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Abstract
Rod photoreceptor synapses use large, ribbon-type active zones for continuous synaptic transmission during light and dark. Since ribbons are physically connected to the active zones, we asked whether illumination-dependent changes of ribbons influence Cav1.4/RIM2 protein clusters at the active zone and whether these illumination-dependent effects at the active zone require the presence of the synaptic ribbon. We found that synaptic ribbon length and the length of presynaptic Cav1.4/RIM2 clusters are tightly correlated. Dark-adaptation did not change the number of ribbons and active zone puncta. However, mean ribbon length and length of presynaptic Cav1.4/RIM2 clusters increased significantly during dark-adaptation when tonic exocytosis is highest. In the present study, we identified by the analyses of synaptic ribbon-deficient RIBEYE knockout mice that synaptic ribbons are (1) needed to stabilize Cav1.4/RIM2 at rod photoreceptor active zones and (2) are required for the darkness-induced active zone enrichment of Cav1.4/RIM2. These data propose a role of the ribbon in active zone stabilization and suggest a homeostatic function of the ribbon in illumination-dependent active zone remodeling.
Highlights
Rod photoreceptor synapses use large, ribbon-type active zones for continuous synaptic transmission during light and dark
In the Cav1.4 knockout retina, RIBEYE immunosignals were still present in the inner plexiform layer (IPL) and -in an altered distribution and abundance- in the outer plexiform layer (OPL) (Fig. 1D,F)
We identified illumination-dependent remodeling of the active zone of photoreceptor ribbon synapses in the outer plexiform layer (OPL) of the retina that is dependent upon the presence of the synaptic ribbon
Summary
Rod photoreceptor synapses use large, ribbon-type active zones for continuous synaptic transmission during light and dark. We identified by the analyses of synaptic ribbon-deficient RIBEYE knockout mice that synaptic ribbons are (1) needed to stabilize Cav1.4/RIM2 at rod photoreceptor active zones and (2) are required for the darkness-induced active zone enrichment of Cav1.4/RIM2. We asked whether alterations in the length of the active zone-associated portion of the ribbon influence Cav1.4/RIM2 protein clusters at the active zone. In order to determine whether the synaptic ribbon itself is directly responsible for the illumination-dependent control of active zone protein clustering, we analyzed photoreceptor synapses from RIBEYE knockout (KO) mice. In these KO mice, ribbons are absent while generally other presynaptic structures appear unaffected[15]. Our data propose that the synaptic ribbon is involved in the illumination-dependent remodeling of the active zones most likely by controlling the delivery and removal of active zone material
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