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in which the type II receptor phosphorylates the type I receptor for activation. Nodal
and Vg1 require an EGF-CFC co-receptor, called One-eyed Pinhead (Oep)/Cripto/
FRL, for binding and activation of the receptor complex (Ding et al. 1998 ; Gritsman
et al. 1999 ; Yeo and Whitman 2001 ; Cheng et al. 2003 ). The activated type I recep-
tor phosphorylates cytoplasmic effector proteins called the Smad proteins. The
name Smad is a historical relic of how the genes were discovered. Genetic screens
identified the mothers against decapentaplegic (mad) genes and the sma-2, sma-3
genes as components of the TGF-β signaling pathway in flies and worms, respec-
tively (Sekelsky et al. 1995 ; Savage et al. 1996 ). For simplicity, the nomenclature of
the gene family was revised, combining sma and mad to produce gene family name,
smad. The Smad proteins fall into two categories: Receptor-regulated smads
(R-smads) are phosphorylated by the receptors and coverted to an active state. There
are four main R-smads, called Smad1, Smad2, Smad3, and Smad5 (Massague et al.
2005 ). The TGF-β/Activin/Vg1 class of ligands binds to the Type I Receptor,
ActR1b/ACVR1b and the Type II Receptor ActR2a/ACVR2a or ActR2b/ACVR2b,
and activates Smad2 and Smad3. The BMPs bind to different Type I and Type II
Receptors and activate Smad1 and Smad5. Smad4 is common (co-Smad) to all
TGF-β signal transduction pathways. Smad4 binds to phosphorylated R-Smad pro-
teins, forming a complex that translocates to the nucleus and activates transcription
of target genes (Wu and Hill 2009 ).
7.6.3 Requirement for TGF-β Signals in Germ Layer
Formation
Loss of function studies indicated that signaling through the Activin/TGF-β path-
way is required for germ layer formation. A truncated version of the Type II Activin
Receptor, ActR2b/ACVR2b can bind Activin but this mutant receptor cannot acti-
vate the cognate Type I receptor and downstream Smad effector proteins (Hemmati-
Brivanlou et al. 1992 ). Expression of this mutant receptor blocks the ability of
exogenous molecules to induce mesoderm in Xenopus animal caps, and blocks the
formation of all mesodermal derivatives in whole embryos (Hemmati-Brivanlou
and Melton 1992 ; Schulte-Merker et al. 1994 ). Similar results were obtained by
overexpression of a dominant negative version of the Type I Activin Receptor,
ActR1b/ACVR1b (Chang et al. 1997 ). Expression of the dominant negative Activin-
receptor inhibits endoderm formation in vegetal explants, indicating that this path-
way is also required for endoderm formation in frogs (Cornell et al. 1995 ).
Expression of a dominant negative variant Type II Activin receptor also blocks for-
mation of mesoderm and endoderm in zebrafish (Rodaway et al. 1999 ). In addition,
actR1b/aCVR1b−/− mutant cells are unable to contribute to mesoderm in chimeric
mice, consistent with idea that TGF-β signaling is essential for germ layer forma-
tion (Gu et al. 1998 ). These studies show that activation of the Activin receptor
complex is required for mesoderm and endoderm formation in vertebrates, but do
not identify the essential ligand(s).
W. Tseng et al.