Pheomelanin production in the epidermis from newborn agouti mice is induced by the expression of the agouti gene in the dermis

How are proliferation and differentiation of melanocytes regulated?

Coat colors are determined by melanin (eumelanin and pheomelanin). Melanin is synthesized in melanocytes and accumulates in special organelles, melanosomes, which upon maturation are transferred to keratinocytes. Melanocytes differentiate from undifferentiated precursors, called melanoblasts, which are derived from neural crest cells. Melanoblast/melanocyte proliferation and differentiation are regulated by the tissue environment, especially by keratinocytes, which synthesize endothelins, steel factor, hepatocyte growth factor, leukemia inhibitory factor and granulocyte-macrophage colony-stimulating factor. Melanocyte differentiation is also stimulated by alpha-melanocyte stimulating hormone; in the mouse, however, this hormone is likely carried through the bloodstream and not produced locally in the skin. Melanoblast migration, proliferation and differentiation are also regulated by many coat color genes otherwise known for their ability to regulate melanosome formation and maturation, pigment type switching and melanosome distribution and transfer. Thus, melanocyte proliferation and differentiation are not only regulated by genes encoding typical growth factors and their receptors but also by genes classically known for their role in pigment formation.

Endothelin 3 Induces Skin Pigmentation in a Keratin-Driven Inducible Mouse Model

Endothelin 3 (Edn3) encodes a ligand important to developing neural crest cells and is allelic to the spontaneous mouse mutation occurring at the lethal spotting (ls) locus. Edn3(ls/ls) mutants exhibit a spotted phenotype due to reduced numbers of neural crest-derived melanocyte precursors in the skin. In this study, we show that when Edn3 is driven by the keratin 5 promoter and thereby placed proximal to melanocyte lineage cells, adult mice manifest pigmented skin harboring dermal melanocytes. Using a tetracycline inducible system, we show that the postnatal expression of Edn3 is required to maintain these dermal melanocytes, and that early expression of the Edn3 transgene is important to the onset of the hyperpigmentation phenotype. Crosses into Edn3(ls/ls) mutants demonstrate that the Edn3 transgene expression does not fully compensate for the endogenous expression pattern. Crosses into tyrosine kinase receptor Kit(Wv) mutants indicate that Edn3 can partially compensate for Kit's role in early development. Crosses into A(y) mutant mice considerably darkened their yellow coat color suggesting a previously unreported role for endothelin signaling in pigment switching. These results demonstrate that exogenous Edn3 affects both precursors and differentiated melanocytes, leading to a phenotype with characteristics similar to the human skin condition dermal melanocytosis.

Recent developments in our understanding of the avian melanocortin system: its involvement in the regulation of pigmentation and energy homeostasis

The mammalian melanocortin system has been established as a crucial regulatory component in an extraordinarily diverse number of physiological functions. In contrast, comparatively little is known about the avian melanocortin system: interest in the physiological role of alpha-MSH in birds has been limited by the fact that birds lack the intermediate lobe of the pituitary, the main source of circulating alpha-MSH in most vertebrates. Recently, however, the main avian melanocortin system genes, including POMC, AGRP, and all the melanocortin receptors, have been cloned and their physiological roles are the beginning to be elucidated. This review outlines our improved understanding of the avian melanocortin system, particularly in relation to two of the most widely studied physiological functions of the melanocortin system in mammals, the regulation of pigmentation and energy homeostasis. The data reviewed here indicate that the melanocortin system has been strongly conserved during vertebrate evolution and that alpha-MSH is produced locally in birds to act as an autocrine/paracrine hormone.

Slc7a11 gene controls production of pheomelanin pigment and proliferation of cultured cells

In mammals, >100 genes regulate pigmentation by means of a wide variety of developmental, cellular, and enzymatic mechanisms. Nevertheless, genes that directly regulate pheomelanin production have not been described. Here, we demonstrate that the subtle gray (sut) mouse pigmentation mutant arose by means of a mutation in the Slc7a11 gene, encoding the plasma membrane cystine/glutamate exchanger xCT [Kanai, Y. & Endou, H. (2001) Curr. Drug Metab. 2, 339-354]. A resulting low rate of extracellular cystine transport into sut melanocytes reduces pheomelanin production. We show that Slc7a11 is a major genetic regulator of pheomelanin pigment in hair and melanocytes, with minimal or no effects on eumelanin. Furthermore, transport of cystine by xCT is critical for normal proliferation, glutathione production, and protection from oxidative stress in cultured cells. Thus, we have found that the Slc7a11 gene controls the production of pheomelanin pigment directly. Cells from sut mice provide a model for oxidative stress-related diseases and their therapies.