Genetic regulation of muscle growth: effects of overexpression of the growth hormone receptor (GHR) in a transgenic fish model
Aquaculture has grown significantly in recent decades due to the increase in the demand for fish in the world and the stagnation of the fishing sector. However, this growth depends on the development of new technological packages aimed at increasing productivity. An alternative is genetic manipulation (transgenics), with growth hormone (GH) being the main target of research with transgenic fish. However, it is proven that the excess of GH causes a series of adverse effects due to its pleiotropic action. The activation of the somatotrophic axis in a tissue-specific way and independent of the excess of circulating hormone can overcome these problems. In this context,the objective of this thesis was to overexpress the GH receptor gene (GHR) in the skeletal muscle tissue of the zebrafish (Danio rerio) and to study the effects of this manipulation on the mechanisms involved in the gene regulation of muscle growth. The stable transgenic strain obtained expresses GHR specifically in muscle tissue 100 times more than non-transgenic ones. These transgenics did not show a significant increase in growth, probably due to the decrease in the expression of growth factor type insulin I (IGF-I). This drop was probably related to the action of the main modulators of GH signaling (SOCS1 and 3), which were increased in transgenics. In addition, a decrease in the expression of the main structural muscle proteins (Acta1, myhc4 and mylz2) was observed,which explains the absence of hypertrophy in transgenics. On the other hand, the increase in the expression of the main myogenic regulatory factors (myod, myf5 and myog) explains the hyperplasia observed in histological analyzes. To verify how the overexpression of GHR activated the transcription of myogenic regulatory factors (MRFs) and, consequently, hyperplasia, the possible mechanisms involved in this process were studied. Among these, both the proliferative pathway (MEK/ERK) and the pathway related to protein synthesis (PI3K/Akt), had no change in the expression of their genes. However, an increase in the expression of transport proteins to the nucleus (importins 1, 3 and 1) was observed, and it can be concluded that the activation of MRFs is related to the transport of GHR to the nucleus of muscle cells. Thus,it can be concluded that hyperplasia and hypertrophy follow two distinct intracellular signaling pathways, both triggered by GH, but regulated by different mechanisms.