Calcium-permeable AMPA channels in neurodegenerative disease and ischemia

Neuroprotective Mechanisms of Taurine against Ischemic Stroke

Ischemic stroke exhibits a multiplicity of pathophysiological mechanisms. To address the diverse pathophysiological mechanisms observed in ischemic stroke investigators seek to find therapeutic strategies that are multifaceted in their action by either investigating multipotential compounds or by using a combination of compounds. Taurine, an endogenous amino acid, exhibits a plethora of physiological functions. It exhibits antioxidative properties, stabilizes membrane, functions as an osmoregulator, modulates ionic movements, reduces the level of pro-inflammators, regulates intracellular calcium concentration; all of which contributes to its neuroprotective effect. Data are accumulating that show the neuroprotective mechanisms of taurine against stroke pathophysiology. In this review, we describe the neuroprotective mechanisms employed by taurine against ischemic stroke and its use in clinical trial for ischemic stroke.

A role of TARPs in the expression and plasticity of calcium-permeable AMPARs: evidence from cerebellar neurons and glia

The inclusion of GluA2 subunits has a profound impact on the channel properties of AMPA receptors (AMPARs), in particular rendering them impermeable to calcium. While GluA2-containing AMPARs are the most abundant in the central nervous system, GluA2-lacking calcium-permeable AMPARs are also expressed in wide variety of neurons and glia. Accumulating evidence suggests that the dynamic control of the GluA2 content of AMPARs plays a critical role in development, synaptic plasticity, and diverse neurological conditions ranging from ischemia-induced brain damage to drug addiction. It is thus important to understand the molecular mechanisms involved in regulating the balance of AMPAR subtypes, particularly the role of their co-assembled auxiliary subunits. The discovery of transmembrane AMPAR regulatory proteins (TARPs), initially within the cerebellum, has transformed the field of AMPAR research. It is now clear that these auxiliary subunits play a key role in multiple aspects of AMPAR trafficking and function in the brain. Yet, their precise role in AMPAR subtype-specific regulation has only recently received particular attention. Here we review recent findings on the differential regulation of calcium-permeable (CP-) and -impermeable (CI-) AMPARs in cerebellar neurons and glial cells, and discuss the critical involvement of TARPs in this process.

This article is part of the Special Issue entitled ‘Glutamate Receptor-Dependent Synaptic Plasticity’.

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Calcium-permeable AMPARs are present in various cerebellar neurons and glial cells.

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The contribution of calcium-permeable AMPARs to transmission is dynamically regulated.

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TARPs influence the relative expression of AMPAR subtypes.

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Evidence suggests that TARPs play a role in calcium-permeable AMPAR plasticity.

Disruption of the GluR2/GAPDH complex protects against ischemia-induced neuronal damage

BACKGROUND

Excitotoxicity and neuronal death following ischemia involve AMPA (α-amino-3hydroxy-5-methylisoxazole-4-propionic acid) glutamate receptors. We have recently reported that the GluR2 subunit of AMPA receptors (AMPARs) forms a protein complex with GAPDH (glyceraldehyde-3-phosphate dehydrogenase). The GluR2/GAPDH complex co-internalizes upon activation of AMPA receptors. Disruption of the GluR2/GAPDH interaction with an interfering peptide protects cells against AMPAR-mediated excitotoxicity and protects against damage induced by oxygen-glucose deprivation (OGD), an in vitro model of brain ischemia.

OBJECTIVE

We sought to test the hypothesis that disruption of the GluR2/GAPDH interaction with an interfering peptide would protect against ischemia-induced neuronal damage in vivo.

METHOD

The rat 4-vessel occlusion (4-VO) model was used to investigate whether the GluR2/GAPDH interaction was enhanced in the hippocampus, and if our newly developed interfering peptide could protect against neuronal death in the ischemic brain area. The transient rat middle cerebral artery occlusion (tMCAo) model was used to determine whether our peptide could reduce infarction volume and improve neurological function. Finally, GAPDH lentiviral shRNA was injected into the brain to reduce GAPDH expression one week prior to tMCAo, to confirm the role of GAPDH in the pathophysiology of brain ischemia.

RESULTS

The GluR2/GAPDH interaction is upregulated in the hippocampus of rats subjected to transient global ischemia. Administration of an interfering peptide that is able to disrupt the GluR2/GAPDH interaction in vivo protects against ischemia-induced cell death in rat models of global ischemia and decreases the infarct volume as well as neurological score in a rat model focal ischemia. Consistent with these observations, decreased GAPDH expression also protects against ischemia-induced cell death in an animal model of focal ischemia.

CONCLUSION

Disruption of the GluR2/GAPDH interaction protects against ischemia-induced neuronal damage in vivo. The GluR2/GAPDH interaction may be a novel therapeutic target for development of treatment for ischemic stroke.

Epigenetic mechanisms in stroke and epilepsy

Epigenetic remodeling and modifications of chromatin structure by DNA methylation and histone modifications represent central mechanisms for the regulation of neuronal gene expression during brain development, higher-order processing, and memory formation. Emerging evidence implicates epigenetic modifications not only in normal brain function, but also in neuropsychiatric disorders. This review focuses on recent findings that disruption of chromatin modifications have a major role in the neurodegeneration associated with ischemic stroke and epilepsy. Although these disorders differ in their underlying causes and pathophysiology, they share a common feature, in that each disorder activates the gene silencing transcription factor REST (repressor element 1 silencing transcription factor), which orchestrates epigenetic remodeling of a subset of 'transcriptionally responsive targets' implicated in neuronal death. Although ischemic insults activate REST in selectively vulnerable neurons in the hippocampal CA1, seizures activate REST in CA3 neurons destined to die. Profiling the array of genes that are epigenetically dysregulated in response to neuronal insults is likely to advance our understanding of the mechanisms underlying the pathophysiology of these disorders and may lead to the identification of novel therapeutic strategies for the amelioration of these serious human conditions.

Piperphentonamine (PPTA) attenuated cerebral ischemia-induced memory deficits via neuroprotection associated with anti-apoptotic activity

The calcium sensitizers levosimendan and piperphentonamine hydrochloride (PPTA) are used as cardiovascular drugs for treatment of heart failure. Given that levosimendan has been reported to exhibit a neuroprotective profile in a model of traumatic brain injury, it was interesting to know whether PPTA, a new calcium sensitizer recently developed in China, exerts a similar effect. The objective of this study was to determine whether PPTA exhibited neuroprotective effects and whether these properties were associated with memory. Four-vessel occlusion (4-VO) was used to induce global cerebral ischemia/reperfusion injury in rats treated with or without PPTA (5, 10 mg/kg, i.p., 2 h after the onset of reperfusion and then once a day for 15 consecutive days). Memory was measured using the step-through passive avoidance test. Neurochemical changes were examined in rat PC12 cells treated with oxygen-glucose deprivation (OGD) for 4 h followed by reoxygenation (OGD-R) for 24 h, in the absence or presence of PPTA. In vehicle-treated animals, 4-VO for 10 min produced memory deficits, as demonstrated by decreased retention in step-through passive avoidance, and massive neuron loss in the hippocampal CA1 subregion. These effects were attenuated by PPTA. The results were consistent with those observed in PC12 cells. PPTA treatment increased cell viability, as indicated by MTT assay, inhibited apoptosis, and decreased extracellular lactate dehydrogenase levels in Na(2)S(2)O(4)-treated PC12 cells. These results provide novel demonstration for the ability of PPTA to attenuate cerebral ischemia-induced memory deficits via neuroprotection in the hippocampus. The neuroprotective effect of PPTA appears to be associated with its anti-apoptotic activity. PPTA has the therapeutic potential for ischemic stroke.

Steric antisense inhibition of AMPA receptor Q/R editing reveals tight coupling to intronic editing sites and splicing

Adenosine-to-Inosine (A-to-I) RNA editing is a post-transcriptional mechanism, evolved to diversify the transcriptome in metazoa. In addition to wide-spread editing in non-coding regions protein recoding by RNA editing allows for fine tuning of protein function. Functional consequences are only known for some editing sites and the combinatorial effect between multiple sites (functional epistasis) is currently unclear. Similarly, the interplay between RNA editing and splicing, which impacts on post-transcriptional gene regulation, has not been resolved. Here, we describe a versatile antisense approach, which will aid resolving these open questions. We have developed and characterized morpholino oligos targeting the most efficiently edited site—the AMPA receptor GluA2 Q/R site. We show that inhibition of editing closely correlates with intronic editing efficiency, which is linked to splicing efficiency. In addition to providing a versatile tool our data underscore the unique efficiency of a physiologically pivotal editing site.

MicroRNA-223 is neuroprotective by targeting glutamate receptors

Stroke is a major cause of mortality and morbidity worldwide. Extracellular glutamate accumulation leading to overstimulation of the ionotropic glutamate receptors mediates neuronal injury in stroke and in neurodegenerative disorders. Here we show that miR-223 controls the response to neuronal injury by regulating the functional expression of the glutamate receptor subunits GluR2 and NR2B in brain. Overexpression of miR-223 lowers the levels of GluR2 and NR2B by targeting 3'-UTR target sites (TSs) in GluR2 and NR2B, inhibits NMDA-induced calcium influx in hippocampal neurons, and protects the brain from neuronal cell death following transient global ischemia and excitotoxic injury. MiR-223 deficiency results in higher levels of NR2B and GluR2, enhanced NMDA-induced calcium influx, and increased miniature excitatory postsynaptic currents in hippocampal neurons. In addition, the absence of MiR-223 leads to contextual, but not cued memory deficits and increased neuronal cell death following transient global ischemia and excitotoxicity. These data identify miR-223 as a major regulator of the expression of GluR2 and NR2B, and suggest a therapeutic role for miR-223 in stroke and other excitotoxic neuronal disorders.

Calcium-permeable AMPA receptors are expressed in a rodent model of status epilepticus

OBJECTIVE

A study was undertaken to characterize the plasticity of AMPA receptor (AMPAR)-mediated neurotransmission in the hippocampus during status epilepticus (SE).

METHODS

SE was induced by pilocarpine, and animals were studied 10 minutes (refractory SE) or 60 minutes (late SE) after the onset of the first grade 5 seizures. AMPAR-mediated currents were recorded from CA1 pyramidal neurons and dentate granule cells (DGCs) by voltage clamp technique. The surface expression of GluA2 subunit on hippocampal membranes was determined using a biotinylation assay. GluA2 internalization and changes in intracellular calcium ([Ca](i)) levels were studied in hippocampal cultures using immunocytochemical and live-imaging techniques. AMPAR antagonist treatment of SE was evaluated by video and electroencephalography.

RESULTS

AMPAR-mediated currents recorded from CA1 neurons from refractory and late SE animals were inwardly rectifying, and philanthotoxin-sensitive; similar changes were observed in recordings obtained from DGCs from refractory SE animals. GluA2 subunit surface expression was reduced in the hippocampus during refractory and late SE. In cultured hippocampal pyramidal neurons, recurrent bursting diminished surface expression of the GluA2 subunit and enhanced its internalization rate. Recurrent bursting-induced increase in [Ca](i) levels was reduced by selective inhibition of GluA2-lacking AMPARs. GYKI-52466 terminated diazepam-refractory SE.

INTERPRETATION

During SE, there is rapid, ongoing plasticity of AMPARs with the expression of GluA2-lacking AMPARs. These receptors provide another source of Ca(2+) entry into the principal neurons. Benzodiazepam-refractory SE can be terminated by AMPAR antagonism. The data identify AMPARs as a potential therapeutic target for the treatment of SE.

Calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors trigger neuronal nitric-oxide synthase activation to promote nerve cell death in an Src kinase-dependent fashion

In the retina information decoding is dependent on excitatory neurotransmission and is critically modulated by AMPA glutamate receptors. The Src-tyrosine kinase has been implicated in modulating neurotransmission in CNS. Thus, our main goal was to correlate AMPA-mediated excitatory neurotransmission with the modulation of Src activity in retinal neurons. Cultured retinal cells were used to access the effects of AMPA stimulation on nitric oxide (NO) production and Src phosphorylation. 4-Amino-5-methylamino-2',7'-difluorofluorescein diacetate fluorescence mainly determined NO production, and immunocytochemistry and Western blotting evaluated Src activation. AMPA receptors activation rapidly up-regulated Src phosphorylation at tyrosine 416 (stimulatory site) and down-regulated phosphotyrosine 527 (inhibitory site) in retinal cells, an effect mainly mediated by calcium-permeable AMPA receptors. Interestingly, experiments confirmed that neuronal NOS was activated in response to calcium-permeable AMPA receptor stimulation. Moreover, data suggest NO pathway as a key regulatory signaling in AMPA-induced Src activation in neurons but not in glial cells. The NO donor SNAP (S-nitroso-N-acetyl-DL-penicillamine) and a soluble guanylyl cyclase agonist (YC-1) mimicked AMPA effect in Src Tyr-416 phosphorylation, reinforcing that Src activation is indeed modulated by the NO pathway. Gain and loss-of-function data demonstrated that ERK is a downstream target of AMPA-induced Src activation and NO signaling. Furthermore, AMPA stimulated NO production in organotypic retinal cultures and increased Src activity in the in vivo retina. Additionally, AMPA-induced apoptotic retinal cell death was regulated by both NOS and Src activity. Because Src activity is pivotal in several CNS regions, the data presented herein highlight that Src modulation is a critical step in excitatory retinal cell death.

TNF-α triggers rapid membrane insertion of Ca(2+) permeable AMPA receptors into adult motor neurons and enhances their susceptibility to slow excitotoxic injury

Excitotoxicity (caused by over-activation of glutamate receptors) and inflammation both contribute to motor neuron (MN) damage in amyotrophic lateral sclerosis (ALS) and other diseases of the spinal cord. Microglial and astrocytic activation in these conditions results in release of inflammatory mediators, including the cytokine, tumor necrosis factor-alpha (TNF-α). TNF-α has complex effects on neurons, one of which is to trigger rapid membrane insertion of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type glutamate receptors, and in some cases, specific insertion of GluA2 lacking, Ca(2+) permeable AMPA receptors (Ca-perm AMPAr). In the present study, we use a histochemical stain based upon kainate stimulated uptake of cobalt ions ("Co(2+) labeling") to provide the first direct demonstration of the presence of substantial numbers of Ca-perm AMPAr in ventral horn MNs of adult rats under basal conditions. We further find that TNF-α exposure causes a rapid increase in the numbers of these receptors, via a phosphatidylinositol 3 kinase (PI3K) and protein kinase A (PKA) dependent mechanism. Finally, to assess the relevance of TNF-α to slow excitotoxic MN injury, we made use of organotypic spinal cord slice cultures. Co(2+) labeling revealed that MNs in these cultures possess Ca-perm AMPAr. Addition of either a low level of TNF-α, or of the glutamate uptake blocker, trans-pyrrolidine-2,4-dicarboxylic acid (PDC) to the cultures for 48 h resulted in little MN injury. However, when combined, TNF-α+PDC caused considerable MN degeneration, which was blocked by the AMPA/kainate receptor blocker, 2,3-Dihydroxy-6-nitro-7-sulfamoylbenzo (F) quinoxaline (NBQX), or the Ca-perm AMPAr selective blocker, 1-naphthyl acetylspermine (NASPM). Thus, these data support the idea that prolonged TNF-α elevation, as may be induced by glial activation, acts in part by increasing the numbers of Ca-perm AMPAr on MNs to enhance injurious excitotoxic effects of deficient astrocytic glutamate transport.

Dysregulation of the autophagy-endolysosomal system in amyotrophic lateral sclerosis and related motor neuron diseases

Amyotrophic lateral sclerosis (ALS) is a heterogeneous group of incurable motor neuron diseases (MNDs) characterized by a selective loss of upper and lower motor neurons in the brain and spinal cord. Most cases of ALS are sporadic, while approximately 5–10% cases are familial. More than 16 causative genes for ALS/MNDs have been identified and their underlying pathogenesis, including oxidative stress, endoplasmic reticulum stress, excitotoxicity, mitochondrial dysfunction, neural inflammation, protein misfolding and accumulation, dysfunctional intracellular trafficking, abnormal RNA processing, and noncell-autonomous damage, has begun to emerge. It is currently believed that a complex interplay of multiple toxicity pathways is implicated in disease onset and progression. Among such mechanisms, ones that are associated with disturbances of protein homeostasis, the ubiquitin-proteasome system and autophagy, have recently been highlighted. Although it remains to be determined whether disease-associated protein aggregates have a toxic or protective role in the pathogenesis, the formation of them results from the imbalance between generation and degradation of misfolded proteins within neuronal cells. In this paper, we focus on the autophagy-lysosomal and endocytic degradation systems and implication of their dysfunction to the pathogenesis of ALS/MNDs. The autophagy-endolysosomal pathway could be a major target for the development of therapeutic agents for ALS/MNDs.

Comparative impact of voltage-gated calcium channels and NMDA receptors on mitochondria-mediated neuronal injury

Glutamate excitotoxicity, a major component of many neurodegenerative disorders, is characterized by excessive calcium influx selectively through NMDARs. However, there is a substantial uncertainty concerning why other known routes of significant calcium entry, in particular, VGCCs, are not similarly toxic. Here, we report that in the majority of neurons in rat hippocampal and cortical cultures, maximal L-type VGCC activation induces much lower calcium loading than toxic NMDAR activation. Consequently, few depolarization-activated neurons exhibit calcium deregulation and cell death. Activation of alternative routes of calcium entry induced neuronal death in proportion to the degree of calcium loading. In a small subset of neurons, depolarization evoked stronger calcium elevations, approaching those induced by toxic NMDA. These neurons were characterized by elevated expression of VGCCs and enhanced voltage-gated calcium currents, mitochondrial dysfunction and cell death. Preventing VGCC-dependent mitochondrial calcium loading resulted in stronger cytoplasmic calcium elevations, whereas inhibiting mitochondrial calcium clearance accelerated mitochondrial depolarization. Both observations further implicate mitochondrial dysfunction in VGCC-mediated cell death. Results indicate that neuronal vulnerability tracks the extent of calcium loading but does not appear to depend explicitly on the route of calcium entry.

Expression of AMPA receptor subunits in hippocampus after status convulsion

OBJECTIVE

The expression of 2-amino-3-(5-methyl-3-oxo-1, 2-oxazol-4-yl) propanoic acid receptor (AMPAR) subunits in the hippocampus of naive immature and adult rats (IRs, ARs) was investigated after status convulsion (SC).

METHODS

Seizures were induced in IRs and ARs with intraperitoneal injections of lithium and pilocarpine. Rats were killed at four time points (3 h, 1 day, 3 days, and 7 days) after SC. The proportion of apoptotic cells was quantified by Annexin V-FITC apoptosis detection. The location and type of apoptotic cells were assessed by using terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling staining. Immunoblotting techniques were used to demonstrate changes in AMPAR subunit expression.

RESULTS

Severe seizures induced neuronal apoptosis in the hippocampus. The proportion of apoptotic cells in IRs was consistently lower than that in ARs after SC. The expressions of four AMPAR subunits in IRs were consistently lower than those in ARs before and after SC. SC for 1 h inhibited the expression of glutamate receptors (GluR1-4) in the hippocampus of IRs and ARs and altered the subunit composition of AMPARs. GluR2 was the predominant AMPAR subunit in the hippocampus of normal ARs, while the GluR2/3 subunits were predominantly expressed 7 days after SC. GluR3/4 subunits were mainly expressed in the hippocampus of normal IRs, which had the lowest levels of GluR2.

CONCLUSIONS

Immature brain was more resistant to seizure-induced neural damage. The time course of reduction and recovery differed for each subunit and was dependent on developmental stage. The increased expression of GluR2 could confer early but transient protection in the immature brain after SC.

Channel properties reveal differential expression of TARPed and TARPless AMPARs in stargazer neurons

Dynamic regulation of calcium-permeable AMPA receptors (CP-AMPARs) is important for normal synaptic transmission, plasticity and pathological changes. Although the involvement of transmembrane AMPAR regulatory proteins (TARPs) in trafficking of calcium-impermeable AMPARs (CI-AMPARs) has been extensively studied, their role in the surface expression and function of CP-AMPARs remains unclear. We examined AMPAR-mediated currents in cerebellar stellate cells from stargazer mice, which lack the prototypical TARP stargazin (g-2). We found a marked increase in the contribution of CP-AMPARs to synaptic responses, indicating that, unlike CI-AMPARs, these can localize at synapses in the absence of g-2. In contrast with CP-AMPARs in extrasynaptic regions, synaptic CP-AMPARs displayed an unexpectedly low channel conductance and strong block by intracellular spermine, suggesting that they were ‘TARPless’. As a proof of principle that TARP association is not an absolute requirement for AMPAR clustering at synapses, miniature excitatory postsynaptic currents mediated by TARPless AMPARs were readily detected in stargazer granule cells following knockdown of their only other TARP, g-7.

Remodeling of synapses in the CA1 area of the hippocampus after transient global ischemia

Synapses are essential to neuronal functions. Synaptic changes occur under physiological and pathological conditions. Here we report the remodeling of synapses in the CA1 area of the hippocampus after transient global ischemia using electron microscopy. Much electron-dense material appeared in the cytoplasm of dendrites at 24h after ischemia. Many dark axons or terminals were found in the CA1 neuropil; some of which were phagocytized by dendrites. Interestingly autophagosomes appeared in many axons or dendrites at 48 h after ischemia. In addition, postsynaptic density (PSD) - like structures or synaptic - like structures were found inside spines and dendrites. Statistical analysis demonstrated that the thickness of PSDs in the CA1 neuropil increased from 12 to 48 h after ischemia. The frequency of autophagosomes appeared to escalate from 12 to 48 h after ischemia. The frequency of asymmetric synapses was significantly increased at 12h and 24h after ischemia in stratum oriens, proximal and distal stratum radiatum. Among asymmetric synapses, the number of perforated synapses consistently increased and reached a peak (approximately 10-fold increase) at 48 h after ischemia. On the other hand, the number of multiple synaptic boutons decreased after ischemia reaching a two to fourfold decrease at 48 h after ischemia. These results have shown that ischemia induces an increase of asymmetric synapses as well as synaptic autophagy, which may contribute to the neuronal death in the CA1 area after transient global ischemia.

Maintenance of synaptic stability requires calcium-independent phospholipase A₂ activity

Phospholipases A₂ (PLA₂s) represent one of the largest groups of lipid-modifying enzymes. Over the years, significant advances have been made in understanding their potential physiological and pathological functions. Depending on their calcium requirement for activation, PLA₂s are classified into calcium dependent and independent. This paper mainly focuses on brain calcium-independent PLA₂ (iPLA₂) and on the mechanisms by which they influence neuronal function and regulate synaptic plasticity. Particular attention will be given to the iPLA₂γ isoform and its role in the regulation of synaptic glutamate receptors. In particular, the paper discusses the possibility that brain iPLA₂γ deficiencies could destabilise normal synaptic operation and might contribute to the aetiology of some brain disorders. In this line, the paper presents new data indicating that iPLA₂γ deficiencies accentuate AMPA receptor destabilization and tau phosphorylation, which suggests that this iPLA₂ isoform should be considered as a potential target for the treatment of Tau-related disorders.

Direct interaction between GluR2 and GAPDH regulates AMPAR-mediated excitotoxicity

Over-activation of AMPARs (α−amino-3-hydroxy-5-methylisoxazole-4-propionic acid subtype glutamate receptors) is implicated in excitotoxic neuronal death associated with acute brain insults, such as ischemic stroke. However, the specific molecular mechanism by which AMPARs, especially the calcium-impermeable AMPARs, induce neuronal death remains poorly understood. Here we report the identification of a previously unrecognized molecular pathway involving a direct protein-protein interaction that underlies GluR2-containing AMPAR-mediated excitotoxicity. Agonist stimulation of AMPARs promotes GluR2/GAPDH (glyceraldehyde-3-phosphate dehydrogenase) complex formation and subsequent internalization. Disruption of GluR2/GAPDH interaction by administration of an interfering peptide prevents AMPAR-mediated excitotoxicity and protects against damage induced by oxygen-glucose deprivation (OGD), an in vitro model of brain ischemia.

The essential role of AMPA receptor GluR2 subunit RNA editing in the normal and diseased brain

α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are comprised of different combinations of GluA1–GluA4 (also known asGluR1–GluR4 and GluR-A to GluR-D) subunits. The GluA2 subunit is subject to RNA editing by the ADAR2 enzyme, which converts a codon for glutamine (Gln; Q), present in the GluA2 gene, to a codon for arginine (Arg; R) found in the mRNA. AMPA receptors are calcium (Ca²⁺)-permeable if they contain the unedited GluA2(Q) subunit or if they lack the GluA2 subunit. While most AMPA receptors in the brain contain the edited GluA2(R) subunit and are therefore Ca²⁺-impermeable, recent evidence suggests that Ca²⁺-permeable AMPA receptors are important in synaptic plasticity, learning, and disease. Strong evidence supports the notion that Ca²⁺-permeable AMPA receptors are usually GluA2-lacking AMPA receptors, with little evidence to date for a significant role of unedited GluA2 in normal brain function. However, recent detailed studies suggest that Ca²⁺-permeable AMPA receptors containing unedited GluA2 do in fact occur in neurons and can contribute to excitotoxic cell loss, even where it was previously thought that there was no unedited GluA2.This review provides an update on the role of GluA2 RNA editing in the healthy and diseased brain and summarizes recent insights into the mechanisms that control this process. We suggest that further studies of the role of unedited GluA2 in normal brain function and disease are warranted, and that GluA2 editing should be considered as a possible contributing factor when Ca²⁺-permeable AMPA receptors are observed.

The abnormal processing of TDP-43 is not an upstream event of reduced ADAR2 activity in ALS motor neurons

TDP-43 pathology in motor neurons is a hallmark of ALS. In addition, the reduced expression of an RNA editing enzyme, adenosine deaminase acting on RNA 2 (ADAR2), increases the expression of GluA2 with an unedited Q/R site in the motor neurons of patients with sporadic ALS. As the occurrence of these two disease-specific abnormalities in the same motor neurons suggests a molecular link between them, we examined the effects of altered TDP-43 processing on ADAR2 activity in TetHeLaG2m and Neuro2a cells. We found that ADAR2 activity did not consistently change due to the overexpression or knockdown of TDP-43 or the expression of abnormal TDP-43, including caspase-3-cleaved fragments, truncated TDP-43 lacking either nuclear localization or export signals and ALS-linked TDP-43 mutants. These results suggest that the abnormal processing of TDP-43 is not an upstream event of inefficient GluA2 Q/R site editing in the motor neurons of sporadic ALS patients.

Repressor element-1 silencing transcription factor (REST)-dependent epigenetic remodeling is critical to ischemia-induced neuronal death

Dysregulation of the transcriptional repressor element-1 silencing transcription factor (REST)/neuron-restrictive silencer factor is important in a broad range of diseases, including cancer, diabetes, and heart disease. The role of REST-dependent epigenetic modifications in neurodegeneration is less clear. Here, we show that neuronal insults trigger activation of REST and CoREST in a clinically relevant model of ischemic stroke and that REST binds a subset of "transcriptionally responsive" genes (gria2, grin1, chrnb2, nefh, nfκb2, trpv1, chrm4, and syt6), of which the AMPA receptor subunit GluA2 is a top hit. Genes with enriched REST exhibited decreased mRNA and protein. We further show that REST assembles with CoREST, mSin3A, histone deacetylases 1 and 2, histone methyl-transferase G9a, and methyl CpG binding protein 2 at the promoters of target genes, where it orchestrates epigenetic remodeling and gene silencing. RNAi-mediated depletion of REST or administration of dominant-negative REST delivered directly into the hippocampus in vivo prevents epigenetic modifications, restores gene expression, and rescues hippocampal neurons. These findings document a causal role for REST-dependent epigenetic remodeling in the neurodegeneration associated with ischemic stroke and identify unique therapeutic targets for the amelioration of hippocampal injury and cognitive deficits.

Profound downregulation of the RNA editing enzyme ADAR2 in ALS spinal motor neurons

Amyotrophic lateral sclerosis (ALS) is the most common adult-onset fatal motor neuron disease. In spinal motor neurons of patients with sporadic ALS, normal RNA editing of GluA2, a subunit of the L-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor, is inefficient. Adenosine deaminase acting on RNA 2 (ADAR2) specifically mediates RNA editing at the glutamine/arginine (Q/R) site of GluA2 and motor neurons expressing Q/R site-unedited GluA2 undergo slow death in conditional ADAR2 knockout mice. Therefore, investigation into whether inefficient ADAR2-mediated GluA2 Q/R site-editing occurs universally in motor neurons of patients with ALS would provide insight into the pathogenesis of ALS. We analyzed the extents of GluA2 Q/R site-editing in an individual laser-captured motor neuron of 29 ALS patients compared with those of normal and disease control subjects. In addition, we analyzed the enzymatic activity of three members of the ADAR family (ADAR1, ADAR2 and ADAR3) in ALS motor neurons expressing unedited GluA2 mRNA and those expressing only edited GluA2 mRNA. Q/R site-unedited GluA2 mRNA was expressed in a significant proportion of motor neurons from all of the ALS cases examined. Conversely, motor neurons of the normal and disease control subjects expressed only edited GluA2 mRNA. ADAR2, but not ADAR1 or ADAR3, was significantly downregulated in all the motor neurons of ALS patients, more extensively in those expressing Q/R site-unedited GluA2 mRNA than those expressing only Q/R site-edited GluA2 mRNA. These results indicate that ADAR2 downregulation is a profound pathological change relevant to death of motor neurons in ALS.

Redox regulation of intracellular zinc: molecular signaling in the life and death of neurons

Zn(2+) has emerged as a major regulator of neuronal physiology, as well as an important signaling agent in neural injury. The intracellular concentration of this metal is tightly regulated through the actions of Zn(2+) transporters and the thiol-rich metal binding protein metallothionein, closely linking the redox status of the cell to cellular availability of Zn(2+). Accordingly, oxidative and nitrosative stress during ischemic injury leads to an accumulation of neuronal free Zn(2+) and the activation of several downstream cell death processes. While this Zn(2+) rise is an established signaling event in neuronal cell death, recent evidence suggests that a transient, sublethal accumulation of free Zn(2+) can also play a critical role in neuroprotective pathways activated during ischemic preconditioning. Thus, redox-sensitive proteins, like metallothioneins, may play a critical role in determining neuronal cell fate by regulating the localization and concentration of intracellular free Zn(2+).

Specific mechanism of use-dependent channel block of calcium-permeable AMPA receptors provides activity-dependent inhibition of glutamatergic neurotransmission

This study examined the blocking action of the selective channel blocker of calcium-permeable (CP) AMPA receptors, N1-(1-phenylcyclohexyl)pentane-1,5-diaminium bromide (IEM-1925), on excitatory postsynaptic currents in rat neostriatal and cortical neurons and in fly neuromuscular junctions. In both preparations, the blocking of CP-AMPA receptor currents increased along with the stimulation frequency. The continuous presence of kainate, which activates AMPA receptors, in the external solution also caused an enhanced blocking effect. Likewise, decrease of the synaptic release by lowering calcium concentration resulted in significant reduction of the blocking action. The activity dependence of the block is explained using the guarded receptor model. The drug molecule can only bind if the channel is open. After the channel has closed, the drug molecule remains trapped inside. However, the trapped molecule slowly egresses from closed channels to the cytoplasm. The total block effect is determined by the equilibrium between accumulation of the drug in the open channels and relief from the closed channels. Therefore, the conditions that favour the open state result in enhanced inhibition. This significant finding reveals a new way to modulate CP-AMPAR-mediated transmission using a physiologically relevant approach. Moreover, it allows the involvement of CP-AMPARs in the physiological and pathological processes – such as high-frequency synaptic activity or increase of the steady-state glutamate concentration – to be examined.

Exposure of neurons to excitotoxic levels of glutamate induces cleavage of the RNA editing enzyme, adenosine deaminase acting on RNA 2, and loss of GLUR2 editing

AMPA receptors are glutamate receptors that are tetramers of various combinations of GluR1-4 subunits. AMPA receptors containing GluR1, 3 and 4 are Ca2+ permeable, however, AMPA receptors containing even a single subunit of GluR2 are Ca2+ impermeable. Most AMPA receptors are Ca2+ impermeable due to the presence of GluR2. GluR2 confers special properties on AMPA receptors through the presence of arginine at the pore apex; other subunits (GluR1, 3, 4) contain glutamine at the pore apex and allow Ca2+ influx. Normally, an RNA editing step changes DNA-encoded glutamine to arginine, introduces arginine in the GluR2 pore apex. GluR2 RNA editing is carried out by an RNA-dependent adenosine deaminase (ADAR2). Loss of GluR2 editing leads to the formation of highly excitotoxic AMPA channels [Mahajan and Ziff (2007) Mol Cell Neurosci 35:470-481] and is shown to contribute to loss of motor neurons in amyotrophic lateral sclerosis (ALS). Relatively higher levels of Ca2+-permeable AMPA receptors are found in motor neurons and this has been correlated with lower GluR2 mRNA levels. However, the reason for loss of GluR2 editing is not known. Here we show that exposure of neurons to excitotoxic levels of glutamate leads to specific cleavage of ADAR2 that leads to generation of unedited GluR2. We demonstrate that cleaved ADAR2 leads to a decrease or loss of GluR2 editing, which will further result in high Ca2+ influx and excitotoxic neuronal death.

The role of glutamate receptors in traumatic brain injury: implications for postsynaptic density in pathophysiology

Traumatic brain injury (TBI) is the major cause of death and disability, and the incidence of TBI continues to increase rapidly. In recent years, increasing attention has been paid to an important structure at the postsynaptic membrane: the postsynaptic density (PSD). Glutamate receptors, as major components of the PSD, are highly responsive to alterations in the glutamate concentration at excitatory synapses and activate intracellular signal transduction via calcium and other second messengers following TBI. PSD scaffold proteins (PSD-95, Homer, and Shank), which anchor glutamate receptors and form a network structure, also have potential effects on these downstream signaling pathways. The changes in the function and structure of these major PSD proteins are also induced by TBI, indicating that there is a more complicated mechanism associated with PSD proteins in the pathophysiological process of TBI.

Activation of the mitochondrial permeability transition pore modulates Ca2+ responses to physiological stimuli in adult neurons

The participation of mitochondria in cellular and neuronal Ca(2+) homeostatic networks is now well accepted. Yet, critical tests of specific mitochondrial pathways in neuronal Ca(2+) responses have been hampered because the identity of mitochondrial proteins that must be integrated within this dynamic system remain uncertain. One putative pathway for Ca(2+) efflux from mitochondria exists through the formation of the permeability transition pore (PTP) that is often associated with cellular and neuronal death. Here, we have evaluated neuronal Ca(2+) dynamics and the PTP in single adult neurons in wild-type mice and those missing cyclophilin D (CyPD), a key regulator of the PTP. Using high-resolution time-lapse imaging, we demonstrate that PTP opening only follows simultaneous activation with two physiological stimuli that generate critical threshold levels of cytosolic and mitochondrial Ca(2+) . Our results are the first to demonstrate CyPD-dependent PTP opening in normal neuronal Ca(2+) homeostatic mechanisms not leading to activation of cell death pathways. As neurons in mice lacking CyPD are protected in a number of neurodegenerative disease models, the results suggest that improved viability of CyPD-knockout animals in these pathological states may be due to the transient, rather than persistent, activation of the PTP in mutant mitochondria, thereby shielding neurons from cytoplasmic Ca(2+) overload.

Excitatory synaptic transmission and network activity are depressed following mechanical injury in cortical neurons

In vitro and in vivo traumatic brain injury (TBI) alter the function and expression of glutamate receptors, yet the combined effect of these alterations on cortical excitatory synaptic transmission is unclear. We examined the effect of in vitro mechanical injury on excitatory synaptic function in cultured cortical neurons by assaying synaptically driven intracellular free calcium ([Ca(2+)](i)) oscillations in small neuronal networks as well as spontaneous and miniature excitatory postsynaptic currents (mEPSCs). We show that injury decreased the incidence and frequency of spontaneous neuronal [Ca(2+)](i) oscillations for at least 2 days post-injury. The amplitude of the oscillations was reduced immediately and 2 days post-injury, although a transient rebound at 4 h post-injury was observed due to increased activity of N-methyl-d-aspartate (NMDARs) and calcium-permeable α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (CP-AMPARs). Increased CP-AMPAR function was abolished by the inhibition of protein synthesis. In parallel, mEPSC amplitude decreased immediately, 4 h, and 2 days post-injury, with a transient increase in the contribution of synaptic CP-AMPARs observed at 4 h post-injury. Decreased mEPSC amplitude was evident after injury, even if NMDARs and CP-AMPARs were blocked pharmacologically, suggesting the decrease reflected alterations in synaptic Glur2-containing, calcium-impermeable AMPARs. Despite the transient increase in CP-AMPAR activity that we observed, the overriding effect of mechanical injury was long-term depression of excitatory neurotransmission that would be expected to contribute to the cognitive deficits of TBI.

Subunit-selective N-terminal domain associations organize the formation of AMPA receptor heteromers

The assembly of AMPA-type glutamate receptors (AMPARs) into distinct ion channel tetramers ultimately governs the nature of information transfer at excitatory synapses. How cells regulate the formation of diverse homo- and heteromeric AMPARs is unknown. Using a sensitive biophysical approach, we show that the extracellular, membrane-distal AMPAR N-terminal domains (NTDs) orchestrate selective routes of heteromeric assembly via a surprisingly wide spectrum of subunit-specific association affinities. Heteromerization is dominant, occurs at the level of the dimer, and results in a preferential incorporation of the functionally critical GluA2 subunit. Using a combination of structure-guided mutagenesis and electrophysiology, we further map evolutionarily variable hotspots in the NTD dimer interface, which modulate heteromerization capacity. This 'flexibility' of the NTD not only explains why heteromers predominate but also how GluA2-lacking, Ca(2+)-permeable homomers could form, which are induced under specific physiological and pathological conditions. Our findings reveal that distinct NTD properties set the stage for the biogenesis of functionally diverse pools of homo- and heteromeric AMPAR tetramers.

Knockout of Zn transporters Zip-1 and Zip-3 attenuates seizure-induced CA1 neurodegeneration

CA1 pyramidal neurons are the final integrators of information flow leaving the hippocampus, yet are singularly vulnerable to activity-dependent cell death. Zinc (Zn) entry into cells may add to this vulnerability. Here, we find that Slc39a1 and Slc39a3, members of the Zip (Zrt/Irt-like protein) plasmalemmal Zn transporter family, are predominantly expressed in the hippocampus. We examined Zip-1,3-deficient mice to investigate their role in neurodegeneration following intense synaptic activation. When isolated by blockade of NMDA receptors and voltage-gated calcium channels, the absence of both transporters slowed passive Zn uptake into CA1 neurons measured with intracellular fluorescent Zn dyes. In vivo CA1 cell damage following kainic acid exposure was greatly attenuated. Consistent with the hypothesis that Zn entry contributes to neurodegeneration, Znt-3-deficient mice lacking synaptic Zn also show less hippocampal cell damage following kainic acid injection. Zip transporters may provide selective therapeutic targets to protect these neurons from early Zn-induced neurodegeneration following injury.

GluA2-lacking, calcium-permeable AMPA receptors--inducers of plasticity?

AMPA receptors (AMPARs) are heterotetromeric complexes composed of GluA1-4 subunits. They are glutamate-gated channels traditionally considered solely as ion carriers for postsynaptic depolarization. However, the existence and dynamic regulation of GluA2-lacking, calcium-permeable AMPARs (Cp-AMPARs) enable these special receptors to serve also as signaling molecules presumably via calcium influx. Recent studies have implicated Cp-AMPARs in several types of synaptic plasticity, including homeostatic synaptic regulation and Hebbian synaptic plasticity. Cp-AMPARs are usually expressed transiently at an early stage of synaptic plasticity, but are then replaced with normal GluA2-containing receptors, indicating a role for Cp-AMPARs in induction, rather than the maintenance, of synaptic plasticity.

Inflammation alters trafficking of extrasynaptic AMPA receptors in tonically firing lamina II neurons of the rat spinal dorsal horn

Peripheral inflammation alters AMPA receptor (AMPAR) subunit trafficking and increases AMPAR Ca(2+) permeability at synapses of spinal dorsal horn neurons. However, it is unclear whether AMPAR trafficking at extrasynaptic sites of these neurons also changes under persistent inflammatory pain conditions. Using patch-clamp recording combined with Ca(2+) imaging and cobalt staining, we found that, under normal conditions, an extrasynaptic pool of AMPARs in rat substantia gelatinosa (SG) neurons of spinal dorsal horn predominantly consists of GluR2-containing Ca(2+)-impermeable receptors. Maintenance of complete Freund's adjuvant (CFA)-induced inflammation was associated with a marked enhancement of AMPA-induced currents and [Ca(2+)](i) transients in SG neurons, while, as we previously showed, the amplitude of synaptically evoked AMPAR-mediated currents was not changed 24 h after CFA. These findings indicate that extrasynaptic AMPARs are upregulated and their Ca(2+) permeability increases dramatically. This increase occurred in SG neurons characterized by intrinsic tonic firing properties, but not in those exhibited strong adaptation. This increase was also accompanied by an inward rectification of AMPA-induced currents and enhancement of sensitivity to a highly selective Ca(2+)-permeable AMPAR blocker, IEM-1460. Electron microcopy and biochemical assays additionally showed an increase in the amount of GluR1 at extrasynaptic membranes in dorsal horn neurons 24h post-CFA. Taken together, our findings indicate that CFA-induced inflammation increases functional expression and proportion of extrasynaptic GluR1-containing Ca(2+)-permeable AMPARs in tonically firing excitatory dorsal horn neurons, suggesting that the altered extrasynaptic AMPAR trafficking might participate in the maintenance of persistent inflammatory pain.

Characterizing single-channel behavior of GluA3 receptors

AMPA receptors play a major role in excitatory neurotransmission in the CNS and are involved in numerous neurological disorders. Agonists bind to each of four bilobed LBDs of this tetrameric receptor, and upon binding, the lobes close to envelope the agonist, leading to channel activation. However, AMPA receptors exhibit complex activation kinetics, the mechanism of which has not yet been determined. We report here single-channel studies of a homomeric AMPA receptor (GluA3) activated by the full agonist, glutamate, and a partial agonist, fluorowillardiine. Both agonists activate the channel to the same three open conductance levels but with different open probabilities in each level. The closed probability (P(c)) varied within records, particularly at low agonist concentrations. By sorting discrete segments of the record according to P(c) using the X-means algorithm, we defined five modes of activity. The kinetic behavior could then be analyzed for both agonists over a range of agonist concentrations with a relatively simple model (three closed states and two open states for each open conductance level). The structural mechanism underlying the modal behavior is not clear; however, it occurs on a timescale consistent with hydrogen bonding across the lobe interface in the LBD.

A dominant mutation in a neuronal acetylcholine receptor subunit leads to motor neuron degeneration in Caenorhabditis elegans

Inappropriate or excessive activation of ionotropic receptors can have dramatic consequences for neuronal function and, in many instances, leads to cell death. In Caenorhabditis elegans, nicotinic acetylcholine receptor (nAChR) subunits are highly expressed in a neural circuit that controls movement. Here, we show that heteromeric nAChRs containing the acr-2 subunit are diffusely localized in the processes of excitatory motor neurons and act to modulate motor neuron activity. Excessive signaling through these receptors leads to cell-autonomous degeneration of cholinergic motor neurons and paralysis. C. elegans double mutants lacking calreticulin and calnexin-two genes previously implicated in the cellular events leading to necrotic-like cell death (Xu et al. 2001)-are resistant to nAChR-mediated toxicity and possess normal numbers of motor neuron cell bodies. Nonetheless, excess nAChR activation leads to progressive destabilization of the motor neuron processes and, ultimately, paralysis in these animals. Our results provide new evidence that chronic activation of ionotropic receptors can have devastating degenerative effects in neurons and reveal that ion channel-mediated toxicity may have distinct consequences in neuronal cell bodies and processes.

Modes of Neuronal Calcium Entry and Homeostasis following Cerebral Ischemia

One of the major instigators leading to neuronal cell death and brain damage following cerebral ischemia is calcium dysregulation. The neuron's inability to maintain calcium homeostasis is believed to be a result of increased calcium influx and impaired calcium extrusion across the plasma membrane. The need to better understand the cellular and biochemical mechanisms of calcium dysregulation contributing to neuronal loss following stroke/cerebral ischemia is essential for the development of new treatments in order to reduce ischemic brain injury. The aim of this paper is to provide a concise overview of the various calcium influx pathways in response to ischemia and how neuronal cells attempts to overcome this calcium overload.

Voltage-dependent and -independent block of α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor channels

Polyamine-containing toxins and synthetic dicationic derivatives of adamantane and phenylcyclohexyl selectively antagonize Ca(2+)-permeable α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor channels. These compounds demonstrate voltage-dependent open-channel block and are trapped by closed channels. In this study, we describe an alternative mechanism of non-competitive AMPA receptor inhibition caused by 9-aminoacridine and some of its derivatives. These compounds exhibit similar potency against Ca(2+)-permeable and Ca(2+)-impermeable AMPA receptors. The inhibition is largely voltage-independent, binding and unbinding do not require presence of agonist. We conclude that 9-aminoacridine binds to a shallow site in the AMPA receptor, which is located above the activation gate. A comparison of three-dimensional structures of the antagonists suggests that the 'V-like' shape of the hydrophobic headgroup favors voltage-dependent binding to the deep site in the channel pore, whereas the compounds possessing flat aromatic headgroups preferably bind to the shallow site. The characterization of the novel mechanism of AMPA receptor channel antagonism opens a way to develop a new family of pharmacological agents, which can be of scientific and practical importance.

Excitotoxicity through Ca2+-permeable AMPA receptors requires Ca2+-dependent JNK activation

The GluA4-containing Ca(2+)-permeable α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (Ca-AMPARs) were previously shown to mediate excitotoxicity through mechanisms involving the activator protein-1 (AP-1), a c-Jun N-terminal kinase (JNK) substrate. To further investigate JNK involvement in excitotoxic pathways coupled to Ca-AMPARs we used HEK293 cells expressing GluA4-containing Ca-AMPARs (HEK-GluA4). Cell death induced by overstimulation of Ca-AMPARs was mediated, at least in part, by JNK. Importantly, JNK activation downstream of these receptors was dependent on the extracellular Ca(2+) concentration. In our quest for a molecular link between Ca-AMPARs and the JNK pathway we found that the JNK interacting protein-1 (JIP-1) interacts with the GluA4 subunit of AMPARs through the N-terminal domain. In vivo, the excitotoxin kainate promoted the association between GluA4 and JIP-1 in the rat hippocampus. Taken together, our results show that the JNK pathway is activated by Ca-AMPARs upon excitotoxic stimulation and suggest that JIP-1 may contribute to the propagation of the excitotoxic signal.

Calcium-dependent mitochondrial function and dysfunction in neurons

Calcium is an extraordinarily versatile signaling ion, encoding cellular responses to a wide variety of external stimuli. In neurons, mitochondria can accumulate enormous amounts of calcium, with the consequence that mitochondrial calcium uptake, sequestration and release play pivotal roles in orchestrating calcium-dependent responses as diverse as gene transcription and cell death. In this review, we consider the basic chemistry of calcium as a 'sticky' cation, which leads to extremely high bound/free ratios, and discuss areas of current interest or controversy. Topics addressed include methodologies for measuring local intracellular calcium, mitochondrial calcium buffering and loading capacity, mitochondrially directed spatial calcium gradients, and the role of calcium overload-dependent mitochondrial dysfunction in glutamate-evoked excitotoxic injury and neurodegeneration. Finally, we consider the relationship between delayed calcium de-regulation, the mitochondrial permeability transition and the generation of reactive oxygen species, and propose a unified view of the 'source specificity' and 'calcium overload' models of N-methyl-d-aspartate (NMDA) receptor-dependent excitotoxicity. Non-NMDA receptor mechanisms of excitotoxicity are discussed briefly.

TDP-43 pathology in sporadic ALS occurs in motor neurons lacking the RNA editing enzyme ADAR2

Both the appearance of cytoplasmic inclusions containing phosphorylated TAR DNA-binding protein (TDP-43) and inefficient RNA editing at the GluR2 Q/R site are molecular abnormalities observed specifically in motor neurons of patients with sporadic amyotrophic lateral sclerosis (ALS). The purpose of this study is to determine whether a link exists between these two specific molecular changes in ALS spinal motor neurons. We immunohistochemically examined the expression of adenosine deaminase acting on RNA 2 (ADAR2), the enzyme that specifically catalyzes GluR2 Q/R site-editing, and the expression of phosphorylated and non-phosphorylated TDP-43 in the spinal motor neurons of patients with sporadic ALS. We found that all motor neurons were ADAR2-positive in the control cases, whereas more than half of them were ADAR2-negative in the ALS cases. All ADAR2-negative neurons had cytoplasmic inclusions that were immunoreactive to phosphorylated TDP-43, but lacked non-phosphorylated TDP-43 in the nucleus. Our results suggest a molecular link between reduced ADAR2 activity and TDP-43 pathology.

Low extracellular zinc increases neuronal oxidant production through nadph oxidase and nitric oxide synthase activation

A decrease in zinc (Zn) levels increases the production of cell oxidants, affects the oxidant defense system and triggers oxidant sensitive signals in neuronal cells. However, the underlying mechanisms are still unclear. This work tested the hypothesis that the increase in neuronal oxidants that occurs when cellular Zn decreases is mediated by the activation of the NMDA receptor. Differentiated PC12 cells were cultured in control, Zn-deficient or Zn-repleted media. The incubation in Zn deficient media led to a rapid increase in cellular calcium levels, which was prevented by a NMDA receptor antagonist (MK-801). Cellular calcium accumulation was associated with NADPH oxidase and nitric oxide synthase (NOS) activation, an increase in cell oxidant levels, and an associated activation of a redox-sensitive signal (AP-1). In cells incubated in the Zn deficient medium, NADPH oxidase activation was prevented by MK-801 and by a protein kinase C inhibitor. The rise in cell oxidants was prevented by inhibitors of NADPH oxidase, of the NOS and by MK-801. A similar pattern of inhibitor action was observed for zinc deficiency-induced AP-1 activation. Results demonstrate that a decrease in extracellular Zn leads to an increase in neuronal oxidants through the activation of the NMDAR that leads to calcium influx and to a calcium-mediated activation of protein kinase C/NADPH oxidase and NOS. Changes in extracellular Zn concentrations can be sensed by neurons, which using reactive oxygen and nitrogen species as second messengers, can regulate signaling involved in neuronal development and function.

Novel etiological and therapeutic strategies for neurodiseases: RNA editing enzyme abnormality in sporadic amyotrophic lateral sclerosis

The motor neurons of patients with sporadic amyotrophic lateral sclerosis (ALS) express abundant Q/R site-unedited GluR2 mRNA, whereas those of patients with other motor neuron diseases including familial ALS associated with mutated SOD1 (ALS1) and those of normal subjects express only Q/R site-edited GluR2 mRNA. Because adenosine deaminase acting on RNA type 2 (ADAR2) specifically catalyzes GluR2 Q/R site-editing, it is likely that ADAR2 activity is not sufficient to edit this site completely in motor neurons of patients with sporadic ALS. Because these molecular abnormalities occur in disease- and motor neuron-specific fashion and induce fatal epilepsy in mice, we have hypothesized that GluR2 Q/R site-underediting due to ADAR2 underactivity is a cause of neuronal death in sporadic ALS. We found that cytoplasmic fragile X mental retardation protein interacting protein 2 (CYFIP2) mRNA had an ADAR2-mediated editing position using RNA interference knockdown. Our review will include a discussion of new ADAR2 substrates that may be useful for research on sporadic ALS.

The effects of alcoholism on the human basolateral amygdala

Alcohol affects gene expression in several brain regions. The amygdala is a key structure in the brain's emotional system and in recent years the crucial importance of the amygdala in drug-seeking and relapse has been increasingly recognized. In this study gene expression screening was used to identify genes involved in alcoholism in the human basolateral amygdala of male patients. The results show that alcoholism affects a broad range of genes and many systems including genes involved in synaptic transmission, neurotransmitter transport, structural plasticity, metabolism, energy production, transcription and RNA processing and the circadian cycle. In particular, genes involved in the glutamate system were affected in the alcoholic patients. In the amygdala the glutamate system is involved in the acquisition, consolidation, expression and extinction of associative learning, which is a vital part of addiction, and in alcohol abusers it is associated with withdrawal anxiety and neurodegeneration. Downregulation of the excitatory amino acid transporters GLAST, GLT-1 and the AMPA glutamate receptor 2 (GluR2) revealed by the microarray were confirmed by Western blots. The decreased expression of GLAST, GLT-1 and GluR2 in the alcoholic patients may increase glutamate tone and activity in the basolateral amygdala and this may contribute to neurodegeneration as well as the expression of associative memories and anxiety which underlie continued drug-seeking and chronic relapse.