Fgf8is required for anterior heart field development

Abstract

In the mouse embryo, the splanchnic mesodermal cells of the anterior heart field (AHF) migrate from the pharynx to contribute to the early myocardium of the outflow tract (OT) and right ventricle (RV). Recent studies have attempted to distinguish the AHF from other precardiac populations, and to determine the genetic and molecular mechanisms that regulate its development. Here, we have used an Fgf8 lacZ allele to demonstrate that Fgf8 is expressed within the developing AHF. In addition, we use both a hypomorphic Fgf8 allele (Fgf8 neo ) and Cre-mediated gene ablation to show that Fgf8 is essential for the survival and proliferation of the AHF. Nkx2.5Cre is expressed in the AHF, primary heart tube and pharyngeal endoderm, while TnT-Cre is expressed only within the specified heart tube myocardium. Deletion of Fgf8 by Nkx2.5Cre results in a significant loss of the Nkx2.5 Cre lineage and severe OT and RV truncations by E9.5, while the remaining heart chambers (left ventricle and atria) are grossly normal. These defects result from significant decreases in cell proliferation and aberrant cell death in both the pharyngeal endoderm and splanchnic mesoderm. By contrast, ablation of Fgf8 in the TnT-Cre domain does not result in OT or RV defects, providing strong evidence that Fgf8 expression is crucial in the pharyngeal endoderm and/or overlying splanchnic mesoderm of the AHF at a stage prior to heart tube elongation. Analysis of downstream signaling components, such as phosphorylated-Erk and Pea3, identifies the AHF splanchnic mesoderm itself as a target for Fgf8 signaling.

Duplication and divergence of fgf8 functions in teleost development and evolution

“…In mouse, Fgf8 is required for several aspects of heart development (Abu-Issa et al, 2002;Ilagan et al, 2006). The role of Fgf8 in heart development seems conserved between ray-fin fish and tetrapods (Reifers et al, 2000b), but in zebrafish, fgf8a plays this role (Reifers et al, 2000b) and in stickleback we infer that fgf8b plays this role because it is expressed in the heart precursor field rather than fgf8a (Fig.…”

Canonical Wnt signaling functions in second heart field to promote right ventricular growth

“…The ␤-catenin loss-of-function embryos had reduced Fgf10 expression in the OFT myocardium, which also likely contributes to diminished proliferation in the mutant OFT. Fgf signaling has been shown to be an important signal that promotes expansion of SHF derivatives (15,16).…”

“…In support of this idea, we observed reduced Fgf10 expression in the SHF of ␤-catenin gain-of-function embryos. As noted above, Fgf signaling in SHF is generally considered to promote proliferation; reduced Fgf signaling would likely result in a shorter OFT (15,16,18). In contrast, once SHF cells have moved into the OFT canonical Wnt signaling has a more direct role in promoting cellular proliferation and myocardial expansion (Fig.…”

“…An enhancer trap into the Fgf10 locus provided early evidence for the existence of the SHF in mice and indicates, along with conditional loss-of-function experiments, the involvement of Fgf signaling in SHF formation (14)(15)(16). Likewise, Bmp signaling has been implicated in SHF development based on data from chick embryos and mouse conditional deletion studies (6,17,18).…”

Tbx5 overexpression favors a first heart field lineage in murine embryonic stem cells and in Xenopus laevis embryos

“…In contrast, cells of the SHF lineage stay longer in a proliferative progenitor state, are integrated into the heart tube later, enter terminal differentiation later, and are fundamental for right ventricle and outflow tract formation (Buckingham et al, 2005;Abu-Issa and Kirby, 2007;Laugwitz et al, 2008). Factors expressed in the SHF lineage before onset of differentiation are among others Isl1 and Tbx1, whereas differentiated derivatives of the SHF such as the outflow tract (OFT) or the right ventricle can be labeled by BMP4 or Hand2, respectively (Cai et al, 2003;Ilagan et al, 2006;Seo and Kume, 2006;Lin et al, 1997;Liu et al, 2004).…”