Characterization of a transient TCF/LEF-responsive progenitor population in the embryonic mouse retina

PURPOSE. High mobility group (HMG) transcription factors of the T-cell-specific transcription factor/lymphoid enhancer binding factor (TCF/LEF) family are a class of intrinsic regulators that are dynamically expressed in the embryonic mouse retina. Activation of TCF/LEFs is a hallmark of the Wnt/β-catenin pathway; however, the requirement for Wnt/β-catenin and noncanonical Wnt signaling during mammalian retinal development remains unclear. The goal of the study was to characterize more fully a TCF/LEF-responsive retinal progenitor population in the mouse embryo and to correlate this with Wnt/β-catenin signaling. METHODS. TCF/LEF activation was analyzed in the TOPgal (TCF optimal promoter) reporter mouse at embryonic ages and compared to Axin2 mRNA expression, an endogenous readout of Wnt/β-catenin signaling. Reporter expression was also examined in embryos with a retina-specific deletion of the β-catenin gene (Ctnnb1), using Six3-Cre transgenic mice. Finally, the extent to which TOPgal cells coexpress cell cycle proteins, basic helix-loop-helix (bHLH) transcription factors, and other retinal cell markers was tested by double immunohistochemistry. RESULTS. TOPgal reporter activation occurred transiently in a subpopulation of embryonic retinal progenitor cells. Axin2 was not expressed in the central retina, and TOPgal reporter expression persisted in the absence of β-catenin. Although a proportion of TOPgal-labeled cells were proliferative, most coexpressed the cyclin-dependent kinase inhibitor p27/Kip1. CONCLUSIONS. TOPgal cells give rise to the four earliest cell types: ganglion, amacrine, horizontal, and photoreceptor. TCF/ LEF activation in the central retina does not correlate with Wnt/β-catenin signaling, pointing to an alternate role for this transcription factor family during retinal development.