Nicotinamide mononucleotide adenylyltransferase promotes hypoxic survival by activating the mitochondrial unfolded protein response.

X R Mao,C M Crowder,D M Kaufman

doi:10.1038/cddis.2016.5

Abstract

Gain-of-function mutations in the mouse nicotinamide mononucleotide adenylyltransferase type 1 (Nmnat1) produce two remarkable phenotypes: protection against traumatic axonal degeneration and reduced hypoxic brain injury. Despite intensive efforts, the mechanism of Nmnat1 cytoprotection remains elusive. To develop a new model to define this mechanism, we heterologously expressed a mouse Nmnat1 non-nuclear-localized gain-of-function mutant gene (m-nonN-Nmnat1) in the nematode Caenorhabditis elegans and show that it provides protection from both hypoxia-induced animal death and taxol-induced axonal pathology. Additionally, we find that m-nonN-Nmnat1 significantly lengthens C. elegans lifespan. Using the hypoxia-protective phenotype in C. elegans, we performed a candidate screen for genetic suppressors of m-nonN-Nmnat1 cytoprotection. Loss of function in two genes, haf-1 and dve-1, encoding mitochondrial unfolded protein response (mitoUPR) factors were identified as suppressors. M-nonN-Nmnat1 induced a transcriptional reporter of the mitoUPR gene hsp-6 and provided protection from the mitochondrial proteostasis toxin ethidium bromide. M-nonN-Nmnat1 was also protective against axonal degeneration in C. elegans induced by the chemotherapy drug taxol. Taxol markedly reduced basal expression of a mitoUPR reporter; the expression was restored by m-nonN-Nmnat1. Taken together, these data implicate the mitoUPR as a mechanism whereby Nmnat1 protects from hypoxic and axonal injury.

Highlights

Caenorhabditis elegans has become an increasingly important model for the study of both hypoxic and axonal injury.[10,11,12,13,14,15,16] In this regard, the strengths of the model lie in its genetic tractability, fully defined anatomy and cellular identity, and the ability to directly observe cell pathology in live animals
Having demonstrated definitively that m-nonNNmnat[1] protected mouse neurons from hypoxia, we wanted to know if this phenotype extended across phyla and is likely a general property of nicotinamide mononucleotide adenylyltransferase (Nmnat)[1]
Given that Nmnat[1] is a biosynthetic enzyme for nicotinamide adenine dinucleotide (NAD), a general mechanism of Nmnat[1] hypoxia protection that we considered was that the m-nonN-Nmnat[1] expression reduces oxygen consumption and thereby reduces the duration of cellular hypoxia and severity of injury

Summary

Introduction

Caenorhabditis elegans has become an increasingly important model for the study of both hypoxic and axonal injury.[10,11,12,13,14,15,16] In this regard, the strengths of the model lie in its genetic tractability, fully defined anatomy and cellular identity, and the ability to directly observe cell pathology in live animals. Screens in C. elegans for genes that control hypoxic sensitivity have implicated multiple distinct pathways as determinants of hypoxic death.[16,17,18,19] In particular, genetic perturbations that improve cellular proteostasis are generally hypoxia protective.[16,17,18,19,20,21] These findings argue that hypoxia perturbs protein folding and that this perturbation contributes to cell death; substantial evidence in C. elegans and in other models indicates that hypoxia/ischemia disrupts protein folding homeostasis.[16,17,19,22,23]. Is hypoxia protection a general feature of Nmnat[1] expression or is it peculiar to the mouse transgenic model previously tested?

Methods

Results

Conclusion