Abstract 255: Loss of Alk5 in Smooth Muscle Cells (SMCs) Causes Aortic Aneurysms Through an Aberrant Tgfbr2 Signal

Abstract

TGF-β signaling disorder has emerged as a cause for aortic aneurysm formation. However, mechanistic details of this disorder remain poorly defined. This study sought to understand the TGF-β signaling propagation and its impact on aortic structural homeostasis under conditions of receptor deficiency. An inducible gene deletion system driven by Myh11-CreER was utilized for selective removal of receptors Alk5, Tgfbr2, or both in SMCs, termed Alk5iko, Tgfbr2iko, and Alk5iko Tgfbr2iko, respectively. Alk5-floxed (Alk5f/f) mice served as controls. Animals (male, 9-11 weeks of age) were followed up to six months following gene deletion. Alk5iko caused progressive aortic enlargement (ultrasound scan). In four weeks, 23% of animals (n=42) died from aortic rupture, with the survivals all showing severe aneurysmal degeneration at locations of ascending (100%), descending (43%) and/or suprarenal (50%) aortas. Surprisingly, Tgfbr2iko in age-matched males (n=26) was well tolerated. Neither rupture nor aortic dilation was noted in four weeks. Evidence of aortic pathology was limited to scattered elastic fiber breaks. To evaluate the role of TGF-β in aortic aneurysm formation in Alk5iko animals, we deleted Tgfbr2 simultaneously with Alk5. The additional Tgfbr2iko completely recused aortic rupture, attenuated aortic dilation from 68% to 25%, and significantly improved aortic histology. Compared to the Alk5f/f controls, Alk5iko, Tgfbr2iko, and Alk5iko Tgfbr2iko aortas all displayed a similar but reduced level of Smad2 phosphorylation at various time points, indicating that loss of the canonical TGF-β signal is not the primary driver for the aortic phenotype of Alk5iko aortas. Meanwhile, the Alk5iko aortas produced 3-fold more pERK than Alk5f/f controls, and this elevation was completely abolished by the additional Tgfbr2 deletion. Pharmaceutical treatment of the Alk5iko mice with ERK inhibitor RDEA-119 significantly inhibited aortic dilation and prevented aortic wall from degeneration while those treated with vehicle control exhibited aortic dilation and intimal/medial tears. In conclusion, loss of Alk5 in SMCs triggers a non-Alk5 but Tgfbr2 dependent signal that promotes aortic aneurysm formation through exacerbating ERK activity.