Endocardial and Epicardial Derived FGF Signals Regulate Myocardial Proliferation and Differentiation In Vivo

Abstract

The epicardium regulates growth and survival of the underlying myocardium. This activity depends on intrinsic retinoic acid (RA) and erythropoietin signals. However, these signals do not act directly on the myocardium and instead are proposed to regulate the production of an unidentified soluble epicardial derived mitogen. Here, we show that Fgf9, Fgf16, and Fgf20 are expressed in the endocardium and epicardium and that RA can induce epicardial expression of Fgf9. Using knockout mice and an embryonic heart organ culture system, we show that endocardial and epicardial derived FGF signals regulate myocardial proliferation during midgestation heart development. We further show that this FGF signal is received by both FGF receptors 1 and 2 acting redundantly in the cardiomyoblast. In the absence of this signal, premature differentiation results in cellular hypertrophy and newborn mice develop a dilated cardiomyopathy. FGFs thus constitute all or part of the epicardial signal regulating myocardial growth and differentiation.

Novel tool to suppress cell proliferation in vivo demonstrates that myocardial and coronary vascular growth represent distinct developmental programs

“…Together with our prior studies (Lavine et al, 2005(Lavine et al, , 2006, the data presented here indicate that FGF signaling to the cardiomyoblast has two distinct roles. The first is to promote myocardial proliferation, which, as a consequence, proportionally controls vascular growth.…”

“…Ablation of the epicardium or loss of the epicardially expressed genes, RXR␣ and BAF180, results in defects in cardiomyoblast proliferation and coronary development (Wang et al, 2004;Merki et al, 2005). Based on these data, we and others have proposed that the epicardium secretes factors that govern both of these processes Lavine et al, 2005).…”

“…To further address this issue, we ectopically expressed p27 in a genetic background sensitized to defects in myocardial growth and coronary development. We have previously reported that hearts lacking myocardial FGF signaling (Fgfr1/2 Mlc2v-cre , double conditional knockout) display ventricular hypoplasia and delays in coronary vascular development (Lavine et al, 2005(Lavine et al, , 2006. To determine whether further reductions in cardiomyoblast proliferation would affect coronary development, we bred one copy of the Rosa26 p27-eGFP allele into the Fgfr1/2 Mlc2v-cre background to generate Fgfr1/2 Mlc2v-cre ; p27 embryos.…”

“…All hearts were relaxed in saturated KCl before fixation. BrdU immunohistochemistry was performed as previously described (Lavine et al, 2005). BrdU (12.5 mg/g) was injected into pregnant mice carrying E12.5 embryos 2 hr before harvest.…”

“…We have previously demonstrated that epicardially derived FGF ligands signal to the cardiomyoblast and are required for both myocardial proliferation and development of the coronary vascular system (Lavine et al, 2005(Lavine et al, , 2006, demonstrating that cell proliferation and vascular growth are coordinately controlled by a single signaling pathway during heart development. We have also shown that Hedgehog (HH) signaling acts downstream of FGF in the coronary vasculogenic pathway controlling the expression of VEGF ligands (Lavine et al, 2006).…”