Kischel P, Bastide B, Muller M, Dubail F, Offredi F, Jin J P, Mounier Y, Martial J
Laboratoire de Biologie Moléculaire et Génie Génétique, Allée de la Chimie 3, Campus du Sart-Tilman, Bât. B6, 4000 Liège, Belgium.
Am J Physiol Cell Physiol. 2005 Aug;289(2):C437-43. doi: 10.1152/ajpcell.00365.2004. Epub 2005 Mar 23.
We investigated the expression and functional properties of slow skeletal troponin T (sTnT) isoforms in rat skeletal muscles. Four sTnT cDNAs were cloned from the slow soleus muscle. Three isoforms were found to be similar to sTnT1, sTnT2, and sTnT3 isoforms described in mouse muscles. A new rat isoform, with a molecular weight slightly higher than that of sTnT3, was discovered. This fourth isoform had never been detected previously in any skeletal muscle and was therefore called sTnTx. From both expression pattern and functional measurements, it appears that sTnT isoforms can be separated into two classes, high-molecular-weight (sTnT1, sTnT2) and low-molecular-weight (sTnTx, sTnT3) isoforms. By comparison to the apparent migration pattern of the four recombinant sTnT isoforms, the newly described low-molecular-weight sTnTx isoform appeared predominantly and typically expressed in fast skeletal muscles, whereas the higher-molecular-weight isoforms were more abundant in slow soleus muscle. The relative proportion of the sTnT isoforms in the soleus was not modified after exposure to hindlimb unloading (HU), known to induce a functional atrophy and a slow-to-fast isoform transition of several myofibrillar proteins. Functional data gathered from replacement of endogenous troponin complexes in skinned muscle fibers showed that the sTnT isoforms modified the Ca(2+) activation characteristics of single skeletal muscle fibers, with sTnT2 and sTnT1 conferring a similar increase in Ca(2+) affinity higher than that caused by low-molecular-weight isoforms sTnTx and sTnT3. Thus we show for the first time the presence of sTnT in fast muscle fibers, and our data show that the changes in neuromuscular activity on HU are insufficient to alter the sTnT expression pattern.
我们研究了大鼠骨骼肌中慢肌肌钙蛋白T（sTnT）亚型的表达及功能特性。从慢肌比目鱼肌中克隆出四个sTnT cDNA。发现其中三种亚型与小鼠肌肉中描述的sTnT1、sTnT2和sTnT3亚型相似。还发现了一种新的大鼠亚型，其分子量略高于sTnT3。此前在任何骨骼肌中均未检测到这种第四种亚型，因此将其命名为sTnTx。从表达模式和功能测量结果来看，sTnT亚型可分为两类，即高分子量（sTnT1、sTnT2）和低分子量（sTnTx、sTnT3）亚型。与四种重组sTnT亚型的表观迁移模式相比，新描述的低分子量sTnTx亚型主要且典型地在快肌中表达，而高分子量亚型在慢肌比目鱼肌中更为丰富。比目鱼肌中sTnT亚型的相对比例在经历后肢卸载（HU）后未发生改变，已知后肢卸载会导致功能性萎缩以及几种肌原纤维蛋白从慢肌亚型向快肌亚型的转变。从对去皮肤肌纤维中内源性肌钙蛋白复合物进行替换所收集到的功能数据表明，sTnT亚型改变了单个骨骼肌纤维的Ca(2+)激活特性，其中sTnT2和sTnT1赋予的Ca(2+)亲和力增加程度相似，高于低分子量亚型sTnTx和sTnT3所引起的增加程度。因此，我们首次证明了快肌纤维中存在sTnT，并且我们的数据表明后肢卸载引起的神经肌肉活动变化不足以改变sTnT的表达模式。
