Normal plasmalogen levels are maintained in tissues from mice with hepatocyte-specific deletion in peroxin 5.

Image of stained human hepatocyte culture.

Werner ER, Swinkels D, Juric V, Dorninger F, Baes M, Keller MA, Berger J, and Watschinger K. (2023) Normal plasmalogen levels are maintained in tissues from mice with hepatocyte-specific deletion in peroxin 5. Brain Research Bulletin

Plasmalogens are a unique class of phospholipid that contain a vinyl-ether bond at the sn-1 position of the glycerol back bone. This double bond causes the chains to have a more compact conformation. Plasmalogens have roles in cell membrane fluidity and structure, vesicular fusion, and have antioxidant properties since the bond can scavenge two radical oxygen species. Deficiency in this class of lipids results in an ultra rare disease called Rhizomelic chondrodysplasia punctata and other diseases such as Alzheimer’s disease, Parkinson’s disease, Gaucher’s disease, and multiple sclerosis have been associated with reduced plasmalogen levels. In the body, the distribution of plasmalogens differs by tissue type with the greatest levels being in the brain. The liver is known to have low plasmalogen levels, but one theory that has been proposed to explain this is that the liver is a major producer of plasmalogens in the body and they are then transported to other organs. This was suggested because of a study that demonstrated that rat hepatocytes secreted plasmalogen-rich lipoproteins in culture and because plasmalogens are found in serum. Werner et al were interested in this theory and wanted to analyze the plasmalogen levels in a mouse line with disrupted peroxisome function in the liver due to a hepatocyte-specific knockout of peroxin 5 (Pex5). The initial steps of plasmalogen biosynthesis occur in the peroxisome, and since Pex5 encodes the receptor which binds new peroxisomal matrix proteins in the cytosol, allowing their translocation into the peroxisome, its knockdown in this cell type in the liver should cause an inability to produce plasmalogens in this tissue.

To determine if plasmalogen levels are altered throughout the body due to knocking down peroxisome function in hepatocytes the forebrain, heart, kidney, liver, lung, testis, white adipose tissue, and plasma were tested from 16 week old hepPex5-/- mice, those without PEX5 function in the hepatocytes, and Pex5fl/fl, mice with functional PEX5 and the littermate controls. They found that the liver had the lowest levels and the highest was in the forebrain. Interestingly, the hepPex5-/- mice showed no effect on the plasmalogens levels across the tissues. To ensure that PEX5 was knocked down in hepPex5-/- mice, Western blots were used to detect the protein. PEX5 protein was not present in hepPex5-/- mice and it was detected in Pex5fl/fl mice.

The authors also compared the mRNA expression of seven genes that encode enzymes in the plasmalogen biosynthetic pathway to determine if there was any difference in expression that could demonstrate a compensatory response to the knockdown in hepatocytes. These genes included Far1, Far2, Gnpat, Agps, Dhrs7b, Selenoi, and Peds1. No significant differences between the two mouse groups, but Agps tended to be higher in hepPex5-/- mice but Peds1 tended to be higher in Pex5fl/fl mice. Interestingly, Far1 was one order of magnitude lower in the liver compared to its expression in other tissues.

Werner et al were interested in testing the theory that although the liver has low plasmalogen levels compared to other tissues, this is because it is a large producer of plasmalogens for the rest of the body. They looked at a mouse line that had peroxisome activity inhibited by knocking down Pex5 and compared it against their littermate controls. If hepatocytes were the location of plasmalogen biosynthesis it would have been expected that the liver of hepPex5-/- mice would be lower than Pex5fl/fl mice. They found that between the lines there was no significant difference in plasmalogen levels in multiple tissues, including the liver. Werner et al suggest that the plasmalogens in the liver must not be from hepatocytes and are from other cell types. After confirming no PEX5 activity could be detected in the hepPex5-/- mice, they also measured mRNA levels of seven genes that encode enzymes in the plasmalogen biosynthetic pathway to determine if there was any compensatory mechanism in the liver. Again, no significant difference was found between the two mouse groups. This study does not support the theory that the liver has low plasmalogen levels because the large amount that is produced is transported to other tissues in the body, and instead that each tissue likely makes its own plasmalogens.