Key role of tissue plasminogen activator in neurovascular coupling

tPA supplementation preserves neurovascular and cognitive function in Tg2576 mice

INTRODUCTION

Amyloid beta (Aβ) impairs the cerebral blood flow (CBF) increase induced by neural activity (functional hyperemia). Tissue plasminogen activator (tPA) is required for functional hyperemia, and in mouse models of Aβ accumulation tPA deficiency contributes to neurovascular and cognitive impairment. However, it remains unknown if tPA supplementation can rescue Aβ-induced neurovascular and cognitive dysfunction.

METHODS

Tg2576 mice and wild-type littermates received intranasal tPA (0.8 mg/kg/day) or vehicle 5 days a week starting at 11 to 12 months of age and were assessed 3 months later.

RESULTS

Treatment of Tg2576 mice with tPA restored resting CBF, prevented the attenuation in functional hyperemia, and improved nesting behavior. These effects were associated with reduced cerebral atrophy and cerebral amyloid angiopathy, but not parenchymal amyloid.

DISCUSSION

These findings highlight the key role of tPA deficiency in the neurovascular and cognitive dysfunction associated with amyloid pathology, and suggest potential therapeutic strategies involving tPA reconstitution.

HIGHLIGHTS

Amyloid beta (Aβ) induces neurovascular dysfunction and impairs the increase of cerebral blood flow induced by neural activity (functional hyperemia). Tissue plasminogen activator (tPA) deficiency contributes to the neurovascular and cognitive dysfunction caused by Aβ. In mice with florid amyloid pathology intranasal administration of tPA rescues the neurovascular and cognitive dysfunction and reduces brain atrophy and cerebral amyloid angiopathy. tPA deficiency plays a crucial role in neurovascular and cognitive dysfunction induced by Aβ and tPA reconstitution may be of therapeutic value.

Cerebrovascular Dysfunction in Alzheimer's Disease and Transgenic Rodent Models

Alzheimer's Disease (AD) and Alzheimer's Disease-Related Dementia (ADRD) are the primary causes of dementia that has a devastating effect on the quality of life and is a tremendous economic burden on the healthcare system. The accumulation of extracellular beta-amyloid (Aβ) plaques and intracellular hyperphosphorylated tau-containing neurofibrillary tangles (NFTs) in the brain are the hallmarks of AD. They are also thought to be the underlying cause of inflammation, neurodegeneration, brain atrophy, and cognitive impairments that accompany AD. The discovery of APP, PS1, and PS2 mutations that increase Aβ production in families with early onset familial AD led to the development of numerous transgenic rodent models of AD. These models have provided new insight into the role of Aβ in AD; however, they do not fully replicate AD pathology in patients. Familial AD patients with mutations that elevate the production of Aβ represent only a small fraction of dementia patients. In contrast, those with late-onset sporadic AD constitute the majority of cases. This observation, along with the failure of previous clinical trials targeting Aβ or Tau and the modest success of recent trials using Aβ monoclonal antibodies, has led to a reappraisal of the view that Aβ accumulation is the sole factor in the pathogenesis of AD. More recent studies have established that cerebral vascular dysfunction is one of the earliest changes seen in AD, and 67% of the candidate genes linked to AD are expressed in the cerebral vasculature. Thus, there is an increasing appreciation of the vascular contribution to AD, and the National Institute on Aging (NIA) and the Alzheimer's Disease Foundation recently prioritized it as a focused research area. This review summarizes the strengths and limitations of the most commonly used transgenic AD animal models and current views about the contribution of Aβ accumulation versus cerebrovascular dysfunction in the pathogenesis of AD.

Two decades of astrocytes in neurovascular coupling

The brain is a highly energy demanding organ, which accounts in humans for the 20% of total energy consumption at resting state although comprising only 2% of the body mass. The necessary delivery of nutrients to brain parenchyma is ensured by the cerebral circulatory system, through the exchange of glucose and oxygen (O2) at the capillary level. Notably, a tight spatial and temporal correlation exists between local increases in neuronal activity and the subsequent changes in regional cerebral blood flow. The recognized concept of neurovascular coupling (NVC), also named functional hyperemia, expresses this close relationship and stands at the basis of the modern functional brain imaging techniques. Different cellular and molecular mechanisms have been proposed to mediate this tight coupling. In this context, astrocytes are ideally positioned to act as relay elements that sense neuronal activity through their perisynaptic processes and release vasodilator agents at their endfeet in contact with brain parenchymal vessels. Two decades after the astrocyte involvement in neurovascular coupling has been proposed, we here review the experimental evidence that contributed to unraveling the molecular and cellular mechanisms underlying cerebral blood flow regulation. While traveling through the different controversies that moved the research in this field, we keep a peculiar focus on those exploring the role of astrocytes in neurovascular coupling and conclude with two sections related to methodological aspects in neurovascular research and to some pathological conditions resulting in altered neurovascular coupling.

Blood tissue Plasminogen Activator (tPA) of liver origin contributes to neurovascular coupling involving brain endothelial N-Methyl-D-Aspartate (NMDA) receptors

BACKGROUND

Regulation of cerebral blood flow (CBF) directly influence brain functions and dysfunctions and involves complex mechanisms, including neurovascular coupling (NVC). It was suggested that the serine protease tissue-type plasminogen activator (tPA) could control CNV induced by whisker stimulation in rodents, through its action on N-methyl-D-Aspartate receptors (NMDARs). However, the origin of tPA and the location and mechanism of its action on NMDARs in relation to CNV remained debated.

METHODS

Here, we answered these issues using tPA^Null mice, conditional deletions of either endothelial tPA (VECad-Cre^ΔtPA) or endothelial GluN1 subunit of NMDARs (VECad-Cre^ΔGluN1), parabioses between wild-type and tPA^Null mice, hydrodynamic transfection-induced deletion of liver tPA, hepatectomy and pharmacological approaches.

RESULTS

We thus demonstrate that physiological concentrations of vascular tPA, achieved by the bradykinin type 2 receptors-dependent production and release of tPA from liver endothelial cells, promote NVC, through a mechanism dependent on brain endothelial NMDARs.

CONCLUSIONS

These data highlight a new mechanism of regulation of NVC involving both endothelial tPA and NMDARs.

Endothelial Cells and the Cerebral Circulation

Endothelial cells form the innermost layer of all blood vessels and are the only vascular component that remains throughout all vascular segments. The cerebral vasculature has several unique properties not found in the peripheral circulation; this requires that the cerebral endothelium be considered as a unique entity. Cerebral endothelial cells perform several functions vital for brain health. The cerebral vasculature is responsible for protecting the brain from external threats carried in the blood. The endothelial cells are central to this requirement as they form the basis of the blood-brain barrier. The endothelium also regulates fibrinolysis, thrombosis, platelet activation, vascular permeability, metabolism, catabolism, inflammation, and white cell trafficking. Endothelial cells regulate the changes in vascular structure caused by angiogenesis and artery remodeling. Further, the endothelium contributes to vascular tone, allowing proper perfusion of the brain which has high energy demands and no energy stores. In this article, we discuss the basic anatomy and physiology of the cerebral endothelium. Where appropriate, we discuss the detrimental effects of high blood pressure on the cerebral endothelium and the contribution of cerebrovascular disease endothelial dysfunction and dementia. © 2022 American Physiological Society. Compr Physiol 12:3449-3508, 2022.

Neurovascular coupling mechanisms in health and neurovascular uncoupling in Alzheimer's disease

To match the metabolic demands of the brain, mechanisms have evolved to couple neuronal activity to vasodilation, thus increasing local cerebral blood flow and delivery of oxygen and glucose to active neurons. Rather than relying on metabolic feedback signals such as the consumption of oxygen or glucose, the main signalling pathways rely on the release of vasoactive molecules by neurons and astrocytes, which act on contractile cells. Vascular smooth muscle cells and pericytes are the contractile cells associated with arterioles and capillaries, respectively, which relax and induce vasodilation.

Much progress has been made in understanding the complex signalling pathways of neurovascular coupling, but issues such as the contributions of capillary pericytes and astrocyte calcium signal remain contentious. Study of neurovascular coupling mechanisms is especially important as cerebral blood flow dysregulation is a prominent feature of Alzheimer’s disease. In this article we will discuss developments and controversies in the understanding of neurovascular coupling and finish by discussing current knowledge concerning neurovascular uncoupling in Alzheimer’s disease.

Neuroserpin, a crucial regulator for axogenesis, synaptic modelling and cell-cell interactions in the pathophysiology of neurological disease

Neuroserpin is an axonally secreted serpin that is involved in regulating plasminogen and its enzyme activators, such as tissue plasminogen activator (tPA). The protein has been increasingly shown to play key roles in neuronal development, plasticity, maturation and synaptic refinement. The proteinase inhibitor may function both independently and through tPA-dependent mechanisms. Herein, we discuss the recent evidence regarding the role of neuroserpin in healthy and diseased conditions and highlight the participation of the serpin in various cellular signalling pathways. Several polymorphisms and mutations have also been identified in the protein that may affect the serpin conformation, leading to polymer formation and its intracellular accumulation. The current understanding of the involvement of neuroserpin in Alzheimer's disease, cancer, glaucoma, stroke, neuropsychiatric disorders and familial encephalopathy with neuroserpin inclusion bodies (FENIB) is presented. To truly understand the detrimental consequences of neuroserpin dysfunction and the effective therapeutic targeting of this molecule in pathological conditions, a cross-disciplinary understanding of neuroserpin alterations and its cellular signaling networks is essential.

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.

Retinal vasodilatory responses are inversely associated with plasminogen activator inhibitor-1: The African-PREDICT study

AIMS

Plasminogen activator inhibitor-1 (PAI-1), traditionally associated with fibrinolysis, is increasingly implicated in impaired vascular function. However, studies on its association with microvascular function are limited to the cutaneous and coronary microvascular beds in older and diseased individuals. To better understand its potential involvement in the early stages of disease development, we investigated the associations of retinal vasodilatory responses to flicker light with PAI-1 activity (PAI-1_act) in young and healthy individuals.

METHODS

We included healthy Black and White women and men (n = 518; aged 20-30 years), and measured plasma PAI-1_act and retinal vasodilatory responses to flicker light provocation. We also collected demographic and lifestyle data, measured blood pressure, anthropometry, blood lipids, inflammatory and other biomarkers.

RESULTS

In multivariate regression analyses, maximal retinal venular dilation associated independently and inversely with PAI-1_act (adj. R² = 0.11; β = -0.15; p = 0.001) in the total group. In exploratory subgroup analyses, this association remained in White women (adj. R² = 0.07; β = -0.23; p = 0.005), and was more robust with younger age and lower blood pressure and in non-smokers, but also with greater central adiposity, higher low-density lipoprotein cholesterol and inflammation (all p < 0.05).

CONCLUSIONS

Our data suggest that in young individuals, PAI-1 may already be associated with subclinical microvascular dysfunction.

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).

tPA Deficiency Underlies Neurovascular Coupling Dysfunction by Amyloid-β

The amyloid-β (Aβ) peptide, a key pathogenic factor in Alzheimer's disease, attenuates the increase in cerebral blood flow (CBF) evoked by neural activity (functional hyperemia), a vital homeostatic response in which NMDA receptors (NMDARs) play a role through nitric oxide, and the CBF increase produced by endothelial factors. Tissue plasminogen activator (tPA), which is reduced in Alzheimer's disease and in mouse models of Aβ accumulation, is required for the full expression of the NMDAR-dependent component of functional hyperemia. Therefore, we investigated whether tPA is involved in the neurovascular dysfunction of Aβ. tPA activity was reduced, and the tPA inhibitor plasminogen inhibitor-1 (PAI-1) was increased in male mice expressing the Swedish mutation of the amyloid precursor protein (tg2576). Counteracting the tPA reduction with exogenous tPA or with pharmacological inhibition or genetic deletion of PAI-1 completely reversed the attenuation of the CBF increase evoked by whisker stimulation but did not ameliorate the response to the endothelium-dependent vasodilator acetylcholine. The tPA deficit attenuated functional hyperemia by suppressing NMDAR-dependent nitric oxide production during neural activity. Pharmacological inhibition of PAI-1 increased tPA activity, prevented neurovascular uncoupling, and ameliorated cognition in 11- to 12-month-old tg2576 mice, effects associated with a reduction of cerebral amyloid angiopathy but not amyloid plaques. The data unveil a selective role of the tPA in the suppression of functional hyperemia induced by Aβ and in the mechanisms of cerebral amyloid angiopathy, and support the possibility that modulation of the PAI-1-tPA pathway may be beneficial in diseases associated with amyloid accumulation.SIGNIFICANCE STATEMENT Amyloid-β (Aβ) peptides have profound neurovascular effects that may contribute to cognitive impairment in Alzheimer's disease. We found that Aβ attenuates the increases in blood flow evoked by neural activation through a reduction in tissue plasminogen activator (tPA) caused by upregulation of its endogenous inhibitor plasminogen inhibitor-1 (PAI-1). tPA deficiency prevents NMDA receptors from triggering nitric oxide production, thereby attenuating the flow increase evoked by neural activity. PAI-1 inhibition restores tPA activity, rescues neurovascular coupling, reduces amyloid deposition around blood vessels, and improves cognition in a mouse model of Aβ accumulation. The findings demonstrate a previously unappreciated role of tPA in Aβ-related neurovascular dysfunction and in vascular amyloid deposition. Restoration of tPA activity could be of therapeutic value in diseases associated with amyloid accumulation.

Tau induces PSD95-neuronal NOS uncoupling and neurovascular dysfunction independent of neurodegeneration

Cerebrovascular abnormalities have emerged as a preclinical manifestation of Alzheimer’s disease and frontotemporal dementia, diseases characterized by accumulation of hyperphosphorylated forms of the microtubule associated protein tau. However, it is unclear if tau contributes to these neurovascular alterations independent of neurodegeneration. We report that mice expressing mutated tau exhibit a selective suppression of neural activity-induced cerebral blood flow increases that precedes tau pathology and cognitive impairment. The dysfunction is attributable to reduced vasodilatation of intracerebral arterioles and is reversible by turning down tau production. Mechanistically, the failure of neurovascular coupling involves tau-induced dissociation of neuronal nitric oxide synthase from post synaptic-density-95 and reduced production of the potent vasodilator nitric oxide during glutamatergic synaptic activity. The data identify glutamatergic signaling dysfunction and nitric oxide deficiency as yet-undescribed early manifestations of tau pathobiology independent of neurodegeneration, and provide a mechanism for the neurovascular alterations observed in the preclinical stages of tauopathies.

The Plasminogen Activation System Promotes Neurorepair in the Ischemic Brain

The plasminogen activation (PA) system was originally thought to exclusively promote the degradation of fibrin by catalyzing the conversion of plasminogen into plasmin via two serine proteinases: tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). However, experimental evidence accumulated over the last 30 years indicates that tPA and uPA are also found in the central nervous system (CNS), where they have a plethora of functions that not always require plasmin generation or fibrin degradation. For example, plasminogen-dependent and - independent effects of tPA and uPA play a central role in the pathophysiological events that underlie one of the leading causes of mortality and disability in the world: cerebral ischemia. Indeed, recent work indicates that while the rapid release of tPA from the presynaptic compartment following the onset of cerebral ischemia protects the synapse from the deleterious effects of the ischemic injury, the secretion of uPA and its binding to its receptor (uPAR) during the recovery phase promotes the repair of synapses that have been lost to the acute ischemic insult. This restorative role of uPA has high translational significance because to this date there is no effective approach to induce neurorepair in the ischemic brain. Here we will discuss recent evidence that bridges the gap between basic research in the field of the PA system and the bedside of ischemic stroke patients, indicating that uPA and uPAR are potential targets for the development of therapeutic strategies to promote neurological recovery among ischemic stroke survivors.

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.

Release of IL-6 After Stroke Contributes to Impaired Cerebral Autoregulation and Hippocampal Neuronal Necrosis Through NMDA Receptor Activation and Upregulation of ET-1 and JNK

The sole FDA-approved drug treatment for ischemic stroke is tissue-type plasminogen activator (tPA). However, upregulation of JNK mitogen-activated protein kinase (MAPK) and endothelin 1 (ET-1) by tPA after stroke contributes to impaired cerebrovascular autoregulation. Wild-type (wt) tPA can bind to the lipoprotein-related receptor (LRP), which mediates vasodilation, or NMDA receptors (NMDA-Rs), exacerbating vasoconstriction. Elevations in IL-6, a marker of inflammation that accompanies stroke, are reported to be an adverse prognostic factor. We hypothesized that IL-6 released into CSF after stroke by wt-tPA through activation of NMDA-Rs and upregulation of ET-1 and JNK contribute to impairment of cerebrovascular autoregulation and increased histopathology. Results show that IL-6 was increased post stroke in pigs, which was increased further by wt-tPA. Co-administration of the IL-6 antagonist LMT-28 with wt-tPA prevented impairment of cerebrovascular autoregulation and necrosis of hippocampal cells. wt-tPA co-administered with the JNK inhibitor SP 600125 and the ET-1 antagonist BQ 123 blocked stroke-induced elevation of IL-6. Co-administration of LMT-28 with wt-tPA blocked the augmentation of JNK and ET-1 post stroke. In conclusion, IL-6 released after stroke, which is enhanced by wt-tPA through activation of NMDA-Rs and upregulation of ET-1 and JNK, impairs cerebrovascular autoregulation and increases histopathology. Strategies that promote fibrinolysis while limiting activation of NMDA-Rs and upregulation of IL-6 may improve the benefit/risk ratio compared to wt-tPA in treatment of stroke.

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.

Release of IL-6 After Stroke Contributes to Impaired Cerebral Autoregulation and Hippocampal Neuronal Necrosis Through NMDA Receptor Activation and Upregulation of ET-1 and JNK

The sole FDA-approved drug treatment for ischemic stroke is tissue-type plasminogen activator (tPA). However, upregulation of JNK mitogen-activated protein kinase (MAPK) and endothelin 1 (ET-1) by tPA after stroke contributes to impaired cerebrovascular autoregulation. Wild-type (wt) tPA can bind to the lipoprotein-related receptor (LRP), which mediates vasodilation, or NMDA receptors (NMDA-Rs), exacerbating vasoconstriction. Elevations in IL-6, a marker of inflammation that accompanies stroke, are reported to be an adverse prognostic factor. We hypothesized that IL-6 released into CSF after stroke by wt-tPA through activation of NMDA-Rs and upregulation of ET-1 and JNK contribute to impairment of cerebrovascular autoregulation and increased histopathology. Results show that IL-6 was increased post stroke in pigs, which was increased further by wt-tPA. Co-administration of the IL-6 antagonist LMT-28 with wt-tPA prevented impairment of cerebrovascular autoregulation and necrosis of hippocampal cells. wt-tPA co-administered with the JNK inhibitor SP 600125 and the ET-1 antagonist BQ 123 blocked stroke-induced elevation of IL-6. Co-administration of LMT-28 with wt-tPA blocked the augmentation of JNK and ET-1 post stroke. In conclusion, IL-6 released after stroke, which is enhanced by wt-tPA through activation of NMDA-Rs and upregulation of ET-1 and JNK, impairs cerebrovascular autoregulation and increases histopathology. Strategies that promote fibrinolysis while limiting activation of NMDA-Rs and upregulation of IL-6 may improve the benefit/risk ratio compared to wt-tPA in treatment of stroke.

Worse Outcome in Stroke Patients Treated with rt-PA Without Early Reperfusion: Associated Factors

Based on preclinical studies suggesting that recombinant tissue plasminogen activator (rt-PA) may promote ischemic brain injuries, we investigated in patients the possible risk of worse clinical outcome after rt-PA treatment as a result of its inability to resolve cerebral ischemia. Here, we designed a cohort study using a retrospective analysis of patients who received treatment with intravenous (4.5-h window) or intraarterial rt-PA, without or with thrombectomy. Controls were consecutive patients who did not receive recanalization treatment, who met all inclusion criteria. As a marker of reperfusion, we defined the variable of early neurological improvement as the difference between the score of the National Institute of Health Stroke Scale (NIHSS) (at admission and 24 h). The main variable was worsening of the patient’s functional situation in the first 3 months. To compare quantitative variables, we used Student’s t test or the Mann-Whitney test. To estimate the odds ratios of each independent variable in the patient’s worsening in the first 3 months, we used a logistic regression model. We included 1154 patients; 577 received rt-PA, and 577 served as controls. In the group of patients treated with rt-PA, 39.4% who did not present clinical reperfusion data developed worsening within 3 months after stroke compared with 3.5% of patients with reperfusion (P < 0.0001). These differences were not significant in the control group. In summary, administration of rt-PA intravenously or intraarterially without reperfusion within the first 24 h may be associated with a higher risk of functional deterioration in the first 3 months.

tPA variant tPA-A296-299 Prevents impairment of cerebral autoregulation and necrosis of hippocampal neurons after stroke by inhibiting upregulation of ET-1

Tissue-type plasminogen activator (tPA) is neurotoxic and exacerbates uncoupling of cerebral blood flow (CBF) and metabolism after stroke, yet it remains the sole FDA-approved drug for treatment of ischemic stroke. Upregulation of c-Jun-terminal kinase (JNK) after stroke contributes to tPA-mediated impairment of autoregulation, but the role of endothelin-1 (ET-1) is unknown. Based on the Glasgow Coma Scale, impaired autoregulation is linked to adverse outcomes after TBI, but correlation with hippocampal histopathology after stroke has not been established. We propose that given after stroke, tPA activates N-Methyl-D-Aspartate receptors (NMDA-Rs) and upregulates ET-1 in a JNK dependent manner, imparing autoregulation and leading to histopathology. After stroke, CBF was reduced in the hippocampus and reduced further during hypotension, which did not occur in hypotensive sham pigs, indicating impairment of autoregulation. Autoregulation and necrosis of hippocampal CA1 and CA3 neurons were further impaired by tPA, but were preserved by the ET-1 antagonist BQ 123 and tPA-A,^296-299 a variant that is fibrinolytic but does not bind to NMDA-Rs. Expression of ET-1 was increased by stroke and potentiated by tPA but returned to sham levels by tPA-A^296-299 and the JNK antagonist SP600125. Results show that JNK releases ET-1 after stroke. Tissue-type plasminogen activator -A^296-299 prevents impairment of cerebral autoregulation and histopathology after stroke by inhibiting upregulation of ET-1.

Multi-modal assessment of neurovascular coupling during cerebral ischaemia and reperfusion using remote middle cerebral artery occlusion

Hyperacute changes in cerebral blood flow during cerebral ischaemia and reperfusion are important determinants of injury. Cerebral blood flow is regulated by neurovascular coupling, and disruption of neurovascular coupling contributes to brain plasticity and repair problems. However, it is unknown how neurovascular coupling is affected hyperacutely during cerebral ischaemia and reperfusion. We have developed a remote middle cerebral artery occlusion model in the rat, which enables multi-modal assessment of neurovascular coupling immediately prior to, during and immediately following reperfusion. Male Wistar rats were subjected to remote middle cerebral artery occlusion, where a long filament was advanced intraluminally through a guide cannula in the common carotid artery. Transcallosal stimulation evoked increases in blood flow, tissue oxygenation and neuronal activity, which were diminished by middle cerebral artery occlusion and partially restored during reperfusion. These evoked responses were not affected by administration of the thrombolytic alteplase at clinically used doses. Evoked cerebral blood flow responses were fully restored at 24 h post-middle cerebral artery occlusion indicating that neurovascular dysfunction was not sustained. These data show for the first time that the rat remote middle cerebral artery occlusion model coupled with transcallosal stimulation provides a novel method for continuous assessment of hyperacute neurovascular coupling changes during ischaemia and reperfusion, and offers unique insight into hyperacute ischaemic pathophysiology.

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.

IGF-1 deficiency impairs neurovascular coupling in mice: implications for cerebromicrovascular aging

Aging is associated with marked deficiency in circulating IGF‐1, which has been shown to contribute to age‐related cognitive decline. Impairment of moment‐to‐moment adjustment of cerebral blood flow (CBF) via neurovascular coupling is thought to play a critical role in the genesis of age‐related cognitive impairment. To establish the link between IGF‐1 deficiency and cerebromicrovascular impairment, neurovascular coupling mechanisms were studied in a novel mouse model of IGF‐1 deficiency (Igf1^f/f‐TBG‐Cre‐AAV8) and accelerated vascular aging. We found that IGF‐1‐deficient mice exhibit neurovascular uncoupling and show a deficit in hippocampal‐dependent spatial memory test, mimicking the aging phenotype. IGF‐1 deficiency significantly impaired cerebromicrovascular endothelial function decreasing NO mediation of neurovascular coupling. IGF‐1 deficiency also impaired glutamate‐mediated CBF responses, likely due to dysregulation of astrocytic expression of metabotropic glutamate receptors and impairing mediation of CBF responses by eicosanoid gliotransmitters. Collectively, we demonstrate that IGF‐1 deficiency promotes cerebromicrovascular dysfunction and neurovascular uncoupling mimicking the aging phenotype, which are likely to contribute to cognitive impairment.

tPA Deficiency in Mice Leads to Rearrangement in the Cerebrovascular Tree and Cerebroventricular Malformations

The serine protease tissue-type plasminogen activator (tPA) is used as a thrombolytic agent in the management of ischemic stroke, but concerns for hemorrhagic conversion greatly limits the number of patients that receive this treatment. It has been suggested that the bleeding complications associated with thrombolytic tPA may be due to unanticipated roles of tPA in the brain. Recent work has suggested tPA regulation of neurovascular barrier integrity, mediated via platelet derived growth factor (PDGF)-C/PDGF receptor-α (PDGFRα) signaling, as a possible molecular mechanism affecting the outcome of stroke. To better understand the role of tPA in neurovascular regulation we conducted a detailed analysis of the cerebrovasculature in brains from adult tPA deficient (tPA^−/−) mice. Our analysis demonstrates that life-long deficiency of tPA is associated with rearrangements in the cerebrovascular tree, including a reduction in the number of vascular smooth-muscle cell covered, large diameter, vessels and a decrease in vessel-associated PDGFRα expression as compared to wild-type (WT) littermate controls. In addition, we found that ablation of tPA results in an increased number of ERG-positive endothelial cells and increased junctional localization of the tight junction protein ZO1. This is intriguing since ERG is an endothelial transcription factor implicated in regulation of vascular integrity. Based on these results, we propose that the protection of barrier properties seen utilizing these tPA^−/− mice might be due, at least in part, to these cerebrovascular rearrangements. In addition, we found that tPA^−/− mice displayed mild cerebral ventricular malformations, a feature previously associated with ablation of PDGF-C, thereby providing an in vivo link between tPA and PDGF signaling in central nervous system (CNS) development. Taken together, the data presented here will advance our understanding of the role of tPA within the CNS and in regulation of cerebrovascular permeability.

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.

Spreading Depression, Spreading Depolarizations, and the Cerebral Vasculature

Spreading depression (SD) is a transient wave of near-complete neuronal and glial depolarization associated with massive transmembrane ionic and water shifts. It is evolutionarily conserved in the central nervous systems of a wide variety of species from locust to human. The depolarization spreads slowly at a rate of only millimeters per minute by way of grey matter contiguity, irrespective of functional or vascular divisions, and lasts up to a minute in otherwise normal tissue. As such, SD is a radically different breed of electrophysiological activity compared with everyday neural activity, such as action potentials and synaptic transmission. Seventy years after its discovery by Leão, the mechanisms of SD and its profound metabolic and hemodynamic effects are still debated. What we did learn of consequence, however, is that SD plays a central role in the pathophysiology of a number of diseases including migraine, ischemic stroke, intracranial hemorrhage, and traumatic brain injury. An intriguing overlap among them is that they are all neurovascular disorders. Therefore, the interplay between neurons and vascular elements is critical for our understanding of the impact of this homeostatic breakdown in patients. The challenges of translating experimental data into human pathophysiology notwithstanding, this review provides a detailed account of bidirectional interactions between brain parenchyma and the cerebral vasculature during SD and puts this in the context of neurovascular diseases.

Identification of a neurovascular signaling pathway regulating seizures in mice

OBJECTIVE

A growing body of evidence suggests that increased blood-brain barrier (BBB) permeability can contribute to the development of seizures. The protease tissue plasminogen activator (tPA) has been shown to promote BBB permeability and susceptibility to seizures. In this study, we examined the pathway regulated by tPA in seizures.

METHODS

An experimental model of kainate-induced seizures was used in genetically modified mice, including mice deficient in tPA (tPA (-/-) ), its inhibitor neuroserpin (Nsp (-/-) ), or both (Nsp:tPA (-/-) ), and in mice conditionally deficient in the platelet-derived growth factor receptor alpha (PDGFRα).

RESULTS

Compared to wild-type (WT) mice, Nsp (-/-) mice have significantly reduced latency to seizure onset and generalization; whereas tPA (-/-) mice have the opposite phenotype, as do Nsp:tPA (-/-) mice. Furthermore, interventions that maintain BBB integrity delay seizure propagation, whereas osmotic disruption of the BBB in seizure-resistant tPA (-/-) mice dramatically reduces the time to seizure onset and accelerates seizure progression. The phenotypic differences in seizure progression between WT, tPA (-/-) , and Nsp (-/-) mice are also observed in electroencephalogram recordings in vivo, but absent in ex vivo electrophysiological recordings where regulation of the BBB is no longer necessary to maintain the extracellular environment. Finally, we demonstrate that these effects on seizure progression are mediated through signaling by PDGFRα on perivascular astrocytes.

INTERPRETATION

Together, these data identify a specific molecular pathway involving tPA-mediated PDGFRα signaling in perivascular astrocytes that regulates seizure progression through control of the BBB. Inhibition of PDGFRα signaling and maintenance of BBB integrity might therefore offer a novel clinical approach for managing seizures.

Pericyte protection by edaravone after tissue plasminogen activator treatment in rat cerebral ischemia

Pericytes play a pivotal role in contraction, mediating inflammation and regulation of blood flow in the brain. In this study, changes of pericytes in the neurovascular unit (NVU) were examined in relation to the effects of exogenous tissue plasminogen activator (tPA) and a free radical scavenger, edaravone. Immunohistochemistry and Western blot analyses showed that the overlap between platelet-derived growth factor receptor β-positive pericytes and N-acetylglucosamine oligomers (NAGO)-positive endothelial cells increased significantly at 4 days after 90 min of transient middle cerebral artery occlusion (tMCAO). The number of pericytes and the overlap with NAGO decreased with tPA but recovered with edaravone 4 days after tMCAO with proliferation. Thus, tPA treatment damaged pericytes, resulting in the detachment from astrocytes and a decrease in glial cell line-derived neurotrophic factor secretion. However, treatment with edaravone greatly improved tPA-induced damage to pericytes. The present study demonstrates that exogenous tPA strongly damages pericytes and destroys the integrity of the NVU, but edaravone treatment can greatly ameliorate such damage after acute cerebral ischemia in rats.

Water deprivation induces neurovascular and cognitive dysfunction through vasopressin-induced oxidative stress

Adequate hydration is essential for normal brain function and dehydration induces cognitive deterioration. In addition, dehydration has emerged as a stroke risk factor. However, it is unknown whether alterations in cerebrovascular regulation are responsible for these effects. To address this issue, C57Bl/6 mice were water deprived for 24 or 48 hours and somatosensory cortex blood flow was assessed by laser-Doppler flowmetry in a cranial window. Dehydration increased plasma osmolality and vasopressin levels, and suppressed the increase in blood flow induced by neural activity, by the endothelium-dependent vasodilator acetylcholine and the smooth muscle relaxant adenosine. The cerebrovascular dysfunction was associated with oxidative stress and cognitive deficits, assessed using the Y maze. The vasopressin 1a receptor antagonist SR49059 improved the dehydration-induced oxidative stress and vasomotor dysfunction. Dehydration upregulated endothelin-1 in cerebral blood vessels, an effect blocked by SR49059. Furthermore, the endothelin A receptor antagonist BQ123 ameliorated cerebrovascular function. These findings show for the first time that dehydration alters critical mechanisms regulating the cerebral circulation through vasopressin and oxidative stress. The ensuing cerebrovascular dysregulation may alter cognitive function and increase the brain's susceptibility to cerebral ischemia.

The role of pericytes in neurovascular unit remodeling in brain disorders

Neurons are extremely vulnerable cells that tightly rely on the brain’s highly dynamic and complex vascular network that assures an accurate and adequate distribution of nutrients and oxygen. The neurovascular unit (NVU) couples neuronal activity to vascular function, controls brain homeostasis, and maintains an optimal brain microenvironment adequate for neuronal survival by adjusting blood-brain barrier (BBB) parameters based on brain needs. The NVU is a heterogeneous structure constituted by different cell types that includes pericytes. Pericytes are localized at the abluminal side of brain microvessels and contribute to NVU function. Pericytes play essential roles in the development and maturation of the neurovascular system during embryogenesis and stability during adulthood. Initially, pericytes were described as contractile cells involved in controlling neurovascular tone. However, recent reports have shown that pericytes dynamically respond to stress induced by injury upon brain diseases, by chemically and physically communicating with neighboring cells, by their immune properties and by their potential pluripotent nature within the neurovascular niche. As such, in this paper, we would like to review the role of pericytes in NVU remodeling, and their potential as targets for NVU repair strategies and consequently neuroprotection in two pathophysiologically distinct brain disorders: ischemic stroke and Alzheimer’s disease (AD).

Alteplase treatment does not increase brain injury after mechanical middle cerebral artery occlusion in the rat

Recanalization of an occluded vessel with recombinant tissue plasminogen activator is an effective strategy for treating acute ischemic stroke. Recombinant tissue plasminogen activator is administered as alteplase, a formulation containing many excipients including L-arginine, the substrate for nitric oxide production. Most studies fail to compare the effects of alteplase on brain injury to its L-arginine carrier solution. This study aimed to verify the previously reported detrimental effects of alteplase after cerebral ischemia and delineate the contribution of L-arginine. Male Wistar rats, subjected to 90 minutes of intraluminal middle cerebral artery occlusion (MCAO), were administered alteplase, the carrier solution or saline upon reperfusion. Neither alteplase nor the carrier affected cerebral blood flow (CBF) restoration throughout the first 60 minutes of reperfusion. Alteplase treatment was associated with increased mortality after MCAO. Twenty-four hours after MCAO, neurologic function and infarct volume did not differ between rats treated with alteplase, the carrier solution, or saline. Irrespective of treatment group, infarct volume was correlated with CBF during reperfusion, neuroscore, and peri-infarct depolarizations. These results suggest that alteplase treatment, independent of thrombolysis, does not cause increased ischemic injury compared with its appropriate carrier solution, supporting the continued use of alteplase in eligible ischemic stroke patients.

tPA-S(481)A prevents impairment of cerebrovascular autoregulation by endogenous tPA after traumatic brain injury by upregulating p38 MAPK and inhibiting ET-1

Traumatic brain injury (TBI) is associated with loss of cerebrovascular autoregulation, which leads to cerebral hypoperfusion. Mitogen activated protein kinase (MAPK) isoforms ERK, p38, and JNK and endothelin-1 (ET-1) are mediators of impaired cerebral hemodynamics after TBI. Excessive tissue plasminogen activator (tPA) released after TBI may cause loss of cerebrovascular autoregulation either by over-activating N-methyl-D-aspartate receptors (NMDA-Rs) or by predisposing to intracranial hemorrhage. Our recent work shows that a catalytically inactive tPA variant (tPA-S(481)A) that competes with endogenous wild type (wt) tPA for binding to NMDA-R through its receptor docking site but that cannot activate it, prevents activation of ERK by wt tPA and impairment of autoregulation when administered 30 min after fluid percussion injury (FPI). We investigated the ability of variants that lack proteolytic activity but bind/block activation of NMDA-Rs by wt tPA (tPA-S(481)A), do not bind/block activation of NMDA-Rs but are proteolytic (tPA-A(296-299)), or neither bind/block NMDA-Rs nor are proteolytic (tPA-A(296-299)S(481)A) to prevent impairment of autoregulation after TBI and the role of MAPK and ET-1 in such effects. Results show that tPA-S(481)A given 3 h post-TBI, but not tPA-A(296-299) or tPA-A(296-299)S(481)A prevents impaired autoregulation by upregulating p38 and inhibiting ET-1, suggesting that tPA-S(481)A has a realistic therapeutic window and focuses intervention on NMDA-Rs to improve outcome.

The complex contribution of NOS interneurons in the physiology of cerebrovascular regulation

Following the discovery of the vasorelaxant properties of nitric oxide (NO) by Furchgott and Ignarro, the finding by Bredt and coll. of a constitutively expressed NO synthase in neurons (nNOS) led to the presumption that neuronal NO may control cerebrovascular functions. Consequently, numerous studies have sought to determine whether neuraly-derived NO is involved in the regulation of cerebral blood flow (CBF). Anatomically, axons, dendrites, or somata of NO neurons have been found to contact the basement membrane of blood vessels or perivascular astrocytes in all segments of the cortical microcirculation. Functionally, various experimental approaches support a role of neuronal NO in the maintenance of resting CBF as well as in the vascular response to neuronal activity. Since decades, it has been assumed that neuronal NO simply diffuses to the local blood vessels and produce vasodilation through a cGMP-PKG dependent mechanism. However, NO is not the sole mediator of vasodilation in the cerebral microcirculation and is known to interact with a myriad of signaling pathways also involved in vascular control. In addition, cerebrovascular regulation is the result of a complex orchestration between all components of the neurovascular unit (i.e., neuronal, glial, and vascular cells) also known to produce NO. In this review article, the role of NO interneuron in the regulation of cortical microcirculation will be discussed in the context of the neurovascular unit.

tPA-S481A prevents neurotoxicity of endogenous tPA in traumatic brain injury

Traumatic brain injury (TBI) is associated with loss of autoregulation due to impaired responsiveness to cerebrovascular dilator stimuli, which leads to cerebral hypoperfusion and neuronal impairment or death. Upregulation of tissue plasminogen activator (tPA) post-TBI exacerbates loss of cerebral autoregulation and NMDA-receptor-mediated impairment of cerebral hemodynamics, and enhances excitotoxic neuronal death. However, the relationship between NMDA-receptor activation, loss of autoregulation, and neurological dysfunction is unclear. Here, we evaluated the potential therapeutic efficacy of a catalytically inactive tPA variant, tPA S481A, that acts by competing with wild-type tPA for binding, cleavage, and activation of NMDA receptors. Lateral fluid percussion brain injury was produced in anesthetized piglets. Pial artery reactivity was measured via a closed cranial window, and cerebrospinal fluid (CSF) extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) was quantified by enzyme-linked immunosorbent assay (ELISA). tPA-S481A prevented impairment of cerebral autoregulation and reduced histopathologic changes after TBI by inhibiting upregulation of the ERK isoform of MAPK. Treatment with this tPA variant provides a novel approach for limiting neuronal toxicity caused by untoward NMDA-receptor activation mediated by increased tPA and glutamate following TBI.

Combination therapy with glucagon and a novel plasminogen activator inhibitor-1-derived peptide enhances protection against impaired cerebrovasodilation during hypotension after traumatic brain injury through inhibition of ERK and JNK MAPK

OBJECTIVE

Outcome of traumatic brain injury (TBI) is impaired by hypotension and glutamate, and TBI-associated release of endogenous tissue plasminogen activator (tPA) impairs cerebral autoregulation. Glucagon decreases central nervous system glutamate, lessens neuronal cell injury, and improves neurological score in mice after TBI. Glucagon partially protects against impaired cerebrovasodilation during hypotension after TBI in piglets by upregulating cAMP which decreases release of tPA. Pial artery dilation during hypotension is due to release of cAMP-dependent dilator prostaglandins (PG), such as PGE2 and PGI2. TBI impairs PGE2 and PGI2-mediated pial artery dilation, which contributes to disturbed cerebral autoregulation post-insult, by upregulating mitogen-activated protein kinase (MAPK). This study was designed to investigate relationships between tPA, prostaglandins, and MAPK as a mechanism to improve the efficacy of glucagon-mediated preservation of cerebrovasodilation during hypotension after TBI.

METHODS

Lateral fluid percussion brain injury (FPI) was induced in piglets equipped with a closed cranial window. ERK and JNK MAPK concentrations in cerebrospinal fluid were quantified by enzyme-linked immunosorbent assay.

RESULTS

Cerebrospinal fluid JNK MAPK was increased by FPI, but blunted by glucagon and the novel plasminogen activator inhibitor-1-derived peptide (PAI-1DP), Ac-RMAPEEIIMDRPFLYVVR-amide. FPI modestly increased, while glucagon and PAI-1DP decreased ERK MAPK. PGE2, PGI2, N-methyl-D-aspartate, and hypotension-induced pial artery dilation was blunted after FPI, partially protected by glucagon, and fully protected by glucagon+PAI-1DP, glucagon+JNK antagonist SP600125 or glucagon+ERK inhibitor U 0126.

DISCUSSION

Glucagon+PAI-1DP act in concert to protect against impairment of cerebrovasodilation during hypotension after TBI via inhibition of ERK and JNK MAPK.

Central cardiovascular circuits contribute to the neurovascular dysfunction in angiotensin II hypertension

Hypertension, a powerful risk factor for stroke and dementia, has damaging effects on the brain and its vessels. In particular, hypertension alters vital cerebrovascular control mechanisms linking neural activity to cerebral perfusion. In experimental models of slow-developing hypertension, free radical signaling in the subfornical organ (SFO), one of the forebrain circumventricular organs, is critical for the hormonal release and sympathetic activation driving the elevation in arterial pressure. However, the contribution of this central mechanism to the cerebrovascular alterations induced by hypertension remains uncertain. We tested the hypothesis that free radical production in the SFO is involved in the alterations in cerebrovascular regulation produced by hypertension. In a mouse model of gradual hypertension induced by chronic administration of subpressor doses of angiotensin II (AngII), suppression of free radicals in the SFO by overexpression of CuZn-superoxide dismutase (CuZnSOD) prevented the alteration in neurovascular coupling and endothelium-dependent responses in somatosensory cortex induced by hypertension. The SFO mediates the dysfunction via two signaling pathways. One involves SFO-dependent activation of the paraventricular hypothalamic nucleus, elevations in plasma vasopressin, upregulation of endothelin-1 in cerebral resistance arterioles and activation of endothelin type A receptors. The other pathway depends on activation of cerebrovascular AngII type 1 (AT1) receptors by AngII. Both pathways mediate vasomotor dysfunction by inducing vascular oxidative stress. The findings implicate for the first time the SFO and its efferent hypothalamic pathways in the cerebrovascular alterations induced by AngII, and identify vasopressin and endothelin-1 as potential therapeutic targets to counteract the devastating effects of hypertension on the brain.

Relative contribution of cyclooxygenases, epoxyeicosatrienoic acids, and pH to the cerebral blood flow response to vibrissal stimulation

The increase in cerebral blood flow (CBF) during neuronal activation can be only partially attenuated by individual inhibitors of epoxyeicosatrienoic acids (EETs), cyclooxgenase-2, group I metabotropic glutamate receptors (mGluR), neuronal nitric oxide synthase (nNOS), N-methyl-D-aspartate receptors, or adenosine receptors. Some studies that used a high concentration (500 μM) of the cyclooxygenase-1 inhibitor SC-560 have implicated cyclooxygenase-1 in gliovascular coupling in certain model systems in the mouse. Here, we found that increasing the concentration of SC-560 from 25 μM to 500 μM over whisker barrel cortex in anesthetized rats attenuated the CBF response to whisker stimulation. However, exogenous prostaglandin E(2) restored the response in the presence of 500 μM SC-560 but not in the presence of a cyclooxygenase-2 inhibitor, thereby suggesting a limited permissive role for cyclooxygenase-1. Furthermore, inhibition of the CBF response to whisker stimulation by an EET antagonist persisted in the presence of SC-560 or a cyclooxygenase-2 inhibitor, thereby indicating that the EET-dependent component of vasodilation did not require cyclooxygenase-1 or -2 activity. With combined inhibition of cyclooxygenase-1 and -2, mGluR, nNOS, EETs, N-methyl-D-aspartate receptors, and adenosine 2B receptors, the CBF response was reduced by 60%. We postulated that the inability to completely block the CBF response was due to tissue acidosis resulting from impaired clearance of metabolically produced CO2. We tested this idea by increasing the concentration of superfused bicarbonate from 25 to 60 mM and found a markedly reduced CBF response to hypercapnia. However, increasing bicarbonate had no effect on the initial or steady-state CBF response to whisker stimulation with or without combined inhibition. We conclude that the residual response after inhibition of several known vasodilatory mechanisms is not due to acidosis arising from impaired CO2 clearance when the CBF response is reduced. An unidentified mechanism apparently is responsible for the rapid, residual cortical vasodilation during vibrissal stimulation.

Modifying neurorepair and neuroregenerative factors with tPA and edaravone after transient middle cerebral artery occlusion in rat brain

Changes in expression of neurorepair and neuroregenerative factors were examined after transient cerebral ischemia in relation to the effects of tissue plasminogen activator (tPA) and the free radical scavenger edaravone. Physiological saline or edaravone was injected twice during 90 min of transient middle cerebral artery occlusion (tMCAO) in rats, followed by the same saline or tPA at reperfusion. Sizes of the infarct and protein factors relating to neurorepair and neuroregeneration were examined at 4d after tMCAO. The protein factors examined were: a chondroitin sulfate proteoglycan neurocan, semaphorin type 3A (Sema3A), a myelin-associated glycoprotein receptor (Nogo receptor, Nogo-R), a synaptic regenerative factor (growth associated protein-43, GAP43), and a chemotropic factor netrin receptor (deleted in colorectal cancer, DCC). Two groups treated by edaravone only or edaravone plus tPA showed a reduction in infarct volume compared to the two groups treated by vehicle only or vehicle plus tPA. Immunohistochemistry and western blot analyses indicated that protein expression of neurocan, Sema3A, Nogo-R, GAP43, and DCC was decreased with tPA, but recovered with edaravone. Additive edaravone prevented the reductions of these five proteins induced by tPA. The present study demonstrates for the first time that exogenous tPA reduced protein factors involved in inhibiting and promoting axonal growth, but that edaravone ameliorated such damage in brain repair after acute ischemia.

Positive association of phencyclidine-responsive genes, PDE4A and PLAT, with schizophrenia

As schizophrenia-like symptoms are produced by administration of phencyclidine (PCP), a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptors, PCP-responsive genes could be involved in the pathophysiology of schizophrenia. We injected PCP to Wistar rats and isolated five different parts of the brain in 1 and 4 hr after the injection. We analyzed the gene expression induced by the PCP treatment of these tissues using the AGILENT rat cDNA microarray system. We observed changes in expression level in 90 genes and 21 ESTs after the treatment. Out of the 10 genes showing >2-fold expressional change evaluated by qRT-PCR, we selected 7 genes as subjects for the locus-wide association study to identify susceptibility genes for schizophrenia in the Japanese population. In haplotype analysis, significant associations were detected in combinations of two SNPs of BTG2 (P = 1.4 × 10(-6) ), PDE4A (P = 1.4 × 10(-6) ), and PLAT (P = 1 × 10(-3) ), after false discovery rate (FDR) correction. Additionally, we not only successfully replicated the haplotype associations in PDE4A (P = 6.8 × 10(-12) ) and PLAT (P = 0.015), but also detected single-point associations of one SNP in PDE4A (P = 0.0068) and two SNPs in PLAT (P = 0.0260 and 0.0104) in another larger sample set consisting of 2,224 cases and 2,250 controls. These results indicate that PDE4A and PLAT may be susceptibility genes for schizophrenia in the Japanese population.

tPA contributes to impaired NMDA cerebrovasodilation after traumatic brain injury through activation of JNK MAPK

OBJECTIVE

N-methyl-D-aspartate (NMDA)-induced pial artery dilation (PAD) is reversed to vasoconstriction after fluid percussion brain injury (FPI). Tissue type plasminogen activator (tPA) is up-regulated and the tPA antagonist, EEIIMD, prevents impaired NMDA PAD after FPI. Mitogen-activated protein kinase (MAPK), a family of at least three kinases, ERK, p38, and JNK, is also up-regulated after traumatic brain injury (TBI). We hypothesize that tPA impairs NMDA-induced cerebrovasodilation after FPI in a MAPK isoform-dependent mechanism.

METHODS

Lateral FPI was induced in newborn pigs. The closed cranial window technique was used to measure pial artery diameter and to collect cerebrospinal fluid (CSF). ERK, p38, and JNK MAPK concentrations in CSF were quantified by ELISA.

RESULTS

CSF JNK MAPK was increased by FPI, increased further by tPA, but blocked by JNK antagonists SP600125 and D-JNKI1. FPI modestly increased p38 and ERK isoforms of MAPK. NMDA-induced PAD was reversed to vasoconstriction after FPI, whereas dilator responses to papaverine were unchanged. tPA, in post-FPI CSF concentration, potentiated NMDA-induced vasoconstriction while papaverine dilation was unchanged. SP 600125 and D-JNKI1, blocked NMDA-induced vasoconstriction and fully restored PAD. The ERK antagonist U 0126 partially restored NMDA-induced PAD, while the p38 inhibitor SB203580 aggravated NMDA-induced vasoconstriction observed in the presence of tPA after FPI.

DISCUSSION

These data indicate that tPA contributes to impairment of NMDA-mediated cerebrovasodilation after FPI through JNK, while p38 may be protective. These data suggest that inhibition of the endogenous plasminogen activator system and JNK may improve cerebral hemodynamic outcome post-TBI.

tPA in the injured central nervous system: different scenarios starring the same actor?

When in 1947, Astrup and Permin reported that animal tissues contain fibrinokinase, a plasminogen activator, and when Pennica and colleagues (Pennica et al., 1983) cloned and expressed human tissue plasminogen activator (tPA) in Escherichia coli in 1983, they might did not realize how much their pioneer work would impact the life of millions of patients suffering from myocardial infarction or ischemic stroke. Some years after, accumulating evidence shows that tPA is not just a plasminogen activator of endothelial origin. Indeed, the main function of tPA released from the endothelium is to convert fibrin-bound plasminogen into active plasmin, thus dissolving the fibrin meshwork of blood clots. But this serine protease is also expressed by several cell types, and its beneficial and deleterious actions stand beyond fibrinolysis or even proteolysis. We will review here the reported effects and mechanisms of action of tPA in the course of three different pathologies of the central nervous system (CNS): spinal cord injury, ischemic stroke and multiple sclerosis. While these three disorders have distinct aetiologies, they share some pathogenic mechanisms. We will depict the main "good" and "bad" sides of tPA described to date during each of these pathological situations, as well as the proposed mechanisms explaining these effects. We speculate that due to common pathogenic pathways, tPA's actions described in one particular disease could in fact occur in the others. Finally, we will evaluate if tPA could be a therapeutic target for these pathologies. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

The neurovascular unit in brain function and disease

Neurovascular coupling, or functional hyperaemia, refers to complex mechanisms of communication between neurons, astrocytes and cerebral vessels which form the neurovascular unit that spatially and temporally adjusts blood supply to the needs in energy and oxygen of activated neurons. Neurovascular coupling is so precise that it underlies neuroimaging techniques to map changes in neuronal activity. Therefore, understanding its basis is indispensable for the proper interpretation of imaging signals from functional magnetic resonance imaging and positron emission tomography, routinely used in humans. Although neurovascular coupling mechanisms are not yet fully understood, considerable progress has been made over the last decade. In this review, we present recent knowledge from in vivo studies on the cortical cellular network involved in neurovascular coupling responses and the mediators implicated in these haemodynamic changes. Recent findings have emphasized the intricate interplay between both excitatory and inhibitory neurons in neurovascular coupling, together with an intermediary role of astrocytes, which are ideally positioned between neurons and microvessels. Finally, we describe latest findings on the alterations of neurovascular function encountered in neurodegenerative conditions such as Alzheimer's disease.

Neuronal nitric oxide contributes to neuroplasticity-associated protein expression through cGMP, protein kinase G, and extracellular signal-regulated kinase

Nitric oxide (NO) synthesized by neuronal NO synthase (nNOS) has long been implicated in brain plasticity. However, it is unclear how this short-lived mediator contributes to the long-term molecular changes underlying neuroplasticity, which typically require activation of the mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) signaling pathway and gene expression. To address this issue, we used a neuroplasticity model based on treatment of neuronal cultures with bicuculline and a model of experience-dependent plasticity in the barrel cortex. In neuronal cultures, NOS inhibition attenuated the bicuculline-induced activation of ERK and the expression of c-Fos, Egr-1, Arc, and brain-derived neurotrophic factor (BDNF), proteins essential for neuroplasticity. Furthermore, inhibition of the NO target soluble guanylyl cyclase or of the cGMP effector kinase protein kinase G (PKG) reduced both ERK activation and plasticity-related protein expression. NOS inhibition did not affect phosphorylation of cAMP response element-binding protein (CREB), a well-established ERK nuclear target, but it attenuated the nuclear accumulation of the CREB coactivator TORC1 and suppressed the activation of Elk-1, another transcription factor target of ERK. Consistent with these in vitro observations, induction of c-Fos, Egr-1, and BDNF was attenuated in the D1 cortical barrel of nNOS(-/-) mice subjected to single whisker experience. These results establish nNOS-derived NO as a key factor in the expression of proteins involved in neuroplasticity, an effect mediated through cGMP, PKG, and ERK signaling. These actions of NO do not depend on CREB phosphorylation but may involve TORC1 and Elk-1. Our data unveil a previously unrecognized link between neuronal NO and the molecular machinery responsible for the sustained synaptic changes underlying neuroplasticity.

Cyclosporine A, FK506, and NIM811 ameliorate prolonged CBF reduction and impaired neurovascular coupling after cortical spreading depression

Cortical spreading depression (CSD) is associated with mitochondrial depolarization, increasing intracellular Ca(2+), and the release of free fatty acids, which favor opening of the mitochondrial permeability transition pore (mPTP) and activation of calcineurin (CaN). Here, we test the hypothesis that cyclosporine A (CsA), which blocks both mPTP and CaN, ameliorates the persistent reduction of cerebral blood flow (CBF), impaired vascular reactivity, and a persistent rise in the cerebral metabolic rate of oxygen (CMRO(2)) following CSD. In addition to CsA, we used the specific mPTP blocker NIM811 and the specific CaN blocker FK506. Cortical spreading depression was induced in rat frontal cortex. Electrocortical activity was recorded by glass microelectrodes, CBF by laser Doppler flowmetry, and tissue oxygen tension with polarographic microelectrodes. Electrocortical activity, basal CBF, CMRO(2), and neurovascular and neurometabolic coupling were unaffected by all three drugs under control conditions. NIM811 augmented the rise in CBF observed during CSD. Cyclosporine A and FK506 ameliorated the persistent decrease in CBF after CSD. All three drugs prevented disruption of neurovascular coupling after CSD; the rise in CMRO(2) was unchanged. Our data suggest that blockade of mPTP formation and CaN activation may prevent persistent CBF reduction and vascular dysfunction after CSD.

Cerebral blood flow alteration in neuroprotection following cerebral ischaemia

The best neuroprotectant for acute ischaemic stroke would always be the rapid return of oxygen and glucose to physiological levels. This is currently provided by thrombolysis which restores blood flow to the ischaemic region. The attempt to confer neuroprotection by targeting the brain parenchyma has shown promise in experimental stroke models, but has unequivocally failed to translate to the clinic. Neuroprotective therapy primarily targets the biochemical cascade that produces cell death following cerebral ischaemia. However, these agents may also alter signal transduction that controls cerebral blood flow, for example glutamate, which may affect the outcome after ischaemia. In these cases, neuroprotection may potentially be due to the improved access to oxygen and glucose rather than biochemical prevention of cell death. Improvement in cerebral blood flow is an important but often overlooked effect of neuroprotective therapy, analogous to the protective effects of drug-induced hypothermia. This short review will discuss cerebral blood flow alteration and protection of the brain in the context of ischaemic preconditioning, oxygen sensing and thrombolysis. Future neuroprotection studies in cerebral ischaemia require stringent monitoring of cerebral blood flow, plus other physiological parameters. This will increase the chances that any protection observed may be able to translate to human therapy.

Involvement of endothelial-derived relaxing factors in the regulation of cerebral blood flow

Despite numerous researches and advances in the present times, delayed cerebral vasospasm remains a severe complication leading to a high mortality and morbidity in patients with subarachnoid hemorrhage (SAH). Since the discovery of endothelium-derived relaxing factor (EDRF) in 1980, its role in delayed cerebral vasospasm after SAH has been widely investigated as well as in regulation of basic cerebral blood flow, pathophysiology of vasoconstriction and application on prevention and treatment of cerebral vasospasm. Among all the EDRFs, nitric oxide has caught the most attention, and the other substances which display similar properties with characteristics of EDRF such as carbon monoxide (CO), hydrogen sulfide (H(2)S), hydrogen peroxide (H(2)O(2)), potassium ion (K(+)) and methane (CH(4)) have also evoked great interest in the research field. This review provides an overview of recent advances in investigations on the involvement of EDRFs in the regulation of cerebral blood flow, especially in cerebral vasospasm after SAH. Possible therapeutic measures and potential clinical implications for cerebral vasospasm are also summarized.