The impact of neuronal Nampt on motor function of mice

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Nicotinamide phosphoribosyltransferase (Nampt), also named visfatin and pre-B cell colony-enhancing factor (PBEF), is the rate-limiting enzyme converting nicotinamide (NAM) into nicotinamide mononucleotide (NMN) and eventually transferring to nicotinamide adenine dinucleotide (NAD+) by nicotinamide mononucleotide adenylyltransferase (Nmnat1-3) in the salvage pathway of mammalian NAD+biosynthesis. In the central nervous system (CNS), Nampt is mainly expressed in neurons. Due to the indispensable role of Nampt in energy supply and inflammatory cytokine secretion in the immune system, Nampt is involved in pathogenesis of different diseases including ischemic stroke, cancer, diabetes, aging, inflammation and so on. However, the effect of Nampt on the function of projection neurons remains to be determined. In this present study, we crossed floxed Nampt (Namptf/f) mice with SLICK (singleneuron labeling with inducible Cre-mediated knockout)-H transgenic line controlled by Thy1 promoters, based on site-specific CreERT2/Lox recombinase system, to generate Thy1-YFP-CreERT2: Namptf/f mice. The deletion of Nampt gene in projection neurons was induced by tamoxifen administration to obtain the neuron-specific Nampt homozygous conditional knockout (cKO) mice, i.e., Thy1-YFP-Nampt-/- cKO mice. Western blotting analysis confirmed that the Nampt levels were decreased by more than 50% in the cortex and hippocampus of the Thy1-YFP-Nampt-/- mice. Surprisingly, Thy1-YFP-Nampt-/- cKO mice showed reactive astrogliosis in the CNS and suffered from gradual body weight loss, hypothermia, progressive paralysis, and muscle atrophy, with an average lifespan of three weeks post tamoxifen administration. These mice also exhibited worse performance in the motor functions than wild-type (WT) mice based on behavioral tests, indicating motor dysfunction in Thy1-YFP-Nampt-/- mice. In addition, widespread abnormalities of the neuromuscular junctions were detected in these mice, including fragmented or faint AChR clusters, non-innervated or multi-innervated postsynaptic sites, partially or completely denervated presynapses, and swollen or sprouted axons, which provide further evidence for motor dysfunction. Interestingly, the symptoms in our Nampt cKO mice are considerably similar to those observed in the mouse model of Amyotrophic lateral sclerosis (ALS), an incurable neurodegenerative disease. On the other hand, NMN treatment slightly but significantly improved the health conditions of homozygous Nampt cKO mice at the end stage. Taken together, our findings demonstrate that the deletion of neuronal Nampt causes general physiological abnormalities, progressive paralysis, muscle wasting and motor behavior dysfunction as well as widespread alterations at both presynaptic and postsynaptic levels in the neuromuscular system of adult mice.