The transforming growth factor-β (TGF-β) signals are mediated by a family of at least nine SMAD proteins, of which SMAD5 is thought to relay signals of the bone morphogenetic protein (BMP)

The cardiovascular system is the first functional organ system to develop in the vertebrate embryo. It starts out with differentiation of mesodermal cells into hemangioblasts, bipotential stem cells capable of development into hematopoietic and endothelial lineages. Angioblasts form a primitive vascular plexus de novo in a process known as vasculogenesis. New blood vessels form by angiogenesis, sprouting or splitting from the pre-existing primary blood vessels, extending this network throughout the embryo. The emerging vascular plexus undergoes rapid pruning and remodeling to form a mature, tree-like network with larger vessels feeding into many smaller capillaries (reviewed in Folkman and D’Amore, 1996; Risau, 1997). Multiple signaling molecules, including fibroblast growth factor-2 (Flamme and Risau, 1992), vascular endothelial growth factor (Carmeliet et al., 1996; Ferrara et al., 1996), angiopoietin-1 (Suri et al., 1996), platelet-derived growth factor (Leveen et al., 1994), Ephrin-B2 (Wang et al., 1998) and transforming growth factor-β1 (TGF-β1) (Dickson et al., 1995), as well as their receptors (Dumont et al., 1994; Fong et al., 1995; Oshima et al., 1996; Sato et al., 1995; Shalaby et al., 1995; Soriano, 1994), are implicated in vasculogenesis and angiogenesis. The TGF-β superfamily consists of over 40 secreted signaling molecules that are known to have important roles in controlling cell fate by regulating proliferation, differentiation and apoptosis (reviewed in Kingsley, 1994; Roberts and Sporn, 1990). TGF-β homologs, particularly TGFβ1, have been shown to regulate new blood vessel formation, both in vivo and in vitro (reviewed in Beck and D’Amore, 1997; Pepper, 1997). Recently, SMAD proteins have been identified as important components of TGF-β signaling pathways. They function downstream of TGF-β receptors and directly transduce signals from the cell membrane into the nucleus. SMAD proteins are highly conserved across species from invertebrates to man. There are nine vertebrate SMADs, including the pathwayspecific SMADs, SMADs 1-3, 5 and 8, MADH6, the common mediator SMAD4, and the inhibitory SMADs, SMAD6 and 7 (Heldin et al., 1997; Kawabata et al., 1998; Watanabe et al., 1997; Massague, 1998). The pathway-specific SMADs are phosphorylated by ligand-activated type I receptors on a conserved Ser-Ser-X-Ser motif in the most C-terminal region, called the MH2 domain (Abdollah et al., 1997; Souchelnytskyi et al., 1997). This releases the MH2 domain from inhibition by the N-terminal MH1 domain, and allows the pathway-specific SMADs to form hetero-oligomers with SMAD4, and translocate into the nucleus, triggering transcriptional responses (Lagna et al., 1996). In vitro studies suggest that SMAD2 and SMAD3 act downstream of the TGF-β and activin receptors (Baker and Harland, 1996; Graff et al., 1996; Zhang et al., 1996), while SMAD1 and SMAD5 respond to BMP signals (Kretzschmar et al., 1997; Lagna et al., 1996; Kawabata 1571 Development 126, 1571-1580 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV4123