Ether lipids transfer across the blood-brain and placental barriers does not improve by inactivation of the most abundant ABC transporters.

Dorninger F, Vaz FM, Waterham HR, van Klinken JB, Zeitler G, Forss-Petter S, Berger J, and Wiesinger C. (2022) Ether lipids transfer across the blood-brain and placental barriers does not improve by inactivation of the most abundant ABC transporters. Brain Research Bulletin

The blood-brain barrier (BBB) is a highly restrictive border composed of endothelial cells lining cerebral vasculature, pericytes, and end-feet of astrocytes. The purpose of the BBB is to be a protective layer and regulate what can enter the brain, however this causes challenges when developing therapeutics with central nervous system targets. Particularly, lipids are very important for brain function, but are also highly restricted by the BBB. One class of lipid that is very important for the brain are plasmalogens, a class of phospholipids that contain a vinyl-ether bond at the sn-1 position. However, when genetic defects in lipid metabolism prevent the production of lipids and they must be provided by the periphery, the BBB hinders this transport. As their previous work had shown that treatment with batyl alcohol, a precursor of plasmalogens, cannot alter brain plasmalogens, Dorninger et al wanted to determine how to increase cerebral plasmalogen uptake across the BBB or placental barrier.

There are a number of transporters at the BBB that could allow plasmalogens or plasmalogen precursors to cross the BBB. However, ether lipids are substrates at ATP-binding cassette (ABC) transporters at both the BBB and another barrier in animals, the placental barrier, and because of this, Dorninger et al theorized that ABC transporters may be the main regulatory step that prevents the entry of ether lipids and other essential lipids. A mouse model with an inability to produce plasmalogens due to a knockout of glyceronephosphate-O-acyltransferase (Gnpat-/-) and a deficiency in the main ABC transporters, Mdr1a-/- /Mdr1b-/- /Bcrp-/- to make a triple knockout (KO) animal to determine if batyl alcohol can augment plasmalogen levels under these conditions. Some mice were also treated with MK-571 which is a multidrug resistance-associated protein (MRP) inhibitor and can inactivate ABC transporters at the BBB. Mice were supplemented with batyl alcohol to determine if lacking the ABC transporters affected its ability to rescue plasmalogen levels, and they found that levels did increase in the heart, but brain levels were still undetectable in the brain, and no functional improvement was seen in the open field test. To ensure the absence of any other ABC transporters outside of MDR1 and BCRP, MK-571 was also administered to the animals along with the batyl alcohol to inhibit any other ABC transporters. Again, no increase in plasmalogens was seen in the brain of these animals with the combined treatment, concluding that the treatment had no effect in the brain.

As the placental barrier is another blockade for the treatment of plasmalogen deficiency, it was thought that the placental barrier likely also prevents the transport of ether lipids. To test whether removing the major ABC transporters at the placental barrier would allow a batyl alcohol treatment to improve plasmalogen levels in the offspring, they used pregnant Gnpat+/- females impregnated by Gnpat+/- males and pregnant Mdr1-/- /Bcrp-/- females mated by Mdr1-/- /Bcrp-/- males. As controls the same design was used with animals with normal ABC transporter expression. Plasmalogen levels were measured in the wild-type and Gnpat KO embryos and newborns and in the control animals with function ABC transporters, the batyl alcohol treatment increased plasmalogen levels in the heart, liver, and kidneys in the Gnpat KO offspring, with a small effect also seen in the brain. However, the ABC transporter-deficient mice did not produce offspring with any increased levels.

Dorninger et al were interested in determining whether plasmalogen uptake could be increased across the BBB or placental barrier by inhibiting the transporters that counteract the uptake of ether lipids. Mouse models were developed that could not produce plasmalogens and also lacked the main ABC transporters to demonstrate whether batyl alcohol supplementation could augment plasmalogen levels in the brain if the transporters did not counteract the uptake of the precursor. The BBB provides many challenges in drug development since it is successful at regulating what can and cannot enter the brain, thus targeting something that could increase uptake in addition to the target drug could help distribution into the brain. However, their results did not show increased plasmalogen levels in the animals with ABC transporter deficiency, leading the authors to conclude that the main ABC transporters must not be the reason plasmalogen levels cannot be rescued with treatment of a plasmalogen precursor. Further work looking into the regulation of ether lipids being transported into the brain could provide a future target for drugs to enable passage into the brain for diseases such as Alzheimer’s disease, Rhizomelia chondrodysplasia punctata, Parkinson’s disease, and multiple sclerosis.