Ikaros Confers Early Temporal Competence to Mouse Retinal Progenitor Cells

Abstract

In the developing mouse retina, multipotent retinal progenitor cells (RPCs) give rise to specific retinal cell types at different times, but the molecular mechanisms regulating how RPCs change over time remain unclear. In the Drosophila neuroblast lineage, the zinc finger transcription factor Hunchback (Hb) is both necessary and sufficient to specify early-born neuronal identity. We show here that Ikaros, a mouse ortholog of Hb, is expressed in all early embryonic RPCs, which then give rise to Ikaros-negative RPCs at later stages in the lineage. Remarkably, misexpression of Ikaros in late RPCs is sufficient to confer competence to generate early-born neurons. Conversely, Ikaros mutant mice have reduced numbers of early-born cell types, whereas late-born cell types are not affected. These results suggest a model in which Ikaros expression is both necessary and sufficient to confer early temporal competence to RPCs and raise the possibility that a similar strategy might be used to control the sequential order of cell birth in other parts of the nervous system.

In vivo evidence for unbiased ikaros retinal lineages using an ikaros‐cre mouse line driving clonal recombination

“…In the eye, we consistently found that Ikaros is expressed ubiquitously (Fig. 2C, D), as we reported previously (Elliott et al, 2008). Consistent with widespread Ikaros expression in embryonic tissue, Cremediated recombination did not display any clear tissue-specificity (see Discussion section).…”

“…Overall, this expression pattern indicates that both lines A and F express Cre in early progenitor cells, although with different efficiencies. Line A is closer to the apparently ubiquitous Ikaros expression in RPCs at early stages (Elliott et al, 2008), whereas the low subset of Ikaros-positive RPCs achieving Cre-mediated deletion of the reporter in line F leads to sparse arrays of YFP progenies. These expression patterns match what was observed with the b-Gal reporter in these lines.…”

“…1E), we could not detect Venus protein expression despite repeated attempts using a variety of approaches. This could stem from the low expression levels of Ikaros mRNA in the developing retina (Elliott et al, 2008) (see Discussion section). Similarly, low expression from the transgene likely translates into inefficient and/or infrequent Cre-mediated deletion, and thus probably accounts for the small subset of Ikaros-positive cells labeled in some lines.…”

“…Recently, we identified the zinc finger transcription factor Ikaros as a regulator of temporal competence in RPCs (Elliott et al, 2008). Ikaros is expressed in early, but not late-stage, RPCs and mice lacking Ikaros generate fewer early-born cell types such as ganglion, horizontal, and amacrine cells, whereas they have normal numbers of late-born cell types.…”

“…To get preliminary insight into this question, we initially used retinal explant cultures to show that retrovirus-infected clones can bear both Ikaros-positive and Ikaros-negative RPCs (Elliott et al, 2008). Here, to further challenge this prediction in vivo, we generated BAC transgenic mouse lines that express Cre recombinase under the regulatory elements of the Ikaros locus, and crossed them to Cre reporter mouse lines.…”