Unravelling the Mystery of Episodic Ataxia Type 6

Sunday - May 14 2023, 12:59 UTC - 1 year ago

Episodic ataxia type 6 (EA6) is a rare neurological disorder caused by a mutation that alters a single amino acid in a protein responsible for transporting the neurotransmitter glutamate across neural cell membranes. University of Groningen scientists have uncovered a mechanism by which this mutation causes malfunction in these cells. The mutation changes a proline amino acid in one of the helical transmembrane domains into an arginine with effects such as a reduced transport rate and formation of anion channels.

Episodic ataxia type 6 is a rare neurological disorder that affects only a small number of individuals globally. It leads to temporary loss of muscle coordination and is caused by a mutation that alters a single amino acid in a protein responsible for transporting the neurotransmitter glutamate across neural cell membranes. Scientists from the University of Groningen in the Netherlands have uncovered the mechanism by which this mutation causes malfunction in these cells. Their findings were recently published in the journal Nature Communications.

Individuals suffering from ataxia experience a loss of muscle control, which can result in difficulties with movements and speech. Among the various forms of ataxia, episodic ataxia type 6 (EA6) is a particularly rare condition, characterized by episodes of muscle control loss. Currently, only a small number of individuals, including one family in the Netherlands, have been identified as having EA6 worldwide, with the total number of known patients numbering just over a dozen.

It is known that EA6 is caused by a single mutation, but how this mutation can have such a dramatic effect was thus far a mystery. ‘This protein transports glutamate across the membrane of neural cells,’ explains structural biologist Albert Guskov. The protein is inserted in the cell membrane, and the mutation changes a proline amino acid in one of the helical transmembrane domains into an arginine.

"A proline in a helix typically causes a kink," explains Guskov. "If a proline is changed into an arginine, we would expect this kink to disappear. To test this, we studied the structure of the mutated protein." .

Since the human transport protein is difficult to study in the lab, Guskov and his colleagues used an analogous protein from archaea, an ancient form of unicellular organism.

"This archaeal protein has been well conserved throughout evolution, and we know from previous work that it is a good model for the human transport protein, even though it transports aspartate and not glutamate," explains Guskov.

Using cryo-electron microscopy on normal and mutated proteins placed in lipid nanodiscs, the team was able to compare the shape of the mutated protein to the normal version. In previous studies, the team had shown that part of the protein moves up and down through the membrane, much like an elevator. The hypothesis was that the mutation would cause the transmembrane kink in the protein to disappear, and that this would change the protein’s shape and block the elevator movement.

However, that was not the case. Guskov: "To our surprise, the kink was still there." .

Nevertheless, the mutation did affect the functioning of the protein. "The transport rate was reduced by a factor of two, compared to the normal protein." Furthermore, during the transport of the aspartate, the protein transiently formed an anion channel. "And in the mutated protein, ion transport was three times higher." .

Somehow, the arginine that replaced the proline did not alter the shape of the transport protein, but it did affect its function. Therefore, the researchers performed molecular dynamics simulations, which show all the interactions of the amino acids of the protein with their surroundings. "What we noticed is that a salt bridge is formed between the arginine amino acid an a neighbouring glutamate. This results in an additional stabilization of the protein, and a deliberate slowing down of the elevator," says Guskov.