title: Early steps in the assembly of photoreceptor ribbon synapses in the mouse retina: the involvement of precursor spheres.
abstract: The retinal photoreceptor ribbon synapse is a chemical synapse structurally and functionally specialized for the tonic release of neurotransmitter. It is characterized by the presynaptic ribbon, an electron-dense organelle at the active zone covered by hundreds of synaptic vesicles. In conventional synapses, dense-core transport vesicles carrying a set of active zone proteins are implicated in early steps of synapse formation. In photoreceptor ribbon synapses, synaptic spheres are suggested to be involved in ribbon synapse assembly, but nothing is known about the molecular composition of these organelles. With light, electron, and stimulated emission depletion microscopy and immunocytochemistry, we investigated a series of presynaptic proteins during photoreceptor synaptogenesis. The cytomatrix proteins Bassoon, Piccolo, RIBEYE, and RIM1 appear early in synaptogenesis. They are transported in nonmembranous, electron-dense, spherical transport units, which we called precursor spheres, to the future presynaptic site. Other presynaptic proteins, i.e., Munc13, CAST1, RIM2, and an L-type Ca(2+) channel alpha1 subunit are not associated with the precursor spheres. They cluster directly at the active zone some time after the first set of cytomatrix proteins has arrived. By quantitative electron microscopy, we found an inverse correlation between the numbers of spheres and synaptic ribbons in the postnatally developing photoreceptor synaptic terminals. From these results, we suggest that the precursor spheres are the transport units for proteins of the photoreceptor ribbon compartment and are involved in the assembly of mature synaptic ribbons.
authors.affiliation: Department of Biology, Animal Physiology, University of Erlangen-Nuremberg, Erlangen, Germany.
authors.forename: Hanna
authors.initials: H
authors.lastname: Regus-Leidig
authors.forename: Susanne
authors.initials: S
authors.lastname: Tom Dieck
authors.forename: Dana
authors.initials: D
authors.lastname: Specht
authors.forename: Lars
authors.initials: L
authors.lastname: Meyer
authors.forename: Johann Helmut
authors.initials: JH
authors.lastname: Brandstätter
mesh_terms: D000429:Alcohol Oxidoreductases; D000818:Animals; D056970:Co-Repressor Proteins; D003598:Cytoskeletal Proteins / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000378:metabolism; D051379:Mice; D009419:Nerve Tissue Proteins / Q000378:metabolism; D009479:Neuropeptides / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D020419:Photoreceptor Cells, Vertebrate / Q000502:physiology / Q000648:ultrastructure; D013569:Synapses / Q000502:physiology / Q000648:ultrastructure; D022161:Transport Vesicles / Q000378:metabolism / Q000648:ultrastructure
chemical_list: C498441:Bsn protein, mouse; D056970:Co-Repressor Proteins; D003598:Cytoskeletal Proteins; D004268:DNA-Binding Proteins; D009419:Nerve Tissue Proteins; D009479:Neuropeptides; C498631:Pclo protein, mouse; D010750:Phosphoproteins; C491490:Rims1 protein, mouse; D000429:Alcohol Oxidoreductases; C111341:Ctbp2 protein, mouse; D019204:GTP-Binding Proteins