Researchers successfully crystallize and map novel conformation of LeuT transporter

Researchers at the University of Copenhagen have mapped a novel structure known as a “neurotransmitter: sodium symporter.” The finding further empowered the scientists’ understanding of neurotransmitters in the brain and could eventually result in improved drugs for epilepsy, depression, and ADHD, apart from others.

Four stages of the LeuT transporter put together for a full cycle. Image Credit: University of Copenhagen.

The Copenhagen researchers spent five years on experimentation and have successfully crystallized and mapped a novel conformation of LeuT—a bacterial protein that falls under the same class of proteins as the brain’s so-called neurotransmitter transporters.

These transporters are unique proteins found in the cell membrane. They function similar to a vacuum cleaner; to reuptake certain neurotransmitters released by the nerve cells, while transferring signals between each other.

Certain drugs or substances perform their functions by blocking the transporters, thereby increasing the amount of some of the neurotransmitters outside the nerve cells. For instance, a narcotic substance like cocaine prevents the reuptake of the neurotransmitter dopamine, while antidepressants prevent the reuptake of serotonin, another neurotransmitter.

“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 Kamil Gotfryd, first author and Associate Professor at the Department of Biomedical Sciences who, at the time of the research, was a postdoc at the Department of Neuroscience.

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.”

Kamil Gotfryd, Study First Author and Associate Professor, Department of Biomedical Sciences, University of Copenhagen

From bacteria to human brains

Transporters are evolutionary, originating from the most primitive bacteria that have developed them to ingest nutrients such as amino acids from the environment to survive.

Various specialized transporters have evolved to carry out a range of functions—for instance, some of them transport neurotransmitters into neurons in the human brain. However, the fundamental principle remains the same—the transporter works by alternately closing and opening to the exterior and interior of a cell, respectively.

A transporter may capture amino acids or transmitter substances when it is open. Then, the protein makes use of sodium ions to modify its structure, enabling it to close outwardly and rather open to the cell’s interior where the transported substance is released and absorbed.

Full cycle

In the recent past, using X-ray crystallography, researchers have been able to map three stages of the transporter mechanism: outwardly open, inwardly open, and outwardly occluded.

Long before, researchers concluded that an inwardly occluded stage of the protein is essential for the cycle to be complete. But so far, it has been challenging to freeze it and thus to map this as its structure is unstable.

However, following several trials, scientists from the University of Copenhagen have been successful in retaining a transporter for the transmitter leucine—a LeuT—exactly in 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. 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.”

Ulrik Gether, Professor and Head, Department of Neuroscience, University of Copenhagen

A key to more discoveries

According to Ulrik Gether, the long-standing mystery could be solved partially by a mutation of the transporter and partially a replacement of leucine by the associated, but somewhat larger, phenylalanine molecule.

As it were, the combination retained the transporter for a sufficiently long time in the preferred position for scientists to purify, crystallize, and map its structure.

Meanwhile, Ulrik Gether notes that there is a high degree of similarity between various types of transporters, which enables researchers to draw parallels to the transporters of an extensive array 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 generalise and create better models for, in example, dopamine, serotonin and GABA transporters which are targets for drugs to treat ADHD, depression and epilepsy, respectively.”

Ulrik Gether, Professor and Head, Department of Neuroscience, University of Copenhagen

Gether says that the next step is to continue their research on the transporters existing in human nerve cells.