Glycine Receptor Activity in β Cells Is Downregulated in Type 2 Diabetes and After High Glucose Culture

Objectives: Glycine acts in an autocrine positive feedback loop in human β cells through its ionotropic receptors (GlyRs). In type 2 diabetes (T2D), islet GlyR activity is impaired by unknown mechanisms. We sought to investigate if the GlyR dysfunction in T2D is replicated by hyperglycemia per se, and to further characterize its action in β cells and the islets. Methods: GlyR-mediated currents were measured using whole-cell patch-clamp in human β cells from donors with or without T2D, or after high glucose culture. We also correlated glycine-induced current amplitude with transcript expression levels through patch-seq. The expression of the GlyR α1, α3, and β subunit mRNA splice variants was compared between islets from donors with and without T2D, and after high glucose culture. Insulin secretion from human islets was measured in the presence or absence of the GlyR antagonist strychnine. Results: Gene expression of GlyRs was decreased in islets from T2D donors along with smaller GlyR-mediated currents in the β cells. Glycine-induced currents are also reduced in islets from donors without diabetes after 48 hours of culture in high glucose, along with reducedα1 subunit expression and increased α3 subunit expression. Glycine-evoked currents are highly heterogeneous between different β cells within and between donors; inversely correlated with donor HbA1c; and significantly correlated to the expression of 99 different transcripts. Finally, glucose-stimulated insulin secretion is decreased in the presence of strychnine.