The endothelin receptors that mediate aggregation of pigment in fish melanophores

Colour changes in Labeo rohita (Ham.) due to pigment translocation in melanophores, on exposure to municipal wastewater of Tung Dhab drain, Amritsar, India

The present study was aimed to evaluate the effect of municipal wastewater of Tung Dhab drain on morphology of scale melanophores in freshwater fish Labeo rohita (Ham.). Chronic, non-renewal toxicity tests were performed with sub lethal concentrations (17.7, 26.6 and 35.4%) of wastewater for exposure durations of 15, 30 and 60 days. Recovery experiments were also performed for duration of 60 days. The scales were removed, processed and diameters of melanophores were measured using an ocular micrometer. The results showed concentration and duration dependent changes in melanophore morphology (size and dendricity) in experimental fish as compared to control fish. The values of mean melanophore size index (MMSI) varied from 5.37 ± 0.49 (17.7%) to 12.12 ± 0.81 (35.5%) in comparison to control values of 4.32 ± 0.32 and 4.55 ± 0.29 for 15 and 60 days respectively. The recovery experiments suggested that observed dark colouration due to pigment translocation is reversible, even after chronic exposure for 60 days.

Endothelin modulates the circadian expression of non-visual opsins

The non-visual opsin, melanopsin, expressed in the mammalian retina, is considered a circadian photopigment because it is responsible to entrain the endogenous biological clock. This photopigment is also present in the melanophores of Xenopus laevis, where it was first described, but its role in these cells is not fully understood. X. laevis melanophores respond to light with melanin granule dispersion, the maximal response being achieved at the wavelength of melanopsin maximal excitation. Pigment dispersion can also be triggered by endothelin-3 (ET-3). Here we show that melanin translocation is greater when a blue light pulse was applied in the presence of ET-3. In addition, we demonstrated that mRNA levels of the melanopsins Opn4x and Opn4m exhibit temporal variation in melanophores under light/dark (LD) cycles or constant darkness, suggesting that this variation is clock-driven. Moreover, under LD cycles the oscillations of both melanopsins show a circadian profile suggesting a role for these opsins in the photoentrainment mechanism. Blue-light pulse decreased Opn4x expression, but had no effect on Opn4m. ET-3 abolishes the circadian rhythm of expression of both opsins; in addition the hormone increases Opn4x expression in a dose-, circadian time- and light-dependent way. ET-3 also increases the expression of its own receptor, in a dose-dependent manner. The variation of melanopsin levels may represent an adaptive mechanism to ensure greater melanophore sensitivity in response to environmental light conditions with ideal magnitude in terms of melanin granule dispersion, and consequently color change.

Rapid color change in fish and amphibians - function, regulation, and emerging applications

Physiological color change is important for background matching, thermoregulation as well as signaling and is in vertebrates mediated by synchronous intracellular transport of pigmented organelles in chromatophores. We describe functions of and animal situations where color change occurs. A summary of endogenous and external factors that regulate this color change in fish and amphibians is provided, with special emphasis on extracellular stimuli. We describe not only color change in skin, but also highlight studies on color change that occurs using chromatophores in other areas such as iris and on the inside of the body. In addition, we discuss the growing field that applies melanophores and skin color in toxicology and as biosensors, and point out research areas with future potential.

Expression of endothelin receptors in frog, chicken, mouse and human pigment cells

Several reports have shown the participation of vasoactive endothelins (ETs) in the regulation of vertebrate pigment cells. In the present study, we identified ET receptors in pigment cells of vertebrate species by RT-PCR assays, and compared the differential expression of the various subtypes in each species by quantitative PCR. RT-PCR was performed with specific primers for ETC, ETA(X) or ETA in Xenopus laevis melanophores, ETA or ETB(2) in chicken melanocytes, ETA or ETB in murine (B-16, S-91 or Melan-A) or human (SK-Mel 23 or SK-Mel 28) melanoma cells, and the products obtained were confirmed by cloning and sequencing. The results showed the presence of ETA(X), but not ETA mRNA, and confirmed the expression of ETC in X. laevis melanophores. ETA and ETB(2) mRNAs were also demonstrated in chicken melanocytes. ETA and ETB receptor were identified in S-91, B16 and Melan-A murine cells. In human melanoma cells, SK-Mel 23 and SK-Mel 28, we confirmed the presence of ETB mRNA, and also found ETA mRNA. The comparison between the two subtypes present in the pigment cell of each species and among species demonstrated that the expression of ETAs in chicken, mouse, and human melanocytes is negligible, as is the expression of ETA(X) in Xenopus melanophores. The relative expression, as determined by quantitative PCR, was as follows: chicken ETB>SK-Mel 23 ETB>S91 ETB>>>Xenopus ETC, suggesting that the endothelin system plays a major role in avian and mammalian pigment cell regulation, as compared to lower vertebrates. The phylogenetic analysis revealed that subtype A receptors were probably the most primitive ET receptors, directly deriving from the ancestral type; all the other receptors, B subtypes and C, originated from diverse derivative molecules.

Chromatic effects of endothelin family peptides in non-innervated fish,Synbranchus marmoratus, melanophores

The biological activity of endothelins (ETs) in non-innervated Synbranchus marmoratus melanophores was demonstrated. These peptides induced a dose-dependent pigment aggregation (lightening skin) in these cells. However, they presented EC50's (effective concentration required to produce 50% of response) 26, 106 and 35 times higher than, respectively, the melanin concentrating hormone (MCH) EC50, and exhibited a characteristic temporal and dose-dependent autodessensibilization of the aggregative effect on the melanophores of this fish. The receptor characterization suggested the presence of the ET(B) subtype, since BQ-788 (selective antagonist of ET(B)) but not BQ-485 (selective antagonist of ET(A)) blocked the aggregative effect of the hormones. Confirming these data, sarafotoxin (SRTX) S6c, a toxin selective for ET(B), induced maximal aggregation of pigment granules. S6c presented an EC50 6.8 times higher than the MCH EC50, and 3.9, 15.6 and 5.1 times lower than the EC50's ETs, respectively. The melanotropic effect of SRTX S6b and vasoactive intestinal contractor (VIC) were demonstrated for the first time in this work. SRTX S6b induced a dose-dependent pigment aggregation and presented an EC50 2.54 and 17.2 times higher than the S6c and MCH EC50's, respectively. Compared to the ETs it was 1.53, 6.19 and 2.03 times lower, respectively.

The Endothelin/Sarafotoxin-Induced Increase of the Proliferation of Undifferentiated and DMSO-Differentiated GEM-81 Goldfish Erythrophoroma Cells is Mediated by ETB Receptors

Endothelins (ETs) and sarafotoxins (SRTXs) have been reported to exert ET(B)-mediated effects on vertebrate pigment cells. GEM-81 cell line, a red pigment cell-derived cutaneous tumor of the teleost Carassius auratus, expresses ET(B) receptors and can be differentiated with 1.5% DMSO treatment, thus constituting an useful model to investigate ET and SRTX effects on cultured fish pigment cells. Our aim was to characterize the pharmacology and biological effects mediated by ET receptors in DMSO-differentiated and undifferentiated cells. ET subtype receptors and their respective Ki values in both cell types were determined by competitive binding assays using (125)I ET-1 and BQ-485 (an ET(A) antagonist) or BQ-788 (an ET(B) antagonist). BQ-788, but not BQ-485, significantly reduced (125)I-ET-1 binding in both cell types, with similar low (Ki > nM) affinities. To determine the proliferation effects of ETs/SRTXs, cells were treated for 72 h with the hormones, and counted in a hemocytometer. The proliferation assays were repeated for SRTX S6c in the presence or absence of BQ-788. The results demonstrated that, with the exception of ET-1 (biphasic effect) and ET-3 (no significant effect) in undifferentiated GEM-81 cells, all the tested hormones induced increases in the proliferation of both types of cells. The hormones were equipotent in DMSO-differentiated cells, which exhibited increased sensitivity to ETs, but not to SRTXs, as compared with undifferentiated cells. The BQ-788 antagonistic effect was also exerted on the proliferation responses to SRTX S6c. These results corroborate the long and important evolutionary history of the ET/SRTX receptor system in vertebrate pigment cells.