Association of HERG and SCN5A Transcripts Regulates Ion Channel Expression and Function in Stem Cell Derived Cardiomyocytes

Abstract

The balance of ion currents that generate the cardiac electrical impulse is crucial for proper heart function. Perturbation in the expression of either the rapid delayed rectifying potassium channel gene (KCNH2) or the sodium channel gene (SCN5A) can result in dramatic cardiac arrhythmias and sudden death, but how cardiac cells coordinate expression of these channels is unknown. We recently demonstrated that alternate transcripts from the KCNH2 gene encoding hERG1a and 1b subunits are physically associated. This allows a proximity that promotes co-translational assembly and ensures proper subunit constituency. Here, we report that hERG transcripts are also associated with SCN5A mRNAs. Using single-molecule fluorescence in-situ hybridization (smFISH) in cardiomyocytes derived from induced pluripotentent stem cells (iPSC-CMs), we found that 45% of the actively translated hERG1a transcripts are associated with SCN5A mRNAs. Of those complexes, 70% were located within 10 μm from the nucleus, consistent with a distribution in active protein synthesis domains. RT-PCR after immunoprecipitation of either hERG or NaV1.5 channels revealed association of hERG1a, hERG1b, and SCN5A mRNAs and their corresponding proteins hERG1a, hERG1b and NaV1.5, indicative of a co-translational complex. Furthermore, the transcripts can be co-regulated in iPSC-CMs as indicated by a coordinate decrease of SCN5A, hERG1a and hERG1b transcript levels upon hERG1b-specific silencing. Whole-cell patch clamp revealed a corresponding reduction in the magnitude of the potassium IKr and sodium INa currents. The co-translational association and co-regulation of transcripts may represent a general mechanism by which cardiac cells coordinate expression and thus activity of different ion channel types. This mechanism may shed new light on cardiac rhythm maintenance and arrhythmogenic disorders, as well as other ion channel-related diseases.