Cardiac valve thickening is a major cardiovascular manifestation of connective tissue disorders (e.g., Marfan syndrome (MFS) and Ehlers-Danlos syndrome (EDS)), and non-syndromic valvular diseases. In humans POSTN protein is reduced in pediatric patients with bicuspid aortic valve. In adult valvular diseases mutations are found in TGF2 receptor genes (syndromic valve disease) and in the SMAD2-interacting gene Filamin-A (non-syndromic valve disease). We have shown that Periostin (Postn) knockout mice exhibit anomalous ECM remodeling and thickened valves, and that Postn protein is down-regulated in both outflow (OFT) and atrioventricular (AV) cushions in the absence of Tgf22. Conversely, Postn-/- mice exhibit reduced pSmad2,3 levels and blunted Tgf2 responsiveness, indicating that POSTN may also augment TGF2 signaling. Moreover, reduced Postn in the thickened Tgfb2-/- valves correlates with increased Tgf21 ligand gene expression and increased Bmp signaling. What is unclear from these data is whether the Tgf2 and Bmp activities are upstream of Postn or whether Tgf21 is downstream of Postn. The signaling mediating these complex interactions and the functional outcomes of interfering with those pathways will be examined in Aim 1. We have shown that Postn OFT transcriptional elements are located within a 3.9kb promoter. Consistent with our model, this sequence contains both Tgf2 (TRE) and Bmp (BRE) response elements. That promoter will be used in Aim 2 to test whether the aberrant valve muscularization seen in human connective tissue disease is regulated by POSTN. Lineage analysis demonstrates that the 3.9kb promoter marks a multipotent subpopulation of OFT cushion cells which can also contribute to the AV valves. We have isolated the OFT enhancer to a 304bp-Postn minimal sequence that contains the TREs and not the BREs. It is unclear whether additional cis-elements are necessary for AV cushion recruitment of the multipotent OFT 304bp-Postn marked cells, and whether their AV cushion recruitment is Tgf2-dependent. These questions will be answered in Aim 3. The overall scheme of the grant is to determine what is up- and down-stream of Postn (Aim 1), to set up a functional test for an important function of Postn that has relevance to a human problem (Aim 2), and to examine the fate of Postn-expressing cells and the role of Tgf2-regulation in establishing that fate (Aim 3). In summary, the complex relationship between POSTN and TGF2 &BMP signaling, and POSTN function, need clarification to add precision to therapeutic intervention in human connective tissue disorders of the heart.
The pivotal role of extracellular matrix and regulated differentiation is highlighted in a recent study in which the secreted chondromodulin-I glycoprotein was deleted in mice, as the normal avascularity of viable null cardiac valves was abrogated which resulted in enhanced Vascular endothelial factor-A expression, angiogenesis and calcification in aged valves. Thus, valves do not even have to look abnormal to cause clinical problems (e.g. Marfan's syndrome, Ehlers-Danlos syndrome, floppy mitral valve syndrome and mitral valve prolapse). These proposed studies will largely focus on determining what role the secreted extracellular matrix protein Periostin (POSTN) plays during valve matrix remodeling;and the part that TGF2 and BMP signaling plays in regulating POSTN.
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