Math5 (Atoh7) gene dosage limits retinal ganglion cell genesis

Abstract

The basic helix-loop-helix factor Math5 (Atoh7) is critical for the determination of retinal ganglion cell (RGC) fate in mice. Recently, genome-wide association studies have identified the ATOH7 locus as a major determinant of variation in the human optic disc area, which is directly correlated with the RGC number. These studies suggest that the level of Math5 expression may determine the ultimate number of RGCs. To test this hypothesis, we systematically compared optic nerve area and RGC axon number in C57BL/6J congenic Math5+/-and +/+ mice at young adult and neonatal ages by transmission electron microscopy. Optic disc area and RGC abundance were not significantly different in adults, but heterozygotes had thinner optic nerves and 25-30% fewer RGCs at birth than wild-type littermates (P < 0.05). Our results suggest that Math5 dosage is important for the genesis, but not the ultimate number, of RGCs. Our findings highlight the importance of ganglion cell culling as a compensatory mechanism for retinal homeostasis, and support a quantitative role for Math5 in RGC specification.

TheAtoh7remote enhancer provides transcriptional robustness during retinal ganglion cell development

“…Our WT C57BL/6 data match those obtained via other automated counting approaches (59,60). Consistent with previous analyses (19), there was no significant difference in the number or density of RGCs in adult Atoh7 KO/+ and SE/+ retinas compared to WT, and KO/KO retinas had scant RGCs. However, both SE/KO and SE/SE retinas had significantly fewer RGCs.…”

“…Postnatal Atoh7 SE Mutants Have Fewer RGCs. In a previous study, we showed that a 50% reduction in Atoh7 dosage impairs RGC genesis but does not alter the final number of RGCs, by comparing optic nerve axons in neonatal and adult Atoh7 +/+ and KO/+ mice (19). The developing nervous system thus compensates for defective RGC genesis by decreasing the extent of neonatal RGC culling (55,56), a normal process that matches the number of RGCs to postsynaptic targets in the brain.…”

“…RGC Genesis Depends Strictly on Atoh7 Activity. Using the hypomorphic SE allele, we extended previous analysis (19) to define Atoh7 dosage thresholds for RGC phenotypes. Two findings are striking.…”

“…Ultrathin optic nerve sections from P30 and P1 mice (n = 3 +/+, SE/SE, SE/ KO) were stained with toluidine blue or processed for electron microscopy to determine cross-section areas and myelinated axon counts (SI Appendix, Supplemental Methods) (19). P30 and P1 optic nerve areas were calculated by perimeter tracing (Fiji).…”

“…At the cellular level, mouse Atoh7 expression begins during the terminal S/G2 phase and ends as postmitotic daughter cells reach their final laminar position in the retina; however, the onset of Atoh7 expression within the last cell cycle is progressively delayed from E11 to P0 (8,(12)(13)(14). Analyses of mutant mice and zebrafish show ATOH7 is an essential, rate-limiting factor for ganglion cell genesis (15)(16)(17)(18)(19). Atoh7 mutants lack RGCs and optic nerves but retain all other cell types.…”