REGULATION OF CARDIAC POTASSIUM CHANNEL GENE EXPRESSION

Kv4亚家族基因编码心肌细胞瞬时外向钾电流(Ito)的很大一部分。这些基因的不同表达显著地导致了位于心脏不同区域的心肌细胞动作电位波形的差异。例如，在左室壁，心外膜区的动作电位时程比心内膜区短，Kv4.2mRNA和Ito的表达与动作电位波形的这种差异有很好的相关性。除了不同的区域表达外，Ito的表达在病理条件下也会发生变化：在包括慢性血压升高在内的各种条件下，肥大的心肌细胞中Ito的表达显著减少。我们和其他人的研究表明，这种Ito的减少可能是由于Kv4.x通道基因表达的减少。因此，阐明控制Kv4亚家族基因表达的分子机制为了解心肌细胞如何实现区域特异性以及病理条件如何产生电异常提供了基本的见解。我们的初步结果表明，Kv4.2基因转录在不同组织中的不同位置开始。具体地说，骨骼肌型起始点在右心室和左室心外膜区显著，而脑型起始点在心房和心内膜区占主导地位。此外，体内实验表明，血管紧张素转换酶抑制剂卡托普利选择性地降低心外膜区Kv4.2mRNA的表达，而不是心内膜区。利用新生儿心肌细胞培养，我们还发现血管紧张素II(AngII)比α-肾上腺素能激动剂苯肾上腺素产生更显著的Kv4.3 mRNA的减少。此外，这种激素诱导的Kv4.3基因表达下调不受蛋白激酶C或钙调神经磷酸酶的抑制。这些发现提示Kv4.x基因的表达可能受不同转录因子的区域选择性调控，Angii可能介导高血压诱导的Kv4.x基因表达下调，而不依赖于肥厚。因此，这项建议是利用高血压动物模型和新生儿心肌细胞培养来测试这些可能性。

Kv4 subfamily genes encode a large portion of transient outward K+ current (Ito) in cardiac myocytes. Differential expression of these genes significantly contributes to differences in action potential waveforms in myocytes located in distinct regions of the heart. For example, in left ventricular wall, action potential duration is shorter in epicardial region than endocardial region, and expression of Kv4.2 mRNA and Ito are well correlated with this difference in action potential waveform. In addition to the differential regional expression, expression of Ito is altered under pathological conditions: a significant reduction in Ito is seen in hypertrophied myocytes produced by various conditions including a chronic increase in blood pressure. Our and others' studies indicate that this decrease in Ito is likely to be due to reductions in Kv4.x channel gene expression. Thus, elucidating the molecular mechanisms controlling expression of Kv4 subfamily genes provide fundamental insights into how regional specification of myocytes is achieved and how pathological conditions produce electrical abnormality. Our preliminary results indicate that Kv4.2 gene transcription starts at distinct sites in different tissues. Specifically, skeletal muscle-type start sites are significant in right ventricle and epicardial region of the left ventricle, whereas brain-type start sites predominate in atria and endocardial region. Furthermore, in vivo experiment revealed that administration of the angiotensin-converting enzyme inhibitor captopril selectively reduces Kv4.2 mRNA in epicardial region, but not in endocardial region. Using neonatal myocyte cultures, we also found that angiotensin II (AngII) produce more dramatic decrease in Kv4.3 mRNA than does the alpha-adrenergic agonist phenylephrine. Moreover, this hormone-induced down-regulation of Kv4.3 mRNA was resistant to inhibition of protein kinase C or calcineurin. These findings suggest that expression of Kv4.x gene expression may be region-selectively controlled by distinct sets of transcription factors and that AngII may mediate hypertension-induced down-regulation of Kv4.x gene expression independently of hypertrophy. Hence, this proposal is to test these possibilities using hypertension animal models and neonatal myocyte cultures.