Pushing the envelope of retinal ganglion cell genesis: Context dependent function of Math5 (Atoh7)

• Published in: Developmental biology Vol. 368; no. 2; pp. 214 - 230
• Main Authors: Prasov, Lev, Glaser, Tom
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 15.08.2012
• Subjects: adults; Animals; Atoh7; Atonal; Axon pathfinding; BAC transgene; bacterial artificial chromosomes; Basic helix-loop-helix (bHLH); Basic Helix-Loop-Helix Transcription Factors - genetics; Basic Helix-Loop-Helix Transcription Factors - metabolism; Cell Cycle; cell death; Cell Differentiation; Cell fate specification; Cell Lineage; Cell Proliferation; Cone–rod homeodomain; Crx; Embryo, Mammalian - cytology; Embryo, Mammalian - embryology; Embryo, Mammalian - metabolism; explants; ganglia; Gene Expression Regulation, Developmental; Green Fluorescent Proteins - genetics; Green Fluorescent Proteins - metabolism; Heterochronic; histology; homeotic genes; Immunohistochemistry; Math5; Mice; Mice, Knockout; Mice, Transgenic; nerve tissue; Nerve Tissue Proteins - genetics; Nerve Tissue Proteins - metabolism; neurons; Optic nerve; photoreceptors; population; retina; Retinal explants; Retinal ganglion cell (RGC); Retinal Ganglion Cells - cytology; Retinal Ganglion Cells - metabolism; Reverse Transcriptase Polymerase Chain Reaction; Time Factors; Transgene rescue; transgenes; Atoh7; Optic nerve; Retinal explants; Math5; Crx; Cone–rod homeodomain; Axon pathfinding; BAC transgene; Cell fate specification; Transgene rescue; Basic helix-loop-helix (bHLH); Heterochronic; Atonal; Retinal ganglion cell (RGC)
• ISSN: 0012-1606; 1095-564X
• DOI: 10.1016/j.ydbio.2012.05.005

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 2.4kb 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. ► Gain-of-function test of Math5 bHLH factor action in mice. ► BAC and conventional Crx>Math5-ires-Cre transgenes, with Crx>Cre controls. ► Temporal shift in retinal ganglion cell (RGC) birthdates but no change in overall time envelope. ► Math5 is not the sole determinant of RGC fate competence. ► Heterochronic rescue of RGC agenesis phenotype in Math5 KO mice, with axonal pathfinding defects.