The majority of genetic risk variants for type 2 diabetes (T2D) affect insulin secretion, but the mechanisms through which they influence pancreatic islet function remain largely unknown. We functionally characterized human islets to determine secretory, biophysical, and ultrastructural features in relation to genetic risk profiles in diabetic and nondiabetic donors. Islets from donors with T2D exhibited impaired insulin secretion, which was more pronounced in lean than obese diabetic donors. We assessed the impact of 14 disease susceptibility variants on measures of glucose sensing, exocytosis, and structure. Variants near TCF7L2 and ADRA2A were associated with reduced glucose-induced insulin secretion, whereas susceptibility variants near ADRA2A, KCNJ11, KCNQ1, and TCF7L2 were associated with reduced depolarization-evoked insulin exocytosis. KCNQ1, ADRA2A, KCNJ11, HHEX/IDE, and SLC2A2 variants affected granule docking. We combined our results to create a novel genetic risk score for β-cell dysfunction that includes aberrant granule docking, decreased Ca²⁺ sensitivity of exocytosis, and reduced insulin release. Individuals with a high risk score displayed an impaired response to intravenous glucose and deteriorating insulin secretion over time. Our results underscore the importance of defects in β-cell exocytosis in T2D and demonstrate the potential of cellular phenotypic characterization in the elucidation of complex genetic disorders.

Insulin secretion is biphasic in response to a step in glucose stimulation. Recent experiments suggest that 2 different mechanisms operate during the 2 phases, with transient first-phase secretion due to exocytosis of docked granules but the second sustained phase due largely to newcomer granules. Another line of research has shown that there exist 2 pools of releasable granules with different Ca ²⁺ sensitivities. An immediately releasable pool (IRP) is located in the vicinity of Ca ²⁺ channels, whereas a highly Ca ²⁺ -sensitive pool (HCSP) resides mainly away from Ca ²⁺ channels. We extend a previous model of exocytosis and insulin release by adding an HCSP and show that the inclusion of this pool naturally leads to insulin secretion mainly from newcomer granules during the second phase of secretion. We show that the model is compatible with data from single cells on the HCSP and from stimulation of islets by glucose, including L- and R-type Ca ²⁺ channel knockouts, as well as from Syntaxin-1A-deficient cells. We also use the model to investigate the relative contribution of calcium signaling and pool depletion in controlling biphasic secretion.

Electrical activity plays a pivotal role in glucose-stimulated insulin secretion from pancreatic -cells. Recent findings have shown that the electrophysiological characteristics of human -cells differ from their rodent counterparts. We show that the electrophysiological responses in human -cells to a range of ion channels antagonists are heterogeneous. In some cells, inhibition of small-conductance potassium currents has no effect on action potential firing, while it increases the firing frequency dramatically in other cells. Sodium channel block can sometimes reduce action potential amplitude, sometimes abolish electrical activity, and in some cells even change spiking electrical activity to rapid bursting. We show that, in contrast to L-type -channels, P/Q-type -currents are not necessary for action potential generation, and, surprisingly, a P/Q-type -channel antagonist even accelerates action potential firing. By including SK-channels and dynamics in a previous mathematical model of electrical activity in human -cells, we investigate the heterogeneous and nonintuitive electrophysiological responses to ion channel antagonists, and use our findings to obtain insight in previously published insulin secretion measurements. Using our model we also study paracrine signals, and simulate slow oscillations by adding a glycolytic oscillatory component to the electrophysiological model. The heterogenous electrophysiological responses in human -cells must be taken into account for a deeper understanding of the mechanisms underlying insulin secretion in health and disease, and as shown here, the interdisciplinary combination of experiments and modeling increases our understanding of human -cell physiology. Insulin is a glucose-lowering hormone secreted from the pancreatic -cells in response to raised plasma glucose levels, and it is now well-established that defective insulin secretion plays a pivotal role in the development of diabetes. The -cells are electrically active, and use electrical activity to transduce an increase in glucose metabolism to calcium influx, which triggers insulin release. Experimental and theoretical studies on -cells from rodents have provided valuable insight in their electrophysiology. However, human -cells differ from their rodent counterparts in several aspects including their electrophysiological characteristics. We show that the electrophysiological responses in human -cells to a range of experimental manipulations are heterogeneous. We extend a previous mathematical model of electrical activity in human -cells to investigate such heterogeneous and nonintuitive electrophysiological responses, and use our findings to obtain insight in previously published insulin secretion measurements. By adding a glycolytic component to the electrophysiological model, we show that oscillations in glucose metabolism might underlie slow oscillations in electrical activity, calcium levels and insulin secretion observed experimentally. We conclude that the interdisciplinary combination of experiments and modeling increases our understanding of human -cell physiology and provides new insight in -cell heterogeneity.