Expression of the Vesicular Inhibitory Amino Acid Transporter in Pancreatic Islet Cells

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Abstract
γ-Aminobutyric acid (GABA) is stored in microvesicles in pancreatic islet cells. Because GAD65 and GAD67, which catalyze the formation of GABA, are cytoplasmic, the existence of an islet vesicular GABA transporter has been postulated. Here, we test the hypothesis that the putative transporter is the vesicular inhibitory amino acid transporter (VIAAT), a neuronal transmembrane transporter of GABA and glycine. We sequenced the human VIAAT gene and determined that the human and rat proteins share over 98% sequence identity. In vitro expression of VIAAT and immunoblotting of brain and islet lysates revealed two forms of the protein: an ∼52-kDa and an ∼57-kDa form. By immunoblotting and immunohistochemistry, we detected VIAAT in rat but not human islets. Immunohistochemical staining showed that in rat islets, the distribution of VIAAT expression parallels that of GAD67, with increased expression in the mantle. GABA, too, was found to be present in islet non-β-cells. We conclude that VIAAT is expressed in rat islets and is more abundant in the mantle and that expression in human islets is very low or nil. The rat islet mantle differs from rat and human β-cells in that it contains only GAD67 and relatively increased levels of VIAAT. Cells that express only GAD67 may require higher levels of VIAAT expression.