Transcription of the tyrosinase gene family in an Atlantic salmon leukocyte cell line (SHK-1) is influenced by temperature, but not by virus infection or bacterin stimulation

Melanisation in Salmonid Skeletal Muscle: A Review

Melanisation can occur in the musculature of fish. A well-known form is the melanised focal changes, or 'black spots', in the fillet of farmed Atlantic salmon (Salmo salar). The aetiology of black spots has not been fully determined, though recent research has emphasised the role of fat necrosis in their development. The initial stages of the changes are observed as focal haemorrhages or 'red spots', and these can progress into melanised focal changes (MFCs). The focal haemorrhages are acute changes characterised by necrotic myocytes and adipocytes and diffuse haemorrhage in the tissue. These changes evolve into a chronic inflammation dominated by fibrosis, encapsulated lipid droplets or pseudocysts, presence of epithelioid cells, granulomas of varying character, giant cells and melano-macrophages, whose presence accounts for the discolouration. The inflammation ranges from mild to severe, and the severity of the lesion has been associated with localised piscine orthoreovirus 1 (PRV 1) replication in macrophages and melano-macrophages within granulomas. The possibility of a genetic impact on the condition has not been supported by available data. The lipid composition and the antioxidative properties of the feed have been shown to affect the development of the changes. Physiological and environmental factors are also believed to influence the prevalence and severity of the condition. Here, we review the current state of knowledge concerning melanisation in fish skeletal musculature, with a special emphasis on the MFCs in Atlantic salmon.

Analysis of morphology, histology characteristics, and circadian clock gene expression of Onychostoma macrolepis at the overwintering period and the breeding period

Fish typically adapt to their environment through evolutionary traits, and this adaptive strategy plays a critical role in promoting species diversity. Onychostoma macrolepis is a rare and endangered wild species that exhibits a life history of overwintering in caves and breeding in mountain streams. We analyzed the morphological characteristics, histological structure, and expression of circadian clock genes in O. macrolepis to elucidate its adaptive strategies to environmental changes in this study. The results showed that the relative values of O. macrolepis eye diameter, body height, and caudal peduncle height enlarged significantly during the breeding period. The outer layer of the heart was dense; the ventricular myocardial wall was thickened; the fat was accumulated in the liver cells; the red and white pulp structures of the spleen, renal tubules, and glomeruli were increased; and the goblet cells of the intestine were decreased in the breeding period. In addition, the spermatogenic cyst contained mature sperm, and the ovaries were filled with eggs at various stages of development. Throughout the overwintering period, the melano-macrophage center is located between the spleen and kidney, and the melano-macrophage center in the cytoplasm has the ability to synthesize melanin, and is arranged in clusters to form cell clusters or white pulp scattered in it. Circadian clock genes were identified in all organs, exhibiting significant differences between the before/after overwintering period and the breeding period. These findings indicate that the environment plays an important role in shaping the behavior of O. macrolepis, helping the animals to build self-defense mechanisms during cyclical habitat changes. Studying the morphological, histological structure and circadian clock gene expression of O. macrolepis during the overwintering and breeding periods is beneficial for understanding its unique hibernation behavior in caves. Additionally, it provides an excellent biological sample for investigating the environmental adaptability of atypical cavefish species.

The melano-macrophage: The black leukocyte of fish immunity

Melanin and the process of melanin synthesis or melanogenesis have central roles in the immune system of insects, and production of melanin-synthesizing enzymes from their haemocytes may be induced following activation through danger signals. Melanin-containing macrophage-like cells have been extensively studied in amphibians and they are also present in reptiles. In fish, melano-macrophages are especially recognized with respect to melano-macrophage centres (MMCs), hypothesized to be analogues of germinal centres in secondary lymphoid organs of mammals and some birds. Melano-macrophages are in addition present in several inflammatory conditions, in particular melanised focal changes, or black spots, in the musculature of farmed Atlantic salmon, Salmo salar. Melanins are complex compounds that may be divided into different forms which all have the ability to absorb and scatter light. Other functions include the quenching of free radicals and a direct effect on the immune system. According to the common view held in the pigment cell community, vertebrate melanin synthesis with melanosome formation may only occur in cells of ectodermal origin. However, abundant information suggests that also myeloid cells of ectothermic vertebrates may be classified as melanocytes. Here, we discuss these opposing views and review relevant literature. Finally, we review the current status on the research concerning melanised focal muscle changes that represent the most severe quality problem in Norwegian salmon production, but also other diseases where melano-macrophages play important roles.

Cellular heterogeneity in red and melanized focal muscle changes in farmed Atlantic salmon (Salmo salar) visualized by spatial transcriptomics

Spatial transcriptomics is a technique that provides insight into gene expression profiles in tissue sections while retaining structural information. We have employed this method to study the pathological conditions related to red and melanized focal changes in farmed Atlantic salmon (Salmo salar). Our findings support a model where similar molecular mechanisms are involved in both red and melanized filet discolorations and genes associated with several relevant pathways show distinct expression patterns in both sample types. Interestingly, there appears to be significant cellular heterogeneity in the foci investigated when looking at gene expression patterns. Some of the genes that show differential spatial expression are involved in cellular processes such as hypoxia and immune responses, providing new insight into the nature of muscle melanization in Atlantic salmon.

Anatomy of teleost fish immune structures and organs

The function of a tissue is determined by its construction and cellular composition. The action of different genes can thus only be understood properly when seen in the context of the environment in which they are expressed and function. We now experience a renaissance in morphological research in fish, not only because, surprisingly enough, large structures have remained un-described until recently, but also because improved methods for studying morphological characteristics in combination with expression analysis are at hand. In this review, we address anatomical features of teleost immune tissues. There are approximately 30,000 known teleost fish species and only a minor portion of them have been studied. We aim our review at the Atlantic salmon (Salmo salar) and other salmonids, but when applicable, we also present information from other species. Our focus is the anatomy of the kidney, thymus, spleen, the interbranchial lymphoid tissue (ILT), the newly discovered salmonid cloacal bursa and the naso-pharynx associated lymphoid tissue (NALT).

Administration of a polyphenol-enriched feed to farmed sea bass (Dicentrarchus labrax L.): Kidney melanomacrophages response

The reinforcement of the defense mechanism of fish, through the administration of immunostimulants, is considered as a promising alternative to vaccines. Natural immunostimulants such as polyphenols, flavanoids, pigments and essential oils can modulate the innate immune response. In lower vertebrates, melano-macrophage centres, i.e. clusters of pigment-containing cells forming the extracutaneous pigment system, are wide-spread 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. In the present study, we evaluated the effect of a polyphenol-enriched diet on the health status of European sea bass (Dicentrarchus labrax L.). Farmed sea bass were administered a feed containing a phytocomplex, rich in catechins and epigallocatechins, which was obtained from the seeds of Canosina Nero di Troia Vitis vinifera and mixed with conventional feed at two different concentrations. The effects of such a diet were investigated in juvenile and commercial size samples, i.e. undergoing a short- and long-term period of diet, respectively, focusing on their extracutaneous pigmentary system and, in more detail, on the enzymatic activities leading to melanin biosynthesis. Our results show that prolonged dietary treatments with higher concentration of polyphenols might modulate tyrosinase activity and gene expression in commercial size fishes. An increase of melano-macrophage activity is correlated to a stimulation of cytoprotective functions against antigenic stimulants and pathogens, as an expression of a robust and protective adaptive immune response.

Development and characterization of a cell line from tilapia head kidney with melanomacrophage characteristics

A novel cell line THK, derived from the tilapia head kidney, was developed and characterized. The THK cell line comprised fibroblastoid cells that markedly proliferated in Leibovitz L-15 medium containing 2%-15% fetal bovine serum (FBS) at 20 °C-35 °C. Cell proliferation was dependent on the FBS concentration, and the optimal temperature for proliferation ranged between 25 °C and 30 °C. THK cells were characterized for the presence of phagocytic activity, acid phosphatase, alkaline phosphatase, α-naphthyl acetate esterase, lipofuscin, and tyrosinase. Transcripts of CD33, CD53, CD82, CD205, macrophage colony stimulating factor receptor, GATA2, and GATA3 that are specific for leucocytes or monocytes/macrophages or both were detected in the THK cells through PCR. However, THK cells lacked for CD83, a specific marker for dendritic cells. The results indicated that the fibroblastoid THK cells were melanomacrophage-related progenitors. PCR revealed that the THK cells exhibited the transcripts of toll-like receptor 1 (TLR1), TLR2, TLR3, and CD200, of which concern with immunity as well as the transcripts of vascular endothelial growth factor receptor 3, angiomotin, and angiopoietin-like protein 2 that associate with angiogenesis regulation and macrophage proliferation. THK cells were subcultured more than 90 times and can be useful for investigating the development and functioning of the teleostean innate immune system.

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.

Pathological pigmentation in cardiac tissues of Atlantic salmon (Salmo salar L.) with cardiomyopathy syndrome

It is widely accepted that melanin formation may play an immunologic role in invertebrates and ectothermic vertebrates. In farmed Atlantic salmon, cardiomyopathy syndrome (CMS) is a common viral disease associated with severe cardiac inflammation that may be accompanied by heavy melanisation of the heart. By the use of histology, laser capture microdissection and transcription analysis of tyrosinase genes, we here show that this melanisation is linked to de novo melanogenesis by melanomacrophages, suggesting an active part in the inflammatory reaction. No general systemic activation of the extracutaneous pigmentary system in response to viral infections with affinity to the heart was observed.