Encapsulation of Pancreatic Islets Within Nano-Thin Functional Polyethylene Glycol Coatings for Enhanced Insulin Secretion

Abstract

Covalent attachment of polymers to cells and tissues could be used to solve a variety of problems associated with cellular therapies. Insulin-dependent diabetes mellitus is a disease resulting from the autoimmune destruction of the b cells of the islets of Langerhans in the pancreas. Transplantation of islets into diabetic patients is an attractive form of treatment, provided that the islets could be protected from the host's immune system to prevent graft rejection, and smaller numbers of islets transplanted in smaller volumes could be sufficient to reverse diabetes. Therefore, a need exists to develop islet encapsulation strategies that minimize transplant volume. In this study, we demonstrate the formation of nano-thin, poly(ethylene glycol) (PEG)-rich functional conformal coatings on individual islets via layer-by-layer assembly technique. The surface of the islets is modified with biotin-PEG-N-hydroxysuccinimide (NHS), and the islets are further covered by streptavidin (SA) and biotin-PEG-peptide conjugates using the layer-by-layer method. An insulinotropic ligand, glucagon-like peptide-1 (GLP-1), is conjugated to biotin-PEG-NHS. The insulinotropic effect of GLP-1 is investigated through layer-by-layer encapsulation of islets using the biotin-PEG-GLP-1 conjugate. The effect of islet surface modification using the biotin-PEG-GLP-1 conjugate on insulin secretion in response to glucose challenge is compared via static incubation and dynamic perifusion assays. The results show that islets coated with the functional PEG conjugate are capable of secreting more insulin in response to high glucose levels compared to control islets. Finally, the presence of SA is confirmed by indirect fluorescent staining with SA-Cy3, and the presence of PEGpeptide on the surface of the islets after treatment with biotin-PEG-GLP-1 is confirmed by indirect fluorescent staining with biotin-PEG-fluorescein isothiocyanate (FITC) and separately with an anti-GLP-1 antibody. This work demonstrates the feasibility of treating pancreatic islets with reactive polymeric segments and provides the foundation for a novel means of potential immunoisolation. With this technique, it may be possible to encapsulate and/or modify islets before portal vein transplantation and reduce transplantation volume significantly, and promote islet viability and insulin secretion due to the presence of insulinotropic peptides on the islet surface. Layer-by-layer self-assembly of PEG-GLP-1 offers a unique approach to islet encapsulation to stimulate insulin secretion in response to high glucose levels.

Recent Advances in the Modeling of PEG Hydrogel Membranes for Biomedical Applications

“…Recently, it was shown that, GLP-1 coated islets exhibited a higher response to glucose challenge, in terms of insulin secretion, compared to the untreated islets in vitro. (Kizilel, Scavone, Liu, Nothias, Ostrega, Witkowski and Millis 2010) This suggested that similar effect could be observed when GLP-1 is immobilized within the PEG hydrogel capsule around the islet. Therefore, it was important to design PEG hydrogel coatings with high GLP-1 concentrations at points closer to the surface in the case of islet microencapsulation within PEG hydrogel.…”

Encapsulation and Surface Engineering of Pancreatic Islets: Advances and Challenges

“…The LbL coating of islets based on streptavidin/biotin anchoring as a driving force for the LBL was also developed (Dai et al, 2007;Kizilel et al, 2010). PEGylated multilayers were grown using LbL assembly of biotinylated PLL and streptavidin.…”

“…In another example, the rat islet cell membranes were first biotinylated through covalent reaction of the membrane proteins and NHS-ester groups of NHS-PEG-biotin. The second layer of streptavidin was followed by a layer of a biotin-PEG conjugate with the insulinotropic Glucagon-like Ppetide 1 (GLP-1) (Kizilel et al, 2010). GLP-1 is a 30-amino acid peptide hormone produced in the intestinal epithelial endocrine L-cells.…”

Nano-scale encapsulation enhances allograft survival and function of islets transplanted in a mouse model of diabetes

“…Several methods of conformal coating have been developed recently, including covalent surface attachment of polyethylene glycol (PEG), known as 'PEGylation' [18], and layer-by-layer encapsulation [19][20][21][22][23][24][25][26][27][28]. Nano-layers with different combinations of components, including streptavidin and biotin-PEG derivatives [19,24], complement receptor 1 and heparin [25], and PEG-lipid and poly(vinyl alcohol) [26,27], have been previously researched as islet surface coatings.…”