Combinación de metodologías ómicas y mea-ómicas para estudiar el efecto de la suplementación de selenio y su interacción con la microbiota intestinal

Abstract

Selenium (Se) is an essential element for human health and crucial for body functions. It is naturally present in the environment in both organic and inorganic forms. In the body, it is incorporated in the form of selenocysteine (Sec) during the biosynthesis of the polypeptide chain, deriving to selenoproteins. The biological effects of selenium are largely mediated by selenoproteins, as it has been recognized to be a key factor for the immune system, thyroid hormone metabolism, and neurological development, among others. The main source of Se in humans is the diet, being Brazil nuts, fish, and meat foods rich in this element. A deficiency of Se can cause various pathophysiological conditions such as heart disease, neurological disease, cancer, infertility, and inflammatory processes. For this reason, sometimes it is necessary a Se supplementation directly through the consumption of food supplements. From the point of view of human reproduction, Se is important for normal testicular development and spermatogenesis. At the neurological level, Se protects the brain from oxidative stress through selenoproteins, which in turn have numerous functions in the central nervous system (CNS) such as the production of neurotransmitters or neuronal signalling. Se is a well-known antagonist against many toxic heavy metals such as mercury, arsenic, or cadmium. On the other hand, Se interact with the gut microbiota, which acts as a barrier to absorption, interferes in the metabolization of Se species, and certain genera incorporate Se into their selenoproteins. . The intestinal microbiota constitutes the set of microorganisms (bacteria, viruses, fungi, and archaea) present in the human body. The intestinal microbiota is involved in immunological, pathophysiological, and metabolic functions of the host and interacts with other organs and tissues. Recently, the interaction between the intestine and the brain, known as the gut-brain axis, has been gaining special interest, demonstrating that numerous neurological problems such as Alzheimer's or depression are related with alteration of the gut microbiota. Simultaneously, the gut microbiota is related to the production of sexual hormones and testicular dysfunctions, having a great impact on male reproduction through the gut microbiota-hypothalamic-pituitary-gonadal axis. During the development of this PhD Thesis, different analytical strategies have been carried out to study the importance of Se, mainly focused on the role of selenoproteins, against toxic metals such as cadmium (Cd) and the modulation of the microbiota through the gut microbiota-brain and hypothalamic-pituitary-gonadal axes. The evaluation of the protective role of Se against Cd has been carried out by an exposure assay in the liver cell line HepG2, as it is one of the most metabolically active organs and on which previous in vivo studies employing Mus musculus mice have already been carried out. To this end, the cells were exposed for 24h to different concentrations of Cd in the form of CdCl2 and in combination with Se in the form of selenomethionine SeMet. Cell viability and the concentration of Cd and Se were determined both in the cells and in the culture media. The content of selenoproteins and selenometabolites in HepG2 cells was analyzed using the column switching technique and isotopic dilution of non-specific species. To determine the impact of Se supplementation and the intestinal microbiota on the selenoproteome of brain and testis tissues, an animal experiment using Mus musculus mice was carried out. Mice were pre-treated with antibiotics to obtain a depressed microbiota and subsequently treated with a Se-supplemented diet. The intestinal microbiota has been characterized by metataxonomic analysis. In both tissues, brain, and testes, a selenoprotein extraction method has been optimized for subsequent analysis. Testicular damage after microbiota depression and the effect of Se supplementation were determined by histopathological analysis of the tissues. Given the importance of the gut-brain axis, the study of brain samples has been completed with an untargeted metabolomic analysis using two analytical platforms covering a wide range of metabolites. In addition, the expression levels of the entire mouse selenoproteome have been analyzed by transcriptomic analysis. Correlation analysis between gut microbiota abundance, selenoproteins, and metabolites have shown associations with essential bacteria for reproduction and neurodevelopment. The analytical techniques applied in this Doctoral Thesis has provided new information about the biological response of the organism to Se supplementation, through changes in selenoproteins both in vitro and in vivo models. Likewise, studies of the microbiota have made it possible to delve into the mechanisms of interaction between the gut-brain and intestine-testis.