Abstract 12568: Angptl2 is Essential for Aortic Valve Development in Mice

Introduction: Angiopoietin-like 2 (Angptl2) is a pro-inflammatory anti-apoptotic protein, secreted by senescent cells. Angptl2 also contributes to maintain tissue homeostasis, and we previously reported that adult Angptl2-knockdown (KD) mice exhibit spontaneous Aortic Valve (AoV) stenosis with thickened leaflets compared to wild-type (WT) littermates. It is unknown whether and how Angptl2 contributes to AoV development. Hypothesis: Angptl2 is expressed in the AoV at the embryonic stage and is essential for its development, by contributing to programmed apoptosis/senescence and proliferation occurring during organogenesis. Methods: Embryonic hearts aged 14.5 (E14.5) and 18 days (E18) of Angptl2 -KD and WT mice were dissected. Protein expression (immunofluorescence) and leaflets/hinges thickness of AoV (hematoxylin & eosin) were quantified in histological sections. Data are mean±SEM of n embryos. Results: Compared to WT mice, in AoV leaflets from Angptl2 -KD mice at E14.5, we observed a decrease in apoptotic cells (determined by TUNEL;-47%, p<0.05, n=4), mostly in Valve Interstitial Cells (VICs), a decreased number of Valve Endothelial Cells (VECs) expressing the senescence marker p21 (-48%, p<0.05, n=4), together with a trend towards an increased cell proliferation (Ki67; +396%, p=0.13, n≥6). In turn, a premature thickening of both the leaflets (from 73±18 to 91±33μm, p<0.05, n≥8) and the hinges (from 38±10 to 49±20μm, p<0.01, n≥8) of the valves was measured in Angptl2 -KD mice at E18. Concomitantly, Angptl2 is dynamically expressed during AoV development in WT mice: at E14.5, Angptl2 is expressed in all cell types, suggesting that it could effectively contribute to regulate apoptosis/senescence in both VICs and VECs. At E18, Angptl2 expression in WT mice is restricted to VICs at the border of the leaflet, where the mechanical stress is high, a pattern also observed in adult WT mice. Conclusions: Knockdown of Angptl2 is deleterious to AoV development by disturbing the fine balance between embryonic cell apoptosis/senescence and proliferation, which is essential to adequate AoV maturation; this unbalance progresses to severe AoV stenosis in adult mice. Angptl2 -KD is a new mouse model of spontaneous AoV stenosis, a disease in high medical needs.