Discussion
Belonging to the fatty acid binding protein gene family, fabp1mainly expressed in liver and intestine with critical functions in intracellular transport and fatty acid uptake (Wang et al., 2015). In intestine, Fabp1 binds to endoplasmic reticulum, participates in budding of the pre-chylomicron, and transfers the pre-chylomicron transport vesicle to Golgi (Cifarelli & Abumrad, 2011). Revolving in the fatty acid dependent activation of PPAR, Fabp1 also plays an important role in regulation of lipid metabolism and cellular signaling pathways (Georgiadi & Kersten, 2012). In addition, Fabp1 is proved to be related to obesity (Atshaves et al., 2010), and overexpression offabp1 may cause diseases, such as nonalcoholic fatty liver disease (NAFLD), that are relevant to excessive accumulation of fat (Pi et al., 2019). In conclusion, the tandem triplication of fabp1gene in the striped catfish genome may make a great difference on fatty acid trafficking and storage in this fish species.
PPAR signaling pathway is an important regulatory mechanism offabp1 . With different PPAR-selectivity, various promoter regulatory elements (PPRE) may lead to differential control of duplicated fabp1 genes (Laprairie et al., 2016; Taylor & Raes, 2004). Compared to the fact that PPARβ/δ is ubiquitously expressed, the expression of PPARα and PPARγ varies in different tissues. PPARα is highly expressed in the liver (Kim et al., 2015); PPARγ is divided into two isoforms (PPARγ1 and PPARγ2), in which PPARγ1 is expressed in many tissues, whereas PPARγ2 is exclusively expressed in adipose tissue (Desvergne & Wahli, 1999). In addition, PPARα and PPARγ were proved to selectively bind to the PPREs in the liver and the adipose tissue, respectively (Shimizu et al., 2004). A recent study (Venkatachalam et al., 2017) reports that retention ofiLBP genes in the zebrafish genome may be caused by subfunctionalization of PPREs in the iLBP promoters. In our present study, we detected PPAR-selectivity for the PPREs offabp1-1 , -2 and -3 in the striped catfish genome by silico analyses, and all putative PPREs were predicted to be PPARγ-selective, suggesting similar regulation of the three fabp1isotypes based on the putative PPAR regulatory mechanism. In addition, subfunctionalization of PPRE was not detected in the promoters of various fabp1 isotypes, implying that there may be some other factors for the retention of fabp1 genes in the striped catfish genome.
According to the duplication-degeneration-complementation (DDC) model, there are three possible consequences of duplicated genes. First, with the accumulation of mutations, one or more copies of the duplicated genes will be lost or become non-functional (Force et al., 1999). Second, some of the duplicates may acquire a novel function (Lynch & Conery, 2000). Third, the subdivision of functions between the duplicated genes will occur (Taylor & Raes, 2004). From our RNA-Seq data (Table 3), the mRNA levels offabp1-1and -3 were detectable while fabp1-2 was not. It is possible that fabp1-2 lost the original function, although this proposal is still waiting for more investigations. Although both fabp1-1 and -3 were predicted to be PPARγ-selective, the R126T mutation of Fabp1-3 may potentially affect the fatty acid binding capacity of the protein, suggest that subfunctionalization may be differential betweenfabp1-1 and -3 .
With great economic significance, striped catfish has become an important aquaculture species for worldwide consumption. In the process of breeding and trading, the high-fat trait of striped catfish has both advantages and disadvantages. On one hand, the good ability of fat storage may enhance adaptability of striped catfish to a harsh condition with insufficient food sources, and these striped catfishes with a high fat content are good resources for fish-oil extraction (De Silva & Soto, 2009). On the other hand, during the preparation of fish products for current consumer markets, visceral mass is usually discarded as waste instead of being utilized, once they are commonly rich in fat (De Silva et al., 2006). The increasement of fat content in striped catfish fillet, therefore, may affect its economic value (Islami et al., 2014). Our detailed investigation on fabp1 genes may support further molecular breeding to select individuals with low fat content.
Fatty-acid trafficking and storage in cells are complicated and dynamic, with a combined regulation by multiple genes. Meanwhiles, the fatty acid storage is also influenced by environment, especially temperature can affect lipid content and fatty acid composition of fish oil (Hemung et al., 2010). Since the high-fat trait of striped catfish is related to various factors, in-depth researches are necessary.