Dynamic expression of the basic helix‐loop‐helix transcription factor neuroD in the rod and cone photoreceptor lineages in the retina of the embryonic and larval zebrafish

Abstract

NeuroD is a basic helix-loop-helix (bHLH) transcription factor critical for determining neuronal cell fate and regulating withdrawal from the cell cycle. We showed previously that, in goldfish, neuroD is expressed in the rod photoreceptor lineage, and we inferred that neuroD is also expressed in a subset of amacrine cells and nascent cone photoreceptors. Here we extended that study by examining the temporal and spatial expression pattern of neuroD in the embryonic and larval zebrafish and by identifying the cell types that express this gene. NeuroD expression in the developing zebrafish retina is dynamic, spanning early retinogenesis and the maturation of cone photoreceptors. In early retinogenesis neuroD expression expands from a small patch in the ventronasal retina, through the remaining retinal neuroepithelium. As retinogenesis progresses, neuroD expression becomes restricted to amacrine cells, immature cones, and cells of rod and cone lineages. This expression achieves an adult pattern by 96 hours postfertilization (hpf), whereupon the temporal pattern of neuroD expression in central retina is spatially recapitulated at the germinative margin. The cellular pattern of expression suggests that neuroD regulates aspects of rod and cone genesis, but through separate cellular lineages. Furthermore, neuroD is coexpressed with the cone-rod-homeobox transcription factor (Crx) in putative cone progenitors and nascent cone photoreceptors, suggesting that, in the zebrafish retina, as in other vertebrate retinas, similar genetic cascades regulate photoreceptor genesis and maturation.

The bHLH Transcription Factor NeuroD Governs Photoreceptor Genesis and Regeneration Through Delta-Notch Signaling

“…1,[70][71][72][73] During retinal development in zebrafish the bHLH transcription factor NeuroD is expressed in mitotic photoreceptor progenitors and nascent photoreceptors, 13,14,39 and within the photoreceptor lineage, NeuroD governs cell cycle exit and photoreceptor maturation. 7 Photoreceptor genesis normally is complete by 70 hpf 63 ; however, in the absence of NeuroD, even though cellular laminae form, photoreceptor progenitors fail to exit the cell cycle and mature photoreceptors are absent.…”

“…12 In medaka fish, NeuroD and Six6 coregulate each other to control amacrine cell and photoreceptor genesis. 6 In embryonic zebrafish, neurod is expressed in mitotic photoreceptor progenitors, 13,14 and this expression is regulated by the zinc finger protein, Insm1a. 15 Within the photoreceptor lineages NeuroD governs cell cycle exit and photoreceptor maturation.…”

Proper differentiation of photoreceptors and amacrine cells depends on a regulatory loop between NeuroD and Six6

“…For example, in Xenopus, NeuroD promotes the generation of amacrine cells (Kanekar et al, 1997;Moore et al, 2002), whereas in zebrafish, it initiates photoreceptor progenitor withdrawal from the cell cycle and, when ectopically expressed, favours photoreceptor generation at the expenses of Müller glial cells (Ochocinska and Hitchcock, 2007;Ochocinska and Hitchcock, 2009). In medaka fish, NeuroD and Six6 are coexpressed in differentiating amacrine cells as well as in photoreceptor progenitors, although in the latter, co-expression is only transient (Fig.…”

“…For example, Ath5, a bHLH-TF, renders postmitotic retinal precursors competent to generate RGCs and activates the expression of HD-TFs such as Brn3 and Islet 1, which are required for RGC differentiation (Hatakeyama and Kageyama, 2004). Similarly, the bHLH TF NeuroD is expressed in postmitotic retinal cells that originate amacrine and photoreceptor cells, although with notable variations among vertebrate species (Liu et al, 2008;Moore et al, 2002;Ochocinska and Hitchcock, 2007;Ochocinska and Hitchcock, 2009;Yan and Wang, 1998). Co-expression of the HD-TF Crx drives NeuroD-positive progenitors toward the photoreceptor fate, activating rod-or cone-specific determinants (Hennig et al, 2008).…”

Rod Progenitor Cells in the Mature Zebrafish Retina

“…However, IGF-I regulation of rod progenitor cells has not yet been demonstrated in vivo. Recently, expression of the bHLH transcription factor NeuroD was demonstrated in the rod progenitors and putative INL stem cells of the adult goldfish and larval zebrafish retina Hitchcock and Kakuk-Atkins, 2004;Ochocinska and Hitchcock, 2007). However, not all of the rod progenitor cells were positive for NeuroD expression, so it may not be a ubiquitous marker of rod progenitors, nor is it known whether NeuroD plays a role in regulating rod progenitor proliferation or differentiation.…”