Abstract
Purpose: :
During development, approximately half of the neurons initially produced undergo programmed cell death (apoptosis). Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) are produced in excess during development. What is the functional consequence of such overproduction and why do ipRGC neurons die subsequently? Bax null mice provide a good model to study this phenomenon because they lack apoptosis in many neuronal populations, including retinal ganglion cells (RGCs). The ipRGCs provide a tractable model system within the CNS because of distinct morphological characteristics, well-defined axonal targets, and easily measurable behavioral outputs including circadian rhythms and pupil constriction.
Methods: :
We used several mouse genetic models such as melanopsin knockout, melanopsin cre and both conventional and conditional bax knockout animals in combination with behavior and histological analysis to determine the role of apoptosis on ipRGCs spacing and function in the retina.
Results: :
Antibody labeling of ipRGCs in adult retinas revealed that bax null mice contain clumps of two or more ipRGCs with tangled dendrites, unlike the evenly spaced mosaic and dendritic network of ipRGCs in the wild type. These clumps of ipRGCs are present during normal development but are subsequently removed via bax-mediated apoptosis. Genetic labeling of ipRGC axons in bax knockout mice revealed normal targeting of both the suprachiasmatic nucleus (SCN), which regulates circadian rhythms, and the olivary pretectal nucleus (OPN), which controls pupil constriction. Although bax and melanopsin single knockouts photoentrain to an external light cycle similar to wild type, bax/melanopsin double knockout mice failed to photoentrain. This indicates that the rod/cone input to ipRGCs is dysfunctional in the bax knockout.
Conclusions: :
Our data reveal that bax-mediated apoptosis is not involved in ipRGC targeting of the SCN and OPN and does not influence the ability of ipRGCs to act as photoreceptors. However, apoptosis plays a crucial role in allowing ipRGCs to receive inputs from rods and cones as well as forming the evenly spaced photoreceptive network of ipRGCs across the retina. This suggests a role for apoptosis during development in regulating cell spacing and dendritic formation.
Keywords: ganglion cells • apoptosis/cell death • circadian rhythms