The Impact of Melanocortin Receptor Ligands on Astrocyte Signalling and Metabolism

Melanocortin peptides, which act on melanocortin receptors (MCRs), play multifaceted roles in the body through various tissues and mechanisms that regulate physiological functions. MCRs, comprising a family of five subtypes (MC1R to MC5R), are 7-transmembrane receptors that can couple with G proteins or function independently of them. These receptors are present in the central nervous system (CNS) as well as other body tissues. This thesis concentrates on the role of MCRs within the CNS and explores their functional role in astrocytes. Astrocytes, once underestimated glial cells, are now recognised as crucial contributors to brain function. Although considered non-excitable cells, they increase their intracellular calcium ion levels in response to a stimulus or signalling event. They maintain brain homeostasis through the regulation of diverse cellular signals, extracellular potassium ion buffering, and metabolic activities. While MC3R and MC4R were initially considered the main MCR subtypes expressed in CNS neurons, recent evidence suggests that other MCR subtypes may also be present, although their expression and function in astrocytes remain poorly defined. This thesis addresses this gap by investigating the expression of all MCRs and the impact of pharmacological modulation on astrocytic signalling pathways, ion dynamics and metabolism. It explores how melanocortin peptides influence metabolic functions in astrocytes, particularly focusing on mitochondrial and glycolytic activity, ERK1/2, PKA and PLC signalling, and calcium and potassium ion flux, in these cells. Through experimental approaches including RT-PCR, immunoblotting, real-time fluorescence imaging, cell bioenergetics and metabolic assays, this work provides new insights into the complex interplay between melanocortin signalling, metabolic and ion changes in astrocytic function. The findings enhance our understanding of how MCRs contribute to brain homeostasis and highlight their potential as therapeutic targets for metabolic and neurological disorders. The overarching hypothesis of this thesis speculates that MCRs are expressed in astrocytes and exhibit functional signalling, alterations in potassium and calcium ion dynamics, and changes in metabolic activity in response to synthetic MCR ligands. This hypothesis was assessed in two cell models: neonatal mouse primary cortical astrocytes and the human astroglioma cell line U373. The findings of this thesis indicate that MCRs are expressed in both cell models examined. Pharmacological modulation of MCRs led to increased ERK1/2 phosphorylation and enhanced cell metabolism in the human U373 astroglioma cell line, though no changes in intracellular calcium ion levels were observed. Melanocortin ligands did not alter ERK1/2 and PKA signalling, and calcium ion influx in mouse primary astrocytes. Despite exploration, melanocortin ligand induced modulation of potassium channels in U373 cells remains unclear. In conclusion, MCRs were found to be present on astrocytes and showed active signalling via ERK1/2 in U373 cells following treatment with melanocortin agonists, leading (directly or indirectly) to the regulation of cell metabolism. MCRs on astrocytes may play a significant role in maintaining brain/systemic homeostasis and may be a therapeutic target for mitigating pathological disorders. Although this thesis does not focus on a disease model, the data generated from this thesis supports further investigation of astrocyte melanocortin signalling in the context of pathophysiology.<p></p>

Author dc:creator

• A Al-Khalidi (21972392)

• melanocortin peptides

Statement dc:rights

• All rights reserved

Identifier

10871/139798

OAI identifier oai:identifier

oai:figshare.com:article/29813267