Abstract 15568: Selective Deletion of Smad4 in Smooth Muscle Cells Causes Thoracic Aortic Aneurysm in Mice
TGFβ signaling have been implicated in the onset of thoracic aortic aneurysms (TAAs). Recently, it has been described that mutations in SMAD4, an intracellular transducer required in the TGFβ canonical pathway, present aortic disease in humans. Although transgenic models recapitulating mutations of TGFβ pathway have been studied, the role of Smad4 in the onset of TAA has never been explored.
Smad4 homozygotes mice are embryonically lethal, thus, we postnatally targeted the gene, using a tamoxifen (TAM)-inducible Cre recombinase under the smooth muscle myosin heavy chain promoter. Activation by TAM suppressed Smad4 in aorta SMCs of Smadf/f;Myh11-CreERT2 mice. Most of these mice died 4/6 months later from hemothorax due to rupture of thoracic aorta, while untreated animals survived. Echocardiography showed a progressive dilatation of aorta and TAA formation. Histological analysis revealed progressive fragmentation of elastic lamellae with increased inflammatory infiltrates at sites of disarrangement, rich in macrophages. At the molecular level, we found that lack of Smad4 was followed by increased levels of P-Smad2 and P-Smad1/5/8 early after TAM. This finding suggested an enhancement of TGFβ receptors activity, a condition that could increase non-canonical TGFβ signaling, the canonical one being certainly depressed. We actually found a significant increase in one of the non-canonical TGFβ signaling, i.e. the p65 subunit of NFkB, a protein complex involved in regulating responses to different stress stimuli, including cytokines. Given the particular relevance that the latter molecules may have in TAA, we tested expression of some of them, finding a selective IL-1β upregulation, produced and stored as an inactive precursor and released in its active form by caspase-1 cleavage. In turn, caspase-1 requires the function of NLRP3 inflammasome, a multiprotein platform that assembles in response to infection and tissue damage and is responsible for activation of inflammatory caspases. Therefore, we also tested NLRP3 mRNA levels and found them to be significantly upregulated. Overall, we demonstrated that selective Smad4 deletion in the SMC of adult mice is sufficient to induce ultrastructural damage and immune reaction culminating in the onset of TAA.
Author Disclosures: D. Carnevale: None. R. Carnevale: None. F. Da Ros: None. G. Cifelli: None. D. Bizzotto: None. P. Braghetta: None. G. Bressan: None. G. Lembo: None.
- © 2015 by American Heart Association, Inc.