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Anuradha Dhingra, Marie Fina, Noga Vardi; Gß3 is Expressed in Retinal ON Bipolar Neurons and is Required for Light ON Responses. Invest. Ophthalmol. Vis. Sci. 2011;52(14):698. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
ON bipolar neurons in retina detect glutamate released by photoreceptors via a signal transduction cascade mediated by a metabotropic glutamate receptor 6 (Grm6). The downstream components of the cascade include a heterotrimeric G protein and a recently identified non-selective cation channel TrpM1. While the alpha subunit of the G protein has been identified as Galphao, the identity of the beta and gamma partners is not yet established. This study is aimed at testing the expression and function of Gß3 (GNB3) in ON bipolar neurons.
Immunocytochemistry was performed on monkey, wild type mouse, Grm6-GFP transgenic mouse (which expresses GFP under mGluR6 promoter and shows expression of GFP in all and only ON bipolar cells), and GNB3 -/- . GNB3 -/-mouse was generated by complete elimination of the Gß3 coding region. Antibodies against PKC, Galphao and Ggamma13 were used for testing expression of different proteins in GNB3-/- retina. Electroretinographic (ERG) recordings under scotopic and photopic condtions were obtained for WT and GNB3-/- mice.
In both mouse and monkey retina, the expression of Gß3 was strong in the outer plexiform layer (OPL) and outer segment of cones, and moderate in inner nuclear layer and in the ON sublamina of inner plexiform layer. Immunostaining of monkey retina showed co-localization of Galphao and Gß3 in somas and their dendrites, thus establishing that Gß3 is expressed in all ON bipolar cells, both ON cone and rod bipolar, but not OFF bipolar cells. Similarly, in Grm6-GFP transgenic mice, staining with anti-Gß3 was present in all and only GFP-positive cells. Staining for Gß3 was eliminated in GNB3-/- mouse retina. Retinal layering and key cell types appeared normal, but certain components of the ON bipolar cascade, such as Galphao and Ggamma13, appeared downregulated. Functionally, the electroretinogram a-wave of the dark-adapted mouse, which represents the rod and cone photocurrents, was normal. However, the electroretinogram b-wave, which represents the pooled response of ON bipolar cells, was completely missing.
We conclude that beta3 subunit of G protein is required for the light ON response, and that it likely is Galphao’s partner in the heterotrimer.
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