Chris van der Poel

Adipose triacylglycerol lipase deletion alters whole body energy metabolism and impairs exercise performance in mice.

Adipose triacylglycerol lipase (ATGL) and hormone-sensitive lipase (HSL) are essential for efficient lipolysis in adipose tissue and skeletal muscle. Herein, we utilized whole body knockout mice to address the importance of ATGL and HSL for metabolic function and exercise performance. ATGL deletion severely disrupts whole-body substrate partitioning at rest; reducing plasma free fatty acid (FFA) availability (WT: 0.49 +/- 0.06 vs. ATGL(-/-) 0.34 +/- 0.03 mM), which in turn enhances carbohydrate oxidation during fasting (mean RER, WT: 0.86 +/- 0.02, ATGL(-/-) 0.90 +/- 0.01) and is associated with depleted muscle and liver glycogen stores. While plasma FFA was modestly reduced (23%) and whole body carbohydrate metabolism increased in HSL(-/-) mice, resting glycogen storage was not compromised. Studies in isolated muscles revealed that the capacity of ATGL and HSL(-/-) muscle to transport exogenous fatty acids is not compromised and the capacity to oxidize fatty acids is actually increased (3.7- and 1.3-fold above WT for ATGL and HSL). The exercise-induced increase in plasma FFA and glycerol was blunted with ATGL or HSL deletion, demonstrating an impaired capacity for exercise-induced lipolysis in these mice. Carbohydrate oxidation was increased concomitantly during exercise in ATGL(-/-) and HSL(-/-) mice, resulting in more muscle and liver glycogen depletion. Maximal running velocity and endurance capacity were reduced by 42% and 46% in ATGL(-/-) mice, but not in HSL(-/-) mice. The reduction in performance in ATGL(-/-) mice was not due to defective muscle contractile performance. These results demonstrate an essential role for both ATGL and HSL in maintaining adequate FFA supply to sustain normal substrate metabolism at rest and during exercise.

Deletion of Skeletal Muscle SOCS3 Prevents Insulin Resistance in Obesity

Obesity is associated with chronic low-grade inflammation that contributes to defects in energy metabolism and insulin resistance. Suppressor of cytokine signaling (SOCS)-3 expression is increased in skeletal muscle of obese humans. SOCS3 inhibits leptin signaling in the hypothalamus and insulin signal transduction in adipose tissue and the liver. Skeletal muscle is an important tissue for controlling energy expenditure and whole-body insulin sensitivity; however, the physiological importance of SOCS3 in this tissue has not been examined. Therefore, we generated mice that had SOCS3 specifically deleted in skeletal muscle (SOCS MKO). The SOCS3 MKO mice had normal muscle development, body mass, adiposity, appetite, and energy expenditure compared with wild-type (WT) littermates. Despite similar degrees of obesity when fed a high-fat diet, SOCS3 MKO mice were protected against the development of hyperinsulinemia and insulin resistance because of enhanced skeletal muscle insulin receptor substrate 1 (IRS1) and Akt phosphorylation that resulted in increased skeletal muscle glucose uptake. These data indicate that skeletal muscle SOCS3 does not play a critical role in regulating muscle development or energy expenditure, but it is an important contributing factor for inhibiting insulin sensitivity in obesity. Therapies aimed at inhibiting SOCS3 in skeletal muscle may be effective in reversing obesity-related glucose intolerance and insulin resistance.

Separation of fast from slow anabolism by site-specific PEGylation of insulin-like growth factor I (IGF-I).

Insulin-like growth factor I (IGF-I) has important anabolic and homeostatic functions in tissues like skeletal muscle, and a decline in circulating levels is linked with catabolic conditions. Whereas IGF-I therapies for musculoskeletal disorders have been postulated, dosing issues and disruptions of the homeostasis have so far precluded clinical application. We have developed a novel IGF-I variant by site-specific addition of polyethylene glycol (PEG) to lysine 68 (PEG-IGF-I). In vitro, this modification decreased the affinity for the IGF-I and insulin receptors, presumably through decreased association rates, and slowed down the association to IGF-I-binding proteins, selectively limiting fast but maintaining sustained anabolic activity. Desirable in vivo effects of PEG-IGF-I included increased half-life and recruitment of IGF-binding proteins, thereby reducing risk of hypoglycemia. PEG-IGF-I was equipotent to IGF-I in ameliorating contraction-induced muscle injury in vivo without affecting muscle metabolism as IGF-I did. The data provide an important step in understanding the differences of IGF-I and insulin receptor contribution to the in vivo activity of IGF-I. In addition, PEG-IGF-I presents an innovative concept for IGF-I therapy in diseases with indicated muscle dysfunction.

Cytoskeletal tropomyosin Tm5NM1 is required for normal Excitation- contraction coupling in skeletal muscle.

The functional diversity of the actin microfilaments relies in part on the actin binding protein tropomyosin (Tm). The muscle-specific Tms regulate actin-myosin interactions and hence contraction. However, there is less known about the roles of the numerous cytoskeletal isoforms. We have shown previously that a cytoskeletal Tm, Tm5NM1, defines a Z-line adjacent cytoskeleton in skeletal muscle. Recently, we identified a second cytoskeletal Tm in this region, Tm4. Here we show that Tm4 and Tm5NM1 define separate actin filaments; the former associated with the terminal sarcoplasmic reticulum (SR) and other tubulovesicular structures. In skeletal muscles of Tm5NM1 knockout (KO) mice, Tm4 localization was unchanged, demonstrating the specificity of the membrane association. Tm5NM1 KO muscles exhibit potentiation of T-system depolarization and decreased force rundown with repeated T-tubule depolarizations consistent with altered T-tubule function. These results indicate that a Tm5NM1-defined actin cytoskeleton is required for the normal excitation-contraction coupling in skeletal muscle.

Therapeutic potential of PEGylated insulin-like growth factor I for skeletal muscle disease evaluated in two murine models of muscular dystrophy

OBJECTIVE Duchenne muscular dystrophy (DMD) is a fatal monogenetic disease with affected males displaying severe and progressive muscle wasting and weakness eventually leading to premature death. Possible therapeutic benefits of insulin-like growth factor I (IGF-I) have been studied extensively in various models of muscle disease and DMD with IGF-I as a mediator of improved skeletal muscle regeneration by enhancing myoblast proliferation and differentiation. DESIGN We tested the efficacy of a novel IGF-I analogue, a polyethylene glycol modified IGF-I (PEG-IGF-I), to ameliorate the pathophysiology of muscular dystrophy in two mouse models of DMD. We used mdx mice which lack dystrophin (as in DMD) but exhibit only a relatively mild phenotype, and the dko mouse which is a transgenic model lacking utrophin in addition to dystrophin, and which exhibits a more severe, lethal phenotype like that in DMD. RESULTS In young mdx mice, twice-weekly PEG-IGF-I s.c. injections for 6 weeks protected the diaphragm muscle against fatigue and the tibialis anterior (TA) muscle against contraction-induced injury. However, this beneficial effect of PEG-IGF-I was less pronounced in mdx mice when treatment was initiated later in adulthood. In severely affected dko mice PEG-IGF-I treatment did not affect pathophysiological parameters including animal survival. CONCLUSIONS These data suggest a therapeutic benefit with PEG-IGF-I treatment only in mild muscle pathologies, since its potential to ameliorate the pathophysiology in models of severe muscular dystrophies was limited. Treatment should be initiated only for mild muscle pathologies if functional benefits are to be realised and therefore may be relevant as a short-term therapy to hasten the functional repair of otherwise healthy muscles after injury.

EFFECT OF TEMPERATURE-INDUCED REACTIVE OXYGEN SPECIES PRODUCTION ON EXCITATION–CONTRACTION COUPLING IN MAMMALIAN SKELETAL MUSCLE

1 Here we review evidence obtained recently by us indicating that the poor longevity of isolated mammalian skeletal muscle preparations at temperatures in the normal physiological range is related to the increased production of reactive oxygen species (ROS) in the resting muscle. 2 Temperature‐induced ROS production increases markedly above 32°C in isolated, resting skeletal muscle and is associated with the gradual and irreversible functional deterioration of the muscle. 3 The majority of the temperature‐induced muscle ROS originates in the mitochondria and acts on various sites involved in excitation–contraction coupling.