Effect of Dietary Ketosis and Nicotinamide Riboside on Hippocampal Krebs Cycle Intermediates and Mitochondrial Energetics in a DNA Repair-Deficient 3xTg/POLβ+/- Alzheimer Disease Mouse Model

Abstract

Alzheimer disease is a neurodegenerative pathology-modifying mitochondrial metabolism with energy impairments where the effects of biological sex and DNA repair deficiencies are unclear. We investigated the therapeutic potential of dietary ketosis alone or with supplemental nicotinamide riboside (NR) on hippocampal intermediary metabolism and mitochondrial bioenergetics in older male and female wild-type (Wt) and 3xTgAD-DNA polymerase-β-deficient (3xTg/POLβ^+/-) (AD) mice. DNA polymerase-β is a key enzyme in DNA base excision repair (BER) of oxidative damage that may also contribute to mitochondrial DNA repair. Metabolic alterations imparted by ketosis and/or NR were assessed in 16 male and female groups, 4 Wt and 4 AD. At 73 weeks of age, mice were divided into: (A) carbohydrate diet (Carb); (B) Carb diet with NR (Carb-NR); (C) Ket diet (Ket); and (D) Ket diet with NR (Ket-NR) groups and remained on their respective treatments for 12 weeks. Mice were euthanized and hippocampi were rapidly removed and frozen. Glycolytic and TCA cycle intermediates were determined by quantitative GC-MS and the ratios of the mitochondrial free [NAD_ox]/[NADH_red] and coenzyme ubiquinone (CoQ/CoQH₂) couples and the Gibbs free energy of the Complex I-II system of the electron transport chain (ETC) ( ${∆G}_{\text{mitochondrial Complex}I-\mathrm{II}}^{\text{'}}$ ) were calculated from selected metabolites. Mice in Groups C and D had elevated blood ketones (1-2 mM). In most groupings, male mice had higher concentrations of TCA cycle intermediates than females. Moreover, higher concentrations of fumarate in Wt males were associated with elevations in the ΔG' of Complex I-II compared to females. In Wt males, NR treatments were associated with elevated concentrations of α-ketoglutarate and malate and linked to increased energy of Complex I-II. In AD males, both NR treatment and dietary ketosis restored the ΔG' of Complex I-II, where the ratio of the CoQ/CoQH₂ couple was oxidized and the [NAD_ox]/[NADH_red] couple was reduced. In AD females, only mice in the Ket diet group had a sufficiently reduced [NAD_ox]/[NADH_red] couple to restore the free energy profile.

Keywords: Alzheimer disease; NAD; TCA cycle; hippocampus; ketones; mitochondrial energetics; nicotinamide riboside.