Basal autophagy prevents autoactivation or enhancement of inflammatory signals by targeting monomeric MyD88

Takeshi Into,Shumpei Niida,Yukitaka Murakami,Jun-Ichiro Inoue,Megumi Inomata,Toshi Horie,Jin Gohda

doi:10.1038/s41598-017-01246-w

Abstract

Autophagy, the processes of delivery of intracellular components to lysosomes, regulates induction of inflammation. Inducible macroautophagy degrades inflammasomes and dysfunctional mitochondria to downregulate inflammatory signals. Nonetheless, the effects of constitutive basal autophagy on inflammatory signals are largely unknown. Here, we report a previously unknown effect of basal autophagy. Lysosomal inhibition induced weak inflammatory signals in the absence of a cellular stimulus and in the presence of a nutrient supply, and their induction was impaired by MyD88 deficiency. During lysosomal inhibition, MyD88 was accumulated, and overabundant MyD88 autoactivated downstream signaling or enhanced TLR/IL-1R-mediated signaling. MyD88 is probably degraded via basal microautophagy because macroautophagy inhibitors, ATG5 deficiency, and an activator of chaperone-mediated autophagy did not affect MyD88. Analysis using a chimeric protein whose monomerization/dimerization can be switched revealed that monomeric MyD88 is susceptible to degradation. Immunoprecipitation of monomeric MyD88 revealed its interaction with TRAF6. In TRAF6-deficient cells, degradation of basal MyD88 was enhanced, suggesting that TRAF6 participates in protection from basal autophagy. Thus, basal autophagy lowers monomeric MyD88 expression, and thereby autoactivation of inflammatory signals is prevented. Given that impairment of lysosomes occurs in various settings, our results provide novel insights into the etiology of inflammatory signals that affect consequences of inflammation.

Highlights

Inflammation is an immunovascular response of tissues to a wide range of stimuli, including microbial pathogens and endogenously generated molecules, where innate immune recognition plays key roles[1, 2]
Expression of Tnf was induced by concanamycin A, another lysosomal vacuolar-type H+-ATPase inhibitor, and by ammonium chloride that accumulates inside lysosomes and neutralizes their intrinsic acidic pH for inhibition of lysosomal protease activities[17] (Fig. 1b)
Our results indicated that lysosomal impairment weakly activates inflammatory signals, including activation of transcription factors nuclear factor (NF)-κB and AP-1 and transcription of proinflammatory cytokines

Summary

Introduction

Inflammation is an immunovascular response of tissues to a wide range of stimuli, including microbial pathogens and endogenously generated molecules, where innate immune recognition plays key roles[1, 2]. TLRs induce a homotypic interaction with an adaptor protein termed myeloid differentiation primary response 88 (MyD88) and initiate intracellular signaling via activation of the E3 ubiquitin ligase tumor necrosis factor receptor-associated factor 6 (TRAF6)[3]. Failure of autophagy causes accumulation of damaged mitochondria and increases production of ROS, resulting in activation or enhancement of inflammatory signals accompanied by inflammasome activation[9, 11]. We report that pharmacological inhibition of lysosomes triggers activation of weak inflammatory signals in macrophages Of note, they are impaired by MyD88 deficiency and yield intracellular accumulation of MyD88. Accumulated MyD88 during lysosomal inhibition enhances TLR- or IL-1 receptor (IL-1R)-activated inflammatory signals These responses are intrinsically prevented via basal autophagic degradation of MyD88. Our findings highlight a previously unknown preventive effect of basal autophagy on inflammatory signals and imply the involvement of basal authophagy in the regulation of inflammatory processes

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Conclusion