Twist1 promotes heart valve cell proliferation and extracellular matrix gene expression during development in vivo and is expressed in human diseased aortic valves

Abstract

During embryogenesis the heart valves develop from undifferentiated mesenchymal endocardial cushions (EC), and activated interstitial cells of adult diseased valves share characteristics of embryonic valve progenitors. Twist1, a class II basic-helix-loop-helix (bHLH) transcription factor, is expressed during early EC development and is downregulated later during valve remodeling. The requirements for Twist1 down-regulation in the remodeling valves and the consequences of prolonged Twist1 activity were examined in transgenic mice with persistent expression of Twist1 in developing and mature valves. Persistent Twist1 expression in the remodeling valves leads to increased valve cell proliferation, increased expression of Tbx20, and increased extracellular matrix (ECM) gene expression, characteristic of early valve progenitors. Among the ECM genes predominant in the EC, Col2a1 was identified as a direct transcriptional target of Twist1. Increased Twist1 expression also leads to dysregulation of fibrillar collagen and periostin expression, as well as enlarged hypercellular valve leaflets prior to birth. In human diseased aortic valves, increased Twist1 expression and cell proliferation are observed adjacent to nodules of calcification. Overall, these data implicate Twist1 as a critical regulator of valve development and suggest that Twist1 influences ECM production and cell proliferation during disease.

Persistent expression of Twist1 in chondrocytes causes growth plate abnormalities and dwarfism in mice

“…We used a conditional Cre-induced transgene approach to constitutively express Twist1 in collagen II-expressing cells and their progeny. Activation of the transgene resulted in a modest (2.6 fold) increase in Twist1 transcripts over endogenous Twist1 expression, which was the same level as found using this transgene in other tissues (Chakraborty et al, 2010), yet the expression of the transgene was maintained as chondrocytes matured. Sustained expression of Twist1 in cartilage led to a growth phenotype, characterized by shortening of the limbs and reduced body mass.…”

“…1 decreased expression of Twist1 may be required for proper chondrocyte maturation. To test this, we used a Twist1 transgene that is conditionally activated by Cre recombinase (Chakraborty et al, 2010, Connerney et al, 2008, Connerney et al, 2006. Twist1 mice were crossed with Col2a1-Cre mice, which express Cre in proliferating chondrocytes, to activate constitutive Twist1 expression in these cells and their progeny.…”

SOX9 modulates the expression of key transcription factors required for heart valve development

“…TWIST1 is highly expressed during the formation of the cardiac cushion mesenchyme and plays important roles in valve mesenchyme proliferation and differentiation (Shelton and Yutzey, 2008;Chakraborty et al, 2010b). Owing to the key role of TWIST1 in heart valve development, we focused on characterizing the transcript levels of Twist1 in the Sox9 cKO valves.…”

“…In the absence of SOX9, Twist1 mRNA expression was reduced by approximately threefold in the valve mesenchyme. TWIST1 can induce proliferation and migration of valve mesenchyme during early valve formation (Shelton and Yutzey, 2008;Chakraborty et al, 2010b) and, following EMT, TWIST1 plays a role in regulating differentiation of the AVC mesenchyme (Vrljicak et al, 2012). When TWIST1 persists at later stages of valve development, it leads to increased mesenchyme proliferation, increased TBX20 expression, and primitive ECM (Chakraborty et al, 2010b).…”

“…TWIST1 can induce proliferation and migration of valve mesenchyme during early valve formation (Shelton and Yutzey, 2008;Chakraborty et al, 2010b) and, following EMT, TWIST1 plays a role in regulating differentiation of the AVC mesenchyme (Vrljicak et al, 2012). When TWIST1 persists at later stages of valve development, it leads to increased mesenchyme proliferation, increased TBX20 expression, and primitive ECM (Chakraborty et al, 2010b). TWIST1 directly regulates Tbx20 (Lee and Yutzey, 2011), but, of note, Twist1-null hearts do not show a difference in Tbx20 levels in the AVC compared with WT (Vincentz et al, 2008).…”