Expression of glutamate transporters in human and rat retina and rat optic nerve

Abstract

l-Glutamate is the major excitatory transmitter in the vertebrate retina and plays a central role in the transmission of the various retinal neurons. Glutamate is removed from the extracellular space by at least five different glutamate transporters. The cellular distribution of these has been studied so far mainly using immunocytochemistry. In the present study non-radioactive in situ hybridisation using complementary RNA probes was applied in order to identify the cell types of rat retina and optic nerve expressing generic GLT1, GLT1 variant (GLT1v or GLT1B), GLAST and EAAC1. The results were compared with immunocytochemical data achieved using affinity-purified antibodies against transporter peptides. In the immunohistochemical studies the human retina was included. The study showed that in the rat retina GLT1v and EAAC1 were coexpressed in various cell types, i.e. photoreceptor, bipolar, horizontal, amacrine, ganglion and Müller cells, whereas GLAST was only detected in Müller cells and astrocytes. In the rat optic nerve GLT1v and EAAC1 were preferentially expressed in oligodendrocytes, whereas GLAST was revealed to be present mainly in astrocytes. Generic GLT1 could not be detected in the retina or optic nerve. The cellular distribution of glutamate transporters (only immunocytochemistry) in the human retina was very similar to that of the rat retina. Remarkable results of our studies were that generic GLT1 was not detectable in the rat (and human) retina and that GLT1v and EAAC1 were demonstrable in most cell types of the retina (including photoreceptor cells and their terminals).

Functional glutamate transport in rodent optic nerve axons and glia

“…reported GLAST expression in interfascicular oligodendrocytes, EAAC1 in a population of unidentified glia and GLT-1 in astrocytes. In contrast, Kugler and Beyer (2003) found GLAST expression in most astrocytes, a population of EAAC1 (+) astrocytes and GLT-1 expression in oligodendrocytes and axons. Kugler and Beyer (2003) used -tubulin as an oligodendrocyte marker, which may also be expressed in astrocytes (e.g., Medrano and Steward, 2001), and it is possible that some cells identified in their study as oligodendrocytes were in fact astrocytes.…”

“…In contrast, Kugler and Beyer (2003) found GLAST expression in most astrocytes, a population of EAAC1 (+) astrocytes and GLT-1 expression in oligodendrocytes and axons. Kugler and Beyer (2003) used -tubulin as an oligodendrocyte marker, which may also be expressed in astrocytes (e.g., Medrano and Steward, 2001), and it is possible that some cells identified in their study as oligodendrocytes were in fact astrocytes. In addition to optic nerve, high levels of GLAST, GLT-1 and EAAC1 expression have been reported in other white matter structures such as sub-cortical white matter, in particular during post-natal development (Furuta et al, 1997a).…”

“…Prior studies have shown EAAC1, GLAST and GLT-1 expression in mature white matter (Choi and Chiu, 1997;Furuta et al, 1997a;Kugler and Beyer, 2003), although the cellular distribution of these transporters is controversial, and little is known regarding their cellular distribution in neonatal white matter. Several studies have shown GLT-1 expression in neonatal central axons using light microscopy (Kugler and Beyer 2003;Furuta et al, 1997a), but the close apposition of axonal and glial membrane casts the axonal expression of this "glial" transporter into doubt. However, membrane expression of GLT-1 in embryonic spinal cord axons before the appearance of astrocytes has been shown using immuno-electron microscopy (Yamada et al, 1998).…”

EAAT2 regulation and splicing: relevance to psychiatric and neurological disorders

“…24,25 As neither group had previously been able to detect the main GLT1 isoform in neurons, as GLT1 immunoreactivity in the retina is confined to neurons, 52 and because GLT1b is the major isoform in the retina, 53 it seemed plausible at the time that GLT1b might be a primarily neuronal variant. However, more recent studies have established that GLT1 is far more abundant than GLT1b in both neurons and astrocytes, indicating that GLT1b is neither unique to neurons, nor is it the predominant isoform in neurons.…”

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

“…EAAT1/GLAST mediates the uptake of glutamate into Müller cells (2)(3)(4)(5)(6)(7)(8)(9)(10), whereas EAAT2/GLT-1 is responsible for the glutamate transport into photoreceptors, bipolar and amacrine cells (4,9,10). Experiments with knockout animals demonstrated profound effects on visual signal transmission upon EAAT1/GLAST removal, but only slight changes in EAAT2/GLT-1 knock-out animals (2).…”