Deciphering the human platelet sheddome

Semaphorin7A promotes tumor growth and exerts a pro-angiogenic effect in macrophages of mammary tumor-bearing mice

Semaphorins are a large family of molecules involved in axonal guidance during the development of the nervous system and have been recently shown to have both angiogenic and anti-angiogenic properties. Specifically, semaphorin 7A (SEMA7A) has been reported to have a chemotactic activity in neurogenesis and to be an immune modulator through α1β1integrins. SEMA7A has been shown to promote monocyte chemotaxis and induce them to produce proinflammatory mediators. In this study we explored the role of SEMA7A in a murine model of breast cancer. We show that SEMA7A is highly expressed by DA-3 murine mammary tumor cells in comparison to normal mammary cells (EpH4), and that peritoneal elicited macrophages from mammary tumor-bearing mice also express SEMA7A at higher levels compared to those derived from normal mice. We also show that murine macrophages treated with recombinant murine SEMA7A significantly increased their expression of proangiogenic molecule CXCL2/MIP-2. Gene silencing of SEMA7A in peritoneal elicited macrophages from DA-3 tumor-bearing mice resulted in decreased CXCL2/MIP-2 expression. Mice implanted with SEMA7A silenced tumor cells showed decreased angiogenesis in the tumors compared to the wild type tumors. Furthermore, peritoneal elicited macrophages from mice bearing SEMA7A-silenced tumors produce significantly (p < 0.01) lower levels of angiogenic proteins, such as CXCL2/MIP-2, CXCL1, and MMP-9, compared to those from control DA-3 mammary tumors. We postulate that SEMA7A in mammary carcinomas may skew monocytes into a pro-tumorigenic phenotype to support tumor growth. SEMA7A could prove to be valuable in establishing new research avenues toward unraveling important tumor-host immune interactions in breast cancer patients.

Testing platelet function

Platelet function tests have been traditionally used to aid in the diagnosis and management of patients with bleeding problems. Given the role of platelets in atherothrombosis, several dedicated platelet function instruments are now available that are simple to use and can be used as point-of-care assays. These can provide rapid assessment of platelet function within whole blood without the requirement of sample processing. Some tests can be used to monitor antiplatelet therapy and assess risk of bleeding and thrombosis, although current guidelines advise against this. This article discusses the potential utility of tests/instruments that are available.

Polo-like kinase 2, a novel ADAM17 signaling component, regulates tumor necrosis factor α ectodomain shedding

ADAM17 (a disintegrin and metalloprotease 17) controls pro- and anti-inflammatory signaling events by promoting ectodomain shedding of cytokine precursors and cytokine receptors. Despite the well documented substrate repertoire of ADAM17, little is known about regulatory mechanisms, leading to substrate recognition and catalytic activation. Here we report a direct interaction of the acidophilic kinase Polo-like kinase 2 (PLK2, also known as SNK) with the cytoplasmic portion of ADAM17 through the C-terminal noncatalytic region of PLK2 containing the Polo box domains. PLK2 activity leads to ADAM17 phosphorylation at serine 794, which represents a novel phosphorylation site. Activation of ADAM17 by PLK2 results in the release of pro-TNFα and TNF receptors from the cell surface, and pharmacological inhibition of PLK2 leads to down-regulation of LPS-induced ADAM17-mediated shedding on primary macrophages and dendritic cells. Importantly, PLK2 expression is up-regulated during inflammatory conditions increasing ADAM17-mediated proteolytic events. Our findings suggest a new role for PLK2 in the regulation of inflammatory diseases by modulating ADAM17 activity.

The Utility of Platelet and Coagulation Testing of Antithrombotics: Fusing Science with Patient Care

[Table: see text] There is an increasing need for the standardization of platelet function and coagulation testing for the assessment of antithrombotic therapies. Investigators continue to strive to identify ideal laboratory testing and monitoring procedures for acquired and inherited platelet function defects as well as for evaluating patient status when treated with existing or emerging antithrombotics. These therapies are used primarily in the treatment of ischemic complications. In patients receiving antithrombotic therapy, the balance between hemostasis and thrombosis is a challenge as there is an ongoing risk for bleeding when patients are receiving antiplatelet agents or anticoagulants to lessen their risk for secondary thrombotic events. There are several diverse tests for monitoring anticoagulant therapy; however, as new agents are developed, more specific tests will be required to directly assess these agents in relationship to overall coagulation status. Research in the platelet biology field is ongoing to provide point-of-care methodologies for the assessment of platelet reactivity in terms of both bleeding and thrombosis risk. Currently there are no instruments that reliably assess the risk of bleeding. The challenges that routinely faced are the complexity of physiology, the need for standardization of platelet testing methodology, and the necessity for appropriate interpretation of the test results.

Bench-to-bedside review: Platelets and active immune functions - new clues for immunopathology?

Platelets display a number of properties besides the crucial function of repairing damaged vascular endothelium and stopping bleeding; these are exploited to benefit patients receiving platelet component transfusions, which might categorize them as innate immune cells. For example, platelets specialize in pro-inflammatory activities, and can secrete a large number of molecules, many of which display biological response modifier functions. Platelets also express receptors for non-self-infectious and possibly non-infectious danger signals, and can engage infectious pathogens by mechanisms barely explained beyond observation. This relationship with infectious pathogens may involve other innate immune cells, especially neutrophils. The sophisticated interplay of platelets with bacteria may culminate in sepsis, a severe pathology characterized by significant reductions in platelet count and platelet dysfunction. How this occurs is still not fully understood. Recent findings from in-depth platelet signaling studies reveal the complexity of platelets and some of the ways they evolve along the immune continuum, from beneficial functions exemplified in endothelium repair to deleterious immunopathology as in systemic inflammatory response syndrome and acute vascular diseases. This review discusses the extended role of platelets as immune cells to emphasize their interactions with infectious pathogens sensed as potentially dangerous.

Immune-reactive soluble OX40 ligand, soluble CD40 ligand, and interleukin-27 are simultaneously oversecreted in platelet components associated with acute transfusion reactions

BACKGROUND

Leukoreduction of labile blood components dramatically decreases the frequency of minor, intermediate, and severe adverse events (AEs), referred to as acute transfusion reactions (ATRs), especially after transfusion of platelet components (PCs). The pathophysiology of AEs may result from accumulation of soluble, secreted, platelet (PLT) factors with proinflammatory functions stored in PCs. Thus, several cosynergizing factors associated with PLT accumulation in PCs may contribute to clinically reported ATRs with inflammatory symptoms.

STUDY DESIGN AND METHODS

We screened for 65 PLT-associated secretory products in PCs that caused ATRs and identified PLT molecules associated with ATRs and inflammation. A functional in vitro study using PC supernatants assayed on reporting immune cells was performed to indicate relevance.

RESULTS

Among 10,600 apheresis PCs, 30 caused inflammatory ATRs and contained significantly elevated levels of soluble CD40 ligand (sCD40L), interleukin (IL)-27, and soluble OX40 ligand (sOX40L). Normal PLTs secreted IL-27 and sOX40L at bioactive concentrations upon thrombin stimulation and were up regulated in association with ATRs, similar to sCD40L. Other secreted products were identified but not investigated further as their positivity was not consistent.

CONCLUSIONS

This study demonstrates the putative participation of PLT-derived sOX40L, IL-27, and sCD40L, which accumulate in PC supernatants, with inflammatory-type ATRs. Further studies are required to determine the clinical significance of these findings to forecast preventive measures whenever possible.

Increased circulating platelet microparticles as a potential biomarker in asthma

BACKGROUND

Endothelial (EMPs) and platelet microparticles (PMPs) have been studied as biomarkers in several inflammatory diseases and as central players in intercellular communication.

METHODS

In this cross-sectional study, we aimed to assess microparticle levels in asthma. Circulating microparticles and inflammatory and angiogenic markers were assessed by clinical and laboratorial evaluation, flow cytometry, and immunoassays, in a group of 20 asthmatic and 15 nonasthmatic subjects.

RESULTS

Circulating levels of PMPs (either CD31+/42b+ or CD31+/42b+/AnV+) were significantly increased in asthmatics (P = 0.021) even after adjustment for confounders. Apoptotic EMPs (CD31+/42b--/AnV+) were significantly increased before (P = 0.005) but not after adjustments (P = 0.117).

CONCLUSIONS

We propose that PMPs may be putative asthma biomarkers, playing a role in asthma pathophysiology.

Aging- and activation-induced platelet microparticles suppress apoptosis in monocytic cells and differentially signal to proinflammatory mediator release

BACKGROUND

Platelet microparticles (PM) are the most abundant cell-derived microparticles in the blood, and accumulate in thrombo-inflammatory diseases. Platelets produce PM upon aging via an apoptosis-like process and by activation with strong agonists. We previously showed that long-term treatment of monocytic cells with apoptosis-induced PM (PMap) promotes their differentiation into resident macrophages. Here we investigated shorter term effects of various types of PM on monocyte signalling and function.

METHODS AND RESULTS

Flow cytometry and scanning electron microscopy revealed that PM formed upon platelet aging (PMap) or ultra-sonication (PMsonic) expressed activated αIIbβ3 integrins and tended to assemble into aggregates. In contrast, PM formed upon platelet activation with thrombin (PMthr) or Ca(2+) ionophore (PMiono) had mostly non-activated αIIbβ3 and little aggregate formation, but had increased CD63 expression. PM from activated and sonicated platelets expressed phosphatidylserine at their surface, while only the latter were enriched in the receptors CD40L and CX3CR1. All PM types expressed P-selectin, interacted with monocytic cells via this receptor, and were internalised into these cells. The various PM types promoted actin cytoskeletal rearrangements and hydrogen peroxide production by monocytic cells. Markedly, both aging- and activation-induced PM types stimulated the phosphoinositide 3-kinase/Akt pathway, suppressing apoptosis induced by several agonists, in a P-selectin-dependent manner. On the other hand, the PM types differentially influenced monocyte signalling in eliciting Ca(2+) fluxes (particularly PMap) and in releasing secondary mediators (complement factor C5a with PMap, and pro-inflammatory tumour necrosis factor-α with PMthr).

CONCLUSIONS

In spite of their common anti-apoptotic potential via Akt activation, aging- and activation-induced PM cause different Ca(2+) signalling events and mediator release in monocytic cells. By implication, aging and activated platelets may modulate monocyte function in different way by the shedding of different PM types.

Identification of a calmodulin-binding domain in Sema4D that regulates its exodomain shedding in platelets

Semaphorin 4D (Sema4D) is a transmembrane protein that supports contact-dependent amplification of platelet activation by collagen before being gradually cleaved by the metalloprotease ADAM17, as we have previously shown. Cleavage releases a soluble 120-kDa exodomain fragment for which receptors exist on platelets and endothelial cells. Here we have examined the mechanism that regulates Sema4D exodomain cleavage. The results show that the membrane-proximal cytoplasmic domain of Sema4D contains a binding site for calmodulin within the polybasic region Arg762-Lys779. Coprecipitation studies show that Sema4D and calmodulin are associated in resting platelets, forming a complex that dissociates upon platelet activation by the agonists that trigger Sema4D cleavage. Inhibiting calmodulin with W7 or introducing a membrane-permeable peptide corresponding to the calmodulin-binding site is sufficient to trigger the dissociation of Sema4D from calmodulin and initiate cleavage. Conversely, deletion of the calmodulin-binding site causes constitutive shedding of Sema4D. These results show that (1) Sema4D is a calmodulin-binding protein with a site of interaction in its membrane-proximal cytoplasmic domain, (2) platelet agonists cause dissociation of the calmodulin-Sema4D complex, and (3) dissociation of the complex is sufficient to trigger ADAM17-dependent cleavage of Sema4D, releasing a bioactive fragment.

Soluble E-cadherin: a critical oncogene modulating receptor tyrosine kinases, MAPK and PI3K/Akt/mTOR signaling

E-cadherin, a cell-cell adhesion glycoprotein, is frequently downregulated with tumorigenic progression. The extracellular domain of E-cadherin is cleaved by proteases to generate a soluble ectodomain fragment, termed sEcad, which is elevated in the urine or serum of cancer patients. In this study, we explored the functional role of sEcad in the progression of skin squamous cell carcinomas (SCCs). We found that full-length E-cadherin expression was decreased and sEcad increased in human clinical tumor samples as well as in ultraviolet (UV)-induced SCCs in mice. Interestingly, sEcad associated with members of the human epidermal growth factor receptor (HER) and insulin-like growth factor-1 (IGF-1R) family of receptors in human and UV-induced mouse tumors. Moreover, in both E-cadherin-positive (E-cadherin(+)) and -negative (E-cadherin(-)) cells in vitro, sEcad activated downstream mitogen-activated protein (MAP) kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signaling and enhanced tumor growth, motility and invasion, the latter via activation of matrix metalloproteinase-2 (MMP-2) and MMP-9. To this end, HER, PI3K or MEK inhibitors suppressed sEcad's tumorigenic effects, including proliferation, migration and invasion. Taken together, our data suggest that sEcad contributes to skin carcinogenesis via association with the HER/IGF-1R-family of receptors and subsequent activation of the MAPK and PI3K/Akt/mTOR pathways, thereby implicating sEcad as a putative therapeutic target in cutaneous SCCs.

The SLAM family member CD84 is regulated by ADAM10 and calpain in platelets

BACKGROUND AND OBJECTIVE

Ectodomain shedding is a major mechanism to modulate platelet receptor signaling and to downregulate platelet reactivity. Proteins of the a disintegrin and metalloproteinase (ADAM) family are implicated in the shedding of various platelet receptors. The signaling lymphocyte activation molecule (SLAM) family receptor CD84 is highly expressed in platelets and immune cells, but its role in platelet physiology is not well explored. Because of its ability to form homodimers, CD84 has been suggested to mediate contact-dependent signaling and contribute to thrombus stability. However, nothing is known about the cellular regulation of CD84.

METHODS

We studied the regulation of CD84 in murine platelets by biochemical approaches and use of three different genetically modified mouse lines. Regulation of CD84 in human platelets was studied using inhibitors and biochemical approaches.

RESULTS

We show that CD84 is cleaved from the surface of human and murine platelets in response to different shedding inducing agents and platelet receptor agonists. CD84 downregulation occurs through ectodomain-shedding and intracellular cleavage. Studies in transgenic mice identified ADAM10 as the principal sheddase responsible for CD84 cleavage, whereas ADAM17 was dispensable. Western blot analyses revealed calpain-mediated intracellular cleavage of the CD84 C-terminus, occurring simultaneously with, but independently of, ectodomain shedding. Furthermore, analysis of plasma and serum samples from transgenic mice demonstrated that CD84 is constitutively shed from the platelet surface by ADAM10 in vivo.

CONCLUSIONS

These results reveal a dual regulation mechanism for platelet CD84 by simultaneous extra- and intracellular cleavage that may modulate platelet-platelet and platelet-immune cell interactions.

Neutrophil extracellular traps (NETs) and infection-related vascular dysfunction

The innate immune system orchestrated by leukocytes primarily neutrophils, serves to remove dead and dying host cells and to provide protection against invasion by pathogens. Failure of this system results in the onset of sepsis leading to grave consequences for the host. Together with mechanical methods to physically isolate and remove the pathogen, neutrophils also release an important set of proinflammatory biological modulators that mediate recruitment of additional cells to a site of infection and amplify the innate protective response. Additionally, neutrophils release highly charged mixtures of DNA and nuclear proteins named neutrophil extracellular traps (NETs). These electrostatically-charged adhesive networks trigger intrinsic coagulation, limit dispersion and entrap the pathogens. NETs also contain the neutrophil secretary granule-derived serine proteases, neutrophil elastase and cathepsin G, known to regulate the reactivity of both neutrophils and platelets. Since the characterization of NETs in 2004, new studies of their functional effect in vivo continue to expand upon unexpected extracellular roles for DNA, and in doing so renew attention to the haemostatic role of the leukocyte. This review will provide a basic description of NETs and examine current knowledge of this important system of defense, including recent work illustrating a role for NETs in activation of thrombosis.

Calpain inhibition stabilizes the platelet proteome and reactivity in diabetes

Platelets from patients with diabetes are hyperreactive and demonstrate increased adhesiveness, aggregation, degranulation, and thrombus formation, processes that contribute to the accelerated development of vascular disease. Part of the problem seems to be dysregulated platelet Ca2+ signaling and the activation of calpains, which are Ca2+-activated proteases that result in the limited proteolysis of substrate proteins and subsequent alterations in signaling. In the present study, we report that the activation of μ- and m-calpain in patients with type 2 diabetes has profound effects on the platelet proteome and have identified septin-5 and the integrin-linked kinase (ILK) as novel calpain substrates. The calpain-dependent cleavage of septin-5 disturbed its association with syntaxin-4 and promoted the secretion of α-granule contents, including TGF-β and CCL5. Calpain was also released by platelets and cleaved CCL5 to generate a variant with enhanced activity. Calpain activation also disrupted the ILK-PINCH-Parvin complex and altered platelet adhesion and spreading. In diabetic mice, calpain inhibition reversed the effects of diabetes on platelet protein cleavage, decreased circulating CCL5 levels, reduced platelet-leukocyte aggregate formation, and improved platelet function. The results of the present study indicate that diabetes-induced platelet dysfunction is mediated largely by calpain activation and suggest that calpain inhibition may be an effective way of preserving platelet function and eventually decelerating atherothrombosis development.

Semaphorin 7a links nerve regeneration and inflammation in the cornea

PURPOSE

We determined Semaphorin 7a (Sema7a) localization and abundance in naive corneas and in corneas after nerve-transecting lamellar flap surgery, and determined the effect of Sema7a supplementation on corneal nerve regeneration and inflammation.

METHODS

Immunolocalization and Western blot analyses were performed to evaluate the abundance of Sema7a in naive corneas and corneas undergoing nerve regeneration after lamellar corneal surgery in thy1-YFP+ neurofluorescent mice. We used compartmental cultures of dissociated trigeminal ganglion cells to determine the effect of Sema7a exposure on neurite outgrowth in vitro. Finally, a Sema7a pellet was implanted under the corneal flap after lamellar transection surgery to determine the neuronal and inflammatory effects of Sema7a supplementation in vivo.

RESULTS

Sema7a was expressed in the corneal epithelium and stromal keratocytes, but was more abundant in the epithelium (74.3%) compared to the stroma (25.7%, P = 0.02). Sema7a expression was increased significantly in the cornea after lamellar corneal surgery and was localized to stromal cells near the regenerating nerve fronds. Exposure of trigeminal neurites to Sema7a (20 nM) in the side compartment increased neurite length significantly. The implanted Sema7a pellet increased significantly YFP+ inflammatory cell influx into the cornea as well as increased corneal nerve length.

CONCLUSIONS

Sema7a is expressed constitutively in the cornea, and potently stimulates nerve regeneration and inflammatory cell influx. Therefore, this immune semaphorin links nerve regeneration and inflammatory processes in the cornea.

Increased phospholipase A2 activity with phosphorylation of peroxiredoxin 6 requires a conformational change in the protein

We have shown previously and confirmed in this study that the phospholipase A(2) (PLA(2)) activity of peroxiredoxin 6 (Prdx6) is markedly increased by phosphorylation. This report evaluates the conformation and thermodynamic stability of Prdx6 protein after phosphorylation to understand the physical basis for increased activity. Phosphorylation resulted in decreased negative far-UV CD, strengthened ANS binding, and a lack of rigid tertiary structure, compatible with a change in conformation to that of a molten globule. The ΔG°(D) was 3.3 ± 0.3 kcal mol(-1) for Prdx6 and 1.7 ± 0.7 kcal mol(-1) for pPrdx6, suggesting that phosphorylation destabilizes the protein. Phosphorylation of Prdx6 changed the conformation of the N-terminal domain exposing Trp 33, as determined by tryptophan fluorescence and NaI fluorescence quenching. The kinetics of interaction of proteins with unilamellar liposomes (50:25:15:10 DPPC:egg PC:cholesterol:PG molar ratio) were evaluated with tryptophan fluorescence. pPrdx6 bound to liposomes with a higher affinity (K(d) = 5.6 ± 1.2 μM) than Prdx6 (K(d) = 24.9 ± 4.5 μM). By isothermal titration calorimetry, pPrdx6 bound to liposomes with a large exothermic heat loss (ΔH = -31.49 ± 0.22 kcal mol(-1)). Correlating our conformational studies with the published crystal structure of oxidized Prdx6 suggests that phosphorylation results in exposure of hydrophobic residues, thereby providing accessibility to the sites for liposome binding. Because binding of the enzyme to the phospholipid substrate interface is a requirement for PLA(2) activity, these results indicate that a change in the conformation of Prdx6 upon its phosphorylation is the basis for enhancement of PLA(2) enzymatic activity.

Platelets and cytokines: How and why?

For patients with platelet deficiencies, platelet components are therapeutic products for which there is no substitute. However, transfusion complications are more frequent with this labile blood product than with others. This is attributable to products secreted by the platelets themselves, including a variety of cytokines, chemokines, and biological response modifiers, some of which are secreted in large quantities following platelet activation. Why platelets are activated and prone to releasing these molecules during certain inflammatory and innate immune responses is not yet fully understood, but it could be due to several parameters including incompatibilities between blood donors and recipients, the process of platelet preparation and preservation, and the ability of the donor's immune system to sense danger presented by external stimuli during the blood donation process. This review presents our current knowledge of how the platelets that constitute the platelet component for transfusion are sources of cytokines and biological response modifiers and discusses methods to improve the quality of blood transfusion products and safety for patients.

Semaphorin 7A inhibits platelet production from CD34+ progenitor cells

BACKGROUND

The multifunctional protein semaphorin 7A (Sema7A) may have regulatory effects on blood cell differentiation via its receptors β1-integrin and plexin C1. As thrombocytopenia can be treated with transfusion of ex vivo CD34(+) cell-derived megakaryocytes, we investigated the effect of Sema7A on differentiation of CD34(+) progenitor cells into megakaryocytes and platelets.

METHODS

Megakaryocytes and platelets were differentiated with a specific cytokine cocktail (CC) from CD34(+) progenitor cells in the presence or absence of Sema7A. Expression of cell markers CD41, CD42a and CD61 or detection of the activation of the signal mediator focal adhesion kinase (FAK) was performed by flow cytometry, cytokine secretion by Luminex technology, and megakaryocyte cell density and morphology by microscopic studies. Sema7A levels in vivo were assessed by real-time PCR and ELISA in hematological patients undergoing chemotherapy.

RESULTS

CD34(+) progenitor cells expressed the receptors for Sema7A. Expression of CD41, CD42a and CD61 was markedly reduced in the presence of Sema7A, after CC-dependent platelet production from CD34(+) progenitor cells. As revealed by microscopic analysis, megakaryocyte cell density was significantly lower in the presence of Sema7A as compared with controls. Blocking of CD29 abrogated the Sema7A-mediated inhibition. Sema7A activated FAK in CD34(+) progenitor cells and significantly increased secretion of the proinflammatory cytokines IL-6, IL-8 and GM-CSF. Finally, Sema7A levels were up-regulated in 50% of patients after chemotherapy.

CONCLUSIONS

Sema7A markedly reduces the production rates of megakaryocytes and platelets from CD34(+) progenitor cells. Hence, up-regulation of Sema7A may be a major risk factor for a reduced platelet repopulation after hematopoietic stem cell transplantation.

Pathologic shear triggers shedding of vascular receptors: a novel mechanism for down-regulation of platelet glycoprotein VI in stenosed coronary vessels

Ligand-induced ectodomain shedding of glycoprotein VI (GPVI) is a metalloproteinase-dependent event. We examined whether shear force, in the absence of GPVI ligand, was sufficient to induce shedding of GPVI. Human-citrated platelet-rich plasma or washed platelets were subjected to increasing shear rates in a cone-plate viscometer, and levels of intact and cleaved GPVI were examined by Western blot and ELISA. Pathophysiologic shear rates (3000-10 000 seconds(-1)) induced platelet aggregation and metalloproteinase-dependent appearance of soluble GPVI ectodomain, and GPVI platelet remnant. Shedding of GPVI continued after transient exposure to shear. Blockade of α(IIb)β(3), GPIbα, or intracellular signaling inhibited shear-induced platelet aggregation but minimally affected shear-induced shedding of GPVI. Shear-induced GPVI shedding also occurred in platelet-rich plasma or washed platelets isolated from a von Willebrand disease type 3 patient with no detectable VWF, implying that shear-induced activation of platelet metalloproteinases can occur in the absence of GPVI and GPIbα ligands. Significantly elevated levels of sGPVI were observed in 10 patients with stable angina pectoris, with well-defined single vessel coronary artery disease and mean intracoronary shear estimates at 2935 seconds(-1) (peak shear, 19 224 seconds(-1)). Loss of GPVI in platelets exposed to shear has potential implications for the stability of a forming thrombus at arterial shear rates.

Proteomic analysis of palmitoylated platelet proteins

Protein palmitoylation is a dynamic process that regulates membrane targeting of proteins and protein-protein interactions. We have previously demonstrated a critical role for protein palmitoylation in platelet activation and have identified palmitoylation machinery in platelets. Using a novel proteomic approach, Palmitoyl Protein Identification and Site Characterization, we have begun to characterize the human platelet palmitoylome. Palmitoylated proteins were enriched from membranes isolated from resting platelets using acyl-biotinyl exchange chemistry, followed by identification using liquid chromatography-tandem mass spectrometry. This global analysis identified > 1300 proteins, of which 215 met criteria for significance and represent the platelet palmitoylome. This collection includes 51 known palmitoylated proteins, 61 putative palmitoylated proteins identified in other palmitoylation-specific proteomic studies, and 103 new putative palmitoylated proteins. Of these candidates, we chose to validate the palmitoylation of triggering receptors expressed on myeloid cell (TREM)-like transcript-1 (TLT-1) as its expression is restricted to platelets and megakaryocytes. We determined that TLT-1 is a palmitoylated protein using metabolic labeling with [³H]palmitate and identified the site of TLT-1 palmitoylation as cysteine 196. The discovery of new platelet palmitoyl protein candidates will provide a resource for subsequent investigations to validate the palmitoylation of these proteins and to determine the role palmitoylation plays in their function.

ADAM17: a molecular switch to control inflammation and tissue regeneration

A disintegrin and metalloproteinase 17 (ADAM17), also known as tumor necrosis factor-α converting enzyme (TACE), is a membrane-bound enzyme that cleaves cell surface proteins, such as cytokines (e.g. TNFα), cytokine receptors (e.g. IL-6R and TNF-R), ligands of ErbB (e.g. TGFα and amphiregulin) and adhesion proteins (e.g. L-selectin and ICAM-1). Here we examine how ectodomain shedding of these molecules can alter their biology and impact on immune and inflammatory responses and cancer development. Gene targeting of Adam17 is embryonic lethal, highlighting the importance of ectodomain shedding during development. Tissue-specific deletion, or hypomorphic knock-in, of Adam17 demonstrates an in vivo role for ADAM17 in controlling inflammation and tissue regeneration. The potential of ADAM17 as therapeutic target is also discussed.

Regulating thrombus growth and stability to achieve an optimal response to injury

An optimal platelet response to injury can be defined as one in which blood loss is restrained and haemostasis is achieved without the penalty of further tissue damage caused by unwarranted vascular occlusion. This brief review considers some of the ways in which thrombus growth and stability can be regulated so that an optimal platelet response can be achieved in vivo. Three related topics are considered. The first focuses on intracellular mechanisms that regulate the early events of platelet activation downstream of G protein coupled receptors for agonists such as thrombin, thromboxane A(2) and ADP. The second considers the ways in which signalling events that are dependent on stable contacts between platelets can influence the state of platelet activation and thus affect thrombus growth and stability. The third focuses on the changes that are experienced by platelets as they move from their normal environment in freely-flowing plasma to a very different environment within the growing haemostatic plug, an environment in which the narrowing gaps and junctions between platelets not only facilitate communication, but also increasingly limit both the penetration of plasma and the exodus of platelet-derived bioactive molecules.

The role of semaphorins and their receptors in platelets: Lessons learned from neuronal and immune synapses

During thrombus formation, activated platelets come into close and increasingly stable contact with each other. This produces a microenvironment in which soluble agonists can accumulate, and proteins on the surface of adjacent platelets can directly interact with each other, potentially modulating subsequent thrombus growth and stability. In some ways, this microenvironment resembles the synapses that support signal propagation between neurons and the exchange of information between T-cells, B-cells, and dendritic cells. Drawing on this analogy, this brief review discusses the role of semaphorins and their receptors in platelets, two protein families that have previously been defined by their role at cell:cell contacts, in both the developing nervous system and adaptive immunity.

Cooperation between integrins and growth factor receptors in signaling and endocytosis

All multicellular animals express receptors for growth factors (GFs) and extracellular matrix (ECM) molecules. Integrin-type ECM receptors anchor cells to their surroundings and concomitantly activate intracellular signal transduction pathways. The same signaling mechanisms are regulated by GF receptors (GFRs). Recently, intensive research efforts have revealed novel mechanisms describing how the two receptor systems collaborate at many different levels. Integrins can directly bind to GFs and promote their activation. Adhesion receptors also organize signaling platforms and assist GFRs or even activate them via ligand-independent mechanisms. Furthermore, integrins can orchestrate endocytosis and recycling of GFRs. Here, we review the present knowledge about the interplay between integrins and GFRs and discuss recent ideas of how this collaboration may explain some previous controversies in integrin research.

Membrane vesicles, current state-of-the-art: emerging role of extracellular vesicles

Release of membrane vesicles, a process conserved in both prokaryotes and eukaryotes, represents an evolutionary link, and suggests essential functions of a dynamic extracellular vesicular compartment (including exosomes, microparticles or microvesicles and apoptotic bodies). Compelling evidence supports the significance of this compartment in a broad range of physiological and pathological processes. However, classification of membrane vesicles, protocols of their isolation and detection, molecular details of vesicular release, clearance and biological functions are still under intense investigation. Here, we give a comprehensive overview of extracellular vesicles. After discussing the technical pitfalls and potential artifacts of the rapidly emerging field, we compare results from meta-analyses of published proteomic studies on membrane vesicles. We also summarize clinical implications of membrane vesicles. Lessons from this compartment challenge current paradigms concerning the mechanisms of intercellular communication and immune regulation. Furthermore, its clinical implementation may open new perspectives in translational medicine both in diagnostics and therapy.

Platelets at work in primary hemostasis

When platelet numbers are low or when their function is disabled, the risk of bleeding is high, which on the one hand indicates that in normal life vascular damage is a rather common event and that hence the role of platelets in maintaining a normal hemostasis is a continuously ongoing physiological process. Upon vascular injury, platelets instantly adhere to the exposed extracellular matrix resulting in platelet activation and aggregation to form a hemostatic plug. This self-amplifying mechanism nevertheless requires a tight control to prevent uncontrolled platelet aggregate formation that eventually would occlude the vessel. Therefore endothelial cells produce inhibitory compounds such as prostacyclin and nitric oxide that limit the growth of the platelet thrombus to the damaged area. With this review, we intend to give an integrated survey of the platelet response to vascular injury in normal hemostasis.