GABAergic system in β-cells: from autoimmunity target to regeneration tool

Abstract

The γ-aminobutyric acid (GABA) is a product of decarboxylation of the amino acid glutamate mediated by the synthesizing enzyme glutamic acid decarboxylase (GAD) (1–3). Although GABA is a major inhibitory neurotransmitter of the brain, it is produced at high levels in pancreatic islets (4). β-Cells store GABA in synaptic-like microvesicles, and upon its secretion, GABA exerts many paracrine functions in pancreatic islets (4). While the total function of GABA in β-cells is incompletely understood (4), its synthesizing enzyme GAD is possibly one of the most significant pancreatic islet β-cell autoantigens (5). GAD is a primary target of autoantibodies, and anti-GAD antibodies are associated with the development of type 1 diabetes (T1D) (5). GABA activates three types of membrane receptors: GABAA and GABAC, which are ligand-gated Cl− channels, and GABAB, a ligand-gated Ca2+ or K+ channel (6). It has been demonstrated that β-cells mainly express the GABAB receptor (GABABR) and the GABAA receptor (GABAAR) and produce GABA through GAD (Fig. 1) (4); GABA colocalizes with insulin as shown by confocal microscopy (4,7). Activation of GABA receptors in islet β-cells increases insulin release (8), exerts protective and regenerative effects on islet β-cells (9), and reduces apoptosis in cultured islets (9). GABA has also been shown to increase DNA synthesis in the pancreatic cell line INS-1, and when injected in vivo it increased the number of Ki67+ islet β-cells (Fig. 1). Thus, GABA increases β-cell proliferation in vivo and in vitro, protects INS-1 cells from streptozotocin (STZ)-induced apoptosis, and prevents hyperglycemia in murine models of diabetes (9). FIG. 1. Regenerative and immunological abilities of the inhibitory neurotransmitter GABA. Extracellular glutamate, the precursor of …