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

Heterotrimeric G-proteins (comprising G and G 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 G(o), G3 and a G. The best G candidate is G13, although functional data to support this are lacking. Thus, we tested G13 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 G3 and for the GTPase activating proteins RGS7, RGS11, R9AP and G5 decreased significantly in rod bipolar cells but not in ON cone bipolar cells. Staining for G(o) and certain other cascade elements decreased only slightly. Analysis of our ON bipolar cDNA library showed that these cells express mRNAs for G5, G10 and G11. 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 G13 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.