Annu. Rev. Cell Bioi. Copyright ©
1991.
7:
ANNUAL REVIEWS
16/�90
1991 by Annual Reviews Inc.
All rights reserved
Further
Quick links to online content
SIGNAL TRANSDUCTION IN
Annu. Rev. Cell. Biol. 1991.7:161-190. Downloaded from www.annualreviews.org Access provided by University of Waterloo on 11/02/15. For personal use only.
THE VISUAL SYSTEM OF DROSOPHILA Dean P. Smith, Mark A. Stamnes, and Charles S. Zuker Howard Hughes Medical Institute, and Departments of Biology and
Neurosciences, University of California, San Diego, La Jolla, California 92093-0649 KEY WORDS:
Drosophila, G proteins, second messengers, phototransduction, visual system
CONTENTS INTRODUCTION.............................................................................................................. GENETIC DISSECTION OF VISUAL TRANSDUCTION IN DROSOPHILA......................................
Genetic Screens . ....... ..
....
....... . ......... ............................................................ ......... . .
.
.
GENES INVOLVED IN PHOTOTRANSDUCTION .....................................................................
162
1 63 1 64 1 69 169
Rhodopsins ............................... .......... .................................................................... G Proteins ................................................................................................... ......... '" norpA Encodes a Phospholipase C .................................................. ........ ....... . . ........ Second Messengers and Light-activated Channels .....................................................
1 73
... Arrestin .................................................................................... ....................... ......... Photoreceptor-Specific Protein Kinase C ................... ...............................................
1 75 1 76 1 76
.
.
.
REGULATION OF THE PHOTOTRANSDUCTION CASCADE ........................................... . . .....
172
173
trp ............................................................................................................................
1 77 1 78
................................................................... ............ rdgA......................................................................................................................... rdgB ............................................................. ........................................................... rdgC ......................................................................................................................... ninaC........................................................................................................................ Chaoptin...................................................................................................................
1 79 1 80 181 1 82 1 82
FUTURE PROSPECTS........................................................................................................
1 82
OTHER GENES AFFECTING THE VISUAL RESPONSE..............................................................
RETINAL DEGENERATION MUTANTS
.
.
179
161 0743-4634/9 1 / 1 1 1 5--0 1 6 1$02.00
1 62
SMITH, STAMNES & ZUKER
Annu. Rev. Cell. Biol. 1991.7:161-190. Downloaded from www.annualreviews.org Access provided by University of Waterloo on 11/02/15. For personal use only.
INTRODUCTION Although vision is a highly specialized and extraordinarily complex process, the primary event converting light into a receptor potential is achieved by the activation of a biochemical cascade similar to that used for a great variety of signal transduction processes (reviewed by Stryer 1 986). Both vertebrates and invertebrates carry out phototransduction via a specialized form of a G protein-coupled receptor cascade. Indeed, since many aspects of the phototransduction cascade for vertebrate rod photo receptor cells are well understood, it serves as the prototype for all such transduction cascades (reviewed by Hurley 1 987; Stryer & Bourne 1 986; Kuhn 1 984). Vertebrate photoreceptors hyperpolarize in response to light by closing cation channels on the plasma membrane. The cascade leading to the closure of these channels is initiated by the absorption of light by the photopigment rhodopsin. Rhodopsin consists of an apoprotein, opsin, which is an integral membrane protein with seven transmembrane domains, attached to the chromophore II-cis-retinal. The absorption of a photon of light causes the retinal to be photoisomerized from the II-cis to the all-trans configuration, thus leading to an activated state of the rhodopsin molecule. Activated rhodopsin acts on the heterotrimeric G protein, transducin (TallY) ' which causes the exchange of GTP for GDP on Ta and the subsequent dissociation ofTa from TIly- Active Ta-GTP activates a cGMP phosphodiesterase (PDE), which hydrolyzes cGMP to GMP. The decrease in cGMP levels causes the transient closure of cGMP-gated cation channels, which leads to the hyperpolarization of the cell. Several mechanisms are likely to be responsible for inactivating the phototransduction cascade. Ta, like other G proteins, is a GTPase that hydrolyzes the bound GTP to GDP and inactivates the protein (reviewed by Bourne ct al 1990). Other molecules thought to be involved in this process, arrestin and rhodopsin kinase, have less well-defined in vivo roles (see below). A key feature of this cascade is its exquisite sensitivity. The signal from the absorption of a single photon can be amplified into a detectable membrane potential called a quantum bump (Yeandle 1 957; Baylor et al 1 979). This amplification is achieved because each rhodopsin is able to activate over 500 transducin molecules, and each molecule of PDE that is activated by transducin is capable of hydrolyzing about 1 000 cGMP molecules (Stryer 1 986). Although many components of the vertebrate transduction process have been identified and their biochemical roles elucidated, several aspects of
Annu. Rev. Cell. Biol. 1991.7:161-190. Downloaded from www.annualreviews.org Access provided by University of Waterloo on 11/02/15. For personal use only.
DROSOPHILA PHOTOTRANSDUCTION
1 63
this process have eluded biochemical characterization. The regulation of this cascade is very poorly understood, and the process through which adaptation occurs is still a mystery. It is very likely that several molecules that have important roles in this signaling process remain to be identified. A combined genetic, physiological, and biochemical approach will be required to more fully understand this complicated set of reactions. As elaborated below, Drosophila melanogaster has proven to be an ideal model system for such a combined approach. Drosophila (like other invertebrates) carries out phototransduction through a biochemical cascade similar to that for vertebrates, yet with several interesting differences. The invertebrate visual cascade, like that in vertebrates, begins with the light activation of rhodopsin molecules and subsequent activation of a G protein (reviewed by Saibil 1 990; Payne 1 986). In contrast to vertebrates, the end result of this activation is the depolarization rather than the hyperpolarization of the photoreceptor cell. This comes about because cation channels are opened instead of closed in response to light. The second messenger responsible for triggering the opening of the light-sen sitive cation channels has eluded firm identification. Available data have implicated IP}, calcium, and cGMP (see below). The purpose of this review is to present an up-to-date view of our present understanding of this process in invertebrates and to provide a working model for the continued dissection of this signaling cascade. Several aspects of this process have been reviewed recently (O'Tousa 1 990).
GENETIC DISSECTION OF VISUAL TRANSDUCTION IN DROSOPHILA A complete understanding of the phototransduction process may not be available until the pathway is completely dissected at a genetic level, i.e. until all the gene products that have a role in this process are identified and studied by the physiological effect of their loss or misfunction. This is not feasible in most phototransduction model systems because of the difficulty in obtaining the relevant mutants in organisms such as the horse shoe crab, frogs, or cows. This approach, however, has been applied with tremendous success in the fruit fly, Drosophila melanogaster, where it has yielded remarkable new insights into the machinery of phototransduction. Genetic analysis of Drosophila began nearly 1 00 years ago when Morgan began to isolate and study the first mutant flies (Morgan 19 1 0). Since that time, thousands of mutants defining hundreds of genes have been characterized and mapped to specific regions on the four chromosomes that make up the Drosophila genome (Lindsley & Zimm 1 985, 1 990).
Annu. Rev. Cell. Biol. 1991.7:161-190. Downloaded from www.annualreviews.org Access provided by University of Waterloo on 11/02/15. For personal use only.
1 64
SMITH, STAMNES & ZUKER
The relatively small size of the Drosophila genome, ease of growth, and rapid generation time make this system ideally suited for screening large numbers of mutagenized individuals for defects in virtually any pheno typically observable or measurable trait. The creation of balancer chro mosomes, containing dominant markers and multiple inversions (which essentially eliminate recombination with wild-type chromosomes), allows any mutation, once recognized, to be rapidly isolated and maintained (Lindsley & Zimm 1 990). Finally, germ line transformation using P element transposition (Rubin & Spradling 1982), combined with the avail ability of tissue specific promoters (e.g. Mismer & Rubin 1 987), allows the introduction of cloned genes back into the organism, thus providing a way to study in vitro modified gene products in their actual cellular and organismal environment (Zuker et a1 1 988; Feiler et al 1 988). Genetic Screens
The Drosophila adult visual system is composed of compound eyes and ocelli. The ocelli are simple eyes located on the vertex of the head and are involved in visual guidance. Each of the two compound eyes is made up of approximately 800 ommatidia or unit eyes. An ommatidium consists of 20 cells, eight of which are photoreceptor neurons (see Figure 1 ; reviewed by Tomlinson 1 988). Each photoreceptor cell has a specialized organelle
co psC PC CC R7 Rl-6 RB PC
I) • 5 6
2
1
5
2
1
8
Figure 1 The rhabdomeres (microvilli containing the visual pigments) of R l -R6 form an asymmetrical trapezoidal shape around the central rhabdomeres of the R 7 and R8 cells. The R8 cell is located below the R7 cells and extends through the proximal half of the retina. The inset shows cross-sections through the distal (upper) and proximal (lower) region of the retina. CO, corneal lens; psC, pseudo-cone; PC, pigment cells; CC, cone cells; R I-6, R7, and R8, photoreceptor cells. Figure adapted from Tomlinson 1990.
Annu. Rev. Cell. Biol. 1991.7:161-190. Downloaded from www.annualreviews.org Access provided by University of Waterloo on 11/02/15. For personal use only.
DROSOPHILA PHOTOTRANSDUCTION
1 65
consisting of a stack of microvilli known as a rhabdomere. This rhab domere is where the phototransduction machinery is localized, and it may be seen as the functional equivalent of vertebrate rod outer segment discs (Fein & Szuts 1 982). The eight photoreceptors can be divided into three classes according to their spectral specificity, position of the rhabdomere within the ommatidium, and synaptic connections in the optic lobes (reviewed by Hardie 1 983; Franceschini 1 985). The RI-R6 cells represent the major class of photoreceptors in the retina (see Figure 1). These cells have peripherally located rhabdomeres, and express a blue-absorbing rhodopsin (Rh 1 ) ; this rhodopsin represents over 90% of the visual pigment present in the compound eyes. R 1 -R6 cells send axons that synapse in the first optic lobe (lamina). In Drosophila, all optomotor behavior is mediated via visual input through R 1 -R6 cells (reviewed by Heisenberg & Wolf 1 984). The other two classes of photoreceptors, R 7 and R8, are each represented by a single cell per ommatidium and have centrally located rhabdomeres. The R7 cell is located distally in the retina and expresses one of two opsins, Rh3 or Rh4. These cells are ultraviolet-sensitive and send axons to the second optic lobe (medulla). Fast and some types of slow phototactic responses are mediated through input via R 7 cells (Harris et a1 I976; Heisenberg & Wolf 1 984). The R8 photoreceptor cell is located proximally in the retina, just beneath the R7 cell and also synapses in the medulla. The opsin expressed in the R8 cells has not been isolated, but the action spectrum of these cells suggests that they express a blue-absorbing rhodopsin (Harris et al 1 976). Systematic genetic screens that identify genes encoding products involved in visual behavior or photoreceptor cell function have been used by several laboratories over many years. These screens have been designed to isolate mutants with defects in optomotor responses (flight orientation changes in response to motion in their visual field) (Heisenberg & Wolf 1 984), phototaxis (Benzer 1 967), light-coincident receptor potential (LCRP) (Pak 1 979; Koenig & Merriam 1 977), or defects in the prolonged depolarizing afterpotential (Pak 1 979). The latter two methods, although extremely labor intensive, have been especially successful in the isolation of large numbers of mutants. Optomotor tests designed to identify mutants with defects in visual input-mediated behavior have been used to characterize existing visual mutants and to screen for new mutations (reviewed by Hall 1 982; Hei senberg & Wolf 1 984). In some of the assays, a fly to be tested is attached to a sensitive torquemeter suspended within a rotating drum, and the fly's response to the rotation of the drum is measured by the torquemeter. Normal flies (and humans) attempt to keep the patterns that are painted within the rotating drum fixed in their visual space. Several mutants defec-
Annu. Rev. Cell. Biol. 1991.7:161-190. Downloaded from www.annualreviews.org Access provided by University of Waterloo on 11/02/15. For personal use only.
1 66
SMITH, STAMNES & ZUKER
tive in optomotor responses were characterized in these screens, e.g. omb: optomotor blind (Heisenberg et al 1 978), and nonA: no-an-transients A (Heisenberg 1 972; Jones & Rubin 1 990). As expected, most of these behavioral mutants affect the function of the optic lobes and visual pro cessing centers rather than genes involved in phototransduction. Phototaxis screens were initiated by Benzer and co-workers using countercurrent distribution devices (Benzer 1 967). This procedure sep arates phototactic from non-phototactic flies by knocking flies to the bottom of a clear tube and then allowing them to run toward a light source. A variation of this scheme was to give the flies a choice between ultraviolet and visible light (color choice screens); wild-type flies prefer the ultraviolet light (Harris et al 1 976). These phototaxis screens generated a collection of mutants that eventually were shown to define genes necessary for development of the visual system, photoreceptor cell integrity, or photo transduction (reviewed by Hall 1 982). Examples are sevenless (Harris et al 1 976; Hafen et al 1 987; Banerjee et al 1 987) and some of the retinal degeneration mutants (see below). In order to find mutants that specifically affect the phototransduction cascade and to identify those with subtle phenotypes which may evade the screening strategies described above, a number of groups, pioneered by Pak and co-workers, began to use extracellular recordings of the electrical activity of the Drosophila eye (electroretinogram or ERG) as a screening assay. ERG recordings represent the summation of the entire electrical output of all photoreceptors, pigment cells, and neurons in the first optic lobe (sec below). Figure 2 (upper left panel) displays an ERG trace of a wild-type fly in response to a pulse of white light; shown is the light-coincident receptor potential (LCRP). In response to a pulse of light, the photoreceptor cells depolarize. The maintained component of the ERG (Figure 2, me) represents the summation of all photoreceptor cell currents and is normally maintained as long as the light pulse is on (Pak 1 975). The on-transients (Figure 2, on) are of laminar origin and are induced only by activation of RI-R6 cells (Heisenberg 1 97 1 ; Coombe 1 986). The on-transients are missing when R I-R6 cells are missing or inactivated. Drosophila photoreceptors, like those of many invertebrates, undergo a prolonged depolarizing afterpotential (PDA) that persists after cessation of the light stimulus whenever a substantial amount of rhodopsin (R) has been converted to the dark-stable metarhodopsin (M) (Minke et a1 1 975a; Hillman et al 1 983) (Figure 2, lower panel). The physiological basis of the PDA is not known, but it may be the result of the continued activity of an internal transmitter in the transduction cascade (Minke 1 986). During a PDA, photoreceptor cells become refractory to subsequent PDA-inducing
DROSOPHILA PHOTOTRANSDUCTION
Wild-type
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1.0 sec --11...-.
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Annu. Rev. Cell. Biol. 1991.7:161-190. Downloaded from www.annualreviews.org Access provided by University of Waterloo on 11/02/15. For personal use only.
off t/ m.e.
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trp
slrp
nonA
ninaD
0.5 sec
30 sec
0.5 sec
0.5 sec
0.5 sec
--11...-.
--
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-y-
--11...-.
--11...-.
� 11
1 67
--11...-.
y
1991.
7:
ANNUAL REVIEWS
16/�90
1991 by Annual Reviews Inc.
All rights reserved
Further
Quick links to online content
SIGNAL TRANSDUCTION IN
Annu. Rev. Cell. Biol. 1991.7:161-190. Downloaded from www.annualreviews.org Access provided by University of Waterloo on 11/02/15. For personal use only.
THE VISUAL SYSTEM OF DROSOPHILA Dean P. Smith, Mark A. Stamnes, and Charles S. Zuker Howard Hughes Medical Institute, and Departments of Biology and
Neurosciences, University of California, San Diego, La Jolla, California 92093-0649 KEY WORDS:
Drosophila, G proteins, second messengers, phototransduction, visual system
CONTENTS INTRODUCTION.............................................................................................................. GENETIC DISSECTION OF VISUAL TRANSDUCTION IN DROSOPHILA......................................
Genetic Screens . ....... ..
....
....... . ......... ............................................................ ......... . .
.
.
GENES INVOLVED IN PHOTOTRANSDUCTION .....................................................................
162
1 63 1 64 1 69 169
Rhodopsins ............................... .......... .................................................................... G Proteins ................................................................................................... ......... '" norpA Encodes a Phospholipase C .................................................. ........ ....... . . ........ Second Messengers and Light-activated Channels .....................................................
1 73
... Arrestin .................................................................................... ....................... ......... Photoreceptor-Specific Protein Kinase C ................... ...............................................
1 75 1 76 1 76
.
.
.
REGULATION OF THE PHOTOTRANSDUCTION CASCADE ........................................... . . .....
172
173
trp ............................................................................................................................
1 77 1 78
................................................................... ............ rdgA......................................................................................................................... rdgB ............................................................. ........................................................... rdgC ......................................................................................................................... ninaC........................................................................................................................ Chaoptin...................................................................................................................
1 79 1 80 181 1 82 1 82
FUTURE PROSPECTS........................................................................................................
1 82
OTHER GENES AFFECTING THE VISUAL RESPONSE..............................................................
RETINAL DEGENERATION MUTANTS
.
.
179
161 0743-4634/9 1 / 1 1 1 5--0 1 6 1$02.00
1 62
SMITH, STAMNES & ZUKER
Annu. Rev. Cell. Biol. 1991.7:161-190. Downloaded from www.annualreviews.org Access provided by University of Waterloo on 11/02/15. For personal use only.
INTRODUCTION Although vision is a highly specialized and extraordinarily complex process, the primary event converting light into a receptor potential is achieved by the activation of a biochemical cascade similar to that used for a great variety of signal transduction processes (reviewed by Stryer 1 986). Both vertebrates and invertebrates carry out phototransduction via a specialized form of a G protein-coupled receptor cascade. Indeed, since many aspects of the phototransduction cascade for vertebrate rod photo receptor cells are well understood, it serves as the prototype for all such transduction cascades (reviewed by Hurley 1 987; Stryer & Bourne 1 986; Kuhn 1 984). Vertebrate photoreceptors hyperpolarize in response to light by closing cation channels on the plasma membrane. The cascade leading to the closure of these channels is initiated by the absorption of light by the photopigment rhodopsin. Rhodopsin consists of an apoprotein, opsin, which is an integral membrane protein with seven transmembrane domains, attached to the chromophore II-cis-retinal. The absorption of a photon of light causes the retinal to be photoisomerized from the II-cis to the all-trans configuration, thus leading to an activated state of the rhodopsin molecule. Activated rhodopsin acts on the heterotrimeric G protein, transducin (TallY) ' which causes the exchange of GTP for GDP on Ta and the subsequent dissociation ofTa from TIly- Active Ta-GTP activates a cGMP phosphodiesterase (PDE), which hydrolyzes cGMP to GMP. The decrease in cGMP levels causes the transient closure of cGMP-gated cation channels, which leads to the hyperpolarization of the cell. Several mechanisms are likely to be responsible for inactivating the phototransduction cascade. Ta, like other G proteins, is a GTPase that hydrolyzes the bound GTP to GDP and inactivates the protein (reviewed by Bourne ct al 1990). Other molecules thought to be involved in this process, arrestin and rhodopsin kinase, have less well-defined in vivo roles (see below). A key feature of this cascade is its exquisite sensitivity. The signal from the absorption of a single photon can be amplified into a detectable membrane potential called a quantum bump (Yeandle 1 957; Baylor et al 1 979). This amplification is achieved because each rhodopsin is able to activate over 500 transducin molecules, and each molecule of PDE that is activated by transducin is capable of hydrolyzing about 1 000 cGMP molecules (Stryer 1 986). Although many components of the vertebrate transduction process have been identified and their biochemical roles elucidated, several aspects of
Annu. Rev. Cell. Biol. 1991.7:161-190. Downloaded from www.annualreviews.org Access provided by University of Waterloo on 11/02/15. For personal use only.
DROSOPHILA PHOTOTRANSDUCTION
1 63
this process have eluded biochemical characterization. The regulation of this cascade is very poorly understood, and the process through which adaptation occurs is still a mystery. It is very likely that several molecules that have important roles in this signaling process remain to be identified. A combined genetic, physiological, and biochemical approach will be required to more fully understand this complicated set of reactions. As elaborated below, Drosophila melanogaster has proven to be an ideal model system for such a combined approach. Drosophila (like other invertebrates) carries out phototransduction through a biochemical cascade similar to that for vertebrates, yet with several interesting differences. The invertebrate visual cascade, like that in vertebrates, begins with the light activation of rhodopsin molecules and subsequent activation of a G protein (reviewed by Saibil 1 990; Payne 1 986). In contrast to vertebrates, the end result of this activation is the depolarization rather than the hyperpolarization of the photoreceptor cell. This comes about because cation channels are opened instead of closed in response to light. The second messenger responsible for triggering the opening of the light-sen sitive cation channels has eluded firm identification. Available data have implicated IP}, calcium, and cGMP (see below). The purpose of this review is to present an up-to-date view of our present understanding of this process in invertebrates and to provide a working model for the continued dissection of this signaling cascade. Several aspects of this process have been reviewed recently (O'Tousa 1 990).
GENETIC DISSECTION OF VISUAL TRANSDUCTION IN DROSOPHILA A complete understanding of the phototransduction process may not be available until the pathway is completely dissected at a genetic level, i.e. until all the gene products that have a role in this process are identified and studied by the physiological effect of their loss or misfunction. This is not feasible in most phototransduction model systems because of the difficulty in obtaining the relevant mutants in organisms such as the horse shoe crab, frogs, or cows. This approach, however, has been applied with tremendous success in the fruit fly, Drosophila melanogaster, where it has yielded remarkable new insights into the machinery of phototransduction. Genetic analysis of Drosophila began nearly 1 00 years ago when Morgan began to isolate and study the first mutant flies (Morgan 19 1 0). Since that time, thousands of mutants defining hundreds of genes have been characterized and mapped to specific regions on the four chromosomes that make up the Drosophila genome (Lindsley & Zimm 1 985, 1 990).
Annu. Rev. Cell. Biol. 1991.7:161-190. Downloaded from www.annualreviews.org Access provided by University of Waterloo on 11/02/15. For personal use only.
1 64
SMITH, STAMNES & ZUKER
The relatively small size of the Drosophila genome, ease of growth, and rapid generation time make this system ideally suited for screening large numbers of mutagenized individuals for defects in virtually any pheno typically observable or measurable trait. The creation of balancer chro mosomes, containing dominant markers and multiple inversions (which essentially eliminate recombination with wild-type chromosomes), allows any mutation, once recognized, to be rapidly isolated and maintained (Lindsley & Zimm 1 990). Finally, germ line transformation using P element transposition (Rubin & Spradling 1982), combined with the avail ability of tissue specific promoters (e.g. Mismer & Rubin 1 987), allows the introduction of cloned genes back into the organism, thus providing a way to study in vitro modified gene products in their actual cellular and organismal environment (Zuker et a1 1 988; Feiler et al 1 988). Genetic Screens
The Drosophila adult visual system is composed of compound eyes and ocelli. The ocelli are simple eyes located on the vertex of the head and are involved in visual guidance. Each of the two compound eyes is made up of approximately 800 ommatidia or unit eyes. An ommatidium consists of 20 cells, eight of which are photoreceptor neurons (see Figure 1 ; reviewed by Tomlinson 1 988). Each photoreceptor cell has a specialized organelle
co psC PC CC R7 Rl-6 RB PC
I) • 5 6
2
1
5
2
1
8
Figure 1 The rhabdomeres (microvilli containing the visual pigments) of R l -R6 form an asymmetrical trapezoidal shape around the central rhabdomeres of the R 7 and R8 cells. The R8 cell is located below the R7 cells and extends through the proximal half of the retina. The inset shows cross-sections through the distal (upper) and proximal (lower) region of the retina. CO, corneal lens; psC, pseudo-cone; PC, pigment cells; CC, cone cells; R I-6, R7, and R8, photoreceptor cells. Figure adapted from Tomlinson 1990.
Annu. Rev. Cell. Biol. 1991.7:161-190. Downloaded from www.annualreviews.org Access provided by University of Waterloo on 11/02/15. For personal use only.
DROSOPHILA PHOTOTRANSDUCTION
1 65
consisting of a stack of microvilli known as a rhabdomere. This rhab domere is where the phototransduction machinery is localized, and it may be seen as the functional equivalent of vertebrate rod outer segment discs (Fein & Szuts 1 982). The eight photoreceptors can be divided into three classes according to their spectral specificity, position of the rhabdomere within the ommatidium, and synaptic connections in the optic lobes (reviewed by Hardie 1 983; Franceschini 1 985). The RI-R6 cells represent the major class of photoreceptors in the retina (see Figure 1). These cells have peripherally located rhabdomeres, and express a blue-absorbing rhodopsin (Rh 1 ) ; this rhodopsin represents over 90% of the visual pigment present in the compound eyes. R 1 -R6 cells send axons that synapse in the first optic lobe (lamina). In Drosophila, all optomotor behavior is mediated via visual input through R 1 -R6 cells (reviewed by Heisenberg & Wolf 1 984). The other two classes of photoreceptors, R 7 and R8, are each represented by a single cell per ommatidium and have centrally located rhabdomeres. The R7 cell is located distally in the retina and expresses one of two opsins, Rh3 or Rh4. These cells are ultraviolet-sensitive and send axons to the second optic lobe (medulla). Fast and some types of slow phototactic responses are mediated through input via R 7 cells (Harris et a1 I976; Heisenberg & Wolf 1 984). The R8 photoreceptor cell is located proximally in the retina, just beneath the R7 cell and also synapses in the medulla. The opsin expressed in the R8 cells has not been isolated, but the action spectrum of these cells suggests that they express a blue-absorbing rhodopsin (Harris et al 1 976). Systematic genetic screens that identify genes encoding products involved in visual behavior or photoreceptor cell function have been used by several laboratories over many years. These screens have been designed to isolate mutants with defects in optomotor responses (flight orientation changes in response to motion in their visual field) (Heisenberg & Wolf 1 984), phototaxis (Benzer 1 967), light-coincident receptor potential (LCRP) (Pak 1 979; Koenig & Merriam 1 977), or defects in the prolonged depolarizing afterpotential (Pak 1 979). The latter two methods, although extremely labor intensive, have been especially successful in the isolation of large numbers of mutants. Optomotor tests designed to identify mutants with defects in visual input-mediated behavior have been used to characterize existing visual mutants and to screen for new mutations (reviewed by Hall 1 982; Hei senberg & Wolf 1 984). In some of the assays, a fly to be tested is attached to a sensitive torquemeter suspended within a rotating drum, and the fly's response to the rotation of the drum is measured by the torquemeter. Normal flies (and humans) attempt to keep the patterns that are painted within the rotating drum fixed in their visual space. Several mutants defec-
Annu. Rev. Cell. Biol. 1991.7:161-190. Downloaded from www.annualreviews.org Access provided by University of Waterloo on 11/02/15. For personal use only.
1 66
SMITH, STAMNES & ZUKER
tive in optomotor responses were characterized in these screens, e.g. omb: optomotor blind (Heisenberg et al 1 978), and nonA: no-an-transients A (Heisenberg 1 972; Jones & Rubin 1 990). As expected, most of these behavioral mutants affect the function of the optic lobes and visual pro cessing centers rather than genes involved in phototransduction. Phototaxis screens were initiated by Benzer and co-workers using countercurrent distribution devices (Benzer 1 967). This procedure sep arates phototactic from non-phototactic flies by knocking flies to the bottom of a clear tube and then allowing them to run toward a light source. A variation of this scheme was to give the flies a choice between ultraviolet and visible light (color choice screens); wild-type flies prefer the ultraviolet light (Harris et al 1 976). These phototaxis screens generated a collection of mutants that eventually were shown to define genes necessary for development of the visual system, photoreceptor cell integrity, or photo transduction (reviewed by Hall 1 982). Examples are sevenless (Harris et al 1 976; Hafen et al 1 987; Banerjee et al 1 987) and some of the retinal degeneration mutants (see below). In order to find mutants that specifically affect the phototransduction cascade and to identify those with subtle phenotypes which may evade the screening strategies described above, a number of groups, pioneered by Pak and co-workers, began to use extracellular recordings of the electrical activity of the Drosophila eye (electroretinogram or ERG) as a screening assay. ERG recordings represent the summation of the entire electrical output of all photoreceptors, pigment cells, and neurons in the first optic lobe (sec below). Figure 2 (upper left panel) displays an ERG trace of a wild-type fly in response to a pulse of white light; shown is the light-coincident receptor potential (LCRP). In response to a pulse of light, the photoreceptor cells depolarize. The maintained component of the ERG (Figure 2, me) represents the summation of all photoreceptor cell currents and is normally maintained as long as the light pulse is on (Pak 1 975). The on-transients (Figure 2, on) are of laminar origin and are induced only by activation of RI-R6 cells (Heisenberg 1 97 1 ; Coombe 1 986). The on-transients are missing when R I-R6 cells are missing or inactivated. Drosophila photoreceptors, like those of many invertebrates, undergo a prolonged depolarizing afterpotential (PDA) that persists after cessation of the light stimulus whenever a substantial amount of rhodopsin (R) has been converted to the dark-stable metarhodopsin (M) (Minke et a1 1 975a; Hillman et al 1 983) (Figure 2, lower panel). The physiological basis of the PDA is not known, but it may be the result of the continued activity of an internal transmitter in the transduction cascade (Minke 1 986). During a PDA, photoreceptor cells become refractory to subsequent PDA-inducing
DROSOPHILA PHOTOTRANSDUCTION
Wild-type
Jc:;-
1.0 sec --11...-.
smv1
on
Annu. Rev. Cell. Biol. 1991.7:161-190. Downloaded from www.annualreviews.org Access provided by University of Waterloo on 11/02/15. For personal use only.
off t/ m.e.
norpA
trp
slrp
nonA
ninaD
0.5 sec
30 sec
0.5 sec
0.5 sec
0.5 sec
--11...-.
--
�
-y-
--11...-.
--11...-.
� 11
1 67
--11...-.
y
