Blockade of visual excitation and adaptation in Limulus photoreceptor by GDP-beta-S

The history of the Drosophila TRP channel: the birth of a new channel superfamily

Transient receptor potential (TRP) channels are polymodal cellular sensors involved in a wide variety of cellular processes, mainly by changing membrane voltage and increasing cellular Ca(2+). This review outlines in detail the history of the founding member of the TRP family, the Drosophila TRP channel. The field began with a spontaneous mutation in the trp gene that led to a blind mutant during prolonged intense light. It was this mutant that allowed for the discovery of the first TRP channels. A combination of electrophysiological, biochemical, Ca(2+) measurements, and genetic studies in flies and in other invertebrates pointed to TRP as a novel phosphoinositide-regulated and Ca(2+)-permeable channel. The cloning and sequencing of the trp gene provided its molecular identity. These seminal findings led to the isolation of the first mammalian homologues of the Drosophila TRP channels. We now know that TRP channel proteins are conserved through evolution and are found in most organisms, tissues, and cell-types. The TRP channel superfamily is classified into seven related subfamilies: TRPC, TRPM, TRPV, TRPA, TRPP, TRPML, and TRPN. A great deal is known today about participation of TRP channels in many biological processes, including initiation of pain, thermoregulation, salivary fluid secretion, inflammation, cardiovascular regulation, smooth muscle tone, pressure regulation, Ca(2+) and Mg(2+) homeostasis, and lysosomal function. The native Drosophila photoreceptor cells, where the founding member of the TRP channels superfamily was found, is still a useful preparation to study basic features of this remarkable channel.

Drosophila photoreceptors and signaling mechanisms

Fly eyes have been a useful biological system in which fundamental principles of sensory signaling have been elucidated. The physiological optics of the fly compound eye, which was discovered in the Musca, Calliphora and Drosophila flies, has been widely exploited in pioneering genetic and developmental studies. The detailed photochemical cycle of bistable photopigments has been elucidated in Drosophila using the genetic approach. Studies of Drosophila phototransduction using the genetic approach have led to the discovery of novel proteins crucial to many biological processes. A notable example is the discovery of the inactivation no afterpotential D scaffold protein, which binds the light-activated channel, its activator the phospholipase C and it regulator protein kinase C. An additional protein discovered in the Drosophila eye is the light-activated channel transient receptor potential (TRP), the founding member of the diverse and widely spread TRP channel superfamily. The fly eye has thus played a major role in the molecular identification of processes and proteins with prime importance.

Photosensitive neurons in mollusks

In addition to regular photoreceptors, some invertebrates possess simple extra ocular photoreceptors. For ex?ample, the central ganglia of mollusks contain photosensitive neurons. These neurons are located on the dorsal surface of the ganglia and based on their electrophysiological properties it has been postulated that they are internal photoreceptors. Besides the eye, transduction of light also occurs in these extra-ocular photoreceptors. In the present work, we analyze the reactivity of these nerve cells to light and describe the underlying mechanism mediating the light-induced response. <br><br><font color="red"><b> This article has been retracted. Link to the retraction <u><a href="http://dx.doi.org/10.2298/ABS150317025E">10.2298/ABS150317025E</a><u></b></font>

The excitation cascade of Limulus ventral photoreceptors: guanylate cyclase as the link between InsP3-mediated Ca2+ release and the opening of cGMP-gated channels

Background

Early stages in the excitation cascade of Limulus photoreceptors are mediated by activation of G_q by rhodopsin, generation of inositol-1,4,5-trisphosphate by phospholipase-C and the release of Ca²⁺. At the end of the cascade, cGMP-gated channels open and generate the depolarizing receptor potential. A major unresolved issue is the intermediate process by which Ca²⁺ elevation leads to channel opening.

Results

To explore the role of guanylate cyclase (GC) as a potential intermediate, we used the GC inhibitor guanosine 5'-tetraphosphate (GtetP). Its specificity in vivo was supported by its ability to reduce the depolarization produced by the phosphodiesterase inhibitor IBMX. To determine if GC acts subsequent to InsP₃ production in the cascade, we examined the effect of intracellular injection of GtetP on the excitation caused by InsP₃ injection. This form of excitation and the response to light were both greatly reduced by GtetP, and they recovered in parallel. Similarly, GtetP reduced the excitation caused by intracellular injection of Ca²⁺. In contrast, this GC inhibitor did not affect the excitation produced by injection of a cGMP analog.

Conclusion

We conclude that GC is downstream of InsP3-induced Ca2+ release and is the final enzymatic step of the excitation cascade. This is the first invertebrate rhabdomeric photoreceptor for which transduction can be traced from rhodopsin photoisomerization to ion channel opening.

Photosensitive neurons in mollusks

In addition to regular photoreceptors, some invertebrates possess simple extraocular photoreceptors. For example, the central ganglia of mollusks contain photosensitive neurons. These neurons are located on the dorsal surface of the ganglia and based on their electrophysiological properties, it has been postulated that they are the internal photoreceptors. However, besides the eye, transduction of the light also occurs in these extra-ocular photoreceptors. In the present work, we analyse the reactivity of these nerve cells to light and describe the underlying mechanism mediating the light-induced response.

The TRP channel and phospholipase C-mediated signaling

Drosophila photoreceptors use a phospholipase C-mediated signaling for phototransduction. This pathway begins by light activation of a G-protein-coupled photopigment and ends by activation of the TRP and TRPL channels. The Drosophila TRP protein is essential for the high Ca2+ permeability and constitutes the major component of the light-induced current, thereby affecting both excitation and adaptation of the photoreceptor cell. TRP is the prototype of a large and diverse multigene family whose members are sharing a structure, which is conserved through evolution from the worm Caenorhabditis elegans to humans. TRP-related channel proteins are found in a variety of cells and tissues and show a large functional diversity although the gating mechanism of Drosophila TRP and of other TRP-related channels is still unknown.

Retinal phospholipase C from squid is a regulator of Gq alpha GTPase activity

The phospholipase C (PLC) pathway is the major signaling mechanism of photoactivation in invertebrate photoreceptors. Here we report the cloning of a cDNA encoding a 140-kDa retinal PLC that is uniquely expressed in squid photoreceptors. This cDNA encodes a protein with multiple distinct modular domains: PH, X and Y catalytic, and C2 domains, as well as G- and P-box motifs and two GTP/ATP binding motifs. The PLC was stimulated by activated squid Gq alpha but not by squid Gq beta gamma or mammalian beta gamma subunits. The PLC was inhibited by monophosphate, diphosphate and triphosphate nucleotides but not cyclic nucleosides. We also tested the ability of PLC-140 to regulate the GTPase activity of Gq alpha in the rhabdomeric membranes. Depletion of PLC-140 from the rhabdomeric membranes decreased the GTP hydrolysis but not GTP gamma S binding to the membranes. Reconstitution of purified PLC-140 with membranes accelerated Gq alpha GTPase activity by fivefold at a concentration of 2.5 microM. Our data suggest that PLC-140 plays an important role in both the activation and inactivation pathways of invertebrate visual transduction.

The Limulus ventral photoreceptor: light response and the role of calcium in a classic preparation

The ventral nerve photoreceptor of the horseshoe crab Limulus polyphemus has been used for many years to investigate basic mechanisms of invertebrate phototransduction. The activation of rhodopsin leads in visual cells of invertebrates to an enzyme cascade at the end of which ion channels in the plasma membrane are transiently opened. This allows an influx of cations resulting in a depolarization of the photoreceptor cell. The receptor current of the Limulus ventral photoreceptor consists of three components which differ in several aspects, such as the time course of activation, the time course of recovery from light adaptation, and the reversal potential. Each component is influenced in a different, characteristic way by various pharmacological manipulations. In addition, at least two types of single photon-evoked events (bumps) and three elementary channel conductances are observed in this photoreceptor cell. These findings suggest that the receptor current components are controlled by three different light-activated enzymatic pathways using three different ligands to increase membrane conductance. Probably one of these ligands is cyclic GMP, another one is activated via the IP3-cascade and calcium, the third one might be cyclic AMP. Calcium ions are very important for the excitation and adaptation of visual cells in invertebrates. The extracellular and intracellular calcium concentrations determine the functional state of the visual cell. A rise in the cytosolic calcium concentration appears to be an essential step in the excitatory transduction cascade. Cytosolic calcium is the major intracellular mediator of adaptation. If the cytosolic calcium level exceeds a certain threshold value after exposure to light it causes the desensitization of the visual cell. On the other hand, from a slight rise in cytosolic calcium facilitation results, i.e. increased sensitivity of the photoreceptor.

Rapid coupling of calcium release to depolarization in Limulus polyphemus ventral photoreceptors as revealed by microphotolysis and confocal microscopy

Microphotolysis and confocal microscopy were used to investigate the timing of calcium release and of the electrical response in Limulus polyphemus ventral photoreceptors. The fluorescent dyes Fluo-3 and Calcium Green-5N were used to monitor local Ca2+ elevations. Photolysis of caged inositol trisphosphate (InsP3) close to the plasma membrane of the light-sensitive rhabdomeral (R-) lobe resulted in Ca2+ elevation within 10-20 msec, 20-45 msec before the physiological response to light normally would be detected. Inward ionic current flow and depolarization followed InsP3-induced calcium release within 2.5 +/- 3.3 msec. Voltage-clamping the cells and removal of extracellular Ca2+ did not affect the timing of the Ca2+ elevation that followed the photolysis of caged InsP3 or its relationship to the electrical response. In contrast to the physiological response to light, which only released calcium within the R-lobe, photolysis of InsP3 elevated Cai in both lobes, although with much greater effect in the R-lobe, as compared with the bulk of the A-lobe, suggesting the presence of InsP3-sensitive calcium stores in both lobes. Photolysis of caged calcium [o-nitrophenyl EGTA (NPE)] at the edge of the R-lobe activated an inward ionic current within 1.8 +/- 0.7 msec. This NPE-induced current reversed at a membrane potential of 10 +/- 6 mV in the range typical of that of the light-activated current under physiological conditions. Calcium release, therefore, activates an inward current rapidly enough to contribute to the electrical response to light.

Immunological demonstration of Gq-protein in Limulus photoreceptors

The phototransduction cascade in invertebrates involves the coupling of rhodopsin activation to the action of the enzyme phospholipase C. This step is performed by G-proteins. An antibody against the alpha-subunit of a mouse Gq type G-protein recognized protein bands in Western blots of lateral eye and ventral nerve photoreceptors of Limulus. The protein bands had an apparent molecular mass of about 42 kDa. The antibody also recognized protein bands of a similar molecular mass in immunoblots of brain and intestine tissue. Immunoreactivity was found in lateral eye frozen sections where it was confined to the rhabdom region. When the antibody was applied to ultrathin sections of ventral nerve photoreceptors, the highest density of labeling was found on the rhabdomeral microvilli, but gold particles were also scattered in the cytoplasm. We conclude that a G-protein of the type Gq participates in the phototransduction of Limulus.

Latencies of calcium elevation and depolarization in Limulus ventral photoreceptors injected with GDP-beta S

We have used confocal fluorescence microscopy and fluorescent calcium indicators to investigate the relationship between light-induced elevation of intracellular calcium ion concentration (Cai) and depolarization in small volumes of cytosol close to the microvillar plasma membrane of the ventral photoreceptor of Limulus polyphemus. We prolonged the latency of the light response by treatment of cells with hydroxylamine and injection of the G-protein blocker, guanosine-5'-O-(2-thiodiphosphate (GDP-beta S). Such treatment increased the latency of the cell's response from approximately 20 to 50 ms. In both treated and untreated cells we observed a close correlation between the times at which we first detected the electrical response and the elevation of Cai. We obtained 18 out of 54 and 12 out of 22 recordings, in untreated and treated cells respectively, for which the elevation of Cai was detected simultaneous with, or 1-4 ms prior to, the electrical response to light. The prolonged latent period exhibited by treated cells may make possible future investigation of the effects on the initial response to light of local photolytic release of caged compounds at the microvillar membrane.

G protein control of Drosophila photoreceptor phospholipase C

Light stimulates phosphatidylinositol bisphosphate phospholipase C (PLC) activity in Drosophila photoreceptors. We have investigated the mechanism of this reaction by assaying PLC activity in Drosophila head membranes using exogenous phospholipid substrates. PLC activation depends on the photoconversion of rhodopsin to metarhodopsin and is reduced in norpAEE5 PLC and ninaEP332 rhodopsin mutants. NorpA PLC is stimulated by light at free Ca2+ concentrations between 10 nM and 1 microM. This finding is consistent with a Ca(2+)-mediated positive feedback mechanism that contributes to the rapid temporal response of invertebrate photoreceptor cells. The guanyl nucleotide dependence of light-stimulated PLC activity indicates that a G protein regulates NorpA. This was confirmed by the observation that light stimulation of PLC activity is deficient in mutants that lack the eye-specific G protein beta subunit G beta e. These results indicate that G beta e functions as the beta subunit of the G protein coupling rhodopsin to NorpA PLC.

Purification, characterization, and partial amino acid sequence of a G protein-activated phospholipase C from squid photoreceptors

Invertebrate visual transduction is thought to be initiated by photoactivation of rhodopsin and its subsequent interaction with a guanyl nucleotide-binding protein (G protein). The identities of the G protein and its target effector have remained elusive, although evidence suggests the involvement of a phospholipase C (PLC). We have identified a phosphatidylinositol-specific PLC from the cytosol of squid retina. The enzyme was purified to near-homogeneity by a combination of carboxymethyl-Sepharose and heparin-Sepharose chromatography. The purified PLC, identified as an approximately 140-kDa protein by sodium dodecyl sulfate-polyacrylamide gels, hydrolyzed phosphatidylinositol 4,5-bisphosphate (PIP2) at a rate of 10-15 mumol/min/mg of protein with 1 microM Ca2+. The partial amino acid sequence of the protein showed homology with a PLC cloned from a Drosophila head library (PLC21) and lesser homology with Drosophila norpA protein and mammalian PLC beta isozymes. Reconstitution of purified squid PLC with an AlF(-)-activated 44-kDa G protein alpha subunit extracted from squid photoreceptor membranes resulted in a significant increase in PIP2 hydrolysis over a range of Ca2+ concentrations while reconstitution with mammalian Gt alpha or Gi 1 alpha was without effect. These results suggest that cephalopod phototransduction is mediated by G alpha-44 activation of a 140-kDa cytosolic PLC.

Ca2+ is an obligatory intermediate in the excitation cascade of limulus photoreceptors

We have investigated the role of Ca2+ in the excitation of Limulus photoreceptors by intracellular injection of the Ca2+ buffer, 5,5'-dibromo-BAPTA. Buffer with free Ca2+ of 0.5 or 5 microM slowed the rising edge of the light response over 100-fold and greatly reduced both the transient and plateau phases of the light response, as expected if Ca2+ elevation is necessary for all phases of excitation. Injection of buffers with free Ca2+ of 5 or 45 microM, levels normally reached during light, evoked sustained inward current as expected if Ca2+ is sufficient for excitation. The transduction cascade appears due to a single pathway that sequentially involves 1,4,5-trisphosphate inositol, Ca2+, and cyclic GMP.

Specific second messengers activate the caudal photoreceptor of crayfish

The caudal photoreceptor (CPR) found in the last abdominal ganglion of crayfish is a well-known example of a non-retinal photosensitive element. In addition to light sensitivity, this cell has been assigned a command role for a walking behavior. The molecular mechanism of transduction in this cell has not been previously studied. The involvement of an intermediate messenger substance is suggested by its long latency to response, its prolonged afterdischarge, and by the requirement for an amplification process for the efficient transduction of light. We tested the effect of some putative second messengers by pressure injecting them into the CPR and noting the physiological response. Here we report that intracellular injection of inositol 1,4,5-trisphosphate (IP3), calcium, and the guanosine nucleotide GTP mimics the light response, while cAMP, IP1 and IP2 have no effect on the firing rate. The key intermediate in transduction in vertebrate photoreceptors, cGMP, was ineffective in this system. This work adds to the growing body of evidence that IP3 plays a role in invertebrate phototransduction.

The identification and purification of the heterotrimeric GTP-binding protein from squid (Loligo forbesi) photoreceptors

The principal GTP-binding protein (G-protein) from squid (Loligo forbesi) photoreceptor membranes has been identified by amino acid sequencing. The heterotrimeric protein was purified by detergent solubilization and ion-exchange chromatography. The amino acid sequence of the G-protein alpha-subunit (G-alpha) indicates that this subunit is closely related to the recently characterized Gq subgroup, whereas the G-gamma subunit varies widely in sequence from other homologues.

A G beta protein in the Drosophila compound eye is different from that in the brain

A G protein beta subunit gene (Gbe) is expressed only in the eyes of adult D. melanogaster. This gene was identified by probing a Drosophila head cDNA expression library with monoclonal antibodies to a previously characterized Drosophila G protein beta subunit (Gbb). Immunoblot and Northern analyses demonstrate that Gbe protein and mRNA is not present in Drosophila mutants that lack eyes. Immunocytochemical and in situ hybridization analyses further demonstrate that Gbe is expressed in the eyes but not in the brain, whereas Gbb is abundantly expressed in the brain. The Gbe product is approximately 45% identical to previously identified G beta subunits and defines a new G beta class. Its localization suggests a possible role in phototransduction.

Roles of cyclic GMP and inositol trisphosphate in phototransduction of the molluscan extraocular photoreceptor

The internal messengers mediating the photocurrent of the molluscan extraocular photoreceptor, A-P-1, were examined. In the dark, pressure-injection of cGMP into the A-P-1, voltage-clamped at resting levels, produced a rapid outward current, associated with an increase in conductance. However, the cGMP-induced current and increase in conductance were suppressed by subsequent photostimulation, suggesting hydrolysis of cGMP by light. The steady-state I/V relation for the cGMP-induced current was non-linear. The I/V relation for the instantaneous cGMP-induced current, measured 50 ms after the beginning of a voltage step, was linear, and reversed at the membrane potential, -67 mV, which corresponded to the K+ equilibrium potential of A-P-1 in 10 mM K+ normal saline. These findings indicate that the internal cGMP induces a voltage- and time-dependent K+ current. Since the photocurrent results from the suppression of a voltage- and time-dependent K+ current similar to above, the photocurrent is considered to be equivalent to the suppression of the cGMP-induced current. Short pressure-injection of GDP-beta-S into A-P-1 reduced the subsequent photocurrent. The photocurrent was also suppressed after an external application of Pertussis toxin. On the other hand, the photocurrent was amplified by prior pressure-injection of inositol 1,4,5-trisphosphate (IP3). However, a short pressure-injection of neomycin into A-P-1 depressed the subsequent photocurrent. These results suggested that the cGMP-induced (dark) current is mediated by cGMP, and that hydrolysis of cGMP by light leads to the photocurrent, then being modified by another messenger, IP3, to be amplified.(ABSTRACT TRUNCATED AT 250 WORDS)

The mouse S-antigen gene. Comparison with human and Drosophila

We have characterized a gene for mouse S-antigen and compared its sequence with that of corresponding human and two recently published Drosophila S-antigen genes. The mouse S-antigen gene was approximately 50 kbp in length and consisted of 16 exons and 15 introns. The length of most exons was less than 100 bp and the smallest one was only 10 bp. In contrast, the length of most introns was larger than 2 kbp and the gene consisted of 97% intron and 3% exon. Both splice sites for donor and accepter were in good agreement with the GT/AG rule. S-antigen genes in human and mouse were highly conserved. In contrast, genes for the Drosophila 49-kDa arrestin homolog and arrestin consist of three introns and four exons and two introns and three exons, respectively. The 5'-flanking region of the mouse S-antigen gene, approximately 1.0 kbp long, had no regulatory elements for transcription such as the TATA, CAAT and GC boxes, while a Drosophila arrestin gene has TATA and CAAT boxes. Interestingly, the 5'-flanking region of the mouse gene had promoter activity in an in vitro transcription assay using a nuclear extract of rat brain. A major transcription start site was found at 387 bp upstream from the translation start codon ATG in mouse. From our results, and those of others, we suggest that the S-antigen gene has evolved from a common ancestor gene by either insertion or deletion of introns. Such an alteration of gene structure may have played a role in the evolution of the S-antigen.

Biophysical processes in invertebrate photoreceptors: recent progress and a critical overview based on Limulus photoreceptors

Limulus ventral nerve photoreceptor, a classical preparation for the study the phototransduction in invertebrate eyes, seems to have a very complex mechanism to transform light energy into a physiological signal. Although the main function of the photoreceptor is to change the membrane conductance according to the illumination, the cell has voltage-activated conductances as well. The voltage-gated conductances are matched to the light-activated ones in the sense that they make the function of the cell more efficient. The complex mechanism of phototransduction and the presence of four different voltage-gated conductance in Limulus ventral nerve photoreceptors indicate that these cells are far less differentiated than the photoreceptor cells of vertebrates. Indications accumulated in recent years support the view that the ventral photoreceptor of Limulus has different light-activated macroscopic current components, ion channels and terminal transmitters. After conclusions from macroscopic current measurements (Payne, 1986; Payne et al. 1986 a, b), direct evidence was presented by single-channel (Nagy & Stieve, 1990 a, b; Nagy, 1990 a, b) and macroscopic current measurements (Deckert et al. 1991 a, b) for three different light-activated conductances. It has been shown that two of these conductances are stimulated by two different excitation mechanisms. The two mechanisms, having different kinetics, release probably two different transmitters. One of them might be the cGMP (Johnson et al. 1986), the other one the calcium ion (Payne et al. 1986 a, b). However, the biochemical processes which link the rhodopsin molecules and the ion channels are not known. The unknown chemical details of the phototransduction result in a delay for the mathematical description of the biophysical mechanisms. More biochemical details are known about the adaptation mechanism. It was found that inositol 1,4,5-trisphosphate is a messenger for the release of calcium ions from the intracellular stores and that calcium ions are the messengers for adaptation (Payne et al. 1986 b; Payne & Fein, 1987). Concerning the mechanism of calcium release, it was revealed that a negative feedback acts on the enzyme cascade to regulate the internal calcium level and to protect the stores against complete emptying (Payne et al. 1988, 1990). Calcium ions also play an important role in the excitation mechanism. (a) In [Ca2+]i-depleted cells the light-induced current was increased after intracellular Ca2+ injection, suggesting that calcium is necessary for the transduction mechanism (Bolsover & Brown, 1985).(ABSTRACT TRUNCATED AT 400 WORDS)

The role of the inositol phosphate cascade in visual excitation of invertebrate microvillar photoreceptors

The identity of the transmitter(s) involved in visual transduction in invertebrate microvillar photoreceptors remains unresolved. In this study, the role of inositol 1,4,5-trisphosphate (IP3) was examined in Limulus ventral photoreceptors by studying the effects on the light response of heparin and neomycin, agents that inhibit the production or action of IP3. Both heparin and neomycin reduce responses to brief flashes of light and the transient component of responses to steps of light, and also inhibit IP3-induced calcium release, indicating that IP3 plays a direct role in invertebrate visual excitation. The effects of BAPTA, a calcium buffer, were also examined and shown to be consistent with a role for IP3-mediated calcium release in visual excitation. However, all three agents fail to block the plateau component of the response to a step of light, indicating that a single pathway involving IP3 and calcium cannot solely be responsible for visual excitation in invertebrates. We suggest that the inositol phosphate cascade and a second parallel process that is not dependent on IP3 are involved in the production of the light response.

Sequence of the beta-subunit of the phosphatidylinositol-specific phospholipase C-directed GTP-binding protein from squid (Loligo forbesi) photoreceptors

The beta-subunit (G-beta) of the squid (Loligo forbesi) visual GTP-binding protein (G-protein), thought to be associated with a phosphatidylinositol-specific phospholipase C, has been identified and the sequence of the protein determined from its cDNA. The predicted polypeptide has a very marked sequence similarity with its mammalian counterparts (80-90% identity). Squid G-beta also has somewhat lower similarity to the yeast protein STE4 (approx. 40% identity). The role of G-beta in signal transduction is discussed in the light of its pronounced structural conservation.

Occurrence, quaternary structure and function of G protein subunits in an insect endocrine gland

The occurrence, structure and function of the alpha and beta subunits of GTP-binding proteins (G proteins) were investigated in the Manduca sexta prothoracic gland, a tissue which possesses a hormonally regulated adenylate cyclase. Subunit-specific antibodies were utilized in immunoblotting studies of tissue from Manduca prothoracic glands, brain, eyes and antennae, and compared to the substrates present in the heads of Drosophila, as well as in a mammalian cell line. All Manduca tissues examined showed putative G beta subunits of 37 and 38 kDa, an unidentified alpha subunit of 41 kDa, in addition to an eye specific alpha subunit of 42 kDa. Manduca tissues also produced putative Gs alpha subunits of 48 and 51 kDa which were coupled to prothoracic gland adenylate cyclase as demonstrated by immunoprecipitation. Prothoracic gland G proteins have a definite and limited quaternary structure, consistent with a heterotrimeric model, as demonstrated by crosslinking of prothoracic gland membrane preparations followed by immunoblotting. These studies also yielded data on relative titers of alpha subunits, and suggest that Gs alpha is present in lower amounts than other alpha subunits. The G protein subunits studied in the prothoracic gland appear strikingly similar in molecular weight, function and structure to their mammalian counterparts.

Light-dependent GTP-binding proteins in squid photoreceptors

Previous biochemical and electrophysiological evidence suggests that in invertebrate photoreceptors, a GTP-binding protein (G-protein) mediates the actions of photoactivated rhodopsin in the initial stages of transduction. We find that squid photoreceptors contain more than one protein (molecular masses 38, 42 and 46 kDa) whose ADP-ribosylation by bacterial exotoxins is light-sensitive. Several lines of evidence suggest that these proteins represent distinct alpha subunits of G-proteins. (1) Pertussis toxin and cholera toxin react with distinct subsets of these polypeptides. (2) Only the 42 kDa protein immunoreacts with the monoclonal antibody 4A, raised against the alpha subunit of the G-protein of vertebrate rods [Hamm & Bownds (1984) J. Gen. Physiol. 84. 265-280]. (3) In terms of ADP-ribosylation, the 42 kDa protein is the least labile to freezing. (4) Of the 38 kDa and 42 kDa proteins, the former is preferentially extracted with hypo-osmotic solutions, as demonstrated by the solubility of its ADP-ribosylated state and by the solubility of the light-dependent binding of guanosine 5'-[gamma-thio]triphosphate. The specific target enzymes for the observed G-proteins have not been established.

Phosphatidylinositol-4,5-bisphosphate phospholipase C in bovine rod outer segments

Preparations of rod outer segments from cattle retinas contained soluble and particulate phospholipase C activities which hydrolyzed phosphatidylinositol 4,5-bisphosphate (PIP2) and the other phosphoinositides. Ca2+ was required for PIP2 hydrolysis, but high (greater than 300 microM) concentrations were inhibitory. Mg2+ and spermine at low concentrations stimulated the particulate activity but inhibited the soluble. Mn2+ inhibited both. High (greater than 100 microM) concentrations of the nonhydrolyzable GTP analogue guanylyl beta,gamma-methylenediphosphonate inhibited PIP2 hydrolysis by both the soluble and particulate activities, but guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), fluoride, and cholera and pertussis toxins were without effect. Overall phospholipase C activity in ROS was unaffected by light. Evidence was found for multiple forms of the enzyme, requiring isolation and separate characterization before ruling out regulation by light or G-protein.

Microvillar components of light adaptation in blowflies

The process of light adaptation in blowfly photoreceptors was analyzed using intracellular recording techniques and double and triple flash stimuli. Adapting flashes of increasing intensity caused a progressive reduction in the excitability of the photoreceptors, which became temporarily suppressed when 3 x 10(6) quanta were absorbed by the cell. This suppression was confirmed by subsequently applying an intense test flash that photoactivated a considerable fraction of the 10(8) visual pigment molecules in the cell. The period of temporary desensitization is referred to as the refractory period. The stimulus intensity to render the receptor cell refractory was found to be independent of the extracellular calcium concentration over a range of 10(-4) and 10(-2) M. During the refractory period (30-40 ms after the adapting flash) the cell appears to be "protected" against further light adaptation since light absorption during this period did not affect the recovery of the cell's excitability. Calculations showed that the number of quantum absorptions necessary to induce receptor refractoriness is just sufficient to photoactivate every microvillus of the rhabdomere. This coincidence led to the hypothesis that the refractoriness of the receptor cells is due to the refractoriness of the individual microvilli. The sensitivity of the receptor cells after relatively weak adapting flashes was reduced considerably more than could be accounted for by the microvilli becoming refractory. A quantitative analysis of these results suggests that a photoactivated microvillus induces a local adaptation over a relatively small area of the rhabdomere around it, which includes several tens of microvilli. After light adaptation with an intense flash, photoactivation of every microvillus by the absorption of a few quanta produced only a small receptor response whereas photoactivation of every rhodopsin molecule in every microvillus produced the maximum response. The excitatory efficiency of the microvilli therefore increases with the number of quanta that are absorbed simultaneously.

Two distinct light regulated G-proteins in octopus photoreceptors

Two distinct light-regulated G-proteins were found in octopus photoreceptors. Gip, a 41 kDa protein from washed microvilli, was ADP ribosylated by pertussis toxin in the presence of GDP in the dark. Light and GTP analogues were inhibitory as with transducin (Gt; G-protein in vertebrate photoreceptors). G34, a 34 kDa protein from fresh octopus retina, was ADP ribosylated by both cholera and pertussis toxin in the dark. Light inhibited labeling of the 34 kDa protein by both toxins. Unlike Gip, G34 is soluble and is very labile to heat, freezing and thawing. Prolonged incubation of octopus retina with cholera toxin and labeled NAD produced an additional radioactive band at 46 kDa. Labeling of the 46 kDa protein, Gsp, was greatly enhanced by GTP analogues, but inhibited by a GDP analogue as with Gs in hormone-sensitive adenylate cyclase. In contrast to Gip and G34, labeling of the 46 kDa protein (Gsp) was not influenced by light. The two distinct light-regulated G-proteins, Gip and G34, found in octopus photoreceptors might be involved in either phototransduction or photoadaptation. The function of Gsp is not known.

Phorbol ester induces photoreceptor-specific degeneration in a Drosophila mutant

In the retinal degeneration B (rdgB) mutant of Drosophila, the major class of photoreceptors degenerate when the fly is raised in the light for several days; raising the fly in the dark largely prevents the degeneration. Thus, the rdgB is a conditional mutant that requires the operation of some stages of the phototransduction cascade to express its characteristic phenotype. We report here experiments that examine the ability of chemical agents to mimic light by causing photoreceptor-specific degeneration in the dark. Application of a specific activator of protein kinase C, phorbol ester, to eyes of rdgB flies led to a degeneration of the photoreceptors that was indistinguishable from that caused by light: both light and phorbol ester-induced degeneration were characterized by (i) selective degeneration of one class of photoreceptors; (ii) a unique pattern of degeneration; and (iii) the appearance of light-induced regenerative spikes at early stages of degeneration. Application of phorbol ester to the eyes of wild-type flies had no effect. We suggest that light or phorbol ester activates a protein kinase C and results in a sustained or excessive phosphorylation of proteins in the rdgB mutant, leading to photoreceptor degeneration. Furthermore, the results are consistent with identification of the rdgB gene product as a phosphoprotein phosphatase that is nonfunctional or absent in the mutant.

The light-suppressible K+ conductance and evaluation of internal messenger candidates in the molluscan extraocular photoreceptor

A photoreceptor potential produced by a decrease in membrane conductance was not thought to occur in any invertebrate photoreceptors. However, we have found that the molluscan extraocular photoreceptor, A-P-1 responds to light with a depolarizing receptor potential due to a decrease in K+ conductance, so that the photoresponse associated with a decrease in membrane conductance is not unique to the vertebrate photoreceptor. The properties that the light-suppressible K+ conductance is time- and voltage-dependent are explained by comparison with those of the single channel conductance obtained in patch-clamp of both vertebrate and invertebrate photoreceptors. The noise analysis of the light-induced current suggest that this macroscopic light-suppressible conductance consists of channels. It is concluded that the light-suppressible K+ conductance is mediated by hydrolysis of cGMP which reduces internal cGMP, in agreement with the cGMP hypothesis of vertebrate phototransduction and that the hydrolysis may be modified by IP3.

Chemical excitation and inactivation in photoreceptors of the fly mutants trp and nss

The Drosophila and Lucilia photoreceptor mutants, trp and nss, respond like wild-type flies to a short pulse of intense light or prolonged dim light; however, upon continuous intense illumination, the trp and nss mutants are unable to maintain persistent excitation. This defect manifests itself by a decline of the receptor potential toward baseline during prolonged intense illumination with little change in the shape or amplitude of the quantal responses to single photons (quantum bumps). Previous work on the trp and nss mutants suggests that a negative feedback loop may control the rate of bump production. Chemical agents affecting different steps of the phototransduction cascade were used in conjunction with light to identify a possible branching point of the feedback loop and molecular stages which are affected by the mutation. Fluoride ions, which in the dark both excite and adapt the photoreceptors of wild-type flies, neither excite nor adapt the photoreceptors of the trp and nss mutants. The hydrolysis-resistant analogue, GTP gamma S, which excites the photoreceptors of wild-type flies, resulting in noisy depolarization, markedly reduces the light response of both mutant flies. Intracellular recordings revealed, however, that the inhibitory effect of GTP gamma S on the nss mutant was accompanied neither by any significant depolarization nor by an increase in the noise, and thus was very different from the effect of a dim background light. The combination of inositol trisphosphate and diphosphoglycerate (InsP3 + DPG), which efficiently excites the photoreceptors of wild-type Lucilia, also excites the photoreceptors of nss Lucilia mutant. The InsP3 + DPG together act synergistically with light to accelerate the decline of the response to light in the mutant flies. These results suggest that the fly phototransduction pathway involves a feedback regulatory loop, which branches subsequent to InsP3 production and regulates guanine nucleotide-binding protein (G protein)-phospholipase C activity. A defect in this regulatory loop, which may cause an unusually low level of intracellular Ca2+, severely reduces the triggering of bumps in the mutants during intense prolonged illumination.

Characterization of guanylate cyclase in squid photoreceptors

Light causes a rapid, 1.7-fold increase in cyclic GMP concentration in intact squid retinas (Johnson et al. (1986]. To determine whether light-induced changes in cyclic GMP concentration result from activation of guanylate cyclase, we have studied the regulation of guanylate cyclase activity in squid (Loligo pealei) photoreceptors. The enzyme is membrane-associated and activity is enhanced by the detergents Triton X-100 or digitonin. The enzyme requires divalent cations, Mn2+ being preferred over Mg2+. The dependence of enzyme activity on the MnGTP concentration deviates from simple Michaelis-Menten kinetics. Under conditions where a light-induced binding of GTP to the guanine nucleotide regulatory protein can be observed, no light-induced change in guanylate cyclase could be detected.

Chemically induced retinal degeneration in the rdgB (retinal degeneration B) mutant of Drosophila

Chemicals which affect different steps of the phototransduction cascade were used to identify the site of action of the rdgB gene product of Drosophila. In the rdgB mutant, the photoreceptor cells degenerate after several days of exposure to light, whereas raising the flies in the dark largely prevents the degeneration. In the rdgBKS222 mutant, which was used in the present studies, the light-induced degeneration is characterized by (1) selective degeneration of the peripheral but not the central photoreceptor cells; (2) random distribution of degenerated cells among ommatidia; and (3) the degeneration is specific to the rdgB but not the wild-type photoreceptor cells. In the present study, we show that application of specific chemical agents to the eyes of rdgBKS222 flies in the dark mimics the effects of light and causes retinal degeneration indistinguishable from light. The agents used in these studies are the metabolically stable GTP analogs GTP gamma S and Gpp(NH)p as well as fluoride ions, which are known to activate the transducing guanine nucleotide binding protein (G-protein of fly photoreceptors). It is unlikely that the chemically induced retinal degeneration is mediated by effects on energy metabolism, since application of the metabolic inhibitors CN- and 2-deoxy-D-glucose did not increase the extent of retinal degeneration over that observed in control flies treated with Ringer solution. The GDP analog GDP beta S, which inhibits G-protein activity, greatly reduced the extent of retinal degeneration in the dark, over that observed in control flies treated with Ringer solution. These results suggest that activation of the G-protein precedes the step in the transduction cascade that leads to retinal degeneration and provides a powerful tool to investigate the molecular mechanism of light-induced degeneration in the rdgB mutant.

An estimate of the number of G regulator proteins activated per excited rhodopsin in living Limulus ventral photoreceptors

Previous work by others on Limulus photoreceptors has shown that application of a variety of guanine nucleotide-binding regulatory protein (G protein) activators produces discrete waves of depolarization similar to those generated by single photos, but smaller in size. We investigated whether these events might originate at a site other than the G protein. Initiation of the events did not depend on the state of the visual pigment, suggesting that the events do not originate at the pigment level. The events could be blocked by the G-protein blocker guanosine 5'-[beta-thio]diphosphate (GDP[betaS]) and thus support the conclusion that these discrete events are due to the activation of G protein itself. Quantitative measurements indicate that the average size of these events is approximately 8 times smaller than that evoked by single photons under the same conditions. Given certain reasonable assumptions, these results imply that the gain of the first stage of transduction in vivo is approximately 8, a value considerably lower than that measured in vitro in vertebrate rods (gain, 100-500). Furthermore, independent evidence for a low first-stage gain in Limulus is derived from the observation that GDP[betaS] barely affects the size of the response to single photons, but greatly reduces the probability that a photon evokes a response. These results can be explained if rhodopsin normally activates only a few G proteins.

Incorporation of analogues of GTP and GDP into rod photoreceptors isolated from the tiger salamander

1. Analogues of GTP and GDP were introduced into isolated rod photoreceptors using the whole-cell patch clamp technique, while simultaneously recording the photocurrent with a suction pipette. After several minutes of whole-cell recording the patch pipette was disengaged, thus trapping the analogue inside the cell. 2. During the introduction of the hydrolysis-resistant GTP analogues guanosine-5'-O-(3-thio-triphosphate) (GTP-gamma-S) and guanylyl-imidodiphosphate (GMP-PNP) the dark current progressively declined, and the duration of responses to flashes of light which had previously been just-saturating increased slightly. The form of the rising phases of the responses to dim or bright flashes was little affected. 3. Following the incorporation of these GTP analogues the response to an intense flash was prolonged by a factor of up to 300, and the circulating current remained suppressed for up to 1 h. Ultimately the circulating current recovered and the duration of the flash response returned to near its control value. 4. Superfusion of the outer segment with the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX) during the extended period of saturation resulted in a rapid increase in the circulating current, suggesting that the analogues had their major effect on the duration of phosphodiesterase activation by light. 5. Introduction of the phosphorylation-resistant GDP analogue guanosine-5'-O-(2-thio-diphosphate) (GDP-beta-S) resulted in a decrease in light sensitivity and a reduction in the slope of the rising phase of the flash response. 6. The response to an intense flash was also prolonged in cells containing GDP-beta-S, recovery becoming progressively slower on successive presentations of the flash following the withdrawal of the patch pipette. This observation suggests that GDP-beta-S may be slowly converted within the cell to form a hydrolysis-resistant product. 7. These results indicate that the presence of a hydrolysis-resistant analogue of GTP within the cell causes light activation of the transduction mechanism for an extended period. Our interpretation of this finding is that hydrolysis of the bound guanosine nucleotide is necessary for the quenching of activated GTP-binding protein.

The localization of calcium release by inositol trisphosphate in Limulus photoreceptors and its control by negative feedback

Microvillar photoreceptors of invertebrates exhibit a light-induced rise in the intracellular concentration of free calcium (Cai) that results in part from release of calcium from an intracellular compartment. This light-induced release of calcium appears to result from a cascade of reactions that involve rhodopsin, a GTP-binding protein and a phospholipase-C which releases inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) from the plasma membrane; the Ins(1,4,5)P3 acts to release calcium from smooth endoplasmic reticulum. In the ventral photoreceptor of the horseshoe crab Limulus polyphemus not all of the endoplasmic reticulum is subject to calcium release by Ins(1,4,5)P3. Only endoplasmic reticulum in the light-sensitive region of the cell is competent to release calcium in response to Ins(1,4,5)P3. The release of calcium by Ins(1,4,5)P3 in ventral photoreceptors appears to be subject to feedback inhibition through elevated Cai. We suggest that this feedback inhibition contributes to sensory adaptation in the photoreceptor and may account for oscillatory membrane responses sometimes observed with large injections of Ins(1,4,5)P3.

Induction of neurite formation in PC12 cells by microinjection of proto-oncogenic Ha-ras protein preincubated with guanosine-5'-O-(3-thiotriphosphate)

Rat pheochromocytoma (PC12) cells differentiate to neuronal cells in response to nerve growth factor. It has been shown that microinjection of oncogenic but not proto-oncogenic p21 protein induces morphological differentiation in PC12 cells (D. Bar-Sagi and J. R. Feramisco, Cell 42:841-848, 1985). In this paper we describe a recombinant human proto-oncogenic Ha-ras protein which can effectively induce neurite extension of PC12 cells when microinjected as a complex with guanosine-5'-O-(3-thiotriphosphate). The protein was found to be less effective when complexed with GTP. On the other hand, an oncogenic ras protein coinjected with guanosine-5'-O-(2-thiodiphosphate) was entirely inactive. These results indicate that the binary p21-GTP complex, but not the p21-GDP complex, is effective in inducing differentiation in PC12 cells, irrespective of the oncogenic or the proto-oncogenic protein.

Application of an invariant spectral form to the visual pigments of crustaceans: implications regarding the binding of the chromophore

Visual pigment absorption spectra were measured in single photoreceptors of a stomatopod, a crayfish, a hermit crab, and five species of brachyuran crab. All fitted a Mansfield (1985) invariant form for visual pigment, the form also fitted by vertebrate retinal-based visual pigments. This is consistent with a theoretical model based on the structure of visual pigment molecules (Greenberg et al., 1975; Honig et al., 1976) which predicts that spectral bandwidth decreases as lambda max increases. The conformation to the invariant form implies that for any given chromophore bandwidth times lambda max is a constant.

Induction of neurite formation in PC12 cells by microinjection of proto-oncogenic Ha-ras protein preincubated with guanosine-5'-O-(3-thiotriphosphate)

Rat pheochromocytoma (PC12) cells differentiate to neuronal cells in response to nerve growth factor. It has been shown that microinjection of oncogenic but not proto-oncogenic p21 protein induces morphological differentiation in PC12 cells (D. Bar-Sagi and J. R. Feramisco, Cell 42:841-848, 1985). In this paper we describe a recombinant human proto-oncogenic Ha-ras protein which can effectively induce neurite extension of PC12 cells when microinjected as a complex with guanosine-5'-O-(3-thiotriphosphate). The protein was found to be less effective when complexed with GTP. On the other hand, an oncogenic ras protein coinjected with guanosine-5'-O-(2-thiodiphosphate) was entirely inactive. These results indicate that the binary p21-GTP complex, but not the p21-GDP complex, is effective in inducing differentiation in PC12 cells, irrespective of the oncogenic or the proto-oncogenic protein.

Light-induced changes in the content of inositol phosphates in squid (Loligo pealei) retina

Illumination induced an increase in inositol polyphosphates, inositol 1,4-bisphosphate and inositol 1,4,5-trisphosphate, in the photoreceptors of the squid, Loligo pealei. There was a concomitant decrease in phosphatidylinositol 4,5-bisphosphate, but no light-induced change in any other phosphoinositide. None of these stimulus-induced changes were altered by treatment in vivo with pertussis toxin, which ADP-ribosylated a Mr-39000-peptide. These findings support the hypothesis that inositol 1,4,5-trisphosphate participates as either a messenger or a modulator in transduction in invertebrate photoreceptors.

Coupling of photoexcited rhodopsin to inositol phospholipid hydrolysis in fly photoreceptors

Fly photoreceptor membranes were used to test the effect on defined biochemical reactions of light and of compounds causing photoreceptor excitation. Complementary electrophysiological studies examined whether putative second messengers excite the fly photoreceptor cells. This analysis revealed the following sequence of events: photoexcited rhodopsin activates a G protein by facilitating GTP binding. The G protein then activates a phospholipase C that generates inositol trisphosphate, which in turn acts as an internal messenger to bring about depolarization of the photoreceptor cell. Binding assays of GTP analogs and measurements of GTPase activity showed that there are 1.6 million copies of G protein per photoreceptor cell. The GTP binding component is a 41-kDa protein, and the light-activated GTPase is dependent on photoconversion of rhodopsin to metarhodopsin. Analysis of phospholipase C activity revealed that this enzyme is under stringent control of the G protein, that the major product formed is inositol trisphosphate, and that this product is rapidly hydrolyzed by a specific phosphomonoesterase. Introduction of inositol trisphosphate to the intact photoreceptor cell mimics the effect of light, and bisphosphoglycerate, which inhibits inositol trisphosphate hydrolysis, enhances the effects of inositol trisphosphate and of dim light. The interaction of photoexcited rhodopsin with a G protein is thus similar in both vertebrate and invertebrate photoreceptors. These G proteins, however, activate different photoreceptor enzymes: phospholipase C in invertebrates and cGMP phosphodiesterase in vertebrates.