The Plus End-Directed Microtubule (Kinesin-3 Family) Motor Protein KIF13B Is Associated with the Photoreceptor Synaptic Ribbon Complex

TL;DR

This study shows that the motor protein KIF13B is located at photoreceptor synaptic ribbons, suggesting it helps with vesicle transport in the retina.

Contribution

The novel finding is the localization of KIF13B at photoreceptor synaptic ribbons and its dependency on the synaptic ribbon structure.

Findings

KIF13B is strongly enriched at photoreceptor synaptic ribbons in addition to the primary cilium.

Synaptic ribbon deficiency in RIBEYE knockout mice reduces KIF13B synaptic enrichment.

KIF13B likely mediates vesicle trafficking at the synaptic ribbon-microtubule interface.

Abstract

Retinal ribbon synapses are continuously active chemical synapses. The eponymous synaptic ribbon is anchored to the active zone neurotransmitter release sites of ribbon synapses, recruits synaptic vesicles and guides ribbon-associated synaptic vesicles to the release sites. RIBEYE is the major protein component of synaptic ribbons. But likely, additional proteins contribute to ribbon synapse function. The synaptic ribbon of photoreceptor synapses is embedded into a highly polarized microtubule cytoskeleton. Interestingly, proteins of the photoreceptor primary cilium, such as NPHP4 and other ciliary proteins, including KIF3A, were shown to be localized to photoreceptor synaptic ribbons. Previous studies demonstrated that the microtubule motor protein KIF13B catalyzes secretory vesicle transport to the plus ends of microtubules and identified an interaction of KIF13B with NPHP4 at primary cilia. However, the localization of KIF13B, a kinesin-3 family motor protein, in the retina is still unknown. In the present study, we used two different antibodies against KIF13B and high-resolution confocal microscopy, super-resolution structured illumination microscopy (SR-SIM), and post-embedding immunogold electron microscopy to determine the localization of KIF13B in retinal photoreceptors. Apart from its localization at the primary photoreceptor cilium, we found a strong enrichment of KIF13B at photoreceptor synaptic ribbons. The synaptic ribbon is needed for the synaptic enrichment of KIF13B as shown by analyses of synaptic ribbon-deficient RIBEYE knockout mice. These findings suggest that KIF13B performs vesicle trafficking functions at the photoreceptor synaptic ribbon complex at the interface between the synaptic ribbon and the presynaptic microtubule transport system.

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