A molecular characterization of the agonist binding site of a nematode cys-loop GABA receptor

Characterization of adjacent charged residues near the agonist binding site of the nematode UNC-49 GABA receptor

The UNC-49 receptor is a Cys-loop GABA receptor that is unique to the nematode phylum. The receptor differs from mammalian GABA receptors both in amino acid sequence and pharmacology which highlights its potential as a novel anthelmintic target. Sequence differences within and near the various ligand-binding loops of the nematode receptor suggest that there could be structural differences compared to mammalian receptors that result in different pharmacological and functional features. Here we investigated three residues in the UNC-49 receptor from the parasitic nematode Haemonchus contortus: K181, E183, and T230. Analysis of these residues was conducted via site-directed mutagenesis, electrophysiology, MD simulations, and mutant cycling analysis. In the UNC-49 receptor, E183 lies in close proximity to K181 where together they appear to play a role in GABA sensitivity and pharmacology, possibly interacting via an ionic bond. While the introduction of single alanine residues at each position separately had a negative impact on GABA EC₅₀, the double alanine mutant (K181A/E183A) exhibited wildtype-level GABA EC₅₀ and some differences in pharmacology. Overall, this study has revealed a potentially novel role for these two residues in nematode UNC-49 GABA receptors that could aid in understanding their function.

Advances in our understanding of nematode ion channels as potential anthelmintic targets

Ion channels are specialized multimeric proteins that underlie cell excitability. These channels integrate with a variety of neuromuscular and biological functions. In nematodes, the physiological behaviors including locomotion, navigation, feeding and reproduction, are regulated by these protein entities. Majority of the antinematodal chemotherapeutics target the ion channels to disrupt essential biological functions. Here, we have summarized current advances in our understanding of nematode ion channel pharmacology. We review cys-loop ligand gated ion channels (LGICs), including nicotinic acetylcholine receptors (nAChRs), acetylcholine-chloride gated ion channels (ACCs), glutamate-gated chloride channels (GluCls), and GABA (γ-aminobutyric acid) receptors, and other ionotropic receptors (transient receptor potential (TRP) channels and potassium ion channels). We have provided an update on the pharmacological properties of these channels from various nematodes. This article catalogs the differences in ion channel composition and resulting pharmacology in the phylum Nematoda. This diversity in ion channel subunit repertoire and pharmacology emphasizes the importance of pursuing species-specific drug target research. In this review, we have provided an overview of recent advances in techniques and functional assays available for screening ion channel properties and their application.

Molecular cloning and pharmacology of Min-UNC-49B, a GABA receptor from the southern root-knot nematode Meloidogyne incognita

BACKGROUND

Root-knot nematodes are plant-parasitic nematodes that cause immense damage to a broad range of cultivated crops by forming root galls, resulting in yield losses in crops. To facilitate the development of faster-acting selective nematicides, we cloned three cDNAs encoding UNC-49B proteins from the southern root-knot nematode Meloidogyne incognita and examined their functional and pharmacological properties by two-electrode voltage clamp electrophysiology using a Xenopus oocyte expression system.

RESULTS

The three cloned cDNAs encoded Min-UNC-49B, Min-UNC-49B-L and Min-UNC-49B-XL; the last two proteins have longer N-terminal regions than the first protein. When expressed in Xenopus oocytes, these proteins responded to γ-aminobutyric acid (GABA) to activate currents with high-micromolar or low-millimolar half-maximal effective concentration (EC₅₀ ) values, indicating the formation of functional homo-pentameric GABA receptors. Fipronil and picrotoxinin inhibited GABA-induced currents with high-nanomolar and low-micromolar half-maximal inhibitory concentration (IC₅₀ ) values, respectively, in oocytes expressing Min-UNC-49B. The G2'A and T6'M mutations in the second transmembrane domain of Min-UNC-49B enhanced and reduced the sensitivity of Min-UNC-49B to these two antagonists, respectively. Samaderine B and SF-14 inhibited GABA responses in oocytes expressing Min-UNC-49B with low-micromolar and high-nanomolar IC₅₀ values, respectively. Ivermectin, α-terpineol, thymol and methyl eugenol exerted dual effects on Min-UNC-49B by potentiating currents induced by low concentrations of GABA and inhibiting currents induced by high concentrations of GABA.

CONCLUSION

We have shown that structurally diverse compounds act at Min-UNC-49B GABA receptors. Our results may serve as a starting point to decipher the molecular function of native GABA receptors of plant-parasitic nematodes, which could aid in the structure-based design of novel nematicides. © 2020 Society of Chemical Industry.

A mutational and molecular dynamics study of the cys-loop GABA receptor Hco-UNC-49 from Haemonchus contortus: Agonist recognition in the nematode GABA receptor family

The UNC-49 receptor is a unique nematode γ-aminobutyric acid (GABA)-gated chloride channel that may prove to be a novel target for the development of nematocides. Here we have characterized various charged amino acid residues in and near the agonist binding site of the UNC-49 receptor from the parasitic nematode Haemonchus contorts. Utilizing the Caenorhabditis elegans GluCl crystal structure as a template, a model was generated and various charged residues [D83 (loop D), E131 (loop A), H137 (pre-loop E), R159 (Loop E), E185 (Loop B) and R241 (Loop C)] were investigated based on their location and conservation. These residues may contribute to structure, function, and molecular interactions with agonists. It was found that all residues chosen were important for receptor function to varying degrees. Results of the mutational analysis and molecular simulations suggest that R159 may be interacting with D83 by an ionic interaction that may be crucial for general GABA receptor function. We have used the results from this study as well as knowledge of residues involved in GABA receptor binding to identify sequence patterns that may assist in understanding the function of lesser known GABA receptor subunits from parasitic nematodes.

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Key functional residues were investigated in the Hco-UNC-49 receptor.

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A glutamic acid in the binding pocket (loop B) is essential for agonist recognition.

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Interaction between residues in different binding loops are important for function.

Molecular Characterization of Binding Loop E in the Nematode Cys-Loop GABA Receptor

Nematodes exhibit a vast array of cys-loop ligand-gated ion channels with unique pharmacologic characteristics. However, many of the structural components that govern the binding of various ligands are unknown. The nematode cys-loop GABA receptor uncoordinated 49 (UNC-49) is an important receptor found at neuromuscular junctions that plays an important role in the sinusoidal movement of worms. The unique pharmacologic features of this receptor suggest that there are structural differences in the agonist binding site when compared with mammalian receptors. In this study, we examined each amino acid in one of the main agonist binding loops (loop E) via the substituted cysteine accessibility method (SCAM) and analyzed the interaction of various residues by molecular dynamic simulations. We found that of the 18 loop E mutants analyzed, H142C, R147C, and S157C had significant changes in GABA EC₅₀ and were accessible to modification by a methanethiosulfonate reagent (MTSET) resulting in a change in I _GABA In addition, the residue H142, which is unique to nematode UNC-49 GABA receptors, appears to play a negative role in GABA sensitivity as its mutation to cysteine increased sensitivity to GABA and caused the UNC-49 receptor partial agonist 5-aminovaleric acid (DAVA) to behave as a full agonist. Overall, this study has revealed potential differences in the agonist binding pocket between nematode UNC-49 and mammalian GABA receptors that could be exploited in the design of novel anthelmintics.

Investigation of Agonist Recognition and Channel Properties in a Flatworm Glutamate-Gated Chloride Channel

Glutamate-gated chloride channels (GluCls) are neurotransmitter receptors that mediate crucial inhibitory signaling in invertebrate neuromuscular systems. Their role in invertebrate physiology and their absence from vertebrates make GluCls a prime target for antiparasitic drugs. GluCls from flatworm parasites are substantially different from and are much less understood than those from roundworm and insect parasites, hindering the development of potential therapeutics targeting GluCls in flatworm-related diseases such as schistosomiasis. Here, we sought to dissect the molecular and chemical basis for ligand recognition in the extracellular glutamate binding site of SmGluCl-2 from Schistosoma mansoni, using site-directed mutagenesis, noncanonical amino acid incorporation, and electrophysiological recordings. Our results indicate that aromatic residues in ligand binding loops A, B, and C are important for SmGluCl-2 function. Loop C, which differs in length compared to other pentameric ligand-gated ion channels (pLGICs), contributes to ligand recognition through both an aromatic residue and two vicinal threonine residues. We also show that, in contrast to other pLGICs, the hydrophobic channel gate in SmGluCl-2 extends from the 9' position to the 6' position in the channel-forming M2 helix. The 6' and 9' positions also seem to control sensitivity to the pore blocker picrotoxin.

Efficient synthesis of the GABAA receptor agonist trans-4-aminocrotonic acid (TACA)

Investigations into the pharmacology of different types of cys-loop GABA receptor have relied for years on the chemical modification of GABA-like compounds. The GABA metabolite GABOB is an attractive molecule to modify due to its convenient chemical structure. In the process of developing new GABA-mimic compounds from GABOB as a starting compound three small molecule GABA derivatives were synthesized using a variety of chemical transformations. Amongst these, a new and reliable method to synthesize TACA (trans-4-aminocrotonic acid) is reported.

Pharmacological characterization of the excitatory 'Cys-loop' GABA receptor family in Caenorhabditis elegans

BACKGROUND AND PURPOSE

Ionotropic GABA receptors are evolutionarily conserved proteins that mediate cellular and network inhibition in both vertebrates and invertebrates. A unique class of excitatory GABA receptors has been identified in several nematode species. Despite well-characterized functions in Caenorhabditis elegans, little is known about the pharmacology of the excitatory GABA receptors EXP-1 and LGC-35. Using a panel of compounds that differentially activate and modulate ionotropic GABA receptors, we investigated the agonist binding site and allosteric modulation of EXP-1 and LGC-35.

EXPERIMENTAL APPROACH

We used two-electrode voltage clamp recordings to characterize the pharmacological profile of EXP-1 and LGC-35 receptors expressed in Xenopus laevis oocytes.

KEY RESULTS

The pharmacology of EXP-1 and LGC-35 is different from that of GABA_A and GABA_A -ρ receptors. Both nematode receptors are resistant to the competitive orthosteric antagonist bicuculline and to classical ionotropic receptor pore blockers. The GABA_A -ρ specific antagonist, TPMPA, was the only compound tested that potently inhibited EXP-1 and LGC-35. Neurosteroids have minimal effects on GABA-induced currents, but ethanol selectively potentiates LGC-35.

CONCLUSIONS AND IMPLICATIONS

The pharmacological properties of EXP-1 and LGC-35 more closely resemble the ionotropic GABA_A -ρ family. However, EXP-1 and LGC-35 exhibit a unique profile that differs from vertebrate GABA_A and GABA_A -ρ receptors, insect GABA receptors and nematode GABA receptors. As a pair, EXP-1 and LGC-35 may be utilized to further understand the differential molecular mechanisms of agonist, antagonist and allosteric modulation at ionotropic GABA receptors and may aid in the design of new and more specific anthelmintics that target GABA neurotransmission.