Reduced aerobic capacity, as measured by maximal oxygen uptake, is a hallmark in cardiovascular diseases and strongly predicts poor prognosis and higher mortality rates in heart failure patients. While exercise capacity is poorly correlated with cardiac function in this population, skeletal muscle abnormalities present a striking association with maximal oxygen uptake. This fact draws substantial attention to the clinical relevance of targeting skeletal myopathy in heart failure. Considering that skeletal muscle is highly responsive to aerobic exercise training, we addressed the benefits of aerobic exercise training to combat skeletal myopathy in heart failure, focusing on the mechanisms by which aerobic exercise training counteracts skeletal muscle atrophy.
What is the topic of this review?
Genetic modifiers act on many different physiological aspects of muscle disease. Understanding and identifying such modifiers is important because their discovery may help to predict the course of muscle disease and also indicate pathways to be exploited in designing new therapeutics.
What advances does it highlight?
Genetic modifiers have been identified that act primarily on limb skeletal muscles. Newer modifiers, where the responsible gene has yet to be identified, alter the course of cardiopulmonary dysfunction in muscular dystrophy. Distinct modifiers that act differentially on limb skeletal muscles versus heart and respiratory muscles reflect underlying physiological differences of these muscle groups.
Many single-gene disorders are associated with a range of symptoms that cannot be explained solely by the primary genetic mutation. Muscular dystrophy is a genetic disorder associated with variable outcomes that arise from both the primary genetic mutation and the contribution from environmental and genetic modifiers. Disruption of the dystrophin complex occurs in Duchenne muscular dystrophy and limb girdle muscular dystrophy, producing heart and muscle disease through a cellular injury process characterized by plasma membrane disruption and fibrosis. Multiple modifier loci have been mapped by using a mouse model of muscular dystrophy. These modifiers exert their effect often on specific muscle groups targeted by the muscular dystrophy process, possibly reflecting distinct pathophysiological processes among muscle groups. Genetic modifiers act on both cardiac and respiratory muscle parameters, suggesting genetic and physiological integration of cardiopulmonary function. Skeletal muscles of the limbs are modified by a locus on mouse chromosome 7. This region of chromosome 7 harbours an insertion/deletion polymorphism in Ltbp4, the gene encoding latent transforming growth factor β binding protein 4. LTBP4 exerts its effect in muscle disease by acting on plasma membrane stability and fibrosis, thereby linking instability of the sarcolemma directly to fibrosis. In the human muscle disease Duchenne muscular dystrophy, protein coding single-nucleotide polymorphisms in LTBP4 associate with prolonged ambulation, demonstrating that modifiers identified from mouse studies translate to human disease.
Autonomic, locomotor and cardiac abnormalities in a mouse model of muscular dystrophy: targeting the renin-angiotensin system
What is the topic of this review?
This symposium report summarizes autonomic, cardiac and skeletal muscle abnormalities in sarcoglycan--deficient mice (Sgcd–/–), a mouse model of limb girdle muscular dystrophy, with emphasis on the roles of autonomic dysregulation and activation of the renin–angiotensin system at a young age.
What advances does it highlight?
The contributions of the autonomic nervous system and the renin–angiotensin system to the pathogenesis of muscular dystrophy are highlighted. Results demonstrate that autonomic dysregulation precedes and predicts later development of cardiac dysfunction in Sgcd–/– mice and that treatment of young Sgcd–/– mice with the angiotensin type 1 receptor antagonist losartan or with angiotensin-(1–7) abrogates the autonomic dysregulation, attenuates skeletal muscle pathology and increases spontaneous locomotor activity.
Muscular dystrophies are a heterogeneous group of genetic muscle diseases characterized by muscle weakness and atrophy. Mutations in sarcoglycans and other subunits of the dystrophin–glycoprotein complex cause muscular dystrophy and dilated cardiomyopathy in animals and humans. Aberrant autonomic signalling is recognized in a variety of neuromuscular disorders. We hypothesized that activation of the renin–angiotensin system contributes to skeletal muscle and autonomic dysfunction in mice deficient in the sarcoglycan- (Sgcd) gene at a young age and that this early autonomic dysfunction contributes to the later development of left ventricular (LV) dysfunction and increased mortality. We demonstrated that young Sgcd–/– mice exhibit histopathological features of skeletal muscle dystrophy, decreased locomotor activity and severe autonomic dysregulation, but normal LV function. Autonomic regulation continued to deteriorate in Sgcd–/– mice with age and was accompanied by LV dysfunction and dilated cardiomyopathy at older ages. Autonomic dysregulation at a young age predicted later development of LV dysfunction and higher mortality in Sgcd–/– mice. Treatment of Sgcd–/– mice with the angiotensin type 1 receptor blocker losartan for 8–9 weeks, beginning at 3 weeks of age, decreased fibrosis and oxidative stress in skeletal muscle, increased locomotor activity and prevented autonomic dysfunction. Chronic infusion of the counter-regulatory peptide angiotensin-(1–7) resulted in similar protection. We conclude that activation of the renin–angiotensin system, at a young age, contributes to skeletal muscle and autonomic dysfunction in muscular dystrophy. We speculate that the latter is mediated via abnormal sensory nerve and/or cytokine signalling from dystrophic skeletal muscle to the brain and contributes to age-related LV dysfunction, dilated cardiomyopathy, arrhythmias and premature death. Therefore, correcting the early autonomic dysregulation and renin–angiotensin system activation may provide a novel therapeutic approach in muscular dystrophy.
Muscle disuse and starvation are often associated with a catabolic response leading to a dramatic loss of skeletal muscle mass. Hibernating animals represent a unique situation where muscle mass is maintained despite prolonged periods of immobilization and lack of nutrition. We analysed the molecular pathways upregulated during hibernation in an obligate hibernator, the 13-lined ground squirrel (Ictidomys tridecemlineatus). Although Akt has an established role in skeletal muscle maintenance, we found that activated Akt was decreased in skeletal muscle of hibernating squirrels. Another serine–threonine kinase, serum- and glucocorticoid-regulated kinase 1 (SGK1), was upregulated during hibernation and contributed to protection from loss of muscle mass via downregulation of proteolysis and autophagy and via an increase in protein synthesis. We extended our observations to non-hibernating animals and demonstrated that SGK1-null mice developed muscle atrophy. These mice displayed an exaggerated response to immobilization and starvation. Furthermore, SGK1 overexpression prevented immobilization-induced muscle atrophy. Taken together, our results identify SGK1 as a novel therapeutic target to combat skeletal muscle loss in acquired and inherited forms of muscle atrophy.
Role of the carotid body chemoreceptors in baroreflex control of blood pressure during hypoglycaemia in humans
Activation of the carotid body chemoreceptors with hypoxia alters baroreceptor-mediated responses. We aimed to examine whether this relationship can be translated to other chemoreceptor stimuli (i.e. hypoglycaemia) by testing the following hypotheses: (i) activation of the carotid body chemoreceptors with hypoglycaemia would reduce spontaneous cardiac baroreflex sensitivity (sCBRS) in healthy humans; and (ii) desensitization of the carotid chemoreceptors with hyperoxia would restore sCBRS to baseline levels during hypoglycaemia. Ten young healthy adults completed two 180 min hyperinsulinaemic [2 mU (kg fat-free mass)–1 min–1], hypoglycaemic (~3.2 μmol ml–1) clamps, separated by at least 1 week and randomized to normoxia (arterial partial pressure of O2, 122 ± 10 mmHg) or hyperoxia (arterial partial pressure of O2, 424 ± 123 mmHg; to blunt activation of the carotid body glomus cells). Changes in heart rate, blood pressure, plasma catecholamines, heart rate variability (HRV) and sCBRS were assessed. During hypoglycaemia, HRV and sCBRS were reduced (P < 0.05) and the baroreflex working range was shifted to higher heart rates. When hyperoxia was superimposed on hypoglycaemia, there was a greater reduction in blood pressure and a blunted rise in heart rate when compared with normoxic conditions (P < 0.05); however, there was no detectable effect of hyperoxia on sCBRS or HRV during hypoglycaemia (P > 0.05). In summary, hypoglycaemia-mediated changes in HRV and sCBRS cannot be attributed exclusively to the carotid chemoreceptors; however, the chemoreceptors appear to play a role in resetting the baroreflex working range during hypoglycaemia.
Angiotensin-converting enzyme (ACE and ACE2) imbalance correlates with the severity of cerulein-induced acute pancreatitis in mice
Angiotensin-converting enzyme (ACE) and its effector peptide angiotensin II (Ang II) have been implicated in the pathogenesis of pancreatitis. Angiotensin-converting enzyme 2 (ACE2) degrades Ang II to angiotensin-(1–7) [Ang-(1–7)] and has recently been described to have an antagonistic effect on ACE signalling. However, the specific underlying role of ACE2 in the pathogenesis of severe acute pancreatitis (SAP) is unclear. In the present study, the local imbalance of ACE and ACE2, as well as Ang II and Ang-(1–7) expression, was compared in wild-type (WT) and ACE2 knock-out (KO) or ACE2 transgenic (TG) mice subjected to cerulein-induced SAP. Serum amylase, tumour necrosis factor-α, interleukin (IL)-1β, IL-6 and IL-10 levels and histological morphometry were used to determine the severity of pancreatitis. In WT mice, pancreatic ACE and Ang II and serum Ang II expression increased (P < 0.05), while pancreatic ACE2 and Ang-(1–7) and serum Ang-(1–7) levels were also significantly elevated (P < 0.05) from 2 to 72 h after the onset of SAP. However, the ratio of pancreatic ACE2 to ACE expression was significantly reduced (from 1.46 ± 0.09 to 0.27 ± 0.05, P < 0.001) and paralleled the severity of pancreatitis. The Ace2 KO mice exhibited increased levels of tumour necrosis factor-α, IL-1β, IL-6, multifocal coagulative necrosis and inflammatory infiltrate, and lower levels of serum IL-10 and pancreatic Ang-(1–7) (4.70 ± 2.13 versus 10.87 ± 2.51, P < 0.001) compared with cerulein-treated WT mice at the same time point. Conversely, Ace2 TG mice with normal ACE expression were more resistant to SAP challenge as evidenced by a decreased inflammatory response, attenuated pathological changes and increased survival rates. These data suggest that the ACE2–ACE imbalance plays an important role in the pathogenesis of SAP and that pancreatic ACE2 is an important factor in determining the severity of SAP.
Muscles of mdx mice are known to be more susceptible to contraction-induced damage than wild-type muscle. However, it is not clear whether this is because of dystrophin deficiency or because of the abnormal branching morphology of dystrophic muscle fibres. This distinction has an important bearing on our traditional understanding of the function of dystrophin as a mechanical stabilizer of the sarcolemma. In this study, we address the question: ‘Does dystrophin-positive, regenerated muscle containing branched fibres also show an increased susceptibility to contraction-induced damage?’ We produced a model of fibre branching by injecting dystrophin-positive extensor digitorum longus muscles with notexin. The regenerated muscle was examined at 21 days postinjection. Notexin-injected muscle contained 29% branched fibres and was not more susceptible to damage from mild eccentric contractions than contralateral saline-injected control muscle. Regenerated muscles also had greater mass, greater cross-sectional area and lower specific force than control muscles. We conclude that the number of branched fibres in this regenerated muscle is below the threshold needed to increase susceptibility to damage. However, it would serve as an ideal control for muscles of young mdx mice, allowing for clearer differentiation of the effects of dystrophin deficiency from the effects of fibre regeneration and morphology.
Alterations in Notch signalling in skeletal muscles from mdx and dko dystrophic mice and patients with Duchenne muscular dystrophy
What is the central question of this study?
The Notch signalling pathway plays an important role in muscle regeneration, and activation of the pathway has been shown to enhance muscle regeneration in aged mice. It is unknown whether Notch activation will have a similarly beneficial effect on muscle regeneration in the context of Duchenne muscular dystrophy (DMD).
What is the main finding and its importance?
Although expression of Notch signalling components is altered in both mouse models of DMD and in human DMD patients, activation of the Notch signalling pathway does not confer any functional benefit on muscles from dystrophic mice, suggesting that other signalling pathways may be more fruitful targets for manipulation in treating DMD.
In Duchenne muscular dystrophy (DMD), muscle damage and impaired regeneration lead to progressive muscle wasting, weakness and premature death. The Notch signalling pathway represents a central regulator of gene expression and is critical for cellular proliferation, differentiation and apoptotic signalling during all stages of embryonic muscle development. Notch activation improves muscle regeneration in aged mice, but its potential to restore regeneration and function in muscular dystrophy is unknown. We performed a comprehensive examination of several genes involved in Notch signalling in muscles from dystrophin-deficient mdx and dko (utrophin- and dystrophin-null) mice and DMD patients. A reduction of Notch1 and Hes1 mRNA in tibialis anterior muscles of dko mice and quadriceps muscles of DMD patients and a reduction of Hes1 mRNA in the diaphragm of the mdx mice were observed, with other targets being inconsistent across species. Activation and inhibition of Notch signalling, followed by measures of muscle regeneration and function, were performed in the mouse models of DMD. Notch activation had no effect on functional regeneration in C57BL/10, mdx or dko mice. Notch inhibition significantly depressed the frequency–force relationship in regenerating muscles of C57BL/10 and mdx mice after injury, indicating reduced force at each stimulation frequency, but enhanced the frequency–force relationship in muscles from dko mice. We conclude that while Notch inhibition produces slight functional defects in dystrophic muscle, Notch activation does not significantly improve muscle regeneration in murine models of muscular dystrophy. Furthermore, the inconsistent expression of Notch targets between murine models and DMD patients suggests caution when making interspecies comparisons.
Respiratory muscle dysfunction documented in sleep apnoea patients is perhaps due to oxidative stress secondary to chronic intermittent hypoxia (CIH). We sought to explore the effects of different CIH protocols on respiratory muscle form and function in a rodent model. Adult male Wistar rats were exposed to CIH (n = 32) consisting of 90 s normoxia–90 s hypoxia (either 10 or 5% oxygen at the nadir; arterial O2 saturation ~90 or 80%, respectively] for 8 h per day or to sham treatment (air–air, n = 32) for 1 or 2 weeks. Three additional groups of CIH-treated rats (5% O2 for 2 weeks) had free access to water containing N-acetyl cysteine (1% NAC, n = 8), tempol (1 mm, n = 8) or apocynin (2 mm, n = 8). Functional properties of the diaphragm muscle were examined ex vivo at 35°C. The myosin heavy chain and sarco(endo)plasmic reticulum Ca2+-ATPase isoform distribution, succinate dehydrogenase and glyercol phosphate dehydrogenase enzyme activities, Na+–K+-ATPase pump content, concentration of thiobarbituric acid reactive substances, DNA oxidation and antioxidant capacity were determined. Chronic intermittent hypoxia (5% oxygen at the nadir; 2 weeks) decreased diaphragm muscle force and endurance. All three drugs reversed the deleterious effects of CIH on diaphragm endurance, but only NAC prevented CIH-induced diaphragm weakness. Chronic intermittent hypoxia increased diaphragm muscle myosin heavy chain 2B areal density and oxidized glutathione/reduced glutathione (GSSG/GSH) ratio. We conclude that CIH-induced diaphragm dysfunction is reactive oxygen species dependent. N-Acetyl cysteine was most effective in reversing CIH-induced effects on diaphragm. Our results suggest that respiratory muscle dysfunction in sleep apnoea may be the result of oxidative stress and, as such, antioxidant treatment could prove a useful adjunctive therapy for the disorder.
The possible mechanisms by which maternal hypothyroidism impairs insulin secretion in adult male offspring in rats
Previous studies have recently shown that maternal hypothyroidism leads to impaired glucose metabolism and reduced insulin secretion in adult offspring in rats. The aim of this study was to locate the defect in the insulin secretion pathway induced by maternal hypothyroidism. Pregnant Wistar rats were divided into two groups; the control group consumed water, while the hypothyroid (FH) group received water containing 0.025% 6-propyl-2-thiouracil during gestation. An intravenous glucose tolerance test was carried out on 5-month-old male offspring. In in vitro studies, the effects of various secretagogues and inhibitors acting at different levels of the insulin secretion cascade were investigated, and insulin content, insulin secretion and glucokinase activity of the islets were compared. Although insulin content of the FH islets did not differ from that of control islets, insulin secretion from FH islets was reduced when it was challenged by glucose or arginine. Compared with control islets, activities of both hexokinase and glucokinase were also significantly decreased in the FH islets. Although, in both groups, increasing glibenclamide and nifedipine concentrations in the presence of 16.7 mmol l–1 glucose increased and decreased insulin secretion, respectively, the percentage of changes in secretion of FH islets was significantly lower compared with control islets. The response of FH islets to high extracellular potassium concentration and diazoxide was also significantly lower than that of the control islets. These findings demonstrate that impaired insulin secretion in the FH group is probably related to alterations in different steps of the insulin secretion pathway and not in the insulin pool of β-cells.
The combined influence of fat consumption and repeated mental stress on brachial artery flow-mediated dilatation: a preliminary study
Experienced separately, both acute mental stress and high-fat meal consumption can transiently impair endothelial function, and the purpose of the present study was to investigate their combined impact. On four separate days, 10 healthy men (23 years old) underwent brachial artery flow-mediated dilatation (FMD) tests, before and hourly for 4 h post-consumption of a high-fat (HFM; 54 g fat) or low-fat meal (LFM; 0 g fat; each meal ~1000 calories), with hourly mental stress (mental arithmetic, speech) or control (counting) tasks (conditions HFM+S, LFM+S, HFM and LFM). Data are presented as means ± SD. Plasma triglycerides increased and remained elevated after the high-fat but not the low-fat meal (P = 0.004) and were not affected by mental stress (P = 0.329). Indices of stress reactivity increased during mental stress tasks (mean arterial pressure, ~20 mmHg; heart rate, ~22 beats min–1; salivary cortisol, ~2.37 nmol l–1; and plasma noradrenaline, ~0.17 ng ml–1) and were not influenced by meal (P > 0.05). There was no effect of the type of meal on FMD (P = 0.562); however, FMD was 4.5 ± 0.5% in the control conditions and 5.8 ± 0.6% in the mental stress conditions (P = 0.087), and this difference was significant when normalized for the shear stress stimulus (FMD/area under the curve of shear stress, P = 0.045). Overall, these preliminary data suggest that postprandial FMD was augmented with mental stress irrespective of meal type. These results are contrary to previous reports of impaired endothelial function after mental stress or fat consumption independently and highlight the need to further investigate the mechanisms underlying the interactions between these factors.
Acute inflammation reduces flow-mediated vasodilatation and increases arterial stiffness in young healthy individuals. However, this response has not been studied in older adults. The aim of this study, therefore, was to evaluate the effect of acute induced systemic inflammation on endothelial function and wave reflection in older adults. Furthermore, an acute bout of moderate-intensity aerobic exercise can be anti-inflammatory. Taken together, we tested the hypothesis that acute moderate-intensity endurance exercise, immediately preceding induced inflammation, would be protective against the negative effects of acute systemic inflammation on vascular function. Fifty-nine healthy volunteers between 55 and 75 years of age were randomized to an exercise or a control group. Both groups received a vaccine (induced inflammation) and sham (saline) injection in a counterbalanced crossover design. Inflammatory markers, endothelial function (flow-mediated vasodilatation) and measures of wave reflection and arterial stiffness were evaluated at baseline and at 24 and 48 h after injections. There were no significant differences in endothelial function and arterial stiffness between the exercise and control group after induced inflammation. The groups were then analysed together, and we found significant differences in the inflammatory markers 24 and 48 h after induction of acute inflammation compared with sham injection. However, flow-mediated vasodilatation, augmentation index normalized for heart rate (AIx75) and β-stiffness did not change significantly. Our results suggest that acute inflammation induced by influenza vaccination did not affect endothelial function in older adults.
Until we turned our nights into days and began to travel in aircraft across multiple time zones, we were largely unaware that we possess a ‘day within’ driven by an internal body clock. Yet the striking impairment of our abilities in the early hours of the morning soon reminds us that we are slaves to our biology. Our ability to perform mathematical calculations or other intellectual tasks between 04.00 and 06.00 h is worse than if we had consumed several shots of whisky and would be classified as legally drunk. Biological clocks drive or alter our sleep patterns, alertness, mood, physical strength, blood pressure and every other aspect of our physiology and behaviour. Our emerging understanding of how these 24 h rhythms are generated and regulated is not only one of the great success stories of modern biology, but is also informing many areas of human health. Sleep and circadian rhythm disruption (SCRD) is a feature shared by some of the most challenging diseases of our time, including neuropsychiatric illness and serious disorders of the eye. Sleep and circadian rhythm disruption is also commonly seen across many sectors of society, from teenagers to shift workers. We also now appreciate that SCRD is far more than feeling sleepy at an inappropriate time. It promotes multiple illnesses ranging across abnormal metabolism, heart disease, reduced immunity, increased stress and abnormal cognition and mood states. This short review considers how 24 h rhythms are generated and regulated, the consequences of working against our body clock and the emerging relationship between SCRD and mental illness.
In the last 20 years there has been mounting evidence that chronic heart failure (CHF) has a complex pathophysiology, which begins with an abnormality of the heart as a ‘primum movens’, but involves adaptive changes in many body parts, including the cardiovascular, musculoskeletal, renal, neuroendocrine, haemostatic, immune and inflammatory systems. Alterations in skeletal muscle are also of importance in limiting functional capacity in patients with CHF, because reduced physical activity plays some part in the muscle alterations in CHF. On the whole, these abnormalities resemble those induced by physical deconditioning. Moreover, the overactivation of signals originating from skeletal muscle receptors (mechano-metaboreceptors) is an intriguing hypothesis proposed to explain the origin of symptoms and the beneficial effect of exercise training in the CHF syndrome. These reflexes may contribute to sympathetic overactivation, to exercise intolerance and to the progression of CHF syndrome. The so-called metaboreflex has been reported to be hyperactive in CHF and to be responsible for a paradoxical increase in systemic vascular resistance and decrease in cardiac output whenever activated in these patients. This report is a brief summary of the latest news in this area of research.
We are endlessly fascinated by memory; we desire to improve it and fear its loss. While it has long been recognized that brain regions such as the hippocampus are vital for supporting memories of our past experiences (autobiographical memories), we still lack fundamental knowledge about the mechanisms involved. This is because the study of specific neural signatures of autobiographical memories in vivo in humans presents a significant challenge. However, recent developments in high-resolution structural and functional magnetic resonance imaging coupled with advanced analytical methods now permit access to the neural substrates of memory representations that has hitherto been precluded in humans. Here, I describe how the application of ‘decoding’ techniques to brain-imaging data is beginning to disclose how individual autobiographical memory representations evolve over time, deepening our understanding of systems-level consolidation. In particular, this prompts new questions about the roles of the hippocampus and ventromedial prefrontal cortex and offers new opportunities to interrogate the elusive memory trace that has for so long confounded neuroscientists.