NLRX1 Regulation Following Acute Mitochondrial Injury.

Xiaogang Chu,Raghavan Raju,Songwei Wu

doi:10.3389/fimmu.2019.02431

Abstract

Several metabolic, cardiovascular, and neurological disorders are characterized by mitochondrial dysfunction followed by dysregulation of cellular energetics. Mitochondria play an important role in ATP production and cell death regulation. NLRX1, a mitochondria-targeted protein, is known to negatively regulate innate immunity, and cell death responses. However, the role of this protein in cellular homeostasis following mitochondrial injury is not well-understood. To understand the mechanisms underlying the effect of acute injury in regulating NLRX1 signaling pathways, we used an in vitro model of mitochondrial injury wherein, rat pulmonary microvascular endothelial cells were subjected to sodium azide treatment or glucose starvation. Both sodium azide and glucose starvation activated NF-κB and TBK1 associated innate immune response. Moreover, increased TBK1, IKK, IκB, and TRAF6 were recruited to mitochondria and interacted with NLRX1. Depletion of endogenous NLRX1 resulted in exacerbated NF-κB and TBK1 associated innate immune response and apoptosis. Our results suggest that NLRX1 participates in the regulation of innate immune response in mitochondria, and plays an important role in the maintenance of cellular homeostasis following acute mitochondrial injury. We propose that the mitochondrial recruitment of inflammatory mediators and their interaction with NLRX1 are protective responses to maintain cellular homeostasis following injury.

Highlights

Mitochondria are the main source of ATP in the cell
oxygen consumption rate (OCR) was decreased at all concentrations of NaN3 tested with the PMVECs, an increase in extracellular acidification rate (ECAR) was observed only at 5 and 10 mM concentrations of NaN3, Abbreviations: nucleotide-binding domain and leucine-rich repeat–containing protein X1 (NLRX1), Nucleotide-binding domain and leucine-rich repeat– containing protein X1; damage”-associated molecular patterns (DAMPs), Damage associated molecular patterns; pathogen-associated molecular patterns (PAMPs), Pathogen associated molecular patterns; PRR, Pattern recognition receptor; CLR, C-type lectin receptors; PMVEC, pulmonary microvascular endothelial cell
Apoptosis following azide treatment is well-recognized [23] and our experiments demonstrate that necroptosis contributes to azide-induced cell death (Supplementary Figure S1).The mitochondrial injury by glucose starvation led to similar results (Figures 2C,D)

Summary

INTRODUCTION

Recent studies suggest that mitochondria is a master regulator of inflammation [1] and play a key role in injury response [2,3,4]. Mitochondria participate in a broad range of innate immune pathways, functioning as signaling platforms in cell death and contributing to effector responses [5, 6]. They decode incoming danger signals and translate them into appropriate adaptive responses [1]. Recent studies suggest that NLRX1 functions as a negative regulator of NF-κB and IRF3 signaling pathway in infection and inflammation [13,14,15]. In this manuscript we sought to determine the role of NLRX1 following acute mitochondrial injury and unravel the molecular pathways relating to NLRX1 in the maintenance of cellular homeostasis following injury

RESULTS

DISCUSSION

EXPERIMENTAL PROCEDURES

DATA AVAILABILITY STATEMENT