Characterization and expression analysis of FGF6 (fibroblast growth factor 6) genes of grass carp (Ctenopharyngodon idellus) reveal their regulation on muscle growth.

The present study was conducted to investigate the regulative function of FGF6 in the muscle growth of grass carp (Ctenopharyngodon idellus) by the bioinformatics analysis and expression pattern analyses of FGF6 genes in different developmental stages and tissues, as well as the correlation analysis between muscle growth and FGF6 expression after fish were fed with different levels of dietary lotus leaf flavonoids (LLF) (0, 0.03%, 0.06%, 0.09%). Results showed that the FGF6a and FGF6b genes are two homologs of the FGF6 family, encoding 205 and 209 amino acids, respectively. Alignment of amino acid sequences and phylogenetic analysis demonstrated that FGF6a and FGF6b are highly conserved with other vertebrates. Quantitative RT-PCR analysis showed both FGF6a and FGF6b expressions were high in brain and muscle but low in other examined tissues. During embryonic development, FGF6a and FGF6b mRNA expressions could be detected as early as at fertilized egg stage and displayed the highest value at cleavage stage. Dietary LLF affected the gene expression of FGF6 in white muscle. The relative expression of FGF6a of 0.06% LLF group was significantly higher than that of 0.09% LLF group, while FGF6b expression of 0.06% LLF group was higher than those of other groups (P < 0.05). The muscle fiber diameter was significantly higher in 0.06% LLF group in comparison with other groups, while the fiber density in this group was lower (P < 0.05). Both FGF6a and FGF6b expressions were positively correlated with fiber diameter but negatively correlated with fiber density. These results collectively suggest that FGF6a and FGF6b play an important role in muscle growth regulation in grass carp.

Total substitution of dietary fish oil by vegetable oils stimulates muscle hypertrophic growth in Senegalese sole and the upregulation of fgf6.

The long term effects of fish oil (FO) substitution by increasing the levels of vegetable oils (VO), 0% (CTR), 50% (VO50) and 100% (VO100), in diets for Senegalese sole were evaluated in terms of skeletal muscle cellularity and expression of related genes. After 140 days of feeding, all fish had similar body weight and length. The inclusion of 50% VO did not result in differences in muscle cellularity, but dorsal muscle cross-sectional area and fast-twitch fibre diameter increased in fish fed total FO substitution, whilst fibre density was reduced (P < 0.05) in relation to CTR. The total number of fibres was similar in all treatments. FO substitution did not affect the transcript levels of myogenic genes (myf5, mrf4, myog, myod1, myod2), but resulted in a two-fold increase of fgf6 transcript levels compared to CTR (P < 0.05). The relative expression of igf-I was higher in VO100 than in VO50, but was similar to CTR. FO substitution resulted in cellularity changes related to the stimulation of muscle hypertrophic growth, but not hyperplastic growth, and associated with a nutritional modulation of fgf6 by dietary VO. This study indicates that 50% VO does not affect the muscle phenotype, but total FO substitution stimulates muscle hypertrophy.

Eccentric stimulation reveals an involvement of FGF6 in muscle resistance to mechanical stress.

The objective of this report was to analyse a potential role for FGF6 in muscle resistance to mechanical stress. Normal or regenerating muscles of FGF6 (-/-) mice versus wild-type mice were submitted to different protocols of damaging eccentric contractions (eccentric electrostimulation and intermittent downhill exercise). Then muscular structural properties were analysed by histological and immunochemistry techniques to evaluate the post-injury muscle recovery; their muscle contractile parameters (maximal tetanic force, kinetics properties and fatigue resistance) were assessed. The absence of FGF6 causes (1) a fast-to-slow myofibre type switch in adult control and regenerating Tibialis anterior (TA) muscle; (2) muscle weakness in regenerating muscles in animals submitted to eccentric exercise protocols due to aberrant extensive necrotic zones. These observations point out a crucial and unexpected role for FGF6 in muscle integrity and muscle protection against mechanical stress.