Lotte Leick

Evidence for a release of brain‐derived neurotrophic factor from the brain during exercise

Brain‐derived neurotrophic factor (BDNF) has an important role in regulating maintenance, growth and survival of neurons. However, the main source of circulating BDNF in response to exercise is unknown. To identify whether the brain is a source of BDNF during exercise, eight volunteers rowed for 4 h while simultaneous blood samples were obtained from the radial artery and the internal jugular vein. To further identify putative cerebral region(s) responsible for BDNF release, mouse brains were dissected and analysed for BDNF mRNA expression following treadmill exercise. In humans, a BDNF release from the brain was observed at rest (P < 0.05), and increased two‐ to threefold during exercise (P < 0.05). Both at rest and during exercise, the brain contributed 70–80% of circulating BDNF, while that contribution decreased following 1 h of recovery. In mice, exercise induced a three‐ to fivefold increase in BDNF mRNA expression in the hippocampus and cortex, peaking 2 h after the termination of exercise. These results suggest that the brain is a major but not the sole contributor to circulating BDNF. Moreover, the importance of the cortex and hippocampus as a source for plasma BDNF becomes even more prominent in response to exercise.

The role of PGC-1α on mitochondrial function and apoptotic susceptibility in muscle

Mitochondria are critical for cellular bioenergetics, and they mediate apoptosis within cells. We used whole body peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) knockout (KO) animals to investigate its role on organelle function, apoptotic signaling, and cytochrome-c oxidase activity, an indicator of mitochondrial content, in muscle and other tissues (brain, liver, and pancreas). Lack of PGC-1alpha reduced mitochondrial content in all muscles (17-44%; P < 0.05) but had no effect in brain, liver, and pancreas. However, the tissue expression of proteins involved in mitochondrial DNA maintenance [transcription factor A (Tfam)], import (Tim23), and remodeling [mitofusin 2 (Mfn2) and dynamin-related protein 1 (Drp1)] did not parallel the decrease in mitochondrial content in PGC-1alpha KO animals. These proteins remained unchanged or were upregulated (P < 0.05) in the highly oxidative heart, indicating a change in mitochondrial composition. A change in muscle organelle composition was also evident from the alterations in subsarcolemmal and intermyofibrillar mitochondrial respiration, which was impaired in the absence of PGC-1alpha. However, endurance-trained KO animals did not exhibit reduced mitochondrial respiration. Mitochondrial reactive oxygen species (ROS) production was not affected by the lack of PGC-1alpha, but subsarcolemmal mitochondria from PGC-1alpha KO animals released a greater amount of cytochrome c than in WT animals following exogenous ROS treatment. Our results indicate that the lack of PGC-1alpha results in 1) a muscle type-specific suppression of mitochondrial content that depends on basal oxidative capacity, 2) an alteration in mitochondrial composition, 3) impaired mitochondrial respiratory function that can be improved by training, and 4) a greater basal protein release from subsarcolemmal mitochondria, indicating an enhanced mitochondrial apoptotic susceptibility.

PGC-1α mediates exercise-induced skeletal muscle VEGF expression in mice

The aim of the present study was to test the hypothesis that PGC-1alpha is required for exercise-induced VEGF expression in both young and old mice and that AMPK activation leads to increased VEGF expression through a PGC-1alpha-dependent mechanism. Whole body PGC-1alpha knockout (KO) and littermate wild-type (WT) mice were submitted to either 1) 5 wk of exercise training, 2) lifelong (from 2 to 13 mo of age) exercise training in activity wheel, 3) a single exercise bout, or 4) 4 wk of daily subcutaneous AICAR or saline injections. In skeletal muscle of PGC-1alpha KO mice, VEGF protein expression was approximately 60-80% lower and the capillary-to-fiber ratio approximately 20% lower than in WT. Basal VEGF mRNA expression was similar in WT and PGC-1alpha KO mice, but acute exercise and AICAR treatment increased the VEGF mRNA content in WT mice only. Exercise training of young mice increased skeletal muscle VEGF protein expression approximately 50% in WT mice but with no effect in PGC-1alpha KO mice. Furthermore, a training-induced prevention of an age-associated decline in VEGF protein content was observed in WT but not in PGC-1alpha KO muscles. In addition, repeated AICAR treatments increased skeletal muscle VEGF protein expression approximately 15% in WT but not in PGC-1alpha KO mice. This study shows that PGC-1alpha is essential for exercise-induced upregulation of skeletal muscle VEGF expression and for a training-induced prevention of an age-associated decline in VEGF protein content. Furthermore, the findings suggest an AMPK-mediated regulation of VEGF expression through PGC-1alpha.

PGC-1α is required for training-induced prevention of age-associated decline in mitochondrial enzymes in mouse skeletal muscle

The aim of the present study was to test the hypothesis that exercise training prevents an age-associated decline in skeletal muscle mitochondrial enzymes through a PGC-1alpha dependent mechanism. Whole body PGC-1alpha knock-out (KO) and littermate wildtype (WT) mice were submitted to long term running wheel exercise training or a sedentary lifestyle from 2 to 13 month of age. Furthermore, a group of approximately 4-month-old mice was used as young untrained controls. There was in both genotypes an age-associated approximately 30% decrease in citrate synthase (CS) activity and superoxide dismutase (SOD)2 protein content in 13-month-old untrained mice compared with young untrained mice. However, training prevented the age-associated decrease in CS activity and SOD2 protein content only in WT mice, but long term exercise training did increase HKII protein content in both genotypes. In addition, while CS activity and protein expression of cytc and SOD2 were 50-150% lower in skeletal muscle of PGC-1alpha mice than WT mice, the expression of the pro-apoptotic protein Bax and the anti-apoptotic Bcl2 was approximately 30% elevated in PGC-1alpha KO mice. In conclusion, the present findings indicate that PGC-1alpha is required for training-induced prevention of an age-associated decline in CS activity and SOD2 protein expression in skeletal muscle.

PGC-1α is required for AICAR-induced expression of GLUT4 and mitochondrial proteins in mouse skeletal muscle

We tested the hypothesis that repeated activation of AMP-activated protein kinase (AMPK) induces mitochondrial and glucose membrane transporter mRNA/protein expression via a peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha)-dependent mechanism. Whole body PGC-1alpha-knockout (KO) and littermate wild-type (WT) mice were given either single or repeated subcutaneous injections of the AMPK activator AICAR or saline. Skeletal muscles were removed either 1 or 4 h after the single AICAR treatment or 24 h after the last injection following repeated AICAR treatment. Repeated AICAR treatment increased GLUT4, cytochrome (cyt) c oxidase I, and (cyt) c protein expression approximately 10-40% relative to saline in white muscles of WT but not of PGC-1alpha-KO mice, whereas fatty acid translocase/CD36 (FAT/CD36) protein expression was unaffected by AICAR treatment in both genotypes. GLUT4, cyt c, and FAT/CD36 mRNA content increased 30-60% 4 h after a single AICAR injection relative to saline in WT, and FAT/CD36 mRNA content decreased in PGC-1alpha-KO mice. One hour after a single AICAR treatment, phosphorylation of AMPK and the downstream target acetyl-coenzyme A carboxylase increased in all muscles investigated independent of genotype, indicating normal AICAR-induced AMPK signaling in the absence of PGC-1alpha. The hexokinase II (HKII) mRNA and protein response was similar in muscles of WT and PGC-1alpha-KO mice after single and repeated AICAR treatments, respectively, confirming that HKII is regulated independently of PGC-1alpha in response to AICAR. In conclusion, here we provide genetic evidence for a role of PGC-1alpha in AMPK-mediated regulation of mitochondrial and glucose membrane transport protein expression in skeletal muscle.

Relative Workload Determines Exercise-Induced Increases in PGC-1α mRNA

INTRODUCTION The hypothesis that brief intermittent exercise-induced increases in human skeletal muscle metabolic mRNA is dependent on relative workload was investigated. METHODS Trained (n = 10) and untrained (n = 8) subjects performed exhaustive intermittent cycling exercise (4 x 4 min at 85% of VO(2peak), interspersed by 3 min). Trained subjects also performed the intermittent exercise at the same absolute workload as the untrained subjects, corresponding to 70% of VO(2peak) (n = 6). RESULTS Exercise at 85% of V(O2peak) elevated (P < 0.001) venous plasma lactate to 10.1 +/- 0.4 and 10.8 +/- 0.5 mM in the trained and untrained subjects, respectively. Peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) mRNA expression was increased (P < 0.001) approximately four- to fivefold for several hours after exercise in both groups. After exercise at 70% of VO(2peak), venous plasma lactate was less (P < 0.001) elevated (3.1 +/- 0.7 mM) and PGC-1alpha mRNA content was less (P < 0.05) increased (approximately threefold) than after exercise at 85% of VO(2peak). Likewise, pyruvate dehydrogenase kinase 4 and hexokinase II mRNA expressions were increased (P < 0.05) only after exercise performed at 85% of VO(2peak) in the trained subjects. Hypoxia-inducible factor 2alpha mRNA only increased (P < 0.05) 3 h into recovery in trained subjects, with no difference between the 70% and 85% of VO(2peak) trial. No change in hypoxia-inducible factor 1alpha, phosphofructokinase, citrate synthase, or lactate dehydrogenase, heart and muscle isoforms, mRNA expressions was detected after any of the exercise trials. CONCLUSIONS The relative intensity of brief intermittent exercise is of major importance for the exercise-induced increase of several mRNA, including PGC-1alpha.

Adipose Tissue Interleukin-18 mRNA and Plasma Interleukin-18: Effect of Obesity and Exercise

Objectives: Obesity and a physically inactive lifestyle are associated with increased risk of developing insulin resistance. The hypothesis that obesity is associated with increased adipose tissue (AT) interleukin (IL)‐18 mRNA expression and that AT IL‐18 mRNA expression is related to insulin resistance was tested. Furthermore, we speculated that acute exercise and exercise training would regulate AT IL‐18 mRNA expression.

AMP‐activated protein kinase regulates nicotinamide phosphoribosyl transferase expression in skeletal muscle

•  NAD is a substrate for sirtuins (SIRTs), which regulate gene transcription in response to specific metabolic stresses. •  Nicotinamide phosphoribosyl transferase (Nampt) is the rate‐limiting enzyme in the NAD salvage pathway. •  Using transgenic mouse models, we tested the hypothesis that skeletal muscle Nampt protein abundance would increase in response to metabolic stress in a manner dependent on the cellular nucleotide sensor, AMP‐activated protein kinase (AMPK). •  Exercise training, as well as repeated pharmacological activation of AMPK by 5‐amino‐1‐β‐d‐ribofuranosyl‐imidazole‐4‐carboxamide (AICAR), increased Nampt protein abundance. However, only the AICAR‐mediated increase in Nampt protein abundance was dependent on AMPK. •  Our results suggest that cellular energy charge and nutrient sensing by SIRTs may be mechanistically related, and that Nampt may play a key role for cellular adaptation to metabolic stress.

Role of PGC-1α in exercise and fasting-induced adaptations in mouse liver

The transcriptional coactivator peroxisome proliferator-activated receptor (PPAR)-γ coactivator (PGC)-1α plays a role in regulation of several metabolic pathways. By use of whole body PGC-1α knockout (KO) mice, we investigated the role of PGC-1α in fasting, acute exercise and exercise training-induced regulation of key proteins in gluconeogenesis and metabolism in the liver. In both wild-type (WT) and PGC-1α KO mice liver, the mRNA content of the gluconeogenic proteins glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK) was upregulated during fasting. Pyruvate carboxylase (PC) remained unchanged after fasting in WT mice, but it was upregulated in PGC-1α KO mice. In response to a single exercise bout, G6Pase mRNA was upregulated in both genotypes, whereas no significant changes were detected in PEPCK or PC mRNA. While G6Pase and PC protein remained unchanged, liver PEPCK protein content was higher in trained than untrained mice of both genotypes. The mRNA content of the mitochondrial proteins cytochrome c (Cyt c) and cytochrome oxidase (COX) subunit I was unchanged in response to fasting. The mRNA and protein content of Cyt c and COXI increased in the liver in response to a single exercise bout and prolonged exercise training, respectively, in WT mice, but not in PGC-1α KO mice. Neither fasting nor exercise affected the mRNA expression of antioxidant enzymes in the liver, and knockout of PGC-1α had no effect. In conclusion, these results suggest that PGC-1α plays a pivotal role in regulation of Cyt c and COXI expression in the liver in response to a single exercise bout and prolonged exercise training, which implies that exercise training-induced improvements in oxidative capacity of the liver is regulated by PGC-1α.

Endurance exercise induces mRNA expression of oxidative enzymes in human skeletal muscle late in recovery

Exercise‐induced adaptations in skeletal muscle oxidative enzymes are suggested to result from the cumulative effects of transient changes in gene expression after each single exercise session. However, for several oxidative enzymes, no changes in mRNA expression are detected up to 8 h after exercise. To test the hypothesis that mRNA expression of many oxidative enzymes is up‐regulated late in recovery (10–24 h) after exercise, male subjects (n=8) performed a 90‐min cycling exercise (70% VO2‐max), with muscle biopsies obtained before exercise (pre), and after 10, 18 and 24 h of recovery. The mRNA expression of carnitine‐palmitoyltransferase (CPT)I, CD36, 3‐hydroxyacyl‐CoA‐dehydrogenase (HAD), cytochrome (Cyt)c, aminolevulinate‐delta‐synthase (ALAS)1 and GLUT4 was 100–200% higher at 10–24 h of recovery from exercise than in a control trial. Exercise induced a 100–300% increase in peroxisome proliferator‐activated receptor γ co‐activator (PGC)‐1α, citrate synthase (CS), CPTI, CD36, HAD and ALAS1 mRNA contents at 10–24 h of recovery relative to before exercise. No protein changes were detected in Cytc, ALAS1 or GLUT4. This shows that mRNA expression of several training‐responsive oxidative enzymes is up‐regulated in human skeletal muscle at 10–24 h of recovery, supporting that exercise‐induced adaptations of these oxidative enzymes can be the result of the cumulative effects of transient changes in mRNA expression.