Melanogenesis in Visceral Tissues of Salmo salar. A link between Immunity and Pigment production?

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 (1) adversely affects the quality of fish products and is a problem for the aquaculture industry. Moreover, a close relationship between nutrition products and human health has been widely demonstrated. 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 (2). In teleosts, melanin-containing cells, named melano-macrophages, occur in the reticulo-endothelial system and are mainly located in kidney and spleen. In these organs, the pigment content may increase in different physiological conditions and as a response to pathological and inflammatory states (2). With the aim to reveal novel functions and behaviour of the poorly understood teleost extracutaneous pigment system, we undertook the investigation of aspects of the melanogenic systems of Atlantic salmon tissues as well as of leukocyte salmon head-kidney cells (SHK-1 cells). SHK-1 cells is a long-term cell line derived from macrophages of the Atlantic salmon head-kidney (3). Using a radiometric method we demonstrated that a tyrosinase activity is present in SHK-1 cells and salmon tissues. Western blotting showed the presence of a tyrosinase protein. Moreover differing melanin-producing activities in salmon tissues were demonstrated suggesting that pigmented cells in the organs undergo a tissue-specific differentiation. These results were confirmed by a tyrosinase protein expression in lymphoid organs, also highlighted by immunohistochemical analysis. As teleosts lack lymph nodes and Peyer’s patches, the head-kidney and spleen are regarded as the most important secondary lymphoid organs (4). The detection of tyrosinase activity in lymphoid organs indicates that a link may exist between the extracutaneous pigmentary system and the immune system in salmon. The extracutaneous melanogenetic system could be used as a biomarker for the health status of farmed fishes. References: 1. Koppang, E.O., Haugarvoll, E., Hordvik, I., Aune, L., Poppe, T. (2005) Journal of Fish Diseases 28: 13-22. 2. Agius, C., Roberts, R.J. (2003) Journal of Fish Diseases 26: 499-509. 3. Dannevig, B., Falk, K., Narmork, E. (1995) Journal of General Virology 76: 1353-1359. 4. Zapata, A., Amemiya, CT. (2000) Curr Top Microbiol Immunol 248: 67-107