Abstract 357: Expression of a Constitutively Active TGF-β Receptor-1 in Mouse Aortic Smooth Muscle Cells Does Not Cause Severe Aortopathy

Abstract

Background: Altered transforming growth factor beta (TGF-β) signaling in aortic SMC is implicated as a cause of aortic aneurysms and dissections (AAD), with AAD attributed both to increased and decreased TGF-β signaling. Loss of TGF-β signaling in aortic SMC of young mice unequivocally causes AAD. However, it is unclear whether increased aortic SMC TGF-β signaling is sufficient to cause AAD. To answer this question, we induced expression of a conditional constitutively active Type I TGF-β receptor (TBR1-CA) in adult mouse aortic SMC. Methods: Experimental mice (Acta2 -CreER T2 +/o TBR1-CA +/o ) were injected with tamoxifen at 6 weeks of age and compared to tamoxifen-injected controls ( Acta2 -CreER T2 o/o TBR1-CA +/o , Acta2 -CreER T2 +/o TBR1-CA o/o mice, or Acta2 -CreER T2 o/o TBR1-CA o/o ). TBR1-CA was detected by immunoblotting; ascending aortas were analyzed by sectioning, histology, planimetry, and wire myography. TBR1-CA +/o SMC were explanted, cultured, and treated with adenovirus-Cre (Ad-Cre; to induce TBR1-CA expression) or control AdNull. Extracts were analyzed by western blotting. Results: Expression of TBR1-CA in SMC of experimental mice was detected by immunoblotting of aortic media extracts 10 days and 4 months after tamoxifen injection. Ad-Cre-treatment of cultured TBR1-CA +/o SMC significantly increased phosphorylation of SMAD2 and SMAD3 (2-3-fold; P < 0.002 for both). Gross aortic hemorrhage was present in 1 of 68 experimental and 0 of 89 control mice (P=0.4). Wire myography (4 weeks after tamoxifen) showed no differences in vasomotor function. There were no penetrating aortic ulcers, intramural hematomas, or Prussian blue staining in any of 42 experimental or 52 control mice harvested either 10 days or 10 weeks after tamoxifen. 10 weeks after tamoxifen, aortic diameters of experimental mice were normal, but medial thickness was mildly increased (45.2 μm ± 6 vs 39 ± 4.7 μm; P= 0.04), and medial area trended higher (0.15 ± 0.03 mm 2 vs 0.13 ± 0.02 mm 2 ; P=0.1). Conclusion: Expression of a constitutively active type I TGF-β receptor in aortic SMC of postnatal mice appears to cause—at most—a mild and indolent aortopathy. Further studies are ongoing; however, elevated SMC TGF-β signaling mediated by TBR1 may be insufficient to cause severe aortopathy.