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Adiponectin belongs to the CTRP C q TNF related
Adiponectin belongs to the CTRP (C1q/TNF-related protein) superfamily. Except for adiponectin, 15 additional members have been identified (CTRP1-15) (Schaffler and Buechler, 2012). Of all the CTRPs, CTRP9 has the highest identity to adiponectin in globular domain (Wong et al., 2009). The structure of CTRP9 is similar to adiponectin, consisting of a signal peptide, a variable region, a collagen domain and a globular domain (Wong et al., 2009). Moreover, CTRP9 can form heterotrimers, heterohexamers and hetero-oligomeric complexes with adiponectin (Peterson et al., 2009). Previous study showed that CTRP9 was mainly expressed in adipose tissue (Wong et al., 2009). However, recent studies showed that the expression of Ctrp9 was predominant in heart, in which expression levels of Ctrp9 was greater than adipocyte (Appari et al., 2017, Breitbart et al., 2013). In overexpressed CTRP9 mice, serum insulin and glucose levels were markedly reduced (Peterson et al., 2013, Wong et al., 2009). Ctrp9-KO (knockout) mice increasedfoodintake, due to upregulated expression of hypothalamic orexigenic neuropeptide (NPY) to control energy intake (Wei et al., 2014). In muscle, CTRP9 stimulates AMPK phosphorylation and increases fatty V5 Epitope oxidation and mitochondrion-specific genes expression, leading to muscle fat oxidation and mitochondrial biogenesis (Yang et al., 2016). CTRP9 may exert vasculoprotective effects through the AdipoR1/AMPK/eNOS dependent/NO mediated pathway (Zheng et al., 2011). In addition, CTRP9 might protect high glucose-induced endothelial oxidative damage via AdipoR1-SIRT1-PGC1α signaling pathway (Cheng et al., 2016). The presence of Adiponectin in fish has been previously reported in zebrafish and rainbow trout, and the function of Adiponectin may lead to an increase in glucose and fatty acid uptake (Nishio et al., 2008, Sanchez-Gurmaches et al., 2012). The reports of Ctrp9 in fish was only in lampreys (Eudontomyzon morii) and grouper (Epinephelus coioides). The report of Ctrp9 in lampreys shows that the Ctrp9 is invovled in immune response (Chen et al., 2016). In grouper, the Ctrp9 plays a role in the regulation of metabolism and food intake (Yang et al., 2017).
Although adiponectin has been documented to be involved in regulation of reproduction (Kawwass et al., 2015, Lagaly et al., 2008, Rodriguez-Pacheco et al., 2007, Wen et al., 2012, Wen et al., 2008), the functional role of CTRP9 in reproduction remains to be established, especially in teleost. To address this issue, Nile tilapia (Oreochromis niloticus) was employed as a model in this study. After the molecular cloning of Ctrp9, Adipor1 and Adipor2, the effects of CTRP9 on the mRNA expression of genes related to the HPG (Hypothalamic–pituitary–gonadal) axis as well as the levels of gonadal steroid hormones in serum were studied.
Materials and Methods
Results
Discussion
In the present study, Ctrp9 and two adiponectin receptors were identified from Nile tilapia. The CTRP9 gene is located on chromosome 13q12.12, and the length is 12.6 kb in human (Wong et al., 2009). One isoform of CTRP9 is found in the majority of species whose genomes have been sequenced, while human and primates possess two isoforms, namely CTRP9A and CTRP9B (Peterson et al., 2009). In our study, only one isoform of Ctrp9 was cloned in tilapia. Analysis of the structure showed that tilapia Ctrp9 contained a signal peptide, a short variable region, a collagen domain and a N-terminal globular domain. The structure of tilapia Ctrp9 was similar to the counterparts of fish and mammals (Chen et al., 2016, Schaffler and Buechler, 2012). A number of phosphorylation sites were located in tilapia Ctrp9, while the glycosylation site was not predicted. Wong and colleagues demonstrated that post-translational modifications were an indispensable process in CTRP9, including prolines and lysines hydroxylated and glycosylated (Wong et al., 2009). In addition, a recent study demonstrated that several phosphorylation sites existed in the grouper Ctrp9 protein (Yang et al., 2017). Sequence alignment showed that the identity of Ctrp9 was high between tilapia with other fish in perciformes. Phylogenetic analysis revealed that tilapia Ctrp9 grouped with the clade of fish Ctrp9. Based on these analyses of amino acids sequences, the sequence we obtained is the putative Ctrp9 of tilapia. Furthermore, two Adipors were also identified from tilapia in our study. The amino acids sequences analysis showed that the Adipor1 and Adipor2 were seven transmembrane proteins. Previous studies indicated that the AdipoRs belong to the PAQR superfamily, and the structure of AdipoRs were contrary to the conventional GPCRs (Kadowaki and Yamauchi, 2005, Tang et al., 2005). The predicted results indicated that the N-terminal of tilapia Adipors were intracellular and C-terminal were extracellular. In the PAQR superfamily, all the members contain a Hly III-like domain, and the Hly III-like domain encodes the seven transmembrane helix bundle (Qin et al., 2014, Tang et al., 2005). Our study showed that Hly III-like domain exist in Adipor1 and Adipor2 protein. AdipoR1 and AdipoR2 share 66.7% identity in mouse (Yamauchi et al., 2003). In tilapia, the identity of Adipor1 and Adipor2 is 69.67%. Analysis of structure revealed that Adipors of tilapia are the PAQR members. The identity of Adipor1 and Adipor2 are more than 79% between tilapia with other fish. Phylogenetic analysis based on amino acids sequences has confirmed that tilapia Adipor1 and Adipor2 can be grouped in the clade of fish Adipor1 and Adipor2, respectively. Based on these analysis of amino acids sequences, the sequences we obtained are the prospective Adipors of tilapia.