Mechanisms of L-selectin-induced activation of the nuclear factor of activated T lymphocytes (NFAT)

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.

Small GTPase and regulation of inflammation response in atherogenesis

Small GTPases are key signal transducers from extracellular stimuli to the nucleus that regulate a variety of cellular responses, including changes in gene expression and cell adhesion and migration. Accumulating data have demonstrated that abnormal activation of these small GTPases plays a critical role in the atherosclerosis characterized by vascular abnormalities, especially endothelial dysfunction and inflammation. Here, we discuss the linkage between small GTPases, inflammation, and atherogenesis. First, small GTPases affect gene expression of inflammatory cytokines through proinflammatory signaling pathways, such as nuclear factor-κB, vascular cell adhesion molecule-1, intercellular adhesion molecule-1, interlukin-8, and monocyte chemoattractant protein-1. Then, these molecules regulate the vascular inflammation through cell adhesion and migration. In turn, small GTPases are also regulated by extracellular stimuli, such as L-selectin, thrombin, oxidized phospholipids, and interleukins. Thus, these inflammatory cytokines generate a vicious cycle for small GTPases and inflammatory responses in the atherogenesis.

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.

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

Recruitment of leukocytes onto inflamed tissues is an important physiological event, in which L-selectin plays an essential role in initial leukocyte capture and at the same time, triggers cell signaling. Lck is a member of the Src family of protein tyrosine kinases and is critical for T cell activation triggered by receptor ligation. Here, we demonstrated that Lck was associated directly with and phosphorylated the L-selectin cytoplasmic tail upon L-selectin engagement with sulfatides. Through the direct interaction with ZAP-70 and c-Abl via its Src homology 2 (SH2) and SH3 domains, Lck organized a signaling complex at the cytoplasmic tail of L-selectin. In the cells with Lck knockdown by small interfering RNA treatment, L-selectin signaling was suppressed dramatically, as indicated by reduced phosphorylation of c-Abl and ZAP-70. Re-expression of wild-type or constitutively active but not kinase-dead murine Lck rescued the phosphorylation completely, but the SH2 domain mutant or the SH3/SH2 double mutant of murine Lck had no effect. These results suggest that Lck plays a critical role in L-selectin signaling upon sulfatides stimulation.

L-selectin ligation-induced CSF-1 gene transcription is regulated by AP-1 in a c-Abl kinase-dependent manner

L-selectin is a cell adhesion molecule that plays an important role both in mediating the initial capture and subsequent rolling of leukocytes along the endothelial cells and in the signal transduction for leukocyte activation. In our previous studies, we reported that L-selectin ligation could increase macrophage colony-stimulating factor (CSF)-1 gene transcription, in which c-Abl acts as a crucial cytoplasmic kinase. Here we investigated the function of the nuclear c-Abl kinase in the CSF-1 gene transcriptional events triggered by L-selectin ligation. We determined that c-Abl kinase recruits to the nucleus following L-selectin ligation, and the nuclear c-Abl kinase can phosphorylate c-Jun and regulate activator protein (AP)-1 activity. Furthermore, the activated c-Abl kinase interacts with AP-1 and forms a complex in the CSF-1 promoter region to regulate CSF-1 gene transcription in the L-selectin ligation-activated cells. These results indicate that nuclear c-Abl kinase can activate CSF-1 gene transcription by regulating AP-1 activity in the signaling events induced by L-selectin ligation.

c-Abl is required for the signaling transduction induced by L-selectin ligation

Lymphocyte recruitment onto inflamed tissues requires cells tethering to and rolling on vascular surfaces under flow. L-selectin is constitutively expressed on leukocytes to mediate the leukocytes' initial capture and subsequent rolling along the vessel. Apart from its adhesive function, engagement of L-selectin also results in cell activation, which is related to the completed signaling transduction. Here we show that ligation of L-selectin with its mAb increases c-Abl kinase activity, and that the activated c-Abl kinase can be recruited to and phosphorylate the cytoplasmic domain of L-selectin. In addition, the activated c-Abl kinase can regulate Zap70 kinase by increasing the phosphorylation of the Y319 site of Zap70 kinase and connect with Zap70 kinase through its SH2 domain. These results indicate that c-Abl kinase plays an important role in accepting and transferring the upstream activation events induced by L-selectin ligation.

c-Abl is involved in the F-actin assembly triggered by L-selectin crosslinking

L-selectin is a cell adhesion molecule mediating the initial capture and subsequent rolling of leukocytes along the endothelial cells expressing L-selectin ligands. In addition to its action in adhesion, an intracellular signaling role for L-selectin has been recognized. Its cytoplasmic domain is involved in signal transduction following antibody crosslinking and in the regulation of receptor binding activity in response to intracellular signals. In this work, we demonstrated that L-selectin crosslinking led to F-actin polymerization and redistribution in human neutrophils. Using immuno-fluorescence microscopy, we observed that F-actin redistribution spatiotemporally related to the polarization of L-selectin. STI571, a specific inhibitor for cytoplasmic tyrosine kinase c-Abl, can inhibit F-actin polymerization and c-Abl redistribution in the activated neutrophils. Furthermore, we determined that c-Abl redistributed to the region where L-selectin polarized and associated with L-selectin in the activated neutrophils. The association between L-selectin and c-Abl was reduced by cytochalasin B. These results suggested that c-Abl was involved in the F-actin alteration triggered by L-selectin crosslinking in human neutrophils.

Evaluation of CD62L expression as a marker for vaccine-elicited memory cytotoxic T lymphocytes

The development of successful vaccination strategies for eliciting cytotoxic T lymphocytes (CTLs) will be facilitated by the definition of strategies for subdividing CTLs into functionally distinct subpopulations. We assessed whether surface expression of a number of cell-surface proteins could be used to define functionally distinct subpopulations of memory CTLs in mice immunized with a recombinant vaccinia virus expressing human immunodeficiency virus (HIV)-1 envelope (Env). We found changes in cell-surface expression of CD11a, CD44, CD45RB, CD49d, CD54 and CD62L on Env-specific CD8(+) T cells that appeared to differentiate them from other CD8(+) T cells within 1 week to 1 month following immunization. Further, we saw an up-regulation of CD62L surface expression on Env-specific CD8(+) memory T cells several months after immunization. However, CD62L expression did not correlate with differences in the abilities of CTLs to proliferate or produce interferon gamma (IFN-gamma) and tumour necrosis factor alpha (TNF-alpha) in vitro in response to Env peptide stimulation. Moreover, the expression of CD62L did not allow differentiation of CTLs into subpopulations with distinct expansion kinetics in vivo after adoptive transfer into naïve mice and subsequent boosting of these mice with a recombinant adenovirus expressing HIV-1 Env. Therefore, the definition of memory CD8(+) T-cell subpopulations on the basis of CD62L expression in mice does not allow the delineation of functionally distinct CTL subpopulations.

Frutalin, a galactose-binding lectin, induces chemotaxis and rearrangement of actin cytoskeleton in human neutrophils: Involvement of tyrosine kinase and phosphoinositide 3-kinase

Several lectin-like molecules have been shown as potent activators of leukocytes. Galactose-binding lectins are of special interest since they could interact with several endogenous molecules involved in the innate and specific immune responses. The effects of Frutalin (FTL), an alpha-D-galactose (Gal)-binding plant lectin, on the modulation of neutrophil (PMN) functions were investigated. FTL induced a dose-dependent PMN migration in mice pleural cavity. Moreover, FTL was also a potent direct chemotactic for human PMN, in vitro, and triggered oxidative burst in these cells. These effects were accompanied by a rearrangement of the actin cytoskeleton dynamic, activation of tyrosine kinase (TK) pathways, increase in focal adhesion kinase (FAK) phosphorylation, and its subsequent association to phosphoinositide3-kinase (PI3K). All those effects were inhibited in the presence of Gal, suggesting specific carbohydrate recognition for FTL effects. The activations of TK and PI3K pathways are essential events for FTL-induced chemotaxis, since inhibitors of these pathways, genistein and LY294002, inhibited neutrophil migration in vitro. The data indicate that sugar-protein interactions between a soluble lectin and galacto-components on neutrophil surface trigger the TK pathway, inducing FAK and PI3K activation, interfering with cell motility and oxidative response.

L-selectin: adhesion, signalling and its importance in pathologic posttraumatic endotoxemia and non-septic inflammation

The leucocyte expressed surface-bound L-selectin belongs to the selectin family of adhesion molecules. It exhibits adhesive as well as signalling functions. Mainly, it is of importance in lymphocyte homing and in the extravasation of leucocytes into the surrounding tissue during inflammation. Acting in the initial step of the cell adhesion cascade, L-selectin is responsible for the rolling of leucocytes on endothelial layers. Therefore, L-selectin is thought to be an adequate target for pharmacological interventions. Beneath the discussion of the molecules' general features like molecule structure and its regulation, the review focuses firstly on L-selectin in the context of posttraumatic inflammatory disorders, and secondly on the importance of L-selectin specific signalling events.

Molecular mechanisms of L-selectin-induced co-localization in rafts and shedding [corrected]

Leukocyte recruitment to lymph nodes or inflammatory sites is regulated by adhesion and activation. L-selectin (CD62L) is expressed on leukocytes and mediates tethering and rolling of leukocytes on endothelial cells. Upon stimulation L-selectin is down-regulated by proteolytic cleavage but the molecular mechanisms regulating this shedding step are poorly defined. To study intracellular mechanisms, we induced shedding of L-selectin by cross-linking with an immobilized L-selectin antibody (Dreg56) in Jurkat cells. The loss of surface expression was quantitated by flow cytometry and the increase of soluble L-selectin was determined by Western blot analysis. We find that Jurkat and p56(lck)-deficient JCaM1.6 cells released L-selectin to similar extent (18+/-4% and 17+/-3%, respectively) and revealed comparable inhibition with the src-tyrosine kinase inhibitor PP2. Glutathione (GSH), an inhibitor of the neutral sphingomyelinase, PD98059, a MAP-kinase (MAP-K) inhibitor and metalloprotease inhibitors (MPI) (TAPI, Ro 31-9790, and BB-3103) reduced significantly L-selectin-induced shedding by 60-80%. In Jurkat cells, L-selectin was present in Triton X-100 insoluble membrane rafts and was constitutively tyr-phosphorylated. Dreg56 cross-linking enhanced phosphorylation and recruitment of L-selectin into rafts which was significantly decreased by pretreatment of cells with PD98059. We conclude, that the metalloproteinase-mediated cleavage of L-selectin from cell surface is triggered by intracellular signaling pathways that are independent of p56(lck) tyrosine kinase activity, but require other tyrosine kinases and the neutral sphingomyelinase. The cleavage of L-selectin might involve membrane rafts as signaling platform.