Sandrine Arbogast

CLOCK and BMAL1 regulate MyoD and are necessary for maintenance of skeletal muscle phenotype and function

MyoD, a master regulator of myogenesis, exhibits a circadian rhythm in its mRNA and protein levels, suggesting a possible role in the daily maintenance of muscle phenotype and function. We report that MyoD is a direct target of the circadian transcriptional activators CLOCK and BMAL1, which bind in a rhythmic manner to the core enhancer of the MyoD promoter. Skeletal muscle of ClockΔ19 and Bmal1−/− mutant mice exhibited ∼30% reductions in normalized maximal force. A similar reduction in force was observed at the single-fiber level. Electron microscopy (EM) showed that the myofilament architecture was disrupted in skeletal muscle of ClockΔ19, Bmal1−/−, and MyoD−/− mice. The alteration in myofilament organization was associated with decreased expression of actin, myosins, titin, and several MyoD target genes. EM analysis also demonstrated that muscle from both ClockΔ19 and Bmal1−/− mice had a 40% reduction in mitochondrial volume. The remaining mitochondria in these mutant mice displayed aberrant morphology and increased uncoupling of respiration. This mitochondrial pathology was not seen in muscle of MyoD−/− mice. We suggest that altered expression of both Pgc-1α and Pgc-1β in ClockΔ19 and Bmal1−/− mice may underlie this pathology. Taken together, our results demonstrate that disruption of CLOCK or BMAL1 leads to structural and functional alterations at the cellular level in skeletal muscle. The identification of MyoD as a clock-controlled gene provides a mechanism by which the circadian clock may generate a muscle-specific circadian transcriptome in an adaptive role for the daily maintenance of adult skeletal muscle.

TNF-α acts via TNFR1 and muscle-derived oxidants to depress myofibrillar force in murine skeletal muscle

Tumor necrosis factor-alpha (TNF) diminishes specific force of skeletal muscle. To address the mechanism of this response, we tested the hypothesis that TNF acts via the type 1 (TNFR1) receptor subtype to increase oxidant activity and thereby depress myofibrillar function. Experiments showed that a single intraperitoneal dose of TNF (100 microg/kg) increased cytosolic oxidant activity (P < 0.05) and depressed maximal force of male ICR mouse diaphragm by approximately 25% within 1 h, a deficit that persisted for 48 h. Pretreating animals with the antioxidant Trolox (10 mg/kg) lessened oxidant activity (P < 0.05) and abolished contractile losses in TNF-treated muscle (P < 0.05). Genetic TNFR1 deficiency prevented the rise in oxidant activity and fall in force stimulated by TNF; type 2 TNF receptor deficiency did not. TNF effects on muscle function were evident at the myofibrillar level. Chemically permeabilized muscle fibers from TNF-treated animals had lower maximal Ca2+-activated force (P < 0.02) with no change in Ca2+ sensitivity or shortening velocity. We conclude that TNF acts via TNFR1 to stimulate oxidant activity and depress specific force. TNF effects on force are caused, at least in part, by decrements in function of calcium-activated myofibrillar proteins.

Selenoproteins and Protection against Oxidative Stress: Selenoprotein N as a Novel Player at the Crossroads of Redox Signaling and Calcium Homeostasis

Healthy cells continually produce low levels of reactive oxygen species (ROS), which are buffered by multiple antioxidant systems. Imbalance between ROS production and elimination results in oxidative stress, which has been implicated in aging and in numerous human diseases, including cancer and diabetes. Selenoproteins are a family of proteins that contain the amino acid selenocysteine, encoded by an in-frame UGA. Those selenoproteins whose function is identified are catalytically active in redox processes, representing one of the main enzymatic antioxidant systems and important mediators of the beneficial role of selenium in human health. Nevertheless, the function of most selenoproteins remains unknown; this included Selenoprotein N (SelN), the only selenoprotein directly associated with a human genetic disease. Mutations of the SelN gene cause SEPN1-related myopathy, a particular early-onset muscle disorder. Recent studies have identified SelN as a key protein in cell protection against oxidative stress and redox-related calcium homeostasis. Furthermore, an effective ex vivo treatment of SelN deficiency has been identified, paving the way to a clinical therapy. In this review we discuss the physiological and pathophysiological role of SelN and the interest of SEPN1-related myopathy as a model paradigm to understand and target therapeutically other selenoproteins involved in human health and disease.

Progressive nuclear factor-κB activation resistant to inhibition by contraction and curcumin in mdx mice

Skeletal muscle of patients with Duchenne‐type muscular dystrophy and mdx mice exhibits elevated activity of the transcription factor NF‐κB (nuclear factor‐κB), which may play a role in muscle catabolism. We measured skeletal muscle NF‐κB activity in mdx mice at three ages (10 days, 4 weeks, and 8 weeks) to test the hypothesis that NF‐κB activity is elevated in an age‐dependent manner in these mice. In addition, we tested the hypothesis that NF‐κB activity could be reduced in mdx skeletal muscle by dietary supplementation with curcumin (1% w/v) or by fatiguing muscle contractions. We found that NF‐κB activity was elevated at 4 and 8 weeks of age but not at 10 days, and was resistant to inhibition by either fatiguing contractions or dietary curcumin. We conclude that NF‐κB activity is elevated in dystrophic skeletal muscle in an age‐related manner and is resistant to inhibition by physiological and pharmacological means. These findings are consistent with a role for NF‐κB activation in dystrophic muscle wasting but suggest that predicted interventions such as exercise or inhibitors of the early steps in the NF‐κ activation pathway may not be effective and that targeted research is needed to identify novel therapeutic strategies. Muscle Nerve, 2006

Effects of endogenous nitric oxide in activation of group IV muscle afferents.

Based on previous observations that acute hypoxemia, which enhances nitric oxide (NO) production, depresses the activation of group IV afferents after repetitive low‐frequency muscle stimulation (MS), we hypothesized that endogenous NO modulates the response of these nerve endings to their specific stimuli. The present study in rabbits examined the effects of a blocker of NO synthase (NG‐nitro‐L‐arginine methyl ester L, L‐NAME) and an exogenous NO donor (3‐morpholinosydnonimine, SIN‐1) on the group IV afferents of tibialis anterior. The efficacy of the two test agents was judged by their effects on systemic blood pressure. L‐NAME markedly elevated (+46%) the resting discharge rate of group IV afferents but abolished their activation after repetitive MS. After SIN‐1 injection, there was a transient decrease in blood pressure, which correlated well with a lowered resting discharge rate of group IV afferents. SIN‐1 infusion caused a stable reduction of blood pressure; the resting afferent nerve discharge rate began first to decrease but then recovered control mean values. SIN‐1 infusion abolished the activation of group IV afferents after MS. This study indicates that endogenous NO production in a resting or contracting muscle attenuates the baseline activity of group IV muscle afferents and their activation after repetitive muscle contractions.