Restoring connexin-36 function in diabetogenic environments precludes mouse and human islet dysfunction

Abstract

The secretion of insulin from β-cells in the islet of Langerhans is governed by a series of metabolic and electrical events, which can fail during the progression of type 2 diabetes (T2D). β-cells are electrically coupled via connexin-36 (Cx36) gap junction channels, which coordinates the pulsatile dynamics of [Ca²⁺ ] and insulin release across the islet. Factors such as pro-inflammatory cytokines and free fatty acids disrupt gap junction coupling under in vitro conditions. Here we test whether gap junction coupling and coordinated [Ca²⁺ ] dynamics are disrupted in T2D, and whether recovery of gap junction coupling can recover islet function. We examine islets from donors with T2D, from db/db mice, and islets treated with pro-inflammatory cytokines (TNF-α, IL-1β, IFN-ɣ) or free fatty acids (palmitate). We modulate gap junction coupling using Cx36 over-expression or pharmacological activation via modafinil. We also develop a peptide mimetic (S293) of the c-terminal regulatory site of Cx36 designed to compete against its phosphorylation. Cx36 gap junction permeability and [Ca²⁺ ] dynamics were disrupted in islets from both human donors with T2D and db/db mice, and in islets treated with pro-inflammatory cytokines or palmitate. Cx36 over-expression, modafinil treatment and S293 peptide all enhanced Cx36 gap junction coupling and protected against declines in coordinated [Ca²⁺ ] dynamics. Cx36 over-expression and S293 peptide also reduced apoptosis induced by pro-inflammatory cytokines. Critically, S293 peptide rescued gap junction coupling and [Ca²⁺ ] dynamics in islets from both db/db mice and a sub-set of T2D donors. Thus, recovering or enhancing Cx36 gap junction coupling can improve islet function in diabetes. KEY POINTS: Connexin-36 (Cx36) gap junction permeability and associated coordination of [Ca²⁺ ] dynamics is diminished in human type 2 diabetes (T2D) and mouse models of T2D. Enhancing Cx36 gap junction permeability protects against disruptions to the coordination of [Ca²⁺ ] dynamics. A novel peptide mimetic of the Cx36 c-terminal regulatory region protects against declines in Cx36 gap junction permeability. Pharmacological elevation in Cx36 or Cx36 peptide mimetic recovers [Ca²⁺ ] dynamics and glucose-stimulated insulin secretion in human T2D and mouse models of T2D.

Keywords: Ca2+; diabetes; gap junctions; islet of Langerhans; peptide mimetic.