How to make a synaptic ribbon: RIBEYE deletion abolishes ribbons in retinal synapses and disrupts neurotransmitter release

Abstract

Synaptic ribbons are large proteinaceous scaffolds at the active zone of ribbon synapses that are specialized for rapid sustained synaptic vesicles exocytosis. A single ribbon-specific protein is known, RIBEYE, suggesting that ribbons may be constructed from RIBEYE protein. RIBEYE knockdown in zebrafish, however, only reduced but did not eliminate ribbons, indicating a more ancillary role. Here, we show in mice that full deletion of RIBEYE abolishes all presynaptic ribbons in retina synapses. Using paired recordings in acute retina slices, we demonstrate that deletion of RIBEYE severely impaired fast and sustained neurotransmitter release at bipolar neuron/AII amacrine cell synapses and rendered spontaneous miniature release sensitive to the slow Ca 2+-buffer EGTA, suggesting that synaptic ribbons mediate nano-domain coupling of Ca 2+ channels to synaptic vesicle exocytosis. Our results show that RIBEYE is essential for synaptic ribbons as such, and may organize presynaptic nano-domains that position release-ready synaptic vesicles adjacent to Ca 2+ channels.

Synaptic Ribbons Require Ribeye for Electron Density, Proper Synaptic Localization, and Recruitment of Calcium Channels

“…It is noteworthy that some of the features we report here differ from those found in a recent report characterizing retinal ribbons in Ribeye knockout mice (Maxeiner et al, 2016). In that study, the authors found ribbons completely lacking from the retina and a general decrease in vesicles residing near active zones.…”

“…Morpholino oligonucleotide (MO) driven knock-down of ribeye leads to loss of calcium channel clusters in both retina (Lv et al, 2012) and neuromast hair cells (Sheets et al, 2012), whereas animals overexpressing Ribeye exhibit ectopic calcium channel clusters that colocalize with Ribeye protein aggregates (Sheets et al, 2012). More recently, knockout of Ribeye in mouse was found to cause a subtle change in the size of calcium channel clusters in photoreceptors (Maxeiner et al, 2016). Notably, in contrast to the previous MO results in retina (Lv et al, 2012), calcium channels remained clustered in discrete locations in the retina.…”

“…Ribeye can be subdivided into two domains, a ribbon-specific A-domain and a B-domain, which is nearly identical to CtBP2 (Schmitz et al, 2000). Genetic deletion of mouse Ribeye leads to a loss of synaptic ribbons in the retina, which is accompanied by a reduction in synaptic transmission from bipolar cells without observable changes in the kinetic features of release (Maxeiner et al, 2016), whereas morpholino oligonucleotide driven knockdown of Ribeye expression leads to the mislocalization of calcium channels (Sheets et al, 2011, Lv et al, 2012) and a reduction of spiking rates in afferent neurons postsynaptic to zebrafish hair cells (Sheets et al, 2011). …”

Innervation regulates synaptic ribbons in lateral line mechanosensory hair cells

“…They are thought to store a ready-releasable pool of synaptic vesicles and coordinate synchronous vesiclerelease at the synapse (Khimich et al, 2005;Buran et al, 2010;Matthews and Fuchs, 2010;Maxeiner et al, 2016). Voltage gated Ltype Ca 2+ channels are found to drive exocytosis at ribbon synapses and are localized at the base of ribbons (Platzer et al, 2000;Brandt et al, 2003;Dou et al, 2004;Sidi et al, 2004;Sheets et al, 2012;Wong et al, 2014).…”

“…Zebrafish larvae in which ribeye b, a gene coding for a major scaffolding protein in synaptic ribbons (Schmitz et al, 2000;Zenisek et al, 2004;Wan et al, 2005;Magupalli et al, 2008), is knocked down show reduction in the number of spikes of afferent neurons of the lateral line mechanosensory hair cells when the hair cells are stimulated (Sheets et al, 2011). Similarly, knockout mice for the ribeye gene (also known as Ctbp2) show impaired neurotransmitter release at retinal bipolar cells (Maxeiner et al, 2016).…”