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Joseph Laird, Vasily Kerov, Sarah Gardner, Josue Lopez, Mei-Ling Joiner, Amy Lee, Sheila A Baker; Organization of the photoreceptor presynaptic complex. Invest. Ophthalmol. Vis. Sci. 2016;57(12):4804.
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© ARVO (1962-2015); The Authors (2016-present)
Cav1.4 is the voltage-gated calcium channel localized beneath the synaptic ribbon in rods and cones. It mediates the inward flux of calcium current that triggers neurotransmitter release in the dark adapted photoreceptor. Loss of Cav1.4 function presents in a variety of disorders including congenital stationary night blindness, cone-rod dystrophy, and retinitis pigmentosa. The goal of this study was to investigate how Cav1.4 contributes to synaptogenesis and how it is tethered in the arciform density beneath the defining structural feature of rod synapses – the ribbon.
In vivo electroporation was used to transfect rods of Cav1.4 knockout mice with FLAG-Cav1.4 WT or its mutants. Conditional RIM1/2 knockout mice were electroporated with Cre. In adult animals, transfected photoreceptors were identified based on co-expression of fluorescent markers and characterized by immunostaining for synaptic proteins. To determine if the restored synapses function, animals were challenged in a vision-guided water maze.
Restoring expression of FLAG-Cav1.4 supported the development of morphologically normal ribbon synapses. These animals had improved performance in a visually-guided water maze. Two mutant channels were generated, FLAG-Cav1.4-3EQ which is incapable of conducting Ca2+, and FLAG-Cav1.4-G369insert, which fails to open in response to voltage. Ribbon development was supported by expression of either of these mutants. Fewer mature ribbons developed upon expression of either FLAG-Cav1.4-3EQ or FLAG-Cav1.4 G369I compared to the wildtype channel and accordingly these animals performed poorly in the water maze. This indicates that the physical presence of Cav1.4 is a triggering event for synaptogenesis leading us to investigate how Cav1.4 localizes to the synapse. A candidate is RIM1/2, another component of the arciform density that in conventional synapses contributes to the localization of Cav2.1 andCav2.2. Deleting RIM1/2 from rods resulted in a concomitant loss Cav1.4.
Expression of non-active Cav1.4 channels restores synaptic development in Cav1.4 KO rods revealing a novel aspect of Cav1.4 function: to scaffold assembly of the arciform density. RIM1/2 is a putative central organizer of synapses but its role in photoreceptors is controversial. Our data indicate that RIM is essential for expression of Cav1.4 in the synapse.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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