Reduced cortical inhibition in a mouse model of familial childhood absence epilepsy

Abstract

Mutations in the GABAA receptor ␥2 subunit are associated with childhood absence epilepsy and febrile seizures. To understand better the molecular basis of absence epilepsy in man, we developed a mouse model harboring a ␥2 subunit point mutation (R43Q) found in a large Australian family. Mice heterozygous for the mutation demonstrated behavioral arrest associated with 6-to 7-Hz spike-and-wave discharges, which are blocked by ethosuximide, a first-line treatment for absence epilepsy in man. Seizures in the mouse showed an abrupt onset at around age 20 days corresponding to the childhood nature of this disease. Reduced cell surface expression of ␥2(R43Q) was seen in heterozygous mice in the absence of any change in ␣1 subunit surface expression, ruling out a dominant-negative effect. GABA Amediated synaptic currents recorded from cortical pyramidal neurons revealed a small but significant reduction that was not seen in the reticular or ventrobasal thalamic nuclei. We hypothesize that a subtle reduction in cortical inhibition underlies childhood absence epilepsy seen in humans harboring the R43Q mutation.GABAA receptor ͉ genetics ͉ electroencephalogram ͉ trafficking ͉ synapse G ABA A receptors in the adult brain are important for inhibiting the activity of neurons in which they reside. Dysfunction of these receptors caused by familial mutations can give rise to febrile seizures (FS) and a variety of generalized epilepsy phenotypes (1-3). To date, five mutations have been reported in the GABA A ␥2 subunit gene with an array of seizure types seen in patients (1, 4-7). Childhood absence epilepsy (CAE) and FS were the main phenotypes in a large Australian family with an arginine to glutamine mutation at position 43 (R43Q) in the GABA A ␥2 subunit gene (1,8).Understanding how the GABA A ␥2(R43Q) mutation causes epilepsy is difficult. GABA A receptors themselves serve several roles. They regulate moment-to-moment brain function (9), play an important role in brain development (10), and have key roles in neuronal plasticity (11, 12) and response to brain injury (13-15). Epilepsy itself is a complex phenomenon involving the interaction of multiple cell types in networks within and between different brain regions that are likely to be influenced by GABA A receptor dysfunction caused by the R43Q mutation. Furthermore, in vitro analyses of the consequences of this mutation have shown inconsistent findings with a range of deficits in receptor pharmacology, trafficking, kinetics, or assembly (16-23) potentially implicated in disease pathogenesis.Clearly, the complex nature of epileptogenesis demands in vivo investigation. Genetic epilepsies provide a framework on which to investigate the consequences of causative mutations at a range of organizational levels within the brain, creating a chain of understanding from molecules to behavior. Linking this chain is impossible in humans because of the highly invasive methodology required and is severely limited in heterologous expression systems that lack necessary complexity. Mice mod...

Deletion of phospholipase C β4 in thalamocortical relay nucleus leads to absence seizures

“…SWDs are generalized over wide cortical areas, requiring a massive synchronization of the thalamo-cortico-thalamic loop. Some studies proposed a leading role of the cortex in the generation of SWDs (10)(11)(12). In one such study using GAERS, a genetic rat model of absence seizures, infusion of ethosuximide into the cortex produced an immediate cessation of SWDs (13).…”

“…Abnormal hypersynchronized oscillatory activities in the thalamocortical network, consisting of feedforward and feedback connections between the cortex and the thalamus, have been implicated as an underlying mechanism for the generation of SWDs (5)(6)(7)(8)(9). Some studies using rat models of absence seizures have suggested that the cortex plays a leading role in the generation of SWDs (10)(11)(12)(13). Other studies support the hypothesis that massive thalamocortical synchronization is driven from recurrent oscillatory activities in the network between reticular thalamic nucleus (nRT) and thalamocortical (TC) relay nucleus (3,8,9,14,15).…”

Stabilization of GABAAReceptors at Endocytic Zones Is Mediated by an AP2 Binding Motif within the GABAAReceptor β3 Subunit

“…Disrupted GABA A R trafficking contributes to altered information processing in pathological conditions such as epilepsy and ischemia, in which acute receptor surface downmodulation and loss of synaptic GABA A Rs leads to compromised neuronal inhibition and altered excitability states, but the underlying mechanisms remain unclear (Mielke and Wang, 2005;Naylor et al, 2005;Tan et al, 2007;). Indeed, although both clathrin-dependent and -independent (e.g., caveolar-dependent) pathways for GABA A R endocytosis have been described (Kittler et al, 2000;Bradley et al, 2008), the specific contribution of AP2-dependent pathways for synaptic receptor loss in epilepsy or ischemia remains unclear.…”

Validation criteria for genetic animal models of epilepsy

“…They include autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) [7][8][9], benign familial neonatal convulsion (BFNC) [10,11], childhood absence epilepsy (CAE) [12,13], severe myoclonic epilepsy in infancy (SMEI) [14] and febrile seizures (FS) [15]. However, their phenotypic features have not been determined fully.…”