MUSCLE FUNCTION & HOMEOSTASIS / MOLECULAR THERAPEUTIC APPROACHES: P.76 Validation of in vivo and ex vivo readouts in a murine model of hind limb unloading to optimize translational research on skeletal muscle atrophy

Abstract

Skeletal muscle atrophy is a major consequence of several pathophysiological conditions, including neuromuscular diseases, disuse (limb immobilization, space flight), aging (sarcopenia) and other wasting conditions (i.e. cancer cachexia). Basically, a disequilibrium between anabolic/catabolic pathways leads to progressive loss of muscle mass and functional impairment. The aim of translational research in this field is to identify anabolic compounds able to support protein synthesis and muscle mass, restoring muscle function and metabolism. Therefore, the validation of adequate readout parameters to assess atrophy and efficacy of therapeutics in reliable animal models is pivotal. Here, we focused on a detailed preclinical investigation of the hind limb unloading (HU) murine model, in which disuse leads to severe atrophy, dysfunction and damage in postural soleus (SOL) muscle. 10-week-old, male C57BL/6J mice underwent a 2-week HU protocol (T0–T2), in comparison to age-, sex-matched non-HU mice for a wide array of in vivo/ex vivo outcomes. At T0 (pre-HU), all mice had comparable body weight (BW). At T2, HU mice BW was significantly lower with respect to T0 (-9.7%) and to non-HU mice (-17.3%). We disclosed for the first time that in vivo torque of hind limb plantar flexor muscles (i.e. SOL, gastrocnemius) was severely impaired in HU mice, as well as SOL muscle contractile and elastic properties, measured ex vivo. This was paralleled by a significant decrease in SOL muscle mass (-39%) and total protein content (-47%). SOL muscle histology was also seriously compromised by HU protocol, with clear signs of myofiber damage and atrophy (significant decrease in fiber cross-sectional area vs non-HU condition). Finally, we found a remarkable -61% reduction of salivary IgA levels, which suggests a HU-induced negative immune response modulation, in line with previous observations from other models of hind limb suspension. Overall, our findings support the usefulness of the HU mouse model for preclinical translational research on skeletal muscle atrophy.