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Mechanisms of microRNA-mediated regulation of the rapid delayed rectifier potassium current, IKr, during sustained beta-adrenergic receptor stimulation
Heart failure (HF) is a chronic clinical syndrome characterized by symptoms including breathlessness, fatigue, swelling of the ankles, and signs such as edema pulmonary crackles etc. During HF, pathogenic mechanisms including hemodynamic overload, ventricular remodeling, aberrant calcium handling, excessive neurohormonal stimulation contribute to the worsening and progression of the condition. Ventricular arrhythmias are the common cause of sudden cardiac death (SCD) in HF patients.
Hyperactivation of the sympathetic nervous system (SNS), a characteristic of HF, causes an increase in circulating catecholamines which becomes detrimental to-adrenergic receptors (-AR) leading to signaling dysfunction, and decrease in contractility and the ionotropic reserve. Expression of calcium/calmodulin-dependent protein kinase II (CaMKII), a downstream effector of-AR and a key regulator of calcium homeostasis, has been shown to be enhanced in HF. CaMKII-mediated mechanisms have been demonstrated to contribute to cardiac remodeling, arrhythmias by pathological regulation of ion channels, and contractile dysfunction.
The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the voltage-gated potassium channel that conduct the rapid component of the delayed rectifier potassium current, IKr. The gating kinetics of IKr makes it a crucial determinant of the duration of the plateau phase of atrial and ventricular action potential (AP). Reduced IKr density due to loss-of-function mutations or pharmacological blockage of hERG channels precipitate arrhythmias. Downregulation of IKr density and protein have been reported in HF. Recent studies suggest that microRNAs (miRNAs) are involved in pathological downregulation of hERG.
miRNA are small non-coding RNAs of approximately 22 nucleotides in length that function as gene expression regulatory elements by repression translation. Aberrant miRNA expression has associated with cancer, cardiovascular, autoimmune, and inflammatory disorders.
The overarching objective of this study is to investigate the mechanisms of CaMKII-mediated regulation of hERG function, including assessment of an interplay with miR-362-3p during sustained β-AR stimulation. In Specific Aim 1, the effect of CaMKII activation through sustained β-AR stimulation on hERG function and miR-362-3p expression will be assessed. The mechanism of miR-362-3p upregulation will be evaluated in Specific Aim 2, and in Specific Aim 3, the interactome of miR-362-3p and binding sites will be characterized and predicted, respectively.
Whole-cell, voltage clamp electrophysiology experiments were performed in HEK 293 cells stably expressing hERG (hERG-HEK) and both hERG and wild-type CaMKIIδ
(hERG/CaMKII-HEK) following treatment with isoproterenol for 48 hours, and after transfection with miR-362-3p. The effect of CaMKII activation on miR-362-3p was assessed using real-time quantitative polymerase chain reaction (RT-qPCR). Total RNA was isolated 48 hours after isoproterenol treatment and the TaqMan assay was used to reverse transcribe and analyze miR-362-3p expression. Cells were transfected with cJun siRNA and precursor miR-362-3p to assess the role of cJun miR-362-3p upregulation during sustained β-AR stimulation with isoproterenol. The interactome of miR-362-3p was assessed in both cell lines using enhanced crosslinking immunoprecipitation (eCLIP) assay. miR-362-3p binding sites were predicted using RNAStructure Duplexfold after identification of miR-362-3p chimeric molecules from eCLIP experiment. Interaction analysis was performed using GeneMania in Cytoscape to identify genes that were potentially downregulated by miR-362-3p and been reported to interact with hERG.
In Specific Aim 1, the effect of sustained β-AR stimulation on hERG currents and endogenous miR-362-3p was assessed in hERG-HEK and hERG/CaMKII-HEK cells. Using whole-cell voltage clamp electrophysiology, we demonstrated that 48 hours treatment with 100 nM isoproterenol reduced hERG currents in hERG/CaMKII-HEK cells (p = 0.032) but had no effect on the voltage dependence of activation (p = 0.61) relative to control vehicle. Isoproterenol treatment for 48 hours, however, had no effect on hERG currents (p = 0.58) and the voltage dependence of activation (p = 0.99) in hERG-HEK cells. The effect of sustained isoproterenol treatment on miR-362-3p was also assessed using RT-qPCR. In hERG/CaMKII cells, 48 hours isoproterenol treatment increased miR-362-3p expression (2.3 folds; p = 0.038) relative to control vehicle. hERG/CaMKII-HEK cells were also treated with 500 nM KN-93 or its inactive analogue, KN-92, in an attempt to reverse CaMKII effect on miR-362-3p expression. Treatment with KN-93 decreased miR-362-3p expression (0.5-fold; p = 0.002) relative KN-92 treatment. Isoproterenol treatment had no effect on miR-362-3p expression in hERG-HEK cells (p = 0.38).
The regulatory mechanism of miR-362-3p expression was evaluated in Specific Aim 2. The role of an activator protein-1 (AP-1)-like sequence located at 98 base pairs upstream of miR-362-3p transcription start site was probed using siRNA inhibition of cJun, a central protein of the AP-1 complex, and deletion of the site sequence. The effect of exogenous miR-362-3p on hERG currents were first assessed. Precursor miR-362-3p decreased hERG currents (p = 0.003) compared to control plasmid. The effect of CaMKII overexpression was also assessed on exogenous miR-363-3p expression. Isoproterenol treatment in hERG/CaMKII-HEK cells transfected with precursor miR-362-3p increased mature miR-362-3p expression (0.029) compared to control vehicle treatment. Inhibition of cJun inhibition with cJun-specific siRNA decreased mature miR-362-3p expression (0.5-fold; p = 0.027) compared to scramble siRNA in hERG-HEK cells. In hERG-HEK cells transfected with mutated precursor miR-362-3p (AP-1-like site deleted), cJun inhibition with siRNA had no effect on miR-362-3p expression (p = 0.40).
The focus of Specific Aim 3 was to characterize the interactome of miR-362-3p as well as predict the miRNA response element (MRE) of its target mRNAs using enhanced crosslinking immunoprecipitation. A network analysis was also performed to identify miR-362-3p targets that have been reported to interact with hERG. Approximately 23% of miR-362-3p mRNA targets from the eCLIP assay have also been catalogued in miRNA database, TargetScanHuman, as miR-362-3p targets. miR-362-3p chimeric molecules with 853 unique targets, of which 75 were identified to interact with hERG through the network analysis. Four unique chimeric molecules between miR-362-3p and hERG mRNA were identified, but the interactions were non-canonical (located in the coding sequence of hERG and outside the seed region of miR-362-3p). Thirty five of the 75 miR-362-3p targets that were identified to interact had a chimeric read ≥ 3, a cutoff number indicating non-random chimeric formation. Using RNAStructure DuplexFold, miR-362-3p was predicted to form canonical binding with 12 of 35 mRNA targets. HSPA4, a heat shock protein involved in the maturation and trafficking of hERG, was identified in a canonical interaction (8-mer) with miR-362-3p.
Sustained β-AR stimulation increases miR-362-3p expression and decreases hERG currents in CaMKII overexpressing cells. cJun mediates miR-362-3p upregulation by interacting with an AP-1-like sequence upstream of miR-362-3p transcription start site. Pathological regulation of IKr by CaMKII mediated by miR-362-3p during sustained-AR may contribute to increased risk of arrhythmias in states of increase catecholaminergic activity, such as HF.
- Doctor of Philosophy
- Pharmacy Practice
- West Lafayette