The initiation of puberty in Atlantic salmon brings about large changes in testicular gene expression that are modulated by the energy status.

BACKGROUND: When puberty starts before males reach harvest size, animal welfare and sustainability issues occur in Atlantic salmon (Salmo salar) aquaculture. Hallmarks of male puberty are an increased proliferation activity in the testis and elevated androgen production. Examining transcriptional changes in salmon testis during the transition from immature to maturing testes may help understanding the regulation of puberty, potentially leading to procedures to modulate its start. Since differences in body weight influence, via unknown mechanisms, the chances for entering puberty, we used two feed rations to create body weight differences. RESULTS: Maturing testes were characterized by an elevated proliferation activity of Sertoli cells and of single undifferentiated spermatogonia. Pituitary gene expression data suggest increased Gnrh receptor and gonadotropin gene expression, potentially responsible for the elevated circulating androgen levels in maturing fish. Transcriptional changes in maturing testes included a broad variety of signaling systems (e.g. Tgfß, Wnt, insulin/Igf, nuclear receptors), but also, activation of metabolic pathways such as anaerobic metabolism and protection against ROS. Feed restriction lowered the incidence of puberty. In males maturing despite feed restriction, plasma androgen levels were higher than in maturing fish receiving the full ration. A group of 449 genes that were up-regulated in maturing fully fed fish, was up-regulated more prominently in testis from fish maturing under caloric restriction. Moreover, 421 genes were specifically up-regulated in testes from fish maturing under caloric restriction, including carbon metabolism genes, a pathway relevant for nucleotide biosynthesis and for placing epigenetic marks. CONCLUSIONS: Undifferentiated spermatogonia and Sertoli cell populations increased at the beginning of puberty, which was associated with the up-regulation of metabolic pathways (e.g. anaerobic and ROS pathways) known from other stem cell systems. The higher androgen levels in males maturing under caloric restriction may be responsible for the stronger up-regulation of a common set of (449) maturation-associated genes, and the specific up-regulation of another set of (421) genes. The latter opened regulatory and/or metabolic options for initiating puberty despite feed restriction. As a means to reduce the incidence of male puberty in salmon, however, caloric restriction seems unsuitable.

A metabolomics approach to elucidate effects of food deprivation in juvenile rainbow trout (Oncorhynchus mykiss).

The aim of this study was to evaluate whether NMR-based metabolomics is a suitable method to generate an integrated view on metabolic processes during food deprivation in salmonid fish. Juvenile rainbow trout (Oncorhynchus mykiss) weighing 43-115 g were either fed ad libitum or deprived of food for 28 days at 10°C to investigate catabolic effects on energy reserves and metabolite patterns. The NMR analysis of plasma, liver, and muscle extracts revealed significant fasting-induced changes in the metabolome. Altered plasma lipoprotein levels and tissue-specific patterns of fatty acid mobilization were the most prominent responses, emphasizing the role of lipids as the primary energy source during fasting. In plasma, very-low-density lipoprotein levels increased in food-deprived fish compared with fed fish, whereas levels of high-density lipoprotein decreased. Signs of muscle protein catabolism were also observed as changes in the alanine dynamics. The results further suggest that mechanisms to preserve liver glycogen are present in the food-deprived fish. This study thus demonstrates the utility of NMR-based metabolomics of tissue extracts and plasma to describe the integrated metabolic status of fish.