Critical role of Lck in L-selectin signaling induced by sulfatides engagement

L-selectin: A Major Regulator of Leukocyte Adhesion, Migration and Signaling

L-selectin (CD62L) is a type-I transmembrane glycoprotein and cell adhesion molecule that is expressed on most circulating leukocytes. Since its identification in 1983, L-selectin has been extensively characterized as a tethering/rolling receptor. There is now mounting evidence in the literature to suggest that L-selectin plays a role in regulating monocyte protrusion during transendothelial migration (TEM). The N-terminal calcium-dependent (C-type) lectin domain of L-selectin interacts with numerous glycans, including sialyl Lewis X (sLe^x) for tethering/rolling and proteoglycans for TEM. Although the signals downstream of L-selectin-dependent adhesion are poorly understood, they will invariably involve the short 17 amino acid cytoplasmic tail. In this review we will detail the expression of L-selectin in different immune cell subsets, and its influence on cell behavior. We will list some of the diverse glycans known to support L-selectin-dependent adhesion, within luminal and abluminal regions of the vessel wall. We will describe how each domain within L-selectin contributes to adhesion, migration and signal transduction. A significant focus on the L-selectin cytoplasmic tail and its proposed contribution to signaling via the ezrin-radixin-moesin (ERM) family of proteins will be outlined. Finally, we will discuss how ectodomain shedding of L-selectin during monocyte TEM is essential for the establishment of front-back cell polarity, bestowing emigrated cells the capacity to chemotax toward sites of damage.

Neutrophil Mechanosignaling Promotes Integrin Engagement With Endothelial Cells and Motility Within Inflamed Vessels

Neutrophils are the most motile of mammalian cells, a feature that enables them to protect the host against the rapid spread of pathogens from tissue into the circulatory system. A critical process is the recruitment of neutrophils to inflamed endothelium within post-capillary venules. This occurs through cooperation between at least four families of adhesion molecules and G-protein coupled signaling receptors. These adhesion molecules convert the drag force induced by blood flow acting on the cell surface into bond tension that resists detachment. A common feature of selectin-glycoprotein tethering and integrin-ICAM bond formation is the mechanics by which force acting on these specific receptor-ligand pairs influences their longevity, strength, and topographic organization on the plasma membrane. Another distinctly mechanical aspect of neutrophil guidance is the capacity of adhesive bonds to convert external mechanical force into internal biochemical signals through the transmission of force from the outside-in at focal sites of adhesive traction on inflamed endothelium. Within this region of the plasma membrane, we denote the inflammatory synapse, Ca²⁺ release, and intracellular signaling provide directional cues that guide actin assembly and myosin driven motive force. This review provides an overview of how bond formation and outside-in signaling controls neutrophil recruitment and migration relative to the hydrodynamic shear force of blood flow.

A head-to-tail view of L-selectin and its impact on neutrophil behaviour

L-selectin is a type I transmembrane cell adhesion molecule expressed on most circulating leukocytes, including neutrophils. Engagement of L-selectin with endothelial-derived ligands initiates neutrophil tethering and rolling behaviour along luminal walls of post-capillary venules, constituting the first step of the multi-step adhesion cascade. There is a large body of evidence to suggest that signalling downstream of L-selectin can influence neutrophil behaviour: adhesion, migration and priming. This review will cover aspects of L-selectin form and function and introduce the “triad of L-selectin regulation”, highlighting the inextricable links between adhesion, signalling and ectodomain shedding and also highlighting the cytosolic proteins that interconnect them. Recent advances in how L-selectin impacts priming, transendothelial migration (TEM) and cell polarity will also be discussed.

Selectin catch-bonds mechanotransduce integrin activation and neutrophil arrest on inflamed endothelium under shear flow

E-selectin extends from the plasma membrane of inflamed endothelium and serves to capture leukocytes from flowing blood via long-lived catch-bonds that support slow leukocyte rolling under shear stress. Its ligands are glycosylated with the tetrasaccharide sialyl Lewis^x (sLe^x), which contributes to bond affinity and specificity. E-selectin-mediated rolling transmits signals into neutrophils that trigger activation of high-affinity β₂-integrins necessary for transition to shear-resistant adhesion and transendothelial migration. Rivipansel is a glycomimetic drug that inhibits E-selectin-mediated vaso-occlusion induced by integrin-dependent sickle-red blood cell-leukocyte adhesion. How Rivipansel antagonizes ligand recognition by E-selectin and blocks outside-in signaling of integrin-mediated neutrophil arrest while maintaining rolling immune-surveillance is unknown. Here, we demonstrate that sLe^x expressed on human L-selectin is preferentially bound by E-selectin and, on ligation, initiates secretion of MRP8/14 that binds TLR4 to elicit the extension of β₂-integrin to an intermediate affinity state. Neutrophil rolling over E-selectin at precise shear stress transmits tension and catch-bond formation with L-selectin via sLe^x, resulting in focal clusters that deliver a distinct signal to upshift β₂-integrins to a high-affinity state. Rivipansel effectively blocked formation of selectin catch-bonds, revealing a novel mechanotransduction circuit that rapidly converts extended β₂-integrins to high-affinity shear-resistant bond clusters with intracellular adhesion molecule 1 on inflamed endothelium.

The cytoplasmic tail of L-selectin interacts with the adaptor-protein complex AP-1 subunit μ1A via a novel basic binding motif

L-selectin regulates leukocyte adhesion and rolling along the endothelium. Proteins binding to the cytoplasmic tail of L-selectin regulate L-selectin functions. We used L-selectin cytoplasmic tail peptide pulldown assays combined with high sensitivity liquid chromatography/mass spectrometry to identify novel L-selectin tail-binding proteins. Incubation of the L-selectin tail with cell extracts from phorbol 12-myristate 13-acetate-stimulated Raw 264.7 macrophages resulted in the binding of μ1A of the clathrin-coated vesicle AP-1 complex. Furthermore, full-length GST-μ1A and the GST-μ1A C-terminal domain, but not the GST-μ1A N-terminal domain, bind to L-selectin tail peptide, and the intracellular pool of L-selectin colocalizes with AP-1 at the trans-Golgi network. We identified a novel basic protein motif consisting of a cluster of three dibasic residues (³⁵⁶RR³⁵⁷, ³⁵⁹KK³⁶⁰, and ³⁶²KK³⁶³) in the membrane-proximal domain of the L-selectin tail as well as a doublet of aspartic acid residues (³⁶⁹DD³⁷⁰) in the membrane-distal end of the L-selectin tail involved in μ1A binding. Stimulation of Raw 264.7 macrophages with PMA augmented the amount of μ1A associated with anti-L-selectin immunoprecipitates. However, full-length GST-μ1A did not bind to the phospho-L-selectin tail or phospho-mimetic S364D L-selectin tail. Accordingly, we propose that phosphorylation of μ1A is required for interaction with the L-selectin tail and that L-selectin tail phosphorylation may regulate this interaction in vivo Molecular docking of the L-selectin tail to μ1A was used to identify the μ1A surface domain binding the L-selectin tail and to explain how phosphorylation of the L-selectin tail abrogates μ1A interaction. Our findings indicate that L-selectin is transported constitutively by the AP-1 complex, leading to the formation of a trans-Golgi network reserve pool and that phosphorylation of the L-selectin tail blocks AP-1-dependent retrograde transport of L-selectin.

p85-RhoGDI2, a novel complex, is required for PSGL-1-induced β1 integrin-mediated lymphocyte adhesion to VCAM-1

P-selectin glycoprotein ligand-1 and β1 integrin play essential roles in T cell trafficking during inflammation. E-selectin and vascular cell adhesion molecule-1 are their ligands expressed on inflammation-activated endothelium. During the tethering and rolling of lymphocytes on endothelium, P-selectin glycoprotein ligand-1 binds E-selectin and induces signals. Subsequently, β1 integrin is activated and mediates stable adhesion. However, the intracellular signal pathways from PSGL-1 to β1 integrin have not yet been fully understood. Here, we find that p85, a regulatory subunit of phosphoinositide 3-kinase, forms a novel complex with Rho-GDP dissociation inhibitor-2, a lymphocyte-specific RhoGTPases dissociation inhibitor. Phosporylations of the p85-bound Rho-GDP dissociation inhibitor-2 on 130 and 153 tyrosine residues by c-Abl and Src were required for the complex to be recruited to P-selectin glycoprotein ligand-1 and thereby regulate β1 integrin-mediated T cell adhesion to vascular cell adhesion molecule-1. Both shRNAs to Rho-GDP dissociation inhibitor-2 and p85 and over-expression of Rho-GDP dissociation inhibitor-2 Y130F and Y153F significantly reduced the above-mentioned adhesion. Although Rho-GDP dissociation inhibitor-2 in the p85-Rho-GDP dissociation inhibitor-2 complex was also phosphorylated on 24 tyrosine residue by Syk, the phosphorylation is not required for the adhesion. Taken together, we find that specific phosphorylations on 130 and 153 tyrosine residues of p85-bound Rho-GDP dissociation inhibitor-2 are pivotal for P-selectin glycoprotein ligand-1-induced β1 integrin-mediated lymphocyte adhesion to vascular cell adhesion molecule-1. This will shed new light on the mechanisms that connect leukocyte initial rolling with subsequent adhesion.

Structural insights into calmodulin-regulated L-selectin ectodomain shedding

The L-selectin glycoprotein receptor mediates the initial steps of leukocyte migration into secondary lymphoid organs and sites of inflammation. Following cell activation through the engagement of G-protein-coupled receptors or immunoreceptors, the extracellular domains of L-selectin are rapidly shed, a process negatively controlled via the binding of the ubiquitous eukaryotic calcium-binding protein calmodulin to the cytoplasmic tail of L-selectin. Here we present the solution structure of calcium-calmodulin bound to a peptide encompassing the cytoplasmic tail and part of the transmembrane domain of L-selectin. The structure and accompanying biophysical study highlight the importance of both calcium and the transmembrane segment of L-selectin in the interaction between these two proteins, suggesting that by binding this region, calmodulin regulates in an "inside-out" fashion the ectodomain shedding of the receptor. Our structure provides the first molecular insight into the emerging new role for calmodulin as a transmembrane signaling partner.

Signaling through L-selectin mediates enhanced chemotaxis of lymphocyte subsets to secondary lymphoid tissue chemokine

L-selectin functions as an important adhesion molecule that mediates tethering and rolling of lymphocytes by binding to high endothelial venule (HEV)-expressed ligands during recirculation. Subsequent lymphocyte arrest and transmigration require activation through binding of HEV-decorated homeostatic chemokines such as secondary lymphoid tissue chemokine (SLC; CCL21) to its counterreceptor, CCR7. Importantly, L-selectin also functions as a signaling molecule. In this study, signaling induced by ligation of L-selectin using mAb or endothelial cell-expressed ligand significantly enhanced the chemotaxis of murine T cells and B cells to SLC but not to other homeostatic chemokines. Consistent with the expression levels of L-selectin in different lymphocyte subsets, L-selectin-mediated enhancement of chemotaxis to SLC was observed for all naive lymphocytes and effector/memory CD8(+) T cells, whereas only a subpopulation of effector/memory CD4(+) T cells responded. During in vivo mesenteric lymph node migration assays, the absence of L-selectin on lymphocytes significantly attenuated both their ability to migrate out of the HEV and their chemotaxis away from the vessel wall. Notably, ligation of L-selectin and/or CCR7 did not result in increased CCR7 expression levels, internalization, or re-expression. Pharmacologic inhibitor studies showed that L-selectin-mediated enhanced chemotaxis to SLC required intact intracellular kinase function. Furthermore, treatment of lymphocytes with the spleen tyrosine kinase family inhibitor piceatannol reduced their ability to migrate across the HEV in peripheral lymph nodes. Therefore, these results suggest that "cross-talk" in the signaling pathways initiated by L-selectin and CCR7 provides a novel mechanism for functional synergy between these two molecules during lymphocyte migration.

L-selectin--a dynamic regulator of leukocyte migration

The leukocytic cell adhesion receptor L-selectin mediates the initial step of the adhesion cascade, the capture and rolling of leukocytes on endothelial cells. This event enables leukocytes to migrate out of the vasculature into surrounding tissues during inflammation and immune surveillance. Distinct domains of L-selectin contribute to proper leukocyte migration. In this review, we discuss the contributions of these domains with respect to L-selectin function: the regulation by serine phosphorylation of the cytoplasmic tail, the role of the transmembrane domain in receptor positioning on the cell surface as well as the N-glycosylation of the extracellular part and the identification of novel binding partners.

c-Abl kinase is required for beta 2 integrin-mediated neutrophil adhesion

Integrin regulation in neutrophil adhesion is essential for innate immune response. c-Abl kinase is a nonreceptor tyrosine kinase and is critical for signaling transduction from various receptors in leukocytes. Using neutrophils and dHL-60 (neutrophil-like differentiation of HL-60) cells, we show that c-Abl kinase is activated by beta(2) integrin engagement and is required for beta(2) integrin-dependent neutrophil sustained adhesion and spreading. The expression of beta(2) integrin on neutrophils induced by TNF-alpha is not affected by c-Abl kinase inhibitor STI571, suggesting that c-Abl kinase is not involved in TNF-alpha-induced integrin activation. The recruitment of c-Abl kinase to beta(2) integrin is dependent on talin head domain, which constitutively interacts with beta(2) integrin cytoplasmic domain. After activated, c-Abl kinase increases the tyrosine phosphorylation of Vav. The SH3 domain of c-Abl kinase is involved in its interaction with talin and Vav. Thus, c-Abl kinase plays an essential role in the activation of Vav induced by beta(2) integrin ligation and in regulating neutrophil-sustained adhesion and spreading.