Changing patterns of spatial buffering of glutamate in developing rat retinae are mediated by the Müller cell glutamate transporter GLAST

Abstract

The patterns of expression of the glutamate transporter GLAST were compared with the patterns of uptake of exogenous D-aspartate, which is a substrate for all glutamate transporters. At postnatal day 0, fine radial processes and end feet of presumptive Müller cells were weakly immunoreactive for GLAST. At postnatal day 3, intense labelling was associated with astrocytes enveloping newly formed blood vessels on the vitread surface of the retina. Between postnatal days 7 and 10, there was a rapid increase in the intensity of labelling in the Müller cells but clear stratification of GLAST-immunoreactive processes in the inner plexiform layer was not observed until postnatal day 14. By comparison, D-aspartate uptake was initially associated with a wide variety of cellular elements including most neuroblasts, presumptive Müller cells, and astrocytes associated with blood vessels but was absent from the somata of many neurons in the ganglion cell layer and amacrine cell layer. There was a gradual contraction in the numbers of cells that were able to take up D-aspartate, such that, by adulthood, uptake was restricted mainly to Müller cells and astrocytes. We conclude that, during early retinal development, the low levels of GLAST expression by Müller cells permit D-aspartate, and by inference, glutamate, to permeate the retina freely, thus allowing uptake by other glutamate transporters on other cell types. As the retina matures, increased expression of GLAST by Müller cells restricts the access of D-aspartate to other cellular compartments in the retina. This changing pattern of spatial buffering of glutamate by GLAST probably has significant implications regarding our understanding of the role of glutamate during processes such as retinal synaptogenesis.

Impact of SLC6A Transporters in Physiological Taurine Transport at the Blood–Retinal Barrier and in the Liver

“…Müller cells are the glial cells in the retina and assumed to have a considerable role in osmolarity regulation of the retinal microenvironment, since they contribute to the homeostasis of low molecular-weight molecules to protect the retinal neurons from excitotoxicity. 45) Earlier studies showed the transport activity of taurine, the expression of taurine transporters, such as TAUT, GAT1 and GAT3, and high accumulation of taurine in Müller cells, 27,[46][47][48] implying the key role of Müller cells in regulating osmolarity at the retinal ECF. Studies using an in vitro Müller cell model, a conditionally immortalized rat Müller cell line (TR-MUL5 cells), 49,50) suggested that Müller cells are involved in carriermediated taurine transport in an Na + -and Cl − -dependent manner, with a K m value estimated to be 37.9 µM, similar to that of TauT.…”

Expression of multiple glutamate transporter splice variants in the rodent testis

“…Detection of D-aspartate uptake was carried out according to our standard methods. [28][29][30] Briefly, testes were removed from rats as described and immediately dissected to remove the capsule around the testis. Small (,2 mm diameter) pieces of tissue were then dissected from each testis and placed into oxygenated physiological media (Ames media; Sigma, Castle Hill, NSW, Australia) or Ames media containing 20 mmol l 21 D-aspartate, at 35 uC for 30 min.…”

Na+–H+ exchanger regulatory factor 1 is a PDZ scaffold for the astroglial glutamate transporter GLAST

“…Antibodies against NHERF1 and NHERF2 (Kanjhan et al, 2006), GLAST (Pow and Barnett, 1999), and GLT1b (Reye et al, 2002) have been described previously. Antibody specific for ezrin (residues 362-585 of human ezrin, GenBank ™ accession number NM_003379) was purchased from Zymed Laboratories (San Francisco, CA, USA).…”