Activation and inhibition of G protein-coupled receptors by cell-penetrating membrane-tethered peptides

Anticoagulant and antithrombotic properties of intracellular protease-activated receptor antagonists

BACKGROUND

Blockade of the thrombin receptors protease-activated receptor (PAR)1 and PAR4 with pepducins, cell-penetrating lipopeptides based on the third intracellular loop of PAR1 and PAR4, effectively inhibits platelet aggregation. We have previously shown that PAR1 pepducin also exerts an anticoagulant activity by partial inhibition of the thrombin plus collagen-induced externalization of phosphatidylserine (PS) at the platelet plasma membrane.

OBJECTIVE

In the present study we examined the effects of PAR1 and PAR4 pepducins on tissue factor (TF)-initiated thrombin generation in platelet-rich plasma (PRP) and the interaction between PAR4 pepducin-loaded mouse platelets and a growing thrombus to confirm the relevance of the in vitro data.

RESULTS

Localization of pepducins at the inner leaflet of the plasma membrane was confirmed with a fluorescence resonance energy transfer assay. Both the PAR1 pepducin, P1pal12, and the PAR4 pepducin, P4pal10, inhibited TF-initiated thrombin generation in PRP. Concentrations of P1pal12 and P4pal10, which blocked the thrombin-induced influx of extracellular calcium ions and inhibited platelet aggregation, reduced the rate of thrombin generation during the propagation phase by 38% and 36%, respectively. Whether this anticoagulant effect is relevant in inhibiting in vivo arterial thrombin growth is uncertain because P4pal10 prevented the incorporation of platelets in a growing thrombus.

CONCLUSIONS

Our findings suggest that in spite of their potential anticoagulant activities the in vivo antithrombotic effect of intracellular PAR pepducins is mainly based on inhibiting platelet-platelet interactions.

Inhibition of IL-6-dependent growth of myeloma cells by an acidic peptide repressing the gp130-mediated activation of Src family kinases

An acidic domain (AD) of gp130 was previously found to interact with the Src family kinase (SFK) Hck. Here, the influence of myristoylated peptides derived from this AD was assessed in the mouse myeloma cell line, 7TD1. The IL-6-dependent growth of 7TD1 cells was reduced by approximately 75%, if 100 microM of myristoylated 18mer peptide (18AD) was included in the growth medium, but was unaffected by a control peptide with scrambled sequence (18sc). A similar differential inhibition by peptides 18AD and 18sc was observed for the erythropoietin-dependent growth of BaF-EH cells expressing chimeric erythropoietin receptor-gp130 and human Hck and for the human myeloma cell line INA-6. While the peptide 18AD concentration inhibiting 50% was approximately 30 microM in 7TD1 and BaF-EH cells, peptide 18AD did not significantly inhibit growth of IL-6-independent MM1.S myeloma and OKT1 hybridoma cells or of BaF-EH cells supplied with IL-3. Treatment with 100 microM peptide 18AD caused the same degree or 60% of apoptosis induction as IL-6 deprivation in 7TD1 or INA-6 cells, respectively. Co-immunoprecipitation experiments revealed that peptide 18AD interfered with the association of Hck and gp130 in 7TD1 lysates in a concentration-dependent manner. IL-6-treatment of INA-6 cells induced the kinase activities of Fyn, Lyn and Hck, but not Src, and the IL-6-induced SFK activities were inhibited by peptide 18AD. Expression in 7TD1 cells of a kinase-inactive Hck mutant (K269R) elicited a dominant-negative effect on cell number increases providing further evidence that SFKs are required for gp130 signalling in myeloma cells.

PAR1, but not PAR4, activates human platelets through a Gi/o/phosphoinositide-3 kinase signaling axis

Thrombin-mediated activation of platelets is critical for hemostasis, but the signaling pathways responsible for this process are not completely understood. In addition, signaling within this cascade can also lead to thrombosis. In this study, we have defined a new signaling pathway for the thrombin receptor protease activated receptor-1 (PAR1) in human platelets. We show that PAR1 couples to G(i/o) in human platelets and activates phosphoinositide-3 kinase (PI3K). PI3K activation regulates platelet integrin alphaIIbbeta3 activation and platelet aggregation and potentiates the PAR1-mediated increase in intraplatelet calcium concentration. PI3K inhibitors eliminated these effects downstream of PAR1, but they had no effect on PAR4 signaling. This study has identified an important role for the direct activation of G(i/o) by PAR1 in human platelets. Given the efficacy of clopidogrel, which blocks the G(i/o)-coupled P2Y purinoceptor 12, as an antiplatelet/antithrombotic drug, our data suggest that specifically blocking only PAR1-mediated G(i/o) signaling could also be an effective therapeutic approach with the possibility of less unwanted bleeding.

Peptides targeting protein kinases: strategies and implications

Protein kinases are important key regulators in most, if not all, biological processes and are linked with many human diseases. Protein kinases thus became attractive targets for drug design. Intracellularly active peptides that selectively interfere with kinase function and or kinase-mediated signaling pathways are potential drug compounds with therapeutic implications.

The roles of proteinase-activated receptors in the vascular physiology and pathophysiology

Proteinase-activated receptors (PARs) belong to a family of G protein-coupled receptors, thus mediating the cellular effects of proteinases. In the vascular system, thrombin and other proteinases in the coagulation-fibrinolysis system are considered to be the physiologically relevant agonists, whereas PARs are among the most important mechanisms mediating the interaction between the coagulation-fibrinolysis system and the vascular wall. Under physiological conditions, PARs are mainly expressed in endothelial cells, and participate in the regulation of vascular tone, mostly by inducing endothelium-dependent relaxation. PARs in endothelial cells are also suggested to contribute to a proinflammatory phenotypic conversion and an increase in the permeability of vascular lesions. In smooth muscle cells, PARs mediate contraction, migration, proliferation, hypertrophy, and production of the extracellular matrix, thereby contributing to the development of vascular lesions and the pathophysiology of such vascular diseases as atherosclerosis. However, the expression of PARs in the smooth muscle of normal arteries is limited. The upregulation of PARs in the smooth muscle is thus considered to be a key step for PARs to participate in the pathogenesis of vascular lesions. Elucidating the molecular mechanism regulating the PARs expression is therefore important to develop new strategies for the prevention and treatment of vascular diseases.

Protease-activated receptors in cardiovascular diseases

Thrombosis associated with the pathophysiological activation of platelets and vascular cells has brought thrombin and its receptors to the forefront of cardiovascular medicine. Thrombin signaling through the protease-activated receptors (PARs) has been shown to influence a wide range of physiological responses including platelet activation, intimal hyperplasia, inflammation, and maintenance of vascular tone and barrier function. The thrombin receptors PAR1 and PAR4 can be effectively targeted in animals in which acute or prolonged exposure to thrombin leads to thrombosis and/or restenosis. In the present study, we describe the molecular and pharmacological basis of small-molecule inhibitors that target PAR1. In addition, we discuss a new class of cell-penetrating inhibitors, termed pepducins, that provide insight into previously unidentified roles of PAR1 and PAR4 in protease signaling.

Therapeutic targeting of molecules involved in leukocyte-endothelial cell interactions

Inflammation is traditionally viewed as a physiological reaction to tissue injury. Leukocytes contribute to the inflammatory response by the secretion of cytotoxic and pro-inflammatory compounds, by phagocytotic activity and by targeted attack of foreign antigens. Leukocyte accumulation in tissues is important for the initial response to injury. However, the overzealous accumulation of leukocytes in tissues also contributes to a wide variety of diseases, such as atherosclerosis, chronic inflammatory bowel disease, rheumatoid arthritis, multiple sclerosis, vasculitis, systemic inflammatory response syndrome, juvenile diabetes and psoriasis. Many therapeutic interventions target immune cells after they have already migrated to the site of inflammation. This review addresses different therapeutic strategies, used to reduce or prevent leukocyte-endothelial cell interactions and communication, in order to limit the progression of inflammatory diseases.

Probing the regulation of M (Kv7) potassium channels in intact neurons with membrane-targeted peptides

M-type (Kv7) potassium channels are closed by Gq/11 G-protein-coupled receptors. Several membrane- or channel-associated molecules have been suggested to contribute to this effect, including depletion of phosphatidylinositol-4,5-bisphosphate (PIP2) and activation of Ca2+/calmodulin and protein kinase C. To facilitate further study of these pathways in intact neurons, we have devised novel membrane-targeted probes that can be applied from the outside of the neuron, by attaching a palmitoyl group to site-directed peptides ("palpeptides") (cf. Covic et al., 2002a,b). A palpeptide incorporating the 10-residue C terminus of Galphaq/11 reduced Gq/11-mediated M-current inhibition in sympathetic neurons by the muscarinic acetylcholine receptor (mAChR) agonist oxotremorine-M but not Go-mediated inhibition of the N-type Ca2+ current by norepinephrine. Instead, the latter was inhibited by the corresponding Go palpeptide. A PIP2 palpeptide, based on the putative PIP2 binding domain of the Kv7.2 channel, inhibited M current (IC50 = approximately 1.5 microm) and enhanced inhibition by oxotremorine-M. Inhibition could not be attributed to activation of mAChRs, calcium influx, or block of M channels but was antagonized by intracellular diC8-PIP2 (dioctanoyl-phosphatidylinositol-4,5-bisphosphate), suggesting that it disrupted PIP2-M channel gating. A fluorescently tagged PIP2 palpeptide was highly targeted to the plasma membrane but did not accumulate in the cytoplasm. We suggest that these palpeptides are anchored in the plasma membrane via the palmitoyl group, such that the peptide moiety can interact with target molecules on the inner face of the membrane. The G-protein-replicating palpeptides were sequence specific and probably compete with the receptor for the cognate G-protein. The PIP2 palpeptide was not sequence specific so probably interacts electrostatically with anionic PIP2 head groups.

Blocking the protease-activated receptor 1-4 heterodimer in platelet-mediated thrombosis

BACKGROUND

Thrombin is the most potent agonist of platelets and plays a critical role in the development of arterial thrombosis. Human platelets express dual thrombin receptors, protease-activated receptor (PAR) 1 and PAR4; however, there are no therapeutic strategies that effectively target both receptors.

METHODS AND RESULTS

Platelet aggregation studies demonstrated that PAR4 activity is markedly enhanced by thrombin-PAR1 interactions. A combination of bivalirudin (hirulog) plus a novel PAR4 pepducin antagonist, P4pal-i1, effectively inhibited aggregation of human platelets to even high concentrations of thrombin and prevented occlusion of carotid arteries in guinea pigs. Likewise, combined inhibition of PAR1 and PAR4 with small-molecule antagonists and pepducins was effective against carotid artery occlusion. Coimmunoprecipitation and fluorescence resonance energy transfer studies revealed that PAR1 and PAR4 associate as a heterodimeric complex in human platelets and fibroblasts. PAR1-PAR4 cofactoring was shown by acceleration of thrombin cleavage and signaling of PAR4 on coexpression with PAR1.

CONCLUSIONS

We show that PAR1 and PAR4 form a stable heterodimer that enables thrombin to act as a bivalent functional agonist. These studies suggest that targeting the PAR1-PAR4 complex may present a novel therapeutic opportunity to prevent arterial thrombosis.

Role of the PAR1 receptor 8th helix in signaling: the 7-8-1 receptor activation mechanism

The protease-activated receptors are tethered ligand G protein-coupled receptors that are activated by proteolytic cleavage of the extracellular domain of the receptor. The archetypic protease-activated receptor PAR1 strongly activates G(q) signaling pathways, but very little is known regarding the mechanism of signal transference between receptor and internally located G protein. The recent x-ray structure of rhodopsin revealed the presence of a highly conserved amphipathic 8th helix that is likely to be physically interposed between receptor and G protein. We found that the analogous 8th helix region of PAR1 was critical for activation of G(q)-dependent signaling. Engineering an 8th helix alpha-aneurysm with a downwards-directed alanine residue markedly interfered with signal transference to G(q). The 8th helix-anchoring cysteine palmitoylation sites were important for the affinity of ligand-dependent G protein coupling but did not affect the maximal signal. A network of H-bond and ionic interactions was found to connect the N-terminal portion of the 8th helix to the nearby NPXXY motif on transmembrane helix 7 and also to the adjacent intracellular loop-1. Disruption of these pairwise interactions caused additive defects in coupling to G protein, indicating that the transmembrane 7-8th helix-i1 loop may move in a coordinated manner to transfer the signal from PAR1 to G protein. This "7-8-1" interaction network was found to be prevalent in G protein-coupled receptors involved in endothelial signaling and angiogenesis.

Pepducins: an effective means to inhibit GPCR signaling by neutrophils

G-protein-coupled receptors (GPCRs) have a central role not only in the competent development of an innate myeloid response to foreign pathogens but, if dysregulated, might contribute to phagocyte-mediated organ injury. Here, recent findings from a study in which neutrophil trafficking is inhibited by using a novel family of GPCR signaling inhibitors, known as pepducins, are discussed.

Synergistic Effect of Thrombin on Collagen-Induced Platelet Procoagulant Activity Is Mediated Through Protease-Activated Receptor-1

OBJECTIVE

In the blood coagulation process, the rate of thrombin formation is critically dependent on phosphatidylserine (PtdSer) at the surface of activated platelets. Thrombin synergistically enhances the collagen-induced platelet procoagulant response. The objective of this study is to elucidate the mechanism of this synergistic action with a focus on the intracellular Ca2+ concentration ([Ca2+]i) and the various platelet receptors for thrombin.

METHODS AND RESULTS

We demonstrate that procoagulant activity is related to a sustained increased [Ca2+]i, which in turn depends on extracellular Ca2+ influx. Increased PtdSer exposure coincides with increased [Ca2+]i and was observed in a subpopulation (approximately 14%) of the platelets after stimulation with thrombin plus collagen. 2D2-Fab fragments against the thrombin binding site on GPIbalpha made clear that this receptor did not signal for platelet procoagulant activity. Inhibition of protease-activated receptor 1 (PAR-1) and PAR-4 by selective intracellular inhibitors and selective desensitization of these receptors revealed that PAR-1, but not PAR-4, activation is a prerequisite for both sustained elevations in [Ca2+]i and procoagulant activity induced by collagen plus thrombin.

CONCLUSIONS

The interaction of thrombin with PAR-1 mediates a synergistic effect on collagen-induced procoagulant activity by inducing a sustained elevation in [Ca2+]i in a subpopulation of platelets.

Reversing systemic inflammatory response syndrome with chemokine receptor pepducins

We describe a new therapeutic approach for the treatment of lethal sepsis using cell-penetrating lipopeptides-termed pepducins-that target either individual or multiple chemokine receptors. Interleukin-8 (IL-8), a ligand for the CXCR1 and CXCR2 receptors, is the most potent endogenous proinflammatory chemokine in sepsis. IL-8 levels rise in blood and lung fluids to activate neutrophils and other cells, and correlate with shock, lung injury and high mortality. We show that pepducins derived from either the i1 or i3 intracellular loops of CXCR1 and CXCR2 prevent the IL-8 response of both receptors and reverse the lethal sequelae of sepsis, including disseminated intravascular coagulation and multi-organ failure in mice. Conversely, pepducins selective for CXCR4 cause a massive leukocytosis that does not affect survival. CXCR1 and CXCR2 pepducins conferred nearly 100% survival even when treatment was postponed, suggesting that our approach might be beneficial in the setting of advanced disease.

D2 dopamine receptor activation of potassium channels is selectively decoupled by Galpha-specific GoLoco motif peptides

The GoLoco motif is a short polypeptide sequence found in G-protein signaling regulators such as regulator of G-protein signaling proteins type 12 and 14 and activator of G-protein signaling protein type 3. A unique property of the GoLoco motifs from these three proteins is their preferential interaction with guanosine diphosphate (GDP)-bound Galpha(i1), Galpha(i3) and, sometimes, Galpha(i2) subunits over Galpha(o) subunits. This interaction prevents both spontaneous guanine nucleotide release and reassociation of Galpha(i)-GDP with Gbetagamma. We utilized this property of the GoLoco motif to examine dopamine (D2 and D3) and somatostatin receptor coupling to G-protein-regulated inwardly rectifying potassium (GIRK) channels in mouse AtT20 cells. GoLoco motif peptides had no effect on either basal channel activity or the initial responses to agonists, suggesting that the GoLoco motif cannot disrupt pre-formed G-protein heterotrimers. GoLoco motif peptides did, however, interfere with human D2((short)) receptor coupling to GIRK channels as demonstrated by the progressively diminished responses after repeated agonist application. This behavior is consistent with some form of compartmentalization of D2 receptors and GIRK channels such that Gbetagamma subunits, freed by local receptor activation and prevented from reforming a heterotrimeric complex, are not functionally constrained within the receptor-channel complex and thus are unable to exert a persistent activating effect. In contrast, GoLoco motif peptides had no effect on either D3 or somatostatin coupling to GIRK channels. Our results suggest that GoLoco motif-based peptides will be useful tools in examining the specificity of G-protein-coupled receptor-effector coupling.

PAR1 is a matrix metalloprotease-1 receptor that promotes invasion and tumorigenesis of breast cancer cells

Protease-activated receptors (PARs) are a unique class of G protein-coupled receptors that play critical roles in thrombosis, inflammation, and vascular biology. PAR1 is proposed to be involved in the invasive and metastatic processes of various cancers. However, the protease responsible for activating the proinvasive functions of PAR1 remains to be identified. Here, we show that expression of PAR1 is both required and sufficient to promote growth and invasion of breast carcinoma cells in a xenograft model. Further, we show that the matrix metalloprotease, MMP-1, functions as a protease agonist of PAR1 cleaving the receptor at the proper site to generate PAR1-dependent Ca2+ signals and migration. MMP-1 activity is derived from fibroblasts and is absent from the breast cancer cells. These results demonstrate that MMP-1 in the stromal-tumor microenvironment can alter the behavior of cancer cells through PAR1 to promote cell migration and invasion.

Purification and in vitro functional analyses of RGS12 and RGS14 GoLoco motif peptides

The GoLoco motif is a short polypeptide sequence that binds to heterotrimeric G-protein alpha subunits of the adenylyl cyclase-inhibitory (Galpha(i/o)) subclass in a nucleotide-dependent manner (i.e., solely to the GDP-bound ground state). This article describes methods used for the expression, purification, and in vitro evaluation of membrane-permeant tag fusion peptides derived from the GoLoco motif regions of "regulator of G-protein signaling" proteins type 12 (RGS12) and 14 (RGS14) and a consensus GoLoco sequence from the multiple GoLoco motif protein AGS3. Three different fluorescence-based assays are described for evaluating the in vitro function of these GoLoco peptides as guanine nucleotide dissociation inhibitors, including measurements of GTPgammaS binding and Galpha subunit activation by the planar ion aluminum tetrafluoride.

Proteinase-activated receptors: transducers of proteinase-mediated signaling in inflammation and immune response

Serine proteinases such as thrombin, mast cell tryptase, trypsin, or cathepsin G, for example, are highly active mediators with diverse biological activities. So far, proteinases have been considered to act primarily as degradative enzymes in the extracellular space. However, their biological actions in tissues and cells suggest important roles as a part of the body's hormonal communication system during inflammation and immune response. These effects can be attributed to the activation of a new subfamily of G protein-coupled receptors, termed proteinase-activated receptors (PARs). Four members of the PAR family have been cloned so far. Thus, certain proteinases act as signaling molecules that specifically regulate cells by activating PARs. After stimulation, PARs couple to various G proteins and activate signal transduction pathways resulting in the rapid transcription of genes that are involved in inflammation. For example, PARs are widely expressed by cells involved in immune responses and inflammation, regulate endothelial-leukocyte interactions, and modulate the secretion of inflammatory mediators or neuropeptides. Together, the PAR family necessitates a paradigm shift in thinking about hormone action, to include proteinases as key modulators of biological function. Novel compounds that can modulate PAR function may be potent candidates for the treatment of inflammatory or immune diseases.

Platelets and anti-platelet therapy

Platelets play a central role in the hemostatic process and consequently are similarly involved in the pathological counterpart, thrombosis. They adhere to various subendothelial proteins, exposed either by injury or disease, and subsequently become activated by the thrombogenic surface or locally produced agonists. These activated platelets aggregate to form a platelet plug, release agonists which recruit more platelets to the growing thrombus, and provide a catalytic surface for thrombin generation and fibrin formation. These platelet-rich thrombi are responsible for the acute occlusion of stenotic vessels and ischemic injury to heart and brain. A range of anti-platelet drugs are currently used, both prophylactically and therapeutically, in regimens to manage thrombo-embolic disorders. These include inhibitors of the generation, or effects, of locally produced agonists; several large clinical trials have supported roles for cyclooxygenase inhibitors, which prevent thromboxane generation, and thienopyridine derivatives, which antagonize ADP receptors. Similarly intravenous alpha IIb beta 3 antagonists have been shown to be effective anti-thrombotics, albeit in highly selective situations; in contrast, to date studies with their oral counterparts have been disappointing. Recent advances in understanding of platelet physiology have suggested several novel, if yet untested, targets for anti-platelet therapy. These include the thrombin receptor, the serotonin handling system, and the leptin receptor.

Advances in antiplatelet therapy

Cardiovascular diseases (CVDs) are the leading cause of morbidity and mortality in the western world. These disorders share a common pathophysiology -- atherosclerosis, which affects various arterial beds, leading to protean manifestations (coronary artery disease [CAD], stroke, peripheral arterial disease [PAD]). The platelet plays a pivotal role in the perpetuation and clinical expression of these disorders. The platelet, once believed to have a role confined to modulation of thrombosis and haemostasis, also plays an active role in vascular inflammation. Antiplatelet agents have become first-line therapy for CVD, and their unequivocal benefits are demonstrated in various basic and experimental models and supported by overwhelming evidence from clinical trials. Search is underway for more effective and safer antiplatelet therapy. Novel therapies are emerging to target the redundant pathways of platelet adhesion, activation and aggregation. Efforts are also ongoing to enhance implementation of existent therapy, target therapy selectively to high-risk patients and to those likely to respond (pharmacogenomics), and study the incremental benefits and safety of various antiplatelet combinations and their interaction with other medications in patients with CVD treated with polypharmacy.

A cyclic peptide mimicking the third intracellular loop of the V2 vasopressin receptor inhibits signaling through its interaction with receptor dimer and G protein

In this study, we investigated the mechanism by which a peptide mimicking the third cytoplasmic loop of the vasopressin V2 receptor inhibits signaling. This loop was synthesized as a cyclic peptide (i3 cyc) that adopted defined secondary structure in solution. We found that i3 cyc inhibited the adenylyl cyclase activity induced by vasopressin or a nonhydrolyzable analog of GTP, guanosine 5'-O-(3-thio)triphosphate. This peptide also affected the specific binding of [3H]AVP by converting vasopressin binding sites from a high to a low affinity state without any effect on the global maximal binding capacity. The inhibitory actions of i3 cyc could also be observed in the presence of maximally uncoupling concentration of guanosine 5'-O-(3-thio)triphosphate, indicating a direct effect on the receptor itself and not exclusively on the interaction between the Gs protein and the V2 receptor (V2-R). Bioluminescence resonance energy-transfer experiments confirmed this assumption, because i3 cyc induced a significant inhibition of the bioluminescence resonance energy-transfer signal between the Renilla reniformis luciferase and the enhanced yellow fluorescent protein fused V2-R. This suggests that the proper arrangement of the dimer could be an important prerequisite for triggering Gs protein activation. In addition to its effect on the receptor itself, the peptide exerted some of its actions at the G protein level, because it could also inhibit guanosine 5'-O-(3-thio)triphosphate-stimulated AC activity. Taken together, the data demonstrate that a peptide mimicking V2-R third intracellular loop affects both the dimeric structural organization of the receptor and has direct inhibitory action on Gs.

Protease-activated receptors: contribution to physiology and disease

Proteases acting at the surface of cells generate and destroy receptor agonists and activate and inactivate receptors, thereby making a vitally important contribution to signal transduction. Certain serine proteases that derive from the circulation (e.g., coagulation factors), inflammatory cells (e.g., mast cell and neutrophil proteases), and from multiple other sources (e.g., epithelial cells, neurons, bacteria, fungi) can cleave protease-activated receptors (PARs), a family of four G protein-coupled receptors. Cleavage within the extracellular amino terminus exposes a tethered ligand domain, which binds to and activates the receptors to initiate multiple signaling cascades. Despite this irreversible mechanism of activation, signaling by PARs is efficiently terminated by receptor desensitization (receptor phosphorylation and uncoupling from G proteins) and downregulation (receptor degradation by cell-surface and lysosomal proteases). Protease signaling in tissues depends on the generation and release of proteases, availability of cofactors, presence of protease inhibitors, and activation and inactivation of PARs. Many proteases that activate PARs are produced during tissue damage, and PARs make important contributions to tissue responses to injury, including hemostasis, repair, cell survival, inflammation, and pain. Drugs that mimic or interfere with these processes are attractive therapies: selective agonists of PARs may facilitate healing, repair, and protection, whereas protease inhibitors and PAR antagonists can impede exacerbated inflammation and pain. Major future challenges will be to understand the role of proteases and PARs in physiological control mechanisms and human diseases and to develop selective agonists and antagonists that can be used to probe function and treat disease.

Protease-activated receptor-4 uses dual prolines and an anionic retention motif for thrombin recognition and cleavage

Thrombin activation of human platelets is mediated by the high-affinity PAR1 (protease-activated receptor-1) and the low-affinity PAR4 receptor. PAR1 and PAR4 exhibit markedly disparate kinetics of activation that likely reflect differences in the macromolecular association of thrombin with their respective N-terminal extracellular domains (exodomains). Here we examine the mechanism of initial thrombin binding and cleavage of the high- and low-affinity PAR exodomains using steady-state kinetic analyses. We showed that the PAR4 exodomain lacks the functional hirudin-like sequence found in PAR1 and does not bind exosite I to cause allosteric activation or inhibition of thrombin. Instead, PAR4 contains an anionic cluster, Asp(57)...Asp(59) ...Glu(62)...Asp(65) (DDED), in its exodomain, which slows the dissociation of PAR4 from the cationic thrombin. The analogous anionic residues in the PAR1 exodomain do not influence affinity for thrombin. Although PAR4 is cleaved more slowly than PAR1 on the cell surface, peptides containing the PAR4 P(4)-P(1) active-site-interacting sequence, Pro(45)-Ala-Pro-Arg (PAPR), are efficiently cleaved due to the optimal placement of dual prolines at positions P(4) and P(2). In comparison, thrombin has low affinity and slow cleavage rates for peptides that have a P(3) proline as occurs in human PAR3. Thus, to compensate for the lack of exosite I binding, PAR4 utilizes proline residues in its P(4)-P(1) sequence to provide high-affinity interactions with the active site and an anionic cluster to slow dissociation from the cationic thrombin.

G Protein–Coupled Receptor Oligomerization

The cardiovascular system is richly endowed with G protein–coupled receptors (GPCRs), members of the largest family of plasma membrane-localized receptors. During the last 10 years, it has become increasingly clear that many, if not all, GPCRs function in oligomeric complexes, as either homo- or hetero-oligomers. This review explores the mechanistic implications of GPCR dimerization and/or oligomerization on receptor activation and interactions with G proteins. The effects of GPCR oligomerization on receptor pharmacology, GPCR-mediated signaling, and potential contributions to GPCR crosstalk will be considered in the context of receptors important in the cardiovascular system. Our evolving understanding of the structural and functional consequences of GPCR oligomerization may provide novel and more selective sites for pharmacological tuning of cardiovascular function.

Blocking receptors on the inside: pepducin-based intervention of PAR signaling and thrombosis

Transmembrane signaling through G-protein coupled receptors (GPCRs) controls a remarkably diverse array of cellular processes including metabolism, growth, motility, adhesion, neuronal signaling, and blood coagulation. The large number of GPCRs and their important roles in normal physiology and in disease have made them the target for more than 50% of prescribed drugs. GPCR agonists and antagonists invariably act on the extracellular surface of the receptors, whereas the intracellular surface has not yet been exploited for development of new therapeutic agents. Here, we demonstrate the utility of novel cell-penetrating peptides, termed pepducins, that act as intracellular inhibitors and/or agonists of signal transference from receptor to G protein. The pepducins require the presence of their cognate receptor for activity and are highly selective for receptor type. Mutational analysis of both intact receptor and pepducins demonstrates that the cell-penetrating agonists do not activate G proteins by the same mechanism as the intact receptor i3 loop, but instead require the C-tail of the receptor. Attachment of a palmitate lipid to shorter i3 loop peptides derived from protease-activated receptors PAR1 and PAR4 created potent inhibitors of thrombin-mediated aggregation of human platelets. Infusion of the anti-PAR4 pepducin into mice extended bleeding time and protected against systemic platelet activation, consistent with the phenotype of a mouse with genetic deficiency of PAR4. These data show that pepducins may be used to ascertain the physiological roles of GPCRs and rapidly determine the potential therapeutic value of blockade of a particular signaling pathway.

Structural Basis for Thrombin Activation of a Protease-Activated Receptor

Protease-activated G protein-coupled receptors (PAR1-4) are tethered-ligand receptors that are activated by proteolytic cleavage of the extracellular domain (exodomain) of the receptor. PAR1, the prototypic member of the PAR family, is the high-affinity thrombin receptor of platelets and vascular endothelium and plays a critical role in blood coagulation, thrombosis, and inflammation. Here, we describe the solution structure of the thrombin-cleaved exodomain of PAR1. The side chains of a hydrophobic hirudin-like (Hir) sequence and adjacent anionic motif project into solution. Docking of the exodomain Hir sequence to exosite I of thrombin reveals that the tethered ligand in the cleaved exodomain bends away from thrombin, leaving its active site available to another large macromolecular substrate. The N-terminal ligand is longer than anticipated and forms an intramolecular complex with a region located in the C terminus of the exodomain. Mutational analysis confirmed that this C-terminal region is a ligand binding site for both intra- and intermolecular ligands. A lipidated-ligand binding site peptide was found to be an effective inhibitor of thrombin-induced platelet aggregation.

Induction of pro-angiogenic signaling by a synthetic peptide derived from the second intracellular loop of S1P3 (EDG3)

The G-protein-coupled receptors of the endothelial differentiation gene (EDG) family mediate pro-angiogenic activities, such as endothelial cell proliferation, chemotaxis, and vessel morphogenesis. We synthesized and tested the effects of a 9-amino acid peptide (KRX-725), derived from the second intracellular loop of S1P3 (EDG3). KRX-725 mimics the effects of sphingosine 1-phosphate (S1P), the natural ligand of S1P3, by triggering a Gi-dependent MEK-ERK (mitogen-activated protein kinase kinase and extracellular signal-regulated kinase) signal transduction pathway. Using aortic rings as an ex vivo model of angiogenesis, vascular sprouting was assessed in the presence of KRX-725 or S1P. KRX-725 induced extensive and dense vascular sprouts, which contain an elaborated organization of endothelial and smooth muscle layers, including lumen formation. When KRX-725 or S1P was combined with proangiogenic factors, such as basic fibroblast growth factor (bFGF), stem cell factor, or vascular endothelial growth factor, the effect was synergistic, leading to further enhancement of vascular sprouting. KRX-725 also initiated neovascularization in a mouse corneal pocket assay in vivo and showed synergism with bFGF. The specificity of KRX-725 was demonstrated via peptide-induced receptor internalization of S1P3 but not S1P1. The ability of a short peptide to stimulate extensive angiogenesis and to synergize with pro-angiogenic factors suggests that KRX-725 may serve as a useful agent in treating pathologic conditions such as peripheral vascular disease, cardiac ischemia, or tissue grafts.

Targeting of proteins to membranes through hedgehog auto-processing

Hedgehog proteins use an auto-processing strategy to generate cholesterol-conjugated peptide products that act as extracellular ligands in a number of developmental signaling pathways. We describe an approach that takes advantage of the hedgehog auto-processing reaction to carry out intracellular modification of heterologous proteins, resulting in their localization to cell membranes. Such processing occurs spontaneously, without accessory proteins or modification by other enzymes. Using the green fluorescent protein (GFP) and the product of the Hras as model proteins, we demonstrate the use of hedgehog auto-processing to process heterologous N-terminal domains and direct the resulting biologically active products to cell membranes. This system represents a tool for targeting functional peptides and proteins to cell membranes, and may also offer a means of directing peptides or other small molecules to components of cholesterol metabolism or regulation.

[Physiological functions of protease-activated receptor-2]

Protease-activated receptors (PARs) are a family of G-protein-coupled-seven-trans-membrane-domain-receptors activated by specific proteases, consisting of four family members. PAR-2, a receptor activated by trypsin, tryptase or coagulation factors VIIa and Xa, is unevenly distributed throughout the mammalian body, modulating multiple physiological functions. In the gastrointestinal tract, PAR-2 is involved in gastric mucosal cytoprotection, smooth muscle motility modulation, salivary and pancreatic exocrine secretion, intestinal ionic transport, etc. In the circulatory system, endothelial PAR-2, upon activation, induces vascular relaxation by mechanisms dependent on nitric oxide or endothelium-derived hyperpolarizing factor (EDHF), resulting in hypotension in vivo. In the respiratory system, PAR-2 appears to play a dual role, being pro- and anti-inflammatory. In the nervous system, PAR-2 present in capsaicin-sensitive sensory neurons participates in processing of pain information. PAR-2 is thus involved in a variety of physiological and pathophysiological functions. PAR-2 is now considered one of the most important molecules as a target for drug development.

Molecular determinants of melanocortin 4 receptor ligand binding and MC4/MC3 receptor selectivity

The molecular basis of ligand recognition by the melanocortin 4 receptor (MC4R) has not been fully defined. In this study, we investigated the molecular determinants of MC4R ligand binding, employing a large array of ligands, using three approaches. First, molecular modeling of the receptor was used to identify Phe284, in transmembrane (TM) 7, as a potential site of ligand interaction. Mutation of Phe284 to alanine reduced binding affinity and potency of peptides containing L-Phe by up to 71-fold but did not appreciably affect binding of linear peptides containing D-Phe, consistent with a hydrophobic interaction between the Phe7 of alpha-melanocyte-stimulating hormone and Phe284. Second, we examined the effect of a naturally occurring mutation in TM3 (I137T) that is linked to obesity. This mutation decreased affinity and potency of cyclic, rigid peptides but not more flexible peptides, consistent with an indirect effect of the mutation on the tertiary structure of the receptor. Third, we examined the residues that support ligand selectivity for the MC4R over the MC3R. Mutation of Ile125 (TM3) of the MC4R to the equivalent residue of the MC3R (phenylalanine) selectively decreased affinity and potency of MC4R-selective ligands. This effect was mirrored by the reciprocal MC3R mutation F157I. The magnitude of this effect indicates that this locus is not of major importance. However, it is considered that an isoleucine/phenylalanine mutation may affect the orientation of Asp122, which has been identified as a major determinant of ligand binding affinity. Thus, this study provides further characterization of the MC4R binding pocket.

Therapeutic potential of protease-activated receptor-1 antagonists

The serine protease thrombin (EC 3.4.21.5) is central to the maintenance of haemostatic balance through its coagulant, anticoagulant and platelet activating properties. In addition, this enzyme affects numerous cellular responses in a wide variety of cells, such as cell proliferation, cytokine and growth factor release, lipid metabolism and tissue remodelling. A family of G-protein-coupled protease-activated receptors (PARs) mediates these cellular actions of thrombin. While thrombin can activate three of the four PAR family members, PAR-1 represents the primary thrombin-responsive receptor in human cells. The expression of PAR-1 in platelets, the vasculature and myocardium, in cells within atherosclerotic plaque and tissues after vascular injury, indicates that this receptor plays an important role during the response to tissue injury and associated inflammatory processes. With the development of PAR-deficient mice and small-molecule antagonists, it is now clear that intervening in processes mediated by PAR-1 presents a new approach to treating a variety of disorders dependent on thrombin generation, including thrombosis and restenosis. The full potential of PAR-1 antagonists has yet to be realised, but the promise of novel therapeutics that modulate receptor function rather than thrombin's proteolytic activity, provides an alternative and, perhaps, more desirable means to dampen the pathological effects of thrombin.

Scientific and therapeutic advances in antiplatelet therapy

Over the past decade, the platelet has emerged as a pivotal entity in cardiovascular diseases. Indeed, the 'preeminence of the platelet' has spawned a variety of drugs that have been shown in large-scale randomized trials to improve patient outcomes in acute coronary syndromes and percutaneous revascularization procedures. Although the platelet was initially viewed only as a bystander in haemostasis, it is now evident that the platelet is in fact a key mediator of thrombosis as well as of inflammation. New insights at the cellular and genomic levels will probably generate novel drugs to inhibit platelet function more effectively and safely than previously possible.

Pepducin-based intervention of thrombin-receptor signaling and systemic platelet activation

Transmembrane signaling through G protein-coupled receptors (GPCRs) controls a diverse array of cellular processes including metabolism, growth, motility, adhesion, neuronal signaling and blood coagulation. The numerous GPCRs and their key roles in both normal physiology and disease have made them the target for more than 50% of all prescribed drugs. GPCR agonists and antagonists act on the extracellular side of the receptors, whereas the intracellular surface has not yet been exploited for development of new therapeutic agents. Here, we demonstrate the utility of novel cell-penetrating peptides, termed 'pepducins', that act as intracellular inhibitors of signal transference from receptors to G proteins. Attachment of a palmitate lipid to peptides based on the third intracellular loop of protease-activated receptor 1 (PAR1) or PAR4 (refs. 3-5) yielded potent inhibitors of thrombin-mediated aggregation of human platelets. Infusion of the anti-PAR4 pepducin into mice extended bleeding time and protected against systemic platelet activation, consistent with the phenotype of PAR4-deficient mice. We show that pepducins might be used to ascertain the physiological roles of GPCRs and rapidly determine the potential therapeutic value of blockade of a particular signaling pathway.