Titin force is enhanced in actively stretched skeletal muscle

Abstract

The sliding filament theory of muscle contraction is widely accepted as the means by which muscles generate force during activation. Within the constraints of this theory, isometric, steady-state force produced during muscle activation is proportional to the amount of filament overlap. Previous studies from our laboratory demonstrated enhanced titin-based force in myofibrils that were actively stretched to lengths which exceeded filament overlap. This observation cannot be explained by the sliding filament theory. The aim of the present study was to further investigate the enhanced state of titin during active stretch. Specifically, we confirm that this enhanced state of force is observed in a mouse model and quantify the contribution of calcium to this force. Titin-based force was increased by up to four times that of passive force during active stretch of isolated myofibrils. Enhanced titin-based force has now been demonstrated in two distinct animal models, suggesting that modulation of titin-based force during active stretch is an inherent property of skeletal muscle. Our results also demonstrated that 15% of the enhanced state of titin can be attributed to direct calcium effects on the protein, presumably a stiffening of the protein upon calcium binding to the E-rich region of the PEVK segment and selected Ig domain segments. We suggest that the remaining unexplained 85% of this extra force results from titin binding to the thin filament. With this enhanced force confirmed in the mouse model, future studies will aim to elucidate the proposed titin-thin filament interaction in actively stretched sarcomeres.

Titin force enhancement following active stretch of skinned skeletal muscle fibres

“…These initial observations of 'passive force enhancement' demonstrated that, following active stretch, titin-based force remained greater after deactivation than its passive force at corresponding lengths in cat soleus muscle (Herzog and Leonard, 2002). Titin force enhancement was investigated in rabbit (and later, mouse) psoas myofibrils stretched actively beyond filament overlap (Leonard and Herzog, 2010;Powers et al, 2014). These studies showed a 400% increase in titin-based force in sarcomeres stretched actively to 6.0 μm, when compared with titinbased force in sarcomeres passively stretched to the same length, greatly exceeding the 20% increase in titin force previously measured following deactivation of actively stretched myofibrils (Joumaa et al, 2008).…”

“…Nevertheless, active sarcomeres are routinely stretched during movement and generate more force following stretch than can be explained by cross-bridges alone (Herzog and Leonard, 2002). Because this additional force depends on the magnitude but not the speed of stretch, and because it results in increased passive force ( passive residual force enhancement), it has been speculated that titin contributes to this unexplained extra force (Herzog and Leonard, 2002;Leonard and Herzog, 2010;Nishikawa et al, 2012;Powers et al, 2014Powers et al, , 2016.…”

“…It is well established that titin becomes stiffer in the presence of calcium (DuVall et al, 2013;Labeit et al, 2003). However, the calcium-based increase in titin force in intact sarcomeres is too small to explain the force increase after active stretch (Joumaa et al, 2008;Powers et al, 2014). Early investigations demonstrated that the magnitude of titin force enhancement depends on sarcomere length at activation; titin force enhancement increases with increased initial contractile force, suggesting that titin-based force is proportional to cross-bridge force (Leonard and Herzog, 2010).…”

Shiver me titin! Elucidating titin's role in shivering thermogenesis

“…Leonard and Herzog (2010) and Powers et al (2014) observed that titinbased muscle stiffness increases upon activation. It is plausible that the mechanism by which titin increases active muscle stiffness is affected by the mdm mutation (Nishikawa et al, 2012;Herzog, 2014).…”

“…However, Nishikawa et al (2012) and Herzog (2014) have suggested that titin's contribution to muscle stiffness may increase upon muscle activation by binding to the thin filament. Leonard and Herzog (2010) and Powers et al (2014) have shown that titin-based stiffness increases during muscle activation.…”