Two mutations linked to nocturnal frontal lobe epilepsy cause use‐dependent potentiation of the nicotinic ACh response

Abstract

Two mutations in the M2 region of the human á4 neuronal nicotinic subunit -á4(S248F) and á4(776ins3) -have been linked to autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) (Steinlein et al. 1995(Steinlein et al. , 1997. The á4(S248F) mutation is a serine to phenylalanine substitution at position 248 in the human á4 nicotinic subunit. The á4(776ins3) mutation is a 3-base pair insertion that adds a leucine at position 259 in the amino acid sequence of the human á4 subunit. Photo-affinity labelling and structurefunction experiments show that the M2 region of the nicotinic subunits forms the conducting pore of the receptor (reviewed in Karlin & Akabas, 1995). Thus, both ADNFLE mutations lie in the conducting pore of the nicotinic receptor. ADNFLE patients suffer from brief and occasionally violent nocturnal seizures . However, the physiological mechanism responsible for these seizures has not been established. Previous studies show that the predominant brain nicotinic receptor subtype is á4â2 (Whiting & Lindstrom, 1987;Flores et al. 1991;Whiting et al. 1991). Receptors formed by co-expressing á4(S248F) or á4(776ins3) subunits with wild-type (WT) â2 subunits in Xenopus oocytes (Weiland et al. 1996; Steinlein et al. 1997;Kuryatov et al. 1997) differ from the WT receptor in several ways but no common effects of the two mutations on the acetylcholine (ACh) response have been reported previously. To determine whether the ADNFLE mutations have any common effects on the ACh response, we constructed two rat homologues (S252F and +L264) of the human ADNFLE mutations á4(S248F) and á4(777ins3), co-expressed them with rat â2 subunits in Xenopus oocytes, and studied the properties of the expressed receptors. We also constructed the rat double mutation V247I:S252F, which combined the S252F mutation with a second V247I mutation that converted the only rat/human residue substitution in the á4 M2 region to the corresponding human residue. All three Journal of Physiology (1998) 1. We constructed rat homologues (S252F and +L264) of two human á4 nicotinic mutations -á4(S248F) and á4(777ins3) -that have been linked to autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) and co-expressed them with wild-type rat â2 subunits in Xenopus oocytes. 2. The S252F and +L264 mutations had three common effects on the ACh response. First, they caused use-dependent potentiation of the response during a train of brief 100 nÒ ACh pulses. Second, they delayed the rise times of the 5-15 nÒ (+L264) and 30 nÒ (S252F) ACh responses. Third, they reduced extracellular Ca¥-induced increases in the 30 ìÒ ACh response. 3. Beside these shared effects, the S252F mutation also reduced the channel burst duration measured from voltage-jump relaxations, enhanced steady-state desensitization and reduced the single-channel conductance. In contrast, the +L264 mutation prolonged the channel burst duration, did not affect desensitization and slightly increased single-channel conductance. Neither mutation affected the number of surface receptors measured b...

Nicotinic receptors in circuit excitability and epilepsy

“…First, current amplitudes were measured in a large number of cells . At variance with previous reports performed in "homozygous" conditions (Weiland et al, 1996;Kuryatov et al, 1997;Steinlein et al, 1997;Figl et al, 1998), no significant variation in the maximal AChevoked currents could be observed in control versus ADNFLE conditions. Second, determination of the receptor sensitivity, over a broad range of agonist concentrations, revealed that all mutant nAChRs displayed an increased ACh sensitivity with respect to wild-type receptors (Bertrand et al, , 2002Moulard et al, 2001).…”

Validation criteria for genetic animal models of epilepsy

“…Although the functional abnormalities of both mutant nAChR with S280F and insL are loss-of-function (enhanced steady-state desensitization) with gain-of-function (enhanced ACh sensitivity and use-dependent potentiation) [41][42][43], both knock-in mice; pS280F-KM and insL-KM, are considerably more sensitive to nicotine-induced seizures than wild-type mice. Indeed, in nicotine-induced seizure tests, these two types of knock-in mice show a lower threshold dose of nicotine for generalized seizures, shorter latencies to seizure onset, and longer seizure durations compared with their wild-type littermates [8].…”

Nicotine-Induced Dystonic Arousal Complex in a Mouse Line Harboring a Human Autosomal-Dominant Nocturnal Frontal Lobe Epilepsy Mutation

“…We used an ACh concentration (2 M) near the previously reported rat WT and ␣4(S252F)␤2 EC 50 for ACh (Kuryatov et al, 1997;Figl et al, 1998) and a mecamylamine concentration (0.5 M) near the range of IC 50 values previously reported for human ␣4␤2-nAChRs (Papke et al, 2001). As reported previously (Weiland et al, 1996;Kuryatov et al, 1997;Bertrand et al, 1998;Figl et al, 1998), the ␣4(S252F) mutation enhanced agonist-induced desensitization. Successive applications of 2 M ACh at 3 min intervals had no significant effect on the peak WT response but reduced the peak mutant response significantly ( p Ͻ 0.05) (Fig.…”