Regulation of pigmentation in zebrafish melanophores

The genetic basis of divergent melanic pigmentation in benthic and limnetic threespine stickleback

Pigmentation is an excellent trait to examine patterns of evolutionary change because it is often under natural selection. Benthic and limnetic threespine stickleback (Gasterosteus aculeatus) exhibit distinct pigmentation phenotypes, likely an adaptation to occupation of divergent niches. The genetic architecture of pigmentation in vertebrates appears to be complex. Prior QTL mapping of threespine stickleback pigmentation phenotypes has identified several candidate loci. However-relative to other morphological phenotypes (e.g., spines or lateral plates)-the genetic architecture of threespine stickleback pigmentation remains understudied. Here, we performed QTL mapping for two melanic pigmentation traits (melanophore density and lateral barring) using benthic-limnetic F2 crosses. The two traits mapped to different chromosomes, suggesting a distinct genetic basis. The resulting QTLs were additive, but explained a relatively small fraction of the total variance (~6%). QTLs maps differed by F1 family, suggesting variation in genetic architecture or ability to detect loci of small effect. Functional analysis identified enriched pathways for candidate loci. Several of the resulting candidate loci for pigmentation, including three loci in enriched pathways (bco1, sulf1, and tyms) have been previously indicated to affect pigmentation in other vertebrates. These findings add to a growing body of evidence suggesting pigmentation is often polygenic.

Identification of pigmentation genes in skin, muscle and tail of a Thai-flag variety of Siamese fighting fish Betta splendens

Pigmentation genes expressed in skin, body muscle and tail of Thai-flag compared with Blue, White and Red varieties of Siamese fighting fish Betta splendens were identified. In total, 22,919 new unigenes were found. Pearson correlation and PCA analysis revealed that expression profiles of genes in muscle, skin and tail across solid color variety were similar. In contrast, those in skin and red tail part of Thai-flag were closely related but they showed different expression profiles with the white tail part. Moreover, 21,347-64,965 SNPs were identified in exonic regions of identified genes. In total, 28,899 genes were differentially expressed between paired comparisons of libraries where 13,907 genes (48.12 %) were upregulated and 14,992 genes (51.88 %) were downregulated. DEGs between paired libraries were 106-5775 genes relative to the compared libraries (56-2982 and 50-2782 for upregulated and downregulated DEGs). Interestingly, 432 pigmentation genes of B. splendens were found. Of these, 297 DEGs showed differential expression between varieties. Many DEGs in melanogenesis (Bsmcr1r, Bsmcr5r, and Bsslc2a15b), tyrosine metabolism (Bstyr, Bstyrp1b and Bsdct), stripe repressor (BsAsip1 and BsAsip2b), pteridine (Bsgch2) and carotenoid (BsBco2) biosynthesis were downregulated in the Thai-flag compared with solid color varieties. Expression of Bsbco1l, Bsfrem2b, Bskcnj13, Bszic2a and Bspah in skin, muscle and tail of Thai-flag, Blue, Red and White varieties was analyzed by qRT-PCR and revealed differential expression between fish varieties and showed anatomical tissue-preferred expression patterns in the same fish variety. The information could be applied to assist genetic-based development of new B. splendens varieties in the future.

Effects of Water Temperature on the Body Color and Expression of the Genes Related to Body Color Regulation in the Goldfish

Melanin-concentrating hormone (MCH), melanocyte-stimulating hormone (MSH), and somatolactin (SL) in the hypothalamus-pituitary axis are associated with body color regulation in teleost fish. Although these hormones' production and secretion respond well to light environments, such as background color, little is known about the effects of different water temperatures. We investigated the effects of water temperature, 10°C, 20°C, and 30°C, on body color and the expression of these genes and corresponding receptor genes in goldfish. The body color in white background (WBG) becomes paler at the higher water temperature, although no difference was observed in black background (BBG). Brain mRNA contents of proMCH genes (pmch1 and pmch2) increased at 30°C and 20°C compared to 10°C in WBG, respectively. Apparent effects of background color and temperature on the pituitary mRNA contents of a POMC gene (pomc) were not observed. The pituitary mRNA contents of the SLα gene were almost double those on a WBG at any temperature, while those of the SLβ gene (slb) at 30°C tended to be higher than those at 10°C and 20°C on WBG and BBG. The scale mRNA contents of the MCH receptor gene (mchr2) in WBG were higher than those in BBG at 30°C. The highest scale mRNA contents of MSH receptor (mc1r and mc5r) on BBG were observed at 20°C, while the lowest respective mRNA levels were observed at 30°C on WBG. These results highlight the importance of temperature for the endocrinological regulation of body color, and darker background color may stabilize those endocrine functions.

Light-specific wavelengths differentially affect the exploration rate, opercular beat, skin color change, opsin transcripts, and the oxi-redox system of the longsnout seahorse Hippocampus reidi

Light is a strong stimulus for the sensory and endocrine systems. The opsins constitute a large family of proteins that can respond to specific light wavelengths. Hippocampus reidi is a near-threatened seahorse that has a diverse color pattern and sexual dimorphism. Over the years, H. reidi's unique characteristics, coupled with its high demand and over-exploitation for the aquarium trade, have raised concerns about its conservation, primarily due to their significant impact on wild populations. Here, we characterized chromatophore types in juvenile and adult H. reidi in captivity, and the effects of specific light wavelengths with the same irradiance (1.20 mW/cm2) on color change, growth, and survival rate. The xanthophores and melanophores were the major components of H. reidi pigmentation with differences in density and distribution between life stages and sexes. In the eye and skin of juveniles, the yellow (585 nm) wavelength induced a substantial increase in melanin levels compared to the individuals kept under white light (WL), blue (442 nm), or red (650 nm) wavelengths. In addition, blue and yellow wavelengths led to a higher juvenile mortality rate in comparison to the other treatments. Adult seahorses showed a rhythmic color change over 24 h, the highest reflectance values were obtained in the light phase, representing a daytime skin lightening for individuals under WL, blue and yellow wavelength, with changes in the acrophase. The yellow wavelength was more effective on juvenile seahorse pigmentation, while the blue wavelength exerted a stronger effect on the regulation of adult physiological color change. Dramatic changes in the opsin mRNA levels were life stage-dependent, which may infer ontogenetic opsin functions throughout seahorses' development. Exposure to specific wavelengths differentially affected the opsins mRNA levels in the skin and eyes of juveniles. In the juveniles, skin transcripts of visual (rh1, rh2, and lws) and non-visual opsins (opn3 and opn4x) were higher in individuals under yellow light. While in the juvenile's eyes, only rh1 and rh2 had increased transcripts influenced by yellow light; the lws and opn3 mRNA levels were higher in juveniles' eyes under WL. Prolonged exposure to yellow wavelength stimulates a robust increase in the antioxidant enzymes sod1 and sod2 mRNA levels. Our findings indicate that changes in the visible light spectrum alter physiological processes at different stages of life in H. reidi and may serve as the basis for a broader discussion about the implications of artificial light for aquatic species in captivity.

Standardization of zebrafish drug testing parameters for muscle diseases

Skeletal muscular diseases predominantly affect skeletal and cardiac muscle, resulting in muscle weakness, impaired respiratory function and decreased lifespan. These harmful outcomes lead to poor health-related quality of life and carry a high healthcare economic burden. The absence of promising treatments and new therapies for muscular disorders requires new methods for candidate drug identification and advancement in animal models. Consequently, the rapid screening of drug compounds in an animal model that mimics features of human muscle disease is warranted. Zebrafish are a versatile model in preclinical studies that support developmental biology and drug discovery programs for novel chemical entities and repurposing of established drugs. Due to several advantages, there is an increasing number of applications of the zebrafish model for high-throughput drug screening for human disorders and developmental studies. Consequently, standardization of key drug screening parameters, such as animal husbandry protocols, drug compound administration and outcome measures, is paramount for the continued advancement of the model and field. Here, we seek to summarize and explore critical drug treatment and drug screening parameters in the zebrafish-based modeling of human muscle diseases. Through improved standardization and harmonization of drug screening parameters and protocols, we aim to promote more effective drug discovery programs.

Comparative Transcriptome Analysis of the Skin and the Peritoneal Wall Layer of Triplophysa stenura Distributed in High Elevations

A total of 81,868 All-Unigenes were sequenced and assembled by the transcriptome in the dorsal skin, the lateral skin, and the peritoneal wall layer of Triplophysa stenura with a total assembly length of 123,827,585 bp, and 68,750 unigenes were annotated to seven functional databases. A total of 588 DEGs were screened between the dorsal and lateral skin, 17,097 DEGs were screened between the dorsal skin and the peritoneal wall layer, and 16,598 DEGs were screened between the lateral skin and the peritoneal wall layer. Most of DEGs in three tissues were annotated to GO terms related to cellular structures, binding, cellular processes, and catalytic activity. They were also annotated to KEGG pathways such as the MAPK signaling pathway, PI3K-Akt signaling pathway, Wnt signaling pathway, melanogenesis, tyrosine metabolism, and cell cycle. A total of twenty-three DEGs were found to be enriched in the melanin synthesis pathway by a local Blast comparison, of which nine DEGs were significantly upregulated in the peritoneal wall layer and six DEGs were significantly upregulated in the dorsal and lateral skin. The results suggest that these genes may be associated with the molecular mechanism of melanin synthesis in T. stenura, and the differential regulation of genes may be related to the differences in UVR intensity and tissue sites of melanin synthesis. Further investigation is needed on how these genes specifically regulate melanin synthesis.

Novel Insights on 6:6 Perfluoroalkyl Phosphonic Acid-Induced Melanin Synthesis Disorders Leading to Pigmentation in Tadpoles

As alternatives to traditional per- and polyfluoroalkyl substances, perfluoroalkyl phosphonic acids (PFPiAs) are widely present in aquatic environments and can potentially harm aquatic organisms. Pigmentation affects the probability of aquatic organisms being preyed on and serves as an important toxic endpoint of development, but little is known about the impacts of PFPiAs on the development of aquatic organisms. In this study, Xenopus laevis embryos were exposed to 6:6 PFPiA (1, 10, and 100 nM) for 14 days. The developed tadpoles exhibited evident pigmentation with increased melanin particle size and density on the skin. Pathological and behavioral experiments revealed that the retinal layers became thinner, reducing the photosensitivity and disturbing the circadian rhythm of the tadpoles. Compared to the control group, the exposed tadpoles showed higher levels but less changes of melanin throughout the light/dark cycle, as well as distinct oxidative damage. Consequently, the expression level of microphthalmia-associated transcription factor (MITF), a key factor inducing melanin synthesis, increased significantly. Molecular docking analysis suggested that 6:6 PFPiA forms strong interactions in the binding pocket of MITF, implying that it could activate MITF directly. The activation of MITF ultimately promotes melanin synthesis, resulting in pigmentation on tadpoles.

Characterising the Effect of Wnt/β-Catenin Signalling on Melanocyte Development and Patterning: Insights from Zebrafish (Danio rerio)

Zebrafish (Danio rerio) is a well-established model organism for studying melanocyte biology due to its remarkable similarity to humans. The Wnt signalling pathway is a conserved signal transduction pathway that plays a crucial role in embryonic development and regulates many aspects of the melanocyte lineage. Our study was designed to investigate the effect of Wnt signalling activity on zebrafish melanocyte development and patterning. Stereo-microscopic examinations were used to screen for changes in melanocyte count, specific phenotypic differences, and distribution in zebrafish, while microscopic software tools were used to analyse the differences in pigment dispersion of melanocytes exposed to LiCl (Wnt enhancer) and W-C59 (Wnt inhibitor). Samples exposed to W-C59 showed low melanocyte densities and defects in melanocyte phenotype and patterning, whereas LiCl exposure demonstrated a stimulatory effect on most aspects of melanocyte development. Our study demonstrates the crucial role of Wnt signalling in melanocyte lineage and emphasises the importance of a balanced Wnt signalling level for proper melanocyte development and patterning.

Effects of monochromatic lights on the melanophores arrangement in the spotted snakehead fish Channa punctata (Bloch, 1793)

Some freshwater teleost fish have pigment cells whose arrangement and shape are affected by the environment. Natural light has a wide range of light intensity. Fish are sensitive to the background and exposed light colour. Fish body colour is a significant criterion in fixing its market value, whether it is ornamental or edible. By favourable light exposure, a culturist may get a good market value of fish on most ethical grounds. In this study, we recorded the changes in melanophore response with the changes in light colour on Channa punctata. Adult fish were treated with monochromatic lights (darkness, white, blue and red light) for 5 and 28 days. After treatment, their body colour and melanophore size, number, length and the number of dendrites were studied. The results showed a significant influence of monochromatic light on melanophore arrangement in fish skin. The data showed that blue light is appropriate for the overall species colour of photic C. punctata. Continuous black or white light caused severe damage to the fish's appearance.

Mitochondrial calcium signaling mediated transcriptional regulation of keratin filaments is a critical determinant of melanogenesis

Mitochondria are versatile organelles that regulate several physiological functions. Many mitochondria-controlled processes are driven by mitochondrial Ca2+ signaling. However, role of mitochondrial Ca2+ signaling in melanosome biology remains unknown. Here, we show that pigmentation requires mitochondrial Ca2+ uptake. In vitro gain and loss of function studies demonstrated that Mitochondrial Ca2+ Uniporter (MCU) is crucial for melanogenesis while the MCU rheostats, MCUb and MICU1 negatively control melanogenesis. Zebrafish and mouse models showed that MCU plays a vital role in pigmentation in vivo. Mechanistically, MCU controls activation of transcription factor NFAT2 to induce expression of three keratins (keratin 5, 7 and 8), which we report as positive regulators of melanogenesis. Interestingly, keratin 5 in turn modulates mitochondrial Ca2+ uptake thereby this signaling module acts as a negative feedback loop that fine-tunes both mitochondrial Ca2+ signaling and melanogenesis. Mitoxantrone, an FDA approved drug that inhibits MCU, decreases physiological melanogenesis. Collectively, our data demonstrates a critical role for mitochondrial Ca2+ signaling in vertebrate pigmentation and reveal the therapeutic potential of targeting MCU for clinical management of pigmentary disorders. Given the centrality of mitochondrial Ca2+ signaling and keratin filaments in cellular physiology, this feedback loop may be functional in a variety of other pathophysiological conditions.

Coordinated microRNA/mRNA Expression Profiles Reveal Unique Skin Color Regulatory Mechanisms in Chinese Giant Salamander (Andrias davidianus)

The Chinese giant salamander (Andrias davidianus) has been increasingly popular in the aquaculture market in China in recent years. In the breeding process of Andrias davidianus, we found that some albino individuals were extremely rare and could not be inherited stably, which severely limits their commercialization in the aquaculture market. In this study, we performed transcriptome and small RNA (sRNA) sequencing analyses in the skin samples of wild-type (WT) and albino (AL) Andrias davidianus. In total, among 5517 differentially expressed genes (DEGs), 2911 DEGs were down-regulated in AL, including almost all the key genes involved in melanin formation. A total of 25 miRNAs were differentially expressed in AL compared to WT, of which 17 were up-regulated. Through the integrated analysis, no intersection was found between the target genes of the differentially expressed miRNAs and the key genes for melanin formation. Gene Ontology (GO) and KEGG pathway analyses on DEGs showed that these genes involved multiple processes relevant to melanin synthesis and the key signal pathway MAPK. Interestingly, the transcription factors SOX10 and PAX3 and the Wnt signaling pathway that play a key role in other species were not included, while the other two transcription factors in the SOX family, SOX21 and SOX7, were included. After analyzing the key genes for melanin formation, it was interesting to note an alternative splicing form of the MITF in WT and a critical mutation of the SLC24A5 gene in AL, which might be the main reason for the skin color change of Andrias davidianus. The results contributed to understanding the molecular mechanism of skin pigmentation in Andrias davidianus and accelerating the acquisition process of individuals with specific body colors by genetic means.

Piebaldism and chromatophore development in reptiles are linked to the tfec gene

Reptiles display great diversity in color and pattern, yet much of what we know about vertebrate coloration comes from classic model species such as the mouse and zebrafish.1,2,3,4 Captive-bred ball pythons (Python regius) exhibit a remarkable degree of color and pattern variation. Despite the wide range of Mendelian color phenotypes available in the pet trade, ball pythons remain an overlooked species in pigmentation research. Here, we investigate the genetic basis of the recessive piebald phenotype, a pattern defect characterized by patches of unpigmented skin (leucoderma). We performed whole-genome sequencing and used a case-control approach to discover a nonsense mutation in the gene encoding the transcription factor tfec, implicating this gene in the leucodermic patches in ball pythons. We functionally validated tfec in a lizard model (Anolis sagrei) using the gene editing CRISPR/Cas9 system and TEM imaging of skin. Our findings show that reading frame mutations in tfec affect coloration and lead to a loss of iridophores in Anolis, indicating that tfec is required for chromatophore development. This study highlights the value of captive-bred ball pythons as a model species for accelerating discoveries on the genetic basis of vertebrate coloration.

Bisphenol S disrupts opsins gene expression and impairs the light-sensing function via antagonizing TH-TRβ signaling pathway in zebrafish larvae

Bisphenol S (BPS) is extensively used in "bisphenol A-free" products such as baby bottles. Although the visual toxicity of BPS has been reported, the underlying mechanism was largely unknown. In the present study, zebrafish were exposed to 0, 4 and 400 nM BPS from 2 h post-fertilization (hpf) to 120 hpf to further explore the thyroid disruption mechanism underlying the BPS induced impairment of visual function. The results showed that BPS decreased T3 levels in larval eyes, induced retinal expression of thyroid hormone receptor β (TRβ), and thereby down-regulated the expression of TH-mediated opsin genes (opn1lw1, opn1lw2, opn1mw1, opn1mw2, opn1mw3, and opn1sw2) and impaired subsequent phototransduction pathways, leading to decreased visually mediated phototactic response and body color adaptation but stimulated visual motor response (VMR). Combining exposure of exogenous T3 or 1-850 (antagonist for TRβ) with BPS could partly compensate the inhibited expression of opsin genes (opn1mw2, opn1lw1, and opn1lw2) and alleviate the hyperactivity of larval VMR caused by BPS alone, suggesting that BPS disrupted the opsins expression and also light-sensing function via antagonizing TH-TRβ signaling pathway. This study underlined the importance of TH signaling in regulating the proper vision and proposed a novel mechanism for the visual toxicity of BPS.

Anti-Melanogenic Potential of Natural and Synthetic Substances: Application in Zebrafish Model

Melanogenesis is a biosynthetic pathway for the formation of the pigment melanin in human skin. A key enzyme in the process of pigmentation through melanin is tyrosinase, which catalyzes the first and only limiting step in melanogenesis. Since the discovery of its methanogenic properties, tyrosinase has been the focus of research related to the anti-melanogenesis. In addition to developing more effective and commercially safe inhibitors, more studies are required to better understand the mechanisms involved in the skin depigmentation process. However, in vivo assays are necessary to develop and validate new drugs or molecules for this purpose, and to accomplish this, zebrafish has been identified as a model organism for in vivo application. In addition, such model would allow tracking and studying the depigmenting activity of many bioactive compounds, important to genetics, medicinal chemistry and even the cosmetic industry. Studies have shown the similarity between human and zebrafish genomes, encouraging their use as a model to understand the mechanism of action of a tested compound. Interestingly, zebrafish skin shares many similarities with human skin, suggesting that this model organism is suitable for studying melanogenesis inhibitors. Accordingly, several bioactive compounds reported herein for this model are compared in terms of their molecular structure and possible mode of action in zebrafish embryos. In particular, this article described the main metabolites of Trichoderma fungi, in addition to substances from natural and synthetic sources.

Redundancy analysis, genome-wide association studies and the pigmentation of brown trout (Salmo trutta L.)

The association of molecular variants with phenotypic variation is a main issue in biology, often tackled with genome-wide association studies (GWAS). GWAS are challenging, with increasing, but still limited, use in evolutionary biology. We used redundancy analysis (RDA) as a complimentary ordination approach to single- and multitrait GWAS to explore the molecular basis of pigmentation variation in brown trout (Salmo trutta) belonging to wild populations impacted by hatchery fish. Based on 75,684 single nucleotide polymorphic (SNP) markers, RDA, single- and multitrait GWAS allowed the extraction of 337 independent colour patterning loci (CPLs) associated with trout pigmentation traits, such as the number of red and black spots on flanks. Collectively, these CPLs (i) mapped onto 35 out of 40 brown trout linkage groups indicating a polygenic genomic architecture of pigmentation, (ii) were found to be associated with 218 candidate genes, including 197 genes formerly mentioned in the literature associated to skin pigmentation, skin patterning, differentiation or structure notably in a close relative, the rainbow trout (Onchorhynchus mykiss), and (iii) related to functions relevant to pigmentation variation (e.g., calcium- and ion-binding, cell adhesion). Annotated CPLs include genes with well-known pigmentation effects (e.g., PMEL, SLC45A2, SOX10), but also markers associated with genes formerly found expressed in rainbow or brown trout skins. RDA was also shown to be useful to investigate management issues, especially the dynamics of trout pigmentation submitted to several generations of hatchery introgression.

Characterization and functional analysis of pax3 in body color transition of polychromatic Midas cichlids (Amphilophus citrinellus)

As the representative genetic and economic trait of ornamental fish, skin color has a strong impact on speciation and adaptation. However, the genetic basis of skin color pigmentation, differentiation and change is still not understood. The Midas cichlid fish with three typical body color transition stages of "black-gray‑gold" is an ideal model system for investigating the formation and change of fish body color. In this study, to investigate the regulatory role of the pair box 3 (pax3) gene in the early body color fading process of Midas cichlids, the complete cDNA sequence (3513 bp) of pax3 was successfully isolated from Midas cichlids (Amphilophus Citrinellus), and found to encode polypeptides of 491 amino acids. Expression patterns of the pax3 gene in tissues of Midas cichlids during different periods, including embryonic development and body color fading stages were detected by quantitative real-time PCR. The qRT-PCR analysis showed that pax3 was expressed in all tissues of adult fish, with a higher expression level in muscle and skin. The highest expression level in muscle tissue was significantly higher than that in other tissues (P < 0.05). During embryonic development, the expression tendency of pax3 was first increased and then decreased. In the three typical stages of early skin color fading from black to gold, pax3 expression in skin, caudal fin and scales all showed a downward trend. The expression level in the black stage was significantly higher than that in other stages (P < 0.05). Positive signal of pax3 protein was detected in the three typical skin color conversion stages, and the highest positive signal intensity was detected in the black stage, which was consistent with qRT-PCR results. After pax3 RNA interference, pax3 and the downstream genes mitf and tyr all decreased, while dct mRNA expression increased in the skin of fish. Western blotting also showed a decrease in pax3 protein concentration. Those results suggest that pax3 plays an important role in skin color formation, distribution and change in Midas cichlids through the melanogenesis pathway.

Requirement of Zebrafish Adcy3a and Adcy5 in Melanosome Dispersion and Melanocyte Stripe Formation

cAMP-PKA signaling plays a pivotal role in melanin synthesis and melanosome transport by responding to the binding of the α-melanocyte-stimulating hormone (α-MSH) to melanocortin-1 receptor (MC1R). Adenylate cyclases (ADCYs) are the enzymes responsible for the synthesis of cAMP from ATP, which comprises nine transmembrane isoforms (ADCYs 1-9) and one soluble adenylate cyclase (ADCY 10) in mammals. However, little is known about which and how ADCY isoforms regulate melanocyte generation, melanin biosynthesis, and melanosome transport in vivo. In this study, we have generated a series of single and double mutants of Adcy isoforms in zebrafish. Among them, adcy3a^-/- and adcy5^-/- double mutants cause defects in melanosome dispersion but do not impair melanoblast differentiation and melanocyte regeneration during the embryonic or larval stages. Activation of PKA, the main effector of cAMP signaling, significantly ameliorates the defects in melanosome dispersion in adcy3a^-/- and adcy5^-/- double mutants. Mechanistically, Adcy3a and Adcy5 regulate melanosome dispersion by activating kinesin-1 while inhibiting cytoplasmic dynein-1. In adult zebrafish, Adcy3a and Adcy5 participate in the regulation of the expression of microphthalmia transcription factor (Mitfa) and melanin synthesis enzymes Tyr, Dct, and Trp1b. The deletion of Adcy3a and Adcy5 inhibits melanin production and reduces pigmented melanocyte numbers, causing a defect in establishing adult melanocyte stripes. Hence, our studies demonstrate that Adcy3a and Adcy5 play essential but redundant functions in mediating α-MSH-MC1R/cAMP-PKA signaling for regulating melanin synthesis and melanosome dispersion.

Design and Synthesis of (Z)-5-(Substituted benzylidene)-3-cyclohexyl-2-thioxothiazolidin-4-one Analogues as Anti-Tyrosinase and Antioxidant Compounds: In Vitro and In Silico Insights

Many compounds containing the β-phenyl-α,β-unsaturated carbonyl (PUSC) scaffold, including cinnamamide derivatives, have been shown to inhibit tyrosinase potently in vitro and in vivo. Structural changes to cinnamamide derivatives were produced by adding a dithionate functional group to provide eight (Z)-5-(substituted benzylidene)-3-cyclohexyl-2-thioxothiazolidin-4-one analogs with high log p values for skin. These analogs were synthesized using a two-step reaction, and their stereochemistry was confirmed using the ³J_C4-Hβ values of C4 measured in proton-coupled ¹³C mode. Analogs 2 (IC₅₀ = 5.21 ± 0.86 µM) and 3 (IC₅₀ = 1.03 ± 0.14 µM) more potently inhibited mushroom tyrosinase than kojic acid (IC₅₀ = 25.26 ± 1.10 µM). Docking results showed 2 binds strongly to the active site of tyrosinase, while 3 binds strongly to an allosteric site. Kinetic studies using l-tyrosine as substrate indicated 2 and 3 competitively and non-competitively inhibit tyrosinase, respectively, which was supported by our docking results. In B16F10 cells, 3 significantly and concentration-dependently reduced α–MSH plus IBMX induced increases in cellular tyrosinase activity and melanin production and the similarity between these inhibitory patterns implied that the anti-melanogenic effect of 3 might be due to its tyrosinase-inhibitory ability. In addition, 2 and 3 exhibited strong antioxidant effects; for example, they reduced ROS and ONOO^– levels and exhibited radical scavenging activities, suggesting that these effects might underlie their anti-melanogenic effects. Furthermore, 3 suppressed the expressions of melanogenesis-associated proteins and genes in B16F10 cells. These results suggest (Z)-5-(substituted benzylidene)-3-cyclohexyl-2-thioxothiazolidin-4-one analogs offer a means of producing novel anti-melanogenesis agents.

Transcriptome comparison for identification of pigmentation-related genes in different color varieties of Siamese fighting fish Betta splendens

Transcriptome comparison was performed to identify genes expressed in skin, muscle and tails of mono-color (Red, Blue, Black, White and Yellow), bi-color (Cambodian) and multi-color (Marble) varieties of Siamese fighting fish Betta splendens. In total, 163,140 unigenes covering 26.348 Gb were found. Of these, 93,899 (57.55 %) unigenes significantly matched at least one database. In total, 5039 differentially expressed genes (DEGs) were found where 2415 genes (47.93 %) showed higher expression and 2624 genes (52.07 %) showed lower expression for all pairwise comparisons. DEGs between paired color varieties were 133-443. Of these, 38-220 genes were more highly expressed while 37-280 genes were more lowly expressed relative to the compared varieties. A total of 897 sequences (148 genes) significantly matched pigmentation-related genes of Danio rerio (E-value < 1e-06). Of these, 19 DEGs were identified. Examples are tyrosinase-related protein 1a (BsTyrp1a), epidermal growth factor receptor (BsEgfr) and neurofibronin 1a (BsNf1a). Moreover, 711,123 SNPs were identified and 1365 of these were located in pigmentation-related genes. Interestingly, an A > C₄₇₄ SNP in the gene BsTrpm7 and an indel (position 3571) in the BsItgb1a gene were found only in Cambodian. A C > T₂₅₂₀ SNP in BsFzd4 and 10 of 11 SNPs in BsTyrp1a were found only in Black. Different expression levels (P < 0.05) were found for tyrosinase (BsTyr), BsTyrp1a, BsNf1a and BsEgf1 among skin, body muscle and tails of the same variety and among the same tissues of different varieties (Red, Green, Blue, Black, Cambodian and Multi-colors, N = 5 each).

Exploring the anticancer and antioxidant properties of Vicia faba L. pods extracts, a promising source of nutraceuticals

Background

Pulse crops are considered the major sources of proteins, dietary fiber, micronutrients, and bioactive phytochemicals. Among the numerous pulse crops, broad beans (Vicia faba L.) have received particular attention due to their nutraceutical, functional and economic importance. Our attention was mainly focused on the broad bean pods (VFs), which are the primary by-product of the domestic and industrial processing of broad beans and an attractive source of valuable ingredients.

Methods

In order to investigate the VFs properties, the flours from broad beans of three different harvest periods were extracted with acetone, methanol and 70% aqueous ethanol and the dried extracts were analyzed, qualitatively and quantitatively, and tested for their antioxidant through DPPH and ABTS assay and anticancer activities using the MTT assay and immunofluorescence analysis.

Results

The VF extracts demonstrated a good in vitro radical scavenging activity from the first stage of collection of all the V. faba L. extracts. Additionally, the extracts were tested for their cytotoxicity against a panel of cancer and normal cells and the outcomes indicated the ethanol extract as the most active against the melanoma cell line Sk-Mel-28, without affecting the viability of the normal cells. Finally, we found out that the ethanol extract interfered with the microtubules organization, leading to the cancer cells death by apoptosis.

Analysis of pigment cell composition, pigment content, tyrosinase content and activity of three kinds of loaches Misgurnus anguillicaudatus from Poyang Lake

Pigment cell composition, pigment content, tyrosinase content and activity analysis were investigated on three kinds of loaches Misgurnus anguillicaudatus: big blackspot loaches (BBL), small blackspot loaches (SBL) and non-blackspot loaches (NBL), from Poyang Lake. Results showed that there were three types of skin pigment cells, namely melanophores, xanthophores and iridophores. Melanophores in dorsum were more than those in abdomen. Melanophore cytosomes in BBL were larger than those in SBL and NBL, and melanosomes were the largest in stage four. The melanophores in dorsal skin of SBL or NBL were small cell bodies, spindle-like and in chain distribution. There was an extremely significant difference in melanin content in BBL between the dorsum and abdomen (P < 0.01). There were no significant differences in melanin abdominal content, lutein and carotenoid contents among three kinds of loaches (P > 0.05). In dorsal skin, tyrosinase content was the highest in BBL, and it was significantly lower in NBL than in BBL and SBL (P < 0.01). This study reveals the differences in pigment and tyrosinase content in three kinds of loaches and provides a theoretical basis for further study of the mechanism of black spot formation.

Whole-organism 3D quantitative characterization of zebrafish melanin by silver deposition micro-CT

We previously described X-ray histotomography, a high-resolution, non-destructive form of X-ray microtomography (micro-CT) imaging customized for three-dimensional (3D), digital histology, allowing quantitative, volumetric tissue and organismal phenotyping (Ding et al., 2019). Here, we have combined micro-CT with a novel application of ionic silver staining to characterize melanin distribution in whole zebrafish larvae. The resulting images enabled whole-body, computational analyses of regional melanin content and morphology. Normalized micro-CT reconstructions of silver-stained fish consistently reproduced pigment patterns seen by light microscopy, and further allowed direct quantitative comparisons of melanin content across wild-type and mutant samples, including subtle phenotypes not previously noticed. Silver staining of melanin for micro-CT provides proof-of-principle for whole-body, 3D computational phenomic analysis of a specific cell type at cellular resolution, with potential applications in other model organisms and melanocytic neoplasms. Advances such as this in whole-organism, high-resolution phenotyping provide superior context for studying the phenotypic effects of genetic, disease, and environmental variables.

Comparative Transcriptome Analysis Identifies Candidate Genes Related to Black-Spotted Pattern Formation in Spotted Scat (Scatophagus argus)

Simple Summary

Spotted scat (Scatophagus argus) is a commercially important marine aquaculture and ornamental fish species in China and East Asian countries. There are dozens of black spots on each side of the body, and the caudal fin, which is yellow and black, is appreciated in ornamental fish markets. To explore the genetic mechanisms of its pattern formation, we found 2357 differentially expressed genes (DEGs) by comparing the transcriptome in the black-spotted skin, non-spotted skin and caudal fin in S. argus. The results will expand our knowledge about the molecular mechanism of important genes and pathways associated with pigment pattern formation and provide a certain theoretical basis for the molecular breeding in S. argus.

Abstract

Spotted scat (Scatophagus argus) is an economically important marine aquaculture and ornamental fish species in Asia, especially in southeast China. In this study, skin transcriptomes of S. argus were obtained for three types of skin, including black-spotted skin (A), non-spotted skin (B) and caudal fin (C). A total of nine complementary DNA (cDNA) libraries were obtained by Illumina sequencing. Bioinformatics analysis revealed that 1358, 2086 and 487 genes were differentially expressed between A and B, A and C, and B and C, respectively. The results revealed that there were 134 common significantly differentially expressed genes (DEGs) and several key genes related to pigment synthesis and pigmentation, including tyrp1, mitf, pmel, slc7a2, tjp1, hsp70 and mart-1. Of these, some DEGs were associated with pigmentation-related Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, such as tyrosine metabolism, melanogenesis, the Wnt signaling pathway and the mitogen-activated protein kinase (MAPK) signaling pathway. The results will facilitate understanding the molecular mechanisms of skin pigmentation differentiation in S. argus and provide valuable information for skin coloration, especially the formation of spotted patterns on other marine fish species.

The Effect of an Anthropogenic Magnetic Field on the Early Developmental Stages of Fishes-A Review

The number of sources of anthropogenic magnetic and electromagnetic fields generated by various underwater facilities, industrial equipment, and transferring devices in aquatic environment is increasing. These have an effect on an array of fish life processes, but especially the early developmental stages. The magnitude of these effects depends on field strength and time of exposure and is species-specific. We review studies on the effect of magnetic fields on the course of embryogenesis, with special reference to survival, the size of the embryos, embryonic motor function, changes in pigment cells, respiration hatching, and directional reactions. We also describe the effect of magnetic fields on sperm motility and egg activation. Magnetic fields can exert positive effects, as in the case of the considerable extension of sperm capability of activation, or have a negative influence in the form of a disturbance in heart rate or developmental instability in inner ear organs.

Genetic deciphering of the antagonistic activities of the melanin-concentrating hormone and melanocortin pathways in skin pigmentation

The genetic origin of human skin pigmentation remains an open question in biology. Several skin disorders and diseases originate from mutations in conserved pigmentation genes, including albinism, vitiligo, and melanoma. Teleosts possess the capacity to modify their pigmentation to adapt to their environmental background to avoid predators. This background adaptation occurs through melanosome aggregation (white background) or dispersion (black background) in melanocytes. These mechanisms are largely regulated by melanin-concentrating hormone (MCH) and α-melanocyte–stimulating hormone (α-MSH), two hypothalamic neuropeptides also involved in mammalian skin pigmentation. Despite evidence that the exogenous application of MCH peptides induces melanosome aggregation, it is not known if the MCH system is physiologically responsible for background adaptation. In zebrafish, we identify that MCH neurons target the pituitary gland-blood vessel portal and that endogenous MCH peptide expression regulates melanin concentration for background adaptation. We demonstrate that this effect is mediated by MCH receptor 2 (Mchr2) but not Mchr1a/b. mchr2 knock-out fish cannot adapt to a white background, providing the first genetic demonstration that MCH signaling is physiologically required to control skin pigmentation. mchr2 phenotype can be rescued in adult fish by knocking-out pomc, the gene coding for the precursor of α-MSH, demonstrating the relevance of the antagonistic activity between MCH and α-MSH in the control of melanosome organization. Interestingly, MCH receptor is also expressed in human melanocytes, thus a similar antagonistic activity regulating skin pigmentation may be conserved during evolution, and the dysregulation of these pathways is significant to our understanding of human skin disorders and cancers.

Melanocytes produce melanin, a natural skin pigment, for body coloration which helps to protect and camouflage an organism and to attract mates. Melanocytes are ubiquitous pigment cells in vertebrates and the genes underlying their development are well conserved, making fishes that possess the ability to modify their pigmentation, biologically relevant and successful models for human skin disorders. Many human skin diseases including albinism, vitiligo, and melanoma are derived from mutations in conserved pigmentation genes. However, much of the conserved molecular mechanisms behind these diseases and human pigmentation remain unknown. For instance, melanin concentrating hormone (MCH) was originally identified as a peptide that when injected, could make fish paler by promoting melanin aggregation but no mutants demonstrating an endogenous function for MCH in pigmentation have been reported. Here, we use zebrafish mutants of MCH and the MCH receptor to determine their specific genetic function in pigmentation. Additionally, we demonstrate that MCH has an antagonistic pigmentation function to the melanocortin system, where MCH expression promotes lighter pigmentation and melanocortin activity promotes darkening. Thus, we find that the balance between the MCH and melanocortin system activities are likely required for skin pigmentation and dysregulation of these pathways could underlie adverse human skin conditions.

Skin transcriptional profiles in Oophaga poison frogs

Aposematic organisms advertise their defensive toxins to predators using a variety of warning signals, including bright coloration. While most Neotropical poison frogs (Dendrobatidae) rely on crypsis to avoid predators, Oophaga poison frogs from South America advertise their chemical defenses, a complex mix of diet-derived alkaloids, by using conspicuous hues. The present study aimed to characterize the skin transcriptomic profiles of South American Oophaga poison frogs. Our analyses showed very similar transcriptomic profiles for these closely related species in terms of functional annotation and relative abundance of gene ontology terms expressed. Analyses of expression profiles of Oophaga and available skin transcriptomes of cryptic anurans allowed us to propose initial hypotheses for the active sequestration of alkaloid-based chemical defenses and to highlight some genes that may be potentially involved in resistance mechanisms to avoid self-intoxication and skin coloration. In doing so, we provide an important molecular resource for the study of warning signals that will facilitate the assembly and annotation of future poison frog genomes.

Molecular Plasticity in Animal Pigmentation: Emerging Processes Underlying Color Changes

Animal coloration has been rigorously studied and has provided morphological implications for fitness with influences over social behavior, predator–prey interactions, and sexual selection. In vertebrates, its study has developed our understanding across diverse fields ranging from behavior to molecular biology. In the search for underlying molecular mechanisms, many have taken advantage of pedigree-based and genome-wide association screens to reveal the genetic architecture responsible for pattern variation that occurs in early development. However, genetic differences do not provide a full picture of the dynamic changes in coloration that are most prevalent across vertebrates at the molecular level. Changes in coloration that occur in adulthood via phenotypic plasticity rely on various social, visual, and dietary cues independent of genetic variation. Here, I will review the contributions of pigment cell biology to animal color changes and recent studies describing their molecular underpinnings and function. In this regard, conserved epigenetic processes such as DNA methylation play a role in lending plasticity to gene regulation as it relates to chromatophore function. Lastly, I will present African cichlids as emerging models for the study of pigmentation and molecular plasticity for animal color changes. I posit that these processes, in a dialog with environmental stimuli, are important regulators of variation and the selective advantages that accompany a change in coloration for vertebrate animals.

Melanosome transport and regulation in development and disease

Melanosomes are specialized membrane-bound organelles that synthesize and organize melanin, ultimately providing color to the skin, hair, and eyes. Disorders in melanogenesis and melanosome transport are linked to pigmentary diseases, such as Hermansky-Pudlak syndrome, Chediak-Higashi syndrome, and Griscelli syndrome. Clinical cases of these pigmentary diseases shed light on the molecular mechanisms that control melanosome-related pathways. However, only an improved understanding of melanogenesis and melanosome transport will further the development of diagnostic and therapeutic approaches. Herein, we review the current literature surrounding melanosomes with particular emphasis on melanosome membrane transport and cytoskeleton-mediated melanosome transport. We also provide perspectives on melanosome regulatory mechanisms which include hormonal action, inflammation, autophagy, and organelle interactions.

Zebrafish as an Animal Model for Ocular Toxicity Testing: A Review of Ocular Anatomy and Functional Assays

Xenobiotics make their way into organisms from diverse sources including diet, medication, and pollution. Our understanding of ocular toxicities from xenobiotics in humans, livestock, and wildlife is growing thanks to laboratory animal models. Anatomy and physiology are conserved among vertebrate eyes, and studies with common mammalian preclinical species (rodent, dog) can predict human ocular toxicity. However, since the eye is susceptible to toxicities that may not involve a histological correlate, and these species rely heavily on smell and hearing to navigate their world, discovering visual deficits can be challenging with traditional animal models. Alternative models capable of identifying functional impacts on vision and requiring minimal amounts of chemical are valuable assets to toxicology. Human and zebrafish eyes are anatomically and functionally similar, and it has been reported that several common human ocular toxicants cause comparable toxicity in zebrafish. Vision develops rapidly in zebrafish; the tiny larvae rely on visual cues as early as 4 days, and behavioral responses to those cues can be monitored in high-throughput fashion. This article describes the comparative anatomy of the zebrafish eye, the notable differences from the mammalian eye, and presents practical applications of this underutilized model for assessment of ocular toxicity.

Ishophloroglucin A Isolated from Ishige okamurae Suppresses Melanogenesis Induced by α-MSH: In Vitro and In Vivo

Diphlorethohydroxycarmalol (DPHC) isolated from Ishige okamurae (IO) showed potential whitening effects against UV-B radiation. However, the components of IO as well as their molecular mechanism against α-melanocyte-stimulating hormone (α-MSH) have not yet been investigated. Thus, this study aimed to investigate the inhibitory effects of Ishophloroglucin A (IPA), a phlorotannin isolated from brown algae IO, and its crude extract (IOE), in melanogenesis in vivo in an α-MSH-induced zebrafish model and in B16F10 melanoma cells in vitro. Molecular docking studies of the phlorotannins were carried out to determine their inhibitory effects and to elucidate their mode of interaction with tyrosinase, a glycoprotein related to melanogenesis. In addition, morphological changes and melanin content decreased in the α-MSH-induced zebrafish model after IPA and IOE treatment. Furthermore, Western blotting results revealed that IPA upregulated the extracellular related protein expression in α-MSH-stimulated B16F10 cells. Hence, these results suggest that IPA isolated from IOE has a potential for use in the pharmaceutical and cosmetic industries.

From extraocular photoreception to pigment movement regulation: a new control mechanism of the lanternshark luminescence

The velvet belly lanternshark, Etmopterus spinax, uses counterillumination to disappear in the surrounding blue light of its marine environment. This shark displays hormonally controlled bioluminescence in which melatonin (MT) and prolactin (PRL) trigger light emission, while α-melanocyte-stimulating hormone (α-MSH) and adrenocorticotropic hormone (ACTH) play an inhibitory role. The extraocular encephalopsin (Es-Opn3) was also hypothesized to act as a luminescence regulator. The majority of these compounds (MT, α-MSH, ACTH, opsin) are members of the rapid physiological colour change that regulates the pigment motion within chromatophores in metazoans. Interestingly, the lanternshark photophore comprises a specific iris-like structure (ILS), partially composed of melanophore-like cells, serving as a photophore shutter. Here, we investigated the role of (i) Es-Opn3 and (ii) actors involved in both MT and α-MSH/ACTH pathways on the shark bioluminescence and ILS cell pigment motions. Our results reveal the implication of Es-Opn3, MT, inositol triphosphate (IP₃), intracellular calcium, calcium-dependent calmodulin and dynein in the ILS cell pigment aggregation. Conversely, our results highlighted the implication of the α-MSH/ACTH pathway, involving kinesin, in the dispersion of the ILS cell pigment. The lanternshark luminescence then appears to be controlled by the balanced bidirectional motion of ILS cell pigments within the photophore. This suggests a functional link between photoreception and photoemission in the photogenic tissue of lanternsharks and gives precious insights into the bioluminescence control of these organisms.

Regression of dark color in subterranean fishes involves multiple mechanisms: response to hormones and neurotransmitters

ABSTRACT Organisms with source-populations restricted to the subterranean biotope (troglobites) are excellent models for comparative evolutionary studies, due to their specialization to permanent absence of light. Eye and dark pigment regression are characteristics of most troglobites. In spite of the advance in knowledge on the mechanisms behind eye regression in cave fishes, very little is known about pigmentation changes. Studies were focused on three species of the genus Pimelodella. Exemplars of the troglobitic P. spelaea and P. kronei were compared with the epigean (surface) P. transitoria, putative sister-species of the latter. Melanophore areas and densities are significantly lower in the troglobitic species. Evaluating the in vitro response of these cells to adrenaline, acetylcholine and MCH, we observed a reduced response in both troglobites to adrenaline. The same trend was observed with MCH, but not statistically significant. No response to acetilcholine was detected in all the three. Contrary to expectations, even though eye-regression in P. spelaea was much lower than in P. kronei, pigmentation regression was more advanced. Multiple mechanisms of loss showing a mosaic of traits in troglobitic fishes are discussed here.

Does soil color affect fish evolution? Differences in color change rate between lineages of the sailfin tetra

ABSTRACT Several organisms match their skin color to the prevalent background color, granting crypsis against predators. The rate at which body color changes occur varies among organisms as a result of physiological constraints and adaptation to variation in contrasts between objects and the environmental background. Faster darkening of body color is favored in environments that show higher amounts of contrast between common objects and the prevailing background. Soil types in Amazon forest streams (igarapés) create distinct environments with respect to the amount of contrast, a result of the amount of sand and clay, which offers different contrasts against dead leaves. Here, we investigated differences in the rates of color change among populations of the sailfin tetra (Crenuchus spilurus) that represent lineages that live in regions of different soil types. Populations inserted into blackwaters (sandy soil) showed higher rates of color darkening in response to exposure to a dark environment composed by dead leaves. We propose that natural selection stemming from predation can favor faster color change rate in environments where there is higher variability of contrasts between leaf litter and soil, which is common in most blackwater streams.

Zebrafish Pigment Pattern Formation: Insights into the Development and Evolution of Adult Form

Vertebrate pigment patterns are diverse and fascinating adult traits that allow animals to recognize conspecifics, attract mates, and avoid predators. Pigment patterns in fish are among the most amenable traits for studying the cellular basis of adult form, as the cells that produce diverse patterns are readily visible in the skin during development. The genetic basis of pigment pattern development has been most studied in the zebrafish, Danio rerio. Zebrafish adults have alternating dark and light horizontal stripes, resulting from the precise arrangement of three main classes of pigment cells: black melanophores, yellow xanthophores, and iridescent iridophores. The coordination of adult pigment cell lineage specification and differentiation with specific cellular interactions and morphogenetic behaviors is necessary for stripe development. Besides providing a nice example of pattern formation responsible for an adult trait of zebrafish, stripe-forming mechanisms also provide a conceptual framework for posing testable hypotheses about pattern diversification more broadly. Here, we summarize what is known about lineages and molecular interactions required for pattern formation in zebrafish, we review some of what is known about pattern diversification in Danio, and we speculate on how patterns in more distant teleosts may have evolved to produce a stunningly diverse array of patterns in nature.

The effects of commonly used anaesthetics on colour measurements across body regions in the poeciliid fish, Girardinus metallicus

The effects of common anaesthetics on the hue, saturation and brightness measurements of the poeciliid fish Girardinus metallicus were investigated in two experiments. For both experiments the coloration of four body regions was measured from digital images of the same males obtained under three conditions: (1) control (in a water-filled chamber); (2) anaesthetised with MS-222; and (3) anaesthetised with eugenol (clove oil). In experiment 1 anaesthetised fish were photographed out of water. In experiment 2 all photographs were taken in a water-filled chamber. Anaesthetics altered coloration in both experiments. In the more methodologically consistent experiment 2 we found significantly different hue, increased saturation and decreased brightness in anaesthetic v. control conditions, consistent with darkening caused by the anaesthetics. The body regions differed in coloration consistent with countershading but did not differentially change in response to anaesthesia. These findings suggest that photographing fish in a water-filled chamber without anaesthetic is preferable for obtaining digital images for colour analysis and that multiple body regions of fish should be measured when assessing coloration patterns meaningful in behavioural contexts, to account for the gradients caused by countershading. We are encouraged that some researchers employ such methods already and caution against using anaesthetics except when absolutely necessary for immobilisation.

Neural signatures of sleep in zebrafish

Slow-wave sleep and rapid eye movement (or paradoxical) sleep have been found in mammals, birds and lizards, but it is unclear whether these neuronal signatures are found in non-amniotic vertebrates. Here we develop non-invasive fluorescence-based polysomnography for zebrafish, and show-using unbiased, brain-wide activity recording coupled with assessment of eye movement, muscle dynamics and heart rate-that there are at least two major sleep signatures in zebrafish. These signatures, which we term slow bursting sleep and propagating wave sleep, share commonalities with those of slow-wave sleep and paradoxical or rapid eye movement sleep, respectively. Further, we find that melanin-concentrating hormone signalling (which is involved in mammalian sleep) also regulates propagating wave sleep signatures and the overall amount of sleep in zebrafish, probably via activation of ependymal cells. These observations suggest that common neural signatures of sleep may have emerged in the vertebrate brain over 450 million years ago.

Synthesis of arbutin-gold nanoparticle complexes and their enhanced performance for whitening

Arbutin, a natural polyphenol, possesses numerous biological activities including whitening, anti-oxidant, anti-cancer, anti-inflammatory activities, as well as strong reducing power, making it an ideal bioactive ingredient for preparing gold nanoparticles (GNPs). Previously, we developed a novel green, mild synthetic method for GNPs using glycosides such as arbutin as reducing agents and stabilizers. Herein, we optimized the synthetic method for glycoside-GNPs using arbutin, methyl β-D-glucoside, and phenyl β-D-glucoside and validated their whitening efficacy in vitro and in vivo. The resulting glycoside-GNPs were predominantly mono-dispersed and spherical (10.30-17.13 nm diameter). Compared with arbutin itself, arbutin-GNP complexes (GNP-A1 and GNP-P2) displayed enhanced whitening capabilities. Furthermore, GNP-P2 exhibited enhanced anti-inflammatory activity and lacked the toxicity associated with arbutin. Bioactive glycoside-GNP complexes may open new directions for cosmeceuticals, and GNP-P2 may serve as a useful whitening ingredient in future cosmeceutical applications.

Whole-Cell Photoacoustic Sensor Based on Pigment Relocalization

Photoacoustic (optoacoustic) imaging can extract molecular information with deeper tissue penetration than possible by fluorescence microscopy techniques. However, there is currently still a lack of robust genetically controlled contrast agents and molecular sensors that can dynamically detect biological analytes of interest with photoacoustics. In a biomimetic approach, we took inspiration from cuttlefish who can change their color by relocalizing pigment-filled organelles in so-called chromatophore cells under neurohumoral control. Analogously, we tested the use of melanophore cells from Xenopus laevis, containing compartments (melanosomes) filled with strongly absorbing melanin, as whole-cell sensors for optoacoustic imaging. Our results show that pigment relocalization in these cells, which is dependent on binding of a ligand of interest to a specific G protein-coupled receptor (GPCR), can be monitored in vitro and in vivo using photoacoustic mesoscopy. In addition to changes in the photoacoustic signal amplitudes, we could furthermore detect the melanosome aggregation process by a change in the frequency content of the photoacoustic signals. Using bioinspired engineering, we thus introduce a photoacoustic pigment relocalization sensor (PaPiReS) for molecular photoacoustic imaging of GPCR-mediated signaling molecules.

From the Cover: Inhibitors of Nicotinamide Phosphoribosyltransferase Cause Retinal Damage in Larval Zebrafish

Nicotinamide phosphoribosyltransferase (NAMPT) has been investigated as a target for oncology because it catalyzes a rate-limiting step in cellular energy metabolism to produce nicotinamide adenine dinucleotide. Small molecule inhibitors of NAMPT have been promising drug candidates but preclinical development has been hindered due to associated retinal toxicity. Here we demonstrate that larval zebrafish can predict retinal toxicity associated with this mechanism revealing an attractive alternative method for identifying such toxicities. Zebrafish permit higher throughput testing while using far lower quantities of test article compared with mammalian systems. NAMPT inhibitor-associated toxicity manifested in zebrafish as a loss of response to visual cues compared with auditory cues. Zebrafish retinal damage associated with NAMPT inhibitor treatment was confirmed through histopathology. Ranking 6 NAMPT inhibitors according to their impact on zebrafish vision revealed a positive correlation with their in vitro potencies on human tumor cells. This correlation indicates translatable pharmacodynamics between zebrafish and human NAMPT and is consistent with on-target activity as the cause of retinal toxicity associated with NAMPT inhibition. Together, these data illustrate the utility of zebrafish for identifying compounds that may cause ocular toxicity in mammals, and, likewise, for accelerating development of compounds with improved safety margins.

Developmental toxicity of synthetic phenolic antioxidants to the early life stage of zebrafish

Synthetic phenolic antioxidants (SPAs) have gained high concerns due to their extensive usages and unintended environmental release via various routes. Their contamination in water system could pose potential threat to aquatic organisms, therefore, the studies on the aquatic toxicology of this kind of chemicals are of high importance. In this research, the developmental toxicities of four commonly used SPAs, including butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tert-butyl hydroquinone (TBHQ), and 2,2'-methylenebis (6-tert-butyl-4-methylphenol) (AO2246) were investigated using the zebrafish embryo toxicity test (ZFET). The results showed that these four SPAs exerted different acute toxicities to zebrafish, and the toxic order, based on their 96 h LC₅₀ values, was AO2246 > TBHQ > BHA > BHT, and decreased hatching rates were induced for the embryos in BHA, TBHQ and AO2246 exposure groups. Non-lethal exposures of BHA (≤20 μM), TBHQ (≤20 μM), BHT (≤200 μM) and AO2246 (≤2 μM) decreased the heart rates and body lengths of zebrafish in exposure concentration-dependent manners. Diverse morphological deformities, including uninflated swim bladder, pericardial edema, spinal curvature, severe yolk deformation, or abnormal pigmentation, were induced in zebrafish larvae upon SPA treatments. The transcriptional levels of the related genes, examined by quantitative PCR, indicated that the interferences of SPAs with hypothalamic-pituitary-thyroid axis (HPT axis), GH/PRL synthesis and Hedgehog (hh) pathway contributed to their developmental toxicities in zebrafish. The up-regulation of pluripotency biomarker, Oct4, caused the developmental retardation during the early stages of zebrafish embryos in BHA and TBHQ exposure groups. The results obtained herein provided important information on the developmental toxicity of SPAs, which could be very helpful in guiding the risk assessment on their aquatic toxicology.

Social Status-Related Differences in Motor Activity Between Wild-Type and Mutant Zebrafish

Use of zebrafish as a model organism in biomedical research has led to the generation of many genetically modified mutant lines to investigate various aspects of developmental and cellular processes. However, the broader effects of the underlying mutations on social and motor behavior remain poorly examined. Here, we compared the dynamics of social interactions in the Tüpfel long-fin nacre mutant line, which lacks skin pigmentation, to wild-type zebrafish; and we determined whether status-dependent differences in escape and swimming behavior existed within each strain. We show that despite similarities in aggressive activity, Tüpfel long-fin nacre pairs exhibit unstable social relationships characterized by frequent reversals in social dominance compared to wild-type pairs. The lack of strong dominance relationships in Tüpfel long-fin nacre pairs correlates with weak territoriality and overlapping spatial distribution of dominants and subordinates. Conversely, wild-type dominants displayed strong territoriality that severely limited the movement of subordinates. Additionally, the sensitivity of the startle escape response was significantly higher in wild-type subordinates compared to dominants. However, status-related differences in sensitivity of escape response in Tüpfel long-fin nacre pairs were absent. Finally, we present evidence suggesting that these differences could be a consequence of a disruption of proper visual social signals. We show that in wild-type pairs dominants are more conspicuous, and that in wild-type and Tüpfel long-fin nacre pairings wild-type fish are more likely to dominate Tüpfel long-fin nacres. Our results serve as a cautionary note in research design when morphologically engineered zebrafish for color differences are utilized in the study of social behavior and central nervous system function.

Distant Insulin Signaling Regulates Vertebrate Pigmentation through the Sheddase Bace2

Patterning of vertebrate melanophores is essential for mate selection and protection from UV-induced damage. Patterning can be influenced by circulating long-range factors, such as hormones, but it is unclear how their activity is controlled in recipient cells to prevent excesses in cell number and migration. The zebrafish wanderlust mutant harbors a mutation in the sheddase bace2 and exhibits hyperdendritic and hyperproliferative melanophores that localize to aberrant sites. We performed a chemical screen to identify suppressors of the wanderlust phenotype and found that inhibition of insulin/PI3Kγ/mTOR signaling rescues the defect. In normal physiology, Bace2 cleaves the insulin receptor, whereas its loss results in hyperactive insulin/PI3K/mTOR signaling. Insulin B, an isoform enriched in the head, drives the melanophore defect. These results suggest that insulin signaling is negatively regulated by melanophore-specific expression of a sheddase, highlighting how long-distance factors can be regulated in a cell-type-specific manner.

Transcriptomics reveals the molecular processes of light-induced rapid darkening of the non-obligate cave dweller Oreolalax rhodostigmatus (Megophryidae, Anura) and their genetic basis of pigmentation strategy

BACKGROUND

Vertebrates use different pigmentation strategies to adapt to various environments. A large amount of research has been done on disclosing the mechanisms of pigmentation strategies in vertebrates either under light, or, living in constant darkness. However, less attention has been paid to non-obligate, darkness dwellers. Red-spotted toothed toads Oreolalax rhodostigmatus (Megophryidae; Anura) from the karst mountainous region of southwestern China are non-obligate cave dwellers. Most tadpoles of the species possess transparent skin as they inhabit the dark karst caves. But remarkably, the transparent tadpoles can darken just within 15 h once exposed to light. Obviously, it is very significant to reveal molecular mechanisms of the unexpected rapid-darkening phenomenon.

RESULTS

We compared the transcriptomes of O. rhodostigmatus tadpoles with different durations of light exposure to investigate the cellular processes and potential regulation signals for their light-induced rapid darkening. Genes involved in melanogenesis (i.e. TYR, TYRP1 and DCT) and melanocyte proliferation, as well as their transcriptional factor (MITF), showed light-induced transcription, suggesting a dominating role of morphological color change (MCC) in this process. Transcription of genes related to growth factor, MAPK and PI3K-Akt pathways increased with time of light exposure, suggesting that light could induce significant growth signal, which might facilitate the rapid skin darkening. Most importantly, an in-frame deletion of four residues was identified in O. rhodostigmatus melanocortin-1 receptor (MC1R), a critical receptor in MCC. This deletion results in a more negatively charged ligand pocket with three stereo-tandem aspartate residues. Such structural changes likely decrease the constitutive activity of MC1R, but increase its ligands-dependent activity, thus coordinating pigment regression and rapid melanogenesis in the dark and light, respectively.

CONCLUSION

Our study suggested that rapid MCC was responsible for the light-induced rapid darkening of O. rhodostigmatus tadpoles. Genetic mutations of MC1R in them could explain how these non-obligate cave dwellers coordinate pigment regression and robust melanogenesis in darkness and light, respectively. To our knowledge, this is the first study that reports the association between pigmentation phenotype adaptation and MC1R mutations in amphibians and/or in non-obligate cave dwellers.

A Zebrafish Embryo as an Animal Model for the Treatment of Hyperpigmentation in Cosmetic Dermatology Medicine

For years, clinical studies involving human volunteers and several known pre-clinical in vivo models (i.e., mice, guinea pigs) have demonstrated their reliability in evaluating the effectiveness of a number of depigmenting agents. Although these models have great advantages, they also suffer from several drawbacks, especially involving ethical issues regarding experimentation. At present, a new depigmenting model using zebrafish has been proposed and demonstrated. The application of this model for screening and studying the depigmenting activity of many bioactive compounds has been given great attention in genetics, medicinal chemistry and even the cosmetic industry. Depigmenting studies using this model have been recognized as noteworthy approaches to investigating the antimelanogenic activity of bioactive compounds in vivo. This article details the current knowledge of zebrafish pigmentation and its reliability as a model for the screening and development of depigmenting agents. Several methods to quantify the antimelanogenic activity of bioactive compounds in this model, such as phenotype-based screening, melanin content, tyrosinase inhibitory activity, other related proteins and transcription genes, are reviewed. Depigmenting activity of several bioactive compounds which have been reported towards this model are compared in terms of their molecular structure and possible mode of actions. This includes patented materials with regard to the application of zebrafish as a depigmenting model, in order to give an insight of its intellectual value. At the end of this article, some limitations are highlighted and several recommendations are suggested for improvement of future studies.