A study of potential health consequences of excessive fat deposition in Atlantic salmon

The shift in composition of traditional Atlantic salmon diets from low fat diets rich in marine ingredients to today's high fat diets rich in plant ingredients has resulted in increased fat depots. Although not known, excessive fat deposition in certain body depots may create a less robust fish and thereby contribute to the 15 to 20% mortalities found in the seawater phase of salmon life cycle. Salmon fat deposition and mobilization are dynamic processes that are affected by processes such as fish maturation and seasonal conditions. In salmon, adipose tissue develops within different depots, including but not limited to intra-abdominal (VAT) and subcutaneous (SAT). In mammals, these different depots are functionally and biologically different from each other and they present different associations with health and disease. However, no information concerning this topic exists for Atlantic salmon. In order to gain insight into human obesity research and lifestyle related disorders and to transfer this knowledge to salmon, a four month stay at The Oxford University was done during the development of this project. One of the specific aims of the project was to describe how the change in diet composition affects fat deposition, number, and size of fat cells in the different body depots, and further evaluate the potential impact on salmon health. Furthermore, we aimed to characterize visceral and subcutaneous adipose tissue through transcriptional and functional studies. To do this, both in vivo and in vitro approaches were used. Results from the in vivo approaches demonstrated that the populations of adipocytes from VAT and those from SAT differed in size. Thus, VAT contained larger cells than SAT depots. A transcriptome analysis comparing VAT and SAT revealed the existence of tissue-specific expression profiles. The interpretation of the results indicates that SAT possess characteristics of a connective-like tissue, whereas VAT resembles an immune organ. In addition, SAT seemed to be more dynamic, changing the transcriptional profile due to changes in season to a higher degree than VAT. Interestingly, the transcriptional changes within each tissue due to sex were negligible. These two fat depots were also characterized in vitro. To this end, the cell culture of primary adipocytes isolated from SAT was established and compared to the well-stablished cultured of primary adipocytes from VAT. Differences in cell morphology were observed during all the developmental process. Cells isolated from the SAT presented a round shape during the first days of culture, whereas cells from VAT had the characteristic fibroblastic appearance and they grew forming clusters to a higher degree. After inducing differentiation, both cell types changed their morphology and started accumulating lipids, however, cells from VAT had a higher capacity to accumulate lipids than cells isolated from the SAT indicating that the main functions of these cells might be different. In order to better understand how lipids are mobilized, the responses to VAT adipocytes to an early stage of fasting were also examined in vitro. Interestingly, most of the secreted lipids were esterified into triglycerides and phospholipids. This suggests that more mechanistic studies regarding lipid mobilization should be performed when evaluating fish lipolysis instead of relying only on the traditional measurements of glycerol and non-esterified fatty acids release. Leptin responded to the energy status of the adipocytes following the model described in most mammalian species, highlighting the role of VAT as an endocrine organ. The Mitochondrial area increased after 3 hours of fasting but after 18 hours it went back to basal levels highlighting the huge plasticity of salmon adipose tissue. In the project, several different adipose tissue deposits have been identified in salmon. These depots have different properties and potentially different functions related to, for example, the deposition and mobilization of fat, in regulating energy metabolism, recruitment of stem cells and in the immune system.

This project has provided new data that allow the industry to gain more detailed knowledge of the basal biological responses of the fish to new diets, fasting, and changing seasons. This knowledge is needed in order to secure production of a healthy salmon in the future. Additionally, the development of a new cell culture model from subcutaneous adipose tissue provided the framework for future mechanistic studies. A better understanding of adipocyte cell biology and adipogenesis may lead to new strategies to modulate the distribution of body fat and potentially determining metabolic health, fish robustness in general, and the role of adipose tissue in reproduction.

This project will increase the knowledge on how the levels of total fat and omega-3 fatty acids in diets influence the risk of developing obesity related disorders in Atlantic salmon at different life stages and locations along the coast. The project addresses a major challenge for the Norwegian aquaculture industry; the potential health consequences for Atlantic salmon when fed high fat diets with more than 70% plant ingredients. This recent shift in ingredients of salmon diet has resulted in increased fat depots and reduced omega-3 fatty acid levels in tissues and organs. Excessive fat deposition may result in a less robust fish and contribute to the 15- 20% mortalities found in the seawater phase. We will describe how high fat/ low protein ratios and reduced EPA+DHA levels in the diet influence the lipid deposition in different body depots of adipose tissues, liver and heart of Atlantic salmon. In addition, we will determine if the fish possess different types of adipocytes with different functions at different body locations, determine how the high lipid diets influence the number and size of fat cells, lipid filling, degree of new requirement of cells, number of mitochondria and risk of inflammation and oxidative stress. Finally, we will cultivate pre-adipocytes isolated from different fat depots in vitro and study how they are influenced by hormones, fatty acids and lipid level, and elucidate their potential role in regulation of sexual maturation and inflammatory disease. The new knowledge created in the project will give the industry knowledge of the basal biological responses of the fish to new diets, knowledge required in order to secure the production of a healthy salmon in the future.