Recognition Mechanism of a Novel Gabapentinoid Drug, Mirogabalin, for Recombinant Human α2δ1, a Voltage-Gated Calcium Channel Subunit.

Mirogabalin is a novel gabapentinoid drug with a hydrophobic bicyclo substituent on the γ-aminobutyric acid moiety that targets the voltage-gated calcium channel subunit α2δ1. Here, to reveal the mirogabalin recognition mechanisms of α2δ1, we present structures of recombinant human α2δ1 with and without mirogabalin analyzed by cryo-electron microscopy. These structures show the binding of mirogabalin to the previously reported gabapentinoid binding site, which is the extracellular dCache_1 domain containing a conserved amino acid binding motif. A slight conformational change occurs around the residues positioned close to the hydrophobic group of mirogabalin. Mutagenesis binding assays identified that residues in the hydrophobic interaction region, in addition to several amino acid binding motif residues around the amino and carboxyl groups of mirogabalin, are critical for mirogabalin binding. The A215L mutation introduced to decrease the hydrophobic pocket volume predictably suppressed mirogabalin binding and promoted the binding of another ligand, L-Leu, with a smaller hydrophobic substituent than mirogabalin. Alterations of residues in the hydrophobic interaction region of α2δ1 to those of the α2δ2, α2δ3, and α2δ4 isoforms, of which α2δ3 and α2δ4 are gabapentin-insensitive, suppressed the binding of mirogabalin. These results support the importance of hydrophobic interactions in α2δ1 ligand recognition.

Development of a deep learning-based method to identify "good" regions of a cryo-electron microscopy grid.

Recent advances in cryo-electron microscopy (cryo-EM) have enabled protein structure determination at atomic resolutions. Cryo-EM specimens are prepared by rapidly freezing a protein solution on a metal grid coated with a holey carbon film; this results in the formation of an ice film on each hole. The thickness of the ice film is a critical factor for high-resolution structure determination; ice that is too thick degrades the contrast of the protein image while ice that is too thin excludes the protein from the hole or denatures the protein. Therefore, trained researchers need to manually select "good" regions with appropriate ice thicknesses for imaging. To reduce the time spent on such tasks, we developed a deep learning program consisting of a "detector" and a "classifier" to identify good regions from low-magnification EM images. In our method, the holes in a low-magnification EM image are detected via a detector, and the ice image on each hole is classified as either good or bad via a classifier. The detector detected more than 95% of the holes regardless of the type of samples. The classifier was trained for different types of samples because the appropriate ice thickness varies between sample types. The accuracies of the classifiers were 93.8% for a soluble protein sample (ß-galactosidase) and 95.3% for a membrane protein sample (bovine heart cytochrome c oxidase). In addition, we found that a training data set containing ~ 2100 hole images from 300 low-magnification EM images was sufficient to obtain good accuracy, such as higher than 90%. We expect that the throughput of the cryo-EM data collection step will be greatly improved by using our method.

Different Contribution of Redox-Sensitive Transient Receptor Potential Channels to Acetaminophen-Induced Death of Human Hepatoma Cell Line.

Acetaminophen (APAP) is a safe analgesic antipyretic drug at prescribed doses. Its overdose, however, can cause life-threatening liver damage. Though, involvement of oxidative stress is widely acknowledged in APAP-induced hepatocellular death, the mechanism of this increased oxidative stress and the associated alterations in Ca(2+) homeostasis are still unclear. Among members of transient receptor potential (TRP) channels activated in response to oxidative stress, we here identify that redox-sensitive TRPV1, TRPC1, TRPM2, and TRPM7 channels underlie Ca(2+) entry and downstream cellular damages induced by APAP in human hepatoma (HepG2) cells. Our data indicate that APAP treatment of HepG2 cells resulted in increased reactive oxygen species (ROS) production, glutathione (GSH) depletion, and Ca(2+) entry leading to increased apoptotic cell death. These responses were significantly suppressed by pretreatment with the ROS scavengers N-acetyl-L-cysteine (NAC) and 4,5-dihydroxy-1,3-benzene disulfonic acid disodium salt monohydrate (Tiron), and also by preincubation of cells with the glutathione inducer Dimethylfumarate (DMF). TRP subtype-targeted pharmacological blockers and siRNAs strategy revealed that suppression of either TRPV1, TRPC1, TRPM2, or TRPM7 reduced APAP-induced ROS formation, Ca(2+) influx, and cell death; the effects of suppression of TRPV1 or TRPC1, known to be activated by oxidative cysteine modifications, were stronger than those of TRPM2 or TRPM7. Interestingly, TRPV1 and TRPC1 were labeled by the cysteine-selective modification reagent, 5,5'-dithiobis (2-nitrobenzoic acid)-2biotin (DTNB-2Bio), and this was attenuated by pretreatment with APAP, suggesting that APAP and/or its oxidized metabolites act directly on the modification target cysteine residues of TRPV1 and TRPC1 proteins. In human liver tissue, TRPV1, TRPC1, TRPM2, and TRPM7 channels transcripts were localized mainly to hepatocytes and Kupffer cells. Our findings strongly suggest that APAP-induced Ca(2+) entry and subsequent hepatocellular death are regulated by multiple redox-activated cation channels, among which TRPV1 and TRPC1 play a prominent role.

Identification of a prostaglandin D2 metabolite as a neuritogenesis enhancer targeting the TRPV1 ion channel.

Mast cells play important roles in allergic inflammation by secreting various mediators. In the present study, based on the finding that the medium conditioned by activated RBL-2H3 mast cells enhanced the nerve growth factor (NGF)-induced neuritogenesis of PC12 cells, we attempted to isolate an active compound from the mast cell conditioned culture medium. Our experiment identified 15-deoxy-Δ(12,14)-PGJ2 (15d-PGJ2), one of the PGD2 metabolites, as a potential enhancer of neuritogenesis. 15d-PGJ2 strongly enhanced the neuritogenesis elicited by a low-concentration of NGF that alone was insufficient to induce the neuronal differentiation. This 15d-PGJ2 effect was exerted in a Ca(2+)-dependent manner, but independently of the NGF receptor TrkA. Importantly, 15d-PGJ2 activated the transient receptor potential vanilloid-type 1 (TRPV1), a non-selective cation channel, leading to the Ca(2+) influx. In addition, we observed that (i) NGF promoted the insertion of TRPV1 into the cell surface membrane and (ii) 15d-PGJ2 covalently bound to TRPV1. These findings suggest that the NGF/15d-PGJ2-induced neuritogenesis may be regulated by two sets of mechanisms, one for the translocation of TRPV1 into the cell surface by NGF and one for the activation of TRPV1 by 15d-PGJ2. Thus, there is most likely a link between allergic inflammation and activation of the neuronal differentiation.

Deciphering Subtype-Selective Modulations in TRPA1 Biosensor Channels.

The transient receptor potential (TRP) proteins are a family of ion channels that act as cellular sensors. Several members of the TRP family are sensitive to oxidative stress mediators. Among them, TRPA1 is remarkably susceptible to various oxidants, and is known to mediate neuropathic pain and respiratory, vascular and gastrointestinal functions, making TRPA1 an attractive therapeutic target. Recent studies have revealed a number of modulators (both activators and inhibitors) that act on TRPA1. Endogenous mediators of oxidative stress and exogenous electrophiles activate TRPA1 through oxidative modification of cysteine residues. Non-electrophilic compounds also activate TRPA1. Certain non-electrophilic modulators may act on critical non-cysteine sites in TRPA1. However, a method to achieve selective modulation of TRPA1 by small molecules has not yet been established. More recently, we found that a novel N-nitrosamine compound activates TRPA1 by S-nitrosylation (the addition of a nitric oxide (NO) group to cysteine thiol), and does so with significant selectivity over other NO-sensitive TRP channels. It is proposed that this subtype selectivity is conferred through synergistic effects of electrophilic cysteine transnitrosylation and molecular recognition of the non-electrophilic moiety on the N-nitrosamine. In this review, we describe the molecular pharmacology of these TRPA1 modulators and discuss their modulatory mechanisms.

Transnitrosylation directs TRPA1 selectivity in N-nitrosamine activators.

S-Nitrosylation, the addition of a nitrosyl group to cysteine thiols, regulates various protein functions to mediate nitric oxide (NO) bioactivity. Recent studies have demonstrated that selectivity in protein S-nitrosylation signaling pathways is conferred through transnitrosylation, a transfer of the NO group, between proteins via interaction. We previously demonstrated that sensitivity to activation by synthetic NO-releasing agents via S-nitrosylation is a common feature of members of the transient receptor potential (TRP) family of Ca(2+)-permeable cation channels. However, strategies to confer subtype selectivity to nitrosylating agents targeted to TRP channels are yet to be developed. Here, we show selective activation of TRPA1 channels by novel NO donors derived from the ABBH (7-azabenzobicyclo[2.2.1]heptane) N-nitrosamines, which exhibit transnitrosylation reactivity to thiols without releasing NO. The NNO-ABBH1 (N-nitroso-2-exo,3-exo-ditrifluoromethyl-7-azabenzobicyclo[2.2.1]heptane) elicits S-nitrosylation of TRPA1 proteins, and dose-dependently induces robust Ca(2+) influx via both recombinant and native TRPA1 channels, but not via other NO-activated TRP channels. TRPA1 activation by NNO-ABBH1 is suppressed by specific cysteine mutations but not by NO scavenging, suggesting that cysteine transnitrosylation underlies the activation of TRPA1 by NNO-ABBH1. This is supported by the correlation of N-NO bond reactivity and TRPA1-activating potency in a congeneric series of ABBH N-nitrosamines. Interestingly, nonelectrophilic derivatives of ABBH also activate TRPA1 selectively, but less potently, compared with NNO-ABBH1. Thus, ABBH N-nitrosamines confer subtype selectivity on S-nitrosylation in TRP channels through synergetic effects of two chemical processes: cysteine transnitrosylation and molecular recognition of the nonelectrophilic moiety.

Redox regulation of transient receptor potential channels.

SIGNIFICANCE: Environmental and endogenous reactive species such as reactive oxygen species (ROS), reactive nitrogen species (RNS), and other electrophiles are not only known to exert toxic effects on organisms, but are also emerging as molecules that mediate cell signaling responses. However, the mechanisms underlying this cellular redox signaling by reactive species remains largely uncharacterized. RECENT ADVANCES: Ca2+-permeable cation channels encoded by the transient receptor potential (trp) gene superfamily are characterized by a wide variety of activation triggers that act from outside and inside the cell. Recent studies have revealed that multiple TRP channels sense reactive species and induce diverse physiological and pathological responses, such as cell death, chemokine production, and pain transduction. TRP channels sense reactive species either indirectly through second messengers or directly via oxidative modification of cysteine residues. In this review, we describe the activation mechanisms and biological roles of redox-sensitive TRP channels, including TRPM2, TRPM7, TRPC5, TRPV1, and TRPA1. CRITICAL ISSUES: The sensitivity of TRP channels to reactive species in vitro has been well characterized using molecular and pharmacological approaches. However, the precise activation mechanism(s) and in vivo function(s) of ROS/RNS-sensitive TRP channels remain elusive. FUTURE DIRECTIONS: Redox sensitivity of TRP channels has been shown to mediate previously unexplained biological phenomena and is involved in various pathologies. Understanding the physiological significance and activation mechanisms of TRP channel regulation by reactive species may lead to TRP channels becoming viable pharmacological targets, and modulators of these channels may offer therapeutic options for previously untreatable diseases.

TRP channels: sensors and transducers of gasotransmitter signals.

The transient receptor potential (trp) gene superfamily encodes cation channels that act as multimodal sensors for a wide variety of stimuli from outside and inside the cell. Upon sensing, they transduce electrical and Ca(2+) signals via their cation channel activities. These functional features of TRP channels allow the body to react and adapt to different forms of environmental changes. Indeed, members of one class of TRP channels have emerged as sensors of gaseous messenger molecules that control various cellular processes. Nitric oxide (NO), a vasoactive gaseous molecule, regulates TRP channels directly via cysteine (Cys) S-nitrosylation or indirectly via cyclic GMP (cGMP)/protein kinase G (PKG)-dependent phosphorylation. Recent studies have revealed that changes in the availability of molecular oxygen (O(2)) also control the activation of TRP channels. Anoxia induced by O(2)-glucose deprivation and severe hypoxia (1% O(2)) activates TRPM7 and TRPC6, respectively, whereas TRPA1 has recently been identified as a novel sensor of hyperoxia and mild hypoxia (15% O(2)) in vagal and sensory neurons. TRPA1 also detects other gaseous molecules such as hydrogen sulfide (H(2)S) and carbon dioxide (CO(2)). In this review, we focus on how signaling by gaseous molecules is sensed and integrated by TRP channels.

TRPA1 underlies a sensing mechanism for O2.

Oxygen (O(2)) is a prerequisite for cellular respiration in aerobic organisms but also elicits toxicity. To understand how animals cope with the ambivalent physiological nature of O(2), it is critical to elucidate the molecular mechanisms responsible for O(2) sensing. Here our systematic evaluation of transient receptor potential (TRP) cation channels using reactive disulfides with different redox potentials reveals the capability of TRPA1 to sense O(2). O(2) sensing is based upon disparate processes: whereas prolyl hydroxylases (PHDs) exert O(2)-dependent inhibition on TRPA1 activity in normoxia, direct O(2) action overrides the inhibition via the prominent sensitivity of TRPA1 to cysteine-mediated oxidation in hyperoxia. Unexpectedly, TRPA1 is activated through relief from the same PHD-mediated inhibition in hypoxia. In mice, disruption of the Trpa1 gene abolishes hyperoxia- and hypoxia-induced cationic currents in vagal and sensory neurons and thereby impedes enhancement of in vivo vagal discharges induced by hyperoxia and hypoxia. The results suggest a new O(2)-sensing mechanism mediated by TRPA1.

Roles of TRPM2 in oxidative stress.

Reactive oxygen species (ROS) play critical roles in cell death, diseases, and normal cellular processes. TRPM2 is a member of transient receptor potential (TRP) protein superfamily and forms a Ca(2+)-permeable nonselective cation channel activated by ROS, specifically by hydrogen peroxide (H(2)O(2)), and at least in part via second-messenger mechanisms. Accumulating evidence has indicated that TRPM2 mediates multiple cellular responses, after our finding that Ca(2+) influx via TRPM2 regulates H(2)O(2)-induced cell death. Recently, we have demonstrated that Ca(2+) influx through TRPM2 induces chemokine production in monocytes and macrophages, which aggravates inflammatory neutrophil infiltration in mice. However, understanding is still limited for in vivo physiological or pathophysiological significance of ROS-induced TRPM2 activation. In this review, we summarize mechanisms underlying activation of TRPM2 channels by oxidative stress and downstream biological responses, and discuss the biological importance of oxidative stress-activated TRP channels.

Differential redox regulation of ORAI ion channels: a mechanism to tune cellular calcium signaling.

Reactive oxygen species (ROS) are involved in many physiological and pathophysiological cellular processes. We used lymphocytes, which are exposed to highly oxidizing environments during inflammation, to study the influence of ROS on cellular function. Calcium ion (Ca(2+)) influx through Ca(2+) release-activated Ca(2+) (CRAC) channels composed of proteins of the ORAI family is essential for the activation, proliferation, and differentiation of T lymphocytes, but whether and how ROS affect ORAI channel function have been unclear. Here, we combined Ca(2+) imaging, patch-clamp recordings, and measurements of cell proliferation and cytokine secretion to determine the effects of hydrogen peroxide (H(2)O(2)) on ORAI channel activity and human T helper lymphocyte (T(H) cell) function. ORAI1, but not ORAI3, channels were inhibited by oxidation by H(2)O(2). The differential redox sensitivity of ORAI1 and ORAI3 channels depended mainly on an extracellularly located reactive cysteine, which is absent in ORAI3. T(H) cells became progressively less redox-sensitive after differentiation into effector cells, a shift that would allow them to proliferate, differentiate, and secrete cytokines in oxidizing environments. The decreased redox sensitivity of effector T(H) cells correlated with increased expression of Orai3 and increased abundance of several cytosolic antioxidants. Knockdown of ORAI3 with small-interfering RNA rendered effector T(H) cells more redox-sensitive. The differential expression of Orai isoforms between naïve and effector T(H) cells may tune cellular responses under oxidative stress.

Molecular characterization of TRPA1 channel activation by cysteine-reactive inflammatory mediators.

TRPA1 is a member of the transient receptor potential (TRP) cation channel family, and is predominantly expressed in nociceptive neurons of dorsal root ganglia (DRG) and trigeminal ganglia. Activation of TRPA1 by environmental irritants such as mustard oil, allicin and acrolein causes acute pain. However, the endogenous ligands that directly activate TRPA1 remain elusive in inflammation. Here, we show that a variety of inflammatory mediators (15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)), nitric oxide (NO), hydrogen peroxide (H(2)O(2)), and proton (H(+))) activate human TRPA1 heterologously expressed in HEK cells. These inflammatory mediators induced robust Ca(2+) influx in a subset of mouse DRG neurons. The TRP channel blocker ruthenium red almost completely inhibited neuronal responses by 15d-PGJ(2) and NO, but partially suppressed responses to H(2)O(2) and H(+). Functional characterization of site-directed cysteine mutants of TRPA1 in combination with labeling experiments using biotinylated 15d-PGJ(2) demonstrated that modifications of cytoplasmic N-terminal cysteines (Cys421 and Cys621) were responsible for the activation of TRPA1 by 15d-PGJ(2). In TRPA1 responses to other cysteine-reactive inflammatory mediators, such as NO and H(2)O(2), the extent of impairment by respective cysteine mutations differed from those in TRPA1 responses to 15d-PGJ(2). Interestingly, the Cys421 mutation critically impaired the TRPA1 response to H(+) as well. Our findings suggest that TRPA1 channels are targeted by an array of inflammatory mediators to elicit inflammatory pain in the nervous system.