Biogenesis of hERG1a/1b ion channels in health and disease model cardiomyocytes

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PROJECT SUMMARY/ABSTRACT Cardiac IKr is a critical repolarizing potassium current shaping the human ventricular action potential. It is conducted by heteromeric assemblies of the human ether-à-go-go-related gene (hERG1) 1a and 1b subunits. These subunits are encoded by alternate transcripts of the hERG/KCNH2 gene and differ only in their amino- terminal regions. hERG1a/1b heteromerization is vital for normal CM function, as the imbalance of subunit expression and/or function results in cellular pro-arrhythmic behaviors. hERG1a/1b assembly is mediated by the co-translational association of the encoding mRNAs in HEK293 cells, cardiomyocytes derived from human induced pluripotent stem cells (hiPSC-CMs), and human myocardium. Evidence suggests that interaction between the nascent proteins is not required for the co-translational complex assembly. This grant's preliminary findings indicate that this complex assembly occurs post-transcriptionally and is promoted by direct interactions between hERG1a and 1b mRNAs governed by their secondary structures. In preliminary studies, RNA binding proteins DDX3X and DDX5 were identified as part of the complex, and purified DDX3X promoted hERG1a/1b mRNAs' association in vitro. In the K99 phase, I will define the mRNA structural features promoting the co-translational association and determine the affinity and energies of the RNA/RNA interaction using in vitro systems, isothermal calorimetry (ITC), mutagenesis, hybrid protein-RNA immunoprecipitation (RIP), and live-cell imaging. I will also determine whether DDX3X and DDX5 affect hERG1a and 1b mRNAs stability, translation, and association in hiPSC-CMs using qPCR, electrophysiology, Western Blot, ribosome profiling, RIP, and single molecule fluorescent in situ hybridization (smFISH). I will use quantitative ITC and in vitro reconstitution approaches to determine the specificity, affinity, and energies of the interaction between purified DDX3X and DDX5 with hERG1a and 1b mRNAs. I will also evaluate if DDX3X and DDX5 promote the association of the mRNAs in in vitro systems. In the R00 phase, I will determine whether the stability, translation, and association of hERG1a and 1b mRNAs are impaired in arrhythmias associated with type 2 long QT syndrome (LQT2). I will use hiPSC-CM disease models to evaluate half-life, translation rate, and association of the mRNAs with qPCR, ribosome profiling, RIP, and smFISH. These experiments will contribute to understanding ion channel biogenesis and elucidate molecular mechanisms underlying LQT2 related arrhythmias. This proposal is designed to fulfill my short-term goals of expanding my skills in cardiovascular research and biophysics and transitioning into the independent phase of my career. This will ultimately allow me to obtain my long-term purpose of linking RNA and ion channel biophysics to translational cardiovascular research.