Dissecting intersubunit contacts in cyclic nucleotide-gated ion channels

Epithelia-Sensory Neuron Cross Talk Underlies Cholestatic Itch Induced by Lysophosphatidylcholine

BACKGROUND & AIMS

Limited understanding of pruritus mechanisms in cholestatic liver diseases hinders development of antipruritic treatments. Previous studies implicated lysophosphatidic acid (LPA) as a potential mediator of cholestatic pruritus.

METHODS

Pruritogenicity of lysophosphatidylcholine (LPC), LPA's precursor, was examined in naïve mice, cholestatic mice, and nonhuman primates. LPC's pruritogenicity involving keratinocyte TRPV4 was studied using genetic and pharmacologic approaches, cultured keratinocytes, ion channel physiology, and structural computational modeling. Activation of pruriceptor sensory neurons by microRNA-146a (miR-146a), secreted from keratinocytes, was identified by in vitro and ex vivo Ca2+ imaging assays. Sera from patients with primary biliary cholangitis were used for measuring the levels of LPC and miR-146a.

RESULTS

LPC was robustly pruritic in mice. TRPV4 in skin keratinocytes was essential for LPC-induced itch and itch in mice with cholestasis. Three-dimensional structural modeling, site-directed mutagenesis, and channel function analysis suggested a TRPV4 C-terminal motif for LPC binding and channel activation. In keratinocytes, TRPV4 activation by LPC induced extracellular release of miR-146a, which activated TRPV1+ sensory neurons to cause itch. LPC and miR-146a levels were both elevated in sera of patients with primary biliary cholangitis with itch and correlated with itch intensity. Moreover, LPC and miR-146a were also increased in sera of cholestatic mice and elicited itch in nonhuman primates.

CONCLUSIONS

We identified LPC as a novel cholestatic pruritogen that induces itch through epithelia-sensory neuron cross talk, whereby it directly activates skin keratinocyte TRPV4, which rapidly releases miR-146a to activate skin-innervating TRPV1+ pruriceptor sensory neurons. Our findings support the new concept of the skin, as a sensory organ, playing a critical role in cholestatic itch, beyond liver, peripheral sensory neurons, and central neural pathways supporting pruriception.

Different agonists induce distinct single-channel conductance states in TRPV1 channels

TRPV1 is a polymodal ion channel that can be activated by lysophosphatidic acid (LPA), resulting in pain. Here we show that TRPV1 activation by LPA promotes a distinct open state with a different single-channel conductance from that induced by capsaicin.

The TRPV1 ion channel is a membrane protein that is expressed in primary afferent nociceptors, where it is activated by a diverse array of stimuli. Our prior work has shown that this channel is activated by lysophosphatidic acid (LPA), an unsaturated lysophospholipid that is produced endogenously and released under certain pathophysiological conditions, resulting in the sensation of pain. Macroscopic currents activated by saturating concentrations of LPA applied to excised membrane patches are larger in magnitude than those activated by saturating concentrations of capsaicin, which causes near-maximal TRPV1 open probability. Here we show that activation of TRPV1 by LPA is associated with a higher single-channel conductance than activation by capsaicin. We also observe that the effects of LPA on TRPV1 are not caused by an increase in the surface charge nor are they mimicked by a structurally similar lipid, ruling out the contribution of change in membrane properties. Finally, we demonstrate that the effects of LPA on the unitary conductance of TRPV1 depend upon the presence of a positively charged residue in the C terminus of the channel, suggesting that LPA induces a distinct conformational change.

Structural Heterogeneity of CNGA1 Channels Revealed by Electrophysiology and Single-Molecule Force Spectroscopy

The determination at atomic resolution of the three-dimensional molecular structure of membrane proteins such as receptors and several ion channels has been a major breakthrough in structural biology. The molecular structure of several members of the superfamily of voltage-gated ionic channels such as K⁺ and Na⁺ is now available. However, despite several attempts, the molecular structure at atomic resolution of the full cyclic nucleotide-gated (CNG) ion channel, although a member of the same superfamily of voltage-gated ion channels, has not been obtained yet, neither by X-ray crystallography nor by electron cryomicroscopy (cryo-EM). It is possible that CNG channels have a high structural heterogeneity, making difficult crystallization and single-particle analysis. To address this issue, we have combined single-molecule force spectroscopy (SMFS) and electrophysiological experiments to characterize the structural heterogeneity of CNGA1 channels expressed in Xenopus laevis oocytes. The unfolding of the cytoplasmic domain had force peaks, occurring with a probability from 0.2 to 0.96. Force peaks during the unfolding of the transmembrane domain had a probability close to 1, but the distribution of the increase in contour length between two successive force peaks had multiple maxima differing by tens of nanometers. Concomitant electrophysiological experiments showed that the rundown in mutant channels S399C is highly variable and that the effect of thiol reagents when specific residues were mutated was consistent with a dynamic structural heterogeneity. These results show that CNGA1 channels have a wide spectrum of native conformations that are difficult to detect with X-ray crystallography and cryo-EM.

Inhibition of TRPV1 channels by a naturally occurring omega-9 fatty acid reduces pain and itch

The transient receptor potential vanilloid 1 (TRPV1) ion channel is mainly found in primary nociceptive afferents whose activity has been linked to pathophysiological conditions including pain, itch and inflammation. Consequently, it is important to identify naturally occurring antagonists of this channel. Here we show that a naturally occurring monounsaturated fatty acid, oleic acid, inhibits TRPV1 activity, and also pain and itch responses in mice by interacting with the vanilloid (capsaicin)-binding pocket and promoting the stabilization of a closed state conformation. Moreover, we report an itch-inducing molecule, cyclic phosphatidic acid, that activates TRPV1 and whose pruritic activity, as well as that of histamine, occurs through the activation of this ion channel. These findings provide insights into the molecular basis of oleic acid inhibition of TRPV1 and also into a way of reducing the pathophysiological effects resulting from its activation.

TRPV1 channels are known to mediate pathological pain and itch. Here, the authors find a naturally occurring fatty acid, oleic acid, acts as a TRPV1 antagonist and can modulate capsaicin and histamine-mediated pain and itch response in mouse models.

Conformational rearrangements in the transmembrane domain of CNGA1 channels revealed by single-molecule force spectroscopy

Cyclic nucleotide-gated (CNG) channels are activated by binding of cyclic nucleotides. Although structural studies have identified the channel pore and selectivity filter, conformation changes associated with gating remain poorly understood. Here we combine single-molecule force spectroscopy (SMFS) with mutagenesis, bioinformatics and electrophysiology to study conformational changes associated with gating. By expressing functional channels with SMFS fingerprints in Xenopus laevis oocytes, we were able to investigate gating of CNGA1 in a physiological-like membrane. Force spectra determined that the S4 transmembrane domain is mechanically coupled to S5 in the open state, but S3 in the closed state. We also show there are multiple pathways for the unfolding of the transmembrane domains, probably caused by a different degree of α-helix folding. This approach demonstrates that CNG transmembrane domains have dynamic structure and establishes SMFS as a tool for probing conformational change in ion channels.

Cyclic nucleotide gated channels are activated after binding cyclic nucleotides. Here, using single molecule force spectroscopy, the authors reveal that cyclic nucleotide binding causes conformational changes and tighter coupling of the S4 helix to the pore forming domain.

Structural determinants of the transient receptor potential 1 (TRPV1) channel activation by phospholipid analogs

The transient receptor potential vanilloid 1 (TRPV1) ion channel is a polymodal protein that responds to various stimuli, including capsaicin (the pungent compound found in chili peppers), extracellular acid, and basic intracellular pH, temperatures close to 42 °C, and several lipids. Lysophosphatidic acid (LPA), an endogenous lipid widely associated with neuropathic pain, is an agonist of the TRPV1 channel found in primary afferent nociceptors and is activated by other noxious stimuli. Agonists or antagonists of lipid and other chemical natures are known to possess specific structural requirements for producing functional effects on their targets. To better understand how LPA and other lipid analogs might interact and affect the function of TRPV1, we set out to determine the structural features of these lipids that result in the activation of TRPV1. By changing the acyl chain length, saturation, and headgroup of these LPA analogs, we established strict requirements for activation of TRPV1. Among the natural LPA analogs, we found that only LPA 18:1, alkylglycerophosphate 18:1, and cyclic phosphatidic acid 18:1, all with a monounsaturated C18 hydrocarbon chain activate TRPV1, whereas polyunsaturated and saturated analogs do not. Thus, TRPV1 shows a more restricted ligand specificity compared with LPA G-protein-coupled receptors. We synthesized fatty alcohol phosphates and thiophosphates and found that many of them with a single double bond in position Δ9, 10, or 11 and Δ9 cyclopropyl group can activate TRPV1 with efficacy similar to capsaicin. Finally, we developed a pharmacophore and proposed a mechanistic model for how these lipids could induce a conformational change that activates TRPV1.

Two structural components in CNGA3 support regulation of cone CNG channels by phosphoinositides

Cyclic nucleotide-gated (CNG) channels in retinal photoreceptors play a crucial role in vertebrate phototransduction. The ligand sensitivity of photoreceptor CNG channels is adjusted during adaptation and in response to paracrine signals, but the mechanisms involved in channel regulation are only partly understood. Heteromeric cone CNGA3 (A3) + CNGB3 (B3) channels are inhibited by membrane phosphoinositides (PIP(n)), including phosphatidylinositol 3,4,5-triphosphate (PIP(3)) and phosphatidylinositol 4,5-bisphosphate (PIP(2)), demonstrating a decrease in apparent affinity for cyclic guanosine monophosphate (cGMP). Unlike homomeric A1 or A2 channels, A3-only channels paradoxically did not show a decrease in apparent affinity for cGMP after PIP(n) application. However, PIP(n) induced an ∼2.5-fold increase in cAMP efficacy for A3 channels. The PIP(n)-dependent change in cAMP efficacy was abolished by mutations in the C-terminal region (R643Q/R646Q) or by truncation distal to the cyclic nucleotide-binding domain (613X). In addition, A3-613X unmasked a threefold decrease in apparent cGMP affinity with PIP(n) application to homomeric channels, and this effect was dependent on conserved arginines within the N-terminal region of A3. Together, these results indicate that regulation of A3 subunits by phosphoinositides exhibits two separable components, which depend on structural elements within the N- and C-terminal regions, respectively. Furthermore, both N and C regulatory modules in A3 supported PIP(n) regulation of heteromeric A3+B3 channels. B3 subunits were not sufficient to confer PIP(n) sensitivity to heteromeric channels formed with PIP(n)-insensitive A subunits. Finally, channels formed by mixtures of PIP(n)-insensitive A3 subunits, having complementary mutations in N- and/or C-terminal regions, restored PIP(n) regulation, implying that intersubunit N-C interactions help control the phosphoinositide sensitivity of cone CNG channels.

CNGA3 achromatopsia-associated mutation potentiates the phosphoinositide sensitivity of cone photoreceptor CNG channels by altering intersubunit interactions

Cyclic nucleotide-gated (CNG) channels are critical for sensory transduction in retinal photoreceptors and olfactory receptor cells; their activity is modulated by phosphoinositides (PIPn) such as phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylinositol 3,4,5-trisphosphate (PIP3). An achromatopsia-associated mutation in cone photoreceptor CNGA3, L633P, is located in a carboxyl (COOH)-terminal leucine zipper domain shown previously to be important for channel assembly and PIPn regulation. We determined the functional consequences of this mutation using electrophysiological recordings of patches excised from cells expressing wild-type and mutant CNG channel subunits. CNGA3-L633P subunits formed functional channels with or without CNGB3, producing an increase in apparent cGMP affinity. Surprisingly, L633P dramatically potentiated PIPn inhibition of apparent cGMP affinity for these channels. The impact of L633P on PIPn sensitivity depended on an intact amino (NH2) terminal PIPn regulation module. These observations led us to hypothesize that L633P enhances PIPn inhibition by altering the coupling between NH2- and COOH-terminal regions of CNGA3. A recombinant COOH-terminal fragment partially restored normal PIPn sensitivity to channels with COOH-terminal truncation, but L633P prevented this effect. Furthermore, coimmunoprecipitation of channel fragments, and thermodynamic linkage analysis, also provided evidence for NH2-COOH interactions. Finally, tandem dimers of CNGA3 subunits that specify the arrangement of subunits containing L633P and other mutations indicated that the putative interdomain interaction occurs between channel subunits (intersubunit) rather than exclusively within the same subunit (intrasubunit). Collectively, these studies support a model in which intersubunit interactions control the sensitivity of cone CNG channels to regulation by phosphoinositides. Aberrant channel regulation may contribute to disease progression in patients with the L633P mutation.

Lysophosphatidic acid directly activates TRPV1 through a C-terminal binding site

Since 1992, there has been growing evidence that the bioactive phospholipid lysophosphatidic acid (LPA), whose amounts are increased upon tissue injury, activates primary nociceptors resulting in neuropathic pain. The TRPV1 ion channel is expressed in primary afferent nociceptors and is activated by physical and chemical stimuli. Here we show that in control mice LPA produces acute pain-like behaviors, which are substantially reduced in Trpv1-null animals. Our data also demonstrate that LPA activates TRPV1 through a unique mechanism that is independent of G protein-coupled receptors, contrary to what has been widely shown for other ion channels, by directly interacting with the C terminus of the channel. We conclude that TRPV1 is a direct molecular target of the pain-producing molecule LPA and that this constitutes, to our knowledge, the first example of LPA binding directly to an ion channel to acutely regulate its function.

Two-pore channels form homo- and heterodimers

Two-pore channels (TPCs) have been recently identified as NAADP-regulated Ca²⁺ release channels, which are localized on the endolysosomal system. TPCs have a 12-transmembrane domain (TMD) structure and are evolutionary intermediates between the 24-TMD α-subunits of Na⁺ or Ca²⁺ channels and the transient receptor potential channel superfamily, which have six TMDs in a single subunit and form tetramers with 24 TMDs as active channels. Based on this relationship, it is predicted that TPCs dimerize to form functional channels, but the dimerization of human TPCs has so far not been studied. Using co-immunoprecipitation studies and a mass spectroscopic analysis of the immunocomplex, we show the presence of homo- and heteromeric complexes for human TPC1 and TPC2. Despite their largely distinct localization, we identified a discrete number of endosomes that coexpressed TPC1 and TPC2. Homo- and heteromerization were confirmed by a FRET study, showing that both proteins interacted in a rotational (N- to C-terminal/head-to-tail) symmetry. This is the first report describing the presence of homomultimeric TPC1 channels and the first study showing that TPCs are capable of forming heteromers.

Identification of a binding motif in the S5 helix that confers cholesterol sensitivity to the TRPV1 ion channel

The TRPV1 ion channel serves as an integrator of noxious stimuli with its activation linked to pain and neurogenic inflammation. Cholesterol, a major component of cell membranes, modifies the function of several types of ion channels. Here, using measurements of capsaicin-activated currents in excised patches from TRPV1-expressing HEK cells, we show that enrichment with cholesterol, but not its diastereoisomer epicholesterol, markedly decreased wild-type rat TRPV1 currents. Substitutions in the S5 helix, rTRPV1-R579D, and rTRPV1-F582Q, decreased this cholesterol response and rTRPV1-L585I was insensitive to cholesterol addition. Two human TRPV1 variants, with different amino acids at position 585, had different responses to cholesterol with hTRPV1-Ile(585) being insensitive to this molecule. However, hTRPV1-I585L was inhibited by cholesterol addition similar to rTRPV1 with the same S5 sequence. In the absence of capsaicin, cholesterol enrichment also inhibited TRPV1 currents induced by elevated temperature and voltage. These data suggest that there is a cholesterol-binding site in TRPV1 and that the functions of TRPV1 depend on the genetic variant and membrane cholesterol content.

Uncoupling charge movement from channel opening in voltage-gated potassium channels by ruthenium complexes

The Kv2.1 channel generates a delayed-rectifier current in neurons and is responsible for modulation of neuronal spike frequency and membrane repolarization in pancreatic β-cells and cardiomyocytes. As with other tetrameric voltage-activated K(+)-channels, it has been proposed that each of the four Kv2.1 voltage-sensing domains activates independently upon depolarization, leading to a final concerted transition that causes channel opening. The mechanism by which voltage-sensor activation is coupled to the gating of the pore is still not understood. Here we show that the carbon-monoxide releasing molecule 2 (CORM-2) is an allosteric inhibitor of the Kv2.1 channel and that its inhibitory properties derive from the CORM-2 ability to largely reduce the voltage dependence of the opening transition, uncoupling voltage-sensor activation from the concerted opening transition. We additionally demonstrate that CORM-2 modulates Shaker K(+)-channels in a similar manner. Our data suggest that the mechanism of inhibition by CORM-2 may be common to voltage-activated channels and that this compound should be a useful tool for understanding the mechanisms of electromechanical coupling.

Role of the S4-S5 linker in CNG channel activation

Cyclic nucleotide-gated (CNG) channels mediate sensory signal transduction in retinal and olfactory cells. The channels are activated by the binding of cyclic nucleotides to a cyclic nucleotide-binding domain (CNBD) in the C-terminus that is located at the intracellular side. The molecular events translating the ligand binding to the pore opening are still unknown. We investigated the role of the S4-S5 linker in the activation process by quantifying its interaction with other intracellular regions. To this end, we constructed chimeric channels in which the N-terminus, the S4-S5 linker, the C-linker, and the CNBD of the retinal CNGA1 subunit were systematically replaced by the respective regions of the olfactory CNGA2 subunit. Macroscopic concentration-response relations were analyzed, yielding the apparent affinity to cGMP and the Hill coefficient. The degree of functional coupling of intracellular regions in the activation gating was determined by thermodynamic double-mutant cycle analysis. We observed that all four intracellular regions, including the relatively short S4-S5 linker, are involved in controlling the apparent affinity of the channel to cGMP and, moreover, in determining the degree of cooperativity between the subunits, as derived from the Hill coefficient. The interaction energies reveal an interaction of the S4-S5 linker with both the N-terminus and the C-linker, but no interaction with the CNBD.

Structural determinants of gating in the TRPV1 channel

Transient receptor potential vanilloid 1 (TRPV1) channels mediate several types of physiological responses. Despite the importance of these channels in pain detection and inflammation, little is known about how their structural components convert different types of stimuli into channel activity. To localize the activation gate of these channels, we inserted cysteines along the S6 segment of mutant TRPV1 channels and assessed their accessibility to thiol-modifying agents. We show that access to the pore of TRPV1 is gated by S6 in response to both capsaicin binding and increases in temperature, that the pore-forming S6 segments are helical structures and that two constrictions are present in the pore: one that impedes the access of large molecules and the other that hampers the access of smaller ions and constitutes an activation gate of these channels.

Conformational rearrangements in the S6 domain and C-linker during gating in CNGA1 channels

This work completes previous findings and, using cysteine scanning mutagenesis (CSM) and biochemical methods, provides detailed analysis of conformational changes of the S6 domain and C-linker during gating of CNGA1 channels. Specific residues between Phe375 and Val424 were mutated to a cysteine in the CNGA1 and CNGA1(cys-free) background and the effect of intracellular Cd(2+) or cross-linkers of different length in the open and closed state was studied. In the closed state, Cd(2+) ions inhibited mutant channels A406C and Q409C and the longer cross-linker reagent M-4-M inhibited mutant channels A406C(cys-free) and Q409C(cys-free). Cd(2+) ions inhibited mutant channels D413C and Y418C in the open state, both constructed in a CNGA1 and CNGA1(cys-free) background. Our results suggest that, in the closed state, residues from Phe375 to approximately Ala406 form a helical bundle with a three-dimensional (3D) structure similar to those of the KcsA; furthermore, in the open state, residues from Ser399 to Gln409 in homologous subunits move far apart, as expected from the gating in K(+) channels; in contrast, residues from Asp413 to Tyr418 in homologous subunits become closer in the open state.

Molecular mechanisms of cyclic nucleotide-gated ion channel gating

Cyclic nucleotide-gated ion channels (CNGs) are distributed most widely in the neuronal cell. Great progress has been made in molecular mechanisms of CNG channel gating in the recent years. Results of many experiments have indicated that the stoichiometry and assembly of CNG channels affect their property and gating. Experiments of CNG mutants and analyses of cysteine accessibilities show that cyclic nucleotide-binding domains (CNBD) bind cyclic nucleotides and subsequently conformational changes occurred followed by the concerted or cooperative conformational change of all four subunits during CNG gating. In order to provide theoretical assistances for further investigation on CNG channels, especially regarding the disease pathogenesis of ion channels, this paper reviews the latest progress on mechanisms of CNG channels, functions of subunits, processes of subunit assembly, and conformational changes of subunit regions during gating.

The analysis of desensitizing CNGA1 channels reveals molecular interactions essential for normal gating

The pore region of cyclic nucleotide–gated (CNG) channels acts as the channel gate. Therefore, events occurring in the cyclic nucleotide–binding (CNB) domain must be coupled to the movements of the pore walls. When Glu363 in the pore region, Leu356 and Thr355 in the P helix, and Phe380 in the upper portion of the S6 helix are mutated into an alanine, gating is impaired: mutant channels E363A, L356A, T355A, and F380A desensitize in the presence of a constant cGMP concentration, contrary to what can be observed in wild-type (WT) CNGA1 channels. Similarly to C-type inactivation of K⁺ channels, desensitization in these mutant channels is associated with rearrangements of residues in the outer vestibule. In the desensitized state, Thr364 residues in different subunits become closer and Pro366 becomes more accessible to extracellular reagents. Desensitization is also observed in the mutant channel L356C, but not in the double-mutant channel L356C+F380C. Mutant channels L356F and F380K did not express, but cGMP-gated currents with a normal gating were observed in the double-mutant channels L356F+F380L and L356D+F380K. Experiments with tandem constructs with L356C, F380C, and L356C+F380C and WT channels indicate that the interaction between Leu356 and Phe380 is within the same subunit. These results show that Leu356 forms a hydrophobic interaction with Phe380, coupling the P helix with S6, whereas Glu363 could interact with Thr355, coupling the pore wall to the P helix. These interactions are essential for normal gating and underlie the transduction between the CNB domain and the pore.

Oxidant regulated inter-subunit disulfide bond formation between ASIC1a subunits

The acid-sensing ion channel-1a (ASIC1a) is composed of 3 subunits and is activated by a decrease in extracellular pH. It plays an important role in diseases associated with a reduced pH and production of oxidants. Previous work showed that oxidants reduce ASIC1a currents. However, the effects on channel structure and composition are unknown. We found that ASIC1a formed inter-subunit disulfide bonds and the oxidant H(2)O(2) increased this link between subunits. Cys-495 in the ASIC1a C terminus was particularly important for inter-subunit disulfide bond formation, although other C-terminal cysteines contributed. Inter-subunit disulfide bonds also produced some ASIC1a complexes larger than trimers. Inter-subunit disulfide bond formation reduced the proportion of ASIC1a located on the cell surface and contributed to the H(2)O(2)-induced decrease in H(+)-gated current. These results indicate that channel function is controlled by disulfide bond formation between intracellular residues on distinct ASIC1a subunits. They also suggest a mechanism by which the redox state can dynamically regulate membrane protein activity by forming intracellular bridges.

Movements of native C505 during channel gating in CNGA1 channels

We investigated conformational changes occurring in the C-linker and cyclic nucleotide-binding (CNB) domain of CNGA1 channels by analyzing the inhibition induced by thiol-specific reagents in mutant channels Q409C and A414C in the open and closed state. Cd(2+) (200 microM) inhibited irreversibly mutant channels Q409C and A414C in the closed but not in the open state. Cd(2+) inhibition was abolished in the mutant A414C(cys-free), in the double mutant A414C + C505T and in the tandem construct A414C + C505T/CNGA1, but it was present in the construct A414C + C505(cys-free). The cross-linker reagent M-2-M inhibited mutant channel Q409C in the open state. M-2-M inhibition in the open state was abolished in the double mutant Q409C + C505T and in the tandem construct Q409C + C505T/CNGA1. These results show that C(alpha) of C505 in the closed state is located at a distance between 4 and 10.5 A from the C(alpha) of A414 of the same subunit, but in the open state C505 moves towards Q409 of the same subunit at a distance that ranges from 10.5 to 12.3 A from C(alpha) of this residue. These results are not consistent with a 3-D structure of the CNGA1 channel homologous to the structure of HCN2 channels either in the open or in the closed state.

A comparison of electrophysiological properties of the CNGA1, CNGA1tandem and CNGA1cys-free channels

Three constructs are used for the analysis of biophysical properties of CNGA1 channels: the WT CNGA1 channel, a CNGA1 channel where all endogenous cysteines were removed (CNGA1cys-free) and a construct composed of two CNGA1 subunits connected by a small linker (CNGA1tandem). So far, it has been assumed, but not proven, that the molecular structure of these ionic channels is almost identical. The I/V relations, ionic selectivity to alkali monovalent cations, blockage by tetracaine and TMA+ were not significantly different. The cGMP dose response and blockage by TEA+ and Cd2+ were instead significantly different in CNGA1 and CNGA1cys-free channels, but not in CNGA1 and CNGA1tandem channels. Cd2+ blocked irreversibly the mutant channel A406C in the absence of cGMP. By contrast, Cd2+ did not block the mutant channel A406C in the CNGA1cys-free background (A406Ccys-free), but an irreversible and almost complete blockage was observed in the presence of the cross-linker M-4-M. Results obtained with different MTS cross-linkers and reagents suggest that the 3D structure of the CNGA1cys-free differs from that of the CNGA1 channel and that the distance between homologous residues at position 406 in CNGA1cys-free is longer than in the WT CNGA1 by several Angstroms.

A single N-terminal cysteine in TRPV1 determines activation by pungent compounds from onion and garlic

Some members of the transient receptor potential (TRP) family of cation channels mediate sensory responses to irritant substances. Although it is well known that TRPA1 channels are activated by pungent compounds found in garlic, onion, mustard and cinnamon extracts, activation of TRPV1 by these extracts remains controversial. Here we establish that TRPV1 is activated by pungent extracts from onion and garlic, as well as by allicin, the active compound in these preparations, and participates together with TRPA1 in the pain-related behavior induced by this compound. We found that in TRPV1 these agents act by covalent modification of cysteine residues. In contrast to TRPA1 channels, modification of a single cysteine located in the N-terminal region of TRPV1 was necessary and sufficient for all the effects we observed. Our findings point to a conserved mechanism of activation in TRP channels, which provides new insights into the molecular basis of noxious stimuli detection.

Thermosensitive TRPV channel subunits coassemble into heteromeric channels with intermediate conductance and gating properties

Heat-sensitive transient receptor potential (TRP) channels (TRPV1–4) form the major cellular sensors for detecting temperature increases. Homomeric channels formed by thermosensitive TRPV subunits exhibit distinct temperature thresholds. While these subunits do share significant sequence similarity, whether they can coassemble into heteromeric channels has been controversial. In the present study we investigated the coassembly of TRPV subunits using both spectroscopy-based fluorescence resonance energy transfer (FRET) and single-channel recordings. Fluorescent protein–tagged TRPV subunits were coexpressed in HEK 293 cells; FRET between different subunits was measured as an indication of the formation of heteromeric channels. We observed strong FRET when fluorescence signals were collected selectively from the plasma membrane using a “spectra FRET” approach but much weaker or no FRET from intracellular fluorescence. In addition, no FRET was detected when TRPV subunits were coexpressed with members of the TRPM subfamily or CLC-0 chloride channel subunits. These results indicate that a substantial fraction of TRP channels in the plasma membrane of cotransfected cells were heteromeric. Single-channel recordings confirmed the existence of multiple heteromeric channel forms. Interestingly, heteromeric TRPV channels exhibit intermediate conductance levels and gating kinetic properties. As these subunits coexpress both in sensory neurons and in other tissues, including heart and brain, coassembly between TRPV subunits may contribute to greater functional diversity.

Functional consequences of progressive cone dystrophy-associated mutations in the human cone photoreceptor cyclic nucleotide-gated channel CNGA3 subunit

Progressive cone dystrophies are a genetically heterogeneous group of disorders characterized by early deterioration of visual acuity and color vision, together with psychophysical and electrophysiological evidence of abnormal cone function and cone degeneration. Recently, three mutations in the gene encoding the CNGA3 subunit of cone photoreceptor cyclic nucleotide-gated (CNG) channels have been linked to progressive cone dystrophy in humans. To investigate the functional consequences of these mutations, we expressed mutant human CNGA3 subunits in Xenopus oocytes, alone or together with human CNGB3, and studied these channels using patch-clamp recording. Compared with wild-type channels, homomeric and heteromeric channels containing CNGA3-N471S or CNGA3-R563H subunits exhibited an increase in apparent affinity for cGMP and an increase in the relative agonist efficacy of cAMP compared with cGMP. In contrast, R277C subunits did not form functional homomeric or heteromeric channels. Cell surface expression levels, determined using confocal microscopy of green fluorescent protein-tagged subunits and patch-clamp recording, were significantly reduced for both R563H and R277C but unchanged for N471S. Overall, these results suggest that the plasma membrane localization and gating properties of cone CNG channels are altered by progressive cone dystrophy-associated mutations, providing evidence that supports the pathogenicity of these mutations.

The carboxyl-terminal region of cyclic nucleotide-modulated channels is a gating ring, not a permeation path

The recent elucidation of the structure of the carboxyl-terminal region of the hyperpolarization-activated cyclic nucleotide-modulated (HCN2) channel has prompted us to investigate a curious feature of this structure in HCN2 channels and in the related CNGA1 cyclic nucleotide-gated (CNG) channels. The crystallized fragment of the HCN2 channel contains both the cyclic nucleotide-binding domain (CNBD) and the C-linker region, which connects the CNBD to the pore. At the center of the fourfold-symmetric structure is a tunnel that runs perpendicular to the membrane. The narrowest part of the tunnel is approximately 10 A in diameter and is lined by a ring of negatively charged amino acids: D487, E488, and D489. Many ion channels have "charge rings" that focus permeant ions at the mouth of the pore and increase channel conductance. We used nonstationary fluctuation analysis and single-channel recording, coupled with site-directed mutagenesis and cysteine modification, to determine whether this part of HCN and CNG channels might be an extension of the permeation pathway. Our results indicate that modifying charge-ring amino acids affects gating but not ion permeation in HCN2 and CNG channels. Thus, this portion of the channel is not an obligatory part of the ion path but instead acts as a "gating ring." The carboxyl-terminal region of these channels must hang below the pore much like the "hanging gondola" of voltage-gated potassium channels, but the permeation pathway must exit the protein before the level of the ring of charged amino acids.

Structural insights into the regulatory mechanism of IP3 receptor

Inositol 1,4,5-trisphosphate receptors (IP(3)R) are intracellular Ca(2+) release channels whose opening requires binding of two intracellular messengers IP(3) and Ca(2+). The regulation of IP(3)R function has also been shown to involve a variety of cellular proteins. Recent biochemical and structural analyses have deepened our understanding of how the IP(3)-operated Ca(2+) channel functions. Specifically, the atomic resolution structure of the IP(3)-binding region has provided a sound structural basis for the receptor interaction with the natural ligand. Electron microscopic studies have also shed light on the overall shape of the tetrameric receptor. This review aims to provide comprehensive overview of the current information available on the structure and function relationship of IP(3)R.

Oxidizing reagent copper-o-phenanthroline is an open channel blocker of the vanilloid receptor TRPV1

The TRPV1 channel plays an important role in generating nociceptive signals in mammalian primary sensory neurons. It consists of 838 amino acids with six transmembrane segments (TM1-TM6), a pore-forming loop between TM5 and TM6 and N- and C- terminals located intracellularly. It is a homotetramer and forms a nonselective cationic channel that can be opened by capsaicin, weak acids and noxious heat. There are 18 cysteines (Cys), three of which are located on the extracellular side of the receptor in and around the region of the pore-forming loop. We report that the TRPV1 channel in transfected HEK293T cells and in cultured rat DRG neurons is blocked in the open state by an oxidizing agent Cu-o-phenanthroline complex (Cu:Phe). The effects of Cu:Phe are concentration dependent ( IC50 = 5.2 : 20.8 microm ) and fully reversible. Cu:Phe applied immediately before exposure to an acidic solution, capsaicin or noxious heat is without effect. Substitutions of the extracellular Cys residues (616, 621, 634) by glycine individually or together do not alter the blocking effects of Cu:Phe suggesting that disulfide cross-linking does not represent the underlying mechanism. It is suggested that the complex Cu:Phe, a bulky, positively charged molecule, represents a very effective and reversible open channel blocker of TRPV1.

Subunit configuration of heteromeric cone cyclic nucleotide-gated channels

Cone photoreceptor cyclic nucleotide-gated (CNG) channels are thought to be tetrameric assemblies of CNGB3 (B3) and CNGA3 (A3) subunits. We have used functional and biochemical approaches to investigate the stoichiometry and arrangement of these subunits in recombinant channels. First, tandem dimers of linked subunits were used to constrain the order of CNGB3 and CNGA3 subunits; the properties of channels formed by B3/B3+A3/A3 dimers, or A3/B3+B3/A3 dimers, closely resembled those of channels arising from B3+A3 monomers. Functional markers in B3/B3 (or A3/A3) dimers confirmed that both B3 subunits (and both A3 subunits) gained membership into the pore-forming tetramer and that like subunits were positioned adjacent to each other. Second, chemical crosslinking and co-immunoprecipitation studies using epitope-tagged monomer subunits both demonstrated the presence of two CNGB3 subunits in cone channels. Together, these data support a preferred subunit arrangement for cone CNG channels (B3-B3-A3-A3) that is distinct from the 3A:1B configuration of rod channels.

Subunit arrangement in P2X receptors

ATP-gated ionotropic receptors (P2X receptors) are distributed widely in the nervous system. For example, a hetero-oligomeric receptor containing both P2X2 and P2X3 subunits is involved in primary afferent sensation. Each subunit has two membrane-spanning domains. We have used disulfide bond formation between engineered cysteines to demonstrate close proximity between the outer ends of the first transmembrane domain of one subunit and the second transmembrane domain of another. After expression in HEK 293 cells of such modified P2X2 or P2X4 subunits, the disulfide bond formation is evident because an ATP-evoked channel opening requires previous reduction with dithiothreitol. In the hetero-oligomeric P2X2/3 receptor the coexpression of doubly substituted subunits with wild-type partners allows us to deduce that the hetero-oligomeric channel contains adjacent P2X3 subunits but does not contain adjacent P2X2 subunits. The results suggest a "head-to-tail" subunit arrangement in the quaternary structure of P2X receptors and show that a trimeric P2X2/3 receptor would have the composition P2X2(P2X3)2.

Achromatopsia-associated mutation in the human cone photoreceptor cyclic nucleotide-gated channel CNGB3 subunit alters the ligand sensitivity and pore properties of heteromeric channels

Cone photoreceptor cyclic nucleotide-gated (CNG) channels are thought to form by assembly of two different subunit types, CNGA3 and CNGB3. Recently, mutations in the gene encoding the CNGB3 subunit have been linked to achromatopsia in humans. Here we describe the functional consequences of two achromatopsia-associated mutations in human CNGB3 (hCNGB3). Co-expression in Xenopus oocytes of human CNGA3 (hCNGA3) subunits with hCNGB3 subunits containing an achromatopsia-associated mutation in the S6 transmembrane domain (S435F) generated functional heteromeric channels that exhibited an increase in apparent affinity for both cAMP and cGMP compared with wild type heteromeric channels. In contrast, co-expression of a presumptive null mutation of hCNGB3 (T383f.s.Delta C) with hCNGA3 produced channels with properties indistinguishable from homomeric hCNGA3 channels. The effect of hCNGB3 S435F subunits on cell-surface expression of green fluorescent protein-tagged hCNGA3 subunits and of non-tagged hCNGA3 subunits on surface expression of green fluorescent protein-hCNGB3 S435F subunits were similar to those observed for wild type hCNGB3 subunits, suggesting that the mutation does not grossly disturb subunit assembly or plasma membrane targeting. The S435F mutation was also found to produce changes in the pore properties of the channel, including decreased single channel conductance and decreased sensitivity to block by l-cis-diltiazem. Overall, these results suggest that the functional properties of cone CNG channels may be altered in patients with the S435F mutation, providing evidence supporting the pathogenicity of this mutation in humans. Thus, achromatopsia may arise from a disturbance of cone CNG channel gating and permeation or from the absence of functional CNGB3 subunits.

Intracellular disulfide bond that affects ATP responsiveness of P2X2 receptor/channel

The role of intracellular cysteine residues in P2X(2) receptor/channel was investigated. When dithiothreitol was intracellularly applied, both the maximal response and the sensitivity of the wild-type channel to ATP were decreased. On the other hand, Cu(2+) phenanthroline did not affect the responsiveness. When two intracellular cysteine residues (Cys(9) and Cys(430)) were replaced with alanine, both the maximal response and the sensitivity was decreased with the replacement at Cys(9), whereas no such decrease was observed with the replacement at Cys(430). These results suggest that an intracellular disulfide bond involving Cys(9) regulates the responsiveness of P2X(2) receptor/channel to ATP.

Critical regions for activation gating of the inositol 1,4,5-trisphosphate receptor

To understand the molecular mechanism of ligand-induced gating of the inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R)/Ca(2+) release channel, we analyzed the channel properties of deletion mutants retaining both the IP(3)-binding and channel-forming domains of IP(3)R1. Using intrinsically IP(3)R-deficient cells as the host cells for receptor expression, we determined that six of the mutants, those lacking residues 1-223, 651-1130, 1267-2110, 1845-2042, 1845-2216, and 2610-2748, did not exhibit any measurable Ca(2+) release activity, whereas the mutants lacking residues 1131-1379 and 2736-2749 retained the activity. Limited trypsin digestion showed that not only the IP(3)-gated Ca(2+)-permeable mutants lacking residues 1131-1379 and 2736-2749, but also two nonfunctional mutants lacking residues 1-223 and 651-1130, retained the normal folding structure of at least the C-terminal channel-forming domain. These results indicate that two regions of IP(3)R1, viz. residues 1-223 and 651-1130, are critical for IP(3)-induced gating. We also identified a highly conserved cysteine residue at position 2613, which is located within the C-terminal tail, as being essential for channel opening. Based on these results, we propose a novel five-domain structure model in which both N-terminal and internal coupling domains transduce ligand-binding signals to the C-terminal tail, which acts as a gatekeeper that triggers opening of the activation gate of IP(3)R1 following IP(3) binding.

Subunit stoichiometry of the CNG channel of rod photoreceptors

Cyclic nucleotide-gated (CNG) channels play a central role in the conversion of sensory stimuli into electrical signals. CNG channels form heterooligomeric complexes built of A and B subunits. Here, we study the subunit stoichiometry of the native rod CNG channel by chemical crosslinking. The apparent molecular weight (M(w)) of each crosslink product was determined by SDS-PAGE, and its composition was analyzed by Western blotting using antibodies specific for the A1 or B1 subunit. The number of crosslink products and their M(w) as well as the immunological identification of A1 and B1 subunits in the crosslink products led us to conclude that the native rod CNG channel is a tetramer composed of three A1 and one B1 subunit. This is an example of violation of symmetry in tetrameric channels.

Multimerization of the ligand binding domains of cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels comprise four subunits and are activated by the direct binding of cyclic nucleotide to an intracellular domain on each subunit. This ligand binding domain is thought to contain a beta roll followed by two alpha helices, designated the B and C helices. To examine the quaternary structure of CNG channels and how it changes during ion channel gating, we introduced single cysteines along the C helix of each subunit in an otherwise cysteineless channel. We found that cysteines on the C helices could form intersubunit disulfide bonds, even between diagonal subunits. Disulfide bond formation occurred primarily in closed channels and inhibited channel opening. These data suggest that the C helices from all four channel subunits are in close proximity in the closed state and move apart during channel opening.