Novel tools for studying the fish growth hormone axis in vivo

Growth hormone (Gh) is the main regulator of fish growth, making it vital to understand how Gh is controlled. However, studying Gh regulation has been limited by the lack of suitable fish models. We identified conserved regulatory elements in tilapia <i>gh</i> and cloned a functional promoter. By using this promoter, we generated reporter transgenic fish lines of zebrafish and tilapia with labeled somatotrophs. Our in vitro and in vivo investigation revealed that the first intron of the tilapia <i>gh</i> contains functional enhancer elements crucial for transgene expression. Tilapia transgenic lines expressing the calcium-sensitive protein GCaMP6s allow the visualization of calcium activity in somatotrophs within intact pituitary glands. Network plots derived from the cell coactivation revealed distinct patterns of activity, showing cell groups with synchronized behavior, suggesting the presence of functional clusters within the somatotroph population. Furthermore, our zebrafish reporter lines revealed higher spontaneous calcium activity of Gh cells in juveniles compared with adults, suggesting increased somatotroph activity in rapidly growing young fish. Overall, our study demonstrates the functionality of tilapia and zebrafish Gh transgenic lines for investigating Gh cell activity in vivo. Combined with previously established methodologies, these transgenic lines serve as valuable tools for studying the regulation and release of Gh in the largest group of living vertebrates.<b>NEW & NOTEWORTHY</b> This study describes the first transgenic reporter tools for real-time imaging of calcium activity and reporter gene expression in fish growth hormone cells. By identifying critical enhancer elements in <i>gh</i>, we successfully targeted somatotroph cells and performed functional analysis of their behavior within the physiological context. These tools could provide valuable insights into growth hormone (Gh) regulation, with important implications in aquaculture and our understanding of the evolution of vertebrate endocrine systems.