Regulation of Fyn through translocation of activated Lck into lipid rafts

Models of spatially restricted biochemical reaction systems

Many reactions within the cell occur only in specific intracellular regions. Such local reaction networks give rise to microdomains of activated signaling components. The dynamics of microdomains can be visualized by live cell imaging. Computational models using partial differential equations provide mechanistic insights into the interacting factors that control microdomain dynamics. The mathematical models show that, for membrane-initiated signaling, the ratio of the surface area of the plasma membrane to the volume of the cytoplasm, the topology of the signaling network, the negative regulators, and kinetic properties of key components together define microdomain dynamics. Thus, patterns of locally restricted signaling reaction systems can be considered an emergent property of the cell.

Lck-dependent Fyn activation requires C terminus-dependent targeting of kinase-active Lck to lipid rafts

Mechanisms regulating the activation and delivery of function of Lck and Fyn are central to the generation of the most proximal signaling events emanating from the T cell antigen receptor (TcR) complex. Recent results demonstrate that lipid rafts (LR) segregate Lck and Fyn and play a fundamental role in the temporal and spatial coordination of their activation. Specifically, TcR-CD4 co-aggregation-induced Lck activation outside LR results in Lck translocation to LR where the activation of LR-resident Fyn ensues. Here we report a structure-function analysis toward characterizing the mechanism supporting Lck partitioning to LR and its capacity to activate co-localized Fyn. Using NIH 3T3 cells ectopically expressing FynT, we demonstrate that only LR-associated, kinase-active (Y505F)Lck reciprocally co-immunoprecipitates with and activates Fyn. Mutational analyses revealed a profound reduction in the formation of Lck-Fyn complexes and Fyn activation, using kinase domain mutants K273R and Y394F of (Y505F)Lck, both of which have profoundly compromised kinase activity. The only kinase-active Lck mutants tested that revealed impaired physical and enzymatic engagement with Fyn were those involving truncation of the C-terminal sequence YQPQP. Remarkably, sequential truncation of YQPQP resulted in an increasing reduction of kinase-active Lck partitioning to LR, in both fibroblasts and T cells. This in turn correlated with an ablation of the capacity of these truncates to enhance TcR-mediated interleukin-2 production. Thus, Lck-dependent Fyn activation is predicated by proximity-mediated transphosphorylation of the Fyn kinase domain, and targeting kinase-active Lck to LR is dependent on the C-terminal sequence QPQP.

Protein interactions between CD2 and Lck are required for the lipid raft distribution of CD2

In T lymphocytes, lipid rafts are preferred sites for signal transduction initiation and amplification. Many cell membrane receptors, such as the TCR, coreceptors, and accessory molecules associate within these microdomains upon cell activation. However, it is still unclear in most cases whether these receptors interact with rafts through lipid-based amino acid modifications or whether raft insertion is driven by protein-protein interactions. In murine T cells, a significant fraction of CD2 associates with membrane lipid rafts. We have addressed the mechanisms that control the localization of rat CD2 at the plasma membrane, and its redistribution within lipid rafts induced upon activation. Following incubation of rat CD2-expressing cells with radioactive-labeled palmitic acid, or using CD2 mutants with Cys226 and Cys228 replaced by alanine residues, we found no evidence that rat CD2 was subjected to lipid modifications that could favor the translocation to lipid rafts, discarding palmitoylation as the principal mechanism for raft addressing. In contrast, using Jurkat cells expressing different CD2 and Lck mutants, we show that the association of CD2 with the rafts fully correlates with CD2 capacity to bind to Lck. As CD2 physically interacts with both Lck and Fyn, preferentially inside lipid rafts, and reflecting the increase of CD2 in lipid rafts following activation, CD2 can mediate the interaction between the two kinases and the consequent boost in kinase activity in lipid rafts.

Deletion of the LIME adaptor protein minimally affects T and B cell development and function

LIME (Lck-interacting membrane protein) is a transmembrane adaptor that associates with the Lck and Fyn protein tyrosine kinases and with the C-terminal Src kinase (Csk). To delineate the role of LIME in vivo, LIME-deficient mice were generated. Although Lime transcripts were expressed in immature and mature B and T cells, the absence of LIME impeded neither the development nor the function of B and T cells. TCR transgenic mice deprived of LIME showed, however, a 1.8-fold enhancement in positive selection. Since B cells and activated T cells express LIME and the related adaptor NTAL, mice lacking both adaptors were generated. Double-deficient mice showed no defect in the development and function of B and T cells, and the lack of LIME had no effect on the autoimmune syndrome that develops in aged NTAL-deficient mice. In contrast to a previous report, we further showed that this autoimmune syndrome develops in the absence of T cells. Therefore, our in vivo results refute all the previous roles postulated for LIME on the basis of studies of transformed B and T cells and demonstrate that LIME has no seminal role in the signaling cassette operated by antigen receptors and coreceptors.

SRC family kinases and receptors: analysis of three activation mechanisms by dynamic systems modeling

Src family kinases (SFKs) interact with a number of cellular receptors. They participate in diverse signaling pathways and cellular functions. Most of the receptors involved in SFK signaling are characterized by similar modes of regulation. This computational study discusses a general kinetic model of SFK-receptor interaction. The analysis of the model reveals three major ways of SFK activation: release of inhibition by C-terminal Src kinase, weakening of the inhibitory intramolecular phosphotyrosine-SH2 interaction, and amplification of a stimulating kinase activity. The SFK model was then extended to simulate interaction with growth factor and T-cell receptors. The modular SFK signaling system was shown to adapt to the requirements of specific signaling contexts and yield qualitatively different responses in the different simulated environments. The model also provides a systematic overview of the major interactions between SFKs and various cellular signaling systems and identifies their common properties.

Fyn regulates the duration of TCR engagement needed for commitment to effector function

In naive T cells, engagement of the TCR with agonist peptide:MHC molecules leads to phosphorylation of key intracellular signaling intermediates within seconds and this peaks within minutes. However, the cell does not commit to proliferation and IL-2 cytokine production unless receptor contact is sustained for several hours. The biochemical basis for this transition to full activation may underlie how T cells receive survival signals while maintaining tolerance, and is currently not well understood. We show here that for CD8 T cells commitment to proliferation and cytokine production requires sustained activation of the Src family kinase Lck and is opposed by the action of Fyn. Thus, in the absence of Fyn, commitment to activation occurs more rapidly, the cells produce more IL-2, and undergo more rounds of division. Our data demonstrate a role for Fyn in modulating the response to Ag in primary T cells.

Coreceptor CD8-driven modulation of T cell antigen receptor specificity

The CD8 coreceptor modulates the interaction between the T cell antigen receptor (TCR) and peptide-major histocompatibility class I (pMHCI). We present evidence that CD8 not only modifies the affinity of cognate TCR/pMHCI binding by altering both the association rate and the dissociation rate of the TCR/pMHCI interaction, but modulates the sensitivity (triggering threshold) of the TCR as well, by recruiting TCR/pMHCI complexes to membrane microdomains at a rate which depends on the affinity of MHCI/CD8 binding. Mathematical analysis of these modulatory effects indicates that a T cell can alter its functional avidity for its agonists by regulating CD8 expression, and can rearrange the relative potencies of each of its potential agonists. Thus we propose that a T cell can specifically increase its functional avidity for one agonist, while decreasing its functional avidity for other potential ligands. This focussing mechanism means that TCR degeneracy is inherently dynamic, allowing each TCR clonotype to have a wide range of agonists while avoiding autorecognition. The functional diversity of the TCR repertoire would therefore be greatly augmented by coreceptor-mediated ligand focussing.

Differential function of PTPalpha and PTPalpha Y789F in T cells and regulation of PTPalpha phosphorylation at Tyr-789 by CD45

CD45 is a major membrane protein tyrosine phosphatase (PTP) expressed in T cells where it regulates the activity of Lck, a Src family kinase important for T cell receptor-mediated activation. PTPalpha is a more widely expressed transmembrane PTP that has been shown to regulate the Src family kinases, Src and Fyn, and is also present in T cells. Here, PTPalpha was phosphorylated at Tyr-789 in CD45(-) T cells but not in CD45(+) T cells suggesting that CD45 could regulate the phosphorylation of PTPalpha at this site. Furthermore, CD45 could directly dephosphorylate PTPalpha in vitro. Expression of PTPalpha and PTPalpha-Y789F in T cells revealed that the mutant had a reduced ability to decrease Fyn and Cbp phosphorylation, to regulate the kinase activity of Fyn, and to restore T cell receptor-induced signaling events when compared with PTPalpha. Conversely, this mutant had an increased ability to prevent Pyk2 phosphorylation and CD44-mediated cell spreading when compared with PTPalpha. These data demonstrate distinct activities of PTPalpha and PTPalpha-Y789F in T cells and identify CD45 as a regulator of PTPalpha phosphorylation at tyrosine 789 in T cells.

Homology modeling of human Fyn kinase structure: discovery of rosmarinic acid as a new Fyn kinase inhibitor and in silico study of its possible binding modes

Tyrosine phosphorylation represents a unique signaling process that controls metabolic pathways, cell activation, growth and differentiation, membrane transport, apoptosis, neural, and other functions. We present here the three-dimensional structure of Fyn tyrosine kinase, a Src-family enzyme involved in T-cell receptor signal transduction. The structure of Fyn was modeled for homology using the Sybyl-Composer suite of programs for modeling. Procheck and Prosa II programs showed the high quality of the obtained three-dimensional model. Rosmarinic acid, a secondary metabolite of herbal plants, was discovered as a new Fyn kinase inhibitor using immunochemical and in silico methods. Two possible binding modes of rosmarinic acid were evaluated here, i.e., near to or in the ATP-binding site of kinase domain of Fyn. Enzyme kinetic experiments revealed that Fyn is inhibited by a linear-mixed noncompetitive mechanism of inhibition by rosmarinic acid. This indicates that rosmarinic acid binds to the second "non-ATP" binding site of the Fyn tyrosine kinase.

Lck regulates the threshold of activation in primary T cells, while both Lck and Fyn contribute to the magnitude of the extracellular signal-related kinase response

The src family kinases p56lck (Lck) and p59fyn (Fyn) are the most proximal signaling molecules to be activated downstream of the T-cell receptor. Using an inducible transgenic model, we can regulate the expression of Lck in primary T cells and ask how the signaling cascade and differentiation potential are affected by the absence or the presence of reduced levels of Lck. We show that in naïve T cells, Lck controls the threshold of activation by preferentially regulating multiple signaling pathways that result in the mobilization of Ca2+ through activation of phospholipase C-gamma and protein kinase C as well as activation of the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway. Fyn is also able to stimulate the ERK/MAPK pathway in primary T cells but has little influence on the mobilization of Ca2+. Only Lck efficiently stimulates production of diacylglycerol and therefore RasGRP1 recruitment to the plasma membrane and phosphorylation of Shc, suggesting that Fyn activates ERK via a different upstream signaling route. Finally, we show that signals through Lck are essential for the development of T-cell-effector potential, particularly for effective cytokine transcription.

Palmitoylation of ligands, receptors, and intracellular signaling molecules

Palmitate, a 16-carbon saturated fatty acid, is attached to more than 100 proteins. Modification of proteins by palmitate has pleiotropic effects on protein function. Palmitoylation can influence membrane binding and membrane targeting of the modified proteins. In particular, many palmitoylated proteins concentrate in lipid rafts, and enrichment in rafts is required for efficient signal transduction. This Review focuses on the multiple effects of palmitoylation on the localization and function of ligands, receptors, and intracellular signaling proteins. Palmitoylation regulates the trafficking and function of transmembrane proteins such as ion channels, neurotransmitter receptors, heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors, and integrins. In addition, immune receptor signaling relies on protein palmitoylation at many levels, including palmitoylated co-receptors, Src family kinases, and adaptor or scaffolding proteins. The localization and signaling capacities of Ras and G proteins are modulated by dynamic protein palmitoylation. Cycles of palmitoylation and depalmitoylation allow H-Ras and G protein alpha subunits to reversibly bind to and signal from different intracellular cell membranes. Moreover, secreted ligands such as Hedgehog, Wingless, and Spitz use palmitoylation to regulate the extent of long- or short-range signaling. Finally, palmitoylation can alter signaling protein function by direct effects on enzymatic activity and substrate specificity. The identification of the palmitoyl acyltransferases has provided new insights into the biochemistry of this posttranslational process and permitted new substrates to be identified.

A novel approach to examining compositional heterogeneity of detergent-resistant lipid rafts

Lipid rafts play an important role in cell signalling, cell adhesion and other cellular functions. Compositional heterogeneity of lipid rafts provides one mechanism of how lipid rafts provide the spatial and temporal regulation of cell signalling and cell adhesion. The constitutive presence of some signalling receptors/molecules and accumulation of others in the lipid raft allows them to interact with each other and thereby facilitate relay of signals from the plasma membrane to the cell interior. Devising a method that can analyze these lipid microdomains for the presence of signalling receptors/molecules on an individual raft basis is required to address the issue of lipid raft heterogeneity. SDS-PAGE analysis, currently used for analyses of detergent-resistant lipid rafts, does not address this question. We have designed a cell-free assay that captures detergent-resistant lipid rafts with an antibody against a raft-resident molecule and detects the presence of another lipid raft molecule. Our results suggest that detergent-resistant lipid rafts, also known as detergent-resistant membranes, are heterogeneous populations on an immortalized mouse T-cell plasma membrane with respect to antigen receptor/signalling complex and other signalling/adhesion proteins. This cell-free assay provides a simple and quick way to examine the simultaneous presence of two proteins in the lipid rafts and has the potential to estimate trafficking of molecules in and out of the lipid microdomains during cell signalling on a single detergent-resistant lipid raft basis.

CD2BP1 modulates CD2-dependent T cell activation via linkage to protein tyrosine phosphatase (PTP)-PEST

Human CD2 regulates T cell activation and adhesion via mechanisms yet to be fully understood. This study focuses on CD2BP1, a CD2 cytoplasmic tail-binding protein preferentially expressed in hematopoetic cells. Structural and functional analyses suggest that CD2BP1 acts as a scaffold protein, participating in regulation of the actin cytoskeleton. In this study, using a murine Ag-specific primary T cell transduction system to assess CD69, IL-2, and IFN-gamma expression, we provide evidence that CD2BP1 directly and negatively impacts T cell activation via isolated CD2 triggering or TCR stimulation dependent on coordinate CD2 engagement. Disruption of protein tyrosine phosphatase-PEST and/or CD2BP1 association with the CD2 signalsome rescues T cells from the inhibitory effect of CD2 crosslinking. The overexpression of CD2BP1 selectively attenuates phospholipase Cgamma1, ERK1/2, and p38 phosphorylation without abrogating CD2-independent TCR stimulation. This study provides new insight on the regulation of T cell activation and may have implications for autoimmune processes known to be associated with CD2BP1 mutations.

Protein Tyrosine Phosphatase α Regulates Fyn Activity and Cbp/PAG Phosphorylation in Thymocyte Lipid Rafts

A role for the receptor protein tyrosine phosphatase alpha (PTPalpha) in immune cell function and regulation of Src family kinases was investigated using thymocytes from PTPalpha-deficient mice. PTPalpha-null thymocytes develop normally, but unstimulated PTPalpha-/- cells exhibit increased tyrosine phosphorylation of specific proteins, increased Fyn activity, and hyperphosphorylation of Cbp/PAG that promotes its association with C-terminal Src kinase. Elevated Fyn activity in the absence of PTPalpha is due to enhanced phosphorylation of Fyn tyrosines 528 and 417. Some PTPalpha is localized in lipid rafts of thymocytes, and raft-associated Fyn is specifically activated in PTPalpha-/- cells. PTPalpha is not a Cbp/PAG phosphatase, because it is not required for Cbp/PAG dephosphorylation in unstimulated or anti-CD3-stimulated thymocytes. Together, our results indicate that PTPalpha, likely located in lipid rafts, regulates the activity of raft Fyn. In the absence of PTPalpha this population of Fyn is activated and phosphorylates Cbp/PAG to enhance association with C-terminal Src kinase. Although TCR-mediated tyrosine phosphorylation was apparently unaffected by the absence of PTPalpha, the long-term proliferative response of PTPalpha-/- thymocytes was reduced. These findings indicate that PTPalpha is a component of the complex Src family tyrosine kinase regulatory network in thymocytes and is required to suppress Fyn activity in unstimulated cells in a manner that is not compensated for by the major T cell PTP and SFK regulator, CD45.

Tuning immune responses: diversity and adaptation of the immunological synapse

The onset and regulation of a specific immune response results from communication between T cells and antigen-presenting cells (APCs), which form molecular interactions at the site of cell-cell contact--and this is known as the immunological synapse. Initially, the immunological synapse was viewed as a stereotypical adhesion and signalling device with a defined molecular structure and signalling processes. However, as we discuss here, T-cell-APC interactions comprise a diverse range of contact modes and distinct molecular arrangements. These diverse interaction modes might define a molecular code, in which the differences in timing, spacing and molecular composition of the signalling platform determine the outcome of T-cell-APC interactions.

Prion protein recruits its neuronal receptor NCAM to lipid rafts to activate p59fyn and to enhance neurite outgrowth

In spite of advances in understanding the role of the cellular prion protein (PrP) in neural cell interactions, the mechanisms of PrP function remain poorly characterized. We show that PrP interacts directly with the neural cell adhesion molecule (NCAM) and associates with NCAM at the neuronal cell surface. Both cis and trans interactions between NCAM at the neuronal surface and PrP promote recruitment of NCAM to lipid rafts and thereby regulate activation of fyn kinase, an enzyme involved in NCAM-mediated signaling. Cis and trans interactions between NCAM and PrP promote neurite outgrowth. When these interactions are disrupted in NCAM-deficient and PrP-deficient neurons or by PrP antibodies, NCAM/PrP-dependent neurite outgrowth is arrested, indicating that PrP is involved in nervous system development cooperating with NCAM as a signaling receptor.

Localization of Src homology 2 domain-containing phosphatase 1 (SHP-1) to lipid rafts in T lymphocytes: functional implications and a role for the SHP-1 carboxyl terminus

The protein tyrosine phosphatase Src homology 2 domain-containing phosphatase 1 (SHP-1) has previously been shown to be a negative regulator of signaling mediated via the TCR. A growing body of evidence indicates that the regulated localization of proteins within certain membrane subdomains, referred to as lipid rafts, is important for the successful transduction of signaling events downstream of the TCR. However, considerably less is known about the localization of negative regulators during these lipid raft-dependent signaling events. In this study we have investigated the subcellular localization of SHP-1 and its role in regulation of TCR-mediated signaling. Our studies demonstrate that in a murine T cell hybridoma as well as in primary murine thymocytes, a fraction of SHP-1 localizes to the lipid rafts, both basally and after TCR stimulation. Interestingly, although SHP-1 localized in the nonraft fractions is tyrosine phosphorylated, the SHP-1 isolated from the lipid rafts lacks the TCR-induced tyrosine phosphorylation, suggesting physical and/or functional differences between these two subpopulations. We identify a requirement for the C-terminal residues of SHP-1 in optimal localization to the lipid rafts. Although expression of SHP-1 that localizes to lipid rafts potently inhibits TCR-mediated early signaling events and IL-2 production, the expression of lipid raft-excluded SHP-1 mutants fails to elicit any of the inhibitory effects. Taken together these studies reveal a key role for lipid raft localization of SHP-1 in mediating the inhibitory effects on T cell signaling events.

The proto-oncogene Fgr regulates cell migration and this requires its plasma membrane localization

Fgr participates in integrin signaling in myeloid leukocytes. To examine the role of its specific domains in regulating cell migration, we expressed various Fgr molecules in COS-7 cells. Full-length, membrane-bound Fgr, but not an N-terminal truncation mutant that distributed to an intracellular compartment, increased cell migration on fibronectin and enhanced phosphorylation of the p85 subunit of phosphatidylinositol 3-kinase (PI3K), cortactin and focal adhesion kinase (FAK) at Y397 and Y576. Fgr increased Rac GTP loading, and phosphorylation of the Rac GEF Vav2, and bound to a protein complex formed by the Rho inhibitor p190RhoGAP and FAK, increasing p190RhoGAP phosphorylation, in a manner absolutely dependent on membrane localization. A kinase-defective truncation mutant of Fgr increased cell migration, albeit to a much lower extent than full-length Fgr, and was found to associate with the plasma membrane, to activate Rac and to form complexes with p190RhoGAP/FAK. Formation of complexes between p190RhoGAP, Fgr, and the FAK-related protein Pyk2 were also detected in murine macrophages. These findings suggest that the proto-oncogene Fgr regulates cell migration impinging on a signaling pathway implicating FAK/Pyk2 and leading to activation of Rac and the Rho inhibitor p190RhoGAP.

RPTPalpha is essential for NCAM-mediated p59fyn activation and neurite elongation

The neural cell adhesion molecule (NCAM) forms a complex with p59^fyn kinase and activates it via a mechanism that has remained unknown. We show that the NCAM140 isoform directly interacts with the intracellular domain of the receptor-like protein tyrosine phosphatase RPTPα, a known activator of p59^fyn. Whereas this direct interaction is Ca²⁺ independent, formation of the complex is enhanced by Ca²⁺-dependent spectrin cytoskeleton–mediated cross-linking of NCAM and RPTPα in response to NCAM activation and is accompanied by redistribution of the complex to lipid rafts. Association between NCAM and p59^fyn is lost in RPTPα-deficient brains and is disrupted by dominant-negative RPTPα mutants, demonstrating that RPTPα is a link between NCAM and p59^fyn. NCAM-mediated p59^fyn activation is abolished in RPTPα-deficient neurons, and disruption of the NCAM–p59^fyn complex in RPTPα-deficient neurons or with dominant-negative RPTPα mutants blocks NCAM-dependent neurite outgrowth, implicating RPTPα as a major phosphatase involved in NCAM-mediated signaling.

Function of the Src-family kinases, Lck and Fyn, in T-cell development and activation

The function of the Src-family kinases (SFKs) Lck and Fyn in T cells has been intensively studied over the past 15 years. Animal models and cell line studies both indicate a critical role for Lck and Fyn in proximal T-cell antigen receptor (TCR) signal transduction. Recruited SFKs phosphorylate TCR ITAMs (immunoreceptor tyrosine-based activation motifs) in the CD3 and zeta chains, which then serve as docking sites for Syk-family kinases. SFKs then phosphorylate and activate the recruited Syk-family kinase. Lck and Fyn are spatially segregated in cell membranes due to differential lipid raft localization, and may undergo sequential activation. In addition to the CD4 and CD8 coreceptors, a recently described adaptor, Unc119, may link SFKs to the TCR. CD45 and Csk provide positive and negative regulatory control of SFK functions, respectively, and Csk is constitutively bound to the transmembrane adapter protein, PAG/Cbp. TCR-based signaling is required at several stages of T-cell development, including at least pre-TCR signaling, positive selection, peripheral maintenance of naive T cells, and lymphopenia-induced proliferation. SFKs are required for each of these TCR-based signals, and Lck seems to be the major contributor.

SAP regulates T(H)2 differentiation and PKC-theta-mediated activation of NF-kappaB1

XLP is caused by mutations affecting SAP, an adaptor that recruits Fyn to SLAM family receptors. SAP-deficient mice recapitulate features of XLP, including increased T cell activation and decreased humoral responses post-infection. SAP-deficient T cells also show increased TCR-induced IFN-gamma and decreased T(H)2 cytokine production. We demonstrate that the defect in IL-4 secretion in SAP-deficient T cells is independent of increased IFN-gamma production. SAP-deficient cells respond normally to polarizing cytokines, yet show impaired TCR-mediated induction of GATA-3 and IL-4. Examination of TCR signaling revealed normal Ca(2+) mobilization and ERK activation in SAP-deficient cells, but decreased PKC-theta recruitment, Bcl-10 phosphorylation, IkappaB-alpha degradation, and nuclear NF-kappaB1/p50 levels. Similar defects were observed in Fyn-deficient cells. SLAM engagement amplified PKC-theta recruitment in wt but not SAP- or Fyn-deficient cells, arguing that a SAP/Fyn-mediated pathway enhances PKC-theta/NF-kappaB1 activation and suggesting a role for this pathway in T(H)2 regulation.

Transmembrane adapters: structure, biochemistry and biology

Transmembrane adapter proteins (TRAPs) represent a relatively new and unique group of signalling molecules in hematopoetic cells. They differ from other signalling proteins as they lack any enzymatic or transcriptional activity, instead they possesses multiple tyrosine-based signalling motifs (TBSMs). Triggering of immunoreceptors induces tyrosine phosphorylation of these motifs by members of the Src-, Syk- or Tec-family of protein tyrosine kinases thus enabling the TRAPs to recruit cytosolic adapter and/or effector molecules via their SH2-domains into close proximity to the immunoreceptors, a position from which they can coordinate and modulate signal transduction pathways important for lymphocyte function.

Regulation of T-cell receptor signalling by membrane microdomains

There is now considerable evidence suggesting that the plasma membrane of mammalian cells is compartmentalized by functional lipid raft microdomains. These structures are assemblies of specialized lipids and proteins and have been implicated in diverse biological functions. Analysis of their protein content using proteomics and other methods revealed enrichment of signalling proteins, suggesting a role for these domains in intracellular signalling. In T lymphocytes, structure/function experiments and complementary pharmacological studies have shown that raft microdomains control the localization and function of proteins which are components of signalling pathways regulated by the T-cell antigen receptor (TCR). Based on these studies, a model for TCR phosphorylation in lipid rafts is presented. However, despite substantial progress in the field, critical questions remain. For example, it is unclear if membrane rafts represent a homogeneous population and if their structure is modified upon TCR stimulation. In the future, proteomics and the parallel development of complementary analytical methods will undoubtedly contribute in further delineating the role of lipid rafts in signal transduction mechanisms.

Bad-Dependent Rafts Alteration Is a Consequence of an Early Intracellular Signal Triggered by Interleukin-4 Deprivation

Many molecules are inducibly localized in lipid rafts, and their alteration inhibits early activation events, supporting a critical role for these domains in signaling. Using confocal microscopy and cellular fractionation, we have shown that the pool of Bad, attached to lipid rafts in proliferating cells, is released when cells undergo apoptosis. Kinetic studies indicate that rafts alteration is a consequence of an intracellular signal triggered by interleukin-4 deprivation. Growth factor deprivation in turn induces PP1α phosphatase activation, responsible for cytoplasmic Bad dephosphorylation as well as caspase-9 and caspase-3 activation. Caspases translocate to rafts and induce their modification followed by translocation of Bad from rafts to mitochondria, which correlates with apoptosis. Taken together, our results suggest that alteration of lipid rafts is an early event in the apoptotic cascade indirectly induced by interleukin-4 deprivation via PP1α activation, dephosphorylation of cytoplasmic Bad, and caspase activation.

Molecular interdiction of Src-family kinase signaling in hematopoietic cells

The ability of Src-family kinases (SFKs) to mediate signaling from cell surface receptors in hematopoietic cells is a function of their catalytic activity, location and binding partners. Kinase activity is regulated in the cell by kinases and phosphatases that alter the state of phosphorylation of key tyrosine residues and by protein binding partners that stabilize the kinase in active or inactive conformations or localize the enzyme to specific subcellular or submembrane domains. Kinase activity and function can be modulated experimentally through the use of small molecule inhibitors designed to directly target catalytic or binding domains or regulate the location of the protein by altering its state of acylation.

Activation of naïve CD4 T cells by anti-CD3 reveals an important role for Fyn in Lck-mediated signaling

Although there was no impairment in IL-2 secretion and proliferation of Fyn-deficient naïve CD4 cells after stimulation with antigen and antigen-presenting cells, stimulation of these cells with anti-CD3 and anti-CD28 revealed profound defects. Crosslinking of purified wild-type naïve CD4 cells with anti-CD3 activated Lck and initiated the signaling cascade downstream of Lck, including phosphorylation of ZAP-70, LAT, and PLC-gamma1; calcium flux; and dephosphorylation and nuclear translocation of the nuclear factor of activated T cells (NFAT)p. All of these signaling events were diminished severely in Fyn-deficient naïve cells activated by CD3 crosslinking. Coaggregation of CD3 and CD4 reconstituted this Lck-dependent signaling pathway in Fyn(-/-) T cells. These results suggest that when signaling of naïve T cells is restricted to the T cell antigen receptor, Fyn plays an essential role by positive regulation of Lck activity.

Lipid rafts-protein association and the regulation of protein activity

Lipid rafts are membrane microdomains enriched in saturated phospholipids, sphingolipids, and cholesterol. They have a varied but distinct protein composition and have been implicated in diverse cellular processes including polarized traffic, signal transduction, endo- and exo-cytoses, entrance of obligate intracellular pathogens, and generation of pathological forms of proteins associated with Alzheimer's and prion diseases. Raft proteins can be permanently or temporarily associated to lipid rafts. Here, we review recent advances on the biochemical and cell biological characterization of rafts, and on the emerging concept of the temporary residency of proteins in rafts as a regulatory mechanism of their biological activity.

Fine-tuning lymphocyte regulation: what’s new with tyrosine kinases and phosphatases?

Although the basic mechanisms of lymphocyte signaling have been established, recent studies have provided new insights into how fine-tuning the regulation of tyrosine kinases and phosphatases contributes to the delicate balance required for appropriate lymphocyte activation. Recent studies include new work on the roles of the immune synapse in regulating T-cell receptor signaling, the discovery of new functions for the Src-family kinase Fyn and the Tec kinase Itk, particularly in regulation of the actin cytoskeleton, and new insights into positive and negative feedback mechanisms in antigen receptor signaling.

Prion protein is a component of the multimolecular signaling complex involved in T cell activation

In this study we analyzed the interaction of prion protein PrP(C) with components of glycosphingolipid-enriched microdomains in lymphoblastoid T cells. PrP(C) was distributed in small clusters on the plasma membrane, as revealed by immunoelectron microscopy. PrP(C) is present in microdomains, since it coimmunoprecipitates with GM3 and the raft marker GM1. A strict association between PrP(C) and Fyn was revealed by scanning confocal microscopy and coimmunoprecipitation experiments. The phosphorylation protein ZAP-70 was immunoprecipitated by anti-PrP after T cell activation. These results demonstrate that PrP(C) interacts with ZAP-70, suggesting that PrP(C) is a component of the multimolecular signaling complex within microdomains involved in T cell activation.

Enrichment of Lck in Lipid Rafts Regulates Colocalized Fyn Activation and the Initiation of Proximal Signals through TCRαβ

Recent results provide insight into the temporal and spatial relationship governing lck-dependent fyn activation and demonstrate TCR/CD4-induced activation and translocation of lck into lipid rafts and the ensuing activation of colocalized fyn. The prediction follows that directly targeting lck to lipid rafts will bypass the requirement for juxtaposing TCR and CD4-lck, and rescue cellular activation mediated by Ab specific for the constant region of TCRbeta chain. The present study uses a family of murine IL-2-dependent CD4(+) T cell clonal variants in which anti-TCRCbeta signaling is impaired in an lck-dependent fashion. Importantly, these variants respond to Ag- and mAb-mediated TCR-CD4 coaggregation, both of which enable the coordinated interaction of CD4-associated lck with the TCR/CD3 complex. We have previously demonstrated that anti-TCRCbeta responsiveness in this system correlates with the presence of kinase-active, membrane-associated lck and preformed hypophosphorylated TCRzeta:zeta-associated protein of 70 kDa complexes, a phenotype recapitulated in primary resting CD4(+) T cells. We show in this study that forced expression of wild-type lck achieved the same basal composition of the TCR/CD3 complex and yet did not rescue anti-TCRCbeta signaling. In contrast, forced expression of C20S/C23S-mutated lck (double-cysteine lck), unable to bind CD4, rescues anti-TCRCbeta proximal signaling and cellular growth. Double-cysteine lck targets lipid rafts, colocalizes with >98% of cellular fyn, and results in a 7-fold increase in basal fyn kinase activity. Coaggregation of CD4 and TCR achieves the same outcome. These results underscore the critical role of lipid rafts in spatially coordinating the interaction between lck and fyn that predicates proximal TCR/CD3 signaling.

Physiological T cell activation starts and propagates in lipid rafts

Lipid rafts are plasma membrane compartments enriched in key signaling molecules. We have previously shown that in T lymphocytes anti-CD3 stimulation is insensitive to cholesterol extraction by methyl-beta-cyclodextrin (MbetaCD), suggesting that anti-CD3 induced signal transduction is independent of raft integrity. Here we show that, in contrast to T cell stimulation by anti-CD3 antibodies, T cell activation by a physiological ligand is mediated by signaling events taking place in lipid raft. Indeed, cholesterol depletion by MbetaCD resulted in reduced T cell activation in response to Epstein Barr Virus (EBV)-transformed B cells pulsed with a bacterial superantigen. Moreover, T cell stimulation by pulsed EBV-B cells, but not by anti-CD3 antibodies, induced recruitment of active Lck in detergent-resistant membranes, where the signal transduction is organized and amplified.

LIME: a new membrane Raft-associated adaptor protein involved in CD4 and CD8 coreceptor signaling

Lymphocyte membrane rafts contain molecules critical for immunoreceptor signaling. Here, we report identification of a new raft-associated adaptor protein LIME (Lck-interacting molecule) expressed predominantly in T lymphocytes. LIME becomes tyrosine phosphorylated after cross-linking of the CD4 or CD8 coreceptors. Phospho-LIME associates with the Src family kinase Lck and its negative regulator, Csk. Ectopic expression of LIME in Jurkat T cells results in an increase of Csk in lipid rafts, increased phosphorylation of Lck and higher Ca²⁺ response to CD3 stimulation. Thus, LIME appears to be involved in regulation of T cell activation by coreceptors.