Real-time imaging of leukotriene B₄ mediated cell migration and BLT1 interactions with β-arrestin

Abstract

G-protein coupled receptors (GPCRs) belong to the seven transmembrane protein family and mediate the transduction of extracellular signals to intracellular responses. GPCRs control diverse biological functions such as chemotaxis, intracellular calcium release, gene regulation in a ligand dependent manner via heterotrimeric G-proteins(1-2). Ligand binding induces a series of conformational changes leading to activation of heterotrimeric G-proteins that modulate levels of second messengers such as cyclic adenosine monophosphate (cAMP), inositol triphosphate (IP3) and diacyl glycerol (DG). Concomitant with activation of the receptor ligand binding also initiates a series of events to attenuate the receptor signaling via desensitization, sequestration and/or internalization. The desensitization process of GPCRs occurs via receptor phosphorylation by G-protein receptor kinases (GRKs) and subsequent binding of β-arrestins(3). β-arrestins are cytosolic proteins and translocate to membrane upon GPCR activation, binding to phosphorylated receptors (most cases) there by facilitating receptor internalization (4-6). Leukotriene B(4;) (LTB(4;)) is a pro-inflammatory lipid molecule derived from arachidonic acid pathway and mediates its actions via GPCRs, LTB(4;) receptor 1 (BLT1; a high affinity receptor) and LTB(4;) receptor 2 (BLT2; a low affinity receptor)(7-9). The LTB(4;)-BLT1 pathway has been shown to be critical in several inflammatory diseases including, asthma, arthritis and atherosclerosis(10-17). The current paper describes the methodologies developed to monitor LTB(4;)-induced leukocyte migration and the interactions of BLT1 with β-arrestin and , receptor translocation in live cells using microscopy imaging techniques(18-19). Bone marrow derived dendritic cells from C57BL/6 mice were isolated and cultured as previously described (20-21). These cells were tested in live cell imaging methods to demonstrate LTB(4;) induced cell migration. The human BLT1 was tagged with red fluorescent protein (BLT1-RFP) at C-terminus and β-arrestin1 tagged with green fluorescent protein (β-arr-GFP) and transfected the both plasmids into Rat Basophilic Leukomia (RBL-2H3) cell lines(18-19). The kinetics of interaction between these proteins and localization were monitored using live cell video microscopy. The methodologies in the current paper describe the use of microscopic techniques to investigate the functional responses of G-protein coupled receptors in live cells. The current paper also describes the use of Metamorph software to quantify the fluorescence intensities to determine the kinetics of receptor and cytosolic protein interactions.