Recently, global aquaculture production is estimated to have reached about 82 million tons in 2018, accounting for 52 percent of total fish production for human consumption around the worldwide [1]. However, with the improvement of consumer health awareness, people pay more and more attention to the quality and nutritional composition of meat [2]. Thus, the aquaculture industry seems to be a growing interest in concerning about fish quality, which is influenced by a number of external factors, such as feeding management, dietary composition, amino acids ration level, and so on [3–6]. Among nutrients, especially amino acids, are considered as primary contributors [7, 8]. As a member of branched-chain amino acids (BCAA), isoleucine (Ile) is not only the building block of protein but also the essential nutrient in aquafeeds, which plays a vital role in growth and flesh quality [9]. Our previous study indicated that dietary Ile improved the growth performance [10], and enhanced the intestinal mucosal and physical barrier functions of hybrid bagrid catfish[11]. However, the exact mechanism concerning dietary Ile on flesh quality of hybrid bagrid catfish has not been extensively elucidated.
Fish flesh quality is affected by nutritional content, physicochemical, texture parameters, and so on [12, 13]. The nutritional content was mainly reflected by muscle protein and lipid contents. Emerging shreds of evidence showed that Ile could increase muscle protein and lipid contents of grass carp Ctenopharyngodon idella [9] and fingerling Channa punctatus (Bolch) [14]. These results provide possible evidence that Ile could affect muscle nutritional content and merit further study. Additionally, physicochemical (pH value, water holding capacity, lactate, cathepsin, and hydroxyproline content) and texture (hardness, springiness, cohesiveness, gumminess, chewiness, and resilience) parameters are also the important indices for flesh quality [15]. Previous study demonstrated that diet had a potential benefit to improve muscle flesh quality by regulating the physicochemical parameters of grass carp [9]. However, knowledge about the effects of dietary Ile on muscle texture parameters is scarce, which deserves further research. Some studies also have shown that flesh quality was negatively linked to the serious oxidative stress in different species fish [16, 9, 3]. Oxidative damage was primarily caused by excessive accumulation of reactive oxygen species (ROS), which could be effectively eliminated by the antioxidant system in fish [3, 8]. Our previous study found that Ile improved intestine function by increasing antioxidant ability via Keap1-Nrf2 signaling pathway in hybrid bagrid catfish[10]. However, it remains unknown whether dietary Ile could enhance the quality of flesh by regulating Keap1-Nrf2 signaling pathway in hybrid bagrid catfish.
The muscle is the complex and heterogeneous tissue, which comprises the primary edible part of the trunk of fish [17]. Fish muscle growth is a dynamic process involving both the hyperplasia of muscle fibers and hypertrophy of existing muscle fibers [18], which is under the control of a series of transcription factors such as insulin-like growth factors (IGFs), proliferating cell nuclear antigen (PCNA), myogenic regulatory factors (MRFs), and myostatin (MSTN) [19, 20]. The IGFs, including IGF-1 and IGF-2, play pivotal roles in the growth and functional maturation of fish muscle [21]. Several studies have revealed that IGFs improve muscle growth of Japanese amberjack Seriola quinqueradiata [22], hybrid bagrid catfish [23], and gilthead sea bream Sparus aurata [24]. The MRFs, including myogenic determination factor (MyoD), myogenic factor 5 (Myf5), myogenic regulatory factor 4 (MRF4), and myogenin (MyoG), play a critical role both in myoblast proliferation and myogenic differentiation [21]. The MSTN acts as a negative regulator of muscle growth by inhibiting the myogenesis in fish [25, 26]. Despite muscle growth induced by these factors have reported in some species[24, 27, 28]. Several studies in our laboratory also reported that dietary threonine [23] and leucine [29] could promote the muscle growth of hybrid bagrid catfish by regulating these factors. However, there was unclear whether dietary Ile improved muscle growth through impacting hyperplasia and hypertrophy in fish.
Fish muscle growth primarily associated with protein deposition of muscle [30]. Muscle protein deposition is the result of a balance between protein synthesis and protein degradation. The IGF-1 and IGF-2 mediate muscle growth by blinding to IGF-1 receptor (IGF-1R) at the cell membrane [31, 32]. Several studies have demonstrated that IGF-1 promotes muscle growth via the IGF-1R- phosphoinositide 3-kinase (PI3K)- serine and/or threonine kinase (AKT) pathway [33, 34, 32]. As a response element of the PI3K-AKT pathway, target of rapamycin (TOR) acts a central role in cell growth, development, and metabolism. The TOR could promote protein synthesis by phosphorylating ribosomal S6 kinase 1 (S6K1) and the eukaryotic translation initiation factor 4E binding protein 1 (4EBP1) in fish [35, 36]. Previous studies have reported that Ile could activate TOR signaling pathway and regulate the phosphorylation of S6K1 and 4EBP1 in bovine [37] and goat [38] mammary tissue, ultimately stimulating protein synthesis. [9] reported that dietary Ile increased the mRNA expressions of TOR and S6K1 in grass carp muscle. On the other hand, protein synthesis and degradation pathways are also interlinked [39]. The PI3K-AKT signaling pathway also regulates protein degradation by activating the forkhead box protein 3 (FOXO3a) and subsequently inhibiting the ubiquitin-proteasome system (UPS), which mediates 80–90% protein degradation [40, 41]. The UPS associated with two key ubiquitin ligases (muscle atrophy F box, MAFBX and muscle RING finger 1, MURF-1) are responsible for specifically induced the muscle protein degradation [42–44]. In previous studies, BCAA decreased amount of MURF-1 and MAFBX during proliferation and differentiation in mice myocytes [45] and protein levels of MURF-1 and MAFBX in piglet muscle [46]. Previous study reported that dietary leucine could regulate protein synthesis and degradation through the target of protein synthesis and degradation signaling pathways in fish muscle[29]. However, whether dietary Ile could affect protein synthesis and protein degradation involving in IGF-1-AKT-TOR and IGF-1-AKT-FOXO3a signaling pathways in fish muscle remain unknown.
Recently, hybrid bagrid catfish has been widely farmed in China [11], and optimal dietary threonine [23] and leucine [29] levels based on the optimal muscle growth have been investigated. The current study was performed to explore the effect of dietary Ile on flesh quality, muscle growth, protein synthesis, and degradation-related signaling pathways in hybrid bagrid catfish.