Persistent increased DNA-binding and expression of serum response factor occur with epilepsy-associated long-term plasticity changes

Molecular Genetics of Acquired Temporal Lobe Epilepsy

An epilepsy diagnosis reduces a patient's quality of life tremendously, and it is a fate shared by over 50 million people worldwide. Temporal lobe epilepsy (TLE) is largely considered a nongenetic or acquired form of epilepsy that develops in consequence of neuronal trauma by injury, malformations, inflammation, or a prolonged (febrile) seizure. Although extensive research has been conducted to understand the process of epileptogenesis, a therapeutic approach to stop its manifestation or to reliably cure the disease has yet to be developed. In this review, we briefly summarize the current literature predominately based on data from excitotoxic rodent models on the cellular events proposed to drive epileptogenesis and thoroughly discuss the major molecular pathways involved, with a focus on neurogenesis-related processes and transcription factors. Furthermore, recent investigations emphasized the role of the genetic background for the acquisition of epilepsy, including variants of neurodevelopmental genes. Mutations in associated transcription factors may have the potential to innately increase the vulnerability of the hippocampus to develop epilepsy following an injury-an emerging perspective on the epileptogenic process in acquired forms of epilepsy.

Impaired Glutamate Receptor Function Underlies Early Activity Loss of Ipsilesional Motor Cortex after Closed-Head Mild Traumatic Brain Injury

Although mild traumatic brain injury (mTBI) accounts for the majority of TBI patients, the effects and cellular and molecular mechanisms of mTBI on cortical neural circuits are still not well understood. Given the transient and non-specific functional deficits after mTBI, it is important to understand whether mTBI causes functional deficits of the brain and the underlying mechanism, particularly during the early stage after injury. Here, we used in vivo optogenetic motor mapping to determine longitudinal changes in cortical motor map and in vitro calcium imaging to study how changes in cortical excitability and calcium signals may contribute to the motor deficits in a closed-head mTBI model. In channelrhodopsin 2 (ChR2)-expressing transgenic mice, we recorded electromyograms (EMGs) from bicep muscles induced by scanning blue laser on the motor cortex. There were significant decreases in the size and response amplitude of motor maps of the injured cortex at 2 h post-mTBI, but an increase in motor map size of the contralateral cortex in 12 h post-mTBI, both of which recovered to baseline level in 24 h. Calcium imaging of cortical slices prepared from green fluorescent calmodulin proteins-expressing transgenic mice showed a lower amplitude, but longer duration, of calcium transients of the injured cortex in 2 h post-mTBI. Blockade of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or N-methyl-d-aspartate receptors resulted in smaller amplitude of calcium transients, suggesting impaired function of both receptor types. Imaging of calcium transients evoked by glutamate uncaging revealed reduced response amplitudes and longer duration in 2, 12, and 24 h after mTBI. Higher percentages of neurons of the injured cortex had a longer latency period after uncaging than that of the uninjured neurons. The results suggest that impaired glutamate neurotransmission contributes to functional deficits of the motor cortex in vivo, which supports enhancing glutamate neurotransmission as a potential therapeutic approach for the treatment of mTBI.

SRF modulates seizure occurrence, activity induced gene transcription and hippocampal circuit reorganization in the mouse pilocarpine epilepsy model

A hallmark of temporal lobe epilepsy (TLE) is hippocampal neuronal demise and aberrant mossy fiber sprouting. In addition, unrestrained neuronal activity in TLE patients induces gene expression including immediate early genes (IEGs) such as Fos and Egr1.

We employed the mouse pilocarpine model to analyze the transcription factor (TF) serum response factor (SRF) in epileptogenesis, seizure induced histopathology and IEG induction. SRF is a neuronal activity regulated TF stimulating IEG expression as well as nerve fiber growth and guidance. Adult conditional SRF deficient mice (Srf^CaMKCreERT2) were more refractory to initial status epilepticus (SE) acquisition. Further, SRF deficient mice developed more spontaneous recurrent seizures (SRS). Genome-wide transcriptomic analysis uncovered a requirement of SRF for SE and SRS induced IEG induction (e.g. Fos, Egr1, Arc, Npas4, Btg2, Atf3). SRF was required for epilepsy associated neurodegeneration, mossy fiber sprouting and inflammation. We uncovered MAP kinase signaling as SRF target during epilepsy. Upon SRF ablation, seizure evoked induction of dual specific phosphatases (Dusp5 and Dusp6) was reduced. Lower expression of these negative ERK kinase regulators correlated with altered P-ERK levels in epileptic Srf mutant animals.

Overall, this study uncovered an SRF contribution to several processes of epileptogenesis in the pilocarpine model.

Adult Deletion of SRF Increases Epileptogenesis and Decreases Activity-Induced Gene Expression

Although the transcription factor serum response factor (SRF) has been suggested to play a role in activity-dependent gene expression and mediate plasticity-associated structural changes in the hippocampus, no unequivocal evidence has been provided for its role in brain pathology, such as epilepsy. A genome-wide program of activity-induced genes that are regulated by SRF also remains unknown. In the present study, we show that the inducible and conditional deletion of SRF in the adult mouse hippocampus increases the epileptic phenotype in the kainic acid model of epilepsy, reflected by more severe and frequent seizures. Moreover, we observe a robust decrease in activity-induced gene transcription in SRF knockout mice. We characterize the genetic program controlled by SRF in neurons and using functional annotation, we find that SRF target genes are associated with synaptic plasticity and epilepsy. Several of these SRF targets function as regulators of inhibitory or excitatory balance and the structural plasticity of neurons. Interestingly, mutations in those SRF targets have found to be associated with such human neuropsychiatric disorders, as autism and intellectual disability. We also identify novel direct SRF targets in hippocampus: Npas4, Gadd45g, and Zfp36. Altogether, our data indicate that proteins that are highly upregulated by neuronal stimulation, identified in the present study as SRF targets, may function as endogenous protectors against overactivation. Thus, the lack of these effector proteins in SRF knockout animals may lead to uncontrolled excitation and eventually epilepsy.

Overexpression of Serum Response Factor in Neurons Restores Ocular Dominance Plasticity in a Model of Fetal Alcohol Spectrum Disorders

BACKGROUND

Deficits in neuronal plasticity underlie many neurobehavioral and cognitive problems presented in fetal alcohol spectrum disorder (FASD). Our laboratory has developed a ferret model showing that early alcohol exposure leads to a persistent disruption in ocular dominance plasticity (ODP). For instance, a few days of monocular deprivation results in a robust reduction of visual cortex neurons' responsiveness to stimulation of the deprived eye in normal animals, but not in ferrets with early alcohol exposure. Previously our laboratory demonstrated that overexpression of serum response factor (SRF) exclusively in astrocytes can improve neuronal plasticity in FASD. Here, we test whether neuronal overexpression of SRF can achieve similar effects.

METHODS

Ferrets received 3.5 g/kg alcohol intraperitoneally (25% in saline) or saline as control every other day between postnatal day 10 to 30, which is roughly equivalent to the third trimester of human gestation. Animals were given intracortical injections of a Herpes Simplex Virus-based vector to express either green fluorescent protein or a constitutively active form of SRF in infected neurons. They were then monocularly deprived by eyelid suture for 4 to 5 days after which single-unit recordings were conducted to determine whether changes in ocular dominance had occurred.

RESULTS

Overexpression of a constitutively active form of SRF by neurons restored ODP in alcohol-treated animals. This effect was observed only in areas near the site of viral infection.

CONCLUSIONS

Overexpression of SRF in neurons can restore plasticity in the ferret model of FASD, but only in areas near the site of infection. This contrasts with SRF overexpression in astrocytes which restored plasticity throughout the visual cortex.

Calcium signaling and epilepsy

Calcium signaling is involved in a multitude of physiological and pathophysiological mechanisms. Over the last decade, it has been increasingly recognized as an important factor in epileptogenesis, and it is becoming obvious that the excess synchronization of neurons that is characteristic for seizures can be linked to various calcium signaling pathways. These include immediate effects on membrane excitability by calcium influx through ion channels as well as delayed mechanisms that act through G-protein coupled pathways. Calcium signaling is able to cause hyperexcitability either by direct modulation of neuronal activity or indirectly through calcium-dependent gliotransmission. Furthermore, feedback mechanisms between mitochondrial calcium signaling and reactive oxygen species are able to cause neuronal cell death and seizures. Unravelling the complexity of calcium signaling in epileptogenesis is a daunting task, but it includes the promise to uncover formerly unknown targets for the development of new antiepileptic drugs.

Statistical parametric mapping reveals regional alterations in cannabinoid CB1 receptor distribution and G-protein activation in the 3D reconstructed epileptic rat brain

PURPOSE

  The endocannabinoid system is known to modulate seizure activity in several in vivo and in vitro models, and CB(1) -receptor activation is anticonvulsant in the rat pilocarpine model of acquired epilepsy (AE). In these epileptic rats, a unique redistribution of the CB(1) receptor occurs within the hippocampus; however, an anatomically inclusive analysis of the effect of status epilepticus (SE)-induced AE on CB(1) receptors has not been thoroughly evaluated. Therefore, statistical parametric mapping (SPM), a whole-brain unbiased approach, was used to study the long-term effect of pilocarpine-induced SE on CB(1) -receptor binding and G-protein activation in rats with AE.

METHODS

  Serial coronal sections from control and epileptic rats were cut at equal intervals throughout the neuraxis and processed for [(3) H]WIN55,212-2 (WIN) autoradiography, WIN-stimulated [(35) S]GTPγS autoradiography, and CB(1) -receptor immunohistochemistry (IHC). The autoradiographic techniques were evaluated with both region of interest (ROI) and SPM analyses.

KEY FINDINGS

  In rats with AE, regionally specific increases in CB(1) -receptor binding and activity were detected in cortex, discrete thalamic nuclei, and other regions including caudate-putamen and septum, and confirmed by IHC. However, CB(1) receptors were unaltered in several brain regions, including substantia nigra and cerebellum, and did not exhibit regional decreases in rats with AE.

SIGNIFICANCE

  This study provides the first comprehensive evaluation of the regional distribution of changes in CB(1) -receptor expression, binding, and G-protein activation in the rat pilocarpine model of AE. These regions may ultimately serve as targets for cannabinomimetic compounds or manipulation of the endocannabinoid system in epileptic brain.

Role of serum response factor in the pathogenesis of disease

Serum response factor (SRF) is a ubiquitously expressed transcription factor that binds to a DNA cis element known as the CArG box, which is found in the proximal regulatory regions of over 200 experimentally validated target genes. Genetic deletion of SRF is incompatible with life in a variety of animals from different phyla. In mice, loss of SRF throughout the early embryo results in gastrulation defects precluding analyses in individual organ systems. Genetic inactivation studies using conditional or inducible promoters directing the expression of the bacteriophage Cre recombinase have shown a vital role for SRF in such cellular processes as contractility, cell migration, synaptic activity, inflammation, and cell survival. A growing number of experimental and human diseases are associated with changes in SRF expression, suggesting that SRF has a role in the pathogenesis of disease. This review summarizes data from experimental model systems and human pathology where SRF expression is either deliberately or naturally altered.

Functional versatility of transcription factors in the nervous system: the SRF paradigm

Individual transcription factors in the brain frequently display broad functional versatility, thereby controlling multiple cellular outputs. In accordance, neuron-restricted mutagenesis of the murine Srf gene, encoding the transcription factor serum response factor (SRF), revealed numerous SRF functions in the nervous system. First, SRF controls immediate early gene (IEG) activation associated with perception of synaptic activity, learning and memory. Second, processes linked to actin cytoskeletal dynamics are mediated by SRF, such as developmental neuronal migration, outgrowth and pathfinding of neurites, as well as synaptic targeting. Therefore, SRF seems to be instrumental in converting synaptic activity into plasticity-associated structural changes in neuronal connectivities. This highlights the decisive role of SRF in integrating cytoskeletal actin dynamics and nuclear gene expression. Finally, we relate SRF to the multi-functional transcription factor CREB and point out overlapping, distinct and concerted functions of these two transcriptional regulators in the brain.

Postsynaptic mechanisms of excitotoxicity: Involvement of postsynaptic density proteins, radicals, and oxidant molecules

Traditional models of neuronal excitotoxicity focused on the overactivation of receptors such as the ionotropic N-methyl-D-aspartate (NMDA)-subtype glutamate receptor. Recent developments have shifted focus to downstream neurotoxic signaling molecules with exciting implications to specific strategies for treating excitotoxic disorders. This review outlines these developments and introduces newly emerging evidence implicating the involvement of the melastatin subfamily in anoxic neuronal death. Both of these converge on the production of reactive oxygen species (ROS), including superoxide, nitric oxide (NO) and the oxidant peroxynitrite.

Traumatic brain injury causes a long-lasting calcium (Ca2+)-plateau of elevated intracellular Ca levels and altered Ca2+ homeostatic mechanisms in hippocampal neurons surviving brain injury

Traumatic brain injury (TBI) survivors often suffer chronically from significant morbidity associated with cognitive deficits, behavioral difficulties and a post-traumatic syndrome and thus it is important to understand the pathophysiology of these long-term plasticity changes after TBI. Calcium (Ca2+) has been implicated in the pathophysiology of TBI-induced neuronal death and other forms of brain injury including stroke and status epilepticus. However, the potential role of long-term changes in neuronal Ca2+ dynamics after TBI has not been evaluated. In the present study, we measured basal free intracellular Ca2+ concentration ([Ca2+](i)) in acutely isolated CA3 hippocampal neurons from Sprague-Dawley rats at 1, 7 and 30 days after moderate central fluid percussion injury. Basal [Ca2+](i) was significantly elevated when measured 1 and 7 days post-TBI without evidence of neuronal death. Basal [Ca2+](i) returned to normal when measured 30 days post-TBI. In contrast, abnormalities in Ca2+ homeostasis were found for as long as 30 days after TBI. Studies evaluating the mechanisms underlying the altered Ca2+ homeostasis in TBI neurons indicated that necrotic or apoptotic cell death and abnormalities in Ca2+ influx and efflux mechanisms could not account for these changes and suggested that long-term changes in Ca2+ buffering or Ca2+ sequestration/release mechanisms underlie these changes in Ca2+ homeostasis after TBI. Further elucidation of the mechanisms of altered Ca2+ homeostasis in traumatized, surviving neurons in TBI may offer novel therapeutic interventions that may contribute to the treatment and relief of some of the morbidity associated with TBI.

Expression changes in mouse brains following nicotine-induced seizures: the modulation of transcription factor networks

Nicotine, acting through the neuronal nicotinic acetylcholine receptors (nAChRs), can induce seizures in mice. We aimed to study brain transcriptional response to seizure and to identify genes whose expression is altered after nicotine-induced seizures. Whole brains of untreated mice were compared with brains 1 h after seizure activity, using Affymetrix U74Av2 microarrays. Experimental groups included wild-type mice and both nicotine-induced seizure-sensitive and -resistant nAChR mutant mice. Each genotype group received different nicotine doses to generate seizures. This approach allowed the identification of significantly changed genes whose expression was dependent on seizure activity, nicotine administration, or both but not on the type of nAChR subunit mutation or the amount of nicotine injected. Significant expression changes were detected in 62 genes ( P < 0.05, false discovery rate correction). Among them, gene ontology functional annotation analysis determined that the most significantly overrepresented categories were of genes encoding MAP kinase phosphatases, regulators of transcription and nucleosome assembly proteins. In silico bioinformatic analysis of the promoter regions of the 62 changed genes detected significant enrichments of 16 transcription regulatory elements (TREs), creating a network of transcriptional regulatory responses to seizures. The TREs for activating transcription factor and serum response factor were most significantly enriched, supporting their association with seizure activity. Our data suggest that nicotine-induced seizure in mice is a useful model to study seizure activity and its global brain transcriptional response. The differentially expressed genes detected here can help us to understand the molecular mechanisms underlying seizures in animal models and may also serve as candidate genes to study epilepsy in humans.

An in vitro model of Stroke‐Induced Epilepsy: Elucidation of The roles of Glutamate and Calcium in The induction and Maintenance of Stroke‐Induced Epileptogenesis

Stroke is a major risk factor for developing acquired epilepsy (AE). Although the underlying mechanisms of ischemia-induced epileptogenesis are not well understood, glutamate has been found to be associated with both epileptogenesis and ischemia-induced injury in several research models. This chapter discusses the development of an in vitro model of epileptogenesis induced by glutamate injury in hippocampal neurons, as found in a clinical stroke, and the implementation of this model of stroke-induced AE to evaluate calcium's role in the induction and maintenance of epileptogenesis. To monitor the acute effects of glutamate on neurons and chronic alterations in neuronal excitability up to 8 days after glutamate exposure, whole-cell current-clamp electrophysiology was employed. Various durations and concentrations of glutamate were applied to primary hippocampal cultures. A single 30-min, 5-microM glutamate exposure produced a subset of neurons that died or had a stroke-like injury, and a larger population of injured neurons that survived. Neurons that survived the injury manifested spontaneous, recurrent, epileptiform discharges (SREDs) in neural networks characterized by paroxysmal depolarizing shifts (PDSs) and high-frequency spike firing that persisted for the life of the culture. The neuronal injury produced in this model was evaluated by determining the magnitude of the prolonged, reversible membrane depolarization, loss of synaptic activity, and neuronal swelling. The permanent epileptiform phenotype expressed as SREDs that resulted from glutamate injury was found to be dependent on the presence of extracellular calcium. The "epileptic" neurons manifested elevated intracellular calcium levels when compared to control neurons, independent of neuronal activity and seizure discharge, demonstrating that alterations in calcium homeostatic mechanisms occur in association with stroke-induced epilepsy. Findings from this investigation present the first in vitro model of glutamate injury-induced epileptogenesis that may help elucidate some of the mechanisms that underlie stroke-induced epilepsy.

Erratum to "Cellular mechanisms underlying acquired epilepsy: the calcium hypothesis of the induction and maintenance of epilepsy." [Pharmacol. Ther. 105(3) (2005) 229-266]

Epilepsy is one of the most common neurological disorders. Although epilepsy can be idiopathic, it is estimated that up to 50% of all epilepsy cases are initiated by neurological insults and are called acquired epilepsy (AE). AE develops in 3 phases: (1) the injury [central nervous system (CNS) insult]. (2) epileptogenesis (latency), and (3) the chronic epileptic (spontaneous recurrent seizure) phases. Status epilepticus (SE), stroke, and traumatic brain injury (TBI) are 3 major examples of common brain injuries that can lead to the development of AE. It is especially important to understand the molecular mechanisms that cause AE because it may lead to innovative strategies to prevent or cure this common condition. Recent studies have offered new insights into the cause of AE and indicate that injury-induced alterations in intracellular calcium concentration levels ([Ca(2+)](i)) and calcium homeostatic mechanisms play a role in the development and maintenance of AE. The injuries that cause AE are different, but the share a common molecular mechanism for producing brain damage--an increase in extracellular glutamate and are exposed to increased [Ca(2+)](i) are the cellular substrates to develop epilepsy because dead cells do not seize. The neurons that survive injury sustain permanent long-term plasticity changes in [Ca(2+)](i) and calcium homeostatic mechanisms that are permanent and are a prominent feature of the epileptic phenotype. In the last several years, evidence has accumulated indicating that the prolonged alteration in neuronal calcium dynamics plays an important role in the induction and maintenance of the prolonged neuroplasticity changes underlying the epileptic phenotype. Understanding the role of calcium as a second messenger in the induction and maintenance of epilepsy may provide novel insights into therapeutic advances that will prevent and even cure AE.

Increased neuronal nuclear calcium influx in neonatal seizures

We hypothesized that neonatal seizures lead to increased Ca(2+) influx (nCa(2+)I) in neuronal nuclei of newborn rats and that such increase is nitric-oxide mediated. Neuronal nuclear (45)Ca(2+) influx (nCa(2+)I) was measured in neuronal nuclei of 25 10-day-old male rat-pups newborn brains. They were divided into five groups (n = 5/group). (I) control; (II) hypoxia without seizures; (III) hypoxia with seizures; (IV) kainate, 2 mg/kg intraperitoneal (i.p.)-induced seizures and (V) 7-nitroindazole (7-NINA), 1 mg/kg i.p. pretreated, kainate-induced seizures. nCa(2+)I was significantly (P < 0.05) increased following hypoxia or seizures (hypoxic- or kainate-induced). Post-hypoxic seizures further enhanced nCa(2+)I increase induced by hypoxia (P < 0.05). 7-NINA abated the nCa(2+)I increase induced by kainate. We conclude that (1) kainate or hypoxia-induced seizures in newborn rats modify the neuronal nuclear membrane function, resulting in increased nCa(2+)I, (2) seizures exacerbate the hypoxia-induced increased nCa(2+)I incurred after hypoxia and (3) intranuclear calcium surges during kainate-induced neonatal seizures are nitric oxide-mediated.

Cellular mechanisms underlying acquired epilepsy: the calcium hypothesis of the induction and maintainance of epilepsy

Epilepsy is one of the most common neurological disorders. Although epilepsy can be idiopathic, it is estimated that up to 50% of all epilepsy cases are initiated by neurological insults and are called acquired epilepsy (AE). AE develops in 3 phases: (1) the injury (central nervous system [CNS] insult), (2) epileptogenesis (latency), and (3) the chronic epileptic (spontaneous recurrent seizure) phases. Status epilepticus (SE), stroke, and traumatic brain injury (TBI) are 3 major examples of common brain injuries that can lead to the development of AE. It is especially important to understand the molecular mechanisms that cause AE because it may lead to innovative strategies to prevent or cure this common condition. Recent studies have offered new insights into the cause of AE and indicate that injury-induced alterations in intracellular calcium concentration levels [Ca(2+)](i) and calcium homeostatic mechanisms play a role in the development and maintenance of AE. The injuries that cause AE are different, but they share a common molecular mechanism for producing brain damage-an increase in extracellular glutamate concentration that causes increased intracellular neuronal calcium, leading to neuronal injury and/or death. Neurons that survive the injury induced by glutamate and are exposed to increased [Ca(2+)](i) are the cellular substrates to develop epilepsy because dead cells do not seize. The neurons that survive injury sustain permanent long-term plasticity changes in [Ca(2+)](i) and calcium homeostatic mechanisms that are permanent and are a prominent feature of the epileptic phenotype. In the last several years, evidence has accumulated indicating that the prolonged alteration in neuronal calcium dynamics plays an important role in the induction and maintenance of the prolonged neuroplasticity changes underlying the epileptic phenotype. Understanding the role of calcium as a second messenger in the induction and maintenance of epilepsy may provide novel insights into therapeutic advances that will prevent and even cure AE.

Evidence that injury-induced changes in hippocampal neuronal calcium dynamics during epileptogenesis cause acquired epilepsy

Alterations in hippocampal neuronal Ca(2+) and Ca(2+)-dependent systems have been implicated in mediating some of the long-term neuroplasticity changes associated with acquired epilepsy (AE). However, there are no studies in an animal model of AE that directly evaluate alterations in intracellular calcium concentration ([Ca(2+)](i)) and Ca(2+) homeostatic mechanisms (Ca(2+) dynamics) during the development of AE. In this study, Ca(2+) dynamics were evaluated in acutely isolated rat CA1 hippocampal, frontal, and occipital neurons in the pilocarpine model by using [Ca(2+)](i) imaging fluorescence microscopy during the injury (acute), epileptogenesis (latency), and chronic-epilepsy phases of the development of AE. Immediately after status epilepticus (SE), hippocampal neurons, but not frontal and occipital neurons, had significantly elevated [Ca(2+)](i) compared with saline-injected control animals. Hippocampal neuronal [Ca(2+)](i) remained markedly elevated during epileptogenesis and was still elevated indefinitely in the chronic-epilepsy phase but was not elevated in SE animals that did not develop AE. Inhibiting the increase in [Ca(2+)](i) during SE with the NMDA channel inhibitor MK801 was associated in all three phases of AE with inhibition of the changes in Ca(2+) dynamics and the development of AE. Ca(2+) homeostatic mechanisms in hippocampal neurons also were altered in the brain-injury, epileptogenesis, and chronic-epilepsy phases of AE. These results provide evidence that [Ca(2+)](i) and Ca(2+)-homeostatic mechanisms are significantly altered during the development of AE and suggest that altered Ca(2+) dynamics may play a role in the induction and maintenance of AE and underlie some of the neuroplasticity changes associated with the epileptic phenotype.

Long-lasting alterations in neuronal calcium homeostasis in an in vitro model of stroke-induced epilepsy

Altered calcium homeostatic mechanisms have been implicated in the development of acquired epilepsy in numerous models. Stroke is one of the leading brain injuries that cause acquired epilepsy, yet little is known concerning the molecular mechanisms underlying stroke-induced epileptogenesis. Recently an in vitro model of stroke-induced epilepsy was developed and characterized as a powerful tool to study the pathophysiology of injury and stroke-induced epileptogenesis. Using this glutamate injury-induced epileptogenesis model, we have investigated the role of altered calcium homeostatic mechanisms in the development and maintenance of stroke-induced epilepsy. Epileptic neurons manifested elevated intracellular calcium levels compared to control neurons independent of neuronal activity and seizure discharge for the remainder of the life of the neurons in culture. In addition, epileptic neurons were found to have alterations in the ability to reduce intracellular calcium levels following a calcium load. These long-term epileptic changes in calcium homeostasis were dependent on calcium during the initial glutamate injury. The data demonstrate that significant alterations in calcium homeostatic mechanisms occur in association with stroke-induced epilepsy and suggest that these changes may play a role in both the induction and maintenance of the epileptic phenotype in this model.

Rapid electrical and delayed molecular signals regulate the serum response element after nerve injury: convergence of injury and learning signals

Axotomy elicits changes in gene expression, but little is known about how information from the site of injury is communicated to the cell nucleus. We crushed nerves in Aplysia californica and the sciatic nerve in the mouse and found short- and long-term activation of an Elk1-SRF transcription complex that binds to the serum response element (SRE). The enhanced short-term binding appeared rapidly and was attributed to the injury-induced activation of an Elk1 kinase that phosphorylates Elk1 at ser383. This kinase is the previously described Aplysia (ap) ERK2 homologue, apMAPK. Nerve crush evoked action potentials that propagated along the axon to the cell soma. Exposing axons to medium containing high K(+), which evoked a similar burst of spikes, or bathing the ganglia in 20 microM serotonin (5HT) for 20 min, activated the apMAPK and enhanced SRE binding. Since 5HT is released in response to electrical activity, our data indicate that the short-term process is initiated by an injury-induced electrical discharge that causes the release of 5HT which activates apMAPK. 5HT is also released in response to noxious stimuli for aversive learning. Hence, apMAPK is a point of convergence for injury signals and learning signals. The delay before the onset of the long-term SRE binding was reduced when the crush was closer to the ganglion and was attributed to an Elk1 kinase that is activated by injury in the axon and retrogradely transported to the cell body. Although this Elk1 kinase phosphorylates mammalian rElk1 at ser383, it is distinct from apMAPK.

Calcium-dependent epileptogenesis in an in vitro model of stroke-induced "epilepsy"

PURPOSE

Stroke is the most common cause of acquired epilepsy. The purpose of this investigation was to characterize the role of calcium in the in vitro, glutamate injury-induced epileptogenesis model of stoke-induced epilepsy.

METHODS

Fura-2 calcium imaging and whole-cell current clamp electrophysiology techniques were used to measure short-term changes in neuronal free intracellular calcium concentration and long-term alterations in neuronal excitability in response to epileptogenic glutamate injury (20 microM, 10 min) under various extracellular calcium conditions and in the presence of different glutamate-receptor antagonists.

RESULTS

Glutamate injury-induced epileptogenesis was associated with prolonged, reversible elevations of free intracellular calcium concentration during and immediately after injury and chronic hyperexcitability manifested as spontaneous recurrent epileptiform discharges for the remaining life of the cultures. Epileptogenic glutamate exposure performed in solutions containing low extracellular calcium, barium substituted for calcium, or N-methyl-d-aspartate (NMDA)-receptor antagonists reduced the duration of intracellular calcium elevation and inhibited epileptogenesis. Antagonism of non-NMDA-receptor subtypes had no effect on glutamate injury-induced calcium changes or the induction epileptogenesis. The duration of the calcium elevation and the total calcium load statistically correlated with the development of epileptogenesis. Comparable elevations in neuronal calcium induced by non-glutamate receptor-mediated pathways did not cause epileptogenesis.

CONCLUSIONS

This investigation indicates that the glutamate injury-induced epileptogenesis model of stroke-induced epilepsy is calcium dependent and requires NMDA-receptor activation. Further, these experiments suggest that prolonged, reversible elevations in neuronal free intracellular calcium initiate the long-term plasticity changes that underlie the development of injury-induced epilepsy.

Developmental expression of serum response factor in the rat central nervous system

Serum response factor (SRF), a transcription factor known to be essential for early embryonic development as well as post-natal regulation of both cellular proliferation and myogenic differentiation, is expressed broadly in neurons within the adult mammalian central nervous system (CNS). The function of SRF within the developing CNS is not well established, but it is likely to play an important role in neuraxial development and neuronal function, since many of its known target genes (e.g., c-fos) and transcriptional partners (e.g., Elk-1) are also highly expressed in neurons. Immunohistochemical survey of the post-natal developing rat brain revealed a progressive increase in SRF immunoreactivity in neurons of the cerebral and cerebellar cortices, and in selective subcortical regions from birth (P0) through post-natal day 28 (P28). SRF immunoreactivity stabilized from P28 into adulthood. A few loci, such as the nucleus of cranial nerve VII, showed the reverse expression pattern (strong immunoreactivity at P0-P7, declining by P28). The developmental expression pattern of SRF overlaps significantly with that of myotonic dystrophy protein kinase, a potential upstream regulator, and of the LIM-only genes Lmo1, Lmo2 and Lmo3, whose products belong to a family of proteins known to be strong positive regulators of SRF's transcriptional activity. These data suggest that SRF has a significant function in the early post-natal development of the CNS.

Chronic inhibition of cortex microsomal Mg(2+)/Ca(2+) ATPase-mediated Ca(2+) uptake in the rat pilocarpine model following epileptogenesis

In the rat pilocarpine model, 1 h of status epilepticus caused significant inhibition of Mg(2+)/Ca(2+) ATPase-mediated Ca(2+) uptake in cortex endoplasmic reticulum (microsomes) isolated immediately after the status episode. The rat pilocarpine model is also an established model of acquired epilepsy. Several weeks after the initial status epilepticus episode, the rats develop spontaneous recurrent seizures, or epilepsy. To determine whether inhibition of Ca(2+) uptake persists after the establishment of epilepsy, Ca(2+) uptake was studied in cortical microsomes isolated from rats displaying spontaneous recurrent seizures for 1 year. The initial rate and total Ca(2+) uptake in microsomes from epileptic animals remained significantly inhibited 1 year after the expression of epilepsy compared to age-matched controls. The inhibition of Ca(2+) uptake was not due to individual seizures nor an artifact of increased Ca(2+) release from epileptic microsomes. In addition, the decreased Ca(2+) uptake was not due to either selective isolation of damaged epileptic microsomes from the homogenate or decreased Mg(2+)/Ca(2+) ATPase protein in the epileptic microsomes. The data demonstrate that inhibition of microsomal Mg(2+)/Ca(2+) ATPase-mediated Ca(2+) uptake in the pilocarpine model may underlie some of the long-term plasticity changes associated with epileptogenesis.

Epileptogenesis induces long-term alterations in intracellular calcium release and sequestration mechanisms in the hippocampal neuronal culture model of epilepsy

Calcium and calcium-dependent processes have been hypothesized to be involved in the induction of epilepsy. It has been shown that epileptic neurons have altered calcium homeostatic mechanisms following epileptogenesis in the hippocampal neuronal culture (HNC) and pilocarpine models of epilepsy. To investigate the mechanisms causing these alterations in [Ca2+]i homeostatic processes following epileptogenesis, we utilized the HNC model of in vitro 'epilepsy' which produces spontaneous recurrent epileptiform discharges (SREDs). Using [Ca2+]i imaging, studies were initiated to evaluate the mechanisms mediating these changes in [Ca2+]i homeostasis. 'Epileptic' neurons required much longer to restore a glutamate induced [Ca2+]i load to baseline levels than control neurons. Inhibition of Ca2+ entry through voltage and receptor gated Ca2+ channels and stretch activated Ca2+ channels had no effect on the prolonged glutamate induced increase in [Ca2+]i in epileptic neurons. Employing thapsigargin, an inhibitor of the sarco/endoplasmic reticulum calcium ATPase (SERCA), it was shown that thapsigargin inhibited sequestration of [Ca2+]i by SERCA was significantly decreased in 'epileptic' neurons. Using Ca2+ induced Ca2+ release (CICR) cell permeable inhibitors for the ryanodine receptor (dantrolene) and the IP3 receptor (2-amino-ethoxydiphenylborate, 2APB) mediated CICR, we demonstrated that CICR was significantly augmented in the 'epileptic' neurons, and determined that the IP3 receptor mediated CICR was the major release mechanism altered in epileptogenesis. These data indicate that both inhibition of SERCA and augmentation of CICR activity contribute to the alterations accounting for the impaired calcium homeostatic processes observed in 'epileptic' neurons. The results suggest that persistent changes in [Ca2+]i levels following epileptogenesis may contribute to the long-term plasticity changes manifested in epilepsy and that understanding the basic mechanisms mediating these changes may provide an insight into the development of novel therapeutic approaches to treat epilepsy and prevent or reverse epileptogenesis.

Long-term alteration of calcium homeostatic mechanisms in the pilocarpine model of temporal lobe epilepsy

The pilocarpine model of temporal lobe epilepsy is an animal model that shares many of the clinical and pathophysiological characteristics of temporal lobe or limbic epilepsy in humans. This model of acquired epilepsy produces spontaneous recurrent seizure discharges following an initial brain injury produced by pilocarpine-induced status epilepticus. Understanding the molecular mechanisms mediating these long lasting changes in neuronal excitability would provide an important insight into developing new strategies for the treatment and possible prevention of this condition. Our laboratory has been studying the role of alterations in calcium and calcium-dependent systems in mediating some of the long-term neuroplasticity changes associated with epileptogenesis. In this study, [Ca(2+)](i) imaging fluorescence microscopy was performed on CA1 hippocampal neurons acutely isolated from control and chronically epileptic animals at 1 year after the induction of epileptogenesis with two different fluorescent dyes (Fura-2 and Fura-FF) having high and low affinities for [Ca(2+)](i). The high affinity Ca(2+) indicator Fura-2 was utilized to evaluate [Ca(2+)](i) levels up to 900 nM and the low affinity indicator Fura-FF was employed for evaluating [Ca(2+)](i) levels above this range. Baseline [Ca(2+)](i) levels and the ability to restore resting [Ca(2+)](i) levels after a brief exposure to several glutamate concentrations in control and epileptic neurons were evaluated. Epileptic neurons demonstrated a statistically significantly higher baseline [Ca(2+)](i) level in comparison to age-matched control animals. This alteration in basal [Ca(2+)](i) levels persisted up to 1 year after the induction of epileptogenesis. In addition, the epileptic neurons were unable to rapidly restore [Ca(2+)](i) levels to baseline following the glutamate-induced [Ca(2+)](i) loads. These changes in Ca(2+) regulation were not produced by a single seizure and were not normalized by controlling the seizures in the epileptic animals with anticonvulsant treatment. Peak [Ca(2+)](i) levels in response to different concentrations of glutamate were the same in both epileptic and control neurons. Thus, glutamate produced the same initial [Ca(2+)](i) load in both epileptic and control neurons. Characterization of the viability of acutely isolated neurons from control and epileptic animals utilizing standard techniques to identify apoptotic or necrotic neurons demonstrated that epileptic neurons had no statistically significant difference in viability compared to age-matched controls. These results provide the first direct measurement of [Ca(2+)](i) levels in an intact model of epilepsy and indicate that epileptogenesis in this model produced long-lasting alterations in [Ca(2+)](i) homeostatic mechanisms that persist for up to 1 year after induction of epileptogenesis. These observations suggest that altered [Ca(2+)](i) homeostatic mechanisms may underlie some aspects of the epileptic phenotype and contribute to the persistent neuroplasticity changes associated with epilepsy.

Induction of spontaneous recurrent epileptiform discharges causes long-term changes in intracellular calcium homeostatic mechanisms

Calcium and calcium-dependent systems have been long implicated in the induction of epilepsy. We have previously observed that intracellular calcium ([Ca2+]i) levels remain elevated in cells undergoing epileptogenesis in the hippocampal neuronal culture (HNC) model. In this study, we employed the hippocampal neuronal culture (HNC) model of in vitro 'epilepsy' which produces spontaneous recurrent epileptiform discharges (SREDs) for the life of the neurons in culture to investigate alterations in [Ca2+]i homeostatic mechanisms that may be associated with the 'epileptic' phenotype. [Ca2+]i imaging fluorescence microscopy was performed on control and 'epileptic' neurons with two different fluorescent dyes ranging from high to low affinities for [Ca2+]i. We measured baseline [Ca2+]i levels and the ability to restore resting [Ca2+]i levels after a brief 2-min exposure to the excitatory amino acid glutamate in control neurons and neurons with SREDs. Neurons manifesting SREDs had statistically significantly higher baseline [Ca2+]i levels that persisted for the life of the culture. In addition, the 'epileptic' phenotype was associated with an inability to rapidly restore [Ca2+]i levels to baseline following a glutamate induced [Ca2+]i load. The use of the low affinity dye Fura-FF demonstrated that the difference in restoring baseline [Ca2+]i levels was not due to saturation of the high affinity dye Indo-1, which was utilized for evaluating the [Ca2+]i kinetics at lower [Ca2+]i levels. Peak [Ca2+]i levels in response to glutamate were the same in both 'epileptic' and control neurons. While [Ca2+]i levels recovered in approximately 30 min in control cells, it took more than 90 min to reach baseline levels in cells manifesting SREDs. Alterations of [Ca2+]i homeostatic mechanisms observed with the 'epileptic' phenotype were shown to be independent of the presence of continuous SREDs and persisted for the life of the neurons in culture. Epileptogenesis was shown not to affect the degree or duration of glutamate induced neuronal depolarization in comparing control and 'epileptic' neurons. The results indicate that epileptogenesis in this in vitro model produced long-lasting alterations in [Ca2+]i regulation that may underlie the 'epileptic' phenotype and contribute to the persistent neuroplasticity changes associated with epilepsy.

Pilocarpine-induced status epilepticus causes N-methyl-D-aspartate receptor-dependent inhibition of microsomal Mg(2+)/Ca(2+) ATPase-mediated Ca(2+) uptake

Status epilepticus is associated with sustained and elevated levels of cytosolic Ca(2+). To elucidate the mechanisms associated with changes of cytosolic Ca(2+) after status epilepticus, this study was initiated to evaluate the effect of pilocarpine-induced status epilepticus on Mg(2+)/Ca(2+) ATPase-mediated Ca(2+) uptake in microsomes isolated from rat cortex, because the Ca(2+) uptake mechanism plays a major role in regulating intracellular Ca(2+) levels. The data demonstrated that the initial rate and overall Ca(2+) uptake in microsomes from pilocarpine treated animals were significantly inhibited compared with those in microsomes from saline-treated control animals. It was also shown that the inhibition of Ca(2+) uptake caused by status epilepticus was not an artifact of increased Ca(2+) release from microsomes, selective isolation of damaged microsomes from the homogenate, or decreased Mg(2+)/Ca(2+) ATPase protein in the microsomes. Pretreatment with the NMDA antagonist dizocilpine maleate blocked status epilepticus-induced inhibition of the initial rate and overall Ca(2+) uptake. The data suggest that inhibition of microsomal Mg(2+)/Ca(2+) ATPase Ca(2+) uptake is involved in NMDA-dependent deregulation of cytosolic Ca(2+) homeostasis associated with status epilepticus.

Chronic DeltaFosB expression and increased AP-1 transcription factor binding are associated with the long term plasticity changes in epilepsy

NMDA receptor activation during status epilepticus (SE) has previously been shown to be required for epileptogenesis as well as the persistent upregulation of serum response factor (SRF) in the in vivo pilocarpine model of epilepsy. SRF is established as a regulator of the FosB gene which expresses FosB and DeltaFosB components of the AP-1 transcription factor complex. Therefore we investigated whether DeltaFosB expression and AP-1 DNA binding were also persistently elevated in pilocarpine-treated rats which chronically displayed spontaneous seizures. Using hippocampal nuclear extracts, DeltaFosB expression and AP-1 DNA binding were significantly elevated for up to one year in the epileptic animals. The expression of other fos and jun proteins was not persistently altered in epilepsy. Neuronal upregulation of DeltaFosB was correlated with regions of the brain that were involved in seizure generation and propagation. The increase in AP-1 DNA binding was shown to be dependent on NMDA receptor activation during SE. Hippocampal DeltaFosB immunostaining was seen predominately in the neuronal nuclei as opposed to other cell types. The data indicate that recurrent seizures which persistently occur in this model were not responsible for the increased DeltaFosB expression. Chronic DeltaFosB expression in epilepsy may be playing a role in the altered expression of other genes in this model and may be involved in some of the neuronal plasticity changes associated with epileptogenesis.