Abstract 251: 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 (TAA). 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, Smad4 role 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. We found that, most of these mice died 4/6 months later from hemothorax due to rupture of thoracic aorta, while untreated mice survived. Echocardiography showed a progressive dilation of aorta and TAA formation. Histology 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 PSmad2 and PSmad1/5/8 early after TAM, suggesting an enhancement of TGFβ receptors activity, a condition that could increase non-canonical TGFβ signaling, the canonical one being 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 controlling DNA transcription and 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 in cells as an inactive precursor and released in its active form by caspase-1 cleavage. In turn, caspase-1 requires 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.
We demonstrated for the first time 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. F. Da Ros: None. R. Carnevale: None. G. Cifelli: None. D. Bizzotto: None. P. Braghetta: None. G. Bressan: None. G. Lembo: None.
- © 2015 by American Heart Association, Inc.