Akt phosphorylation and arrhythmogenic modification of the cardiac sodium channel

AKT是一种丝氨酸-苏氨酸磷酸酶，参与多种信号转导，包括细胞增殖和凋亡。Akt基因在小鼠心脏中的选择性过表达会导致心肌肥大，但偶尔也会有一些转基因小鼠在心肌肥厚完全建立之前突然死亡，原因不明，提示在Akt诱导过程中出现了危及生命的心律失常。本研究的目的是阐明Akt诱导的心肌肥厚和致死性心律失常的生物物理和生化机制。我们关注的是心脏钠通道Nav1.5，因为Nav1.5与其他心脏离子通道不同，它有两个Akt磷酸化识别序列(RXRXXT/S)。在HEK 293细胞中，Navl.5与Akt或其磷酸化缺陷突变体Akt-3A(K179A+T308A+S478A)共表达，并用全细胞膜片钳技术测定其生物物理性质。Akt可显著增加Nav.15的电流密度，但Akt-3A对其无明显影响。激活曲线的电压依赖关系，但不是失活，显著地向去极化方向移动。此外，标志标记的Nav.15与抗磷酸化Akt抗体的免疫共沉淀显示Akt而不是Akt-3A增加了Akt与心肌钠通道的联系。这些结果表明Akt改变了Na通道的生物物理性质和表达水平，导致了致死性心律失常的发生。为了阐明心脏Akt信号通路与致死性心律失常之间的机制联系，还需要进一步的研究，包括在AID转基因小鼠体内记录心电。

Akt is a serine-threonine phosphatase implicated in wide variety of signal transductions including cell proliferation and apoptosis. Selective overexpression of Akt in mice heart results in cardiac hypertrophy, but occasionally, some transgenic mice die suddenly with unknown causes before the cardiac hypertrophy is fully established, suggesting life-threatening arrhythmias during Akt-induction. The purpose of the study was to elucidate the biophysical and biochemical mechanisms underlying Akt-induced cardiac hypertrophy and lethal arrhythmias. We focused on cardiac Na channel Navl.5, because Navl.5, unlike other cardiac ion channels, has two Akt phosphorylation recognition sequences (RXRXXT/S). Navl.5 was coexpressed in HEK 293 cells together with Akt or its phosphorylation deficient mutant Akt-3A (K179A+T308A+S478A), and the biophysical properties were determined by whole-cell patch clamp technique. Current density of Nav.15 was significantly increased by Akt but not by Akt-3A. Voltage-dependence of the activation curve, but not inactivation, was significantly shifted in the depolarizing direction. Furthermore, coimmunoprecipitation of FLAG-tagged Nav.15 with anti-phosphorylated-Akt antibodies revealed that Akt but not Akt-3A increased the association of Akt with cardiac Na channel. These results suggest that Akt modified the biophysical properties and expression levels of Na channel, leading to induce of lethal arrhythmias. Further study including in vivo ECG recording in Aid transgenic mice are required to elucidate the mechanistic link between cardiac Akt signaling pathway and the lethal arrhythmias.