Excess of Gbetae over Gqalphae in vivo prevents dark, spontaneous activity of Drosophila photoreceptors

The latency of the light response is modulated by the phosphorylation state of Drosophila TRP at a specific site

Drosophila photoreceptors respond to oscillating light of high frequency (∼100 Hz), while increasing the oscillating light intensity raises the maximally detected frequency. Recently, we reported that dephosphorylation of the light-activated TRP ion channel at S936 is a fast, graded, light-, and Ca²⁺-dependent process. We further found that this process affects the detection limit of high frequency oscillating light. Accordingly, transgenic Drosophila, which do not undergo phosphorylation at the S936-TRP site (trp^S936A), revealed a short time-interval before following the high stimulus frequency (oscillation-lock response) in both dark- and light-adapted flies. In contrast, the trp^S936D transgenic flies, which mimic constant phosphorylation, showed a long-time interval to oscillation-lock response in both dark- and light-adapted flies. Here we extend these findings by showing that dark-adapted trp^S936A flies reveal light-induced current (LIC) with short latency relative to trp^WT or trp^S936D flies, indicating that the channels are a limiting factor of response kinetics. The results indicate that properties of the light-activated channels together with the dynamic light-dependent process of TRP phosphorylation at the S936 site determine response kinetics.

Phototransduction in Drosophila Is Compromised by Gal4 Expression but not by InsP3 Receptor Knockdown or Mutation

Drosophila phototransduction is mediated by phospholipase C, leading to activation of transient receptor potential (TRP) and TRP-like (TRPL) channels by mechanisms that are unresolved. A role for InsP₃ receptors (IP₃Rs) had been excluded because IP₃R mutants (itpr) appeared to have normal light responses; however, this was recently challenged by Kohn et al. (“Functional cooperation between the IP3 receptor and phospholipase C secures the high sensitivity to light of Drosophila photoreceptors in vivo,” Journal of Neuroscience 35:2530), who reported defects in phototransduction after IP₃R-RNAi knockdown. They concluded that InsP₃-induced Ca²⁺ release plays a critical role in facilitating channel activation, and that previous failure to detect IP₃R phenotypes resulted from trace Ca²⁺ in electrodes substituting for InsP₃-induced Ca²⁺ release. In an attempt to confirm this, we performed electroretinograms, whole-cell recordings, and GCaMP6f Ca²⁺ imaging from both IP₃R-RNAi flies and itpr-null mutants. Like Kohn et al., we used GMRGal4 to drive expression of UAS-IP₃R-RNAi, but we also used controls expressing GMRGal4 alone. We describe several GMRGal4 phenotypes suggestive of compromised development, including reductions in sensitivity, dark noise, potassium currents, and cell size and capacitance, as well as extreme variations in sensitivity between cells. However, we found no effect of IP₃R RNAi or mutation on photoreceptor responses or Ca²⁺ signals, indicating that the IP₃R plays little or no role in Drosophila phototransduction.

Electrophysiological Method for Whole-cell Voltage Clamp Recordings from Drosophila Photoreceptors

Whole-cell voltage clamp recordings from Drosophila melanogaster photoreceptors have revolutionized the field of invertebrate visual transduction, enabling the use of D. melanogaster molecular genetics to study inositol-lipid signaling and Transient Receptor Potential (TRP) channels at the single-molecule level. A handful of labs have mastered this powerful technique, which enables the analysis of the physiological responses to light under highly controlled conditions. This technique allows control over the intracellular and extracellular media; the membrane voltage; and the fast application of pharmacological compounds, such as a variety of ionic or pH indicators, to the intra- and extracellular media. With an exceptionally high signal-to-noise ratio, this method enables the measurement of dark spontaneous and light-induced unitary currents (i.e. spontaneous and quantum bumps) and macroscopic Light-induced Currents (LIC) from single D. melanogaster photoreceptors. This protocol outlines, in great detail, all the key steps necessary to perform this technique, which includes both electrophysiological and optical recordings. The fly retina dissection procedure for the attainment of intact and viable ex vivo isolated ommatidia in the bath chamber is described. The equipment needed to perform whole-cell and fluorescence imaging measurements are also detailed. Finally, the pitfalls in using this delicate preparation during extended experiments are explained.

The Phosphorylation State of the Drosophila TRP Channel Modulates the Frequency Response to Oscillating Light In Vivo

Drosophila photoreceptors respond to oscillating light of high frequency (∼100 Hz), while the detected maximal frequency is modulated by the light rearing conditions, thus enabling high sensitivity to light and high temporal resolution. However, the molecular basis for this adaptive process is unclear. Here, we report that dephosphorylation of the light-activated transient receptor potential (TRP) ion channel at S936 is a fast, graded, light-dependent, and Ca²⁺-dependent process that is partially modulated by the rhodopsin phosphatase retinal degeneration C (RDGC). Electroretinogram measurements of the frequency response to oscillating lights in vivo revealed that dark-reared flies expressing wild-type TRP exhibited a detection limit of oscillating light at relatively low frequencies, which was shifted to higher frequencies upon light adaptation. Strikingly, preventing phosphorylation of the S936-TRP site by alanine substitution in transgenic Drosophila (trp^S936A ) abolished the difference in frequency response between dark-adapted and light-adapted flies, resulting in high-frequency response also in dark-adapted flies. In contrast, inserting a phosphomimetic mutation by substituting the S936-TRP site to aspartic acid (trp^S936D ) set the frequency response of light-adapted flies to low frequencies typical of dark-adapted flies. Light-adapted rdgC mutant flies showed relatively high S936-TRP phosphorylation levels and light-dark phosphorylation dynamics. These findings suggest that RDGC is one but not the only phosphatase involved in pS936-TRP dephosphorylation. Together, this study indicates that TRP channel dephosphorylation is a regulatory process that affects the detection limit of oscillating light according to the light rearing condition, thus adjusting dynamic processing of visual information under varying light conditions.SIGNIFICANCE STATEMENTDrosophila photoreceptors exhibit high temporal resolution as manifested in frequency response to oscillating light of high frequency (≤∼100 Hz). Light rearing conditions modulate the maximal frequency detected by photoreceptors, thus enabling them to maintain high sensitivity to light and high temporal resolution. However, the precise mechanisms for this process are not fully understood. Here, we show by combination of biochemistry and in vivo electrophysiology that transient receptor potential (TRP) channel dephosphorylation at a specific site is a fast, light-activated and Ca²⁺-dependent regulatory process. TRP dephosphorylation affects the detection limit of oscillating light according to the adaptation state of the photoreceptor cells by shifting the detection limit to higher frequencies upon light adaptation. This novel mechanism thus adjusts dynamic processing of visual information under varying light conditions.

A Quantitative Model of the GIRK1/2 Channel Reveals That Its Basal and Evoked Activities Are Controlled by Unequal Stoichiometry of Gα and Gβγ

G protein-gated K⁺ channels (GIRK; Kir3), activated by Gβγ subunits derived from G_i/o proteins, regulate heartbeat and neuronal excitability and plasticity. Both neurotransmitter-evoked (I_evoked) and neurotransmitter-independent basal (I_basal) GIRK activities are physiologically important, but mechanisms of I_basal and its relation to I_evoked are unclear. We have previously shown for heterologously expressed neuronal GIRK1/2, and now show for native GIRK in hippocampal neurons, that I_basal and I_evoked are interrelated: the extent of activation by neurotransmitter (activation index, R_a) is inversely related to I_basal. To unveil the underlying mechanisms, we have developed a quantitative model of GIRK1/2 function. We characterized single-channel and macroscopic GIRK1/2 currents, and surface densities of GIRK1/2 and Gβγ expressed in Xenopus oocytes. Based on experimental results, we constructed a mathematical model of GIRK1/2 activity under steady-state conditions before and after activation by neurotransmitter. Our model accurately recapitulates I_basal and I_evoked in Xenopus oocytes, HEK293 cells and hippocampal neurons; correctly predicts the dose-dependent activation of GIRK1/2 by coexpressed Gβγ and fully accounts for the inverse I_basal-R_a correlation. Modeling indicates that, under all conditions and at different channel expression levels, between 3 and 4 Gβγ dimers are available for each GIRK1/2 channel. In contrast, available Gα_i/o decreases from ~2 to less than one Gα per channel as GIRK1/2's density increases. The persistent Gβγ/channel (but not Gα/channel) ratio support a strong association of GIRK1/2 with Gβγ, consistent with recruitment to the cell surface of Gβγ, but not Gα, by GIRK1/2. Our analysis suggests a maximal stoichiometry of 4 Gβγ but only 2 Gα_i/o per one GIRK1/2 channel. The unique, unequal association of GIRK1/2 with G protein subunits, and the cooperative nature of GIRK gating by Gβγ, underlie the complex pattern of basal and agonist-evoked activities and allow GIRK1/2 to act as a sensitive bidirectional detector of both Gβγ and Gα.

Many neurotransmitters and hormones inhibit the electric activity of excitable cells (such as cardiac cells and neurons) by activating a K⁺ channel, GIRK (G protein-gated Inwardly Rectifying K⁺ channel). GIRK channels also possess constitutive “basal” activity which contributes to regulation of neuronal and cardiac excitability and certain disorders, but the mechanism of this activity and its interrelation with the neurotransmitter-evoked activity are poorly understood. In this work we show that key features of basal and neurotransmitter-evoked activities are similar in cultured hippocampal neurons and in two model systems (mammalian HEK293 cells and Xenopus oocytes). Using experimental data of the neuronal GIRK1/2 channel function upon changes in GIRK and G protein concentrations, we constructed a mathematical model that quantitatively accounts for basal and evoked activity, and for the inverse correlation between the two. Our analysis suggests a novel and unexpected mechanism of interaction of GIRK1/2 with the G protein subunits, where the tetrameric GIRK channel can assemble with 4 molecules of the Gβγ subunits but only 2 molecules of Gα. GIRK is a prototypical effector of Gβγ, and the unequal stoichiometry of interaction with G protein subunits may have general implications for G protein signaling.

Functional cooperation between the IP3 receptor and phospholipase C secures the high sensitivity to light of Drosophila photoreceptors in vivo

Drosophila phototransduction is a model system for the ubiquitous phosphoinositide signaling. In complete darkness, spontaneous unitary current events (dark bumps) are produced by spontaneous single Gqα activation, while single-photon responses (quantum bumps) arise from synchronous activation of several Gqα molecules. We have recently shown that most of the spontaneous single Gqα activations do not produce dark bumps, because of a critical phospholipase Cβ (PLCβ) activity level required for bump generation. Surpassing the threshold of channel activation depends on both PLCβ activity and cellular [Ca(2+)], which participates in light excitation via a still unclear mechanism. We show here that in IP3 receptor (IP3R)-deficient photoreceptors, both light-activated Ca(2+) release from internal stores and light sensitivity were strongly attenuated. This was further verified by Ca(2+) store depletion, linking Ca(2+) release to light excitation. In IP3R-deficient photoreceptors, dark bumps were virtually absent and the quantum-bump rate was reduced, indicating that Ca(2+) release from internal stores is necessary to reach the critical level of PLCβ catalytic activity and the cellular [Ca(2+)] required for excitation. Combination of IP3R knockdown with reduced PLCβ catalytic activity resulted in highly suppressed light responses that were partially rescued by cellular Ca(2+) elevation, showing a functional cooperation between IP3R and PLCβ via released Ca(2+). These findings suggest that in contrast to the current dogma that Ca(2+) release via IP3R does not participate in light excitation, we show that released Ca(2+) plays a critical role in light excitation. The positive feedback between PLCβ and IP3R found here may represent a common feature of the inositol-lipid signaling.

Speed and sensitivity of phototransduction in Drosophila depend on degree of saturation of membrane phospholipids

Drosophila phototransduction is mediated via a G-protein-coupled PLC cascade. Recent evidence, including the demonstration that light evokes rapid contractions of the photoreceptors, suggested that the light-sensitive channels (TRP and TRPL) may be mechanically gated, together with protons released by PLC-mediated PIP2 hydrolysis. If mechanical gating is involved we predicted that the response to light should be influenced by altering the physical properties of the membrane. To achieve this, we used diet to manipulate the degree of saturation of membrane phospholipids. In flies reared on a yeast diet, lacking polyunsaturated fatty acids (PUFAs), mass spectrometry showed that the proportion of polyunsaturated phospholipids was sevenfold reduced (from 38 to ∼5%) but rescued by adding a single species of PUFA (linolenic or linoleic acid) to the diet. Photoreceptors from yeast-reared flies showed a 2- to 3-fold increase in latency and time to peak of the light response, without affecting quantum bump waveform. In the absence of Ca(2+) influx or in trp mutants expressing only TRPL channels, sensitivity to light was reduced up to ∼10-fold by the yeast diet, and essentially abolished in hypomorphic G-protein mutants (Gαq). PLC activity appeared little affected by the yeast diet; however, light-induced contractions measured by atomic force microscopy or the activation of ectopic mechanosensitive gramicidin channels were also slowed ∼2-fold. The results are consistent with mechanosensitive gating and provide a striking example of how dietary fatty acids can profoundly influence sensory performance in a classical G-protein-coupled signaling cascade.

Common mechanisms regulating dark noise and quantum bump amplification in Drosophila photoreceptors

Absolute visual thresholds are limited by "dark noise," which in Drosophila photoreceptors is dominated by brief (∼10 ms), small (∼2 pA) inward current events, occurring at ∼2/s, believed to reflect spontaneous G protein activations. These dark events were increased in rate and amplitude by a point mutation in myosin III (NINAC), which disrupts its interaction with the scaffolding protein, INAD. This phenotype mimics that previously described in null mutants of ninaC (no inactivation no afterpotential; encoding myosin III) and an associated protein, retinophilin (rtp). Dark noise was similarly increased in heterozygote mutants of diacylglycerol kinase (rdgA/+). Dark noise in ninaC, rtp, and rdgA/+ mutants was greatly suppressed by mutations of the Gq α-subunit (Gαq) and the major light-sensitive channel (trp) but not rhodopsin. ninaC, rtp, and rdgA/+ mutations also all facilitated residual light responses in Gαq and PLC hypomorphs. Raising cytosolic Ca(2+) in the submicromolar range increased dark noise, facilitated activation of transient receptor potential (TRP) channels by exogenous agonist, and again facilitated light responses in Gαq hypomorphs. Our results indicate that RTP, NINAC, INAD, and diacylglycerol kinase, together with a Ca(2+)-dependent threshold, share common roles in suppressing dark noise and regulating quantum bump generation; consequently, most spontaneous G protein activations fail to generate dark events under normal conditions. By contrast, quantum bump generation is reliable but delayed until sufficient G proteins and PLC are activated to overcome threshold, thereby ensuring generation of full-size bumps with high quantum efficiency.

Compartmentalization and Ca2+ buffering are essential for prevention of light-induced retinal degeneration

Fly photoreceptors are polarized cells, each of which has an extended interface between its cell body and the light-signaling compartment, the rhabdomere. Upon intense illumination, rhabdomeric calcium concentration reaches millimolar levels that would be toxic if Ca(2+) diffusion between the rhabdomere and cell body was not robustly attenuated. Yet, it is not clear how such effective attenuation is obtained. Here we show that Ca(2+) homeostasis in the photoreceptor cell relies on the protein calphotin. This unique protein functions as an immobile Ca(2+) buffer localized along the base of the rhabdomere, separating the signaling compartment from the cell body. Generation and analyses of transgenic Drosophila strains, in which calphotin-expression levels were reduced in a graded manner, showed that moderately reduced calphotin expression impaired Ca(2+) homeostasis while calphotin elimination resulted in severe light-dependent photoreceptor degeneration. Electron microscopy, electrophysiology, and optical methods revealed that the degeneration was rescued by prevention of Ca(2+) overload via overexpression of CalX, the Na(+)-Ca(2+) exchanger. In addition, Ca(2+)-imaging experiments showed that reduced calphotin levels resulted in abnormally fast kinetics of Ca(2+) elevation in photoreceptor cells. Together, the data suggest that calphotin functions as a Ca(2+) buffer; a possibility that we directly demonstrate by expressing calphotin in a heterologous expression system. We propose that calphotin-mediated compartmentalization and Ca(2+) buffering constitute an effective strategy to protect cells from Ca(2+) overload and light-induced degeneration.

Phospholipase C-mediated suppression of dark noise enables single-photon detection in Drosophila photoreceptors

Drosophila photoreceptor cells use the ubiquitous G-protein-mediated phospholipase C (PLC) cascade to achieve ultimate single-photon sensitivity. This is manifested in the single-photon responses (quantum bumps). In photoreceptor cells, dark activation of G(q)α molecules occurs spontaneously and produces unitary dark events (dark bumps). A high rate of spontaneous G(q)α activation and dark bump production potentially hampers single-photon detection. We found that in wild-type flies the in vivo rate of spontaneous G(q)α activation is very high. Nevertheless, this high rate is not manifested in a substantially high rate of dark bumps. Therefore, it is unclear how phototransduction suppresses dark bump production arising from spontaneous G(q)α activation, while still maintaining high-fidelity representation of single photons. In this study we show that reduced PLC catalytic activity selectively suppressed production of dark bumps but not light-induced bumps. Manipulations of PLC activity using PLC mutant flies and Ca(2+) modulations revealed that a critical level of PLC activity is required to induce bump production. The required minimal level of PLC activity selectively suppressed random production of single G(q)α-activated dark bumps despite a high rate of spontaneous G(q)α activation. This minimal PLC activity level is reliably obtained by photon-induced synchronized activation of several neighboring G(q)α molecules activating several PLC molecules, but not by random activation of single G(q)α molecules. We thus demonstrate how a G-protein-mediated transduction system, with PLC as its target, selectively suppresses its intrinsic noise while preserving reliable signaling.

Phototransduction in Drosophila

The Drosophila visual transduction is the fastest known G protein-coupled signaling cascade and has been served as a model for understanding the molecular mechanisms of other G protein-coupled signaling cascades. Numbers of components in visual transduction machinery have been identified. Based on the functional characterization of these genes, a model for Drosophila phototransduction has been outlined, including rhodopsin activation, phosphoinoside signaling, and the opening of TRP and TRPL channels. Recently, the characterization of mutants, showing slow termination, revealed the physiological significance and the mechanism of rapid termination of light response.

G protein trafficking

The classical view of heterotrimeric G protein signaling places G -proteins at the cytoplasmic surface of the cell's plasma membrane where they are activated by an appropriate G protein-coupled receptor. Once activated, the GTP-bound Gα and the free Gβγ are able to regulate plasma membrane-localized effectors, such as adenylyl cyclase, phospholipase C-β, RhoGEFs and ion channels. Hydrolysis of GTP by the Gα subunit returns the G protein to the inactive Gαβγ heterotrimer. Although all of these events in the G protein cycle can be restricted to the cytoplasmic surface of the plasma membrane, G protein localization is dynamic. Thus, it has become increasingly clear that G proteins are able to move to diverse subcellular locations where they perform non-canonical signaling functions. This chapter will highlight our current understanding of trafficking pathways that target newly synthesized G proteins to the plasma membrane, activation-induced and reversible translocation of G proteins from the plasma membrane to intracellular locations, and constitutive trafficking of G proteins.

Retinophilin is a light-regulated phosphoprotein required to suppress photoreceptor dark noise in Drosophila

Photoreceptor cells achieve high sensitivity, reliably detecting single photons, while limiting the spontaneous activation events responsible for dark noise. We used proteomic, genetic, and electrophysiological approaches to characterize Retinophilin (RTP) (CG10233) in Drosophila photoreceptors and establish its involvement in dark-noise suppression. RTP possesses membrane occupation and recognition nexus (MORN) motifs, a structure shared with mammalian junctophilins and other membrane-associated proteins found within excitable cells. We show the MORN repeats, and both the N- and C-terminal domains, are required for RTP localization in the microvillar light-gathering organelle, the rhabdomere. RTP exists in multiple phosphorylated isoforms under dark conditions and is dephosphorylated by light exposure. An RTP deletion mutant exhibits a high rate of spontaneous membrane depolarization events in dark conditions but retains the normal kinetics of the light response. Photoreceptors lacking neither inactivation nor afterpotential C (NINAC) myosin III, a motor protein/kinase, also display a similar dark-noise phenotype as the RTP deletion. We show that NINAC mutants are depleted for RTP. These results suggest the increase in dark noise in NINAC mutants is attributable to lack of RTP and, furthermore, defines a novel role for NINAC in the rhabdomere. We propose that RTP is a light-regulated phosphoprotein that organizes rhabdomeric components to suppress random activation of the phototransduction cascade and thus increases the signaling fidelity of dark-adapted photoreceptors.

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.

Mutation of a TADR protein leads to rhodopsin and Gq-dependent retinal degeneration in Drosophila

The Drosophila photoreceptor is a model system for genetic study of retinal degeneration. Many gene mutations cause fly photoreceptor degeneration, either because of excessive stimulation of the visual transduction (phototransduction) cascade, or through apoptotic pathways that in many cases involve a visual arrestin Arr2. Here we report a gene named tadr (for torn and diminished rhabdomeres), which, when mutated, leads to photoreceptor degeneration through a different mechanism. Degeneration in the tadr mutant is characterized by shrunk and disrupted rhabdomeres, the light sensory organelles of photoreceptor. The TADR protein interacted in vitro with the major light receptor Rh1 rhodopsin, and genetic reduction of the Rh1 level suppressed the tadr mutation-caused degeneration, suggesting the degeneration is Rh1-dependent. Nonetheless, removal of phospholipase C (PLC), a key enzyme in phototransduction, and that of Arr2 failed to inhibit rhabdomeral degeneration in the tadr mutant background. Biochemical analyses revealed that, in the tadr mutant, the G(q) protein of Rh1 is defective in dissociation from the membrane during light stimulation. Importantly, reduction of G(q) level by introducing a hypomorphic allele of G(alphaq) gene greatly inhibited the tadr degeneration phenotype. These results may suggest that loss of a potential TADR-Rh1 interaction leads to an abnormality in the G(q) signaling, which in turn triggers rhabdomeral degeneration independent of the PLC phototransduction cascade. We propose that TADR-like proteins may also protect photoreceptors from degeneration in mammals including humans.

Translocation of Gq alpha mediates long-term adaptation in Drosophila photoreceptors

Light adaptation is a process that enables photoreceptor cells to operate over a wide range of light intensities without saturation. In invertebrate photoreceptors, fast adaptation is mediated by a Ca2+-dependent negative-feedback mechanism, which mainly affects the terminal steps of the cascade. Therefore, the response to each photon is smaller as light intensity increases, accommodating both high sensitivity and a vast dynamic range. Here, we describe a novel type of adaptation, which is mediated by one of the first steps in the phototransduction cascade affecting the sensitivity to absorbed photons. Long exposure to light resulted in dramatic reduction in the probability of each absorbed photon to elicit a response, whereas the size and shape of each single photon response did not change. To dissect the molecular mechanism underlying this form of adaptation we used a series of Drosophila mutants. Genetic dissection showed a pivotal role for light-induced translocation of Gq alpha between the signaling membrane and the cytosol. Biochemical studies revealed that the sensitivity to light depends on membrane Gq alpha concentration, which was modulated either by light or by mutations that impaired its targeting to the membrane. We conclude that long-term adaptation is mediated by the movement of Gq alpha from the signaling membrane to the cytosol, thereby reducing the probability of each photon to elicit a response. The slow time scale of this adaptation fits well with day/night light intensity changes, because there is no need to maintain single photon sensitivity during daytime.

Assembly and trafficking of heterotrimeric G proteins

To be activated by cell surface G protein-coupled receptors, heterotrimeric G proteins must localize at the cytoplasmic surface of plasma membranes. Moreover, some G protein subunits are able to traffic reversibly from the plasma membrane to intracellular locations upon activation. This current topic will highlight new insights into how nascent G protein subunits are assembled and how they arrive at plasma membranes. In addition, recent reports have increased our knowledge of activation-induced trafficking of G proteins. Understanding G protein assembly and trafficking will lead to a greater understanding of novel ways that cells regulate G protein signaling.

Phototransduction and retinal degeneration in Drosophila

Drosophila visual transduction is the fastest known G-protein-coupled signaling cascade and has therefore served as a genetically tractable animal model for characterizing rapid responses to sensory stimulation. Mutations in over 30 genes have been identified, which affect activation, adaptation, or termination of the photoresponse. Based on analyses of these genes, a model for phototransduction has emerged, which involves phosphoinoside signaling and culminates with opening of the TRP and TRPL cation channels. Many of the proteins that function in phototransduction are coupled to the PDZ containing scaffold protein INAD and form a supramolecular signaling complex, the signalplex. Arrestin, TRPL, and G alpha(q) undergo dynamic light-dependent trafficking, and these movements function in long-term adaptation. Other proteins play important roles either in the formation or maturation of rhodopsin, or in regeneration of phosphatidylinositol 4,5-bisphosphate (PIP2), which is required for the photoresponse. Mutation of nearly any gene that functions in the photoresponse results in retinal degeneration. The underlying bases of photoreceptor cell death are diverse and involve mechanisms such as excessive endocytosis of rhodopsin due to stable rhodopsin/arrestin complexes and abnormally low or high levels of Ca2+. Drosophila visual transduction appears to have particular relevance to the cascade in the intrinsically photosensitive retinal ganglion cells in mammals, as the photoresponse in these latter cells appears to operate through a remarkably similar mechanism.

TRP channels and Ca2+ signaling

There is a rapidly growing interest in the family of transient receptor potential (TRP) channels because TRP channels are not only important for many sensory systems, but they are crucial components of the function of neurons, epithelial, blood and smooth muscle cells. These facts make TRP channels important targets for treatment of diseases arising from the malfunction of these channels in the above cells and for treatment of inflammatory pain. TRP channels are also important for a growing number of genetic diseases arising from mutations in various types of TRP channels. The Minerva-Gentner Symposium on TRP channels and Ca(2+) signaling, which took place in Eilat, Israel (February 24-28, 2006) has clearly demonstrated that the study of TRP channels is a newly emerging field of biomedicine with prime importance. In the Eilat symposium, investigators who have contributed seminal publications and insight into the TRP field presented their most recent, and in many cases still unpublished, studies. The excellent presentations and excitement generated by them demonstrated that much progress has been achieved. Nevertheless, it was also evident that the field of TRP channels is still in its infancy in comparison to other fields of ion channels, and even the fundamental knowledge of the gating mechanism of TRP channels is still unsolved. The beautiful location of the symposium, together with informal intensive discussions among the participants, contributed to the success of this meeting.

Subcellular translocation of the eGFP-tagged TRPL channel in Drosophila photoreceptors requires activation of the phototransduction cascade

Signal-mediated translocation of transient receptor potential (TRP) channels is a novel mechanism to fine tune a variety of signaling pathways including neuronal path finding and Drosophila photoreception. In Drosophila phototransduction the cation channels TRP and TRP-like (TRPL) are the targets of a prototypical G protein-coupled signaling pathway. We have recently found that the TRPL channel translocates between the rhabdomere and the cell body in a light-dependent manner. This translocation modifies the ion channel composition of the signaling membrane and induces long-term adaptation. However, the molecular mechanism underlying TRPL translocation remains unclear. Here we report that eGFP-tagged TRPL expressed in the photoreceptor cells formed functional ion channels with properties of the native channels, whereas TRPL-eGFP translocation could be directly visualized in intact eyes. TRPL-eGFP failed to translocate to the cell body in flies carrying severe mutations in essential phototransduction proteins, including rhodopsin, Galphaq, phospholipase Cbeta and the TRP ion channel, or in proteins required for TRP function. Our data, furthermore, show that the activation of a small fraction of rhodopsin and of residual amounts of the Gq protein is sufficient to trigger TRPL-eGFP internalization. In addition, we found that endocytosis of TRPL-eGFP occurs independently of dynamin, whereas a mutation of the unconventional myosin III, NINAC, hinders complete translocation of TRPL-eGFP to the cell body. Altogether, this study revealed that activation of the phototransduction cascade is mandatory for TRPL internalization, suggesting a critical role for the light induced conductance increase and the ensuing Ca2+ -influx in the translocation process. The critical role of Ca2+ influx was directly demonstrated when the light-induced TRPL-eGFP translocation was blocked by removing extracellular Ca2+.