Apoptosis Regulates Iprgc Spacing And Rod-cone Signaling For Circadian Photoentrainment

Abstract

Purpose: : Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) are produced in excess during development with approximately half of them undergo programmed cell death (apoptosis). However, it is unclear why ipRGCs were overproduced and die subsequently, and what is the functional consequence of this apoptosis.

Methods: : To understand the functional consequence of ipRGCs apoptosos, we study mouse lines with mutated pro-apoptotic protein Bax. Bax null mice lack apoptosis in many neuronal populations, including retinal ganglion cells (RGCs). In addition, we also crossed conditional Bax mouse line with our melanopsin driven cre mouse line to specifically knockout Bax in ipRGCs. We perform behavior and histological analysis to determine the role of apoptosis on ipRGCs spacing and function in the retina.

Results: : With melanopsin antibody and Opn4-Tau-LacZ labeling, we show that bax null mice contain clumps of ipRGCs with tangled dendrites, unlike the evenly spaced mosaic and dendritic network of ipRGCs in the wild type. Although bax and melanopsin single knockouts photoentrain to an external light cycle similar to wild type, ipRGCs in Bax/melanopsin double knockout mice have reduced light response and they failed to photoentrain. Similar disruption in ipRGCs spatial distribution and the inability to photoentrain was also observed in the conditional Bax/melanopsin double knockout mice.

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, in cell autonomous level, apoptosis plays a crucial role in allowing ipRGCs to form the evenly spaced network across the retina and to convey rod and cone signal for circadian photoentrainment. This suggests a role for apoptosis during development in regulating cell spacing and the ability of ipRGCs to convey light information from outer retina to the brain.

Keywords: apoptosis/cell death • retinal connections, networks, circuitry • ganglion cells