The importance of trehalose metabolism for proper Drosophila immune response during parasitoid infection
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The primary circulating sugar in insects is trehalose, which is usually present in hemolymph in much higher concentration than glucose. Trehalose is a non-reducing sugar, which can be rapidly cleaved to two glucose molecules by the trehalase enzyme. In Drosophila, there are two versions of trehalase, cytoplasmic and secreted. The secreted version is used to maintain circulating glucose, by hydrolyzing trehalose, at a stable level and appears to be part of systemic regulation of carbohydrate metabolism. However, this systemic regulation cannot distinguish the supply of different organs. The activated immune system becomes privileged in the acquisition of nutrients, and this raises extremely interesting question of whether activated insect immune cells (hemocytes) favor direct uptake of trehalose and conversion to glucose by cytoplasmic trehalase, thereby being independent of systemic regulation. Therefore, we test the role of trehalose and its metabolism during the immune response. Experiments with 13C-labeled trehalose show that hemocytes during parasitoid infection increase the uptake of trehalose, which is partially metabolized by glycolysis but mainly by the pentose phosphate pathway. In addition, transcript-specific expression analysis, immune cell-specific RNAi and mitotic recombination have shown us the importance of cytoplasmic trehalase for immune cells. The absence of the trehalase enzyme, specifically in hemocytes, significantly reduces resistance to parasitoid. These results are in agreement with the privileged access of immune cells to nutrients during infection, which is required for efficient resistance, and demonstrate an alternative evolutionary strategy for such a privileged access in insects.