Thrombin receptor peptides induce shape change in neonatal murine astrocytes in culture

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 Evolving Concept of Neuro-Thromboinflammation for Neurodegenerative Disorders and Neurotrauma: A Rationale for PAR1-Targeting Therapies

Interest in the role of coagulation and fibrinolysis in the nervous system was active in several laboratories dating back before cloning of the functional thrombin receptor in 1991. As one of those, our attention was initially on thrombin and plasminogen activators in synapse formation and elimination in the neuromuscular system, with orientation towards diseases such as amyotrophic lateral sclerosis (ALS) and how clotting and fibrinolytic pathways fit into its pathogenesis. This perspective is on neuro-thromboinflammation, emphasizing this emerging concept from studies and reports over more than three decades. It underscores how it may lead to novel therapeutic approaches to treat the ravages of neurotrauma and neurodegenerative diseases, with a focus on PAR1, ALS, and parmodulins.

Astrocytes and the Warning Signs of Intracerebral Hemorrhagic Stroke

Two decades into the two thousands, intracerebral hemorrhagic stroke (ICH) continues to reap lives across the globe. In the US, nearly 12,000 people suffer from ICH every year. Half of them survive, but many are left with permanent physical and cognitive disabilities, the severity of which depends on the location and broadness of the brain region affected by the hemorrhage. The ongoing efforts to identify risk factors for hemorrhagic stroke have been instrumental for the development of new medical practices to prevent, aid the recovery and reduce the risk of recurring ICH. Recent efforts approach the study of ICH from a different angle, providing information on how we can limit brain damage by manipulating astrocyte receptors. These results provide a novel understanding of how astrocytes contribute to brain injury and recovery from small ICH. Here, we discuss current knowledge on the risk factors and molecular pathology of ICH and the functional properties of astrocytes and their role in ICH. Last, we discuss candidate astrocyte receptors that may prove to be valuable therapeutic targets to treat ICH. Together, these findings provide basic and clinical scientists useful information for the future development of strategies to improve the detection of small ICH, limit brain damage, and prevent the onset of more severe episodes of brain hemorrhage.

The role of proteases in pain

Proteinase-activated receptors (PARs) are a family of G protein-coupled receptor that are activated by extracellular cleavage of the receptor in the N-terminal domain. This slicing of the receptor exposes a tethered ligand which binds to a specific docking point on the receptor surface to initiate intracellular signalling. PARs are expressed by numerous tissues in the body, and they are involved in various physiological and pathological processes such as food digestion, tissue remodelling and blood coagulation. This chapter will summarise how serine proteinases activate PARs leading to the development of pain in several chronic pain conditions. The potential of PARs as a drug target for pain relief is also discussed.

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 activation of astrocytes in mixed rat hippocampal cultures is inhibited by soluble thrombomodulin

Thrombin, a serine protease known for its role in coagulation, also induces a variety of protease activated receptor (PAR)-mediated responses in the central nervous system that contribute to many brain pathologies. Since the proteolytic specificity of thrombin is uniquely controlled by thrombomodulin (TM), we investigated the mechanisms by which thrombin and a recombinant soluble form of human TM (Solulin, INN: sothrombomodulin alpha; rhsTM) could influence rat hippocampal cultures. Treatment of hippocampal cultures with thrombin for up to 48h resulted in a significant morphological rearrangement with the formation of expansive cell-free areas (CFAs) and a reduction in cell viability; both effects were blocked by rhsTM. Treatment with the selective PAR-1 agonist, TRAP (SFLLRN) caused the formation of CFAs, suggesting that CFA formation involved PAR-1 signaling. Astrocytes prepared from PAR-1(-/-) mice also had an attenuated CFA response to thrombin. Thrombin-induced CFA formation was a consequence of cell movement and substantial changes in cell morphology, rather than due to cell detachment. Immunocytochemical and functional analyses revealed that the thrombin-sensitive cells within these hippocampal cultures were astrocytes. The effects of thrombin on CFA development were mediated by astrocyte-specific release of intracellular calcium and signalling through ERK1/2. rhsTM was able to attenuate thrombin-induced ERK1/2 phosphorylation. Finally, astrocytes were shown to maintain thrombin-sensitivity following neuronal depletion with NMDA, a result which was confirmed with pure astrocyte cultures. Hence thrombin mediates PAR-1-induced activation of hippocampal astrocytes, but not neurons, in a process that can be modulated by rhsTM.

Involvement of PGE2 and PGDH but not COX-2 in thrombin-induced cortical neuron apoptosis

The pathways that contribute to thrombin-induced neuron death have been incompletely defined. Induction of cyclooxygenase 2 (COX-2), the enzyme that catalyzes the first step in prostaglandin synthesis, promotes neuronal injury. PGE2, a downstream product of COX-2 metabolism, is neurotoxic in vitro and in vivo, and is thought to be the bioactive mediator responsible for COX-2 neurotoxicity. The objective of this study is to determine the ability of thrombin to affect PGE2 metabolism in cultured neurons. The data show that in thrombin-induced apoptosis of cultured neurons, PGE2 release increases when COX-2 is absent, and is regulated by prostaglandin dehydrogenase (PGDH), a key enzyme that degrades PGE2. NS398, a COX-2 specific inhibitor, protects neurons against thrombin toxicity, by inducing active PGDH. These data implicate PGDH in thrombin-mediated neuronal cell death.

Cystatin C expression in ischemic white matter lesions

OBJECTIVES

To study the involvement of cystatin C in the progression of ischemic white matter lesions (WMLs).

MATERIALS AND METHODS

Cystatin C levels in the cerebrospinal fluid (CSF) of patients with cerebrovascular disease, and also in primary and established human neural cell cultures were investigated. For pathologic analysis, cystatin C immunoreactivity was investigated in the white matter of patients with severe WMLs, mild WMLs or controls.

RESULTS

Cystatin C levels in the CSF of patients with Fazekas WML grade 3 [14 with hypertension; W/HT(+) and nine without hypertension; W/HT(-)] were lower than those in 38 patients with grade 0-1 (P = 0.0022 and P < 0.0001 respectively). Immunohistochemical study showed that the cystatin C immunoreactivity was found in astrocytes, and the number of astrocytes in the white matter in the severe WML group was decreased when compared with that in controls (P = 0.0027) and in the mild WML group (P = 0.0024). In human neural cell cultures, treatments with thrombin, matrix metalloproteinases and interleukin 1 beta increased the expression of cystatin C mRNA in human astrocytes and hybrid neurons, but an enzyme-linked immunosorbent assay revealed that only thrombin significantly increased the production and secretion of cystatin C in astrocytes.

CONCLUSIONS

These results suggest that low levels of CSF cystatin C in ischemic WMLs might be due to the decreased number of astrocytes that secrete cystatin C in response to the stimuli of proteases and inflammatory cytokines.

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.

The effect of thrombin on astrocyte stellation with regional specificity

In this study, we have examined the possible existence of astrocyte regional heterogeneity in thrombin effect on astrocyte stellation. Neonatal astrocytes were cultured for 2 weeks from six different regions of the neonatal rat brain, including the cerebral cortex, hippocampus, brainstem, midbrain, hypothalamus and cerebellum. Culture medium was changed to DMEM containing 8-CPT-cyclic AMP (cAMP) or isoproterenol plus various concentrations of thrombin for 2 hr. Thrombin effectively blocked both cAMP- and isoproterenol-induced cell stellation in a dose-dependent manner in all regional astrocytes except cerebellar astrocytes. RT-PCR analysis showed that thrombin receptor mRNA was expressed in all regional astrocytes, suggesting that cerebellar astrocytes may maintain a unique signaling pathway downstream of the thrombin receptor.

Evidence that protease-activated receptor-2 mediates trypsin-induced reversal of stellation in cultured rat astrocytes

Serine proteases such as thrombin and trypsin play a key role in the development and repair processes in the central nervous system. Molecular actions of serine proteases include multiple cellular events like activation of protease-activated receptors (PARs). PARs belong to a family of G protein-coupled receptors that can be stimulated through their proteolytic cleavage by ligands. PAR-2 has been implicated in neurodegenerative diseases including astrogliosis. Although recent studies have shown that low concentration of trypsin activates PAR-2, its role in morphological changes in primary astrocytes has not been studied. In the present study, we investigated the effects of PAR-2 in astrocyte stellation in rat primary astrocyte culture. Both trypsin (0.1-1 U/ml) and a PAR-2-activating peptide SLIGRL-NH2 (1-50 microM) significantly reversed the stellation induced by serum deprivation in rat astrocytes. Treatment of astrocytes with trypsin or SLIGRL-NH2 resulted in a transient rise of the intracellular Ca2+ level and trypsin-induced morphological changes were blocked by BAPTA, a Ca2+ chelator. In addition, a protein kinase C (PKC) inhibitor, bisindolylmaleimide significantly inhibited the trypsin-induced morphological changes, whereas activation of PKC by phorbol-12-myristate-13-acetate acted as trypsin. Taken together, these results suggest that activation of PAR-2 by trypsin caused reversal of stellation in cultured astrocytes, in part, via the mobilization of intracellular Ca2+ and activation of PKC.

Expression of mRNA for four subtypes of the proteinase-activated receptor in rat dorsal root ganglia

Proteinase-activated receptors (PARs) are members of the superfamily of G-protein coupled receptors that initiate intracellular signaling by the proteolytic activity of extracellular serine proteases. Three member of this family (PAR-1, PAR-3, and PAR-4) are considered thrombin receptors, whereas PAR-2 is activated by trypsin and tryptase. Recently, activation of PAR-2 signal was identified as a pro-inflammatory factor that mediates peripheral sensitization of nociceptors. Activation of PAR-1 in the periphery is also considered to be a neurogenic mediator of inflammation that is involved in peptide release. Here, we investigated the expression of these four members of PARs in the adult rat dorsal root ganglia (DRG) using radioisotope-labeled in situ hybridization histochemistry. We detected mRNA for all subtypes of PARs in the DRG. Histological analysis revealed the specific expression patterns of the PARs. PAR-1, PAR-2, and PAR-3 mRNA was expressed in 29.0+/-4.0%, 16.0+/-3.2%, and 40.9+/-1.3% of DRG neurons, respectively. In contrast, PAR-4 mRNA was mainly observed in non-neuronal cells. A double-labeling study of PARs with NF-200 and alpha calcitonin gene-related peptide (CGRP) also revealed the distinctive expression of PARs mRNA in myelinated or nociceptive neurons. This study shows the precise expression pattern of PARs mRNA in the DRG and indicates that the cells in DRG can receive modulation with different types of proteinase-activated receptors.

Activation of protease-activated receptor-1 triggers astrogliosis after brain injury

We have studied the involvement of the thrombin receptor [protease-activated receptor-1 (PAR-1)] in astrogliosis, because extravasation of PAR-1 activators, such as thrombin, into brain parenchyma can occur after blood-brain barrier breakdown in a number of CNS disorders. PAR1-/- animals show a reduced astrocytic response to cortical stab wound, suggesting that PAR-1 activation plays a key role in astrogliosis associated with glial scar formation after brain injury. This interpretation is supported by the finding that the selective activation of PAR-1 in vivo induces astrogliosis. The mechanisms by which PAR-1 stimulates glial proliferation appear to be related to the ability of PAR-1 receptor signaling to induce sustained extracellular receptor kinase (ERK) activation. In contrast to the transient activation of ERK by cytokines and growth factors, PAR-1 stimulation induces a sustained ERK activation through its coupling to multiple G-protein-linked signaling pathways, including Rho kinase. This sustained ERK activation appears to regulate astrocytic cyclin D1 levels and astrocyte proliferation in vitro and in vivo. We propose that this PAR-1-mediated mechanism underlying astrocyte proliferation will operate whenever there is sufficient injury-induced blood-brain barrier breakdown to allow extravasation of PAR-1 activators.

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.

Characterization of the prothrombin gene expression during nerve differentiation

The transcriptional regulation of the prothrombin gene expression in neuroblastomas was investigated because of the interest in non-hepatic thrombin expression and function in the nervous system. The data indicated that the murine prothrombin gene was distinctively transcribed in proliferating murine N2a cells and that the transcripts were decreased during the differentiation of N2a cells. The gene transcription in proliferating N2a cells was due to the C-I nuclear complex formation in the promoter region, -248/-140. Mutation analyses indicated that nucleotides from -237 to -231 are the core C-I binding site while the longer sequence -248/-140 is needed for the C-I binding. The C-I binding to the promoter -248/-140 could be inhibited by the presence of competitor probe -187/-166, and the mutation in nucleotides from -186 to -179 significantly diminished not only the formation of C-I binding in the promoter region but also the promoter activity in proliferating neuroblastoma cells. Cyclic AMP response element (CRE) modulator, CREM, appeared to selectively bind to the sequence encompassing -186/-179. Taken together, the results indicate that the prothrombin gene transcription in proliferating N2a cells was critically dependent on the cooperative interaction between the factor(s) binding to the C-1 cis-acting element (-237/-231) and the putative CRE site (-186/-179) in the prothrombin promoter, and that the lack of prothrombin expression that coincided with nerve differentiation was mainly due to the lack of C-I complex formation in the promoter.

Prothrombin overexpressed in post-natal neurones requires blood factors for activation in the mouse brain

Thrombin is thought to mediate, through protease-activated receptors, both protective as well as cytotoxic effects. As thrombin receptors are expressed in the CNS, an important question arises as to whether the intact nervous system is able to generate thrombin by activation of its precursor prothrombin, derived endogenously or only upon extravasation following brain injury. To address this question, transgenic mice that express C-terminally haemagglutinin tagged human prothrombin in post-mitotic neurones were generated. In situ hybridization and immunohistochemical analysis showed abundant and widespread cerebral expression of the transgene. Amidolytic assays of brain homogenates and hippocampal slice cultures demonstrated that activation of transgenic prothrombin required added factors, such as snake venom or blood components. This strongly suggests that any possible action of thrombin in the adult CNS depends on blood-derived factors that activate prothrombin. Furthermore, the results are consistent with the idea that in the non-pathological situation an as yet unidentified ligand activates thrombin receptors in the nervous system.

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.

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 downregulates muscle acetylcholine receptors via an IP3 signaling pathway by activating its G-protein-coupled protease-activated receptor-1

Regulation of thrombin activity may be required during skeletal muscle differentiation since the thrombin tissue inhibitor protease nexin-1 appears at the myotube stage before being localized at the neuromuscular synapse. Here, we have used a model of rat fetal myotube primary cultures to study the effect of thrombin on acetylcholine receptor (AChR) expression, which is enhanced at the myotube stage. Our results show that thrombin decreases both the number of surface AChRs (AChRn) and AChR alpha-subunit gene expression. Using the agonist peptide SFLLRN, we establish that the AChRn decrease is mediated by the G protein-coupled thrombin receptor "protease-activated receptor-1" (PAR-1). Moreover, the specific thrombin inhibitor hirudin increases AChRn by inhibiting the thrombin intrinsically present in the cultures. We further demonstrate that the activation of PAR-1 by thrombin induces intracellular calcium movements that are blocked by 2-APB, an inhibitor of inositol 1,4,5-triphosphate (IP3)-induced calcium release. These calcium signals are more intense in nuclei than in the cytoplasm and are consistent with the intracellular distribution of IP3 receptor that we find in the cytoplasm in a cross-striated pattern and at a high level in the nuclear envelope zone. Finally, we show that the blockade of these IP3-induced calcium signals by 2-APB prevents the AChRn decrease induced by thrombin. Our results thus demonstrate that thrombin downregulates AChR expression by activating PAR-1 and that this effect is mediated via an IP3 signaling pathway.

Role and expression of thrombin receptor PAR-1 in muscle cells and neuromuscular junctions during the synapse elimination period in the neonatal rat

A role for thrombin and its receptor (ThR) during mammalian skeletal muscle cell differentiation and neuromuscular junction (NMJ) formation has been suggested. Previously, we found that the synapse elimination process in the neonatal rat muscle was accelerated by thrombin and blocked by hirudin, its specific inhibitor (Lanuza et al. [2001] J. Neurosci. Res. 63:330-340). To test whether this process resulted from a signal transduction cascade initiated by activation of ThR, in particular PAR-1, we applied to the levator auris longus (LAL) muscle of newborn rats two synthetic peptides (SFLL and FSLL). SFLL is a potent specific agonist for activation of PAR-1, whereas FSLL is an inactive peptide. We have demonstrated that the activation of PAR-1 by SFLL produced acceleration of the presynaptic loss of connections and the postsynaptic maturation of NMJs. Moreover, Western blot analysis showed that PAR-1 was present in the skeletal muscle, and by immunohistochemistry we detected PAR-1 in muscle fibers concentrated in the synaptic area but also in satellite cells. Several lines of evidence suggested that PAR-1 is localized in the postsynaptic membrane: PAR-1 immunofluorescence was concentrated at denervated synaptic sites and was present in the myotube membrane in vitro in the absence of neurons and in dissociated single muscle fibers from which nerve terminals and Schwann cells had been removed. Taken together, these results indicate that thrombin mediates certain stages of activity-dependent synapse elimination in the skeletal muscle and does so through its action on the thrombin receptor PAR-1 localized, at least in part, on the postsynaptic membrane.

Stage-specific gene expression in early differentiating oligodendrocytes

The screening of a differential library from precursor and differentiated oligodendrocytes, obtained through the representational difference analysis (RDA) technique, has generated a number of cDNA recombinants corresponding to mRNA coding for known and unknown proteins: (1) mRNA coding for proteins involved in protein synthesis, (2) mRNA coding for proteins involved in the organization of the cytoskeleton, and (3) mRNA coding for proteins of unknown function. The expression profile of the mRNA was studied by Northern blot hybridization to the poly-A(+) mRNA from primary rat progenitor and differentiated oligodendrocytes. In most cases, hybridization to the precursor was higher than hybridization to the differentiated mRNA, supporting the validity of the differential screening. Hybridization of the cDNA to rat cerebral hemisphere and brain stem poly-A(+) mRNA, isolated from 1- to 90-day-old rats, confirms the results obtained with the mRNA from differentiating oligodendrocytes. The intensity of the hybridization bands decreases as differentiation proceeds. The pattern of expression observed in oligodendrocytes is different from that found in the brain only in the case of the nexin-1 mRNA, the level of which remains essentially constant throughout differentiation both in the brain stem and in the cerebral hemispheres, in agreement with the published data. In contrast, the intensity of hybridization to the oligodendrocyte mRNA is dramatically lower in the differentiated cells compared with the progenitor oligodendrocyte cells. Some of the recombinant cDNA represent mRNA sequences present at high frequency distribution in the cells, while others belong to the rare sequences group. Six recombinants code for proteins of the ribosomal family, suggesting that of approximately 70 known ribosomal proteins, only a few are upregulated during oligodendrocyte differentiation. The third category of open reading frame (ORF) is represented by rare messengers coding for proteins of unknown functions and includes six clones: RDA 279, 11, 95, 96, 254, and 288.

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.

Thrombin-receptor activation and thrombin-induced brain tolerance

The authors previously found that pretreatment with a low dose of thrombin attenuates the brain edema induced by a large dose of thrombin or an intracerebral hemorrhage, and reduces infarct volume after focal cerebral ischemia (i.e., thrombin preconditioning). This study investigated whether thrombin preconditioning is caused by activation of the thrombin receptor, also called protease-activated receptor. In the in vivo studies, thrombin-induced brain tolerance was eliminated by RPPGF (Arg-Pro-Pro-Gly-Phe), a thrombin-receptor antagonist. Pretreatment with a thrombin-receptor agonist reduced the amount of edema induced by a large dose of thrombin infused into the ipsilateral basal ganglia 7 days later (81.3 +/- 0.7% vs. 82.6 +/- 0.8% in the control, P < 0.05). In the in vitro study, low doses of thrombin (1 or 2 U/mL) did not induce cell death. However, doses greater than 5 U/mL resulted in dose-dependent lactate dehydrogenase release (P < 0.01). Thrombin and thrombin receptor-activating peptide preconditioning reduced lactate dehydrogenase release induced by a high dose of thrombin (10 and 20 U/mL), whereas RPPGF blocked the effect of thrombin preconditioning in vitro. Western blots indicated that p44/42 mitogen-activated protein kinases were activated after thrombin preconditioning. Finally, inhibition of p44/42 mitogen-activated protein kinases activation by PD98059 abolished the thrombin-preconditioning effect. Results indicate that thrombin-induced brain tolerance is in part achieved through activation of the thrombin receptor. Activation of the thrombin receptor in the brain may be neuroprotective. The protective effect of thrombin preconditioning is achieved through the p44/42 mitogen-activated protein kinase signal-transduction pathway.

Four different types of protease-activated receptors are widely expressed in the brain and are up-regulated in hippocampus by severe ischemia

A variety of extracellular serine proteases are expressed in the central nervous system or might permeate the blood-brain barrier under pathological conditions. However, their intracerebral targets and physiological functions are largely unknown. Here, we show that four distinct subtypes of protease-activated receptors (PARs) are abundantly expressed in the adult rat brain and in organotypic hippocampal slice cultures. PAR-1 expression was significant in the hippocampus, cortex and amygdala. Highest densities of PAR-2 and PAR-3 were observed in hippocampus, cortex, amygdala, thalamus, hypothalamus and striatum. Apart from the striatum, a similar localization was found for PAR-4. Within the hippocampal formation, each PAR subtype was predominantly localized in the pyramidal cell layers. Additionally, we identified PAR-2 in mossy fibers between dentate gyrus and CA3, PAR-3 in the subiculum and PAR-4 in CA3 and in mossy fibres as well as in the stratum lacunosum moleculare. After exposing hippocampal slice cultures to a severe experimental ischemia (oxygen-glucose deprivation), the expression of PARs 1-3 was up-regulated with subtype-specific kinetics. The localization of PARs in brain regions particularly vulnerable to ischemic insults as well as distinct alterations in the expression pattern after experimental ischemia support the notion of an important role of extracellular serine proteases and PARs in cerebral ischemia.

Protease-activated receptors in inflammation, neuronal signaling and pain

The ability of proteases to regulate cell function via protease-activated receptors (PARs) has led to new insights about the potential physiological functions of these enzymes. Several studies suggest that PARs play roles in both inflammation and tissue repair, depending on the cellular environment in which they act. The recent detection of PARs on peripheral and central neurons suggests that neuronal PARs might be involved not only in neurogenic inflammation and neurodegenerative processes, but also in nociception. Thus, the list of potential roles for PARs has lengthened considerably and their physiological course of action might be much broader than initially anticipated.

A 295-kDA intermediate filament-associated protein in radial glia and developing muscle cells in vivo and in vitro

The RC2 antibody is frequently used to label mouse radial glial cells in all parts of the nervous system where neuronal migration occurs during embryonic and early postnatal life. The antigen recognized by this antibody still needs to be identified. We have characterized further its localization in vivo, its expression and subcellular localization in vitro, as well as its molecular nature. Histologic investigations of whole mouse embryos reveal an equally intense expression of RC2 immunostaining in radial glial cells in brain and spinal cord and in skeletal muscle. In glial cells cultures, the RC2 antibody recognizes an epitope located on the glial cytoskeleton and identified as an intermediate filament associated protein (IFAP) at the ultrastructural level. RC2 immunostaining in those cells is strongly dependent on the presence of a serum-derived activity. Serum-removal causes a decrease of the staining while adding serum back to the cells induces reexpression of RC2 immunoreactivity. By Western blotting, we find that in intermediate filament (IF) preparations obtained from cultured cerebellar glia, the RC2 antibody recognizes a 295-kDa protein whose expression is also dependent on the presence of serum in culture medium. In developing muscle cells, RC2 immunostaining is observed from the myoblast stage and disappears after complete myotube fusion. Both in vivo and in vitro, staining is first seen as a loose capping around myoblasts nuclei and progressively concentrates into Z-disks in association with the muscle IF protein desmin. The RC2 antibody also recognizes a 295-kDa protein band in muscle tissue protein extracts. Thus, the RC2 antibody recognizes a developmentally regulated cytoskeletal protein that is expressed, like other previously identified IFAPs, by cells of the glial and myogenic lineages and whose expression in vitro seems to be controlled by a signaling mechanism known to modulate astroglial morphology.

Inward rectifier K(+) channel Kir2.3 (IRK3) in reactive astrocytes from adult rat brain

Astrocytic inward rectifying K(+) channels that participate in K(+) spatial buffering in the central nervous system have been extensively investigated, but specific gene products have not been fully identified. We studied primary cultured reactive astrocytes of stellate and polygonal morphology from adult rat brains, as well as stellate astrocytes from neonatal rat brains. Single-channel recordings of cell-attached patches revealed that polygonal reactive astrocytes expressed only one hyperpolarization-activated single-channel conductance of 11-15 pS whose open probability was independent of voltage, whereas stellate reactive and stellate neonatal astrocytes exhibited two conductances, 11-15 pS and 24-27 pS. All three subtypes of astrocytes exhibited a hyperpolarization-activated macroscopic inward K(+) current that was strongly rectifying and was abrogated by 1 mM intracellular Mg(2+) introduced during conventional but not perforated patch whole-cell recording. This Mg(2+)-sensitive current comprised the total inward rectifier current in polygonal reactive astrocytes, but only a fraction of the inward rectifier current in stellate reactive and stellate neonatal astrocytes. Because a strongly rectifying, inward rectifier K(+) channel with a single-channel conductance of 11-15 pS that is voltage independent is consistent with features of Kir2.3 (IRK3), we performed immunofluorescence experiments with anti-Kir2.3 and anti-glial fibrillary acidic protein antibodies. Both antibodies co-localized to all three subtypes of astrocytes in primary culture and to reactive astrocytes in situ within brain and gelatin sponge implants. Our data indicate that astrocytes of both polygonal and stellate morphology, from both adult and neonatal rat brain, express Kir2.3 both in vivo and in vitro. Constitutive expression of Kir2.3 regardless of cell morphology or age of origin of the source tissue suggests an important functional role for this channel in astrocytes.

Desensitisation of protease-activated receptor-1 (PAR-1) in rat astrocytes: evidence for a novel mechanism for terminating Ca2+ signalling evoked by the tethered ligand

Protease-activated receptor-1 (PAR-1), a G-protein-coupled receptor, is activated when thrombin cleaves its N-terminal exodomain, thereby regulating morphology, growth and survival of neurones and astrocytes. We have investigated the mechanism of PAR-1 desensitisation and resensitisation after proteolytic or non-proteolytic stimulation with thrombin or thrombin receptor agonist peptide (TRag), respectively. In rat primary astrocytes, short-term stimulation with thrombin resulted in a single [Ca2+]i transient and a dose-dependent de- and resensitisation, as assessed by single-cell Ca2+ imaging of fura-2-loaded astrocytes. An initial proteolytic activation of astrocyte PAR-1 by exposure to thrombin strongly decreased the response elicited by subsequent application of a second dose of thrombin or of TRag. In contrast, after an initial non-proteolytic activation of astrocyte PAR-1 by TRag, the subsequent response to thrombin, but not to an additional application of TRag, was strongly attenuated, and the time course for desensitisation was slower. Based on this finding we hypothesised that after PAR-1 activation, the 'tethered ligand' is proteolytically destroyed. As a consequence, the receptor becomes unresponsive to a subsequent thrombin stimulus but is still capable of responding to TRag. This hypothesis was supported by applying thermolysin, which is known to cleave PAR-1 within its tethered-ligand domain, and was confirmed by incubation with soybean trypsin inhibitor. PAR-1 resensitisation occurs via new PAR-1 synthesis since resensitisation was inhibited by cycloheximide and brefeldin A. From these results, we derive a novel model wherein activation of PAR-1, in addition to initiating signal transduction, activates a protease mechanism that cleaves the N-terminus of the receptor, thus terminating the signal and probably inducing receptor internalisation.

Thrombomodulin as a new marker of lesion-induced astrogliosis: involvement of thrombin through the G-protein-coupled protease-activated receptor-1

Because injury of the CNS causes an astrogliosis, characterized by cell swelling and proliferation, similar to the effects of the serine protease thrombin on astrocytes, we hypothesized that a high level of thrombin at the site of injury might initially induce an astrocyte reaction and later increase the expression of its specific inhibitor, thrombomodulin. Thrombomodulin could then stabilize the astroglial scar through its adhesive properties. Here, we studied the in vivo injury response of astrocytes in the anterior medullary velum of adult rat by immunostaining and in situ hybridization of thrombomodulin. Thrombomodulin was poorly expressed on astrocytes in normal tissue, increased up to 2 d after injury, and was still highly expressed at 6 d. To check that thrombin had a direct effect on thrombomodulin expression by astrocytes, we used brain cortical astrocyte primary cultures treated with either thrombin or the agonist peptide thrombin receptor-activating peptide-6, known to activate directly the thrombin G-protein-coupled receptor (GPCR) protease-activated receptor-1 (PAR-1). Modification of thrombomodulin expression was studied by Western blotting and quantitative reverse transcription-PCR. There was a dose-dependent increase in thrombomodulin after 48 hr of treatment, with gene expression peaking at 24 hr but falling to control levels by 48 hr. Together, these results show the following: (1) injury increases astrocyte thrombomodulin expression; (2) thrombin might mediate thrombomodulin expression via the specific receptor PAR-1; and (3) serine proteases, their inhibitors, and the new family of GPCR, PARs, are active on astrogliosis.

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.

Myofibroblasts. I. Paracrine cells important in health and disease

Myofibroblasts are a unique group of smooth-muscle-like fibroblasts that have a similar appearance and function regardless of their tissue of residence. Through the secretion of inflammatory and anti-inflammatory cytokines, chemokines, growth factors, both lipid and gaseous inflammatory mediators, as well as extracellular matrix proteins and proteases, they play an important role in organogenesis and oncogenesis, inflammation, repair, and fibrosis in most organs and tissues. Platelet-derived growth factor (PDGF) and stem cell factor are two secreted proteins responsible for differentiating myofibroblasts from embryological stem cells. These and other growth factors cause proliferation of myofibroblasts, and myofibroblast secretion of extracellular matrix (ECM) molecules and various cytokines and growth factors causes mobility, proliferation, and differentiation of epithelial or parenchymal cells. Repeated cycles of injury and repair lead to organ or tissue fibrosis through secretion of ECM by the myofibroblasts. Transforming growth factor-beta and the PDGF family of growth factors are the key factors in the fibrotic response. Because of their ubiquitous presence in all tissues, myofibroblasts play important roles in various organ diseases and perhaps in multisystem diseases as well.

Thrombin and mast cell tryptase regulate guinea-pig myenteric neurons through proteinase-activated receptors-1 and -2

1. Proteases regulate cells by cleaving proteinase-activated receptors (PARs). Thrombin and trypsin cleave PAR-1 and PAR-2 on neurons and astrocytes of the brain to regulate morphology, growth and survival. We hypothesized that thrombin and mast cell tryptase, which are generated and released during trauma and inflammation, regulate enteric neurons by cleaving PAR-1 and PAR-2. 2. We detected immunoreactive PAR-1 and PAR-2 in > 60 % of neurons from the myenteric plexus of guinea-pig small intestine in primary culture. A large proportion of neurons that expressed substance P, vasoactive intestinal peptide or nitric oxide synthase also expressed PAR-1 and PAR-2. We confirmed expression of PAR-1 and PAR-2 in the myenteric plexus by RT-PCR using primers based on sequences of cloned guinea-pig receptors. 3. Thrombin, trypsin, tryptase, a filtrate from degranulated mast cells, and peptides corresponding to the tethered ligand domains of PAR-1 and PAR-2 increased [Ca2+]i in > 50 % of cultured myenteric neurons. Approximately 60 % of neurons that responded to PAR-1 agonists responded to PAR-2 agonists, and > 90 % of PAR-1 and PAR-2 responsive neurons responded to ATP. 4. These results indicate that a large proportion of myenteric neurons that express excitatory and inhibitory neurotransmitters and purinoceptors also express PAR-1 and PAR-2. Thrombin and tryptase may excite myenteric neurons during trauma and inflammation when prothrombin is activated and mast cells degranulate. This novel action of serine proteases probably contributes to abnormal neurotransmission and motility in the inflamed intestine.

Regulation of astrocyte morphology by RhoA and lysophosphatidic acid

Astrocytes in the CNS undergo morphological changes and start to proliferate after breakdown of the blood-brain barrier. In culture, proliferating astrocytes have a flat, polygonal shape. When treated with cAMP-raising agents, astrocytes adopt a stellate, process-bearing morphology resembling their in vivo appearance. Stellation is accompanied by loss of actin stress fibers and focal adhesions. Lysophosphatidic acid (LPA), a blood-borne mitogen that signals through its cognate G protein-coupled receptor, stimulates DNA synthesis in astrocytes and causes rapid reversal of cAMP-induced stellation. LPA reversal of stellation is initiated by f-actin reassembly and tyrosine phosphorylation of focal adhesion proteins such as paxillin. Botulinum C3 toxin, which inactivates the Rho GTPase, mimics cAMP-raising agents in inducing stellation, f-actin disassembly, paxillin dephosphorylation, and growth arrest. However, unlike cAMP-induced stellation, C3-induced stellation cannot be reversed by LPA. Conversely, astrocytes expressing activated RhoA fail to undergo cAMP-induced stellation. Thus, RhoA controls astrocyte morphology in that active RhoA directs LPA reversal of stellation, while inactivation of RhoA is sufficient to induce stellation.

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.

Apoptosis in cellular compartments of rat spinal cord after severe contusion injury

Following a controlled, severe contusion lesion to the lower thoracic spinal cord in adult rats, we found that apoptosis occurred in cells located in both gray and white matter. This suggested that both nonneuronal cells, including astrocytes, oligodendroglia and microglia, as well as neurons, might participate in programmed cell death (PCD) following spinal cord injury (SCI). Determination of which cell populations participate, and the kinetics and extent of their involvement might reveal new paradigms for approaches to therapy. Consequently, we assessed the functional deficit, comparing a comprehensive locomotor rating scale (LRS) with the inclined plane test at various times after injury. Using standard histology, along with cell-specific markers, we assessed PCD in different spinal cord segments using several parameters of apoptosis. Our results indicate that hind limb motor function was lost at day 1, and then only gradually and ineffectively (about 10-15%) recovered over the next month. Evidence for increased cell number was present for astrocytes and microglia beginning at day 1 after injury. Over the postinjury time period, apoptotic cells appeared (from day 1 to 14), and peaked (in terms of apoptotic index) on day 3. About one-third were microglia, whereas neurons, both large and small, also underwent apoptosis, again peaking at day 3. However, neurons continued to die and were not replaced by proliferation, so that at day 7, three times as many neurons (as a percentage) underwent PCD compared with the glial compartment. Oligodendrocytes also underwent apoptosis, with a biphasic curve, both at days 3 and 14 following injury. Thus, in addition to immediate, passive necrosis, delayed and apoptotic PCD also occurred in all cell populations in severely injured spinal cord.

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.

Proteinase-activated receptors: novel mechanisms of signaling by serine proteases

Although serine proteases are usually considered to act principally as degradative enzymes, certain proteases are signaling molecules that specifically regulate cells by cleaving and triggering members of a new family of proteinase-activated receptors (PARs). There are three members of this family, PAR-1 and PAR-3, which are receptors for thrombin, and PAR-2, a receptor for trypsin and mast cell tryptase. Proteases cleave within the extracellular NH2-terminus of their receptors to expose a new NH2-terminus. Specific residues within this tethered ligand domain interact with extracellular domains of the cleaved receptor, resulting in activation. In common with many G protein-coupled receptors, PARs couple to multiple G proteins and thereby activate many parallel mechanisms of signal transduction. PARs are expressed in multiple tissues by a wide variety of cells, where they are involved in several pathophysiological processes, including growth and development, mitogenesis, and inflammation. Because the cleaved receptor is physically coupled to its agonist, efficient mechanisms exist to terminate signaling and prevent uncontrolled stimulation. These include cleavage of the tethered ligand, receptor phosphorylation and uncoupling from G proteins, and endocytosis and lysosomal degradation of activated receptors.

Changes in the expression of protease-activated receptor 1 and protease nexin-1 mRNA during rat nervous system development and after nerve lesion

HDs racI Thrombin causes profound metabolic and morphological changes in cultured neural cells via activation of the thrombin receptor, also called protease-activated receptor 1 (PAR1). PAR1 mRNA is present in the rat brain, but the role of this receptor in the nervous system remains elusive. The expression of PAR1 and the potent thrombin inhibitor protease nexin-1 (PN-1) was investigated in the developing rat brain and spinal cord and after peripheral nerve lesion. As seen by in situ hybridization, the PAR1 mRNA signal in the late embryonic and early postnatal nervous system was widespread, but generally of low intensity whereas in the adult it was more pronounced and confined to particular neuronal cells. These include the mesencephalic dopaminergic neurons, several thalamic and brainstem nuclei, the mitral cells in the olfactory bulb and the Purkinje cells in the cerebellum. In the spinal cord, PAR1 mRNA was abundant in motoneurons and a particularly high expression was detected in the preganglionic neurons of the autonomic nervous system. High PAR1 mRNA expression was also found in the dorsal root ganglia. Interestingly, strong immunoreactivity for the protease inhibitor PN-1 was present in spinal motoneuron cell bodies, although its transcript was undetectable there. In response to sciatic nerve transection, the signal intensity of PAR1 mRNA as seen by Northern analysis increased in the proximal and the distal part of the lesioned nerve and in the denervated muscle, whereas the PN-1 mRNA signal strongly increased only in the distal part of the nerve but remained unchanged in the proximal part and in the muscle. After facial nerve transection, PAR1 mRNA expression substantially decreased in facial motoneurons. No PAR1 transcript was detected in reactive astrocytes. Similar to PAR1, PN-1 mRNA which was expressed in interneurons within the facial nucleus was also decreased following facial nerve transection.