Application of Large-Scale Transiently Transfected Cells to Functional Assays of Ion Channels: Different Targets and Assay Formats

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

Corneal pain and experimental model development

The cornea is a valuable tissue for studying peripheral sensory nerve structure and regeneration due to its avascularity, transparency, and dense innervation. Somatosensory innervation of the cornea serves to identify changes in environmental stimuli at the ocular surface, thereby promoting barrier function to protect the eye against injury or infection. Due to regulatory demands to screen ocular safety of potential chemical exposure, a need remains to develop functional human tissue models to predict ocular damage and pain using in vitro-based systems to increase throughput and minimize animal use. In this review, we summarize the anatomical and functional roles of corneal innervation in propagation of sensory input, corneal neuropathies associated with pain, and the status of current in vivo and in vitro models. Emphasis is placed on tissue engineering approaches to study the human corneal pain response in vitro with integration of proper cell types, controlled microenvironment, and high-throughput readouts to predict pain induction. Further developments in this field will aid in defining molecular signatures to distinguish acute and chronic pain triggers based on the immune response and epithelial, stromal, and neuronal interactions that occur at the ocular surface that lead to functional outcomes in the brain depending on severity and persistence of the stimulus.

Novel selective, potent naphthyl TRPM8 antagonists identified through a combined ligand- and structure-based virtual screening approach

Transient receptor potential melastatin 8 (TRPM8), a nonselective cation channel, is the predominant mammalian cold temperature thermosensor and it is activated by cold temperatures and cooling compounds, such as menthol and icilin. Because of its role in cold allodynia, cold hyperalgesia and painful syndromes TRPM8 antagonists are currently being pursued as potential therapeutic agents for the treatment of pain hypersensitivity. Recently TRPM8 has been found in subsets of bladder sensory nerve fibres, providing an opportunity to understand and treat chronic hypersensitivity. However, most of the known TRPM8 inhibitors lack selectivity, and only three selective compounds have reached clinical trials to date. Here, we applied two virtual screening strategies to find new, clinics suitable, TRPM8 inhibitors. This strategy enabled us to identify naphthyl derivatives as a novel class of potent and selective TRPM8 inhibitors. Further characterization of the pharmacologic properties of the most potent compound identified, compound 1, confirmed that it is a selective, competitive antagonist inhibitor of TRPM8. Compound 1 also proved itself active in a overreactive bladder model in vivo. Thus, the novel naphthyl derivative compound identified here could be optimized for clinical treatment of pain hypersensitivity in bladder disorders but also in different other pathologies.

TRPA1 agonist activity of probenecid desensitizes channel responses: consequences for screening

The transient receptor potential channel subtype A member 1 (TRPA1) is a nonselective cation channel widely viewed as having therapeutic potential, particularly for pain-related indications. Realization of this potential will require potent, selective modulators; however, currently the pharmacology of TRPA1 is poorly defined. As TRPA1 is calcium permeable, calcium indicators offer a simple assay format for high-throughput screening. In this report, we show that probenecid, a uricosuric agent used experimentally in screening to increase loading of calcium-sensitive dyes, activates TRPA1. Prolonged probenecid incubation during the dye-loading process reduces agonist potency upon subsequent challenge. When Chinese Hamster Ovary (CHO)-hTRPA1 or STC-1 cells, which endogenously express TRPA1, were dye loaded in the presence of 2 mM probenecid TRPA1, agonists appeared less potent; EC(50) for allyl isothiocyante agonists in CHO-hTRPA1 was increased from 1.5±0.19 to 7.32±1.20 μM (P<0.01). No significant effect on antagonist potency was observed when using the agonist EC(80) concentration determined under the appropriate dye-loading conditions. We suggest an alternative protocol for calcium imaging using another blocker of anion transport, sulfinpyrazone. This blocker significantly augments indicator dye loading and the screening window, but is not a TRPA1 agonist and has no effect on agonist potency.

A Real-Time Screening Assay for GIRK1/4 Channel Blockers

The cardiac acetylcholine-activated K+ channel (IK,Ach) represents a novel target for drug therapy in the treatment of atrial fibrillation (AF). This channel is a member of the G-protein-coupled inward rectifier K+ (GIRK) channel superfamily and is composed of the GIRK1/4 (Kir3.1 and Kir3.4) subunits. The goal of this study was to develop a cell-based screening assay for identifying new blockers of the GIRK1/4 channel. The mouse atrial HL-1 cell line, expressing the GIRK1/4 channel, was plated in 96-well plate format, loaded with the fluorescent membrane potential-sensitive dye bis-(1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC4(3)) and measured using a fluorescent imaging plate reader (FLIPR). Application of the muscarinic agonist carbachol to the cells caused a rapid, time-dependent decrease in the fluorescent signal, indicative of K+ efflux through the GIRK1/4 channel (carbachol vs. control solution, Z′ factor = 0.5-0.6). The GIRK1/4 channel fluorescent signal was blocked by BaCl2 and enhanced by increasing the driving force for K+ across the cell membrane. To test the utility of the assay for screening GIRK1/4 channel blockers, cells were treated with a small compound library of Na+ and K+ channel modulators. Analogues of amiloride and propafenone were identified as channel blockers at concentrations less than 1 µM. Thus, the GIRK1/4 channel assay may be used in the development of new and selective agents for treating AF.

Recombinant cell lines stably expressing functional ion channels

Ion channels are membrane proteins that gate the flow of ions into and out of a cell. They are present in the membranes of human, animal, plant, and bacterial cells. They are profoundly involved in diverse tasks ranging from neuronal functions to hormonal secretion and cell division. Biophysical characterization and modulation of ion channel targets are important approaches in modern drug discovery. With the heterologous expression of the nicotinic acetylcholine receptor alpha7 (nAChRalpha7) in a host cell, we show a way to construct and use such a stable cell-based expression system for electrophysiological assays.

Aequorin functional assay for characterization of G-protein-coupled receptors: implementation with cryopreserved transiently transfected cells

Assay technologies that measure intracellular Ca(2+) release are among the predominant methods for evaluation of GPCR function. These measurements have historically been performed using cell-permeable fluorescent dyes, although the use of the recombinant photoprotein aequorin (AEQ) as a Ca(2+) sensor has gained popularity with recent advances in instrumentation. The requirement of the AEQ system for cells expressing both the photoprotein and the GPCR target of interest has necessitated the labor-intensive development of cell lines stably expressing both proteins. With the goal of streamlining this process, transient transfections were used to either (1) introduce AEQ into cells stably expressing the GPCR of interest or (2) introduce the GPCR into cells stably expressing the AEQ protein, employing the human muscarinic M(1) receptor as a model system. Robust results were obtained from cryopreserved cells prepared by both strategies, yielding agonist and antagonist pharmacology in good agreement with literature values. Good reproducibility was observed between multiple transient transfection events. These results indicate that transient transfection is a viable and efficient method for production of cellular reagents for use in AEQ assays.

Generation of Site-Specific Retargeting Platform Cell Lines for Drug Discovery Using phiC31 and R4 Integrases

One of the challenges in developing cell lines for high-throughput screening in drug discovery is the labor- and time-intensive process required to create stable clonal cell lines that express specific reporters or drug targets. The authors report here the generation of a site-specific retargeting platform in 3 different cell lines: adherent HEK293, suspension CHO-S, and a human embryonic cell line (BGO1V). These platform cell lines were generated by using a combination of 2 site-specific integrases to develop a system that allows one to efficiently target a gene of interest to a specific locus and generates rapid production of homogeneous cell pools that stably express the gene of interest. The phiC31 integrase was used to create a platform line by placing a target site for the R4 integrase into a pseudo attP site, and then the R4 integrase was used to place a gene of interest into specific R4 target site. The authors demonstrate the successful and rapid retargeting of a G-protein-coupled receptor (cholecystokinin receptor A, CCKAR), an ion channel (the transient receptor potential cation channel, subfamily M, member 8, TRPM8), and a GFP-c-Jun(1-79) fusion protein into the specific loci in these cell lines and show that these retargeted cell lines exhibit functional and pharmacological responses consistent with those reported in the literature.

Model systems for the discovery and development of antiarrhythmic drugs

Cardiovascular diseases are the leading cause of mortality worldwide and about 25% of cardiovascular deaths are due to disturbances in cardiac rhythm or "arrhythmias". Arrhythmias were traditionally treated with antiarrhythmic drugs, but increasing awareness of the risks of presently available antiarrhythmic agents has greatly limited their usefulness. Most common treatment algorithms still involve small molecule drugs, and antiarrhythmic agents with improved efficacy and safety are sorely needed. This paper reviews the model systems that are available for discovery and development of new antiarrhythmic drugs. We begin with a presentation of screening methods used to identify specific channel-interacting agents, with a particular emphasis on high-throughput screens. Traditional manual electrophysiological methods, automated electrophysiology, fluorescent dye methods, flux assays and radioligand binding assays are reviewed. We then discuss a variety of relevant arrhythmia models. Two models are widely used in testing for arrhythmogenic actions related to excess action potential prolongation, an important potential adverse effect of chemical entities affecting cardiac rhythm: the methoxamine-sensitized rabbit and the dog with chronic atrioventricular block. We then go on to review models used to assess potential antiarrhythmic actions. For ventricular arrhythmias, chemical induction methods, cardiac or neural electrical stimulation, ischaemic heart models and models of cardiac channelopathies can be used to identify effective antiarrhythmic agents. For atrial arrhythmias, potentially useful models include vagally-maintained atrial fibrillation, acute asphyxia with atrial burst-pacing, sterile pericarditis, Y-shaped atria surgical incisions, chronic atrial dilation models, atrial electrical remodelling due to sustained atrial tachycardia, heart failure-related atrial remodelling, and acute atrial ischaemia. It is hoped that the new technologies now available and the recently-developed models for arrhythmia-response assessment will permit the introduction of newer and more effective antiarrhythmic therapies in the near future.

Mapping the human protein interactome

Interactions are the essence of all biomolecules because they cannot fulfill their roles without interacting with other molecules. Hence, mapping the interactions of biomolecules can be useful for understanding their roles and functions. Furthermore, the development of molecular based systems biology requires an understanding of the biomolecular interactions. In recent years, the mapping of protein-protein interactions in different species has been reported, but few reports have focused on the large-scale mapping of protein-protein interactions in human. Here, we review the developments in protein interaction mapping and we discuss issues and strategies for the mapping of the human protein interactome.

Molecular determinants of species-specific activation or blockade of TRPA1 channels

TRPA1 is an excitatory, nonselective cation channel implicated in somatosensory function, pain, and neurogenic inflammation. Through covalent modification of cysteine and lysine residues, TRPA1 can be activated by electrophilic compounds, including active ingredients of pungent natural products (e.g., allyl isothiocyanate), environmental irritants (e.g., acrolein), and endogenous ligands (4-hydroxynonenal). However, how covalent modification leads to channel opening is not understood. Here, we report that electrophilic, thioaminal-containing compounds [e.g., CMP1 (4-methyl-N-[2,2,2-trichloro-1-(4-nitro-phenylsulfanyl)-ethyl]-benzamide)] covalently modify cysteine residues but produce striking species-specific effects [i.e., activation of rat TRPA1 (rTRPA1) and blockade of human TRPA1 (hTRPA1) activation by reactive and nonreactive agonists]. Through characterizing rTRPA1 and hTRPA1 chimeric channels and point mutations, we identified several residues in the upper portion of the S6 transmembrane domains as critical determinants of the opposite channel gating: Ala-946 and Met-949 of rTRPA1 determine channel activation, whereas equivalent residues of hTRPA1 (Ser-943 and Ile-946) determine channel block. Furthermore, side-chain replacements at these critical residues profoundly affect channel function. Therefore, our findings reveal a molecular basis of species-specific channel gating and provide novel insights into how TRPA1 respond to stimuli.

(R)-(5-tert-butyl-2,3-dihydro-1H-inden-1-yl)-3-(1H-indazol-4-yl)-urea (ABT-102) blocks polymodal activation of transient receptor potential vanilloid 1 receptors in vitro and heat-evoked firing of spinal dorsal horn neurons in vivo

The transient receptor potential vanilloid (TRPV) 1 receptor, a nonselective cation channel expressed on peripheral sensory neurons and in the central nervous system, plays a key role in pain. TRPV1 receptor antagonism is a promising approach for pain management. In this report, we describe the pharmacological and functional characteristics of a structurally novel TRPV1 antagonist, (R)-(5-tert-butyl-2,3-dihydro-1H-inden-1-yl)-3-(1H-indazol-4-yl)-urea (ABT-102), which has entered clinical trials. At the recombinant human TRPV1 receptor ABT-102 potently (IC(50) = 5-7 nM) inhibits agonist (capsaicin, N-arachidonyl dopamine, anandamide, and proton)-evoked increases in intracellular Ca(2+) levels. ABT-102 also potently (IC(50) = 1-16 nM) inhibits capsaicin-evoked currents in rat dorsal root ganglion (DRG) neurons and currents evoked through activation of recombinant rat TRPV1 currents by capsaicin, protons, or heat. ABT-102 is a competitive antagonist (pA(2) = 8.344) of capsaicin-evoked increased intracellular Ca(2+) and shows high selectivity for blocking TRPV1 receptors over other TRP receptors and a range of other receptors, ion channels, and transporters. In functional studies, ABT-102 blocks capsaicin-evoked calcitonin gene-related peptide release from rat DRG neurons. Intraplantar administration of ABT-102 blocks heat-evoked firing of wide dynamic range and nociceptive-specific neurons in the spinal cord dorsal horn of the rat. This effect is enhanced in a rat model of inflammatory pain induced by administration of complete Freund's adjuvant. Therefore, ABT-102 potently blocks multiple modes of TRPV1 receptor activation and effectively attenuates downstream consequences of receptor activity. ABT-102 is a novel and selective TRPV1 antagonist with pharmacological and functional properties that support its advancement into clinical studies.