Objective
The aim of this study is to determine the molecular mechanisms by which PTG promotes the engraftment of islets and the reversal of diabetes. Insights gained from this study will help to develop a PTG-inspired islet engraftment cocktail to enhance islet graft survival and function and durable reversal of type 1 diabetes. The new knowledge and technology from this study may be used to improve stem cell-derived β-cell transplantation in the future.
Background Rationale
Currently, type 1 diabetes (T1D) is managed by daily insulin injections, but the precise dose and timing of insulin can be difficult to achieve, resulting in erratic blood glucose levels in some patients. Severe hypoglycemia can be acutely fatal whereas chronic hyperglycemia can lead to tissue and organ damages. Insulin-producing β-cell replacement therapies such as islet transplantation and stem cell-derived β-cell transplantation, are the most promising therapies to restore natural islet function. However, a major limitation of such therapies is the poor graft survival, resulting in inadequate function. This is mainly due to the long time required to re-establish a good blood connection between the graft and the recipient after transplantation, leading to prolonged ischemic damage and massive islet cell death. Therefore, solving this issue has the potential to greatly improve the efficacy of islet transplantation.
Our approach to improving islet survival after transplantation is inspired by another highly vascularized endocrine tissue, the parathyroid glands (PTG). PTG transplantation have a success rate of more than 90%. Co-transplantation of PTG in mice improved islet graft survival and function. A phase I clinical trial based on this is underway. While the clinical trial aims to replicate the successful outcomes observed in animal studies, the reliance on deceased PTG donors limits the wide application of this therapy. My project will address this challenge by finding out the molecules responsible for PTG protection of islet transplantation so that these molecules can be used to support islet transplants in place of PTG.
Experiments in my laboratory have found that co-culture PTG protected human islets under hypoxic conditions, suggesting that PTG may directly protect β-cells by releasing active components. Furthermore, in mice co-transplanted with PTG and islets, we observed the formation of new blood vessel networks within and around the transplanted islets that was not observed in islets transplanted alone. This indicates that co-transplantation of PTG promotes the revascularization of the transplanted islets. Interestingly, our analysis of the proteins secreted by PTG and islets revealed several proteins predominantly secreted by PTG, some of which have previously been shown to protect β-cells from stress-induced cell death or promote blood vessel growth. In my proposed study, I will systematically investigate which components are responsible for the beneficial effects of PTG co-transplantation on pancreatic islets.
Description of Project
Although islet transplantation that replaces insulin-producing β-cell is a promising therapy for curing type 1 diabetes (T1D), its therapeutic potential is limited by the poor survival of islets after transplantation. The islets are disconnected from the circulation during isolation, and it takes several days after transplantation for the islets to revascularize (re-establish blood supply). During this time, the islet cells suffocate resulting in loss of more than 80% of the transplanted graft mass within a few days. Furthermore, the few islets that survived the initial wave of cell death never achieve the same vascular density as the islets in their native environment in the pancreas, resulting in impaired function.
In contrast to islets, another vascular-rich endocrine tissue, the parathyroid gland (PTG), has a transplant success rate of over 90%. Inspired by the highly effective revascularization of PTG after transplantation, we explored whether PTG co-transplantation could improve islet engraftment. Indeed, we found that more than 80% of the initial islet graft mass could be preserved when co-transplanted with PTG. Furthermore, our in vitro and in vivo experimental data suggest that PTG helps islets in at least two ways - supporting islet survival before they re-establish blood supply and accelerating islet revascularization. In support of these ideas, we found that PTG produces many known proteins for islet survival and angiogenesis.
The project described in this fellowship application builds on these novel insights to systematically dissect the molecular mechanisms of PTG-mediated enhancement of islet transplantation. Knowledge gained from these experiments will help to develop a PTG-inspired “islet engraftment cocktail” to improve the efficacy of islet transplantation. I propose the following specific aims: 1. identify PTG-produced molecules that protect β-cells from ischemia and determine the composition of a combination of factors that promotes early β-cell survival after transplantation; 2. screen and identify a PTG-derived molecular combination that shortens the time needed for revascularization of grafts after transplantation and rebuild vascular connection with the recipients. In the final step of this project, I will develop an islet engraftment cocktail by combining the molecules identified in these two aims, mimicking PTG, thereby enhancing islet engraftment and diabetes reversal.
Overall, I aim to discover new biological pathways of islet survival and angiogenesis and in the meantime develop novel approaches to realize the full potential of β-cell replacement therapies for T1D.
Anticipated Outcome
At the end of this study, I expect to define a mixture of several molecules that can at least partially recapitulate the function of PTG in promoting β-cell survival and islet revascularization. This mixture can be used instead of PTG to improve the efficacy of diabetes reversal after islet transplantation. The study will also reveal molecular pathways that control β-cell survival and islet revascularization. These insights will be instrumental for engineering more effective stem cell derived β-cell replacement therapy in the future.
Relevance to T1D
This project is directly related to type 1 diabetes (T1D) as it aims to develop new strategies to improve efficacy of β-cell replacement therapy for T1D.