The neuronal K⁺Cl^- co-transporter 2 (Slc12a5) modulates insulin secretion

Intracellular chloride concentration ([Cl^-]_i) in pancreatic β-cells is kept above electrochemical equilibrium due to the predominant functional presence of Cl^- loaders such as the Na⁺K⁺2Cl^- co-transporter 1 (Slc12a2) over Cl^-extruders of unidentified nature. Using molecular cloning, RT-PCR, Western blotting, immunolocalization and in vitro functional assays, we establish that the "neuron-specific" K⁺Cl^- co-transporter 2 (KCC2, Slc12a5) is expressed in several endocrine cells of the pancreatic islet, including glucagon secreting α-cells, but particularly in insulin-secreting β-cells, where we provide evidence for its role in the insulin secretory response. Three KCC2 splice variants were identified: the formerly described KCC2a and KCC2b along with a novel one lacking exon 25 (KCC2a-S25). This new variant is undetectable in brain or spinal cord, the only and most abundant known sources of KCC2. Inhibition of KCC2 activity in clonal MIN6 β-cells increases basal and glucose-stimulated insulin secretion and Ca²⁺ uptake in the presence of glibenclamide, an inhibitor of the ATP-dependent potassium (K_ATP)-channels, thus suggesting a possible mechanism underlying KCC2-dependent insulin release. We propose that the long-time considered "neuron-specific" KCC2 co-transporter is expressed in pancreatic islet β-cells where it modulates Ca²⁺-dependent insulin secretion.