Phosphoinositide-mediated phototransduction in Drosophila photoreceptors: the role of Ca 2+ and trp

Abstract

Drosophila photoreceptors represent a paradigm for the genetic dissection of phototransduction and, more generally for Ca 2+ signalling. As in most invertebrates, phototransduction in Drosophila is mediated by the phosphoinositide (PI) cascade and is completely blocked by null mutations of the norpA gene which encodes a phospholipase C-β isoform. The light-activated conductance in Drosophila is normally highly permeable to Ca 2+, but in null mutants of the trp gene Ca 2+ permeability is great reduced. Furthermore, the trp gene sequence shows homologies with voltage gated Ca 2+ channels, suggesting that trp encodes a light-sensitive channel subunit. Ca 2+ influx via these channels is instrumental in light adaptation, and profoundly influences phototransduction via positive and negative feedback at multiple molecular targets including protein kinase C. The mechanism of activation of the light-sensitive channels remains unresolved. A requirement for Ca 2+ release from internal stores is suggested by the finding that Drosophila photoreceptors cannot sustain a maintained response under various conditions which might be erected to result in depletion of Ca 2+ stores. However, Ca 2+ release cannot be detected by Ca 2+ indicator dyes and raising Ca 2+ by photorelease of caged Ca 2+ fails to mimic excitation. Recent studies, both in situ and with heterologously expressed trp protein, suggest that the trp-dependent channels may be activated by a process analogous to ‘capacitative Ca 2+ entry’, a widespread, but poorly understood mode of PI-regulated Ca 2+ influx in vertebrate cells.