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Fig. 8. Model for FGF-dependent activation of Pea3 and Erm, and
subsequent induction of Scx in the somite. (A) FGF signaling leads to
expression of Ets transcription factors Pea3 and Erm and
inhibitors Mkp3, Sef and Spry in the anterior and posterior
sclerotome and dermomyotome. FGFs secreted by the myotome bind to and activate
an FGFR (green arrow). Receptor activation results in series of
phosphorylation events (red arrows), culminating in direct or indirect
transcriptional activation (black arrow) of target genes such as Pea3,
Erm, Mkp3, Sef and Spry. (B) Once Pea3 and Erm
expression domains have been established, further FGF signaling triggers
phosphorylation and subsequent activation of Pea3 and Erm,
which, in turn, activate transcription of target genes resulting in
Scx expression. (C) Schematic of four somites, frontal view:
dermomyotomes are beige; myotomes are yellow; sclerotomes are aqua. FGFs
expressed in center of myotome (orange) can diffuse to surrounding
dermomyotome, myotome and dorsal sclerotome (arrows). FGF signaling here
results in expression of Pea3 (red) and Erm (green), in a
nested pattern, within anterior and posterior dermomyotome and dorsal
sclerotome. Scx expression (purple) is induced when myotomal FGFs
signal to the Pea3- and Erm-expressing dermomyotome and
sclerotome.
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