Decreased metabolic response to visual stimulation in the superior colliculus of mice lacking the glial glutamate transporter GLT-1

Activation of Glutamate Transport Increases Arteriole Diameter in v ivo: Implications for Neurovascular Coupling

In order to meet the energetic demands of cell-to-cell signaling, increases in local neuronal signaling are matched by a coordinated increase in local blood flow, termed neurovascular coupling. Multiple different signals from neurons, astrocytes, and pericytes contribute to this control of blood flow. Previously, several groups demonstrated that inhibition/ablation of glutamate transporters attenuates the neurovascular response. However, it was not determined if glutamate transporter activation was sufficient to increase blood flow. Here, we used multiphoton imaging to monitor the diameter of fluorescently labeled cortical arterioles in anesthetized C57/B6J mice. We delivered vehicle, glutamate transporter substrates, or a combination of a glutamate transporter substrate with various pharmacologic agents via a glass micropipette while simultaneously visualizing changes in arteriole diameter. We developed a novel image analysis method to automate the measurement of arteriole diameter in these time-lapse analyses. Using this workflow, we first conducted pilot experiments in which we focally applied L-glutamate, D-aspartate, or L-threo-hydroxyaspartate (L-THA) and measured arteriole responses as proof of concept. We subsequently applied the selective glutamate transport substrate L-THA (applied at concentrations that do not activate glutamate receptors). We found that L-THA evoked a significantly larger dilation than that observed with focal saline application. This response was blocked by co-application of the potent glutamate transport inhibitor, L-(2S,3S)-3-[3-[4-(trifluoromethyl)-benzoylamino]benzyloxy]-aspartate (TFB-TBOA). Conversely, we were unable to demonstrate a reduction of this effect through co-application of a cocktail of glutamate and GABA receptor antagonists. These studies provide the first direct evidence that activation of glutamate transport is sufficient to increase arteriole diameter. We explored potential downstream mechanisms mediating this transporter-mediated dilation by using a Ca2+ chelator or inhibitors of reversed-mode Na+/Ca2+ exchange, nitric oxide synthetase, or cyclo-oxygenase. The estimated effects and confidence intervals suggested some form of inhibition for a number of these inhibitors. Limitations to our study design prevented definitive conclusions with respect to these downstream inhibitors; these limitations are discussed along with possible next steps. Understanding the mechanisms that control blood flow are important because changes in blood flow/energy supply are implicated in several neurodegenerative disorders and are used as a surrogate measure of neuronal activity in widely used techniques such as functional magnetic resonance imaging (fMRI).

Computational singular perturbation analysis of brain lactate metabolism

Lactate in the brain is considered an important fuel and signalling molecule for neuronal activity, especially during neuronal activation. Whether lactate is shuttled from astrocytes to neurons or from neurons to astrocytes leads to the contradictory Astrocyte to Neuron Lactate Shuttle (ANLS) or Neuron to Astrocyte Lactate Shuttle (NALS) hypotheses, both of which are supported by extensive, but indirect, experimental evidence. This work explores the conditions favouring development of ANLS or NALS phenomenon on the basis of a model that can simulate both by employing the two parameter sets proposed by Simpson et al. (J Cereb. Blood Flow Metab., 27:1766, 2007) and Mangia et al. (J of Neurochemistry, 109:55, 2009). As most mathematical models governing brain metabolism processes, this model is multi-scale in character due to the wide range of time scales characterizing its dynamics. Therefore, we utilize the Computational Singular Perturbation (CSP) algorithm, which has been used extensively in multi-scale systems of reactive flows and biological systems, to identify components of the system that (i) generate the characteristic time scale and the fast/slow dynamics, (ii) participate to the expressions that approximate the surfaces of equilibria that develop in phase space and (iii) control the evolution of the process within the established surfaces of equilibria. It is shown that a decisive factor on whether the ANLS or NALS configuration will develop during neuronal activation is whether the lactate transport between astrocytes and interstitium contributes to the fast dynamics or not. When it does, lactate is mainly generated in astrocytes and the ANLS hypothesis is realised, while when it doesn't, lactate is mainly generated in neurons and the NALS hypothesis is realised. This scenario was tested in exercise conditions.

Region-specific deletions of the glutamate transporter GLT1 differentially affect seizure activity and neurodegeneration in mice

Glial glutamate transporter GLT1 plays a key role in the maintenance of extracellular glutamate homeostasis. Recent human genetic studies have suggested that de novo mutations in GLT1 (EAAT2) cause early-onset epilepsy with multiple seizure types. Consistent with these findings, global GLT1 null mice show lethal spontaneous seizures. The consequences of GLT1 dysfunction vary between different brain regions, suggesting that the role of GLT1 dysfunction in epilepsy may also vary with brain regions. In this study, we generated region-specific GLT1 knockout mice by crossing floxed-GLT1 mice with mice that express the Cre recombinase in a particular domain of the ventricular zone. Selective deletion of GLT1 in the diencephalon, brainstem and spinal cord is sufficient to reproduce the phenotypes (excess mortality, decreased body weight, and lethal spontaneous seizure) of the global GLT1 null mice. By contrast, dorsal forebrain-specific GLT1 knockout mice showed nonlethal complex seizures including myoclonic jerks, hyperkinetic running, spasm and clonic convulsion via the activation of NMDA receptors during a limited period from P12 to P14 and selective neuronal death in cortical layer II/III and the hippocampus. Thus, GLT1 dysfunction in the dorsal forebrain is involved in the pathogenesis of infantile epilepsy and GLT1 in the diencephalon, brainstem and spinal cord may play a critical role in preventing seizure-induced sudden death.

Glutamine and Glutamate Complex, as Measured by Functional Magnetic Resonance Spectroscopy, Alters During Face-Name Association Task in Patients with Mild Cognitive Impairment and Alzheimer's Disease

BACKGROUND

The metabolite response during a memory task in Alzheimer's disease (AD) patients is unknown.

OBJECTIVE

To investigate the metabolite changes in subjects with AD, amnestic mild cognitive impairment (aMCI), and cognitively normal (CN) elderly during a memory task using functional magnetic resonance spectroscopy (fMRS).

METHODS

This study involved 23 young normal controls (YC), 24 CN elderly, 24 aMCI, and 24 mild and probable AD individuals. fMRS data were acquired at the precuneus and posterior cingulate brain regions during a face-name association task. Statistical analyses of quantified metabolites were performed to evaluate differences of the metabolite values between the stimulation conditions and among the four subject groups. Receiver operating curve analysis was performed to evaluate whether the metabolic changes after functional activations can differentiate the subject groups.

RESULT

Glutamine and glutamate complex (Glx) was statistically significantly different between the fixation and repeat conditions in aMCI (p = 0.0492) as well as between the fixation and the novel conditions in the AD (p = 0.0412) group. The total N-acetylaspartate (tNAA) was statistically significantly different among the four subject groups in the fixation condition (DF = 3, F = 7.673, p <  0.001), the novel condition (DF = 3, F = 6.945, p <  0.001), and the repeat condition (DF = 3, F = 7.127, p <  0.001). tNAA, tCr, and mIns could be used to differentiate CN from aMCI. Furthermore, tNAA, tCr, Glx, and Glu could also differentiate CN from AD, and aMCI from AD.

CONCLUSION

Glx was altered during a stimulation that may be used to evaluate neuronal dysfunction in a demented patient. tNAA and tCr were reduced in patients with AD.

Partial Loss of the Glutamate Transporter GLT-1 Alters Brain Akt and Insulin Signaling in a Mouse Model of Alzheimer's Disease

The glutamate transporter GLT-1 (also called EAAT2 in humans) plays a critical role in regulating extracellular glutamate levels in the central nervous system (CNS). In Alzheimer's disease (AD), EAAT2 loss is associated with neuropathology and cognitive impairment. In keeping with this, we have reported that partial GLT-1 loss (GLT-1+/-) causes early-occurring cognitive deficits in mice harboring familial AD AβPPswe/PS1ΔE9 mutations. GLT-1 plays important roles in several molecular pathways that regulate brain metabolism, including Akt and insulin signaling in astrocytes. Significantly, AD pathogenesis also involves chronic Akt activation and reduced insulin signaling in the CNS. In this report we tested the hypothesis that GLT-1 heterozygosity (which reduces GLT-1 to levels that are comparable to losses in AD patients) in AβPPswe/PS1ΔE9 mice would induce sustained activation of Akt and disturb components of the CNS insulin signaling cascade. We found that partial GLT-1 loss chronically increased Akt activation (reflected by increased phosphorylation at serine 473), impaired insulin signaling (reflected by decreased IRβ phosphorylation of tyrosines 1150/1151 and increased IRS-1 phosphorylation at serines 632/635 - denoted as 636/639 in humans), and reduced insulin degrading enzyme (IDE) activity in brains of mice expressing familial AβPPswe/PS1ΔE9 AD mutations. GLT-1 loss also caused an apparent compensatory increase in IDE activity in the liver, an organ that has been shown to regulate peripheral amyloid-β levels and expresses GLT-1. Taken together, these findings demonstrate that partial GLT-1 loss can cause insulin/Akt signaling abnormalities that are in keeping with those observed in AD.

Alteration of sensory-evoked metabolic and oscillatory activities in the olfactory bulb of GLAST-deficient mice

Astrocytes are key cellular elements in both the tripartite synapse and the neurovascular unit. To fulfill this dual role in synaptic activity and metabolism, they express a panel of receptors and transporters that sense glutamate. Among them, the GLT-1 and GLAST transporters are known to regulate extracellular glutamate concentrations at excitatory synapses and consequently modulate glutamate receptor signaling. These major uptake systems are also involved in energy supply to neurons. However, the functional role of GLAST in concurrent regulation of metabolic and neuronal activity is currently unknown. We took advantage of the attractive structural and functional features of the main olfactory bulb to explore the impact of GLAST on sensory information processing while probing both glutamate uptake and neuronal activity in glomeruli and deeper cellular layers, respectively. Using odor-evoked 2-deoxyglucose imaging and local field potential recordings in GLAST knockout mice, we show in vivo that deletion of GLAST alters both glucose uptake and neuronal oscillations in olfactory bulb networks.

Brain energy metabolism: focus on astrocyte-neuron metabolic cooperation

The energy requirements of the brain are very high, and tight regulatory mechanisms operate to ensure adequate spatial and temporal delivery of energy substrates in register with neuronal activity. Astrocytes-a type of glial cell-have emerged as active players in brain energy delivery, production, utilization, and storage. Our understanding of neuroenergetics is rapidly evolving from a "neurocentric" view to a more integrated picture involving an intense cooperativity between astrocytes and neurons. This review focuses on the cellular aspects of brain energy metabolism, with a particular emphasis on the metabolic interactions between neurons and astrocytes.

Differential expression of the glutamate transporter GLT-1 in pancreas

The glutamate uptake transporter GLT-1 is best understood for its critical role in preventing brain seizures. Increasing evidence argues that GLT-1 also modulates, and is modulated by, metabolic processes that influence glucose homeostasis. To investigate further the potential role of GLT-1 in these regards, the authors examined GLT-1 expression in pancreas and found that mature multimeric GLT-1 protein is stably expressed in the pancreas of wild-type, but not GLT-1 knockout, mice. There are three primary functional carboxyl-terminus GLT-1 splice variants, called GLT-1a, b, and c. Brain and liver express all three variants; however, the pancreas expresses GLT-1a and GLT-1b but not GLT-1c. Quantitative real time-PCR further revealed that while GLT-1a is the predominant GLT-1 splice variant in brain and liver, GLT-1b is the most abundant splice variant expressed in pancreas. Confocal microscopy and immunohistochemistry showed that GLT-1a and GLT-1b are expressed in both islet β- and α-cells. GLT-1b was also expressed in exocrine ductal domains. Finally, glutamine synthetase was coexpressed with GLT-1 in islets, which suggests that, as with liver and brain, one possible role of GLT-1 in the pancreas is to support glutamine synthesis.

A combination of atlas-based and voxel-wise approaches to analyze metabolic changes in autoradiographic data from Alzheimer's mice

Murine models are commonly used in neuroscience research to improve our knowledge of disease processes and to test drug effects. To accurately study brain glucose metabolism in these animals, ex vivo autoradiography remains the gold standard. The analysis of 3D-reconstructed autoradiographic volumes using a voxel-wise approach allows clusters of voxels representing metabolic differences between groups to be revealed. However, the spatial localization of these clusters requires careful visual identification by a neuroanatomist, a time-consuming task that is often subject to misinterpretation. Moreover, the large number of voxels to be computed in autoradiographic rodent images leads to many false positives. Here, we proposed an original automated indexation of the results of a voxel-wise approach using an MRI-based 3D digital atlas, followed by the restriction of the statistical analysis using atlas-based segmentation, thus taking advantage of the specific and complementary strengths of these two approaches. In a preliminary study of transgenic Alzheimer's mice (APP/PS1), and control littermates (PS1), we were able to achieve prompt and direct anatomical indexation of metabolic changes detected between the two groups, revealing both hypo- and hypermetabolism in the brain of APP/PS1 mice. Furthermore, statistical results were refined using atlas-based segmentation: most interesting results were obtained for the hippocampus. We thus confirmed and extended our previous results by identifying the brain structures affected in this pathological model and demonstrating modified glucose uptake in structures like the olfactory bulb. Our combined approach thus paves the way for a complete and accurate examination of functional data from cerebral structures involved in models of neurodegenerative diseases.

Validation of MRI-based 3D digital atlas registration with histological and autoradiographic volumes: an anatomofunctional transgenic mouse brain imaging study

Murine models are commonly used in neuroscience to improve our knowledge of disease processes and to test drug effects. To accurately study neuroanatomy and brain function in small animals, histological staining and ex vivo autoradiography remain the gold standards to date. These analyses are classically performed by manually tracing regions of interest, which is time-consuming. For this reason, only a few 2D tissue sections are usually processed, resulting in a loss of information. We therefore proposed to match a 3D digital atlas with previously 3D-reconstructed post mortem data to automatically evaluate morphology and function in mouse brain structures. We used a freely available MRI-based 3D digital atlas derived from C57Bl/6J mouse brain scans (9.4T). The histological and autoradiographic volumes used were obtained from a preliminary study in APP(SL)/PS1(M146L) transgenic mice, models of Alzheimer's disease, and their control littermates (PS1(M146L)). We first deformed the original 3D MR images to match our experimental volumes. We then applied deformation parameters to warp the 3D digital atlas to match the data to be studied. The reliability of our method was qualitatively and quantitatively assessed by comparing atlas-based and manual segmentations in 3D. Our approach yields faster and more robust results than standard methods in the investigation of post mortem mouse data sets at the level of brain structures. It also constitutes an original method for the validation of an MRI-based atlas using histology and autoradiography as anatomical and functional references, respectively.

Activation of astrocytes by CNTF induces metabolic plasticity and increases resistance to metabolic insults

High energy demands of neurons make them vulnerable to adverse effects of energy impairment. Recently, astrocytes were shown to regulate the flux of energy substrates to neurons. In pathological situations, astrocytes are activated but the consequences on brain energy metabolism are still poorly characterized. We found that local lentiviral-mediated gene transfer of ciliary neurotrophic factor (CNTF), a cytokine known to activate astrocytes, induced a stable decrease in the glycolytic flux in the rat striatum in vivo as measured by 2-[18F]-2-deoxy-D-glucose autoradiography and micro-positron emission tomography imaging. The activity of the mitochondrial complex IV enzyme cytochrome oxidase was not modified, suggesting maintenance of downstream oxidative steps of energy production. CNTF significantly increased the phosphorylation level of the intracellular energy sensor AMP-activated protein kinase (AMPK), supporting a specific reorganization of brain energy pathways. Indeed, we found that different key enzymes/transporters of fatty acids beta-oxidation and ketolysis were overexpressed by CNTF-activated astrocytes within the striatum. In primary striatal neuron/astrocyte mixed cultures exposed to CNTF, the AMPK pathway was also activated, and the rate of oxidation of fatty acids and ketone bodies was significantly enhanced. This metabolic plasticity conferred partial glial and neuronal protection against prolonged palmitate exposure and glycolysis inhibition. We conclude that CNTF-activated astrocytes may have a strong protective potential to face severe metabolic insults.

Neuron-astrocyte interactions in the regulation of brain energy metabolism: a focus on NMR spectroscopy

An adequate and timely production of ATP by brain cells is of cardinal importance to support the major energetic cost of the rapid processing of information via synaptic and action potentials. Recently, evidence has been accumulated to support the view that the regulation of brain energy metabolism is under the control of an intimate dialogue between astrocytes and neurons. In vitro studies on cultured astrocytes and in vivo studies on rodents have provided evidence that glutamate and Na(+) uptake in astrocytes is a key triggering signal regulating glucose use in the brain. With the advent of NMR spectroscopy, it has been possible to provide experimental evidence to show that energy consumption is mainly devoted to glutamatergic neurotransmission and that glutamate-glutamine cycling is coupled in a approximately 1 : 1 molar stoichiometry to glucose oxidation, at least in the cerebral cortex. This improved understanding of neuron-astrocyte metabolic interactions offers the potential for developing novel therapeutic strategies for many neurological disorders that include a metabolic deficit.

Activity-dependent regulation of energy metabolism by astrocytes: An update

Astrocytes play a critical role in the regulation of brain metabolic responses to activity. One detailed mechanism proposed to describe the role of astrocytes in some of these responses has come to be known as the astrocyte-neuron lactate shuttle hypothesis (ANLSH). Although controversial, the original concept of a coupling mechanism between neuronal activity and glucose utilization that involves an activation of aerobic glycolysis in astrocytes and lactate consumption by neurons provides a heuristically valid framework for experimental studies. In this context, it is necessary to provide a survey of recent developments and data pertaining to this model. Thus, here, we review very recent experimental evidence as well as theoretical arguments strongly supporting the original model and in some cases extending it. Aspects revisited include the existence of glutamate-induced glycolysis in astrocytes in vitro, ex vivo, and in vivo, lactate as a preferential oxidative substrate for neurons, and the notion of net lactate transfer between astrocytes and neurons in vivo. Inclusion of a role for glycogen in the ANLSH is discussed in the light of a possible extension of the astrocyte-neuron lactate shuttle (ANLS) concept rather than as a competing hypothesis. New perspectives offered by the application of this concept include a better understanding of the basis of signals used in functional brain imaging, a role for neuron-glia metabolic interactions in glucose sensing and diabetes, as well as novel strategies to develop therapies against neurodegenerative diseases based upon improving astrocyte-neuron coupled energetics.

Differential effects of NMDA and AMPA glutamate receptors on functional magnetic resonance imaging signals and evoked neuronal activity during forepaw stimulation of the rat

Most of the currently used methods for functional brain imaging do not visualize neuronal activity directly but rather rely on the elicited hemodynamic and/or metabolic responses. Glutamate, the major excitatory neurotransmitter, plays an important role in the neurovascular/neurometabolic coupling, but the specific mechanisms are still poorly understood. To investigate the role of the two major ionotropic glutamate receptors [NMDA receptors (NMDA-Rs) and AMPA receptors (AMPA-Rs)] for the generation of functional magnetic resonance imaging (fMRI) signals, we used fMRI [measurements of blood oxygenation level-dependent (BOLD), perfusion-weighted imaging (PWI), and cerebral blood volume (CBV)] together with recordings of somatosensory evoked potentials (SEPs) during electrical forepaw stimulation in the alpha-chloralose anesthetized rat. Intravenous injection of the NMDA-R antagonist MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate] (0.06 mg/kg plus 3.6 microg x kg(-1) x h(-1)) significantly decreased BOLD (-51 +/- 19%; n = 5) and PWI (-57 +/- 26%; n = 5) responses but reduced the SEPs only mildly (approximately -10%). Systemic application of the AMPA-R antagonist GYKI-53655 [1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7,8-methylenedioxy-3,4-dihydro-5H-2,3-benzodiazepine] significantly decreased both the hemodynamic response (BOLD, -49 +/- 13 and -65 +/- 15%; PWI, -22 +/- 48 and -68 +/- 4% for 5 and 7 mg/kg, i.v., respectively; CBV, -80 +/- 7% for 7 mg/kg; n = 4) and the SEPs (up to -60%). These data indicate that the interaction of glutamate with its postsynaptic and/or glial receptors is necessary for the generation of blood flow and BOLD responses and illustrate the differential role of NMDA-Rs and AMPA-Rs in the signaling chain leading from increased neuronal activity to the hemodynamic response in the somatosensory cortex.

Ciliary neurotrophic factor activates astrocytes, redistributes their glutamate transporters GLAST and GLT-1 to raft microdomains, and improves glutamate handling in vivo

To study the functional role of activated astrocytes in glutamate homeostasis in vivo, we used a model of sustained astrocytic activation in the rat striatum through lentiviral-mediated gene delivery of ciliary neurotrophic factor (CNTF). CNTF-activated astrocytes were hypertrophic, expressed immature intermediate filament proteins and highly glycosylated forms of their glutamate transporters GLAST and GLT-1. CNTF overexpression produced a redistribution of GLAST and GLT-1 into raft functional membrane microdomains, which are important for glutamate uptake. In contrast, CNTF had no detectable effect on the expression of a number of neuronal proteins and on the spontaneous glutamatergic transmission recorded from striatal medium spiny neurons. These results were replicated in vitro by application of recombinant CNTF on a mixed neuron/astrocyte striatal culture. Using microdialysis in the rat striatum, we found that the accumulation of extracellular glutamate induced by quinolinate (QA) was reduced threefold with CNTF. In line with this result, CNTF significantly increased QA-induced [(18)F]-fluoro-2-deoxyglucose uptake, an indirect index of glutamate uptake by astrocytes. Together, these data demonstrate that CNTF activation of astrocytes in vivo is associated with marked phenotypic and molecular changes leading to a better handling of increased levels of extracellular glutamate. Activated astrocytes may therefore be important prosurvival agents in pathological conditions involving defects in glutamate homeostasis.