Characterization of the first honeybee Ca²⁺ channel subunit reveals two novel species- and splicing-specific modes of regulation of channel inactivation

Honeybee CaV4 has distinct permeation, inactivation, and pharmacology from homologous NaV channels

DSC1, a Drosophila channel with sequence similarity to the voltage-gated sodium channel (NaV), was identified over 20 years ago. This channel was suspected to function as a non-specific cation channel with the ability to facilitate the permeation of calcium ions (Ca2+). A honeybee channel homologous to DSC1 was recently cloned and shown to exhibit strict selectivity for Ca2+, while excluding sodium ions (Na+), thus defining a new family of Ca2+ channels, known as CaV4. In this study, we characterize CaV4, showing that it exhibits an unprecedented type of inactivation, which depends on both an IFM motif and on the permeating divalent cation, like NaV and CaV1 channels, respectively. CaV4 displays a specific pharmacology with an unusual response to the alkaloid veratrine. It also possesses an inactivation mechanism that uses the same structural domains as NaV but permeates Ca2+ ions instead. This distinctive feature may provide valuable insights into how voltage- and calcium-dependent modulation of voltage-gated Ca2+ and Na+ channels occur under conditions involving local changes in intracellular calcium concentrations. Our study underscores the unique profile of CaV4 and defines this channel as a novel class of voltage-gated Ca2+ channels.

An ancestral MAGUK protein supports the modulation of mammalian voltage-gated Ca2+ channels through a conserved CaVβ-like interface

Eukaryote voltage-gated Ca²⁺ channels of the Ca_V2 channel family are hetero-oligomers formed by the pore-forming Ca_Vα1 protein assembled with auxiliary Ca_Vα2δ and Ca_Vβ subunits. Ca_Vβ subunits are formed by a Src homology 3 (SH3) domain and a guanylate kinase (GK) domain connected through a HOOK domain. The GK domain binds a conserved cytoplasmic region of the pore-forming Ca_Vα1 subunit referred as the "AID". Herein we explored the phylogenetic and functional relationship between Ca_V channel subunits in distant eukaryotic organisms by investigating the function of a MAGUK protein (XM_004990081) cloned from the choanoflagellate Salpingoeca rosetta (Sro). This MAGUK protein (Sroβ) features SH3 and GK structural domains with a 25% primary sequence identity to mammalian Ca_Vβ. Recombinant expression of its cDNA with mammalian high-voltage activated Ca²⁺ channel Ca_V2.3 in mammalian HEK cells produced robust voltage-gated inward Ca²⁺ currents with typical activation and inactivation properties. Like Ca_Vβ, Sroβ prevents fast degradation of total Ca_V2.3 proteins in cycloheximide assays. The three-dimensional homology model predicts an interaction between the GK domain of Sroβ and the AID motif of the pore-forming Ca_Vα1 protein. Substitution of AID residues Trp (W386A) and Tyr (Y383A) significantly impaired co-immunoprecipitation of Ca_V2.3 with Sroβ and functional upregulation of Ca_V2.3 currents. Likewise, a 6-residue deletion within the GK domain of Sroβ, similar to the locus found in mammalian Ca_Vβ, significantly reduced peak current density. Altogether our data demonstrate that an ancestor MAGUK protein reconstitutes the biophysical and molecular features responsible for channel upregulation by mammalian Ca_Vβ through a minimally conserved molecular interface.

Heterogeneous expression of GABA receptor-like subunits LCCH3 and GRD reveals functional diversity of GABA receptors in the honeybee Apis mellifera

BACKGROUND AND PURPOSE

Despite a growing awareness, annual losses of honeybee colonies worldwide continue to reach threatening levels for food safety and global biodiversity. Among the biotic and abiotic stresses probably responsible for these losses, pesticides, including those targeting ionotropic GABA receptors, are one of the major drivers. Most insect genomes include the ionotropic GABA receptor subunit gene, Rdl, and two GABA-like receptor subunit genes, Lcch3 and Grd. Most studies have focused on Rdl which forms homomeric GABA-gated chloride channels, and a complete analysis of all possible molecular combinations of GABA receptors is still lacking.

EXPERIMENTAL APPROACH

We cloned the Rdl, Grd, and Lcch3 genes of Apis mellifera and systematically characterized the resulting GABA receptors expressed in Xenopus oocytes, using electrophysiological assays, fluorescence microscopy and co-immunoprecipitation techniques.

KEY RESULTS

The cloned subunits interacted with each other, forming GABA-gated heteromeric channels with particular properties. Strikingly, these heteromers were always more sensitive than AmRDL homomer to all the pharmacological agents tested. In particular, when expressed together, Grd and Lcch3 form a non-selective cationic channel that opens at low concentrations of GABA and with sensitivity to insecticides similar to that of homomeric Rdl channels.

CONCLUSION AND IMPLICATIONS

For off-target species like the honeybee, chronic sublethal exposure to insecticides constitutes a major threat. At these concentration ranges, homomeric RDL receptors may not be the most pertinent target to study and other ionotropic GABA receptor subtypes should be considered in order to understand more fully the molecular mechanisms of sublethal toxicity to insecticides.

Honey bees long-lasting locomotor deficits after exposure to the diamide chlorantraniliprole are accompanied by brain and muscular calcium channels alterations

Diamides belong to one of the newest insecticides class. We characterized cellular effects of the first commercialized diamide, chlorantraniliprole (ChlorAnt). ChlorAnt not only induces a dose-dependent calcium release from internal stores of honey bee muscle cells, but also a dose-dependent blockade of the voltage-gated calcium current involved in muscles and brain excitability. We measured a long lasting impairment in locomotion after exposure to a sublethal dose and despite an apparent remission, bees suffer a critical relapse seven days later. A dose that was sublethal when applied onto the thorax turned out to induce severe mortality when applied on other body parts. Our results may help in filling the gap in the toxicological evaluation of insecticides that has recently been pointed out by international instances due to the lack of suitable tests to measure sublethal toxicity. Intoxication symptoms in bees with ChlorAnt are consistent with a mode of action on intracellular calcium release channels (ryanodine receptors, RyR) and plasma membrane voltage-gated calcium channels (CaV). A better coupling of in vitro and behavioral tests may help in more efficiently anticipating the intoxication symptoms.

Honeybee locomotion is impaired by Am-CaV3 low voltage-activated Ca2+ channel antagonist

Voltage‐gated Ca²⁺ channels are key transducers of cellular excitability and participate in several crucial physiological responses. In vertebrates, 10 Ca²⁺ channel genes, grouped in 3 families (Ca_V1, Ca_V2 and Ca_V3), have been described and characterized. Insects possess only one member of each family. These genes have been isolated in a limited number of species and very few have been characterized although, in addition to their crucial role, they may represent a collateral target for neurotoxic insecticides. We have isolated the 3 genes coding for the 3 Ca²⁺ channels expressed in Apis mellifera. This work provides the first detailed characterization of the honeybee T-type Ca_V3 Ca²⁺ channel and demonstrates the low toxicity of inhibiting this channel. Comparing Ca²⁺ currents recorded in bee neurons and myocytes with Ca²⁺ currents recorded in Xenopus oocytes expressing the honeybee Ca_V3 gene suggests native expression in bee muscle cells only. High‐voltage activated Ca²⁺ channels could be recorded in the somata of different cultured bee neurons. These functional data were confirmed by in situ hybridization, immunolocalization and in vivo analysis of the effects of a Ca_V3 inhibitor. The biophysical and pharmacological characterization and the tissue distribution of Ca_V3 suggest a role in honeybee muscle function.

A Locomotor Deficit Induced by Sublethal Doses of Pyrethroid and Neonicotinoid Insecticides in the Honeybee Apis mellifera

The toxicity of pesticides used in agriculture towards non-targeted organisms and especially pollinators has recently drawn the attention from a broad scientific community. Increased honeybee mortality observed worldwide certainly contributes to this interest. The potential role of several neurotoxic insecticides in triggering or potentiating honeybee mortality was considered, in particular phenylpyrazoles and neonicotinoids, given that they are widely used and highly toxic for insects. Along with their ability to kill insects at lethal doses, they can compromise survival at sublethal doses by producing subtle deleterious effects. In this study, we compared the bee's locomotor ability, which is crucial for many tasks within the hive (e.g. cleaning brood cells, feeding larvae…), before and after an acute sublethal exposure to one insecticide belonging to the two insecticide classes, fipronil and thiamethoxam. Additionally, we examined the locomotor ability after exposure to pyrethroids, an older chemical insecticide class still widely used and known to be highly toxic to bees as well. Our study focused on young bees (day 1 after emergence) since (i) few studies are available on locomotion at this stage and (ii) in recent years, pesticides have been reported to accumulate in different hive matrices, where young bees undergo their early development. At sublethal doses (SLD48h, i.e. causing no mortality at 48 h), three pyrethroids, namely cypermethrin (2.5 ng/bee), tetramethrin (70 ng/bee), tau-fluvalinate (33 ng/bee) and the neonicotinoid thiamethoxam (3.8 ng/bee) caused a locomotor deficit in honeybees. While the SLD48h of fipronil (a phenylpyrazole, 0.5 ng/bee) had no measurable effect on locomotion, we observed high mortality several days after exposure, an effect that was not observed with the other insecticides. Although locomotor deficits observed in the sublethal range of pyrethroids and thiamethoxam would suggest deleterious effects in the field, the case of fipronil demonstrates that toxicity evaluation requires information on multiple endpoints (e.g. long term survival) to fully address pesticides risks for honeybees. Pyrethroid-induced locomotor deficits are discussed in light of recent advances regarding their mode of action on honeybee ion channels and current structure-function studies.

Molecular characterization and functional expression of the Apis mellifera voltage-dependent Ca2+ channels

Voltage-gated Ca(2+) channels allow the influx of Ca(2+) ions from the extracellular space upon membrane depolarization and thus serve as a transducer between membrane potential and cellular events initiated by Ca(2+) transients. Most insects are predicted to possess three genes encoding Cavα, the main subunit of Ca(2+) channels, and several genes encoding the two auxiliary subunits, Cavβ and Cavα2δ; however very few of these genes have been cloned so far. Here, we cloned three full-length cDNAs encoding the three Cavα subunits (AmelCav1a, AmelCav2a and AmelCav3a), a cDNA encoding a novel variant of the Cavβ subunit (AmelCavβc), and three full-length cDNAs encoding three Cavα2δ subunits (AmelCavα2δ1 to 3) of the honeybee Apis mellifera. We identified several alternative or mutually exclusive exons in the sequence of the AmelCav2 and AmelCav3 genes. Moreover, we detected a stretch of glutamine residues in the C-terminus of the AmelCav1 subunit that is reminiscent of the motif found in the human Cav2.1 subunit of patients with Spinocerebellar Ataxia type 6. All these subunits contain structural domains that have been identified as functionally important in their mammalian homologues. For the first time, we could express three insect Cavα subunits in Xenopus oocytes and we show that AmelCav1a, 2a and 3a form Ca(2+) channels with distinctive properties. Notably, the co-expression of AmelCav1a or AmelCav2a with AmelCavβc and AmCavα2δ1 produces High Voltage-Activated Ca(2+) channels. On the other hand, expression of AmelCav3a alone leads to Low Voltage-Activated Ca(2+) channels.

Molecular mechanisms underlying formation of long-term reward memories and extinction memories in the honeybee (Apis mellifera)

The honeybee (Apis mellifera) has long served as an invertebrate model organism for reward learning and memory research. Its capacity for learning and memory formation is rooted in the ecological need to efficiently collect nectar and pollen during summer to ensure survival of the hive during winter. Foraging bees learn to associate a flower's characteristic features with a reward in a way that resembles olfactory appetitive classical conditioning, a learning paradigm that is used to study mechanisms underlying learning and memory formation in the honeybee. Due to a plethora of studies on appetitive classical conditioning and phenomena related to it, the honeybee is one of the best characterized invertebrate model organisms from a learning psychological point of view. Moreover, classical conditioning and associated behavioral phenomena are surprisingly similar in honeybees and vertebrates, suggesting a convergence of underlying neuronal processes, including the molecular mechanisms that contribute to them. Here I review current thinking on the molecular mechanisms underlying long-term memory (LTM) formation in honeybees following classical conditioning and extinction, demonstrating that an in-depth analysis of the molecular mechanisms of classical conditioning in honeybees might add to our understanding of associative learning in honeybees and vertebrates.

Gene splicing of an invertebrate beta subunit (LCavβ) in the N-terminal and HOOK domains and its regulation of LCav1 and LCav2 calcium channels

The accessory beta subunit (Ca(v)β) of calcium channels first appear in the same genome as Ca(v)1 L-type calcium channels in single-celled coanoflagellates. The complexity of this relationship expanded in vertebrates to include four different possible Ca(v)β subunits (β1, β2, β3, β4) which associate with four Ca(v)1 channel isoforms (Ca(v)1.1 to Ca(v)1.4) and three Ca(v)2 channel isoforms (Ca(v)2.1 to Ca(v)2.3). Here we assess the fundamentally-shared features of the Ca(v)β subunit in an invertebrate model (pond snail Lymnaea stagnalis) that bears only three homologous genes: (LCa(v)1, LCa(v)2, and LCa(v)β). Invertebrate Ca(v)β subunits (in flatworms, snails, squid and honeybees) slow the inactivation kinetics of Ca(v)2 channels, and they do so with variable N-termini and lacking the canonical palmitoylation residues of the vertebrate β2a subunit. Alternative splicing of exon 7 of the HOOK domain is a primary determinant of a slow inactivation kinetics imparted by the invertebrate LCa(v)β subunit. LCa(v)β will also slow the inactivation kinetics of LCa(v)3 T-type channels, but this is likely not physiologically relevant in vivo. Variable N-termini have little influence on the voltage-dependent inactivation kinetics of differing invertebrate Ca(v)β subunits, but the expression pattern of N-terminal splice isoforms appears to be highly tissue specific. Molluscan LCa(v)β subunits have an N-terminal "A" isoform (coded by exons: 1a and 1b) that structurally resembles the muscle specific variant of vertebrate β1a subunit, and has a broad mRNA expression profile in brain, heart, muscle and glands. A more variable "B" N-terminus (exon 2) in the exon position of mammalian β3 and has a more brain-centric mRNA expression pattern. Lastly, we suggest that the facilitation of closed-state inactivation (e.g. observed in Ca(v)2.2 and Ca(v)β3 subunit combinations) is a specialization in vertebrates, because neither snail subunit (LCa(v)2 nor LCa(v)β) appears to be compatible with this observed property.