title: Shifting boundaries of retinoic acid activity control hindbrain segmental gene expression.
abstract: Retinoic acid (RA) generated by Raldh2 in paraxial mesoderm is required for specification of the posterior hindbrain, including restriction of Hoxb1 expression to presumptive rhombomere 4 (r4). Hoxb1 expression requires 3' and 5' RA response elements for widespread induction up to r4 and for r3/r5 repression, but RA has previously been detected only from r5-r8, and vHnf1 is required for repression of Hoxb1 posterior to r4 in zebrafish. We demonstrate in mouse embryos that an RA signal initially travels from the paraxial mesoderm to r3, forming a boundary next to the r2 expression domain of Cyp26a1 (which encodes an RA-degrading enzyme). After Hoxb1 induction, the RA boundary quickly shifts to r4/r5, coincident with induction of Cyp26c1 in r4. A functional role for Cyp26c1 in RA degradation was established through examination of RA-treated embryos. Analysis of Raldh2-/- and vHnf1-/- embryos supports a direct role for RA in Hoxb1 induction up to r4 and repression in r3/r5, as well as an indirect role for RA in Hoxb1 repression posterior to r4 via RA induction of vHnf1 up to the r4/r5 boundary. Our findings suggest that Raldh2 and Cyp26 generate shifting boundaries of RA activity, such that r3-r4 receives a short pulse of RA and r5-r8 receives a long pulse of RA. These two pulses of RA activity function to establish expression of Hoxb1 and vHnf1 on opposite sides of the r4/r5 boundary.
authors.affiliation: OncoDevelopmental Biology Program, Burnham Institute, 10901 North Torrey Pines Road, La Jolla, CA 92037, USA.
authors.forename: Ioan Ovidiu
authors.initials: IO
authors.lastname: Sirbu
authors.forename: Lionel
authors.initials: L
authors.lastname: Gresh
authors.forename: Jacqueline
authors.initials: J
authors.lastname: Barra
authors.forename: Gregg
authors.initials: G
authors.lastname: Duester
mesh_terms: D000445:Aldehyde Oxidoreductases / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D000072500:Cytochrome P450 Family 26; D018507:Gene Expression Regulation, Developmental / Q000187:drug effects / Q000502:physiology*; D018398:Homeodomain Proteins / Q000378:metabolism; D017403:In Situ Hybridization; D051379:Mice / Q000196:embryology*; D008822:Mice, Transgenic; D012249:Rhombencephalon / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology*; D014212:Tretinoin / Q000378:metabolism* / Q000494:pharmacology
chemical_list: C101121:HOXB1 homeodomain protein; D018398:Homeodomain Proteins; D014212:Tretinoin; D003577:Cytochrome P-450 Enzyme System; C495026:Cyp26c1 protein, mouse; D000072500:Cytochrome P450 Family 26; D000445:Aldehyde Oxidoreductases; C470579:RALDH2 protein, mouse