Joanne E. Mallinson

Blunted Akt/FOXO signalling and activation of genes controlling atrophy and fuel use in statin myopathy

Statins are used clinically for cholesterol reduction, but statin therapy is associated with myopathic changes through a poorly defined mechanism. We used an in vivo model of statin myopathy to determine whether statins up‐regulate genes associated with proteasomal‐ and lysosomal‐mediated proteolysis and whether PDK gene expression is simultaneously up‐regulated leading to the impairment of muscle carbohydrate oxidation. Animals were dosed daily with 80 mg kg−1 day−1 simvastatin for 4 (n= 6) and 12 days (n= 5), 88 mg kg−1 day−1 simvastatin for 12 days (n= 4), or vehicle (0.5% w/v hydroxypropyl‐methylcellulose and 0.1% w/v polysorbate 80; Control, n= 6) for 12 days by oral gavage. We found, in biceps femoris muscle, decreased AktSer473, FOXO1Ser253 and FOXO3aSer253 phosphorylation in the cytosol (P < 0.05, P < 0.05, P < 0.001, respectively) and decreased phosphorylation of FOXO1 in the nucleus after 12 days simvastatin when compared to Control (P < 0.05). This was paralleled by a marked increase in the transcription of downstream targets of FOXO, i.e. MAFbx (P < 0.001), MuRF‐1 (P < 0.001), cathepsin‐L (P < 0.05), PDK2 (P < 0.05) and PDK4 (P < 0.05). These changes were accompanied by increased PPARα (P < 0.05), TNFα (P < 0.01), IL6 (P < 0.01), Mt1A (P < 0.01) mRNA and increased muscle glycogen (P < 0.05) compared to Control. RhoA activity decreased after 4 days simvastatin (P < 0.05); however, activity was no different from Control after 12 days. Simvastatin down‐regulated PI3k/Akt signalling, independently of RhoA, and up‐regulated FOXO transcription factors and downstream gene targets known to be implicated in proteasomal‐ and lysosomal‐mediated muscle proteolysis, carbohydrate oxidation, oxidative stress and inflammation in an in vivo model of statin‐induced myopathy. These changes occurred in the main before evidence of extensive myopathy or a decline in the muscle protein to DNA ratio.

Fetal exposure to a maternal low-protein diet during mid-gestation results in muscle-specific effects on fibre type composition in young rats

This study assessed the impact of reduced dietary protein during specific periods of fetal life upon muscle fibre development in young rats. Pregnant rats were fed a control or low-protein (LP) diet at early (days 0-7 gestation, LPEarly), mid (days 8-14, LPMid), late (days 15-22, LPLate) or throughout gestation (days 0-22, LPAll). The muscle fibre number and composition in soleus and gastrocnemius muscles of the offspring were studied at 4 weeks of age. In the soleus muscle, both the total number and density of fast fibres were reduced in LPMid females (P = 0.004 for both, Diet x Sex x Fibre type interactions), while both the total number and density of glycolytic (non-oxidative) fibres were reduced in LPEarly, LPMid and LPLate (but not LPAll) offspring compared with controls (P < 0.001 for both, Diet x Fibre type interaction). In the gastrocnemius muscle, only the density of oxidative fibres was reduced in LPMid compared with control offspring (P = 0.019, Diet x Fibre type interaction), with the density of slow fibres being increased in LPAll males compared with control (P = 0.024, Diet x Sex x Fibre type interaction). There were little or no effects of maternal diet on fibre type diameters in the two muscles. In conclusion, a maternal low-protein diet mainly during mid-pregnancy reduced muscle fibre number and density in 4-week-old rats, but there were muscle-specific differences in the fibre types affected.

Obesity appears to be associated with altered muscle protein synthetic and breakdown responses to increased nutrient delivery in older men, but not reduced muscle mass or contractile function.

Obesity is increasing, yet despite the necessity of maintaining muscle mass and function with age, the effect of obesity on muscle protein turnover in older adults remains unknown. Eleven obese (BMI 31.9 ± 1.1 kg · m−2) and 15 healthy-weight (BMI 23.4 ± 0.3 kg · m−2) older men (55–75 years old) participated in a study that determined muscle protein synthesis (MPS) and leg protein breakdown (LPB) under postabsorptive (hypoinsulinemic-euglycemic clamp) and postprandial (hyperinsulinemic hyperaminoacidemic-euglycemic clamp) conditions. Obesity was associated with systemic inflammation, greater leg fat mass, and patterns of mRNA expression consistent with muscle deconditioning, whereas leg lean mass, strength, and work done during maximal exercise were no different. Under postabsorptive conditions, MPS and LPB were equivalent between groups, whereas insulin and amino acid administration increased MPS in only healthy-weight subjects and was associated with lower leg glucose disposal (LGD) (63%) in obese men. Blunting of MPS in the obese men was offset by an apparent decline in LPB, which was absent in healthy-weight subjects. Lower postprandial LGD in obese subjects and blunting of MPS responses to amino acids suggest that obesity in older adults is associated with diminished muscle metabolic quality. This does not, however, appear to be associated with lower leg lean mass or strength.

Mechanisms responsible for disuse muscle atrophy: Potential role of protein provision and exercise as countermeasures

Muscle disuse is often observed after injury or during periods of illness, resulting in the loss of muscle mass and strength, with sometimes debilitating consequences. Although substantial advancements have been made in determining the mechanisms responsible for the etiology of muscle disuse atrophy in rodents, only in recent years have studies of any significant number focused on reaffirming these findings in humans. In this review, we discuss the processes responsible for disuse atrophy as based on current evidence and highlight where gaps in our knowledge persist. Furthermore, given the emphasis placed on resistance exercise and nutrition as potential therapeutic countermeasures, we consider recent advancements in the study of resistance exercise and nutrition in the stimulation of muscle protein synthesis and the associated implications when devising effective treatment strategies.

Pharmacological activation of the pyruvate dehydrogenase complex reduces statin-mediated upregulation of FOXO gene targets and protects against statin myopathy in rodents

•  Statin myopathy impairs phosphatidylinositol 3‐kinase/Akt signalling and activates forkhead box protein O (FOXO) transcription factors in vivo in rodent skeletal muscle. This is associated with upregulation of downstream gene targets known to increase proteasomal and lysosomal‐mediated protein breakdown, oxidative stress and inflammation, and inhibit muscle carbohydrate (CHO) oxidation. •  We hypothesised that forcibly increasing muscle CHO oxidation in vivo, using the pyruvate dehydrogenase complex activator, dichloroacetate (DCA), would blunt statin‐mediated increases in mRNA expression of these FOXO gene targets, thereby reducing statin myopathy. •  Chronic administration of DCA with simvastatin dampened statin‐mediated increases in muscle atrophy F‐box (MAFbx), cathepsin‐L and pyruvate dehydrogenase kinase‐4 mRNA in a dose‐dependent manner, which was corroborated by protein activity and expression measurements, and blunted statin myopathy. •  These results provide convincing evidence that pharmacologically increasing muscle CHO oxidation reduces simvastatin‐induced myopathy by dampening the upregulation of genes known to increase proteasomal and lysosomal protein breakdown and inhibit CHO oxidation.

Chapter 2. Calcineurin signaling and the slow oxidative skeletal muscle fiber type.

Calcineurin, also known as protein phosphatase 2B (PP2B), is a calcium-calmodulin-dependent phosphatase. It couples intracellular calcium to dephosphorylate selected substrates resulting in diverse biological consequences depending on cell type. In mammals, calcineurins functions include neuronal growth, development of cardiac valves and hypertrophy, activation of lymphocytes, and the regulation of ion channels and enzymes. This chapter focuses on the key roles of calcineurin in skeletal muscle differentiation, regeneration, and fiber type conversion to an oxidative state, all of which are crucial to muscle development, metabolism, and functional adaptations. It seeks to integrate the current knowledge of calcineurin signaling in skeletal muscle and its interactions with other prominent regulatory pathways and their signaling intermediates to form a molecular overview that could provide directions for possible future exploitations in human metabolic health.

Statin myalgia is not associated with reduced muscle strength, mass or protein turnover in older male volunteers, but is allied with a slowing of time to peak power output, insulin resistance and differential muscle mRNA expression

Statins cause muscle‐specific side effects, most commonly muscle aches/weakness (myalgia), particularly in older people. Furthermore, evidence has linked statin use to increased risk of type 2 diabetes. However, the mechanisms involved are unknown. This is the first study to measure muscle protein turnover rates and insulin sensitivity in statin myalgic volunteers and age‐matched, non‐statin users under controlled fasting and fed conditions using gold standard methods. We demonstrate in older people that chronic statin myalgia is not associated with deficits in muscle strength and lean mass or the dysregulation of muscle protein turnover compared to non‐statin users. Furthermore, there were no between‐group differences in blood or muscle inflammatory markers. Statin users did, however, show blunting of muscle power output at the onset of dynamic exercise, increased abdominal adiposity, whole body and leg insulin resistance, and clear differential expression of muscle genes linked to mitochondrial dysfunction and apoptosis, which warrant further investigation.

Muscle thickness correlates to muscle cross-sectional area in the assessment of strength training-induced hypertrophy

Muscle thickness (MT) measured by ultrasound has been used to estimate cross‐sectional area (measured by CT and MRI) at a single time point. We tested whether MT could be used as a valid marker of MRI determined muscle anatomical cross‐sectional area (ACSA) and volume changes following resistance training (RT). Nine healthy, young, male volunteers (24 ± 2 y.o., BMI 24.1 ± 2.8 kg/m2) had vastus lateralis (VL) muscle volume (VOL) and ACSAmid (at 50% of femur length, FL) assessed by MRI, and VL MT measured by ultrasound at 50% FL. Measurements were taken at baseline and after 12 weeks of isokinetic RT. Differences between baseline and post‐training were assessed by Students paired t test. The relationships between MRI and ultrasound measurements were tested by Pearsons correlation. After RT, MT increased by 7.5 ± 6.1% (P < .001), ACSAmid by 5.2 ± 5% (P < .001), and VOL by 5.0 ± 6.9% (P < .05) (values: means ± SD). Positive correlations were found, at baseline and 12 weeks, between MT and ACSAmid (r = .82, P < .001 and r = .73, P < .001, respectively), and between MT and VOL (r = .76, P < .001 and r = .73, P < .001, respectively). The % change in MT with training was correlated with % change in ACSAmid (r = .69, P < .01), but not % change in VOL (r = .33, P > .05). These data support evidence that MT is a reliable index of muscle ACSAmid and VOL at a single time point. MT changes following RT are associated with parallel changes in muscle ACSAmid but not with the changes in VOL, highlighting the impact of RT on regional hypertrophy.

Eccentric exercise increases circulating fibroblast activation protein α but not bioactive fibroblast growth factor 21 in healthy humans

What is the central question of this study? The role of FGF21 as an exercise‐induced myokine remains controversial. The aim of this study was to determine whether eccentric exercise would augment the release of FGF21 and/or its regulatory enzyme, fibroblast activation protein α (FAP), from skeletal muscle tissue into the systemic circulation of healthy human volunteers. What is the main finding and its importance? Eccentric exercise does not release total or bioactive FGF21 from human skeletal muscle. However, exercise releases its regulatory enzyme, FAP, from tissue(s) other than muscle, which might play a role in the inactivation of FGF21.