Excitatory Amino Acid Transporters of the Salamander Retina: Identification, Localization, and Function

Abstract

The rapid re-uptake of extracellular glutamate mediated by a family of high-affinity glutamate transporter proteins is essential to continued glutamatergic signaling and neuronal viability, but the contributions of individual transporter subtypes toward cellular physiology are poorly understood. Because the physiology of glutamate transport in the salamander retina has been well described, we have examined the expression and function of glutamate transporter subtypes in this preparation. cDNAs encoding five distinct salamander excitatory amino acid transporter (sEAAT) subtypes were isolated, and their molecular properties and distributions of expression were compared. We report evidence that at least four distinct sEAAT subtypes are expressed in glial (Mü ller) cells. In addition, four of the five transporter subtypes are localized in neurons throughout the retina. The brightest immunostaining was seen in the synaptic regions of the inner and outer plexiform layers and in the outer nuclear layer. Using electrophysiological measurements in the Xenopus oocyte expression system, we also examined the pharmacology and ionic dependence of the four expressing transporter subtypes that make it possible to distinguish, on the basis of functional behavior, among the various subtypes. Although no simple correlation between transporter subtype and retinal cell physiology can be made, the diverse population of sEAAT transporter subtypes with unique localization and functional properties indicates that glutamate transporters play a wide variety of roles in retinal function and are likely to underlie both the uptake of glutamate by Mü ller cells and the glutamateelicited chloride conductance involved in signal transduction by photoreceptors and bipolar cells. Key words: glutamate transporter; uptake; amino acid; retina; chloride; molecular cloning; photoreceptors; Mü ller cells; bipolar cells; immunohistochemistryGlutamate is the predominant excitatory neurotransmitter in the vertebrate retina and is released from photoreceptors, bipolar cells, and ganglion cells (for review, see Massey, 1990). Termination of glutamatergic synaptic transmission requires uptake of glutamate by means of plasma membrane glutamate transporters, which may be present presynaptically and postsynaptically as well as in neighboring glial cells. These transporters are of particular importance in the retina, where glutamate is released continuously and at a high rate in the dark.Electrophysiological studies performed primarily in the salamander retina have demonstrated the existence of glutamate transporters in retinal glial (Müller) cells (Brew and Attwell,

Synaptic transmission at retinal ribbon synapses

“…However, subsequent experimentation established that VGlut1, 2 and 3 are present on vesicular membranes and function as glutamate transporters (Bellocchio et al, 2000;Takamori et al, 2000). By contrast with the plasma membrane glutamate transporters (Eliasof et al, 1998), vesicular glutamate transporters are sodium-independent and exhibit a more than 10-fold lower Km (2 mM: Bellocchio et al, 2000). In place of sodium-dependence, glutamate uptake into vesicles relies primarily on the electrical component (ΔΨ) of the electrochemical gradient across the vesicle membrane.…”

Glial modulation of synaptic transmission in the retina

“…Glutamate transport into glia is well characterized and is mediated by GLAST (EAAT1) and GLT-1 (EAAT2) transporters in Müller cells (Lehre et al, 1997;Eliasof et al, 1998).…”

Glutamate transport by retinal Müller cells in glutamate/aspartate transporter‐knockout mice

“…For example, Arriza et al (1997) demonstrated that a large part of the glutamateevoked current mediated by EAAT5 is due to its anion conductance. Mü ller cells of some species have been reported to express EAAT5 (Eliasof et al, 1998a). However, the existence of EAAT5 in Mü ller cells from a mammalian retina remains to be demonstrated (Pow, 2001a).…”

“…EAAC1 (EAAT3) is present in neurons in the inner retina (Rauen et al, 1996;Schultz and Stell, 1996). EAAT4 has been reported in retinal astrocytes (Ward et al, 2004), while EAAT5 is localized to photoreceptors, bipolar cells, and possibly some Mü ller cells (Arriza et al, 1997;Eliasof et al, 1998a;Barnett and Pow, 2000).…”