UBC Theses and Dissertations
Activity-regulated genes in pancreatic islets and neurons of the arcuate nucleus Yoon, Ji Soo
Both pancreatic islets and hypothalamic arcuate nucleus neurons are critical for energy homeostasis and depend on activity-regulated genes to respond to stimuli. NPAS4 is one such gene that is expressed in both sites and is important for proper function of islets and neurons. The overall goals of my thesis are to profile activity-regulated genes in pancreatic islets and arcuate nucleus neurons, and characterize the functional role of NPAS4 in arcuate nucleus neurons in order to reveal novel mechanisms in cells important in energy homeostasis. To identify calcium-dependent activity-regulated genes in human islet cell types, I used single cell RNA sequencing on islets that had experimentally induced or inhibited intracellular calcium signalling. In addition to identifying activity-regulated genes in each islet cell type, I found that beta cells with the most calcium-regulated genes expressed the marker PCDH7 and had enhanced glucose-stimulated insulin secretion. In order to study the role of NPAS4 in the arcuate nucleus, I first determined that Npas4 mRNA was expressed and was induced in AgRP and POMC neurons in the arcuate nucleus in response to peripheral nutrient and endocrine stimuli. Next, I characterized mice with NPAS4 knockout in either AgRP or POMC neurons, with or without high-fat diet, and found that mice with POMC-specific NPAS4 knockout gained less body weight on high-fat diet due to an early reduction in food intake. To determine the molecular mechanism behind this phenotype, I performed single cell RNA sequencing on arcuate nuclei from fasted and refed NPAS4 knockout and control mice fed high-fat diet. I found that the POMC neurons with NPAS4 knockout had reduced Pomc expression, increased feeding-regulated genes, and reduced inhibitory GABAergic synapse genes, which could explain the reduced body weight over time. Future experiments would involve assessing electrophysiological changes in POMC neurons from NPAS4 knockout mice to confirm functional alterations as a result of reduced inhibitory synapses. Overall, this thesis shows the importance of activity-regulated genes in both pancreatic islets and arcuate nucleus neurons, and shows a novel role of the transcription factor NPAS4 in POMC neurons in regulating energy homeostasis.
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