Fast and Reversible Stimulation of Astrocytic Glycolysis by K⁺and a Delayed and Persistent Effect of Glutamate

Abstract

Synaptic activity is followed within seconds by a local surge in lactate concentration, a phenomenon that underlies functional magnetic resonance imaging and whose causal mechanisms are unclear, partly because of the limited spatiotemporal resolution of standard measurement techniques. Using a novel Förster resonance energy transfer-based method that allows real-time measurement of the glycolytic rate in single cells, we have studied mouse astrocytes in search for the mechanisms responsible for the lactate surge. Consistent with previous measurements with isotopic 2-deoxyglucose, glutamate was observed to stimulate glycolysis in cultured astrocytes, but the response appeared only after a lag period of several minutes. Na ϩ overloads elicited by engagement of the Na ϩ -glutamate cotransporter with D-aspartate or application of the Na ϩ ionophore gramicidin also failed to stimulate glycolysis in the short term. In marked contrast, K ϩ stimulated astrocytic glycolysis by fourfold within seconds, an effect that was observed at low millimolar concentrations and was also present in organotypic hippocampal slices. After removal of the agonists, the stimulation by K ϩ ended immediately but the stimulation by glutamate persisted unabated for Ͼ20 min. Both stimulations required an active Na ϩ /K ϩ ATPase pump. By showing that small rises in extracellular K ϩ mediate short-term, reversible modulation of astrocytic glycolysis and that glutamate plays a long-term effect and leaves a metabolic trace, these results support the view that astrocytes contribute to the lactate surge that accompanies synaptic activity and underscore the role of these cells in neurometabolic and neurovascular coupling.

NH4+ triggers the release of astrocytic lactate via mitochondrial pyruvate shunting

“…This calculated value, which is close to the physiological range, is probably an overestimate as it does not take into account NH + 4 clearance by peripheral tissues and by astrocytes. Aerobic glycolysis may be triggered by stimulation of astrocytic glycolysis, such as that observed in response to glutamate or to high K + (11,12). To assess glycolysis as a possible target for NH + 4 , astrocytic glucose was measured using the genetically encoded glucose nanosensor FLII 12 Pglu700μΔ6 (28).…”

“…Aerobic glycolysis and its associated lactate surge are causally linked to diverse functions of the brain in health and disease (3-10). Two signals are known to trigger aerobic glycolysis in brain tissue: glutamate and K + , which are released by active neurons and stimulate glycolysis in astrocytes (11,12).Neurons produce as much NH + 4 as they produce glutamate, both molecules being stoichiometrically linked in the glutamateglutamine cycle (13). Brain tissue NH + 4 increases within seconds of neural activation (14-16) and is quickly released to the interstitium (17, 18) to be captured by astrocytes through K + channels and transporters (19).…”

“…Aerobic glycolysis and its associated lactate surge are causally linked to diverse functions of the brain in health and disease (3)(4)(5)(6)(7)(8)(9)(10). Two signals are known to trigger aerobic glycolysis in brain tissue: glutamate and K + , which are released by active neurons and stimulate glycolysis in astrocytes (11,12).…”

“…In the short term (seconds), glutamate stimulates the glucose transporter GLUT1 via Na + and Ca 2+ transients mediated by the Na + -glutamate cotransporter (44,45). Acting in parallel, K + stimulates glycolysis via the Na + -HCO 3 − cotransporter NBCe1 (12,46) and also opens a lactate permeable ion channel at the cell surface (27). By suppressing the uptake of pyruvate by astrocytic mitochondria within seconds, NH + 4 adds an additional strategy for short-term induction of aerobic glycolysis.…”

“…The production of lactate by astrocytes is also stimulated by neuronal signals in the long term (minutes). Glutamate activates glycolysis via the Na + -glutamate cotransporter (11,12) and K + stimulates glycogen degradation via the HCO 3 − -sensitive soluble adenylyl cyclase (47). Also within minutes, nitric oxide has been shown to stimulate astrocytic glycolysis through inhibition of cytochrome c oxidase leading to activation of 5′-AMP-activated protein kinase and PFKFB3 (48).…”