Depletion of PtdIns(4,5)P2 underlies retinal degeneration in Drosophila trp mutants

Abstract

SummaryThe prototypical transient receptor potential (TRP) channel is the major light-sensitive, and Ca 2+ -permeable channel in the microvillar photoreceptors of Drosophila. TRP channels are activated following hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P 2 ] by the key effector enzyme phospholipase C (PLC). Mutants lacking TRP channels undergo light-dependent retinal degeneration, as a consequence of the reduced Ca 2+ influx. It has been proposed that degeneration is caused by defects in the Ca 2+ -dependent visual pigment cycle, which result in accumulation of toxic phosphorylated metarhodopsin-arrestin complexes (M PP -Arr2). Here we show that two interventions, which prevent accumulation of M PP -Arr2, namely rearing under red light or eliminating the C-terminal rhodopsin phosphorylation sites, failed to rescue degeneration in trp mutants. Instead, degeneration in trp mutants reared under red light was rescued by mutation of PLC. Degeneration correlated closely with the light-induced depletion of PtdIns(4,5)P 2 that occurs in trp mutants due to failure of Ca 2+ -dependent inhibition of PLC. Severe retinal degeneration was also induced in the dark in otherwise wild-type flies by overexpression of a bacterial PtdInsP n phosphatase (SigD) to deplete PtdIns(4,5)P 2 . In degenerating trp photoreceptors, phosphorylated Moesin, a PtdIns(4,5)P 2 -regulated membrane-cytoskeleton linker essential for normal microvillar morphology, was found to delocalize from the rhabdomere and there was extensive microvillar actin depolymerisation. The results suggest that compromised light-induced Ca 2+ influx, due to loss of TRP channels, leads to PtdIns(4,5)P 2 depletion, resulting in dephosphorylation of Moesin, actin depolymerisation and disintegration of photoreceptor structure.

ERM proteins at a glance

“…The rhabdomere is a microvillusdense membrane that forms through the periscopic extension and 90-degree rotation of the photoreceptor apical membrane; the amplified membrane houses the light-sensitive signaling apparatus and holds it perpendicular to incoming light. The single Drosophila ERM, Moesin, localizes to the rhabdomere terminal web and is required for rhabdomere architecture and function (Karagiosis and Ready, 2004;Sengupta et al, 2013). Moesin associates with photosensitive transient receptor potential (TRP) channels only in the dark; this association is transiently relieved by light-induced dephosphorylation and translocation of Moesin from the cell membrane to the cytoplasm (Chorna-Ornan et al, 2005).…”

“…Moesin associates with photosensitive transient receptor potential (TRP) channels only in the dark; this association is transiently relieved by light-induced dephosphorylation and translocation of Moesin from the cell membrane to the cytoplasm (Chorna-Ornan et al, 2005). Recent studies suggest that light-induced control of membrane PtdIns(4,5)P 2 levels downstream of TRP channel activation is essential for maintaining the membrane localization of Drosophila Moesin and preventing degeneration of the rhabdomere microvillar membrane (Sengupta et al, 2013), providing an elegant example of how the ERMs can act at the nexus of receptor activity and membrane architecture.…”

Negative Regulation of the Novel norpAP24 Suppressor, diehard4, in the Endo-lysosomal Trafficking Underlies Photoreceptor Cell Degeneration

“…These results suggest that the blockage of endo-lysosomal trafficking by Osi21 is not the sole cause of retinal degeneration in rdgC 306 and trp 1 mutants. Recently, Sengupta et al [45] proposed that PI(4,5)P 2 depletion by NORPA underlies retinal degeneration in trp CM and trp 343 mutants. Interestingly, both mutants exhibit faster light-dependent retinal degeneration than trp 1 mutants.…”

Loss of Na+/K+-ATPase in Drosophila photoreceptors leads to blindness and age-dependent neurodegeneration

“…On the microscope stage, a spotlight was shone into the head from the neck side to antidromically illuminate the compound eye. After the images were acquired with a CoolSNAP ES2 CCD Camera, the number of rhabdomeres that appeared as bright dots resulting from a high transmission of light were counted for each upright ommatidium (Sengupta et al, 2013). Degeneration of photoreceptors was assessed directly with this method (Lessing and Bonini, 2009).…”