Valvular Leaflets Are Not Innocent Bystanders: Divergent Fibrotic Remodeling Accompanies Functional Mitral and Tricuspid Regurgitation.

BACKGROUND

Functional valve regurgitation (FVR) progression has traditionally been attributed to external anatomic alterations, without consideration of leaflet-intrinsic pathology. Emerging evidence now implicates valvular leaflets as active contributors rather than passive bystanders in FVR pathogenesis. Systematic investigations into leaflet-specific pathomechanisms remain absent for major FVR subtypes, particularly functional mitral regurgitation (FMR) and functional tricuspid regurgitation (FTR).

METHODS

The association of FVR with clinical parameters was analyzed in a heart transplantation cohort. Comprehensive microscopic pathology evaluated fibrotic remodeling in mild, moderate, and severe FMR/FTR leaflets, validated in an independent bulk RNA sequencing cohort (FMR/FTR: n=41 each). Single-cell RNA sequencing was performed on 19 FVR leaflets (FMR: 3 mild/2 moderate/3 severe; FTR: 5 mild/3 moderate/3 severe). Subsequent analyses (cluster annotation, cellular proportions, trajectory inference, and cell-cell communication) explored cellular mechanisms of fibrotic remodeling in FVR, focusing on unique and shared changes between FMR and FTR. Histopathology and bulk transcriptomics validated single-cell RNA sequencing findings. Primary valvular endothelial cells and valvular interstitial cells from FVR patients underwent pharmacological intervention. A monocrotaline-induced rat pulmonary hypertension model established FTR, followed by pharmacological treatment to assess leaflet-directed therapy efficacy.

RESULTS

Fibrotic leaflet remodeling scores independently predicted FMR/FTR severity with high precision and significantly improved prediction beyond anatomic anomalies. Fibrotic remodeling showed divergent mechanisms between FMR and FTR. In FMR, suppressed retinoic acid metabolism drove antifibrotic-to-neutral valvular interstitial cell transition. In FTR, impaired IFN (interferon) signaling promoted antifibrotic-to-profibrotic valvular interstitial cell transdifferentiation, worsened by endothelial-to-mesenchymal transition-derived profibrotic valvular endothelial cells. Targeted PDK4 (pyruvate dehydrogenase kinase 4) upregulation or IFN signaling activation reduced FTR severity in vitro and in vivo.

CONCLUSIONS

Leaflet-specific organic fibrotic remodeling actively involves FVR beyond functional adaptation, with distinct fibrotic mechanisms in FMR versus FTR. PDK4 and IFN modulation demonstrate therapeutic potential for FTR.