*For medical professionals only
Choline is an important essential nutrient for the human body.
The synthesis of cell membranes and the transmission of nerve signals require large amounts of choline.
The brain has a very high demand for choline and cannot be synthesized effectively de novo in the body, so most of the choline needed by the brain is absorbed from the foods we eat every day. The question of how choline, an important component of lecithin, is absorbed by the brain has puzzled scientists for more than half a century.
Recently, a research team led by Filippo Mancia and Rosemary J. Cater of Columbia University and Thomas Arnold of the University of California, San Francisco, published a blockbuster research paper in the top journal Nature [1], which solved this academic problem that has puzzled scientists for more than 50 years.
For the first time, researchers have discovered that FLVCR2, the main cotransporter, is the main choline transporter in the blood-brain barrier, and most of the choline absorbed by the brain is done through FLVCR2. They also explored the process of choline transport by FLVCR2 with the help of cryo-electron microscopy.
The significance of the discovery of this transporter is not only to discover the mechanism of choline absorption by the brain, but also to develop more drugs that can enter the brain with the help of this absorption process in the future.
▲ Screenshot of the first page of the paper
For nearly 20 years, scientists have been searching for potential transporters for choline, but to date, only two choline transporters have been identified, one is ChT (also known as SLC5A7), which is expressed almost exclusively in cholinergic neurons [2]; The other is FLVCR1, which has been identified in the last two years, and is expressed in most cell types but not in brain endothelial cells [3-5].
As the name suggests, FLVCR1 and FLVCR2 may be "one family", but they are actually close relatives, with only 55% homology. Unlike FLVCR1, FLVCR2 is predominantly expressed in endothelial cells of the blood-brain barrier. Although some studies have found that FLVCR2 is essential for cerebral angiogenesis and normal brain development, the true physiological function of FLVCR2 is still unknown.
After the choline transporter identity of FLVCR1 was revealed in 2023 [3,4], the function of the close relative FLVCR2 was revealed. Mancia's team soon confirmed that FLVCR2 was indeed only present in endothelial cells in all cerebral vascular segments (arteries, capillaries, and veins) based on a mouse model.
Moreover, FLVCR2 was expressed on both sides of the plasma membrane of brain endothelial cells, and the expression level of the cell membrane on the lumen side was relatively high. They found a similar distribution in adult brains. In terms of the site of expression, FLVCR2 is indeed in an ideal position to absorb choline from the bloodstream into the brain.
▲ Spatial distribution of FLVCR2
The next step is to explore whether FLVCR2 can transport choline.
Mancia's team developed a mouse model in which the FLVCR2-encoding gene Flvcr2 was conditionally knocked out (Flvcr2-cKO) and then observed the absorption of radiolabeled choline in vivo. The results showed that the absorption of choline in the brains of mice with gene deletion was significantly reduced when the cerebral vascular density, morphology, and barrier function were intact.
In vitro cell line-based studies have also confirmed these findings, and all the evidence suggests that FLVCR2 is indeed a choline transporter in the blood-brain barrier and is responsible for choline uptake by the brain. In addition, FLVCR2 binds choline more easily under alkaline conditions (above pH 7.5) than under acidic conditions (below pH 7.5).
▲ Schematic diagram of FLVCR2 in the blood-brain barrier
At the end of the study, Mancia's team used cryo-EM to study the interaction sites between FLVCR2 when it transports choline.
In simple terms, FLVCR2, a member of the major cotransporter superfamily, transports choline in a similar way to other transporter transport substrates. When choline is about to cross the blood-brain barrier, FLVCR2 opens the lumen side of the endothelial cell and closes the intracellular side; Choline then enters the lumen and binds to specific sites, causing a conformational change in FLVCR2 – the intracellular side opens and the extracellular side closes, completing choline trafficking.
Another study published in Nature three weeks after the publication of Mancia's team's paper, led by Schara Safarian, Di Wu, and Gerhard Hummer from the Max Planck Institute for Biophysics in Germany, and Long N. Nguyen from the National University of Singapore, revealed in more detail the process of choline transport by FLVCR2 [6].
▲ The process of choline transport by FLVCR2[6]
Overall, the Mancia team's research identified for the first time a key transporter of choline in the brain, solving a puzzle that has puzzled the academic community for more than half a century. This discovery also provides a new idea for the development of brain-targeted drugs.
In addition, this study reminds us to eat more choline-rich foods (such as eggs, vegetables, meat, nuts and legumes, etc.), after all, the brain needs choline very much, and a special absorption system is prepared for this.
Bibliography:
[1]. Cater RJ, Mukherjee D, Gil-Iturbe E, et al. Structural and molecular basis of choline uptake into the brain by FLVCR2. Nature. 2024; 629(8012):704-709. doi:10.1038/s41586-024-07326-y
[2]. Okuda T, Haga T. High-affinity choline transporter. Neurochem Res. 2003; 28(3-4):483-488. doi:10.1023/a:1022809003997
[3]. Kenny TC, Khan A, Son Y, et al. Integrative genetic analysis identifies FLVCR1 as a plasma-membrane choline transporter in mammals. Cell Metab. 2023; 35(6):1057-1071.e12. doi:10.1016/j.cmet.2023.04.003
[4]. Tsuchiya M, Tachibana N, Nagao K, Tamura T, Hamachi I. Organelle-selective click labeling coupled with flow cytometry allows pooled CRISPR screening of genes involved in phosphatidylcholine metabolism. Cell Metab. 2023; 35(6):1072-1083.e9. doi:10.1016/j.cmet.2023.02.014
[5]. Son Y, Kenny TC, Khan A, Birsoy K, Hite RK. Structural basis of lipid head group entry to the Kennedy pathway by FLVCR1. Nature. 2024; 629(8012):710-716. doi:10.1038/s41586-024-07374-4
[6]. Ri K, Weng TH, Claveras Cabezudo A, et al. Molecular mechanism of choline and ethanolamine transport in humans. Nature. 2024; 630(8016):501-508. doi:10.1038/s41586-024-07444-7
本文作者丨BioTalker