Control of the segmentation process by graded MAPK/ERK activation in the chick embryo

Abstract

The regular spacing of somites during vertebrate embryogenesis involves a dynamic gradient of FGF signaling that controls the timing of maturation of cells in the presomitic mesoderm (PSM). How the FGF signal is transduced by PSM cells is unclear. Here, we first show that the FGF gradient is translated into graded activation of the extracellular signal-regulated kinase (ERK)͞mitogen-activated protein kinase (MAPK) pathway along the PSM in the chicken embryo. Using in ovo electroporation of PSM cells, we demonstrate that constitutive activation of ERK signaling in the PSM blocks segmentation by preventing maturation of PSM cells, thus phenocopying the overexpression of FGF8. Conversely, inhibition of ERK phosphorylation mimics a loss of function of FGF signaling in the PSM. Interestingly, video microscopy analysis of cell movements shows that ERK regulates the motility of PSM cells, suggesting that the decrease of cell movements along the PSM enables mesenchymal PSM cells to undergo proper segmentation. Together, our data demonstrate that ERK is the effector of the gradient of FGF in the PSM that controls the segmentation process. extracellular signal-regulated kinase ͉ fibroblast growth factor ͉ somite S omitogenesis leads to the subdivision of the paraxial mesoderm into transient epithelial metameric units, called somites (1). Somite formation begins anteriorly and proceeds posteriorly, passing like a wave through the paraxial mesoderm as successive groups of cells segregate from the PSM at regular intervals, in concert with extension of the body axis (2). Somite formation involves an oscillator (the segmentation clock) that drives cyclic gene expression. This periodic signal is converted into the repeated array of somite boundaries by a spacing mechanism relying on a traveling threshold of FGF8 and Wnt3a that regresses with body axis extension (3). FGF8 mRNA and protein are distributed in a graded fashion in the caudal presomitic mesoderm (PSM) of vertebrate embryos (4-6), and overexpression of FGF8 in the PSM cells prevents them from differentiating (5). These data led to the idea that high concentrations of FGF8 are required to actively maintain newly formed PSM cells in an immature state. Thus, because of the progressive decrease of FGF8 expression during maturation of the PSM, cells reach a threshold of FGF signaling at a given level in the PSM, called the ''determination front'', where they activate their segmentation program. This determination front marks a molecular transition for PSM cells, as shown by the down-regulation of posterior genes such as brachyury and the activation of new sets of genes such as paraxis in the anterior PSM (5). In chick and fish embryos, FGFR1 is the only FGF receptor to be expressed in the PSM, and in mouse, the FGFR1 knockout disrupts somite formation, suggesting that FGFR1 mediates FGF8 activity in somitogenesis (4,5,7,8).FGF signaling activates a variety of downstream effectors, such has mitogen-activated protein kinase (MAPK)͞extracellu-lar signal-regulated kinase (ERK)...

Random cell movement promotes synchronization of the segmentation clock

“…In zebrafish, the synchronization of the segmentation clock is realized by intercellular interaction via Delta-Notch signaling (11-13, 30, 31). Previous models have confirmed that this interaction successfully leads to the synchronization of oscillation among cells (24,25,30).However, previous analyses did not answer the following questions: (i) In the actual segmentation process, dynamic rearrangement of relative cell positions due to cell proliferation and movement is observed in the PSM (12,32,33). Can the synchronization of the segmentation clock be achieved and stably sustained under such random cell movement?…”

Tbx6-mediated Notch signaling controls somite-specific Mesp2 expression

“…It is likely therefore that the Fgf8 and RA signals also participate in the regulation of Mesp2 expression. Recently, Delfini et al (36) reported an intriguing result suggesting that Fgf signaling represses Mesp expression. Using in ovo electroporation, they demonstrated that the up-regulation of Fgf in the PSM diminishes the endogenous expression of cMeso, the chick Mesp homolog.…”

The long and short of it: Somite formation in mice

“…The function of the FGF8 gradient in determining the spacing of somatic boundaries is refined by a gradient of retinoic acid signaling activity that runs in the opposite direction (Diez del Corral et al, 2003;Moreno and Kintner, 2004). It has been demonstrated in the chicken embryo that the FGF8 gradient is translated into graded activation of the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway in the presomitic mesoderm (Delfini et al, 2005).…”