Differential function of Gγ13 in rod bipolar and ON cone bipolar cells

Abstract

Key pointsr The G-protein-mediated metabotropic glutamate receptor 6 (mGluR6) signalling cascade is critical for light responses in all retinal ON bipolar cells. The present study evaluates the role of G-protein subunit Gγ13 in ON bipolar cells by deleting the mouse Gng13 gene.r Gγ13 is a partner of Gβ3 in all types of retinal ON bipolar cells, and contributes to generating the light response.r Gγ13 also contributes to maintenance because, in its absence, the light response decreases with age.r Three independent functionality assessments (i.e. light response as measured by electroretinogram b-waves, localization of GTPase activating proteins and deterioration of the light response with age) show that the contribution of Gγ13 is much greater in rod bipolar cells than in ON cone bipolar cells.r Our data also suggest that the contribution of Gγ13 to coupling mGluR6 to its effector is smaller than its contribution to maintaining GTPase activating protein localization.Abstract Heterotrimeric G-proteins (comprising Gα and Gβγ subunits) are critical for coupling of metabotropic receptors to their downstream effectors. In the retina, glutamate released from photoreceptors in the dark activates metabotropic glutamate receptor 6 (mGluR6) receptors in ON bipolar cells; this leads to activation of G o , closure of transient receptor potential melastatin 1 channels and hyperpolarization of these cells. G o comprises Gα o , Gβ3 and a Gγ. The best Gγ candidate is Gγ13, although functional data to support this are lacking. Thus, we tested Gγ13 function by generating Gng13 −/− knockout (KO) mice, recording electroretinograms (ERG) and performing immunocytochemical staining. The amplitude of scotopic ERG b-waves in KO mice was lower than in wild-type (WT) mice. Furthermore, in both KO and WT mice, the ERG b-wave decreased with age; this decrease was much more pronounced in KO mice. By contrast, the photopic ERG b-waves in KO mice were hardly affected at any age. In KO mice retinas, immunostaining for Gβ3 and for the GTPase activating proteins RGS7, RGS11, R9AP and Gβ5 decreased significantly in rod bipolar cells but not in ON cone bipolar cells. Staining for Gα o and certain other cascade elements decreased only slightly. Analysis of our ON bipolar cDNA library showed that these cells express mRNAs for Gγ5, Gγ10 and Gγ11. Quantitative RT-PCR of retinal cDNA showed greater values for these transcripts in retinas of KO mice, although the difference was not significant. Our results suggest that Gγ13 contributes to mGluR6 signalling in rod bipolar cells more than in ON cone bipolar cells, and that this contribution includes both coupling the receptor and maintaining a stable localization of the mGluR6-related cascade elements.

A group I metabotropic glutamate receptor controls synaptic gain between rods and rod bipolar cells in the mouse retina

“…; Ramakrishnan et al. ). Although Rampino and Nawy () found that TrpM1‐evoked currents were regulated by the PIP 2 ‐PLC‐PKC signaling pathway, gain control mechanisms by group I mGluRs have not been reported.…”

Lack of mGluR6‐related cascade elements leads to retrograde trans‐synaptic effects on rod photoreceptor synapses via matrix‐associated proteins

“…We have previously shown that in ON bipolar cells, deleting Gβ3 dramatically reduces Gα o and Gγ13 (Dhingra et al ., ). Surprisingly, deleting Gγ13 greatly reduces Gβ3, but hardly affects the expression of Gα o (Ramakrishnan et al ., ). To test whether these relationships are reciprocal, we tested how the deletion of Gα o1 , the primary splice variant in ON bipolar cells, affects its partners.…”

The Transduction Cascade in Retinal ON-Bipolar Cells: Signal Processing and Disease

“…Similarly, Gγ13 was described as the most prominent isoform present in ON-BCs (Dhingra et al 2008, Huang et al 2003). However, elimination of Gγ13 in mice only partially affected ON-RBC responses with no appreciable effects in ON-CBC (Ramakrishnan et al 2015). Thus, while Gβ3γ13 appears to be the major species involved in mGluR6 signaling in ON-RBCs, other Gβγ dimers might play a role in signaling in both ON-RBCs and to a greater extent in ON-CBCs.…”

“…Thus, while Gβ3γ13 appears to be the major species involved in mGluR6 signaling in ON-RBCs, other Gβγ dimers might play a role in signaling in both ON-RBCs and to a greater extent in ON-CBCs. Indeed, several additional proteins have been identified to be enriched in ON-BCs including Gβ4, γ5, γ10, and γ11 (Huang et al 2003, Ramakrishnan et al 2015). It seems possible that this functional heterogeneity of Gβγ combinations may contribute to distinct response properties of ON-BC populations.…”