Two Transcription Factors Can Direct Three Photoreceptor Outcomes from Rod Precursor Cells in Mouse Retinal Development

Abstract

The typical mammalian visual system is based upon three photoreceptor types: rods for dim light vision and two types of cones (M and S) for color vision in daylight. However, the process that generates photoreceptor diversity and the cell type in which diversity arises remain unclear. Mice deleted for thyroid hormone receptor ®2 (TR®2) and neural retina leucine zipper factor (NRL) lack M cones and rods, respectively, but gain S cones. We therefore tested the hypothesis that NRL and TR®2 direct a common precursor to a rod, M cone or S cone outcome using Nrlb2/b2 “knock-in” mice that express TR®2 instead of NRL from the endogenous Nrl gene. Nrlb2/b2 mice lacked rods and produced excess M cones in contrast to the excess S cones in Nrl−/− mice. Notably, the presence of both factors yielded rods in Nrl+/b2 mice. The results demonstrate innate plasticity in post-mitotic rod precursors that allows these cells to form three functional photoreceptor types in response to NRL or TRβ2. We also detected precursor cells in normal embryonic retina that transiently co-expressed Nrl and TRβ2, suggesting that some precursors may originate in a plastic state. The plasticity of the precursors revealed in Nrlb2/b2 mice suggests that a two-step transcriptional switch can direct three photoreceptor fates: first, rod versus cone identity dictated by NRL and secondly, if NRL fails to act, M versus S cone identity dictated by TR®2.

Mobilizing Endogenous Stem Cells for Retinal Repair

“…Currently, no treatments are available that can effectively reverse vision loss due to photoreceptor degeneration. Various sources of cells, including neural precursors 110 , embryonic and postnatal RPCs 111 , neural stem cell lines 112 and bone marrow stem cells 113 , have been tested for their ability to differentiate and replace photoreceptors. An appropriate source of precursor cells is a key for photoreceptor cell replacement therapy.…”

American Thyroid Association Guide to Investigating Thyroid Hormone Economy and Action in Rodent and Cell Models

“…For example, in Western blot analysis, evidence for the specificity and sensitivity of the antibody used is essential. Also, as a qualitative control for the amount and integrity of protein sample loaded, the specific protein band detected may be compared to an internal control (reference) protein such as actin or RNA polymerase II, if these proteins themselves do not vary substantially in response to altered T 3 or TR status in the tissue being studied (82,398). If necessary, the specific protein band detected may also be quantified relative to the internal control protein band.…”

“…The dorso-ventral axis of the globe is marked. In situ hybridization and immunohistochemistry are used to assess expression of candidate target genes and are typically performed on 10-16 lm paraformaldehyde-fixed cryosections (279,398,503,504). Protein expression analyses can be performed on protein extracts prepared from dissected retina to investigate various opsin and rhodopsin proteins using Western blot and immunohistochemical studies (279,398,503,504,506).…”

“…Given the critical functions of thyroid hormone in different cone types, it is important to analyze photopic (light-adapted) cone responses in response to specific light wavelengths that are selective for the different cone populations, namely, those with peak sensitivity to mediumlong (M, or ''green'' *520 nm) and short (S, or ''blue'' *367 nm) wavelengths of light. Scotopic (dark-adapted) rod responses should also be determined (279,398).…”

Conditional knockdown of DNA methyltransferase 1 reveals a key role of retinal pigment epithelium integrity in photoreceptor outer segment morphogenesis

“…Specific cell fates are determined by combinatorial actions of a select set of transcription factors. OTX2, RORβ, BLIMP1 (PRDM1) and CRX are among the regulatory proteins that are crucial for photoreceptor development ; however, two key transcription factors -NRL and TRβ2 (encoded by Thrb) -together determine the generation of three distinct types of photoreceptors (rods, S-cones and M-cones) from postmitotic precursors (Ng et al, 2011). Downstream targets of NRL and CRX, as well as signaling proteins that modify their activity, further modulate the expression of photoreceptor genes (Oh et al, 2008;Onishi et al, 2009;Onishi et al, 2010;Roger et al, 2010;Hao et al, 2011).…”