Molecular cloning and characterization of secretogranin II in the catfish Heteropneustes fossilis: Sex and seasonal brain regional variations and its gonadotropin regulation.

Secretoneurin (SN) is a novel functional peptide derived from secretogranin II, a protein belonging to the class of chromogranins. Catfish Heteropneustes fossilis is an economically important species of the Asian subcontinent and holds an important place in reproductive physiology study. In the present study, the full-length cDNA encoding secretogranin IIb (SgIIb) was cloned from the brain of catfish H. fossilis. Sequence analysis showed that a 33 amino acid SN peptide (SNb) is present in SgIIb proprotein. The full-length sequence of SgIIb is 2912 bp with open-reading frame 1761 bp long. The 5'UTR is 681 bp long upstream and the 3'UTRis 470 bp long downstream. The ORF encodes 586 amino acid residues comprising signal peptide (24 amino acids) and SNb (33 amino acids), EM66 (66 amino acids). Catfish SgIIb showed 88% nucleotide homology and 90% protein identity with Ictalurus punctatus. Tissue expression showed that it is highly expressed in the brain among all tissues studied. In situ hybridization and quantitative real-time PCR showed significant brain regional distribution with highest transcript abundance in the pituitary. In ovary tissue SgIIb transcripts were localized in the granulosa layer. In the brain, hCG administration stimulated expression highly at 16 h. In ovary, hCG treatment under in vivo and in vitro condition decreased SgIIb expression at all doses. Thus, in the present study in situ localization of SgIIb mRNAs in different regions of brain and pituitary, and ovary suggest its putative role in regulating hypothalamic-pituitary-gonadal (HPG) axis and also suggest that the SgIIb expression pattern in the brain and ovary could be related to reproductive activity and may be involved in the neuroendocrine role of SgIIb in the catfish H. fossilis.