Mouse models of human KCNQ2 and KCNQ3 mutations for benign familial neonatal convulsions show seizures and neuronal plasticity without synaptic reorganization.

The childhood epilepsy syndrome of benign familial neonatal convulsions (BFNC) exhibits the remarkable feature of clinical remission within a few weeks of onset and a favourable prognosis, sparing cognitive abilities despite persistent expression of the mutant KCNQ2 or KCNQ3 potassium channels throughout adulthood. To better understand such dynamic neuroprotective plasticity within the developing brain, we introduced missense mutations that underlie human BFNC into the orthologous murine Kcnq2 (Kv7.2) and Kcnq3 (Kv7.3) genes. Mutant mice were examined for altered thresholds to induced seizures, spontaneous seizure characteristics, hippocampal histology, and M-current properties of CA1 hippocampal pyramidal neurons. Adult Kcnq2(A306T/+) and Kcnq3(G311V/+) heterozygous knock-in mice exhibited reduced thresholds to electrically induced seizures compared to wild-type littermate mice. Both Kcnq2(A306T/A306T) and Kcnq3(G311V/G311V) homozygous mutant mice exhibited early onset spontaneous generalized tonic-clonic seizures concurrent with a significant reduction in amplitude and increased deactivation kinetics of the neuronal M-current. Mice had recurrent seizures into adulthood that triggered molecular plasticity including ectopic neuropeptide Y (NPY) expression in granule cells, but without hippocampal mossy fibre sprouting or neuronal loss. These novel knocking mice recapitulate proconvulsant features of the human disorder yet show that inherited M-current defects spare granule cells from reactive changes in adult hippocampal networks. The absence of seizure-induced pathology found in these epileptic mouse models parallels the benign neurodevelopmental cognitive profile exhibited by the majority of BFNC patients.

BK channel beta4 subunit reduces dentate gyrus excitability and protects against temporal lobe seizures.

Synaptic inhibition within the hippocampus dentate gyrus serves a 'low-pass filtering' function that protects against hyperexcitability that leads to temporal lobe seizures. Here we demonstrate that calcium-activated potassium (BK) channel accessory beta4 subunits serve as key regulators of intrinsic firing properties that contribute to the low-pass filtering function of dentate granule cells. Notably, a critical beta4 subunit function is to preclude BK channels from contributing to membrane repolarization and thereby broaden action potentials. Longer-duration action potentials secondarily recruit SK channels, leading to greater spike frequency adaptation and reduced firing rates. In contrast, granule cells from beta4 knockout mice show a gain-of-function for BK channels that sharpens action potentials and supports higher firing rates. Consistent with breakdown of the dentate filter, beta4 knockouts show distinctive seizures emanating from the temporal cortex, demonstrating a unique nonsynaptic mechanism for gate control of hippocampal synchronization leading to temporal lobe epilepsy.

Mice lacking Dlx1 show subtype-specific loss of interneurons, reduced inhibition and epilepsy.

Dlx homeodomain transcription factors are essential during embryonic development for the production of forebrain GABAergic interneurons. Here we show that Dlx1 is also required for regulating the functional longevity of cortical and hippocampal interneurons in the adult brain. We demonstrate preferential Dlx1 expression in a subset of cortical and hippocampal interneurons which, in postnatal Dlx1 mutants, show a time-dependent reduction in number. This reduction preferentially affects calretinin(+) (bipolar cells) and somatostatin(+) subtypes (for example, bitufted cells), whereas parvalbumin(+) subpopulations (basket cells and chandelier cells) seem to be unaffected. Cell transplantation analysis demonstrates that interneuron loss reflects cell-autonomous functions of Dlx1. The decrease in the number of interneurons was associated with a reduction of GABA-mediated inhibitory postsynaptic current in neocortex and hippocampus in vitro and cortical dysrhythmia in vivo. Dlx1 mutant mice show generalized electrographic seizures and histological evidence of seizure-induced reorganization, linking the Dlx1 mutation to delayed-onset epilepsy associated with interneuron loss.

Mice lacking sodium channel beta1 subunits display defects in neuronal excitability, sodium channel expression, and nodal architecture.

Sodium channel beta1 subunits modulate alpha subunit gating and cell surface expression and participate in cell adhesive interactions in vitro. beta1-/- mice appear ataxic and display spontaneous generalized seizures. In the optic nerve, the fastest components of the compound action potential are slowed and the number of mature nodes of Ranvier is reduced, but Na(v)1.6, contactin, caspr 1, and K(v)1 channels are all localized normally at nodes. At the ultrastructural level, the paranodal septate-like junctions immediately adjacent to the node are missing in a subset of axons, suggesting that beta1 may participate in axo-glial communication at the periphery of the nodal gap. Sodium currents in dissociated hippocampal neurons are normal, but Na(v)1.1 expression is reduced and Na(v)1.3 expression is increased in a subset of pyramidal neurons in the CA2/CA3 region, suggesting a basis for the epileptic phenotype. Our results show that beta1 subunits play important roles in the regulation of sodium channel density and localization, are involved in axo-glial communication at nodes of Ranvier, and are required for normal action potential conduction and control of excitability in vivo.

Cathepsin B but not cathepsins L or S contributes to the pathogenesis of Unverricht-Lundborg progressive myoclonus epilepsy (EPM1).

The inherited epilepsy Unverricht-Lundborg disease (EPM1) is caused by loss-of-function mutations in the cysteine protease inhibitor, cystatin B. Because cystatin B inhibits a class of lysosomal cysteine proteases called cathepsins, we hypothesized that increased proteolysis by one or more of these cathepsins is likely to be responsible for the seizure, ataxia, and neuronal apoptosis phenotypes characteristic of EPM1. To test this hypothesis and to identify which cysteine cathepsins contribute to EPM1, we have genetically removed three candidate cathepsins from cystatin B-deficient mice and tested for rescue of their EPM1 phenotypes. Whereas removal of cathepsins L or S from cystatin B-deficient mice did not ameliorate any aspect of the EPM1 phenotype, removal of cathepsin B resulted in a 36-89% reduction in the amount of cerebellar granule cell apoptosis depending on mouse age. The incidence of an incompletely penetrant eye phenotype was also reduced upon removal of cathepsin B. Because the apoptosis and eye phenotypes were not abolished completely and the ataxia and seizure phenotypes experienced by cystatin B-deficient animals were not diminished, this suggests that another molecule besides cathepsin B is also responsible for the pathogenesis, or that another molecule can partially compensate for cathepsin B function. These findings establish cathepsin B as a contributor to the apoptotic phenotype of cystatin B-deficient mice and humans with EPM1. They also suggest that the identification of cathepsin B substrates may further reveal the molecular basis for EPM1.