Fish pigmentation and the melanocortin system

Abstract

The melanocortin system is a complex neuroendocrine signaling mechanism involved in numerous physiological processes in vertebrates, including pigmentation, steroidogenesis and metabolic control. This review focuses at one of its most fascinating function in fish, its regulatory role in the control of pigmentation, in which the melanocortin 1 receptor (Mc1r), its agonist α-melanocyte stimulating hormone (α-Msh), and the endogenous antagonist agouti signaling protein (Asip1) are the main players. Functional control of Mc1r, which is highly expressed in fish skin and whose activation stimulates melanin production and melanosome dispersion in fish melanophores, is considered a key mechanism for vertebrate pigment phenotypes. The α-Msh peptide, the most documented Mc1r agonist involved in pigmentation, is produced in the pituitary gland, activating melanin synthesis by binding to Mc1r in fish melanophores. Finally, Asip1 is the putative factor for establishing the evolutionarily conserved dorso-ventral pigment pattern found across vertebrates. However, we are just starting to understand how other melanocortin system components are acting in this complex regulatory network.

Introduction

Pigment patterns exhibit a wide functional spectrum in vertebrates, including camouflage (Belk and Smith, 1996, Protas and Patel, 2008, Rudh and Qvarnström, 2013), thermoregulation (Ellis, 1980, Rudh and Qvarnström, 2013), photo-protection (Rudh and Qvarnström, 2013), mate choice (Kodric-Brown and Nicoletto, 2001, Bajer et al., 2011, Maan and Sefc, 2013), and others. Pigment patterns are primarily generated by the organized distribution of neural crest-derived pigment cells or chromatophores. Mammals have only one type of chromatophore – termed melanocytes – that produce both eumelanin, a melanin pigment that generates dark black, brown or grey colorations, and pheomelanin, a yellow to reddish melanin pigment. The ratio of pheomelanin to eumelanin production and the distribution of these pigment types in turn generate mammalian pigment patterns. In birds, probably the most diverse group of animals regarding melanin-based coloration, the integument (avian skin) also contains both eumelanin and pheomelanin (Galván and Solano, 2016).

However, the diversity and complexity of pigment patterns are exceedingly more complex in fish, which possess up to six different types of chromatophores with pigments of different chemical identities. Fish melanophores only produce eumelanin, but no pheomelanin (Fujii, 1993, Bagnara, 1998, Kottler et al., 2015), whereas xanthophores and erythrophores display pteridine and/or carotenoid pigments (Schartl et al., 2015). Iridophores and leucophores contain stacked crystalline platelets composed of purines, mainly of guanine, that are responsible for reflection of light (Fujii, 1993), while cyanophores contain blue pigmentary organelles of unknown chemical composition (Goda and Fujii, 1995, Schartl et al., 2015). The genetic basis of the chromatophore diversity in fish, particularly teleosts, remains poorly understood, but functional diversification of pigment pathway components following gene duplication in a teleost-specific genome duplication (TGD) event has been suggested as a potential contributing factor (Braasch et al., 2007, Braasch et al., 2008, Braasch et al., 2009a). Pigment patterns also depend on chromatophore interactions such as the striped pattern of the zebrafish (Danio rerio) (Maderspacher, 2003, Takahashi and Kondo, 2008, Patterson and Parichy, 2013, Singh et al., 2014, Eom et al., 2015). Zebrafish iridophores and melanophores show short-range repulsion, which causes melanophore aggregation in the presence of iridophores, while iridophores and xanthophores exhibit mutual attraction, and xanthophores and melanophores repel each other (Frohnhöfer et al., 2013). Chromatophores in another teleost fish, the sole (Solea senegalensis), show similar cell type interactions to build a spotted pigment pattern (Darias et al., 2013).

In fish, most studies on pigment patterning have addressed teleost fishes, which represent around 96% of all living fish species and about half of all living vertebrates (Nelson, 2006), and have focused on stripe formation using zebrafish as model system, identifying a core striping mechanism dependent on interactions between different pigment cell types. It has recently been demonstrated that zebrafish uses two distinct adult pigment patterning mechanisms: an ancient dorso-ventral patterning mechanism, which is present in all bony vertebrates and which is controlled by regulating melanin chemistry, pigment cell distribution, and development, and an evolutionary more recent striping mechanism based on the aforementioned pigment cell-cell interactions that provides zebrafish with its eponymous adult pattern of dark and light stripes. It has been determined that these two pigment patterning mechanisms function largely independently, with resultant patterns superimposed to generate the full, combinatory pattern (Ceinos et al., 2015). However, these pigment patterning mechanisms may change in response to different environmental stimuli, making it one of the most captivating features of fish pigmentation.

Fish color changes are accomplished by two different mechanisms: physiological and morphological color change. Physiological color change is caused by short-term stimuli and is based on pigment organelle aggregation or dispersion within skin chromatophores. Physiological color change is controlled by both sympathetic (noradrenalin) and endocrine (α-melanocyte stimulating hormone, α-Msh, and melanin concentrating hormone, Mch) systems (Fujii, 2000). Noradrenalin and Mch released from chromaffin tissue and the pituitary have been shown to induce pigment-aggregation on light backgrounds (Kawauchi et al., 1983, Logan et al., 2006, Sugimoto, 2002). By contrast, α-Msh induces pigment dispersion and its plasma level is increased on dark backgrounds (Logan et al., 2006, Mizusawa et al., 2013). Morphological color change, on the other hand, is caused by long-term stimuli and is mediated by apoptosis or proliferation of skin chromatophores, as well as changes in their cellular morphology. Several studies have demonstrated that fish also can morphologically change their color by decreasing or increasing the number and size of melanophores during long-term adaptation to light or dark background, respectively (Sugimoto et al., 2005, van der Salm et al., 2005). The opposite response, a decrease in dark backgrounds and increase in white backgrounds, is observed in the number of iridophores (Sugimoto et al., 2005). Both physiological and morphological color changes appear to be controlled by similar mechanisms, which at the molecular-cellular level include the melanocortin system.

The teleost fish melanocortin system consists of five melanocortin receptor subtypes (Mc1r, Mc2r, Mc3r, Mc4r, Mc5r), the endogenous agonists alpha and beta melanocyte stimulating hormone (Msh) and adrenocorticotropic hormone (Acth), melanocortin peptides derived from the proopiomelanocortin (pomc) gene, and the endogenous antagonists of the agouti signaling protein family (Asip1, Asip2, Agrp1 and Agrp2). All these components in their cellular context are illustrated in Fig. 1.