Motifs and Facets of Phototransduction in Image-Forming Vision and Non-Visual Photoreception in Metazoa

Photoreception is the first key step in seeing (i.e. the perception of color, shape and motion). It takes place in photoreceptor cells (ciliary and rhabdomeric, in vertebrates and invertebrates respectively) capable 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 amazing ubiquity, 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 luminous stimuli. Basically, although in a frame of different structure-function relationships, in vertebrate and invertebrate visual cells photoreception starts with the photoisomerization of the retinal cromophore of the photopigment, usually an opsin-based pigment. This process triggers the binding of the opsin with a G-protein which leads an enzymatic visual cascade culminating in the production of a second messenger, which gates light-sensitive ion channels in order 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. 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, timing and photoentrainment of circadian rhythms). NVP photoreceptors, formerly named extraretinal or extraocular, are currently 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: above all the same superfamily of opsin-based photopigment although with variations in structural motifs and related phototransductive elements. The searching for novel opsins supplying non-image-forming photoreception is a new challenging field in photosensory biology and in vision research. Recently, these pigments have been identified in cells beyond the retinal photoreceptors in several vertebrates and also in extraretinal tissues of some invertebrates. Focusing the attention on both opsin-based pigments and how almost similar structural motifs and biophysical mechanisms could serve different physiological processes, this lecture will survey similarities and differences among visual and non-visual systems in both vertebrates and invertebrates. Finally optogenetics approaches using genetically engineered opsin photoreceptors for the manipulation and the control of biosystems will be outlined.