Dual effects of thrombin and a 14-amino acid peptide agonist of the thrombin receptor on septal cholinergic neurons

Protease activated receptor-1 and brain edema formation in glioma models

OBJECTIVE

Our previous studies showed that thrombin contributes to brain edema in gliomas. The present study investigated the role of a thrombin receptor, protease activated receptor-1 (PAR-1), in edema formation in glioma models.

METHODS

These experiments were performed in Fischer 344 rats, PAR-1 knockout mice, and wild-type C57BL/6 mice controls. F98 glioma cells were infused into the right caudate. Animals were euthanized and the brains were used for measurements of brain edema and PAR-1 expression.

RESULTS

In rats, implantation of glioma cells resulted in significant brain edema in the ipsilateral hemisphere (82.6 ± ;1.4 vs. 78.1 ± 0.9 % in the contralateral hemisphere, p < 0.01). By Western blot analysis and RT-PCR, we found that both protein and mRNA levels of PAR-1 were upregulated in the glioma (p < 0.01). In mice, implantation of glioma cells also caused brain edema in the ipsilateral hemisphere (p < 0.05). Glioma-induced brain edema was less in PAR-1 knockout mice (day 12: 79.4 ;± 1.3 vs. 81.5 ± 1.1 % in the wild-type mice, p < ;0.05).

CONCLUSION

PAR-1 plays a role in glioma-induced brain edema. Clarification of the role of PAR-1 in edema formation should help to develop new therapeutic strategies for gliomas.

Real-Time Analysis of Agonist-Induced Activation of Protease-Activated Receptor 1/Gαi1Protein Complex Measured by Bioluminescence Resonance Energy Transfer in Living Cells

G protein-coupled receptors transmit extracellular signals into the cells by activating heterotrimeric G proteins, a process that is often followed by receptor desensitization. Monitoring such a process in real time and in living cells will help better understand how G protein activation occurs. Energy transfer-based approaches [fluorescence resonance energy transfer (FRET) and bioluminescence resonance energy transfer (BRET)] were recently shown to be powerful methods to monitor the G protein-coupled receptors (GPCRs)-G protein association in living cells. Here, we used a BRET technique to monitor the coupling between the protease-activated receptor 1 (PAR1) and Galpha(i1) protein. A specific constitutive BRET signal can be measured between nonactivated PAR1 and the Galpha(i1) protein expressed at a physiological level. This signal is insensitive to pertussis toxin (PTX) and probably reflects the preassembly of these two proteins. The BRET signal rapidly increases upon receptor activation in a PTX-sensitive manner. The BRET signal then returns to the basal level after few minutes. The desensitization of the BRET signal is concomitant with beta-arrestin-1 recruitment to the receptor, consistent with the known rapid desensitization of PARs. The agonist-induced BRET increase was dependent on the insertion site of fluorophores in proteins. Taken together, our results show that BRET between GPCRs and Galpha proteins can be used to monitor the receptor activation in real time and in living cells. Our data also revealed that PAR1 can be part of a preassembled complex with Galpha(i1) protein, resulting either from a direct interaction between these partners or from their colocalization in specific microdomains, and that receptor activation probably results in rearrangements within such complexes.

Mechanisms of brain injury after intracerebral haemorrhage

The past decade has resulted in a rapid increase in knowledge of mechanisms underlying brain injury induced by intracerebral haemorrhage (ICH). Animal studies have suggested roles for clot-derived factors and the initial physical trauma and mass effect as a result of haemorrhage. The coagulation cascade (especially thrombin), haemoglobin breakdown products, and inflammation all play a part in ICH-induced injury and could provide new therapeutic targets. Human imaging has shown that many ICH continue to expand after the initial ictus. Rebleeding soon after the initial haemorrhage is common and forms the basis of a current clinical trial using factor VIIa to prevent rebleeding. However, questions about mechanisms of injuries remain. There are conflicting data on the role of ischaemia in ICH and there is uncertainty over the role of clot removal in ICH therapy. The next decade should bring further information about the underlying mechanisms of ICH-induced brain injury and new therapeutic interventions for this severe form of stroke. This review addresses our current understanding of the mechanisms underlying ICH-induced brain injury.

PAR-1 deficiency protects against neuronal damage and neurologic deficits after unilateral cerebral hypoxia/ischemia

Cardiovascular and neurologic surgeries often involve a temporary reduction in cerebral blood flow. In these conditions, as well as during cerebral ischemia and traumatic brain injury, the temporary loss of oxygen and glucose initiates a cascade of cellular events that culminate in neuronal death and damage. Understanding the mechanisms that contribute to neuronal death after hypoxia/ischemia is critically important for treatment of such brain injury. Here, we use a model of combined cerebral hypoxia/ischemia (H/I) to examine the role of protease-activated receptor-1 (PAR-1) in hypoxic/ischemic neuronal damage. Our data show that PAR-1-deficient mice have smaller lesion volumes than wild-type controls after 45 minutes of H/I. The results of the genetic block of PAR-1 were corroborated using a PAR-1 antagonist, which decreased infarct volume in wild-type C57Bl6 mice. Examination of cellular responses to H/I reveals that PAR-1 -/- animals have less cellular death and diminished glial fibrillary acidic protein expression. Additionally, PAR-1 -/- mice exhibit less motor behavior impairment in rotorod and inverted wire-hang tests. These data suggest that PAR-1 contributes to hypoxic/ischemic brain injury and are consistent with other studies that implicate serine proteases and their receptors in neuropathology after cerebral insults.

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.

Proteinase-activated receptors: novel signals for peripheral nerves

The discovery of proteinase-activated receptors (PARs) in the nervous system has led to new insights about the potential physiological functions of these enzymes, which were traditionally considered merely as degradative molecules. This review summarizes evidence that proteinases, through activation of PARs, interact with the peripheral nervous system (PNS), playing roles in neurogenic inflammation, pain perception, secretory and motor functions, as well as in the response to nerve injuries. Activation of PARs interferes with numerous physiological events that are under tight neural control, in addition to modulating nerve survival. New potential roles are suggested for members of the PAR family, highlighting proteinases and their receptors as potential therapeutic targets for diseases associated with PNS activation.

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.

On the mechanism of thrombin-induced angiogenesis: involvement of alphavbeta3-integrin

Thrombin has been reported to be a potent angiogenic factor both in vitro and in vivo, and many of the cellular effects of thrombin may contribute to activation of angiogenesis. In this report we show that thrombin-treatment of human endothelial cells increases mRNA and protein levels of alphavbeta3-integrin. This thrombin-mediated effect is specific, dose dependent, and requires the catalytic site of thrombin. In addition, thrombin interacts with alphavbeta3 as demonstrated by direct binding of alphavbeta3 protein to immobilized thrombin. This interaction of thrombin with alphavbeta3-integrin, which is an angiogenic marker in vascular tissue, is of functional significance. Immobilized thrombin promotes endothelial cells attachment, migration, and survival. Antibody to alphavbeta3 or a specific peptide antagonist to alphavbeta3 can abolish all these alphavbeta3-mediated effects. Furthermore, in the chick chorioallantoic membrane system, the antagonist peptide to alphavbeta3 diminishes both basal and the thrombin-induced angiogenesis. These results support the pivotal role of thrombin in activation of endothelial cells and angiogenesis and may be related to the clinical observation of neovascularization within thrombi.

Extracellular proteolysis in brain injury and inflammation: role for plasminogen activators and matrix metalloproteinases

The role of intracellular proteases (e.g., calpains and caspases) in the pathophysiology of neuronal cell death has been extensively investigated. More recently, accumulating data have suggested that extracellular proteolysis also plays a critical role. The two major systems that modify the extracellular matrix in brain are the plasminogen activator (PA) and matrix metalloproteinase (MMP) axes. This Mini-Review delineates major pathways of PA and MMP action after stroke, brain trauma, and chronic inflammation. Deleterious effects include the disruption of blood-brain barrier integrity, amplification of inflammatory infiltrates, demyelination, and possibly interruption of cell-cell and cell-matrix interactions that may trigger cell death. In contrast, PA-MMP actions may contribute to extracellular proteolysis that mediates parenchymal and angiogenic recovery after brain injury. As the mechanisms of deleterious vs. potentially beneficial PA and MMP actions become better defined, it is hoped that new therapeutic targets will emerge for ameliorating the sequelae of brain injury and inflammation.

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.

T cell-mediated neuroprotection involves antithrombin activity

Functional loss after injury to the mammalian central nervous system (CNS) has been attributed not only to the immediate loss of neurons but also to secondary neuronal degeneration caused by the toxicity of physiological compounds present in abnormally high amounts as a result of the injury. One such compound appears to be the protease thrombin. Here we show that the beneficial effect of T cells directed against myelin self-antigens can be attributed, at least in part, to the ability of these 'autoimmune' T cells to produce antithrombin III. Using transgenic mice lacking the thrombin receptor PAR-1, we also present molecular evidence indicating that down-regulation of PAR-1 by genetic manipulation leads to increased post-traumatic survival of CNS neurons. We further show that the ability of autoimmune T cells to produce thrombin inhibitors and to exert post-traumatic neuroprotection are both independent of their PAR-1 expression. These findings suggest that thrombin plays a key role in post-injury neuronal survival, and that its post-traumatic toxicity can be down-regulated by appropriate alteration of the amounts of PAR-1 receptors or of antithrombin III, the latter achieved by T cell-mediated autoimmunity.

Thrombin attenuation is neuroprotective in the injured rat optic nerve

The functional loss that often follows injury of the mammalian CNS has been attributed not only to the immediate neural loss, but also to secondary neuronal degeneration caused by toxic biochemical mediators in the environment of the injured nerve. We report here that a high thrombin content, produced as a result of injury-induced activation of prothrombin, appears to be an important mediator of secondary damage. Measurement of post-traumatic neuronal survival in vivo revealed that post-traumatic local application of the thrombin inhibitor N-alpha-(2-naphthylsulphonylglycyl)-4-(D,L)-amidinophenylalanine piperidide acetate in the rat optic nerve subjected to mild partial crush injury left twice as many retinal ganglion cells with functioning axons as in controls. Thus, by readjusting thrombin activity, thereby possibly obtaining a moderate post-traumatic increase and thus gaining the benefit of thrombin without its toxic effects, it may be possible to create an environment that is more favourable for post-traumatic survival.

Glial cells as targets and producers of neurotrophins

Glial cells fulfill important tasks within the neural network of the central and peripheral nervous systems. The synthesis and secretion of various polypeptidic factors (cytokines) and a number of receptors, with which glial cells are equipped, allow them to communicate with their environment. Evidence has accumulated during recent years that neurotrophins play an important role not only for neurons but also for glial cells. This brief update of some morphological, immunocytochemical, and biochemical characteristics of glial cell lineages conveys our present knowledge about glial cells as targets and producers of neurotrophins under normal and pathological conditions. The chapter discusses the presence of neurotrophin receptors on glial cells, glial cells as producers of neurotrophins, signaling pathways downstream Trk and p75NTR, and the significance of neurotrophins and their receptors for glial cells during development, in cell death and survival, and in neurological disorders.

The effects of thrombin preconditioning on focal cerebral ischemia in rats

Our previous studies have shown that prior intracerebral infusion of a low dose of thrombin (thrombin preconditioning; TPC) reduces the brain edema that follows a subsequent intracerebral infusion of a high dose of thrombin or an intracerebral hemorrhage. In vitro studies have also demonstrated that low concentrations of thrombin protect neurons and astrocytes from hypoglycemia and oxidative stress-induced damage. This study, therefore, examines the hypothesis that TPC would offer protection from ischemic brain damage in vivo. This was a blinded design study. The rat brain was preconditioned with 1 U thrombin by direct infusion into the left caudate nucleus. Seven days after thrombin pretreatment, permanent middle cerebral artery occlusion (MCAO) was induced. Twenty-four hours post-ischemia, neurological deficit was evaluated and infarction volume, brain water and ion contents were measured. Compared to saline-treated rats, thrombin pretreatment significantly attenuated brain infarction in cortex (90+/-33 vs. 273+/-22 mm(3); P<0.05) and basal ganglia (56+/-17 vs. 119+/-12 mm(3); P<0.05) that followed 24 h of permanent MCAO. TPC also reduced the brain edema in cortex and basal ganglia by 50 and 53% (P<0.05). Neurological deficit was improved in thrombin pretreatment group (P<0.05). These effects of TPC were, in part, prevented by co-injection of hirudin, a thrombin inhibitor, indicating that the protection was indeed thrombin mediated. Cerebral TPC significantly reduces ischemic brain damage, perhaps by activation of the thrombin receptor. This finding provides a new mechanism by which to study ischemic tolerance.

Vasoactive intestinal peptide. Link between electrical activity and glia-mediated neurotrophism

Vasoactive intestinal peptide has neurotrophic and neuroprotective properties that influence the survival of activity-dependent neurons in the central nervous system. Investigations of the mechanism of this neurotrophic peptide indicated that these actions are contingent on interactions with astroglia. The complex mixture of neurotrophic mediators released from astroglia include cytokines, a protease inhibitor, and activity-dependent neurotrophic factor, a protein with apparent structural similarities to hsp60. Investigations of ADNF resulted in the discovery of active peptides of extraordinary potency and broad neuroprotective properties. These studies indicate that a nine-amino acid core peptide of ADNF had significantly greater neuroprotective properties in comparison to the parent growth factor and these advantages identify ADNF-9 as an attractive lead compound for drug development.

A nerve growth factor mimetic TrkA antagonist causes withdrawal of cortical cholinergic boutons in the adult rat

Cholinergic neurons respond to the administration of nerve growth factor (NGF) in vivo with a prominent and selective increase of choline acetyl transferase activity. This suggests the possible involvement of endogenous NGF, acting through its receptor TrkA, in the maintenance of central nervous system cholinergic synapses in the adult rat brain. To test this hypothesis, a small peptide, C(92-96), that blocks NGF-TrkA interactions was delivered stereotactically into the rat cortex over a 2-week period, and its effect and potency were compared with those of an anti-NGF monoclonal antibody (mAb NGF30). Two presynaptic antigenic sites were studied by immunoreactivity, and the number of presynaptic sites was counted by using an image analysis system. Synaptophysin was used as a marker for overall cortical synapses, and the vesicular acetylcholine transporter was used as a marker for cortical cholinergic presynaptic sites. No significant variations in the number of synaptophysin-immunoreactive sites were observed. However, both mAb NGF30 and the TrkA antagonist C(92-96) provoked a significant decrease in the number and size of vesicular acetylcholine transporter-IR sites, with the losses being more marked in the C(92-96) treated rats. These observations support the notion that endogenously produced NGF acting through TrkA receptors is involved in the maintenance of the cholinergic phenotype in the normal, adult rat brain and supports the idea that NGF normally plays a role in the continual remodeling of neural circuits during adulthood. The development of neurotrophin mimetics with antagonistic and eventually agonist action may contribute to therapeutic strategies for central nervous system degeneration and trauma.

Injury-induced gelatinase and thrombin-like activities in regenerating and nonregenerating nervous systems

It is now widely accepted that injured nerves, like any other injured tissue, need assistance from their extracellular milieu in order to heal. We compared the postinjury activities of thrombin and gelatinases, two types of proteolytic activities known to be critically involved in tissue healing, in nonregenerative (rat optic nerve) and regenerative (fish optic nerve and rat sciatic nerve) neural tissue. Unlike gelatinases, whose induction pattern was comparable in all three nerves, thrombin-like activity differed clearly between regenerating and nonregenerating nervous systems. Postinjury levels of this latter activity seem to dictate whether it will display beneficial or detrimental effects on the capacity of the tissue for repair. The results of this study further highlight the fact that tissue repair and nerve regeneration are closely linked and that substances that are not unique to the nervous system, but participate in wound healing in general, are also crucial for regeneration or its failure in the nervous system.

TrkA antagonists decrease NGF-induced ChAT activity in vitro and modulate cholinergic synaptic number in vivo

Cholinergic neurons are known to respond in vivo to the administration of nerve growth factor (NGF) by a prominent and selective increase of choline acetyl transferase activity and by cholinergic synaptogenesis in the rat brain. By using a synthetic TrkA antagonist we demonstrated that endogenously produced NGF is involved in the continual re-modeling of cholinergic neuronal connections during adulthood, acting through TrkA receptors.

Involvement of protease-activated receptor-1 in the in vitro development of mesencephalic dopaminergic neurons

In situ hybridization studies have revealed high levels of protease (thrombin)-activated receptor-1 messenger RNA in the mesencephalon of rats, suggesting that dopaminergic neurons are a target for thrombin's actions. We have evaluated the effect of thrombin receptor activation, either by thrombin or by thrombin receptor agonist peptide, a 14 amino acid agonist of protease-activated receptor-1, on tyrosine hydroxylase-positive neurons. Pure cultures of rat mesencephalic neurons or co-cultures of mesencephalic neurons and glial cells were treated with either thrombin or thrombin receptor agonist peptide the day after plating. Tyrosine hydroxylase-positive cell counting, [3H]dopamine uptake and morphometric analysis were performed on day 5. Thrombin and thrombin receptor agonist peptide influenced neurite elongation, branching and the number of primary, secondary and tertiary neurites of tyrosine hydroxylase-positive neurons. In pure cultures, the most significant effects of thrombin and thrombin receptor agonist peptide were to delay branching and to increase the centrifugal growth of neurites without affecting the total neuritic length. Thrombin (up to 10 nM) and thrombin receptor agonist peptide did not affect the number of tyrosine hydroxylase-positive neurons or [3H]dopamine uptake. Neurotrophin-4 also influenced the morphology of tyrosine hydroxylase-positive neurons. The increase of neuritic length initiated by this neurotrophin is complementary to the radial elongation induced by protease-activated receptor-1 activation. When neurons were cultured in the presence of glial cells, the effects of thrombin and thrombin receptor agonist peptide on most of these parameters were larger than those observed with pure cultures. Thus, thrombin is able to initiate a complex remodelling of the architecture of tyrosine hydroxylase-positive neurons through the activation of protease-activated receptor-1. These results provide further support for the involvement of protease-activated receptor-1 activation in the development and differentiation of the central nervous system.

Transforming growth factor-alpha's effects on astroglial-cholinergic cell interactions in the medial septal area in vitro are mediated by alpha 2-macroglobulin

We reported previously that two epidermal growth factor receptor ligands, epidermal growth factor and transforming growth factor-alpha, inhibit medial septal cholinergic cell phenotypic expression (choline acetyltransferase and acetylcholinesterase activities) in vitro indirectly via (a) soluble molecule(s) released from astrocytes [Kenigsberg R. L. et al. (1992) Neuroscience 50, 85-97; Kenigsberg R. L. and Mazzoni I. E. (1995) J. Neurosci. Res. 41, 734-744; Mazzoni I. E. and Kenigsberg R. L. (1996) Brain Res. 707, 88-99]. In the present study, we found that this response to transforming growth factor-alpha is mediated, for the most part, by alpha 2-macroglobulin, a potent protease inhibitor with a wide spectrum of biological activities. In this regard, the effects of transforming growth factor-alpha on cholinergic cells can be blocked with immunoneutralizing antibodies raised against alpha 2-macroglobulin. Furthermore, western blot analysis reveals that although alpha 2-macroglobulin is present in conditioned media from control septal cultures, it is more abundant in those treated with transforming growth factor-alpha. In addition, exogenous alpha 2-macroglobulin, both in its native and trypsin-activated forms, can mimic transforming growth factor-alpha's effects on septal cholinergic cell expression. However, while the native antiprotease can slightly but significantly decrease choline acetyltransferase activity, trypsin-activated alpha 2-macroglobulin, in the nanomolar range, induces as marked a decrease in this enzyme activity as that noted with transforming growth factor-alpha. Furthermore, trypsin-activated alpha 2-macroglobulin, like epidermal growth factor/transforming growth factor-alpha, decreases choline acetyltransferase activity by arresting its spontaneous increase that occurs with time in culture, does so in a reversible manner and is not neurotoxic. In addition, trypsin-activated alpha 2-macroglobulin, in the nanomolar range, can affect choline acetyltransferase in a dual manner, up-regulating it at low concentrations while down-regulating it at higher ones. These responses are identical in mixed neuronal-glial and pure neuronal septal cultures. Furthermore, when concentrations of trypsin-activated alpha 2-macroglobulin, which alone decrease choline acetyltransferase, are added simultaneously with nerve growth factor, they serve to potentiate the nerve growth factor-induced increase in enzymatic activity. As GABAergic cell expression is not affected by alpha 2-macroglobulin, it appears that the effects of this protease inhibitor on medial septal neuronal expression are neurotransmitter-specific. Finally, trypsin-activated but not native alpha 2-macroglobulin promotes a dose-dependent aggregation of the septal neurons. This change in morphology, however, is not related to those noted in choline acetyltransferase activity. In summary, these data suggest that the expression of alpha 2-macroglobulin in astroglia from the medial septal nucleus can be controlled by epidermal growth factor receptor ligands to impact the functioning of basal forebrain cholinergic neurons.

Thrombin induces apoptosis in cultured neurons and astrocytes via a pathway requiring tyrosine kinase and RhoA activities

Thrombin activity is a factor in acute CNS trauma and may contribute to such chronic neurodegenerative diseases as Alzheimer's disease. Thrombin is a multifunctional serine protease that catalyses the final steps in blood coagulation. However, increasing evidence indicates that thrombin also elicits a variety of cellular and inflammatory responses, including responses from neural cells. Most recently, high concentrations of thrombin were shown to cause cell death in both astrocyte and hippocampal neuron cultures. The purpose of this study was to determine the mechanisms underlying thrombin-induced cell death. Our data show that thrombin appears to cause apoptosis as evidenced by cleavage of DNA into oligonucleosomal-sized fragments, fragmentation of nuclei, and prevention of death by inhibition of protein synthesis. Synthetic peptides that directly activate the thrombin receptor also induced apoptosis, indicating that thrombin-induced cell death occurred via activation of the thrombin receptor. The signal transduction cascade involves tyrosine and serine/threonine kinases and an intact actin cytoskeleton. Additional study revealed the involvement of the small GTP-binding protein RhoA. Thrombin induced RhoA activity in both astrocytes and hippocampal neurons, and inhibition of RhoA activity with exoenzyme C3 attenuated cell death, indicating that thrombin activation of RhoA was necessary for thrombin-induced cell death. Tyrosine kinase inhibitors blocked thrombin induction of RhoA, indicating that tyrosine kinase activity was required upstream of RhoA. These data suggest a sequential linkage of cellular events from which we propose a model for the second messenger cascade induced by thrombin in neural cells that can lead to apoptosis.

Pharmacological characterization of protease-activated receptor (PAR-1) in rat astrocytes

The proteolytic action of thrombin on its receptor (protease-activated receptor-1 or PAR-1) results in a conformational change in which the new N-terminal sequence auto-activates the receptor. Peptide analogs of this N-terminal sequence (TRAPs) are able to mimic the effect of thrombin and an extensive search has led to the definition of the structural requirement for the agonist and antagonist activity on thrombin receptors in several peripheral systems. Thrombin plays an important role in central and peripheral nervous system development and PAR-1 is present in neurons and astrocytes. We have now characterized thrombin receptors pharmacologically in cultured rat astrocytes by using [3H]thymidine incorporation and reversal of stellation induced by Bt2cAMP as end-points. Thrombin increased [3H]thymidine incorporation into DNA with an EC50 of 1 nM and induced a complete reversion of cell stellation. The effects of thrombin on [3H]thymidine incorporation were mimicked by TRAP-14 (EC50 = 3 microM) and a peptide containing non-natural amino acids Ala-Phe(p-F)-Arg-Cha-HArg-Tyr-NH2 (A6Y; EC50 = 0.8 microM). Similarly, these two peptides reversed Bt2cAMP-induced stellation. The effect of thrombin, TRAP-14 and A6Y on [3H]thymidine incorporation into DNA was significantly prevented by L9R, a 9-amino-acid peptide (Leu-Val-Arg-D-Cys-Gly-Lys-His-Ser-Arg; IC50 = 180 microM against thrombin and TRAP-14 and 800 microM against A6Y) previously described as an antagonist in human platelet aggregation. L9R antagonized also thrombin effects on astrocyte morphology. These results demonstrate that rat astrocytes express PAR-1 receptors which are pharmacologically similar to those previously characterized in human platelets.