Postgastrulation Smad2- deficient embryos show defects in embryo turning and anterior morphogenesis

Abstract

SMAD2 is a member of the transforming growth factor ␤ and activin-signaling pathway. To examine the role of Smad2 in postgastrulation development, we independently generated mice with a null mutation in this gene. Smad2-deficient embryos die around day 7.5 of gestation because of failure of gastrulation and failure to establish an anterior-posterior (A-P) axis. Expression of the homeobox gene Hex (the earliest known marker of the A-P polarity and the prospective head organizer) was found to be missing in Smad2-deficient embryos. Homozygous mutant embryos and embryonic stem cells formed mesoderm derivatives revealing that mesoderm induction is SMAD2 independent. In the presence of wild-type extraembryonic tissues, Smad2-deficient embryos developed beyond 7.5 and up to 10.5 days postcoitum, demonstrating a requirement for SMAD2 in extraembryonic tissues for the generation of an A-P axis and gastrulation. The rescued postgastrulation embryos showed malformation of head structures, abnormal embryo turning, and cyclopia. Our results show that Smad2 expression is required at several stages during embryogenesis. T he transforming growth factor ␤ (TGF-␤) superfamily is a group of signaling molecules that play essential roles in development and tissue homeostasis in vertebrates and invertebrates (1-3). Members of this family include the TGF-␤s, activins, and bone morphogenetic proteins.A family of proteins known as SMADs has been shown to function downstream of TGF family type I receptors (reviewed in ref. 4). The TGF-␤ and activin type I receptors interact with SMAD2 and the closely related SMAD3 (5-9). After being phosphorylated by type I receptors, pathway-restricted SMADs associate with SMAD4 (10), translocate to the nucleus, and activate, in conjunction with several specific transcription factors, their respective target genes (4, 11).The SMAD2 protein consists of three domains (4), the Mad homology domain 1 (MH1) and the MH2 domain that harbors the DNA-binding site (12). The third linker region domain connects the MH1 and MH2 domains, is phosphorylated, and may serve as a molecular hinge. The MH2 domain interacts with the type I receptors and is phosphorylated on ligand activation. The SMAD2-type I receptor interaction is also regulated by a newly discovered protein named SARA (13). The MH2 domain also exhibits transactivation properties.The human SMAD2 gene is located on chromosome 18q21, in close proximity to the tumor suppressors DCC and SMAD4͞DPC4 (14-16). Mutations that disrupt the function of SMAD2 have been identified in colorectal cancers (16,17) and lung cancer (18), suggesting its role as a tumor suppressor.Smad2 is expressed almost ubiquitously and has an important role in early development (19)(20)(21)(22). Targeted inactivation of Smad2 resulted in early lethality before gastrulation. Though all these studies showed that Smad2-deficient embryos die because of gastrulation failure, two studies did not describe mesoderm formation (21,22), whereas the other reported transient mesoderm induction but failur...

Interaction between Hex and GATA Transcription Factors in Vascular Endothelial Cells Inhibits flk-1/KDR-mediated Vascular Endothelial Growth Factor Signaling

“…Moreover, Smad-mediated signaling has been implicated in the control of Hex expression (47). Mice that are null for Smad-2 deficiency are embryonic lethal at E7.5 and lack detectable levels of Hex (48). Taken together, these observations raise the possibility that TGF-␤1 signaling is coupled to Hex-GATA-mediated inhibition of flk-1/KDR through a Smad-2-dependent pathway.…”

TGF-β1 inhibition of IFN-γ-induced signaling and Th1 gene expression in CD4+ T cells is Smad3 independent but MAP kinase dependent

“…Smad2 or Smad4). As mice with targeted disruption of either the SMAD2 or SMAD4 genes do not survive to birth (Dunker and Krieglstein, 2000;Heyer et al, 1999), the mouse reagents to test their roles in mediating effects of TGF-β1 in primary T cells are not readily available. The development of mice with T cell specific conditional deletions of these genes should provide a way to experimentally circumvent this problem (Kim et al, 2006).…”

Regulation of cell proliferation by Smad proteins

“…Mice lacking the Smad2 gene show failure in egg cylinder elongation, mesoderm formation, gastrulation, and establishment of an anterior-posterior (A-P) axis and die between E7.5 and E8.5. Analysis of chimeric embryos demonstrated that Smad2 also plays an important role in extraembryonic tissue formation (Nomura and Li, 1998;Waldrip et al, 1998;Weinstein et al, 1998;Heyer et al, 1999). In contrast to Smad2-knockout mice, Smad3-de®cient mice do not die before birth.…”