Turnover of membrane and opsin in visual receptors of normal and mutant Drosophila

Light-Driven Processes Control Both Rhodopsin Maturation and Recycling in Mosquito Photoreceptors

Many invertebrates carry out a daily cycle of shedding and rebuilding of the photoreceptor's photosensitive rhabdomeric membranes. The mosquito Aedes aegypti shows a robust response, losing nearly all Aaop1 rhodopsin from the rhabdomeric membranes during the shedding process at dawn. Here, we made use of Aaop1 antibodies capable of distinguishing newly synthesized, glycosylated rhodopsin from mature nonglycosylated rhodopsin to characterize the fate of Aaop1 during the shedding and rebuilding processes. The rhabdomeric rhodopsin is moved into large cytoplasmic vesicles at dawn and is subsequently degraded during the standard 12 h daytime period. The endocytosed rhodopsin is trafficked back to the photosensitive membranes if animals are shifted back to dark conditions during the morning hours. During the daytime period, small vesicles containing newly synthesized and glycosylated Aaop1 rhodopsin accumulate within the cytoplasm. At dusk, these vesicles are lost as the newly synthesized Aaop1 is converted to the nonglycosylated form and deposited in the rhabdomeres. We demonstrate that light acts though a novel signaling pathway to block rhodopsin maturation, thus inhibiting the deglycosylation and rhabdomeric targeting of newly synthesized Aaop1 rhodopsin. Therefore, light controls two cellular processes responsible for the daily renewal of rhodopsin: rhodopsin endocytosis at dawn and inhibition of rhodopsin maturation until dusk.

SIGNIFICANCE STATEMENT

Organisms use multiple strategies to maximize visual capabilities in different light conditions. Many invertebrates show a daily cycle of shedding the photoreceptor's rhabdomeric membranes at dawn and rebuilding these during the following night. We show here that the Aedes aegypti mosquito possesses two distinct light-driven cellular signaling processes for modulating rhodopsin content during this cycle. One of these, endocytosis of rhabdomeric rhodopsin, has been described previously. The second, a light-activated cellular pathway acting to inhibit the anterograde movement of newly synthesized rhodopsin, is revealed here for the first time. The discovery of this cellular signaling pathway controlling a G-protein-coupled receptor is of broad interest due to the prominent role of this receptor family across all areas of neuroscience.

chaoptin, prominin, eyes shut and crumbs form a genetic network controlling the apical compartment of Drosophila photoreceptor cells

The apical surface of epithelial cells is often highly specialised to fulfil cell type-specific functions. Many epithelial cells expand their apical surface by forming microvilli, actin-based, finger-like membrane protrusions. The apical surface of Drosophila photoreceptor cells (PRCs) forms tightly packed microvilli, which are organised into the photosensitive rhabdomeres. As previously shown, the GPI-anchored adhesion protein Chaoptin is required for the stability of the microvilli, whereas the transmembrane protein Crumbs is essential for proper rhabdomere morphogenesis. Here we show that chaoptin synergises with crumbs to ensure optimal rhabdomere width. In addition, reduction of crumbs ameliorates morphogenetic defects observed in PRCs mutant for prominin and eyes shut, known antagonists of chaoptin. These results suggest that these four genes provide a balance of adhesion and anti-adhesion to maintain microvilli development and maintenance. Similar to crumbs mutant PRCs, PRCs devoid of prominin or eyes shut undergo light-dependent retinal degeneration. Given the observation that human orthologues of crumbs, prominin and eyes shut result in progressive retinal degeneration and blindness, the Drosophila eye is ideally suited to unravel the genetic and cellular mechanisms that ensure morphogenesis of PRCs and their maintenance under light-mediated stress.

Light-mediated control of rhodopsin movement in mosquito photoreceptors

Multiple mechanisms contribute to a photoreceptor's ability to adapt to ambient light conditions. The mosquito Aedes aegypti expresses the long-wavelength rhodopsin Aaop1 in all R1-R6 photoreceptors and most R8 photoreceptors. These photoreceptors alter the cellular location of Aaop1 and reorganize their photosensitive rhabdomeric membranes on a daily basis. During daylight periods, Aaop1 is excluded from the light-sensitive rhabdomeres and localized to multivesicular bodies (MVBs) within the photoreceptor cytoplasm. In the dark, Aaop1 accumulates in the rhabdomeres and no Aaop1-containing MVBs are present in the cytoplasm. Manipulation of light treatments shows the cellular movement of Aaop1 in and out of the rhabdomere is directly controlled by light. In a separate process, the photoreceptors reduce Aaop1 protein content during a time period spanning from late afternoon into the first 2 h of the dark period. Aaop1 levels then gradually increase through the dark period and remain high following movement of Aaop1 to the cytoplasm at dawn. These results demonstrate that mosquito photoreceptors control rhodopsin availability during the daily light-dark cycle by novel mechanisms not discerned from analysis of traditional invertebrate models. These mechanisms will maximize a photoreceptor's light sensitivity range and therefore may be common in organisms active in low light conditions.

Dynamin- and Rab5-dependent endocytosis is required to prevent Drosophila photoreceptor degeneration

In Drosophila photoreceptors, Rhodopsin 1 (ninaE, Rh1) is required for proper morphogenesis and maintenance of the apical light-gathering organelle, the rhabdomere. It has been proposed that Rh1, coupled to the Rho GTPases Rac1 and Cdc42, promotes the morphogenesis of a sub-rhabdomeric F-actin meshwork or rhabdomere terminal web (RTW). The RTW provides mechanical support to the apical microvilli and is likely to guide Rab11-dependent delivery of Rh1-rich membrane to the rhabdomere from the trans Golgi network. However, the nature and function of the molecular pathway involved in RTW morphogenesis remains incomplete. Here, we show that Rh1 function in promoting RTW morphogenesis is light-independent and is conserved throughout evolution. This Rh1 function does not require G(q)α(e), which is required for phototransduction. Finally, we show that interfering with Dynamin- and Rab5-dependent endocytosis leads to a phenotype that is undistinguishable from that of the ninaE-null mutant. Importantly, the corresponding endocytic activity is essential to prevent early onset of rhabdomere degeneration. In conclusion, we propose that Rh1 function in promoting RTW morphogenesis is not only needed to sustain apical membrane delivery but is also required for proper rhabdomeric membrane endocytosis and turnover.

A dynamic role for the mushroom bodies in promoting sleep in Drosophila

The fruitfly, Drosophila melanogaster, exhibits many of the cardinal features of sleep, yet little is known about the neural circuits governing its sleep. Here we have performed a screen of GAL4 lines expressing a temperature-sensitive synaptic blocker shibire(ts1) (ref. 2) in a range of discrete neural circuits, and assayed the amount of sleep at different temperatures. We identified three short-sleep lines at the restrictive temperature with shared expression in the mushroom bodies, a neural locus central to learning and memory. Chemical ablation of the mushroom bodies also resulted in reduced sleep. These studies highlight a central role for the mushroom bodies in sleep regulation.

Ceramidase expression facilitates membrane turnover and endocytosis of rhodopsin in photoreceptors

Transgenic expression of ceramidase suppresses retinal degeneration in Drosophila arrestin and phospholipase C mutants. Here, we show that expression of ceramidase facilitates the dissolution of incompletely formed and inappropriately located elements of rhabdomeric membranes in ninaE(I17) mutants lacking the G protein receptor Rh1 in R1-R6 photoreceptor cells. Ceramidase expression facilitates the endocytic turnover of Rh1. Although ceramidase expression aids the removal of internalized rhodopsin, it does not affect the turnover of Rh1 in photoreceptors maintained in dark, where Rh1 is not activated and thus has a slower turnover and a long half-life. Therefore, the phenotypic consequence of ceramidase expression in photoreceptors is caused by facilitation of endocytosis. This study provides mechanistic insight into the sphingolipid biosynthetic pathway-mediated modulation of endocytosis and suppression of retinal degeneration.

Protein kinase C-induced disorganization and endocytosis of photosensitive membrane in Limulus ventral photoreceptors

Protein kinase C (PKC) desensitizes the light response in photoreceptors from the ventral optic nerve of the horseshoe crab Limulus. Photoisomerization of Limulus rhodopsin leads to phosphoinositide hydrolysis, resulting in the production of inositol trisphosphate and diacylglycerol (DAG). Inositol trisphosphate mobilizes intracellular stores of Ca(2+), resulting in photoreceptor excitation in Limulus, while DAG may activate PKC. We investigated whether PKC-mediated desensitization of the photoresponse is accompanied by ultrastructural changes in the rhodopsin-bearing photosensitive membrane (rhabdom) in Limulus ventral photoreceptors. PKC activation by (-)-indolactam V in darkness induces disorganization and swelling of the rhodopsin-containing microvilli and endocytosis of rhabdomeral membrane. The effects of (-)-indolactam V on dark-adapted photoreceptor ultrastructure are reversible, are stereospecific, are blocked by coapplication of PKC inhibitors, and closely match those induced by continuous, bright light. Rhabdom disorganization and endocytosis via PKC activation may, therefore, contribute to desensitization of the light-adapted photoreceptor.

A molecular pathway for light-dependent photoreceptor apoptosis in Drosophila

Light-induced photoreceptor apoptosis occurs in many forms of inherited retinal degeneration resulting in blindness in both vertebrates and invertebrates. Though mutations in several photoreceptor signaling proteins have been implicated in triggering this process, the molecular events relating light activation of rhodopsin to photoreceptor death are yet unclear. Here, we uncover a pathway by which activation of rhodopsin in Drosophila mediates apoptosis through a G protein-independent mechanism. This process involves the formation of membrane complexes of phosphorylated, activated rhodopsin and its inhibitory protein arrestin, and subsequent clathrin-dependent endocytosis of these complexes into a cytoplasmic compartment. Together, these data define the proapoptotic molecules in Drosophila photoreceptors and indicate a novel signaling pathway for light-activated rhodopsin molecules in control of photoreceptor viability.

Vitamin A, visual pigments, and visual receptors in Drosophila

The fly visual system has served for decades as a model for receptor spectral multiplicity and vitamin A utilization. A diverse armamentarium of structural techniques has dovetailed with convenient electrophysiology, photochemistry, genetics, and molecular biology in Drosophila to facilitate recent progress, which is reviewed here. New data are also presented. Ultrastructure of retinula cells of carotenoid-deprived flies shows that organelles associated with protein biosynthesis, i.e., rough endoplasmic reticulum and Golgi apparatus, are present, while organelles associated with rhabdomere turnover, i.e., multivesicular bodies (MVBs), are rare. Ultrastructure and morphometry suggest that retinoic acid-rearing stimulates membrane export and rhabdomere buildup, even though functional rhodopsin is missing. Confocal microscopy suggests that RH4, one of the ultraviolet rhodopsins, may reside in the previously-described pale fluorescent R7 cells with RH3 in the yellow fluorescent R7 cells.

Carotenoid replacement in Drosophila: freeze-fracture electron microscopy

Because of the consequent lack of photopigment chromophore, carotenoid/ retinoid (vitamin A) deprivation during the larval period of Drosophila leads to decreased rhodopsin in adult photoreceptors. Decreased density of P-face particles in photoreceptor membrane (rhabdomeric microvilli) is a prominent ultrastructural feature of this rhodopsin deficiency. When adults are fed carotenoid, the rhabdomeric P-face particle density-which reflects the concentration of rhodopsin-increases halfway to the replete control level during the first 12 hours, and is fully restored by 2 days. Based on freeze-fracture replicas, there is a continuity of membrane between rhabdomeric microvilli and the parent retinula cell. That confluence is relevant to turnover of photoreceptive membrane. Microvillar and retinula cell P-face particle densities covary. The relevance of the demonstration of rapid recovery from chromophore depletion is discussed in relation to hypotheses that the chromophore and/or related retinoids regulate opsin gene transcription, and/or post-translational processing and deployment from the endoplasmic reticulum to the rhabdomere.

Daily and circadian rhythms of synaptic frequency in the first visual neuropile of the housefly's (Musca domestica L.) optic lobe

Photoreceptors of the fly's compound eye generally show no very obvious daily or circadian rhythms, a lack which prompted us to examine whether their function might be regulated not in the retina, but at the site of transmission in the first visual neuropile, or lamina. Here, photoreceptor terminals (R1-R6) are reciprocally interconnected with one class of lamina monopolar cell, L2: L2 receives input from R1-R6 at so-called tetrad synapses, and in turn is presynaptic to R1-R6 at feedback synapses. We have calculated the mean frequencies of these synaptic profiles in electron micrographs of single lamina sections. L2 feedback synapses were more numerous at night than during the day, whereas the number of tetrads showed only small modulations between day and night. These changes persisted amongst feedback synapses in flies held in constant darkness, and are thus circadian. In contrast to the slow modulations during a 24 h cycle, the number of L2 feedback synapses after 1 h light pulse in flies held in constant darkness showed no clear change, whereas it increased the number of tetrad profiles. These findings support the occurrence of cyclical daily and circadian changes amongst the two lamina synaptic populations, with tetrads showing rather weak modulations in frequency, but more pronounced responses to the light pulse than feedback synapses.

Localization of subrhabdomeric haemolymph lacunae in the retina of Drosophila melanogaster and Calliphora erythrocephala

The haemolymph of flies and other insects contains tyrosinase (EC 1.14.18.1), the rate-limiting enzyme in mammalian melanogenesis. After incubation with 5 mM L-DOPA for several hours the endogenous tyrosinase of the haemolymph forms an electron dense reaction product. This method was used to localize spaces in the retina of the wild type and the white (w) mutant of Drosophila melanogaster that are filled with haemolymph. A network of subrhabdomeric haemolymph lacunae was found. Moreover it was found that these haemolymph lacunae also form extensions into the photoreceptor cells and are connected with small haemolymph lacunae that cross the retinal basement membrane. In a second set of experiments L-DOPA was injected into the thoraces of Calliphora erythrocephala. Half-an-hour after injections the flies were killed and the eyes were embedded for electron microscopy. The small molecule of DOPA or its product dopachrome, surprisingly, penetrated the retinal basement membrane and reached the subrhabdomeric haemolymph lacunae and the ommatidial cavity.

Atypical granules in the eyes of the white mutant of Drosophila melanogaster are lysosome-related organelles

In the pigment cells of the white mutant of Drosophila melanogaster, as described earlier, two types of abnormal granules are found by conventional electron microscopy. However, both types of abnormal granules, in addition to those in pigment cell invaginations, are also present in the cytoplasm of the photoreceptor cells. Three enzymes (acid phosphatase, peroxidase, and tyrosinase) are localized within the eyes of wild type and white mutant Drosophila melanogaster by electron microscopy. Peroxidase activity is present in lamellar bodies close to the rhabdomeral microvilli of both fly types. However the organelles containing peroxidase activity are 6-fold more frequent in the wild type than in the mutant. Acid phosphatase is present in lamellar bodies between and at the bases of the rhabdomeral microvilli of the wild type, as well as in ommochrome granules of the photoreceptor cells. In the white mutant, however, acid phosphatase was located in electron lucent vacuoles in the cytoplasm of the receptor cells. These acid phosphatase-positive vacuoles also contained both types of abnormal granules. The latter result indicates that abnormal granules in the receptor cells originate from lysosomal degradation and that targeting of lysosomal enzymes is altered in the white mutant. Due to the tyrosinase activity in the hemolymph of flies, the extracellular spaces are electron dense after DOPA incubation. Since some abnormal granules within the photoreceptor cells are not surrounded by an extracellular space, they can be assumed to originate within the photoreceptor cells.

Phospholipids in Drosophila heads: effects of visual mutants and phototransduction manipulations

A procedure was developed to label phospholipids in Drosophila heads by feeding radioactive phosphate (32Pi). High-performance thin-layer chromatography showed label incorporation into various phospholipids. After 24 h of feeding, major phospholipids labeled were phosphatidylethanolamine (PE), 47%; phosphatidylcholine (PC), 24%; and phosphatidylinositol (PI), 12%. Drosophila heads have virtually no sphingomyelin as compared with mammalian tissues. Notable label was in ethanolamine plasmalogen, lysophosphatidylethanolamine, lysophosphatidylcholine and lysophosphatidylinositol. Less than 1% of the total label was in phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. Other lipids labeled included phosphatidylserine, phosphatidic acid and some unidentified lipids. A time course (3-36 h) study revealed a gradual decrease in proportion of labeled PI, an increase in proportion of labeled PC and no obvious change in labeled PE. There were no significant differences in phospholipid labeling comparing the no receptor potential (norpA) visual mutant and wild type under light vs. dark conditions. However, overall 32P labeling was higher in the wild type fed in the light as compared to the dark and to norpA either in light or dark. This suggests that functional vision facilitates incorporation of label. Differences in phospholipid labeling were observed between young and aged flies, particularly in lysophospholipids and poly-PI, implicating phospholipase A2 function in recycling. v Manipulations such as the outer rhabdomeres absent and eyes absent mutants and carotenoid deprivation failed to yield notable differences in phospholipid labeling pattern, suggesting that phospholipids important to vision may constitute only a minor portion of the total labeled pool in the head.

Electrophysiological sensitivity of carotenoid deficient and replaced Drosophila

R1-6 dominated electroretinographic (ERG) spectral sensitivities were determined as a function of days posteclosion from carotenoid deprived and replaced white-eyed Drosophila. The sensitivity of flies deprived from egg to adult waxed (about 1.5 log units by day 3), and then waned gradually from 3-11 days (over 2 log units by day 11). Carotenoid replacement (feeding nothing but carrot juice) effected recovery to near the replete controls' level in about 1 day throughout (tested at 0, 4, and 11 days). The normal yellow cornmeal-agar-molasses-brewers yeast fly food (in our laboratory, supplemented with beta-carotene) renders a slower recovery (requiring 7-9 days) since it is a medium designed largely for larval growth. Placing replete adults on deprivational medium did not create a deprivational syndrome in over 11 days. At 3-7 days, deprived flies reared and maintained in constant darkness had substantially enhanced sensitivity, beyond the 1.5 log unit increment already described for cyclic light rearing conditions. All spectral analyses are consistent with the ultraviolet (UV) sensitization of the blue (480 nm) rhodopsin by a replacement-dependent retinoid including two unexpected findings: (1) sensitivity recovery with carrot juice was so fast that the UV peak was already high at 6 h; and (2) the waxing of the deprived fly's sensitivity in dark rearing was so great that the UV peak was present at 4-7 days.

Visual receptor cycle in normal and period mutant Drosophila: microspectrophotometry, electrophysiology, and ultrastructural morphometry

Visual pigment, sensitivity, and rhabdomere size were measured throughout a 12-h light/12-h dark cycle in Drosophila. Visual pigment and sensitivity were measured during subsequent constant darkness [dark/dark (D/D)]. MSP (microspectrophotometry) and the ERG (electroretinogram) revealed a cycling of visual pigment and sensitivity, respectively. A visual pigment decrease of 40% was noted at 4 h after light onset that recovered 2-4 h later in white-eyed (otherwise wild-type, w per+) flies. The ERG sensitivity [in w per+ flies in light/dark (L/D)] decreased by 75% at 4 h after light onset, more than expected if mediated by visual pigment (MSP) changes alone. ERG sensitivity begins decreasing 8 h before light onset while decreases in visual pigment begin 2 h after light onset. These cycles continue in constant darkness (D/D), suggesting a circadian rhythm. White-eyed period (per) mutants show similar cycles of visual pigment level and sensitivity in L/D; per's alterations, if any on the D/D cycles were subtle. The cross-sectional areas of rhabdomeres in w per+ were measured using electron micrographic (EM) morphometry. Area changed little through the L/D cycle.