Nampt

Nampt

Nicotinamide phosphoribosyltransferase (NAmPRTase or Nampt) also known as pre-B-cell colony-enhancing factor 1 (PBEF1) or visfatin is an enzyme that in humans is encoded by the PBEF1 gene. This protein has also been reported to be a cytokine (PBEF) that promotes B cell maturation and inhibits neutrophil apoptosis.

NAD is an essential cofactor in many fundamental cellular reactions due to its electron transferring ability during redox reactions, its ability to regulate the activity of cellular longevity mediators and its role as a substrate for the production of other critically important molecules. In mammals, NAD generation can occur by two central pathways: de novo synthesis or salvage through NAD degradation products. De novo synthesis occurs through conversion of the amino acid tryptophan to quinolinic acid via quinolate phosphoribosyltransferase (Qprt), which leads to nicotinic acid mononucleotide (NaMN) generation. Through the adenylation activity of Nmnat, deamido-NAD is produced and NAD synthetase then catalyzes amidation of the molecule resulting in its conversion to NAD.

Of the four , and aging. Aging decreases Nampt protein in skeletal muscle (40% to 80%), WAT (50%), hippocampus (43%), cortex (32%), cerebellum, heart, lung, liver, and kidney; however, these results are contraversial for the cortex, cerebellum, striatum, liver, heart, serum, or microglia. Conversely, Nampt expression is increased by conditions of energy deprivation including fasting in the liver, glucose restriction in adipocytes and skeletal myoblasts, hypoxia in cardiomyocytes, and ischemia in the brain but see.

And finally, although Nampt is expressed in all mammalian tissues examined, its expression level ranges considerably. Nampt expression is highest in blood leukocytes, the liver, and the lung in humans, the liver and heart in rats, and the brown adipose tissue, liver, and kidney in mice. However, Nampt expression is almost undetectable in thebrain and pancreas, thus raising the question as to what pathway of NAD+ biosynthesis these organs use. Moreover, even though the majority of mammalian tissues and organs predominantly use Nampt-mediated NAD+ biosynthesis, different tissues have differential dependence upon Nampt as seen both by knockdown of Nampt and inhibition of Nampt with FK866. FK866 decreases NAD+ levels in a wide range of cell types, but at variable time courses.

Another factor that makes the importance of Nampt for the brain and pancreas unclear is the fact that Nampt is secreted from brown adipocytes, white adipocytes, mesangial cells, and stimulated neutrophils, to generate extracellular Nampt (eNampt). eNampt secretion can be extensive; adipocytes secrete almost 30 fold more eNampt than leptin. The mechanism by which eNampt is secreted remains unclear how as Nampt does not have a cleavable signal sequence and is not released by the endoplasmic reticulum-Golgi pathway of secretion nor via microvescicles. Plausible alternatives include non-classical secretory pathways such as blebbing of the cell membrane or a transporter system. eNampt also has phosphoribosyltransferase activity, and is hypothesized to serve tissues with low intracellular Nampt expression. Whether eNampt and/or plasma NAD+ can cross the blood-brain barrier, and thus the functional importance of eNampt for brain NAD+ levels, remain central questions.

References:

Stein, L. L. R. (2014). The Role of Nampt-Mediated NAD+ Biosynthesis in Hippocampal Neural Stem Cells and Excitatory Neurons.

Malam, Z. (2011). Pre-B Cell Colony-enhancing Factor (PBEF) Promotes Neutrophil Inflammatory Function through Enzymatic and Non-enzymatic Mechanisms (Doctoral dissertation).

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