DSC1 channel-dependent developmental regulation of pyrethroid susceptibility in Drosophila melanogaster

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.

Periplocoside P affects synaptic transmission at the neuromuscular junction and reduces synaptic excitability in Drosophila melanogaster by inhibiting V-ATPase

BACKGROUND

Periplocoside P (PSP) is a major component of Periploca sepium Bunge known for its potent insecticidal activity. V-Type adenosine triphosphatase (V-ATPase), which is widely distributed in the cytoplasmic membranes and organelles of eukaryotic cells, plays a crucial role in synaptic excitability conduction. Previous research has shown that PSP targets the apical membrane of goblet cells in the insect midgut. However, the effects of PSP on synaptic transmission at the neuromuscular junction are often overlooked.

RESULTS

The bioassay revealed that Drosophila adults with different genetic backgrounds showed varying levels of susceptibility to PSP in the order: parats1  > parats1 ;DSC1-/-  ≈ w1118  > DSC1-/- . Intracellular electrode recording demonstrated that PSP, similar to bafilomycin A1, had an impact on the amplitude of the excitatory junction potential (EJP) and accelerated excitability decay. Furthermore, the alteration in EJP amplitude is concentration-dependent. Another surprising discovery was that the knockout DSC1 channel showed insensitivity to PSP.

CONCLUSION

Our findings confirm that PSP can influence synaptic transmission at the neuromuscular junction of Drosophila larvae by targeting V-ATPase. These results provide a basis for investigating the mechanism of action of PSP and its potential application in designing novel insecticides. © 2023 Society of Chemical Industry.

Role of DSC1 in Drosophila melanogaster synaptic activities in response to haedoxan A

Drosophila sodium channel 1 (DSC1) encodes a voltage-gated divalent cation channel that mediates neuronal excitability in insects. Previous research revealed that DSC1 knockout Drosophila melanogaster conferred different susceptibility to insecticides, which indicated the vital regulation role of DSC1 under insecticide stress. Haedoxan A (HA) is a lignan compound isolated from Phryma leptostachya, and we found that HA has excellent insecticidal activity and is worthy of further study as a botanical insecticide. Herein, we performed bioassay and electrophysiological experiments to test the biological and neural changes in the larval Drosophila with/without DSC1 knockout in response to HA. Bioassay results showed that knockout of DSC1 reduced the sensitivity to HA in both w1118 (a common wild-type strain in the laboratory) and parats1 (a pyrethroid-resistant strain) larvae. Except for parats1 /DSC1-/- , electrophysiology results implicated that HA delayed the decay rate and increased the frequency of miniature excitatory junctional potentials of Drosophila from w1118 , parats1 , and DSC1-/- strains. Moreover, the neuromuscular synapse excitatory activities of parats1 /DSC1-/- larvae were more sensitive to HA than DSC1-/- larvae, which further confirmed the functional contribution of DSC1 to neuronal excitability. Collectively, these results indicated that the DSC1 channel not only regulated the insecticidal activity of HA, but also maintained the stability of neural circuits through functional interaction with voltage-gated sodium channels. Therefore, our study provides useful information for elucidating the regulatory mechanism of DSC1 in the neural system of insects involving the action of HA derived from P. leptostachya.

Effects of periplocoside T isolated from Periploca sepium on behavior and sensory-CNS-motor circuits in Drosophila melanogaster larvae

Periplocoside T (PST) from Periploca sepium has insecticidal activity against some lepidopterans, which can significantly inhibit the activity of vacuolar-type H⁺-ATPases (V-ATPase). V-ATPase is involved in the release of neurotransmitters in vesicles during nerve signal transduction. However, there are actions of PST on behavior and sensory-central nervous system (CNS)-motor neural circuit which are commonly overlooked. After exposure to 500 mg/L PST for 48 h, the difference of the proportion of larvae responding to stimuli in the four Drosophila strains was not significant as compared to controls, but larval mouth hook movement and body wall motion were significantly decreased as compared to controls, and the decrease was more obvious in para^ts1; DSC1^-/- and DSC1^-/- strains, especially in para^ts1; DSC1^-/- strain. Compared with control (DMSO), the excitatory junction potential (EJP) frequencies of sensory-CNS-motor circuits in the four Drosophila strains after PST or bafiloymcin A1 (BA1, a V-ATPase specific inhibitor) treatment gradually decreased with time, and the decreasing amplitude of BA1 treatment was greater than that of PST treatment, but both were higher than that of the control. The decay amplitude of EJP frequency in two strains with DSC1 channel knockout was lower than that of w¹¹¹⁸ and para^ts1 strains without DSC1 channel knockout. Thus, the results indicated that PST, similar to BA1, could inhibit the transmission of sensory-CNS-motor circuit excitability of Drosophila larvae by inhibiting the activity of V-ATPase, and DSC1 channel play a role of in regulating the stability of nervous system.

Negative cross-resistance of a pyrethroid-resistant Drosophila mutant to Phryma leptostachya-derived haedoxan A

Voltage-gated sodium channels are the primary target of pyrethroid insecticides. Mutations in sodium channel confer knockdown resistance (kdr) to pyrethroids in various arthropod pests. Haedoxan A (HA) is the major insecticidal component from Phryma leptostachya. It has been shown that HA alters electrical responses at the Drosophila neuromuscular junction and modifies the gating properties of cockroach sodium channels expressed in Xenopus oocytes. However, whether sodium channel mutations that confer pyrethroid resistance also affect the action of HA is unknown. In this study, we conducted bioassays using HA and permethrin in two Drosophila melanogaster strains: w¹¹¹⁸ , an insecticide-susceptible strain, and para^ts1 , a pyrethroid-resistant strain due to a I265N mutation in the sodium channel, and identified a new case of negative cross-resistance (NCR) between permethrin and HA. Both para^ts1 larvae and adults were more resistant to permethrin, as expected. However, both para^ts1 larvae and adults were more sensitive to HA compared to w¹¹¹⁸ . We confirmed that the I265N mutation reduced the sensitivity to permethrin of a Drosophila sodium channel variant, DmNa_v 22, expressed in Xenopus oocytes. Interestingly, the I265N mutation also abolished the effect of HA on sodium channels. Further characterization showed that I265 on the sodium channels is critical for the action of both pyrethroids and HA on sodium channels, pointing to an overlapping mode of action between pyrethroids and HA on the sodium channel. Overall, our results suggest an I265N-independnt mechanism(s) in para^ts1 flies that is responsible for the NCR between permethrin and HA at the whole insect level.

How varying parameters impact insecticide resistance bioassay: An example on the worldwide invasive pest Drosophila suzukii

Monitoring pesticide resistance is essential for effective and sustainable agricultural practices. Bioassays are the basis for pesticide-resistance testing, but devising a reliable and reproducible method can be challenging because these tests are carried out on living organisms. Here, we investigated five critical parameters and how they affected the evaluation of resistance to the organophosphate phosmet or the pyrethroid lambda-cyhalothrin using a tarsal-contact protocol on Drosophila suzukii, a worldwide invasive pest. Three of the parameters were related to insect biology: (i) sex, (ii) age of the imago (adult stage) and (iii) genetic diversity of the tested population. The two remaining parameters were linked to the experimental setup: (iv) the number of individuals tested per dose and (v) the duration of exposure to the active ingredient. Results showed that response to insecticide differed depending on sex, males being twice as susceptible to phosmet as females. Age principally affected young females' susceptibility to phosmet, because 0-24 hour-old flies were twice as susceptible as 24-48 hour-old and 72-96 hour-old females. Genetic diversity had no observable effect on resistance levels. The precision and accuracy of the median lethal dose (LD50) were greatly affected by the number of individuals tested per dose with a threshold effect. Finally, optimal duration of exposure to the active ingredient was 24 h, as we found an underestimation of mortality when assessed between 1 and 5 h after exposure to lambda-cyhalothrin. None of the main known point mutations on the para sodium channel gene associated with a knockdown effect were observed. Our study demonstrates the importance of calibrating the various parameters of a bioassay to develop a reliable method. It also provides a valuable and transferable protocol for monitoring D. suzukii resistance worldwide.