Charged residues at the pore extracellular half of the glycine receptor facilitate channel gating: a potential role played by electrostatic repulsion

N318L Blocks the Interaction of Fluralaner but Not Broflanilide or Fipronil with the Insect GABA Receptor In Vivo

Fluralaner is a novel insecticide targeting the ionotropic GABA receptor (GABAR) subunit, RDL. A recent study revealed that N316L, a substitution of asparagine (N) with leucine (L), in the second transmembrane (M2)-spanning region reduced the antagonist action of fluralaner on the housefly Musca domestica RDL (MdRDL) in vitro. To verify the impact of N316L in vivo, the corresponding mutation (N318L) in the fruitfly Drosophila melanogaster RDL (DmRDL) was constructed using CRISPR/Cas9 genome editing. The homozygous DmRDLN318L mutant showed a 9.87-fold resistance to fluralaner compared with w1118 while still being highly sensitive to broflanilide and fipronil, which is consistent with those findings observed in the electrophysiology assays of the homomeric DmRDLWT or DmRDLN318L channel. Moreover, DmRDLN318L led to malformed ovaries, stunted eggs, and sterility in homozygous females. These results highlighted N318 as a molecular site for fluralaner in vivo and in vitro and might elucidate the resistance mechanisms of insects against fluralaner.

Exploring the Activation Process of the Glycine Receptor

Glycine receptors (GlyR) conduct inhibitory glycinergic neurotransmission in the spinal cord and the brainstem. They play an important role in muscle tone, motor coordination, respiration, and pain perception. However, the mechanism underlying GlyR activation remains unclear. There are five potential glycine binding sites in α1 GlyR, and different binding patterns may cause distinct activation or desensitization behaviors. In this study, we investigated the coupling of protein conformational changes and glycine binding events to elucidate the influence of binding patterns on the activation and desensitization processes of α1 GlyRs. Subsequently, we explored the energetic distinctions between the apical and lateral pathways during α1 GlyR conduction to identify the pivotal factors in the ion conduction pathway preference. Moreover, we predicted the mutational effects of the key residues and verified our predictions using electrophysiological experiments. For the mutants that can be activated by glycine, the predictions of the mutational directions were all correct. The strength of the mutational effects was assessed using Pearson's correlation coefficient, yielding a value of -0.77 between the calculated highest energy barriers and experimental maximum current amplitudes. These findings contribute to our understanding of GlyR activation, identify the key residues of GlyRs, and provide guidance for mechanistic studies on other pLGICs.

Asymmetric gating of a human hetero-pentameric glycine receptor

Hetero-pentameric Cys-loop receptors constitute a major type of neurotransmitter receptors that enable signal transmission and processing in the nervous system. Despite intense investigations into their working mechanism and pharmaceutical potentials, how neurotransmitters activate these receptors remains unclear due to the lack of high-resolution structural information in the activated open state. Here we report near-atomic resolution structures resolved in digitonin consistent with all principle functional states of the human α1β GlyR, which is a major Cys-loop receptor that mediates inhibitory neurotransmission in the central nervous system of adults. Glycine binding induces cooperative and symmetric structural rearrangements in the neurotransmitter-binding extracellular domain but asymmetrical pore dilation in the transmembrane domain. Symmetric response in the extracellular domain is consistent with electrophysiological data showing cooperative glycine activation and contribution from both α1 and β subunits. A set of functionally essential but differentially charged amino acid residues in the transmembrane domain of the α1 and β subunits explains asymmetric activation. These findings provide a foundation for understanding how the gating of the Cys-loop receptor family members diverges to accommodate specific physiological environments.

External amino acid residues of insect GABA receptor channels dictate the action of the isoxazoline ectoparasiticide fluralaner

BACKGROUND

Fluralaner is the first isoxazoline ectoparasiticide developed to protect companion animals from fleas and ticks. Fluralaner primarily inhibits arthropod γ-aminobutyric acid receptors (GABARs), which are ligand-gated ion channels comprising five subunits arranged around the channel pore. We previously reported that the action site of fluralaner resides at the M1-M3 transmembrane interface between adjacent GABAR subunits. To investigate whether fluralaner interacts with the second transmembrane segment (M2) located deep in the interface, we generated four housefly RDL GABAR mutants with non-conservative amino acid substitutions in the M2 region.

RESULTS

Electrophysiological analysis of GABARs expressed in Xenopus oocytes indicated that S313A and S314A mutants exhibited fluralaner sensitivities similar to that of the wild type. M312S mutant exhibited approximately seven-fold lower sensitivity than that of the wild type. Notably, the N316L mutant was almost insensitive to fluralaner.

CONCLUSION

The findings of this study indicate that the conserved external amino acid residues of insect GABAR channels play a critical role in the antagonistic effect of fluralaner. © 2023 Society of Chemical Industry.

Computational Investigation of Mechanisms for pH Modulation of Human Chloride Channels

Many transmembrane proteins are modulated by intracellular or extracellular pH. Investigation of pH dependence generally proceeds by mutagenesis of a wide set of amino acids, guided by properties such as amino-acid conservation and structure. Prediction of pKas can streamline this process, allowing rapid and effective identification of amino acids of interest with respect to pH dependence. Commencing with the calcium-activated chloride channel bestrophin 1, the carboxylate ligand structure around calcium sites relaxes in the absence of calcium, consistent with a measured lack of pH dependence. By contrast, less relaxation in the absence of calcium in TMEM16A, and maintenance of elevated carboxylate sidechain pKas, is suggested to give rise to pH-dependent chloride channel activity. This hypothesis, modulation of calcium/proton coupling and pH-dependent activity through the extent of structural relaxation, is shown to apply to the well-characterised cytosolic proteins calmodulin (pH-independent) and calbindin D9k (pH-dependent). Further application of destabilised, ionisable charge sites, or electrostatic frustration, is made to other human chloride channels (that are not calcium-activated), ClC-2, GABAA, and GlyR. Experimentally determined sites of pH modulation are readily identified. Structure-based tools for pKa prediction are freely available, allowing users to focus on mutagenesis studies, construct hypothetical proton pathways, and derive hypotheses such as the model for control of pH-dependent calcium activation through structural flexibility. Predicting altered pH dependence for mutations in ion channel disorders can support experimentation and, ultimately, clinical intervention.

Asymmetric gating of a human hetero-pentameric glycine receptor

Hetero-pentameric Cys-loop receptors constitute a major type of neurotransmitter receptors that enable signal transmission and processing in the nervous system. Despite intense investigations in their working mechanism and pharmaceutical potentials, how neurotransmitters activate these receptors remain unclear due to the lack of high-resolution structural information in the activated open state. Here we report near-atomic resolution structures in all principle functional states of the human α1β GlyR, which is a major Cys-loop receptor that mediates inhibitory neurotransmission in the central nervous system of adults. Glycine binding induced cooperative and symmetric structural rearrangements in the neurotransmitter-binding extracellular domain, but asymmetrical pore dilation in the transmembrane domain. Symmetric response in the extracellular domain is consistent with electrophysiological data showing similar contribution to activation from all the α1 and β subunits. A set of functionally essential but differentially charged amino-acid residues in the transmembrane domain of the α1 and β subunits explains asymmetric activation. These findings point to a gating mechanism that is distinct from homomeric receptors but more compatible with heteromeric GlyRs being clustered at synapses through β subunit-scaffolding protein interactions. Such mechanism provides foundation for understanding how gating of the Cys-loop receptor members diverge to accommodate specific physiological environment.

Different Behaviors of a Glycine Receptor Channel Pore Residue between Wild-Type-Mimicking and Disease-Type-Mimicking Formats

The glycine receptor (GlyR) is a neurotransmitter-gated chloride channel that mediates fast inhibitory neurotransmission, predominantly in the spinal cord and brain stem. Mutations of the GlyR are the major cause of hereditary hyperekplexia. Site-specific cysteine substitution followed by labeling with a fluorophore has previously been used to explore the behaviors of the hyperekplexia-related 271 (19') residue of the GlyR. However, this manipulation dramatically compromises sensitivity toward the agonist glycine and alters the pharmacological effects of various agents in manners similar to those of the hyperekplexia-causing R19'Q/L mutations, raising the question whether what is reported by the substituted and modified residue faithfully reflects what actually happens to the wild-type (WT) residue. In this study, a mechanism-rescuing second-site mutation was introduced to create a WT-mimicking GlyR (with the 19' residue cysteine substitution and modification still in place), in which the sensitivity toward glycine and pharmacological effects of various agents were restored. Further experiments revealed stark differences in the behaviors upon the various pharmacological treatments and consequently the underlying mechanisms of the 19' residue between this WT-mimicking GlyR and the GlyR without the mechanism rescue, which is correspondingly defined as the disease-type (DT)-mimicking GlyR. The data presented in this study warn generally that caution is required when attempting to deduce the behaviors of a WT residue from data based on substituted or modified residues that alter protein structure and function. Extra measures, such as rescuing mechanisms via alternative means as presented in this study, are needed to mitigate this challenge.

Pair of Residue Substitutions at the Outer Mouth of the Channel Pore Act as Inputs for a Boolean Logic "OR" Gate Based on the Glycine Receptor

The glycine receptor (GlyR) is a ligand-activated chloride channel, whose mutations are the major cause of hereditary hyperekplexia. The hyperekplexia-causing R271Q mutation, which is located at the extracellular outer mouth of the channel pore, dramatically impairs the GlyR function manifesting a reduced sensitivity toward glycine. This study reports that a second mutation, S273D, rescues the function of the R271Q GlyR to that of the wild-type (WT) GlyR. Surprisingly, the S273D mutation, when introduced to the WT GlyR, does not further increase the receptor function. In other words, the compromised function of the 271Q 273S GlyR (i.e., the R271Q GlyR) can be rescued to WT levels by the introduction of either, or both, of the Q271R and S273D substitutions. From the perspective of Boolean logic gates, the Q271R and S273D substitutions act as inputs for an OR gate based on the GlyR. Further experiments revealed that the negative-charge carried by the 273 residue is essential for the expression of the OR gate and that the expression of the OR gate is residue-position-specific. In addition, mechanistic investigation implied that the 273 residue influences the 271 residue, which might underpin the unique nonadditive OR gate relationship between these two residues. Such an ion-channel-based OR gate, expressing output in the form of electrical current, could potentially be developed to digitally manipulate neuronal activity.