Cloaked Caged Compounds: Chemical Probes for Two-Photon Optoneurobiology

Seeing and Cleaving: Turn-Off Fluorophore Uncaging and Its Application in Hydrogel Photopatterning and Traceable Neurotransmitter Photocages

The advancements in targeted drug release and experimental neuroscience have amplified the scientific interest in photolabile protecting groups (PPGs) and photouncaging. The growing need for the detection of uncaging events has led to the development of reporters with fluorescence turn-on upon uncaging. In contrast, fluorescent tags with turn-off properties have been drastically underexplored, although there are applications where they would be sought after. In this work, a rhodamine-based fluorescent tag is developed with signal turn-off following photouncaging. One-photon photolysis experiments reveal a ready loss of red fluorescence signal upon UV (365 nm) irradiation, while no significant change is observed in control experiments in the absence of PPG or with irradiation around the absorption maximum of the fluorophore (595 nm). The two-photon photolysis of the turn-off fluorescent tag is explored in hydrogel photolithography experiments. The hydrogel-bound tag enables the power-, dwell time-, and wavelength-dependent construction of intricate patterns and gradients. Finally, a prominent caged neurotransmitter (MNI-Glu) is modified with the fluorescent tag, resulting in the glutamate precursor named as GlutaTrace with fluorescence traceability and turn-off upon photouncaging. GlutaTrace is successfully applied for the visualization of glutamate precursor distribution following capillary microinjection and for the selective excitation of neurons in a mouse brain model.

Optical Control of TRPV1 Channels In Vitro with Tethered Photopharmacology

Transient receptor potential vanilloid 1 (TRPV1) is a nonselective cation channel that is important for nociception and inflammatory pain and is activated by a variety of nociceptive stimuli─including lipids such as capsaicin (CAP) and endocannabinoids. TRPV1's role in physiological systems is often studied by activating it with externally perfused ligands; however, this approach is plagued by poor spatiotemporal resolution. Lipid agonists are insoluble in physiological buffers and can permeate membranes to accumulate nonselectively inside cells, where they can have off-target effects. To increase the spatiotemporal precision with which we can activate lipids on cells and tissues, we previously developed optically cleavable targeted (OCT) ligands, which use protein tags (SNAP-tags) to localize a photocaged ligand on a target cellular membrane. After enrichment, the active ligand is released on a flash of light to activate nearby receptors. In our previous work, we developed an OCT-ligand to control a cannabinoid-sensitive GPCR. Here, we expand the scope of OCT-ligand technology to target TRPV1 ion channels. We synthesize a probe, OCT-CAP, that tethers to membrane-bound SNAP-tags and releases a TRPV1 agonist when triggered by UV-A irradiation. Using Ca2+ imaging and electrophysiology in HEK293T cells expressing TRPV1, we demonstrate that OCT-CAP uncaging activates TRPV1 with superior spatiotemporal precision when compared to standard diffusible ligands or photocages. This study is the first example of an OCT-ligand designed to manipulate an ion-channel target. We anticipate that these tools will find many applications in controlling lipid signaling pathways in various cells and tissues.

Intracellular delivery and deep tissue penetration of nucleoside triphosphates using photocleavable covalently bound dendritic polycations

Nucleoside triphosphates (NTPs) are essential in various biological processes. Cellular or even organismal controlled delivery of NTPs would be highly desirable, yet in cellulo and in vivo applications are hampered owing to their negative charge leading to cell impermeability. NTP transporters or NTP prodrugs have been developed, but a spatial and temporal control of the release of the investigated molecules remains challenging with these strategies. Herein, we describe a general approach to enable intracellular delivery of NTPs using covalently bound dendritic polycations, which are derived from PAMAM dendrons and their guanidinium derivatives. By design, these modifications are fully removable through attachment on a photocage, ready to deliver the native NTP upon irradiation enabling spatiotemporal control over nucleotide release. We study the intracellular distribution of the compounds depending on the linker and dendron generation as well as side chain modifications. Importantly, as the polycation is bound covalently, these molecules can also penetrate deeply into the tissue of living organisms, such as zebrafish.

Towards a Light-mediated Gene Therapy for the Eye using Caged Ethinylestradiol and the Inducible Cre/lox System

Increasingly, retinal pathologies are being treated with virus-mediated gene therapies. To be able to target viral transgene expression specifically to the pathological regions of the retina with light, we established an in vivo photoactivated gene expression paradigm for retinal tissue. Based on the inducible Cre/lox system, we discovered that ethinylestradiol is a suitable alternative to Tamoxifen as ethinylestradiol is more amenable to modification with photosensitive protecting compounds, i.e., "caging." Identification of ethinylestradiol as a ligand for the mutated human estradiol receptor was supported by in silico binding studies showing the reduced binding of caged ethinylestradiol. Caged ethinylestradiol was injected into the eyes of double transgenic GFAP-CreERT2 mice with a Cre-dependent tdTomato reporter transgene followed by irradiation with light of 450 nm. Photoactivation significantly increased retinal tdTomato expression compared to controls. We thus demonstrated a first step towards the development of a targeted, light-mediated gene therapy for the eyes.

Aza-Residue Modulation of Cyclic d,l-α-Peptide Nanotube Assembly with Enhanced Anti-Amyloidogenic Activity

Transient soluble oligomers of amyloid-β (Aβ) are considered among the most toxic species in Alzheimer's disease (AD). Soluble Aβ oligomers accumulate early prior to insoluble plaque formation and cognitive impairment. The cyclic d,l-α-peptide CP-2 (1) self-assembles into nanotubes and demonstrates promising anti-amyloidogenic activity likely by a mechanism involving engagement of soluble oligomers. Systematic replacement of the residues in peptide 1 with aza-amino acid counterparts was performed to explore the effects of hydrogen bonding on propensity to mitigate Aβ aggregation and toxicity. Certain azapeptides exhibited improved ability to engage, alter the secondary structure, and inhibit aggregation of Aβ. Moreover, certain azapeptides disassembled preformed Aβ fibrils and protected cells from Aβ-mediated toxicity. Substitution of the l-norleucine³ and d-serine⁶ residues in peptide 1 with aza-norleucine and aza-homoserine provided, respectively, nontoxic [azaNle³]-1 (4) and [azaHse⁶]-1 (7), that significantly abated symptoms in a transgenic Caenorhabditis elegans AD model by decreasing Aβ oligomer levels.

Reverse Engineering Caged Compounds: Design Principles for their Application in Biology

Light passes through biological tissue, and so it is used for imaging biological processes in situ. Such observation is part of the very essence of science, but mechanistic understanding requires intervention. For more than 50 years a "second function" for light has emerged; namely, that of photochemical control. Caged compounds are biologically inert signaling molecules that are activated by light. These optical probes enable external instruction of biological processes by stimulation of an individual element in complex signaling cascades in its native environment. Cause and effect are linked directly in spatial, temporal, and frequency domains in a quantitative manner by their use. I provide a guide to the basic properties required to make effective caged compounds for the biological sciences.

Electrophysiological study of the effects of side products of RuBi-GABA uncaging on GABAA receptors in cerebellar granule cells

The study of the GABAA receptor itself and its pharmacology is of paramount importance for shedding light on the role of this receptor in the central nervous system. Caged compounds have emerged as powerful tools to support research in this field, as they allow to control, in space and time, the release of neurotransmitters enabling, for example, to map receptors’ distribution and dynamics. Here we focus on γ-aminobutyric acid (GABA)-caged compounds, particularly on a commercial complex called RuBi-GABA, which has high efficiency of uncaging upon irradiation at visible wavelengths. We characterized, by electrophysiological measurements, the effects of RuBi-GABA on GABAA receptors of rat cerebellar granule cells in vitro. In particular, we evaluated the effects of side products obtained after RuBi-GABA photolysis. For this purpose, we developed a procedure to separate the “RuBi-cage” from GABA after uncaging RuBi-GABA with a laser source; then, we compared electrophysiological measurements acquired with and without administering the RuBi-cage in the perfusing bath. In conclusion, to investigate the role of the “cage” molecules both near and far from the cell soma, we compared experiments performed changing the distance of the uncaging point from the cell.

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.

Two-Photon Sensitive Coumarinyl Photoremovable Protecting Groups with Rigid Electron-Rich Cycles Obtained by Domino Reactions Initiated by a 5-exo-Dig Cyclocarbopalladation

We herein report the design, synthesis, and photophysical characterization of extended and rigid coumarinyl derivatives showing large two-photon sensitivities (δ_aΦ_u ≤ 125 GM) at 740 and 800 nm. To efficiently synthesize these complex photoremovable protecting groups (PPGs), we used step-economic domino reactions. Moreover, those new coumarinyl PPGs display unique bathochromic shifts (≤100 nm) of the uncaging subproducts as a result of the formation of a more conjugated fulvene moiety.

Photosensitive MRI biosensor for BCRP-Targeted uptake and light-induced inhibition of tumor cells

Riboflavin and its derivatives are the most important coenzymes in vivo metabolism, and are closely related to life activities. In this paper, the first photolysis ¹²⁹Xe biosensor was developed by combining cryptophane-A with riboflavin moiety, which showed photosensitivity recorded by hyperpolarized ¹²⁹Xe NMR/MRI technology with an obvious chemical shift change of 5.3 ppm in aqueous solution. Cellular fluorescence imaging confirmed that the biosensor could be enriched in MCF-7 cells, and MTT assays confirmed that the cytotoxicity was enhanced after irradiation. Findings suggested that the biosensor has a potential application in tumor targeting and the inhibition of tumor cell proliferation after photodegradation.

Targeting drug delivery with light: A highly focused approach

Light is a uniquely powerful tool for controlling molecular events in biology. No other external input (e.g., heat, ultrasound, magnetic field) can be so tightly focused or so highly regulated as a clinical laser. Drug delivery vehicles that can be photonically activated have been developed across many platforms, from the simplest "caging" of therapeutics in a prodrug form, to more complex micelles and circulating liposomes that improve drug uptake and efficacy, to large-scale hydrogel platforms that can be used to protect and deliver macromolecular agents including full-length proteins. In this Review, we discuss recent innovations in photosensitive drug delivery and highlight future opportunities to engineer and exploit such light-responsive technologies in the clinical setting.

Long wavelength single photon like driven photolysis via triplet triplet annihilation

Photolysis has enabled the occurrence of numerous discoveries in chemistry, drug discovery and biology. However, there is a dearth of efficient long wavelength light mediated photolysis. Here, we report general and efficient long wavelength single photon method for a wide array of photolytic molecules via triplet-triplet annihilation photolysis. This method is versatile and “LEGO”-like. The light partners (the photosensitizers and the photolytic molecules) can be energetically matched to adapt to an extensive range of electromagnetic spectrum wavelengths and the diversified chemical structures of photoremovable protecting groups, photolabile linkages, as well as a broad array of targeted molecules. Compared to the existing photolysis methods, our strategy of triplet-triplet annihilation photolysis not only exhibits superior reaction yields, but also resolves the photodamage problem, regardless of whether they are single photon or multiple photon associated. Furthermore, the biological promise of this “LEGO” system was illustrated via developing ambient air-stable nanoparticles capable of triplet-triplet annihilation photolysis.

The use of short wavelength light in photolysis applications in chemistry and biology is limited by photolytic reaction yields, photodamage and photobleaching. Here, the authors report a general long wavelength single photon driven photolysis method using a triplet-triplet annihilation process.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Thiocoumarin Caged Nucleotides: Synthetic Access and Their Photophysical Properties

Photocages have been successfully applied in cellular signaling studies for the controlled release of metabolites with high spatio-temporal resolution. Commonly, coumarin photocages are activated by UV light and the quantum yields of uncaging are relatively low, which can limit their applications in vivo. Here, syntheses, the determination of the photophysical properties, and quantum chemical calculations of 7-diethylamino-4-hydroxymethyl-thiocoumarin (thio-DEACM) and caged adenine nucleotides are reported and compared to the widely used 7-diethylamino-4-hydroxymethyl-coumarin (DEACM) caging group. In this comparison, thio-DEACM stands out as a phosphate cage with improved photophysical properties, such as red-shifted absorption and significantly faster photolysis kinetics.

Optical Control of GABAA Receptors with a Fulgimide-Based Potentiator

Optogenetic and photopharmacological tools to manipulate neuronal inhibition have limited efficacy and reversibility. We report the design, synthesis, and biological evaluation of Fulgazepam, a fulgimide derivative of benzodiazepine that behaves as a pure potentiator of ionotropic γ‐aminobutyric acid receptors (GABA_ARs) and displays full and reversible photoswitching in vitro and in vivo. The compound enables high‐resolution studies of GABAergic neurotransmission, and phototherapies based on localized, acute, and reversible neuroinhibition.

Monitoring of uncaging processes by designing photolytical reactions

The use of photolabile protecting groups (PPGs) has been growing in emphasis for decades, and nowadays they enable cutting-edge results in numerous fields ranging from organic synthesis to neurosciences. PPGs are chemical entities that can be conjugated to a biomolecule to hide its biological activity, forming a stable so called "caged compound". This conjugate can be simply cleaved by light and therefore, the functionality of the biomolecule is restored with the formation of a PPG by-product. However, there is a sizeable need for PPGs that are able to quantify the "uncaging" process. In this review, we will discuss several strategies leading to an acute quantification of the uncaging events by fluorescence. In particular, we will focus on how molecular engineering of PPG could open new opportunities by providing easy access to photoactivation protocols.

Useful Caged Compounds for Cell Physiology

Light has been instrumental in the study of living cells since its use helped in their discovery in the late 17th century. Further, combining chemical technology with light microscopy was an essential part of the Nobel Prize for Physiology in 1906. Such landmark scientific findings involved passive observation of cells. However, over the past 50 years, a "second use" of light has emerged in cell physiology, namely one of rational control. The seminal method for this emerged in late 1970s with the invention of caged compounds. This was the point when "caged compounds" were defined as optical probes in which the active functionality of a physiological signaling molecule was blocked with a photochemical protecting group. Caged compounds are analogous to prodrugs; in both, the activity of the effector is latent. However, caged compounds, unlike prodrugs, use a trigger that confers the power of full temporal and spatial manipulation of the effects of release of its latent biological cargo. Light is distinct because it is bio-orthogonal, passes through living tissue (even into the cell interior), and initiates rapid release of the "caged" biomolecule. Further, because light can be directed to broad areas or focused to small points, caged compounds offer an array of timing scenarios for physiologists to dissect virtually any type of cellular process.The collaborative interaction between chemists and physiologists plays a fundamental role in the development of caged compounds. First, the physiologists must define the problem to be addressed; then, with the help of chemists, decide if a caged compound would be useful. For this, structure-activity relationships of the potential optical probe and receptor must be determined. If rational targets seem feasible, synthetic organic chemistry is used to make the caged compound. The crucial property of prephotolysis bio-inertness relies on physiological or biochemical assays. Second, detailed optical characterization of the caged compound requires the skill of photochemists because the rate and efficiency of uncaging are also crucial properties for a useful caged compound. Often, these studies reveal limitations in the caged compound which has been developed; thus, chemists and physiologists use their abilities for iterative development of even more powerful optical probes. A similar dynamic will be familiar to scientists in the pharmaceutical industry. Therefore, caged compound development provides an excellent training framework for (young) chemists both intellectually and professionally. In this Account, I draw on my long experience in the field of making useful caged compounds for cell physiology by showing how each probe I have developed has been defined by an important physiological problem. Fundamental to this process has been my initial training by the pioneers in aromatic photochemistry, Derek Bryce-Smith and Andrew Gilbert. I discuss making a range of "caged calcium" probes, ones which went on to be the most widely used of all caged compounds. Then, I describe the development of caged neurotransmitters for two-photon uncaging microscopy. Finally, I survey recent work on making new photochemical protecting groups for wavelength orthogonal, two-color, and ultraefficient two-photon uncaging.

Optofluidic control of rodent learning using cloaked caged glutamate

Glutamate is the major excitatory neurotransmitter in the brain, and photochemical release of glutamate (or uncaging) is a chemical technique widely used by biologists to interrogate its physiology. A basic prerequisite of these optical probes is bio-inertness before photolysis. However, all caged glutamates are known to have strong antagonism toward receptors of γ-aminobutyric acid, the major inhibitory transmitter. We have developed a caged glutamate probe that is inert toward these receptors at concentrations that are effective for photolysis with violet light. Pharmacological tests in vitro revealed that attachment of a fifth-generation (G5) dendrimer (i.e., cloaking) to the widely used 4-methoxy-7-nitro-indolinyl(MNI)-Glu probe prevented such off-target effects while not changing the photochemical properties of MNI-Glu significantly. G5-MNI-Glu was used with optofluidic delivery to stimulate dopamine neurons of the ventral tegmental area of freely moving mice in a conditioned place-preference protocol so as to mediate Pavlovian conditioning.

Near Infrared-Triggered Liposome Cages for Rapid, Localized Small Molecule Delivery

Photolabile chelating cages or protecting groups need complex chemical syntheses and require UV, visible, or two-photon NIR light to trigger release. Different cages have different solubilities, reaction rates,  and energies required for triggering. Here we show that liposomes containing calcium, adenosine triphosphate, or carboxyfluorescein are tethered to plasmon-resonant hollow gold nanoshells (HGN) tuned to absorb light from 650-950 nm. Picosecond pulses of near infrared (NIR) light provided by a two-photon microscope, or by a stand-alone laser during flow through microfluidic channels, trigger contents release with spatial and temporal control. NIR light adsorption heats the HGN, inducing vapor nanobubbles that rupture the liposome, releasing cargo within milliseconds. Any water-soluble molecule can be released at essentially the same rate from the liposome-HGN. By using liposomes of different composition, or HGN of different sizes or shapes with different nanobubble threshold fluences, or irradiating on or off resonance, two different cargoes can be released simultaneously, one before the other, or in a desired ratio. Calcium release from liposome-HGN can be spatially patterned to crosslink alginate gels and trap living cells. Liposome-HGN provide stable, biocompatible isolation of the bioactive compound from its surroundings with minimal interactions with the local environment.

Photopotentiation of the GABAA receptor with caged diazepam

As the inhibitory γ-aminobutyric acid-ergic (GABAergic) transmission has a pivotal role in the central nervous system (CNS) and defective forms of its synapses are associated with serious neurological disorders, numerous versions of caged GABA and, more recently, photoswitchable ligands have been developed to investigate such transmission. While the complementary nature of these probes is evident, the mechanisms by which the GABA receptors can be photocontrolled have not been fully exploited. In fact, the ultimate need for specificity is critical for the proper synaptic exploration. No caged allosteric modulators of the GABA_A receptor have been reported so far; to introduce such an investigational approach, we exploited the structural motifs of the benzodiazepinic scaffold to develop a photocaged version of diazepam (CD) that was tested on basolateral amygdala (BLa) pyramidal cells in mouse brain slices. CD is devoid of any intrinsic activity toward the GABA_A receptor before irradiation. Importantly, CD is a photoreleasable GABA_A receptor-positive allosteric modulator that offers a different probing mechanism compared to caged GABA and photoswitchable ligands. CD potentiates the inhibitory signaling by prolonging the decay time of postsynaptic GABAergic currents upon photoactivation. Additionally, no effect on presynaptic GABA release was recorded. We developed a photochemical technology to individually study the GABA_A receptor, which specifically expands the toolbox available to study GABAergic synapses.

Dendrimer Conjugation Enables Multiphoton Chemical Neurophysiology Studies with an Extended π-Electron Caging Chromophore

We have developed a caged neurotransmitter using an extended π-electron chromophore for efficient multiphoton uncaging on living neurons. Widely studied in a chemical context, such chromophores are inherently bioincompatible due to their highly lipophilic character. Attachment of two polycarboxylate dendrimers, a method we call "cloaking", to a bisstyrylthiophene (or BIST) core effectively transformed the chromophore into a water-soluble optical probe, whilst maintaining the high two-photon absorption of over 500 GM. Importantly, the cloaked caged compound was biologically inert at the high concentrations required for multiphoton chemical physiology. Thus, in contrast to non-cloaked BIST compounds, the BIST-caged neurotransmitter can be safely delivered onto neurons in acutely isolated brain slices, thereby enabling high-resolution two-photon uncaging without any side effects. We expect that our cloaking method will enable the development of new classes of cell-compatible photolabile probes using a wide variety of extended π-electron caging chromophores.

Development of photolabile protecting groups and their application to the optochemical control of cell signaling

Many biological processes are naturally regulated with spatiotemporal control. In order to perturb and investigate them, optochemical tools have been developed that convey similar spatiotemporal precision. Pivotal to optochemical probes are photolabile protecting groups, so called caging groups, and recent developments have enabled new applications to cellular processes, including cell signaling. This review focuses on the advances made in the field of caging groups and their application in cell signaling through caged molecules such as neurotransmitters, lipids, secondary messengers, and proteins.

Two-Photon Uncaging of Glutamate

Two-photon microscopy produces the excited singlet state of a chromophore with wavelengths approximately double that used for normal excitation. Two photons are absorbed almost simultaneously, via a virtual state, and this makes the excitation technique inherently non-linear. It requires ultra-fast lasers to deliver the high flux density needed to access intrinsically very short lived intermediates, and in combination with lenses of high numerical aperture, this confines axial excitation highly. Since the two-photon excitation volume is similar to a large spine head, the technique has been widely used to study glutamatergic transmission in brain slices. Here I describe the principles of two-photon uncaging of glutamate and provide a practical guide to its application.

Improved Synthesis of Caged Glutamate and Caging Each Functional Group

Glutamate is an excitatory neurotransmitter that controls numerous pathways in the brain. Neuroscientists make use of photoremovable protecting groups, also known as cages, to release glutamate with precise spatial and temporal control. Various cage designs have been developed and among the most effective has been the nitroindolinyl caging of glutamate. We, hereby, report an improved synthesis of one of the current leading molecules of caged glutamate, 4-carboxymethoxy-5,7-dinitroindolinyl glutamate (CDNI-Glu), which possesses efficiencies with the highest reported quantum yield of at least 0.5. We present the shortest route, to date, for the synthesis of CDNI-Glu in 4 steps, with a total reaction time of 40 h and an overall yield of 20%. We also caged glutamate at the other two functional groups, thereby, introducing two new cage designs: α-CDNI-Glu and N-CDNI-Glu. We included a study of their photocleavage properties using UV-vis, NMR, as well as a physiology experiment of a two-photon uncaging of CDNI-Glu in acute hippocampal brain slices. The newly introduced cage designs may have the potential to minimize the interference that CDNI-Glu has with the GABA_A receptor. We are broadly disseminating this to enable neuroscientists to use these photoactivatable tools.

Optical probing of acetylcholine receptors on neurons in the medial habenula with a novel caged nicotine drug analogue

KEY POINTS

A new caged nicotinic acetylcholine receptor (nAChR) agonist was developed, ABT594, which is photolysed by one- and two-photon excitation. The caged compound is photolysed with a quantum yield of 0.20. One-photon uncaging of ABT594 elicited large currents and Ca²⁺ transients at the soma and dendrites of medial habenula (MHb) neurons of mouse brain slices. Unexpectedly, uncaging of ABT594 also revealed highly Ca²⁺ -permeable nAChRs on axons of MHb neurons.

ABSTRACT

Photochemical release of neurotransmitters has been instrumental in the study of their underlying receptors, with acetylcholine being the exception due to its inaccessibility to photochemical protection. We caged a nicotinic acetylcholine receptor (nAChR) agonist, ABT594, via its secondary amine functionality. Effective photolysis could be carried out using either one- or two-photon excitation. Brief flashes (0.5-3.0 ms) of 410 nm light evoked large currents and Ca²⁺ transients on cell bodies and dendrites of medial habenula (MHb) neurons. Unexpectedly, photorelease of ABT594 also revealed nAChR-mediated Ca²⁺ signals along the axons of MHb neurons.

Advances in Nanomaterials for Brain Microscopy

Microscopic imaging of the brain continues to reveal details of its structure, connectivity, and function. To further improve our understanding of the emergent properties and functions of neural circuits, new methods are necessary to directly visualize the relationship between brain structure, neuron activity, and neurochemistry. Advances in engineering the chemical and optical properties of nanomaterials concurrent with developments in deep-tissue microscopy hold tremendous promise for overcoming the current challenges associated with in vivo brain imaging, particularly for imaging the brain through optically-dense brain tissue, skull, and scalp. To this end, developments in nanomaterials offer much promise toward implementing tunable chemical functionality for neurochemical targeting and sensing, and fluorescence stability for long-term imaging. In this review, we summarize current brain microscopy methods and describe the diverse classes of nanomaterials recently leveraged as contrast agents and functional probes for microscopic optical imaging of the brain.

A red-shifted two-photon-only caging group for three-dimensional photorelease

Based on nitrodibenzofuran (NDBF) a new photocage with higher two-photon action cross section and red-shifted absorption was developed. Due to calculations, a dimethylamino functionality (DMA) was added at ring position 7. The uncaging of nucleobases after two-photon excitation (2PE) could be visualized via double-strand displacement in a hydrogel. With this assay we achieved three-dimensional photorelease of DMA-NDBF-protected DNA orthogonal to NDBF-protected strands. While being an excellent 2P-cage, DMA-NDBF is surprisingly stable under visible-light one-photon excitation (1PE). This case of excitation-specific photochemistry enhances the scope of orthogonal photoregulation.

Two-color, one-photon uncaging of glutamate and GABA

Neuronal cells receive a variety of excitatory and inhibitory signals which they process to generate an output signal. In order to study the interaction between excitatory and inhibitory receptors with exogenously applied transmitters in the same preparation, two caging chromophores attached to glutamate and GABA were developed that were selectively photolyzed by different wavelengths of light. This technique has the advantage that the biologically inactive caged compound can be applied at equilibrium prior to the near instantaneous release of the transmitters. This method therefore mimics the kinetics of endogenously released transmitters that is otherwise not possible in brain slice preparations. Repeated photolysis with either of the two wavelengths resulted in GABA- or glutamate-induced activation of both ionotropic and metabotropic receptors to evoke reproducible currents. With these compounds, the interaction between inhibitory and excitatory receptors was examined using whole field photolysis.

Intracellular Uncaging of cGMP with Blue Light

We have made a new caged cGMP that is photolyzed with blue light. Using our recently developed derivative of 7-diethylaminocourmarin (DEAC) called DEAC450, we synthesized coumarin phosphoester derivatives of cGMP with two negative charges appended to the DEAC450 moiety. DEAC450-cGMP is freely soluble in physiological buffer without the need for any organic cosolvents. With a photolysis quantum yield of 0.18 and an extinction coefficient of 43 000 M^-1 cm^-1 at 453 nm, DEAC450-cGMP is the most photosensitive caged cGMP made to date. In patch-clamped neurons in acutely isolated brain slices, blue light effectively uncaged cGMP from DEAC450 and facilitated activation of hyperpolarization and cyclic nucleotide gated cation (HCN) channels in cholinergic interneurons. Thus, DEAC450-cGMP has a unique set of optical and chemical properties that make it a useful addition to the optical arsenal available to neurobiologists.