An Expanding Role for Nonvisual Opsins in Extraocular Light Sensing Physiology

Abstract

We live on a planet that is bathed in daily and seasonal sunlight cycles. In this context, terrestrial life forms have evolved mechanisms that directly harness light energy (plants) or decode light information for adaptive advantage. In animals, the main light sensors are a family of G protein–coupled receptors called opsins. Opsin function is best described for the visual sense. However, most animals also use opsins for extraocular light sensing for seasonal behavior and camouflage. While it has long been believed that mammals do not have an extraocular light sensing capacity, recent evidence suggests otherwise. Notably, encephalopsin (OPN3) and neuropsin (OPN5) are both known to mediate extraocular light sensing in mice. Examples of this mediation include photoentrainment of circadian clocks in skin (by OPN5) and acute light-dependent regulation of metabolic pathways (by OPN3 and OPN5). This review summarizes current findings in the expanding field of extraocular photoreception and their relevance for human physiology. Expected final online publication date for the Annual Review of Vision Science, Volume 9 is September 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A newOpn4crerecombinase mouse line to target intrinsically photosensitive retinal ganglion cells (ipRGCs)

“…Photopigments are expressed in cells beyond the neural retina and the eye 59 . For example, Opn3 and Opn5 are distributed in tissues outside the eye and have had reported functions within the brain [60][61][62] .…”

Biophysical control of plasticity and patterning in regeneration and cancer

“…Thus, light emissions are readily observed in stressed, aging, diseased, or highly metabolic systems [ 237 , 238 ]. As excited electrons return to ground state, photons are released with emission frequencies and patterns reflective of molecular events within the cell [ 239 ]. Though optical signaling among cells has not received significant attention, the widespread expression of non-visual opsins (e.g., OPN3, OPN5) in tissues of the eyes, skin, brain, testis, spinal cord, lung, liver, and kidney among others [ 240 ] suggest that they may serve a physiological role.…”

Differential Photosensitivity of Fibroblasts Obtained from Normal Skin and Hypertrophic Scar Tissues

“…There are five known types: OPN1 (cone opsin, short wave: SW; middle wave: MW; long wave: LW), OPN2 (rhodopsin), OPN3 (encephalopsin/panopsin), OPN4 (melanopsin), and OPN5 (neuropsin). Each has its own sensitive wavelength range, and although there is some variation in reports, the peak sensitivities are generally considered to be OPN1 M/LW: 510-570 nm (red to green light); OPN1 SW: 420 nm (blue light); OPN2: 500 nm (red light); OPN3: 460-470 nm (blue light); OPN4: 460-480 nm (blue light); and OPN5: 360-380 nm (ultraviolet light) [10,11]. Opsins are G protein-coupled receptors, with OPN1 and OPN2 coupling with Gt, OPN3 with Gi/o, OPN4 with Gq, and OPN5 with Gi.…”

“…Opsins are G protein-coupled receptors, with OPN1 and OPN2 coupling with Gt, OPN3 with Gi/o, OPN4 with Gq, and OPN5 with Gi. Upon light reception, they convert the signal into an electrical signal through a second messenger, the G protein, thus activating their respective signaling pathways [10][11][12][13][14]. Human skin tissues express all opsin types [10,15]; however, their functions are not yet fully understood.…”

“…Upon light reception, they convert the signal into an electrical signal through a second messenger, the G protein, thus activating their respective signaling pathways [10][11][12][13][14]. Human skin tissues express all opsin types [10,15]; however, their functions are not yet fully understood. In skin tissues, OPN3 demonstrates the highest reported expression among these OPN types [13,14].…”