Expression of UV-Sensitive Parapinopsin in the Iguana Parietal Eyes and Its Implication in UV-Sensitivity in Vertebrate Pineal-Related Organs

Abstract

The pineal-related organs of lower vertebrates have the ability to discriminate different wavelengths of light. This wavelength discrimination is achieved through antagonistic light responses to UV or blue and visible light. Previously, we demonstrated that parapinopsin underlies the UV reception in the lamprey pineal organ and identified parapinopsin genes in teleosts and frogs of which the pineal-related organs were reported to discriminate light. In this study, we report the first identification of parapinopsin in the reptile lineage and show its expression in the parietal eye of the green iguana. Spectroscopic analysis revealed that iguana parapinopsin is a UV-sensitive pigment, similar to lamprey parapinopsin. Interestingly, immunohistochemical analyses using antibodies specific to parapinopsin and parietopsin, a parietal eye green-sensitive pigment, revealed that parapinopsin and parietopsin are colocalized in the outer segments of the parietal eye photoreceptor cells in iguanas. These results strongly suggest that parapinopsin underlies the wavelength discrimination involving UV reception in the iguana parietal eye. The current findings support the idea that parapinopsin is a common photopigment underlying the UV-sensitivity in wavelength discrimination of the pineal-related organs found from lampreys to reptiles.

Signaling and Gene Regulatory Networks in Mammalian Lens Development

“…The parietal eye is found in most lizards, frogs, lamprey, and some species of fish and is photosensitive via an unorthodox phototransduction mechanism [111], which includes parapinopsin [112], non-visual green-light-sensitive opsin called parietopsin, as well as blue-sensitive pinopsin [113]. In contrast to the ectoderm-derived lateral lenses, the parietal lens is formed from the neuroectoderm, and is comprised of a monolayer of elongated cells; nevertheless, it expresses normal crystallins [114, 115].…”

Functional characterisation of the chromatically antagonistic photosensitive mechanism of erythrophores in the tilapiaOreochromis niloticus

“…For example, the parietal (third) eye of lizards displays chromatically dependent responses consisting of short-wavelength-sensitive hyperpolarisation and green-sensitive depolarisation (Solessio and Engbretson, 1993). This chromatically dependent antagonistic mechanism in the lizard parietal eye photoreceptors is composed of two non-visual photopigments, UVsensitive parapinopsin and green-sensitive parietopsin, and is suggested to assess cyclic spectral variation and enhance twilight detection (Shand and Foster, 1999;Wada et al, 2012). Tilapia lightsensitive erythrophores may also use a colour-opponent system for wavelength discrimination.…”

The Pineal Gland from Development to Function

“…This parietal eye contains all the cell types described in the pineal gland: PhRs, PNs, and glia (Engbretson & Anderson, 1990;Engbretson & Linser, 1991). Interestingly, the lizard parietal eye expresses parapinopsin and parietopsin, which are not found in their pineal gland (Wada et al, 2012). Along the same line, the parietal eye plays a unique role during thermic regulation in reptiles (see Section 1.3), suggesting a great extent of functional specialization for the main and accessory pineal organs.…”

“…Expression of parapinopsin, pinopsin, and parietopsin has been reported in the reptile pineal complex (Frigato, Vallone, Bertolucci, & Foulkes, 2006;Wada, Kawano-Yamashita, Koyanagi, & Terakita, 2012).…”