Removal of Ig Domains of Titin Alters Contractility in Mouse Soleus Muscle

Abstract

The giant protein titin provides elasticity to the skeletal muscle sarcomere by acting as a molecular spring. We created a mouse model in which 9 immunoglobulin (Ig) domains (Ig3-11) have been deleted from titin's spring region, providing a novel model to test the effect of a shorter spring on the physiology of skeletal muscle. The soleus muscle was studied by using gel electrophoresis and functional measurements on contractility made via an intact muscle mechanics system on six month old male mice. An additional set of mice were exposed to exercise training on a treadmill to determine exercise capacity and response to exercise. Soleus mass was lower in the IGKO (8.3±0.2mg) vs WT (10.2±1.2mg). IGKO mice showed a significant decrease in titin size on 1% agarose gels greater than the 88KDa loss predicted by the deletion if Ig3-11. Furthermore, IGKO mice have a reduced myosin II/ myosin I ratio (2.1 vs. 8.0, p=0.002). Intact muscle mechanics measurements probed both passive and active tension. IG KO muscles produced an increased passive tension (19% p=0.02) at 20% above slack length of the muscle. Active force measurements probed at the optimal twitch length (Lo) revealed a significantly decreased twitch/tetanus ratio in IGKO's (12.4% vs 8.1%, p<0.001) and decreased active force at all frequencies examined (1-150Hz). IGKO also experienced less fatigue (28% vs 67% decrease, p=0.01). Exercise testing in mice showed a significantly reduced maximum running speed between IGKO and WT (18.8 vs 22.0 m/min, p=0.005), which persisted after two weeks of training. The IgKO mouse provides a unique model for elucidating the effect of titin's spring elements on contractility in skeletal muscle.