The Drosophila light-activated TRP and TRPL channels - Targets of the phosphoinositide signaling cascade

The Drosophila light-activated Transient Receptor Potential (TRP) channel is the founding member of a large and diverse family of channel proteins. It is now established that TRP channels are evolutionarily conserved and are found in many organisms and tissues. This review outlines the progress made in our understanding of Drosophila phototransduction with a focus on the light sensitive TRP channels. The visual system of Drosophila has remarkable capabilities, such as single photon sensitivity, low dark noise, wide dynamic range of responses to changing ambient light intensities and an unusually wide range of frequency responses to modulated lights. These capabilities are obtained by a unique cellular structure called rhabdomere, which contains 40,000 microvilli, harboring a sophisticated molecular machinery performing phototransduction. The phototransduction cascade was discovered mainly by using the power of Drosophila molecular genetics and the ability to generate mutations in virtually every gene of the cascade. This allowed a detailed functional analysis and mechanistic description of the phototransduction cascade. Drosophila phototransduction has been a model system, instrumental for studying phosphoinositide signaling and its participation in TRP channel activation. Accordingly, the phosphoinositide signaling cascade activates the TRP/TRPL channels via Gq-protein-mediated PLC beta, while the gating mechanism of the channels following PLC activation is still under debate. Detailed studies of the single photon response (quantum bump) and the spontaneous dark bump has given important tools to investigate critical features of channel activation and regulation including: synchronization in channel activity, the existence of a Ca2+ regulated threshold of channel activation, positive and negative feedback and refractory period in bump generation. We anticipate that studies in Drosophila photoreceptors will continue shed light on mechanisms that operate in mammalian TRP channels.