Department of Cell Biology and Anatomy, Chicago Medical School, Rosalind Franklin University of Medicine and Science, 3333 Green Bay Road, North Chicago, IL 60064, USA.
Exp Cell Res. 2010 Apr 1;316(6):1039-49. doi: 10.1016/j.yexcr.2010.01.008. Epub 2010 Jan 11.
Vertebrate skeletal muscle fiber types are defined by a broad array of differentially expressed contractile and metabolic protein genes. The mechanisms that establish and maintain these different fiber types vary throughout development and with changing functional demand. Chicken skeletal muscle fibers can be generally categorized as fast and fast/slow based on expression of the slow myosin heavy chain 2 (MyHC2) gene in fast/slow muscle fibers. To investigate the cellular and molecular mechanisms that control fiber type formation in secondary or fetal muscle fibers, myoblasts from the fast pectoralis major (PM) and fast/slow medial adductor (MA) muscles were isolated, allowed to differentiate in vitro, and electrically stimulated. MA muscle fibers were induced to express the slow MyHC2 gene by electrical stimulation, whereas PM muscle fibers did not express the slow MyHC2 gene under identical stimulation conditions. However, PM muscle fibers did express the slow MyHC2 gene when electrical stimulation was combined with inhibition of inositol triphosphate receptor (IP3R) activity. Electrical stimulation was sufficient to increase nuclear localization of expressed nuclear-factor-of-activated-T-cells (NFAT), NFAT-mediated transcription, and slow MyHC2 promoter activity in MA muscle fibers. In contrast, both electrical stimulation and inhibitors of IP3R activity were required for these effects in PM muscle fibers. Electrical stimulation also increased levels of peroxisome-proliferator-activated receptor-gamma co-activator-1 (PGC-1alpha) protein in PM and MA muscle fibers. These results indicate that MA muscle fibers can be induced by electrical stimulation to express the slow MyHC2 gene and that fast PM muscle fibers are refractory to stimulation-induced slow MyHC2 gene expression due to fast PM muscle fiber specific cellular mechanisms involving IP3R activity.
脊椎动物骨骼肌纤维类型由广泛表达的收缩和代谢蛋白基因定义。在发育过程中，随着功能需求的变化，建立和维持这些不同纤维类型的机制也在不断变化。根据慢肌球蛋白重链 2（MyHC2）基因在快/慢肌纤维中的表达，鸡骨骼肌纤维通常可分为快肌和快/慢肌。为了研究控制次级或胎儿肌肉纤维形成的细胞和分子机制，从快胸大肌（PM）和快/慢内收肌（MA）肌肉中分离出成肌细胞，允许其在体外分化，并进行电刺激。电刺激可诱导 MA 肌肉纤维表达慢肌 MyHC2 基因，而 PM 肌肉纤维在相同的刺激条件下不表达慢肌 MyHC2 基因。然而，当电刺激与肌醇三磷酸受体（IP3R）活性抑制相结合时，PM 肌肉纤维确实会表达慢肌 MyHC2 基因。电刺激足以增加表达核因子活化 T 细胞（NFAT）的核定位、NFAT 介导的转录和慢肌 MyHC2 启动子活性在 MA 肌肉纤维中。相比之下，电刺激和 IP3R 活性抑制剂都需要这些作用在 PM 肌肉纤维中。电刺激还增加了 PM 和 MA 肌肉纤维中过氧化物酶体增殖物激活受体-γ共激活因子 1（PGC-1alpha）蛋白的水平。这些结果表明，电刺激可以诱导 MA 肌肉纤维表达慢肌 MyHC2 基因，而快 PM 肌肉纤维由于涉及 IP3R 活性的快 PM 肌肉纤维特异性细胞机制而对刺激诱导的慢肌 MyHC2 基因表达产生抗性。
