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Aaron J. Mercer, Wallace B. Thoreson; Cholesterol Regulates Calcium Channel Mobility and Vesicle Release at Cone Photoreceptor Synapses. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4803.
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© ARVO (1962-2015); The Authors (2016-present)
Voltage-gated L-type Ca2+ (CaV) channels are positioned near vesicle release sites to maintain efficient transmission at CNS synapses, including cone synapses. Findings in our laboratory indicate that CaV channels at cone terminals show confined mobility. In the present study, we characterized the role of membrane cholesterol on CaV channel mobility and neurotransmission at cone synapses.
Experiments were performed using Ambystoma tigrinum retina slices. To examine CaV channel mobility, channels were imaged after conjugating quantum dots to extracellular CaV α2Δ4 subunits. Effects of cholesterol depletion on synaptic release were studied by obtaining paired whole cell recordings from cones and OFF bipolar or horizontal cells.
By staining retinal slices with FITC-cholera toxin B, we found evidence for cholesterol-rich lipid rafts in the OPL, and showed that cholesterol could be depleted from the OPL using methyl-β-cyclodextrin (MβCD) or cholesterol oxidase (COase). Cholesterol depletion significantly expanded the confinement area of CaV channels (control, N=36; MβCD N=17, COase, N=16; P<0.001). If CaV channels move farther from vesicle release sites after cholesterol depletion, coupling between Ca2+ influx and exocytosis may be impaired. We found that the time to peak of post-synaptic currents (PSCs) evoked in horizontal or bipolar cells by depolarizing steps (-70 to -10 mV) applied to cones was significantly delayed by cholesterol depletion (PSC peak vs. cone ICa peak: control = -6.49 + 1.4 ms, N=9; MβCD: +2.19 + 0.9 ms, N=9; P<0.001). Currents evoked by flash photolysis of MNI-glutamate were not appreciably altered by cholesterol depletion, suggesting that PSC changes did not result from changes in post-synaptic glutamate receptor properties.
Our findings indicate that cholesterol helps to confine CaV channels near release sites. Depleting cholesterol from the cone synapse allows channels to move farther from release sites, which reduces Ca2+/exocytosis coupling efficiency and slows release. These findings highlight the dynamic nature of cone synapses, and suggest novel mechanisms by which alterations in lipid metabolism might result in retinal dysfunction.
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