Required, tissue-specific roles for Fgf8 in outflow tract formation and remodeling

Abstract

Fibroblast growth factor 8 (Fgf8) is a secreted signaling protein expressed in numerous temporospatial domains that are potentially relevant to cardiovascular development. However, the pathogenesis of complex cardiac and outflow tract defects observed in Fgf8-deficient mice, and the specific source(s) of Fgf8 required for outflow tract formation and subsequent remodeling are unknown. A detailed examination of the timing and location of Fgf8 production revealed previously unappreciated expression in a subset of primary heart field cells; Fgf8 is also expressed throughout the anterior heart field (AHF) mesoderm and in pharyngeal endoderm at the crescent and early somite stages. We used conditional mutagenesis to examine the requirements for Fgf8 function in these different expression domains during heart and outflow tract morphogenesis. Formation of the primary heart tube and the addition of right ventricular and outflow tract myocardium depend on autocrine Fgf8 signaling in cardiac crescent mesoderm. Loss of Fgf8 in this domain resulted in decreased expression of the Fgf8 target gene Erm,and aberrant production of Isl1 and its target Mef2c in the anterior heart field, thus linking Fgf8 signaling with transcription factor networks that regulate survival and proliferation of the anterior heart field. We further found that mesodermal- and endodermal-derived Fgf8 perform specific functions during outflow tract remodeling: mesodermal Fgf8 is required for correct alignment of the outflow tract and ventricles, whereas activity of Fgf8 emanating from pharyngeal endoderm regulates outflow tract septation. These findings provide a novel insight into how the formation and remodeling of primary and anterior heart field-derived structures rely on Fgf8 signals from discrete temporospatial domains.

Gestational stress induces the unfolded protein response, resulting in heart defects

“…In the SHF, reduction or loss of FGF signaling causes reduced cell proliferation and OFT defects (Hutson et al, 2006;Park et al, 2006Park et al, , 2008Zhang et al, 2008). We therefore examined the expression of direct target genes of FGF signaling in the SHF.…”

“…The simplest model is that receptor or ligand levels are reduced directly in response to cellular hypoxia. FGF signaling in the SHF involves the ligands FGF8 (Brown et al, 2004;Ilagan et al, 2006;Park et al, 2006) and FGF10 (Marguerie et al, 2006;Watanabe et al, 2010) and the receptors FGFR1 and FGFR2 (Marguerie et al, 2006;Park et al, 2008;Zhang et al, 2008). We focused on FGF8 and FGFR1, as these are prominent in SHF formation (Marguerie et al, 2006;Park et al, 2008;Watanabe et al, 2010).…”

“…The proliferation of SHF cells is controlled by FGF and β-catenin/WNT signaling, and their subsequent differentiation is controlled by BMP and non-canonical WNT pathways. Perturbation of proliferation, or premature activation of differentiation or apoptosis, for example by conditional deletion of FGF signaling components in the SHF, results in OFT elongation, alignment and septation defects (Hutson et al, 2006;Ilagan et al, 2006;Park et al, 2006). In humans such defects include membranous ventricular septal defects (VSDs), persistent truncus arteriosus (PTA), double outlet right ventricle (DORV), transposition of the great arteries (TGA), overriding aorta (OA) and tetralogy of Fallot (TOF), and account for as much as 30-60% of all human CHD (Thom et al, 2006;Bruneau, 2008).…”

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

“…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).…”

Expression of Id2 in the second heart field and cardiac defects in Id2 knock‐out mice

“…Although initially cells from both heart fields may express Isl-1 (Park et al, 2006;Prall et al, 2007), cells from the first heart field lose this expression very early while the Isl-1-expressing SHF-derived cells incorporate into the heart later in development after which some cells are still recognized by Isl-1 expression (Cai et al, 2003).…”