Retinoic acid and TGFβ signaling regulate cardiovascular development

Congenital heart disease (CHD) occurs in approximately twelve out of 1000 live births (Hoffman et al, 2004), and is the main source of infant mortality and morbidity. Additionally, about three per 1000 live births will require some intervention during the first year of life (Gruber and Epstein 2004). Thus, it is very important to maintain a tight regulation of the various signaling pathways and different interactions between cell lineages in order to develop a healthy and properly functional four chambered heart. With a better understanding of genetic regulation, we will be able to identify the mechanisms behind this complex morphogenesis and ultimately provide better insight into cardiovascular diseases. We used genetically modified mouse models to primarily study the malformation called common arterial trunk (CAT) throughout this thesis. ❧ Retinoic acid (RA), the biologically active derivative of vitamin A, is an important morphogen during development. Deficiency in nutritional vitamin A and mutation in the nuclear receptor for RA will result in CHD. Nearly one third of the CHD involves outflow tract (OFT) malformations. For example, in retinoic acid receptor (RAR) mutants, SHF defects (elongation and misalignment defects) occurred due to the deficiency in recruitment surrounding progenitor cells into the OFT . Consequently, the OFT of RAR mutants has a misspecified proximal-distal axis, and the proximal markers MLC2v and TGFβ2 are both ectopically expressed in the distal OFT. In addition, the RAR mutants also experience a variety of pharyngeal arch artery and great vessel defects. Using various tissue-specific genetic rescue approaches, we show that the endothelial specific reduction of TGFβ receptor 2 (Tgfbr2) gene dosage in the RAR mutant background restores proper septation in the OFT, but does not rescue arch artery defects and alignment defects, demonstrating that the endocardium is the responsive tissue to excess TGFβ2, and that arch artery formation and alignment defects are both independent of TGFβ signaling. ❧ In addition to the migration of the SHF into the OFT, cardiac neural crest cells also migrate towards the OFT at about E9.5-E10.0. These cells are required to initiate the formation of the aorticopulmonary (A/P) septum (Jiang et al, 2000), which divides the aortic sac into aorta and pulmonary vessels. Upon deletion of Tgfbr2 in neural crest cells (NCC) in our mouse model, OFT septation fails and results in CAT, despite the normal migration pattern of NCC into the OFT. Although this model also involves altered TGFβ signaling, the cellular basis of the CAT phenotype in this model is quite different from the retinoic acid mutant. CAT in this mutant background may be mediated by the non-canonical TGFβ signaling pathway. Using a neural crest tissue specific mutant approach, we were able to recapitulate the CAT phenotype in the Wnt1Cre;Tgfbr2 mutant by knocking out TAK1 protein in a tissue-specific way. This result suggested that TGFβ response in the neural crest cell may be mediated by TAK signaling.