Restoring Connexin-36 Function in Diabetogenic Environments Precludes Mouse and Human Islet Dysfunction

Abstract

Type2 diabetes results from failure of the β-cell to compensate for insulin resistance, such as in obesity. Insulin secretion is governed by a series of metabolic and electrical events which can fail during the progression of diabetes. β-cells are electrically coupled via Cx36 gap junction channels, thereby coordinating the pulsatile dynamics of electrical activity, Ca2+ and insulin release across the islet, enhancing insulin action. Pulsatile insulin release is disrupted in human type2 diabetes, although whether this disruption results from diminished gap junction coupling is unclear. Factors such as pro-inflammatory cytokines and free fatty acids disrupt gap junction coupling under invitro conditions. Here we test whether gap junction coupling and coordinated Ca2+ dynamics are disrupted in type2 diabetes, and whether recovery of gap junction coupling can recover islet function. We examine islets from healthy donors and those with type2 diabetes, as well as islets from db/db mice and islets treated with a cocktail of proinflammatory 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 tail of Cx36 designed to compete against its phosphorylation and downregulation. Cx36 gap junction permeability and coordinated Ca2+ dynamics were disrupted in islets from human donors with type2 diabetes, as well as in islets from db/db mice or treated with proinflammatory 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 proinflammatory 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.

Beta-cell Metabolic Activity Rather than Gap Junction Structure Dictates Subpopulations in the Islet Functional Network

“…This implies that as structural gap junction connections are removed, the islet loses efficient signal propagation and high ability to synchronize. This decreased functional small worldness likely contributes to the lack of islet coordination and pulsatile insulin in diseases such as diabetes, where gap junction coupling has been shown to decrease (26, 27, 50).…”

“…Diabetes Mellitus is marked by changes in both the gap junction-based structural 1,12,[41][42][43][44] and [Ca 2+ ] synchronization-based functional 11,12,45,46 networks in the pancreatic islet. However, the relationship between these two network representations is not well understood and has caused confusion in our understanding of islet communication 26,29,[31][32][33] .…”

“…Further, the significant decrease in both clustering coefficient and global efficiency of the functional network with decreasing Cx36 expression and gcoup indicates that as structural gap junction connections are removed, the islet loses efficient signal propagation and high ability to synchronize, which likely has pathophysiological implications. For example, this decreased functional small worldness likely contributes to the lack of islet coordination and pulsatile insulin, in diseases such as diabetes, where gap junction coupling has been shown to decrease (25,26,51). Islet coordination and pulsatile insulin release are essential for proper glucose homeostasis (19)(20)(21).…”

“…This electrical coupling allows the 𝛽-cell network to synchronize its electrical dynamics and secrete insulin in discrete pulses (19), which are efficacious for the maintenance of glucose homeostasis (20)(21)(22)(23). Interestingly, both type 1 and type 2 diabetes are preceded by the loss of this islet synchronization and disruption to Cx36 gap junction coupling (20,(24)(25)(26)(27). Therefore, understanding the link between the islet structural network formed by gap junction channels, the functional network defined by Ca 2+ activity patterns, and how these networks are disrupted in diabetes is highly relevant for understanding the progression of islet dysfunction in diabetes.…”

“…Loss of intra-islet coordination in diabetes is associated with changes in the functional 𝛽-cell network. However, it is unclear how disruption in the functional network is related to changes in individual 𝛽-cell function and disruptions in the structural network, as type 1 and some presentations of type 2 diabetes are also associated with dysfunction or death of insulin-producing 𝛽-cells and loss of gap junction coupling (22)(23)(24)(25)(26). Therefore, understanding the interaction between structure, function, and system dynamics is important.…”