[Biogenesis of melanosomes - the chessboard of pigmentation]

TRPA1 promotes UVB-induced skin pigmentation by regulating melanosome luminal pH

Melanocytes stimulated by ultraviolet radiation (UVR) produce melanin and melanosomes, which causes skin pigmentation and acts as an important physiological defence process for photoprotection. Neutral luminal pH of melanosomes is critical for providing optimal conditions for the rate-limiting, pH-sensitive melanin synthesizing enzyme tyrosinase (TYR). As a major component of extraocular phototransduction pathway, transient receptor potential ankyrin1 (TRPA1) can be activated by ultraviolet B (UVB) and reported to be expressed in melanocytes. However, whether TRPA1 is involved in the regulation of melanogenesis remains unclear. Melanogenic activity of TRPA1 was evaluated in primary normal human epidermal melanocytes (HEMs) and murine B16-F10 cell cultures, and the effects of topical applications of TRPA1 specific agonist and antagonist on UVB-induced skin pigmentation were confirmed on in vivo guinea pig models. Calcium (Ca²⁺ ) imaging and pH imaging were performed to analyse the effects of TRPA1 on intracellular Ca²⁺ concentration ([Ca²⁺ ]_ic ) and melanosome luminal pH. TRPA1 regulated melanin synthesis, UVB-induced Ca²⁺ influx and melanosome luminal pH in HEMs and B16-F10 cells. Topical treatment of TRPA1 specific agonist JT010 increased UVB-induced skin pigmentation in guinea pigs, while topical using of TRPA1 selective antagonist HC-030031 mitigated such pigmentation. Our results indicated that TRPA1 activated by UVB enhanced the skin pigmentation, most likely by regulating the [Ca²⁺ ]_ic and the melanosomal pH, consequently influencing the enzymatic activity of TYR. Therefore, the results suggest TRPA1 as a potential therapeutic target in the treatment of skin pigmented disorders that are at high risk under UVB irradiation.

A Novel Method for Visualizing Melanosome and Melanin Distribution in Human Skin Tissues

Melanin incorporated into keratinocytes plays an important role in photoprotection; however, abnormal melanin accumulation causes hyperpigmentary disorders. To understand the mechanism behind the accumulation of excess melanin in the skin, it is essential to clarify the spatial distribution of melanosomes or melanin in the epidermis. Although several markers have been used to detect melanosomes or melanin, no suitable markers to determine the precise localization of melanin in the epidermis have been reported. In this study, we showed that melanocore-interacting Kif1c-tail (M-INK), a recently developed fluorescent probe for visualizing mature melanosomes, binds to purified melanin in vitro, and applied it for detecting melanin in human skin tissues. Frozen skin sections from different phototypes were co-stained for the hemagglutinin (HA)-tagged M-INK probe and markers of melanocytes or keratinocytes, and a wide distribution of melanin was observed in the epidermis. Analysis of the different skin phototypes indicated that the fluorescent signals of HA-M-INK correlated well with skin color. The reconstruction of three-dimensional images of epidermal sheets enabled us to observe the spatial distribution of melanin in the epidermis. Thus, the HA-M-INK probe is an ideal tool to individually visualize melanin (or melanosome) distribution in melanocytes and in keratinocytes in skin tissues.

Elucidation of Melanogenesis Cascade for Identifying Pathophysiology and Therapeutic Approach of Pigmentary Disorders and Melanoma

Melanogenesis is the biological and biochemical process of melanin and melanosome biosynthesis. Melanin is formed by enzymic reactions of tyrosinase family proteins that convert tyrosine to form brown-black eumelanin and yellow-red pheomelanin within melanosomal compartments in melanocytes, following the cascades of events interacting with a series of autocrine and paracrine signals. Fully melanized melanosomes are delivered to keratinocytes of the skin and hair. The symbiotic relation of a melanocyte and an associated pool of keratinocytes is called epidermal melanin unit (EMU). Microphthalmia-associated transcription factor (MITF) plays a vital role in melanocyte development and differentiation. MITF regulates expression of numerous pigmentation genes for promoting melanocyte differentiation, as well as fundamental genes for maintaining cell homeostasis. Diseases involving alterations of EMU show various forms of pigmentation phenotypes. This review introduces four major topics of melanogenesis cascade that include (1) melanocyte development and differentiation, (2) melanogenesis and intracellular trafficking for melanosome biosynthesis, (3) melanin pigmentation and pigment-type switching, and (4) development of a novel therapeutic approach for malignant melanoma by elucidation of melanogenesis cascade.

The SLC9A-C Mammalian Na+/H+ Exchanger Family: Molecules, Mechanisms, and Physiology

Na⁺/H⁺ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na⁺ and H⁺ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na⁺/H⁺ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.

[Tetraspanins in cutaneous physiopathology]

Tetraspanins are transmembrane proteins that interact laterally with each other and with different partners such as integrins, immunoglobulin (Ig)-domain-containing proteins, growth factors and cytokine receptors. Such tetraspanin-partner complexes help to organize dynamic membrane networks called "tetraspanin web", which trigger different signalling pathways. Despite the fact that tetraspanins seem abundantly and widely expressed, their function remained unclear. However, it is well established that they control fundamental cellular processes including cell survival, adhesion, migration, invasion or viral infection, but the underlying molecular mechanisms are not well elucidated. This review focuses on tetraspanins that are expressed in epidermis and the roles they play in normal and pathological conditions, specifically in skin cancer.

Amyloidosis in alkaptonuria

Alkaptonuria (AKU) is an ultra-rare inborn error of metabolism developed from the lack of homogentisic acid oxidase activity, causing homogentisic acid (HGA) accumulation that produces an HGA-melanin ochronotic pigment, of hitherto unknown composition. Besides the accumulation of HGA, the potential role and presence of unidentified proteins has been hypothesized as additional causal factors involved in ochronotic pigment deposition. Evidence has been provided on the presence of serum amyloid A (SAA) in several AKU tissues, which allowed classifying AKU as a novel secondary amyloidosis. In this paper, we will briefly review all direct and indirect lines of evidence related to the presence of amyloidosis in AKU. We also report the first data on abnormal SAA serum levels in a cohort of AKU patients.