Abstract
Purpose :
The rod’s single ribbon facilitates a high rate of exocytosis, which is complemented by endocytosis. Endocytosis commonly requires the scission protein dynamin, encoded by three genes (Dnm1-3). Dnm1 and Dnm3 are neuron-specific and can act redundantly; Dnm2 is ubiquitous and mediates a house-keeping role. Here, we evaluated the impact of deleting dynamins on rod structure and function.
Methods :
Dnm1- and Dnm2-floxed mice were bred with transgenic iCre75 mice to generate rod-specific knockouts (KOs). These mice were crossed with germline Dnm3-/- to generate double and triple KOs. ERG recordings tested photoreceptor function. Photoreceptor structure was evaluated by immunohistochemistry and confocal microscopy and ultrastructure by electron microscopy.
Results :
The KO of Dnm1-3 from rods was confirmed by immunostaining of retinal cross-sections with specific antibodies at P28. ERG recordings from single KO mice were intact, consistent with normal photoreceptor morphology, indicating redundant function of dynamins 1, 2 and 3 in rods. Combined deletion of Dnm1/3 in rods impaired synaptic transmission to bipolar cells and caused progressive degeneration of rod synapses. Formation of enlarged endosome-like vesicles and reduction in synaptic proteins (e.g., RIBEYE and VGlut1) indicated impaired vesicle cycling at the rod terminal, which depends redundantly on Dnm1 and Dnm3. Combined deletion of Dnm2/3 had no effect on function and structure of rods, indicating that Dnm1 can perform both synapse-specific and housekeeping functions. Triple KO of all dynamins lead to fast degeneration of rods, as well as cones at P28. Scotopic and photopic ERG responses were absent and one layer of the outer nuclear layer remained. Rods expressing one copy of Dnm2 degenerated at a slower rate than the triple KO; at P28, mice showed a 50% reduction in ERG responses, consistent with a reduction in the outer nuclear layer, and degenerating outer segments and synaptic terminals.
Conclusions :
Rod structure and function were normal in single KOs. Combined Dnm1/3 KO from rods caused functional impairment and degeneration of the synapses. Combined Dnm2/3 KO had no effect on rods. However, eliminating either one or two copies of Dnm2 in the Dnm1/3 double KO in rods caused either slow or fast photoreceptor degeneration affecting both rods and cones. We conclude that rod synapse function depends on either dynamin 1 or 3 and rod survival depends on either dynamin 1 or 2.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.