[Pharmacology of excitatory amino acid transporters (EAATs and VGLUTs)]

Regulation Mechanisms of the Glutamate Transporter in the Response of Pacific Oyster upon High-Temperature Stress

Glutamate transporters (GLTs) are integral to the glutamatergic system, modulating glutamate homeostasis to enhance resilience and resistance against environmental stress. There are six GLTs identified in the Pacific oyster (Crassostrea gigas), which were categorized into two subfamilies: excitatory amino acid transporters (CgEAATs) and vesicular glutamate transporters (CgVGLUTs). The CgEAATs harbor a GltP domain, while CgVGLUTs feature an MFS domain, both with conserved sequence and structural characteristics. The expression of CgGLTs is elevated during the planktonic larval stage compared to the fertilized egg stage and is constitutively expressed in various tissues of adult oysters, suggesting its critical role in both larval development and the physiological processes of adult oysters. Transcriptomic analysis revealed diverse expression patterns of GLTs in oyster gills after 7 days of high-temperature stress, with CgEAAT3 showing a significant upregulation. A KEGG pathway enrichment analysis identified glutathione metabolism and ferroptosis as prominently enriched pathways. At 48 h after high-temperature stress, the expression levels of Glutathione Peroxidase 4 (CgGPX4) and CgEAAT3, along with elevated Fe content in the gills, significantly increased. Moreover, the RNAi-mediated the inhibition of CgEAAT3 expression under high-temperature stress, resulting in a significant reduction in CgGPX4 expression and a further increase in Fe accumulation in oyster gills. These results indicate that CgEAAT3 contributes to the regulation of ferroptosis and redox homeostasis by modulating CgGPX4 expression. This study provides new insights into the adaptive mechanisms of bivalves to environmental stress.

VGLUT substrates and inhibitors: A computational viewpoint

The vesicular glutamate transporters (VGLUTs) bind and move glutamate (Glu) from the cytosol into the lumen of synaptic vesicles using a H⁺-electrochemical gradient (ΔpH and Δψ) generated by the vesicular H⁺-ATPase. VGLUTs show very low Glu binding and to date, no three-dimensional structure has been elucidated. Prior studies have attempted to identify the key residues involved in binding VGLUT substrates and inhibitors using homology models and docking experiments. Recently, the inward and outward oriented crystal structures of d-galactonate transporter (DgoT) emerged as possible structure templates for VGLUT. In this review, a new homology model for VGLUT2 based on DgoT has been developed and used to conduct docking experiments to identify and differentiate residues and binding orientations involved in ligand interactions. This review describes small molecule-ligand interactions including docking using a VGLUT2 homology model derived from DgoT.

Demonstration of vesicular glutamate transporter-1 in corticotroph cells in the anterior pituitary of the rat

Recent immunohistochemical studies of the rat adenohypophysis identified type-2 vesicular glutamate transporter (VGLUT2), a marker for glutamatergic neuronal phenotype, in high percentages of adenohypophysial gonadotrophs and thyrotrophs. The presence and molecular identity of amino acid neurotransmitters in the remaining hormone producing cell types are unknown. In the present study we addressed the putative synthesis of another glutamatergic marker, VGLUT1 by adenohypophysial cells. Immunohistochemical studies revealed VGLUT1 immunoreactivity in a small subset of polygonal medium-sized cells in the anterior lobe. Western blot analysis revealed a single major 60 kDa protein band in the adenohypophysis. Furthermore, the expression of VGLUT1 mRNA was confirmed by reverse transcription-polymerase chain reaction followed by sequence analysis of the amplicon. In contrast with rats which only showed VGLUT1 signal in the anterior lobe of the pituitary, mice contained high levels of VGLUT1 immunoreactivity in the intermediate, in addition to the anterior lobe. No signal was present in VGLUT1-knockout mice, providing evidence for specificity. In rats, results of colocalization studies with dual-immunofluorescent labeling provided evidence for VGLUT1 immunoreactivity in 45.9% of corticotrophs and 7.7% of luteinizing hormone beta-immunopositive gonadotrophs. Cells of the other peptide hormone phenotypes were devoid of VGLUT1 signal. A few cells in the adenohypophysis expressed both VGLUT1 and VGLUT2 immunoreactivities. The presence of the glutamate markers VGLUT1 and VGLUT2 in distinct populations of peptide hormone-secreting hypophysial cells highly indicates the involvement of endogenous glutamate release in autocrine/paracrine regulatory mechanisms. The biological function of adenohypophysial glutamate will require clarification.

Conformationally-restricted amino acid analogues bearing a distal sulfonic acid show selective inhibition of system x(c)(-) over the vesicular glutamate transporter

A panel of amino acid analogs and conformationally-restricted amino acids bearing a sulfonic acid were synthesized and tested for their ability to preferentially inhibit the obligate cysteine-glutamate transporter system x(c)(-) versus the vesicular glutamate transporter (VGLUT). Several promising candidate molecules were identified: R/S-4-[4'-carboxyphenyl]-phenylglycine, a biphenyl substituted analog of 4-carboxyphenylglycine and 2-thiopheneglycine-5-sulfonic acid both of which reduced glutamate uptake at system x(c)(-) by 70-75% while having modest to no effect on glutamate uptake at VGLUT.

Glutamate-induced exocytosis of glutamate from astrocytes

Recent studies indicate that astrocytes can play a much more active role in neuronal circuits than previously believed, by releasing neurotransmitters such as glutamate and ATP. Here we report that local application of glutamate or glutamine synthetase inhibitors induces astrocytic release of glutamate, which activates a slowly decaying transient inward current (SIC) in CA1 pyramidal neurons and a transient inward current in astrocytes in hippocampal slices. The occurrence of SICs was accompanied by an appearance of large vesicles around the puffing pipette. The frequency of SICs was positively correlated with [glutamate]o. EM imaging of anti-glial fibrillary acid protein-labeled astrocytes showed glutamate-induced large astrocytic vesicles. Imaging of FM 1-43 fluorescence using two-photon laser scanning microscopy detected glutamate-induced formation and fusion of large vesicles identified as FM 1-43-negative structures. Fusion of large vesicles, monitored by collapse of vesicles with a high intensity FM 1-43 stain in the vesicular membrane, coincided with SICs. Glutamate induced two types of large vesicles with high and low intravesicular [Ca2+]. The high [Ca2+] vesicle plays a major role in astrocytic release of glutamate. Vesicular fusion was blocked by infusing the Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, or the SNARE blocker, tetanus toxin, suggesting Ca2+- and SNARE-dependent fusion. Infusion of the vesicular glutamate transport inhibitor, Rose Bengal, reduced astrocytic glutamate release, suggesting the involvement of vesicular glutamate transports in vesicular transport of glutamate. Our results demonstrate that local [glutamate]o increases induce formation and exocytotic fusion of glutamate-containing large astrocytic vesicles. These large vesicles could play important roles in the feedback control of neuronal circuits and epileptic seizures.