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Stephanie Niklaus, Simon Früh, Matthias Gesemann, Stella Glasauer, Stephan C F Neuhauss; EAAT5 and EAAT6 in the Zebrafish outer Retina. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4173.
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Excitatory amino acid transporters (EAATs) play a crucial role in retinal signaling. EAATs are capable of removing glutamate from the synaptic cleft, thus contributing to precise signal termination and avoidance of excitotoxicity. Furthermore, transport induces an uncoupled Cl- conductance. Here, we analyze the expression and function of the EAAT5 and EAAT6 paralogs in the zebrafish retina. In addition, we examine a potential wave-length dependency of the different glutamate signaling pathways.
Expression of eaat5a, 5b, 6a and 6b was assessed with in situ hybridization and localization of the proteins with immunohistochemistry on whole mount larvae and adult retinal sections using custom made peptide antibodies. Functional analysis is being performed by morpholino mediated gene knock-down and subsequent electroretinogram (ERG) measurements.
In the adult zebrafish retina, all four genes are expressed in photoreceptors. Eaat5a and eaat5b are additionally expressed in the inner nuclear layer, and eaat5a in ganglion cells. Two different splice variants of eaat5b are found, that probably differ in their Cl- conductance. Protein expression reveals localization of EAAT5a, 5b and 6a in the outer plexiform layer, while EAAT6b is found at the base of the photoreceptor outer segments. EAAT5b localizes to all cone to ON-bipolar cell synapses but not to synapses between rods and ON-bipolar cells.
The obtained results indicate an involvement of EAAT5b in photopic but not in scotopic vision. This would be in line with studies that suggest that the scotopic ON-response is mediated by the mGluR6 pathway while EAATs seem to be important in the photopic ON-response. An involvement of EAAT5b in the cone ON-response remains to be assessed by ERG recordings along with the examination of wavelength-dependent differences in the activated glutamate signaling pathways.
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