In silico simulations and functional cell studies evidence similar potency and distinct binding of Pacific and Caribbean ciguatoxins

Abstract

Ciguatoxins (CTX) cause ciguatera fish poisoning, which is the most common reported hu-man food poisoning related to natural marine toxins. Pacific ciguatoxins are the most abundant and studied CTX analogues, however, the growing distribution of Caribbean analogues and the limited data available on their biological effects makes necessary to re-evaluate their relative potency. For decades, the guidelines established by regulatory agencies have assumed that the potency of the Caribbean CTXs were 10-fold lower than the Pacific CTXs. We present here an integrated study involving Neuro-2a MTT and electrophysiological assays and in silico simulations that evidence the similar cytotoxicity of both toxins and the asymmetry binding of CTXs with sodium channels. The simulation studies explain their distinct impact on the activation potential of the channel as experimentally observed and provided a detailed picture of the effects caused by these toxins on an atomic scale.

Synergistic Effect of Brevetoxin BTX-3 and Ciguatoxin CTX3C in Human Voltage-Gated Nav1.6 Sodium Channels

“…The combined effects of both food contaminants on sodium channel functionality have not been previously studied. CTXs decreased the maximum peak inward sodium currents at very low concentrations and cause a negative change in the activation voltage of up to 20 mV of VGSC, which is in accordance to previous reports 7,11,17 and also their fast inactivation state. 35,38 BTXs also caused an important negative displacement in the activation voltage of sodium channels of up to 13 mV not modifying the maximum I Na 4,5 and hyperpolarizing their fast inactivation voltage.…”

“…3 However, despite being similar in their mechanism of action, there are differences in the reported effects and potency between the two groups of toxins that indicate that BTXs act at micromolar concentrations, while CTXs present physiological effects at nanomolar concentrations. 4,5 CTXs and BTXs bind to a common site of the sodium channel located at the cleft created by segment S5 of domain IV, segment S6 of domain I, and the P-loop (P1) that connects segments S5 and S6 of domain IV of the VGSC α-subunit 6,7 but with different affinities. Previous studies indicated that CTXs interact specifically and with higher affinity with sodium channels; 8,9 however, the differences in the chemical structures between the CTX analogues generate modifications in their binding and activity over VGSC.…”

“…The functional consequence of the interaction of CTXs and BTXs with VGSC is a hyperpolarizing shift in the sodium channel activation curve 5,7,15 as well as a negative shift of their half inactivation voltage. 16 CTXs are known to have a significant effect on the activation and inactivation state of VGSC at concentrations of 1 nM and higher.…”

“…7,11,17 As a consequence, the channels will remain permanently open at resting cell membrane potentials, resulting in depolarization of the membrane and spontaneous and/or repetitive action potential discharges in excitable cells. 6,9 Although both groups of toxins interact with the same site of VGSC, there are important differences between the observed cellular effect of each group since CTXs cause a remarkable decrease in the maximum peak inward sodium current amplitude (I Na ), 7,11,17 while no significant effects were reported for BTXs. 4,5 It is important to note that the effects and the affinity of CTXs or BTXs for VGSC depend on the analogue and the channel subtype.…”

“…16 CTXs are known to have a significant effect on the activation and inactivation state of VGSC at concentrations of 1 nM and higher. 7,11,17 As a consequence, the channels will remain permanently open at resting cell membrane potentials, resulting in depolarization of the membrane and spontaneous and/or repetitive action potential discharges in excitable cells. 6,9 Although both groups of toxins interact with the same site of VGSC, there are important differences between the observed cellular effect of each group since CTXs cause a remarkable decrease in the maximum peak inward sodium current amplitude (I Na ), 7,11,17 while no significant effects were reported for BTXs.…”