Interlocked Feedforward Loops Control Cell-Type-Specific Rhodopsin Expression in the Drosophila Eye
Highlights
Dve is a key transcription factor in the network regulating Rhodopsins in the fly eye
Dve is a shared component of two opposing, interlocked feedforward loops (FFLs)
An incoherent FFL represses expression of Rhodopsins in outer photoreceptors
A coherent FFL relieves Dve repression and activates Rhodopsins in inner photoreceptors
Summary
How complex networks of activators and repressors lead to exquisitely specific cell-type determination during development is poorly understood. In the Drosophila eye, expression patterns of Rhodopsins define at least eight functionally distinct though related subtypes of photoreceptors. Here, we describe a role for the transcription factor gene defective proventriculus (dve) as a critical node in the network regulating Rhodopsin expression. dve is a shared component of two opposing, interlocked feedforward loops (FFLs). Orthodenticle and Dve interact in an incoherent FFL to repress Rhodopsin expression throughout the eye. In R7 and R8 photoreceptors, a coherent FFL relieves repression by Dve while activating Rhodopsin expression. Therefore, this network uses repression to restrict and combinatorial activation to induce cell-type-specific expression. Furthermore, Dve levels are finely tuned to yield cell-type- and region-specific repression or activation outcomes. This interlocked FFL motif may be a general mechanism to control terminal cell-fate specification.
The INAD Scaffold Is a Dynamic, Redox-Regulated Modulator of Signaling in the Drosophila EyeHighlights
PDZ domains 4 and 5 of Drosophila scaffold protein INAD form a supramodule
Coupling between PDZ4 and 5 elevates the redox potential of the PDZ5 disulfide bond
The release of protons by PIP2 hydrolysis regulates the coupling of PDZ4 and 5
The redox potential cycle of the PDZ5 disulfide bond regulates fly visual signaling
Summary
INAD is a scaffolding protein that regulates signaling in Drosophila photoreceptors. One of its PDZ domains, PDZ5, cycles between reduced and oxidized forms in response to light, but it is unclear how light affects its redox potential. Through biochemical and structural studies, we show that the redox potential of PDZ5 is allosterically regulated by its interaction with another INAD domain, PDZ4. Whereas isolated PDZ5 is stable in the oxidized state, formation of a PDZ45 supramodule locks PDZ5 in the reduced state by raising the redox potential of its Cys606/Cys645 disulfide bond by 330 mV. Acidification, potentially mediated via light and PLCβ-mediated hydrolysis of PIP2, disrupts the interaction between PDZ4 and PDZ5, leading to PDZ5 oxidation and dissociation from the TRP Ca2+ channel, a key component of fly visual signaling. These results show that scaffolding proteins can actively modulate the intrinsic redox potentials of their disulfide bonds to exert regulatory roles in signaling.
