Abstract 83: Molecular Mechanism of Aortic Aneurysm Formation in Fibulin-4--Deficient Mice

Abstract

Background: Fibulin-4 (Fbln4) is a secreted extracellular matrix protein, which is expressed in blood vessels and associated with microfibrils surrounding elastic fibers. Fbln4-null mice exhibit perinatal lethality with rupture of aortic aneurysm and a marked disruption of elastic fibers, and these phenotypes were recapitulated by smooth muscle cell (SMC)-specific deletion of Fbln4 in vivo (Fbln4SMKO). Severe aneurysms developed exclusively in the ascending aorta at postnatal day 90 (P90) and vascular SMCs were hyper-proliferative and failed to fully differentiate in the Fbln4SMKO aorta. Objective: To elucidate the molecular mechanism that initiates the aneurysmal change in Fbln4SMKO mice, we performed proteomics analysis of Fbln4-deficient (Fbln4-/-) SMCs and compared with wild-type SMCs. Methods: The early stage of ascending aneurysm was evaluated in Fbln4SMKO mice using microscopic methods and immunohistochemistry. The protein expression profiles were obtained from postnatal day (P) 7 wild-type and Fbln4 -/- SMCs by two-dimensional fluorescence difference gel electrophoresis. Several differentially expressed proteins were identified by mass spectrometry and validated using qPCR and Western blot analysis. Results: Macroscopic observation revealed that the ascending aorta dilatation was not observed at P7 but became detectable at P14. Histologically, however, focal lesion of SMC proliferation was already noticeable in the sub-endothelial region at P7 and became evident at P14 with a thickened aortic media. Differential proteomic analysis showed that molecules involved in actin dynamics, including gelsolin, ezrin/radixin/moesin (ERM) protein, cofilin-2, and Rho GDI were upregulated in Fbln4 -/- SMCs. In addition, phosphorylation of cofilin and ERM was increased in Fbln4 -/- SMCs. Interestingly, SMC differentiation markers were normally expressed in Fbln4SMKO aortae until P7. Conclusion: Our study indicated that the phenotypic change of SMCs occurred after P7 in the Fbln4SMKO aorta and Fbln4 deficiency contributed to aneurysm formation by facilitating the alteration in actin dynamics. This information will shed light on the molecular mechanism of aneurysm initiation in the Fbln4 deficient SMCs.