N the developing heart. (A) Immunoblot for Mef2c protein. Lane 1, Mef2c binding to control Mef2 consensus sites from the muscle creatine kinase gene. Lane 2, negative control oligo with mutated Mef2 sites fails to bind to Mef2c. Lanes 3 and 5, Mef2c interacts with the Mef2 consensus sites in the Crtl1 promoter region at 2913 to 2923 (Site 1, Lane 3) and at 2698 to 2707 (Site 2, Lane 5). Lanes 4 and 6, mutation of the Mef2 consensus sites in the Crtl1 promoter region blocks Mef2 binding at both consensus sites. (B) Mef2c and Sox9 bind the Crtl1 Promoter in vivo. Chromatin Immunoprecipitation (ChIP) was performed using embryonic hearts stage ED10.5?1.0. PCR of the DNA input, Mef2c immunoprecipitate, and IgG control precipitate was performed using primers flanking both Mef2 consensus sites of the Crtl1 promoter. For the Sox9 ChIP, the 
initial DNA input, Sox9 immunoprecipitate, and IgG control precipitate were PCR amplified using primers specific for the Sox9 consensus site on the Crtl1 promoter. doi:10.1371/journal.pone.0057073.gMef2 consensus site (Mef2 Site 1) from 2922 to 2913 was mutated from 59-ttataaataa-39 to 59-ttatagcgaa-39 (Crtl1-Mutant 1). Mutation of Mef2 Site 2, results in a 69-25-0 diminished response with an approximately 50 reduction in Crtl1 promoter activity at basal levels (p = 7.061026). The reduced promoter activity remained in the presence of both 50 ng and 100 ng of exogenous Mef2c (p = 0.0007 and p = 0.014 respectively) (Figure 5D). Mutation of Mef2 site 1, however, only results in an approximately 25 reduction in Crtl1 promoter activity at basal levels (p = 0.084) and in the presence of high SMER-28 chemical information concentrations of Mef2c (p = 0.008) (Figure 5D). This data further validates that Crtl1 activity is dependent on Mef2c interaction with the promoter.DiscussionThe development of the atrioventricular valves begins with the swelling of the endocardial lining of the AV junction and the formation of the AV cushions. The cushions then become populated by endocardially derived cells resulting from endocardial-to-mesenchymal transition (EMT). At this point in time, the endocardial cushion mesenchyme contains a variety of ECM components, including Crtl1, hyaluronan, and versican. These ECM components play important roles in cushion development [2,31?3]. In previous work, we have shown that Crtl1 is widely expressed in the endocardial cushions and AV valves during stages of mesenchymal cell proliferation and differentiation [2]. As the valves elongate and mature, the composition of the ECM changes. This change is characterized by an increase in expression of proteins such as fibronectin and collagen-1 [34] whereas the expression of Crtl1 becomes largely restricted to the atrial and ventricular aspects of the valve leaflets [12].While there is 15900046 an increasing knowledge of the role of ECM in valvulogenesis, relatively little is known regarding the spatiotemporal regulation of the ECM during cardiac development. Previously, it was demonstrated that Mef2c expression is found in the cardiomyocytes of the developing heart where it is considered pro-myogenic as it is known to regulate contractileproteins 
such as cardiac alpha-actin and alpha-myosin heavy chain [16?8]. Here we show that Mef2c expression is not restricted to the myocardium and that it is also found in the endocardium and endocardially-derived mesenchyme of the AV valves from mid to late gestation where it is co-expressed 22948146 with Crtl1, suggesting that Mef2c may act as an important regulato.N the developing heart. (A) Immunoblot for Mef2c protein. Lane 1, Mef2c binding to control Mef2 consensus sites from the muscle creatine kinase gene. Lane 2, negative control oligo with mutated Mef2 sites fails to bind to Mef2c. Lanes 3 and 5, Mef2c interacts with the Mef2 consensus sites in the Crtl1 promoter region at 2913 to 2923 (Site 1, Lane 3) and at 2698 to 2707 (Site 2, Lane 5). Lanes 4 and 6, mutation of the Mef2 consensus sites in the Crtl1 promoter region blocks Mef2 binding at both consensus sites. (B) Mef2c and Sox9 bind the Crtl1 Promoter in vivo. Chromatin Immunoprecipitation (ChIP) was performed using embryonic hearts stage ED10.5?1.0. PCR of the DNA input, Mef2c immunoprecipitate, and IgG control precipitate was performed using primers flanking both Mef2 consensus sites of the Crtl1 promoter. For the Sox9 ChIP, the initial DNA input, Sox9 immunoprecipitate, and IgG control precipitate were PCR amplified using primers specific for the Sox9 consensus site on the Crtl1 promoter. doi:10.1371/journal.pone.0057073.gMef2 consensus site (Mef2 Site 1) from 2922 to 2913 was mutated from 59-ttataaataa-39 to 59-ttatagcgaa-39 (Crtl1-Mutant 1). Mutation of Mef2 Site 2, results in a diminished response with an approximately 50 reduction in Crtl1 promoter activity at basal levels (p = 7.061026). The reduced promoter activity remained in the presence of both 50 ng and 100 ng of exogenous Mef2c (p = 0.0007 and p = 0.014 respectively) (Figure 5D). Mutation of Mef2 site 1, however, only results in an approximately 25 reduction in Crtl1 promoter activity at basal levels (p = 0.084) and in the presence of high concentrations of Mef2c (p = 0.008) (Figure 5D). This data further validates that Crtl1 activity is dependent on Mef2c interaction with the promoter.DiscussionThe development of the atrioventricular valves begins with the swelling of the endocardial lining of the AV junction and the formation of the AV cushions. The cushions then become populated by endocardially derived cells resulting from endocardial-to-mesenchymal transition (EMT). At this point in time, the endocardial cushion mesenchyme contains a variety of ECM components, including Crtl1, hyaluronan, and versican. These ECM components play important roles in cushion development [2,31?3]. In previous work, we have shown that Crtl1 is widely expressed in the endocardial cushions and AV valves during stages of mesenchymal cell proliferation and differentiation [2]. As the valves elongate and mature, the composition of the ECM changes. This change is characterized by an increase in expression of proteins such as fibronectin and collagen-1 [34] whereas the expression of Crtl1 becomes largely restricted to the atrial and ventricular aspects of the valve leaflets [12].While there is 15900046 an increasing knowledge of the role of ECM in valvulogenesis, relatively little is known regarding the spatiotemporal regulation of the ECM during cardiac development. Previously, it was demonstrated that Mef2c expression is found in the cardiomyocytes of the developing heart where it is considered pro-myogenic as it is known to regulate contractileproteins such as cardiac alpha-actin and alpha-myosin heavy chain [16?8]. Here we show that Mef2c expression is not restricted to the myocardium and that it is also found in the endocardium and endocardially-derived mesenchyme of the AV valves from mid to late gestation where it is co-expressed 22948146 with Crtl1, suggesting that Mef2c may act as an important regulato.