GluN2C selective inhibition is a target to develop new antiepileptic compounds

OBJECTIVE

Many early-onset epilepsies present as developmental and epileptic encephalopathy associated with refractory seizures, altered psychomotor development, and disorganized interictal cortical activity. Abnormal upregulation of specific N-methyl-d-aspartate receptor (NMDA-R) subunits is being disentangled as one of the mechanisms of severe early-onset epilepsies. In tuberous sclerosis complex (TSC), upregulation of the GluN2C subunit of the NMDA-R with slow deactivation kinetic results in increased neuronal excitation and synchronization.

METHODS

Starting from an available GluN2C/D antagonist, NMDA-R-modulating compounds were developed and screened using a patch clamp on neuronal culture to select those with the strongest inhibitory effect on glutamatergic NMDA currents. For these selected compounds, blood pharmacokinetics and passage through the blood-brain barrier were studied. We tested the effect of the most promising compounds on epileptic activity in Tsc1^+/- mice brain slices with multielectrode array, and then in vivo at postnatal ages P14-P17, comparable with the usual age at epilepsy onset in human TSC.

RESULTS

Using a double-electrode voltage clamp on isolated NMDA currents, we identified the most prominent antagonists of the GluN2C subunit with no effect on GluN2A as a means of preventing side effects. The best compound passing through the blood-brain barrier was selected. Applied in vivo in six Tsc1^+/- mice at P14-P17, this compound reduced or completely stopped spontaneous seizures in four of them, and decreased the background activity disorganization. Furthermore, ictal-like discharges stopped on a human brain sample from an infant with epilepsy due to TSC.

INTERPRETATION

Subunit-selective inhibition is a valuable target for developing drugs for severe epilepsies resulting from an upregulation of NMDA-R subunit-mediated transmission.

Effects of gap junction blockers on human neocortical synchronization

Field potentials and intracellular recordings were obtained from human neocortical slices to study the role of gap junctions (GJ) in neuronal network synchronization. First, we examined the effects of GJ blockers (i.e., carbenoxolone, octanol, quinine, and quinidine) on the spontaneous synchronous events (duration = 0.2-1.1 s; intervals of occurrence = 3-27 s) generated by neocortical slices obtained from temporal lobe epileptic patients during application of 4-aminopyridine (4AP, 50 muM) and glutamatergic receptor antagonists. The synchronicity of these potentials (recorded at distances up to 5 mm) was decreased by GJ blockers within 20 min of application, while prolonged GJ blockers treatment at higher doses made them disappear with different time courses. Second, we found that slices from patients with focal cortical dysplasia (FCD) could generate in normal medium spontaneous synchronous discharges (duration = 0.4-8 s; intervals of occurrence = 0.5-90 s) that were (i) abolished by NMDA receptor antagonists and (ii) slowed down by carbenoxolone. Finally, octanol or carbenoxolone blocked 4AP-induced ictal-like discharges (duration = up to 35 s) in FCD slices. These data indicate that GJ play a role in synchronizing human neocortical networks and may implement epileptiform activity in FCD.

Heterogeneous distribution of polyamines in temporal lobe epilepsy

Polyamine contents were determined in human temporal lobe epilepsy. In the seven patients studied, stereoelectroencephalography (SEEG) located the epileptogenic focus in Ammon's horn and neuropathological findings were limited to hippocampal gliosis and sclerosis. Each polyamine exhibited a specific regional distribution. The most important variations were observed for spermidine and spermine while putrescine levels varied less. The regional variation was predominant in middle > posterior > anterior parts of the temporal lobe. Spermine contents and the spermidine/spermine (SPD/SPM) index varied especially in the middle and posterior parts of the hippocampus. Metabolic SPD/SPM index and spermidine levels were found to be drastically increased in almost all limbic parts when compared to neocortical regions. The opposite was observed for spermine. The heterogeneous distribution of polyamines was compared to abnormal electrical activities recorded by SEEG: SPD/SPM index and spermidine levels were sharply increased in seizure onset areas and high levels of spermine were detected in temporal cortex propagation areas. The presently reported heterogeneity of polyamine contents might contribute to modulate differentially the local control of excitability in human temporal epilepsy.

[Intervention of GABAergic neurotransmission in partial epilepsies]

The gamma-aminobutyric acid (GABA) is one of the most important inhibitory transmitter in the CNS. When GABA is released in the synaptic cleft, it can act on two types of receptors, type A (GABAA-R) and type B. The GABAA-R is an ionotropic receptor whose subunits form a chloride channel. It contains specific binding sites at least for GABA, benzodiazepines, picrotoxin, barbiturates, anesthetic steroids, divalent cations such as Zn2+ and other compounds. Neurotransmitters and neuropeptides that regulate intracellular second messengers may modulate the responses of GABAA-R in the post-synaptic membrane and thus affect the synaptic plasticity. While consensus sites for several kinases are present on many subunit-subtypes, the functional consequences of these phosphorylations are unclear. However, the maintenance of normal GABA currents required the activity of a unique kinase specific for the GABAA-R. This intracellular regulation site might be involved in synaptic plasticity and considered as a site of vulnerability for epileptogenesis. The generation of epileptic discharge, synchronized burst firing and interictal spikes, can be subsequent to the alteration of GABAA-R function. A consequence of GABAergic disinhibition is the formation of new polysynaptic pathways leading to a network of neurons that were previously not connected. Cell loss and plasticity are currently observed in most patients with temporal lobe epilepsy. CA1 pyramidal cells are missing and mossy fibers of dentate granule cells project back through the granule cell layer to form recurrent terminals on granule cell dendrites. This mossy fiber sprouting leads to the destruction of most dentate hilar somatostatine interneurons. Nevertheless, local circuit neurons containing glutamic acid decarboxylase survive in this layer and in all regions of the sclerotic hippocampus. A decrease of the GABA release has been proposed as a basis for disinhibition temporal-lobe epilepsy is partially characterized by a loss of glutamate-stimulated GABA release that is secondary to a reduction in the number of GABA transporters. A molecular reorganization of GABAA-R subunits has been suggested in the kindling model of temporal lobe epilepsy because the zinc released from abberantly sprouted mossy fiber terminals is responsible for a collapse of augmented inhibition by GABA. These results support the concept of a loss of inhibition in chronic epilepsy models and probably in human epilepsies.

Astroglial cells express large amounts of GABAA receptor proteins in mature brain

GABAA receptors were characterized in cellular fractions isolated from adult bovine brain. The fraction enriched in cortical astrocytes is very rich in high-affinity binding sites for [3H]flunitrazepam and other "central-type" benzodiazepine ligands. The amount of specific [3H]flunitrazepam binding was more than five times higher in the glial fraction than in synaptosomal and perikaryal fractions. [3H]Flunitrazepam was displaced by low concentrations of clonazepam and other specific ligands for central GABAA receptors. Specific binding sites for GABA, flunitrazepam, barbiturates, and picrotoxin-like convulsants were characterized. Allosteric interactions between the different sites were typical of central-type GABAA receptors. The presence of alpha-subunit(s), as revealed by [3H]flunitrazepam photoaffinity labeling, was demonstrated in all brain fractions at molecular mas 51-53 kDa. Photoaffinity labeling was highest in the glial fraction. However, in primary cultured astrocytes from neonate rat cortex, no photoaffinity labeling was detected. Information obtained from astrocytes in culture should thus be taken with caution when extrapolated to differentiated astroglial cells. Our results actually show that, in mature brain, most of the fully pharmacologically active GABAA receptors are extrasynaptic and expressed in astroglia.

Thiamine deficiency in cultured neuroblastoma cells: effect on mitochondrial function and peripheral benzodiazepine receptors

When neuroblastoma cells were transferred to a medium of low (6 nM) thiamine concentration, a 16-fold decrease in total intracellular thiamine content occurred within 8 days. Respiration and ATP levels were only slightly affected, but addition of a thiamine transport inhibitor (amprolium) decreased ATP content and increased lactate production. Oxygen consumption became low and insensitive to oligomycin and uncouplers. At least 25% of mitochondria were swollen and electron translucent. Cell mortality increased to 75% within 5 days. [3H]PK 11195, a specific ligand of peripheral benzodiazepine receptors (located in the outer mitochondrial membrane) binds to the cells with high affinity (KD = 1.4 +/- 0.2 nM). Thiamine deficiency leads to an increase in both Bmax and KD. Changes in binding parameters for peripheral benzodiazepine receptors may be related to structural or permeability changes in mitochondrial outer membranes. In addition to the high-affinity (nanomolar range) binding site for peripheral benzodiazepine ligands, there is a low-affinity (micromolar range) saturable binding for PK 11195. At micromolar concentrations, peripheral benzodiazepines inhibit thiamine uptake by the cells. Altogether, our results suggest that impairment of oxidative metabolism, followed by mitochondrial swelling and disorganization of cristae, is the main cause of cell mortality in severely thiamine-deficient neuroblastoma cells.

[Comparative biochemistry of the triune brain]

The contribution of the phylogenesis to the study of the functional organisation of brain constitutes an essential component in the integration of the current data regarding the knowledge of the central nervous system. The three evolutive structures evidenced by McLean allowed manifestation of a biological diversity particularly expressed in the neommamalian brain. Studying this biological diversity constitutes one of the field of comparative biochemistry. In this article, the authors attempt to characterize evolution through the study of GABA-receptors.

Polyamine metabolism in epileptic cortex

Polyamine (tissue) concentrations have been studied in hippocampus and temporal neocortex from patients with temporal lobe epilepsy. Depth electrode recordings demonstrated hippocampal origin of the seizures, the temporal neocortex being involved during the discharge propagation. Neuropathological examination of excised tissues showed glial proliferation or glioma in Ammon's horn (CA), whereas the temporal neocortex did not exhibit any histological abnormality. Polyamine (putrescine or PUT, spermidine or SPD, spermine or SPM) concentrations were determined on surgical samples from the hippocampus and various areas of temporal neocortex. Human post-mortem tissue from temporal lobe regions was used for controls. In post-mortem controls and temporal neocortex specimens from epileptic patients, polyamine levels were similar (in nmol/g wet weight: PUT = 40-100; SPD = 200-350; SPM = 100-200). In CA, polyamine levels exhibited striking changes: SPD content was significantly increased (350-700 nmol/g) while SPM was lowered (50-100). PUT was only increased in CA invaded by the tumoral process (100-180). Accordingly, a very high SPD/SPM molar ratio in the abnormal CA region was observed, indicating an acceleration of polyamine neosynthesis which is usually related to ornithine decarboxylase induction. Metabolic changes in polyamines appear to be selective of human epileptic hippocampus. A relationship between glial proliferation (gliosis or neoplasia), epileptic firing and polyamines is discussed.

Characteristics of putrescine uptake and subsequent GABA formation in primary cultured astrocytes from normal C57BL/6J and epileptic DBA/2J mouse brain cortices

Brain maturation and GABA metabolism are known to play a key role in epileptogenesis. The metabolism of the polyamines (putrescine, spermidine and spermine) is closely linked to the process of brain maturation. Putrescine has been shown to be catabolized to GABA in brain tissue and astrocytes. In order to better understand the importance of glial putrescine transport and metabolism, a model of age-dependent epilepsy was used to study the kinetic properties of [14C]putrescine uptake into cultured astrocytes from normal C57/BL and audiogenic DBA/2 newborn mice, and the subsequent GABA formation. (1) Putrescine uptake exhibited non-Michaelian allosteric kinetics with positive co-operativity (Hill factor = 2), suggesting a physiological importance of putrescine uptake by astrocytes. (2) The Vmax of putrescine uptake was significantly higher in C57/BL astrocytes than in DBA/2J, but the uptake affinity for putrescine was higher in DBA/2J than in C57/BL. (3) Higher K+ concentrations (18 mM) had little effect on putrescine uptake in either strain. (4) Ten-micromolar N-acetylputrescine, the first putrescine metabolite, stimulated putrescine uptake into astrocytes of both strains, but to a different degree: +46% in C57/BL and + 102% in DBA/2J. (5) The specific radioactivity of the GABA formed from labelled putrescine was four times higher in astrocytes from DBA/2J than from C57/BL mice. (6) The molar ratio of glutamate/GABA in the cerebral cortex of the DBA/2J mice was significantly higher during the period of audiogenic seizure susceptibility than in age-matched C57/BL mice. Our results show characteristics of putrescine uptake into astrocytes; we demonstrated distinct kinetic properties between normal and epileptic strains of mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of brain (Na+,K+)-ATPase alpha catalytic subunits in normal and epileptic cerebral cortex: I. The audiogenic mice and the cat with a freeze lesion

Partially purified (Na+,K+)-ATPase (E.C. 3.6.1.3.) was investigated in the epileptic cortex of audiogenic DBA/2 mice and in the primary and secondary foci of cats with acute or chronic freeze lesions. No differences in specific activities measured at 3 mM K+ were observed between epileptic and control cortex, except an increase of enzymic activities in the primary focus of acutely lesioned cats. The (Na+,K+)-ATPase catalytic subunits were resolved by SDS-gel electrophoresis and their phosphorylation levels were measured in presence of K+ ions and phenytoin. K+ was more effective in inducing maximal dephosphorylation of (Na+,K+)-ATPase in C57/BL, with identical affinity in the two strains. Phenytoin decreased the net phosphorylation level of (Na+,K+)-ATPase by about 50% in C57/BL mice, but only by 20% in DBA/2 mice. Both K+ and phenytoin dephosphorylating influences were decreased in primary and secondary foci of acutely lesioned cats. Those changes were limited to the alpha(-) subunit. In chronic cats, the dephosphorylating step of the (Na+,K+)-ATPase catalytic subunit recovered a normal affinity to K+, but its sensitivity to phenytoin remained decreased. Those differences in K+ and phenytoin influences on brain (Na+,K+)-ATPases between control and epileptic cortex might be responsible for the ictal transformation and seizure spread. In cats, the alteration of the alpha(-) isoform could mainly affect the glial cells.

Phosphorylation of brain (Na+,K+)-ATPase alpha catalytic subunits in normal and epileptic cerebral cortex: II. Partial seizures in human epilepsy

We examined the activity and phosphorylation level of (Na+,K+)-ATPase (E.C. 3.6.1.3) partially purified from normal and epileptic human cortices. Control patients (n = 11) were operated on for a non-epileptogenic deep brain lesion, while epileptic patients (n = 10) were operated on for temporal or frontal originating partial seizures, resistant to medications or secondary to evolutive brain tumors. No differences in the specific activity of microsomal (Na+,K+)-ATPase were observed between the two groups of patients. After partial purification of the enzyme followed by SDS-polyacrylamide gel electrophoresis, (Na+,K+)-ATPase catalytic subunit had a decreased affinity for K+ in human epileptic cortex and lost its sensitivity to phenytoin dephosphorylation. Indirect evidence suggests that those abnormalities of (Na+,K+)-ATPase in human epileptic cortex hold preferentially true for the alpha(-) enzymatic subunit. Those results indicate that, in human epileptic cortex, (Na+,K+)-ATPase and most probably its glial subtype is altered in its K+ regulation and phenytoin sensitivity and could be responsible for ictal transformation and seizure spread.

Effect of milacemide on audiogenic seizures and cortical (Na+, K+)-ATPase of DBA/2J mice

Milacemide (MLM, CP 1552 S, 2-N-pentylaminoacetamide), a glycinamide derivative, is currently being evaluated clinically for antiepileptic activity. Anticonvulsant properties have been shown in various animal models, but the mechanism of action of MLM is unclear. We studied its activity in audiogenic seizures of DBA/2J mice. MLM was effective in inhibiting the convulsions induced by sound with a biphasic dose-effect relation. The ED50 was 109 mg/kg orally against tonic extension. Higher doses were necessary to abolish clonic convulsion and running response. Because impaired cerebral (Na+, K+)-ATPase activity is supposed to play a role in epileptogenesis, we tested MLM on in vitro cortical enzymatic activity of DBA/2J mice. Basal (Na+, K+)-ATPase activity was unchanged by several concentrations of MLM in normal C57BL/6J and audiogenic DBA/2J mice. K+ activation (from 3 to 18 mM) of (Na+, K+)-ATPase is abolished in DBA/2J mice as compared with C57BL/6J mice, suggesting impaired glial (Na+, K+)-ATPase. In the presence of MLM (from 30 to 1000 mg/L), cortical (Na+, K+)-ATPase of DBA/2J mice is activated by high concentrations of K+, as in C57BL/6J mice. Results suggest that the antiepileptic activity of MLM in audiogenic mice may be secondary to an activation of a deficient glial (Na+, K+)-ATPase.

Milacemide stimulates deficient glial Na+, K(+)-ATPase in freezing-induced epileptogenic cortex of cats

We investigated the influence of milacemide, a glycinamide derivative with putative antiepileptic activity, on the K(+)-activation of Na+,K(+)-ATPase in bulk isolated glial cells and synaptosomes of control and epileptogenic cortex of cats with a chronic freeze lesion. In the primary and secondary epileptic foci of non-treated animals, glial Na+,K(+)-ATPase lost its physiological K(+)-activation, while the synaptosomal enzyme was unchanged. These data reproduced previous work done on the kinetic measurement of the enzymic activities. In treated animals (500 mg/kg milacemide given orally for 2 weeks after the freeze lesion), the glial enzyme showed a normal K(+)-activation in the epileptic foci. These results confirm the existence of an abnormal glial Na+,K(+)-ATPase in cold-induced focal epilepsy and suggest that the antiepileptic activity of milacemide might be secondary to an activation of glial Na+,K(+)-ATPase, contributing to antagonize ictal transformation and seizure spread.

Two isoenzymes of Na+,K+-ATPase have different kinetics of K+ dephosphorylation in normal cat and human brain cortex

Analysis of purified Na+,K+-ATPase from cat and human cortex by sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals two large catalytic subunits called alpha (-) (lower molecular weight) and alpha (+) (higher molecular weight). Differences in K+ dephosphorylation of these two molecular forms have been investigated by measuring the phosphorylation level of each protein after their separation on sodium dodecyl sulfate gels. In the presence of Na+, Mg2+, and ATP, both subunits are phosphorylated. Increasing concentrations (from 0 to 3 mM) of K+ induce progressive dephosphorylation of both alpha-subunits, although the phosphoprotein content of alpha (-) is decreased significantly less than that of alpha (+). Ka values of alpha (-) for K+ are 40% and 50% greater in cat and human cortex, respectively, than values of alpha (+). alpha (-) and alpha (+) are thought to be localized in specific cell types of the brain: alpha (-) is the exclusive form of nonneuronal cells (astrocytes), whereas alpha (+) is the only form of axolemma. Our results support the hypothesis that glial and neuronal Na+,K+-ATPases are different molecular entities differing at least by their K+ sensitivity. Results are discussed in relation to the role of glial cells in the regulation of extracellular K+ in brain.

Analysis of dansyl derivatives of di- and polyamines in mouse brain, human serum and duodenal biopsy specimens by high-performance liquid chromatography on a standard reversed-phase column

The concentrations of putrescine, spermine and spermidine were measured in human serum, children's duodenal biopsy specimens and mouse brain homogenates by high-performance liquid chromatography. The chromatographic analysis was performed on dansyl derivatives of the polyamines using a reverse-phase system with an ion-pairing retention mechanism (heptane sulphonate). Capacity factors were determined at different concentrations of acetonitrile. Simple linear gradients were set up for fast (15 min) or routine (25 min) analysis. Three fluorescence detectors were compared for these determinations and their detection limits determined. The minimum detectable amount of polyamines was 25 fmol compared to 500 fmol with standard detectors. While samples prepared from tissues did not require a high sensitivity, a detector of better performance was needed to assay the polyamines in human serum.