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

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.

Azobenzene-isoxazoline as photopharmacological ligand for optical control of insect GABA receptor and behavior

BACKGROUND

Photopharmacology is a fast-growing photonics-based technology, which realizes the high-resolution regulation of drugs in time and space through light. The purpose of this research was to introduce photochromic groups into the isoxazoline structure to realize the regulation of γ-Aminobutyric acid receptors (GABARs) targeting insect behavior.

RESULTS

Azobenzene-Fluralaner analogs ABF02, ABF03 and ABF04 have been proven to have larvicidal activity against mosquito larvae. Cis-ABF03 had excellent larvicidal activity against mosquito larvae with a median lethal concentration (LC₅₀ ) value of 1.63, which was better than that of trans-ABF03 (LC₅₀  = 3.90). In particular, ABF03 also showed insecticidal activity against Mythimna separata. Further experiments showed that ABF03 (1 μm) induced depolarization of dorsal unpaired median neurons after ultraviolet light irradiation, enhanced affinity to the receptor, and blocked ligand-gated chloride channels of GABARs. ABF03 (1 μm) realized the real-time photoregulation of the behavior of mosquito larvae, which indicated that the synthesized ligand can complete the binding and off-target action of drugs and targets in vivo under the regulation of light.

CONCLUSION

Azobenzene-Isoxazoline as photopharmacological ligand was synthesized and evaluated for optical control of insect GABARs and behavior for the first time. ABF03 completed the differential regulation of cockroach neurons and the real-time reversible regulation of insect behavior. The establishment of photochromic ligands provides a new strategy for basic and convenience-oriented research on GABARs in invertebrates. © 2021 Society of Chemical Industry.

Photochromic meta-diamides for optical modulation of ligand activity and neuron function†

Photopharmacology offers facile solutions for spatiotemporal control over ligand activity and receptor function. The meta-diamide insecticide acts on insect GABA receptors (GABARs) as an antagonist that causes firing of a neuron. We present here photochromic GABAR ligands azobenzene-meta-diamides (ABMDAs) by incorporating photoswitchable azobenzene with meta-diamides. ABMDAs showed good isomerization efficiency and fatigue resistance. Among them, ABMDA7 shows a 1.5-fold insecticidal activity difference towards mosquito larvae (Aedes albopictus) before and after UV illumination. We translated this light-dependent activity difference to the optical modulation of the membrane potential of American cockroach (Periptaneta americana) DUM neurons. This light-responsive meta-diamide-based GABAR ligand allows for optical regulation of insecticidal activity and DUM neurons.

Multiple combinations of RDL subunits diversify the repertoire of GABA receptors in the honey bee parasite Varroa destructor

In insects, γ-aminobutyric acid (GABA) is the major inhibitory neurotransmitter, and GABA-gated ion channels are the target of different classes of insecticides, including fipronil. We report here the cloning of six subunits (four RDL, one LCCH3, and one GRD) that constitute the repertoire of the GABA-gated ion channel family of the Varroa mite (Varroa destructor), a honey bee ectoparasite. We also isolated a truncated GRD subunit with a premature stop codon. We found that when expressed in Xenopus laevis oocytes, three of the four RDL subunits (VdesRDL1, VdesRDL2, and VdesRDL3) formed functional, homomultimeric anionic receptors, whereas GRD and LCCH3 produced heteromultimeric cationic receptors. These receptors displayed specific sensitivities toward GABA and fipronil, and VdesRDL1 was the most resistant to the insecticide. We identified specific residues in the VdesRDL1 pore-lining region that explain its high resistance to fipronil. VdesRDL4 did not form a functional receptor when expressed alone, but it assembled with VdesRDL1 to form a heteromultimeric receptor with properties distinct from those of the VdesRDL1 homomultimeric receptor. Moreover, VdesRDL1 physically interacted with VdesRDL3, generating a heteromultimeric receptor combining properties of both subunits. On the other hand, we did not detect any functional interaction between VdesLCCH3 and the VdesRDL subunits, an observation that differed from what was previously reported for Drosophila melanogaster In conclusion, this study provides insights relevant to improve our understanding of the precise role of GABAergic signaling in insects and new tools for the development of Varroa mite-specific insecticidal agents that do not harm honey bees.

Highly Efficient, Rapid and Co-CRISPR-Independent Genome Editing in Caenorhabditis elegans

We describe a rapid and highly efficient method to generate point mutations in Caenorhabditis elegans using direct injection of CRISPR-Cas9 ribonucleoproteins. This versatile method does not require sensitized genetic backgrounds or co-CRISPR selection-based methods, and represents a single strategy that can be used for creating genomic point mutations, regardless of location. As proof of principle, we show that knock-in mutants more faithfully report variant-associated phenotypes as compared to transgenic overexpression. Data for nine knock-in mutants across five genes are presented that demonstrate high editing efficiencies (60%), a reduced screening workload (24 F1 progeny), and a rapid timescale (4–5 d). This optimized method simplifies genome engineering and is readily adaptable to other model systems.

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.