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

Minimal Traumatic Brain Injury in Mice: Protease-Activated Receptor 1 and Thrombin-Related Changes

Minimal traumatic brain injury (mTBI) is partially defined by the existence of retrograde amnesia and is associated with microscopic bleeds containing activated coagulation factors. In a previous study, we have found that mTBI immediately releases thrombin-like activity in the brain, which induces amnesia by activating protease-activated receptor 1 (PAR-1) and blocking long-term potentiation (LTP). In the present study, we assessed the effects of mTBI on thrombin and PAR-1 levels in the brain using the same model. After the immediate elevation, thrombin activity returned to baseline 1 h post-trauma and increased again 72 h later (42% relative to control; p < 0.005). These changes were associated with a significant increase in PAR-1 levels 24 (17%; p < 0.05) and 72 h (20%; p < 0.05) post-trauma. Interestingly, the late elevation in thrombin-like activity was also associated with elevation of the major central nervous system thrombin inhibitor, protease nexin-1, 72 h post-mTBI (10%; p < 0.005). When thrombin was injected into brain ventricles, an increased sensitivity to seizure-like activity was detected at 72 h post-mTBI. The results are compatible with astrocyte activation post-mTBI resulting in increased thrombin secretion, PAR-1 expression, and seizure sensitivity.

Novel pharmaceutical treatments for minimal traumatic brain injury and evaluation of animal models and methodologies supporting their development

BACKGROUND

The need for effective pharmaceuticals within animal models of traumatic brain injury (TBI) continues to be paramount, as TBI remains the major cause of brain damage for children and young adults. While preventative measures may act to reduce the incidence of initial blunt trauma, well-tolerated drugs are needed to target the neurologically damaging internal cascade of molecular mechanisms that follow. Such processes, known collectively as the secondary injury phase, include inflammation, excitotoxicity, and apoptosis among other changes still subject to research. In this article positive treatment findings to mitigate this secondary injury in rodent TBI models will be overviewed, and include recent studies on Exendin-4, N-Acetyl-l-cycteine, Salubrinal and Thrombin.

CONCLUSIONS

These studies provide representative examples of methodologies that can be combined with widely available in vivo rodent models to evaluate therapeutic approaches of translational relevance, as well as drug targets and biochemical cascades that may slow or accelerate the degenerative processes induced by TBI. They employ well-characterized tests such as the novel object recognition task for assessing cognitive deficits. The application of such methodologies provides both decision points and a gateway for implementation of further translational studies to establish the feasibility of clinical efficacy of potential therapeutic interventions.

Possible role of proteases in preconditioning of brain cells to pathological conditions

Preconditioning (PC) is one of the most effective strategies to reduce the severity of cell damage, in particular of nervous tissue cells. Although PC mechanisms are studied insufficiently, it is clear that proteases are involved in them, but their role has yet been not studied in detail. In this work, some mechanisms of a potential recruiting of proteases in PC are considered. Our attention is mainly focused on the protease families of caspases and cathepsins and on protease receptors. We present evidence that just these proteins are involved in the PC of brain cells. A hypothesis is proposed that secreted cathepsin B is involved in the realization of PC through activation of PAR2 receptor.

Protease activated receptor-1 antagonist ameliorates the clinical symptoms of experimental autoimmune encephalomyelitis via inhibiting breakdown of blood-brain barrier

To evaluate the question of whether protease activated receptor-1 (PAR-1) antagonist is a potential therapeutic target in multiple sclerosis, we treated experimental autoimmune encephalomyelitis (EAE) mice with two PAR-1 antagonists, KC-A0590 and SCH-530348. Treatment with both antagonists resulted in a significant decrease in the clinical characteristics of EAE mice by suppressing demyelination and infiltration of inflammatory cells in the spinal cord and brain, as well as a significantly reducing the increased thrombin and tumor necrosis factor-α. Profound leakage of dextran was observed in the brain of EAE mice. However, treatment with PAR-1 antagonists resulted in the stabilization of vascular endothelial cells and reduced blood-brain barrier breakdown with suppression of inflammatory response. Treatment with PAR-1 antagonists also resulted in down-regulated expression of matrix metalloproteinase-9 and preserved expression of occludin and zonula occludens (ZO)-1 in the brain and their significant expression was confirmed in neurons, astrocytes, and vascular endothelial cells. Finally, endothelial cells and primary cultured astrocytes were treated with PAR-1 antagonists; both antagonists suppressed thrombin-induced breakdown of ZO-1 in endothelial cells and secretion of matrix metalloproteinase-9 in astrocytes. Collectively, our results suggest that PAR-1 antagonist is effective in attenuation of the clinical symptoms of EAE mice by stabilizing the blood-brain barrier and may have therapeutic potential for treatment of multiple sclerosis.

Thrombin decreases expression of the glutamate transporter GLAST and inhibits glutamate uptake in primary cortical astrocytes via the Rho kinase pathway

Astrocyte glutamate transporters GLAST and GLT1 play a key role in regulating neuronal excitation and their levels are altered in patients with epilepsy, and after traumatic brain injury. The mechanisms which regulate their expression are not well understood. We tested the hypothesis that exposure of astrocytes to high levels of thrombin, as may occur after a compromise of the blood-brain barrier, would reduce astrocyte glutamate transporter levels. In isolated rat cortical astrocytes we examined the effects of thrombin on the expression and function of glutamate transporters, and the signaling pathways involved in these responses by using Western blotting and selective inhibitors. Thrombin induced a selective decrease in the expression of GLAST but not GLT1, with a corresponding decrease in the capacity of astrocytes to take up glutamate. Activation of the thrombin receptor PAR-1 with an activating peptide induced a similar decrease in the expression of GLAST and compromise of glutamate uptake. The downregulation of GLAST induced by thrombin was mediated by the mitogen activated protein kinases p38 MAPK, ERK and JNK, but inhibition of these kinases did not prevent the decrease in glutamate uptake induced by thrombin. In contrast, inhibition of the Rho kinase pathway using the specific inhibitor, Y27632, suppressed both the decrease in the expression of GLAST and the decrease in glutamate uptake induced by thrombin. In hippocampal astrocyte cultures, thrombin caused a decrease in both GLAST and GLT1. In tissue resected from brains of children with intractable epilepsy, we found a decrease in the integrity of the blood-brain barrier along with a reduction in immunoreactivity for both transporters which was associated with an increase in cleaved thrombin and reactive astrogliosis. The in vitro results suggest a specific mechanism by which thrombin may lead to a compromise of astrocyte function and enhanced synaptic excitability after the blood-brain barrier is compromised. The human in vivo results provide indirect support evidence linking the compromise of the blood-brain barrier to thrombin-induced reduction in glutamate transporter expression and an increase in neuronal excitation.

Genetic targeting of protease activated receptor 2 reduces inflammatory astrogliosis and improves recovery of function after spinal cord injury

Inflammatory-astrogliosis exacerbates damage in the injured spinal cord and limits repair. Here we identify Protease Activated Receptor 2 (PAR2) as an essential regulator of these events with mice lacking the PAR2 gene showing greater improvements in motor coordination and strength after compression-spinal cord injury (SCI) compared to wild type littermates. Molecular profiling of the injury epicenter, and spinal segments above and below, demonstrated that mice lacking PAR2 had significantly attenuated elevations in key hallmarks of astrogliosis (glial fibrillary acidic protein (GFAP), vimentin and neurocan) and in expression of pro-inflammatory cytokines (interleukin-6 (IL-6), tumor necrosis factor (TNF) and interleukin-1 beta (IL-1β)). SCI in PAR2-/- mice was also accompanied by improved preservation of protein kinase C gamma (PKCγ)-immunopositive corticospinal axons and reductions in GFAP-immunoreactivity, expression of the pro-apoptotic marker BCL2-interacting mediator of cell death (BIM), and in signal transducer and activator of transcription 3 (STAT3). The potential mechanistic link between PAR2, STAT3 and astrogliosis was further investigated in primary astrocytes to reveal that the SCI-related serine protease, neurosin (kallikrein 6) promotes IL-6 secretion in a PAR2 and STAT3-dependent manner. Data point to a signaling circuit in primary astrocytes in which neurosin signaling at PAR2 promotes IL-6 secretion and canonical STAT3 signaling. IL-6 promotes expression of GFAP, vimentin, additional IL-6 and robust increases in both neurosin and PAR2, thereby driving the PAR2-signaling circuit forward. Given the significant reductions in astrogliosis and inflammation as well as superior neuromotor recovery observed in PAR2 knockout mice after SCI, we suggest that this receptor and its agonists represent new drug targets to foster neuromotor recovery.

Pharmacological Targeting of Protease-Activated Receptor 2 Affords Protection from Bleomycin-Induced Pulmonary Fibrosis

Idiopathic pulmonary fibrosis is the most devastating diffuse fibrosing lung disease that remains refractory to therapy. Despite increasing evidence that protease-activated receptor 2 (PAR-2) contributes to fibrosis, its importance in pulmonary fibrosis is under debate. We addressed whether PAR-2 deficiency persistently reduces bleomycin-induced pulmonary fibrosis or merely delays disease progression and whether pharmacological PAR-2 inhibition limits experimental pulmonary fibrosis. Bleomycin was instilled intranasally into wild-type or PAR-2-deficient mice in the presence/absence of a specific PAR-2 antagonist (P2pal-18S). Pulmonary fibrosis was consistently reduced in PAR-2-deficient mice throughout the fibrotic phase, as evident from reduced Ashcroft scores (29%) and hydroxyproline levels (26%) at d 28. Moreover, P2pal-18S inhibited PAR-2-induced profibrotic responses in both murine and primary human pulmonary fibroblasts (p < 0.05). Once daily treatment with P2pal-18S reduced the severity and extent of fibrotic lesions in lungs of bleomycin-treated wild-type mice but did not further reduce fibrosis in PAR-2-deficient mice. Importantly, P2pal-18S treatment starting even 7 d after the onset of fibrosis limits pulmonary fibrosis as effectively as when treatment was started together with bleomycin instillation. Overall, PAR-2 contributes to the progression of pulmonary fibrosis, and targeting PAR-2 may be a promising therapeutic strategy for treating pulmonary fibrosis.

Protease-activated receptors and their biological role - focused on skin inflammation

OBJECTIVES

For several years, protease-activated receptors (PARs) are targets of science regarding to various diseases and platelet aggregation. In the past, a number of publications related to PARs have been published, which refer to a variety of aspects. An important point of view is the inflammation of the skin, which has not been reported in detail yet. This review will provide an overview of the current knowledge on PARs, and in particular, on the involvement of PARs in terms of skin inflammation.

KEY FINDINGS

Wound healing is an important step after skin injury and is connected with involvement of PARs and inflammation. An important point in skin inflammation is the coagulation-dependent skin inflammation.

SUMMARY

PARs are a special kind of receptors, being activated by proteolytic cleavage or chemical agonists. They may play an important role in various physiological processes. It is shown that the proteases are involved in many diseases for example Parkinson's disease and Alzheimer's disease. The fact, that proteases regulate the coagulation, and are involved in interleukin and cytokine release leads to the conclusion that they are involved in inflammation processes.

Brain pericytes are the most thrombin-sensitive matrix metalloproteinase-9-releasing cell type constituting the blood-brain barrier in vitro

In the acute phase of intracerebral hemorrhage (ICH), hemorrhagic transformation and brain edema are associated with blood-brain barrier (BBB) disruption. Elevated levels of thrombin, a coagulation factor, contribute to the development of brain edema during ICH through matrix metalloproteinase (MMP)-9 production. Thrombin directly induces a variety of cellular responses through its specific receptors known as protease-activated receptors (PARs). However, it remains unclear which cell types constituting the BBB mainly produce MMP-9 in response to thrombin. Here, we compared the MMP-9 release induced by thrombin using primary cultures of rat brain microvascular endothelial cells, astrocytes, and pericytes. Brain pericytes exhibited the highest levels of MMP-9 release due to thrombin stimulation among the BBB cells. The pattern of PAR mRNA expression in pericytes was characterized by high expression of PAR1 and moderate expression of PAR4. Heat-inactivated thrombin failed to stimulate pericytes to release MMP-9. A selective PAR1 inhibitor SCH79797 blocked the thrombin-induced MMP-9 release from pericytes. These findings suggest that both PAR1 and PAR4 mediate thrombin-induced MMP-9 release from pericytes. The present study raises the possibility that brain pericytes could play a pivotal role as a highly thrombin-sensitive and MMP-9-producing cell type at the BBB in brain damage including ICH.

Thrombin regulation of synaptic transmission and plasticity: implications for health and disease

Thrombin, a serine protease involved in the blood coagulation cascade has been shown to affect neural function following blood-brain barrier breakdown. However, several lines of evidence exist that thrombin is also expressed in the brain under physiological conditions, suggesting an involvement of thrombin in the regulation of normal brain functions. Here, we review ours’ as well as others’ recent work on the role of thrombin in synaptic transmission and plasticity through direct or indirect activation of Protease-Activated Receptor-1 (PAR1). These studies propose a novel role of thrombin in synaptic plasticity, both in physiology as well as in neurological diseases associated with increased brain thrombin/PAR1 levels.

Contribution of protease-activated receptor 1 in status epilepticus-induced epileptogenesis

Clinical observations and studies on different animal models of acquired epilepsy consistently demonstrate that blood-brain barrier (BBB) leakage can be an important risk factor for developing recurrent seizures. However, the involved signaling pathways remain largely unclear. Given the important role of thrombin and its major receptor in the brain, protease-activated receptor 1 (PAR1), in the pathophysiology of neurological injury, we hypothesized that PAR1 may contribute to status epilepticus (SE)-induced epileptogenesis and that its inhibition shortly after SE will have neuroprotective and antiepileptogenic effects. Adult rats subjected to lithium-pilocarpine SE were administrated with SCH79797 (a PAR1 selective antagonist) after SE termination. Thrombin and PAR1 levels and neuronal cell survival were evaluated 48h following SE. The effect of PAR1 inhibition on animal survival, interictal spikes (IIS) and electrographic seizures during the first two weeks after SE and behavioral seizures during the chronic period was evaluated. SE resulted in a high mortality rate and incidence of IIS and seizures in the surviving animals. There was a marked increase in thrombin, decrease in PAR1 immunoreactivity and hippocampal cell loss in the SE-treated rats. Inhibition of PAR1 following SE resulted in a decrease in mortality and morbidity, increase in neuronal cell survival in the hippocampus and suppression of IIS, electrographic and behavioral seizures following SE. These data suggest that the PAR1 signaling pathway contributes to epileptogenesis following SE. Because breakdown of the BBB occurs frequently in brain injuries, PAR1 inhibition may have beneficial effects in a variety of acquired injuries leading to epilepsy.

Proteinase-activated receptor 2 and disease biomarkers in cerebrospinal fluid in cases with autopsy-confirmed prion diseases and other neurodegenerative diseases

BACKGROUND

Proteinase-activated receptor 2 (PAR-2) has been shown to promote both neurotoxic and neuroprotective effects. Similarly, other routinely used nonspecific markers of neuronal damage can be found in cerebrospinal fluid (CSF) and can be used as biomarkers for different neurodegenerative disorders.

METHODS

Using enzyme-linked immunosorbent assays and western blotting we assessed PAR-2, total-tau, phospho-tau, beta-amyloid levels, and protein 14-3-3 in the CSF of former patients who had undergone a neuropathological autopsy after death and who had been definitively diagnosed with a prion or other neurodegenerative disease.

RESULTS

We did not find any significant correlation between levels of PAR-2 and other biomarkers, nor did we find any differences in PAR-2 levels between prion diseases and other neurodegenerative conditions. However, we confirmed that very high total-tau levels were significantly associated with definitive prion diagnoses and exhibited greater sensitivity and specificity than protein 14-3-3, which is routinely used as a marker.

CONCLUSIONS

Our study showed that PAR-2, in CSF, was not specifically altered in prion diseases compared to other neurodegenerative conditions. Our results also confirmed that very high total-tau protein CSF levels were significantly associated with a definitive Creutzfeldt-Jakob disease (CJD) diagnosis and should be routinely tested as a diagnostic marker. Observed individual variability in CSF biomarkers provide invaluable feedback from neuropathological examinations even in "clinically certain" cases.

Thrombin Activity and Thrombin Receptor in Rat Glioblastoma Model: Possible Markers and Targets for Intervention?

High-grade gliomas constitute a group of aggressive CNS cancers that have high morbidity and mortality rates. Despite extensive research, current therapeutic approaches enable survival beyond 2 years in rare cases only. Thrombin and its main CNS target, protease-activated receptor-1, have been implicated in tumor progression and brain edema. Our aim was to study protease-activated receptor-1 (PAR-1) protein expression and thrombin-like activity levels in both in vitro and in vivo models of glioblastoma and correlate them with the volume of the surrounding edema. We measured the presence of PAR-1 protein using fluorescence immunohistochemistry and assessed thrombin activity in various glial and non-glial cell lines and in a CNS-1 glioma rat model using a thrombin-specific fluorescent assay. Thrombin activity was found to be highly elevated in various high-grade glioma cell lines as well as in non-glial malignant cell lines. In the CNS-1 glioma model, the level of PAR-1 fluorescence in the tumor was significantly elevated compared to adjacent regions of reactive gliosis or distant brain areas. The elevated level of thrombin activity observed in the high-grade glioma positively correlated with tumor-induced brain edema. In conclusion, thrombin is secreted from glioma cells and PAR-1 may be a new biological marker for high-grade gliomas.

Proteinases, their receptors and inflammatory signalling: the Oxford South Parks Road connection

In keeping with the aim of the Paton Memorial Lecture to 'facilitate the historical study of pharmacology', this overview, which is my distinct honour to write, represents a 'Janus-like' personal perspective looking both backwards and forwards at the birth and growth of 'receptor molecular pharmacology' with special relevance to inflammatory diseases. The overview begins in the Oxford Department of Pharmacology in the mid-1960s and then goes on to provide a current perspective of signalling by proteinases. Looking backwards, the synopsis describes the fruitful Oxford Pharmacology Department infrastructure that Bill Paton generated in keeping with the blueprint begun by his predecessor, J H Burn. Looking forwards, the overview illustrates the legacy of that environment in generating some of the first receptor ligand-binding data and providing the inspiration and vision for those like me who were training in the department at the same time. With apologies, I mention only in passing a number of individuals who benefitted from the 'South Parks Road connection' using myself as one of the 'outcome study' examples. It is also by looking forward that I can meet the complementary aim of summarizing the lecture presented at a 'BPS 2014 Focused Meeting on Cell Signalling' to provide an overview of the role of proteinases and their signalling mechanisms in the setting of inflammation.

Retinal neurons curb inflammation and enhance revascularization in ischemic retinopathies via proteinase-activated receptor-2

Ischemic retinopathies are characterized by sequential vaso-obliteration followed by abnormal intravitreal neovascularization predisposing patients to retinal detachment and blindness. Ischemic retinopathies are associated with robust inflammation that leads to generation of IL-1β, which causes vascular degeneration and impairs retinal revascularization in part through the liberation of repulsive guidance cue semaphorin 3A (Sema3A). However, retinal revascularization begins as inflammation culminates in ischemic retinopathies. Because inflammation leads to activation of proteases involved in the formation of vasculature, we hypothesized that proteinase-activated receptor (Par)-2 (official name F2rl1) may modulate deleterious effects of IL-1β. Par2, detected mostly in retinal ganglion cells, was up-regulated in oxygen-induced retinopathy. Surprisingly, oxygen-induced retinopathy-induced vaso-obliteration and neovascularization were unaltered in Par2 knockout mice, suggesting compensatory mechanisms. We therefore conditionally knocked down retinal Par2 with shRNA-Par2-encoded lentivirus. Par2 knockdown interfered with normal revascularization, resulting in pronounced intravitreal neovascularization; conversely, the Par2 agonist peptide (SLIGRL) accelerated normal revascularization. In vitro and in vivo exploration of mechanisms revealed that IL-1β induced Par2 expression, which in turn down-regulated sequentially IL-1 receptor type I and Sema3A expression through Erk/Jnk-dependent processes. Collectively, our findings unveil an important mechanism by which IL-1β regulates its own endothelial cytotoxic actions by augmenting neuronal Par2 expression to repress sequentially IL-1 receptor type I and Sema3A expression. Timely activation of Par2 may be a promising therapeutic avenue in ischemic retinopathies.

KLKs and their hormone-like signaling actions: a new life for the PSA-KLK family

Human kallikrein-related peptidases (KLKs), including the well-known prostate cancer biomarker, prostate-specific antigen (PSA-KLK3), along with 14 other serine proteinase KLK family members are now known to regulate cells by cleaving and activating members of the G-protein-coupled proteinase-activated receptor (PAR) family. This hormone-like signaling action of the KLKs has provided a new perspective for understanding the biological roles that KLKs may play in normal and pathophysiological settings. This overview summarizes the circumstances leading up to the discovery of this action of the KLKs and provides an overview of the diverse impact on tissue function that may result from KLK-triggered PAR activation.

Nafamostat mesilate attenuates neuronal damage in a rat model of transient focal cerebral ischemia through thrombin inhibition

Evidence suggests that thrombin, a blood coagulation serine protease, mediates neuronal injury in experimental cerebral ischemia. Here, we test the hypothesis that nafamostat mesilate, a serine protease inhibitor, may ameliorate ischemia-induced neuronal damage through thrombin inhibition after ischemic stroke. Focal ischemia was induced in adult Sprague-Dawley rats by occlusion of the middle cerebral artery for 2 hours followed by 22 hours of reperfusion. The administration of nafamostat mesilate during ischemia and reperfusion reduced the brain infarct volume, edema volume and neurological deficit. Thrombin expression and activity in the ipsilateral striatum were increased after ischemia, whereas the administration of nafamostat mesilate significantly inhibited thrombin expression and activity. Immunostaining showed that the majority of thrombin was expressed in neurons. TUNEL staining showed that nafamostat mesilate reduced the number of dying cells during ischemia. A rat behavioral test showed that nafamostat mesilate treatment significantly improved the learning ability of ischemic rats. These results suggest that nafamostat mesilate may have a potential therapeutic role for neuroprotection against focal cerebral ischemia through thrombin inhibition.

The anticoagulant activated protein C (aPC) promotes metaplasticity in the hippocampus through an EPCR-PAR1-S1P1 receptors dependent mechanism

Thrombin and other clotting factors regulate long-term potentiation (LTP) in the hippocampus through the activation of the protease activated receptor 1 (PAR1) and consequent potentiation of N-methyl-d-aspartate receptor (NMDAR) functions. We have recently shown that the activation of PAR1 either by thrombin or the anticoagulant factor activated protein C (aPC) has differential effects on LTP. While thrombin activation of PAR1 induces an NMDAR-mediated slow onset LTP, which saturates the ability to induce further LTP in the exposed network, aPC stimulation of PAR1 enhances tetanus induced LTP through a voltage-gated calcium channels mediated mechanism. In this study, we addressed the mechanisms by which aPC enhances LTP in hippocampal slices. Using extracellular recordings, we show that a short tetanic stimulation, which does not induce LTP, is able to enhance plasticity in the presence of aPC through a mechanism that requires the activation of sphingosine-1 phosphate receptor 1 and intracellular Ca(2+) stores. These data identify aPC as a "metaplastic molecule", capable of shifting the threshold of LTP towards further potentiation. Our findings propose novel strategies to enhance plasticity in neurological diseases associated with the breakdown of the blood brain barrier and alterations in synaptic plasticity.

Epileptic focus and alteration of metabolism

Epilepsy is one of the most common neurologic disorders affecting a substantial part of the population worldwide. Epileptic seizures represent the situation of increased neuronal activity associated with the enhanced demands for sufficient energy supply. For that purpose, very efficient regulatory mechanisms have to operate to ensure that cerebral blood flow, delivery of oxygen, and nutrients are continuously adapted to the local metabolic needs. The sophisticated regulation has to function in concert at several levels (systemic, tissue, cellular, and subcellular). Particularly, mitochondria play a key role not only in the energy production, but they are also central to many other processes including those leading to neuronal death. Impairment of any of the involved pathways can result in serious functional alterations, neurodegeneration, and potentially in epileptogenesis. The present review will address some of the important issues concerning vascular and metabolic changes in pathophysiology of epilepsy.

Reversal of trauma-induced amnesia in mice by a thrombin receptor antagonist

Minimal traumatic brain injury (mTBI) is associated with the existence of retrograde amnesia and microscopic bleeds containing activated coagulation factors. In an mTBI model, we report that thrombin induces amnesia through its receptor protease-activated receptor 1 (PAR-1). Thrombin activity was significantly elevated (32 %, p < 0.05) 5 min following mTBI compared to controls. Amnesia was assessed by the novel object recognition test in mTBI animals and in animals injected intracerebroventricularly (ICV) with either thrombin or a PAR-1 agonist 1 h after the acquisition phase. Saline-injected controls had a preference index of over 0.3 while mTBI animals and those injected with thrombin or the PAR-1 agonist spent equal time with both objects indicating no recall of the object presented to them 24 h previously (p < 0.05). Co-injecting a PAR-1 antagonist (SCH79797) completely blocked the amnestic effects of mTBI, thrombin, and the PAR-1 agonist. Long-term potentiation, measured in hippocampal slices 24 h after mTBI, ICV thrombin or the PAR-1 agonist, was significantly impaired and this effect was completely reversed by the PAR-1 antagonist. The results support a crucial role for PAR-1 in the generation of amnesia following mTBI, revealing a novel therapeutic target for the cognitive effects of brain trauma.

Salmon and human thrombin differentially regulate radicular pain, glial-induced inflammation and spinal neuronal excitability through protease-activated receptor-1

Chronic neck pain is a major problem with common causes including disc herniation and spondylosis that compress the spinal nerve roots. Cervical nerve root compression in the rat produces sustained behavioral hypersensitivity, due in part to the early upregulation of pro-inflammatory cytokines, the sustained hyperexcitability of neurons in the spinal cord and degeneration in the injured nerve root. Through its activation of the protease-activated receptor-1 (PAR1), mammalian thrombin can enhance pain and inflammation; yet at lower concentrations it is also capable of transiently attenuating pain which suggests that PAR1 activation rate may affect pain maintenance. Interestingly, salmon-derived fibrin, which contains salmon thrombin, attenuates nerve root-induced pain and inflammation, but the mechanisms of action leading to its analgesia are unknown. This study evaluates the effects of salmon thrombin on nerve root-mediated pain, axonal degeneration in the root, spinal neuronal hyperexcitability and inflammation compared to its human counterpart in the context of their enzymatic capabilities towards coagulation substrates and PAR1. Salmon thrombin significantly reduces behavioral sensitivity, preserves neuronal myelination, reduces macrophage infiltration in the injured nerve root and significantly decreases spinal neuronal hyperexcitability after painful root compression in the rat; whereas human thrombin has no effect. Unlike salmon thrombin, human thrombin upregulates the transcription of IL-1β and TNF-α and the secretion of IL-6 by cortical cultures. Salmon and human thrombins cleave human fibrinogen-derived peptides and form clots with fibrinogen with similar enzymatic activities, but salmon thrombin retains a higher enzymatic activity towards coagulation substrates in the presence of antithrombin III and hirudin compared to human thrombin. Conversely, salmon thrombin activates a PAR1-derived peptide more weakly than human thrombin. These results are the first to demonstrate that salmon thrombin has unique analgesic, neuroprotective and anti-inflammatory capabilities compared to human thrombin and that PAR1 may contribute to these actions.

Regulation of neuronal plasticity and fear by a dynamic change in PAR1-G protein coupling in the amygdala

Fear memories are acquired through neuronal plasticity, an orchestrated sequence of events regulated at circuit and cellular levels. The conventional model of fear acquisition assumes unimodal (for example, excitatory or inhibitory) roles of modulatory receptors in controlling neuronal activity and learning. Contrary to this view, we show that protease-activated receptor-1 (PAR1) promotes contrasting neuronal responses depending on the emotional status of an animal by a dynamic shift between distinct G protein-coupling partners. In the basolateral amygdala of fear-naive mice PAR1 couples to Gαq/11 and Gαo proteins, while after fear conditioning coupling to Gαo increases. Concurrently, stimulation of PAR1 before conditioning enhanced, but afterwards it inhibited firing of basal amygdala neurons. An initial impairment of the long-term potentiation (LTP) in PAR1-deficient mice was transformed into an increase in LTP and enhancement of fear after conditioning. These effects correlated with more frequent 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid (AMPA) receptor-mediated miniature post synaptic events and increased neuronal excitability. Our findings point to experience-specific shifts in PAR1-G protein coupling in the amygdala as a novel mechanism regulating neuronal excitability and fear.

Critical role for PAR1 in kallikrein 6-mediated oligodendrogliopathy

Kallikrein 6 (KLK6) is a secreted serine protease preferentially expressed by oligodendroglia in CNS white matter. Elevated levels of KLK6 occur in actively demyelinating multiple sclerosis (MS) lesions and in cases of spinal cord injury (SCI), stroke, and glioblastoma. Taken with recent evidence establishing KLK6 as a CNS-endogenous activator of protease-activated receptors (PARs), we hypothesized that KLK6 activates a subset of PARs to regulate oligodendrocyte physiology and potentially pathophysiology. Here, primary oligodendrocyte cultures derived from wild type or PAR1-deficient mice and the murine oligodendrocyte cell line, Oli-neu, were used to demonstrate that Klk6 (rodent form) mediates loss of oligodendrocyte processes and impedes morphological differentiation of oligodendrocyte progenitor cells (OPCs) in a PAR1-dependent fashion. Comparable gliopathy was also elicited by the canonical PAR1 agonist, thrombin, as well as PAR1-activating peptides (PAR1-APs). Klk6 also exacerbated ATP-mediated oligodendrogliopathy in vitro, pointing to a potential role in augmenting excitotoxicity. In addition, Klk6 suppressed the expression of proteolipid protein (PLP) RNA in cultured oligodendrocytes by a mechanism involving PAR1-mediated Erk1/2 signaling. Microinjection of PAR1 agonists, including Klk6 or PAR1-APs, into the dorsal column white matter of PAR1(+/+) but not PAR1(-/-) mice promoted vacuolating myelopathy and a loss of immunoreactivity for myelin basic protein (MBP) and CC-1(+) oligodendrocytes. These results demonstrate a functional role for Klk6-PAR1 signaling in oligodendroglial pathophysiology and suggest that antagonists of PAR1 or its protease agonists may represent new modalities to moderate demyelination and to promote myelin regeneration in cases of CNS white matter injury or disease.

Activation of protease-activated receptor 2-mediated signaling by mast cell tryptase modulates cytokine production in primary cultured astrocytes

Protease-activated receptor 2 (PAR-2), which is abundantly expressed in astrocytes, is known to play major roles in brain inflammation. However, the influence of the natural agonist of PAR-2, tryptase, on proinflammatory mediator releasedfrom astrocytes remains uninvestigated. In the present study, we found that tryptase at lower concentrations modestly reduced intracellular ROS production but significantly increased IL-6 and TNF-α secretion at higher concentrations without affecting astrocytic viability and proliferation. The actions of tryptase were alleviated by specific PAR-2 antagonist FSLLRY-NH2 (FS), indicating that the actions of tryptase were via PAR-2. PI3K/AKT inhibitor LY294002 reversed the effect of tryptase on IL-6 production, whereas inhibitors specific for p38, JNK, and ERK1/2 abolished the effect of tryptase on TNF-α production, suggesting that different signaling pathways are involved. Moreover, tryptase-induced activation of MAPKs and AKT was eliminated by FS, implicating that PAR-2 is responsible for transmitting tryptase biosignals to MAPKs and AKT. Tryptase provoked also expression of TGF-β and CNTF in astrocytes. The present findings suggest for the first time that tryptase can regulate the release of cytokines from astrocytes via PAR-2-MAPKs or PAR-2-PI3K/AKT signaling pathways, which reveals PAR-2 as a new target actively participating in the regulation of astrocytic functions.

Treating seizures and epilepsy with anticoagulants?

Thrombin is a serine protease playing an essential role in the blood coagulation cascade. Recent work, however, has identified a novel role for thrombin-mediated signaling pathways in the central nervous system. Binding of thrombin to protease-activated receptors (PARs) in the brain appears to have multiple actions affecting both health and disease. Specifically, thrombin has been shown to lead to the onset of seizures via PAR-1 activation. In this perspective article, we review the putative mechanisms by which thrombin causes seizures and epilepsy. We propose a potential role of PAR-1 antagonists and novel thrombin inhibitors as new, possible antiepileptic drugs.

Thrombin regulation of synaptic plasticity: implications for physiology and pathology

Thrombin, a serine protease involved in the coagulation cascade has been recently shown to affect neuronal function following blood-brain barrier breakdown. Several lines of evidence have shown that thrombin may exist in the brain parenchyma under normal physiological conditions, yet its role in normal brain functions and synaptic transmission has not been established. In an attempt to shed light on the physiological functions of thrombin and Protease Activated Receptor 1 (PAR1) in the brain, we studied the effects of thrombin and a PAR1 agonist on long term potentiation (LTP) in mice hippocampal slices. Surprisingly, different concentrations of thrombin affect LTP through different molecular routes converging on PAR1. High thrombin concentrations induced an NMDA dependent, slow onset LTP, whereas low concentrations of thrombin promoted a VGCCs, mGluR-5 dependent LTP through activated Protein C (aPC). Remarkably, aPC facilitated LTP by activating PAR1 through an Endothelial Protein C Receptor (EPCR)-mediated mechanism which involves intracellular calcium stores. These findings reveal a novel mechanism by which PAR1 may regulate the threshold for synaptic plasticity in the hippocampus and provide additional insights into the role of this receptor in normal and pathological conditions.

Modulation of PAR(1) signalling by benzimidazole compounds

BACKGROUND AND PURPOSE

Recently, a small molecule (Q94) was reported to selectively block PAR(1) /Gα(q) interaction and signalling. Here, we describe the pharmacological properties of Q94 and two analogues that share its benzimidazole scaffold (Q109, Q89). Q109 presents a modest variation from Q94 in the substituent group at the 2-position, while Q89 has quite different groups at the 1- and 2-positions.

EXPERIMENTAL APPROACH

Using human microvascular endothelial cells, we examined intracellular Ca(2+) mobilization and inositol 1,4,5-trisphosphate accumulation as well as isoprenaline- or forskolin-stimulated cAMP production in response to thrombin.

KEY RESULTS

Q89 (10 µM) produced a leftward shift in the thrombin-mediated intracellular Ca(2+) mobilization concentration-response curve while having no effect on the E(max) . Both Q94 (10 µM) and Q109 (10 µM) reduced intracellular Ca(2+) mobilization, leading to a decrease in E(max) and an increase in EC(50) values. Experiments utilizing receptor-specific activating peptides confirmed that Q94 and Q109 were selective for PAR(1) as they did not alter the Ca(2+) response mediated by a PAR(2) activating peptide. Consistent with our Ca(2+) results, micromolar concentrations of either Q94 or Q109 significantly reduced thrombin-induced inositol 1,4,5-trisphosphate production. Neither Q94 nor Q109 diminished the inhibitory effects of thrombin on cAMP production, indicating they inhibit signalling selectively through the G(q) pathway. Our results also suggest the 1,2-disubstituted benzimidazole derivatives act as 'allosteric agonists' of PAR(1) .

CONCLUSIONS AND IMPLICATIONS

The Q94 and Q109 benzimidazole derivatives represent a novel scaffold for the development of new PAR(1) inhibitors and provide a starting point to develop dual signalling pathway-selective positive/negative modulators of PAR(1) .

Neuroprotective effect of protease-activated receptor-2 in the hypoxia-induced apoptosis of rat RGC-5 cells

Hypoxia-induced apoptosis of retinal ganglion cells (RGCs) is regarded as a pivotal pathological process in various ocular diseases. Protease-activated receptor-2 (PAR-2) is involved in the regulation of cell inflammation, differentiation, and apoptosis in many cell types and tissues, but the role of PAR-2 in RGCs under pathological conditions remains unknown. The purpose of this study was to investigate the role of PAR-2 in the apoptosis of RGCs under hypoxic stress. An immortalized rat RGC line (RGC-5) was exposed to hypoxia (5 % O₂). The expression and location of PAR-2 in RGC-5 cells under hypoxia stress were investigated using real-time PCR, western blotting and immunocytochemistry. Cell viability was determined using the Cell Counting Kit-8 assay. Apoptosis was detected using Hoechst 33342 staining and AnnexinV-FITC/PI assays. The role of Bcl-2, Bax, and the active subunit of caspase-3 was also investigated. The results showed that PAR-2 was functionally expressed in RGC-5 cells and up-regulated at both mRNA and protein levels under hypoxic stress. The PAR-2 selective agonist, SLIGRL, rescued RGC-5 cells from hypoxia-induced apoptosis through up-regulation of the Bcl-2/Bax ratio and down-regulation of caspase-3 activation. This study provides the first evidence that PAR-2 has a protective effect against the hypoxia-induced apoptosis of RGC-5 cells.

Deletion of protease-activated receptor 2 prolongs survival of scrapie-inoculated mice

Proteinase-activated receptor 2 (PAR2) has recently been identified to be a possible modulator of neurodegeneration. To investigate whether PAR2 plays a role in prion infection, we inoculated PAR2-deficient (PAR2−/−) and wild-type (WT) mice intracerebrally with the Rocky Mountain Laboratory strain of scrapie. PAR2−/− mice demonstrated a delayed onset of clinical symptoms, including weight loss, and demonstrated moderate but highly significant prolongation of survival over WT controls. Concomitantly, no apparent differences in brain pathology, infectivity or features of brain prion protein between deceased WT and PAR2−/− mice were found. Our study suggests that PAR2 deletion modulates dynamics of the disease without gross perturbation of its pathogenesis.

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.

Chronic compression or acute dissociation of dorsal root ganglion induces cAMP-dependent neuronal hyperexcitability through activation of PAR2

Chronic compression (CCD) or dissociation of dorsal root ganglion (DRG) can induce cyclic adenosine monophosphate (cAMP)-dependent DRG neuronal hyperexcitability and behaviorally expressed hyperalgesia. Here, we report that protease-activated receptor 2 (PAR2) activation after CCD or dissociation mediates the increase of cAMP activity and protein kinase A (PKA) and cAMP-dependent hyperexcitability and hyperalgesia in rats. CCD and dissociation, as well as trypsin (a PAR2 activator) treatment, increased level of cAMP concentration, mRNA, and protein expression for PKA subunits PKA-RII and PKA-c and protein expression of PAR2, in addition to producing neuronal hyperexcitability and, in CCD rats, thermal hyperalgesia. The increased expression of PAR2 was colocalized with PKA-c subunit. A PAR2 antagonistic peptide applied before and/or during the treatment, prevented or largely diminished the increased activity of cAMP and PKA, neuronal hyperexcitability, and thermal hyperalgesia. However, posttreatment with the PAR2 antagonistic peptide failed to alter either hyperexcitability or hyperalgesia. In contrast, an adenylyl cyclase inhibitor, SQ22536, administrated after dissociation or CCD, successfully suppressed hyperexcitability and hyperalgesia, in vitro and/or in vivo. Trypsin-induced increase of the intracellular calcium [Ca(2+)](i) was prevented in CCD or dissociation DRG neurons. These alterations were further confirmed by knockdown of PAR2 with siRNA. In addition, trypsin and PAR2 agonistic peptide-induced increase of cAMP was prevented by inhibition of PKC, but not Gαs. These findings suggest that PAR2 activation is critical to induction of nerve injury-induced neuronal hyperexcitability and cAMP-PKA activation. Inhibiting PAR2 activation may be a potential target for preventing/suppressing development of neuropathic pain.

The Molecular Mechanisms that Promote Edema After Intracerebral Hemorrhage

Intracerebral hemorrhage (ICH) is a devastating type of stroke with no effective therapies. Clinical advances in ICH treatment are limited by an incomplete understanding of the molecular mechanisms responsible for secondary injury and poor outcome. Increasing evidence suggests that cerebral edema is a major contributor to secondary injury and poor outcome in ICH. ICH activates specific signaling pathways that promote edema and damage neuronal tissue. By increasing our understanding of these pathways, we may be able to target them pharmaceutically to reduce edema in ICH patients. In this review, we focus on three major signaling pathways that promote edema after ICH: (1) the coagulation cascade and thrombin, (2) the inflammatory response and matrix metalloproteinases, and (3) the complement cascade and hemoglobin toxicity. We will describe the experimental evidence that confirms these pathways promote edema in ICH, discuss potential targets for new therapies, and comment on important directions for future research.

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.

Specific Immunoassay Reveals Increased Serum Trypsinogen 3 in Acute Pancreatitis

BACKGROUND

Trypsinogen 3 is a minor trypsinogen isoform in the pancreas. In contrast with trypsin 1 and 2, trypsin 3 degrades pancreatic secretory trypsin inhibitor, which may lead to an excess of active trypsin and acute pancreatitis (AP). We developed an immunoassay for trypsinogen 3 and studied whether an assay of serum trypsinogen 3 is of clinical utility in the diagnosis of AP.

METHODS

Monoclonal antibodies were generated using recombinant human trypsinogen 3 as the antigen and used to establish a sandwich-type immunoassay. We analyzed serum trypsinogen 3 concentrations in 82 patients with AP and 63 patients with upper abdominal pain (controls). The reference interval was determined using serum samples from 172 apparently healthy individuals.

RESULTS

The measuring range of the trypsinogen 3 assay was 1.0–250 μg/L. Intra- and interassay CVs were &lt;11%, and cross-reactivity with other trypsinogen isoenzymes was &lt;0.1%. The median trypsinogen 3 concentration in serum from healthy individuals was &lt;1.0 μg/L, and the upper reference limit was 4.4 μg/L. We observed increased trypsinogen 3 concentrations in patients with mild (median 9.5 μg/L) and severe (15.0 μg/L) AP; in both groups, the concentrations were significantly higher than in controls (median &lt;1.0 μg/L) (P &lt; 0.0001). In ROC analysis, the area under the curve of trypsinogen 3 for separation between AP and controls was 0.90 (P &lt; 0.0001).

CONCLUSIONS

We established for the first time a specific immunoassay for trypsinogen 3 using monoclonal antibodies. Patients with AP were found to have increased serum concentrations of trypsinogen 3. The availability of this assay will be useful for studies of the clinical utility of trypsinogen 3.

Low concentrations of methamphetamine can protect dopaminergic cells against a larger oxidative stress injury: mechanistic study

Mild stress can protect against a larger insult, a phenomenon termed preconditioning or tolerance. To determine if a low intensity stressor could also protect cells against intense oxidative stress in a model of dopamine deficiency associated with Parkinson disease, we used methamphetamine to provide a mild, preconditioning stress, 6-hydroxydopamine (6-OHDA) as a source of potentially toxic oxidative stress, and MN9D cells as a model of dopamine neurons. We observed that prior exposure to subtoxic concentrations of methamphetamine protected these cells against 6-OHDA toxicity, whereas higher concentrations of methamphetamine exacerbated it. The protection by methamphetamine was accompanied by decreased uptake of both [(3)H] dopamine and 6-OHDA into the cells, which may have accounted for some of the apparent protection. However, a number of other effects of methamphetamine exposure suggest that the drug also affected basic cellular survival mechanisms. First, although methamphetamine preconditioning decreased basal pERK1/2 and pAkt levels, it enhanced the 6-OHDA-induced increase in these phosphokinases. Second, the apparent increase in pERK1/2 activity was accompanied by increased pMEK1/2 levels and decreased activity of protein phosphatase 2. Third, methamphetamine upregulated the pro-survival protein Bcl-2. Our results suggest that exposure to low concentrations of methamphetamine cause a number of changes in dopamine cells, some of which result in a decrease in their vulnerability to subsequent oxidative stress. These observations may provide insights into the development of new therapies for prevention or treatment of PD.

Systemic administration of argatroban inhibits protease-activated receptor-1 expression in perihematomal tissue in rats with intracerebral hemorrhage

The present study investigated the role of thrombin in the expression of protease-activated receptor-1 (PAR-1), and the effect of argatroban (Arg) a direct thrombin inhibitor, on PAR-1 expression in perihematomal tissue with intracerebral hemorrhage (ICH). For these experiments 90 rats were divided into 5 groups: sham, ICH, argatroban-treated ICH (ICH+Arg), thrombin (TM) and argatroban-treated thrombin (TM+Arg). The ICH model or thrombin injection models were established by injecting autologous blood or thrombin, respectively. Rats in TM+Arg and ICH+Arg groups were administered argatroban (0.9mg/kg) after models were established for 3h and 12h, intraperitoneally. All rats were killed to harvest brains after models were established for 24h. The levels of PAR-1 protein and PAR-1 mRNA expression were detected by Western blot and RT-PCR, respectively. Brain water content was also measured. Our results showed that the levels of PAR-1 protein or PAR-1 mRNA in ICH and TM groups were up-regulated compared to that observed for the sham group; while the levels observed in ICH+Arg group and TM+Arg group were significantly lower than that observed for the ICH group and TM group (P<0.01 or P<0.05). The intraperitoneal administration argatroban also significantly reduced edema in ICH or TM group (P<0.05). Our observations suggested that the production of thrombin following ICH play a key role in the up-regulation of PAR-1 and anti-PAR-1 by systemic administration of argatroban, and may be a potential strategy for ICH therapy.

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.

Serine proteases, serine protease inhibitors, and protease-activated receptors: roles in synaptic function and behavior

Serine proteases, serine protease inhibitors, and protease-activated receptors have been intensively investigated in the periphery and their roles in a wide range of processes-coagulation, inflammation, and digestion, for example-have been well characterized (see Coughlin, 2000; Macfarlane et al., 2001; Molinari et al., 2003; Wang et al., 2008; Di Cera, 2009 for reviews). A growing number of studies demonstrate that these protein systems are widely expressed in many cell types and regions in mammalian brains. Accumulating lines of evidence suggest that the brain has co-opted the activities of these interesting proteins to regulate various processes underlying synaptic activity and behavior. In this review, we discuss emerging roles for serine proteases in the regulation of mechanisms underlying synaptic plasticity and memory formation.

Neuronal protease-activated receptor 1 drives synaptic retrograde signaling mediated by the endocannabinoid 2-arachidonoylglycerol

Protease-activated receptor 1 (PAR1) is a member of the G-protein coupled receptors that are proteolytically activated by serine proteases. Recent studies suggest a definite contribution of PAR1 to brain functions, including learning and memory. However, cellular mechanisms by which PAR1 activation influences neuronal activity are not well understood. Here we show that PAR1 activation drives retrograde endocannabinoid signaling and thereby regulates synaptic transmission. In cultured hippocampal neurons from rat, PAR1 activation by thrombin or PAR1-specific peptide agonists transiently suppressed inhibitory transmission at cannabinoid-sensitive, but not cannabinoid-insensitive, synapses. The PAR1-induced suppression of synaptic transmission was accompanied by an increase in paired-pulse ratio, and was blocked by a cannabinoid CB(1) receptor antagonist. The PAR1-induced suppression was blocked by pharmacological inhibition of postsynaptic diacylglycerol lipase (DGL), a key enzyme for biosynthesis of the major endocannabinoid 2-arachidonoylglycerol (2-AG), and was absent in knock-out mice lacking the α isoform of DGL. The PAR1-induced IPSC suppression remained intact under the blockade of metabotropic glutamate receptors and was largely resistant to the treatment that blocked Ca(2+) elevation in glial cells following PAR1 activation, which excludes the major contribution of glial PAR1 in IPSC suppression. We conclude that activation of neuronal PAR1 triggers retrograde signaling mediated by 2-AG, which activates presynaptic CB(1) receptors and suppresses transmitter release at hippocampal inhibitory synapses.

Binding of alkaloids into the S1 specificity pocket of α-chymotrypsin: evidence from induced circular dichroism spectra

Non-covalent binding of planar aromatic molecules into the S1 specificity pocket of the serine protease α-chymotrypsin (αCHT) can be detected by measuring induced circular dichroism (CD) spectroscopic signals. Utilizing this phenomenon, αCHT association of proflavine (PRF), the well known serine protease inhibitor has been investigated together with plant-derived compounds including isoquinoline, pyridocarbazole and indoloquinoline alkaloids, of which αCHT binding has never been reported. Non-degenerate exciton coupling between π-π* transitions of the ligand molecules and two tryptophan residues (Trp172 and Trp215) near to the binding site is proposed to be responsible for the induced CD activity. The association constants calculated from CD titration data indicated strong αCHT association of sanguninarine, ellipticine, desmethyl-isocryptolepine and isoneocryptolepine (K(a) ≈ 10(5) M(-1)) while berberine, coptisine and chelerythrine bind to the enzyme with lower, PRF-like affinity (K(a) ≈ 10(4) M(-1)). PRF-trypsin and ellipticine-trypsin binding interactions have also been demonstrated. The binding of the alkaloids into the S1 pocket of αCHT has been confirmed by CD competition experiments. Molecular docking calculations showed the inclusion of PRF as well as the alkaloid molecules in the S1 cavity where they are stabilized by hydrophobic and H-bonding interactions. These novel nonpeptidic scaffolds can be used for developing selective inhibitors of serine proteases having chymotrypsin-like folds. Furthermore, the results provide a novel, CD spectroscopic based approach for probing the ligand binding of αCHT and related proteases.

Neurovascular dysfunction, inflammation and endothelial activation: implications for the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is an age-related disorder characterized by progressive cognitive decline and dementia. Alzheimer's disease is an increasingly prevalent disease with 5.3 million people in the United States currently affected. This number is a 10 percent increase from previous estimates and is projected to sharply increase to 8 million by 2030; it is the sixth-leading cause of death. In the United States the direct and indirect costs of Alzheimer's and other dementias to Medicare, Medicaid and businesses amount to more than $172 billion each year. Despite intense research efforts, effective disease-modifying therapies for this devastating disease remain elusive. At present, the few agents that are FDA-approved for the treatment of AD have demonstrated only modest effects in modifying clinical symptoms for relatively short periods and none has shown a clear effect on disease progression. New therapeutic approaches are desperately needed. Although the idea that vascular defects are present in AD and may be important in disease pathogenesis was suggested over 25 years ago, little work has focused on an active role for cerebrovascular mechanisms in the pathogenesis of AD. Nevertheless, increasing literature supports a vascular-neuronal axis in AD as shared risk factors for both AD and atherosclerotic cardiovascular disease implicate vascular mechanisms in the development and/or progression of AD. Also, chronic inflammation is closely associated with cardiovascular disease, as well as a broad spectrum of neurodegenerative diseases of aging including AD. In this review we summarize data regarding, cardiovascular risk factors and vascular abnormalities, neuro- and vascular-inflammation, and brain endothelial dysfunction in AD. We conclude that the endothelial interface, a highly synthetic bioreactor that produces a large number of soluble factors, is functionally altered in AD and contributes to a noxious CNS milieu by releasing inflammatory and neurotoxic species.

Increased prothrombin, apolipoprotein A-IV, and haptoglobin in the cerebrospinal fluid of patients with Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disease caused by an unstable CAG trinucleotide repeat expansion. The need for biomarkers of onset and progression in HD is imperative, since currently reliable outcome measures are lacking. We used two-dimensional electrophoresis and mass spectrometry to analyze the proteome profiles in cerebrospinal fluid (CSF) of 6 pairs of HD patients and controls. Prothrombin, apolipoprotein A-IV (Apo A-IV) and haptoglobin were elevated in CSF of the HD patients in comparison with the controls. We used western blot as a semi-quantified measurement for prothrombin and Apo A-IV, as well as enzyme linked immunosorbent assay (ELISA) for measurement of haptoglobin, in 9 HD patients and 9 controls. The albumin quotient (Qalb), a marker of blood-brain barrier (BBB) function, was not different between the HD patients and the controls. The ratios of CSF prothrombin/albumin (prothrombin/Alb) and Apo A-IV/albumin (Apo A-IV/Alb), and haptoglobin level were significantly elevated in HD. The ratio of CSF prothrombin/Alb significantly correlated with the disease severity assessed by Unified Huntington's Disease Rating Scale (UHDRS). The results implicate that increased CSF prothrombin, Apo A-IV, and haptoglobin may be involved in pathogenesis of HD and may serve as potential biomarkers for HD.