Light regulation of transcripts of classic and unconventional opsins in the eyeless cnidarian Hydra

Opsins play a key role in the ability to sense light both in image-forming processes and in non-visual photoreception or circadian vision (i.e., photoentrainment: the entrainment of a cellular oscillator by the light/dark cycle) [1]. Regardless of these two modalities, in the animal phyla the photoreceptor protein in the great majority of cases is an opsin-based protein binding to a light reactive chromophore by a Lys residue (base of Schiff) [2]. So far, GPCRs containing Lys have been discovered neither in sponges nor in porifera while new classes of opsins have been identified in the photoresponsive Hydra, an eyeless cnidarian considered the evolutionary sister species to bilaterians [3-5]. Aimed to verify whether light influences and modulates the gene expression of known opsins in Hydra, we utilized four opsin EST sequences, belonging to two classic opsins and two non-visual opsins, to investigate, by means of a quantitative RT-PCR, the expression patterns during both photic Zeitgeber (from the German for "time-giver", an entrainment signal) and circadian time. In particular, we utilized the mRNA sequences expressed by the following opsin genes: Melanopsin-like, GenBank: DT617488; SW Rhodopsin-like, GenBank: CN554795; SW Blue sensitive opsin-like, GenBank: CN775258 and Peropsin-like, GenBank: CB073527.1. The expression levels followed the light hours of diurnal cycle with respect to the darkness ones and in constant dark condition the relative expression increased. The monophasic behaviour in L12:D12 cycle turned into a triphasic expression profile during the continuous darkness condition. Consequently, while the diurnal opsin-like expression reveals a close dependence on light hours, the highest transcript levels in D12:D12 leaving us to hypothesize an "internal" biological clock which autonomously provided the opsins expression during the circadian time. Therefore, the irradiance, acting as the main temporal indicator of the day-time, would downregulate the circadian mechanism to better optimize the opsins expression to the varying demands of night and day. In conclusion, in Hydra the irradiance would regulate the diurnal expression of visual and non-visual opsins as already demonstrated in higher invertebrate and vertebrate species. Our data confirm that the eyeless cnidarian Hydra represents a suitable model for studying ancestral precursor of both visual and non-visual photosensitive modalities providing useful hints on the evolution of visual and photosensory systems [6]. [1] Musio C & Santillo S (2012) Nonvisual photosensitivity and circadian vision. In: CRC Handbook Organic Photochem Photobiol (Griesbeck A, Oelgemöller M, Ghetti F, eds), Boca Raton: CRC Press, pp. 1195-1210. [2] Hunt DM, Hankins MW, Collin SP, Marshall NJ, eds (2014) Evolution of Visual and Non-Visual Pigments, Springer New York, NY, pages viii-276. [3] Musio C, Santillo S, et al. (2001) First identification and localization of a visual pigment in Hydra (Cnidaria, Hydrozoa), J. Comp. Physiol. A 187: 79-81. [4] Suga H, Schmid V, Gehring WJ (2008) Evolution and functional diversity of jellyfish opsins, Curr Biol. 18: 51-5. [5] Plachetzki DC, Fong CR, Oakley TH. (2010) The evolution of phototransduction from an ancestral cyclic nucleotide gated pathway, Proc Biol Sci. 277(1690): 1963-9. [6] Santillo S, P. Orlando, et al. (2006) Evolving visual pigments: Hints from the opsin-based proteins in a phylogenetically old eyeless invertebrate, BioSystems 86: 3-17.