Evolutionary conservation of amacrine cell types in the vertebrate retina

PROJECT SUMMARY Neural circuits derive most of their computational power from inhibitory interneurons, which balance excitation, enhance contrast, and provide temporal-spatial precision. In the retina, amacrine cells (ACs) are the largest class of inhibitory interneurons, providing critical inhibition and modulation to bipolar cells and ganglion cells through their release of GABA or glycine. ACs are incredibly heterogeneous, comprising as many as 60 types that perform distinct computations and expand the repertoire of visual features relayed to the brain. However, over 80% of these AC types have not been functionally characterized. This is largely due to a lack of molecular markers to genetically target these types for morphological and physiological experiments in animal models. Furthermore, since AC types are also morphologically complex, they often lack clear orthologs in other species, making it difficult to identify their conserved biological function. The overarching objective of this proposal is to map AC types across species and identify conserved molecular markers with which to target these types for functional experiments. Recent work in our lab showed that bipolar and ganglion cell types in the retina are highly conserved across mammals. Although this study omitted AC types due to their complexity, our preliminary computational analysis suggests that AC types can be molecularly aligned between primates and rodents. Consistent with these findings, more recent experimental and genomic studies revealed that the direction- selective and scotopic circuits – key AC-driven circuits – are present in fish. Based on these observations, we hypothesize that AC diversity is evolutionarily ancient and that AC types are broadly conserved across vertebrates. To test this hypothesis, we will use single-cell RNA-sequencing atlases of ACs recently generated in our lab to guide a series of computational and experimental studies. Aim 1 will investigate the molecular conservation of ACs across 20 vertebrates by aligning single-cell atlases and building upon existing comparative transcriptomic methods. Aim 2 will identify novel, conserved molecular markers of AC types and label them in primates, rodents, and chicken using histology. Experiments in Aim 3 will investigate the circuitry and function of a highly conserved, novel AC type marked by PDGFRA. Together, these data will describe the molecular and structural conservation level of ACs and reveal new aspects about their cellular properties. Importantly, we will identify and test novel molecular markers of specific AC types that will be useful for targeting them in future studies. This proposed work has broad implications for uncovering contributions of inhibitory interneurons to retinal processing.