Relative importance of redox buffers GSH and NAD(P)H in age‐related neurodegeneration and Alzheimer disease‐like mouse neurons

Debolina Ghosh,Kelsey R Levault,Gregory J Brewer

doi:10.1111/acel.12216

Abstract

Aging, a major risk factor in Alzheimer’s disease (AD), is associated with an oxidative redox shift, decreased redox buffer protection, and increased free radical reactive oxygen species (ROS) generation, probably linked to mitochondrial dysfunction. While NADH is the ultimate electron donor for many redox reactions, including oxidative phosphorylation, glutathione (GSH) is the major ROS detoxifying redox buffer in the cell. Here, we explored the relative importance of NADH and GSH to neurodegeneration in aging and AD neurons from nontransgenic and 3xTg-AD mice by inhibiting their synthesis to determine whether NADH can compensate for the GSH loss to maintain redox balance. Neurons stressed by either depleting NAD(P)H or GSH indicated that NADH redox control is upstream of GSH levels. Further, although depletion of NAD(P)H or GSH correlated linearly with neuron death, compared with GSH depletion, higher neurodegeneration was observed when NAD(P)H was extrapolated to zero, especially in old age, and in the 3xTg-AD neurons. We also observed an age-dependent loss of gene expression of key redox-dependent biosynthetic enzymes, NAMPT (nicotinamide phosphoribosyltransferase), and NNT (nicotinamide nucleotide transhydrogenase). Moreover, age-related correlations between brain NNT or NAMPT gene expression and NADPH levels suggest that these genes contribute to the age-related declines in NAD(P)H. Our data indicate that in aging and more so in AD-like neurons, NAD(P)H redox control is upstream of GSH and an oxidative redox shift that promotes neurodegeneration. Thus, NAD(P)H generation may be a more efficacious therapeutic target upstream of GSH and ROS.

Highlights

While the major electron transfer currencies for oxidation and reduction reactions in cells are the redox couples NADH/NAD+ and NADPH/NADP+, Accepted for publication 19 February 2014 glutathione (GSH) at 1–2 mM is the major redox buffer in neurons (Johnson et al, 2012) and other cells
If NAD+ is merely recycled without consumption, inhibition of nicotinamide phosphoribosyltransferase (NAMPT) will have no effect on NAD(P)H levels
The increase in starting levels of NAD(P)H in middle age followed by the decline in old age indicates the importance of NAD(P)H for aging, as previously described and discussed by us in Ghosh et al Fig. 1 Inhibition of NAMPT decreases NAD(P)H and glutathione levels in both non-Tg and 3xTg-Alzheimer’s disease (AD) neurons

Summary

Introduction

While the major electron transfer currencies for oxidation and reduction reactions in cells are the redox couples NADH/NAD+ and NADPH/NADP+, Accepted for publication 19 February 2014 glutathione (GSH) at 1–2 mM is the major redox buffer in neurons (Johnson et al, 2012) and other cells. NADH is the major electron donor to power the mitochondrial electron transport chain for ATP synthesis. There are several pathways for NADH generation including glycolysis, Kreb’s cycle dehydrogenases, and the NAMPTdependent salvage pathway. While two molecules of NADH are generated in glycolysis, the Kreb’s cycle dehydrogenases including pyruvate dehydrogenase, isocytrate dehydrogenase, a-ketogluterate dehydrogenase, and malate dehydrogenase synthesize NADH and feed most of the NADH required for the electron transport chain. A decline of NAMPT in aging or age-associated Alzheimer’s disease (AD) brain may decrease NADH levels that in turn could cause an oxidized redox shift, lower GSH levels and promote neurodegeneration (Brewer, 2010; Ghosh et al, 2012)

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