Activation of C-terminal Src kinase (Csk) by phosphorylation at serine-364 depends on the Csk-Src homology 3 domain

Development of FRET Biosensor to Characterize CSK Subcellular Regulation

C-terminal Src kinase (CSK) is the major inhibitory kinase for Src family kinases (SFKs) through the phosphorylation of their C-tail tyrosine sites, and it regulates various types of cellular activity in association with SFK function. As a cytoplasmic protein, CSK needs be recruited to the plasma membrane to regulate SFKs' activity. The regulatory mechanism behind CSK activity and its subcellular localization remains largely unclear. In this work, we developed a genetically encoded biosensor based on fluorescence resonance energy transfer (FRET) to visualize the CSK activity in live cells. The biosensor, with an optimized substrate peptide, confirmed the crucial Arg107 site in the CSK SH2 domain and displayed sensitivity and specificity to CSK activity, while showing minor responses to co-transfected Src and Fyn. FRET measurements showed that CSK had a relatively mild level of kinase activity in comparison to Src and Fyn in rat airway smooth muscle cells. The biosensor tagged with different submembrane-targeting signals detected CSK activity at both non-lipid raft and lipid raft microregions, while it showed a higher FRET level at non-lipid ones. Co-transfected receptor-type protein tyrosine phosphatase alpha (PTPα) had an inhibitory effect on the CSK FRET response. The biosensor did not detect obvious changes in CSK activity between metastatic cancer cells and normal ones. In conclusion, a novel FRET biosensor was generated to monitor CSK activity and demonstrated CSK activity existing in both non-lipid and lipid raft membrane microregions, being more present at non-lipid ones.

CSK-mediated signalling by integrins in cancer

Cancer progression and metastasis are processes heavily controlled by the integrin receptor family. Integrins are cell adhesion molecules that constitute the central components of mechanosensing complexes called focal adhesions, which connect the extracellular environment with the cell interior. Focal adhesions act as key players in cancer progression by regulating biological processes, such as cell migration, invasion, proliferation, and survival. Src family kinases (SFKs) can interplay with integrins and their downstream effectors. SFKs also integrate extracellular cues sensed by integrins and growth factor receptors (GFR), transducing them to coordinate metastasis and cell survival in cancer. The non-receptor tyrosine kinase CSK is a well-known SFK member that suppresses SFK activity by phosphorylating its specific negative regulatory loop (C-terminal Y⁵²⁷ residue). Consequently, CSK may play a pivotal role in tumour progression and suppression by inhibiting SFK oncogenic effects in several cancer types. Remarkably, CSK can localise near focal adhesions when SFKs are activated and even interact with focal adhesion components, such as phosphorylated FAK and Paxillin, among others, suggesting that CSK may regulate focal adhesion dynamics and structure. Even though SFK oncogenic signalling has been extensively described before, the specific role of CSK and its crosstalk with integrins in cancer progression, for example, in mechanosensing, remain veiled. Here, we review how CSK, by regulating SFKs, can regulate integrin signalling, and focus on recent discoveries of mechanotransduction. We additionally examine the cross talk of integrins and GFR as well as the membrane availability of these receptors in cancer. We also explore new pharmaceutical approaches to these signalling pathways and analyse them as future therapeutic targets.

Regulation, targets and functions of CSK

The Src family kinases (SFK) plays an important role in multiple signal transduction pathways. Aberrant activation of SFKs leads to diseases such as cancer, blood disorders, and bone pathologies. By phosphorylating and inactivating SFKs, the C-terminal Src kinase (CSK) serves as the key negative regulator of SFKs. Similar to Src, CSK is composed of SH3, SH2, and a catalytic kinase domain. However, while the Src kinase domain is intrinsically active, the CSK kinase domain is intrinsically inactive. Multiple lines of evidence indicate that CSK is involved in various physiological processes including DNA repair, permeability of intestinal epithelial cells (IECs), synaptic activity, astrocyte-to-neuron communication, erythropoiesis, platelet homeostasis, mast cell activation, immune and inflammation responses. As a result, dysregulation of CSK may lead to many diseases with different underlying molecular mechanisms. Furthermore, recent findings suggest that in addition to the well-established CSK-SFK axis, novel CSK-related targets and modes of CSK regulation also exist. This review focuses on the recent progress in this field for an up-to-date understanding of CSK.

Dissection of the catalytic and regulatory structure-function relationships of Csk protein tyrosine kinase

Protein tyrosine kinases (PTKs) are a large enzyme family that regulates many cellular processes. The key to their broad role in signaling is their tunable substrate specificity and regulatory mechanisms that allow each to respond to appropriate regulatory signals and phosphorylate the correct physiological protein substrates. Thus, in addition to the general PTK catalytic platform, each PTK acquires unique structural motifs that confer a unique combination of catalytic and regulatory properties. Understanding the structural basis for these properties is essential for understanding and manipulating the PTK-based signaling networks in normal and cancer cells. C-terminal Src kinase (Csk) and its homolog, Csk-homologous kinase (Chk), phosphorylate Src family kinases on a C-terminal Tyr residue and negatively regulate their kinase activity. While this regulatory function is biologically essential, Csk and Chk have also been excellent model PTKs for dissecting the structural basis of PTK catalysis and regulation. In this article, we review the structure-function studies of Csk and Chk that shed light on the regulatory and catalytic mechanisms of protein tyrosine kinases in general.

A multiscale model of the regulation of aquaporin 2 recycling

The response of cells to their environment is driven by a variety of proteins and messenger molecules. In eukaryotes, their distribution and location in the cell are regulated by the vesicular transport system. The transport of aquaporin 2 between membrane and storage region is a crucial part of the water reabsorption in renal principal cells, and its malfunction can lead to Diabetes insipidus. To understand the regulation of this system, we aggregated pathways and mechanisms from literature and derived three models in a hypothesis-driven approach. Furthermore, we combined the models to a single system to gain insight into key regulatory mechanisms of Aquaporin 2 recycling. To achieve this, we developed a multiscale computational framework for the modeling and simulation of cellular systems. The analysis of the system rationalizes that the compartmentalization of cAMP in renal principal cells is a result of the protein kinase A signalosome and can only occur if specific cellular components are observed in conjunction. Endocytotic and exocytotic processes are inherently connected and can be regulated by the same protein kinase A signal.

Sprouty2 positively regulates T cell function and airway inflammation through regulation of CSK and LCK kinases

The function of Sprouty2 (Spry2) in T cells is unknown. Using 2 different (inducible and T cell-targeted) knockout mouse strains, we found that Spry2 positively regulated extracellular signal-regulated kinase 1/2 (ERK1/2) signaling by modulating the activity of LCK. Spry2-/- CD4+ T cells were unable to activate LCK, proliferate, differentiate into T helper cells, or produce cytokines. Spry2 deficiency abrogated type 2 inflammation and airway hyperreactivity in a murine model of asthma. Spry2 expression was higher in blood and airway CD4+ T cells from patients with asthma, and Spry2 knockdown impaired human T cell proliferation and cytokine production. Spry2 deficiency up-regulated the lipid raft protein caveolin-1, enhanced its interaction with CSK, and increased CSK interaction with LCK, culminating in augmented inhibitory phosphorylation of LCK. Knockdown of CSK or dislodgment of caveolin-1-bound CSK restored ERK1/2 activation in Spry2-/- T cells, suggesting an essential role for Spry2 in LCK activation and T cell function.

Transforming growth factor β (TGF-β) receptor signaling regulates kinase networks and phosphatidylinositol metabolism during T-cell activation

The cytokine content in tissue microenvironments shapes the functional capacity of a T cell. This capacity depends on the integration of extracellular signaling through multiple receptors, including the T-cell receptor (TCR), co-receptors, and cytokine receptors. Transforming growth factor β (TGF-β) signals through its cognate receptor, TGFβR, to SMAD family member proteins and contributes to the generation of a transcriptional program that promotes regulatory T-cell differentiation. In addition to transcription, here we identified specific signaling networks that are regulated by TGFβR. Using an array of biochemical approaches, including immunoblotting, kinase assays, immunoprecipitation, and flow cytometry, we found that TGFβR signaling promotes the formation of a SMAD3/4-protein kinase A (PKA) complex that activates C-terminal Src kinase (CSK) and thereby down-regulates kinases involved in proximal TCR activation. Additionally, TGFβR signaling potentiated CSK phosphorylation of the P85 subunit in the P85-P110 phosphoinositide 3-kinase (PI3K) heterodimer, which reduced PI3K activity and down-regulated the activation of proteins that require phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3) for their activation. Moreover, TGFβR-mediated disruption of the P85-P110 interaction enabled P85 binding to a lipid phosphatase, phosphatase and tensin homolog (PTEN), aiding in the maintenance of PTEN abundance and thereby promoting elevated PtdIns(4,5)P2 levels in response to TGFβR signaling. Taken together, these results highlight that TGF-β influences the trajectory of early T-cell activation by altering PI3K activity and PtdIns levels.

Regulation of the firing activity by PKA-PKC-Src family kinases in cultured neurons of hypothalamic arcuate nucleus

The cAMP‐dependent protein kinase A family (PKAs), protein kinase C family (PKCs), and Src family kinases (SFKs) are found to play important roles in pain hypersensitivity. However, more detailed investigations are still needed in order to understand the mechanisms underlying the actions of PKAs, PKCs, and SFKs. Neurons in the hypothalamic arcuate nucleus (ARC) are found to be involved in the regulation of pain hypersensitivity. Here we report that the action potential (AP) firing activity of ARC neurons in culture was up‐regulated by application of the adenylate cyclase activator forskolin or the PKC activator PMA, and that the forskolin or PMA application‐induced up‐regulation of AP firing activity could be blocked by pre‐application of the SFK inhibitor PP2. SFK activation also up‐regulated the AP firing activity and this effect could be prevented by pre‐application of the inhibitors of PKCs, but not of PKAs. Furthermore, we identified that forskolin or PMA application caused increases in the phosphorylation not only in PKAs at T197 or PKCs at S660 and PKCα/βII at T638/641, but also in SFKs at Y416. The forskolin or PMA application‐induced increase in the phosphorylation of PKAs or PKCs was not affected by pre‐treatment with PP2. The regulations of the SFK and AP firing activities by PKCs were independent upon the translocation of either PKCα or PKCβII. Thus, it is demonstrated that PKAs may act as an upstream factor(s) to enhance SFKs while PKCs and SFKs interact reciprocally, and thereby up‐regulate the AP firing activity in hypothalamic ARC neurons.

SUMOylation of Csk Negatively Modulates its Tumor Suppressor Function

Csk, a non-receptor tyrosine kinase, serves as an indispensable negative regulator of the Src family kinases (SFKs). However, little is known about regulation of Csk expression so far. SUMOylation, a reversible post-translational modification, has been shown to regulate many biological processes especially in tumor progression. Here we report that Csk is covalently modified by SUMO1 at lysine 53 (K53) both in vitro and in vivo. Treatment with hydrogen peroxide inhibited this modification to a certain extent, but PIAS3, identified as the main specific SUMO E3 ligase for Csk, could significantly enhance SUMO1-Csk level. In addition, phosphorylation at Ser364, the active site in Csk, had no effect on this modification. Ectopic expression of SUMO-defective mutant, Csk ^K53R, inhibited tumor cell growth more potentially than Csk wild-type. Consistent with the biological phenotype, the SUMO modification of Csk impaired its activity to interact with Cbp (Csk binding protein) leading to decreased c-Src phosphorylation at Y527. Our results suggest that SUMOylation of Csk mainly at lysine 53 negatively modulates its tumor suppressor function by reducing its binding with Cbp and consequently, inducing c-Src activation.

Evaluation of resistance to HIV-1 infection ex vivo of PBMCs isolated from patients with chronic myeloid leukemia treated with different tyrosine kinase inhibitors

Current antiretroviral treatment (ART) may control HIV-1 replication but it cannot cure the infection due to the formation of a reservoir of latently infected cells. CD4+ T cell activation during HIV-1 infection eliminates the antiviral function of the restriction factor SAMHD1, allowing proviral integration and the reservoir establishment. The role of tyrosine kinases during T-cell activation is essential for these processes. Therefore, the inhibition of tyrosine kinases could control HIV-1 infection and restrict the formation of the reservoir. A family of tyrosine kinase inhibitors (TKIs) is successfully used in clinic for treating chronic myeloid leukemia (CML). The safety and efficacy against HIV-1 infection of five TKIs was assayed in PBMCs isolated from CML patients on prolonged treatment with these drugs that were infected ex vivo with HIV-1. We determined that the most potent and safe TKI against HIV-1 infection was dasatinib, which preserved SAMHD1 antiviral function and avoid T-cell activation through TCR engagement and homeostatic cytokines. Imatinib and nilotinib showed lower potency and bosutinib was quite toxic in vitro. Ponatinib presented similar profile to dasatinib but as it has been associated with higher incidence of arterial ischemic events, dasatinib would be the better choice of TKI to be used as adjuvant of ART in order to avoid the establishment and replenishment of HIV-1 reservoir and move forward towards an HIV cure.

TRAF3 enhances TCR signaling by regulating the inhibitors Csk and PTPN22

The adaptor protein TNF receptor associated factor (TRAF) 3 is required for effective TCR signaling and normal T cell effector functions, and associates with the CD3/CD28 complex upon activation. To determine how TRAF3 promotes proximal TCR signaling, we studied TRAF3-deficient mouse and human T cells, which showed a marked reduction in activating phosphorylation of the TCR-associated kinase Lck. The impact of TRAF3 on this very early signaling event led to the hypothesis that TRAF3 restrains one or both of two known inhibitors of Lck, C-terminal Src kinase (Csk) and protein tyrosine phosphatase N22 (PTPN22). TRAF3 associated with Csk, promoting the dissociation of Csk from the plasma membrane. TRAF3 also associated with and regulated the TCR/CD28 induced localization of PTPN22. Loss of TRAF3 resulted in increased amounts of both Csk and PTPN22 in T cell membrane fractions and decreased association of PTPN22 with Csk. These findings identify a new role for T cell TRAF3 in promoting T cell activation, by regulating localization and functions of early TCR signaling inhibitors.

Dopamine promotes NMDA receptor hypofunction in the retina through D1 receptor-mediated Csk activation, Src inhibition and decrease of GluN2B phosphorylation

Dopamine and glutamate are critical neurotransmitters involved in light-induced synaptic activity in the retina. In brain neurons, dopamine D₁ receptors (D₁Rs) and the cytosolic protein tyrosine kinase Src can, independently, modulate the behavior of NMDA-type glutamate receptors (NMDARs). Here we studied the interplay between D₁Rs, Src and NMDARs in retinal neurons. We reveal that dopamine-mediated D₁R stimulation provoked NMDAR hypofunction in retinal neurons by attenuating NMDA-gated currents, by preventing NMDA-elicited calcium mobilization and by decreasing the phosphorylation of NMDAR subunit GluN2B. This dopamine effect was dependent on upregulation of the canonical D₁R/adenylyl cyclase/cAMP/PKA pathway, of PKA-induced activation of C-terminal Src kinase (Csk) and of Src inhibition. Accordingly, knocking down Csk or overexpressing a Csk phosphoresistant Src mutant abrogated the dopamine-induced NMDAR hypofunction. Overall, the interplay between dopamine and NMDAR hypofunction, through the D₁R/Csk/Src/GluN2B pathway, might impact on light-regulated synaptic activity in retinal neurons.

Identification and characterization of the role of c-terminal Src kinase in dengue virus replication

We screened a siRNA library targeting human tyrosine kinases in Huh-7 cells and identified c-terminal Src kinase (Csk) as one of the kinases involved in dengue virus replication. Knock-down of Csk expression by siRNAs or inhibition of Csk by an inhibitor reduced dengue virus RNA levels but did not affect viral entry. Csk partially colocalized with viral replication compartments. Dengue infection was drastically reduced in cells lacking the three ubiquitous src family kinases, Src, Fyn and Yes. Csk knock-down in these cells failed to block dengue virus replication suggesting that the effect of Csk is via regulation of Src family kinases. Csk was found to be hyper-phosphorylated during dengue infection and inhibition of protein kinase A led to a block in Csk phosphorylation and dengue virus replication. Overexpression studies suggest an important role for the kinase and SH3 domains in this process. Our results identified a novel role for Csk as a host tyrosine kinase involved in dengue virus replication and provide further insights into the role of host factors in dengue replication.

Roflumilast inhibits leukocyte-platelet interactions and prevents the prothrombotic functions of polymorphonuclear leukocytes and monocytes

ESSENTIALS: Thrombosis is a major comorbidity in patients with chronic obstructive pulmonary disease (COPD). Roflumilast is a selective phosphodiesterase type-4 (PDE4) inhibitor approved for treatment of severe COPD. PDE4 blockade by roflumilast inhibits prothrombotic functions of neutrophils and monocytes. PDE4 inhibitors may reduce thrombotic risk in COPD as well as in other vascular diseases.

BACKGROUND

Roflumilast, an oral selective phosphodiesterase type 4 inhibitor, is approved for the treatment of severe chronic obstructive pulmonary disease (COPD). A recent meta-analysis of trials on COPD revealed that treatment with roflumilast was associated with a significant reduction in the rate of major cardiovascular events. The mechanisms of this effect remain unknown.

OBJECTIVES

We tested the hypothesis that roflumilast N-oxide (RNO), the active metabolite of roflumilast, curbs the molecular mechanisms required for leukocyte-platelet (PLT) interactions and prevents the prothrombotic functions of polymorphonuclear leukocytes (PMNs) and monocytes (MNs).

METHODS

Using well-characterized in vitro models, we analysed the effects of RNO on: (i) PMN adhesiveness; (ii) the release of neutrophil extracellular traps (NETs); and (iii) tissue factor expression in MNs. Key biochemical events underlying the inhibitory effects of RNO were defined.

RESULTS AND CONCLUSIONS

In PMNs, RNO prevented phosphoinositide 3-kinase (PI3K)-dependent phosphorylation of Akt on Ser473, and Src family kinase (SFK)-mediated Pyk2 phosphorylation on Tyr579-580, while inducing protein kinase A-mediated phosphorylation of C-terminal Src kinase, the major negative regulator of SFKs. Modulation of these signaling pathways by RNO resulted in a significant impairment of PMN adhesion to activated PLTs or human umbilical vein endothelial cells, mainly mediated by inhibition of the adhesive function of Mac-1. Moreover RNO curbed SFK/PI3K-mediated NET release by PMNs adherent on fibrinogen-coated surfaces. In MNs interacting with activated PLTs, RNO curbed PI3K-mediated expression of tissue factor. The efficacy of RNO was significantly potentiated by formoterol, a long acting β-adrenergic receptor agonist. This study reveals novel antithrombotic activities by which roflumilast may exert protective effects against cardiovascular comorbodities in COPD.

Phosphodiesterase type 4 blockade prevents platelet-mediated neutrophil recruitment at the site of vascular injury

OBJECTIVE

Platelet-neutrophil interactions play a key role in cardiovascular disease and inflammatory processes. Src family kinases mediate P-selectin glycoprotein ligand-1-Mac-1 cross talk necessary for firm platelet-neutrophil adhesion. Because Src family kinase activity can be regulated by cAMP-dependent pathways, in this work, we evaluated the role of phosphodiesterases in the signaling events that are required to sustain platelet-neutrophil interactions and neutrophil recruitment at the site of vascular injury.

APPROACH AND RESULTS

In neutrophils exposed to P-selectin, selective phosphodiesterase 4 (PDE4) inhibition prevented Src family kinase-mediated phosphorylation of the proline-rich tyrosine kinase 2 on Tyr579/Tyr580. The effects of PDE4 inhibition required protein kinase A, likely through protein kinase A-mediated activation of COOH-terminal Src kinase, a major negative regulator of Src family kinases. PDE4, but not other phosphodiesterase inhibitors, reduced platelet-neutrophil conjugates as well as neutrophil firm adhesion on spread platelets under flow conditions. The effect of PDE4 inhibition on neutrophil adhesion was primarily mediated by downregulation of P-selectin-induced activation of Mac-1. In a murine model of endovascular injury, selective inhibition of PDE4 significantly reduced neutrophil recruitment at the site of vascular damage.

CONCLUSIONS

This study identifies PDE4 as a central node in the signaling network that mediates platelet-neutrophil adhesion and suggests that pharmacological inhibition of PDE4 may represent a novel therapeutic avenue for the treatment of cardiovascular disease.

Immunoreceptor tyrosine-based inhibitory motif (ITIM)-mediated inhibitory signaling is regulated by sequential phosphorylation mediated by distinct nonreceptor tyrosine kinases: a case study involving PECAM-1

The activation state of many blood and vascular cells is tightly controlled by a delicate balance between receptors that contain immunoreceptor tyrosine-based activation motifs (ITAMs) and those that contain immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Precisely how the timing of cellular activation by ITAM-coupled receptors is regulated by ITIM-containing receptors is, however, poorly understood. Using platelet endothelial cell adhesion molecule 1 (PECAM-1) as a prototypical ITIM-bearing receptor, we demonstrate that initiation of inhibitory signaling occurs via a novel, sequential process in which Src family kinases phosphorylate the C-terminal ITIM, thereby enabling phosphorylation of the N-terminal ITIM of PECAM-1 by other Src homology 2 domain-containing nonreceptor tyrosine kinases (NRTKs). NRTKs capable of mediating the second phosphorylation event include C-terminal Src kinase (Csk) and Bruton's tyrosine kinase (Btk). Btk and Csk function downstream of phosphatidylinositol 3-kinase (PI3K) activation during ITAM-dependent platelet activation. In ITAM-activated platelets that were treated with a PI3K inhibitor, PECAM-1 was phosphorylated but did not bind the tandem SH2 domain-containing tyrosine phosphatase SHP-2, indicating that it was not phosphorylated on its N-terminal ITIM. Csk bound to and phosphorylated PECAM-1 more efficiently than did Btk and required its SH2 domain to perform these functions. Additionally, the phosphorylation of the N-terminal ITIM of Siglec-9 by Csk is enhanced by the prior phosphorylation of its C-terminal ITIM, providing evidence that the ITIMs of other dual ITIM-containing receptors are also sequentially phosphorylated. On the basis of these findings, we propose that sequential ITIM phosphorylation provides a general mechanism for precise temporal control over the recruitment and activation of tandem SH2 domain-containing tyrosine phosphatases that dampen ITAM-dependent signals.

Regulation of the SRC family kinases by Csk

The non-receptor tyrosine kinase Csk serves as an indispensable negative regulator of the Src family tyrosine kinases (SFKs) by specifically phosphorylating the negative regulatory site of SFKs, thereby suppressing their oncogenic potential. Csk is primarily regulated through its SH2 domain, which is required for membrane translocation of Csk via binding to scaffold proteins such as Cbp/PAG1. The binding of scaffolds to the SH2 domain can also upregulate Csk kinase activity. These regulatory features have been elucidated by analyses of Csk structure at the atomic levels. Although Csk itself may not be mutated in human cancers, perturbation of the regulatory system consisting of Csk, Cbp/PAG1, or other scaffolds, and certain tyrosine phosphatases may explain the upregulation of SFKs frequently observed in human cancers. This review focuses on the molecular bases for the function, structure, and regulation of Csk as a unique regulatory tyrosine kinase for SFKs.

Substrate-specific reorganization of the conformational ensemble of CSK implicates novel modes of kinase function

Protein kinases use ATP as a phosphoryl donor for the posttranslational modification of signaling targets. It is generally thought that the binding of this nucleotide induces conformational changes leading to closed, more compact forms of the kinase domain that ideally orient active-site residues for efficient catalysis. The kinase domain is oftentimes flanked by additional ligand binding domains that up- or down-regulate catalytic function. C-terminal Src kinase (Csk) is a multidomain tyrosine kinase that is up-regulated by N-terminal SH2 and SH3 domains. Although the X-ray structure of Csk suggests the enzyme is compact, X-ray scattering studies indicate that the enzyme possesses both compact and open conformational forms in solution. Here, we investigated whether interactions with the ATP analog AMP-PNP and ADP can shift the conformational ensemble of Csk in solution using a combination of small angle x-ray scattering and molecular dynamics simulations. We find that binding of AMP-PNP shifts the ensemble towards more extended rather than more compact conformations. Binding of ADP further shifts the ensemble towards extended conformations, including highly extended conformations not adopted by the apo protein, nor by the AMP-PNP bound protein. These ensembles indicate that any compaction of the kinase domain induced by nucleotide binding does not extend to the overall multi-domain architecture. Instead, assembly of an ATP-bound kinase domain generates further extended forms of Csk that may have relevance for kinase scaffolding and Src regulation in the cell.

The Src protein kinases are integral members of numerous signaling pathways involved in cellular growth and differentiation. The master regulator of the Src family is the protein kinase Csk, which adds a phosphate to the C-terminal tail, inhibiting Src Kinase function. Proper regulation of these signaling pathways by Csk is essential as unregulated activity in these pathways is correlated with the development of various cancers and autoimmune diseases. Understanding the nature of the mechanism and structure of Csk may lead to therapeutics and a better understanding of Src signaling pathways. Conformational changes associated with nucleotide binding and release have been shown to regulate the efficiency of Src down-regulation by Csk. To obtain insights into the nature of these nucleotide-induced structural changes, we examined the conformation of Csk in solution while bound to the ATP analog AMP-PNP and product ADP using a combination of small angle x-ray scattering and molecular dynamics. Surprisingly, both nucleotides induce extended conformations of Csk compared to the apo-enzyme, suggesting a novel mode of function. Further understanding of this mode of function may aid in the design of cancer therapeutics that act by regulating Src signaling pathways by modulating the function of Csk.

Integrating signals from the T-cell receptor and the interleukin-2 receptor

T cells orchestrate the adaptive immune response, making them targets for immunotherapy. Although immunosuppressive therapies prevent disease progression, they also leave patients susceptible to opportunistic infections. To identify novel drug targets, we established a logical model describing T-cell receptor (TCR) signaling. However, to have a model that is able to predict new therapeutic approaches, the current drug targets must be included. Therefore, as a next step we generated the interleukin-2 receptor (IL-2R) signaling network and developed a tool to merge logical models. For IL-2R signaling, we show that STAT activation is independent of both Src- and PI3-kinases, while ERK activation depends upon both kinases and additionally requires novel PKCs. In addition, our merged model correctly predicted TCR-induced STAT activation. The combined network also allows information transfer from one receptor to add detail to another, thereby predicting that LAT mediates JNK activation in IL-2R signaling. In summary, the merged model not only enables us to unravel potential cross-talk, but it also suggests new experimental designs and provides a critical step towards designing strategies to reprogram T cells.

Structural elements and allosteric mechanisms governing regulation and catalysis of CSK-family kinases and their inhibition of Src-family kinases

C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK) are endogenous inhibitors constraining the activity of the oncogenic Src-family kinases (SFKs) in cells. Both kinases suppress SFKs by selectively phosphorylating their consensus C-terminal regulatory tyrosine. In addition to phosphorylation, CHK can suppress SFKs by a unique non-catalytic inhibitory mechanism that involves tight binding of CHK to SFKs to form stable complexes. In this review, we discuss how allosteric regulators, phosphorylation, and inter-domain interactions interplay to govern the activity of CSK and CHK and their ability to inhibit SFKs. In particular, based upon the published results of structural and biochemical analysis of CSK and CHK, we attempt to chart the allosteric networks in CSK and CHK that govern their catalysis and ability to inhibit SFKs. We also discuss how the published three-dimensional structure of CSK complexed with an SFK member sheds light on the structural basis of substrate recognition by protein kinases.

Hydrophobic interaction between the SH2 domain and the kinase domain is required for the activation of Csk

The protein tyrosine kinase C-terminal Src kinase (Csk) is activated by the engagement of its Src homology (SH) 2 domain. However, the molecular mechanism required for this is not completely understood. The crystal structure of the active Csk indicates that Csk could be activated by contact between the SH2 domain and the beta3-alphaC loop in the N-terminal lobe of the kinase domain. To study the importance of this interaction for the SH2-domain-mediated activation of Csk, we mutated the amino acid residues forming the contacts between the SH2 domain and the beta3-alphaC loop. The mutation of the beta3-alphaC loop Ala228 to glycine and of the SH2 domain Tyr116, Tyr133, Leu138, and Leu149 to alanine resulted in the inability of the SH2 domain ligand to activate Csk. Furthermore, the overexpressed Csk mutants A228G, Y133A/Y116A, L138A, and L149A were unable to efficiently inactivate endogenous Src in human embryonic kidney 293 cells. The results suggest that the SH2-domain-mediated activation of Csk is dependent on the binding of the beta3-alphaC loop Ala228 to the hydrophobic pocket formed by the side chains of Tyr116, Tyr133, Leu138, and Leu149 on the surface of the SH2 domain.

Interaction of triosephosphate isomerase from the cell surface of Staphylococcus aureus and alpha-(1->3)-mannooligosaccharides derived from glucuronoxylomannan of Cryptococcus neoformans

The glycolytic enzyme triosephosphate isomerase (TPI; EC 5.3.1.1) of Staphylococcus aureus is a candidate adhesion molecule for the interaction between the bacterium and the fungal pathogen Cryptococcus neoformans. TPI may recognize the mannan backbone of glucuronoxylomannan (GXM) of C. neoformans. We purified TPI from extracts of S. aureus surface proteins to investigate its binding by surface plasmon resonance analysis. The immobilized TPI reacted with GXM in a dose-dependent manner. Furthermore, the interactions between staphylococcal TPI and α-(1→3)-mannooligosaccharides derived from GXM were examined. The oligosaccharides exhibited binding with TPI; however, monomeric mannose did not. Differences in the slopes of the sensorgrams were observed between oligosaccharides with an even number of residues versus those with an odd number. A heterogeneous ligand-parallel reaction model revealed the existence of at least two binding sites on TPI. The enzymic activities of TPI were inhibited in a dose-dependent manner by α-(1→3)-mannooligosaccharides larger than triose. The binding of TPI and α-(1→3)-mannotriose near the substrate-binding site was predicted in silico (AutoDock 3.05). An oligosaccharide of size equal to or greater than triose could bind to the site, affecting enzymic activities. Moreover, affinities were indicated, especially for biose and tetraose, to another binding pocket, which would not affect enzymic activity. These data suggest a novel role for TPI, in addition to glycolysis, on the surface of S. aureus.

Control of excitatory synaptic transmission by C-terminal Src kinase

The induction of long-term potentiation at CA3-CA1 synapses is caused by an N-methyl-d-aspartate (NMDA) receptordependent accumulation of intracellular Ca(2+), followed by Src family kinase activation and a positive feedback enhancement of NMDA receptors (NMDARs). Nevertheless, the amplitude of baseline transmission remains remarkably constant even though low frequency stimulation is also associated with an NMDAR-dependent influx of Ca(2+) into dendritic spines. We show here that an interaction between C-terminal Src kinase (Csk) and NMDARs controls the Src-dependent regulation of NMDAR activity. Csk associates with the NMDAR signaling complex in the adult brain, inhibiting the Src-dependent potentiation of NMDARs in CA1 neurons and attenuating the Src-dependent induction of long-term potentiation. Csk associates directly with Src-phosphorylated NR2 subunits in vitro. An inhibitory antibody for Csk disrupts this physical association, potentiates NMDAR mediated excitatory postsynaptic currents, and induces long-term potentiation at CA3-CA1 synapses. Thus, Csk serves to maintain the constancy of baseline excitatory synaptic transmission by inhibiting Src kinase-dependent synaptic plasticity in the hippocampus.

A weak Lck tail bite is necessary for Lck function in T cell antigen receptor signaling

Src family kinases are suppressed by a "tail bite" mechanism, in which the binding of a phosphorylated tyrosine in the C terminus of the protein to the Src homology (SH) 2 domain in the N-terminal half of the protein forces the catalytic domain into an inactive conformation stabilized by an additional SH3 interaction. In addition to this intramolecular suppressive function, the SH2 domain also mediates intermolecular interactions, which are crucial for T cell antigen receptor (TCR) signaling. To better understand the relative importance of these two opposite functions of the SH2 domain of the Src family kinase Lck in TCR signaling, we created three mutants of Lck in which the intramolecular binding of the C terminus to the SH2 domain was strengthened. The mutants differed from wild-type Lck only in one to three amino acid residues following the negative regulatory tyrosine 505, which was normally phosphorylated by Csk and dephosphorylated by CD45 in the mutants. In the Lck-negative JCaM1 cell line, the Lck mutants had a much reduced ability to transduce signals from the TCR in a manner that directly correlated with SH2-Tyr(P)(505) affinity. The mutant with the strongest tail bite was completely unable to support any ZAP-70 phosphorylation, mitogen-activated protein kinase activation, or downstream gene activation in response to TCR ligation, whereas other mutants had intermediate abilities. Lipid raft targeting was not affected. We conclude that Lck is regulated by a weak tail bite to allow for its activation and service in TCR signaling, perhaps through a competitive SH2 engagement mechanism.

G protein-coupled receptors control NMDARs and metaplasticity in the hippocampus

Long-term potentiation (LTP) and long-term depression (LTD) are the major forms of functional synaptic plasticity observed at CA1 synapses of the hippocampus. The balance between LTP and LTD or "metaplasticity" is controlled by G-protein coupled receptors (GPCRs) whose signal pathways target the N-methyl-D-asparate (NMDA) subtype of excitatory glutamate receptor. We discuss the protein kinase signal cascades stimulated by Galphaq and Galphas coupled GPCRs and describe how control of NMDAR activity shifts the threshold for the induction of LTP.

Surface plasmon resonance label-free monitoring of antibody antigen interactions in real time

Detection of biologically active compounds is one of the most important topics in molecular biology and biochemistry. One of the most promising detection methods is based on the application of surface plasmon resonance for label-free detection of biologically active compounds. This method allows one to monitor binding events in real time without labeling. The system can therefore be used to determine both affinity and rate constants for interactions between various types of molecules. Here, we describe the application of a surface plasmon resonance biosensor for label-free investigation of the interaction between an immobilized antigen bovine serum albumin (BSA) and antibody rabbit anti-cow albumin IgG1 (anti-BSA). The formation of a self-assembled monolayer (SAM) over a gold surface is introduced into this laboratory training protocol as an effective immobilization method, which is very promising in biosensing systems based on detection of affinity interactions. In the next step, covalent attachment via artificially formed amide bonds is applied for the immobilization of proteins on the formed SAM surface. These experiments provide suitable experience for postgraduate students to help them understand immobilization of biologically active materials via SAMs, fundamentals of surface plasmon resonance biosensor applications, and determination of non-covalent biomolecular interactions. The experiment is designed for master and/or Ph.D. students. In some particular cases, this protocol might be adoptable for bachelor students that already have completed an extended biochemistry program that included a background in immunology.

Biomolecular interaction analysis in functional proteomics

To understand the function of highly complex eukaryotic tissues like the human brain, in depth knowledge about cellular protein networks is required. Biomolecular interaction analysis (BIA), as a part of functional proteomics, aims to quantify interaction patterns within a protein network in detail. We used the cAMP dependent protein kinase (PKA) as a model system for the binding analysis between small natural ligands, cAMP and cAMP analogues, with their physiological interaction partner, the regulatory subunit of PKA. BIA comprises a variety of methods based on physics, biochemistry and molecular biology. Here we compared side by side real time SPR (surface plasmon resonance, Biacore), a bead based assay (AlphaScreen), a fluorescence based method (Fluorescence polarisation) and ITC (isothermal titration calorimetry). These in vitro methods were complemented by an in cell reporter assay, BRET(2) (bioluminescence resonance energy transfer), allowing to test the effects of cAMP analogues in living cells.

Immune modulatory effects of cyclooxygenase type 2 inhibitors in HIV patients on combination antiretroviral treatment

OBJECTIVES

To examine the immune modulating effects of cyclooxygenase type 2 (COX-2) inhibitors (COX-2i) in HIV-infected patients on combination antiretroviral treatment (CART).

DESIGN

In-depth substudy from an approved, open, controlled, randomized study comparing the immune modulating effects of CART in combination with COX-2i after 12 weeks.

METHODS

Patients (n = 38) on long-term CART with stable viral load (VL) < 50,000 copies/ml and CD4+ T-cell counts > 100/microl were randomized to CART and rofecoxib 25 mg bid (n = 12) or celecoxib 400 mg bid (n = 12), or CART only without placebo (n = 14). Routine clinical chemistry, CD4+ and CD8+ counts and VL were safety parameters. Immunological parameters included C-reactive protein, beta2-microglobulin, Ig isotypes and IgG subclasses as well as several T-lymphocyte subsets. Non-parametric analyses were used throughout.

RESULTS

Prestudy experiments showed higher median intracellular expression of COX-2 in CD4+ (P = 0.048) and possibly CD8+ (P = 0.09) T cells from patients on CART compared with uninfected controls. In the clinical study, increased CD4+ T-cell counts were observed only in patients on COX-2i with VL < 50 copies/ml (P = 0.02). Decreased expression of CD38+ on CD8+ T cells and subsets as well as reductions in IgA and IgM (P < 0.03) were most pronounced in patients on COX-2i who had detectable VL (n = 6). COX-2i treatment enhanced the perforin content particularly in the differentiated CD27-/CD8+ T-cell subsets compared with controls (P = 0.05).

CONCLUSIONS

COX-2i together with CART improved markers for persistent immune activation, particularly in patients with viraemia, as well as enhanced perforin expression, and thereby strengthened COX-2 as a potential therapeutic target in HIV infection.

C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK)--endogenous negative regulators of Src-family protein kinases

C-terminal Src kinase (CSK) and CSK-homologous kinase (CHK) are endogenous inhibitors of the Src-family protein tyrosine kinases (SFKs). Since constitutive activation of SFKs contributes to cancer formation and progression, to prevent excessive activation of SFKs, their activity in normal cells is kept at the basal level by CSK and CHK. CSK and CHK inactivate SFKs by specifically phosphorylating a consensus tyrosine (called Y(T)) near their C-termini. Upon phosphorylation, the phospho-Y(T) engages in intramolecular interactions that lock the SFK molecule in an inactive conformation. SFKs are anchored to the plasma membrane, while CSK and CHK are localized predominantly in the cytosol. To inhibit SFKs, CSK and CHK need to translocate to the plasma membrane. Recruitment of CSK and CHK to the plasma membrane is mediated by the binding of their SH2, SH3 and/or kinase domains to specific transmembrane proteins, G-proteins and adaptor proteins located near the plasma membrane. For CSK, membrane recruitment often accompanies activation. CSK and CHK employ two types of direct interactions with SFKs to achieve efficient Y(T) phosphorylation: (i) short-range interactions involving binding of the active sites of CSK and CHK to specific residues near Y(T), (ii) long-range non-catalytic interactions involving binding of SFKs to motifs located distally from the active sites of CSK and CHK. The interactions between CSK and SFKs are transient in nature. Unlike CSK, CHK binds tightly to SFKs to form stable protein complexes. The binding is non-catalytic as it is independent of Y(T). More importantly, the tight binding alone is sufficient to completely inhibit SFKs. This non-catalytic inhibitory binding represents a novel mechanism employed by CHK to inhibit SFKs. Given that SFKs are implicated in cancer development, compounds mimicking the non-catalytic inhibitory mechanism of CHK are potential anti-cancer therapeutics.

Compartmentalisation of phosphodiesterases and protein kinase A: opposites attract

Understanding the molecular organisation of intracellular signalling pathways is a topic of considerable research interest. Since many signalling enzymes are widely distributed and have several substrates, a critical component in signal transduction is the control of specificity. This is achieved, in part by the assembly of multiprotein complexes where clusters of signalling enzymes create focal points to disseminate the intracellular action of many hormones. This is particularly true for the cAMP dependent protein kinase (PKA) that is localised throughout the cell via its association with A-kinase anchoring proteins (AKAPs). Recent data suggest that some AKAPs also interact with phosphodiesterases (PDEs). Compartmentalisation of PDEs not only provides an elegant means to control PKA activation by monitoring the local cAMP flux, but also serves to concentrate and segregate the action of these important regulatory enzymes.

Direct optical detection of protein-ligand interactions

Direct optical detection provides an excellent means to investigate interactions of molecules in biological systems. The dynamic equilibria inherent to these systems can be described in greater detail by recording the kinetics of a biomolecular interaction. Optical biosensors allow direct detection of interaction patterns without the need for labeling. An overview covering several commercially available biosensors is given, with a focus on instruments based on surface plasmon resonance (SPR) and reflectometric interference spectroscopy (RIFS). Potential assay formats and experimental design, appropriate controls, and calibration procedures, especially when handling low molecular weight substances, are discussed. The single steps of an interaction analysis combined with practical tips for evaluation, data processing, and interpretation of kinetic data are described in detail. In a practical example, a step-by-step procedure for the analysis of a low molecular weight compound interaction with serum protein, determined on a commercial SPR sensor, is presented.

Survey of the year 2003 commercial optical biosensor literature

In the year 2003 there was a 17% increase in the number of publications citing work performed using optical biosensor technology compared with the previous year. We collated the 962 total papers for 2003, identified the geographical regions where the work was performed, highlighted the instrument types on which it was carried out, and segregated the papers by biological system. In this overview, we spotlight 13 papers that should be on everyone's 'must read' list for 2003 and provide examples of how to identify and interpret high-quality biosensor data. Although we still find that the literature is replete with poorly performed experiments, over-interpreted results and a general lack of understanding of data analysis, we are optimistic that these shortcomings will be addressed as biosensor technology continues to mature.