A molecular phenotype atlas of the zebrafish retina

The rasborine cyprinid Danio rerio (the zebrafish) has become a popular model of retinal function and development. Its value depends, in part, on validation of homologies with retinal cell populations of cyprinine cyprinids. This atlas provides raw and interpreted molecular phenotype data derived from computationally classified sets of small molecule signals from different cell types in the zebrafish retina: L-alanine, L-aspartate, L-glutamine, L-glutamate, glutathione, glycine, taurine and gamma-aminobutyrate. This basis set yields an 8-dimensional signature for every retinal cell and formally establishes molecular signature homologies with retinal neurons, glia, epithelia and endothelia of other cyprinids. Zebrafish photoreceptor classes have been characterized previously: we now show their metabolic profiles to be identical to those of the corresponding photoreceptors in goldfish. The inner nuclear layer is partitioned into precise horizontal, bipolar and amacrine cell layers. The horizontal cell layer contains at least three and perhaps all four known classes of cyprinine horizontal cells. Homologues of cyprinid glutamatergic ON-center and OFF-center mixed rod-cone bipolar cells are present and it appears likely that all five classes are present in zebrafish. The cone bipolar cells defy simple analysis but comprise the largest fraction of bipolar cells, as in all cyprinids. Signature analysis reveals six molecular phenotypes in the bipolar cell cohort: most are superclasses. The amacrine cell layer is composed of approximately equal 64% GABA+ and 35% glycine+ amacrine cells, with the remainder being sparse dopaminergic interplexiform cells and other rare unidentified neurons. These different amacrine cell types are completely distinct in the dark adapted retina, but light adapted retinas display weak leakage of GABA signals into many glycinergic amacrine cells, suggesting widespread heterocellular coupling. The composition of the zebrafish ganglion cell layer is metabolically indistinguishable from that in other cyprinids, and the signatures of glial and non-neuronal cells display strong homologies with those in mammals. As in most vertebrates, zebrafish Müller cells possess a high glutamine, low glutamate signature and contain the dominant pool of glutathione in the neural retina. The retinal pigmented epithelium shows a general mammalian signature but also has exceptional glutathione content (5-10 mM), perhaps required by the unusually high oxygen tensions of teleost retinas. The optic nerve and the marginal zone of the retina reveal characteristic metabolic specializations. The marginal zone is strongly laminated and its nascent neurons display their characteristic signatures before taking their place in the retina proper.