Biochemical and biophysical characterization of insulin granules isolated from rat pancreatic islets by an iso-osmotic gradient

ZIGIR, a Granule-Specific Zn2+ Indicator, Reveals Human Islet α Cell Heterogeneity

Numerous mammalian cells contain abundant Zn²⁺ in their secretory granules, yet available Zn²⁺ sensors lack the desired specificity and sensitivity for imaging granular Zn²⁺. We developed a fluorescent zinc granule indicator, ZIGIR, that possesses numerous desired properties for live cell imaging, including >100-fold fluorescence enhancement, membrane permeability, and selective enrichment to acidic granules. The combined advantages endow ZIGIR with superior sensitivity and specificity for imaging granular Zn²⁺. ZIGIR enables separation of heterogenous β cells based on their insulin content and sorting of mouse islets into pure α cells and β cells. In human islets, ZIGIR facilitates sorting of endocrine cells into highly enriched α cells and β cells, reveals unexpectedly high Zn²⁺ activity in the somatostatin granule of some δ cells, and uncovers variation in the glucagon content among human α cells. We expect broad applications of ZIGIR for studying Zn²⁺ biology and Zn²⁺-rich secretory granules and for engineering β cells with high insulin content for treating diabetes.

Novel standards in the measurement of rat insulin granules combining electron microscopy, high-content image analysis and in silico modelling

AIMS/HYPOTHESIS

Knowledge of number, size and content of insulin secretory granules is pivotal for understanding the physiology of pancreatic beta cells. Here we re-evaluated key structural features of rat beta cells, including insulin granule size, number and distribution as well as cell size.

METHODS

Electron micrographs of rat beta cells fixed either chemically or by high-pressure freezing were compared using a high-content analysis approach. These data were used to develop three-dimensional in silico beta cell models, the slicing of which would reproduce the experimental datasets.

RESULTS

As previously reported, chemically fixed insulin secretory granules appeared as hollow spheres with a mean diameter of ∼350 nm. Remarkably, most granules fixed by high-pressure freezing lacked the characteristic halo between the dense core and the limiting membrane and were smaller than their chemically fixed counterparts. Based on our analyses, we conclude that the mean diameter of rat insulin secretory granules is 243 nm, corresponding to a surface area of 0.19 μm(2). Rat beta cells have a mean volume of 763 μm(3) and contain 5,000-6,000 granules.

CONCLUSIONS/INTERPRETATION

A major reason for the lower mean granule number/rat beta cell relative to previous accounts is a reduced estimation of the mean beta cell volume. These findings imply that each granule contains about twofold more insulin, while its exocytosis increases membrane capacitance about twofold less than assumed previously. Our integrated approach defines new standards for quantitative image analysis of beta cells and could be applied to other cellular systems.

Detection of exocytosis at individual pancreatic beta cells by amperometry at a chemically modified microelectrode

Amperometry at a carbon fiber microelectrode modified with a composite of ruthenium oxide and cyanoruthenate was used to monitor chemical secretions of single pancreatic beta cells from rats and humans. When the insulin secretagogues glucose, tolbutamide, and K+ were applied to the cell, a series of randomly occurring current spikes was observed. The current spikes were shown to be due to the detection of chemical substances secreted from the cell. Chromatography showed that the primary secreted substance detected by the electrode was insulin. The current spikes were strongly dependent on external Ca2+, had an average area that was independent of the stimulation method, and had an area distribution which corresponded to the distribution of vesicle sizes in beta cells. It was concluded that the spikes were due to the detection of concentration pulses of insulin secreted by exocytosis.

Insulin and C-peptide (proinsulin) packaging: a quantitative immunocytochemical approach

C-peptide and insulin antigenic sites in rat pancreatic beta cells were both labeled by the protein A and immunogold techniques. Gold bead density in the secretory granules was correlated with granule size distribution. Our results demonstrate that the distribution patterns of antigenic sites for C-peptide (which also represents proinsulin sites) and insulin are similar when granule size distribution is correlated to the density of antigenic sites. It was found that the ratio of C-peptide to insulin in granules is independent of granule size. Examination of frequency histograms by a Kolmogorov-Smirnov analysis excluded the possibility that the antigenic sites of C-peptide and insulin are distributed in the same manner. Mean gold bead density for insulin antigen (97 +/- 24) was slightly higher than C-peptide density (64 +/- 18). The efficiency of labeling was found to be very low; less than 0.1% of the theoretically estimated antigenic sites were labeled.

A small-scale method for the isolation of insulin-containing secretory granules from islets of Langerhans

A method has been developed which uses small-scale (400 microliter) Percoll gradients and an inexpensive bench-top microcentrifuge for the rapid isolation of insulin-containing secretory granules from islets of Langerhans available from a single rat pancreas. Granule fractions were prepared from homogenates of isolated rat islets by a differential centrifugation step (10 min) to produce a granule-enriched membrane pellet, followed by a further centrifugation (10 min) on a discontinuous Percoll gradient to produce a granule fraction. Measurement of membrane-marker enzyme activities suggested that the yield and purity of granule fractions prepared by this method were comparable to those reported for other methods involving longer centrifugation times in ultracentrifuges. Further purification of the granule fractions by removing lysosomal contamination was achieved by an additional centrifugation (10 min) on another small-scale gradient of higher Percoll concentration. The method proved useful for isolating biosynthetically labeled secretory granule membranes and contents from islets of Langerhans which had been cultured in the presence of 35S-labeled amino acids. The speed and simplicity of this method suggest that it will prove useful in studies requiring the rapid isolation of insulin-containing secretory granules from isolated islets.

Properties of the Ca-activated K+ channel in pancreatic beta-cells

The existence of [Ca2+]i-activated K+-channels in the pancreatic beta-cell membrane is based in two observations: quinine inhibits K+-permeability and, increasing intracellular Ca2+ stimulates it. The changes in K+-permeability of the beta-cell have been monitored electrically by combining measurements of the dependence of the membrane potential on external K+ concentration and input resistance. The changes in the passive 42K and 86Rb efflux from the whole islet have been measured directly. Intracellular Ca2+ has been increased by various means, including increasing extracellular Ca2+, addition of the Ca2+-ionophore A23187 or noradrenaline and application of mitochondrial uncouplers and blockers. In addition to quinine, many other substances have been found to inhibit or modulate the [Ca2+]i-activated K+-channel. The most important of these is the natural stimulus for insulin secretion, glucose. Glucose may inhibit K+-permeability by lowering intracellular Ca2+. Glibenclamide, a hypoglycaemic sulphonylurea, is about 25 times more active than quinine in blocking the K+-channel in beta-cells. The methylxanthines, c-AMP, various calmodulin inhibitors and Ba2+ also inhibit K+-permeability. Genetically diabetic mice have been studied and show an alteration in the [Ca2+]i-activated K+-channel. It is concluded that the [Ca2+]i-activated K+-channel plays a major role in the normal function of the pancreatic beta-cell. The study of its properties should prove valuable for the understanding and treatment of diabetes.

Purification of mitochondria and secretory granules isolated from pancreatic beta cells using Percoll and Sephacryl S-1000 superfine

Mitochondria and secretory granules were isolated from beta-cell-rich pancreatic islets of ob/ob mice. Crude fractions obtained by differential centrifugation were subjected to centrifugation on isotonic Percoll. The gradient medium was removed from the resulting fractions by gel filtration on Sephacryl S-1000 Superfine. When compared to the crude fractions, the purified mitochondrial fraction exhibited a 4.5-fold increase in citrate synthase activity, a 70% reduction of lysosomal arylsulfatase, and a 40% decrease of contamination with granular insulin. In the purified secretory granule fraction, the insulin content was as high as 60% of the total protein (albumin standard) with arylsulfatase unchanged and no detectable citrate synthase activity.