Increased expression of the neuronal glutamate transporter (EAAT3/EAAC1) in hippocampal and neocortical epilepsy

Rapid trafficking of the neuronal glutamate transporter, EAAC1: evidence for distinct trafficking pathways differentially regulated by protein kinase C and platelet-derived growth factor

The neuronal glutamate transporter, EAAC1, appears to both limit spillover between excitatory synapses and provide precursor for the synthesis of the inhibitory neurotransmitter, gamma-aminobutyric acid. There is evidence for a large intracellular pool of EAAC1 from which transporter is redistributed to the cell surface following activation of protein kinase C (PKC) or platelet-derived growth factor (PDGF) receptor by seemingly independent pathways. A variety of biotinylation strategies were employed to measure trafficking of EAAC1 to and from the plasma membrane and to examine the effects of phorbol ester and PDGF on these events. Biotinylation of cell surface protein under trafficking-permissive conditions (37 degrees C) resulted in a 2-fold increase in the amount of biotinylated EAAC1 within 15 min in C6 glioma and in primary neuronal cultures, suggesting that EAAC1 has a half-life of approximately 5-7 min for residence at the plasma membrane. Both phorbol ester and PDGF increased the amount of transporter labeled under these conditions. Using a reversible biotinylation strategy, a similarly rapid internalization of EAAC1 was observed in C6 glioma. Phorbol ester, but not PDGF, blocked this measure of internalization. Incubation at 18 degrees C, which blocks some forms of intracellular membrane trafficking, inhibited PKC- and PDGF-dependent redistribution of EAAC1 but had no effect on basal trafficking of EAAC1. These studies suggest that both PKC and PDGF accelerate delivery of EAAC1 to the cell surface and that PKC has an additional effect on endocytosis. The data also suggest that basal and regulated pools of EAAC1 exist in distinct compartments.

Glutamate transporters: animal models to neurologic disease

Glutamate is the primary excitatory amino acid neurotransmitter in the central nervous system and its activity is carefully modulated in the synaptic cleft by glutamate transporters. A number of glutamate transporters have been identified in the central nervous system and each has a unique physiologic property and distribution. Glutamate transporter dysfunction may either be an initiating event or part of a cascade leading to cellular dysfunction and ultimately cell death. Animal models of glutamate transporter dysfunction have revealed a significant role for these proteins in pathologic conditions such as neurodegenerative diseases, epilepsy, stroke, and central nervous system tumors. Recent work has focused on glutamate transporter biology in human diseases with an emphasis on how manipulation of these transporter proteins may lead to therapeutic interventions in neurologic disease.

Cortical dysplasia and epilepsy: animal models

Cortical dysplasia syndromes--those conditions of abnormal brain structure/organization that arise during aberrant brain development--frequently involve epileptic seizures. Neuropathological and neuroradiological analyses have provided descriptions and categorizations based on gross anatomical and cellular histological features (e.g., lissencephaly, heterotopia, giant cells), as well as on the developmental mechanisms likely to be involved in the abnormality (e.g., cell proliferation, migration). Recently, the genes responsible for several cortical dysplastic conditions have been identified and the underlying molecular processes investigated. However, it is still unclear how the various structural abnormalities associated with cortical dysplasia are related to (i.e., "cause") chronic seizures. To elucidate these relationships, a number of animal models of cortical dysplasia have been developed in rats and mice. Some models are based on laboratory manipulations that injure the brain (e.g., freeze, undercut, irradiation, teratogen exposure) of immature animals; others are based on spontaneous genetic mutations or on gene manipulations (knockouts/transgenics) that give rise to abnormal cortical structures. Such models of cortical dysplasia provide a means by which investigators can not only study the developmental mechanisms that give rise to these brain lesions, but also examine the cause-effect relationships between structural abnormalities and epileptogenesis.

Long-term alterations in glutamate receptor and transporter expression following early-life seizures are associated with increased seizure susceptibility

Prolonged seizures in early childhood are associated with an increased risk of development of epilepsy in later life. The mechanism(s) behind this susceptibility to later development of epilepsy is unclear. Increased synaptic activity during development has been shown to permanently alter excitatory neurotransmission and could be one of the mechanisms involved in this increased susceptibility to the development of epilepsy. In the present study we determine the effect of status-epilepticus induced by lithium/pilocarpine at postnatal day 10 (P10 SE) on the expression of glutamate receptor and transporter mRNAs in hippocampal dentate granule cells and protein levels in dentate gyrus of these animals in adulthood. The results revealed a decrease in glutamate receptor 2 (GluR2) mRNA expression and protein levels as well as an increase in protein levels for the excitatory amino acid carrier 1 (EAAC1) in P10 SE rats compared to controls. Expression of glutamate receptor 1 (GluR1) mRNA was decreased in both P10 SE rats and identically handled, lithium-injected littermate controls compared to naive animals, and GluR1 protein levels were significantly lower in lithium-controls than in naive rats, suggesting an effect of either the handling or the lithium on GluR1 expression. These changes in EAA receptors and transporters were accompanied by an increased susceptibility to kainic acid induced seizures in P10 SE rats compared to controls. The current data suggest that early-life status-epilepticus can result in permanent alterations in glutamate receptor and transporter gene expression, which may contribute to a lower seizure threshold.

Glutamate uptake in blood platelets from epileptic patients

Glutamate, the major excitatory amino acid neurotransmitter, is involved in epileptogenesis and initiation and spread of seizures. We studied glutamate uptake into blood platelets from patients with distinct epileptic syndromes: included were 20 patients with temporal lobe epilepsy and hippocampal sclerosis (TLE+HS), 20 with juvenile myoclonus epilepsy (JME) and 20 healthy volunteers matched for age and sex. The affinity of glutamate for the transporters was highest in patients with TLE+HS, but the maximal velocity of transport was highest in controls. There were no differences in the plasma levels of glutamate. Carbamazepine (CBZ), valproate (VPA) and lamotrigine (LTG) did not affect the uptake in vitro. The alterations observed in the uptake of glutamate in TLE+HS patients may reflect an up-regulated uptake of glutamate in the brain.

Metabolic parameters of epilepsy: adjuncts to established antiepileptic drug therapy

Hughlings Jackson at the turn of the century defined epilepsy as a disorder originating in a "morbid nutrition" of the neuron. With the advances in modern neurochemistry, it is becoming increasingly clear that a chronic seizure predisposition or a lowering of the brain's discharge threshold can be demarcated by a number of biochemical markers. They include a tendency for an increased release of glutamate with or without GABAergic impairment, (intra)neural tissue alterations in water redistribution/osmolarity or other distortions of the cytoarchitecture, and an elevation of ionic calcium inside the cell. These changes are dominantly shared parameters of the seizure prone brain. Magnetic resonance spectroscopy (MRS) shows that cerebral levels of glutamate + glutamine (Glx) are increased interictally in epileptogenic regions in human partial epilepsy; other findings using this technique suggest damage to (cellular/mitochondrial) membranes, denoted by N-acetyl-aspartic acid (NAA) changes and a decreased energy capability. The merging of previous in vitro and ex vivo findings in neurophysiology and neurochemistry with magnetic resonance spectroscopy technology provides a powerful new methodology to interpret and to obtain clinical insight into the metabolic alterations that underlie an epileptogenic process. In this review some of these basic neurochemical and electrophysiological mechanisms are discussed. In addition, certain adjuncts to established antiepileptic drug therapy are suggested in the hope that over the long term they may help in correcting the primary metabolic deficits.