ANDERS BUCH-LARSEN-COPENHAGEN

A new map of a neurotransmitter may lead to better drugs for ADHD, depression, epilepsy, and more, researchers report.

The discovery also adds to the researchers’ knowledge of neurotransmitters in the brain.

The map is of a new conformation of LeuT, a bacterial protein that belongs to the same family of proteins as the brain’s neurotransmitter transporters.

These transporters are special proteins that sit in the cell membrane. As a kind of vacuum cleaner, they reuptake some of the neurotransmitters that nerve cells release when sending a signal to one another.

The coiled LeuT transporter shifts into different positions against a white backgroundAnimation of the four stages of the LeuT transporter put together for a full cycle. (Credit: U. Copenhagen)
Some drugs or substances work by blocking the transporters, increasing the amount of certain neurotransmitters outside the nerve cells. For example, antidepressants inhibit the reuptake of the neurotransmitter serotonin, while a narcotic such as cocaine inhibits the reuptake of the neurotransmitter dopamine.

“Transporters are extremely important for regulating the signaling between neurons in the brain and thus the balance of how the whole system works. You cannot do without them,” says first author Kamil Gotfryd, associate professor in the biomedical sciences department at the University of Copenhagen and a postdoc in the neuroscience department during the study.

“Not only does the new discovery give us additional basic scientific knowledge about the complex transporter proteins. It also has perspectives in relation to developing pharmacological methods, with which we can change the function of transporters. In other words, the discovery may lead to better drugs,” he adds.

Evolutionary, transporters derive from the most primitive bacteria, which have developed them to absorb nutrients, such as amino acids, from the environment in order to survive.

Since then, specialized transporters have developed to perform a variety of functions. For example, to transport neurotransmitters into neurons in the human brain. Still, the basic principle is the same, namely that the transporter functions by alternately opening and closing to the interior and exterior of a cell, respectively.

When a transporter is open outwardly, it may capture transmitter substances or amino acids. Thereafter, the protein uses sodium ions to change its structure so that it will close outwardly and instead open to the interior of the cell where the transported substance is released and absorbed.

In recent years, X-ray crystallography has enabled researchers to map three stages of the transporter mechanism: Outwardly open, outwardly occluded, and inwardly open.

In order for the cycle to be complete, researchers have long concluded that there must also be an inwardly occluded stage of the protein. However, since this structure is unstable, it has long been difficult to freeze it and thus be able to map it.

But now, after many trials, the researchers have succeeded in retaining a transporter for the transmitter leucine—a LeuT—in precisely that stage.

“We have been working on this for five years, and no matter what we did, we never got the structure we wanted. But suddenly it happened,” says Ulrik Gether, a professor in and head of the neuroscience department.

“Our study is in fact—I would say—’the missing link.’ This structure has been missing and it has been important to understand the entire cycle which the transporter is going through,” he adds.

Gether explains that the key to solving the long-standing mystery was partly a mutation of the transporter and partly a replacement of the substance leucine by the related, but slightly larger phenylalanine molecule.

The combination held the transporter in the desired position long enough for researchers to purify, crystallize, and map its structure.

At the same time, Gether explains that the high degree of similarity between different types of transporters allows researchers to draw parallels to the transporters of a wide range of other neurotransmitters.

“Now that we know more about LeuT, the result may be transferred to other transporters of other neurotransmitters. We believe that we can generalize and create better models for, in example, dopamine, serotonin and GABA transporters which are targets for drugs to treat ADHD, depression, and epilepsy, respectively,” says Gether.

The next step is to continue working with the transporters in human nerve cells, according to Gether.

The research appears in Nature Communications. Additional researchers from Aarhus University in Denmark, Columbia University, and Cornell University contributed to the work.

Support for the study came from, among others, the Independent Research Fund Denmark, the Lundbeck Foundation, the Carlsberg Foundation, and the EU.

Source: University of Copenhagen

Original Study DOI: 10.1038/s41467-020-14735-w