A single amino acid change within the ion-channel domain of the gamma-aminobutyric acid rho1 receptor accelerates desensitization and increases taurine agonism

An outline of desensitization in pentameric ligand-gated ion channel receptors

Pentameric ligand-gated ion channel (pLGIC) receptors exhibit desensitization, the progressive reduction in ionic flux in the prolonged presence of agonist. Despite its pathophysiological importance and the fact that it was first described over half a century ago, surprisingly little is known about the structural basis of desensitization in this receptor family. Here, we explain how desensitization is defined using functional criteria. We then review recent progress into reconciling the structural and functional basis of this phenomenon. The extracellular-transmembrane domain interface is a key locus. Activation is well known to involve conformational changes at this interface, and several lines of evidence suggest that desensitization involves a distinct conformational change here that is incompatible with activation. However, major questions remain unresolved, including the structural basis of the desensitization-induced agonist affinity increase and the mechanism of pore closure during desensitization.

Functional impact of serial deletions at the C-terminus of the human GABArho1 receptor

GABArho1 receptors are formed by homopentameric assemblies that gate a chloride ion-channel upon activation by the neurotransmitter. Very little is known about the structural and functional roles played by the different domains that form each subunit; but one of them, the fourth transmembrane segment (TM4), is known to form a hydrophobic bundle together with three other TM segments that are necessary to stabilize the structure of the receptor. In this study we progressively removed amino acid residues from the C-terminus of the human GABArho1 and studied the functional properties of the receptor mutants expressed in X. laevis oocytes. We found that deletions of up to the last four residues gave rise to receptors that were still functional, generating currents of 3.92 microA for the wt, 5.75 microA for S479X, 1.82 microA for F478X, 0.52 microA for I477X and 0.27 microA for S476X when exposed to 5 microM GABA; surprisingly, the mutant with one residue removed resulted more sensitive to the agonists. Further deletions, up to residue W475, resulted in receptors that did not gate an ion-channel. In addition, deleting the signal sequence, from R2-A15, in the N-terminus produced non-functional receptors. This study reveals that GABArho1 can tolerate removal of several residues that form the fourth transmembrane segment up to a critical point, signaled by W475, beyond which the mutant protein is translated but does not form functional receptors. A comparative study is presented of some electrophysiological and pharmacological properties of the deletion mutants that were able to generate GABA currents.

Role of neurotransmitter receptors in mediating light-evoked responses in retinal interplexiform cells

Interplexiform (IP) cells are a long-neglected group of retinal neurons the function of which is yet to be determined. Anatomical study indicates that IP cells are located in the inner nuclear layer, juxtaposed with the third-order neurons. However, the synaptic transmission of IP cells in the inner retina is poorly understood. Using whole cell patch-clamp and pharmacological techniques, we extensively studied synaptic receptors in IP cells. The IP cells in amphibian retinal slices were identified by electrical and morphological properties with voltage-clamp recording and Lucifer yellow dialysis. We find that light-evoked excitatory postsynaptic currents (L-EPSCs) are mediated by AMPA and N-methyl-d-aspartate receptors in IP cells. Although both receptors contributed to the amplitude and kinetics of L-EPSCs, AMPA receptor desensitization substantially shaped L-EPSCs in the neurons, similar to those found in the third-order neurons. The light-evoked inhibitory postsynaptic currents (L-IPSCs) in IP cells were primarily mediated by strychnine-sensitive glycine receptors with a small component of GABA(C) receptors. GABA(C) receptor rho2 subunits were detected in IP cells with single-cell RT-PCR assays. Expression of GABA(C) receptors is one of the special features for IP cells, distinct from most of the third-order neurons that depend on GABA(A) and glycine receptors to relay the inhibitory signals. However, GABA(A) receptors in IP cells acted like nonsynaptic receptors that were activated by exogenous GABA application. Furthermore, L-IPSCs in IP cells were inhibited by the serial inhibitions between amacrine cells in the inner retina. In addition, application of neurotransmitters on the axon terminals of IP cells had no significant current generated in the cells, indicating that the synaptic inputs of IP cells are mainly from the inner retina. This study demonstrates the important role that light signals are encoded by both experiment of inhibitory receptors in IP cells.

A single amino acid in the second transmembrane domain of GABA rho receptors regulates channel conductance

GABAC receptors, expressed predominately in vertebrate retina, are thought to be formed mainly by GABA rho subunits, each of which exhibits distinct physiological and pharmacological properties. In this study, the receptors formed by perch GABA rho subunits were expressed in HEK cells, and their single channel conductances were determined using noise analysis techniques. The receptors formed by the perch rho1A subunit gate a channel with a conductance of 0.2 pS, whereas the receptors formed by GABA rho2 subunits exhibit much higher channel conductances, i.e., 3.2 and 3.5 pS for perch rho2A and rho2B receptors, respectively. A comparison of the amino acid sequences of the channel-forming TMII regions of the various subunits suggested that a single amino acid at position 2' was a potential site for the large differential in conductance. We found that switching the serine residue at that site in the GABA rho2 subunit to the proline residue present in the rho1 subunit reduced the channel conductance to a level similar to that of the wild type rho1 receptor. Conversely, mutating proline to serine in the amino acid sequence of the rho1 receptor significantly increased its unitary conductance. These results indicate that a single amino acid in the TMII region plays an important role in determining the single channel conductance of the GABAC receptors.

Inhibitory effect of taurine on veratridine-evoked D-[3H]aspartate release from murine corticostriatal slices: involvement of chloride channels and mitochondria

We have previously shown that the inhibitory neuromodulator taurine attenuates the release of preloaded D-[3H]aspartate from murine corticostriatal slices evoked by ischemic conditions or by application of the sodium channel agonist veratridine. The release of D-[3H]aspartate (a non-metabolized analog of glutamate) was used as an index of glutamate release. The aim of the present study was to reveal the molecular mechanisms responsible for this inhibitory effect of taurine. It was shown that 10 mM taurine suppresses D-[3H]aspartate release evoked by 0.1 mM veratridine, but does not affect the high-K+ -(50 mM) or ouabain- (0.1 mM) evoked release. Taurine had no effect in Ca2+ -free medium when the synaptic exocytosis of D-[3H]aspartate was inhibited. Nor did it suppress the release from slices preloaded with the competitive glutamate uptake blocker DL-threo-beta-hydroxyaspartate (THBA), which inhibits D-[3H]aspartate release mediated by the reverse action of glutamate transporters. Omission of Cl- from the incubation medium reduced the effect of taurine, signifying the involvement of a Cl- channel. The glycine receptor antagonist strychnine and the GABA(A) receptor antagonist bicuculline did not block the taurine effect, although picrotoxin, a less specific blocker of agonist-gated chloride channels, completely prevented the effect of taurine on veratridine-induced D-[3H]aspartate release. The respiratory chain blocker rotenone or mitochondrial protonophore carbonyl cyanide 3-chlorophenylhydrazone (CCCP) in combination with the mitochondrial ATPase inhibitor oligomycin, which inhibits the mitochondrial Ca2+ uniporter, also reduced the effect of taurine. The results obtained in the present study show that taurine acts specifically on the release of preloaded D-[3H]aspartate evoked by veratridine, but not on that evoked by other depolarizing agents, and affects the release mediated both by synaptic exocytosis and the reverse action of glutamate transporter. Taurine may attenuate D-[3H]aspartate release by regulation of mitochondrial Ca2+ sequestration and by activation of a chloride channel, but not that governed by GABA(A) or strychnine-sensitive glycine receptors.

Pharmacology of GABAC receptors: responses to agonists and antagonists distinguish A- and B-subtypes of homomeric rho receptors expressed in Xenopus oocytes

GABA(C) receptors, expressed predominantly in vertebrate retina, are thought to be formed mainly by GABA rho subunits. Five GABA rho subunits have been cloned from white perch retina, four of which form functional homooligomeric receptors when expressed in Xenopus oocytes. These rho subtypes, classified as rho1A, rho1B, rho2A and rho2B receptors based on amino acid sequence alignment, exhibit distinct temporal and pharmacological properties. To examine further the pharmacological properties associated with the various rho receptor subtypes, we investigated the effects of a selective GABA(C) receptor antagonist, TPMPA, on the GABA-mediated activity of receptors formed in Xenopus oocytes by the four GABA rho subunits. In addition, we recorded the activation profiles of beta-alanine, taurine, and glycine, three amino acids that modulate neuronal activity in various parts of the CNS and are purported to be rho receptor agonists. TPMPA effectively inhibited GABA-elicited responses on A-type receptors, whereas B-type receptors exhibited a relatively low sensitivity to the drug. A-type and B-type receptors also displayed distinctly different reactions to agonists. Both taurine and glycine-activated the B-type receptors, whereas these agents had no detectable effect on A-type receptors. Similarly, beta-alanine evoked large responses from B-type receptors, but was far less effective on A-type receptors. These results indicate that, in addition to the characteristic response properties identified previously, there is a pattern of pharmacological reactions that further distinguishes the A- and B-subtypes of GABA rho receptor.