Charge selectivity of the Cys-loop family of ligand-gated ion channels

Anion-cation permeability correlates with hydrated counterion size in glycine receptor channels

The functional role of ligand-gated ion channels depends critically on whether they are predominantly permeable to cations or anions. However, these, and other ion channels, are not perfectly selective, allowing some counterions to also permeate. To address the mechanisms by which such counterion permeation occurs, we measured the anion-cation permeabilities of different alkali cations, Li(+) Na(+), and Cs(+), relative to either Cl(-) or NO(3)(-) anions in both a wild-type glycine receptor channel (GlyR) and a mutant GlyR with a wider pore diameter. We hypothesized and showed that counterion permeation in anionic channels correlated inversely with an equivalent or effective hydrated size of the cation relative to the channel pore radius, with larger counterion permeabilities being observed in the wider pore channel. We also showed that the anion component of conductance was independent of the nature of the cation. We suggest that anions and counterion cations can permeate through the pore as neutral ion pairs, to allow the cations to overcome the large energy barriers resulting from the positively charged selectivity filter in small GlyR channels, with the permeability of such ion pairs being dependent on the effective hydrated diameter of the ion pair relative to the pore diameter.

Structural determinants of Ca2+ permeability and conduction in the human 5-hydroxytryptamine type 3A receptor

Cation-selective cysteine (Cys)-loop transmitter-gated ion channels provide an important pathway for Ca2+ entry into neurones. We examined the influence on Ca2+ permeation of amino acids located at intra- and extracellular ends of the conduction pathway of the human 5-hydroxytryptamine type 3A (5-HT3A) receptor. Mutation of cytoplasmic arginine residues 432, 436, and 440 to glutamine, aspartate, and alanine (the aligned residues of the human 5-HT3B subunit (yielding 5-HT3A(QDA)) increased PCa/PCs from 1.4 to 3.7. The effect was attributable to the removal of an electrostatic influence of the Arg-436 residue. Despite its relatively high permeability to Ca2+, the single channel conductance of the 5-HT3A(QDA) receptor was depressed in a concentration-dependent and voltage-independent manner by extracellular Ca2+. A conserved aspartate, located toward the extracellular end of the conduction pathway and known to influence ionic selectivity, contributed to the inhibitory effect of Ca2+ on macroscopic currents mediated by 5-HT3A receptors. We introduced a D293A mutation into the 5-HT3A(QDA) receptor (yielding the 5-HT3A(QDA D293A) construct) to determine whether the aspartate is required for the suppression of single channel conductance by Ca2+. The D293A mutation decreased the PCa/PCs ratio to 0.25 and reduced inwardly directed single channel conductance from 41 to 30 pS but did not prevent suppression of single channel conductance by Ca2+. The D293A mutation also reduced PCa/PCs when engineered into the wild-type 5-HT3A receptor. The data helped to identify key residues in the cytoplasmic domain (Arg-436) and extracellular vestibule (Asp-293) that markedly influence PCa/PCs and additionally directly demonstrated a depression of single channel conductance by Ca2+.

The cys-loop ligand-gated ion channel gene superfamily of the nematode, Caenorhabditis elegans

The nematode, Caenorhabditis elegans, possesses the most extensive known superfamily of cys-loop ligand-gated ion channels (cys-loop LGICs) consisting of 102 subunit-encoding genes. Less than half of these genes have been functionally characterised which include cation-permeable channels gated by acetylcholine (ACh) and γ-aminobutyric acid (GABA) as well as anion-selective channels gated by ACh, GABA, glutamate and serotonin. Following the guidelines set for genetic nomenclature for C. elegans, we have designated unnamed subunits as lgc genes (ligand-gated ion channels of the cys-loop superfamily). Phylogenetic analysis shows that several of these lgc subunits form distinct groups which may represent novel cys-loop LGIC subtypes.

Dynamics of heteropentameric nicotinic acetylcholine receptor: implications of the gating mechanism

The dynamics characteristics of the currently available structure of Torpedo nicotinic acetylcholine receptor (nAChR), including the extracellular, transmembrane, and intracellular domains (ICDs), were analyzed using the Gaussian Network Model (GNM) and Anisotropic Network Model (ANM). We found that a symmetric quaternary twist motion, reported previously in the literature in a homopentameric receptor (Cheng et al. J Mol Biol 2006;355:310-324; Taly et al. Biophys J 2005;88:3954-3965), occurred also in the heteropentameric Torpedo nAChR. We believe, however, that the symmetric twist alone is not sufficient to explain a large body of experimental data indicating asymmetry and subunit nonequivalence during gating. Here we report our results supporting the hypothesis that a combination of symmetric and asymmetric motions opens the gate. We show that the asymmetric motion involves tilting of the TM2 helices. Furthermore, our study reveals three additional aspects of channel dynamics: (1) loop A serves as an allosteric mediator between the ligand binding loops and those at the domain interface, particularly the linker between TM2 and TM3; (2) the ICD can modulate the pore dynamics and thus should not be neglected in gating studies; and (3) the F loops, which are peculiarly longer and poorly-conserved in non-alpha-subunits, have important dynamical implications.

The cys-loop ligand-gated ion channel gene superfamily of the red flour beetle, Tribolium castaneum

BACKGROUND

Members of the cys-loop ligand-gated ion channel (cys-loop LGIC) superfamily mediate chemical neurotransmission and are studied extensively as potential targets of drugs used to treat neurological disorders such as Alzheimer's disease. Insect cys-loop LGICs are also of interest as they are targets of highly successful insecticides. The red flour beetle, Tribolium castaneum, is a major pest of stored agricultural products and is also an important model organism for studying development.

RESULTS

As part of the T. castaneum genome sequencing effort, we have characterized the beetle cys-loop LGIC superfamily which is the third insect superfamily to be described after those of Drosophila melanogaster and Apis mellifera, and also the largest consisting of 24 genes. As with Drosophila and Apis, Tribolium possesses ion channels gated by acetylcholine, gamma-amino butyric acid (GABA), glutamate and histamine as well as orthologs of the Drosophila pH-sensitive chloride channel subunit (pHCl), CG8916 and CG12344. Similar to Drosophila and Apis, Tribolium cys-loop LGIC diversity is broadened by alternative splicing although the beetle orthologs of RDL and GluCl possess more variants of exon 3. Also, RNA A-to-I editing was observed in two Tribolium nicotinic acetylcholine receptor subunits, Tcasalpha6 and Tcasbeta1. Editing in Tcasalpha6 is evolutionarily conserved with D. melanogaster, A. mellifera and Heliothis virescens, whereas Tcasbeta1 is edited at a site so far only observed in the beetle.

CONCLUSION

Our findings reveal that in diverse insect species the cys-loop LGIC superfamily has remained compact with only minor changes in gene numbers. However, alternative splicing, RNA editing and the presence of divergent subunits broadens the cys-loop LGIC proteome and generates species-specific receptor isoforms. These findings on Tribolium castaneum enhance our understanding of cys-loop LGIC functional genomics and provide a useful basis for the development of improved insecticides that target an important agricultural pest.

Acidic amino acids impart enhanced Ca2+ permeability and flux in two members of the ATP-gated P2X receptor family

P2X receptors are ATP-gated cation channels expressed in nerve, muscle, bone, glands, and the immune system. The seven family members display variable Ca²⁺ permeabilities that are amongst the highest of all ligand-gated channels (Egan and Khakh, 2004). We previously reported that polar residues regulate the Ca²⁺ permeability of the P2X₂ receptor (Migita et al., 2001). Here, we test the hypothesis that the formal charge of acidic amino acids underlies the higher fractional Ca²⁺ currents (Pf%) of the rat and human P2X₁ and P2X₄ subtypes. We used patch-clamp photometry to measure the Pf% of HEK-293 cells transiently expressing a range of wild-type and genetically altered receptors. Lowering the pH of the extracellular solution reduced the higher Pf% of the P2X₁ receptor but had no effect on the lower Pf% of the P2X₂ receptor, suggesting that ionized side chains regulate the Ca²⁺ flux of some family members. Removing the fixed negative charges found at the extracellular ends of the transmembrane domains also reduced the higher Pf% of P2X₁ and P2X₄ receptors, and introducing these charges at homologous positions increased the lower Pf% of the P2X₂ receptor. Taken together, the data suggest that COO⁻ side chains provide an electrostatic force that interacts with Ca²⁺ in the mouth of the pore. Surprisingly, the glutamate residue that is partly responsible for the higher Pf% of the P2X₁ and P2X₄ receptors is conserved in the P2X₃ receptor that has the lowest Pf% of all family members. We found that neutralizing an upstream His⁴⁵ increased Pf% of the P2X₃ channel, suggesting that this positive charge masks the facilitation of Ca²⁺ flux by the neighboring Glu⁴⁶. The data support the hypothesis that formal charges near the extracellular ends of transmembrane domains contribute to the high Ca²⁺ permeability and flux of some P2X receptors.

The cellular, molecular and ionic basis of GABA(A) receptor signalling

GABA(A) receptors mediate fast synaptic inhibition in the CNS. Whilst this is undoubtedly true, it is a gross oversimplification of their actions. The receptors themselves are diverse, being formed from a variety of subunits, each with a different temporal and spatial pattern of expression. This diversity is reflected in differences in subcellular targetting and in the subtleties of their response to GABA. While activation of the receptors leads to an inevitable increase in membrane conductance, the voltage response is dictated by the distribution of the permeant Cl(-) and HCO(3)(-) ions, which is established by anion transporters. Similar to GABA(A) receptors, the expression of these transporters is not only developmentally regulated but shows cell-specific and subcellular variation. Untangling all these complexities allows us to appreciate the variety of GABA-mediated signalling, a diverse set of phenomena encompassing both synaptic and non-synaptic functions that can be overtly excitatory as well as inhibitory.

Molecular modelling of the GABAA ion channel protein

The GABAA ion channel protein is central to the mechanism of action of general anaesthetics and thus to the phenomenon of human consciousness. A molecular model of the alpha1beta2gamma2 gamma-aminobutyric acid type-A (GABAA) ligand-gated ion channel protein has been constructed. The cryo-electron microscopy structure of the nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata and the X-ray crystal structure of the acetylcholine binding protein (AChBP) from Lymnaea stagnalis were used as starting templates for comparative modelling. Features of the modelling approach used in the development of this GABAA model include: (1) multiple sequence alignment of members of the Cys-loop superfamily; (2) the design and implementation of a quasi-ab initio loop modelling algorithm; (3) expansion of the transmembrane domain (TMD) ion pore to model the open-state of the GABAA channel; (4) hydrophobicity analysis of the TMD to refine the structure in regions involved in general anaesthetic binding. The final model of the alpha1beta2gamma2 GABAA protein agrees with available experimental data concerning general anaesthetics.

Identification of molluscan nicotinic acetylcholine receptor (nAChR) subunits involved in formation of cation- and anion-selective nAChRs

Acetylcholine (ACh) is a neurotransmitter commonly found in all animal species. It was shown to mediate fast excitatory and inhibitory neurotransmission in the molluscan CNS. Since early intracellular recordings, it was shown that the receptors mediating these currents belong to the family of neuronal nicotinic acetylcholine receptors and that they can be distinguished on the basis of their pharmacology. We previously identified 12 Lymnaea cDNAs that were predicted to encode ion channel subunits of the family of the neuronal nicotinic acetylcholine receptors. These Lymnaea nAChRs can be subdivided in groups according to the residues supposedly contributing to the selectivity of ion conductance. Functional analysis in Xenopus oocytes revealed that two types of subunits with predicted distinct ion selectivities form homopentameric nicotinic ACh receptor (nAChR) subtypes conducting either cations or anions. Phylogenetic analysis of the nAChR gene sequences suggests that molluscan anionic nAChRs probably evolved from cationic ancestors through amino acid substitutions in the ion channel pore, a mechanism different from acetylcholine-gated channels in other invertebrates.

Molecular determinants of single-channel conductance and ion selectivity in the Cys-loop family: insights from the 5-HT3 receptor

The molecular determinants of the ionic selectivity and single-channel conductance of the Cys-loop family of transmitter-gated ion channels are beginning to be understood with increasing precision, in part, as a result of the recent availability of refined ultrastructural information for the archetype of the family, the nicotinic acetylcholine receptor (nAChR). Studies of another member of this family, the 5-HT(3) receptor, have now provided insight into the structure of its channel pore, the location of its gate and mechanisms of ion selectivity and translocation. The anomaly of the extremely low single-channel conductance of the homo-oligomeric 5-HT(3A) receptor has recently been solved, revealing that an intracellular domain of the protein is an important determinant of single-channel conductance. Such data are interpreted, in this article, in light of the most recent developments in structural characterization of the nAChR.