Lack Of Mglur6-Related Cascade Elements Leads To Retrograde Trans-Synaptic Effects On Rod Photoreceptor Synapses Via Matrix-Associated Proteins

Heterotrimeric G-proteins couple metabotropic receptors to downstream effectors. In retinal ON bipolar cells, G(o) couples the metabotropic receptor mGluR6 to the TRPM1 channel and closes it in the dark, thus hyperpolarizing the cell. Light, via GTPase-activating proteins, deactivates G(o), opens TRPM1 and depolarizes the cell. G(o) comprises G(o1), G3 and G13; all are necessary for efficient coupling. In addition, G3 contributes to trafficking of certain cascade proteins and to maintaining the synaptic structure. The goal of this study was to determine the role of G(o1) in maintaining the cascade and synaptic integrity. Using mice lacking G(o1), we quantified the immunostaining of certain mGluR6-related components. Deleting G(o1) greatly reduced staining for G3, G13, G5, RGS11, RGS7 and R9AP. Deletion of G(o1) did not affect mGluR6, TRPM1 or PCP2. In addition, deleting G(o1) reduced the number of rod bipolar dendrites that invaginate the rod terminal, similar to the effect seen in the absence of mGluR6, G3 or the matrix-associated proteins, pikachurin, dystroglycan and dystrophin, which are localized presynaptically to the rod bipolar cell. We therefore tested mice lacking mGluR6, G(o1) and G3 for expression of these matrix-associated proteins. In all three genotypes, staining intensity for these proteins was lower than in wild type, suggesting a retrograde trans-synaptic effect. We propose that the mGluR6 macromolecular complex is connected to the presynaptic rod terminal via a protein chain that includes the matrix-associated proteins. When a component of the macromolecular chain is missing, the chain may fall apart and loosen the dendritic tip adherence within the invagination.