Dysregulated endocardial TGFβ signaling and mesenchymal transformation result in heart outflow tract septation failure

Abstract

Heart outflow tract septation in mouse embryos carrying mutations in retinoic acid receptor genes fails with complete penetrance. In this mutant background, ectopic TGFβ signaling in the distal outflow tract is responsible for septation failure, but it was uncertain what tissue was responsive to ectopic TGFβ and why this response interfered with septation. By combining RAR gene mutation with tissue-specific Cre drivers and a conditional type II TGFβ receptor (Tgfbr2) allele, we determined that ectopic activation of TGFβ signaling in the endocardium is responsible for septation defects. Ectopic TGFβ signaling results in ectopic mesenchymal transformation of the endocardium and thereby in improperly constituted distal OFT cushions. Our analysis highlights the interactions between myocardium, endocardium, and neural crest cells in outflow tract morphogenesis, and demonstrates the requirement for proper TGFβ signaling in outflow tract cushion organization and septation.

Recent insights on the role and regulation of retinoic acid signaling during epicardial development

“…At the beginning of cardiogenesis, RA restricts the specification of cardiogenic progenitors from the lateral mesoderm controlling the size of the cardiogenic progenitor pool (Keegan, Feldman, Begemann, Ingham, & Yelon, 2005). RA then promotes the differentiation of a sublineage of the SHF that gives rise to the outflow tract (Li, Pashmforoush, & Sucov, 2010;Ma et al, 2016). The more posterior precursors from both the PHF (Tbx5+) and SHF (Isl1+) are exposed to higher levels of RA and commit toward a more sino-atrial fate to give rise to the inflow tract (Hochgreb, 2003 #3470;Niederreither et al, 2001;Ryckebusch et al, 2008;Sirbu, Zhao, & Duester, 2008;Yutzey, Rhee, & Bader, 1994).…”

“…Next, a caudo-rostral gradient of RA patterns the inflow-outflow partition of the heart field, RA being produced by the posterior mesoderm and later by cardiac precursors. Later in gestation, RA influences the migration and differentiation of cardiac neural crest cells and endocardial cells that contribute to the cushions and aorticopulmonary septum (El Robrini et al, 2016;Ma et al, 2016). RA then promotes the differentiation of a sublineage of the SHF that gives rise to the outflow tract (Li, Pashmforoush, & Sucov, 2010;Ma et al, 2016).…”

“…The more posterior precursors from both the PHF (Tbx5+) and SHF (Isl1+) are exposed to higher levels of RA and commit toward a more sino-atrial fate to give rise to the inflow tract (Hochgreb, 2003 #3470;Niederreither et al, 2001;Ryckebusch et al, 2008;Sirbu, Zhao, & Duester, 2008;Yutzey, Rhee, & Bader, 1994). RA then promotes the differentiation of a sublineage of the SHF that gives rise to the outflow tract (Li, Pashmforoush, & Sucov, 2010;Ma et al, 2016). Later in gestation, RA influences the migration and differentiation of cardiac neural crest cells and endocardial cells that contribute to the cushions and aorticopulmonary septum (El Robrini et al, 2016;Ma et al, 2016).…”

“…RA then promotes the differentiation of a sublineage of the SHF that gives rise to the outflow tract (Li, Pashmforoush, & Sucov, 2010;Ma et al, 2016). Later in gestation, RA influences the migration and differentiation of cardiac neural crest cells and endocardial cells that contribute to the cushions and aorticopulmonary septum (El Robrini et al, 2016;Ma et al, 2016).…”

Making a heart: advances in understanding the mechanisms of cardiac development

“…These findings extend the list of roles of BMP signaling in the developing heart to include structural elements within cardiac muscle. Another study used a complex combination of mutations and partial rescue to determine that BMP signaling in the endocardium (using Tie2Cre), not in neural crest (using Wnt1Cre), is responsible for the outflow tract septation defects observed in a previously described model of truncus arteriosus [9]. Combination strategies such as this allow a more precise understanding of cardiac structural phenotypes that have been described, but not fully explained, using more global knockout strategies.…”