LC3-associated endocytosis facilitates extracellular Tau aggregate internalization and degradation in microglia

Abstract Background Microglia, which are resident phagocytic cells in the brain, are involved in the active clearance of microbes, misfolded proteins, and cell debris, among others. In Alzheimer’s disease, microglia play a pivotal role in clearing extracellular amyloid-β (Aβ) plaques and intracellular Tau aggregates from the brain. Microglial cells have several mechanisms for Tau internalization, including macropinocytosis, heparan sulfate proteoglycans (HSPGs), dynamin- dependent endocytosis, and receptor-mediated endocytosis. Internalized Tau seeds either undergo proteasomal or lysosomal degradation or are exocytosed into the extracellular space via exosomes. Microtubule-associated protein 1 light chain 3 (LC3)-associated endocytosis (LANDO) is a recently discovered microglial mechanism for an effective clearance of Aβ aggregates and alleviates neurodegeneration in murine Alzheimer’s disease. Several microglial receptors are reported to be involved in misfolded Aβ and Tau aggregate internalization such as triggering receptor expressed on myeloid cell 2 (TREM-2), P2Y purinoceptor 12 (P2Y12R), C- X3-C motif chemokine receptor 1 (CX3CR1), etc. Methods In this study, we report the LANDO of Tau monomers and aggregates by murine microglial cells, which are further degraded by lysosomal fusion. LANDO of extracellular Tau is demonstrated by several biochemical and cell-biology studies including western blotting, fluorescence and confocal imaging of microglial cells. Results We analyzed microglial activation by measuring the upregulation of Ionized Calcium- binding Adapter Molecule 1 (Iba-1) expression. Later, we demonstrated the LANDO of human full-length Tau species, where LC3 colocalizes with internalized Tau, followed by lysosomal degradation by microglia. The accumulation of internalized Tau in the perinuclear region of the cell in the presence of chloroquine supports phagolysosomal fusion and degradation. Conclusion Hence, we concluded that microglia internalize extracellular Tau species via LANDO, which further undergoes degradation via the lysosomal pathway.