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In vivo knockdown of astroglial glutamate transporters GLT-1 and GLAST increases excitatory neurotransmission in mouse infralimbic cortex: Relevance for depressive-like phenotypes.

ABSTRACT: Alterations of energy metabolism and of astrocyte number/function in ventral anterior cingulate cortex (vACC) have been reported in major depressive disorder (MDD) patients and may contribute to MDD pathophysiology. We recently developed a mouse model of MDD mimicking these alterations. We knocked down the astroglial glutamate transporters GLAST and GLT-1 in infralimbic cortex (IL, rodent equivalent of vACC) using small interfering RNA (siRNA). GLAST and GLT-1 siRNA microinfusion in IL evoked a depressive-like phenotype, associated with a reduced serotonergic function and reduced forebrain BDNF expression. Neither effect occurred after siRNA application in the adjacent prelimbic cortex (PrL), thus emphasizing the critical role of vACC/IL in MDD pathogenesis. Here we examined the cellular/network basis of the changes induced in IL using intracellular recordings of layer V pyramidal neurons from mice microinjected with siRNA 24?h before. We analyzed (i) the electrophysiological characteristics of neurons; (ii) the synaptic transmission properties, by monitoring miniature, spontaneous and evoked EPSCs, and (iii) the gliotransmission, by monitoring slow inward currents (SICs), mediated by astrocytic glutamate release and activation of extra-synaptic NMDA receptors. GLT-1 and GLAST knockdown led to a more depolarized membrane potential and increased action potential firing rate of layer V pyramidal neurons, and enhanced excitatory synaptic transmission, as shown by the enhanced amplitude/frequency of spontaneous EPSCs. Gliotransmission was also increased, as indicated by the enhanced SIC amplitude/frequency. Hence, the depressive-like phenotype is associated with IL hyperactivity, likely leading to an excessive top-down inhibitory control of serotonergic activity through IL-midbrain descending pathways.

SUBMITTER: Fullana MN

PROVIDER: S-EPMC6911366 | biostudies-literature | 2019 Nov

REPOSITORIES: biostudies-literature

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In vivo knockdown of astroglial glutamate transporters GLT-1 and GLAST increases excitatory neurotransmission in mouse infralimbic cortex: Relevance for depressive-like phenotypes.

Fullana Mª Neus MN Covelo Ana A Bortolozzi Analía A Araque Alfonso A Artigas Francesc F

European neuropsychopharmacology : the journal of the European College of Neuropsychopharmacology 20191001 11

Alterations of energy metabolism and of astrocyte number/function in ventral anterior cingulate cortex (vACC) have been reported in major depressive disorder (MDD) patients and may contribute to MDD pathophysiology. We recently developed a mouse model of MDD mimicking these alterations. We knocked down the astroglial glutamate transporters GLAST and GLT-1 in infralimbic cortex (IL, rodent equivalent of vACC) using small interfering RNA (siRNA). GLAST and GLT-1 siRNA microinfusion in IL evoked a ...[more]

PMID: 31582286

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Project description:Glutamate is the primary excitatory neurotransmitter in the central nervous system (CNS) which initiates rapid signal transmission in the synapse before its re-uptake into the surrounding glia, specifically astrocytes. The astrocytic glutamate transporters glutamate-aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) and their human homologs excitatory amino acid transporter 1 (EAAT1) and 2 (EAAT2), respectively, are the major transporters which take up synaptic glutamate to maintain optimal extracellular glutamic levels, thus preventing accumulation in the synaptic cleft and ensuing excitotoxicity. Growing evidence has shown that excitotoxicity is associated with various neurological disorders, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), Parkinson's disease (PD), manganism, ischemia, schizophrenia, epilepsy, and autism. While the mechanisms of neurological disorders are not well understood, the dysregulation of GLAST/GLT-1 may play a significant role in excitotoxicity and associated neuropathogenesis. The expression and function of GLAST/GLT-1 may be dysregulated at the genetic, epigenetic, transcriptional or translational levels, leading to high levels of extracellular glutamate and excitotoxicity. Consequently, understanding the regulatory mechanisms of GLAST/GLT-1 has been an area of interest in developing therapeutics for the treatment of neurological disorders. Pharmacological agents including ?-lactam antibiotics, estrogen/selective estrogen receptor modulators (SERMs), growth factors, histone deacetylase inhibitors (HDACi), and translational activators have shown significant efficacy in enhancing the expression and function of GLAST/GLT-1 and glutamate uptake both in vitro and in vivo. This comprehensive review will discuss the regulatory mechanisms of GLAST/GLT-1, their association with neurological disorders, and the pharmacological agents which mediate their expression and function. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Project description:Historically, glutamate uptake in the CNS was mainly attributed to glial cells for three reasons: 1) none of the glutamate transporters were found to be located in presynaptic terminals of excitatory synapses; 2) the putative glial transporters, GLT-1 and GLAST are expressed at high levels in astrocytes; 3) studies of the constitutive GLT-1 knockout as well as pharmacological studies demonstrated that >90% of glutamate uptake into forebrain synaptosomes is mediated by the operation of GLT-1. Here we summarize the history leading up to the recognition of GLT-1a as a presynaptic glutamate transporter. A major issue now is understanding the physiological and pathophysiological significance of the expression of GLT-1 in presynaptic terminals. To elucidate the cell-type specific functions of GLT-1, a conditional knockout was generated with which to inactivate the GLT-1 gene in different cell types using Cre/lox technology. Astrocytic knockout led to an 80% reduction of GLT-1 expression, resulting in intractable seizures and early mortality as seen also in the constitutive knockout. Neuronal knockout was associated with no obvious phenotype. Surprisingly, synaptosomal uptake capacity (Vmax) was found to be significantly reduced, by 40%, in the neuronal knockout, indicating that the contribution of neuronal GLT-1 to synaptosomal uptake is disproportionate to its protein expression (5-10%). Conversely, the contribution of astrocytic GLT-1 to synaptosomal uptake was much lower than expected. In contrast, the loss of uptake into liposomes prepared from brain protein from astrocyte and neuronal knockouts was proportionate with the loss of GLT-1 protein, suggesting that a large portion of GLT-1 in astrocytic membranes in synaptosomal preparations is not functional, possibly because of a failure to reseal. These results suggest the need to reinterpret many previous studies using synaptosomal uptake to investigate glutamate transport itself as well as changes in glutamate homeostasis associated with normal functions, neurodegeneration, and response to drugs.

Dataset Information

In vivo knockdown of astroglial glutamate transporters GLT-1 and GLAST increases excitatory neurotransmission in mouse infralimbic cortex: Relevance for depressive-like phenotypes.

Publications

In vivo knockdown of astroglial glutamate transporters GLT-1 and GLAST increases excitatory neurotransmission in mouse infralimbic cortex: Relevance for depressive-like phenotypes.

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