Multiple Potentials of Retinal Progenitors Transiently Expressing Neurogenin2

Abstract

Abstract: : Purpose: To investigate whether retinal cell fate specification employs bHLH gene neurogenin2 (ngn2) by determining whether retinal cells transiently expressing ngn2 will develop into a particular type of cells or many different types of cells. Methods: We examined ngn2–CreER^TM mice, which have the ngn2 coding sequence replaced with Cre recombinase linked to an estrogen receptor inducible with 4–hydroxy tamoxifen (Txf). Using the conditional, binary CreER^TM – LacZ fate mapping system, we traced the final fates of cells that transiently expressed ngn2 during a window of a few hours in the developing retina when administered Txf was available to the retinal cells. Results: We identified and scored the number of LacZ⁺ cells in P13–P17 retinas with Txf administered at different time points, from E10 to P10. No LacZ⁺ cells were detected when Txf was administered at E10, likely because few cells expressed ngn2 at this time as shown with in situ hybridization. Overall, LacZ⁺ cells were detected in all three nuclear layers of the retina and included all major types of retinal cells, including Müller glia. In addition, the overall distribution of LacZ⁺ cells among the three nuclear layers remarkably resembled the overall representation of retinal cells by the three layers. The temporal window in which a particular cell type was marked during development appeared nonrandom, but was similar to its birthdate, and the order of LacZ labeling of major cell types closely resembled their birth sequence. Furthermore, earlier administration of Txf did not result in more LacZ⁺ cells. Simultaneous administration of Txf and BrdU resulted in LacZ⁺/BrdU⁺ cells, indicating that ngn2 was expressed in proliferating cells. Conclusions: Our data suggest that transient ngn2 expression likely takes place as progenitors approach the end of their proliferation activity, and such expression has little impact on retinal cell type specification. We propose that ngn2 is expressed in a step prior to terminal mitosis and is involved in the development of multipotent progenitors whose fates are subsequently specified by other factors.

Keywords: retinal development • retinal degenerations: cell biology • gene/expression