A nonsense mutation in the alpha1 subunit of the inhibitory glycine receptor associated with bovine myoclonus.

Inherited congenital myoclonus of Poll Hereford calves is an autosomal recessive disease characterized by hyperesthesia and myoclonic jerks of the skeletal musculature that occur both spontaneously and in response to sensory stimuli. Binding studies have previously shown that myoclonus is associated with specific loss of [(3)H]strychnine-binding sites from spinal cord and brain stem in affected calves. In order to identify the mutation responsible for myoclonus, we examined the candidate genes, glycine receptor alpha1 (Glra1) and beta (Glrb) subunits, in affected and normal cattle. A nonsense mutation was found at amino acid 24, located in exon 2 of the Glra1 gene in both cDNA and genomic sequences from affected but not control animals. Immunohistochemistry, with a monoclonal antibody to alpha and beta subunits of the glycine receptor, revealed a loss of cell surface immunoreactivity in myoclonic animals, suggesting a failure in the assembly of the receptor that could explain the characteristic phenotype of the disease.

Identification of intracellular and extracellular domains mediating signal transduction in the inhibitory glycine receptor chloride channel.

Fast synaptic neurotransmission is mediated by transmitter-activated conformational changes in ligand-gated ion channel receptors, culminating in opening of the integral ion channel pore. Human hereditary hyperekplexia, or startle disease, is caused by mutations in both the intracellular or extracellular loops flanking the pore-lining M2 domain of the glycine receptor alpha1 subunit. These flanking domains are designated the M1-M2 loop and the M2-M3 loop respectively. We show that four startle disease mutations and six additional alanine substitution mutations distributed throughout both loops result in uncoupling of the ligand binding sites from the channel activation gate. We therefore conclude that the M1-M2 and M2-M3 loops act in parallel to activate the channel. Their locations strongly suggest that they act as hinges governing allosteric control of the M2 domain. As the members of the ligand-gated ion channel superfamily share a common structure, this signal transduction model may apply to all members of this superfamily.

The human glycine receptor beta subunit: primary structure, functional characterisation and chromosomal localisation of the human and murine genes.

The inhibitory glycine receptor (GlyR) is a pentameric receptor comprised of alpha and beta subunits, of which the beta subunit has not been characterised in humans. A 2106 bp cDNA, isolated from a human hippocampal cDNA library, contained an open reading frame of 497 amino acids which encodes the beta subunit of the human GlyR. The mature human GlyR beta polypeptide displays 99% amino acid identity with the rat GlyR beta subunit and 48% identity with the human GlyR alpha 1 subunit. Neither [3H]strychnine binding nor glycine-gated currents were detected when the human GlyR beta subunit cDNA was expressed in the human embryonic kidney 293 cell line. However, co-expression of the beta subunit cDNA with the alpha 1 subunit cDNA resulted in expression of functional GlyRs which showed a 4-fold reduction in the EC50 values when compared to alpha 1 homomeric GlyRs. Glycine-gated currents of alpha 1/beta GlyRs were 17-fold less sensitive than homomeric alpha 1 GlyRs to the antagonists picrotoxin, picrotoxinin and picrotin, providing clear evidence that heteromeric alpha 1/beta GlyRs were expressed. The beta subunit appears to play a structural rather than ligand binding role in GlyR function. Fluorescence in situ hybridisation was used to localise the gene encoding the human GlyR beta subunit (GLRB) to chromosome 4q32, a position syntenic with mouse chromosome 3. In situ hybridisation using the human GlyR beta subunit cDNA showed that the murine GlyR beta subunit gene (Glrb) maps to the spastic (spa) locus on mouse chromosome 3 at bands E3-F1. This is consistent with the recent finding that a mutation in the murine GlyR beta subunit causes the spa phenotype. It also raises the possibility that mutations in the human beta subunit gene may cause inherited disorders of the startle response.

A missense mutation in the gene encoding the alpha 1 subunit of the inhibitory glycine receptor in the spasmodic mouse.

Hereditary hyperekplexia, an autosomal dominant neurologic disorder characterized by an exaggerated startle reflex and neonatal hypertonia, can be caused by mutations in the gene encoding the alpha 1 subunit of the inhibitory glycine receptor (GLRA1). Spasmodic (spd), a recessive neurologic mouse mutant, resembles hyperekplexia phenotypically, and the two disease loci map to homologous chromosomal regions. Here we describe a Glra1 missense mutation in spd that results in reduced agonist sensitivity in glycine receptors expressed in vitro. We conclude that spd is a murine homologue of hyperekplexia and that mutations in GLRA1/Glra1 can produce syndromes with different inheritance patterns.

Distinct agonist- and antagonist-binding sites on the glycine receptor.

The distinction between receptor-binding sites for agonists and antagonists underpins the pharmacological differences between these two classes of ligands. In the glycine receptor, antagonist (strychnine) binding requires an interaction with residues Lys-200 and Tyr-202. We now demonstrate that the agonist-binding site of this receptor is located at the residue Thr-204. The agonist-binding site interaction is thus likely to be mediated by hydrogen bonding and not by ionic interactions. Our results demonstrate that, in contrast to other studies of ligand-gated ion channel receptors, agonist- and antagonist-binding sites are composed of distinct amino acid residues.