A photochromic agonist of AMPA receptors

In vivo photopharmacological inhibition of hippocampal activity via multimodal probes - perspective and opening steps on experimental and computational challenges

Neurological conditions such as epilepsy can have a significant impact on people's lives. Here, we discuss a new perspective for the study/treatment of these conditions using photopharmacology. A multimodal, intracranial implant that incorporates fluidic channels for localised drug delivery, electrodes for recording and stimulation, and a light source for photoswitching is used for in vivo administration and deactivation of a photoresponsive AMPA antagonist. We review current advancements in the relevant disciplines and show experimentally that the inhibition of seizure-like events induced in the hippocampus by electrical stimulation can be altered upon switching the drug with light. We discuss the interconnection of the drug's photopharmacological properties with the design of the device by modelling light penetration into the rat brain with Monte Carlo simulations. This work delivers a new perspective, including initial experimental and computational efforts on in vivo photopharmacology to understand and eventually treat neurological conditions.

Reversible Control of Native GluN2B-Containing NMDA Receptors with Visible Light

NMDA receptors (NMDARs) are glutamate-gated ion channels playing a central role in synaptic transmission and plasticity. NMDAR dysregulation is linked to various neuropsychiatric disorders. This is particularly true for GluN2B-containing NMDARs (GluN2B-NMDARs), which have major pro-cognitive, but also pro-excitotoxic roles, although their exact involvement in these processes remains debated. Traditional GluN2B-selective antagonists suffer from slow and irreversible effects, limiting their use in native tissues. We therefore developed OptoNAM-3, a photoswitchable negative allosteric modulator selective for GluN2B-NMDARs. OptoNAM-3 provided light-induced reversible inhibition of GluN2B-NMDAR activity with precise temporal control both in vitro and in vivo on the behavior of freely moving Xenopus tadpoles. When bound to GluN2B-NMDARs, OptoNAM-3 displayed remarkable red-shifting of its photoswitching properties allowing the use of blue light instead of UV light to turn-off its activity, which we attributed to geometric constraints imposed by the binding site onto the azobenzene moiety of the ligand. This study therefore highlights the importance of the binding site in shaping the photochemical properties of azobenzene-based photoswitches. In addition, by enabling selective, fast, and reversible photocontrol of native GluN2B-NMDARs with in vivo compatible photochemical properties (visible light), OptoNAM-3 should be a useful tool for the investigation of the GluN2B-NMDAR physiology in native tissues.

Light-Sensitive Open Channel Block of Ionotropic Glutamate Receptors by Quaternary Ammonium Azobenzene Derivatives

Glutamate ionotropic receptors mediate fast excitation processes in the central nervous system of vertebrates and play an important role in synaptic plasticity, learning, and memory. Here, we describe the action of two azobenene-containing compounds, AAQ (acrylamide-azobenzene-quaternary ammonium) and QAQ (quaternary ammonium-azobenzene-quaternary ammonium), which produced rapid and fully reversible light-dependent inhibition of glutamate ionotropic receptors. The compounds demonstrated voltage-dependent inhibition with only minor voltage-independent allosteric action. Calcium-impermeable AMPA receptors had weaker sensitivity compared to NMDA and calcium-permeable AMPA receptors. We further revealed that the compounds bound to NMDA and calcium-permeable AMPA receptors in different modes. They were able to enter the wide selectivity filter of AMPA receptors, and strong negative voltages caused permeation into the cytoplasm. The narrow selectivity filter of the NMDA receptors did not allow the molecules to bypass them; therefore, QAQ and AAQ bound to the shallow channel site and prevented channel closure by a foot-in-the-door mechanism. Computer simulations employing available AMPA and NMDA receptor structures readily reproduced the experimental findings, allowing for the structure-based design of more potent and selective drugs in the future. Thus, our work creates a framework for the development of light-sensitive blockers of calcium-permeable AMPA receptors, which are desirable tools for neuroscience.

Synthesis and photochromism of catechol-containing symmetrical azobenzene compounds

Symmetrical azobenzene derivatives with two catechol groups, 1d-4d, were synthesized as kinds of novel compounds, and the structures were confirmed using mass spectrometry and nuclear magnetic resonance spectroscopy. These compounds could attain photostationary state rapidly in solution upon UV irradiation, and their photochromism had good reversibility. Substituents and their positions on azobenzene chromophore had obvious influence on the maximum absorption and photochromic performances of these as-synthesized compounds. Electron-donating group on ortho positions could contribute to the redshift π-π* band. The sulfonamide group that is bonded to dopamine molecules and azobenzene rings caused a negligible n-π* transition of cis isomer, resulting in photobleaching upon UV irradiation. Among the four compounds, 4d had the strongest electron-donating ortho-methoxy substituents and lower planarity; thus its absorption could decrease more significantly upon UV irradiation of the same intensity, and its cis-to-trans conversion could be up to 63%. Furthermore, owing to the presence of catechol groups, 4d showed an effective affinity and adhesion to substrate, and on the surface of substrate, a weak colour change could be observed upon UV irradiation, but the reversibility was poorer than that in solution.

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.

Deciphering Molecular Mechanisms and Intervening in Physiological and Pathophysiological Processes of Ca2+ Signaling Mechanisms Using Optogenetic Tools

Calcium ion channels are involved in numerous biological functions such as lymphocyte activation, muscle contraction, neurotransmission, excitation, hormone secretion, gene expression, cell migration, memory, and aging. Therefore, their dysfunction can lead to a wide range of cellular abnormalities and, subsequently, to diseases. To date various conventional techniques have provided valuable insights into the roles of Ca2+ signaling. However, their limited spatiotemporal resolution and lack of reversibility pose significant obstacles in the detailed understanding of the structure-function relationship of ion channels. These drawbacks could be partially overcome by the use of optogenetics, which allows for the remote and well-defined manipulation of Ca2+-signaling. Here, we review the various optogenetic tools that have been used to achieve precise control over different Ca2+-permeable ion channels and receptors and associated downstream signaling cascades. We highlight the achievements of optogenetics as well as the still-open questions regarding the resolution of ion channel working mechanisms. In addition, we summarize the successes of optogenetics in manipulating many Ca2+-dependent biological processes both in vitro and in vivo. In summary, optogenetics has significantly advanced our understanding of Ca2+ signaling proteins and the used tools provide an essential basis for potential future therapeutic application.

Photopharmacology of Ion Channels through the Light of the Computational Microscope

The optical control and investigation of neuronal activity can be achieved and carried out with photoswitchable ligands. Such compounds are designed in a modular fashion, combining a known ligand of the target protein and a photochromic group, as well as an additional electrophilic group for tethered ligands. Such a design strategy can be optimized by including structural data. In addition to experimental structures, computational methods (such as homology modeling, molecular docking, molecular dynamics and enhanced sampling techniques) can provide structural insights to guide photoswitch design and to understand the observed light-regulated effects. This review discusses the application of such structure-based computational methods to photoswitchable ligands targeting voltage- and ligand-gated ion channels. Structural mapping may help identify residues near the ligand binding pocket amenable for mutagenesis and covalent attachment. Modeling of the target protein in a complex with the photoswitchable ligand can shed light on the different activities of the two photoswitch isomers and the effect of site-directed mutations on photoswitch binding, as well as ion channel subtype selectivity. The examples presented here show how the integration of computational modeling with experimental data can greatly facilitate photoswitchable ligand design and optimization. Recent advances in structural biology, both experimental and computational, are expected to further strengthen this rational photopharmacology approach.

Pyrrole Hemithioindigo Antimitotics with Near-Quantitative Bidirectional Photoswitching that Photocontrol Cellular Microtubule Dynamics with Single-Cell Precision*

We report the first cellular application of the emerging near-quantitative photoswitch pyrrole hemithioindigo, by rationally designing photopharmaceutical PHTub inhibitors of the cytoskeletal protein tubulin. PHTubs allow simultaneous visible-light imaging and photoswitching in live cells, delivering cell-precise photomodulation of microtubule dynamics, and photocontrol over cell cycle progression and cell death. This is the first acute use of a hemithioindigo photopharmaceutical for high-spatiotemporal-resolution biological control in live cells. It additionally demonstrates the utility of near-quantitative photoswitches, by enabling a dark-active design to overcome residual background activity during cellular photopatterning. This work opens up new horizons for high-precision microtubule research using PHTubs and shows the cellular applicability of pyrrole hemithioindigo as a valuable scaffold for photocontrol of a range of other biological targets.

Optical Control of N-Methyl-d-aspartate Receptors by Azobenzene Quaternary Ammonium Compounds

Azobenzene-based quaternary ammonium compounds provide optical control of ion channels and are considered promising agents for regulation of neuronal excitability and for restoration of the photosensitivity of retinal cells. However, the selectivity of the action of these compounds remains insufficiently known. We studied the action of DENAQ (diethylamine-azobenzene-quaternary ammonium) and DMNAQ (dimethylamine-azobenzene-quaternary ammonium) on ionotropic glutamate receptors in rat brain neurons. In the dark, both compounds applied extracellularly caused fast and reversible inhibition of NMDA (N-methyl-d-aspartate) receptor-mediated currents with IC₅₀ values of 10 and 5 μM, respectively. Light-induced transformation of DENAQ and DMNAQ to their cis forms caused the IC₅₀ values to increase to 30 and 27 μM, respectively. Detailed analysis of this action revealed a complex nature consisting of fast inhibitory and slower potentiating effects. The AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors were only weakly affected independently on illumination. We conclude that, in addition to their long-lasting intracellular action, which persists after washout, azobenzene-based quaternary ammonium compounds should affect glutamatergic transmission and synaptic plasticity during treatment. Our findings also extend the list of soluble photoswitchable inhibitors of NMDA receptors. While the site(s) and mechanisms of action are unclear, the effect of DENAQ demonstrates strong pH dependence. At acidic pH values, DENAQ potentiates both NMDA and AMPA receptors.

The Nature of Azo-Substituted Carbocations: N-N π-Electron Stabilization versus Nitrogen Nonbonding Electron Stabilization

Computational and experimental studies reveal two different modes of cation stabilization by the phenylazo group. The first mode involves a relatively weak conjugative interaction with the azo π-bond, while the second mode involves an interaction with the nitrogen nonbonding electrons. The 4-phenylazo group is slightly rate-retarding in the solvolysis of cumyl chloride and benzyl mesylate derivatives but rate-enhancing in the solvolysis of α-CF₃ benzylic analogs. The phenylazo group can become a potent electron-donating group in cations such as [Me₂C-N═N-Ph]⁺. Nonbonding electron stabilization can be strong enough to offset the very powerful γ-silyl stabilization. In aromatic cyclopropenium and tropylium cations, the demand for stabilization is quite low, and the mode of phenylazo stabilization reverts back to the less-effective π-stabilization. The solvolysis of cis-4-phenylazo benzyl mesylate is faster than that of trans-4-phenylazo benzyl mesylate. Products formed suggest a stepwise ionization, cation isomerization, and nucleophile capture mechanism. Computational studies indicate a vanishingly small barrier for the isomerization of the cis-cation intermediate to the trans-cation.

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.

Photoisomerization of azobenzene units drives the photochemical reaction cycles of proteorhodopsin and bacteriorhodopsin analogues

In this study we substituted the retinal units in proteorhodopsin (PR) and bacteriorhodopsin (BR) with azo chromophores to investigate the mechanism of photoinduced proton pumping in rhodopsins and potentially develop new artificial molecular pumps. We used an indium tin oxide electrode to investigate the photoinduced proton transfer of the azo analogues of PR and BR. We also employed flash photolysis to determine the characteristic photocycles, comprising multiple transient intermediates, of the azo chromophore-bound PR and BR. Moreover, our studies of the photoinduced proton pumping functions of azo-proteoopsin and azo-bacterioopsin complexes revealed that they did not pump protons upon illumination, even though they underwent photoinduced proton transfer and the characteristic photocycle. Mutational analysis suggested that the proton pumping malfunction of the azo analogues of PR and BR resulted from the absence of proton transfer reactions through cytoplasmic channels, even though these reactions were evoked in extracellular channels. Based on our experimental findings, we propose herein a putative model of the proton transfer reaction mechanism for the azo analogues of PR and BR.

A Photoswitchable Inhibitor of the Human Serotonin Transporter

The human serotonin transporter (hSERT) terminates serotonergic signaling through reuptake of neurotransmitter into presynaptic neurons and is a target for many antidepressant drugs. We describe here the development of a photoswitchable hSERT inhibitor, termed azo-escitalopram, that can be reversibly switched between trans and cis configurations using light of different wavelengths. The dark-adapted trans isomer was found to be significantly less active than the cis isomer, formed upon irradiation.

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.

Persistent remodeling and neurodegeneration in late-stage retinal degeneration

Retinal remodeling is a progressive series of negative plasticity revisions that arise from retinal degeneration, and are seen in retinitis pigmentosa, age-related macular degeneration and other forms of retinal disease. These processes occur regardless of the precipitating event leading to degeneration. Retinal remodeling then culminates in a late-stage neurodegeneration that is indistinguishable from progressive central nervous system (CNS) proteinopathies. Following long-term deafferentation from photoreceptor cell death in humans, and long-lived animal models of retinal degeneration, most retinal neurons reprogram, then die. Glial cells reprogram into multiple anomalous metabolic phenotypes. At the same time, survivor neurons display degenerative inclusions that appear identical to progressive CNS neurodegenerative disease, and contain aberrant α-synuclein (α-syn) and phosphorylated α-syn. In addition, ultrastructural analysis indicates a novel potential mechanism for misfolded protein transfer that may explain how proteinopathies spread. While neurodegeneration poses a barrier to prospective retinal interventions that target primary photoreceptor loss, understanding the progression and time-course of retinal remodeling will be essential for the establishment of windows of therapeutic intervention and appropriate tuning and design of interventions. Finally, the development of protein aggregates and widespread neurodegeneration in numerous retinal degenerative diseases positions the retina as a ideal platform for the study of proteinopathies, and mechanisms of neurodegeneration that drive devastating CNS diseases.

Reversible silencing of endogenous receptors in intact brain tissue using 2-photon pharmacology

The physiological activity of proteins is often studied with loss-of-function genetic approaches, but the corresponding phenotypes develop slowly and can be confounding. Photopharmacology allows direct, fast, and reversible control of endogenous protein activity, with spatiotemporal resolution set by the illumination method. Here, we combine a photoswitchable allosteric modulator (alloswitch) and 2-photon excitation using pulsed near-infrared lasers to reversibly silence metabotropic glutamate 5 (mGlu5) receptor activity in intact brain tissue. Endogenous receptors can be photoactivated in neurons and astrocytes with pharmacological selectivity and with an axial resolution between 5 and 10 µm. Thus, 2-photon pharmacology using alloswitch allows investigating mGlu5-dependent processes in wild-type animals, including synaptic formation and plasticity, and signaling pathways from intracellular organelles.

Azologization of serotonin 5-HT3 receptor antagonists

The serotonin 5-hydroxytryptamine 3 receptor (5-HT3R) plays a unique role within the seven classes of the serotonin receptor family, as it represents the only ionotropic receptor, while the other six members are G protein-coupled receptors (GPCRs). The 5-HT3 receptor is related to chemo-/radiotherapy provoked emesis and dysfunction leads to neurodevelopmental disorders and psychopathologies. Since the development of the first serotonin receptor antagonist in the early 1990s, the range of highly selective and potent drugs expanded based on various chemical structures. Nevertheless, on-off-targeting of a pharmacophore's activity with high spatiotemporal resolution as provided by photopharmacology remains an unsolved challenge bearing additionally the opportunity for detailed receptor examination. In the presented work, we summarize the synthesis, photochromic properties and in vitro characterization of azobenzene-based photochromic derivatives of published 5-HT3R antagonists. Despite reported proof of principle of direct azologization, only one of the investigated derivatives showed antagonistic activity lacking isomer specificity.

Selective Single-Step Oxidation of Amine to Cross-Azo Compounds with an Unhampered Primary Benzyl Alcohol Functionality

This is the first report of a single-step synthesis of primary benzyl alcohol containing different cross-azo compounds (14 examples) by Cu(II) in the presence of a newly synthesized amino-ether heteroditopic macrobicycle cage. Interestingly, even with extreme conditions, the benzyl alcohol remains unoxidized by the Cu(II) catalyst due to the protective etherial pocket of the cage.

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.

Light-induced regulation of ligand-gated channel activity

The control of ligand-gated receptors with light using photochromic compounds has evolved from the first handcrafted examples to accurate, engineered receptors, whose development is supported by rational design, high-resolution protein structures, comparative pharmacology and molecular biology manipulations. Photoswitchable regulators have been designed and characterized for a large number of ligand-gated receptors in the mammalian nervous system, including nicotinic acetylcholine, glutamate and GABA receptors. They provide a well-equipped toolbox to investigate synaptic and neuronal circuits in all-optical experiments. This focused review discusses the design and properties of these photoswitches, their applications and shortcomings and future perspectives in the field.

LINKED ARTICLES

This article is part of a themed section on Nicotinic Acetylcholine Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.11/issuetoc.

Photopharmacological control of bipolar cells restores visual function in blind mice

Photopharmacological control of neuronal activity using synthetic photochromic ligands, or photoswitches, is a promising approach for restoring visual function in patients suffering from degenerative retinal diseases. Azobenzene photoswitches, such as AAQ and DENAQ, have been shown to restore the responses of retinal ganglion cells to light in mouse models of retinal degeneration but do not recapitulate native retinal signal processing. Here, we describe diethylamino-azo-diethylamino (DAD), a third-generation photoswitch that is capable of restoring retinal ganglion cell light responses to blue or white light. In acute brain slices of murine layer 2/3 cortical neurons, we determined that the photoswitch quickly relaxes to its inactive form in the dark. DAD is not permanently charged, and the uncharged form enables the photoswitch to rapidly and effectively cross biological barriers and thereby access and photosensitize retinal neurons. Intravitreal injection of DAD restored retinal light responses and light-driven behavior to blind mice. Unlike DENAQ, DAD acts upstream of retinal ganglion cells, primarily conferring light sensitivity to bipolar cells. Moreover, DAD was capable of generating ON and OFF visual responses in the blind retina by utilizing intrinsic retinal circuitry, which may be advantageous for restoring visual function.

Synapses in the spotlight with synthetic optogenetics

Membrane receptors and ion channels respond to various stimuli and relay that information across the plasma membrane by triggering specific and timed processes. These include activation of second messengers, allowing ion permeation, and changing cellular excitability, to name a few. Gaining control over equivalent processes is essential to understand neuronal physiology and pathophysiology. Recently, new optical techniques have emerged proffering new remote means to control various functions of defined neuronal populations by light, dubbed optogenetics. Still, optogenetic tools do not typically address the activity of receptors and channels native to neurons (or of neuronal origin), nor gain access to their signaling mechanisms. A related method-synthetic optogenetics-bridges this gap by endowing light sensitivity to endogenous neuronal receptors and channels by the appending of synthetic, light-receptive molecules, or photoswitches. This provides the means to photoregulate neuronal receptors and channels and tap into their native signaling mechanisms in select regions of the neurons, such as the synapse. This review discusses the development of synthetic optogenetics as a means to study neuronal receptors and channels remotely, in their natural environment, with unprecedented spatial and temporal precision, and provides an overview of tool design, mode of action, potential clinical applications and insights and achievements gained.

Optical control of AMPA receptors using a photoswitchable quinoxaline-2,3-dione antagonist

We have developed the first photoswitchable AMPA receptor antagonist, termed ShuBQX-3. It permits the precise optical control of AMPA receptors and exhibits a remarkable red-shifting of its photoswitching properties when bound to the receptor.

AMPA receptors respond to the neurotransmitter glutamate and play a critical role in excitatory neurotransmission. They have been implicated in several psychiatric disorders and have rich pharmacology. Antagonists of AMPA receptors have been explored as drugs and one has even reached the clinic. We now introduce a freely diffusible photoswitchable antagonist that is selective for AMPA receptors and endows them with light-sensitivity. Our photoswitch, ShuBQX-3, is active in its dark-adapted trans-isoform but is significantly less active as its cis-isoform. ShuBQX-3 exhibits a remarkable red-shifting of its photoswitching properties through interactions with the AMPA receptor ligand binding site. Since it can be used to control action potential firing with light, it could emerge as a powerful tool for studying synaptic transmission with high spatial and temporal precision.

Tweaking Subtype Selectivity and Agonist Efficacy at (S)-2-Amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propionic acid (AMPA) Receptors in a Small Series of BnTetAMPA Analogues

A series of analogues of the (S)-2-Amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propionic acid (AMPA) receptor agonist BnTetAMPA (5b) were synthesized and characterized pharmacologically in radioligand binding assays at native and cloned AMPA receptors and functionally by two-electrode voltage clamp electrophysiology at the four homomeric AMPA receptors expressed in Xenopus laevis oocytes. The analogues 6 and 7 exhibit very different pharmacological profiles with binding affinity preference for the subtypes GluA1 and GluA3, respectively. X-ray crystal structures of three ligands (6, 7, and 8) in complex with the agonist binding domain (ABD) of GluA2 show that they induce full domain closure despite their low agonist efficacies. Trp767 in GluA2 ABD could be an important determinant for partial agonism of this compound series at AMPA receptors, since agonist efficacy also correlated with the location of the Trp767 side chain.

Restoring Light Sensitivity in Blind Retinae Using a Photochromic AMPA Receptor Agonist

Retinal degenerative diseases can have many possible causes and are currently difficult to treat. As an alternative to therapies that require genetic manipulation or the implantation of electronic devices, photopharmacology has emerged as a viable approach to restore visual responses. Here, we present a new photopharmacological strategy that relies on a photoswitchable excitatory amino acid, ATA. This freely diffusible molecule selectively activates AMPA receptors in a light-dependent fashion. It primarily acts on amacrine and retinal ganglion cells, although a minor effect on bipolar cells has been observed. As such, it complements previous pharmacological approaches based on photochromic channel blockers and increases the potential of photopharmacology in vision restoration.

Orthogonal Optical Control of a G Protein-Coupled Receptor with a SNAP-Tethered Photochromic Ligand

The covalent attachment of synthetic photoswitches is a general approach to impart light sensitivity onto native receptors. It mimics the logic of natural photoreceptors and significantly expands the reach of optogenetics. Here we describe a novel photoswitch design-the photoswitchable orthogonal remotely tethered ligand (PORTL)-that combines the genetically encoded SNAP-tag with photochromic ligands connected to a benzylguanine via a long flexible linker. We use the method to convert the G protein-coupled receptor mGluR2, a metabotropic glutamate receptor, into a photoreceptor (SNAG-mGluR2) that provides efficient optical control over the neuronal functions of mGluR2: presynaptic inhibition and control of excitability. The PORTL approach enables multiplexed optical control of different native receptors using distinct bioconjugation methods. It should be broadly applicable since SNAP-tags have proven to be reliable, many SNAP-tagged receptors are already available, and photochromic ligands on a long leash are readily designed and synthesized.

Optical control of NMDA receptors with a diffusible photoswitch

N-methyl-D-aspartate receptors (NMDARs) play a central role in synaptic plasticity, learning and memory, and are implicated in various neuronal disorders. We synthesized a diffusible photochromic glutamate analogue, azobenzene-triazole-glutamate (ATG), which is specific for NMDARs and functions as a photoswitchable agonist. ATG is inactive in its dark-adapted trans-isoform, but can be converted into its active cis-isoform using one-photon (near UV) or two-photon (740 nm) excitation. Irradiation with violet light photo-inactivates ATG within milliseconds, allowing agonist removal on the timescale of NMDAR deactivation. ATG is compatible with Ca²⁺ imaging and can be used to optically mimic synaptic coincidence detection protocols. Thus, ATG can be used like traditional caged glutamate compounds, but with the added advantages of NMDAR specificity, low antagonism of GABAR-mediated currents, and precise temporal control of agonist delivery.

N-methyl-D-aspartate receptors (NMDARs) play a central role in synaptic plasticity, learning and memory. Here the authors describe azobenzene-triazole-glutamate (ATG), a new diffusible photoswitchable agonist that allows precise temporal control over NMDAR activity.

Molecular Dynamics Investigation of gluazo, a Photo-Switchable Ligand for the Glutamate Receptor GluK2

Photochromic ligands (PCLs), defined as photoswitchable molecules that are able to endow native receptors with a sensitivity towards light, have become a promising photopharmacological tool for various applications in biology. In general, PCLs consist of a ligand of the target receptor covalently linked to an azobenzene, which can be reversibly switched between two configurations upon light illumination. Gluazo, as a PCL that targets excitatory amino acid receptors, in its dark-adapted trans iso-form was characterized to be a partial agonist of the kainate glutamate receptor GluK2. Application of UV light leads to the formation of the cis form, with remarkedly reduced affinity towards GluK2. The mechanism of the change of ligand affinity induced by the photoisomerization was unresolved. The presented computational study explains how the isomerization of such a PCL affects the structural changes in the target receptor that lead to its activation.

A roadmap to success in photopharmacology

Light is a fascinating phenomenon that ties together physics, chemistry, and biology. It is unmatched in its ability to confer information with temporal and spatial precision and has been used to map objects on the scale of tens of nanometers (10(-8) m) to light years (10(16) m). This information, gathered through super-resolution microscopes or space-based telescopes, is ultimately funneled through the human visual system, which is a miracle in itself. It allows us to see the Andromeda galaxy at night, an object that is 2.5 million light years away and very dim, and ski the next day in bright sunlight at an intensity that is 12 orders of magnitude higher. Human vision is only one of many photoreceptive systems that have evolved on earth and are found in all kingdoms of life. These systems rely on molecular photoswitches, such as retinal or tetrapyrrols, which undergo transient bond isomerizations or bond formations upon irradiation. The set of chromophores that have been employed in Nature for this purpose is surprisingly small. Nevertheless, they control a wide variety of biological functions, which have recently been significantly increased through the rapid development of optogenetics. Optogenetics originated as an effort to control neural function with genetically encoded photoreceptors that use abundant chromophores, in particular retinal. It now covers a variety of cellular functions other than excitability and has revolutionized the control of biological pathways in neuroscience and beyond. Chemistry has provided a large repertoire of synthetic photoswitches with highly tunable properties. Like their natural counterparts, these chromophores can be attached to proteins to effectively put them under optical control. This approach has enabled a new type of synthetic photobiology that has gone under various names to distinguish it from optogenetics. We now call it photopharmacology. Here we trace our involvement in this field, starting with the first light-sensitive potassium channel (SPARK) and concluding with our most recent work on photoswitchable fatty acids. Instead of simply providing a historical account of our efforts, we discuss the design criteria that guided our choice of molecules and receptors. As such, we hope to provide a roadmap to success in photopharmacology and make a case as to why synthetic photoswitches, properly designed and made available through well-planned and efficient syntheses, should have a bright future in biology and medicine.

Selective photoregulation of the activity of glycogen synthase and glycogen phosphorylase, two key enzymes in glycogen metabolism

Glycogen is a polymer of α-1,4- and α-1,6-linked glucose units that provides a readily available source of energy in living organisms. Glycogen synthase (GS) and glycogen phosphorylase (GP) are the two enzymes that control, respectively, the synthesis and degradation of this polysaccharide and constitute adequate pharmacological targets to modulate cellular glycogen levels, by means of inhibition of their catalytic activity. Here we report on the synthesis and biological evaluation of a selective inhibitor that consists of an azobenzene moiety glycosidically linked to the anomeric carbon of a glucose molecule. In the ground state, the more stable (E)-isomer of the azobenzene glucoside had a slight inhibitory effect on rat muscle GP (RMGP, IC50 = 4.9 mM) and Escherichia coli GS (EcGS, IC50 = 1.6 mM). After irradiation and subsequent conversion to the (Z)-form, the inhibitory potency of the azobenzene glucoside did not significantly change for RMGP (IC50 = 2.4 mM), while its effect on EcGS increased 50-fold (IC50 = 32 μM). Sucrose synthase 4 from potatoes, a glycosyltransferase that does not operate on glycogen, was only slightly inhibited by the (E)-isomer (IC50 = 0.73 mM). These findings could be rationalized on the basis of kinetic and computer-aided docking analysis, which indicated that both isomers of the azobenzene glucoside mimic the EcGS acceptor substrate and exert their inhibitory effect by binding to the glycogen subsite in the active center of the enzyme. The ability to selectively photoregulate the catalytic activity of key enzymes of glycogen metabolism may represent a new approach for the treatment of glycogen metabolism disorders.

AzoCholine Enables Optical Control of Alpha 7 Nicotinic Acetylcholine Receptors in Neural Networks

Nicotinic acetylcholine receptors (nAChRs) are essential for cellular communication in higher organisms. Even though a vast pharmacological toolset to study cholinergic systems has been developed, control of endogenous neuronal nAChRs with high spatiotemporal precision has been lacking. To address this issue, we have generated photoswitchable nAChR agonists and re-evaluated the known photochromic ligand, BisQ. Using electrophysiology, we found that one of our new compounds, AzoCholine, is an excellent photoswitchable agonist for neuronal α7 nAChRs, whereas BisQ was confirmed to be an agonist for the muscle-type nAChR. AzoCholine could be used to modulate cholinergic activity in a brain slice and in dorsal root ganglion neurons. In addition, we demonstrate light-dependent perturbation of behavior in the nematode, Caenorhabditis elegans.

Photoreversible prodrugs and protags: switching the release of maleimides by using light under physiological conditions

A water-soluble furyl-substituted diarylethene derivative has been prepared that can undergo reversible Diels-Alder reactions with maleimides to yield photoswitchable Diels-Alder adducts. Employing bioorthogonal visible light, the release of therapeutically effective concentrations of maleimide-based reactive inhibitors or labels from these "prodrugs" or "protags" could be photoreversibly triggered in buffered, aqueous solution at body temperature. It is shown how the release properties can be fine-tuned and a thorough investigation of the release dynamics is presented. Our system should allow for spatiotemporal control over the inhibition and labeling of specific protein targets and is ready to be surveyed in living organisms.

Controlling ionotropic and metabotropic glutamate receptors with light: principles and potential

Light offers unique advantages for studying and manipulating biomolecules and the cellular processes that they control. Optical control of ionotropic and metabotropic glutamate receptors has garnered significant interest, since these receptors are central to signaling at neuronal synapses and only optical approaches provide the spatial and temporal resolution required to directly probe receptor function in cells and tissue. Following the classical method of glutamate photo-uncaging, recently developed methods have added other forms of remote control, including those with high molecular specificity and genetic targeting. These tools open the door to the direct optical control of synaptic transmission and plasticity, as well as the probing of native receptor function in intact neural circuits.

Understanding the fast thermal isomerisation of azophenols in glassy and liquid-crystalline polymers

The good solubility of azophenols in low molar mass liquid crystals together with the ability of their related polymers to form homogeneous nematic and glassy thin films make such azoderivatives valuable chromophores to get a great variety of photoactivatable systems with fast switching speeds under ambient conditions. In fact, the final applicability of these systems is mainly determined by the thermal cis-to-trans isomerisation rate of the photoactive azophenol used, in other words, by the intimate mechanism the reaction goes through. The kinetico-mechanistic study reported herein shows that the rate of the thermal back reaction for azophenols is very sensitive to the local environment where the azo chromophore is located, mainly to its capability to establish hydrogen bonding with its surroundings. With a proper design, azophenol-based polymers can exhibit thermal isomerisation rates as fast as those of the monomers in solution even without the presence of any solvent.