Direct evidence for activity-dependent glucose phosphorylation in neurons with implications for the astrocyte-to-neuron lactate shuttle

Abstract

Previous 13C magnetic resonance spectroscopy experiments have shown that over a wide range of neuronal activity, approximately one molecule of glucose is oxidized for every molecule of glutamate released by neurons and recycled through astrocytic glutamine. The measured kinetics were shown to agree with the stoichiometry of a hypothetical astrocyte-to-neuron lactate shuttle model, which predicted negligible functional neuronal uptake of glucose. To test this model, we measured the uptake and phosphorylation of glucose in nerve terminals isolated from rats infused with the glucose analog, 2-fluoro-2-deoxy-D-glucose (FDG) in vivo. The concentrations of phosphorylated FDG (FDG 6P ), normalized with respect to known neuronal metabolites, were compared in nerve terminals, homogenate, and cortex of anesthetized rats with and without bicuculline-induced seizures. The increase in FDG 6P in nerve terminals agreed well with the increase in cortical neuronal glucose oxidation measured previously under the same conditions in vivo, indicating that direct uptake and oxidation of glucose in nerve terminals is substantial under resting and activated conditions. These results suggest that neuronal glucose-derived pyruvate is the major oxidative fuel for activated neurons, not lactate-derived from astrocytes, contradicting predictions of the original astrocyte-to-neuron lactate shuttle model under the range of study conditions. neuroenergetics | glutamate−glutamine cycle | neuronal glucose phosphorylation | synaptoneurosomes | 2-fluorodeoxyglucose M etabolic and neurophysiological research has experimentally related brain energy consumption, in the form of glucose oxidation, to the brain work supporting neuronal firing. Carbon-13 magnetic resonance spectroscopy (MRS) measurements (1, 2) of the associated fluxes in cerebral cortex of anesthetized rats over a range of electrical activity revealed, surprisingly, a near 1:1 relationship (in molar equivalent units) between increments in the glutamate−glutamine neurotransmitter cycle and neuronal glucose oxidation. Subsequent studies of rat and human cerebral cortex have been consistent with this finding (3, 4). The near 1:1 flux relation was consistent with a cellular/ molecular model, originally proposed by Pellerin and Magistretti (5), and subsequently expanded to include the glutamate/glutamine cycle (1, 6). Evidence for the astrocyte-to-neuron lactate shuttle (ANLS) model is summarized in ref. 7. In this model (Fig. 1A), glutamate released from neurons is taken up by astrocytes and converted to glutamine using ATP derived from glycolysis. Lactate produced by this process is transferred to neurons where oxidation occurs. This ANLS model predicts a 1:1 relationship between increments in astrocytic glutamate uptake and glycolysis. Glycolytically derived ATP might provide for more rapid clearance of glutamate from the synaptic cleft into astrocyte processes devoid of mitochondria (8).The ANLS hypothesis has been challenged on biochemical, in vivo, in situ, and in vitro experimental a...

The Role of Mitochondrially Derived ATP in Synaptic Vesicle Recycling

“…Much of this ATP may be generated from lactate produced by adjacent glial cells and then imported into neurons via the astrocyteneuron lactate shuttle, where it is converted into pyruvate (28,59). However, recent evidence suggests that neurons (not astrocytes) directly metabolize much of the glucose entering the brain (60,61), suggesting that glycolysis may actually contribute more to supporting neuronal energy needs than previously suspected. Importantly, we must better understand the role of energy failure in neurodegeneration because mitochondria have other functions; they buffer calcium, produce reactive oxygen species, synthesize lipids, and regulate apoptosis (62).…”

Fluctuations in Cytosolic Calcium Regulate the Neuronal Malate–Aspartate NADH Shuttle: Implications for Neuronal Energy Metabolism

“…As mentioned above, a well-known proposal is that during brain activation, lactate is mainly produced by astrocytes, and consumed in neurons [33,48]. However, other studies have concluded that neurons still consume glucose during brain activation, and that the excess of lactate produced is probably removed from the brain through the circulation [45,46,49,50]. The results of Bak et al [28,29] show that neurons may be one of the sites of lactate production during activation, perhaps due to intermittent MAS inhibition under high activating conditions, consistent with the role of matrix Ca 2?…”

The Contribution of Ketone Bodies to Basal and Activity-Dependent Neuronal Oxidation in Vivo

“…An important role for neuronal glycolysis as a source of the reducing equivalents is supported by our recent findings of significant activity-dependent glucose phosphorylation in nerve terminals in vivo. 40 For the awake rat cortex oxidizing ketone bodies at the maximum (saturating) rate of B0.7 mmol/g per minute, V pdhN would be 1.12 À 0.70 ¼ 0.42 mmol/g per minute. With V cyc ¼ 0.55 mmol/g per minute, V pdhN :V cyc B0.76:1, which is lower than the 2:1 ratio found between DV tcaN and DV cyc , reflecting the substantial contribution of BHB to total neuronal metabolism under saturating conditions.…”