Functional characterisation of the chromatically antagonistic photosensitive mechanism of erythrophores in the tilapia Oreochromis niloticus.

Non-visual photoreceptors with diverse photopigments allow organisms to adapt to changing light conditions. Whereas visual photoreceptors are involved in image formation, non-visual photoreceptors mainly undertake various non-image-forming tasks. They form specialised photosensory systems that measure the quality and quantity of light and enable appropriate behavioural and physiological responses. Chromatophores are dermal non-visual photoreceptors directly exposed to light and they not only receive ambient photic input but also respond to it. These specialised photosensitive pigment cells enable animals to adjust body coloration to fit environments, and play an important role in mate choice, camouflage and ultraviolet (UV) protection. However, the signalling pathway underlying chromatophore photoresponses and the physiological importance of chromatophore colour change remain under-investigated. Here, we characterised the intrinsic photosensitive system of red chromatophores (erythrophores) in tilapia. Like some non-visual photoreceptors, tilapia erythrophores showed wavelength-dependent photoresponses in two spectral regions: aggregations of inner pigment granules under UV and short-wavelengths and dispersions under middle- and long-wavelengths. The action spectra curve suggested that two primary photopigments exert opposite effects on these light-driven processes: SWS1 (short-wavelength sensitive 1) for aggregations and RH2b (rhodopsin-like) for dispersions. Both western blot and immunohistochemistry showed SWS1 expression in integumentary tissues and erythrophores. The membrane potential of erythrophores depolarised under UV illumination, suggesting that changes in membrane potential are required for photoresponses. These results suggest that SWS1 and RH2b play key roles in mediating intrinsic erythrophore photoresponses in different spectral ranges and this chromatically dependent antagonistic photosensitive mechanism may provide an advantage to detect subtle environmental photic change.

Light-sensitive motile iridophores and visual pigments in the neon tetra, Paracheirodon innesi.

Although motile iridophores in the longitudinal stripes of neon tetra skin are under control of the sympathetic nervous system, they also respond to light directly and show circadian color changes. Using neon tetra skin, we found that the photoresponse of iridophores depends on light intensity, and that light near 500 nm is most effective. RT-PCR demonstrated the expression of mRNAs encoding rhodopsin and two kinds of cone opsins (Pi-green1 and Pi-green2) in neon tetra skin where the light-sensitive iridophores exist. These mRNAs are also expressed in the lateral eyes. The cone opsin genes, Pi-green1 and Pi-green2, show high similarity with the g101 and g103 genes of unique green cone opsins (belonging to the MWS/LWS group) of the blind Mexican cavefish. These results show that Pi-green1, Pi-green2, and/or rhodopsin may play important roles in the photoresponse of neon tetra iridophores, which are most sensitive to light near 500 nm.

The signaling pathway in photoresponses that may be mediated by visual pigments in erythrophores of Nile tilapia.

The ability to increase the synthesis or vary the distribution of pigment in response to light is an important feature of many pigment cells. Unlike other light-sensitive pigment cells, erythrophores of Nile tilapia change the direction of pigment migration depending on the peak wavelength of incident light: light near 365, 400 or 600 nm induces pigment aggregation, while dispersion occurs in response to light at 500 nm. How these phenomena are achieved is currently unknown. In the present study, the phototransduction involved in the pigment dispersion caused by light at 500 nm or the aggregation by light at 600 nm was examined, using pertussis toxin, cholera toxin, blockers of ion channels, various chemicals affecting serial steps of signaling pathways and membrane-permeable cAMP analog. The results show that light-induced bidirectional movements in tilapia erythrophores may be controlled by cytosolic cAMP levels via Gi- or Gs-type G proteins. In addition, RT-PCR demonstrated for the first time the expression of mRNAs encoding red and green opsins in tilapia fins, only where erythrophores exist. Here, we suggest that multiple cone-type visual pigments may be present in the erythrophores, and that unique cascades in which such opsins couple to Gi or Gs-type G proteins are involved in the photoresponses in these pigment cells. Thus, tilapia erythrophore system seems to be a nice model for understanding the photoresponses of cells other than visual cells.

Further evidence for synthesis of screening pigment granules involved in the photosensory membrane turnover of the crayfish photoreceptor.

Photosensory membrane degradation in crayfish occurs at first in multi-vesicular bodies (MVBs) and then, with the aid of lysosomal enzymes, in lysosome related lamellar bodies. In organ culture experiments with the isolated crayfish retina (Orconectes limosus) small screening pigment-like granules became visible under the electron microscope in such lamellar bodies and suggested a possible relation of photosensory membrane degradation and screening pigment granule synthesis. Chloroquine, an inhibitor of lysosomal activity, when added to the culture medium reduced the appearance of screening pigment-like granules in lamellar bodies, but led to the appearance of these granules in mature MVB's, indicating the involvement of lysosomal enzymes in the formation of pigmented lamellar bodies. In a second set of experiments the effect of bright light on the screening pigment granule ultrastructure of crayfish phoreceptors was investigated. It was found that after bright light exposure large numbers of little screening pigment granules (0.15-0.3 microns) were located between or close to rhabdomeral microvilli that were not at these sites in crayfish kept under natural light. MVB's were also reduced in size, and among the little screening pigmentary organelles granules of different electron density and morphology appeared. Additionally, vesicle flux to little screening pigment granules was detected. The screening pigment granules of the little type did not seem to be transported close to or between the microvilli, but appeared to be synthesized at these sites within little MVBs.

Comparison of the contribution from different energy-linked reactions to the function of a membrane potential in photosynthetic bacteria.

The steady-state membrane potentials generated by light, PP(i), ATP or the reverse transhydrogenase reaction were studied in chromatophores from two different phototrophic bacteria, Rhodospirillum rubrum and Rhodopseudomonas viridis. The membrane potentials generated by the different energy-linked reactions were evaluated by a tetraphenylboron(TPB(-)) ion-selective electrode. The generated by light was estimated to be 110 mV and 50 mV in R. rubrum and Rps. viridis chromatophores, respectively. In the dark, PP (i), ATP and reversed transhydrogenase generated membrane potentials in R. rubrum and Rps. viridis chromatophores 50, 60 and 35 mV, and 14, 35 and 25 mV,respectively. The effect of magnesium ion on the membrane potential generated by different energy-linked reactions was also studied. The induced by different energy-generating reactions in R. rubrum and Rps. viridis chromatophores and the possible relationship to the chromatophore structures are discussed.