Ascl1 expression defines a subpopulation of lineage-restricted progenitors in the mammalian retina

Abstract

The mechanisms of cell fate diversification in the retina are not fully understood. The seven principal cell types of the neural retina derive from a population of multipotent progenitors during development. These progenitors give rise to multiple cell types concurrently, suggesting that progenitors are a heterogeneous population. It is thought that differences in progenitor gene expression are responsible for differences in progenitor competence (i.e. potential) and, subsequently, fate diversification. To elucidate further the mechanisms of fate diversification, we assayed the expression of three transcription factors made by retinal progenitors: Ascl1 (Mash1), Ngn2 (Neurog2) and Olig2. We observed that progenitors were heterogeneous, expressing every possible combination of these transcription factors. To determine whether this progenitor heterogeneity correlated with different cell fate outcomes, we conducted Ascl1- and Ngn2-inducible expression fate mapping using the CreER™/LoxP system. We found that these two factors gave rise to markedly different distributions of cells. The Ngn2 lineage comprised all cell types, but retinal ganglion cells (RGCs) were exceedingly rare in the Ascl1 lineage. We next determined whether Ascl1 prevented RGC development. Ascl1-null mice had normal numbers of RGCs and, interestingly, we observed that a subset of Ascl1+ cells could give rise to cells expressing Math5 (Atoh7), a transcription factor required for RGC competence. Our results link progenitor heterogeneity to different fate outcomes. We show that Ascl1 expression defines a competence-restricted progenitor lineage in the retina, providing a new mechanism to explain fate diversification.

Conserved microRNA pathway regulates developmental timing of retinal neurogenesis

“…Of these, 200 genes were increased by more than 1.5-fold in the Dicer-CKO retinal cells (Table S1) whereas 382 genes were down-regulated by a similar amount. Many of the genes that were down-regulated in the Dicer CKO cells (Table S2) have been previously shown to be expressed in the late progenitors, e.g., Ascl1, Dll3, Hes6 and Sox9 (3,14,19) further confirming that loss of Dicer blocked the developmental progression of the progenitor cells.…”

“…Lin28b is a known target of miR-125 (26) and let-7 (27), and the increase in expression of LP-miRNAs is consistent with the observed decline in Lin28b. By contrast, Ascl1 is expressed in progenitor cells only after ganglion cell production is complete (3,28) and is strongly up-regulated over these same ages. We confirmed that Prtg and Lin28b are expressed in early progenitors by immunostaining (Fig.…”

“…Generation and genotyping of Dicer-CKO and Nrl-GFP mice have been described previously (9,15,54). Immunofluorescence labeling and antibodies used in this study have been described previously (3,9) and are described more fully in SI Materials and Methods. Western blotting was carried out with standard protocols.…”

“…S8 A and D, arrows). The retina develops in a central-to-peripheral pattern, and E14 defines the initial onset of Ascl1 in a subset of late progenitors in the central retina (3). By E14, Prtg and Lin28b were down-regulated in the central retina but still highly expressed in peripheral regions (Fig.…”

“…One of the first steps in this developmental progression is the shift from progenitors that generate retinal ganglion cells to those that do not. In the mouse, this transition occurs with the onset of Ascl1; the earliest retinal progenitors do not express Ascl1, and, with its onset, the progenitors no longer produce ganglion cells (3). The other early-generated cell types, the cones and horizontal cells, are included in the Ascl1 + progenitor lineage, but are overrepresented in clones from Olig2 + progenitors (4).…”