Real-Time In Vivo Imaging of Whole Islet Ca²⁺ Dynamics Reveals Glucose-Induced Changes in Beta-Cell Connectivity in Mouse and Human Islets

Imaging of Ca²⁺ dynamics in the intact islet in vitro has recently been subjected to connectivity analysis informed by similar approaches in neuronal systems. This has revealed the existence of intercellular networks between individual β cells which are increased by high glucose. Here, we sought to investigate if network connectivity also exists in islets in vivo under conditions of near-normal vascularisation and innervation. Donor mouse (C57BL/6J) islets were infected with adenovirus expressing the Ca²⁺ sensor GCaMP6m and transplanted into the anterior chamber of the eye in syngeneic recipients. Under isofluorane anaesthesia, fully implanted islets were imaged using a spinning disk confocal microscope (Nikon Ti-E, 25 x water dipping 1.0 NA objective, 300ms exposure time, 1 frame per second, 488nm excitation). Mean baseline blood glucose was 19mM and rose to >30mM following an IP bolus of 1.5g/L glucose. Human islets were infected with AV-GCaMP6m, transplanted into a nu/nu mouse recipient and imaged under conditions of normoglycaemia (blood glucose 3.9-4.9 mM followed by hyperglycaemia (blood glucose >30 mM). As circulating blood glucose increased, the proportion of connected cells in mouse islets rose from 65.5% to 88.3% (n=3 islets in 3 different animals, p=0.02) whilst the coefficient of correlation (a measure of the strength of the connectivity) showed a tendency to increase from 0.34 to 0.44 (p=NS). Data from human islets also revealed a tendency towards a rise in connectivity from 58.3% to 63.9% in response to rising circulating glucose. These data demonstrate that functional connectivity of Ca²⁺ dynamics between β cells is conserved in vivo in both mouse and human islets, with enhanced connectivity patterns observed in response to rising blood glucose levels. This technique, which allows β cell connectivity to be examined prospectively over time, will provide a powerful new approach for assessing its significance in health and disease.