Photoregulation of biological activity by photocromic reagents. II. Inhibitors of acetylcholinesterase

Disequilibrating azobenzenes by visible-light sensitization under confinement

Photoisomerization of azobenzenes from their stable E isomer to the metastable Z state is the basis of numerous applications of these molecules. However, this reaction typically requires ultraviolet light, which limits applicability. In this study, we introduce disequilibration by sensitization under confinement (DESC), a supramolecular approach to induce the E-to-Z isomerization by using light of a desired color, including red. DESC relies on a combination of a macrocyclic host and a photosensitizer, which act together to selectively bind and sensitize E-azobenzenes for isomerization. The Z isomer lacks strong affinity for and is expelled from the host, which can then convert additional E-azobenzenes to the Z state. In this way, the host-photosensitizer complex converts photon energy into chemical energy in the form of out-of-equilibrium photostationary states, including ones that cannot be accessed through direct photoexcitation.

Photochromic Fentanyl Derivatives for Controlled μ-Opioid Receptor Activation

Photoswitchable ligands as biological tools provide an opportunity to explore the kinetics and dynamics of the clinically relevant μ-opioid receptor. These ligands can potentially activate or deactivate the receptor when desired by using light. Spatial and temporal control of biological activity allows for application in a diverse range of biological investigations. Photoswitchable ligands have been developed in this work, modelled on the known agonist fentanyl, with the aim of expanding the current "toolbox" of fentanyl photoswitchable ligands. In doing so, ligands have been developed that change geometry (isomerize) upon exposure to light, with varying photophysical and biochemical properties. This variation in properties could be valuable in further studying the functional significance of the μ-opioid receptor.

Reversible Regulation of Succinate Dehydrogenase by Tools of Photopharmacology

Succinate dehydrogenase (SDH) is extremely important in metabolic function and biological processes. Modulation of SDH has been reported to be a promising therapeutic target to SDH mutations. Current measures for the regulation of SDH are scarce, and precise and reversible modulation of SDH still remains challenging. Herein, a powerful tool for reversible optical control of SDH was proposed and evaluated utilizing the technology of photopharmacology. We reported photochromic ligands (PCLs), azobenzene-pyrazole amides (APAs), that exert light-dependent inhibition effects on SDH. Physicochemical property tests and biological assays were conducted to demonstrate the feasibility of modulating SDH. In this paper, common agricultural pathogens were used to develop a procedure by which our PCLs could reversibly and precisely control SDH utilizing green light. This research would help us to understand the target-ligand interactions and provide new insights into modulation of SDH.

Future-Oriented Advanced Diarylethene Photoswitches: From Molecular Design to Spontaneous Assembly Systems

Diarylethene (DAE) photoswitch is a new and promising family of photochromic molecules and has shown superior performance as a smart trigger in stimulus-responsive materials. During the past few decades, the DAE family has achieved a leap from simple molecules to functional molecules and developed toward validity as a universal switching building block. In recent years, the introduction of DAE into an assembly system has been an attractive strategy that enables the photochromic behavior of the building blocks to be manifested at the level of the entire system, beyond the DAE unit itself. This assembly-based strategy will bring many unexpected results that promote the design and manufacture of a new generation of advanced materials. Here, recent advances in the design and fabrication of diarylethene as a trigger in materials science, chemistry, and biomedicine are reviewed.

A guide to designing photocontrol in proteins: methods, strategies and applications

Light is essential for various biochemical processes in all domains of life. In its presence certain proteins inside a cell are excited, which either stimulates or inhibits subsequent cellular processes. The artificial photocontrol of specifically proteins is of growing interest for the investigation of scientific questions on the organismal, cellular and molecular level as well as for the development of medicinal drugs or biocatalytic tools. For the targeted design of photocontrol in proteins, three major methods have been developed over the last decades, which employ either chemical engineering of small-molecule photosensitive effectors (photopharmacology), incorporation of photoactive non-canonical amino acids by genetic code expansion (photoxenoprotein engineering), or fusion with photoreactive biological modules (hybrid protein optogenetics). This review compares the different methods as well as their strategies and current applications for the light-regulation of proteins and provides background information useful for the implementation of each technique.

Advances and opportunities in the exciting world of azobenzenes

Azobenzenes are archetypal molecules that have a central role in fundamental and applied research. Over the course of almost two centuries, the area of azobenzenes has witnessed great achievements; azobenzenes have evolved from simple dyes to 'little engines' and have become ubiquitous in many aspects of our lives, ranging from textiles, cosmetics, food and medicine to energy and photonics. Despite their long history, azobenzenes continue to arouse academic interest, while being intensively produced for industrial purposes, owing to their rich chemistry, versatile and straightforward design, robust photoswitching process and biodegradability. The development of azobenzenes has stimulated the production of new coloured and light-responsive materials with various applications, and their use continues to expand towards new high-tech applications. In this Review, we highlight the latest achievements in the synthesis of red-light-responsive azobenzenes and the emerging application areas of photopharmacology, photoswitchable adhesives and biodegradable materials for drug delivery. We show how the synthetic versatility and adaptive properties of azobenzenes continue to inspire new research directions, with limits imposed only by one's imagination.

Diarylethene moiety as an enthalpy-entropy switch: photoisomerizable stapled peptides for modulating p53/MDM2 interaction

Analogs of the known inhibitor (peptide pDI) of the p53/MDM2 protein-protein interaction are reported, which are stapled by linkers bearing a photoisomerizable diarylethene moiety. The corresponding photoisomers possess significantly different affinities to the p53-interacting domain of the human MDM2. Apparent dissociation constants are in the picomolar-to-low nanomolar range for those isomers with diarylethene in the "open" configuration, but up to eight times larger for the corresponding "closed" isomers. Spectroscopic, structural, and computational studies showed that the stapling linkers of the peptides contribute to their binding. Calorimetry revealed that the binding of the "closed" isomers is mostly enthalpy-driven, whereas the "open" photoforms bind to the protein stronger due to their increased binding entropy. The results suggest that conformational dynamics of the protein-peptide complexes may explain the differences in the thermodynamic profiles of the binding.

Two-photon absorption and two-photon-induced isomerization of azobenzene compounds

The process of two-photon-induced isomerization occurring in various organic molecules, among which azobenzene derivatives hold a prominent position, offers a wide range of functionalities, which can be used in both material and life sciences. This review provides a comprehensive description of nonlinear optical (NLO) properties of azobenzene (AB) derivatives whose geometries can be switched through two-photon absorption (TPA). Employing the nonlinear excitation process allows for deeper penetration of light into the tissues and provides opportunities to regulate biological systems in a non-invasive manner. At the same time, the tight focus of the beam needed to induce nonlinear absorption helps to improve the spatial resolution of the photoinduced structures. Since near-infrared (NIR) wavelengths are employed, the lower photon energies compared to usual one-photon excitation (typically, the azobenzene geometry change from trans to cis form requires the use of UV photons) cause less damage to the biological samples. Herein, we present an overview of the strategies for optimizing azobenzene-based photoswitches for efficient two-photon excitation (TPE) and the potential applications of two-photon-induced isomerization of azobenzenes in biological systems: control of ion flow in ion channels or control of drug release, as well as in materials science, to fabricate data storage media, optical filters, diffraction elements etc., based on phenomena like photoinduced anisotropy, mass transport and phase transition. The extant challenges in the field of two-photon switchable azomolecules are discussed.

Axitinib: A Photoswitchable Approved Tyrosine Kinase Inhibitor

The goal of photopharmacology is to develop photoswitchable enzyme modulators as tunable (pro-)drugs that can be spatially and temporally controlled by light. In this context, the tyrosine kinase inhibitor axitinib, which contains a photosensitive stilbene-like moiety that allows for E/Z isomerization, is of interest. Axitinib is an approved drug that targets the vascular endothelial growth factor receptor 2 (VEGFR2) and is licensed for second-line therapy of renal cell carcinoma. The photoinduced E/Z isomerization of axitinib has been investigated to explore if its inhibitory effect can be turned "on" and "off", as triggered by light. Under controlled light conditions, (Z)-axitinib is 43 times less active than that of the E isomer in an VEGFR2 assay. Furthermore, it was proven that kinase activity in human umbilical vein cells (HUVECs) was decreased by (E)-axitinib, but only weakly affected by (Z)-axitinib. By irradiating (Z)-axitinib in vitro with UV light (λ=385 nm), it is possible to switch it almost quantitatively into the E isomer and to completely restore the biological activity of (E)-axitinib. However, switching the biological activity off from (E)- to (Z)-axitinib was not possible in aqueous solution due to a competing irreversible [2+2]-photocycloaddition, which yielded a biologically inactive axitinib dimer.

Efficiently Photocontrollable or Not? Biological Activity of Photoisomerizable Diarylethenes

Diarylethene derivatives, the biological activity of which can be reversibly changed by irradiation with light of different wavelengths, have shown promise as scientific tools and as candidates for photocontrollable drugs. However, examples demonstrating efficient photocontrol of their biological activity are still relatively rare. This concept article discusses the possible reasons for this situation and presents a critical analysis of existing data and hypotheses in this field, in order to extract the design principles enabling the construction of efficient photocontrollable diarylethene-based molecules. Papers addressing biologically relevant interactions between diarylethenes and biomolecules are analyzed; however, in most published cases, the efficiency of photocontrol in living systems remains to be demonstrated. We hope that this article will encourage further discussion of design principles, primarily among pharmacologists, synthetic and medicinal chemists.

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.

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.

Photoregulation of α-Chymotrypsin Activity by Spiropyran-Based Inhibitors in Solution and Attached to an Optical Fiber

Here the synthesis and characterization of a new class of spiropyran-based protease inhibitor is reported that can be reversibly photoswitched between an active spiropyran (SP) isomer and a less active merocyanine (MC) isomer upon irradiation with UV and visible light, respectively, both in solution and on a surface of a microstructured optical fiber (MOF). The most potent inhibitor in the series (SP-3 b) has a C-terminal phenylalanyl-based α-ketoester group and inhibits α-chymotrypsin with a Ki of 115 nM. An analogue containing a C-terminal Weinreb amide (SP-2 d) demonstrated excellent stability and photoswitching in solution and was attached to the surface of a MOF. The SP isomer of Weinreb amide 2 d is a competitive reversible inhibitor in solution and also on fiber, while the corresponding MC isomer was significantly less active in both media. The ability of this new class of spiropyran-based protease inhibitor to modulate enzyme activity on a MOF paves the way for sensing applications.

Design, Synthesis and Inhibitory Activity of Photoswitchable RET Kinase Inhibitors

REarranged during Transfection (RET) is a transmembrane receptor tyrosine kinase required for normal development and maintenance of neurons of the central and peripheral nervous systems. Deregulation of RET and hyperactivity of the RET kinase is intimately connected to several types of human cancers, most notably thyroid cancers, making it an attractive therapeutic target for small-molecule kinase inhibitors. Novel approaches, allowing external control of the activity of RET, would be key additions to the signal transduction toolbox. In this work, photoswitchable RET kinase inhibitors based on azo-functionalized pyrazolopyrimidines were developed, enabling photonic control of RET activity. The most promising compound displays excellent switching properties and stability with good inhibitory effect towards RET in cell-free as well as live-cell assays and a significant difference in inhibitory activity between its two photoisomeric forms. As the first reported photoswitchable small-molecule kinase inhibitor, we consider the herein presented effector to be a significant step forward in the development of tools for kinase signal transduction studies with spatiotemporal control over inhibitor concentration in situ.

Electrodes modified with the phase transition polymer and heme peptide: biocatalysis and biosensing with tunable activity and dynamic range

An electrode was modified with a phase transition polymer, poly(N-isopropylacrylamide), and the polymer was further modified with a peroxidase model compound, heme peptide (HP). As the polymer layer shrank at temperatures above 30-40 degrees C, the catalytic activity of the HP molecules for H(2)O(2) reduction improved, and simultaneously, the number of HP molecules that can communicate electrochemically with the electrode increased. As a result, the catalytic current for H(2)O(2) reduction in the shrunken state was 4 times larger than that in the swollen state. This reversible change was exploited for tuning the sensitivity and dynamic range of the HP electrode in H(2)O(2) biosensing. The dynamic range in inhibition-based biosensing of imidazole derivatives was also tunable.

Photoregulated ion binding

A photoregulated chelating agent has been synthesized. It is a photochromic azobenzene compound containing two iminodiacetic acid groups and can exists as cis and trans stereoisomers. The planar trans isomer does not bind zinc ions. On exposure to light of 320 nanometers, the trans isomer is converted to a nonplanar cis isomer, which, because of cooperativity between the two iminodiacetic acid groups, binds zinc ions with the value of the binding constant estimated to be 1.1 x 10(5) +/- 9.2 x 10(5) liters per mole at a ratio of one molecule of chelating agent to one zinc ion. The interconversion of the cis and trans isomers is reversible, suggesting possible application of this class of compounds as photoresponsive ion pumps.

Agents related to a potent activator of the acetylcholine receptor of Electrophorus electricus

The synthesis of a number of compounds related to trans-3,3'-bis[alpha-(trimethylammonium)methyl]azobenzene dibromide (trans-3,3'-BisQ) (1) is described. Among the compounds are: [14C]-trans-3,3'-BisQ (1) diiodide, cis-3,3'-BisQ (2) dibromide, the trans-2,2' (7) and 4,4' (11) isomers of BisQ, 2,2', (12), 3,3' (13) and 4,4' (14) isomers of bis-benzyldimethylammonium analogues, and related compounds in which the azo bridge between the two aromatic rings is replaced by diketo and amide bridges. Of them all trans-3,3'-BisQ (1) was the most active cholinergic compound in the electroplax system of Electrophorus electricus; the pure cis isomer (2) was without activity. Intermediate activities were found for some of the other compounds and others were inhibitors. The relationship of the structure of these agents to a proposed conformation and topography of the binding site of the acetylcholine receptor (AChR) is discussed.

New concepts in therapeutic photomedicine: photochemistry, optical targeting and the therapeutic window

Advances in optics technology, synthetic photochemistry, and the science of photobiology make it possible to think beyond phototherapy and photochemotherapy which is dependent on direct photochemical alteration of metabolites or direct phototoxic insult to cells. This report discusses another gender of photomedicine therapy which includes in vivo photoactivation of medicines, photon-dependent drug delivery, and manipulation of host and exposure source to maximize therapeutic index. These therapeutic manipulations are made possible because the skin is highly overperfused and because non-ionizing electromagnetic radiation that enters skin and blood has adequate photon energy to cause electronic excitation. Radiation of 320-800 nm is not very directly phototoxic, is absorbed by a variety of relatively nontoxic photolabile molecules and has an internal dosimetric depth profile. This radiation can therefore be used to activate, deactivate, bind, release or biotransform medications in vivo in skin or other organs. The photochemist, synthetic chemist and photobiologist can collaborate to significantly increase therapeutic possibilities.

Choline acetyltransferase

Acetylcholine is essential to neural function. It synthesis is catalyzed by choline acetyltransferase, the enzyme responsible for the acetylation of choline by acetyl coenzye A, a reaction favored slightly thermodymodynamically and not at all kinetically. An analytically pure enzyme still has not been obtained; however, method of purification have been greatly improved recently. Numerous inhibitors of the enzyme have been synthesized and their structure-action relationships examained. Evidence has been accumulated showing the essential involvement of an imidazole group in the active site of choline acetyltransferase. The literature regarding the controversial role to thiol groups in choline acetyltransferase is reviewed. Recently, derivatives of coenzyme A have been introduced as inhibitors of this enzyme and the specificity of coenzyme A binding has been examined. Possible mechanisms responsible for the control fo acetylcholine synthesis are discussed.

Photochromic activators of the acetylcholine receptor

Two photochromic activators of the electrogenic membrane of the electroplax of Electrophorus electricus are described. Trans-3,3'-bis[alpha-(trimethylammonium)methyl]azobenzene dibromide (Bis-Q), one of the most potent ever reported, is active at concentrations of less than 10(-7) M. Its cis isomer, which is obtained from the trans by exposure to light of 330 nm, is practically devoid of activity. Photoregulation of the potential of the membrane takes place in the presence of Bis-Q, presumably because of the conversion of the active trans isomer to the inactive cis isomer in the single-cell electroplax system. The second activator, 3-(alpha-bromomethyl)-3'-[alpha-(trimethylammonium)methyl]azobenzene bromide (QBr) can be covalently attached to the electroplax membrane after reduction of the membrane with dithiothreitol. Activation of the membrane is induced by the covalently linked reagent. Its cis isomer, obtained from the trans by exposure to light of 330 nm, is, like cis-Bis-Q, of very low activity. Both isomers of Bis-Q are equally active as inhibitors of acetylcholinesterase, 50% inhibition occurring at a concentration of 10(-5) M. The possibility of using trans-Bis-Q and trans-QBr to characterize and isolate the receptor protein is discussed.

Evidence for the regulation of phytochrome-mediated processes in bean roots by the neurohumor, acetylcholine

Using pharmacological and chromatographic techniques, it was shown that acetylcholine was present in all organs of both light- and dark-grown mung bean seedings (Phaseolus aureus). The highest concentrations were found in tissues containing active growing points: buds and secondary roots. Within 4 minutes, red light caused an increase in the efflux of acetylcholine from secondary root tips as well as a significant increase in the endogenous titer. Four minutes of subsequent far red light reduced the latter to a level comparable to that in the dark.Acetylcholine, given for 4 minutes in the dark, was able to substitute for red light in reducing the formation of secondary roots, inducing increased H(+) efflux, and causing the root tips to adhere to a negatively charged glass surface. Acetylcholine-esterase and atropine inhibited the latter phenomenon, whereas eserine inhibited the far red-induced release from glass. THESE AND OTHER DATA SUPPORT THE CONCLUSION THAT ACETYLCHOLINE FUNCTIONS IN MUNG BEAN ROOTS AS IT DOES IN ANIMAL SYSTEMS: by mediating changes in ion flux across cell membranes. It also seems probable that acetylcholine acts as a local hormone which regulates these phytochrome-mediated phenomena.

Photoregulation of biological activity by photochromic reagents, IV. A model for diurnal variation of enzymic activity

Levels of acetylcholinesterase activity can be made to vary in response to the presence or absence of sunlight in a system that can be considered as a model for photoperiodic processes found in nature. The enzyme is rendered photosensitive by the presence of a photochromic inhibitor, N-p-phenylazophenylcarbamyl choline, which changes from a trans to a cis isomer under the influence of the light of the sun and reverts back to the trans isomer in the dark. The two isomers differ in their ability acetylcholinesterase, thus rendering the enzyme system responsive to sunlight. The relationship of this system to photoresponsive processes in nature is discussed, and a possible role in photoregulation is suggested for naturally occurring carotenoids.

Photoregulation of biological activity by photochromic reagents. 3. Photoregulation of bioelectricity by acetylcholine receptor inhibitors

The photochromic compounds N-p-phenylazophenyl-N-phenylcarbamylcholine chloride and p-phenylazophenyltrimethylammonium chloride inhibit the carbamylcholine-produced depolarization of the excitable membrane of the monocellular electroplax preparation of Electrophorus. The trans isomer of each predominates in the light of a photoflood (420 mmu) lamp; they are stronger inhibitors than the cis isomers, which predominate under ultraviolet (320 mmu) irradiation. The potential difference across the excitable membrane may be photoregulated by exposing an electroplax in the presence of a solution of carbamylcholine and either of the two compounds to light of appropriate wavelengths, since light shifts the cis-trans equilibrium. The system may be considered as a model illustrating how one may link a cis-trans isomerization, the first step in the initiation of a visual impulse, with substantial changes (20-30 mv) in the potential difference across an excitable membrane.