Extracellular Degradation into Adenosine and the Activities of Adenosine Kinase and AMPK Mediate Extracellular NAD+-produced increases in the Adenylate Pool of BV2 Microglia under Basal Conditions

Abstract

Cumulating evidence has indicated NAD+ deficiency as a common central pathological factor of multiple diseases and aging. NAD+ supplement is highly protective in various disease and aging models, while two key questions remain unanswered: 1) Does extracellular NAD+ also produce its effects through its degradation product adenosine? 2) Does extracellular NAD+ produce the protective effects by affecting cells under pathological insults only, or by affecting both normal cell and cells under pathological insults? Since extracellular NAD+ can be degraded into adenosine, and endogenous adenosine levels are in the nanomolar range under physiological conditions, extracellular NAD+ may produce its effects through its degradation into adenosine. In this study we used BV2 microglia as a cellular model to test our hypothesis that NAD+ treatment can increase the intracellular adenylate pool under basal conditions through its extracellular degradation into adenosine. Our study has shown that extracellular NAD+ increases the adenylate pool of BV2 microglia under basal conditions through its degradation into adenosine that enters the cells through equilibrative nucleoside transporters. The intracellular adenosine is converted to AMP by adenosine kinase, which increases intracellular ATP by both activating AMPK and increasing ADP that drives mitochondrial FoF1-ATP synthase. Collectively, our study has suggested that extracellular NAD+ can enhance defensive capacity of normal cells through a novel pathway, which includes extracellular NAD+ degradation into adenosine and the activities of adenosine kinase and AMPK. Our findings have also suggested that NAD+ administration in various disease and aging models may significantly affect the microglia under basal conditions.