Reduction of ether-type glycerophospholipids, plasmalogens, by NF-ĸB signal leading to microglial activation.

Hossain MS, Abe Y, Ali F, Youssef M, Honsho M, Fujiki Y, and Karafuchi T. (20217) Reduction of ether-type glycerophospholipids, plasmalogens, by NF-ĸB signal leading to microglial activation. Journal of Neuroscience

Plasmalogens are a class of phospholipid that contain a vinyl-ether bond at the sn-1 position. This double bond causes these lipids to have a compact structure, making them important in cellular membrane structure and fluidity and because of this, they are necessary for proper vesicular fusion. As well, plasmalogens have roles in myelination, lipid raft formation, and they have antioxidative properties which protect from oxidative stress. Interestingly, previous work by Hossain et al has demonstrated that plasmlaogens can suppress lipopolysaccharide (LPS)-induced accumulation of beta-amyloid (Aβ), the activation of glial cells within the hippocampus (discussed further in this blog), and prevents neuronal cell death. In addition, glycerone phosphate O-acyltransferase (GNPAT), an enzyme in the plasmalogen biosynthetic pathway, knockout mice have shown a reduction in protein kinase B (AKT) signalling in Schwann cells and this caused defective myelination. Knockdown of Gnpat locally has also been found to cause microglial activation in vitro and in vivo as well as increase the expression of proinflammatory cytokines. Although the mechanisms that lead to a reduction in plasmalogens are not well understood, there is an association between inflammatory stimuli, stress, and aging in the activation of NF-ĸB in glial and neuronal cells that ultimately leads to neuroinflammation. As neuroinflammation is also closely associated with neurodegenerative diseases such as Alzheimer’s disease, Hossain et al were interested in determining the relationship between inflammatory signals, aging, stress, and how these factors alter plasmalogen levels in glial cells by the downregulation of Gnpat.

To study the process that can reduce plasmalogens in the brain, microglial (MG6) and astrocyte (A1) cell lines were treated with a major inflammatory stimuli including LPS, Poly I:C, or IL-1β. A significant reduction in mRNA and protein expression of Gnpat was found along with significant reduction in ethanolamine plasmalogens, a specific class of plasmalogens with an ethanolamine headgroup at sn-3. Interestingly, these were not altered in primary neurons and a neuronal cell line, N2A, that were also administered these stimuli. To determine if these inflammatory stimuli activated NF-ĸB, the major subunit p65 was detected. They found that the three stimuli increased nuclear translocation within MG6 and A1 cells, however, again this was not increased in the neuronal cell line or the primary neuronal cell culture. These results suggest that inflammatory stimuli reduce Gnpat expression through activation of the NF-ĸB pathway in microglia an astrocytes but not in neuronal cells.

Hossain et al were also interested in identifying the transcription factors that regulate Gnpat expression through the NF-ĸB pathway. To do this, the Gnpat promoter was cloned and the maximum promoter activity in glial and neuronal cells as determined. This activity was found to be reduced in glial cells after exposure to the three inflammatory stimuli, but this was not seen in the neuronal cells possibly because of low expression of receptors for LPS, Poly I:C, and IL-1β. Through an analysis of the Gnpat promoter sequence it was determined that there are two possible Myc protein-binding sites in the 5’-untranslated region including c-Myc and Mycn, but only c-Myc expression was found to be increased following treatment with the three inflammatory stimuli.

To determine the role of plasmalogens in neuroinflammation, a mouse model was used by inducing neuroinflammation with LPS. They found that plasmalogen levels and Gnpat expression in the hippocampus were reduced, but expression of IL-1β mRNA and nuclear localization of p65 in microglia and astrocytes were increased, however this increase was not seen in neurons. As well, aged mice that were subjected to chronic restraint stress to induce inflammation through stress also had a reduction in Gnpat expression and plasmalogens levels in the hippocampus while IL-1β expression was increased.

Hossain et al were interested in studying the relationship between components consistent with neurodegenerative diseases such as inflammatory stimuli, aging, and stress, how they alter Gnpat expression, and how this in turn affects plasmalogen levels. They demonstrated for the first time that different inflammatory stimuli reduced plasmalogen levels in glial cells from mouse models through activating NF-ĸB, which then downregulated Gnpat through increased c-Myc recruitment onto the Gnpat promoter. Interestingly, this was not seen in neuronal cells. An LPS mouse model was used to confirm that neuroinflammation or aging and stress caused a reduction of plasmalogens and Gnpat expression and increased expression of IL-1β in the hippocampus. This work supports the theory that plasmalogens are reduced in neurodegenerative diseases such as Alzheimer’s disease. Further work will help elucidate the role of plasmalogens in cellular signaling and whether a plasmalogen treatment could repair neurodegeneration and cellular signaling.