Departments of Anatomy and Cell Biology and Internal Medicine, College of Medicine, University of Iowa, Iowa City, IA, 52242, USA,
J Muscle Res Cell Motil. 2013 Aug;34(3-4):295-310. doi: 10.1007/s10974-013-9343-z. Epub 2013 May 23.
Tropomyosin (Tm) is the key regulatory component of the thin-filament and plays a central role in the cardiac muscle's cooperative activation mechanism. Many mutations of cardiac Tm are related to hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM), and left ventricular noncompaction (LVNC). Using the thin-filament extraction/reconstitution technique, we are able to incorporate various Tm mutants and protein isoforms into a muscle fiber environment to study their roles in Ca(2+) regulation, cross-bridge kinetics, and force generation. The thin-filament reconstitution technique poses several advantages compared to other in vitro and in vivo methods: (1) Tm mutants and isoforms are placed into the real muscle fiber environment to exhibit their effect on a level much higher than simple protein complexes; (2) only the primary and immediate effects of Tm mutants are studied in the thin-filament reconstituted myocardium; (3) lethal mutants of Tm can be studied without causing a problem; and (4) inexpensive. In transgenic models, various secondary effects (myocyte disarray, ECM fibrosis, altered protein phosphorylation levels, etc.) also affect the performance of the myocardium, making it very difficult to isolate the primary effect of the mutation. Our studies on Tm have demonstrated that: (1) Tm positively enhances the hydrophobic interaction between actin and myosin in the "closed state", which in turn enhances the isometric tension; (2) Tm's seven periodical repeats carry distinct functions, with the 3rd period being essential for the tension enhancement; (3) Tm mutants lead to HCM by impairing the relaxation on one hand, and lead to DCM by over inhibition of the AM interaction on the other hand. Ca(2+) sensitivity is affected by inorganic phosphate, ionic strength, and phosphorylation of constituent proteins; hence it may not be the primary cause of the pathogenesis. Here, we review our current knowledge regarding Tm's effect on the actomyosin interaction and the early molecular pathogenesis of Tm mutation related to HCM, DCM, and LVNC.
原肌球蛋白（Tm）是细肌丝的关键调节成分，在心肌协同激活机制中发挥核心作用。许多心脏 Tm 的突变与肥厚型心肌病（HCM）、扩张型心肌病（DCM）和左心室致密化不全（LVNC）有关。通过细肌丝提取/重组技术，我们能够将各种 Tm 突变体和蛋白异构体纳入肌纤维环境中，以研究它们在 Ca（2+）调节、横桥动力学和力产生中的作用。与其他体外和体内方法相比，细肌丝重组技术具有以下几个优势：（1）Tm 突变体和异构体被置于真实的肌纤维环境中，以显示它们在比简单蛋白复合物更高的水平上对 Ca（2+）调节、横桥动力学和力产生的影响；（2）在重组心肌中只研究 Tm 突变体的主要和直接影响；（3）可以研究致死性突变体而不会造成问题；（4）成本低廉。在转基因模型中，各种次级效应（肌原纤维排列紊乱、细胞外基质纤维化、蛋白磷酸化水平改变等）也会影响心肌的性能，使得很难分离突变的主要影响。我们对 Tm 的研究表明：（1）Tm 阳性增强肌球蛋白在“关闭状态”下与肌动蛋白的疏水相互作用，进而增强等长张力；（2）Tm 的七个周期性重复具有不同的功能，第 3 个周期对张力增强至关重要；（3）Tm 突变体一方面通过损害舒张，另一方面通过过度抑制 AM 相互作用，导致 HCM 和 DCM。Ca（2+）敏感性受无机磷酸盐、离子强度和组成蛋白磷酸化的影响；因此，它可能不是发病机制的主要原因。在这里，我们回顾了我们目前对 Tm 对肌球蛋白相互作用的影响以及与 HCM、DCM 和 LVNC 相关的 Tm 突变的早期分子发病机制的认识。
