Spatially Organized β-Cell Subpopulations Control Electrical Dynamics across Islets of Langerhans

Abstract

Understanding how heterogeneous cells within a multicellular system interact and affect overall function is difficult without a means of perturbing individual cells or subpopulations. Here we apply optogenetics to understand how subpopulations of β-cells control the overall [Ca²⁺]_i response and insulin secretion dynamics of the islets of Langerhans. We spatiotemporally perturbed electrical activity in β-cells of channelrhodopsin2-expressing islets, mapped the [Ca²⁺]_i response, and correlated this with the cellular metabolic activity and an in silico electrophysiology model. We discovered organized regions of metabolic activity across the islet, and these affect the way in which β-cells electrically interact. Specific regions acted as pacemakers by initiating calcium wave propagation. Our findings reveal the functional architecture of the islet, and show how distinct subpopulations of cells can disproportionality affect function. These results also suggest ways in which other neuroendocrine systems can be regulated, and demonstrate how optogenetic tools can discern their functional architecture.

Figure 1

ChR2 activation creates local [Ca²⁺]_i elevation in pancreatic islets. (A) A model of spatial activation of ChR2 in pancreatic islets. Activation regions within the islet are defined, where ChR2 is activated by 458 nm illumination leading to depolarization and opening of V-gated Ca²⁺ channels. (B) An activation region defined over the whole islet (left, green dashes) generates islet-wide [Ca²⁺]_i influx (left, orange). [Ca²⁺]_i increases rapidly after ChR2 activation, as measured by Rhod-2 fluorescence (right). (C) An activation region defined over a quadrant of the islet (left, green dashes) generates local [Ca²⁺]_i elevation, which extends outside of the activation region (left, orange). More distant areas of the islet show no [Ca²⁺]_i influx (right). (D) Mean ± SE ChR2-activated [Ca²⁺] resulting from quadrant activation regions, presented in rank order, normalized to islet size. (E) Mean ± SE intraislet variation in ChR2-activated [Ca²⁺] relative to the islet average. The relative area of ChR2-activated [Ca²⁺]_i over each quadrant (as in D) was expressed relative to the mean ChR2-activated [Ca²⁺]_i of each islet, and sorted from least to most. (F) Activation of smaller single-cell regions within islets similar to (C). [Ca²⁺] is elevated within and outside the activation region (right). (G) Distribution of the ChR2-activated area upon single-cell activation regions (n = 96 regions). (H) Mean ± SE ChR2-activated area of [Ca²⁺]_i elevation upon varying sizes of activation region. (I) Repeated activation of one quadrant is shown at 0, 10, 20, and 30 min. (J) Mean ± SE intraislet variation in ChR2-activated [Ca²⁺] as in (E), at time 0 and after 30 min of repeated ChR2 stimulation, sorted from least to most based on time-0 measurements. (K) As in (I), for minimum and maximum quadrants at 0, 10, 20, and 30 min during ChR2 stimulation. (L) Mean ± SE ChR2-activated area upon single-cell activation at time 0 and after 10 min of repeated ChR2 stimulation, averaged over the bottom and top 25% areas, and under a second stimulation sorted based on time-0 measurements. Scale bar in (B), (C), and (F) indicates 2% change in fluorescence. Data in (D) and (E) were averaged over n = 25 islets from six mice; data in (G) was averaged over n = 96 regions, 26 islets from five mice. Data in (I–L) were averaged over n = 10 islets from two mice. ^∗∗∗∗ indicates p < 0.0001, ^∗∗ indicates p < 0.01, ^∗ indicates p < 0.05 comparing experimental groups indicated. In (K), maximal values are compared to the minimal values at each time point. Image scale bars, 100 μm. To see this figure in color, go online.

Figure 2

Spatial variations in metabolic activity control electrical activity. (A) Representative images of NAD(P)H autofluorescence at glucose levels indicated, or change in NAD(P)H autofluorescence between indicated glucose levels. (B) Mean ± SE ChR2-activated area at 5 mM glucose for corresponding rank order quadrants of NAD(P)H response between 5 and 11 mM glucose. (C) Mean ± SE NAD(P)H response between 5 and 11 mM glucose for single-cell activation regions in which ChR2-activated area was low (<0.1) or high (>0.1). (D) As in (B) for ChR2-activated area at 5 mM glucose plus 150 μM diazoxide, for corresponding NAD(P)H response between 5 and 11 mM glucose plus 150 μM diazoxide. Data in (B) were averaged over n = 25 islets from six mice; data in (C) were averaged over n = 96 regions, 26 islets from five mice; data in (D) were averaged over n = 10 islets from three mice. ^∗ indicates p < 0.05, ^∗∗ indicates p < 0.01, comparing experimental groups indicated. For (B), this includes a Benjamini-Hochberg procedure and a Bonferroni correction (together with data in Fig. S3, D–H). † indicates p < 0.05 comparing experimental groups indicated, including a Benjamini-Hochberg procedure (together with unpublished data for four further conditions). Student t-test was used in (C) to calculate statistical significance. Scale bar in (A) indicates 100 μm. To see this figure in color, go online.

Figure 3

Spatial domains in an islet model recapitulates ex vivo excitability. (A) Representative false-color map shows cellular GK rate over a simulated islet, where subregions of similar heterogeneity in GK rate are applied (other variables randomly distributed). Each sphere represents a simulated β-cell. (B) Representative time-course of [Ca²⁺] in five cells of a simulated islet is given following ChR2 protocol: glucose is held at a concentration of 5 mM for quadrant ChR2 activation, and then increased to 11 mM to stimulate [Ca²⁺] oscillations. (C) Mean ± SE of ChR2-activated area, given in terms of number of cells activated, by quadrant in rank order. (D) Mean ± SE intraislet variation in ChR2-activated area given relative to the simulated islet average. The relative area of ChR2-activated [Ca²⁺] over each quadrant was expressed relative to the mean ChR2-activated [Ca²⁺] of each simulated islet, and sorted from least to most. (E) Mean ± SE intraislet variation in GK rate given relative to the islet average, calculated in the same way as in (D). (F) Mean ± SE ChR2-activated area given relative to the islet average at 2 mM glucose, for each corresponding quadrant GK rate (as in D). (G) Mean ± SE ChR2-activated area given relative to the islet average at 5 mM glucose, for each corresponding quadrant GK rate at 5 mM glucose (as in F). (H) Mean ± SE ChR2-activated area given relative to the islet average at 5 mM glucose, for each corresponding ascending rank-ordered quadrant GK rate (as in D) under the addition of 10% K_ATP open channel conductance in all cells. (I) Representative false-color map shows cellular GK rate over a simulated islet, where random spatial distribution of heterogeneity is applied. (J) As in (D), for simulated islets with randomly distributed heterogeneity. (K) As in (G), for simulated islets with randomly distributed heterogeneity. Data in (C–H), (J–K) were averaged over n = 30 simulated islets. ^∗ indicates p < 0.05, ^∗∗ indicates p < 0.01, ^∗∗∗ indicates p < 0.001 comparing experimental groups indicated. To see this figure in color, go online.

Figure 4

Spatial analysis of NAD(P)H and ChR2-activated [Ca²⁺] responses. (A) Representative NAD(P)H intensity image, displaying how spatial correlations in the NAD(P)H response (between 5 and 11 mM glucose) were quantified. The ΔNAD(P)H response was calculated by the absolute difference taken between an initial NAD(P)H response measurement (purple), and the NAD(P)H response averaged over an outwardly expanding region (white circles) with radius Δr. (B) ΔNAD(P)H response shown as a function of radial distance Δr, as described in (A), normalized to the ΔNAD(P)H at the largest Δr. (C) Equivalent analysis to that in (B), calculated from GK rates in simulated islets with random spatial distribution of GK rate (gray) or subregions of similar GK rates (black). (D) Representative image showing how spatial differences in NAD(P)H response were quantified. The absolute differences in NAD(P)H response were calculated for several small regions (purple circles) separated by distance Δr, in a pairwise manner. (E) Shown here are mean absolute differences in NAD(P)H response between regions as a function of spatial separation Δr, as described in (D), normalized to the difference in response at maximal separation. (F) Equivalent analysis to that in (E), calculated from GK rates in simulated islets with random spatial distribution of GK rate (gray) or subregions of similar GK rates (black). Dashed lines in (B), (C), (E), and (F) indicate a distance of ∼6 cells. (G) Representative map of ChR2-activated [Ca²⁺]_i in an islet at 5 mM glucose (orange, blue), where activation regions are applied individually over a single cell. (H) Mean ± SE pairwise absolute differences in ChR2-activated [Ca²⁺] area were averaged by close (Δr < 50 μm) and far (Δr > 50 μm) differences (12–16 locations per islet), then normalized to the average ChR2-activated area for each islet. (I) Equivalent analysis to that in (H) calculated from single-cell ChR2-activated [Ca²⁺] in simulated islets with subregions of similar or random GK rates. Student t-test was used in (H) and (I). ^∗∗ indicates p < 0.01, comparing experimental groups indicated. Scale bar in (A) indicates 100 μm. To see this figure in color, go online.

Figure 5

Calcium wave origin corresponds to less excitable and metabolically active regions. (A) A representative phase map of [Ca²⁺] oscillations within an islet at 11 mM glucose, as calculated through Fourier analysis, which indicates a wave emerging in region of minimum phase (dark blue) and terminating in a region of maximum phase (dark red). (B) Representative time-courses from (A), showing phase lag of [Ca²⁺] waves. (C) Mean ± SE ChR2-activated area of [Ca²⁺] relative to the islet average, in quadrant of wave origin and wave end. (D) Mean ± SE NAD(P)H response (between 5 and 11 mM glucose) relative to the islet average, in selected regions of minimum and maximum phase, as indicated in (A). (E) Mean ± SE [Ca²⁺] oscillation amplitude (normalized to average Rhod-2AM fluorescence) in quadrants of wave origin and wave end. (F) A representative false-color map of [Ca²⁺] wave in a simulated islet, as in (A). (G) Representative time-courses from (F), showing phase lag of [Ca²⁺] waves. (H) Mean ± SE ChR2-activated area in quadrants of wave origin and wave-end in simulated islet, as in (C). (I) Mean ± SE GK rate in quadrants of wave origin and wave-end in simulated islet, as in (D). (J) Mean ± SE [Ca²⁺] oscillation amplitude in quadrants of wave origin and wave-end in simulated islet, as in (E). Data in (C–E) were averaged over n = 16 islets from four mice. Data in (H–J) were averaged over n = 30 simulated islets. ^∗ indicates p < 0.05, ^∗∗ indicates p < 0.01, ^∗∗∗ indicates p < 0.001, ^∗∗∗∗ indicates p < 0.0001 comparing experimental groups indicated. Student t-test was used in (C–E), (H–J) to calculate statistical significance. Vertical scale bars indicate 2% change in fluorescence in experiment or 100 nM change in [Ca²⁺]_i in simulated islets. Scale bar in (A) indicates 100 μm. To see this figure in color, go online.

Figure 6

Metabolic activity connects intrinsic oscillatory frequency with wave origin. (A) A representative false-color map of β-cell [Ca²⁺]_i oscillatory frequency in a Cx36^−/− islet. (B) The relationship of NAD(P)H response between 5 and 11 mM glucose and the [Ca²⁺] oscillation frequency of individual β-cells within Cx36^−/− islets, together with linear regression (±95% confidence intervals). (C) Mean ± SE [Ca²⁺] oscillation frequency before (−) and after (+) addition of the GK inhibitor 5 mM D-mannoheptulose in individual β-cells within Cx36^−/− islets. (D) Representative time-courses of two uncoupled β-cells in the islet model with high or low GK rate. (E) Pearson correlation coefficient for the effect of heterogeneity in the indicated parameters on [Ca²⁺] oscillation frequency within the islet model. (F) Natural oscillation frequency of β-cells was averaged over regions corresponding to wave origin and wave-end, in electrically uncoupled islets that are simulated to have subregions of similar GK rates (see Fig. 3). Data in (B) represent n = 8 islets from three mice. Data in (C) represent n = 5 islets from two mice. ^∗∗ indicates p < 0.01, ^∗∗∗ indicates p < 0.001 comparing experimental groups indicated. Scale bar (A) indicates 100 μm. To see this figure in color, go online.