Anne-Marie Koop

6 241 such as miR-17/92, 39,40 miR-27a, 41 miR-96, 42 miR-126, 27 miR-130, 43 miR-143/145 44 and miR-210 45 , all known to be involved either in cell proliferation, vascular remodelling or apoptosis, were identified as playing important roles in the pathogenesis of PAH. miR-126 downregulation in the skeletal muscle of PAH patients is associated with a decrease in RV vascular endothelial growth factor (VEGF)-induced angiogenesis and in exercise tolerance. 27 Reestablishment of miR-126 levels resulted in improved RV function and increased microvascular density in experimental PAH. 27 Furthermore, Reddy and colleagues 34 identified miR-34a, miR-28, miR-93 and miR- 148 to be upregulated in the pressure-overloaded RV and further inducing changes in oxidative metabolism, cell cycle and calcium homeostasis. Nevertheless, the differences between RV and LV response to pressure overload regarding microRNA- dependent regulatory mechanisms has not yet been explored. Thus, the present study provides evidence for distinct roles of a specific microRNA, depending on which chamber is under direct stress. Our previous work and that from others has established that miR-199b is upregulated in both ventricles throughout the different remodelling stages induced by pressure overload. 33,34 Although common mechanisms are suggested to be involved in both RV and LV remodelling, the current study indicates their regulation to be different, as well as their vulnerability to distinctive types of stress. RV response to increased pressure load, as for the LV, also includes changes in myocardial hypertrophy and fibrosis, as well as adaptation of the cardiac capillary network. 46 In the present study, we show that increased levels of miR-199b in the pressure-overloaded RV result in increased hypertrophic growth of the RV when compared to wildtype animals under similar RV pressure. We were able to induce chronic pressure load of the RV progressively in time in both wildtype and TG mice. Decreased RV contractility in TG mice after 4 and 6 weeks of pressure overload reflected in the TAPSE, a measure of the longitudinal contraction of the RV. Furthermore, cardiac output and RV ejection fraction inversely correlated with miR-199b expression. Altogether, these data strongly suggest a pathologic effect of increased miR-199b levels on the hemodynamic function of the RV. Regarding fibrosis, while there was a significant increase in collagen deposition after PAB, no differences betweenWT and miR-199b TG animals were observed, at the histological level. Nevertheless, after RV pressure we observed increased transcript levels of collagen type I alpha 1 chain ( Col1A1 ) in TG animals compared to wildtype, suggesting a more fibrotic profile of the TG RVs. Similar to what we have previously described for the LV, we also observed increased NFAT activity in the stressed RV, which was reflected by increased transcript levels of RCAN1-4. 33 However, we could not directly correlate this with miR-199b expression levels. Upon RV pressure overload, a switch in myosin heavy chain (MHC) isoforms from the fast α MHC to the slower and energetically favorable, β MHC, is induced in