Abstract
Aortic valve disease (AVD) is characterized by elastic fiber fragmentation (EFF), fibrosis and aberrant angiogenesis. Emilin1 is an elastin-binding glycoprotein that regulates elastogenesis and inhibits TGF-β signaling, but the role of Emilin1 in valve tissue is unknown. We tested the hypothesis that Emilin1 deficiency results in AVD, mediated by non-canonical (MAPK/phosphorylated Erk1 and Erk2) TGF-β dysregulation. Using histology, immunohistochemistry, electron microscopy, quantitative gene expression analysis, immunoblotting and echocardiography, we examined the effects of Emilin1 deficiency (Emilin1−/−) in mouse aortic valve tissue. Emilin1 deficiency results in early postnatal cell-matrix defects in aortic valve tissue, including EFF, that progress to latent AVD and premature death. The Emilin1−/− aortic valve displays early aberrant provisional angiogenesis and late neovascularization. In addition, Emilin1−/− aortic valves are characterized by early valve interstitial cell activation and proliferation and late myofibroblast-like cell activation and fibrosis. Interestingly, canonical TGF-β signaling (phosphorylated Smad2 and Smad3) is upregulated constitutively from birth to senescence, whereas non-canonical TGF-β signaling (phosphorylated Erk1 and Erk2) progressively increases over time. Emilin1 deficiency recapitulates human fibrotic AVD, and advanced disease is mediated by non-canonical (MAPK/phosphorylated Erk1 and Erk2) TGF-β activation. The early manifestation of EFF and aberrant angiogenesis suggests that these processes are crucial intermediate factors involved in disease progression and therefore might provide new therapeutic targets for human AVD.
Highlights
Aortic valve disease (AVD) affects more than 2% of the general population and typically manifests later in life (Nkomo et al, 2006)
This results in early elastase-mediated elastic fiber fragmentation and aberrant angiogenesis in association with early valve interstitial cell (VIC) activation
Emilin1−/− aortic valve tissue is characterized by early elastic fiber fragmentation (EFF) and progressive elastolysis Juvenile Emilin1−/− aortic valve tissue exhibits EFF, which is characterized by decreased elastic fiber content and dispersion of elastic fiber components to all cusp layers, namely the fibrosa, spongiosa and ventricularis, as well as the annulus, indicating faulty elastic fiber assembly (Fig. 1A,B)
Summary
Aortic valve disease (AVD) affects more than 2% of the general population and typically manifests later in life (Nkomo et al, 2006). AVD is characterized by extracellular matrix (ECM). The prevailing view has been that injury and inflammation result in AVD, but there is increasing evidence that the cell-ECM changes that characterize AVD are mediated by abnormalities in molecular programs that regulate cardiac development (Markwald et al, 2010; Hinton and Yutzey, 2011). Our understanding of the molecular mechanisms underlying AVD pathogenesis, especially disease progression, has been limited, in part, by a lack of animal models that recapitulate the natural history of human AVD (Schoen, 2008; Rajamannan et al, 2011)
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