A live cell assay of GPCR coupling allows identification of optogenetic tools for controlling Go and Gi signaling

Gz Enhanced Signal Transduction assaY (G Z ESTY) for GPCR deorphanization

G protein-coupled receptors (GPCRs) are key pharmacological targets, yet many remain underutilized due to unknown activation mechanisms and ligands. Orphan GPCRs, lacking identified natural ligands, are a high priority for research, as identifying their ligands will aid in understanding their functions and potential as drug targets. Most GPCRs, including orphans, couple to G i/o/z family members, however current assays to detect their activation are limited, hindering ligand identification efforts. We introduce G Z ESTY, a highly sensitive, cell-based assay developed in an easily deliverable format designed to study the pharmacology of G i/o/z -coupled GPCRs and assist in deorphanization. We optimized assay conditions and developed an all-in-one vector employing novel cloning methods to ensure the correct expression ratio of G Z ESTY components. G Z ESTY successfully assessed activation of a library of ligand-activated GPCRs, detecting both full and partial agonism, as well as responses from endogenous GPCRs. Notably, with G Z ESTY we established the presence of endogenous ligands for GPR176 and GPR37 in brain extracts, validating its use in deorphanization efforts. This assay enhances the ability to find ligands for orphan GPCRs, expanding the toolkit for GPCR pharmacologists.

A bistable inhibitory optoGPCR for multiplexed optogenetic control of neural circuits

Information is transmitted between brain regions through the release of neurotransmitters from long-range projecting axons. Understanding how the activity of such long-range connections contributes to behavior requires efficient methods for reversibly manipulating their function. Chemogenetic and optogenetic tools, acting through endogenous G-protein-coupled receptor pathways, can be used to modulate synaptic transmission, but existing tools are limited in sensitivity, spatiotemporal precision or spectral multiplexing capabilities. Here we systematically evaluated multiple bistable opsins for optogenetic applications and found that the Platynereis dumerilii ciliary opsin (PdCO) is an efficient, versatile, light-activated bistable G-protein-coupled receptor that can suppress synaptic transmission in mammalian neurons with high temporal precision in vivo. PdCO has useful biophysical properties that enable spectral multiplexing with other optogenetic actuators and reporters. We demonstrate that PdCO can be used to conduct reversible loss-of-function experiments in long-range projections of behaving animals, thereby enabling detailed synapse-specific functional circuit mapping.

Eyeless razor clam Sinonovacula constricta discriminates light spectra through opsins to guide Ca2+ and cAMP signaling pathways

Phototransduction is based on opsins that drive distinct types of Gα cascades. Although nonvisual photosensitivity has long been known in marine bivalves, the underlying molecular basis and phototransduction mechanism are poorly understood. Here, we introduced the eyeless razor clam Sinonovacula constricta as a model to clarify this issue. First, we showed that S. constricta was highly diverse in opsin family members, with a significant expansion in xenopsins. Second, the expression of putative S. constricta opsins was highly temporal-spatio specific, indicating their potential roles in S. constricta development and its peripheral photosensitivity. Third, by cloning four S. constricta opsins with relatively higher expression (Sc_opsin1, 5, 7, and 12), we found that they exhibited different expression levels in response to different light environments. Moreover, we demonstrated that these opsins (excluding Sc_opsin7) couple with Gαq and Gαi cascades to mediate the light-dependent Ca2+ (Sc_opsin1 and 5) and cAMP (Sc_opsin12) signaling pathways. The results indicated that Sc_opsin1 and 5 belonged to Gq-opsins, Sc_opsin12 belonged to Gi-opsins, while Sc_opsin7 might act as a photo-isomerase. Furthermore, we found that the phototransduction function of S. constricta Gq-opsins was dependent on the lysine at the seventh transmembrane domain, and greatly influenced by the external light spectra in a complementary way. Thus, a synergistic photosensitive system mediated by opsins might exist in S. constricta to rapidly respond to the transient or subtle changes of the external light environment. Collectively, our findings provide valuable insights into the evolution of opsins in marine bivalves and their potential functions in nonvisual photosensitivity.

Optimized design and in vivo application of optogenetically functionalized Drosophila dopamine receptors

Neuromodulatory signaling via G protein-coupled receptors (GPCRs) plays a pivotal role in regulating neural network function and animal behavior. The recent development of optogenetic tools to induce G protein-mediated signaling provides the promise of acute and cell type-specific manipulation of neuromodulatory signals. However, designing and deploying optogenetically functionalized GPCRs (optoXRs) with accurate specificity and activity to mimic endogenous signaling in vivo remains challenging. Here we optimize the design of optoXRs by considering evolutionary conserved GPCR-G protein interactions and demonstrate the feasibility of this approach using two Drosophila Dopamine receptors (optoDopRs). These optoDopRs exhibit high signaling specificity and light sensitivity in vitro. In vivo, we show receptor and cell type-specific effects of dopaminergic signaling in various behaviors, including the ability of optoDopRs to rescue the loss of the endogenous receptors. This work demonstrates that optoXRs can enable optical control of neuromodulatory receptor-specific signaling in functional and behavioral studies.

Modulating signalling lifetime to optimise a prototypical animal opsin for optogenetic applications

Animal opsins are light activated G-protein-coupled receptors, capable of optogenetic control of G-protein signalling for research or therapeutic applications. Animal opsins offer excellent photosensitivity, but their temporal resolution can be limited by long photoresponse duration when expressed outside their native cellular environment. Here, we explore methods for addressing this limitation for a prototypical animal opsin (human rod opsin) in HEK293T cells. We find that the application of the canonical rhodopsin kinase (GRK1)/visual arrestin signal termination mechanism to this problem is complicated by a generalised suppressive effect of GRK1 expression. This attenuation can be overcome using phosphorylation-independent mutants of arrestin, especially when these are tethered to the opsin protein. We further show that point mutations targeting the Schiff base stability of the opsin can also reduce signalling lifetime. Finally, we apply one such mutation (E122Q) to improve the temporal fidelity of restored visual responses following ectopic opsin expression in the inner retina of a mouse model of retinal degeneration (rd1). Our results reveal that these two strategies (targeting either arrestin binding or Schiff-base hydrolysis) can produce more time-delimited opsin signalling under heterologous expression and establish the potential of this approach to improve optogenetic performance.

Highly sensitive visual restoration and protection via ectopic expression of chimeric rhodopsin in mice

Photoreception requires amplification by mammalian rhodopsin through G protein activation, which requires a visual cycle. To achieve this in retinal gene therapy, we incorporated human rhodopsin cytoplasmic loops into Gloeobacter rhodopsin, thereby generating Gloeobacter and human chimeric rhodopsin (GHCR). In a murine model of inherited retinal degeneration, we induced retinal GHCR expression by intravitreal injection of a recombinant adeno-associated virus vector. Retinal explant and visual thalamus electrophysiological recordings, behavioral tests, and histological analysis showed that GHCR restored dim-environment vision and prevented the progression of retinal degeneration. Thus, GHCR may be a potent clinical tool for the treatment of retinal disorders.

A bistable inhibitory OptoGPCR for multiplexed optogenetic control of neural circuits

Information is transmitted between brain regions through the release of neurotransmitters from long-range projecting axons. Understanding how the activity of such long-range connections contributes to behavior requires efficient methods for reversibly manipulating their function. Chemogenetic and optogenetic tools, acting through endogenous G-protein coupled receptor (GPCRs) pathways, can be used to modulate synaptic transmission, but existing tools are limited in sensitivity, spatiotemporal precision, or spectral multiplexing capabilities. Here we systematically evaluated multiple bistable opsins for optogenetic applications and found that the Platynereis dumerilii ciliary opsin (PdCO) is an efficient, versatile, light-activated bistable GPCR that can suppress synaptic transmission in mammalian neurons with high temporal precision in-vivo. PdCO has superior biophysical properties that enable spectral multiplexing with other optogenetic actuators and reporters. We demonstrate that PdCO can be used to conduct reversible loss-of-function experiments in long-range projections of behaving animals, thereby enabling detailed synapse-specific functional circuit mapping.

A self-inactivating invertebrate opsin optically drives biased signaling toward Gβγ-dependent ion channel modulation

Animal opsins, light-sensitive G protein-coupled receptors, have been used for optogenetic tools to control G protein-dependent signaling pathways. Upon G protein activation, the Gα and Gβγ subunits drive different intracellular signaling pathways, leading to complex cellular responses. For some purposes, Gα- and Gβγ-dependent signaling needs to be separately modulated, but these responses are simultaneously evoked due to the 1:1 stoichiometry of Gα and Gβγ Nevertheless, we show temporal activation of G protein using a self-inactivating invertebrate opsin, Platynereis c-opsin1, drives biased signaling for Gβγ-dependent GIRK channel activation in a light-dependent manner by utilizing the kinetic difference between Gβγ-dependent and Gα-dependent responses. The opsin-induced transient Gi/o activation preferentially causes activation of the kinetically fast Gβγ-dependent GIRK channels rather than slower Gi/oα-dependent adenylyl cyclase inhibition. Although similar Gβγ-biased signaling properties were observed in a self-inactivating vertebrate visual pigment, Platynereis c-opsin1 requires fewer retinal molecules to evoke cellular responses. Furthermore, the Gβγ-biased signaling properties of Platynereis c-opsin1 are enhanced by genetically fusing with RGS8 protein, which accelerates G protein inactivation. The self-inactivating invertebrate opsin and its RGS8-fusion protein can function as optical control tools biased for Gβγ-dependent ion channel modulation.

Probing the orphan receptors: Tools and directions

The endogenous ligands activating a large fraction of the G Protein Coupled Receptor (GPCR) family members have yet to be identified. These receptors are commonly labeled as orphans (oGPCRs), and because of the absence of available pharmacological tools they are currently understudied. Nonetheless, genome wide association studies, together with research using animal models identified many physiological functions regulated by oGPCRs. Similarly, mutations in some oGPCRs have been associated with rare genetic disorders or with an increased risk of developing pathologies. The once underestimated pharmacological potential of targeting oGPCRs is increasingly being exploited by the development of novel tools to understand their biology and by drug discovery endeavors aimed at identifying new modulators of their activity. Here, we summarize recent advancements in the field of oGPCRs and future directions.

Functional optimization of light-activatable Opto-GPCRs: Illuminating the importance of the proximal C-terminus in G-protein specificity

Introduction: G-protein coupled receptors (GPCRs) are the largest family of human receptors that transmit signals from natural ligands and pharmaceutical drugs into essentially every physiological process. One main characteristic of G-protein coupled receptors is their ability to specifically couple with different families of G-proteins, thereby triggering specific downstream signaling pathways. While an abundance of structural information is available on G-protein coupled receptorn interactions with G-proteins, little is known about the G-protein coupled receptor domains functionally mediating G-protein specificity, in particular the proximal C-terminus, the structure which cannot be predicted with high confidentiality due to its flexibility. Methods: In this study, we exploited OptoGPCR chimeras between lightgated G-protein coupled receptors (opsins) and ligand-gated G-protein coupled receptors to systematically investigate the involvement of the C-terminus steering G-protein specificity. We employed rhodopsin-beta2-adrenoceptor and melanopsin-mGluR6 chimeras in second messenger assays and developed structural models of the chimeras. Results: We discovered a dominant role of the proximal C-terminus, dictating G-protein selectivity in the melanopsin-mGluR6 chimera, whereas it is the intracellular loop 3, which steers G-protein tropism in the rhodopsin-beta2-adrenoceptor. From the functional results and structural predictions, melanopsin and mGluR6 use a different mechanism to bovine rhodopsin and b2AR to couple to a selective G-protein. Discussion: Collectively, this work adds knowledge to the G-protein coupled receptor domains mediating G-protein selectivity, ultimately paving the way to optogenetically elicited specific G-protein signaling on demand.

Optogenetic Prosthetization of Retinal Bipolar Cells

Abstract

Although the experience of optogenetic retinal prosthetics in animal models dates back to more than 16 years, the first results obtained on humans have only been reported in the last year. Over this period, the main challenges of prosthetics became clear and the approaches to their solution were proposed. In this review, we aim to present the achievements in the field of optogenetic prosthetization of retinal bipolar cells with a focus mainly on relatively recent publications. The review addresses the advantages and disadvantages of bipolar cell prosthetics as compared to the alternative target, retinal ganglion cells, and provides a comparative analysis of the effectiveness of ionotropic light-sensitive proteins (channelrhodopsins) or metabotropic receptors (rhodopsins) as prosthetic tools.

Optogenetics for visual restoration: From proof of principle to translational challenges

Degenerative retinal disorders are a diverse family of diseases commonly leading to irreversible photoreceptor death, while leaving the inner retina relatively intact. Over recent years, innovative gene replacement therapies aiming to halt the progression of certain inherited retinal disorders have made their way into clinics. By rendering surviving retinal neurons light sensitive optogenetic gene therapy now offers a feasible treatment option that can restore lost vision, even in late disease stages and widely independent of the underlying cause of degeneration. Since proof-of-concept almost fifteen years ago, this field has rapidly evolved and a detailed first report on a treated patient has recently been published. In this article, we provide a review of optogenetic approaches for vision restoration. We discuss the currently available optogenetic tools and their relative advantages and disadvantages. Possible cellular targets will be discussed and we will address the question how retinal remodelling may affect the choice of the target and to what extent it may limit the outcomes of optogenetic vision restoration. Finally, we will analyse the evidence for and against optogenetic tool mediated toxicity and will discuss the challenges associated with clinical translation of this promising therapeutic concept.

Optogenetic Stimulation of Gi Signaling Enables Instantaneous Modulation of Cardiomyocyte Pacemaking

G-protein signaling pathways are central in the regulation of cardiac function in physiological and pathophysiological conditions. Their functional analysis through optogenetic techniques with selective expression of opsin proteins and activation by specific wavelengths allows high spatial and temporal precision. Here, we present the application of long wavelength-sensitive cone opsin (LWO) in cardiomyocytes for activation of the G_i signaling pathway by red light. Murine embryonic stem (ES) cells expressing LWO were generated and differentiated into beating cardiomyocytes in embryoid bodies (EBs). Illumination with red light (625 nm) led to an instantaneous decrease up to complete inhibition (84–99% effectivity) of spontaneous beating, but had no effect on control EBs. By using increasing light intensities with 10 s pulses, we determined a half maximal effective light intensity of 2.4 μW/mm² and a maximum effect at 100 μW/mm². Pre-incubation of LWO EBs with pertussis toxin completely inhibited the light effect proving the specificity for G_i signaling. Frequency reduction was mainly due to the activation of GIRK channels because the specific channel blocker tertiapin reduced the light effect by ~80%. Compared with pharmacological stimulation of M₂ receptors with carbachol with slow kinetics (>30 s), illumination of LWO had an identical efficacy, but much faster kinetics (<1 s) in the activation and deactivation demonstrating the temporal advantage of optogenetic stimulation. Thus, LWO is an effective optogenetic tool for selective stimulation of the G_i signaling cascade in cardiomyocytes with red light, providing high temporal precision.

Rhodopsins at a glance

Rhodopsins are photoreceptive membrane proteins consisting of a common heptahelical transmembrane architecture that contains a retinal chromophore. Rhodopsin was first discovered in the animal retina in 1876, but a different type of rhodopsin, bacteriorhodopsin, was reported to be present in the cell membrane of an extreme halophilic archaeon, Halobacterium salinarum, 95 years later. Although these findings were made by physiological observation of pigmented tissue and cell bodies, recent progress in genomic and metagenomic analyses has revealed that there are more than 10,000 microbial rhodopsins and 9000 animal rhodopsins with large diversity and tremendous new functionality. In this Cell Science at a Glance article and accompanying poster, we provide an overview of the diversity of functions, structures, color discrimination mechanisms and optogenetic applications of these two rhodopsin families, and will also highlight the third distinctive rhodopsin family, heliorhodopsin.

Luciferase-based GloSensor™ cAMP assay: Temperature optimization and application to cell-based kinetic studies

G protein-coupled receptors (GPCRs) are an important receptor superfamily and common therapeutic targets. The second messenger cyclic adenosine monophosphate (cAMP) is a key mediator in many GPCR signaling pathways. Monitoring intracellular cAMP levels can help identify orthosteric agonists and antagonists, as well as allosteric modulators. In this regard, luminescence-based biosensors have revolutionized our ability to monitor GPCR signaling kinetics. The GloSensor™ cAMP assay enables real-time monitoring of signaling downstream of many GPCRs. However, it is crucial to optimize assay conditions such as temperature. As well, it has not been reported whether the effects of temperature on biosensor activity are reversible. Here, we describe the temperature sensitivity and reversibility of the GloSensor™ cAMP assay, and which GloSensor™ version is optimal for measuring cytosolic cAMP. We also present a detailed protocol for monitoring cAMP levels in live cells expressing endogenous or exogenous GPCRs. Temperature optimization studies were carried out using HEK293H cells transiently transfected with the adenosine receptor A2a and the GloSensor™ plasmid (pGloSensor-20F or -22F). We found that preincubation and luminescence reading at room temperature were optimal as compared to higher temperatures. As well, the GloSensor-22F biosensor had a superior signal-to-background ratio and the effect of temperature on biosensor activity was reversible. However, thermal instability of the biosensor may pose a problem for in vivo studies. Nevertheless, the GloSensor™ cAMP assay can be applied to analyze signaling by a wide range of GPCRs for drug discovery purposes.

Chimeric human opsins as optogenetic light sensitisers

Human opsin-based photopigments have great potential as light-sensitisers, but their requirement for phototransduction cascade-specific second messenger proteins may restrict their functionality in non-native cell types. In this study, eight chimeric human opsins were generated consisting of a backbone of either a rhodopsin (RHO) or long-wavelength-sensitive (LWS) opsin and intracellular domains from G_q/11-coupled human melanopsin. Rhodopsin/melanopsin chimeric opsins coupled to both G_i and G_q/11 pathways. Greater substitution of the intracellular surface with corresponding melanopsin domains generally showed greater G_q/11 activity with a decrease in G_i activation. Unlike melanopsin, rhodopsin and rhodopsin/melanopsin chimeras were dependent upon exogenous chromophore to function. By contrast, wild-type LWS opsin and LWS opsin/melanopsin chimeras showed only weak G_i activation in response to light, whilst G_q/11 pathway activation was not detected. Immunocytochemistry (ICC) demonstrated that chimeric opsins with more intracellular domains of melanopsin were less likely to be trafficked to the plasma membrane. This study demonstrates the importance of G_α coupling efficiency to the speed of cellular responses and created human opsins with a unique combination of properties to expand the range of customised optogenetic biotools for basic research and translational therapies.

Summary: Combining different domains of human visual opsins and melanopsin creates functionally unique chimeric opsins with potential optogenetic applications.

Decoding LncRNAs

Non-coding RNAs (ncRNAs) have been considered as unimportant additions to the transcriptome. Yet, in light of numerous studies, it has become clear that ncRNAs play important roles in development, health and disease. Long-ignored, long non-coding RNAs (lncRNAs), ncRNAs made of more than 200 nucleotides have gained attention due to their involvement as drivers or suppressors of a myriad of tumours. The detailed understanding of some of their functions, structures and interactomes has been the result of interdisciplinary efforts, as in many cases, new methods need to be created or adapted to characterise these molecules. Unlike most reviews on lncRNAs, we summarize the achievements on lncRNA studies by taking into consideration the approaches for identification of lncRNA functions, interactomes, and structural arrangements. We also provide information about the recent data on the involvement of lncRNAs in diseases and present applications of these molecules, especially in medicine.

Efficient optogenetic silencing of neurotransmitter release with a mosquito rhodopsin

Information is carried between brain regions through neurotransmitter release from axonal presynaptic terminals. Understanding the functional roles of defined neuronal projection pathways requires temporally precise manipulation of their activity. However, existing inhibitory optogenetic tools have low efficacy and off-target effects when applied to presynaptic terminals, while chemogenetic tools are difficult to control in space and time. Here, we show that a targeting-enhanced mosquito homolog of the vertebrate encephalopsin (eOPN3) can effectively suppress synaptic transmission through the G_i/o signaling pathway. Brief illumination of presynaptic terminals expressing eOPN3 triggers a lasting suppression of synaptic output that recovers spontaneously within minutes in vitro and in vivo. In freely moving mice, eOPN3-mediated suppression of dopaminergic nigrostriatal afferents induces a reversible ipsiversive rotational bias. We conclude that eOPN3 can be used to selectively suppress neurotransmitter release at presynaptic terminals with high spatiotemporal precision, opening new avenues for functional interrogation of long-range neuronal circuits in vivo.

In vitro profiling of orphan G protein coupled receptor (GPCR) constitutive activity

BACKGROUND AND PURPOSE

Members of the GPCR family are targeted by a significant fraction of the available FDA-approved drugs. However, the physiological role and pharmacological properties of many GPCRs remain unknown, representing untapped potential in drug design. Of particular interest are ~100 less-studied GPCRs known as orphans because their endogenous ligands are unknown. Intriguingly, disease-causing mutations identified in patients, together with animal studies, have demonstrated that many orphan receptors play crucial physiological roles and, thus, represent attractive drug targets.

EXPERIMENTAL APPROACH

The majority of deorphanized GPCRs demonstrate coupling to G_i/o . However, a limited number of techniques allow the detection of intrinsically small constitutive activity associated with G_i/o protein activation, which represents a significant barrier in our ability to study orphan GPCR signalling. Using luciferase reporter assays, we effectively detected constitutive G_s , G_q and G_12/13 protein signalling by unliganded receptors and introducing various G protein chimeras, we provide a novel, highly sensitive tool capable of identifying G_i/o coupling in unliganded orphan GPCRs.

KEY RESULTS

Using this approach, we measured the constitutive activity of the entire class C GPCR family that includes eight orphan receptors and a subset of 20 prototypical class A GPCR members, including 11 orphans. Excitingly, this approach illuminated the G protein coupling profile of eight orphan GPCRs (GPR22, GPR137b, GPR88, GPR156, GPR158, GPR179, GPRC5D and GPRC6A) previously linked to pathophysiological processes.

CONCLUSION AND IMPLICATIONS

We provide a new platform that could be utilized in ongoing studies in orphan receptor signalling and de-orphanization efforts.

Seasonal variation in UVA light drives hormonal and behavioural changes in a marine annelid via a ciliary opsin

The right timing of animal physiology and behaviour ensures the stability of populations and ecosystems. To predict anthropogenic impacts on these timings, more insight is needed into the interplay between environment and molecular timing mechanisms. This is particularly true in marine environments. Using high-resolution, long-term daylight measurements from a habitat of the marine annelid Platynereis dumerilii, we found that temporal changes in ultraviolet A (UVA)/deep violet intensities, more than longer wavelengths, can provide annual time information, which differs from annual changes in the photoperiod. We developed experimental set-ups that resemble natural daylight illumination conditions, and automated, quantifiable behavioural tracking. Experimental reduction of UVA/deep violet light (approximately 370-430 nm) under a long photoperiod (16 h light and 8 h dark) significantly decreased locomotor activities, comparable to the decrease caused by a short photoperiod (8 h light and 16 h dark). In contrast, altering UVA/deep violet light intensities did not cause differences in locomotor levels under a short photoperiod. This modulation of locomotion by UVA/deep violet light under a long photoperiod requires c-opsin1, a UVA/deep violet sensor employing G_i signalling. C-opsin1 also regulates the levels of rate-limiting enzymes for monogenic amine synthesis and of several neurohormones, including pigment-dispersing factor, vasotocin (vasopressin/oxytocin) and neuropeptide Y. Our analyses indicate a complex inteplay between UVA/deep violet light intensities and photoperiod as indicators of annual time.

Optogenetic Control of the BMP Signaling Pathway

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor β (TGFβ) superfamily and have crucial roles during development; including mesodermal patterning and specification of renal, hepatic, and skeletal tissues. In vitro developmental models currently rely upon costly and unreliable recombinant BMP proteins that do not enable dynamic or precise activation of the BMP signaling pathway. Here, we report the development of an optogenetic BMP signaling system (optoBMP) that enables rapid induction of the canonical BMP signaling pathway driven by illumination with blue light. We demonstrate the utility of the optoBMP system in multiple human cell lines to initiate signal transduction through phosphorylation and nuclear translocation of SMAD1/5, leading to upregulation of BMP target genes including Inhibitors of DNA binding ID2 and ID4. Furthermore, we demonstrate how the optoBMP system can be used to fine-tune activation of the BMP signaling pathway through variable light stimulation. Optogenetic control of BMP signaling will enable dynamic and high-throughput intervention across a variety of applications in cellular and developmental systems.

An integrated 3D fluidic device with bubble guidance mechanism for long-term primary and secondary cell recordings on multi-electrode array platform

A 3D fluidic device (3D-FD) is designed and developed with the capability of auto bubble guidance via a helical pathway in a 3D geometry. This assembly is integrated to a multi-electrode array (MEA) to maintain secondary cell lines, primary cells and primary retinal tissue explants of chick embryos for continuous monitoring of the growth and electrophysiology recording. The ability to maintain the retinal tissue explant, extracted from day 14 (E-14) and day 21 (E-21) chick embryos in an integrated 3D-FD MEA for long duration (>100 h) and study the development is demonstrated. The enhanced duration of monitoring offered by this device is due to the controlled laminar flow and the maintenance of a stable microenvironment. The spontaneous electrical activity of the retina, including the spike recordings from the retinal ganglion layer, was monitored over a long duration. Specifically, the spiking activity in embryonic chick retinas of different days (E-14 to 21) is studied, and the presence of light-stimulated firings along with a distinct electroretinogram for E-21 mature retina provides the evidence of a stable microenvironment over a sustained period.

Extraocular, rod-like photoreceptors in a flatworm express xenopsin photopigment

Animals detect light using opsin photopigments. Xenopsin, a recently classified subtype of opsin, challenges our views on opsin and photoreceptor evolution. Originally thought to belong to the Gαi-coupled ciliary opsins, xenopsins are now understood to have diverged from ciliary opsins in pre-bilaterian times, but little is known about the cells that deploy these proteins, or if they form a photopigment and drive phototransduction. We characterized xenopsin in a flatworm, Maritigrella crozieri, and found it expressed in ciliary cells of eyes in the larva, and in extraocular cells around the brain in the adult. These extraocular cells house hundreds of cilia in an intra-cellular vacuole (phaosome). Functional assays in human cells show Maritigrella xenopsin drives phototransduction primarily by coupling to Gαi. These findings highlight similarities between xenopsin and c-opsin and reveal a novel type of opsin-expressing cell that, like jawed vertebrate rods, encloses the ciliary membrane within their own plasma membrane.

Eyes are elaborate organs that many animals use to detect light and see, but light can also be sensed in other, simpler ways and for purposes other than seeing. All animals that perceive light rely on cells called photoreceptors, which come in two main types: ciliary or rhabdomeric. Sometimes, an organism has both types of photoreceptors, but one is typically more important than the other. For example, most vertebrates see using ciliary photoreceptors, while rhabdomeric photoreceptors underpin vision in invertebrates.

Flatworms are invertebrates that have long been studied due to their ability to regenerate following injuries. These worms have rhabdomeric photoreceptors in their eyes, but they also have unusual cells outside their eyes that have cilia – slender protuberances from the cell body - and could potentially be light sensitive. One obvious way to test if a cell is a photoreceptor is to see if it produces any light-sensing proteins, such as opsins. Until recently it was thought that each type of photoreceptor produced a different opsin, which were therefore classified into rhabdomeric of ciliary opsins. However, recent work has identified a new type of opsin, called xenopsin, in the ciliary photoreceptors of the larvae of some marine invertebrates.

To determine whether the cells outside the flatworm’s eye were ciliary photoreceptors, Rawlinson et al. examined the genetic code of 30 flatworm species looking for ciliary opsin and xenopsin genes. This search revealed that all the flatworm species studied contained the genetic sequence for xenopsin, but not for the ciliary opsin.

Rawlinson et al. chose the tiger flatworm to perform further experiments. First, they showed that, in this species, xenopsin genes are active both in the eyes of larvae and in the unusual ciliary cells found outside the eyes of the adult. Next, they put the xenopsin from the tiger flatworm into human embryonic kidney cells, and found that when the protein is present these cells can respond to light. This demonstrates that the newly discovered xenopsin is light-sensitive, suggesting that the unusual ciliary cells found expressing this protein outside the eyes in flatworms are likely photoreceptive cells.

It is unclear why flatworms have developed these unusual ciliary photoreceptor cells or what their purpose is outside the eye. Often, photoreceptor cells outside the eyes are used to align the ‘body clock’ with the day-night cycle. This can be a factor in healing, hinting perhaps that these newly found cells may have a role in flatworms’ ability to regenerate.