Effect Of The Reduction Of Neuronal NAD On Neuron-Astrocyte Metabolic Coupling

The energy needs of the brain are very large.In order to meet the metabolic needs of the brain in different states,astrocytes and neurons have a wide range of metabolic couplings,such as "glutamate-glutamine cycle" and "lactate shuttle." Nicotinamide adenine dinucleotide(NAD)is widely involved in this metabolic coupling.NAD levels were significantly reduced in the brain of aged mice,or in the presence of brain cryoinjury and cerebral ischemia.And simultaneously,the metabolism of the brain would change.It is not clear whether neuronal NAD levels would affect the metabolic coupling of neurons and astrocytes.FK866 is a highly specific inhibitor of the rate-limiting enzyme for NAD salvaging biosynthesis pathway-nicotinamide phosphate ribose transferase(NAMPT).We used the intracerebroventricular injection to administer FK866,which mimicked the reduction of NAD levels in both physiological and pathological conditions.The metabolism of small molecules in the brain can be acquired and analyzed by using a classical metabolic assay(NMR).At the same time,we supplied neurons and astrocytes selectively 13C isotopically labeled lactic acid and acetate,respectively.The relative metabolites derived from the given lactic acid and acetate would be detected using 13C based NMR spectra,which were subsequently used to evaluate the metabolic capacity of neurons or astrocytes,and explore the consequences of these metabolic changes.We found that even mild decreases in NAD levels can alter the metabolic coupling of neurons to astrocytes.A mild decrease in NAD levels would destroy the glutamate-glutamine cycle and increase lactate shuttling.More importantly,a mild decrease in NAD increases the susceptibility of neurons to ischemic injury.Early supplementation of NAD reverses the metabolic coupling changes between neurons and astrocytes,and protects against cerebral ischemia.