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Boff JM, Shrestha AP, Madireddy S, Viswaprakash N, Della Santina L, Vaithianathan T. The Interplay between Neurotransmitters and Calcium Dynamics in Retinal Synapses during Development, Health, and Disease. Int J Mol Sci 2024; 25:2226. [PMID: 38396913 PMCID: PMC10889697 DOI: 10.3390/ijms25042226] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/11/2024] [Revised: 02/05/2024] [Accepted: 02/06/2024] [Indexed: 02/25/2024] Open
Abstract
The intricate functionality of the vertebrate retina relies on the interplay between neurotransmitter activity and calcium (Ca2+) dynamics, offering important insights into developmental processes, physiological functioning, and disease progression. Neurotransmitters orchestrate cellular processes to shape the behavior of the retina under diverse circumstances. Despite research to elucidate the roles of individual neurotransmitters in the visual system, there remains a gap in our understanding of the holistic integration of their interplay with Ca2+ dynamics in the broader context of neuronal development, health, and disease. To address this gap, the present review explores the mechanisms used by the neurotransmitters glutamate, gamma-aminobutyric acid (GABA), glycine, dopamine, and acetylcholine (ACh) and their interplay with Ca2+ dynamics. This conceptual outline is intended to inform and guide future research, underpinning novel therapeutic avenues for retinal-associated disorders.
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Affiliation(s)
- Johane M Boff
- Department of Pharmacology, Addiction Science, and Toxicology, College of Medicine, University of Tennessee Health Science Center, Memphis, TN 38163, USA
| | - Abhishek P Shrestha
- Department of Pharmacology, Addiction Science, and Toxicology, College of Medicine, University of Tennessee Health Science Center, Memphis, TN 38163, USA
| | - Saivikram Madireddy
- College of Medicine, The University of Tennessee Health Science Center, Memphis, TN 38163, USA
| | - Nilmini Viswaprakash
- Department of Medical Education, College of Medicine, University of Tennessee Health Science Center, Memphis, TN 38163, USA
| | | | - Thirumalini Vaithianathan
- Department of Pharmacology, Addiction Science, and Toxicology, College of Medicine, University of Tennessee Health Science Center, Memphis, TN 38163, USA
- Department of Ophthalmology, Hamilton Eye Institute, University of Tennessee Health Science Center, Memphis, TN 38163, USA
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2
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Piniella D, Zafra F. Functional crosstalk of the glycine transporter GlyT1 and NMDA receptors. Neuropharmacology 2023; 232:109514. [PMID: 37003571 DOI: 10.1016/j.neuropharm.2023.109514] [Citation(s) in RCA: 7] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/29/2022] [Revised: 03/10/2023] [Accepted: 03/20/2023] [Indexed: 04/03/2023]
Abstract
NMDA-type glutamate receptors (NMDARs) constitute one of the main glutamate (Glu) targets in the central nervous system and are involved in synaptic plasticity, which is the molecular substrate of learning and memory. Hypofunction of NMDARs has been associated with schizophrenia, while overstimulation causes neuronal death in neurodegenerative diseases or in stroke. The function of NMDARs requires coincidental binding of Glu along with other cellular signals such as neuronal depolarization, and the presence of other endogenous ligands that modulate their activity by allosterism. Among these allosteric modulators are zinc, protons and Gly, which is an obligatory co-agonist. These characteristics differentiate NMDARs from other receptors, and their structural bases have begun to be established in recent years. In this review we focus on the crosstalk between Glu and glycine (Gly), whose concentration in the NMDAR microenvironment is maintained by various Gly transporters that remove or release it into the medium in a regulated manner. The GlyT1 transporter is particularly involved in this task, and has become a target of great interest for the treatment of schizophrenia since its inhibition leads to an increase in synaptic Gly levels that enhances the activity of NMDARs. However, the only drug that has completed phase III clinical trials did not yield the expected results. Notwithstanding, there are additional drugs that continue to be investigated, and it is hoped that knowledge gained from the recently published 3D structure of GlyT1 may allow the rational design of more effective new drugs.
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Affiliation(s)
- Dolores Piniella
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain; IdiPAZ, Institute of Health Carlos III (ISCIII), Spain
| | - Francisco Zafra
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, Madrid, Spain; IdiPAZ, Institute of Health Carlos III (ISCIII), Spain.
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3
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Berry MH, Moldavan M, Garrett T, Meadows M, Cravetchi O, White E, Leffler J, von Gersdorff H, Wright KM, Allen CN, Sivyer B. A melanopsin ganglion cell subtype forms a dorsal retinal mosaic projecting to the supraoptic nucleus. Nat Commun 2023; 14:1492. [PMID: 36932080 PMCID: PMC10023714 DOI: 10.1038/s41467-023-36955-6] [Citation(s) in RCA: 6] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2022] [Accepted: 02/24/2023] [Indexed: 03/19/2023] Open
Abstract
Visual input to the hypothalamus from intrinsically photosensitive retinal ganglion cells (ipRGCs) influences several functions including circadian entrainment, body temperature, and sleep. ipRGCs also project to nuclei such as the supraoptic nucleus (SON), which is involved in systemic fluid homeostasis, maternal behavior, social behaviors, and appetite. However, little is known about the SON-projecting ipRGCs or their relationship to well-characterized ipRGC subtypes. Using a GlyT2Cre mouse line, we show a subtype of ipRGCs restricted to the dorsal retina that selectively projects to the SON. These ipRGCs tile a dorsal region of the retina, forming a substrate for encoding ground luminance. Optogenetic activation of their axons demonstrates they release the neurotransmitter glutamate in multiple regions, including the suprachiasmatic nucleus (SCN) and SON. Our results challenge the idea that ipRGC dendrites overlap to optimize photon capture and suggests non-image forming vision operates to sample local regions of the visual field to influence diverse behaviors.
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Affiliation(s)
- Michael H Berry
- Department of Ophthalmology, Casey Eye Institute, Oregon Health & Science University, Portland, OR, USA
- Medical Scientist Training Program, Oregon Health & Science University, Portland, OR, USA
| | - Michael Moldavan
- Oregon Institute of Occupational Health Sciences, Oregon Health and Science University, Portland, OR, USA
- Department of Behavioral Neuroscience, Oregon Health & Science University, Portland, OR, USA
| | - Tavita Garrett
- Department of Ophthalmology, Casey Eye Institute, Oregon Health & Science University, Portland, OR, USA
- Neuroscience Graduate program, Oregon Health & Science University, Portland, OR, USA
| | - Marc Meadows
- Neuroscience Graduate program, Oregon Health & Science University, Portland, OR, USA
- Vollum Institute, Oregon Health & Science University, Portland, OR, USA
| | - Olga Cravetchi
- Oregon Institute of Occupational Health Sciences, Oregon Health and Science University, Portland, OR, USA
- Department of Behavioral Neuroscience, Oregon Health & Science University, Portland, OR, USA
| | - Elizabeth White
- Department of Ophthalmology, Casey Eye Institute, Oregon Health & Science University, Portland, OR, USA
| | - Joseph Leffler
- Department of Ophthalmology, Casey Eye Institute, Oregon Health & Science University, Portland, OR, USA
| | - Henrique von Gersdorff
- Department of Ophthalmology, Casey Eye Institute, Oregon Health & Science University, Portland, OR, USA
- Vollum Institute, Oregon Health & Science University, Portland, OR, USA
- Department of Chemical Physiology and Biochemistry, Oregon Health & Science University, Portland, OR, USA
| | - Kevin M Wright
- Vollum Institute, Oregon Health & Science University, Portland, OR, USA
| | - Charles N Allen
- Oregon Institute of Occupational Health Sciences, Oregon Health and Science University, Portland, OR, USA
- Department of Behavioral Neuroscience, Oregon Health & Science University, Portland, OR, USA
| | - Benjamin Sivyer
- Department of Ophthalmology, Casey Eye Institute, Oregon Health & Science University, Portland, OR, USA.
- Department of Chemical Physiology and Biochemistry, Oregon Health & Science University, Portland, OR, USA.
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Flux coupling, not specificity, shapes the transport and phylogeny of SLC6 glycine transporters. Proc Natl Acad Sci U S A 2022; 119:e2205874119. [PMID: 36191186 PMCID: PMC9564218 DOI: 10.1073/pnas.2205874119] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
ATB[Formula: see text] (SLC6A14) is a member of the amino acid transporter branch of the SLC6 family along with GlyT1 (SLC6A9) and GlyT2 (SLC6A5), two glycine-specific transporters coupled to 2:1 and 3:1 Na[Formula: see text]:Cl[Formula: see text], respectively. In contrast, ATB[Formula: see text] exhibits broad substrate specificity for all neutral and cationic amino acids, and its ionic coupling remains unsettled. Using the reversal potential slope method, we demonstrate a 3:1:1 Na[Formula: see text]:Cl[Formula: see text]:Gly stoichiometry for ATB[Formula: see text] that is consistent with its 2.1 e/Gly charge coupling. Like GlyT2, ATB[Formula: see text] behaves as a unidirectional transporter with virtually no glycine efflux at negative potentials after uptake, except by heteroexchange as remarkably shown by leucine activation of NMDARs in Xenopus oocytes coexpressing both membrane proteins. Analysis and computational modeling of the charge movement of ATB[Formula: see text] reveal a higher affinity for sodium in the absence of substrate than GlyT2 and a gating mechanism that locks Na[Formula: see text] into the apo-transporter at depolarized potentials. A 3:1 Na[Formula: see text]:Cl[Formula: see text] stoichiometry justifies the concentrative transport properties of ATB[Formula: see text] and explains its trophic role in tumor growth, while rationalizing its phylogenetic proximity to GlyT2 despite their extreme divergence in specificity.
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Eulenburg V, Hülsmann S. Synergistic Control of Transmitter Turnover at Glycinergic Synapses by GlyT1, GlyT2, and ASC-1. Int J Mol Sci 2022; 23:ijms23052561. [PMID: 35269698 PMCID: PMC8909939 DOI: 10.3390/ijms23052561] [Citation(s) in RCA: 10] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/04/2022] [Revised: 02/21/2022] [Accepted: 02/23/2022] [Indexed: 01/25/2023] Open
Abstract
In addition to being involved in protein biosynthesis and metabolism, the amino acid glycine is the most important inhibitory neurotransmitter in caudal regions of the brain. These functions require a tight regulation of glycine concentration not only in the synaptic cleft, but also in various intracellular and extracellular compartments. This is achieved not only by confining the synthesis and degradation of glycine predominantly to the mitochondria, but also by the action of high-affinity large-capacity glycine transporters that mediate the transport of glycine across the membranes of presynaptic terminals or glial cells surrounding the synapses. Although most cells at glycine-dependent synapses express more than one transporter with high affinity for glycine, their synergistic functional interaction is only poorly understood. In this review, we summarize our current knowledge of the two high-affinity transporters for glycine, the sodium-dependent glycine transporters 1 (GlyT1; SLC6A9) and 2 (GlyT2; SLC6A5) and the alanine–serine–cysteine-1 transporter (Asc-1; SLC7A10).
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Affiliation(s)
- Volker Eulenburg
- Department for Anesthesiology and Intensive Care, Faculty of Medicine, University of Leipzig, Liebigstraße 20, D-04103 Leipzig, Germany
- Correspondence: (V.E.); (S.H.)
| | - Swen Hülsmann
- Department for Anesthesiology, University Medical Center, Georg-August University, Humboldtallee 23, D-37073 Göttingen, Germany
- Correspondence: (V.E.); (S.H.)
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Lux UT, Ehrenberg J, Joachimsthaler A, Atorf J, Pircher B, Reim K, Kremers J, Gießl A, Brandstätter JH. Cell Types and Synapses Expressing the SNARE Complex Regulating Proteins Complexin 1 and Complexin 2 in Mammalian Retina. Int J Mol Sci 2021; 22:ijms22158131. [PMID: 34360929 PMCID: PMC8348166 DOI: 10.3390/ijms22158131] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/05/2021] [Revised: 07/20/2021] [Accepted: 07/27/2021] [Indexed: 11/16/2022] Open
Abstract
Complexins (Cplxs) 1 to 4 are components of the presynaptic compartment of chemical synapses where they regulate important steps in synaptic vesicle exocytosis. In the retina, all four Cplxs are present, and while we know a lot about Cplxs 3 and 4, little is known about Cplxs 1 and 2. Here, we performed in situ hybridization experiments and bioinformatics and exploited Cplx 1 and Cplx 2 single-knockout mice combined with immunocytochemistry and light microscopy to characterize in detail the cell type and synapse-specific distribution of Cplx 1 and Cplx 2. We found that Cplx 2 and not Cplx 1 is the main isoform expressed in normal and displaced amacrine cells and ganglion cells in mouse retinae and that amacrine cells seem to operate with a single Cplx isoform at their conventional chemical synapses. Surprising was the finding that retinal function, determined with electroretinographic recordings, was altered in Cplx 1 but not Cplx 2 single-knockout mice. In summary, the results provide an important basis for future studies on the function of Cplxs 1 and 2 in the processing of visual signals in the mammalian retina.
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Affiliation(s)
- Uwe Thorsten Lux
- Division of Animal Physiology, Department of Biology, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany; (U.T.L.); (J.E.); (B.P.)
| | - Johanna Ehrenberg
- Division of Animal Physiology, Department of Biology, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany; (U.T.L.); (J.E.); (B.P.)
| | - Anneka Joachimsthaler
- Department of Ophthalmology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany; (A.J.); (J.A.); (J.K.); (A.G.)
| | - Jenny Atorf
- Department of Ophthalmology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany; (A.J.); (J.A.); (J.K.); (A.G.)
| | - Bianca Pircher
- Division of Animal Physiology, Department of Biology, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany; (U.T.L.); (J.E.); (B.P.)
| | - Kerstin Reim
- Department of Molecular Neurobiology, Max Planck Institute of Experimental Medicine, 37075 Göttingen, Germany;
| | - Jan Kremers
- Department of Ophthalmology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany; (A.J.); (J.A.); (J.K.); (A.G.)
| | - Andreas Gießl
- Department of Ophthalmology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91054 Erlangen, Germany; (A.J.); (J.A.); (J.K.); (A.G.)
| | - Johann Helmut Brandstätter
- Division of Animal Physiology, Department of Biology, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany; (U.T.L.); (J.E.); (B.P.)
- Correspondence:
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7
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Harsing LG, Szénási G, Zelles T, Köles L. Purinergic-Glycinergic Interaction in Neurodegenerative and Neuroinflammatory Disorders of the Retina. Int J Mol Sci 2021; 22:ijms22126209. [PMID: 34201404 PMCID: PMC8228622 DOI: 10.3390/ijms22126209] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/25/2021] [Revised: 06/03/2021] [Accepted: 06/04/2021] [Indexed: 12/26/2022] Open
Abstract
Neurodegenerative–neuroinflammatory disorders of the retina seriously hamper human vision. In searching for key factors that contribute to the development of these pathologies, we considered potential interactions among purinergic neuromodulation, glycinergic neurotransmission, and microglia activity in the retina. Energy deprivation at cellular levels is mainly due to impaired blood circulation leading to increased release of ATP and adenosine as well as glutamate and glycine. Interactions between these modulators and neurotransmitters are manifold. First, P2Y purinoceptor agonists facilitate reuptake of glycine by glycine transporter 1, while its inhibitors reduce reverse-mode operation; these events may lower extracellular glycine levels. The consequential changes in extracellular glycine concentration can lead to parallel changes in the activity of NR1/NR2B type NMDA receptors of which glycine is a mandatory agonist, and thereby may reduce neurodegenerative events in the retina. Second, P2Y purinoceptor agonists and glycine transporter 1 inhibitors may indirectly inhibit microglia activity by decreasing neuronal or glial glycine release in energy-compromised retina. These inhibitions may have a role in microglia activation, which is present during development and progression of neurodegenerative disorders such as glaucomatous and diabetic retinopathies and age-related macular degeneration or loss of retinal neurons caused by thromboembolic events. We have hypothesized that glycine transporter 1 inhibitors and P2Y purinoceptor agonists may have therapeutic importance in neurodegenerative–neuroinflammatory disorders of the retina by decreasing NR1/NR2B NMDA receptor activity and production and release of a series of proinflammatory cytokines from microglial cells.
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Affiliation(s)
- Laszlo G. Harsing
- Department of Pharmacology and Pharmacotherapy, Semmelweis University, H-1089 Budapest, Hungary; (T.Z.); (L.K.)
- Correspondence: ; Tel.: +36-1-210-4416
| | - Gábor Szénási
- Institute of Translational Medicine, Semmelweis University, H-1089 Budapest, Hungary;
| | - Tibor Zelles
- Department of Pharmacology and Pharmacotherapy, Semmelweis University, H-1089 Budapest, Hungary; (T.Z.); (L.K.)
- Department of Oral Biology, Semmelweis University, H-1089 Budapest, Hungary
| | - László Köles
- Department of Pharmacology and Pharmacotherapy, Semmelweis University, H-1089 Budapest, Hungary; (T.Z.); (L.K.)
- Department of Oral Biology, Semmelweis University, H-1089 Budapest, Hungary
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Jiménez E, Piniella D, Giménez C, Zafra F. Regulation of the Glycine Transporter GLYT1 by microRNAs. Neurochem Res 2021; 47:138-147. [PMID: 33484385 DOI: 10.1007/s11064-021-03228-x] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2020] [Revised: 12/18/2020] [Accepted: 01/02/2021] [Indexed: 12/28/2022]
Abstract
The glycine transporter GLYT1 participates in inhibitory and excitatory neurotransmission by controlling the reuptake of this neuroactive substance from synapses. Over the past few years, microRNAs have emerged as potent negative regulators of gene expression. In this report, we investigate the possible regulation of GLYT1 by microRNAs. TargetScan software predicted the existence of multiple targets for microRNAs within the 3' UTR of the human GLYT1 (miR-7, miR-30, miR-96, miR-137 and miR-141), and as they are all conserved among mammalian orthologues, their effects on GLYT1 expression were determined experimentally. Dual reporter bioluminescent assays showed that only miR-96 and miR-137 down-regulated expression of the Renilla reporter fused to the 3' UTR of GLYT1. Mutations introduced into the target sequences blocked this inhibitory effect. Consistently, these two microRNAs downregulated the uptake of [3H]glycine into glial C6 cells, a cell line where GLYT1 is the main carrier for glycine. Moreover, the expression of endogenous GLYT1 in primary mixed cultures from rat spinal cord was decreased upon lentiviral expression of miR-96 and miR-137. Although the bulk of GLYT1 is glial, it is abundantly expressed in glycinergic neurons of the retina and in smaller amounts in glutamatergic neurons though the brain. Since miR-96 in the retina is strongly downregulated by light exposure, when rats were maintained in darkness for a few hours we observed a concomitant increase of GLYT1 expression, suggesting that at least miR-96 might be an important negative regulator of GLYT1 under physiological conditions.
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Affiliation(s)
- Esperanza Jiménez
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, C / Nicolás Cabrera 1, Madrid, 28049, Spain.,IdiPAZ, Instituto de Salud Carlos III, Madrid, Spain
| | - Dolores Piniella
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, C / Nicolás Cabrera 1, Madrid, 28049, Spain.,IdiPAZ, Instituto de Salud Carlos III, Madrid, Spain
| | - Cecilio Giménez
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, C / Nicolás Cabrera 1, Madrid, 28049, Spain.,IdiPAZ, Instituto de Salud Carlos III, Madrid, Spain
| | - Francisco Zafra
- Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, C / Nicolás Cabrera 1, Madrid, 28049, Spain. .,IdiPAZ, Instituto de Salud Carlos III, Madrid, Spain.
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Heterogeneous Presynaptic Distribution of Munc13 Isoforms at Retinal Synapses and Identification of an Unconventional Bipolar Cell Type with Dual Expression of Munc13 Isoforms: A Study Using Munc13-EXFP Knock-in Mice. Int J Mol Sci 2020; 21:ijms21217848. [PMID: 33105896 PMCID: PMC7660176 DOI: 10.3390/ijms21217848] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2020] [Revised: 10/19/2020] [Accepted: 10/20/2020] [Indexed: 12/19/2022] Open
Abstract
Munc13 isoforms are constituents of the presynaptic compartment of chemical synapses, where they govern important steps in preparing synaptic vesicles for exocytosis. The role of Munc13-1, -2 and -3 is well documented in brain neurons, but less is known about their function and distribution among the neurons of the retina and their conventional and ribbon-type chemical synapses. Here, we examined the retinae of Munc13-1-, -2-, and -3-EXFP knock-in (KI) mice with a combination of immunocytochemistry, physiology, and electron microscopy. We show that knock-in of Munc13-EXFP fusion proteins did not affect overall retinal anatomy or synapse structure, but slightly affected synaptic transmission. By labeling Munc13-EXFP KI retinae with specific antibodies against Munc13-1, -2 and -3, we found that unlike in the brain, most retinal synapses seem to operate with a single Munc13 isoform. A surprising exception to this rule was type 6 ON bipolar cells, which expressed two Munc13 isoforms in their synaptic terminals, ubMunc13-2 and Munc13-3. The results of this study provide an important basis for future studies on the contribution of Munc13 isoforms in visual signal processing in the mammalian retina.
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