Spatial and temporal coordination of insulin granule exocytosis in intact human pancreatic islets
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Insulin secretion is widely studied because it plays a central role in glucose homeostasis and diabetes. Processes from insulin granule fusion in beta cells to in vivo insulin secretion have been elucidated, but data at the cellular level do not fully account for several aspects of the macroscopic secretory pattern. Here we investigated how individual secretory events are coordinated spatially and temporally within intact human islets.
We used the fluorescent probe neuropeptide Y (NPY)–pHluorin to visualise insulin granule secretion in isolated intact human islets.
We found that individual beta cells respond to increases in glucose concentration by releasing insulin granules in very discrete bursts with periods consistent with in vivo pulsatile insulin secretion. In successive secretory bursts during prolonged exposure to high glucose levels, secretory events progressively localised to preferential release sites, coinciding with the transition to second phase insulin secretion. Granule secretion was very synchronised in neighbouring beta cells, forming discrete regional clusters of activity.
These results reveal how individual secretory events are coordinated to produce pulsatile insulin secretion from human islets.
KeywordsExocytosis Human islets Insulin granule Pulsatile secretion
Green fluorescent protein
Regions of interest
Soluble N-ethylmaleimide-sensitive factor attachment protein receptor
Total internal reflection fluorescence
The beta cell of the pancreatic islet is the only cell in the body that secretes insulin, which in turn is the only hormone able to lower plasma glucose levels. Death or dysfunction of beta cells thus disturbs glucose homeostasis and leads to diabetes. Not surprisingly, beta cell function has been studied so exhaustively that the signalling pathways coupling glucose detection to insulin granule release are part of the physiological canon. Different aspects of insulin secretion, such as its dose–response relationship for glucose and its dynamic temporal profile, have been examined in vitro and in vivo in many species including humans. A major feature of insulin secretion is its pulsatile pattern, in which most insulin is released in discrete bursts with periods ranging from 4 to 10 min [1, 2]. At the molecular level, insulin is packed in granules that are released in a Ca2+-dependent process similar to that in neurons, which is Ca2+-dependent and requires soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins .
Granule exocytosis in beta cells has been studied using electrophysiological approaches, such as membrane capacitance measurements and amperometry [4, 5, 6], and microscopic approaches such as two-photon or total internal reflection fluorescence (TIRF) microscopy in combination with fluorescent probes and vesicle cargo proteins [7, 8, 9, 10, 11]. Data have mainly been obtained from clonal beta cell lines or dispersed beta cells, which do not allow the study of how insulin release is coordinated in the islet. Attempts at measuring granule exocytosis in beta cells within intact islets have been made only with rodent islets [9, 12, 13], which differ structurally and functionally from human islets [14, 15].
For these reasons, it is still unclear how individual granule secretory events are coordinated in individual beta cells within an islet to produce a concerted secretory response that is pulsatile. Indeed, most studies show that insulin granules are released at a steady rate. Only two studies report a pulsatile pattern of granule fusion, though at a frequency not related to that of in vivo secretion [11, 13]. We therefore investigated insulin granule exocytosis by using the fluorescent reporter neuropeptide Y (NPY)–pHluorin and confocal microscopy to visualise granule secretion in real time in beta cells within intact human islets. This technique allowed us to study the spatial and temporal patterns of granule secretion in individual beta cells as well as in beta cell populations throughout the human islet.
We obtained human pancreatic islets from the Integrated Islet Distribution Program (NIDDK, NIH (Bethesda, MD, USA)). We received 11 healthy human islet preparations for this study but we observed responses to KCl and high glucose in only seven islet preparations that we used for the experiments included in the manuscript (male and female donors, 44.6 ± 6.2 years old, BMI 26.0 ± 3.1 kg/m2). Experiments have been approved by the ethics committee of the University of Miami.
Adenoviral production and islet infection
Human islets were infected with adenoviruses containing NPY fused to the pH-dependent green fluorescent protein (GFP) pHluorin for 48 h and cultured at 37°C for 5–7 days in CMRL 1066 (Cellgro, Herndon, Virginia, USA), 10% (vol./vol.) FBS and 2 mmol/l l-glutamine . Approximately 20–40% of the islet cells were infected. Most of the infected cells showed no background fluorescence, with few granules visible at basal glucose concentration (3 mmol/l) in the absence of stimulation. Some cells had increased background fluorescence in the cytoplasm but did not show secretory events upon stimulation and were therefore not included in the analyses.
Confocal imaging and data quantification
Human islets were placed on a coverslip in an imaging chamber (Warner instruments, Hamden, CT, USA) for imaging on a Leica TCS SP5 upright laser-scanning confocal microscope (Leica Microsystems, Wetzlar, Germany). Islets were continuously perfused with extracellular solution (in mmol/l: 125 NaCl, 5.9 KCl, 2.56 CaCl2, 1 MgCl2, 25 HEPES, 0.1% BSA, 3 mmol/l glucose, pH 7.4, 37°C) and confocal images were acquired with LAS AF software (Leica Microsystems) using a 63× water immersion objective (HCX APO L 63×/0.9 NA). We used a resonance scanner for fast image acquisition to produce time-lapse recordings spanning 50 μm of the islet (z-step: 5 μm, stack of ten confocal images with a size of 512 × 512 pixels) at 1.5 s resolution (XYZT imaging). NPY-pHluorin fluorescence was excited at 488 nm and emission detected at 535–550 nm. Insulin granule exocytosis was stimulated by increasing glucose concentration from 3 to 16 mmol/l, in the presence of cAMP-raising agents 3-isobutyl-1-methylxanthine (IBMX) (100 μmol/l) and forskolin (10 μmol/l)  or by KCl depolarisation (30 mmol/l). cAMP-raising agents increased the consistency of secretory responses but did not change temporal patterns of granule secretion so that secretory events appeared in discrete bursts synchronised with the average cell response (see below) and with similar burst periods (range 1.4–6.6 min with IBMX/forskolin vs 1.5–10 min without). Intracellular pH was increased by using 50 mmol/l NH4Cl (to replace NaCl on an equimolar basis) and the plasma membrane was labelled with di-8-ANEPP (2 μmol/l for 1 h at 37°C; Invitrogen), excited at 488 nm and detected at 620 nm.
ImageJ (http://imagej.nih.gov/ij/) was used to determine changes in fluorescence intensity over time in regions of interest (ROI) placed around single secretory events in confocal planes (see electronic supplementary material [ESM] Methods for a complete description of imaging data analysis). To identify sites of preferential secretion, we plotted fluorescence intensity along cell membranes and determined the number of secretory events that reappeared at the same site in two secretory bursts (adaptation from the birthday problem). Temporal stacks of images acquired during 30 s (∼20 images) were used to calculate the CV of the fluorescence signal in the cell , according to the formula: CV = SD / mean. Higher CV values indicate higher concentration of fluorescence signals. Pearson’s correlation coefficient was used to determine co-localisation of secretory events in 30 s temporal stacks from two bursts.
Insulin secretion from non-infected or NPY–pHluorin-infected islets was measured 5 days after infection with adenoviruses as described previously .
Human islets infected for 5 days with NPY-pHluorin were fixed with 4% (vol./vol.) paraformaldehyde for 30–60 min on a microscope slide and immunostained for insulin, glucagon or somatostatin and GFP (Invitrogen, Carlsbad, CA, USA) using protocols previously described . To assess co-localisation of NPY–pHluorin with islet hormones, we used ImageJ co-localisation analysis plugin ‘Intensity Correlation Analysis’ and calculated Mander’s overlap coefficients (R). R values over 0.6 denote co-localisation .
We used Prism 5.0 software (GraphPad Software, version 5, San Diego, CA, USA) to perform one-way analysis of variance (ANOVA) followed by a Tukey’s multiple comparison test and considered statistical significance when p values were lower than 0.05. Bar graphs show means ± SEM.
NPY–pHluorin works as a reporter for insulin secretory events
Most NPY–pHluorin-expressing cells were beta cells (92 ± 3%) and only a small proportion of glucagon-containing alpha and somatostatin-containing delta cells were GFP-labelled (11 ± 3% and 13 ± 2% for alpha and delta cells, respectively, n = 5 islets). Insulin co-localised with NPY–pHluorin, confirming that NPY fusion protein was stored in granules containing endogenous insulin (Fig. 1g–i). Although a subset of alpha cells was also infected with NPY–pHluorin (∼10%), lowering the glucose concentration from 16 to 1 mmol/l, a specific stimulus for alpha cells, did not elicit detectable granule fusion. Given their small number in the islet (∼10%), the contribution of delta cells is negligible. While part of the exogenous NPY–pHluorin could segregate to granules other than those containing insulin , their glucose dependency and co-localisation with insulin indicate that most secretory events under high glucose conditions represent cargo release from insulin granules.
Although NPY has been shown to decrease insulin secretion from rodent islets [22, 23] and to be present in human islets , the total amount of glucose-stimulated insulin released from NPY–pHluorin-infected human islets was not different from that released from non-infected islets (AUC during 15 min stimulation with 16 mmol/l glucose was 0.14 ± 0.01 ng/μl × min for non-infected islets and 0.11 ± 0.005 ng/μl × min for NPY–pHluorin-infected islets, p = 0.11, n = 3 perifusions).
Secretory events are highly synchronised and generate discrete secretory bursts
In healthy people insulin is secreted in pulses. Pulsatile secretion from the islet is crucial for the action of hormones in target tissues as it maintains a higher expression of insulin receptors at the membrane and facilitates post-insulin receptor signalling [2, 25]. Insulin secretion oscillates in periods of 4–10 min in portal blood [26, 27]; a similar range has been observed in vitro with perfused pancreases or isolated islets from dogs (10 min), rhesus monkeys (∼6 min) and humans (4–7 min [28, 29, 30, 31, 32, 33, 34]).
Spatial pattern of granule secretion
Release sites become more localised with each subsequent secretory burst
Secretory patterns reveal clusters of synchronous cells
The order in which beta cells showed exocytotic responses changed from burst to burst, indicating that there was no dominant cell serving as a pacemaker within a cluster (Fig. 6b).
Our results show that imaging of NPY–pHluorin can be used to visualise insulin granule exocytosis in beta cells within intact human islets. We determined that in response to glucose stimulation insulin granules in individual beta cells are secreted in distinct bursts with exquisite synchronisation. These bursts appear in periods similar to those of pulsatile in vivo insulin secretion. With prolonged glucose stimulation and increasing pulse number, secretory events within a beta cell become localised to regions of preferred secretion. We further demonstrated that neighbouring human beta cells form distinct clusters comprised of ∼5–10 cells in which activity is synchronised. These clusters of synchronised activity are preserved from burst to burst and differ from surrounding clusters in their timing within the global secretory pulse of the islet.
Most imaging studies on insulin exocytosis have been performed on either cell lines or isolated beta cells, mainly using TIRF microscopy (e.g. [8, 11, 35, 38, 39, 40, 41]). Islet dissociation and cell culture induce cellular rearrangements that not only disrupt cell-to-cell communication, but may also affect granule pool distribution. Dispersed beta cells show marked variability in their responses to glucose due to metabolic heterogeneity and different activation thresholds. Because TIRF microscopy does not allow imaging of cells within intact islets, other imaging modalities have been used to measure insulin secretion in situ in the intact islet [9, 12, 13, 37]. These experiments, however, were conducted on rodent islets, which are structurally different from human islets [14, 15]. Not surprisingly, our results with intact human islets have revealed features in insulin exocytosis not predicted by previous studies (see below).
Our findings show that multiple secretory events in individual beta cells occur almost simultaneously, within a narrow temporal window of 8 s, and in discrete bursts. Few secretory events (<9%) occurred in the periods between bursts. This pattern is in contrast to that found in other studies in which insulin granules underwent exocytosis at a steady rate, most likely as a result of averaging the behaviour of asynchronous dispersed cells. Our results are in line with those of a study on small clusters of human beta cells showing that insulin granules are secreted in recurrent discrete bursts . However, bursts appeared with a periodicity of 15–45 s, which is different from the periods we determined and from those of insulin secretion in vitro and in vivo [26, 27, 29, 31, 33, 34, 42].
Our findings further indicate that prolonged stimulation with high glucose changes the spatial pattern of granule secretion. With each subsequent secretory burst, granule secretion becomes more localised to preferred sites of exocytosis. We currently do not have a morphological or molecular correlate for the observed sites of preferred secretion, but they could have a high density of P/Q-type Ca2+ channels , be areas where SNARE proteins cluster  or be plasma membrane hubs for the delivery of components of the exocytotic machinery orientated towards the vasculature . Interestingly, granule secretion becomes more confined at a time that coincides with the transition from first phase to the sustained second phase of insulin secretion (∼5–10 min into glucose stimulation) [45, 46]. The transition to the second phase of insulin secretion can be explained by the depletion of primed granules and the recruitment of different functional pools of insulin granules with different release competence [3, 4]. Our results now suggest that this transition is also associated with changes in the spatial pattern of insulin granule secretion.
We also found that neighbouring beta cells form highly synchronised functional clusters of up to ten cells, consistent with the six to seven cells that are coupled through gap junctions in human islets . There was no preferential sequence in which cells within a cluster were activated, which is consistent with the notion that electrical coupling lessens the effects of physiological cell-to-cell variations [48, 49]. In contrast to the fast coupling seen within clusters, longer time delays in activation and their spatial segregation within the islet suggest that coordination between clusters requires paracrine signalling. The looser coupling between cell clusters may explain why the overall secretory pulse of the islet is less discrete than that of granule secretion from individual cells or clusters [33, 34].
Characterising insulin exocytosis in vitro is a first step towards fully understanding the cellular mechanisms that orchestrate individual secretory events to achieve the complex oscillatory pattern of insulin release. The ability to measure insulin granule secretion in real time from multiple cells in intact human islets provides an exceptional tool that can be combined with mouse models of in vivo imaging of vascularised and innervated islets . It is important to determine whether the subcellular spatial pattern of insulin granule secretion we report here is associated with vascular arrangements .
This work was funded as follows: NIH grants R56DK084321 (AC) and R01DK084321 (AC); the Institute for Basic Science IBS-R013-D1 (HGN); the Juvenile Diabetes Research Foundation (P-OB); the Swedish Research Council; the Novo Nordisk Foundation; the Swedish Diabetes Association; the Family Erling-Persson Foundation; the Skandia Insurance Company Ltd; Strategic Research Program in Diabetes at Karolinska Institutet; the Berth von Kantzow’s Foundation; VIBRANT Grant FP7-2288933; the Knut and Alice Wallenberg Foundation and Lee Kong Chien School of Medicine, Nanyang Technical University, Singapore and Imperial College, London, UK ERC-2013-AdG 338936-BetaImage. JA is a recipient of a postdoctoral fellowship from the American Heart Association (14POST20380499).
Duality of interest
The authors declare that there is no duality of interest associated with this manuscript.
JA and AC conceived and designed the study and wrote the manuscript. P-OB, HGN and AC raised funding. JA acquired the data, JA and AC analysed the data and all authors interpreted the data. All authors revised the article and approved its final version. JA is the guarantor of this work.
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