Functional and biochemical analysis of a sodium channel beta1 subunit mutation responsible for generalized epilepsy with febrile seizures plus type 1.
Generalized epilepsy with febrile seizures plus type 1 is an inherited human epileptic syndrome, associated with a cysteine-to-tryptophan (C121W) mutation in the extracellular immunoglobin domain of the auxiliary beta1 subunit of the voltage-gated sodium channel. The mutation disrupts beta1 function, but how this leads to epilepsy is not understood. In this study, we make several observations that may be relevant for understanding why this beta1 mutation results in seizures. First, using electrophysiological recordings from mammalian cell lines, coexpressing sodium channel alpha subunits and either wild-type beta1 or C121Wbeta1, we show that loss of beta1 functional modulation, caused by the C121W mutation, leads to increased sodium channel availability at hyperpolarized membrane potentials and reduced sodium channel rundown during high-frequency channel activity, compared with channels coexpressed with wild-type beta1. In contrast, neither wild-type beta1 nor C121Wbeta1 significantly affected sodium current time course or the voltage dependence of channel activation. We also show, using a Drosophila S2 cell adhesion assay, that the C121W mutation disrupts beta1-beta1 homophilic cell adhesion, suggesting that the mutation may alter the ability of beta1 to mediate protein-protein interactions critical for sodium channel localization. Finally, we demonstrate that neither functional modulation nor cell adhesion mediated by wild-type beta1 is occluded by coexpression of C121Wbeta1, arguing against the idea that the mutant beta1 acts as a dominant-negative subunit. Together, these data suggest that C121Wbeta1 causes subtle effects on channel function and subcellular distribution that bias neurons toward hyperexcitabity and epileptogenesis.
Meadows
Laurence S
LS
Department of Neurology and Neurosurgery, Montreal Neurological Institute, McGill University, Montreal, Quebec H3A 2B4, Canada.
Malhotra
Jyoti
J
Loukas
Andrew
A
Thyagarajan
Veena
V
Kazen-Gillespie
Kristin A
KA
Koopman
Matthew C
MC
Kriegler
Steven
S
Isom
Lori L
LL
Ragsdale
David S
DS
D000818:Animals; D016466:CHO Cells; D002448:Cell Adhesion; D002449:Cell Aggregation; D002460:Cell Line; D002478:Cells, Cultured; D006224:Cricetinae; D004330:Drosophila / Q000166:cytology; D004553:Electric Conductivity; D004829:Epilepsy, Generalized / Q000235:genetics; D020022:Genetic Predisposition to Disease; D006801:Humans; D007700:Kinetics; D008564:Membrane Potentials; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D017354:Point Mutation; D021122:Protein Subunits; D051381:Rats; D003294:Seizures, Febrile / Q000235:genetics; D015222:Sodium Channels / Q000032:analysis / Q000235:genetics* / Q000502:physiology*; D013577:Syndrome; D062547:Voltage-Gated Sodium Channel beta-1 Subunit; D014981:Xenopus
D021122:Protein Subunits; C470379:SCN1B protein, human; D015222:Sodium Channels; D062547:Voltage-Gated Sodium Channel beta-1 Subunit