(DCP011) GLYCINE RECEPTOR ACTIVITY IN Β CELLS IS DOWNREGULATED IN TYPE 2 DIABETES AND AFTER HIGH GLUCOSE CULTURE

Background: Glycine receptors (GlyRs) are present in human β cells and contribute to cell depolarization, increasing insulin secretion. However, in type 2 diabetes (T2D), islet GlyR activity is impaired by unknown mechanisms, and the GlyR subunit GLRA1 is reported as one of the strongest downregulated transcripts in islets, with increasing donor HbA1c. We aimed to further investigate how GlyRs can influence islet function, and if the GlyR dysfunction in T2D is caused by hyperglycemia.

METHODS AND RESULTS: GlyR-mediated currents were measured using whole-cell patch-clamp in human β cells from donors with or without T2D, or after culture in 5.5 or 15 mmol/L glucose for 2 days. The expression of the GlyR α1, α3, and β subunits mRNA splice variants was compared between islets from donors with and without T2D. Insulin secretion from human islets was measured in the presence or absence of the GlyR antagonist strychnine (10 µM) and quantified by ELISA.
The glycine-evoked currents in β cells from donors with T2D (-0.81±0.51 pA/pF, n=7) were smaller than those measured in cells from donors without diabetes (-12.72±2.38 pA/pF, n=29; p< 0.001). The β cells cultured in 15 mmo l/L glucose for 2 days had smaller glycine-evoked currents (-9.18±1.86 pA/pF, n=18) than those in control media (-17.63±3.53 pA/pF, n =14; p< 0.05). The expression of most GlyR subunit mRNA splice variants was decreased in islets of donors with T2D, with no evidence of a shift in alternative splicing towards ‘non-functional’ receptor isoforms. Glucose-stimulated insulin secretion was reduced when 10 µM strychnine was present (AUC = 13.58 ±1.10, versus 23.83 ±2.43 without strychnine; p< 0.05).

Conclusion: Glycine-evoked currents in β cells are decreased after 2 days of culture with high glucose, showing that hyperglycemia is capable of down-regulating GlyRs. This is similar to the reduced glycine-evoked current in T2D, where we find a decrease in overall GlyR gene expression, but not a shift in GlyR mRNA splicing. In addition, inhibiting the GlyR reduces glucose-stimulated insulin secretion.