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M. Kamermans, C. R. Shields, I. Fahrenfort, Y. Claassen, L. J. Klaassen, G. Zoidl, J. Klooster; Feedback From Horizontal Cells to Cones Is Reduced in Cx55.5-/- Zebrafish. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1041.
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© ARVO (1962-2015); The Authors (2016-present)
Horizontal cells feed back negatively to cones. The mechanism involved remains an issue of intense debate. According to one hypothesis, hemichannels in the tips of the horizontal cell dendrites formed by Cx55.5 play a fundamental role in this feedback pathway. Morphological and pharmacological studies have generated support for this hypothesis. In the present study, we aimed to test this hypothesis in an independent manner and examined the properties of feedback in zebrafish lacking Cx55.5 protein.
The "tilling" technique was used to generate a zebrafish with an early stop codon in the first extracellular loop of Cx55.5. This protein can not form functional channels, rendering this mutant a functional knockout (Cx55.5-/-). Morphological and QPCR techniques were used to determine changes in connexin expression at both the mRNA and protein levels in the Cx55.5-/- line. Whole cell patch-clamp techniques were used to study feedback responses in cones.
In Cx55.5-/- zebrafish, Cx55.5 mRNA was downregulated and the protein was absent. Differing results were observed for Cx52.6, Cx52.7, and Cx52.9, the other connexins expressed by horizontal cells (Abstract Klooster et al.). Cx52.6 mRNA was downregulated and the protein was absent, whereas the mRNA and protein for Cx52.7 and Cx52.9 remained present. Feedback responses measured in cones were reduced in the Cx55.5-/- zebrafish compared to WT. The feedback responses induced by hyperpolarization of horizontal cells were absent, whereas those induced by depolarization of horizontal cells were fully intact. These results can be accounted for by differences in the IV relations of the hemichannel currents measured in dissociated horizontal cells from WT and Cx55.5-/- fish (Abstract Sun et al.).
Feedback from horizontal cells to cones depends strongly on Cx55.5. Removing Cx55.5 from horizontal cells results in a severe and very specific inhibition of feedback. The difference in feedback properties is in accordance with the change in the hemichannel IV relation measured in dissociated horizontal cells.
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