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Regulation of Hoxb4 induction after neurulation by somite signal and neural competence.

Amirthalingam GS, Howard S, Alvarez S, de Lera AR, Itasaki N - BMC Dev. Biol. (2009)

Bottom Line: We also show that the dorsal side of the neural tube has a greater susceptibility to expressing Hoxb4 than the ventral region, a feature associated with dorsalization of the neural tube by BMP signals.BMP4 is additionally able to up-regulate Hoxb4 ventrally, but the effect is restricted to the axial levels at which Hoxb4 is normally expressed, and only in the presence of retinoic acid (RA) or somites, suggesting a role for BMP in rendering the neural tube competent to express Hoxb4 in response to RA or somite signals.In identifying the collaboration between somites and neural tube competence in the induction of Hoxb4, this study demonstrates interplay between A-P and dorsal-ventral (D-V) patterning systems, whereby a specific feature of D-V polarity may be a prerequisite for proper A-P patterning by Hox genes.

View Article: PubMed Central - HTML - PubMed

Affiliation: Division of Developmental Neurobiology, MRC National Institute for Medical Research, The Ridgeway, London, NW7 1AA, UK. saroshi7@yahoo.co.uk

ABSTRACT

Background: While the body axis is largely patterned along the anterior-posterior (A-P) axis during gastrulation, the central nervous system (CNS) shows dynamic changes in the expression pattern of Hox genes during neurulation, suggesting that the CNS refines the A-P pattern continuously after neural tube formation. This study aims at clarifying the role of somites in up-regulating Hoxb4 expression to eventually establish its final pattern and how the neural tube develops a competence to respond to extrinsic signals.

Results: We show that somites are required for the up-regulation of Hoxb4 in the neural tube at the level of somites 1 to 5, the anterior-most domain of expression. However, each somite immediately adjacent to the neural tube is not sufficient at each level; planar signaling is additionally required particularly at the anterior-most segments of the expression domain. We also show that the dorsal side of the neural tube has a greater susceptibility to expressing Hoxb4 than the ventral region, a feature associated with dorsalization of the neural tube by BMP signals. BMP4 is additionally able to up-regulate Hoxb4 ventrally, but the effect is restricted to the axial levels at which Hoxb4 is normally expressed, and only in the presence of retinoic acid (RA) or somites, suggesting a role for BMP in rendering the neural tube competent to express Hoxb4 in response to RA or somite signals.

Conclusion: In identifying the collaboration between somites and neural tube competence in the induction of Hoxb4, this study demonstrates interplay between A-P and dorsal-ventral (D-V) patterning systems, whereby a specific feature of D-V polarity may be a prerequisite for proper A-P patterning by Hox genes.

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The explant culture system recapitulates the initiation of Hoxb4 expression in the neural tube and reveals a stage-dependent requirement of somites. (A-F) As schematized in the left of the panel, explants of neural tube were taken with flanking somites at the level of somites 1 to 5, from embryos at 2 (A, D), 6 (B, E) or 8 (C, F) somite stages, and either fixed immediately (A-C) or cultured for 24 hours (D-F). Immediately fixed explants show no or faint expression of Hoxb4 depending on the stage of explanting, reflecting the normal expression (A-C, n = 3/3 at each stage). After 24 hours of incubation, the explants show strong expression of Hoxb4 throughout the length of the neural tube (D-F, n = 6/6 at each stage). Indicated above each panel are the somite stages (S) at which explants were dissected. (G-R) Neural tube explants were taken either with (G-L) or without (M-R) flanking somites, at stages between 3 and 8 somites (3S to 8S) and cultured for 24 hours. All explants that included somites strongly express Hoxb4 homogenously in the neural tube (G-L, n = 6/6 at each stage). When somites are removed, Hoxb4 expression is absent in explants from 3 to 5 somite stages (M-O, n = 6/6 at each stage). In explants taken at the 6 somite stage, weak expression is observed (P, n = 8/8). It is at the 7 and 8 somite stages that the explants express Hoxb4 strongly (Q, R, n = 6/6 at each stage). Scale bar; 200 μm.
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Figure 2: The explant culture system recapitulates the initiation of Hoxb4 expression in the neural tube and reveals a stage-dependent requirement of somites. (A-F) As schematized in the left of the panel, explants of neural tube were taken with flanking somites at the level of somites 1 to 5, from embryos at 2 (A, D), 6 (B, E) or 8 (C, F) somite stages, and either fixed immediately (A-C) or cultured for 24 hours (D-F). Immediately fixed explants show no or faint expression of Hoxb4 depending on the stage of explanting, reflecting the normal expression (A-C, n = 3/3 at each stage). After 24 hours of incubation, the explants show strong expression of Hoxb4 throughout the length of the neural tube (D-F, n = 6/6 at each stage). Indicated above each panel are the somite stages (S) at which explants were dissected. (G-R) Neural tube explants were taken either with (G-L) or without (M-R) flanking somites, at stages between 3 and 8 somites (3S to 8S) and cultured for 24 hours. All explants that included somites strongly express Hoxb4 homogenously in the neural tube (G-L, n = 6/6 at each stage). When somites are removed, Hoxb4 expression is absent in explants from 3 to 5 somite stages (M-O, n = 6/6 at each stage). In explants taken at the 6 somite stage, weak expression is observed (P, n = 8/8). It is at the 7 and 8 somite stages that the explants express Hoxb4 strongly (Q, R, n = 6/6 at each stage). Scale bar; 200 μm.

Mentions: Although it has been previously shown that somites are capable of inducing Hoxb4 in ectopic locations [16], it is not known whether somites are required for the endogenous up-regulation of Hoxb4. If so, at which stage are they required and are they sufficient for the correct patterning of the neural tube in normal development? In order to address these questions, the somite level 1–5 region was analyzed using the explant culture system. First, in order to test whether the initiation of Hoxb4 expression in the neural tube is recapitulated in the explant culture system, the neural tube at somite level 1–5 including flanking somites, as well as surface ectoderm, notochord and endoderm, were dissected from embryos between 2 and 8 somite stages. It was found that, in all explants dissected at the above stages, Hoxb4 expression was up-regulated in the neural tube along the axial length after 24 hours of culture (Fig. 2A–F). Presence or absence of notochord did not have an effect on the result (data not shown). Next, in order to investigate the requirement of somites, the neural tube was dissected without somites (Fig. 2M–R) and compared to a neural tube that included flanking somites (Fig. 2G–L). The results show that neural explants taken between somite stages 3–5 did not show any up-regulation of Hoxb4 after 24 hours (Fig. 2M–O). Some weak expression was observed in the posterior half when the explant was taken at the 6 somite stage (Fig. 2P). Conversely, from the 7 somite stage onwards, the removal of somites did not affect the level of expression (Fig. 2Q, R), indicating that somites are not required at these stages.


Regulation of Hoxb4 induction after neurulation by somite signal and neural competence.

Amirthalingam GS, Howard S, Alvarez S, de Lera AR, Itasaki N - BMC Dev. Biol. (2009)

The explant culture system recapitulates the initiation of Hoxb4 expression in the neural tube and reveals a stage-dependent requirement of somites. (A-F) As schematized in the left of the panel, explants of neural tube were taken with flanking somites at the level of somites 1 to 5, from embryos at 2 (A, D), 6 (B, E) or 8 (C, F) somite stages, and either fixed immediately (A-C) or cultured for 24 hours (D-F). Immediately fixed explants show no or faint expression of Hoxb4 depending on the stage of explanting, reflecting the normal expression (A-C, n = 3/3 at each stage). After 24 hours of incubation, the explants show strong expression of Hoxb4 throughout the length of the neural tube (D-F, n = 6/6 at each stage). Indicated above each panel are the somite stages (S) at which explants were dissected. (G-R) Neural tube explants were taken either with (G-L) or without (M-R) flanking somites, at stages between 3 and 8 somites (3S to 8S) and cultured for 24 hours. All explants that included somites strongly express Hoxb4 homogenously in the neural tube (G-L, n = 6/6 at each stage). When somites are removed, Hoxb4 expression is absent in explants from 3 to 5 somite stages (M-O, n = 6/6 at each stage). In explants taken at the 6 somite stage, weak expression is observed (P, n = 8/8). It is at the 7 and 8 somite stages that the explants express Hoxb4 strongly (Q, R, n = 6/6 at each stage). Scale bar; 200 μm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2667173&req=5

Figure 2: The explant culture system recapitulates the initiation of Hoxb4 expression in the neural tube and reveals a stage-dependent requirement of somites. (A-F) As schematized in the left of the panel, explants of neural tube were taken with flanking somites at the level of somites 1 to 5, from embryos at 2 (A, D), 6 (B, E) or 8 (C, F) somite stages, and either fixed immediately (A-C) or cultured for 24 hours (D-F). Immediately fixed explants show no or faint expression of Hoxb4 depending on the stage of explanting, reflecting the normal expression (A-C, n = 3/3 at each stage). After 24 hours of incubation, the explants show strong expression of Hoxb4 throughout the length of the neural tube (D-F, n = 6/6 at each stage). Indicated above each panel are the somite stages (S) at which explants were dissected. (G-R) Neural tube explants were taken either with (G-L) or without (M-R) flanking somites, at stages between 3 and 8 somites (3S to 8S) and cultured for 24 hours. All explants that included somites strongly express Hoxb4 homogenously in the neural tube (G-L, n = 6/6 at each stage). When somites are removed, Hoxb4 expression is absent in explants from 3 to 5 somite stages (M-O, n = 6/6 at each stage). In explants taken at the 6 somite stage, weak expression is observed (P, n = 8/8). It is at the 7 and 8 somite stages that the explants express Hoxb4 strongly (Q, R, n = 6/6 at each stage). Scale bar; 200 μm.
Mentions: Although it has been previously shown that somites are capable of inducing Hoxb4 in ectopic locations [16], it is not known whether somites are required for the endogenous up-regulation of Hoxb4. If so, at which stage are they required and are they sufficient for the correct patterning of the neural tube in normal development? In order to address these questions, the somite level 1–5 region was analyzed using the explant culture system. First, in order to test whether the initiation of Hoxb4 expression in the neural tube is recapitulated in the explant culture system, the neural tube at somite level 1–5 including flanking somites, as well as surface ectoderm, notochord and endoderm, were dissected from embryos between 2 and 8 somite stages. It was found that, in all explants dissected at the above stages, Hoxb4 expression was up-regulated in the neural tube along the axial length after 24 hours of culture (Fig. 2A–F). Presence or absence of notochord did not have an effect on the result (data not shown). Next, in order to investigate the requirement of somites, the neural tube was dissected without somites (Fig. 2M–R) and compared to a neural tube that included flanking somites (Fig. 2G–L). The results show that neural explants taken between somite stages 3–5 did not show any up-regulation of Hoxb4 after 24 hours (Fig. 2M–O). Some weak expression was observed in the posterior half when the explant was taken at the 6 somite stage (Fig. 2P). Conversely, from the 7 somite stage onwards, the removal of somites did not affect the level of expression (Fig. 2Q, R), indicating that somites are not required at these stages.

Bottom Line: We also show that the dorsal side of the neural tube has a greater susceptibility to expressing Hoxb4 than the ventral region, a feature associated with dorsalization of the neural tube by BMP signals.BMP4 is additionally able to up-regulate Hoxb4 ventrally, but the effect is restricted to the axial levels at which Hoxb4 is normally expressed, and only in the presence of retinoic acid (RA) or somites, suggesting a role for BMP in rendering the neural tube competent to express Hoxb4 in response to RA or somite signals.In identifying the collaboration between somites and neural tube competence in the induction of Hoxb4, this study demonstrates interplay between A-P and dorsal-ventral (D-V) patterning systems, whereby a specific feature of D-V polarity may be a prerequisite for proper A-P patterning by Hox genes.

View Article: PubMed Central - HTML - PubMed

Affiliation: Division of Developmental Neurobiology, MRC National Institute for Medical Research, The Ridgeway, London, NW7 1AA, UK. saroshi7@yahoo.co.uk

ABSTRACT

Background: While the body axis is largely patterned along the anterior-posterior (A-P) axis during gastrulation, the central nervous system (CNS) shows dynamic changes in the expression pattern of Hox genes during neurulation, suggesting that the CNS refines the A-P pattern continuously after neural tube formation. This study aims at clarifying the role of somites in up-regulating Hoxb4 expression to eventually establish its final pattern and how the neural tube develops a competence to respond to extrinsic signals.

Results: We show that somites are required for the up-regulation of Hoxb4 in the neural tube at the level of somites 1 to 5, the anterior-most domain of expression. However, each somite immediately adjacent to the neural tube is not sufficient at each level; planar signaling is additionally required particularly at the anterior-most segments of the expression domain. We also show that the dorsal side of the neural tube has a greater susceptibility to expressing Hoxb4 than the ventral region, a feature associated with dorsalization of the neural tube by BMP signals. BMP4 is additionally able to up-regulate Hoxb4 ventrally, but the effect is restricted to the axial levels at which Hoxb4 is normally expressed, and only in the presence of retinoic acid (RA) or somites, suggesting a role for BMP in rendering the neural tube competent to express Hoxb4 in response to RA or somite signals.

Conclusion: In identifying the collaboration between somites and neural tube competence in the induction of Hoxb4, this study demonstrates interplay between A-P and dorsal-ventral (D-V) patterning systems, whereby a specific feature of D-V polarity may be a prerequisite for proper A-P patterning by Hox genes.

Show MeSH