Astrocyte-axon interactions in central white matter energy metabolism: the roles of glycogen and lactate
The goal of this research was to investigate the role of astrocytic glycogen in central nervous system (CNS) white matter (WM) metabolism. Experiments were designed to test two hypotheses: (1) Metabolites other than glucose can support function of WM axons. (2) During glucose withdrawal in CNS WM, astrocytes supply energy substrate to axons in the form of lactate, derived from glycogen; and axon function and survival depend on astrocytic glycogen in the absence of exogenous glucose. Experiments were performed on the isolated rat optic nerve, a central WM tract, using electrophysiological and biochemical techniques. Axon function was determined by quantitatively monitoring the area under the stimulus-evoked compound action potential (CAP). To address the first hypothesis, we tested compounds that were able to enter energy pathways. The CAP was maintained for 2 hr in physiological saline containing 10 mM glucose. 20 mM lactate, 20 mM pyruvate, 10 mM fructose, or 10 mM mannose supported axon function as effectively as did glucose; 10 mM glutamine provided partial support. In the next phase of the study we examined whether astrocytic glycogen sustains axon function during, and enhances axon survival after, 60 min of glucose deprivation. Exposure of nerves to glucose-free perfusate had no effect on CAP area for 30 min, after which the CAP rapidly failed. Irreversible injury, measured as incomplete recovery of the CAP compared with control, was sustained. Glycogen content of the tissue fell 30 min after glucose withdrawal, compatible with rapid turnover in the absence of glucose. Up-regulation of glycogen increased latency to CAP failure and improved CAP recovery; down-regulation of glycogen decreased latency to CAP failure and reduced recovery. Lactate transport blockers were used to determine whether lactate represented the fuel derived from glycogen and shuttled to axons. The blockers, when applied during glucose withdrawal, decreased latency to CAP failure and decreased CAP recovery. These results indicated that in the absence of glucose, astrocytic glycogen was metabolized to lactate, which was subsequently transferred to axons for fuel. This study represents the first demonstration of the importance of astrocytic glycogen for axon function and survival under conditions of glucose withdrawal.