Dynamic expression of ganglion cell markers in retinal progenitors during the terminal cell cycle

Department of Human Genetics, University of Michigan, Ann Arbor, MI 48109, United States.
Molecular and Cellular Neuroscience (Impact Factor: 3.73). 05/2012; 50(2):160-8. DOI: 10.1016/j.mcn.2012.05.002
Source: PubMed

ABSTRACT The vertebrate neural retina contains seven major cell types, which arise from a common multipotent progenitor pool. During neurogenesis, these cells stop cycling, commit to a single fate, and differentiate. The mechanism and order of these steps remain unclear. The first-born type of retinal neurons, ganglion cells (RGCs), develop through the actions of Math5 (Atoh7), Brn3b (Pou4f2) and Islet1 (Isl1) factors, whereas inhibitory amacrine and horizontal precursors require Ptf1a for differentiation. We have examined the link between these markers, and the timing of their expression during the terminal cell cycle, by nucleoside pulse-chase analysis in the mouse retina. We show that G2 phase lasts 1-2 h at embryonic (E) 13.5 and E15.5 stages. Surprisingly, we found that cells expressing Brn3b and/or Isl1 were frequently co-labeled with EdU after a short chase (<1 h) in early embryos (<E14), indicating that these factors, which mark committed RGCs, can be expressed during S or G2 phases. However, during late development (>E15), Brn3b and Isl1 were exclusively expressed in post-mitotic cells, even as new RGCs are still generated. In contrast, Ptf1a and amacrine marker AP2α were detected only after terminal mitosis, at all developmental stages. Using a retroviral tracer in embryonic retinal explants (E12-E13), we identified two-cell clones containing paired ganglion cells, consistent with RGC fate commitment prior to terminal mitosis. Thus, although cell cycle exit and fate determination are temporally correlated during retinal neurogenesis, the order of these events varies according to developmental stage and final cell type.

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Available from: Tom Glaser, May 19, 2015
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    • "Atoh7 was shown to be capable of inducing differentiation of ganglion cells if expressed in cells restricted to amacrine and photoreceptor cell fates. However, Atoh7 was not sufficient to alter differentiation significantly if expressed in cells committed to the photoreceptor lineage (Prasov and Glaser, 2012a; Mao et al., 2013). Together these findings suggest a differential susceptibility of RPCs and differentiating RPCs towards Atoh7 function. "
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    ABSTRACT: During vertebrate eye development retinal progenitor cells (RPCs) differentiate into all neural cell types of the retina. Retinal ganglion cells (RGCs) represent the first cell type to be generated. For their development, Atoh7, a basic Helix Loop Helix (bHLH) transcription factor is crucial. Atoh7 loss of function results in a massive reduction or even a total loss of RGCs. However, inconsistent results have been obtained in atoh7 gain of function experiments with respect to ganglion cell genesis, implying that the effect of Atoh7 is likely to be dependent on the competence state of the RPC. In this study we addressed the differential susceptibilities of early RPCs to Atoh7 in vivo, using medaka. Unexpectedly, we observed a largely normal development of the dorsal retina, although atoh7 was precociously expressed. However, the development of the retina close to the optic nerve head (part of the ventral retina) was disturbed severely. Photoreceptors were largely absent and the Müller glia cell number was reduced significantly. The majority of cells in this domain were ganglion cells and the abnormal development of this area affected the closure of the optic fissure resulting in coloboma.
    Mechanisms of Development 08/2014; 133. DOI:10.1016/j.mod.2014.08.002 · 2.24 Impact Factor
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    • "In the human retina, RGC genesis begins around fetal week (Fwk) 5 and ends around Fwk 18 (Pacal and Bremner, 2014), whereas in fish and amphibians, new RGCs can be added throughout life. Commitment of progenitors to a RGC fate occurs during, or just shortly after, the terminal cell division (McLoon and Barnes, 1989; Waid and McLoon, 1995; Prasov and Glaser, 2012; Pacal and Bremner, 2014) and is controlled by a combination of intrinsic factors and cell–cell signals. Differentiation of RGCs begins in the central retina and spreads peripherally in a roughly concentric fashion (Drager, 1985; Hu and Easter, 1999; McCabe et al., 1999; Neumann and Nuesslein-Volhard, 2000; Martinez-Morales et al., 2005; Hufnagel et al., 2010). "
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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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    ABSTRACT: The basic-helix-loop helix factor Math5 (Atoh7) is required for retinal ganglion cell (RGC) development. However, only 10% of Math5-expressing cells adopt the RGC fate, and most become photoreceptors. In principle, Math5 may actively bias progenitors towards RGC fate or passively confer competence to respond to instructive factors. To distinguish these mechanisms, we misexpressed Math5 in a wide population of precursors using a Crx BAC or 2.4 kb promoter, and followed cell fates with Cre recombinase. In mice, the Crx cone-rod homeobox gene and Math5 are expressed shortly after cell cycle exit, in temporally distinct, but overlapping populations of neurogenic cells that give rise to 85% and 3% of the adult retina, respectively. The Crx>Math5 transgenes did not stimulate RGC fate or alter the timing of RGC births. Likewise, retroviral Math5 overexpression in retinal explants did not bias progenitors towards the RGC fate or induce cell cycle exit. The Crx>Math5 transgene did reduce the abundance of early-born (E15.5) photoreceptors two-fold, suggesting a limited cell fate shift. Nonetheless, retinal histology was grossly normal, despite widespread persistent Math5 expression. In an RGC-deficient (Math5 knockout) environment, Crx>Math5 partially rescued RGC and optic nerve development, but the temporal envelope of RGC births was not extended. The number of early-born RGCs (before E13) remained very low, and this was correlated with axon pathfinding defects and cell death. Together, these results suggest that Math5 is not sufficient to stimulate RGC fate. Our findings highlight the robust homeostatic mechanisms, and role of pioneering neurons in RGC development.
    Developmental Biology 05/2012; 368(2):214-30. DOI:10.1016/j.ydbio.2012.05.005 · 3.64 Impact Factor
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