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. 2018 Jan;115(1):232-245.
doi: 10.1002/bit.26442. Epub 2017 Sep 19.

Glucose-stimulated insulin release: Parallel perifusion studies of free and hydrogel encapsulated human pancreatic islets

Peter Buchwald  1   2 , Alejandro Tamayo-Garcia  1 , Vita Manzoli  1   3 , Alice A Tomei  1   4 , Cherie L Stabler  5
Affiliations

Glucose-stimulated insulin release: Parallel perifusion studies of free and hydrogel encapsulated human pancreatic islets

Peter Buchwald et al. Biotechnol Bioeng. 2018 Jan.

Abstract

To explore the effects immune-isolating encapsulation has on the insulin secretion of pancreatic islets and to improve our ability to quantitatively describe the glucose-stimulated insulin release (GSIR) of pancreatic islets, we conducted dynamic perifusion experiments with isolated human islets. Free (unencapsulated) and hydrogel encapsulated islets were perifused, in parallel, using an automated multi-channel system that allows sample collection with high temporal resolution. Results indicated that free human islets secrete less insulin per unit mass or islet equivalent (IEQ) than murine islets and with a less pronounced first-phase peak. While small microcapsules (d = 700 µm) caused only a slightly delayed and blunted first-phase insulin response compared to unencapsulated islets, larger capsules (d = 1,800 µm) completely blunted the first-phase peak and decreased the total amount of insulin released. Experimentally obtained insulin time-profiles were fitted with our complex insulin secretion computational model. This allowed further fine-tuning of the hormone-release parameters of this model, which was implemented in COMSOL Multiphysics to couple hormone secretion and nutrient consumption kinetics with diffusive and convective transport. The results of these GSIR experiments, which were also supported by computational modeling, indicate that larger capsules unavoidably lead to dampening of the first-phase insulin response and to a sustained-release type insulin secretion that can only slowly respond to changes in glucose concentration. Bioartificial pancreas type devices can provide long-term and physiologically desirable solutions only if immunoisolation and biocompatibility considerations are integrated with optimized nutrient diffusion and insulin release characteristics by design.

Keywords: FEM model; alginate; diabetes mellitus; fluid dynamics; glucose-stimulated insulin secretion; islet encapsulation.

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Conflict of interest statement

Competing interests

The author(s) declare that they have no competing interests.

Figures

Figure 1
Figure 1. Schematic concept of the dynamic perifusion assay used to assess glucose-stimulated insulin release (GSIR) of free or encapsulated pancreatic islet
An idealized spherical islet (red) is shown with a hydrogel capsule (dark blue). As indicated, the perifusing solution, which has fully adjustable incoming glucose concentration, flows from left to right, and it is collected at the output to assess its insulin content.
Figure 2
Figure 2. Conceptual representation of the present FEM computational model of insulin release
Insulin secretion is determined by the local glucose concentration, cgluc, and its rate of change, ∂cgluc/∂t, but it is also influenced by the local oxygen concentration, coxy. The model uses a total of five coupled modules for glucose, oxygen, and insulin concentrations with incorporated fluid dynamics, as indicated by the corresponding equations on the left side.
Figure 3
Figure 3. Dynamic GSIR in isolated human and murine islets
Summary of all experimental data collected for free murine and human islets perifused using a low (3 mM; G3, 5 min) → high (11 mM; G11, 20 min) → low (3 mM; G3, 15 min) incoming glucose stimulation (plus 10 min KCl followed by G3) as shown. Automated PERI4-02 multichannel perifusion apparatus used (samples collected every minute; 0.1 mL/min flow rate, ~100 IEQ per channel). Data are average ± SEM from multiple isolations (single or duplicate channels perifused; n = 12 total samples for both human and murine). Dashed lines are calculated values using the present COMSOL Multiphysics model.
Figure 4
Figure 4. Glucose-induced insulin secretion in free and encapsulated human islets obtained in parallel perifusion experiments
Average of experimental data for free, as well as small and large alginate-encapsulated (d = 700 & 1800 μm) human islets perifused using the same protocol as described in Figure 3. Data are average ± SEM from multiple isolations (n = 3 for each condition sourced from the same islet preparation and conducted in parallel). The average of all free human islet perifusions shown in Figure 3 is included for reference (dashed line), and a set of illustrative phase contrast microscope images of free and encapsulated islet samples are shown at right.
Figure 5
Figure 5. Additional dose-response GSIR data used for further calibration of the present computational model
Dynamic perifusion data from three different dose-response experiments from Henquin and co-workers (Henquin et al., 2006a; Henquin et al., 2015) used for fine-tuning the first- and second-phase insulin response calculated by the present COMSOL Multiphysics model. A variety of incoming glucose step-up challenges were used, as indicated by color-coded symbols: sequential steps of G1 → G3 → G5 → G7 → G10 and then G10 → G15 → G20 → G30 → G7 for the same preparation (A), different single-step challenges of G3 → G6/G8/G10/G15/G30 (B), and different single-step challenges of G3/G6/G8/G10 → G15 (C). Data (symbols) were obtained on a pg/min/IEQ scale here by using the average percent secretion values published with the average insulin content per islet from the same work (17.8 ng/islet; (Henquin et al., 2015)). Continuous thicker lines of corresponding colors are values calculated with the present model.
Figure 6
Figure 6. Fit of experimental GSIR data (free and encapsulated islets) with the present computational model
Experimental data are reproduced from Figure 4, calculated values were obtained as described in the text and converted to pg/min/IEQ for ease in comparison.
Figure 7
Figure 7. Model-calculated insulin production rates in response to increasing glucose concentrations
(i.e., during the first-phase response). Calculated insulin production rates for two islets (d = 100 & 150 μm – at the top and bottom, respectively) with small and large capsules (lcaps = 100 and 400 μm; left and right, respectively) under normoxic conditions. Data shown as surface plot are insulin production rates (color-coded from green for low to red for high). Streamlines show the flow of the perifusion fluid (color-coded for velocity; flow from left to right) and colored contour lines show isolevels for the perifusing glucose (from light blue for low to light red for high). Model calculated values are shown during the increase of the glucose concentration from 3 mM to 11 mM; first phase response is noticeably delayed and blunted in the large capsule.
Figure 8
Figure 8. Diffusion limitation of oxygen versus glucose in avascular islets
Model-calculated oxygen (top) and glucose (bottom) concentrations during high glucose (G11) exposure in two encapsulated islets with d = 100 & 150 μm, lcaps = 400 μm (with offset) under normoxic conditions. Data shown as surface plots that are also color-coded from blue for high to red for low to illustrate that oxygen diffusion is the main limiting factor as it decreases more severely inside the islets than glucose. Data are shown at 11 mM glucose exposure under normoxic conditions (atmospheric oxygen; ~20% ≈ 0.2 mM). Oxygen limitations are even more severe for transplanted islets exposed to tissue oxygen concentrations only (~5% ≈ 0.06 mM).
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