Báo cáo khoa học: Endothelial signalling events during leukocyte transmigration pptx

MINIREVIEW Endothelial signalling events during leukocyte transmigration Peter L. Hordijk Department of Molecular Cell Biology, Sanquin Research and Landsteiner Laboratory, Academic Medical Center, University of Amsterdam, the Netherlands Transendothelial migration (TEM) is an essential aspect of the trafficking of leukocytes, as well as of malignant cells. Much pathology is associated with uncontrolled TEM, for instance in chronic inflamma- tory disorders (asthma, rheumatoid arthritis, psoriasis) and in metastasis. On the other hand, regulated leuko- cyte trafficking is required for immune surveillance and stem cell homing following transplantation proce- dures. The multistep model for TEM [1] is well estab- lished and probably applies to most transmigration events, albeit that tissue specificity may result in addi- tional complexity. For example, in the brain, the endo- thelium constitutes the blood–brain barrier, which is tighter than the endothelium in other tissues, resulting in additional requirements for both leukocytes and the endothelium to allow efficient transmigration. Seminal work by the groups of Silverstein & Bender [2,3] has triggered an ever-growing list of studies [4] confirming the notion that endothelial cells (EC) not only mediate leukocyte adhesion, but also actively par- ticipate in the transmigration event. Most of these studies have focussed on regulation of the paracellular pathway, although, recently, analysis of transcellular migration has also become fashionable. This review will focus on events in the EC that have been implica- ted in these different routes of TEM. Keywords adhesion; ICAM-1; migration; transendothelial migration; VCAM-1 Correspondence P. L. Hordijk, Department of Molecular Cell Biology, Sanquin Research and Landsteiner Laboratory, Academic Medical Center, University of Amsterdam, Plesmanlaan 125, 1066 CX Amsterdam, the Netherlands Fax: +31 20 5123474 Tel: +31 20 5123263 E-mail: p.hordijk@sanquin.nl (Received 15 May 2006, accepted 17 July 2006) doi:10.1111/j.1742-4658.2006.05440.x The notion that it takes two to tango is certainly true for leukocyte trans- endothelial migration. A growing pallet of leukocyte adhesion-induced signaling events in endothelial cells have been identified, mediating both short-term (i.e. permeability) as well as long-term (i.e. regulation of tran- scription) effects. Efficient paracellular migration (i.e. through endothelial cell–cell junctions) requires both intracellular calcium and the actin cyto- skeleton, but also involves small GTPases, reactive oxygen species and pro- tein kinases. The alternative route of transcellular migration appears to depend on components such as caveolae and intermediate filaments. This minireview discusses our current knowledge on the regulation of leukocyte transmigration through endothelial signalling. Abbreviations CAM, cellular adhesion molecule; EC, endothelial cells; ERK, extracellular signal regulated kinase; ICAM, intercellular adhesion molecule; JAM, junctional adhesion molecule; MAPK, mitogen-activated protein kinase; NOX, NADPH oxidase; PECAM-1, platelet ⁄ endothelial cell adhesion molecule-1; ROS, reactive oxygen species; TEM, transendothelial migration; VCAM-1, vascular cell adhesion molecule-1; VE-cadherin, vascular endothelial cadherin. 4408 FEBS Journal 273 (2006) 4408–4415 ª 2006 The Author Journal compilation ª 2006 FEBS Endothelial docking structures Rolling and adhesion of leukocytes over activated endothelium (i.e. at sites of inflammation) is accompan- ied by a complex response from the endothelial cells. Initially, this comprises engagement and subsequent clustering of endothelial adhesion molecules. These include E-selectin and Ig-like cell adhesion molecules, such as intercellular adhesion molecule (ICAM)-1, ICAM-2, vascular cell adhesion molecule-1 (VCAM-1), platelet ⁄ endothelial cell adhesion molecule-1 (PECAM-1) and members of the junctional adhesion molecule (JAM) subfamily. Subsequent to leukocyte adhesion, the EC show a pronounced morphological response by forming ‘docking structures’ [5] or ‘trans- migratory cups’ [6]. These are actin-rich membrane extensions that form around the adherent leukocyte. In these structures, not only are integrin ligands such as ICAM-1 and VCAM-1 concentrated, but also adaptor and linker molecules, such as ERM (ezrin, radixin, moesin) proteins, vinculin, talin and a-actinin [5]. For- mation of these structures requires calcium and, accord- ing to some [5], but not to others [7], activation of the Rho ⁄ p160ROCK pathway. As a result of the concen- tration of adhesion and signaling molecules, docking structures represent the main signaling ‘platforms’ from which intracellular signaling into the EC is initiated. There are indications that docking structures, and the proteins therein, remain associated with the leukocytes throughout the transmigration process [5,6]. This might well be important for the sustained signaling that is required for efficient crossing of the endothelial barrier. Transcellular versus paracellular migration Leukocyte TEM has classically been considered to occur at cell–cell junctions. It is now clear that, next to this paracellular pathway, transcellular migration (i.e. through the endothelial cell body) can also be observed [8]. In vivo analysis showed that neutrophils can cross the endothelial monolayer in a transcellular manner [9]. Recently, a series of studies reported that transcellular migration can also be observed in vitro [6,10–13]. In particular, ICAM-1 has been associated with transcellular migration [6,10,11,13]. It is obvious that ICAM-1, being the main endothelial ligand for b2-integrins, is crucial for TEM in general and for polymorphonuclear cell transmigration in particular [14]. Yet, Yang et al. [10] showed that prolonged tumor necrosis factor-a treatment, or expression of an ICAM-1–green fluorescent protein fusion on immortal- ized EC, increases the relative contribution of transcel- lular migration to polymorphonuclear cell diapedesis, suggesting that ICAM-1 plays an active role in deter- mining whether polymorphonuclear cells use the para- cellular or the transcellular route. Whether VCAM-1 plays a similar role, for example, for monocytes, is not known. Additional regulatory factors that might pro- mote transcellular migration are the polygonal shape of the EC or the levels of b2-integrin occupancy, shear and the presence of chemokines on the EC [10,13]. The endothelial structures that mediate transcellular migration were initially suggested to be vesiculo-vacuo- lar organelles, which are abundant in EC and could align to form a channel for macromolecules [15] and perhaps even for migrating leukocytes. More recently, transcellular migration was linked to caveolae, a sub- class of membrane lipid rafts that may, by invagination, detach from the membrane and mediate vesicular trans- port. The protein caveolin, a key marker for caveolae, was found to be enriched at the site of leukocyte–endo- thelial cell contact [6]. Using a caveolin knockdown approach, Millan et al. [11] showed that caveolin was required for transcellular, but not for paracellular, TEM. Another study, by Nieminen et al. [12], has implicated the intermediate filament protein, vimentin, in the process of lymphocyte transcellular migration. However, its regulation and precise role in the transmi- gration process remains to be determined. Although these studies show that the paracellular and transcellular pathways co-exist, considerable vari- ation in the relative contribution of the transcellular pathway to leukocyte TEM has been noted. This may depend on the type of leukocyte, as Yang et al. repor- ted efficient transcellular migration (up to 50% of the total transmigration events) for neutrophils, whereas T lymphocytes exclusively used the paracellular route [10]. Contrasting findings have also been described (i.e. that lymphocytes would preferentially use the transcellular route) [12]. Similarly, the source of the endothelium (microvascular versus macrovascular [11]), and the state of activation of the endothelium or the leukocytes [10,12,13], may also affect the relat- ive importance of one route over the other. Yet, in most of these studies, the contribution of the transcel- lular pathway was only 10–30% to the total trans- migration events. Intriguingly, down-regulation of caveolin expression in human umbilical vein endothel- ial cells blocked transcellular migration by T lympho- blasts, but did not reduce the overall TEM, suggesting that cells can switch from the transcellular to the paracellular route without a significant reduction in TEM efficiency [11]. The factors that determine the choice of leukocytes for one or the other pathway remain to be established. P. L. Hordijk Endothelial signalling events FEBS Journal 273 (2006) 4408–4415 ª 2006 The Author Journal compilation ª 2006 FEBS 4409 Signaling by endothelial adhesion molecules As mentioned above, leukocyte adhesion and the for- mation of endothelial docking structures is associated with the clustering of adhesion and signaling mole- cules. It is very likely that this clustering is required for efficient signal transduction into the EC which, at least for some types of leukocyte, is important for effi- cient TEM. Many cell surface (adhesion) proteins have been implicated in leukocyte TEM, in particular Ig family members. However, for only some of these has the induction of intracellular signaling been causally related to leukocyte transmigration. E-selectin Although in the classical multistep model for TEM, selectins are usually depicted as mediating low-affinity interactions to allow rolling, there is ample evidence for the signaling capacity of E-selectin (CD62 E), both towards the actin cytoskeleton [16] as well as to p42 mi- togen-activated protein kinase (MAPK) ⁄ extracellular signal regulated kinase (ERK) activation and the induc- tion of c-fos [17]. Clustering of E-selectin, which is an adhesion receptor for neutrophils and memory T cells, results in its association with the actin cytoskeleton. In addition, clustered E-selectin associates, through its intracellular domain, with Ras, Raf and MAPK ⁄ ERK kinase (MEK). These proteins trigger the downstream signaling towards MAPK and c-fos [17,18]. Later stud- ies showed that tyrosine phosphorylation of the E-se- lectin intracellular tail is instrumental in these events through the recruitment and activation of the SHP-2 phosphatase, which signals, via Shc and Grb2 adapter proteins, to the Ras-MAPK pathway [19]. In addition, E-selectin resides in caveolin-containing lipid rafts and associates with phospholipase C gamma [20]. Raft dis- ruption ablates the activation of phospholipase C gamma, but not of MAPK, indicating that the activa- tion of different signaling pathways can occur in dis- tinct membrane subdomains [20]. PECAM-1 The Ig-like CAM, PECAM-1, mediates homotypic interactions between leukocytes and EC and between EC themselves at intercellular junctions. PECAM-1 has been implicated in cell survival, angiogenesis, lung development and experimental autoimmune encephalo- myelitis [21,22]. Blocking antibodies to PECAM-1 inhi- bit neutrophil and lymphocyte TEM in vitro [23,24] (also see review by Petri & Bixel, this issue of FEBS). In contrast to most other Ig-like CAMs, PECAM-1 has an extended intracellular tail that encodes two immuno- receptor tyrosine-based inhibition motifs and which is subject to tyrosine phosphorylation by src-like kinases, primarily in response to cell stimulation or PECAM-1 cross-linking [21]. These immunoreceptor tyrosine-based inhibition motifs mediate, following phosphorylation, association with the SHP-1 and SHP-2 tyrosine phos- phatases, with the SH2 domain-containing inositol 5-phosphatase, SHIP, with adapter, proteins such as Grb2, and with b- and c-catenin. PECAM-1 stimulates integrin adhesion by activating Rap1 [25] and has been associated with cell survival. Similarly to E-selectin, PECAM-1 can activate MAPK via its association with SHP2. Remarkably, although the signaling capacities of PECAM-1 have been extensively studied, its relevance as a signaling molecule in TEM is not clear. This may also relate to the fact that PECAM-1-deficient mice showed only limited problems in models of inflamma- tion, although later studies reported that this result may depend on the mice strain used [26]. Recently, however, the modulation of cell–cell adhesion by PECAM-1 has been proposed, based on studies in transfected epithelial cells [27]. Whether these data can be readily translated to EC remains to be seen. JAMs The family of JAM molecules concentrate in endothel- ial tight junctions [28]. In addition, JAM proteins are expressed by leukocytes. Several studies have clearly shown that JAM family members are essential for leu- kocyte TEM [8,29] (also see the review by Petri & Bixel, this issue of FEBS). In addition, JAM proteins have been implicated in cell signaling towards cell polarity and the formation of cell–cell contact. JAMs can associate, through C-terminal PDZ-binding motifs, with a series of proteins, including ZO1, AF6, Par 3 and MUPP1 [28]. Despite their role in the regulation of cell–cell adhesion and the fact that the JAMs clearly have relevant signaling capacities, it is, as for PECAM-1, not yet known whether they in fact trans- mit signals into the EC that promote TEM. ICAM-1 ICAM-1 is one of the main integrin ligands involved in leukocyte TEM, in particular of lymphocytes and neutrophils. ICAM-1 is expressed on resting endo- thelium, but up-regulated upon activation by inflam- matory stimuli. ICAM-1 has a short cytoplasmic tail of 29 amino acids that associates to ERM (ezrin, ra- dixin, moesin) proteins, as well as to a-actinin [30,31]. Endothelial signalling events P. L. Hordijk 4410 FEBS Journal 273 (2006) 4408–4415 ª 2006 The Author Journal compilation ª 2006 FEBS ICAM-1 acts as an adhesion molecule and a signal transducer in EC. ICAM-1 activates the p60src kinase, which leads to phosphorylation of cortactin [32], trig- gers release of intracellular calcium and activates the Rho GTPase, which explains the effects of ICAM-1 on the actin cytoskeleton and on contractility in EC [16]. These effects are mediated by the C-terminus of ICAM-1 and are required for efficient TEM of lymphocytes [31,33,34]. Moreover, ICAM-1 has been shown to activate p60src via the activation of xanthine oxidase, in a SHP2-dependent manner, leading to tyro- sine phosphorylation of ezrin and p38 MAPK [35]. Finally, cell-permeable versions of the cytoplasmic tail of ICAM-1 were found to block leukocyte TEM [10,34,36]. In conclusion, ICAM-1 activates a series of signaling events through its intracellular C-terminal tail that are likely to increase endothelial permeability, resulting in enhanced leukocyte TEM. VCAM-1 The main b1-integrin ligand on the endothelium, VCAM-1, is, in contrast to ICAM-1, absent from rest- ing cells but greatly up-regulated by inflammatory stimuli. Similarly to ICAM-1, VCAM-1 not only acts as an adhesion receptor, but also as a signal transducer upon binding of leukocytes. The cytoplasmic domain of VCAM-1 is only 19 amino acids long and comprises a type I PDZ-binding motif. However, whether specific interactions are mediated by this motif is unknown; to date, only ezrin and moesin have been shown to asso- ciate with the cytoplasmic domain of VCAM-1 [5]. VCAM-1 clustering leads to the activation of Rac1, production of reactive oxygen species (ROS), activa- tion of p38 MAPK and changes in the actin cytoskele- ton (i.e. stress fiber formation) (Fig. 1). These events have all been associated with the increased endothelial permeability (as measured by tracer molecules or trans- endothelial resistance) that is induced by VCAM-1 cross-linking. VCAM-1-mediated leukocyte TEM is also dependent on some of these signaling events, including Rac1 and Rho activation [37]. Of particular interest is the role of VCAM-1-induced production of ROS. ROS are known to impair cell–cell adhesion in EC and are important regulators of endothelial integ- rity through their indirect stimulation of tyrosine kinase activity. In addition, vascular ROS play an important role in the development of cardiovascular disease [38]. Conversely, scavenging ROS preserves endothelial barrier function, prevents endothelial cell migration and angiogenesis, and is atheroprotective. The source of endothelial ROS has been suggested to be the NADPH oxidase 2 (NOX2) which is, like its relative NOX4, also expressed in EC [39,40]. NOX2 is supposedly localized in the endothelial plasma mem- brane, resulting in an extracellular release of ROS. These have been proposed to activate metalloproteases that would promote endothelial permeability by proteolytic degradation of vascular endothelial cadher- in (VE-cadherin) or of the extracellular matrix (Fig. 2A) [39]. Our laboratory has shown that ROS production in response to VCAM-1-mediated activation of Rac1 can be observed intracellularly, and we have proposed a role for the redox sensitive proline-rich tyrosine kinase 2 in the control of endothelial integrity through the phosphorylation of b-catenin [41] (Fig. 2B). However, our present knowledge of this pathway remains limited. The molecular mechanism of VCAM-1-triggered acti- vation of Rac1 is completely unknown, as is the poten- tial role for the relatively abundant NOX4 protein in VCAM-1 signalling. Also, the mechanism of VE-cadh- erin inactivation through ROS (i.e. either by proteolytic breakdown, or by reducing its homophilic adhesion through reduction of its link to the actin cytoskeleton), be it from the inside or the outside of the cells, requires further analysis. In addition to ROS signaling, endo- thelial integrity is also subject to regulation by the Rap1 GTPase, microtubule dynamics and by proteins that control VE-cadherin internalization. To what extent these events are also part of the process of leukocyte TEM is presently unclear. It is important to underscore that engaged, clustered ICAM-1 and VCAM-1 may be in very close proximity on the endothelial cell surface, in particular following adhesion of leukocytes that use b1 and b2 integrins for transmigration. This means that the signaling which is induced by these molecules may also be intercon- nected. The extent of cross-talk between ICAM-1 and Fig. 1. Clustered vascular cell adhesion molecule-1 (VCAM-1) aligns with actin stress fibers. Transient expression of the VCAM-1–green fluorescent protein (GFP) fusion (green) shows its diffuse distribu- tion over the endothelial cell surface (left panel). Cross-linking by a VCAM-1 antibody induces clustering of the protein and alignment of the VCAM-1–GFP clusters with actin stress fibers (right panel; F-actin in blue). P. L. Hordijk Endothelial signalling events FEBS Journal 273 (2006) 4408–4415 ª 2006 The Author Journal compilation ª 2006 FEBS 4411 VCAM-1 induced signalling events is thus an import- ant issue for future research. Effects of shear EC in various parts of the vasculature are exposed to different levels of fluid shear stress. There is no doubt that this shear force triggers and modulates endothel- ial cell signaling and affects endothelial permeability, proliferation, migration and gene expression [42]. Shear stress is strongly associated with the develop- ment of atherosclerosis, which is an arterial disease that occurs predominantly at sites of disturbed lami- nar blood flow. VCAM-1 and E-selectin, in conjunc- tion with the actin cytoskeleton, have been shown to activate ERK2 in a shear-dependent manner [18]. Recently, the vascular endothelial growth factor receptor, VE-cadherin and PECAM-1 were identified as components of a shear detecting complex in EC [43]. This complex mediates shear induced and ligand- independent activation of src and of the phosphatidyl- inositol-3-Kinase ⁄ Akt pathway and is required for the activation of nuclear factor-jB at sites of disturbed flow. Apart from affecting the EC, shear also pro- motes chemokine-induced lymphocyte TEM, an effect coined ‘chemorheotaxis’ [13]. Thus, shear force repre- sents an additional level of regulation of both leuko- cyte migration as well as endothelial signalling. Targets of endothelial signaling There appears to be at least two classes of endothelial target downstream of the signaling that is initiated by leukocyte binding. There are rapid effects on the actin cytoskeleton and the VE–cadherin–catenin complex and these appear to co-operate in mediating efficient transendothelial migration. On the other hand, there is evidence for activation of transcription factors, such as nuclear factor-jB and c-fos [17]. The subsequent up-regulation of cell adhesion molecules or metallo- proteases may have important effects on the amplifi- cation and ⁄ or duration of the inflammatory response. Activation of ERK may well play a role in both pathways, as ERK has been implicated in the regula- tion of cell adhesion and migration [44]. In addition, ERK is involved in the activation of c-fos and of nuclear factor-jB. Thus, it appears that there is co- operativity between the ERK and p38 MAPK path- ways, as well as the ROS that are produced in the EC, in altering the gene expression profile of activated endothelium. Concluding remarks Along with the increased knowledge on the control of endothelial integrity, the number of signaling compo- nents that are implicated in the efficient transmigration of leukocytes is also growing. The key players appear to be small GTPases and the actin cytoskeleton, ROS, MAPKs, cell-matrix adhesion molecules, transcription A B Fig. 2. Models for the action of vascular cell adhesion molecule-1 (VCAM-1)-derived reactive oxygen species (ROS) in controlling transendothelial migration (TEM). According to one model (Fig. 2A), VCAM-1 activates NADPH oxidase 2 (NOX2), which resides in the plasma membrane, in a Rac1-dependent manner. Extracellularly produced ROS activate metalloproteases, which degrade junctional and ⁄ or matrix proteins. A second model (Fig. 2B) proposes ROS- mediated activation of the proline-rich tyrosine kinase 2 and phos- phorylation of b-catenin as instrumental in the transient loss of vascular endothelial cadherin-mediated cell-to-cell contact, which follows VCAM-1 engagment. Intercellular adhesion molecule-1- mediated RhoA activation is required in both models, for providing enhanced endothelial contractility. See the text for details. Endothelial signalling events P. L. Hordijk 4412 FEBS Journal 273 (2006) 4408–4415 ª 2006 The Author Journal compilation ª 2006 FEBS factors, and also enzymes such as calpain or activated metalloproteases. A recurrent theme is that the type of leukocyte, source of the EC, inflammatory stimulus and the absence or presence of shear, will affect the responses measured and thus the consequent implica- tion of a particular event in TEM. The complexity of this field is further boosted by studies on the transcellular pathway. 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J Biol Chem 280, 21129– 21136. 42 Tzima E (2006) Role of small GTPases in endothelial cytoskeletal dynamics and the shear stress response. Circ Res 98, 176–185. 43 Tzima E, Irani-Tehrani M, Kiosses WB, Dejana E, Sch- ultz DA, Engelhardt B, Cao G, DeLisser H & Schwartz MA (2005) A mechanosensory complex that mediates the endothelial cell response to fluid shear stress. Nature 437, 426–431. 44 Howe AK, Aplin AE & Juliano RL (2002) Anchorage- dependent ERK signaling – mechanisms and conse- quences. Curr Opin Genet Dev 12, 30–35. P. L. Hordijk Endothelial signalling events FEBS Journal 273 (2006) 4408–4415 ª 2006 The Author Journal compilation ª 2006 FEBS 4415 . MINIREVIEW Endothelial signalling events during leukocyte transmigration Peter L. Hordijk Department of Molecular. is certainly true for leukocyte trans- endothelial migration. A growing pallet of leukocyte adhesion-induced signaling events in endothelial cells have