Signaling Pathways Coordinating Mesoderm Induction and Segmentation

As part of its role in axial extension, Brachyury regulates Wnt and retinoic acid signaling. These pathways, along with the FGF pathway, which is also involved in NMP maintenance and differentiation, simultaneously impact the somitogenesis process (Martin, 2016). The addition of new progenitor cells to the growing vertebrate body axis requires these cells to exit the tailbud and join the presomitic mesoderm. This is an active process that begins with an epithelial-to-mesenchymal transition (EMT) and subsequent directional migration into the mesodermal compartment (Figure 5.4A). The NMPs exist as an epithelium, where they undergo a canonical Wnt signaling-induced apical constriction and separation from the epithelium, and enter into a partial EMT state (Goto et al., 2017). Cells in the partial EMT state are migratory but lack directionality, and remain in the tailbud until canonical Wnt and FGF signaling induce the expression of the t-box transcription factors Tbx6 (in mouse) or tbxl6 and tbx6l in zebrafish, as well as bHLH transcription factor msgnl in both mouse and zebrafish (Chapman and Papaioannou, 1998; Fior et al., 2012; Goto et al., 2017; Griffin et al., 1998; Ho and Kane, 1990; Kimmel et al., 1989; Manning and Kimelman, 2015; Morrow et al., 2017; Row et al., 2011; Yabe and Takada, 2012; Yoon and Wold, 2000). The induction of these t-box and bHLH transcription factors causes the cells to undergo a directional migration and exit into the mesodermal compartment, thus completing the EMT (Goto et al., 2017; Manning and Kimelman, 2015; Row et al., 2011).

Exogenously activating the Wnt pathway can accelerate the exit of NMP-derived mesoderm into the presomitic mesoderm through promotion of the EMT event (Bouldin et al., 2015; Goto et al., 2017). Wnt and FGF, which is also required for

Secreted signals control the rate of mesoderm production and establish the position of segment formation

FIGURE 5.4 Secreted signals control the rate of mesoderm production and establish the position of segment formation. (A) NMPs undergo a two-step epithelial-to-mesenchymal transition as they become mesoderm, where Wnt signaling initiates the EMT (yellow), and FGF signaling promotes the completion of the EMT (orange). The process affects the rate at which new mesoderm joins the presomitic mesoderm. (B) The posteriorly localized Wnt and FGF signaling source creates a posterior-to-anterior gradient of signal across the presomitic mesoderm, which establishes the position of segment formation. A reciprocal anterior-to- posterior gradient of retinoic acid signaling represses FGF signaling, refining the FGF gradient across the presomitic mesoderm.

the EMT event, are the same signals that are involved in positioning the wavefront that establishes when a segment boundary forms. When FGF and Wnt signaling drop below a threshold level, segment boundary formation is initiated (Aulehla and Pourquie, 2010). The posteriorly localized expression of FGF and Wnt ligands creates a posterior-to-anterior gradient of signal through the presomitic mesoderm (Figure 5.4B). The dual activities of Wnt and FGF serve as a potential mechanism allowing somitogenesis to scale with presomitic mesoderm size. With higher Wnt and FGF, more cells will enter the presomitic mesoderm, creating a larger presomitic mesoderm. The higher amount of FGF and Wnt signal means that cells will have to be located farther away from the tailbud to reach the threshold level that induces border formation (Aulehla and Pourquie, 2010). Thus, border formation should scale with presomitic mesoderm size since the signals responsible for regulating NMP- derived mesoderm exit into the presomitic mesoderm, and therefore regulating PSM size, are the same signals establishing the wavefront position that dictates the location of border formation. A model has been developed to describe the scaling of somite size with the size of the presomitic mesoderm, which is termed a clock and scaled gradient model (Ishimatsu et al., 2018). This model explains a striking phenomenon, where size-reduced embryos generate the exact number of somites appropriate for that species despite the overall size of the embryo being reduced by nearly half (Cooke, 1975, 1981; Ishimatsu et al., 2018)

As previously mentioned, Brachyury maintains a low retinoic acid environment in the tailbud through direct regulation of cyp26al expression (Martin and Kimelman, 2010). The signaling sink created by posteriorly localized cyp26al expression and the location of the signal source in the newly formed somites creates a retinoic acid gradient across the presomitic mesoderm that is inversely correlated with the FGF and Wnt gradients (Diez del Corral et al., 2003; Shimozono et al., 2013). This RA gradient also helps position the somite boundary by negatively regulating the FGF gradient. In the mouse, RA signaling directly represses Fgf8 expression, and FGF8 along with FGF4 establish the wavefront position (Kumar and Duester, 2014; Naiche et al., 2011). In the absence of RA signaling, FGF signaling increases, shifting the wavefront to a more anterior position and causing the formation of smaller somites (Diez del Corral et al., 2003). Interestingly, in zebrafish RA activates the expression °f fgf8 and fgfl7, yet loss of RA signaling results in smaller somites just as in the mouse (Begemann et al., 2001; Hamade et al., 2006; Kawakami et al., 2005).

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