Introduction: Cardiac ion channel dysregulation is a hallmark of heart failure. Consistently, the disease yields dramatic decline in Ito through loss in Kv4 and its auxiliary partner KChIP2. Notably, transcriptional changes in heart failure can be elicited through KChIP2 silencing without disease signaling, suggesting potential transcriptional capacity for KChIP2. Further, disparity between resulting transcript and protein patterns suggests a mechanism compatible with modified miRNA activity. Considering other members of the KChIP family behave as transcriptional repressors, we hypothesize that KChIP2 regulates discrete miRNAs which in turn regulate cardiac excitability.
Methods and Results: A miRNA microarray was conducted on neonatal rat ventricular myocytes (NRVM) following in vitro silencing of KChIP2 by siRNA, identifying the miR-34 family as potential transcriptional targets of KChIP2. Regulation, confirmed by quantitative PCR, showed reduction in miR-34a/b/c when over-expressing KChIP2 and increase following silencing. Luciferase assays were performed on the cloned promoter for miR-34b/c which reinforced direct KChIP2 repression on the miR-34b/c promoter. Furthermore, chromatin immunoprecipitation followed by PCR identified physical interaction of KChIP2 to the promoter site. Previous studies show modified expression of KChIP2 can lead to changes in several ion channel subunits. Therefore, we investigated if this was the consequence of KChIP2 regulation via miR-34. miR-34a/b/c precursors were expressed in NRVM which reduced transcript levels of Nav1.5 and Navβ1, and reduced protein levels for Kv4.3. Reflecting these changes, peak INa was reduced following miR precursor treatment. NRVMs were exposed to 100 μM phenylephrine for 48 hrs, significantly reducing KChIP2, Nav1.5, Navβ1, and Kv4.3, while elevating miR-34b/c. Returning KChIP2 expression by adenovirus normalized these changes back towards baseline, implicating the physiologic relevance of this pathway.
Conclusion: These observations describe a novel mechanism where KChIP2 regulates a host of cardiac genes through transcriptional control of miRNAs, potentially explaining electrical remodeling observed in disease states where KChIP2 is reduced.