Genetic Approaches To Drosophila Phototransduction

To identify the molecules and mechanisms underlying Drosophila phototransduction, electrophysio-logical, cell biological, and pharmacological approaches have been combined with genetic approaches. Many mutations affecting phototransduc-tion have been identified using a relatively simple electrophysiological assay, electroretinogram recordings (ERGs). ERGs are extracellular recordings that measure the summed responses of all the cells in the retina to light. There are two main components of the ERG, the on- and off-transients, which emanate from activity postsynaptic to the photoreceptor cells in the optic lobes and the maintained component (or light coincident receptor potential, LCRP) which is principally a summed response of all the cells in the retina (Fig. 3A). Mutations have been identified which affect each of these two components; however, only those mutations that affect the LCRP are considered here (Table 1). Some of the mutations, such as trp, disrupted proteins that were not previously predicted to function in visual transduction and led to unexpected insights into Drosophila visual transduction. However, the screens did not saturate the genome for loci critical in phototransduction. Thus, mutations in other genes predicted to function in Drosophila visual

Fig. 1. Comparison between vertebrate and Drosophila phototransduction. (A) Vertebrate phototransduction. (B) Model of Drosophila phototransduction. See text for details. The question marks in (B) indicate the uncertainties as to whether TRP is activated by release of Ca2+ from the internal stores or by IP3 and/or DAG through a mechanism independent of the stores. The extracellular (out) and cytoplasmic sides (in) of the membrane are indicated. 5'-GMP, 3':5'-cyclic guanosine monophosphate; CaM, calmodulin; cGMP, 5'-guanosine monophosphate; DAG, diacylglycerol; Ga-, y-, P-subunits of transducin; Gqa, P and y, subunits of Gq; IP3, inositol-1,4,5-trisphosphate; PDE, phosphodiesterase; PIP2, phosphatidylinositol-4,5-bisphosphate; PLC, phospholipase CP.

Fig. 1. Comparison between vertebrate and Drosophila phototransduction. (A) Vertebrate phototransduction. (B) Model of Drosophila phototransduction. See text for details. The question marks in (B) indicate the uncertainties as to whether TRP is activated by release of Ca2+ from the internal stores or by IP3 and/or DAG through a mechanism independent of the stores. The extracellular (out) and cytoplasmic sides (in) of the membrane are indicated. 5'-GMP, 3':5'-cyclic guanosine monophosphate; CaM, calmodulin; cGMP, 5'-guanosine monophosphate; DAG, diacylglycerol; Ga-, y-, P-subunits of transducin; Gqa, P and y, subunits of Gq; IP3, inositol-1,4,5-trisphosphate; PDE, phosphodiesterase; PIP2, phosphatidylinositol-4,5-bisphosphate; PLC, phospholipase CP.

Fig. 2. Structure of Drosophila compound eye. (A) Scanning electron micrograph of the surface of the compound eye. Each eye contains ~800 ommatidia. (B) Schematic representation of a single ommatidium. Shown is an ommatidium at a 90° angle relative to the surface of the eye in the scanning electron micrograph. The ommatidia are ~0.1 mm in length. The corneal lens cell (CO) lies at the top of each ommatidium. Below the CO are the pseudocone (psC), pigment cells (PC), and cone cells (CC). The retina, which is comprised of the photoreceptor cells (R1-6, R7, and R8) and pigment cells (PC), is immediately beneath the cone layer. The microvillar portion of the photoreceptor cells, the rhabdomeres, of the R1-6, R7, and R8 photoreceptor cells are indicated by the columns without shading, with densely spaced diagonal lines and sparsely spaced diagonal lines, respectively. Indicated to the right are sections through the upper and lower portions of the omma-tidia. In the cross-sections through the distal (upper) and proximal (lower) regions of the retina, shown to the right, the rhabdomeres are depicted as black circles. Note that the R7 and R8 cells occupy only the top and bottom portions of the retina respectively. Thus, only seven photoreceptor cells are present in any given plane.

Fig. 3. Examples of mutants with altered electroretinogram recordings. (A) trp and norpA show defects in the light coincident receptor potential (LCRP). norpA flies are unresponsive to light. The trp mutant is characterized by a transient (LCRP). Flies were dark adapted for 2 min and then exposed to a white-light stimulus. The duration of the light pulses was 5 s. The initiation and cessation of the light stimuli are indicated below and a 5 mV scale is included to the right. (B) nina (neither-inactivation-nor-afterpotential) mutants display reduced PDAs. Wild-type and nina mutant flies (ninaDP245) were exposed to pulses of intense blue (B; 480 nm) and orange (O; 580 nm) light. The black and white boxes indicate the initiation and cessation of the blue and orange stimuli, respectively. The stimulus duration was 4 s and the calibration pulse before each response was 5 mV. (Adapted with permission by the Cambridge University Press from Stephenson et al., 1983.)

transduction were generated using genetic strategies that targeted specific loci for mutagenesis.

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