Abstract 300: Pathogenetic Mechanisms of Thoracic Aortic Aneurysm in a Smad4 Mutant Mouse Model: Identification of New Molecular Targets for Pharmacological Therapy

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TGFβ signaling have been implicated in the onset of thoracic aortic aneurysms (TAAs). It has been described cases of aortic disease in humans associated to mutations in SMAD4, an intracellular transducer required in the TGFβ canonical pathway. Interestingly, Smad4 homozygotes mice are embryonically lethal and embryonic inactivation of Smad4 in smooth muscle cells leads to early death because of aortic aneurysms. In order to study mechanistic insights and potential translational target of Smad4-dependent aneurysms, we postnatally targeted the gene, using a tamoxifen (TAM) inducible Cre recombinase under the smooth muscle myosin heavy chain promoter. 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. Most of these mice died 4/6 months because of rupture of thoracic aorta. We further characterized the infiltrate of monocytes in the aorta by flow cytometry, finding a significant increase of CD11b + Ly6C hi cells and F4/80 + CD169 + resident macrophages. Thus, we hypothesized that the deletion of Smad4 should activate an immune/inflammatory mechanism in the aorta, which could be in the end responsible for the pathological phenotype. At the molecular level, we actually found a significant increase in one of the noncanonical TGFβ signaling, i.e. the p65 subunit of NFkB, a protein complex involved in regulating responses to different stressors, including cytokines. Among these, we found a selective IL1β upregulation, produced as an inactive precursor and released upon caspase1 cleavage. In turn, caspase1 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 an immune reaction starting from the molecular alterations downstream to TGFβ signaling and culminating in the onset of TAA.