Editors' ChoiceCell Biology

Dynamically Regulated by a Common Smad

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Science Signaling  17 Jul 2012:
Vol. 5, Issue 233, pp. ec189
DOI: 10.1126/scisignal.2003400

Transforming growth factor–β (TGF-β) signaling, essential for development, has two branches: TGF-β and activin induce phosphorylation of the receptor-activated transcription factor Smad2, whereas bone morphogenetic proteins (BMPs) activate Smad1. The interaction of Smad1 or Smad2 with the common Smad, Smad4, enables the complexes to function as transcriptional coactivators, following translocation to the nucleus. Warmflash et al. tested the role of nuclear localization of receptor-activated Smads and the common Smad, Smad4, in mediating TGF-β signaling by examining the transcriptional activity and localization of Smad2 and Smad4. Smad2 fused to red fluorescent protein (RFP-Smad2) localized to the nucleus of C2C12 cells when they were stimulated with TGF-β1, and nuclear localization was reduced when an inhibitor of TGF-β receptors was added after stimulation with the ligand, suggesting that Smad2 nuclear localization reflects activity of the pathway. In reporter gene assays, addition of a TGF-β receptor inhibitor at various times after stimulation of the pathway with TGF-β1 showed that the inhibitor was ineffective if added 4 hours after the ligand, suggesting that the pathway was no longer active. The transient nature of the transcriptional response was also observed for endogenous TGF-β target genes in C2C12 cells continuously exposed to ligand. Smad4 fused to green fluorescent protein (GFP-Smad4) also showed transient nuclear accumulation in response to continuous ligand exposure, and the temporal dynamics of the Smad4 nucleocytoplasmic trafficking were not affected by subsequent inhibition of the receptor after initial stimulation with ligand. In cells exposed to TGF-β1, treatment with the protein synthesis inhibitor cyclohexamide eliminated the transient accumulation of GFP-Smad4 in the nucleus, eliminated the consistent accumulation of RFP-Smad2 in the nucleus, and converted the transient transcriptional response into a sustained one. In Xenopus animal cap explants, the authors observed uniform nuclear localization from receptor-activated Smads and multiple pulses of localization to the nucleus for Smad4 over time in response to ligands. Thus, contrary to expectations, the dynamics of TGF-β pathway transcriptional activity in response to continuous ligand exposure appear to depend on the nucleocytoplasmic trafficking of the common Smad, Smad4.

A. Warmflash, Q. Zhang, B. Sorre, A. Vonica, E. D. Siggia, A. H. Brivanlou, Dynamics of TGF-β signaling reveal adaptive and pulsatile behaviors reflected in the nuclear localization of transcription factor Smad4. Proc. Natl. Acad. Sci. U.S.A. 109, E1947–E1956 (2012). [Abstract] [Full Text]

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