Expression profile of AMPA receptor subunit mRNA in single adult rat brain and spinal cord neurons in situ

Motor Neuron Susceptibility in ALS/FTD

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the death of both upper and lower motor neurons (MNs) in the brain, brainstem and spinal cord. The neurodegenerative mechanisms leading to MN loss in ALS are not fully understood. Importantly, the reasons why MNs are specifically targeted in this disorder are unclear, when the proteins associated genetically or pathologically with ALS are expressed ubiquitously. Furthermore, MNs themselves are not affected equally; specific MNs subpopulations are more susceptible than others in both animal models and human patients. Corticospinal MNs and lower somatic MNs, which innervate voluntary muscles, degenerate more readily than specific subgroups of lower MNs, which remain resistant to degeneration, reflecting the clinical manifestations of ALS. In this review, we discuss the possible factors intrinsic to MNs that render them uniquely susceptible to neurodegeneration in ALS. We also speculate why some MN subpopulations are more vulnerable than others, focusing on both their molecular and physiological properties. Finally, we review the anatomical network and neuronal microenvironment as determinants of MN subtype vulnerability and hence the progression of ALS.

Attenuation of ketamine-induced impairment in verbal learning and memory in healthy volunteers by the AMPA receptor potentiator PF-04958242

There is a need to develop treatments for cognitive impairment associated with schizophrenia (CIAS). The significant role played by N-methyl-d-aspartate receptors (NMDARs) in both the pathophysiology of schizophrenia and in neuronal plasticity suggests that facilitation of NMDAR function might ameliorate CIAS. One strategy to correct NMDAR hypofunction is to stimulate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) as AMPAR and NMDAR functioning are coupled and interdependent. In rats and nonhuman primates (NHP), AMPAR potentiators reduce spatial working memory deficits caused by the nonselective NMDAR antagonist ketamine. The current study assessed whether the AMPAR potentiator PF-04958242 would attenuate ketamine-induced deficits in verbal learning and memory in humans. Healthy male subjects (n=29) participated in two randomized treatment periods of daily placebo or PF-04958242 for 5 days separated by a washout period. On day 5 of each treatment period, subjects underwent a ketamine infusion for 75 min during which the effects of PF-04958242/placebo were assessed on ketamine-induced: (1) impairments in verbal learning and recall measured by the Hopkins Verbal Learning Test; (2) impairments in working memory on a CogState battery; and (3) psychotomimetic effects measured by the Positive and Negative Syndrome Scale and Clinician-Administered Dissociative Symptoms Scale. PF-04958242 significantly reduced ketamine-induced impairments in immediate recall and the 2-Back and spatial working memory tasks (CogState Battery), without significantly attenuating ketamine-induced psychotomimetic effects. There were no pharmacokinetic interactions between PF-04958242 and ketamine. Furthermore, PF-04958242 was well tolerated. 'High-impact' AMPAR potentiators like PF-04958242 may have a role in the treatment of the cognitive symptoms, but not the positive or negative symptoms, associated with schizophrenia. The excellent concordance between the preclinical (rat, NHP) and human studies with PF-04958242, and in silico modeling of AMPAR-NMDAR interactions in the hippocampus, highlights the translational value of this study.

Conformational transitions in the glycine-bound GluN1 NMDA receptor LBD via single-molecule FRET

The N-methyl-D-aspartate receptor (NMDAR) is a member of the glutamate receptor family of proteins and is responsible for excitatory transmission. Activation of the receptor is thought to be controlled by conformational changes in the ligand binding domain (LBD); however, glutamate receptor LBDs can occupy multiple conformations even in the activated form. This work probes equilibrium transitions among NMDAR LBD conformations by monitoring the distance across the glycine-bound LBD cleft using single-molecule Förster resonance energy transfer (smFRET). Recent improvements in photoprotection solutions allowed us to monitor transitions among the multiple conformations. Also, we applied a recently developed model-free algorithm called "step transition and state identification" to identify the number of states, their smFRET efficiencies, and their interstate kinetics. Reversible interstate conversions, corresponding to transitions among a wide range of cleft widths, were identified in the glycine-bound LBD, on much longer timescales compared to channel opening. These transitions were confirmed to be equilibrium in nature by shifting the distribution reversibly via denaturant. We found that the NMDAR LBD proceeds primarily from one adjacent smFRET state to the next under equilibrium conditions, consistent with a cleft-opening/closing mechanism. Overall, by analyzing the state-to-state transition dynamics and distributions, we achieve insight into specifics of long-lived LBD equilibrium structural dynamics, as well as obtain a more general description of equilibrium folding/unfolding in a conformationally dynamic protein. The relationship between such long-lived LBD dynamics and channel function in the full receptor remains an open and interesting question.

PICK1 expression in reactive astrocytes within the spinal cord of amyotrophic lateral sclerosis (ALS) rats

AIMS

The protein interacting with C kinase 1 (PICK1), a PDZ domain-containing protein mainly expressed in the central nervous system, interacts with the glutamate receptor subunit GluR2, with the glutamate transporter GLT-1b and with the enzyme serine racemase. These three proteins appear as key actors in the glutamate-mediated excitotoxicity associated with amyotrophic lateral sclerosis (ALS), in both patients and animal models of the disease. In this study, we examined the expression of PICK1 in the spinal cord of transgenic rats expressing a mutated form of the human superoxide dismutase 1 (hSOD1(G93A) ) during the progression of the disease.

METHODS

Expression of PICK1 was examined by real-time qPCR at presymptomatic and symptomatic stages as well as at end-stage. The expression of PICK1 in the different cell types of the spinal cord was examined by immunohistochemistry.

RESULTS

The overall expression of PICK1 is not modified in cervical and lumbar spinal cord of transgenic (hSOD1(G93A) ) rats during the progression of the disease. Nonetheless, immunohistochemical studies of lumbar ventral horns revealed a shift of PICK1 expression from motor neurones in healthy rats to activated astrocytes in end-stage hSOD1(G93A) animals.

CONCLUSIONS

Considering the documented influence of PICK1 expression on d-serine release and glutamate transport in astrocytes, these findings point to a potential implication of PICK1 in the progression of ALS.

Unravelling the enigma of selective vulnerability in neurodegeneration: motor neurons resistant to degeneration in ALS show distinct gene expression characteristics and decreased susceptibility to excitotoxicity

A consistent clinical feature of amyotrophic lateral sclerosis (ALS) is the sparing of eye movements and the function of external sphincters, with corresponding preservation of motor neurons in the brainstem oculomotor nuclei, and of Onuf’s nucleus in the sacral spinal cord. Studying the differences in properties of neurons that are vulnerable and resistant to the disease process in ALS may provide insights into the mechanisms of neuronal degeneration, and identify targets for therapeutic manipulation. We used microarray analysis to determine the differences in gene expression between oculomotor and spinal motor neurons, isolated by laser capture microdissection from the midbrain and spinal cord of neurologically normal human controls. We compared these to transcriptional profiles of oculomotor nuclei and spinal cord from rat and mouse, obtained from the GEO omnibus database. We show that oculomotor neurons have a distinct transcriptional profile, with significant differential expression of 1,757 named genes (q < 0.001). Differentially expressed genes are enriched for the functional categories of synaptic transmission, ubiquitin-dependent proteolysis, mitochondrial function, transcriptional regulation, immune system functions, and the extracellular matrix. Marked differences are seen, across the three species, in genes with a function in synaptic transmission, including several glutamate and GABA receptor subunits. Using patch clamp recording in acute spinal and brainstem slices, we show that resistant oculomotor neurons show a reduced AMPA-mediated inward calcium current, and a higher GABA-mediated chloride current, than vulnerable spinal motor neurons. The findings suggest that reduced susceptibility to excitotoxicity, mediated in part through enhanced GABAergic transmission, is an important determinant of the relative resistance of oculomotor neurons to degeneration in ALS.

When Does ALS Start? ADAR2-GluA2 Hypothesis for the Etiology of Sporadic ALS

Amyotrophic lateral sclerosis (ALS) is the most common adult-onset motor neuron disease. More than 90% of ALS cases are sporadic, and the majority of sporadic ALS patients do not carry mutations in genes causative of familial ALS; therefore, investigation specifically targeting sporadic ALS is needed to discover the pathogenesis. The motor neurons of sporadic ALS patients express unedited GluA2 mRNA at the Q/R site in a disease-specific and motor neuron-selective manner. GluA2 is a subunit of the AMPA receptor, and it has a regulatory role in the Ca²⁺-permeability of the AMPA receptor after the genomic Q codon is replaced with the R codon in mRNA by adenosine–inosine conversion, which is mediated by adenosine deaminase acting on RNA 2 (ADAR2). Therefore, ADAR2 activity may not be sufficient to edit all GluA2 mRNA expressed in the motor neurons of ALS patients. To investigate whether deficient ADAR2 activity plays pathogenic roles in sporadic ALS, we generated genetically modified mice (AR2) in which the ADAR2 gene was conditionally knocked out in the motor neurons. AR2 mice showed an ALS-like phenotype with the death of ADAR2-lacking motor neurons. Notably, the motor neurons deficient in ADAR2 survived when they expressed only edited GluA2 in AR2/GluR-B^R/R (AR2res) mice, in which the endogenous GluA2 alleles were replaced by the GluR-B^R allele that encoded edited GluA2. In heterozygous AR2 mice with only one ADAR2 allele, approximately 20% of the spinal motor neurons expressed unedited GluA2 and underwent degeneration, indicating that half-normal ADAR2 activity is not sufficient to edit all GluA2 expressed in motor neurons. It is likely therefore that the expression of unedited GluA2 causes the death of motor neurons in sporadic ALS. We hypothesize that a progressive downregulation of ADAR2 activity plays a critical role in the pathogenesis of sporadic ALS and that the pathological process commences when motor neurons express unedited GluA2.

Ca2+-permeable AMPA receptors and intracellular Ca2+ determine motoneuron vulnerability in rat spinal cord in vivo

Excitotoxicity mediated by overactivation of glutamate receptors, particularly the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) type, has been implicated in motoneuron degeneration. AMPA receptors lacking the GluR2 subunit are permeable to Ca(2+) and the entrance of this cation might be responsible for the selective vulnerability of spinal motoneurons in amyotrophic lateral sclerosis (ALS). To evaluate this hypothesis in vivo, we have used a model of motoneuron death in which AMPA, perfused by microdialysis in the rat lumbar spinal cord, produces ipsilateral paralysis and a remarkable loss of spinal motoneurons. Perfusion of 1-naphthyl acetyl spermine, a selective blocker of the Ca(2+)-permeable AMPA receptors, and of the intracellular Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxymethyl ester) (BAPTA-AM), prevented the AMPA-induced paralysis and reduced by about 50% the loss of motoneurons. In addition, perfusion of pyruvate, an energy metabolic substrate, similarly prevented the paralysis and the motoneuron death. These results suggest that functional AMPA receptors lacking the GluR2 subunit are present in the rat spinal cord, and that motoneuron death is triggered by an increase of intracellular Ca(2+) via such Ca(2+)-permeable AMPA receptors. Our finding that pyruvate also protected against the excitotoxic effects of AMPA suggests that the increased intracellular Ca(2+) probably interferes with the mitochondrial energetic metabolism.

Slow and selective death of spinal motor neurons in vivo by intrathecal infusion of kainic acid: implications for AMPA receptor-mediated excitotoxicity in ALS

Excitotoxicity mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors has been proposed to play a major role in the selective death of motor neurons in sporadic amyotrophic lateral sclerosis (ALS), and motor neurons are more vulnerable to AMPA receptor-mediated excitotoxicity than are other neuronal subclasses. On the basis of the above evidence, we aimed to develop a rat model of ALS by the long-term activation of AMPA receptors through continuous infusion of kainic acid (KA), an AMPA receptor agonist, into the spinal subarachnoid space. These rats displayed a progressive motor-selective behavioral deficit with delayed loss of spinal motor neurons, mimicking the clinicopathological characteristics of ALS. These changes were significantly ameliorated by co-infusion with 6-nitro-7-sulfamobenso(f)quinoxaline-2,3-dione (NBQX), but not with d(-)-2-amino-5-phosphonovaleric acid (APV), and were exacerbated by co-infusion with cyclothiazide, indicative of an AMPA receptor-mediated mechanism. Among the four AMPA receptor subunits, expression of GluR3 mRNA was selectively up-regulated in motor neurons but not in dorsal horn neurons of the KA-infused rats. The up-regulation of GluR3 mRNA in this model may cause a molecular change that induces the selective vulnerability of motor neurons to KA by increasing the proportion of GluR2-lacking (i.e. calcium-permeable) AMPA receptors. This rat model may be useful in investigating ALS etiology.

Calcium-permeable AMPA channels in neurodegenerative disease and ischemia

Compelling evidence supports contributions of glutamate receptor overactivation ('excitotoxicity') to neurodegeneration in both acute conditions, such as stroke, and chronic neurodegenerative conditions, such as amyotrophic lateral sclerosis. However, anti-excitotoxic therapeutic trials, which have generally targeted highly Ca2+ permeable NMDA-type glutamate channels, have to date failed to demonstrate impressive efficacy. Whereas most AMPA type glutamate channels are Ca2+ impermeable, an evolving body of evidence supports the contention that relatively unusual Ca2+ permeable AMPA channels might be crucial contributors to injury in these conditions. These channels are preferentially expressed in discrete neuronal subpopulations, and their numbers appear to be upregulated in amyotrophic lateral sclerosis and stroke. In addition, unlike NMDA channels, Ca2+ permeable AMPA channels are not blocked by Mg2+, but are highly permeable to another potentially harmful endogenous cation, Zn2+. The targeting of these channels might provide efficacious new avenues in the therapy of certain neurological diseases.