Temporal identity factors regulate the competence of neural progenitors to generate specific cell types in a time-dependent manner, but how they operate remains poorly defined. In the developing mouse retina, the Ikaros zinc finger transcription factor Ikzf1 regulates the production of early-born cell types, except cone photoreceptors. In this study we show that Ikzf4, another Ikaros family protein, cooperates with Ikzf1 to control cone photoreceptor production during early stages of retinal development, whereas at late stages, when Ikzf1 is no longer expressed in progenitors, Ikzf4 is instead required for Muller glia production. Using CUT&RUN sequencing, we find that both Ikzf1 and Ikzf4 generally bind to the same genes involved in cone development and other early-born fates, but at different cis-regulatory elements. In late-stage progenitors, Ikzf4 re-localizes to bind target genes involved in Muller glia development and regulate their expression. Specifically, we show that Ikzf4 maintains Hes1 expression in differentiating cells using two Ikzf GGAA binding sites at the Hes1 promoter, thereby favouring Muller glia fate commitment. These results uncover a combinatorial role for Ikaros family members in nervous system development and provide mechanistic insights on how they temporally regulate cell fate output.
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