Mecanismos centrales y periféricos sensores de ácidos grasos en la trucha arco iris como modelo de pez teleósteoimplicación en el control de la ingesta de alimento

Resumen

En el presente proyecto, usando como modelo de pez teleósteo la trucha arco iris, pretendemos caracterizar la posible existencia de un mecanismo que informe de cambios en los niveles circulantes de ácidos grasos tanto a nivel central (distintas regiones cerebrales) como periférico (cuerpos de Brockmann, hígado). Evaluaremos la presencia en dichos tejidos de metabolitos (niveles de ácidos grasos, triglicéridos, lípidos totales y malonil-CoA), enzimas (ácido graso sintetasa , 3-hidroxiacil-CoA, ATP citrato liasa, malonil-CoA descarboxilasa) y la expresión de genes relacionadas con la respuesta a cambios en los niveles de ácidos grasos en otros modelos de vertebrados. Una vez demostrada su existencia profundizaremos en sus mecanismos y abordaremos la relación del mismo con el control de la ingesta de alimento en peces.We have shown for the first time in fish, that several metabolic and molecular components of the main lipid sensing system described in mammals are sensitive to the high circulating fatty acid (FA) levels. The increase of long-chain fatty acid (LCFA) or medium-chain fatty acid (MCFA levels) in vivo or in vitro in rainbow trout are sensed in the hypothalamus, Brockmann bodies and liver, through a complex mechanism in which FA metabolism, binding to CD36 and mitochondrial activity are involved. The mechanisms are quite similar to those suggested in mammals except for the capacity of rainbow trout to detect increases not only in LCFA but also in MCFA, which could be related to a higher importance of MCFA in teleosts. Changes in the hypothalamic pathways could be related to the control of food intake since food intake was inhibited when the FA metabolism was perturbed (using FAS or ACC inhibitors) and changes in mRNA levels of specific neuropeptides such as NPY , POMC and CART were also noticed in hypothalamus. This response seems to be exclusive of the hypothalamus, since the other centre controlling food intake (hindbrain) was unaffected by treatments. The results obtained support direct FA sensing of rainbow trout hypothalamus to increased levels of oleate and octanoate through reduced potential of lipogenesis and FA oxidation, and decreased potential of K+ATP. The FA sensing through binding to FAT /CD36 and subsequent expression of transcription factors appears to be also direct but an interaction induced by changes in levels of peripheral hormones cannot be rejected. In contrast, FA sensing in liver and in Brockmann bodies is apparently indirect and be the consequence of previous hypothalamic FA sensing followed by vagal and/or sympathetic outflow or can be influenced by changes in the levels of peripheral hormones. We have obtained evidence, about the existence of a counter-regulatory response to a fall in circulating FA levels. The response is apparently associated with food intake control and the activation of HPI axis and this activation probably not related to the control of food intake through FA sensor systems but to the modulation of lipolysis in peripheral tissues to restore FA levels in plasma. This counter-regulatory response initiated in the hypothalamus, probably through changes in CRF, and this activation would arrive to peripheral tissues such as liver where metabolic changes would occur accordingly. In BB the response to a fall in circulating FA levels induce changes in several parameters compatible with the function of FA sensing systems informing about the decrease in circulating levels to match these changes with the production and release of pancreatic hormones, probably insulin. In general, the effects of insulin in the modulation of FA-sensing in hypothalamus were very scarce, and no general trend could be concluded. However, this is in contrast with the effects observed by insulin treatment on food intake, clearly suggesting a potential effect of FA on the anorectic effects of FA. In contrast, the results obtained in liver and BB clearly support the modulatory action of insulin on the FA sensing capacity of peripheral tissues like liver and BB where the responses to OL or OCT alone observed in parameters related to FA sensing were further potentiated by insulin. The changes observed in the activity of neurons involved in the production of anorexigenic and orexigenic factors ultimately leading to a further decrease in food intake are translated into the liver and BB to further potentiate the effects of FA alone. The increased levels of FA in hypothalamus and liver of rainbow trout fed with a high levels of crude lipid (HF) diet only partially activated FA-sensing systems with no changes in food intake allowing us to suggest that FA-sensing response in fish to increased levels of FA is more dependent on the presence of specific FA such as oleate or octanoate rather than to the global increase in FA. However, we also found evidences for the presence and functioning in hypothalamus of energy sensors like AMPK and proteins involved in cellular signaling like mTOR, Akt and S6. These proteins in hypothalamus and liver are generally activated in fish fed the HF diet , suggesting an enhanced response of the cellular signaling pathways to the increased availability of FA. This response, however, do not exactly coincide with changes observed in mRNA abundance of parameters that are normally related to them suggesting a complex interaction of these systems with FA-sensing and mechanisms related, including the control of food intake. As a general conclusion, the results obtained in the present PhD Thesis provide evidence for a specific role for FA (MCFA and LCFA) as metabolic signals in hypothalamus, Brockmann bodies and liver where the detection of those FA can be associated with the control of food intake and hormone release.