Low editing efficiency of GluR2 mRNA is associated with a low relative abundance of ADAR2 mRNA in white matter of normal human brain

Familial Alzheimer's Disease Neurons Bearing Mutations in PSEN1 Display Increased Calcium Responses to AMPA as an Early Calcium Dysregulation Phenotype

Familial Alzheimer's disease (FAD) can be caused by mutations in PSEN1 that encode presenilin-1, a component of the gamma-secretase complex that cleaves amyloid precursor protein. Alterations in calcium (Ca2+) homeostasis and glutamate signaling are implicated in the pathogenesis of FAD; however, it has been difficult to assess in humans whether or not these phenotypes are the result of amyloid or tau pathology. This study aimed to assess the early calcium and glutamate phenotypes of FAD by measuring the Ca2+ response of induced pluripotent stem cell (iPSC)-derived neurons bearing PSEN1 mutations to glutamate and the ionotropic glutamate receptor agonists NMDA, AMPA, and kainate compared to isogenic control and healthy lines. The data show that in early neurons, even in the absence of amyloid and tau phenotypes, FAD neurons exhibit increased Ca2+ responses to glutamate and AMPA, but not NMDA or kainate. Together, this suggests that PSEN1 mutations alter Ca2+ and glutamate signaling as an early phenotype of FAD.

Preferential potentiation of AMPA-mediated currents in brainstem hypoglossal motoneurons by subchronic exposure of mice expressing the human superoxide dismutase 1 G93A gene mutation to neurotoxicant methylmercury in vivo

The exact causes of Amyotrophic lateral sclerosis (ALS), a progressive and fatal neurological disorder due to loss of upper and/or lower motoneurons, remain elusive. Gene-environment interactions are believed to be an important factor in the development of ALS. We previously showed that in vivo exposure of mice overexpressing the human superoxide dismutase 1 (hSOD1) gene mutation (hSOD1G93A; G93A), a mouse model for ALS, to environmental neurotoxicant methylmercury (MeHg) accelerated the onset of ALS-like phenotype. Here we examined the time-course of effects of MeHg on AMPA receptor (AMPAR)-mediated currents in hypoglossal motoneurons in brainstem slices prepared from G93A, hSOD1wild-type (hWT) and non-carrier WT mice following in vivo exposure to MeHg. Mice were exposed daily to 3 ppm (approximately 0.7 mg/kg/day) MeHg via drinking water beginning at postnatal day 28 (P28) and continued until P47, 64 or 84, then acute brainstem slices were prepared, and spontaneous excitatory postsynaptic currents (sEPSCs) or AMPA-evoked currents were examined using whole cell patch-clamp recording technique. Brainstem slices of untreated littermates were prepared at the same time points to serve as control. MeHg exposure had no significant effect on either sEPSCs or AMPA-evoked currents in slices from hWT or WT mice during any of those exposure time periods under our experimental conditions. MeHg also did not cause any significant effect on sEPSCs or AMPA-currents in G93A hypoglossal motoneurons at P47 and P64. However, at P84, MeHg significantly increased amplitudes of both sEPSCs and AMPA-evoked currents in hypoglossal motineurons from G93A mice (p < 0.05), but not the sEPSC frequency, suggesting a postsynaptic action on AMPARs. MeHg exposure did not cause any significant effect on GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs). Therefore, MeHg exposure in vivo caused differential effects on AMPARs in hypoglossal motoneurons from mice with different genetic backgrounds. MeHg appears to preferentially stimulate the AMPAR-mediated currents in G93A hypoglossal motoneurons in an exposure time-dependent manner, which may contribute to the AMPAR-mediated motoneuron excitotoxicity, thereby facilitating development of ALS-like phenotype.

Postsynaptic Proteins at Excitatory Synapses in the Brain—Relationship with Depressive Disorders

Depressive disorders (DDs) are an increasingly common health problem that affects all age groups. DDs pathogenesis is multifactorial. However, it was proven that stress is one of the most important environmental factors contributing to the development of these conditions. In recent years, there has been growing interest in the role of the glutamatergic system in the context of pharmacotherapy of DDs. Thus, it has become increasingly important to explore the functioning of excitatory synapses in pathogenesis and pharmacological treatment of psychiatric disorders (including DDs). This knowledge may lead to the description of new mechanisms of depression and indicate new potential targets for the pharmacotherapy of illness. An excitatory synapse is a highly complex and very dynamic structure, containing a vast number of proteins. This review aimed to discuss in detail the role of the key postsynaptic proteins (e.g., NMDAR, AMPAR, mGluR5, PSD-95, Homer, NOS etc.) in the excitatory synapse and to systematize the knowledge about changes that occur in the clinical course of depression and after antidepressant treatment. In addition, a discussion on the potential use of ligands and/or modulators of postsynaptic proteins at the excitatory synapse has been presented.

Testing of the therapeutic efficacy and safety of AMPA receptor RNA aptamers in an ALS mouse model

In motor neurons of sporadic amyotrophic lateral sclerosis (ALS) patients, the RNA editing at the glutamine/arginine site of the GluA2 subunit of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors is defective or incomplete. As a result, AMPA receptors containing the abnormally expressed, unedited isoform of GluA2 are highly Ca²⁺-permeable, and are responsible for mediating abnormal Ca²⁺ influx, thereby triggering motor neuron degeneration and cell death. Thus, blocking the AMPA receptor-mediated, abnormal Ca²⁺ influx is a potential therapeutic strategy for treatment of sporadic ALS. Here, we report a study of the efficacy and safety of two RNA aptamers targeting AMPA receptors on the ALS phenotype of AR2 mice. A 12-wk continuous, intracerebroventricular infusion of aptamers to AR2 mice reduced the progression of motor dysfunction, normalized TDP-43 mislocalization, and prevented death of motor neurons. Our results demonstrate that the use of AMPA receptor aptamers as a novel class of AMPA receptor antagonists is a promising strategy for developing an ALS treatment approach.

RNA Editing: A New Therapeutic Target in Amyotrophic Lateral Sclerosis and Other Neurological Diseases

The conversion of adenosine to inosine in RNA editing (A-to-I RNA editing) is recognized as a critical post-transcriptional modification of RNA by adenosine deaminases acting on RNAs (ADARs). A-to-I RNA editing occurs predominantly in mammalian and human central nervous systems and can alter the function of translated proteins, including neurotransmitter receptors and ion channels; therefore, the role of dysregulated RNA editing in the pathogenesis of neurological diseases has been speculated. Specifically, the failure of A-to-I RNA editing at the glutamine/arginine (Q/R) site of the GluA2 subunit causes excessive permeability of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors to Ca2+, inducing fatal status epilepticus and the neurodegeneration of motor neurons in mice. Therefore, an RNA editing deficiency at the Q/R site in GluA2 due to the downregulation of ADAR2 in the motor neurons of sporadic amyotrophic lateral sclerosis (ALS) patients suggests that Ca2+-permeable AMPA receptors and the dysregulation of RNA editing are suitable therapeutic targets for ALS. Gene therapy has recently emerged as a new therapeutic opportunity for many heretofore incurable diseases, and RNA editing dysregulation can be a target for gene therapy; therefore, we reviewed neurological diseases associated with dysregulated RNA editing and a new therapeutic approach targeting dysregulated RNA editing, especially one that is effective in ALS.

Calcium Permeable-AMPA Receptors and Excitotoxicity in Neurological Disorders

Excitotoxicity is one of the primary mechanisms of cell loss in a variety of diseases of the central and peripheral nervous systems. Other than the previously established signaling pathways of excitotoxicity, which depend on the excessive release of glutamate from axon terminals or over-activation of NMDA receptors (NMDARs), Ca²⁺ influx-triggered excitotoxicity through Ca²⁺-permeable (CP)-AMPA receptors (AMPARs) is detected in multiple disease models. In this review, both acute brain insults (e.g., brain trauma or spinal cord injury, ischemia) and chronic neurological disorders, including Epilepsy/Seizures, Huntington’s disease (HD), Parkinson’s disease (PD), Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), chronic pain, and glaucoma, are discussed regarding the CP-AMPAR-mediated excitotoxicity. Considering the low expression or absence of CP-AMPARs in most cells, specific manipulation of the CP-AMPARs might be a more plausible strategy to delay the onset and progression of pathological alterations with fewer side effects than blocking NMDARs.

Profiling neural editomes reveals a molecular mechanism to regulate RNA editing during development

Adenosine (A) to inosine (I) RNA editing contributes to transcript diversity and modulates gene expression in a dynamic, cell type–specific manner. During mammalian brain development, editing of specific adenosines increases, whereas the expression of A-to-I editing enzymes remains unchanged, suggesting molecular mechanisms that mediate spatiotemporal regulation of RNA editing exist. Herein, by using a combination of biochemical and genomic approaches, we uncover a molecular mechanism that regulates RNA editing in a neural- and development-specific manner. Comparing editomes during development led to the identification of neural transcripts that were edited only in one life stage. The stage-specific editing is largely regulated by differential gene expression during neural development. Proper expression of nearly one-third of the neurodevelopmentally regulated genes is dependent on adr-2, the sole A-to-I editing enzyme in C. elegans. However, we also identified a subset of neural transcripts that are edited and expressed throughout development. Despite a neural-specific down-regulation of adr-2 during development, the majority of these sites show increased editing in adult neural cells. Biochemical data suggest that ADR-1, a deaminase-deficient member of the adenosine deaminase acting on RNA (ADAR) family, is competing with ADR-2 for binding to specific transcripts early in development. Our data suggest a model in which during neural development, ADR-2 levels overcome ADR-1 repression, resulting in increased ADR-2 binding and editing of specific transcripts. Together, our findings reveal tissue- and development-specific regulation of RNA editing and identify a molecular mechanism that regulates ADAR substrate recognition and editing efficiency.

AMPA receptor contribution to methylmercury-mediated alteration of intracellular Ca2+ concentration in human induced pluripotent stem cell motor neurons

α motor neurons (MNs) are a target of the environmental neurotoxicant methylmercury (MeHg), accumulating MeHg and subsequently degenerating. In mouse spinal cord MN cultures, MeHg increased intracellular Ca²⁺ [Ca²⁺]_i; the AMPA receptor (AMPAR) antagonist CNQX delayed the increase in [Ca²⁺]_i, implicating the role of AMPARs in this response. Here we used human induced pluripotent stem cell-derived MNs (hiPSC-MNs), to characterize the role of MN AMPARs in MeHg neurotoxicity. Acute exposure to MeHg (0.1, 0.2, 0.5, 1 and 1.5 μM), fura-2 microfluorimetry, and a standard cytotoxicity assay, were used to examine MN regulation of [Ca²⁺]_i, and cytotoxicity, respectively. Contribution of Ca²⁺-permeable and impermeable AMPARs was compared using either CNQX, or the Ca²⁺-permeable AMPAR antagonist N-acetyl spermine (NAS). MeHg-induced cytotoxicity was evaluated following a 24 h delay subsequent to 1 h exposure of hiPSC-MNs. MeHg caused a characteristic biphasic increase in [Ca²⁺]_i, the onset of which was concentration-dependent; higher MeHg concentrations hastened onset of both phases. CNQX significantly delayed MeHg's effect on onset time of both phases. In contrast, NAS significantly delayed only the 2nd phase increase in fura-2 fluorescence. Exposure to MeHg for 1 h followed by a 24 h recovery period caused a concentration-dependent incidence of cell death. These results demonstrate for the first time that hiPSC-derived MNs are highly sensitive to effects of MeHg on [Ca²⁺]_i, and cytotoxicity, and that both Ca²⁺-permeable and impermeable AMPARs contribute the elevations in [Ca²⁺]_i.

AMPAR/TARP stoichiometry differentially modulates channel properties

AMPARs control fast synaptic communication between neurons and their function relies on auxiliary subunits, which importantly modulate channel properties. Although it has been suggested that AMPARs can bind to TARPs with variable stoichiometry, little is known about the effect that this stoichiometry exerts on certain AMPAR properties. Here we have found that AMPARs show a clear stoichiometry-dependent modulation by the prototypical TARP γ2 although the receptor still needs to be fully saturated with γ2 to show some typical TARP-induced characteristics (i.e. an increase in channel conductance). We also uncovered important differences in the stoichiometric modulation between calcium-permeable and calcium-impermeable AMPARs. Moreover, in heteromeric AMPARs, γ2 positioning in the complex is important to exert certain TARP-dependent features. Finally, by comparing data from recombinant receptors with endogenous AMPAR currents from mouse cerebellar granule cells, we have determined a likely presence of two γ2 molecules at somatic receptors in this cell type.

Seizure-induced neuronal apoptosis is related to dysregulation of the RNA-edited GluR2 subunit in the developing mouse brain

Ca²⁺-permeable AMPA receptors (AMPAR) which crucially modify maturational programs of the developing brain are involved in seizure-induced glutamate excitotoxicity and apoptosis. Regulatory effects on AMPAR subunit composition and RNA-editing in the developing brain and their significance as therapeutic targets are not well understood. Here, we analyzed acute effects of recurrent pilocarpine-induced neonatal seizures on age- and region-specific expression of AMPAR subunits and adenosine deaminases (ADAR) in the developing mouse brain (P10). After recurrent seizure activity and regeneration periods of 6-72 h cerebral mRNA levels of GluR (glutamate receptor subunit) 1, GluR2, GluR3, and GluR4 were unaffected compared to controls. However, ratio of GluR2 and GluR4 to pooled GluR1-4 mRNA concentration significantly decreased in seizure-exposed brains in comparison to controls. After a regeneration period of 24-72 h ADAR1 and ADAR2 mRNA expression was significantly lower in seizure-exposed brains than in those of controls. This was confirmed at the protein level in the hippocampal CA3 region. We observed a regionally increased apoptosis (TUNEL+ and CC3+ cells) in the hippocampus, parietal cortex and subventricular zone of seizure-exposed brains in comparison to controls. Together, present in vivo data demonstrate the maturational age-specific, functional role of RNA-edited GluR2 in seizure-induced excitotoxicity in the developing mouse brain. In response to recurrent seizure activity, we observed reduced expression of GluR2 and the GluR2 mRNA-editing enzymes ADAR1 and ADAR2 accompanied by increased apoptosis in a region-specific manner. Thus, AMPA receptor subtype-specific mRNA editing is assessed as a promising target of novel neuroprotective treatment strategies in consideration of age-related developmental mechanisms.

Emerging intersections between neuroscience and glioma biology

The establishment of neuronal and glial networks in the brain depends on the activities of neural progenitors, which are influenced by cell-intrinsic mechanisms, interactions with the local microenvironment and long-range signaling. Progress in neuroscience has helped identify key factors in CNS development. In parallel, studies in recent years have increased our understanding of molecular and cellular factors in the development and growth of primary brain tumors. To thrive, glioma cells exploit pathways that are active in normal CNS progenitor cells, as well as in normal neurotransmitter signaling. Furthermore, tumor cells of incurable gliomas integrate into communicating multicellular networks, where they are interconnected through neurite-like cellular protrusions. In this Review, we discuss evidence that CNS development, organization and function share a number of common features with glioma progression and malignancy. These include mechanisms used by cells to proliferate and migrate, interact with their microenvironment and integrate into multicellular networks. The emerging intersections between the fields of neuroscience and neuro-oncology considered in this review point to new research directions and novel therapeutic opportunities.

Electrical and synaptic integration of glioma into neural circuits

High-grade gliomas are lethal brain cancers whose progression is robustly regulated by neuronal activity. Activity-regulated release of growth factors promotes glioma growth, but this alone is insufficient to explain the effect that neuronal activity exerts on glioma progression. Here we show that neuron and glioma interactions include electrochemical communication through bona fide AMPA receptor-dependent neuron-glioma synapses. Neuronal activity also evokes non-synaptic activity-dependent potassium currents that are amplified by gap junction-mediated tumour interconnections, forming an electrically coupled network. Depolarization of glioma membranes assessed by in vivo optogenetics promotes proliferation, whereas pharmacologically or genetically blocking electrochemical signalling inhibits the growth of glioma xenografts and extends mouse survival. Emphasizing the positive feedback mechanisms by which gliomas increase neuronal excitability and thus activity-regulated glioma growth, human intraoperative electrocorticography demonstrates increased cortical excitability in the glioma-infiltrated brain. Together, these findings indicate that synaptic and electrical integration into neural circuits promotes glioma progression.

ADAR2-dependent A-to-I RNA editing in the extracellular linear and circular RNAs

Currently, no reliable biomarkers of amyotrophic lateral sclerosis (ALS) exist. In sporadic ALS, RNA editing at the glutamine/arginine site of GluA2 mRNA is specifically reduced in the motor neurons due to the downregulation of adenosine deaminase acting on RNA 2 (ADAR2). Furthermore, TDP-43 pathology, the pathological hallmark of ALS, is observed in the ADAR2-lacking motor neurons in ALS patients and conditional ADAR2 knockout mice, suggesting a pivotal role of ADAR2 downregulation in the ALS pathogenesis. Extracellular RNAs were shown to represent potential disease biomarkers and the editing efficiencies at their ADAR2-dependent sites may reflect cellular ADAR2 activity, suggesting that these RNAs isolated from the body fluids may represent the biomarkers of ALS. We searched for ADAR2-dependent sites in the mouse motor neurons and human-derived cultured cells and found 10 sites in five host RNAs expressed in SH-SY5Y cells and their culture medium. Of these, the arginine/glycine site of SON mRNA was newly identified as an ADAR2-dependent site. Furthermore, we detected a circular RNA with an ADAR2-dependent site in the SH-SY5Y cells and their culture medium. Therefore, the changes in the editing efficiencies at the identified host RNA sites isolated from the body fluids may represent potential biomarkers of ALS.

Synaptic dysfunction and altered excitability in C9ORF72 ALS/FTD

Amyotrophic lateral sclerosis (ALS) is characterized by a progressive degeneration of upper and lower motor neurons, resulting in fatal paralysis due to denervation of the muscle. Due to genetic, pathological and symptomatic overlap, ALS is now considered a spectrum disease together with frontotemporal dementia (FTD), the second most common cause of dementia in individuals under the age of 65. Interestingly, in both diseases, there is a large prevalence of RNA binding proteins (RBPs) that are mutated and considered disease-causing, or whose dysfunction contribute to disease pathogenesis. The most common shared genetic mutation in ALS/FTD is a hexanucleuotide repeat expansion within intron 1 of C9ORF72 (C9). Three potentially overlapping, putative toxic mechanisms have been proposed: loss of function due to haploinsufficient expression of the C9ORF72 mRNA, gain of function of the repeat RNA aggregates, or RNA foci, and repeat-associated non-ATG-initiated translation (RAN) of the repeat RNA into toxic dipeptide repeats (DPRs). Regardless of the causative mechanism, disease symptoms are ultimately caused by a failure of neurotransmission in three regions: the brain, the spinal cord, and the neuromuscular junction. Here, we review C9 ALS/FTD-associated synaptic dysfunction and aberrant neuronal excitability in these three key regions, focusing on changes in morphology and synapse formation, excitability, and excitotoxicity in patients, animal models, and in vitro models. We compare these deficits to those seen in other forms of ALS and FTD in search of shared pathways, and discuss the potential targeting of synaptic dysfunctions for therapeutic intervention in ALS and FTD patients.

Pedunculopontine glutamatergic neurons control spike patterning in substantia nigra dopaminergic neurons

Burst spiking in substantia nigra pars compacta (SNc) dopaminergic neurons is a key signaling event in the circuitry controlling goal-directed behavior. It is widely believed that this spiking mode depends upon an interaction between synaptic activation of N-methyl-D-aspartate receptors (NMDARs) and intrinsic oscillatory mechanisms. However, the role of specific neural networks in burst generation has not been defined. To begin filling this gap, SNc glutamatergic synapses arising from pedunculopotine nucleus (PPN) neurons were characterized using optical and electrophysiological approaches. These synapses were localized exclusively on the soma and proximal dendrites, placing them in a good location to influence spike generation. Indeed, optogenetic stimulation of PPN axons reliably evoked spiking in SNc dopaminergic neurons. Moreover, burst stimulation of PPN axons was faithfully followed, even in the presence of NMDAR antagonists. Thus, PPN-evoked burst spiking of SNc dopaminergic neurons in vivo may not only be extrinsically triggered, but extrinsically patterned as well.

Detection of a Subset of Posttranscriptional Transfer RNA Modifications in Vivo with a Restriction Fragment Length Polymorphism-Based Method

Transfer RNAs (tRNAs) are among the most heavily modified RNA species. Posttranscriptional tRNA modifications (ptRMs) play fundamental roles in modulating tRNA structure and function and are being increasingly linked to human physiology and disease. Detection of ptRMs is often challenging, expensive, and laborious. Restriction fragment length polymorphism (RFLP) analyses study the patterns of DNA cleavage after restriction enzyme treatment and have been used for the qualitative detection of modified bases on mRNAs. It is known that some ptRMs induce specific and reproducible base "mutations" when tRNAs are reverse transcribed. For example, inosine, which derives from the deamination of adenosine, is detected as a guanosine when an inosine-containing tRNA is reverse transcribed, amplified via polymerase chain reaction (PCR), and sequenced. ptRM-dependent base changes on reverse transcription PCR amplicons generated as a consequence of the reverse transcription reaction might create or abolish endonuclease restriction sites. The suitability of RFLP for the detection and/or quantification of ptRMs has not been studied thus far. Here we show that different ptRMs can be detected at specific sites of different tRNA types by RFLP. For the examples studied, we show that this approach can reliably estimate the modification status of the sample, a feature that can be useful in the study of the regulatory role of tRNA modifications in gene expression.

Differing intrinsic biological properties between forebrain and spinal oligodendroglial lineage cells

Differentiation of oligodendroglial progenitor cells (OPCs) into myelinating oligodendrocytes is known to be regulated by the microenvironment where they differentiate. However, current research has not verified whether or not oligodendroglial lineage cells (OLCs) derived from different anatomical regions of the central nervous system (CNS) respond to microenvironmental cues in the same manner. Here, we isolated pure OPCs from rat neonatal forebrain (FB) and spinal cord (SC) and compared their phenotypes in the same in vitro conditions. We found that although FB and SC OLCs responded differently to the same external factors; they were distinct in proliferation response to mitogens, oligodendrocyte phenotype after differentiation, and cytotoxic responses to α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-type glutamate receptor-mediated excitotoxicity at immature stages of differentiation in a cell-intrinsic manner. Moreover, transcriptome analysis identified genes differentially expressed between these OPC populations, including those encoding transcription factors (TFs), cell surface molecules, and signaling molecules. Particularly, FB and SC OPCs retained the expression of FB- or SC-specific TFs, such as Foxg1 and Hoxc8, respectively, even after serial passaging in vitro. Given the essential role of these TFs in the regional identities of CNS cells along the rostrocaudal axis, our results suggest that CNS region-specific gene regulation by these TFs may cause cell-intrinsic differences in cellular responses between FB and SC OLCs to extracellular molecules. Further understanding of the regional differences among OPC populations will help to improve treatments for demyelination in different CNS regions and to facilitate the development of stem cell-derived OPCs for cell transplantation therapies for demyelination. Cover Image for this issue: doi. 10.1111/jnc.13809.

Effects of methylmercury on spinal cord afferents and efferents-A review

Methylmercury (MeHg) is an environmental neurotoxicant of public health concern. It readily accumulates in exposed humans, primarily in neuronal tissue. Exposure to MeHg, either acutely or chronically, causes severe neuronal dysfunction in the central nervous system and spinal neurons; dysfunction of susceptible neuronal populations results in neurodegeneration, at least in part through Ca2+-mediated pathways. Biochemical and morphologic changes in peripheral neurons precede those in central brain regions, despite the fact that MeHg readily crosses the blood-brain barrier. Consequently, it is suggested that unique characteristics of spinal cord afferents and efferents could heighten their susceptibility to MeHg toxicity. Transient receptor potential (TRP) ion channels are a class of Ca2+-permeable cation channels that are highly expressed in spinal afferents, among other sensory and visceral organs. These channels can be activated in numerous ways, including directly via chemical irritants or indirectly via Ca2+ release from intracellular storage organelles. Early studies demonstrated that MeHg interacts with heterologous TRP channels, though definitive mechanisms of MeHg toxicity on sensory neurons may involve more complex interaction with, and among, differentially-expressed TRP populations. In spinal efferents, glutamate receptors of the N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and possibly kainic acid (KA) classes are thought to play a major role in MeHg-induced neurotoxicity. Specifically, the Ca2+-permeable AMPA receptors, which are abundant in motor neurons, have been identified as being involved in MeHg-induced neurotoxicity. In this review, we will describe the mechanisms that could contribute to MeHg-induced spinal cord afferent and efferent neuronal degeneration, including the possible mediators, such as uniquely expressed Ca2+-permeable ion channels.

Adenosine Deaminase That Acts on RNA 3 (ADAR3) Binding to Glutamate Receptor Subunit B Pre-mRNA Inhibits RNA Editing in Glioblastoma

RNA editing is a cellular process that precisely alters nucleotide sequences, thus regulating gene expression and generating protein diversity. Over 60% of human transcripts undergo adenosine to inosine RNA editing, and editing is required for normal development and proper neuronal function of animals. Editing of one adenosine in the transcript encoding the glutamate receptor subunit B, glutamate receptor ionotropic AMPA 2 (GRIA2), modifies a codon, replacing the genomically encoded glutamine (Q) with arginine (R); thus this editing site is referred to as the Q/R site. Editing at the Q/R site of GRIA2 is essential, and reduced editing of GRIA2 transcripts has been observed in patients suffering from glioblastoma. In glioblastoma, incorporation of unedited GRIA2 subunits leads to a calcium-permeable glutamate receptor, which can promote cell migration and tumor invasion. In this study, we identify adenosine deaminase that acts on RNA 3 (ADAR3) as an important regulator of Q/R site editing, investigate its mode of action, and detect elevated ADAR3 expression in glioblastoma tumors compared with adjacent brain tissue. Overexpression of ADAR3 in astrocyte and astrocytoma cell lines inhibits RNA editing at the Q/R site of GRIA2 Furthermore, the double-stranded RNA binding domains of ADAR3 are required for repression of RNA editing. As the Q/R site of GRIA2 is specifically edited by ADAR2, we suggest that ADAR3 directly competes with ADAR2 for binding to GRIA2 transcript, inhibiting RNA editing, as evidenced by the direct binding of ADAR3 to the GRIA2 pre-mRNA. Finally, we provide evidence that both ADAR2 and ADAR3 expression contributes to the relative level of GRIA2 editing in tumors from patients suffering from glioblastoma.

Synaptic AMPA receptor composition in development, plasticity and disease

AMPA receptors (AMPARs) are assemblies of four core subunits, GluA1-4, that mediate most fast excitatory neurotransmission. The component subunits determine the functional properties of AMPARs, and the prevailing view is that the subunit composition also determines AMPAR trafficking, which is dynamically regulated during development, synaptic plasticity and in response to neuronal stress in disease. Recently, the subunit dependence of AMPAR trafficking has been questioned, leading to a reappraisal of this field. In this Review, we discuss what is known, uncertain, conjectured and unknown about the roles of the individual subunits, and how they affect AMPAR assembly, trafficking and function under both normal and pathological conditions.

Chronic glutamate treatment selectively modulates AMPA RNA editing and ADAR expression and activity in primary cortical neurons

Adenosine-to-inosine RNA editing is a post-transcriptional process, catalyzed by ADAR enzymes, with an important role in diversifying the number of proteins derived from a single gene. In neurons, editing of ionotropic AMPA glutamate receptors has been shown to be altered under several experimental conditions, including severe pathologies, thus highlighting the potential significance of its modulation. In this study, we treated rat primary cortical cell cultures with a sub-lethal dose of glutamate (10 μM), focusing on RNA editing and ADAR activity. We found that chronic glutamate treatment down-regulates RNA editing levels at the R/G site of GluA2-4 subunits of AMPA receptors and at the K/E site of CYFIP2. These changes are site-specific since they were not observed either for the GluA2 Q/R site or for other non-glutamatergic sites. Glutamate treatment also down-regulates the protein expression levels of both ADAR1 and ADAR2 enzymes, through a pathway that is Ca(2+)- and calpain-dependent. Given that AMPA receptors containing unedited subunits show a slower recovery rate from desensitization compared to those containing edited forms, the reduced editing at the R/G site may, at least in part, compensate for glutamate over-stimulation, perhaps through the reduced activation of postsynaptic receptors. In summary, our data provide direct evidence of the involvement of ADAR1 and ADAR2 activity as a possible compensatory mechanism for neuronal protection following glutamate over-stimulation.

ADAR2-dependent GluA2 editing regulates cocaine seeking

Activation of AMPA receptors (AMPARs) in the nucleus accumbens is necessary for the reinstatement of cocaine-seeking behavior, an animal model of drug craving and relapse. AMPARs are tetrameric protein complexes that consist of GluA1-4 subunits, of which GluA2 imparts calcium permeability. Adenosine deaminase acting on RNA 2 (ADAR2) is a nuclear enzyme that is essential for editing GluA2 pre-mRNA at Q/R site 607. Unedited GluA2(Q) subunits form calcium-permeable AMPARs (CP-AMPARs), whereas edited GluA2(R) subunits form calcium-impermeable channels (CI-AMPARs). Emerging evidence suggests that the reinstatement of cocaine seeking is associated with increased synaptic expression of CP-AMPARs in the nucleus accumbens. However, the role of GluA2 Q/R site editing and ADAR2 in cocaine seeking is unclear. In the present study, we investigated the effects of forced cocaine abstinence on GluA2 Q/R site editing and ADAR2 expression in the nucleus accumbens. Our results demonstrate that 7 days of cocaine abstinence is associated with decreased GluA2 Q/R site editing and reduced ADAR2 expression in the accumbens shell, but not core, of cocaine-experienced rats compared with yoked saline controls. To examine the functional significance of ADAR2 and GluA2 Q/R site editing in cocaine seeking, we used viral-mediated gene delivery to overexpress ADAR2b in the accumbens shell. Increased ADAR2b expression in the shell attenuated cocaine priming-induced reinstatement of drug seeking and was associated with increased GluA2 Q/R site editing and surface expression of GluA2-containing AMPARs. Taken together, these findings support the novel hypothesis that an increased contribution of accumbens shell CP-AMPARs containing unedited GluA2(Q) promotes cocaine seeking. Therefore, CP-AMPARs containing unedited GluA2(Q) represent a novel target for cocaine addiction pharmacotherapies.

GluA2 is rapidly edited at the Q/R site during neural differentiation in vitro

The majority of AMPA receptors in the adult brain contain GluA2 subunits, which can be edited at the Q/R site, changing a glutamine to an arginine within the ion pore. Q/R editing renders AMPARs virtually Ca²⁺-impermeable, which is important for normal AMPA receptor function. Thus, all GluA2 subunits are Q/R-edited in the adult brain. However, it has remained controversial precisely when editing sets in during development. In the present study, we show that GluA2 mRNA is very rapidly Q/R-edited immediately after its appearance, which is after 4.5 days of differentiation from 46C embryonic stem cells (ESCs) to neuroepithelial precursor cells (NEPs). At this time point, most of the GluA2 transcripts were already edited, with only a small fraction remaining unedited, and half a day later all GluA2 transcripts were edited. This can be explained by the observation that the enzyme that Q/R-edits GluA2 transcripts, ADAR2, is already expressed in the cell well before GluA2 transcription starts, and later is not significantly upregulated any more. Editing at another site works differently: The R/G site within the ligand-binding domain was never completely edited at any of the developmental stages tested, and the enzyme that performs this editing, ADAR1, was significantly upregulated during neural differentiation. This confirms previous data suggesting that R/G editing, in contrast to Q/R editing, progresses gradually during development.

Novel RNA modifications in the nervous system: form and function

Modified RNA molecules have recently been shown to regulate nervous system functions. This mini-review and associated mini-symposium provide an overview of the types and known functions of novel modified RNAs in the nervous system, including covalently modified RNAs, edited RNAs, and circular RNAs. We discuss basic molecular mechanisms involving RNA modifications as well as the impact of modified RNAs and their regulation on neuronal processes and disorders, including neural fate specification, intellectual disability, neurodegeneration, dopamine neuron function, and substance use disorders.

Unique nuclear vacuoles in the motor neurons of conditional ADAR2-knockout mice

A reduction in adenosine deaminase acting on RNA 2 (ADAR2) activity causes the death of spinal motor neurons specifically via the GluA2 Q/R site-RNA editing failure in sporadic amyotrophic lateral sclerosis (ALS). We studied, over time, the spinal cords of ADAR2-knockout mice, which are the mechanistic model mice for sporadic ALS, using homozygous ADAR2(flox/flox)/VAChT-Cre.Fast (AR2), homozygous ADAR2(flox/flox)/VAChT-Cre.Slow (AR2Slow), and heterozygous ADAR2(flox/+)/VAChT-Cre.Fast (AR2H) mice. The conditional ADAR2-knockout mice were divided into 3 groups by stage: presymptomatic (AR2H mice), early symptomatic (AR2 mice, AR2H mice) and late symptomatic (AR2Slow mice). Light-microscopically, some motor neurons in AR2 and AR2H mice (presymptomatic) showed simple neuronal atrophy and astrogliosis, and AR2H (early symptomatic) and AR2Slow mice often showed vacuoles predominantly in motor neurons. The number of vacuole-bearing anterior horn neurons decreased with the loss of anterior horn neurons in AR2H mice after 40 weeks of age. Electron-microscopically, in AR2 mice, while the cytoplasm of normal-looking motor neurons was almost always normal-appearing, the interior of dendrites was frequently loose and disorganized. In AR2H and AR2Slow mice, large vacuoles without a limiting membrane were observed in the anterior horns, preferentially in the nuclei of motor neurons, astrocytes and oligodendrocytes. Nuclear vacuoles were not observed in AR2res (ADAR2(flox/flox)/VAChT-Cre.Fast/GluR-B(R/R)) mice, in which motor neurons express edited GluA2 in the absence of ADAR2. These findings suggest that ADAR2-reduction is associated with progressive deterioration of nuclear architecture, resulting in vacuolated nuclei due to a Ca(2+)-permeable AMPA receptor-mediated mechanism.

Kainate receptor RNA editing is markedly altered by acute spinal cord injury

We have previously observed changes in the RNA editing of AMPA receptors after acute spinal cord injury (SCI); this implies that post-transcriptional modifications are capable of affecting the physiological properties of glutamate receptor channels and related signal transduction in this neurodegenerative condition. Here, we report that the editing of the ionotropic KAR is markedly decreased at both GluK1 and GluK2 Q/R sites in the epicenter of the lesion and with distinct magnitude and kinetics also in the caudal and rostral portions of the injured cord. These effects are persistent, being observed as late as 30 days after lesioning. In addition, also the I/V and Y/C sites of GluK2 were severely affected after SCI. These findings add novel information to the relevance of editing of glutamate receptors following acute SCI, thus expanding the recently emerged role of post-transcriptional mechanisms under these experimental conditions.

The ADAR RNA editing enzyme controls neuronal excitability in Drosophila melanogaster

RNA editing by deamination of specific adenosine bases to inosines during pre-mRNA processing generates edited isoforms of proteins. Recoding RNA editing is more widespread in Drosophila than in vertebrates. Editing levels rise strongly at metamorphosis, and Adar^5G1 null mutant flies lack editing events in hundreds of CNS transcripts; mutant flies have reduced viability, severely defective locomotion and age-dependent neurodegeneration. On the other hand, overexpressing an adult dADAR isoform with high enzymatic activity ubiquitously during larval and pupal stages is lethal. Advantage was taken of this to screen for genetic modifiers; Adar overexpression lethality is rescued by reduced dosage of the Rdl (Resistant to dieldrin), gene encoding a subunit of inhibitory GABA receptors. Reduced dosage of the Gad1 gene encoding the GABA synthetase also rescues Adar overexpression lethality. Drosophila Adar^5G1 mutant phenotypes are ameliorated by feeding GABA modulators. We demonstrate that neuronal excitability is linked to dADAR expression levels in individual neurons; Adar-overexpressing larval motor neurons show reduced excitability whereas Adar^5G1 null mutant or targeted Adar knockdown motor neurons exhibit increased excitability. GABA inhibitory signalling is impaired in human epileptic and autistic conditions, and vertebrate ADARs may have a relevant evolutionarily conserved control over neuronal excitability.

Rescue of amyotrophic lateral sclerosis phenotype in a mouse model by intravenous AAV9-ADAR2 delivery to motor neurons

Amyotrophic lateral sclerosis (ALS) is the most common adult-onset motor neuron disease, and the lack of effective therapy results in inevitable death within a few years of onset. Failure of GluA2 RNA editing resulting from downregulation of the RNA-editing enzyme adenosine deaminase acting on RNA 2 (ADAR2) occurs in the majority of ALS cases and causes the death of motor neurons via a Ca²⁺-permeable AMPA receptor-mediated mechanism. Here, we explored the possibility of gene therapy for ALS by upregulating ADAR2 in mouse motor neurons using an adeno-associated virus serotype 9 (AAV9) vector that provides gene delivery to a wide array of central neurons after peripheral administration. A single intravenous injection of AAV9-ADAR2 in conditional ADAR2 knockout mice (AR2), which comprise a mechanistic mouse model of sporadic ALS, caused expression of exogenous ADAR2 in the central neurons and effectively prevented progressive motor dysfunction. Notably, AAV9-ADAR2 rescued the motor neurons of AR2 mice from death by normalizing TDP-43 expression. This AAV9-mediated ADAR2 gene delivery may therefore enable the development of a gene therapy for ALS.

Co-occurrence of TDP-43 mislocalization with reduced activity of an RNA editing enzyme, ADAR2, in aged mouse motor neurons

TDP-43 pathology in spinal motor neurons is a neuropathological hallmark of sporadic amyotrophic lateral sclerosis (ALS) and has recently been shown to be closely associated with the downregulation of an RNA editing enzyme called adenosine deaminase acting on RNA 2 (ADAR2) in the motor neurons of sporadic ALS patients. Because TDP-43 pathology is found more frequently in the brains of elderly patients, we investigated the age-related changes in the TDP-43 localization and ADAR2 activity in mouse motor neurons. We found that ADAR2 was developmentally upregulated, and its mRNA expression level was progressively decreased in the spinal cords of aged mice. Motor neurons normally exhibit nuclear ADAR2 and TDP-43 immunoreactivity, whereas fast fatigable motor neurons in aged mice demonstrated a loss of ADAR2 and abnormal TDP-43 localization. Importantly, these motor neurons expressed significant amounts of the Q/R site-unedited AMPA receptor subunit 2 (GluA2) mRNA. Because expression of unedited GluA2 has been demonstrated as a lethality-causing molecular abnormality observed in the motor neurons, these results suggest that age-related decreases in ADAR2 activity play a mechanistic role in aging and serve as one of risk factors for ALS.

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.

RNA editing of the Q/R site of GluA2 in different cultured cell lines that constitutively express different levels of RNA editing enzyme ADAR2

Adenosine deaminase acting on RNA 2 (ADAR2) catalyzes RNA editing at the glutamine/arginine (Q/R) site of GluA2, and an ADAR2 deficiency may play a role in the death of motor neurons in ALS patients. The expression level of ADAR2 mRNA is a determinant of the editing activity at the GluA2 Q/R site in human brain but not in cultured cells. Therefore, we investigated the extent of Q/R site-editing in the GluA2 mRNA and pre-mRNA as well as the ADAR2 mRNA and GluA2 mRNA and pre-mRNA levels in various cultured cell lines. The extent of the GluA2 mRNA editing was 100% except in SH-SY5Y cells, which have a much lower level of ADAR2 than the other cell lines examined. The ADAR2 activity at the GluA2 pre-mRNA Q/R site correlated with the ADAR2 mRNA level relative to the GluA2 pre-mRNA. SH-SY5Y cells expressed higher level of the GluA2 mRNA in the cytoplasm compared with other cell lines. These results suggest that the ADAR2 expression level reflects editing activity at the GluA2 Q/R site and that although the edited GluA2 pre-mRNA is readily spliced, the unedited GluA2 pre-mRNA is also spliced and transported to the cytoplasm when ADAR2 expression is low.

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.

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.

Laser microdissection and pressure catapulting of single human motor neurons for RNA editing analysis

Glutamate is the major excitatory neurotransmitter in the mammalian nervous system. The properties of their ionotropic glutamate receptors largely determine how different neurons respond to glutamate. RNA editing in pre-mRNAs encoding subunits of glutamate receptors, particularly the GluR 2 subunit of AMPA receptors, controls calcium permeability, response time, and total ion flow in individual receptors as well as the density of AMPA receptors at synapses through effects on ER assembly, sorting, and plasma membrane insertion. When RNA editing fails in a neuron, calcium influx through AMPA receptors may cause neuron death by glutamate excitotoxicity, as in the case of vulnerable hippocampal CA1 pyramidal neurons that die after transient forebrain ischemia. Elevated cerebrospinal glutamate is common in ALS and loss of GluR 2 Q/R site RNA editing has been reported to occur selectively in lower motor neurons in a majority of Japanese sporadic ALS patients. We describe our methods for laser microdissection followed by RT-PCR analysis to study RNA editing in single motor neurons.

Induced loss of ADAR2 engenders slow death of motor neurons from Q/R site-unedited GluR2

GluR2 is a subunit of the AMPA receptor, and the adenosine for the Q/R site of its pre-mRNA is converted to inosine (A-to-I conversion) by the enzyme called adenosine deaminase acting on RNA 2 (ADAR2). Failure of A-to-I conversion at this site affects multiple AMPA receptor properties, including the Ca(2+) permeability of the receptor-coupled ion channel, thereby inducing fatal epilepsy in mice (Brusa et al., 1995; Feldmeyer et al., 1999). In addition, inefficient GluR2 Q/R site editing is a disease-specific molecular dysfunction found in the motor neurons of sporadic amyotrophic lateral sclerosis (ALS) patients (Kawahara et al., 2004). Here, we generated genetically modified mice (designated as AR2) in which the ADAR2 gene was conditionally targeted in motor neurons using the Cre/loxP system. These AR2 mice showed a decline in motor function commensurate with the slow death of ADAR2-deficient motor neurons in the spinal cord and cranial motor nerve nuclei. Notably, neurons in nuclei of oculomotor nerves, which often escape degeneration in ALS, were not decreased in number despite a significant decrease in GluR2 Q/R site editing. All cellular and phenotypic changes in AR2 mice were prevented when the mice carried endogenous GluR2 alleles engineered to express edited GluR2 without ADAR2 activity (Higuchi et al., 2000). Thus, loss of ADAR2 activity causes AMPA receptor-mediated death of motor neurons.

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.

Expression of glutamate receptor subunits in human cancers

Emerging evidence suggests a role for glutamate and its receptors in the biology of cancer. This study was designed to systematically analyze the expression of ionotropic and metabotropic glutamate receptor subunits in various human cancer cell lines, compare expression levels to those in human brain tissue and, using electrophysiological techniques, explore whether cancer cells respond to glutamate receptor agonists and antagonists. Expression analysis of glutamate receptor subunits NR1-NR3B, GluR1-GluR7, KA1, KA2 and mGluR1-mGluR8 was performed by means of RT-PCR in human rhabdomyosarcoma/medulloblastoma (TE671), neuroblastoma (SK-NA-S), thyroid carcinoma (FTC 238), lung carcinoma (SK-LU-1), astrocytoma (MOGGCCM), multiple myeloma (RPMI 8226), glioma (U87-MG and U343), lung carcinoma (A549), colon adenocarcinoma (HT 29), T cell leukemia cells (Jurkat E6.1), breast carcinoma (T47D) and colon adenocarcinoma (LS180). Analysis revealed that all glutamate receptor subunits were differentially expressed in the tumor cell lines. For the majority of tumors, expression levels of NR2B, GluR4, GluR6 and KA2 were lower compared to human brain tissue. Confocal imaging revealed that selected glutamate receptor subunit proteins were expressed in tumor cells. By means of patch-clamp analysis, it was shown that A549 and TE671 cells depolarized in response to application of glutamate agonists and that this effect was reversed by glutamate receptor antagonists. This study reveals that glutamate receptor subunits are differentially expressed in human tumor cell lines at the mRNA and the protein level, and that their expression is associated with the formation of functional channels. The potential role of glutamate receptor antagonists in cancer therapy is a feasible goal to be explored in clinical trials.

ADAR1 and ADAR2 expression and editing activity during forebrain development

The conversion of adenosine to inosine within RNA transcripts is regulated by a family of double-stranded RNA-specific adenosine deaminases referred to as adenosine deaminases that act on RNA (ADARs). Little is known regarding the developmental expression of ADAR family members or the mechanisms responsible for the specific patterns of editing observed for ADAR substrates. We have examined the spatiotemporal expression patterns for ADAR1 and ADAR2 in mouse forebrain. ADAR1 and ADAR2 are broadly distributed in most regions of the mouse forebrain by P0, including the cerebral cortex, hippocampus, and diencephalon. High expression levels were maintained into adulthood. Colocalization studies demonstrated ADAR1 and ADAR2 expression in neurons but not astrocytes. Editing for specific ADAR mRNA targets precedes high expression of ADAR proteins, suggesting that region-specific differences in editing patterns may not be mediated solely by ADAR expression levels.

Calcium-permeable AMPA receptors containing Q/R-unedited GluR2 direct human neural progenitor cell differentiation to neurons

We identify calcium-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors on human neural progenitor cells (NPCs) and present a physiological role in neurogenesis. RNA editing of the GluR2 subunit at the Q/R site is responsible for making most AMPA receptors impermeable to calcium. Because a single-point mutation could eliminate the need for editing at the Q/R site and Q/R-unedited GluR2 exists during embryogenesis, the Q/R-unedited GluR2 subunit presumably has some important actions early in development. Using calcium imaging, we found that NPCs contain calcium-permeable AMPA receptors, whereas NPCs differentiated to neurons and astrocytes express calcium-impermeable AMPA receptors. We utilized reverse-transcription polymerase chain reaction and BbvI digestion to demonstrate that NPCs contain Q/R-unedited GluR2, and differentiated cells contain Q/R-edited GluR2 subunits. This is consistent with the observation that the nuclear enzyme responsible for Q/R-editing, adenosine deaminase (ADAR2), is increased during differentiation. Activation of calcium-permeable AMPA receptors induces NPCs to differentiate to the neuronal lineage and increases dendritic arbor formation in NPCs differentiated to neurons. AMPA-induced differentiation of NPCs to neurons is abrogated by overexpression of ADAR2 in NPCs. This elucidates the role of AMPA receptors as inductors of neurogenesis and provides a possible explanation for why the Q/R editing process exists.

Determination of editors at the novel A-to-I editing positions

A-to-I RNA editing modifies a variety of biologically important mRNAs, and is specifically catalyzed by either adenosine deaminase acting on RNA type 1 (ADAR1) or type 2 (ADAR2) in mammals including human. Recently several novel A-to-I editing sites were identified in mRNAs abundantly expressed in mammalian organs by means of computational genomic analysis, but which enzyme catalyzes these editing sites has not been determined. Using RNA interference (RNAi) knockdowns, we found that cytoplasmic fragile X mental retardation protein interacting protein 2 (CYFIP2) mRNA had an ADAR2-mediated editing position and bladder cancer associated protein (BLCAP) mRNA had an ADAR1-mediated editing position. In addition, we found that ADAR2 forms a complex with mRNAs with ADAR2-mediated editing positions including GluR2, kv1.1 and CYFIP2 mRNAs, particularly when the editing sites were edited in human cerebellum by means of immunoprecipitation (IP) method. CYFIP2 mRNA was ubiquitously expressed in human tissues with variable extents of K/E site editing. Because ADAR2 underactivity may be a causative molecular change of death of motor neurons in sporadic amyotrophic lateral sclerosis (ALS), this newly identified ADAR2-mediated editing position may become a useful tool for ALS research.

A real-time PCR method for the quantitative analysis of RNA editing at specific sites

In this study, a quantitative PCR (qPCR) method was developed to determine the A-to-I RNA editing frequencies at specific sites. The A-to-I RNA editing of nuclear transcripts exerts profound effects on the biological activities of gene products. RNA editing of nuclear gene transcripts have been shown to be developmentally regulated and tissue specific, and alternations of RNA editing activities have been observed under pathological conditions. Two sites of ionotropic glutamate receptor subunits, the Q/R site of zebrafish gria2alpha and the Y/C site of grik2alpha, were chosen in this study to demonstrate the applicability of the SYBR Green detection-based real-time PCR method to measure RNA editing activities during zebrafish development. The results obtained by qPCR were consistent with those obtained by the limited primer extension. However, the qPCR method has the advantages of easy handling and low cost.

Novel toxicity of the unedited GluR2 AMPA receptor subunit dependent on surface trafficking and increased Ca2+-permeability

RNA editing modifies the GluR2 AMPA receptor subunit pore loop at the Q/R site and limits receptor Ca(2+) permeability. Editing failure is implicated in neurodegenerative diseases, including amyotrophic lateral sclerosis. We show that channels with unedited GluR2 are highly toxic in cultured hippocampal neurons. Toxicity exceeds that of other Ca(2+)-permeable AMPA receptor types and is influenced by agonist binding site mutations, ability to desensitize, and extracellular Ca(2+) levels. Significantly, toxicity also depends on GluR2's constitutive surface trafficking, a function dependent on GluR2 C-terminal domain interaction with NSF, a specialized chaperone. We have exploited the interaction between unedited GluR2 and NSF to reduce GluR2 surface levels. We show that a peptide that blocks the GluR2-NSF interaction reduces unedited GluR2 toxicity by reducing receptor surface expression. Peptide block of trafficking provides a model for design of drugs to reduce unedited GluR2 excitotoxicity in neurodegenerative diseases that result from editing failure.

Calcium mediated excitotoxicity in neurofilament aggregate-bearing neurons in vitro is NMDA receptor dependant

We have previously shown that the co-localization of neuronal nitric oxide synthase (nNOS) with neurofilament (NF) aggregates in motor neurons derived from transgenic mice over-expressing the human low molecular weight NF protein (hNFL+/+) is associated with a deregulation of calcium influx via the N-methyl-d-aspartate (NMDA) receptor, resulting in apoptosis. Because the absence of the GluR2 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA) receptor confers calcium permeability and has been implicated in the process of excitotoxicity in ALS, we have examined the role of the AMPA receptor in this model. GluR2 protein expression and mRNA were examined in hNFL+/+ and wild-type motor neurons (wt). Live cell calcium imaging was performed using Oregon-Green Bapta and Fura-2 calcium dyes. For apoptotic studies, neurons were treated with glutamate, with or without glutamate receptor antagonists [6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX) or (+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801)] and examined for active caspase-3 or phospholipid inversion. We observed that although both GluR2 mRNA and protein levels were decreased in hNFL+/+ motor neurons compared to wt, there was no appreciable calcium influx via the AMPA receptor. These studies demonstrate that calcium mediated excitotoxicity in NF aggregate-bearing neurons is NMDA receptor dependant.

Excitotoxicity and ALS: what is unique about the AMPA receptors expressed on spinal motor neurons?

It has been repeatedly reported that spinal motor neurons are selectively vulnerable to AMPA receptor-mediated excitotoxicity. Therefore, identifying the uniqueness of AMPA receptors that are expressed on motor neurons, especially in individuals affected with sporadic amyotrophic lateral sclerosis (ALS) is essential for elucidating the etiology of this disorder. The mechanism that initiates motor neuronal death appears to be an exaggerated influx of Ca(2+) through AMPA receptors. The determinants that affect this Ca(2+) influx are Ca(2+) permeability, which is regulated by the presence of the GluR2 subunit and by RNA editing at the Q/R site of GluR2; channel desensitization, which is regulated by alternative splicing at the flip/flop site and by RNA editing at the R/G site of GluR subunits; and receptor density on the cell surface, which is controlled by many factors including regulatory proteins, direct phosphorylation and RNA editing at the Q/R site. This review focuses on recent progress on the molecular dynamics of AMPA receptors and discusses the pathophysiology of selective motor neuron death mediated by AMPA receptors in individuals affected with sporadic ALS.

Underediting of GluR2 mRNA, a neuronal death inducing molecular change in sporadic ALS, does not occur in motor neurons in ALS1 or SBMA

Deficient RNA editing of the AMPA receptor subunit GluR2 at the Q/R site is a primary cause of neuronal death and recently has been reported to be a tightly linked etiological cause of motor neuron death in sporadic amyotrophic lateral sclerosis (ALS). We quantified the RNA editing efficiency of the GluR2 Q/R site in single motor neurons of rats transgenic for mutant human Cu/Zn-superoxide dismutase (SOD1) as well as patients with spinal and bulbar muscular atrophy (SBMA), and found that GluR2 mRNA was completely edited in all the motor neurons examined. It seems likely that the death cascade is different among the dying motor neurons in sporadic ALS, familial ALS with mutant SOD1 and SBMA.