Investigating the role of nuclear receptor NR4A1 in human cardiac fibroblasts and atrial fibrillation

Thesis

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia, posing clinical and therapeutic challenges. AF is characterised by electrical and structural remodelling in the atria. The latter involves atrial fibrosis that promotes AF maintenance and progression while hampering treatment. Atrial cardiac fibroblasts (ACFs) are central drivers of structural remodelling, existing in a heterogenous state. We recently identified three transcriptional ACF subpopulations in the human left atrial appendage, one highly enriched for nuclear receptor subfamily 4 group A member 1 (NR4A1). Although NR4A1 regulates fibrosis across multiple organs, its role in human ACF fibrotic responses and relevance to persistent AF remain unclear, forming the rationale of my thesis.

NR4A1 knockdown in human ACFs reduced fibrotic marker expression and suppressed proliferation, while pharmacological activation increased fibrotic marker protein levels. Bulk RNA-sequencing of NR4A1-deficient ACFs revealed downregulation of gene signatures for ECM remodelling and immunomodulation, and upregulation of those linked to metabolic and stress-responsive signalling. Interleukin-6 (IL-6), a top-ranked hub among NR4A1 knockdown differentially expressed genes, promoted collagen secretion in human ACFs. Prostaglandin A2 (PGA2), an endogenous NR4A1 ligand, upregulated proinflammatory cytokines (IL-6 and interleukin-1β) – an effect abolished by NR4A1 knockdown. ACFs from persistent AF patients exhibited higher total NR4A1 protein levels and increased cytoplasmic accumulation compared with sinus rhythm controls. IL-6 and PGA2 levels were also elevated in ACF secretomes from AF patients. Proteomics revealed distinct NR4A1 interactome profiles between sinus rhythm and persistent AF ACFs. Nucleoporin 214 was identified as an AF-specific NR4A1 interactor, potentially driving increased nucleocytoplasmic translocation of NR4A1 in AF ACFs.

Collectively, my findings identify NR4A1 has a pro-fibrotic role in human ACFs, acting through ECM remodelling and pro-inflammatory pathways, with elevated protein abundance and cytoplasmic enrichment in persistent AF. Targeting NR4A1 and its associated signalling pathways may represent a novel fibroblast-targeted therapeutic strategy for persistent AF.

Authors: Yiu, CHK

• DOI: 10.5287/ora-pzmzygzak
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: atrial fibrillation; NR4A1