Melanocore uptake by keratinocytes occurs through phagocytosis and involves Protease-activated receptor-2 internalization

Melanin's Journey from Melanocytes to Keratinocytes: Uncovering the Molecular Mechanisms of Melanin Transfer and Processing

Skin pigmentation ensures efficient photoprotection and relies on the pigment melanin, which is produced by epidermal melanocytes and transferred to surrounding keratinocytes. While the molecular mechanisms of melanin synthesis and transport in melanocytes are now well characterized, much less is known about melanin transfer and processing within keratinocytes. Over the past few decades, distinct models have been proposed to explain how melanin transfer occurs at the cellular and molecular levels. However, this remains a debated topic, as up to four different models have been proposed, with evidence presented supporting each. Here, we review the current knowledge on the regulation of melanin exocytosis, internalization, processing, and polarization. Regarding the different transfer models, we discuss how these might co-exist to regulate skin pigmentation under different conditions, i.e., constitutive and facultative skin pigmentation or physiological and pathological conditions. Moreover, we discuss recent evidence that sheds light on the regulation of melanin exocytosis by melanocytes and internalization by keratinocytes, as well as how melanin is stored within these cells in a compartment that we propose be named the melanokerasome. Finally, we review the state of the art on the molecular mechanisms that lead to melanokerasome positioning above the nuclei of keratinocytes, forming supranuclear caps that shield the nuclear DNA from UV radiation. Thus, we provide a comprehensive overview of the current knowledge on the molecular mechanisms regulating skin pigmentation, from melanin exocytosis by melanocytes and internalization by keratinocytes to processing and polarization within keratinocytes. A better knowledge of these molecular mechanisms will clarify long-lasting questions in the field that are crucial for the understanding of skin pigmentation and can shed light on fundamental aspects of organelle biology. Ultimately, this knowledge can lead to novel therapeutic strategies to treat hypo- or hyper-pigmentation disorders, which have a high socio-economic burden on patients and healthcare systems worldwide, as well as cosmetic applications.

The journey from melanocytes to melanoma

Over the past decade, melanoma has led the field in new cancer treatments, with impressive gains in on-treatment survival but more modest improvements in overall survival. Melanoma presents heterogeneity and transcriptional plasticity that recapitulates distinct melanocyte developmental states and phenotypes, allowing it to adapt to and eventually escape even the most advanced treatments. Despite remarkable advances in our understanding of melanoma biology and genetics, the melanoma cell of origin is still fiercely debated because both melanocyte stem cells and mature melanocytes can be transformed. Animal models and high-throughput single-cell sequencing approaches have opened new opportunities to address this question. Here, we discuss the melanocytic journey from the neural crest, where they emerge as melanoblasts, to the fully mature pigmented melanocytes resident in several tissues. We describe a new understanding of melanocyte biology and the different melanocyte subpopulations and microenvironments they inhabit, and how this provides unique insights into melanoma initiation and progression. We highlight recent findings on melanoma heterogeneity and transcriptional plasticity and their implications for exciting new research areas and treatment opportunities. The lessons from melanocyte biology reveal how cells that are present to protect us from the damaging effects of ultraviolet radiation reach back to their origins to become a potentially deadly cancer.

A Comprehensive Review of Mammalian Pigmentation: Paving the Way for Innovative Hair Colour-Changing Cosmetics

The natural colour of hair shafts is formed at the bulb of hair follicles, and it is coupled to the hair growth cycle. Three critical processes must happen for efficient pigmentation: (1) melanosome biogenesis in neural crest-derived melanocytes, (2) the biochemical synthesis of melanins (melanogenesis) inside melanosomes, and (3) the transfer of melanin granules to surrounding pre-cortical keratinocytes for their incorporation into nascent hair fibres. All these steps are under complex genetic control. The array of natural hair colour shades are ascribed to polymorphisms in several pigmentary genes. A myriad of factors acting via autocrine, paracrine, and endocrine mechanisms also contributes for hair colour diversity. Given the enormous social and cosmetic importance attributed to hair colour, hair dyeing is today a common practice. Nonetheless, the adverse effects of the long-term usage of such cosmetic procedures demand the development of new methods for colour change. In this context, case reports of hair lightening, darkening and repigmentation as a side-effect of the therapeutic usage of many drugs substantiate the possibility to tune hair colour by interfering with the biology of follicular pigmentary units. By scrutinizing mammalian pigmentation, this review pinpoints key targetable processes for the development of innovative cosmetics that can safely change the hair colour from the inside out.

The UV-induced uptake of melanosome by skin keratinocyte is triggered by α7 nicotinic acetylcholine receptor-mediated phagocytosis

The melanosome is an organelle that produces melanin for skin pigmentation, which is synthesized by epidermal melanocytes, subsequently transported and internalized by epidermal keratinocytes. Exposure to ultraviolet (UV) from sunlight radiation is a major stimulator of melanosome uptake by keratinocytes. Acetylcholine (ACh) is known to be released by keratinocytes under UV exposure, which regulates melanin production in melanocytes by participating in which has been named as 'skin synapse'. Here, the role of cholinergic molecules, i.e. ACh and α7 nicotinic acetylcholine receptor (nAChR), in regulating melanosome uptake through phagocytosis by keratinocytes was illustrated. In cultured keratinocytes (HaCaT cells), the fluorescent beads at different sizes imitating melanosomes, or melanosomes, were phagocytosed under UV exposure. The UV-induced phagocytosis in keratinocytes was markedly increased by applied ACh, an acetylcholinesterase (AChE) inhibitor or an α7 nAChR agonist. By contrast, the antagonist of α7 nAChR was able to fully block the UV-induced phagocytosis, suggesting the role of α7 nAChR in this event. The intracellular Ca⁺⁺ mobilization was triggered by UV exposure, accounting for the initiation of phagocytosis. The blockage of UV-mediated Ca⁺⁺ mobilization, triggered by BAPTA-AM or α7 nAChR antagonist, resulted in a complete termination of phagocytosis. Besides, the phosphorylation of cofilin, as well as expression and activation of RhoA, accounting for phagocytosis was induced by UV exposure: the phosphorylation was blocked by BAPTA-AM or α7 nAChR antagonist. The result suggests that the cholinergic system, especially α7 nAChR, is playing a regulatory role in modulating melanosome uptake in keratinocytes being induced by UV exposure.

RAB3A Regulates Melanin Exocytosis and Transfer Induced by Keratinocyte-Conditioned Medium

Skin pigmentation is imparted by melanin and is crucial for photoprotection against UVR. Melanin is synthesized and packaged into melanosomes within melanocytes and is then transferred to keratinocytes (KCs). Although the molecular players involved in melanogenesis have been extensively studied, those underlying melanin transfer remain unclear. Previously, our group proposed that coupled exocytosis/phagocytosis is the predominant mechanism of melanin transfer in human skin and showed an essential role for RAB11B and the exocyst tethering complex in this process. In this study, we show that soluble factors present in KC-conditioned medium stimulate melanin exocytosis from melanocytes and transfer to KCs. Moreover, we found that these factors are released by differentiated KCs but not by basal layer KCs. Furthermore, we found that RAB3A regulates melanin exocytosis and transfer stimulated by KC-conditioned medium. Indeed, KC-conditioned medium enhances the recruitment of RAB3A to melanosomes in melanocyte dendrites. Therefore, our results suggest the existence of two distinct routes of melanin exocytosis: a basal route controlled by RAB11B and a RAB3A-dependent route, stimulated by KC-conditioned medium. Thus, this study provides evidence that soluble factors released by differentiated KCs control skin pigmentation by promoting the accumulation of RAB3A-positive melanosomes in melanocyte dendrites and their release and subsequent transfer to KCs.