We described a novel mechanism for the hydrolysis of cholesteryl esters in retained and aggregated LDL (agLDL), which is the predominant form of lipoprotein in atherosclerotic lesions. Macrophages (MΦ) create tightly sealed compartments that surround the agLDL. They acidify these compartments and secrete lysosomal enzymes into them, creating a lysosomal synapse. This leads to formation of unesterified cholesterol outside the cell, which can be delivered to the plasma membrane leading to changes in signal transduction and foam cell formation. We hypothesize that this mechanism for degrading agLDL has significant differences compared to phagocytic or endocytic mechanisms and that these differences have important consequences for the pathophysiology and treatment of atherosclerosis. We will study the cellular mechanisms that regulate this process (which we call exophagy); better characterize exophagy in vivo; and explore a novel role for HDL and cyclodextrins in clearing the cholesterol produced by exophagy. (1) Characterize the mechanisms for extracellular hydrolysis of agLDL. We have used MΦ from knockout mice, RNAi, and pharmacological agents to identify a role for TLR4, Myd88, SYK, Akt, PI3 kinases, and other signaling molecules in exophagy. We use three main quantitative assays: lysosome secretion, formation of F- actin where MΦ contact agLDL, and formation of lipid droplets. We will identify the Rab and SNARE proteins required for lysosome secretion, and we will identify genes that are activated during exophagy. (2) Determine the role of lysosomal synapses in atherosclerotic lesions. We will use optical and electron microscopy to analyze the activity of lysosomal synapses in mouse models of atherosclerosis. We will use bone marrow transplants into LDL receptor knockout mice to determine the importance of signaling processes. (3) Characterize HDL interactions with agLDL in contact with MΦ. AgLDL in contact with MΦ has very high levels of unesterified cholesterol. HDL or cholesterol-balanced cyclodextrins can remove excess cholesterol with no loss of cholesterol from cells. We will characterize this process and its impact on foam cell formation.

Relevance An important aspect of atherosclerosis is the interaction of immune system cells (macrophages) with lipoprotein deposits in major blood vessels. We are characterizing the cell biology of macrophage-lipoprotein interactions with the long-term goal of basing new therapies on improved understanding of the biological processes.