Role of microglial lysosomes in amyloid-A-beta degradation

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We have characterized a process used by phagocytic cells to digest objects that are too large to be phagocytosed. We call this process digestive exophagy, and we have demonstrated its role in digestion of large aggregates of lipoproteins in atherosclerosis and also in the digestion of dead adipocytes. We hypothesize that microglia use digestive exophagy to trim and partially degrade β-amyloid. This may be the mechanism by which microglia limit the expansion of β-amyloid plaques. Digestive exophagy involves the formation of a tight seal on the object to be degraded (a lysosomal synapse), secretion of late endosomes and lysosomes (LE/Ly) into the lysosomal synapse, and acidification by V-ATPase proton pumps. We have evidence that primary microglia in culture form lysosomal synapses upon contact with β-amyloid. In Aim 1 we will continue to characterize this. First, we will confirm our preliminary data that microglia create acidified, F-actin-rich contact regions when they interact with β-amyloid and that they secrete lysosomal contents into these areas. We will then quantify time-dependent degradation of amyloid in these compartments. We will determine if anti- Aβ IgG increases digestion of the plaques by microglia. We will also use optical and electron microscopy to examine evidence for digestive exophagy in AD model mice. We have partially characterized the signaling mechanisms required for digestive exophagy by macrophages, and we will use this as a model for studies in microglia. We note that the signaling pathways we described for digestive exophagy overlap signaling pathways implicated in genetic studies of Alzheimer’s disease. In Aim 2 we will explore the effects of some of these mutations on digestive exophagy. These will include Trem2R47H, Dap12-/-, and Trem2-/- mouse microglia. We have shown that pharmacological manipulation of PI3-kinase and Akt as well as other signaling molecules alter digestive exophagy in macrophages, and we will extend our preliminary studies on the effects of these drugs on digestive exophagy of amyloid by microglia. Microglia have been proposed to play a role in the spread of neuropathic fibrils, including tau and undigested Aβ fibrils. LE/Ly secretion, which is an essential component of digestive exophagy, may contribute to the spread of partially digested fragments of tau filaments or Aβ fibrils out of microglial cells, and we will examine this in the third Aim. Specifically, we will determine whether the enhanced LE/Ly secretion when microglia contact β-amyloid leads to enhanced secretion of previously endocytosed fibrils. We will then test the effects of treatments described in Aims 1&2 on endocytosis and digestion of fibrils and their secretion upon contact with amyloid. In our tissue culture studies, we will use primary mouse microglia and human iPSC-derived microglial cells.