Benign familial neonatal epilepsy with mutations in two potassium channel genes

Epilepsy as a neurodevelopmental disorder

Epilepsy is characterized by spontaneous recurrent seizures and comprises a diverse group of syndromes with different etiologies. Epileptogenesis refers to the process whereby the brain becomes epileptic and can be related to several factors, such as acquired structural brain lesions, inborn brain malformations, alterations in neuronal signaling, and defects in maturation and plasticity of neuronal networks. In this review, we will focus on alterations of brain development that lead to an hyperexcitability phenotype in adulthood, providing examples from both animal and human studies. Malformations of cortical development (including focal cortical dysplasia, lissencephaly, heterotopia, and polymicrogyria) are frequently epileptogenic and result from defects in cell proliferation in the germinal zone and/or impaired neuronal migration and differentiation. Delayed or reduced arrival of inhibitory interneurons into the cortical plate is another possible cause of epileptogenesis. GABAergic neurons are generated during early development in the ganglionic eminences, and failure to pursue migration toward the cortex alters the excitatory/inhibitory balance resulting in aberrant network hyperexcitability. More subtle defects in the developmental assembly of excitatory and inhibitory synapses are also involved in epilepsy. For example, mutations in the presynaptic proteins synapsins and SNAP-25 cause derangements of synaptic transmission and plasticity which underlie appearance of an epileptic phenotype. Finally, there is evidence that defects in synapse elimination and remodeling during early "critical periods" can trigger hyperexcitability later in life. Further clarification of the developmental pathways to epilepsy has important implications for disease prevention and therapy.

Inhibitory effect of tungsten carbide nanoparticles on voltage-gated potassium currents of hippocampal CA1 neurons

The effects of tungsten carbide nanoparticles (nano-WC) on the properties of voltage-dependent potassium currents and evoked action potentials were studied in the hippocampal CA1 pyramidal neurons of rats at the ages of postnatal days 10-14 using the whole-cell patch-clamp technique. The results indicated that: (1) the amplitudes of transient outward potassium current (I(A)) and delayed rectifier potassium current (I(K)) were significantly decreased by 10(-7) g/ml nano-WC, while the current-voltage curves of I(A) and I(K) were significantly decreased by nano-WC from +10 to +90 mV. (2) Nano-WC produced a depolarizing shift in the steady-state activation curve of I(A) and I(K) with increased slope factors, and delayed the recovery of I(A) from inactivation, but no significant effects were found on the inactivation of I(A). (3) Nano-WC prolonged the evoked action potential duration and lowered the firing rate. These results suggest that 10(-7) g/ml nano-WC can decrease the amplitudes of I(A) and I(K) currents by reducing the opening number of voltage-gated potassium channels and delaying the recovery of I(A) from inactivation, which indicate that nano-WC has the potential neurotoxicity.

Channeling studies in yeast: yeast as a model for channelopathies?

Regulation of the concentration of ions within a cell is mediated by their specific transport and sequestration across cellular membranes. This regulation constitutes a major factor in the maintenance of correct cellular homeostasis, with the transport occurring through the action of a large number of different channel proteins localized to the plasma membrane as well as to various organelles. These ion channels vary in specificity from broad (cationic vs anionic) to highly selective (chloride vs sodium). Mutations in many of these channels result in a large number of human diseases, collectively termed channelopathies. Characterization of many of these channels has been undertaken in a variety of both prokaryotic and eukaryotic organisms. Among these organisms is the budding yeast Saccharomyces cerevisiae. Possessing a fully annotated genome, S. cerevisiae would appear to be an ideal organism in which to study this class of proteins associated to diseases. We have compiled and reviewed a list of yeast ion channels, each possessing a human homolog implicated in a channelopathy. Although yeast has been used for the study of other human disease, it has been under utilized for channelopathy research. The utility of using yeast as a model system for studying ion channels associated to human disease is illustrated using yeast lacking the GEF1 gene product that encodes the human homolog to the chloride channel CLC-3.

Epileptiform activity induced by pharmacologic reduction of M-current in the developing hippocampus in vitro

PURPOSE

Benign familial neonatal convulsions (BFNCs), an inheritable epilepsy that occurs in neonates but not in adults, is caused by hypofunctional mutations in genes codifying for the M-type K+ current. In an attempt to develop an in vitro model of this disease, we tested whether blocking M-current with linopirdine induces epileptiform activity in brain slices from animals of different ages.

METHODS

Horizontal hippocampus-entorhinal cortex slices were obtained from neonatal (1-2 weeks after birth) and adult (8-9 weeks after birth) rats. Extracellular field recordings of the CA1 region were performed. After recording control conditions, linopirdine was added to the bath, and field activity was recorded continuously for 3 h. 4-Aminopyridine, a drug commonly used to induce epileptiform activity in vitro, was used as a control for our experimental conditions.

RESULTS

Bath perfusion of linopirdine induced epileptiform activity only in slices from neonatal rats. Epileptiform activity consisted of interictal-like and ictal-like activity. In slices from adult rats, linopirdine induced erratic interictal-like activity. In contrast, 4-aminopyridine was able to induce epileptiform activity in slices from both neonatal and adult rats.

CONCLUSIONS

We demonstrated that blockade of M-current in vitro produces epileptiform activity with a developmental pattern similar to that observed in BNFCs. This could be an in vitro model that can be used to study the cellular mechanisms of epileptogenesis and the developmental features of BFNCs, as well as to develop some therapeutic strategies.

Effects of retigabine on the neurodegeneration and extracellular glutamate changes induced by 4-aminopyridine in rat hippocampus in vivo

We have previously shown that microdialysis perfusion of the K+ channel blocker 4-aminopyridine (4-AP) in rat hippocampus induces convulsions and neurodegeneration, due to the stimulation of glutamate release from synaptic terminals. Retigabine is an opener of the KCNQ2/Q3-type K+ channel that possesses antiepileptic action and may be neuroprotective, and we have therefore studied its effect on the hyperexcitation, the neuronal damage and the changes in extracellular glutamate induced by 4-AP. Retigabine and 4-AP were co-administered by microdialysis in the hippocampus of anesthetized rats, with simultaneous recording of the EEG, and the extracellular concentration of glutamate was measured in the microdialysis fractions. In 70-80% of the rats tested retigabine reduced the 4-AP-induced stimulation of glutamate release and prevented the neuronal damage observed at 24 h in the CA1 hippocampal region. However, retigabine did not block the EEG epileptic discharges and their duration was reduced in only 20-25% of the tested animals. We conclude that the neuroprotective action of retigabine is probably due to the blockade of the 4-AP-induced stimulation of glutamate release. This inhibition, however, was not sufficient to block the epileptic activity.

Structure and function of Kv4-family transient potassium channels

Shal-type (Kv4.x) K(+) channels are expressed in a variety of tissue, with particularly high levels in the brain and heart. These channels are the primary subunits that contribute to transient, voltage-dependent K(+) currents in the nervous system (A currents) and the heart (transient outward current). Recent studies have revealed an enormous degree of complexity in the regulation of these channels. In this review, we describe the surprisingly large number of ancillary subunits and scaffolding proteins that can interact with the primary subunits, resulting in alterations in channel trafficking and kinetic properties. Furthermore, we discuss posttranslational modification of Kv4.x channel function with an emphasis on the role of kinase modulation of these channels in regulating membrane properties. This concept is especially intriguing as Kv4.2 channels may integrate a variety of intracellular signaling cascades into a coordinated output that dynamically modulates membrane excitability. Finally, the pathophysiology that may arise from dysregulation of these channels is also reviewed.

Clues to the genetic influences of drug responsiveness in epilepsy

Despite the introduction of newer antiepileptic drugs (AEDs), nonresponsiveness to AEDs remains a common problem in epilepsy clinics. There may be important genetic determinants for responsiveness, and this discussion focused on some potential areas: drug transporters; drug-metabolizing enzymes, and ion channels. We review the literature and speculate the contribution of each of these factors in management of patients with epilepsy in the future.

The biology of epilepsy genes

Mutations in over 70 genes now define biological pathways leading to epilepsy, an episodic dysrhythmia of the cerebral cortex marked by abnormal network synchronization. Some of the inherited errors destabilize neuronal signaling by inflicting primary disorders of membrane excitability and synaptic transmission, whereas others do so indirectly by perturbing critical control points that balance the developmental assembly of inhibitory and excitatory circuits. The genetic diversity is now sufficient to discern short- and long-range functional convergence of epileptogenic molecular pathways, reducing the broad spectrum of primary molecular defects to a few common processes regulating cortical synchronization. Synaptic inhibition appears to be the most frequent target; however, each gene mutation retains unique phenotypic features. This review selects exemplary members of several gene families to illustrate principal categories of the disease and trace the biological pathways to epileptogenesis in the developing brain.

Genetic and environmental factors in epilepsy: a population-based study of 11900 Danish twin pairs

The contribution of genetic and environmental factors to the occurrence of epilepsy was examined in an unselected sample of twins recruited from the population-based Danish Twin Registry. Information on the occurrence of epilepsy in both members of a twin pair was obtained from 11900 pairs whose ages ranged from 12 to 41 years. Concordance rates, odds ratios and tetrachoric correlations were used to quantify the similarity of monozygotic (MZ) and dizygotic (DZ) twins. The sample was stratified by sex and separated into two age cohorts for analysis. Significantly higher probandwise concordance rates were found for MZ compared with DZ twins (0.37 and 0.08, P<0.01). Odds ratios and tetrachoric correlation showed similar pattern. An etiological model including additive genetic and individual specific environmental factors provided the best overall fit to the data, with 70 and 88% of the liability to develop epilepsy being accounted for by genetic factors in the younger and older cohorts, respectively. Individual specific environmental factors explained the remaining 30 and 12%, respectively. In conclusion, this study has confirmed the substantial impact, which genetic factors have in the etiology of epilepsy. The heritability of epilepsy is high and seems to increase with age.