Retinoic Acid Regulates Differentiation of the Secondary Heart Field and TGFβ-Mediated Outflow Tract Septation

Abstract

SUMMARY In many experimental models and clinical examples, defects in the differentiation of the second heart field (SHF) and heart outflow tract septation defects are combined, although the mechanistic basis for this relationship has been unclear. We found that as the initial SHF population incorporates into the outflow tract, it is replenished from the surrounding progenitor territory. In retinoic acid (RA) receptor mutant mice, this latter process fails, and the outflow tract is shortened and misaligned as a result. As an additional consequence, the outflow tract is misspecified along its proximal-distal axis, which results in ectopic expression of TGFβ2 and ectopic mesenchymal transformation of the endocardium. Reduction of TGFβ2 gene dosage in the RA receptor deficient background restores septation but does not rescue alignment defects, indicating that excess TGFβ causes septation defects. This may be a common pathogenic pathway when second heart field and septation defects are coupled.

Ectopic retinoic acid signaling affects outflow tract cushion development through suppression of the myocardial Tbx2-Tgfβ2 pathway

“…In general, the SHF contributes to the OFT, but not the AVC myocardium (Harvey, 2002;Laugwitz et al, 2008). Recent studies using RA synthetase (Raldh2) or RA receptor knockout mice have indicated that SHF but not first heart field (FHF) formation is affected in RA-defective mutant embryos (Vermot et al, 2003;Ryckebusch et al, 2008;Li et al, 2010). Our results also showed that the RARE-reporter gene in RA-treated embryos was more highly expressed in the OFT region compared with other regions of the developing heart (Fig.…”

Retinoids and Cardiac Development

“…Interestingly, mouse embryos lacking Raldh2 show a failure in the deployment of SHF cells, which subsequently causes abnormal formation of the outflow tract as revealed by the absence of the y96-Myf5-nlacZ-16 transgene, a marker of the inferior wall of the outflow tract [71,88]. Studies using RARE reporter lines, demonstrated that the RARE-hsp68-lacZ and RARE-Cre;R26R-lacZ transgenes are expressed at E9.5 in the inferior wall of the outflow tract suggesting a differential contribution of RA exposed cells to the formation of this region of the heart [90,94]. Moreover, analysis of the RARE-hsp68-lacZ transgene in the Rarα1/Rarβ mutant background, in which outflow tract defects are observed, revealed the absence of X-gal+ cells in this region [94], demonstrating a role of RA signaling during outflow tract development.…”

“…Studies using RARE reporter lines, demonstrated that the RARE-hsp68-lacZ and RARE-Cre;R26R-lacZ transgenes are expressed at E9.5 in the inferior wall of the outflow tract suggesting a differential contribution of RA exposed cells to the formation of this region of the heart [90,94]. Moreover, analysis of the RARE-hsp68-lacZ transgene in the Rarα1/Rarβ mutant background, in which outflow tract defects are observed, revealed the absence of X-gal+ cells in this region [94], demonstrating a role of RA signaling during outflow tract development.…”

Clonal analysis reveals common lineage relationships between head muscles and second heart field derivatives in the mouse embryo

“…The Mef2c-AHF-enhancer-Cre is activated in the common progenitor of right ventricular myocardium and first-branchial-arch-derived head muscles, but marks only arterial pole myocardium and not head muscles derived from the second branchial arch, presumably reflecting a later activation of the enhancer in this lineage. This may correspond to the retinoic-acid-sensitive population that contributes later to the outflow tract described by Li et al (Li et al, 2010).…”