Ankyrin-G isoform imbalance and interneuronopathy link epilepsy and bipolar disorder.

ANK3, encoding the adaptor protein Ankyrin-G (AnkG), has been implicated in bipolar disorder by genome-wide association studies. ANK3 has multiple alternative first exons, and a bipolar disorder-associated ANK3 variant has been shown to reduce the expression of exon 1b. Here we identify mechanisms through which reduced ANK3 exon 1b isoform expression disrupts neuronal excitation-inhibition balance. We find that parvalbumin (PV) interneurons and principal cells differentially express ANK3 first exon subtypes. PV interneurons express only isoforms containing exon 1b, whereas excitatory principal cells express exon 1e alone or both 1e and 1b. In transgenic mice deficient for exon 1b, PV interneurons lack voltage-gated sodium channels at their axonal initial segments and have increased firing thresholds and diminished action potential dynamic range. These mice exhibit an Ank3 gene dosage-dependent phenotype including behavior changes modeling bipolar disorder, epilepsy and sudden death. Thus ANK3's important association with human bipolar susceptibility may arise from imbalance between AnkG function in interneurons and principal cells and resultant excessive circuit sensitivity and output. AnkG isoform imbalance is a novel molecular endophenotype and potential therapeutic target.

Progressive ataxia, myoclonic epilepsy and cerebellar apoptosis in cystatin B-deficient mice.

Loss-of-function mutations in the gene (CSTB) encoding human cystatin B, a widely expressed cysteine protease inhibitor, are responsible for a severe neurological disorder known as Unverricht-Lundborg disease (EPM1). The primary cellular events and mechanisms underlying the disease are unknown. We found that mice lacking cystatin B develop myoclonic seizures and ataxia, similar to symptoms seen in the human disease. The principal cytopathology appears to be a loss of cerebellar granule cells, which frequently display condensed nuclei, fragmented DNA and other cellular changes characteristic of apoptosis. This mouse model of EPM1 provides evidence that cystatin B, a non-caspase cysteine protease inhibitor, has a role in preventing cerebellar apoptosis.

Large expansion of the ATTCT pentanucleotide repeat in spinocerebellar ataxia type 10.

Spinocerebellar ataxia type 10 (SCA10; MIM 603516; refs 1,2) is an autosomal dominant disorder characterized by cerebellar ataxia and seizures. The gene SCA10 maps to a 3.8-cM interval on human chromosome 22q13-qter (refs 1,2). Because several other SCA subtypes show trinucleotide repeat expansions, we examined microsatellites in this region. We found an expansion of a pentanucleotide (ATTCT) repeat in intron 9 of SCA10 in all patients in five Mexican SCA10 families. There was an inverse correlation between the expansion size, up to 22.5 kb larger than the normal allele, and the age of onset (r2=0.34, P=0.018). Analysis of 562 chromosomes from unaffected individuals of various ethnic origins (including 242 chromosomes from Mexican persons) showed a range of 10 to 22 ATTCT repeats with no evidence of expansions. Our data indicate that the new SCA10 intronic ATTCT pentanucleotide repeat in SCA10 patients is unstable and represents the largest microsatellite expansion found so far in the human genome.

Inherited epilepsy: spike-wave and focal motor seizures in the mutant mouse tottering.

Mice with the mutant gene tottering (tg, chromosome 8, autosomal recessive) show, in adolescence, abnormal bursts of bilaterally synchronous spike waves as revealed in electrocorticograms recorded over long periods. The spike waves are accompanied by behavioral "absence" attacks and intermittent focal motor seizures showing somatotopic progression. Cerebral metabolic activity during seizures was assayed by autoradiography of brain sections from mice injected intravenously with 14C-labeled 2-deoxyglucose. Metabolic activity was increased bilaterally in selected brainstem structures. Spontaneous electrocorticographic and clinical seizures of this general pattern were recognized hitherto only in humans.

Hypomyelination alters K+ channel expression in mouse mutants shiverer and Trembler.

Voltage-gated K+ channels are localized to juxtaparanodal regions of myelinated axons. To begin to understand the role of normal compact myelin in this localization, we examined mKv1.1 and mKv1.2 expression in the dysmyelinating mouse mutants shiverer and Trembler. In neonatal wild-type and shiverer mice, the focal localization of both proteins in axon fiber tracts is similar, suggesting that cues other than mature myelin can direct initial K+ channel localization in shiverer mutants. In contrast, K+ channel localization is altered in hypomyelinated axonal fiber tracts of adult mutants, suggesting that abnormal myelination leads to channel redistribution. In shiverer adult, K+ channel expression is up-regulated in both axons and glia, as revealed by immunocytochemistry, RNase protection, and in situ hybridization studies. This up-regulation of K+ channels in hypomyelinated axon tracts may reflect a compensatory reorganization of ionic currents, allowing impulse conduction to occur in these dysmyelinating mouse mutants.

Mutation in AP-3 delta in the mocha mouse links endosomal transport to storage deficiency in platelets, melanosomes, and synaptic vesicles.

The mouse mutant mocha, a model for the Hermansky-Pudlak storage pool deficiency syndrome, is characterized by defective platelets, coat and eye color dilution, lysosomal abnormalities, inner ear degeneration, and neurological deficits. Here, we show that mocha is a null allele of the delta subunit of the adaptor-like protein complex AP-3, which is associated with coated vesicles budding from the trans-Golgi network, and that AP-3 is missing in mocha tissues. In mocha brain, the ZnT-3 transporter is reduced, resulting in a lack of zinc-associated Timm historeactivity in hippocampal mossy fibers. Our results demonstrate that the AP-3 complex is responsible for cargo selection to lysosome-related organelles such as melanosomes and platelet dense granules as well as to neurotransmitter vesicles.

Mutation of the Angelman ubiquitin ligase in mice causes increased cytoplasmic p53 and deficits of contextual learning and long-term potentiation.

The E6-AP ubiquitin ligase (human/mouse gene UBE3A/Ube3a) promotes the degradation of p53 in association with papilloma E6 protein, and maternal deficiency causes human Angelman syndrome (AS). Ube3a is imprinted with silencing of the paternal allele in hippocampus and cerebellum in mice. We found that the phenotype of mice with maternal deficiency (m-/p+) for Ube3a resembles human AS with motor dysfunction, inducible seizures, and a context-dependent learning deficit. Long-term potentiation (LTP) was severely impaired in m-/p+ mice despite normal baseline synaptic transmission and neuroanatomy, indicating that ubiquitination may play a role in mammalian LTP and that LTP may be abnormal in AS. The cytoplasmic abundance of p53 was increased in postmitotic neurons in m-/p+ mice and in AS, providing a potential biochemical basis for the phenotype through failure to ubiquitinate and degrade various effectors.

Ion channels and epilepsy in man and mouse.

Inherited disorders of voltage-gated ion channels are a recently recognized etiology of epilepsy in the developing and mature central nervous system. Two human epilepsy syndromes, benign familial neonatal convulsions and generalized epilepsy with febrile seizures plus, represent K+ and Na+ channelopathies, and other newly defined syndromes have now been mapped to chromosomal regions that are rich in ion channel genes. Experimental mouse models promise a resolution of their intriguing pathophysiology, which includes a diverse array of cellular phenotypes consistent with the differential contributions of individual channels to excitability in neural networks.

Persistent hypersynchronization of neocortical neurons in the mocha mutant of mouse.

A recessive mutation in the mouse at the mocha locus (mh, chromosome 10) modulates the synchronous synaptic activation of neocortical neurons, resulting in a constant 6-7 Hz (theta) wave pattern in the electrocorticogram. The gene-linked brain rhythm is unaffected by motor behavior and cannot be desynchronized by sensory stimuli. This exemplary neurological mutation affecting cortical excitability is the first to reveal clearly that the predominance of a specific pattern of spontaneous brain wave activity can be inherited as a recessive trait.

Presynaptic Ca2+ channels and neurotransmitter release at the terminal of a mouse cortical neuron.

Regional variation in synaptic efficacy is an important determinant of associative processing as information flows through major circuits of the brain. The perforant path is the principal route of entry from cortex to the hippocampus and contains the first synapse in the cortical-hippocampal projection pathway. We used optical imaging techniques to analyze presynaptic Ca(2+) entry and neurotransmitter release at synapses in the medial perforant path linking stellate neurons located in layer II of the entorhinal cortex to granule cells in the dentate gyrus. Similar to other excitatory central synapses, the relationship between neurotransmitter release and the amount of Ca(2+) influx can be best described by a Hill equation with a Hill coefficient of 3.5. Our Ca(2+) channel toxin studies indicate that P/Q-type channels are the predominant Ca(2+) source triggering neurotransmitter release in this pathway, as shown by a potent inhibition of Ca(2+) entry and synaptic transmission by the P/Q-type channel blocker omega-agatoxin IVA. However, compared with the downstream hippocampal pyramidal neuron CA3-CA1 synapse, neurotransmitter release was less sensitive to the N-type Ca(2+) channel blocker omega-conotoxin GVIA, although the amount of N-type Ca(2+) current is comparable. The contribution of N-type channels to neurotransmitter release approximates that found at the CA3-CA1 synapse when tested under lower [Ca(2+)](o), which effectively reduces the size of the Ca(2+) microdomain surrounding each channel. These results suggest that P/Q-type channels are more closely associated with release machinery then N-type channels at this synapse and that cooperativity differences for each channel subtype may characterize variations in signaling at central synapses.

Presynaptic Ca(2+) influx at a mouse central synapse with Ca(2+) channel subunit mutations.

Genetic alterations in Ca(2+) channel subunits can be used to study the interaction among channel subunits and their roles in channel function. P/Q- and N-type Ca(2+) channels reside at the presynaptic terminal and control the release of neurotransmitter at mammalian central synapses. We used fluorescence imaging techniques to investigate presynaptic Ca(2+) currents and neurotransmitter release at hippocampal Schaffer collateral synapses in both tottering (tg, alpha(1A) subunit) and lethargic (lh, beta(4) subunit) mutant mice. Application of selective toxins revealed a large reduction in presynaptic P/Q-type Ca(2+) transients, from 39% of total in +/+ mice to 6% in tg/tg mice, whereas the proportion of N-type increased from 35 to 68%, respectively. Neurotransmitter release in the tg/tg mutant relied almost exclusively on N-type channels, as shown by the complete blockade of synaptic transmission with omega-conotoxin GVIA. Remarkably, loss of beta4, a subunit predicted to regulate the subcellular targeting and modulation of both P/Q- and N-type channels, resulted in no significant difference in the ratio of Ca(2+) channel subtypes or Ca(2+) dependence of neurotransmitter release in lethargic mice. G-protein-mediated inhibition of Ca(2+) channels was also unaltered. These results indicate that a profound decrease in presynaptic P/Q-type currents leads to dependence of neurotransmitter release on N-type channels. In contrast, absence of beta(4) appears not to compromise either P/Q- or N-type channel function at this hippocampal synapse, implicating rescue of presynaptic Ca(2+) currents by other available beta subunits. The present study reveals compensatory molecular mechanisms in the regulation of presynaptic Ca(2+) entry and neurotransmitter release.

Rocker is a new variant of the voltage-dependent calcium channel gene Cacna1a.

Rocker (gene symbol rkr), a new neurological mutant phenotype, was found in descendents of a chemically mutagenized male mouse. Mutant mice display an ataxic, unstable gait accompanied by an intention tremor, typical of cerebellar dysfunction. These mice are fertile and appear to have a normal life span. Segregation analysis reveals rocker to be an autosomal recessive trait. The overall cytoarchitecture of the young adult brain appears normal, including its gross cerebellar morphology. Golgi-Cox staining, however, reveals dendritic abnormalities in the mature cerebellar cortex characterized by a reduction of branching in the Purkinje cell dendritic arbor and a "weeping willow" appearance of the secondary branches. Using simple sequence length polymorphism markers, the rocker locus was mapped to mouse chromosome 8 within 2 centimorgans of the calcium channel alpha1a subunit (Cacna1a, formerly known as tottering) locus. Complementation tests with the leaner mutant allele (Cacna1a(la)) produced mutant animals, thus identifying rocker as a new allele of Cacna1a (Cacna1a(rkr)). Sequence analysis of the cDNA revealed rocker to be a point mutation resulting in an amino acid exchange: T1310K between transmembrane regions 5 and 6 in the third homologous domain. Important distinctions between rocker and the previously characterized alleles of this locus include the absence of aberrant tyrosine hydroxylase expression in Purkinje cells and the separation of the absence seizures (spike/wave type discharges) from the paroxysmal dyskinesia phenotype. Overall these findings point to an important dissociation between the seizure phenotypes and the abnormalities in catecholamine metabolism, and they emphasize the value of allelic series in the study of gene function.

Impaired fast-spiking, suppressed cortical inhibition, and increased susceptibility to seizures in mice lacking Kv3.2 K+ channel proteins.

Voltage-gated K(+) channels of the Kv3 subfamily have unusual electrophysiological properties, including activation at very depolarized voltages (positive to -10 mV) and very fast deactivation rates, suggesting special roles in neuronal excitability. In the brain, Kv3 channels are prominently expressed in select neuronal populations, which include fast-spiking (FS) GABAergic interneurons of the neocortex, hippocampus, and caudate, as well as other high-frequency firing neurons. Although evidence points to a key role in high-frequency firing, a definitive understanding of the function of these channels has been hampered by a lack of selective pharmacological tools. We therefore generated mouse lines in which one of the Kv3 genes, Kv3.2, was disrupted by gene-targeting methods. Whole-cell electrophysiological recording showed that the ability to fire spikes at high frequencies was impaired in immunocytochemically identified FS interneurons of deep cortical layers (5-6) in which Kv3.2 proteins are normally prominent. No such impairment was found for FS neurons of superficial layers (2-4) in which Kv3.2 proteins are normally only weakly expressed. These data directly support the hypothesis that Kv3 channels are necessary for high-frequency firing. Moreover, we found that Kv3.2 -/- mice showed specific alterations in their cortical EEG patterns and an increased susceptibility to epileptic seizures consistent with an impairment of cortical inhibitory mechanisms. This implies that, rather than producing hyperexcitability of the inhibitory interneurons, Kv3.2 channel elimination suppresses their activity. These data suggest that normal cortical operations depend on the ability of inhibitory interneurons to generate high-frequency firing.

Apoptosis caused by cathepsins does not require Bid signaling in an in vivo model of progressive myoclonus epilepsy (EPM1).

Apoptosis can be mediated by mechanisms other than the traditional caspase-mediated cleavage cascade. There is growing recognition that alternative proteolytic enzymes such as the lysosomal cathepsin proteases can initiate or propagate proapoptotic signals, but it is currently unclear how cathepsins achieve these actions. Recent in vitro evidence suggests that cathepsins cleave the proapoptotic Bcl-2 family member Bid, thereby activating it and allowing it to induce the mitochondrial release of cytochrome c and subsequent apoptosis. We have tested this hypothesis in vivo by breeding mice that lack cathepsin inhibition (cystatin B-deficient mice) to Bid-deficient mice, to determine whether the apoptosis caused by cathepsins is dependent on Bid signaling. We found that cathepsins are still able to promote apoptosis even in the absence of Bid, indicating that these proteases mediate apoptosis via a different pathway, or that some other molecule can functionally substitute for Bid in this system.

Expression of apoptosis inhibitor protein Mcl1 linked to neuroprotection in CNS neurons.

Mcl1 is a Bcl2-related antiapoptotic protein originally isolated from human myeloid leukemia cells. Unlike Bcl2, expression has not been reported in CNS neurons. We isolated Mcl1 in a direct screen for candidate modifier genes of neuronal vulnerability by differential display of mRNAs upregulated following prolonged seizures in two mouse strains with contrasting levels of hippocampal cell death. Mcl1 is widely expressed in neurons, and transcription is rapidly induced in both strains. In resistant C57Bl/6J mice, Mcl1 protein levels remain persistently elevated in hippocampal pyramidal neurons after seizures, but fall rapidly in C3H/HeJ hippocampus, coinciding with extensive neuronal apoptosis. DNA damage and caspase-mediated cell death were strikingly increased in Mcl1-deficient mice when compared to +/+ littermates after similar seizures. We identify Mcl1 as a neuronal gene responsive to excitotoxic insult in the brain, and link relative levels of Mcl1 expression to inherited differences in neuronal thresholds for apoptosis.

Genetic epilepsy model derived from common inbred mouse strains.

The recombinant inbred mouse strain, SWXL-4, exhibits tonic-clonic and generalized seizures similar to the commonest epilepsies in humans. In SWXL-4 animals, seizures are observed following routine handling at about 80 days of age and may be induced as early as 55 days by rhythmic gentle tossing. Seizures are accompanied by rapid, bilateral high frequency spike cortical discharges and followed by a quiescent post-ictal phase. Immunohistochemistry of the immediate early gene products c-Fos and c-Jun revealed abnormal activation within cortical and limbic structures. The seizure phenotype of SWXL-4 can be explained and replicated fully by the inheritance of susceptibility alleles from its progenitor strains, SWR/J and C57L/J. Outcrosses of SWXL-4 with most other common inbred strains result in F1 hybrids that have seizure at least as frequently as SWXL-4 itself. Quantitative trait locus mapping reveals a seizure frequency determinant, Szf1, near the pink-eyed dilution locus on chromosome 7, accounting for up to 32% of the genetic variance in an F2 intercross between SWXL-4 and the linkage testing strain ABP/Le. These studies demonstrate that common strains of mice such as SWR and C57L contain latent epilepsy susceptibility alleles. Although the inheritance of susceptibility may be complex, these results imply that a number of potentially important and practical, noninvasive models for this disorder can be constructed and studied in crosses between common mouse strains.

Molecular characterization of a high-affinity mouse glutamate transporter.

The complete coding sequence of a mouse glutamate transporter (mEAAT2) has been cloned by polymerase chain reaction (PCR) from adult whole-brain total RNA. Southern hybridization analysis of PCR products amplified from templates derived from various murine adult tissues demonstrated that the transcript for mEAAT2 was specific to the central nervous system. High-affinity transport of D-aspartate, Km value (17 +/- 5 microM), was determined in a vaccinia/T7 RNA polymerase expression system. The deduced amino-acid sequence of mEAAT2 shares 96 and 93% identity with the rat and human EAAT2 homologues, respectively.

Males with epilepsy, complete subcortical band heterotopia, and somatic mosaicism for DCX.

Subcortical band heterotopia (SBH) is seen predominantly in females, resulting from mutations in the X-linked doublecortin (DCX) gene, and can present with mild mental retardation and epilepsy. Males carrying DCX mutations usually demonstrate lissencephaly and are clinically much more severely affected. This article reports two cases of males with SBH indistinguishable from the female phenotype, both resulting from somatic mosaicism for DCX mutation.

Future directions for epilepsy research.

The authors propose that epilepsy research embark on a revitalized effort to move from targeting control of symptoms to strategies for prevention and cure. The recent advances that make this a realistic goal include identification of genes mutated in inherited epilepsy syndromes, molecular characterization of brain networks, better imaging of sites of seizure origin, and developments in seizure prediction by quantitative EEG analysis. Research directions include determination of mechanisms of epilepsy development, identification of genes for common epilepsy syndromes through linkage analysis and gene chip technology, and validation of new models of epilepsy and epileptogenesis. Directions for therapeutics include identification of new molecular targets, focal methods of drug delivery tied to EEG activity, gene and cell therapy, and surgical and nonablative therapies. Integrated approaches, such as coupling imaging with electrophysiology, are central to progress in localizing regions of epilepsy development in people at risk and better seizure prediction and treatment for people with epilepsy.

Impaired neurotransmitter release and elevated threshold for cortical spreading depression in mice with mutations in the alpha1A subunit of P/Q type calcium channels.

The P/Q type voltage-gated Ca2+ channels are involved in membrane excitability and Ca2+-dependent neurotransmitter release within the CNS. Mutations in the CacnalA gene encoding the alpha1A subunit of the P/Q type Ca2+ channel have recently been reported in tottering mice and a more severely affected allele, leaner. Here we show using in vivo cortical microdialysis that evoked increases of extracellular glutamate levels are markedly attenuated in both mutants upon KCl-induced depolarization compared with wild-type mice. Tottering and leaner mice also show a 10-fold resistance to cortical spreading depression induced by cortical electrical stimulation or KCl application to the pial surface. A slower transcortical propagation speed and failure to sustain regenerative spread of the depolarizing wave were more pronounced in leaner neocortex. Both signaling defects appeared unrelated to the developmental history of repeated cortical spike-wave discharges, since neither were observed in the stargazer mouse, a Ca2+ channel gamma2 subunit mutant with a similar seizure phenotype. These data demonstrate two cortical excitability defects revealed by prolonged depolarization in cerebral networks expressing mutant P/Q type Ca2+ channels, and are the first to identify a gene linked to a spreading depression phenotype.