Titin–Actin Interaction in Mouse Myocardium: Passive Tension Modulation and Its Regulation by Calcium/S100A1

Abstract

Passive tension in striated muscles derives primarily from the extension of the giant protein titin. However, several studies have suggested that, in cardiac muscle, interactions between titin and actin might also contribute to passive tension. We expressed recombinant fragments representing the subdomains of the extensible region of cardiac N2B titin (tandem-Ig segments, the N2B splice element, and the PEVK domain), and assayed them for binding to F-actin. The PEVK fragment bound F-actin, but no binding was detected for the other fragments. Comparison with a skeletal muscle PEVK fragment revealed that only the cardiac PEVK binds actin at physiological ionic strengths. The significance of PEVK-actin interaction was investigated using in vitro motility and single-myocyte mechanics. As F-actin slid relative to titin in the motility assay, a dynamic interaction between the PEVK domain and F-actin retarded filament sliding. Myocyte results suggest that a similar interaction makes a significant contribution to the passive tension. We also investigated the effect of calcium on PEVK-actin interaction. Although calcium alone had no effect, S100A1, a soluble calcium-binding protein found at high concentrations in the myocardium, inhibited PEVK-actin interaction in a calcium-dependent manner. Gel overlay analysis revealed that S100A1 bound the PEVK region in vitro in a calcium-dependent manner, and S100A1 binding was observed at several sites along titin's extensible region in situ, including the PEVK domain. In vitro motility results indicate that S100A1-PEVK interaction reduces the force that arises as F-actin slides relative to the PEVK domain, and we speculate that S100A1 may provide a mechanism to free the thin filament from titin and reduce titin-based tension before active contraction.

Targeting S100A1 in heart failure

“…Periodically, it appears at the Z-and the M-line as well as within both the I-and A-band. [50][51][52] Within the I-band, S100A1 seems to interact with the PEVK domain of the giant protein titin in a Ca 2+ -dependent manner. 50 Titin is mainly responsible for the preservation of passive tension in CM and a single titin molecule spans half the length of a sarcomere.…”

“…[50][51][52] Within the I-band, S100A1 seems to interact with the PEVK domain of the giant protein titin in a Ca 2+ -dependent manner. 50 Titin is mainly responsible for the preservation of passive tension in CM and a single titin molecule spans half the length of a sarcomere. 53 In diastole, the PEVK domain of titin interacts with actin, contributing to the diastolic myocardial stiffness.…”

“…53 In diastole, the PEVK domain of titin interacts with actin, contributing to the diastolic myocardial stiffness. In the presence of systolic [Ca 2+ ], Yamasaki et al 50 were the first ones to show that binding of S100A1 to titin results in an inhibition of the titin-actin interaction. They furthermore proposed that this interaction frees the actin filaments from titin before active contraction, thereby reducing pre-contractile passive tension and facilitating initiation of contraction.…”

“…They furthermore proposed that this interaction frees the actin filaments from titin before active contraction, thereby reducing pre-contractile passive tension and facilitating initiation of contraction. 50,51 If S100A1 also interacts with titin at the A-band or with another sarcomeric protein remains to be established.…”

“…[39][40][41]45 In addition, facilitated dissociation of Ca 2+ from the myofilaments and a decreased sarcomeric passive tension contributes to the diastolic improvements in CM. 50,51 In systole, S100A1-mediated increase in SR Ca 2+ load together with increased RyR2 opening and SR Ca 2+ release is responsible for increased cellular Ca 2+ transients, thereby supporting contractile performance. 40,[42][43][44] Regarding the increased energy demand that originates from increased SR and myofilament ATPase activity, S100A1 might be able to meet this demand by increasing ATP generation and cytoplasmic availability.…”