2017 Cardiovascular Research Day Abstract Book

27
LRP1 Deletion in Smooth Muscle Cells of the Outer Aortic Media Promotes Angiotensin IIinduced
Thoracic Aortic Aneurysm
Hisashi Sawada, MD, PhD 1 • Debra Rateri 1 • Mark Majesky, PhD 2 • Alan Daugherty, PhD 1
1Saha CVRC, University of Kentucky • 2 Center for Cardiovascular Biology, University of Washington
Postdoc
Objective: Low-density lipoprotein receptor-related protein 1 (LRP1) is a multifunctional protein
that is linked to several vascular pathologies. LRP1 deletion in smooth muscle cells (SMCs)
accelerates angiotensin II (AngII)-induced thoracic aortic aneurysm (TAA). In association with TAA
formation, there is medial thickening that is characterized by a transmural gradient in which
pathology progressively increases from lumen to adventitial aspect. We hypothesized that deletion
of LRP1 in the outer medial layers of the proximal thoracic aorta has a pivotal role in the
pathogenesis of TAA. The aim of this study was to determine whether LRP1 deletion in the outer
media accelerates AngII-induced TAA formation.
Methods and Results: SMCs in the outer media of the ascending aorta are derived from the second
heart field, as demonstrated by lineage tracing studies using Cre under the control of Mef2c.
Therefore, we used Mef2c-driven Cre to delete LRP1 in SMCs of the outer medial layers. Female
LRP1 flox/flox mice were bred to male Mef2c-Cre1/0 mice to generate study mice. We first
confirmed LRP1 deletion in Cre1/0 mice by both immunostaining and Western blot. LRP1 was
expressed ubiquitously across smooth muscle cells of all aortic medial layers in Cre 0/0 mice. In
mice expressing Mef2c-Cre, aortic LRP1 protein was detected only in SMCs of the inner laminar
medial layers. Western blotting demonstrated LRP1 protein abundance in Cre expressing mice was
reduced by 43%. Saline or AngII (1,000 ng/kg/min) was infused by subcutaneous osmotic pumps
for 28 days into 12 - 14 week-old male Cre0/0 and 1/0 mice. As expected, systolic blood pressure
increased similarly in both AngII-infused Cre 0/0 and 1/0 mice compared to saline-infused mice.
Aortic rupture occurred within 3 to 10 days after AngII infusion in 17% of AngII-infused Cre 0/0
mice, while LRP1 deletion in Cre 1/0 mice increased aortic rupture to 27%. Aortic diameter in the
survivors was significantly increased in Cre1/0 mice compared to Cre0/0 mice. Histologically,
elastin fragmentation was detected in the aorta of AngII-infused Cre 0/0 mice and greater in Cre1/0
mice.
Conclusion: LRP1 in second heart field-derived SMCs of the outer media may play a critical role in
the pathogenesis of TAA.
43