Gluca-gone : an examination of glucagon secretion from human islets and stem cell-derived alpha-like cells transplanted in a novel glucagon-null mouse

Glucose homeostasis is a common process throughout the animal kingdom. The importance of glucose control is made apparent by the mortality of diseases that affect energy management tissues, like diabetes, if left untreated. The cause of diabetes is largely rooted in the dysfunction of hormone signaling axes between endocrine micro-organs, called the islets of Langerhans, and the tissue targets of their hormones. Islets of Langerhans are clusters of heterogeneous endocrine cells dispersed throughout the pancreas. Islets maintain glucose homeostasis via hormone regulation of glucose metabolism and glucose uptake in target tissues. Islet endocrine cells are glucose-sensing, and hormone release from islet cells is tightly coupled to circulating glucose levels. Islet α cells secrete glucagon under low glucose conditions, which drives hepatic glucose output through glycogenolysis and gluconeogenesis. In contrast, increases in blood glucose trigger islet β cell insulin secretion, which promotes glucose uptake in target organs. Diabetes is characterized by insufficient or dysregulated hormone output by α and β cells. While both glucagon and insulin secretion are dysregulated in diabetes, compared to β cell insulin secretion, little is known about mechanisms regulating α cell glucagon secretion. Our lack of knowledge about glucagon regulation may be partly due to the lack of tools to study human α cells in a physiological setting, like after transplantation into immunocompromised mice. Distinct mouse and human insulin peptides permit studies of human β cells after xenotransplantation into immunocompromised mice. However, evolutionary conservation of the glucagon peptide sequence amongst placental mammals prevents accurate measurements of glucagon release from transplanted human tissue after xenotransplantation, where variations in endogenous rodent-derived glucagon can obscure measurements. In this thesis, I present a novel immunocompromised transplantation mouse model that lacks endogenous glucagon and thus permits studies of regulated glucagon secretion from human islets. Using this mouse model, we provide evidence that hyperglucagonemia exhibited by patients with type 2 diabetes (T2D) is an islet intrinsic defect that partly contributes to T2D fasting hyperglycemia. Additionally, I present evidence that this glucagon-null mouse model provides a physiological setting to allow longitudinal maturation studies of human embryonic stem cell-derived α-like cells. Along with these in vivo studies, I present a preliminary in vitro study of lentivirus-mediated genetic screens in primary human islets to identify genes that play a role in regulating human α cell function. Overall, the data presented in this thesis underscores the importance of tightly regulated glucagon secretion, provides a new tool to study human α cells in vivo, and will aid in the identification of genes that impact glucagon secretion

Keywords: Developmental Biology.
Advisor: Kim Seung K.
Annes Justin 
Barna Maria (Professor of developmental biology)
Nusse Roel