title: Differential distribution of synaptotagmin immunoreactivity among synapses in the goldfish, salamander, and mouse retina.
abstract: Synaptotagmin I is the leading candidate for the calcium sensor that triggers exocytosis at conventional synapses. However, physiological characterization of the calcium sensor for phasic release at the ribbon-style synapses of the goldfish Mb1 bipolar cell demonstrates a lower than predicted affinity for calcium, suggesting that a modified or different sensor triggers exocytosis at this synapse. We examined synaptotagmin immunolabeling in goldfish retina using two different antibodies directed against synaptotagmin epitopes that specifically labeled the expected 65-kDa protein on western blots of goldfish and mouse retinal membranes. The first antiserum strongly labeled conventional synapses in the inner plexiform layer (IPL), but did not label the ribbon-style synapse-containing synaptic terminals of goldfish Mb1 bipolar cells or photoreceptors. The second antibody also specifically labeled the expected 65-kDa protein on western blots but did not label any synapses in the goldfish retina. A third synaptotagmin antibody that performed poorly on western blots selectively labeled goldfish photoreceptor terminals. These results suggest that synaptotagmin may exist in at least three distinct "forms" in goldfish retinal synapses. These forms, which are differentially localized to conventional synapses, bipolar cell, and photoreceptor terminals, may represent differences in isoform, posttranslational modifications, epitope availability, and protein-binding partners. Labeling with these antibodies in the salamander and mouse retina revealed species-specific differences, indicating that synaptotagmin epitopes can vary across species as well as among synapses.
authors.affiliation: Department of Neurobiology and Anatomy and the W.M. Keck Center for the Neurobiology of Learning and Memory, University of Texas-Houston Medical School, Houston, USA.
authors.forename: Ruth
authors.initials: R
authors.lastname: Heidelberger
authors.forename: Meng M
authors.initials: MM
authors.lastname: Wang
authors.forename: David M
authors.initials: DM
authors.lastname: Sherry
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002135:Calcium-Binding Proteins; D000939:Epitopes / Q000276:immunology / Q000378:metabolism; D005456:Fluorescent Dyes / Q000378:metabolism; D006054:Goldfish; D015151:Immunoblotting; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D007211:Indoles / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000145:classification / Q000276:immunology / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D018613:Microscopy, Confocal / Q000379:methods; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000145:classification / Q000276:immunology / Q000378:metabolism*; D010446:Peptide Fragments / Q000276:immunology / Q000378:metabolism; D020033:Protein Isoforms / Q000378:metabolism; D011493:Protein Kinase C / Q000378:metabolism; D012160:Retina / Q000033:anatomy & histology / Q000166:cytology / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D013045:Species Specificity; D013569:Synapses / Q000145:classification / Q000378:metabolism*; D050863:Synaptotagmin I; D050857:Synaptotagmins; D014446:Tyrosine 3-Monooxygenase / Q000378:metabolism; D014562:Caudata
chemical_list: D002135:Calcium-Binding Proteins; D000939:Epitopes; D005456:Fluorescent Dyes; D007211:Indoles; D008562:Membrane Glycoproteins; D009419:Nerve Tissue Proteins; D010446:Peptide Fragments; D020033:Protein Isoforms; C489614:Sv2a protein, mouse; D050863:Synaptotagmin I; C494311:Syt1 protein, mouse; D050857:Synaptotagmins; C007293:DAPI; D014446:Tyrosine 3-Monooxygenase; D011493:Protein Kinase C