Characterization of islet cells during development and after transplantation

Abstract

Diabetes Mellitus is a disease in which patients are not able to maintain blood glucose levels. This is caused by dysfunction or destruction of the beta cells in the islets of Langerhans, located in the pancreas. Beta cells are responsible for the production of insulin, a hormone that decreases the amount of available glucose in the blood when it becomes too high. Therapeutically, diabetes patients inject themselves with insulin as an alternative, but this treatment is symptomatic. The only available cure at the moment is transplantation of donor tissue, in the form of a whole pancreas, or the isolated islets of Langerhans. Novel therapies are being developed, where stem cells are driven to beta-like phenotypes. The pathways used to drive these cells strongly resemble embryonic development. Many characteristics of embryonic development are of yet not completely understood, and in this thesis we aimed to better understand how beta cells develop from embryonic progenitors into adult cells, and how these beta cells behave after transplantation. To do this, we have developed a novel technique that allows intravital imaging of transplanted pancreatic tissue under the kidney capsule. We transplanted embryonic pancreases to see how these tissues develop into adult organs. Transplanted embryonic pancreases developed very similarly to endogenous embryonic pancreases. We found that the organ increases more than 6 fold in size, and islets of Langerhans are being formed, in two weeks time after transplantation. The first islet-like structures can already be observed after seven days. Islets formed in these organs consist not only of beta, but also of glucagon producing alpha cells. Three days after transplantation, mesenchymal like cells that are positive for insulin were observed in the transplanted tissue, and these cells displayed a migratory phenotype. Cells were not directionally mmigrating, but rather moved around randomly. This behavioral trait was strongly diminished seven days after transplantation, when islets were starting to form. We also transplanted adult islets of Langerhans under the kidney capsule. These islets did not show any plasticity after transplantation, only a small increase of total transplanted mass, which can be caused in part due to co-transplanted endothelial and mesenchymal cells. Islets were also subcutaneously transplanted in alginate-based scaffolds. Intravital imaging revealed that these islets lost functionality within seven days after transplantation. Finally, we characterized the developing pancreas using single cell transcriptome sequencing. Here, we found all cell types that are normally present in the embryonic pancreas: tip and trunk epithelium, endocrine progenitors and all endocrine adult cell types (alpha, beta, gamma and delta cells). We found that cells became more mature through development, as illustrated by a decreasing transcriptomic entropy. A pseudo-temporal map shows the developmental time line for alpha and beta cell maturation, and we show how genes are temporally regulated during development. Taken together, we provide new insight into maturation of endocrine cell types and how islets of Langerhans are formed. Also, we show the dynamic behavior of islets of Langerhans after transplantation under various conditions.