title: Slow emergence of a multithreshold response to activin requires cell-contact-dependent sharpening but not prepattern.
abstract: The growth factor activin elicits mesodermal fates when applied to prospective ectodermal cells of the Xenopus blastula stage embryo. Previous experiments with dissociated cells showed that there are at least five different responses separated by closely spaced, sharp dose thresholds. Here we investigate this multithreshold activin response further using probes for genes expressed at early gastrula stages, namely Xbra, goosecoid, noggin, Xwnt-8 and Mix.1. We show that initial dose-response profiles are broad and smooth in contrast to the later threshold-bound patterns. For Xbra, goosecoid and noggin, the later expression ranges are subsets of earlier ones. Unexpectedly, Xwnt-8 is initially induced at high doses only, but later appears only in cells that have received a low dose of activin. Keeping the cells dissociated after activin treatment, rather than allowing them to reaggregate, prevents sustained expression of Xbra and Xwnt-8 but allows that of goosecoid and noggin. However, cell contact is required for sharpening the dose-response threshold of goosecoid. Finally, we show that a previously reported dorsoventral prepattern in the animal cap is also cell-contact dependent and it is not required for the multi-threshold response to activin.
authors.affiliation: Department of Molecular and Cell Biology, University of California, Berkeley 94720.
authors.forename: J B
authors.initials: JB
authors.lastname: Green
authors.forename: J C
authors.initials: JC
authors.lastname: Smith
authors.forename: J C
authors.initials: JC
authors.lastname: Gerhart
mesh_terms: D028341:Activins; D000818:Animals; D002352:Carrier Proteins; D002450:Cell Communication / Q000502:physiology; D002454:Cell Differentiation / Q000187:drug effects; D004268:DNA-Binding Proteins / Q000235:genetics; D004305:Dose-Response Relationship, Drug; D004475:Ectoderm / Q000187:drug effects*; D004627:Embryonic Induction / Q000502:physiology; D015870:Gene Expression / Q000187:drug effects; D051956:Goosecoid Protein; D006133:Growth Substances / Q000494:pharmacology*; D018398:Homeodomain Proteins; D007265:Inhibins / Q000494:pharmacology*; D008648:Mesoderm / Q000166:cytology* / Q000502:physiology; D011506:Proteins / Q000235:genetics; D012097:Repressor Proteins; D014157:Transcription Factors; D014981:Xenopus; D000099176:Noggin Protein
chemical_list: D028341:Activins; D002352:Carrier Proteins; D004268:DNA-Binding Proteins; D051956:Goosecoid Protein; D006133:Growth Substances; D018398:Homeodomain Proteins; D007265:Inhibins; D011506:Proteins; D012097:Repressor Proteins; D014157:Transcription Factors; D000099176:Noggin Protein