Tilapia glycoprotein hormone α subunit: cDNA cloning and hypothalamic regulation

The cDNA encoding the glycoprotein α (GPα) subunit of tilapia (Oreochromis mossambicus) was partially cloned using RACE-polymerase chain reaction (PCR) technique. The amplified cDNA was found to be 583 bases long, and to consist of a portion of the signal peptide, the full sequence encoding the mature peptide (94 amino acids) and the 3′ untranslated region. Northern blot analysis revealed a single band of approximately 600 bp. Alignment of the deduced amino acids of the mature protein showed that the tilapia GPα subunit shares more than 80% identity with that of other perciform fish (i.e. striped bass, sea bream and yellowfin porgy) and less than 70% with that of more taxonomically remote fish and other vertebrates. Exposure of dispersed tilapia pituitary cells to salmon gonadotropin-releasing hormone (sGnRH) elevated GPα mRNA levels via both PKC and cAMP-protein kinase A (PKA) pathways. The transcript levels were also regulated by pituitary adenylate cyclase activating polypeptide (PACAP) and neuropeptide Y (NPY), both acting through PKC and PKA pathways. Moreover, a combined treatment of PACAP or NPY with GnRH seems to have an additive effect on the GPα subunit gene transcription. These results suggest that in tilapia the expression of GPα subunit is regulated by GnRH mainly via PKC and PKA pathways. Furthermore, PACAP and NPY can elevate the GnRH-stimulated GPα subunit transcription and can directly affect the subunit mRNA levels, via the same transduction pathways.