Functional Properties of the Retinal Glutamate Transporters GLT-1c and EAAT5

Abstract

Background: GLT-1c and EAAT5 are two excitatory amino acid transporters co-expressed in retinal neurons. Results: GLT-1c and EAAT5 differ in glutamate and Na+ affinity, individual transport rates as well as in unitary anion current amplitudes. Conclusion: GLT-1c and EAAT5 are optimized to fulfill different physiological tasks. Significance: Identification of unitary channel properties underlying separate anion conductances associated with GLT-1c and EAAT5. In the mammalian retina, glutamate uptake is mediated by members of a family of glutamate transporters known as excitatory amino acid transporters (EAATs). Here we cloned and functionally characterized two retinal EAATs from mouse, the GLT-1/EAAT2 splice variant GLT-1c, and EAAT5. EAATs are glutamate transporters and anion-selective ion channels, and we used heterologous expression in mammalian cells, patch-clamp recordings and noise analysis to study and compare glutamate transport and anion channel properties of both EAAT isoforms. We found GLT-1c to be an effective glutamate transporter with high affinity for Na+ and glutamate that resembles original GLT-1/EAAT2 in all tested functional aspects. EAAT5 exhibits glutamate transport rates too low to be accurately measured in our experimental system, with significantly lower affinities for Na+ and glutamate than GLT-1c. Non-stationary noise analysis demonstrated that GLT-1c and EAAT5 also differ in single-channel current amplitudes of associated anion channels. Unitary current amplitudes of EAAT5 anion channels turned out to be approximately twice as high as single-channel amplitudes of GLT-1c. Moreover, at negative potentials open probabilities of EAAT5 anion channels were much larger than for GLT-1c. Our data illustrate unique functional properties of EAAT5, being a low-affinity and low-capacity glutamate transport system, with an anion channel optimized for anion conduction in the negative voltage range.