Molecular genetics of retinal degeneration: A Drosophila perspective

Differential activation of rhodopsin triggers distinct endocytic trafficking and recycling in vivo via differential phosphorylation

Activated GPCRs are phosphorylated and internalized mostly via clathrin-mediated endocytosis (CME), which are then sorted for recycling or degradation. We investigated how differential activation of the same GPCR affects its endocytic trafficking in vivo using rhodopsin as a model in pupal photoreceptors of flies expressing mCherry-tagged rhodopsin 1 (Rh1-mC) or GFP-tagged arrestin 1 (Arr1-GFP). Upon blue light stimulation, activated Rh1 recruited Arr1-GFP to the rhabdomere, which became co-internalized and accumulated in cytoplasmic vesicles of photoreceptors. This internalization was eliminated in shits1 mutants affecting dynamin. Moreover, it was blocked by either rdgA or rdgB mutations affecting the PIP2 biosynthesis. Together, the blue light-initiated internalization of Rh1 and Arr1 belongs to CME. Green light stimulation also triggered the internalization and accumulation of activated Rh1-mC in the cytoplasm but with faster kinetics. Importantly, Arr1-GFP was also recruited to the rhabdomere but not co-internalized with Rh1-mC. This endocytosis was not affected in shits1 nor rdgA mutants, indicating it is not CME. We explored the fate of internalized Rh1-mC following CME and observed it remained in cytoplasmic vesicles following 30 min of dark adaptation. In contrast, in the non-CME Rh1-mC appeared readily recycled back to the rhabdomere within five min of dark treatment. This faster recycling may be regulated by rhodopsin phosphatase, RdgC. Together, we demonstrate two distinct endocytic and recycling mechanisms of Rh1 via two light stimulations. It appears that each stimulation triggers a distinct conformation leading to different phosphorylation patterns of Rh1 capable of recruiting Arr1 to rhabdomeres. However, a more stable interaction leads to the co-internalization of Arr1 that orchestrates CME. A stronger Arr1 association appears to impede the recycling of the phosphorylated Rh1 by preventing the recruitment of RdgC. We conclude that conformations of activated rhodopsin determine the downstream outputs upon phosphorylation that confers differential protein-protein interactions.

Phototactic preference and its genetic basis in the planulae of the colonial Hydrozoan Hydractinia symbiolongicarpus

Background

Marine organisms with sessile adults commonly possess motile larval stages that make settlement decisions based on integrating environmental sensory cues. Phototaxis, the movement toward or away from light, is a common behavioral characteristic of aquatic and marine metazoan larvae, and of algae, protists, and fungi. In cnidarians, behavioral genomic investigations of motile planulae larvae have been conducted in anthozoans (corals and sea anemones) and scyphozoans (true jellyfish), but such studies are presently lacking in hydrozoans. Here, we examined the behavioral genomics of phototaxis in planulae of the hydrozoan Hydractinia symbiolongicarpus.

Results

A behavioral phototaxis study of day 3 planulae indicated preferential phototaxis to green (523 nm) and blue (470 nm) wavelengths of light, but not red (625 nm) wavelengths. A developmental transcriptome study where planula larvae were collected from four developmental time points for RNA-seq revealed that many genes critical to the physiology and development of ciliary photosensory systems are dynamically expressed in planula development and correspond to the expression of phototactic behavior. Microscopical investigations using immunohistochemistry and in situ hybridization demonstrated that several transcripts with predicted function in photoreceptors, including cnidops class opsin, CNG ion channel, and CRX-like transcription factor, localize to ciliated bipolar sensory neurons of the aboral sensory neural plexus, which is associated with the direction of phototaxis and the site of settlement.

Conclusions

The phototactic preference displayed by planulae is consistent with the shallow sandy marine habitats they experience in nature. Our genomic investigations add further evidence of similarities between cnidops-mediated photoreceptors of hydrozoans and other cnidarians and ciliary photoreceptors as found in the eyes of humans and other bilaterians, suggesting aspects of their shared evolutionary history.

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.

Dietary restriction and the transcription factor clock delay eye aging to extend lifespan in Drosophila Melanogaster

Many vital processes in the eye are under circadian regulation, and circadian dysfunction has emerged as a potential driver of eye aging. Dietary restriction is one of the most robust lifespan-extending therapies and amplifies circadian rhythms with age. Herein, we demonstrate that dietary restriction extends lifespan in Drosophila melanogaster by promoting circadian homeostatic processes that protect the visual system from age- and light-associated damage. Altering the positive limb core molecular clock transcription factor, CLOCK, or CLOCK-output genes, accelerates visual senescence, induces a systemic immune response, and shortens lifespan. Flies subjected to dietary restriction are protected from the lifespan-shortening effects of photoreceptor activation. Inversely, photoreceptor inactivation, achieved via mutating rhodopsin or housing flies in constant darkness, primarily extends the lifespan of flies reared on a high-nutrient diet. Our findings establish the eye as a diet-sensitive modulator of lifespan and indicates that vision is an antagonistically pleiotropic process that contributes to organismal aging.

Aging and Light Stress Result in Overlapping and Unique Gene Expression Changes in Photoreceptors

Advanced age is one of the leading risk factors for vision loss and eye disease. Photoreceptors are the primary sensory neurons of the eye. The extended photoreceptor cell lifespan, in addition to its high metabolic needs due to phototransduction, makes it critical for these neurons to continually respond to the stresses associated with aging by mounting an appropriate gene expression response. Here, we sought to untangle the more general neuronal age-dependent transcriptional signature of photoreceptors with that induced by light stress. To do this, we aged flies or exposed them to various durations of blue light, followed by photoreceptor nuclei-specific transcriptome profiling. Using this approach, we identified genes that are both common and uniquely regulated by aging and light induced stress. Whereas both age and blue light induce expression of DNA repair genes and a neuronal-specific signature of death, both conditions result in downregulation of phototransduction. Interestingly, blue light uniquely induced genes that directly counteract the overactivation of the phototransduction signaling cascade. Lastly, unique gene expression changes in aging photoreceptors included the downregulation of genes involved in membrane potential homeostasis and mitochondrial function, as well as the upregulation of immune response genes. We propose that light stress contributes to the aging transcriptome of photoreceptors, but that there are also other environmental or intrinsic factors involved in age-associated photoreceptor gene expression signatures.

Application of Fluorescent Proteins for Functional Dissection of the Drosophila Visual System

The Drosophila eye has been used extensively to study numerous aspects of biological systems, for example, spatio-temporal regulation of differentiation, visual signal transduction, protein trafficking and neurodegeneration. Right from the advent of fluorescent proteins (FPs) near the end of the millennium, heterologously expressed fusion proteins comprising FPs have been applied in Drosophila vision research not only for subcellular localization of proteins but also for genetic screens and analysis of photoreceptor function. Here, we summarize applications for FPs used in the Drosophila eye as part of genetic screens, to study rhodopsin expression patterns, subcellular protein localization, membrane protein transport or as genetically encoded biosensors for Ca2+ and phospholipids in vivo. We also discuss recently developed FPs that are suitable for super-resolution or correlative light and electron microscopy (CLEM) approaches. Illustrating the possibilities provided by using FPs in Drosophila photoreceptors may aid research in other sensory or neuronal systems that have not yet been studied as well as the Drosophila eye.

Identification of Genes Required for Apical Protein Trafficking in Drosophila Photoreceptor Cells

Drosophila melanogaster photoreceptor cells are highly polarized epithelial cells. Their apical membrane is further subdivided into the stalk membrane and the light-sensing rhabdomere. The photo-pigment Rhodopsin1 (Rh1) localizes to the rhabdomere, whereas the apical determinant Crumbs (Crb) is enriched at the stalk membrane. The proteoglycan Eyes shut (Eys) is secreted through the apical membrane into an inter-rhabdomeral space. Rh1, Crb, and Eys are essential for the development of photoreceptor cells, normal vision, and photoreceptor cell survival. Human orthologs of all three proteins have been linked to retinal degenerative diseases. Here, we describe an RNAi-based screen examining the importance of 237 trafficking-related genes in apical trafficking of Eys, Rh1, and Crb. We found 28 genes that have an effect on the localization and/or levels of these apical proteins and analyzed several factors in more detail. We show that the Arf GEF protein Sec71 is required for biosynthetic traffic of both apical and basolateral proteins, that the exocyst complex and the microtubule-based motor proteins dynein and kinesin promote the secretion of Eys and Rh1, and that Syntaxin 7/Avalanche controls the endocytosis of Rh1, Eys, and Crb.

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.

Drosophila melanogaster White Mutant w1118 Undergo Retinal Degeneration

Key scientific discoveries have resulted from genetic studies of Drosophila melanogaster, using a multitude of transgenic fly strains, the majority of which are constructed in a genetic background containing mutations in the white gene. Here we report that white mutant flies from w1118 strain undergo retinal degeneration. We observed also that w1118 mutants have progressive loss of climbing ability, shortened life span, as well as impaired resistance to various forms of stress. Retinal degeneration was abolished by transgenic expression of mini-white+ in the white null background w1118 . We conclude that beyond the classical eye-color phenotype, mutations in Drosophila white gene could impair several biological functions affecting parameters like mobility, life span and stress tolerance. Consequently, we suggest caution and attentiveness during the interpretation of old experiments employing white mutant flies and when planning new ones, especially within the research field of neurodegeneration and neuroprotection. We also encourage that the use of w1118 strain as a wild-type control should be avoided.

Drosophila Rhodopsin 7 can partially replace the structural role of Rhodopsin 1, but not its physiological function

Rhodopsin 7 (Rh7), a new invertebrate Rhodopsin gene, was discovered in the genome of Drosophila melanogaster in 2000 and thought to encode for a functional Rhodopsin protein. Indeed, Rh7 exhibits most hallmarks of the known Rhodopsins, except for the G-protein-activating QAKK motif in the third cytoplasmic loop that is absent in Rh7. Here, we show that Rh7 can partially substitute Rh1 in the outer receptor cells (R1–6) for rhabdomere maintenance, but that it cannot activate the phototransduction cascade in these cells. This speaks against a role of Rh7 as photopigment in R1–6, but does not exclude that it works in the inner photoreceptor cells.

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.

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.

Loss of Na(+)/K(+)-ATPase in Drosophila photoreceptors leads to blindness and age-dependent neurodegeneration

The activity of Na(+)/K(+)-ATPase establishes transmembrane ion gradients and is essential to cell function and survival. Either dysregulation or deficiency of neuronal Na(+)/K(+)-ATPase has been implicated in the pathogenesis of many neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease and rapid-onset dystonia Parkinsonism. However, genetic evidence that directly links neuronal Na(+)/K(+)-ATPase deficiency to in vivo neurodegeneration has been lacking. In this study, we use Drosophila photoreceptors to investigate the cell-autonomous effects of neuronal Na(+)/K(+) ATPase. Loss of ATPα, an α subunit of Na(+)/K(+)-ATPase, in photoreceptors through UAS/Gal4-mediated RNAi eliminated the light-triggered depolarization of the photoreceptors, rendering the fly virtually blind in behavioral assays. Intracellular recordings indicated that ATPα knockdown photoreceptors were already depolarized in the dark, which was due to a loss of intracellular K(+). Importantly, ATPα knockdown resulted in the degeneration of photoreceptors in older flies. This degeneration was independent of light and showed characteristics of apoptotic/hybrid cell death as observed via electron microscopy analysis. Loss of Nrv3, a Na(+)/K(+)-ATPase β subunit, partially reproduced the signaling and degenerative defects observed in ATPα knockdown flies. Thus, the loss of Na(+)/K(+)-ATPase not only eradicates visual function but also causes age-dependent degeneration in photoreceptors, confirming the link between neuronal Na(+)/K(+) ATPase deficiency and in vivo neurodegeneration. This work also establishes Drosophila photoreceptors as a genetic model for studying the cell-autonomous mechanisms underlying neuronal Na(+)/K(+) ATPase deficiency-mediated neurodegeneration.

Drosophila king tubby (ktub) mediates light-induced rhodopsin endocytosis and retinal degeneration

Background

The tubby (tub) and tubby-like protein (tulp) genes encode a small family of proteins found in many organisms. Previous studies have shown that TUB and TULP genes in mammalian involve in obesity, neural development, and retinal degeneration. The purpose of this study was to investigate the role of Drosophila king tubby (ktub) in rhodopsin 1 (Rh1) endocytosis and retinal degeneration upon light stimulation.

Results

Drosophila ktub mutants were generated using imprecise excision. Wild type and mutant flies were raised in dark or constant light conditions. After a period of light stimulation, retinas were dissected, fixed and stained with anti-Rh1 antibody to reveal Rh1 endocytosis. Confocal and transmission electron microscope were used to examine the retinal degeneration. Immunocytochemical analysis shows that Ktub is expressed in the rhabdomere domain under dark conditions. When flies receive light stimulation, the Ktub translocates from the rhabdomere to the cytoplasm and the nucleus of the photoreceptor cells. Wild type photoreceptors form Rh1-immunopositive large vesicles (RLVs) shortly after light stimulation. In light-induced ktub mutants, the majority of Rh1 remains at the rhabdomere, and only a few RLVs appear in the cytoplasm of photoreceptor cells. Mutation of norpA allele causes massive Rh1 endocytosis in light stimulation. In ktub and norpA double mutants, however, Rh1 endocytosis is blocked under light stimulation. This study also shows that ktub and norpA double mutants rescue the light-induced norpA retinal degeneration. Deletion constructs further demonstrate that the Tubby domain of the Ktub protein participates in an important role in Rh1 endocytosis.

Conclusions

The results in this study delimit the novel function of Ktub in Rh1 endocytosis and retinal degeneration.

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.

Drosophila fatty acid transport protein regulates rhodopsin-1 metabolism and is required for photoreceptor neuron survival

Tight regulation of the visual response is essential for photoreceptor function and survival. Visual response dysregulation often leads to photoreceptor cell degeneration, but the causes of such cell death are not well understood. In this study, we investigated a fatty acid transport protein (fatp) null mutation that caused adult-onset and progressive photoreceptor cell death. Consistent with fatp having a role in the retina, we showed that fatp is expressed in adult photoreceptors and accessory cells and that its re-expression in photoreceptors rescued photoreceptor viability in fatp mutants. The visual response in young fatp-mutant flies was abnormal with elevated electroretinogram amplitudes associated with high levels of Rhodopsin-1 (Rh1). Reducing Rh1 levels in rh1 mutants or depriving flies of vitamin A rescued photoreceptor cell death in fatp mutant flies. Our results indicate that fatp promotes photoreceptor survival by regulating Rh1 abundance.