Strategies for sterols (or surrogates) acquisition in protists

UTTARO, ANTONIO D.

Santa Fe

Congreso; XXVIII Reunión anual de la SAP; 2016

Sterols are the hallmark of eukaryotic cells, being essential for several physiological processes. They play a key role in regulating membrane fluidity and permeability and participate in different signaling pathways as precursors of steroid hormones and vitamins. Sterols are also fundamentals in the formation of lipid rafts, involved in concerted functions such as cell-cell recognition, adhesion, communication and phagocytosis. Several strategies were selected in Nature for sterol acquisition, with de novo synthesis being present in numerous organisms like mammals, fungi, protozoa and plants. However, the synthesis of sterols is energetically expensive and oxygen dependent. It makes anaerobic parasites absolutely dependent of cholesterol uptake from their host. Anaerobic non-parasitic protozoa and some derived extant aerobic ones adapted to synthesize triterpenoids in one step, by direct cyclization of the squalene, without need of oxygen. It is the case of tetrahymanol in Tetrahymena ciliates, a surrogate of cholesterol with similar structural functions. Other ciliates like Paramecium, are absolutely dependent of phytosterols taken from their phagocytosed diet. Interestingly, Tetrahymena are also able to take up sterols from the media, which modify by dealkylation and desaturations, and incorporate to their membranes. Additionally, sterols represent an environmental signal which is sensed by Tetrahymena and transduced for the complete repression of tetrahymanol synthesis and induction of the enzymes involved in sterol bioconversion. Helminths and insects are also auxotrophic for sterols, which are precursors for the synthesis of essential hormones in these organisms. Trypanosomatids synthesize de novo ergosterol, which has sparking functions even in the mammalian stages, although the host cholesterol is avidly incorporated by the parasites, with concomitant reduction of ergosterol biosynthesis. The aim of this conference is to describe our contribution in the understanding of sterol bioconversion and tetrahymanol repression in Tetrahymena thermophila. In addition, I will describe a novel strategy for sterols acquisition detected in Capsaspora owczarzaki, a symbiont of the freshwater snail Biomphalaria glabrata (the invertebrate vector of Schistosoma sp.). This filasterean protozoa shows features that resemble both fungi and animals, having the capacity for the synthesis of 7-dehydrocholesterol or ergosterol along its life cycle.