Reduction of Ether-Type Glycerophospholipids, Plasmalogens, by NF-κB Signal Leading to Microglial Activation

Abstract

Neuroinflammation characterized by activation of glial cells is observed in various neurodegenerative diseases including Alzheimer's disease (AD). Although the reduction of ether-type glycerophospholipids, plasmalogens (Pls), in the brain is reported in AD patients, the mechanism of the reduction and its impact on neuroinflammation remained elusive. In the present study, we found for the first time that various inflammatory stimuli reduced Pls levels in murine glial cells via NF-κB activation, which then downregulated a Pls-synthesizing enzyme, glycerone phosphate O-acyltransferase (Gnpat) through increased c-Myc recruitment onto the promoter. We also found that systemic injection of lipopolysaccharide, aging, and chronic restraint stress reduced brain Pls contents that were associated with glial NF-κB activation, an increase in c-Myc expression, and downregulation of in the mouse cortex and hippocampus. More interestingly, the reduction of Pls contents in the murine cortex itself could increase the activated phenotype of microglial cells and the expression of proinflammatory cytokines, suggesting further acceleration of neuroinflammation by reduction of brain Pls. A similar mechanism of reduction was also found in human cell lines, triple-transgenic AD mouse brain, and postmortem human AD brain tissues. These findings suggest a novel mechanism of neuroinflammation that may explain prolonged progression of AD and help us to explore preventive and therapeutic strategies to treat neurodegenerative diseases. Ether-type glycerophospholipids, plasmalogens (Pls), are reduced in the brain of Alzheimer disease (AD) patients. We found that inflammatory stimuli reduced Pls contents by downregulation of the Pls-synthesizing enzyme glycerone phosphate O-acyltransferase (Gnpat) through NF-κB-mediated recruitment of c-Myc onto the promoter in both murine and human cell lines. Murine brains after systemic lipopolysaccharide, chronic stress, and aging, as well as triple-transgenic AD mice and postmortem human AD brain tissues all showed increased c-Myc and reduced expression. Interestingly, knockdown of itself activated NF-κB in glial cell lines and microglia in mouse cortex. Our findings provide a new insight into the mechanism of neuroinflammation and may help to develop a novel therapeutic approach for neurodegenerative diseases such as AD.

From peroxisomal disorders to common neurodegenerative diseases – the role of ether phospholipids in the nervous system

“…However, we do not exclude that in constitutive KO models adaptive compensatory processes may occur, as we have previously argued in the case of neutropenia, which seems to be present in an inducible KO model of ether lipid deficiency but not in (constitutive) Gnpat KO mice . Acute reduction in plasmalogen levels, as evaluated after in vivo knockdown of Gnpat , was described to promote microglia activation and upregulation of inflammatory cytokines in adult mice . On the other hand, the lack of proinflammatory mediators like PAF or alkyl‐LPA may dampen the immune response and protect neurons from death signaling in the ether lipid‐deficient nervous system.…”

“…Increasing evidence indicates that several key signal transduction pathways are impacted by the lack of ether lipids. Recent reports have shown that phosphorylation of both AKT (protein kinase B) and extracellular signal‐regulated kinase (ERK; a mitogen‐activated protein kinase, MAPK) is reduced in nervous tissue of ether lipid‐deficient mice. In Schwann cells, the defect in ether lipid biosynthesis was demonstrated to impair the recruitment of AKT to the plasma membrane thereby inhibiting its proper phosphorylation, which in turn causes aberrant phosphorylation and overt activation of the downstream kinase glycogen synthase kinase (GSK) 3β and finally impaired myelin formation and maintenance .…”

“…However, a recent study of mice with Schwann cell‐selective KO of Pex5 did not reveal such abnormalities in myelination suggesting that the inherent defect in Schwann cells is only one aspect of the myelination problem and that also interaction with other compartments (e.g., axons) with additional disturbances is involved in mice with global ether lipid deficiency. Another study found decreased phosphorylation of both AKT and ERK phosphorylation after injection of lentiviral shRNAs against the Gnpat mRNA into the cerebral cortex of mice ; while exogenous supplementation with plasmalogens in cultured neuronal cells was suggested to enhance AKT and ERK phosphorylation via the induction of G protein‐coupled receptors . AKT as well as MAPK signaling play indispensable roles in nervous system development and function .…”

“…Others suggest that amyloid‐beta (Aβ) is responsible for the plasmalogen reduction, either by stimulation of plasmalogen‐selective phospholipase A2 or by destabilization of the ADHAPS protein, thus limiting plasmalogen synthesis . Alternatively, based on the neuroinflammation present in AD and results obtained in an AD mouse model, a recent report claims that the inflammatory response downregulates Gnpat expression via NF‐κB signaling and c‐Myc binding to the Gnpat promoter .…”

“…Other authors hypothesize that the elevated activity of plasmalogen‐selective phospholipase A2 stimulates the production of lysoplasmalogens, resulting in excessive vesicular fusion and eventually causing synaptic failure . Furthermore, signaling defects resulting from plasmalogen reduction (like those discovered in the ERK and AKT pathways) may cause additional damage in AD pathology, as suggested based on experiments involving RNAi to downregulate Gnpat mRNA levels in the cortex of mice . Intriguing, in this context, is the impaired phosphorylation of AKT and, therefore, loss of inhibitory phosphorylation of GSK 3β observed in the PNS of plasmalogen‐deficient mice .…”