Regulation of neurons and astrocytes by thrombin and protease nexin-1. Relationship to brain injury

Thrombin and protein C pathway in peripheral nerve Schwann cells

Thrombin and activated protein C (aPC) bound to the endothelial protein C receptor (EPCR) both activate protease-activated receptor 1 (PAR1) generating either harmful or protective signaling respectively. In the present study we examined the localization of PAR-1 and EPCR and thrombin activity in Schwann glial cells of normal and crushed peripheral nerve and in Schwannoma cell lines. In the sciatic crush model nerves were excised 1h, 1, 4, and 7days after the injury. Schwannoma cell lines produced high levels of prothrombin which is converted to active thrombin and expressed both EPCR and PAR-1 which co-localized. In the injured sciatic nerve thrombin levels were elevated as early as 1h after injury, reached their peak 1day after injury which was significantly higher (24.4±4.1mU/ml) compared to contralateral uninjured nerves (2.6±7mU/ml, t-test p<0.001) and declined linearly reaching baseline levels by day 7. EPCR was found to be located at the microvilli of Schwann cells at the node of Ranvier and in cytoplasm surrounding the nucleus. Four days after sciatic injury, EPCR levels increased significantly (57,785±16602AU versus 4790±1294AU in the contralateral uninjured nerves, p<0.001 by t-test) mainly distal to the site of injury, where axon degeneration is followed by proliferation of Schwann cells which are diffusely stained for EPCR. EPCR seems to be located to cytoplasmic component of Schwann cells and not to compact myelin component, and is highly increased following injury.

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.

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.

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.

The role of thrombin and thrombin receptors in ischemic, hemorrhagic and traumatic brain injury: deleterious or protective?

In the last two decades it has become apparent that thrombin has many extravascular effects that are mediated by a family of protease-activated receptors (PARs). PAR-1, -3 and -4 are activated via cleavage by thrombin. The importance of extravascular thrombin in modulating ischemic, hemorrhagic and traumatic injury in brain has recently become clear. Thus, in vitro, thrombin at low concentration protects neurons and astrocytes from cell death caused by a number of different insults. In vivo, pretreating the brain with a low dose of thrombin (thrombin preconditioning), attenuates the brain injury induced by a large dose of thrombin, an intracerebral hemorrhage or by focal cerebral ischemia. Thrombin may also be an important mediator of ischemic preconditioning. In contrast, high doses of thrombin kill neurons and astrocytes in vitro and cause disruption of the blood-brain barrier, brain edema and seizures in vivo. This review examines the role of thrombin in brain injury and the molecular mechanisms and signaling cascades involved.

Protease nexin 1 is a potent urinary plasminogen activator inhibitor in the presence of collagen type IV

Protease nexin 1 (PN1) in solution forms inhibitory complexes with thrombin or urokinase, which have opposing effects on the blood coagulation cascade. An initial report provided data supporting the idea that PN1 target protease specificity is under the influence of collagen type IV (1). Although collagen type IV demonstrated no effect on the association rate between PN1 and thrombin, the study reported that the association rate between PN1 and urokinase was allosterically reduced 10-fold. This has led to the generally accepted idea that the primary role of PN1 in the brain is to act as a rapid thrombin inhibition and clearance mechanism during trauma and loss of vascular integrity. In studies to identify the structural determinants of PN1 that mediate the allosteric interaction with collagen type IV, we found that protease specificity was only affected after transient exposure of PN1 to acidic conditions that mimic the elution protocol from a monoclonal antibody column. Because PN1 used in previous studies was purified over a monoclonal antibody column, we propose that the allosteric regulation of PN1 target protease specificity by collagen type IV is a result of the purification protocol. We provide both biochemical and kinetic data to support this conclusion. This finding is significant because it implies that PN1 may play a much larger role in the modeling and remodeling of brain tissues during development and is not simply an extravasated thrombin clearance mechanism as previously suggested.

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.

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.

Induction of colligin may attenuate brain edema following intracerebral hemorrhage

Brain edema plays an important role in the secondary brain injury following intracerebral hemorrhage (ICH). Edema formation after ICH has been linked to thrombin toxicity. Therefore, the induction of endogenous serine protease inhibitors, which inhibit thrombin prior to ICH may limit edema formation. This study examines whether injection of a low dose of thrombin upregulates such inhibitors and induces tolerance to subsequent ICH. Rats received intracerebral infusions of either one unit thrombin or saline into the right caudate nucleus. After seven days, the rats were either (A) used to examine colligin (a serine protease inhibitor) induction by Western blot analysis, immunohistochemistry and immunofluorescent double labeling, (B) to determine brain water content, or (C) they received a second injection of 50 microL blood and brain edema was determined one day later. Intracerebral infusion of thrombin caused a marked upregulation of colligin, a serine protease inhibitor, in the ipsilateral basal ganglia. Immunocytochemistry and immunofluorescent double labeling showed that colligin was induced in astrocytes. Infusion of this dose of thrombin alone did not affect brain water content but it significantly attenuated subsequent ICH-induced brain edema (79.0 +/- 0.5 vs. 81.4 +/- 0.9%, P < 0.01). Our results demonstrate that low doses of thrombin upregulate brain colligin levels and attenuate edema formation induced by ICH.

Therapeutic potential of anti-inflammatory drugs in focal stroke

The importance of cytokines, especially TNF-alpha and IL-1beta, are emphasised in the propagation and maintenance of the brain inflammatory response to injury. Much data supports the case that ischaemia and trauma elicit an inflammatory response in the injured brain. This inflammatory response consists of mediators (cytokines, chemokines and adhesion molecules) followed by cells (neutrophils early after the onset of brain injury and then a later monocyte infiltration). De novo upregulation of pro-inflammatory cytokines, chemokines and endothelial-leukocyte adhesion molecules occurs soon after focal ischaemia and trauma, as well as at the time when the tissue injury is evolving. The significance of this brain inflammatory response and its contribution to brain injury is now becoming more understood. In this review, we discuss the role of TNF-alpha and IL-1beta in traumatic and ischaemic brain injury and associated inflammation and the co-operative actions of chemokines and adhesion molecules in this process. We also address novel approaches to target cytokines and reduce the brain inflammatory response and thus brain injury, in stroke and neurotrauma. The mitogen-activated protein kinase (MAPK), p38, has been linked to inflammatory cytokine production and cell death following cellular stress. Stroke-induced p38 enzyme activation in the brain has been demonstrated and treatment with a second generation p38 MAPK inhibitor, SB-239063, provides a significant reduction in infarct size, neurological deficits and inflammatory cytokine expression produced by focal stroke. SB-239063 can also provide direct protection of cultured brain tissue to in vitro ischaemia. This robust SB-239063-induced neuroprotection emphasises a significant opportunity for targeting MAPK pathways in ischaemic stroke injury and also suggests that p38 inhibition should be evaluated for protective effects in other experimental models of nervous system injury and neurodegeneration.

Roles of the heparin and low density lipid receptor-related protein-binding sites of protease nexin 1 (PN1) in urokinase-PN1 complex catabolism. The PN1 heparin-binding site mediates complex retention and degradation but not cell surface binding or internalization

We have previously described thrombin (Th)-protease nexin 1 (PN1) inhibitory complex binding to cell surface heparins and subsequent low density lipid receptor-related protein (LRP)-mediated internalization. Our present studies examine the catabolism of urinary plasminogen activator (uPA)-PN1 inhibitory complexes, which, unlike Th.PN1 complexes, bind almost exclusively through the uPA receptor. In addition, the binding site in PN1 required for the LRP-mediated internalization of Th.PN1 complexes is not required for the LRP-mediated internalization of uPA.PN1 complexes. Thus, the protease moiety of the complex partially determines the mechanistic route of entry. Because cell surface heparins are only minimally involved in the binding and internalization of uPA.PN1 complexes, we then predicted that complexes between uPA and the heparin binding-deficient PN1 variant, PN1(K7E), should be catabolized at the same rate as complexes formed with native PN1. Surprisingly, the uPA.PN1(K7E) complexes were degraded at only a fraction of the rate of native complexes. Internalization studies revealed that both uPA. PN1(K7E) and native uPA.PN1 complexes were initially internalized at the same rate, but uPA.PN1(K7E) complexes were rapidly retro-endocytosed in an intact form. By examining the pH dependence of complex binding in the range of 4.0-7.0, it was determined that the uPA.PN1 inhibitory complexes must specifically bind to endosomal heparins at pH 5.5 to be retained and sorted to lysosomes. These studies are the first to document a role for heparins in the catabolism of SERPIN-protease complexes at a point further in the pathway than cell surface binding, and this role may extend to other heparin-binding LRP-internalized ligands.

Injury induces neuropsin mRNA in the central nervous system

We have shown that neuropsin is expressed in the neurons of the limbic system in the adult mouse. After the central nervous system was injured by incision or intraperitoneal kainate injection, neuropsin mRNA was induced in the peri-lesioned region. The cells in which neuropsin mRNA was induced were localized mainly in axon fiber pathways and closely associated to proteolipid protein (PLP) mRNA expressing oligodendrocytes.