Epimedin B exhibits pigmentation by increasing tyrosinase family proteins expression, activity, and stability

Epimedin B (EB) is one of the main flavonoid ingredients present in Epimedium brevicornum Maxim., a traditional herb widely used in China. Our previous study showed that EB was a stronger inducer of melanogenesis and an activator of tyrosinase (TYR). However, the role of EB in melanogenesis and the mechanism underlying the regulation remain unclear. Herein, as an extension to our previous investigation, we provide comprehensive evidence of EB-induced pigmentation in vivo and in vitro and elucidate the melanogenesis mechanism by assessing its effects on the TYR family of proteins (TYRs) in terms of expression, activity, and stability. The results showed that EB increased TYRs expression through microphthalmia-associated transcription factor-mediated p-Akt (referred to as protein kinase B (PKB))/glycogen synthase kinase 3β (GSK3β)/β-catenin, p-p70 S6 kinase cascades, and protein 38 (p38)/mitogen-activated protein (MAP) kinase (MAPK) and extracellular regulated protein kinases (ERK)/MAPK pathways, after which EB increased the number of melanosomes and promoted their maturation for melanogenesis in melanoma cells and human primary melanocytes/skin tissues. Furthermore, EB exerted repigmentation by stimulating TYR activity in hydroquinone- and N-phenylthiourea-induced TYR inhibitive models, including melanoma cells, zebrafish, and mice. Finally, EB ameliorated monobenzone-induced depigmentation in vitro and in vivo through the enhancement of TYRs stability by inhibiting TYR misfolding, TYR-related protein 1 formation, and retention in the endoplasmic reticulum and then by downregulating the ubiquitination and proteolysis processes. These data conclude that EB can target TYRs and alter their expression, activity, and stability, thus stimulating their pigmentation function, which might provide a novel rational strategy for hypopigmentation treatment in the pharmaceutical and cosmetic industries.

Intermittent inhibition of FYVE finger-containing phosphoinositide kinase induces melanosome degradation in B16F10 melanoma cells

BACKGROUND

Melanosomes are lysosome-related organelles that contain melanogenic factors and synthesize melanin as they mature. FYVE finger-containing phosphoinositide kinase (PIKfyve) regulates late endosome and lysosome morphology, vesicle trafficking, and autophagy. In melanocytes, PIKfyve inhibition has been reported to induce hypopigmentation due to impairments in the metabolism of early-stage melanosomes.

METHODS AND RESULTS

Here, we report a new type of melanosome metabolism: post-PIKfyve inhibition, which was found during the characterization of the endosome/lysosome fluoroprobe GIF-2250. In B16F10 mouse melanoma cells, GIF-2250 highlighted vesicles positive for lysosomal-associated membrane protein 1 (lysosome marker) and other endosome/lysosome markers (CD63 and Rab7/9). When cells were continuously treated with PIKfyve inhibitors, intracellular vacuoles formed, while GIF-2250 fluorescence signals diminished and were diffusely distributed in the vacuoles. After removal of the PIKfyve inhibitors, the GIF-2250 signal intensity was restored, and some GIF-2250-positive vesicles wrapped the melanosomes, which spun at high speed. In addition, intermittent PIKfyve inhibition caused melanin diffusion in the vacuoles and possible leakage into the cytoplasmic compartments, and melanosome degradation was detected by a transmission electron microscope. Melanosome degradation was accompanied by decreased levels of melanin synthesis enzymes and increased levels of the autophagosome maker LC3BII, which is also associated with early melanosomes. However, the protein levels of p62, which is degraded during autophagy, were increased, suggesting an impairment in autophagy flux during intermittent PIKfyve inhibition. Moreover, the autophagy inhibitor 3-methyladenine does not affect these protein levels, suggesting that the melanosome degradation by the intermittent inhibition of PIKfyve is not mediated by canonical autophagy.

CONCLUSIONS

In conclusion, disturbance of PIKfyve activity induces melanosome degradation in a canonical autophagy-independent manner.

A novel gain of function mutation in TPC2 reiterates pH-pigmentation interplay: Emerging role of ionic homeostasis as a master pigmentation regulator

Pigmentation is a complex physiological phenomenon that protects from UV induced damage. Perturbations in pigmentation pathways lead to pigmentary disorders such as vitiligo, albinism and Darier...s disease. Emerging literature implicates a critical role of ionic homeostasis and pH in regulating pigmentation. In a recent study, Wang et al. identified a novel gain of function mutation in a non-selective cation channel "Two Pore Channel 2" (TPC2) that is responsible for albinism in a human patient. The authors demonstrate that this mutation leads to constitutive activation of TPC2 thereby modulating cellular calcium dynamics and inducing changes in the lysosomal pH. Further, authors generated a knock in mice with homologous TPC2 mutation and corroborated a causative role for this mutation in albinism. It is an exciting study that reports a novel TPC2 mutation, which is responsible for albinism in an autosomal dominant inheritance fashion. Since TPC2 is localized on melanosomes as well, going forward it would be interesting to investigate the role of this mutation on melanosomal calcium dynamics and alterations in melanosomal pH.

Effects of MITF on marker protein expression of multivesicular bodies and miRNA omics of extracellular vesicles of mice melanocyte cell line

Extracellular vesicles (EVs) are heterogeneous membrane-bound complexes of cell-derived and nanosized structures originating from the endosomal system and subsequently released from the plasma membrane. EVs contribute significantly to intercellular communication and are involved in pigmentation processes that rely on tight communication between keratinocytes and melanocytes in the epidermis. Microphthalmia-associated transcription factor (MITF) induces melanogenesis and modulates the expression factors involved in melanosome biogenesis, maturation and dispersal in melanocytes. Here, we evaluated the effects of MITF on the fate of multivesicular bodies and the biogenesis of extracellular vesicles of melanocytes. It was found that MITF increased the expression of subunits of the endosomal sorting complex, required for transport (ESCRT), including VPS37, VPS36B, and tetraspanin CD81, which are key mediators of multivesicular body biogenesis. Over 110 miRNAs, including miR-211-5p, miR-335-5p, let-7g-5p and miR-28a-3p, were differentially expressed in melanocyte-derived EVs after overexpression of MITF in melanocytes. These miRNAs have been reported to be key regulators of plasma protein binding, changes in the cell membrane system and transferase activity. These results suggest that while enhancing melanogenesis, melanocytes may mediate intercellular communication with surrounding cells by serving as EV delivery vehicles.

Structural analysis of melanosomes in living mammalian cells using scanning electron-assisted dielectric microscopy with deep neural network

Melanins are the main pigments found in mammals. Their synthesis and transfer to keratinocytes have been widely investigated for many years. However, analysis has been mainly carried out using fixed rather than live cells. In this study, we have analysed the melanosomes in living mammalian cells using newly developed scanning electron-assisted dielectric microscopy (SE-ADM). The melanosomes in human melanoma MNT-1 cells were observed as clear black particles in SE-ADM. The main structure of melanosomes was toroidal while that of normal melanocytes was ellipsoidal. In tyrosinase knockout MNT-1 cells, not only the black particles in the SE-ADM images but also the Raman shift of melanin peaks completely disappeared suggesting that the black particles were really melanosomes. We developed a deep neural network (DNN) system to automatically detect melanosomes in cells and analysed their diameter and roundness. In terms of melanosome morphology, the diameter of melanosomes in melanoma cells did not change while that in normal melanocytes increased during culture. The established DNN analysis system with SE-ADM can be used for other particles, e.g. exosomes, lysosomes, and other biological particles.

Mechanisms of cellular retention of melanin bound drugs: Experiments and computational modeling

Melanin binding of drugs is known to increase drug concentrations and retention in pigmented eye tissues. Even though the correlation between melanin binding in vitro and exposure to pigmented eye in vivo has been shown, there is a discrepancy between rapid drug release from melanin particles in vitro and the long in vivo retention in the pigmented tissues. We investigated mechanisms and kinetics of pigment-related drug retention experimentally using isolated melanin particles from porcine retinal pigment epithelium and choroid, isolated porcine eye melanosomes, and re-pigmented ARPE-19 cells in a dynamic flow system. The experimental studies were supplemented with kinetic simulations. Affinity and capacity of levofloxacin, terazosin, papaverine, and timolol binding to melanin revealed K_d values of ≈ 50-150 μM and B_max ≈ 40-112 nmol.mg^-1. The drugs were released from melanin in <1 h (timolol) or in 6-12 h (other drugs). The drugs were released slower from the melanosomes than from melanin; the experimental differences ranged from 1.2-fold (papaverine) to 7.4-fold (timolol). Kinetic simulations supported the role of the melanosomal membrane in slowing down the release of melanin binders. In release studies from the pigmented ARPE-19 cells, drugs were released from the cellular melanin to the extracellular space in ≈ 1 day (timolol) and ≈ 11 days (levofloxacin), i.e., much slower than the release from melanin or melanosomes. Simulations of drug release from pigmented cells in the flow system matched the experimental data and enabled further sensitivity analyses. The simulations demonstrated a significant prolongation of drug retention in the cells as a function of decreasing drug permeability in the melanosomal membranes and increasing melanin content in the cells. Overall, we report the impact of cellular factors in prolonging drug retention and release from melanin-containing cells. These data and simulations will facilitate the design of melanin binding drugs with prolonged ocular actions.

Kaempferol, the melanogenic component of Sanguisorba officinalis, enhances dendricity and melanosome maturation/transport in melanocytes

Kaempferol, a representative flavonoid constituent of Sanguisorba officinalis, promotes melanogenesis, but the underlying mechanisms remain unknown. Here, we evaluated the effects of kaempferol on melanocytes morphology and behavior and determined the mechanisms regulating kaempferol-induced pigmentation. We observed that kaempferol increased melanin contents and dendritic length and stimulated melanocyte migration both in vitro and vivo. It significantly enhanced the expression of microphthalmia-associated transcription factor (MITF) and downstream enzymes of melanin biosynthesis-tyrosinase (TYR), tyrosinase-related protein (TRP-1), and dopachrome tautomerase (DCT). It also induced melanosome maturation (increased stage III and IV melanosomes) and melanin transfer to dendritic tips; this was evidenced as follows: kaempferol-treated melanocytes exhibited the perimembranous accumulation of HMB45-positive melanosomes and increased the expression of Rab27A, RhoA, and Cdc42, which improved melanosome transport to perimembranous actin filaments. These results jointly indicated that kaempferol promotes melanogenesis and melanocyte growth. Additionally, kaempferol stimulated the phosphorylation of P38/ERK MAPK and downregulated p-PI3K, p-AKT, and p-P70s6K expression. Pre-incubation with P38 (SB203580) and ERK (PD98059) signaling inhibitors reversed the melanogenic and dendritic effects and MITF expression. PI3K/AKT inhibitor augmented kaempferol-induced melanin content and dendrite length. In summary, kaempferol regulated melanocytes' dendritic growth and melanosome quantity, maturation, and transport via P38/ERK MAPK and PI3K/AKT signaling pathways.

The regional distribution of melanosomes in the epidermis affords a localized intensive photoprotection for basal keratinocyte stem cells

Human skin is a highly efficient self-renewing barrier that is critical to withstanding environmental insults. Undifferentiated keratinocyte stem cells reside in the basal layer of the epidermis and in hair follicles that continuously give rise to progenies ensuring epidermal turnover and renewal. Ultraviolet (UV) radiation is a proven cause of skin keratinocyte cancers, which preferentially occur at sun-exposed areas of the skin. Fortunately, melanocytes produce melanin that is packaged in specific organelles (termed melanosomes) that are then delivered to nearby keratinocytes, endowing the recipient cells with photoprotection. It has long been thought that melanosome transfer takes place stochastically from melanocytes to keratinocytes via an as-yet-unrecognized manner. However, recent studies have indicated that melanosomes are distributed regionally in the basal layer of the skin, affording localized intensive photoprotection for progenitor keratinocytes and stem cells that reside in the microenvironment of the basal epidermis. In this review, we summarize current knowledge about molecular and cellular mechanisms that are responsible for the selective transfer and exclusive degradation of melanosomes in the epidermis, emphasizing implications for skin carcinogenesis.

Single organelle measurements of melanosome pH using the novel ratiometric indicator RpHiMEL

Melanocytes are specialized cells that produce melanin pigments responsible for skin, hair, and eye pigmentation. The synthesis and storage of melanin occurs in unique lysosome-related organelles called melanosomes, which regulate melanin production via complex regulatory mechanisms. Maintenance of the melanosome luminal ionic environment and pH is crucial for proper function of the main melanogenic enzymes. Defects in genes encoding pH-regulating melanosomal proteins result in oculocutaneous albinism, which is characterized by hypopigmentation, impaired vision, and increased susceptibility to skin and eye cancers. We recently uncovered several ion channels and transporters that modulate melanin synthesis by acidifying or neutralizing the luminal pH of melanosomes. However, our understanding of how melanosomes and other related organelles maintain their luminal pH is far from complete. The study of melanosome pH regulation requires robust imaging and quantification tools. Despite recent advances in the development of such methods, many limitations remain, particularly for quantitative analysis of individual organelle pH. In this chapter, we will provide an overview of the available methods used for melanosome pH determination, including their advantages, limitations, and challenges. To address the critical, unmet need for reliable melanosome pH quantification tools, we engineered a novel genetically encoded, ratiometric pH sensor for melanosomes that we named RpHiMEL. Here, we describe the design and optimization of RpHiMEL, and provide a pH quantification method for individual melanosomes in live cells. We demonstrate that RpHiMEL is a highly versatile tool with the potential to advance our understanding of pH regulation in melanosomes and related organelles.

Whitening effect of novel peptide mixture by regulating melanosome biogenesis, transfer and degradation

Peptides are short chain of amino acids linked by peptide bonds. They are widely used as effective and biocompatible active ingredients in cosmetic industry. In this study, we developed novel peptide mixture and identified its anti-pigmentation effect on melanocytes and keratinocytes. Our results revealed that peptide mixture inhibited melanosome biogenesis through the regulation of microphthalmia-associated transcription factor, a key factor of melanogenesis in melanocytes. And we observed that peptide mixture inhibited melanosome uptake through the reduction of protease-activated receptor 2, a phagocytosis-related receptor in keratinocytes. Furthermore, peptide mixture activated autophagy system resulting in degradation of transferred melanosomes in keratinocytes. The anti-pigmentation effect of multi-targeting peptide mixture was assessed in a human skin equivalent model (MelanoDerm). Melanin contents in epidermal layer were significantly decreased by topical treatment of peptide mixture, suggesting that it can be applied as a novel cosmetics material having a whitening function.

Ursolic acid inhibits pigmentation by increasing melanosomal autophagy in B16F1 cells

Melanosomes are specialized membrane-bound organelles that are involved in melanin synthesis. Unlike melanosome biogenesis, the melanosome degradation pathway is poorly understood. Among the cellular processes, autophagy controls degradation of intracellular components by cooperating with lysosomes. In this study, we showed that ursolic acid inhibits skin pigmentation by promoting melanosomal autophagy, or melanophagy, in melanocytes. We found that B16F1 cells treated with ursolic acid suppressed alpha-melanocyte stimulating hormone (α-MSH) stimulated increase in melanin content and activated autophagy. In addition, we found that treatment with ursolic acid promotes melanosomal degradation, and bafilomycin A1 inhibition of autophagosome-lysosome fusion blocked the removal of melanosomes in α-MSH-stimulated B16F1 cells. Furthermore, depletion of the autophagy-related gene 5 (ATG5) resulted in significant suppression of ursolic acid-mediated anti-pigmentation activity and autophagy in α-MSH-treated B16F1 cells. Taken together, our results suggest that ursolic acid inhibits skin pigmentation by increasing melanosomal degradation in melanocytes.

Membrane trafficking in the retinal pigment epithelium at a glance

The retinal pigment epithelium (RPE) is a highly specialised pigmented monolayer sandwiched between the choroid and the photoreceptors in the retina. Key functions of the RPE include transport of nutrients to the neural retina, removal of waste products and water from the retina to the blood, recycling of retinal chromophores, absorption of scattered light and phagocytosis of the tips of the photoreceptor outer segments. These functions place a considerable membrane trafficking burden on the RPE. In this Cell Science at a Glance article and the accompanying poster, we focus on RPE-specific adaptations of trafficking pathways. We outline mechanisms underlying the polarised expression of membrane proteins, melanosome biogenesis and movement, and endocytic trafficking, as well as photoreceptor outer segment phagocytosis and degradation. We also briefly discuss theories of how dysfunction in trafficking pathways contributes to retinal disease.

Microtubule motor transport in the delivery of melanosomes to the actin-rich apical domain of the retinal pigment epithelium

Melanosomes are motile, light-absorbing organelles that are present in pigment cells of the skin and eye. It has been proposed that melanosome localization, in both skin melanocytes and the retinal pigment epithelium (RPE), involves melanosome capture from microtubule motors by an unconventional myosin, which dynamically tethers the melanosomes to actin filaments. Recent studies with melanocytes have questioned this cooperative capture model. Here, we test the model in RPE cells by imaging melanosomes associated with labeled actin filaments and microtubules, and by investigating the roles of different motor proteins. We found that a deficiency in cytoplasmic dynein phenocopies the lack of myosin-7a, in that melanosomes undergo fewer of the slow myosin-7a-dependent movements and are absent from the RPE apical domain. These results indicate that microtubule-based motility is required for the delivery of melanosomes to the actin-rich apical domain and support a capture mechanism that involves both microtubule and actin motors.

A method for quantifying pigment position in retinal pigment epithelium

In wildtype mice, the pigment granules in the retinal pigment epithelium aggregate in the dark towards Bruch's membrane and disperse towards the photoreceptors in the light. We have developed a repeatable method amenable for quantifying pigment position in the RPE from wild type mice by estimating the population density of pigment granules, or pigment density, within 4 μm² areas in the basal part of cells examined by transmission electron microscopy. To measure pigment position, 2 μm × 2 μm squares were aligned along the apical ends of the basal microvilli. The pigment granules within each 4 μm² area were counted, and the average pigment density was calculated for each mouse. The average pigment density for light-adapted mice (n = 3 mice) was 1.3 pigment granules/μm² (± 0.2 pigment granules/μm²). For dark-adapted wildtype mice (n = 3 mice), pigment density was 1.9 pigment granules/μm² (± 0.3 pigment granules/μm²). Pigment density was statistically significantly different (p < 0.02) between light-adapted and dark-adapted mice, with pigment density higher in the dark-adapted mice. This method was implemented by four observers and their results were compared. No statistically significant differences were found in the measurements acquired by the different observers, illustrating the repeatability of the method.

TLR3 stimulation induces melanosome endo/phagocytosis through RHOA and CDC42 in human epidermal keratinocyte

BACKGROUND

Keratinocytes and melanocytes in human epidermis express Toll-like receptors (TLR) and induce immune responses. We previously reported that TLR3 stimulation increases melanosome transport from perinuclear to cell membrane in melanocytes and enhanced release of melanosome from melanocytes, which were followed by increase in melanosome uptake into keratinocytes.

OBJECTIVE

In this study, we investigated whether TLR3 stimuli directly affect keratinocytes to enhance melanosome uptake.

METHODS

To observe keratinocyte's melanosome uptake ability precisely without melanocytes influences, we isolated melanosomes from human melanocytes and applied isolated melanosomes to keratinocytes stimulated by Poly(I:C).

RESULTS

Poly(I:C)-stimulated keratinocytes enhanced uptake of isolated melanosome-rich globules five-times as much as control. Poly(I:C) increases the RNA and protein expressions of RHOA and CDC42, which are small GTP-binding proteins inducing the endocytosis. Pull-down assay showed that Poly(I:C) increased the GTP-binding RHOA and CDC42, suggesting TLR3 stimulation activated RHOA and CDC42. The knockdown of TLR3 suppressed RHOA and CDC42 induction by Poly(I:C). Consistently, the knockdown of RHOA and CDC42 significantly suppressed the melanosome-rich globules uptake by Poly(I:C)-stimulated keratinocytes.

CONCLUSION

Because RHOA and CDC42 activation induces endocytosis by modification of actin stress fiber and filopodia formation, respectively, these results suggested that TLR3 stimulation enhances melanosome uptake into keratinocytes through endocytosis mechanisms. Combining with the data of our previous publications, TLR3, which signal is activated by sensing viral molecules, enhance pigmentation by controlling both melanin transport system by RAB GTPases induction in melanocytes and uptake system by RHOA and CDC42 in keratinocytes.

Mitochondrial NCKX5 regulates melanosomal biogenesis and pigment production

Oculocutaneous albinism (OCA) is a heterogeneous and autosomal recessive hypopigmentation disorder, which is caused by mutations of genes involved in pigment biosynthesis or melanosome biogenesis. We have previously identified NCKX5 (also known as SLC24A5) as a causative gene for OCA type 6 (OCA6). However, the pathogenesis of OCA6 is unknown. We found that NCKX5 is localized to mitochondria, not to melanosomes. Pharmacological inhibition of mitochondrial function or NCKX exchanger activity reduced pigment production. Loss of NCKX5 attenuated Ca2+ enrichment in melanosomes, which compromised PMEL fibril formation, melanosome maturation and pigment production. Thus, we have defined a new class of hypopigmentation attributable to dysfunctional mitochondria and an impairment of mitochondrial Ca2+ transfer into melanosomes. Thus, it is possible that mitochondrial function could have a role in the graying of hair in older people and formation of hypopigmented lesions in vitiligo patients.

The PIKfyve complex regulates the early melanosome homeostasis required for physiological amyloid formation

The metabolism of PI(3,5)P2 is regulated by the PIKfyve, VAC14 and FIG4 complex, mutations in which are associated with hypopigmentation in mice. These pigmentation defects indicate a key, but as yet unexplored, physiological relevance of this complex in the biogenesis of melanosomes. Here, we show that PIKfyve activity regulates formation of amyloid matrix composed of PMEL protein within the early endosomes in melanocytes, called stage I melanosomes. PIKfyve activity controls the membrane remodeling of stage I melanosomes, which regulates PMEL abundance, sorting and processing. PIKfyve activity also affects stage I melanosome kiss-and-run interactions with lysosomes, which are required for PMEL amyloidogenesis and the establishment of melanosome identity. Mechanistically, PIKfyve activity promotes both the formation of membrane tubules from stage I melanosomes and their release by modulating endosomal actin branching. Taken together, our data indicate that PIKfyve activity is a key regulator of the melanosomal import-export machinery that fine tunes the formation of functional amyloid fibrils in melanosomes and the maintenance of melanosome identity.This article has an associated First Person interview with the first author of the paper.

pH-Dependent fibril maturation of a Pmel17 repeat domain isoform revealed by tryptophan fluorescence

The pre-melanosomal protein (Pmel17) aggregates within melanosomes to form functional amyloid fibrils that facilitate melanin polymerization. The repeat domain (RPT) of Pmel17 fibrillates under strict acidic melanosomal pH. Alternative splicing results in a shortened repeat domain (sRPT), which also forms amyloid fibrils. Here, we explored the effects of pH and protein concentration on sRPT aggregation by monitoring the intrinsic fluorescence of the sole tryptophan at position 381 (³⁸¹W). ³⁸¹W emission properties revealed changes of local environment polarity for sRPT fibrils formed at different pH. At pH 4, fibrils formed rapidly with no lag phase. A high ³⁸¹W intensity was observed with a slight blue shift (10 nm). These fibrils underwent further structural rearrangements at intermediate pH (5-6), mirroring that of melanosome maturation, which initiates at pH 4 and increases to near neutral pH. In contrast, typical sigmoidal kinetics were observed at pH 6 with slower rates and ³⁸¹W exhibited quenched emission. Interestingly, biphasic kinetics were observed at pH 5 in a protein concentration-dependent manner. A large ³⁸¹W blue shift (23 nm) was measured, indicating a more hydrophobic environment for fibrils made at pH 5. Consistent with ³⁸¹W fluorescence, Raman spectroscopy revealed molecular level perturbations in sRPT fibrils that were not evident from circular dichroism, transmission electron microscopy, or limited proteolysis analysis. Finally, sRPT fibrils did not form at pH ≥7 and preformed fibrils rapidly disaggregated under these solution conditions. Collectively, this work yields mechanistic insights into pH-dependent sRPT aggregation in the context of melanosome maturation.

Unusual light-reflecting pigment cells appear in the Xenopus neural tube culture system in the presence of guanosine

Isolation and culture of Xenopus laevis neural tubes resulted in differentiation of melanophores and iridophores from neural crest cells; the differentiated melanophores and iridophores were then maintained in culture for more than 6 months. Guanosine has been reported to promote reflecting platelet formation in melanin-producing pigment cells; however, the process of pigment organellogenesis is still unclear. In the present study, unusual light-reflecting pigment cells were observed upon addition of guanosine to the neural tube culture system, which contained melanosomes specific to melanophores, and reflecting platelets specific to iridophores. Ultrastructural studies suggested that irregularly shaped reflecting platelets were formed from stage II melanosomes (the early stage of melanosome formation) in these unusual pigment cells.

Silencing of PMEL attenuates melanization via activating lysosomes and degradation of tyrosinase by lysosomes

The functionally specialized melanosome is a membrane-enclosed lysosome-related organelle, which coexists with lysosomes in melanocytes. Pre-melanosomal protein (PMEL) initiates pre-melanosome morphogenesis and is the only cell-specific pigment protein required for the formation of fibrils on which melanin is deposited in melanosomes. But the effects of PMEL on melanin synthesis and lysosome activity remain unclear. In the study, PMEL was silenced in human epidermal melanocytes by siRNA transfection. Compared to the non-treated group, melanin content in the transfected cells was greatly reduced. Real-time qPCR, Western blotting and immunofluorescence analyses all showed that PMEL-siRNA transfection reduced protein level of tyrosinase, a key enzyme in melanogenesis, but it does not affect tyrosinase gene expression. Moreover, in the absence of PMEL, lysosomal activation was manifested by an increase in the number of lysosomes and activity of hydrolysis enzymes. The lysosome inhibitors restored tyrosinase expression after PMEL silencing, indicating that tyrosinase was degradated by lysosomes. The data collectively showed that silencing of PMEL suppressed melanization through activating lysosomes and degradation of tyrosinase by lysosomes. Our findings provide novel insight into the interaction between the melanosome and its related organelle, the lysosome, supplying a new idea for the pathogenesis and clinical treatment of pigmented diseases.

Spiders have rich pigmentary and structural colour palettes

Elucidating the mechanisms of colour production in organisms is important for understanding how selection acts upon a variety of behaviours. Spiders provide many spectacular examples of colours used in courtship, predation, defence and thermoregulation, but are thought to lack many types of pigments common in other animals. Ommochromes, bilins and eumelanin have been identified in spiders, but not carotenoids or melanosomes. Here, we combined optical microscopy, refractive index matching, confocal Raman microspectroscopy and electron microscopy to investigate the basis of several types of colourful patches in spiders. We obtained four major results. First, we show that spiders use carotenoids to produce yellow, suggesting that such colours may be used for condition-dependent courtship signalling. Second, we established the Raman signature spectrum for ommochromes, facilitating the identification of ommochromes in a variety of organisms in the future. Third, we describe a potential new pigmentary-structural colour interaction that is unusual because of the use of long wavelength structural colour in combination with a slightly shorter wavelength pigment in the production of red. Finally, we present the first evidence for the presence of melanosomes in arthropods, using both scanning and transmission electron microscopy, overturning the assumption that melanosomes are a synapomorphy of vertebrates. Our research shows that spiders have a much richer colour production palette than previously thought, and this has implications for colour diversification and function in spiders and other arthropods.

A polymethoxyflavone mixture extracted from orange peels, mainly containing nobiletin, 3,3',4',5,6,7,8-heptamethoxyflavone and tangeretin, suppresses melanogenesis through the acidification of cell organelles, including melanosomes

BACKGROUND

Skin color is determined by melanin contents and its distribution. Melanin is synthesized in melanosomes of melanocytes, catalyzed by tyrosinase, melanogenic enzymes. Regarding the process of melanin synthesis, melanosomal pH is considered to play an important role, because it has been reported to differ between Caucasian and Black melanocytes.

OBJECTIVE

Although polymethoxyflavone (PMF) has many beneficial effects, it has not been reported which PMF suppresses melanogenesis. In this study, we identified the mechanism underlying the effect of PMF on melanogenesis METHODS: We determined the effects of a PMF mixture extracted from orange peels on melanogenesis, on tyrosinase expression, on the localization of tyrosinase and on the acidification of organelles, including melanosomes, in HM3KO human melanoma cells. RESULTS TREATMENT: with the PMF mixture elicited the suppression of melanogenesis, the degradation of tyrosinase in lysosomes and the mislocalization of tyrosinase associated with the acidification of intracellular organelles, including melanosomes. The neutralization of cell organelle pH by ammonium chloride restored melanogenesis and the correct localization of tyrosinase to melanosomes, which had been suppressed by the PMF mixture.

CONCLUSION

These results suggest that the PMF mixture suppresses the localization of tyrosinase to melanosomes and consequently inhibits melanogenesis due to the acidification of cell organelles, including melanosomes.

Differences in the melanosome distribution within the epidermal melanin units and its association with the impairing background of leukoderma in vitiligo and halo nevi: a retrospective study

Skin color is determined by the number of melanin granules produced by melanocytes that are transferred to keratinocytes. Melanin synthesis and the distribution of melanosomes to keratinocytes within the epidermal melanin unit (EMU) within the skin of vitiligo patients have been poorly studied. The ultrastructure and distribution of melanosomes in melanocytes and surrounding keratinocytes in perilesional vitiligo and normal skin were investigated using transmission electron microscopy (TEM). Furthermore, we performed a quantitative analysis of melanosome distribution within the EMUs with scatter plot. Melanosome count within keratinocytes increased significantly compared with melanocytes in perilesional stable vitiligo (P < 0.001), perilesional halo nevi (P < 0.01) and the controls (P < 0.01), but not in perilesional active vitiligo. Furthermore, melanosome counts within melanocytes and their surrounding keratinocytes in perilesional active vitiligo skin decreased significantly compared with the other groups. In addition, taking the means-standard error of melanosome count within melanocytes and keratinocytes in healthy controls as a normal lower limit, EMUs were graded into 3 stages (I-III). Perilesional active vitiligo presented a significantly different constitution in stages compared to other groups (P < 0.001). The distribution and constitution of melanosomes were normal in halo nevi. Impaired melanin synthesis and melanosome transfer are involved in the pathogenesis of vitiligo. Active vitiligo varies in stages and in stage II, EMUs are slightly impaired, but can be resuscitated, providing a golden opportunity with the potential to achieve desired repigmentation with an appropriate therapeutic choice. Adverse milieu may also contribute to the low melanosome count in keratinocytes.

Lysosomal Trafficking Regulator (LYST)

Regulation of vesicle trafficking to lysosomes and lysosome-related organelles (LROs) as well as regulation of the size of these organelles are critical to maintain their functions. Disruption of the lysosomal trafficking regulator (LYST) results in Chediak-Higashi syndrome (CHS), a rare autosomal recessive disorder characterized by oculocutaneous albinism, prolonged bleeding, severe immunodeficiency, recurrent bacterial infection, neurologic dysfunction and hemophagocytic lympohistiocytosis (HLH). The classic diagnostic feature of the syndrome is enlarged LROs in all cell types, including lysosomes, melanosomes, cytolytic granules and platelet dense bodies. The most striking CHS ocular pathology observed is an enlargement of melanosomes in the retinal pigment epithelium (RPE), which leads to aberrant distribution of eye pigmentation, and results in photophobia and decreased visual acuity. Understanding the molecular function of LYST and identification of its interacting partners may provide therapeutic targets for CHS and other diseases associated with the regulation of LRO size and/or vesicle trafficking, such as asthma, urticaria and Leishmania amazonensis infections.

Quantitative characterization and comparative study of feather melanosome internal morphology using surface analysis

A successful feather development implies in a precise orchestration of cells in the follicle, which culminates in one of the most complex epidermal structures in nature. Melanocytes contribute to the final structure by delivering melanosomes to the barb and barbule cells. Disturbance to the tissue during the feather growth can damage the final structure. Here, melanosomes seen in an unusual outgrowth on the barb cortex of a flight feather are reported and compared to commonly observed melanosomes embedded in the cortex. Transmission Electron Microscopy in scanning-transmission mode (STEM) generated images coupled with secondary electron detection. The two classes of melanosomes were registered on images combining transmitted and secondary electron signals. Image processing allowed surface analyses of roughness and texture of the internal morphology of these organelles. Results showed that the two classes of melanosomes are significantly distinct internally, indicating that different physiological processes up to feather maturation could have occurred. Surface analysis methods are not regularly used in cell biology studies, but here it is shown that it has great potential for microscopic image analysis, which could add robust information to studies of cell biology events.

Cooperation of endothelin-1 signaling with melanosomes plays a role in developing and/or maintaining human skin hyperpigmentation

Skin hyperpigmentation is characterized by increased melanin synthesis and deposition that can cause significant psychosocial and psychological distress. Although several cytokine-receptor signaling cascades contribute to the formation of ultraviolet B-induced cutaneous hyperpigmentation, their possible involvement in other types of skin hyperpigmentation has never been clearly addressed. Since our continuous studies using skin specimens from more than 30 subjects with ethnic skin diversity emphasized a consistent augmentation in the expression of endothelin-1 (ET-1) and its receptor (Endothelin B receptor, ET-B) in hyperpigmented lesions, including senile lentigos (SLs), the precise function of ET-1 signaling was investigated in the present study. In line with previous studies, ET-1 significantly induced melanogenesis followed by increases in melanosome transport in melanocytes and in its transfer to keratinocytes while inhibition of ET-B function substantially depressed melanogenic ability in tissue-cultured SLs. Additionally, in agreement with a previous report that the formation of autophagosomes rather than melanosomes is stimulated according to starvation or defective melanosome production, ET-1 was found to remarkably augment the expression of components necessary for early melanosome formation, indicating its counteraction against autophagy-targeting melanosome degradation in melanocytes. Despite the lack of substantial impact of ET-1 on keratinocyte melanogenic functions, the expression of ET-1 was enhanced following melanosome uptake by keratinocytes. Taken together, our data suggest that ET-1 plays a substantial role in the development and/or maintenance of skin hyperpigmentation in reciprocal cooperation with increased melanosome incorporation.

Understanding the ultrastructural aspects of berberine-induced skin-darkening activity in the toad, Bufo melanostictus, melanophores

Berberine is an active compound of Berberis vulgaris (Daruhaldi) with known multiple pharmacological activities, including antimicrobial, antiviral, anti-inflammatory, cholesterol-lowering, and anticancer effects. The present work aimed to study the ultrastructural effects of berberine to determine its skin-darkening potential using Bufo melanostictus melanophores, which has not been done to date. Light and electron microscopic analysis of isolated dorsal skin melanophores of B. melanostictus has been done after treatment with various concentrations of berberine, along with specific antagonists and agonists of β-adrenoceptors in order to explore the mechanism of action of berberine-induced skin darkening. The results showed that the number of melanophores with melanin-loaded dendrites increased in the subepidermal layer significantly in berberine-treated skin pieces in a dose-dependent manner leading to skin darkening. Highly electron-dense melanosomes of Stage IV increased considerably due to the enhanced process of melanization. These effects were found to be antagonized by propranolol, and were also found to be highly potentiated by isoprenaline, which is a specific β-adrenoceptor agonist. The findings show that berberine possesses a skin-darkening potential and could be used as a safe melanogenic agent for the treatment of hypopigmentation disorders or vitiligo.

Scaling and Root Planning decreases the Number of Melanosomes within Keratinocytes in Human Gingiva: Ultrastructural Analysis of Three Cases

AIM

The aim of the present report was to evaluate the number of melanosomes within keratinocytes on pigmented gingiva, after and before scaling and root planning.

MATERIALS AND METHODS

Inflamed gingiva biopsies were taken from three patients (group 1). Forty days after scaling and root planning, biopsies were collected from the homologous contralateral areas (group 2). Samples were fixed in 2% glutaraldehyde-2.5% formaldehyde (freshly prepared from paraformaldehyde) in 0.1 M sodium cacodylate buffer, pH 7.4 for 4 hours, and then processed for transmission electron microscopy. Eighty electron micrographs were evaluated for recording the number of granules by a cross-section grid. The granules that were on intersections were recorded as well as the points that appeared on the cytoplasm for calculating the volumetric density (Vd), i.e the volume that the melanosomes occupied into the cytoplasm of keratinocytes. The presence of melanosomes in different stages of maturation and distribution into the cells were recorded with the aid of a magnifying glass. For the statistical analysis, a student t-test was applied.

RESULTS

Results of the present report showed that melanosomes within keratinocytes were present in a higher number in inflamed gingiva A (11.08 ± 1.47), B (3.16 ± 0.38) and C (4.92 ± 0.89) and decreased after resolving of gingival inflammation A (9.46 ± 0.88), B (1.73 ± 0.25) and C (0.76 ± 0.18).

CONCLUSION

There is a possibility that inflammation influences the intensity of gingival melanin pigmentation.

CLINICAL SIGNIFICANCE

The periodontal treatment appears to have an effect on gingival melanin pigmentation.

ADAM protease inhibitors reduce melanogenesis by regulating PMEL17 processing in human melanocytes

BACKGROUND

ADAMs (a disintegrin and metalloprotease) are a family of proteases involved in ectodomain shedding that play a role in various biological processes such as cell adhesion and migration. ADAM10 and ADAM17 are suggested to be involved in pigmentary disorders.

OBJECTIVE

We examined the effect of ADAM protease inhibitors on the modulation of melanogenesis in normal human epidermal melanocytes (NHEM).

METHODS

NHEMs and B16F10 treated with ADAM protease inhibitors were analyzed. AlamarBlue cell proliferation assay, melanin content assay, tyrosinase activity assay, Western blotting analysis, electron microscopic analysis, and RNA interference were employed.

RESULTS

In NHEMs, melanin content was reduced by treatment with ADAM protease inhibitors. The inhibitors did not change the protein expression of tyrosinase, TRP-1, and MITF. In B16F10 cells, treatment of the cells with ADAM protease inhibitor diminished the α-MSH-induced increase in melanin content. Electron microscopy showed that the number of fibrillar and mature melanosomes was significantly reduced and that the vacuolar compartments were filled with dense unstructured aggregates after treatment with ADAM protease inhibitors. We therefore focused on the processing of PMEL17, a melanosomal glycoprotein that forms a fibrillar matrix on which melanin gets deposited. Proteolytic processing of PMEL17 is required to form functional fibrils during melanogenesis. Recently, γ-secretase and β-site amyloid precursor protein-cleaving enzyme 2 (BACE2) were found to cleave PMEL17. We found that ADAM protease inhibitors exerted effects on the processing of C-terminal and N-terminal fragments of PMEL17. Using BACE2 siRNA and γ-secretase inhibitor, we showed that ADAM protease inhibitor affected PMEL17 processing in a γ-secretase and BACE2-independent mechanism.

CONCLUSION

Several proteases, including ADAM proteases, can contribute to the formation of fibrils and their assembly into sheets in melanosomes.

Regulation of melanosome number, shape and movement in the zebrafish retinal pigment epithelium by OA1 and PMEL

Analysis of melanosome biogenesis in the retinal pigment epithelium (RPE) is challenging because it occurs predominantly in a short embryonic time window. Here, we show that the zebrafish provides an ideal model system for studying this process because in the RPE the timing of melanosome biogenesis facilitates molecular manipulation using morpholinos. Morpholino-mediated knockdown of OA1 (also known as GPR143), mutations in the human homologue of which cause the most common form of human ocular albinism, induces a major reduction in melanosome number, recapitulating a key feature of the mammalian disease where reduced melanosome numbers precede macromelanosome formation. We further show that PMEL, a key component of mammalian melanosome biogenesis, is required for the generation of cylindrical melanosomes in zebrafish, which in turn is required for melanosome movement into the apical processes and maintenance of photoreceptor integrity. Spherical and cylindrical melanosomes containing similar melanin volumes co-exist in the cell body but only cylindrical melanosomes enter the apical processes. Taken together, our findings indicate that melanosome number and shape are independently regulated and that melanosome shape controls a function in the RPE that depends on localisation in the apical processes.

Iron upregulates melanogenesis in cultured retinal pigment epithelial cells

The purpose of our studies was to examine the relationship between iron and melanogenesis in retinal pigment epithelial cells, as prior observations had suggested that iron may promote melanogenesis. This relationship has potential clinical importance, as both iron overload and hyperpigmentation are associated with age-related macular degeneration (AMD). Human fetal retinal pigment epithelial cells and ARPE-19 cells were treated with iron in the form of ferric ammonium citrate, after which quantitative RT-PCR and electron microscopy were performed. Melanogenesis genes tyrosinase, tyrosinase-related protein 1, Hermansky-Pudlak Syndrome 3, premelanosome protein and dopachrome tautomerase were upregulated, as was the melanogenesis-controlling transcription factor, microphthalmia-associated transcription factor (MITF). Iron-treated cells had increased pigmentation and melanosome number. Multiple transcription factors upstream of MITF were upregulated by iron.

Ion transport in pigmentation

Skin melanocytes and ocular pigment cells contain specialized organelles called melanosomes, which are responsible for the synthesis of melanin, the major pigment in mammals. Defects in the complex mechanisms involved in melanin synthesis and regulation result in vision and pigmentation deficits, impaired development of the visual system, and increased susceptibility to skin and eye cancers. Ion transport across cellular membranes is critical for many biological processes, including pigmentation, but the molecular mechanisms by which it regulates melanin synthesis, storage, and transfer are not understood. In this review we first discuss ion channels and transporters that function at the plasma membrane of melanocytes; in the second part we consider ion transport across the membrane of intracellular organelles, with emphasis on melanosomes. We discuss recently characterized lysosomal and endosomal ion channels and transporters associated with pigmentation phenotypes. We then review the evidence for melanosomal channels and transporters critical for pigmentation, discussing potential molecular mechanisms mediating their function. The studies investigating ion transport in pigmentation physiology open new avenues for future research and could reveal novel molecular mechanisms underlying melanogenesis.

A simple assay method for melanosome transfer

Pigmentation is induced by production of melanin in specialized organelles termed melanosomes and by transfer of these organelles from melanocytes to surrounding keratinocytes. The chemical basis of melanogenesis is relatively well known but the mechanism of melanosome transfer is not well studied. Various pigmentary disorders and cosmetic applications require the use of depigmenting agents. Currently available topical agents used for the reduction of pigmentation mainly include tyrosinase inhibitors and/or melanocyte-cytotoxic agents. Recently, several agents have been introduced to inhibit melanosome transfer from melanocytes to keratinocytes. However, an experimental model for melanosome transfer is not well established. In this study, a simple assay method using flow cytometry is described.