Session 1: What is Diabetes? Pathways to beta cell failure

Type 1 and type 2 diabetes are distinct clinical entities primarily driven by autoimmunity and metabolic dysfunction, respectively. However, there is a growing appreciation that they may share an etiopathological factor, namely the role of variation in beta-cell sensitivity to stress factors. Increased sensitivity increases the risk of beta-cell death or insulin secretion dysfunction. The beta-cell fragility model proposes that this variation contributes to the risk of developing either type 1 or type 2 diabetes, in the presence of immunological and/or metabolic stress factors. Therapeutics that increase the resistance of beta cells to these factors and decreasing fragility may constitute a new class of anti-diabetogenics, with potential use across both diseases.

Pancreatic beta-cells are the main regulators of glucose levels in higher organisms. However, our understanding of beta-cell function comes with two caveats. First, monitoring beta-cell function with single-cell resolution in their native environment, in which beta-cells are innervated and vascularized and in constant communication with the metabolic state of the organism, remains very challenging. Second, it is difficult to mimic in an ex vivo setting the progressive increase in glucose levels that takes place after a meal in vivo. Thus, how beta-cell respond to endogenous blood sugar remains a mystery due to the difficulties to visualize beta-cell activity in vivo. We have developed a high-resolution, high speed protocol for calcium imaging of beta-cells in vivo using transgenic zebrafish expressing GCAMP6s under the insulin promoter. Remarkably, we find that endogenously, beta-cells exhibit rapid oscillations in calcium. Several experimental manipulations support the role of these oscillations in regulating glucose levels. First, they are rapidly inhibited in response to glucose lowering via insulin injection. Secondly, beta-cell activity ceases rapidly in response to temporal silencing of the circulation due to optogenetic and mechanical block of the heartbeat. Moreover, blocking the calcium oscillations leads to hyperglycemia. To test whether beta-cells are equal in their glucose-responsiveness in vivo, we developed a setup for temporally controlled increases in glucose levels. Using high-speed imaging, our results document the presence of leader and follower beta-cell subpopulations. Currently, we are applying optogenetics and selective cell ablations to define whether the leader cells control the activity of the rest of the beta-cells in the islet. Thus, our approach shines light on the true response of beta-cells to increases in glucose concentrations in vivo.