An asparagine residue regulating conductance through P2X2 receptor/channels

Voltage-clamp fluorometry analysis of structural rearrangements of ATP-gated channel P2X2 upon hyperpolarization

Gating of the ATP-activated channel P2X2 has been shown to be dependent not only on [ATP] but also on membrane voltage, despite the absence of a canonical voltage-sensor domain. We aimed to investigate the structural rearrangements of rat P2X2 during ATP- and voltage-dependent gating, using a voltage-clamp fluorometry technique. We observed fast and linearly voltage-dependent fluorescence intensity (F) changes at Ala337 and Ile341 in the TM2 domain, which could be due to the electrochromic effect, reflecting the presence of a converged electric field. We also observed slow and voltage-dependent F changes at Ala337, which reflect structural rearrangements. Furthermore, we determined that the interaction between Ala337 in TM2 and Phe44 in TM1, which are in close proximity in the ATP-bound open state, is critical for activation. Taking these results together, we propose that the voltage dependence of the interaction within the converged electric field underlies the voltage-dependent gating.

Allosteric modulation of Ca2+ flux in ligand-gated cation channel (P2X4) by actions on lateral portals

Human P2X receptors are a family of seven ATP-gated ion channels that transport Na(+), K(+), and Ca(2+) across cell surface membranes. The P2X4 receptor is unique among family members in its sensitivity to the macrocyclic lactone, ivermectin, which allosterically modulates both ion conduction and channel gating. In this paper we show that removing the fixed negative charge of a single acidic amino acid (Glu(51)) in the lateral entrance to the transmembrane pore markedly attenuates the effect of ivermectin on Ca(2+) current and channel gating. Ca(2+) entry through P2X4 receptors is known to trigger downstream signaling pathways in microglia. Our experiments show that the lateral portals could present a novel target for drugs in the treatment of microglia-associated disease including neuropathic pain.

Pore-opening mechanism in trimeric P2X receptor channels

The opening of ion channels in response to ligand binding, voltage or membrane stretch underlies electrical and chemical signalling throughout biology. Two structural classes of pore-opening mechanisms have been established, including bending of pore-lining helices in the case of tetrameric cation channels, or tilting of such helices in mechanosensitive channels. In this paper, we explore how the structure of the pore changes during opening in P2X receptors by measuring the modification of introduced cysteine residues in transmembrane helices by thiol-reactive reagents, and by engineering metal bridges. Our results are consistent with the X-ray structure of the closed state, and demonstrate that expansion of the gate region in the external pore is accompanied by a significant narrowing of the inner pore, indicating that pore-forming helices straighten on ATP binding to open the channel. This unique pore-opening mechanism has fundamental implications for the role of subunit interfaces in the gating mechanism of P2X receptors and points to a role of the internal pore in ion permeation.

A recent X-ray structure revealed the closed state of a P2X receptor ion channel. Here, Li and colleagues probe the structural rearrangements that take place during channel opening by measuring the effects of covalent modification of engineered cysteines.

On the role of the first transmembrane domain in cation permeability and flux of the ATP-gated P2X2 receptor

P2X receptors are a family of seven ligand-gated ion channels (P2X1-P2X7) that open in the presence of ATP. We used alanine-scanning mutagenesis and patch clamp photometry to study the role of the first transmembrane domain of the rat P2X2 receptor in cation permeability and flux. Three alanine-substituted mutants did not respond to ATP, and 19 of the 22 functional receptors resembled the wild-type receptor with regard to the fraction of the total ATP-gated current carried by calcium or the permeability of calcium relative to cesium. The remaining three mutants showed modest changes in calcium dynamics. Two of these occurred at sites (Gly30 and Phe44) that are unlikely to interact with permeating cations in a meaningful way. The third was a conserved tyrosine (Tyr43) that may form an inter-pore binding site for calcium. The data suggest that, with the possible exception of Tyr43, the first transmembrane domain contributes little to the permeation properties of the P2X2 receptor.

Participation of the Lys313-Ile333 Sequence of the Purinergic P2X4 Receptor in Agonist Binding and Transduction of Signals to the Channel Gate

To study the roles of the Lys(313)-Ile(333) ectodomain sequence of the rat P2X(4) receptor in ATP binding and transduction of signals to the channel gate, the conserved Lys(313), Tyr(315), Gly(316), Ike(317), Arg(318), Asp(320), Val(323), Lys(329), Phe(330), and Ile(333) residues were mutated. Current recordings were done on lifted cells and ATP was applied using an ultrafast solution-switching system. The rates of wild type channel opening and closing in the presence of ATP, but not the rate of washout-induced closing, were dependent on agonist concentration. All mutants other than I317A were expressed in the plasma membrane at comparable levels. The majority of mutants showed significant changes in the peak amplitude of responses and the EC(50) values for ATP. When stimulated with the supramaximal (1.4 mm) ATP concentration, mutants also differed in the kinetics of their activation, deactivation, and/or desensitization. The results suggest a critical role of the Lys(313) residue in receptor function other than coordination of the phosphate group of ATP and possible contribution of the Tyr(315) residue to the agonist binding module. The pattern of changes of receptor function by mutation of other residues was consistent with the operation of the Gly(316)-Ile(333) sequence as a signal transduction module between the ligand binding domain and the channel gate in the second transmembrane domain.

Biophysics of P2X receptors

The P2X receptor is the baby brother of the ligand-gated ion channel super-family. An understanding of its role in human physiology is still developing, and no one truly knows how it works to transport ions across the membrane. In this study, we review some aspects of P2X channel biophysics, concentrating on ion permeation and gating. P2X channels transport both small and large cations and anions across cell membranes in a manner that depends on both the subunit composition of the receptor and the experimental conditions. We describe the pore properties of wild-type receptors and use the altered phenotypes of mutant receptors to point the way towards a structural model of the pore.

Secondary structure and gating rearrangements of transmembrane segments in rat P2X4 receptor channels

P2X receptors are cation selective channels that are activated by extracellular nucleotides. These channels are likely formed by three identical or related subunits, each having two transmembrane segments (TM1 and TM2). To identify regions that undergo rearrangement during gating and to probe their secondary structure, we performed tryptophan scanning mutagenesis on the two putative TMs of the rat P2X₄ receptor channel. Mutant channels were expressed in Xenopus oocytes, concentration–response relationships constructed for ATP, and the EC₅₀ estimated by fitting the Hill equation to the data. Of the 22 mutations in TM1 and 24 in TM2, all but one in TM1 and seven in TM2 result in functional channels. Interestingly, the majority of the functional mutants display an increased sensitivity to ATP, and in general these perturbations are more pronounced for TM2 when compared with TM1. For TM1 and for the outer half of TM2, the perturbations are consistent with these regions adopting α-helical secondary structures. In addition, the greatest perturbations in the gating equilibrium occur for mutations near the outer ends of both TM1 and TM2. Surface biotinylation experiments reveal that all the nonfunctional mutants traffic to the surface membrane at levels comparable to the WT channel, suggesting that these mutations likely disrupt ion conduction or gating. Taken together, these results suggest that the outer parts of TM1 and TM2 are helical and that they move during activation. The observation that the majority of nonconducting mutations are clustered toward the inner end of TM2 suggests a critical functional role for this region.

Gain and loss of channel function by alanine substitutions in the transmembrane segments of the rat ATP-gated P2X2 receptor

ATP opens ionotropic P2X channels through a process that is poorly understood. We made an array of mutant rat P2X2 channels containing unique alanine substitutions in the transmembrane segments with the goal of identifying possible secondary structure and mapping gating domains in the pore. Alteration of channel function was measured as a change in ATP potency, 2'-3'-O-(4-benzoylbenzoyl)ATP (BzATP) efficacy, and deactivation kinetics. Four mutants (V45A, Y47A, V51A, and D349A) failed to respond to ATP. Seven (H33A, Q37A, I40A, L41A, Y43A, F44A, and I50A) of 22 mutations in the first transmembrane segment (TM1) produced channels with altered potencies, and two mutants (Y43A and F44A) were active in the absence of agonist. The pattern of hits was consistent with a helical secondary structure. In contrast, nine (I328A, P329A, N333A, L338A, T339A, S340A, G342A, G344A, and S345A) of 24 mutations in the second transmembrane segment (TM2) resulted in a change in potency, but no regular pattern of impact was apparent. Many of the same mutations that altered ATP potency also changed the relative efficacy of the partial agonist BzATP. Together, these data suggest that both TM1 and TM2 participate in the conformational changes that occur during receptor activation and help to define domains involved in conformational switching within or near the pore.

Size of side-chain at channel pore mouth affects Ca(2+) block of P2X(2) receptor

Effects of amino acid replacement at the channel pore mouth of P2X(2) receptor/channel on multivalent cation channel block were investigated. When Asn(333) was replaced with various amino acid residues with neutral side chains (Gly, Ala, Val, Leu and Ile), the block by Ca(2+) was attenuated according to the sizes of the side chains. The block by La(3+) was also greatest with the Gly-substituted mutant, but this preference was not found for the block by other multivalent cations tested. The side chain at the channel pore mouth may interfere with the access of Ca(2+) block by steric hindrance.

Polar residues of the second transmembrane domain influence cation permeability of the ATP-gated P2X(2) receptor

P2X receptors are simple polypeptide channels that mediate fast purinergic depolarizations in both nerve and muscle. Although the depolarization results mainly from the influx of Na(+), these channels also conduct a significant Ca(2+) current that is large enough to evoke transmitter release from presynaptic neurons. We sought to determine the molecular basis of this Ca(2+) conductance by a mutational analysis of recombinant P2X(2) receptors. Wild type and 31 mutant P2X(2) receptors were expressed in HEK-293 cells and studied under voltage-clamp. We found that the relative Ca(2+) permeability measured from the reversal potentials of ATP-gated currents was unaffected by neutralizing fixed charge (Asp(315), Asp(349)) near the mouths of the channel pore. By contrast, mutations that changed the character or side chain volume of three polar residues (Thr(336), Thr(339), Ser(340)) within the pore led to significant changes in P(Ca)/P(Cs). The largest changes occurred when Thr(339) and Ser(340) were replaced with tyrosine; these mutations almost completely abolished Ca(2+) permeability, reduced P(Li)/P(Cs) by about one-half, and shifted the relative permeability sequence of Cs(+), Rb(+), K(+), and Na(+) to their relative mobility in water. Our results suggest that the permeability sequence of the P2X(2) receptor arises in part from interactions of permeating cations with the polar side chains of three amino acids located in a short stretch of the second transmembrane domain.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Block and unblock by imipramine of cloned and mutated P2X2 receptor/channel expressed in Xenopus oocytes

Effects of imipramine on the cloned P2X2 receptor/channel and its mutants expressed in Xenopus oocytes were examined. Imipramine (100 microM) partially blocked an ionic current mediated through the wild-type P2X2 receptor/channel. With a higher concentration (300 microM) of imipramine, the current block was attenuated, suggesting that the second, lower affinity, 'unblocking' binding-site for imipramine exists in addition to the 'blocking' binding-site. These profiles of the modulation by imipramine were influenced by the substitution of negatively charged or polarized amino acid residues near the outer mouth of the channel pore (Asp315, Thr330 and Asn333) with neutral amino acid residues (Val or Ile). With the neutralization of Asp315, the current 'block' by 100 microM imipramine was attenuated. With the neutralization of Thr330, the current 'block' by 100 microM imipramine was enhanced. With the neutralization of Asn333, the 'unblock' by 300 microM imipramine disappeared. The results suggest that imipramine modulates P2X2 receptor/channels by interacting these amino acid residues.