The sinus node (SN) is the heart’s primary pacemaker and has a unique expression of pacemaking ion channels and immune cell markers. The role of microribonucleic acids (miRNAs) in control of ion channels and immune function of the sinus node is not well understood. We have recently shown that hsa-miR-486-3p downregulates the main pacemaking channel HCN4 in the SN. In addition, we recently demonstrated that immune cells are significantly more abundant in the SN compared to the right atrium. The aim of this study was to validate the previously predicted interactions between miRNAs and mRNAs of key Ca²⁺ ion channels (involved in peacemaking) and mRNA of TPSAB1—(a mast cells marker) using luciferase assay. We now show that miR-486 significantly downregulates Ca_v1.3, Ca_v3.1, and TPSAB1-mediated luciferase activity, while miR-938 significantly downregulates only TPSAB1-mediated luciferase activity. This makes miR-486-3p a potential therapeutic target in the treatment of SN dysfunctions.

(1) Background: The sinus node (SN) is the main pacemaker of the heart. It is characterized by pacemaker cells that lack mitochondria and contractile elements. We investigated the possibility that transcription factors (TFs) and microRNAs (miRs) present in the SN can regulate gene expression that affects SN morphology and function. (2) Methods: From human next-generation sequencing data, a list of mRNAs that are expressed at lower levels in the SN compared with the right atrium (RA) was compiled. The mRNAs were then classified into contractile, mitochondrial or glycogen mRNAs using bioinformatic software, RStudio and Ingenuity Pathway Analysis. The mRNAs were combined with TFs and miRs to predict their interactions. (3) Results: From a compilation of the 1357 mRNAs, 280 contractile mRNAs and 198 mitochondrial mRNAs were identified to be expressed at lower levels in the SN compared with RA. TFs and miRs were shown to interact with contractile and mitochondrial function-related mRNAs. (4) Conclusions: In human SN, TFs (MYCN, SOX2, NUPR1 and PRDM16) mainly regulate mitochondrial mRNAs (COX5A, SLC25A11 and NDUFA8), while miRs (miR-153-3p, miR-654-5p, miR-10a-5p and miR-215-5p) mainly regulate contractile mRNAs (RYR2, CAMK2A and PRKAR1A). TF and miR-mRNA interactions provide a further understanding of the complex molecular makeup of the SN and potential therapeutic targets for cardiovascular treatments.