title: Intracellular mediators of transforming growth factor beta superfamily signaling localize to endosomes in chicken embryo and mouse lenses in vivo.
abstract: BACKGROUND
  Endocytosis is a key regulator of growth factor signaling pathways. Recent studies showed that the localization to endosomes of intracellular mediators of growth factor signaling may be required for their function. Although there is substantial evidence linking endocytosis and growth factor signaling in cultured cells, there has been little study of the endosomal localization of signaling components in intact tissues or organs.
  
  
  RESULTS
  Proteins that are downstream of the transforming growth factor-beta superfamily signaling pathway were found on endosomes in chicken embryo and postnatal mouse lenses, which depend on signaling by members of the TGFbeta superfamily for their normal development. Phosphorylated Smad1 (pSmad1), pSmad2, Smad4, Smad7, the transcriptional repressors c-Ski and TGIF and the adapter molecules Smad anchor for receptor activation (SARA) and C184M, localized to EEA-1- and Rab5-positive vesicles in chicken embryo and/or postnatal mouse lenses. pSmad1 and pSmad2 also localized to Rab7-positive late endosomes. Smad7 was found associated with endosomes, but not caveolae. Bmpr1a conditional knock-out lenses showed decreased nuclear and endosomal localization of pSmad1. Many of the effectors in this pathway were distributed differently in vivo from their reported distribution in cultured cells.
  
  
  CONCLUSIONS
  Based on the findings reported here and data from other signaling systems, we suggest that the localization of activated intracellular mediators of the transforming growth factor-beta superfamily to endosomes is important for the regulation of growth factor signaling.
authors.affiliation: Dept. Ophthalmology and Visual Sciences, USA. rrajagop@artsci.wustl.edu <rrajagop@artsci.wustl.edu>
authors.forename: Ramya
authors.initials: R
authors.lastname: Rajagopal
authors.forename: Shunsuke
authors.initials: S
authors.lastname: Ishii
authors.forename: David C
authors.initials: DC
authors.lastname: Beebe
mesh_terms: D000818:Animals; D015415:Biomarkers; D019485:Bone Morphogenetic Proteins / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism; D002642:Chick Embryo; D011992:Endosomes / Q000378:metabolism*; D007908:Lens, Crystalline / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D015398:Signal Transduction; D051896:Smad Proteins, Receptor-Regulated / Q000378:metabolism; D013997:Time Factors; D016212:Transforming Growth Factor beta / Q000145:classification / Q000378:metabolism*; D033921:Vesicular Transport Proteins / Q000378:metabolism
chemical_list: D015415:Biomarkers; D019485:Bone Morphogenetic Proteins; D008565:Membrane Proteins; D051896:Smad Proteins, Receptor-Regulated; D016212:Transforming Growth Factor beta; D033921:Vesicular Transport Proteins; C093846:early endosome antigen 1