The Role of C-Terminus and Nuclear Localization Sequence of Parathyroid Hormone-Related Protein (PTHrP) in Pancreatic Islet Morphology and Glucose Homeostasis.

Parathyroid Hormone-related protein (PTHrP) is an important polyhormone with multiple functions in development and cell regulation. PTHrP is expressed by pancreatic beta cells which are responsible for insulin secretion. Previous studies have focused on the ability of N-terminal PTHrP to stimulate proliferation in beta cells. We have developed a knock-in mouse model lacking the C-terminal and nuclear localization sequence of PTHrP, retaining only N-terminal PTHrP. These mice die at ~day 5, are severely stunted in growth (weigh 54% less than control mice), have chondrodystrophy, hypoinsulinemia, hypoglycemia (~40-60mg/dL), hypotriglyceridemia, and lack body fat. Percentage stomach weight (including ingested milk) measurements in relation to body weight were consistent among all mouse genotypes indicating that inanition was not the cause of death. To characterize the pancreatic islets in these mice, islets (~10-20) were isolated from 2-5 day-old-mice using collagenase digestion and a glucose-stimulated intracellular calcium assay was conducted at 0, 4, 8, 12, 16 and 20mM glucose. Results showed higher intracellular calcium response in knock-in islets compared to control islets and increased insulin secretion after overnight incubation. Data from immunofluorescence also showed less glucagon staining in knock-in islets compared to control islets. This study revealed that the C-terminus and nuclear localization sequence of PTHrP are crucial to life, including regulation of glucose homeostasis and islet response to glucose metabolism. It also revealed that these portions of PTHrP may be important for the developmental stage of islets and differentiation into alpha or beta cells.