Signaling pathways in melanosome biogenesis and pathology

Abstract

Melanosomes are the specialized intracellular organelles of pigment cells devoted to the synthesis, storage and transport of melanin pigments, which are responsible for most visible pigmentation in mammals and other vertebrates. As a direct consequence, any genetic mutation resulting in alteration of melanosomal function, either because affecting pigment cell survival, migration and differentiation, or because interfering with melanosome biogenesis, transport and transfer to keratinocytes, is immediately translated into color variations of skin, fur, hair or eyes. Thus, over 100 genes and proteins have been identified as pigmentary determinants in mammals, providing us with a deep understanding of this biological system, which functions by using mechanisms and processes that have parallels in other tissues and organs. In particular, many genes implicated in melanosome biogenesis have been characterized, so that melanosomes represent an incredible source of information and a model for organelles belonging to the secretory pathway. Furthermore, the function of melanosomes can be associated with common physiological phenotypes, such as variation of pigmentation among individuals, and with rare pathological conditions, such as albinism, characterized by severe visual defects. Among the most relevant mechanisms operating in melanosome biogenesis are the signal transduction pathways mediated by two peculiar G protein-coupled receptors: the melanocortin-1 receptor (MC1R), involved in the fair skin/red hair phenotype and skin cancer; and OA1 (GPR143), whose loss-of-function results in X-linked ocular albinism. This review will focus on the most recent novelties regarding the functioning of these two receptors, by highlighting emerging signaling mechanisms and general implications for cell biology and pathology.

Figure 1

Optical and ultrastructural features of melanosomes. (A) Representative picture of wild-type mouse melanocytes in culture as observed by bright-field optical microscopy. In this condition, melanosomes represent the only visible dark objects, thanks to the melanin pigment, and show an even distribution and size throughout the cell. (B) Bright-field picture of Oa1-KO mouse melanocytes, showing reduction in melanosome number, presence of macromelanosomes and displacement of the organelles towards the cell periphery. A′ and B′ represent a 3x magnification of delimited areas in A and B, respectively. Asterisks indicate the position of the cell nuclei. Scale bars 10 μm (1 μm for 3x magnifications). (C, D) Conventional electron microscopy of wild-type mouse melanocytes, showing melanosomes at various maturation stages, as indicated. Stages II and III are characterized by longitudinal striatures, without or with progressive melanin deposition (black), respectively, while stage IV melanosomes show no recognizable internal structures and a completely electron dense melanized lumen. Scale bar, 1 μm.

Figure 2

Schematical overview of the signaling pathway mediated by MC1R. MC1R resides at the plasma membrane of melanocytes and is activated by melanocortins, in particular α-MSH, and inactivated by Agouti protein. To be fully effective, Agouti also needs Attractin, which helps Agouti binding to the receptor, and Mahogunin, which acts at the cytosolic side of the plasma membrane, most likely by competing with Gs for receptor binding. In addition, MC1R has a neutral agonist, β-Defensin, which interferes with both agonist and antagonist binding, thereby highlighting the high degree of constitutive activity of the receptor. The canonical pathway activated by MC1R leads to cAMP increase via Gs and adenylyl cyclase, and proceeds with CREB phosphorylation by PKA and transcription of downstream targets, in particular MITF. Increased MITF activity is further reinforced and regulated by cAMP-independent ERK activation, probably resulting from cross-talk between MC1R and the cKIT receptor. MITF binds to promoters and stimulates transcription of genes coding for melanosomal proteins implicated in the eumelanogenesis pathway, thus inducing a switch from pheomelanin to eumelain synthesis and increasing melanosome number, size and transport. MC1R signaling also reduces the generation of reactive oxygen species and enhances DNA repair mechanisms by still poorly understood mechanisms (not shown). PM, plasma membrane.

Figure 3

Schematical overview of the signaling pathway mediated by OA1. Endogenous OA1 resides on the membrane of late endosomes/lysosomes and melanosomes, where it could function as a “sensor” of melanosome maturation and be activated by molecules belonging to the melanin biosynthetic pathway, possibly L-DOPA. It appears coupled to Gi proteins, through which it regulates proper biogenesis of melanosomes and restrains their transport toward the cell periphery by still unknown effectors. While a role in melanosome biogenesis and transport is supported by numerous lines of evidence, it is unclear whether the lysosomal localization of endogenous OA1 in melanocytes has a functional or downregulation role. In addition, a minor fraction of endogenous OA1 might travel to the cell surface of RPE cells, where it could become activated by L-DOPA and in turn trigger a Gq-mediated signaling pathway eventually leading to the secretion of neurotrophic factors. In contrast, exogenous OA1 overexpressed in non-melanocytic cells can be substantially missorted to the plasma membrane, downregulated by arrestins and coupled to Gq proteins, leading to phospholipase C activation and inositol phosphate or intracellular calcium increase (not shown). PM, plasma membrane; EE, early endosomes; LE, late endosomes; MVB, multivesicular bodies.