Complement activation in experimental and human temporal lobe epilepsy

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Abstract

We investigated the involvement of the complement cascade during epileptogenesis in a rat model of temporal lobe epilepsy (TLE), and in the chronic epileptic phase in both experimental as well as human TLE. Previous rat gene expression analysis using microarrays indicated prominent activation of the classical complement pathway which peaked at 1 week after SE in CA3 and entorhinal cortex. Increased expression of C1q, C3 and C4 was confirmed in CA3 tissue using quantitative PCR at 1 day, 1 week and 3–4 months after status epilepticus (SE). Upregulation of C1q and C3d protein expression was confirmed mainly to be present in microglia and in a few hippocampal neurons. In human TLE with hippocampal sclerosis, astroglial, microglial and neuronal (5/8 cases) expression of C1q, C3c and C3d was observed particularly within regions where neuronal cell loss occurs. The membrane attack protein complex (C5b-C9) was predominantly detected in activated microglial cells. The persistence of complement activation could contribute to a sustained inflammatory response and could destabilize neuronal networks involved.

Introduction

Temporal lobe epilepsy (TLE) represents a severe and common epilepsy syndrome in patients with medically intractable epilepsy. The epileptogenic focus involves often the hippocampal formation, and hippocampal sclerosis (HS), also known as Ammon's horn sclerosis, is the most common neuropathological finding in these patients (Armstrong, 1993, Blumcke et al., 2002, Mathern et al., 2002, Thom et al., 1999). The major histopathological features of HS include selective neuronal cell loss, gliosis and synaptic reorganization of the mossy fibers (mossy fiber sprouting) (Wieser and Epilepsy, 2004). However, an increasing number of evidence shows that the immune response plays a prominent role in the affected brain tissue in TLE (Vezzani and Granata, 2005).

An important component of the immune response is the activation of the complement system. The complement system consists of about 35 proteins including soluble, as well as membrane-bound proteins, which, when activated result in a complex cascade of processes leading to activation of microglia, secretion of proinflammatory cytokines, recruitment of macrophages, activation of phagocytosis and increased vascular permeability. The complement cascade is activated by three pathways, the classical, the alternative and the lectin pathway, which lead to the formation of the C5b-C9 membrane attack protein complex (MAC; for reviews see Holers, 2003, Liszewski et al., 1996, Mayer, 1973). Complement activation in CNS is increasingly recognized to be associated with exacerbation and progression of tissue injury in different degenerative and inflammatory diseases (Bellander et al., 2001, Bonifati and Kishore, 2006, Hartung et al., 1992, Holers, 2003). Evidence of complement activation has been found in serum of epileptic patients (Basaran et al., 1994). Upregulation of several complement related factors was reported in an animal model for TLE (Lukasiuk et al., 2003) and more recently in the entorhinal cortex of MTLE patients (Jamali et al., 2006). Moreover, complement activation appears to play a major role in Rasmussen's syndrome (Whitney and McNamara, 2000). Interestingly, sequential infusion of individual proteins of the membrane attack pathway (C5b6, C7, C8, and C9) into the hippocampus of awake, freely moving rats induces both behavioral and electrographic seizures as well as cytotoxicity (Xiong et al., 2003), suggesting a role for the complement system in epileptogenesis.

In a recent large-scale microarray study in the rat we found that, among many different processes, the immune response was most significantly increased during the latent period (Gorter et al., 2006). Especially the classical complement cascade was prominently activated (Fig. 1). The aim of the present study was to investigate the dynamics of the complement cascade during epileptogenesis in a rat model of TLE in more detail, to investigate the cellular distribution of several complement proteins and to compare this with complement expression and distribution in the chronic phase of spontaneous seizures in specimens of TLE patients with HS.

Section snippets

Experimental animals

Adult male Sprague–Dawley rats (Harlan CPB laboratories, Zeist, The Netherlands) weighing 300–500 g were used in this study which was approved by the University Animal Welfare committee. The rats were housed individually in a controlled environment (21 ± 1 °C; humidity 60%; lights on 08:00 AM–8:00 PM; food and water available ad libitum).

Electrode implantation and seizure induction

In order to record hippocampal EEG, a pair of insulated stainless steel electrodes (70-μm wire diameter, tips were 0.8 mm apart) were implanted into the left

Replication of gene profile in the rat using QPCR

Using quantitative PCR we could validate the upregulation of several complement genes (C1qa, C3 and C4) as shown in Fig. 2A. We normalized the expression on the array and in the PCR to control values and calculated the fold change. Although the exact fold change could differ from the array data, essentially a similar gene activation profile was obtained with PCR with largest activation of the three genes at 1 week after SE.

Replication of gene profiles in the rat by immunostaining

To determine whether the protein expression corresponds with transcript

Discussion

The main findings of the present study are that during epileptogenesis, and in the chronic epileptic phase, in the post-SE rat there is a prominent activation of the classical complement pathway in cellular and perivascular elements; similar observations were made in human TLE with HS.

Acknowledgments

This work was supported by the National Epilepsy Fund—“Power of the Small”/Hersenstichting Nederland (NEF 02-10; NEF 05-11, E. Aronica and K. Boer; NEF 03-03, J.A. Gorter) and by the Epilepsy Institute of The Netherlands (Heemstede, The Netherlands; E. Aronica). We thank Professor Andrea Tenner, Center for Immunology University of California, Irvine, USA, for providing the C1q antibody.

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