Mammalian cardiac outflow tract (OFT) and aortic arch morphogenesis is characterized by the tightly coupled development of multiple cell types. The complexity of this process is underscored by the prevalence of congenital anomalies of great vessel patterning in humans.1
The heart, OFT and aortic arch are formed by several developmentally distinct cell populations, including cardiomyocytes (derived from the first and second heart fields), vascular smooth muscle cells (derived from the second heart field and neural crest), and endothelial cells.2
These populations of cells interact within the maturing pharyngeal arches - a dynamic milieu in which cross-regulation between these cell types, as well as the pharyngeal endoderm and ectoderm, results in the coordinated development of the OFT, great vessels and neighboring structures.3, 4
Many of the processes that regulate cell fate decisions and patterning during OFT development are also active in vascular remodeling in adult disease states in humans.5
The OFT initially develops as a single vessel, the truncus arteriosus, comprised of myocardium and smooth muscle cells emerging from a single, unseptated ventricle. Neural crest cells, which make up the bulk of the pharyngeal arch mesenchyme, contribute to the smooth muscle of the OFT and also condense to form much of the smooth muscle of the aortic arch arteries - a series of paired vessels that connect the developing truncus arteriosus to the dorsal aortae.6
The aortic arch arteries are then extensively remodeled, eventually giving rise to the mature aortic arch and several great vessels. Neural crest cells also contribute to septation and rotation of the OFT, with secondary effects on septation of the ventricles. These processes are highly dependent on the migration, expansion and differentiation of cardiac neural crest in response to cues from neighboring cells.1
Murine models of congenital cardiovascular abnormalities have shed new light on the interplay between cell populations in the pharyngeal arches, as well as on the pathogenesis of human congenital cardiac disease. For instance, human mutations in components of the Notch signaling pathway can result in Alagille syndrome, a heterogeneous disorder that can include cardiac OFT defects. It has been demonstrated in mice that active Notch signaling in both neural crest cells and the second heart field is critical for OFT morphogenesis.7, 8
Notch signaling from endothelial cells to neural crest in the pharyngeal mesenchyme via the Notch ligand Jagged1 is required to initiate smooth muscle differentiation in the aortic arch arteries; in the absence of such signaling, mice develop cardiac abnormalities reminiscent of those found in Alagille patients.7
In addition to Notch, other signaling modalities including the Wnt, FGF and BMP pathways, as well as activation of the MAPK pathway by integrin signaling have been shown to be important in OFT formation or remodeling.4, 9-12
However, in spite of our increasing understanding of the signaling pathways that influence OFT formation, the mechanisms by which progenitor cells become competent to respond to external stimuli and the means by which these stimuli are coupled to changes in gene expression remain largely unknown.
Class I Hdacs are important regulators of transcription, via both chromatin-mediated (epigenetic) transcriptional repression and direct deacetylation of key transcriptional regulators.13, 14
Class I Hdacs include Hdac1, Hdac2, Hdac3 and Hdac8. During embryogenesis, these proteins are widely expressed, but play specific roles as modulators of early developmental processes and organogenesis.15-19
As such, class I Hdacs are intriguing candidates for studying the regulation of gene expression during cardiac neural crest development.
Interestingly, neural crest-specific deletion of Hdac1, Hdac2 or Hdac8 alone does not affect patterning of the great vessels.19
Hdac3, however, is unique among class I Hdacs in that it associates with NCoR or SMRT in a well-characterized transcriptional repressor complex.20
Global deletion of Hdac3 results in lethality at the gastrulation stage.15
In a tissue-specific context, Hdac3 has been shown to regulate differentiation in osteoblastic and myocytic precursors.21, 22
Additionally, Hdac3 has been shown to regulate cell cycle progression in cell lines and cardiomyocytes.15, 23-26
Hdac3 also plays important roles in metabolic regulation in multiple tissues, including the heart, through its action as a transcriptional repressor.15, 25, 27, 28
In order to study the role of Hdac3 in cardiac neural crest, we genetically inactivated Hdac3 in premigratory neural crest cells. Using in vivo and ex vivo approaches, we show that Hdac3 plays a critical role in smooth muscle differentiation of neural crest cells.