Persistent protease-activated receptor 4 signaling mediates thrombin-induced microglial activation

Elevated protease-activated receptor 4 (PAR4) gene expression in Alzheimer's disease predicts cognitive decline

Platelet activation of protease-activated receptor 4 (PAR4) and thrombin are at the top of a chain of events leading to fibrin deposition, microinfarcts, blood-brain barrier disruption, and inflammation. We evaluated mRNA expression of the PAR4 gene F2RL3 in human brain and global cognitive performance in participants with and without cognitive impairment or dementia. Data were acquired from the Religious Orders Study (ROS) and the Rush Memory and Aging Project (MAP). F2RL3 mRNA was elevated in AD cases and was associated with worse retrospective longitudinal cognitive performance. Moreover, F2RL3 expression interacted with clinical AD diagnosis on longitudinal cognition whereas this relationship was attenuated in individuals without cognitive impairment. Additionally, when adjusting for the effects of AD neuropathology, F2RL3 expression remained a significant predictor of cognitive decline. F2RL3 expression correlated positively with transcript levels of proinflammatory markers including TNFα, IL-1β, NFκB, and fibrinogen α/β/γ. Together, these results reveal that F2RL3 mRNA expression is associated with multiple AD-relevant outcomes and its encoded product, PAR4, may play a role in disease pathogenesis.

Receptors on Microglia

Microglial cells are the most receptive cells in the central nervous system (CNS), expressing several classes of receptors reflecting their immune heritage and newly acquired neural specialisation. Microglia possess, depending on the particular context, receptors to neurotransmitters and neuromodulators as well as immunocompetent receptors. This rich complement allows microglial cells to monitor the functional status of the nervous system, contribute actively to the regulation of neural activity and plasticity and homeostasis, and guard against pathogens as well as other challenges to the CNS's integrity and function.

Low-intensity pulsed ultrasound ameliorates glia-mediated inflammation and neuronal damage in experimental intracerebral hemorrhage conditions

BACKGROUND

Intracerebral hemorrhage (ICH) is a condition associated with high morbidity and mortality, and glia-mediated inflammation is a major contributor to neurological deficits. However, there is currently no proven effective treatment for clinical ICH. Recently, low-intensity pulsed ultrasound (LIPUS), a non-invasive method, has shown potential for neuroprotection in neurodegenerative diseases. This study aimed to investigate the neuroprotective effects and potential mechanisms of LIPUS on glia-mediated inflammation in ICH.

METHODS

This study used 289 mice to investigate the effects of LIPUS on ICH. ICH was induced by injecting bacterial collagenase (type VII-S; 0.0375 U) into the striatum of the mice. LIPUS was applied noninvasively for 3 days, including a 2-h-delayed intervention to mimic clinical usage. The study evaluated neurological function, histology, brain water content, hemoglobin content, MRI, and protein expression of neurotrophic factors, inflammatory molecules, and apoptosis. In vitro studies investigated glia-mediated inflammation by adding thrombin (10 U/mL) or conditioned media to primary and cell line cultures. The PI3K inhibitor LY294002 was used to confirm the effects of PI3K/Akt signaling after LIPUS treatment.

RESULTS

LIPUS treatment improved neurological deficits and reduced tissue loss, edema, and neurodegeneration after ICH. The protective effects of LIPUS resulted from decreased glia-mediated inflammation by inhibiting PI3K/Akt-NF-κB signaling, which reduced cytokine expression and attenuated microglial activation-induced neuronal damage in vitro.

CONCLUSIONS

LIPUS treatment improved neurological outcomes and reduced glia-mediated inflammation by inhibiting PI3K/Akt-NF-κB signaling after ICH. LIPUS may provide a non-invasive potential management strategy for ICH.

Intracerebral hemorrhage and thrombin-induced alterations in cerebral microvessel matrix

Four phase III clinical trials of oral direct factor Xa or thrombin inhibitors demonstrated significantly lower intracranial hemorrhage compared to warfarin in patients with nonvalvular-atrial fibrillation. This is counter-intuitive to the principle that inhibiting thrombosis should increase hemorrhagic risk. We tested the novel hypothesis that anti-thrombin activity decreases the risk of intracerebral hemorrhage by directly inhibiting thrombin-mediated degradation of cerebral microvessel basal lamina matrix, responsible for preventing hemorrhage. Collagen IV, laminin, and perlecan each contain one or more copies of the unique α-thrombin cleavage site consensus sequence. In blinded controlled experiments, α-thrombin significantly degraded each matrix protein in vitro and in vivo in a concentration-dependent fashion. In vivo stereotaxic injection of α-thrombin significantly increased permeability, local IgG extravasation, and hemoglobin (Hgb) deposition together with microvessel matrix degradation in a mouse model. In all formats the direct anti-thrombin dabigatran completely inhibited matrix degradation by α-thrombin. Fourteen-day oral exposure to dabigatran etexilate-containing chow completely inhibited matrix degradation, the permeability to large molecules, and cerebral hemorrhage associated with α-thrombin. These experiments demonstrate that thrombin can degrade microvessel matrix, leading to hemorrhage, and that inhibition of microvessel matrix degradation by α-thrombin decreases cerebral hemorrhage. Implications for focal ischemia and other conditions are discussed.

Are apolipoprotein E fragments a promising new therapeutic target for Alzheimer’s disease?

Human apolipoprotein E (ApoE) is a 299-amino acid secreted glycoprotein that binds cholesterol and phospholipids. ApoE exists as three common isoforms (ApoE2, ApoE3, and ApoE4) and heterozygous carriers of the ε4 allele of the gene encoding ApoE (APOE) have a fourfold greater risk of developing Alzheimer’s disease (AD). The enzymes thrombin, cathepsin D, α-chymotrypsin-like serine protease, and high-temperature requirement serine protease A1 are responsible for ApoE proteolytic processing resulting in bioactive C-terminal-truncated fragments that vary depending on ApoE isoforms, brain region, aging, and neural injury. The objectives of the present narrative review were to describe ApoE processing, discussing current hypotheses about the potential role of various ApoE fragments in AD pathophysiology, and reviewing the current development status of different anti-ApoE drugs. The exact mechanism by which APOE gene variants increase/decrease AD risk and the role of ApoE fragments in the deposition are not fully understood, but APOE is known to directly affect tau-mediated neurodegeneration. ApoE fragments co-localize with neurofibrillary tangles and amyloid β (Aβ) plaques, and may cause neurodegeneration. Among anti-ApoE approaches, a fascinating strategy may be to therapeutically overexpress ApoE2 in APOE ε4/ε4 carriers through vector administration or liposomal delivery systems. Another approach involves reducing ApoE4 expression by intracerebroventricular antisense oligonucleotides that significantly decreased Aβ pathology in transgenic mice. Differences in the proteolytic processing of distinct ApoE isoforms and the use of ApoE fragments as mimetic peptides in AD treatment are also under investigation. Treatment with peptides that mimic the structural and biological properties of native ApoE may reduce Aβ deposition, tau hyperphosphorylation, and glial activation in mouse models of Aβ pathology. Alternative strategies involve the use of ApoE4 structure correctors, passive immunization to target a certain form of ApoE, conversion of the ApoE4 aminoacid sequence into that of ApoE3 or ApoE2, and inhibition of the ApoE-Aβ interaction.

Discovery and Optimization of a Novel Series of Competitive and Central Nervous System-Penetrant Protease-Activated Receptor 4 (PAR4) Inhibitors

The detailed pharmacology and therapeutic potential of the central PAR4 receptors are poorly understood due to a lack of potent, selective, and brain-penetrant tool compounds. Despite this, robust data with biochemical and genetic tools show the therapeutic potential of PAR4 antagonists in traumatic brain injury, Alzheimer's disease, Parkinson's disease, and other neurodegenerative disorders with a neuroinflammatory component. Thus, we performed a functional HTS campaign, identified a fundamentally new PAR4 competitive inhibitor chemotype, optimized this new series (increased potency >45-fold), discovered enantiospecific activity (though opposing preference for human versus mouse PAR4), and engendered high central nervous system penetration (rat K_p's of 0.52 to 4.2 and K_p,uu's of 0.52 to 1.2).

Treatment with Edoxaban Attenuates Acute Stroke Severity in Mice by Reducing Blood-Brain Barrier Damage and Inflammation

Patients with atrial fibrillation and previous ischemic stroke (IS) are at increased risk of cerebrovascular events despite anticoagulation. In these patients, treatment with non-vitamin K oral anticoagulants (NOAC) such as edoxaban reduced the probability and severity of further IS without increasing the risk of major bleeding. However, the detailed protective mechanism of edoxaban has not yet been investigated in a model of ischemia/reperfusion injury. Therefore, in the current study we aimed to assess in a clinically relevant setting whether treatment with edoxaban attenuates stroke severity, and whether edoxaban has an impact on the local cerebral inflammatory response and blood–brain barrier (BBB) function after experimental IS in mice. Focal cerebral ischemia was induced by transient middle cerebral artery occlusion in male mice receiving edoxaban, phenprocoumon or vehicle. Infarct volumes, functional outcome and the occurrence of intracerebral hemorrhage were assessed. BBB damage and the extent of local inflammatory response were determined. Treatment with edoxaban significantly reduced infarct volumes and improved neurological outcome and BBB function on day 1 and attenuated brain tissue inflammation. In summary, our study provides evidence that edoxaban might exert its protective effect in human IS by modulating different key steps of IS pathophysiology, but further studies are warranted.

Promising drug repurposing approach targeted for cytokine storm implicated in SARS-CoV-2 complications

A global threat has emerged in 2019 due to the rapid spread of Coronavirus disease (COVID-19). As of January 2021, the number of cases worldwide reached 103 million cases and 2.22 million deaths which were confirmed as the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This global pandemic galvanized the scientific community to study the causative virus (SARS-CoV2) pathogenesis, transmission, and clinical symptoms. Remarkably, the most common complication associated with this disease is the cytokine storm which is responsible for COVID-19 mortality. Thus, targeting the cytokine storm with new medications is needed to hamper COVID-19 complications where the most prominent strategy for the treatment is drug repurposing. Through this strategy, several steps are skipped especially those required for testing drug safety and thus may help in reducing the dissemination of this pandemic. Accordingly, the aim of this review is to outline the pathogenesis, clinical features, and immune complications of SARS-CoV2 in addition to suggesting several repurposed drugs with their plausible mechanism of action for possible management of severe COVID-19 cases.

Role of Thrombin in Central Nervous System Injury and Disease

Thrombin is a Na+-activated allosteric serine protease of the chymotrypsin family involved in coagulation, inflammation, cell protection, and apoptosis. Increasingly, the role of thrombin in the brain has been explored. Low concentrations of thrombin are neuroprotective, while high concentrations exert pathological effects. However, greater attention regarding the involvement of thrombin in normal and pathological processes in the central nervous system is warranted. In this review, we explore the mechanisms of thrombin action, localization, and functions in the central nervous system and describe the involvement of thrombin in stroke and intracerebral hemorrhage, neurodegenerative diseases, epilepsy, traumatic brain injury, and primary central nervous system tumors. We aim to comprehensively characterize the role of thrombin in neurological disease and injury.

Thrombin generation and activity in multiple sclerosis

The coagulation cascade and immune system are intricately linked, highly regulated and respond cooperatively in response to injury and infection. Increasingly, evidence of hyper-coagulation has been associated with autoimmune disorders, including multiple sclerosis (MS). The pathophysiology of MS includes immune cell activation and recruitment to the central nervous system (CNS) where they degrade myelin sheaths, leaving neuronal axons exposed to damaging inflammatory mediators. Breakdown of the blood-brain barrier (BBB) facilitates the entry of peripheral immune cells. Evidence of thrombin activity has been identified within the CNS of MS patients and studies using animal models of experimental autoimmune encephalomyelitis (EAE), suggest increased thrombin generation and activity may play a role in the pathogenesis of MS as well as inhibit remyelination processes. Thrombin is a serine protease capable of cleaving multiple substrates, including protease activated receptors (PARs), fibrinogen, and protein C. Cleavage of all three of these substrates represent pathways through which thrombin activity may exert immuno-regulatory effects and regulate permeability of the BBB during MS and EAE. In this review, we summarize evidence that thrombin activity directly, through PARs, and indirectly, through fibrin formation and activation of protein C influences neuro-immune responses associated with MS and EAE pathology.

Involvement of Kallikrein-Related Peptidases in Nervous System Disorders

Kallikrein-related peptidases (KLKs) are a family of serine proteases that when dysregulated may contribute to neuroinflammation and neurodegeneration. In the present review article, we describe what is known about their physiological and pathological roles with an emphasis on KLK6 and KLK8, two KLKs that are highly expressed in the adult central nervous system (CNS). Altered expression and activity of KLK6 have been linked to brain physiology and the development of multiple sclerosis. On the other hand, altered levels of KLK6 in the brain and serum of people affected by Alzheimer's disease and Parkinson's disease have been documented, pointing out to its function in amyloid metabolism and development of synucleinopathies. People who have structural genetic variants of KLK8 can suffer mental illnesses such as intellectual and learning disabilities, seizures, and autism. Increased expression of KLK8 has also been implicated in schizophrenia, bipolar disorder, and depression. Also, we discuss the possible link that exists between KLKs activity and certain viral infections that can affect the nervous system. Although little is known about the exact mechanisms that mediate KLKs function and their participation in neuroinflammatory and neurodegenerative disorders will open a new field to develop novel therapies to modulate their levels and/or activity and their harmful effects on the CNS.

Microglia and macrophage phenotypes in intracerebral haemorrhage injury: therapeutic opportunities

The prognosis of intracerebral haemorrhage continues to be devastating despite much research into this condition. A prominent feature of intracerebral haemorrhage is neuroinflammation, particularly the excessive representation of pro-inflammatory CNS-intrinsic microglia and monocyte-derived macrophages that infiltrate from the circulation. The pro-inflammatory microglia/macrophages produce injury-enhancing factors, including inflammatory cytokines, matrix metalloproteinases and reactive oxygen species. Conversely, the regulatory microglia/macrophages with potential reparative and anti-inflammatory roles are outcompeted in the early stages after intracerebral haemorrhage, and their beneficial roles appear to be overwhelmed by pro-inflammatory microglia/macrophages. In this review, we describe the activation of microglia/macrophages following intracerebral haemorrhage in animal models and clinical subjects, and consider their multiple mechanisms of cellular injury after haemorrhage. We review strategies and medications aimed at suppressing the pro-inflammatory activities of microglia/macrophages, and those directed at elevating the regulatory properties of these myeloid cells after intracerebral haemorrhage. We consider the translational potential of these medications from preclinical models to clinical use after intracerebral haemorrhage injury, and suggest that several approaches still lack the experimental support necessary for use in humans. Nonetheless, the preclinical data support the use of deactivator or inhibitor of pro-inflammatory microglia/macrophages, whilst enhancing the regulatory phenotype, as part of the therapeutic approach to improve the prognosis of intracerebral haemorrhage.

Destabilization of β-amyloid aggregates by thrombin derived peptide: plausible role of thrombin in neuroprotection

β-amyloid (Aβ) aggregates involved in Alzheimer's disease (AD) are resistant to proteases but could be destabilized by small peptides designed to target specific hydrophobic regions of Aβ that take part in aggregate assembly. Since thrombin and AD are intricately connected, and elastase modulates thrombin activity, elastase-digested thrombin peptides were verified for intervention in the Aβ-aggregation pathway. Intact or elastase-digested thrombin destabilized Aβ fibril, as demonstrated by thioflavin T assay. Peptides were synthesized employing thrombin as a template, of which, a hexapeptide (T3) showed maximum destabilization at 1 µm. ExPASy peptide cutter software coupled with mass spectrometric analysis confirmed the generation of T3 peptide from elastase-digested thrombin. TEM micrographs revealed that 30-day incubation of preformed Aβ fibrils or monomers with T3 resulted in destabilization or inhibition, respectively, leading mostly to particles of 1.74 ± 0.17 nm, which roughly corresponded to Aβ monomer. Surface plasmon resonance employing CM5 chip coupled with Aβ40 mouse monoclonal antibody showed a drop in response when T3 was incubated with Aβ fibrils between 2 and 8 h. 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide and confocal microscopy demonstrated the ability of T3 to rescue neuroblastoma cells from Aβ oligomer-induced cytotoxic damage. Although no [Aβ-T3] adduct could be detected by mass spectrometry, an initial interaction appeared to facilitate the process of destabilization/inhibition of aggregation. T3 was comparable to standard β-sheet breaker peptides, LPFFD and KLVFF in terms of Aβ aggregate destabilization. High hydrophobicity values coupled with recognition and breaking elements make T3 a potential candidate for future therapeutic applications.

Using PAR4 Inhibition as an Anti-Thrombotic Approach: Why, How, and When?

Protease-activated receptors (PARs) are a family of four GPCRs with a variety of cellular functions, yet the only advanced clinical endeavours to target these receptors for therapeutic gain to date relates to the impairment of platelet function for anti-thrombotic therapy. The only approved PAR antagonist is the PAR1 inhibitor, vorapaxar—the sole anti-platelet drug against a new target approved in the past 20 years. However, there are two PARs on human platelets, PAR1 and PAR4, and more recent efforts have focused on the development of the first PAR4 antagonists, with first-in-class agents recently beginning clinical trial. Here, we review the rationale for this approach, outline the various modes of PAR4 inhibition, and speculate on the specific therapeutic potential of targeting PAR4 for the prevention of thrombotic conditions.

Reactive microglia and astrocytes in neonatal intraventricular hemorrhage model are blocked by mesenchymal stem cells

Severe intraventricular hemorrhage (IVH) in premature infants triggers reactive gliosis, causing acute neuronal death and glial scar formation. Transplantation of mesenchymal stem cells (MSCs) has often showed improved CNS recovery in an IVH model, but whether this response is related to reactive glial cells is still unclear. Herein, we suggest that MSCs impede the response of reactive microglia rather than astrocytes, thereby blocking neuronal damage. Astrocytes alone showed mild reactiveness under hemorrhagic conditions mimicked by thrombin treatment, and this was not blocked by MSC-conditioned medium (MSC-CM) in vitro. In contrast, thrombin-induced microglial activation and release of proinflammatory cytokines were inhibited by MSC-CM. Interestingly, astrocytes showed greater reactive response when co-cultured with microglia, and this was abolished in the presence of MSC-CM. Gene expression profiles in microglia revealed that transcript levels of genes for immune response and proinflammatory cytokines were altered by thrombin treatment. This result coincided with the robust phosphorylation of STAT1 and p38 MAPK, which might be responsible for the production and release of proinflammatory cytokines. Furthermore, application of MSC-CM diminished thrombin-mediated phosphorylation of STAT1 and p38 MAPK, supporting the acute anti-inflammatory role of MSCs under hemorrhagic conditions. In line with this, activation of microglia and consequent cytokine release were impaired in Stat1-null mice. However, reactive response in Stat1-deficient astrocytes was maintained. Taken together, our results demonstrate that MSCs mainly block the activation of microglia involving STAT1-mediated cytokine release and subsequent reduction of reactive astrocytes.

Protease-activated receptor 4 protects mice from Coxsackievirus B3 and H1N1 influenza A virus infection

PAR4 is expressed by a variety of cells, including platelets, cardiac, lung and immune cells. We investigated the contribution of PAR4 to viral infections of the heart and lung. Toll-like receptor (TLR) 3-dependent immune responses were analyzed after co-stimulation of PAR4 in murine bone-marrow derived macrophages, embryonic fibroblasts and embryonic cardiomyocytes. In addition, we analyzed Coxsackievirus B3 (CVB3) or H1N1 influenza A virus (H1N1 IAV) infection of PAR4^-/- (ΔPAR4) and wild-type (WT) mice. Lastly, we investigated the effect of platelet inhibition on H1N1 IAV infection. In vitro experiments revealed that PAR4 stimulation enhances the expression of TLR3-dependent CXCL10 expression and decreases TLR3-dependent NFκB-mediated proinflammatory gene expression. Furthermore, CVB3-infected ΔPAR4 mice exhibited a decreased anti-viral response and increased viral genomes in the heart leading to more pronounced CVB3 myocarditis compared to WT mice. Similarly, H1N1 IAV-infected ΔPAR4 mice had increased immune cell numbers and inflammatory mediators in the lung, and increased mortality compared with infected WT controls. The study showed that PAR4 protects mice from viral infections of the heart and lung.

Effect of thrombin injection on cerebral vascular in rats with subarachnoid hemorrhage

Objective

To evaluate the effect of thrombin (TM) injection via the cerebellomedullary cistern on cerebral vessels in rats with subarachnoid hemorrhage (SAH).

Methods

Eighteen rats were randomly divided into three groups. In the A1 group, physiological saline was injected via the cerebellomedullary cistern; in the A2 group, 3 U of TM was injected into the subarachnoid space; and in the A3 group, SAH models were established and 3 U of TM was injected with the first injection of whole blood. Three days later, basilar artery specimens were collected for pathological examination.

Results

The basilar arterial lumen cross-sectional area was significantly smaller in the A2 versus the A1 group, and proteinase-activated receptor (PAR)-1 and tumor necrosis factor (TNF)-α average optical densities were significantly higher (all P < 0.05). Basilar arterial lumen cross-sectional areas were significantly smaller in the A3 than the A2 group and average TNF-α optical densities were significantly lower (both P < 0.05), while those of PAR-1 did not differ significantly.

Conclusions

There was no significant difference in the extent of cerebral vasospasm between SAH and non-SAH model groups following TM injection into the subarachnoid space, so TM was considered to be an independent factor affecting cerebral vasospasm.

Epithelial cell adhesion efficacy of a novel peptide identified by panning on a smooth titanium surface

Epithelial attachment via the basal lamina on the tooth surface provides an important structural defence mechanism against bacterial invasion in combating periodontal disease. However, when considering dental implants, strong epithelial attachment does not exist throughout the titanium-soft tissue interface, making soft tissues more susceptible to peri-implant disease. This study introduced a novel synthetic peptide (A10) to enhance epithelial attachment. A10 was identified from a bacterial peptide display library and synthesized. A10 and protease-activated receptor 4-activating peptide (PAR4-AP, positive control) were immobilized on commercially pure titanium. The peptide-treated titanium showed high epithelial cell migration ability during incubation in platelet-rich plasma. We confirmed the development of dense and expanded BL (stained by Ln5) with pericellular junctions (stained by ZO1) on the peptide-treated titanium surface. In an adhesion assay of epithelial cells on A10-treated titanium, PAR4-AP-treated titanium, bovine root and non-treated titanium, A10-treated titanium and PAR4-AP-treated titanium showed significantly stronger adhesion than non-treated titanium. PAR4-AP-treated titanium showed significantly higher inflammatory cytokine release than non-treated titanium. There was no significant difference in inflammatory cytokine release between A10-treated and non-treated titanium. These results indicated that A10 could induce the adhesion and migration of epithelial cells with low inflammatory cytokine release. This novel peptide has a potentially useful application that could improve clinical outcomes with titanium implants and abutments by reducing or preventing peri-implant disease.

Pretreatment with rivaroxaban attenuates stroke severity in rats by a dual antithrombotic and anti-inflammatory mechanism

Stroke outcome is more favourable in patients receiving oral anticoagulants compared with non-anticoagulated patients. The reasons for this “stroke-attenuating” property of oral anticoagulants are largely unknown. This study examined whether prestroke anticoagulation with rivaroxaban, a novel direct factor Xa inhibitor, influences stroke severity, thrombin-mediated intracerebral thrombus formation and pro-inflammatory processes in a rat model of brain ischaemia/reperfusion injury. Male Wistar rats were anticoagulated with rivaroxaban and subjected to 90 minutes of transient middle cerebral artery occlusion. Infarct size, functional outcome and the occurrence of intracranial haemorrhage (ICH) were assessed until day 7. Thrombin generation was determined by measuring the amount of thrombin/antithrombin complex. Intracerebral thrombus formation was evaluated by histology and Western blot. CD68-immunoreactivity and the expression of cytokines and adhesion molecules were investigated to assess postischaemic inflammation. The integrity of the blood–brain barrier was analysed using fluorescein isothiocyanate-dextran. Rats pretreated with rivaroxaban developed significantly smaller strokes and less severe functional deficits compared with controls. Although rivaroxaban strongly reduced thrombin-mediated thrombus formation, this was not accompanied by an increased risk of ICH. In addition, rivaroxaban dampened the inflammatory response in the ischaemic brain by downregulating ICAM-1 expression and the activation of CD68+-immune cells. In contrast, rivaroxaban had no effect on the integrity of the blood–brain barrier after stroke. Here, we identified reduced thrombo-inflammation as a major determinant of the stroke-protective property of rivaroxaban in rats. Further studies are needed to assess the therapeutic potential of novel oral anticoagulants in the acute phase after a stroke.

Proximate Mediators of Microvascular Dysfunction at the Blood-Brain Barrier: Neuroinflammatory Pathways to Neurodegeneration

Current projections are that by 2050 the numbers of people aged 65 and older with Alzheimer's disease (AD) in the US may increase threefold while dementia is projected to double every 20 years reaching ~115 million by 2050. AD is clinically characterized by progressive dementia and neuropathologically by neuronal and synapse loss, accumulation of amyloid plaques, and neurofibrillary tangles (NFTs) in specific brain regions. The preclinical or presymptomatic stage of AD-related brain changes may begin over 20 years before symptoms occur, making development of noninvasive biomarkers essential. Distinct from neuroimaging and cerebrospinal fluid biomarkers, plasma or serum biomarkers can be analyzed to assess (i) the presence/absence of AD, (ii) the risk of developing AD, (iii) the progression of AD, or (iv) AD response to treatment. No unifying theory fully explains the neurodegenerative brain lesions but neuroinflammation (a lethal stressor for healthy neurons) is universally present. Current consensus is that the earlier the diagnosis, the better the chance to develop treatments that influence disease progression. In this article we provide a detailed review and analysis of the role of the blood-brain barrier (BBB) and damage-associated molecular patterns (DAMPs) as well as coagulation molecules in the onset and progression of these neurodegenerative disorders.

Thrombin-induced, TNFR-dependent miR-181c downregulation promotes MLL1 and NF-κB target gene expression in human microglia

BACKGROUND

Controlling thrombin-driven microglial activation may serve as a therapeutic target for intracerebral hemorrhage (ICH). Here, we investigated microRNA (miRNA)-based regulation of thrombin-driven microglial activation using an in vitro thrombin toxicity model applied to primary human microglia.

METHODS

A miRNA array identified 22 differential miRNA candidates. Quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) identified miR-181c as the most significantly downregulated miRNA. TargetScan analysis identified mixed lineage leukemia-1 (MLL1) as a putative gene target for miR-181c. qRT-PCR was applied to assess tumor necrosis factor-alpha (TNF-α), miR-181c, and MLL1 levels following thrombin or proteinase-activated receptor-4-specific activating peptide (PAR4AP) exposure. Anti-TNF-α antibodies and tumor necrosis factor receptor (TNFR) silencing were employed to test TNF-α/TNFR dependence. A dual-luciferase reporter system and miR-181c mimic transfection assessed whether mir-181c directly binds to and negatively regulates MLL1. Nuclear factor kappa-B (NF-κB)-dependent luciferase reporter assays and NF-κB target gene expression were assessed in wild-type (MLL1+) and MLL1-silenced cells.

RESULTS

Thrombin or PAR4AP-induced miR-181c downregulation (p < 0.05) and MLL1 upregulation (p < 0.05) that were dependent upon TNF-α/TNFR. miR-181c decreased wild-type MLL1 3'-UTR luciferase reporter activity (p < 0.05), and a miR-181c mimic suppressed MLL1 expression (p < 0.05). Thrombin treatment increased, while miR-181c reduced, NF-κB activity and NF-κB target gene expression in both wild-type (MLL1+) and MLL1-silenced cells (p < 0.05).

CONCLUSIONS

Thrombin-induced, TNF-α/TNFR-dependent miR-181c downregulation promotes MLL1 expression, increases NF-κB activity, and upregulates NF-κB target gene expression. As miR-181c opposes thrombin's stimulation of pro-inflammatory NF-κB activity, miR-181c mimic therapy may show promise in controlling thrombin-driven microglial activation following ICH.

Modulatory Role of Nurr1 Activation and Thrombin Inhibition in the Neuroprotective Effects of Dabigatran Etexilate in Rotenone-Induced Parkinson's Disease in Rats

Recently, it has been shown that both decreased nuclear receptor-related 1 (Nurr1) expression and thrombin accumulation are involved in the degeneration of dopaminergic neurons in Parkinson's disease (PD). The new anticoagulant dabigatran etexilate (DE) is a direct thrombin inhibitor that owns benzimidazole group, which has been proposed to activate Nurr1. In the present study, we examined the neuroprotective effects of DE in rotenone model of PD. Rotenone was injected subcutaneously at a dose of 1.5 mg/kg every other day for 21 days. An oral regimen of DE (15 mg/kg) was started after the 5th rotenone injection following the manifestations of PD. Treatment of PD rats with DE mitigated rotenone-induced neuronal degeneration and restored striatal dopamine level with motor recovery. As well, DE enhanced Nurr1 expression in substantia nigra along with increasing transcriptional activation of Nurr1-controlled genes namely tyrosine hydroxylase, vascular monoamine transporter, glial cell line-derived neurotrophic factor, and its receptor gene c-Ret, which are critical for development and maintenance of dopaminergic neurons. DE also suppressed thrombin accumulation in substantia nigra. Both effects probably contributed to repressing neurotoxic proinflammatory cytokines, which was manifested by decreased level of nuclear factor kappa beta and tumor necrosis factor alpha. In conclusion, the present results suggest that DE could possess significant neuroprotective and regenerative effects in a rotenone-induced PD animal model as consequence of Nurr1 activation and thrombin inhibition.

Proinflammatory signaling functions of thrombin in cancer

Thrombin-induced activation of protease-activated receptors (PARs) represents a link between inflammation and cancer. Proinflammatory signaling functions of thrombin are associated with several inflammatory diseases including neurodegenerative, cardiovascular, and of special interest in this review cancer. Thrombin-induced inflammatory responses up-regulates expression of cytokines, adhesion molecules, angiogenic factors, and matrix-degrading proteases that facilitate tumor cells proliferation, angiogenesis, invasion, and metastasis. This review summarizes the current knowledge about the mechanisms of thrombin-mediated proinflammatory responses in cancer pathology for a better understanding and hence a better management of this disease.

Reduction of intracerebral hemorrhage by rivaroxaban after tPA thrombolysis is associated with downregulation of PAR-1 and PAR-2

This study aimed to assess the risk of intracerebral hemorrhage (ICH) after tissue-type plasminogen activator (tPA) treatment in rivaroxaban compared with warfarin-pretreated male Wistar rat brain after ischemia in relation to activation profiles of protease-activated receptor-1, -2, -3, and -4 (PAR-1, -2, -3, and -4). After pretreatment with warfarin (0.2 mg/kg/day), low-dose rivaroxaban (60 mg/kg/day), high-dose rivaroxaban (120 mg/kg/day), or vehicle for 14 days, transient middle cerebral artery occlusion was induced for 90 min, followed by reperfusion with tPA (10 mg/kg/10 ml). Infarct volume, hemorrhagic volume, immunoglobulin G leakage, and blood parameters were examined. Twenty-four hours after reperfusion, immunohistochemistry for PARs was performed in brain sections. ICH volume was increased in the warfarin-pretreated group compared with the rivaroxaban-treated group. PAR-1, -2, -3, and -4 were widely expressed in the normal brain, and their levels were increased in the ischemic brain, especially in the peri-ischemic lesion. Warfarin pretreatment enhanced the expression of PAR-1 and PAR-2 in the peri-ischemic lesion, whereas rivaroxaban pretreatment did not. The present study shows a lower risk of brain hemorrhage in rivaroxaban-pretreated compared with warfarin-pretreated rats following tPA administration to the ischemic brain. It is suggested that the relative downregulation of PAR-1 and PAR-2 by rivaroxaban compared with warfarin pretreatment might be partly involved in the mechanism of reduced hemorrhagic complications in patients receiving rivaroxaban in clinical trials. © 2016 Wiley Periodicals, Inc.

Targeting the thrombin receptor modulates inflammation and astrogliosis to improve recovery after spinal cord injury

The deregulation of serine protease activity is a common feature of neurological injury, but little is known regarding their mechanisms of action or whether they can be targeted to facilitate repair. In this study we demonstrate that the thrombin receptor (Protease Activated Receptor 1, (PAR1)) serves as a critical translator of the spinal cord injury (SCI) proteolytic microenvironment into a cascade of pro-inflammatory events that contribute to astrogliosis and functional decline. PAR1 knockout mice displayed improved locomotor recovery after SCI and reduced signatures of inflammation and astrogliosis, including expression of glial fibrillary acidic protein (GFAP), vimentin, and STAT3 signaling. SCI-associated elevations in pro-inflammatory cytokines such as IL-1β and IL-6 were also reduced in PAR1-/- mice and co-ordinate improvements in tissue sparing and preservation of NeuN-positive ventral horn neurons, and PKCγ corticospinal axons, were observed. PAR1 and its agonist's thrombin and neurosin were expressed by perilesional astrocytes and each agonist increased the production of IL-6 and STAT3 signaling in primary astrocyte cultures in a PAR1-dependent manner. In turn, IL-6-stimulated astrocytes increased expression of PAR1, thrombin, and neurosin, pointing to a model in which PAR1 activation contributes to increased astrogliosis by feedforward- and feedback-signaling dynamics. Collectively, these findings identify the thrombin receptor as a key mediator of inflammation and astrogliosis in the aftermath of SCI that can be targeted to reduce neurodegeneration and improve neurobehavioral recovery.

Role of thrombin in the pathogenesis of central nervous system inflammatory diseases

Thrombin initiates proinflammatory signaling responses through activation of protease-activated receptors (PARs) in in vitro and in vivo systems. Proinflammatory signaling function of thrombin increases secretion of proinflammatory cytokines and chemokines, triggers vascular permeability, promotes leukocyte migration, and induces adhesion molecule expression. Thrombin as a potent signaling molecule is strongly implicated in a number of proinflammatory disorders including severe sepsis, cancer, cardiovascular disease, and of special interest in this review neurodegenerative disorders. This review summarizes the role of thrombin in the pathogenesis of central nervous system (CNS) inflammatory diseases, including Alzheimer's disease (AD) and Parkinson's disease (PD), promoting greater understanding and clinical management of these diseases. J. Cell. Physiol. 232: 482-485, 2017. © 2016 Wiley Periodicals, Inc.

HMGB1 and thrombin mediate the blood-brain barrier dysfunction acting as biomarkers of neuroinflammation and progression to neurodegeneration in Alzheimer's disease

Background

The blood-brain barrier (BBB) dysfunction represents an early feature of Alzheimer’s disease (AD) that precedes the hallmarks of amyloid beta (amyloid β) plaque deposition and neuronal neurofibrillary tangle (NFT) formation. A damaged BBB correlates directly with neuroinflammation involving microglial activation and reactive astrogliosis, which is associated with increased expression and/or release of high-mobility group box protein 1 (HMGB1) and thrombin. However, the link between the presence of these molecules, BBB damage, and progression to neurodegeneration in AD is still elusive. Therefore, we aimed to profile and validate non-invasive clinical biomarkers of BBB dysfunction and neuroinflammation to assess the progression to neurodegeneration in mild cognitive impairment (MCI) and AD patients.

Methods

We determined the serum levels of various proinflammatory damage-associated molecules in aged control subjects and patients with MCI or AD using validated ELISA kits. We then assessed the specific and direct effects of such molecules on BBB integrity in vitro using human primary brain microvascular endothelial cells or a cell line.

Results

We observed a significant increase in serum HMGB1 and soluble receptor for advanced glycation end products (sRAGE) that correlated well with amyloid beta levels in AD patients (vs. control subjects). Interestingly, serum HMGB1 levels were significantly elevated in MCI patients compared to controls or AD patients. In addition, as a marker of BBB damage, soluble thrombomodulin (sTM) antigen, and activity were significantly (and distinctly) increased in MCI and AD patients. Direct in vitro BBB integrity assessment further revealed a significant and concentration-dependent increase in paracellular permeability to dextrans by HMGB1 or α-thrombin, possibly through disruption of zona occludins-1 bands. Pre-treatment with anti-HMGB1 monoclonal antibody blocked HMGB1 effects and leaving BBB integrity intact.

Conclusions

Our current studies indicate that thrombin and HMGB1 are causal proximate proinflammatory mediators of BBB dysfunction, while sTM levels may indicate BBB endothelial damage; HMGB1 and sRAGE might serve as clinical biomarkers for progression and/or therapeutic efficacy along the AD spectrum.

Protease-Activated Receptors in Neuronal Development, Neurodegeneration, and Neuroprotection: Thrombin as Signaling Molecule in the Brain

Protease-activated receptors (PARs) belong to the superfamily of seven transmembrane domain G protein-coupled receptors. Four PAR subtypes are known, PAR-1 to -4. PARs are highly homologous between the species and are expressed in a wide variety of tissues and cell types. Of particular interest is the role which these receptors play in the brain, with regard to neuroprotection or degeneration under pathological conditions. The main agonist of PARs is thrombin, a multifunctional serine protease, known to be present not only in blood plasma but also in the brain. PARs possess an irreversible activation mechanism. Binding of agonist and subsequent cleavage of the extracellular N-terminus of the receptor results in exposure of a so-called tethered ligand domain, which then binds to extracellular loop 2 of the receptor leading to receptor activation. PARs exhibit an extensive expression pattern in both the central and the peripheral nervous system. PARs participate in several mechanisms important for normal cellular functioning and during critical situations involving cellular survival and death. In the last few years, research on Alzheimer’s disease and stroke has linked PARs to the pathophysiology of these neurodegenerative disorders. Actions of thrombin are concentration-dependent, and therefore, depending on cellular function and environment, serve as a double-edged sword. Thrombin can be neuroprotective during stress conditions, whereas under normal conditions high concentrations of thrombin are toxic to cells.

Molecular pathophysiology of cerebral edema

Advancements in molecular biology have led to a greater understanding of the individual proteins responsible for generating cerebral edema. In large part, the study of cerebral edema is the study of maladaptive ion transport. Following acute CNS injury, cells of the neurovascular unit, particularly brain endothelial cells and astrocytes, undergo a program of pre- and post-transcriptional changes in the activity of ion channels and transporters. These changes can result in maladaptive ion transport and the generation of abnormal osmotic forces that, ultimately, manifest as cerebral edema. This review discusses past models and current knowledge regarding the molecular and cellular pathophysiology of cerebral edema.

The Importance of Thrombin in Cerebral Injury and Disease

There is increasing evidence that prothrombin and its active derivative thrombin are expressed locally in the central nervous system. So far, little is known about the physiological and pathophysiological functions exerted by thrombin in the human brain. Extra-hepatic prothrombin expression has been identified in neuronal cells and astrocytes via mRNA measurement. The actual amount of brain derived prothrombin is expected to be 1% or less compared to that in the liver. The role in brain injury depends upon its concentration, as higher amounts cause neuroinflammation and apoptosis, while lower concentrations might even be cytoprotective. Its involvement in numerous diseases like Alzheimer’s, multiple sclerosis, cerebral ischemia and haemorrhage is becoming increasingly clear. This review focuses on elucidation of the cerebral thrombin expression, local generation and its role in injury and disease of the central nervous system.

New insights into protease-activated receptor 4 signaling pathways in the pathogenesis of inflammation and neuropathic pain: a literature review

Pain is an unpleasant sensory and emotional experience that is commonly associated with actual or potential tissue damage. Despite decades of pain research, many patients continue to suffer from chronic pain that is refractory to current treatments. Accumulating evidence has indicated an important role of protease-activated receptor 4 (PAR4) in the pathogenesis of inflammation and neuropathic pain. Here we reviewed PAR4 expression and activation via intracellular signaling pathways and the role of PAR4 signaling pathways in the development and maintenance of pain. Understanding PAR4 and its corresponding signaling pathways will provide insight to further explore the molecular basis of pain, which will also help to identify new targets for pharmacological intervention for pain relief.

Protease-activated receptor 4: a critical participator in inflammatory response

Protease-activated receptors (PARs) are G protein-coupled receptors of which four members PAR1, PAR2, PAR3, and PAR4 have been identified, characterized by a typical mechanism of activation involving various related proteases. The amino-terminal sequence of PARs is cleaved by a broad array of proteases, leading to specific proteolytic cleavage which forms endogenous tethered ligands to induce agonist-biased PAR activation. The biological effect of PARs activated by coagulation proteases to regulate hemostasis and thrombosis plays an enormous role in the cardiovascular system, while PAR4 can also be activated by trypsin, cathepsin G, the activated factor X of the coagulation cascade, and trypsin IV. Irrespective of its role in thrombin-induced platelet aggregation, PAR4 activation is believed to be involved in inflammatory lesions, as show by investigations that have unmasked the effects of PAR4 on neutrophil recruitment, the regulation of edema, and plasma extravasation. This review summarizes the roles of PAR4 in coagulation and other extracellular protease pathways, which activate PAR4 to participate in normal regulation and disease.

Interference with protease-activated receptor 1 does not reduce damage to subventricular zone cells of immature rodent brain following exposure to blood or blood plasma

BACKGROUND

Prior work showed that whole blood, plasma, and serum injections are damaging to the neonatal rodent brain in a model of intracerebral/periventricular hemorrhage. Thrombin alone is also damaging. In adult animal models of hemorrhagic stroke, the protease-activated (thrombin) receptor PAR1 mediates some of the brain damage. We hypothesized that PAR1 interference will reduce the adverse effects of blood products on immature rodent brain and cells.

RESULTS

Cultured oligodendrocyte precursor cells from rats and mice were exposed to blood plasma with and without the PAR1 antagonists SCH-79797 or BMS-200261. In concentrations previously shown to have activity on brain cells, neither drug showed evidence of protection against the toxicity of blood plasma. Newborn mice (wild type, heterozygous, and PAR1 knockout) were subjected to intracerebral injection of autologous whole blood into the periventricular region of the frontal lobe. Cell proliferation, measured by Ki67 immunoreactivity in the subventricular zone, was suppressed at 1 and 2 days, and was not normalized in the knockout mice. Cell apoptosis, measured by activated caspase 3 immunoreactivity, was not apparent in the subventricular zone. Increased apoptosis in periventricular striatal cells was not normalized in the knockout mice.

CONCLUSION

Interference with the thrombin-PAR1 system does not reduce the adverse effects of blood on germinal cells of the immature rodent brain. PAR1 interference is unlikely to be a useful treatment for reducing the brain damage that accompanies periventricular (germinal matrix) hemorrhage, a common complication of premature birth.

Targeting secondary injury in intracerebral haemorrhage--perihaematomal oedema

Perihaematomal oedema (PHO) is an important pathophysiological marker of secondary injury in intracerebral haemorrhage (ICH). In this Review, we describe a novel method to conceptualize PHO formation within the framework of Starling's principle of movement of fluid across a capillary wall. We consider progression of PHO through three stages, characterized by ionic oedema (stage 1) and progressive vasogenic oedema (stages 2 and 3). In this context, possible modifiers of PHO volume and their value in identifying patients who would benefit from therapies that target secondary injury are discussed; the practicalities of using neuroimaging to measure PHO volume are also considered. We examine whether PHO can be used as a predictor of neurological outcome following ICH, and we provide an overview of emerging therapies. Our discussion emphasizes that PHO has clinical relevance both as a therapeutic target, owing to its augmentation of the mass effect of a haemorrhage, and as a surrogate marker for novel interventions that target secondary injury.

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.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

Structure, function and pathophysiology of protease activated receptors

Discovered in the 1990s, protease activated receptors(1) (PARs) are membrane-spanning cell surface proteins that belong to the G protein coupled receptor (GPCR) family. A defining feature of these receptors is their irreversible activation by proteases; mainly serine. Proteolytic agonists remove the PAR extracellular amino terminal pro-domain to expose a new amino terminus, or tethered ligand, that binds intramolecularly to induce intracellular signal transduction via a number of molecular pathways that regulate a variety of cellular responses. By these mechanisms PARs function as cell surface sensors of extracellular and cell surface associated proteases, contributing extensively to regulation of homeostasis, as well as to dysfunctional responses required for progression of a number of diseases. This review examines common and distinguishing structural features of PARs, mechanisms of receptor activation, trafficking and signal termination, and discusses the physiological and pathological roles of these receptors and emerging approaches for modulating PAR-mediated signaling in disease.

Thrombin-induced neuronal protection: role of the mitogen activated protein kinase/ribosomal protein S6 kinase pathway

Our previous studies have found that intracerebral pretreatment with a low dose of thrombin (thrombin preconditioning, TPC) reduces infarct volume and attenuates brain edema after focal cerebral ischemia. In this study, we examined whether TPC protects against the neuronal death induced by oxygen glucose deprivation (OGD), and whether the protection is through thrombin receptors and the p44/42 mitogen activated protein kinases (MAPK)/ribosomal protein S6 kinases (p70 S6K) pathway. Expression of protease-activated receptors (PARs) mRNA was detected in cultured primary rat neurons and thrombin upregulated PAR-1 and PAR-4 mRNA expression. TPC reduced OGD-induced neuronal death (e.g. dead cells: 52.5 ± 5.4% vs. 72.3 ± 7.2% in the control group, n=6, p<0.01). Agonists of PAR-1 and PAR-4 mimicked the effects of thrombin and reduced OGD-induced neuronal death. Pretreatment with thrombin or PAR agonists induced the upregulation of activated p44/42 MAPK and p70S6K (Thr 421/Ser 424). PD98059, an inhibitor of p44/42 MAPK kinase, blocked thrombin-induced upregulation of activated p44/42 MAPK and p70S6K. It also reduced TPC-induced neuronal protection (e.g. dead cells: 68.2 ± 5.2% vs. 56.9 ± 4.6% in vehicle+TPC group, n=6, p<0.05). These results suggest that TPC-induced ischemic tolerance is through activation of thrombin receptors and the p44/42 MAPK/p70S6K pathway.

GRK5 deficiency leads to reduced hippocampal acetylcholine level via impaired presynaptic M2/M4 autoreceptor desensitization

G protein-coupled receptor kinase 5 (GRK5) deficiency has been linked recently to early Alzheimer disease (AD), but the mechanism by which GRK5 deficiency may contribute to AD pathogenesis remains elusive. Here we report that overexpression of dominant negative mutant of GRK5 (dnGRK5) in a cholinergic neuronal cell line led to decreased acetylcholine (ACh) release. This reduction was fully corrected by pertussis toxin, atropine (a nonselective muscarinic antagonist), or methoctramine (a selective M2/M4 muscarinic receptor antagonist). Consistent with results in cultured cells, high potassium-evoked ACh release in hippocampal slices from young GRK5 knock-out mice was significantly reduced compared with wild type littermates, and this reduced ACh release was also fully corrected by methoctramine. In addition, following treatment with the nonselective muscarinic agonist oxotremorine-M, M2, and M4 receptors underwent significantly reduced internalization in GRK5KO slices compared with wild type slices, as assessed by plasma membrane retention of receptor immunoreactivity, whereas M1 receptor internalization was not affected by loss of GRK5 expression. Moreover, Western blotting revealed no synaptic or cholinergic degenerative changes in young GRK5 knock-out mice. Altogether, these results suggest that GRK5 deficiency leads to a reduced hippocampal ACh release and cholinergic hypofunction by selective impairment of desensitization of presynaptic M2/M4 autoreceptors. Because this nonstructural cholinergic hypofunction precedes the hippocampal cholinergic hypofunction associated with structural cholinergic degeneration and cognitive decline in aged GRK5 knock-out mice, this nonstructural alteration may be an early event contributing to cholinergic degeneration in AD.

Protease-activated receptors as drug targets in inflammation and pain

Proteases have been shown to signal to cells through the activation of a novel class of receptors coupled to G proteins: the protease-activated receptors (PARs). Those receptors are expressed in a wide range of cells, which ultimately are all involved in mechanisms of inflammation and pain. Numerous studies have considered the role of PARs in cells, organ systems or in vivo, highlighting the fact that PAR activation results in signs of inflammation. A growing body of evidences discussed here suggests that these receptors, and the proteases that activate them, interfere with inflammation and pain processes. Whether a role for PARs has been clearly defined in inflammatory and pain pathologies is discussed in this review. Further, the pros and cons for considering PARs as targets for the development of therapeutic options for the treatment of inflammation and pain are discussed.

Microglial activation and brain injury after intracerebral hemorrhage

Microglial activation and thrombin formation contribute to brain injury after intracerebral hemorrhage (ICH). Tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1beta) are 2 major proinflammatory cytokines. In this study, we investigated whether thrombin stimulates TNF-alpha and IL-1beta secretion in vitro, and whether microglial inhibition reduces ICH-induced brain injury in vivo. There were 2 parts to this study. In the first part, cultured rat microglial cells were treated with vehicle, thrombin (5 and 10U/mL), or thrombin plus tuftsin (0.05 microg/mL), an inhibitor of microglia activation. Levels of TNF-alpha and IL-1beta in culture medium were measured by ELISA at 4, 8, and 24 h after thrombin treatment. In the second part of the study, rats received an intracerebral infusion of 100 microL autologous whole blood with or without 25 microg of tuftsin 1-3 fragment. Rats were killed at day 1 or day 3 for immunohistochemistry and brain water content measurement. We found that thrombin receptors were expressed in cultured microglia cells, and TNF-alpha and IL-1beta levels in the culture medium were increased after thrombin treatment. Tuftsin reduced thrombin-induced upregulation of TNF-alpha and IL-1beta. In vivo, microglia were activated after ICH, and intracerebral injection of tuftsin reduced brain edema in the ipsilateral basal ganglia (81.1 +/- 0.7% vs. 82.7 +/- 1.3% in vehicle-treated group; p < 0.05) after ICH. These results suggest a critical role of microglia activation in ICH-related brain injury.

Minocycline reduces intracerebral hemorrhage-induced brain injury

OBJECTIVES

Microglial activation and thrombin formation contribute to brain injury after intracerebral hemorrhage. Tumor necrosis factor-alpha and interleukin-1beta are two major pro-inflammatory cytokines. The present study investigated if thrombin stimulates tumor necrosis factor-alpha and interleukin-1beta secretion in vitro and if microglial inhibition reduces intracerebral hemorrhage-induced brain injury in vivo.

METHODS

There were two parts in this study. In the first part, cultured rat microglial cells were treated with vehicle, thrombin (10 U/ml) or thrombin plus minocycline (1 or 10 microM), an inhibitor of microglia activation. Levels of tumor necrosis factor-alpha and interleukin-1beta in culture medium were measured by enzyme-linked immunosorbent assay 24 hours after thrombin treatment. In the second part, rats had an intracerebral injection of 100 microl autologous whole blood. Rats received minocycline or vehicle treatment. Brain edema was measured at day 3 and brain atrophy was determined at day 28 after intracerebral hemorrhage.

RESULTS

Thrombin receptors were expressed in cultured microglia cells, and tumor necrosis factor-alpha and interleukin-1beta levels in the culture medium were increased after thrombin treatment. Minocycline reduced thrombin-induced up-regulation of tumor necrosis factor-alpha and interleukin-1beta. In vivo, minocycline reduced perihematomal brain edema, neurological deficits and brain atrophy.

DISCUSSION

Thrombin stimulates microglia to release the pro-inflammatory cytokines, tumor necrosis factor-alpha and interleukin-1beta, and microglial inhibition with minocycline reduces brain injury after intracerebral hemorrhage, suggesting a critical role of microglia activation in intracerebral hemorrhage-related brain injury.

Effect of microgravity on gene expression in mouse brain

Changes in gravitational force such as that experienced by astronauts during space flight induce a redistribution of fluids from the caudad to the cephalad portion of the body together with an elimination of normal head-to-foot hydrostatic pressure gradients. To assess brain gene profile changes associated with microgravity and fluid shift, a large-scale analysis of mRNA expression levels was performed in the brains of 2-week control and hindlimb-unloaded (HU) mice using cDNA microarrays. Although to different extents, all functional categories displayed significantly regulated genes indicating that considerable transcriptomic alterations are induced by HU. Interestingly, the TIC class (transport of small molecules and ions into the cells) had the highest percentage of up-regulated genes, while the most down-regulated genes were those of the JAE class (cell junction, adhesion, extracellular matrix). TIC genes comprised 16% of those whose expression was altered, including sodium channel, nonvoltage-gated 1 beta (Scnn1b), glutamate receptor (Grin1), voltage-dependent anion channel 1 (Vdac1), calcium channel beta 3 subunit (Cacnb3) and others. The analysis performed by GeneMAPP revealed several altered protein classes and functional pathways such as blood coagulation and immune response, learning and memory, ion channels and cell junction. In particular, data indicate that HU causes an alteration in hemostasis which resolves in a shift toward a more hyper-coagulative state with an increased risk of venous thrombosis. Furthermore, HU treatment seems to impact on key steps of synaptic plasticity and learning processes.

Augmented axonal defects and synaptic degenerative changes in female GRK5 deficient mice

Recent studies suggested that G protein-coupled receptor kinase 5 (GRK5) deficiency plays a significant role in early Alzheimer's disease (AD) pathogenesis, and that the GRK5 knockout (GRK5KO) mouse displays an early Alzheimer-like cognitive deficit associated with increased hippocampal axonal defects and synaptic degenerative changes. Gender is known to play a role in AD, with females showing more extensive pathologic changes in brain compared to males. Although GRK5 deficiency is linked to AD, it is unknown whether the pathologic changes solely driven by the GRK5 deficiency are gender-dependent. To determine this, extent of the pathologic changes in aged GRK5KO mice was compared between genders. We find that female GRK5KO mice had a 2.5-fold increase in hippocampal swollen axonal clusters compared to male GRK5KO mice. Moreover, hippocampal levels of several synaptic proteins, including synaptophysin, were significantly lower in the female than the male. In addition, although increased Luteinizing hormone (LH) activity is believed to play a significant role in the gender phenomenon in AD, we found that desensitization of LH receptor is not affected by the GRK5 deficiency. Therefore, the worsened pathologic changes in the female mice cannot be attributed to an impaired LH receptor desensitization. Taken together, this study demonstrates a synergistic interaction between GRK5 deficiency and gender in promoting early AD-like pathologic changes in the female GRK5KO mice, although the underlying molecular mechanisms remain to be elucidated.