Investigating the role of an orphan nuclear receptor NR4A1 in atrial fibrillation

Abstract Introduction Atrial fibrosis is a major remodelling process in atrial fibrillation (AF). The lack of clinically effective drug targets, due to insufficient understanding of the mechanisms of cardiac fibrosis, hampers the treatment of AF. Orphan nuclear receptor subfamily 4 group A member 1 (NR4A1) is linked to fibrotic disease in multiple organs, however, its role in cardiac fibrosis and AF is yet to be demonstrated. Recent transcriptomic profiling revealed that NR4A1 expression is downregulated in human atrial cardiac fibroblasts (ACFs) in the presence of persistent AF. However, its mechanistic role in atrial fibrogenesis and AF is unknown. Purpose To investigate the expression profile and function of NR4A1 in human ACFs and provide insights into the role of NR4A1 in the context of persistent AF. Methods Human ACFs were isolated (enzymatic digestion) from right and left atrial appendages of 24 patients with persistent AF and controls in sinus rhythm (SR), who had elective heart surgery. NR4A1 knockdown in ACFs was achieved using NR4A1-siRNA. Gene and protein expression were determined by qPCR and western blot respectively, while BrdU assay and scratch assay were used to evaluate cell proliferation and migration. Atrial fibroblasts subpopulations were identified by single-nucleus RNA-sequencing. Subcellular localisation of NR4A1 was assessed by immunostaining, analysed by ImageJ. Results Validation of RNA-sequencing data confirmed a reduction of NR4A1 gene expression in human ACFs in persistent AF by 25% (p=0.029, Fig. 1A) and the NR4A1 predominant abundance in one of three identified subclusters of human ACFs (NR4A1+NAMPT+). The siRNA-mediated knockdown of NR4A1 in ACFs suppressed gene and protein expression of important component of extracellular matrix collagen-1 (Fig. 1B and Fig. 1C) and fibronectin protein (but not mRNA). The NR4A1-deficient cells also had decreased alpha-smooth muscle actin (αSMA) protein and gene expression (Fig. 1D and Fig. 1E), reduced periostin gene expression in the absence of changes in its protein. Furthermore, the NR4A1-deficiency reduced proliferation (by 18%, p=0.005, Fig. 1F) and accelerated migration (p=0.019, Fig. 1G) of human ACFs. These effects were independent of TGF-β1 stimulation, unlike reported in rat neonatal cardiac and dermal fibroblasts. Despite decreased NR4A1 gene expression, patients with AF had doubled NR4A1 protein levels compared to SR-ACFs (p=0.049, Fig. 2A). This was associated with abnormal 1.8-fold increase in the receptor subcellular localisation (p=0.032, Fig. 2B and Fig. 2C), as opposed to the physiological nuclear localisation observed in ACFs in the absence of AF. Conclusion While under physiological conditions, NR4A1 potently activates profibrotic processes in human ACFs, persistent AF patients have increased but abnormally distributed NR4A1 protein in atrial fibroblasts, whose consequences are yet to be determined in order to develop new therapies for AF.Figure 1Figure 2