title: Nuclear beta-catenin is required to specify vegetal cell fates in the sea urchin embryo.
abstract: Beta-catenin is thought to mediate cell fate specification events by localizing to the nucleus where it modulates gene expression. To ask whether beta-catenin is involved in cell fate specification during sea urchin embryogenesis, we analyzed the distribution of nuclear beta-catenin in both normal and experimentally manipulated embryos. In unperturbed embryos, beta-catenin accumulates in nuclei that include the precursors of the endoderm and mesoderm, suggesting that it plays a role in vegetal specification. Using pharmacological, embryological and molecular approaches, we determined the function of beta-catenin in vegetal development by examining the relationship between the pattern of nuclear beta-catenin and the formation of endodermal and mesodermal tissues. Treatment of embryos with LiCl, a known vegetalizing agent, caused both an enhancement in the levels of nuclear beta-catenin and an expansion in the pattern of nuclear beta-catenin that coincided with an increase in endoderm and mesoderm. Conversely, overexpression of a sea urchin cadherin blocked the accumulation of nuclear beta-catenin and consequently inhibited the formation of endodermal and mesodermal tissues including micromere-derived skeletogenic mesenchyme. In addition, nuclear beta-catenin-deficient micromeres failed to induce a secondary axis when transplanted to the animal pole of uninjected host embryos, indicating that nuclear beta-catenin also plays a role in the production of micromere-derived signals. To examine further the relationship between nuclear beta-catenin in vegetal nuclei and micromere signaling, we performed both transplantations and deletions of micromeres at the 16-cell stage and demonstrated that the accumulation of beta-catenin in vegetal nuclei does not require micromere-derived cues. Moreover, we demonstrate that cell autonomous signals appear to regulate the pattern of nuclear beta-catenin since dissociated blastomeres possessed nuclear beta-catenin in approximately the same proportion as that seen in intact embryos. Together, these data show that the accumulation of beta-catenin in nuclei of vegetal cells is regulated cell autonomously and that this localization is required for the establishment of all vegetal cell fates and the production of micromere-derived signals.
authors.affiliation: Developmental, Cellular and Molecular Biology Group and the Department of Zoology, Duke University, Durham, NC 27708, USA. cylogan@cmgm.stanford.edu
authors.forename: C Y
authors.initials: CY
authors.lastname: Logan
authors.forename: J R
authors.initials: JR
authors.lastname: Miller
authors.forename: M J
authors.initials: MJ
authors.lastname: Ferkowicz
authors.forename: D R
authors.initials: DR
authors.lastname: McClay
mesh_terms: D000818:Animals; D015820:Cadherins / Q000235:genetics; D002232:Carbocyanines / Q000378:metabolism; D002454:Cell Differentiation / Q000502:physiology*; D003598:Cytoskeletal Proteins / Q000378:metabolism*; D004625:Embryo, Nonmammalian / Q000166:cytology; D047108:Embryonic Development; D004707:Endoderm / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental / Q000235:genetics; D007150:Immunohistochemistry; D018021:Lithium Chloride / Q000494:pharmacology; D008648:Mesoderm / Q000378:metabolism; D008845:Microinjections; D018613:Microscopy, Confocal; D009687:Nuclear Proteins / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics; D012617:Sea Urchins / Q000196:embryology*; D015534:Trans-Activators; D051176:beta Catenin
chemical_list: C035005:3,3'-dihexadecylindocarbocyanine; D015820:Cadherins; D002232:Carbocyanines; D003598:Cytoskeletal Proteins; D009687:Nuclear Proteins; D012333:RNA, Messenger; D015534:Trans-Activators; D051176:beta Catenin; D018021:Lithium Chloride