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

Effects and mechanisms of bisphenols exposure on neurodegenerative diseases risk: A systemic review

Environmental bisphenols (BPs) pose a global threat to human health because of their extensive use as additives in plastic products. BP residues are increasing in various environmental media (i.e., water, soil, and indoor dust) and biological and human samples (i.e., serum and brain). Both epidemiological and animal studies have determined an association between exposure to BPs and an increased risk of neurodegenerative diseases (e.g., Parkinson's disease, Alzheimer's disease, and amyotrophic lateral sclerosis), including cognitive abnormalities and behavioral disturbances. Hence, understanding the biological responses to different BPs is essential for prevention, and treatment. This study provides an overview of the underlying pathogenic molecular mechanisms as a valuable basis for understanding neurodegenerative disease responses to BPs, including accumulation of misfolded proteins, reduction of tyrosine hydroxylase and dopamine, abnormal hormone signaling, neuronal death, oxidative stress, calcium homeostasis, and inflammation. These findings provide new insights into the neurotoxic potential of BPs and ultimately contribute to a comprehensive health risk evaluation.

Calcium-binding protein parvalbumin in the spinal cord and dorsal root ganglia

Parvalbumin is one of the calcium-binding proteins. In the spinal cord, it is mainly expressed in inhibitory neurons; in the dorsal root ganglia, it is expressed in proprioceptive neurons. In contrast to in the brain, weak systematization of parvalbumin-expressing neurons occurs in the spinal cord. The aim of this paper is to provide a systematic review of parvalbumin-expressing neuronal populations throughout the spinal cord and the dorsal root ganglia of mammals, regarding their mapping, co-expression with some functional markers. The data reviewed are mostly concerning rodentia species because they are predominantly presented in literature.

Characterization of Mitochondria Degeneration in Spinal Motor Neurons Triggered by Chronic Over-activation of α-Amino-3-Hydroxy-5-Methylisoxazole-4-Propionic Acid Receptors in the Rat Spinal Cord in Vivo

Mitochondrial damage is a central mechanism involved in neurological disorders as Alzheimer's, and Parkinson's diseases and amyotrophic lateral sclerosis. Energy production is the most studied mitochondrial function; however, mitochondria are also involved in processes like calcium buffering homeostasis, and cell death control during apoptosis and necrosis. Using transmission electron microscopy, in this in vivo study in male rats, we describe ultrastructural mitochondrial alterations of spinal motor neurons along chronic AMPA-induced excitotoxicity, which has been described as one of the most relevant mechanisms in ALS disease. Mitochondrial alterations begin with a crest swelling, which progresses to a full mitochondrial swelling and crest disruption. Changes on the mitochondrial morphology from elongated to a circular shape also occur along the AMPA-excitotoxicity process. In addition, by combining the TUNEL assay and immunohistochemistry for mitochondrial enzymes, we show evidence of mitochondrial DNA damage. Evidence of mitochondrial alterations during an AMPA-excitotoxic event is relevant because resembles the mitochondrial alterations previously reported in ALS patients and in transgenic familial ALS models, suggesting that a chronic excitotoxic model can be related to sporadic ALS (as has been shown in recent papers), which represent more than the 90% of the ALS cases. Understanding the mechanisms involved in motor neuron degenerative process, such as the ultrastructural mitochondrial changes permits to design strategies for MN-degeneration treatments in ALS.

Synaptic Disruption by Soluble Oligomers in Patients with Alzheimer's and Parkinson's Disease

Neurodegenerative diseases are the result of progressive dysfunction of the neuronal activity and subsequent neuronal death. Currently, the most prevalent neurodegenerative diseases are by far Alzheimer's (AD) and Parkinson's (PD) disease, affecting millions of people worldwide. Although amyloid plaques and neurofibrillary tangles are the neuropathological hallmarks for AD and Lewy bodies (LB) are the hallmark for PD, current evidence strongly suggests that oligomers seeding the neuropathological hallmarks are more toxic and disease-relevant in both pathologies. The presence of small soluble oligomers is the common bond between AD and PD: amyloid β oligomers (AβOs) and Tau oligomers (TauOs) in AD and α-synuclein oligomers (αSynOs) in PD. Such oligomers appear to be particularly increased during the early pathological stages, targeting synapses at vulnerable brain regions leading to synaptic plasticity disruption, synapse loss, inflammation, excitation to inhibition imbalance and cognitive impairment. Absence of TauOs at synapses in individuals with strong AD disease pathology but preserved cognition suggests that mechanisms of resilience may be dependent on the interactions between soluble oligomers and their synaptic targets. In this review, we will discuss the current knowledge about the interactions between soluble oligomers and synaptic dysfunction in patients diagnosed with AD and PD, how it affects excitatory and inhibitory synaptic transmission, and the potential mechanisms of synaptic resilience in humans.

Body Weight-Supported Treadmill Training Ameliorates Motoneuronal Hyperexcitability by Increasing GAD-65/67 and KCC2 Expression via TrkB Signaling in Rats with Incomplete Spinal Cord Injury

Spasticity is a typical consequence after spinal cord injury (SCI). The critical reasons are reducing the synthesis of Gamma-Aminobutyric Acid (GABA), glycine and potassium chloride co-transporter 2 (KCC2) inside the distal spinal cord. The current work aimed to test whether exercise training could increase the expression of glutamic acid decarboxylase 65/67 (GAD-65/67, the key enzymes in GABA synthesis) and KCC2 in the distal spinal cord via tropomyosin-related kinase B (TrkB) signaling. The experimental rats were randomly assigned to the following five groups: Sham, SCI/phosphate-buffered saline (PBS), SCI-treadmill training (TT)/PBS, SCI/TrkB-IgG, and SCI-TT/TrkB-IgG. After that, the model of T10 contusion SCI was used, then TrkB-IgG was used to prevent TrkB activity at 7 days post-SCI. Body weight-supported treadmill training started on the 8^th day post-SCI for four weeks. The Hmax/Mmax ratio and the rate-dependent depression of H-reflex were used to assess the excitability of spinal motoneuronal networks. Western blotting and Immunohistochemistry techniques were utilized for measuring the expression of GAD-65, GAD-67, and KCC2. The findings revealed that exercise training could reduce motoneuronal excitability and boost GAD-65, GAD-67, and KCC2 production in the distal region of the spinal cord after SCI. The effects of exercise training were decreased after the TrkB signaling was inhibited. The present exploration demonstrated that exercise training increases GAD-65, GAD-67, and KCC2 expression in the spinal cord via TrkB signaling and that this method could also improve rats with motoneuronal hyperexcitability and spasticity induced by incomplete SCI.

BAPTA, a calcium chelator, neuroprotects injured neurons in vitro and promotes motor recovery after spinal cord transection in vivo

Aim

Despite animal evidence of a role of calcium in the pathogenesis of spinal cord injury, several studies conducted in the past found calcium blockade ineffective. However, those studies involved oral or parenteral administration of Ca++ antagonists. We hypothesized that Ca++ blockade might be effective with local/immediate application (LIA) at the time of neural injury.

Methods

In this study, we assessed the effects of LIA of BAPTA (1,2‐bis (o‐aminophenoxy) ethane‐N, N, N′, N'‐tetraacetic acid), a cell‐permeable highly selective Ca++ chelator, after spinal cord transection (SCT) in mice over 4 weeks. Effects of BAPTA were assessed behaviorally and with immunohistochemistry. Concurrently, BAPTA was submitted for the first time to multimodality assessment in an in vitro model of neural damage as a possible spinal neuroprotectant.

Results

We demonstrate that BAPTA alleviates neuronal apoptosis caused by physical damage by inhibition of neuronal apoptosis and reactive oxygen species (ROS) generation. This translates to enhanced preservation of electrophysiological function and superior behavioral recovery.

Conclusion

This study shows for the first time that local/immediate application of Ca++ chelator BAPTA is strongly neuroprotective after severe spinal cord injury.

Neuroprotective Effect of Vascular Endothelial Growth Factor on Motoneurons of the Oculomotor System

Vascular endothelial growth factor (VEGF) was initially characterized as a potent angiogenic factor based on its activity on the vascular system. However, it is now well established that VEGF also plays a crucial role as a neuroprotective factor in the nervous system. A deficit of VEGF has been related to motoneuronal degeneration, such as that occurring in amyotrophic lateral sclerosis (ALS). Strikingly, motoneurons of the oculomotor system show lesser vulnerability to neurodegeneration in ALS compared to other motoneurons. These motoneurons presented higher amounts of VEGF and its receptor Flk-1 than other brainstem pools. That higher VEGF level could be due to an enhanced retrograde input from their target muscles, but it can also be produced by the motoneurons themselves and act in an autocrine way. By contrast, VEGF’s paracrine supply from the vicinity cells, such as glial cells, seems to represent a minor source of VEGF for brainstem motoneurons. In addition, ocular motoneurons experiment an increase in VEGF and Flk-1 level in response to axotomy, not observed in facial or hypoglossal motoneurons. Therefore, in this review, we summarize the differences in VEGF availability that could contribute to the higher resistance of extraocular motoneurons to injury and neurodegenerative diseases.

Decreased Mitochondrial Function, Biogenesis, and Degradation in Peripheral Blood Mononuclear Cells from Amyotrophic Lateral Sclerosis Patients as a Potential Tool for Biomarker Research

Amyotrophic lateral sclerosis (ALS) is a multifactorial and progressive neurodegenerative disease of unknown etiology. Due to ALS’s unpredictable onset and progression rate, the search for biomarkers that allow the detection and tracking of its development and therapeutic efficacy would be of significant medical value. Considering that alterations of energy supply are one of ALS’s main hallmarks and that a correlation has been established between gene expression in human brain tissue and peripheral blood mononuclear cells (PBMCs), the present work investigates whether changes in mitochondrial function could be used to monitor ALS. To achieve this goal, PBMCs from ALS patients and control subjects were used; blood sampling is a quite non-invasive method and is cost-effective. Different parameters were evaluated, namely cytosolic calcium levels, mitochondrial membrane potential, oxidative stress, and metabolic compounds levels, as well as mitochondrial dynamics and degradation. Altogether, we observed lower mitochondrial calcium uptake/retention, mitochondria depolarization, and redox homeostasis deregulation, in addition to a decrease in critical metabolic genes, a diminishment in mitochondrial biogenesis, and an augmentation in mitochondrial fission and autophagy-related gene expression. All of these changes can contribute to the decreased ATP and pyruvate levels observed in ALS PBMCs. Our data indicate that PBMCs from ALS patients show a significant mitochondrial dysfunction, resembling several findings from ALS’ neural cells/models, which could be exploited as a powerful tool in ALS research. Our findings can also guide future studies on new pharmacological interventions for ALS since assessments of brain samples are challenging and represent a relevant limited strategy.

Altered calcium dynamics and glutamate receptor properties in iPSC-derived motor neurons from ALS patients with C9orf72, FUS, SOD1 or TDP43 mutations

The fatal neurodegenerative disease amyotrophic lateral sclerosis (ALS) is characterized by a profound loss of motor neurons (MNs). Until now only riluzole minimally extends life expectancy in ALS, presumably by inhibiting glutamatergic neurotransmission and calcium overload of MNs. Therefore, the aim of this study was to investigate the glutamate receptor properties and key aspects of intracellular calcium dynamics in induced pluripotent stem cell (iPSC)-derived MNs from ALS patients with C9orf72 (n = 4 cell lines), fused in sarcoma (FUS) (n = 9), superoxide dismutase 1 (SOD1) (n = 3) or transactive response DNA-binding protein 43 (TDP43) (n = 3) mutations as well as healthy (n = 7 cell lines) and isogenic controls (n = 3). Using calcium imaging, we most frequently observed spontaneous transients in mutant C9orf72 MNs. Basal intracellular calcium levels and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-induced signal amplitudes were elevated in mutant TDP43 MNs. Besides, a majority of mutant TDP43 MNs responded to 3.5-dihydroxyphenylglycine as metabotropic glutamate receptor agonist. Quantitative real-time PCR demonstrated significantly increased expression levels of AMPA and kainate receptors in mutant FUS cells compared to healthy and isogenic controls. Furthermore, the expression of kainate receptors and voltage gated calcium channels in mutant C9orf72 MNs as well as metabotropic glutamate receptors in mutant SOD1 cells was markedly elevated compared to controls. Our data of iPSC-derived MNs from familial ALS patients revealed several mutation-specific alterations in glutamate receptor properties and calcium dynamics that could play a role in ALS pathogenesis and may lead to future translational strategies with individual stratification of neuroprotective ALS treatments.

Excitatory Dendritic Mitochondrial Calcium Toxicity: Implications for Parkinson's and Other Neurodegenerative Diseases

Dysregulation of calcium homeostasis has been linked to multiple neurological diseases. In addition to excitotoxic neuronal cell death observed following stroke, a growing number of studies implicate excess excitatory neuronal activity in chronic neurodegenerative diseases. Mitochondria function to rapidly sequester large influxes of cytosolic calcium through the activity of the mitochondrial calcium uniporter (MCU) complex, followed by more gradual release via calcium antiporters, such as NCLX. Increased cytosolic calcium levels almost invariably result in increased mitochondrial calcium uptake. While this response may augment mitochondrial respiration, limiting classic excitotoxic injury in the short term, recent studies employing live calcium imaging and molecular manipulation of calcium transporter activities suggest that mitochondrial calcium overload plays a key role in Parkinson’s disease (PD), Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), and related dementias [PD with dementia (PDD), dementia with Lewy bodies (DLB), and frontotemporal dementia (FTD)]. Herein, we review the literature on increased excitatory input, mitochondrial calcium dysregulation, and the transcriptional or post-translational regulation of mitochondrial calcium transport proteins, with an emphasis on the PD-linked kinases LRRK2 and PINK1. The impact on pathological dendrite remodeling and neuroprotective effects of manipulating MCU, NCLX, and LETM1 are reviewed. We propose that shortening and simplification of the dendritic arbor observed in neurodegenerative diseases occur through a process of excitatory mitochondrial toxicity (EMT), which triggers mitophagy and perisynaptic mitochondrial depletion, mechanisms that are distinct from classic excitotoxicity.

Altered Cerebellar Short-Term Plasticity but No Change in Postsynaptic AMPA-Type Glutamate Receptors in a Mouse Model of Juvenile Batten Disease

Juvenile Batten disease is the most common progressive neurodegenerative disorder of childhood. It is associated with mutations in the CLN3 gene, causing loss of function of CLN3 protein and degeneration of cerebellar and retinal neurons. It has been proposed that changes in granule cell AMPA-type glutamate receptors (AMPARs) contribute to the cerebellar dysfunction. In this study, we compared AMPAR properties and synaptic transmission in cerebellar granule cells from wild-type and Cln3 knock-out mice. In Cln3Δex1-6 cells, the amplitude of AMPA-evoked whole-cell currents was unchanged. Similarly, we found no change in the amplitude, kinetics, or rectification of synaptic currents evoked by individual quanta, or in their underlying single-channel conductance. We found no change in cerebellar expression of GluA2 or GluA4 protein. By contrast, we observed a reduced number of quantal events following mossy-fiber stimulation in Sr2+, altered short-term plasticity in conditions of reduced extracellular Ca2+, and reduced mossy fiber vesicle number. Thus, while our results suggest early presynaptic changes in the Cln3Δex1-6 mouse model of juvenile Batten disease, they reveal no evidence for altered postsynaptic AMPARs.

Excitatory and Inhibitory Neuronal Circuits in the Spinal Cord and Their Role in the Control of Motor Neuron Function and Degeneration

The complex neuronal networks of the spinal cord coordinate a wide variety of motor functions, including walking, running, and voluntary and involuntary movements. This is accomplished by different groups of neurons, called center pattern generators, which control left-right alternation and flexor-extensor patterns. These spinal circuits, located in the ventral horns, are formed by several neuronal types, and the specific function of most of them has been identified by means of studies in vivo and in the isolated spinal cord of mice harboring genetically induced ablation of specific neuronal populations. These studies have shown that the coordinated activity of several interneuron types, mainly GABAergic and glycinergic inhibitory neurons, have a crucial role in the modulation of motor neurons activity that finally excites the corresponding muscles. A pharmacological experimental approach by administering in the spinal cord agonists and antagonists of glutamate, GABA, glycine, and acetylcholine receptors to alter their synaptic action has also produced important results, linking the deficits in the synaptic function with the resulting motor alterations. These results have also increased the knowledge of the mechanisms of motor neuron degeneration, which is characteristic of diseases such as amyotrophic lateral sclerosis, and therefore open the possibility of designing new strategies for the prevention and treatment of these diseases.

PPARγ as a therapeutic target to rescue mitochondrial function in neurological disease

There is increasing evidence for the involvement of mitochondrial dysfunction and oxidative stress in the pathogenesis of many of the major neurodegenerative and neuroinflammatory diseases, suggesting that mitochondrial and antioxidant pathways may represent potential novel therapeutic targets. Recent years have seen a rapidly growing interest in the use of therapeutic strategies that can limit the defects in, or even to restore, mitochondrial function while reducing free radical generation. The peroxisome proliferation-activated receptor gamma (PPARγ), a ligand-activated transcription factor, has a wide spectrum of biological functions, regulating mitochondrial function, mitochondrial turnover, energy metabolism, antioxidant defence and redox balance, immune responses and fatty acid oxidation. In this review, we explore the evidence for potential beneficial effects of PPARγ agonists in a number of neurological disorders, including Parkinson's disease, Alzheimer's disease, Amyotrophic lateral sclerosis and Huntington's disease, ischaemia, autoimmune encephalomyelitis and neuropathic pain. We discuss the mechanisms underlying those beneficial effects in particular in relation to mitochondrial function, antioxidant defence, cell death and inflammation, and suggest that the PPARγ agonists show significant promise as therapeutic agents in otherwise intractable neurological disease.

Polyamines as Snake Toxins and Their Probable Pharmacological Functions in Envenomation

While decades of research have focused on snake venom proteins, far less attention has been paid to small organic venom constituents. Using mostly pooled samples, we surveyed 31 venoms (six elapid, six viperid, and 19 crotalid) for spermine, spermidine, putrescine, and cadaverine. Most venoms contained all four polyamines, although some in essentially trace quantities. Spermine is a potentially significant component of many viperid and crotalid venoms (≤0.16% by mass, or 7.9 µmol/g); however, it is almost completely absent from elapid venoms assayed. All elapid venoms contained larger molar quantities of putrescine and cadaverine than spermine, but still at levels that are likely to be biologically insignificant. As with venom purines, polyamines impact numerous physiological targets in ways that are consistent with the objectives of prey envenomation, prey immobilization via hypotension and paralysis. Most venoms probably do not contain sufficient quantities of polyamines to induce systemic effects in prey; however, local effects seem probable. A review of the pharmacological literature suggests that spermine could contribute to prey hypotension and paralysis by interacting with N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, nicotinic and muscarinic acetylcholine receptors, γ-Aminobutyric acid (GABA) receptors, blood platelets, ryanodine receptors, and Ca²⁺-ATPase. It also blocks many types of cation-permeable channels by interacting with negatively charged amino acid residues in the channel mouths. The site of envenomation probably determines which physiological targets assume the greatest importance; however, venom-induced liberation of endogenous, intracellular stores of polyamines could potentially have systemic implications and may contribute significantly to envenomation sequelae.

Mitochondrial Dysfunction during the Early Stages of Excitotoxic Spinal Motor Neuron Degeneration in Vivo

Glutamate excitotoxicity and mitochondrial dysfunction are involved in motor neuron degeneration process during amyotrophic lateral sclerosis (ALS). We have previously shown that microdialysis perfusion of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) in the lumbar region of the rat spinal cord produces permanent paralysis of the ipsilateral hindlimb and death of motor neurons by a Ca(2+)-dependent mechanism, in a process that starts 2-3 h after AMPA perfusion. Co-perfusion with different energy metabolic substrates, mainly pyruvate, prevented the paralysis and motor neuron degeneration induced by AMPA, suggesting that mitochondrial energetic deficiencies are involved in this excitotoxic motor neuron death. To test this, in the present work, we studied the functional and ultrastructural characteristics of mitochondria isolated from the ventral horns of lumbar spinal cords of rats, at the beginning of the AMPA-induced degeneration process, when motor neurons are still alive. Animals were divided in four groups: perfused with AMPA, AMPA + pyruvate, and pyruvate alone and Krebs-Ringer medium as controls. Mitochondria from the AMPA-treated group showed decreased oxygen consumption rates, respiratory controls, and transmembrane potentials. Additionally, activities of the respiratory chain complexes I and IV were significantly decreased. Electron microscopy showed that mitochondria from AMPA-treated rats presented swelling, disorganized cristae and disrupted membranes. Remarkably, in the animals co-perfused with AMPA and pyruvate all these abnormalities were prevented. We conclude that mitochondrial dysfunction plays a crucial role in spinal motor neuron degeneration induced by overactivation of AMPA receptors in vivo. These mechanisms could be involved in ALS motor neuron degeneration.

Motor Alterations Induced by Chronic 4-Aminopyridine Infusion in the Spinal Cord In vivo: Role of Glutamate and GABA Receptors

Motor neuron (MN) degeneration is the pathological hallmark of MN diseases, a group of neurodegenerative disorders clinically manifested as muscle fasciculations and hyperreflexia, followed by paralysis, respiratory failure, and death. Ample evidence supports a role of glutamate-mediated excitotoxicity in motor death. In previous work we showed that stimulation of glutamate release from nerve endings by perfusion of the K⁺-channel blocker 4-aminopyridine (4-AP) in the rat hippocampus induces seizures and neurodegeneration, and that AMPA infusion in the spinal cord produces paralysis and MN death. On these bases, in this work we have tested the effect of the chronic infusion of 4-AP in the spinal cord, using implanted osmotic minipumps, on motor activity and on MN survival, and the mechanisms underlying this effect. 4-AP produced muscle fasciculations and motor deficits assessed in two motor tests, which start 2–3 h after the implant, which ameliorated spontaneously within 6–7 days, but no neurodegeneration. These effects were prevented by both AMPA and NMDA receptors blockers. The role of GABA_A receptors was also explored, and we found that chronic infusion of bicuculline induced moderate MN degeneration and enhanced the hyperexcitation produced by 4-AP. Unexpectedly, the GABA_AR agonist muscimol also induced motor deficits and failed to prevent the MN death induced by AMPA. We conclude that motor alterations induced by chronic 4-AP infusion in the spinal cord in vivo is due to ionotropic glutamate receptor overactivation and that blockade of GABAergic neurotransmission induces MN death under chronic conditions. These results shed light on the role of glutamatergic and GABAergic neurotransmission in the regulation of MN excitability in the spinal cord.

Impaired Autophagy and Defective Mitochondrial Function: Converging Paths on the Road to Motor Neuron Degeneration

Selective motor neuron degeneration is a hallmark of amyotrophic lateral sclerosis (ALS). Around 10% of all cases present as familial ALS (FALS), while sporadic ALS (SALS) accounts for the remaining 90%. Diverse genetic mutations leading to FALS have been identified, but the underlying causes of SALS remain largely unknown. Despite the heterogeneous and incompletely understood etiology, different types of ALS exhibit overlapping pathology and common phenotypes, including protein aggregation and mitochondrial deficiencies. Here, we review the current understanding of mechanisms leading to motor neuron degeneration in ALS as they pertain to disrupted cellular clearance pathways, ATP biogenesis, calcium buffering and mitochondrial dynamics. Through focusing on impaired autophagic and mitochondrial functions, we highlight how the convergence of diverse cellular processes and pathways contributes to common pathology in motor neuron degeneration.

Degeneration of spinal motor neurons by chronic AMPA-induced excitotoxicity in vivo and protection by energy substrates

INTRODUCTION

Several data suggest that excitotoxicity due to excessive glutamatergic neurotransmission may be an important factor in the mechanisms of motor neuron (MN) death occurring in amyotrophic lateral sclerosis (ALS). We have previously shown that the overactivation of the Ca(2+)-permeable α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) glutamate receptor type, through the continuous infusion of AMPA in the lumbar spinal cord of adult rats during several days, results in progressive rear limb paralysis and bilateral MN degeneration. Because it has been shown that energy failure and oxidative stress are involved in MN degeneration, in both ALS and experimental models of spinal MN degeneration, including excitotoxicity, in this work we tested the protective effect of the energy substrates pyruvate and β-hydroxybutyrate (βHB) and the antioxidants glutathione ethyl ester (GEE) and ascorbate in this chronic AMPA-induced neurodegeneration.

RESULTS

AMPA infusion induced remarkable progressive motor deficits, assessed by two motor tasks, that by day seven reach bilateral rear limb paralysis. These effects correlate with the death of >80% of lumbar spinal MNs in the infused and the neighbor spinal cord segments, as well as with notable astrogliosis in the ventral horns, detected by glial fibrillary acidic protein immunohistochemistry. Co-infusion with pyruvate or βHB notably prevented the motor deficits and paralysis, decreased MN loss to <25% and completely prevented the induction of astrogliosis. In contrast, the antioxidants tested were ineffective regarding all parameters analyzed.

CONCLUSIONS

Chronic progressive excitotoxicity due to AMPA receptors overactivation results in MN death and astrogliosis, with consequent motor deficits and paralysis. Because of the notable protection against these effects exerted by pyruvate and βHB, which are well established mitochondrial energy substrates, we conclude that deficits in mitochondrial energy metabolism are an important factor in the mechanisms of this slowly developed excitotoxic MN death, while the lack of protective effect of the antioxidants indicates that oxidative stress seems to be less significant factor. Because excitotoxicity may be involved in MN degeneration in ALS, these findings suggest possible preventive or therapeutic strategies for the disease.

Ligand-directed delivery of fluorophores to track native calcium-permeable AMPA receptors in neuronal cultures

Subcellular trafficking of neuronal receptors is known to play a key role in synaptic development, homeostasis, and plasticity. We have developed a ligand-targeted and photo-cleavable probe for delivering a synthetic fluorophore to AMPA receptors natively expressed in neurons. After a receptor is bound to the ligand portion of the probe molecule, a proteinaceous nucleophile reacts with an electrophile on the probe, covalently bonding the two species. The ligand may then be removed by photolysis, returning the receptor to its non-liganded state while leaving intact the new covalent bond between the receptor and the fluorophore. This strategy was used to label polyamine-sensitive receptors, including calcium-permeable AMPA receptors, in live hippocampal neurons from rats. Here, we describe experiments where we examined specificity, competition, and concentration on labeling efficacy as well as quantified receptor trafficking. Pharmacological competition during the labeling step with either a competitive or non-competitive glutamate receptor antagonist prevented the majority of labeling observed without a blocker. In other experiments, labeled receptors were observed to alter their locations and we were able to track and quantify their movements. We used a small molecule, ligand-directed probe to deliver synthetic fluorophores to endogenously expressed glutamate receptors for the purpose of tracking these receptors on live, hippocampal neurons. We found that clusters of receptors appear to move at similar rates to previous studies. We also found that the polyamine toxin pharmacophore likely binds to receptors in addition to calcium-permeable AMPA receptors.

Selective vulnerability of motoneuron and perturbed mitochondrial calcium homeostasis in amyotrophic lateral sclerosis: implications for motoneurons specific calcium dysregulation

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disorder characterized by the selective degeneration of defined subgroups of motoneuron in the brainstem, spinal cord and motor cortex with signature hallmarks of mitochondrial Ca²⁺ overload, free radical damage, excitotoxicity and impaired axonal transport. Although intracellular disruptions of cytosolic and mitochondrial calcium, and in particular low cytosolic calcium ([Ca²⁺]_c) buffering and a strong interaction between metabolic mechanisms and [Ca²⁺]_i have been identified predominantly in motoneuron impairment, the causes of these disruptions are unknown. The existing evidence suggests that the mutant superoxide dismutase1 (mtSOD1)-mediated toxicity in ALS acts through mitochondria, and that alteration in cytosolic and mitochondria-ER microdomain calcium accumulation are critical to the neurodegenerative process. Furthermore, chronic excitotoxcity mediated by Ca²⁺-permeable AMPA and NMDA receptors seems to initiate vicious cycle of intracellular calcium dysregulation which leads to toxic Ca²⁺ overload and thereby selective neurodegeneration. Recent advancement in the experimental analysis of calcium signals with high spatiotemporal precision has allowed investigations of calcium regulation in-vivo and in-vitro in different cell types, in particular selectively vulnerable/resistant cell types in different animal models of this motoneuron disease. This review provides an overview of latest advances in this field, and focuses on details of what has been learned about disrupted Ca²⁺ homeostasis and mitochondrial degeneration. It further emphasizes the critical role of mitochondria in preventing apoptosis by acting as a Ca²⁺ buffers, especially in motoneurons, in pathophysiological conditions such as ALS.

Tumor necrosis factor alpha: a link between neuroinflammation and excitotoxicity

Tumor necrosis factor alpha (TNF-α) is a proinflammatory cytokine that exerts both homeostatic and pathophysiological roles in the central nervous system. In pathological conditions, microglia release large amounts of TNF-α; this de novo production of TNF-α is an important component of the so-called neuroinflammatory response that is associated with several neurological disorders. In addition, TNF-α can potentiate glutamate-mediated cytotoxicity by two complementary mechanisms: indirectly, by inhibiting glutamate transport on astrocytes, and directly, by rapidly triggering the surface expression of Ca⁺² permeable-AMPA receptors and NMDA receptors, while decreasing inhibitory GABA_A receptors on neurons. Thus, the net effect of TNF-α is to alter the balance of excitation and inhibition resulting in a higher synaptic excitatory/inhibitory ratio. This review summarizes the current knowledge of the cellular and molecular mechanisms by which TNF-α links the neuroinflammatory and excitotoxic processes that occur in several neurodegenerative diseases, but with a special emphasis on amyotrophic lateral sclerosis (ALS). As microglial activation and upregulation of TNF-α expression is a common feature of several CNS diseases, as well as chronic opioid exposure and neuropathic pain, modulating TNF-α signaling may represent a valuable target for intervention.

Role of energy metabolic deficits and oxidative stress in excitotoxic spinal motor neuron degeneration in vivo

MN (motor neuron) death in amyotrophic lateral sclerosis may be mediated by glutamatergic excitotoxicity. Previously, our group showed that the microdialysis perfusion of AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionate) in the rat lumbar spinal cord induced MN death and permanent paralysis within 12 h after the experiment. Here, we studied the involvement of energy metabolic deficiencies and of oxidative stress in this MN degeneration, by testing the neuroprotective effect of various energy metabolic substrates and antioxidants. Pyruvate, lactate, β-hydroxybutyrate, α-ketobutyrate and creatine reduced MN loss by 50–65%, preserved motor function and completely prevented the paralysis. Ascorbate, glutathione and glutathione ethyl ester weakly protected against motor deficits and reduced MN death by only 30–40%. Reactive oxygen species formation and 3-nitrotyrosine immunoreactivity were studied 1.5–2 h after AMPA perfusion, during the initial MN degenerating process, and no changes were observed. We conclude that mitochondrial energy deficiency plays a crucial role in this excitotoxic spinal MN degeneration, whereas oxidative stress seems a less relevant mechanism. Interestingly, we observed a clear correlation between the alterations of motor function and the number of damaged MNs, suggesting that there is a threshold of about 50% in the number of healthy MNs necessary to preserve motor function.

Mitochondrial energy substrates protect against in vivo excitotoxic spinal motor neuron degeneration and the consequent paralysis, whereas antioxidants are less efficient. These results allowed to establish a minimal threshold number of spinal motor neurons necessary to preserve motor function.

Repeated Baclofen treatment ameliorates motor dysfunction, suppresses reflex activity and decreases the expression of signaling proteins in reticular nuclei and lumbar motoneurons after spinal trauma in rats

The interruption of supraspinal input to the spinal cord leads to motor dysfunction and the development of spasticity. Clinical studies have shown that Baclofen (a GABAB agonist), while effective in modulating spasticity is associated with side-effects and the development of tolerance. The aim of the present study was to assess if discontinued Baclofen treatment and its repeated application leads antispasticity effects, and whether such changes affect neuronal nitric oxide synthase (nNOS) in the brainstem, nNOS and parvalbumin (PV) in lumbar α-motoneurons and glial fibrillary acidic protein in the ventral horn of the spinal cord. Adult male Wistar rats were exposed to Th9 spinal cord transection. Baclofen (30mg/b.w.) diluted in drinking water, was administered for 6 days, starting at week 1 after injury and then repeated till week 4 after injury. The behavior of the animals was tested (tail-flick test, BBB locomotor score) from 1 to 8 weeks. Our results clearly indicate the role of nitric oxide, produced by nNOS in the initiation and the maintenance of spasticity states 1, 6 and 8 weeks after spinal trauma. A considerable decrease of nNOS staining after Baclofen treatment correlates with improvement of motor dysfunction. The findings also show that parvalbumin and astrocytes participate in the regulation of ion concentrations in the sub-acute phase after the injury.

Ca2+-permeable AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors and dopamine D1 receptors regulate GluA1 trafficking in striatal neurons

Regulation of striatal medium spiny neuron synapses underlies forms of motivated behavior and pathological drug seeking. A primary mechanism for increasing synaptic strength is the trafficking of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) into the postsynapse, a process mediated by GluA1 AMPAR subunit phosphorylation. We have examined the role of converging glutamate and dopamine inputs in regulating biochemical cascades upstream of GluA1 phosphorylation. We focused on the role of Ca(2+)-permeable AMPARs (CPARs), which lack the GluA2 AMPAR subunit. Under conditions that prevented depolarization, stimulation of CPARs activated neuronal nitric oxide synthase and production of cGMP. CPAR-dependent cGMP production was sufficient to induce synaptic insertion of GluA1, detected by confocal microscopy, through a mechanism dependent on GluA1 Ser-845 phosphorylation. Dopamine D1 receptors, in contrast, stimulate GluA1 extra synaptic insertion. Simultaneous activation of dopamine D1 receptors and CPARs induced additive increases in GluA1 membrane insertion, but only CPAR stimulation augmented CPAR-dependent GluA1 synaptic insertion. This incorporation into the synapse proceeded through a sequential two-step mechanism; that is, cGMP-dependent protein kinase II facilitated membrane insertion and/or retention, and protein kinase C activity was necessary for synaptic insertion. These data suggest a feed-forward mechanism for synaptic priming whereby an initial stimulus acting independently of voltage-gated conductance increases striatal neuron excitability, facilitating greater neuronal excitation by a subsequent stimulus.

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.

Delayed administration of VEGF rescues spinal motor neurons from death with a short effective time frame in excitotoxic experimental models in vivo

VEGF (vascular endothelial growth factor) prevents neuronal death in different models of ALS (amyotrophic lateral sclerosis), but few studies have addressed the efficacy of VEGF to protect motor neurons after the onset of symptoms, a critical point when considering VEGF as a potential therapeutic target for ALS. We studied the capability of VEGF to protect motor neurons after an excitotoxic challenge in two models of spinal neurodegeneration in rats induced by AMPA (α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) administered either chronically with osmotic minipumps or acutely by microdialysis. VEGF was administered through osmotic minipumps in the chronic model or injected intracerebroventricularly in the acute model, and its effects were assessed by immunohistochemical and histological analyses and motor performance tests. In the chronic model, VEGF stopped the progression of the paralysis and protected motor neurons when administered after AMPA before the onset of the motor symptoms, whereas no protection was observed when administered after the onset. VEGF was also protective in the acute model, but with a short time window, since the protection was effective when administered 1 h but not 2 h after AMPA. Our results indicate that while VEGF has an indubitable neuroprotective effect, its therapeutic potential for halting or delaying the progression of motor neuron loss in ALS would likely have a short effective time frame.

AMPA induced Ca2+ influx in motor neurons occurs through voltage gated Ca2+ channel and Ca2+ permeable AMPA receptor

The rise in intracellular Ca(2+) mediated by AMPA subtype of glutamate receptors has been implicated in the pathogenesis of motor neuron disease, but the exact route of Ca(2+) entry into motor neurons is not clearly known. In the present study, we examined the role of voltage gated calcium channels (VGCCs) in AMPA induced Ca(2+) influx and subsequent intracellular signaling events responsible for motor neuron degeneration. AMPA stimulation caused sodium influx in spinal neurons that would depolarize the plasma membrane. The AMPA induced [Ca(2+)](i) rise in motor neurons as well as other spinal neurons was drastically reduced when extracellular sodium was replaced with NMDG, suggesting the involvement of voltage gated calcium channels. AMPA mediated rise in [Ca(2+)](i) was significantly inhibited by L-type VGCC blocker nifedipine, whereas ω-agatoxin-IVA and ω-conotoxin-GVIA, specific blockers of P/Q type and N-type VGCC were not effective. 1-Napthyl-acetyl spermine (NAS), an antagonist of Ca(2+) permeable AMPA receptors partially inhibited the AMPA induced [Ca(2+)](i) rise but selectively in motor neurons. Measurement of AMPA induced currents in whole cell voltage clamp mode suggests that a moderate amount of Ca(2+) influx occurs through Ca(2+) permeable AMPA receptors in a subpopulation of motor neurons. The AMPA induced mitochondrial calcium loading [Ca(2+)](m), mitochondrial depolarization and neurotoxicity were also significantly reduced in presence of nifedipine. Activation of VGCCs by depolarizing concentration of KCl (30mM) in extracellular medium increased the [Ca(2+)](i) but no change was observed in mitochondrial Ca(2+) and membrane potential. Our results demonstrate that a subpopulation of motor neurons express Ca(2+) permeable AMPA receptors, however the larger part of Ca(2+) influx occurs through L-type VGCCs subsequent to AMPA receptor activation and consequent mitochondrial dysfunction is the trigger for motor neuron degeneration. Nifedipine is an effective protective agent against AMPA induced mitochondrial stress and degeneration of motor neurons.

Reduction in the motoneuron inhibitory/excitatory synaptic ratio in an early-symptomatic mouse model of amyotrophic lateral sclerosis

Excitotoxicity is a widely studied mechanism underlying motoneuron degeneration in amyotrophic lateral sclerosis (ALS). Synaptic alterations that produce an imbalance in the ratio of inhibitory/excitatory synapses are expected to promote or protect against motoneuron excitotoxicity. In ALS patients, motoneurons suffer a reduction in their synaptic coverage, as in the transition from the presymptomatic (2-month-old) to early-symptomatic (3-month-old) stage of the hSOD1(G93A) mouse model of familial ALS. Net synapse loss resulted from inhibitory bouton loss and excitatory synapse gain. Furthermore, in 3-month-old transgenic mice, remaining inhibitory but not excitatory boutons attached to motoneurons showed reduction in the active zone length and in the spatial density of synaptic vesicles in the releasable pool near the active zone. Bouton degeneration/loss seems to be mediated by bouton vacuolization and by mechanical displacement due to swelling vacuolated dendrites. In addition, chronic treatment with a nitric oxide (NO) synthase inhibitor avoided inhibitory loss but not excitatory gain. These results indicate that NO mediates inhibitory loss occurring from the pre- to early-symptomatic stage of hSOD1(G93A) mice. This work contributes new insights on ALS pathogenesis, recognizing synaptic re-arrangement onto motoneurons as a mechanism favoring disease progression rather than as a protective homeostatic response against excitotoxic events.

Zinc pre-treatment enhances NMDAR-mediated excitotoxicity in cultured cortical neurons from SOD1(G93A) mouse, a model of amyotrophic lateral sclerosis

Zn²+ is co-released at glutamatergic synapses throughout the central nervous system and acts as a neuromodulator for glutamatergic neurotransmission, as a key modulator of NMDA receptor functioning. Zn²+ is also implicated in the neurotoxicity associated with several models of acute brain injury and neurodegeneration. Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease affecting motor neurons in the spinal cord and cortex. In this study, we have investigated the modulatory role exerted by Zn²+ in NMDA-mediated neurotoxicity in either near-pure or mixed cortical cultured neurons obtained from either mice over-expressing the G93A mutant form of Cu/Zn superoxide dismutase (SOD1) human gene, a gene linked to familial ALS, or wild type (WT) mice. To that aim, SOD1(G93A) or WT cultures were exposed to either NMDA by itself or to Zn²+ prior to a toxic challenge with NMDA, and neuronal loss evaluated 24 h later. While we failed to observe any significant difference between NMDA and Zn²+/NMDA-mediated toxicity in mixed SOD1(G93A) or WT cortical cultures, different vulnerability to these toxic paradigms was found in near-pure neuronal cultures. In the WT near-pure neuronal cultures, a brief exposure to sublethal concentrations of Zn²+-enhanced NMDA receptor-mediated cell death, an effect that was far more pronounced in the SOD1(G93A) cultures. This increased excitotoxicity in SOD1(G93A) near-pure neuronal cultures appears to be mediated by a significant increase in NMDA-dependent rises of intraneuronal Ca²+ levels as well as enhanced production of cytosolic reactive oxygen species, while the injurious process seems to be unrelated to activation of nNOS or ERK1/2 pathways. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Persistent inward currents in spinal motoneurons: important for normal function but potentially harmful after spinal cord injury and in amyotrophic lateral sclerosis

Meaningful body movements depend on the interplay between synaptic inputs to motoneurons and their intrinsic properties. Injury and disease often alter either or both of these factors and cause motoneuron and movement dysfunction. The ability of the motoneuronal membrane to generate persistent inward currents (PICs) is especially potent in setting the intrinsic excitability of motoneurons and can drastically change the motoneuron output to a given input. In this article, we review the role of PICs in modulating the excitability of spinal motoneurons during health, and their contribution to motoneuron excitability after spinal cord injury (SCI) and in amyotrophic lateral sclerosis (ALS) leading to exaggerated long-lasting reflexes and muscle spasms, and contributing to neuronal degeneration, respectively.

The neurobiology of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a degenerative disease affecting the motor neurons. In spite of our growing insights into its biology, it remains a lethal condition. The identification of the cause of several of the familial forms of ALS allowed generation of models to study this disease both in vitro and in vivo. Here, we summarize what is known about the pathogenic mechanisms of ALS induced by hereditary mutations, and attempt to identify the relevance of these findings for understanding the pathogenic mechanisms of the sporadic form of this disease.

Unraveling the mechanism of β-N-oxalyl-α,β-diaminopropionic acid (β-ODAP) induced excitotoxicity and oxidative stress, relevance for neurolathyrism prevention

β-N-Oxalyl-α,β-diaminopropionic acid (β-ODAP) is a plant metabolite present in Lathyrus sativus (L. Sativus) seeds that is proposed to be responsible for the neurodegenerative disease neurolathyrism. This excitatory amino acid binds to α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors and several lines of evidence indicate that β-ODAP triggers motor neuron degeneration by inducing excitotoxic cell death and increasing oxidative stress. In addition, this toxin is known to disturb the mitochondrial respiration chain and recent data indicate that β-ODAP may inhibit the uptake of cystine thereby compromising the cells' abilities to cope with oxidative stress. Recent work from our group furthermore suggests that β-ODAP disturbs the cellular Ca(2+) homeostasis machinery with increased Ca(2+) loading in the endoplasmic reticulum (ER)-mitochondrial axis. In this review, we aim to integrate the various mechanistic levels of β-ODAP toxicity into a consistent pathophysiological picture. Interestingly, the proposed cascade contains several aspects that are common with other neurodegenerative diseases, for example amyotrophic lateral sclerosis (ALS). Based on these mechanistic insights, we conclude that dietary supplementation with methionine (Met) and cysteine (Cys) may significantly lower the risk for neurolathyrism and can thus be considered, in line with epidemiological data, as a preventive measure for neurolathyrism.

Tumor necrosis factor alpha enhances glutamatergic transmission onto spinal motoneurons

The early stages of spinal cord injury (SCI) start with excitotoxic damage caused by a massive release of glutamate. However, glutamate release is not the only factor to consider. Inflammatory molecules like tumor necrosis factor alpha (TNFalpha), belonging to a group of cytokines initially identified and named for their ability to kill tumor cells, is also a key factor in neuronal death and inflammation. TNFalpha is released from macrophages and activated microglia following a SCI, reaching a peak 1 h after the primary injury. Motoneurons whose survival is necessary for successful rehabilitation are especially vulnerable to the effects of TNFalpha release. While TNFalpha has been postulated to increase glutamatergic synaptic transmission, evidence for this has been indirect. Here, we show using whole-cell recording from lumbar motoneurons that AMPA and NMDA receptor-mediated excitatory postsynaptic currents are rapidly increased following bath application of TNFalpha. Concurrently, the single-channel open probability of AMPA and NMDA channels were also augmented by TNFalpha. Overall, our data lead us to propose the idea that motoneuronal vulnerability to excitotoxicity is not only due to the excessive release of glutamate, but may also be attributable to the increased sensitivity of AMPARs and NMDARs to the proinflammatory factor, TNFalpha, released after SCI.

Transient Recovery in a Rat Model of Familial Amyotrophic Lateral Sclerosis after Transplantation of Motor Neurons Derived from Mouse Embryonic Stem Cells

Embryonic stem (ES) cells can be induced to differentiate into motor neurons (MN). Animal models resembling MN degeneration and paralysis observed in familial amyotrophic lateral sclerosis (ALS) have been previously reported. In this work, we aimed to investigate whether transplanted MN could prevent motor deterioration in transgenic rats expressing a mutant form of human superoxide dismutase 1 (hSOD1G93A) associated with inherited ALS. Mouse ES cells were differentiated to neurons that express green fluorescent protein (GFP) under the promoter of the MN-specific gene hb9, as well as molecular markers indicative of MN identity. Cells were grafted into the lumbar spinal cord of adult wild-type (WT) or hSOD1G93A rats at 10 weeks of age, when transgenic animals are presymptomatic. Grafted cells with MN phenotype can survive for at least 1 week in hSOD1G93A animals. To quantitatively evaluate motor performance of WT and transgenic rats, we carried out weekly rotarod tests starting when the animals were 14 weeks old. Sham and grafted WT animals showed no decline in their ability to sustain themselves on the rotating rod. In contrast, sham hSOD1G93A rats decreased in motor performance from week 16 onwards, reaching paralysis by week 19 of age. In grafted transgenic animals, there was a significant improvement in rotarod competence at weeks 16 and 17 when compared to sham hSOD1G93A. However, in the following weeks, transplanted hSOD1G93A rats showed motor deterioration and eventually exhibited paralysis by week 19. At end-stage, we found only a few endogenous MN in sham and grafted hSOD1G93A rats by cresyl violet staining; no choline acetyl transferase-positive nor GFP-positive MN were present in grafted transgenic subjects. In contrast, WT rats analyzed at the same age possessed grafted GFP-positive MN in their spinal cords. These results strongly suggest that the transgenic hSOD1G93A environment is detrimental to grafted MN in the long term.

Experimental models for the study of neurodegeneration in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease of unknown cause, characterized by the selective and progressive death of both upper and lower motoneurons, leading to a progressive paralysis. Experimental animal models of the disease may provide knowledge of the pathophysiological mechanisms and allow the design and testing of therapeutic strategies, provided that they mimic as close as possible the symptoms and temporal progression of the human disease. The principal hypotheses proposed to explain the mechanisms of motoneuron degeneration have been studied mostly in models in vitro, such as primary cultures of fetal motoneurons, organotypic cultures of spinal cord sections from postnatal rodents and the motoneuron-like hybridoma cell line NSC-34. However, these models are flawed in the sense that they do not allow a direct correlation between motoneuron death and its physical consequences like paralysis. In vivo, the most widely used model is the transgenic mouse that bears a human mutant superoxide dismutase 1, the only known cause of ALS. The major disadvantage of this model is that it represents about 2%-3% of human ALS. In addition, there is a growing concern on the accuracy of these transgenic models and the extrapolations of the findings made in these animals to the clinics. Models of spontaneous motoneuron disease, like the wobbler and pmn mice, have been used aiming to understand the basic cellular mechanisms of motoneuron diseases, but these abnormalities are probably different from those occurring in ALS. Therefore, the design and testing of in vivo models of sporadic ALS, which accounts for >90% of the disease, is necessary. The main models of this type are based on the excitotoxic death of spinal motoneurons and might be useful even when there is no definitive demonstration that excitotoxicity is a cause of human ALS. Despite their difficulties, these models offer the best possibility to establish valid correlations between cellular alterations and motor behavior, although improvements are still necessary in order to produce a reliable and integrative model that accurately reproduces the cellular mechanisms of motoneuron degeneration in ALS.

Glutamate transporters and the excitotoxic path to motor neuron degeneration in amyotrophic lateral sclerosis

Responsible for the majority of excitatory activity in the central nervous system (CNS), glutamate interacts with a range of specific receptor and transporter systems to establish a functional synapse. Excessive stimulation of glutamate receptors causes excitotoxicity, a phenomenon implicated in both acute and chronic neurodegenerative diseases [e.g., ischemia, Huntington's disease, and amyotrophic lateral sclerosis (ALS)]. In physiology, excitotoxicity is prevented by rapid binding and clearance of synaptic released glutamate by high-affinity, Na(+)-dependent glutamate transporters and amplified by defects to the glutamate transporter and receptor systems. ALS pathogenetic mechanisms are not completely understood and characterized, but excitotoxicity has been regarded as one firm mechanism implicated in the disease because of data obtained from ALS patients and animal and cellular models as well as inferred by the documented efficacy of riluzole, a generic antiglutamatergic drug, has in patients. In this article, we critically review the several lines of evidence supporting a role for glutamate-mediated excitotoxicity in the death of motor neurons occurring in ALS, putting a particular emphasis on the impairment of the glutamate-transport system.

Chronic elevation of extracellular glutamate due to transport blockade is innocuous for spinal motoneurons in vivo

Glutamate-mediated excitotoxicity has been considered to play an important role in the mechanism of spinal motoneuron death in amyotrophic lateral sclerosis (ALS), and some reports suggest that this excitotoxicity may be due to a decreased glutamate transport and the consequent elevation of its extracellular level. We have previously shown that short lasting increments in extracellular glutamate due to administration of the non-selective glutamate transport blocker l-2,4-trans-pyrrolidine-dicarboxylate (PDC) by microdialysis in the rat spinal cord do not induce motoneuron damage. In the present work we examined the potential involvement of chronic glutamate transport blockade as a causative factor of spinal motoneuron death and paralysis in vivo. Using osmotic minipumps, we infused directly in the spinal cord for up to 10 days PDC and another glutamate transport blocker, dl-threo-beta-benzyloxyaspartate (TBOA), and we measured by means of microdialysis and HPLC the extracellular concentration of glutamate and other amino acids. We found that after the infusion of both PDC and TBOA the concentration of endogenous extracellular glutamate was 3-4-fold higher than that of the controls. Nevertheless, in spite of this elevation no motoneuron degeneration or gliosis were observed, assessed by histological examination and choline acetyltransferase and glial fibrillary acidic protein immunocytochemistry. In accord with this lack of toxic effect, no motor deficits, assessed by three motor activity tests, were observed. Because we had previously shown that under identical experimental conditions the infusion of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) induced progressive motoneuron death and paralysis, we conclude that prolonged elevation of extracellular glutamate due to its transport blockade in vivo is innocuous for spinal motoneurons and therefore that these results do not support the hypothesis that glutamate transport deficiency plays a crucial role as a causal factor of spinal motoneuron degeneration in ALS.

Altered presymptomatic AMPA and cannabinoid receptor trafficking in motor neurons of ALS model mice: implications for excitotoxicity

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder involving the selective loss of spinal cord motor neurons. Excitotoxicity mediated by glutamate has been implicated as a cause of this progressive degeneration. In this study we examined two types of receptors, the excitatory alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptors (AMPARs) and inhibitory cannabinoid receptor (CB1) with respect to their localization and total expression in spinal cord motor neurons. AMPAR and CB1 represent major excitatory and inhibitory transmission input, respectively, and their expression levels on the plasma membrane have direct relevance to the vulnerability of the motor neurons to glutamatergic excitotoxicity. We used quantitative immunofluorescence microscopy to comparatively measure the total cellular expression and the synaptic localization of specific subclasses of AMPARs [as determined by the presence of the subunits glutamate receptor 1 (GluR1) or glutamate receptor 2 (GluR2)] and CB1 in spinal cord motor neurons during disease progression in a G93ASOD1 mouse model of ALS. We found an increase in synaptic GluR1 and a decrease of synaptic and total GluR2 at early ages (6 weeks, prior to disease onset). Total CB1 receptor levels were decreased at 6 weeks old. We determined the gene expression of CB1, GluR1 and GluR2 using quantitative real-time reverse transcriptase-polymerase chain reaction. The decreased synaptic and total GluR2 and increased synaptic GluR1 levels may result in increased numbers of Ca2+-permeable AMPARs, thus contributing to neuronal death. Early alterations in CB1 expression may also predispose motor neurons to excitotoxicity. To our knowledge, this is the first demonstration of presymptomatic changes in trafficking of receptors that are in direct control of excitotoxicity and death in a mouse model of ALS.

Rapid tumor necrosis factor alpha-induced exocytosis of glutamate receptor 2-lacking AMPA receptors to extrasynaptic plasma membrane potentiates excitotoxicity

The postinjury inflammatory response in the CNS leads to neuronal excitotoxicity. Our previous studies show that a major component of this response, the inflammatory cytokine tumor necrosis factor alpha (TNFalpha), causes a rapid increase in AMPA glutamate receptors (AMPARs) on the plasma membrane of cultured hippocampal neurons. This may potentiate neuron death through an increased vulnerability to AMPAR-dependent excitotoxic stress. Here, we test this hypothesis with an in vitro lactose dehydrogenase death assay and examine in detail the AMPAR surface delivery time course, receptor subtype, and synaptic and extrasynaptic distribution after TNFalpha exposure. These data demonstrate that surface levels of glutamate receptor 2 (GluR2)-lacking Ca2+-permeable AMPARs peak at 15 min after TNFalpha treatment, and the majority are directed to extrasynaptic sites. TNFalpha also induces an increase in GluR2-containing surface AMPARs but with a slower time course. We propose that this activity contributes to excitotoxic neuron death because TNFalpha potentiation of kainate excitotoxicity is blocked by a Ca2+-permeable AMPAR antagonist [NASPM (1-naphthyl acetyl spermine)] and a specific phosphoinositide 3 kinase (PI3 kinase) inhibitor (LY294,002 [2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one]) previously shown to block the TNFalpha-induced increase in AMPAR surface delivery. This information forms the basis for future in vivo studies examining AMPAR-dependent potentiation of excitotoxic neuron death and dysfunction caused by TNFalpha after acute injury and during neurodegenerative or neuropsychiatric disorders.

Glutamate excitotoxicity and therapeutic targets for amyotrophic lateral sclerosis

Two forms of amyotrophic lateral sclerosis (ALS) are known, the familial (FALS), due in part to mutations in superoxide dismutase 1 (SOD1), and the sporadic (SALS), which accounts for > 90% of all cases. The cause of SALS is not known, but excitotoxicity due to overactivation of glutamate receptors may mediate the motor neuron degeneration in the spinal cord, which is the hallmark of this disease. Overactivation of calcium-permeable alpha-amino-3-hydroxy-5-isoxazole propionate receptors lacking the subunit glutamate receptor 2, leading to an increase in calcium cytoplasmic concentration, seems to play an important role in the mechanism of neuronal death. The knowledge of this mechanism, in addition to other factors, provides several possible targets for therapeutic strategies that are reviewed in this article. Some of these strategies have proven to be partially effective in both human mutant superoxide dismutase 1 transgenic rodents (FALS model) and the few existing in vivo models of spinal motor neurodegeneration induced by excitotoxicity (SALS models), although observable benefits are still to be shown in clinical trials.