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Haruhisa Okawa, Benjamin Odermatt, Haining Zhong, Leon Lagnado, Frank Schmitz, Rachel O Wong, Ulf Matti; Dynamics of ribbon synapse assembly in the developing inner retina. Invest. Ophthalmol. Vis. Sci. 2016;57(12):2759.
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© ARVO (1962-2015); The Authors (2016-present)
The proper formation of ribbon synapses is critical for the normal transmission of visual information through the retina. We previously showed that some synapses formed between bipolar cells (BC) and ganglion cells (RGC) are eliminated during development (Morgan et al, 2011), but it remains unknown whether the elimination or stabilization of synapses depends on the localization of ribbons to the nascent synapses. Here, we test the hypothesis that ribbon localization promotes the stability of the synapses, by monitoring the dynamics of ribbons in BC axon terminals apposed to postsynaptic sites on developing RGCs in live mouse retinas.
To visualize ribbons in live retinas, we generated transgenic mice expressing fluorescent protein-tagged RIBEYE (RIBEYE-FP), the main constituent protein of ribbons. All the excitatory postsynaptic sites in the inner retina were visualized using knock-in mice expressing FP-tagged postsynaptic density protein 95 (PSD95-FP). Alternatively, PSD95-FP was expressed in isolated RGCs by biolistic labeling. We performed live imaging ex vivo at postnatal day 10, when the rate of synaptogenesis between BCs and RGCs is high.
RIBEYE-FP puncta were highly dynamic in developing BC axon terminals with ~10% of total RIBEYE-FP puncta turning over every hour. Simultaneous imaging of RIBEYE-FP and PSD95-FP in the inner plexiform layer revealed that the RIBEYE-FP puncta that colocalized with PSD95-FP puncta survived longer than those that did not show colocalization. Likewise, in individual RGCs, PSD95-FP puncta that colocalized with RIBEYE-FP puncta were less likely to be eliminated compared to those that were not apposed to ribbons.
Our findings support the hypothesis that synaptic localization of ribbons stabilizes nascent synapses. However, because PSD95 puncta apposed to ribbons can still be eliminated, the presence of ribbons at synaptic sites does not always guarantee synaptic maintenance. Together, our results suggest that the developmental increase in synapses between BCs and RGCs arises from stabilization of the majority of newly formed ribbon-associated synapses.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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