Hindlimb unloading induces changes in actin sliding velocity of mouse pure myosin isoforms

Disuse muscle atrophy following hindlimb unloading (HU) can cause changes in unloaded shortening velocity (V0) at muscle fiber level (Zhong et al., 2006). The aim of this study was to clarify whether myosin motor function can be affected by HU, accounting for the observed changes in V0. The in vitro motility assay (IVMA) approach was used. In IVMA the properties of myosin motor could be investigated in the absence of other myofibrillar proteins by assessing actin sliding velocity (Vf) on isolated myosin. Vf is widely considered analogous to V0 as significant correlation between V0 and Vf was demonstrated (Canepari et al., 1999). Pure type 1 myosin from soleus (S) and type 2B myosin from gastrocnemius (GAS) muscle of HU and control (C) mice were analyzed. The results show that Vf on myosin 2B from HU was significantly slower (2.70 ± 0.32) then Vf on myosin 2B from C (4.11 ± 0.35), whereas no significant difference was found for myosin 1 (0.84 ± 0.17 vs. 0.89 ± 0.04). The results suggest that type 2B myosin motor could be functionally altered by HU. The effect could be related to a differentMLCisoform composition (Zhong et al., 2007) or a post-translational modification of myosin (Ramamurthy et al., 2003; Coirault et al., 2007). Preliminary analysis of MLC composition show no differences between HU and C