Necrotic neurons enhance microglial neurotoxicity through induction of glutaminase by a MyD88-dependent pathway.
BACKGROUND
Microglia are macrophage-like cells that constantly sense the microenvironment within the central nervous system (CNS). In the event of neuronal stress or injury, microglial cells rapidly react and change their phenotype. This response may lead to a deleterious type of microglial activation, which is often associated with neuroinflammation and neurotoxicity in several neuropathological conditions. We investigated the molecular mechanisms underlying triggering of microglial activation by necrotic neuronal damage.


METHODS
Primary cultures of microglia were used to study the effect of necrotic neurons on microglial inflammatory responses and toxicity towards cerebellar granule neurons (CGN). The mouse hippocampal cell line, HT22, was used in this study as the main source of necrotic neurons to stimulate microglia. To identify the signal transduction pathways activated in microglia, primary microglial cultures were obtained from mice deficient in Toll-like receptor (TLR) -2, -4, or in the TLR adapter protein MyD88.


RESULTS
Necrotic neurons, but not other necrotic cell types, induced microglial activation which was characterized by up-regulation of: i) MHC class II; ii) co-stimulatory molecules, i.e. CD40 and CD24; iii) beta2 integrin CD11b; iii) pro-inflammatory cytokines, i.e. interleukin 6 (IL-6), IL-12p40 and tumor-necrosis factor (TNF); iv) pro-inflammatory enzymes such as nitric oxide synthase (iNOS, type II NOS), indoleamine 2,3-dioxygenase (IDO) and cyclooxygenase-2 (COX-2) and increased microglial motility. Moreover, microglia-conditioned medium (MCM) obtained from cultures of activated microglia showed increased neurotoxicity mediated through the N-methyl-D-aspartate receptor (NMDAR). The activation of microglia by necrotic neurons was shown to be dependent on the TLR-associated adapter molecule myeloid differentiation primary response gene (MyD88). Furthermore, MyD88 mediated enhanced neurotoxicity by activated microglia through up-regulation of the expression and activity of glutaminase, an enzyme that produces glutamate, which is an NMDAR agonist.


CONCLUSIONS
These results show that necrotic neurons activate in microglia a MyD88-dependent pathway responsible for a pro-inflammatory response that also leads to increased neurotoxic activity through induction of glutaminase. This finding contributes to better understanding the mechanisms causing increased neuroinflammation and microglial neurotoxicity in a neurodegenerative environment.
Instituto Gulbenkian de Ciência, Rua da Quinta Grande, 6, 2780-156 Oeiras, Portugal. tfariapais@gmail.com
Teresa F
TF
Pais
Catarina
C
Figueiredo
Rui
R
Peixoto
Maria H
MH
Braz
Sukalyan
S
Chatterjee
D000818:Animals; D002478:Cells, Cultured; D017077:Culture Media, Conditioned / Q000494:pharmacology; D016207:Cytokines / Q000276:immunology / Q000378:metabolism; D004660:Encephalitis / Q000209:etiology* / Q000378:metabolism / Q000503:physiopathology; D005911:Gliosis / Q000209:etiology / Q000378:metabolism / Q000503:physiopathology; D018698:Glutamic Acid / Q000096:biosynthesis; D005972:Glutaminase / Q000378:metabolism*; D018836:Inflammation Mediators / Q000276:immunology / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D017628:Microglia / Q000187:drug effects / Q000276:immunology / Q000378:metabolism*; D053594:Myeloid Differentiation Factor 88 / Q000235:genetics / Q000378:metabolism*; D009336:Necrosis / Q000150:complications* / Q000378:metabolism / Q000503:physiopathology; D009410:Nerve Degeneration / Q000150:complications* / Q000378:metabolism / Q000503:physiopathology; D016194:Receptors, N-Methyl-D-Aspartate / Q000819:agonists / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D051193:Toll-Like Receptors / Q000276:immunology / Q000378:metabolism; D015854:Up-Regulation / Q000187:drug effects / Q000502:physiology
D017077:Culture Media, Conditioned; D016207:Cytokines; D018836:Inflammation Mediators; C507780:Myd88 protein, mouse; D053594:Myeloid Differentiation Factor 88; D016194:Receptors, N-Methyl-D-Aspartate; D051193:Toll-Like Receptors; D018698:Glutamic Acid; D005972:Glutaminase