Ribbon synapses in the retina

Head:

Prof. Dr. med. Frank Schmitz
Anatomy and Cellbiology
Faculty of Medicine
Saarland University
email: frank.schmitz@uniklinik-saarland.de

Ribbon synapses are specialized synapses, which are found in the most highly developed sensory organs of the human body, the eye and the ear. Ribbon synapses are characterized by unique physiological and structural properties. In contrast to the phasic activity of “conventional” synapses, ribbon synapses tonically release neurotransmitter. Synaptic ribbons represent structural specialization of the active zone of ribbon synapses which probably have evolved to cope with the specific requirements of tonic activity. Synaptic ribbons bind large numbers of synaptic vesicles which then serve as a reserve pool of vesicles that can be rapidly activated for exocytosis thereby tuning ribbon synapses to very high levels of synaptic performance (for review, see von Gersdorff, 2001). The rate of vesicle exocytosis is higher in ribbon synapses than in conventional synapses, requiring very potent synaptic vesicle priming mechanisms (Schmitz et al., 2001). The composition of synaptic ribbons was largely unknown. In order to get insight into the molecular composition of ribbons we partially purified synaptic ribbons from the bovine retina (Schmitz et al., 1996).

We identified and cloned the cDNA of a major component of synaptic ribbons that we called RIBEYE (Schmitz et al., 2000). In contrast to Rim (Wang et al., 1997) and Bassoon (tom Dieck et al., 1998; Brandstätter et al., 1999), RIBEYE is a specific component of synaptic ribbons and represents a major structural component of synaptic ribbons. From the characterization of RIBEYE and RIBEYE-associated proteins we hope to understand how synaptic ribbons participate in the synaptic vesicle cycle and how precisely the synaptic vesicle cycle is tuned to the observed high synaptic efficiency in this type of synapse.

References:

Brandstätter J.H., Fletcher E.L., Garner C.C., Gundelfinger E.D., Wässle H. (1999) Differential expression of the cytomatrix protein bassoon among ribbon synapses in the mammalian retina. Europ. J. Neurosci. 11: 3683-3693.

Schmitz F., Bechmann M., Drenckhahn D. (1996) Purification of synaptic ribbons, structural components of the photoreceptor active zone complex. J. Neurosci. 16: 7109-7116.

Schmitz F., Königstorfer A., Südhof T.C. (2000) RIBEYE, a component of synaptic ribbons: a protein's journey through evolution provides insight into synaptic ribbon function. Neuron 28: 857-872.

Schmitz F., Augustin I., Brose N. (2001) The synaptic vesicle priming protein Munc13-1 is absent from tonically active ribbon synapses of the rat retina. Brain Res. 895: 258-263.

tom Dieck S., Sanmarti-Vila L., Langnaese K., Richter K., Kindler S., Soyke A., Wex H., Smalla K.H., Kämpf U., Fränzer J.T., Stumm M., Garner C.C., Gundelfinger E.D. (1998) Bassoon, a novel zinc-finger CAG/glutamine-repeat protein selectively localized at the active zone of presynaptic nerve terminals. J. Cell Biol. 142:499-509.

von Gersdorff H. (2001) Synaptic ribbons: versatile signal transducers. Neuron 29: 7-10.

Wang Y., Okamoto M., Schmitz F., Hofmann K., Südhof T.C. (1997) Rim is a putative rab3 effector in regulating synaptic vesicle fusion. Nature 388: 593-598.