The structural and functional integrity of rod photoreceptor ribbon synapses depends on redundant actions of dynamins 1 and 3

Abstract

Vertebrate vision begins with light absorption by rod and cone photoreceptors, which transmit signals from their synaptic terminals to second-order neurons: bipolar and horizontal cells. In mouse rods, there is a single presynaptic ribbon-type active zone at which release of glutamate occurs tonically in the dark. This tonic glutamatergic signaling requires continuous exo- and endocytosis of synaptic vesicles. At conventional synapses, endocytosis commonly requires dynamins: GTPases encoded by three genes (Dnm1-3), which perform membrane scission. Disrupting endocytosis by dynamin deletions impairs transmission at conventional synapses, but the impact of disrupting endocytosis and the role(s) of specific dynamin isoforms at rod ribbon synapses are understood incompletely. Here, we used cell-specific knockouts of the neuron-specific Dnm1 and Dnm3 to investigate the functional roles of dynamin isoforms in rod photoreceptors in mice of either sex. Analysis of synaptic protein expression, synapse ultrastructure, and retinal function via electroretinograms showed that dynamins 1 and 3 act redundantly and are essential for supporting the structural and functional integrity of rod ribbon synapses. Single Dnm3 knockout showed no phenotype, and single Dnm1 knockout only modestly reduced synaptic vesicle density without affecting vesicle size and overall synapse integrity; whereas, double Dnm1/Dnm3 knockout impaired vesicle endocytosis profoundly, causing enlarged vesicles, reduced vesicle density, reduced ERG responses, synaptic terminal degeneration, and disassembly and degeneration of postsynaptic processes. Concurrently, cone function remained intact. These results show the fundamental redundancy of dynamins 1 and 3 in regulating the structure and function of rod ribbon synapses.Significance Statement The process of vision starts with the capturing of light by rod and cone photoreceptors within the retina. Photoreceptors communicate with downstream retinal neurons at specialized sites called ribbon synapses, where vesicles of the neurotransmitter glutamate are released and recycled. The synaptic vesicle recycling process at conventional synapses commonly requires specialized proteins for membrane scission, typically dynamins 1 and 3. The role of dynamins in vesicle cycling at photoreceptor ribbon synapses, however, is not fully understood. Here, we specifically deleted dynamins 1 and 3 in rod photoreceptors using a conditional gene knockout approach and demonstrated the redundant role of dynamins 1 and 3 in maintaining rod photoreceptor ribbon synapse structure and function.