REGULATION OF CARDIAC POTASSIUM CHANNEL GENE EXPRESSION

Kv4亚家族基因编码心肌细胞瞬时外向钾离子电流（Ito）的很大一部分。这些基因的差异表达显著地导致了位于心脏不同区域的肌细胞中动作电位波形的差异。例如，在左心室壁，心外膜区动作电位持续时间短于心内膜区，Kv4.2 mRNA和Ito的表达与这种动作电位波形的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发生了改变：在包括慢性血压升高在内的各种条件下产生的肥大肌细胞中，Ito的表达显著减少。我们和其他人的研究表明，伊藤的这种减少可能是由于Kv4的减少。X通道基因表达。因此，阐明控制Kv4亚家族基因表达的分子机制，为了解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基因的转录开始于不同组织的不同位点。具体来说，骨骼肌型起跳位点在右心室和左心室心外膜区显著，而脑型起跳位点在心房和心内膜区显著。此外，体内实验显示，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域的Kv4.2 mRNA，而心内膜区域没有。通过新生儿肌细胞培养，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下降。此外，这种激素诱导的Kv4.3 mRNA的下调对蛋白激酶C或钙调磷酸酶的抑制具有抗性。这些发现提示Kv4的表达。x基因的表达可能受到不同转录因子组的区域选择性控制，AngII可能介导高血压诱导的Kv4下调。X基因表达独立于肥大。因此，本研究建议使用高血压动物模型和新生儿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.