Hormonal and nutritional regulation of muscle growth and development in gilthead sea bream (Sparus aurata) = Regulación endocrina y nutricional del crecimiento y desarrollo muscular en dorada (Sparus aurata)

Abstract

In the Mediterranean region the increasing of competitiveness of the aquaculture industry is of great interest, especially in the case of gilthead sea bream. With the ultimate purpose of optimizing the aquaculture production of this species, the overall goal of the present thesis is to increase the knowledge on the regulation of somatic growth, muscle development and flesh quality. This work includes in vitro approaches to investigate the nutritional (i.e., amino acids, AA) and endocrine (i.e., growth factors) regulation of myogenesis, the role in this process of the main endogenous proteolytic systems, as well as of β2-agonists. On the other hand, by means of in vivo studies, the role of GH/IGF axis’ molecules mediating the positive effects of sustained and moderate swimming in growth, as well as the muscular adaptation to exercise in fingerlings of gilthead sea bream was evaluated. In addition, the effects of a sustained-release formulation of recombinant bovine GH (rBGH) on somatic growth, GH/IGF axis and muscle histology was also analyzed. Regarding the first block of studies, the obtained results revealed the important role of endocrine and nutritional factors, and their crosstalk, controlling in vitro myogenesis. Moreover, the nutritional regulation of protein synthesis through TOR signaling pathway was also demonstrated. Besides, the results have shown that the gene expression of calpains is in general higher at the early proliferative stages, whereas cathepsins and the ubiquitin-proteasome (UbP) molecules are more important at the early differentiation phase of in vitro myogenesis. Nevertheless, calpains and UbP members compared to cathepsins are differentially regulated in response to AA availability, supporting the relevance of nutritional factors in muscle development. The in vitro studies also demonstrated that β2-adrenoceptor agonists, mainly salmeterol, can activate both the PKA and PI3K/AKT/TOR signaling pathways. This activation produces an increase in cell proliferation and lipolysis in gilthead sea bream myocytes, therefore, β2-agonists should be considered an effective strategy to stimulate muscle growth by hyperplasia while reducing fat content, thus, to improve flesh quality in fish. Regarding in vivo studies, both the exercise and the rBGH treatment appeared as different useful strategies to stimulate the GH/IGF axis and to enhance muscle growth. Further, the excess of rBGH revealed the existence of systemic and local regulatory mechanisms in this endocrine system to achieve an accurate control of somatic growth. In addition, the obtained results demonstrated that an increased ratio of IGF-I/GH in plasma is an excellent indicator of growth capacity in this species. Also, a combination of enhanced expression of hepatic igf-1 with muscular GH receptor, ghr-1 has appeared as another molecular marker of fish growth potential. Further, the results demonstrated that growth induced by rBGH is a process that combines muscle hypertrophy and hyperplasia, as well as bone development, to maintain the adequate muscle-skeleton balance that will guarantee a quality product with a proper morphology. Additionally, sustained swimming also induced an improvement of muscle quality characteristics, since in response to exercise the protein turnover (cathepsins and UbP) was increased, which in parallel to structural changes and the new vessels formation, are required for hyperplastic growth and enhanced textural parameters. Overall, the works within this thesis demonstrate that it is possible to improve somatic growth and flesh quality in gilthead sea bream, being the moderate and sustained swimming and the adequate balance of nutrients (e.g., AA) in the diet, besides other potential muscle growth stimulators (such as salmeterol), very promising strategies to boost aquaculture production.

El objetivo general de esta tesis es profundizar en la regulación del crecimiento somático, el desarrollo muscular y la calidad del filete de dorada, para contribuir así a un aumento de competitividad de la acuicultura mediterránea. Este trabajo incluye experimentos in vitro sobre la regulación de la miogénesis a nivel nutricional (aminoácidos) y endocrino (factores de crecimiento). Se estudió también el papel de los principales sistemas proteolíticos y los efectos de los agonistas β2-adrenérgicos en la miogénesis, síntesis de proteínas y lipólisis. Por otro lado, mediante estudios in vivo se analizó el papel de las moléculas del eje GH/IGF mediando los efectos positivos de la natación moderada y sostenida en el crecimiento, así como la adaptación muscular al ejercicio en alevines. Además, se analizaron los efectos de una formulación de liberación sostenida de GH recombinante bovina (rBGH) sobre crecimiento, el eje GH/IGF y la histología muscular. Con respecto al primer bloque de estudios, los resultados revelaron la importancia de los factores endocrinos y nutricionales, así como su interconexión, en el control de la miogénesis. También se demostró que los agonistas β2, especialmente salmeterol, pueden producir un aumento en la proliferación celular y la lipólisis, y por lo tanto, los agonistas pueden considerarse como una estrategia interesante para estimular el crecimiento por hiperplasia, reducir el contenido de grasa, y mejorar la calidad del producto. Con respecto a los estudios in vivo, tanto el ejercicio como el tratamiento con rBGH resultaron ser métodos útiles para estimular el eje GH/IGF y mejorar el crecimiento. Los resultados demostraron que un mayor ratio de IGF-I/GH en plasma es un excelente indicador de la capacidad de crecimiento en esta especie. Además, la combinación incrementada de expresión de igf-1 hepático con ghr-1 muscular ha resultado ser otro marcador del potencial de crecimiento. En general, esta tesis demuestra que es posible mejorar el crecimiento y la calidad de la carne en dorada, siendo la natación moderada y sostenida y el equilibrio adecuado de nutrientes en la dieta (AA), además de otros estimuladores potenciales del crecimiento muscular (como el salmeterol), estrategias muy prometedoras para impulsar la producción acuícola.