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#Siamois

2 public questions tagged with this topic.

frog development

Organizer induction requires cooperation between Siamois, Twin, and:

Formation of functional Spemann organizer requires integration of two vegetal signals: dorsal Wnt-beta-catenin via Siamois/Twin and Nodal-related TGF-beta via Smad2. Nodal ligands Xnr1,2,5 activated by VegT phosphorylate Smad2 which enters nucleus and cooperates with Siamois/Twin on enhancers of goosecoid, chordin and Cerberus. Chromatin immunoprecipitation shows Smad2/Siamois co-binding. Either pathway alone insufficient; blocking Smad2 with Cerberus-S or Siamois with dominant-negative abolishes organizer. Therefore cooperation between Siamois, Twin and Smad2 creates robust organizer induction. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: Gilbert, Developmental Biology 12th ed., Chapter 10: Organizer induction by Siamois, Twin and Smad2 cooperation.

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frog development

Siamois and Twin are activated by:

Siamois and Twin are immediate early zygotic genes expressed in Nieuwkoop center downstream of Wnt pathway. Beta-catenin stabilized dorsally enters nucleus displacing Groucho corepressor from Tcf3, forming beta-catenin-Tcf3/Lef activation complex binding specific TCF sites in siamois and twin promoters. Their transcription suppressed by dominant-negative Tcf3 or beta-catenin depletion, superinduced by LiCl or Wnt mRNA. Noggin, Activin or BMP act later or downstream, so primary activator complex responsible for their initiation is beta-catenin-Tcf3 complex, initiating organizer cascade. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: Gilbert, Developmental Biology 12th ed., Chapter 10: Siamois and Twin activation by beta-catenin-Tcf3.

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