Glycolysis and Glutamate Accumulation into Synaptic Vesicles

Abstract

Glucose is the major source of brain energy and is essential for maintaining normal brain and neuronal function. Hypoglycemia causes impaired synaptic trans- mission. This occurs even before significant reduction in global cellular ATP concentration, and relationships among glycolysis, ATP supply, and synaptic transmis- sion are not well understood. We demonstrate that the glycolytic enzymes glyceraldehyde phosphate dehydro- genase (GAPDH) and 3-phosphoglycerate kinase (3-PGK) are enriched in synaptic vesicles, forming a functional complex, and that synaptic vesicles are capable of accumulating the excitatory neurotransmitter glutamate by harnessing ATP produced by vesicle- bound GAPDH/3-PGK at the expense of their substrates. The GAPDH inhibitor iodoacetate suppressed GAPDH/ 3-PGK-dependent, but not exogenous ATP-dependent, [3H]glutamate uptake into isolated synaptic vesicles. It also decreased vesicular [3H]glutamate content in the nerve ending preparation synaptosome; this decrease was reflected in reduction of depolarization-induced [3H]glutamate release. In contrast, oligomycin, a mito- chondrial ATP synthase inhibitor, had minimal effect on any of these parameters. ADP at concentrations above 0.1 mM inhibited vesicular glutamate and dissipated membrane potential. This suggests that the coupled GAPDH/3-PGK system, which converts ADP to ATP, en- sures maximal glutamate accumulation into presynap- tic vesicles. Together, these observations provide in- sight into the essential nature of glycolysis in sustaining normal synaptic transmission