A point mutation associated with episodic ataxia 6 increases glutamate transporter anion currents

Episodic ataxia is a human genetic disease characterized by paroxysmal cerebellar incoordination. There are several genetically and clinically distinct forms of this disease, and one of them, episodic ataxia type 6, is caused by mutations in the gene encoding a glial glutamate transporter, the excitatory amino acid transporter-1. So far, reduced glutamate uptake by mutant excitatory amino acid transporter-1 has been thought to be the main pathophysiological process in episodic ataxia type 6. However, excitatory amino acid transporter-1 does not only mediate secondary-active glutamate transport, but also functions as an ion channel. Here, we examined the effects of a disease-associated point mutation, P290R, on glutamate transport, anion current as well as on the subcellular distribution of excitatory amino acid transporter-1 using heterologous expression in mammalian cells. P290R reduces the number of excitatory amino acid transporter-1 in the surface membrane and impairs excitatory amino acid transporter-1-mediated glutamate uptake. Cells expressing P290R excitatory amino acid transporter-1 exhibit larger anion currents than wild-type cells in the absence as well as in the presence of external l-glutamate, despite a lower number of mutant transporters in the surface membrane. Noise analysis revealed unaltered unitary current amplitudes, indicating that P290R modifies opening and closing, and not anion permeation through mutant excitatory amino acid transporter-1 anion channels. These findings identify gain-of-function of excitatory amino acid transporter anion conduction as a pathological process in episodic ataxia. Episodic ataxia type 6 represents the first human disease found to be associated with altered function of excitatory amino acid transporter anion channels and illustrates possible physiological and pathophysiological impacts of this functional mode of this class of glutamate transporters.