Anne-Marie Koop

232 Cardiac-specific overexpression of miR-199b promotes RV remodelling induced by pressure overload Hypertrophyof the RVwas determined by the Fulton index, the ratio of right ventricular weight to left ventricular plus septumweight (RV/LV+S). An increased Fulton indexwas observed 6 weeks after PAB in both WT and MHC-199b animals, compared to sham ( figure 1d ). Despite the differences in miR-199b expression levels, animals from both groups developed similar degrees of RV hypertrophy when subjected to PAB ( figure 1d ). Histological analysis demonstrated, however, that besides displaying myocardial disarray ( figure 1e ), the banded TG hearts also display increased cross-sectional surface areas of cardiomyocytes when compared to banded-WT hearts ( figure 1e-g ). At six weeks post-PAB, the hearts of WT animals subjected to PAB displayed severe deposition of collagen and formation of fibrotic lesions which, however, did not differ from those found in the RV of TG mice ( figure 1h, 1i ). miR-199b overexpression impairs RV function during pressure overload-induced RV remodelling Although miR-199b TG mice do not display an obvious pathological baseline phenotype nor cardiac dysfunction, these animals were shown to be more sensitive to cardiac stress than WT mice. 33 MHC-199b mice, in the sham group did not show cardiac dilatation or dysfunction. Mice with a pressure-loaded RV, either TG or WT, developed RV dilatation as reflected by increased end-diastolic and end-systolic RV volumes ( figure 2a-c ). Whereas cardiac output remained preserved in WT animals subjected to PAB surgery ( figure 2d ), it decreased significantly in MHC-199b animals after six weeks of RV pressure overload. In fact, miR-199b expression is inversely related to cardiac output ( figure 2e ). When evaluating the effects of PAB on RV EF, we observed more pronounced effects since both WT and TG animals showed decreased ejection fraction (EF) after PAB ( figure 2f ). RVEF also negatively correlated with miR-199b expression levels ( figure 2g ). Similarly, the tricuspid annular plane systolic excursion (TAPSE) was also decreased upon RV pressure overload in both WT and MHC-199b, but more severely in the latter group ( figure 2h ). The slopes of TAPSE over time are indicative of progressive impairment of RV function in MHC-199b compared to WT mice ( figure 2i ). The observed cardiac phenotypes induced by PAB correlated with increased transcript abundance of cardiac stress genes, including atrial natriuretic factor ( Nppa ), brain natriuretic factor ( Nppb ) α -skeletal actin ( Acta1 ) and β -myosin heavy chain ( Myh7 ) ( figure 3a-d ). While α -myosin heavy chain ( Myh6 ) was decreased after PAB in bothWT and MHC-199b hearts, the Myh6/Myh7 ratio is significantly lower in the TG animals, miR-199b overexpression aggravates right ventricular hypertrophy and is associated with decreased cardiac function in right ventricular pressure load.