Regulatory arrestin cycle secures the fidelity and maintenance of the fly photoreceptor cell

Olfactory sensory neuron population expansions influence projection neuron adaptation and enhance odour tracking

The evolutionary expansion of sensory neuron populations detecting important environmental cues is widespread, but functionally enigmatic. We investigated this phenomenon through comparison of homologous olfactory pathways of Drosophila melanogaster and its close relative Drosophila sechellia, an extreme specialist for Morinda citrifolia noni fruit. D. sechellia has evolved species-specific expansions in select, noni-detecting olfactory sensory neuron (OSN) populations, through multigenic changes. Activation and inhibition of defined proportions of neurons demonstrate that OSN number increases contribute to stronger, more persistent, noni-odour tracking behaviour. These expansions result in increased synaptic connections of sensory neurons with their projection neuron (PN) partners, which are conserved in number between species. Surprisingly, having more OSNs does not lead to greater odour-evoked PN sensitivity or reliability. Rather, pathways with increased sensory pooling exhibit reduced PN adaptation, likely through weakened lateral inhibition. Our work reveals an unexpected functional impact of sensory neuron population expansions to explain ecologically-relevant, species-specific behaviour.

Wind gates olfaction-driven search states in free flight

For organisms tracking a chemical cue to its source, the motion of their surrounding fluid provides crucial information for success. Swimming and flying animals engaged in olfaction-driven search often start by turning into the direction of an oncoming wind or water current. However, it is unclear how organisms adjust their strategies when directional cues are absent or unreliable, as is often the case in nature. Here, we use the genetic toolkit of Drosophila melanogaster to develop an optogenetic paradigm to deliver temporally precise "virtual" olfactory experiences for free-flying animals in either laminar wind or still air. We first confirm that in laminar wind flies turn upwind. Furthermore, we show that they achieve this using a rapid (∼100 ms) turn, implying that flies estimate the ambient wind direction prior to "surging" upwind. In still air, flies adopt a remarkably stereotyped "sink and circle" search state characterized by ∼60° turns at 3-4 Hz, biased in a consistent direction. Together, our results show that Drosophila melanogaster assesses the presence and direction of ambient wind prior to deploying a distinct search strategy. In both laminar wind and still air, immediately after odor onset, flies decelerate and often perform a rapid turn. Both maneuvers are consistent with predictions from recent control theoretic analyses for how insects may estimate properties of wind while in flight. We suggest that flies may use their deceleration and "anemometric" turn as active sensing maneuvers to rapidly gauge properties of their wind environment before initiating a proximal or upwind search routine.

Wind Gates Olfaction Driven Search States in Free Flight

For organisms tracking a chemical cue to its source, the motion of their surrounding fluid provides crucial information for success. Swimming and flying animals engaged in olfaction driven search often start by turning into the direction of an oncoming wind or water current. However, it is unclear how organisms adjust their strategies when directional cues are absent or unreliable, as is often the case in nature. Here, we use the genetic toolkit of Drosophila melanogaster to develop an optogenetic paradigm to deliver temporally precise "virtual" olfactory experiences for free-flying animals in either laminar wind or still air. We first confirm that in laminar wind flies turn upwind. Furthermore, we show that they achieve this using a rapid (∼100 ms) turn, implying that flies estimate the ambient wind direction prior to "surging" upwind. In still air, flies adopt remarkably stereotyped "sink and circle" search state characterized by ∼60°turns at 3-4 Hz, biased in a consistent direction. Together, our results show that Drosophila melanogaster assess the presence and direction of ambient wind prior to deploying a distinct search strategy. In both laminar wind and still air, immediately after odor onset, flies decelerate and often perform a rapid turn. Both maneuvers are consistent with predictions from recent control theoretic analyses for how insects may estimate properties of wind while in flight. We suggest that flies may use their deceleration and "anemometric" turn as active sensing maneuvers to rapidly gauge properties of their wind environment before initiating a proximal or upwind search routine.

Sensory neuron population expansion enhances odor tracking without sensitizing projection neurons

The evolutionary expansion of sensory neuron populations detecting important environmental cues is widespread, but functionally enigmatic. We investigated this phenomenon through comparison of homologous neural pathways of Drosophila melanogaster and its close relative Drosophila sechellia , an extreme specialist for Morinda citrifolia noni fruit. D. sechellia has evolved species-specific expansions in select, noni-detecting olfactory sensory neuron (OSN) populations, through multigenic changes. Activation and inhibition of defined proportions of neurons demonstrate that OSN population increases contribute to stronger, more persistent, noni-odor tracking behavior. These sensory neuron expansions result in increased synaptic connections with their projection neuron (PN) partners, which are conserved in number between species. Surprisingly, having more OSNs does not lead to greater odor-evoked PN sensitivity or reliability. Rather, pathways with increased sensory pooling exhibit reduced PN adaptation, likely through weakened lateral inhibition. Our work reveals an unexpected functional impact of sensory neuron expansions to explain ecologically-relevant, species-specific behavior.

In vivo identification of Drosophila rhodopsin interaction partners by biotin proximity labeling

Proteins exert their function through protein-protein interactions. In Drosophila, G protein-coupled receptors like rhodopsin (Rh1) interact with a G protein to activate visual signal transduction and with arrestins to terminate activation. Also, membrane proteins like Rh1 engage in protein-protein interactions during folding within the endoplasmic reticulum, during their vesicular transport and upon removal from the cell surface and degradation. Here, we expressed a Rh1-TurboID fusion protein (Rh1::TbID) in Drosophila photoreceptors to identify in vivo Rh1 interaction partners by biotin proximity labeling. We show that Rh1::TbID forms a functional rhodopsin that mediates biotinylation of arrestin 2 in conditions where arrestin 2 interacts with rhodopsin. We also observed biotinylation of Rh1::TbID and native Rh1 as well as of most visual signal transduction proteins. These findings indicate that the signaling components in the rhabdomere approach rhodopsin closely, within a range of ca. 10 nm. Furthermore, we have detected proteins engaged in the maturation of rhodopsin and elements responsible for the trafficking of membrane proteins, resembling potential interaction partners of Rh1. Among these are chaperons of the endoplasmic reticulum, proteins involved in Clathrin-mediated endocytosis as well as previously unnoticed contributors to rhodopsin transportation, such as Rab32, Vap33, or PIP82.

The Role of Reversible Phosphorylation of Drosophila Rhodopsin

Vertebrate and fly rhodopsins are prototypical GPCRs that have served for a long time as model systems for understanding GPCR signaling. Although all rhodopsins seem to become phosphorylated at their C-terminal region following activation by light, the role of this phosphorylation is not uniform. Two major functions of rhodopsin phosphorylation have been described: (1) inactivation of the activated rhodopsin either directly or by facilitating binding of arrestins in order to shut down the visual signaling cascade and thus eventually enabling a high-temporal resolution of the visual system. (2) Facilitating endocytosis of activated receptors via arrestin binding that in turn recruits clathrin to the membrane for clathrin-mediated endocytosis. In vertebrate rhodopsins the shutdown of the signaling cascade may be the main function of rhodopsin phosphorylation, as phosphorylation alone already quenches transducin activation and, in addition, strongly enhances arrestin binding. In the Drosophila visual system rhodopsin phosphorylation is not needed for receptor inactivation. Its role here may rather lie in the recruitment of arrestin 1 and subsequent endocytosis of the activated receptor. In this review, we summarize investigations of fly rhodopsin phosphorylation spanning four decades and contextualize them with regard to the most recent insights from vertebrate phosphorylation barcode theory.

Exploring Excitotoxicity and Regulation of a Constitutively Active TRP Ca2+ Channel in Drosophila

Unregulated Ca²⁺ influx affects intracellular Ca²⁺ homoeostasis, which may lead to neuronal death. In Drosophila, following the activation of rhodopsin the TRP Ca²⁺ channel is open to mediate the light-dependent depolarization. A constitutively active TRP channel triggers the degeneration of Trp^P365 /+ photoreceptors. To explore retinal degeneration, we employed a multidisciplinary approach including live imaging using GFP tagged actin and arrestin 2. Importantly, we demonstrate that the major rhodopsin (Rh1) was greatly reduced before the onset of rhabdomere degeneration; a great reduction of Rh1 affects the maintenance of rhabdomere leading to degeneration of photoreceptors. Trp^P365 /+ also led to the up-regulation of CaMKII, which is beneficial as suppression of CaMKII accelerated retinal degeneration. We explored the regulation of TRP by investigating the genetic interaction between Trp^P365 /+ and mutants affecting the turnover of diacylglycerol (DAG). We show a loss of phospholipase C in norpA^P24 exhibited a great reduction of the DAG content delayed degeneration of Trp^P365 /+ photoreceptors. In contrast, knockdown or mutations in DAG lipase (InaE) that is accompanied by slightly reduced levels of most DAG but an increased level of DAG 34:1, exacerbated retinal degeneration of Trp^P365 /+. Together, our findings support the notion that DAG plays a role in regulating TRP. Interestingly, DAG lipase is likely required during photoreceptor development as Trp^P365 /+; inaE^N125 double mutants contained severely degenerated rhabdomeres.

Sustained Melanopsin Photoresponse Is Supported by Specific Roles of β-Arrestin 1 and 2 in Deactivation and Regeneration of Photopigment

Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) are indispensable for non-image-forming visual responses that sustain under prolonged illumination. For sustained signaling of ipRGCs, the melanopsin photopigment must continuously regenerate. The underlying mechanism is unknown. We discovered that a cluster of Ser/Thr sites within the C-terminal region of mammalian melanopsin is phosphorylated after a light pulse. This forms a binding site for β-arrestin 1 (βARR1) and β-arrestin 2. β-arrestin 2 primarily regulates the deactivation of melanopsin; accordingly, βαrr2^–/–mice exhibit prolonged ipRGC responses after cessation of a light pulse. β-arrestin 1 primes melanopsin for regeneration. Therefore, βαrr1^–/– ipRGCs become desensitized after repeated or prolonged photostimulation. The lack of either β-arrestin atten-uates ipRGC response under prolonged illumination, suggesting that β-arrestin 2-mediated deactivation and β-arrestin 1-dependent regeneration of melanopsin function in sequence. In conclusion, we discovered a molecular mechanism by which β-arrestins regulate different aspects of melanopsin photoresponses and allow ipRGC-sustained responses under prolonged illumination.

The mechanism by which melanopsin-expressing retinal ganglion cells (mRGCs) tonically respond to continuous illumination is unknown. Mure et al. show that phosphorylation-dependent binding of β-arrestin 1 and 2 coordinately deactivate and regenerate melanopsin photopigment to enable sustained firing of mRGCs in response to prolonged illumination.

Daily Regulation of Phototransduction, Circadian Clock, DNA Repair, and Immune Gene Expression by Heme Oxygenase in the Retina of Drosophila

The daily expression of genes and the changes in gene expression after silencing the heme oxygenase (ho) gene were examined in the retina of Drosophila using microarray and SybrGreen qPCR (quantitative polymerase chain reaction) methods. The HO decrease in the morning upregulated 83 genes and downregulated 57 genes. At night, 80 genes were upregulated and 22 were downregulated. The top 20 genes downregulated after ho silencing in the morning modulate phototransduction, immune responses, autophagy, phagocytosis, apoptosis, the carbon monoxide (CO) response, the oxidative stress/UV response, and translation. In turn, the genes that upregulated at night were involved in translation—the response to oxidative stress, DNA damage, and phototransduction. Among the top 20 genes downregulated at night were genes involved in phototransduction, immune responses, and autophagy. For some genes, a low level of HO had an opposite effect in the morning compared to those at night. Silencing ho also changed the expression of circadian clock genes, while the HO decrease during the night enhanced the expression of immune system genes. The results showed that the cyclic expression of HO is important for controlling several processes in the retina, including neuroprotection and those involved in the innate immune system.

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.

Ectopic Expression of Mouse Melanopsin in Drosophila Photoreceptors Reveals Fast Response Kinetics and Persistent Dark Excitation

The intrinsically photosensitive M1 retinal ganglion cells (ipRGC) initiate non-image-forming light-dependent activities and express the melanopsin (OPN4) photopigment. Several features of ipRGC photosensitivity are characteristic of fly photoreceptors. However, the light response kinetics of ipRGC is much slower due to unknown reasons. Here we used transgenic Drosophila, in which the mouse OPN4 replaced the native Rh1 photopigment of Drosophila R1-6 photoreceptors, resulting in deformed rhabdomeric structure. Immunocytochemistry revealed OPN4 expression at the base of the rhabdomeres, mainly at the rhabdomeral stalk. Measurements of the early receptor current, a linear manifestation of photopigment activation, indicated large expression of OPN4 in the plasma membrane. Comparing the early receptor current amplitude and action spectra between WT and the Opn4-expressing Drosophila further indicated that large quantities of a blue absorbing photopigment were expressed, having a dark stable blue intermediate state. Strikingly, the light-induced current of the Opn4-expressing fly photoreceptors was ∼40-fold faster than that of ipRGC. Furthermore, an intense white flash induced a small amplitude prolonged dark current composed of discrete unitary currents similar to the Drosophila single photon responses. The induction of prolonged dark currents by intense blue light could be suppressed by a following intense green light, suggesting induction and suppression of prolonged depolarizing afterpotential. This is the first demonstration of heterologous functional expression of mammalian OPN4 in the genetically emendable Drosophila photoreceptors. Moreover, the fast OPN4-activated ionic current of Drosophila photoreceptors relative to that of mouse ipRGC, indicates that the slow light response of ipRGC does not arise from an intrinsic property of melanopsin.

Membrane protein trafficking in Drosophila photoreceptor cells

Membrane protein trafficking occurs throughout the lifetime of neurons and includes the initial protein synthesis and anterograde transport to the plasma membrane as well as internalization, degradation, and recycling of plasma membrane proteins. Defects in protein trafficking can result in neuronal degeneration and underlie blinding diseases such as retinitis pigmentosa as well as other neuronal disorders. Drosophila photoreceptor cells have emerged as a model system for identifying the components and mechanisms involved in membrane protein trafficking in neurons. Here we summarize the current knowledge about trafficking of three Drosophila phototransduction proteins, the visual pigment rhodopsin and the two light-activated ion channels TRP (transient receptor potential) and TRPL (TRP-like). Despite some common requirements shared by rhodopsin and TRP, details in the trafficking of these proteins differ considerably, suggesting the existence of several trafficking pathways for these photoreceptor proteins.

The effect of phosphorylation on arrestin-rhodopsin interaction in the squid visual system

Invertebrate visual opsins are G protein-coupled receptors coupled to retinoid chromophores that isomerize reversibly between inactive rhodopsin and active metarhodopsin upon absorption of photons of light. The squid visual system has an arrestin protein that binds to metarhodopsin to block signaling to Gq and activation of phospholipase C. Squid rhodopsin kinase (SQRK) can phosphorylate both metarhodopsin and arrestin, a dual role that is unique among the G protein-coupled receptor kinases. The sites and role of arrestin phosphorylation by SQRK were investigated here using recombinant proteins. Arrestin was phosphorylated on serine 392 and serine 397 in the C-terminus. Unphosphorylated arrestin bound to metarhodopsin and phosphorylated metarhodopsin with similar high affinities (Kd 33 and 21 nM respectively), while phosphorylation of arrestin reduced the affinity 3- to 5-fold (Kd 104 nM). Phosphorylation of metarhodopsin slightly increased the dissociation of arrestin observed during a 1 hour incubation. Together these studies suggest a unique role for SQRK in phosphorylating both receptor and arrestin and inhibiting the binding of these two proteins in the squid visual system. Invertebrate visual systems are inactivated by arrestin binding to metarhodopsin that does not require receptor phosphorylation. Here we show that squid rhodopsin kinase phosphorylates arrestin on two serines (S392,S397) in the C-terminus and phosphorylation decreases the affinity of arrestin for squid metarhodopsin. Metarhodopsin phosphorylation has very little effect on arrestin binding but does increase arrestin dissociation.

Impaired Mitochondrial Energy Production Causes Light-Induced Photoreceptor Degeneration Independent of Oxidative Stress

Two insults often underlie a variety of eye diseases including glaucoma, optic atrophy, and retinal degeneration—defects in mitochondrial function and aberrant Rhodopsin trafficking. Although mitochondrial defects are often associated with oxidative stress, they have not been linked to Rhodopsin trafficking. In an unbiased forward genetic screen designed to isolate mutations that cause photoreceptor degeneration, we identified mutations in a nuclear-encoded mitochondrial gene, ppr, a homolog of human LRPPRC. We found that ppr is required for protection against light-induced degeneration. Its function is essential to maintain membrane depolarization of the photoreceptors upon repetitive light exposure, and an impaired phototransduction cascade in ppr mutants results in excessive Rhodopsin1 endocytosis. Moreover, loss of ppr results in a reduction in mitochondrial RNAs, reduced electron transport chain activity, and reduced ATP levels. Oxidative stress, however, is not induced. We propose that the reduced ATP level in ppr mutants underlies the phototransduction defect, leading to increased Rhodopsin1 endocytosis during light exposure, causing photoreceptor degeneration independent of oxidative stress. This hypothesis is bolstered by characterization of two other genes isolated in the screen, pyruvate dehydrogenase and citrate synthase. Their loss also causes a light-induced degeneration, excessive Rhodopsin1 endocytosis and reduced ATP without concurrent oxidative stress, unlike many other mutations in mitochondrial genes that are associated with elevated oxidative stress and light-independent photoreceptor demise.

Some mitochondrial disorders cause blindness through increased oxidative stress. This study shows that in other such disorders, light-activated photoreceptors degenerate because the shortfall in mitochondrial energy production impairs rhodopsin trafficking and induces toxicity.

Mitochondrial dysfunction is associated with a number of metabolic and neurological diseases such as Leigh syndrome and progressive blindness. Increased oxidative stress, which is often associated with mitochondrial dysfunction, is thought to be a common cause of disease progression. Here, we identified nuclear genes that encode mitochondrial proteins, whose loss causes the demise of photoreceptor neurons. Contrary to the common idea that this degeneration is triggered by elevated levels of oxidative stress, we find no change in the levels of oxidative stress. We show that activating photoreceptor neurons with light significantly increases energy production, and that this process is required to sustain their activity. Mitochondrial dysfunction impairs this capacity and leads to a premature termination of the light response. This in turn impairs the cycling of the light-sensitive receptor Rhodopsin in photoreceptors, and Rhodopsin accumulates in the cell inducing toxicity. This distinct mechanism of degeneration suggests that different mitochondrial diseases may follow different paths of disease progression and would hence respond differently to treatments.

Distinct roles of arrestin 1 protein in photoreceptors during Drosophila development

Arrestin regulates many facets of G-protein coupled receptor signaling. In Drosophila, Arrestin 1 (Arr1) is expressed at a lower level than Arrestin 2 (Arr2), and the role of Arr1 in visual physiology is less understood. Here we generated transgenic flies expressing enhanced green fluorescent protein tagged Arr1 (Arr1-eGFP) and explored its trafficking in live photoreceptors. We show that Arr1-eGFP is localized in the cytoplasm and displays light-dependent translocation to the rhabdomere possibly by interacting with photoactivated rhodopsin 1 (Rh1*). In the adult, translocation of Arr1-eGFP occurs with slower kinetics when compared with that of Arr2-eGFP. This slower kinetic activity may be attributable to a reduced level of phosphorylated Rh1*. Indeed, a reduced level of phosphorylated Rh1* recruits a lower level of Arr1-eGFP to rhabdomeres. To investigate whether Arr1 is required for the deactivation of phosphorylated Rh1*, we show that in flies with reduced Arr1 prolonged depolarizing afterpotential can be triggered with fewer light pulses, indicating that inactivation of phosphorylated Rh1* is compromised when the Arr1 level is reduced. Consistently, Arr1 is no longer required for deactivation of Rh1 in flies expressing phosphorylation-deficient Rh1. Previously it was reported that Arr1 displays light-dependent internalization. Unexpectedly, in adult photoreceptors we failed to observe endocytosis of Arr1-eGFP. In contrast, we show that in pupal photoreceptors Arr1-eGFP becomes internalized and sequestered in vesicles within the cytoplasm. Taken together, we propose that Arr1 plays distinct roles during development and adulthood. Arr1 orchestrates the recycling of phosphorylated Rh1* in pupae whereas it regulates the deactivation in adult.

Fine-tuning somatostatin receptor signalling by agonist-selective phosphorylation and dephosphorylation: IUPHAR Review 5

The biological actions of somatostatin are mediated by a family of five GPCRs, named sst1 to sst5 . Somatostatin receptors exhibit equally high-binding affinities to their natural ligand somatostatin-14 and largely overlapping distributions. The overexpression of somatostatin receptors in human tumours is the molecular basis for diagnostic and therapeutic application of the stable somatostatin analogues octreotide, lanreotide and pasireotide. The efficiency of somatostatin receptor signalling is tightly regulated and ultimately limited by the coordinated phosphorylation and dephosphorylation of intracellular carboxyl-terminal serine and threonine residues. Here, we review and discuss recent progress in the generation and application of phosphosite-specific antibodies for human sst2 and sst5 receptors. These phosphosite-specific antibodies are unique tools to monitor the spatial and temporal dynamics of receptors phosphorylation and dephosphorylation. Using a combined approach of phosphosite-specific antibodies and siRNA knock-down screening, relevant kinases and phosphatases were identified. Emerging evidence suggests distinct mechanisms of agonist-selective fine-tuning for individual somatostatin receptors. The recently uncovered differences in phosphorylation and dephosphorylation of these receptors may hence be of physiological significance in mediating responses to acute, persistent or repeated stimuli in a variety of target tissues.

Photosensitive TRPs

The Drosophila "transient receptor potential" channel is the prototypical TRP channel, belonging to and defining the TRPC subfamily. Together with a second TRPC channel, trp-like (TRPL), TRP mediates the transducer current in the fly's photoreceptors. TRP and TRPL are also implicated in olfaction and Malpighian tubule function. In photoreceptors, TRP and TRPL are localised in the ~30,000 packed microvilli that form the photosensitive "rhabdomere"-a light-guiding rod, housing rhodopsin and the rest of the phototransduction machinery. TRP (but not TRPL) is assembled into multimolecular signalling complexes by a PDZ-domain scaffolding protein (INAD). TRPL (but not TRP) undergoes light-regulated translocation between cell body and rhabdomere. TRP and TRPL are also found in photoreceptor synapses where they may play a role in synaptic transmission. Like other TRPC channels, TRP and TRPL are activated by a G protein-coupled phospholipase C (PLCβ4) cascade. Although still debated, recent evidence indicates the channels can be activated by a combination of PIP2 depletion and protons released by the PLC reaction. PIP2 depletion may act mechanically as membrane area is reduced by cleavage of PIP2's bulky inositol headgroup. TRP, which dominates the light-sensitive current, is Ca(2+) selective (P Ca:P Cs >50:1), whilst TRPL has a modest Ca(2+) permeability (P Ca:P Cs ~5:1). Ca(2+) influx via the channels has profound positive and negative feedback roles, required for the rapid response kinetics, with Ca(2+) rapidly facilitating TRP (but not TRPL) and also inhibiting both channels. In trp mutants, stimulation by light results in rapid depletion of microvillar PIP2 due to lack of Ca(2+) influx required to inhibit PLC. This accounts for the "transient receptor potential" phenotype that gives the family its name and, over a period of days, leads to light-dependent retinal degeneration. Gain-of-function trp mutants with uncontrolled Ca(2+) influx also undergo retinal degeneration due to Ca(2+) cytotoxicity. In vertebrate retina, mice knockout studies suggest that TRPC6 and TRPC7 mediate a PLCβ4-activated transducer current in intrinsically photosensitive retinal ganglion cells, expressing melanopsin. TRPA1 has been implicated as a "photo-sensing" TRP channel in human melanocytes and light-sensitive neurons in the body wall of Drosophila.

Phosphorylation of the Drosophila transient receptor potential ion channel is regulated by the phototransduction cascade and involves several protein kinases and phosphatases

Protein phosphorylation plays a cardinal role in regulating cellular processes in eukaryotes. Phosphorylation of proteins is controlled by protein kinases and phosphatases. We previously reported the light-dependent phosphorylation of the Drosophila transient receptor potential (TRP) ion channel at multiple sites. TRP generates the receptor potential upon stimulation of the photoreceptor cell by light. An eye-enriched protein kinase C (eye-PKC) has been implicated in the phosphorylation of TRP by in vitro studies. Other kinases and phosphatases of TRP are elusive. Using phosphospecific antibodies and mass spectrometry, we here show that phosphorylation of most TRP sites depends on the phototransduction cascade and the activity of the TRP ion channel. A candidate screen to identify kinases and phosphatases provided in vivo evidence for an involvement of eye-PKC as well as other kinases and phosphatases in TRP phosphorylation.

Negative regulation of the novel norpA(P24) suppressor, diehard4, in the endo-lysosomal trafficking underlies photoreceptor cell degeneration

Rhodopsin has been used as a prototype system to investigate G protein-coupled receptor (GPCR) internalization and endocytic sorting mechanisms. Failure of rhodopsin recycling upon light activation results in various degenerative retinal diseases. Accumulation of internalized rhodopsin in late endosomes and the impairment of its lysosomal degradation are associated with unregulated cell death that occurs in dystrophies. However, the molecular basis of rhodopsin accumulation remains elusive. We found that the novel norpA^P24 suppressor, diehard4, is responsible for the inability of endo-lysosomal rhodopsin trafficking and retinal degeneration in Drosophila models of retinal dystrophies. We found that diehard4 encodes Osiris 21. Loss of its function suppresses retinal degeneration in norpA^P24, rdgC³⁰⁶, and trp¹, but not in rdgB², suggesting a common cause of photoreceptor death. In addition, the loss of Osiris 21 function shifts the membrane balance between late endosomes and lysosomes as evidenced by smaller late endosomes and the proliferation of lysosomal compartments, thus facilitating the degradation of endocytosed rhodopsin. Our results demonstrate the existence of negative regulation in vesicular traffic between endosomes and lysosomes. We anticipate that the identification of additional components and an in-depth description of this specific molecular machinery will aid in therapeutic interventions of various retinal dystrophies and GPCR-related human diseases.

Malfunctioning of phototransduction is the major cause of human blindness. Without functional phototransduction, rhodopsin-1, the major visual pigment, is rapidly endocytosed and accumulated in late endosomes. Impaired lysosomal delivery of endocytosed rhodopsin and its degradation has been reported to trigger progressive and light-dependent retinal degeneration in Drosophila models. It is intriguing why endocytosed rhodopsin accumulates in late endosomes instead of being delivered to lysosomes for degradation. Is this attributable to a saturation of rhodopsin endocytosis, which impedes the delivery capacity of the cell? To investigate the underlying mechanisms of rhodopsin accumulation in late endosomes, we used a suppressor of phototransduction mutants, which was identified previously from our unbiased genetic screen. This suppressor, called diehard4, shifts the membrane balance between late endosomes and lysosomes, resulting in the facilitated degradation of endocytosed rhodopsin. Our results clearly demonstrate that a previously unknown mechanism of negative regulation is actively engaged in vesicular traffic between endosomes and lysosomes in fly photoreceptors. We showed that eliminating such blockage alone was enough to rescue retinal degeneration in phototransduction mutants. From these results, we anticipate that the identification of additional components and an in-depth description of this molecular machinery will aid in therapeutic interventions of various retinal dystrophies and neurodegenerative disorders.

Depletion of PtdIns(4,5)P₂ underlies retinal degeneration in Drosophila trp mutants

The prototypical transient receptor potential (TRP) channel is the major light-sensitive, and Ca(2+)-permeable channel in the microvillar photoreceptors of Drosophila. TRP channels are activated following hydrolysis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂] by the key effector enzyme phospholipase C (PLC). Mutants lacking TRP channels undergo light-dependent retinal degeneration, as a consequence of the reduced Ca(2+) influx. It has been proposed that degeneration is caused by defects in the Ca(2+)-dependent visual pigment cycle, which result in accumulation of toxic phosphorylated metarhodopsin-arrestin complexes (MPP-Arr2). Here we show that two interventions, which prevent accumulation of MPP-Arr2, namely rearing under red light or eliminating the C-terminal rhodopsin phosphorylation sites, failed to rescue degeneration in trp mutants. Instead, degeneration in trp mutants reared under red light was rescued by mutation of PLC. Degeneration correlated closely with the light-induced depletion of PtdIns(4,5)P₂ that occurs in trp mutants due to failure of Ca(2+)-dependent inhibition of PLC. Severe retinal degeneration was also induced in the dark in otherwise wild-type flies by overexpression of a bacterial PtdInsPn phosphatase (SigD) to deplete PtdIns(4,5)P₂. In degenerating trp photoreceptors, phosphorylated Moesin, a PtdIns(4,5)P₂-regulated membrane-cytoskeleton linker essential for normal microvillar morphology, was found to delocalize from the rhabdomere and there was extensive microvillar actin depolymerisation. The results suggest that compromised light-induced Ca(2+) influx, due to loss of TRP channels, leads to PtdIns(4,5)P₂ depletion, resulting in dephosphorylation of Moesin, actin depolymerisation and disintegration of photoreceptor structure.

Role of rhodopsin and arrestin phosphorylation in retinal degeneration of Drosophila

Arrestins belong to a family of multifunctional adaptor proteins that regulate internalization of diverse receptors including G-protein-coupled receptors (GPCRs). Defects associated with endocytosis of GPCRs have been linked to human diseases. We used enhanced green fluorescent protein-tagged arrestin 2 (Arr2) to monitor the turnover of the major rhodopsin (Rh1) in live Drosophila. We demonstrate that during degeneration of norpA(P24) photoreceptors the loss of Rh1 is parallel to the disappearance of rhabdomeres, the specialized visual organelle that houses Rh1. The cause of degeneration in norpA(P24) is the failure to activate CaMKII (Ca(2+)/calmodulin-dependent protein kinase II) and retinal degeneration C (RDGC) because of a loss of light-dependent Ca(2+) entry. A lack of activation in CaMKII, which phosphorylates Arr2, leads to hypophosphorylated Arr2, while a lack of activation of RDGC, which dephosphorylates Rh1, results in hyperphosphorylated Rh1. We investigated how reversible phosphorylation of Rh1 and Arr2 contributes to photoreceptor degeneration. To uncover the consequence underlying a lack of CaMKII activation, we characterized ala(1) flies in which CaMKII was suppressed by an inhibitory peptide, and showed that morphology of rhabdomeres was not affected. In contrast, we found that expression of phosphorylation-deficient Rh1s, which either lack the C terminus or contain Ala substitution in the phosphorylation sites, was able to prevent degeneration of norpA(P24) photoreceptors. This suppression is not due to a loss of Arr2 interaction. Importantly, co-expression of these modified Rh1s offered protective effects, which greatly delayed photoreceptor degeneration. Together, we conclude that phosphorylation of Rh1 is the major determinant that orchestrates its internalization leading to retinal degeneration.

Lipid signaling in Drosophila photoreceptors

Drosophila photoreceptors are sensory neurons whose primary function is the transduction of photons into an electrical signal for forward transmission to the brain. Photoreceptors are polarized cells whose apical domain is organized into finger like projections of plasma membrane, microvilli that contain the molecular machinery required for sensory transduction. The development of this apical domain requires intense polarized membrane transport during development and it is maintained by post developmental membrane turnover. Sensory transduction in these cells involves a high rate of G-protein coupled phosphatidylinositol 4,5 bisphosphate [PI(4,5)P(2)] hydrolysis ending with the activation of ion channels that are members of the TRP superfamily. Defects in this lipid-signaling cascade often result in retinal degeneration, which is a consequence of the loss of apical membrane homeostasis. In this review we discuss the various membrane transport challenges of photoreceptors and their regulation by ongoing lipid signaling cascades in these cells. This article is part of a Special Issue entitled Lipids and Vesicular Transport.

The history of the prolonged depolarizing afterpotential (PDA) and its role in genetic dissection of Drosophila phototransduction

In invertebrate photoreceptors, the photopigment exhibits a long-lived and physiologically active photoproduct, called metarhodopsin (M). The long life of invertebrate M implies that under physiological conditions, M and the original pigment state rhodopsin, R, are in photoequilibrium. In many invertebrates, the absorption spectra of R and M states are different, allowing large photopigment conversion between R and M states. These net pigment molecules conversions between R and M are the basis of the prolonged depolarizing afterpotential (PDA) phenomenology, which is the main subject of this review. A large net conversion of R to M disrupts phototransduction termination at the photopigment level, which in turn results in sustained excitation long after the light is turned off. Throughout this period, the photoreceptors are partially desensitized and are insensitive (or less sensitive) to subsequent test lights. In Drosophila, the PDA tests the maximal capacity of the photoreceptor cell to maintain excitation for an extended period and is strictly dependent on the presence of high concentrations of rhodopsin and the transient receptor potential (TRP) channels. Therefore, it detects even minor defects in rhodopsin or TRP biogenesis and easily scores deficient replenishment of phototransduction components, which results in temporary desensitization of the phototransduction process. Indeed, the introduction and use of PDA to screen for phototransduction-defective Drosophila mutants by Pak and colleagues yielded a plethora of new and most interesting visual mutants. Remarkably, to this day, the PDA mutants that Pak and his colleagues isolated are the main source of mutants for analysis of the Drosophila visual system.

Molecular genetics of retinal degeneration: A Drosophila perspective

Inherited retinal degeneration in Drosophila has been explored for insights into similar processes in humans. Based on the mechanisms, I divide these mutations in Drosophila into three classes. The first consists of genes that control the specialization of photoreceptor cells including the morphogenesis of visual organelles  (rhabdomeres) that house the visual signaling proteins. The second class contains genes that regulate the activity or level of the major rhodopsin, Rh1, which is the light sensor and also provides a structural role for the maintenance of rhabdomeres. Some mutations in Rh1 (NinaE) are dominant due to constitutive activity or folding defects, like autosomal dominant retinitis pigmentosa (ADRP) in humans. The third class consists of genes that control the Ca ( 2+) influx directly or indirectly by promoting the turnover of the second messenger and regeneration of PIP 2, or mediate the Ca ( 2+) -dependent regulation of the visual response. These gene products are critical for the increase in cytosolic Ca ( 2+ ) following light stimulation to initiate negative regulatory events. Here I will focus on the signaling mechanisms underlying the degeneration in norpA, and in ADRP-type NinaE mutants that produce misfolded Rh1. Accumulation of misfolded Rh1 in the ER triggers the unfolded protein response (UPR), while endosomal accumulation of activated Rh1 may initiate autophagy in norpA. Both autophagy and the UPR are beneficial for relieving defective endosomal trafficking and the ER stress, respectively. However, when photoreceptors fail to cope with the persistence of these stresses, a cell death program is activated leading to retinal degeneration.

Phosphoinositide 3-kinase signaling in retinal rod photoreceptors

PURPOSE

Phosphoinositide 3-kinase (PI3K) consists of a p110 catalytic protein and a p85α regulatory protein, required for the stabilization and localization of p110-PI3K activity. The biological significance of PI3K was investigated in vertebrate rod photoreceptors by deleting its regulatory p85α protein and examining its role in photoreceptor structure, function, and protein trafficking.

METHODS

Mice that expressed Cre recombinase in rods were bred to mice with a floxed p85α (pik3r1) regulatory subunit of PI3K to generate a conditional deletion of pik3r1 in rods. Functional and structural changes were determined by ERG and morphometric analysis, respectively. PI3K activity was measured in retinal homogenates immunoprecipitated with an anti-PY antibody. Akt activation was determined by Western blot analysis with a pAkt antibody.

RESULTS

Light-induced stress increased PI3K activity in retinal immunoprecipitates and phosphorylation of Akt. There was no effect of pik3r1 deletion on retinal structure. However, twin flash electroretinography revealed a slight delay in recovery kinetics in pik3r1 knockout (KO) mice compared with wild-type controls. The movement of arrestin in the pik3r1 KO mice was slower than that in the wild-type mouse retinas at 5 minutes of exposure to light. At 10 minutes of exposure, the ROS localization of arrestin was almost identical between the wild-type and pik3r1 KO mice.

CONCLUSIONS

The results provide the first direct evidence that rods use PI3K-generated phosphoinositides for photoreceptor function. The lack of phenotype in pik3r1 KO rod photoreceptors suggests a redundant role in controlling PIP(3) synthesis.

The functional cycle of visual arrestins in photoreceptor cells

Visual arrestin-1 plays a key role in the rapid and reproducible shutoff of rhodopsin signaling. Its highly selective binding to light-activated phosphorylated rhodopsin is an integral part of the functional perfection of rod photoreceptors. Structure-function studies revealed key elements of the sophisticated molecular mechanism ensuring arrestin-1 selectivity and paved the way to the targeted manipulation of the arrestin-1 molecule to design mutants that can compensate for congenital defects in rhodopsin phosphorylation. Arrestin-1 self-association and light-dependent translocation in photoreceptor cells work together to keep a constant supply of active rhodopsin-binding arrestin-1 monomer in the outer segment. Recent discoveries of arrestin-1 interaction with other signaling proteins suggest that it is a much more versatile signaling regulator than previously thought, affecting the function of the synaptic terminals and rod survival. Elucidation of the fine molecular mechanisms of arrestin-1 interactions with rhodopsin and other binding partners is necessary for the comprehensive understanding of rod function and for devising novel molecular tools and therapeutic approaches to the treatment of visual disorders.

Rapid dephosphorylation of G protein-coupled receptors by protein phosphatase 1β is required for termination of β-arrestin-dependent signaling

Termination of signaling of activated G protein-coupled receptors (GPCRs) is essential for maintenance of cellular homeostasis. It is well established that β-arrestin redistributes to phosphorylated GPCRs and thereby facilitates desensitization of classical G protein-dependent signaling. β-Arrestin in turn serves as a scaffold to initiate a second wave of signaling. Here, we report a molecular mechanism that regulates the termination of unconventional β-arrestin-dependent GPCR signaling. We identify protein phosphatase 1β (PP1β) as a phosphatase for the cluster of phosphorylated threonines ((353)TTETQRT(359)) within the sst(2A) somatostatin receptor carboxyl terminus that mediates β-arrestin binding using siRNA knock-down screening. We show that PP1β-mediated sst(2A) dephosphorylation is initiated directly after receptor activation at or near the plasma membrane. As a functional consequence of diminished PP1β activity, we find that somatostatin- and substance P-induced but not epidermal growth factor-induced ERK activation was aberrantly enhanced and prolonged. Thus, we demonstrate a novel mechanism for fine tuning unconventional β-arrestin-dependent GPCR signaling in that recruitment of PP1β to activated GPCRs facilitates GPCR dephosphorylation and, hence, leads to disruption of the β-arrestin-GPCR complex.

Role of Phosphorylation in the Control of Clathrin-Mediated Internalization of GPCR

The process by which G protein-coupled receptors (GPCRs) are internalized through the clathrin-coated vesicles involves interactions of multifunctional adaptor proteins. These interactions are tightly controlled by phosphorylation and dephosphorylation mechanisms resulting in the regulation of receptor endocytosis. However, the identities of the kinases involved in this process remained largely unknown until recently. This paper discusses advances in our knowledge of the important role played by protein phosphorylation in the regulation of the endocytic machinery and how phosphorylation controls the coated vesicle cycle.

Arrestin in ciliary invertebrate photoreceptors: molecular identification and functional analysis in vivo

Arrestin was identified in ciliary photoreceptors of Pecten irradians, and its role in terminating the light response was established electrophysiologically. Downstream effectors in these unusual visual cells diverge from both microvillar photoreceptors and rods and cones; the finding that key regulatory mechanisms of the early steps of visual excitation are conserved across such distant lineages of photoreceptors underscores that a common blueprint for phototransduction exists across metazoa. Arrestin was detected by Western blot analysis of retinal lysates, and localized in ciliary photoreceptors by immunostaining of whole-eye cryosections and dissociated cells. Two arrestin isoforms were molecularly identified by PCR; these present the canonical N- and C-arrestin domains, and are identical at the nucleotide level over much of their sequence. A high degree of homology to various β-arrestins (up to 70% amino acid identity) was found. In situ hybridization localized the two transcripts within the retina, but failed to reveal finer spatial segregation, possibly because of insufficient differences between the riboprobes. Intracellular dialysis of anti arrestin antibodies into voltage-clamped ciliary photoreceptors produced a gradual slow-down of the photocurrent falling phase, leaving a tail that decayed over many seconds after light termination. The antibodies also caused spectrally neutral flashes to elicit prolonged aftercurrents in the absence of large metarhodopsin accumulation; such aftercurrents could be quenched by chromatic illumination that photoconverts metarhodopsin back to rhodopsin. These observations indicate that the antibodies depleted functionally available arrestin, and implicate this molecule in the deactivation of the photoresponse at the rhodopsin level.

800 facets of retinal degeneration

In today's world of genomics and large computational analyses, rapid progress has been made in identifying genes associated with human retinal diseases. Nevertheless, before significant advances toward effective therapeutic intervention is made, a clearer understanding of the molecular and cellular role of these gene products in normal and diseased photoreceptor cell biology is required. Given the complexity of the vertebrate retina, these advancements are unlikely to be revealed in isolated human cell lines, but instead, will require the use of numerous model systems. Here, we describe several parallels between vertebrate and invertebrate photoreceptor cell biology that are beginning to emerge and advocate the use of Drosophila melanogaster as a powerful genetic model system for uncovering molecular mechanisms of human retinal pathologies, in particular photoreceptor neurodegeneration.

Light- and drug-activated G-protein-coupled receptors to control intracellular signalling

G-protein-coupled receptors (GPCRs) integrate extracellular cues into intracellular signals to modulate the cellular state. Owing to their diverse modulatory functions, GPCRs represent one of the major drug targets of the pharmaceutical industry. Until now, the characterization and control of GPCRs and their intracellular signalling cascades have mainly relied on chemical compounds, which either activate or inhibit GPCR pathways, albeit with limited receptor and cell-type specificity. Recently, new approaches have been developed to control signalling cascades in cell- and receptor-type-specific ways. The chemical approach focuses on GPCR design and activation by an inert chemical compound, whereas the physical approach uses designer GPCRs and activation by physical stimuli, such as light.

Protein phosphatase with EF-hand domains 2 (PPEF2) is a potent negative regulator of apoptosis signal regulating kinase-1 (ASK1)

The function of protein phosphatases with EF-hand domains (PPEF) in mammals is not known. Large-scale expression profiling experiments suggest that PPEF expression may correlate with stress protective responses, cell survival, growth, proliferation, or neoplastic transformation. Apoptosis signal regulating kinase-1 (ASK1) is a MAP kinase kinase kinase implicated in cancer, cardiovascular and neurodegenerative diseases. ASK1 is activated by oxidative stress and induces pro-apoptotic or inflammatory signalling, largely via sustained activation of MAP kinases p38 and/or JNK. We identify human PPEF2 as a novel interacting partner and a negative regulator of ASK1. In COS-7 or HEK 293A cells treated with H(2)O(2), expression of PPEF2 abrogated sustained activation of p38 and one of the JNK p46 isoforms, and prevented ASK1-dependent caspase-3 cleavage and activation. PPEF2 efficiently suppressed H(2)O(2)-induced activation of ASK1. Overexpessed as well as endogenous ASK1 co-immunoprecipitated with PPEF2. PPEF2 was considerably more potent both as a suppressor of ASK1 activation and as its interacting partner as compared to protein phosphatase 5 (PP5), a well-known negative regulator of ASK1. PPEF2 was found to form complexes with endogenous Hsp70 and to a lesser extent Hsp90, which are also known interacting partners of PP5. These data identify, for the first time, a possible downstream signalling partner of a mammalian PPEF phosphatase, and suggest that, despite structural divergence, PPEF and PP5 phosphatases may share common interacting partners and functions.

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.

Light-dependent phosphorylation of the drosophila transient receptor potential ion channel

The Drosophila phototransduction cascade terminates in the opening of an ion channel, designated transient receptor potential (TRP). TRP has been shown to become phosphorylated in vitro, suggesting regulation of the ion channel through posttranslational modification. However, except for one phosphorylation site, Ser(982), which was analyzed by functional in vivo studies (Popescu, D. C., Ham, A. J., and Shieh, B. H. (2006) J. Neurosci. 26, 8570-8577), nothing is known about the role of TRP phosphorylation in vivo. Here, we report the identification of 21 TRP phosphorylation sites by a mass spectrometry approach. 20 phosphorylation sites are located in the C-terminal portion of the channel, and one site is located near the N terminus. All 21 phosphorylation sites were also identified in the inaC(P209) mutant, indicating that phosphorylation of TRP at these sites occurred independently from the eye-enriched protein kinase C. Relative quantification of phosphopeptides revealed that at least seven phosphorylation sites were predominantly phosphorylated in the light, whereas one site, Ser(936), was predominantly phosphorylated in the dark. We show that TRP phosphorylated at Ser(936) was located in the rhabomere. Light-dependent changes in the phosphorylation state of this site occurred within minutes. The dephosphorylation of TRP at Ser(936) required activation of the phototransduction cascade.

Gene duplication and the origins of morphological complexity in pancrustacean eyes, a genomic approach

BACKGROUND

Duplication and divergence of genes and genetic networks is hypothesized to be a major driver of the evolution of complexity and novel features. Here, we examine the history of genes and genetic networks in the context of eye evolution by using new approaches to understand patterns of gene duplication during the evolution of metazoan genomes. We hypothesize that 1) genes involved in eye development and phototransduction have duplicated and are retained at higher rates in animal clades that possess more distinct types of optical design; and 2) genes with functional relationships were duplicated and lost together, thereby preserving genetic networks. To test these hypotheses, we examine the rates and patterns of gene duplication and loss evident in 19 metazoan genomes, including that of Daphnia pulex - the first completely sequenced crustacean genome. This is of particular interest because the pancrustaceans (hexapods+crustaceans) have more optical designs than any other major clade of animals, allowing us to test specifically whether the high amount of disparity in pancrustacean eyes is correlated with a higher rate of duplication and retention of vision genes.

RESULTS

Using protein predictions from 19 metazoan whole-genome projects, we found all members of 23 gene families known to be involved in eye development or phototransduction and deduced their phylogenetic relationships. This allowed us to estimate the number and timing of gene duplication and loss events in these gene families during animal evolution. When comparing duplication/retention rates of these genes, we found that the rate was significantly higher in pancrustaceans than in either vertebrates or non-pancrustacean protostomes. Comparing patterns of co-duplication across Metazoa showed that while these eye-genes co-duplicate at a significantly higher rate than those within a randomly shuffled matrix, many genes with known functional relationships in model organisms did not co-duplicate more often than expected by chance.

CONCLUSIONS

Overall, and when accounting for factors such as differential rates of whole-genome duplication in different groups, our results are broadly consistent with the hypothesis that genes involved in eye development and phototransduction duplicate at a higher rate in Pancrustacea, the group with the greatest variety of optical designs. The result that these genes have a significantly high number of co-duplications and co-losses could be influenced by shared functions or other unstudied factors such as synteny. Since we did not observe co-duplication/co-loss of genes for all known functional modules (e.g. specific regulatory networks), the interactions among suites of known co-functioning genes (modules) may be plastic at the temporal scale of analysis performed here. Other factors in addition to gene duplication - such as cis-regulation, heterotopy, and co-option - are also likely to be strong factors in the diversification of eye types.

Photoreceptor responses of fruitflies with normal and reduced arrestin content studied by simultaneous measurements of visual pigment fluorescence and ERG

We have simultaneously measured the electroretinogram (ERG) and the metarhodopsin content via fluorescence in white-eyed, wild-type Drosophila and the arrestin2 hypomorphic mutant (w(-);arr2 (3)) at a range of stimulus wavelengths and intensities. Photoreceptor response amplitude and termination (transition between full repolarization and prolonged depolarizing afterpotential, PDA) were related to visual pigment conversions and arrestin concentration. The data were implemented in a kinetic model of the rhodopsin-arrestin cycle, allowing us to estimate the active metarhodopsin concentration as a function of effective light intensity and arrestin concentration. Arrestin reduction in the mutant modestly increased the light sensitivity and decreased the photoreceptor dynamic range. Compared to the wild type, in the mutant the transition between full repolarization and PDA occurred at a lower metarhodopsin fraction and was more abrupt. We developed a steady-state stochastic model to interpret the dependence of the PDA on effective light intensity and arrestin content and to help deduce the arrestin to rhodopsin ratio from the sensitivity and PDA data. The feasibility of different experimental methods for the estimation of arrestin content from ERG and PDA is discussed.

PPEF/PP7 protein Ser/Thr phosphatases

PPEF/PP7 represents one of the five subfamilies of the PPP protein Ser/Thr phosphatases. Studies published in recent years point to a role of plant PP7 at a crossroad of different pathways of light and stress signalling. In animals, PPEFs are highly expressed in sensory neurons, and Drosophila PPEF phosphatase, rdgC, is essential for dephosphorylation of rhodopsin. Expression profiling suggests that mammalian PPEF may play a role in stress-protective responses, cell survival, growth, proliferation, and oncogenesis. Despite structural similarities of the catalytic domains and the fact that some of these phosphatases are involved in light perception both in animals and in plants, the plant and non-plant representatives of this group have distinct domain architecture and appear not to be orthologues.

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.

Role of Ca2+/calmodulin-dependent protein kinase II in Drosophila photoreceptors

Ca(2+) modulates the visual response in both vertebrates and invertebrates. In Drosophila photoreceptors, an increase of cytoplasmic Ca(2+) mimics light adaptation. Little is known regarding the mechanism, however. We explored the role of the sole Drosophila Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) to mediate light adaptation. CaMKII has been implicated in the phosphorylation of arrestin 2 (Arr2). However, the functional significance of Arr2 phosphorylation remains debatable. We identified retinal CaMKII by anti-CaMKII antibodies and by its Ca(2+)-dependent autophosphorylation. Moreover, we show that phosphorylation of CaMKII is greatly enhanced by okadaic acid, and indeed, purified PP2A catalyzes the dephosphorylation of CaMKII. Significantly, we demonstrate that anti-CaMKII antibodies co-immunoprecipitate, and CaMKII fusion proteins pull down the catalytic subunit of PP2A from fly extracts, indicating that PP2A interacts with CaMKII to form a protein complex. To investigate the function of CaMKII in photoreceptors, we show that suppression of CaMKII in transgenic flies affects light adaptation and increases prolonged depolarizing afterpotential amplitude, whereas a reduced PP2A activity brings about reduced prolonged depolarizing afterpotential amplitude. Taken together, we conclude that CaMKII is involved in the negative regulation of the visual response affecting light adaptation, possibly by catalyzing phosphorylation of Arr2. Moreover, the CaMKII activity appears tightly regulated by the co-localized PP2A.

Ca2+-dependent metarhodopsin inactivation mediated by calmodulin and NINAC myosin III

Phototransduction in flies is the fastest known G protein-coupled signaling cascade, but how this performance is achieved remains unclear. Here, we investigate the mechanism and role of rhodopsin inactivation. We determined the lifetime of activated rhodopsin (metarhodopsin = M( *)) in whole-cell recordings from Drosophila photoreceptors by measuring the time window within which inactivating M( *) by photoreisomerization to rhodopsin could suppress responses to prior illumination. M( *) was inactivated rapidly (tau approximately 20 ms) under control conditions, but approximately 10-fold more slowly in Ca2+-free solutions. This pronounced Ca2+ dependence of M( *) inactivation was unaffected by mutations affecting phosphorylation of rhodopsin or arrestin but was abolished in mutants of calmodulin (CaM) or the CaM-binding myosin III, NINAC. This suggests a mechanism whereby Ca2+ influx acting via CaM and NINAC accelerates the binding of arrestin to M( *). Our results indicate that this strategy promotes quantum efficiency, temporal resolution, and fidelity of visual signaling.

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.

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.

Regulation of alpha2AR trafficking and signaling by interacting proteins

The continuing discovery of new G protein-coupled receptor (GPCR) interacting proteins and clarification of the functional consequences of these interactions has revealed multiple roles for these events. Some of these interactions serve to scaffold GPCRs to particular cellular micro-compartments or to tether them to defined signaling molecules, while other GPCR-protein interactions control GPCR trafficking and the kinetics of GPCR-mediated signaling transduction. This review provides a general overview of the variety of GPCR-protein interactions reported to date, and then focuses on one prototypical GPCR, the alpha(2)AR, and the in vitro and in vivo significance of its reciprocal interactions with arrestin and spinophilin. It seems appropriate to recognize the life and career of Arthur Hancock with a summary of studies that both affirm and surprise our preconceived notions of how nature is designed, as his career-long efforts similarly affirmed the complexity of human biology and attempted to surprise pathological changes in that biology with novel, discovery-based therapeutic interventions. Dr. Hancock's love of life, of family, and of commitment to making the world a better place are a model of the life well lived, and truly missed by those who were privileged to know, and thus love, him.