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Neurog2 controls the leading edge of neurogenesis in the mammalian retina

Departments of Pediatrics and Ophthalmology, Division of Developmental Biology, Cincinnati Children's Research Foundation, University of Cincinnati College of Medicine, Cincinnati, OH 45229, USA.
Developmental Biology (Impact Factor: 3.64). 02/2010; 340(2):490-503. DOI: 10.1016/j.ydbio.2010.02.002
Source: PubMed

ABSTRACT In the mammalian retina, neuronal differentiation begins in the dorso-central optic cup and sweeps peripherally and ventrally. While certain extrinsic factors have been implicated, little is known about the intrinsic factors that direct this process. In this study, we evaluate the expression and function of proneural bHLH transcription factors during the onset of mouse retinal neurogenesis. Dorso-central retinal progenitor cells that give rise to the first postmitotic neurons express Neurog2/Ngn2 and Atoh7/Math5. In the absence of Neurog2, the spread of neurogenesis stalls, along with Atoh7 expression and RGC differentiation. However, neurogenesis is eventually restored, and at birth Neurog2 mutant retinas are reduced in size, with only a slight increase in the retinal ganglion cell population. We find that the re-establishment of neurogenesis coincides with the onset of Ascl1 expression, and that Ascl1 can rescue the early arrest of neural development in the absence of Neurog2. Together, this study supports the hypothesis that the intrinsic factors Neurog2 and Ascl1 regulate the temporal progression of retinal neurogenesis by directing overlapping waves of neuron formation.

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    • "In mice lacking Neurog2, the neurogenic wave front stalls temporally resulting in disruption of RGC genesis and smaller retinas. The reinstatement of neurogenesis in Neurog2 mutants correlates with onset of expression of Ascl1, a later expressed bHLH transcription factor, suggesting functional redundancy between Neurog2 and Ascl1 (Hufnagel et al., 2010). FGF1 and the classical morphogen Sonic hedgehog (Shh) have also been implicated in driving the propagation of the RGC differentiation wave across the retina (McCabe et al., 1999; Neumann and Nuesslein-Volhard, 2000). "
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    ABSTRACT: The visual system is beautifully crafted to transmit information of the external world to visual processing and cognitive centers in the brain. For visual information to be relayed to the brain, a series of axon pathfinding events must take place to ensure that the axons of retinal ganglion cells, the only neuronal cell type in the retina that sends axons out of the retina, find their way out of the eye to connect with targets in the brain. In the past few decades, the power of molecular and genetic tools, including the generation of genetically manipulated mouse lines, have multiplied our knowledge about the molecular mechanisms involved in the sculpting of the visual system. Here, we review major advances in our understanding of the mechanisms controlling the differentiation of RGCs, guidance of their axons from the retina to the primary visual centers, and the refinement processes essential for the establishment of topographic maps and eye-specific axon segregation. Human disorders, such as albinism and achiasmia, that impair RGC axon growth and guidance and, thus, the establishment of a fully functioning visual system will also be discussed. The eyes together with their connecting pathways to the brain form the visual system. In the eye, the cornea bends light rays and is primarily responsible for focusing the image on the retina. The lens behind the cornea inverts the image top to bottom and right to left. The retina, the receptive surface inside the back of the eye, is the struc-ture that translates light into nerve signals, and enables us to see under conditions that range from dark to sunlight, discriminate colors, and provide a high degree of visual precision. The retina consists of three layers of nerve cell bodies separated by two layers containing synapses made by the axons and dendrites of these cells. The back of the retina comprises the photoreceptors, the rods, and cones. The medial retinal layer contains three types of nerve cells, bipolar, horizontal, and amacrine cells. Bipolar cells receive input from the photoreceptors, and many of them feed directly into the retinal ganglion cells (RGCs). Horizontal cells connect receptors and bipolar cells by relatively long connections that run parallel to the retinal layers. Amacrine cells link bipolar cells and RGCs, the cells located in the inner retina. RGC axons pass across the surface of the retina and are collected in a bundle at the optic disk to leave the eye and form the optic nerve. There are approximately 20 RGC types that can be classified by morphological, molecular, and func-tional criteria. Each RGC type participates in distinct retinal circuits and projects to a specific set of targets in the brain (Coombs et al., 2007; Schmidt et al., 2011), including the main image-forming nuclei such as the lat-eral geniculate nucleus (LGN), the visual part of the thal-amus, and the superior colliculus (SC), located in the roof of the midbrain, that coordinates rapid movement of the eye (Figure 1). The optic axons from both eyes meet at the optic chiasm, which is located at the base of the hypothalamus. There, RGC axons from the nasal retina cross over to the opposite side of the brain (contralateral or commissural axons) while axons from the temporal retina turn to
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    • "B,C,H) At both ages, increasing ectopic Ascl1 and deceasing Atoh7 gene dosage correlates with an increased numbers of S phase (BrdU+) cells (H). D- E,I) Both Atoh7 Ascl1KI/+ and Atoh7 Ascl1/Ascl retinal lineages exhibit a significant, cell autonomous increase in S phase cells (arrows in D,E; yellow fraction of bars), compared to the Atoh7 +/+ lineage (Brzezinski IV et al., 2012; Hufnagel et al., 2010). By contrast, no Sphase Atoh7 mRNA+ or Atoh7-GFP+ retinal cells have been observed (Brzezinski IV et al., 2012; Hufnagel et al., 2010; Le et al., 2006). "
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    ABSTRACT: Retinal neurons and glia arise from a common progenitor pool in a temporal order, with retinal ganglion cells (RGCs) appearing first, and Müller glia last. The transcription factors Atoh7/Math5 and Ascl1/Mash1 represent divergent bHLH clades, and exhibit distinct spatial and temporal retinal expression patterns, with little overlap during early development. Here, we tested the ability of Ascl1 to change the fate of cells in the Atoh7 lineage when misexpressed from the Atoh7 locus, using an Ascl1-IRES-DsRed2 knock-in allele. In Atoh7Ascl1KI/+ and Atoh7Ascl1KI/Ascl1KI embryos, ectopic Ascl1 delayed cell cycle exit and differentiation, even in cells coexpressing Atoh7. The heterozygous retinas recovered, and eventually produced a normal complement of RGCs, while homozygous substitution of Ascl1 for Atoh7 did not promote postnatal retinal fates precociously, nor rescue Atoh7 mutant phenotypes. However, our analyses revealed two unexpected findings. First, ectopic Ascl1 disrupted cell cycle progression within the marked Atoh7 lineage, but also nonautonomously in other retinal cells. Second, the size of the Atoh7 retinal lineage was unaffected, supporting the idea of a compensatory shift of the non-proliferative cohort to maintain lineage size. Overall, we conclude that Ascl1 acts dominantly to block cell cycle exit, but is incapable of redirecting the fates of early RPCs.
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    ABSTRACT: The Math5 (Atoh7) gene is transiently expressed during retinogenesis by progenitors exiting mitosis, and is essential for ganglion cell (RGC) development. Math5 contains a single exon, and its 1.7 kb mRNA encodes a 149-aa polypeptide. Mouse Math5 mutants have essentially no RGCs or optic nerves. Given the importance of this gene in retinal development, we thoroughly investigated the possibility of Math5 mRNA splicing by Northern blot, 3'RACE, RNase protection assays, and RT-PCR, using RNAs extracted from embryonic eyes and adult cerebellum, or transcribed in vitro from cDNA clones. Because Math5 mRNA contains an elevated G+C content, we used graded concentrations of betaine, an isostabilizing agent that disrupts secondary structure. Although approximately 10% of cerebellar Math5 RNAs are spliced, truncating the polypeptide, our results show few, if any, spliced Math5 transcripts exist in the developing retina (<1%). Rare deleted cDNAs do arise via RT-mediated RNA template switching in vitro, and are selectively amplified during PCR. These data differ starkly from a recent study (Kanadia and Cepko 2010), which concluded that the vast majority of Math5 and other bHLH transcripts are spliced to generate noncoding RNAs. Our findings clarify the architecture of the Math5 gene and its mechanism of action. These results have implications for all members of the bHLH gene family, for any gene that is alternatively spliced, and for the interpretation of all RT-PCR experiments.
    PLoS ONE 08/2010; 5(8):e12315. DOI:10.1371/journal.pone.0012315 · 3.53 Impact Factor
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