Neurodegenerative diseases result in the progressive deterioration of the nervous system, with motor and cognitive impairments being the two most observable problems. Motor dysfunction could be caused by motor neuron diseases (MNDs) characterized by the loss of motor neurons, such as amyotrophic lateral sclerosis and Charcot–Marie–Tooth disease, or other neurodegenerative diseases with the destruction of brain areas that affect movement, such as Parkinson’s disease and Huntington’s disease. Nicotinamide adenine dinucleotide (NAD+) is one of the most abundant metabolites in the human body and is involved with numerous cellular processes, including energy metabolism, circadian clock, and DNA repair. NAD+ can be reversibly oxidized-reduced or directly consumed by NAD+-dependent proteins. NAD+ is synthesized in cells via three different paths: the de novo, Preiss–Handler, or NAD+ salvage pathways, with the salvage pathway being the primary producer of NAD+ in mammalian cells. NAD+ metabolism is being investigated for a role in the development of neurodegenerative diseases. In this review, we discuss cellular NAD+ homeostasis, looking at NAD+ biosynthesis and consumption, with a focus on the NAD+ salvage pathway. Then, we examine the research, including human clinical trials, focused on the involvement of NAD+ in MNDs and other neurodegenerative diseases with motor dysfunction.
Nicotinamide adenine dinucleotide (NAD +) plays a critical role in energy metabolism and bioenergetic homeostasis. Most NAD + in mammalian cells is synthesized via the NAD + salvage pathway, where nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme, converting nicotinamide into nicotinamide mononucleotide (NMN). Using a Thy1-Nampt −/− projection neuron conditional knockout (cKO) mouse, we studied the impact of NAMPT on synaptic vesicle cycling in the neuromuscular junction (NMJ), end-plate structure of NMJs and muscle contractility of semitendinosus muscles. Loss of NAMPT impaired synaptic vesicle endocytosis/exocytosis in the NMJs. The cKO mice also had motor endplates with significantly reduced area and thickness. When the cKO mice were treated with NMN, vesicle endocytosis/exocytosis was improved and endplate morphology was restored. Electrical stimulation induced muscle contraction was significantly impacted in the cKO mice in a frequency dependent manner. The cKO mice were unresponsive to high frequency stimulation (100 Hz), while the NMN-treated cKO mice responded similarly to the control mice. Transmission electron microscopy (TEM) revealed sarcomere misalignment and changes to mitochondrial morphology in the cKO mice, with NMN treatment restoring sarcomere alignment but not mitochondrial morphology. This study demonstrates that neuronal NAMPT is important for pre-/ post-synaptic NMJ function, and maintaining skeletal muscular function and structure. Nicotinamide adenine dinucleotide (NAD +) is found in all cells of the human body and is an important cofactor or co-substrate, used in numerous enzymatic processes including glycolysis, the tricarboxylic acid (TCA) cycle, oxidative phosphorylation, DNA repair, and protein deacetylation 1. NAD + levels may be important for many aspects of health and aging, including cellular metabolism, sarcopenia, and neurodegeneration 2. NAD + can be synthesized through multiple enzymatic pathways. One is a de novo pathway that begins with the amino acid tryptophan, while other pathways utilize different metabolites capable of being converted into NAD +. In mammalian cells, the majority of NAD + is produced from metabolites entering the NAD + salvage pathway 3. The rate limiting enzyme of the salvage pathway is nicotinamide phosphoribosyltransferase (NAMPT), which condenses nicotinamide (NAM) and 5-phosphoribosyl pyrophosphate (PRPP) into nicotinamide mononucleotide (NMN). NMN is subsequently synthesized into NAD + by nicotinamide mononucleotide adenylyltransferases (NMNATs) 4. NAD + levels decline with age and in different diseases. However, administration of NAD + precursor molecules, such as NMN or nicotinamide riboside (NR), are effective at preventing or reversing many age-or disease-related declines 5-10. NAD + and the NAD + salvage pathway are vitally important to maintain bioenergetic homeostasis, the normal health and function of many different organs and tissues in the human body, with neurons and skeletal muscles being impacted greatly. In n...
Nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme in the NAD+ salvage pathway. Our previous study demonstrated that deletion of NAMPT gene in projection neurons using Thy1-NAMPT−/− conditional knockout (cKO) mice causes neuronal degeneration, muscle atrophy, neuromuscular junction abnormalities, paralysis and eventually death. Here we conducted a combined metabolomic and transcriptional profiling study in vivo in an attempt to further investigate the mechanism of neuronal degeneration at metabolite and mRNA levels after NAMPT deletion. Here using steady-state metabolomics, we demonstrate that deletion of NAMPT causes a significant decrease of NAD+ metabolome and bioenergetics, a buildup of metabolic intermediates upstream of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) in glycolysis, and an increase of oxidative stress. RNA-seq shows that NAMPT deletion leads to the increase of mRNA levels of enzymes in NAD metabolism, in particular PARP family of NAD+ consumption enzymes, as well as glycolytic genes Glut1, Hk2 and PFBFK3 before GAPDH. GO, KEGG and GSEA analyses show the activations of apoptosis, inflammation and immune responsive pathways and the inhibition of neuronal/synaptic function in the cKO mice. The current study suggests that increased oxidative stress, apoptosis and neuroinflammation contribute to neurodegeneration and mouse death as a direct consequence of bioenergetic stress after NAMPT deletion.
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