Constitutive Activity of the Light-Sensitive Channels TRP and TRPL in the Drosophila Diacylglycerol Kinase Mutant, rdgA

Abstract

Mutations in the Drosophila retinal degeneration A (rdgA) gene, which encodes diacylglycerol kinase (DGK), result in early onset retinal degeneration and blindness. Whole-cell recordings revealed that light-sensitive Ca2+ channels encoded by the trp gene were constitutively active in rdgA photoreceptors. Early degeneration was rescued in rdgA;trp double mutants, lacking TRP channels; however, the less Ca2+-permeable light-sensitive channels (TRPL) were constitutively active instead. No constitutive activity was seen in rdgA;trpI;trp mutants lacking both classes of channel, although, like rdgA;trp, these still showed a residual slow degeneration. Responses to light were restored in rdgA;trp but deactivated abnormally slowly, indicating that DGK is required for response termination. The findings suggest that early degeneration in rdgA is caused by uncontrolled Ca2+ influx and support the proposal that diacylglycerol or its metabolites are messengers of excitation in Drosophila photoreceptors.

Phototransduction and retinal degeneration in Drosophila

“…Therefore, the increased DAG appears to be the major reason for the retinal degeneration in rdgA. Consistent with the concept that DAG contributes to excitation during the photoresponse, the retinal degeneration in the rdgA mutant is suppressed by mutations in trp or norpA [136,228]. Thus, it is likely that the retinal degeneration in rdgA is caused by excessive Ca 2+ influx through constitutively active TRP channels.…”

“…In support of this conclusion, introduction of either DAG or PUFAs, such as linolenic acid, results in activation of the channels in photoreceptor cells or in vitro [148]. The channels are constitutively active in the rdgA mutant, which is missing the DAG kinase that metabolizes DAG to PA [136]. Conversely, in a mutant background (laza) that removes the second PLD/PAP pathway for production for production of DAG (Fig.…”

“…However, the flies are not able to sustain a light response. Conversely, strong rdgA alleles display uncontrolled activity in the photoreceptor cells, even in the dark [136]. The implications of these findings are discussed below in the context of the mechanism of activation of the TRP and TRPL channels.…”

“…The most profound effects on photoreceptor cell survival result from disruption of the eye-enriched DAG kinase, RDGA, which normally catalyzes the conversion of DAG to PA [133,227]. The failure to metabolize DAG in rdgA induces constitutive activity of TRP and TRPL and light-independent retinal degeneration [28,136]. Overexpression of the eye-enriched PA phosphatase, Lazaro (Laza), which catalyzes dephosphorylation of PA and generation of DAG, enhances both the increased activation of the photoresponse and the retinal degeneration in rdgA mutant, whereas mutation of laza suppresses both aspects of the rdgA phenotype [32,33].…”

Transient Receptor Potential Ion Channels and Animal Sensation: Lessons from Drosophila Functional Research

“…Second, introduction of caged IP3 into the photoreceptors by whole cell recording pipette also failed to activate the channels (Hardie and Raghu, 1998). Most importantly, null mutations in the IP3 receptor gene of Drosophila had no effect on phototransduction (Acharya et al, 1997;Raghu et al, 2000). These results suggest that IP(3)R-mediated store depletion is unlikely to underlie the TRP and TRPL activation in Drosophila photoreceptors.…”