Intrathecal heat shock protein 60 mediates neurodegeneration and demyelination in the CNS through a TLR4- and MyD88-dependent pathway.

Karen Rosenberger,Helmut Kettenmann,Eckart Schott,Paul Dembny,Seija Lehnardt,Christina Krüger,Susanne A Wolf,Andreas Meisel,Odilo Engel,Katja Derkow

doi:10.1186/s13024-015-0003-1

Abstract

BackgroundToll-like receptors (TLR) constitute a highly conserved class of receptors through which the innate immune system responds to both pathogen- and host-derived factors. Although TLRs are involved in a wide range of central nervous system (CNS) disorders including neurodegenerative diseases, the molecular events leading from CNS injury to activation of these innate immune receptors remain elusive. The stress protein heat shock protein 60 (HSP60) released from injured cells is considered an endogenous danger signal of the immune system. In this context, the main objective of the present study was to investigate the impact of extracellular HSP60 on the brain in vivo.ResultsWe show here that HSP60 injected intrathecally causes neuronal and oligodendrocyte injury in the CNS in vivo through TLR4-dependent signaling. Intrathecal HSP60 results in neuronal cell death, axonal injury, loss of oligodendrocytes, and demyelination in the cerebral cortex of wild-type mice. In contrast both mice lacking TLR4 and the TLR adaptor molecule MyD88 are protected against deleterious effects induced by HSP60. In contrast to the exogenous TLR4 ligand, lipopolysaccharide, intrathecal HSP60 does not induce such a considerable inflammatory response in the brain. In the CNS, endogenous HSP60 is predominantly expressed in neurons and released during brain injury, since the cerebrospinal fluid (CSF) from animals of a mouse stroke model contains elevated levels of this stress protein compared to the CSF of sham-operated mice.ConclusionsOur data show a direct toxic effect of HSP60 towards neurons and oligodendrocytes in the CNS. The fact that these harmful effects involve TLR4 and MyD88 confirms a molecular pathway mediated by the release of endogenous TLR ligands from injured CNS cells common to many forms of brain diseases that bi-directionally links CNS injury and activation of the innate immune system to neurodegeneration and demyelination in vivo.Electronic supplementary materialThe online version of this article (doi:10.1186/s13024-015-0003-1) contains supplementary material, which is available to authorized users.

Highlights

Toll-like receptors (TLR) constitute a highly conserved class of receptors through which the innate immune system responds to both pathogen- and host-derived factors
TLR4 expressed in microglia is required for neuronal injury induced by heat shock protein 60 (HSP60) We observed previously that the quantity of viable neurons recovered from forebrains of mutant lpsd mice, in which the TLR4 signaling cascade is disrupted as the result of a naturally occurring mutation, is increased
Increased numbers of TUNEL-positive cells (Figure 1E) and DAPI-stained nuclei displaying apoptotic hallmarks such as shrinkage and fragmentation (Figure 1G) in co-cultures containing with C57BL/6 J (WT) microglia but not in co-cultures supplemented with TLR4−/− microglia treated with HSP60 confirmed toxic effects induced by HSP60 through TLR4 in vitro (Figure 1F, H)

Summary

Introduction

Toll-like receptors (TLR) constitute a highly conserved class of receptors through which the innate immune system responds to both pathogen- and host-derived factors. TLR4 is activated by both lipopolysaccharide (LPS) derived from gram-negative bacteria and host-derived molecules such as heat shock proteins (HSPs) including HSP60 and HSP70, extracellular matrix proteins, such as fibronectin extra domain A and soluble heparin sulphate, and other proteins including β-defensin and high mobility group box 1 protein [2,3,4,5,6,7,8]. Upon binding to their respective ligands, TLRs initiate signaling through their intracellular Toll/interleukin-1 (IL-1) receptor (TIR) domains. TLRs are widely expressed in innate immune cells such as macrophages and microglia in the CNS and dendritic cells, and in non-immune cells such as neurons and epithelial cells [10,11]

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