Excitation in temporal lobe epilepsy : focus on the glutamate-glutamine cycle

Abstract

Epilepsy is a common neurological disorder. Temporal lobe epilepsy (TLE) is the most frequent type of human focal epilepsy. Despite ample availability of anti-epileptic dugs, about 30% of TLE patients are pharmaco-resistant. Surgical removal of the epileptogenic focus, which usually includes the hippocampus, may then be the therapy of choice. The etiology of TLE is largely unknown. One of the key mechanisms implicated is a disbalans between the excitatory and inhibitory neurotransmitters glutamate and GABA. There is compelling evidence that glutamate metabolism is impaired in TLE. Synaptically released glutamate is recycled in the glutamate-glutamine cycle, the local pathway in which neurons and glia cooperate. The cycle is essential to terminate the glutamatergic signal, thereby preventing excitotoxicity. Glutamate is taken up by glial cells, predominantly via excitatory amino acid transporter 2, where it is converted into non-toxic glutamine by the enzyme glutamine synthetase (GS). Glutamine transfer from astrocytes to neurons is probably mediated by neutral amino acid transporters. In the neuron, glutamine is deaminated to glutamate via phosphate-activated glutaminase. Glutamate is loaded in synaptic vesicles by specific vesicular glutamate transporters (VGLUTs), prior to exocytosis. The overall aim of this thesis was to determine the role and contribution of components of the glutamate-glutamine cycle in the development and chronic phase of TLE. Expression of GS, putative glutamine transporters (SNAT1,2,3, ASCT2), and VGLUTs were investigated in human biopsy material obtained during surgical resection of the affected hippocampus. TLE patients without (non-HS) and with (HS) hippocampal sclerosis were compared with autopsy controls. Expression of GS, but also of SNAT1,2, and VGLUT1, was decreased in the hippocampus of HS patients, indicating severe impairment of glutamate metabolism. The expression pattern of SNAT3 and the absence of ASCT2 expression in the human hippocampus argue against a role of these transporters in glutamine efflux from glial cells. Although human tissue is unique, studying the etiology of TLE requires animal studies. Therefore, the juvenile pilocarpine model was used to study early changes in glutamate-glutamine cycle components in the process leading to TLE. In this model, pilocarpine-injection induces acute prolonged seizures (status epilepticus). After a period of 15-18 weeks 44% of these animals developed spontaneous recurrent seizures. Interestingly, GS was the only component of the glutamate-glutamine cycle that progressively decreased before as well as after the occurrence of recurrent seizures. These early changes could be monitored in living animals by magnetic resonance spectroscopy and may be early markers of imminent epilepsy. Subsequently, we investigated the effect of reduced GS expression on seizure susceptibility in heterozygous GS knockout mice (GS+/-) with ~50% reduction of GS expression, thus mimicking the effects in human and animal TLE. These animals were subjected to hyperthermia-induced febrile seizures, which often are an early precipitating event in TLE patients. Febrile seizure susceptibility was increased in GS+/- mice compared to controls. Compensatory mechanisms of other components of the glutamate-glutamine cycle were not detected. Thus, GS is a key factor in regulating seizure susceptibility. These results open possibilities for the early non-invasive detection of TLE and development of new treatment strategies.