Granzier H, Helmes M, Cazorla O, McNabb M, Labeit D, Wu Y, Yamasaki R, Redkar A, Kellermayer M, Labeit S, Trombitás K
Department of Veterinary and Comparative Anatomy, Pharmacology and Physiology, Washington State University, Pullman, USA.
Adv Exp Med Biol. 2000;481:283-300; discussion 300-4. doi: 10.1007/978-1-4615-4267-4_17.
Titin is a giant filamentous polypeptide of multi-domain construction spanning between the Z- and M-lines of the sarcomere. As a result of differential splicing, length variants of titin are expressed in different skeletal and cardiac muscles. Here we first briefly review some of our previous work that has revealed that titin develops force in sarcomeres either stretched beyond their slack length (passive force) or shortened to below the slack length (restoring force) and that titin's force underlies a large fraction of the diastolic force of cardiac muscle. Next we present our mechanical and immunoelectron microscopical (IEM) studies of skeletal and cardiac muscles that express titin isoforms. The previously deduced molecular properties of titin were used to model titin's extensible region in the sarcomere as serially linked WLCs: rigid segments (containing folded Ig/Fn domains) and more flexible segments (PEVK segment). The model was tested on skeletal muscle fibers that express titin isoforms with tandem Ig and PEVK length variants. The model adequately predicts titin's behavior along a wide sarcomere length range in skeletal muscle, but at long sarcome lengths (SLs), predicted forces are much higher than those determined experimentally. IEM reveals that this may result from Ig domain unfolding. Experiments were also performed on cardiac myocytes from mouse and cow that express predominantly a small cardiac titin isoform (N2B titin) or a large isoform (N2BA titin), respectively. The passive tension-SL relation of myocytes was found to increase more steeply with SL in mouse than in cow. IEM revealed an additional source of extensibility within both of these cardiac titins: the unique N2B sequence (absent in skeletal muscle). Furthermore, the PEVK segment of the N2BA isoform extended to a maximal length of approximately 200 nm, as opposed to approximately 60 nm for the N2B isoform. We propose that, along the physiological SL range, the long PEVK segment found in N2BA titins results in a low PEVK fractional extension and that this underlies the lower passive tensions of N2BA-expressing cow myocytes.
肌联蛋白是一种巨大的丝状多结构域多肽，横跨肌节的Z线和M线。由于可变剪接，肌联蛋白的长度变体在不同的骨骼肌和心肌中表达。在此，我们首先简要回顾我们之前的一些工作，这些工作揭示了肌联蛋白在肌节中被拉伸超过其松弛长度时（被动力）或缩短至松弛长度以下时（恢复力）会产生力，并且肌联蛋白的力构成了心肌舒张期力的很大一部分。接下来，我们展示了对表达肌联蛋白异构体的骨骼肌和心肌进行的力学和免疫电子显微镜（IEM）研究。先前推导的肌联蛋白分子特性被用于将肌节中肌联蛋白的可伸展区域建模为串联连接的WLC：刚性片段（包含折叠的Ig/Fn结构域）和更灵活的片段（PEVK片段）。该模型在表达具有串联Ig和PEVK长度变体的肌联蛋白异构体的骨骼肌纤维上进行了测试。该模型能够充分预测骨骼肌在较宽肌节长度范围内肌联蛋白的行为，但在长肌节长度（SL）时，预测的力远高于实验测定的力。IEM显示这可能是由于Ig结构域展开所致。我们还对分别主要表达小的心肌肌联蛋白异构体（N2B肌联蛋白）或大的异构体（N2BA肌联蛋白）的小鼠和牛的心肌细胞进行了实验。发现小鼠心肌细胞的被动张力-SL关系随SL增加比牛的更陡峭。IEM揭示了这两种心肌肌联蛋白中另一个可伸展的来源：独特的N2B序列（骨骼肌中不存在）。此外，N2BA异构体的PEVK片段延伸至最大长度约200nm，而N2B异构体约为60nm。我们提出，在生理SL范围内，N2BA肌联蛋白中发现的长PEVK片段导致PEVK分数延伸较低，这是表达N2BA的牛心肌细胞被动张力较低的原因。
