The Mstn-Cmpt Dl1Abc- Mice. A Mouse Model to Study Muscle Weakness, Fatigue and Soce

Abstract

In our mouse model, a naturally occuring 12-bp deletion in the myostatin gene is considered responsible for the compact phenotype (Mstn(Cmpt-dl1Abc)) labeled by a tremendous increase in body weight along with signs of muscle weakness and easier fatigability.Western blot screenings showed significantly reduced endogenous STIM1 and Orai1 protein levels in the compact mouse muscle samples. As a consequence, we hypothesized that SOCE may be consequently altered. Enzymatically isolated fluo-8 AM loaded FDB fibers from wild type and Mstn(Cmpt-dl1Abc) mice were used. To elicit a massive SR Ca2+-release a RyR1 agonist (4-chloro-meta-cresol) was applied in a Ca2+ free medium and in the presence of the SR Ca2+ pump inhibitor (thapsigargin). The above cocktail triggered a deep transient store depletion which in turn switched on SOCE in a wild type cell. The peak of the Ca2+ transient (triggered by the entry of the external Ca2+ into the cytoplasm following the internal store depletion) was normalized to the peak of the SR Ca2+ release transient (1.06±0.11, n=17 vs 0.8±0.08, n=9). On isolated FDB fibers under voltage clamp, we found that the voltage dependence of the normalized fluorescence of the calcium transients in response to membrane depolarizations ranging between −60 and +30 mV, with a 10 mV increments were well fitted with a Boltzmann distribution (V0.5: −23.22±1.35 mV vs −24.15±0.77 mV with respective k values of 6.14±1.15 vs. 6.93±0.65). Fatigue inducing protocols are being currently tested. Collectively, these data point toward altered Ca2+ homeostasis in the compact mice. Understanding the mechanisms behind them may help developing new strategies in muscle atrophy, ageing and wasting diseases. To our knowledge, we are the first to employ Mstn(Cmpt-dl1Abc)mice and perform a comparative study to characterize their Ca2+ homeostasis in light of SOCE.