Mitral Valve Dystrophy: What role do leukocytes play?

Mitral Valve Dystrophy (MVD) is an outgrowing disease, however little is known regarding the involved pathophysiological mechanisms. We have generated a unique knock-in (KI) animal model for the P637Q mutation on the FLNA gene, the first causal gene associated with MVD identified by our team. We established the relevance of this model to study MVD by confirming the presence of the disease in 3-week-old (D21) KI rats. The molecular phenotyping at this time highlighted specific biological processes such as immune cell recruitment, extracellular matrix remodeling or response to molecular stress, as central players in the development of MVD. The aim of the present study was to analyze earlier time point (7-day post-natal; D7) and the specific role of endothelial, interstitial and immune cells in the onset and the progression of MVD. D7 and D21 WT and KI rats were studied. Classical histology was performed at D7 to evaluate the valve remodeling. Cytometry was performed to determine the nature and the proportion of cell subpopulation at each time-points. Following cell sorting, qPCR experiments allowed the characterization of the molecular signature for each subpopulation. The MV phenotyping at D7 confirmed the presence of MVD, but no difference was observed in myeloid cell proportion at this age (6% vs 6% for WT and KI animals, respectively; P = 0.63). However, concordant with the RNA-seq data at D21, cytometry experiments revealed a 2-fold increase in the proportion of myeloid cells in MV from KI animals compared to WT (13% vs 7% respectively; P < 0.05). CD45+ leukocytes and CD206+ developmental macrophages were located in the medial third and atrial border of the MV leaflet in WT rats, and more diffusely observed in KI animals at D21. Transcript levels of typical inflammatory markers (Esm1 + 4.3x in KI VECs vs WT, n = 4; P = 0.05), ECM markers (Cspg4 + 1.90x in KI VICs vs WT, n = 5; P = 0.008), and cellular activation confirmed the pro-inflammatory environment and the activation of interstitial cells in KI animals. Our results revealed a specific role of myeloid cells in the progression of MVD rather than the onset of the disease. Furthermore, preliminary results on the cell-specific activity shows an activation of myeloid cells, endothelial and interstitial cells in MVD. Cellular cross-talk needs to be further studied to decipher the molecular mechanisms leading to MVD.