Exclusion of CD45 inhibits activity of p56lck associated with glycolipid-enriched membrane domains

p140Cap modulates the mevalonate pathway decreasing cell migration and enhancing drug sensitivity in breast cancer cells

p140Cap is an adaptor protein involved in assembling multi-protein complexes regulating several cellular processes. p140Cap acts as a tumor suppressor in breast cancer (BC) and neuroblastoma patients, where its expression correlates with a better prognosis. The role of p140Cap in tumor metabolism remains largely unknown. Here we study the role of p140Cap in the modulation of the mevalonate (MVA) pathway in BC cells. The MVA pathway is responsible for the biosynthesis of cholesterol and non-sterol isoprenoids and is often deregulated in cancer. We found that both in vitro and in vivo, p140Cap cells and tumors show an increased flux through the MVA pathway by positively regulating the pace-maker enzyme of the MVA pathway, the 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGCR), via transcriptional and post-translational mechanisms. The higher cholesterol synthesis is paralleled with enhanced cholesterol efflux. Moreover, p140Cap promotes increased cholesterol localization in the plasma membrane and reduces lipid rafts-associated Rac1 signalling, impairing cell membrane fluidity and cell migration in a cholesterol-dependent manner. Finally, p140Cap BC cells exhibit decreased cell viability upon treatments with statins, alone or in combination with chemotherapeutic at low concentrations in a synergistic manner. Overall, our data highlight a new perspective point on tumor suppression in BC by establishing a previously uncharacterized role of the MVA pathway in p140Cap expressing tumors, thus paving the way to the use of p140Cap as a potent biomarker to stratify patients for better tuning therapeutic options.

The Membrane Phase Transition Gives Rise to Responsive Plasma Membrane Structure and Function

Several groups have recently reported evidence for the emergence of domains in cell plasma membranes when membrane proteins are organized by ligand binding or assembly of membrane proximal scaffolds. These domains recruit and retain components that favor the liquid-ordered phase, adding to a decades-old literature interrogating the contribution of membrane phase separation in plasma membrane organization and function. Here we propose that both past and present observations are consistent with a model in which membranes have a high compositional susceptibility, arising from their thermodynamic state in a single phase that is close to a miscibility phase transition. This rigorous framework naturally allows for both transient structure in the form of composition fluctuations and long-lived structure in the form of induced domains. In this way, the biological tuning of plasma membrane composition enables a responsive compositional landscape that facilitates and augments cellular biochemistry vital to plasma membrane functions.

Pre-organized landscape of T cell surface

T cell activation is initiated by the recognition of specific antigenic peptides and subsequently accomplished by complex signaling cascades. These aspects have been extensively studied for decades as pivotal factors in the establishment of adaptive immunity. However, how receptors or signaling molecules are organized in the resting state prior to encountering antigens has received less attention. Recent advancements in super-resolution microscopy techniques have revealed topographically controlled pre-formed organization of key molecules involved in antigen recognition and signal transduction on microvillar projections of T cells before activation and substantial effort has been dedicated to characterizing the topological structure of resting T cells over the past decade. This review will summarize our current understanding of how key surface receptors are pre-organized on the T-cell plasma membrane and discuss the potential role of these receptors, which are preassembled prior to ligand binding in the early activation events of T cells.

Coordinating Cytoskeleton and Molecular Traffic in T Cell Migration, Activation, and Effector Functions

Dynamic localization of receptors and signaling molecules at the plasma membrane and within intracellular vesicular compartments is crucial for T lymphocyte sensing environmental cues, triggering membrane receptors, recruiting signaling molecules, and fine-tuning of intracellular signals. The orchestrated action of actin and microtubule cytoskeleton and intracellular vesicle traffic plays a key role in all these events that together ensure important steps in T cell physiology. These include extravasation and migration through lymphoid and peripheral tissues, T cell interactions with antigen-presenting cells, T cell receptor (TCR) triggering by cognate antigen-major histocompatibility complex (MHC) complexes, immunological synapse formation, cell activation, and effector functions. Cytoskeletal and vesicle traffic dynamics and their interplay are coordinated by a variety of regulatory molecules. Among them, polarity regulators and membrane-cytoskeleton linkers are master controllers of this interplay. Here, we review the various ways the T cell plasma membrane, receptors, and their signaling machinery interplay with the actin and microtubule cytoskeleton and with intracellular vesicular compartments. We highlight the importance of this fine-tuned crosstalk in three key stages of T cell biology involving cell polarization: T cell migration in response to chemokines, immunological synapse formation in response to antigen cues, and effector functions. Finally, we discuss two examples of perturbation of this interplay in pathological settings, such as HIV-1 infection and mutation of the polarity regulator and tumor suppressor adenomatous polyposis coli (Apc) that leads to familial polyposis and colorectal cancer.

Role of lipid rafts in neuronal differentiation of dental pulp-derived stem cells

Human dental pulp-derived stem cells (hDPSCs) are characterized by a typical fibroblast-like morphology. They express specific markers for mesenchymal stem cells and are capable of differentiation into osteoblasts, adipoblasts and neurons in vitro. Previous studies showed that gangliosides are involved in the induction of early neuronal differentiation of hDPSCs. This study was undertaken to investigate the role of lipid rafts in this process. Lipid rafts are signaling microdomains enriched in glycosphingolipids, cholesterol, tyrosine kinase receptors, mono- or heterotrimeric G proteins and GPI-anchored proteins. We preliminary showed that established cells expressed multipotent mesenchymal stromal-specific surface antigens. Then, we analyzed the distribution of lipid rafts, revealing plasma membrane microdomains with GM2 and EGF-R enrichment. Following stimulation with EGF/bFGF, neuronal differentiation was observed. To analyze the functional role of lipid rafts in EGF/bFGF-induced hDPSCs differentiation, cells were preincubated with lipid raft affecting agents, i.e. [D]-PDMP or methyl-β-cyclodextrin. These compounds significantly prevented neuronal-specific antigen expression, as well as Akt and ERK 1/2 phosphorylation, induced by EGF/bFGF, indicating that lipid raft integrity is essential for EGF/bFGF-induced hDPSCs differentiation. These results suggest that lipid rafts may represent specific chambers, where multimolecular signaling complexes, including lipids (gangliosides, cholesterol) and proteins (EGF-R), play a role in hDPSCs differentiation.

O-glycans direct selectin ligands to lipid rafts on leukocytes

Palmitoylated cysteines typically target transmembrane proteins to domains enriched in cholesterol and sphingolipids (lipid rafts). P-selectin glycoprotein ligand-1 (PSGL-1), CD43, and CD44 are O-glycosylated proteins on leukocytes that associate with lipid rafts. During inflammation, they transduce signals by engaging selectins as leukocytes roll in venules, and they move to the raft-enriched uropods of polarized cells upon chemokine stimulation. It is not known how these glycoproteins associate with lipid rafts or whether this association is required for signaling or for translocation to uropods. Here, we found that loss of core 1-derived O-glycans in murine C1galt1(-/-) neutrophils blocked raft targeting of PSGL-1, CD43, and CD44, but not of other glycosylated proteins, as measured by resistance to solubilization in nonionic detergent and by copatching with a raft-resident sphingolipid on intact cells. Neuraminidase removal of sialic acids from wild-type neutrophils also blocked raft targeting. C1galt1(-/-) neutrophils or neuraminidase-treated neutrophils failed to activate tyrosine kinases when plated on immobilized anti-PSGL-1 or anti-CD44 F(ab')2. Furthermore, C1galt1(-/-) neutrophils incubated with anti-PSGL-1 F(ab')2 did not generate microparticles. In marked contrast, PSGL-1, CD43, and CD44 moved normally to the uropods of chemokine-stimulated C1galt1(-/-) neutrophils. These data define a role for core 1-derived O-glycans and terminal sialic acids in targeting glycoprotein ligands for selectins to lipid rafts of leukocytes. Preassociation of these glycoproteins with rafts is required for signaling but not for movement to uropods.

Multimodality imaging of coiled-coil mediated self-assembly in a "drug-free" therapeutic system

Two complementary coiled-coil peptides CCE/CCK are used to develop a "drug free" therapeutic system, which can specifically kill cancer cells without a drug. CCE is attached to the Fab' fragment of anti-CD20 1F5 antibody (Fab'-CCE), and CCK is conjugated in multiple grafts to poly[N-(2-hydroxypropyl)methacrylamide] (P-(CCK)x ). Two conjugates are consecutively administered: First, Fab'-CCE coats peptide CCE at CD20 antigen of lymphoma cell surface; second, CCE/CCK biorecognition between Fab'-CCE and P-(CCK)x leads to coiled-coil formation, CD20 crosslinking, membrane reorganization, and ultimately cell apoptosis. To prove that two conjugates can assemble at cell surface, multiple fluorescence imaging studies are performed, including 2-channel FMT, 3D confocal microscopy, and 4-color FACS. Confocal microscopy shows colocalization of two fluorescently labeled conjugates on non-Hodgkin's lymphoma (NHL) Raji cell surface, indicating "two-step" targeting specificity. The fluorescent images also reveal that these two conjugates can disrupt normal membrane lipid distribution and form lipid raft clusters, leading to cancer cell apoptosis. This "two-step" biorecognition capacity is further demonstrated in a NHL xenograft model, using fluorescent images at whole-body, tissue and cell levels. It is also found that delaying injection of P-(CCK)x can significantly enhance targeting efficacy. This high-specificity therapeutics provide a safe option to treat NHL and other B cell malignancies.

Membrane–cytoskeleton interactions in cholesterol-dependent domain formation

Since the inception of the fluid mosaic model, cell membranes have come to be recognized as heterogeneous structures composed of discrete protein and lipid domains of various dimensions and biological functions. The structural and biological properties of membrane domains are represented by CDM (cholesterol-dependent membrane) domains, frequently referred to as membrane ‘rafts’. Biological functions attributed to CDMs include signal transduction. In T-cells, CDMs function in the regulation of the Src family kinase Lck (p56lck) by sequestering Lck from its activator CD45. Despite evidence of discrete CDM domains with specific functions, the mechanism by which they form and are maintained within a fluid and dynamic lipid bilayer is not completely understood. In the present chapter, we discuss recent advances showing that the actomyosin cytoskeleton has an integral role in the formation of CDM domains. Using Lck as a model, we also discuss recent findings regarding cytoskeleton-dependent CDM domain functions in protein regulation.

The role of membrane rafts in Lck transport, regulation and signalling in T-cells

Tyrosine phosphorylation is one of the key covalent modifications that occur in multicellular organisms. Since its discovery more than 30 years ago, tyrosine phosphorylation has come to be understood as a fundamentally important mechanism of signal transduction and regulation in all eukaryotic cells. The tyrosine kinase Lck (lymphocyte-specific protein tyrosine kinase) plays a crucial role in the T-cell response by transducing early activation signals triggered by TCR (T-cell receptor) engagement. These signals result in the phosphorylation of immunoreceptor tyrosine-based activation motifs present within the cytosolic tails of the TCR-associated CD3 subunits that, once phosphorylated, serve as scaffolds for the assembly of a large supramolecular signalling complex responsible for T-cell activation. The existence of membrane nano- or micro-domains or rafts as specialized platforms for protein transport and cell signalling has been proposed. The present review discusses the signals that target Lck to membrane rafts and the importance of these specialized membranes in the transport of Lck to the plasma membrane, the regulation of Lck activity and the phosphorylation of the TCR.

CD28 sensitizes TCR Ca²⁺ signaling during Ag-independent polarization of plasma membrane rafts

T cells become polarized during initial interactions with an APC to form an Ag-independent synapse (AIS) composed of membrane rafts, TCR, and TCR-proximal signaling molecules. AISs occur temporally before TCR triggering, but their role in downstream TCR signaling is not understood. Using both human and murine model systems, we studied the signals that activate AIS formation and the effect of these signals on TCR-dependent responses. We show that CD28 produces AISs detectable by spinning disc confocal microscopy seconds following initial interactions between the T cell and APC. AIS formation by CD28 coincided with costimulatory signaling, evidenced by a cholesterol-sensitive activation of the MAPK ERK that potentiated Ca²⁺ signaling in response to CD3 cross-linking. CD45 also enriched in AISs but to modulate Src kinase activity, because localization of CD45 at the cell interface reduced the activation of proximal Lck. In summary, we show that signaling by CD28 during first encounters between the T cell and APC both sensitizes TCR Ca²⁺ signaling by an Erk-dependent mechanism and drives formation of an AIS that modulates the early signaling until TCR triggering occurs. Thus, early Ag-independent encounters are an important window for optimizing T cell responses to Ag by CD28.

A mechanistic model of early FcεRI signaling: lipid rafts and the question of protection from dephosphorylation

We present a model of the early events in mast cell signaling mediated by FcεRI where the plasma membrane is composed of many small ordered lipid domains (rafts), surrounded by a non-order region of lipids consisting of the remaining plasma membrane. The model treats the rafts as transient structures that constantly form and breakup, but that maintain a fixed average number per cell. The rafts have a high propensity for harboring Lyn kinase, aggregated, but not unaggregated receptors, and the linker for the activation of T cells (LAT). Phosphatase activity in the rafts is substantially reduced compared to the nonraft region. We use the model to analyze published experiments on the rat basophilic leukemia (RBL)-2H3 cell line that seem to contradict the notion that rafts offer protection. In these experiments IgE was cross-linked with a multivalent antigen and then excess monovalent hapten was added to break-up cross-links. The dephosphorylation of the unaggregated receptor (nonraft associated) and of LAT (raft associated) were then monitored in time and found to decay at similar rates, leading to the conclusion that rafts offer no protection from dephosphorylation. In the model, because the rafts are transient, a protein that is protected while in a raft will be subject to dephosphorylation when the raft breaks up and the protein finds itself in the nonraft region of the membrane. We show that the model is consistent with the receptor and LAT dephosphorylation experiments while still allowing rafts to enhance signaling by providing substantial protection from phosphatases.

How does the kinase Lck phosphorylate the T cell receptor? Spatial organization as a regulatory mechanism

T cell signaling begins with the ligation of the T cell antigen receptor (TCR) by a cognate peptide and the phosphorylation of the receptor’s immunoreceptor tyrosine-based activation motif domains by the kinase Lck. However, the canonical receptor model is insufficient to explain how the constitutively active kinase Lck can discriminate between non-ligated and ligated TCRs. Here, we discuss the factors that are thought to regulate the spatial distribution of the TCR and Lck, and therefore critically influence TCR signaling initiation.

Cytoskeletal modulation of lipid interactions regulates Lck kinase activity

The actin cytoskeleton promotes clustering of proteins associated with cholesterol-dependent rafts, but its effect on lipid interactions that form and maintain rafts is not understood. We addressed this question by determining the effect of disrupting the cytoskeleton on co-clustering of dihexadecyl-(C(16))-anchored DiO and DiI, which co-enrich in ordered lipid environments such as rafts. Co-clustering was assayed by fluorescence resonance energy transfer (FRET) in labeled T cells, where rafts function in the phosphoregulation of the Src family kinase Lck. Our results show that probe co-clustering was sensitive to depolymerization of actin filaments with latrunculin B (Lat B), inhibition of myosin II with blebbistatin, and treatment with neomycin to sequester phosphatidylinositol 4,5-bisphosphate. Cytoskeletal effects on lipid interactions were not restricted to order-preferring label because co-clustering of C(16)-anchored DiO with didodecyl (C(12))-anchored DiI, which favors disordered lipids, was also reduced by Lat B and blebbistatin. Furthermore, conditions that disrupted probe co-clustering resulted in activation of Lck. These data show that the cytoskeleton globally modulates lipid interactions in the plasma membrane, and this property maintains rafts that function in Lck regulation.

Control of genetically prescribed protein tyrosine kinase activities by environment-linked redox reactions

Recent observations on environment-linked control of genetically prescribed signaling systems for either cell activation or cell death have been reviewed with a focus on the regulation of activities of protein tyrosine kinases (PTKs). The environment-linked redox reactions seem to primarily affect cell surface receptors and cell membrane lipid rafts, and they induce generation of reactive oxygen species (ROS) in cells. ROS thus generated might upregulate the catalytic activities of PTKs through inactivating protein tyrosine phosphatases that dephosphorylate and inactivate autophosphorylated PTKs. Recent evidence has, however, demonstrated that ROS could also directly oxidize SH groups of genetically conserved specific cysteines on PTKs, sometimes producing disulfide-bonded dimers of PTK proteins, either for upregulation or downregulation of their catalytic activities. The basic role of the redox reaction/covalent bond-mediated modification of protein tertiary structure-linked noncovalent bond-oriented signaling systems in living organisms is discussed.

Inhibition of lymphoid tyrosine phosphatase by benzofuran salicylic acids

The lymphoid tyrosine phosphatase (Lyp, PTPN22) is a critical negative regulator of T cell antigen receptor (TCR) signaling. A single-nucleotide polymorphism (SNP) in the ptpn22 gene correlates with the incidence of various autoimmune diseases, including type 1 diabetes, rheumatoid arthritis, and systemic lupus erythematosus. Since the disease-associated allele is a more potent inhibitor of TCR signaling, specific Lyp inhibitors may become valuable in treating autoimmunity. Using a structure-based approach, we synthesized a library of 34 compounds that inhibited Lyp with IC(50) values between 0.27 and 6.2 μM. A reporter assay was employed to screen for compounds that enhanced TCR signaling in cells, and several inhibitors displayed a dose-dependent, activating effect. Subsequent probing for Lyp's direct physiological targets by immunoblot analysis confirmed the ability of the compounds to inhibit Lyp in T cells. Selectivity profiling against closely related tyrosine phosphatases and in silico docking studies with the crystal structure of Lyp yielded valuable information for the design of Lyp-specific compounds.

Plasma membrane subdomain partitioning of Lck in primary human T lymphocytes

Uncovering the plasma membrane distribution of tyrosine kinase Lck is crucial to understanding T lymphocyte triggering. Several studies of Lck species partitioning have given contradictory results. We decided to re-address this point by using phospho-specific antibodies to characterize active and inactive Lck partitioning in raft and non-raft membranes from primary human peripheral blood T lymphocytes. We show that most inactive Lck was localized in rafts and was associated with nearly all CD4 coreceptors and its negative regulator Csk in resting cells, while T cell receptor (TCR) engagement promoted a sustained dephosphorylation of inactive Lck. In contrast, active Lck had a more discrete distribution interacting with only a small number of CD4 coreceptors, and the kinase showed a rapid and short phosphorylation after TCR triggering. The differences in distribution and kinetics may be related to T lymphocyte signalling threshold modulation by Lck species and suggest how TCR triggering is first initiated. This study furthers our knowledge of the TCR activation model in primary human T lymphocytes.

Functional role of gangliotetraosylceramide in epithelial-to-mesenchymal transition process induced by hypoxia and by TGF-{beta}

The epithelial-to-mesenchymal transition (EMT) is a basic cellular process that plays a key role in normal embryonic development and in cancer progression/metastasis. Our previous study indicated that EMT processes of mouse and human epithelial cells induced by TGF-β display clear reduction of gangliotetraosylceramide (Gg4) and ganglioside GM2, suggesting a close association of glycosphingolipids (GSLs) with EMT. In the present study, using normal murine mammary gland (NMuMG) cells, we found that levels of Gg4 and of mRNA for the UDP-Gal:β1-3galactosyltransferase-4 (β3GalT4) gene, responsible for reduction of Gg4, were reduced in EMT induced by hypoxia (∼1% O(2)) or CoCl(2) (hypoxia mimic), similarly to that for TGF-β-induced EMT. An increase in the Gg4 level by its exogenous addition or by transfection of the β3GalT4 gene inhibited the hypoxia-induced or TGF-β-induced EMT process, including changes in epithelial cell morphology, enhanced motility, and associated changes in epithelial vs. mesenchymal molecules. We also found that Gg4 is closely associated with E-cadherin and β-catenin. These results suggest that the β3GalT4 gene, responsible for Gg4 expression, is down-regulated in EMT; and Gg4 has a regulatory function in the EMT process in NMuMG cells, possibly through interaction with epithelial molecules important to maintain epithelial cell membrane organization.

Phosphorylation of leukocyte PECAM and its association with detergent-resistant membranes regulate transendothelial migration

Leukocyte migration across the endothelial lining is a critical step in the body's response to infection and inflammation. The homophilic interaction between endothelial PECAM and leukocyte PECAM is essential for this process. The molecular events that are triggered in the endothelial cell by PECAM engagement have been well characterized; however, the function of leukocyte PECAM remains to be elucidated. To study this, we first blocked leukocyte transmigration using anti-PECAM Ab and then specifically activated leukocyte PECAM. This was sufficient to overcome the block and promote transmigration, suggesting an active signaling role for leukocyte PECAM. Consistent with this, we found that ligation of leukocyte PECAM induces phosphorylation of two tyrosine residues on its cytoplasmic tail. By performing RNA interference-rescue experiments, we demonstrate that these phosphorylation events are indispensable for transendothelial migration. Finally, we show that leukocyte PECAM translocates to a detergent-resistant membrane (DRM) during transmigration. PECAM localized in DRMs displays reduced phosphorylation and does not support transmigration. Together, these data support a model whereby engagement of leukocyte PECAM induces its transient tyrosine phosphorylation and induction of downstream signals that drive transmigration. These signals are then downregulated following PECAM translocation to DRMs.

Grb2, a simple adapter with complex roles in lymphocyte development, function, and signaling

Lymphocyte development, activation, and tolerance depend on antigen receptor signaling transduced via multiple intracellular signalosomes. These signalosomes are assembled by different adapters. Given that signaling molecules can be either positive or negative regulators for a biochemical target, the complex of a target with different regulator may dictate the final signaling outcome. Grb2 is a simple adapter known to be involved in a variety of growth factor receptor signaling. However, its role in antigen receptor signaling as well as lymphocyte development and function has emerged only recently. Despite its simple molecular structure, recent experiments show that Grb2 may play a complex role in T and B-cell antigen receptor signaling. In this article, we review recent findings about the physiological role of Grb2 in T and B-cell development and activation and summarize the current mechanistic understanding of how Grb2 exerts its function following T and B-cell antigen receptor stimulation.

Glucosylceramide synthase upregulates MDR1 expression in the regulation of cancer drug resistance through cSrc and beta-catenin signaling

Background

Drug resistance is the outcome of multiple-gene interactions in cancer cells under stress of anticancer agents. MDR1 overexpression is most commonly detected in drug-resistant cancers and accompanied with other gene alterations including enhanced glucosylceramide synthase (GCS). MDR1 encodes for P-glycoprotein that extrudes anticancer drugs. Polymorphisms of MDR1 disrupt the effects of P-glycoprotein antagonists and limit the success of drug resistance reversal in clinical trials. GCS converts ceramide to glucosylceramide, reducing the impact of ceramide-induced apoptosis and increasing glycosphingolipid (GSL) synthesis. Understanding the molecular mechanisms underlying MDR1 overexpression and how it interacts with GCS may find effective approaches to reverse drug resistance.

Results

MDR1 and GCS were coincidently overexpressed in drug-resistant breast, ovary, cervical and colon cancer cells; silencing GCS using a novel mixed-backbone oligonucleotide (MBO-asGCS) sensitized these four drug-resistant cell lines to doxorubicin. This sensitization was correlated with the decreased MDR1 expression and the increased doxorubicin accumulation. Doxorubicin treatment induced GCS and MDR1 expression in tumors, but MBO-asGCS treatment eliminated "in-vivo" growth of drug-resistant tumor (NCI/ADR-RES). MBO-asGCS suppressed the expression of MDR1 with GCS and sensitized NCI/ADR-RES tumor to doxorubicin. The expression of P-glycoprotein and the function of its drug efflux of tumors were decreased by 4 and 8 times after MBO-asGCS treatment, even though this treatment did not have a significant effect on P-glycoprotein in normal small intestine. GCS transient transfection induced MDR1 overexpression and increased P-glycoprotein efflux in dose-dependent fashion in OVCAR-8 cancer cells. GSL profiling, silencing of globotriaosylceramide synthase and assessment of signaling pathway indicated that GCS transfection significantly increased globo series GSLs (globotriaosylceramide Gb3, globotetraosylceramide Gb4) on GSL-enriched microdomain (GEM), activated cSrc kinase, decreased β-catenin phosphorylation, and increased nuclear β-catenin. These consequently increased MDR1 promoter activation and its expression. Conversely, MBO-asGCS treatments decreased globo series GSLs (Gb3, Gb4), cSrc kinase and nuclear β-catenin, and suppressed MDR-1 expression in dose-dependent pattern.

Conclusion

This study demonstrates, for the first time, that GCS upregulates MDR1 expression modulating drug resistance of cancer. GSLs, in particular globo series GSLs mediate gene expression of MDR1 through cSrc and β-catenin signaling pathway.

Modulation of immune cell signalling by the leukocyte common tyrosine phosphatase, CD45

CD45 is a leukocyte specific transmembrane glycoprotein and a receptor-like protein tyrosine phosphatase (PTP). CD45 can be expressed as several alternatively spliced isoforms that differ in the extracellular domain. The isoforms are regulated in a cell type and activation state-dependent manner, yet their function has remained elusive. The Src family kinase members Lck and Lyn are key substrates for CD45 in T and B lymphocytes, respectively. CD45 lowers the threshold of antigen receptor signalling, which impacts T and B cell activation and development. CD45 also regulates antigen triggered Fc receptor signalling in mast cells and Toll-like receptor (TLR) signalling in dendritic cells, thus broadening the role of CD45 to other recognition receptors involved in adaptive and innate immunity. In addition, CD45 can affect immune cell adhesion and migration and can modulate cytokine production and signalling. Here we review what is known about the substrate specificity and regulation of CD45 and summarise its effect on immune cell signalling pathways, from its established role in T and B antigen receptor signalling to its emerging role regulating innate immune cell recognition and cytokine production.

T cell signal regulation by the actin cytoskeleton

T cells form an immunological synapse (IS) that sustains and regulates signals for cell stimulation. Enriched in the IS is the Src family kinase Lck. Conversely, the membrane phosphatase CD45, which activates Src family kinases, is excluded, and this is necessary to avoid quenching of T cell receptor phosphosignals. Data suggest that this arrangement occurs by an enrichment of cholesterol-dependent rafts in the IS. However, the role of rafts in structuring the IS remains unclear. To address this question, we used fluorescence resonance energy transfer (FRET) to interrogate the nanoscopic structure of the IS. The FRET probes consisted of membrane-anchored fluorescent proteins with distinct affinities for rafts. Both the raft and nonraft probes exhibited clustering in the IS. However, co-clustering of raft donor-acceptor pairs was 10-fold greater than co-clustering of raft-nonraft pairs. We measured the effect of disrupting rafts in the IS on CD45 localization and Lck regulation by treating stimulated T cells with filipin. The filipin specifically disrupted co-clustering of the raft FRET pairs in the IS and allowed targeting of CD45 to the IS and dephosphorylation of the regulatory tyrosine of Lck. Clustering of the raft probes was also sensitive to latrunctulin B, which disrupts actin filaments. Strikingly, enriching the cortical cytoskeleton using jasplakinolide maintained raft probe co-clustering, CD45 exclusion, and Lck regulation in the IS following the addition of filipin. These data show the actin cytoskeleton maintains a membrane raft environment in the IS that promotes Lck regulation by excluding CD45.

Dynamic regulation of CD45 lateral mobility by the spectrin-ankyrin cytoskeleton of T cells

The leukocyte common antigen, CD45, is a critical immune regulator whose activity is modulated by cytoskeletal interactions. Components of the spectrin-ankyrin cytoskeleton have been implicated in the trafficking and signaling of CD45. We have examined the lateral mobility of CD45 in resting and activated T lymphocytes using single-particle tracking and found that the receptor has decreased mobility caused by increased cytoskeletal contacts in activated cells. Experiments with cells that have disrupted betaI spectrin interactions show decreased cytoskeletal contacts in resting cells and attenuation of receptor immobilization in activated cells. Applying two types of population analyses to single-particle tracking trajectories, we find good agreement between the diffusion coefficients obtained using either a mean squared displacement analysis or a hidden Markov model analysis. Hidden Markov model analysis also reveals the rate of association and dissociation of CD45-cytoskeleton contacts, demonstrating the importance of this analysis for measuring cytoskeleton binding events in live cells. Our findings are consistent with a model in which multiple cytoskeletal contacts, including those with spectrin and ankyrin, participate in the regulation of CD45 lateral mobility. These interactions are a major factor in CD45 immobilization in activated cells. Furthermore, cellular activation leads to CD45 immobilization by reduction of the CD45-cytoskeleton dissociation rate. Short peptides that mimic spectrin repeat domains alter the association rate of CD45 to the cytoskeleton and cause an apparent decrease in dissociation rates. We propose a model for CD45-cytoskeleton interactions and conclude that the spectrin-ankyrin-actin network is an essential determinant of immunoreceptor mobility.

Association of fission proteins with mitochondrial raft-like domains

It was shown that receptor-mediated apoptosis involves a cascade of subcellular events including alterations of mitochondria. Loss of mitochondrial membrane potential that follows death receptor ligation allows the release of apoptogenic factors that result in apoptosis execution. Further important mitochondrial changes have been observed in this regard: mitochondrial remodeling and fission that appear as prerequisites for the occurrence of the cell death program. As it was observed that lipid rafts, glycosphingolipid-enriched structures, can participate in the apoptotic cascade being recruited to the mitochondria under receptor-mediated proapoptotic stimulation, we decided to analyze the possible implication of these microdomains in mitochondrial fission. We found that molecules involved in mitochondrial fission processes are associated with these domains. In particular, although hFis1 was constitutively included in mitochondrial raft-like domains, dynamin-like protein 1 was recruited to these domains on CD95/Fas triggering. Accordingly, the disruption of rafts, for example, by inhibiting ceramide synthase, leads to the impairment of fission molecule recruitment to the mitochondria, reduction of mitochondrial fission and a significant reduction of apoptosis. We hypothesize that under apoptotic stimulation the recruitment of fission-associated molecules to the mitochondrial rafts could have a role in the morphogenetic changes leading to organelle fission.

Opposite effect of membrane raft perturbation on transport activity of KCC2 and NKCC1

In the majority of neurons, the intracellular Cl(-) concentration is set by the activity of the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) and the K(+)-Cl(-) cotransporter (KCC2). Here, we investigated the cotransporters' functional dependence on membrane rafts. In the mature rat brain, NKCC1 was mainly insoluble in Brij 58 and co-distributed with the membrane raft marker flotillin-1 in sucrose density flotation experiments. In contrast, KCC2 was found in the insoluble fraction as well as in the soluble fraction, where it co-distributed with the non-raft marker transferrin receptor. Both KCC2 populations displayed a mature glycosylation pattern. Disrupting membrane rafts with methyl-beta-cyclodextrin (MbetaCD) increased the solubility of KCC2, yet had no effect on NKCC1. In human embryonic kidney-293 cells, KCC2 was strongly activated by a combined treatment with MbetaCD and sphingomyelinase, while NKCC1 was inhibited. These data indicate that membrane rafts render KCC2 inactive and NKCC1 active. In agreement with this, inactive KCC2 of the perinatal rat brainstem largely partitioned into membrane rafts. In addition, the exposure of the transporters to MbetaCD and sphingomyelinase showed that the two transporters differentially interact with the membrane rafts. Taken together, membrane raft association appears to represent a mechanism for co-ordinated regulation of chloride transporter function.

Cytoskeleton-membrane interactions in membrane raft structure

Cell membranes are structurally heterogeneous, composed of discrete domains with unique physical and biological properties. Membrane domains can form through a number of mechanisms involving lipid-lipid and protein-lipid interactions. One type of membrane domain is the cholesterol-dependent membrane raft. How rafts form remains a current topic in membrane biology. We review here evidence of structuring of rafts by the cortical actin cytoskeleton. This includes evidence that the actin cytoskeleton associates with rafts, and that many of the structural and functional properties of rafts require an intact actin cytoskeleton. We discuss the mechanisms of the actin-dependent raft organization, and the properties of the actin cytoskeleton in regulating raft-associated signaling events. We end with a discussion of membrane rafts and the actin cytoskeleton in T cell activation, which function synergistically to initiate the adaptive immune response.

Tetraspanin CD9 negatively regulates lipopolysaccharide-induced macrophage activation and lung inflammation

Tetraspanins facilitate the formation of multiple molecular complexes at specialized membrane microdomains and regulate cell activation and motility. In the present study, the role of tetraspanin CD9 in LPS-induced macrophage activation and lung inflammation was investigated in vitro and in vivo. When CD9 function was ablated with mAb treatment, small interfering RNA transfection, or gene knockout in RAW264.7 cells or bone marrow-derived macrophages, these macrophages produced larger amounts of TNF-alpha, matrix metalloproteinase-2, and -9 upon stimulation with LPS in vitro, when compared with control cells. Sucrose gradient analysis revealed that CD9 partly colocalized with the LPS-induced signaling mediator, CD14, at low-density light membrane fractions. In CD9 knockout macrophages, CD14 expression, CD14 and TLR4 localization into the lipid raft, and their complex formation were increased whereas IkappaBalpha expression was decreased when compared with wild-type cells, suggesting that CD9 prevents the formation of LPS receptor complex. Finally, deletion of CD9 in mice enhanced macrophage infiltration and TNF-alpha production in the lung after intranasal administration of LPS in vivo, when compared with wild-type mice. These results suggest that macrophage CD9 negatively regulates LPS response at lipid-enriched membrane microdomains.

CD45 down-regulates Lck-mediated CD44 signaling and modulates actin rearrangement in T cells

The tyrosine phosphatase CD45 dephosphorylates the negative regulatory tyrosine of the Src family kinase Lck and plays a positive role in TCR signaling. In this study we demonstrate a negative regulatory role for CD45 in CD44 signaling leading to actin rearrangement and cell spreading in activated thymocytes and T cells. In BW5147 T cells, CD44 ligation led to CD44 and Lck clustering, which generated a reduced tyrosine phosphorylation signal in CD45(+) T cells and a more sustained, robust tyrosine phosphorylation signal in CD45(-) T cells. This signal resulted in F-actin ring formation and round spreading in the CD45(+) cells and polarized, elongated cell spreading in CD45(-) cells. The enhanced signal in the CD45(-) cells was consistent with enhanced Lck Y394 phosphorylation compared with the CD45(+) cells where CD45 was recruited to the CD44 clusters. This enhanced Src family kinase-dependent activity in the CD45(-) cells led to PI3K and phospholipase C activation, both of which were required for elongated cell spreading. We conclude that CD45 induces the dephosphorylation of Lck at Y394, thereby preventing sustained Lck activation and propose that the amplitude of the Src family kinase-dependent signal regulates the outcome of CD44-mediated signaling to the actin cytoskeleton and T cell spreading.

Have we become overly reliant on lipid rafts? Talking Point on the involvement of lipid rafts in T-cell activation

During the past decade, the lipid-raft hypothesis has focused attention on the role of membrane domains in controlling cellular functions. Among the best-studied roles of lipid rafts is the regulation of T-cell signalling. In particular, a model has emerged in which lipid rafts regulate protein-protein interactions during signalling in a cholesterol-dependent manner. Does this model provide the best description of what is happening in living cell membranes? Alternatively, has our ability to evaluate this question critically become compromised by the influential nature of the lipid-raft model itself? Here, this issue is explored in the context of two of the major tenets of the lipid-raft model.

Compartmentalization of phosphatidylinositol 4,5-bisphosphate signaling evidenced using targeted phosphatases

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) is a prevalent phosphoinositide in cell membranes, with important functions in cell signaling and activation. A large fraction of PIP(2) associates with the detergent-resistant membrane "raft" fraction, but the functional significance of this association remains controversial. To measure the properties of raft and nonraft PIP(2) in cell signaling, we targeted the PIP(2)-specific phosphatase Inp54p to either the raft or nonraft membrane fraction using minimal membrane anchors. Interestingly, we observed that targeting Inp54p to the nonraft fraction resulted in an enrichment of raft-associated PIP(2) and striking changes in cell morphology, including a wortmannin-sensitive increase in cell filopodia and cell spreading. In contrast, raft-targeted Inp54p depleted the raft pool of PIP(2) and produced smooth T cells void of membrane ruffling and filopodia. Furthermore, raft-targeted Inp54p inhibited capping in T cells stimulated by cross-linking the T cell receptor, but without affecting the T cell receptor-dependent Ca(2+) flux. Altogether, these results provide evidence of compartmentalization of PIP(2)-dependent signaling in cell membranes such as predicted by the membrane raft model.

Altered lipid raft composition and defective cell death signal transduction in glycosylphosphatidylinositol anchor-deficient PIG-A mutant cells

Paroxysmal nocturnal haemoglobinuria (PNH) is a clonal disorder of haematopoietic stem cells caused by somatic PIGA mutations, resulting in a deficiency in glycosylphosphatidylinositol-anchored proteins (GPI-AP). Because GPI-AP associate with lipid rafts (LR), lack of GPI-AP on PNH cells may result in alterations in LR-dependent signalling. Conversely, PNH cells are a suitable model for investigating LR biology. LR from paired, wild-type GPI(+), and mutant GPI(-) cell lines (K562 and TF1) were isolated and analysed; GPI(-) LR contained important anti-apoptotic proteins, not found in LR from GPI(+) cells. When methyl-beta-cyclodextrin (MbetaCD) was utilized to probe for functional differences between normal and GPI(-) LR, increased levels of phospho-p38 mitogen-activated protein kinase (MAPK), and phospho-p65 nuclear factor NF-kappaB were found in control and GPI(-) cells respectively. Subsequent experiments addressing the inhibition of phosphoinositide-3-kinase (PI3K) suggest that the PI3K/AKT pathway may be responsible for the resistance of K562 GPI(-)cells to negative effects of MbetaCD. In addition, transduction of tumour necrosis factor-alpha (TNF-alpha) signals in a LR-dependent fashion increased induction of p38 MAPK in GPI(+) and increased pro-survival NF-kappaB levels in K562 GPI(-) cells. Therefore, we suggest that the altered LR-dependent signalling in PNH-like cells may induce different responses to pro-inflammatory cytokines from those observed in cells with intact GPI-AP.

A mechanism for SRC kinase-dependent signaling by noncatalytic receptors

A fundamental issue in cell biology is how signals are transmitted across membranes. A variety of transmembrane receptors, including multichain immune recognition receptors, lack catalytic activity and require Src family kinases (SFKs) for signal transduction. However, many receptors only bind and activate SFKs after ligand-induced receptor dimerization. This presents a conundrum: How do SFKs sense the dimerization of receptors to which they are not already bound? Most proposals for resolving this enigma invoke additional players, such as lipid rafts or receptor conformational changes. Here we used simple thermodynamics to show that SFK activation is a natural outcome of clustering of receptors with SFK phosphorylation sites, provided that there is phosphorylation-dependent receptor-SFK association and an SFK bound to one receptor can phosphorylate the second receptor or its associated SFK in a dimer. A simple system of receptor, SFK, and an unregulated protein tyrosine phosphatase (PTP) can account for ligand-induced changes in phosphorylation observed in cells. We suggest that a core signaling system comprising a receptor with SFK phosphorylation sites, an SFK, and an unregulated PTP provides a robust mechanism for transmembrane signal transduction. Other events that regulate signaling in specific cases may have evolved for fine-tuning of this basic mechanism.

Clustering of membrane raft proteins by the actin cytoskeleton

Cell membranes are laterally organized into functionally discrete domains that include the cholesterol-dependent membrane "rafts." However, how membrane domains are established and maintained remains unresolved and controversial but often requires the actin cytoskeleton. In this study, we used fluorescence resonance energy transfer to measure the role of the actin cytoskeleton in the co-clustering of membrane raft-associated fluorescent proteins (FPs) and FPs targeted to the nonraft membrane fraction. By fitting the fluorescence resonance energy transfer data to an isothermal binding equation, we observed a specific co-clustering of raft-associated donor and acceptor probes that was sensitive to latrunculin B (Lat B), which disrupts the actin cytoskeleton. Conversely, treating with jasplakinolide to enhance actin polymerization increased co-clustering of the raft-associated FPs over that of the nonraft probes. We also observed by immunoblotting experiments that the actin-dependent co-clustering coincided with regulation of the raft-associated Src family kinase Lck. Specifically, Lat B decreased the phosphorylation of the C-terminal regulatory tyrosine of Lck (Tyr505), and combining the Lat B with filipin further decreased the Tyr505 phosphorylation. Furthermore, the Lat B-dependent changes in Lck regulation required CD45 because no significant changes occurred in treated T cells lacking CD45 expression. These data define a role for the actin cytoskeleton in promoting co-clustering of raft-associated proteins and show that this property is important toward regulating raft-associated signaling proteins such as Lck.

Anti-beta2-glycoprotein I antibodies induce monocyte release of tumor necrosis factor alpha and tissue factor by signal transduction pathways involving lipid rafts

OBJECTIVE

To investigate the association of beta(2)-glycoprotein I (beta(2)GPI) with lipid rafts in monocytic cells and to evaluate the proinflammatory and procoagulant effects of anti-beta(2)GPI binding to its target antigen on the monocyte plasma membrane.

METHODS

Human monocytes were fractionated by sucrose density-gradient centrifugation and analyzed by Western blotting. Immunoprecipitation experiments were performed to analyze the association of beta(2)GPI with lipid rafts and the possible interaction of beta(2)GPI with annexin A2 and Toll-like receptor 4 (TLR-4). Monocytes were then stimulated with affinity-purified anti-beta(2)GPI antibodies from patients with the antiphospholipid syndrome (APS). Interleukin-1 receptor-associated kinase (IRAK) phosphorylation and NF-kappaB activation were evaluated by immunoprecipitation and transcription factor assay, respectively. Supernatants from monocytes were tested for tumor necrosis factor alpha (TNFalpha) and tissue factor (TF) levels by enzyme-linked immunosorbent assay.

RESULTS

We found beta(2)GPI and its putative receptor annexin A2 in lipid raft fractions of human monocytes. Moreover, there was an association between beta(2)GPI and TLR-4, suggesting that it was partially dependent on raft integrity. Triggering with anti-beta(2)GPI antibodies induced IRAK phosphorylation and consequent NF-kappaB activation, which led to the release of TNFalpha and TF.

CONCLUSION

Anti-beta(2)GPI antibodies react with their target antigen, likely in association with annexin A2 and TLR-4, in lipid rafts in the monocyte plasma membrane. Anti-beta(2)GPI binding triggers IRAK phosphorylation and NF-kappaB translocation, leading to a proinflammatory and procoagulant monocyte phenotype characterized by the release of TNFalpha and TF, respectively. These findings provide new insight into the pathogenesis of APS, improving our knowledge of valuable therapeutic targets.

Lipid rafts in T cell signalling and disease

Lipid rafts is a blanket term used to describe distinct areas in the plasma membrane rich in certain lipids and proteins and which are thought to perform diverse functions. A large number of studies report on lipid rafts having a key role in receptor signalling and activation of lymphocytes. In T cells, lipid raft involvement was demonstrated in the early steps during T cell receptor (TCR) stimulation. Interestingly, recent evidence has shown that signalling in these domains differs in T cells isolated from patients with autoimmune diseases such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). Here, we discuss these findings and explore the potential of lipid rafts as targets for the development of a new class of agents to downmodulate immune responses and for the treatment of autoimmune diseases.

Identification of a novel lipid raft-targeting motif in Src homology 2-containing phosphatase 1

The tyrosine phosphatase Src homology 2-containing phosphatase 1 (SHP-1) is a key negative regulator of TCR-mediated signaling. Previous studies have shown that in T cells a fraction of SHP-1 constitutively localizes to membrane microdomains, commonly referred to as lipid rafts. Although this localization of SHP-1 is required for its functional regulation of T cell activation events, how SHP-1 is targeted to the lipid rafts was unclear. In this study, we identify a novel, six-amino acid, lipid raft-targeting motif within the C terminus of SHP-1 based on several biochemical and functional observations. First, mutations of this motif in the context of full-length SHP-1 result in the loss of lipid raft localization of SHP-1. Second, this motif alone restores raft localization when fused to a mutant of SHP-1 (SHP-1 DeltaC) that fails to localize to rafts. Third, a peptide encompassing the 6-mer motif directly binds to phospholipids whereas a mutation of this motif abolishes lipid binding. Fourth, whereas full-length SHP-1 potently inhibits TCR-induced tyrosine phosphorylation of specific proteins, expression of a SHP-1-carrying mutation within the 6-mer motif does not. Additionally, although SHP-1 DeltaC was functionally inactive, the addition of the 6-mer motif restored its functionality in inhibiting TCR-induced tyrosine phosphorylation. Finally, this 6-mer mediated targeting of SHP-1 lipid rafts was essential for the function of this phosphatase in regulating IL-2 production downstream of TCR. Taken together, these data define a novel 6-mer motif within SHP-1 that is necessary and sufficient for lipid raft localization and for the function of SHP-1 as a negative regulator of TCR signaling.

CTLA-4 Differentially Regulates the Immunological Synapse in CD4 T Cell Subsets

Primary murine Th1 and Th2 cells differ in the organization of the immunological synapse, with Th1 cells, but not Th2 cells, clustering signaling molecules at the T cell/B cell synapse site. We sought to determine whether differential costimulatory signals could account for the differences observed. We found that Th2 cells express higher levels of CTLA-4 than Th1 cells, and demonstrated that Th2 cells lacking CTLA-4 are now able to cluster the TCR with the same frequency as Th1 cells. Furthermore, reconstitution of CTLA-4 into CTLA-4-deficient Th2 cells, or into Th1 cells, inhibits the clustering of the TCR. We have also shown that Th2 cells, but not Th1 cells, show variations in the organization of the immunological synapse depending on levels of expression of CD80/CD86 on the APC. These studies demonstrate a unique role for CTLA-4 as a critical regulator of Th2 cells and the immunological synapse.

Redistribution of P-selectin glycoprotein ligand-1 (PSGL-1) in chemokine-treated neutrophils: a role of lipid microdomains

P-selectin glycoprotein ligand-1 (PSGL-1) is a mucin-like cell adhesion molecule expressed on leukocyte plasma membranes and involved in platelet-leukocyte and endothelium-leukocyte interactions. The treatment of neutrophils with a low concentration of IL-8 induced the redistribution of PSGL-1 to one end of the cell to form a cap-like structure. We investigated the role of lipid microdomains in the redistribution of PSGL-1 and its effect on the adhesive characteristics of IL-8-treated neutrophils. The redistribution of PSGL-1 induced by IL-8 was inhibited by cholesterol-perturbing agents such as methyl-beta-cyclodextrin and filipin. Sucrose density gradient centrifugation analysis revealed that PSGL-1 was enriched in a low-density fraction together with the GM1 ganglioside after solubilization of the cell membranes with a nonionic detergent, Brij 58. However, when Triton X-100 was used for the solubilization, PSGL-1 was no longer recovered in the low-density fraction, although GM1 ganglioside remained in the low-density fraction. Furthermore, immunofluorescence microscopic observation demonstrated that the localization of PSGL-1 differed from that of GM1 ganglioside, suggesting that PSGL-1 is associated with a microdomain distinct from that containing the GM1 ganglioside. Treatment of neutrophils with IL-8 increased the formation of microaggregates composed of neutrophils and activated platelets, and this treatment also enhanced reactive oxygen species production in neutrophils induced by the cross-linking of PSGL-1 with antibodies. These results suggest that the association of PSGL-1 with lipid microdomains is essential for its redistribution induced by IL-8 stimulation and that the redistribution modulates neutrophil functions mediated by interactions with P-selectin.

Altered lipid raft-associated proximal signaling and translocation of CD45 tyrosine phosphatase in B lymphocytes from patients with systemic lupus erythematosus

OBJECTIVE

B lymphocytes from patients with systemic lupus erythematosus (SLE) exhibit defective intracellular signaling, hyperactivity, and autoantibody production. Recent evidence indicates a reduced expression of Lyn kinase, a negative regulator of B cell signaling, and reduced translocation of Lyn into membrane signaling domains in SLE. The present study was undertaken to investigate the causes of this altered regulation of Lyn by assessing the expression levels of regulatory molecules and their translocation into the signaling domains of SLE B lymphocytes.

METHODS

Blood was obtained from 48 patients with SLE and 28 healthy controls for the assessment of B lymphocytes. Levels and intracellular distribution of Lyn, CD45, COOH-terminal Src kinase (Csk), and c-Cbl were studied by Western blotting, confocal microscopy, and flow cytometry. The kinetics of signaling molecule translocation to the B cell receptor (BCR)-antigen synapse were investigated by confocal microscopy.

RESULTS

A profound alteration in the expression and translocation of regulatory signaling molecules in membrane domains of B cells from patients with SLE was observed. B lymphocytes from SLE patients, but not those from healthy controls, expressed a low molecular weight isoform of CD45 in lipid raft signaling microdomains. Kinetic studies revealed that translocation of Lyn, CD45, Csk, and c-Cbl led to increased recruitment and retention of Lyn and CD45 in the BCR-antigen synapse in SLE B cells.

CONCLUSION

The results provide evidence of altered expression and translocation/interaction of kinases and phosphatases in membrane signaling microdomains of B cells from patients with SLE. Altered translocation of CD45 correlated with reduced expression of Lyn, indicating that Lyn is a key molecule in the regulation of BCR-mediated signaling.

Changes in the Role of the CD45 Protein Tyrosine Phosphatase in Regulating Lck Tyrosine Phosphorylation during Thymic Development

CD45-dependent dephosphorylation of the negative regulatory C-terminal tyrosine of the Src family kinase Lck, promotes efficient TCR signal transduction. However, despite the role of CD45 in positively regulating Lck activity, the distinct phenotypes of CD45 and Lck/Fyn-deficient mice suggest that the role of CD45 in promoting Lck activity may be differentially regulated during thymocyte development. In this study, we have found that the C-terminal tyrosine of Lck (Y505) is markedly hyperphosphorylated in total thymocytes from CD45-deficient mice compared with control animals. In contrast, regulation of the Lck Y505 phosphorylation in purified, double-negative thymocytes is relatively unaffected in CD45-deficient cells. These changes in the role of CD45 in regulating Lck phosphorylation during thymocyte development correlate with changes in coreceptor expression and the presence of coreceptor-associated Lck. Biochemical analysis of coreceptor-associated and nonassociated Lck in thymocytes, and in cell lines varying in CD4 and CD45 expression, indicate that CD45-dependent regulation of Lck Y505 phosphorylation is most evident within the fraction of Lck that is coreceptor associated. In contrast, Lck Y505 phosphorylation that is not coreceptor associated is less affected by the absence of CD45. These data define distinct pools of Lck that are differentially regulated by CD45 during T cell development.

From tango to quadrilla: current views of the immunological synapse

All T cell functions require establishing contacts with other cells. In the last ten years, the immunological synapse, the contact-site between T cells and their partners, has been the object of numerous investigations and recent advances in imaging technologies have provided significant insights into the mechanism of immunological synapse formation and its functional outcomes. Considering all the available data, the immunological synapse can be defined as a dynamic structure, formed between a T cell and one or more antigen-presenting cells, showing lipid and protein segregation, signaling compartmentalization, and bidirectional information exchange though soluble and membrane-bound transmitters. In this review, we present the current views on the immunological synapse and discuss about some interesting unresolved questions.