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.
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.
Isomerization of amino acids in proteins has recently been identified as a part of the aging process. Increases in d-amino acids as a consequence of isomerization influence the function and structure of proteins. Senescence-related pulmonary diseases, such as chronic obstructive pulmonary disease, are thought to be caused by reductions of lung function with age. We hypothesized that changes of protein structure in lung tissue induced by the isomerization of amino acids could result in decreased lung function. Therefore, we examined whether isomerization of amino acids takes place in the lungs of rats as they age. We measured the content of land d-amino acids in collagen 1 by HPLC using a chiral column. We found that collagen 1 was increasingly racemized with age, so that significantly higher proportions of d-Ser were present in 12-and 24-month-old rats than in 8-week-old rats. d-Asp increased slightly but not significantly. We also investigated the localization of collagen 1 in lung tissue. Stacks of collagen 1 were observed in the parenchyma and airway wall, and age-dependent changes were especially prominent in the airway wall. Racemization of collagen 1 could therefore influence lung function and contribute to pulmonary diseases.
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