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

Abstract

Background: We showed previously that the transcription factor Ikaros is expressed in early but not late retinal progenitors cells (RPCs), and is necessary and sufficient for the production of early-born neurons. Preliminary evidence using retinal explant cultures qualitatively suggested that Ikaros-positive RPCs might share a common lineage with Ikaros-negative RPCs. Results: To explore further this question in vivo in a quantitative manner, we generated BAC transgenic mouse lines expressing Cre recombinase under the regulatory elements of the Ikaros gene, and crossed them with Cre reporter lines. Different transgenic lines labeled a variable number of RPCs, resulting in either dense or sparse radial arrays of reporter-positive progenies. Analysis of over 800 isolated cell arrays, which are most likely clones, confirmed that Ikaros-expressing RPCs generate both early-and late-born cell types in the same lineage, and that the overall cell composition of the arrays closely resembles that of the population of the mature retina. Interestingly, another sparse line did not label arrays, but appeared to specifically reflect Ikaros postmitotic expression in amacrine and ganglion cells. Key words: lineage; competence; temporal identity; cell fate; cell specification; development Key FindingsThe Ikaros retinal lineage gives rise to both early-and late-born cell types and is confirmed unbiased in vivo One Ikaros-Cre mouse line allows sparse recombination in retinal progenitors and is validated as a tool for in vivo lineage tracing A distinct Ikaros-Cre mouse line allows sparse recombination specifically in some postmitotic early-born retinal cell types

Casz1 controls higher-order nuclear organization in rod photoreceptors

“…CD1 mice were obtained from Taconic. Casz1 Flox/Flox ( 21 ), α–Pax6::Cre ( 46 ), LMOP::Cre ( 72 ), IKCre-A ( 78 ), and R26-Stop-EYFP ( 79 ) alleles and genotyping protocols were previously described. (See also SI Appendix, Methods and Table S1 .)…”

Reconciling competence and transcriptional hierarchies with stochasticity in retinal lineages

“…For example, in the mouse, RPCs pass through an early stage in which they express Ikaros, the vertebrate orthologue of Hunchback. Ikaros, when overexpressed, biases the production of early fates, while Ikaros mutant mice have reduced numbers of early-born cell types [ 36 • ,37 ]. One cannot rule out the possibility that an entire sequence of competence factors homologous to those found in Drosophila neuorblasts will be found in the vertebrate retina, but at present there is scant evidence for this.…”

Ikaros promotes early-born neuronal fates in the cerebral cortex

“…If Ikaros is a temporal factor specifying early competence in cortical progenitor cells, the prediction would be that Ikaros-expressing progenitor cells are multipotent and give rise to neurons of all laminar types. To test this hypothesis and to study the neuronal output from Ikaros-expressing progenitor cells, we took advantage of a BAC transgenic mouse line expressing Cre recombinase under the control of the Ikaros promoter (37). On crossing this line to the R26-YFP reporter line (38), Ikaros-Cre-dependent YFP expression was observed in the cortex.…”