Novel roles of cardiac-derived erythropoietin in cardiac development and function.

The role of erythropoietin (EPO) has extended beyond hematopoiesis to include cytoprotection, inotropy, and neurogenesis. Extra-renal EPO has been reported for multiple tissue/cell types, but the physiological relevance remains unknown. Although the EPO receptor is expressed by multiple cardiac cell types and human recombinant EPO increases contractility and confers cytoprotection against injury, whether the heart produces physiologically meaningful amounts of EPO in vivo is unclear. We show a distinct circadian rhythm of cardiac EPO mRNA expression in adult mice and increased mRNA expression during embryogenesis, suggesting physiological relevance to cardiac EPO production throughout life. We then generated constitutive, cardiomyocyte-specific EPO knockout mice driven by the Mlc2v promoter (EPOfl/fl:Mlc2v-cre+/-; EPOΔ/Δ-CM). During cardiogenesis, cardiac EPO mRNA expression and cellular proliferation were reduced in EPOΔ/Δ-CM hearts. However, in adult EPOΔ/Δ- CM mice, total heart weight was preserved through increased cardiomyocyte cross-sectional area, indicating the reduced cellular proliferation was compensated for by cellular hypertrophy. Echocardiography revealed no changes in cardiac dimensions, with modest reductions in ejection fraction, stroke volume, and tachycardia, whereas invasive hemodynamics showed increased cardiac contractility and lusitropy. Paradoxically, EPO mRNA expression in the heart was elevated in adult EPOΔ/Δ-CM, along with increased serum EPO protein content and hematocrit. Using RNA fluorescent in situ hybridization, we found that Epo RNA colocalized with endothelial cells in the hearts of adult EPOΔ/Δ-CM mice, identifying the endothelial cells as a cell responsible for the EPO hyper-expression. Collectively, these data identify the first physiological roles for cardiomyocyte-derived EPO. We have established cardiac EPO mRNA expression is a complex interplay of multiple cell types, where loss of embryonic cardiomyocyte EPO production results in hyper-expression from other cells within the adult heart.

Impairments in left ventricular mitochondrial bioenergetics precede overt cardiac dysfunction and remodelling in Duchenne muscular dystrophy.

KEY POINTS: Ninety-eight per cent of patients with Duchenne muscular dystrophy (DMD) develop cardiomyopathy, with 40% developing heart failure. While increased propensity for mitochondrial induction of cell death has been observed in left ventricle, it remains unknown whether this is linked to impaired mitochondrial respiratory control and elevated H2 O2 emission prior to the onset of cardiomyopathy. Classic mouse models of DMD demonstrate hyper-regeneration in skeletal muscle which may mask mitochondrial abnormalities. Using a model with less regenerative capacity that is more akin to DMD patients, we observed elevated left ventricular mitochondrial H2 O2 and impaired oxidative phosphorylation in the absence of cardiac remodelling or overt cardiac dysfunction at 4 weeks. These impairments were associated with dysfunctions at complex I, governance by ADP and creatine-dependent phosphate shuttling, which results in a less efficient response to energy demands. Mitochondria may be a therapeutic target for the treatment of cardiomyopathy in DMD. ABSTRACT: In Duchenne muscular dystrophy (DMD), mitochondrial dysfunction is predicted as a response to numerous cellular stressors, yet the contribution of mitochondria to the onset of cardiomyopathy remains unknown. To resolve this uncertainty, we designed in vitro assessments of mitochondrial bioenergetics to model mitochondrial control parameters that influence cardiac function. Both left ventricular mitochondrial responsiveness to the central bioenergetic controller ADP and the ability of creatine to facilitate mitochondrial-cytoplasmic phosphate shuttling were assessed. These measurements were performed in D2.B10-DMDmdx /2J mice - a model that demonstrates skeletal muscle atrophy and weakness due to limited regenerative capacities and cardiomyopathy more akin to people with DMD than classic models. At 4 weeks of age, there was no evidence of cardiac remodelling or cardiac dysfunction despite impairments in ADP-stimulated respiration and ADP attenuation of H2 O2 emission. These impairments were seen at both submaximal and maximal ADP concentrations despite no reductions in mitochondrial content markers. The ability of creatine to enhance ADP's control of mitochondrial bioenergetics was also impaired, suggesting an impairment in mitochondrial creatine kinase-dependent phosphate shuttling. Susceptibly to permeability transition pore opening and the subsequent activation of cell death pathways remained unchanged. Mitochondrial H2 O2 emission was elevated despite no change in markers of irreversible oxidative damage, suggesting alternative redox signalling mechanisms should be explored. These findings demonstrate that selective mitochondrial dysfunction precedes the onset of overt cardiomyopathy in D2.mdx mice, suggesting that improving mitochondrial bioenergetics by restoring ADP, creatine-dependent phosphate shuttling and complex I should be considered for treating DMD patients.

Hemodynamic assessment of diastolic function for experimental models.

Traditionally, the evaluation of cardiac function has focused on systolic function; however, there is a growing appreciation for the contribution of diastolic function to overall cardiac health. Given the emerging interest in evaluating diastolic function in all models of heart failure, there is a need for sensitivity, accuracy, and precision in the hemodynamic assessment of diastolic function. Hemodynamics measure cardiac pressures in vivo, offering a direct assessment of diastolic function. In this review, we summarize the underlying principles of diastolic function, dividing diastole into two phases: 1) relaxation and 2) filling. We identify parameters used to comprehensively evaluate diastolic function by hemodynamics, clarify how each parameter is obtained, and consider the advantages and limitations associated with each measure. We provide a summary of the sensitivity of each diastolic parameter to loading conditions. Furthermore, we discuss differences that can occur in the accuracy of diastolic and systolic indices when generated by automated software compared with custom software analysis and the magnitude each parameter is influenced during inspiration with healthy breathing and a mild breathing load, commonly expected in heart failure. Finally, we identify key variables to control (e.g., body temperature, anesthetic, sampling rate) when collecting hemodynamic data. This review provides fundamental knowledge for users to succeed in troubleshooting and guidelines for evaluating diastolic function by hemodynamics in experimental models of heart failure.

Repeatability of exercise-induced changes in mRNA expression and technical considerations for qPCR analysis in human skeletal muscle.

NEW FINDINGS: What is the central question of this study? Are individual changes in exercise-induced mRNA expression repeatable (i.e. representative of the true response to exercise rather than random error)? What is the main finding and its importance? Exercise-induced changes in mRNA expression are not repeatable even under identical experimental conditions, thereby challenging the use of mRNA expression as a biomarker of adaptive potential and/or individual responsiveness to exercise. ABSTRACT: It remains unknown if (1) the observed change in mRNA expression reflects an individual's true response to exercise or random (technical and/or biological) error, and (2) the individual responsiveness to exercise is protocol-specific. We examined the repeatability of skeletal muscle PGC-1α, PDK4, NRF-1, VEGF-A, HSP72 and p53 mRNA expression following two identical endurance exercise (END) bouts (END-1, END-2; 30 min of cycling at 65% of peak work rate (WRpeak ), n = 11) and inter-individual variability in PGC-1α and PDK4 mRNA expression following END and sprint interval training (SIT; 8 × 20 s cycling intervals at ∼170% WRpeak , n = 10) in active young males. The repeatability of key gene analysis steps (RNA extraction, reverse transcription, qPCR) and within-sample fibre-type distribution (n = 8) was also determined to examine potential sources of technical error in our analyses. Despite highly repeatable exercise bout characteristics (work rate, heart rate, blood lactate; ICC > 0.71; CV < 10%; r > 0.85, P < 0.01), gene analysis steps (ICC > 0.73; CV < 24%; r > 0.75, P < 0.01), and similar group-level changes in mRNA expression, individual changes in PGC-1α, PDK4, VEGF-A and p53 mRNA expression were not repeatable (ICC < 0.22; CV > 20%; r < 0.21). Fibre-type distribution in two portions of the same muscle biopsy was highly variable and not significantly related (ICC = 0.39; CV = 26%; r = 0.37, P = 0.37). Since individual changes in mRNA expression following identical exercise bouts were not repeatable, inferences regarding individual responsiveness to END or SIT were not made. Substantial random error exists in changes in mRNA expression following acute exercise, thereby challenging the use of mRNA expression for analysing individual responsiveness to exercise.

Moderate and severe hypoxia elicit divergent effects on cardiovascular function and physiological rhythms.

KEY POINTS: In the present study, we provide evidence for divergent physiological responses to moderate compared to severe hypoxia, addressing an important knowledge gap related to severity, duration and after-effects of hypoxia encountered in cardiopulmonary situations. The physiological responses to moderate and severe hypoxia were not proportional, linear or concurrent with the time-of-day. Hypoxia elicited severity-dependent physiological responses that either persisted or fluctuated throughout normoxic recovery. The physiological basis for these distinct cardiovascular responses implicates a shift in the sympathovagal set point and probably not molecular changes at the artery resulting from hypoxic stress. ABSTRACT: Hypoxia is both a consequence and cause of many acute and chronic diseases. Severe hypoxia causes hypertension with cardiovascular sequelae; however, the rare studies using moderate severities of hypoxia indicate that it can be beneficial, suggesting that hypoxia may not always be detrimental. Comparisons between studies are difficult because of the varied classifications of hypoxic severities, methods of delivery and use of anaesthetics. Thus, to investigate the long-term effects of moderate hypoxia on cardiovascular health, radiotelemetry was used to obtain in vivo physiological measurements in unanaesthetized mice during 24 h of either moderate (FIO2=0.15) or severe (FIO2=0.09) hypoxia, followed by 72 h of normoxic recovery. Systolic blood pressure was decreased during recovery following moderate hypoxia but increased following severe hypoxia. Moderate and severe hypoxia increased haeme oxygenase-1 expression during recovery, suggesting parity in hypoxic stress at the level of the artery. Severe but not moderate hypoxia increased the low/high frequency ratio of heart rate variability 72 h post-hypoxia, indicating a shift in sympathovagal balance. Moderate hypoxia dampened the amplitude of circadian rhythm, whereas severe disrupted rhythm during the entire insult, with perturbations persisting throughout normoxic recovery. Thus, hypoxic severity differentially regulates circadian blood pressure.

SIRT3 gene expression but not SIRT3 subcellular localization is altered in response to fasting and exercise in human skeletal muscle.

NEW FINDINGS: What is the central question of this study? Evidence from cellular and animal models suggests that SIRT3 is involved in regulating aerobic ATP production. Thus, we investigated whether changes in fatty acid and oxidative metabolism known to accompany fasting and exercise occur in association with changes in SIRT3 mitochondrial localization and expression in human skeletal muscle. What is the main finding and its importance? We find that 48 h of fasting and acute endurance exercise decrease SIRT3 mRNA expression but do not alter SIRT3 mitochondrial localization despite marked increases in fatty acid oxidation. This suggests that SIRT3 activity is not regulated by changes in mitochondrial localization in response to cellular energy stress in human skeletal muscle. The present study examined SIRT3 expression and SIRT3 mitochondrial localization in response to acute exercise and short-term fasting in human skeletal muscle. Experiment 1 involved eight healthy men (age, 21.4 ± 2.8 years; peak O2 uptake, 47.1 ± 11.8 ml min(-1)  kg(-1) ) who performed a single bout of exercise at ∼55% of peak aerobic work rate for 1 h. Muscle biopsies were obtained at rest (Rest), immediately after exercise (EX-0) and 3 h postexercise (EX-3). Experiment 2 involved 10 healthy men (age, 22.0 ± 1.5 years; peak O2 uptake, 46.9 ± 6.0 ml min−1 kg−1) who underwent a 48 h fast, with muscle biopsies collected 1 h postprandial (Fed) and after 48 h of fasting (Fast). Mitochondrial respiration was measured using high-resolution respirometry in permeabilized muscle fibre bundles to assess substrate oxidation. Whole body fat oxidation increased after both exercise (Rest, 0.96 ± 0.32 kcal min(-1) ; Exercise, 5.66 ± 1.97 kcal min(-1) ; P < 0.001) and fasting (Fed, 0.87 ± 0.51 kcal min(-1) ; Fast, 1.30 ± 0.37 kcal min(-1) , P < 0.05). SIRT3 gene expression decreased (P < 0.05) after both exercise (-8%) and fasting (-19%); however, SIRT3 whole muscle protein content was unaltered after fasting. No changes were observed in SIRT3 mitochondrial localization following either exercise or fasting. Fasting also decreased the Vmax of glutamate [80 ± 43 versus 50 ± 21 pmol s(-1)  (mg dry weight)(-1) ; P < 0.05]. These findings suggest that SIRT3 does not appear to be regulated by changes in mitochondrial localization at the time points measured in the present study in response to cellular energy stress in human skeletal muscle.

Regulation of plasma agouti-related protein and its relationship with hunger in lean and obese men.

Agouti-related protein (AgRP) is an orexigenic (appetite stimulating) neuropeptide suggested to exert tonic control over long-term energy balance. While some have speculated AgRP is not involved in the episodic (i.e. meal to meal energy intake) control, acute decreases in plasma agouti-related protein (AgRP) following a meal have been observed in humans in a role consistent with episodic control for AgRP. Whether changes in plasma AgRP are associated with episodic, and/or tonic changes in appetite has yet to be directly examined. The present study examined the relationship between agouti-related protein (AgRP), leptin and the regulation of appetite following a 48-h fast and an acute meal challenge. Blood samples were obtained from young lean and obese men before and after a 48 h fast (lean n = 10; obese n = 7). Fasting resulted in an increase in AgRP and a decrease in leptin with these changes being greater in lean than obese. In addition, blood samples were obtained from lean men before and 1, 2, 3 and 4 h after a meal (n = 8). Following a meal, AgRP was reduced from 2 to 4 h, a change that was dissociated from both leptin and subjective measures of hunger and satiety. These results demonstrate that AgRP is not associated with changes in hunger or satiety, and can change without corresponding changes in leptin. This suggests that AgRP may not be involved in the episodic control of appetite and the release of AgRP may involve signals other than leptin.

Intron retention is a mechanism of erythropoietin regulation in brain cell models.

Intron retention is a mechanism of post-transcriptional gene regulation, including genes involved in erythropoiesis. Erythropoietin (EPO) is a hormone without evidence of intracellular vesicle storage that regulates erythropoiesis. We hypothesize that EPO uses intron retention as a mechanism of post-transcriptional regulation in response to hypoxia and ischemia. Cell models of hypoxia and ischemia for kidney, liver, and brain cells were examined for intron retention by real time quantitative PCR. EPO expression increased in most cells except for blood brain barrier and liver cells. The intron retained transcript ratio decreased in brain cells, except for Astrocytes, but showed no change in kidney or liver after 24 h of ischemia. The shift in intron ratio was maintained when using poly (A) enriched cDNA, suggesting that intron retention is not due to immature transcripts. The expression of EPO was elevated at variable time points amongst cell models with the intron ratio also changing over a time course of 2 to 16 h after ischemia. We conclude that intron retention is a mechanism regulating EPO expression in response to ischemia in a tissue specific manner.

The influence of sex, hemoglobin mass, and skeletal muscle characteristics on cycling critical power.

Critical power (CP) represents an important threshold for exercise performance and fatiguability. We sought to determine the extent to which sex, hemoglobin mass (Hbmass), and skeletal muscle characteristics influence CP. Before CP determination (i.e., 3-5 constant work rate trials to task failure), Hbmass and skeletal muscle oxidative capacity (τ) were measured and vastus lateralis (VL) muscle biopsy samples were collected from 12 females and 12 males matched for aerobic fitness relative to fat-free mass (FFM) [means (SD); V̇o2max: 59.2 (7.7) vs. 59.5 (7.1) mL·kg·FFM-1·min-1, respectively]. Males had a significantly greater CP than females in absolute units [225 (28) vs. 170 (43) W; P = 0.001] but not relative to body mass [3.0 (0.6) vs. 2.7 (0.6) W·kg·BM-1; P = 0.267] or FFM [3.6 (0.7) vs. 3.7 (0.8) W·kg·FFM-1; P = 0.622]. Males had significantly greater W' (P ≤ 0.030) and greater Hbmass (P ≤ 0.016) than females, regardless of the normalization approach; however, there were no differences in mitochondrial protein content (P = 0.375), τ (P = 0.603), or MHC I proportionality (P = 0.574) between males and females. Whether it was expressed in absolute or relative units, CP was positively correlated with Hbmass (0.444 ≤ r ≤ 0.695; P < 0.05), mitochondrial protein content (0.413 ≤ r ≤ 0.708; P < 0.05), and MHC I proportionality (0.506 ≤ r ≤ 0.585; P < 0.05), and negatively correlated with τ when expressed in relative units only (-0.588 ≤ r ≤ -0.527; P < 0.05). Overall, CP was independent of sex, but variability in CP was related to Hbmass and skeletal muscle characteristics. The extent to which manipulations in these physiological parameters influence CP warrants further investigation to better understand the factors underpinning CP.NEW & NOTEWORTHY In males and females matched for aerobic fitness [maximal oxygen uptake normalized to fat-free mass (FFM)], absolute critical power (CP) was greater in males, but relative CP (per kilogram body mass or FFM) was similar between sexes. CP correlated with hemoglobin mass, mitochondrial protein content, myosin heavy chain type I proportion, and skeletal muscle oxidative capacity. These findings demonstrate the importance of matching sexes for aerobic fitness, but further experiments are needed to determine causality.

Cardiomyocyte crosstalk with endothelium modulates cardiac structure, function, and ischemia-reperfusion injury susceptibility through erythropoietin.

Erythropoietin (EPO) exerts non-canonical roles beyond erythropoiesis that are developmentally, structurally, and physiologically relevant for the heart as a paracrine factor. The role for paracrine EPO signalling and cellular crosstalk in the adult is uncertain. Here, we provided novel evidence showing cardiomyocyte restricted loss of function in Epo in adult mice induced hyper-compensatory increases in Epo expression by adjacent cardiac endothelial cells via HIF-2α independent mechanisms. These hearts showed concentric cellular hypertrophy, elevated contractility and relaxation, and greater resistance to ischemia-reperfusion injury. Voluntary exercise capacity compared to control hearts was improved independent of any changes to whole-body metabolism or blood O2 content or delivery (i.e., hematocrit). Our findings suggest cardiac EPO had a localized effect within the normoxic heart, which was regulated by cell-specific EPO-reciprocity between cardiomyocytes and endothelium. Within the heart, hyper-compensated endothelial Epo expression was accompanied by elevated Vegfr1 and Vegfb RNA, that upon pharmacological pan-inhibition of VEGF-VEGFR signaling, resulted in a paradoxical upregulation in whole-heart Epo. Thus, we provide the first evidence that a novel EPO-EPOR/VEGF-VEGFR axis exists to carefully mediate cardiac homeostasis via cardiomyocyte-endothelial EPO crosstalk.

The effects of recreational sport on VO2peak, VO2 kinetics and submaximal exercise performance in males and females.

The purpose of the present study was to examine changes in VO(2peak), VO(2) kinetics and steady-state exercise performance following 4 weeks of participation in recreational sport. Subjects (male n = 8, female n = 9) participated in recreational sport (basketball, floor hockey and soccer) four times per week for 4 weeks. Both before and after training, VO(2peak) was measured on a cycle ergometer, VO(2) kinetics was determined as the average of three transitions to 80 W, and heart rate (HR) and respiratory exchange ratio (RER) were measured during 60 min at a work rate corresponding to 50 % of pre-training VO(2peak). HR was also monitored during all training sessions. After training, VO(2peak) was increased in females, but not males, while VO(2) kinetics (τVO(2)) were sped in both males and females. HR during constant load exercise was reduced in both males and females, but exercise RER was only reduced in females. Mean HR during participation in sport was higher in males than females and higher during basketball than both floor hockey and soccer. These results demonstrate that training adaptations traditionally associated with endurance exercise can also be obtained through regular participation in recreational sport.

CARM1 drives mitophagy and autophagy flux during fasting-induced skeletal muscle atrophy.

CARM1 (coactivator associated arginine methyltransferase 1) has recently emerged as a powerful regulator of skeletal muscle biology. However, the molecular mechanisms by which the methyltransferase remodels muscle remain to be fully understood. In this study, carm1 skeletal muscle-specific knockout (mKO) mice exhibited lower muscle mass with dysregulated macroautophagic/autophagic and atrophic signaling, including depressed AMP-activated protein kinase (AMPK) site-specific phosphorylation of ULK1 (unc-51 like autophagy activating kinase 1; Ser555) and FOXO3 (forkhead box O3; Ser588), as well as MTOR (mechanistic target of rapamycin kinase)-induced inhibition of ULK1 (Ser757), along with AKT/protein kinase B site-specific suppression of FOXO1 (Ser256) and FOXO3 (Ser253). In addition to lower mitophagy and autophagy flux in skeletal muscle, carm1 mKO led to increased mitochondrial PRKN/parkin accumulation, which suggests that CARM1 is required for basal mitochondrial turnover and autophagic clearance. carm1 deletion also elicited PPARGC1A (PPARG coactivator 1 alpha) activity and a slower, more oxidative muscle phenotype. As such, these carm1 mKO-evoked adaptations disrupted mitophagy and autophagy induction during food deprivation and collectively served to mitigate fasting-induced muscle atrophy. Furthermore, at the threshold of muscle atrophy during food deprivation experiments in humans, skeletal muscle CARM1 activity decreased similarly to our observations in mice, and was accompanied by site-specific activation of ULK1 (Ser757), highlighting the translational impact of the methyltransferase in human skeletal muscle. Taken together, our results indicate that CARM1 governs mitophagic, autophagic, and atrophic processes fundamental to the maintenance and remodeling of muscle mass. Targeting the enzyme may provide new therapeutic approaches for mitigating skeletal muscle atrophy.

An Evaluation of Cardiac Health in the Spontaneously Hypertensive Rat Colony: Implications of Evolutionary Driven Increases in Concentric Hypertrophy.

BACKGROUND: The Spontaneously Hypertensive Rat (SHR) Colony was established in 1963 and is the most commonly used rodent model for studying heart failure (HF). Ideally, animal models should recapitulate the clinical disease as closely as possible. Any drift in a genetic model may create a new model that no longer adequately represents the human pathology. Further, instability overtime may lead to conflicting data between laboratories and/or irreproducible results. While systolic blood pressure (SBP) is closely monitored during inbreeding, the sequelae of HF (e.g., cardiac hypertrophy) are not. Thus, the object of this review was to investigate whether the hypertension-induced sequelae of HF in the SHR have remained stable after decades of inbreeding. METHODS: A systematic review was performed to evaluate indices of cardiovascular health in the SHR over the past 60 years. For post hoc statistical analyses, studies were separated into 2 cohorts: Initial (mid to late 1900s) and Current (early 2000s to present) Colony SHRs. Wistar-Kyoto rats (WKY) were used as controls. RESULTS: SBP was consistent between Initial and Current Colony SHRs. However, Current Colony SHRs presented with increased concentric hypertrophy (i.e., elevated heart weight and posterior wall thickness) while cardiac output remained consistent. Since these changes were not observed in the WKY controls, cardiac-derived changes in Current Colony SHRs were unlikely due to differences in environmental conditions. CONCLUSIONS: Together, these data firmly establish a cardiac-based phenotypic shift in the SHR model and provide important insights into the beneficial function of concentric hypertrophy in hypertension-induced HF.

A new approach to improve the hemodynamic assessment of cardiac function independent of respiratory influence.

Cardiovascular and respiratory systems are anatomically and functionally linked; inspiration produces negative intrathoracic pressures that act on the heart and alter cardiac function. Inspiratory pressures increase with heart failure and can exceed the magnitude of ventricular pressure during diastole. Accordingly, respiratory pressures may be a confounding factor to assessing cardiac function. While the interaction between respiration and the heart is well characterized, the extent to which systolic and diastolic indices are affected by inspiration is unknown. Our objective was to understand how inspiratory pressure affects the hemodynamic assessment of cardiac function. To do this, we developed custom software to assess and separate indices of systolic and diastolic function into inspiratory, early expiratory, and late expiratory phases of respiration. We then compared cardiac parameters during normal breathing and with various respiratory loads. Variations in inspiratory pressure had a small impact on systolic pressure and function. Conversely, diastolic pressure strongly correlated with negative inspiratory pressure. Cardiac pressures were less affected by respiration during expiration; late expiration was the most stable respiratory phase. In conclusion, inspiration is a large confounding influence on diastolic pressure, but minimally affects systolic pressure. Performing cardiac hemodynamic analysis by accounting for respiratory phase yields more accuracy and analytic confidence to the assessment of diastolic function.

Early myopathy in Duchenne muscular dystrophy is associated with elevated mitochondrial H2 O2 emission during impaired oxidative phosphorylation.

BACKGROUND: Muscle wasting and weakness in Duchenne muscular dystrophy (DMD) causes severe locomotor limitations and early death due in part to respiratory muscle failure. Given that current clinical practice focuses on treating secondary complications in this genetic disease, there is a clear need to identify additional contributions in the aetiology of this myopathy for knowledge-guided therapy development. Here, we address the unresolved question of whether the complex impairments observed in DMD are linked to elevated mitochondrial H2 O2 emission in conjunction with impaired oxidative phosphorylation. This study performed a systematic evaluation of the nature and degree of mitochondrial-derived H2 O2 emission and mitochondrial oxidative dysfunction in a mouse model of DMD by designing in vitro bioenergetic assessments that attempt to mimic in vivo conditions known to be critical for the regulation of mitochondrial bioenergetics. METHODS: Mitochondrial bioenergetics were compared with functional and histopathological indices of myopathy early in DMD (4 weeks) in D2.B10-DMDmdx /2J mice (D2.mdx)-a model that demonstrates severe muscle weakness. Adenosine diphosphate's (ADP's) central effect of attenuating H2 O2 emission while stimulating respiration was compared under two models of mitochondrial-cytoplasmic phosphate exchange (creatine independent and dependent) in muscles that stained positive for membrane damage (diaphragm, quadriceps, and white gastrocnemius). RESULTS: Pathway-specific analyses revealed that Complex I-supported maximal H2 O2 emission was elevated concurrent with a reduced ability of ADP to attenuate emission during respiration in all three muscles (mH2 O2 : +17 to +197% in D2.mdx vs. wild type). This was associated with an impaired ability of ADP to stimulate respiration at sub-maximal and maximal kinetics (-17 to -72% in D2.mdx vs. wild type), as well as a loss of creatine-dependent mitochondrial phosphate shuttling in diaphragm and quadriceps. These changes largely occurred independent of mitochondrial density or abundance of respiratory chain complexes, except for quadriceps. This muscle was also the only one exhibiting decreased calcium retention capacity, which indicates increased sensitivity to calcium-induced permeability transition pore opening. Increased H2 O2 emission was accompanied by a compensatory increase in total glutathione, while oxidative stress markers were unchanged. Mitochondrial bioenergetic dysfunctions were associated with induction of mitochondrial-linked caspase 9, necrosis, and markers of atrophy in some muscles as well as reduced hindlimb torque and reduced respiratory muscle function. CONCLUSIONS: These results provide evidence that Complex I dysfunction and loss of central respiratory control by ADP and creatine cause elevated oxidant generation during impaired oxidative phosphorylation. These dysfunctions may contribute to early stage disease pathophysiology and support the growing notion that mitochondria are a potential therapeutic target in this disease.

Coordination of mitochondrial biogenesis by PGC-1α in human skeletal muscle: A re-evaluation.

The transcriptional co-activator peroxisome proliferator-activated receptor gamma co-activator-1 alpha (PGC-1α) is proposed to coordinate skeletal muscle mitochondrial biogenesis through the integrated induction of nuclear- and mitochondrial-encoded gene transcription. This paradigm is based largely on experiments demonstrating PGC-1α's ability to co-activate various nuclear transcription factors that increase the expression of mitochondrial genes, as well as PGC-1α's direct interaction with mitochondrial transcription factor A within mitochondria to increase the transcription of mitochondrial DNA. While this paradigm is supported by evidence from cellular and transgenic animal models, as well as acute exercise studies involving animals, the up-regulation of nuclear- and mitochondrial-encoded genes in response to exercise does not appear to occur in a coordinated fashion in human skeletal muscle. This review re-evaluates our current understanding of this phenomenon by highlighting evidence from recent studies examining the exercise-induced expression of nuclear- and mitochondrial-encoded genes targeted by PGC-1α. We also highlight several possible theories that may explain the apparent inability of PGC-1α to coordinately up-regulate the expression of genes required for mitochondrial biogenesis in human skeletal muscle, and provide directions for future work exploring mitochondrial biogenic gene expression following exercise.

The efficacy of a whole body sprint-interval training intervention in an office setting: A feasibility study.

BACKGROUND: Whole body sprint-interval training (WB-SIT) represents a mode of exercise training that is both time-efficient and does not require access to an exercise facility. OBJECTIVE: The current study examined the feasibility of implementing a WB-SIT intervention in a workplace setting. METHODS: A total of 747 employees from a large office building were invited to participate with 31 individuals being enrolled in the study. Anthropometrics, aerobic fitness, core and upper body strength, and lower body mobility were assessed before and after a 12-week exercise intervention consisting of 2-4 training sessions per week. Each training session required participants to complete 8, 20-second intervals (separated by 10 seconds of rest) of whole body exercise. RESULTS: Proportion of participation was 4.2% while the response rate was 35% (11/31 participants completed post training testing). In responders, compliance to prescribed training was 83±17%, and significant (p < 0.05) improvements were observed for aerobic fitness, push-up performance and lower body mobility. CONCLUSION: These results demonstrate the efficacy of WB-SIT for improving fitness and mobility in an office setting, but highlight the difficulties in achieving high rates of participation and response in this setting.

Reproducibility of peak oxygen consumption and the impact of test variability on classification of individual training responses in young recreationally active adults.

This study investigated whether VO2 peak is reproducible across repeated tests before (PRE) and after (POST) training, and whether variability across tests impacts how individual responses are classified following 3 weeks of aerobic exercise training (cycle ergometry). Data from 45 young healthy adults (age: 20·1 ± 0·9 years; VO2 peak, 42·0 ± 6·7 ml·min-1 ) from two previously published studies were utilized in the current analysis. Non-responders were classified as individuals who failed to demonstrate an increase or decrease in VO2 peak that was greater than 2·0 times the typical error of measurement (107 ml·min-1 ) away from zero, while responders and adverse responders were above and below this cut-off, respectively. VO2 peak tests at PRE (three total) and POST (three total) were highly reproducible (PRE and POST average and single measures ICCs: range 0·938-0·992), with low coefficients of variation (PRE:4·9 ± 3·1%, POST: 4·8 ± 2·7%). However, a potential learning effect was observed in the VO2 peak tests prior to training, as the initial pretraining test was significantly lower than the third (p = 0·010, PRE 1: 2 946 ± 924 ml·min-1 , PRE 3: 3 042 ± 919 ml·min-1 ). This resulted in fewer individuals classified as adverse responders for Test 3 compared to any combination of tests that included Test 1, suggesting that a single ramp test at baseline may not be sufficient to accurately classify the VO2 peak response in young recreationally active individuals. Thus, it is our recommendation that the initial VO2 peak test be used as a familiarization visit and not included for analysis.

A systematic upregulation of nuclear and mitochondrial genes is not present in the initial postexercise recovery period in human skeletal muscle.

The purpose of the current investigation was to determine if an exercise-mediated upregulation of nuclear and mitochondrial-encoded genes targeted by the transcriptional co-activator peroxisome-proliferator-activated receptor gamma co-activator-1 alpha (PGC-1α) occurs in a systematic manner following different exercise intensities in humans. Ten recreationally active males (age: 23 ± 3 years; peak oxygen uptake: 41.8 ± 6.6 mL·kg-1·min-1) completed 2 acute bouts of work-matched interval exercise at ∼73% (low; LO) and ∼100% (high; HI) of work rate at peak oxygen uptake in a randomized crossover design. Muscle biopsies were taken before, immediately after, and 3 h into recovery following each exercise bout. A main effect of time (p < 0.05) was observed for glycogen depletion. PGC-1α messenger RNA (mRNA) increased following both conditions and was significantly (p < 0.05) higher following HI compared with LO (PGC-1α, LO: +442% vs. HI: +845%). PDK4 mRNA increased following LO whereas PPARα, NRF1, and CS increased following HI. However, a systematic upregulation of nuclear and mitochondrial-encoded genes was not present as TFAM, COXIV, COXI, COXII, ND1, and ND4 mRNA were unchanged. However, changes in COXI, COXII, ND1 and ND4 mRNA were positively correlated following LO and COXI, ND1, and ND4 were positively correlated following HI, which suggests mitochondrial-encoded gene expression was coordinated. PGC-1α and ND4 mRNA, as well as PGC-1α mRNA and the change in muscle glycogen, were positively correlated in response to LO. The lack of observed systematic upregulation of nuclear- and mitochondrial-encoded genes suggests that exercise-induced upregulation of PGC-1α targets are differentially regulated during the initial hours following acute exercise in humans.

Acute upregulation of PGC-1α mRNA correlates with training-induced increases in SDH activity in human skeletal muscle.

The purpose of the present study was to determine if acute responses in PGC-1α, VEGFA, SDHA, and GPD1-2 mRNA expression predict their associated chronic skeletal muscle molecular (SDH-GPD activity and substrate storage) and morphological (fibre-type composition and capillary density) adaptations following training. Skeletal muscle biopsies were collected from 14 recreationally active men (age: 22.0 ± 2.4 years) before (PRE) and 3 h after (3HR) the completion of an acute bout of sprint interval training (SIT) (eight 20-s intervals at ∼170% peak oxygen uptake work rate separated by 10 s of recovery). Participants then completed 6 weeks of SIT 4 times per week with additional biopsies after 2 (MID) and 6 (POST) weeks of training. Acute increases in PGC-1α mRNA strongly predicted increases in SDH activity (a marker of oxidative capacity) from PRE and MID to POST (PRE-POST: r = 0.81, r2 = 0.65, p < 0.01; MID-POST: r = 0.79, r2 = 0.62, p < 0.01) and glycogen content from MID to POST (r = 0.60, r2 = 0.36, p < 0.05). No other significant relationships were found between acute responses in PGC-1α, VEGFA, SDHA, and GPD1-2 mRNA expression and chronic adaptations to training. These results suggest that acute upregulation of PGC-1α mRNA relates to the magnitude of subsequent training-induced increases in oxidative capacity, but not other molecular and morphological chronic skeletal muscle adaptations. Additionally, acute mRNA responses in PGC-1α correlated with VEGFA, but not SDHA, suggesting a coordinated upregulation between PGC-1α and only some of its proposed targets in human skeletal muscle.