QUICK SEARCH:   [advanced] Author: Keyword(s):
Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents

This Article Summary Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Bel-Vialar, S. Articles by Krumlauf, R. Search for Related Content PubMed PubMed Citation Articles by Bel-Vialar, S. Articles by Krumlauf, R.

Initiating Hox gene expression: in the early chick neural tube differential sensitivity to FGF and RA signaling subdivides the HoxB genes in two distinct groups

Sophie Bel-Vialar^1,*, Nobue Itasaki¹ and Robb Krumlauf^1,2,

¹ Division of Developmental Neurobiology, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK
² Stowers Institute for Medical Research, 1000 East 50^th Street, Kansas City, Missouri 64110, USA
^* Present address: Centre de biologie du développement, UMR 5547 CNRS; 118 route de Narbonne 31062 Toulouse cedex 4, France

View larger version (105K): [in a new window]   Fig. 1. Comparison of Hoxb4 and Hoxb9 expression in the chick neural tube. Dorsal views of different stages of embryos hybridized with Hoxb4 (A-E) or Hoxb9 (F-J). Embryos are at stage 4 (A), stage 8 (B,F), stage 9 (C,G), stage 10- (H), stage 10 (D), stage 11 (I), stage 14 (E), or stage 17 (J). Expression of Hoxb4 remains at a fixed AP boundary in the neural tube once activated, whereas that of Hoxb9 regresses posteriorly in the later stages. White arrows indicate the initial boundary of expression. HIN, Hensen's node. Horizontal black bars mark the boundary of expression in the CNS relative to the adjacent somite (s) number or rhombomere (r).

View larger version (105K): [in a new window]   Fig. 2. Effect of RA treatment and somite grafting on Hoxb4 and Hoxb9 expression in the neural tube. Dorsal views of stage 15 (A-C,G) stage 19 (D,H) and stage 14 (E,F) embryos hybridized with Hoxb4 (A-D) or Hoxb9 (E-H). (A,E) Untreated embryos. (B,F) Retinoic acid treated embryos. Exogenous application of retinoic acid causes anterior shift of the expression domain and creates a new anterior limit of Hoxb4 expression (black arrow in B) while Hoxb9 does not show any anterior shift (F). (C,G) Embryos electroporated with a dnRAR expressing construct unilaterally on the left side of the neural tube. dnRAR causes down-regulation of endogenous Hoxb4 expression (white arrowheads in C) while Hoxb9 expression is not affected (G). (D,H) Hoxb4 and Hoxb9 expression in grafted embryos, whereby posterior somites 23-25 of a stage 15 donor embryo were transposed into an anterior region of a stage 15 host embryo at the level of somite 7-9 and cultured for 36hrs. The grafted somites induce upregulation of Hoxb4 (*, D) while there is no change in the pattern of Hoxb9 expression (*, H). Black arrowheads in H show position of graft. OV, otic vesicle; SM, somite grafts.

View larger version (103K): [in a new window]   Fig. 3. Effect of FGF treatment on Hoxb4 and Hoxb9 expression in the neural tube. Dorsal views of stage 14-15 embryos hybridized with Hoxb4 (A-C) or Hoxb9 (D-F). Untreated embryos (A,D), embryos treated overnight with FGF2 in culture (B,E), embryos electroporated with an e-FGF-expressing construct unilaterally in the left side of the neural tube (C,F). In both cases, Hoxb9 is upregulated by exogenous FGF (E,F) while Hoxb4 shows no change (B,C). In E, note that the anterior limit of the Hoxb9 expression reaches the level just posterior to the otic vesicle. Black arrowheads in F show the extended domain of Hoxb9 on the left and the control limit on the right in the neural tube.

View larger version (81K): [in a new window]   Fig. 4. Effect of FGF treatment on cdxA and cdxB expression in the neural tube. Dorsal views of stage 9 embryos hybridized with cdxA (A,B) and stage 15 embryos hybridized with cdxB (C,D). Untreated control embryos (A,C) and embryos treated for 6 hours (B) or overnight (D) with recombinant FGF2 protein in culture. In B and D white arrowheads show the normal boundary of expression and black arrowheads show the anteriorized limit of expression in the neural tube upon FGF2 treatment. In controls black arrowheads show the normal boundary.

View larger version (122K): [in a new window]   Fig. 5. Effect of FGF2 treatment and electroporation of activated and dominant negative cdx variants on Hoxb9 expression. Dorsal views of stage 15 embryos hybridized with Hoxb9 following treatment with FGF2 and/or electroporation of Xcad constructs. D,E,F shows higher magnification of A,B,C, respectively. (A,D) An embryo treated overnight with FGF2 as a control of the effect of FGF. (B,E) Electroporation of an activated form of Xcad (XcadVP16) unilaterally in the left side of the neural tube induces an anterior expansion of Hoxb9 expression (bracket in B and arrowheads in E). (C,F) An embryo electroporated with a dominant negative form of Xcad (XcadEnR) unilaterally on the left side of the neural tube and cultured for overnight in the presence of FGF2 shows that the FGF mediated induction of Hoxb9 is reduced. Note that the anterior boundary of Hoxb9 expression on the right side (non-electroporated side) is just posterior to the otic vesicle (OV) because of the FGF treatment. The bracket in C and white arrowheads in F mark the zone where the ectopic expression of Hoxb9 caused by FGF treatment is down-regulated by electroporation of the XcadEnR construct.

View larger version (51K): [in a new window]   Fig. 6. Effect of retinoic acid and FGF2 treatments expression of Hoxb genes. All embryos are presented in the dorsal view and were hybridized with riboprobes shown on the left of each horizontal panel. Embryos treated with retinoic acid are at stage 15-17. Untreated control embryos are at stage 15 for Hoxb1 and Hoxb3 and stage 11-13 for Hoxb5 to Hoxb8. FGF2-treated embryos are at stage 11-13. Asterisks mark the level of the otic vesicle. Arrowheads indicate the anterior limit of expression of genes in the neural tube. In retinoic acid-treated embryos, an anterior shift in the expression is seen for Hoxb1, Hoxb3 and Hoxb5. FGF2 treatment causes anterior shifts in the expression of Hoxb6, Hoxb7 and Hoxb8.

View larger version (108K): [in a new window]   Fig. 7. Effect of XcadVP16 expression and FGF treatment in the hindbrain. Dorsal views of stage 15 embryos hybridized with Hoxb4 (A) or Hoxb9 (B), after electroporation with the XcadVP16 expressing construct unilaterally on the left side of the hindbrain. The embryos were cultured overnight (A) in the absence of or (B) in the presence of an exogenous FGF. In both cases (A,B), upregulation and anterior expansion of each Hox gene is observed in the anterior hindbrain region (area between arrowheads) in response to FGF treatment (see also Fig. 5A for comparison). The asterisk (*) in A shows that the anterior limit of expression on the non-electroporated side is at the level of otic vesicle. The asterisk in B is just posterior to the OV.

View larger version (23K): [in a new window]   Fig. 8. Models of Hox response to FGF based on different effects of FGF on Cdx expression and Hox accessibility. (A) In normal development, Hox loci (colored boxes) are progressively opened over time in an anterior to posterior direction. During this period, Cdx expression domains (grey shaded areas) are gradually regressing toward the caudal end of the neural tube. As each Hox gene in a complex is believed to be accessible at a slightly different time, when it becomes accessible it is exposed to a different pattern or level of Cdx expression, to which it can respond. Hence the final boundary of a given Hox gene is determined by two parameters: the time when the Hox locus is accessible and the position of the Cdx anterior boundary at this particular time. As the embryo develops, Hox loci become accessible in a domain where Cdx is not expressed so they are not capable of being induced. This sets up the nested patterns of expression shown at the right. (B) The addition of FGF leads to an anterior expansion and maintenance of Cdx domains of expression over time and leads to an extended accessibility of Hox complexes along the entire AP axis. This dual effect induces an anteriorization of Hox domains in the neural tissue.