Characterization of eomesa+ retinal ganglion cells in zebrafish

Abstract

Retinal ganglion cells (RGCs) play a bottleneck role in visual processing, as they are the only output neurons of the eye that allow the flow of all the visual information to the visual centers in the brain. In Kölsch et al. (2021), using zebrafish as the model organism, it was determined by computational analysis of transcriptional profiles that there are around 30 different RGC types in zebrafish. Strikingly, the set of RGCs in larvae differs strongly from that in adults. To see how RGCs develop into different suptypes and how the set of subtypes changes over time, this project focused on the differentiation of one early transcriptomic subcluster, LC13, from Kölsch et al. LC13 expresses both the transcription factors eomesa and onecut1, but differentiates into larval and adult clusters that express either one or both of the transcription factors. To determine a suitable developmental model for LC13, this project aimed to match transcriptomic RGC types to their morphology in a temporal fashion. To achieve this, the morphology of eomesa+ RGCs in the larval and juvenile stages were catalogued by sparse labelling and imaging. This study identified 12 different morphotypes of eomesa+ RGCs in six dpf old zebrafish with seven different types based on dendritic morphology only. In comparison, only five different dendritic morphotypes of eomesa+ RGCs were found in juvenile stages suggesting that two types are removed over the course of development. Additionally, the project aimed to understand the role of LC13 related RGCs in visually guided responses, with phototaxis as an example. Zebrafish adults are photophobic while larvae show positive phototaxis. Further, selective ablation of eomesa+ RGCs in zebrafish larvae led to impaired phototaxis (Kölsch et al. (2021)). Combining these two observations, we hypothesized that the change in the transcriptomic and morphological types of LC13 related RGCs are relevant for the phototactic switch in zebrafish. Chemogenetic ablation of different LC13 related RGCs, followed by phototaxis assay revealed that both onecut1+ and eomesa+ RGCs are relevant for phototactic responses in seven dpf old zebrafish.