Functional expression in Xenopus oocytes of the strychnine binding 48 kd subunit of the glycine receptor.

The inhibitory postsynaptic glycine receptor (GlyR) of rat spinal cord is an oligomeric transmembrane protein which forms an agonist-gated anion channel. Expression in Xenopus oocytes of its mol. wt 48,000 subunit generated glycine-gated chloride channels which were analysed by voltage clamp. The agonist and antagonist response properties as well as the desensitization characteristics of these 48 kd subunit receptors resembled GlyRs expressed from spinal cord poly(A)+ RNA. These data indicate that the 48 kd subunit is capable of assembling into a functional receptor homo-oligomer which displays the pharmacology characteristic of the spinal cord GlyR.     

Dietary glycine blunts lung inflammatory cell influx following acute endotoxin

Mortality associated with endotoxin shock is likely mediated by Kupffer cells, alveolar macrophages, and circulating neutrophils. Acute dietary glycine prevents mortality and blunts increases in serum tumor necrosis factor-alpha (TNF-alpha) following endotoxin in rats. Furthermore, acute glycine blunts activation of Kupffer cells, alveolar macrophages, and neutrophils by activating a glycine-gated chloride channel. However, in neuronal tissue, glycine rapidly downregulates chloride channel function. Therefore, the long-term effects of a glycine-containing diet on survival following endotoxin shock were investigated. Dietary glycine for 4 wk improved survival after endotoxin but did not improve liver pathology, decrease serum alanine transaminase, or effect TNF-alpha levels compared with animals fed control diet. Interestingly, dietary glycine largely prevented inflammation and injury in the lung following endotoxin. Surprisingly, Kupffer cells from animals fed glycine for 4 wk were no longer inactivated by glycine in vitro; however, isolated alveolar macrophages and neutrophils from the same animals were sensitive to glycine. These data are consistent with the hypothesis that glycine downregulates chloride channels on Kupffer cells but not on alveolar macrophages or neutrophils. Importantly, glycine diet for 4 wk protected against lung inflammation due to endotoxin. Chronic glycine improves survival by unknown mechanisms, but reduction of lung inflammation is likely involved.     

Glycine reduces platelet aggregation

It has been demonstrated that a wide variety of white blood cells and macrophages (i.e. Kupffer cells, alveolar and peritoneal macrophages and neutrophils) contain glycine-gated chloride channels. Binding of glycine on the receptor stimulates Cl^? influx causing membrane hyperpolarization that prevents agonist-induced influx of calcium. Since platelet-aggregation is calcium-dependent, this study was designed to test the hypothesis that glycine would inhibit platelet aggregation. Rats were fed diets rich of glycine for 5 days, while controls received isonitrogenous valine. The bleeding time and ADP- and collagen-induced platelet aggregation were measured. Dietary glycine significantly increased bleeding time about twofold compared to valine-treated controls. Furthermore, the amplitude of platelet aggregation stimulated with ADP or collagen was significantly decreased in whole blood drawn from rats fed 2.5 or 5 % dietary glycine by over 50 %. Addition of glycine in vitro (1–10 mM) also blunted rat platelet aggregation in a dose-dependent manner. Strychnine, a glycine receptor antagonist, abrogated the inhibitory effect of glycine on platelet-aggregation in vitro suggesting the glycine works via a glycine receptor. Glycine also blunted aggregation of human platelets. Further, the glycine receptor was detected in both rat and human platelets by western blotting. Based on these data, it is concluded that glycine prevents aggregation of platelets in a dose-dependent manner via mechanisms involving a glycine receptor.     

Agonist binding to purified glycine receptor reconstituted into giant liposomes elicits two types of chloride currents.

Using 'inside-out' membrane patches obtained from reconstituted giant liposomes containing purified glycine receptor from rat spinal cord, we have detected chloride currents elicited in response to the presence of the agonists glycine or beta-alanine. Regardless of the agonist employed, two different patterns of single channel currents could be detected, which differ in their main conductance, complexity of substates and opening frequency. In agreement with the expectations of glycine receptor heterogeneity suggested recently at the mRNA and cDNA level, our results indicate the existence of functionally different glycine receptors in the adult rat spinal cord.     

Glycine-gated chloride channels in neutrophils attenuate calcium influx and superoxide production.

Recently, it was demonstrated that liver injury and TNF-alpha production as a result of endotoxin (lipopolysaccharide, LPS) were attenuated by feeding animals a diet enriched with glycine. This phenomenon was shown to be a result of, at least in part, activation of a chloride channel in Kupffer cells by glycine, which hyperpolarizes the cell membrane and blunts increases in intracellular calcium concentrations ([Ca(2+)](i)) similar to its action in the neuron. It is well known that hepatotoxicity due to LPS has a neutrophil-mediated component and that activation of neutrophils is dependent on increases in [Ca(2+)](i). Therefore, the purpose of this study was to determine if glycine affected agonist-induced increases in [Ca(2+)](i) in rat neutrophils. The effect of glycine on increases in [Ca(2+)](i) elicited either by the bacterial-derived peptide formyl-methionine-leucine-phenylalanine (FMLP) or LPS was studied in individual neutrophils using Fura-2 and fluorescence microscopy. Both FMLP and LPS caused dose-dependent increases in [Ca(2+)](i), which were maximal at 1 microM FMLP and 100 microgram/ml LPS, respectively. LPS increased intracellular calcium in the presence and absence of extracellular calcium. Glycine blunted increases in [Ca(2+)](i) in a dose-dependent manner with an IC(50) of approximately 0.3 mM, values only slightly higher than plasma levels. Glycine was unable to prevent agonist-induced increases in [Ca(2+)](i) in chloride-free buffer. Moreover, strychnine (1 microM), an antagonist of the glycine-gated chloride channel in the central nervous system, reversed the effects of glycine (1 mM) on FMLP- or LPS-stimulated increases in [Ca(2+)](i). To provide hard evidence for a glycine-gated chloride channel in the neutrophil, the effect of glycine on radioactive chloride uptake was determined. Glycine caused a dose-dependent increase in chloride uptake into neutrophils with an ED(50) of approximately 0.4 mM, an effect also prevented by 1 microM strychnine. Glycine also significantly reduced the production of superoxide anion from FMLP-stimulated neutrophils. Taken together, these data provide clear evidence that neutrophils contain a glycine-gated chloride channel that can attenuate increases in [Ca(2+)](i) and diminish oxidant production by this important leukocyte.     

Cloning of a glycine receptor subtype expressed in rat brain and spinal cord during a specific period of neuronal development.

Complementary (c) DNAs encoding a glycine receptor (GlyR) isomer were cloned from libraries constructed in lambda ZAPII with poly (A)+ RNA of neonatal rat spinal cord. Northern blot analysis revealed that RNA hybridized to the cloned cDNA is detectable only for a period of late embryonic/early postnatal stage of the spinal cord. Moreover, other central nervous tissues, such as hippocampus and cerebral cortex, in the infant rats are also rich in this message. The 'neonatal (N) GlyR' has 71% homology to that of another GlyR isoform in which adult rad cord is rich (AGlyR). Injection of a single RNA transcribed from the NGlyr-cDNA into Xenopus oocyte induced functional formation of glycine-gated Cl- channels, however, its pharmacological property differed from that of AGlyR.     

Single-channel currents underlying glycinergic inhibitory postsynaptic responses in spinal neurons.

Single-channel properties of glycine receptors have been characterized so far only in cultured neurons. To characterize the glycine receptor channels in situ, we applied the patch-clamp technique to spinal neurons in slice preparations. Glycine-gated, single-channel currents were recorded in outside-out patches excised from spinal neurons. In the falling phase of glycinergic inhibitory synaptic currents, single-channel currents were resolved as discrete steps. In both cases, the glycine-gated channels showed similar multiple conductance levels. These results suggest that the receptor channel properties are indistinguishable in the synaptic and extrasynaptic sites. We conclude that multiple conductance states of a receptor channel are the native feature of the glycine receptor in situ.     

Ivermectin inhibits activation of Kupffer cells induced by lipopolysaccharide toxin]

Lipopolysaccharide-stimulated liver macrophages (Kupffer cells) secrete many physiologically active substances responsible for inflammatory reaction of the organism. The mechanism by which ivermectin, a macrocyclic lactone possessing a broad antiparasitic activity, modulates basic effects elicited by lipopolysaccharide in the primary culture of rat Kupffer cells was studied. It was found that ivermectin in the absence of endotoxin did not affect a functional state of the Kupffer cells. Preincubation of Kupffer cells with ivermectin (1 mM), however, significantly blocked response to the subsequent administration of lipopolysaccharide (1 mg/ml). In particular, secretion of tumor necrosis factor TNF alpha, nitric oxide NO and prostaglandin E2 was suppressed. Also, an LPS-triggered rise in the intracellular concentration of calcium ions was less pronounced. Removal of chloride anions from the extracellular medium completely abolished inhibitory effects of ivermectin. It is suggested that invermectin exerts its action via binding to the glycine-gated chloride receptors/channels of the Kupffer cells, which may reduce toxic reactions manifestations observed under infections caused by Gram-negative bacteria.     

Sensitive Immunoassay Shows Selective Association of Peripheral and Integral Membrane Proteins of the Inhibitory Glycine Receptor Complex

The inhibitory glycine receptor of mammalian spinal cord is a ligand-gated chloride channel that, on affinity purification, contains two subunits of 48-kilodalton (kD) and 58-kD molecular mass in addition to an associated 93-kD protein. Ligand-binding 48-kD subunit and 93-kD protein were quantified in the CNS of the adult rat using a newly developed dot receptor assay (detection limit less than or equal to 1 fmol/assay) which employs monoclonal antibodies specific for glycine receptor polypeptides. The 93-kD protein was found to codistribute at a fixed stoichiometry with the 48-kD subunit throughout the CNS of the rat. Moreover, the 93-kD protein cofractionated with the ligand-binding subunit on solubilization and affinity chromatography or immunoprecipitation. However, both proteins were separated on sucrose gradient centrifugation of detergent extracts of spinal cord membranes in accord with earlier observations on purified receptor. These data prove that the 93-kD polypeptide is selectively associated with the membrane core of the strychnine-sensitive glycine receptor. The regional distribution of glycine receptor polypeptides was also determined in the CNS of the spastic rat mutant. In contrast to hereditary spasticity in mouse and cattle, no reduction of glycine receptors was found in the spastic rat.     

Functional reconstitution of the glycine receptor

The functional reconstitution of the chloride channel coupled glycine receptor is described. Glycine receptors were purified from the cholate extract of rat spinal cord membranes by affinity chromatography and incorporated into phospholipid vesicles by the addition of phosphatidylcholine and removal of detergent by gel filtration. The reconstituted vesicles showed the same polypeptide composition as the purified receptor (proteins of Mr 48,000 and 58,000). The pharmacological characteristics of the glycine receptor were also preserved in the proteoliposomes, as demonstrated by the displacement of [3H]strychnine binding by several glycinergic ligands and by photoaffinity labeling experiments. In order to observe functional responses (i.e., specific agonist-induced anion translocation), we have developed an assay based on the fluorescence quenching of an anion-sensitive entrapped probe, SPQ [6-methoxy-N-(3-sulfopropyl)quinolinium]. Reconstituted vesicles were loaded with the fluorescent probe during a freeze-thaw-sonication cycle in the presence of added liposomes containing cholesterol. In such a reconstituted system, glycine receptor agonists are able to increase the rate of anion influx into the vesicles. The action of agonists is blocked by the simultaneous presence of strychnine or other glycine antagonists. Our results show that the purified 48,000- and 58,000-dalton polypeptides reconstituted into phospholipid vesicles can bind ligands and promote specific ion translocation in a way similar to the glycine receptor in its native environment.     

The atypical M2 segment of the beta subunit confers picrotoxinin resistance to inhibitory glycine receptor channels.

Purified preparations of the inhibitory glycine receptor (GlyR) contain alpha and beta subunits, which share homologous primary structures and a common transmembrane topology with other members of the ligand-gated ion channel superfamily. Here, a beta subunit-specific antiserum was shown to precipitate the [3H]strychnine binding sites localized on alpha subunits from membrane extracts of both rat spinal cord and mammalian cells co-transfected with alpha and beta cDNAs. Further, inhibition of alpha homo-oligomeric GlyRs by picrotoxinin, a non-competitive blocker of ion flow, was reduced 50- to 200-fold for alpha/beta hetero-oligomeric receptors generated by cotransfection. Site-directed mutagenesis identified residues within the second predicted transmembrane segment (M2) of the beta subunit as major determinants of picrotoxinin resistance. These data implicate the M2 segment in blocker binding to and lining of the GlyR chloride channel.     

Mechanisms of homomeric alpha1 glycine receptor endocytosis

Little is known regarding the mechanism(s) by which glycine receptors are endocytosed. Here we examined the endocytosis of homomeric alpha1 glycine receptors expressed in HEK 293 cells using immunofluorescence/confocal microscopy and whole-cell patch-clamp recordings. Our studies demonstrate that constitutive endocytosis of glycine receptors is blocked by the dominant negative dynamin construct K44A and that intracellular dialysis with peptide P4, a dynamin/amphiphysin-disrupting peptide, increased whole-cell glycine-gated chloride currents. To examine whether receptor endocytosis could be regulated by PKC, experiments with the PKC activator PMA (phorbol 12-myristate 13-acetate) were performed. PMA, but not its inactive analogue PMM (phorbol 12-monomyristate), stimulated receptor endocytosis and inhibited glycine-gated chloride currents. Similar to constitutive endocytosis, PKC-stimulated endocytosis was blocked by dynamin K44A. Mutation of a putative AP2 adaptin dileucine motif (L314A, L315A) present in the receptor cytoplasmic loop blocked PMA-stimulated receptor endocytosis and also prevented PMA inhibition of glycine receptor currents. In patch-clamp experiments, intracellular dialysis of a 12-amino acid peptide corresponding to the region of the receptor containing the dileucine motif prevented PKC modulation of wild-type glycine receptors. Unlike PKC modulation of the receptor, constitutive endocytosis was not affected by mutation of this dileucine motif. These results demonstrate that PKC activation stimulates glycine receptor endocytosis, that both constitutive endocytosis and PKC-stimulated endocytosis are dynamin-dependent, and that PKC-stimulated endocytosis, but not constitutive endocytosis, occurs via the dileucine motif (L314A, L315A) within the cytoplasmic loop of the receptor.     

Expression of the human glycine receptor alpha 1 subunit in Xenopus oocytes: apparent affinities of agonists increase at high receptor density.

The inhibitory glycine receptor (GlyR) is a ligand-gated chloride channel, which mediates post-synaptic inhibition in spinal cord and other brain regions. Heterologous expression of the ligand binding alpha subunits of the GlyR generates functional agonist-gated chloride channels that mimic most of the pharmacological properties of the receptor in vivo. Here, nuclear injection into Xenopus oocytes of a human alpha 1 subunit cDNA, engineered for efficient expression, was used to create GlyR channels over a wide density range, resulting in whole-cell glycine currents of 10 nA to 25 microA. Notably, the pharmacology of these channels changed at high expression levels, with the appearance of a novel receptor subpopulation of 5- to 6-fold higher apparent agonist affinity at current values > 4 microA. The low-affinity receptors were readily blocked by nM concentrations of the competitive antagonist strychnine, whereas the high-affinity receptors were more resistant to antagonism by this alkaloid. Picrotoxinin, a chloride channel blocker, inhibited both GlyR populations with equal potency. Our data suggest that receptor interactions, occurring at high receptor density, modify the agonist response of the GlyR. This phenomenon may contribute to neurotransmitter efficacy at fast synapses.     

Characterization of mice with targeted deletion of glycine receptor alpha 2

Glycine receptors are ligand-gated chloride channels that mediate inhibitory neurotransmission in the adult nervous system. During development, glycine receptor alpha 2 (GlyRalpha2) is expressed in the retina, in the spinal cord, and throughout the brain. Within the cortex, GlyRalpha2 is expressed in immature cells and these receptors have been shown to be active and excitatory. In the developing retina, inhibition of glycine receptor activity prevents proper rod photoreceptor development. These data suggest that GlyRalpha2, the developmentally expressed glycine receptor, may play an important role in neuronal development. We have generated mice with a targeted deletion of glycine receptor alpha 2 (Glra2). Although these mice lack expression of GlyRalpha2, no gross morphological or molecular alterations were observed in the nervous system. In addition, the cerebral cortex does not appear to require glycine receptor activity for proper development, as Glra2 knockout mice did not show any electrophysiological responses to glycine.     

Molecular characterization of rat substance K receptor and its mRNAs

The nucleotide sequence and the amino acid sequence for rat substance K receptor were deduced by molecular cloning and sequence analysis of its cDNAs. The rat substance K receptor consists of 390 amino acid residues and belongs to the family of G protein-coupled receptors. The comparison of the amino acid sequences of the rat and bovine substance K receptors indicated that they are highly homologous in the regions covering seven putative transmembrane domains, and this similarity is particularly remarkable in the transmembrane segments III and VII and their surrounding regions. RNA blot hybridization analysis showed that the rat substance K receptor is encoded by two species of mRNAs which differ in the lengths of the extreme 5' sequence of the 5'-untranslated regions. This analysis also indicated that the substance K receptor mRNAs are expressed in the gastrointestinal tract. Interestingly, no appreciable substance K receptor mRNAs were detected in poly(A)+ RNAs isolated from the brain and spinal cord, even though these tissues are known to not only contain substance K but also express the mRNA encoding the substance K precursor.