Phototransduction in Drosophila melanogaster

Deep Diversity: Extensive Variation in the Components of Complex Visual Systems across Animals

Understanding the molecular underpinnings of the evolution of complex (multi-part) systems is a fundamental topic in biology. One unanswered question is to what the extent do similar or different genes and regulatory interactions underlie similar complex systems across species? Animal eyes and phototransduction (light detection) are outstanding systems to investigate this question because some of the genetics underlying these traits are well characterized in model organisms. However, comparative studies using non-model organisms are also necessary to understand the diversity and evolution of these traits. Here, we compare the characteristics of photoreceptor cells, opsins, and phototransduction cascades in diverse taxa, with a particular focus on cnidarians. In contrast to the common theme of deep homology, whereby similar traits develop mainly using homologous genes, comparisons of visual systems, especially in non-model organisms, are beginning to highlight a "deep diversity" of underlying components, illustrating how variation can underlie similar complex systems across taxa. Although using candidate genes from model organisms across diversity was a good starting point to understand the evolution of complex systems, unbiased genome-wide comparisons and subsequent functional validation will be necessary to uncover unique genes that comprise the complex systems of non-model groups to better understand biodiversity and its evolution.

Sensory TRP Channels in Three Dimensions

Transient receptor potential (TRP) ion channels are sophisticated signaling machines that detect a wide variety of environmental and physiological signals. Every cell in the body expresses one or more members of the extended TRP channel family, which consists of over 30 subtypes, each likely possessing distinct pharmacological, biophysical, and/or structural attributes. While the function of some TRP subtypes remains enigmatic, those involved in sensory signaling are perhaps best characterized and have served as models for understanding how these excitatory ion channels serve as polymodal signal integrators. With the recent resolution revolution in cryo-electron microscopy, these and other TRP channel subtypes are now yielding their secrets to detailed atomic analysis, which is beginning to reveal structural underpinnings of stimulus detection and gating, ion permeation, and allosteric mechanisms governing signal integration. These insights are providing a framework for designing and evaluating modality-specific pharmacological agents for treating sensory and other TRP channel-associated disorders.

C. elegans PEZO-1 is a mechanosensitive ion channel involved in food sensation

PIEZO channels are force sensors essential for physiological processes, including baroreception and proprioception. The Caenorhabditis elegans genome encodes an orthologue gene of the Piezo family, pezo-1, which is expressed in several tissues, including the pharynx. This myogenic pump is an essential component of the C. elegans alimentary canal, whose contraction and relaxation are modulated by mechanical stimulation elicited by food content. Whether pezo-1 encodes a mechanosensitive ion channel and contributes to pharyngeal function remains unknown. Here, we leverage genome editing, genetics, microfluidics, and electropharyngeogram recording to establish that pezo-1 is expressed in the pharynx, including in a proprioceptive-like neuron, and regulates pharyngeal function. Knockout (KO) and gain-of-function (GOF) mutants reveal that pezo-1 is involved in fine-tuning pharyngeal pumping frequency, as well as sensing osmolarity and food mechanical properties. Using pressure-clamp experiments in primary C. elegans embryo cultures, we determine that pezo-1 KO cells do not display mechanosensitive currents, whereas cells expressing wild-type or GOF PEZO-1 exhibit mechanosensitivity. Moreover, infecting the Spodoptera frugiperda cell line with a baculovirus containing the G-isoform of pezo-1 (among the longest isoforms) demonstrates that pezo-1 encodes a mechanosensitive channel. Our findings reveal that pezo-1 is a mechanosensitive ion channel that regulates food sensation in worms.

Melanopsin phototransduction: beyond canonical cascades

Melanopsin is a visual pigment that is expressed in a small subset of intrinsically photosensitive retinal ganglion cells (ipRGCs). It is involved in regulating non-image forming visual behaviors, such as circadian photoentrainment and the pupillary light reflex, while also playing a role in many aspects of image-forming vision, such as contrast sensitivity. Melanopsin was initially discovered in the melanophores of the skin of the frog Xenopus, and subsequently found in a subset of ganglion cells in rat, mouse and primate retinas. ipRGCs were initially thought to be a single retinal ganglion cell population, and melanopsin was thought to activate a single, invertebrate-like Gq/transient receptor potential canonical (TRPC)-based phototransduction cascade within these cells. However, in the 20 years since the discovery of melanopsin, our knowledge of this visual pigment and ipRGCs has expanded dramatically. Six ipRGC subtypes have now been identified in the mouse, each with unique morphological, physiological and functional properties. Multiple subtypes have also been identified in other species, suggesting that this cell type diversity is a general feature of the ipRGC system. This diversity has led to a renewed interest in melanopsin phototransduction that may not follow the canonical Gq/TRPC cascade in the mouse or in the plethora of other organisms that express the melanopsin photopigment. In this Review, we discuss recent findings and discoveries that have challenged the prevailing view of melanopsin phototransduction as a single pathway that influences solely non-image forming functions.

Stage- and sex-specific transcriptome analyses reveal distinctive sensory gene expression patterns in a butterfly

Background

Animal behavior is largely driven by the information that animals are able to extract and process from their environment. However, the function and organization of sensory systems often change throughout ontogeny, particularly in animals that undergo indirect development. As an initial step toward investigating these ontogenetic changes at the molecular level, we characterized the sensory gene repertoire and examined the expression profiles of genes linked to vision and chemosensation in two life stages of an insect that goes through metamorphosis, the butterfly Bicyclus anynana.

Results

Using RNA-seq, we compared gene expression in the heads of late fifth instar larvae and newly eclosed adults that were reared under identical conditions. Over 50 % of all expressed genes were differentially expressed between the two developmental stages, with 4,036 genes upregulated in larval heads and 4,348 genes upregulated in adult heads. In larvae, upregulated vision-related genes were biased toward those involved with eye development, while phototransduction genes dominated the vision genes that were upregulated in adults. Moreover, the majority of the chemosensory genes we identified in the B. anynana genome were differentially expressed between larvae and adults, several of which share homology with genes linked to pheromone detection, host plant recognition, and foraging in other species of Lepidoptera.

Conclusions

These results revealed promising candidates for furthering our understanding of sensory processing and behavior in the disparate developmental stages of butterflies and other animals that undergo metamorphosis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07819-4.

Molecular Evolution of Phototransduction Pathway Genes in Nocturnal and Diurnal Fireflies (Coleoptera: Lampyridae)

Most organisms are dependent on sensory cues from their environment for survival and reproduction. Fireflies (Coleoptera: Lampyridae) represent an ideal system for studying sensory niche adaptation due to many species relying on bioluminescent communication; as well as a diversity of ecologies. Here; using transcriptomics; we examine the phototransduction pathway in this non-model organism; and provide some of the first evidence for positive selection in the phototransduction pathway beyond opsins in beetles. Evidence for gene duplications within Lampyridae are found in inactivation no afterpotential C and inactivation no afterpotential D. We also find strong support for positive selection in arrestin-2; inactivation no afterpotential D; and transient receptor potential-like; with weak support for positive selection in guanine nucleotide-binding protein G(q) subunit alpha and neither inactivation nor afterpotential C. Taken with other recent work in flies; butterflies; and moths; this represents an exciting new avenue of study as we seek to further understand diversification and constraint on the phototransduction pathway in light of organism ecology.

Polymodal Functionality of C. elegans OLL Neurons in Mechanosensation and Thermosensation

Sensory modalities are important for survival but the molecular mechanisms remain challenging due to the polymodal functionality of sensory neurons. Here, we report the C. elegans outer labial lateral (OLL) sensilla sensory neurons respond to touch and cold. Mechanosensation of OLL neurons resulted in cell-autonomous mechanically-evoked Ca²⁺ transients and rapidly-adapting mechanoreceptor currents with a very short latency. Mechanotransduction of OLL neurons might be carried by a novel Na⁺ conductance channel, which is insensitive to amiloride. The bona fide mechano-gated Na⁺-selective degenerin/epithelial Na⁺ channels, TRP-4, TMC, and Piezo proteins are not involved in this mechanosensation. Interestingly, OLL neurons also mediated cold but not warm responses in a cell-autonomous manner. We further showed that the cold response of OLL neurons is not mediated by the cold receptor TRPA-1 or the temperature-sensitive glutamate receptor GLR-3. Thus, we propose the polymodal functionality of OLL neurons in mechanosensation and cold sensation.

Signaling roles of phosphoinositides in the retina

The field of phosphoinositide signaling has expanded significantly in recent years. Phosphoinositides (also known as phosphatidylinositol phosphates or PIPs) are universal signaling molecules that directly interact with membrane proteins or with cytosolic proteins containing domains that directly bind phosphoinositides and are recruited to cell membranes. Through the activities of phosphoinositide kinases and phosphoinositide phosphatases, seven distinct phosphoinositide lipid molecules are formed from the parent molecule, phosphatidylinositol. PIP signals regulate a wide range of cellular functions, including cytoskeletal assembly, membrane budding and fusion, ciliogenesis, vesicular transport, and signal transduction. Given the many excellent reviews on phosphoinositide kinases, phosphoinositide phosphatases, and PIPs in general, in this review, we discuss recent studies and advances in PIP lipid signaling in the retina. We specifically focus on PIP lipids from vertebrate (e.g., bovine, rat, mouse, toad, and zebrafish) and invertebrate (e.g., Drosophila, horseshoe crab, and squid) retinas. We also discuss the importance of PIPs revealed from animal models and human diseases, and methods to study PIP levels both in vitro and in vivo. We propose that future studies should investigate the function and mechanism of activation of PIP-modifying enzymes/phosphatases and further unravel PIP regulation and function in the different cell types of the retina.

Influence of light and temperature cycles on the expression of circadian clock genes in the mussel Mytilus edulis

Clock genes and environmental cues regulate essential biological rhythms. The blue mussel, Mytilus edulis, is an ecologically and economically important intertidal bivalve undergoing seasonal reproductive rhythms. We previously identified seasonal expression differences in M. edulis clock genes. Herein, the effects of light/dark cycles, constant darkness, and daily temperature cycles on the circadian expression patterns of such genes are characterised. Clock genes Clk, Cry1, ROR/HR3, Per and Rev-erb/NR1D1, and Timeout-like, show significant mRNA expression variation, persisting in darkness indicating endogenous control. Rhythmic expression was apparent under diurnal temperature cycles in darkness for all except Rev-erb. Temperature cycles induced a significant expression difference in the non-circadian clock-associated gene aaNAT. Furthermore, Suppression Subtractive Hybridisation (SSH) was used to identify seasonal genes with potential links to molecular clock function and revealed numerous genes meriting further investigation. Understanding the relationship between environmental cues and molecular clocks is crucial in predicting the outcomes of environmental change on fundamental rhythmic processes.

Evolution of Phototransduction Genes in Lepidoptera

Vision is underpinned by phototransduction, a signaling cascade that converts light energy into an electrical signal. Among insects, phototransduction is best understood in Drosophila melanogaster. Comparison of D. melanogaster against three insect species found several phototransduction gene gains and losses, however, lepidopterans were not examined. Diurnal butterflies and nocturnal moths occupy different light environments and have distinct eye morphologies, which might impact the expression of their phototransduction genes. Here we investigated: 1) how phototransduction genes vary in gene gain or loss between D. melanogaster and Lepidoptera, and 2) variations in phototransduction genes between moths and butterflies. To test our prediction of phototransduction differences due to distinct visual ecologies, we used insect reference genomes, phylogenetics, and moth and butterfly head RNA-Seq and transcriptome data. As expected, most phototransduction genes were conserved between D. melanogaster and Lepidoptera, with some exceptions. Notably, we found two lepidopteran opsins lacking a D. melanogaster ortholog. Using antibodies we found that one of these opsins, a candidate retinochrome, which we refer to as unclassified opsin (UnRh), is expressed in the crystalline cone cells and the pigment cells of the butterfly, Heliconius melpomene. Our results also show that butterflies express similar amounts of trp and trpl channel mRNAs, whereas moths express ∼50× less trp, a potential adaptation to darkness. Our findings suggest that while many single-copy D. melanogaster phototransduction genes are conserved in lepidopterans, phototransduction gene expression differences exist between moths and butterflies that may be linked to their visual light environment.

Phototransduction Gene Expression and Evolution in Cave and Surface Crayfishes

In the absence of light in caves, animals have repeatedly evolved reduced eyes and visual systems. Whether the underlying genetic components remain intact in blind species remains unanswered across taxa. The freshwater crayfish have evolved to live in caves multiple times throughout their history; therefore, this system provides an opportunity to probe the genetic patterns and processes underlying repeated vision loss. Using transcriptomic data from the eyes of 14 species of cave and surface crayfishes, we identify the expression of 17 genes putatively related to visual phototransduction. We find a similarly complete repertoire of phototransduction gene families expressed in cave and surface species, but that the expression levels of those transcripts are consistently lower in cave species. We find statistical support for episodic positive selection, increased and decreased selection strength in caves, depending on the gene family. Analyses of gene expression evolution suggest convergent and possibly adaptive downregulation of these genes across eye-reduction events. Our results reveal a combination of evolutionary processes acting on the sequences and gene expression levels of vision-related genes underlying the loss of vision in caves.

Point process analysis of noise in early invertebrate vision

Noise is a prevalent and sometimes even dominant aspect of many biological processes. While many natural systems have adapted to attenuate or even usefully integrate noise, the variability it introduces often still delimits the achievable precision across biological functions. This is particularly so for visual phototransduction, the process responsible for converting photons of light into usable electrical signals (quantum bumps). Here, randomness of both the photon inputs (regarded as extrinsic noise) and the conversion process (intrinsic noise) are seen as two distinct, independent and significant limitations on visual reliability. Past research has attempted to quantify the relative effects of these noise sources by using approximate methods that do not fully account for the discrete, point process and time ordered nature of the problem. As a result the conclusions drawn from these different approaches have led to inconsistent expositions of phototransduction noise performance.

This paper provides a fresh and complete analysis of the relative impact of intrinsic and extrinsic noise in invertebrate phototransduction using minimum mean squared error reconstruction techniques based on Bayesian point process (Snyder) filters. An integrate-fire based algorithm is developed to reliably estimate photon times from quantum bumps and Snyder filters are then used to causally estimate random light intensities both at the front and back end of the phototransduction cascade. Comparison of these estimates reveals that the dominant noise source transitions from extrinsic to intrinsic as light intensity increases. By extending the filtering techniques to account for delays, it is further found that among the intrinsic noise components, which include bump latency (mean delay and jitter) and shape (amplitude and width) variance, it is the mean delay that is critical to noise performance. As the timeliness of visual information is important for real-time action, this delay could potentially limit the speed at which invertebrates can respond to stimuli. Consequently, if one wants to increase visual fidelity, reducing the photoconversion lag is much more important than improving the regularity of the electrical signal.

The invertebrate phototransduction system captures and converts environmental light inputs into electrical signals for use in later visual processing. Consequently, one would expect it to be optimised in some way to ensure that only a minimal amount of environmental information is lost during conversion. Confirming this requires an understanding and quantification of the performance limiting noise sources. Photons, which are inherently random and discrete, introduce extrinsic noise. The phototransduction cascade, which converts photons into electrical bumps possessing non-deterministic shapes and latencies, contributes intrinsic noise. Previous work on characterising the relative impact of all these sources did not account for the discrete, causal, point process nature of the problem and thus results were often inconclusive. Here we use non-linear Poisson process filtering to show that photon noise is dominant at low light intensity and cascade noise limiting at high intensity. Further, our analysis reveals that mean bump delay is the most deleterious aspect of the intrinsic noise. Our work emphasises a new approach to assessing sensory noise and provides the first complete description and evaluation of the relative impact of noise in phototransduction that does not rely on continuity, linearity or Gaussian approximations.

The planarian TRPA1 homolog mediates extraocular behavioral responses to near-ultraviolet light

Although light is most commonly thought of as a visual cue, many animals possess mechanisms to detect light outside of the eye for various functions, including predator avoidance, circadian rhythms, phototaxis and migration. Here we confirm that planarians (like Caenorhabditis elegans, leeches and Drosophila larvae) are capable of detecting and responding to light using extraocular photoreception. We found that, when either eyeless or decapitated worms were exposed to near-ultraviolet (near-UV) light, intense wild-type photophobic behaviors were still observed. Our data also revealed that behavioral responses to green wavelengths were mediated by ocular mechanisms, whereas near-UV responses were driven by extraocular mechanisms. As part of a candidate screen to uncover the genetic basis of extraocular photoreception in the planarian species Schmidtea mediterranea, we identified a potential role for a homolog of the transient receptor potential channel A1 (TRPA1) in mediating behavioral responses to extraocular light cues. RNA interference (RNAi) to Smed-TrpA resulted in worms that lacked extraocular photophobic responses to near-UV light, a mechanism previously only identified in Drosophila These data show that the planarian TRPA1 homolog is required for planarian extraocular-light avoidance and may represent a potential ancestral function of this gene. TRPA1 is an evolutionarily conserved detector of temperature and chemical irritants, including reactive oxygen species that are byproducts of UV-light exposure. Our results suggest that planarians possess extraocular photoreception and display an unconventional TRPA1-mediated photophobic response to near-UV light.

Bidirectional regulation of fragile X mental retardation protein phosphorylation controls rhodopsin homoeostasis

Homoeostatic regulation of the light sensor, rhodopsin, is critical for the maintenance of light sensitivity and survival of photoreceptors. The major fly rhodopsin, Rh1, undergoes light-induced endocytosis and degradation, but its protein and mRNA levels remain constant during light/dark cycles. It is not clear how translation of Rh1 is regulated. Here, we show that adult photoreceptors maintain a constant, abundant quantity of ninaE mRNA, which encodes Rh1. We demonstrate that the Fmr1 protein associates with ninaE mRNA and represses its translation. Further, light exposure triggers a calcium-dependent dephosphorylation of Fmr1, which relieves suppression of Rh1 translation. We demonstrate that Mts, the catalytic subunit of protein phosphatase 2A (PP2A), mediates light-induced Fmr1 dephosphorylation in a regulatory B subunit of PP2A (CKa)-dependent manner. Finally, we show that blocking light-induced Rh1 translation results in reduced light sensitivity. Our results reveal the molecular mechanism of Rh1 homoeostasis and physiological consequence of Rh1 dysregulation.

White - cGMP Interaction Promotes Fast Locomotor Recovery from Anoxia in Adult Drosophila

Increasing evidence indicates that the white (w) gene in Drosophila possesses extra-retinal functions in addition to its classical role in eye pigmentation. We have previously shown that w+ promotes fast and consistent locomotor recovery from anoxia, but how w+ modulates locomotor recovery is largely unknown. Here we show that in the absence of w+, several PDE mutants, especially cyclic guanosine monophosphate (cGMP)-specific PDE mutants, display wildtype-like fast locomotor recovery from anoxia, and that during the night time, locomotor recovery was light-sensitive in white-eyed mutant w1118, and light-insensitive in PDE mutants under w1118 background. Data indicate the involvement of cGMP in the modulation of recovery timing and presumably, light-evoked cGMP fluctuation is associated with light sensitivity of locomotor recovery. This was further supported by the observations that w-RNAi-induced delay of locomotor recovery was completely eliminated by upregulation of cGMP through multiple approaches, including PDE mutation, simultaneous overexpression of an atypical soluble guanylyl cyclase Gyc88E, or sildenafil feeding. Lastly, prolonged sildenafil feeding promoted fast locomotor recovery from anoxia in w1118. Taken together, these data suggest that a White-cGMP interaction modulates the timing of locomotor recovery from anoxia.

The use of in vivo, ex vivo, in vitro, computational models and volunteer studies in vision research and therapy, and their contribution to the Three Rs

Much is known about mammalian vision, and considerable progress has been achieved in treating many vision disorders, especially those due to changes in the eye, by using various therapeutic methods, including stem cell and gene therapy. While cells and tissues from the main parts of the eye and the visual cortex (VC) can be maintained in culture, and many computer models exist, the current non-animal approaches are severely limiting in the study of visual perception and retinotopic imaging. Some of the early studies with cats and non-human primates (NHPs) are controversial for animal welfare reasons and are of questionable clinical relevance, particularly with respect to the treatment of amblyopia. More recently, the UK Home Office records have shown that attention is now more focused on rodents, especially the mouse. This is likely to be due to the perceived need for genetically-altered animals, rather than to knowledge of the similarities and differences of vision in cats, NHPs and rodents, and the fact that the same techniques can be used for all of the species. We discuss the advantages and limitations of animal and non-animal methods for vision research, and assess their relative contributions to basic knowledge and clinical practice, as well as outlining the opportunities they offer for implementing the principles of the Three Rs (Replacement, Reduction and Refinement).

Reprint of "Mechanisms of lipid regulation and lipid gating in TRPC channels"

TRPC proteins form cation channels that integrate and relay cellular signals by mechanisms involving lipid recognition and lipid-dependent gating. The lipohilic/amphiphilic molecules that function as cellular activators or modulators of TRPC proteins span a wide range of chemical structures. In this context, cellular redox balance is likely linked to the lipid recognition/gating features of TRPC channels. Both classical ligand-protein interactions as well as indirect and promiscuous sensory mechanisms have been proposed. Some of the recognition processes are suggested to involve ancillary lipid-binding scaffolds or regulators as well as dynamic protein-protein interactions determined by bilayer architecture. A complex interplay of protein-protein and protein-lipid interactions is likely to govern the gating and/or plasma membrane recruitment of TRPC channels, thereby providing a distinguished platform for signal integration and coincident signal detection. Both the primary molecular event(s) of lipid recognition by TRPC channels as well as the transformation of these events into distinct gating movements is poorly understood at the molecular level, and it remains elusive whether lipid sensing in TRPCs is conferred to a distinct sensor domain. Recent structural information on the molecular action of lipophilic activators in distantly related members of the TRP superfamily encourages speculations on TRPC gating mechanisms involved in lipid recognition/gating. This review aims to provide an update on the current understanding of the lipid-dependent control of TRPC channels with focus on the TRPC lipid sensing, signal-integration hub and a short discussion of potential links to redox signaling.

Mechanisms of lipid regulation and lipid gating in TRPC channels

TRPC proteins form cation channels that integrate and relay cellular signals by mechanisms involving lipid recognition and lipid-dependent gating. The lipohilic/amphiphilic molecules that function as cellular activators or modulators of TRPC proteins span a wide range of chemical structures. In this context, cellular redox balance is likely linked to the lipid recognition/gating features of TRPC channels. Both classical ligand-protein interactions as well as indirect and promiscuous sensory mechanisms have been proposed. Some of the recognition processes are suggested to involve ancillary lipid-binding scaffolds or regulators as well as dynamic protein-protein interactions determined by bilayer architecture. A complex interplay of protein-protein and protein-lipid interactions is likely to govern the gating and/or plasma membrane recruitment of TRPC channels, thereby providing a distinguished platform for signal integration and coincident signal detection. Both the primary molecular event(s) of lipid recognition by TRPC channels as well as the transformation of these events into distinct gating movements is poorly understood at the molecular level, and it remains elusive whether lipid sensing in TRPCs is conferred to a distinct sensor domain. Recent structural information on the molecular action of lipophilic activators in distantly related members of the TRP superfamily encourages speculations on TRPC gating mechanisms involved in lipid recognition/gating. This review aims to provide an update on the current understanding of the lipid-dependent control of TRPC channels with focus on the TRPC lipid sensing, signal-integration hub and a short discussion of potential links to redox signaling.

Diverse Distributions of Extraocular Opsins in Crustaceans, Cephalopods, and Fish

Non-visual and extraocular photoreceptors are common among animals, but current understanding linking molecular pathways to physiological function of these receptors is lacking. Opsin diversity in extraocular tissues suggests that many putative extraocular photoreceptors utilize the "visual" phototransduction pathway-the same phototransduction pathway as photoreceptors within the retina dedicated to light detection for image sensing. Here, we provide a brief overview of the current understanding of non-visual and extraocular photoreceptors, and contribute a synopsis of several novel putative extraocular photoreceptors that use both visual and non-visual phototransduction pathways. Crayfish, cephalopods, and flat fish express opsins in diverse tissues, suggesting the presence of extraocular photoreceptors. In most cases, we find that these animals use the same phototransduction pathway that is utilized in the retinas for image-formation. However, we also find the presence of non-visual phototransduction components in the skin of flounders. Our evidence suggests that extraocular photoreceptors may employ a number of phototransduction pathways that do not appear to correlate with purpose or location of the photoreceptor.

Current advances in invertebrate vision: insights from patch-clamp studies of photoreceptors in apposition eyes

Traditional electrophysiological research on invertebrate photoreceptors has been conducted in vivo, using intracellular recordings from intact compound eyes. The only exception used to be Drosophila melanogaster, which was exhaustively studied by both intracellular recording and patch-clamp methods. Recently, several patch-clamp studies have provided new information on the biophysical properties of photoreceptors of diverse insect species, having both apposition and neural superposition eyes, in the contexts of visual ecology, behavior, and ontogenesis. Here, I discuss these and other relevant results, emphasizing differences between fruit flies and other species, between photoreceptors of diurnal and nocturnal insects, properties of distinct functional types of photoreceptors, postembryonic developmental changes, and relationships between voltage-gated potassium channels and visual ecology.

Prominent role of prominin in the retina

Prominin molecules represent a new family of pentaspan membrane glycoproteins expressed throughout the animal kingdom. The name originates from its localization on membrane protrusion, such as microvilli, filopodia, lamellipodia, and microspikes. Following the original description in mouse and human, representative prominin members were found in fish (e.g., Danio rerio), amphibian (Ambystoma mexicanum, Xenopus laevis), worm (Caenorhabditis elegans), and flies (Drosophila melanogaster). Mammalian prominin-1 was identified as a marker of somatic and cancer stem cells and plays an essential role in the visual system, which contributed to increased interest of the medical field in this molecule. Here we summarize recent data from various fields, including Drosophila, which will aid to our understanding of its still elusive function.

Single-base pair differences in a shared motif determine differential Rhodopsin expression

The final identity and functional properties of a neuron are specified by terminal differentiation genes, which are controlled by specific motifs in compact regulatory regions. To determine how these sequences integrate inputs from transcription factors that specify cell types, we compared the regulatory mechanism of Drosophila Rhodopsin genes that are expressed in subsets of photoreceptors to that of phototransduction genes that are expressed broadly, in all photoreceptors. Both sets of genes share an 11-base pair (bp) activator motif. Broadly expressed genes contain a palindromic version that mediates expression in all photoreceptors. In contrast, each Rhodopsin exhibits characteristic single-bp substitutions that break the symmetry of the palindrome and generate activator or repressor motifs critical for restricting expression to photoreceptor subsets. Sensory neuron subtypes can therefore evolve through single-bp changes in short regulatory motifs, allowing the discrimination of a wide spectrum of stimuli.

Functional genomics identifies regulators of the phototransduction machinery in the Drosophila larval eye and adult ocelli

Sensory perception of light is mediated by specialized Photoreceptor neurons (PRs) in the eye. During development all PRs are genetically determined to express a specific Rhodopsin (Rh) gene and genes mediating a functional phototransduction pathway. While the genetic and molecular mechanisms of PR development is well described in the adult compound eye, it remains unclear how the expression of Rhodopsins and the phototransduction cascade is regulated in other visual organs in Drosophila, such as the larval eye and adult ocelli. Using transcriptome analysis of larval PR-subtypes and ocellar PRs we identify and study new regulators required during PR differentiation or necessary for the expression of specific signaling molecules of the functional phototransduction pathway. We found that the transcription factor Krüppel (Kr) is enriched in the larval eye and controls PR differentiation by promoting Rh5 and Rh6 expression. We also identified Camta, Lola, Dve and Hazy as key genes acting during ocellar PR differentiation. Further we show that these transcriptional regulators control gene expression of the phototransduction cascade in both larval eye and adult ocelli. Our results show that PR cell type-specific transcriptome profiling is a powerful tool to identify key transcriptional regulators involved during several aspects of PR development and differentiation. Our findings greatly contribute to the understanding of how combinatorial action of key transcriptional regulators control PR development and the regulation of a functional phototransduction pathway in both larval eye and adult ocelli.

Sexually dimorphic gene expression in the lateral eyes of Euphilomedes carcharodonta (Ostracoda, Pancrustacea)

Background

The evolution and development of sexual dimorphism illuminates a central question in biology: How do similar genomes produce different phenotypes? In an XX/XO system especially the state of a sexually dimorphic trait is determined by differences in gene expression, as there are no additional genetic loci in either sex. Here, we examine the XX/XO ostracod crustacean species Euphilomedes carcharodonta. This species exhibits radical sexual dimorphism of their lateral eyes, females have only a tiny simple lateral eye while males have elaborate ommatidial eyes.

Results

We find that males express three of nine eye-development gene homologs at significantly higher levels during juvenile eye development, compared to females. We also find that most eye-development genes examined are pleiotropic, with high expression levels during embryonic development as well as during juvenile eye development. Later, in adults, we find that phototransduction genes are expressed at higher levels in males than in females, as we might expect when comparing ommatidial to simple eyes.

Conclusions

We show here that expression changes of a handful of developmental genes may underlie the radical difference in a dimorphic character. This work gives an important point of comparison for studying eye evolution and development in the Pancrustacea.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-015-0026-2) contains supplementary material, which is available to authorized users.

Transcriptome-Wide Differential Gene Expression in Bicyclus anynana Butterflies: Female Vision-Related Genes Are More Plastic

Vision is energetically costly to maintain. Consequently, over time many cave-adapted species downregulate the expression of vision genes or even lose their eyes and associated eye genes entirely. Alternatively, organisms that live in fluctuating environments, with different requirements for vision at different times, may evolve phenotypic plasticity for expression of vision genes. Here, we use a global transcriptomic and candidate gene approach to compare gene expression in the heads of a polyphenic butterfly. Bicyclus anynana have two seasonal forms that display sexual dimorphism and plasticity in eye morphology, and female-specific plasticity in opsin gene expression. Nonchoosy dry season females downregulate opsin expression, consistent with the high physiological cost of vision. To identify other genes associated with sexually dimorphic and seasonally plastic differences in vision, we analyzed RNA-sequencing data from whole head tissues. We identified two eye development genes (klarsicht and warts homologs) and an eye pigment biosynthesis gene (henna) differentially expressed between seasonal forms. By comparing sex-specific expression across seasonal forms, we found that klarsicht, warts, henna, and another eye development gene (domeless) were plastic in a female-specific manner. In a male-only analysis, white (w) was differentially expressed between seasonal forms. Reverse transcription polymerase chain reaction confirmed that warts and white are expressed in eyes only, whereas klarsicht, henna and domeless are expressed in both eyes and brain. We find that differential expression of eye development and eye pigment genes is associated with divergent eye phenotypes in B. anynana seasonal forms, and that there is a larger effect of season on female vision-related genes.

Proteome Analysis Unravels Mechanism Underling the Embryogenesis of the Honeybee Drone and Its Divergence with the Worker (Apis mellifera lingustica)

The worker and drone bees each contain a separate diploid and haploid genetic makeup, respectively. Mechanisms regulating the embryogenesis of the drone and its mechanistic difference with the worker are still poorly understood. The proteomes of the two embryos at three time-points throughout development were analyzed by applying mass spectrometry-based proteomics. We identified 2788 and 2840 proteins in the worker and drone embryos, respectively. The age-dependent proteome driving the drone embryogenesis generally follows the worker's. The two embryos however evolve a distinct proteome setting to prime their respective embryogenesis. The strongly expressed proteins and pathways related to transcriptional-translational machinery and morphogenesis at 24 h drone embryo relative to the worker, illustrating the earlier occurrence of morphogenesis in the drone than worker. These morphogenesis differences remain through to the middle-late stage in the two embryos. The two embryos employ distinct antioxidant mechanisms coinciding with the temporal-difference organogenesis. The drone embryo's strongly expressed cytoskeletal proteins signify key roles to match its large body size. The RNAi induced knockdown of the ribosomal protein offers evidence for the functional investigation of gene regulating of honeybee embryogenesis. The data significantly expand novel regulatory mechanisms governing the embryogenesis, which is potentially important for honeybee and other insects.

Saccadic body turns in walking Drosophila

Drosophila melanogaster structures its optic flow during flight by interspersing translational movements with abrupt body rotations. Whether these “body saccades” are accompanied by steering movements of the head is a matter of debate. By tracking single flies moving freely in an arena, we now discovered that walking Drosophila also perform saccades. Movement analysis revealed that the flies separate rotational from translational movements by quickly turning their bodies by 15 degrees within a tenth of a second. Although walking flies moved their heads by up to 20 degrees about their bodies, their heads moved with the bodies during saccadic turns. This saccadic strategy contrasts with the head saccades reported for e.g., blowflies and honeybees, presumably reflecting optical constraints: modeling revealed that head saccades as described for these latter insects would hardly affect the retinal input in Drosophila because of the lower acuity of its compound eye. The absence of head saccades in Drosophila was associated with the absence of haltere oscillations, which seem to guide head movements in other flies. In addition to adding new twists to Drosophila walking behavior, our analysis shows that Drosophila does not turn its head relative to its body when turning during walking.

UV light activates a Gαq/11-coupled phototransduction pathway in human melanocytes

UV light stimulates a phosphoinositide signaling pathway in human melanocytes similar to those elicited by light in the eye.

While short exposure to solar ultraviolet radiation (UVR) can elicit increased skin pigmentation, a protective response mediated by epidermal melanocytes, chronic exposure can lead to skin cancer and photoaging. However, the molecular mechanisms that allow human skin to detect and respond to UVR remain incompletely understood. UVR stimulates a retinal-dependent signaling cascade in human melanocytes that requires GTP hydrolysis and phospholipase C β (PLCβ) activity. This pathway involves the activation of transient receptor potential A1 (TRPA1) ion channels, an increase in intracellular Ca²⁺, and an increase in cellular melanin content. Here, we investigated the identity of the G protein and downstream elements of the signaling cascade and found that UVR phototransduction is Gα_q/11 dependent. Activation of Gα_q/11/PLCβ signaling leads to hydrolysis of phosphatidylinositol (4,5)-bisphosphate (PIP₂) to generate diacylglycerol (DAG) and inositol 1, 4, 5-trisphosphate (IP₃). We found that PIP₂ regulated TRPA1-mediated photocurrents, and IP₃ stimulated intracellular Ca²⁺ release. The UVR-elicited Ca²⁺ response appears to involve both IP₃-mediated release from intracellular stores and Ca²⁺ influx through TRPA1 channels, showing the fast rising phase of the former and the slow decay of the latter. We propose that melanocytes use a UVR phototransduction mechanism that involves the activation of a Gα_q/11-dependent phosphoinositide cascade, and resembles light phototransduction cascades of the eye.

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.

Loss of gq/11 genes does not abolish melanopsin phototransduction

In mammals, a subset of retinal ganglion cells (RGCs) expresses the photopigment melanopsin, which renders them intrinsically photosensitive (ipRGCs). These ipRGCs mediate various non-image-forming visual functions such as circadian photoentrainment and the pupillary light reflex (PLR). Melanopsin phototransduction begins with activation of a heterotrimeric G protein of unknown identity. Several studies of melanopsin phototransduction have implicated a G-protein of the G_q/11 family, which consists of Gna11, Gna14, Gnaq and Gna15, in melanopsin-evoked depolarization. However, the exact identity of the G_q/11 gene involved in this process has remained elusive. Additionally, whether G_q/11 G-proteins are necessary for melanopsin phototransduction in vivo has not yet been examined. We show here that the majority of ipRGCs express both Gna11 and Gna14, but neither Gnaq nor Gna15. Animals lacking the melanopsin protein have well-characterized deficits in the PLR and circadian behaviors, and we therefore examined these non-imaging forming visual functions in a variety of single and double mutants for G_q/11 family members. All G_q/11 mutant animals exhibited PLR and circadian behaviors indistinguishable from WT. In addition, we show persistence of ipRGC light-evoked responses in Gna11^−/−; Gna14^−/− retinas using multielectrode array recordings. These results demonstrate that G_q, G₁₁, G₁₄, or G₁₅ alone or in combination are not necessary for melanopsin-based phototransduction, and suggest that ipRGCs may be able to utilize a G_q/11-independent phototransduction cascade in vivo.

Activity and coexpression of Drosophila black with ebony in fly optic lobes reveals putative cooperative tasks in vision that evade electroretinographic detection

Drosophila mutants black and ebony show pigmentation defects in the adult cuticle, which disclose their cooperative activity in β-alanyl-dopamine formation. In visual signal transduction, Ebony conjugates β-alanine to histamine, forming β-alanyl-histamine or carcinine. Mutation of ebony disrupts signal transduction and reveals an electroretinogram (ERG) phenotype. In contrast to the corresponding cuticle phenotype of black and ebony, there is no ERG phenotype observed when black expression is disrupted. This discrepancy calls into question the longstanding assumption of Black and Ebony interaction. The purpose of this study was to investigate the role of Black and Ebony in fly optic lobes. We excluded a presynaptic histamine uptake pathway and confirmed histamine recycling via carcinine formation in glia. β-Alanine supply for this pathway is independent of enzymatic synthesis by Black and β-alanine synthase Pyd3. Two versions of Black are expressed in vivo. Black is a specific aspartate decarboxylase with no activity on glutamate. RNA in situ hybridization and anti-Black antisera localized Black expression in the head. Immunolabeling revealed expression in lamina glia, in large medulla glia, in glia of the ocellar ganglion, and in astrocyte-like glia below the ocellar ganglion. In these glia types, Black expression is strictly accompanied by Ebony expression. Activity, localization, and strict coexpression with Ebony strongly indicate a specific mode of functional interaction that, however, evades ERG detection.

Exquisite light sensitivity of Drosophila melanogaster cryptochrome

Drosophila melanogaster shows exquisite light sensitivity for modulation of circadian functions in vivo, yet the activities of the Drosophila circadian photopigment cryptochrome (CRY) have only been observed at high light levels. We studied intensity/duration parameters for light pulse induced circadian phase shifts under dim light conditions in vivo. Flies show far greater light sensitivity than previously appreciated, and show a surprising sensitivity increase with pulse duration, implying a process of photic integration active up to at least 6 hours. The CRY target timeless (TIM) shows dim light dependent degradation in circadian pacemaker neurons that parallels phase shift amplitude, indicating that integration occurs at this step, with the strongest effect in a single identified pacemaker neuron. Our findings indicate that CRY compensates for limited light sensitivity in vivo by photon integration over extraordinarily long times, and point to select circadian pacemaker neurons as having important roles.

We investigate the paradox that fruit flies show exquisite light sensitivity for day/night circadian clock functions, yet the circadian photoreceptor cryptochrome (CRY) responds only to very high light levels in assays requiring immediate responses. Our in vivo behavioral assays are unique in that we expose flies to dim and limiting levels of light. We find that CRY integrates photons efficiently over time periods of at least six hours, with light sensitivity unexpectedly increasing with duration of light exposure. This contrasts with image-forming responses that occur on millisecond time scales in Drosophila. We show that light dependent degradation of the CRY target timeless (TIM) occurs at limiting light levels, closely paralleling behavioral effects, in the circadian pacemaker neurons. One of these neurons shows particularly strong light sensitivity, and a particularly strong temporal integration effect. We have thus identified the precise step at which temporal integration is functioning. The structurally unrelated vertebrate circadian photoreceptor melanopsin also shows the ability to integrate photons over time, though not to the extent of Drosophila CRY. We thus conclude that temporal integration is a universal mechanism to enhance photosensitivity of non-visual photopigments.

Establishing and maintaining gene expression patterns: insights from sensory receptor patterning

In visual and olfactory sensory systems with high discriminatory power, each sensory neuron typically expresses one, or very few, sensory receptor genes, excluding all others. Recent studies have provided insights into the mechanisms that generate and maintain sensory receptor expression patterns. Here, we review how this is achieved in the fly retina and compare it with the mechanisms controlling sensory receptor expression patterns in the mouse retina and in the mouse and fly olfactory systems.

Photoreception and signal transduction in corals: proteomic and behavioral evidence for cytoplasmic calcium as a mediator of light responsivity

Little is known about how corals sense and respond to light. In this report the proteome of coral is explored using 2D protein electrophoresis in two species, Montastraea cavernosa and Acropora millepora. Multiple protein species have major shifts in abundance in both species when sampled in daylight compared to corals sampled late in the night. These changes were observed both in larvae lacking zooxanthellae and in adult tissue containing zooxanthellae, including both Pacific and Caribbean corals. When larvae kept in the dark were treated with either thapsigargin or ionomycin, compounds that raise the level of cytoplasmic calcium, the night pattern of proteins shifted to the day pattern. This implies that photoreceptors responding to light elevate calcium levels and that calcium acts as the second messenger relaying light responses in corals. Corals spawn at night, and spawning can be delayed by exposure to light or pushed forward by early artificial sunsets. In a series of behavioral experiments, treatment of corals with ionomycin or thapsigargin was found to delay broadcast spawning in M. franksi, demonstrating that pharmacologically altering cytoplasmic calcium levels generates the same response as light exposure. Together these results show that the photo-responsive cells of corals detect and respond to light by altering cytoplasmic calcium levels, similarly to the transduction pathways in complex invertebrate eyes. The primacy of cytoplasmic calcium levels in light responsivity has broad implications for coral reproduction, including predicting how different species spawn at different times after sunset and how reproductive isolation is achieved during coral speciation.

Signal coding in cockroach photoreceptors is tuned to dim environments

In dim light, scarcity of photons typically leads to poor vision. Nonetheless, many animals show visually guided behavior with dim environments. We investigated the signaling properties of photoreceptors of the dark active cockroach ( Periplaneta americana) using intracellular and whole-cell patch-clamp recordings to determine whether they show selective functional adaptations to dark. Expectedly, dark-adapted photoreceptors generated large and slow responses to single photons. However, when light adapted, responses of both phototransduction and the nontransductive membrane to white noise (WN)-modulated stimuli remained slow with corner frequencies ∼20 Hz. This promotes temporal integration of light inputs and maintains high sensitivity of vision. Adaptive changes in dynamics were limited to dim conditions. Characteristically, both step and frequency responses stayed effectively unchanged for intensities >1,000 photons/s/photoreceptor. A signal-to-noise ratio (SNR) of the light responses was transiently higher at frequencies <5 Hz for ∼5 s after light onset but deteriorated to a lower value upon longer stimulation. Naturalistic light stimuli, as opposed to WN, evoked markedly larger responses with higher SNRs at low frequencies. This allowed realistic estimates of information transfer rates, which saturated at ∼100 bits/s at low-light intensities. We found, therefore, selective adaptations beneficial for vision in dim environments in cockroach photoreceptors: large amplitude of single-photon responses, constant high level of temporal integration of light inputs, saturation of response properties at low intensities, and only transiently efficient encoding of light contrasts. The results also suggest that the sources of the large functional variability among different photoreceptors reside mostly in phototransduction processes and not in the properties of the nontransductive membrane.

The retinal mosaics of opsin expression in invertebrates and vertebrates

Color vision is found in many invertebrate and vertebrate species. It is the ability to discriminate objects based on the wavelength of emitted light independent of intensity. As it requires the comparison of at least two photoreceptor types with different spectral sensitivities, this process is often mediated by a mosaic made of several photoreceptor types. In this review, we summarize the current knowledge about the formation of retinal mosaics and the regulation of photopigment (opsin) expression in the fly, mouse, and human retina. Despite distinct evolutionary origins, as well as major differences in morphology and phototransduction machineries, there are significant similarities in the stepwise cell-fate decisions that lead from progenitor cells to terminally differentiated photoreceptors that express a particular opsin. Common themes include (i) the use of binary transcriptional switches that distinguish classes of photoreceptors, (ii) the use of gradients of signaling molecules for regional specializations, (iii) stochastic choices that pattern the retina, and (iv) the use of permissive factors with multiple roles in different photoreceptor types.

Transcriptional code and disease map for adult retinal cell types

Brain circuits are assembled from a large variety of morphologically and functionally diverse cell types. It is not known how the intermingled cell types of an individual adult brain region differ in their expressed genomes. Here we describe an atlas of cell type transcriptomes in one brain region, the mouse retina. We found that each adult cell type expressed a specific set of genes, including a unique set of transcription factors, forming a 'barcode' for cell identity. Cell type transcriptomes carried enough information to categorize cells into morphological classes and types. Several genes that were specifically expressed in particular retinal circuit elements, such as inhibitory neuron types, are associated with eye diseases. The resource described here allows gene expression to be compared across adult retinal cell types, experimenting with specific transcription factors to differentiate stem or somatic cells to retinal cell types, and predicting cellular targets of newly discovered disease-associated genes.

Melanopsin and mechanisms of non-visual ocular photoreception

In addition to rods and cones, the mammalian eye contains a third class of photoreceptor, the intrinsically photosensitive retinal ganglion cell (ipRGC). ipRGCs are heterogeneous irradiance-encoding neurons that primarily project to non-visual areas of the brain. Characteristics of ipRGC light responses differ significantly from those of rod and cone responses, including depolarization to light, slow on- and off-latencies, and relatively low light sensitivity. All ipRGCs use melanopsin (Opn4) as their photopigment. Melanopsin resembles invertebrate rhabdomeric photopigments more than vertebrate ciliary pigments and uses a G(q) signaling pathway, in contrast to the G(t) pathway used by rods and cones. ipRGCs can recycle chromophore in the absence of the retinal pigment epithelium and are highly resistant to vitamin A depletion. This suggests that melanopsin employs a bistable sequential photon absorption mechanism typical of rhabdomeric opsins.

Microarray-based transcriptomic analysis of differences between long-term gregarious and solitarious desert locusts

Desert locusts (Schistocerca gregaria) show an extreme form of phenotypic plasticity and can transform between a cryptic solitarious phase and a swarming gregarious phase. The two phases differ extensively in behavior, morphology and physiology but very little is known about the molecular basis of these differences. We used our recently generated Expressed Sequence Tag (EST) database derived from S. gregaria central nervous system (CNS) to design oligonucleotide microarrays and compare the expression of thousands of genes in the CNS of long-term gregarious and solitarious adult desert locusts. This identified 214 differentially expressed genes, of which 40% have been annotated to date. These include genes encoding proteins that are associated with CNS development and modeling, sensory perception, stress response and resistance, and fundamental cellular processes. Our microarray analysis has identified genes whose altered expression may enable locusts of either phase to deal with the different challenges they face. Genes for heat shock proteins and proteins which confer protection from infection were upregulated in gregarious locusts, which may allow them to respond to acute physiological challenges. By contrast the longer-lived solitarious locusts appear to be more strongly protected from the slowly accumulating effects of ageing by an upregulation of genes related to anti-oxidant systems, detoxification and anabolic renewal. Gregarious locusts also had a greater abundance of transcripts for proteins involved in sensory processing and in nervous system development and plasticity. Gregarious locusts live in a more complex sensory environment than solitarious locusts and may require a greater turnover of proteins involved in sensory transduction, and possibly greater neuronal plasticity.

DEG/ENaC but not TRP channels are the major mechanoelectrical transduction channels in a C. elegans nociceptor

Many nociceptors detect mechanical cues, but the ion channels responsible for mechanotransduction in these sensory neurons remain obscure. Using in vivo recordings and genetic dissection, we identified the DEG/ENaC protein, DEG-1, as the major mechanotransduction channel in ASH, a polymodal nociceptor in Caenorhabditis elegans. But DEG-1 is not the only mechanotransduction channel in ASH: loss of deg-1 revealed a minor current whose properties differ from those expected of DEG/ENaC channels. This current was independent of two TRPV channels expressed in ASH. Although loss of these TRPV channels inhibits behavioral responses to noxious stimuli, we found that both mechanoreceptor currents and potentials were essentially wild-type in TRPV mutants. We propose that ASH nociceptors rely on two genetically distinct mechanotransduction channels and that TRPV channels contribute to encoding and transmitting information. Because mammalian and insect nociceptors also coexpress DEG/ENaCs and TRPVs, the cellular functions elaborated here for these ion channels may be conserved.

Shedding new light on opsin evolution

Opsin proteins are essential molecules in mediating the ability of animals to detect and use light for diverse biological functions. Therefore, understanding the evolutionary history of opsins is key to understanding the evolution of light detection and photoreception in animals. As genomic data have appeared and rapidly expanded in quantity, it has become possible to analyse opsins that functionally and histologically are less well characterized, and thus to examine opsin evolution strictly from a genetic perspective. We have incorporated these new data into a large-scale, genome-based analysis of opsin evolution. We use an extensive phylogeny of currently known opsin sequence diversity as a foundation for examining the evolutionary distributions of key functional features within the opsin clade. This new analysis illustrates the lability of opsin protein-expression patterns, site-specific functionality (i.e. counterion position) and G-protein binding interactions. Further, it demonstrates the limitations of current model organisms, and highlights the need for further characterization of many of the opsin sequence groups with unknown function.

Drosophila photoreceptor cells exploited for the production of eukaryotic membrane proteins: receptors, transporters and channels

BACKGROUND

Membrane proteins (MPs) play key roles in signal transduction. However, understanding their function at a molecular level is mostly hampered by the lack of protein in suitable amount and quality. Despite impressive developments in the expression of prokaryotic MPs, eukaryotic MP production has lagged behind and there is a need for new expression strategies. In a pilot study, we produced a Drosophila glutamate receptor specifically in the eyes of transgenic flies, exploiting the naturally abundant membrane stacks in the photoreceptor cells (PRCs). Now we address the question whether the PRCs also process different classes of medically relevant target MPs which were so far notoriously difficult to handle with conventional expression strategies.

PRINCIPAL FINDINGS

We describe the homologous and heterologous expression of 10 different targets from the three major MP classes--G protein-coupled receptors (GPCRs), transporters and channels in Drosophila eyes. PRCs offered an extraordinary capacity to produce, fold and accommodate massive amounts of MPs. The expression of some MPs reached similar levels as the endogenous rhodopsin, indicating that the PRC membranes were almost unsaturable. Expression of endogenous rhodopsin was not affected by the target MPs and both could coexist in the membrane stacks. Heterologous expression levels reached about 270 to 500 pmol/mg total MP, resulting in 0.2-0.4 mg purified target MP from 1 g of fly heads. The metabotropic glutamate receptor and human serotonin transporter--both involved in synaptic transmission--showed native pharmacological characteristics and could be purified to homogeneity as a prerequisite for further studies.

SIGNIFICANCE

We demonstrate expression in Drosophila PRCs as an efficient and inexpensive tool for the large scale production of functional eukaryotic MPs. The fly eye system offers a number of advantages over conventional expression systems and paves the way for in-depth analyses of eukaryotic MPs that have so far not been accessible to biochemical and biophysical studies.