AMPA receptor-mediated neurotoxicity: role of Ca2+ and desensitization

Extracellular Calcium Influx Pathways in Astrocyte Calcium Microdomain Physiology

Astrocytes are complex glial cells that play many essential roles in the brain, including the fine-tuning of synaptic activity and blood flow. These roles are linked to fluctuations in intracellular Ca2+ within astrocytes. Recent advances in imaging techniques have identified localized Ca2+ transients within the fine processes of the astrocytic structure, which we term microdomain Ca2+ events. These Ca2+ transients are very diverse and occur under different conditions, including in the presence or absence of surrounding circuit activity. This complexity suggests that different signalling mechanisms mediate microdomain events which may then encode specific astrocyte functions from the modulation of synapses up to brain circuits and behaviour. Several recent studies have shown that a subset of astrocyte microdomain Ca2+ events occur rapidly following local neuronal circuit activity. In this review, we consider the physiological relevance of microdomain astrocyte Ca2+ signalling within brain circuits and outline possible pathways of extracellular Ca2+ influx through ionotropic receptors and other Ca2+ ion channels, which may contribute to astrocyte microdomain events with potentially fast dynamics.

Overlooked Alzheimer's smoking gun?

Overview of Szutowicz et al. (Neurochem Res 38(8):1523-1542, 2013), is focusing on specific features of acetyl-CoA metabolism in the cholinergic compartment of the brain. Authors are suggesting that deficit of that metabolite can act as a trigger for several cholinergic encephalopathies, with special emphasis on Alzheimer disease (AD). Central role of acetyl-CoA and its metabolic paths in neurodegeneration are charted starting from its synthesis in mitochondria, followed by utilization in energy metabolism, as well as transport into cytoplasm and participation in the synthesis and turnover of neurotransmitter acetylcholine to emergence of diseased states. Various putative pathogenic signals are evaluated that might be responsible for acetyl-CoA deficit ending up in development of neurodegeneration, unraveling exceptional susceptibility of cholinergic system. They are discussed in context of other existing alternative hypotheses on AD etiology. Overview is thoroughly documented (178 references) and is supported by results accomplished by extensive research in Prof. Szutowicz's laboratory (approximately 25 original papers).

Perampanel Study 207: long-term open-label evaluation in patients with epilepsy

OBJECTIVES

Evaluate interim long-term tolerability, safety and efficacy of adjunctive perampanel, a novel α-amino-3-hydroxy-5-methyl-5-isoxazolepropionic acid (AMPA)-receptor antagonist, in patients with refractory partial-onset seizures.

MATERIALS AND METHODS

Study 207, an open-label extension (OLE) study (ClinicalTrials.gov identifier: NCT00368472), enrolled patients (18-70 years) who completed one of two randomized, placebo-controlled, dose-escalation Phase II studies. The OLE Treatment Phase comprised a 12-week Titration Period (2 mg increments of perampanel every 2 weeks to 12 mg/day, maximum) and a Maintenance Period, during which patients continued treatment up to a planned maximum of 424 weeks (~8 years). Interim analysis data cut-off date was 1 December, 2010.

RESULTS

Of 180 patients completing the Phase II studies, 138 enrolled in study 207. At the time of interim analyses (approximately 4 years after study start), over a third (n = 53, 38.4%) remained on perampanel; 41.3% (n = 57) of patients had >3 years of exposure; and 13.0% (n = 18) had at least 4 years' exposure. Mean ± standard deviation (SD) duration of exposure was 116 ± 75 weeks and mean ± SD dose during the OLE Maintenance Period was 7.3 ± 3.3 mg. No new safety signals emerged with long-term treatment. Consistent with previous studies, the most common treatment-emergent adverse events were as follows: dizziness, headache and somnolence. Overall median (range) per cent change from baseline in seizure frequency per 28 days during open-label treatment was -31.5% (-99.2 to 512.2).

CONCLUSIONS

Long-term - up to 4 years - adjunctive perampanel had a favourable tolerability profile in patients with refractory partial-onset seizures. Improvements in seizure control were maintained with long-term treatment.

Acetyl-L-carnitine reduces the infarct size and striatal glutamate outflow following focal cerebral ischemia in rats

Acetyl-L-carnitine (ALCAR), an endogenous water soluble molecule, is synthesized in the brain and is involved in many aspects of neuronal activity, including metabolism and neuronal membrane formation and integrity. To determine ALCAR's neuroprotective effects, focal cerebral ischemia was induced using four models of middle cerebral artery occlusion (MCAO) and treatment with 0-400 mg/kg ALCAR (i.p.) prior to MCAO. While acute doses were without effect, pretreatment with chronic ALCAR (400 mg/kg/day for five days) significantly reduced infarct size. Lower chronic ALCAR doses were not effective. Additionally, elevations in microdialysate glutamate post-MCAO were attenuated by ALCAR treatment.

Evolution of neurotoxins: from research modalities to clinical realities

In the 1950s, the discovery of anti-nerve growth factor, an immunotoxin stunting sympathetic neural development, signaled the advent of neurotoxins as research modalities. Other selective neurotoxins were discovered in rapid succession. In the 1960s, 6-hydroxydopamine and 6-hydroxydopa were shown to destroy noradrenergic and dopaminergic nerves. Excitotoxins (glutamate, aspartate, and analogs) were discovered in the 1970s. DSP-4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine] proved to be selective for noradrenergic destruction, while 5,6- and 5,7-dihydroxytryptamines were relatively selective for serotonin neurons. Additional neurotoxins were discovered, but it was MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) that predominated neurotoxicity research in the 1980s. Eventually, Clostridium botulinum neurotoxin (BoNT), discovered as a "poisonous" principle in the late 1800s, resurfaced in purified and standardized forms as a clinically useful drug. Neurotoxins represent chemical tools, useful not only for discerning neuronal mechanisms and animal modeling of neurological disorders, but also for their use in medicine and potential as treatments for medical disorders. This unit reviews the early discovery of neurotoxins, describes categories of neurotoxins, and finally characterizes their usefulness--first as research tools, and eventually as clinical therapeutic agents.

Common key-signals in learning and neurodegeneration: focus on excito-amino acids, beta-amyloid peptides and alpha-synuclein

In this paper a hypothesis that some special signals ("key-signals" excito-amino acids, beta-amyloid peptides and alpha-synuclein) are not only involved in information handling by the neuronal circuits, but also trigger out substantial structural and/or functional changes in the Central Nervous System (CNS) is introduced. This forces the neuronal circuits to move from one stable state towards a new state, but in doing so these signals became potentially dangerous. Several mechanisms are put in action to protect neurons and glial cells from these potentially harmful signals. However, in agreement with the Red Queen Theory of Ageing (Agnati et al. in Acta Physiol Scand 145:301-309, 1992), it is proposed that during ageing these neuroprotective processes become less effective while, in the meantime, a shortage of brain plasticity occurs together with an increased need of plasticity for repairing the wear and tear of the CNS. The paper presents findings supporting the concept that such key-signals in instances such as ageing may favour neurodegenerative processes in an attempt of maximizing neuronal plasticity.

Hierarchical recruitment by AMPA but not staurosporine of pro-apoptotic mitochondrial signaling in cultured cortical neurons: evidence for caspase-dependent/independent cross-talk

Excitotoxicity mediated via the (S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) subtype of receptor for l-glutamate contributes to various neuropathologies involving acute brain injury and chronic degenerative disorders. In this study, AMPA-induced neuronal injury and staurosporine (STS)-mediated apoptosis were compared in primary neuronal cultures of murine cerebral cortex by analyzing indices up- and downstream of mitochondrial activation. AMPA-mediated apoptosis involved induction of Bax, loss of mitochondrial transmembrane potential (deltapsi(m)), early release of cytochrome c (cyt c), and more delayed release of second mitochondrial activator of caspases (SMAC), Omi, and apoptosis-inducing factor (AIF) with early calpain and minor late activation of caspase 3. STS-induced apoptosis was characterized by a number of differences, a more rapid time course, non-involvement of deltapsi(m), and relatively early recruitment of SMAC and caspase 3. The AMPA-induced rise in intracellular calcium appeared insufficient to evoke feltapsi(m) as release of cyt c preceded mitochondrial depolarization, which was followed by the cytosolic translocation of SMAC, Omi, and AIF. Bax translocation preceded cyt c release for both stimuli inferring its involvement in apoptotic induction. Inclusion of the broad spectrum caspase inhibitor zVAD-fmk reduced the AMPA-induced release of cyt c, SMAC, and AIF, while only affecting the redistribution of Omi and AIF in the STS-treated neurons. Only AIF release was affected by a calpain inhibitor (calpastatin) which exerted relatively minor effects on the progression of cellular injury. AMPA-mediated release of apoptogenic proteins was more hierarchical relative to STS with its calpain activation and caspase-dependent AIF redistribution arguing for a model with cross-talk between caspase-dependent/independent apoptosis.

Neuroprotective and anticonvulsant effects of EGIS-8332, a non-competitive AMPA receptor antagonist, in a range of animal models

BACKGROUND AND PURPOSE

Blockade of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors is a good treatment option for a variety of central nervous system disorders. The present study evaluated the neuroprotective and anticonvulsant effects of EGIS-8332, a non-competitive AMPA receptor antagonist, as a potential drug candidate.

EXPERIMENTAL APPROACH

AMPA antagonist effects of EGIS-8332 were measured using patch-clamp techniques. Neuroprotective and anticonvulsant effects of EGIS-8332 were evaluated in various experimental models, relative to those of GYKI 53405.

KEY RESULTS

EGIS-8332 inhibited AMPA currents in rat cerebellar Purkinje cells and inhibited the AMPA- and quisqualate-induced excitotoxicity in primary cultures of telencephalon neurons (IC(50)=5.1-9.0 microM), in vitro. Good anticonvulsant actions were obtained in maximal electroshock-, sound- and chemically-induced seizures (range of ED(50)=1.4-14.0 mg kg(-1) i.p.) in mice. Four days after transient global cerebral ischaemia, EGIS-8332 decreased neuronal loss in the hippocampal CA1 area in gerbils and rats. EGIS-8332 dose-dependently reduced cerebral infarct size after permanent middle cerebral artery occlusion in mice and rats (minimum effective dose=3 mg kg(-1) i.p.). Side effects of EGIS-8332 emerged much above its pharmacologically active doses. A tendency for better efficacy of GYKI 53405 than that of EGIS-8332 was observed in anticonvulsant tests that reached statistical significance in few cases, while the contrary was perceived in cerebral ischaemia tests.

CONCLUSIONS AND IMPLICATIONS

EGIS-8332 seems suitable for further development for the treatment of epilepsy, ischaemia and stroke based on its efficacy in a variety of experimental disease models, and on its low side effect potential.

Astroglia: important mediators of traumatic brain injury

Traumatic brain injury (TBI) research to date has focused almost exclusively on the pathophysiology of injured neurons with very little attention paid to non-neuronal cells. However in the past decade, exciting discoveries have challenged this century-old view of passive glial cells and have led to a reinterpretation of the role of glial cells in central nervous system (CNS) biology and pathology. In this chapter we review several lines of evidence, indicating that glial cells, particularly astrocytes, are active partners to neurons in the brain, and summarize recent findings that detail the significance of astrocyte pathology in traumatic brain injury.

Block of kainate receptor desensitization uncovers a key trafficking checkpoint

A prominent feature of ionotropic glutamate receptors from the AMPA and kainate subtypes is their profound desensitization in response to glutamate-a process thought to protect the neuron from overexcitation. In AMPA receptors, it is well established that desensitization results from rearrangements of the interface formed between agonist-binding domains of adjacent subunits; however, it is unclear how this mechanism applies to kainate receptors. Here we show that stabilization of the binding domain dimer by the generation of intermolecular disulfide bonds apparently blocked desensitization of the kainate receptor GluR6. This result establishes a common desensitization mechanism in both AMPA and kainate receptors. Surprisingly, however, surface expression of these nondesensitizing mutants was drastically reduced and did not depend on channel activity. Therefore, in addition to its role at the synapse, we now propose an intracellular role for desensitization in controlling maturation and trafficking of glutamate receptors.

Therapeutic time window of neuroprotection by non-competitive AMPA antagonists in transient and permanent focal cerebral ischemia in rats

EGIS-8332 and GYKI 53405 are selective, non-competitive AMPA (2-amino-3[3-hydroxy-5-methyl-4-isoxazolyl] propionic acid) antagonists that effectively protected against tissue injury caused by global and focal cerebral ischemia in laboratory animals. This study evaluated the therapeutic time window of neuroprotection by EGIS-8332 and GYKI 53405 in permanent and transient middle cerebral artery occlusion (MCAO) in Sprague-Dawley rats. Infarct size was measured by TTC staining 48 h after permanent MCAO (electrocoagulation), and 24 h after reperfusion following a 1-h transient MCAO carried out using the intraluminal filament technique. Treatment with EGIS-8332 (10 mg/kg, i.p.) 60 or 120 min after permanent MCAO, decreased infarct size by 30% and 36%, respectively, and the effect of GYKI 53405 (10 mg/kg, i.p.) was similar (30% and 33%, respectively; p<0.01 all). Neither compound was effective if administered 180 or 240 min after permanent MCAO. Both EGIS-8332 and GYKI 53405 (20 mg/kg, i.p.) reduced the core and total (core plus penumbra) volumes of tissue injury in the whole brain and the cerebral cortex when administered 120 or 180 min after transient MCAO. The compounds did not alter tissue damage in the striatum. No neuroprotective effect was obtained at 240 min after transient MCAO. In conclusion, the therapeutic time window of neuroprotection by EGIS-8332 and GYKI 53405 was 2 h in permanent and 3 h in transient focal cerebral ischemia in rats. The results suggest that treatment with non-competitive AMPA antagonists can only be expected to produce a neuroprotective action in humans if administered shortly after the appearance of stroke symptoms.

Response of Purkinje neurons to hypobaric hypoxic exposure as shown by alteration in expression of glutamate receptors, nitric oxide synthases and calcium binding proteins

Hypobaric hypoxia is known to impair muscular coordination. It is not known whether hypobaric hypoxia causes any damage to the Purkinje neurons which may be responsible for impairment of muscular coordination. Expression of ionotropic glutamate receptors N-methyl-d-aspartate receptor subunit 1, amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid GluR2/3, calcium binding proteins and nitric oxide synthases in the Purkinje neurons was examined in rats exposed to hypobaric hypoxia. The mRNA expression of N-methyl-d-aspartate receptor subunit 1, GluR2, GluR3 and nitric oxide synthases [neuronal, endothelial and inducible] was upregulated at 3 h peaking at 24 h after the exposure. This was sustained up to 3 days; thereafter, it was comparable to the controls. Immunohistochemical analysis confirmed a marked expression of N-methyl-d-aspartate receptor subunit 1 and GluR2/3 at the above time intervals. Immunoexpression of calbindin-D28k (calbindin) and parvalbumin was intense in the soma of Purkinje neurons in the control rats. It was, however, drastically downregulated up to 3 days after exposure. At 3 days the neuronal dendrites showed intense expression of calbindin which returned to control levels at 7 days. Expression of neuronal nitric oxide synthase and inducible nitric oxide synthase was markedly upregulated from 3 h to 3 days whereas endothelial nitric oxide synthase expression, localized in the blood vessels and Purkinje neurons, remained elevated up to 24 h after the exposure. A progressive darkening of the Purkinje neuron cell bodies was observed at ultrastructural level up to 3 days but degenerating cells were not observed. A salient alteration was the dilation and stacking of smooth endoplasmic reticulum in the dendrites up to 14 days after the exposure. The present results suggest that hypobaric hypoxia leads to overexpression of N-methyl-d-aspartate receptor subunit 1 and GluR2/3 in Purkinje neurons that may be responsive to altered calcium levels as manifested by decreased expression of calcium binding proteins. This together with excess nitric oxide production may have led to transient ultrastructural changes. We propose that the functions of the Purkinje neurons may be altered in response to an acute exposure to hypobaric hypoxia resulting in impairment of motor coordination.

Glutamate receptor-mediated regulation of c-fos expression in cultured microglia

It has been recently shown that the expression of various types of neurotransmitter receptors is not restricted to neurons but also observed in a majority of glial cells. However, their function in glial cells is not known well in both physiological and pathological conditions. Here, we investigated the role of glutamate receptor on c-fos gene expression in primary cultured and BV-2 microglia. Our results demonstrated that both c-fos mRNA and protein were dramatically induced following treatment with various glutamate receptor agonists (500muM); N-methyl-d-aspartic acid, kainic acid, (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and (RS)-3,5-dihydroxyphenylglycine. The responses were significantly suppressed by specific antagonists and also by calcium chelating agents EGTA and BAPTA-AM. Our results suggest that glutamate receptor activation regulates c-fos gene expression by modifying intracellular calcium levels in microglia. These findings might provide an insight in to understanding the function of microglial glutamate receptors in neuron-to-glial interaction under the excitotoxic conditions.

Role of astrocytic transport processes in glutamatergic and GABAergic neurotransmission

The fine tuning of both glutamatergic and GABAergic neurotransmission is to a large extent dependent upon optimal function of astrocytic transport processes. Thus, glutamate transport in astrocytes is mandatory to maintain extrasynaptic glutamate levels sufficiently low to prevent excitotoxic neuronal damage. In GABA synapses hyperactivity of astroglial GABA uptake may lead to diminished GABAergic inhibitory activity resulting in seizures. As a consequence of this the expression and functional activity of astrocytic glutamate and GABA transport is regulated in a number of ways at transcriptional, translational and post-translational levels. This opens for a number of therapeutic strategies by which the efficacy of excitatory and inhibitory neurotransmission may be manipulated.

Modulation by topiramate of AMPA and kainate mediated calcium influx in cultured cerebral cortical, hippocampal and cerebellar neurons

The effect of the antiepileptic drug topiramate on Ca2+ uptake through (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionate (AMPA) and kainate (KA) receptors was investigated in different cell culture systems consisting of neurons from the cerebral cortex, hippocampus, and cerebellum. Ca2+ influx was assayed using a fluorescent Ca2+ chelator to monitor changes in the intracellular Ca2+ concentration or cobalt staining to assess the effect of topiramate on Ca2+-permeable AMPA/KA receptors. In all types of neuronal cultures studied, AMPA and KA were found to elicit an influx of Ca2+ in a subset of the neuronal population. Topiramate, at concentrations of 30 and 100 microM, inhibited Ca2+ influx by up to 60%. Modulation of AMPA and KA-evoked Ca2+ influx may contribute to both the antiepileptic and neuroprotective properties of topiramate.