Mitral regurgitation induces a unique fibroblast population associated with atrial fibrillation susceptibility

Background: Mitral regurgitation (MR) is a major risk factor for the development of atrial fibrillation (AF), yet the molecular mechanisms linking volume overload to arrhythmogenic remodeling remain poorly understood. Although fibrosis has long been considered the primary substrate for AF, increasing evidence suggests that fibroblast heterogeneity and cell-cell interactions may play important roles in disease progression. Methods: MR was created endovascularly by chordal avulsion in 12 dogs with 6 controls. AF inducibility was assessed by transvenous burst pacing, left atrial volume by echocardiography, and collagen content by Masson trichrome and picrosirius red staining. Single-nucleus RNA sequencing (snRNA-seq) was performed on left atrial posterior wall tissue from control, 4-week, and 6-month MR animals. Fibroblast subpopulations and fibroblast-cardiomyocyte communication were analyzed and markers validated by RNA in situ hybridization in all 18 animals. Results: MR resulted in progressive left atrial dilation, but neither the change in left atrial volume from baseline nor total collagen burden correlated with the inducibility of AF (n=6 each). SnRNA-seq resolved seven major cardiac cell populations and identified four transcriptionally distinct fibroblast populations (NOX4/GRIA4, PCOLCE2, ADRB2/HCN1, PTX3/ICAM1). Fibroblast composition shifted markedly: matrix-associated PCOLCE2 fibroblasts starkly declined by 6 months, whereas inflammatory-associated PTX3/ICAM1 fibroblasts expanded stepwise over time. Cardiomyocyte-to-fibroblast signaling, dominated by PTPRM and LAMA2, was progressively redirected toward PTX3/ICAM1 fibroblasts. RNAscope confirmed a stepwise rise in ICAM1 transcripts and higher ICAM1 in AF-inducible than non-inducible animals. Conclusions: In a canine model of MR, the inducibility of AF was associated with fibroblast state remodeling rather than with atrial dilation or collagen burden. Progressive expansion of inflammatory-associated PTX3/ICAM1 fibroblasts, together with reorganized fibroblast-cardiomyocyte signaling, defines a candidate arrhythmogenic mechanism and therapeutic target in MR.