Abstract 304: Notch Signaling in Coronary Arteries: Notch Ligand Delta-like1 Regulates Coronary Artery Numbers and is Protective in Myocardial Infarction
Introduction: Notch ligands of the Delta-like (Dll) family regulate embryonic and postnatal arteriogenesis. However, the role of notch signaling in coronary arteries, especially in the endothelium, is poorly characterized. We have analyzed the role of Notch ligand Dll1 in coronary arteries and myocardial infarction.
Methods and Results: Analysis of Dll1 expression by immunofluorescence and Dll1-lacZ reporter gene studies in adult mouse hearts revealed selective endocardial and coronary endothelial, but not venous or capillary Dll1 expression. Hearts of mice heterozygous for Dll1 (Dll1+/−) were significantly smaller than wildtype (wt) hearts (heart weight (g)/femur length (cm), wt: 0.13± 0.02 vs. het: 0.09± 0.02, n=10, p<0.05) while body weight and survival over 18 months was comparable. Furthermore, the number of medium (>50 –100 um) and large (>100 um) coronary arteries was significantly reduced in Dll1+/− hearts (p<0.01). Reduced coronary numbers were also apparent in neonatal hearts, suggesting that Dll1 regulates coronary artery development and, consequently, heart size. To define the role of endothelial Dll1 in myocardial infarction in the absence of confounding developmental phenotypes we generated inducible and endothelial-specific Dll1 knockout mice by crossing a transgenic mouse strain with a tamoxifen (TAM)-sensitive Cre mutant driven by the vascular endothelial cadherin promoter with floxed Dll1 mice. The inducible and pan-endothelial specific activity of Cre was verified by RosaR26 lacZ reporter gene studies. After TAM injections mice were subjected to permanent LAD occlusion (MI) and analyzed after 4 weeks. Following MI Dll1 was strongly upregulated in the periinfarct area of control mice, mainly in smooth-muscle positive neo-vessels. However, in endothelial Dll1 knockout mice Dll1 expression was strongly decreased, and infarct size (increase 28%, p<0.05) and heart weight/femur length ratio (increase 10%, p<0.05) significantly increased (n=6).
Conclusion: Dll1 regulates coronary artery numbers and heart size. In the postnatal heart, Dll1 is upregulated in periinfarct neo-vessels and a lack of endothelial Dll1 increases infarct size, suggesting an important role for vascular Dll1 in myocardial arteriogenesis.