CaMello-XR enables visualization and optogenetic control of Gq/11 signals and receptor trafficking in GPCR-specific domains

Molecular Property, Manipulation, and Potential Use of Opn5 and Its Homologs

Animal opsin is a G-protein coupled receptor (GPCR) and binds retinal as a chromophore to form a photopigment. The Opsin 5 (Opn5) group within the animal opsin family comprises a diverse array of related proteins, such as Opn5m, a protein conserved across all vertebrate lineages including mammals, and other members like Opn5L1 and Opn5L2 found in non-mammalian vertebrate genomes, and Opn6 found in non-therian vertebrate genomes, along with Opn5 homologs present in invertebrates. Although these proteins collectively constitute a single clade within the molecular phylogenetic tree of animal opsins, they exhibit markedly distinct molecular characteristics in areas such as retinal binding properties, photoreaction, and G-protein coupling specificity. Based on their molecular features, they are believed to play a significant role in physiological functions. However, our understanding of their precise physiological functions and molecular characteristics is still developing and only partially realized. Furthermore, their unique molecular characteristics of Opn5-related proteins suggest a high potential for their use as optogenetic tools through more specialized manipulations. This review intends to encapsulate our current understanding of Opn5, discuss potential manipulations of its molecular attributes, and delve into its prospective utility in the burgeoning field of animal opsin optogenetics.

Synthetic surprise as the foundation of the psychedelic experience

Psychedelic agents, such as LSD and psilocybin, induce marked alterations in consciousness via activation of the 5-HT2A receptor (5-HT2ARs). We hypothesize that psychedelics enforce a state of synthetic surprise through the biased activation of the 5-HTRs system. This idea is informed by recent insights into the role of 5-HT in signaling surprise. The effects on consciousness, explained by the cognitive penetrability of perception, can be described within the predictive coding framework where surprise corresponds to prediction error, the mismatch between predictions and actual sensory input. Crucially, the precision afforded to the prediction error determines its effect on priors, enabling a dynamic interaction between top-down expectations and incoming sensory data. By integrating recent findings on predictive coding circuitry and 5-HT2ARs transcriptomic data, we propose a biological implementation with emphasis on the role of inhibitory interneurons. Implications arise for the clinical use of psychedelics, which may rely primarily on their inherent capacity to induce surprise in order to disrupt maladaptive patterns.

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.

Optical Approaches for Investigating Neuromodulation and G Protein-Coupled Receptor Signaling

Despite the fact that roughly 40% of all US Food and Drug Administration (FDA)-approved pharmacological therapeutics target G protein-coupled receptors (GPCRs), there remains a gap in our understanding of the physiologic and functional role of these receptors at the systems level. Although heterologous expression systems and in vitro assays have revealed a tremendous amount about GPCR signaling cascades, how these cascades interact across cell types, tissues, and organ systems remains obscure. Classic behavioral pharmacology experiments lack both the temporal and spatial resolution to resolve these long-standing issues. Over the past half century, there has been a concerted effort toward the development of optical tools for understanding GPCR signaling. From initial ligand uncaging approaches to more recent development of optogenetic techniques, these strategies have allowed researchers to probe longstanding questions in GPCR pharmacology both in vivo and in vitro. These tools have been employed across biologic systems and have allowed for interrogation of everything from specific intramolecular events to pharmacology at the systems level in a spatiotemporally specific manner. In this review, we present a historical perspective on the motivation behind and development of a variety of optical toolkits that have been generated to probe GPCR signaling. Here we highlight how these tools have been used in vivo to uncover the functional role of distinct populations of GPCRs and their signaling cascades at a systems level. SIGNIFICANCE STATEMENT: G protein-coupled receptors (GPCRs) remain one of the most targeted classes of proteins for pharmaceutical intervention, yet we still have a limited understanding of how their unique signaling cascades effect physiology and behavior at the systems level. In this review, we discuss a vast array of optical techniques that have been devised to probe GPCR signaling both in vitro and in vivo.

Optogenetic activation of mGluR1 signaling in the cerebellum induces synaptic plasticity

Neuronal plasticity underlying cerebellar learning behavior is strongly associated with type 1 metabotropic glutamate receptor (mGluR1) signaling. Activation of mGluR1 leads to activation of the G_q/11 pathway, which is involved in inducing synaptic plasticity at the parallel fiber-Purkinje cell synapse (PF-PC) in form of long-term depression (LTD). To optogenetically modulate mGluR1 signaling we fused mouse melanopsin (OPN4) that activates the G_q/11 pathway to the C-termini of mGluR1 splice variants (OPN4-mGluR1a and OPN4-mGluR1b). Activation of both OPN4-mGluR1 variants showed robust Ca²⁺ increase in HEK cells and PCs of cerebellar slices. We provide the prove-of-concept approach to modulate synaptic plasticity via optogenetic activation of OPN4-mGluR1a inducing LTD at the PF-PC synapse in vitro. Moreover, we demonstrate that light activation of mGluR1a signaling pathway by OPN4-mGluR1a in PCs leads to an increase in intrinsic activity of PCs in vivo and improved cerebellum driven learning behavior.

5-HT2A receptor dysregulation in a schizophrenia relevant mouse model of NMDA receptor hypofunction

Blockade of N-methyl-D-aspartate receptors (NMDAR) is known to augment cortical serotonin 2A receptors (5-HT2ARs), which is implicated in psychosis. However, the pathways from NMDAR hypofunction to 5-HT2AR up-regulation are unclear. Here we addressed in mice whether genetic deletion of the indispensable NMDAR-subunit Grin1 principally in corticolimbic parvalbumin-positive fast-spiking interneurons, could up-regulate 5-HT2ARs leading to cortical hyper-excitability. First, in vivo local-field potential recording revealed that auditory cortex in Grin1 mutant mice became hyper-excitable upon exposure to acoustic click-train stimuli that release 5-HT in the cortex. This excitability increase was reproduced ex vivo where it consisted of an increased frequency of action potential (AP) firing in layer 2/3 pyramidal neurons of mutant auditory cortex. Application of the 5-HT2AR agonist TCB-2 produced similar results. The effect of click-trains was reversed by the 5-HT2AR antagonist M100907 both in vivo and ex vivo. Increase in AP frequency of pyramidal neurons was also reversed by application of Gαq protein inhibitor BIM-46187 and G protein-gated inwardly-rectifying K⁺ (GIRK) channel activator ML297. In fast-spiking interneurons, 5-HT2AR activation normally promotes GABA release, contributing to decreased excitability of postsynaptic pyramidal neurons, which was missing in the mutants. Moreover, unlike the controls, the GABA_A receptor antagonist (+)-bicuculline had little effect on AP frequency of mutant pyramidal neurons, indicating a disinhibition state. These results suggest that the auditory-induced hyper-excitable state is conferred via GABA release deficits from Grin1-lacking interneurons leading to 5-HT2AR dysregulation and GIRK channel suppression in cortical pyramidal neurons, which could be involved in auditory psychosis.

Selective optogenetic control of Gq signaling using human Neuropsin

G_q proteins are universally important for signal transduction in mammalian cells. The underlying kinetics and transformation from extracellular stimuli into intracellular signaling, however could not be investigated in detail so far. Here we present the human Neuropsin (hOPN5) for specific and repetitive manipulation of G_q signaling in vitro and in vivo with high spatio-temporal resolution. Properties and G protein specificity of hOPN5 are characterized by UV light induced IP₃ generation, Ca²⁺ transients and inhibition of G_IRK channel activity in HEK cells. In adult hearts from a transgenic animal model, light increases the spontaneous beating rate. In addition, we demonstrate light induced contractions in the small intestine, which are not detectable after pharmacological G_q protein block. All-optical high-throughput screening for TRPC6 inhibitors is more specific and sensitive than conventional pharmacological screening. Thus, we demonstrate specific G_q signaling of hOPN5 and unveil its potential for optogenetic applications.

Reverse optogenetics of G protein signaling by zebrafish non-visual opsin Opn7b for synchronization of neuronal networks

Opn7b is a non-visual G protein-coupled receptor expressed in zebrafish. Here we find that Opn7b expressed in HEK cells constitutively activates the Gi/o pathway and illumination with blue/green light inactivates G protein-coupled inwardly rectifying potassium channels. This suggests that light acts as an inverse agonist for Opn7b and can be used as an optogenetic tool to inhibit neuronal networks in the dark and interrupt constitutive inhibition in the light. Consistent with this prediction, illumination of recombinant expressed Opn7b in cortical pyramidal cells results in increased neuronal activity. In awake mice, light stimulation of Opn7b expressed in pyramidal cells of somatosensory cortex reliably induces generalized epileptiform activity within a short (<10 s) delay after onset of stimulation. Our study demonstrates a reversed mechanism for G protein-coupled receptor control and Opn7b as a tool for controlling neural circuit properties.

Bridging the gap between single receptor type activity and whole-brain dynamics

What is the effect of activating a single modulatory neuronal receptor type on entire brain network dynamics? Can such effect be isolated at all? These are important questions because characterizing elementary neuronal processes that influence network activity across the given anatomical backbone is fundamental to guide theories of brain function. Here, we introduce the concept of the cortical 'receptome' taking into account the distribution and densities of expression of different modulatory receptor types across the brain's anatomical connectivity matrix. By modelling whole-brain dynamics in silico, we suggest a bidirectional coupling between modulatory neurotransmission and neuronal connectivity hardware exemplified by the impact of single serotonergic (5-HT) receptor types on cortical dynamics. As experimental support of this concept, we show how optogenetic tools enable specific activation of a single 5-HT receptor type across the cortex as well as in vivo measurement of its distinct effects on cortical processing. Altogether, we demonstrate how the structural neuronal connectivity backbone and its modulation by a single neurotransmitter system allow access to a rich repertoire of different brain states that are fundamental for flexible behaviour. We further propose that irregular receptor expression patterns-genetically predisposed or acquired during a lifetime-may predispose for neuropsychiatric disorders like addiction, depression and anxiety along with distinct changes in brain state. Our long-term vision is that such diseases could be treated through rationally targeted therapeutic interventions of high specificity to eventually recover natural transitions of brain states.

Expression of serotonin 1A and 2A receptors in molecular- and projection-defined neurons of the mouse insular cortex

The serotonin (5-HT) system is the target of multiple anxiolytics, including Buspirone, which is a partial agonist of the serotonin 1A receptor (5-HT1A). Similarly, ligands of the serotonin 2A receptor (5-HT2A) were shown to alter anxiety level. The 5-HT1A and 2A receptors are widely expressed across the brain, but the target region(s) underlying the influence of those receptors on anxiety remain unknown. Interestingly, recent studies in human and non-human primates have shown that the 5-HT1A and 5-HT2A binding potentials within the insular cortex (insula) are correlated to anxiety. As an initial step to define the function of 5-HT transmission in the insula, we quantified the proportion of specific neuronal populations of the insula expressing 5-HT1A or 5-HT2A. We analyzed seven neural populations, including three defined by a molecular marker (putative glutamate, GABA or parvalbumin), and four defined by their projections to different downstream targets. First, we found that more than 70% of putative glutamatergic neurons, and only 30% of GABAergic neurons express the 5-HT1A. Second, within insular projection neurons, 5-HT1A is highly expressed (75-80%) in the populations targeting one sub-nuclei of the amygdala (central or basolateral), or targeting the rostral or caudal sections of the lateral hypothalamus (LH). Similarly, 70% of putative glutamatergic neurons and only 30% of insular GABAergic neurons contain 5-HT2A. Finally, the 5-HT2A is present in a majority of insula-amygdala and insula-LH projection neurons (73-82%). These observations suggest that most glutamatergic neurons can respond to 5-HT through 5-HT1A or 5-HT2A in the insula, and that 5-HT directly affects a limited number of GABAergic neurons. This study defines a molecular and neuroanatomical map of the 5-HT system within the insular cortex, providing ground knowledge to identify the potential role of serotonergic modulation of selective insular populations in anxiety.

Separable gain control of ongoing and evoked activity in the visual cortex by serotonergic input

Controlling gain of cortical activity is essential to modulate weights between internal ongoing communication and external sensory drive. Here, we show that serotonergic input has separable suppressive effects on the gain of ongoing and evoked visual activity. We combined optogenetic stimulation of the dorsal raphe nucleus (DRN) with wide-field calcium imaging, extracellular recordings, and iontophoresis of serotonin (5-HT) receptor antagonists in the mouse visual cortex. 5-HT1A receptors promote divisive suppression of spontaneous activity, while 5-HT2A receptors act divisively on visual response gain and largely account for normalization of population responses over a range of visual contrasts in awake and anesthetized states. Thus, 5-HT input provides balanced but distinct suppressive effects on ongoing and evoked activity components across neuronal populations. Imbalanced 5-HT1A/2A activation, either through receptor-specific drug intake, genetically predisposed irregular 5-HT receptor density, or change in sensory bombardment may enhance internal broadcasts and reduce sensory drive and vice versa.

Signaling in the primary cilium through the lens of the Hedgehog pathway

Cilia are microtubule-based, cell-surface projections whose machinery is evolutionarily conserved. In vertebrates, cilia are observed on almost every cell type and are either motile or immotile. Immotile sensory, or primary cilia, are responsive to extracellular ligands and signals. Cilia can be thought of as compartments, functionally distinct from the cell that provides an environment for signaling cascades. Hedgehog is a critical developmental signaling pathway which is functionally linked to primary cilia in vertebrates. The major components of the vertebrate Hedgehog signaling pathway dynamically localize to the ciliary compartment and ciliary membrane. Critically, G-protein coupled receptor (GPCR) Smoothened, the obligate transducer of the pathway, is enriched and activated in the cilium. While Smoothened is the most intensely studied ciliary receptor, many GPCRs localize within cilia. Understanding the link between Smoothened and cilia defines common features, and distinctions, of GPCR signaling within the primary cilium. This article is categorized under: Signaling Pathways > Global Signaling Mechanisms Signaling Pathways > Cell Fate Signaling.

Lamprey Parapinopsin ("UVLamP"): a Bistable UV-Sensitive Optogenetic Switch for Ultrafast Control of GPCR Pathways

Optogenetics uses light‐sensitive proteins, so‐called optogenetic tools, for highly precise spatiotemporal control of cellular states and signals. The major limitations of such tools include the overlap of excitation spectra, phototoxicity, and lack of sensitivity. The protein characterized in this study, the Japanese lamprey parapinopsin, which we named UVLamP, is a promising optogenetic tool to overcome these limitations. Using a hybrid strategy combining molecular, cellular, electrophysiological, and computational methods we elucidated a structural model of the dark state and probed the optogenetic potential of UVLamP. Interestingly, it is the first described bistable vertebrate opsin that has a charged amino acid interacting with the Schiff base in the dark state, that has no relevance for its photoreaction. UVLamP is a bistable UV‐sensitive opsin that allows for precise and sustained optogenetic control of G protein‐coupled receptor (GPCR) pathways and can be switched on, but more importantly also off within milliseconds via lowintensity short light pulses. UVLamP exhibits an extremely narrow excitation spectrum in the UV range allowing for sustained activation of the G_i/o pathway with a millisecond UV light pulse. Its sustained pathway activation can be switched off, surprisingly also with a millisecond blue light pulse, minimizing phototoxicity. Thus, UVLamP serves as a minimally invasive, narrow‐bandwidth probe for controlling the G_i/o pathway, allowing for combinatorial use with multiple optogenetic tools or sensors. Because UVLamP activated G_i/o signals are generally inhibitory and decrease cellular activity, it has tremendous potential for health‐related applications such as relieving pain, blocking seizures, and delaying neurodegeneration.