Lineage and Functional Analyses of Specific Subsets of Retinal Progenitor Cells

The vertebrate central nervous system (CNS) is made up of a diverse array of cell types. The retina, an accessible part of the CNS with seven major cell types and more than sixty cell subtypes, is an excellent model system to study cell fate determination in the CNS. Previous retroviral lineage tracing experiments have demonstrated that retinal progenitor cells (RPCs) are multipotent, and give rise to clones of variable sizes and cell type compositions. Two models are proposed to explain this variability in clone size and composition: One model states that there are different subsets of RPCs, whose distinctive molecular profiles determine their daughter cell fates, while the other model argues that RPCs are equipotent and daughter cell types are determined by environmental cues and/or stochastic cellular processes. To test the hypothesis on distinct RPCs, we first asked whether we could identify specific RPC subsets that would produce specific daughter cell types. We made use of 10 molecular markers to mark specific RPC subsets, and traced individual subsets' daughter cell fates with Cre-recombinase fate mapping and retroviral lineage tracing. A novel RPC subset, which express the basic helix-loop-helix (bHLH) transcription factor Ngn2, were found to be heavily biased towards generating rod photoreceptors and amacrine cells in terminal divisions in the postnatal mouse retina. Next, we partitioned the postnatal RPC pool into different RPC subsets with molecular marker Ngn2 and Olig2, and probed individual subsets' responsiveness to misexpression of two sets of transcription factors, which are known to play important roles in retinal cell fate determination. We have shown that different RPC subsets respond differently to the same genetic perturbation, which is indicative of their distinctive intrinsic capabilities to generate certain daughter cell types. Together, we have shown that in the mouse retina, the RPC pool is composed of distinct RPC subsets, each of which have unique molecular profiles, give rise to specific daughter cell types, and respond differently to perturbations. This study provides new insight into cell fate determination in the retina, and may shed light on a more general mechanism of cell fate determination in a variety of systems.