title: Cooperative and independent functions of FGF and Wnt signaling during early inner ear development.
abstract: BACKGROUND
  In multiple vertebrate organisms, including chick, Xenopus, and zebrafish, Fibroblast Growth Factor (FGF) and Wnt signaling cooperate during formation of the otic placode. However, in the mouse, although FGF signaling induces Wnt8a expression during induction of the otic placode, it is unclear whether these two signaling pathways functionally cooperate. Sprouty (Spry) genes encode intracellular antagonists of receptor tyrosine kinase signaling, including FGF signaling. We previously demonstrated that the Sprouty1 (Spry1) and Sprouty2 (Spry2) genes antagonize FGF signaling during induction of the otic placode. Here, we investigate cross talk between FGF/SPRY and Wnt signaling during otic placode induction and assess whether these two signaling pathways functionally cooperate during early inner ear development in the mouse.
  
  
  METHODS
  Embryos were generated carrying combinations of a Spry1 null allele, Spry2 null allele, β-catenin null allele, or a Wnt reporter transgene. Otic phenotypes were assessed by in situ hybridization, semi-quantitative reverse transcriptase PCR, immunohistochemistry, and morphometric analysis of sectioned tissue.
  
  
  RESULTS
  Comparison of Spry1, Spry2, and Wnt reporter expression in pre-otic and otic placode cells indicates that FGF signaling precedes and is active in more cells than Wnt signaling. We provide in vivo evidence that FGF signaling activates the Wnt signaling pathway upstream of TCF/Lef transcriptional activation. FGF regulation of Wnt signaling is functional, since early inner ear defects in Spry1 and Spry2 compound mutant embryos can be genetically rescued by reducing the activity of the Wnt signaling pathway. Interestingly, we find that although the entire otic placode increases in size in Spry1 and Spry2 compound mutant embryos, the size of the Wnt-reporter-positive domain does not increase to the same extent as the Wnt-reporter-negative domain.
  
  
  CONCLUSIONS
  This study provides genetic evidence that FGF and Wnt signaling cooperate during early inner ear development in the mouse. Furthermore, our data suggest that although specification of the otic placode may be globally regulated by FGF signaling, otic specification of cells in which both FGF and Wnt signaling are active may be more tightly regulated.
authors.affiliation: Department of Pediatrics, Children's Research Institute, Medical College of Wisconsin, Milwaukee, WI, 53226, USA. kwright@mcw.edu.
authors.forename: Kevin D
authors.initials: KD
authors.lastname: Wright
authors.affiliation: Department of Pediatrics, Children's Research Institute, Medical College of Wisconsin, Milwaukee, WI, 53226, USA. aamahoneyrogers@oakland.edu.
authors.forename: Amanda A
authors.initials: AA
authors.lastname: Mahoney Rogers
authors.affiliation: Department of Pediatrics, Children's Research Institute, Medical College of Wisconsin, Milwaukee, WI, 53226, USA. jzhang@mcw.edu.
authors.forename: Jian
authors.initials: J
authors.lastname: Zhang
authors.affiliation: Department of Pediatrics, Children's Research Institute, Medical College of Wisconsin, Milwaukee, WI, 53226, USA. katherineshim@gmail.com.
authors.forename: Katherine
authors.initials: K
authors.lastname: Shim
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism; D000818:Animals; D002642:Chick Embryo / Q000378:metabolism; D007758:Ear, Inner / Q000196:embryology* / Q000378:metabolism; D004622:Embryo, Mammalian / Q000378:metabolism*; D005346:Fibroblast Growth Factors / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism; D051379:Mice; D010750:Phosphoproteins / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases; D015398:Signal Transduction; D060449:Wnt Signaling Pathway; D014981:Xenopus / Q000196:embryology / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D047908:Intracellular Signaling Peptides and Proteins; D008565:Membrane Proteins; D010750:Phosphoproteins; C425468:Spry1 protein, mouse; D005346:Fibroblast Growth Factors; D017346:Protein Serine-Threonine Kinases; C118389:Spry2 protein, mouse