Phosphorylation of complement factor C3 in vivo

The Human Platelet as an Innate Immune Cell: Interactions Between Activated Platelets and the Complement System

Platelets play an essential role in maintaining homeostasis in the circulatory system after an injury by forming a platelet thrombus, but they also occupy a central node in the intravascular innate immune system. This concept is supported by their extensive interactions with immune cells and the cascade systems of the blood. In this review we discuss the close relationship between platelets and the complement system and the role of these interactions during thromboinflammation. Platelets are protected from complement-mediated damage by soluble and membrane-expressed complement regulators, but they bind several complement components on their surfaces and trigger complement activation in the fluid phase. Furthermore, localized complement activation may enhance the procoagulant responses of platelets through the generation of procoagulant microparticles by insertion of sublytic amounts of C5b9 into the platelet membrane. We also highlight the role of post-translational protein modifications in regulating the complement system and the critical role of platelets in driving these reactions. In particular, modification of disulfide bonds by thiol isomerases and protein phosphorylation by extracellular kinases have emerged as important mechanisms to fine-tune complement activity in the platelet microenvironment. Lastly, we describe disorders with perturbed complement activation where part of the clinical presentation includes uncontrolled platelet activation that results in thrombocytopenia, and illustrate how complement-targeting drugs are alleviating the prothrombotic phenotype in these patients. Based on these clinical observations, we discuss the role of limited complement activation in enhancing platelet activation and consider how these drugs may provide opportunities for further dissecting the complex interactions between complement and platelets.

Ecto-protein kinase CK2, the neglected form of CK2

Ecto-protein kinases, including protein kinase CK2 (former name, casein kinase 2), have been the focus of research for more than 30 years. At the beginning of the ecto-kinase research their identification was performed with substrates and inhibitors whose specificity under the current knowledge was rather limited. Since all currently known ecto-kinases, including ecto-CK2, have intracellular counterparts, one has to exclude that an ecto-localization originates from intracellular counterparts after cell damage. Protein kinase CK2 is involved in cellular key processes such as cell cycle progression, inhibition of apoptosis, DNA damage repair, differentiation and many other processes. CK2 is composed of two catalytic CK2α or CK2α' subunits and two non-catalytic CK2β subunits. Progress in the ecto-kinase and in particular ecto-CK2 studies was made with the use of transfected tagged CK2 subunits, which allowed to follow their individual transport and localization on the cell surface after transfection. Furthermore, immunofluorescence studies with antibodies against CK2 subunits as well as affinity chromatography with a binding partner of CK2 subunits have improved ecto-kinase research. The use of new and more specific inhibitors as well as of substrates, which do not cross the plasma membrane, have further improved the specificity for ecto-CK2. From the various substrates of ecto-CK2, it can be concluded that ecto-CK2 plays a role in Alzheimer disease, cell adhesion, platelet aggregation, immune response and cellular signalling. New tools and techniques, to study ecto-CK2 activity, are required to identify new substrates and thereby new functional implications for ecto-CK2.

Phosphorylation of protein S by platelet kinases enhances its activated protein C cofactor activity

Protein S (PS) is a multifunctional plasma protein of the hemostatic and inflammatory pathways, although mechanisms for its regulation are poorly understood. Since certain plasma proteins are regulated through extracellular phosphorylation, we investigated whether the anticoagulant activity of PS is regulated through phosphorylation by platelet-secreted kinases. PS was phosphorylated on exposure to activated platelets or their releasates, as judged by immunoblotting for phospho-amino acids and PS. PS phosphorylation was reduced by specific inhibitors of casein kinase 1 (CK1) and casein kinase 2 (CK2) (10 μM D4476, 100 μM CK2-inhibitory peptide YNLKSKSSEDIDESS). Involvement of CKs in PS phosphorylation was confirmed using purified CK1/CK2. Phosphorylation of PS by purified CK1 did not affect its activated protein C (APC) cofactor activity in activated partial thromboplastin time assays in PS-depleted plasma. However, phosphorylation of PS by CK2 or by CK1/CK2 increased PS cofactor activity ∼1.5-fold (158.7±4.8%, P<0.01) or ∼2-fold (191.5±6.4%, P<0.0001), respectively. The APC cofactor activity of PS in PS-depleted plasma exposed to platelet-secreted kinases was enhanced, while CK2 but not CK1 inhibitors reduced APC cofactor activity. Mass spectrometry revealed a phosphorylated CK2 site at Thr37 within the N-terminal Gla-domain. Thus, platelet-mediated extracellular phosphorylation of PS is a potential mechanism by which its activity is regulated.

Self-assembled combinatorial encoding nanoarrays for multiplexed biosensing

Multiplexed and sensitive detection of nucleic acids, proteins, or other molecules from a single solution and a small amount of sample is of great demand in biomarker profiling and disease diagnostics. Here we describe a new concept using combinatorial self-assembly of DNA nanotiles into micrometer-sized two-dimensional arrays that carry nucleic acid probes and barcoded fluorescent dyes to achieve multiplexed detection. We demonstrated the specificity and sensitivity of the arrays by detecting multiple DNA sequences and aptamer binding molecules. This DNA tile-array-based sensor platform can be constructed through DNA self-assembly. The attachment of different molecular probes can be achieved by simple DNA hybridization so bioconjugation is not necessary for the labeling. Accurate control of the interprobe distances and solution-based binding reactions ensures fast target binding kinetics.

Laminin-1 is phosphorylated by ecto-protein kinases of monocytes

Monocytes encounter basement membranes and interact with laminins while crossing the vascular barrier. It is known that these cells possess ecto-protein kinase activity on their surface. Several proteins of the extracellular matrix can be phosphorylated by ectokinases. Therefore, it has been hypothesized that monocyte ectokinases could phosphorylate laminins and influence their biological properties. In order to test the above hypothesis, we used intact human monocytes and adenosine triphosphate labeled with radioactive phosphate at the third phosphate ([gamma-32P]-ATP) to phosphorylate laminin-1. Autoradiography after sodium dodecyl sulphate polyacrylamyde gel electrophoresis (SDS-PAGE) electrophoresis indicated phosphorylation of laminin-1 on the beta and/or gamma chains. After phosphorylation, phosphoserine could be detected on Western blots by a specific monoclonal antibody. Phosphorylation was not detected when monocytes were pre-treated with trypsin and was inhibited by a specific ecto-protein kinase inhibitor (K252b). Laminin phosphorylation was also inhibited by heparin, a known inhibitor of casein kinase II and by pretreatment of monocytes by a monoclonal anti-casein kinase II antibody. Heparin binding, cell attachment and proliferation, and monocyte migration were enhanced on the phosphorylated laminin-1 as compared to the non-phosphorylated controls. These data indicate that laminin-1 can be phosphorylated by monocyte casein kinase II type ectokinase. This phosphorylation influences important functions of laminin and therefore could provide an additional means for the interaction of monocytes with basement membranes.

Alterations in C3 Activation and Binding Caused by Phosphorylation by a Casein Kinase Released from Activated Human Platelets

A casein kinase released from activated human platelets phosphorylates a number of plasma proteins extracellularly, and that activation of platelets in systemic lupus erythematosus patients parallels an increase in the phosphate content of plasma proteins, including C3. The present study was undertaken to characterize this platelet protein kinase and to further elucidate the effect(s) on C3 function of phosphorylation by platelet casein kinase. The phosphate content of human plasma C3 was increased from 0.15 to 0.60 mol phosphate/mol of C3 after platelet activation in whole blood or platelet-rich plasma. The platelet casein kinase was distinct from other casein kinases in terms of its dependence on cations, inhibition by specific protein kinase inhibitors, and immunological reactivity. C3 that had been phosphorylated with platelet casein kinase was tested for its susceptibility to cleavage by trypsin or the classical and alternative pathway convertases and its binding to EAC and IgG. Phosphorylation did not affect the cleavage of C3 into C3a and C3b, but the binding of fragments from phosphorylated C3 to EAC14oxy2 cells and to IgG in purified systems and in serum was increased by 1.6–4.5 times over that of unphosphorylated C3. A covariation was seen between the enhanced binding of C3 fragments to IgG after phosphorylation and an increased ratio of glycerol/glycine binding, from 2.0 for unphosphorylated C3 to 4.9 for phosphorylated C3. The present study suggests that an overall effect of phosphorylation of C3 by platelet casein kinase is to enhance the opsonization of immune complexes.

Phosphorylation of the complement component, C9, by an ecto-protein kinase of human leukemic cells

Ecto-protein kinases (ecto-PK) are surface constituents of many, if not all, animal cell types; normal, transformed or malignant. The occurrence of ecto-PK on the surface of human leukemia cell lines was described [Paas, Y., Fishelson, Z., 1995. Shedding of tyrosine and serine/threonine ecto-PK from human leukemic cells. Arch. Biochem. Biophys. 316 780-788.]. These ecto-PKs have been shown to phosphorylate several exogenous substrates, including the complement C9 protein, an essential component of the terminal complement system. C9 is phosphorylated by ecto-PK of K562 cells on serine residue(s). Phosphorylation occurs in the N-terminal C9a portion produced by cleavage of phosphorylated C9 with human alpha-thrombin. C9 polymers generated upon incubation of C9 with ZnCl2 do not serve as substrates for the K562 ecto-PK. In contrast, unfolded C9, obtained by reduction and alkylation, serves as a superior substrate for the K562 ecto-PK. Native C9 phosphorylation produced a rather low stoichiometry of incorporated phosphate (around 3%) per C9. Despite that, the phosphorylated C9 expressed reduced hemolytic activity. The complement-sensitive variant of K562 (K562/S) did not express the C9 phosphorylating activity. Various PK inhibitors tested failed to block C9 phosphorylation. Only heparin and 2,3-diphosphoglycerate (dpGA) prevented C9 phosphorylation, indicating that the ecto-PK is related to the casein kinase CK2. C9 can be phosphorylated by ecto-PK from other tumor cells, including Jurkat, SK-OV-3 and BT-474. These results suggest that extracellular phosphorylation of C9 may serve as a protective mechanism against complement in tumor cells.

Increased phosphate content in complement component C3, fibrinogen, vitronectin, and other plasma proteins in systemic lupus erythematosus: covariation with platelet activation and possible association with thrombosis

OBJECTIVE

To investigate whether extracellular phosphorylation of plasma proteins takes place in vivo in patients with systemic lupus erythematosus (SLE), to determine possible correlations between phosphate levels and clinical and/or laboratory parameters, and to identify individual phosphorylated plasma proteins.

METHODS

Sera from SLE patients were analyzed for total amounts of protein-bound phosphate by a colorimetric technique, and for levels of beta-thromboglobulin by radioimmunoassay. In addition, the ability of these sera to activate platelets, resulting in the release of protein kinase, was tested using an assay in which platelet-rich plasma from healthy blood donors was incubated with sera or immune complexes from SLE patients. In this assay, [gamma-32P]ATP was added, and 32P-labeled C3 was quantified. Phosphate in individual proteins was detected by Western blot analysis.

RESULTS

32P-labeled, activated platelets were able to phosphorylate exogenously added proteins, without the addition of ATP or cations. Platelet-rich plasma from healthy blood donors became activated by sera or by polyethylene glycol-precipitated immune complexes from patients with SLE, which led to the extracellular phosphorylation of plasma proteins, exemplified in the C3 assay. The phosphate content in plasma proteins was increased in SLE patients with previous thrombosis. The degree of phosphorylation increased up to 3-fold in serial samples obtained from 2 SLE patients during periods of disease exacerbation. Substantial phosphate increases were seen in C3 and fibrinogen. The changes were linked to platelet activation because of the observed covariation with the levels of beta-thromboglobulin.

CONCLUSION

In SLE patients, the phosphate content in plasma proteins (including C3 and fibrinogen) increases due to platelet activation.

Phosphorylation of complement component C3 after synthesis in U937 cells by a putative protein kinase, casein kinase 2, which is regulated by CD11b: evidence that membrane-bound proteases preferentially cleave phosphorylated C3

It was our aim in this study to investigate the possibility that the third component of complement (C3) is phosphorylated during synthesis and secretion in U937 cells. Labelling of U937 cells with [32P]Pi, followed by immunoprecipitation of C3 from cell lysates and culture supernatants at different time points, showed that C3 was phosphorylated intracellularly immediately before release into the medium, which initiated cleavage of the protein into an iC3b-like fragment. Stimulation of CD11b/CD18 increased phosphorylation 7-fold, from a basal level of 2%. The phosphorylation sites in C3 did not resemble those described previously for casein kinase (CK) 1, cAMP-dependent protein kinase A or calcium- and phospholipid-dependent protein kinase C. Instead, protein kinase CK2 was suggested inasmuch as: (1) CK2 was detected both on the cell surface and on shed microparticles; (2) phosphorylation of purified C3 by microparticles was abolished by a monoclonal antibody, anti-CK2; (3) the [32P]Pi tag of both phosphorylated C3 (secreted from U937 cells) and of microparticle-phosphorylated C3, which was cleaved either by membrane proteases or by leucocyte elastase, was found in a 40 and a 70 kDa polypeptide; (4) both secreted C3 and C3 phosphorylated in vitro were much more susceptible to cleavage by proteases. Generation of C3 fragments provides a means by which U937 cells can stimulate nearby cells which are expressing complement receptors. The present study demonstrates that the cleavage of C3 is controlled by an intracellular phosphorylation event regulated by CD11b/CD18.

C1q is a nucleotide binding protein and is responsible for the ability of clusterin preparations to promote immune complex formation

Clusterin prepared from human serum by monoclonal antibody affinity chromatography was devoid of the ability to increase the rates of formation of insoluble immune complexes associated with clusterin preparations obtained by polyclonal IgG affinity chromatography. Clusterin did not bind to AMP-Sepharose but the protein responsible for increasing the rates of formation of insoluble immune complexes did bind to this affinity matrix. This protein was identified as complement protein C1q on the basis of its behaviour on SDS/PAGE and reactivity in sandwich ELISA with monoclonal antibodies specific for C1q. C1q (identified from its behaviour on SDS/PAGE, immunoreactivity with C1q-specific monoclonal antibodies and N-terminal sequencing data) was purified from serum by AMP-Sepharose chromatography. The binding of C1q to AMP-Sepharose was inhibited by adenine nucleotides.

Increased phosphate content of fibrinogen in vivo correlates with alteration in fibrinogen behaviour

Fibrinogen was purified from five patients admitted for hip-replacement surgery the day before (day 0), the day after (day 2) and and one week after the operation (day 8). The behaviour of each patient's three fibrinogens was compared in thrombin gelation assays and plasmin degradation experiments to investigate whether the reported increase in protein-bound phosphate at day 2 and day 8 had any effect on the functional behaviour of fibrinogen as has been demonstrated in vitro. It was found that the thickness of the fibrin fibres produced by thrombin increased markedly at day 2 and declined thereafter. Susceptibility to plasmin appeared to decrease post-operatively by 50% and remained at that level on day 8 despite the phosphate content returning to normal. This has also been shown for fibrinogen phosphorylated in vitro. We conclude, after testing the fibrinogens with and without alkaline phosphatase pretreatment, that our data most resemble the published findings for in vitro phosphorylation of fibrinogen by casein kinase II.

Cyclic AMP-dependent protein kinase phosphorylates serine378 in vitronectin

We previously observed that Ser378 in the heparin-binding domain of vitronectin becomes phosphorylated by a protein kinase in plasma upon addition of ATP and divalent cations. We now report that purified plasma vitronectin contains approximately 2.5 mol of phosphate per mol of protein and that vitronectin becomes phosphorylated during biosynthesis in human hepatoma (HepG2) cells. In vitro, rabbit muscle cAMP-dependent protein kinase specifically phosphorylates Ser378 in single-chain (75 kDa) vitronectin but does not phosphorylate the two-chain (65/10 kDa) form cleaved at Arg379. Heparin affects neither the time course nor the extent of phosphorylation of Ser378 at neutral pH. The extent of phosphorylation of Ser378 achieved with cAMP-dependent protein kinase (greater than or equal to 0.3 mol phosphate per mol vitronectin) is greater than that obtainable in plasma and should enable comparisons to be made of the activities of the native and phosphorylated forms.

Clusterin enhances the formation of insoluble immune complexes

Clusterin was purified from human serum by sequential affinity chromatography over IgG-, protein A- and Con A-Sepharose. The protein was approximately 70 kDa by SDS/PAGE under nonreducing conditions and was resolved into approximately 35 kDa bands under reducing conditions. The protein reacted with clusterin-specific Mabs in ELISA and in Western blots. Its N-terminal sequences agreed with those published for clusterin. An antiserum specific for clusterin made by the above method detected it in complement membrane attack complexes on rabbit erythrocyte membranes. The interaction of clusterin with IgG was physiologically relevant because it was found to increase the rate of formation of insoluble immune complexes.

Phosphorylation of coagulation factor II by phospholipid/Ca(2+)-dependent protein kinase (protein kinase C)

Prothrombin is a major constituent of the blood coagulation cascade and requires phospholipid and Ca2+ for its activation. We have found that phospholipid/Ca(2+)-dependent protein kinase (Protein kinase C) phosphorylates prothrombin and the associated apparent Km value for prothrombin (0.86 microM) is comparable to the Km value reported for most known substrates of protein kinase C. A 2-dimension separation analysis revealed that serine residue was apparently phosphorylated by PKC. The phosphorylation was inhibited by such phosphatidylserine- and/or Ca2+ competitive protein kinase C inhibitors as trifluoperazine, palmitoylcarnitine and gossypol. These results suggest that protein kinase C phosphorylation was involved in the regulation of blood coagulation.

Complement C3: a molecular mosaic of binding sites

Many of the biological activities of the complement system are mediated by C3, the third complement component, and its proteolytic fragments. At the same time, several of the molecules which regulate complement activation target their action at the C3 molecule. Accordingly, the C3 molecule is equipped with multiple binding sites for at least 14 other complement or complement-related proteins. As described in this review, major progress has been made recently in the identification of the C3 binding sites and the residues involved. Yet this has exposed only the "tip of the iceberg". A novel technique which may facilitate the elucidation of the active sites in C3 is presented. Finally, based on the current knowledge on the C3 molecule, a hypothetical model of the molecular organization of this molecule and its binding sites is presented.

The effects of in vitro phosphorylation and dephosphorylation on the thrombin-induced gelation and plasmin degradation of fibrinogen

The alpha-chain of human fibrinogen was found to be phosphorylated in EDTA-anticoagulated whole blood when trace amounts of (gamma-32P)ATP and 7.5 mM Mg2+ ions were added. Fibrinogen was not phosphorylated if only the ATP was added. The thrombin-induced gelation of fibrinogen phosphorylated by protein kinase A, casein kinase I or II was studied spectrophotomerically. It was found that phosphorylation by protein kinase A caused the formation of thinner fibrin fibres, whereas phosphorylation by casein kinase II resulted in fibres slightly thicker than those of the control fibrinogen (equivalent to a 20% increase in the control fibrinogen concentration). Phosphorylation with casein kinase I did not significantly affect the fibrin fibre thickness. Dephosphorylation by alkaline phosphatase removed 50% of the 32P-labelled phosphate from protein kinase A-phosphorylated fibrinogen and over 90% from the casein kinase I or II-phosphorylated fibrinogens. This dephosphorylation resulted in a general increase in fibre thickness in the gelation assay in all samples, although the fibres of the phosphorylated fibrinogens remained substantially thinner than the dephosphorylated control fibrinogen. Plasmin digestion of the phosphorylated fibrinogens showed that they were more resistant to cleavage, being cleaved at only 30% to 70% of the rate of control fibrinogen and that this resistance was unaltered by dephosphorylation, in contrast to the thrombin gelation experiments.

An enzymatic assay for vitronectin based on its selective phosphorylation by protein kinase A

The catalytic subunit (C) of cAMP-dependent protein kinase selectively phosphorylates vitronectin, a plasma protein that promotes cell adhesion and platelet aggregation, inhibits the inactivation of thrombin by antithrombin III, and participates in complement function. This specific phosphorylation is used here (a) to develop an enzymatic assay for vitronectin (with C and [gamma-32P]ATP) which can be used to identify the vitronectin-containing fractions at each stage of its purification; (b) to radioactively label vitronectin and differentiate between the intact and the nicked form of this protein in structure-function studies; and (c) to identify possible vitronectin-related proteins in the plasma of other animal species.

Plasmin digestion of human fibrinogen previously phosphorylated by protein kinase C or dephosphorylated by alkaline phosphatase in vitro

Human fibrinogen, either untreated or previously phosphorylated by protein kinase C, was incubated with plasmin generated by streptokinase, urokinase or tissue plasminogen activator and the resulting fragments were separated by gel electrophoresis. Plasmin degradation resulted in the expected X, Y and D fragments, but the degradation rates differed. In vitro phosphorylation of fibrinogen was seen to inhibit the plasmin digestion. Treatment with alkaline phosphatase did not reverse the inhibition.