PROJECT SUMMARY/ABSTRACT Graft integration of microvasculature is a critical next step for cell replacement strategies for type I diabetics. The incorporation of host-connected microvasculature is essential for post-implantation graft survival and over the longer-term impacts the kinetics of glucose response and systemic insulin delivery. Directing angiogenesis into islet-containing synthetic hydrogels would guarantee host-connected microvasculature in the graft, but control over angiogenesis remains limited. Our long-term goal is to understand how physical cues from the cellular microenvironment impinge upon critical steps of angiogenesis and devise engineering methods to incorporate these cues into translatable biomaterials. Angiogenesis involves a series of spatiotemporally controlled cellular programs including endothelial tip cell activation and directed invasion, collective migration of leading tip cells and ensuing stalk cells, and proliferation and lumenization of the multicellular strand. Our prior work demonstrates a critical balance between tip cell migration and stalk cell proliferation during collective migration required for forming functional microvessels, and that hydrogel degradability modulates the collectivity of endothelial cell migration. Further, we have pioneered hydrogel composites containing physical cues in the form of synthetic fibers that promote endothelial-to-mesenchymal transition and cause quiescent endothelial cells to adopt invasive behavior suggestive of tip cells that lead angiogenic sprouts. Together, these observations motivate our central hypothesis: modular control of hydrogel structure can drive the angiogenic formation of microvasculature that supports the function of hPSC-derived pancreatic islet organoids. Using novel composite hydrogels, organotypic tissue models, and assessments of vascular and islet function in vivo, we aim to understand the microenvironmental regulation of endothelial cell decision-making during angiogenesis. In Aim 1, we will utilize hydrogel composites containing cell-adhesive guidance fibers to phenotypically transition quiescent endothelial cells into invasive tip cells. In Aim 2, we will engineer hydrogel crosslinking and microscale porosity to drive endothelial stalk cells proliferation and establish quantitative relationships between collective migration of stalk cells, proliferative events, and microvessel lumenization. In Aim 3, we will use in vitro and in vivo models to examine the impact of material-guided angiogenesis and resulting microvasculature on the function of hydrogel grafts containing hPSC-derived islets. The proposed studies will 1) shed light on the microenvironmental regulation of phenotypic transitions during angiogenesis and 2) identify biomaterial design parameters that support functional angiogenesis. We anticipate the developed strategies to provide microvascular support to engineered pancreatic islet grafts will have bearing on grafts containing other metabolically demanding parenchymal tissues.

PUBLIC HEALTH RELEVANCE The presence and function of vasculature is critical to the success of islet cell replacement therapies for type I diabetes that utilize hydrogel-based scaffolds. The proposed work will use novel techniques to control hydrogel structure with the goal of driving the controlled formation of vasculature around stem cell-derived pancreatic islets. Results from the proposed studies will improve the survival and function of engineered pancreatic islet grafts and help bring these emerging technologies closer to a translational endpoint.