Are melanosome complexes lysosomes?

Recent Advances and Progress on Melanin: From Source to Application

Melanin is a biological pigment formed by indoles and phenolic compounds. It is widely found in living organisms and has a variety of unique properties. Due to its diverse characteristics and good biocompatibility, melanin has become the focus in the fields of biomedicine, agriculture, the food industry, etc. However, due to the wide range of melanin sources, complex polymerization properties, and low solubility of specific solvents, the specific macromolecular structure and polymerization mechanism of melanin remain unclear, which significantly limits the further study and application of melanin. Its synthesis and degradation pathways are also controversial. In addition, new properties and applications of melanin are constantly being discovered. In this review, we focus on the recent advances in the research of melanin in all aspects. Firstly, the classification, source, and degradation of melanin are summarized. Secondly, a detailed description of the structure, characterization, and properties of melanin is followed. The novel biological activity of melanin and its application is described at the end.

Cellular Lysosomes' Activity for Melanin Reduction on Artificial Skin Tissue

All eukaryotes have lysosomes which contain hydrolytic enzymes such as protease to degrade waste materials and cellular fragments. As a cellular organelle, lysosomes function as the digestive system of the cell, serving both to degrade material taken up from outside the cell and to digest obsolete components of the cell itself. Conversely, melanin has photochemical functions to protect tissue from the harmful effects of ultraviolet rays. However, too much of melanin leads to problems such as hyperpigmentation, requiring materials to maintain and control the amount of melanin. In this study, we found evidence of correlation between lysosome and melanin in a new eco-friendly material, MelanoDerm, a reconstituted 3D human skin model containing normal melanocytes and keratinocytes. Melanin content assay and cell viability were measured, using 2% kojic acid as positive control, while MelanoDerm was exposed to various concentrations of lysosome. Our results indicate that lysosome may be a useful cosmetic agent for the treatment of hyperpigmentation.

Unraveling the molecular nature of melanin changes in metastatic cancer

More people die from melanoma after a stage I diagnosis than after a stage IV diagnosis, because the tools available to clinicians do not readily identify which early-stage cancers will be aggressive. Near-infrared pump-probe microscopy detects fundamental differences in melanin structure between benign human moles and melanoma and also correlates with metastatic potential. However, the biological mechanisms of these changes have been difficult to quantify, as many different mechanisms can contribute to the pump-probe signal. We use model systems (sepia, squid, and synthetic eumelanin), cellular uptake studies, and a range of pump and probe wavelengths to demonstrate that the clinically observed effects come from alterations of the aggregated mode from "thick oligomer stacks" to "thin oligomer stacks" (due to changes in monomer composition) and (predominantly) deaggregation of the assembled melanin structure. This provides the opportunity to use pump-probe microscopy for the detection and study of melanin-associated diseases.

Effect of skin surface temperature on skin pigmentation during contact and intralesional cryosurgery of keloids

BACKGROUND

This 15-month study was designed to compare the effect of skin surface temperature on skin pigmentation following a single intralesional or contact cryosurgical treatment of keloids.

PATIENTS/METHODS

Thirty Caucasian patients with 45 keloids present for more than 6 months were included in this study. Twenty-one keloids were treated by the contact method while the remaining 24 scars were managed using an intralesional cryosurgery technique. The skin surface temperature at the keloids was measured and recorded using a Ni/Cd thermocouple. Four variables of the thermal history were evaluated with the contact and the intralesional methods, namely cooling rate, hold time, end temperature and thawing rate. Assessment of the local hypopigmentation was performed 6 months after the treatment using a pigmentation scale.

RESULTS

Significantly slower cooling (6.09 +/- 4.56 degrees C/min) and thawing rates (54.52 +/- 32.17 degrees C/min) were recorded with the intralesional cryosurgery method when compared with the cooling rates (13.47 +/- 9.04 degrees C/min) and thawing rates (89.00 +/- 86.42 degrees C/min) of the contact method (P < 0.000001). The end temperature of the contact technique was significantly cooler (-46.77 +/- 14.74 degrees C) when compared with that of the intralesional method (-15.55 +/- 6.77 degrees C) (P < 0.000001). There was a trend for the hold time of intralesional cryosurgery to be longer (82.67 +/- 138.03 s) than that of the contact method (16.86 +/- 23.49 s) (P < 0.059). A significant difference in skin pigmentation was demonstrated between the two cryosurgical methods. In 91.7% of the keloids treated by the contact technique a significant hypopigmentation was noticed, while no marked hypopigmentation was detected in the skin surface of the keloids treated by the intralesional method (P < 0.0001).

CONCLUSION

We hypothesize that the thermal history of the skin surface during the intralesional cryosurgery technique provides a better survival environment for the melanocytes than the contact method, thus producing a lower rate of permanent hypopigmentation and disfiguring.

Melanosome degradation: fact or fiction

Our mini review summarizes what is known about the (bio)degradation of melanosomes. Unlike melanosome biogenesis where our knowledge enables us to explain it in molecular terms posing many interesting questions on the relation between lysosomes and melanosomes, melanosome degradation has remained 'terra incognita'. Observations at optical and ultrastructural levels describe the disintegration of melanosomes in the lysosomal compartment (in auto- and heterophagosomes). Histochemical studies suggest the participation of acid hydrolases in the process of melanosome degradation. Biochemical data confirm the ability of lysosomal hydrolases to degrade melanosome constituents except the melanin moiety. The similarity of melanin structure to that of polycyclic aromatic hydrocarbons suggests that melanin should be sensitive mainly, if not exclusively, to oxidative breakdown. In vitro melanin can indeed be decomposed by an oxidative attack and the degradation is accompanied by fluorescence and decreasing absorbance. From enzymes engaged in the biotransformation of polycyclic hydrocarbons only phagosomal NADPH oxidase meets the criteria (particularly as for compartmental and catalytic properties) to be involved in melanin biodegradation. The in vivo biodegradation of melanin has so far been clearly demonstrated in Aspergillus and fungi melanins.

The amphibian Kupffer cells build and demolish melanosomes: an ultrastructural point of view

This ultrastructural research was carried out to investigate the nature of the liver pigment cells of anuran and caudate amphibians, the pattern of melanosome ontogenesis, and the demolition processes of old melanosomes. We demonstrate that these liver pigment cells are able to internalize zymosan particles and latex beads; therefore, being professional phagocytes, they, as liver resident macrophages, can be classified as Kupffer cells (KCs). They show "melanosomogenesis centers" in which several maturation stages of premelanosomes are visible; the premelanosomes are formed by two principal components: a filamentous structure that will constitute the "inner" area of the melanosome and a vesicular component, budding from the Trans Golgi Network and that carries enzymes, which will constitute the "cortical area" of the melanosome. Thus the KCs, thanks to the presence of the "melanosomogenesis centers," are also melanosome producing cells. They are also able to demolish melanosomes by heterophagocytosis and, probably, also by autophagocytosis. In conclusion, we propose a classification of vertebrate pigment cells.

Detection of a fine lamellar gridwork after degradation of ocular melanin granules by cultured peritoneal macrophages

In the present study we investigated by electron microscopy whether melanin granules derived from choroidal melanocytes and retinal pigment epithelium of cattle could be degraded in the phagolysosomes of cultured murine macrophages. It was found that degradation of ocular melanin is possible by the lysosomes of these macrophages. During degradation of the melanin granules an internal gridwork of fine concentric, highly ordered membranes, 3-4 nm thick, became visible. These membranes may represent remnants of the melanin polymer in the original melanosome or may result from self-assembly of degradation products. Early-stage melanosome-like structures also appeared during digestion of these melanin granules. Melanin granules that seemed to break down into smaller fragments without any visible internal structure were also observed.

A cytochemical study of dense granules in the rat junctional epithelium

To clarify the details of intracytoplasmic granules in the junctional epithelium (JE) of the rat gingiva, phosphotungstic acid (PTA) staining, acid phosphatase cytochemistry and zinc iodide-osmium (ZIO) staining were carried out. Condensing granules containing homogeneous electron-dense materials, measuring about 260 nm in diameter, have been observed at the peripheral cytoplasm of the JE. These granules reveal PTA-positive, ZIO-positive and acid phosphatase-positive staining. These results indicate that the granules contain glycoprotein, phospholipid, lipoprotein and lysosomal enzymes. From their composition and morphology, it is suggested that these granules in the JE may contribute secretory function and secrete glycoproteins or glycolipids. Although it is unclear whether the dense granules correspond to membrane-coating granules by our staining methods, it is surmised that, if the dense granules can engage in establishment of the barrier function, then this role is impeded by the large intercellular space in the JE.

Gingival pigmentation beneath a metallic crown: light and electron microscopic observations and energy dispersive X-ray analysis

Light and electron microscopic studies and energy dispersive X-ray analysis disclosed that the essential cause of gingival discoloration following the placement of a metallic crown, was marked deposition of melanin pigment. Deposition of melanin pigment was observed in epithelial cells, on basement membranes, and in fibroblasts, macrophages and among intercellular ground substance of the proprial layer. Brown or dark brown colored granules were observed in the deep portion of the proprial layer. Some metallic elements as silver and sulfur were detected. It was presumed that these materials were dental metals accidentally implanted in gingival tissues during the therapeutic procedure. The deposition of melanin pigment closely corresponded with mucosal tissue where these materials were present in the deep portion of the proprial layer. These findings suggested that these materials influenced the physiological metabolism of melanin and induced its pathological deposition in the proprial tissue.

pH-dependent interconversion of two forms of tyrosinase in human skin

1. We have shown that the characteristic lag in cresolase activity of human skin tyrosinase at inhibitory concentration of tyrosine was absent at all pH values studied, i.e. pH 5.2, 5.7, 6.2 and 6.8, if the enzyme solubilized at low pH was used as the source of enzyme, but the same enzyme when dialysed against buffers of various pH values showed linear activity only at pH 5.2 and was not inhibited by excess tyrosine, whereas at higher pH values it exhibited a lag and inhibition by excess tyrosine. 2. However, the enzyme solubilized in buffer/detergent, pH 6.8, when dialysed against buffer of the same pH showed linear activity at pH 5.2 and non-linear activity at pH 6.8. 3. The water/detergent-solubilized enzyme from human skin melanosomes showed linear activity even at inhibitory concentrations of tyrosine at pH 5.2 and 6.8 up to 2 h, but acceleration of rate was observed after 2 h for the enzyme measured at pH 6.8. 4. After dialysis of the water/detergent-solubilized enzyme against double-glass-distilled water, it still exhibits linear activity at inhibitory concentration of tyrosines at pH 6.8 for the first 2 h, but the same enzyme when dialysed against 0.02 M-sodium phosphate buffer, pH 6.8, exhibits negligible activity up to 1/2 h, in contrast with considerable activity before dialysis during the same interval of time, but without any loss of activity at later intervals of incubation time. 5. On the basis of these results, it is concluded that the enzyme exists in at least two interconvertible forms, one without lag and inhibition by excess tyrosine and the other with lag and inhibition by excess tyrosine. These two forms are interconvertible only by gradual change in pH over a period of hours.

pH-dependent interconvertible allosteric forms of murine melanoma tyrosinase. Physiological implications

Murine melanoma melanosomal tyrosinase, solubilised at pH 6.8 and 1% Igepal, exhibits a lag in cresolase activity which increases with increasing concentration of tyrosine. The enzyme, solubilised at pH 5.0 and assayed at pH 5.0, does not exhibit lag even at inhibitory concentrations of tyrosine while the same enzyme when assayed at pH 6.8 exhibits characteristic lag. When the enzyme was solubilised from a melanosomal fraction with detergent/water without any buffer, significant linear activity for 2 h was seen at an inhibitory concentration of tyrosine, indicating for the first time the presence of a form of tyrosinase without lag and inhibition by excess tyrosine. Exposure of the enzyme solubilised in buffer/detergent at pH 6.8 to rapid decrease in pH to 5.0 or 4.7 makes the enzyme remain irreversibly in the form without characteristic lag, even at an inhibitory concentration of tyrosine and at pH 6.8. These results may be interpreted as follows. The enzyme at pH 6.8 exists in the E form with an allosteric site for tyrosine. Decrease of the pH of the enzyme solution from 6.8 to 5.0 or 4.7 by dialysis results in the reversible protonation of the enzyme, which no longer binds tyrosine at its allosteric site and consequently inhibition by excess tyrosine and lag were not observed at acidic pH. However, if the enzyme was rapidly brought to pH 5.0 from 6.8 it remains irreversibly in the protonated form even at pH 6.8. Ascorbic acid acts as an effective reductant for the hydroxylation of tyrosine by tyrosinase, while 3,4-dihydroxyphenylalanine is both an effective reductant and counteracts the inhibition by tyrosine at pH 6.8.

Pigment change and melanocyte distribution in guinea pig skin after cutaneous freeze injury

Cryosurgery has been recommended for the treatment of cutaneous neoplasms, including benign and malignant melanocytic tumors. The main side effect of this treatment is pigment disruption which may be prolonged. There are no quantitative studies on melanocyte distribution after freeze injury. In this study the effect of standardized freeze times on melanocytes in guinea pig skin (tricolored) is described at the anatomic level and at the light and electron microscopic level. Melanocytes are quantified in epidermal sheets following dopa staining at 1-, 2-, and 3-month intervals after freezing. All the lesions were initially hypopigmented with a peripheral rim of hyperpigmentation. This was a transient phenomenon associated with an absence of melanocytes. Pigment migrated into the lesions so that after 3 months all the lesions on black skin were diffusely hyperpigmented. Pigment was slower to return to the red skin. Hair follicles were destroyed in the center of the lesion. The epidermal sheets demonstrated that the hyperpigmentation was an epidermal phenomenon and was associated with increased numbers of melanocytes (p less than 0.001). These melanocytes were distributed evenly throughout the lesion in contrast to the irregular distribution in normal skin.

Acid phosphatase in melanosome formation: a cytochemical study in normal human melanocytes

A cytochemical study of acid phosphatase (AcPase) activity was conducted in normal epidermal melanocytes from both sun-exposed and sun-protected human skin to define the relationship between enzyme activity and melanosome formation. In the perikarya of melanocytes of both sun-exposed and sun-protected skin, it was determined that only a small proportion of stage 1 and 2 melanosomes had AcPase activity (20-33% and 9-26%, respectively). The proportion of AcPase-positive melanosomes in perikarya increased in stage 3 (39-56%), reaching a maximum in stage 4 (67-84%). In the dendrite of melanocytes, where melanosomes were mostly in stages 3 and 4, the vast majority of melanosomes demonstrated AcPase activity (79-87% and 88-93%, respectively). The preferential incorporation of AcPase in the later stages of melanogenesis is more consistent with a possible role for this enzyme in the degradation or transfer of melanosomes, rather than as an essential component in the early process of melanization.

The ultra-structure of a melanotic schwannoma of the skin

The case is described of a 53-year-old female with a pigmented tumour of the skin of the foot. The light and electron-microscopic findings established that the tumour was a melanotic Schwannoma. The ultra-structural features indicated that the Schwann cells, which contained melanosomes in varying stages of maturation, were producing melanin. A second type of cell within the tumour with features of a mesenchymal cell also contained melanosomes. The interrelationships of nerve sheath tumours, their relationship to pigment cells of neural crest origin and to other pigmented tumours of uncertain histogenesis, is discussed in the light of the findings described in the paper.

Trends in electron microscopy of skin

Some trends in electron microscopy of skin have emerged and should be pursued in the future. The fine structure and some basic cellular reaction patterns of epidermal cells are discussed to illustrate the interplay of morphologic, cytochemical, and tracer studies. Intracytoplasmic membranes and secretory granules, lysosomes and endocytic mechanisms, cytomembranes and cell surface specialization are discussed to show how these can be used to arrive at a more meaningful interpretation of structure. Despite all advances, however, a great deal more needs to be done before the details of skin structure are completely elucidated.

Melanotic nerve sheath tumors

✓ A solitary intraspinal, extradural melanotic tumor was subtotally resected in a 59-year-old man who had a 17-year history of radicular pain and later evidence of progressive spinal cord compression. The neoplasm revealed the histological features of a benign nerve sheath tumor with massive but uneven melanin production. In electron micrographs the tumor cells contained masses of melanosomes of the type seen in normal skin melanocytes and in B type melanocarcinomas. In the 16-month postoperative period there has been only minimal radiological indication of local recurrence and no metastases.