Autophagy and phagocytosis: functional crosstalk in microglia
Laboratory of Glial Cell Biology (Achucarro)
Autophagy and phagocytosis are conserved cellular processes in charge of maintaining cell and tissue homeostasis, through the recycling of cytoplasmic content through autophagy, and the degradation of extracellular components, such as apoptotic cells, through phagocytosis. Both autophagy and phagocytosis are part of the endosomal pathway that converges in the lysosome, suggesting a functional relationship between them. In microglia, the professional phagocytes of the brain, the phagocytic function is well characterized but, however, the role of autophagy is yet unclear. Thus, we first studied the importance of basal autophagy using the selective inhibitor MRT68921, which targets ULK1, a key protein of the autophagy initiation complex. MRT reduced microglial survival and microglial phagocytosis of apoptotic cells, suggesting a key role of basal autophagy in microglial physiology. We then studied the role of autophagy induced by stress conditions such as nutrient deprivation. We hypothesized that autophagy dysfunction could be involved in the phagocytosis impairment we had observed in a in vivo model of stroke, where a generalized energetic failure occurs. To demonstrate this hypothesis, we used an in vitro model of oxygen and nutrient deprivation (OND), in which phagocytosis, and particularly the degradation of apoptotic cells, was impaired. This reduction was related to an increased lysosomal pH, possibly as a consequence of alterations in energy-dependent proton pumps that lead to a deficient enzymatic activity. We hypothesized that the energetic failure would also affect autophagy as part of the endosomal pathway. To assess the changes induced by OND, we used electron microscopy and Western blot and observed a remodeling of the autophagy and lysosomal compartments, that resulted in an increase of autophagy-like structures and a decrease in the lysosomal vesicles. In order to revert the OND induced autophagy changes and the phagocytosis impairment, we tested the mTOR inhibitor, rapamycin, both in vitro and in vivo, to restore the autophagy flux and recover the phagocytic activity. Thus, the microglial phagocytic potential opens a novel approach to accelerate the recovery of the ischemic brain by modulating its interrelation with the autophagy pathway.
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