title: Patterning of the third pharyngeal pouch into thymus/parathyroid by Six and Eya1.
abstract: Previous studies have suggested a role of the homeodomain Six family proteins in patterning the developing vertebrate head that involves appropriate segmentation of three tissue layers, the endoderm, the paraxial mesoderm and the neural crest cells; however, the developmental programs and mechanisms by which the Six genes act in the pharyngeal endoderm remain largely unknown. Here, we examined their roles in pharyngeal pouch development. Six1-/- mice lack thymus and parathyroid and analysis of Six1-/- third pouch endoderm demonstrated that the patterning of the third pouch into thymus/parathyroid primordia is initiated. However, the endodermal cells of the thymus/parathyroid rudiments fail to maintain the expression of the parathyroid-specific gene Gcm2 and the thymus-specific gene Foxn1 and subsequently undergo abnormal apoptosis, leading to a complete disappearance of organ primordia by E12.5. This thus defines the thymus/parathyroid defects present in the Six1 mutant. Analyses of the thymus/parathyroid development in Six1-/-;Six4-/- double mutant show that both Six1 and Six4 act synergistically to control morphogenetic movements of early thymus/parathyroid tissues, and the threshold of Six1/Six4 appears to be crucial for the regulation of the organ primordia-specific gene expression. Previous studies in flies and mice suggested that Eya and Six genes may function downstream of Pax genes. Our data clearly show that Eya1 and Six1 expression in the pouches does not require Pax1/Pax9 function, suggesting that they may function independently from Pax1/Pax9. In contrast, Pax1 expression in all pharyngeal pouches requires both Eya1 and Six1 function. Moreover, we show that the expression of Tbx1, Fgf8 and Wnt5b in the pouch endoderm was normal in Six1-/- embryos and slightly reduced in Six1-/-;Six4-/- double mutant, but was largely reduced in Eya1-/- embryos. These results indicate that Eya1 appears to be upstream of very early events in the initiation of thymus/parathyroid organogenesis, while Six genes appear to act in an early differentiation step during thymus/parathyroid morphogenesis. Together, these analyses establish an essential role for Eya1 and Six genes in patterning the third pouch into organ-specific primordia.
authors.affiliation: McLaughlin Research Institute for Biomedical Sciences, Great Falls, MT 59405, USA.
authors.forename: Dan
authors.initials: D
authors.lastname: Zou
authors.forename: Derek
authors.initials: D
authors.lastname: Silvius
authors.forename: Julie
authors.initials: J
authors.lastname: Davenport
authors.forename: Raphaelle
authors.initials: R
authors.lastname: Grifone
authors.forename: Pascal
authors.initials: P
authors.lastname: Maire
authors.forename: Pin-Xian
authors.initials: PX
authors.lastname: Xu
mesh_terms: D000818:Animals; D019521:Body Patterning / Q000235:genetics; D001934:Branchial Region / Q000196:embryology* / Q000378:metabolism; D051524:Fibroblast Growth Factor 8 / Q000235:genetics; D051858:Forkhead Transcription Factors / Q000235:genetics; D005865:Gestational Age; D006023:Glycoproteins / Q000235:genetics; D018398:Homeodomain Proteins / Q000235:genetics* / Q000378:metabolism; D047908:Intracellular Signaling Peptides and Proteins / Q000172:deficiency / Q000235:genetics* / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D008817:Mice, Mutant Strains; D008954:Models, Biological; D009687:Nuclear Proteins / Q000172:deficiency / Q000235:genetics* / Q000378:metabolism; D051761:Paired Box Transcription Factors / Q000235:genetics; D010280:Parathyroid Glands / Q000196:embryology* / Q000378:metabolism; D017027:Protein Tyrosine Phosphatases / Q000172:deficiency / Q000235:genetics* / Q000378:metabolism; D020825:T-Box Domain Proteins / Q000235:genetics; D013950:Thymus Gland / Q000196:embryology* / Q000378:metabolism; D015534:Trans-Activators / Q000172:deficiency / Q000235:genetics* / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics; D051153:Wnt Proteins / Q000235:genetics
chemical_list: C496457:Fgf8 protein, mouse; D051858:Forkhead Transcription Factors; D006023:Glycoproteins; D018398:Homeodomain Proteins; D047908:Intracellular Signaling Peptides and Proteins; D009687:Nuclear Proteins; D051761:Paired Box Transcription Factors; C490947:Six1 protein, mouse; C098490:Six4 protein, mouse; D020825:T-Box Domain Proteins; C491573:Tbx1 protein, mouse; D015534:Trans-Activators; D014157:Transcription Factors; C090140:Whn protein; D051153:Wnt Proteins; C468567:Wnt5b protein, mouse; C070835:PAX1 transcription factor; D051524:Fibroblast Growth Factor 8; C501156:Eya1 protein, mouse; D017027:Protein Tyrosine Phosphatases