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The Role of Nampt-Mediated NAD+ Biosynthesis in Hippocampal Neural Stem Cells and Excitatory Neurons
Date of Award
Doctor of Philosophy (PhD)
Nicotinamide adenine dinucleotide (NAD+) is a classic coenzyme recycled in oxidation-reduction reactions and cellular homeostasis, as well as consumed by several families of enzymes as a cosubstrate. Mammalian cells predominantly synthesize NAD+ from nicotinamide via a pathway rate-limited by the enzyme nicotinamide phosphoribosyltransferase (Nampt). Previous work demonstrated that NAD+ and Nampt levels decrease in peripheral tissues with aging and high fat diet feeding, and loss of Nampt-mediated NAD+ biosynthesis causally contributed to deleterious metabolic phenotypes. Nampt expression was almost undetectable in whole brain extracts, but specifically expressed in hippocampal cells. This expression pattern, the presence of other NAD+ biosynthetic pathways, and the potential of paracrine secretion of NAD+ raised several questions. What pathway of NAD+ biosynthesis do neural cells use? What is the importance of hippocampal Nampt? Does age- or metabolism-associated loss of NAD+ biosynthesis contribute to cognitive dysfunction?
In this thesis, I demonstrate that hippocampal levels of NAD+ and Nampt decrease with age. In contrast, a 12-month Western diet neither altered Nampt-mediated NAD+ biosynthesis nor hippocampal histopathology. To address the importance of intracellular Nampt-mediated NAD+ biosynthesis, I generated adult neural stem/progenitor cell (NSPC)-specific inducible (iNSPC-Nampt-KO mice) and forebrain excitatory neuron (CamKIIαNampt-/- mice) Nampt knockout mice. Analysis of these mice revealed that Nampt is the main source of NAD+ levels for NSPCs and forebrain excitatory neurons. In NSPCs, loss of Nampt resulted in a G1/S stall in the cell cycle, reducing pool size and proliferative ability. Loss of Nampt also prevented NSPC differentiation into oligodendrocytes in vitro as well as upon insult in vivo. The age-related decrease in the NSPC pool was lessened by administration of the end product of Nampt activity, nicotinamide mononucleotide. Mice lacking Nampt in forebrain excitatory neurons (CamKIIαNampt-/- mice) showed hippocampal and cortical atrophy, astrogliosis, microgliosis, and abnormal dendritic morphology. These histological changes presented with altered electrophysiology and behavior including: altered intra-hippocampal connectivity, impaired induction of long-term depression, dysfunction of NR2B (GluN2B)-containing N-methyl-D-aspartate receptors, hyperactivity, memory impairment, and reduced anxiety. These findings demonstrate that intracellular Nampt-mediated NAD+ biosynthesis is critical for neural cells, and suggest a potential therapeutic approach of nicotinamide mononucleotide administration to ameliorate age-related neural pathologies.
Chair and Committee
David Gutmann, Aaron DiAntonio, Robyn Klein, Kelly Monk, Jason Weber
Stein, Liana Leigh Roberts, "The Role of Nampt-Mediated NAD+ Biosynthesis in Hippocampal Neural Stem Cells and Excitatory Neurons" (2014). Arts & Sciences Electronic Theses and Dissertations. 97.