Anne-Marie Koop

7 261 Cardiac-specific silencing of Hand2 exacerbates hypertrophic growth of the RV in response to pressure overload Hypertrophy of the right heart was assessed by the Fulton index, the ratio of right ventricular weight to left ventricular plus septum weight (RV/LV+S). Whereas an increased Fulton index was observed 6 weeks after PAB in both control and knockout animals, compared to sham ( figure 2a, table 2 ), the effect was more profound in the absence of Hand2 expression. These observations were confirmed by increased CSA of cardiomyocytes derived from banded-knockout hearts when compared banded-control hearts ( figure 2b and 2c) . Hypertrophic RV remodelling was associated with significantly increased mRNA expression of hypertrophic “fetal” cardiac genes encoding natriuretic peptides atrial natriuretic factor ( Nppa ) and brain natriuretic peptide ( Nppb ), alpha-skeletal actin 1 ( Acta1 ) and a-myosin heavy chain ( Myh7 ) ( figure 2d-g ). Furthermore, an increase in mRNA levels of regulator of calcineurin isoform 4 ( RCAN1.4 ) confirms activation of calcineurin/NFAT signaling in the RV response to pressure overload which was not affected by Hand2 silencing ( figure 2h ). At 6weeks post-PAB, both control and Hand2 knockout animals displayed similar levels of collagen deposition and formation of fibrotic lesions ( figure 2i,j ). In line, no significant differences were observed for mRNA expression levels of the profibrotic markers collagen type I ( Col1A ), transforming growth factor beta ( Tgfb) and endoglin ( Eng ) between the banded animals ( figure 2k-m ). Remarkably, despite silencing of Hand2 resulting in lower levels of profibrotic markers in the RV of sham animals, these levels significantly rose to control levels upon subjection to PAB. As RV capillary rarefaction is a phenomenon that leads to maladaptive RV remodelling, we assessed the capillary to myocyte ratio of the RV in our different experimental groups. While histochemical analysis and respective quantification reflected higher capillary to myocyte ratio in control mice subjected to RV stress, a significant decrease was observed in Hand2 knockout animals, under the same conditions ( figure 2n ). Vascular endothelial growth factor receptor 2 ( Vegfr2 ) expression levels, revealed angiogenic favoring patterns with increased expression levels in the knockout compared to control animals, either subjected to sham or PAB. Although knockout PAB animals showed increased Vegfr2 expression (compared to controls), this pattern was insufficient to promote adequate angiogenesis ( figure 2n ), and RV stress in the absence of Hand2 expression resulted in inhibition of Vegfr2 (compared to sham, figure 2o ). Taken together, these data indicate that silencing of Hand2 induces exacerbated hypertrophy while also impairing microvascular remodelling under conditions of RV pressure overload.