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Hox genes control vertebrate body elongation by collinear Wnt repression.

Denans N, Iimura T, Pourquié O - Elife (2015)

Bottom Line: Our data indicate that a subset of progressively more posterior Hox genes, which are collinearly activated in vertebral precursors, repress Wnt activity with increasing strength.This leads to a graded repression of the Brachyury/T transcription factor, reducing mesoderm ingression and slowing down the elongation process.Due to the continuation of somite formation, this mechanism leads to the progressive reduction of PSM size.

View Article: PubMed Central - PubMed

Affiliation: Institut de Génétique et de Biologie Moléculaire et Cellulaire, University of Strasbourg, Illkirch, France.

ABSTRACT
In vertebrates, the total number of vertebrae is precisely defined. Vertebrae derive from embryonic somites that are continuously produced posteriorly from the presomitic mesoderm (PSM) during body formation. We show that in the chicken embryo, activation of posterior Hox genes (paralogs 9-13) in the tail-bud correlates with the slowing down of axis elongation. Our data indicate that a subset of progressively more posterior Hox genes, which are collinearly activated in vertebral precursors, repress Wnt activity with increasing strength. This leads to a graded repression of the Brachyury/T transcription factor, reducing mesoderm ingression and slowing down the elongation process. Due to the continuation of somite formation, this mechanism leads to the progressive reduction of PSM size. This ultimately brings the retinoic acid (RA)-producing segmented region in close vicinity to the tail bud, potentially accounting for the termination of segmentation and axis elongation.

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Control of ingression of PM precursors by anterior Hoxgenes is dependent on Pbx1.(A) Pbx1 expression during somitogenesis. Redsquares: region of PM progenitors. White dashed line: level of transversesection shown in bottom left. (B–C)Pbx1 expression in stage 6–7 HH chicken embryoselectroporated with Venus and control siRNA (B) orPbx1 siRNA (C). Left panels: Venusexpression. (D–J) 2-day-old chicken embryosconsecutively electroporated first with Cherry and a control siRNA and thenwith a Pbx1 siRNA and a Venus construct either alone(D) or together with Pbx1 (E),Hoxb7 (F), Hoxb9(G), Hoxc9 (H),Hoxc11 (I), Hoxa13(J). Arrowheads: anterior boundary of Cherry (red) and ofVenus (green) domains. (K) Ratio of Venus over Cherryexpressing domains. Dots: electroporated embryos. Bar indicates mean.(L) Effect of Pbx1 over-expression on axiselongation rate. Stars represent the p value of the two-tailedStudent's t-test applied between the differentconditions. ***p < 0.005. Error bars: standarderror to the mean (SEM).DOI:http://dx.doi.org/10.7554/eLife.04379.012
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fig7: Control of ingression of PM precursors by anterior Hoxgenes is dependent on Pbx1.(A) Pbx1 expression during somitogenesis. Redsquares: region of PM progenitors. White dashed line: level of transversesection shown in bottom left. (B–C)Pbx1 expression in stage 6–7 HH chicken embryoselectroporated with Venus and control siRNA (B) orPbx1 siRNA (C). Left panels: Venusexpression. (D–J) 2-day-old chicken embryosconsecutively electroporated first with Cherry and a control siRNA and thenwith a Pbx1 siRNA and a Venus construct either alone(D) or together with Pbx1 (E),Hoxb7 (F), Hoxb9(G), Hoxc9 (H),Hoxc11 (I), Hoxa13(J). Arrowheads: anterior boundary of Cherry (red) and ofVenus (green) domains. (K) Ratio of Venus over Cherryexpressing domains. Dots: electroporated embryos. Bar indicates mean.(L) Effect of Pbx1 over-expression on axiselongation rate. Stars represent the p value of the two-tailedStudent's t-test applied between the differentconditions. ***p < 0.005. Error bars: standarderror to the mean (SEM).DOI:http://dx.doi.org/10.7554/eLife.04379.012

Mentions: Expression of anterior Hox genes in the primitive streak ismaintained during the fast axis elongation phase occurring during the formation ofthe first ten somites, suggesting that there must be a mechanism blocking theireffect on ingression during this time window (Figure1A). TALE (Three Amino-acid Loop Extension) family members have been shownto differentially interact with anterior and posterior Hox genes(Chang et al., 1995; Moens and Selleri, 2006). In chicken, the onlyTALE gene expressed in PM precursors is Pbx1 which is detected inthe primitive streak from stage 4 to 7 HH (Figure7A, n = 8 embryos for each condition [Coy and Borycki, 2010]). Electroporation of a siRNA targetingPbx1 in the epiblast resulted in strong down-regulation ofPbx1 (Figure 7B–C,n = 4 embryos for each condition). In consecutive electroporations performedfirst with Cherry and a control siRNA and then with Venus and a siRNA targetingPbx1, cells electroporated with the Pbx1 siRNAwere found extending more anteriorly than control cells (Figure 7D,K, n = 19 embryos). The effect ofPbx1 siRNA on ingression could be rescued by co-expressingPbx1 (Figure 7E,K, n= 16 embryos). We compared in consecutive electroporations the effect ofexpressing first a control siRNA with either Hoxb7, Hoxb9,Hoxa9, Hoxc9, Hoxd10, Hoxd11, Hoxc11, Hoxa13, Hoxb13 orHoxc13, and then the Pbx1 siRNA with the sameHox gene. Cells co-expressing Hoxb7 orHoxb9 and the Pbx1 siRNA reached more anteriorlevels than cells co-expressing these Hox genes and the controlsiRNA (Figure 7F–G, K, n = 10and 15 embryos respectively). In contrast, cells co-expressing either a control orthe Pbx1 siRNA together with a posterior Hox genewere found to extend up to the same anterior level (Figure 7H–K and not shown, n > 8 embryos for eachcondition). Over-expression of Pbx1 in PM precursors after the3-somite stage slowed down axis elongation (Figure7L and Video 5, n = 12embryos), suggesting that Pbx1 can restore the effect of anteriorHox genes on ingression during this time window. Thus,Hox-dependent control of ingression in the paraxial mesodermrequires Pbx1 for anterior but not for posteriorHox genes.10.7554/eLife.04379.012Figure 7.Control of ingression of PM precursors by anterior Hoxgenes is dependent on Pbx1.


Hox genes control vertebrate body elongation by collinear Wnt repression.

Denans N, Iimura T, Pourquié O - Elife (2015)

Control of ingression of PM precursors by anterior Hoxgenes is dependent on Pbx1.(A) Pbx1 expression during somitogenesis. Redsquares: region of PM progenitors. White dashed line: level of transversesection shown in bottom left. (B–C)Pbx1 expression in stage 6–7 HH chicken embryoselectroporated with Venus and control siRNA (B) orPbx1 siRNA (C). Left panels: Venusexpression. (D–J) 2-day-old chicken embryosconsecutively electroporated first with Cherry and a control siRNA and thenwith a Pbx1 siRNA and a Venus construct either alone(D) or together with Pbx1 (E),Hoxb7 (F), Hoxb9(G), Hoxc9 (H),Hoxc11 (I), Hoxa13(J). Arrowheads: anterior boundary of Cherry (red) and ofVenus (green) domains. (K) Ratio of Venus over Cherryexpressing domains. Dots: electroporated embryos. Bar indicates mean.(L) Effect of Pbx1 over-expression on axiselongation rate. Stars represent the p value of the two-tailedStudent's t-test applied between the differentconditions. ***p < 0.005. Error bars: standarderror to the mean (SEM).DOI:http://dx.doi.org/10.7554/eLife.04379.012
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fig7: Control of ingression of PM precursors by anterior Hoxgenes is dependent on Pbx1.(A) Pbx1 expression during somitogenesis. Redsquares: region of PM progenitors. White dashed line: level of transversesection shown in bottom left. (B–C)Pbx1 expression in stage 6–7 HH chicken embryoselectroporated with Venus and control siRNA (B) orPbx1 siRNA (C). Left panels: Venusexpression. (D–J) 2-day-old chicken embryosconsecutively electroporated first with Cherry and a control siRNA and thenwith a Pbx1 siRNA and a Venus construct either alone(D) or together with Pbx1 (E),Hoxb7 (F), Hoxb9(G), Hoxc9 (H),Hoxc11 (I), Hoxa13(J). Arrowheads: anterior boundary of Cherry (red) and ofVenus (green) domains. (K) Ratio of Venus over Cherryexpressing domains. Dots: electroporated embryos. Bar indicates mean.(L) Effect of Pbx1 over-expression on axiselongation rate. Stars represent the p value of the two-tailedStudent's t-test applied between the differentconditions. ***p < 0.005. Error bars: standarderror to the mean (SEM).DOI:http://dx.doi.org/10.7554/eLife.04379.012
Mentions: Expression of anterior Hox genes in the primitive streak ismaintained during the fast axis elongation phase occurring during the formation ofthe first ten somites, suggesting that there must be a mechanism blocking theireffect on ingression during this time window (Figure1A). TALE (Three Amino-acid Loop Extension) family members have been shownto differentially interact with anterior and posterior Hox genes(Chang et al., 1995; Moens and Selleri, 2006). In chicken, the onlyTALE gene expressed in PM precursors is Pbx1 which is detected inthe primitive streak from stage 4 to 7 HH (Figure7A, n = 8 embryos for each condition [Coy and Borycki, 2010]). Electroporation of a siRNA targetingPbx1 in the epiblast resulted in strong down-regulation ofPbx1 (Figure 7B–C,n = 4 embryos for each condition). In consecutive electroporations performedfirst with Cherry and a control siRNA and then with Venus and a siRNA targetingPbx1, cells electroporated with the Pbx1 siRNAwere found extending more anteriorly than control cells (Figure 7D,K, n = 19 embryos). The effect ofPbx1 siRNA on ingression could be rescued by co-expressingPbx1 (Figure 7E,K, n= 16 embryos). We compared in consecutive electroporations the effect ofexpressing first a control siRNA with either Hoxb7, Hoxb9,Hoxa9, Hoxc9, Hoxd10, Hoxd11, Hoxc11, Hoxa13, Hoxb13 orHoxc13, and then the Pbx1 siRNA with the sameHox gene. Cells co-expressing Hoxb7 orHoxb9 and the Pbx1 siRNA reached more anteriorlevels than cells co-expressing these Hox genes and the controlsiRNA (Figure 7F–G, K, n = 10and 15 embryos respectively). In contrast, cells co-expressing either a control orthe Pbx1 siRNA together with a posterior Hox genewere found to extend up to the same anterior level (Figure 7H–K and not shown, n > 8 embryos for eachcondition). Over-expression of Pbx1 in PM precursors after the3-somite stage slowed down axis elongation (Figure7L and Video 5, n = 12embryos), suggesting that Pbx1 can restore the effect of anteriorHox genes on ingression during this time window. Thus,Hox-dependent control of ingression in the paraxial mesodermrequires Pbx1 for anterior but not for posteriorHox genes.10.7554/eLife.04379.012Figure 7.Control of ingression of PM precursors by anterior Hoxgenes is dependent on Pbx1.

Bottom Line: Our data indicate that a subset of progressively more posterior Hox genes, which are collinearly activated in vertebral precursors, repress Wnt activity with increasing strength.This leads to a graded repression of the Brachyury/T transcription factor, reducing mesoderm ingression and slowing down the elongation process.Due to the continuation of somite formation, this mechanism leads to the progressive reduction of PSM size.

View Article: PubMed Central - PubMed

Affiliation: Institut de Génétique et de Biologie Moléculaire et Cellulaire, University of Strasbourg, Illkirch, France.

ABSTRACT
In vertebrates, the total number of vertebrae is precisely defined. Vertebrae derive from embryonic somites that are continuously produced posteriorly from the presomitic mesoderm (PSM) during body formation. We show that in the chicken embryo, activation of posterior Hox genes (paralogs 9-13) in the tail-bud correlates with the slowing down of axis elongation. Our data indicate that a subset of progressively more posterior Hox genes, which are collinearly activated in vertebral precursors, repress Wnt activity with increasing strength. This leads to a graded repression of the Brachyury/T transcription factor, reducing mesoderm ingression and slowing down the elongation process. Due to the continuation of somite formation, this mechanism leads to the progressive reduction of PSM size. This ultimately brings the retinoic acid (RA)-producing segmented region in close vicinity to the tail bud, potentially accounting for the termination of segmentation and axis elongation.

Show MeSH
Related in: MedlinePlus