Invertebrate Photoreceptors. A Comparative Analysis

Turnover of Photoreceptor Membrane and Visual Pigment in Invertebrates
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Functional Genomics Experiments. Protein Structures. Gene Ontology GO Terms. Data Citations. Proteomics Data. The latter use a photopigment closely related to vertebrate rod and cone opsins. View PDF.

Extraocular Photoreception

Save to Library. Create Alert. Share This Paper. Figures and Topics from this paper. Citations Publications citing this paper. In contrast, activation of invertebrate photoreceptors, like Drosophila , leads to stimulation of phospholipase C and the generation of a depolarizing receptor potential. The comparative study of these two systems of phototransduction offers the opportunity to understand how similar biological problems may be solved by different molecular mechanisms of signal transduction.

The study of this process in Drosophila , a system ideally suited to genetic and molecular manipulation, allows us to dissect the function and regulation of such a complex signaling cascade in its normal cellular environment. In this manuscript I review some of our recent findings and the strategies used to dissect this process. The Drosophila compound eye is made up of ommatidia or unit eyes.

Each ommatidium is composed of 20 cells including 8 photoreceptor neurons for review, see ref. The eight photoreceptor neurons can be divided into three main classes depending on their spectral sensitivity: the six outer photoreceptors are blue sensitive and contain the major rhodopsin in the retina, Rh1 2 , 3. Of the two central neurons, the distal R7 cells express one of two different UV-sensitive opsins 4 , and the proximal R8 cell expresses a blue-green rhodopsin 5. The light receptor molecule rhodopsin R is composed of a protein, opsin, covalently linked to a chromophore, 3-hydroxycis-retinal.

Upon absorption of a light photon the chromophore is isomerized from the cis to the all- trans configuration. This change in the conformation of the chromophore leads to a change in the conformation of the protein and to the activation of its catalytic properties. Activated rhodopsin, or metarhodopsin M , activates a heterotrimeric G protein of the G q -family 6 , 7 , which in turn activates a phospholipase C PLC encoded by the norpA gene 8.

PLC catalyzes the breakdown of the minor membrane phospholipid phosphatidyl 4,5-bisphosphate PI P 2 into the two intracellular messengers inositol trisphosphate I P 3 and diacylglycerol DAG. This reaction leads to the opening of cation-selective channels and the generation of a depolarizing receptor potential Drosophila photoreceptors, like most invertebrates, depolarize as opposed to hyperpolarize in response to light. In addition to excitation, photoreceptor neurons have evolved sophisticated mechanisms to control termination of the light response deactivation and light and dark adaptation for review, see D.

Baylor in this issue.

Molecular, genetic, and physiological studies suggest that as many as 50 different gene products are dedicated to the functioning and regulation of this one signaling cascade in the fruit fly Drosophila melanogaster 9 — We and others have used three general strategies to identify molecules involved in phototransduction. The first relies on the expectation that many of the proteins involved in this process will be encoded by genes preferentially expressed in the visual system. This is not an unreasonable assumption because the high degree of specialization seen in photoreceptors was most likely accompanied by the evolution of dedicated components.

Thus, by taking advantage of mutants lacking compound eyes and using highly sensitive subtraction hybridization protocols, it has been possible to isolate a large number of genes encoding eye-specific proteins.

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The second strategy, and perhaps the most powerful, relies on classical genetics and functional screens. Nearly 30 yr ago Seymour Benzer isolated the first Drosophila visual mutants by screening for defects in visual behavior this was also the birth of the field of neurogenetics Several years later, Bill Pak and coworkers at Purdue University pioneered the use of electrophysiological screens to search for mutant flies with defects in visual physiology Since then, several groups, including our own, have extended these screens to include a wide range of genetic, physiological, and behavioral assays of photoreceptor function.

The molecular and physiological analysis of many of these mutants is providing fundamental insight into the biology of this process see below.

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The third strategy relies on enhancer trap screens, a technique developed in Walter Gehring's laboratory several years ago. Using combinations of these three strategies we have been able to identify a number of genes involved in phototransduction 9 — Of particular interest are those whose role could have not been predicted on. The publication costs of this article were defrayed in part by page charge payment. Phototransduction in Drosophila photoreceptors. Absorption of a photon of light causes a conformational change in the rhodopsin molecule R and activates its catalytic properties.

INTRODUCTION

Extracellular sodium and calcium enter the cell through the light-activated conductance and cause the depolarization of the photoreceptor cells. The light-activated conductance appears to be composed of at least two types of channels. The trp gene is required for a class of channels with high calcium permeability. DAG is thought to modulate a photoreceptor cell-specific PKC encoded by inaC that regulates deactivation and desensitization of the light response. Metarhodopsin is inactivated via phosphorylation by rhodopsin kinase RoK and arrestin binding encoded by the arr1 and arr2 genes.

Inactive metarhodopsin is photoconverted back to rhodopsin and then presumably dephosphorylated by the rdgC-encoded phosphatase. The box in the upper right indicates a pathway likely to be required for synthesis of PI P 2. Mutations in all gene products highlighted in red are now available refs. Drosophila phototransduction is one of the best model systems for the study of G protein-coupled PLC signaling 10 , 22 , Not only is the system amenable to molecular genetic analysis but also it can report activity with exquisite sensitivity and specificity: photoreceptor cells are sensitive to single photons, and the signaling pathway can be turned on and off with millisecond kinetics phototransduction in Drosophila is the fastest known G protein cascade, taking just a few tens of milliseconds to go from light activation of rhodopsin to the generation of a receptor potential.

IP3 mobilizes calcium from internal stores, which affects and modulates many cellular processes, and DAG activates members of the PKC family of proteins.

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Given the central role of PIP2 in signaling, its levels may be expected to be tightly regulated in the cell. Using enhancer trap technology, we identified an eye-specific form of CDP-DAG and isolated mutations in this gene eyecds To determine whether eye-cds mutants have a defect in their signaling properties, wild-type and mutant animals were assayed for their ability to maintain a continuously activated state of the photoreceptor cells because such a state would require the continuous availability of the second messenger PIP 2.

Our results demonstrated that light activation depletes a pool of PIP2 necessary for excitation that cannot be replenished in eye-cds mutants Fig. This phenotype is due exclusively to a defect in eye-cds, because introduction of the wild-type eye-CDS cDNA into mutant hosts fully restores wild-type physiology Fig. Furthermore, inclusion of PIP 2 in the patch pipette is sufficient to restore signaling in the depleted eye-cds mutants Fig. On the basis of these findings, we reasoned that it should be possible to modulate the output of this cascade by experimentally manipulating the levels of eye-CDS.

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Indeed, we made. PI P 2 regeneration cycle.

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Listed in parentheses are the sites of action of known photoreceptor cell-specific proteins in Drosophila adapted from ref. These results open up the possibility of genetically and pharmacologically manipulating PI P 2 signaling in vivo and highlight three unexpected aspects of PLC signaling.

This is demonstrated by the observation that eye-cds mutants are only defective in signaling and only in response to light activation. This is further demonstrated by the observation that eye-cds mutants display a reduction in the amplitude of their response as a function of the number of light flashes and thus their state of depletion. A search for second-site mutations that enhance or suppress the eye-CDS phenotype should produce mutations in other components of this cycle and make it possible to carry out a comprehensive genetic dissection of the various players required for the functioning and regulation of PI P 2 and its metabolites.

An important and unresolved issue in the study of invertebrate phototransduction has been the identification of the intracellular messenger s that mediate the opening of the lightactivated ion channels. I P 3 , calcium, and cGMP have been implicated in this process 23 , Although the messenger s that actually gates the plasma membrane ion channels remains elusive, patch clamp studies have provided strong evidence implicating calcium in the regulation of the light response 18 , 25 , For example, extracellular calcium influx is both sufficient and necessary to regulate activation and deactivation kinetics of the light-activated conductance.

In the absence of external calcium, photoreceptors display slow activation and deactivation kinetics. Defects shown by eye-cds mutants in photoreceptor cell function. To determine whether eye-cds mutants have a defect in their signaling properties, we assayed wild-type and mutant animals for their ability to maintain a continuous supply of the second-messenger PI P 2.

Control and mutant cells were dissected and transferred to a bath solution with nominally zero calcium. The excitation mechanisms were then depleted before patching by subjecting the cells to 40 min of a light pulse protocol, consisting of 3 sec of intense light pulses followed by 3 min in the dark. If the same depletion protocol is applied to cds mutant cells, the light response does not recover c , d. This phenotype is due exclusively to a defect in eye-cds because introduction of the wild-type eye-CDS cDNA into mutant hosts fully restores wild-type physiology e , f.