Summary Cardiac outflow tract defects in mice lacking ALK 2 in neural crest cells

Conotruncal cardiovascular malformations are a major cause of neonatal mortality (Creazzo et al., 1998). These conditions can be isolated or they can be associated with craniofacial, thymic and parathyroid birth defects as a part of the velocardiofacial/DiGeorge syndrome that is caused by a deletion of the DiGeorge critical region (DGCR) on chromosome 22q11 (Scambler, 2000; Epstein, 2001). Within this region, TBX1 has emerged as a crucial gene for cardiovascular development, and mutations in TBX1 have recently been described in individuals with the DiGeorge phenotype, but without the DGCR deletion (Yagi et al., 2003). Other genes within the DGCR have also been implicated in the DiGeorge phenotype including HIRA and CRKL (Farrell et al., 1999b; Guris et al., 2001). Neural crest cells (NCC) are known to migrate to the arterial pole of the heart and to differentiate into vascular smooth muscle, and outflow tract (OFT) and aortic arch defects, such as those seen in DiGeorge syndrome, can be reproduced in animal models with defects in neural crest. However, Tbx1 is not expressed in NCCs, leading to the hypothesis that crucial signaling pathways between Tbx1expressing cells and migrating NCCs orchestrate specific aspects of cardiovascular development. Members of the FGF, PDGF and TGFβfamily of signaling molecules have been implicated in this process (Lindsay and Baldini, 2001; AbuIssa et al., 2002; Frank et al., 2002; Bachiller et al., 2003; Tallquist and Soriano, 2003; Brown et al., 2004). Aberrant signaling through these pathways may contribute to some forms of genetic and environmentally induced congenital heart disease. The NC is a multipotent population of cells that originates from the dorsal neural tube at all axial levels (LaBonne and Bronner-Fraser, 1999). A subpopulation of NCCs called the cardiac neural crest (CNC) delaminates from the rhombencephalon between the mid-otic placode and the third somite (Kirby et al., 1983; Kirby and Waldo, 1995), and migrates lateroventrally along the pharyngeal arch arteries into pharyngeal arches 3, 4 and 6. Some CNC cells participate in patterning of pharyngeal arch arteries, whereas others continue to migrate to the conotruncus, where they contribute to formation of the aorticopulmonary septum (Knecht and Bronner-Fraser, 2002). BMPs have been shown to play an important role together with other soluble factors, such as WNTs and FGFs, in NC specification, possibly by serving a Cardiac neural crest cells are multipotent migratory cells that contribute to the formation of the cardiac outflow tract and pharyngeal arch arteries. Neural crest-related developmental defects account for a large proportion of congenital heart disorders. Recently, the genetic bases for some of these disorders have been elucidated, and signaling pathways required for induction, migration and differentiation of cardiac neural crest have emerged. Bone morphogenetic proteins comprise a family of secreted ligands implicated in numerous aspects of organogenesis, including heart and neural crest development. However, it has remained generally unclear whether BMP ligands act directly on neural crest or cardiac myocytes during cardiac morphogenesis, or function indirectly by activating other cell types. Studies on BMP receptor signaling during organogenesis have been hampered by the fact that receptor knockouts often lead to early embryonic lethality. We have used a Cre/loxP system for neural crest-specific deletion of the type I receptor, ALK2, in mouse embryos. Mutant mice display cardiovascular defects, including persistent truncus arteriosus, and abnormal maturation of the aortic arch reminiscent of common forms of human congenital heart disease. Migration of mutant neural crest cells to the outflow tract is impaired, and differentiation to smooth muscle around aortic arch arteries is deficient. Moreover, in Alk2 mutants, the distal outflow tract fails to express Msx1, one of the major effectors of BMP signaling. Thus, the type I BMP receptor ALK2 plays an essential cell-autonomous role in the development of the cardiac outflow tract and aortic arch derivatives.