Differentiating the Roles of Metabolic Similarity and Ionic Coupling in Determining the Beta Hub Cell Phenotype

Abstract

Pancreatic beta cells regulate circulating glucose levels by releasing insulin. Beta cells transduce elevated blood glucose into electrical activity through an intrinsic cascade of metabolic and electrophysiologic responses. These responses are synchronized across the electrically connected network of beta cells, such that insulin release is also relatively synchronous. Despite their coordinated behavior, individual beta cells exhibit significant functional heterogeneity. This heterogeneity is thought to provide cells with specific functional phenotypes the ability to control the activity of the broader network. Hub cells, identified by their synchronized [Ca2+] activity, are one such functional subpopulation, and are believed to orchestrate the second, oscillatory phase of insulin release. However, it remains unclear whether the cell-autonomous characteristics of hub cells, such as their metabolic activity and electrophysiologic properties, are more important than their network characteristics (i.e. gap junctional coupling) for their ability to influence broader network activity. In this study, we investigate the roles of intrinsic metabolic and electrophysiologic properties and ionic coupling in determining the beta hub cell phenotype. Using a computational islet model of 1,000 beta cells, our analysis revealed that both intrinsic metabolic properties and structural coupling via gap junctions are crucial for determining the hub cell phenotype. After investigating the intrinsic coupling conductance of neighboring cells and the number of structural (direct electrical) links as independent contributors to the hub cells local electrical coupling, we find that the number of cells to which a beta cell is directly coupled may be a key determinant of its propensity to serve as a hub cell in the model. As predicted for this subpopulation, we also demonstrate that decoupling hub cells impairs the functional connectivity of the entire network. Our findings indicate the importance of both autonomous cellular dynamics and non-autonomous structural coupling for the hub cell phenotype. These insights help build a fundamental understanding of hub cells, which in turn may contribute to identifying potential approaches to preserve or improve beta cell function and thereby manage the progression of diabetes.