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


CONCLUSIONS
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.
Amirthalingam
Gayana S
GS
Division of Developmental Neurobiology, MRC National Institute for Medical Research, The Ridgeway, London, NW7 1AA, UK. saroshi7@yahoo.co.uk
Howard
Sara
S
Alvarez
Susana
S
de Lera
Angel R
AR
Itasaki
Nobue
N
D000818:Animals; D019521:Body Patterning; D002642:Chick Embryo; D018507:Gene Expression Regulation, Developmental; D018398:Homeodomain Proteins / Q000378:metabolism*; D008648:Mesoderm / Q000378:metabolism; D054259:Neural Tube / Q000378:metabolism; D054261:Neurulation; D015398:Signal Transduction; D019170:Somites / Q000378:metabolism*; D014212:Tretinoin / Q000378:metabolism; D015854:Up-Regulation
D018398:Homeodomain Proteins; D014212:Tretinoin