Thrombin, its receptor and protease nexin I, its potent serpin, in the nervous system

MPs-ACT, an Assay to Evaluate the Procoagulant Activity of Microparticles

The procoagulant effect of microparticles (MPs) contributes to hypercoagulability-induced thrombosis. We provide preliminary findings of the MPs-Activated Clotting Time (MPs-ACT) assay to determine the procoagulant activity of MPs. MPs-rich plasma was obtained and recalcified. Changes in plasma viscoelasticity were evaluated and the time to the peak viscoelastic changes was defined as the MPs-ACT. MPs concentration was measured by flow cytometry. Coagulation products produced during plasma clotting were identified by fibrin and fibrinopeptide A. MPs were prepared in vitro and added to standard plasma to simulate pathological samples. In addition, reproducibility and sensitivity were evaluated. We confirmed the linear relationship between MPs-ACT and MP concentrations. Dynamic changes in fibrin production were depicted. We simulated the correlation between MPs-ACT and standard plasma containing MPs prepared in vitro. The reproducibility of high-value and low-value samples was 6.0% and 10.8%, respectively. MPs-ACT sensitively detected hypercoagulable samples from patients with pre-eclampsia, hip fractures, and lung tumors. MPs-ACT largely reflects the procoagulant effect of MPs. MPs-ACT sensitively and rapidly detects hypercoagulability with MPs-rich plasma. It may be promising for the diagnosis of hypercoagulable states induced by MPs.

Protease-activated receptors in health and disease

Proteases are signaling molecules that specifically control cellular functions by cleaving protease-activated receptors (PARs). The four known PARs are members of the large family of G protein-coupled receptors. These transmembrane receptors control most physiological and pathological processes and are the target of a large proportion of therapeutic drugs. Signaling proteases include enzymes from the circulation; from immune, inflammatory epithelial, and cancer cells; as well as from commensal and pathogenic bacteria. Advances in our understanding of the structure and function of PARs provide insights into how diverse proteases activate these receptors to regulate physiological and pathological processes in most tissues and organ systems. The realization that proteases and PARs are key mediators of disease, coupled with advances in understanding the atomic level structure of PARs and their mechanisms of signaling in subcellular microdomains, has spurred the development of antagonists, some of which have advanced to the clinic. Herein we review the discovery, structure, and function of this receptor system, highlight the contribution of PARs to homeostatic control, and discuss the potential of PAR antagonists for the treatment of major diseases.

The Role of Thrombin in Brain Injury After Hemorrhagic and Ischemic Stroke

Thrombin is increased in the brain after hemorrhagic and ischemic stroke primarily due to the prothrombin entry from blood either with a hemorrhage or following blood-brain barrier disruption. Increasing evidence indicates that thrombin and its receptors (protease-activated receptors (PARs)) play a major role in brain pathology following ischemic and hemorrhagic stroke (including intracerebral, intraventricular, and subarachnoid hemorrhage). Thrombin and PARs affect brain injury via multiple mechanisms that can be detrimental or protective. The cleavage of prothrombin into thrombin is the key step of hemostasis and thrombosis which takes place in every stroke and subsequent brain injury. The extravascular effects and direct cellular interactions of thrombin are mediated by PARs (PAR-1, PAR-3, and PAR-4) and their downstream signaling in multiple brain cell types. Such effects include inducing blood-brain-barrier disruption, brain edema, neuroinflammation, and neuronal death, although low thrombin concentrations can promote cell survival. Also, thrombin directly links the coagulation system to the immune system by activating interleukin-1α. Such effects of thrombin can result in both short-term brain injury and long-term functional deficits, making extravascular thrombin an understudied therapeutic target for stroke. This review examines the role of thrombin and PARs in brain injury following hemorrhagic and ischemic stroke and the potential treatment strategies which are complicated by their role in both hemostasis and brain.

Thrombin and the Coag-Inflammatory Nexus in Neurotrauma, ALS, and Other Neurodegenerative Disorders

This review details our current understanding of thrombin signaling in neurodegeneration, with a focus on amyotrophic lateral sclerosis (ALS, Lou Gehrig's disease) as well as future directions to be pursued. The key factors are multifunctional and involved in regulatory pathways, namely innate immune and the coagulation cascade activation, that are essential for normal nervous system function and health. These two major host defense systems have a long history in evolution and include elements and regulators of the coagulation pathway that have significant impacts on both the peripheral and central nervous system in health and disease. The clotting cascade responds to a variety of insults to the CNS including injury and infection. The blood brain barrier is affected by these responses and its compromise also contributes to these detrimental effects. Important molecules in signaling that contribute to or protect against neurodegeneration include thrombin, thrombomodulin (TM), protease activated receptor 1 (PAR1), damage associated molecular patterns (DAMPs), such as high mobility group box protein 1 (HMGB1) and those released from mitochondria (mtDAMPs). Each of these molecules are entangled in choices dependent upon specific signaling pathways in play. For example, the particular cleavage of PAR1 by thrombin vs. activated protein C (APC) will have downstream effects through coupled factors to result in toxicity or neuroprotection. Furthermore, numerous interactions influence these choices such as the interplay between HMGB1, thrombin, and TM. Our hope is that improved understanding of the ways that components of the coagulation cascade affect innate immune inflammatory responses and influence the course of neurodegeneration, especially after injury, will lead to effective therapeutic approaches for ALS, traumatic brain injury, and other neurodegenerative disorders.

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.

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.

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.

Quantitative detection of thrombin activity in an ischemic stroke model

Thrombin, a central factor in thrombogenesis, affects cells in the brain through protease activated receptors. Low levels of thrombin activity are neuroprotective while higher levels are deleterious, and we have therefore developed a new method for its direct quantitative measurement in brain slices following stroke. Thrombin activity was measured by a fluorescent substrate on fresh coronal slices taken from the ipsilateral and contralateral hemispheres 24-72 h following permanent right middle cerebral artery occlusion. Prolyl endopeptidase and aminopeptidases were inhibited as a critical step to insure the specificity of the assay for thrombin detection. Infarct volume was assessed using TTC staining. Thrombin activity in the right ischemic hemisphere was significantly higher compared to the contralateral hemisphere (32 ± 6 and 27 ± 10 mU/ml, mean ± SE in the two most affected slices from the ischemic hemisphere vs. 21 ± 6 and 8 ± 2 mU/ml in corresponding contralateral slices; p < 0.05). Thrombin levels in the ischemic and contralateral hemispheres were significantly higher compared to healthy control mice and were above the range known to be protective to brain cells. A significant correlation was found between thrombin activity in the ischemic hemisphere and the infarct volume. Results of studies based on this method may translate into potential thrombin based therapies.

Localization and expression of prothrombin in rodent osteoclasts and long bones

The serum protein prothrombin (PT) is proteolytically converted to thrombin during the coagulation cascade by the cell-associated prothrombinase complex. In vitro, RANKL-differentiated osteoclasts express tissue factor and coagulation factor Xa, which convert PT to thrombin (Karlstrom et al. Biochem Biophys Res Commun 394:593-599, 2010). The present study investigated the localization of PT in bone as well as the expression of PT mRNA in bone and osteoclasts. Herein, immunoblot analysis detected PT and smaller proteolytically cleaved fragments with sizes consistent with the action of prothrombinase in a protein fraction extracted with guanidine-HCl EDTA from mouse tibia. Light microscopic and ultrastructural immunohistochemistry demonstrated the presence of PT in the newly formed bone matrix of the metaphysis. Furthermore, fluorescent immunohistochemistry on metaphyseal trabecular bone showed that PT colocalized with MMP-9-expressing subepiphyseal osteoclasts, whereas cathepsin K-expressing osteoclasts were closely associated with PT of the bone matrix. RT-qPCR analysis revealed that PT mRNA was detected in tibia. Expression of PT mRNA in the tibia was 0.2% of the level in the liver. In addition, PT mRNA expression was increased by RANKL-induced differentiation of bone marrow macrophages to osteoclasts. The results demonstrate that PT is synthesized and proteolytically processed in bone. Furthermore, PT is present mainly in the newly formed bone matrix of the metaphyseal trabecular bone compartment in close association to osteoclasts. In addition, MMP-9-positive osteoclasts contain PT, and PT expression is increased during osteoclastogenesis.

The expression and the role of protease nexin-1 on brain edema after intracerebral hemorrhage

Brain edema is one of the most frequent and serious complications of intracerebral hemorrhage (ICH), but how the ICH cause brain edema is unknown. Our studies were designed to investigate the regulation and distribution of protease nexin-1 (PN-1), thrombin and aquaporin-4 (AQP-4) in brain edema after ICH in rat and human brain in vivo. Our result showed that the severity of cerebral edema resulted from an acute stage of ICH. The PN-1-thrombin system modulated cerebral edema after ICH. Thrombin and AQP-4 increased to aggregate cerebral edema after ICH. In order to control the deleterious effect of thrombin's overexpression, PN-1 appeared quickly and abundantly to inhibit thrombin and lessen the cerebral edema. PN-1 was distributed in neurons and glial cells of cerebral cortex, hippocampus, thalamencephalon, basal ganglia, cerebellum and circum-encephalocoele in rat and human brain. The expression of AQP-4 is different between human and rat. Thus, we demonstrated that the animal experimental approach was, however, not sufficient by itself and needed to be corroborated by observations on human brains.

Mice lacking protease nexin-1 show delayed structural and functional recovery after sciatic nerve crush

Multiple molecular mechanisms influence nerve regeneration. Because serine proteases were shown to affect peripheral nerve regeneration, we performed nerve crush experiments to study synapse reinnervation in adult mice lacking the serpin protease nexin-1 (PN-1). PN-1 is a potent endogenous inhibitor of thrombin, trypsin, tissue plasminogen activators (tPAs), and urokinase plasminogen activators. Compared with the wild type, a significant delay in synapse reinnervation was detected in PN-1 knock-out (KO) animals, which was associated with both reduced proliferation and increased apoptosis of Schwann cells. Various factors known to affect Schwann cells were also altered. Fibrin deposits, tPA activity, mature BDNF, and the low-affinity p75 neurotrophin receptor were increased in injured sciatic nerves of mutant mice. To test whether the absence of PN-1 in Schwann cells or in the axon caused delay in reinnervation, PN-1 was overexpressed exclusively in the nerves of PN-1 KO mice. Neuronal PN-1 expression did not rescue the delayed reinnervation. The results suggest that Schwann cell-derived PN-1 is crucial for proper reinnervation through its contribution to the autocrine control of proliferation and survival. Thus, the precise balance between distinct proteases and serpins such as PN-1 can modulate the overall impact on the kinetics of recovery.

Protease nexin-1 expression is altered in human breast cancer

Background

Urokinase-type Plasminogen Activator (uPA), a serine protease, plays a pivotal role in human breast cancer metastasis by mediating the degradation of extracellular matrix proteins and promoting cell motility. In more advanced breast cancers, uPA activity is significantly up regulated and serves as a prognostic indicator of poor patient outcome. Classically, regulation of uPA activity, especially in breast cancers, is thought to be mediated by Type 1 Plasminogen Activator Inhibitor (PAI-1). However, we have recently found that a lesser known natural inhibitor of uPA, Protease Nexin 1 (PN-1), is expressed in normal human mammary tissue. Based on this observation, we investigated if PN-1 is also expressed in human breast cancers where it may contribute to the regulation of uPA and participate in the development of a metastatic phenotype.

Results

Using quantitative real-time PCR analysis, we measured PN-1 mRNA expression in tissues obtained from 26 human breast tumor biopsies and compared these values with those obtained from 10 normal breast tissue samples. Since both PAI-1 and uPA expression levels are known to be elevated in metastatic breast cancer, we also measured their levels in our 26 tumor samples for direct comparison with PN-1 expression. We found that PN-1 expression was elevated over that found in normal mammary tissue; an increase of 1.5- to 3.5-fold in 21 of 26 human breast tumors examined. As anticipated, both PAI-1 and uPA mRNA levels were significantly higher in the majority of breast tumors; 19 of 26 tumors for PAI-1 and 22 of 26 tumors for uPA. A quantile box plot of these data demonstrates that the elevated PN-1 expression in breast tumor tissues directly correlates with the increased expression levels found for PAI-1 and uPA.

Conclusion

The fact that PN-1 expression is elevated in human breast cancer, and that its increased expression is directly correlated with increases measured for PAI-1 and uPA, suggests that PN-1 may contribute to the regulation of uPA-mediate tumor cell motility and metastatic spread.

Thrombin in inflammatory brain diseases

Inflammatory brain diseases such as multiple sclerosis (MS) include hyperactivation of the coagulation pathway which includes thrombin. In the experimental autoimmune encephalomyelitis (EAE) model we have found significantly higher levels of thrombin inhibitors which include the very early elevation of protease nexin 1. The physiological importance of excess thrombin in neural tissue is demonstrated by recent experiments which link thrombin with conduction block in the sciatic nerve.

Identification of new CNS-resident macrophage subpopulation molecular markers for the discrimination with murine systemic macrophages

A controversial issue in neurobiology concerns the respective functions of central nervous system (CNS)-resident macrophages and systemic infiltrating macrophages morphologically and phenotypically similar during most of CNS injury processes. In a previous work, we isolated sixteen mRNAs differentially expressed between two microglial EOC clones. By studying their pattern of expression, we found that three of them were not expressed in peripheral macrophages, even after stimulation with IFNgamma, TNFalpha or IL10. These three molecules are physiologically expressed by murine adult microglia and could be used to evaluate in vivo their discriminative potential toward CNS-infiltrating macrophages during inflammatory events.

Increased thrombin inhibition in experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE), are inflammatory diseases of the central nervous system (CNS). Activated coagulation factors are associated with inflammation and are elevated in the plasma of animals with EAE. Thrombin is a key coagulation factor and its major endogenous inhibitors are antithrombin III (ATIII) in the plasma and protease nexin 1 (PN-1) in the brain. We measured the capacity of brain homogenates to inhibit exogenous thrombin and the CNS levels of ATIII and PN-1 during the course of EAE. Acute EAE was induced in SJL/J mice by immunization with mouse spinal cord homogenates. On Days 8, 13, and 22 post-immunization, inhibition of exogenous thrombin activity was measured by a recently developed fluorimetric assay. PN-1 and ATIII were assayed both by immunohistochemistry and by immunoblots in the brain and spinal cord. Total brain thrombin inhibitory activity increased (32%) in EAE mice at the peak of clinical disease (Day 13, P=0.04 compared to controls). Brain ATIII also increased at the peak of disease (2.5-fold higher than controls, P=0.0001), and correlated significantly with clinical scores at all stages of disease (r=0.72, P=0.0068). In contrast, PN-1 elevations were more pronounced at the preclinical stage on Day 8 (3-fold higher than controls, P=0.01) than on Day 13 (1.4-fold higher, P=0.005). Increased brain thrombin inhibition at the clinical peak of EAE probably reflects increased influx of plasma thrombin inhibitors. Early PN-1 changes represent a potential target for thrombin modulating drugs in EAE and MS.

Transglutaminase 2 expression in the salivary myoepithelial cells of mouse embryo

Earlier a strong transient expression of transglutaminase 2 (TGase 2) localized at the anchoring sites of muscle bundles in human embryo was observed. In this study, we report a similar transient expression of the TGase 2 in the salivary myoepithelial cells of mouse embryo by immunohistochemistry, RNA in situ hybridisation, and RT-PCR. From 35 submandibular glands of mouse embryos and postnatal mice, a consistent expression of TGase 2 in the myoepithelial cells via a stage-specific manner was identified by mono-clonal antibody to TGase 2 immunostaining. A similar expression pattern of TGase 2 in the myoepithelial cells was also observed by RNA in situ hybridisation analysis. The expression of TGase 2 in the salivary epithelium and mesenchyme during the prenatal 14.5-15.5 days was found minimally diffusely spread and became intensely focalised in the myoepithelial cells of salivary acini and ducts during the prenatal 16.5-18.5 days but thereafter gradually decreased until postnatal 7 days and remained weak in postnatal 3 weeks. Such transient rise and fall expressions of TGase 2 were also found with the sequential amount of RT-PCR products during the same period. The alpha-smooth muscle actin (alpha-SMA) as a positive control in the myoepithelial cells of mouse submandibular glands was consistently expressed during the prenatal and postnatal period. These results of transient expression of TGase 2 in the myoepithelial cells coincided with the formation of the dendritic basket structure in the periphery of acini and ducts, suggest a possible catalytic role of transglutaminase in a newly formed cellular matrixes during the cytodifferentiating stage of mouse prenatal and neonatal submandibular glands.

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

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

Protease-activated receptors: regulation of neuronal function

Certain serine proteases from the circulation (e.g., coagulation factors), inflammatory cells (e.g., mast-cell tryptase, neutrophil proteinase 3), and from many other cell types (e.g., trypsins) can specifically signal to cells by cleaving protease-activated receptors (PARs), a family of four G protein-coupled receptors. Proteases cleave PARs at specific sites within the extracellular amino-terminus to expose amino-terminal tethered ligand domains that bind to and activate the cleaved receptors. The proteases that activate PARs are often generated and released during injury and inflammation, and activated PARs orchestrate tissue responses to injury, including hemostasis, inflammation, pain, and repair. This review concerns protease and PAR signaling in the nervous system. Neurons of the central and peripheral nervous systems express all four PARs. Proteases that may derive from the circulation, inflammatory cells, or neural tissues can cleave PARs on neurons and thereby activate diverse signaling pathways that control survival, morphology, release of neurotransmitters, and activity of ion channels. In this manner proteases and PARs regulate neurodegeneration, neurogenic inflammation, and pain transmission. Thus, PARs may participate in disease states and PAR antagonists or agonists may be useful therapies for certain disorders.

Neuroprotective Effects of Recombinant Thrombomodulin in Controlled Contusion Spinal Cord Injury Implicates Thrombin Signaling

Although the central nervous system (CNS) of mammals has had poor prospects for regeneration, recent studies suggest this might improve from blocking "secondary cell loss" or apoptosis. In this regard, intravenous activated protein C (aPC) improved neurologic outcomes in a rat compression spinal cord injury (SCI) model. Protein C activation occurs when the serine protease thrombin binds to the cell surface proteoglycan thrombomodulin (TM) forming a complex that halts coagulation. In culture, rTM blocks thrombin's activation of protease-activated receptors (PARs), that mediate thrombin killing of neurons and glial reactivity. Both PAR1 and prothrombin are rapidly upregulated after contusion SCI in rats, prior to peak apoptosis. We now report neuroprotective effects of intraperitoneal soluble recombinant human rTM on open-field locomotor rating scale (BBB) and spinal cord lesion volume when given 1 h after SCI. BBB scores from four separate experiments showed a 7.6 +/- 1.4 absolute score increase (p < 0.05) at 3 days, that lasted throughout the time course. Histological sections at 14 days were even more dramatic where a twofold reduction in lesion volume was quantified in rTM-treated rats. Thionin staining revealed significant preservation of motor neuronal profiles both at, and two segments below, the lesion epicenter. Activated caspase-3 immunocytochemistry indicated apoptosis was quite prominent in motor neurons in vehicle (saline) controls, but was dramatically reduced by rTM. Microglia, increased and activated after injury, were reduced with rTM treatment. Taken together, these and previous results support a prominent role for coagulation-inflammation signaling cascades in the subacute changes following SCI. They identify a neuroprotective role for rTM by its inhibition of thrombin generation and blockade of PAR activation.

Thrombin reduces MuSK and acetylcholine receptor expression along with neuromuscular contact size in vitro

In the course of studies on thrombin and its inhibitor(s) in synaptic plasticity, we addressed the question of their roles in the formation of neuromuscular junctions (NMJ) and used a model of rat neuron-myotube cocultures. We report that the size of acetylcholinesterase (AChE) patches used as a marker of neuromuscular contacts was decreased in the presence of either thrombin or SFLLRN, the agonist peptide of the thrombin receptor PAR-1, whereas it was increased with hirudin, a specific thrombin inhibitor. In an attempt to relate these neuromuscular contact size variations to molecular changes, we studied muscle-specific tyrosine kinase receptor (MuSK), acetylcholine receptor (AChR) and rapsyn expression in the presence of thrombin. We showed that thrombin did not change rapsyn gene and protein expression. However, the expression of MuSK and surface AChR proteins was diminished in both myotube cultures and neuron-myotube cocultures. These reductions in protein expression were associated with a decrease in MuSK and AChR alpha-subunit gene expression in myotube cultures but not in neuron-myotube cocultures. Moreover, the expression of the AChR epsilon-subunit gene, specifically enhanced by neuron-released factors, was not modified by thrombin in neuron-myotube cocultures. This suggests that thrombin did not affect the expression of synaptic AChRs enhanced by neuron-released factors but rather reduced the level of extrasynaptic AChRs. Taken together, these results indicate that thrombin in balance with its inhibitor(s) could modulate the formation of neuromuscular contacts in vitro by affecting the expression of two essential molecules in NMJ postsynaptic differentiation, MuSK and AChR.

Protease-activated receptors: contribution to physiology and disease

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

Proteinase-activated receptors in the nervous system

Recent data point to important roles for proteinases and their cognate proteinase-activated receptors (PARs) in the ontogeny and pathophysiology of the nervous system. PARs are a family of G-protein-coupled receptors that can affect neural cell proliferation, morphology and physiology. PARs also have important roles in neuroinflammatory and degenerative diseases such as human immunodeficiency virus-associated dementia, Alzheimer's disease and pain. These receptors might also influence the pathogenesis of stroke and multiple sclerosis, conditions in which the blood-brain barrier is disrupted. The diversity of effects of PARs on neural function and their widespread distribution in the nervous system make them attractive therapeutic targets for neurological disorders. Here, we review the roles of PARs in the central and peripheral nervous systems during health and disease, with a focus on neuroinflammatory and degenerative disorders.

Reciprocal regulation of urokinase receptor (CD87)-mediated cell adhesion by plasminogen activator inhibitor-1 and protease nexin-1

Protease nexin-1 (PN-1) and plasminogen activator inhibitor-1 (PAI-1) are serine protease inhibitors that bind to the extracellular matrix protein vitronectin (VN) with high affinity. PAI-1 is known to inhibit cell adhesion and migration by binding to VN and inhibiting the interaction with integrins or the urokinase receptor (uPAR). Unexpectedly, PN-1 was found to increase the association between VN and uPAR in the presence of enzymatically active uPA. Through this mechanism PN-1 also stimulated uPAR-dependent cell adhesion to immobilized VN. In contrast to PAI-1, PN-1 did not influence VN binding to integrins or integrin-mediated cell adhesion. Upon adhesion of monocytes to VN there was an accumulation of uPAR and PN-1 at the interface between the cell and the matrix, whereas on fibronectin (FN) both components were distributed evenly over the whole cell as visualized by confocal microscopy. Immunohistochemistry of atherosclerotic vessels indicated that PN-1 was found associated with smooth muscle cells, macrophages and platelets. In some regions of the diseased vessels PN-1 was in close proximity to VN and uPAR, but no PN-1 was present in normal vessels. These results indicate a novel function of PN-1 linked to complex formation with uPA that leads to the regulation of VN-dependent adhesion of leukocytes.

In the cut and thrust of apoptosis, serine proteases come of age

Proteolysis is central to the systematic cellular degradation that occurs during apoptosis. Predominantly, caspases have been studied in this regard. However, increasing evidence suggests that certain serine proteases may also play a significant role in apoptosis. Not only are these serine proteases involved in apoptosis signalling pathways independently, but they may also interact with more classical mediators of apoptosis such as the caspases or Bcl-2 family proteins. Isolation of apoptosis-associated serine proteases and the use of specific inhibitors have helped to shed light on potential pathways in which they are involved. Despite the recent developments in the field, knowledge regarding the role of serine proteases in apoptosis remains limited, but it is clear that investigations are gathering momentum and such studies may herald a new and exciting departure in apoptosis research.

Rapid tau aggregation and delayed hippocampal neuronal death induced by persistent thrombin signaling

Tau hyperphosphorylation, leading to self-aggregation, is widely held to underlie the neurofibrillary degeneration found in Alzheimer's disease (AD) and other tauopathies. However, it is unclear exactly what environmental factors may trigger this pathogenetic tau hyperphosphorylation. From several perspectives, the coagulation serine protease, thrombin, has been implicated in AD and activates several different protein kinase pathways but has not previously been shown how it may contribute to AD pathogenesis. Here we report that nanomolar thrombin induced rapid tau hyperphosphorylation and aggregation in murine hippocampal neurons via protease-activated receptors, which was followed by delayed synaptophysin reduction and apoptotic neuronal death. Mechanistic study revealed that a persistent thrombin signaling via protease-activated receptor 4 and prolonged downstream p44/42 mitogenactivated protein kinase activation are at least in part responsible. These results pathogenetically linked thrombin to subpopulations of AD and other tauopathies associated with cerebrovascular damage. Such knowledge may be instrumental in transforming therapeutic paradigms.

Thrombin inhibits NMDA-mediated nociceptive activity in the mouse: possible mediation by endothelin

The CNS expresses many components of an extracellular protease signalling system, including the protease-activated receptor-1 (PAR-1) whose tethered ligand is generated by thrombin. Activation of PAR-1 potentiates NMDA receptor activity in hippocampal neurons. Because NMDA activity mediates hyperalgesia, we tested the hypothesis that PAR-1 receptors also regulate pain processing. In contrast to the potentiating effect of thrombin in the hippocampus, NMDA-induced behaviours and the transient mechanical hyperalgesia (von Frey fibres) induced by intrathecally injected NMDA in mice were inhibited by thrombin in a dose-related fashion. This anti-hyperalgesic effect was mimicked by SFLLRN, the natural ligand at PAR-1 binding sites, but not SLIGRL-amide, a PAR-2 agonist. The effects of SFLLRN were less potent and shorter in duration than that of thrombin, consistent with its more transient effect on PAR-1 sites. Both thrombin and SFLLRN inhibited acetic acid-induced abdominal stretch (writhing) behaviours, which were also sensitive to NMDA antagonism, but not hot plate or tail flick latencies, which were insensitive to NMDA antagonists. TFLLR-amide, a selective ligand for PAR-1 sites, mimicked the effects of thrombin while RLLFT-amide, an inactive, reverse peptide sequence, did not. In addition, the effect of TFLLR-amide was prevented by RWJ-56110, a PAR-1 antagonist. Thrombin and TFLLR-amide produced no oedema (Evans Blue extravasation) in the spinal cord that would account for these effects. Based on the reported ability of thrombin to mobilize endothelin-1 from astrocytes, we tested the role of this compound in thrombin's activity. BQ123, an endothelin A receptor antagonist, prevented thrombin's inhibition of writhing and NMDA-induced behaviours while BQ788, an endothelin B receptor antagonist, did not. Thus, activation of PAR-1 sites by thrombin in the CNS appears to inhibit NMDA-mediated nociception by a pathway involving endothelin type A receptors.

Role of thrombin in CNS damage associated with intracerebral haemorrhage: opportunity for pharmacological intervention?

Intracerebral haemorrhage (ICH) results in high mortality and morbidity. The most important causes of neurological deterioration after ICH are progression of oedema and injury to nerve cells and axons surrounding the haematoma, as well as haematoma enlargement. Recent studies have indicated that thrombin, formed upon clotting of the haematoma, plays an important role in these processes. As opposed to conventional therapeutic approaches, administration of a thrombin inhibitor could effectively limit oedema formation and neuronal damage, improving survival and functional outcome. A small, preliminary clinical trial has suggested that antithrombin therapy with intravenously administered argatroban may be useful in treatment of ICH. Randomised, controlled studies are needed to confirm these initial findings.

Expression and functional characterization of the serine protease inhibitor neuroserpin in endocrine cells

Serine proteases play essential roles in a wide variety of cellular processes in endocrine cells. There is a growing interest in the roles of serine protease inhibitors, or serpins, as key regulators of their activity. We have cloned two neuroserpin cDNAs from a rat pituitary cDNA library and confirmed tissue plasminogen activator as a potential target for this inhibitor. We show that neuroserpin transcripts are expressed by endocrine cells in the adrenal and pituitary glands and that immunoreactive neuroserpin is stored in densely cored secretory granules in these cells. Overexpression of neuroserpin in an anterior pituitary corticotroph cell line results in the extension of neurite-like processes, suggesting that neuroserpin may play a role in cell communication, cell adhesion, and/or cell migration.

Plasticity and stabilization of neuromuscular and CNS synapses: interactions between thrombin protease signaling pathways and tissue transglutaminase

The first association of the synapse as a potential site of neurodegenerative disease burden was suggested for Alzheimer's disease (AD) almost 30 years ago. Since then protease:protease inhibitor (P:PI) systems were first linked to functional regulation of synaptogenesis and synapse withdrawal at the neuromuscular junction (NMJ) more than 20 years ago. Confirmatory evidence for the involvement of the synapse, the rate-limiting or key unit in neural function, in AD did not become clear until the beginning of the 1990s. However, over the past 15 years evidence for participation of thrombin, related serine proteases and neural PIs, homologous and even identical to those of the plasma clot cascade, has been mounting. Throughout development a balance between stabilization forces, on the one hand, and breakdown influences, on the other, becomes established at synaptic junctions, just as it does in plasma clot proteins. The formation of protease-resistant cross-links by the transglutaminase (TGase) family of enzymes may add to the stability for this balance. The TGase family includes coagulation factor XIIIA and 8 other different genes, some of which may also influence the persistence of neural connections. Synaptic location of protease-activated, G-protein-coupled receptors (PARs) for thrombin and related proteases, their serpin and Kunitz-type PIs such as protease nexin I (PNI), alpha1-antichymotrypsin (alpha-ACT), and the Kunitz protease inhibitor (KPI)-containing secreted forms of beta-amyloid protein precursor (beta-APP), along with the TGases and their putative substrates, have all been amply documented. These findings strongly add to the conclusion that these molecules participate in the eventual structural stability of synaptic connections, as they do in coagulation cascades, and focus trophic activity on surviving terminals during periods of selective contact elimination. In disease states, this imbalance is likely to be shifted in favor of destabilizing forces: increased and/or altered protease activity, enhanced PAR influence, decreased and/or altered protease inhibitor function, reduction and/or alteration in tTG expression and activity, and alteration in its substrate profile. This imbalance further initiates a cascade of events leading to inappropriate programmed cell death and may well be considered evidence of synaptic apoptosis.

Characterisation of two serine protease inhibitors expressed in the pituitary gland

Serine protease inhibitors (serpins) are a family of structurally related proteins that play key roles in the regulation of proteolytic homeostasis. We have isolated a novel intracellular serpin, termed raPIT5a, from the rat pituitary gland. Northern blot analysis indicated raPIT5a mRNA expression in a range of tissues, including the adrenal gland and the brain. In situ hybridisation histochemistry revealed raPIT5a mRNA expression in specific cell populations in the rat pituitary gland, adrenal gland, and pancreas. Based on sequence similarities to other intracellular serpins, we predicted raPIT5a may inhibit the pro-apoptotic serine protease granzyme B. We confirmed this experimentally by identification of a stable inhibitory complex between granzyme B and raPIT5a. To determine whether granzyme B or granzyme B-related enzymes were expressed in the rat pituitary gland, we performed PCR using primers predicted to amplify granzyme B and two other published granzyme sequences. We identified rat natural killer protease-1 (RNKP-1), the rat homologue of granzyme B, and a novel putative serine protease highly similar to granzyme-like protein III (GLP III), which we termed GLP IIIa. These data suggest raPIT5a may regulate apoptosis in the pituitary by inhibition of granzyme B or GLP IIIa, or members of the caspase enzyme family which have similar substrate specificity. We have also identified expression of a second serpin, called neuroserpin, in pituitary tissue and found that it alters the morphology of the AtT20 corticotrope cell line, presumably through changes in cell adhesion. These results identify new roles for serpins in pituitary cell function.

Agonists of proteinase-activated receptor 1 induce plasma extravasation by a neurogenic mechanism

Thrombin, generated in the circulation during injury, cleaves proteinase-activated receptor 1 (PAR1) to stimulate plasma extravasation and granulocyte infiltration. However, the mechanism of thrombin-induced inflammation in intact tissues is unknown. We hypothesized that thrombin cleaves PAR1 on sensory nerves to release substance P (SP), which interacts with the neurokinin 1 receptor (NK1R) on endothelial cells to cause plasma extravasation. PAR1 was detected in small diameter neurons known to contain SP in rat dorsal root ganglia by immunohistochemistry and in situ hybridization. Thrombin and the PAR1 agonist TFLLR-NH(2) (TF-NH(2)) increased [Ca(2+)](i) >50% of cultured neurons (EC(50)s 24 mu ml(-1) and 1.9 microM, respectively), assessed using Fura-2 AM. The PAR1 agonist completely desensitized responses to thrombin, indicating that thrombin stimulates neurons through PAR1. Injection of TF-NH(2) into the rat paw stimulated a marked and sustained oedema. An NK1R antagonist and ablation of sensory nerves with capsaicin inhibited oedema by 44% at 1 h and completely by 5 h. In wild-type but not PAR1(-/-) mice, TF-NH(2) stimulated Evans blue extravasation in the bladder, oesophagus, stomach, intestine and pancreas by 2 - 8 fold. Extravasation in the bladder, oesophagus and stomach was abolished by an NK1R antagonist. Thus, thrombin cleaves PAR1 on primary spinal afferent neurons to release SP, which activates the NK1R on endothelial cells to stimulate gap formation, extravasation of plasma proteins, and oedema. In intact tissues, neurogenic mechanisms are predominantly responsible for PAR1-induced oedema.

Conformational properties of serine proteinase inhibitors (serpins) confer multiple pathophysiological roles

Serine proteinase inhibitors (Serpins) are irreversible suicide inhibitors of proteases that regulate diverse physiological processes such as coagulation, fibrinolysis, complement activation, angiogenesis, apoptosis, inflammation, neoplasia and viral pathogenesis. The molecular structure and physical properties of serpins permit these proteins to adopt a number of variant conformations under physiological conditions including the native inhibitory form and several inactive, non-inhibitory forms, such as complexes with protease or other ligands, cleaved, polymerised and oxidised. Alterations of a serpin which affect its structure and/or secretion and thus reduce its functional levels may result in pathology. Serpin dysfunction has been implicated in thrombosis, emphysema, liver cirrhosis, immune hypersensitivity and mental disorders. The loss of inhibitory activity of serpins necessarily results in an imbalance between proteases and their inhibitors, but it may also have other physiological effects through the generation of abnormal concentrations of modified, non-inhibitory forms of serpins. Although these forms of inhibitory serpins are detected in tissues and fluids recovered from inflammatory sites, the important questions of which conditions result in generation of different molecular forms of serpins, what biological function these forms have, and which of them are directly linked to pathologies and/or may be useful markers for characterisation of disease states, remain to be answered. Elucidation of the biological activities of non-inhibitory forms of serpins may provide useful insights into the pathogenesis of diseases and suggest new therapeutic strategies.

Thrombin-induced activation of cultured rodent microglia

Microglia are the resident immune cells of the CNS. Upon brain damage, these cells are rapidly activated and function as tissue macrophages. The first steps in this activation still remain unclear, but it is widely believed that substances released from damaged brain tissue trigger this process. In this article, we describe the effects of the blood coagulation factor thrombin on cultured rodent microglial cells. Thrombin induced a transient Ca(2+) increase in microglial cells, which persisted in Ca(2+)-free media. It was blocked by thapsigargin, indicating that thrombin caused a Ca(2+) release from internal stores. Preincubation with pertussis toxin did not alter the thrombin-induced [Ca(2+)](i) signal, whereas it was blocked by hirudin, a blocker of thrombin's proteolytic activity. Incubation with thrombin led to the production of nitric oxide and the release of the cytokines tumor necrosis factor-alpha, interleukin-6, interleukin-12, the chemokine KC, and the soluble tumor necrosis factor-alpha receptor II and had a significant proliferative effect. Our findings indicate that thrombin, a molecule that enters the brain at sites of injury, rapidly triggered microglial activation.

Meizothrombin, an intermediate of prothrombin activation, stimulates human glioblastoma cells by interaction with PAR-1-type thrombin receptors

Thrombin induces well-characterized effects on normal and neoplastic brain cells by interaction with protease-activated receptor (PAR)-type thrombin receptors. However, nothing is known about the function of intermediate enzymes of prothrombin activation recently shown to evoke PAR-1-mediated signaling in smooth muscle cells. Therefore, we investigated the effect of recombinant human meizothrombin (rMT), one of thrombin's catalytically active precursor enzymes in the prothrombin cleavage cascade, on calcium mobilization in human SNB-19 glioblastoma cells. By using reverse-transcription polymerase chain reaction, immunofluorescence studies with a monoclonal anti-PAR-1 antibody and calcium measurements, SNB-19 cells were shown to express functional PAR-1-type thrombin receptors. PAR-1 is not only a receptor for thrombin in SNB-19 cells but was also activated by rMT very effectively. Under the conditions used in our experiments, SNB-19 cells stimulated with thrombin after rMT challenge were unable to elicit a new calcium response and vice versa. In addition, both rMT and thrombin induced no further calcium signal after that observed with the PAR-1-activating peptide SFLLRN. Therefore, rMT and thrombin seem to activate calcium signaling by similar mechanisms including PAR-1. Our results demonstrate rMT as a potent activator of PAR-1-type thrombin receptors in SNB-19 glioblastoma cells, suggesting a function of catalytically active thrombin precursor enzymes in cells of glial origin.

The protease thrombin is an endogenous mediator of hippocampal neuroprotection against ischemia at low concentrations but causes degeneration at high concentrations

We have considered the extracellular serine protease thrombin and its receptor as endogenous mediators of neuronal protection against brain ischemia. Exposure of gerbils to prior mild ischemic insults, here two relatively short-lasting occlusions (2 min) of both common carotid arteries applied at 1-day intervals 2 days before a severe occlusion (6 min), caused a robust ischemic tolerance of hippocampal CA1 neurons. This resistance was impaired if the specific thrombin inhibitor hirudin was injected intracerebroventricularly before each short-lasting insult. Thus, efficient native neuroprotective mechanisms exist and endogenous thrombin seems to be involved therein. In vitro experiments using organotypic slice cultures of rat hippocampus revealed that thrombin can have protective but also deleterious effects on hippocampal CA1 neurons. Low concentrations of thrombin (50 pM, 0.01 unit/ml) or of a synthetic thrombin receptor agonist (10 microM) induced significant neuroprotection against experimental ischemia. In contrast, 50 nM (10 units/ml) thrombin decreased further the reduced neuronal survival that follows the deprivation of oxygen and glucose, and 500 nM even caused neuronal cell death by itself. Degenerative thrombin actions also might be relevant in vivo, because hirudin increased the number of surviving neurons when applied before a 6-min occlusion. Among the thrombin concentrations tested, 50 pM induced intracellular Ca(2+) spikes in fura-2-loaded CA1 neurons whereas higher concentrations caused a sustained Ca(2+) elevation. Thus, distinct Ca(2+) signals may define whether or not thrombin initiates protection. Taken together, in vivo and in vitro data suggest that thrombin can determine neuronal cell death or survival after brain ischemia.

Differential cytoskeletal changes during growth cone collapse in response to hSema III and thrombin

Growth cones are known as the site of action of many factors that influence neurite growth behavior. To assess how different collapsing agents influence the growth cone cytoskeleton, we used recombinant human Semaphorin III (hSema III) and the serine protease thrombin. Embryonic chick dorsal root ganglion neurons showed a dramatic depolymerization of actin filaments within 5 min upon hSema III exposure and virtually no influence on microtubules (MT). Only at later time points (20-30 min) was the polymerization/depolymerization rate of MT significantly affected. Thrombin induced a morphologically and kinetically similar growth cone collapse. Moreover, thrombin induced an early and selective depolymerization of dynamic MT, accompanied by the formation of loops of stable MT bundles. Selective changes in the phosphorylation pattern of tau were associated with microtubule assembly in thrombin-induced responses. Our data provide evidence that different signal transduction pathways lead to distinct changes of the growth cone cytoskeleton.

Protease nexin I expression is up-regulated in human skeletal muscle by injury-related factors

Protease nexin I is a 43-50 kDa glycoprotein capable of inhibiting a number of serine proteases. In cultured differentiated human skeletal muscle (myotubes), we previously found that protease nexin I was localized in patches at their surface where it was active and able to inhibit thrombin. To understand the role of skeletal muscle protease nexin I after injury or in inflammatory conditions where thrombin might be extravasated by blood vessels, we examined the role of inflammatory factors on protease nexin I synthesis and secretion by myotubes in culture. By enzyme-linked immunosorbent assay (ELISA) and Western blotting, we found that this serine protease inhibitor is secreted by cultured human myotubes. Protease nexin I secretion is stimulated by tumor necrosis factor-alpha, transforming growth factor-beta and interleukin-1. Complex formation experiments with labeled thrombin reveal active protease nexin I bound to the surface of the treated cells. Secreted protease nexin I-thrombin complex was enhanced in the presence of transforming growth factor-beta and tumor necrosis factor-alpha. Protease nexin I mRNA was detected by reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot analysis. Whatever the conditions, no significantly different levels were observed, indicating that the changes in cell and media protease nexin I concentration are elicited at the translational/posttranslational levels. Immunocytochemical studies on human skeletal muscle biopsies of patients suffering from inflammatory myopathies showed an overexpression of protease nexin I together with the above inflammatory factors. These findings suggest that skeletal muscle protease nexin I might play a role after injury or inflammatory pathologies.

Serpins and regulation of cell death

Proteolysis is a key feature of programmed cell death. Extracellular proteinases can activate cell surface receptors which trigger apoptosis, and the effector machinery requires the activation and activity of numerous intracellular proteinases (primarily caspases). Effective control of proteolysis is essential for homeostasis and can occur at two levels: regulation of proteinase activation, and regulation of the activated proteinase. Serpins control activated proteinases and several have been implicated in the regulation of cell death. Serpins that inhibit intracellular processes include the viral proteins CrmA and SPI-1, as well as the granzyme B inhibitor, PI-9. Another endogenous serpin, PN-I, prevents the delivery of an apoptotic signal by inhibiting an extracellular proteinase from cleaving a cell surface receptor. There is evidence to suggest that PAI-2 may target an extracellular as well as an intracellular proteinase. Much of our knowledge of proteolysis within apoptotic cells has come from studies using the poxvirus serpin CrmA/SPI-2. CrmA prevents cytokine processing by inhibiting caspase-1, and protects against Fas-, TNF- and TRAIL-mediated apoptosis by inhibiting an unidentified proteinase specific to these pathways. Work with CrmA has also clearly demonstrated that there are separable effector mechanisms within cells, and that those triggered by growth factor withdrawal, matrix dissociation or cytotoxic ligands are different in several respects to those triggered by radiation, chemicals or steroid hormones. It is likely that analysis of other poxvirus serpins with different inhibitory profiles (especially SPI-1) will yield further insights into these processes. Prospecting for intracellular serpin genes in other virus species may also be fruitful. Finally, all of the serpins known to regulate intracellular proteolysis are members of the ovalbumin subgroup. It remains to be seen whether the more recently described "orphan" ovalbumin serpins (Riewald and Schleef 1995; Sprecher et al. 1995; Sun et al. 1997) also have roles in the regulation of cell death.

Astrocyte regulation of endothelial tissue plasminogen activator in a blood-brain barrier model

Expression of tissue plasminogen activator (tPA) substantially determines endothelial-dependent fibrinolysis. We used a blood-brain barrier (BBB) model to analyze regulation of brain capillary endothelial tPA and its inhibitor, plasminogen activator inhibitor-1 (PAI-1). This model consists of coculture of murine astrocytes with bovine brain capillary endothelial cells grown as capillary-like structures (CS); after 1 week, astrocytes become extensively associated with CS, and the BBB-associated enzyme gamma-glutamyl transpeptidase is present. We measured tPA and PAI-1 mRNA and tPA activity in this model. Reverse transcription-polymerase chain reaction (RT-PCR) studies showed similar tPA and PAI-1 mRNA levels after 1 day mono-culture (endothelial cells only) versus astrocyte-endothelial coculture preparations. After 7 days (i.e., when elements of the BBB are present), astrocyte-endothelial cocultures (compared with endothelial mono-cultures) showed a 50.7%+/-27.1% (mean +/- SD) reduction in tPA mRNA (P < 0.03) and a 183.3%+/-86.9% increase in PAI-1 mRNA expression (P < 0.02). Moreover, 7-day cocultures demonstrated reduced tPA activity compared with mono-cultures (14.6+/-2.9 IU/mL versus 30.2+/-7.7 IU/mL, P < 0.01); 1-day cocultures and mono-cultures had similar tPA activity. These findings demonstrate that astrocytes regulate brain capillary endothelial expression of tPA when elements of the BBB phenotype are present in this model. These data suggest an important role for astrocytes in the regulation of brain capillary endothelial fibrinolysis.

Serpin=serine protease-like complexes within neurofilament conglomerates of motoneurons in amyotrophic lateral sclerosis

Neurofilamentous conglomerates (NfCg), as axonal spheroids or conglomerates in motoneurons, are the histopathologic hallmarks for early stages of amyotrophic lateral sclerosis (ALS). We hypothesize that NfCg may be formed by post-translational modifications of altered Nf proteins that include: (1) hyperphosphorylation, (2) glycosylation (or glycoxidation), (3) nitration, (4) ubiquitination and/or (5) crosslinking by the Ca++-dependent transglutaminase (TGase). These, as well as other changes, are predicted to be initiated or accentuated by oxidative damage. The damaged Nf proteins then activate cascades of intracellular protein degradation which include ATP-dependent ubiquitin/proteasome proteolysis. Other proteolytic systems, either Ca++-dependent or independent, may also be activated, such as serine and cysteine protease systems. These enzymes, either lysosomal or non-lysosomal may also participate in the degradation of damaged Nf proteins being balanced by their cognate inhibitors. Protein complexes formed by these protease=inhibitor systems, along with damaged Nf proteins, may accumulate within the cell bodies as neuronal inclusions, since a number of intracellular inclusions are found in motor neurons in ALS. In the current study, we investigated the involvement of serine proteases and their serpins in NfCg formation. Pairs of three serine proteases (trypsin, chymotrypsin and thrombin) and their cognate serpins (alpha1-anti-trypsin, alpha1-anti-chymotrypsin, and protease nexin I) were probed in motoneurons with their antibodies for both NfCg and inclusions. Positive immunoreactivities for all serine proteases and their cognate serpins support the contention that the imbalance of serine proteases and internalized serpins may have a role in formation of NfCg and inclusions, and hence, the pathogenesis of ALS.

Thrombin perturbs neurite outgrowth and induces apoptotic cell death in enriched chick spinal motoneuron cultures through caspase activation

Increasing evidence indicates several roles for thrombin-like serine proteases and their cognate inhibitors (serpins) in normal development and/or pathology of the nervous system. In addition to its prominent role in thrombosis and/or hemostasis, thrombin inhibits neurite outgrowth in neuroblastoma and primary neuronal cells in vitro, prevents stellation of glial cells, and induces cell death in glial and neuronal cell cultures. Thrombin is known to act via a cell surface protease-activated receptor (PAR-1), and recent evidence suggests that rodent neurons express PAR-1. Previously, we have shown that the thrombin inhibitor, protease nexin-1, significantly prevents neuronal cell death both in vitro and in vivo. Here we have examined the effects of human alpha-thrombin and the presence and/or activation of PAR-1 on the survival and differentiation of highly enriched cultures of embryonic chick spinal motoneurons. We show that thrombin significantly decreased the mean neurite length, prevented neurite branching, and induced motoneuron death by an apoptosis-like mechanism in a dose-dependent manner. These effects were prevented by cotreatment with hirudin, a specific thrombin inhibitor. Treatment of the cultures with a synthetic thrombin receptor-activating peptide (SFLLRNP) mimicked the deleterious effects of thrombin on motoneurons. Furthermore, cotreatment of the cultures with inhibitors of caspase activities completely prevented the death of motoneurons induced by either thrombin or SFLLRNP. These findings indicate that (1) embryonic avian spinal motoneurons express functional PAR-1 and (2) activation of this receptor induces neuronal cell degeneration and death via stimulation of caspases. Together with previous reports, our results suggest that thrombin, its receptor(s), and endogenous thrombin inhibitors may be important regulators of neuronal cell fate during development, after injury, and in pathology of the nervous system.

Prothrombin in normal human cerebrospinal fluid originates from the blood

In spite of the fact that prothrombin is produced by cells within the central nervous system, its presence in the cerebrospinal fluid (CSF) has not been investigated. We determined the concentration of prothrombin in CSF with reference to the concentration in plasma in paired samples from 18 "normal" control patients and 4 patients with relapsing-remitting type of multiple sclerosis (MS). The newly developed ELISA was very specific (no cross-reactivity with thrombin) and sensitive (detection limit--0.7 ng/ml) with an imprecision of CV = 8.3% (intraseries) and 7.0% (interassay). The mean prothrombin concentration in normal CSF was 0.55 mg/l (CV +/- 33%, range: 0.28-0.93 mg/l), in normal plasma 121.8 mg/l +/- 21%, resulting in a mean CSF/plasma concentration quotient (Q(Proth)--4.5 x 10(-3) (CV +/- 35%, range: 2.1-8.3 x 10(-3)) corresponding to a mean albumin quotient in this group of subjects of Q(Alb) = 5.8 x 10(-3). Due to the Q(Proth) and the molecular weight of prothrombin (72 kDa)--similar to that of albumin--we conclude that prothrombin in normal human CSF originates predominantly (>95%) from blood. The enzymatic activity in CSF is conserved. Comparable results obtained in MS patients with only few small MRI lesions suggest that local chronic inflammatory disease of the central nervous system does not influence prothrombin concentration in the CSF if the blood-CSF barrier function is normal.

Thrombin induced inhibition of neurite outgrowth from dorsal root ganglion neurons

Thrombin is a multifunctional protease. Recent studies on cultured neuronal cells have suggested a function for thrombin in the development and maintenance of the nervous system. Thrombin has been found to induce neurite retraction and reverse stellation in neuroblastoma cell lines and rat astrocytes, respectively. The major focus of our study was to investigate the potential role of thrombin in peripheral nervous system development using the rat embryonic dorsal root ganglion model. We found a dose dependent inhibition of neurite outgrowth from explant dorsal root ganglion cultures upon exposure to 2 to 200 nM thrombin. This effect was reversed by the specific thrombin inhibitor, hirudin. A synthetic peptide that imitates the fully active receptor, thrombin receptor activating peptide, was also found to inhibit neurite outgrowth from dorsal root ganglia. bis-Benzimide stained neuronal cultures did not show any evidence of cell death after exposure to thrombin or thrombin receptor activating peptides. Immunohistochemical studies revealed specific staining of the thrombin receptor on neurons, with intense labeling along neurites. Enriched neuronal cultures exposed to thrombin and thrombin receptor activating peptides revealed rapid activation of phospholipase Cgamma-1, a second messenger associated with the thrombin receptor. These findings are the first to describe the localization of the thrombin receptor to dorsal root ganglion neurons. We propose that receptor activation is associated with thrombin induced inhibition of neurite outgrowth.