An analysis of amacrine cell diversity and developement

A major obstacle for understanding the central nervous system (CNS) is that its fundamental units, the cells of the nervous system, are still poorly enumerated and characterized. The vertebrate retina is an excellent model system for approaching the diversity of cells within the CNS because it is composed of only 7 major cell classes, yet these classes still represent an impressive range of form and function. Within a single cell class there can be a multitude of distinct cell types. Amacrine cells comprise the most diverse class of retinal neurons, with over 28 morphologically distinct cell types. Despite the extensive morphological characterization of amacrine cells, their molecular diversity and how this diversity arises during development is poorly understood. To address these problems we performed microarray-based expression profiling of single amacrine cells. From this analysis we established a classification of amacrine cells according to combinatorial gene expression. Furthermore, we identified specific cohorts of genes that are expressed in individual amacrine cell types during development. This analysis also revealed a mechanism for how amacrine cell diversity arises during development. Developmentally, GABAeric amacrine cells are identifiable prior to glycinergic amacrine cells, suggesting that they may differentiate prior to glycinergic amacrine cells. We confirmed and extended this observation by determining that the window of GABAergic amacrine cell birth is distinct and initiates prior to the glycinergic amacrine cell birth window. Lastly, we examined the role of a class of guidance cues in the morphological development of amacrine cells. We found that the cell surface receptors Robo1 and Robo3 and their ligand Slit2 are expressed in amacrine cells during development. Robo receptors are expressed in the cholinergic amacrine cell type, coincident with the formation of the inner plexiform layer. Constitutive activation of the Robo1 leads to cell death specifically in amacrine cells, but not in other cell classes. These results suggest a distinct role for Slit-Robo signaling in the development of cholinergic retinal amacrine cells. Our experiments help to define amacrine cell type diversity and how this diversity arises during retinal development.