Four subtypes of protease-activated receptors, co-expressed in rat astrocytes, evoke different physiological signaling

Ketorolac reduces spinal astrocytic activation and PAR1 expression associated with attenuation of pain after facet joint injury

Chronic neck pain affects up to 70% of persons, with the facet joint being the most common source. Intra-articular injection of the non-steroidal anti-inflammatory drug ketorolac reduces post-operative joint-mediated pain; however, the mechanism of its attenuation of facet-mediated pain has not been evaluated. Protease-activated receptor-1 (PAR1) has differential roles in pain maintenance depending on the type and location of painful injury. This study investigated if the timing of intra-articular ketorolac injection after painful cervical facet injury affects behavioral hypersensitivity by modulating spinal astrocyte activation and/or PAR1 expression. Rats underwent a painful joint distraction and received an injection of ketorolac either immediately or 1 day later. Separate control groups included injured rats with a vehicle injection at day 1 and sham operated rats. Forepaw mechanical allodynia was measured for 7 days, and spinal cord tissue was immunolabeled for glial fibrillary acidic protein (GFAP) and PAR1 expression in the dorsal horn on day 7. Ketorolac administered on day 1 after injury significantly reduced allodynia (p=0.0006) to sham levels, whereas injection immediately after the injury had no effect compared with vehicle. Spinal astrocytic activation followed behavioral responses and was significantly decreased (p=0.009) only for ketorolac given at day 1. Spinal PAR1 (p=0.0025) and astrocytic PAR1 (p=0.012) were significantly increased after injury. Paralleling behavioral data, astrocytic PAR1 was returned to levels in sham only when ketorolac was administered on day 1. Yet, spinal PAR1 was significantly reduced (p<0.0001) by ketorolac independent of timing. Spinal astrocyte expression of PAR1 appears to be associated with the maintenance of facet-mediated pain.

Activated microglia enhance neurogenesis via trypsinogen secretion

White matter neurons in multiple sclerosis brains are destroyed during demyelination and then replaced in some chronic multiple sclerosis lesions that exhibit a morphologically distinct population of activated microglia [Chang A, et al. (2008) Brain 131(Pt 9):2366-2375]. Here we investigated whether activated microglia secrete factors that promote the generation of neurons from white matter cells. Adult rat brain microglia (resting or activated with lipopolysaccharide) were isolated by flow cytometry and cocultured with neonatal rat optic nerve cells in separate but media-connected chambers. Optic nerve cells cocultured with activated microglia showed a significant increase in the number of cells of neuronal phenotype, identified by neuron-specific class III beta-tubulin (TUJ-1) labeling, compared with cultures with resting microglia. To investigate the possible source of the TUJ-1-positive cells, A2B5-positive oligodendrocyte progenitor cells and A2B5-negative cells were isolated and cocultured with resting and activated microglia. Significantly more TUJ-1-positive cells were generated from A2B5-negative cells (∼70%) than from A2B5-positive cells (~30%). Mass spectrometry analysis of microglia culture media identified protease serine 2 (PRSS2) as a factor secreted by activated, but not resting, microglia. When added to optic nerve cultures, PRSS2 significantly increased neurogenesis, whereas the serine protease inhibitor, secretory leukocyte protease inhibitor, decreased activated microglia-induced neurogenesis. Collectively our data provide evidence that activated microglia increase neurogenesis through secretion of PRSS2.

Novel animal models of acute and chronic cancer pain: a pivotal role for PAR2

Targeted therapy to prevent the progression from acute to chronic pain in cancer patients remains elusive. We developed three novel cancer models in mice that together recapitulate the anatomical, temporal, and functional characteristics of acute and chronic head and neck cancer pain in humans. Using pharmacologic and genetic approaches in these novel cancer models, we identified the interaction between protease-activated receptor 2 (PAR2) and serine proteases to be of central importance. We show that serine proteases such as trypsin induce acute cancer pain in a PAR2-dependent manner. Chronic cancer pain is associated with elevated serine proteases in the cancer microenvironment and PAR2 upregulation in peripheral nerves. Serine protease inhibition greatly reduces the severity of persistent cancer pain in wild-type mice, but most strikingly, the development of chronic cancer pain is prevented in PAR2-deficient mice. Our results demonstrate a direct role for PAR2 in acute cancer pain and suggest that PAR2 upregulation may favor the development and maintenance of chronic cancer pain. Targeting the PAR2-serine protease interaction is a promising approach to the treatment of acute cancer pain and prevention of chronic cancer pain.

Role of protease-activated receptor-1 in brain injury after experimental global cerebral ischemia

BACKGROUND AND PURPOSE

Evidence suggests that the protease-activated receptor-1 (PAR-1), a thrombin receptor, mediates neuronal injury in experimental cerebral ischemia. The present study investigated whether PAR-1 plays a role in brain injury after global cerebral ischemia.

METHODS

Adult male wild-type or PAR-1 knockout mice underwent a 20-minute bilateral common carotid artery occlusion or a sham operation. Behavior tests were performed before ischemia and 1, 2, and 3 days after bilateral common carotid artery occlusion. Mice were euthanized at different time points for thrombin activity, brain edema, Western blot analysis, and brain histology.

RESULTS

Thrombin activity and PAR-1 expression were increased in the brain after bilateral common carotid artery occlusion. Compared with wild-type mice, PAR-1 knockout mice had less brain edema formation, neuronal death, and behavior impairment after bilateral common carotid artery occlusion. In addition, bilateral common carotid artery occlusion-induced activation of mitogen-activated protein kinases was absent in PAR-1 knockout mice.

CONCLUSIONS

PAR-1 contributes to the brain injury induced by global cerebral ischemia, which may be related to activation of mitogen-activated protein kinases.

Transient early neurotrophin release and delayed inflammatory cytokine release by microglia in response to PAR-2 stimulation

Activated microglia exerts both beneficial and deleterious effects on neurons, but the signaling mechanism controlling these distinct responses remain unclear. We demonstrated that treatment of microglial cultures with the PAR-2 agonist, 2-Furoyl-LIGRLO-NH2, evoked early transient release of BDNF, while sustained PAR-2 stimulation evoked the delayed release of inflammatory cytokines (IL-1 β and TNF-α) and nitric oxide. Culture medium harvested during the early phase (at 1 h) of microglial activation induced by 2-Furoyl-LIGRLO-NH2 (microglial conditioned medium, MCM) had no deleterious effects on cultured neurons, while MCM harvested during the late phase (at 72 h) promoted DNA fragmentation and apoptosis as indicated by TUNEL and annexin/PI staining. Blockade of PAR-1 during the early phase of PAR-2 stimulation enhanced BDNF release (by 11%, small but significant) while a PAR-1 agonist added during the late phase (24 h after 2-Furoyl-LIGRLO-NH2 addition) suppressed the release of cytokines and NO. The neuroprotective and neurotoxic effects of activated microglial exhibit distinct temporal profiles that are regulated by PAR-1 and PAR-2 stimulation. It may be possible to facilitate neuronal recovery and repair by appropriately timed stimulation and inhibition of microglial PAR-1 and PAR-2 receptors.

Specific phosphorylation of αA-crystallin is required for the αA-crystallin-induced protection of astrocytes against staurosporine and C2-ceramide toxicity

We previously reported that αA-crystallin and protease-activated receptor are involved in protection of astrocytes against C2-ceramide- and staurosporine-induced cell death (Li et al., 2009). Here, we investigated the molecular mechanism of αA-crystallin-mediated cytoprotection. We found that the expression of mutants mimicking specific phosphorylation of αA-crystallin increases the protection of astrocytes. However, the expression of mutants mimicking unphosphorylation of αA-crystallin results in loss of protection. These data revealed that the phosphorylation of αA-crystallin at Ser122 and Ser148 is required for protection. Furthermore, we explored the mechanism of cytoprotection of astrocytes by αA-crystallin. Application of specific inhibitors of p38 and ERK abrogates the protection of astrocytes by over-expression of αA-crystallin. Thus, p38 and ERK contribute to protective processes by αA-crystallin. This is comparable to our previous results which demonstrated that p38 and ERK regulated protease-activated receptor-2 (PAR-2)/αB-crystallin-mediated cytoprotection. Furthermore, we found that PAR-2 activation increases the expression of αA-crystallin. Thus, endogenous αA-crystallin protects astrocytes via mechanisms, which regulate the expression and/or phosphorylation status of αA-crystallin.

Protease-activated receptor 1 (PAR1) coupling to G(q/11) but not to G(i/o) or G(12/13) is mediated by discrete amino acids within the receptor second intracellular loop

Protease-activated receptor 1 (PAR1) is an unusual GPCR that interacts with multiple G protein subfamilies (G(q/11), G(i/o), and G(12/13)) and their linked signaling pathways to regulate a broad range of pathophysiological processes. However, the molecular mechanisms whereby PAR1 interacts with multiple G proteins are not well understood. Whether PAR1 interacts with various G proteins at the same, different, or overlapping binding sites is not known. Here we investigated the functional and specific binding interactions between PAR1 and representative members of the G(q/11), G(i/o), and G(12/13) subfamilies. We report that G(q/11) physically and functionally interacts with specific amino acids within the second intracellular (i2) loop of PAR1. We identified five amino acids within the PAR1 i2 loop that, when mutated individually, each markedly reduced PAR1 activation of linked inositol phosphate formation in transfected COS-7 cells (functional PAR1-null cells). Among these mutations, only R205A completely abolished direct G(q/11) binding to PAR1 and also PAR1-directed inositol phosphate and calcium mobilization in COS-7 cells and PAR1-/- primary astrocytes. In stark contrast, none of the PAR1 i2 loop mutations disrupted direct PAR1 binding to either G(o) or G(12), or their functional coupling to linked pertussis toxin-sensitive ERK phosphorylation and C3 toxin-sensitive Rho activation, respectively. In astrocytes, our findings suggest that PAR1-directed calcium signaling involves a newly appreciated G(q/11)-PLCε pathway. In summary, we have identified key molecular determinants for PAR1 interactions with G(q/11), and our findings support a model where G(q/11), G(i/o) or G(12/13) each bind to distinct sites within the cytoplasmic regions of PAR1.

Targeting proteinase-activated receptors: therapeutic potential and challenges

Proteinase-activated receptors (PARs), a family of four seven-transmembrane G protein-coupled receptors, act as targets for signalling by various proteolytic enzymes. PARs are characterized by a unique activation mechanism involving the proteolytic unmasking of a tethered ligand that stimulates the receptor. Given the emerging roles of these receptors in cancer as well as in disorders of the cardiovascular, musculoskeletal, gastrointestinal, respiratory and central nervous system, PARs have become attractive targets for the development of novel therapeutics. In this Review we summarize the mechanisms by which PARs modulate cell function and the roles they can have in physiology and diseases. Furthermore, we provide an overview of possible strategies for developing PAR antagonists.

Expression of protease-activated receptor (PAR)-2, but not other PARs, is regulated by inflammatory cytokines in rat astrocytes

Protease-activated receptors (PARs) are widely expressed in the central nervous system (CNS) and are believed to play an important role in normal brain functioning as well as in development of various inflammatory and neurodegenerative disorders. Pathological conditions cause altered expression of PARs in brain cells and therefore altered responsiveness to PAR activation. The exact mechanisms of regulation of PAR expression are not well studied. Here, we evaluated in rat astrocytes the influence of LPS, pro-inflammatory cytokines TNFα and IL-1β and continuous PAR activation by PAR agonists on the expression levels of PARs. These stimuli are important in inflammatory and neurological disorders, where their levels are increased. We report that LPS as well as cytokines TNFα and IL-1β affected only the PAR-2 level, but their effects were opposite. LPS and TNFα increased the functional expression of PAR-2, whereas IL-1β down-regulated the functional response of PAR-2. Agonists of PAR-1 specifically increased mRNA level of PAR-2, but not protein level. Transcript levels of other PARs were not changed after PAR-1 activation. Stimulation of the cells with PAR-2 or PAR-4 agonists did not alter PAR levels. We found that up-regulation of PAR-2 is dependent on PKC activity, mostly via its Ca²⁺-sensitive isoforms. Two transcription factors, NFκB and AP-1, are involved in up-regulation of PAR-2. These findings provide new information about the regulation of expression of PAR subtypes in brain cells. This is of importance for targeting PARs, especially PAR-2, for the treatment of CNS disorders.

The phosphorylation status of extracellular-regulated kinase 1/2 in astrocytes and neurons from rat hippocampus determines the thrombin-induced calcium release and ROS generation

Challenge of protease-activated receptors induces cytosolic Ca(2+) concentration ([Ca(2+) ](c)) increase, mitogen-activated protein kinase activation and reactive oxygen species (ROS) formation with a bandwidth of responses in individual cells. We detected in this study in situ the thrombin-induced [Ca(2+) ](c) rise and ROS formation in dissociated hippocampal astrocytes and neurons in a mixed culture. In identified cells, single cell responses were correlated with extracellular-regulated kinase (ERK)1/2 phosphorylation level. On average, in astrocytes, thrombin induced a transient [Ca(2+) ](c) rise with concentration-dependent increase in amplitude and extrusion rate and high ERK1/2 phosphorylation level. Correlation analysis of [Ca(2+) ](c) response characteristics of single astrocytes reveals that astrocytes with nuclear phosphoERK1/2 localization have a smaller Ca(2+) amplitude and extrusion rate compared with cells with a cytosolic phosphoERK1/2 localization. In naive neurons, without thrombin challenge, variable ERK1/2 phosphorylation patterns are observed. ROS were detected by hydroethidine. Only in neurons with increased ERK1/2 phosphorylation level, we see sustained intracellular rise in fluorescence of the dye lasting over several minutes. ROS formation was abolished by pre-incubation with the NADPH oxidase inhibitor apocynin. Additionally, thrombin induced an immediate, transient hydroethidine fluorescence increase. This was interpreted as NADPH oxidase-mediated O(2) (•-) -release into the extracellular milieu, because it was decreased by pre-incubation with apocynin, and could be eluted by superfusion. In conclusion, the phosphorylation status of ERK1/2 determines the thrombin-dependent [Ca(2+) ](c) increase and ROS formation and, thus, influences the capacity of thrombin to regulate neuroprotection or neurodegeneration.

Protease-activated receptor 2 signaling in inflammation

Protease-activated receptors (PARs) are G protein-coupled receptors that are activated by proteolytical cleavage of the amino-terminus and thereby act as sensors for extracellular proteases. While coagulation proteases activate PARs to regulate hemostasis, thrombosis, and cardiovascular function, PAR2 is also activated in extravascular locations by a broad array of serine proteases, including trypsin, tissue kallikreins, coagulation factors VIIa and Xa, mast cell tryptase, and transmembrane serine proteases. Administration of PAR2-specific agonistic and antagonistic peptides, as well as studies in PAR2 knockout mice, identified critical functions of PAR2 in development, inflammation, immunity, and angiogenesis. Here, we review these roles of PAR2 with an emphasis on the role of coagulation and other extracellular protease pathways that cleave PAR2 in epithelial, immune, and neuronal cells to regulate physiological and pathophysiological processes.

Anatomical localization of protease-activated receptor-1 and protease-mediated neuroglial crosstalk on peri-synaptic astrocytic endfeet

We studied the localization, activation and function of protease-activated receptor 1 (PAR-1) at the CNS synapse utilizing rat brain synaptosomes and slices. Confocal immunofluoresence and transmission electron microscopy in brain slices with pre-embedding diaminobenzidine (DAB) immunostaining found PAR-1 predominantly localized to the peri-synaptic astrocytic endfeet. Structural confocal immunofluorescence microscopy studies of isolated synaptosomes revealed spherical structures stained with anti-PAR-1 antibody which co-stained mainly for glial-filament acidic protein compared with the neuronal markers synaptophysin and PSD-95. Immunoblot studies of synaptosomes demonstrated an appropriate major band corresponding to PAR-1 and activation of the receptor by a specific agonist peptide (SFLLRN) significantly modulated phosphorylated extracellular signal-regulated kinase. A significant membrane potential depolarization was produced by thrombin (1 U/mL) and the PAR-1 agonist (100 μM) and depolarization by high K(+) elevated extracellular thrombin-like activity in the synaptosomes preparation. The results indicate PAR-1 localized to the peri-synaptic astrocytic endfeet is most likely activated by synaptic proteases and induces cellular signaling and modulation of synaptic electrophysiology. A protease mediated neuron-glia pathway may be important in both physiological and pathological regulation of the synapse.

Activation of protease activated receptor 1 increases the excitability of the dentate granule neurons of hippocampus

Protease activated receptor-1 (PAR1) is expressed in multiple cell types in the CNS, with the most prominent expression in glial cells. PAR1 activation enhances excitatory synaptic transmission secondary to the release of glutamate from astrocytes following activation of astrocytically-expressed PAR1. In addition, PAR1 activation exacerbates neuronal damage in multiple in vivo models of brain injury in a manner that is dependent on NMDA receptors. In the hippocampal formation, PAR1 mRNA appears to be expressed by a subset of neurons, including granule cells in the dentate gyrus. In this study we investigate the role of PAR activation in controlling neuronal excitability of dentate granule cells. We confirm that PAR1 protein is expressed in neurons of the dentate cell body layer as well as in astrocytes throughout the dentate. Activation of PAR1 receptors by the selective peptide agonist TFLLR increased the intracellular Ca²⁺ concentration in a subset of acutely dissociated dentate neurons as well as non-neuronal cells. Bath application of TFLLR in acute hippocampal slices depolarized the dentate gyrus, including the hilar region in wild type but not in the PAR1-/- mice. PAR1 activation increased the frequency of action potential generation in a subset of dentate granule neurons; cells in which PAR1 activation triggered action potentials showed a significant depolarization. The activation of PAR1 by thrombin increased the amplitude of NMDA receptor-mediated component of EPSPs. These data suggest that activation of PAR1 during normal function or pathological conditions, such as during ischemia or hemorrhage, can increase the excitability of dentate granule cells.

Indirect modulation of neuronal excitability and synaptic transmission in the hippocampus by activation of proteinase-activated receptor-2

BACKGROUND AND PURPOSE

Proteinase-activated receptor-2 (PAR2) is widely expressed in the CNS under normal physiological conditions. However, its potential role in modulating neuronal excitability and synaptic transmission remains to be determined. Here, we have investigated whether PAR2 activation modulates synaptic activity in the hippocampus.

EXPERIMENTAL APPROACH

PAR2 activation and its effect on the hippocampus were examined in rat primary cultures and acute slices using whole cell patch clamp and standard extracellular recordings, respectively.

KEY RESULTS

PAR2 activation leads to a depolarization of hippocampal neurones and a paradoxical reduction in the occurrence of synaptically driven spontaneous action potentials (APs). PAR2-induced neuronal depolarization was abolished following either the inhibition of astrocytic function or antagonism of ionotropic glutamate receptors whilst the PAR2-induced decrease in AP frequency was also reduced when astrocytic function was inhibited. Furthermore, when examined in acute hippocampal slices, PAR2 activation induced a profound long-term depression of synaptic transmission that was dependent on NMDA receptor activation and was sensitive to disruption of astrocytic function.

CONCLUSIONS AND IMPLICATIONS

These novel findings show that PAR2 activation indirectly inhibits hippocampal synaptic activity and indicate that these receptors may play an active role in modulating normal physiological CNS function, in addition to their role in pathophysiological disorders.

Expression of plasminogen activator inhibitor-1 by olfactory ensheathing glia promotes axonal regeneration

Olfactory ensheathing glia (OEG) cells are known to facilitate repair following axotomy of adult neurons, although the molecular mechanisms involved are not fully understood. We previously identified plasminogen activator inhibitor-1 (PAI-1), proteinase-activated receptor-1 (PAR-1), and thrombomodulin (TM) as candidates to regulate rat OEG-dependent axonal regeneration. In this study, we have validated the involvement of these proteins in promoting axonal regeneration by immortalized human OEGs. We studied the effect of silencing these proteins in OEGs on their capacity to promote the regeneration of severed adult retinal ganglion cells (RGCs) axons. Our results support the role of glial PAI-1 as a downstream effector of PAR-1 in promoting axon regeneration. In contrast, we found that TM inhibits OEG induced-axonal regeneration. We also assessed the signaling pathways downstream of PAR-1 that might modulate PAI-1 expression, observing that specifically inhibiting Gα(i), Rho kinase, or PLC and PKC downregulated the expression of PAI-1 in OEGs, with a concomitant reduction in OEG-dependent axon regeneration in adult RGCs. Our findings support an important role for the thrombin system in regulating adult axonal regeneration by OEGs.

Regulation of human vascular protease-activated receptor-3 through mRNA stabilization and the transcription factor nuclear factor of activated T cells (NFAT)

Thrombin promotes vascular smooth muscle cell (SMC) proliferation and inflammation via protease-activated receptor (PAR)-1. A further thrombin receptor, PAR-3, acts as a PAR-1 cofactor in some cell-types. Unlike PAR-1, PAR-3 is dynamically regulated at the mRNA level in thrombin-stimulated SMC. This study investigated the mechanisms controlling PAR-3 expression. In human vascular SMC, PAR-3 siRNA attenuated thrombin-stimulated interleukin-6 expression and extracellular signal-regulated kinases 1/2 phosphorylation, indicating PAR-3 contributes to net thrombin responses in these cells. Thrombin slowed the decay of PAR-3 but not PAR-1 mRNA in the presence of actinomycin D and induced cytosolic shuttling and PAR-3 mRNA binding of the mRNA-stabilizing protein human antigen R (HuR). HuR siRNA prevented thrombin-induced PAR-3 expression. By contrast, forskolin inhibited HuR shuttling and destabilized PAR-3 mRNA, thus reducing PAR-3 mRNA and protein expression. Other cAMP-elevating agents, including the prostacyclin-mimetic iloprost, also down-regulated PAR-3, accompanied by decreased HuR/PAR-3 mRNA binding. Iloprost-induced suppression of PAR-3 was reversed with a myristoylated inhibitor of protein kinase A and mimicked by phorbol ester, an inducer of cyclooxygenase-2. In separate studies, iloprost attenuated PAR-3 promoter activity and prevented binding of nuclear factor of activated T cells (NFAT2) to the human PAR-3 promoter in a chromatin immunoprecipitation assay. Accordingly, PAR-3 expression was suppressed by the NFAT inhibitor cyclosporine A or NFAT2 siRNA. Thus human PAR-3, unlike PAR-1, is regulated post-transcriptionally via the mRNA-stabilizing factor HuR, whereas transcriptional control involves NFAT2. Through modulation of PAR-3 expression, prostacyclin and NFAT inhibitors may limit proliferative and inflammatory responses to thrombin after vessel injury.

Induction of apoptosis by thrombin in the cultured neurons of dorsal motor nucleus of the vagus

BACKGROUND

A previous study demonstrated the presence of protease-activated receptor (PAR) 1 and 2 in the dorsal motor nucleus of vagus (DMV). The aim of this study is to characterize the effect of thrombin on the apoptosis of DMV neurons.

METHODS

The dorsal motor nucleus of vagus neurons were isolated from neonatal rat brainstems using micro-dissection and enzymatic digestion and cultured. Apoptosis of DMV neurons were examined in cultured neurons. Apoptotic neuron was examined by TUNEL and ELISA. Data were analyzed using anova and Student's t-test.

KEY RESULTS

Exposure of cultured DMV neurons to thrombin (0.1 to 10 U mL(-1)) for 24 h significantly increased apoptosis. Pretreatment of DMV neurons with hirudin attenuated the apoptotic effect of thrombin. Similar induction of apoptosis was observed for the PAR1 receptor agonist SFLLR, but not for the PAR3 agonist TFRGAP, nor for the PAR4 agonist YAPGKF. Protease-activated receptors 1 receptor antagonist Mpr(Cha) abolished the apoptotic effect of thrombin, while YPGKF, a specific antagonist for PAR4, demonstrated no effect. After administration of thrombin, phosphorylation of JNK and P38 occurred as early as 15 min, and remained elevated for up to 45 min. Pretreatment of DMV neurons with SP600125, a specific inhibitor for JNK, or SB203580, a specific inhibitor for P38, significantly inhibited apoptosis induced by thrombin.

CONCLUSIONS & INFERENCES

Thrombin induces apoptosis in DMV neurons through a mechanism involving the JNK and P38 signaling pathways.

Functional identification of neural stem cell-derived oligodendrocytes by means of calcium transients elicited by thrombin

Current immunosuppressive treatments for central nervous system demyelinating diseases fail to prevent long-term motor and cognitive decline in patients. Excitingly, glial cell transplantation arises as a promising complementary strategy to challenge oligodendrocytes loss occurring in myelination disorders. A potential source of new oligodendrocytes is the subventricular zone (SVZ) pool of multipotent neural stem cells. However, this approach has been handicapped by the lack of functional methods for identification and pharmacological analysis of differentiating oligodendrocytes, prior to transplantation. In this study, we questioned whether SVZ-derived oligodendrocytes could be functionally discriminated due to intracellular calcium level ([Ca(2+)](i)) variations following KCl, histamine, and thrombin stimulations. Previously, we have shown that SVZ-derived neurons and immature cells can be discriminated on the basis of their selective [Ca(2+)](i) rise upon KCl and histamine stimulation, respectively. Herein, we demonstrate that O4+ and proteolipid protein-positive (PLP+) oligodendrocytes do not respond to these stimuli, but display a robust [Ca(2+)](i) rise following thrombin stimulation, whereas other cell types are thrombin-insensitive. Thrombin-induced Ca(2+) increase in oligodendrocytes is mediated by protease-activated receptor-1 (PAR-1) activation and downstream signaling through G(q/11) and phospholipase C (PLC), resulting in Ca(2+) recruitment from intracellular compartments. This method allows the analysis of functional properties of oligodendrocytes in living SVZ cultures, which is of major interest for the development of effective grafting strategies in the demyelinated brain. Additionally, it opens new perspectives for the search of new pro-oligodendrogenic factors to be used prior grafting.

Deficiency of PAR4 attenuates cerebral ischemia/reperfusion injury in mice

Stroke is the third leading cause of death in the USA. Antithrombotic therapy targeting platelet activation is one of the treatments for ischemic stroke. Here we investigate the role of one of the thrombin receptors, protease-activated receptor 4 (PAR4), in a mouse transient middle cerebral artery occlusion (MCAO) model. After a 60 min MCAO and 23 h reperfusion, leukocyte and platelet rolling and adhesion on cerebral venules, blood-brain barrier (BBB) permeability, and cerebral edema were compared in PAR4-deficient mice and wild-type mice. Cerebral infarction volume and neuronal death were also measured. PAR4-/- mice had more than an 80% reduction of infarct volume and significantly improved neurologic and motor function compared with wild-type mice after MCAO. Furthermore, deficiency of PAR4 significantly inhibits the rolling and adhesion of both platelets and leukocytes after MCAO. BBB disruption and cerebral edema were also attenuated in PAR4-/- mice compared with wild-type animals. The results of this investigation indicate that deficiency of PAR4 protects mice from cerebral ischemia/reperfusion (I/R) injury, partially through inhibition of platelet activation and attenuation of microvascular inflammation.

Astrocytic activation and an inhibition of MAP kinases are required for proteinase-activated receptor-2-mediated protection from neurotoxicity

Proteinase-activated receptor-2 (PAR-2) expression levels are altered in several CNS disorders with these changes being proposed to either exacerbate or diminish the disease state depending on the cell type in which this occurs. Here we present data investigating the consequence of PAR-2 activation on kainate (KA)-induced neurotoxicity in organotypic hippocampal slices cultures (OHSC). Exposure of OHSC to the PAR-2 activators trypsin or Ser-Leu-Ile-Gly-Arg-Leu (SLIGRL) induced no neurotoxicity when applied alone but was neuroprotective against KA-induced neurotoxicity. SLIGRL-mediated neuroprotection involved astrocytic activation as the neuroprotective effect was abolished following OHSC pre-treatment with fluoroacetate. Moreover, co-application of either reparixin or LY341495, antagonists of the CXCR2 chemokine receptor and metabotropic glutamate receptors respectively, inhibited the SLIGRL-mediated neuroprotection. SLIGRL application inhibited both p38 MAPK and ERK activity in OHSC, but not the JNK 1/2 signalling pathway. Accordingly, the co-application of the p38 MAPK and ERK inhibitors SB203580 and UO126 reduced KA-induced cell death, mimicking PAR-2-mediated neuroprotection. These data indicate that PAR-2 activation is neuroprotective and involves astrocytic activation, gliotransmitter release, and the subsequent inhibition of MAPK signalling cascades, providing further evidence for PAR-2 as an interesting therapeutic target in certain CNS disorders.

Thrombin-stimulated proliferation is mediated by endothelin-1 in cultured rat gingival fibroblasts

Abstract Endothelin-1 (ET-1) appears to be involved in drug-induced proliferation of gingival fibroblasts. Thrombin induces proliferation of human gingival fibroblasts via protease-activated receptor 1 (PAR1). In this study, using cultured rat gingival fibroblasts, we investigated whether thrombin-induced proliferation of gingival fibroblasts is mediated by ET-1. Thrombin-induced proliferation (0.05-2.5 U/mL). Proliferation was also induced by a PAR1-specific agonist (TFLLR-NH(2,) 0.1-30 microm), but not by a PAR2-specific agonist (SLIGRL-NH(2)). Thrombin (2.5 U/mL) induced an increase in immunoreactive ET-1 expression, which was inhibited by cycloheximide (10 microg/mL), and an increase in preproET-1 mRNA expression, as assessed by reverse transcription polymerase chain reaction. TFLLR-NH(2) increased ET-1 release into the culture medium in both a concentration (0.01-10 microm)- and time (6-24 h)-dependent manner, as assessed by solid phase sandwich enzyme-linked immunosorbent assay. The thrombin (2.5 U/mL)-induced proliferation was inhibited by a PAR1-selective inhibitor, SCH79797 (0.1 microm) and an ET(A) antagonist, BQ-123 (1 microm), but not by an ET(B) antagonist, BQ-788 (1 microm). These findings suggest that thrombin, acting via PAR1, induced proliferation of cultured rat gingival fibroblasts that was mediated by ET-1 acting via ET(A).

Alpha A-crystallin and alpha B-crystallin, newly identified interaction proteins of protease-activated receptor-2, rescue astrocytes from C2-ceramide- and staurosporine-induced cell death

Protease-activated receptor-2 (PAR-2) is a G protein-coupled receptor activated by trypsin and other trypsin-like serine proteases. The widely expressed PAR-2 is involved in inflammation response but the physiological/pathological roles of PAR-2 in the nervous system are still uncertain. In the present study, we report novel PAR-2 interaction proteins, alphaA-crystallin and alphaB-crystallin. These 20 kDa proteins have been implicated in neurodegenerative diseases like Alexander's disease, Creutzfeldt-Jacob disease, Alzheimer's disease, and Parkinson's disease. Results from yeast two-hybrid assay using the cytoplasmic C-tail of PAR-2 as bait suggested that alphaA-crystallin interacts with PAR-2. We further demonstrate the in vitro and cellular in vivo interaction of C-tail of PAR-2 as well as of full-length PAR-2 with alphaA(alphaB)-crystallins. We use pull-down, co-immunoprecipitation, and co-localization assays. Analysis of alphaA-crystallin deletion mutants showed that amino acids 120-130 and 136-154 of alphaA-crystallin are required for the interaction with PAR-2. Co-immunoprecipitation experiments ruled out an interaction of alphaA(alphaB)-crystallins with PAR-1, PAR-3, and PAR-4. This demonstrates that alphaA(alphaB)-crystallins are PAR-2-specific interaction proteins. Moreover, we investigated the functional role of PAR-2 and alpha-crystallins in astrocytes. Evidence is presented to show that PAR-2 activation and increased expression of alpha-crystallins reduced C2-ceramide- and staurosporine-induced cell death in astrocytes. Thus, both PAR-2 and alpha-crystallins are involved in cytoprotection in astrocytes.

Essential role of mitogen-activated protein kinase pathways in protease activated receptor 2-mediated nitric-oxide production from rat primary astrocytes

Protease-activated receptors (PARs) play important roles in the regulation of brain function such as neuroinflammation by transmitting the signal from proteolytic enzymes such as thrombin and trypsin. We and others have reported that a member of the family, PAR-2 is activated by trypsin, whose involvement in the neurophysiological process is increasingly evident, and is involved in the neuroinflammatory processes including morphological changes of astrocytes. In this study, we investigated the role of PAR-2 in the production of nitric oxide (NO) in rat primary astrocytes. Treatment of PAR-2 agonist trypsin increased NO production in a dose-dependent manner, which was mediated by the induction of inducible nitric-oxide synthase. The trypsin-mediated production of NO was mimicked by PAR-2 agonist peptide and reduced by either pharmacological PAR-2 antagonist peptide or by siRNA-mediated inhibition of PAR-2 expression, which suggests the critical role of PAR-2 in this process. NO production by PAR-2 was mimicked by PMA, a PKC activator, and was attenuated by Go6976, a protein kinase C (PKC) inhibitor. PAR-2 stimulation activated three subtypes of mitogen-activated protein kinases (MAPKs): extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK. NO production by PAR-2 was blocked by inhibition of ERK, p38, and JNK pathways. PAR-2 stimulation also activated nuclear factor-kappaB (NF-kappaB) DNA binding and transcriptional activity as well as IkappaBalpha phosphorylation. Inhibitors of NF-kappaB pathway inhibited PAR-2-mediated NO production. In addition, inhibitors of MAPK pathways prevented transcriptional activation of NF-kappaB reporter constructs. These results suggest that PAR-2 activation-mediated NO production in astrocytes is transduced by the activation of MAPKs followed by NF-kappaB pathways.

Proteinase-activated receptor-1 activation presynaptically enhances spontaneous glutamatergic excitatory transmission in adult rat substantia gelatinosa neurons

Proteinase-activated receptors (PARs) have a unique activation mechanism in that a proteolytically exposed N-terminal region acts as a tethered ligand. A potential impact of PAR on sensory processing has not been fully examined yet. Here we report that synthetic peptides with sequences corresponding to PAR ligands enhance glutamatergic excitatory transmission in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by using the whole cell patch-clamp technique. The frequency of spontaneous excitatory postsynaptic current (EPSC) was increased by PAR-1 agonist SFLLRN-NH2 (by 47% at 1 microM) with small increases by PAR-2 and -4 agonists (SLIGKV-NH2 and GYPGQV-OH, respectively; at >3 microM); there was no change in its amplitude or in holding current at -70 mV. The PAR-1 peptide action was inhibited by PAR-1 antagonist YFLLRNP-OH. TFLLR-NH2, an agonist which is more selective to PAR-1 than SFLLRN-NH2, dose-dependently increased spontaneous EPSC frequency (EC50=0.32 microM). A similar presynaptic effect was produced by PAR-1 activating proteinase thrombin in a manner sensitive to YFLLRNP-OH. The PAR-1 peptide action was resistant to tetrodotoxin and inhibited in Ca2+-free solution. Primary-afferent monosynaptically evoked EPSC amplitudes were unaffected by PAR-1 agonist. These results indicate that PAR-1 activation increases the spontaneous release of L-glutamate onto SG neurons from nerve terminals in a manner dependent on extracellular Ca2+. Considering that sensory processing within the SG plays a pivotal role in regulating nociceptive transmission to the spinal dorsal horn, the PAR-1-mediated glutamatergic transmission enhancement could be involved in a positive modulation of nociceptive transmission.

Protease-activated receptor 1-dependent neuronal damage involves NMDA receptor function

Protease-activated receptor 1 (PAR1) is a G-protein coupled receptor that is expressed throughout the central nervous system. PAR1 activation by brain-derived as well as blood-derived proteases has been shown to have variable and complex effects in a variety of animal models of neuronal injury and inflammation. In this study, we have evaluated the effects of PAR1 on lesion volume in wild-type or PAR1-/- C57Bl/6 mice subjected to transient occlusion of the middle cerebral artery or injected with NMDA in the striatum. We found that removal of PAR1 reduced infarct volume following transient focal ischemia to 57% of control. Removal of PAR1 or application of a PAR1 antagonist also reduced the neuronal injury associated with intrastriatal injection of NMDA to 60% of control. To explore whether NMDA receptor potentiation by PAR1 activation contributes to the harmful effects of PAR1, we investigated the effect of NMDA receptor antagonists on the neuroprotective phenotype of PAR1-/- mice. We found that MK801 reduced penumbral but not core neuronal injury in mice subjected to transient middle cerebral artery occlusion or intrastriatal NMDA injection. Lesion volumes in both models were not significantly different between PAR1-/- mice treated with and without MK801. Use of the NMDA receptor antagonist and dissociative anesthetic ketamine also renders NMDA-induced lesion volumes identical in PAR1-/- mice and wild-type mice. These data suggest that the ability of PAR1 activation to potentiate NMDA receptor function may underlie its harmful actions during injury.

Protease-activated receptor dependent and independent signaling by kallikreins 1 and 6 in CNS neuron and astroglial cell lines

While protease-activated receptors (PARs) are known to mediate signaling events in CNS, contributing both to normal function and pathogenesis, the endogenous activators of CNS PARs are poorly characterized. In this study, we test the hypothesis that kallikreins (KLKs) represent an important pool of endogenous activators of CNS PARs. Specifically, KLK1 and KLK6 were examined for their ability to evoke intracellular Ca(2+) flux in a PAR-dependent fashion in NSC34 neurons and Neu7 astrocytes. Both KLKs were also examined for their ability to activate mitogen-activated protein kinases (extracellular signal-regulated kinases, C-Jun N-terminal kinases, and p38) and protein kinase B (AKT) intracellular signaling cascades. Cumulatively, these studies show that KLK6, but not KLK1, signals through PARs. KLK6 evoked intracellular Ca(2+) flux was mediated by PAR1 in neurons and both PAR1 and PAR2 in astrocytes. Importantly, both KLK1 and KLK6 altered the activation state of mitogen-activated protein kinases and AKT, suggestive of important roles for each in CNS neuron and glial differentiation, and survival. The cellular specificity of CNS-KLK activity was underscored by observations that both proteases promoted AKT activation in astrocytes, but inhibited such signaling in neurons. PAR1 and bradykinin receptor inhibitors were used to demonstrate that KLK1-mediated activation of extracellular signal-regulated kinases in neurons occurred in a non-PAR, bradykinin 2 (B2) receptor-dependent fashion, while similar signaling by KLK6 was mediated by the combined activation of PAR1 and B2. Cumulatively results indicate KLK6, but not KLK1 is an activator of CNS PARs, and that both KLKs are poised to signal in a B2 receptor-dependent fashion to regulate multiple signal transduction pathways relevant to CNS physiologic function and dysfunction.

Thrombin receptor and ventricular arrhythmias after acute myocardial infarction

The mechanism mediating the development of ventricular arrhythmia (VA) after acute myocardial infarction (AMI) is still uncertain. Thrombin receptor (TR) activation has been proven to be arrhythmogenic in many other situations, and we hypothesize that it may participate in the genesis of post-AMI VA. Using a left coronary artery ligation rat model of AMI, we found that a local injection of hirudin into the left ventricle (LV) significantly reduced the ratio of VA durations to infarction sizing, whereas injection of thrombin receptor-activating peptide (TRAP) increased the ratios of VA duration to infarction sizing. The effects of TR activation on whole-cell currents were investigated in isolated myocytes. TRAP increased a glibenclamide-sensitive outward current. Pretreatment of rats with glibenclamide (4 mg/kg intraperitoneally) eliminated the effects of a local injection of TRAP on the ratios of VA durations to infarction sizing. TR mRNA and protein expression in the ischemic left ventricle had reached its peak by 20 min postligation in the rat AMI model (P < 0.05). TR-immunoreactive myocytes were observed in infarcted LV but were seldom seen in the right ventricle or in the normal heart. By 60 min, TR transcript levels had returned to control levels. We conclude that increased TR activation and expression in the infarcted LV after AMI may contribute to VA through a mechanism involving glibenclamide-sensitive potassium channels.

Plasmin potentiates synaptic N-methyl-D-aspartate receptor function in hippocampal neurons through activation of protease-activated receptor-1

Protease-activated receptor-1 (PAR1) is activated by a number of serine proteases, including plasmin. Both PAR1 and plasminogen, the precursor of plasmin, are expressed in the central nervous system. In this study we examined the effects of plasmin in astrocyte and neuronal cultures as well as in hippocampal slices. We find that plasmin evokes an increase in both phosphoinositide hydrolysis (EC(50) 64 nm) and Fura-2/AM fluorescence (195 +/- 6.7% above base line, EC(50) 65 nm) in cortical cultured murine astrocytes. Plasmin also activates extracellular signal-regulated kinase (ERK1/2) within cultured astrocytes. The plasmin-induced rise in intracellular Ca(2+) concentration ([Ca(2+)](i)) and the increase in phospho-ERK1/2 levels were diminished in PAR1(-/-) astrocytes and were blocked by 1 microm BMS-200261, a selective PAR1 antagonist. However, plasmin had no detectable effect on ERK1/2 or [Ca(2+)](i) signaling in primary cultured hippocampal neurons or in CA1 pyramidal cells in hippocampal slices. Plasmin (100-200 nm) application potentiated the N-methyl-D-aspartate (NMDA) receptor-dependent component of miniature excitatory postsynaptic currents recorded from CA1 pyramidal neurons but had no effect on alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate- or gamma-aminobutyric acid receptor-mediated synaptic currents. Plasmin also increased NMDA-induced whole cell receptor currents recorded from CA1 pyramidal cells (2.5 +/- 0.3-fold potentiation over control). This effect was blocked by BMS-200261 (1 microm; 1.02 +/- 0.09-fold potentiation over control). These data suggest that plasmin may serve as an endogenous PAR1 activator that can increase [Ca(2+)](i) in astrocytes and potentiate NMDA receptor synaptic currents in CA1 pyramidal neurons.

cAMP-dependent regulation of spinesin/TMPRSS5 gene expression in astrocytes

Spinesin/TMPRSS5 is a mosaic type serine protease that is predominantly expressed in the spinal cord. To identify the mechanism of spinesin expression, we investigated its expression in vivo and in vitro using several cell lines. Immunohistochemical and in situ hybridization analyses revealed that mouse spinesin (m-spinesin) was abundantly expressed in white matter astrocytes. Similarly, we confirmed abundant expression of m-spinesin in astrocyte cell lines. Then, we analyzed the expression of variant forms of m-spinesin in these cell lines. Interestingly, a transmembrane type (type 4) variant was expressed in neuroblastoma and astrocyte cell lines, whereas a cytoplasmic type (type 1) variant was specifically expressed in astrocyte cell lines. Furthermore, expression of both variants was up-regulated by dibutyryl-cAMP (dbcAMP) treatment only in astrocyte cell lines. We also analyzed the promoter region of the m-spinesin gene and revealed that the 5'-flanking region from base pairs -224 to -188 was essential for cAMP-dependent regulation of its transcription. These results indicate that m-spinesin is involved in the function of astrocytes in the spinal cord and that there may be astrocyte-specific regulation of its gene expression.

Activation of protease-activated receptor1 mediates induction of matrix metalloproteinase-9 by thrombin in rat primary astrocytes

Thrombin plays an important role in diverse neurological processes such as proliferation, cell migration, differentiation and neuroinflammation. In this study, we investigated the effect of thrombin on matrix metalloprotease-9 (MMP-9) expression in rat primary astrocytes. Thrombin (1-10U/ml) induced a significant increase in MMP-9 activity as measured by gelatin zymography. Thrombin also increased MMP-9 mRNA expression. Among three isotypes of thrombin receptor, i.e. protease-activated receptor (PAR)-1, -3 and -4, PAR1 agonist (1-100muM) but not PAR3 and PAR4 agonist induced MMP-9 expression. Inhibition of thrombin-induced MMP-9 production by SCH 79797 (10-50nM), a selective PAR1 receptor antagonist, confirmed that PAR1 is a main receptor for thrombin-induced MMP-9 expression. In astrocytes, thrombin activated Erk1/2, and it was inhibited by PD98059. In this study, thrombin-induced MMP-9 expression was inhibited by PD98059. PAR1 agonist activated Erk1/2 and PD98059 inhibited PAR1 agonist-induced MMP-9 expression. MMP-9 promoter reporter assay confirmed the positive effect of ERK1/2 on MMP-9 expression. These results suggest that the activation of PAR1 mediates thrombin-induced MMP-9 expression through the regulation of Erk1/2.

Activation of protease-activated receptors in astrocytes evokes a novel neuroprotective pathway through release of chemokines of the growth-regulated oncogene/cytokine-induced neutrophil chemoattractant family

Activation of protease-activated receptors (PARs) is known to exert neuroprotection when low concentrations of the agonist protease thrombin are applied. However, the mechanism of protection is still unclear. Here, we showed that activation of multiple PARs, including PAR-1, PAR-2 and PAR-4, was able to elevate the release of the chemokine cytokine-induced neutrophil chemoattractant (CINC)-3 from rat astrocytes, in addition to evoking CINC-1 secretion. Different molecular mechanisms were identified as being involved in the secretion of CINC-1 and CINC-3, upon activation of different PARs. Importantly, we found that both CINC-1 and CINC-3 could signal to rat cortical neurons. Both chemokines acted via CXCR2 to prevent C2-ceramide-induced cytochrome c release from mitochondria. Consequently CINC-1 and CINC-3 protected neurons from apoptosis. We further revealed that conditioned media obtained from PAR-activated astrocytes similarly protected cortical neurons against C2-ceramide-induced cell death. The neuroprotection was considerably suppressed by a CXCR2 antagonist. CXCR2 is the cognate receptor for CINC. Therefore, our findings demonstrate that PAR-activated astrocytes are able to protect neurons against neurodegeneration and cell death via regulation of the secretion of chemokines CINC-1 and CINC-3. These data indicate a previously unknown mechanism for astrocyte-mediated neuroprotection achieved by PAR activation.

PAR-2 activation and LPS synergistically enhance inflammatory signaling in airway epithelial cells by raising PAR expression level and interleukin-8 release

Protease-activated receptors (PARs) are involved in the contribution of airway epithelial cells to the development of inflammation by release of pro- and anti-inflammatory mediators. Here, we evaluated in epithelial cells the influence of LPS and continuous PAR activation on PAR expression level and the release of the proinflammatory chemokine IL-8. We studied primary human small airway epithelial cells and two airway epithelial cell lines, A549 and HBE cells. LPS specifically upregulated expression of PAR-2 but not of PAR-1. Exposure of epithelial cells to PAR-1 or PAR-2 agonists increased the PAR-1 expression level. The PAR-2 agonist exhibited higher potency than PAR-1 activators. However, the combined exposure of epithelial cells to LPS and PAR agonists abrogated the PAR-1 upregulation. The PAR-2 expression level was also upregulated after exposure to PAR-1 or PAR-2 agonists. This elevation was higher than the effect of PAR agonists on the PAR-1 level. In contrast to the PAR-1 level, the PAR-2 level remained elevated under concomitant stimulation with LPS and PAR-2 agonist. Furthermore, activation of PAR-2, but not of PAR-1, caused production of IL-8 from the epithelial cells. Interestingly, both in the epithelial cell line and in primary epithelial cells, there was a potentiation of the stimulation of the IL-8 synthesis and release by PAR-2 agonist together with LPS. In summary, these results underline the important role of PAR-2 in human lung epithelial cells. Moreover, our study shows an intricate interplay between LPS and PAR agonists in affecting PAR regulation and IL-8 production.

Protease-activated receptors in the brain: receptor expression, activation, and functions in neurodegeneration and neuroprotection

Protease-activated receptors (PARs) are G protein-coupled receptors that regulate the cellular response to extracellular serine proteases, like thrombin, trypsin, and tryptase. The PAR family consists of four members: PAR-1, -3, and -4 as thrombin receptors and PAR-2 as the trypsin/tryptase receptor, which are abundantly expressed in the brain throughout development. Recent evidence has supported the direct involvement of PARs in brain development and function. The expression of PARs in the brain is differentially upregulated or downregulated under pathological conditions in neurodegenerative disorders, like Parkinson's disease, Alzheimer's disease, multiple sclerosis, stroke, and human immunodeficiency virus-associated dementia. Activation of PARs mediates cell death or cell survival in the brain, depending on the amplitude and the duration of agonist stimulation. Interference or potentiation of PAR activation is beneficial in animal models of neurodegenerative diseases. Therefore, PARs mediate either neurodegeneration or neuroprotection in neurodegenerative diseases and represent attractive therapeutic targets for treatment of brain injuries. Here, we review the abnormal expression of PARs in the brain under pathological conditions, the functions of PARs in neurodegenerative disorders, and the molecular mechanisms involved.

The role of calcium in protease-activated receptor-induced secretion of chemokine GRO/CINC-1 in rat brain astrocytes

Our recent data showed that activation of protease-activated receptor (PAR)-1 and PAR-2 in rat astrocytes not only evokes calcium signaling, but also regulates the release of the chemokine growth-regulated oncogene/cytokine-induced neutrophil chemoattractant-1 (GRO/CINC-1), a counterpart of the human GRO. This chemokine provides a feedback to protect astrocytes from toxic insults. Activated PAR-1 and PAR-2 were strong stimuli to induce the release of GRO/CINC-1. The effect was comparable to that induced by TNF-alpha. However, the role of calcium in the PAR-induced GRO/CINC-1 secretion remains unknown. Here, we found that the pharmacological blockade of either calcium release from the intracellular stores, or influx from the extracellular space, increased PAR-1- and PAR-2-induced GRO/CINC-1 secretion. Under calcium-free conditions, the basal mRNA level of GRO/CINC-1 was clearly increased. Further studies revealed that the intracellular GRO/CINC-1 protein level was slightly increased by treatment with thrombin or TRag in calcium-free conditions. However, the amount of protein synthesized was largely reduced in the absence of extracellular calcium as compared to that under normal calcium conditions. Importantly, we found that the intracellularly formed GRO/CINC-1 was not secreted into the cell culture supernatant under calcium-free conditions. These data suggest a dual role of calcium. On the one side, an increase in cytosolic calcium negatively regulates PAR-induced GRO/CINC-1 gene expression in rat astrocytes, but on the other side, the basal level of calcium is the pre-requisite for GRO/CINC-1 protein synthesis and secretion.

Thrombin potently enhances swelling-sensitive glutamate efflux from cultured astrocytes

High concentrations of thrombin (Thr) have been linked to neuronal damage in cerebral ischemia and traumatic brain injury. In the present study we found that Thr markedly enhanced swelling-activated efflux of (3)H-glutamate from cultured astrocytes exposed to hyposmotic medium. Thr (0.5-5 U/mL) elicited small (3)H-glutamate efflux under isosmotic conditions and increased the hyposmotic glutamate efflux by 5- to 10-fold, the maximum effect being observed at 15% osmolarity reduction. These Thr effects involve its protease activity and are fully mimicked by SFFLRN, the synthetic peptide activating protease-activated receptor-1. Thr potentiation of (3)H-glutamate efflux was largely dependent on a Thr-elicited increases in cytosolic Ca(2+) (Ca(2+) (i)) concentration ([Ca(2+)](i)). Preventing Ca(2+) (i) rise by treatment with EGTA-AM or with the phospholipase C blocker U73122 reduced the Thr-increased glutamate efflux by 68%. The protein kinase C blockers Go6976 or chelerythrine reduced the Thr effect by 19%-22%, while Ca/calmodulin blocker W7 caused a 63% inhibition. In addition to this Ca(2+)-sensitive pathway, Thr effect on glutamate efflux also involved activation of phosphoinositide-3 kinase (PI3K), since it was reduced by the PI3K inhibitor wortmannin (51% inhibition). Treating cells with EGTA-AM plus wortmannin essentially abolished Thr-dependent glutamate efflux. Thr-activated glutamate release was potently inhibited by the blockers of the volume-sensitive anion permeability pathway, NPPB (IC(50) 15.8 microM), DCPIB (IC(50) 4.2 microM), and tamoxifen (IC(50) 6.6 microM. These results suggest that Thr may contribute to the excitotoxic neuronal injury by elevating extracellular glutamate release from glial cells. Therefore, this work may aid in search of neuroprotective strategies for treating cerebral ischemia and brain trauma.

A novel therapeutic target in various lung diseases: Airway proteases and protease-activated receptors

Protease-activated receptors (PAR), which are G protein-coupled receptors, have 4 members, PAR-1 to PAR-4. PARs are activated by proteolysis of a peptide bond at the N-terminal domain of the receptor. PARs are widely distributed throughout the airways. Their activity is modulated by airway proteases of endogenous and exogenous origin, which can either activate or disable the receptors. The regulation of PAR activity by proteases is important under pathological conditions when the activity of proteases is increased. Moreover, various inflammatory mediators, such as cytokines, growth factors, or prostanoids, alter the PAR expression level. Elevated PAR levels are observed in various lung disorders, and their significance in the development of pathological situations in the lung is currently intensively investigated. Consequences of PAR activation can be either beneficial or deleterious, depending on the PAR subtype. PAR-1 has been shown to be an important player in the development of pulmonary fibrosis. Thus, PAR-1 represents an exciting target for clinical intervention in fibrotic diseases. PAR-2 contributes to allergic airway inflammation. However, the question whether the impact of PAR-2 is beneficial or deleterious is still under intensive discussion. Therefore, precise information concerning the participation of PAR-2 in various lesions is required. Moreover, it is necessary to generate selective PAR- and organ-targeted approaches for treating the diseases. A thorough understanding of PAR-induced cellular events and the consequences of receptor blockade may help in the development of novel therapeutic strategies targeted to prevent lung destruction and to avoid deterioration of conditions of patients with inflammatory or fibrotic lung diseases.

Exacerbation of Dopaminergic Terminal Damage in a Mouse Model of Parkinson's Disease by the G-Protein-Coupled Receptor Protease-Activated Receptor 1

Protease-activated receptor 1 (PAR1) is a G-protein-coupled receptor activated by serine proteases and expressed in astrocytes, microglia, and specific neuronal populations. We examined the effects of genetic deletion and pharmacologic blockade of PAR1 in the mouse 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease, a neurodegenerative disease characterized by nigrostriatal dopamine damage and gliosis. After MPTP injection, PAR1-/- mice showed significantly higher residual levels of dopamine, dopamine transporter, and tyrosine hydroxylase and diminished microgliosis compared with wild-type mice. Comparable levels of dopaminergic neuroprotection from MPTP-induced toxicity were obtained by infusion of the PAR1 antagonist, BMS-200261 into the right lateral cerebral ventricle. MPTP administration caused changes in the brain protease system, including increased levels of mRNA for two PAR1 activators, matrix metalloprotease-1 and Factor Xa, suggesting a mechanism by which MPTP administration could lead to overactivation of PAR1. We also report that PAR1 is expressed in human substantia nigra pars compacta glia as well as tyrosine hydroxylase-positive neurons. Together, these data suggest that PAR1 might be a target for therapeutic intervention in Parkinson's disease.

The effect of thrombin on a 6-hydroxydopamine model of Parkinson's disease depends on timing

Recent results in animal models suggest that thrombin may modulate brain injury in Parkinson's disease (PD). High doses of thrombin ( approximately 20U) can damage dopaminergic neurons, while we have found that low dose thrombin (1U), given several days before a brain insult (thrombin preconditioning), is protective in models of PD and stroke. However, the effects of such low levels of thrombin at the time of, or after, exposure to the dopamine neurotoxin 6-hydroxydopamine (6-OHDA) have not been examined and are the focus of this study. In the first set of experiments, rats received co-administration of thrombin (1U) or saline and 6-OHDA (5microg) into the medial forebrain bundle. 6-OHDA+thrombin resulted in striking increases in behavioral deficits, compared to 6-OHDA+saline. Similarly, co-administration of an agonist to protease-activated receptor (PAR)-1, a thrombin receptor, also resulted in significantly greater behavioral deficits. In a second set of experiments, thrombin (1U) or saline was administered 1 or 7 days after 6-OHDA to determine the effects of thrombin after 6-OHDA. Surprisingly, the rats that received saline had strikingly increased behavioral and neurochemical deficits resulting from the 6-OHDA lesion, while delayed thrombin administration prevented this effect. The results indicate that thrombin has differential effects in the 6-OHDA model, dependent on the time of administration. The ability of a second cannula insertion with saline infusion to increase dramatically deficits raises questions as to what role physical injury to already susceptible cells might play in the pathogenesis of some cases of PD.

Proteinase-activated receptor-2 (PAR2): a tumor suppressor in skin carcinogenesis

The proteinase-activated receptor PAR(2) has been demonstrated to modulate tumor growth, invasion and metastasis in various tissues. However, the role of PAR(2) in cutaneous cancerogenesis is still unknown. Here we could show a protective role of PAR(2) in the development of epidermal skin tumors: we established a mouse skin tumor model using chemically induced carcinogenesis. Tumors started to appear after eight weeks. After 13 weeks, PAR(2)-deficient mice showed a significantly increased number of skin tumors (14 per animal on the average) in contrast to the wild type (eight tumors per mouse). Analysis of possible signal transduction pathways activated upon PAR(2) stimulation in HaCaT keratinocytes showed an involvement of extracellular signal-regulated kinase 1/2 and profound epidermal growth factor receptor transactivation, leading to secretion of the tumor-suppressing factor transforming growth factor-beta1. Thus, our results provide early experimental evidence for a tumor-protective role of PAR(2).

Astrocytic control of synaptic NMDA receptors

Astrocytes express a wide range of G-protein coupled receptors that trigger release of intracellular Ca2+, including P2Y, bradykinin and protease activated receptors (PARs). By using the highly sensitive sniffer-patch technique, we demonstrate that the activation of P2Y receptors, bradykinin receptors and protease activated receptors all stimulate glutamate release from cultured or acutely dissociated astrocytes. Of these receptors, we have utilized PAR1 as a model system because of favourable pharmacological and molecular tools, its prominent expression in astrocytes and its high relevance to neuropathological processes. Astrocytic PAR1-mediated glutamate release in vitro is Ca2+ dependent and activates NMDA receptors on adjacent neurones in culture. Activation of astrocytic PAR1 in hippocampal slices induces an APV-sensitive inward current in CA1 neurones and causes APV-sensitive neuronal depolarization in CA1 neurones, consistent with release of glutamate from astrocytes. PAR1 activation enhances the NMDA receptor-mediated component of synaptic miniature EPSCs, evoked EPSCs and evoked EPSPs in a Mg2+-dependent manner, which may reflect spine head depolarization and consequent reduction of NMDA receptor Mg2+ block during subsequent synaptic currents. The release of glutamate from astrocytes following PAR1 activation may also lead to glutamate occupancy of some perisynaptic NMDA receptors, which pass current following relief of tonic Mg2+ block during synaptic depolarization. These results suggest that astrocytic G-protein coupled receptors that increase intracellular Ca2+ can tune synaptic NMDA receptor responses.

The emergence of proteinase-activated receptor-2 as a novel target for the treatment of inflammation-related CNS disorders

The signalling molecules that are involved in inflammatory pathways are now thought to play a part in many disorders of the central nervous system (CNS). In common with peripheral chronic inflammatory diseases such a rheumatoid arthritis and ulcerative colitis, evidence now exists for the involvement of inflammatory cytokines, for example tumour necrosis factor (TNF) and interleukins (IL), in neurological disorders. A common factor observed with the up-regulation of these cytokines in peripheral inflammatory diseases, is the increased expression of the proteinase-activated receptor (PAR) subtype PAR-2. Indeed, recent evidence suggests that targeting PAR-2 helps reduce joint swelling observed in animal models of arthritis. So could targeting this receptor prove to be useful in treating those CNS disorders where inflammatory processes are thought to play an intrinsic role? The aim of this review is to summarize the emerging data regarding the role of PAR-2 in neuroinflammation and ischaemic injury and discuss its potential as an exciting new target for the prevention and/or treatment of CNS disorders.

Proteinase-activated receptor-1 and -2 induce the release of chemokine GRO/CINC-1 from rat astrocytes via differential activation of JNK isoforms, evoking multiple protective pathways in brain

Activation of both PAR-1 (proteinase-activated receptor-1) and PAR-2 resulted in release of the chemokine GRO (growth-regulated oncogene)/CINC-1 (cytokine-induced neutrophil chemoattractant-1), a functional counterpart of human interleukin-8, from rat astrocytes. Here, we investigate whether the two PAR receptor subtypes can signal separately. PAR-2-induced GRO/CINC-1 release was independent of protein kinase C, phosphoinositide 3-kinase and MEK (mitogen-activated protein kinase kinase)-1/2 activation, whereas these three kinases were involved in PAR-1-induced GRO/CINC-1 release. Despite such clear differences between PAR-1 and PAR-2 signalling pathways, JNK (c-Jun N-terminal kinase) was identified in both signalling pathways to play a pivotal role. By isoform-specific loss-of-function studies using small interfering RNA against JNK1-3, we demonstrate that different JNK isoforms mediated GRO/CINC-1 secretion, when it was induced by either PAR-1 or PAR-2 activation. JNK2 and JNK3 isoforms were both activated by PAR-1 and essential for chemokine GRO/CINC-1 secretion, whereas PAR-1-mediated JNK1 activation was mainly responsible for c-Jun phosphorylation, which was not involved in GRO/CINC-1 release. In contrast, PAR-2-induced JNK1 activation, which failed to phosphorylate c-Jun, uniquely contributed to GRO/CINC-1 release. Therefore our results show for the first time that JNK-mediated chemokine GRO/CINC-1 release occurred in a JNK isoform-dependent fashion and invoked PAR subtype-specific mechanisms. Furthermore, here we demonstrate that activation of PAR-2, as well as PAR-1, rescued astrocytes from ceramide-induced apoptosis via regulating chemokine GRO/CINC-1 release. Taken together, our results suggest that PAR-1 and PAR-2 have overlapping functions, but can activate separate pathways under certain pathological conditions to rescue neural cells from cell death. This provides new functional insights into PAR/JNK signalling and the protective actions of PARs in brain.

Thrombin inhibits aquaporin 4 expression through protein kinase C-dependent pathway in cultured astrocytes

Aquaporin 4 (AQP4) is a key molecule for maintaining water balance in the central nervous system, and its dysfunction might cause brain edema. However, little is known about the regulation of AQP4 expression. Because thrombin has been implicated in brain edema formation, the purpose of this study is to determine whether thrombin affects expression of AQP4 in astrocytes. Here, the effect of thrombin on AQP4 expression in vitro was evaluated using Western blot analysis and RT-PCR. Meanwhile, we investigated whether the effect of thrombin on AQP4 expression was due to protease-activated receptor 1 (PAR-1). In addition, we examined the role of protein kinase C (PKC) in the effect of thrombin on AQP4 expression using Western blot analysis. We found that thrombin did not affect cell viability at concentrations of 0.05, 0.5, 5, or 50 nM but killed astrocytes at concentrations of 500 nM, with approx 72% of astrocytes surviving at 500 nM thrombin. Our data showed that AQP4 protein expression achieved only 28% of controls in 500 nM thrombin treatment, even if astrocytes survived approx 72% of controls at 500 nM thrombin. Thrombin significantly inhibited AQP4 in a time- and dose dependent manner in vitro (p<0.05). Cathepsin-G, a thrombin PAR-1 inhibitor, reversed significantly (p<0.05) the effect of thrombin on AQP4 mRNA and protein expression in astrocytes. We also observed that PKC inhibitor H-7 or prolonged pretreatment with TPA can rapidly increase AQP4 expression (p<0.05). Thrombin might inhibit AQP4 expression in rat astrocytes, and this effect is possibly mediated by the PKC pathway.

Mesotrypsin, a brain trypsin, activates selectively proteinase-activated receptor-1, but not proteinase-activated receptor-2, in rat astrocytes

Proteinase-activated receptors (PARs), a subfamily of G protein-coupled receptors, which are activated by serine proteases, such as trypsin, play pivotal roles in the CNS. Mesotrypsin (trypsin IV) has been identified as a brain-specific trypsin isoform. However, its potential physiological role concerning PAR activation in the brain is largely unknown. Here, we show for the first time that mesotrypsin, encoded by the PRSS3 (proteinase, serine) gene, evokes a transient and pronounced Ca(2+) mobilization in both primary rat astrocytes and retinal ganglion RGC-5 cells, suggesting a physiological role of mesotrypsin in brain cells. Mesotrypsin mediates Ca(2+) responses in rat astrocytes in a concentration-dependent manner, with a 50% effective concentration (EC(50)) value of 25 nm. The maximal effect of mesotrypsin on Ca(2+) mobilization in rat astrocytes is much higher than that observed in 1321N1 human astrocytoma cells, indicating that the activity of mesotrypsin is species-specific. The pre-treatment of cells with thrombin or the PAR-1-specific peptide TRag (Ala-pFluoro-Phe-Arg-Cha-HomoArg-Tyr-NH(2), synthetic thrombin receptor agonist peptide), but not the PAR-2-specific peptide, reduces significantly the mesotrypsin-induced Ca(2+) response. Treatment with the PAR-1 antagonist SCH79797 confirms that mesotrypsin selectively activates PAR-1 in rat astrocytes. Unlike mesotrypsin, the two other trypsin isoforms, cationic and anionic trypsin, activate multiple PARs in rat astrocytes. Therefore, our data suggest that brain-specific mesotrypsin, via the regulation of PAR-1, is likely to be involved in multiple physiological/pathological processes in the brain.