Biologically active PEG hydrogel microenvironments for improving encapsulated beta-cell survival and function

The transplantation of encapsulated islets is an exciting potential cell replacement therapy for treating type 1 diabetes and avoiding the systemic immune suppression usually required for any transplantation. Semipermeable encapsulation barriers prevent cell-cell contact mediated destruction of transplanted cells and the passage of large immune cell-secreted cytotoxic factors, but permit diffusion of low molecular weight nutrients and metabolites to and from encapsulated cells. To minimize the immune response, encapsulation barrier materials have been designed to be as inert as possible, but inert barriers create an islet extracellular microenvironment that is very different from that of the native pancreas, where islets are extensively vascularized and in contact with basement membrane proteins. Recent studies of islets cultured on extracellular matrix substrates have demonstrated the positive effects of these cell-matrix interactions on islet survival and function in vitro. The overall hypothesis of this research is that the introduction of extracellular signaling found in the native islet environment into the encapsulated islet environment will promote islet survival and function.; Poly(ethylene glycol) (PEG) hydrogels formed via the photoinitiated polymerization of dimethacrylated PEG were applied as a model three-dimensional encapsulation environment for systematically testing the isolated and combined effects of specific extracellular interactions on islet survival and function. Matrix interactions were incorporated into PEG hydrogels in the form of whole proteins and small, covalently tethered adhesive peptide sequences, and the influence of these interactions on individual beta-cell survival and the coordinated function of isolated islets in response to glucose stimulation was investigated. Because the nutrient supply delivered to interior islet cells by intraislet vasculature is lost upon isolation, islets were dissociated in enzyme-free conditions and allowed to re-aggregate into smaller, islet-like clusters that retain islet function in vitro but have reduced intraislet diffusion distances. Finally, the translation of the three-dimensional PEG hydrogel culture platform to a functionalized islet encapsulation barrier material was explored by studying the transport properties of PEG networks with respect to proteins, and the localization of biological functionalities for the spatial control of extracellular signaling within PEG encapsulation barriers.