Filament Compliance Influences Cooperative Activation of Thin Filaments and the Dynamics of Force Production in Skeletal Muscle

Abstract

Striated muscle contraction is a highly cooperative process initiated by Ca2+ binding to the troponin complex, which leads to tropomyosin movement and myosin cross-bridge (XB) formation along thin filaments. Experimental and computational studies suggest skeletal muscle fiber activation is greatly augmented by cooperative interactions between neighboring thin filament regulatory units (RU-RU cooperativity; 1 RU = 7 actin monomers+1 troponin complex+1 tropomyosin molecule). XB binding can also amplify thin filament activation through interactions with RUs (XB-RU cooperativity). Because these interactions occur with a temporal order, they can be considered kinetic forms of cooperativity. Our previous spatially-explicit models illustrated that mechanical forms of cooperativity also exist, arising from XB-induced XB binding (XB-XB cooperativity). These mechanical and kinetic forms of cooperativity are likely coordinated during muscle contraction, but the relative contribution from each of these mechanisms is difficult to separate experimentally. To investigate these contributions we built a multi-filament model of the half sarcomere, allowing RU activation kinetics to vary with the state of neighboring RUs or XBs. Simulations suggest Ca2+ binding to troponin activates a thin filament distance spanning 9 to 11 actins and coupled RU-RU interactions dominate the cooperative force response in skeletal muscle, consistent with measurements from rabbit psoas fibers. XB binding was critical for stabilizing thin filament activation, particularly at submaximal Ca2+ levels, even though XB-RU cooperativity amplified force less than RU-RU cooperativity. Similar to previous studies, XB-XB cooperativity scaled inversely with lattice stiffness, leading to slower rates of force development as stiffness decreased. Including RU-RU and XB-RU cooperativity in this model resulted in the novel prediction that the force-[Ca2+] relationship can vary due to filament and XB compliance. Simulations also suggest kinetic forms of cooperativity occur rapidly and dominate early to get activation, while mechanical forms of cooperativity act more slowly, augmenting XB binding as force continues to develop.

Random myosin loss along thick-filaments increases myosin attachment time and the proportion of bound myosin heads to mitigate force decline in skeletal muscle

“…The computational models used in this work build on a series of spatially-explicit models developed over the past 15 years that simulate muscle contraction within a network of linear springs [9, 10, 37, 38]. In summary, the current model comprises 4 thick-filaments and 8 thin-filaments of half-sarcomere length to model Ca 2+ -regulated actomyosin cross-bridge binding and force production (Fig.…”

“…Monte Carlo algorithms drive kinetic state transitions for both thin-filament activation and cross-bridge binding. This kinetic scheme includes cooperative activation of thin-filaments from neighboring thin-filament regulatory units and bound cross-bridges [38]. As further described below, we have now: i) modified myosin organization along thick-filaments to represent the three-start helix of vertebrate thick-filaments (simple-lattice structure [5]), ii) added a linear elastic element between the free-end of thick-filaments and the Z-line to represent titin (Fig.…”

“…These k xb values were chosen because previous computational modelling efforts have shown that variations in k xb influence the level of cross-bridge binding and cross-bridge kinetics [9, 10, 37, 38], which could have a combined influence on cross-bridge behavior as myosin content was reduced. Generally we focused on simulations when k xb =3 pN nm −1 as these have been most representative of measured experimental force and ATPase values in our prior computational studies [37, 38] and remains consistent with estimates of k xb from cellular experiments [21, 28, 34]. …”

“…Thin-filament structures represent the helical pitch of native vertebrate thin-filaments (rotating 180° over 37.7 nm, [36]) and were modeled identical to prior studies [37, 38]. Regulatory unit activation span was set at 50 nm for all simulations, thereby activating the equivalent length of ~9 actin monomers, or 3 thin-filament nodes along one actin helix, subsequent to Ca 2+ -binding to troponin and tropomyosin movement.…”

“…Similarly, contractility can vary with cross-bridge stiffness and thick-to-thin filament overlap at different sarcomere lengths, as these mechanical and structural properties of the myofilament lattice influence the upper limits of cross-bridge binding throughout the sarcomere. To investigate the roles of varied thick-filament structure, sarcomere-length, and cross-bridge stiffness on ensemble cross-bridge binding, force production, and kinetics, we used a computational model of the half-sarcomere to simulate muscle contraction in a system of Ca 2+ -regulated myosin-actin cross-bridge interactions between multiple thick- and thin-filaments [9, 37, 38]. While cross-bridge stiffness and sarcomere length influenced overall sarcomeric force production by affecting the number and kinetic properties of bound cross-bridges, these measures were influenced more by reductions in myosin content.…”