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

Abstract

Heterotrimeric G-proteins couple metabotropic receptors to downstream effectors. In retinal ON bipolar cells, Go 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 Go, opens TRPM1, and depolarizes the cell. Go comprises Gαo1, Gβ3 and Gγ13; all are necessary for efficient coupling. In addition, Gβ3 contributes to trafficking of certain cascade proteins and to maintaining the synaptic structure. The goal of this study was to determine the role of Gαo1 in maintaining the cascade and synaptic integrity. Using mice lacking Gαo1, we quantified the immunostaining of certain mGluR6-related components. Deleting Gαo1 greatly reduced staining for Gβ3, Gγ13, Gβ5, RGS11, RGS7, and R9AP. Gαo1’s deletion did not affect mGluR6, TRPM1, or PCP2. In addition, deleting Gαo1 reduced the number of rod bipolar dendrites that invaginate the rod terminal, similar to the effect seen in the absence of mGluR6, Gβ3, or the matrix-associated proteins, pikachurin, dystroglycan, and dystrophin, which are localized presynaptically to the rod bipolar cell. We therefore tested mice lacking mGluR6, Gαo1, and Gβ3 for expression of the 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.

LRIT3 Differentially Affects Connectivity and Synaptic Transmission of Cones to ON- and OFF-Bipolar Cells

“…Interestingly, structural defects have already been shown in several cCSNB mouse models or in knockout mouse models lacking proteins involved in the mGluR6 signaling cascade. 35 – 40 However, the present report is the first to observe a structural defect specifically affecting cone, but not rod synapses. These results suggest that LRIT3 may participate in synaptogenesis reactions that are selective for wiring cones.…”

“…Proteins of the dystrophin–glycoprotein complex localized in the synaptic cleft of both rod and cone synapses, including dystroglycan and pikachurin, may be involved in the maintenance of a stable adhesion between rod spherules and rod ON-BC dendrites. 40 We evaluated the localization of pikachurin in the OPL of Lrit3 nob6/nob6 mice and showed that pikachurin normally localizes at the synaptic cleft of both rod and cone synapses, indicating that pikachurin is unlikely to be directly involved in the maintenance of the cone ribbon synapse. Similarly, it has been recently shown that a protein, ELFN1, is specifically expressed at the surface of rod spherules where it transsynaptically interacts with mGluR6, to enable formation of the rod synapse.…”

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

“…In contrast, deletion of the Gα o , Gβ3, or mGluR6 resulted in reduced accumulation of DCG at the synapse (Tummala et al 2016). These observations further support the idea of trans -synaptic coordination between the ON-BC signaling cascade and photoreceptors, and highlight that extracellular matrix may play an essential role in this process.…”

LRIT1 Modulates Adaptive Changes in Synaptic Communication of Cone Photoreceptors

“…Recent findings suggest that the postsynaptic cascade of ON-BCs is further engaged in contacts with the photoreceptor presynaptic release apparatus ( Cao et al, 2015 ; Tummala et al, 2016 ; Wang et al, 2017 ). For example, the mGluR6 is directly recruited by the rod-specific molecule ELFN1 to the Ca V 1.4 channel complex, an interaction that is crucial for the physical assembly of rod synapses and the transmission of rod signals to rod ON-BCs ( Cao et al, 2015 ; Wang et al, 2017 ).…”