title: A Presynaptic Group III mGluR Recruits Gβγ/SNARE Interactions to Inhibit Synaptic Transmission by Cone Photoreceptors in the Vertebrate Retina.
abstract: G-protein βγ subunits (Gβγ) interact with presynaptic proteins and regulate neurotransmitter release downstream of Ca2+influx. To accomplish their roles in sensory signaling, photoreceptor synapses use specialized presynaptic proteins that support neurotransmission at active zone structures known as ribbons. While several G-protein coupled receptors (GPCRs) influence synaptic transmission at ribbon synapses of cones and other retinal neurons, it is unknown whether Gβγ contributes to these effects. We tested whether activation of one particular GPCR, a metabotropic glutamate receptor (mGluR), can reduce cone synaptic transmission via Gβγ in tiger salamander retinas. In recordings from horizontal cells, we found that an mGluR agonist (L-AP4) reduced cone-driven light responses and mEPSC frequency. In paired recordings of cones and horizontal cells, L-AP4 slightly reduced cone ICa(∼10%) and caused a larger reduction in cone-driven EPSCs (∼30%). Proximity ligation assay revealed direct interactions between SNAP-25 and Gβγ subunits in retinal synaptic layers. Pretreatment with the SNAP-25 cleaving protease BoNT/A inhibited L-AP4 effects on synaptic transmission, as did introduction of a peptide derived from the SNAP-25 C terminus. Introducing Gβγ subunits directly into cones reduced EPSC amplitude. This effect was inhibited by BoNT/A, supporting a role for Gβγ/SNAP-25 interactions. However, the mGluR-dependent reduction in ICawas not mimicked by Gβγ, indicating that this effect was independent of Gβγ. The finding that synaptic transmission at cone ribbon synapses is regulated by Gβγ/SNAP-25 interactions indicates that these mechanisms are shared by conventional and ribbon-type synapses. Gβγ liberated from other photoreceptor GPCRs is also likely to regulate synaptic transmission.SIGNIFICANCE STATEMENTDynamic regulation of synaptic transmission by presynaptic G-protein coupled receptors shapes information flow through neural circuits. At the first synapse in the visual system, presynaptic metabotropic glutamate receptors (mGluRs) regulate cone photoreceptor synaptic transmission, although the mechanisms and functional impact of this are unclear. We show that mGluRs regulate light response encoding across the cone synapse, accomplished in part by triggering G-protein βγ subunits (Gβγ) interactions with SNAP-25, a core component of the synaptic vesicle fusion machinery. In addition to revealing a role in visual processing, this provides the first demonstration that Gβγ/SNAP-25 interactions regulate synaptic function at a ribbon-type synapse, contributing to an emerging picture of the ubiquity of Gβγ/SNARE interactions in regulating synaptic transmission throughout the nervous system.
authors.affiliation: Truhlsen Eye Institute, Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, Omaha, Nebraska 68198, matt.vanhook@unmc.edu.
authors.forename: Matthew J
authors.identifier: 0000-0003-1615-4314
authors.initials: MJ
authors.lastname: Van Hook
authors.affiliation: Department of Biology, Animal Physiology, University of Erlangen-Nuremberg, 91058 Erlangen, Germany.
authors.forename: Norbert
authors.identifier: 0000-0002-9348-0546
authors.initials: N
authors.lastname: Babai
authors.affiliation: Department of Pharmacology, Vanderbilt University Medical School, Nashville, Tennessee 37232, and.
authors.forename: Zack
authors.initials: Z
authors.lastname: Zurawski
authors.affiliation: Department of Pharmacology, Vanderbilt University Medical School, Nashville, Tennessee 37232, and.
authors.forename: Yun Young
authors.identifier: 0000-0002-0061-6820
authors.initials: YY
authors.lastname: Yim
authors.affiliation: Department of Pharmacology, Vanderbilt University Medical School, Nashville, Tennessee 37232, and.
authors.forename: Heidi E
authors.identifier: 0000-0001-5437-5287
authors.initials: HE
authors.lastname: Hamm
authors.affiliation: Truhlsen Eye Institute, Department of Ophthalmology and Visual Sciences, University of Nebraska Medical Center, Omaha, Nebraska 68198.|Department of Pharmacology and Experimental Neuroscience, University of Nebraska Medical Center, Omaha, Nebraska 68198.
authors.forename: Wallace B
authors.identifier: 0000-0001-7104-042X
authors.initials: WB
authors.lastname: Thoreson
mesh_terms: D000557:Ambystoma / Q000502:physiology*; D000818:Animals; D019706:Excitatory Postsynaptic Potentials / Q000502:physiology; D005260:Female; D044387:GTP-Binding Protein beta Subunits / Q000378:metabolism*; D044388:GTP-Binding Protein gamma Subunits / Q000378:metabolism*; D008297:Male; D018094:Receptors, Metabotropic Glutamate / Q000187:drug effects / Q000378:metabolism*; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology*; D051248:Retinal Horizontal Cells / Q000378:metabolism / Q000502:physiology; D050600:SNARE Proteins / Q000378:metabolism*; D013569:Synapses / Q000502:physiology*; D009435:Synaptic Transmission / Q000502:physiology*
chemical_list: C118981:G-protein Beta gamma; D044387:GTP-Binding Protein beta Subunits; D044388:GTP-Binding Protein gamma Subunits; D018094:Receptors, Metabotropic Glutamate; D050600:SNARE Proteins
keywords: G-protein coupled receptor; SNARE complex; mGluR; retina; ribbon; synapse