Ubiquitin targeting of rat muscle proteins during short periods of unloading

Aim: The ubiquitin-proteasome system is known to be involved in many situations leading to skeletal muscle atrophy. However, the cellular mechanisms triggering the atrophic process initiation are still poorly understood. For short periods of rate hindlimb unloading, we assessed the specific ubiquitin targeting of sarcoplasmic or myofibrillar proteins in slow and fast rat muscle types. Methods: Adult Sprague Dawley rats were randomly assigned to three groups: control, hindlimb-unloaded for 4 days (HU4) and hindlimb-unloaded for 8 days (HU8). In fractionated extracts from soleus (SOL) and Extensor Digitorum Longus (EDL) muscles, the relative contents of free and conjugated ubiquitin were quantified by immunoblotting. Results: Hindlimb unloading of short durations resulted in a preferential atrophy of slow-twitch fibres and bound ubiquitin levels were increased by 37 and 68% in the soleus myofibrillar fraction after respectively 4 and 8 days. The ubiquitin conjugation was shown to principally affect the high molecular weight proteins. Free and conjugated ubiquitin levels remained unchanged in sarcoplasmic fraction from SOL muscle after 8 days HU. For the fast muscle (EDL), ubiquitin contents were approximately twofold lower in control conditions, and did not significantly change during the hindlimb unloading periods considered. Conclusion: The postural SOL muscle was shown to contain higher constitutive sarcoplasmic ubiquitin levels than the phasic EDL. The high response to unloading of the slow twitch fibres rich SOL muscle was accompanied by a specific conjugation of its myofibrillar proteins that may participate in the initiation of skeletal muscle remodelling consequent to disuse.