Abstract
Purpose: :
The mammalian eye detects light for the purposes of forming images and modulating physiological processes. All light signals are sent to the brain by the retinal ganglion cells (RGCs), which are the sole output neurons of the retina. RGCs receive light input from rods and cones, the classical photoreceptors; however, 1-2% of ganglion cells are intrinsically photosensitive (ipRGCs) and transmit light signals to the brain to modulate non-image forming outputs such as circadian photoentrainment and pupil constriction. In previous developmental studies, we found that ipRGCs are largely born during the later part of RGC neurogenesis in the mouse, from embryonic days 14-18. Conventional RGCs rely on the basic helix-loop-helix transcription factor Math5 for competence to take on the ganglion cell fate; however, Math5 is downregulated during late embryogenesis. We investigated the developmental lineage of the ipRGCs, and the functional consequences of loss of ipRGCs of the Math5 lineage.
Methods: :
We mated Math5Cre animals to Cre reporters and used melanopsin immunohistochemistry to trace the transcriptional lineage of ipRGCs. We used the pupillary light reflex as a behavioral readout of ipRGC function. To determine if non-Math5 ipRGCs die in the Math5 knockout, we mated Math5 knockout animals to Bax null animals, in which neuronal apoptosis is inhibited. Melanopsin immunohistochemistry was used to count remaining ipRGCs.
Results: :
We found that roughly half of ipRGCs are part of the Math5 lineage, which contradicts reports in the literature that 95% of ganglion cells derive from this lineage. Thus, it is possible that a higher percentage of ipRGCs develop independent of Math5; however, in Math5 knockout animals, only 10% of ipRGCs remain. Interestingly, when apoptosis was inhibited in the Math5 knockout animal, 50% of ipRGCs remained, in agreement with our lineage analysis. Whereas Math5 null animals do not have a functional pupillary light reflex, Math5-Bax null animals do retain some pupil function.
Conclusions: :
Some ipRGCs do not rely on Math5 for their specification as RGCs. Additionally, in the Math5 null animal, non-Math5 ipRGCs may die due to secondary effects.
Keywords: ganglion cells • development • transcription factors