Effects of emodepside on single-channel properties of Onchocerca volvulus SLO-1A (BK) potassium channels

Control of onchocerciasis (river blindness of humans due to infection with the filarial nematode, Onchocerca volvulus) remains a challenge because of the lack of effective adulticides and vaccines. Emodepside is a broad-spectrum veterinary anthelmintic that has been found to inhibit nematode muscle activity by activating their tetrameric SLO-1K channels. Emodepside has adulticidal activity and is being trialed for onchocerciasis treatment, but the molecular mode of action of emodepside is still being elucidated. Here, we examine the single-channel currents of Ovo-SLO-1A, a SLO-1K splice variant from O. volvulus, and explore how emodepside modulates the dynamics of the opening of the channel.

Ovo-SLO-1A was expressed in HEK293 cells, and patch clamp electrophysiology techniques were used to record currents. Single-channel currents were recorded to determine open-state channel conductances and kinetics. Emodepside's effects were tested at 0.3 µM and 1.0 µM.

Ovo-SLO-1A had a main open-state conductance of 110 ± 3 pS and frequent flickering subconductance states. The presence of the flickering subconductance states suggests that there is limited cooperativity between the tetrameric channel subunits required for opening to the main open state. Emodepside increased mean current amplitudes. Emodepside also increased open burst times, and open probability. Verruculogen (1 µM) inhibited channel opening in the presence or absence of emodepside.

This study successfully expressed Ovo-SLO-1A in HEK293 cells, measured the conductance of the main open-state, and detected the presence of subconductance states and flickering openings. The increased amplitudes of the single-channel currents, open burst times, and open probabilities provides insights into the complex kinetics by which emodepside increases Slo-1K currents and illustrates the dynamic actions of emodepside on Ovo-SLO-1A.