Aggregates of lipoproteins that are tightly crosslinked to the extracellular matrix are the major type of lipoprotein in atherosclerotic lesions. The majority of the cholesterol in these aggregates is unesterified, but it has been unclear how the cholesteryl esters in the core of retained and aggregated extracellular LDL are hydrolyzed – especially because a lysosomal hydrolase has been reported to be involved. Our recent studies demonstrate a novel mechanism for the hydrolysis of cholesteryl esters in the core of retained and aggregated LDL in which macrophages (M) create tightly sealed compartments that surround portions of the aggregated LDL. They then acidify these compartments and secrete lysosomal enzymes into them, creating a lysosomal synapse. It has been shown that the extracellular hydrolysis of cholesteryl esters by lysosomal acid lipase, in a process called digestive exophagy, leads to production of unesterified cholesterol outside the cell, and transport of this cholesterol into the cell leads to foam cell formation. The high concentrations of cholesterol in the aggregated LDL were observed in a preliminary study to lead to the formation of extracellular cholesterol crystals, which can cause inflammatory responses in M . The overarching hypothesis of this proposal is that this mechanism for degrading lipoproteins has significant differences as compared to conventional phagocytic or endocytic mechanisms and that these differences have important consequences in the pathophysiology of atherosclerosis. Furthermore, understanding this process may lead to improved therapeutic interventions. Work in the first aim will characterize the molecular mechanisms of digestive exophagy. This will include a study of the Rab and SNARE proteins that are required for lysosomal exocytosis. Signaling by Tlr4, Myd88, PI3-kinase, Akt, Syk, Vav, Cdc42, and other molecules has been shown to be important for digestive exophagy, and the roles of additional signaling molecules will be explored. High concentrations of cholesterol are generated in aggregated LDL, and in Aim 2 formation of cholesterol crystals and resulting inflammatory activation of M will be examined. Work in the third aim will use optical imaging and 3-D electron microscopy (FIB-SEM) to examine the 3D structures of lysosomal synapses in a mouse atherosclerosis model. Formation of lysosomal synapses, association of cholesterol crystals with retained and aggregated LDL, and inflammatory activation will be studied in various mouse models of atherosclerosis by optical microscopy. Better understanding of the cellular and molecular events occurring in atherosclerotic lesions can lead to better risk assessments and potentially new therapies.

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 and biochemistry of macrophage-lipoprotein interactions with the long-term goal of basing new therapies on improved understanding of the biological processes.