Light Out of Sight: Signaling Mechanisms for Nonvisual Opsins

Life on earth evolved under daily cycles of sunlight, and all species developed mechanisms for detecting and responding to solar wavelengths reaching the surface of the earth. Early phototransduction studies found that our eyes detect visible wavelengths using light-activated G protein–coupled receptors named opsins. Many years after discovering the mechanisms by which of rhodopsin (opsin 2) and the cone opsins (opsin 1) mediate vision, three other members of the opsin family (opsins 3, 4, and 5) were identified and, surprisingly, found to be expressed in the brain and peripheral organs. Named nonvisual opsins (NVOs), these receptors mediate physiological light responses, such as pupillary light reflex and circadian rhythms. NVOs have been the focus of an increasing number of extraocular phototransduction studies, illuminating novel ways in which light modulates human physiology. This review summarizes our current knowledge on signaling mechanisms mediating nonvisual photoreception and their physiological functions.