Melanogenesis in the pigment cells of Rana esculenta L. liver: evidence for tyrosinase-like activity in the melanosome protein fraction

Ultrastructural Evidence of Melanomacrophagic Centers and Lipofuscin in the Liver of Zebrafish (Denio rerio)

Melanomacrophagic centers (MMCs) were studied in the liver of zebrafish using transmission electron microscope (TEM). The MMCs were located in the space of Disse (SD), and their pseudopodia protruded into the lumen of sinusoids. The degree of extension of body structure of MMCs in the SD was determined by the size of the phagocytosed content. An irregular or amoeboid nucleus was present. Vacuoles were occasionally present, both, in endothelium and MMCs. The cytoplasm of MMCs showed several engulfed structures. The most common structure was the presence of mitochondria of small to giant size and distorted shape with inconspicuous cristae. The product of mitochondrial degeneration accompanied by lysosomes contributed to the formation of lipofuscins. Besides, changes were also observed in rough endoplasmic reticulum (rER), the Golgi complex, and lysosomes. Occasionally, small to large fragments of the erythrocytes were found in the cytoplasm of MMCs. The rER encompassed the mitochondria and lipid droplets forming a membrane-like structure. Golgi complex were dilated. Lysosomes fused with such membrane-bound structures contributed to the formation of the lipofuscin. The results provide evidence of the role of liver-resident MMCs of zebrafish in phagocytosis of damaged organelles, clearance of the worn-out erythrocytes, and lipofuscin formation.

A calcium channel blocker nifedipine distorts the effects of nano-zinc oxide on metal metabolism in the marsh frog Pelophylax ridibundus

Global decline of amphibian populations causes particular concern about their vulnerability to novel environmental pollutants, including engineering nanomaterials and pharmaceutical products. We evaluated the bioavailability of nanoform of zinc oxide (n-ZnO) in frog Pelophylax ridibundus and determined whether co-exposure to a common pharmaceutical, a calcium-channel blocker nifedipine (Nfd) can affect this bioavailability. Male frogs were exposed for 14 days to the tap water (Control) and n-ZnO (3.1 μM), Zn²⁺ (3.1 μM, as a positive control for n-ZnO exposures), Nfd (10 μM), and combination of n-ZnO and Nfd (n-ZnO + Nfd) in environmentally-relevant concentration. Exposure to Zn²⁺ or n-ZnO led to up-regulation of metal-binding proteins, metallothioneins (MTs) in the liver and Zn-carrying vitellogenin-like proteins in the blood plasma. Notably, upregulation of MTs by Zn²⁺ or n-ZnO exposures combined with increased binding of Zn and Cu to MTs. This was associated with the more reducing conditions in the liver tissue indicated by elevated lactate to pyruvate ratio. Nfd suppressed the binding of Zn and Cu to MTs and led to a decrease in Lactate/Pyruvate ratio and elevated protein carbonylation indicating pro-oxidant conditions. Redox status parameters were not directly related to DNA fragmentation, nuclear abnormalities or suppression of cholinesterase activity indicating that factors other than oxidative stress are involved in cytotoxicity of different pollutants and their combinations. Furthermore, activity of Phase I biotransformation enzyme (CYP450 oxidase measured as EROD) was elevated in Nfd-containing exposures and in Zn²⁺ exposed frogs. Tyrosinase-like activity in the frog liver was strongly stimulated by Zn²⁺ but suppressed by n-ZnO, Nfd and n-ZnO + Nfd. These findings show that Nfd modulates homeostasis of essential metals in amphibians and emphasize that physiological consequences of combined n-ZnO and Nfd exposures are difficult to predict based on the mechanisms of single stressors.

A multidisciplinary study of the extracutaneous pigment system of European sea bass (Dicentrarchus labrax L.). A possible relationship between kidney disease and dopa oxidase activity level

Infectious diseases and breeding conditions can influence fish health status. Furthermore it is well known that human and animal health are strongly correlated. In lower vertebrates melano-macrophage centres, clusters of pigment-containing cells forming the extracutaneous pigment system, are widespread in the stroma of the haemopoietic tissue, mainly in kidney and spleen. In fishes, melano-macrophage centres play an important role in the immune response against antigenic stimulants and pathogens. Hence, they are employed as biomarker of fish health status. We have investigated this cell system in the European sea bass (Dicentrarchus labrax L.) following the enzyme activities involved in melanin biosynthesis. We have found a possible relationship between kidney disease of farmed fishes and dopa oxidase activity level, suggesting it as an indicator of kidney disease. Moreover variations of dopa oxidase activity in extracutaneous pigment system have been observed with respect to environmental temperature. At last, for the first time, using femtosecond transient absorption spectroscopy (Femto-TA), we pointed out that pigment-containing cells of fish kidney tissue present melanin pigments.

Melanogenesis in visceral tissues ofSalmo salar. A link between immunity and pigment production?

Melanogenesis is mostly studied in melanocytes and melanoma cells, but much less is known about other pigment cell systems. Liver, spleen, kidney, and other organs of lower vertebrates harbour a visceral pigment cell system with an embryonic origin that differs from that of melanocytes. In teleosts, melanin-containing cells occur in the reticulo-endothelial system and are mainly in the kidney and spleen. The Atlantic salmon ( Salmo salar L.) is an ichthyic breeding species of considerable economic importance. The accumulation of pigments in salmon visceral organs and musculature adversely affects the quality of fish products and is a problem for the aquaculture industry. With the aim to reveal novel functions and behaviour of the salmonid extracutaneous pigment system, we investigated aspects of the melanogenic systems in the tissues of Atlantic salmon, as well as in SHK-1 cells, which is a long-term cell line derived from macrophages of the Atlantic salmon head-kidney. We demonstrate that a melanogenic system is present in SHK-1 cells, head-kidney, and spleen tissues. As teleosts lack lymph nodes and Peyer’s patches, the head-kidney and spleen are regarded as the most important secondary lymphoid organs. The detection of tyrosinase activity in lymphoid organs indicates that a link exists between the extracutaneous pigmentary system and the immune system in salmon.

Melanocytes in the Testes ofEupemphix nattereri (Anura, Leiuperidae): Histological, Stereological, and Ultrastructural Aspects

Ectothermic vertebrates have a well-developed system of melanin-containing cells, which localize in several organs and tissues and compose an extracutaneous pigmentary system. This research aimed at characterizing histological and ultrastructural patterns of pigmented cells in the testes of the anura Eupemphix nattereri (Steindachner, 1963), including the stereological and quantitative evaluation of this cell type in the gonads. Ten adult males were collected in Nova Itapirema, São Paulo, Brazil, and submitted to morphological studies with light and transmission electron microscopy. The testis presents a great number of large cells with many brown granules and long cytoplasmic processes. The pigmented cells found in the testis are structurally similar to melanocytes, characterized by large amounts of melanosomes. The cells may be in intimate contact with the same cell type, with myoid cells surrounded by a large amount of collagen fibers, Leydig cells, and next to fibroblasts. The distribution and amount of extracutaneous melanocytes is variable when other organs and membranes are analyzed, allowing the establishment of species-specific patterns for the extracutaneous pigmentary system.

Pigmentação testicular em Physalaemus nattereri (Steindachner) (Amphibia, Anura) com observações anatômicas sobre o sistema pigmentar extracutâneo

O presente estudo foi realizado com o intuito de relatar a ocorrência e morfologia de células pigmentares viscerais constituintes do "sistema pigmentar extracutâneo" em Physalaemus nattereri (Steindachner, 1863) (Leptodactylidae). Foram utilizados dez exemplares machos para a análise macroscópica e obtenção de fragmentos testiculares incluídos em resina e corados com H/E. Os anuros, dentre outros animais exotérmicos, possuem células especiais, os melanócitos, que se caracteriza por intensa pigmentação e sintetiza melanina, além de melanomacrófagos, que se caracteriza por atividade fagocítica e muitas vezes apresentam intensa pigmentação. A nomenclatura destas células não é consensual e, por isso, várias denominações são apresentadas, principalmente nos seguintes órgãos: fígado (como sinônimo de células de Kupffer), rins, baço e menos freqüentemente em outras localizações, com os termos - células pigmentares, células pigmentares extracutâneas, macrófagos pigmentados, melanomacrófagos, melanófagos, melanóforos e melanócitos. Para os anuros os estudos são recentes e relatam células pigmentares em poucas espécies. Em Physalaemus nattereri e alguns anuros, os pigmentos melânicos são encontrados, além da cútis, em outros órgãos constituindo um sistema pigmentar extracutâneo, com diferentes ocorrências, tipos e quantidade em distintas espécies. Associados ao aparelho reprodutor de P. nattereri, os melanócitos foram observados nas gônadas, na albugínea e no interstício, especialmente associado com vasos sangüíneos. A notória presença de numerosas células com pigmento distribuídas no testículo confere uma coloração que varia do preto mesclado com branco ao preto intenso. Trata-se de uma rara peculiaridade e não há informações sobre seu significado funcional ou valor biológico.

Melanogenic response of the Kupffer cells of Rana esculenta L to melanocyte stimulating hormone

We have previously shown that Kupffer cells (KCs) of Rana esculenta L. possess melanogenic ability. The melanogenic enzyme activities in these cells are different from those described in skin melanocytes, and very little is known about their regulation by extracellular signalling molecules. In order to study this regulation, we analysed the effects of NDP-MSH on the levels of expression of the tyrosinase gene and on dopa-oxidase activity, using primary cultures of KCs. Incubation of the cells with NDP-MSH increases tyrosinase gene transcription, within the first 24 h of stimulation. To gain insight into the signalling mechanism involved in the cell response to the hormone, KCs in culture were incubated with IBMX or forskolin. These agents mimic the effects of alpha-MSH on melanocytes by increasing the intracellular level of cAMP. The experimental results showed that while the hormonal treatment always activated the KC tyrosinase system, treatment with IBMX or forskolin never did. Therefore, in KCs the tyrosinase-stimulating action of NDP-MSH was not mimicked by cAMP elevating agents. Assays of cAMP levels in cells stimulated with NDP-MSH demonstrated that the hormone does not produce significant increases in intracellular cAMP. On the contrary, forskolin produced significant increases in cAMP starting from 30 min of incubation. These results suggest that tyrosinase induction by melanocortins in KCs is not mediated by the cAMP pathway, and highlight the existence of substantial differences in the hormone signal transduction mechanisms between amphibian KCs and melanocytes or melanoma cells.

Amphibia Kupffer cells

Amphibia Kupffer cells (i.e., liver resident macrophages) show many common characteristics when compared with Mammalia Kupffer cells: filopodia, microvillous-like structures, lamellipodia, fuzzy coat, coated vesicles, bristled vacuoles, nonspecific esterase activity, and pinocytotic and phagocytic activity are present both in Amphibia and Mammalia Kupffer cells. On the other hand, some differences are present between Kupffer cells of both zoological classes: phagocytosed red cells and their derivatives, iron-protein complexes, and lipofuscin bodies are normally present in Amphibia Kupffer cells, but absent in the same cells of healthy mammals. Worm-like structures are not seen in Amphibia and endogenous peroxidase activity is very weak in these animals compared with Mammalia. The most important difference lies in the ability of Amphibia Kupffer cells to produce melanins: in fact the tyrosinase gene is expressed, "melanosome centers" are present, and dopa oxidase activity is demonstrable.

Spleen and liver pigmented macrophages of Rana esculenta L. A new melanogenic system?

The present study reports the results of a morpho-functional analysis of spleen pigmented cells from Rana esculenta L. and comparison with liver melanin-synthesizing cells, belonging to the macrophage cell lineage. Cytological and cytochemical analyses show that parenchymal pigmented cells of the spleen, like those of the liver, are positive to peroxidase and lipase reactions and have phagocytic properties. The observation of premelanosomes in various stages of differentiation, together with the demonstration of dopa oxidase activity in the melanosome proteins, indicate that spleen pigmented macrophages have endogenous melanogenic ability as do liver pigmented macrophages. Attempts to demonstrate tyrosinehydroxylase activity in melanosome protein extracts from frog spleen and liver, using the same protocol as for mammalian tyrosinases, gave negative results. As regards the dopa oxidase activity revealed, some of its properties differ from the typical behaviour observed for tyrosinases from different sources. Peroxidase activity is shown in spleen and liver melanosome proteins with p-phenylenediamine-pyrocatechol (PPD-PC), and not with typical peroxidase substrates. Suitable inhibition tests revealed that dopa oxidase and peroxidase activities might be supported by two different proteins. Liver melanosome extracts display a very strong laccase (dimethoxyphenoloxidase) activity but spleen extracts do not. Differences observed in the enzymatic properties of the spleen and liver melanosomes suggest that pigmented macrophages may undergo tissue-specific differentiation. These preliminary data show that the melanin pathway of pigmented macrophages is different from that of melanocytes and may pave the way to identification of a new melanogenic pathway in vertebrates.

Mechanisms of changes to the liver pigmentary component during the annual cycle (activity and hibernation) of Rana esculenta L

The present study was performed to elucidate the mechanisms responsible for the changes of melanin content/ distribution we had previously discovered in the liver parenchyma of Rana esculenta during natural hibernation. Melanomacrophagic component response was analysed using morphocytochemical methods. The results demonstrated that during the prehibernation period (October-November) the melanomacrophages reach the highest proliferative activity (BrdU, PCNA labelling) which is accompanied by an evident melanosynthesis (dopa-oxidase activity). In contrast, after hibernation, the decrease of liver pigmentation was the consequence of a partial cell loss by apoptotic mechanisms (TUNEL labelling, pyknosis-karyorhexis) accompanied by a decrease of melanosome content by autophagy and low melanosynthetic activity. On the basis of these findings, there is evidence that liver melanomacrophages represent a metabolically (melanin synthesis/degradation) and cytokinetically (proliferation/ death) active cell population during the annual cycle of the frog. The results are also discussed in relation to the functional synergism between hepatocytes and pigment cells in the adaptation to environmental changes.

Increase in liver pigmentation during natural hibernation in some amphibians

The amount/distribution of liver melanin in 3 amphibian species (Rana esculenta, Triturus a. apuanus, Triturus carnifex) was studied during 2 periods of the annual cycle (summer activity-winter hibernation) by light and electron microscopy, image analysis and microspectrofluorometry. The increase in liver pigmentation (melanin content) during winter appeared to be correlated with morphological and functional modifications in the hepatocytes, which at this period were characterised by a decrease in metabolic activity. These findings were interpreted according to the functional role (e.g. phagocytosis, cytotoxic substance inactivation) played by the pigment cell component in the general physiology of the heterothermic vertebrate liver and, in particular, in relation to a compensatory engagement of these cells against hepatocellular hypoactivity during the winter period.

Ultrastructural and functional study of the liver pigment cells from Rana esculenta L

A study of the liver pigment cells of Rana esculenta L. has been performed on both liver in toto and cells in culture. Ultrastructural and cytochemical analyses showed a close relationship between this visceral pigment cell system and the cells of hepatic macrophage lineage. Like the latter, the liver pigment cells present phagocytic activity, in the sinusoids and in vitro, and give a positive response to tests for peroxidase and lipase. The liver pigment cells are isolated, together with the Kupffer cells, from the sinusoidal cell fraction of the liver. In culture, they maintain their melanogenetic ability, demonstrated by the presence of dopaoxidase activity in the soluble, membranous, and melanosome fractions. Analysis of the cultures showed that as culture time increased, so did melanosome dopaoxidase activity, the number of pigmented fields, and the level of pigmentation of the cells. The values of dopaoxidase activity of the pigment cells in culture show the same seasonal oscillations as the system in toto, indicating that the cells maintain an internal clock, at least in the first 72 h of culture. There is evidence that the pigment cells are macrophages which can express a melanogenetic function. Our results and other experimental data provide a basis for hypothesizing that the pigment cells in Rana esculenta L. liver may derive from, or have a common origin with, the Kupffer cells.

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.

Melanomacrophage aggregations and their age relationships in the yellow mud turtle, Kinosternon flavescens (Kinosternidae)

Microscopic examination of the pigment cell aggregations in the liver and spleen of mud turtles (Kinosternon flavescens) suggests that the morphology and behavior of these cells is consistent with the melanomacrophages described in teleost fishes and the pigmented "Kupffer cells" described in frogs and reptiles. These cells contain massive amounts of melanin, substantial lipofuscin, and some hemosiderin consistent with their phagocytic function. Similar-appearing isolated pigmented macrophages are solitary in the liver, spleen, lung, and kidney. Number and size of the largest hepatic aggregations increase almost linearly with turtle age so that in old turtles they may constitute up to 20% of the liver volume. This increase may result from hepatic recruitment of macrophages throughout the life of the turtle and suggests that size and number of melanomacrophage aggregations may serve as a marker for senescence in otherwise healthy turtles of this species.

Evidence for melanogenesis in the retinal pigment epithelium of adult cattle and golden hamster

1. The ultrastructure of the retinal pigment epithelium (RPE) of adult Syrian golden hamsters and cattle was examined with respect to pigment granules and phagosomes involved in degradation of disk membranes from rod outer segments. 2. In the RPE of cattle, phagosomes were found that contained an electron-dense melanin-like material that was not autofluorescent and therefore not lipofuscin. 3. Disk membranes of rods are about 4 nm thick and become enlarged (7-20 nm) and electron-dense during degradation in the RPE. 4. Additionally electron-dense vesiculo-globular bodies (10-100 nm) were found in phagosomes during disk membrane degradation and in mature melanin granules. 5. In the RPE of adult hamsters that had been exposed to intense light, premelanosomes containing unmelanised filaments with a striated periodicity were found in the cytoplasm or in association with mature melanin granules. Early and late stage melanosomes were also present. Phagosomes in the RPE contained degraded disk membranes, melanin-like material and melanofilaments. 6. Dopa oxidase was detected ultrastructurally within shed disk membranes that were in close contact with the microvilli of the RPE. 7. The possibility of melanogenesis within phagosomes during disk membrane degradation is discussed.

Distribution of extracutaneous melanin pigment in Sparus auratus, Mugil cephalus, and Dicertranchus labrax (Pisces, Teleostei)

The morphological and biochemical characteristics of pigment accumulations found in the kidney, liver, spleen, and mesentery of three different species of teleost fishes have been studied. There are significant differences in number, distribution, and morphology of pigment accumulations in different organs of the three species. Biochemical studies have shown the existence of tyrosinase activity in the mesentery of Mugil cephalus and in the kidney and mesentery of Sparus auratus. No tyrosinase activity was found in any internal organs of Dicertranchus labrax. That activity was assayed using three methods: tyrosine hidroxylation, dopa oxidation, and melanin formation. The morphological and biochemical observations are in agreement. In those organs in which we have demonstrated melanin synthetic activity, the pigment cells are morphologically and like melanophores, while in the organs that show no melanin synthetic activity, the pigment cells resemble macrophages.

Morpho-functional characterization of cultured pigment cells from Rana esculenta L. liver

A simple method to isolate and culture liver pigment cells from Rana esculenta L. is described which utilizes a pronase digestion of perfused liver, followed by sedimentation on a Ficoll gradient. A first characterization of isolated and cultured cells is also reported. They show both positivity for nonspecific esterases, and phagocytosis ability, like the cells of phagocytic lineage. Furthermore, after stimulation with a phorbol ester, these cells generate superoxide anions. At phase contrast microscope, liver pigment cells present variability in size, morphology, and in their content of dark-brown granules. Inasmuch as a cell extract obtained from cultured cells exhibits a specific protein band with dopa-oxidase activity, when run on nondenaturing polyacrylamide gel electrophoresis, liver pigment cells from Rana esculenta L. should not be considered as melanophages, but as cells that can actively synthesize melanin. The method presented here seems to be useful to more directly investigate this extra-cutaneous melanin-containing cell system and to clarify its physiologic relevance.

Circannual rhythm of the melanin content in frog liver (Rana esculenta L.)

The melanin content of Rana esculenta L. liver varies according to a circannual statistically significant rhythm, as shown by variance and single cosinor analysis. The maximum is found in autumn-winter, the minimum in spring-summer. The linear regression analysis shown a negative correlation between the amount of melanin and the environmental temperature.

Effects of copper on the tyrosinase of liver pigment cells from Rana esculenta L

1. The liver pigment cells of R. esculenta L. constitute a peculiar pigment cell system of histiocytic nature and contain a tyrosinase-like activity localized in the protein component of melanosomes. 2. The effects of addition and/or removal of Cu on the DOPA-oxidase activity of the system were studied. 3. It was concluded that: (a) this tyrosinase behaves as a Cu-enzyme; (b) Cu could be involved in the regulation of the enzyme activity; and (c) mixtures of apoenzyme and active enzyme coexist in the melanosomes.