What question did this study set out to answer?

The study aims to understand how the P312R mutation in EAAT4 affects glutamate uptake and anion channel function.

January 25, 2026Open Access

Apo state pore opening as functional basis of increased EAAT anion channel activity in episodic ataxia 6

Key Points

The study aims to understand how the P312R mutation in EAAT4 affects glutamate uptake and anion channel function.
Analyzed wild type and mutant EAAT4 using cellular electrophysiology and fast substrate application.
Conducted kinetic modeling of transporter states and transitions associated with substrate binding and anion channel opening.
Generated various amino acid substitutions at position P312 and evaluated their functional effects.
P312R mutation creates a new anion conducting state in the apo state of EAAT4.
Functional comparisons reveal that only certain amino acid substitutions at P312 are tolerated and impact anion conduction.
The study provides a quantitative model describing alterations in transporter function due to the mutation.

Abstract

SLC1A2 and SLC1A3 encode the glial glutamate transporters EAAT2 and EAAT1,which are not only the predominant glutamate uptake carriers in our brain, butalso function as anion channels. Two homologous mutations, which predictsubstitutions of prolines in the center of the fifth transmembrane helix byarginine (P289R EAAT2, P290R EAAT1), have been identified in patients withepileptic encephalopathy (SLC1A2) or with episodic ataxia type 6 (SLC1A3).Both mutations have been shown to impair glutamate uptake and to increaseanion conduction. The molecular processes that link the disease-causingmutations to two major alterations of glutamate transporter function remaininsufficiently understood. The mutated proline is conserved in every EAAT.Since the pathogenic changes mainly affect the anion channel function, wehere study the functional consequences of the homologous P312R mutation inthe neuronal glutamate transporter EAAT4, a low capacity glutamate transporterwith predominant anion channel function. To assess the impact of charge andstructure of the inserted amino acid for the observed functional changes, wegenerated and functionally evaluated not only P312R, but also substitutions ofP312 with all other amino acids. However, only exchange of proline by arginine,lysine, histidine and asparagine were functionally tolerated. We compared WT,P312R and P312N EAAT4 using a combination of cellular electrophysiology, fastsubstrate application and kinetic modelling. We found that WT and mutantEAAT4 anion currents can be described with a 11-state model of the transportcycle, in which several states are connected to branching anion channel states toaccount for the EAAT anion channel function. Substitutions of P312 modify varioustransitions describing substrate binding/unbinding, translocation or anion channelopening. Most importantly, P312R generates a new anion conducting state that isaccessible in the outward facing apo state and that is the main determinant of theincreased anion conduction of EAAT transporters carrying this mutation. Our workprovides a quantitative description how a naturally occurring mutation changesglutamate uptake and anion currents in two genetic diseases.

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