Regulation of msx1 by a BMP gradient is essential for neural crest specification

CELESTE TRÍBULO; MANUEL J. AYBAR; VU H. NGUYEN; MARY C. MULLINS; ROBERTO MAYOR

DEVELOPMENT

The Company of Biologists

Lugar: Cambridge, UK; Año: 2003 vol. 130 p. 6441 - 6452

0950-1991

There is evidence that the neural crest/neural folds are specified at the border of the neural plate by a specific threshold concentration of a BMP gradient.  In order to understand the molecular mechanism by which a gradient of BMP is able to specify the neural crest we analyzed how BMP controls its downstream target msx and which is the role that this factor has on neural crest specification. As msx  is an expected direct downstream gene of BMP, we experimentally modified the level of BMP activity by use of a dominant negative BMP4 or BMP receptor in Xenopus and zebrafish embryos and also through BMP pathway component mutants in the zebrafish and msx expression was analyzed. All the results show that a reduction in the level of BMP activity leads to an increase in the expression of msx in the neural plate border.  Most interestingly, by reaching different levels of BMP activity in animal cap ectoderm, we show that  a specific concentration of BMP induces msx1 expression, which is similar to the level required to induce neural crest.  Our results indicate that a gradient of BMP specifies the expression of msx in the neural fold region.  In addition, we have analyzed the role that msx1 plays on neural crest specification. As msx1 has a role in dorso-ventral pattering we have carried out conditional gain and loss of function experiments using different msx-1 constructs fused to a glucocorticoid receptor element to avoid an early effect of this factor. We show that msx1 expression is able to induce all other early neural crest markers tested (Snail, Slug, FoxD3) at the time of neural crest specification. Furthermore, the expression of a dominant negative of msx leads to the inhibition of all the neural crest markers analyzed.  These results indicate that msx play a critical role in neural crest specification. It has been previously shown that Snail is one of the earliest genes acting in the neural crest genetic cascade. In order to study the hierarchical relationship between msx-1 and Snail/Slug we performed several rescue experiments  using dominant negatives for these genes. The rescuing activity by Snail and Slug on neural crest development of the msx-1 dominant negative, together with the inability of Slug or Snail to rescue the dominant negative msx-1 strongly argue that msx-1 is upstream of Snail and Slug  in the genetic cascade that specifies the neural crest in the ectoderm.  We propose a model where a gradient of BMP activity specify the expression of msx in the neural folds, and that this expression is essential for the early specification of the neural crest.