Exploratory transcriptomic analysis in muscle tissue of broilers fed a phytase‐supplemented diet

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Phosphorus (P) is essential for broiler chicken development due to its involvement in many metabolic processes (National Research Council 1994). Phytate (Inositol‐6‐phosphate) is the major P source in feedstuffs of plant origin fed to farm animals. Phytate is considered an antinutritional compound due to its ability to bind to cations, proteins and starch (Cosgrove and Irving, 1980), generating insoluble aggregates that lead to reduced bioavailability in poultry (Olukosi, 2012). Due to endogenous phytase with low activity at small intestinal pH (Denbow et al., 1995), supplementation with microbial phytase has been shown to improve phytate‐P digestibility in farm animals and to reduce environmental P pollution (Bernadet et al., 2005). Increased retention of phytate‐P and the associated beneficial effects on Ca solubility has a range of beneficial effects on feed intake and bone mineralization.
In addition, phytase improves growth performance, energy and amino acid digestibility. These so‐called extra‐phosphoric (or perhaps extra‐mineral) effects are associated with partial elimination of the antinutritional effects of phytate on endogenous amino acid flow mediated via reduced binding between phytate and dietary protein (Nitrayova et al., 2006; Guggenbuhl et al., 2012; Yu et al., 2012). Additionally, the involvement of myo‐inositol (the nucleus of the phytate molecule) in the improved growth efficiency of poultry has already been implied (Zyla et al., 2004; Cowieson et al., 2013, 2014; Walk et al., 2014). Relatively few studies have been reported on the effect of phytase supplementation on gene expression and the interaction with performance. In poultry, Jozefiak et al. (2010) reported a possible relationship of phytase on insulin growth factor 1 (IGF‐1) gene expression in liver. In a study on various marker genes responsible for the maintenance of gut mucosa integrity, Olukosi et al. (2013) demonstrated a decrease in sodium‐dependent phosphate transporter type II‐b (NaPi‐IIb) mRNA at the ileal level in the phytase‐supplemented diet, suggesting different molecular mechanisms responsible for intestinal P transport based on the source of the phosphate. Furthermore, some studies revealed the impact of phytase supplementation on various aspects of metabolism at intestinal or hepatic level as described hereafter: caecal and duodenal transcripts of interferon‐γ, interleukin 4 and 17 were investigated by Shaw et al. (2011), showing an increase of IL‐17 gene expression in chickens fed phytase and vaccinated against coccidiosis. In pigs, expression of several intestinal genes was evaluated (Vigors et al., 2014) in conjunction with phytase supplementation, revealing an upregulation in the expression of the oligopeptide transporter PEPT1. Martinez‐Montemayor et al. (2008) performed microarray analysis on hepatic tissue following zinc‐oxide supplementation in combination with phytase, suggesting an increase of the GSTM4 (glutathione S‐transferase mu 4) gene expression, participating in the oxidative stress response. Therefore, to obtain a complemented view of what has already been observed, this study focused on possible relationship between phytase supplementation and muscle growth metabolism. The present work investigated gene response and potential affected pathways behind the effects seen in the use of phytase which cannot be explained by liberated phosphorus in isolation and focused on the role of esters of inositol and myo‐inositol. In so doing, P digestibility, growth performance, bone mineralization and plasma myo‐inositol concentration were studied and correlated with a transcriptomic analysis on breast muscle.
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