Rapid, specific and active site-catalyzed effect of tissue-plasminogen activator on hippocampus-dependent learning in mice

Impact and Interrelationships of Striatal Proteins, EPHB2, OPRM1, and PER2 on Mild Cognitive Impairment

With the global increase in life expectancy, there has been a rise in the incidence of cognitive impairments attributed to diverse etiologies. Notably, approximately 50% of individuals diagnosed with mild cognitive impairment (MCI) progress to dementia within 3 years. However, the precise mechanisms underlying MCI remain elusive. Therefore, this study aimed to elucidate potential mechanisms implicated in MCI utilizing Per2 knockout (KO) mice, which have previously been shown to have cognitive deficits. Behavioral (Y-maze, Barnes maze) and molecular (electrophysiology, RNA sequencing, western blot, and immunofluorescence) experiments were conducted in Per2 KO and wild-type (WT) mice. Per2 KO mice exhibited impaired spatial working memory in the Y-maze and Barnes maze. However, there were no significant group differences in hippocampal long-term potentiation (LTP) between Per2 KO and WT mice, whereas striatal LTP in Per2 KO mice was lower compared to WT mice. In RNA sequencing analysis, 58 genes were downregulated and 64 genes were upregulated in the striatum of Per2 KO mice compared to WT mice. Among the differentially expressed genes, four genes (Chrm2, EphB2, Htr1b, Oprm1) were identified. Optimal expression levels of EPHB2 and OPRM1 were found to significantly enhance cognitive performance in mice. Additionally, Per2 KO mice exhibited reduced EPHB2-NMDAR-LTP and OPRM-mTOR signaling, along with elevated amyloid beta (Aβ) levels, when compared to WT mice. However, these alterations were reversed upon administration of morphine treatment. Striatal OPRM1-mTOR signaling, EPHB2-NMDAR-LTP signaling, and Aβ expression levels may exert a combined effect on MCI under the control of Per2 expression.

Glucocorticoid-Responsive Tissue Plasminogen Activator (tPA) and Its Inhibitor Plasminogen Activator Inhibitor-1 (PAI-1): Relevance in Stress-Related Psychiatric Disorders

Stressful events trigger a set of complex biological responses which follow a bell-shaped pattern. Low-stress conditions have been shown to elicit beneficial effects, notably on synaptic plasticity together with an increase in cognitive processes. In contrast, overly intense stress can have deleterious behavioral effects leading to several stress-related pathologies such as anxiety, depression, substance use, obsessive-compulsive and stressor- and trauma-related disorders (e.g., post-traumatic stress disorder or PTSD in the case of traumatic events). Over a number of years, we have demonstrated that in response to stress, glucocorticoid hormones (GCs) in the hippocampus mediate a molecular shift in the balance between the expression of the tissue plasminogen activator (tPA) and its own inhibitor plasminogen activator inhibitor-1 (PAI-1) proteins. Interestingly, a shift in favor of PAI-1 was responsible for PTSD-like memory induction. In this review, after describing the biological system involving GCs, we highlight the key role of tPA/PAI-1 imbalance observed in preclinical and clinical studies associated with the emergence of stress-related pathological conditions. Thus, tPA/PAI-1 protein levels could be predictive biomarkers of the subsequent onset of stress-related disorders, and pharmacological modulation of their activity could be a potential new therapeutic approach for these debilitating conditions.

Modulations of the neuronal trafficking of tissue-type plasminogen activator (tPA) influences glutamate release

The discovery of the neuronal expression of the serine protease tissue-type plasminogen activator (tPA) has opened new avenues of research, with important implications in the physiopathology of the central nervous system. For example, the interaction of tPA with synaptic receptors (NMDAR, LRP1, Annexin II, and EGFR) and its role in the maturation of BDNF have been reported to influence synaptic plasticity and neuronal survival. However, the mechanisms regulating the neuronal trafficking of tPA are unknown. Here, using high-resolution live cell imaging and a panel of innovative genetic approaches, we first unmasked the dynamic characteristics of the dendritic and axonal trafficking of tPA-containing vesicles under different paradigms of neuronal activation or inhibition. We then report a constitutive exocytosis of tPA- and VAMP2-positive vesicles, dramatically increased in conditions of neuronal activation, with a pattern which was mainly dendritic and thus post-synaptic. We also observed that the synaptic release of tPA led to an increase of the exocytosis of VGlut1 positive vesicles containing glutamate. Finally, we described alterations of the trafficking and exocytosis of neuronal tPA in cultured cortical neurons prepared from tau-22 transgenic mice (a preclinical model of Alzheimer's disease (AD)). Altogether, these data provide new insights about the neuronal trafficking of tPA, contributing to a better knowledge of the tPA-dependent brain functions and dysfunctions.

Targeting NMDA Receptors at the Neurovascular Unit: Past and Future Treatments for Central Nervous System Diseases

The excitatory neurotransmission of the central nervous system (CNS) mainly involves glutamate and its receptors, especially N-methyl-D-Aspartate receptors (NMDARs). These receptors have been extensively described on neurons and, more recently, also on other cell types. Nowadays, the study of their differential expression and function is taking a growing place in preclinical and clinical research. The diversity of NMDAR subtypes and their signaling pathways give rise to pleiotropic functions such as brain development, neuronal plasticity, maturation along with excitotoxicity, blood-brain barrier integrity, and inflammation. NMDARs have thus emerged as key targets for the treatment of neurological disorders. By their large extracellular regions and complex intracellular structures, NMDARs are modulated by a variety of endogenous and pharmacological compounds. Here, we will present an overview of NMDAR functions on neurons and other important cell types involved in the pathophysiology of neurodegenerative, neurovascular, mental, autoimmune, and neurodevelopmental diseases. We will then discuss past and future development of NMDAR targeting drugs, including innovative and promising new approaches.

PKCδ-positive GABAergic neurons in the central amygdala exhibit tissue-type plasminogen activator: role in the control of anxiety

Tissue plasminogen activator (tPA) is a serine protease expressed in several brain regions and reported to be involved in the control of emotional and cognitive functions. Nevertheless, little is known about the structure-function relationships of these tPA-dependent behaviors. Here, by using a new model of constitutive tPA-deficient mice (tPA^null), we first show that tPA controls locomotor activity, spatial cognition and anxiety. To investigate the brain structures involved in these tPA-dependent behavioral phenotypes, we next generated tPA^flox mice allowing conditional tPA deletion (cKO) following stereotaxic injections of adeno-associated virus driving Cre-recombinase expression (AAV-Cre-GFP). We demonstrate that tPA removal in the dentate gyrus of the hippocampus induces hyperactivity and partial spatial memory deficits. Moreover, the deletion of tPA in the central nucleus of the amygdala, but not in the basolateral nucleus, induces hyperactivity and reduced anxiety-like level. Importantly, we prove that these behaviors depend on the tPA present in the adult brain and not on neurodevelopmental disorders. Also, interestingly, our data show that tPA from Protein kinase-C delta-positive (PKCδ) GABAergic interneurons of the lateral/ capsular part of adult mouse central amygdala controls emotional functions through neuronal activation of the medial central amygdala. Together, our study brings new data about the critical central role of tPA in behavioral modulations in adult mice.

Tissue Plasminogen Activator in Central Nervous System Physiology and Pathology: From Synaptic Plasticity to Alzheimer's Disease

Tissue plasminogen activator's (tPA) fibrinolytic function in the vasculature is well-established. This specific role for tPA in the vasculature, however, contrasts with its pleiotropic activities in the central nervous system. Numerous physiological and pathological functions have been attributed to tPA in the central nervous system, including neurite outgrowth and regeneration; synaptic and spine plasticity; neurovascular coupling; neurodegeneration; microglial activation; and blood–brain barrier permeability. In addition, multiple substrates, both plasminogen-dependent and -independent, have been proposed to be responsible for tPA's action(s) in the central nervous system. This review aims to dissect a subset of these different functions and the different molecular mechanisms attributed to tPA in the context of learning and memory. We start from the original research that identified tPA as an immediate-early gene with a putative role in synaptic plasticity to what is currently known about tPA's role in a learning and memory disorder, Alzheimer's disease. We specifically focus on studies demonstrating tPA's involvement in the clearance of amyloid-β and neurovascular coupling. In addition, given that tPA has been shown to regulate blood–brain barrier permeability, which is perturbed in Alzheimer's disease, this review also discusses tPA-mediated vascular dysfunction and possible alternative mechanisms of action for tPA in Alzheimer's disease pathology.

Circulating tPA contributes to neurovascular coupling by a mechanism involving the endothelial NMDA receptors

The increase of cerebral blood flow evoked by neuronal activity is essential to ensure enough energy supply to the brain. In the neurovascular unit, endothelial cells are ideally placed to regulate key neurovascular functions of the brain. Nevertheless, some outstanding questions remain about their exact role neurovascular coupling (NVC). Here, we postulated that the tissue-type plasminogen activator (tPA) present in the circulation might contribute to NVC by a mechanism dependent of its interaction with endothelial N-Methyl-D-Aspartate Receptor (NMDAR). To address this question, we used pharmacological and genetic approaches to interfere with vascular tPA-dependent NMDAR signaling, combined with laser speckle flowmetry, intravital microscopy and ultrafast functional ultrasound in vivo imaging. We found that the tPA present in the blood circulation is capable of potentiating the cerebral blood flow increase induced by the activation of the mouse somatosensorial cortex, and that this effect is mediated by a tPA-dependent activation of NMDAR expressed at the luminal part of endothelial cells of arteries. Although blood molecules, such as acetylcholine, bradykinin or ATP are known to regulate vascular tone and induce vessel dilation, our present data provide the first evidence that circulating tPA is capable of influencing neurovascular coupling (NVC).

Demystifying the extracellular matrix and its proteolytic remodeling in the brain: structural and functional insights

The extracellular matrix (ECM) plays diverse roles in several physiological and pathological conditions. In the brain, the ECM is unique both in its composition and in functions. Furthermore, almost all the cells in the central nervous system contribute to different aspects of this intricate structure. Brain ECM, enriched with proteoglycans and other small proteins, aggregate into distinct structures around neurons and oligodendrocytes. These special structures have cardinal functions in the normal functioning of the brain, such as learning, memory, and synapse regulation. In this review, we have compiled the current knowledge about the structure and function of important ECM molecules in the brain and their proteolytic remodeling by matrix metalloproteinases and other enzymes, highlighting the special structures they form. In particular, the proteoglycans in brain ECM, which are essential for several vital functions, are emphasized in detail.

The role of endogenous tissue-type plasminogen activator in neuronal survival after ischemic stroke: friend or foe?

Endogenous protease tissue-type plasminogen activator (tPA) has highly efficient fibrinolytic activity and its recombinant variants alteplase and tenecteplase are established as highly effective thrombolytic drugs for ischemic stroke. Endogenous tPA is constituted of five functional domains through which it interacts with a variety of substrates, binding proteins and receptors, thus having enzymatic and cytokine-like effects to act on all cell types of the brain. In the past 2 decades, numerous studies have explored the clinical relevance of endogenous tPA in neurological diseases, especially in ischemic stroke. tPA is released from many cells within the brain parenchyma exposed to ischemia conditions in vitro and in vivo, which is believed to control neuronal fate. Some studies proved that tPA could induce blood-brain barrier disruption, neural excitotoxicity and inflammation, while others indicated that tPA also has anti-excitotoxic, neurotrophic and anti-apoptotic effects on neurons. Therefore, more work is needed to elucidate how tPA mediates such opposing functions that may amplify tPA from a therapeutic means into a key therapeutic target in endogenous neuroprotection after stroke. In this review, we summarize the biological characteristics and pleiotropic functions of tPA in the brain. Then we focus on possible hypotheses about why and how endogenous tPA mediates ischemic neuronal death and survival. Finally, we analyze how endogenous tPA affects neuron fate in ischemic stroke in a comprehensive view.

Increased 5-HT2C receptor editing predisposes to PTSD-like behaviors and alters BDNF and cytokines signaling

Post-traumatic stress disorder (PTSD) is a trauma- and stress-related disorder with dysregulated fear responses and neurobiological impairments, notably at neurotrophic and inflammation levels. Understanding the mechanisms underlying this disease is crucial to develop PTSD models that meet behavioral and neurobiological validity criteria as well as innovative therapeutic approaches. Serotonin 2C receptors (5-HT2CR) are known for their important role in anxiety, and mice having only the fully edited VGV isoform of 5-HT2CR, which thereby overexpressed brain 5-HT2CR, are of special interest to study PTSD predisposition. Innate and conditioned fear-related behaviors were assessed in VGV and wild-type mice. mRNA expression of brain-derived neurotrophic factor (BDNF), tissue-plasminogen activator (tPA), and pro-inflammatory cytokines (IL-6, IL-1β, and calcineurin) were measured by qRT-PCR. The effect of acute and chronic paroxetine was evaluated on both behavior and gene expression. VGV mice displayed greater fear expression, extensive fear extinction deficits, and fear generalization. Paroxetine restored fear extinction in VGV mice when administered acutely and decreased innate fear and fear generalization when administered chronically. In parallel, Bdnf, tPA, and pro-inflammatory cytokines mRNA levels were dysregulated in VGV mice. Bdnf and tPA mRNA expression was decreased in the hippocampus but increased in the amygdala, and chronic paroxetine normalized Bdnf mRNA levels both in the amygdala and the hippocampus. Amygdalar calcineurin mRNA level in VGV mice was also normalized by chronic paroxetine. VGV-transgenic mice displayed behavioral and neurobiological features that could be accessory to the investigation of PTSD and its treatment. Furthermore, these data point out to the role of 5-HT2CR in neuroplasticity and neuroinflammation.

Characterization of Tissue Plasminogen Activator Expression and Trafficking in the Adult Murine Brain

Tissue plasminogen activator (tPA) is an immediate-early gene important for regulating physiological processes like synaptic plasticity and neurovascular coupling. It has also been implicated in several pathological processes including blood-brain barrier (BBB) permeability, seizure progression, and stroke. These varied reports suggest that tPA is a pleiotropic mediator whose actions are highly compartmentalized in space and time. The specific localization of tPA, therefore, can provide useful information about its function. Accordingly, the goal of this study was to provide a detailed characterization of tPA's regional, cellular, and subcellular localization in the brain. To achieve this, two new transgenic mouse lines were utilized: (1) a PlatβGAL reporter mouse, which houses the β-galactosidase gene in the tPA locus and (2) a tPABAC-Cerulean mouse, which has a cerulean-fluorescent protein fused in-frame to the tPA C-terminus. Using these two transgenic reporters, we show that while tPA is expressed throughout most regions of the adult murine brain, it appears to be preferentially targeted to fiber tracts in the limbic system. In the hippocampus, confocal microscopy revealed tPA-Cerulean (tPA-Cer) puncta localized to giant mossy fiber boutons (MFBs) and astrocytes in stratum lucidum. With amplification of the tPA-Cer signal, somatically localized tPA was also observed in the stratum oriens (SO)/alveus layer of both CA1 and CA3 subfields. Coimmunostaining of tPA-Cer and interneuronal markers indicates that these tPA-positive cell bodies belong to a subclass of somatostatin (SST)/oriens-lacunosum moleculare (O-LM) interneurons. Together, these data imply that tPA's localization is differentially regulated, suggesting that its neuromodulatory effects may be compartmentalized and specialized to cell type.

Distant Space Processing is Controlled by tPA-dependent NMDA Receptor Signaling in the Entorhinal Cortex

In humans, spatial cognition and navigation impairments are a frequent situation during physiological and pathological aging, leading to a dramatic deterioration in the quality of life. Despite the discovery of neurons with location-specific activity in rodents, that is, place cells in the hippocampus and later on grid cells in the entorhinal cortex (EC), the molecular mechanisms underlying spatial cognition are still poorly known. Our present data bring together in an unusual combination 2 molecules of primary biological importance: a major neuronal excitatory receptor, N-methyl-D-aspartate receptor (NMDAR), and an extracellular protease, tissue plasminogen activator (tPA), in the control of spatial navigation. By using tPA-deficient mice and a structure-selective pharmacological approach, we demonstrate that the tPA-dependent NMDAR signaling potentiation in the EC plays a key and selective role in the encoding and the subsequent use of distant landmarks during spatial learning. We also demonstrate that this novel function of tPA in the EC is reduced during aging. Overall, these results argue for the concept that encoding of proximal versus distal landmarks is mediated not only by different anatomical pathways but also by different molecular mechanisms, with the tPA-dependent potentiation of NMDAR signaling in the EC that plays an important role.

Can fibrinolytic system components explain cognitive impairment in multiple sclerosis?

BACKGROUND

The fibrinolytic system is capable of modulating inflammatory and degenerative events within the central nervous system. Specifically, the plasminogen activator inhibitor-1 (PAI-1) has been associated with different pathological conditions in multiple sclerosis (MS) and its role in cognitive functioning is also known.

OBJECTIVES AND METHODS

To study the association between plasma levels and the polymorphic variants of the PAI-1 gene and cognitive performance in MS. 176 patients were studied. Neuropsychological evaluation was performed with the Brief Repeatable Battery of Neuropsychological Tests (BRB-N). A Polymerase Chain Reaction (PCR) was used to determine PAI-1 4G/5G polymorphisms and quantification was performed using an Enzyme-Linked ImmunoSorbent Assay (ELISA).

RESULTS

Participants were categorized as not cognitively impaired (NCI; n=114) and cognitively impaired (CI; n=62). The NCI group had a higher percentage of heterozygous subjects but no statistical differences were found between the CI and NCI group. Neuropsychological functioning did not correlate with plasma levels of PAI-1 or its genetic polymorphism. It is noteworthy that PAI-1 plasma levels were related to neurological impairment.

DISCUSSION

Cognitive impairment in MS is due to strategic focal lesions affecting regions and tracts involved in cognitive processes and to diffuse damage in the white and gray matter. This complex etiology could explain the absence of a relationship between the cognitive functioning and PAI-1 in patients with MS that has been found in vascular dementia or Alzheimer's disease. Plasma curves of PAI-1 and its measures in cerebrospinal fluid could help elucidate the role of PAI-1 in MS.

tPA Modulation of the Blood-Brain Barrier: A Unifying Explanation for the Pleiotropic Effects of tPA in the CNS

The plasminogen activation (PA) system is best known for its role in fibrinolysis. However, it has also been shown to regulate many nonfibrinolytic functions in the central nervous system (CNS). In particular, tissue-type plasminogen activator (tPA) is reported to have pleiotropic activities in the CNS, regulating events such as neuronal plasticity, excitotoxicity, and cerebrovascular barrier integrity, whereas urokinase-type plasminogen activator is mainly associated with tissue remodeling and cell migration. It has been suggested that the role tPA plays in controlling barrier integrity may provide a unifying mechanism for the reported diverse, and often opposing, functions ascribed to tPA in the CNS. Here we will review the possibility that the pleiotropic effects reported for tPA in physiologic and pathologic processes in the CNS may be a consequence of its role in the neurovascular unit in regulation of cerebrovascular responses and subsequently parenchymal homeostasis. We propose that this might offer an explanation for the ongoing debate regarding the neurotoxic versus neuroprotective roles of tPA.

Tissue-type plasminogen activator controls neuronal death by raising surface dynamics of extrasynaptic NMDA receptors

N-methyl-d-aspartate receptors (NMDARs) are ion channels whose synaptic versus extrasynaptic localization critically influences their functions. This distribution of NMDARs is highly dependent on their lateral diffusion at the cell membrane. Each obligatory subunit of NMDARs (GluN1 and GluN2) contains two extracellular clamshell-like domains with an agonist-binding domain and a distal N-terminal domain (NTD). To date, the roles and dynamics of the NTD of the GluN1 subunit in NMDAR allosteric signaling remain poorly understood. Using single nanoparticle tracking in mouse neurons, we demonstrate that the extracellular neuronal protease tissue-type plasminogen activator (tPA), well known to have a role in the synaptic plasticity and neuronal survival, leads to a selective increase of the surface dynamics and subsequent diffusion of extrasynaptic NMDARs. This process explains the previously reported ability of tPA to promote NMDAR-mediated calcium influx. In parallel, we developed a monoclonal antibody capable of specifically blocking the interaction of tPA with the NTD of the GluN1 subunit of NMDAR. Using this original approach, we demonstrate that the tPA binds the NTD of the GluN1 subunit at a lysine in position 178. Accordingly, when applied to mouse neurons, our selected antibody (named Glunomab) leads to a selective reduction of the tPA-mediated surface dynamics of extrasynaptic NMDARs, subsequent signaling and neurotoxicity, both in vitro and in vivo. Altogether, we demonstrate that the tPA is a ligand of the NTD of the obligatory GluN1 subunit of NMDAR acting as a modulator of their dynamic distribution at the neuronal surface and subsequent signaling.

Dependence receptor involvement in subtilisin-induced long-term depression and in long-term potentiation

The serine protease subtilisin induces a form of long-term depression (LTD) which is accompanied by a reduced expression of the axo-dendritic guidance molecule Unco-ordinated-5C (Unc-5C). One objective of the present work was to determine whether a loss of Unc-5C function contributed to subtilisin-induced LTD by using Unc-5C antibodies in combination with the pore-forming agents Triton X-100 (0.005%) or streptolysin O in rat hippocampal slices. In addition we have assessed the effect of subtilisin on the related dependence receptor Deleted in Colorectal Cancer (DCC) and used antibodies to this protein for functional studies. Field excitatory postsynaptic potentials (fEPSPs) were analyzed in rat hippocampal slices and protein extracts were used for Western blotting. Subtilisin produced a greater loss of DCC than of Unc-5C, but the antibodies had no effect on resting excitability or fEPSPs and did not modify subtilisin-induced LTD. However, antibodies to DCC but not Unc-5C did reduce the amplitude of theta-burst long-term potentiation (LTP). In addition, two inhibitors of endocytosis - dynasore and tat-gluR2(3Y) - were tested and, although the former compound had no effect on neurophysiological responses, tat-gluR2(3Y) did reduce the amplitude of subtilisin-induced LTD without affecting the expression of DCC or Unc-5C but with some loss of PostSynaptic Density Protein-95. The results support the view that the dependence receptor DCC may be involved in LTP and suggest that the endocytotic removal of a membrane protein or proteins may contribute to subtilisin-induced LTD, although it appears that neither Unc-5C nor DCC are involved in this process.

Impacts of tissue-type plasminogen activator (tPA) on neuronal survival

Tissue-type plasminogen activator (tPA) a serine protease is constituted of five functional domains through which it interacts with different substrates, binding proteins, and receptors. In the last years, great interest has been given to the clinical relevance of targeting tPA in different diseases of the central nervous system, in particular stroke. Among its reported functions in the central nervous system, tPA displays both neurotrophic and neurotoxic effects. How can the protease mediate such opposite functions remain unclear but several hypotheses have been proposed. These include an influence of the degree of maturity and/or the type of neurons, of the level of tPA, of its origin (endogenous or exogenous) or of its form (single chain tPA versus two chain tPA). In this review, we will provide a synthetic snapshot of our current knowledge regarding the natural history of tPA and discuss how it sustains its pleiotropic functions with focus on excitotoxic/ischemic neuronal death and neuronal survival.

Physiological and pathological roles of tissue plasminogen activator and its inhibitor neuroserpin in the nervous system

Although its roles in the vascular space are most well-known, tissue plasminogen activator (tPA) is widely expressed in the developing and adult nervous system, where its activity is believed to be regulated by neuroserpin, a predominantly brain-specific member of the serpin family of protease inhibitors. In the normal physiological state, tPA has been shown to play roles in the development and plasticity of the nervous system. Ischemic damage, however, may lead to excess tPA activity in the brain and this is believed to contribute to neurodegeneration. In this article, we briefly review the physiological and pathological roles of tPA in the nervous system, which includes neuronal migration, axonal growth, synaptic plasticity, neuroprotection and neurodegeneration, as well as a contribution to neurological disease. We summarize tPA's multiple mechanisms of action and also highlight the contributions of the inhibitor neuroserpin to these processes.

BDNF-TrkB signaling through Erk1/2 MAPK phosphorylation mediates the enhancement of fear memory induced by glucocorticoids

Activation of glucocorticoid receptors (GR) by glucocorticoid hormones (GC) enhances contextual fear memories through the activation of the Erk1/2(MAPK) signaling pathway. However, the molecular mechanism mediating this effect of GC remains unknown. Here we used complementary molecular and behavioral approaches in mice and rats and in genetically modified mice in which the GR was conditionally deleted (GR(NesCre)). We identified the tPA-BDNF-TrkB signaling pathway as the upstream molecular effectors of GR-mediated phosphorylation of Erk1/2(MAPK) responsible for the enhancement of contextual fear memory. These findings complete our knowledge of the molecular cascade through which GC enhance contextual fear memory and highlight the role of tPA-BDNF-TrkB-Erk1/2(MAPK) signaling pathways as one of the core effectors of stress-related effects of GC.

Altered behavior in mice with deletion of the alpha2-antiplasmin gene

Background

The α2-antiplasmin (α2AP) protein is known to be a principal physiological inhibitor of plasmin, and is expressed in various part of the brain, including the hippocampus, cortex, hypothalamus and cerebellum, thus suggesting a potential role for α2AP in brain functions. However, the involvement of α2AP in brain functions is currently unclear.

Objectives

The goal of this study was to investigate the effects of the deletion of the α2AP gene on the behavior of mice.

Methods

The motor function was examined by the wire hang test and rotarod test. To evaluate the cognitive function, a repeated rotarod test, Y-maze test, Morris water maze test, passive or shuttle avoidance test and fear conditioning test were performed. An open field test, dark/light transition test or tail suspension test was performed to determine the involvement of α2AP in anxiety or depression-like behavior.

Results and Conclusions

The α2AP knockout (α2AP^−/−) mice exhibited impaired motor function compared with α2AP^+/+ mice. The α2AP^−/− mice also exhibited impairments in motor learning, working memory, spatial memory and fear conditioning memory. Furthermore, the deletion of α2AP induced anxiety-like behavior, and caused an anti-depression-like effect in tail suspension. Therefore, our findings suggest that α2AP is a crucial mediator of motor function, cognitive function, anxiety-like behavior and depression-like behavior, providing new insights into the role of α2AP in the brain functions.

AAV-mediated overexpression of neuroserpin in the hippocampus decreases PSD-95 expression but does not affect hippocampal-dependent learning and memory

Neuroserpin is a serine protease inhibitor, or serpin, that is expressed in the nervous system and inhibits the protease tissue plasminogen activator (tPA). Neuroserpin has been suggested to play a role in learning and memory but direct evidence for such a role is lacking. Here we have used an adeno-associated virus (AAV) vector expression system to investigate the effect of neuroserpin on hippocampal-dependent learning and memory in the young adult rat. A FLAG-tagged neuroserpin construct was initially characterized by in vitro transcription/translation and transfection into HEK293 cells and shown to interact with tPA and be targeted to the secretory pathway. Targeted injection of a chimeric AAV1/2 vector expressing FLAG-neuroserpin resulted in localized overexpression in the dorsal hippocampus. Neuroserpin overexpression led to the appearance of an unstable neuroserpin:tPA complex in zymographic assays consistent with interaction with endogenous tPA in vivo. Rats overexpressing neuroserpin also showed a significant decrease in the levels of postsynaptic density protein 95, a major postsynaptic scaffolding protein. Three weeks after injection, a range of behavioural tests was performed to measure spatial and associative learning and memory, as well as innate and acquired fear. These tests provided no evidence of a role for neuroserpin in hippocampal-dependent learning and memory. In summary this study does not support a role for neuroserpin in hippocampal-dependent learning and memory in young adult rats but does suggest an involvement of neuroserpin in hippocampal synaptic plasticity.

Neural ECM proteases in learning and synaptic plasticity

Recent studies implicate extracellular proteases in synaptic plasticity, learning, and memory. The data are especially strong for such serine proteases as thrombin, tissue plasminogen activator, neurotrypsin, and neuropsin as well as matrix metalloproteinases, MMP-9 in particular. The role of those enzymes in the aforementioned phenomena is supported by the experimental results on the expression patterns (at the gene expression and protein and enzymatic activity levels) and functional studies, including knockout mice, specific inhibitors, etc. Counterintuitively, the studies have shown that the extracellular proteolysis is not responsible mainly for an overall degradation of the extracellular matrix (ECM) and loosening perisynaptic structures, but rather allows for releasing signaling molecules from the ECM, transsynaptic proteins, and latent form of growth factors. Notably, there are also indications implying those enzymes in the major neuropsychiatric disorders, probably by contributing to synaptic aberrations underlying such diseases as schizophrenia, bipolar, autism spectrum disorders, and drug addiction.

Valproic acid induces astrocyte-dependent neurite outgrowth from cultured rat primary cortical neuron via modulation of tPA/PAI-1 activity

Tissue plasminogen activator (tPA) is expressed in several regions of brain and plays regulatory roles such as neurite outgrowth, synaptic plasticity and long term potentiation. The activity of tPA is regulated by an endogenous inhibitor plasminogen activator inhibitor-1 (PAI-1), which is expressed mainly in astrocytes. Valproic acid (VPA), a histone deacetylase inhibitor that is used for the treatment of epilepsy and bipolar disorders, promotes neurite extension, neuronal growth and has neuroprotective effect in neurodegenerative diseases. In this study, we examined whether the neurite extension effects of VPA is mediated by modulating tPA/PAI-1 system. VPA dose-dependently increased tPA activity and decreased PAI-1 activity in rat primary astrocytes but not in neurons. PAI-1 protein level secreted into the culture medium but not tPA per se was decreased by VPA. In co-culture system or in neuronal culture stimulated with astrocyte conditioned media but not in pure neuronal cell culture, VPA induced neurite outgrowth via increased tPA activity due to the decreased PAI-1 activity in astrocytes. The decrease in PAI-1 activity and increased neurite extension was regulated via JNK mediated post-transcriptional pathway. The essential role of tPA/PAI-1 system in the regulation of VPA-mediated neurite extension was further demonstrated by experiments using astrocyte conditioned media obtained from tPA or PAI-1 knockout mice. Regulation of PAI-1 activity in astrocyte by VPA may affect both physiological and pathological processes in brain by upregulating tPA activity.

GluN2D subunit-containing NMDA receptors control tissue plasminogen activator-mediated spatial memory

Tissue plasminogen activator (tPA) is a serine protease with pleiotropic actions in the CNS, such as synaptic plasticity and neuronal death. Some effects of tPA require its interaction with the GluN1 subunit of the NMDA receptor (NMDAR), leading to a potentiation of NMDAR signaling. We have reported previously that the pro-neurotoxic effect of tPA is mediated through GluN2D subunit-containing NMDARs. Thus, the aim of the present study was to determine whether GluN2D subunit-containing NMDARs drive tPA-mediated cognitive functions. To address this issue, a strategy of immunization designed to prevent the in vivo interaction of tPA with NMDARs and GluN2D-deficient mice were used in a set of behavioral tasks. Altogether, our data provide the first evidence that tPA influences spatial memory through its preferential interaction with GluN2D subunit-containing NMDARs.

Protease-activated receptor-1 modulates hippocampal memory formation and synaptic plasticity

Protease-activated receptor-1 (PAR1) is an unusual G-protein coupled receptor (GPCR) that is activated through proteolytic cleavage by extracellular serine proteases. Although previous work has shown that inhibiting PAR1 activation is neuroprotective in models of ischemia, traumatic injury, and neurotoxicity, surprisingly little is known about PAR1's contribution to normal brain function. Here, we used PAR1-/- mice to investigate the contribution of PAR1 function to memory formation and synaptic function. We demonstrate that PAR1-/- mice have deficits in hippocampus-dependent memory. We also show that while PAR1-/- mice have normal baseline synaptic transmission at Schaffer collateral-CA1 synapses, they exhibit severe deficits in N-methyl-d-aspartate receptor (NMDAR)-dependent long-term potentiation (LTP). Mounting evidence indicates that activation of PAR1 leads to potentiation of NMDAR-mediated responses in CA1 pyramidal cells. Taken together, this evidence and our data suggest an important role for PAR1 function in NMDAR-dependent processes subserving memory formation and synaptic plasticity.

Protease-activated receptor 1 (PAR1) coupling to G(q/11) but not to G(i/o) or G(12/13) is mediated by discrete amino acids within the receptor second intracellular loop

Protease-activated receptor 1 (PAR1) is an unusual GPCR that interacts with multiple G protein subfamilies (G(q/11), G(i/o), and G(12/13)) and their linked signaling pathways to regulate a broad range of pathophysiological processes. However, the molecular mechanisms whereby PAR1 interacts with multiple G proteins are not well understood. Whether PAR1 interacts with various G proteins at the same, different, or overlapping binding sites is not known. Here we investigated the functional and specific binding interactions between PAR1 and representative members of the G(q/11), G(i/o), and G(12/13) subfamilies. We report that G(q/11) physically and functionally interacts with specific amino acids within the second intracellular (i2) loop of PAR1. We identified five amino acids within the PAR1 i2 loop that, when mutated individually, each markedly reduced PAR1 activation of linked inositol phosphate formation in transfected COS-7 cells (functional PAR1-null cells). Among these mutations, only R205A completely abolished direct G(q/11) binding to PAR1 and also PAR1-directed inositol phosphate and calcium mobilization in COS-7 cells and PAR1-/- primary astrocytes. In stark contrast, none of the PAR1 i2 loop mutations disrupted direct PAR1 binding to either G(o) or G(12), or their functional coupling to linked pertussis toxin-sensitive ERK phosphorylation and C3 toxin-sensitive Rho activation, respectively. In astrocytes, our findings suggest that PAR1-directed calcium signaling involves a newly appreciated G(q/11)-PLCε pathway. In summary, we have identified key molecular determinants for PAR1 interactions with G(q/11), and our findings support a model where G(q/11), G(i/o) or G(12/13) each bind to distinct sites within the cytoplasmic regions of PAR1.

Tissue-type plasminogen activator-mediated plasminogen activation and contact activation, implications in and beyond haemostasis

Due to its discovery as initiator of fibrinolysis and its well-studied activation by fibrin, tissue-type plasminogen activator (tPA) and the fibrinolytic system are generally associated with the dissolution of blood clots. However, it has been demonstrated over the years that (i) tPA can be activated by multiple proteins, (ii) plasmin has many substrates other than fibrin and (iii) tPA and plasmin have biological functions independent of fibrin and distinct from their role in blood clot lysis. We here review the data with respect to the activation of tPA by fibrin and its multiple other cofactors, in relation to tPA's role in pathophysiology, notably fibrinolysis and amyloidosis, with emphasis on Alzheimer's disease. We demonstrate a common structural element, termed cross-β structure, in misfolded proteins that is causal to tPA activation. The implications for protein misfolding diseases that are known to be associated with the deposition of amyloid and for diseases for which this has not (yet) been established are discussed.

Exercise influences hippocampal plasticity by modulating brain-derived neurotrophic factor processing

Exercise has been shown to impact brain plasticity and function by involving the action of brain-derived neurotrophic factor (BDNF); however, mechanisms involved are poorly understood. Two types of BDNF coexist in the brain, the precursor (proBDNF) and its mature product (mBDNF), which preferentially bind specific receptors and exert distinct functions. It is crucial to understand how exercise affects crucial steps in the BDNF processing and signaling to evaluate therapeutic applications. We found that 7 days of voluntary exercise increased both pro and mature BDNF in the rat hippocampus. Exercise also increased the activity of tissue-type plasminogen activator (tPA), a serine proteinase shown to facilitate proBDNF cleavage into mBDNF. The blockade of tPA activity reduced the exercise effects on proBDNF and mBDNF. The tPA blocking also inhibited the activation of TrkB receptor, and the TrkB signaling downstream effectors phospho-ERK, phospho-Akt, and phospho-CaMKII. The blocking of tPA also counteracted the effects of exercise on the plasticity markers phospho-synapsin I and growth-associated protein 43 (GAP-43). These results indicate that the effects of exercise on hippocampal plasticity are dependent on BDNF processing and subsequent TrkB signaling, with important implications for neuronal function.

Serine proteases, serine protease inhibitors, and protease-activated receptors: roles in synaptic function and behavior

Serine proteases, serine protease inhibitors, and protease-activated receptors have been intensively investigated in the periphery and their roles in a wide range of processes-coagulation, inflammation, and digestion, for example-have been well characterized (see Coughlin, 2000; Macfarlane et al., 2001; Molinari et al., 2003; Wang et al., 2008; Di Cera, 2009 for reviews). A growing number of studies demonstrate that these protein systems are widely expressed in many cell types and regions in mammalian brains. Accumulating lines of evidence suggest that the brain has co-opted the activities of these interesting proteins to regulate various processes underlying synaptic activity and behavior. In this review, we discuss emerging roles for serine proteases in the regulation of mechanisms underlying synaptic plasticity and memory formation.

Tissue-type plasminogen activator induces plasmin-dependent proteolysis of intracellular neuronal nitric oxide synthase

BACKGROUND INFORMATION

Despite its pro-fibrinolytic activity, tPA (tissue plasminogen activator) is a serine protease known to influence a number of physiological and pathological functions in the central nervous system. Accordingly, tPA was reported to mediate some of its functions in the central nervous system through NMDA (N-methyl-D-aspartate) receptors, LRP (low-density lipoprotein receptor-related protein) or annexin II.

RESULTS

We provide here both in vitro and in vivo evidence that tPA could mediate proteolysis and subsequent delocalization of neuronal nitric oxide synthase, thereby reducing endogenous neuronal nitric oxide release. We also demonstrate that although this effect is independent of NMDA receptors, LRP signalling and calpain-mediated proteolysis, it is dependent on the ability of tPA to promote the conversion of plasminogen into plasmin.

CONCLUSION

Altogether, these results demonstrate a new function for tPA in the central nervous system, which most likely contributes to its pleiotropic functions.

Ventral striatal plasticity and spatial memory

Spatial memory formation is a dynamic process requiring a series of cellular and molecular steps, such as gene expression and protein translation, leading to morphological changes that have been envisaged as the structural bases for the engram. Despite the role suggested for medial temporal lobe plasticity in spatial memory, recent behavioral observations implicate specific components of the striatal complex in spatial information processing. However, the potential occurrence of neural plasticity within this structure after spatial learning has never been investigated. In this study we demonstrate that blockade of cAMP response element binding protein-induced transcription or inhibition of protein synthesis or extracellular proteolytic activity in the ventral striatum impairs long-term spatial memory. These findings demonstrate that, in the ventral striatum, similarly to what happens in the hippocampus, several key molecular events crucial for the expression of neural plasticity are required in the early stages of spatial memory formation.

Decreased serotonin levels associated with behavioral disinhibition in tissue plasminogen activator deficient (tPA-/-) mice

Tissue Plasminogen Activator (tPA) is a serine protease expressed in different areas of the mammalian brain. It has been used clinically to dissolve clots and shown to have a role in neurodegeneration. Early studies suggested that tPA plays an important role in the processes of learning and memory, demonstrated at the level of behavior and synaptic plasticity. Herein, we extend the behavioral characterization of these mice to the related dimension of exploratory-related behavior using an extensive battery of behavioral tests as well as the neurotransmitter metabolism associated with the behavioral measures. Our results indicate a behavior tendency in these mice consistent with "impulsivity" or reduced exploratory inhibition. These patterns are accompanied by decreased levels of serotonin in several brain regions important in behavioral regulation in the tPA(-/-) mice compared to control animals. Systemic administration of fluoxetine reversed the behavioral disinhibition of tPA(-/-) mice, further supporting an important alteration in behavior regulation mediated by serotonin systems as underappreciated but important element of the behavioral phenotype of these animals.

Tissue-type plasminogen activator requires a co-receptor to enhance NMDA receptor function

Glutamate is the main excitatory neurotransmitter of the CNS. Tissue-type plasminogen activator (tPA) is recognized as a modulator of glutamatergic neurotransmission. This attribute is exemplified by its ability to potentiate calcium signaling following activation of the glutamate-binding NMDA receptor (NMDAR). It has been hypothesized that tPA can directly cleave the NR1 subunit of the NMDAR and thereby potentiate NMDA-induced calcium influx. In contrast, here we show that this increase in NMDAR signaling requires tPA to be proteolytically active, but does not involve cleavage of the NR1 subunit or plasminogen. Rather, we demonstrate that enhancement of NMDAR function by tPA is mediated by a member of the low-density lipoprotein receptor (LDLR) family. Hence, this study proposes a novel functional relationship between tPA, the NMDAR, a LDLR and an unknown substrate which we suspect to be a serpin. Interestingly, whilst tPA alone failed to cleave NR1, cell-surface NMDARs did serve as an efficient and discrete proteolytic target for plasmin. Hence, plasmin and tPA can affect the NMDAR via distinct avenues. Altogether, we find that plasmin directly proteolyses the NMDAR whilst tPA functions as an indirect modulator of NMDA-induced events via LDLR engagement.

Retinoids and activation of PKC induce tissue-type plasminogen activator expression and storage in human astrocytes

BACKGROUND

Emerging data demonstrate important roles for tissue-type plasminogen activator (t-PA) in the central nervous system (CNS). In contrast to endothelial cells, little is known about the regulation of t-PA gene expression and secretion in astrocytes.

OBJECTIVES

The aims of the present study were to investigate whether t-PA gene expression is regulated by retinoids and the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) in human astrocytes, and to study whether t-PA is stored and subject to regulated release from these cells, as with endothelial cells.

METHODS

Native human astrocytes were treated with RA and/or PMA. mRNA was quantified by real-time RT-PCR and protein secretion determined by ELISA. Intracellular t-PA immunoreactivity in astrocytes was examined by immunocyto- and histochemistry.

RESULTS

RA and/or PMA induced a time-dependent increase in t-PA mRNA and protein levels in astrocytes, reaching 10-fold after combined treatment. This was associated with increased amounts of t-PA storage in intracellular granular structures. Both forskolin and histamine induced regulated release of t-PA. The presence of t-PA in reactive astrocytes was confirmed in human brain tissue.

CONCLUSIONS

These data show that RA and PKC activation induce a strong up-regulation of t-PA expression in astrocytes, and increased intracellular storage pools. Moreover, a regulated release of t-PA can be induced from these cells. This raises the possibility that astrocytes contribute to the regulation of extracellular t-PA levels in the CNS.

Plasmin potentiates synaptic N-methyl-D-aspartate receptor function in hippocampal neurons through activation of protease-activated receptor-1

Protease-activated receptor-1 (PAR1) is activated by a number of serine proteases, including plasmin. Both PAR1 and plasminogen, the precursor of plasmin, are expressed in the central nervous system. In this study we examined the effects of plasmin in astrocyte and neuronal cultures as well as in hippocampal slices. We find that plasmin evokes an increase in both phosphoinositide hydrolysis (EC(50) 64 nm) and Fura-2/AM fluorescence (195 +/- 6.7% above base line, EC(50) 65 nm) in cortical cultured murine astrocytes. Plasmin also activates extracellular signal-regulated kinase (ERK1/2) within cultured astrocytes. The plasmin-induced rise in intracellular Ca(2+) concentration ([Ca(2+)](i)) and the increase in phospho-ERK1/2 levels were diminished in PAR1(-/-) astrocytes and were blocked by 1 microm BMS-200261, a selective PAR1 antagonist. However, plasmin had no detectable effect on ERK1/2 or [Ca(2+)](i) signaling in primary cultured hippocampal neurons or in CA1 pyramidal cells in hippocampal slices. Plasmin (100-200 nm) application potentiated the N-methyl-D-aspartate (NMDA) receptor-dependent component of miniature excitatory postsynaptic currents recorded from CA1 pyramidal neurons but had no effect on alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate- or gamma-aminobutyric acid receptor-mediated synaptic currents. Plasmin also increased NMDA-induced whole cell receptor currents recorded from CA1 pyramidal cells (2.5 +/- 0.3-fold potentiation over control). This effect was blocked by BMS-200261 (1 microm; 1.02 +/- 0.09-fold potentiation over control). These data suggest that plasmin may serve as an endogenous PAR1 activator that can increase [Ca(2+)](i) in astrocytes and potentiate NMDA receptor synaptic currents in CA1 pyramidal neurons.

Learning and memory deficits in mice lacking protease activated receptor-1

The roles of serine proteases and protease activated receptors have been extensively studied in coagulation, wound healing, inflammation, and neurodegeneration. More recently, serine proteases have been suggested to influence synaptic plasticity. In this context, we examined the role of protease activated receptor 1 (PAR1), which is activated following proteolytic cleavage by thrombin and plasmin, in emotionally motivated learning. We were particularly interested in PAR1 because its activation enhances the function of NMDA receptors, which are required for some forms of synaptic plasticity. We examined several baseline behavioral measures, including locomotor activity, expression of anxiety-like behavior, motor task acquisition, nociceptive responses, and startle responses in C57Bl/6 mice in which the PAR1 receptor has been genetically deleted. In addition, we evaluated learning and memory in these mice using two memory tasks, passive avoidance and cued fear-conditioning. Whereas locomotion, pain response, startle, and measures of baseline anxiety were largely unaffected by PAR1 removal, PAR1-/- animals showed significant deficits in a passive avoidance task and in cued fear conditioning. These data suggest that PAR1 may play an important role in emotionally motivated learning.

The extracellular protease matrix metalloproteinase-9 is activated by inhibitory avoidance learning and required for long-term memory

Matrix metalloproteinases (MMPs) are a family of extracellularly acting proteolytic enzymes with well-recognized roles in plasticity and remodeling of synaptic circuits during brain development and following brain injury. However, it is now becoming increasingly apparent that MMPs also function in normal, nonpathological synaptic plasticity of the kind that may underlie learning and memory. Here, we extend this idea by investigating the role and regulation of MMP-9 in an inhibitory avoidance (IA) learning and memory task. We demonstrate that following IA training, protein levels and proteolytic activity of MMP-9 become elevated in hippocampus by 6 h, peak at 12-24 h, then decline to baseline values by approximately 72 h. When MMP function is abrogated by intrahippocampal infusion of a potent gelatinase (MMP-2 and MMP-9) inhibitor 3.5 h following IA training, a time prior to the onset of training-induced elevation in levels, IA memory retention is significantly diminished when tested 1-3 d later. Animals impaired at 3 d exhibit robust IA memory when retrained, suggesting that such impairment is not likely attributed to toxic or other deleterious effects that permanently disrupt hippocampal function. In anesthetized adult rats, the effective distance over which synaptic plasticity is impaired by a single intrahippocampal infusion of the MMP inhibitor of the kind that blocks IA memory is approximately 1200 microm. Taken together, these data suggest that IA training induces a slowly emerging, but subsequently protracted period of MMP-mediated proteolysis critical for enabling long-lasting synaptic modification that underlies long-term memory consolidation.

Modulation of NR2B-regulated contextual fear in the hippocampus by the tissue plasminogen activator system

Contextual fear conditioning is regulated by the hippocampus, and NR2B, a subunit of the NMDA receptor (NR), is involved in this process. We show that acute stress modulates tissue plasminogen activator (tPA) activity in the hippocampus by inducing expression of its inhibitor, plasminogen activator inhibitor-1. Acute stress increases NR2B expression and ERK1/2 phosphorylation, a classical marker of postsynaptic plasticity, in the hippocampus. tPA forms a complex with NR2B and is necessary for binding NR2B to postsynaptic density-95, allowing for NR activation and membrane anchoring. Acute stress increases the interaction between NR2B and RACK-1, which is also dependent on tPA, further suggesting that tPA is an important factor in NMDA signaling and plasticity in the hippocampus. Finally, acutely stressed tPA(-/-) mice show a decrease in contextual fear conditioning compared with stressed WT mice. These results indicate that tPA is a key modulator in stabilizing the NR complex during stress and participates in changes in behavior and synaptic plasticity.

Two conserved regions within the tissue-type plasminogen activator gene promoter mediate regulation by brain-derived neurotrophic factor

Tissue-type plasminogen activator (t-PA) has recently been identified as a modulator of neuronal plasticity and can initiate conversion of the pro-form of brain-derived neurotrophic factor (BDNF) into its mature form. BDNF also increases t-PA gene expression implicating t-PA as a downstream effector of BDNF function. Here we demonstrate that BDNF-mediated induction of t-PA mRNA requires an increase in t-PA gene transcription. Reporter constructs harboring 9.5 kb of the human t-PA promoter conferred BDNF-responsiveness in transfected mouse primary cortical neurons. This regulation was recapitulated in HEK 293 cells coexpressing the TrkB neurotrophin receptor. t-PA promoter-deletion analysis revealed the presence of two BDNF-responsive domains, one located between -3.07 and -2.5 kb and the other within the proximal promoter. The upstream region was shown to confer BDNF responsiveness in a TrkB-dependent manner when attached to a heterologous promoter. We also identify homologous regions within the murine and bovine t-PA gene promoters and demonstrate that the equivalent upstream murine sequence functions as a BDNF-responsive enhancer when inserted 5' of the human proximal t-PA promoter. Hence, BDNF-mediated induction of t-PA transcription relies on conserved modular promoter elements including a novel upstream BDNF-responsive domain and the proximal t-PA gene promoter.

Astrocytic control of synaptic NMDA receptors

Astrocytes express a wide range of G-protein coupled receptors that trigger release of intracellular Ca2+, including P2Y, bradykinin and protease activated receptors (PARs). By using the highly sensitive sniffer-patch technique, we demonstrate that the activation of P2Y receptors, bradykinin receptors and protease activated receptors all stimulate glutamate release from cultured or acutely dissociated astrocytes. Of these receptors, we have utilized PAR1 as a model system because of favourable pharmacological and molecular tools, its prominent expression in astrocytes and its high relevance to neuropathological processes. Astrocytic PAR1-mediated glutamate release in vitro is Ca2+ dependent and activates NMDA receptors on adjacent neurones in culture. Activation of astrocytic PAR1 in hippocampal slices induces an APV-sensitive inward current in CA1 neurones and causes APV-sensitive neuronal depolarization in CA1 neurones, consistent with release of glutamate from astrocytes. PAR1 activation enhances the NMDA receptor-mediated component of synaptic miniature EPSCs, evoked EPSCs and evoked EPSPs in a Mg2+-dependent manner, which may reflect spine head depolarization and consequent reduction of NMDA receptor Mg2+ block during subsequent synaptic currents. The release of glutamate from astrocytes following PAR1 activation may also lead to glutamate occupancy of some perisynaptic NMDA receptors, which pass current following relief of tonic Mg2+ block during synaptic depolarization. These results suggest that astrocytic G-protein coupled receptors that increase intracellular Ca2+ can tune synaptic NMDA receptor responses.

Anti-NR1 N-terminal-domain vaccination unmasks the crucial action of tPA on NMDA-receptor-mediated toxicity and spatial memory

Fine-tuning of NMDA glutamatergic receptor signalling strategically controls crucial brain functions. This process depends on several ligands and modulators, one of which unexpectedly includes the serine protease tissue-type plasminogen activator (tPA). In vitro, tPA increases NMDA-receptor-mediated calcium influx by interacting with, and then cleaving, the NR1 subunit within its N-terminal domain. Owing to lack of in vivo evidence of the relevance and contribution of this mechanism in physiological and pathological brain processes, active immunisation was developed here in mice, to allow transient and specific prevention of the interaction of tPA with the NR1 subunit. Immunisation significantly reduced the severity of ischemic and excitotoxic insults in the mouse brain. Cognitive function was altered in some, but not all behavioural tasks affected in tPA-deficient mice. Our data demonstrate that in vivo, tPA controls neurotoxicity and the encoding of novel spatial experiences by binding to and cleaving the NMDA receptor NR1 subunit. Interesting therapeutic possibilities for several brain pathologies that involve excitotoxicity may now be envisaged.

Increased expression and activity of urokinase-type plasminogen activator during epileptogenesis

Our recent large-scale molecular profiling study revealed a sevenfold upregulation in the expression of urokinase-type plasminogen activator (uPA) during epileptogenesis. uPA is a member of the plasminogen activation system, which is a major contributor to the reorganization of neuronal circuits after trauma. Here, we investigated the expression and activity of uPA in normal and epileptogenic rat hippocampus to test a hypothesis that the expression of uPA is altered in brain areas that undergo epilepsy-related circuitry reorganization. Epileptogenesis was triggered by inducing status epilepticus (SE) with electrical stimulation of the amygdala in rats. Continuous video-electroencephalogram recordings were used to monitor the development of SE and the occurrence of spontaneous seizures. Animals were killed at 1, 4 or 14 days after SE, and brains were processed for immunohistochemistry or protein extraction. Confocal microscopy analysis of double-immunolabelled preparations indicated that SE triggered an increased expression of uPA in hippocampal astrocytes, neurons, white matter and blood vessels. Zymography revealed that the expression of uPA protein is associated with increased levels of enzymatically active uPA during epileptogenesis. uPA expression and enzymatic activity peaked within 1-4 days after SE, that is, before the occurrence of spontaneous seizures, and remained elevated for at least 2 weeks. These data suggest that uPA is involved in the reorganization of neuronal tissue during the epileptogenic process.

Plasmin deficiency in Alzheimer's disease brains: causal or casual?

Substantial recent evidence suggests that defects in amyloid peptide degradation can be at the base of cases of sporadic Alzheimer's disease (AD). Among the discovered brain enzymes with the capacity to degrade amyloid peptide, the serine protease plasmin acquires special physiological relevance because of its low levels in areas of AD human brains with a high susceptibility to amyloid plaque accumulation. In this article we comment on a series of observations supporting the fact that plasmin paucity in the brain is not simply a secondary event in the disease but rather a primary defect in certain cases of sporadic AD. We also refer to recent data pointing to alterations in raft membrane domains and diminished membrane cholesterol as the underlying cause. Finally, we discuss the possibility that plasmin deficiency in the brain could lead to AD symptomatology because of amyloid aggregation and the triggering of cell death signaling cascades.