Alternaria alternata-induced airway epithelial signaling and inflammatory responses via protease-activated receptor-2 expression.

Inhalation of the fungus Alternaria alternata is associated with an increased risk of allergic asthma development and exacerbations. Recent work in acute exposure animal models suggests that A. alternata-induced asthma symptoms, which include inflammation, mucus overproduction and airway hyperresponsiveness, are due to A. alternata proteases that act via protease-activated receptor-2 (PAR2). However, because other active components present in A. alternata may be contributing to asthma pathophysiology through alternative signaling, the specific role PAR2 plays in asthma initiation and maintenance remains undefined. Airway epithelial cells provide the first encounter with A. alternata and are thought to play an important role in initiating the physiologic response. To better understand the role for PAR2 airway epithelial signaling we created a PAR2-deficient human bronchial epithelial cell line (16HBEPAR-/-) from a model bronchial parental line (16HBE14o-). Comparison of in vitro physiologic responses in these cell lines demonstrated a complete loss of PAR2 agonist (2at-LIGRL-NH2) response and significantly attenuated protease (trypsin and elastase) and A. alternata responses in the 16HBEPAR-/- line. Apical application of A. alternata to 16HBE14o- and 16HBEPAR2-/- grown at air-liquid interface demonstrated rapid, PAR2-dependent and independent, inflammatory cytokine, chemokine and growth factor basolateral release. In conclusion, the novel human PAR2-deficient cell line allows for direct in vitro examination of the role(s) for PAR2 in allergen challenge with polarized human airway epithelial cells.

Protease-activated receptor-2 signaling through ß-arrestin-2 mediates Alternaria alkaline serine protease-induced airway inflammation.

Alternaria alternata is a fungal allergen associated with severe asthma and asthma exacerbations. Similarly to other asthma-associated allergens, Alternaria secretes a serine-like trypsin protease(s) that is thought to act through the G protein-coupled receptor protease-activated receptor-2 (PAR2) to induce asthma symptoms. However, specific mechanisms underlying Alternaria-induced PAR2 activation and signaling remain ill-defined. We sought to determine whether Alternaria-induced PAR2 signaling contributed to asthma symptoms via a PAR2/ß-arrestin signaling axis, identify the protease activity responsible for PAR2 signaling, and determine whether protease activity was sufficient for Alternaria-induced asthma symptoms in animal models. We initially used in vitro models to demonstrate Alternaria-induced PAR2/ß-arrestin-2 signaling. Alternaria filtrates were then used to sensitize and challenge wild-type, PAR2-/- and ß-arrestin-2-/- mice in vivo. Intranasal administration of Alternaria filtrate resulted in a protease-dependent increase of airway inflammation and mucin production in wild-type but not PAR2-/- or ß-arrestin-2-/- mice. Protease was isolated from Alternaria preparations, and select in vitro and in vivo experiments were repeated to evaluate sufficiency of the isolated Alternaria protease to induce asthma phenotype. Administration of a single isolated serine protease from Alternaria, Alternaria alkaline serine protease (AASP), was sufficient to fully activate PAR2 signaling and induce ß-arrestin-2-/--dependent eosinophil and lymphocyte recruitment in vivo. In conclusion, Alternaria filtrates induce airway inflammation and mucus hyperplasia largely via AASP using the PAR2/ß-arrestin signaling axis. Thus, ß-arrestin-biased PAR2 antagonists represent novel therapeutic targets for treating aeroallergen-induced asthma.

Cofilin under control of ß-arrestin-2 in NMDA-dependent dendritic spine plasticity, long-term depression (LTD), and learning.

Dendritic spines are dynamic, actin-rich structures that form the postsynaptic sites of most excitatory synapses in the brain. The F-actin severing protein cofilin has been implicated in the remodeling of dendritic spines and synapses under normal and pathological conditions, by yet unknown mechanisms. Here we report that ß-arrestin-2 plays an important role in NMDA-induced remodeling of dendritic spines and synapses via translocation of active cofilin to dendritic spines. NMDAR activation triggers cofilin activation through calcineurin and phosphatidylinositol 3-kinase (PI3K)-mediated dephosphorylation and promotes cofilin translocation to dendritic spines that is mediated by ß-arrestin-2. Hippocampal neurons lacking ß-arrestin-2 develop mature spines that fail to remodel in response to NMDA. ß-Arrestin-2-deficient mice exhibit normal hippocampal long-term potentiation, but significantly impaired NMDA-dependent long-term depression and spatial learning deficits. Moreover, ß-arrestin-2-deficient hippocampal neurons are resistant to Aß-induced dendritic spine loss. Our studies demonstrate unique functions of ß-arrestin-2 in NMDAR-mediated dendritic spine and synapse plasticity through spatial control over cofilin activation.

ß-Arrestin-2 mediates the proinflammatory effects of proteinase-activated receptor-2 in the airway.

Proteinase-Activated receptor-2 (PAR(2)), a G-protein-coupled Receptor, activated by serine proteinases, is reported to have both protective and proinflammatory effects in the airway. Given these opposing actions, both inhibitors and activators of PAR(2) have been proposed for treating asthma. PAR(2) can signal through two independent pathways: a ß-arrestin-dependent one that promotes leukocyte migration, and a G-protein/Ca(2+) one that is required for prostaglandin E(2) (PGE(2)) production and bronchiolar smooth muscle relaxation. We hypothesized that the proinflammatory responses to PAR(2) activation are mediated by ß-arrestins, whereas the protective effects are not. Using a mouse ovalbumin model for PAR(2)-modulated airway inflammation, we observed decreased leukocyte recruitment, cytokine production, and mucin production in ß-arrestin-2(-/-) mice. In contrast, PAR(2)-mediated PGE(2) production, smooth muscle relaxation, and decreased baseline airway resistance (measures of putative PAR(2) "protective" effects) were independent of ß-arrestin-2. Flow cytometry and cytospins reveal that lung eosinophil and CD4 T-cell infiltration, and production of IL-4, IL-6, IL-13, and TNFα, were enhanced in wild-type but not ß-arrestin-2(-/-) mice. Using the forced oscillation technique to measure airway resistance reveals that PAR(2) activation protects against airway hyperresponsiveness by an unknown mechanism, possibly involving smooth muscle relaxation. Our data suggest that the PAR(2)-enhanced inflammatory process is ß-arrestin-2 dependent, whereas the protective anticonstrictor effect of bronchial epithelial PAR(2) may be ß-arrestin independent.

Molecular mechanisms underlying beta-arrestin-dependent chemotaxis and actin-cytoskeletal reorganization.

ß-Arrestins play a crucial role in cell migration downstream of multiple G-protein-coupled receptors (GPCRs) through multiple mechanisms. There is considerable evidence that ß-arrestin-dependent scaffolding of actin assembly proteins facilitates the formation of a leading edge in response to a chemotactic signal. Conversely, there is substantial support for the hypothesis that ß-arrestins facilitate receptor turnover through their ability to desensitize and internalize GPCRs. This chapter discusses both theories for ß-arrestin-dependent chemotaxis in the context of recent studies, specifically addressing known actin assembly proteins regulated by ß-arrestins, chemokine receptors, and signaling by chemotactic receptors.

Protease-Activated Receptor 2 (PAR2) Expressed in Sensory Neurons Contributes to Signs of Pain and Neuropathy in Paclitaxel Treated Mice.

Chemotherapy-induced peripheral neuropathy (CIPN) is a common, dose-limiting side effect of cancer therapy. Protease-activated receptor 2 (PAR2) is implicated in a variety of pathologies, including CIPN. In this study, we demonstrate the role of PAR2 expressed in sensory neurons in a paclitaxel (PTX)-induced model of CIPN in mice. PAR2 knockout/wildtype (WT) mice and mice with PAR2 ablated in sensory neurons were treated with PTX administered via intraperitoneal injection. In vivo behavioral studies were done in mice using von Frey filaments and the Mouse Grimace Scale. We then examined immunohistochemical staining of dorsal root ganglion (DRG) and hind paw skin samples from CIPN mice to measure satellite cell gliosis and intra-epidermal nerve fiber (IENF) density. The pharmacological reversal of CIPN pain was tested with the PAR2 antagonist C781. Mechanical allodynia caused by PTX treatment was alleviated in PAR2 knockout mice of both sexes. In the PAR2 sensory neuronal conditional knockout (cKO) mice, both mechanical allodynia and facial grimacing were attenuated in mice of both sexes. In the DRG of the PTX-treated PAR2 cKO mice, satellite glial cell activation was reduced compared to control mice. IENF density analysis of the skin showed that the PTX-treated control mice had a reduction in nerve fiber density while the PAR2 cKO mice had a comparable skin innervation as the vehicle-treated animals. Similar results were seen with satellite cell gliosis in the DRG, where gliosis induced by PTX was absent in PAR cKO mice. Finally, C781 was able to transiently reverse established PTX-evoked mechanical allodynia. PERSPECTIVE: Our work demonstrates that PAR2 expressed in sensory neurons plays a key role in PTX-induced mechanical allodynia, spontaneous pain, and signs of neuropathy, suggesting PAR2 as a possible therapeutic target in multiple aspects of PTX CIPN.

C781, a ß-Arrestin Biased Antagonist at Protease-Activated Receptor-2 (PAR2), Displays in vivo Efficacy Against Protease-Induced Pain in Mice.

Given the limited options and often harmful side effects of current analgesics and the suffering caused by the opioid crisis, new classes of pain therapeutics are needed. Protease-activated receptors (PARs), particularly PAR2, are implicated in a variety of pathologies, including pain. Since the discovery of the role of PAR2 in pain, development of potent and specific antagonists has been slow. In this study, we describe the in vivo characterization of a novel small molecule/peptidomimetic hybrid compound, C781, as a ß-arrestin-biased PAR2 antagonist. In vivo behavioral studies were done in mice using von Frey filaments and the Mouse Grimace Scale. Pharmacokinetic studies were done to assess pharmacokinetic/pharmacodynamic relationship in vivo. We used both prevention and reversal paradigms with protease treatment to determine whether C781 could attenuate protease-evoked pain. C781 effectively prevented and reversed mechanical and spontaneous nociceptive behaviors in response to small molecule PAR2 agonists, mast cell activators, and neutrophil elastase. The ED50 of C781 (intraperitoneal dosing) for inhibition of PAR2 agonist (20.9 ng 2-AT)-evoked nociception was 6.3 mg/kg. C781 was not efficacious in the carrageenan inflammation model. Pharmacokinetic studies indicated limited long-term systemic bioavailability for C781 suggesting that optimizing pharmacokinetic properties could improve in vivo efficacy. Our work demonstrates in vivo efficacy of a biased PAR2 antagonist that selectively inhibits ß-arrestin/MAPK signaling downstream of PAR2. Given the importance of this signaling pathway in PAR2-evoked nociception, C781 exemplifies a key pharmacophore for PAR2 that can be optimized for clinical development. PERSPECTIVE: Our work provides evidence that PAR2 antagonists that only block certain aspects of signaling by the receptor can be effective for blocking protease-evoked pain in mice. This is important because it creates a rationale for developing safer PAR2-targeting approaches for pain treatment.

ß-Arrestin-biased proteinase-activated receptor-2 antagonist C781 limits allergen-induced airway hyperresponsiveness and inflammation.

BACKGROUND AND PURPOSE: Asthma is a heterogenous disease strongly associated with inflammation that has many different causes and triggers. Current asthma treatments target symptoms such as bronchoconstriction and airway inflammation. Despite recent advances in biological therapies, there remains a need for new classes of therapeutic agents with novel, upstream targets. The proteinase-activated receptor-2 (PAR2) has long been implicated in allergic airway inflammation and asthma and it remains an intriguing target for novel therapies. Here, we describe the actions of C781, a newly developed low MW PAR2 biased antagonist, in vitro and in vivo in the context of acute allergen exposure. EXPERIMENTAL APPROACH: A human bronchial epithelial cell line expressing PAR2 (16HBE14o- cells) was used to evaluate the modulation in vitro, by C781, of physiological responses to PAR2 activation and downstream ß-arrestin/MAPK and Gq/Ca2+ signalling. Acute Alternaria alternata sensitized and challenged mice were used to evaluate C781 as a prophylactically administered modulator of airway hyperresponsiveness, inflammation and mucus overproduction in vivo. KEY RESULTS: C781 reduced in vitro physiological signalling in response to ligand and proteinase activation. C781 effectively antagonized ß-arrestin/MAPK signalling without significant effect on Gq/Ca2+ signalling in vitro. Given prophylactically, C781 modulated airway hyperresponsiveness, airway inflammation and mucus overproduction of the small airways in an acute allergen-challenged mouse model. CONCLUSION AND IMPLICATIONS: Our work demonstrates the first biased PAR2 antagonist for ß-arrestin/MAPK signalling. C781 is efficacious as a prophylactic treatment for allergen-induced airway hyperresponsiveness and inflammation in mice. It exemplifies a key pharmacophore for PAR2 that can be optimized for clinical development.

Neutrophil elastase acts as a biased agonist for proteinase-activated receptor-2 (PAR2).

Human neutrophil proteinases (elastase, proteinase-3, and cathepsin-G) are released at sites of acute inflammation. We hypothesized that these inflammation-associated proteinases can affect cell signaling by targeting proteinase-activated receptor-2 (PAR(2)). The PAR family of G protein-coupled receptors is triggered by a unique mechanism involving the proteolytic unmasking of an N-terminal self-activating tethered ligand (TL). Proteinases can either activate PAR signaling by unmasking the TL sequence or disarm the receptor for subsequent enzyme activation by cleaving downstream from the TL sequence. We found that none of neutrophil elastase, cathepsin-G, and proteinase-3 can activate G(q)-coupled PAR(2) calcium signaling; but all of these proteinases can disarm PAR(2), releasing the N-terminal TL sequence, thereby preventing G(q)-coupled PAR(2) signaling by trypsin. Interestingly, elastase (but neither cathepsin-G nor proteinase-3) causes a TL-independent PAR(2)-mediated activation of MAPK that, unlike the canonical trypsin activation, does not involve either receptor internalization or recruitment of ß-arrestin. Cleavage of synthetic peptides derived from the extracellular N terminus of PAR(2), downstream of the TL sequence, demonstrated distinct proteolytic sites for all three neutrophil-derived enzymes. We conclude that in inflammation, neutrophil proteinases can modulate PAR(2) signaling by preventing/disarming the G(q)/calcium signal pathway and, via elastase, can selectively activate the p44/42 MAPK pathway. Our data illustrate a new mode of PAR regulation that involves biased PAR(2) signaling by neutrophil elastase and a disarming/silencing effect of cathepsin-G and proteinase-3.

Proteinase-activated receptor-2 antagonist C391 inhibits Alternaria-induced airway epithelial signalling and asthma indicators in acute exposure mouse models.

BACKGROUND AND PURPOSE: Despite the availability of a variety of treatment options, many asthma patients have poorly controlled disease with frequent exacerbations. Proteinase-activated receptor-2 (PAR2) has been identified in preclinical animal models as important to asthma initiation and progression following allergen exposure. Proteinase activation of PAR2 raises intracellular Ca2+ , inducing MAPK and ß-arrestin signalling in the airway, leading to inflammatory and protective effects. We have developed C391, a potent PAR2 antagonist effective in blocking peptidomimetic- and trypsin-induced PAR2 signalling in vitro as well as reducing inflammatory PAR2-associated pain in vivo. We hypothesized that PAR2 antagonism by C391 would attenuate allergen-induced acutely expressed asthma indicators in murine models. EXPERIMENTAL APPROACH: We evaluated the ability of C391 to alter Alternaria alternata-induced PAR2 signalling pathways in vitro using a human airway epithelial cell line that naturally expresses PAR2 (16HBE14o-) and a transfected embryonic cell line (HEK 293). We next evaluated the ability for C391 to reduce A. alternata-induced acutely expressed asthma indicators in vivo in two murine strains. KEY RESULTS: C391 blocked A. alternata-induced, PAR2-dependent Ca2+ and MAPK signalling in 16HBE14o- cells, as well as ß-arrestin recruitment in HEK 293 cells. C391 effectively attenuated A. alternata-induced inflammation, mucus production, mucus cell hyperplasia and airway hyperresponsiveness in acute allergen-challenged murine models. CONCLUSIONS AND IMPLICATIONS: To our best knowledge, this is the first demonstration of pharmacological intervention of PAR2 to reduce allergen-induced asthma indicators in vivo. These data support further development of PAR2 antagonists as potential first-in-class allergic asthma drugs.

Apical and basolateral pools of proteinase-activated receptor-2 direct distinct signaling events in the intestinal epithelium.

Studies suggest that there are two distinct pools of proteinase-activated receptor-2 (PAR2) present in intestinal epithelial cells: an apical pool accessible from the lumen, and a basolateral pool accessible from the interstitial space and blood. Although introduction of PAR2 agonists such as 2-furoyl-LIGRL-O-NH2 (2fAP) to the intestinal lumen can activate PAR2, the presence of accessible apical PAR2 has not been definitively shown. Furthermore, some studies have suggested that basolateral PAR2 responses in the intestinal epithelium are mediated indirectly by neuropeptides released from enteric nerve fibers, rather than by intestinal PAR2 itself. Here we identified accessible pools of both apical and basolateral PAR2 in cultured Caco2-BBe monolayers and in mouse ileum. Activation of basolateral PAR2 transiently increased short-circuit current by activating electrogenic Cl⁻ secretion, promoted dephosphorylation of the actin filament-severing protein, cofilin, and activated the transcription factor, AP-1, whereas apical PAR2 did not. In contrast, both pools of PAR2 activated extracellular signal-regulated kinase 1/2 (ERK1/2) via temporally and mechanistically distinct pathways. Apical PAR2 promoted a rapid, biphasic PLCß/Ca²(+)/PKC-dependent ERK1/2 activation, resulting in nuclear localization, whereas basolateral PAR2 promoted delayed ERK1/2 activation which was predominantly restricted to the cytosol, involving both PLCß/Ca²(+) and ß-arrestin-dependent pathways. These results suggest that the outcome of PAR2 activation is dependent on the specific receptor pool that is activated, allowing for fine-tuning of the physiological responses to different agonists.

beta-Arrestins scaffold cofilin with chronophin to direct localized actin filament severing and membrane protrusions downstream of protease-activated receptor-2.

Protease-activated receptor-2 (PAR-2) mediates pro-inflammatory signals in a number of organs, including enhancing leukocyte recruitment to sites of injury and infection. At the cellular level, PAR-2 promotes activation of the actin filament-severing protein cofilin, which is crucial for the reorganization of the actin cytoskeleton and chemotaxis. These responses require the scaffolding functions of beta-arrestins; however, the mechanism by which beta-arrestins spatially regulate cofilin activity and the role of this pathway in primary cells has not been investigated. Here, using size-exclusion chromatography and co-immunoprecipitation, we demonstrate that PAR-2 promotes the formation of a complex containing beta-arrestins, cofilin, and chronophin (CIN) in primary leukocytes and cultured cells. Both association of cofilin with CIN and cell migration are inhibited in leukocytes from beta-arrestin-2(-/-) mice. We show that, in response to PAR-2 activation, beta-arrestins scaffold cofilin with its upstream activator CIN, to facilitate the localized generation of free actin barbed ends, leading to membrane protrusion. These studies suggest that a major role of beta-arrestins in chemotaxis is to spatially regulate cofilin activity to facilitate the formation of a leading edge, and that this pathway may be important for PAR-2-stimulated immune cell migration.

Focal adhesion kinase acts downstream of EphB receptors to maintain mature dendritic spines by regulating cofilin activity.

Dendritic spines are the postsynaptic sites of most excitatory synapses in the brain and are highly enriched in polymerized F-actin, which drives the formation and maintenance of mature dendritic spines and synapses. We propose that suppressing the activity of the actin-severing protein cofilin plays an important role in the stabilization of mature dendritic spines, and is accomplished through an EphB receptor-focal adhesion kinase (FAK) pathway. Our studies revealed that Cre-mediated knock-out of loxP-flanked fak prompted the reversion of mature dendritic spines to an immature filopodial-like phenotype in primary hippocampal cultures. The effects of FAK depletion on dendritic spine number, length, and morphology were rescued by the overexpression of the constitutively active FAK(Y397E), but not FAK(Y397F), indicating the significance of FAK activation by phosphorylation on tyrosine 397. Our studies demonstrate that FAK acts downstream of EphB receptors in hippocampal neurons and EphB2-FAK signaling controls the stability of mature dendritic spines by promoting cofilin phosphorylation, thereby inhibiting cofilin activity. While constitutively active nonphosphorylatable cofilin(S3A) induced an immature spine profile, phosphomimetic cofilin(S3D) restored mature spine morphology in neurons with disrupted EphB activity or lacking FAK. Further, we found that EphB-mediated regulation of cofilin activity at least partially depends on the activation of Rho-associated kinase (ROCK) and LIMK-1. These findings indicate that EphB2-mediated dendritic spine stabilization relies, in part, on the ability of FAK to activate the RhoA-ROCK-LIMK-1 pathway, which functions to suppress cofilin activity and inhibit cofilin-mediated dendritic spine remodeling.

Beta-arrestin inhibits CAMKKbeta-dependent AMPK activation downstream of protease-activated-receptor-2.

BACKGROUND: Proteinase-activated-receptor-2 (PAR2) is a seven transmembrane receptor that can activate two separate signaling arms: one through Gαq and Ca2+ mobilization, and a second through recruitment of ß-arrestin scaffolds. In some cases downstream targets of the Gαq/Ca2+ signaling arm are directly inhibited by ß-arrestins, while in other cases the two pathways are synergistic; thus ß-arrestins act as molecular switches capable of modifying the signal generated by the receptor. RESULTS: Here we demonstrate that PAR2 can activate adenosine monophosphate-activated protein kinase (AMPK), a key regulator of cellular energy balance, through Ca2+-dependent Kinase Kinase ß (CAMKKß), while inhibiting AMPK through interaction with ß-arrestins. The ultimate outcome of PAR2 activation depended on the cell type studied; in cultured fibroblasts with low endogenous ß-arrestins, PAR2 activated AMPK; however, in primary fat and liver, PAR2 only activated AMPK in ß-arrestin-2-/- mice. ß-arrestin-2 could be co-immunoprecipitated with AMPK and CAMKKß under baseline conditions from both cultured fibroblasts and primary fat, and its association with both proteins was increased by PAR2 activation. Addition of recombinant ß-arrestin-2 to in vitro kinase assays directly inhibited phosphorylation of AMPK by CAMKKß on Thr172. CONCLUSIONS: Studies have shown that decreased AMPK activity is associated with obesity and Type II Diabetes, while AMPK activity is increased with metabolically favorable conditions and cholesterol lowering drugs. These results suggest a role for ß-arrestin in the inhibition of AMPK signaling, raising the possibility that ß-arrestin-dependent PAR2 signaling may act as a molecular switch turning a positive signal to AMPK into an inhibitory one.

Differential regulation of class IA phosphoinositide 3-kinase catalytic subunits p110 alpha and beta by protease-activated receptor 2 and beta-arrestins.

PAR-2 (protease-activated receptor 2) is a GPCR (G-protein-coupled receptor) that can elicit both G-protein-dependent and -independent signals. We have shown previously that PAR-2 simultaneously promotes Galphaq/Ca2+-dependent activation and beta-arrestin-1-dependent inhibition of class IA PI3K (phosphoinositide 3-kinase), and we sought to characterize further the role of beta-arrestins in the regulation of PI3K activity. Whereas the ability of beta-arrestin-1 to inhibit p110alpha (PI3K catalytic subunit alpha) has been demonstrated, the role of beta-arrestin-2 in PI3K regulation and possible differences in the regulation of the two catalytic subunits (p110alpha and p110beta) associated with p85alpha (PI3K regulatory subunit) have not been examined. In the present study we have demonstrated that: (i) PAR-2 increases p110alpha- and p110beta-associated lipid kinase activities, and both p110alpha and p110beta are inhibited by over-expression of either beta-arrestin-1 or -2; (ii) both beta-arrestin-1 and -2 directly inhibit the p110alpha catalytic subunit in vitro, whereas only beta-arrestin-2 directly inhibited p110beta; (iii) examination of upstream pathways revealed that PAR-2-induced PI3K activity required the small GTPase Cdc (cell-division cycle)42, but not tyrosine phosphorylation of p85; and (iv) beta-arrestins inhibit PAR-2-induced Cdc42 activation. Taken together, these results indicated that beta-arrestins could inhibit PAR-2-stimulated PI3K activity, both directly and through interference with upstream pathways, and that the two beta-arrestins differ in their ability to inhibit the p110alpha and p110beta catalytic subunits. These results are particularly important in light of the growing interest in PAR-2 as a pharmacological target, as commonly used biochemical assays that monitor G-protein coupling would not screen for beta-arrestin-dependent signalling events.

The novel PAR2 ligand C391 blocks multiple PAR2 signalling pathways in vitro and in vivo.

BACKGROUND AND PURPOSE: Proteinase-activated receptor-2 (PAR2) is a GPCR linked to diverse pathologies, including acute and chronic pain. PAR2 is one of the four PARs that are activated by proteolytic cleavage of the extracellular amino terminus, resulting in an exposed, tethered peptide agonist. Several peptide and peptidomimetic agonists, with high potency and efficacy, have been developed to probe the functions of PAR2, in vitro and in vivo. However, few similarly potent and effective antagonists have been described. EXPERIMENTAL APPROACH: We modified the peptidomimetic PAR2 agonist, 2-furoyl-LIGRLO-NH2 , to create a novel PAR2 peptidomimetic ligand, C391. C391 was evaluated for PAR2 agonist/antagonist activity to PAR2 across Gq signalling pathways using the naturally expressing PAR2 cell line 16HBE14o-. For antagonist studies, a highly potent and specific peptidomimetic agonist (2-aminothiazo-4-yl-LIGRL-NH2 ) and proteinase agonist (trypsin) were used to activate PAR2. C391 was also evaluated in vivo for reduction of thermal hyperalgesia, mediated by mast cell degranulation, in mice. KEY RESULTS: C391 is a potent and specific peptidomimetic antagonist, blocking multiple signalling pathways (Gq -dependent Ca2+ , MAPK) induced following peptidomimetic or proteinase activation of human PAR2. In a PAR2-dependent behavioural assay in mice, C391 dose-dependently (75 µg maximum effect) blocked the thermal hyperalgesia, mediated by mast cell degranulation. CONCLUSIONS AND IMPLICATIONS: C391 is the first low MW antagonist to block both PAR2 Ca2+ and MAPK signalling pathways activated by peptidomimetics and/or proteinase activation. C391 represents a new molecular structure for PAR2 antagonism and can serve as a basis for further development for this important therapeutic target.

Arrestins in actin reorganization and cell migration.

Arrestins have emerged as important regulators of actin reorganization and cell migration. Both in their classical roles as mediators of receptor desensitization and internalization, and in their newer role as signaling scaffolds, ß-arrestins help orchestrate the cellular response to chemotactic signals. However, there is still a considerable amount to be learned about the precise molecular mechanisms underlying these processes. This review discusses how, by regulating receptor internalization and by scaffolding of signaling molecules in discrete cellular locations, arrestins facilitate gradient sensing and cytoskeletal reorganization, ultimately resulting in cell migration. In addition, putative new targets of ß-arrestin regulation that may play important roles in cell migration are discussed, as continued research on these targets may provide important details to fill in the current gaps in our understanding of these processes.

ß-Arrestin-kinase scaffolds: turn them on or turn them off?

G-protein-coupled receptors (GPCRs) can signal through heterotrimeric G-proteins or through ß-arrestins to elicit responses to a plethora of extracellular stimuli. While the mechanisms underlying G-protein signaling is relatively well understood, the mechanisms by which ß-arrestins regulate the diverse set of proteins with which they associate remain unclear. Multi-protein complexes are a common feature of ß-arrestin-dependent signaling. The first two such complexes discovered were the mitogen-activated kinases modules associated with extracellular regulated kinases (ERK1/2) and Jnk3. Subsequently a number of other kinases have been shown to undergo ß-arrestin-dependent regulation, including Akt, phosphatidylinositol-3kinase (PI3K), Lim-domain-containing kinase (LIMK), calcium calmodulin kinase II (CAMKII), and calcium calmodulin kinase kinase ß (CAMKKß). Some are positively and some negatively regulated by ß-arrestin association. One of the missing links to understanding these pathways is the molecular mechanisms by which the activity of these kinases is regulated. Do ß-arrestins merely serve as scaffolds to bring enzyme and substrate together or do they have a direct effect on the enzymatic activities of target kinases? Recent evidence suggests that both mechanisms are involved and that the mechanisms by which ß-arrestins regulate kinase activity varies with the target kinase. This review discusses recent advances in the field focusing on 5 kinases for which considerable mechanistic detail and specific sites of interaction have been elucidated.

Beta-arrestins as regulators of signal termination and transduction: how do they determine what to scaffold?

Over the last decade ß-arrestins have emerged as pleiotropic scaffold proteins, capable of mediating numerous diverse responses to multiple agonists. Most well characterized are the G-protein-coupled receptor (GPCR) stimulated ß-arrestin signals, which are sometimes synergistic with, and sometimes independent of, heterotrimeric G-protein signals. ß-arrestin signaling involves the recruitment of downstream signaling moieties to ß-arrestins; in many cases specific sites of interaction between ß-arrestins and the downstream target have been identified. As more information unfolds about the nature of ß-arrestin scaffolding interactions, it is evident that these proteins are capable of adopting multiple conformations which in turn reveal a specific set of interacting domains. Recruitment of ß-arrestin to a specific GPCR can promote formation of a specific subset of available ß-arrestin scaffolds, allowing for a higher level of specificity to given agonists. This review discusses recent advances in ß-arrestin signaling, discussing the molecular details of a subset of known ß-arrestin scaffolds and the significance of specific binding interactions on the ultimate cellular response.

Stop that cell! Beta-arrestin-dependent chemotaxis: a tale of localized actin assembly and receptor desensitization.

Beta-arrestins have recently emerged as key regulators of directed cell migration or chemotaxis. Given their traditional role as mediators of receptor desensitization, one theory is that beta-arrestins contribute to cell polarity during chemotaxis by quenching the signal at the trailing edge of the cell. A second theory is that they scaffold signaling molecules involved in cytoskeletal reorganization to promote localized actin assembly events leading to the formation of a leading edge. This review addresses both models. It discusses studies demonstrating the involvement of beta-arrestins in chemotaxis both in vivo and in vitro as well as recent evidence that beta-arrestins directly bind and regulate proteins involved in actin reorganization.