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R.M. Duvoisin, W. Liu, B. Bearnot, T.G. Wensel, C.W. Morgans; The Gbeta5–RGS7 Complex Is Localized to the Dendrites of On–Bipolar Cells in the Retina . Invest. Ophthalmol. Vis. Sci. 2006;47(13):387.
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© ARVO (1962-2015); The Authors (2016-present)
Glutamate release from retinal photoreceptors hyperpolarizes ON–bipolar cells (ON–BPCs) through a unique metabotropic glutamate receptor, mGluR6, exclusively located at the tips of ON–BPC dendrites. It is now established that the α subunit of the G protein activated by mGluR6 is Gαo, but the other elements of the mGluR6 signaling cascade are unknown. We present evidence that the Gß5–RGS7 complex is a component of the mGluR6 transduction pathway.
Immunohistochemistry was performed as described previously (Berntson and Morgans, 2003). A Zeiss LSM510 confocal microscope was used to visualize the labeling. Immunoprecipitations were done as described by Hu and Wensel (2004).
The distribution of Gß5 in the retina was examined by immunofluorescent staining of mouse retina sections. Bright, punctate staining was observed in the OPL associated with both rod and cone terminals, as well as weak staining of photoreceptor outer segments. Double labeling with a synaptic ribbon antibody revealed that pairs of Gß5 puncta are contained within the arc of a rod synaptic ribbon, suggesting that Gß5 is likely to be localized post–synaptically. Little overlap was observed between Gß5 and the calbindin–labeled tips of the horizontal cell processes. In contrast, double labeling for Gß5 and mGluR6 indicated near perfect overlap, a strong indication that Gß5 is localized to the tips of ON–BPC dendrites. Identical staining was observed for RGS7, a GGL–domain containing RGS protein that forms a tightly bound complex with Gß5. We further find that R9AP, a protein known to anchor the Gß5–RGS9 complex in photoreceptor outer segments, is also co–localized with mGluR6 in the OPL suggesting that it may serve as a membrane anchor for a Gß5–RGS7 complex in ON–BPC dendrites. Co–immunoprecipitations indicate that the short isoform of Gß5, Gß5s, forms a complex with RGS7 in the retina. Further experiments are planned to determine if R9AP is part of this complex.
The precise co–localization of the Gß5–RGS7 complex with mGluR6 in ON–BPC dendritic tips argues that Gß5–RGS7 functions specifically in the mGluR6 signal transduction pathway. One possibility is that this complex accelerates the GTPase activity of Gαo, thus accelerating the opening of ON–BPC cation channels and the depolarizing response to light.
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