Abstract
Our previous study indicated that nicotinamide phosphoribosyltransferase (NAMPT) is released from cells and might be an important extracellular neuroprotective factor in brain ischemia. Here, we tested whether NAMPT protects against ischemic brain injury when administered directly into the intracerebroventricular (ICV) compartment of the cranium. Recombinant NAMPT protein (2 μg) was delivered ICV in mice subjected to 45-min middle cerebral artery occlusion (MCAO), and the effects on infarct volume, sensorimotor function, microglia/macrophage polarization, neutrophil infiltration, and BBB integrity were analyzed. The results indicate that ICV administration of NAMPT significantly reduced infarct volume, retained its beneficial properties even when ICV administration was delayed by 6 h after MCAO, and improved neurological outcomes. NAMPT treatment inhibited pro-inflammatory microglia/macrophages, promoted microglia/macrophage polarization toward the anti-inflammatory phenotype, and reduced the infiltration of neutrophils into the perilesional area after brain ischemia. In vitro studies indicated that multiple pro-inflammatory microglial markers/cytokines were downregulated while multiple anti-inflammatory microglial markers/cytokines were induced in primary microglial cultures treated with NAMPT protein. NAMPT treatment also fortified the blood-brain barrier (BBB), as shown by reduced extravascular leakage of the small-molecule tracer Alexa Fluor 555 Cadaverine and larger-sized endogenous IgGs into brain parenchyma. Thus, NAMPT may protect against ischemic brain injury partly through a novel anti-inflammatory mechanism, which in turn maintains BBB integrity and reduces the infiltration of peripheral inflammatory cells. Taken together, these results provide validation of recombinant NAMPT delivery into the extracellular space as a potential neuroprotective strategy for stroke.