Dual optical control and mechanistic insights into photoswitchable group II and III metabotropic glutamate receptors

Projection-Targeted Photopharmacology Reveals Distinct Anxiolytic Roles for Presynaptic mGluR2 in Prefrontal- and Insula-Amygdala Synapses

Dissecting how membrane receptors regulate neural circuit function is critical for deciphering basic principles of neuromodulation and mechanisms of therapeutic drug action. Classical pharmacological and genetic approaches are not well-equipped to untangle the roles of specific receptor populations, especially in long-range projections which coordinate communication between brain regions. Here we use viral tracing, electrophysiological, optogenetic, and photopharmacological approaches to determine how presynaptic metabotropic glutamate receptor 2 (mGluR2) activation in the basolateral amygdala (BLA) alters anxiety-related behavior. We find that mGluR2-expressing neurons from the ventromedial prefrontal cortex (vmPFC) and posterior insular cortex (pIC) preferentially target distinct cell types and subregions of the BLA to regulate different forms of avoidant behavior. Using projection-specific photopharmacological activation, we find that mGluR2-mediated presynaptic inhibition of vmPFC-BLA, but not pIC-BLA, connections can produce long-lasting decreases in spatial avoidance. In contrast, presynaptic inhibition of pIC-BLA connections decreased social avoidance, novelty-induced hypophagia, and increased exploratory behavior without impairing working memory, establishing this projection as a novel target for the treatment of anxiety disorders. Overall, this work reveals new aspects of BLA neuromodulation with therapeutic implications while establishing a powerful approach for optical mapping of drug action via photopharmacology.

A "double-edged" role for type-5 metabotropic glutamate receptors in pain disclosed by light-sensitive drugs

Knowing the site of drug action is important to optimize effectiveness and address any side effects. We used light-sensitive drugs to identify the brain region-specific role of mGlu5 metabotropic glutamate receptors in the control of pain. Optical activation of systemic JF-NP-26, a caged, normally inactive, negative allosteric modulator (NAM) of mGlu5 receptors, in cingulate, prelimbic and infralimbic cortices and thalamus inhibited neuropathic pain hypersensitivity. Systemic treatment of alloswitch-1, an intrinsically active mGlu5 receptor NAM, caused analgesia, and the effect was reversed by light-induced drug inactivation in in the prelimbic and infralimbic cortices, and thalamus. This demonstrates that mGlu5 receptor blockade in the medial prefrontal cortex and thalamus is both sufficient and necessary for the analgesic activity of mGlu5 receptor antagonists. Surprisingly, when light was delivered in the basolateral amygdala, local activation of systemic JF-NP-26 reduced pain thresholds, whereas inactivation of alloswitch-1 enhanced analgesia. Electrophysiological analysis showed that alloswitch-1 increased excitatory synaptic responses in prelimbic pyramidal neurons evoked by stimulation of BLA input, and decreased feedforward inhibition of amygdala output neurons by BLA. Both effects were reversed by optical silencing and reinstated by optical reactivation of alloswitch-1. These findings demonstrate for the first time that the action of mGlu5 receptors in the pain neuraxis is not homogenous, and suggest that blockade of mGlu5 receptors in the BLA may limit the overall analgesic activity of mGlu5 receptor antagonists. This could explain the suboptimal effect of mGlu5 NAMs on pain in human studies and validate photopharmacology as an important tool to determine ideal target sites for systemic drugs.

Development of Photoswitchable Tethered Ligands that Target the μ-Opioid Receptor

Converting known ligands into photoswitchable derivatives offers the opportunity to modulate compound structure with light and hence, biological activity. In doing so, these probes provide unique control when evaluating G-protein-coupled receptor (GPCR) mechanism and function. Further conversion of such compounds into covalent probes, known as photoswitchable tethered ligands (PTLs), offers additional advantages. These include localization of the PTLs to the receptor binding pocket. Covalent localization increases local ligand concentration, improves site selectivity and may improve the biological differences between the respective isomers. This work describes chemical, photophysical and biochemical characterizations of a variety of PTLs designed to target the μ-opioid receptor (μOR). These PTLs were modeled on fentanyl, with the lead disulfide-containing agonist found to covalently interact with a cysteine-enriched mutant of this medically-relevant receptor.

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.

Optical Control of Dopamine D2-like Receptors with Cell-Specific Fast-Relaxing Photoswitches

Dopamine D2-like receptors (D2R, D3R, and D4R) control diverse physiological and behavioral functions and are important targets for the treatment of a variety of neuropsychiatric disorders. Their complex distribution and activation kinetics in the brain make it difficult to target specific receptor populations with sufficient precision. We describe a new toolkit of light-activatable, fast-relaxing, covalently taggable chemical photoswitches that fully activate, partially activate, or block D2-like receptors. This technology combines the spatiotemporal precision of a photoswitchable ligand (P) with cell type and spatial specificity of a genetically encoded membrane anchoring protein (M) to which the P tethers. These tools set the stage for targeting endogenous D2-like receptor signaling with molecular, cellular, and spatiotemporal precision using only one wavelength of light.

Modulating cell signalling in vivo with magnetic nanotransducers

Weak magnetic fields offer nearly lossless transmission of signals within biological tissue. Magnetic nanomaterials are capable of transducing magnetic fields into a range of biologically relevant signals in vitro and in vivo. These nanotransducers have recently enabled magnetic control of cellular processes, from neuronal firing and gene expression to programmed apoptosis. Effective implementation of magnetically controlled cellular signalling relies on careful tailoring of magnetic nanotransducers and magnetic fields to the responses of the intended molecular targets. This primer discusses the versatility of magnetic modulation modalities and offers practical guidelines for selection of appropriate materials and field parameters, with a particular focus on applications in neuroscience. With recent developments in magnetic instrumentation and nanoparticle chemistries, including those that are commercially available, magnetic approaches promise to empower research aimed at connecting molecular and cellular signalling to physiology and behaviour in untethered moving subjects.

Photopharmacology for vision restoration

Blinding diseases that are caused by degeneration of rod and cone photoreceptor cells often spare the rest of the retinal circuit, from bipolar cells, which are directly innervated by photoreceptor cells, to the output ganglion cells that project axons to the brain. A strategy for restoring vision is to introduce light sensitivity to the surviving cells of the retina. One approach is optogenetics, in which surviving cells are virally transfected with a gene encoding a signaling protein that becomes sensitive to light by binding to the biologically available chromophore retinal, the same chromophore that is used by the opsin photo-detectors of rods and cones. A second approach uses photopharmacology, in which a synthetic photoswitch associates with a native or engineered ion channel or receptor. We review these approaches and look ahead to the next generation of advances that could reconstitute core aspects of natural vision.

Optogenetic Methods to Investigate Brain Alterations in Preclinical Models

Investigating the neuronal dynamics supporting brain functions and understanding how the alterations in these mechanisms result in pathological conditions represents a fundamental challenge. Preclinical research on model organisms allows for a multiscale and multiparametric analysis in vivo of the neuronal mechanisms and holds the potential for better linking the symptoms of a neurological disorder to the underlying cellular and circuit alterations, eventually leading to the identification of therapeutic/rescue strategies. In recent years, brain research in model organisms has taken advantage, along with other techniques, of the development and continuous refinement of methods that use light and optical approaches to reconstruct the activity of brain circuits at the cellular and system levels, and to probe the impact of the different neuronal components in the observed dynamics. These tools, combining low-invasiveness of optical approaches with the power of genetic engineering, are currently revolutionizing the way, the scale and the perspective of investigating brain diseases. The aim of this review is to describe how brain functions can be investigated with optical approaches currently available and to illustrate how these techniques have been adopted to study pathological alterations of brain physiology.

Photoswitchable Serotonins for Optical Control of the 5-HT2A Receptor

Serotonin receptors play central roles in neuromodulation and are critical drug targets for psychiatric disorders. Optical control of serotonin receptor subtypes has the potential to greatly enhance our understanding of the spatiotemporal dynamics of receptor function. While other neuromodulatory receptors have been successfully rendered photoswitchable, reversible photocontrol of serotonin receptors has not been achieved, representing a major gap in GPCR photopharmacology. Herein, we develop the first tools that allow for such control. Azo5HT-2 shows light-dependent 5-HT_2A R agonism, with greater activity in the cis-form. Based on docking and test compound analysis, we also develop photoswitchable orthogonal, remotely-tethered ligands (PORTLs). These BG-Azo5HTs provide rapid, reversible, and repeatable optical control following conjugation to SNAP-tagged 5-HT_2A R. Overall, this study provides a foundation for the broad extension of photopharmacology to the serotonin receptor family.

Chemogenetics of cell surface receptors: beyond genetic and pharmacological approaches

Cell surface receptors transmit extracellular information into cells. Spatiotemporal regulation of receptor signaling is crucial for cellular functions, and dysregulation of signaling causes various diseases. Thus, it is highly desired to control receptor functions with high spatial and/or temporal resolution. Conventionally, genetic engineering or chemical ligands have been used to control receptor functions in cells. As the alternative, chemogenetics has been proposed, in which target proteins are genetically engineered to interact with a designed chemical partner with high selectivity. The engineered receptor dissects the function of one receptor member among a highly homologous receptor family in a cell-specific manner. Notably, some chemogenetic strategies have been used to reveal the receptor signaling of target cells in living animals. In this review, we summarize the developing chemogenetic methods of transmembrane receptors for cell-specific regulation of receptor signaling. We also discuss the prospects of chemogenetics for clinical applications.

Advances in tethered photopharmacology for precise optical control of signaling proteins

To overcome the limitations of traditional pharmacology, the field of photopharmacology has developed around the central concept of using light to endow drug action with spatiotemporal precision. Tethered photopharmacology, where a photoswitchable ligand is covalently attached to a target protein, offers a particularly high degree of spatiotemporal control, as well as the ability to genetically target drug action and limit effects to specific protein subtypes. In this review, we describe the core engineering concepts of tethered pharmacology and highlight recent advances in harnessing the power of tethered photopharmacology for an expanded palette of targets and conjugation modes using new, complementary strategies. We also discuss the various applications, including mechanistic studies from the molecular biophysical realm to in vivo studies in behaving animals, that demonstrate the power of tethered pharmacology.

Melding Synthetic Molecules and Genetically Encoded Proteins to Forge New Tools for Neuroscience

Unraveling the complexity of the brain requires sophisticated methods to probe and perturb neurobiological processes with high spatiotemporal control. The field of chemical biology has produced general strategies to combine the molecular specificity of small-molecule tools with the cellular specificity of genetically encoded reagents. Here, we survey the application, refinement, and extension of these hybrid small-molecule:protein methods to problems in neuroscience, which yields powerful reagents to precisely measure and manipulate neural systems. Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The emergence of molecular systems neuroscience

Systems neuroscience is focused on how ensemble properties in the brain, such as the activity of neuronal circuits, gives rise to internal brain states and behavior. Many of the studies in this field have traditionally involved electrophysiological recordings and computational approaches that attempt to decode how the brain transforms inputs into functional outputs. More recently, systems neuroscience has received an infusion of approaches and techniques that allow the manipulation (e.g., optogenetics, chemogenetics) and imaging (e.g., two-photon imaging, head mounted fluorescent microscopes) of neurons, neurocircuits, their inputs and outputs. Here, we will review novel approaches that allow the manipulation and imaging of specific molecular mechanisms in specific cells (not just neurons), cell ensembles and brain regions. These molecular approaches, with the specificity and temporal resolution appropriate for systems studies, promise to infuse the field with novel ideas, emphases and directions, and are motivating the emergence of a molecularly oriented systems neuroscience, a new discipline that studies how the spatial and temporal patterns of molecular systems modulate circuits and brain networks, and consequently shape the properties of brain states and behavior.

Neurophotonic tools for microscopic measurements and manipulation: status report

Neurophotonics was launched in 2014 coinciding with the launch of the BRAIN Initiative focused on development of technologies for advancement of neuroscience. For the last seven years, Neurophotonics' agenda has been well aligned with this focus on neurotechnologies featuring new optical methods and tools applicable to brain studies. While the BRAIN Initiative 2.0 is pivoting towards applications of these novel tools in the quest to understand the brain, this status report reviews an extensive and diverse toolkit of novel methods to explore brain function that have emerged from the BRAIN Initiative and related large-scale efforts for measurement and manipulation of brain structure and function. Here, we focus on neurophotonic tools mostly applicable to animal studies. A companion report, scheduled to appear later this year, will cover diffuse optical imaging methods applicable to noninvasive human studies. For each domain, we outline the current state-of-the-art of the respective technologies, identify the areas where innovation is needed, and provide an outlook for the future directions.

Photo-isomerizable tweezers to probe ionotropic receptor mechanisms

Ligand-gated ion channels (LGIC, also referred to as ionotropic receptors) are important transmembrane proteins that open to allow ions to flow across the membrane and locally modify the membrane potential in response to the binding of a ligand. For more than a decade, a tremendous effort has been carried out in the determination of many LGIC structures in high resolution, leading to an unprecedented molecular description of channel gating. However, it is sometimes difficult to classify experimentally derived structures to their corresponding functional states, and alternative methods may help resolve or refine this issue. In this review, we focus on the application of photo-isomerizable tweezers (PIT) as a powerful strategy to interrogate molecular mechanisms of LGIC while assessing their functionality by electrophysiology.

Membrane trafficking and positioning of mGluRs at presynaptic and postsynaptic sites of excitatory synapses

The plethora of functions of glutamate in the brain are mediated by the complementary actions of ionotropic and metabotropic glutamate receptors (mGluRs). The ionotropic glutamate receptors carry most of the fast excitatory transmission, while mGluRs modulate transmission on longer timescales by triggering multiple intracellular signaling pathways. As such, mGluRs mediate critical aspects of synaptic transmission and plasticity. Interestingly, at synapses, mGluRs operate at both sides of the cleft, and thus bidirectionally exert the effects of glutamate. At postsynaptic sites, group I mGluRs act to modulate excitability and plasticity. At presynaptic sites, group II and III mGluRs act as auto-receptors, modulating release properties in an activity-dependent manner. Thus, synaptic mGluRs are essential signal integrators that functionally couple presynaptic and postsynaptic mechanisms of transmission and plasticity. Understanding how these receptors reach the membrane and are positioned relative to the presynaptic glutamate release site are therefore important aspects of synapse biology. In this review, we will discuss the currently known mechanisms underlying the trafficking and positioning of mGluRs at and around synapses, and how these mechanisms contribute to synaptic functioning. We will highlight outstanding questions and present an outlook on how recent technological developments will move this exciting research field forward.

Reagents and models for detecting endogenous GLP1R and GIPR

Glucagon-like peptide-1 receptor (GLP1R) agonists target the GLP1R, whereas dual GLP1R/ gastric inhibitory polypeptide receptor (GIPR) agonists target both the GLP1R and GIPR. Despite the importance of these drug classes for the treatment of diabetes and obesity, still very little is known about the localization of GLP1R and GIPR themselves. Complicating matters is the low abundance of GLP1R and GIPR mRNA/protein, as well as a lack of specific and validated reagents for their detection. Without knowing where GLP1R and GIPR are located, it is difficult to propose mechanisms of action in the various target organs, and whether this is indirect or direct. In the current review, we will explain the steps needed to properly validate reagents for endogenous GLP1R/GIPR detection, describe the available approaches to visualize GLP1R/GIPR, and provide an update on the state-of-art. The overall aim is to provide a reference resource for researchers interested in GLP1R and GIPR signaling.

The evidence for and consequences of metabotropic glutamate receptor heterodimerization

Metabotropic glutamate receptors (mGluRs) are an essential component of the mammalian central nervous system. These receptors modulate neuronal excitability in response to extracellular glutamate through the activation of intracellular heterotrimeric G proteins. Like most other class C G protein-coupled receptors, mGluRs function as obligate dimer proteins, meaning they need to form dimer complexes before becoming functional receptors. All mGluRs possess the ability to homodimerize, but studies over the past ten years have demonstrated these receptors are also capable of forming heterodimers in specific patterns. These mGluR heterodimers appear to have their own unique biophysical behavior and pharmacology with both native and synthetic compounds with few rules having been identified that allow for prediction of the consequences of any particular mGluR pair forming heterodimers. Here, we review the relevant literature demonstrating the existence and consequences of mGluR heterodimerization. By collecting biophysical and pharmacological data of several mGluR heterodimers we demonstrate the lack of generalizable behavior of these complexes indicating that each individual dimeric pair needs to be investigated independently. Additionally, by combining sequence alignment and structural analysis, we propose that interactions between the β4-A Helix Loop and the D Helix in the extracellular domain of these receptors are the structural components that dictate heterodimerization compatibility. Finally, we discuss the potential implications of mGluR heterodimerization from the viewpoints of further developing our understanding of neuronal physiology and leveraging mGluRs as a therapeutic target for the treatment of pathophysiology.

Cell specific photoswitchable agonist for reversible control of endogenous dopamine receptors

Dopamine controls diverse behaviors and their dysregulation contributes to many disorders. Our ability to understand and manipulate the function of dopamine is limited by the heterogenous nature of dopaminergic projections, the diversity of neurons that are regulated by dopamine, the varying distribution of the five dopamine receptors (DARs), and the complex dynamics of dopamine release. In order to improve our ability to specifically modulate distinct DARs, here we develop a photo-pharmacological strategy using a Membrane anchored Photoswitchable orthogonal remotely tethered agonist for the Dopamine receptor (MP-D). Our design selectively targets D1R/D5R receptor subtypes, most potently D1R (MP-D1_ago), as shown in HEK293T cells. In vivo, we targeted dorsal striatal medium spiny neurons where the photo-activation of MP-D1_ago increased movement initiation, although further work is required to assess the effects of MP-D1_ago on neuronal function. Our method combines ligand and cell type-specificity with temporally precise and reversible activation of D1R to control specific aspects of movement. Our results provide a template for analyzing dopamine receptors.

Tethering-based chemogenetic approaches for the modulation of protein function in live cells

Proteins are the workhorse molecules performing various tasks to sustain life. To investigate the roles of a protein under physiological conditions, the rapid modulation of the protein with high specificity in a living system would be ideal, but achieving this is often challenging. To address this challenge, researchers have developed chemogenetic strategies for the rapid and selective modulation of protein function in live cells. Here, the target protein is modified genetically to become sensitive to a designer molecule that otherwise has no effect on other cellular biomolecules. One powerful chemogenetic strategy is to introduce a tethering point into the target protein, allowing covalent or non-covalent attachment of the designer molecule. In this tutorial review, we focus on tethering-based chemogenetic approaches for modulating protein function in live cells. We first describe genetic, optogenetic and chemical means to study protein function. These means lay the basis for the chemogenetic concept, which is explained in detail. The next section gives an overview, including advantages and limitations, of tethering tactics that have been employed for modulating cellular protein function. The third section provides examples of the modulation of cell-surface proteins using tethering-based chemogenetics through non-covalent tethering and covalent tethering for irreversible modulation or functional switching. The fourth section presents intracellular examples. The last section summarizes key considerations in implementing tethering-based chemogenetics and shows perspectives highlighting future directions and other applications of this burgeoning research field.

Optogenetic Techniques for Manipulating and Sensing G Protein-Coupled Receptor Signaling

G protein-coupled receptors (GPCRs) form the largest class of membrane receptors in the mammalian genome with nearly 800 human genes encoding for unique subtypes. Accordingly, GPCR signaling is implicated in nearly all physiological processes. However, GPCRs have been difficult to study due in part to the complexity of their function which can lead to a plethora of converging or diverging downstream effects over different time and length scales. Classic techniques such as pharmacological control, genetic knockout and biochemical assays often lack the precision required to probe the functions of specific GPCR subtypes. Here we describe the rapidly growing set of optogenetic tools, ranging from methods for optical control of the receptor itself to optical sensing and manipulation of downstream effectors. These tools permit the quantitative measurements of GPCRs and their downstream signaling with high specificity and spatiotemporal precision.

A fine-tuned azobenzene for enhanced photopharmacology in vivo

Despite the power of photopharmacology for interrogating signaling proteins, many photopharmacological systems are limited by their efficiency, speed, or spectral properties. Here, we screen a library of azobenzene photoswitches and identify a urea-substituted "azobenzene-400" core that offers bistable switching between cis and trans with improved kinetics, light sensitivity, and a red-shift. We then focus on the metabotropic glutamate receptors (mGluRs), neuromodulatory receptors that are major pharmacological targets. Synthesis of "BGAG_12,400," a photoswitchable orthogonal, remotely tethered ligand (PORTL), enables highly efficient, rapid optical agonism following conjugation to SNAP-tagged mGluR2 and permits robust optical control of mGluR1 and mGluR5 signaling. We then produce fluorophore-conjugated branched PORTLs to enable dual imaging and manipulation of mGluRs and highlight their power in ex vivo slice and in vivo behavioral experiments in the mouse prefrontal cortex. Finally, we demonstrate the generalizability of our strategy by developing an improved soluble, photoswitchable pore blocker for potassium channels.

Development of a Novel SNAP-Epitope Tag/Near-Infrared Imaging Assay to Quantify G Protein-Coupled Receptor Degradation in Human Cells

We have developed a novel reporter assay that leverages SNAP-epitope tag/near-infrared (NIR) imaging technology to monitor G protein-coupled receptor (GPCR) degradation in human cell lines. N-terminal SNAP-tagged GPCRs were subcloned and expressed in human embryonic kidney (HEK) 293 cells and then subjected to 24 h of cycloheximide (CHX)-chase degradation assays to quantify receptor degradation half-lives (t_1/2) using LICOR NIR imaging-polyacrylamide gel electrophoresis (PAGE) analysis. Thus far, we have used this method to quantify t_1/2 for all nine adrenergic (ADRA1A, ADRA1B, ADRA1D, ADRA2A, ADRA2B, ADRA2C, ADRB1, ADRB2, ADRB3), five somatostatin (SSTR1, SSTR2, SSTR3, SSTR4, SSTR5), four chemokine (CXCR1, CXCR2, CXCR3, CXCR5), and three 5-HT2 (5HT2A, 5HT2B, 5HT2C) receptor subtypes. SNAP-GPCR-CHX degradation t_1/2 values ranged from 0.52 h (ADRA1D) to 5.5 h (SSTR3). On the contrary, both the SNAP-tag alone and SNAP-tagged and endogenous β-actin were resistant to degradation with CHX treatment. Treatment with the proteasome inhibitor bortezomib produced significant but variable increases in SNAP-GPCR protein expression levels, indicating that SNAP-GPCR degradation primarily occurs through the proteasome. Remarkably, endogenous β2-adrenergic receptor/ADRB2 dynamic mass redistribution functional responses to norepinephrine were significantly decreased following CHX treatment, with a time course equivalent to that observed with the SNAP-ADRB2 degradation assay. We subsequently adapted this assay into a 96-well glass-bottom plate format to facilitate high-throughput GPCR degradation screening. t_1/2 values quantified for the α₁-adrenergic receptor subtypes (ADRA1A, ADRA1B, ADR1D) using the 96-well-plate format correlated with t_1/2 values quantified using NIR-PAGE imaging analysis. In summary, this novel assay permits precise quantitative analysis of GPCR degradation in human cells and can be readily adapted to quantify degradation for any membrane protein of interest.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

Recent advances in bioelectronics chemistry

Research in bioelectronics is highly interdisciplinary, with many new developments being based on techniques from across the physical and life sciences. Advances in our understanding of the fundamental chemistry underlying the materials used in bioelectronic applications have been a crucial component of many recent discoveries. In this review, we highlight ways in which a chemistry-oriented perspective may facilitate novel and deep insights into both the fundamental scientific understanding and the design of materials, which can in turn tune the functionality and biocompatibility of bioelectronic devices. We provide an in-depth examination of several developments in the field, organized by the chemical properties of the materials. We conclude by surveying how some of the latest major topics of chemical research may be further integrated with bioelectronics.

In Vivo Photopharmacology Enabled by Multifunctional Fibers

Photoswitchable ligands can add an optical switch to a target receptor or signaling cascade and enable reversible control of neural circuits. The application of this approach, termed photopharmacology, to behavioral experiments has been impeded by a lack of integrated hardware capable of delivering both light and compounds to deep brain regions in moving subjects. Here, we devise a hybrid photochemical genetic approach to target neurons using a photoswitchable agonist of the capsaicin receptor TRPV1, red-AzCA-4. Using multifunctional fibers with optical and microfluidic capabilities, we delivered a transgene coding for TRPV1 into the ventral tegmental area (VTA). This sensitized excitatory VTA neurons to red-AzCA-4, allowing us to optically control conditioned place preference in mice, thus extending applications of photopharmacology to behavioral experiments. Applied to endogenous receptors, our approach may accelerate future studies of molecular mechanisms underlying animal behavior.

Photocontrol of Metabotropic Glutamate Receptors: When One Agonist Is Not Enough, Make It Two

A current major challenge lies in controlling molecularly defined brain receptor and channel populations to investigate their function in vivo. In this issue of Neuron, Acosta-Ruiz et al. (2020) developed a highly versatile molecular toolkit to efficiently manipulate specific metabotropic glutamate receptor subtypes in brain circuits with light.

Optical control of neuronal ion channels and receptors

Light-controllable tools provide powerful means to manipulate and interrogate brain function with relatively low invasiveness and high spatiotemporal precision. Although optogenetic approaches permit neuronal excitation or inhibition at the network level, other technologies, such as optopharmacology (also known as photopharmacology) have emerged that provide molecular-level control by endowing light sensitivity to endogenous biomolecules. In this Review, we discuss the challenges and opportunities of photocontrolling native neuronal signalling pathways, focusing on ion channels and neurotransmitter receptors. We describe existing strategies for rendering receptors and channels light sensitive and provide an overview of the neuroscientific insights gained from such approaches. At the crossroads of chemistry, protein engineering and neuroscience, optopharmacology offers great potential for understanding the molecular basis of brain function and behaviour, with promises for future therapeutics.

Conformational pathway provides unique sensitivity to a synaptic mGluR

Metabotropic glutamate receptors (mGluRs) are dimeric G-protein-coupled receptors that operate at synapses. Macroscopic and single molecule FRET to monitor structural rearrangements in the ligand binding domain (LBD) of the mGluR7/7 homodimer revealed it to have an apparent affinity ~4000-fold lower than other mGluRs and a maximal activation of only ~10%, seemingly too low for activation at synapses. However, mGluR7 heterodimerizes, and we find it to associate with mGluR2 in the hippocampus. Strikingly, the mGluR2/7 heterodimer has high affinity and efficacy. mGluR2/7 shows cooperativity in which an unliganded subunit greatly enhances activation by agonist bound to its heteromeric partner, and a unique conformational pathway to activation, in which mGluR2/7 partially activates in the Apo state, even when its LBDs are held open by antagonist. High sensitivity and an unusually broad dynamic range should enable mGluR2/7 to respond to both glutamate transients from nearby release and spillover from distant synapses.

Branched Photoswitchable Tethered Ligands Enable Ultra-efficient Optical Control and Detection of G Protein-Coupled Receptors In Vivo

The limitations of classical drugs have spurred the development of covalently tethered photoswitchable ligands to control neuromodulatory receptors. However, a major shortcoming of tethered photopharmacology is the inability to obtain optical control with an efficacy comparable with that of the native ligand. To overcome this, we developed a family of branched photoswitchable compounds to target metabotropic glutamate receptors (mGluRs). These compounds permit photo-agonism of G_i/o-coupled group II mGluRs with near-complete efficiency relative to glutamate when attached to receptors via a range of orthogonal, multiplexable modalities. Through a chimeric approach, branched ligands also allow efficient optical control of G_q-coupled mGluR5, which we use to probe the spatiotemporal properties of receptor-induced calcium oscillations. In addition, we report branched, photoswitch-fluorophore compounds for simultaneous receptor imaging and manipulation. Finally, we demonstrate this approach in vivo in mice, where photoactivation of SNAP-mGluR2 in the medial prefrontal cortex reversibly modulates working memory in normal and disease-associated states.

Next-generation interfaces for studying neural function

Monitoring and modulating the diversity of signals used by neurons and glia in a closed-loop fashion is necessary to establish causative links between biochemical processes within the nervous system and observed behaviors. As developments in neural-interface hardware strive to keep pace with rapid progress in genetically encoded and synthetic reporters and modulators of neural activity, the integration of multiple functional features becomes a key requirement and a pressing challenge in the field of neural engineering. Electrical, optical and chemical approaches have been used to manipulate and record neuronal activity in vivo, with a recent focus on technologies that both integrate multiple modes of interaction with neurons into a single device and enable bidirectional communication with neural circuits with enhanced spatiotemporal precision. These technologies not only are facilitating a greater understanding of the brain, spinal cord and peripheral circuits in the context of health and disease, but also are informing the development of future closed-loop therapies for neurological, neuro-immune and neuroendocrine conditions.

Reversible Photocontrolled Nanopore Assembly

Self-assembly is a fundamental feature of biological systems, and control of such processes offers fascinating opportunities to regulate function. Fragaceatoxin C (FraC) is a toxin that upon binding to the surface of sphingomyelin-rich cells undergoes a structural metamorphosis, leading to the assembly of nanopores at the cell membrane and causing cell death. In this study we attached photoswitchable azobenzene pendants to various locations near the sphingomyelin binding pocket of FraC with the aim of remote controlling the nanopore assembly using light. We found several constructs in which the affinity of the toxin for biological membranes could be activated or deactivated by irradiation, thus enabling reversible photocontrol of pore formation. Notably, one construct was completely inactive in the thermally adapted state; it however induced full lysis of cultured cancer cells upon light irradiation. Selective irradiation also allowed isolation of individual nanopores in artificial lipid membranes. Photocontrolled FraC might find applications in photopharmacology for cancer therapeutics and has potential to be used for the fabrication of nanopore arrays in nanopore sensing devices, where the reconstitution, with high spatiotemporal precision, of single nanopores must be controlled.

Cell-Specific Neuropharmacology

Neuronal communication involves a multitude of neurotransmitters and an outstanding diversity of receptors and ion channels. Linking the activity of cell surface receptors and ion channels in defined neural circuits to brain states and behaviors has been a key challenge in neuroscience, since cell targeting is not possible with traditional neuropharmacology. We review here recent technologies that enable the effect of drugs to be restricted to specific cell types, thereby allowing acute manipulation of the brain's own proteins with circuit specificity. We highlight the importance of developing cell-specific neuropharmacology strategies for decoding the nervous system with molecular and circuit precision, and for developing future therapeutics with reduced side effects.

Glutamatergic Signaling in the Central Nervous System: Ionotropic and Metabotropic Receptors in Concert

Glutamate serves as both the mammalian brain's primary excitatory neurotransmitter and as a key neuromodulator to control synapse and circuit function over a wide range of spatial and temporal scales. This functional diversity is decoded by two receptor families: ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs). The challenges posed by the complexity and physiological importance of each of these subtypes has limited our appreciation and understanding of how these receptors work in concert. In this review, by comparing both receptor families with a focus on their crosstalk, we argue for a more holistic understanding of neural glutamate signaling.

Genetically Targeted Optical Control of an Endogenous G Protein-Coupled Receptor

G protein-coupled receptors (GPCRs) are membrane proteins that play important roles in biology. However, our understanding of their function in complex living systems is limited because we lack tools that can target individual receptors with sufficient precision. State-of-the-art approaches, including DREADDs, optoXRs, and PORTL gated-receptors, control GPCR signaling with molecular, cell type, and temporal specificity. Nonetheless, these tools are based on engineered non-native proteins that may (i) express at nonphysiological levels, (ii) localize and turnover incorrectly, and/or (iii) fail to interact with endogenous partners. Alternatively, membrane-anchored ligands (t-toxins, DARTs) target endogenous receptors with molecular and cell type specificity but cannot be turned on and off. In this study, we used a combination of chemistry, biology, and light to control endogenous metabotropic glutamate receptor 2 (mGluR2), a Family C GPCR, in primary cortical neurons. mGluR2 was rapidly, reversibly, and selectively activated with photoswitchable glutamate tethered to a genetically targeted-plasma membrane anchor (membrane anchored Photoswitchable Orthogonal Remotely Tethered Ligand; maPORTL). Photoactivation was tuned by adjusting the length of the PORTL as well as the expression level and geometry of the membrane anchor. Our findings provide a template for controlling endogenous GPCRs with cell type specificity and high spatiotemporal precision.

GPCR photopharmacology

New technologies for spatial and temporal remote control of G protein-coupled receptors (GPCRs) are necessary to unravel the complexity of GPCR signalling in cells, tissues and living organisms. An effective approach, recently developed, consists on the design of light-operated ligands whereby light-dependent GPCR activity regulation can be achieved. In this context, the use of light provides an advantage as it combines safety, easy delivery, high resolution and it does not interfere with most cellular processes. In this review we summarize the most relevant successful achievements in GPCR photopharmacology. These recent findings constitute a significant advance in research studies on the molecular dynamics of receptor activation and their physiological roles in vivo. Moreover, these molecules hold potential toward clinical uses as light-operated drugs, which can overcome some of the problems of conventional pharmacology.

Restoration of high-sensitivity and adapting vision with a cone opsin

Inherited and age-related retinal degenerative diseases cause progressive loss of rod and cone photoreceptors, leading to blindness, but spare downstream retinal neurons, which can be targeted for optogenetic therapy. However, optogenetic approaches have been limited by either low light sensitivity or slow kinetics, and lack adaptation to changes in ambient light, and not been shown to restore object vision. We find that the vertebrate medium wavelength cone opsin (MW-opsin) overcomes these limitations and supports vision in dim light. MW-opsin enables an otherwise blind retinitis pigmenotosa mouse to discriminate temporal and spatial light patterns displayed on a standard LCD computer tablet, displays adaption to changes in ambient light, and restores open-field novel object exploration under incidental room light. By contrast, rhodopsin, which is similar in sensitivity but slower in light response and has greater rundown, fails these tests. Thus, MW-opsin provides the speed, sensitivity and adaptation needed to restore patterned vision.

Designing azobenzene-based tools for controlling neurotransmission

Chemical and electrical signaling at the synapse is a dynamic process that is crucial to neurotransmission and pathology. Traditional pharmacotherapy has found countless applications in both academic labs and the clinic; however, diffusible drugs lack spatial and temporal precision when employed in heterogeneous tissues such as the brain. In the field of photopharmacology, chemical attachment of a synthetic photoswitch to a bioactive ligand allows cellular signaling to be controlled with light. Azobenzenes have remained the go-to photoswitch for biological applications due to their tunable photophysical properties, and can be leveraged to achieve reversible optical control of numerous receptors and ion channels. Here, we discuss the most recent advances in photopharmacology which will improve the use of azobenzene-based probes for neuroscience applications.

A light in the dark: state of the art and perspectives in optogenetics and optopharmacology for restoring vision

In the last decade, innovative therapeutic strategies against inherited retinal degenerations (IRDs) have emerged. In particular, chemical- and opto-genetics approaches or a combination of them have been identified for modulating neuronal/optical activity in order to restore vision in blinding diseases. The ‘chemical-genetics approach’ (optopharmacology) uses small molecules (exogenous photoswitches) for restoring light sensitivity by activating ion channels. The ‘opto-genetics approach’ employs light-activated photosensitive proteins (exogenous opsins), introduced by viral vectors in injured tissues, to restore light response. These approaches offer control of neuronal activities with spatial precision and limited invasiveness, although with some drawbacks. Currently, a combined therapeutic strategy (optogenetic pharmacology) is emerging. This review describes the state of the art and provides an overview of the future perspectives in vision restoration.

Light-Switchable Antagonists for the Histamine H1 Receptor at the Isolated Guinea Pig Ileum

The histamine H₁ G protein-coupled receptor (GPCR) plays an important role in allergy and inflammation. Existing drugs that address the H₁ receptor differ in their chemical structure, pharmacology, and side effects. Light-controllable spatial and temporal activity regulation of photochromic H₁ ligands may contribute to a better mechanistic understanding and the development of improved correlations between ligand structure and pharmacologic effects. We report photochromic H₁ receptor ligands, which were investigated in an organ-pharmacological assay. Initially, five photochromic azobenzene derivatives of reported dual H₁ -H₄ receptor antagonists were designed, synthesized, photochemically characterized, and organ-pharmacologically tested on the isolated guinea pig ileum. Among them, one compound [trans-19: (Z)-1-(4-chlorophenyl)-1-(4-methylpiperazin-1-yl)-N-(4-((E)-phenyldiazenyl)phenyl)methanimine] retained the antagonistic activity of its non-photochromic lead, and trans-cis isomerization by irradiation induced a fourfold difference in the pharmacological response. Further structural optimization resulted in two bathochromically shifted derivatives of 19 [NO₂ -substituted 35 {(Z)-1-(4-chlorophenyl)-1-(4-methylpiperazin-1-yl)-N-(4-((E)-(4-nitrophenyl)diazenyl)phenyl)methanimine} and SO₃ ^- -substituted 41 {4-((E)-(4-(((Z)-(4-chlorophenyl)(4-methylpiperazin-1-yl)methylene)amino)phenyl)diazenyl)benzenesulfonate}], which do not require the use of UV light for photoisomerization and which also have improved solubility and show reduced tissue impairment. The trans isomers of both compounds showed a remarkable increase in antagonistic activity relative to their lead trans-19; furthermore, a 46-fold difference in activity on the isolated guinea pig ileum was observed between trans- and cis-35.

Optical Regulation of Class C GPCRs by Photoswitchable Orthogonal Remotely Tethered Ligands

G protein-coupled receptors (GPCRs) respond to a wide range of extracellular cues to initiate complex downstream signaling cascades that control myriad aspects of cell function. Despite a long-standing appreciation of their importance to both basic physiology and disease treatment, it remains a major challenge to understand the dynamic activation patterns of GPCRs and the mechanisms by which they modulate biological processes at the molecular, cellular, and tissue levels. Unfortunately, classical methods of pharmacology and genetic knockout are often unable to provide the requisite precision needed to probe such questions. This is an especially pressing challenge for the class C GPCR family which includes receptors for the major excitatory and inhibitory neurotransmitters, glutamate and GABA, which signal in a rapid, spatially-delimited manner and contain many different subtypes whose roles are difficult to disentangle. The desire to manipulate class C GPCRs with spatiotemporal precision, genetic targeting, and subtype specificity has led to the development of a variety of photopharmacological tools. Of particular promise are the photoswitchable orthogonal remotely tethered ligands ("PORTLs") which attach to self-labeling tags that are genetically encoded into full length, wild-type metabotropic glutamate receptors (mGluRs) and allow the receptor to be liganded and un-liganded in response to different wavelengths of illumination. While powerful for studying class C GPCRs, a number of detailed considerations must be made when working with these tools. The protocol included here should provide a basis for the development, characterization, optimization, and application of PORTLs for a wide range of GPCRs.

SNAP-Tagged Nanobodies Enable Reversible Optical Control of a G Protein-Coupled Receptor via a Remotely Tethered Photoswitchable Ligand

G protein-coupled receptors (GPCRs) mediate the transduction of extracellular signals into complex intracellular responses. Despite their ubiquitous roles in physiological processes and as drug targets for a wide range of disorders, the precise mechanisms of GPCR function at the molecular, cellular, and systems levels remain partially understood. To dissect the function of individual receptor subtypes with high spatiotemporal precision, various optogenetic and photopharmacological approaches have been reported that use the power of light for receptor activation and deactivation. Here, we introduce a novel and, to date, most remote way of applying photoswitchable orthogonally remotely tethered ligands by using a SNAP-tag fused nanobody. Our nanobody-photoswitch conjugates can be used to target a green fluorescent protein-fused metabotropic glutamate receptor by either gene-free application of purified complexes or coexpression of genetically encoded nanobodies to yield robust, reversible control of agonist binding and subsequent downstream activation. By harboring and combining the selectivity and flexibility of both nanobodies and self-labeling proteins (or suicide enzymes), we set the stage for targeting endogenous receptors in vivo.

The role of beta cell heterogeneity in islet function and insulin release

It is becoming increasingly apparent that not all insulin-secreting beta cells are equal. Subtle differences exist at the transcriptomic and protein expression levels, with repercussions for beta cell survival/proliferation, calcium signalling and insulin release. Notably, beta cell heterogeneity displays plasticity during development, metabolic stress and type 2 diabetes mellitus (T2DM). Thus, heterogeneity or lack thereof may be an important contributor to beta cell failure during T2DM in both rodents and humans. The present review will discuss the molecular and cellular features of beta cell heterogeneity at both the single-cell and islet level, explore how this influences islet function and insulin release and look into the alterations that may occur during obesity and T2DM.

Restoring Vision to the Blind with Chemical Photoswitches

Degenerative retinal diseases such as retinitis pigmentosa (RP) and age-related macular degeneration (AMD) affect millions of people around the world and lead to irreversible vision loss if left untreated. A number of therapeutic strategies have been developed over the years to treat these diseases or restore vision to already blind patients. In this Review, we describe the development and translational application of light-sensitive chemical photoswitches to restore visual function to the blind retina and compare the translational potential of photoswitches with other vision-restoring therapies. This therapeutic strategy is enabled by an efficient fusion of chemical synthesis, chemical biology, and molecular biology and is broadly applicable to other biological systems. We hope this Review will be of interest to chemists as well as neuroscientists and clinicians.