The bHLH transcription factor Tcf21 is required for lineage-specific EMT of cardiac fibroblast progenitors.

The basic helix-loop-helix (bHLH) family of transcription factors orchestrates cell-fate specification, commitment and differentiation in multiple cell lineages during development. Here, we describe the role of a bHLH transcription factor, Tcf21 (epicardin/Pod1/capsulin), in specification of the cardiac fibroblast lineage. In the developing heart, the epicardium constitutes the primary source of progenitor cells that form two cell lineages: coronary vascular smooth muscle cells (cVSMCs) and cardiac fibroblasts. Currently, there is a debate regarding whether the specification of these lineages occurs early in the formation of the epicardium or later after the cells have entered the myocardium. Lineage tracing using a tamoxifen-inducible Cre expressed from the Tcf21 locus demonstrated that the majority of Tcf21-expressing epicardial cells are committed to the cardiac fibroblast lineage prior to initiation of epicardial epithelial-to-mesenchymal transition (EMT). Furthermore, Tcf21 null hearts fail to form cardiac fibroblasts, and lineage tracing of the null cells showed their inability to undergo EMT. This is the first report of a transcription factor essential for the development of cardiac fibroblasts. We demonstrate a unique role for Tcf21 in multipotent epicardial progenitors, prior to the process of EMT that is essential for cardiac fibroblast development.

Platelet-derived growth factor receptor beta signaling is required for efficient epicardial cell migration and development of two distinct coronary vascular smooth muscle cell populations.

The epicardium plays an essential role in coronary artery formation and myocardial development, but signals controlling the development and differentiation of this tissue are not well understood. To investigate the role of platelet-derived growth factor receptor (PDGFR)beta in development of epicardial-derived vascular smooth muscle cells (VSMCs), we examined PDGFRbeta(-/-) and PDGFRbeta epicardial mutant hearts. We found that PDGFRbeta(-/-) hearts failed to form dominant coronary vessels on the ventral heart surface, had a thinned myocardium, and completely lacked coronary VSMCs (cVSMCs). This constellation of defects was consistent with a primary defect in the epicardium. To verify that these defects were specific to epicardial derivatives, we generated mice with an epicardial deletion of PDGFRbeta that resulted in reduced cVSMCs distal to the aorta. The regional absence of cVSMCs suggested that cVSMCs could arise from 2 sources, epicardial and nonepicardial, and that both were dependent on PDGFRbeta. In the absence of PDGFRbeta signaling, epicardial cells adopted an irregular actin cytoskeleton, leading to aberrant migration of epicardial cells into the myocardium in vivo. In addition, PDGF receptor stimulation promoted epicardial cell migration, and PDGFRbeta-driven phosphoinositide 3'-kinase signaling was critical for this process. Our data demonstrate that PDGFRbeta is required for the formation of 2 distinct cVSMC populations and that loss of PDGFRbeta-PI3K signaling disrupts epicardial cell migration.

Disruption of PDGFRalpha-initiated PI3K activation and migration of somite derivatives leads to spina bifida.

Spina bifida, or failure of the vertebrae to close at the midline, is a common congenital malformation in humans that is often synonymous with neural tube defects (NTDs). However, it is likely that other etiologies exist. Genetic disruption of platelet-derived growth factor receptor (PDGFR) alpha results in spina bifida, but the underlying mechanism has not been identified. To elucidate the cause of this birth defect in PDGFRalpha mutant embryos, we examined the developmental processes involved in vertebrae formation. Exposure of chick embryos to the PDGFR inhibitor imatinib mesylate resulted in spina bifida in the absence of NTDs. We next examined embryos with a tissue-specific deletion of the receptor. We found that loss of the receptor from chondrocytes did not recapitulate the spina bifida phenotype. By contrast, loss of the receptor from all sclerotome and dermatome derivatives or disruption of PDGFRalpha-driven phosphatidyl-inositol 3' kinase (PI3K) activity resulted in spina bifida. Furthermore, we identified a migration defect in the sclerotome as the cause of the abnormal vertebral development. We found that primary cells from these mice exhibited defects in PAK1 activation and paxillin localization. Taken together, these results indicate that PDGFRalpha downstream effectors, especially PI3K, are essential for cell migration of a somite-derived dorsal mesenchyme and disruption of receptor signaling in these cells leads to spina bifida.