Retinal cell fate determination and bHLH factors
Introduction
During retinal development, progenitors change their competency over time under the control of extrinsic (such as neurotrophic factors) and intrinsic regulators (such as transcription factors) [1], [2], [3]. In mouse, retinal progenitors initially proliferate extensively to increase the cell number but, from embryonic day (E) 10.5 onward, proliferating progenitors start cell differentiation. In the neural retina, there are six types of neurons and one type of glial cells (Müller glial cells), which constitute three cellular layers: rod and cone photoreceptors in the outer nuclear layer (ONL), horizontal, bipolar, and amacrine interneurons and Müller glial cells in the inner nuclear layer (INL), and ganglion and displaced amacrine cells in the ganglion cell layer (GCL) (Fig. 1B). These seven types of cells are differentiated from common progenitors in an order conserved among many species: ganglion cells first and Müller glial cells last (Fig. 1A). Thus, retinal development consists of three successive processes: (i) proliferation of progenitors, (ii) neurogenesis, and (iii) gliogenesis. It has been shown that these processes are controlled by multiple basic helix–loop–helix (bHLH) genes, which function as intrinsic regulators [4], [5].
There are two functionally distinct groups of bHLH genes: repressors and activators. bHLH repressors such as Hes1 and Hes5, homologs of Drosophila hairy and Enhancer of split genes, are expressed by common progenitors and inhibit neuronal differentiation. These bHLH repressors promote maintenance of progenitors in the embryonal retina and glial differentiation in the postnatal retina. When the bHLH repressors are downregulated, then bHLH activators such as Mash1 and Math5, homologs of Drosophila proneural genes acheate-scute and atonal, promote neuronal differentiation. In this review, we discuss roles of bHLH repressors and activators in maintenance of progenitors and specification of neurons and glial cells.
Section snippets
Maintenance of progenitors by bHLH repressors
During retinal development, progenitors proliferate extensively to increase the cell number while giving rise to distinct subtypes of cells over time by changing their competency. Thus, it is essential to maintain progenitors until late stages to have not only the enough number of cells but also a full range of cell types. It has been shown that the bHLH factor Hes1 plays an essential role in maintenance of retinal progenitors. Hes1 is a transcriptional repressor that recruits the corepressor
Generation of Müller glia by bHLH repressors
In the postnatal retina, Hes1 and Hes5 expression does not disappear but is observed in differentiating Müller glial cells, the last cell fate (Fig. 1A). This expression is transient and soon disappears by postnatal day (P) 10. Misexpression of Hes1 or Hes5 in the postnatal retina promotes generation of Müller glial cells [14], [15]. Similar gliogenic activities are also reported for Notch [14], [16], [17] and the homeodomain gene rax [14]. rax is known to promote proliferation of retinal
Neuronal cell fate determination by bHLH activators
bHLH activators such as the Achaete-Scute homolog Mash1 and Atonal homologs Math3, Math5 and NeuroD are expressed initially in the ventricular zone but later by subsets of differentiating neurons (Fig. 1A). bHLH activators are known to promote the neuronal fate and inhibit the glial fate in the brain [24]. Misexpression of these bHLH activators in the developing retina also generates neurons only [25]. The precise mechanism by which bHLH activators suppress bHLH repressors during neuronal
Bipolar cell fate specification by the bHLH activators Mash1/Math3 and the homeodomain factor Chx10
Mash1 is transiently expressed by differentiating bipolar cells (Fig. 1A), and Mash1-null mutation decreases bipolar cells while increasing Müller glial cells [27]. Math3 is also expressed by bipolar cells (Fig. 1A), but Math3-null mutation does not affect bipolar cell development [27]. In contrast, in Mash1–Math3-double mutations, virtually all bipolar cells are abolished, and those that would normally differentiate into bipolar cells adopt the Müller glial cell fate (Fig. 3B(b)) [27]. Thus,
Amacrine cell fate specification by the bHLH activators NeuroD/Math3 and the homeodomain factors Pax6/Six3
The bHLH gene NeuroD is transiently expressed by differentiating amacrine cells (Fig. 1A) [30]. Although NeuroD-null mutation delays amacrine cell development, the amacrine cell number is recovered to the normal level by P12, indicating that NeuroD alone is not sufficient for amacrine cell development [30]. Similarly, although Math3 is transiently expressed by differentiating amacrine cells, Math3-null mutation does not affect amacrine cell development [27]. However, in NeuroD–Math3-double
Specification of other neuronal subtypes by bHLH and homeodomain factors
In addition to bipolar and amacrine cells, other retinal neurons have been also analyzed for dependency upon bHLH and homeodomain genes. Recent studies revealed that the bHLH activator Math5 plays an essential role in ganglion cell development. Among the bHLH activators, Math5 is expressed earlier in the ventricular zone than others. The expression starts from the boundary between the optic stalk and the neural retina, spreads circumferentially and expands throughout the neural retina [32]. It
Conclusion
The three processes of retinal development (proliferation of progenitors, neurogenesis and gliogenesis) are all controlled by bHLH genes. During these processes, progenitors are known to change their competency over time. Early progenitors have a competency to become early-born cell types such as ganglion and amacrine cells, and they adopt the ganglion cell fate when Math5 is on and the amacrine cell fate when NeuroD or Math3 is on (Fig. 4). Late progenitors have a competency to become
Acknowledgements
This work was supported by research grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan and Japan Society for the Promotion of Science. J.H. was supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists.
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