title: Mice exclusively expressing the short isoform of Smad2 develop normally and are viable and fertile.
abstract: Smad2 and Smad3 are closely related effectors of TGFbeta/Nodal/Activin-related signaling. Smad3 mutant mice develop normally, whereas Smad2 plays an essential role in patterning the embryonic axis and specification of definitive endoderm. Alternative splicing of Smad2 exon 3 gives rise to two distinct protein isoforms. The short Smad2(Deltaexon3) isoform, unlike full-length Smad2, Smad2(FL), retains DNA-binding activity. Here, we show that Smad2(FL) and Smad2(Deltaexon3) are coexpressed throughout mouse development. Directed expression of either Smad2(Deltaexon3) or Smad3, but not Smad2(FL), restores the ability of Smad2-deficient embryonic stem (ES) cells to contribute descendants to the definitive endoderm in wild-type host embryos. Mice engineered to exclusively express Smad2(Deltaexon3) correctly specify the anterior-posterior axis and definitive endoderm, and are viable and fertile. Moreover, introducing a human Smad3 cDNA into the mouse Smad2 locus similarly rescues anterior-posterior patterning and definitive endoderm formation and results in adult viability. Collectively, these results demonstrate that the short Smad2(Deltaexon3) isoform or Smad3, but not full-length Smad2, activates all essential target genes downstream of TGFbeta-related ligands, including those regulated by Nodal.
authors.affiliation: Department of Molecular and Cellular Biology, Harvard University, Cambridge, Massachusetts 02138, USA.
authors.forename: N Ray
authors.initials: NR
authors.lastname: Dunn
authors.forename: Chad H
authors.initials: CH
authors.lastname: Koonce
authors.forename: Dorian C
authors.initials: DC
authors.lastname: Anderson
authors.forename: Ayesha
authors.initials: A
authors.lastname: Islam
authors.forename: Elizabeth K
authors.initials: EK
authors.lastname: Bikoff
authors.forename: Elizabeth J
authors.initials: EJ
authors.lastname: Robertson
mesh_terms: D000818:Animals; D019521:Body Patterning; D004268:DNA-Binding Proteins / Q000235:genetics* / Q000502:physiology*; D004622:Embryo, Mammalian; D004707:Endoderm; D005298:Fertility / Q000235:genetics; D005328:Fetal Viability / Q000235:genetics; D018507:Gene Expression Regulation, Developmental / Q000502:physiology*; D048788:Growth and Development / Q000235:genetics*; D006801:Humans; D051379:Mice; D008817:Mice, Mutant Strains; D008822:Mice, Transgenic; D020033:Protein Isoforms / Q000502:physiology; D051899:Smad2 Protein; D051900:Smad3 Protein; D013234:Stem Cells; D015534:Trans-Activators / Q000235:genetics* / Q000502:physiology*
chemical_list: D004268:DNA-Binding Proteins; D020033:Protein Isoforms; C497782:SMAD2 protein, human; C497790:SMAD3 protein, human; D051899:Smad2 Protein; C497783:Smad2 protein, mouse; D051900:Smad3 Protein; C497791:Smad3 protein, mouse; D015534:Trans-Activators