Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ, 86011-4185, USA.
Edgewise Therapeutics Inc, 3415 Colorado Ave, Boulder, CO, 80303, USA.
J Muscle Res Cell Motil. 2020 Mar;41(1):125-139. doi: 10.1007/s10974-019-09540-y. Epub 2019 Jul 9.
Gaps in our understanding of muscle mechanics demonstrate that the current model is incomplete. Increasingly, it appears that a role for titin in active muscle contraction might help to fill these gaps. While such a role for titin is increasingly accepted, the underlying molecular mechanisms remain unclear. The goals of this paper are to review recent studies demonstrating Ca-dependent interactions between N2A titin and actin in vitro, to explore theoretical predictions of muscle behavior based on this interaction, and to review experimental data related to the predictions. In a recent study, we demonstrated that Ca increases the association constant between N2A titin and F-actin; that Ca increases rupture forces between N2A titin and F-actin; and that Ca and N2A titin reduce sliding velocity of F-actin and reconstituted thin filaments in motility assays. Preliminary data support a role for Ig83, but other Ig domains in the N2A region may also be involved. Two mechanical consequences are inescapable if N2A titin binds to thin filaments in active muscle sarcomeres: (1) the length of titin's freely extensible I-band should decrease upon muscle activation; and (2) binding between N2A titin and thin filaments should increase titin stiffness in active muscle. Experimental observations demonstrate that these properties characterize wild type muscles, but not muscles from mdm mice with a small deletion in N2A titin, including part of Ig83. Given the new in vitro evidence for Ca-dependent binding between N2A titin and actin, it is time for skepticism to give way to further investigation.
我们对肌肉力学的理解存在差距，这表明目前的模型并不完整。越来越多的迹象表明，肌联蛋白在肌肉主动收缩中的作用可能有助于填补这些空白。虽然肌联蛋白在这方面的作用越来越被接受，但潜在的分子机制仍不清楚。本文的目的是回顾最近的研究，这些研究表明肌联蛋白 N2A 结构域与肌动蛋白在体外具有 Ca 依赖性相互作用，探讨基于这种相互作用的肌肉行为的理论预测，并回顾与预测相关的实验数据。在最近的一项研究中，我们证明了 Ca 增加了 N2A 肌联蛋白与 F-肌动蛋白之间的结合常数；Ca 增加了 N2A 肌联蛋白与 F-肌动蛋白之间的断裂力；Ca 和 N2A 肌联蛋白降低了 F-肌动蛋白和重组成的薄丝在运动检测中的滑动速度。初步数据支持 Ig83 发挥作用，但 N2A 结构域中的其他 Ig 结构域也可能参与其中。如果 N2A 肌联蛋白在活跃的肌节中结合到薄丝上，那么会产生两个不可避免的力学后果：（1）肌联蛋白的自由伸展 I 带的长度在肌肉激活时应该减少；（2）N2A 肌联蛋白和薄丝之间的结合应该增加肌联蛋白在活跃肌肉中的刚度。实验观察表明，这些特性是野生型肌肉的特征，但不是 N2A 肌联蛋白中有小缺失的 mdm 肌肉的特征，包括 Ig83 的一部分。鉴于新的体外证据表明 Ca 依赖性结合存在于 N2A 肌联蛋白和肌动蛋白之间，怀疑论现在应该让位于进一步的研究。
