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Abstract
Irx7, a member in the zebrafish iroquois transcription factor (TF) family, has been shown to control brain patterning. During retinal development, irx7's expression was found to appear exclusively in the inner nuclear layer (INL) as soon as the prospective INL cells withdraw from the cell cycle and during retinal lamination. In Irx7-deficient retinas, the formation of a proper retinal lamination was disrupted and the differentiation of INL cell types, including amacrine, horizontal, bipolar and Muller cells, was compromised. Despite irx7's exclusive expression in the INL, photoreceptors differentiation was also compromised in Irx7-deficient retinas. Compared with other retinal cell types, ganglion cells differentiated relatively well in these retinas, except for their dendritic projections into the inner plexiform layer (IPL). In fact, the neuronal projections of amacrine and bipolar cells into the IPL were also diminished. These indicate that the retinal lamination issue in the Irx7-deficient retinas is likely caused by the attenuation of the neurite outgrowth. Since the expression of known TFs that can specify specific retinal cell type was also altered in Irx7-deficient retinas, thus the irx7 gene network is possibly a novel regulatory circuit for retinal development and lamination.
Highlights
The vertebrate retina has six types of neuron and one major type of glial cell
The ganglion cell layer (GCL) and inner nuclear layer (INL) are separated by a synaptic layer called inner plexiform layer (IPL), while INL and outer nuclear layer (ONL) are separated by outer plexiform layer (OPL)
Additional irx7 expression was observed in the anterior dorsal retina (Figure 1B, black arrowhead) by 43 hpf, when most cells in the prospective INL have withdrawn from the cell cycle
Summary
The vertebrate retina has six types of neuron and one major type of glial cell. All of which originate from the same progenitors in a conserved order [1]. Ganglion cells (GCs) are born first, followed by overlapping births of horizontal cells (HCs), cone photoreceptors (cones), amacrine cells (ACs), bipolar cells (BCs), rod photoreceptors (rods) and Muller cells (MCs) During their differentiation, these cell types are organized into three cellular layers, including ganglion cell layer (GCL) that contains primarily GCs, INL that contains ACs, BCs, MCs and HCs, and outer nuclear layer (ONL) that contains rods and cones. Smarca4-regulated genes may play important roles in these terminal differentiation, retinal lamination and patterning processes [8]. Several members in the iroquois (irx) TFs family, including irx1a, 3a, 4a, 4b, 5a and 7, are transcriptionally activated by Smarca4 These TFs share a similar structure that consists of a homeodomain and an Iro box [10]. It has been shown that irx regulates the compartmentalization of midbrain and hindbrain [10,12,13]
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