Abnormal morphological and functional organization of the hippocampus in a p35 mutant model of cortical dysplasia associated with spontaneous seizures

Granule cell dispersion in two mouse models of temporal lobe epilepsy and reeler mice is associated with changes in dendritic orientation and spine distribution

Temporal lobe epilepsy is characterized by hippocampal neuronal death in CA1 and hilus. Dentate gyrus granule cells survive but show dispersion of the compact granule cell layer. This is associated with decrease of the glycoprotein Reelin, which regulates neuron migration and dendrite outgrow. Reelin-deficient (reeler) mice show no layering, their granule cells are dispersed throughout the dentate gyrus. We studied granule cell dendritic orientation and distribution of postsynaptic spines in reeler mice and two mouse models of temporal lobe epilepsy, namely the p35 knockout mice, which show Reelin-independent neuronal migration defects, and mice with unilateral intrahippocampal kainate injection. Granule cells were Golgi-stained and analyzed, using a computerized camera lucida system. Granule cells in naive controls exhibited a vertically oriented dendritic arbor with a small bifurcation angle if positioned proximal to the hilus and a wider dendritic bifurcation angle, if positioned distally. P35 knockout- and kainate-injected mice showed a dispersed granule cell layer, granule cells showed basal dendrites with wider bifurcation angles, which lost position-specific differences. Reeler mice lacked dendritic orientation. P35 knockout- and kainate-injected mice showed increased dendritic spine density in the granule cell layer. Molecular layer dendrites showed a reduced spine density in kainate-injected mice only, whereas in p35 knockouts no reduced spine density was seen. Reeler mice showed a homogenous high spine density. We hypothesize that granule cells migrate in temporal lobe epilepsy, develop new dendrites which show a spread of the dendritic tree, create new spines in areas proximal to mossy fiber sprouting, which is present in p35 knockout- and kainate-injected mice and loose spines on distal dendrites if mossy cell death is present, as it was in kainate-injected mice only. These results are in accordance with findings in epilepsy patients.

Inducible Knockout of the Cyclin-Dependent Kinase 5 Activator p35 Alters Hippocampal Spatial Coding and Neuronal Excitability

p35 is an activating co-factor of Cyclin-dependent kinase 5 (Cdk5), a protein whose dysfunction has been implicated in a wide-range of neurological disorders including cognitive impairment and disease. Inducible deletion of the p35 gene in adult mice results in profound deficits in hippocampal-dependent spatial learning and synaptic physiology, however the impact of the loss of p35 function on hippocampal in vivo physiology and spatial coding remains unknown. Here, we recorded CA1 pyramidal cell activity in freely behaving p35 cKO and control mice and found that place cells in the mutant mice have elevated firing rates and impaired spatial coding, accompanied by changes in the temporal organization of spiking both during exploration and rest. These data shed light on the role of p35 in maintaining cellular and network excitability and provide a physiological correlate of the spatial learning deficits in these mice.

Structural alterations in fast-spiking GABAergic interneurons in a model of posttraumatic neocortical epileptogenesis

Electrophysiological experiments in the partial cortical isolation ("undercut" or "UC") model of injury-induced neocortical epileptogenesis have shown alterations in GABAergic synaptic transmission attributable to abnormalities in presynaptic terminals. To determine whether the decreased inhibition was associated with structural abnormalities in GABAergic interneurons, we used immunocytochemical techniques, confocal microscopy and EM in UC and control sensorimotor rat cortex to analyze structural alterations in fast-spiking parvalbumin-containing interneurons and pyramidal (Pyr) cells of layer V. Principle findings were: 1) there were no decreases in counts of parvalbumin (PV)- or GABA-immunoreactive interneurons in UC cortex, however there were significant reductions in expression of VGAT and GAD-65 and -67 in halos of GABAergic terminals around Pyr somata in layer V. 2) Consistent with previous results, somatic size and density of Pyr cells was decreased in infragranular layers of UC cortex. 3) Dendrites of biocytin-filled FS interneurons were significantly decreased in volume. 4) There were decreases in the size and VGAT content of GABAergic boutons in axons of biocytin-filled FS cells in the UC, together with a decrease in colocalization with postsynaptic gephyrin, suggesting a reduction in GABAergic synapses. Quantitative EM of layer V Pyr somata confirmed the reduction in inhibitory synapses. 5) There were marked and lasting reductions in brain derived neurotrophic factor (BDNF)-IR and -mRNA in Pyr cells and decreased TrkB-IR on PV cells in UC cortex. 6) Results lead to the hypothesis that reduction in trophic support by BDNF derived from Pyr cells may contribute to the regressive changes in axonal terminals and dendrites of FS cells in the UC cortex and decreased GABAergic inhibition.

SIGNIFICANCE

Injury to cortical structures is a major cause of epilepsy, accounting for about 20% of cases in the general population, with an incidence as high as ~50% among brain-injured personnel in wartime. Loss of GABAergic inhibitory interneurons is a significant pathophysiological factor associated with epileptogenesis following brain trauma and other etiologies. Results of these experiments show that the largest population of cortical interneurons, the parvalbumin-containing fast-spiking (FS) interneurons, are preserved in the partial neocortical isolation model of partial epilepsy. However, axonal terminals of these cells are structurally abnormal, have decreased content of GABA synthetic enzymes and vesicular GABA transporter and make fewer synapses onto pyramidal neurons. These structural abnormalities underlie defects in GABAergic neurotransmission that are a key pathophysiological factor in epileptogenesis found in electrophysiological experiments. BDNF, and its TrkB receptor, key factors for maintenance of interneurons and pyramidal neurons, are decreased in the injured cortex. Results suggest that supplying BDNF to the injured epileptogenic brain may reverse the structural and functional abnormalities in the parvalbumin FS interneurons and provide an antiepileptogenic therapy.

Functional characterization of CDK5 and CDK5R1 mutations identified in patients with non-syndromic intellectual disability

Cyclin-dependent kinase 5 (CDK5) and cyclin-dependent kinase 5, regulatory subunit 1 (CDK5R1), encoding CDK5 activator p35, have a fundamental role in central nervous system (CNS) development and function, and are involved in the pathogenesis of several neurodegenerative disorders, thus constituting strong candidate genes for the onset of intellectual disability (ID). We carried out a mutation screening of CDK5 and CDK5R1 coding regions and CDK5R1 3'-UTR on a cohort of 360 patients with non-syndromic ID (NS-ID) using denaturing high performance liquid chromatography (DHPLC) and direct sequencing. We found one novel silent mutation in CDK5 and one novel silent mutation in CDK5R1 coding regions, three novel intronic variations in CDK5, not causing any splicing defect, and four novel heterozygous variations in CDK5R1 3'-UTR. None of these variations was present in 450 healthy controls and single-nucleotide polymorphism (SNP) databases. The functional study of CDK5R1 p.A108V mutation evidenced an impaired p35 cleavage by the calcium-dependent protease calpain. Moreover, luciferase constructs containing the CDK5R1 3'-UTR mutations showed altered gene expression levels. Eight known polymorphisms were also identified displaying different frequencies in NS-ID patients compared with the controls. In particular, the minor allele of CDK5R1 3'-UTR rs735555 polymorphism was associated with increased risk for NS-ID. In conclusion, our data suggest that mutations and polymorphisms in CDK5 and CDK5R1 genes may contribute to the onset of the NS-ID phenotype.

Neuronal migration disorders: Focus on the cytoskeleton and epilepsy

A wide spectrum of focal, regional, or diffuse structural brain abnormalities, collectively known as malformations of cortical development (MCDs), frequently manifest with intellectual disability (ID), epilepsy, and/or autistic spectrum disorder (ASD). As the acronym suggests, MCDs are perturbations of the normal architecture of the cerebral cortex and hippocampus. The pathogenesis of these disorders remains incompletely understood; however, one area that has provided important insights has been the study of neuronal migration. The amalgamation of human genetics and experimental studies in animal models has led to the recognition that common genetic causes of neurodevelopmental disorders, including many severe epilepsy syndromes, are due to mutations in genes regulating the migration of newly born post-mitotic neurons. Neuronal migration genes often, though not exclusively, code for proteins involved in the function of the cytoskeleton. Other cellular processes, such as cell division and axon/dendrite formation, which similarly depend on cytoskeletal functions, may also be affected. We focus here on how the susceptibility of the highly organized neocortex and hippocampus may be due to their laminar organization, which involves the tight regulation, both temporally and spatially, of gene expression, specialized progenitor cells, the migration of neurons over large distances and a birthdate-specific layering of neurons. Perturbations in neuronal migration result in abnormal lamination, neuronal differentiation defects, abnormal cellular morphology and circuit formation. Ultimately this results in disorganized excitatory and inhibitory activity leading to the symptoms observed in individuals with these disorders.

Neuronal migration and its disorders affecting the CA3 region

In this review, we focus on CA3 neuronal migration disorders in the rodent. We begin by introducing the main steps of hippocampal development, and we summarize characteristic hippocampal malformations in human. We then describe various mouse mutants showing structural hippocampal defects. Notably, genes identified in human cortical neuronal migration disorders consistently give rise to a CA3 phenotype when mutated in the mouse. We successively describe their molecular, physiological and behavioral phenotypes that together contribute to a better understanding of CA3-dependent functions. We finally discuss potential factors underlying the CA3 vulnerability revealed by these mouse mutants and that may also contribute to other human neurological and psychiatric disorders.

Does mossy fiber sprouting give rise to the epileptic state?

Many patients with temporal lobe epilepsy display structural changes in the seizure initiating zone, which includes the hippocampus. Structural changes in the hippocampus include granule cell axon (mossy fiber) sprouting. The role of mossy fiber sprouting in epileptogenesis is controversial. A popular view of temporal lobe epileptogenesis contends that precipitating brain insults trigger transient cascades of molecular and cellular events that permanently enhance excitability of neuronal networks through mechanisms including mossy fiber sprouting. However, recent evidence suggests there is no critical period for mossy fiber sprouting after an epileptogenic brain injury. Instead, findings from stereological electron microscopy and rapamycin-delayed mossy fiber sprouting in rodent models of temporal lobe epilepsy suggest a persistent, homeostatic mechanism exists to maintain a set level of excitatory synaptic input to granule cells. If so, a target level of mossy fiber sprouting might be determined shortly after a brain injury and then remain constant. Despite the static appearance of synaptic reorganization after its development, work by other investigators suggests there might be continual turnover of sprouted mossy fibers in epileptic patients and animal models. If so, there may be opportunities to reverse established mossy fiber sprouting. However, reversal of mossy fiber sprouting is unlikely to be antiepileptogenic, because blocking its development does not reduce seizure frequency in pilocarpine-treated mice. The challenge remains to identify which, if any, of the many other structural changes in the hippocampus are epileptogenic.

The development of hippocampal cellular assemblies

The proper assembly of a cohort of distinct cell types is a prerequisite for building a functional hippocampus. In this review, we describe the major molecular events of the developmental program leading to the cellular construction of the hippocampus. Data from rodent studies are used here to elaborate on our understanding of these processes.

Blood-brain barrier, epileptogenesis, and treatment strategies in cortical dysplasia

Cortical dysplasia (CD) is one of the most important causes of intractable epilepsy. The precise mechanisms of epileptogenesis in CD are not known. Using CD animal models, we attempted to understand the mechanisms and efficacy of various antiepileptic drugs. In two separate studies, we assessed (1) the effects of levetiracetam (LEV) and vagus nerve stimulation (VNS) on pentylenetetrazol (PTZ)-kindled rats, and (2) the effects of LEV and topiramate (TPM) on rats with CD and hyperthermia (HT). In the HT-induced rats with CD study, LEV and TPM decreased both the intensity of seizures and the number of rats with seizure. In these studies, we used immunocytochemistry (occludin, glial fibrillary acidic protein [GFAP], and P-glycoprotein [Pgp antibodies] and electron microscopy (EM) (sodium fluorescein [NaFlu]) and horseradish peroxidase [HRP]) to assess blood-brain barrier (BBB) integrity. Both LEV and TPM protected BBB. In PTZ- kindled rats with CD, both LEV and VNS reduced the duration of seizures. Immunocytochemistry and EM revealed no BBB impairment in any of the treatment groups. In a second set of experiments, we assessed the relationship between disruption of vascular components and epileptogenesis. Astrocytic albumin uptake in focal epileptogenic lesions with vascular components suggested that dysfunction of the BBB contributes immediately to epileptogenesis, rather than simply resulting from seizure activity. Hemosiderin deposits were seen as potential epileptogenic triggers in vascular malformations (e.g., cavernomas [CA] or arteriovenous malformations [AVMs] with or without a dysplastic cortical component). However, we found strikingly high accumulation of astrocytic albumin deposits in surgically removed brain parenchyma in the vicinity of CAs and AVMs from patients with pharmacoresistant epilepsy, which suggests different pathophysiologic dispersion pathways for hemosiderin and albumin in vascular lesions.

Matrix metalloproteinase-9 (MMP-9) in human intractable epilepsy caused by focal cortical dysplasia

Focal cortical dysplasia (FCD) is a developmental brain disorder characterized by localized abnormalities of cortical layering and neuronal morphology. It is associated with pharmacologically intractable forms of epilepsy in both children and adults. The mechanisms that underlie FCD-associated seizures and lead to the progression of the disease are unclear. Matrix metalloproteinases (MMPs) are enzymes that are able to influence neuronal function through extracellular proteolysis in various normal and pathological conditions. The results of experiments that have used rodent models showed that extracellular MMP-9 can play an important role in epileptogenesis. However, no studies have shown that MMP-9 is involved in the pathogenesis of human epilepsy. The aim of the present study was to determine whether MMP-9 plays a role in intractable epilepsy. Using an unbiased antibody microarray approach, we found that up regulation of MMP-9 is prominent and consistent in FCD tissue derived from epilepsy surgery, regardless of the patient's age. Additionally, an up regulation of MMP-1, -2, -8, -10, and -13 was found but was either less pronounced or limited only to adult cases. In the dysplastic cortex, immunohistochemistry revealed that the highest MMP-9 immuno reactivity occurred in the cytoplasm of abnormal neurons and balloon cells. The neuronal over expression of MMP-9 also occurred in sclerotic hippocampi that were excised together with the dysplastic cortex, but sclerotic hippocampi were free of dysplastic features. In both locations, MMP-9 was also found in reactive astrocytes, albeit to a lesser extent. At the subcellular level, increased MMP-9 immunoreactivity was prominently upregulated at synapses. Thus, although upregulation of the enzyme in FCD is not causally linked to the developmental malformation, it may be a result of ongoing abnormal synaptic plasticity. The present findings support the hypothesis of the pathogenic role of MMP-9 in human epilepsy and may stimulate discussions about whether MMPs could be novel therapeutic targets for intractable epilepsy.

Synaptic connections of hilar basal dendrites of dentate granule cells in a neonatal hypoxia model of epilepsy

Numerous animal models of epileptogenesis demonstrate neuroplastic changes in the hippocampus. These changes occur not only for the mature neurons and glia, but also for the newly generated granule cells in the dentate gyrus. One of these changes, the sprouting of mossy fiber axons, is derived predominantly from newborn granule cells in adult rats with pilocarpine-induced temporal lobe epilepsy. Newborn granule cells also mainly contribute to another neuroplastic change, hilar basal dendrites (HBDs), which are synaptically targeted by mossy fibers in the hilus. Both sprouted mossy fibers and HBDs contribute to recurrent excitatory circuitry that is hypothesized to be involved in increased seizure susceptibility and the development of spontaneous recurrent seizures (SRS) that occur following the initial pilocarpine-induced status epilepticus. Considering the putative role of these neuroplastic changes in epileptogenesis, a critical question is whether similar anatomic phenomena occur after epileptogenic insults to the immature brain, where the proportion of recently born granule cells is higher due to ongoing maturation. The current study aimed to determine if such neuroplastic changes could be observed in a standardized model of neonatal seizure-inducing hypoxia that results in development of SRS. We used immunoelectron microscopy for the immature neuronal marker doublecortin to label newborn neurons and their HBDs following neonatal hypoxia. Our goal was to determine whether synapses form on HBDs from neurons born after neonatal hypoxia. Our results show a robust synapse formation on HBDs from animals that experienced neonatal hypoxia, regardless of whether the animals experienced tonic-clonic seizures during the hypoxic event. In both cases, the axon terminals that synapse onto HBDs were identified as mossy fiber terminals, based on the appearance of dense core vesicles. No such synapses were observed on HBDs from newborn granule cells obtained from sham animals analyzed at the same time points. This aberrant circuit formation may provide an anatomic substrate for increased seizure susceptibility and the development of epilepsy.

Acute, but reversible, kainic acid-induced DNA damage in hippocampal CA1 pyramidal cells of p53-deficient mice

p53 plays an essential role in mediating apoptotic responses to cellular stress, especially DNA damage. In a kainic acid (KA)-induced seizure model in mice, hippocampal CA1 pyramidal cells undergo delayed neuronal death at day 3-4 following systemic KA administration. We previously demonstrated that CA1 neurons in p53(-/-) animals are protected from such apoptotic neuronal loss. However, extensive morphological damage associated with DNA strand breaks in CA1 neurons was found in a fraction of p53(-/-) animals at earlier time points (8 h to 2 days). No comparable acute damage was observed in wild-type animals. Stereological counting confirmed that there was no significant loss of CA1 pyramidal cells in p53(-/-) animals at 7 days post-KA injection. These results suggest that seizure-induced DNA strand breaks are accumulated to a greater extent but do not lead to apoptosis in the absence of p53. In wild-type animals, therefore, p53 appears to stimulate DNA repair and also mediate apoptosis in CA1 neurons in this excitotoxicity model. These results also reflect remarkable plasticity of neurons in recovery from injury.

Inhibition and interneuron distribution in the dentate gyrus of p35 knockout mice

The p35 knockout (p35-/-) mouse is an animal model of temporal lobe epilepsy that recapitulates key neuroanatomic abnormalities-granule cell dispersion and mossy fiber sprouting-observed in the hippocampal formation of humans, as well as spontaneous seizure activity. It is a useful model in which to study the relationship between the abnormal neuronal structure and seizure activity to further our understanding of cortical dysplasia in epileptogenesis. Our previous work using this mouse model characterized the anatomic features of the dentate granule cells and the functional implications of these abnormalities on increased recurrent excitation. These data also suggested that there might be compromised inhibition in this animal model. We pursued this possibility, focusing our investigation on inhibitory circuitry. In preliminary investigations using neuroanatomic tools (immunocytochemistry, camera lucida reconstructions of individually labeled interneurons, and electron microscopy) combined with intracellular electrophysiology, we observed no significant reduction in the number of symmetric versus asymmetric synaptic contacts on dentate granule cell somata, and no statistically significant changes in evoked early or late inhibition. Although there were some abnormalities in the morphology/distribution of inhibitory interneurons (as well as a larger population of dentate granule cells) of the dentate gyrus, overall inhibition in the p35 knockout mouse appeared to be largely intact.

The importance of immunohistochemical analyses in evaluating the phenotype of Kv channel knockout mice

To gain insights into the phenotype of voltage-gated potassium (Kv)1.1 and Kv4.2 knockout mice, we used immunohistochemistry to analyze the expression of component principal or α subunits and auxiliary subunits of neuronal Kv channels in knockout mouse brains. Genetic ablation of the Kv1.1 α subunit did not result in compensatory changes in the expression levels or subcellular distribution of related ion channel subunits in hippocampal medial perforant path and mossy fiber nerve terminals, where high levels of Kv1.1 are normally expressed. Genetic ablation of the Kv4.2 α subunit did not result in altered neuronal cytoarchitecture of the hippocampus. Although Kv4.2 knockout mice did not exhibit compensatory changes in the expression levels or subcellular distribution of the related Kv4.3 α subunit, we found dramatic decreases in the cellular and subcellular expression of specific Kv channel interacting proteins (KChIPs) that reflected their degree of association and colocalization with Kv4.2 in wild-type mouse and rat brains. These studies highlight the insights that can be gained by performing detailed immunohistochemical analyses of Kv channel knockout mouse brains.

Are developmental dysplastic lesions epileptogenic?

Cortical dysplasia of various types, reflecting abnormalities of brain development, have been closely associated with epileptic activities. Yet, there remains considerable discussion about if/how these structural lesions give rise to seizure phenomenology. Animal models have been used to investigate the cause-effect relationships between aberrant cortical structure and epilepsy. In this article, we discuss three such models: (1) the Eker rat model of tuberous sclerosis, in which a gene mutation gives rise to cortical disorganization and cytologically abnormal cellular elements; (2) the p35 knockout mouse, in which the genetic dysfunction gives rise to compromised cortical organization and lamination, but in which the cellular elements appear normal; and (3) the methylazoxymethanol-exposed rat, in which time-specific chemical DNA disruption leads to abnormal patterns of cell formation and migration, resulting in heterotopic neuronal clusters. Integrating data from studies of these animal models with related clinical observations, we propose that the neuropathologic features of these cortical dysplastic lesions are insufficient to determine the seizure-initiating process. Rather, it is their interaction with a more subtly disrupted cortical "surround" that constitutes the circuitry underlying epileptiform activities as well as seizure propensity and ictogenesis.

Mossy cell dendritic structure quantified and compared with other hippocampal neurons labeled in rats in vivo

Mossy cells are likely to contribute to normal hippocampal function and to the pathogenesis of neurologic disorders that involve the hippocampus, including epilepsy. Mossy cells are the least well-characterized excitatory neurons in the hippocampus. Their somatic and dendritic morphology has been described qualitatively but not quantitatively. In the present study rat mossy cells were labeled intracellularly with biocytin in vivo. Somatic and dendritic structure was reconstructed three-dimensionally. For comparison, granule cells, CA3 pyramidal cells, and CA1 pyramidal cells were labeled and analyzed using the same approach. Among the four types of hippocampal neurons, granule cells had the smallest somata, fewest primary dendrites and dendritic branches, and shortest total dendritic length. CA1 pyramidal cells had the most dendritic branches and longest total dendritic length. Mossy cells and CA3 pyramidal cells both had large somata and similar total dendritic lengths. However, mossy cell dendrites branched less than CA3 pyramidal cells, especially close to the soma. These findings suggest that mossy cells have dendritic features that are not identical to any other type of hippocampal neuron. Therefore, electrotonic properties that depend on soma-dendritic structure are likely to be distinct in mossy cells compared to other neurons.

Abnormalities of granule cell dendritic structure are a prominent feature of the intrahippocampal kainic acid model of epilepsy despite reduced postinjury neurogenesis

PURPOSE

  Aberrant plastic changes among adult-generated hippocampal dentate granule cells are hypothesized to contribute to the development of temporal lobe epilepsy. Changes include formation of basal dendrites projecting into the dentate hilus. Innervation of these processes by granule cell mossy fiber axons leads to the creation of recurrent excitatory circuits within the dentate. The destabilizing effect of these recurrent circuits may contribute to hyperexcitability and seizures. Although basal dendrites have been identified in status epilepticus models of epilepsy associated with increased neurogenesis, we do not know whether similar changes are present in the intrahippocampal kainic acid model of epilepsy, which is associated with reduced neurogenesis.

METHODS

  In the present study, we used Thy1-YFP-expressing transgenic mice to determine whether hippocampal dentate granule cells develop hilar-projecting basal dendrites in the intrahippocampal kainic acid model. Brain sections were examined 2 weeks after treatment. Tissue was also examined using ZnT-3 immunostaining for granule cell mossy fiber terminals to assess recurrent connectivity. Adult neurogenesis was assessed using the proliferative marker Ki-67 and the immature granule cell marker calretinin.

KEY FINDINGS

  Significant numbers of cells with basal dendrites were found in this model, but their structure was distinct from basal dendrites seen in other epilepsy models, often ending in complex tufts of short branches and spines. Even more unusual, a subset of cells with basal dendrites had an inverted appearance; they completely lacked apical dendrites. Spines on basal dendrites were found to be apposed to ZnT-3 immunoreactive puncta, suggestive of recurrent mossy fiber input. Finally, YFP-expressing abnormal granule cells did not colocalize Ki-67 or calretinin, indicating that these cells were more than a few weeks old, but were found almost exclusively in proximity to the neurogenic subgranular zone, where the youngest granule cells are located.

SIGNIFICANCE

  Recent studies have demonstrated in other models of epilepsy that dentate pathology develops following the aberrant integration of immature, adult-generated granule cells. Given these findings, one might predict that the intrahippocampal kainic acid model of epilepsy, which is associated with a dramatic reduction in adult neurogenesis, would not exhibit these changes. Herein we demonstrate that hilar basal dendrites are a common feature of this model, with the abnormal cells likely resulting from the disruption of juvenile granule cell born in the weeks before the insult. These studies demonstrate that postinjury neurogenesis is not required for the accumulation of large numbers of abnormal granule cells.

Cyclin-dependent kinases in brain development and disease

Cyclin-dependent kinase 5 (Cdk5) is a multifaceted serine/threonine kinase protein with important roles in the nervous system. Two related proteins, p35 and p39, activate Cdk5 upon direct binding. Over the past decade, Cdk5 activity has been demonstrated to regulate many events during brain development, including neuronal migration as well as axon and dendrite development. Recent evidence also suggests a pivotal role for Cdk5 in synaptic plasticity, behavior, and cognition. Dysfunction of Cdk5 has been implicated in a number of neurological disorders and neurodegenerative diseases including Alzheimer's disease, amyotrophic lateral sclerosis, Niemann-Pick type C disease, and ischemia. Hyperactivation of Cdk5 due to the conversion of p35 to p25 by the calcium-dependent protease calpain during neurotoxicity also contributes to the pathological state. This review surveys recent literature surrounding Cdk5 in synaptic plasticity and homeostasis, with particular emphasis on Cdk5 kinase activity under neurodegenerative conditions.

Stereologic estimation of hippocampal GluR2/3- and calretinin-immunoreactive hilar neurons (presumptive mossy cells) in two mouse models of temporal lobe epilepsy

PURPOSE

Hippocampal mossy cells receive dense innervation from dentate granule cells and, in turn, mossy cells innervate both granule cells and interneurons. Mossy cell loss is thought to trigger granule cell mossy fiber sprouting, which may affect granule cell excitability. The aim of this study was to quantify mossy cell loss in two animal models of temporal lobe epilepsy, and determine whether there exists a relationship between mossy cell loss, mossy fiber sprouting, and granule cell dispersion.

METHODS

Representative hippocampal sections from p35 knockout mice and mice with unilateral intrahippocampal kainate injection were immunolabeled for GluR2/3, two subunits of the amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor and calretinin to identify mossy cells. Mossy fibers were immunostained against synaptoporin.

KEY FINDINGS

p35 Knockout mice showed no hilar cell death, but moderate mossy fiber sprouting and granule cell dispersion. In the kainate-injected hippocampus, there was an 80% and 85% reduction of GluR2/3- and GluR2/3/calretinin-positive hilar neurons, respectively, and dense mossy fiber sprouting and significant granule cell dispersion. In the contralateral hippocampus there was a 52% loss of GluR2/3-, but only a 20% loss of GluR2/3-calretinin-immunoreactive presumptive mossy cells, and granule cell dispersion; no mossy fiber sprouting was observed.

SIGNIFICANCE

These results indicate a probable lack of causality between mossy cell death and mossy fiber sprouting.

Making a neuron: Cdk5 in embryonic and adult neurogenesis

Cyclin-dependent kinase 5 (Cdk5) has been implicated in the migration, maturation and survival of neurons born during embryonic development. New evidence suggests that Cdk5 has comparable but also distinct functions in adult neurogenesis. Here we summarize accumulating evidence on the role of Cdk5 in regulation of the cell cycle, migration, survival, maturation and neuronal integration. We specifically highlight the many similarities and few tantalizing differences in the roles of Cdk5 in the embryonic and adult brain. We discuss the signaling pathways that might contribute to Cdk5 action in regulating embryonic and adult neurogenesis, highlighting future research directions that will help to clarify the mechanisms underlying lifelong neurogenesis in the mammalian brain.

Pharmacogenetic analysis reveals a post-developmental role for Rac GTPases in Caenorhabditis elegans GABAergic neurotransmission

The nerve-cell cytoskeleton is essential for the regulation of intrinsic neuronal activity. For example, neuronal migration defects are associated with microtubule regulators, such as LIS1 and dynein, as well as with actin regulators, including Rac GTPases and integrins, and have been thought to underlie epileptic seizures in patients with cortical malformations. However, it is plausible that post-developmental functions of specific cytoskeletal regulators contribute to the more transient nature of aberrant neuronal activity and could be masked by developmental anomalies. Accordingly, our previous results have illuminated functional roles, distinct from developmental contributions, for Caenorhabditis elegans orthologs of LIS1 and dynein in GABAergic synaptic vesicle transport. Here, we report that C. elegans with function-altering mutations in canonical Rac GTPase-signaling-pathway members demonstrated a robust behavioral response to a GABA(A) receptor antagonist, pentylenetetrazole. Rac mutants also exhibited hypersensitivity to an acetylcholinesterase inhibitor, aldicarb, uncovering deficiencies in inhibitory neurotransmission. RNA interference targeting Rac hypomorphs revealed synergistic interactions between the dynein motor complex and some, but not all, members of Rac-signaling pathways. These genetic interactions are consistent with putative Rac-dependent regulation of actin and microtubule networks and suggest that some cytoskeletal regulators cooperate to uniquely govern neuronal synchrony through dynein-mediated GABAergic vesicle transport in C. elegans.

Inhibitory inputs to hippocampal interneurons are reorganized in Lis1 mutant mice

Epilepsy and brain malformation are commonly associated with excessive synaptic excitation and decreased synaptic inhibition of principal neurons. However, few studies have examined the state of synaptic inhibition of interneurons in an epileptic, malformed brain. We analyzed inhibitory inputs, mediated by gamma-aminobutyric acid (GABA), to hippocampal interneurons in a mouse model of type 1 lissencephaly, a neurological disorder linked with severe seizures and brain malformation. In the disorganized hippocampal area CA1 of Lis1(+/-) mice, we initially observed a selective displacement of fast-spiking, parvalbumin-positive basket-type interneurons from stratum oriens (SO) locations to s. radiatum and s. lacunosum-moleculare (R/LM). Next, we recorded spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) onto visually identified interneurons located in SO or R/LM of Lis1(+/-) mice and age-matched littermate controls. We observed significant, layer-specific reorganizations in GABAergic inhibition of interneurons in Lis1 mutant mice. Spontaneous IPSC frequency onto SO interneurons was significantly increased in hippocampal slices from Lis1(+/-) mice, whereas mIPSC mean amplitude onto these interneurons was significantly decreased. In addition, the weighted decay times of sIPSCs and mIPSCs were significantly increased in R/LM interneurons. Taken together, these findings illustrate the extensive redistribution and reorganization of inhibitory connections between interneurons that can take place in a malformed brain.

Aberrant dentate gyrus cytoarchitecture and fiber lamination in Lis1 mutant mice

Mutant mice with a heterozygous deletion of LIS1, show varying degrees of hippocampal abnormality and enhanced excitability. To examine how LIS1 affects cytoarchitecture and fiber lamination in dentate gyrus (DG), we performed a series of immunohistochemistry studies. By using different neuronal- and glial-specific antibodies, we found that the majority of hippocampal cell populations were affected by heterozygous mutation of LIS1; some reelin-positive Cajal-Retzius cells were left undisturbed. Granule cell dispersion was significant in hippocampal sections from Lis1-deficient mice. However, the fiber termination of commissural/associational fibers and mossy fibers appeared relatively compact despite obvious granule cell dispersion and CA1-CA3 pyramidal cell disorganization. vGlut1-immunoreactive axon terminals were found aberrantly traversing the dispersed granule cell layer. Consistent with previous observations, we also found that immature granule cells in Lis1 mutants, here stained with antibodies to doublecortin (DCX) and Mash-1, are aberrantly located and bear an abnormal cellular morphology. Our findings suggest that LIS1 mutants exhibit abnormal cell positioning and aberrant hippocampal neurogenesis, but maintain relatively normal fiber termination patterns. The functional consequences of hippocampal granule cell dispersion could offer critical insight to the epileptic and cognitive disorder associated with LIS1 haploinsufficiency.

Unravelling the complex role of Cdk5 in the developing cerebral cortex

The normal development of the mammalian CNS is entirely dependent on the coordinated behavior of its cellular components. Particular importance is attributed to the correct morphology, migration and communication of neurons. Recent years have seen the identification of many extracellular, cell surface and intracellular signaling molecules that are important for normal CNS development, consequently triggering huge progress in our understanding of the complex processes involved. A key molecule to emerge is Cdk5. To date, Cdk5 has been functionally linked with controlled neuronal morphology, migration, synaptic function, cognition, drug addiction, neuronal death and neurodegeneration. The complexity of its function has been confirmed by the ever increasing number of diverse upstream regulators, protein substrates and biological consequences of altered catalytic function. The aim of this review is to consolidate recent findings concerning the role of Cdk5 in the developing nervous system, particularly the cerebral cortex, where its importance is most clearly evidenced.

Epilepsy in Dcx knockout mice associated with discrete lamination defects and enhanced excitability in the hippocampus

Patients with Doublecortin (DCX) mutations have severe cortical malformations associated with mental retardation and epilepsy. Dcx knockout (KO) mice show no major isocortical abnormalities, but have discrete hippocampal defects. We questioned the functional consequences of these defects and report here that Dcx KO mice are hyperactive and exhibit spontaneous convulsive seizures. Changes in neuropeptide Y and calbindin expression, consistent with seizure occurrence, were detected in a large proportion of KO animals, and convulsants, including kainate and pentylenetetrazole, also induced seizures more readily in KO mice. We show that the dysplastic CA3 region in KO hippocampal slices generates sharp wave-like activities and possesses a lower threshold for epileptiform events. Video-EEG monitoring also demonstrated that spontaneous seizures were initiated in the hippocampus. Similarly, seizures in human patients mutated for DCX can show a primary involvement of the temporal lobe. In conclusion, seizures in Dcx KO mice are likely to be due to abnormal synaptic transmission involving heterotopic cells in the hippocampus and these mice may therefore provide a useful model to further study how lamination defects underlie the genesis of epileptiform activities.

The potential role of cyclin-dependent kinase 5 in focal cortical dysplasia

Focal cortical dysplasia (FCD) is the most common malformation of cortical development found in epilepsy surgical series. Characterised by cortical mislamination, dysplastic neurons and, in a subgroup of cases, balloon cells, FCD is potently epileptogenic. Despite decades of study, the underlying aetiology of FCD remains uncertain and research has been hampered by the lack of a good animal model in which to simulate the condition. In this article we review some of the potential molecular mechanisms that might underpin human FCD. In particular we examine the potential role of cyclin-dependent kinase 5 and its principal activator p35 in FCD and estimate the contribution that deregulation of cyclin-dependent kinase 5 might make to the pathogenesis of this condition.

Alternative roles for Cdk5 in learning and synaptic plasticity

Protein kinases mediate the intracellular signal transduction pathways controlling synaptic plasticity in the central nervous system. While the majority of protein kinases achieve this function via the phosphorylation of synaptic substrates, some kinases may contribute through alternative mechanisms in addition to enzymatic activity. There is growing evidence that protein kinases may often play structural roles in plasticity as well. Cyclin-dependent kinase 5 (Cdk5) has been implicated in learning and synaptic plasticity. Initial scrutiny focused on its enzymatic activity using pharmacological inhibitors and genetic modifications of Cdk5 cofactors. Quite recently Cdk5 has been shown to govern learning and plasticity via regulation of glutamate receptor degradation, a function that may not dependent on phosphorylation of downstream effectors. From these new studies, two roles emerge for Cdk5 in plasticity: one in which it controls structural plasticity via phosphorylation of synaptic substrates, and a second where it regulates functional plasticity via protein-protein interactions.

Structural consequences of Kcna1 gene deletion and transfer in the mouse hippocampus

PURPOSE

Mice lacking the Kv1.1 potassium channel alpha subunit encoded by the Kcna1 gene develop recurrent behavioral seizures early in life. We examined the neuropathological consequences of seizure activity in the Kv1.1(-/-) (knock-out) mouse, and explored the effects of injecting a viral vector carrying the deleted Kcna1 gene into hippocampal neurons.

METHODS

Morphological techniques were used to assess neuropathological patterns in hippocampus of Kv1.1(-/-) animals. Immunohistochemical and biochemical techniques were used to monitor ion channel expression in Kv1.1(-/-) brain. Both wild-type and knockout mice were injected (bilaterally into hippocampus) with an HSV1 amplicon vector that contained the rat Kcna1 subunit gene and/or the E. coli lacZ reporter gene. Vector-injected mice were examined to determine the extent of neuronal infection.

RESULTS

Video/EEG monitoring confirmed interictal abnormalities and seizure occurrence in Kv1.1(-/-) mice. Neuropathological assessment suggested that hippocampal damage (silver stain) and reorganization (Timm stain) occurred only after animals had exhibited severe prolonged seizures (status epilepticus). Ablation of Kcna1 did not result in compensatory changes in expression levels of other related ion channel subunits. Vector injection resulted in infection primarily of granule cells in hippocampus, but the number of infected neurons was quite variable across subjects. Kcna1 immunocytochemistry showed "ectopic" Kv1.1 alpha channel subunit expression.

CONCLUSIONS

Kcna1 deletion in mice results in a seizure disorder that resembles--electrographically and neuropathologically--the patterns seen in rodent models of temporal lobe epilepsy. HSV1 vector-mediated gene transfer into hippocampus yielded variable neuronal infection.

Newborn neurons with hilar basal dendrites hallmark epileptogenic networks

Basal dendrites are transiently present on developing newly generated granule cells in adult rats. In epileptic rats, basal dendrites often fail to retract, resulting in the generation of persisting hilar basal dendrites that have been proposed to contribute to hyperexcitability. Here, we demonstrate that the formation of hilar basal dendrites also occurs in the absence of seizures following induction of an epileptogenic neuronal network by amygdala kindling. The number of newly generated neurons with hilar basal dendrites was significantly increased 6-8 weeks after the last kindled seizure. Thus, even in periods without seizure activity, persistence of hilar basal dendrites presents a hallmark of hyperexcitable epileptogenic networks in the rodent brain and may contribute to disease progression.

Magnetic resonance imaging and histological studies of corpus callosal and hippocampal abnormalities linked to doublecortin deficiency

Mutated doublecortin (DCX) gives rise to severe abnormalities in human cortical development. Adult Dcx knockout mice show no major neocortical defects but do have a disorganized hippocampus. We report here the developmental basis of these hippocampal abnormalities. A heterotopic band of neurons was identified starting at E17.5 in the CA3 region and progressing throughout the CA1 region by E18.5. At neonatal stages, the CA1 heterotopic band was reduced, but the CA3 band remained unchanged, continuing into adulthood. Thus, in mouse, migration of CA3 neurons is arrested during development, whereas CA1 cell migration is retarded. On the Sv129Pas background, magnetic resonance imaging (MRI) also suggested abnormal dorsal hippocampal morphology, displaced laterally and sometimes rostrally and associated with medial brain structure abnormalities. MRI and cryosectioning showed agenesis of the corpus callosum in Dcx knockout mice on this background and an intermediate, partial agenesis in heterozygote mice. Wild-type littermates showed no callosal abnormalities. Hippocampal and corpus callosal abnormalities were also characterized in DCX-mutated human patients. Severe hippocampal hypoplasia was identified along with variable corpus callosal defects ranging from total agenesis to an abnormally thick or thin callosum. Our data in the mouse, identifying roles for Dcx in hippocampal and corpus callosal development, might suggest intrinsic roles for human DCX in the development of these structures.

Dentate development in organotypic hippocampal slice cultures from p35 knockout mice

Abnormal brain development, induced by genetic influences or resulting from a perinatal trauma, has been recognized as a cause of seizure disorders. To understand how and when these structural abnormalities form, and how they are involved in epileptogenesis, it is important to generate and investigate animal models. We have studied one such model, a mouse in which deletion of the p35 gene (p35-/-) gives rise to both structural disorganization and seizure-like function. We now report that aberrant dentate development can be recognized in the organotypic hippocampal slice culture preparation generated from p35-/- mouse pups. In these p35-/- cultures, an abnormally high proportion of dentate granule cells migrates into the hilus and molecular layer, and develops aberrant dendritic and axonal morphology. In addition, astrocyte formation in the dentate gyrus is disturbed, as is the distribution of GABAergic interneurons. Although the p35-/- brain shows widespread abnormalities, the disorganization of the hippocampal dentate region is particularly intriguing since a similar pathology is often found in hippocampi of temporal lobe epilepsy patients. The abnormal granule cell features occur early in development, and are independent of seizure activity. Further, these aberrant patterns and histopathological features of p35-/- culture preparations closely resemble those observed in p35 knockout mice in vivo. This culture preparation thus provides an experimentally accessible window for studying abnormal developmental factors that can result in seizure propensity.

Cyclin-dependent kinase 5 in synaptic plasticity, learning and memory

Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine kinase with a multitude of functions. Although Cdk5 is widely expressed, it has been studied most extensively in neurons. Since its initial characterization, the fundamental contribution of Cdk5 to an impressive range of neuronal processes has become clear. These phenomena include neural development, dopaminergic function and neurodegeneration. Data from different fields have recently converged to provide evidence for the participation of Cdk5 in synaptic plasticity, learning and memory. In this review, we consider recent data implicating Cdk5 in molecular and cellular mechanisms underlying synaptic plasticity. We relate these findings to its emerging role in learning and memory. Particular attention is paid to the activation of Cdk5 by p25, which enhances hippocampal synaptic plasticity and memory, and suggests formation of p25 as a physiological process regulating synaptic plasticity and memory.

Differential growth factor regulation of aspartyl-(asparaginyl)-beta-hydroxylase family genes in SH-Sy5y human neuroblastoma cells

Background

Aspartyl (asparaginyl)-β-hydroxylase (AAH) hydroxylates Asp and Asn residues within EGF-like domains of Notch and Jagged, which mediate cell motility and differentiation. This study examines the expression, regulation and function of AAH, and its related transcripts, Humbug and Junctin, which lack catalytic domains, using SH-Sy5y neuroblastoma cells.

Results

Real time quantitative RT-PCR demonstrated 8- or 9-fold higher levels of Humbug than AAH and Junctin, and lower levels of all 3 transcripts in normal human brains compared with neuroblastic tumor cells. AAH and Humbug expression were significantly increased in response to insulin and IGF-I stimulation, and these effects were associated with increased directional motility. However, over-expression of AAH and not Humbug significantly increased motility. Treatment with chemical inhibitors of Akt, Erk MAPK, or cyclin-dependent kinase 5 (Cdk-5) significantly reduced IGF-I stimulated AAH and Humbug expression and motility relative to vehicle-treated control cells. In addition, significantly increased AAH and Humbug expression and directional motility were observed in cells co-transfected with Cdk-5 plus its p35 or p25 regulatory partner. Further studies demonstrated that activated Cdk-5 mediated its stimulatory effects on AAH through Erk MAPK and PI3 kinase.

Conclusion

AAH and Humbug are over-expressed in SH-Sy5y neuroblastoma cells, and their mRNAs are regulated by insulin/IGF-1 signaling through Erk MAPK, PI3 kinase-Akt, and Cdk-5, which are known mediators of cell migration. Although AAH and Humbug share regulatory signaling pathways, AAH and not Humbug mediates directional motility in SH-Sy5y neuroblastoma cells.

Molecular signaling mechanisms underlying epileptogenesis

Epilepsy, a disorder of recurrent seizures, is a common and frequently devastating neurological condition. Available therapy is only symptomatic and often ineffective. Understanding epileptogenesis, the process by which a normal brain becomes epileptic, may help identify molecular targets for drugs that could prevent epilepsy. A number of acquired and genetic causes of this disorder have been identified, and various in vivo and in vitro models of epileptogenesis have been established. Here, we review current insights into the molecular signaling mechanisms underlying epileptogenesis, focusing on limbic epileptogenesis. Study of different models reveals that activation of various receptors on the surface of neurons can promote epileptogenesis; these receptors include ionotropic and metabotropic glutamate receptors as well as the TrkB neurotrophin receptor. These receptors are all found in the membrane of a discrete signaling domain within a particular type of cortical neuron--the dendritic spine of principal neurons. Activation of any of these receptors results in an increase Ca2+ concentration within the spine. Various Ca2+-regulated enzymes found in spines have been implicated in epileptogenesis; these include the nonreceptor protein tyrosine kinases Src and Fyn and a serine-threonine kinase [Ca2+-calmodulin-dependent protein kinase II (CaMKII)] and phosphatase (calcineurin). Cross-talk between astrocytes and neurons promotes increased dendritic Ca2+ and synchronous firing of neurons, a hallmark of epileptiform activity. The hypothesis is proposed that limbic epilepsy is a maladaptive consequence of homeostatic responses to increases of Ca2+ concentration within dendritic spines induced by abnormal neuronal activity.

In vivo inactivation of pRb, p107 and p130 in murine neuroprogenitor cells leads to major CNS developmental defects and high seizure rates

Nestin-positive cells were targeted for pRb, p107 and p130 (pRb(f)) inactivation by expression of T(121), a truncated SV40 large T antigen that selectively binds to and inactivates pRb(f). Cre expression was initiated under GFAP control, resulting in T(121) expression restricted to neuroprogenitor cells beginning at embryonic day 11.5 (E11.5). Bi-transgenic embryos showed aberrant central nervous system (CNS) cell proliferation and apoptosis by E13.5. Defects in cortical development were evident with primary effects resulting in depletion of neural progenitors and aberrant cellular migration. Consequently, juvenile and adult brain morphology was reproducibly abnormal, including disorganization of neocortical, hippocampal and cerebellar regions. These aberrations resulted in behavioral phenotypes, including ataxia and seizures. The data indicate that inactivation of pRb(f) in radial glial cells, a population of neuroprogenitor cells, leads to specific disruptions in CNS patterning. The neuroprogenitor-restricted transgene expression provides a model in which to explore both developmental mechanisms and functional neurological outcomes.

Epilepsy: a review of selected clinical syndromes and advances in basic science

Epilepsy is a common neurologic disorder that manifests in diverse ways. There are numerous seizure types and numerous mechanisms by which the brain generates seizures. The two hallmarks of seizure generation are hyperexcitability of neurons and hypersynchrony of neural circuits. A large variety of mechanisms alters the balance between excitation and inhibition to predispose a local or widespread region of the brain to hyperexcitability and hypersynchrony. This review discusses five clinical syndromes that have seizures as a prominent manifestation. These five syndromes differ markedly in their etiologies and clinical features, and were selected for discussion because the seizures are generated at a different 'level' of neural dysfunction in each case: (1) mutation of a specific family of ion (potassium) channels in benign familial neonatal convulsions; (2) deficiency of the protein that transports glucose into the CNS in Glut-1 deficiency; (3) aberrantly formed local neural circuits in focal cortical dysplasia; (4) synaptic reorganization of limbic circuitry in temporal lobe epilepsy; and (5) abnormal thalamocortical circuit function in childhood absence epilepsy. Despite this diversity of clinical phenotype and mechanism, these syndromes are informative as to how pathophysiological processes converge to produce brain hyperexcitability and seizures.

Laminating the hippocampus

Lamination of neurons and fibre projections is a fundamental organizational principle of the mammalian cerebral cortex. A laminated organization is likely to be essential for cortical function, as studies in mutant mice have revealed causal relationships between lamination defects and functional deficits. Unveiling the determinants of the laminated cortical architecture will contribute to our understanding of how cortical functions have evolved in phylogenetic and ontogenetic development. Recently, the hippocampus, with its clearly segregated cell and fibre layers, has become a major subject of studies on cortical lamination.

Reeler homozygous mice exhibit enhanced susceptibility to epileptiform activity

PURPOSE

Seizures are observed frequently in humans with diffuse neuronal migration disorders. The reeler mutant mouse also exhibits a diffuse disruption of migration, yet no pro-epileptic phenotype has been reported for this model. Whether this disparity reflects a phenotypic difference that can be used to delineate the mechanisms associated with increasing seizure susceptibility or reflects a paucity of knowledge is unclear. Consequently, this study examined whether seizure susceptibility is altered in reeler mutant mice.

METHODS

In vivo (minimal electroshock delivered transcorneally) and in vitro techniques (field-potential recordings in neocortical and hippocampal brain slice preparations exposed to bicuculline methiodide) were used to determine whether the susceptibility to epileptiform activity is enhanced in reeler homozygous mice relative to controls. Adult (3-7 months) male reeler homozygotes (rl/rl) and controls (+/?) were identified based on their behavioral phenotype and were used in all experiments.

RESULTS

Minimal electroshock revealed that rl/rl mice, compared with controls, exhibited a lower threshold for electroshock-induced seizures (4.5 +/- 0.52 vs. 6.7 +/- 0.35 mA), and a higher incidence of behavioral seizures (median seizure score, class 4 vs. class 0) when animals were subjected to a 5-mA electroshock stimulus. Additionally, neocortical and hippocampal slices from rl/rl mice were more likely to generate spontaneous epileptiform activity after bicuculline application, compared with controls, and the duration of the epileptiform events elicited in 10-30 muM bicuculline was longer in slices from rl/rl mice.

CONCLUSIONS

These data demonstrate that rl/rl mice have enhanced seizure susceptibility that is in part intrinsic to the malformed neocortex and hippocampus. Thus in contrast to prior belief, most animal models of diffuse neuronal migration disorders do exhibit a pro-epileptic phenotype.

Cellular and molecular mechanisms of epilepsy in the human brain

Animal models have provided invaluable data for identifying the pathogenesis of epileptic disorders. Clearly, the relevance of these experimental findings would be strengthened by the demonstration that similar fundamental mechanisms are at work in the human epileptic brain. Epilepsy surgery has indeed opened the possibility to directly study the functional properties of human brain tissue in vitro, and to analyze the mechanisms underlying seizures and epileptogenesis. Here, we summarize the findings obtained over the last 40 years from electrophysiological, histochemical and molecular experiments made with the human brain tissue. In particular, this review will focus on (i) the synaptic and non-synaptic properties of neocortical neurons along with their ability to produce synchronous activity; (ii) the anatomical and functional alterations that characterize limbic structures in patients presenting with mesial temporal lobe epilepsy; (iii) the issue of antiepileptic drug action and resistance; and (iv) the pathophysiology of seizure genesis in Taylor's type focal cortical dysplasia. Finally, we will address some of the problems that are inherent to this type of experimental approach, in particular the lack of proper controls and possible strategies to obviate this limitation.

When good Cdk5 turns bad

The cyclin-dependent kinase-5 (Cdk5) is critical to normal mammalian development and has been implicated in synaptic plasticity, learning, and memory in the adult brain. But Cdk-5 activity has also been linked to neurodegenerative diseases. Could a single protein have opposing effects? A new study shows that production of a neuronal protein capable of regulating Cdk-5 activity can turn Cdk-5 from "good" to "bad." The findings may have implications for the development and treatment of conditions like Alzheimer's disease.

Seizures induce proliferation and dispersion of doublecortin-positive hippocampal progenitor cells

One neuropathological hallmark of temporal lobe epilepsy is granule cell dispersion, a widening of the hippocampal granule cell layer (GCL) with abnormally positioned excitatory neurons. The finding that seizure activity also induces adult hippocampal neurogenesis was taken largely as indicative of a regenerative attempt, not as part of the pathology. The aim of our study was to characterize a potential relationship between granule cell dispersion and seizure-induced neurogenesis. Kainic acid (KA)-induced seizures in mice led to increased cell proliferation and new neurons persisted for months after the seizures. We show that the proliferative stimulus did not affect nestin-expressing early precursor cells that primarily respond to physiologic mitogenic stimuli, but stimulated the division of late type-3 progenitor cells, which express doublecortin (DCX), a protein associated with cell migration. This delayed proliferation presumably interfered with migration, leading to a significant dispersion of DCX-positive progenitors and early postmitotic neurons within the dentate gyrus granule cell layer. We propose that initial seizures induce ectopic precursor cell proliferation resulting in the dispersion of immature neurons within the adult granule cell layer. Thus, seizure-generated neurons might contribute to the disease process of epilepsy.

Impairment of hippocampal long-term depression and defective spatial learning and memory in p35-/- mice

Cdk5 (cyclin-dependent kinase 5) activity is dependent upon association with one of two neuron-specific activators, p35 or p39. Genetic deletion of Cdk5 causes perinatal lethality with severe defects in corticogenesis and neuronal positioning. p35(-/-) mice are viable with milder histological abnormalities. Although substantial evidence implicates Cdk5 in synaptic plasticity, its role in learning and memory has not been evaluated using mutant mouse models. We report here that p35(-/-) mice have deficiencies in spatial learning and memory. Close examination of hippocampal circuitry revealed subtle histological defects in CA1 pyramidal cells. Furthermore, p35(-/-) mice exhibit impaired long-term depression and depotentiation of long-term potentiation in the Schaeffer collateral CA1 pathway. Moreover, the Cdk5-dependent phosphorylation state of protein phosphatase inhibitor-1 was increased in 4-week-old mice due to increased levels of p39, which co-localized with inhibitor-1 and Cdk5 in the cytoplasm. These results demonstrate that p35-dependent Cdk5 activity is important to learning and synaptic plasticity. Deletion of p35 may shift the substrate specificity of Cdk5 due to compensatory expression of p39.

Patterns of dentate granule cell responses to perforant path stimulation in epileptic mice with granule cell dispersion

In adult mice, intrahippocampal administration of kainic acid induces a structural modification of the granule cell layer reminiscent of granule cell dispersion (GCD) seen in humans with temporal lobe epilepsy. We tested that GCD might be involved in the patterns of granule cell responses to perforant path stimulation by recording field potentials in vivo after kainic acid-induced status epilepticus until the phase of chronic seizure activity in presence of GCD or after its alteration by K252a co-treatment, an inhibitor of tyrosine kinase activities. Stimulation triggered bursts of multiple population spikes, the number of which progressively increased with time whereas their amplitude decreased in parallel with the progressive decrease in granule cell density. The population spike threshold was reached for a lower excitatory synaptic drive than in controls, as assessed by the initial slope of the field excitatory post-synaptic potential. This indicates that, for identical synaptic responses, granule cells were closer to the firing threshold. Fast inhibition, assessed by paired pulse stimulation, was compromised immediately after the initial status epilepticus, consistent with the rapid loss of most hilar cells. Neither the epileptic course nor the epileptiform responses of the granule cells were modified and manipulation by alteration following GCD manipulation while granule cell neuropeptide-Y immunostaining was substantially decreased. In this mouse model of TLE, granule cells display a progressive increase in epileptiform responses to afferent input until the occurrence of spontaneous seizures. The population spike amplitude decreases in parallel with GCD while the granule cell excitability is enhanced. Consequently, data from field potentials in epilepsy experiments should be interpreted with care, taking into account the possible variations in the neuronal density in the recorded area.

IRM des malformations de l’hippocampe dans l’épilepsie temporale réfractaire

OBJECTIVE

In some patients with temporal lobe epilepsy, recent MRI studies have revealed several morphological features indicative of discrete hippocampal malformation (HM). Its prevalence is unknown and the relationship between the HM and the origin of seizures has never been investigated. Our purpose is to define the MRI findings of this new entity and to determine its incidence in a group of patients and in a control group in order to evaluate its clinical significance.

MATERIALS AND METHODS

MR imaging findings in 97 patients suffering from medically intraceable temporal epilepsy were prospectively evaluated during the preoperative evaluation of surgical candidates. The MR-imaging protocol included oblique coronal slices perpendicular to the temporal lobes using high resolution T2 weighted (HR TSE T2), Fluid attenuated inversion recovery (FLAIR) and inversion-images. This protocol has been completed by axial FLAIR images and axial and sagittal IR images of the whole brain. Coronal HR TSE T2 images were performed in 50 healthy control subjects. Cerebral lesion and hippocampal morphology were evaluated in both groups.

RESULTS

Fourteen patients (14%) showed hippocampal morphological modification. The most frequent and specific findings were lack of visualization of the internal hippocampal (lack of linear T2 hypointensity within the hippocampus) and the abnormal shape (pyramidal, vertically oriented or globular-shaped). Other signs were: abnormal position of the hippocampus (medically located hippocampus) and vertical collateral sulcus. Cases without visualization of the internal structure of the hippocampus were considered as a complete form of HM and were correlated with temporal epilepsy. A vertical collateral sulcus was observed in some control group subjects.

CONCLUSION

Complete forms of HM could be considered as epileptogenic lesions. Nevertheless, interpretation of the incomplete form is delicate: the abnormal angle of the collateral sulcus can be encountered in healthy subjects and could therefore be considered a normal variant.