Role of melanopsin in outer segment renewal

Cleveland Clinic Lerner Com-Cwru, Cleveland OH

Investigators

Abstract

PROJECT SUMMARY The non-image-forming visual system is responsible for mediating a range of light-driven processes, including circadian photoentrainment, the pupillary light reflex, masking, mood modulation, and hormonal regulation. These myriad functions are thought to be primarily mediated by melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs). Our previous research has demonstrated that melanopsin signaling is essential for regulating vascular events during eye development. Melanopsin is also expressed in iris, cornea, and retinal pigment epithelium (RPE), but physiological roles of melanopsin in non-retinal tissues are unknown. Here, we expand on our exciting preliminary findings supporting (1) a role of melanopsin and (2) a direct effect of the dark-light transition at light onset in photoreceptor outer segment renewal. Outer segment renewal is a fundamental homeostatic mechanism maintaining lifelong photoreceptor health and function in vision. New formation of outer segment disks by photoreceptors is balanced with removal of distal tips of outer segments, a process originally termed “disk shedding”. Recent work illustrated that disposal of distal outer segment tips requires complex molecular collaboration of photoreceptors and neighboring RPE such that spent outer segment tips are recognized and phagocytosed by the RPE without photoreceptor autonomous tip shedding. RPE dysfunction including abnormal phagocytosis is associated with several retinal diseases. Mechanistically, molecular signaling couples externalization of the “eat-me” signal phosphatidylserine (PS) by photoreceptors precisely at distal outer segment tips to engagement of the RPE’s phagocytic machinery. Outer segment renewal in general and both PS exposure and RPE phagocytosis specifically follow a diurnal, circadian rhythm in entrained animals with a burst of outer segment tip engulfment after light onset. Our preliminary data show abnormal diurnal burst of PS exposure and attenuated RPE phagocytosis in mice with global or RPE- specific deletion of melanopsin. Moreover, our experiments reveal a direct requirement for light in the diurnal burst of outer segment renewal. Intriguingly, this light dependence is lost in mice lacking melanopsin or the PS-inducer galectin-1, which in WT mice is released to the subretinal space in a light-dependent manner. These results support a novel molecular mechanism of direct light-sensitive regulation of outer segment renewal that was hitherto not recognized. Based on these exciting preliminary data, we hypothesize that light-activated melanopsin signaling, which occurs at least in part in the RPE, controls the characteristic diurnal burst of outer segment renewal by regulating galectin-1. Knowledge gap: Non-RGC functions of melanopsin remain enigmatic. Triggers of the diurnal burst of outer segment renewal are not understood. This grant will fill these knowledge gaps by exploring new cell-type specific deletion models of melanopsin combined with innovative quantification of outer segment renewal processes in vivo and ex vivo. In this proposal, we will identify and define the molecular links of melanopsin, light, and the molecular machinery for outer segment renewal.