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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 G?o1 , G?3 and G?13; all are necessary for efficient coupling. In addition, G?3 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 G?3, G?13, G?5, 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, G?3 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 G?3 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.

SUBMITTER: Tummala SR

PROVIDER: S-EPMC4899192 | biostudies-literature | 2016 Jun

REPOSITORIES: biostudies-literature

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

Tummala Shanti R SR Dhingra Anuradha A Fina Marie E ME Li Jian J JJ Ramakrishnan Hariharasubramanian H Vardi Noga N

The European journal of neuroscience 20160510 11

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 sy ...[more]

PMID: 27037829

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Project description:α2δ-4 is an auxiliary subunit of voltage-gated Cav1.4 L-type channels that regulate the development and mature exocytotic function of the photoreceptor ribbon synapse. In humans, mutations in the CACNA2D4 gene encoding α2δ-4 cause heterogeneous forms of vision impairment in humans, the underlying pathogenic mechanisms of which remain unclear. To investigate the retinal function of α2δ-4, we used genome editing to generate an α2δ-4 knock-out (α2δ-4 KO) mouse. In male and female α2δ-4 KO mice, rod spherules lack ribbons and other synaptic hallmarks early in development. Although the molecular organization of cone synapses is less affected than rod synapses, horizontal and cone bipolar processes extend abnormally in the outer nuclear layer in α2δ-4 KO retina. In reconstructions of α2δ-4 KO cone pedicles by serial block face scanning electron microscopy, ribbons appear normal, except that less than one-third show the expected triadic organization of processes at ribbon sites. The severity of the synaptic defects in α2δ-4 KO mice correlates with a progressive loss of Cav1.4 channels, first in terminals of rods and later cones. Despite the absence of b-waves in electroretinograms, visually guided behavior is evident in α2δ-4 KO mice and better under photopic than scotopic conditions. We conclude that α2δ-4 plays an essential role in maintaining the structural and functional integrity of rod and cone synapses, the disruption of which may contribute to visual impairment in humans with CACNA2D4 mutations.SIGNIFICANCE STATEMENT In the retina, visual information is first communicated by the synapse formed between photoreceptors and second-order neurons. The mechanisms that regulate the structural integrity of this synapse are poorly understood. Here we demonstrate a role for α2δ-4, a subunit of voltage-gated Ca2+ channels, in organizing the structure and function of photoreceptor synapses. We find that presynaptic Ca2+ channels are progressively lost and that rod and cone synapses are disrupted in mice that lack α2δ-4. Our results suggest that alterations in presynaptic Ca2+ signaling and photoreceptor synapse structure may contribute to vision impairment in humans with mutations in the CACNA2D4 gene encoding α2δ-4.

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Dataset Information

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

Publications

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

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