Cardiac hypertrophy and the subsequent heart failure (HF) kill more than 260,000 lives a year in The United States. Recent studies demonstrated that the pathological increase of intracellular calcium [Ca2+]i is a primary trigger for cardiac hypertrophy. Interestingly, inhibition of transient outward K+ channel (Ito) also causes cell growth and cardiac hypertrophy. Thus, Ito may participate in the functional regulation of [Ca2+]I in myocytes. We recently reported that blocking of Ito significantly facilitates L-type calcium current (ICa) in ventricular myocytes and we suggested that there is a functional association between Ito and ICa and that Ca2+/Calmodulin- dependent kinase II (CaMKII) is the underlying regulatory signal. The general hypothesis for this proposal is that a significant amount of CaMKII forms a molecular complex with Ito channel in its inactive form. The coupled CaMKII, however, will be dissociated from the complex and subsequently activated when Ito channel blockers binds to the channel or when Ito channel is knocked down, causing an increased ICa channel phosphorylation by CaMKII. Here, we extend our work to test these hypotheses. Experiments in specific aim 1 will test whether molecular coupling of Ito channel subunits prevents CaMKII from activation (i.e. that the CaMKII-Ito complex serves as a CaMKII reservoir) by over-expression of Kv4.3 in myocytes. The Kv4.3 binding sites in CaMKII molecule will be determined to test whether Kv4.3 couples to the functional sites of CaMKII, preventing CaMKII from activation.
In specific aim 2, we will test whether reduction of Ito channel expression or binding of 4-AP to Ito channel causes CaMKII dissociation and activation.
Specific aims 3 will demonstrate that the dissociation of CaMKII from the complex is an important mechanism for the functional activation of CaMKII and ICa in cardiomyocytes. This work will link Ito alteration to the calcium channel function and subsequent intracellular Ca2+ homeostasis and Ca2+-related signaling. The anticipated results from this study may decipher an important mechanism that implicates Ito down-regulation in the activation of CaMKII and ICa, the important contributors to the triggering and development of cardiac hypertrophy, HF, and lethal ventricular arrhythmias.
Currently, five million Americans are suffering from cardiac hypertrophy and heart failure, and the incidence of this morbid disease is increasing rapidly. Owing to the limited understanding of the mechanisms, the therapeutic means for treating these diseases are disappointingly ineffective. The proposed study implicates a cell membrane potassium channel (Ito) in regulating the cellular calcium influx channel (ICa) function, which may decipher a new mechanism for this morbid disease.
