Localized expression of amphiphysin Ir, a retina‐specific variant of amphiphysin I, in the ribbon synapse and its functional implication

Abstract

In the vertebrate retina, presynaptic terminals of photoreceptors and bipolar cells form ribbon synapses and release neurotransmitter continuously. Endocytic machinery in the ribbon synapse is likely to differ from that in conventional synapses because of the much higher rate of synaptic vesicle recycling. However, protein components of the ribbon synapse identified so far are quite similar to those of the conventional synapses. Recently we identified amphiphysin I splice variants, termed amphiphysin Ir, that are transcribed specifically in retina from the authentic amphiphysin I gene [Y. Terada et al. (2002) FEBS Lett., 519, 185-190]. Amphiphysin I is a nerve terminal-enriched protein, and involved in synaptic vesicle endocytosis as heterodimer with amphiphysin II, an isoform of amphiphysin I. We report here that the retina-specific amphiphysin Ir is expressed exclusively in the ribbon synapse and not in conventional synapses. This is the first endocytosis-related, ribbon synapse-specific protein identified in the retina. By immunoprecipitation and double-immunolabelling, amphiphysin Ir was shown to be associated not only with amphiphysin II, but also with dynamin, clathrin and alpha-adaptin that are involved in synaptic vesicle recycling. The results suggest that endocytosis of the synaptic vesicle membrane in retinal ribbon synapses proceeds through a pathway similar to the one that is used in conventional synapses, although amphiphysin Ir is substituted for amphiphysin I. Amphiphysin Ir may play an essential role in the avid endocytic activity of ribbon synapses by associating with yet unknown protein partner(s) through its large insertional domain, which is absent from the conventional amphiphysin I.

Conical Tomography of a Ribbon Synapse: Structural Evidence for Vesicle Fusion

“…The presence of coated pits and vesicles, a feature of clathrin-mediated mechanisms, is reported in isolated photoreceptors, and in photoreceptor terminals in situ and in vitro (retinal slices) [11], [100]–[105]. Consistent with these observations is a high level of expression of clathrin and clathrin accessory proteins, including dynamin and amphiphysin in photoreceptor terminals [106], [107]. Furthermore, the omega figures are not likely to be synaptic vesicles undergoing a “kiss-and-run” exocytosis, where synaptic vesicles have a transient fusion with the plasma membrane [108], [109].…”

Ribbon synapses of the retina

“…Differences between the two synapse types originate from differentially expressed protein isoforms and variable protein amounts, rather than from two fundamentally different organelles. This has been shown, for example, for the syntaxins (Morgans et al 1996), the calcium channels (Heidelberger et al 2005), the amphiphysins (Hosoya et al 2004) and the complexins (Reim et al 2005). Moreover, the various retinal ribbon synapses differ from each other in their expression of protein isoforms and in their release kinetics (Dick et al 2001;Heidelberger et al 2003;Rabl et al 2005;Reim et al 2005).…”

The Molecular Architecture of Ribbon Presynaptic Terminals

“…For example, syntaxin 1 is present in hair cells [144] and pinealocytes [142, 143, 162], but absent from retinal ribbon synapses [100, 106, 111, 138, 174]. Instead, photoreceptor and bipolar cell terminals express the b isoform [188] of syntaxin 3 [100, 102, 138, 174]. It remains to be determined how syntaxin 3b and other specific SNARE protein isoforms contribute to homotypic and heterotypic vesicle fusion events at ribbon terminals.…”

“…For example, Sherry and Heidelberger [103] localized clathrin, amphiphysin, and dynamin to photoreceptors and bipolar cells, although dynamin was only highly expressed in mouse rod bipolar cell terminals. Retina-specific isoforms of unknown function have been identified for amphiphysin I [102, 216] and dynamin 1 [217]. Among other major players in endocytosis, AP180 [104] and synaptojanin [8] are enriched at retinal ribbon terminals.…”