic Signaling Modulates Mitochondrial Function and Morphology in Skeletal Muscle in Response to Aerobic Exercise

Regenerative failure of sympathetic axons contributes to deficits in functional recovery after nerve injury

Renewed scientific interest in sympathetic modulation of muscle and neuromuscular junctions has spurred a flurry of new discoveries with major implications for motor diseases. However, the role sympathetic axons play in the persistent dysfunction that occurs after nerve injuries remains to be explored. Peripheral nerve injuries are common and lead to motor, sensory, and autonomic deficits that result in lifelong disabilities. Given the importance of sympathetic signaling in muscle metabolic health and maintaining bodily homeostasis, it is imperative to understand the regenerative capacity of sympathetic axons after injury. Therefore, we tested sympathetic axon regeneration and functional reinnervation of skin and muscle, both acute and long-term, using a battery of anatomical, pharmacological, chemogenetic, cell culture, analytical chemistry, and electrophysiological techniques. We employed several established growth-enhancing interventions, including electrical stimulation and conditioning lesion, as well as an innovative tool called bioluminescent optogenetics. Our results indicate that sympathetic regeneration is not enhanced by any of these treatments and may even be detrimental to sympathetic regeneration. Despite the complete return of motor reinnervation after sciatic nerve injury, gastrocnemius muscle atrophy and deficits in muscle cellular energy charge, as measured by relative ATP, ADP, and AMP concentrations, persisted long after injury, even with electrical stimulation. We suggest that these long-term deficits in muscle energy charge and atrophy are related to the deficiency in sympathetic axon regeneration. New studies are needed to better understand the mechanisms underlying sympathetic regeneration to develop therapeutics that can enhance the regeneration of all axon types.

Phenylacetylglutamine produced from injury lung alveolar epithelial cells promotes the function of BMSCs by regulating NONRATT006276.2/Mapt pathway

Mesenchymal stem cell (MSC)-based therapy regenerates damaged structures of the respiratory system and restores lung function, thus providing a promising therapeutic approach for chronic obstructive pulmonary disease. Understanding the communication between injured alveolar cells and MSCs can improve the efficiency of MSC-based therapies. The present study analyzed the untargeted metabolomics of the supernatant of AEC-II injury induced by cigarette smoke extract and identified 205 differential metabolites. Phenotypic assays indicated that phenylacetylglutamine (PAG) significantly promoted the migration and mitochondrial function of bone marrow MSCs (BMSCs). Whole-transcriptome sequencing (WT-seq) was used to analyze the long noncoding RNA (lncRNA) and mRNA expression profiles of BMSCs treated with PAG. The upregulated lncNRA NONRATT006276.2 (NRT6276.2) and its trans-regulated gene, microtubule-associated protein tau (Mapt), were identified based on the lncRNA-mRNA co-expression network and bioinformatics analysis. The knockdown of NRT6276.2 or Mapt inhibited the positive effects of PAG on BMSCs. Furthermore, Mapt overexpression reversed the phenotype of BMSCs inhibited by silencing NRT6276.2. In conclusion, PAG enhanced the migration and mitochondrial function of BMSCs by regulating the NRT6276.2/Mapt pathway. This study clarified the positive effects of PAG produced by injured lung cells on transplanted MSCs, providing a potential new strategy to enhance the efficiency of MSC-based therapies.

Cardiac autoantibodies promote a fibrotic transcriptome and reduced ventricular recovery in human myocarditis

Myocarditis leads to dilated cardiomyopathy (DCM) with one-third failing to recover normal ejection fraction (EF 50%). Our previous studies have supported a Th17 autoimmune pathogenesis where IL17A and IL-6 are elevated in myocarditis patients who do not recover normal EF. In the non-recovered group, autoantibody mechanisms of pathogenesis in myocardial injury and systolic dysfunction are not fully understood. Furthermore, in our myocarditis cohort, cardiac myosin (CM) autoantibodies (AAbs) were elevated and cross-reactive with the β-adrenergic receptor (βAR). Here we studied cross-reactive CM/βAR serum AAbs and human myocarditis-derived monoclonal antibodies (mAbs) to define their potential pathogenic mechanisms and to identify unique human CM epitopes associated with non-recovery in a longitudinal (n=41) cohort. Elevated CM IgG AAbs in the non-recovered phenotype correlated with reduced EF and poor outcomes. Human CM epitopes unique to the non-recovered phenotype shared strong amino acid sequence homology with extracellular loops of βARs and supported molecular mimicry and cross-reactivity between CM and βAR. Myocarditis-derived IgG and human mAb 2C.4 activated protein kinase A (PKA) in an IgG, CM, and βAR-dependent manner in H9c2 heart myoblast cell line, and transcriptomic analysis revealed mAb 2C.4 induced fibrosis pathways which were highly similar pathways seen with isoproterenol, a beta receptor agonist. Our data translate into new mechanistic insights from our small longitudinal group of myocarditis/DCM patients and into potential therapeutic targets and biomarkers for future studies.

Advancing β-adrenoreceptor agonism for recovery after volumetric muscle loss through regenerative rehabilitation and biomaterial delivery approaches

Volumetric muscle loss (VML) injury results in the unrecoverable loss of muscle mass and contractility. Oral delivery of formoterol, a β2-adrenergic receptor agonist, produces a modest recovery of muscle mass and contractility in VML-injured mice. The objective of this study was to determine if a regenerative rehabilitation paradigm or a regenerative medicine paradigm could enhance the recovery of VML-injured muscle. Regenerative rehabilitation involved oral formoterol delivery combined with voluntary wheel running. Regenerative medicine involved direct delivery of formoterol to VML-injured muscle using a non-biodegradable poly(ethylene glycol) biomaterial. To determine if the regenerative rehabilitation or regenerative medicine approaches were effective at 8 weeks post-injury, muscle mass, contractile function, metabolic function, and histological evaluations were used. One model of regenerative rehabilitation, in which rehabilitation was delayed until 1 month post-injury, resulted in greater muscle mass, muscle contractility, and permeabilized muscle fiber mitochondrial respiration compared to untreated VML-injured mice. Histologically, these mice had evidence of greater total muscle fiber number and oxidative fibers; however, they also had a greater percentage of densely packed collagen. The regenerative medicine model produced greater permeabilized muscle fiber mitochondrial respiration compared to untreated VML-injured mice; however, the non-biodegradable biomaterial was associated with fewer total muscle fibers and lower muscle quality (i.e. lower muscle mass-normalized contractility). The conclusions reached from this study are: (i) regenerative rehabilitation and regenerative medicine strategies utilizing formoterol require further optimization but showed promising outcomes; and (ii) in general, β-adrenergic receptor agonism continues to be a physiologically supportive intervention to improve muscle contractile and metabolic function after VML injury.

Regenerative failure of sympathetic axons contributes to deficits in functional recovery after nerve injury

Renewed scientific interest in sympathetic modulation of muscle and neuromuscular junctions has spurred a flurry of new discoveries with major implications for motor diseases. However, the role sympathetic axons play in the persistent dysfunction that occurs after nerve injuries remains to be explored. Peripheral nerve injuries are common and lead to motor, sensory, and autonomic deficits that result in lifelong disabilities. Given the importance of sympathetic signaling in muscle metabolic health and maintaining bodily homeostasis, it is imperative to understand the regenerative capacity of sympathetic axons after injury. Therefore, we tested sympathetic axon regeneration and functional reinnervation of skin and muscle, both acute and long-term, using a battery of anatomical, pharmacological, chemogenetic, cell culture, analytical chemistry, and electrophysiological techniques. We employed several established growth-enhancing interventions, including electrical stimulation and conditioning lesion, as well as an innovative tool called bioluminescent optogenetics. Our results indicate that sympathetic regeneration is not enhanced by any of these treatments and may even be detrimental to sympathetic regeneration. Despite the complete return of motor reinnervation after sciatic nerve injury, gastrocnemius muscle atrophy and deficits in muscle cellular energy charge, as measured by relative ATP, ADP, and AMP concentrations, persisted long after injury, even with electrical stimulation. We suggest that these long-term deficits in muscle energy charge and atrophy are related to the deficiency in sympathetic axon regeneration. New studies are needed to better understand the mechanisms underlying sympathetic regeneration to develop therapeutics that can enhance the regeneration of all axon types.

Possible Extracellular Signals to Ameliorate Sarcopenia in Response to Medium-Chain Triglycerides (8:0 and 10:0) in Frail Older Adults

In frail older adults (mean age 85 years old), a 3-month supplementation with a low dose (6 g/day) of medium-chain triglycerides (MCTs; C8:0 and C10:0) given at a meal increased muscle mass and function, relative to supplementation with long-chain triglycerides (LCTs), but it decreased fat mass. The reduction in fat mass was partly due to increased postprandial energy expenditure by stimulation of the sympathetic nervous system (SNS). However, the extracellular signals to ameliorate sarcopenia are unclear. The following three potential extracellular signals to increase muscle mass and function after MCT supplementation are discussed: (1) Activating SNS-the hypothesis for this is based on evidence that a beta2-adrenergic receptor agonist acutely (1-24 h) markedly upregulates isoforms of peroxisomal proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) mRNAs, promotes mitochondrial biogenesis, and chronically (~1 month) induces muscle hypertrophy. (2) An increased concentration of plasma acyl-ghrelin stimulates growth hormone secretion. (3) A nitrogen-sparing effect of ketone bodies, which fuel skeletal muscle, may promote muscle protein synthesis and prevent muscle protein breakdown. This review will help guide clinical trials of using MCTs to treat primary (age-related) sarcopenia.

Mitochondrial network dynamics in pulmonary disease: Bridging the gap between inflammation, oxidative stress, and bioenergetics

Once thought of in terms of bioenergetics, mitochondria are now widely accepted as both the orchestrator of cellular health and the gatekeeper of cell death. The pulmonary disease field has performed extensive efforts to explore the role of mitochondria in regulating inflammation, cellular metabolism, apoptosis, and oxidative stress. However, a critical component of these processes needs to be more studied: mitochondrial network dynamics. Mitochondria morphologically change in response to their environment to regulate these processes through fusion, fission, and mitophagy. This allows mitochondria to adapt their function to respond to cellular requirements, a critical component in maintaining cellular homeostasis. For that reason, mitochondrial network dynamics can be considered a bridge that brings multiple cellular processes together, revealing a potential pathway for therapeutic intervention. In this review, we discuss the critical modulators of mitochondrial dynamics and how they are affected in pulmonary diseases, including chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF), acute lung injury (ALI), and pulmonary arterial hypertension (PAH). A dysregulated mitochondrial network plays a crucial role in lung disease pathobiology, and aberrant fission/fusion/mitophagy pathways are druggable processes that warrant further exploration. Thus, we also discuss the candidates for lung disease therapeutics that regulate mitochondrial network dynamics.

Formoterol Acting via β2-Adrenoreceptor Restores Mitochondrial Dysfunction Caused by Parkinson's Disease-Related UQCRC1 Mutation and Improves Mitochondrial Homeostasis Including Dynamic and Transport

Formoterol, a β2-adrenergic receptor (β2AR) agonist, shows promise in various diseases, but its effectiveness in Parkinson's disease (PD) is debated, with unclear regulation of mitochondrial homeostasis. This study employed a cell model featuring mitochondrial ubiquinol-cytochrome c reductase core protein 1 (UQCRC1) variants associated with familial parkinsonism, demonstrating mitochondrial dysfunction and dynamic imbalance, exploring the therapeutic effects and underlying mechanisms of formoterol. Results revealed that 24-h formoterol treatment enhanced cell proliferation, viability, and neuroprotection against oxidative stress. Mitochondrial function, encompassing DNA copy number, repatriation, and complex III-linked respiration, was comprehensively restored, along with the dynamic rebalance of fusion/fission events. Formoterol reduced extensive hypertubulation, in contrast to mitophagy, by significantly upregulating protein Drp-1, in contrast to fusion protein Mfn2, mitophagy-related protein Parkin. The upstream mechanism involved the restoration of ERK signaling and the inhibition of Akt overactivity, contingent on the activation of β2-adrenergic receptors. Formoterol additionally aided in segregating healthy mitochondria for distribution and transport, therefore normalizing mitochondrial arrangement in mutant cells. This study provides preliminary evidence that formoterol offers neuroprotection, acting as a mitochondrial dynamic balance regulator, making it a promising therapeutic candidate for PD.

β2-Adrenoceptors activation regulates muscle trophic-related genes following acute resistance exercise in mice

Resistance exercise (RE) training and pharmacological stimulation of β2-Adrenoceptors (β2-ARs) alone can promote muscle hypertrophy and prevent muscle atrophy. Although the activation of the sympathetic nervous system (SNS) is a well-established response during RE, the physiological contribution of the endogenous catecholamines and β2-ARs to the RE-induced changes on skeletal muscle protein metabolism remains unclear. This study investigated the effects of the β2-ARs blockade on the acute molecular responses induced by a single bout of RE in rodent skeletal muscles. Male C57BL6/J mice were subjected to a single bout of progressive RE (until exhaustion) on a vertical ladder under β2-AR blockade with ICI 118,551 (ICI; 10 mg kg-1, i. p.), or vehicle (sterile saline; 0.9%, i. p.), and the gene expression was analyzed in gastrocnemius (GAS) muscles by qPCR. We demonstrated that a single bout of RE acutely increased the circulating levels of stress-associated hormones norepinephrine (NE) and corticosterone (CORT), as well as the muscle phosphorylation levels of AMPK, p38 MAPK and CREB, immediately after the session. The acute increase in the phosphorylation levels of CREB was followed by the upregulation of CREB-target genes Sik1, Ppargc1a and Nr4a3 (a central regulator of the acute RE response), 3 h after the RE session. Conversely, β2-AR blockade reduced significantly the Sik1 and Nr4a3 mRNA levels in muscles of exercised mice. Furthermore, a single bout of RE stimulated the mRNA levels of the atrophic genes Map1lc3b and Gabarapl1 (autophagy-related genes) and Mstn (a well-known negative regulator of muscle growth). Unexpectedly, the gene expression of Igf-1 or Il-6 were not affected by RE, while the atrophic genes Murf1/Trim63 and Atrogin-1/Mafbx32 (ubiquitin-ligases) were increased only in muscles of exercised mice under β2-AR blockade. Interestingly, performing a single bout of RE under β2-AR blockade increased the mRNA levels of Mstn in muscles of exercised mice. These data suggest that β2-ARs stimulation during acute RE stimulates the hypertrophic gene Nr4a3 and prevents the overexpression of atrophic genes such as Mstn, Murf1/Trim63, and Atrogin-1/Mafbx32 in the first hours of postexercise recovery, indicating that he SNS may be physiologically important to muscle adaptations in response to resistance training.

Anaphylatoxin C5a receptor signaling induces mitochondrial fusion and sensitizes retinal pigment epithelial cells to oxidative stress

Mitochondrial dynamics is a morphological balance between fragmented and elongated shapes, reflecting mitochondrial metabolic status, cellular damage, and mitochondrial dysfunction. The anaphylatoxin C5a derived from complement component 5 cleavage, enhances cellular responses involved in pathological stimulation, innate immune responses, and host defense. However, the specific response of C5a and its receptor, C5a receptor (C5aR), in mitochondria is unclear. Here, we tested whether the C5a/C5aR signaling axis affects mitochondrial morphology in human-derived retinal pigment epithelial cell monolayers (ARPE-19). C5aR activation with the C5a polypeptide induced mitochondrial elongation. In contrast, oxidatively stressed cells (H2O2) responded to C5a with an enhancement of mitochondrial fragmentation and an increase in the number of pyknotic nuclei. C5a/C5aR signaling increased the expression of mitochondrial fusion-related protein, mitofusin-1 (MFN1) and - 2 (MFN2), as well as enhanced optic atrophy-1 (Opa1) cleavage, which are required for mitochondrial fusion events, whereas the mitochondrial fission protein, dynamin-related protein-1 (Drp1), and mitogen-activated protein kinase (MAPK)-dependent extracellular signal-regulated protein kinase (Erk1/2) phosphorylation were not affected. Moreover, C5aR activation increased the frequency of endoplasmic reticulum (ER)-mitochondria contacts. Finally, oxidative stress induced in a single cell within an RPE monolayer (488 nm blue laser spot stimulation) induced a bystander effect of mitochondrial fragmentation in adjacent surrounding cells only in C5a-treated monolayers. These results suggest that C5a/C5aR signaling produced an intermediate state, characterized by increased mitochondrial fusion and ER-mitochondrial contacts, that sensitizes cells to oxidative stress, leading to mitochondrial fragmentation and cell death.

Moderate-Intensity Exercise Maintains Redox Homeostasis for Cardiovascular Health

It is well known that exercise is beneficial for cardiovascular health. Oxidative stress is the common pathological basis of many cardiovascular diseases. The overproduction of free radicals, both reactive oxygen species and reactive nitrogen species, can lead to redox imbalance and exacerbate oxidative damage to the cardiovascular system. Maintaining redox homeostasis and enhancing anti-oxidative capacity are critical mechanisms by which exercise protects against cardiovascular diseases. Moderate-intensity exercise is an effective means to maintain cardiovascular redox homeostasis. Moderate-intensity exercise reduces the risk of cardiovascular disease by improving mitochondrial function and anti-oxidative capacity. It also attenuates adverse cardiac remodeling and enhances cardiac function. This paper reviews the primary mechanisms of moderate-intensity exercise-mediated redox homeostasis in the cardiovascular system. Exploring the role of exercise-mediated redox homeostasis in the cardiovascular system is of great significance to the prevention and treatment of cardiovascular diseases.

Shared and unique phosphoproteomics responses in skeletal muscle from exercise models and in hyperammonemic myotubes

Skeletal muscle generation of ammonia, an endogenous cytotoxin, is increased during exercise. Perturbations in ammonia metabolism consistently occur in chronic diseases, and may blunt beneficial skeletal muscle molecular responses and protein homeostasis with exercise. Phosphorylation of skeletal muscle proteins mediates cellular signaling responses to hyperammonemia and exercise. Comparative bioinformatics and machine learning-based analyses of published and experimentally derived phosphoproteomics data identified differentially expressed phosphoproteins that were unique and shared between hyperammonemic murine myotubes and skeletal muscle from exercise models. Enriched processes identified in both hyperammonemic myotubes and muscle from exercise models with selected experimental validation included protein kinase A (PKA), calcium signaling, mitogen-activated protein kinase (MAPK) signaling, and protein homeostasis. Our approach of feature extraction from comparative untargeted "omics" data allows for selection of preclinical models that recapitulate specific human exercise responses and potentially optimize functional capacity and skeletal muscle protein homeostasis with exercise in chronic diseases.

Adrenergic Receptor Regulation of Mitochondrial Function in Cardiomyocytes

ABSTRACT

Adrenergic receptors (ARs) are G protein-coupled receptors that are stimulated by catecholamines to induce a wide array of physiological effects across tissue types. Both α1- and β-ARs are found on cardiomyocytes and regulate cardiac contractility and hypertrophy through diverse molecular pathways. Acute activation of cardiomyocyte β-ARs increases heart rate and contractility as an adaptive stress response. However, chronic β-AR stimulation contributes to the pathobiology of heart failure. By contrast, mounting evidence suggests that α1-ARs serve protective functions that may mitigate the deleterious effects of chronic β-AR activation. Here, we will review recent studies demonstrating that α1- and β-ARs differentially regulate mitochondrial biogenesis and dynamics, mitochondrial calcium handling, and oxidative phosphorylation in cardiomyocytes. We will identify potential mechanisms of these actions and focus on the implications of these findings for the modulation of contractile function in the uninjured and failing heart. Collectively, we hope to elucidate important physiological processes through which these well-studied and clinically relevant receptors stimulate and fuel cardiac contraction to contribute to myocardial health and disease.

Targeting skeletal muscle mitochondrial health in obesity

Metabolic demands of skeletal muscle are substantial and are characterized normally as highly flexible and with a large dynamic range. Skeletal muscle composition (e.g., fiber type and mitochondrial content) and metabolism (e.g., capacity to switch between fatty acid and glucose substrates) are altered in obesity, with some changes proceeding and some following the development of the disease. Nonetheless, there are marked interindividual differences in skeletal muscle composition and metabolism in obesity, some of which have been associated with obesity risk and weight loss capacity. In this review, we discuss related molecular mechanisms and how current and novel treatment strategies may enhance weight loss capacity, particularly in diet-resistant obesity.

Hepatokine ERAP1 Disturbs Skeletal Muscle Insulin Sensitivity Via Inhibiting USP33-Mediated ADRB2 Deubiquitination

Chronic inflammation in liver induces insulin resistance systemically and in other tissues, including the skeletal muscle (SM); however, the underlying mechanisms remain largely unknown. RNA sequencing of primary hepatocytes from wild-type mice fed long-term high-fat diet (HFD), which have severe chronic inflammation and insulin resistance revealed that the expression of hepatokine endoplasmic reticulum aminopeptidase 1 (ERAP1) was upregulated by a HFD. Increased ERAP1 levels were also observed in interferon-γ-treated primary hepatocytes. Furthermore, hepatic ERAP1 overexpression attenuated systemic and SM insulin sensitivity, whereas hepatic ERAP1 knockdown had the opposite effects, with corresponding changes in serum ERAP1 levels. Mechanistically, ERAP1 functions as an antagonist-like factor, which interacts with β2 adrenergic receptor (ADRB2) and reduces its expression by decreasing ubiquitin-specific peptidase 33-mediated deubiquitination and thereby interrupts ADRB2-stimulated insulin signaling in the SM. The findings of this study indicate ERAP1 is an inflammation-induced hepatokine that impairs SM insulin sensitivity. Its inhibition may provide a therapeutic strategy for insulin resistance-related diseases, such as type 2 diabetes.

MCU-complex-mediated mitochondrial calcium signaling is impaired in Barth syndrome

Calcium signaling via mitochondrial calcium uniporter (MCU) complex coordinates mitochondrial bioenergetics with cellular energy demands. Emerging studies show that the stability and activity of the pore-forming subunit of the complex, MCU, is dependent on the mitochondrial phospholipid, cardiolipin (CL), but how this impacts calcium-dependent mitochondrial bioenergetics in CL-deficiency disorder like Barth syndrome (BTHS) is not known. Here we utilized multiple models of BTHS including yeast, mouse muscle cell line, as well as BTHS patient cells and cardiac tissue to show that CL is required for the abundance and stability of the MCU-complex regulatory subunit MICU1. Interestingly, the reduction in MICU1 abundance in BTHS mitochondria is independent of MCU. Unlike MCU and MICU1/MICU2, other subunit and associated factor of the uniporter complex, EMRE and MCUR1, respectively, are not affected in BTHS models. Consistent with the decrease in MICU1 levels, we show that the kinetics of MICU1-dependent mitochondrial calcium uptake is perturbed and acute stimulation of mitochondrial calcium signaling in BTHS myoblasts fails to activate pyruvate dehydrogenase, which in turn impairs the generation of reducing equivalents and blunts mitochondrial bioenergetics. Taken together, our findings suggest that defects in mitochondrial calcium signaling could contribute to cardiac and skeletal muscle pathologies observed in BTHS patients.

Health benefits of aerobic exercise: known mechanisms and research potential

The pandemic of noncommunicable diseases, which is currently one of the main threats to health and well-being of mankind, makes us look for ways to prevent their development early. Low cardiorespiratory endurance due to a sedentary lifestyle is associated with high cardiovascular risk, all-cause and cancer mortality. Skeletal muscles are the most important secretory organ and is characterized by outstanding metabolic performance and endurance. Exercise-induced low-dose stress contributes to mitochondrial biogenesis and remodeling of not only the muscular system, but also other systems involved in maintaining muscle activity, including regulating glucose and fat metabolism, maintaining immunity, and stimulating angiogenesis. These and other effects of physical activity are implements through the myokine system discovered in recent years. Shutting off the paracrine, exocrine and endocrine functions of muscles cannot be replenished in any other way and leads to disruption of vital adaptive processes. This review describes currently available evidence of unique role of aerobic physical activity in maintaining the human health, as well as to define the chain of pathological reactions during physical inactivity. The search was carried out in the Medline and PubMed Central databases for the keywords: cardiorespiratory endurance, non-communicable diseases, maximum oxygen consumption, myokines, interleukin-6, aerobic exercise.