Astrocytic extracelluar vesicles and miRNA dynamicsModulating metablism and neuroendocrine functions in the hypothalamus

Abstract

The thesis investigates the role of astrocytes in metabolic regulation, particularly through their release of extracellular vesicles EVs carrying microRNAs miRNAs that influence neuronal function in the hypothalamus. Astrocytes, glial cells that support neurons in the brain, have recently gained attention for their involvement in maintaining energy homeostasis. This research centers on understanding how astrocytes communicate with hypothalamic neurons via EVs, particularly under different dietary conditions, to control processes like energy intake, expenditure, and overall metabolic balance. The study focuses on two fatty acids - palmitic acid PA, a saturated fatty acid, and oleic acid OA, a monounsaturated fatty acid-as representative dietary inputs that impact astrocytes' metabolic responses and the EVs they release. Hypothalamic astrocytes exposed to these fatty acids show distinct metabolic changes, especially in the Krebs cycle and glutamate/glutamine cycling, pathways essential for energy production and neurotransmitter metabolism. These metabolic shifts in astrocytes lead to alterations in the cargo of EVs, including miRNAs and cytokines, which in turn can affect hypothalamic neurons. The thesis specifically examines how EVs from astrocytes exposed to PA or OA influence POMC and AgRP neurons in the arcuate nucleus of the hypothalamus. POMC neurons play a key role in suppressing appetite and increasing energy expenditure, while AgRP neurons stimulate hunger and reduce energy expenditure. The research found that EVs from PA-treated astrocytes altered POMC neuron activity by impacting mitochondrial functions and glycolysis, affecting energy metabolism at the cellular level. This indicates that dietary fatty acids can modulate neuronal activity and gene expression via astrocytic EVs, suggesting a pathway by which dietary components influence hypothalamic function and, consequently, energy balance. Further, the study identifies specific miRNAs within these EVs - such as miR-199a-3p and miR-145-5p - that impact crucial metabolic signaling pathways. For instance, miR-199a-3p modulates the mTOR signaling pathway, associated with cellular growth and energy homeostasis, while miR-145-5p influences the PI3K-Akt pathway, which is essential for glucose metabolism and insulin sensitivity. These miRNAs were found to influence POMC expression and activity, demonstrating that astrocytic EVs regulate neuronal metabolic responses by delivering specific molecular signals. The thesis also includes in vivo experiments to confirm these findings and explore their broader physiological implications. When astrocytic EVs were administered centrally in mice, they affected food intake, energy expenditure, and even circadian metabolic rhythms, underscoring the impact of astrocyte-derived EVs on systemic energy homeostasis. Additionally, changes in hypothalamic neuropeptides and miRNA expression patterns following EV administration provide further evidence that astrocytes, through EVs, influence hypothalamic circuits in ways that reflect dietary fatty acid inputs. The research offers significant insights into astrocyte-neuron communication and presents a new perspective on how diet can impact brain metabolism and energy regulation. By revealing that EVs from astrocytes carry molecular signals (miRNAs) responsive to dietary cues, this study opens up potential avenues for therapeutic intervention in metabolic disorders. Specifically, targeting astrocytic EVs or their miRNA content could help modulate hypothalamic function to treat or prevent conditions like obesity and type 2 diabetes. The findings emphasize the complex, dynamic interaction between dietary factors, astrocytic signaling, and neuronal metabolic control, offering a foundation for future studies aimed at translating these insights into clinical strategies for metabolic health improvement.