Mechanical factors are important in the regulation of muscle phenotype. Adaptation of structural and mechanical proteins to withstand increased load and cope with increased work is needed in a muscle exposed to mechanical stimuli such as stretch.
Small muscle protein X-chromosome (SMPX) is a protein upregulated in stretched skeletal muscle, and could therefore work as a structural protein, a signalling factor in a mechanotransduction pathway, or a combination of both.
In my project I have studied the in vivo and ex vivo localization of SMPX in skeletal muscle, as well as effects of overexpressing SMPX in the fast-twitch muscle extensor digitorum longus (EDL) and the slow twitch muscle soleus in adult mice.
Overexpression of SMPX for 14 days gave no significant changes in fibre type distribution or cross sectional area in EDL. In soleus the results were variable, but there was a shift towards a faster fibre type and a less pronounced increase in cross sectional area.
Our expriments do not support the idea that SMPX works as a major regulatory protein or a signalling molecule related to force transduction in the I-band. If it serves as a regulatory protein, our results show that this could be true for soleus, but not for EDL.