Basic science shows how a single mutation causes ataxia

Worldwide, only a small number of patients are known to have symptomatic ataxia type 6, a neurological disease that causes a transient loss of muscle control. The cause lies in a mutation that changes a single amino acid in a protein that transports the neurotransmitter glutamate across the membrane of neurons. Researchers from the University of Groningen (Netherlands) explain how the mutation causes these cells to malfunction. Their results appear in Nature Communications.

Patients with ataxia lose control of their muscles, which can affect for example how they move or speak. One extremely rare form of this disease is episodic ataxia type 6 (EA6), in which patients have ataxia episodes. There are more than a dozen known patients worldwide, including one family in the Netherlands. EA6 is known to be caused by a single mutation, but how this mutation could have such a dramatic effect has been a mystery until now.

“This protein transports glutamate across the membrane of neurons,” explains structural biologist Albert Guskoff. The protein is inserted into the cell membrane, and the mutation changes the amino acid proline in one of the transmembrane domains of the helix to arginine.

“Usually, proline in the helix causes twisting,” Guskov explains. “If proline is changed to arginine, we would expect that twist to disappear. To test this, we studied the structure of the mutated protein.”

Because the human transport protein is difficult to study in the lab, Guskov and his colleagues used a similar protein from archaea, an ancient form of single-celled organisms. “This primitive protein has been well conserved throughout evolution, and we know from previous work that it is a good model for a human transport protein, even though it transports aspartate but not glutamate,” explains Guskoff.

Using cryo-electron microscopy on normal and mutant proteins placed in lipid nanodiscs, the team was able to compare the shape of the mutant protein to the normal one. In previous studies, the group Show that a portion of the protein moves up and down across the membraneLike an elevator. The hypothesis was that the mutation would lead to the disappearance of membrane buckling in the protein, and that this would change the shape of the protein and hinder the movement of the elevator.

However, this was not the case. Gustov says, “To our surprise, the nets were still there.” However, the mutation affected the functioning of the protein. “The transport rate is reduced by two times compared to normal protein.” Moreover, during aspartate transport, the protein transiently formed an anion channel. “And in the mutated protein, the ionic transport was three times higher.”

Somehow, the arginine that replaced proline didn’t change the shape of the transport protein, but it did affect its function. Therefore, the researchers ran molecular dynamics simulations, which show all the interactions of a protein’s amino acids with their surroundings. “What we’ve noticed is that a salt bridge forms between the amino acid arginine and the membrane lipid.” This salt bridge, a form of intermolecular attraction, appears to slow down the movement of the sublimated portion of the protein.

Gustov says, “If this anode is moving more slowly, it explains the decrease in aspartate transport, but it also means that the temporary ion channel stays open longer, thus enabling more anions to pass through.” In human neurons, this will result in decreased transmission of the neurotransmitter glutamate, and increased anion imbalance. These findings explain how this mutation causes ataxia. “Both have very serious consequences for the functioning of neurons.”

However, there is no simple way to treat the effect of mutation. Furthermore, this transporter is present throughout the body, Gustoff says, so any drug that affects it is likely to have serious side effects. Also, since there are so few patients, no pharmaceutical company will invest in a cure. “Although there may be more patients. Because it is an episodic disease and symptoms can be mild, many people may not be aware of it. They are simply used to feeling unwell for a few days at a time, just like anyone who He suffers from migraine.”

For the scientific community, these findings raise a number of interesting questions. Gustov says, “The protein has been well preserved throughout evolutionary history. So why did this transient anion channel appear, and did it turn out to be so useful to archaea that it was carried over time into our neurons? We’d like to understand.”