Induction of proepicardial marker gene expression by the liver bud

Abstract

Cells of the coronary vessels arise from a unique extracardiac mesothelial cell population, the proepicardium, which develops posterior to the sinoatrial region of the looping-stage heart. Although contribution of the proepicardial cells to cardiac development has been studied extensively, it remains unresolved how the proepicardium is induced and specified in the mesoderm during embryogenesis. It is known, however, that the proepicardium develops from the mesothelium that overlays the liver bud. Here, we show that the expression of proepicardial marker genes -Wt1, capsulin (epicardin, pod1, Tcf21) and Tbx18, can be induced in naïve mesothelial cells by the liver bud, both in vitro and in vivo. Lateral embryonic explants, when co-cultured with the liver bud, were induced to express these proepicardial marker genes. The same induction of the marker genes was detected in vivo when a quail liver bud was implanted in the posterior-lateral regions of a chick embryo. This ectopic induction of marker gene expression was not evident when other endodermal tissues, such as the lung bud or stomach, were implanted. This inductive response to the liver bud was not detectable in host embryos before stage 12 (16-somite stage). These results suggest that, after a specific developmental stage, a large area of the mesothelium becomes competent to express proepicardial marker genes in response to localized liver-derived signal(s). The developmentally regulated competency of mesothelium and a localized inductive signal might play a role in restricting the induction of the proepicardial marker gene expression to a specific region of the mesothelium. The data might also provide a foundation for future engineering of a coronary vascular progenitor population.

Identification of a novel developmental mechanism in the generation of mesothelia

“…Liver bud transplanted ectopically into the lateral embryo distal to the heart induced Wt1 in the closely adjoining tissue. Interestingly, the lung bud and stomach did not have similar inductive capabilities in that system (Ishii et al, 2007). For the majority of the mesothelium not in contact with the liver bud, alternative inductive tissues and signals must be involved.…”

Role of fibroblast growth factor signaling during proepicardium formation in the chick embryo

“…7A,B,E,F,I-L). Similarly the expression domain of Hex, a marker for the liver primordium (Ishii et al, 2007), was unaffected by the FGFR implant (Fig. 7C,D,G,H).…”

“…Bilateral asymmetric PE development is controlled by FGF8 and Snai1 (Schlueter and Brand, 2009). The PE develops from mesodermal cells adjacent to the liver bud, which might produce some unknown inducing signaling factor (Mä nner et al, 2001;Ishii et al, 2007). BMP2 and -4 are expressed in the PE and the adjacent sinus horn myocardium, and BMP signaling is required to maintain PE marker gene expression (Schlueter et al, 2006).…”

“…Sinoatrial myocardium and the liver primordium have both been implicated in PE induction (Ishii et al, 2007;Mikawa and Brand, 2010); hence, we focused our attention on these tissues. We analyzed AMHC, myosin heavy chain, and smooth muscle a-actin expression.…”

“…The exact mechanism that induces PE formation at the venous pole region is currently unknown. Because the PE is closely apposed to the liver bud and sinoatrial myocardium, both of these tissues might play a role in the induction process (Mä nner, 2006;Ishii et al, 2007).…”