Protein kinase C is required for light adaptation in Drosophila photoreceptors

Visual transduction in Drosophila

The brain's capacity to analyse and interpret information is limited ultimately by the input it receives. This sets a premium on information capacity of sensory receptors, which can be maximized by optimizing sensitivity, speed and reliability of response. Nowhere is selection pressure for information capacity stronger than in the visual system, where speed and sensitivity can mean the difference between life and death. Phototransduction in flies represents the fastest G-protein-signalling cascade known. Analysis in Drosophila has revealed many of the underlying molecular strategies, leading to the discovery and characterization of signalling molecules of widespread importance.

Scaffolding proteins organize multimolecular protein complexes for sensory signal transduction

Scaffolding proteins composed of protein-protein interaction domains have emerged as organizers of multiprotein complexes in diverse cellular compartments, including neuronal synapses, cell-cell junctions of epithelial cells, and the stimulus perceiving structures of sensory neurons. This review focuses on the INAD-assembled signalling complex of Drosophila photoreceptors, which organizes key components of the phototransduction cascade into a multiprotein signal transduction unit. The structure, the physiological consequences, and the assembly and targeting of the members of the INAD signalling complex will be described. In addition, the existence of signalling complexes in vertebrate photoreceptors, olfactory neurons and mechanosensitive hair cells will be discussed.

The second PDZ domain of INAD is a type I domain involved in binding to eye protein kinase C. Mutational analysis and naturally occurring variants

INAD is a scaffolding protein containing five PSD95/dlg/zonular occludens-1 (PDZ) domains that tether NORPA (phospholipase Cbeta(4)), the TRP calcium channel, and eye-PKC in Drosophila photoreceptors. We previously showed that eye-PKC interacted with the second PDZ domain (PDZ2) of INAD. Sequence comparison with a prototypical type I PDZ domain predicts that PDZ2 is the best candidate among the five PDZ domains to recognize eye-PKC that contains a type I PDZ ligand, Ile-Thr-Ile-Ile, at its carboxyl terminus. Replacement of Ile(-3) in eye-PKC with charged residues resulted in a drastic reduction of the PDZ2 interaction. Substitution of a conserved His with Arg at the second alpha-helix of PDZ2 led to a reduced binding; however, a Leu replacement resulted in an enhanced eye-PKC association. We isolated and sequenced the InaD gene. The coding sequence of InaD contains nine exons spanning 3 kilobases. Translation of coding sequences from three wild-type alleles revealed three SNPs affecting residues, 282, 319, and 333 of INAD. These polymorphisms are localized in PDZ2. Interestingly, we found two of three PDZ2 variants displayed a greater affinity for eye-PKC. In summary, we evaluated the molecular basis of the eye-PKC and PDZ2 association by mutational analysis and concluded that PDZ2 of INAD is a type I domain important for the eye-PKC interaction.

Caenorhabditis elegans PIAK, a phospholipid-independent kinase that activates the AKT/PKB survival kinase

Phospholipid-dependent kinase 1 (PDK 1) is a 3'-phospholipid-responsive serine/threonine kinase that plays a critical role in cell survival by phosphorylating and activating the anti-apoptotic AKT/PKB kinase. While PDK 1 is clearly an important component of the cell survival machinery, the potential for phospholipid-independent activation of the AKT/PKB survival pathway has not been extensively examined at the molecular level. We have identified a second form of PDK 1 in the nematode Caenorhabditis elegans that we have termed PIAK (phospholipid-independent AKT/PKB kinase). PIAK is highly homologous to C. elegans and mammalian PDK 1 with the exception that the novel kinase lacks a phospholipid binding pleckstrin homology domain. The domain structure of PIAK suggests that it might be a phospholipid-independent kinase, and PIAK phosphorylates mammalian AKT/PKB at the activating Thr(308) residue in the presence of the phosphatidylinositol (PI) 3-kinase inhibitors as well as in the absence of growth factors. In addition, PIAK is capable of inducing the phospholipid-independent, AKT/PKB-induced phosphorylation of the AFX-type forkhead transcription factor, resulting in its cytoplasmic localization. Because the nuclear localization of this transcription factor induces an apoptotic state, this PIAK-mediated cytoplasmic sequestration allows for cell survival. Finally, PIAK activity appears to be induced by various inhibitors of cell cycle G(1) progression. These data suggest an alternate, phosphatidylinositol 3-kinase-independent mechanism for the activation of the AKT/PKB survival pathway that may be utilized during periods of cellular quiescence.

The quantitative distribution of a putative PKC epsilon mRNA in Limulus central nervous system by modified competitive RT-PCR

Recently, a full length cDNA for the epsilon (epsilon) isoform of protein kinase C (PKC) was cloned and sequenced from a cDNA library for the horseshoe crab, Limulus polyphemus. This multifunctional enzyme has been implicated in the modulation of the choline cotransporter in Limulus and the epsilon isoform has been identified in homogenates from its central nervous system (CNS). RT-PCR has proven to be a very useful method for quantifying even a few molecules of mRNA in tissue samples. A modified competitive RT-PCR was used here to quantify a putative PKC epsilon mRNA in Limulus CNS preparations. First, we replaced normally used oligo dT and random primers generated from mRNA with a PKC epsilon specific (3' end) primer P4. Then we used modified nucleotides to extend sample life in storage and finally, we used only annealing and denaturing temperatures during PCR to reduce background. The modified method was used for the first time to quantify PKC epsilon mRNA from three distinct areas of the CNS in Limulus. Results revealed high levels of PKC epsilon mRNA in the corpora pedunculata, in the abdominal ganglia and in the brain ring. These results indicate that PKC epsilon mRNA is broadly distributed throughout the Limulus CNS. Importantly, this modified competitive RT-PCR technique was successfully applied to the quantitation of specific mRNA from Limulus nervous tissue for which no internal standard is available commercially.

Independent anchoring and assembly mechanisms of INAD signaling complexes in Drosophila photoreceptors

In Drosophila photoreceptors the multivalent PDZ protein INAD organizes the phototransduction cascade into a macromolecular signaling complex containing the effector PLC, the light-activated TRP channels, and a regulatory PKC. Previously, we showed that the subcellular localization of INAD signaling complexes is critical for signaling. Now we have examined how INAD complexes are anchored and assembled in photoreceptor cells. We find that trp mutants, or transgenic flies expressing inaD alleles that disrupt the interaction between INAD and TRP, cause the mislocalization of the entire transduction complex. The INAD-TRP interaction is not required for targeting but rather for anchoring of complexes, because INAD and TRP can be targeted independently of each other. We also show that, in addition to its scaffold role, INAD functions to preassemble transduction complexes. Preassembly of signaling complexes helps to ensure that transduction complexes with the appropriate composition end up in the proper location. This may be a general mechanism used by cells to target different signaling machinery to the pertinent subcellular location.

Light adaptation in Drosophila photoreceptors: I. Response dynamics and signaling efficiency at 25 degrees C

Besides the physical limits imposed on photon absorption, the coprocessing of visual information by the phototransduction cascade and photoreceptor membrane determines the fidelity of photoreceptor signaling. We investigated the response dynamics and signaling efficiency of Drosophila photoreceptors to natural-like fluctuating light contrast stimulation and intracellular current injection when the cells were adapted over a 4-log unit light intensity range at 25 degrees C. This dual stimulation allowed us to characterize how an increase in the mean light intensity causes the phototransduction cascade and photoreceptor membrane to produce larger, faster and increasingly accurate voltage responses to a given contrast. Using signal and noise analysis, this appears to be associated with an increased summation of smaller and faster elementary responses (i.e., bumps), whose latency distribution stays relatively unchanged at different mean light intensity levels. As the phototransduction cascade increases, the size and speed of the signals (light current) at higher adapting backgrounds and, in conjunction with the photoreceptor membrane, reduces the light-induced voltage noise, and the photoreceptor signal-to-noise ratio improves and extends to a higher bandwidth. Because the voltage responses to light contrasts are much slower than those evoked by current injection, the photoreceptor membrane does not limit the speed of the phototransduction cascade, but it does filter the associated high frequency noise. The photoreceptor information capacity increases with light adaptation and starts to saturate at approximately 200 bits/s as the speed of the chemical reactions inside a fixed number of transduction units, possibly microvilli, is approaching its maximum.

TRP and the PDZ protein, INAD, form the core complex required for retention of the signalplex in Drosophila photoreceptor cells

The light response in Drosophila photoreceptor cells is mediated by a series of proteins that assemble into a macromolecular complex referred to as the signalplex. The central player in the signalplex is inactivation no afterpotential D (INAD), a protein consisting of a tandem array of five PDZ domains. At least seven proteins bind INAD, including the transient receptor potential (TRP) channel, which depends on INAD for localization to the phototransducing organelle, the rhabdomere. However, the determinants required for localization of INAD are not known. In this work, we showed that INAD was required for retention rather than targeting of TRP to the rhabdomeres. In addition, we demonstrated that TRP bound to INAD through the COOH terminus, and this interaction was required for localization of INAD. Other proteins that depend on INAD for localization, phospholipase C and protein kinase C, also mislocalized. However, elimination of any other member of the signalplex had no impact on the spatial distribution of INAD. A direct interaction between TRP and INAD did not appear to have a role in the photoresponse independent of localization of multiple signaling components. Rather, the primary function of the TRP/ INAD complex is to form the core unit required for localization of the signalplex to the rhabdomeres.

Reversible phosphorylation of the signal transduction complex in Drosophila photoreceptors

In the Drosophila visual cascade, the transient receptor potential (TRP) calcium channel, phospholipase Cbeta (no-receptor-potential A), and an eye-specific isoform of protein kinase C (eye-PKC) comprise a multimolecular signaling complex via their interaction with the scaffold protein INAD. Previously, we showed that the interaction between INAD and eye-PKC is a prerequisite for deactivation of a light response, suggesting eye-PKC phosphorylates proteins in the complex. To identify substrates of eye-PKC, we immunoprecipitated the complex from head lysates using anti-INAD antibodies and performed in vitro kinase assays. Wild-type immunocomplexes incubated with [(32)P]ATP revealed phosphorylation of TRP and INAD. In contrast, immunocomplexes from inaC mutants missing eye-PKC, displayed no phosphorylation of TRP or INAD. We also investigated protein phosphatases that may be involved in the dephosphorylation of proteins in the complex. Dephosphorylation of TRP and INAD was partially suppressed by the protein phosphatase inhibitors okadaic acid, microcystin, and protein phosphatase inhibitor-2. These phosphatase activities were enriched in the cytosol of wild-type heads, but drastically reduced in extracts prepared from glass mutants, which lack photoreceptors. Our findings indicate that INAD functions as RACK (receptor for activated PKC), allowing eye-PKC to phosphorylate INAD and TRP. Furthermore, dephosphorylation of INAD and TRP is catalyzed by PP1/PP2A-like enzymes preferentially expressed in photoreceptor cells.

Constitutive activity of the light-sensitive channels TRP and TRPL in the Drosophila diacylglycerol kinase mutant, rdgA

Mutations in the Drosophila retinal degeneration A (rdgA) gene, which encodes diacylglycerol kinase (DGK), result in early onset retinal degeneration and blindness. Whole-cell recordings revealed that light-sensitive Ca2+ channels encoded by the trp gene were constitutively active in rdgA photoreceptors. Early degeneration was rescued in rdgA;trp double mutants, lacking TRP channels; however, the less Ca2+-permeable light-sensitive channels (TRPL) were constitutively active instead. No constitutive activity was seen in rdgA;trpI;trp mutants lacking both classes of channel, although, like rdgA;trp, these still showed a residual slow degeneration. Responses to light were restored in rdgA;trp but deactivated abnormally slowly, indicating that DGK is required for response termination. The findings suggest that early degeneration in rdgA is caused by uncontrolled Ca2+ influx and support the proposal that diacylglycerol or its metabolites are messengers of excitation in Drosophila photoreceptors.

Single photon responses in Drosophila photoreceptors and their regulation by Ca2+

1. Discrete events (quantum bumps) elicited by dim light were analysed in whole-cell voltage clamp of photoreceptors from dissociated Drosophila ommatidia. Bumps were automatically detected and analysed for amplitude, rise and decay times, and latency. 2. The bump interval and amplitude distributions, and the 'frequency of seeing' curve conformed to Poisson predictions for the absorption of single photons. 3. At resting potential (-70 mV), bumps averaged 10 pA in peak amplitude with a half-width of ca 20 ms, representing simultaneous activation of ca 15 channels. 4. The macroscopic response to flashes containing up to at least 750 photons were predicted by the linear summation of quantum bumps convolved with their latency dispersion. 5. Bump duration was unaffected by lowering the extracellular Ca2+ concentration ([Ca2+]o) from 1.5 to 0.5 mM, but increased >10-fold between 0.5 mM Ca2+ and 0 Ca2+. Bump amplitude was constant over the range 1.5-100 microM, but decreased ca 5- to 10-fold at lower Ca2+ concentrations. Bump latency increased by ca 50 % between 1.5 mM and 100 microM Ca2+o but returned to near control levels in Ca2+-free solutions. At intermediate [Ca2+]o bumps were biphasic with a slow rising phase followed by rapid amplification and inactivation. This behaviour was mimicked in high [Ca2+]o by internal buffering with BAPTA, but not EGTA. This suggests that Ca2+ influx through the light-sensitive channels must first raise cytosolic Ca2+ to a threshold level before initiating a cycle of positive and negative feedback mediated by molecular targets within the same microvillus. Quantum bumps in trp mutants lacking the major class of light-sensitive channel were reduced in size (mean 3.5 pA) representing simultaneous activation of only one or two channels; however, a second rarer (10 %) class of large bump had an amplitude similar to wild-type (WT) bumps. Bumps in trpl mutants lacking the second class of light-sensitive channel were very similar to WT bumps, but with slightly slower decay times. In InaDP215 mutants, in which the association of the TRP channels with the INAD scaffolding molecule is disrupted, bumps showed a defect in quantum bump termination, but their amplitudes and latencies were near normal.

Calcium transients in the rhabdomeres of dark- and light-adapted fly photoreceptor cells

The light response of fly photoreceptor cells is modulated by changes in free Ca(2+) concentration. Fly phototransduction and most processes regulating it take place in or very close to the rhabdomere. We therefore measured the kinetics and the absolute values of the free Ca(2+) concentration in the rhabdomere of fly photoreceptor cells in vivo by making use of the natural optics of the fly's eye. We show that Ca(2+) flowing into the rhabdomere after light stimulation of dark-adapted cells causes fast Ca(2+) transients that reach peak values higher than 200 microM in <20 msec. Approximately 500 msec later, the free Ca(2+) concentration has declined again to approximately 20 microM. The duration of the Ca(2+) transients becomes still shorter, and their size reduced, when the photoreceptor cell is light-adapted. This reduction in duration and size of the Ca(2+) transients is graded with the intensity of the adapting light. The kinetics and absolute values of the free calcium concentration found to occur in the rhabdomere are suitable to mediate the fast feedback signals known to act on the fly phototransduction cascade.

Visual transduction in Drosophila

The Drosophila phototransduction cascade has emerged as an attractive paradigm for understanding the molecular mechanisms underlying visual transduction, as well as other G protein-coupled signaling cascades that are activated and terminated with great rapidity. A large collection of mutants affecting the fly visual cascade have been isolated, and the nature and function of many of the affected gene products have been identified. Virtually all of the proteins, including those that were initially classified as novel, are highly related to vertebrate homologs. Recently, it has become apparent that most of the proteins central to Drosophila phototransduction are coupled into a supramolecular signaling complex, signalplex, through association with a PDZ-containing scaffold protein. The characterization of this complex has led to a re-evaluation of the mechanisms underlying the activation and deactivation of the phototransduction cascade.

Protein kinase C activators inhibit the visual cascade in Limulus ventral photoreceptors at an early stage

The phosphoinositide cascade mediates visual transduction in invertebrate photoreceptors. Phospholipase C (PLC) catalyzes the hydrolysis of phosphatidylinositol bisphosphate, producing inositol trisphosphate (InsP(3)) and diacylglycerol (DAG). Protein kinase C (PKC) is a major target of DAG in many cell types. We have used PKC activators to investigate the function of the kinase in the phototransduction cascade in Limulus polyphemus ventral photoreceptors. Extracellular application of (-)-indolactam V (0. 03-30 microM) or phorbol-12,13-dibutyrate (10 microM) reversibly reduced the sensitivity of the electrical response of the photoreceptors to light by up to 1000-fold. The inert stereoisomer (+)-indolactam V and 4alpha-phorbol had no effect. The effect of (-)-indolactam V was antagonized by the PKC inhibitors bisindolylmaleimide I and Gö 6976. Coapplication of bisindolylmaleimide V, used as a negative control compound for PKC inhibition, did not reduce the effectiveness of (-)-indolactam V. These findings are consistent with (-)-indolactam V activating PKC and desensitizing the light response. Furthermore, our pharmacological results indicate that PKC activation does not appear to play a role in light adaptation. We localized the position of the target of PKC in the visual cascade. We chemically excited the cascade at various stages to determine the kinase's target. PKC activation by (-)-indolactam V decreased the light-induced elevation of intracellular calcium but had no effect on the photoreceptor's excitatory response to intracellular injection of InsP(3). However, the PKC activator greatly reduced the excitation caused by GTP-gamma-S injection. We propose that PKC inhibits the visual transduction cascade at the G-protein and/or PLC stage.

The organization of INAD-signaling complexes by a multivalent PDZ domain protein in Drosophila photoreceptor cells ensures sensitivity and speed of signaling

Phototransduction in Drosophila has emerged as an attractive model system for studying the organization of signaling cascades in vivo. In photoreceptor neurons, the multivalent PDZ protein INAD serves as a scaffold to assemble different components of the phototransduction pathway, including the effector PLC, the light-activated ion channel TRP, and a protein kinase C involved in deactivation of the light response. INAD is required for organizing and maintaining signaling complexes in the rhabdomeres of photoreceptors. This macromolecular organization endows photoreceptors with many of their signaling properties, including high sensitivity, fast activation and deactivation kinetics, and exquisite feedback regulation by small localized changes in [Ca2+]i. Assembly of transduction components into signaling complexes is also an important cellular strategy for ensuring specificity of signaling while minimizing unwanted cross-talk. In this report, we review INAD's role as a signal transduction scaffold and its role in the assembly and localization of photoreceptor complexes.

Does Ca2+ reach millimolar concentrations after single photon absorption in Drosophila photoreceptor microvilli?

The quantum bump, the elementary event of fly phototransduction induced by the absorption of a single photon, is a small, transient current due to the opening of cation-channels permeable to Ca2+. These channels are located in small, tube-like protrusions of the cell membrane, the microvilli. Using a modeling approach, we calculate the changes of free Ca2+ concentration inside the microvilli, taking into account influx and diffusion of Ca2+. Independent of permeability ratios and Ca2+ buffering, we find that the free Ca2+ concentrations rise to millimolar values, as long as we assume that all activated channels are located in a single microvillus. When we assume that as much as 25 microvilli participate in a single bump, the free Ca2+ concentration still reaches values higher than 80 microM. These very high concentrations show that the microvilli of fly photoreceptors are unique structures in which the Ca2+ signaling is even more extreme than in calcium concentration microdomains very close to Ca2+ channels.

Reduced capacitative calcium entry correlates with vesicle accumulation and apoptosis

A preneoplastic variant of Syrian hamster embryo cells, sup(+), exhibits decreased endoplasmic reticulum calcium levels and subsequently undergoes apoptosis in low serum conditions (Preston, G. A., Barrett, J. C., Biermann, J. A., and Murphy, E. (1997) Cancer Res. 57, 537-542). This decrease in endoplasmic reticulum calcium appears to be due, at least in part, to reduced capacitative calcium entry at the plasma membrane. Thus we investigated whether inhibition of capacitative calcium entry per se could reduce endoplasmic reticulum calcium and induce apoptosis of cells. We find that treatment with either SKF96365 (30-100 microM) or cell-impermeant 1,2-bis(o-amino-5-bromophenoxy)ethane-N,N,N', N'-tetraacetic acid (5-10 mM) is able to induce apoptosis of cells in conditions where apoptosis does not normally occur. Because previous work has implicated vesicular trafficking as a mechanism of regulating capacitative calcium entry, we investigated whether disruption of vesicular trafficking could lead to decreased capacitative calcium entry and subsequent apoptosis of cells. Coincident with low serum-induced apoptosis, we observed an accumulation of vesicles within the cell, suggesting deregulated vesicle trafficking. Treatment of cells with bafilomycin (30-100 nM), an inhibitor of the endosomal proton ATPase, produced an accumulation of vesicles, decreased capacitative entry, and induced apoptosis. These data suggest that deregulation of vesicular transport results in reduced capacitative calcium entry which in turn results in apoptosis.

Isolation and characterization of Drosophila retinal degeneration B suppressors

The Drosophila retinal degeneration B protein (RdgB) is a novel integral membrane phosphatidylinositol transfer protein required for photoreceptor cell viability and light response. We isolated one intragenic suppressor (rdgBsu100) and four autosomal suppressors of the hypomorphic rdgBKS222 retinal degeneration phenotype. The rdgBsu100 suppressor dramatically slowed rdgBKS222's photoreceptor degeneration without significantly improving the electroretinogram (ERG) light response. One autosomal recessive suppressor [su(rdgB)69] significantly slowed rdgBKS222 retinal degeneration and restored the ERG light response near to that of the wild type. Unlike all the previously characterized rdgB suppressors, the four new autosomal suppressors do not affect the ERG light response in rdgB+ flies. Only Su(rdgB)116 exhibited a mutant phenotype in a rdgB+ background, which was smaller R1-6 rhabdomeres. We also examined the extent to which two previously identified visual transduction mutations suppressed rdgB retinal degeneration. Absence of one of the light-activated calcium channels (trpCM) slowed the onset of rdgB-dependent degeneration. However, loss of protein kinase C (inaC209), which blocks photoreceptor cell deactivation, desensitization, and light adaptation, failed to suppress rdgB degeneration under normal light conditions. This demonstrates that TRP activity, but not INAC, is required for rapid rdgB-dependent degeneration.

Polyunsaturated fatty acids activate the Drosophila light-sensitive channels TRP and TRPL

Phototransduction in invertebrate microvillar photoreceptors is thought to be mediated by the activation of phospholipase C (PLC), but how this leads to gating of the light-sensitive channels is unknown. Most attention has focused on inositol-1,4,5-trisphosphate, a second messenger produced by PLC from phosphatidylinositol-4,5-bisphosphate; however, PLC also generates diacylglycerol, a potential precursor for several polyunsaturated fatty acids, such as arachidonic acid and linolenic acid. Here we show that both of these fatty acids reversibly activate native light-sensitive channels (transient receptor potential (TRP) and TRP-like (TRPL)) in Drosophila photoreceptors as well as recombinant TRPL channels expressed in Drosophila S2 cells. Recombinant channels are activated rapidly in both whole-cell recordings and inside-out patches, with a half-maximal effector concentration for linolenic acid of approximately 10 microM. Four different lipoxygenase inhibitors, which might be expected to lead to build-up of endogenous fatty acids, also activate native TRP and TRPL channels in intact photoreceptors. As arachidonic acid may not be found in Drosophila, we suggest that another polyunsaturated fatty acid, such as linolenic acid, may be a messenger of excitation in Drosophila photoreceptors.

Requirement for the NINAC kinase/myosin for stable termination of the visual cascade

Activation of the Drosophila photoresponse is a rapid process that results in plasma membrane Ca2+ and Na+ conductances. Ca2+ functions in negative feedback regulation of Drosophila vision including deactivation. Protein kinase C (PKC) binds directly to Ca2+ and is required for deactivation. However, the consequences of disrupting phosphorylation of any individual PKC substrate in the Drosophila retina have not been addressed. In the current work, we show that NINAC p174, which consists of a protein kinase domain joined to the head region of myosin heavy chain, is a phosphoprotein and is phosphorylated in vitro by PKC. Mutation of either of two PKC sites in the p174 tail resulted in an unusual defect in deactivation that had not been detected previously for other ninaC alleles or other loci. After cessation of the light stimulus, there appeared to be a transient reactivation of the visual cascade. This phenotype suggests that a mechanism exists to prevent reactivation of the visual cascade and that p174 participates in this process.

Retinal targets for calmodulin include proteins implicated in synaptic transmission

Ca2+ influxes regulate multiple events in photoreceptor cells including phototransduction and synaptic transmission. An important Ca2+ sensor in Drosophila vision appears to be calmodulin since a reduction in levels of retinal calmodulin causes defects in adaptation and termination of the photoresponse. These functions of calmodulin appear to be mediated, at least in part, by four previously identified calmodulin-binding proteins: the TRP and TRPL ion channels, NINAC and INAD. To identify additional calmodulin-binding proteins that may function in phototransduction and/or synaptic transmission, we conducted a screen for retinal calmodulin-binding proteins. We found eight additional calmodulin-binding proteins that were expressed in the Drosophila retina. These included six targets that were related to proteins implicated in synaptic transmission. Among these six were a homolog of the diacylglycerol-binding protein, UNC13, and a protein, CRAG, related to Rab3 GTPase exchange proteins. Two other calmodulin-binding proteins included Pollux, a protein with similarity to a portion of a yeast Rab GTPase activating protein, and Calossin, an enormous protein of unknown function conserved throughout animal phylogeny. Thus, it appears that calmodulin functions as a Ca2+ sensor for a broad diversity of retinal proteins, some of which are implicated in synaptic transmission.

Coordination of an array of signaling proteins through homo- and heteromeric interactions between PDZ domains and target proteins

The rapid activation and feedback regulation of many G protein signaling cascades raises the possibility that the critical signaling proteins may be tightly coupled. Previous studies show that the PDZ domain containing protein INAD, which functions in Drosophila vision, coordinates a signaling complex by binding directly to the light-sensitive ion channel, TRP, and to phospholipase C (PLC). The INAD signaling complex also includes rhodopsin, protein kinase C (PKC), and calmodulin, though it is not known whether these proteins bind to INAD. In the current work, we show that rhodopsin, calmodulin, and PKC associate with the signaling complex by direct binding to INAD. We also found that a second ion channel, TRPL, bound to INAD. Thus, most of the proteins involved directly in phototransduction appear to bind to INAD. Furthermore, we found that INAD formed homopolymers and the homomultimerization occurred through two PDZ domains. Thus, we propose that the INAD supramolecular complex is a higher order signaling web consisting of an extended network of INAD molecules through which a G protein-coupled cascade is tethered.

Membrane current induced by protein kinase C activators in rhabdomeric photoreceptors: implications for visual excitation

Visual excitation in rhabdomeric photoreceptors is thought to be mediated by activation of a light-regulated phospholipase C (PLC) and the consequent hydrolysis of phosphatidylinositol bisphosphate. Whereas much attention has been devoted to inositol trisphosphate (IP3) production and intracellular Ca2+ release, little is known about the possible role of the DAG branch in the generation of the light response. We have tested the effect of chemically distinct surrogates of DAG on isolated Lima photoreceptors. Application of the phorbol ester PMA (0.5-10 microM) or the alkaloid (-)-indolactam (20-100 microM) from a holding potential of -50 mV elicited an inward current, several hundred picoamperes in amplitude, accompanied by a pronounced increase in membrane conductance. The stereoisomers 4alpha-PMA and (+)-indolactam were both inactive, arguing for the specificity of the effects. Elevation of cytosolic Ca2+ by intracellular dialysis accelerated this current, whereas chelerythrine antagonized it, suggesting the involvement of PKC. The reversal potential of the membrane current induced by PKC activators was approximately +10 mV; replacement of extracellular Na with impermeant N-methyl-D-glucamine decreased its amplitude and shifted the reversal potential in the negative direction. Stimulation by PMA and (-)-indolactam was accompanied by a pronounced depression of light responsiveness; conversely, steady illumination reduced the size of the current elicited by these PKC activators. Taken together, these results support the notion that the DAG branch of the PLC cascade, in addition to its suggested participation in visual adaptation, may play a role in the activation of the photoresponse or a component thereof, probably in synergy with IP3-mediated Ca2+ release.

Interaction of eye protein kinase C and INAD in Drosophila. Localization of binding domains and electrophysiological characterization of a loss of association in transgenic flies

Drosophila eye-specific protein kinase C (eye-PKC) is involved in light adaptation and deactivation. eye-PKC, NORPA (phospholipase Cbeta), and transient-receptor-potential (TRP) (calcium channel) are integral components of a signal transduction complex organized by INAD, a protein containing five PDZ domains. We previously demonstrated the direct association between the third PDZ domain of INAD with TRP in addition to the carboxyl-terminal half of INAD with the last three residues of NORPA. In this work, the molecular interaction between eye-PKC and INAD is defined via the yeast two-hybrid and ligand overlay assays. We show that the second PDZ domain of INAD interacts with the last three residues in the carboxyl-terminal tail of eye-PKC, Thr-Ile-Ile. The association between eye-PKC and INAD is disrupted by an amino acid substitution (Ile-700 to Asp) at the final residue of eye-PKC. In flies lacking endogenous eye-PKC (inaCp215), normal visual physiology is restored upon expression of wild-type eye-PKC, whereas the eye-PKCI700D mutant is completely inactive. Flies homozygous for inaCp209 and InaDp215, a mutation that causes a loss of the INAD-TRP association, were generated. These double mutants display a more severe response inactivation than either of the single mutants. Based on these findings, we conclude that the in vivo activity of eye-PKC depends on its association with INAD and that the sensitivity of photoreceptors is cooperatively regulated by the presence of both eye-PKC and TRP in the signaling complex.

TRP trapped in fly signaling web

In the Drosophila eye, photoactivation of rhodopsin leads to the opening of the light-sensitive cation influx channels TRP and TRPL. This response is extremely rapid and results in depolarization of the photoreceptor cells followed by Ca(2+)-mediated feedback regulation of the visual signaling cascade. The mechanisms that facilitate the rapid kinetics of activation and feedback regulation are poorly understood. However, the recent discovery that most of the proteins that function in fly phototransduction associate into a supramolecular complex permits a re-evaluation of the mechanisms underlying the activation and regulation of the cascade. The central player in the signaling complex is INAD, a protein with five protein-interaction motifs known as PDZ domains. The INAD complex does not appear to be a particle, but a massive signaling web composed of an INAD polymer with which some of the target proteins associate through complex multivalent interactions.

TRP, TRPL and trouble in photoreceptor cells

Calcium influx through the TRP and TRPL light-activated channels triggers a complex regulatory hierarchy resulting in positive and negative feedback regulation of the phototransduction cascade. Recent studies have begun to elucidate the function of TRP and TRPL in vivo, and to examine their relationship to intracellular calcium changes during the light response.

The TRP Ca2+ channel assembled in a signaling complex by the PDZ domain protein INAD is phosphorylated through the interaction with protein kinase C (ePKC)

Photoreceptors which use a phospholipase C-mediated signal transduction cascade harbor a signaling complex in which the phospholipase Cbeta (PLCbeta), the light-activated Ca2+ channel TRP, and an eye-specific protein kinase C (ePKC) are clustered by the PDZ domain protein INAD. Here we investigated the function of ePKC by cloning the Calliphora homolog of Drosophila ePKC, by precipitating the TRP signaling complex with anti-ePKC antibodies, and by performing phosphorylation assays in isolated signaling complexes and in intact photoreceptor cells. The deduced amino acid sequence of Calliphora ePKC comprises 685 amino acids (MW = 78 036) and displays 80.4% sequence identity with Drosophila ePKC. Immunoprecipitations with anti-ePKC antibodies led to the coprecipitation of PLCbeta, TRP, INAD and ePKC but not of rhodopsin. Phorbolester- and Ca2+-dependent protein phosphorylation revealed that, apart from the PDZ domain protein INAD, the Ca2+ channel TRP is a substrate of ePKC. TRP becomes phosphorylated in isolated signaling complexes. TRP phosphorylation in intact photoreceptor cells requires the presence of extracellular Ca2+ in micromolar concentrations. It is proposed that ePKC-mediated phosphorylation of TRP is part of a negative feedback loop which regulates Ca2+ influx through the TRP channel.

The role of calmodulin-binding sites in the regulation of the Drosophila TRPL cation channel expressed in Xenopus laevis oocytes by ca2+, inositol 1,4,5-trisphosphate and GTP-binding proteins

The roles of calmodulin-binding sites in the regulation by Ca2+, inositol 1,4,5-trisphosphate (InsP3) and GTP-binding regulatory proteins (G-proteins) of the Drosophila melanogaster TRPL (transient-receptor-potential-like) non-specific Ca2+ channel were investigated. Wild-type TRPL protein and two mutant forms, TRPL (W713G) and TRPL (W814G), in which a key tryptophan residue in each of the two putative calmodulin-binding sites (Sites 1 and 2, respectively) was replaced by glycine, were expressed heterologously in Xenopus laevis oocytes. Immunofluorescence studies indicated that the expressed TRPL, TRPL (W713G) and TRPL (W814G) proteins are located at the plasma membrane. TRPL oocytes (oocytes injected with trpl cRNA) and TRPL (W814G) oocytes [oocytes injected with trpl (W814G) cRNA] exhibited substantially greater rates of basal (constitutive) Ca2+ inflow (measured using fluo-3 and the Ca2+ add-back protocol) than mock-injected oocytes (mock oocytes). In TRPL (W713G) oocytes, this difference was abolished. In TRPL and TRPL (W814G) [oocytes injected with trpl (W713G) cRNA], but not in TRPL (W713G) oocytes, basal Ca2+ inflow was inhibited by W13, an inhibitor of calmodulin action. Calmodulin (3 muM intracellular) inhibited basal Ca2+ inflow in TRPL but not in TRPL (W713G) or TRPL (W814G) oocytes. Staurosporin, an inhibitor of protein kinase C (PKC), inhibited, while PMA (an activator of PKC) stimulated, basal Ca2+ inflow in TRPL oocytes. In oocytes incubated in the presence of PMA (to suppress Ca2+ inflow through endogenous receptor-activated Ca2+ channels), the InsP3-induced stimulation of Ca2+ inflow through TRPL channels was more clearly evident than in oocytes incubated in the absence of PMA. InsP3 caused a significant stimulation of Mn2+ inflow in TRPL but not in mock oocytes. Rates of InsP3-stimulated Ca2+ inflow through the TRPL, TRPL (W713G) and TRPL (W814G) channels were similar. The ability of GTPgammaS to stimulate Ca2+ inflow through TRPL channels was inhibited by 50% in TRPL (W713G) oocytes but was unaffected in TRPL (W814G) oocytes. It is concluded that, in the environment of the Xenopus oocyte, the Drosophila TRPL channel is activated by (a) interaction with Ca2+/calmodulin at calmodulin-binding Site 1; (b) PKC; (c) InsP3 in a process that does not involve Ca2+ and calmodulin; and (d) a trimeric G-protein(s) through both a Ca2+/calmodulin-dependent and a Ca2+/calmodulin-independent mechanism.

Distribution of Na+,K(+)-ATPase in photoreceptor cells of insects

Light stimulation of insect photoreceptors causes opening of cation channels and an inward current that is partially carried by Na+ ions. There is also an efflux of K+ ions upon photostimulation. Na+ and K+ gradients across the photoreceptor membrane are reestablished by the activity of the enzyme Na+,K(+)-ATPase. About two-thirds of the total amount of ATP consumed in response to a light stimulus is attributed to the activity of this ion pump, demonstrating the importance of this enzyme for photoreceptor function. Insect photoreceptor cells are polarized epithelial cells; their plasma membrane is organized into two domains having a distinct morphology, molecular composition, and function. The visual pigment rhodopsin and the molecular components of the transduction machinery are localized in the rhabdomere, an array of densely packed microvilli, whereas Na+,K(+)-ATPase resides in the nonrhabdomeric membrane. Comparative immunolocalization studies on compound eyes of diverse insect species have demonstrated subtle variations in the distribution patterns of Na+,K(+)-ATPase. These may be accounted for by differences in the mechanisms responsible for Na+,K(+)-ATPase positioning.

Association of INAD with NORPA is essential for controlled activation and deactivation of Drosophila phototransduction in vivo

Visual transduction in Drosophila is a G protein-coupled phospholipase C-mediated process that leads to depolarization via activation of the transient receptor potential (TRP) calcium channel. Inactivation-no-afterpotential D (INAD) is an adaptor protein containing PDZ domains known to interact with TRP. Immunoprecipitation studies indicate that INAD also binds to eye-specific protein kinase C and the phospholipase C, no-receptor-potential A (NORPA). By overlay assay and site-directed mutagenesis we have defined the essential elements of the NORPA-INAD association and identified three critical residues in the C-terminal tail of NORPA that are required for the interaction. These residues, Phe-Cys-Ala, constitute a novel binding motif distinct from the sequences recognized by the PDZ domain in INAD. To evaluate the functional significance of the INAD-NORPA association in vivo, we generated transgenic flies expressing a modified NORPA, NORPAC1094S, that lacks the INAD interaction. The transgenic animals display a unique electroretinogram phenotype characterized by slow activation and prolonged deactivation. Double mutant analysis suggests a possible inaccessibility of eye-specific protein kinase C to NORPAC1094S, undermining the observed defective deactivation, and that delayed activation may similarly result from NORPAC1094S being unable to localize in close proximity to the TRP channel. We conclude that INAD acts as a scaffold protein that facilitates NORPA-TRP interactions required for gating of the TRP channel in photoreceptor cells.

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.

The phosphatidylinositol transfer protein domain of Drosophila retinal degeneration B protein is essential for photoreceptor cell survival and recovery from light stimulation

The Drosophila retinal degeneration B (rdgB) gene encodes an integral membrane protein involved in phototransduction and prevention of retinal degeneration. RdgB represents a nonclassical phosphatidylinositol transfer protein (PITP) as all other known PITPs are soluble polypeptides. Our data demonstrate roles for RdgB in proper termination of the phototransduction light response and dark recovery of the photoreceptor cells. Expression of RdgB's PITP domain as a soluble protein (RdgB-PITP) in rdgB2 mutant flies is sufficient to completely restore the wild-type electrophysiological light response and prevent the degeneration. However, introduction of the T59E mutation, which does not affect RdgB-PITP's phosphatidylinositol (PI) and phosphatidycholine (PC) transfer in vitro, into the soluble (RdgB-PITP-T59E) or full-length (RdgB-T59E) proteins eliminated rescue of retinal degeneration in rdgB2 flies, while the light response was partially maintained. Substitution of the rat brain PITPalpha, a classical PI transfer protein, for RdgB's PITP domain (PITPalpha or PITPalpha-RdgB chimeric protein) neither restored the light response nor maintained retinal integrity when expressed in rdgB2 flies. Therefore, the complete repertoire of essential RdgB functions resides in RdgB's PITP domain, but other PITPs possessing PI and/or PC transfer activity in vitro cannot supplant RdgB function in vivo. Expression of either RdgB-T59E or PITPalpha-RdgB in rdgB+ flies produced a dominant retinal degeneration phenotype. Whereas RdgB-T59E functioned in a dominant manner to significantly reduce steady-state levels of rhodopsin, PITPalpha-RdgB was defective in the ability to recover from prolonged light stimulation and caused photoreceptor degeneration through an unknown mechanism. This in vivo analysis of PITP function in a metazoan system provides further insights into the links between PITP dysfunction and an inherited disease in a higher eukaryote.

Protein kinase C activity and light sensitivity of single amphibian rods

Biochemical experiments by others have indicated that protein kinase C activity is present in the rod outer segment, with potential or demonstrated targets including rhodopsin, transducin, cGMP-phosphodiesterase (PDE), guanylate cyclase, and arrestin, all of which are components of the phototransduction cascade. In particular, PKC phosphorylations of rhodopsin and the inhibitory subunit of PDE (PDE ) have been studied in some detail, and suggested to have roles in downregulating the sensitivity of rod photoreceptors to light during illumination. We have examined this question under physiological conditions by recording from a single, dissociated salamander rod with a suction pipette while exposing its outer segment to the PKC activators phorbol-12-myristate,13-acetate (PMA) or phorbol-12,13-dibutyrate (PDBu), or to the PKC-inhibitor GF109203X. No significant effect of any of these agents on rod sensitivity was detected, whether in the absence or presence of a background light, or after a low bleach. These results suggest that PKC probably does not produce any acute downregulation of rod sensitivity as a mechanism of light adaptation, at least for isolated amphibian rods.

A role for hydrolysis of inositol 1,4,5-trisphosphate in terminating the response to inositol 1,4,5-trisphosphate and to a flash of light in Limulus ventral photoreceptors

Injection of inositol 1,4,5-trisphosphate (Ins 1,4,5-P3) into Limulus ventral photoreceptors produces excitation similar to that produced by light. One process which might contribute to rapid termination of the responses to Ins 1,4,5-P3 and to light is the hydrolysis of Ins 1,4,5-P3 by an InsP3-5-phosphatase to form inositol 1,4-bisphosphate. Inositol 2,4,5-trisphosphate (Ins 2,4,5-P3) is known to be less hydrolysable by the InsP3-5-phosphatase than is Ins 1,4,5-P3. Whereas ventral photoreceptors respond to an injection of Ins 1,4,5-P3 with a single wave of depolarization, the response to Ins 2,4,5-P3 is a burst of waves of depolarization. Our hypothesis is that it is the resistance to hydrolysis by the InsP3-5-phosphatase which accounts for the burst of waves produced by Ins 2,4,5-P3. To test this idea we injected ventral photoreceptors with Ins 1,4,5-P3 in the presence of the non-specific phosphatase inhibitors, vanadate and fluoride, which prolong the response to a flash of light in ventral photoreceptors (D.W. Corson, A. Fein, W.W. Walthall, J. Gen. Physiol. 82 (1983) 659-677). In the presence of fluoride or vanadate the response to Ins 1,4,5-P3 was composed of a burst of waves rather than a single wave of depolarization. We conclude that hydrolysis of Ins 1,4,5-P3 by the InsP3-5-phosphatase plays a role in terminating the ventral photoreceptors response to Ins 1,4,5-P3 and also to light.

Feedback regulation of ATP-induced Ca2+ signaling in HL-60 cells is mediated by protein kinase A- and C-mediated changes in capacitative Ca2+ entry

Extracellular ATP increases intracellular Ca2+ ([Ca2+]i) in HL-60 cells. When cells are stimulated with supramaximal concentrations of ATP, although the initial [Ca2+]i increase is similar over a range of 30, 100, and 300 microM ATP, the rate of the return to basal [Ca2+]i level is faster in cells treated with higher concentrations of ATP. This probably results from differences in Ca2+ influx rather than Ca2+ release, since the influx of the unidirectional Ca2+ surrogates Ba2+ and Mn2+ also exhibit similar responses. Furthermore, while 300 microM ATP had an inhibitory effect on the thapsigargin-induced capacitative Ca2+ entry, 30 microM ATP potentiated the response. However, the inhibitory action of 300 microM ATP was blocked by protein kinase C (PKC) inhibitors, such as GF 109203X and chelerythrine, and the potentiating action of 30 microM ATP was blocked by protein kinase A (PKA) inhibitors H89 and Rp-cAMPS. The PKC inhibitors also slowed the decay rate of the Ca2+ response induced by 300 microM ATP, and the PKA inhibitors increased it when induced by 30 microM ATP. In the measurements of PKA and PKC activity, 30 microM ATP activates only PKA, while 300 microM ATP activates both kinases. Taken together, these data suggest that the changes in the ATP-induced Ca2+ response result from differential modulation of ATP-induced capacitative Ca2+ entry by PKC and PKA in HL-60 cells.

Phospholipases and arachidonic acid contribute independently to sensory transduction and associative neuronal facilitation in Hermissenda type B photoreceptors

During contiguous pairings of light and rotation, B photoreceptors in the Hermissenda eye undergo an increase in excitability that contributes to a modification of several light-elicited behaviors. This excitability increase requires a light-induced rise in intracellular Ca2+ in the photoreceptor concomitant with transmitter binding to G protein-coupled receptors as a result of presynaptic vestibular hair cell stimulation. Phospholipases and arachidonic acid (ArA) are here reported to be involved in independent signal transduction pathways that underlie both receptor function and activity-dependent facilitation of the B photoreceptor. 4-Bromophenacyl bromide (BPB), an inhibitor of phospholipases A2 (PLA2) and C (PLC), blocked the generation of light-induced depolarizing generator potentials, but had no affect on the inhibitory postsynaptic potential (IPSP) in the B cell that results from hair cell stimulation. Quinacrine, which predominantly blocks the activity of PLA2 in neurons, had no affect on either the light response or the IPSP, but did block increases in excitability (i.e. increased input resistance and elicited spike rate) of the B cell that results from pairings of light and presynaptic vestibular stimulation (i.e., in vitro associative conditioning). Neither nordihydroquararetic acid (NDGA), which inhibits metabolism of ArA by cyclooxygenase, nor indomethacin, which inhibits lipoxygenase metabolism of ArA, affected the light response or IPSP, but both blocked the increases in excitability in the B cell that accompanied in vitro conditioning. In combination with earlier results, these data suggest that ArA activates PKC in a synergistic fashion with Ca2+ and diacylglycerol in the B cell, and suggest that PLA2-induced ArA release, though not necessary for transduction of light or the hair cell-induced IPSP in the B cell, is a critical component of the convergence of signals that precipitates associative facilitation in this system.

Light adaptation in Pecten hyperpolarizing photoreceptors. Insensitivity to calcium manipulations

The ability of scallop hyperpolarizing photoreceptors to respond without attenuation to repetitive flashes, together with their low light sensitivity, lack of resolvable quantum bumps and fast photoresponse kinetics, had prompted the suggestion that these cells may be constitutively in a state akin to light adaptation. We here demonstrate that their photocurrent displays all manifestations of sensory adaptation: (a) The response amplitude to a test flash is decreased in a graded way by background or conditioning lights. This attenuation of the response develops with a time constant of 200-800 ms, inversely related to background intensity. (b) Adapting stimuli shift the stimulus-response curve and reduce the size of the saturating photocurrent. (c) The fall kinetics of the photoresponse are accelerated by light adaptation, and the roll-of of the modulation transfer function is displaced to higher frequencies. This light-induced desensitization exhibits a rapid recovery, on the order of a few seconds. Based on the notion that Ca mediates light adaptation in other cells, we examined the consequences of manipulating this ion. Removal of external Ca reversibly increased the photocurrent amplitude, without affecting light sensitivity, photoresponse kinetics, or susceptibility to background adaptation; the effect, therefore, concerns ion permeation, rather than the regulation of the visual response. Intracellular dialysis with 10 mM BAPTA did not reduce the peak-to-plateau decay of the photocurrent elicited by prolonged light steps, not the background-induced compression of the response amplitude range and the acceleration of its kinetics. Conversely, high levels of buffered free [Ca]i (10 microM) only marginally shifted the sensitivity curve (delta sigma = 0.3 log) and spared all manifestations of light adaptation. These results indicate that hyperpolarizing invertebrate photoreceptors adapt to light, but the underlying mechanisms must utilize pathways that are largely independent of changes in cytosolic Ca. The results are discussed in terms of aspects of commonalty to other ciliary sensory receptor cells.

Evidence for a phorbol ester-insensitive phosphorylation step in capacitative calcium entry in rat thymic lymphocytes

Experiments were undertaken to investigate the regulation of capacitative Ca2+ entry by phorbol ester-sensitive protein kinase C and serine/threonine protein phosphatase activity. The thapsigargin-activated Ca2+ entry pathway was probed in control cells and cells treated with phosphatase type 1/2A inhibitors, okadaic acid and calyculin A, or with the phorbol ester, phorbol 12-myristate 13-acetate. The permeability state of this pathway was monitored in the presence or absence of these agents using fluorometric measurements of intracellular Ca2+ concentration, unidirectional Mn2+ entry, and membrane potential and unidirectional measurements of Ca2+ uptake using 45Ca2+. The results of these studies demonstrate that modification of the phosphorylation state of target protein(s) on serine/threonine amino acid residues by inhibition of phosphatase type 1/2A inhibits the capacitative Ca2+ entry pathway in rat thymic lymphocytes. Importantly, the capacitative Ca2+ entry pathway in rat thymic lymphocytes is not modulated by activation of phorbol ester-sensitive protein kinase C.

Differential effects of ninaC proteins (p132 and p174) on light-activated currents and pupil mechanism in Drosophila photoreceptors

The Drosophila ninaC locus encodes two retinal specific proteins (p132 and p174) both consisting of a protein kinase joined to a myosin head domain and a C terminal with a calmodulin-binding domain. The role of p132 and p174 was studied via whole-cell recording and through measurements of the pupil mechanism, i.e. the pigment migration in the photoreceptor cells, in the ninaC mutants, P[ninaC delta 132] (p132 absent), P[ninaC delta 174] (p174 absent), and ninaCP235 (null mutant). Voltage-clamped flash responses in P[ninaC delta 174] and ninaCP235 showed delayed response termination. In response to steady light, plateau responses in both P[ninaC delta 174] and ninaCP235 were also large. In both cases the defect was significantly more severe in ninaCP235. Responses in P[ninaC delta 132] were apparently normal. P[ninaC delta 174] and ninaCP235 were also characterized by spontaneous quantum bump-like activity in the dark and by larger and longer light-induced quantum bumps. The turn-off of the pupil mechanism in P[ninaC delta 174] and ninaCP235 was also defective, although in this case the rate of return to baseline in both mutants was more or less the same. In all ninaC mutants, the amplitudes of the pupillary pigment migration were distinctly smaller than that in the wild type. The reduction of the amplitude was largest in P[ninaC delta 174]. The light sensitivity of the pupil mechanism of P[ninaC delta 174] compared to that of wild type was reduced by 1.3 log units. Remarkably, the light sensitivity of P[ninaC delta 132] and ninaCP235 was ca. 0.5 log units higher than that of the wild type. The results suggest that the p174 protein is required for normal termination of the transduction cascade. The diverse phenotypes observed may suggest multiple roles calmodulin distribution for controlling response termination and regulating pigment migration in Drosophila photoreceptors.

Calcium/calmodulin-dependent protein kinase II and arrestin phosphorylation in Limulus eyes

In rhabdomeral photoreceptors, light stimulates the phosphorylation of arrestin, a protein critical for quenching the photoresponse, by activating a calcium/calmodulin-dependent protein kinase (CaM PK). Here we present biochemical evidence that a CaM PK that phosphorylates arrestin in Limulus eyes is structurally similar to mammalian CaM PK II. In addition, cDNAs encoding proteins homologous to mammalian and Drosophila CaM PK II in the catalytic and regulatory domains were cloned and sequenced from a Limulus lateral eye cDNA library. The Limulus sequences are unique, however, in that they lack most of the association domain. The proteins encoded by these sequences may phosphorylate arrestin.

A quantitative estimate of the maximum amount of light-induced Ca2+ release in Drosophila photoreceptors

Simultaneous measurements of the light-induced current (LIC) and cytosolic Ca2+ (using INDO-1) were made in Drosophila photoreceptors. In the presence of 1.5 mM Cao2+, the UV light used to measure INDO-1 fluorescence saturated the LIC and induced a large Ca2+ rise. In the absence of extracellular Ca2+ and with Na+ replaced by N-methyl-D-glucamine, the light-induced Ca2+ rise was virtually abolished. A residual rise of about 20 nM is regarded as an upper estimate of Ca2+ released from internal stores. To estimate the Ca2+ flux required to generate such a rise, Ca2+ influx signals in response to weak light steps (500 ms LED stimulus) were measured in the presence of external Ca2+. The relationship between [Ca(in)] and the total charge carried during the LIC had a slope of 2.7 nM pC-1. Assuming that 50% of the LIC is carried by Ca2+ and that the single-channel Ca2+ current carried by the InsP3 receptor is 0.04 pA, it was estimated that about 350 InsP3 receptors should have been sufficient to generate a Ca2+ rise of 20 nM within 500 ms. By contrast, the current activated by the UV measuring light was equivalent to the activation of at least 5000 quantum bumps, making it unlikely that InsP3-induced Ca2+ release could have been the causal event for excitation under these conditions.

Matched filtering by a photoreceptor membrane

This study demonstrates how phototransduction cascades and membranes tune photoreceptor response dynamics to image quality, and eliminate noise introduced in cell signalling. Intracellular recordings from intact retina confirm that the light-adapted photoreceptors of the crane fly Tipula paludosa (Diptera; Tipulidae) have a slow response, appropriate for their visual ecology. To provide a slow response, the phototransduction cascade's impulse response fails to narrow with light-adaptation, despite reductions in the timescales of latency and quantum bumps. The photoreceptor membrane acts as a passive RC-filter, because light induced depolarization inactivates voltage-gated potassium currents. The frequency response of the membrane equals the cascade's and, as a result, the membrane is a matched filter that suppresses photon shot noise. This type of broad-band filter, matched to the predictable dynamics of preceding processes to remove noise, could be widely employed in vision and in many other chains of cellular communication.

Phosphorylation of the InaD gene product, a photoreceptor membrane protein required for recovery of visual excitation

In an approach directed to isolate and characterize key proteins of the transduction cascade in photoreceptors using the phosphoinositide signaling pathway, we have isolated the Calliphora homolog of the Drosophila InaD gene product, which in Drosophila InaD mutants causes slow deactivation of the light response. By screening a retinal cDNA library with antibodies directed against photoreceptor membrane proteins, we have isolated a cDNA coding for an amino acid sequence of 665 residues (Mr = 73,349). The sequence displays 65.3% identity (77.3% similarity) with the Drosophila InaD gene product. Probing Western blots with monospecific antibodies directed against peptides comprising amino acids 272-542 (anti-InaD-(272-542)) or amino acids 643-655 (anti-InaD-(643-655)) of the InaD gene product revealed that the Calliphora InaD protein is specifically associated with the signal-transducing rhabdomeral photoreceptor membrane from which it can be extracted by high salt buffer containing 1.5 M NaCl. As five out of eight consensus sequences for protein kinase C phosphorylation reside within stretches of 10-16 amino acids that are identical in the Drosophila and Calliphora InaD protein, the InaD gene product is likely to be a target of protein kinase C. Phosphorylation studies with isolated rhabdomeral photoreceptor membranes followed by InaD immunoprecipitation revealed that the InaD protein is a phosphoprotein. In vitro phosphorylation is, at least to some extent, Ca 2+ dependent and activated by phorbol 12-myristate 13-acetate. The inaC-encoded eye-specific form of a protein kinase C (eye-PKC) is co-precipitated by antibodies specific for the InaD protein from detergent extracts of rhabdomeral photoreceptor membranes, suggesting that the InaD protein and eye-PKC are interacting in these membranes. Co-precipitating with the InaD protein and eye-PKC are two other key components of the transduction pathway, namely the trp protein, which is proposed to form a Ca2+ channel, and the norpA-encoded phospholipase C, the primary target enzyme of the transduction pathway. It is proposed that the rise of the intracellular Ca2+ concentration upon visual excitation initiates the phosphorylation of the InaD protein by eye-PKC and thereby modulates its function in the control of the light response.

Regulation of the TRP Ca2+ channel by INAD in Drosophila photoreceptors

Drosophila vision involves a G protein-coupled phospholipase C-mediated signaling pathway that leads to membrane depolarization through activation of Na+ and Ca2+ channels. InaD mutant flies have a M442K point mutation and display a slow recovery of the Ca2+ dependent current. We report that anti-INAD antibodies coimmunoprecipitate TRP, identified by its electrophoretic mobility, cross reactivity with anti-TRP antibody, and absence in a null allele trp mutant. This interaction is abolished by the InaD point mutation in vitro and in vivo. Interaction was localized to the 19 amino acid C-terminus of TRP by overlay assays, and to the PDZ domain of INAD, encompassing the point mutation. Given the impaired electrophysiology of the InaD mutant, this novel interaction suggests that INAD functions as a regulatory subunit of the TRP Ca2+ channel.

Role of Drosophila TRP in inositide-mediated Ca2+ entry

Inositol lipid signaling relies on an InsP3-induced Ca2+ release from intracellular stores and on extracellular Ca2+ entry, which takes place when the Ca2+ stores become depleted of Ca2+. This interplay between Ca2+ release and Ca2+ entry has been termed capacitative Ca2+ entry and the inward current calcium release activated current (CRAC) to indicate gating of Ca2+ entry by Ca2+-store depletion. The signaling pathway and the gating mechanism of capacitative Ca2+ entry, however, are largely unknown and the molecular participants in this process have not been identified. In this article we review genetic, molecular, and functional studies of wild-type and mutant Drosophila photoreceptors, suggesting that the transient receptor potential mutant (trp) is the first putative capacitative Ca2+ entry mutant. Furthermore, several lines of evidence suggest that the trp gene product TRP is a candidate subunit of the plasma membrane channel that is activated by Ca2+ store depletion.

Calcium homeostasis in photoreceptor cells of Drosophila mutants inaC and trp studied with the pupil mechanism

The light-driven pupil mechanism, consisting of an assembly of mobile pigment granules inside the photoreceptor cells, has been investigated by in vivo reflection microspectrophotometry in wild type (WT) Drosophila and in the photoreceptor mutants inaC and trp. The pupillary response of a dark-adapted WT eye to a step in light is a monophasic reflectance increase reaching a plateau after ca. 15-s light adaptation. This reflectance change is due to photoreceptor pigment granules that accumulate near the tips of the rhabdomeres under light adaptation and that are withdrawn towards the periphery in the dark (Franceschini & Kirschfeld, 1976). The step response of the pupil mechanism of inaC is triphasic. Strikingly, the reflectance level at light onset is distinctly higher than that in WT, due to a partly aggregated state of the photoreceptor pigment granules near the rhabdomere tips that persists in the dark-adapted state, in line with direct calcium measurements of Peretz et al. (1994b). The step response of the pupil mechanism of inaC is slightly elevated compared to that of WT. The step response in trp is a transient, biphasic reflectance change, approximating a log normal function. This function is also a good approximation of the pulse response in WT and inaC. The intensity range of pupillary sensitivity is about 4 log unit. The range of inaC compared to that of WT is slightly (approximately 0.5 log unit) shifted towards lower intensities, but that in trp is strongly shifted to higher intensities (approximately 2.5 log unit). The results can be interpreted with the present knowledge of the primary steps in fly phototransduction and the hypothesis that the local intracellular calcium concentration determines the position of the pigment granules, and hence are in line with the notion that the pupil can be used as a qualitative Ca2+ probe.

Molecular, biochemical, and electrophysiological characterization of Drosophila norpA mutants

Inositol phosphate signaling has been implicated in a wide variety of eukaryotic cellular processes. In Drosophila, the phototransduction cascade is mediated by a phosphoinositide-specific phospholipase C (PLC) encoded by the norpA gene. We have characterized eight norpA mutants by electroretinogram (ERG), Western, molecular, and in vitro PLC activity analyses. ERG responses of the mutants show allele-dependent reductions in amplitudes and retardation in kinetics. The mutants also exhibit allele-dependent reductions in in vitro PLC activity levels and greatly reduced or undetectable NorpA protein levels. Three carry a missense mutation and five carry a nonsense mutation within the norpA coding sequence. In missense mutants, the amino acid substitution occurs at residues highly conserved among PLCs. These substitutions reduce the levels of both the NorpA protein and the PLC activity, with the reduction in PLC activity being greater than can be accounted for simply by the reduction in protein. The effects of the mutations on the amount and activity of the protein are much greater than their effects on the ERG, suggesting an amplification of the transduction signal at the effector (NorpA) protein level. Transgenic flies were generated by germline transformation of a null norpA mutant using a P-element construct containing the wild-type norpA cDNA driven by the ninaE promoter. Transformed flies show rescue of the electrophysiological phenotype in R1-R6 photoreceptors, but not in R7 or R8. The degeneration phenotype of R1-R6 photoreceptors is also rescued.