Effect of alginate matrix engineered to mimic the pancreatic microenvironment on encapsulated islet function

Abstract

Islet transplantation is emerging as a therapeutic option for type 1 diabetes, albeit, only a small number of patients meeting very stringent criteria are eligible for the treatment because of the side effects of the necessary immunosuppressive therapy and the relatively short time frame of normoglycemia that most patients achieve. The challenge of the immune-suppressive regimen can be overcome through microencapsulation of the islets in a perm-selective coating of alginate microbeads with poly-l-lysine or poly- l-ornithine. In addition to other issues including the nutrient supply challenge of encapsulated islets a critical requirement for these cells has emerged as the need to engineer the microenvironment of the encapsulation matrix to mimic that of the native pancreatic scaffold that houses islet cells. That microenvironment includes biological and mechanical cues that support the viability and function of the cells. In this study, the alginate hydrogel was modified to mimic the pancreatic microenvironment by incorporation of extracellular matrix (ECM). Mechanical and biological changes in the encapsulating alginate matrix were made through stiffness modulation and incorporation of decellularized ECM, respectively. Islets were then encapsulated in this new biomimetic hydrogel and their insulin production was measured after 7 days in vitro. We found that manipulation of the alginate hydrogel matrix to simulate both physical and biological cues for the encapsulated islets enhances the mechanical strength of the encapsulated islet constructs as well as their function. Our data suggest that these modifications have the potential to improve the success rate of encapsulated islet transplantation.

Keywords: alginate; diabetes; extracellular matrix; hydrogel; islet transplantation; microencapsulation.

FIGURE 1

Storage modulus of alginate hydrogels with either Sr²⁺ or Ca²⁺ crosslinkers ranging from 100 to 12.5 mM. Human pancreas tissue (native) also measured for comparisons. Student's t test. Error bars indicate standard deviation. *p < 0.05, ****p < 0.0001, φ = p > 0.05 compared with native, n = 4

FIGURE 2

Storage modulus of alginate ± solubilized extracellular matrix (ECM). Student's t test. No significance was shown within crosslinker concentrations (mean ± SD, p < 0.05, n = 4)

FIGURE 3

(a) Inverted microscopy images of alginate beads following 36 h of mechanical agitation. The top row and bottom row are ×2 and ×10 magnification, respectively. From left to right rows, the beads are crosslinked in 25 mM SrCl₂, 25 mM SrCl₂ with ECM-alginate, 50 mM SrCl₂, and 50 mM SrCl₂ with extracellular matrix (ECM)-alginate. Scale bar: 500 μm for ×2 and 100 μm for ×10. (b) Number of beads intact after mechanical agitation. Initial count of 100 beads were made with or without ECM and crosslinked with either 25 or 50 mM SrCl₂. Student's t test (mean ± SD, n = 5, *p < 0.05, **p < 0.01, ****p < 0.0001)

FIGURE 4

Islet glucose-stimulated insulin secretion (GSIS) results on Day 7 postencapsulation. One-way analysis of variance (mean ± SD, *p < 0.05, **p < 0.01, ***p < 0.001, n = 3)

FIGURE 5

Glucose stimulation index (GSI) for both Days 4 and 7 postpost encapsulation. Each bar represents the average GSI per group, which is defined as the insulin secretion during the high glucose period divided by the average of the insulin secretion during the low glucose phase. One-way analysis of variance. Error bars represent the SD. **p < 0.01, n = 3