Effect of Dietary Methionine Level on Muscle Growth Mechanisms in Juvenile Rainbow Trout (Oncorhynchus Mykiss)

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anglais. The aim of the study was to analyse the effects of different dietary methionine (Met) concentrations on the mechanisms of fish muscle growth (hyperplasia and hypertrophy) by using cellular and molecular approaches. Three plant-based diets differing by their Met level (MD, Met-deficient; MC, Met-control, corresponding to established Met requirement; MH, Met-high) were fed to rainbow trout juveniles for 12 weeks. Met deficiency led to a reduced somatic growth right from the first 3 weeks of feeding onwards, while feeding the MH-diet led to an increased body growth in the 6 last weeks of feeding. The protein gain of the juveniles increased with dietary Met content. The protein retention efficiency of the MD-fed juveniles was half that of the MC- and MH-fed juveniles which were statistically similar. The changes induced by MD- and MH-feeding at muscle molecular and cellular levels were different. Compared to control, dietary Met deficiency altered the expression of myogenic, muscle growth- and metabolism-related genes by increasing Myf5, MyoD1, Mrf4, Mef2a, βact, CathD, GSTπ and TNFα expressions and decreasing Col1α1 and IGF1 expressions, whereas a high level of dietary Met decreased Pax7a2, Mef2c, βact, CathD, GDH2 and NF-κB expressions. The Met-control fed fish were discrimated according to HGF, Pax7a1 and Pax7a2 expressions. Dietary Met level thus acted on the expression of genes regulating specific transition points of myogenesis, and affected the expression of muscle structural genes and growth factors involved in satellite cell activation and muscle growth. Molecular results suggested that MD-diet favours satellite cell return to quiescence, while MC-diet favours satellite cell activation and MH-diet myogenic differentiation. At cellular level, the muscle total cross-sectional area of the juveniles fed the MD-diet was 28% lower than that of the juveniles fed the MC-diet despite a similar total number of white muscle fibres, and the number of white muscle fibres with a large diameter was lower in MD-fed fish than in MC-fed ones. The muscle total cross-sectional area of the juveniles fed the MH-diet was 32% higher than that of the juveniles fed the MC-diet, their total number of white muscle fibres 25% higher, and their number of white muscle fibres with a large diameter similar. Increasing dietary methionine above established requirement thus promotes white muscle hyperplasia and muscle growth in rainbow trout juveniles, while methionine deficiency dysregulates the expression of myogenic and muscle growth-related genes and impairs white fibre hypertrophy. Statement of relevance: Determining the effects of dietary methionine levels on muscle growth mechanisms will help develop low fish meal feeds.