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Hypothalamic glucose-excited (GE) neurons contribute to whole-body glucose homeostasis and participate in the detection of hypoglycaemia. This system appears defective in type 1 diabetes, in which hypoglycaemia commonly occurs. Unfortunately, it is at present unclear which molecular components required for glucose sensing are produced in individual neurons and how these are functionally linked. We used the GT1-7 mouse hypothalamic cell line to address these issues.
Electrophysiological recordings, coupled with measurements of gene expression and protein levels and activity, were made from unmodified GT1-7 cells and cells in which AMP-activated protein kinase (AMPK) catalytic subunit gene expression and activity were reduced.
Hypothalamic GT1-7 neurons express the genes encoding glucokinase and ATP-sensitive K+ channel (K-ATP) subunits K (ir) 6.2 and Sur1 and exhibit GE-type glucose-sensing behaviour. Lowered extracellular glucose concentration hyperpolarised the cells in a concentration-dependent manner, an outcome that was reversed by tolbutamide. Inhibition of glucose uptake or metabolism hyperpolarised cells, showing that energy metabolism is required to maintain their resting membrane potential. Short hairpin (sh)RNA directed to Ampk alpha 2 (also known as Prkaa2) reduced GT1-7 cell AMPK alpha 2, but not AMPK alpha 1, activity and lowered the threshold for hypoglycaemia-induced hyperpolarisation. shAmpk alpha 1 (also known as Prkaa1) had no effect on glucose-sensing or AMPK alpha 2 activity. Decreased uncoupling protein 2 (Ucp2) mRNA was detected in AMPK alpha 2-reduced cells, suggesting that AMPK alpha 2 regulates UCP2 levels.
We have demonstrated that GT1-7 cells closely mimic GE neuron glucose-sensing behaviour, and reducing AMPK alpha 2 blunts their responsiveness to hypoglycaemic challenge, possibly by altering UCP2 activity. These results show that suppression of AMPK alpha 2 activity inhibits normal glucose-sensing behaviour and may contribute to defective detection of hypoglycaemia.