Intracellular lipid flux and membrane microdomains as organizing principles in inflammatory cell signaling

Abstract

Lipid rafts and caveolae play a pivotal role in organization of signaling by TLR4 and several other immune receptors. Beyond the simple cataloguing of signaling events compartmentalized by these membrane microdomains, recent studies have revealed the surprisingly central importance of dynamic remodeling of membrane lipid domains to immune signaling. Simple interventions upon membrane lipid, such as changes in cholesterol loading or crosslinking of raft lipids, are sufficient to induce micrometer-scale reordering of membranes and their protein cargo with consequent signal transduction. In this review, using TLR signaling in the macrophage as a central focus, we discuss emerging evidence that environmental and genetic perturbations of membrane lipid regulate protein signaling, illustrate how homeostatic flow of cholesterol and other lipids through rafts regulates the innate immune response, and highlight recent attempts to harness these insights toward therapeutic development.

Figure 1. Intracellular cholesterol trafficking regulates macrophage rafts

Cholesterol synthesized in the endoplasmic reticulum (ER) by HMG CoA reductase (HMGCR) or internalized via scavenger receptors (CD36, SR-A) or low density lipoprotein receptor (LDLR), is assembled into nascent rafts in the Golgi apparatus for caveolin- and Niemann Pick C1 protein (NPC1)-dependent transfer to the plasma membrane. NPC1 together with NPC2 also regulates endosomal recycling of cholesterol to the plasma membrane. In turn, cholesterol is effluxed either by simple diffusion or via transporters (ATP Binding Cassette [ABC]A1, ABCG1, SR-BI) to extracellular acceptors (apolipoprotein [apo]A-I, high density lipoprotein [HDL]), and also likely equilibrates with non-raft regions. Cholesterol esterification is regulated in the cytosol by cholesterol ester hydrolase (CEH) and in the ER by acyl-coenzyme A:cholesterol acyltransferase 1 (ACAT1). Raft cholesterol/abundance and abundance of LPS recognition proteins (CD14, TLR4) are regulated by cholesterol flux through this pathway.

Figure 2. ATP Binding Cassette (ABC)A1-deficient macrophages have enlarged lipid rafts

(A) Rafts were imaged in peritoneal macrophages from WT and Abca1 null mice with the use two raft cholesterol probes, BCθ toxin (red) and fPEG-chol (green). Nuclei were stained with 4’6-diaminophenylindole (blue). Reprinted from (59) with permission. (B) Peritoneal macrophages from wild type (+/+) or macrophage-specific Abca1 null (-M/-M) mice were cholesterol-depleted with mβCD or cholesterol-loaded with mβCD-cholesterol, stained with fPEG-chol, and then quantified by flow cytometry. Data are mean +/− SEM. *, p<.05; **, p<.01. Reprinted from (43).