Functional architecture of the pancreatic islets reveals first responder cells which drive the first-phase [Ca²⁺] response

Abstract

Insulin-secreting β-cells are functionally heterogeneous. Subpopulations of β-cells can control islet function and the regulation of hormone release, such as driving the second (oscillatory) phase of free-calcium ([Ca2+]) following glucose elevation. Whether there exists a subpopulation that drives the first-phase response, critical for effective insulin secretion and disrupted early in diabetes, has not been examined. Here, we examine a ‘first responder’ cell population, defined by the earliest [Ca2+] response during first-phase [Ca2+] elevation. We record [Ca2+] dynamics in intact mouse islets, via β-cell specific expression of the [Ca2+] indicator GCamP6s. We identify multiple β-cell subpopulations based on signatures of their [Ca2+] dynamics and investigate the role of ‘first responder’ cells in islet function by means of 2-photon laser ablation. We further characterize the functional properties of ‘first responder’ cells by NAD(P)H autofluorescence, fluorescent recovery after photobleaching, glibenclamide stimulation, and network analysis. We also investigate which functional characteristics of these cells are critical by a computational model of islet electrophysiology. Based on the dynamics of [Ca2+] responses, first responder cells are distinct from previously identified functional subpopulations. The first-phase response time of β-cells is spatially organized, dependent on the cell’s distance to the first responder cells, and consistent over time up to ~24 h. First responder cells showed characteristics of high membrane excitability and slightly lower than average coupling to their neighbors. When first responder cells were ablated, the first-phase [Ca2+] diminished, compared to ablating a random cell. We also observed a hierarchy of the first-phase response time, where cells that were next earliest to respond often take over the role of the first responder cell upon ablation. In summary, we discover and characterize a distinct first responder β-cell subpopulation, based on [Ca2+] response timing, which is critical for the islet first-phase response to glucose.

Glucose-dependent activation, activity, and deactivation of beta cell networks in acute mouse pancreas tissue slices

“…Our data do not imply that the first-responders are in any way necessary for islets to respond to glucose concentrations and that their role is fixed in time. It is reasonable to speculate that the removal or dysfunction of first-responders would make other cells become first-responders (33,41,104,105,122), and recent experimental data corroborated this (54). Even without removal or dysfunction, the metabolic status and physiological features change as b cells mature (123).…”

“…Physiologically, an earlier response to a stronger stimulus may help prevent too large excursions of glucose in vivo. Interestingly, during the first couple of minutes following activation and before a sustained plateau phase was established, an exceedingly high frequency of fast oscillations or a [Ca 2 þ ] IC signal that could be explained by continuous bursting of electrical activity were observed, indicating that during this time, there is a transient "overshoot" of activity, as observed before (2,7,18,29,34,54). Further studies are needed to understand this phenomenon, but from a homeostatic perspective, this behavior could indicate that b cells behave as both phasic and tonic sensors, responding differently to an increasing and constantly increased glucose, again ameliorating excursions of glycemia in vivo (113)(114)(115)(116).…”

“…In dissociated b cells in culture, glucose thresholds differed substantially among individual b cells, ranging from low to supraphysiological glucose concentrations. Increasing stimulus intensity progressively recruited additional b cells into a secretory response (34,47,(52)(53)(54)(55). However, b cells in intact islets are coupled, and this narrows the concentration range over which the whole islet activates.…”

Reduced synchroneity of intra-islet Ca2+ oscillations in vivo in Robo-deficient β cells

“…β cells coupled by these gap junctions display varying levels of excitability and metabolic rates, aspects which themselves are thought to display a non-random spatial organization ( Hraha et al, 2014 ). This coupling of spatially organized heterogenous β cells populations creates an electrical syncytium that responds homogenously to glucose such that, at low glucose levels, insulin secretion is inhibited through repression of [Ca 2+ ] i oscillations across the islet, but at high glucose levels, pulsatile insulin secretion occurs through synchronous [Ca 2+ ] i oscillations that spread in fast waves across the islet from distinct initiation sites ( Benninger et al, 2008 ; Farnsworth et al, 2014 ; Hraha et al, 2014 ; Skelin Klemen et al, 2017 ; Benninger and Piston, 2014 ; Westacott et al, 2017a ; Benninger et al, 2014 ; Kravets, 2020 ; Ravier et al, 2005 ; Speier et al, 2007 ; Head et al, 2012 ). Indeed, in silico modeling experiments showed that decreasing the ratio of homotypic β cell-β cell nearest neighbors is predicted to result in perturbation to synchronous [Ca 2+ ] i oscillations ( Hoang et al, 2014 ; Head et al, 2012 ; Nittala et al, 2007 ; Hoang et al, 2016 ).…”

Partners in Crime: Beta-Cells and Autoimmune Responses Complicit in Type 1 Diabetes Pathogenesis

“…In 2020, the Benninger lab defined yet another subpopulation that is functionally distinct from those previously described (102). These cells are termed "first-responder" beta-cells, as defined by calcium dynamics.…”