α- Linoleanic acid modulates phagocytosis of extracellular Tau and induces microglial migration by actin-remodeling
Authors: Desale Smita Eknath , Chinnathambi Subashchandrabose 
Published: April 2020

Abstract
Background Seeding effect of extracellular Tau species is an emerging aspect to study the Tauopathies in Alzheimer’s disease. Tau seeds enhance the propagation of disease along with its contribution to microglia-mediated inflammation. Omega-3 fatty acids are known to exert the anti-inflammatory property to microglia by modulating cell membrane compositions. The immunomodulatory function of omega-3 fatty acids exerts anti-inflammatory property to microglia. Owing to the imparted anti-inflammatory nature enhance phagocytosis and increased migration property has been observed in microglia. The dietary omega-3 fatty acids are found to change the lipid composition of the cell membrane that predominated many signaling cascade and by modulating specific receptor response. Thus the omega-3 fatty acids influence microglial response in Tauopathy.Method N9 microglia cells were exposed to extracellular full-length Tau monomer and aggregates along with ALA (α- Linolenic acid) to study the internalization of exposed Tau. The degradation of internalized Tau studied with the endosomal markers Rab5 and Rab7. The final degradation step in phagocytosis has been studied with LAMP-2A as lysosomal markers. The changes in the rate of migration of microglia were assessed by wound-scratch assay along with Microtubule organizing center (MTOC) reorientation were studied after exposure of Tau and ALA as the property of highly migratory microglia. The role of actin in phagocytosis and migration was observed with the study of actin structures lamellipodia, filopodia, and membrane ruffling. The formation of extensive actin branching in lamellipodia and membrane ruffling was studied with the help of ARP2/3 complex for nucleating actin network.Results The increased phagocytosis of extracellular Tau monomer and aggregates has been observed upon ALA exposure to microglia cells. The intracellular degradation of internalized Tau species was targeted by early and late endosomal markers Rab5 and Rab7. The increase levels of LAMP-2A and colocalization with internalized Tau indicated the degradation via lysosome. These results indicate the degradation of internalized Tau species in the presence of ALA instead of getting accumulated in the cell. The enhanced migratory ability of microglia in the presence of ALA induces the MTOC repolarization and reduces the nuclear-centrosomal axis polarity and favorable anterior positioning of MTOC. The increased migration also complememnted with the enhance actin remodeling through lamellipodia, filopodia and membrane ruffles formation along with Iba-1 protein. The high desnity of ARP2/3 complex at the leading ends of migratory microglia confirmed the extensive branching of actin filaments on ALA exposure.Conclusions Tau seeds greatly contributes to the spread of disease, one way to reduce the spreading is to reduce the presence of extracellular Tau seed. Microglia could be influenced to reduce extracellular Tau seed with dietary fatty acids. Our results suggest that dietary fatty acids ALA significantly enhance phagocytosis and intracellular degradation of internalized Tau. The actin dynamics and enhanced migration supports the phagocytosis process. Our approach provides the insights of beneficial role of ALA as anti-inflammatory dietary supplement to treat AD.