Résumé :
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INTRODUCTION: Nebulin is a giant protein expressed in skeletal muscle which plays a major role in both the organization of the sarcomeric structure and the regulation of cross-bridge cycling kinetics2,5. Indeed, reduced force production and calcium sensitivity have been reported in nebulin knockout mice (NEB-KO)2. Interestingly, in heterozygous animals for a null actin or myosin heavy chain mutation1,4, a haploinsufficiency phenotype and a reduced expression of the corresponding protein have been reported. Considering that in NEB-KO heterozygous (Het) mice, the protein level is unaffected, one could hypothesize that muscle function would not be altered. We combine in vivo and in vitro investigations of muscle function in Het mice in order to document the functional impact of the expression of a single nebulin allele. MATERIALS AND METHODS: Three-month-old Het and wild-type (WT) mice were used for both in vivo and in vitro experiments. In vivo investigations were performed as previously described3 i.e. allowing measurements of hindlimb muscle volume and tetanic force of gastrocnemius muscles. In vitro experiments were performed on intact whole soleus muscle. Once the muscle was dissected, muscle tendons were attached to a force transducer and a length motor and activated by field stimulation. The muscle was exposed to various stimulation protocols and muscle cross sectional area was determined as previously described2. RESULTS: In vivo maximal tetanic force was similar in both groups whereas muscle volume was significantly smaller in Het (347±6 mm3) as compared to controls (379±7 mm3). Specific force was then higher in Het (Figure 1). Muscle force at 1, 20 and 30 Hz was significantly lower in Het as compared to WT. The force-frequency relationship was also shifted to the right in Het. In vitro maximal tetanic stress was significantly reduced in Het. The relative force-frequency relationship was shifted to the left in Het as compared to controls with a significantly higher force in Het at 10 and 20 Hz (Figure 1). CONCLUSION: We clearly demonstrated that the expression of a single functional nebulin allele in Het mice resulted in abnormal and opposite results in the soleus (in vitro) and gastrocnemius muscles (in vivo). The corresponding differences could be related to fiber-type composition and warrant further investigations in order to determine the cellular and/or molecular mechanisms accounting for the muscle-specific alterations in Het mice. REFERENCES:1. Beall et al. Genes Dev:131-140, 1989.2. Chandra et al. J Biol Chem:30889-30896, 2009.3. Giannesini et al. Magn Reson Med:262-270, 2010.4. Jones et al. J Clin Invest:1906-1917, 1996.5. Ottenheijm et al. Physiology (Bethesda):304-310, 2010.
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