Fibroblast glutamate transport in aging and in AD: correlations with disease severity

Metabolomics to Study Human Aging: A Review

In the last years, with the increase in the average life expectancy, the world's population is progressively aging, which entails social, health and economic problems. In this sense, the need to better understand the physiology of the aging process becomes an urgent need. Since the study of aging in humans is challenging, cellular and animal models are widely used as alternatives. Omics, namely metabolomics, have emerged in the study of aging, with the aim of biomarker discovering, which may help to uncomplicate this complex process. This paper aims to summarize different models used for aging studies with their advantages and limitations. Also, this review gathers the published articles referring to biomarkers of aging already discovered using metabolomics approaches, comparing the results obtained in the different studies. Finally, the most frequently used senescence biomarkers are described, along with their importance in understanding aging.

Complex Processes Underlying the Dynamic Changes of D-serine Levels in AD Brains

BACKGROUND

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by extracellular β-amyloid (Aβ) plaques and cognitive impairments. D-Serine, produced by the enzyme serine racemase (SR) in the brain, functions as an endogenous co-agonist at the glycine-binding site of N-methyl-D-aspartate receptor (NMDAR), has been implicated in the pathophysiological progression of AD.

OBJECTIVES

Evidence regarding the understanding of the role and dynamic modulation of D-serine during AD progression remains controversial. This literature review aims to offer novel research directions for studying the functions and metabolisms of D-serine in AD brains.

METHODS

We searched PubMed, using D-serine/SR and AD as keywords. Studies related to NMDAR dysfunction, neuronal excitotoxicity, D-serine dynamic changes and inflammatory response were included.

RESULTS

This review primarily discusses: (i) Aβ oligomers' role in NMDAR dysregulation, and the subsequent synaptic dysfunction and neuronal damage in AD, (ii) D-serine's role in NMDAR-elicited excitotoxicity, and (iii) the involvement of D-serine and SR in AD-related inflammatory pathological progression.

CONCLUSION

We also presented supposed metabolism and dynamic changes of D-serine during AD progression and hypothesized that: (i) the possible modulation of D-serine levels or SR expression as an effective method of alleviating neurotoxicity during AD pathophysiological progression, and (ii) the dynamic changes of D-serine levels in AD brains possibly resulting from complex processes.

Human Dermal Fibroblast: A Promising Cellular Model to Study Biological Mechanisms of Major Depression and Antidepressant Drug Response

BACKGROUND

Human dermal fibroblasts (HDF) can be used as a cellular model relatively easily and without genetic engineering. Therefore, HDF represent an interesting tool to study several human diseases including psychiatric disorders. Despite major depressive disorder (MDD) being the second cause of disability in the world, the efficacy of antidepressant drug (AD) treatment is not sufficient and the underlying mechanisms of MDD and the mechanisms of action of AD are poorly understood.

OBJECTIVE

The aim of this review is to highlight the potential of HDF in the study of cellular mechanisms involved in MDD pathophysiology and in the action of AD response.

METHODS

The first part is a systematic review following PRISMA guidelines on the use of HDF in MDD research. The second part reports the mechanisms and molecules both present in HDF and relevant regarding MDD pathophysiology and AD mechanisms of action.

RESULTS

HDFs from MDD patients have been investigated in a relatively small number of works and most of them focused on the adrenergic pathway and metabolism-related gene expression as compared to HDF from healthy controls. The second part listed an important number of papers demonstrating the presence of many molecular processes in HDF, involved in MDD and AD mechanisms of action.

CONCLUSION

The imbalance in the number of papers between the two parts highlights the great and still underused potential of HDF, which stands out as a very promising tool in our understanding of MDD and AD mechanisms of action.

Circadian entrainment by food and drugs of abuse

Circadian rhythms organize behavior and physiological processes to be appropriate to the predictable cycle of daily events. These rhythms are entrained by stimuli that provide time of day cues (zeitgebers), such as light, which regulates the sleep-wake cycle and associated rhythms. But other events, including meals, social cues, and bouts of locomotor activity, can act as zeitgebers. Recent evidence shows that most organs and tissues contain cells that are capable of some degree of independent circadian cycling, suggesting the circadian system is broadly and diffusely distributed. Within laboratory studies of behavior, circadian rhythms tend to be treated as a complication to be minimized, but they offer a useful model of predictable shifts in behavioral tendencies. In the present review, we summarize the evidence that formed the basis for a hypothesis that drugs of abuse can entrain circadian rhythms and describe the outcome of a series of experiments designed to test that hypothesis. We propose that such drug-entrained rhythms may contribute to demonstrated daily variations in drug metabolism, tolerance, and sensitivity to drug reward. Of particular importance, these rhythms may be evoked by a single episode of drug taking, strengthen with repeated episodes, and re-emerge after long periods of abstinence, thereby contributing to drug abuse, addiction, and relapse.

Circadian Regulation of Glutamate Transporters

L-glutamate is the major excitatory amino acid in the mammalian central nervous system (CNS). This neurotransmitter is essential for higher brain functions such as learning, cognition and memory. A tight regulation of extra-synaptic glutamate levels is needed to prevent a neurotoxic insult. Glutamate removal from the synaptic cleft is carried out by a family of sodium-dependent high-affinity transporters, collectively known as excitatory amino acid transporters. Dysfunction of glutamate transporters is generally involved in acute neuronal injury and neurodegenerative diseases, so characterizing and understanding the mechanisms that lead to the development of these disorders is an important goal in the design of novel treatments for the neurodegenerative diseases. Increasing evidence indicates glutamate transporters are controlled by the circadian system in direct and indirect manners, so in this contribution we focus on the mechanisms of circadian regulation (transcriptional, translational, post-translational and post-transcriptional regulation) of glutamate transport in neuronal and glial cells, and their consequence in brain function.

Vitamin E-Mediated Modulation of Glutamate Receptor Expression in an Oxidative Stress Model of Neural Cells Derived from Embryonic Stem Cell Cultures

Glutamate is the primary excitatory neurotransmitter in the central nervous system. Excessive concentrations of glutamate in the brain can be excitotoxic and cause oxidative stress, which is associated with Alzheimer's disease. In the present study, the effects of vitamin E in the form of tocotrienol-rich fraction (TRF) and alpha-tocopherol (α-TCP) in modulating the glutamate receptor and neuron injury markers in an in vitro model of oxidative stress in neural-derived embryonic stem (ES) cell cultures were elucidated. A transgenic mouse ES cell line (46C) was differentiated into a neural lineage in vitro via induction with retinoic acid. These cells were then subjected to oxidative stress with a significantly high concentration of glutamate. Measurement of reactive oxygen species (ROS) was performed after inducing glutamate excitotoxicity, and recovery from this toxicity in response to vitamin E was determined. The gene expression levels of glutamate receptors and neuron-specific enolase were elucidated using real-time PCR. The results reveal that neural cells derived from 46C cells and subjected to oxidative stress exhibit downregulation of NMDA, kainate receptor, and NSE after posttreatment with different concentrations of TRF and α-TCP, a sign of neurorecovery. Treatment of either TRF or α-TCP reduced the levels of ROS in neural cells subjected to glutamate-induced oxidative stress; these results indicated that vitamin E is a potent antioxidant.

Glutaminase C overexpression in the brain induces learning deficits, synaptic dysfunctions, and neuroinflammation in mice

Glutaminolysis, a metabolic process that converts glutamine to glutamate, is particularly important for the central nervous system since glutamate is the major transmitter of excitatory synapses. Glutaminase is the mitochondrial enzyme that catalyzes the first step of glutaminolysis. Two genes encode at least four isoforms of glutaminase in humans. Gls1 gene encodes isoforms kidney-type glutaminase (KGA) and glutaminase C (GAC) through alternative splicing, whereas Gls2 gene encodes liver-type glutaminase isoforms. KGA and GAC have been associated with several neurological diseases. However, it remains unclear whether changes in their expressions can directly cause brain abnormalities. Using a transgenic approach, we generated mice that overexpressed GAC in the brain. The resulting transgenic mice had severe impairments in spatial and fear learning compared with littermate controls. The learning deficits were consistent with diminished hippocampal long-term potentiation in the hippocampal slices of the GAC transgenic mice. Furthermore, we found increases in astrocyte and microglia markers, inflammatory factors, and a decrease in synapse marker synaptophysin, suggesting neuroinflammation and synaptic changes in the GAC transgenic mouse brains. In conclusion, these findings provide the first evidence that GAC overexpression in the brain has deleterious effects on learning and synaptic integrity in vivo.

Sleep deprivation induces differential morphological changes in the hippocampus and prefrontal cortex in young and old rats

Sleep is a fundamental state necessary for maintenance of physical and neurological homeostasis throughout life. Several studies regarding the functions of sleep have been focused on effects of sleep deprivation on synaptic plasticity at a molecular and electrophysiological level, and only a few studies have studied sleep function from a structural perspective. Moreover, during normal aging, sleep architecture displays some changes that could affect normal development in the elderly. In this study, using a Golgi-Cox staining followed by Sholl analysis, we evaluate the effects of 24 h of total sleep deprivation on neuronal morphology of pyramidal neurons from Layer III of the prefrontal cortex (PFC) and the dorsal hippocampal CA1 region from male Wistar rats at two different ages (3 and 22 months). We found no differences in total dendritic length and branching length in both analyzed regions after sleep deprivation. Spine density was reduced in the CA1 of young-adults, and interestingly, sleep deprivation increased spine density in PFC of aged animals. Taken together, our results show that 24 h of total sleep deprivation have different effects on synaptic plasticity and could play a beneficial role in cognition during aging.

Gene expression patterns in the hippocampus during the development and aging of Glud1 (Glutamate Dehydrogenase 1) transgenic and wild type mice

Background

Extraneuronal levels of the neurotransmitter glutamate in brain rise during aging. This is thought to lead to synaptic dysfunction and neuronal injury or death. To study the effects of glutamate hyperactivity in brain, we created transgenic (Tg) mice in which the gene for glutamate dehydrogenase (Glud1) is over-expressed in neurons and in which such overexpression leads to excess synaptic release of glutamate. In this study, we analyzed whole genome expression in the hippocampus, a region important for learning and memory, of 10 day to 20 month old Glud1 and wild type (wt) mice.

Results

During development, maturation and aging, both Tg and wt exhibited decreases in the expression of genes related to neurogenesis, neuronal migration, growth, and process elongation, and increases in genes related to neuro-inflammation, voltage-gated channel activity, and regulation of synaptic transmission. Categories of genes that were differentially expressed in Tg vs. wt during development were: synaptic function, cytoskeleton, protein ubiquitination, and mitochondria; and, those differentially expressed during aging were: synaptic function, vesicle transport, calcium signaling, protein kinase activity, cytoskeleton, neuron projection, mitochondria, and protein ubiquitination. Overall, the effects of Glud1 overexpression on the hippocampus transcriptome were greater in the mature and aged than the young.

Conclusions

Glutamate hyperactivity caused gene expression changes in the hippocampus at all ages. Some of these changes may result in premature brain aging. The identification of these genomic expression differences is important in understanding the effects of glutamate dysregulation on neuronal function during aging or in neurodegenerative diseases.

Metabolism changes during aging in the hippocampus and striatum of glud1 (glutamate dehydrogenase 1) transgenic mice

The decline in neuronal function during aging may result from increases in extracellular glutamate (Glu), Glu-induced neurotoxicity, and altered mitochondrial metabolism. To study metabolic responses to persistently high levels of Glu at synapses during aging, we used transgenic (Tg) mice that over-express the enzyme Glu dehydrogenase (GDH) in brain neurons and release excess Glu in synapses. Mitochondrial GDH is important in amino acid and carbohydrate metabolism and in anaplerotic reactions. We monitored changes in nineteen neurochemicals in the hippocampus and striatum of adult, middle aged, and aged Tg and wild type (wt) mice, in vivo, using proton ((1)H) magnetic resonance spectroscopy. Significant differences between adult Tg and wt were higher Glu, N-acetyl aspartate (NAA), and NAA + NAA-Glu (NAAG) levels, and lower lactate in the Tg hippocampus and striatum than those of wt. During aging, consistent changes in Tg and wt hippocampus and striatum included increases in myo-inositol and NAAG. The levels of glutamine (Gln), a key neurochemical in the Gln-Glu cycle between neurons and astroglia, increased during aging in both the striatum and hippocampus of Tg mice, but only in the striatum of the wt mice. Age-related increases of Glu were observed only in the striatum of the Tg mice.

Dysregulation of synaptic and extrasynaptic N-methyl-D-aspartate receptors induced by amyloid-β

The toxicity of amyloid-beta (Aβ) is strongly associated with Alzheimer's disease (AD), which has a high incidence in the elderly worldwide. Recent evidence showed that alteration in the activity of N-methyl-D-aspartate receptors (NMDARs) plays a key role in Aβ-induced neurotoxicity. However, the activation of synaptic and extrasynaptic NMDARs has distinct consequences for plasticity, gene regulation, neuronal death, and Aβ production. This review focuses on the dysregulation of synaptic and extrasynaptic NMDARs induced by Aβ. On one hand, Aβ downregulates the synaptic NMDAR response by promoting NMDAR endocytosis, leading to either neurotoxicity or neuroprotection. On the other hand, Aβ enhances the activation of extrasynaptic NMDARs by decreasing neuronal glutamate uptake and inducing glutamate spillover, subsequently causing neurotoxicity. In addition, selective enhancement of synaptic activity by low doses of NMDA, or reduction of extrasynaptic activity by memantine, a non-competitive NMDAR antagonist, halts Aβ-induced neurotoxicity. Therefore, future neuroprotective drugs for AD should aim at both the enhancement of synaptic activity and the disruption of extrasynaptic NMDAR-dependent death signaling.

Dock3 attenuates neural cell death due to NMDA neurotoxicity and oxidative stress in a mouse model of normal tension glaucoma

Dedicator of cytokinesis 3 (Dock3), a new member of the guanine nucleotide exchange factors for the small GTPase Rac1, promotes axon regeneration following optic nerve injury. In the present study, we found that Dock3 directly binds to the intracellular C-terminus domain of NR2B, an N-methyl-D-aspartate (NMDA) receptor subunit. In transgenic mice overexpressing Dock3 (Dock3 Tg), NR2B expression in the retina was significantly decreased and NMDA-induced retinal degeneration was ameliorated. In addition, overexpression of Dock3 protected retinal ganglion cells (RGCs) from oxidative stress. We previously reported that glutamate/aspartate transporter (GLAST) is a major glutamate transporter in the retina, and RGC degeneration due to glutamate neurotoxicity and oxidative stress is observed in GLAST-deficient (KO) mice. In GLAST KO mice, the NR2B phosphorylation rate in the retina was significantly higher compared with Dock3 Tg:GLAST KO mice. Consistently, glaucomatous retinal degeneration was significantly improved in GLAST KO:Dock3 Tg mice compared with GLAST KO mice. These results suggest that Dock3 overexpression prevents glaucomatous retinal degeneration by suppressing both NR2B-mediated glutamate neurotoxicity and oxidative stress, and identifies Dock3 signaling as a potential therapeutic target for both neuroprotection and axonal regeneration.

IL-1β and TNF-α induce neurotoxicity through glutamate production: a potential role for neuronal glutaminase

Glutaminase 1 is the main enzyme responsible for glutamate production in mammalian cells. The roles of macrophage and microglia glutaminases in brain injury, infection, and inflammation are well documented. However, little is known about the regulation of neuronal glutaminase, despite neurons being a predominant cell type of glutaminase expression. Using primary rat and human neuronal cultures, we confirmed that interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), two pro-inflammatory cytokines that are typically elevated in neurodegenerative disease states, induced neuronal death and apoptosis in vitro. Furthermore, both intracellular and extracellular glutamate levels were significantly elevated following IL-1β and/or TNF-α treatment. Pre-treatment with N-Methyl-D-aspartate (NMDA) receptor antagonist MK-801 blocked cytokine-induced glutamate production and alleviated the neurotoxicity, indicating that IL-1β and/or TNF-α induce neurotoxicity through glutamate. To determine the potential source of excess glutamate production in the culture during inflammation, we investigated the neuronal glutaminase and found that treatment with IL-1β or TNF-α significantly upregulated the kidney-type glutaminase (KGA), a glutaminase 1 isoform, in primary human neurons. The up-regulation of neuronal glutaminase was also demonstrated in situ in a murine model of HIV-1 encephalitis. In addition, IL-1β or TNF-α treatment increased the levels of KGA in cytosol and TNF-α specifically increased KGA levels in the extracellular fluid, away from its main residence in mitochondria. Together, these findings support neuronal glutaminase as a potential component of neurotoxicity during inflammation and that modulation of glutaminase may provide therapeutic avenues for neurodegenerative diseases.

Isolation and characterization of living primary astroglial cells using the new GLAST-specific monoclonal antibody ACSA-1

Astrocytes show large morphological and functional heterogeneity and are involved in many aspects of neural function. Progress in defining astrocyte subpopulations has been hampered by the lack of a suitable antibody for their direct detection and isolation. Here, we describe a new monoclonal antibody, ACSA-1, which was generated by immunization of GLAST1 knockout mice. The antibody specifically detects an extracellular epitope of the astrocyte-specific L-glutamate/L-aspartate transporter GLAST (EAAT1, Slc1a3). As shown by immunohistochemistry, immunocytochemistry, and flow cytometry, ACSA-1 was cross-reactive for mouse, human, and rat. It labeled virtually all astrocytes positive for GFAP, GS, BLBP, RC2, and Nestin, including protoplastic, fibrous, and reactive astrocytes as well as Bergmann glia, Müller glia, and radial glia. Oligodendrocytes, microglia, neurons, and neuronal progenitors were negative for ACSA-1. Using an immunomagnetic approach, we established a method for the isolation of GLAST-positive cells with high purity. Binding of the antibody to GLAST and subsequent sorting of GLAST-positive cells neither interfered with cellular glutamate transport nor compromised astrocyte viability in vitro. The ACSA-1 antibody is not only a valuable tool to identify and track astrocytes by immunostaining, but also provides the possibility of separation and further analysis of pure astrocytes.

Role of thiamine in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia in elderly individuals and is associated with progressive neurodegeneration of the human neocortex. Thiamine levels and the activity of thiamine-dependent enzymes are reduced in the brains and peripheral tissues of patients with AD. Genetic studies have provided the opportunity to determine what proteins link thiamine to AD pathology (ie, transketolase, apolipoprotein E, α-1-antitrypsin, pyruvate dehydrogenase complex, p53, glycogen synthetase kinase-3β, c-Fos gene, the Sp1 promoter gene, and the poly(ADP-ribosyl) polymerase-1 gene). We reviewed the association between histopathogenesis and neurotransmitters to understand the relationship between thiamine and AD pathology. Oral thiamine trials have been shown to improve the cognitive function of patients with AD; however, absorption of thiamine is poor in elderly individuals. In the early stage of thiamine-deficient encephalopathy (Wernicke's encephalopathy), however, parental thiamine has been used successfully. Therefore, further studies are needed to determine the benefits of using parental thiamine as a treatment for AD.

Nonfibrillar Abeta 1-42 inhibits glutamate uptake and phosphorylates p38 in human fibroblasts

Alzheimer disease (AD) is the most prevalent neurodegenerative disease, characterized by an increased deposition of β-amyloid (Abeta) within the central nervous system, leading to neuronal death. The availability of effective models, in which confirming novel pathogenic hypotheses and developing therapeutic targets, represents a very important goal for the field of AD. Fibroblasts from these patients may be relevant models in which addressing these issues, as they display biochemical alterations mirroring SNC ones. In this work, fibroblasts obtained from controls were studied after exposure to nonfibrillar Abeta 1-42, showing decreased glutamate uptake, similar to that observed in AD cells, in absence of transporters modifications. Nonfibrillar Abeta 1-42 was able to induce in control cells mitochondrial alterations and p38-phosphorylation, mirroring similar alterations found in AD fibroblasts. Under our experimental conditions, this treatment induced neither apoptosis nor necrosis. To investigate a putative role of p38-modulation in mediating nonfibrillar Abeta 1-42 toxicity, fibroblasts from controls were pretreated with retinoic-acid, and SB203580, a p38-inhibitor. These pretreatments prevented both p38-phosphorylation and glutamate uptake inhibition. Our results suggest that nonfibrillar Abeta 1-42 downregulates glutamate transporters activity interfering with p38-activation and mitochondrial stress. Thus, modulating complex kinase signaling pathway might represent a future therapeutic target in AD.

The selective and competitive N-methyl-D-aspartate receptor antagonist, (-)-6-phosphonomethyl-deca-hydroisoquinoline-3-carboxylic acid, prevents synaptic toxicity induced by amyloid-β in mice

The toxicity of amyloid β (Aβ) is highly associated with Alzheimer's disease (AD), which has a high incidence in elderly people worldwide. While the current treatment for moderate and severe AD includes blockage of the N-methyl-d-aspartate receptor (NMDAR), the molecular mechanisms of its effect are still poorly understood. Herein, we report that a single i.p. administration of the selective and competitive (NMDAR) antagonist LY235959 reduced Aβ neurotoxicity by preventing the down-regulation of glial glutamate transporters (glutamate-aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1)), the decrease in glutamate uptake, and the production of reactive oxygen species (ROS) induced by Aβ(1-40). Importantly, the blockage of NMDAR restored the Aβ(1-40)-induced synaptic dysfunction and cognitive impairment. However, LY235959 failed to prevent the inflammatory response associated with Aβ(1-40) treatment. Altogether, our data indicate that the acute administration of Aβ promotes oxidative stress, a decrease in glutamate transporter expression, and neurotoxicity. Our results reinforce the idea that NMDAR plays a critical regulatory action in Aβ toxicity and they provide further pre-clinical evidence for the potential role of the selective and competitive NMDAR antagonists in the treatment of AD.

Expression, distribution and glutamate uptake activity of high affinity-excitatory aminoacid transporters in in vitro cultures of embryonic rat dorsal root ganglia

Glutamate is the major mediator of excitatory signalling in the mammalian central nervous system, but it has recently been shown to play a role in the transduction of sensory input at the periphery and in peripheral neuropathies. New advances in research have demonstrated that rat peripheral sensory terminals and dorsal root ganglia (DRG) express molecules involved in glutamate signalling, including high-affinity membrane-bound glutamate transporters (GLAST [glutamate aspartate transporter], GLT1 [glutamate transporter 1], EAAC1 [excitatory aminoacid transporter 1]) and that alterations in their expression and/or functionality can be implicated in several models of peripheral neuropathy, neuropathic pain and hyperalgesia. Here we describe, through immunoblotting, immunofluorescence assays and β-counter analysis of [(3)H] l-glutamate uptake, the expression, distribution and activity of the glutamate transporters in in vitro cultures of embryonic dorsal root ganglia sensory neurons, sensory neurons+satellite cells and satellite cells. In this work we demonstrated that glutamate transporters are expressed in all cultures with a peculiar pattern of distribution. Even if GLAST is strongly detected in satellite cells, it is slightly expressed also in sensory neurons. GLT1 immunostaining is very weak in DRG neurons, but it was evident in the satellite cells. Finally, EAAC1 is localized in the soma and in the neuritis of sensory neurons, while it is not detectable in satellite cells. Moreover, all the cell cultures showed a strong sodium-energy-dependent glutamate uptake activity and it is more marked in neurons alone or in co-culture with satellite cells compared to satellite cells alone. Finally, we show that the complete or partial pharmacological inhibition of glutamate transporters virtually completely or partially abolish glutamate uptake in all cell culture. These results, that demonstrate that functionally active glutamate transporters can be studied in dorsal root ganglia cell cultures, provide further evidence for a role of glutamatergic transport in the peripheral nervous system and will be useful for testing whether any changes occur in in vitro models of peripheral nervous system damage.

Transcriptomic responses in mouse brain exposed to chronic excess of the neurotransmitter glutamate

Background

Increases during aging in extracellular levels of glutamate (Glu), the major excitatory neurotransmitter in the brain, may be linked to chronic neurodegenerative diseases. Little is known about the molecular responses of neurons to chronic, moderate increases in Glu levels. Genome-wide gene expression in brain hippocampus was examined in a unique transgenic (Tg) mouse model that exhibits moderate Glu hyperactivity throughout the lifespan, the neuronal Glutamate dehydrogenase (Glud1) mouse, and littermate 9 month-old wild type mice.

Results

Integrated bioinformatic analyses on transcriptomic data were used to identify bio-functions, pathways and gene networks underlying neuronal responses to increased Glu synaptic release. Bio-functions and pathways up-regulated in Tg mice were those associated with oxidative stress, cell injury, inflammation, nervous system development, neuronal growth, and synaptic transmission. Increased gene expression in these functions and pathways indicated apparent compensatory responses offering protection against stress, promoting growth of neuronal processes (neurites) and re-establishment of synapses. The transcription of a key gene in the neurite growth network, the kinase Ptk2b, was significantly up-regulated in Tg mice as was the activated (phosphorylated) form of the protein. In addition to genes related to neurite growth and synaptic development, those associated with neuronal vesicle trafficking in the Huntington's disease signalling pathway, were also up-regulated.

Conclusions

This is the first study attempting to define neuronal gene expression patterns in response to chronic, endogenous Glu hyperactivity at brain synapses. The patterns observed were characterized by a combination of responses to stress and stimulation of nerve growth, intracellular transport and recovery.

Early brain response to low-dose radiation exposure involves molecular networks and pathways associated with cognitive functions, advanced aging and Alzheimer's disease

Understanding the cognitive and behavioral consequences of brain exposures to low-dose ionizing radiation has broad relevance for health risks from medical radiation diagnostic procedures, radiotherapy and environmental nuclear contamination as well as for Earth-orbit and space missions. Analyses of transcriptome profiles of mouse brain tissue after whole-body irradiation showed that low-dose exposures (10 cGy) induced genes not affected by high-dose radiation (2 Gy) and that low-dose genes were associated with unique pathways and functions. The low-dose response had two major components: pathways that are consistently seen across tissues and pathways that were specific for brain tissue. Low-dose genes clustered into a saturated network (P < 10(-53)) containing mostly down-regulated genes involving ion channels, long-term potentiation and depression, vascular damage, etc. We identified nine neural signaling pathways that showed a high degree of concordance in their transcriptional response in mouse brain tissue after low-dose irradiation, in the aging human brain (unirradiated), and in brain tissue from patients with Alzheimer's disease. Mice exposed to high-dose radiation did not show these effects and associations. Our findings indicate that the molecular response of the mouse brain within a few hours after low-dose irradiation involves the down-regulation of neural pathways associated with cognitive dysfunctions that are also down-regulated in normal human aging and Alzheimer's disease.

Enhanced folate binding of cultured fibroblasts from Alzheimer's disease patients

We compared the levels of serum folate from Alzheimer's disease (AD) patients and from age-matched healthy subjects and used primary cultures of fibroblasts, obtained from the two groups, to assess possible differences in their ability to bind folate. The results show that the levels of circulating folate are significantly (p<0.01; n=30) lower in AD patients than in controls (4.91+/-2.44 and 7.56+/-2.5 ng/mL, respectively). Moreover, the folate binding of AD fibroblasts is significantly (p<0.01; n=8) higher (2-4-fold) with respect to controls. RT-PCR experiments suggest that the higher folate binding could be due to an enhanced expression in AD fibroblasts of folate receptor alpha.

Neuroinflammation and regulation of glial glutamate uptake in neurological disorders

Oxidative stress, neuroinflammation, and excitotoxicity are frequently considered distinct but common hallmarks of several neurological disorders, including Parkinson's disease, amyotrophic lateral sclerosis, multiple sclerosis, and Alzheimer's disease. Although neuron degeneration and death are the ultimate consequences of these pathological processes, it is now widely accepted that alterations in the function of surrounding glial cells are key features in the progression of these diseases. In response to alteration in their local environment, microglia, commonly considered the resident immune cells of the nervous parenchyma, become activated and release a variety of soluble factors. Among these, proinflammatory cytokines and free radicals actively participate in the degenerative insults. In addition, excitotoxic neuronal damage resulting from excessive glutamate is frequently associated with impaired handling of extracellular glutamate by gliotic astrocytes. Although several research projects have focused on the biochemical mechanisms of the regulation of glial glutamate transporters, a relationship between activation of microglia and modulation of astrocytic glutamate uptake is now suggested. The aim of this review is to summarize and discuss the data showing an influence of inflammatory mediators and related free radicals on the expression and activity of glial glutamate transporters.

The role of glutamate transporters in neurodegenerative diseases and potential opportunities for intervention

Extracellular concentrations of the predominant excitatory neurotransmitter, glutamate, and related excitatory amino acids are maintained at relatively low levels to ensure an appropriate signal-to-noise ratio and to prevent excessive activation of glutamate receptors that can result in cell death. The latter phenomenon is known as 'excitotoxicity' and has been associated with a wide range of acute and chronic neurodegenerative disorders, as well as disorders that result in the loss of non-neural cells such as oligodendroglia in multiple sclerosis. Unfortunately clinical trials with glutamate receptor antagonists that would logically seem to prevent the effects of excessive receptor activation have been associated with untoward side effects or little clinical benefit. In the mammalian CNS, the extracellular concentrations of glutamate are controlled by two types of transporters; these include a family of Na(+)-dependent transporters and a cystine-glutamate exchange process, referred to as system X(c)(-). In this review, we will focus primarily on the Na(+)-dependent transporters. A brief introduction to glutamate as a neurotransmitter will be followed by an overview of the properties of these transporters, including a summary of the presumed physiologic mechanisms that regulate these transporters. Many studies have provided compelling evidence that impairing the function of these transporters can increase the sensitivity of tissue to deleterious effects of aberrant activation of glutamate receptors. Over the last decade, it has become clear that many neurodegenerative disorders are associated with a change in localization and/or expression of some of the subtypes of these transporters. This would suggest that therapies directed toward enhancing transporter expression might be beneficial. However, there is also evidence that glutamate transporters might increase the susceptibility of tissue to the consequences of insults that result in a collapse of the electrochemical gradients required for normal function such as stroke. In spite of the potential adverse effects of upregulation of glutamate transporters, there is recent evidence that upregulation of one of the glutamate transporters, GLT-1 (also called EAAT2), with beta-lactam antibiotics attenuates the damage observed in models of both acute and chronic neurodegenerative disorders. While it seems somewhat unlikely that antibiotics specifically target GLT-1 expression, these studies identify a potential strategy to limit excitotoxicity. If successful, this type of approach could have widespread utility given the large number of neurodegenerative diseases associated with decreases in transporter expression and excitotoxicity. However, given the massive effort directed at developing glutamate receptor agents during the 1990s and the relatively modest advances to date, one wonders if we will maintain the patience needed to carefully understand the glutamatergic system so that it will be successfully targeted in the future.

The potential role of glutamate transporters in the pathogenesis of normal tension glaucoma

Glaucoma, a progressive optic neuropathy due to retinal ganglion cell (RGC) degeneration, is one of the leading causes of irreversible blindness. Although glaucoma is often associated with elevated intraocular pressure (IOP), IOP elevation is not detected in a significant subset of glaucomas, such as normal tension glaucoma (NTG). Moreover, in some glaucoma patients, significant IOP reduction does not prevent progression of the disease. Thus, understanding IOP-independent mechanisms of RGC loss is important. Here, we show that mice deficient in the glutamate transporters GLAST or EAAC1 demonstrate spontaneous RGC and optic nerve degeneration without elevated IOP. In GLAST-deficient mice, the glutathione level in Müller glia was decreased; administration of glutamate receptor blocker prevented RGC loss. In EAAC1-deficient mice, RGCs were more vulnerable to oxidative stress. These findings suggest that glutamate transporters are necessary both to prevent excitotoxic retinal damage and to synthesize glutathione, a major cellular antioxidant and tripeptide of glutamate, cysteine, and glycine. We believe these mice are the first animal models of NTG that offer a powerful system for investigating mechanisms of neurodegeneration in NTG and developing therapies directed at IOP-independent mechanisms of RGC loss.

Sustained metabolic inhibition induces an increase in the content and phosphorylation of the NR2B subunit of N-methyl-d-aspartate receptors and a decrease in glutamate transport in the rat hippocampus in vivo

The concentration of glutamate is regulated to ensure neurotransmission with a high temporal and local resolution. It is removed from the extracellular medium by high-affinity transporters, dependent on the maintenance of the Na(+) gradient through the activity of Na(+),K(+)-ATPases. Failure of glutamate clearance can lead to neuronal damage, named excitotoxic damage, due to the prolonged activation of glutamate receptors. Severe impairment of glycolytic metabolism during ischemia and hypoglycemia, leads to glutamate transport dysfunction inducing the elevation of extracellular glutamate and aspartate, and neuronal damage. Altered glucose metabolism has also been associated with some neurodegenerative diseases such as Alzheimer's and Huntington's, and a role of excitotoxicity in the neuropathology of these disorders has been raised. Alterations in glutamate transporters and N-methyl-D-aspartate (NMDA) receptors have been observed in these patients, suggesting altered glutamatergic neurotransmission. We hypothesize that inhibition of glucose metabolism might induce changes in glutamatergic neurotransmission rendering neurons more vulnerable to excitotoxicity. We have previously reported that sustained glycolysis impairment in vivo induced by inhibition of glyceraldehyde 3-phosphate dehydrogenase (GAPDH), facilitates glutamate-mediated neuronal damage. We have now investigated whether this facilitating effect involves altered glutamate uptake, and/or NMDA receptors in the rat hippocampus in vivo. Results indicate that metabolic inhibition leads to the progressive elevation of extracellular glutamate and aspartate levels in the hippocampus, which correlates with decreased content of the GLT-1 glutamate transporter and diminished glutamate uptake. In addition, we observed increased Tyr(1472) phosphorylation and protein content of the NR2B subunit of the NMDA receptor. Results suggest that moderate sustained glycolysis inhibition alters glutamatergic neurotransmission.

Calcium-mediated transient phosphorylation of tau and amyloid precursor protein followed by intraneuronal amyloid-beta accumulation

Intraneuronal accumulation of hyperphosphorylated protein tau in paired helical filaments together with amyloid-beta peptide (Abeta) deposits confirm the clinical diagnosis of Alzheimer disease. A common cellular mechanism leading to the production of these potent toxins remains elusive. Here we show that, in cultured neurons, membrane depolarization induced a calcium-mediated transient phosphorylation of both microtubule-associated protein tau and amyloid precursor protein (APP), followed by a dephosphorylation of these proteins. Phosphorylation was mediated by glycogen synthase kinase 3 and cyclin-dependent kinase 5 protein kinases, while calcineurin was responsible for dephosphorylation. Following the transient phosphorylation of APP, intraneuronal Abeta accumulated and induced neurotoxicity. Phosphorylation of APP on Thr-668 was indispensable for intraneuronal accumulation of Abeta. Our data demonstrate that an increase in cytosolic calcium concentration induces modifications of neuronal metabolism of APP and tau, similar to those found in Alzheimer disease.

Reduced plasma membrane surface expression of GLAST mediates decreased glutamate regulation in the aged striatum

Extracellular L-glutamate poses a severe excitotoxic threat to neurons and glia when unregulated, therefore low synaptic levels of this neurotransmitter must be maintained via a rapid and robust transport system. A recent study from our laboratory showed a reduced glutamate uptake rate in the striatum of the aged Fischer 344 (F344) rat, yet the mechanism underlying this phenomenon is unknown. The current study utilized in vivo electrochemical recordings, immunoblotting and biotinylation in young (6 months), late-middle aged (18 months) and aged (24 months) F344 rats to elucidate the potential role that glutamate transporters (GLT-1, GLAST, and EAAC1) may play in this mechanism. Here we show that the time necessary to clear glutamate from the late-middle aged and aged striatum is significantly prolonged in comparison to the young striatum. In addition, an analysis of various sub-regions of the striatum revealed a marked dorsoventral gradient in terms of glutamate clearance times in the aged striatum, a phenomenon which was not present in the striatum of the animals of the remaining age groups. We also found that the decreased glutamate clearance time observed in the late-middle aged and aged rats is not due to a decrease in the production of total transporter protein among these three transporters. Rather, a significant reduction in the amount of GLAST expressed on the plasma membrane surface in the aged animals (approximately 55% when compared to young rats) may contribute to this phenomenon. These age-related alterations in extracellular l-glutamate regulation may be key contributors to the increased susceptibility of the aged brain to excitotoxic insults such as stroke and hypoxia.

Chlorpromazine, clozapine and olanzapine inhibit anionic amino acid transport in cultured human fibroblasts

We report here that chlorpromazine, a first generation antipsychotic drug, inhibits anionic amino acid transport mediated by system X(-) (AG) (EAAT transporters) in cultured human fibroblasts. With 30 microM chlorpromazine, transport inhibition is detectable after 3 h of treatment, maximal after 48 h (>60%), and referable to a decrease in V(max). Chlorpromazine effect is not dependent upon changes of membrane potential and is selective for system X(-) (AG) since transport systems A and y(+) are not affected. Among antipsychotic drugs, the inhibitory effect of chlorpromazine is shared by two dibenzodiazepines, clozapine and olanzapine, while other compounds, such as risperidon, zuclopentixol, sertindol and haloperidol, are not effective. Transport inhibition by clozapine and olanzapine, but not by chlorpromazine, is reversible, suggesting that the mechanisms involved are distinct. These results indicate that a subset of antipsychotic drugs inhibits EAAT transporters in non-nervous tissues and prompt further investigation on possible alterations of glutamate transport in peripheral tissues of schizophrenic patients.

Hyaluronic acid attenuates osteoarthritis development in the anterior cruciate ligament-transected knee: Association with excitatory amino acid release in the joint dialysate

We previously reported increased release of the excitatory amino acid (EAA) neurotransmitters, glutamate and aspartate, during the early stage of experimental osteoarthritis (OA). Our present objective was to study the effect of intraarticular injection of hyaluronic acid (HA) on OA development, and to analyze concomitant changes in EAA levels in dialysates of anterior cruciate ligament-transected (ACLT) knee joints. OA was induced in Wistar rats by ACLT of one hindlimb; the knee of the other hindlimb was used as the sham-operated control. HA group (n = 12) were injected intraarticularly in the ACLT knee with 1 mg of HA once a week for 5 consecutive weeks, starting at 8 weeks after surgery. Saline group (n = 12) were injected as above with normal saline. The sham-operated group, underwent arthrotomy, but not ACLT, and received no treatment (n = 14). Twenty weeks after surgery, knee joint dialysates were collected by microdialysis and EAA levels assayed by high-performance liquid chromatography, and gross morphological examination and histopathological evaluation were performed on the medial femoral condyles and synovia. Rats receiving intraarticular HA injections showed a significantly lower degree of cartilage degeneration on the medial femoral condyle at both the macroscopic level and on the Mankin grading scale than rats receiving saline injections. Intraarticular HA treatment also suppressed synovitis. Moreover, glutamate and aspartate levels were significantly reduced in the HA group compared to the saline group. Intraarticular injection of HA limits articular cartilage and synovium damage and OA formation, and, in parallel, reduces EAA levels in ACLT joint dialysates. This study suggests that the underlying mechanism of the anti-inflammatory effect of HA is to inhibit glutamate and aspartate release in ACLT knee joints, which attenuates the early development of OA.

The search for disease-modifying drugs for neurodegenerative disorders

Neurodegenerative disorders (NDs) are a complex group of illnesses that possibly share common mechanisms. The onset and progression of NDs may depend upon genetic traits as well as complex interactions between individual genetic backgrounds and environmental factors. The exact role of the risk factors involved, and how they influence the onset and pacing of the disease, is not yet fully understood. Similarly, the relationship between the rate of neuronal death and clinical expression of the disease is a matter for discussion. The knowledge of the various molecules and mechanisms that accompany NDs as primary or secondary events is increasing steadily, but the distinction between their physiological and pathological roles is still uncertain. These premises highlight the difficulties underlying the design of disease modifying strategies. However, it is possible to identify several disease modifying strategies based on various approaches, including the targeting of putative causal elements or of general mechanisms of degeneration and reparative strategies. The purpose of this review is to examine the biological rationale behind attempted protective, restorative and therapeutic strategies.