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.

Hierarchical organization in the hemostatic response and its relationship to the platelet-signaling network.

Achieving hemostasis following vascular injury requires the rapid accumulation of platelets and fibrin. Here we used a combination of confocal intravital imaging, genetically engineered mice, and antiplatelet agents to determine how variations in the extent of platelet activation following vascular injury arise from the integration of different elements of the platelet-signaling network. Two forms of penetrating injury were used to evoke the hemostatic response. Both produced a hierarchically organized structure in which a core of fully activated platelets was overlaid with an unstable shell of less-activated platelets. This structure emerged as hemostasis was achieved and persisted for at least 60 minutes following injury, its organization at least partly reflecting agonist concentration gradients. Thrombin activity and fibrin formation were found primarily in the innermost core. As proposed previously, greater packing density in the core facilitated contact-dependent signaling and limited entry of plasma-borne molecules visualized with fluorophores coupled to dextran and albumin. Blocking contact-dependent signaling or inhibiting thrombin reduced the size of the core, while the shell was heavily influenced by adenosine 5'-diphosphate and regulators of Gi2-mediated signaling. Thus, the hemostatic response is shown to produce a hierarchical structure arising, in part, from distinct elements of the platelet-signaling network.

Deciphering the human platelet sheddome.

Activated platelets shed surface proteins, potentially modifying platelet function as well as providing a source of bioactive fragments. Previous studies have identified several constituents of the platelet sheddome, but the full extent of shedding is unknown. Here we have taken a global approach, analyzing protein fragments in the supernate of activated platelets using mass spectroscopy and looking for proteins originating from platelet membranes. After removing plasma proteins and microparticles, 1048 proteins were identified, including 69 membrane proteins. Nearly all of the membrane proteins had been detected previously, but only 10 had been shown to be shed in platelets. The remaining 59 are candidates subject to confirmation. Based on spectral counts, protein representation in the sheddome varies considerably. As proof of principle, we validated one of the less frequently detected proteins, semaphorin 7A, which had not previously been identified in platelets. Surface expression, cleavage, and shedding of semaphorin 7A were demonstrated, as was its association with α-granules. Finally, cleavage of semaphorin 7A and 12 other proteins was substantially reduced by an inhibitor of ADAM17, a known sheddase. These results define a subset of membrane proteins as sheddome candidates, forming the basis for further studies examining the impact of ectodomain shedding on platelet function.

Diminished contact-dependent reinforcement of Syk activation underlies impaired thrombus growth in mice lacking Semaphorin 4D.

We recently reported that Semaphorin 4D (Sema4D) and its receptors are expressed on the platelet surface and showed that Sema4D((-/-)) mice have a selective defect in collagen-induced platelet aggregation and an impaired vascular injury response. Here we investigated the mechanisms involved, tested the role of platelet-platelet contacts in Sema4D-mediated events, and examined the relationship between Sema4D-dependent signaling and integrin α(IIb)ß(3) outside-in signaling. The results show that spleen tyrosine kinase (Syk) activation, an early step in collagen signaling via the glycoprotein VI (GPVI)/FcRγ complex, is greatly reduced in Sema4D((-/-)) platelets and can be restored by adding soluble Sema4D. Earlier events, including FcRγ phosphorylation, occur normally; later events are impaired. In contrast, when engagement of α(IIb)ß(3) was blocked, Sema4D((-/-)) and control platelets were indistinguishable in assays of Syk activation, adhesion, spreading on collagen, and activation of α(IIb)ß(3). Finally, we found that, unlike the Sema4D knockout, α(IIb)ß(3) blockade inhibited FcRγ phosphorylation and that stimulating aggregation with Mn(2+) failed to normalize Syk activation in the absence of Sema4D. Collectively, these results show that α(IIb)ß(3) and Sema4D jointly promote collagen responses by amplifying Syk activation, partly by forming integrin-mediated contacts that enable the binding of Sema4D to its receptors and partly through integrin outside-in signaling. These 2 processes are interdependent, but distinguishable.

Platelet signaling.

This chapter summarizes current ideas about the intracellular signaling that drives platelet responses to vascular injury. After a brief overview of platelet activation intended to place the signaling pathways into context, the first section considers the early events of platelet activation leading up to integrin activation and platelet aggregation. The focus is on the G protein-mediated events utilized by agonists such as thrombin and ADP, and the tyrosine kinase-based signaling triggered by collagen. The second section considers the events that occur after integrin engagement, some of which are dependent on close physical contact between platelets. A third section addresses the regulatory events that help to avoid unprovoked or excessive platelet activation, after which the final section briefly considers individual variations in platelet reactivity and the role of platelet signaling in the innate immune response and embryonic development.

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.

Novel delta opioid receptor agonists exhibit differential stimulation of signaling pathways.

A novel family of 1,3,5-trisubstituted 1,2,4-triazoles was discovered as potent and selective ligands for the delta opioid receptor by rational design. Compound 5b exhibited low-nanomolar in vitro binding affinity (IC(50)=5.8 nM), excellent selectivity for the delta opioid receptor over the alternative mu and kappa opioid receptors, full agonist efficacy in receptor down-regulation and MAP kinase activation assays, and low-efficacy partial agonist activity in stimulation of GTPgammaS binding. The apparent discrepancy observed in these functional assays may stem from different signaling pathways involved in each case, as found previously for other G-protein coupled receptors. More biological studies are underway to better understand the differential stimulation of signaling pathways by these novel compounds.

Purification and mass spectrometric analysis of the kappa opioid receptor.

A clonal human embryonic kidney (HEK) 293 cell line was established that stably expressed the rat kappa-opioid receptor (rKOR) with a FLAG epitope at the amino terminus. The Kd for [3H]diprenorphine was 1.1+/-0.2 nM, and the Bmax was 2.6+/-0.4 pmol/mg. Dynorphin A (1-13), U69,593 and naloxone competitively inhibited [3H]diprenorphine binding with Ki values of 2.0, 18 and 18 nM, respectively, in good agreement with previously reported affinities for the unmodified receptor. U69,593 stimulated [35S]GTPgammaS binding in a concentration-dependent manner and caused phosphorylation of mitogen-activated protein (MAP) kinase, indicating that the activated epitope-tagged receptor triggered appropriate signaling pathways. Immunoblot analysis demonstrated that two immunoreactive receptor species with apparent molecular masses of 42 and 52 kDa were expressed. Previous studies indicated that the 42 kDa protein was localized intracellularly and was a precursor of the 52 kDa receptor, which was present at the cell surface. rKOR was extracted from transfected HEK 293 cell membranes with n-dodecyl-beta-D-maltopyranoside. Sequential use of wheat germ agglutinin chromatography, Sephacryl S300 gel filtration chromatography, anti-FLAG immunoaffinity chromatography and SDS/PAGE permitted purification of the 52 kDa receptor. MALDI-TOF mass spectrometry was used to identify peptides derived from rKOR following sequential in-gel digestion with trypsin and cyanogen bromide. Eighteen rKOR peptides were detected, corresponding to 27.1% coverage of the receptor. Precursor-selective MS/MS confirmed the identity of most of these peptides. In addition, we have identified heat shock protein 70 (HSP70) as a rKOR-interacting protein.

Search of the human proteome for endomorphin-1 and endomorphin-2 precursor proteins.

Based on the promising opioid pharmacological profile of the peptide, Tyr-Pro-Trp-Gly-NH(2) (Tyr-W-MIF), Zadina et al. [Zadina, J.E., Hackler, L., Ge, L.-J., Kastin, A.J., 1997. A potent and selective endogenous agonist for the mu-opiate receptor. Nature 386, 499-5502] synthesized and screened other Gly(4)-substituted peptides, culminating in the synthesis of Tyr-Pro-Trp-Phe-NH(2) (endomorphin-1), which displayed high affinity and selectivity for the mu-opioid receptor. The amidated peptide was then isolated from bovine brain frontal cortex, as was a related peptide, Tyr-Pro-Phe-Phe-NH(2) (endomorphin-2), that displayed similar high affinity and selectivity for the mu-opioid receptor. The biosynthesis of the endomorphins in the brain remains obscure, since the putative precursor proteins for the peptides have not been identified. With the completion of the human genome sequencing project, we hypothesized that we should uncover the biological precursors of the peptides using a bioinformatic approach to search the current human proteome for proteins that contained the endomorphin peptide sequences followed by Gly-Lys/Arg, the consensus sequence for peptide alpha-amidation and precursor cleavage. Twelve proteins were identified that contained the endomorphin-1 Tyr-Pro-Trp-Phe sequence, however none contained the Tyr-Pro-Trp-Phe-Gly sequence necessary for alpha-amidation. Twenty-two distinct proteins contained the endomorphin-2 tetrapeptide sequence, and two of those contained the sequence, Tyr-Pro-Phe-Phe-Gly, however, none contained the requisite peptide-Gly-Lys/Arg sequence. Western blot analysis using an endomorphin-2 antibody detected 4 prominent proteins in mouse brain, necessitating reinterpretation of previous immunocytolocalization studies in the brain. Screening of the current human proteome yielded no evidence for endomorphin precursor proteins based on accepted biochemical criteria.

A select set of opioid ligands induce up-regulation by promoting the maturation and stability of the rat kappa-opioid receptor in human embryonic kidney 293 cells.

Ligand-induced regulation of the rat kappa-opioid receptor (rKOR) was investigated in human embryonic kidney 293 cells stably expressing the FLAG-tagged rKOR. Incubation of rKOR cells with naltrexone for 24 h increased the B(max) >3-fold, with no change in the affinity of [(3)H]diprenorphine. Two immunoreactive receptor species were present in cell lysates: naltrexone treatment caused a >3-fold increase in the 52-kDa species while decreasing the level of the 42-kDa species. Dynorphin(1-13), U69,593 [(5alpha,7alpha,8beta)-(+)-N-methyl-N-(7-[1-pyrrolidinyl]-1-oxaspiro[4,5]dec-8-yl)benzeneacetamide], or salvinorin A [2S,4aR,6aR,7R,9S,10aS, 10bR)-9-(acetyloxy)-2-(3-furanyl)dodecahydro-6a,10b-dimethyl-4,10-dioxo-2H-naphtho[2,1c]pyran-7-carboxylic acid methyl ester] treatment did not alter the level of immunoreactive rKOR protein, whereas etorphine, cyclazocine, naloxone, and naloxone methiodide increased the 52-kDa and decreased the 42-kDa rKOR bands. Receptor up-regulation was associated with an increase in the number of cell surface receptors and a 2-fold increase in the E(max) for guanosine 5'-O-(3-[(35)S]thio)triphosphate binding. Glycosidase digestion indicated that the 52- and 42-kDa receptors contained complex and high-mannose N-glycans, respectively, Pulse-chase analysis and glycosidase digestion sensitivities suggested that the 42-kDa rKOR species was a precursor of the 52-kDa species. Naltrexone did not alter rKOR mRNA levels or translational efficiency, and rKOR up-regulation was not inhibited by cycloheximide. Brefeldin A caused accumulation of intracellular rKOR intermediates, and coincubation with naltrexone increased the levels of the brefeldin-induced species significantly. These results suggest that select opioid ligands up-regulate rKOR by enhancing the rate of receptor folding and maturation and by protecting the receptor from degradation, resulting in an increase in the number of rKOR binding sites, immunoreactive protein, and functional receptors.