The glutamate transporter GLT1/EAAT2 in islets of Langerhans : a key player in the control of β-cell function and integrity in health and disease

Background and aim: The clinical course of Diabetes Mellitus is characterized by a progressive decline in β-cell function and mass. Understanding the causes of β-cell failure and death is of capital importance to develop new and more effective therapeutic strategies. We have shown that glutamate represents a new insult for β-cells. Indeed, chronic exposure to elevated extracellular glutamate concentrations causes apoptosis in clonal β-cell lines and in human islet β-cells, but not in α-cells. In the islet, L-glutamate is released by α-cells with glucagon and it modulates hormone secretion and β-cell viability by acting on ionotropic and metabotropic glutamate receptors. Its extracellular concentration is locally controlled by high affinity glutamate transporters of the solute carrier 1 family (SLC1), in particular by GLT1/EAAT2 (glutamate transporter 1/excitatory amino acid transporters 2). GLT1 is prevalently localized on β-cell membrane and its normal function is essential to prevent glutamate-induced β-cell death. Given the particular role of GLT1 in β-cell protection, aim of the proposed research was to verify whether chronic hyperglycemia might modulate GLT1 expression and/or its activity in human islets of Langerhans. Material and methods: Human isolated islets were exposed for three days to 5.5 mM (normoglycemia) or 16.7 mM (hyperglycemia) glucose. Transporters expression, localization and function were assessed by RT-PCR, western blotting, indirect immunofluorescence and [3H]-D glutamate uptake. Results: Quantitative PCR analysis showed a 40±3% reduction in the total ASCT2/SLC1A5 expression, after incubation in chronic hyperglycemia. No changes in the total GLT1/SLC1A2 mRNA and protein expression in human islets were found. Immunofluorescence experiments performed on human islets exposed to hyperglycemia revealed GLT1 relocalization into intracellular vesicular compartments of β-cells. Because of this relocalization, the GLT1-mediated surface activity measured by [3H]-D-glutamate uptake was inhibited by 31±5% relative to normoglycemic conditions (p<0.05; n=4 in triplicate). Chronic hyperglycemia induced a downregulation of the PI3K/Akt pathway in human β-cells (35±3% downregulation of P-Akt expression, n=5 islet preparations), suggesting a possible involvement of this pathway in the modulation of GLT1 trafficking. According to this possibility, PI3K inhibition with 100 μM LY294002 in human islets caused the GLT1 relocalization in intracellular compartments and a 75±8% downregulation of its activity (p<0.001; n=3 different islet isolations, in quadruplicate). Chronic treatment with 10 nM ceftriaxone, a drug able to upregulate GLT1 expression, increases the glutamate transport activity and significantly prevented hyperglycemia-induced apoptosis in human islets (65±12% reduction of apoptosis. P<0.001; n=2 preparations, in quadruplicate). Conclusion: Our data indicate that hyperglycemia alters glutamate signaling in human islets and this may further contribute to β-cell dysfunction and death. Glutamate signalling system components may be promising therapeutic targets to prevent β-cell dysfunction and to regulate glucose homeostasis in diabetes.