Transforming growth factor alpha-induced expression of type 1 plasminogen activator inhibitor in astrocytes rescues neurons from excitotoxicity

Crosstalk between small-cell lung cancer cells and astrocytes mimics brain development to promote brain metastasis

Brain metastases represent an important clinical problem for patients with small-cell lung cancer (SCLC). However, the mechanisms underlying SCLC growth in the brain remain poorly understood. Here, using intracranial injections in mice and assembloids between SCLC aggregates and human cortical organoids in culture, we found that SCLC cells recruit reactive astrocytes to the tumour microenvironment. This crosstalk between SCLC cells and astrocytes drives the induction of gene expression programmes that are similar to those found during early brain development in neurons and astrocytes. Mechanistically, the brain development factor Reelin, secreted by SCLC cells, recruits astrocytes to brain metastases. These astrocytes in turn promote SCLC growth by secreting neuronal pro-survival factors such as SERPINE1. Thus, SCLC brain metastases grow by co-opting mechanisms involved in reciprocal neuron-astrocyte interactions during brain development. Targeting such developmental programmes activated in this cancer ecosystem may help prevent and treat brain metastases.

Deficiency in Retinal TGFβ Signaling Aggravates Neurodegeneration by Modulating Pro-Apoptotic and MAP Kinase Pathways

Transforming growth factor β (TGFβ) signaling has manifold functions such as regulation of cell growth, differentiation, migration, and apoptosis. Moreover, there is increasing evidence that it also acts in a neuroprotective manner. We recently showed that TGFβ receptor type 2 (Tgfbr2) is upregulated in retinal neurons and Müller cells during retinal degeneration. In this study we investigated if this upregulation of TGFβ signaling would have functional consequences in protecting retinal neurons. To this end, we analyzed the impact of TGFβ signaling on photoreceptor viability using mice with cell type-specific deletion of Tgfbr2 in retinal neurons and Müller cells (Tgfbr2^ΔOC) in combination with a genetic model of photoreceptor degeneration (VPP). We examined retinal morphology and the degree of photoreceptor degeneration, as well as alterations of the retinal transcriptome. In summary, retinal morphology was not altered due to TGFβ signaling deficiency. In contrast, VPP-induced photoreceptor degeneration was drastically exacerbated in double mutant mice (Tgfbr2^ΔOC; VPP) by induction of pro-apoptotic genes and dysregulation of the MAP kinase pathway. Therefore, TGFβ signaling in retinal neurons and Müller cells exhibits a neuroprotective effect and might pose promising therapeutic options to attenuate photoreceptor degeneration in humans.

Low Density Receptor-Related Protein 1 Interactions With the Extracellular Matrix: More Than Meets the Eye

The extracellular matrix (ECM) is a biological substrate composed of collagens, proteoglycans and glycoproteins that ensures proper cell migration and adhesion and keeps the cell architecture intact. The regulation of the ECM composition is a vital process strictly controlled by, among others, proteases, growth factors and adhesion receptors. As it appears, ECM remodeling is also essential for proper neuronal and glial development and the establishment of adequate synaptic signaling. Hence, disturbances in ECM functioning are often present in neurodegenerative diseases like Alzheimer’s disease. Moreover, mutations in ECM molecules are found in some forms of epilepsy and malfunctioning of ECM-related genes and pathways can be seen in, for example, cancer or ischemic injury. Low density lipoprotein receptor-related protein 1 (Lrp1) is a member of the low density lipoprotein receptor family. Lrp1 is involved not only in ligand uptake, receptor mediated endocytosis and lipoprotein transport—functions shared by low density lipoprotein receptor family members—but also regulates cell surface protease activity, controls cellular entry and binding of toxins and viruses, protects against atherosclerosis and acts on many cell signaling pathways. Given the plethora of functions, it is not surprising that Lrp1 also impacts the ECM and is involved in its remodeling. This review focuses on the role of Lrp1 and some of its major ligands on ECM function. Specifically, interactions with two Lrp1 ligands, integrins and tissue plasminogen activator are described in more detail.

Increased extracellular clusterin in the prefrontal cortex in schizophrenia

The expression of the gene that encodes clusterin, a glycoprotein that has been implicated in the regulation of many cellular processes, has previously been found in gene expression profiling studies to be among the most significantly differentially expressed genes in pyramidal and parvalbumin-containing inhibitory neurons in the cerebral cortex in subjects with schizophrenia. In this study, we investigated whether clusterin may also be dysregulated at the protein level in schizophrenia subjects. We found that, although the intracellular amount of clusterin may be unchanged, the level of extracellular, secreted clusterin appears to be significantly increased in schizophrenia subjects. It is speculated that this finding may represent a neuroprotective response to pathophysiological events that underlie schizophrenia.

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.

Molecular profiles of pyramidal neurons in the superior temporal cortex in schizophrenia

Disrupted synchronized oscillatory firing of pyramidal neuronal networks in the cerebral cortex in the gamma frequency band (i.e., 30-100 Hz) mediates many of the cognitive deficits and symptoms of schizophrenia. In fact, the density of dendritic spines and the average somal area of pyramidal neurons in layer 3 of the cerebral cortex, which mediate both long-range (associational) and local (intrinsic) corticocortical connections, are decreased in subjects with this illness. To explore the molecular pathophysiology of pyramidal neuronal dysfunction, we extracted ribonucleic acid (RNA) from laser-captured pyramidal neurons from layer 3 of Brodmann's area 42 of the superior temporal gyrus (STG) from postmortem brains from schizophrenia and normal control subjects. We then profiled the messenger RNA (mRNA) expression of these neurons, using microarray technology. We identified 1331 mRNAs that were differentially expressed in schizophrenia, including genes that belong to the transforming growth factor beta (TGF-β) and the bone morphogenetic proteins (BMPs) signaling pathways. Disturbances of these signaling mechanisms may in part contribute to the altered expression of other genes found to be differentially expressed in this study, such as those that regulate extracellular matrix (ECM), apoptosis, and cytoskeletal and synaptic plasticity. In addition, we identified 10 microRNAs (miRNAs) that were differentially expressed in schizophrenia; enrichment analysis of their predicted gene targets revealed signaling pathways and gene networks that were found by microarray to be dysregulated, raising an interesting possibility that dysfunction of pyramidal neurons in schizophrenia may in part be mediated by a concerted dysregulation of gene network functions as a result of the altered expression of a relatively small number of miRNAs. Taken together, findings of this study provide a neurobiological framework within which specific hypotheses about the molecular mechanisms of pyramidal cell dysfunction in schizophrenia can be formulated.

Vascular inflammatory, oxidative and protease-based processes: implications for neuronal cell death in Alzheimer's disease

A substantial literature demonstrates activation of inflammatory processes in the Alzheimer's disease (AD) brain and an association between inflammation and oxidative stress. We have shown that brain microvessels from AD patients express high levels of inflammatory proteins and that these proteins evoke release of the neurotoxic protease thrombin from brain endothelial cells. The objective of this study was to determine the effects of inflammatory proteins on brain endothelial cell reactive oxygen species generation, protease release and cell apoptosis. Also, the effects of inflammatory proteins on neuronal reactive oxygen species generation, injury and apoptosis were assessed. Treatment of cultured brain endothelial cells with inflammatory proteins (LPS, IL-1beta, IL-6, IFN-gamma, TNF-alpha) resulted in a significant increase (p < 0.01) in intracellular levels of reactive oxygen species by 1 h. Inflammatory proteins also caused release of tissue plasminogen activator and increased apoptosis by 24 h in these cells. In cultured neurons, inflammatory proteins caused an increase in reactive oxygen species, membrane fluidity, and apoptosis by 24 h, as detected by fluorescent microscopy. Taken together, these data support the hypothesis that vascular inflammatory, oxidative and protease-based processes contribute to neuronal cell death, and suggest that therapies targeted at these mediators and processes could be effective in AD.

Plasminogen activator inhibitor type 1 regulates microglial motility and phagocytic activity

Background

Plasminogen activator inhibitor type 1 (PAI-1) is the primary inhibitor of urokinase type plasminogen activators (uPA) and tissue type plasminogen activators (tPA), which mediate fibrinolysis. PAI-1 is also involved in the innate immunity by regulating cell migration and phagocytosis. However, little is known about the role of PAI-1 in the central nervous system.

Methods

In this study, we identified PAI-1 in the culture medium of mouse mixed glial cells by liquid chromatography and tandem mass spectrometry. Secretion of PAI-1 from glial cultures was detected by ELISA and western blotting analysis. Cell migration was evaluated by in vitro scratch-wound healing assay or Boyden chamber assay and an in vivo stab wound injury model. Phagocytic activity was measured by uptake of zymosan particles.

Results

The levels of PAI-1 mRNA and protein expression were increased by lipopolysaccharide and interferon-γ stimulation in both microglia and astrocytes. PAI-1 promoted the migration of microglial cells in culture via the low-density lipoprotein receptor-related protein (LRP) 1/Janus kinase (JAK)/signal transducer and activator of transcription (STAT)1 axis. PAI-1 also increased microglial migration in vivo when injected into mouse brain. PAI-1-mediated microglial migration was independent of protease inhibition, because an R346A mutant of PAI-1 with impaired PA inhibitory activity also promoted microglial migration. Moreover, PAI-1 was able to modulate microglial phagocytic activity. PAI-1 inhibited microglial engulfment of zymosan particles in a vitronectin- and Toll-like receptor 2/6-dependent manner.

Conclusion

Our results indicate that glia-derived PAI-1 may regulate microglial migration and phagocytosis in an autocrine or paracrine manner. This may have important implications in the regulation of brain microglial activities in health and disease.

Activity deprivation induces neuronal cell death: mediation by tissue-type plasminogen activator

Spontaneous activity is an essential attribute of neuronal networks and plays a critical role in their development and maintenance. Upon blockade of activity with tetrodotoxin (TTX), neurons degenerate slowly and die in a manner resembling neurodegenerative diseases-induced neuronal cell death. The molecular cascade leading to this type of slow cell death is not entirely clear. Primary post-natal cortical neurons were exposed to TTX for up to two weeks, followed by molecular, biochemical and immunefluorescence analysis. The expression of the neuronal marker, neuron specific enolase (NSE), was down-regulated, as expected, but surprisingly, there was a concomitant and striking elevation in expression of tissue-type plasminogen activator (tPA). Immunofluorescence analysis indicated that tPA was highly elevated inside affected neurons. Transfection of an endogenous tPA inhibitor, plasminogen activator inhibitor-1 (PAI-1), protected the TTX-exposed neurons from dying. These results indicate that tPA is a pivotal player in slowly progressing activity deprivation-induced neurodegeneration.

Biphasic regulation of tissue plasminogen activator activity in ischemic rat brain and in cultured neural cells: essential role of astrocyte-derived plasminogen activator inhibitor-1

In brain, the serine protease tissue plasminogen activator (tPA) and its endogenous inhibitor plasminogen activator inhibitor-1 (PAI-1) have been implicated in the regulation of various neurophysiological and pathological responses. In this study, we investigated the differential role of neurons and astrocytes in the regulation of tPA/PAI-1 activity in ischemic brain. The activity of tPA peaked transiently and then decreased in cortex and striatum along with delayed induction of PAI-1 in the inflammatory stage after MCAO/reperfusion injury. In cultured primary cells, glutamate stimulation increased tPA activity in neurons but not in other cells such as microglia and astrocytes. With LPS stimulation, a model of neuroinflammatory insults, robust PAI-1 induction was observed in astrocytes but not in neurons and microglia. The upregulation of PAI-1 by LPS in astrocytes was also verified by RT-PCR analysis as well as PAI-1 promoter reporter assay. Lastly, we checked the effects of hypoxia on tPA/PAI-1 activity. Hypoxia increased tPA release from neurons without effects on microglia, while the activity of tPA in astrocyte was decreased consistent with increased PAI-1 activity in astrocyte. Taken together, the results from the present study suggest that neurons are the major source of tPA and that the glutamate-induced stimulated release is mainly governed by neurons in the acute phase. In contrast, the massive up-regulation of PAI-1 in astrocytes during subchronic and chronic inflammatory conditions, leads to decreased tPA activity in the later stages of MCAO. Differential regulation of tPA and PAI-1 in neurons, astrocytes and microglia suggest more attention is required to understand the role of local tPA activity in the vicinity of individual cell types.

Expression of plasminogen activator inhibitor-1 and protease nexin-1 in human astrocytes: Response to injury-related factors

Astrocytes play a diverse role in central nervous system (CNS) injury. Production of the serine protease inhibitors (serpins) plasminogen activator inhibitor-1 (PAI-1) and protease nexin-1 (PN-1) by astrocytes may counterbalance excessive serine protease activity associated with CNS pathologies such as ischemic stroke. Knowledge regarding the regulation of these genes in the brain is limited, so the objective of the present study was to characterize the effects of injury-related factors on serpin expression in human astrocytes. Native human astrocytes were exposed to hypoxia or cytokines, including interleukin-6 (IL-6), IL-1beta, tumor necrosis factor-alpha (TNF-alpha), IL-10, transforming growth factor-alpha (TGF-alpha), and TGF-beta for 0-20 hr. Serpin mRNA expression and protein secretion were determined by real-time RT-PCR and ELISA, respectively. Localization of PAI-1 and PN-1 in human brain tissue was examined by immunohistochemistry. Hypoxia and all assayed cytokines induced a significant increase in PAI-1 expression, whereas prolonged treatment with IL-1beta or TNF-alpha resulted in a significant down-regulation. The most pronounced induction of both PAI-1 and PN-1 was observed following early treatment with TGF-alpha. In contrast to PAI-1, the PN-1 gene did not respond to hypoxia. Positive immunoreactivity for PAI-1 in human brain tissue was demonstrated in reactive astrocytes within gliotic areas of temporal cortex. We show here that human astrocytes express PAI-1 and PN-1 and demonstrate that this astrocytic expression is regulated in a dynamic manner by injury-related factors.

The regulation of the CNS innate immune response is vital for the restoration of tissue homeostasis (repair) after acute brain injury: a brief review

Neurons and glia respond to acute injury by participating in the CNS innate immune response. This involves the recognition and clearance of "not self " pathogens and "altered self " apoptotic cells. Phagocytic receptors (CD14, CD36, TLR-4) clear "not self" pathogens; neurons and glia express "death signals" to initiate apoptosis in T cells.The complement opsonins C1q, C3, and iC3b facilitate the clearance of apoptotic cells by interacting with CR3 and CR4 receptors. Apoptotic cells are also cleared by the scavenger receptors CD14, Prs-R, TREM expressed by glia. Serpins also expressed by glia counter the neurotoxic effects of thrombin and other systemic proteins that gain entry to the CNS following injury. Complement pathway and T cell activation are both regulated by complement regulatory proteins expressed by glia and neurons. CD200 and CD47 are NIRegs expressed by neurons as "don't eat me" signals and they inhibit microglial activity preventing host cell attack. Neural stem cells regulate T cell activation, increase the Treg population, and suppress proinflammatory cytokine expression. Stem cells also interact with the chemoattractants C3a, C5a, SDF-1, and thrombin to promote stem cell migration into damaged tissue to support tissue homeostasis.

ErbB receptor signaling in astrocytes: a mediator of neuron-glia communication in the mature central nervous system

Astrocytes are now recognized as active players in the developing and mature central nervous system. Each astrocyte contacts vascular structures and thousands of synapses within discrete territories. These cells receive a myriad of inputs and generate appropriate responses to regulate the function of brain microdomains. Emerging evidence has implicated receptors of the ErbB tyrosine kinase family in the integration and processing of neuronal inputs by astrocytes: ErbB receptors can be activated by a wide range of neuronal stimuli; they control critical steps of glutamate-glutamine metabolism; and they regulate the biosynthesis and release of various glial-derived neurotrophic factors, gliomediators and gliotransmitters. These key properties of astrocytic ErbB signaling in neuron-glia interactions have significance for the physiology of the mature central nervous system, as exemplified by the central control of reproduction within the hypothalamus, and are also likely to contribute to pathological situations, since both dysregulation of ErbB signaling and glial dysfunction occur in many neurological disorders.

Increasing tPA activity in astrocytes induced by multipotent mesenchymal stromal cells facilitate neurite outgrowth after stroke in the mouse

We demonstrate that tissue plasminogen activator (tPA) and its inhibitors contribute to neurite outgrowth in the central nervous system (CNS) after treatment of stroke with multipotent mesenchymal stromal cells (MSCs). In vivo, administration of MSCs to mice subjected to middle cerebral artery occlusion (MCAo) significantly increased activation of tPA and downregulated PAI-1 levels in the ischemic boundary zone (IBZ) compared with control PBS treated mice, concurrently with increases of myelinated axons and synaptophysin. In vitro, MSCs significantly increased tPA levels and concomitantly reduced plasminogen activator inhibitor 1 (PAI-1) expression in astrocytes under normal and oxygen and glucose deprivation (OGD) conditions. ELISA analysis of conditioned medium revealed that MSCs stimulated astrocytes to secrete tPA. When primary cortical neurons were cultured in the conditioned medium from MSC co-cultured astrocytes, these neurons exhibited a significant increase in neurite outgrowth compared to conditioned medium from astrocytes alone. Blockage of tPA with a neutralizing antibody or knock-down of tPA with siRNA significantly attenuated the effect of the conditioned medium on neurite outgrowth. Addition of recombinant human tPA into cortical neuronal cultures also substantially enhanced neurite outgrowth. Collectively, these in vivo and in vitro data suggest that the MSC mediated increased activation of tPA in astrocytes promotes neurite outgrowth after stroke.

The intranigral injection of tissue plasminogen activator induced blood-brain barrier disruption, inflammatory process and degeneration of the dopaminergic system of the rat

Tissue-type plasminogen activator (tPA) is the only drug approved for the treatment of thromboembolic stroke, but it might lead to some neurotoxic side effects. tPA is a highly specific serine proteinase, one of the two principal plasminogen activators and one of the three trypsin-like serine proteinases of the tissue kallikrein family. We have observed that tPA injection in the SN leads to the degeneration of the dopaminergic neurons in a dose-dependent manner, without affecting the GABAergic neurons. We also found that tPA injected in the substantia nigra of rats produced the disruption of the blood-brain barrier (BBB) integrity, the induction of microglial activation, the loss of astroglia and the expression of aquaporin 4 (AQP4), as well as an increase in the expression of NMDA receptors and the brain derived neurothrophic factor (BDNF). All these effects, along with the changes produced in the phosphorylated forms of several MAP kinases and the transcription factor CREB, and the increase in the expression of nNOS and iNOS observed under our experimental conditions, could be involved in the loss of dopaminergic neurons.

Innate immunity and protective neuroinflammation: new emphasis on the role of neuroimmune regulatory proteins

Brain inflammation due to infection, hemorrhage, and aging is associated with activation of the local innate immune system as expressed by infiltrating cells, resident glial cells, and neurons. The innate immune response relies on the detection of "nonself" and "danger-self" ligands behaving as "eat me signals" by a plethora of pattern recognition receptors (PRRs) expressed by professional and amateur phagocytes to promote the clearance of pathogens, toxic cell debris (amyloid fibrils, aggregated synucleins, prions), and apoptotic cells accumulating within the brain parenchyma and the cerebrospinal fluid (CSF). These PRRs (e.g., complement, TLR, CD14, scavenger receptors) are highly conserved between vertebrates and invertebrates and may represent the most ancestral innate scavenging system involved in tissue homeostasis. However, in some diseases, these protective mechanisms lead to neurodegeneration on the ground that several innate immune molecules have neurocytotoxic activities. The response is a "double-edged sword" representing a fine balance between protective and detrimental effects. Several key regulatory mechanisms have now been evidenced in the control of CNS innate immunity, and these could be harnessed to explore novel therapeutic avenues. We will herein provide new emphasis on the role of neuroimmune regulatory proteins (NIRegs), such as CD95L, TNF, CD200, CD47, sialic acids, CD55, CD46, fH, C3a, HMGB1, which are involved in silencing innate immunity at the cellular and molecular levels and suppression of inflammation. For instance, NIRegs may play an important role in controlling lymphocyte/macrophage/microglia hyperinflammatory responses, while sparing host defense and repair mechanisms. Moreover, NIRegs have direct beneficial effects on neurogenesis and contributing to brain tissue remodeling.

TGFβ1 regulates the inflammatory response during chronic neurodegeneration

The ME7 model of murine prion disease shows an atypical inflammatory response characterized by morphologically activated microglia and an anti-inflammatory cytokine profile with a marked expression of TGFbeta1. The investigation of the role of TGFbeta1 during a time course disease shows that its expression is correlated with (i) the onset of behavioral abnormalities, (ii) increased activated microglia, (iii) thickening of the basement membrane, and (iv) is associated with increased PrP(sc) deposition. Increasing TGFbeta1 using an adenoviral vector has no significant impact on prion-associated behavioral impairments or on neuropathology. In contrast, inhibition of TGFbeta1 activity using an adenovirus expressing decorin induces severe cerebral inflammation, expression of inducible nitric oxide synthase and acute neuronal death in prion-diseased animals only. These data suggest that TGFbeta1 plays a critical role in the downregulation of microglial responses minimizing brain inflammation and thus avoiding exacerbation of brain damage.

Interleukin-1β enhances the angiotensin-induced expression of plasminogen activator inhibitor-1 through angiotensin receptor upregulation in human astrocytes

Plasminogen activator inhibitor-1 (PAI-1) regulates not only fibrinolysis but extracellular matrix remodeling, and angiotensin II is known to play an important role in controlling the expression of PAI-1 in astrocytes. We have studied the effect of interleukin-1beta (IL-1beta), one of major cytokines also active in the nervous system, on the angiotensin II-induced expression of PAI-1 in human astrocytes. Cultures of normal human astrocytes were stimulated with IL-1beta and angiotensin II, and the expression of mRNAs for angiotensin II type 1 receptor (AT1) and PAI-1 was analyzed by reverse transcription-polymerase chain reaction (RT-PCR) or real-time quantitative PCR. PAI-1 protein in astrocyte-conditioned medium was measured by enzyme-linked immunosorbent assay (ELISA). IL-1beta enhanced the expression of AT1 in astrocytes in time- and concentration-dependent manners. After 24-h stimulation, 10 ng/ml IL-1beta and 10 nM angiotensin II increased the levels of PAI-1 protein in astrocyte-conditioned medium by 1.9-fold and 1.8-fold of the basal value, respectively. There was no synergistic effect when the cells were stimulated simultaneously with IL-1beta and angiotensin II. When the cells were stimulated, with angiotensin II, 16 h after the stimulation with IL-1beta, the production of PAI-1 was enhanced by 1.4-fold as compared to the cells stimulated only with IL-1beta. CV-11794, an AT1 antagonist, inhibited the enhanced PAI-1 production in response to angiotensin II. We conclude that IL-1beta increases angiotensin II-induced PAI-1 secretion by astrocytes through the induction of AT1, and the enhanced secretion of PAI-1 may modulate functions of plasminogen activators in the nervous system.

The complexity of tissue-type plasminogen activator: can serine protease inhibitors help in stroke management?

Stroke, the third leading cause of death in industrialised countries, represents a major burden on healthcare authorities. The elucidation of molecular events sustaining infarct evolution in experimental models has allowed the development of putative therapeutic agents. However, despite marked benefits in animals, most of them have failed in clinical trials. At present, the only approved therapy for stroke is early reperfusion by intravenous injection of the thrombolytic agent, tissue-type plasminogen activator (tPA). tPA-dependent thrombolysis sometimes promotes haemorrhage, but improves neurological outcome in a great proportion of patients, provided it is performed within the recommended therapeutic window. In addition to the benefit of tPA injection in the vascular compartment, this endogenously produced serine protease could also promote excitotoxic processes within the cerebral parenchyma. This article reviews the various aspects of tPA during stroke, and discusses potential improvements to current clinical management, with a particular emphasis on targeting the deleterious actions of tPA through endogenous serine protease inhibitors (serpins).

Gene expression pathways induced by axotomy and decentralization of rat superior cervical ganglion neurons

To identify genes potentially involved in remodelling synaptic connections, we induced the temporary detachment of pre- and post-synaptic elements by axotomy or denervation of rat superior cervical ganglion neurons. cDNA microarray analysis followed by stringent selection criteria allowed the identification of a panel of genes whose expression was modulated by axotomy at various time points after injury. Among these genes, 11 were validated by real-time reverse transcriptase-polymerase chain reaction on independently prepared samples after superior cervical ganglion neuron axotomy (1, 3 and 6 days) and compared with the effect of decentralization (8 h, 1 and 3 days). These genes code for extracellular matrix/space [apolipoprotein D (apoD), decorin, collagen alpha1 type I, collagen alpha1 type III] and intermediate filament (vimentin) proteins, for modulators of neurite outgrowth (thrombin receptor, plasminogen activator inhibitor-1, bone morphogenetic protein 4, annexin II and S-100-related protein, clone 42C) and for a nerve cell transcription factor (brain finger protein). Eight of these 11 genes showed significant and persistent modulations after both types of injury. Finally, protein levels of apoD were shown to increase in superior cervical ganglion after axotomy. Our results identify hitherto unrecorded genes responsive to axotomy and decentralization of superior cervical ganglion neurons, and probably involved in synapse formation, remodelling and elimination.

The brain-specific tissue-type plasminogen activator inhibitor, neuroserpin, protects neurons against excitotoxicity both in vitro and in vivo

Considering its brain-specific expression, neuroserpin (NS), a potent inhibitor of tissue-type plasminogen activator (tPA), might be a good therapeutic target to limit the pro-excitotoxic effects of tPA within the cerebral parenchyma, without affecting the benefit from thrombolysis in stroke patients. Here, we aimed at determining the mechanisms of action responsible for the previously reported neuroprotective activity of NS in rodent experimental cerebral ischemia. First, we show in vivo that exogenous NS protects the cortex and the striatum against NMDA-induced injury. Then, the cellular mechanisms of this neuroprotection were investigated in primary cultures of cortical neurons. We show that NS fails to prevent serum deprivation-induced apoptotic neuronal death, while it selectively prevents NMDA- but not AMPA-induced excitotoxicity. This beneficial effect is associated to a decrease in NMDA receptor-mediated intracellular calcium influx. Altogether, these data suggest that an overexpression of neuroserpin in the brain parenchyma might limit the deleterious effect of tPA on NMDA receptor-mediated neuronal death, which occurs following experimental ischemia.

Fibrin-modifying serine proteases thrombin, tPA, and plasmin in ischemic stroke: a review

Ischemic stroke is a sudden loss of circulation to a portion of the brain that results in a loss of neurologic function. Many ischemic strokes are embolic. They result from a thrombus traveling into the central circulation and occluding a blood vessel. Treatment of ischemic stroke with recombinant tissue plasminogen activator (tPA) can improve patient outcomes. However, tPA must be used during a specific time window after the stroke onset to be effective and it risks converting an ischemic stroke into a hemorrhagic one. We explore the basic effects of fibrin-modifying proteases on neurons, astrocytes, and microglia during ischemia. tPA, thrombin, and plasmin can initiate microglial activation and change both neuronal and astrocytic survival. As a result of these functions and of their role in blood homeostasis, all three of these proteases have profound effects on neurons and glial cells in the brain and are capable of altering the development and severity of ischemic stroke.

Astrocyte-derived transforming growth factor-{beta} mediates the neuroprotective effects of 17{beta}-estradiol: involvement of nonclassical genomic signaling pathways

17beta-Estradiol (E2) and selective estrogen receptor modulators (SERMs), such as tamoxifen, mediate numerous effects in the brain, including neurosecretion, neuroprotection, and the induction of synaptic plasticity. Astrocytes, the most abundant cell type in the brain, influence many of these same functions and thus may represent a mediator of estrogen action. The present study examined the regulatory effect and underlying cell signaling mechanisms of E2-induced release of neurotropic growth factors from primary rat cortical astrocyte cultures. The results revealed that E2 (0.5, 1, and 10 nm) and tamoxifen (1 mum) increased both the expression and release of the neuroprotective cytokines, TGF-beta1 and TGF-beta2 (TGF-beta), from cortical astrocytes. The stimulatory effect of E2 was attenuated by the estrogen receptor (ER) antagonist, ICI182,780, suggesting ER dependency. The effect of E2 also appeared to involve mediation by the phosphotidylinositol 3-kinase (PI3K)/Akt signaling pathway, because E2 rapidly induced Akt phosphorylation, and pharmacological or molecular inhibition of the PI3K/Akt pathway prevented E2-induced release of TGF-beta. Additionally, the membrane-impermeant conjugate, E2-BSA, stimulated the release of TGF-beta, suggesting the potential involvement of a membrane-bound ER. Finally, E2, tamoxifen, and E2-BSA were shown to protect neuronal-astrocyte cocultures from camptothecin-induced neuronal cell death, effects that were attenuated by ICI182,780, Akt inhibition, or TGF-beta immunoneutralization. As a whole, these studies suggest that E2 induction of TGF-beta release from cortical astrocytes could provide a mechanism of neuroprotection, and that E2 stimulation of TGF-beta expression and release from astrocytes occurs via an ER-dependent mechanism involving mediation by the PI3K/Akt signaling pathway.

Tissue plasminogen activator and glial function

Tissue plasminogen activator (tPA) is the only FDA-approved treatment of thrombotic stroke and is a major parenchymal serine protease in the brain. However, it has been implicated in a plethora of brain pathologies, raising concern about its use as a safe therapeutic. tPA is thought to regulate physiological processes that entail tissue remodeling and plasticity, purportedly due to its ability to initiate the degradation of extracellular matrix proteins and possibly other substrates. Understanding the physiological role(s) of tPA promises to both elucidate important aspects of brain function and improve the available therapies for neurological disease. In this context, the effects of tPA on glial cells, mainly microglial cells, but also astrocytes and Schwann cells, appear to be of particular importance, given the increasing awareness of the significance of glia in brain physiology and pathology

The amyloid-beta peptide suppresses transforming growth factor-beta1-induced matrix metalloproteinase-2 production via Smad7 expression in human monocytic THP-1 cells

Accumulation of the amyloid-beta (Abeta) peptide in the brain is a crucial factor in the development of Alzheimer disease. Expression of transforming growth factor-beta1 (TGF-beta1), an immunosuppressive cytokine, has been associated in vivo with Abeta accumulation in transgenic mice and recently with Abeta clearance by activated microglia, suggesting its deleterious and beneficial effects in neuronal cells. In this study, we demonstrated that TGF-beta1 stimulated the production of matrix metalloproteinase-2 (MMP-2) in a time- and dose-dependent manner in a human monocytic THP-1 cell line. Notably, we found that Abeta1-42 consistently inhibited the TGF-beta1-induced production of MMP-2, the endogenous gene containing Smad response elements, whereas the reverse peptide, Abeta42-1, evidenced little effect. Additionally, Abeta1-42 reduced TGF-beta1-induced increase in plasminogen activator inhibitor-1 (PAI-1). This inhibitory effect of Abeta1-42 was also seen in human astroglial T98G cell line. Furthermore, Abeta1-42 significantly induced the expression of Smad7, which appears in turn to mediate the Abeta suppression of the TGF-beta1-induced MMP-2 production. Indeed, Smad7 overexpression mimicked the inhibitory effect of Abeta1-42 on TGF-beta1-induced MMP-2 production. Importantly, Abeta1-42 markedly suppressed the transactivation of the transfected reporter construct, p3TP-Lux, which contains TGF-beta1-inducible Smad response elements. This was concomitant with a decreased MMP-2 production in TGF-beta1-treated cells. Inhibition of cellular Smad7 levels via the small interference RNA method significantly ameliorated the Abeta1-42-mediated suppression of TGF-beta1-inducible transcription reporter activity, thereby restoring MMP-2 induction, whereas Smad7 transfection down-regulated TGF-beta1-inducible transcription reporter activity. Collectively, these data suggest that Abeta1-42 may play an important role in the negative regulation of TGF-beta1-induced MMP-2 production via Smad7 expression.

Extracellular proteases: biological and behavioral roles in the mammalian central nervous system

Extracellular proteases and their inhibitors have been implicated in both physiological and pathological states in the central nervous system (CNS). Given the presence of several classes of proteases, it is believed that each enzyme may undertake distinct biological roles. Some are indispensible for neuronal migration, neurite outgrowth and pathfinding, and synaptic plasticity. Others are required for neuronal death and tumor growth and invasion. Furthermore, studies from transgenic animals lacking or overexpressing one or more of the proteases have suggested that functional compensations and redundance among different members do exist. Normally, protease activity is tightly regulated by specific inhibitors to prevent disastrous proteolysis. Various insults can disrupt the fine control of proteolysis and caise pathological changes. Novel strategies have been attempted to maintain or restore protease-inhibitors homeostasis, thus minimizing damages to the CNS. They may provide us with effective therapeutic tools for fighting certain neurological disorders.

Inhibition of apoptosis and caspase-3 in vascular smooth muscle cells by plasminogen activator inhibitor type-1

Increased expression of plasminogen activator inhibitor type 1 (PAI-1) is associated with decreased apoptosis of neoplastic cells. We sought to determine whether PAI-1 alters apoptosis in vascular smooth muscle cells (VSMC) and, if so, by what mechanisms. A twofold increase in the expression of PAI-1 was induced in VSMC from transgenic mice with the use of the SM-22alpha gene promoter (SM22-PAI+). Cultured VSMC from SM22-PAI+ mice were more resistant to apoptosis induced by tumor necrosis factor plus phorbol myristate acetate or palmitic acid compared with VSMC from negative control littermates. Both wild type (WT) and a stable active mutant form of PAI-1 (Active) inhibited caspase-3 amidolytic activity in cell lysates while a serpin-defective mutant (Mut) PAI-1 did not. Similarly, both WT and Active PAI-1 decreased amidolytic activity of purified caspase-3, whereas Mut PAI-1 did not. WT but not Mut PAI-1 decreased the cleavage of poly-[ADP-ribose]-polymerase (PARP), the physiological substrate of caspase-3. Noncovalent physical interaction between caspase-3 and PAI-1 was demonstrable with the use of both qualitative and quantitative in vitro binding assays. High affinity binding was eliminated by mutations that block PAI-1 serpin activity. Accordingly, attenuated apoptosis resulting from elevated expression of PAI-1 by VSMC may be attributable, at least in part, to reversible inhibition of caspase-3 by active PAI-1.

2,7-Bis-(4-amidinobenzylidene)-cycloheptan-1-one dihydrochloride, tPA stop, prevents tPA-enhanced excitotoxicity both in vitro and in vivo

Tissue-type plasminogen activator (tPA) is available for the treatment of thromboembolic stroke in humans. However, adverse effects of tPA have been observed in animal models of ischemic brain injuries. In the present study, we have used a synthetic tPA inhibitor, named 2,7-bis-(4-amidino-benzylidene)-cycloheptan-1-one dihydrochloride (tPA stop), to investigate the role of endogenous tPA in the cerebral parenchyma. In mouse cortical cell cultures, we observed that although tPA stop reduced N-methyl-D-aspartic acid (NMDA)-mediated excitotoxic neuronal death, it failed to modulate alpha-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazole propanoic acid or kainate-mediated necrosis. In addition, we found that tPA stop could prevent the deleterious effects of both endogenous and exogenous tPA during NMDA exposure. At the functional level, tPA stop was found to prevent tPA-dependent potentiation of NMDA receptor-evoked calcium influx. The relevance of those findings was strengthened by the observation of a massive reduction of NMDA-induced excitotoxic lesion in rats when tPA stop was co-injected. Altogether, these data demonstrate that the blockade of the endogenous proteolytic activity of tPA in the cerebral parenchyma could be a powerful neuroprotective strategy raised against brain pathologies associated with excitotoxicity.

Role of extracellular signal regulated kinases 1 and 2 in neuronal survival

Extracellular signal regulated kinases 1 and 2 (ERK1/2) regulate cellular responses to a variety of extracellular stimuli. In the nervous system, ERK1/2 is critical for neuronal differentiation, plasticity and may also modulate neuronal survival. In this minireview, we present evidence that supports prosurvival activity of ERK1/2 in neurons. Several reports suggest that ERK1/2 mediates neuroprotective activity of extracellular factors, including neurotrophins. In addition, ERK1/2 is activated by neuronal injury. In damaged cells, ERK1/2 activation may act as a defensive mechanism that helps to compensate for the deleterious effects of a damaging insult. The emerging mechanisms of ERK1/2-mediated neuroprotection may involve transcriptional regulation and/or direct inhibition of cell death machinery.

Equivocal roles of tissue-type plasminogen activator in stroke-induced injury

Stroke represents a major health problem in the ever-ageing population of industrialized nations. Each year, over three million people in the USA alone suffer from this affliction. Stroke, which results from the obstruction of an intra- or extra-cerebral artery, induces irreversible neuronal damage. The clot-busting drug tissue-type plasminogen activator (tPA) is the only FDA-approved therapy for acute stroke. Although tPA has been successfully used to treat myocardial infarction due to clot formation, its use in the treatment of occlusive cerebrovascular diseases remains controversial. Indeed, tPA is clearly beneficial as a thrombolytic agent. However, increasing evidence suggests that tPA could have direct and deleterious effects on neurons and glial cells.

Bax mediates the apoptosis-sensitizing effect of maspin

Maspin, a serine protease inhibitor (serpin), can suppress tumor growth and metastasis in vivo and tumor cell motility and invasion in vitro. This may occur through maspin-mediated inhibition of pericellular proteolysis. In a recent report, we provided evidence that maspin may also suppress tumor progression by enhancing cellular sensitivity to apoptotic stimuli. To our knowledge, maspin is the only proapoptotic serpin among all of the serpins implicated thus far in apoptosis regulation. The goal of the present study is to identify the specific target molecule(s), the modification of which by maspin renders tumor cells sensitive to chemotherapeutic agents. Our cellular, molecular, and biochemical studies demonstrate an essential role of Bax in the proapoptotic effect of maspin. First, Bax was up-regulated in maspin-transfected prostate and breast tumor cells, whereas the levels of other Bcl-2 family members including Bcl-2, Bcl-xl, and Bak remained unchanged. Second, on apoptosis induction, a greater amount of Bax was translocated from cytosol to mitochondria in maspin-transfected cells. After treatment with a Bax-silencing small interfering RNA, maspin-transfected cells became significantly more resistant to drug-induced apoptosis. Consistently, the release of cytochrome c and Smac/DIABLO from mitochondria was more responsive to apoptosis stimuli in maspin-transfected cells than in the mock-transfected cells. Third, the apoptosis induction of maspin-transfected cells was associated with increased activation of both caspase-8 and caspase-9. However, a caspase-9-specific inhibitor blocked the sensitization effect of maspin in a dose-dependent and time-dependent manner, demonstrating a rate-limiting role for caspase-9. In line with the central role of the Bax-mediated mitochondrial apoptotic pathway, maspin sensitized the apoptotic response of breast and prostate carcinoma cells to various drugs, ranging from death ligands to endoplasmic reticulum stress. The link between maspin and Bax up-regulation explains the loss of maspin-expressing tumor cells in invasive breast and prostate carcinomas. Our data reveal a novel mechanism for tumor suppressive maspin and suggest that maspin may be used as a modifier for apoptosis-based cancer therapy.

Astrocyte apoptosis: implications for neuroprotection

Astrocytes, the most abundant glial cell types in the brain, provide metabolic and trophic support to neurons and modulate synaptic activity. Accordingly, impairment in these astrocyte functions can critically influence neuronal survival. Recent studies show that astrocyte apoptosis may contribute to pathogenesis of many acute and chronic neurodegenerative disorders, such as cerebral ischemia, Alzheimer's disease and Parkinson's disease. We found that incubation of cultured rat astrocytes in a Ca(2+)-containing medium after exposure to a Ca(2+)-free medium causes an increase in intracellular Ca(2+) concentration followed by apoptosis, and that NF-kappa B, reactive oxygen species, and enzymes such as calpain, xanthine oxidase, calcineurin and caspase-3 are involved in reperfusion-induced apoptosis. Furthermore, we demonstrated that heat shock protein, mitogen-activated protein/extracellular signal-regulated kinase, phosphatidylinositol-3 kinase and cyclic GMP phosphodiesterase are target molecules for anti-apoptotic drugs. This review summarizes (1) astrocytic functions in neuroprotection, (2) current evidence of astrocyte apoptosis in both in vitro and in vivo studies including its molecular pathways such as Ca(2+) overload, oxidative stress, NF-kappa B activation, mitochondrial dysfunction, endoplasmic reticulum stress, and protease activation, and (3) several drugs preventing astrocyte apoptosis. As a whole, this article provides new insights into the potential role of astrocytes as targets for neuroprotection. In addition, the advance in the knowledge of molecular mechanisms of astrocyte apoptosis may lead to the development of novel therapeutic strategies for neurodegenerative disorders.

Excitotoxic and excitoprotective mechanisms: abundant targets for the prevention and treatment of neurodegenerative disorders

Activation of glutamate receptors can trigger the death of neurons and some types of glial cells, particularly when the cells are coincidentally subjected to adverse conditions such as reduced levels of oxygen or glucose, increased levels of oxidative stress, exposure to toxins or other pathogenic agents, or a disease-causing genetic mutation. Such excitotoxic cell death involves excessive calcium influx and release from internal organelles, oxyradical production, and engagement of programmed cell death (apoptosis) cascades. Apoptotic proteins such as p53, Bax, and Par-4 induce mitochondrial membrane permeability changes resulting in the release of cytochrome c and the activation of proteases, such as caspase-3. Events occurring at several subcellular sites, including the plasma membrane, endoplasmic reticulum, mitochondria and nucleus play important roles in excitotoxicity. Excitotoxic cascades are initiated in postsynaptic dendrites and may either cause local degeneration or plasticity of those synapses, or may propagate the signals to the cell body resulting in cell death. Cells possess an array of antiexcitotoxic mechanisms including neurotrophic signaling pathways, intrinsic stress-response pathways, and survival proteins such as protein chaperones, calcium-binding proteins, and inhibitor of apoptosis proteins. Considerable evidence supports roles for excitotoxicity in acute disorders such as epileptic seizures, stroke and traumatic brain and spinal cord injury, as well as in chronic age-related disorders such as Alzheimer's, Parkinson's, and Huntington's disease and amyotrophic lateral sclerosis. A better understanding of the excitotoxic process is not only leading to the development of novel therapeutic approaches for neurodegenerative disorders, but also to unexpected insight into mechanisms of synaptic plasticity.

Emotions are building up in the field of extracellular proteolysis

Activity-dependent remodeling of neural connections might require localized extracellular proteolysis. The tissue-type plasminogen activator (tPA)-plasmin proteolytic system is expressed in different regions of the central nervous system, in the context of a variety of physiological and pathological processes. Accumulating evidence regarding the expression and role of tPA and its inhibitors suggests that extracellular proteolysis is a key player in the biology of memory, emotions and neurodegeneration.