Decreased agonist affinity and chloride conductance of mutant glycine receptors associated with human hereditary hyperekplexia.

Hereditary hyperekplexia is a dominant neurological disorder associated with point mutations at the channel-forming segment M2 of the glycine receptor alpha 1 subunit. Voltage-clamp recordings from the heterologously expressed mutants (alpha 1R271L or alpha 1R271Q) revealed 146- to 183-fold decreased potencies of glycine to activate the chloride channel, and significantly reduced maximal whole-cell currents as compared with wild-type receptors. In contrast, the ability of the competitive antagonist strychnine to block glycine-induced currents was similar in all cases. Radioligand binding assays showed a 90- to 1365-fold reduction in the ability of glycine to displace [3H]strychnine from its binding site on the mutant receptors. Paralleling the reductions in whole-cell current, the elementary main-state conductances of the mutants (alpha 1R271L, 64 pS; alpha 1R271Q, 14 pS) were lower than that of the wild-type receptor (86 pS). The decreased agonist affinities and chloride conductances of the mutants are likely to cause neural hyperexcitability of affected patients by impairing glycinergic inhibition. In addition, our data reveal that structural modifications of the ion-channel region can affect agonist binding to the glycine receptor

Modulation by zinc ions of native rat and recombinant human inhibitory glycine receptors.

1. The effect of the divalent cation Zn2+ on inhibitory glycine receptor (GlyR) currents was investigated in rat embryonic spinal cord neurons and Xenopus oocytes expressing recombinant GlyRs. 2. In cultured spinal neurons, Zn2+ potentiated glycine-induced whole-cell currents about 3-fold when applied extracellularly at concentrations of 0.5-10 microM. In contrast, higher concentrations (> 100 microM) of Zn2+ decreased the glycine response. 3. A similar biphasic modulation of glycine-induced currents by Zn2+ was also found with recombinant homo- and hetero-oligomeric GlyRs generated in Xenopus oocytes. Dose-response analysis showed that both the potentiating and inhibitory effects of Zn2+ result from changes in apparent agonist affinity. 4. Analysis of chimeric constructs of the GlyR alpha 1- and beta-subunits revealed that the positive and negative modulatory effects of Zn2+ are mediated by different regions of the alpha 1-subunit. 5. Our data indicate the existence of distinct high- and low-affinity Zn2+ binding sites on the ligand-binding alpha-subunits of the GlyR. These sites may be implicated in the regulation of synaptic efficacy within glycinergic pathways

Structure, diversity and synaptic localization of inhibitory glycine receptors.

The inhibitory glycine receptor (GlyR) mediates postsynaptic inhibition in spinal cord, brain stem and other regions of the vertebrate central nervous system. Biochemical and molecular approaches have identified different developmentally and regionally regulated GlyR isoforms that result from the differential expression of at least four genes coding for different variants of the ligand-binding alpha subunit. Molecular studies have allowed identification of GlyR subunit domains implicated in ligand binding, channel formation and receptor assembly. At the postsynaptic membrane, the GlyR colocalizes with a 93-kDa tubulin-binding peripheral membrane protein, gephyrin. Antisense inhibition of gephyrin expression prevents GlyR accumulation at postsynaptic membrane specialization. Thus, gephyrin is essential for postsynaptic receptor topology