The Impact of Glucose Variation on Human Astrocytes

Diabetes is a metabolic disorder dysregulating glucose homeostasis. The role of astrocytes in central glucose sensing is poorly understood. But it is recognised they take part in whole-body energy homeostasis, specifically as glucose sensors necessary for the counterregulatory response (CRR) to hypoglycaemia. Iatrogenic hypoglycaemia is the limiting factor to glycaemic control in people with type 1 or type 2 diabetes. Severe hypoglycaemia occurs approximately once per year, whereas, the incidence of minor hypoglycaemia is much greater. Hypoglycaemia impairs awareness of future hypoglycaemia and blunts the CRR, eventually causing hypoglycaemia-associated autonomic failure. The mechanisms of this process are poorly understood. This thesis utilised isolated human astrocytes exposed to acute or recurrent low glucose (RLG) in vitro to mimic glucose variation in diabetes. Cellular responses were characterised of three key astrocyte functions. Firstly, is astrocyte metabolism altered by acute and RLG treatment? Secondly, do isolated human astrocytes become activated by low glucose treatment, and is this affected by RLG? Thirdly, are astrocytic inflammatory pathways altered by acute or RLG? The key findings from this thesis shows for the first time that astrocytic mitochondrial oxidation is increased following RLG, with a concurrent increase in fatty acid dependency but decreased coupling efficiency; glycolytic function is also enhanced. Together, this indicates that astrocytes successfully adapt to low glucose to sustain intracellular nucleotide ratios. Contrary to previous work, these human astrocytes do not respond to low glucose by Ca2+-dependent activation. However, the astrocytes do increase inflammatory cytokine release following acute and RLG. Lastly, for the first time an RNA-sequencing approach has been used to identify low glucose-induced differential gene expression. Together these findings support the argument that astrocytes are sensitive to low glucose and may be important in glucose sensation and the CRR.