Phototransduction motifs in Metazoa: From non-visual photoreception to image-forming vision

Photoreception is the first key step in seeing (i.e. the perception of color, shape and motion). It takes place in photoreceptor cells able to sense directly ambient light. Visual pigments are integral membrane proteins of photoreceptor cells, which absorb photon energy and finally convert it into an electrical signal toward the central nervous system. Photoreception is phylogenetically one of the oldest sensory systems due to the remarkable presence, in all animal phyla, of morphological, functional and molecular systems (from simple invertebrate light-sensitive cells to more complex vertebrate eyes) that respond to environmental photic inputs. In vertebrate and invertebrate basically, although in a frame of different structure-function relationship, photoreception starts with the photoisomerization of the retinal cromophore of the photopigment, usually an opsin. This process triggers the binding of the opsin with a G-protein and leads an enzymatic visual cascade producing a second messenger able to gate light-sensitive ion channels which to modulate and shape the electric signal toward the nervous system. In addition to conventional eyed-structures, vertebrates and invertebrates have supplementary non-visual photoreceptor (NVP) systems for non-image forming function. Photic information mediated by NVP integrates visual activity and is involved in temporal (time-of-day) and behavioral physiology of the animal (e.g., photoperiodism, photoentrainment of circadian rhythms). NVP cells are termed as non-visual (non-image-forming) photosensitive cells in invertebrates and non-rod non-cone photoreceptors in vertebrates, after the discovery of photosensitive retinal ganglion cells ipRGCs. NVP cells are mainly located within nervous system districts and share with retinal photoreceptors common evolutionary origin and light-sensing modalities constituted, above all, by the same superfamily of opsin-based photopigment. The searching for novel opsins (e.g., melanopsin) supplying non-image-forming photoreception is a new challenging field in photosensory biology. Surprisingly, these pigments have been identified in cells beyond the retinal photoreceptors in several vertebrates and in extraretinal tissues of invertebrates. Recently, genetically-engineered visual (rhodopsin), non-visual (melanopsin), and microbial opsins (e.g., channelrhodopsin), have been successfully proved as suitable optogenetic tools to control the physiology and the behavior of cells and organisms. This lecture, focusing on the role of opsins, will survey comparisons between visual and non-visual systems, show how almost similar structural/functional mechanisms serve different physiological tasks, and report some insights on optogenetic control and manipulation by opsins.