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Fig. 7. A model of FoxE3 activation in Xenopus. (A)
Integration of Otx2 and Notch inputs on the FoxE3 enhancer. Otx2 and
Su(H) are proposed to bind to the FoxE3 enhancer prior to receiving
Notch signaling, but remain in a quiescent state. In response to stimuli from
Delta ligands, the Notch intracellular domain (NICD) is cleaved from its
extracellular domain and translocated into the nucleus. Although this model
shows a direct interaction between membrane-bound Notch and Delta, we do not
know the exact mechanism by which this signaling may occur in this system (see
Discussion). In the nucleus, NICD forms a complex with Su(H) and activates
transcription by recruiting a co-activator (Co-A). Otx2 synergistically
stimulates this transcription. An unidentified factor(s) (indicated with a
question mark) may contribute to boosting and refining the PLE-specific
expression. (B) Otx2 is broadly expressed in the head ectoderm that
includes PLE, while localized Notch signaling is provided to the lens-field
from the underlying optic vesicle (upper). FoxE3 is expressed in the
PLE, which is the region where there is both Otx2 expression and
Notch signaling (beneath).