Time-dependent analysis of plasmalogens in the hippocampus of an Alzheimer’s disease mouse model: A role of ethanolamine plasmalogen.

Neurons with accumulated beta-amyloid plaques.

Azad AK, Sheikh AM, Haque MA, Osago H, Sakai H, Shibly AZ, Yano S, Michikawa M, Hossain S, Tabassum S, Zhou GAX, Zhang Y, and Nagai A. (2021) Time-dependent analysis of plasmalogens in the hippocampus of an Alzheimer’s disease mouse model: A role of ethanolamine plasmalogen. Brain Sciences, 11, 1603

Plasmalogens are a class of phospholipid that contain a vinyl-ether bond at the sn-1 position on a glycerol backbone. This double bond allows plasmalogens to have antioxidative properties and influences membrane fluidity, structure, and is important in vesicular fusion. A reduction in plasmalogens is associated with a number of disorders including Parkinson’s disease, multiple sclerosis, and Alzheimer’s disease. Alzheimer’s disease (AD) is a neurodegenerative disease characterized by memory loss and cognitive decline. No singular cause of AD has been found but many factors are thought to increase the risk for it including the accumulation of beta-amyloid and hyperphosphorylated tau, the apolipoprotein allele ε4, poor diet, high alcohol consumption, mental illness, lack of physical activity, and reduced plasmalogen levels. It is thought that plasmalogen levels progressively increase with age until mid-forties, then begin to decline. Because of this, Azad et al were interested in studying the changes in plasmalogen levels over time in an AD mouse model and the effects that any changes may have in the animals.

In this study, Azad et al used male J20 mice which have the human amyloid precursor protein (hAPP) transgene causing them to over-express the Swedish-K670N/M671L and Indiana-V717F mutations and compared them to their wild-type littermates. These mutations cause the production of beta-amyloid and leads to progressive behavioural changes. To determine if ethanolamine plasmalogen levels change in the hippocampus across age in these animals they were analyzed at 3, 6, 9, 12, and 15 months of age. In the J20 mice, plasmalogen levels increased to 9 months, then progressively dropped off until 15 months, while the wild-type mice show the highest plasmalogen levels at 15 months. Ethanolamine plasmalogen levels in J20 mice were significantly different from the levels seen in wild-type mice at 9 months and 15 months. Choline plasmalogens did not change across any of the timepoints in the J20 or wild-type mice and were not significantly different between the two groups.

Glyceronephosphate O-acyltransferase (GNPAT) is the first enzyme in the plasmalogen biosynthetic pathway, therefore GNPAT protein expression and its location was also evaluated. Consistent with the trend seen in plasmalogen levels in J20 mice, GNPAT levels increased to 9 months then decreased at 15 months. At 6 and 12 months, GNPAT expression was similar in the J20 mice and wild-type mice. Azad et al also determined if this enzyme localized to glial cells by double immunofluorescence staining with the microglia marker, Iba1+, and the astrocyte marker, GFAP+. GNPAT was found in both microglia and astrocytes in the J20 mice. Interestingly, it was also found that GFAP+ and Iba1+ positive cells increased at 9 months while neuronal cells were decreased.

To determine the level of oxidative stress in the animals, reactive oxygen species (ROS) levels were measured. ROS were increased in J20 mice at 3, 6, 12, and 15 months compared to the wild-type mice, but no difference was seen at 9 months.

Azad et al were interested in determining whether temporal changes could be seen in the AD model, J20, and if these changes were associated with other alterations in the mice. Plasmalogen levels were found to increase until 9 months of age, but began to degrease at 12 and 15 months, matching the presumed etiology in people with AD, where they develop with normal levels of plasmalogens and these continue to increase until adulthood, then a quick decrease begins, possibly leading to AD pathology. Interestingly, this decrease matches perfectly with the trend in GNPAT expression, where the expression of this protein increases to 9 months, then begins to decrease at 12 and 15 months. As this enzyme is the first step in plasmalogen biosynthesis, it reasons that its levels should be closely related to plasmalogen levels. The mutation that causes these animals to be an AD model is the over-expression of hAPP leading to an accumulation of beta-amyloid. Another link to the high GNPAT and plasmalogen levels could be the ability to protect against ROS since the only timepoint where ROS was not higher than wild-type mice was at 9 months, also the timepoint with the greatest GNPAT and plasmalogen levels. The authors suggest that more information regarding the interaction between beta-amyloid, GNPAT expression, and plasmalogen levels may elucidate the role of changes in plasmalogens early in AD onset.