Transient Ectopic Overexpression of Agouti-Signalling Protein 1 (Asip1) Induces Pigment Anomalies in Flatfish

Abstract

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.

Morphological and Molecular Characterization of Dietary-Induced Pseudo-Albinism during Post-Embryonic Development of Solea senegalensis (Kaup, 1858)

“…Our results suggest that ARA affected normal gene expression in a way which blocked the pigment patterning that should have been established on the ocular side of the fish at metamorphosis via asip1 gene expression; then, the excessive asip1 expression levels could alter normal chromatoblast differentiation leading to an inhibition of differentiation of new post-metamorphic iridophores. This hypothesis is supported by recent findings on asip1 regulation in adult Senegalese sole pseudo-albinos that showed a higher expression of this gene within unpigmented patches of the dorsal skin than in pigmented regions, but similar to the levels found in the blind side [14]. In developing Senegalese sole larvae, asip1 seemed to be responsible for the inhibition of melanophore differentiation on the blind side concomitant with the induction of iridophore differentiation on the ocular side before the differentiation of iridophores on the blind side [15].…”

“…The molecular mechanisms underlying these processes need to be further explored. Taking into account the well-known role of asip1 in the establishment of the dorsal-ventral pigment patterning in this and other species [12], [14], and the results obtained in this study, the role of asip1 in xanthophore physiology deserves investigation. These findings show that the genetically determined pigmentation in larvae can be modulated by environmental factors (i.e., nutrition) and thereby affect changes in the normal molecular signatures occurring during the process of metamorphosis.…”

“…Indeed, fish have been used as models for melanoma research because it has been shown tissues within fish share molecular signatures and histopathological features with human cancers [6]. The genetics of pigmentation have been explored in several model teleost fish including zebrafish [7], [8], medaka [9], fugu [10], goldfish [11], [12] and, recently, in flatfish [13], [14], [15]. Flatfish are particularly useful to analyze the origin of pigmentation disorders during the ontogeny because altered pigmentation can be induced under intensive rearing conditions [16], [17], [18], [19].…”

Coordinated Regulation of Chromatophore Differentiation and Melanogenesis during the Ontogeny of Skin Pigmentation of Solea senegalensis (Kaup, 1858)

“…Due to the similarities shared with humans, fish have become extremely valuable experimental animal models for vertebrate developmental studies, especially those with underlying genetic components [60]. The genetics of pigmentation have been explored in several model teleost fish including zebrafish [5], [65], medaka [66], fugu [67], goldfish [35], [68] and, recently, in flatfish [27], [69]. To our knowledge, this is the first study that combines the characterization of the morphological ontogeny of skin pigmentation with the analysis of the expression profile of a set of key pigmentation-related genes during the larval development of a fish.…”

Neural Crest Cells and Pigmentation

“…Furthermore, knockdown experiments have indicated a role of Mc1r in melanosome dispersal in zebrafish [187]. Recent evidence indicates an unexpected role for Agouti in fish, and dissection of the underlying mechanisms will be particularly fascinating [188].…”