Lab Research Focus
The focus of our laboratory is to understand how bioactive phospholipids produced by oxidative and enzymatic modification of lipoproteins and cellular membranes regulate inflammatory and immune processes associated with the development and progression of atherosclerosis.
Background
Atherosclerosis is the primary cause of cardiovascular heart disease and as such remains the leading cause of death in Western societies. Atherosclerosis is a chronic inflammatory disease of the large arteries characterized by the accumulation of low-density lipoprotein (LDL) in the arterial wall with progressive vascular inflammation elicited by phospholipid products of its oxidative modification. Both innate (monocyte-derived macrophages and dendritic cells) and adaptive (T cells) immune cells contribute to vascular inflammation in atherosclerosis. Monocytes infiltrate the vascular wall at atherosclerotic foci and differentiate into macrophages that secrete pro-inflammatory cytokines. Dendritic cells are also present within atherosclerotic lesions and can emigrate to draining lymph nodes where they may present antigen derived form oxidized LDL and other atherosclerosis-related antigens. Subsequent adaptive immune responses modulate atherosclerosis by generating antigen-specific CD4+ T cells that influence B cell antibody responses to atherosclerosis-related antigens, infiltrate developing atherosclerotic lesions, and promote vascular inflammation through secretion of inflammatory cytokines (interferon-gamma).
Projects
A major area of interest in our laboratory is the role of a recently discovered G protein-coupled receptor named G2A which mediates biochemical and cellular responses of macrophages and lymphocytes to a bioactive lysophospholipid, lysophosphatidylcholine (LPC). LPC accumulates within atherosclerotic lesions and increases in the blood during atherogenesis as a result of lipoprotein oxidation, cholesterol esterification, and the action of macrophage-derived pro-inflammatory phospholipases. Recent work in our laboratory showed that mice lacking G2A develop significantly less atherosclerosis compared to normal mice. The ability of G2A to accelerate atherosclerosis development in mice is associated with wide-ranging effects of this receptor both locally (vascular wall) and systemically (circulation). For example, G2A increases macrophage infiltration at lesion-prone arterial sites and reduces plasma levels of high-density lipoprotein (HDL)-cholesterol in hypercholesterolemic mice. More recent studies in our laboratory have revealed an important regulatory role for G2A in peripheral lymphocyte trafficking. Current projects seek to establish the mechanisms underlying these effects of G2A at the cellular and molecular level, and to determine their individual consequences for atherosclerosis.
Experimental approaches
We employ hypercholesterolemic mouse models of atherosclerosis (LDL receptor knockout: LDLR-/- and apolipoprotein E knockout: apoE-/- mice) harboring targeted deletions of G2A to determine its impact on vascular inflammation, atherosclerosis and lymphocyte immune responses. To determine the cellular and molecular mechanisms of G2A-mediated pro-atherogenic action, we employ chimeric mice with blood cell-restricted deficiency or expression of G2A generated by bone marrow transplantation. These in vivo experimental approaches are complimented by in vitro approaches addressing specific mechanisms of G2A-mediated action (monocyte chemotaxis, macrophage cholesterol efflux, LDL/HDL metabolism). To determine the impact of G2A activity in specific cell-types relevant to atherosclerosis, we are generating tissue-specific G2A knockout mice which will allow direct and separate assessment of G2A-mediated effects in individual cell-types such as macrophages, T cells and endothelial cells. We also use macrophage-specific retroviruses to overexpress G2A in lesional macrophages of LDLR-/- mice and thereby directly address how modulating G2A activity affects macrophage functions related to atherosclerosis (cholesterol efflux, inflammation) within developing atherosclerotic lesions. The role of key macrophage secretory phospholipase enzymes involved in bioactive lipid and LPC generation is also being investigated using macrophage-specific retroviruses; in conjunction with mass spectrometric lipid quantification, we aim to test whether modulation of the activity of these enzymes can attenuate vascular inflammation and thus retard the development or progression of atherosclerotic plaques. Finally, we are studying the role of G2A in modulating peripheral lymphocyte trafficking in response to elevations in circulating LPC levels during chronic inflammation and hypercholesterolemia (using adoptive lymphocyte transfer into hypercholesterolemic and inflammatory mouse models). This study also addresses how G2A-mediated regulation of peripheral lymphocyte trafficking impacts adaptive immune responses during atherogenesis and investigates its contribution to the synergistic inter-relationship between autoimmunity and atherosclerosis (using autoimmune mouse strains on hypercholesterolemic backgrounds).
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