New roles for interleukin-1 Beta in the mechanisms of epilepsy

Sphingolipids and gangliosides of the nervous system in membrane function and dysfunction

Simple sphingolipids such as ceramide and sphingomyelin (SM) as well as more complex glycosphingolipids play very important roles in cell function under physiological conditions and during disease development and progression. Sphingolipids are particularly abundant in the nervous system. Due to their amphiphilic nature they localize to cellular membranes and many of their roles in health and disease result from membrane reorganization and from lipid interaction with proteins within cellular membranes. In this review we discuss some of the functions of sphingolipids in processes that entail cellular membranes and their role in neurodegenerative diseases, with an emphasis on SM, ceramide and gangliosides.

CCR3, CCR2A and macrophage inflammatory protein (MIP)-1a, monocyte chemotactic protein-1 (MCP-1) in the mouse hippocampus during and after pilocarpine-induced status epilepticus (PISE)

AIMS

To investigate protein and gene expressions of chemokine subtypes CCR3, CCR2A and their respective ligands macrophage inflammatory protein 1-alpha (MIP-1alpha), monocyte chemotactic protein-1 (MCP-1) in the normal mouse central nervous system (CNS) and in the hippocampus at different time points during and after pilocarpine-induced status epilepticus (PISE).

METHODS

CCR3 and MIP-1alpha protein expressions were mapped in the mouse CNS. The protein and gene expressions of CCR3 and CCR2A and their respective ligands MIP-1alpha, MCP-1 in the hippocampus were studies by immunocytochemical and quantitative real-time RT-PCR during and after PISE.

RESULTS

CCR3 and MIP-1alpha gene expression and immunopositive neurones were broadly distributed in the CNS. CCR3 and CCA2A gene and their protein expression were downregulated in the hippocampus at 1 h during PISE. The protein expression of MIP-1alpha, MCP-1 decreased but gene expression increased at 2 h during PISE. In the hilus of the dentate gyrus, significant reduction of the numbers of CCR3, CCR2A, MCP-1 immunopositive neurones occurred from 1 h during to 2 months after PISE, but the number of MIP-1alpha neurones reduced from 2 h during to 2 months after PISE. Induced expression of CCR3 at 1 week, CCR2A, MCP-1 or MIP-1alpha at 1 week and 2 months after PISE was found in reactive astrocytes. MCP-1 was also demonstrated in the blood vessels of the hippocampus at 2 months after PISE.

CONCLUSIONS

CCR3 and MIP-1alpha may play important functional roles in the mouse brain. The downregulation of CCR3, CCR2A, MIP-1alpha and MCP-1 in the hippocampal neurones at the acute stage during and after PISE may weaken the neuroprotective mechanisms. However, induced expression of MCP-1 in hippocampal blood vessel may be related to changes in permeability of the blood-brain barrier during epileptogenesis.

Enhanced microglial activation and proinflammatory cytokine upregulation are linked to increased susceptibility to seizures and neurologic injury in a 'two-hit' seizure model

Early-life seizures result in increased susceptibility to seizures and greater neurologic injury with a second insult in adulthood. The mechanisms which link seizures in early-life to increased susceptibility to neurologic injury following a 'second hit' are not known. We examined the contribution of microglial activation and increased proinflammatory cytokine production to the subsequent increase in susceptibility to neurologic injury using a kainic acid (KA)-induced, established 'two-hit' seizure model in rats. Postnatal day (P)15 rats were administered intraperitoneal KA (early-life seizures) or saline, followed on P45 with either a 'second hit' of KA, a first exposure to KA (adult seizures), or saline. We measured the levels of proinflammatory cytokines (IL-1 beta, TNF-alpha, and S100B), the chemokine CCL2, microglial activation, seizure susceptibility and neuronal outcomes in adult rats 12 h and 10 days after the second hit on P45. The 'two-hit' group exposed to KA on both P15 and P45 had higher levels of cytokines, greater microglial activation, and increased susceptibility to seizures and neurologic injury compared to the adult seizures group. Treatment after early-life seizures with Minozac, a small molecule experimental therapeutic that targets upregulated proinflammatory cytokine production, attenuated the enhanced microglial and cytokine responses, the increased susceptibility to seizures, and the greater neuronal injury in the 'two-hit' group. These results implicate microglial activation as one mechanism by which early-life seizures contribute to increased vulnerability to neurologic insults in adulthood, and indicate the potential longer term benefits of early-life intervention with therapies that target up-regulation of proinflammatory cytokines.

Age-dependent vascular changes induced by status epilepticus in rat forebrain: implications for epileptogenesis

Brain inflammation, angiogenesis and increased blood-brain barrier (BBB) permeability occur in adult rodent and human epileptogenic brain tissue. We addressed the role of these events in epileptogenesis using a developmental approach since the propensity to develop spontaneous seizures, therefore the induction of epileptogenesis, is age-dependent and increases with brain maturation. Inflammation, angiogenesis and BBB permeability were studied in postnatal day (PN)9 and PN21 rats, 1 week and 4 months after pilocarpine-induced status epilepticus. Brain inflammation was evaluated by interleukin(IL)-1beta immunohistochemistry; angiogenesis was quantified by measuring the density of microvessels identified by an anti-laminin antibody or by the intraluminal signal of FITC-albumin; BBB integrity was assessed by extravascular IgG immunostaining or detection of parenchymal extravasation of FITC-albumin. Neither inflammation nor angiogenesis or changes in BBB permeability were detected in PN9 rats after status epilepticus, and these rats did not develop spontaneous seizures in adulthood as assessed by video-EEG monitoring. Differently, status epilepticus in PN21 rats induced chronic inflammation, angiogenesis and BBB leakage in the hippocampus in 62% of rats, while in the remaining rats only transient inflammation in forebrain was observed. Epilepsy developed in about 62% of PN21 rats exposed to SE and these epileptic rats showed the three phenomena concomitantly in the hippocampus. PN21 rats that did not develop epilepsy 4 months after status epilepticus, as assessed by video-EEG monitoring, they did not show inflammation, angiogenesis or BBB damage in forebrain at this time. Our data show that age-dependent vascular changes and brain inflammation induced by status epilepticus are associated with epileptogenesis, suggesting that these phenomena are implicated in the mechanisms underlying the occurrence of spontaneous seizures.

The role of interleukin-1beta in febrile seizures

Febrile seizures (FS) occur in children as a result of fever. Despite their prevalence, the pathophysiology of FS has remained unclear. Recent evidence from clinical and experimental studies has highlighted a potential role of immune generated products in the genesis of FS. Of particular interest are the pro-inflammatory cytokine, interleukin-1beta (IL-1beta) and its naturally occurring antagonist, interleukin 1 receptor antagonist (IL-1ra). Using a novel animal model of FS, involving the generation of physiological fever, we investigated the role of the IL-1beta/IL-1ra system in the genesis of FS. We found that animals with FS had increased hippocampal and hypothalamic IL-1beta compared to equally treated animals without FS, which was first evident at onset of FS in the hippocampus. There were no differences in IL-1ra levels. ICV IL-1beta increased the number of animals with FS while IL-1ra had an opposite anti-convulsant effect. The data from these studies, in combination with recent results from other laboratories, have established a putative role for the IL-1beta/IL-1ra system in the genesis of FS.

Blood-brain barrier breakdown-inducing astrocytic transformation: novel targets for the prevention of epilepsy

Epileptogenesis is common following brain insults such as trauma, ischemia and infection. However, the mechanisms underlying injury-related epileptogenesis remain unknown. Recent studies demonstrated impaired integrity of the blood-brain barrier (BBB) during epileptogenesis. Here we review accumulating experimental evidence supporting the potential involvement of primary BBB lesion in epileptogenesis. Data from animal experiments demonstrate that primary breakdown of the BBB prone animals to develop focal neocortical epilepsy that is followed by neuronal loss and impaired functions. The extravasation of albumin from the circulation into the brain neuropil was found to be sufficient for the induction of epileptogenesis. Albumin binds to transforming growth factor beta receptor 2 (TGFbetaR2) in astrocytes and induces rapid transcriptional modifications, astrocytic transformation and dysfunction. We highlight a novel cascade of events which is initiated by increased BBB permeability, eventually leading to neuronal dysfunction, epilepsy and cell loss. We review potential mechanisms and existing experimental evidence for the important role of astrocytes and the TGFbeta pathway in epileptogenesis. Finally, we review evidence from human clinical data supporting the involvement of BBB lesion in epilepsy. We propose that primary vascular injury, and specifically BBB breakdown and repair, are key elements in altered interactions within the neurovascular unit and thus may serve as new therapeutic targets.

Curing epilepsy: progress and future directions

During the past decade, substantial progress has been made in delineating clinical features of the epilepsies and the basic mechanisms responsible for these disorders. Eleven human epilepsy genes have been identified and many more are now known from animal models. Candidate targets for cures are now based upon newly identified cellular and molecular mechanisms that underlie epileptogenesis. However, epilepsy is increasingly recognized as a group of heterogeneous syndromes characterized by other conditions that co-exist with seizures. Cognitive, emotional and behavioral co-morbidities are common and offer fruitful areas for study. These advances in understanding mechanisms are being matched by the rapid development of new diagnostic methods and therapeutic approaches. This article reviews these areas of progress and suggests specific goals that once accomplished promise to lead to cures for epilepsy.

The direct excitatory effect of IL-1beta on cerebellar Purkinje cell

Interleukin (IL)-1beta is one of the important proinflammatory cytokines in neural as well as immune systems, and plays a pivotal role in the neuroinflammation. We previously demonstrated that cerebellar IL-1beta is involved in kainate-induced ataxia, i.e., IL-1beta was activated in the cerebellum with systemic administration of kainate, and its type I receptor (IL-1R) was expressed at a soma of cerebellar Purkinje cells. In this study, we examined the effect of IL-1beta on cerebellar Purkinje cell function by recording extracellular neuronal activities in anesthetized mice. Systemic administration of kainate increased the firing rates of cerebellar Purkinje cells in normal mice but showed little effect in IL-1R-knockout (IL-1R-KO) mice. Moreover, microiontophoretic administration of IL-1beta to cerebellar Purkinje cells increased the firing rates promptly in response to IL-1beta. The present results demonstrate that IL-1 system exerts a direct modulatory effect on cerebellar Purkinje cells.

Molecular and cellular basis of epileptogenesis in symptomatic epilepsy

Epileptogenesis refers to a process in which an initial brain-damaging insult triggers a cascade of molecular and cellular changes that eventually lead to the occurrence of spontaneous seizures. Cellular alterations include neurodegeneration, neurogenesis, axonal sprouting, axonal injury, dendritic remodeling, gliosis, invasion of inflammatory cells, angiogenesis, alterations in extracellular matrix, and acquired channelopathies. Large-scale molecular profiling of epileptogenic tissue has provided information about the molecular pathways that can initiate and maintain cellular alterations. Currently we are learning how these pathways contribute to postinjury epileptogenesis and recovery process and whether they could be used as treatment targets.

The role of interleukin-1 in seizures and epilepsy: a critical review

Interleukin-1 (IL-1) has a multitude of functions in the central nervous system. Some of them involve mechanisms that are related to epileptogenesis. The role of IL-1 in seizures and epilepsy has been investigated in both patients and animal models. This review aims to synthesize, based on the currently available literature, the consensus role of IL-1 in epilepsy. Three lines of evidence suggest a role for IL-1: brain tissue from epilepsy patients and brain tissue from animal models shows increased IL-1 expression after seizures, and IL-1 has proconvulsive properties when applied exogeneously. However, opposing results have been published as well. More research is needed to fully establish the role of IL-1 in seizure generation and epilepsy, and to explore possible new treatment strategies that are based on interference with intracellular signaling cascades that are initiated when IL-1 binds to its receptor.

Cell-specific and concentration-dependent actions of interleukin-1 in acute brain inflammation

Interleukin (IL)-1 is a pivotal pro-inflammatory cytokine and an important mediator of both acute and chronic central nervous system (CNS) injuries. Despite intense research in CNS IL-1 biology over the past two decades, its precise mechanism of action in inflammatory responses to acute brain disorders remains largely unknown. In particular, much effort has been focussed on using in vitro approaches to better understand the cellular and signalling mechanisms of actions of IL-1, yet some discrepancies in the literature regarding the effects produced by IL-1beta in in vitro paradigms of injury still exist, particularly as to whether IL-1 exerts neurotoxic or neuroprotective effects. Here we aim to review the cell-specific and concentration-dependent actions of IL-1 in brain cells, to depict the mechanism by which this cytokine induces neurotoxicity or neuroprotection in acute brain injury.

Microglial activation and TNFalpha production mediate altered CNS excitability following peripheral inflammation

Peripheral inflammation leads to a number of centrally mediated physiological and behavioral changes. The underlying mechanisms and the signaling pathways involved in these phenomena are not yet well understood. We hypothesized that peripheral inflammation leads to increased neuronal excitability arising from a CNS immune response. We induced inflammation in the gut by intracolonic administration of 2,4,6-trinitrobenzene sulfonic acid (TNBS) to adult male rats. To examine the excitability of the brain in vivo, we administered pentylenetetrazole (PTZ; a GABAergic antagonist) intravenously to evoke clonic seizures. Rats treated with TNBS showed increased susceptibility to PTZ seizures that was strongly correlated with the severity and progression of intestinal inflammation. In vitro hippocampal slices from inflamed, TNBS-treated rats showed increased spontaneous interictal burst firing following application of 4-aminopyridine, indicating increased intrinsic excitability. The TNBS-treated rats exhibited a marked, reversible inflammatory response within the hippocampus, characterized by microglial activation and increases in tumor necrosis factor alpha (TNFalpha) levels. Central antagonism of TNFalpha using a monoclonal antibody or inhibition of microglial activation by i.c.v. injection of minocycline prevented the increase in seizure susceptibility. Moreover, i.c.v. infusion of TNFalpha in untreated rats for 4 days also increased seizure susceptibility and thus mimicked the changes in seizure threshold observed with intestinal inflammation. Our finding of a microglia-dependent TNFalpha-mediated increase in CNS excitability provides insight into potential mechanisms underlying the disparate neurological and behavioral changes associated with chronic inflammation.

Postnatal inflammation increases seizure susceptibility in adult rats

There are critical postnatal periods during which even subtle interventions can have long-lasting effects on adult physiology. We asked whether an immune challenge during early postnatal development can alter neuronal excitability and seizure susceptibility in adults. Postnatal day 14 (P14) male Sprague Dawley rats were injected with the bacterial endotoxin lipopolysaccharide (LPS), and control animals received sterile saline. Three weeks later, extracellular recordings from hippocampal slices revealed enhanced field EPSP slopes after Schaffer collateral stimulation and increased epileptiform burst-firing activity in CA1 after 4-aminopyridine application. Six to 8 weeks after postnatal LPS injection, seizure susceptibility was assessed in response to lithium-pilocarpine, kainic acid, and pentylenetetrazol. Rats treated with LPS showed significantly greater adult seizure susceptibility to all convulsants, as well as increased cytokine release and enhanced neuronal degeneration within the hippocampus after limbic seizures. These persistent increases in seizure susceptibility occurred only when LPS was given during a critical postnatal period (P7 and P14) and not before (P1) or after (P20). This early effect of LPS on adult seizures was blocked by concurrent intracerebroventricular administration of a tumor necrosis factor alpha (TNFalpha) antibody and mimicked by intracerebroventricular injection of rat recombinant TNFalpha. Postnatal LPS injection did not result in permanent changes in microglial (Iba1) activity or hippocampal cytokine [IL-1beta (interleukin-1beta) and TNFalpha] levels, but caused a slight increase in astrocyte (GFAP) numbers. These novel results indicate that a single LPS injection during a critical postnatal period causes a long-lasting increase in seizure susceptibility that is strongly dependent on TNFalpha.

The role of cytokines in the pathophysiology of epilepsy

Recent findings in experimental models and in the clinical setting highlight the possibility that inflammatory processes in the brain contribute to the etiopathogenesis of seizures and to the establishment of a chronic epileptic focus. Prototypical inflammatory cytokines such as IL-1 beta, TNF-alpha and IL-6 have been shown to be overexpressed in experimental models of seizures in brain areas of seizure generation and propagation, prominently by glia and to a lesser extent by neurons. Cytokines receptors are also upregulated, and the related intracellular signalling is activated, in both cell populations highlighting autocrine and paracrine actions of cytokines in the brain. Cytokines have been shown to profoundly affect seizures in rodents; in particular, IL-1 beta is endowed of proconvulsant activity in a large variety of seizure models. The recent demonstration of functional interactions between cytokines and classical neurotransmitters such as glutamate and GABA, suggest the possibility that these interactions underlie the cytokine-mediated changes in neuronal excitability, thus promoting seizure phenomena and the associated neuropathology. These findings point out at novel glio-neuronal communications in diseased conditions and highlight potential new targets for therapeutic intervention.

Exogenous corticosterone reduces L-DOPA-induced dyskinesia in the hemi-parkinsonian rat: role for interleukin-1beta

While the etiology of Parkinson's disease (PD) remains unknown, there is overwhelming evidence that neuroinflammation plays a critical role in the progressive loss of dopamine (DA) neurons. Because nearly all persons suffering from PD receive l-DOPA, it is surprising that inflammation has not been examined as a potential contributor to the abnormal involuntary movements (AIMs) that occur as a consequence of chronic l-DOPA treatment. As an initial test of this hypothesis, we examined the effects of exogenously administered corticosterone (CORT), an endogenous anti-inflammatory agent, on the expression and development of l-DOPA-induced dyskinesia (LID) in unilateral DA-depleted rats. To do this, male Sprague-Dawley rats received unilateral medial forebrain bundle 6-hydroxydopamine lesions. Three weeks later, l-DOPA primed rats received acute injections of CORT (0-3.75 mg/kg) prior to l-DOPA to assess the expression of LID. A second group of rats was used to examine the development of LID in l-DOPA naïve rats co-treated with CORT and l-DOPA for 2 weeks. AIMs and rotations were recorded. Exogenous CORT dose-dependently attenuated both the expression and development of AIMs without affecting rotations. Real-time reverse-transcription polymerase chain reaction of striatal tissue implicated a role for interleukin-1 (IL-1) beta in these effects as its expression was increased on the lesioned side in rats treated with l-DOPA (within the DA-depleted striatum) and attenuated with CORT. In the final experiment, interleukin-1 receptor antagonist (IL-1ra) was microinjected into the striatum of l-DOPA-primed rats to assess the impact of IL-1 signaling on LID. Intrastriatal IL-1ra reduced the expression of LID without affecting rotations. These findings indicate a novel role for neuroinflammation in the expression of LID, and may implicate the use of anti-inflammatory agents as a potential adjunctive therapy for the treatment of LID.

Human herpes virus 6B: a possible role in epilepsy?

Human herpes virus 6 (HHV6) infection is nearly ubiquitous in childhood and may include central nervous system invasion. There are two variants, HHV6A and HHV6B. Usually asymptomatic, it is associated with the common, self-limited childhood illness roseola infantum and rarely with more severe syndromes. In patients with immune compromise, subsequent reactivation of viral activity may lead to severe limbic encephalitis. HHV6 has been identified as a possible etiologic agent in multiple sclerosis, myocarditis, and encephalitis. A preponderance of evidence supports an association between HHV6 and febrile seizures. An ongoing multicenter study is investigating possible links between HHV6 infection, febrile status epilepticus, and development of mesial temporal sclerosis (MTS). Investigation of temporal lobectomy specimens showed evidence of active HHV6B but not HHV6A replication in hippocampal astrocytes in about two-thirds of patients with MTS but not other causes of epilepsy. It has been suggested that HHV6B may cause "excitotoxicity" by interfering with astrocyte excitatory amino acid transport. Although conventional inflammatory changes are not found in most MTS specimens, inflammatory modulators may play a role in neuronal injury leading to MTS as well. If the link between early viral infection, complex or prolonged febrile seizures, and later development of intractable temporal lobe epilepsy is confirmed, new therapeutic approaches to a common intractable epilepsy syndrome may be possible.

Glia as a source of cytokines: implications for neuronal excitability and survival

In the last decade, preclinical studies have provided a better characterization of the homeostatic and maladaptive mechanisms occurring either during the process of epileptogenesis or after the permanent epileptic state has emerged. Experimental evidence supported by clinical observations highlighted the possibility that brain inflammation is a common factor contributing, or predisposing, to the occurrence of seizures and cell death, in various forms of epilepsy of different etiologies. Expression of proinflammatory cytokines, as a hallmark of brain inflammation, has been demonstrated in glia in various experimental models of seizures and in human epilepsies. Experimental studies in rodents with perturbed cytokine systems indicate that these inflammatory mediators can alter neuronal excitability and affect cell survival by activating transcriptional and posttranslational intracellular pathways. This paper will provide an overview on the current knowledge in this field to discuss mechanistic hypotheses into the study of pathogenesis of epilepsy and recognize new potential therapeutic options.

Epileptogenesis in the developing brain: what can we learn from animal models?

Knowledge of the processes by which epilepsy is generated (epileptogenesis) is incomplete and has been a topic of major research efforts. Animal models can inform us about these processes. We focus on the distinguishing features of epileptogenesis in the developing brain and model prolonged febrile seizures (FS) that are associated with human temporal lobe epilepsy. In the animal model of FS, epileptogenesis occurs in approximately 35% of rats. Unlike the majority of acquired epileptogeneses in adults, this process early in life (in the febrile seizures model as well as in several others) does not require "damage" (cell death). Rather, epileptogenesis early in life involves molecular mechanisms including seizure-evoked, long-lasting alterations of the expression of receptors and ion channels. Whereas transient changes in gene expression programs are common after early-life seizures, enduring effects, such as found after experimental FS, are associated with epileptogenesis. The ability of FS to generate long-lasting molecular changes and epilepsy suggests that mechanisms, including cytokine activation that are intrinsic to FS generation, may play a role also in the epileptogenic consequences of these seizures.

Fever, febrile seizures and epilepsy

Seizures induced by fever (febrile seizures) are the most common type of pathological brain activity in infants and children. These febrile seizures and their potential contribution to the mechanisms of limbic (temporal lobe) epilepsy have been a topic of major clinical and scientific interest. Key questions include the mechanisms by which fever generates seizures, the effects of long febrile seizures on neuronal function and the potential contribution of these seizures to epilepsy. This review builds on recent advances derived from animal models and summarizes our current knowledge of the mechanisms underlying febrile seizures and of changes in neuronal gene expression and function that facilitate the enduring effects of prolonged febrile seizures on neuronal and network excitability. The review also discusses the relevance of these findings to the general mechanisms of epileptogenesis during development and points out gaps in our knowledge, including the relationship of animal models to human febrile seizures and epilepsy.

Innate and adaptive immunity during epileptogenesis and spontaneous seizures: evidence from experimental models and human temporal lobe epilepsy

We investigated the activation of the IL-1 beta system and markers of adaptive immunity in rat brain during epileptogenesis using models of temporal lobe epilepsy (TLE). The same inflammatory markers were studied in rat chronic epileptic tissue and in human TLE with hippocampal sclerosis (HS). IL-1 beta was expressed by both activated microglia and astrocytes within 4 h from the onset of status epilepticus (SE) in forebrain areas recruited in epileptic activity; however, only astrocytes sustained inflammation during epileptogenesis. Activation of the IL-1 beta system during epileptogenesis was associated with neurodegeneration and blood-brain barrier breakdown. In rat and human chronic epileptic tissue, IL-1 beta and IL-1 receptor type 1 were broadly expressed by astrocytes, microglia and neurons. Granulocytes appeared transiently in rat brain during epileptogenesis while monocytes/macrophages were present in the hippocampus from 18 h after SE onset until chronic seizures develop, and they were found also in human TLE hippocampi. In rat and human epileptic tissue, only scarce B- and T-lymphocytes and NK cells were found mainly associated with microvessels. These data show that specific inflammatory pathways are chronically activated during epileptogenesis and they persist in chronic epileptic tissue, suggesting they may contribute to the etiopathogenesis of TLE.