ROLE OF M-CHANNELS IN SEIZURE GENERATION: A STUDY IN ZEBRAFISH
Epilepsy is a neurological disorder that is characterized by recurrent seizures that result from an overall increase in neuronal excitability. A subset of epilepsies are caused by gene mutations majority of which result in ion channel dysfunction, alteration of neurotransmitter receptors or cause brain deformities. For this thesis, we examined a particular subset of potassium channels (M-channels) in which mutations of the corresponding KCNQ genes results in a seizure disorder known as benign familial neonatal convulsions (BFNC).
M-channels produce an inhibitory potassium current that allows firing of single action potentials but opposes sustained depolarization and repetitive firing of action potentials. Suppression of this channel thus results in continuous firing of action potentials and may cause general network hyperexcitation. We sought to study the role of these M-channels in seizure generation using zebrafish larvae as the model organism. First we determined which of the M-channel genes KCNQ2, KCNQ3 and KCNQ5 were expressed in Zebrafish. We were able to confirm the expression of KCNQ3 and KCNQ5 but not KCNQ2 due to sequence assembly errors. We then determined the developmental expression and localization of both KCNQ3 and KCNQ5 in the zebrafish larvae. At the channel level, we examined the effect of pharmacological alteration of M-channel function in vivo. We were able to show in that blocking M-channels using linopirdine was sufficient to cause seizure-like electrical bursting in the brain, as well as stereotyped seizure behaviors. These seizure phenotypes could be blocked and also reversed by application of the M-channel enhancer Retigabine.
We then knocked down the expression of KCNQ3 using targeted morpholinos, and were able to confirm that knock-down of KCNQ3 in zebrafish larvae was sufficient to produce an electrical seizure phenotype in the brain. Overall, we confirmed the specific hypothesis that suppression of M-channel activity in vivo alters neuronal function in a manner that leads to hyperexcitability and seizures. Since the zebrafish expressed specific M-channel genes and disruption of either gene function or channel function (pharmacologically) consistently produced a seizure phenotype, we were able to show that the zebrafish is a viable model system in which to study M-channels and BFNC.