A Role for Glutamate Transporters in the Regulation of Insulin Secretion

Abstract

In the brain, glutamate is an extracellular transmitter that mediates cell-to-cell communication. Prior to synaptic release it is pumped into vesicles by vesicular glutamate transporters (VGLUTs). To inactivate glutamate receptor responses after release, glutamate is taken up into glial cells or neurons by excitatory amino acid transporters (EAATs). In the pancreatic islets of Langerhans, glutamate is proposed to act as an intracellular messenger, regulating insulin secretion from β-cells, but the mechanisms involved are unknown. By immunogold cytochemistry we show that insulin containing secretory granules express VGLUT3. Despite the fact that they have a VGLUT, the levels of glutamate in these granules are low, indicating the presence of a protein that can transport glutamate out of the granules. Surprisingly, in β-cells the glutamate transporter EAAT2 is located, not in the plasma membrane as it is in brain cells, but exclusively in insulin-containing secretory granules, together with VGLUT3. In EAAT2 knock out mice, the content of glutamate in secretory granules is higher than in wild type mice. These data imply a glutamate cycle in which glutamate is carried into the granules by VGLUT3 and carried out by EAAT2. Perturbing this cycle by knocking down EAAT2 expression with a small interfering RNA, or by over-expressing EAAT2 or a VGLUT in insulin granules, significantly reduced the rate of granule exocytosis. Simulations of granule energetics suggest that VGLUT3 and EAAT2 may regulate the pH and membrane potential of the granules and thereby regulate insulin secretion. These data suggest that insulin secretion from β-cells is modulated by the flux of glutamate through the secretory granules.

Beta-cell mitochondrial carriers and the diabetogenic stress response

“…Accordingly, insulin-secreting cells express vesicular glutamate transporters, allowing glutamate transport similarly to neurons [82]. Moreover, β-cells express both VGLUT3 and the excitatory amino acid transporter EAAT2 on insulin-containing secretory vesicles, favouring respectively import and export of glutamate [83]. Interestingly, a link has been established between glutamate and cAMP, a strong GLP1-dependent enhancer of insulin secretion described above.…”

Proteome Analysis and Conditional Deletion of the EAAT2 Glutamate Transporter Provide Evidence against a Role of EAAT2 in Pancreatic Insulin Secretion in Mice

“…The Number of Secretory Granules in ␤-Cells Is High-How many EAAT2 and VGLUT3 molecules would be expected if these proteins were localized to secretory granules as suggested (26)? To answer this question, we determined the average granular surface area per ␤-cell volume in two wild-type C57Bl/6 mice (Fig.…”

“…For instance, if postembedding immunogold labeling (for method description, see Ref. 43) is specific and results in several gold particles per micrometer of granule membrane length as reported (26), thereby approaching labeling intensities obtained along astrocyte membranes in the brain (70,71), then difficulties in detecting EAAT2 by immunoperoxidase or in situ hybridization in ␤-cells in tissue sections would be unexpected considering the similar densities of membranes per volume of tissue and the extremely high EAAT2 levels in the brain (49). As FIGURE 9.…”

“…Some investigators have reported expression in the plasma membranes but have arrived at different conclusions with respect to cell types and functional roles (10,(22)(23)(24)(25). Another study (26) proposed a radically new model in which EAAT2 colocalizes with the VGLUT3 in the membranes of insulincontaining secretory granules. VGLUT3 is suggested to transport glutamate into granules, and EAAT2 is suggested to carry glutamate out again.…”

Elevation of transcription factor Islet-1 levels in vivo increases β-cell function but not β-cell mass

“…3D), although neither of these genes was detected in the microarray study, likely due to sensitivity issues with gene expression arrays. 32 We also attempted to confirm an increase in the expression of several other genes that may be involved in secretion, including glutamate receptor (Grin2c), 33 non voltage-gated sodium channel (Scnn1a), 34,35 and sperm acrosome associated 1 (Spaca1), 36,37 but were unable to validate upregulation of these genes. Taken together, our results suggest that Isl-1 may mediate increased β-cell functionality through a subset of genes involved in protein trafficking, metabolism, development, signal transduction and transcriptional regulation.…”