Growth morphometry, muscle structure and expression of myogenic factors in a model of transgenic fish (Danio rerio) for growth hormone (GH)
Growth hormone (GH) is produced and secreted by the adenohypophysis with the main effect of promoting somatic growth in vertebrates. In the present study, growth morphometry, muscle tissue structure and expression of myogenic factors were analyzed in male and female individuals of the F0104 strain of transgenic zebrafish for GH. The morphometric indices suggest that the excess of circulating GH is promoting an anticipation of the sexual maturation age. Also, a lower condition factor was observed in transgenics of both sexes (P <0.05) and a change in morphometric patterns over time evident in transgenic males. Multivariate analyzes showed two distinct phenotypes of transgenic males. The first resembles transgenic females (large size and low condition factor),and the second to non-transgenic males (smaller size and low condition factor). This heterogeneous growth pattern in transgenic males can be explained by the change in the GH secretion profile associated with individual genetic variability in response to the excess of circulating GH. Histological analyzes have shown that transgenics show marked muscle hypertrophy when compared to non-transgenics, with transgenic females being more hypertrophic than transgenic males. The expression of genes related to muscle growth showed that the muscle hypertrophy observed in transgenics is independent of insulin-like growth factor I (IGF-I). Additionally, in transgenic males a significant induction in the expression of myogenin was observed,indicating that this protein may be mediating, at least in part, hypertrophic growth in this group. Gene expression also showed an induction of α-actin in males only, regardless of transgenics. However, there was no change in the total muscle protein content. Within the context of the results obtained in the present study, it was clear that the excess of GH in F0104 transgenic fish is probably promoting early sexual maturation, IGF-I-independent muscle hypertrophy, and heterogeneous growth in transgenic males due to alteration of the hormone secretion pattern as an effect of the constitutive expression of the transgene. This model is an interesting tool for the study of fish with limited growth.