The Melanosome: Dark Pigment Granule Shines Bright Light on Vesicle Biogenesis and More

Ultrastructural and clinical features of central nervous system melanoma:Analysis of nine cases

To investigate the ultrastructural and clinical characteristics of melanoma of the central nervous system (CNS). The clinical and electron microscopy pathology data of nine patients with melanoma surveyed from 1993 to 2017 were analyzed. All the CNS melanomas were confirmed by transmission electron microscopy (TEM), including eight cases of primary melanomas and one case of metastatic melanoma. In this study, four stage II melanosomes were intracranial space-occupying, three of which were malignant melanoma, the other one was melanoma. Among the five stage IV melanosomes, four cases were intraspinal space-occupying, the other one was intracranial space-occupying, and the pathological diagnoses were all melanoma. At present, TEM is an important tool for the diagnosis of CNS melanomas. Malignant melanoma has high malignancy and recurrence rate and poor prognosis, while benign melanoma with relatively low recurrence rate, so we speculate that patients with mainly immature melanosomes are more likely to exhibit recurrence.

A novel role for microphthalmia-associated transcription factor-regulated pigment epithelium-derived factor during melanoma progression

Microphthalmia-associated transcription factor (MITF) acts via pigment epithelium-derived factor (PEDF), an antiangiogenic protein, to regulate retinal pigment epithelium migration. PEDF expression and/or regulation during melanoma development have not been investigated previously. Using immunohistochemistry, we determined expression of PEDF in common and dysplastic melanocytic nevi, melanoma in situ, invasive melanoma, and metastatic melanoma (n = 102). PEDF expression was consistently decreased in invasive and metastatic melanoma, compared with nevi and melanoma in situ (P < 0.0001). PEDF was lost in thicker melanomas (P = 0.003), and correlated with depth of invasion (P = 0.003) and distant metastasis (P = 0.0331), but only marginally with mitotic index, AJCC stage, nodal metastasis, or blood vascular density (0.05 < P < 0.10). Quantitative real-time PCR and microarray analyses confirmed PEDF down-regulation at the mRNA level in several melanoma lines, compared with melanocytes. MITF positively correlated with PEDF expression in invasive melanomas (P = 0.0003). Searching for PEDF regulatory mechanisms revealed two occupied conserved E-boxes (DNA recognition elements) in the first intron of the human and mouse PEDF promoter regions, confirmed by binding assays. Dominant-negative and siRNA approaches in vivo demonstrated direct transcriptional influence of MITF on PEDF, establishing the PEDF gene (SERPINF1) as a MITF target in melanocytes and melanoma cells. These findings suggest that loss of PEDF expression promotes early invasive melanoma growth.

Ultrastructural observations of human epidermal melanocytes cultured on polyethylene terephthalate film

OBJECTIVE

To assess the application of polyethylene terephthalate film as a supporting material of cultured human epidermal melanocytes and to observe the ultrastructure of human epidermal melanocytes in vitro.

METHODS

Human epidermal melanocytes were isolated from 7- to 14-years old children foreskins and were cultured in M254 culture medium containing human melanocyte growth supplement. Cultured melanocytes were purified via a differential trypsinization method. Purified melanocytes were cultured on a film and prepared for transmission electron microscopy.

RESULTS

In comparing with the cellular supporting materials polyvinylidene chloride and cellophane, human epidermal melanocytes only attached and grew on polyethylene terephthalate film. Melanocytes grown on polyethylene terephthalate film maintained an intact shape, and retained special ultrastructures characteristic of melanocytes such as dendrites.

CONCLUSIONS

Polyethylene terephthalate film proved to be an ideal cellular supporting material for the cultivation of human epidermal melanocytes. Ultrastructure of melanocytes showed melanosomes are transferred both from the tip and middle section of dendrites.

Inhibitory effects of Asterina pectinifera extracts on melanin biosynthesis through tyrosinase activity

The control of melanogenesis is an important strategy in the treatment of abnormal skin pigmentation for cosmetic purposes. The aim of the present study was to investigate the anti-melanogenic effect of Asterina pectinifera (A. pectinifera) extracts by cell-free mushroom tyrosinase assay, cellular tyrosinase assay, melanin content assay and the analysis of related protein expression in melan-a cells. A. pectinifera was extracted with 80% methanol (80-MAP) and further fractionated with hexane (He-AP) and ethyl acetate (EA-AP). In addition, the enzyme extract (En-AP) of A. pectinifera, to which protease was added, was processed. EA-AP and En-AP among A. pectinifera extracts showed strong inhibitory activity against the cell-free mushroom tyrosinase activity. EA-AP and En-AP induced significant inhibition of melanin production and cellular tyrosinase activity. In the action of EA-AP and En-AP on melanogenesis, they reduced the expression of melanogenic genes and proteins including tyrosinase, tyrosinase-related protein-1 (TRP-1) and dopachrome tautomerase (Dct). These results showed that EA-AP and En-AP inhibited melanogenesis by reducing tyrosinase activity and melanin production via subsequent downregulation of tyrosinase-related proteins. The overall results suggest that EA-AP and En-AP among A. pectinifera extracts may be promising candidates for the treatment of hyperpigmentation disorder and useful for self-tanning cosmetic products.

Neurocutaneous syndrome with mental delay, autism, blockage in intracellular vescicular trafficking and melanosome defects

We evaluated a 11-year-old male patient with mental delay, autism and brownish and whitish skin spots. The former resembled those of neurofibromatosis, the latter those of tuberous sclerosis. The patient received a complete clinical work-up to exclude neurofibromatosis, tuberous sclerosis, or any other known neurocutaneous disease, with biochemistry, chromosome analysis and analysis of skin specimens. Being all the other tests not significant, two main ultrastructural defects were observed. The first was a blockage in intracellular vescicular trafficking with sparing of the mitochondria; the second an aberrant presence of melanosomes in vacuoles of several cell lines and abnormal transfer of these organelles to keratinocytes. This patient presented with a unique clinical picture distinct from neurofibromatosis or tuberous sclerosis or any other known neurocutaneous disease. The ultrastructural abnormalities suggested a defect in cell trafficking involving several cell lines and compartments.

Tyrosine-induced melanogenesis shows differences in morphologic and melanogenic preferences of melanosomes from light and dark skin types

The quality, quantity and distribution of melanosomes in epidermis play a crucial role in the determination of skin color and its sensitivity to UV radiation. Melanocyte cultures originating from individuals with light and dark skin types were grown in media with varying concentration of L-tyrosine. Melanosomal melanin content and the size of the organelles were measured after subcellular fractionation. In light-skin type cells, increased melanin production resulted in a more elliptical shape of melanosomes. In melanosomes that constitutively produce more melanin, the tyrosine-induced melanogenesis caused enlargement in all dimensions. X-ray microanalysis provided evidence that the increase in sulfur content induced by high tyrosine concentration was more prominent in the melanosomes from light skin types. A ratio between pheomelanin and eumelanin found in light-skin type melanosomes by HPLC was increased more markedly than that in melanosomes from dark skin melanocytes. These findings suggest that the melanocytes of light-skinned individuals exhibit a preference for pheomelanogenesis. Pheomelanin production is a thiol-consuming process and that might increase the risk of oxidation stress in these cells. This fact, together with the limited ability of pheomelanin to absorb UV radiation may lead to an elevated skin cancer risk among light-skinned individuals.

Epitope mapping of the melanosomal matrix protein gp100 (PMEL17): rapid processing in the endoplasmic reticulum and glycosylation in the early Golgi compartment

Melanosomes, specific organelles produced only by melanocytes, undergo a unique maturation process that involves their transition form amorphous rounded vesicles to fibrillar ellipsoid organelles, during which they move from the perinuclear to the distal areas of the cells. This depends upon the trafficking and processing of gp100 (also known as Pmel17 and the silver protein), a protein of great interest, because it elicits immune responses in melanoma patients but in which specific function(s) remains elusive. In this study, we have used biochemical and immunochemical approaches to more critically assess the synthesis, processing, glycosylation, and trafficking of gp100. We now report that gp100 is processed and sorted in a manner distinct from other melanosomal proteins (such as tyrosinase, Tyrp1 and Dct) and is predominantly delivered directly to immature melanosomes following its rapid processing in the endoplasmic reticulum and cis-Golgi. Following its arrival, gp100 is cleaved at the amino and at the carboxyl termini in a series of specific steps that result in the reorganization of immature melanosomes to the fibrillar mature melanosomes. Once this structural reorganization occurs, melanogenic enzymes begin to be targeted to the melanosomes, which are then competent to synthesize melanin pigment.