GABA blocks pathological but not acute TRPV1 pain signals.

Sensitization of the capsaicin receptor TRPV1 is central to the initiation of pathological forms of pain, and multiple signaling cascades are known to enhance TRPV1 activity under inflammatory conditions. How might detrimental escalation of TRPV1 activity be counteracted? Using a genetic-proteomic approach, we identify the GABAB1 receptor subunit as bona fide inhibitor of TRPV1 sensitization in the context of diverse inflammatory settings. We find that the endogenous GABAB agonist, GABA, is released from nociceptive nerve terminals, suggesting an autocrine feedback mechanism limiting TRPV1 sensitization. The effect of GABAB on TRPV1 is independent of canonical G protein signaling and rather relies on close juxtaposition of the GABAB1 receptor subunit and TRPV1. Activating the GABAB1 receptor subunit does not attenuate normal functioning of the capsaicin receptor but exclusively reverts its sensitized state. Thus, harnessing this mechanism for anti-pain therapy may prevent adverse effects associated with currently available TRPV1 blockers.

Sensory mechanotransduction at membrane-matrix interfaces.

Sensory cells specialized to detect extremely small mechanical changes are common to the auditory and somatosensory systems. It is widely accepted that mechanosensitive channels form the core of the mechanoelectrical transduction in hair cells as well as the somatic sensory neurons that underlie the sense of touch and mechanical pain. Here, we will review how the activation of such channels can be measured in a meaningful physiological context. In particular, we will discuss the idea that mechanosensitive channels normally occur in transmembrane complexes that are anchored to extracellular matrix components (ECM) both in vitro and in vivo. One component of such complexes in sensory neurons is the integral membrane scaffold protein STOML3 which is a robust physiological regulator of native mechanosensitive currents. In order to better characterize such channels in transmembrane complexes, we developed a new electrophysiological method that enables the quantification of mechanosensitive current amplitude and kinetics when activated by a defined matrix movement in cultured cells. The results of such studies strongly support the idea that ion channels in transmembrane complexes are highly tuned to detect movement of the cell membrane in relation to the ECM.

The α1K276E startle disease mutation reveals multiple intermediate states in the gating of glycine receptors.

Loss-of-function mutations in human glycine receptors cause hyperekplexia, a rare inherited disease associated with an exaggerated startle response. We have studied a human disease mutation in the M2-M3 loop of the glycine receptor α1 subunit (K276E) using direct fitting of mechanisms to single-channel recordings with the program HJCFIT. Whole-cell recordings from HEK293 cells showed the mutation reduced the receptor glycine sensitivity. In single-channel recordings, rat homomeric α1 K276E receptors were barely active, even at 200 mM glycine. Coexpression of the ß subunit partially rescued channel function. Heteromeric mutant channels opened in brief bursts at 300 µM glycine (a concentration that is near-maximal for wild type) and reached a maximum one-channel open probability of about 45% at 100 mm glycine (compared to 96% for wild type). Distributions of apparent open times contained more than one component in high glycine and, therefore, could not be described by mechanisms with only one fully liganded open state. Fits to the data were much better with mechanisms in which opening can also occur from more than one fully liganded intermediate (e.g., "primed" models). Brief pulses of glycine (∼3 ms, 30 mM) applied to mutant channels in outside-out patches activated currents with a slower rise time (1.5 ms) than those of wild-type channels (0.2 ms) and a much faster decay. These features were predicted reasonably well by the mechanisms obtained from fitting single-channel data. Our results show that, by slowing and impairing channel gating, the K276E mutation facilitates the detection of closed reaction intermediates in the activation pathway of glycine channels.

The kinetic properties of the α3 rat glycine receptor make it suitable for mediating fast synaptic inhibition.

Glycine receptors mediate fast synaptic inhibition in spinal cord and brainstem. Two α subunits are present in adult neurones, α1, which forms most of the synaptic glycine receptors, and α3. The physiological role of α3 is not known, despite the fact that α3 expression is concentrated in areas involved in nociceptive processing, such as the superficial dorsal horn. In the present study, we characterized the kinetic properties of rat homomeric α3 glycine receptors heterologously expressed in HEK293 cells. We analysed steady state single channel activity at a range of different glycine concentrations by fitting kinetic schemes and found that α3 channels resemble α1 receptors in their high maximum open probability (99.1% cf. 98% for α1), but differ in that maximum open probability is reached when all five binding sites are occupied by glycine (cf. three out of five sites for α1). α3 activation was best described by kinetic schemes that allow the channel to open also when partially liganded and that contain more than the minimum number of shut states, either as desensitized distal states (Jones and Westbrook scheme) or as pre-open gating intermediates (flip scheme). We recorded also synaptic-like α3 currents elicited by the rapid application of 1 ms pulses of high concentration glycine to outside-out patches. These currents had fast deactivation, with a time constant of decay of 9 ms. Thus, if native synaptic currents can be mediated by α3 glycine receptors, they are likely to be very close in their kinetics to α1-mediated synaptic events.

Chloride ions in the pore of glycine and GABA channels shape the time course and voltage dependence of agonist currents.

In the vertebrate CNS, fast synaptic inhibition is mediated by GABA and glycine receptors. We recently reported that the time course of these synaptic currents is slower when intracellular chloride is high. Here we extend these findings to measure the effects of both extracellular and intracellular chloride on the deactivation of glycine and GABA currents at both negative and positive holding potentials. Currents were elicited by fast agonist application to outside-out patches from HEK-293 cells expressing rat glycine or GABA receptors. The slowing effect of high extracellular chloride on current decay was detectable only in low intracellular chloride (4 mm). Our main finding is that glycine and GABA receptors "sense" chloride concentrations because of interactions between the M2 pore-lining domain and the permeating ions. This hypothesis is supported by the observation that the sensitivity of channel gating to intracellular chloride is abolished if the channel is engineered to become cation selective or if positive charges in the external pore vestibule are eliminated by mutagenesis. The appropriate interaction between permeating ions and channel pore is also necessary to maintain the channel voltage sensitivity of gating, which prolongs current decay at depolarized potentials. Voltage dependence is abolished by the same mutations that suppress the effect of intracellular chloride and also by replacing chloride with another permeant ion, thiocyanate. These observations suggest that permeant chloride affects gating by a foot-in-the-door effect, binding to a channel site with asymmetrical access from the intracellular and extracellular sides of the membrane.

In glycine and GABA(A) channels, different subunits contribute asymmetrically to channel conductance via residues in the extracellular domain.

Single-channel conductance in Cys-loop channels is controlled by the nature of the amino acids in the narrowest parts of the ion conduction pathway, namely the second transmembrane domain (M2) and the intracellular helix. In cationic channels, such as Torpedo ACh nicotinic receptors, conductance is increased by negatively charged residues exposed to the extracellular vestibule. We now show that positively charged residues at the same loop 5 position boost also the conductance of anionic Cys-loop channels, such as glycine (α1 and α1ß) and GABA(A) (α1ß2γ2) receptors. Charge reversal mutations here produce a greater decrease on outward conductance, but their effect strongly depends on which subunit carries the mutation. In the glycine α1ß receptor, replacing Lys with Glu in α1 reduces single-channel conductance by 41%, but has no effect in the ß subunit. By expressing concatameric receptors with constrained stoichiometry, we show that this asymmetry is not explained by the subunit copy number. A similar pattern is observed in the α1ß2γ2 GABA(A) receptor, where only mutations in α1 or ß2 decreased conductance (to different extents). In both glycine and GABA receptors, the effect of mutations in different subunits does not sum linearly: mutations that had no detectable effect in isolation did enhance the effect of mutations carried by other subunits. As in the nicotinic receptor, charged residues in the extracellular vestibule of anionic Cys-loop channels influence elementary conductance. The size of this effect strongly depends on the direction of the ion flow and, unexpectedly, on the nature of the subunit that carries the residue.

Additional acetylcholine (ACh) binding site at alpha4/alpha4 interface of (alpha4beta2)2alpha4 nicotinic receptor influences agonist sensitivity.

Nicotinic acetylcholine receptor (nAChR) α4 and ß2 subunits assemble in two alternate stoichiometries to produce (α4ß2)(2)α4 and (α4ß2)(2)ß2, which display different agonist sensitivities. Functionally relevant agonist binding sites are thought to be located at α4(+)/ß2(-) subunit interfaces, but because these interfaces are present in both receptor isoforms, it is unlikely that they account for differences in agonist sensitivities. In contrast, incorporation of either α4 or ß2 as auxiliary subunits produces isoform-specific α4(+)/α4(-) or ß2(+)/ß2(-) interfaces. Using fully concatenated (α4ß2)(2)α4 nAChRs in conjunction with structural modeling, chimeric receptors, and functional mutagenesis, we have identified an additional site at the α4(+)/α4(-) interface that accounts for isoform-specific agonist sensitivity of the (α4ß2)(2)α4 nAChR. The additional site resides in a region that also contains a potentiating Zn(2+) site but is engaged by agonists to contribute to receptor activation. By engineering α4 subunits to provide a free cysteine in loop C at the α4(+)α4(-) interface, we demonstrated that the acetylcholine responses of the mutated receptors are attenuated or enhanced, respectively, following treatment with the sulfhydryl reagent [2-(trimethylammonium)ethyl]methanethiosulfonate or aminoethyl methanethiosulfonate. The findings suggest that agonist occupation of the site at the α4(+)/(α4(-) interface leads to channel gating through a coupling mechanism involving loop C. Overall, we propose that the additional agonist site at the α4(+)/α4(-) interface, when occupied by agonist, contributes to receptor activation and that this additional contribution underlies the agonist sensitivity signature of (α4ß2)(2)α4 nAChRs.

TMEM87a/Elkin1, a component of a novel mechanoelectrical transduction pathway, modulates melanoma adhesion and migration.

Mechanoelectrical transduction is a cellular signalling pathway where physical stimuli are converted into electro-chemical signals by mechanically activated ion channels. We describe here the presence of mechanically activated currents in melanoma cells that are dependent on TMEM87a, which we have renamed Elkin1. Heterologous expression of this protein in PIEZO1-deficient cells, that exhibit no baseline mechanosensitivity, is sufficient to reconstitute mechanically activated currents. Melanoma cells lacking functional Elkin1 exhibit defective mechanoelectrical transduction, decreased motility and increased dissociation from organotypic spheroids. By analysing cell adhesion properties, we demonstrate that Elkin1 deletion is associated with increased cell-substrate adhesion and decreased homotypic cell-cell adhesion strength. We therefore conclude that Elkin1 supports a PIEZO1-independent mechanoelectrical transduction pathway and modulates cellular adhesions and regulates melanoma cell migration and cell-cell interactions.

When cells receive signals about their surrounding environment, this initiates a chain of signals which generate a response. Some of these signalling pathways allow cells to sense physical and mechanical forces via a process called mechanotransduction. There are different types of mechanotransduction. In one pathway, mechanical forces open up specialized channels on the cell surface which allow charged particles to move across the membrane and create an electrical current. Mechanoelectrical transduction plays an important role in the spread of cancer: as cancer cells move away from a tumour they use these signalling pathways to find their way between cells and move into other parts of the body. Understanding these pathways could reveal ways to stop cancer from spreading, making it easier to treat. However, it remains unclear which molecules regulate mechanoelectrical transduction in cancer cells. Now, Patkunarajah, Stear et al. have studied whether mechanoelectrical transduction is involved in the migration of skin cancer cells. To study mechanoelectrical transduction, a fine mechanical input was applied to the skin cancer cells whilst measuring the flow of charged molecules moving across the membrane. This experiment revealed that a previously unknown protein named Elkin1 is required to convert mechanical forces into electrical currents. Deleting this newly found protein caused skin cancer cells to move more slowly and dissociate more easily from tumour-like clusters of cells. These findings suggest that Elkin1 is part of a newly identified mechanotransduction pathway that allows cells to sense mechanical forces from their surrounding environment. More work is needed to determine what role Elkin1 plays in mechanoelectrical transduction and whether other proteins are also involved. This could lead to new approaches that prevent cancer cells from dissociating from tumours and spreading to other body parts.

SUMOylation of Enzymes and Ion Channels in Sensory Neurons Protects against Metabolic Dysfunction, Neuropathy, and Sensory Loss in Diabetes.

Diabetic peripheral neuropathy (DPN) is a highly frequent and debilitating clinical complication of diabetes that lacks therapies. Cellular oxidative stress regulates post-translational modifications, including SUMOylation. Here, using unbiased screens, we identified key enzymes in metabolic pathways and ion channels as novel molecular targets of SUMOylation that critically regulated their activity. Sensory neurons of diabetic patients and diabetic mice demonstrated changes in the SUMOylation status of metabolic enzymes and ion channels. In support of this, profound metabolic dysfunction, accelerated neuropathology, and sensory loss were observed in diabetic gene-targeted mice selectively lacking the ability to SUMOylate proteins in peripheral sensory neurons. TRPV1 function was impaired by diabetes-induced de-SUMOylation as well as by metabolic imbalance elicited by de-SUMOylation of metabolic enzymes, facilitating diabetic sensory loss. Our results unexpectedly uncover an endogenous post-translational mechanism regulating diabetic neuropathy in patients and mouse models that protects against metabolic dysfunction, nerve damage, and altered sensory perception.

Non-agonist-binding subunit interfaces confer distinct functional signatures to the alternate stoichiometries of the alpha4beta2 nicotinic receptor: an alpha4-alpha4 interface is required for Zn2+ potentiation.

The alpha4beta2 subtype is the most abundant nicotinic acetylcholine receptor (nAChR) in the brain and possesses the high-affinity binding site for nicotine. The alpha4 and beta2 nAChR subunits assemble into two alternate stoichiometries, (alpha4)(2)(beta2)(3) and (alpha4)(3)(beta2)(2), which differ in their functional properties and sensitivity to chronic exposure to nicotine. Here, we investigated the sensitivity of both receptor stoichiometries to modulation by Zn2+. We show that Zn2+ exerts an inhibitory modulatory effect on (alpha4)(2)(beta2)(3) receptors, whereas it potentiates or inhibits, depending on its concentration, the function of (alpha4)(3)(beta2)(2) receptors. Furthermore, Zn2+ inhibition on (alpha4)(2)(beta2)(3) nAChRs is voltage-dependent, whereas it is not on (alpha4)(3)(beta2)(2) receptors. We used molecular modeling in conjunction with alanine substitution and functional studies to identify two distinct sets of residues that determine these effects and may coordinate Zn(2+). Zn(2+) inhibition is mediated by a site located on the beta2(+)/alpha4(-) subunit interfaces on both receptor stoichiometries. alpha4(H195) and beta2(D218) are key determinants of this site. Zn2+ potentiation on (alpha4)(3)(beta2)(2) nAChRs is exerted by a site that resides on the alpha4(+)/alpha4(-) of this receptor stoichiometry. alpha4(H195) on the (-) side of the ACh-binding alpha4 subunit and alpha4(E224) on the (+) side of the non-ACh-binding alpha4 subunit critically contribute to this site. We also identified residues within the beta2 subunit that confer voltage dependency to Zn2+ inhibition on (alpha4)(2)(beta2)(3), but not on (alpha4)(3)(beta2)(2) nAChRs.

Sazetidine-A is a potent and selective agonist at native and recombinant alpha 4 beta 2 nicotinic acetylcholine receptors.

Sazetidine-A has been recently proposed to be a "silent desensitizer" of alpha4beta2 nicotinic acetylcholine receptors (nAChRs), implying that it desensitizes alpha4beta2 nAChRs without first activating them. This unusual pharmacological property of sazetidine-A makes it, potentially, an excellent research tool to distinguish between the role of activation and desensitization of alpha4beta2 nAChRs in mediating the central nervous system effects of nicotine itself, as well as those of new nicotinic drugs. We were surprised to find that sazetidine-A potently and efficaciously stimulated nAChR-mediated dopamine release from rat striatal slices, which is mediated by alpha4beta2(*) and alpha6beta2(*) subtypes of nAChR. The agonist effects on native striatal nAChRs prompted us to re-examine the effects of sazetidine-A on recombinant alpha4beta2 nAChRs in more detail. We expressed the two alternative stoichiometries of alpha4beta2 nAChR in Xenopus laevis oocytes and investigated the agonist properties of sazetidine-A on both alpha4(2)beta2(3) and alpha4(3)beta2(2) nAChRs. We found that sazetidine-A potently activated both stoichiometries of alpha4beta2 nAChR: it was a full agonist on alpha4(2)beta2(3) nAChRs, whereas it had an efficacy of only 6% on alpha4(3)beta2(2) nAChRs. In contrast to what has been published before, we therefore conclude that sazetidine-A is an agonist of native and recombinant alpha4beta2 nAChRs but shows differential efficacy on alpha4beta2 nAChRs subtypes.

Voltage gating of mechanosensitive PIEZO channels.

Mechanosensitive PIEZO ion channels are evolutionarily conserved proteins whose presence is critical for normal physiology in multicellular organisms. Here we show that, in addition to mechanical stimuli, PIEZO channels are also powerfully modulated by voltage and can even switch to a purely voltage-gated mode. Mutations that cause human diseases, such as xerocytosis, profoundly shift voltage sensitivity of PIEZO1 channels toward the resting membrane potential and strongly promote voltage gating. Voltage modulation may be explained by the presence of an inactivation gate in the pore, the opening of which is promoted by outward permeation. Older invertebrate (fly) and vertebrate (fish) PIEZO proteins are also voltage sensitive, but voltage gating is a much more prominent feature of these older channels. We propose that the voltage sensitivity of PIEZO channels is a deep property co-opted to add a regulatory mechanism for PIEZO activation in widely different cellular contexts.

Experimentally induced aggressiveness in heroin-dependent patients treated with buprenorphine: comparison of patients receiving methadone and healthy subjects.

Objective measures of experimentally induced aggressiveness were evaluated in heroin-dependent patients (HDP), 15 receiving buprenorphine (BUP) and 15 receiving methadone (METH) treatment. HDP were randomly assigned to BUP and METH groups. Fifteen healthy subjects (CONT) were included in the study as controls. During a laboratory task, the Point Subtraction Aggression Paradigm, subjects earned monetary reinforcement and could respond by ostensibly subtracting money from a fictitious subject (the aggressive response). Money-earning (points maintained) responses did not differ in BUP patients and in controls. In contrast, point-maintained responses were significantly lower in the group of HDP treated with METH than in both the BUP and CONT groups. Aggressive responses were significantly higher in the HDP group than in the CONT group. No significant differences in aggressive responses were found between the BUP and METH groups. Baseline concentrations of plasma adrenocorticotropic hormone (ACTH) and cortisol (CORT) were higher in HDP than in CONT. During the experimental task, ACTH and CORT increased significantly less in METH patients than in BUP patients and CONT. Norepinephrine (NE) and epinephrine (EPI) levels increased significantly more in HDP than in CONT, without any difference between the METH and BUP patients. PSAP aggressive responses positively correlated with NE and EPI changes, as well as with Buss-Durkee Hostility Inventory (BDHI) scores in both METH and BUP patients and also in CONT subjects. No correlation was found between the extent of heroin exposure, drug doses and aggressiveness levels. BUP, similarly to METH, does not seem to affect outward-directed aggressiveness, as aggressive responses related more to monoamine levels and personality traits than to the action of opioid agonists. Money-earning responses seemed to be unimpaired in BUP patients.

Direct measurement of TRPV4 and PIEZO1 activity reveals multiple mechanotransduction pathways in chondrocytes.

The joints of mammals are lined with cartilage, comprised of individual chondrocytes embedded in a specialized extracellular matrix. Chondrocytes experience a complex mechanical environment and respond to changing mechanical loads in order to maintain cartilage homeostasis. It has been proposed that mechanically gated ion channels are of functional importance in chondrocyte mechanotransduction; however, direct evidence of mechanical current activation in these cells has been lacking. We have used high-speed pressure clamp and elastomeric pillar arrays to apply distinct mechanical stimuli to primary murine chondrocytes, stretch of the membrane and deflection of cell-substrate contacts points, respectively. Both TRPV4 and PIEZO1 channels contribute to currents activated by stimuli applied at cell-substrate contacts but only PIEZO1 mediates stretch-activated currents. These data demonstrate that there are separate, but overlapping, mechanoelectrical transduction pathways in chondrocytes.

Small-molecule inhibition of STOML3 oligomerization reverses pathological mechanical hypersensitivity.

The skin is equipped with specialized mechanoreceptors that allow the perception of the slightest brush. Indeed, some mechanoreceptors can detect even nanometer-scale movements. Movement is transformed into electrical signals via the gating of mechanically activated ion channels at sensory endings in the skin. The sensitivity of Piezo mechanically gated ion channels is controlled by stomatin-like protein-3 (STOML3), which is required for normal mechanoreceptor function. Here we identify small-molecule inhibitors of STOML3 oligomerization that reversibly reduce the sensitivity of mechanically gated currents in sensory neurons and silence mechanoreceptors in vivo. STOML3 inhibitors in the skin also reversibly attenuate fine touch perception in normal mice. Under pathophysiological conditions following nerve injury or diabetic neuropathy, the slightest touch can produce pain, and here STOML3 inhibitors can reverse mechanical hypersensitivity. Thus, small molecules applied locally to the skin can be used to modulate touch and may represent peripherally available drugs to treat tactile-driven pain following neuropathy.

Regulation of body weight and energy homeostasis by neuronal cell adhesion molecule 1.

Susceptibility to obesity is linked to genes regulating neurotransmission, pancreatic beta-cell function and energy homeostasis. Genome-wide association studies have identified associations between body mass index and two loci near cell adhesion molecule 1 (CADM1) and cell adhesion molecule 2 (CADM2), which encode membrane proteins that mediate synaptic assembly. We found that these respective risk variants associate with increased CADM1 and CADM2 expression in the hypothalamus of human subjects. Expression of both genes was elevated in obese mice, and induction of Cadm1 in excitatory neurons facilitated weight gain while exacerbating energy expenditure. Loss of Cadm1 protected mice from obesity, and tract-tracing analysis revealed Cadm1-positive innervation of POMC neurons via afferent projections originating from beyond the arcuate nucleus. Reducing Cadm1 expression in the hypothalamus and hippocampus promoted a negative energy balance and weight loss. These data identify essential roles for Cadm1-mediated neuronal input in weight regulation and provide insight into the central pathways contributing to human obesity.

Phosphorylation and function of alpha4beta2 receptor.

The neuronal nicotinic acetylcholine receptor (nAChR) alpha4 and beta2 subunits expressed in heterologous expression systems assemble into high- and low-affinity receptors (Zwart and Vijverberg, 1998; Buisson and Bertrand, 2001; Houlihan et al., 2001; Nelson et al., 2003), which reflects the assembly of two distinct subunit stoichiometries of alpha4beta2 receptor (Nelson et al., 2003). The high-affinity receptor ([alpha4]2[beta2]3) is about 100-fold more sensitive to ACh than the low-affinity receptor ([alpha4]3[beta2]2) (Zwart and Vijverberg, 1998; Buisson and Bertrand, 2001; Houlihan et al., 2001; Nelson et al., 2003). Recent evidence implicated 14-3-3 proteins as modulators of the relative abundance of nAChR subunits in the endoplasmic reticulum (ER), where ligand-gated ion channels assemble. The 14-3-3 proteins influence ER-to-plasma membrane trafficking of multimeric cell-surface proteins (O'Kelly et al., 2002). 14-3-3 proteins bind components of these multimeric proteins, and this interaction overrides dibasic COP1 retention signal to permit forward transport of the protein (O'Kelly et al., 2002). In the case of alpha4beta2 nAChRs, 14-3-3 binds the alpha4 subunit, and this association is dependent on phosphorylation of a serine residue within a protein kinase A(PKA) consensus sequence in the large cytoplasmic domain of the alpha4 subunit, which is also a binding motif recognized by 14-3-3 (Jeancloss et al., 2001; O'Kelly et al., 2002). The interplay among PKA, alpha4 subunits, and 14-3-3 proteins increases cell-surface expression of alpha4beta2 nAChRs by increasing steady-state levels of the alpha4 subunit available for assembly with beta2 subunits (Jeancloss et al., 2001). Because it is not known how 14-3-3-dependent changes in the steady-state levels of the alpha4 subunit might affect the functional type of alpha4beta2 receptors, we have investigated the effects of mutations of the 14-3-3 binding motif in the alpha4 subunit on alpha4beta2 nAChR function.

Stoichiometry and pharmacology of two human alpha4beta2 nicotinic receptor types.

The alpha4beta2 nicotinic acetylcholine receptor (nAChR) is the most abundant nAChR subtype in the brain, where it forms the high-affinity binding site for nicotine. The alpha4beta2 nAChR belongs to a gene family of ligand-gated ion channels that also includes muscle nAChRs, GABAA receptors, and glycine receptors and that assembles into pentameric structures. alpha4 and beta2 nAChR subunits expressed heterologously in Xenopus laevis oocytes assemble into a mixture of high- and low-affinity functional receptors, giving rise to biphasic ACh concentration-response curves (Zwart and Vijverberg, 1998; Buisson and Bertrand, 2001; Houlihan et al., 2001). High- and low-affinity alpha4beta2 nAChRs differ significantly in their functional and pharmacological properties (Zwart and Vijverberg, 1998; Buisson and Bertrand, 2001; Houlihan et al., 2001; Nelson et al., 2003) and result from the assembly of alpha4 and beta2 subunits into two distinct stoichiometric arrangements: (alpha4)2(beta2)3(high-affinity subtype) and (alpha4)3(beta2)2 (low-affinity subtype) (Nelson et al., 2003). In this study we have examined the functional and pharmacological properties of high- and low-affinity alpha4beta2 receptors using two-electrode voltage clamp procedures on Xenopus oocytes transfected with high (1:10) or low (10:1) ratios of alpha4/beta2 cDNAs, which yield high (1:10)- or low (10:1)- affinity receptors with monophasic ACh concentration- response curves. Furthermore, to determine the stoichiometry of high- and low-affinity receptors expressed heterologously by Xenopus oocytes, we have determined the stoichiometry of high- and low-affinity alpha4beta2 receptors by mutating a highly conserved hydrophobic residue in the middle (position 9') of the pore-lining domain, which increases agonist potency in a manner that allows predictions on subunit composition (Cooper et al., 1991; Revah et al., 1991; Labarca et al., 1995; Boorman et al., 2000).

5-I A-85380 and TC-2559 differentially activate heterologously expressed alpha4beta2 nicotinic receptors.

The neuronal nicotinic acetylcholine receptor alpha4 and beta2 subunits expressed in heterologous expression systems assemble into at least two distinct subunit stoichiometries of alpha4beta2 receptor. The (alpha4)2(beta2)3 stoichiometry is about 100-fold more sensitive to acetylcholine than the (alpha4)3(beta2)2 stoichiometry. In order to investigate if agonists in general distinguish high- and low-affinity alpha4beta2 nicotinic acetylcholine receptors, we have expressed human alpha4 and beta2 nicotinic acetylcholine receptor subunits in two different expression systems. The relative amounts of alpha4beta2 nicotinic acetylcholine receptors with high- and low-affinity for acetylcholine were manipulated by (a) injecting the subunit cDNAs at different alpha:beta ratios into Xenopus oocytes and (b) by culturing HEK-293 cells stably expressing alpha4beta2 nicotinic acetylcholine receptors overnight at different temperatures. The sensitivities of the alpha4beta2 nicotinic acetylcholine receptors to the agonists acetylcholine, 5-I A-85380, and TC-2559 were investigated using the voltage-clamp technique on Xenopus oocytes and using a fluorescent imaging plate reader to measure calcium responses from HEK-293 cells. Like acetylcholine, 5-I A-85380 produced biphasic concentration-response curves and the high-affinity component became larger when the cells were manipulated to produce a greater proportion of (alpha4)2(beta2)3 nicotinic acetylcholine receptors. Interestingly, under all circumstances, TC-2559 produced monophasic concentration-response curves. In oocytes injected with alpha4 and beta2 subunits in the 1:1 ratio the maximum effect of TC-2559 was 28% of that of acetylcholine. The EC50 for TC-2559 was not changed when oocytes were manipulated to express exclusively (alpha4)2(beta2)3 nicotinic acetylcholine receptors, however, the maximum effect of TC-2559 was dramatically enhanced. These results suggest that TC-2559 is a selective agonist of the (alpha4)2(beta2)3 nicotinic acetylcholine receptor stoichiometry.

Hypofunctional TrkA Accounts for the Absence of Pain Sensitization in the African Naked Mole-Rat.

The naked mole-rat is a subterranean rodent lacking several pain behaviors found in humans, rats, and mice. For example, nerve growth factor (NGF), an important mediator of pain sensitization, fails to produce thermal hyperalgesia in naked mole-rats. The sensitization of capsaicin-sensitive TRPV1 ion channels is necessary for NGF-induced hyperalgesia, but naked mole-rats have fully functional TRPV1 channels. We show that exposing isolated naked mole-rat nociceptors to NGF does not sensitize TRPV1. However, the naked mole-rat NGF receptor TrkA displays a reduced ability to engage signal transduction pathways that sensitize TRPV1. Between one- and three-amino-acid substitutions in the kinase domain of the naked mole-rat TrkA are sufficient to render the receptor hypofunctional, and this is associated with the absence of heat hyperalgesia. Our data suggest that evolution has selected for a TrkA variant that abolishes a robust nociceptive behavior in this species but is still compatible with species fitness.