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The DAF-3 Smad protein antagonizes TGF-beta-related receptor signaling in the Caenorhabditis elegans dauer pathway
G I Patterson1, A Koweek, A Wong
1Department of Molecular Biology, Massachusetts General Hospital, Department of Genetics, Harvard Medical School, Boston, Massachusetts 02114 USA.
Abstract:
Signals from TGF-beta superfamily receptors are transduced to the nucleus by Smad proteins, which transcriptionally activate target genes. In Caenorhabditis elegans, defects in a TGF-beta-related pathway cause a reversible developmental arrest and metabolic shift at the dauer larval stage. Null mutations in daf-3 suppress mutations in genes encoding this TGF-beta signal, its receptors, and associated Smad signal transduction proteins. daf-3 encodes a Smad protein that is most closely related to mammalian DPC4, and is expressed throughout development in many of the tissues that are remodeled during dauer development. DAF-4, the type II TGF-beta receptor in this pathway, is also expressed in remodeled tissues. These data suggest that the DAF-7 signal from sensory neurons acts as a neuroendocrine signal throughout the body to directly regulate developmental and metabolic shifts in tissues that are remodeled during dauer formation. A full-length functional DAF-3/GFP fusion protein is predominantly cytoplasmic, and this localization is independent of activity of the upstream TGF-beta-related pathway. However, this fusion protein is associated with chromosomes in mitotic cells, suggesting that DAF-3 binds DNA directly or indirectly. DAF-3 transgenes also interfere with dauer formation, perhaps attributable to a dosage effect. A truncated DAF-3/GFP fusion protein that is predominantly nuclear interferes with dauer formation, implying a role for DAF-3 in the nucleus. These data suggest that DAF-7 signal transduction antagonizes or modifies DAF-3 Smad activity in the nucleus to induce reproductive development; when DAF-7 signals are disabled, unmodified DAF-3 Smad activity mediates dauer arrest and its associated metabolic shift. Therefore, daf-3 is unique in that it is antagonized, rather than activated, by a TGF-beta pathway.
Insights
The daf-3 gene in C. elegans encodes a Smad protein that regulates developmental arrest. TGF-beta signaling antagonizes DAF-3 activity in the nucleus, controlling the dauer larval stage and metabolic shifts.
Area of Science:
- Developmental Biology
- Molecular Biology
- Genetics
Background:
- TGF-beta superfamily signaling pathways regulate gene transcription via Smad proteins.
- In C. elegans, disruptions in a TGF-beta pathway lead to a reversible dauer larval arrest and metabolic changes.
Purpose of the Study:
- To investigate the role of the daf-3 gene, encoding a Smad protein, in the TGF-beta-related pathway controlling dauer development in C. elegans.
- To elucidate the mechanism by which DAF-7 signaling influences DAF-3 activity and dauer formation.
Main Methods:
- Genetic analysis using null mutations in daf-3 to suppress mutations in the TGF-beta pathway.
- Expression analysis of daf-3 and DAF-4 (type II TGF-beta receptor).
- Localization studies using DAF-3/GFP fusion proteins (full-length and truncated) in mitotic and interphase cells.
Main Results:
- daf-3 encodes a Smad protein homologous to mammalian DPC4, expressed in tissues undergoing remodeling during dauer development.
- DAF-3/GFP fusion proteins indicated cytoplasmic localization but association with chromosomes in mitotic cells.
- Truncated, nuclear-localized DAF-3/GFP interfered with dauer formation, suggesting a nuclear function.
- DAF-7 signaling antagonizes DAF-3 nuclear activity to promote reproductive development.
Conclusions:
- DAF-3 acts as a Smad protein that mediates dauer arrest and metabolic shifts when TGF-beta (DAF-7) signaling is absent.
- DAF-3's activity is uniquely antagonized, not activated, by the TGF-beta pathway, highlighting a novel regulatory mechanism.
- Nuclear localization and function of DAF-3 are critical for mediating dauer arrest.