ISL1 is required for the terminal differentiation and maintenance of Starburst Amacrine Cells

During mammalian nervous system development, multipotent progenitor cells differentiate into hundreds of distinct neuronal subtypes. Neuronal subtype terminal differentiation is regulated by transcription factors that directly control the acquisition and maintenance of terminal identity. Using the mouse retina as a model system, we show that the transcription factor ISL1 is required for the terminal differentiation and maintenance of starburst amacrine cells (SACs), the sole cholinergic neuron subtype in the retina. We conditionally deleted Isl1 selectively from amacrine and horizontal cell precursors, allowing us to assess its cell-autonomous role in SAC development without disrupting its critical function in bipolar cell and retinal ganglion cell development. We find that ISL1 is not required for initial SAC specification but is required for their terminal differentiation. Isl1-deficient SACs retain a GABAergic amacrine cell identity but fail to adopt many SAC-specific features including expression of genes required for cholinergic transmission. In the absence of ISL1, SACs fail to migrate properly, with both OFF- and ON- SACs localizing to the inner nuclear layer. Deletion of Isl1 from post-migratory SACs at later stages of development revealed that it is also required for maintaining SAC identity and proper dendrite stratification. Loss of Isl1 from SACs ultimately caused abnormal development of the direction selective circuit. Together, these findings establish that ISL1 is required for the induction and maintenance of SAC-specific identity from a generic amacrine state.