Melanization stimulating factor (MSF) and melanization inhibiting factor (MIF) in the integument of fish

Countershading in zebrafish results from an Asip1 controlled dorsoventral gradient of pigment cell differentiation

Dorso-ventral (DV) countershading is a highly-conserved pigmentary adaptation in vertebrates. In mammals, spatially regulated expression of agouti-signaling protein (ASIP) generates the difference in shading by driving a switch between the production of chemically-distinct melanins in melanocytes in dorsal and ventral regions. In contrast, fish countershading seemed to result from a patterned DV distribution of differently-coloured cell-types (chromatophores). Despite the cellular differences in the basis for counter-shading, previous observations suggested that Agouti signaling likely played a role in this patterning process in fish. To test the hypotheses that Agouti regulated counter-shading in fish, and that this depended upon spatial regulation of the numbers of each chromatophore type, we engineered asip1 homozygous knockout mutant zebrafish. We show that loss-of-function asip1 mutants lose DV countershading, and that this results from changed numbers of multiple pigment cell-types in the skin and on scales. Our findings identify asip1 as key in the establishment of DV countershading in fish, but show that the cellular mechanism for translating a conserved signaling gradient into a conserved pigmentary phenotype has been radically altered in the course of evolution.

Left-right pigmentation pattern of Japanese flounder corresponds to expression levels of melanocortin receptors (MC1R and MC5R), but not to agouti signaling protein 1 (ASIP1) expression

Body coloration in flatfish is one of the most distinctive asymmetries in the animal kingdom, although the fundamental molecular mechanism of the pigmentation is unclear. In the dorso-ventral coloration (countershading) of other teleost fishes, ventral-specific expression of agouti signaling protein 1 (ASIP1), an endogenous antagonist of melanocortin 1 receptor (MC1R), has been reported to play a pivotal role. Contribution of ASIP1 is also suggested in the asymmetrical pigmentation of flatfish. In order to confirm the contribution of ASIP1 and further examine receptor function in the body coloration of Japanese flounder, expression levels of asip1, mc1r, melanocortin 5 receptor (mc5r), and melanin-concentrating hormone receptor 2 (mchr2) were measured in the normally pigmented area of the left side, the normally non-pigmented area of the right side, and the abnormally pigmented (exhibiting hypermelanosis) area of the right side. Measurement was also carried out under conditions of hypermelanosis stimulated by cortisol and during the transition from non-pigmentation to pigmentation in areas of hypermelanosis. Contrary to our expectations, no difference was detected in asip1 expression between pigmented and non-pigmented areas. There was also no difference between normal and hormonally stimulated pigmented conditions in areas of hypermelanosis or during the transition process. Instead, the expression levels of mc1r, mc5r, and mchr2 were consistently higher in pigmented areas, and were especially increased under hormonally stimulated conditions. In addition, expressions of these receptor genes increased prior to pigmentation in areas of future hypermelanosis. Our results suggest that MC1Rand MC5R, but not necessarily ASIP1, contribute to pigmentation and hypermelanosis in Japanese flounder. We propose a yet unknown molecular mechanism for asymmetrical pigmentation in flatfish that is distinct from that of countershading in other vertebrates.

Claudin tight junction proteins in rainbow trout (Oncorhynchus mykiss) skin: Spatial response to elevated cortisol levels

This study examined regional distribution and corticosteroid-induced alterations of claudin (cldn) transcript abundance in teleost fish skin. Regional comparison of mRNA encoding 20 Cldns indicated that 12 exhibit differences in abundance along the dorsoventral axis of skin. However, relative abundance of cldns (i.e. most to least abundant) remained similar in different skin regions. Several cldns appear to be present in the epidermis and dermal vasculature whereas others are present only in the epidermis. Increased circulating cortisol levels significantly altered mRNA abundance of 10 cldns in a region specific manner, as well as corticosteroid receptors and 11β-hydroxysteroid dehydrogenase (type 2). Epidermis and epidermal mucous cell morphometrics also altered in response to cortisol, exhibiting changes that appear to enhance skin barrier properties. Taken together, data provide a first look at spatial variation in the molecular physiology of the teleost fish integument TJ complex and region-specific sensitivity to an endocrine factor.

Pigment patterns in adult fish result from superimposition of two largely independent pigmentation mechanisms

Dorso-ventral pigment pattern differences are the most widespread pigmentary adaptations in vertebrates. In mammals, this pattern is controlled by regulating melanin chemistry in melanocytes using a protein, agouti-signalling peptide (ASIP). In fish, studies of pigment patterning have focused on stripe formation, identifying a core striping mechanism dependent upon interactions between different pigment cell types. In contrast, mechanisms driving the dorso-ventral countershading pattern have been overlooked. Here, we demonstrate that, in fact, zebrafish utilize two distinct adult pigment patterning mechanisms - an ancient dorso-ventral patterning mechanism, and a more recent striping mechanism based on cell-cell interactions; remarkably, the dorso-ventral patterning mechanism also utilizes ASIP. These two mechanisms function largely independently, with resultant patterns superimposed to give the full pattern.

Hyperpigmentation in North Sea dab Limanda limanda. II. Macroscopic and microscopic characteristics and pathogen screening

An increasing trend in the prevalence of hyperpigmentation in the common dab Limanda limanda from the North Sea prompted us to investigate the potential role of infectious agents as causes or contributing factors to the condition. Dab representing 3 severity grades of hyperpigmentation were sampled for virology, bacteriology, histopathology and ultrastructure assessments. No cytopathic effect was recorded during virology testing, and bacteriological results showed no differences between normal and hyperpigmented dab. Histopathological assessment showed that the most significant changes occurred in the dermis as a result of chromatophore hyperplasia, namely melanophores and iridophores, alongside loose melanin granules. Dermal lymphocytic infiltration occasionally expanding into the epidermis and the underlying musculature was more frequent in highly pigmented dab than in normal fish, suggesting an active immune response. Ultrastructure studies showed additional disruption of the epithelial layer, with loose melanin granules between cells and a number of single or aggregated melanocytes. Dab representing different grades of hyperpigmentation kept in the laboratory alongside normal fish for a monitoring period of 18 mo showed no changes in their pigment distribution pattern, nor occurrence of new pigment in the normal fish. The current investigation found no association of hyperpigmentation in the common dab with infectious agents; therefore, understanding the cause of the condition remains a challenge which can now more reliably focus on a non-infectious origin hypothesis.

Transient ectopic overexpression of agouti-signalling protein 1 (asip1) induces pigment anomalies in flatfish

While flatfish in the wild exhibit a pronounced countershading of the dorso-ventral pigment pattern, malpigmentation is commonly observed in reared animals. In fish, the dorso-ventral pigment polarity is achieved because a melanization inhibition factor (MIF) inhibits melanoblast differentiation and encourages iridophore proliferation in the ventrum. A previous work of our group suggested that asip1 is the uncharacterized MIF concerned. In order to further support this hypothesis, we have characterized asip1 mRNAs in both turbot and sole and used deduced peptide alignments to analyze the evolutionary history of the agouti-family of peptides. The putative asip precursors have the characteristics of a secreted protein, displaying a putative hydrophobic signal. Processing of the potential signal peptide produces mature proteins that include an N-terminal region, a basic central domain with a high proportion of lysine residues as well as a proline-rich region that immediately precedes the C-terminal poly-cysteine domain. The expression of asip1 mRNA in the ventral area was significantly higher than in the dorsal region. Similarly, the expression of asip1 within the unpigmented patches in the dorsal skin of pseudoalbino fish was higher than in the pigmented dorsal regions but similar to those levels observed in the ventral skin. In addition, the injection/electroporation of asip1 capped mRNA in both species induced long term dorsal skin paling, suggesting the inhibition of the melanogenic pathways. The data suggest that fish asip1 is involved in the dorsal-ventral pigment patterning in adult fish, where it induces the regulatory asymmetry involved in precursor differentiation into mature chromatophore. Adult dorsal pseudoalbinism seems to be the consequence of the expression of normal developmental pathways in an inaccurate position that results in unbalanced asip1 production levels. This, in turn, generates a ventral-like differentiation environment in dorsal regions.

Pigmentation development, defects, and patterning in summer flounder (Paralichthys dentatus)

Flounders offer unique opportunities to study the cytological basis of vertebrate pigmentation. Individual skin pigment cells are clearly visible at hatching, and flounder ontogeny includes a dramatic shift in overall pigmentation (from symmetrical to asymmetrical) during metamorphosis. Moreover, several types of malpigmentation occur in hatchery populations; although much effort has gone into reducing the frequency of such defects, their etiology remains poorly understood, and they have rarely been described at the cellular level. In this paper, we use light and fluorescence microscopy to describe the cytological basis of normal developmental changes and of common types of malpigmentation. We then discuss the implications of these observations for underlying patterning mechanisms.