Vali S, Carlsen R, Pessah I, Gorin F
Department of Neurology, Center for the Neurosciences, School of Medicine, University of California, Davis, California, USA.
J Cell Physiol. 2000 Nov;185(2):184-99. doi: 10.1002/1097-4652(200011)185:2<184::AID-JCP3>3.0.CO;2-T.
Nerve-evoked contractile activity in skeletal muscle regulates transcript and protein levels of many metabolic genes in a coordinate fashion, including the muscle isozyme of glycogen phosphorylase (MGP). Cellular signaling mechanisms mediating the activity-dependent modulation of MGP transcript levels were investigated in a spontaneously contractile rat skeletal muscle cell line (Rmo). Mechanisms regulating MGP mRNA levels in Rmo myotubes were compared with those previously shown to modulate the gene encoding the alpha subunit of the acetylcholine receptor (alphaAChR). Reducing the resting membrane potential from -78 to -30 mV, either electrochemically (KCl) or by increasing Na(+) permeability (veratridine): (1) prevented activation of transverse tubules, (2) impeded calcium release by the sarcoplasmic reticulum (SR), and (3) blocked Rmo contractility. MGP mRNA levels decreased to 30% of control levels and alphaAChR levels increased to 350% following 24 h of depolarization. Differing mechanisms appear to mediate this voltage-dependent regulation of MGP and alphaAChR. Inhibition of SR calcium efflux selectively decreased MGP mRNA levels by 30-50% when using dantrolene, thapsigargin, or a dose of ryanodine shown to inactivate Ca(2+)-induced SR Ca(2+) release (CICR). By contrast, blockade of voltage sensors in transverse tubules with nifedipine, a dihydroaminopyridine (DHAP) antagonist, selectively increased alphaAChR mRNA levels by twofold. These data indicate that the voltage-dependent regulation of AChR gene expression differs from that modulating the MGP gene. KCl-induced depolarization and dantrolene both inhibit pulsatile SR Ca(2+) efflux in Rmo myotubes, but by differing mechanisms. Depolarization and dantrolene comparably reduced MGP mRNA levels and decreased MGP transcript stability from a t(1/2) of 24 h to 14.5 and 16 h, respectively. Reduced transcript stability can account for the observed reduction in mRNA levels of MGP in noncontractile Rmo myotubes and could be a significant regulatory mechanism in skeletal muscle that coordinates the activity-dependent expression of MGP with other glycogenolytic genes.
骨骼肌中神经诱发的收缩活动以协调的方式调节许多代谢基因的转录本和蛋白质水平，包括糖原磷酸化酶的肌肉同工酶（MGP）。在一种自发收缩的大鼠骨骼肌细胞系（Rmo）中，研究了介导MGP转录本水平活性依赖性调节的细胞信号机制。将Rmo肌管中调节MGP mRNA水平的机制与先前显示可调节乙酰胆碱受体α亚基（αAChR）编码基因的机制进行了比较。通过电化学方法（KCl）或将膜电位从-78 mV降低到-30 mV，或通过增加Na(+)通透性（藜芦碱）：（1）阻止横管的激活，（2）阻碍肌浆网（SR）释放钙，以及（3）阻断Rmo的收缩性。去极化24小时后，MGP mRNA水平降至对照水平的30%，而αAChR水平升至对照水平的350%。不同的机制似乎介导了MGP和αAChR的这种电压依赖性调节。当使用丹曲林、毒胡萝卜素或一定剂量的ryanodine（已证明可使Ca(2+)诱导的SR Ca(2+)释放（CICR）失活）时，抑制SR钙外流选择性地使MGP mRNA水平降低30 - 50%。相比之下，用二氢氨基吡啶（DHAP）拮抗剂硝苯地平阻断横管中的电压传感器，可使αAChR mRNA水平选择性增加两倍。这些数据表明，AChR基因表达的电压依赖性调节与调节MGP基因的调节不同。KCl诱导的去极化和丹曲林均抑制Rmo肌管中脉动性SR Ca(2+)外流，但机制不同。去极化和丹曲林同等程度地降低MGP mRNA水平，并使MGP转录本稳定性分别从24小时的半衰期降至14.5小时和16小时。转录本稳定性降低可以解释在非收缩性Rmo肌管中观察到的MGP mRNA水平降低，并且可能是骨骼肌中一种重要的调节机制，可将MGP的活性依赖性表达与其他糖原分解基因协调起来。
