TRPswitch-A Step-Function Chemo-optogenetic Ligand for the Vertebrate TRPA1 Channel

A Bright Future for Photopharmaceuticals Addressing Central Nervous System Disorders: State of the Art and Challenges Toward Clinical Translation

Photopharmacology is an innovative approach that uses light to activate drugs. This method offers the potential for highly localized and precise drug activation, making it particularly promising for the treatment of neurological disorders. Despite the enticing prospects of photopharmacology, its application to treat human central nervous system (CNS) diseases remains to be demonstrated. In this review, we provide an overview of prominent strategies for the design and activation of photopharmaceutical agents in the field of neuroscience. Photocaged and photoswitchable drugs and bioactive molecules are discussed, and an instructive list of examples is provided to highlight compound design strategies. Special emphasis is placed on photoactivatable compounds for the modulation of glutamatergic, GABAergic, dopaminergic, and serotonergic neurotransmission for the treatment of neurological conditions, as well as various photoresponsive molecules with potential for improved pain management. Compounds holding promise for clinical translation are discussed in-depth and their potential for future applications is assessed. Neurophotopharmaceuticals have yet to achieve breakthrough in the clinic, as both light delivery and drug design have not reached full maturity. However, by describing the current state of the art and providing illustrative case studies, we offer a perspective on future opportunities in the field of neurophotopharmacology focused on addressing CNS disorders.

ON-OFF nanopores for optical control of transmembrane ionic communication

Nanoscale photoswitchable proteins could facilitate precise spatiotemporal control of transmembrane communication and support studies in synthetic biology, neuroscience and bioelectronics. Here, through covalent modification of the α-haemolysin protein pore with arylazopyrazole photoswitches, we produced 'photopores' that transition between iontronic resistor and diode modes in response to irradiation at orthogonal wavelengths. In the diode mode, a low-leak OFF-state nanopore exhibits a reversible increase in unitary conductance of more than 20-fold upon irradiation at 365 nm. A rectification ratio of >5 was achieved with photopores in the diode state by either direct or alternating voltage input. Unlike conventional electronic phototransistors with intensity-dependent photoelectric responses, the photopores regulated current output solely based on the wavelength(s) of monochromatic or dual-wavelength irradiation. Dual-wavelength irradiation at various relative intensities allowed graded adjustment of the photopore conductance. By using these properties, photonic signals encoding text or graphic messages were converted into ionic signals, highlighting the potential applications of photopores as components of smart devices in synthetic biology.

Optical Control of TRPM8 Channels with Photoswitchable Menthol

Transient receptor potential melastatin 8 (TRPM8) channels are well known as sensors for cold temperatures and cooling agents such as menthol and icilin and these channels are tightly regulated by the membrane lipid phosphoinositol-4,5-bisphosphate (PIP2). Since TRPM8 channels emerged as promising drug targets for treating pain, itching, obesity, cancer, dry eye disease, and inflammation, we aimed at developing a high-precision TRPM8 channel activator, to achieve spatiotemporal control of TRPM8 activity with light. In this study, we designed, synthesized and characterized the first photoswitchable TRPM8 activator azo-menthol (AzoM). AzoM enables optical control of endogenously and heterologously expressed TRPM8 channels with UV and blue light which is demonstrated by performing patch-clamp experiments. Moreover, AzoM facilitates the reliable determination of activation, inactivation, and deactivation kinetics thereby providing further insights into the channel gating. Using AzoM, the specific roles of individual amino acids for AzoM or PIP2 binding and for sensitization by PIP2 can be elucidated. Altogether, AzoM represents as a high-precision pharmaceutical tool for reversible control of TRPM8 channel function that enhances our biophysical understanding of TRPM8 channels and holds the potential to support the development of novel pharmaceuticals.

New Molecular Photoswitch Based on the Conformational Transition of Phenothiazine Derivatives and Corresponding Triplet Emission Properties

Molecular photoswitch research has drawn much attention in the last century owing to its great potential in the development of smart materials. However, photoswitches suitable for constructing light-responsive luminescent materials remain limited, especially those involving triplet-state phosphorescence. Herein, we designed a novel molecular photoswitch based on the conformation transition of phenothiazine derivatives, minimizing steric hindrance (-CH3 > -Cl > -F) to regulate the conformation transition process while introducing a cyanobenzene acceptor to promote phosphorescence emission potential. When they were doped into a polymer matrix, varying photoswitch rates were achieved by incorporating different steric hindrance groups into phenothiazine or cyanobenzene groups, accompanied by photoresponsive room-temperature phosphorescence. This study is expected to greatly expand the diversity and applications of organic photoswitch molecules.

Chemo-optogenetic Dimerization Dissects Complex Biological Processes

Light offers superior control in terms of high temporal precision, high spatial precision, and non-invasiveness for the regulation of cellular functions. In recent years, chemical biologists have adopted chemo-optogenetic dimerization approaches, such as photo-triggered chemical inducers of dimerization (pCIDs), as a general tool for spatiotemporal regulation of cellular functions. Traditional chemo-optogenetic dimerization triggers either a single ON or a single OFF of cellular activity. However, more sophisticated approaches are introduced in recent years. These include the ability to turn ON and OFF using different wavelengths of light, tools enabling multi-layer control of cellular activities, and nanobody-tethered photodimerizers. These advancements not only shed light on the study of ubiquitously existing multi-functional proteins but also create new opportunities for investigating complex cellular activity networks.

A Multimodal, In Vivo Approach for Assessing Structurally and Phenotypically Related Neuroactive Molecules

A recently reported behavioral screen in larval zebrafish for phenocopiers of known anesthetics and associated drugs yielded an isoflavone. Related isoflavones have also been reported as GABAA potentiators. From this, we synthesized a small library of isoflavones and incorporated an in vivo phenotypic approach to perform structure-behavior relationship studies of the screening hit and related analogs via behavioral profiling, patch-clamp experiments, and whole brain imaging. This revealed that analogs effect a range of behavioral responses, including sedation with and without enhancing the acoustic startle response. Interestingly, a subset of compounds effect sedation and enhancement of motor responses to both acoustic and light stimuli. Patch clamp recordings of cells with a human GABAA receptor confirmed that behavior-modulating isoflavones modify the GABA signaling. To better understand these molecules' nuanced effects on behavior, we performed whole brain imaging to reveal that analogs differentially effect neuronal activity. These studies demonstrate a multimodal approach to assessing activities of neuroactives.

Tuning the Thermal Stability of Tetra-o-chloroazobenzene Derivatives by Transforming Push-Pull to Push-Push Systems

Molecular photoswitches provide interesting tools to reversibly control various biological functions with light. Thanks to its small size and easy introduction into the biomolecules, azobenzene derivatives have been widely employed in the field of photopharmacology. All visible-light switchable azobenzenes with controllable thermostability are highly demanded. Based on the reported tetra-o-chloroazobenzenes, we synthesized push-pull systems, by introducing dialkyl amine and nitro groups as strong electron-donating and electron-withdrawing groups on the para-positions, and then transformed to push-push systems by a simple reduction step. The developed push-pull and push-push tetra-o-chloroazobenzene derivatives displayed excellent photoswitching properties, as previously reported. The half-life of the Z-isomers can be tuned from milliseconds for the push-pull system to several hours for the push-push system. The n-π* and π-π* transitions have better resolution in the push-push molecules, and excitation at different wavelengths can tune the E/Z ratio at the photostationary state. For one push-pull molecule, structure and absorption spectra obtained from theoretical calculations are compared with experimental data, along with data on the push-push counterpart.

Optogenetics Neuromodulation of the Nose

Recently developed optogenetic technology, which allows high-fidelity control of neuronal activity, has been applied to investigate the neural circuits underlying sensory processing and behavior. The nasal cavity is innervated by the olfactory nerve and trigeminal nerve, which are closely related to common symptoms of rhinitis, such as impairment of smell, itching, and sneezing. The olfactory system has an amazing ability to distinguish thousands of odorant molecules at trace levels. However, there are many issues in olfactory sensing mechanisms that need to be addressed. Optogenetics offers a novel technical approach to solve this dilemma. Therefore, we review the recent advances in olfactory optogenetics to clarify the mechanisms of chemical sensing, which may help identify the mechanism of dysfunction and suggest possible treatments for impaired smell. Additionally, in rhinitis patients, alterations in the other nerve (trigeminal nerve) that innervates the nasal cavity can lead to hyperresponsiveness to various nociceptive stimuli and central sensitization, causing frequent and persistent itching and sneezing. In the last several years, the application of optogenetics in regulating nociceptive receptors, which are distributed in sensory nerve endings, and amino acid receptors, which are distributed in vital brain regions, to alleviate overreaction to nociceptive stimuli, has gained significant attention. Therefore, we focus on the progress in optogenetics and its application in neuromodulation of nociceptive stimuli and discuss the potential clinical translation for treating rhinitis in the future.

Remote neurostimulation through an endogenous ion channel using a near-infrared light-activatable nanoagonist

The development of noninvasive approaches to precisely control neural activity in mammals is highly desirable. Here, we used the ion channel transient receptor potential ankyrin-repeat 1 (TRPA1) as a proof of principle, demonstrating remote near-infrared (NIR) activation of endogenous neuronal channels in mice through an engineered nanoagonist. This achievement enables specific neurostimulation in nongenetically modified mice. Initially, target-based screening identified flavins as photopharmacological agonists, allowing for the photoactivation of TRPA1 in sensory neurons upon ultraviolet A/blue light illumination. Subsequently, upconversion nanoparticles (UCNPs) were customized with an emission spectrum aligned to flavin absorption and conjugated with flavin adenine dinucleotide, creating a nanoagonist capable of NIR activation of TRPA1. Following the intrathecal injection of the nanoagonist, noninvasive NIR stimulation allows precise bidirectional control of nociception in mice through remote activation of spinal TRPA1. This study demonstrates a noninvasive NIR neurostimulation method with the potential for adaptation to various endogenous ion channels and neural processes by combining photochemical toolboxes with customized UCNPs.

Visible-Light-Activated Heteroaryl Azoswitches: Toward a More Colorful Future

Azobenzenes (Ph-N═N-Ph) are known as the most widely studied molecular photoswitches, and the recent rise of azoheteroarenes (Het-N═N-Ph or Het-N═N-Het) offers great opportunities to advance this already mature field. A common limitation is that azo-switches generally require harmful UV light for activation, which hinders their application across various fields. Despite great efforts in developing visible-light azobenzenes over the past few decades, the potential of visible-light heteroaryl azoswitches remains largely unexplored. This Perspective summarizes the state-of-the-art advancements in visible-light heteroaryl azoswitches, covering molecular design strategies, the structure-property relationship, and potential applications. We highlight the distinctive advantages of azoheteroarenes over azobenzenes in the research and development of visible-light switches. Furthermore, we discuss the opportunities and challenges in this emerging field and propose potential solutions to address crucial issues such as spectral red-shift and thermal half-life. Through this Perspective paper, we aim to provide inspiration for further exploration in this field, in anticipation of the growing prosperity and bright future of visible-light azoheteroarene photoswitches.

Visual analysis of global research on the transient receptor potential ankyrin 1 channel: A literature review from 2002 to 2022

Background and aims

The transient receptor potential ankyrin 1 (TRPA1) channel has become a focus in pain research. However, there are no bibliometric studies that systematically analyze the existing research in this area. This study aimed to provide a systematic review of the existing literature on TRPA1 using a bibliometric analysis.

Methods

Published literature in the field of TRPA1 was collected from the Web of Science Core Collection database. Quantitative and qualitative analyses of publications, countries, institutions, authors, journals, and other entries were conducted using Excel, VOSview, and Citespace software to provide insight into global research hotspots and trends in the TRPA1 field.

Results

This study included 1189 scientific products published in 398 journals from 52 countries. The United States of America (n = 367) had the most publications, ahead of Japan (n = 212) and China (n = 199). The University of Florence (n = 55) was the most productive institution and Pierangelo Geppetti (n = 46) was the most productive author. PLoS One (n = 40) published the most articles on TRPA1. Pain, cold, inflammation, covalent modification, hyperalgesia, and oxidative stress were the most common keywords used in the studies.

Conclusion

This study provides the first bibliometric analysis of TRPA1 publications. The physiological functions of TRPA1, TRPA1, and neuropathic pain, TRPA1 as a therapeutic target, and agonists of TRPA1 are trending in TRPA1 research. Neuropathic pain, apoptosis, and sensitization could be focus areas of future research. This study provides important insight in the field of TRPA1 research.

Photoactivatable Nanobody Conjugate Dimerizer Temporally Resolves Tiam1-Rac1 Signaling Axis

The precise spatiotemporal dynamics of protein activities play a crucial role in cell signaling pathways. To control cellular functions in a spatiotemporal manner, a powerful method called photoactivatable chemically induced dimerization (pCID) is used. In this study, photoactivatable nanobody conjugate inducers of dimerization (PANCIDs) is introduced, which combine pCID with nanobody technology. A PANCID consists of a nanobody module that directly binds to an antigenic target, a photocaged small molecule ligand, and a cyclic decaarginine (cR10 *) cell-penetrating peptide (CPP) for efficient nonendocytic intracellular delivery. Therefore, PANCID photodimerizers also benefit from nanobodies, such as their high affinities (in the nm or pm range), specificities, and ability to modulate endogenous proteins. Additionally it is demonstrated that the nanobody moiety can be easily replaced with alternative ones, expanding the potential applications. By using PANCIDs, the dynamics of the Tiam1-Rac1 signaling cascade is investigated and made an interesting finding. It is found that Rac1 and Tiam1 exhibit distinct behaviors in this axis, acting as time-resolved "molecular oscillators" that transit between different functions in the signaling cascade when activated either slowly or rapidly.

The conducting state of TRPA1 modulates channel lateral mobility

We have studied Danio rerio (Zebrafish) TRPA1 channel using a method that combines single channel electrophysiological and optical recordings to evaluate lateral mobility and channel gating simultaneously in single channels. TRPA1 channel activation by two distinct chemical ligands: allyl isothiocyanate (AITC) and TRPswitch B, results in substantial reduction of channel lateral mobility at the plasma membrane. Incubation with the cholesterol sequestering agent methyl-β-cyclodextrin (MβCD), prevents the reduction on lateral mobility induced by the two chemical agonists. This results strongly suggest that the open conformation of TRPA1 modulates channel lateral mobility probably by facilitating the insertion of the channel into cholesterol-enriched domains at the plasma membrane.

Azoheteroarene and Diazocine Molecular Photoswitches: Self-Assembly, Responsive Materials and Photopharmacology

Aromatic units tethered with an azo (-N=N-) functionality comprise a unique class of compounds, known as molecular photoswitches, exhibiting a reversible transformation between their E- and Z-isomers in response to photo-irradiation. Photoswitches have been explored extensively in the recent past to prepare dynamic self-assembled materials, optoelectronic devices, responsive biomaterials, and more. Most of such materials involve azobenzenes as the molecular photoswitch and to date, SciFinder lists more than 7000 articles and 1000 patents. Subsequently, a great deal of effort has been invested to improve the photo-isomerization efficiency and related mesoscopic properties of azobenzenes. Recently, azoheteroarenes and cyclic azobenzenes, such as arylazopyrazoles, arylazoisoxazoles, arylazopyridines, and diazocines, have emerged as second generation molecular photoswitches beyond conventional azobenzenes. These photoswitches offer distinct photoswitching behavior and responsive properties which make them highly promising candidates for multifaceted applications ranging from photoresponsive materials to photopharmacophores. In this minireview, we introduce the structural refinement and photoresponsive properties of azoheteroarenes and diazocines and summarize the state-of-the-art on utilizing these photoswitches as responsive building blocks in supramolecular assembly, material science and photopharmacology, highlighting their versatile photochemical behavior, enhanced functionality, and latest applications.

Bistable Aryl Azopyrazolium Ionic Photoswitches in Water

We report a new class of arylazopyrazolium-based ionic photoswitches (AAPIPs). These AAPIPs with different counter ions have been accessed through a modular synthetic approach in high yields. More importantly, the AAPIPs exhibit excellent reversible photoswitching and exceptional thermal stability in water. The effects of solvents, counter ions, substitutions, concentration, pH, and glutathione (GSH) have been evaluated using spectroscopic investigations. The results revealed that the bistability of studied AAPIPs is robust and near quantitative. The thermal half-life of Z isomers is extremely high in water (up to years), and it can be lowered electronically by the electron-withdrawing groups or highly basic pH.

ThermoTRP channels in pain sexual dimorphism: new insights for drug intervention

Chronic pain is a major burden for the society and remains more prevalent and severe in females. The presence of chronic pain is linked to persistent alterations in the peripheral and the central nervous system. One of the main types of peripheral pain transducers are the transient receptor potential channels (TRP), also known as thermoTRP channels, which intervene in the perception of hot and cold external stimuli. These channels, and especially TRPV1, TRPA1 and TRPM8, have been subjected to profound investigation because of their role as thermosensors and also because of their implication in acute and chronic pain. Surprisingly, their sensitivity to endogenous signaling has been far less studied. Cumulative evidence suggests that the function of these channels may be differently modulated in males and females, in part through sexual hormones, and this could constitute a significant contributor to the sex differences in chronic pain. Here, we review the exciting advances in thermoTRP pharmacology for males and females in two paradigmatic types of chronic pain with a strong peripheral component: chronic migraine and chemotherapy-induced peripheral neuropathy (CIPN). The possibilities of peripheral druggability offered by these channels and the differential exploitation for men and women represent a development opportunity that will lead to a significant increment of the armamentarium of analgesic medicines for personalized chronic pain treatment.

BTDAzo: A Photoswitchable TRPC5 Channel Activator

Photoswitchable reagents can be powerful tools for high-precision biological control. TRPC5 is a Ca2+ -permeable cation channel with distinct tissue-specific roles, from synaptic function to hormone regulation. Reagents giving spatiotemporally-resolved control over TRPC5 activity may be key to understanding and harnessing its biology. Here we develop the first photoswitchable TRPC5-modulator, BTDAzo, to address this goal. BTDAzo can photocontrol TRPC5 currents in cell culture, as well as controlling endogenous TRPC5-based neuronal Ca2+ responses in mouse brain slices. BTDAzos are also the first reported azo-benzothiadiazines, an accessible and conveniently derivatised azoheteroarene with strong two-colour photoswitching. BTDAzo's ability to control TRPC5 across relevant channel biology settings makes it suitable for a range of dynamically reversible photoswitching studies in TRP channel biology, with the aim to decipher the various biological roles of this centrally important ion channel.

Atomistic mechanisms of human TRPA1 activation by electrophile irritants through molecular dynamics simulation and mutual information analysis

The ion channel TRPA1 is a promiscuous chemosensor, with reported response to a wide spectrum of noxious electrophilic irritants, as well as cold, heat, and mechanosensation. It is also implicated in the inception of itch and pain and has hence been investigated as a drug target for novel analgesics. The mechanism of electrophilic activation for TRPA1 is therefore of broad interest. TRPA1 structures with the pore in both open and closed states have recently been published as well as covalent binding modes for electrophile agonists. However, the detailed mechanism of coupling between electrophile binding sites and the pore remains speculative. In addition, while two different cysteine residues (C621 and C665) have been identified as critical for electrophile bonding and activation, the bound geometry has only been resolved at C621. Here, we use molecular dynamics simulations of TRPA1 in both pore-open and pore-closed states to explore the allosteric link between the electrophile binding sites and pore stability. Our simulations reveal that an open pore is structurally stable in the presence of open ‘pockets’ in the C621/C665 region, but rapidly collapses and closes when these pockets are shut. Binding of electrophiles at either C621 or C665 provides stabilisation of the pore-open state, but molecules bound at C665 are shown to be able to rotate in and out of the pocket, allowing for immediate stabilisation of transient open states. Finally, mutual information analysis of trajectories reveals an informational path linking the electrophile binding site pocket to the pore via the voltage-sensing-like domain, giving a detailed insight into the how the pore is stabilized in the open state.

Bistable Photoswitch Allows in Vivo Control of Hematopoiesis

Optical control has enabled functional modulation in cell culture with unparalleled spatiotemporal resolution. However, current tools for in vivo manipulation are scarce. Here, we design and implement a genuine on-off optochemical probe capable of achieving hematopoietic control in zebrafish. Our photopharmacological approach first developed conformationally strained visible light photoswitches (CS-VIPs) as inhibitors of the histone methyltransferase MLL1 (KMT2A). In blood homeostasis MLL1 plays a crucial yet controversial role. CS-VIP 8 optimally fulfils the requirements of a true bistable functional system in vivo under visible-light irradiation, and with unprecedented stability. These properties are exemplified via hematopoiesis photoinhibition with a single isomer in zebrafish. The present interdisciplinary study uncovers the mechanism of action of CS-VIPs. Upon WDR5 binding, CS-VIP 8 causes MLL1 release with concomitant allosteric rearrangements in the WDR5/RbBP5 interface. Since our tool provides on-demand reversible control without genetic intervention or continuous irradiation, it will foster hematopathology and epigenetic investigations. Furthermore, our workflow will enable exquisite photocontrol over other targets inhibited by macrocycles.

Photosensitive and Photoswitchable TRPA1 Agonists Optically Control Pain through Channel Desensitization

Transient receptor potential ankyrin 1 (TRPA1) channel, as a nonselective ligand-gated cation channel robustly in dorsal root ganglion sensory neurons, is implicated in sensing noxious stimuli and nociceptive signaling. However, small-molecule tools targeting TRPA1 lack temporal and spatial resolution, limiting their use for validation of TRPA1 as a therapeutic target for pain. In our previous work, we found that 4,4'-(diazene-1,2-diyl)dianiline (AB1) is a photoswitchable TRPA1 agonist, but the poor water solubility and activity hinder its further development. Here, we report a series of specific and potent azobenzene-derived photoswitchable TRPA1 agonists (series 1 and 2) that enable optical control of the TRPA1 channel. Two representative compounds 1g and 2c can alleviate capsaicin-induced pain in the cheek model of mice through channel desensitization but not in TRPA1 knockout mice. Taken together, our findings demonstrate that photoswitchable TRPA1 agonists can be used as pharmacological tools for study of pain signaling.

A Photosensitive Polymeric Carrier with a Renewable Singlet Oxygen Reservoir Regulated by Two NIR Beams for Enhanced Antitumor Phototherapy

Photodynamic therapy (PDT), which utilizes photosensitizer to convert molecular oxygen into singlet oxygen (¹ O₂ ) upon laser irradiation to ablate tumors, will exacerbate the already oxygen shortage of most solid tumors and is thus self-limiting. Herein, a sophisticated photosensitive polymeric material (An-NP) that allows sustained ¹ O₂ generation and sufficient oxygen supply during the entire phototherapy is engineered by alternatively applying PDT and photothermal therapy (PTT) controlled by two NIR laser beams. In addition to a photosensitizer that generates ¹ O₂ , An-NP consists of two other key components: a molecularly designed anthracene derivative capable of trapping/releasing ¹ O₂ with superior reversibility and a dye J-aggregate with superb photothermal performance. Thus, in 655 nm laser-triggered PDT process, An-NP generates abundant ¹ O₂ with extra ¹ O₂ being trapped via the conversion into EPO-NP; while in the subsequent 785 nm laser-driven PTT process, the converted EPO-NP undergoes thermolysis to liberate the captured ¹ O₂ and regenerates An-NP. The intratumoral oxygen level can be replenished during the PTT cycle for the next round of PDT to generate ¹ O₂ . The working principle and phototherapy efficacy are preliminarily demonstrated in living cells and tumor-bearing mice, respectively.

Optical Assessment of Nociceptive TRP Channel Function at the Peripheral Nerve Terminal

Free nerve endings are key structures in sensory transduction of noxious stimuli. In spite of this, little is known about their functional organization. Transient receptor potential (TRP) channels have emerged as key molecular identities in the sensory transduction of pain-producing stimuli, yet the vast majority of our knowledge about sensory TRP channel function is limited to data obtained from in vitro models which do not necessarily reflect physiological conditions. In recent years, the development of novel optical methods such as genetically encoded calcium indicators and photo-modulation of ion channel activity by pharmacological tools has provided an invaluable opportunity to directly assess nociceptive TRP channel function at the nerve terminal.