Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - ColourFish (Zebrafish colour vision: a functional approach to studying outer retinal wiring strategies)

Teaser

Vertebrate colour vision starts with the use of spectrally distinct cone-photoreceptors. Loss of one or more cone types leads to partial or complete colour blindness. To cure colour blindness, gene and stem cell therapy methods have been developed as a means to introduce or...

Summary

Vertebrate colour vision starts with the use of spectrally distinct cone-photoreceptors. Loss of one or more cone types leads to partial or complete colour blindness. To cure colour blindness, gene and stem cell therapy methods have been developed as a means to introduce or replace the lost cone types. The next essential step towards clinical application of these methods is to understand how the downstream retinal circuits of cone-photoreceptors process chromatic information. In this project, we studied the chromatic information processing at the first synaptic layer of the visual system. Using highly visual model animal with robust colour vision, zebrafish, and by harnessing it’s genetic accessibility and transparent larvae, we achieved to record cone functions in live animal for the first time. With this tool, we studied how cone signals are combined and modulated to compute contrasts in wavelength in the retina’s outer plexiform layer. Further, we investigated how loss of one cone type impact the chromatic information processing in the retina.

Work performed

First, to lay the basis of what cone inputs bipolar cells receive and how it is formed during development, we examined the distribution patterns of cones across the eye and compared these with bipolar cell connectivity patterns with cones. We unexpectedly found that there is a series of striking anisotropies across photoreceptor types with retinal position. This specialisation aligns well with the major spectral trends in their visual world and their behavioral demands. We observed that these anisotropies are already formed during cone photoreceptor genesis but becomes more prominent as the eye matures.
Next, we used two-photon functional imaging of light-driven synaptic release from cones as well as both dendritic and axonal imaging in bipolar cells to study how chromatically distinct signals are functionally integrated. Recordings of more than 6000 bipolar cell terminals revealed two major axes of functional organisation of the inner retina. First, different chromatic and achromatic channels are separated in distinct sublaminar of the inner plexiform layer (IPL). Achromatic channels are mostly localised at the upper layer of ON or OFF bands, whereas chromatic channels are distributed throughout the IPL. The other axis of the organisation is the regional specialisation. Different regions of the retina see different parts in the visual field. We found that the balance of colour preferences among BC populations varies with the position in the eye and this variation matches the colour distribution in the natural scene in zebrafish natural habitat. The retinal region that looks downwards toward the ground composes predominantly red/green encoding chromatic channels – in line with the colour of the ground. In contrast, the temporal-ventral region is dominated by UV channels, likely to support detection of prey, UV-bright micro-organisms. The region looking towards the horizon is the most balanced, consisting of achromatic and chromatic channels equally. We further explored if this anisotropic functional organisation is reflected in the anatomical organisation and molecular expression by immunolabeling BC axon terminals and the molecular marker. We found that larval zebrafish retina employs two design strategies which underly the set of observed BC functional anisotropies; first, BC types with specific function exist in only certain parts of the eye, and second genetically defined BC types exist all across the retina, but more large-scale outer- and/or inner-retinal circuits can “override” this genetically defined functional organisation. Taken together, we discovered that retinal circuits are structurally and functionally fine-tuned depends on the parts of the retina match animals specific requirement for their visual tasks. These results were published as an article Current Biology (2018) 28: 1-15.
Finally, to investigate how disruption of cone compositions in the retina impacts the way colour information is processed in the retinal circuits, we generated transgenic animals where specific type of cones can be ablated. We are currently measuring cone spectrum tunings after ablating any one type of cones.

Final results

Zebrafish are the perhaps the best genetically accessible vertebrate model animals that have robust colour vision. We have established genetic tools for the identification and manipulation of their outer retinal circuits as well as for functional recording. These enabled us to study the retinal function in live animal for the first time. Further, our experiments of disrupting cone photoreceptor compositions provide insights into what happens to the retinal functions in disease conditions where one or more cone types are lost. For example, several conditions result from a complete lack of S-cones in the human retina, and using zebrafish we were able to study how chromatic circuits can rearrange in this type of situation. and our findings will be a critical basis for future studies towards the treatment of retinal disease in human.

Website & more info

More info: https://badenlab.org.