Role of endothelin-1 in astrocyte responses after acute brain damage

Sovateltide (ILR-1620) Improves Motor Function and Reduces Hyperalgesia in a Rat Model of Spinal Cord Injury

BACKGROUND

Spinal cord injury (SCI) presents a major global health challenge, with rising incidence rates and substantial disability. Although progress has been made in understanding SCI's pathophysiology and early management, there is still a lack of effective treatments to mitigate long-term consequences. This study investigates the potential of sovateltide, a selective endothelin B receptor agonist, in improving clinical outcomes in an acute SCI rat model.

METHODS

Thirty male Sprague-Dawley rats underwent sham surgery (group A) or SCI and treated with vehicle (group B) or sovateltide (group C). Clinical tests, including Basso, Beattie, and Bresnahan scoring, inclined plane, and allodynia testing with von Frey hair, were performed at various time points. Statistical analyses assessed treatment effects.

RESULTS

Sovateltide administration significantly improved motor function, reducing neurological deficits and enhancing locomotor recovery compared with vehicle-treated rats, starting from day 7 post injury. Additionally, the allodynic threshold improved, suggesting antinociceptive properties. Notably, the sovateltide group demonstrated sustained recovery, and even reached preinjury performance levels, whereas the vehicle group plateaued.

CONCLUSIONS

This study suggests that sovateltide may offer neuroprotective effects, enhancing neurogenesis and angiogenesis. Furthermore, it may possess anti-inflammatory and antinociceptive properties. Future clinical trials are needed to validate these findings, but sovateltide shows promise as a potential therapeutic strategy to improve functional outcomes in SCI. Sovateltide, an endothelin B receptor agonist, exhibits neuroprotective properties, enhancing motor recovery and ameliorating hyperalgesia in a rat SCI model. These findings could pave the way for innovative pharmacological interventions for SCI in clinical settings.

Propitious Therapeutic Modulators to Prevent Blood-Spinal Cord Barrier Disruption in Spinal Cord Injury

The blood-spinal cord barrier (BSCB) is a specialized protective barrier that regulates the movement of molecules between blood vessels and the spinal cord parenchyma. Analogous to the blood-brain barrier (BBB), the BSCB plays a crucial role in maintaining the homeostasis and internal environmental stability of the central nervous system (CNS). After spinal cord injury (SCI), BSCB disruption leads to inflammatory cell invasion such as neutrophils and macrophages, contributing to permanent neurological disability. In this review, we focus on the major proteins mediating the BSCB disruption or BSCB repair after SCI. This review is composed of three parts. Section 1. SCI and the BSCB of the review describes critical events involved in the pathophysiology of SCI and their correlation with BSCB integrity/disruption. Section 2. Major proteins involved in BSCB disruption in SCI focuses on the actions of matrix metalloproteinases (MMPs), tumor necrosis factor alpha (TNF-α), heme oxygenase-1 (HO-1), angiopoietins (Angs), bradykinin, nitric oxide (NO), and endothelins (ETs) in BSCB disruption and repair. Section 3. Therapeutic approaches discusses the major therapeutic compounds utilized to date for the prevention of BSCB disruption in animal model of SCI through modulation of several proteins.

Endothelin-1 induces LIMK2-mediated programmed necrotic neuronal death independent of NOS activity

BACKGROUND

Recently, we have reported that LIM kinase 2 (LIMK2) involves programmed necrotic neuronal deaths induced by aberrant cyclin D1 expression following status epilepticus (SE). Up-regulation of LIMK2 expression induces neuronal necrosis by impairment of dynamin-related protein 1 (DRP1)-mediated mitochondrial fission. However, we could not elucidate the upstream effecter for LIMK2-mediated neuronal death. Thus, we investigated the role of endothelin-1 (ET-1) in LIMK2-mediated neuronal necrosis, since ET-1 involves neuronal death via various pathways.

RESULTS

Following SE, ET-1 concentration and its mRNA were significantly increased in the hippocampus with up-regulation of ETB receptor expression. BQ788 (an ETB receptor antagonist) effectively attenuated SE-induced neuronal damage as well as reduction in LIMK2 mRNA/protein expression. In addition, BQ788 alleviated up-regulation of Rho kinase 1 (ROCK1) expression and impairment of DRP1-mediated mitochondrial fission in CA1 neurons following SE. BQ788 also attenuated neuronal death and up-regulation of LIMK2 expression induced by exogenous ET-1 injection.

CONCLUSION

These findings suggest that ET-1 may be one of the upstream effectors for programmed neuronal necrosis through abnormal LIMK2 over-expression by ROCK1.

Selective astrocytic endothelin-1 overexpression contributes to dementia associated with ischemic stroke by exaggerating astrocyte-derived amyloid secretion

Endothelin-1 (ET-1) is synthesized by endothelial cells and astrocytes in stroke and in brains of Alzheimer's disease patients. Our transgenic mice with ET-1 overexpression in the endothelial cells (TET-1) showed more severe blood-brain barrier (BBB) breakdown, neuronal apoptosis, and glial reactivity after 2-hour transient middle cerebral artery occlusion (tMCAO) with 22-hour reperfusion and more severe cognitive deficits after 30 minutes tMCAO with 5 months reperfusion. However, the role of astrocytic ET-1 in contributing to poststroke cognitive deficits after tMCAO is largely unknown. Therefore, GET-1 mice were challenged with tMCAO to determine its effect on neurologic and cognitive deficit. The GET-1 mice transiently displayed a sensorimotor deficit after reperfusion that recovered shortly, then more severe deficit in spatial learning and memory was observed at 3 months after ischemia compared with that of the controls. Upregulation of TNF-α, cleaved caspase-3, and Thioflavin-S-positive aggregates was observed in the ipsilateral hemispheres of the GET-1 brains as early as 3 days after ischemia. In an in vitro study, ET-1 overexpressing astrocytic cells showed amyloid secretion after hypoxia/ischemia insult, which activated endothelin A (ETA) and endothelin B (ETB) receptors in a PI3K/AKT-dependent manner, suggesting role of astrocytic ET-1 in dementia associated with stroke by astrocyte-derived amyloid production.

Increasing the Permeability of the Blood-brain Barrier in Three Different Models in vivo

AIMS

Blood-brain barrier (BBB) plays significant roles in the circumstance maintains for the central nervous system (CNS). The dysfunction of the BBB could occur in all pathological conditions of CNS diseases, such as ischemic stroke, cerebral edema, or inflammatory disorders. However, the comparisons among different animal models with a broken BBB in vivo are still need to be further studied.

METHODS

Here we used three different mice models in vivo, including MCAO induce, LPS treatment, and cold injury to mimic the situation in clinic. The permeability of BBB in three models was detected by perfusion of Evan's blue dye. The functional proteins of the BBB including claudin-5, VE-cadherin, and caveolin-1 were compared in three different models in vivo.

RESULTS

With the hyperpermeability of Evan's blue in the three models, both claudin-5 and VE-cadherin were decreased, while the expression of caveolin-1 was increased. Our study showed that BBB dysfunction induced by MCAO in mice was relatively stable, reliable, and moderate compared with LPS or cold injury-induced BBB permeability models, although the procedural time was generally long and operation complexity was hard. Moreover, our study also found that the model of the increased BBB permeability by cold injury was severe in the regional cerebral tissue and the model treated with LPS was mild in the global cerebral tissue. The operation of the two models in vivo was easy, quick, and stable.

CONCLUSION

The MCAO model was the most suitable for studying the permeability of BBB among the three models in vivo.

p38α controls self-renewal and fate decision of neurosphere-forming cells in adult hippocampus

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Neural stem cells (NSC) from the adult hippocampus easily lose their activity in vitro.

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Inhibition of p38α enables successful long-term culture of adult hippocampus NSC.

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Inhibition of p38α can maintain a high neurogenic capacity for NSC.

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Neurogenic competence-related microRNAs are upregulated in NSC by p38α inhibition.

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In vitro expanded NSC by p38α inhibition are beneficial against brain damage.

Neural stem cells (NSC) from the adult hippocampus easily lose their activity in vitro. Efficient in vitro expansion of adult hippocampus-derived NSC is important for generation of tools for research and cell therapy. Here, we show that a single copy disruption or pharmacological inhibition of p38α enables successful long-term neurosphere culture of adult mouse hippocampal cells. Expanded neurospheres with high proliferative activity differentiated into the three neuronal lineages under differentiating conditions. Thus, inhibition of p38α can maintain adult hippocampal NSC activity in vitro.

Activation of either the ETA or the ETB receptors is involved in the development of electrographic seizures following intrahippocampal infusion of the endothelin-1 in immature rats

The period around birth is a risky time for stroke in infants, which is associated with two major acute and subacute processes: anatomical damage and seizures. It is unclear as to what extent each of these processes independently contributes to poor outcome. Furthermore, it is unclear whether there is an interaction between the two processes - does seizure activity cause additional brain damage beyond that produced by ischemia and/or does brain damage foster seizures? The model of focal cerebral ischemia induced by the intrahippocampal infusion of endothelin-1 (ET-1) in 12-day-old rat was used to examine the role of the endothelin receptors in the development of focal ischemia, symptomatic acute seizures and neurodegeneration. ET-1 (40pmol/μl) was infused either alone or co-administered with selective antagonists of ETA (BQ123; 70nmol/μl) or ETB receptors (BQ788; 70nmol/1μl). Effects of activation of ETB receptors were studied using selective agonist 4-Ala-ET-1 (40pmol/1μl). Regional cerebral blood flow (rCBF) and tissue oxygenation (pO2) were measured in anesthetized animals with a Doppler-flowmeter and a pO2-sensor, respectively. Seizure development was assessed with video-EEG in freely moving rats. Controls received the corresponding volume of the appropriate vehicle (10mM PBS or 0.01% DMSO-PBS solution; pH7.4). The extent of hippocampal lesion was determined using FluoroJade B staining performed 24h after ET-1 infusion. Infusion of ET-1 or ET-1+ETB receptor antagonist reduced rCBF to ~25% and pO2 to ~10% for about 1.5h, whereas selective ETB agonist, ET-1+ETA antagonist and the PBS vehicle had only negligible effect on the rCBF and pO2 levels. Reduction of rCBF was associated with the development of lesion in the injected hippocampus. In all groups, except sham operated and PBS controls, epileptiform activity was observed after activation of the ETA or the ETB receptors. The data revealed a positive correlation between the severity of morphological damage and all the measured seizure parameters (seizure frequency, average and total seizure duration) in the ET-1 group. In addition, the severity of morphological damage positively correlated with the average seizure duration in animals after infusion of ET-1+ETA receptor antagonist or after infusion of ET-1+ETB receptor antagonist. Our results indicate that the activation of ETA receptors is crucial for ischemia development, however either ETA or ETB receptors mediate the development of seizures following the application of ET-1 in immature rats. The dissociation between the ischemic-producing and seizure-producing processes suggests that damage is not necessary to induce seizures, although it may exacerbate them.

Amelioration of cold injury-induced cortical brain edema formation by selective endothelin ETB receptor antagonists in mice

Brain edema is a potentially fatal pathological condition that often occurs in stroke and head trauma. Following brain insults, endothelins (ETs) are increased and promote several pathophysiological responses. This study examined the effects of ET_B antagonists on brain edema formation and disruption of the blood-brain barrier in a mouse cold injury model (Five- to six-week-old male ddY mice). Cold injury increased the water content of the injured cerebrum, and promoted extravasation of both Evans blue and endogenous albumin. In the injury area, expression of prepro-ET-1 mRNA and ET-1 peptide increased. Intracerebroventricular (ICV) administration of BQ788 (ET_B antagonist), IRL-2500 (ET_B antagonist), or FR139317 (ET_A antagonist) prior to cold injury significantly attenuated the increase in brain water content. Bolus administration of BQ788, IRL-2500, or FR139317 also inhibited the cold injury-induced extravasation of Evans blue and albumin. Repeated administration of BQ788 and IRL-2500 beginning at 24 h after cold injury attenuated both the increase in brain water content and extravasation of markers. In contrast, FR139317 had no effect on edema formation when administrated after cold injury. Cold injury stimulated induction of glial fibrillary acidic protein-positive reactive astrocytes in the injured cerebrum. Induction of reactive astrocytes after cold injury was attenuated by ICV administration of BQ788 or IRL-2500. These results suggest that ET_B receptor antagonists may be an effective approach to ameliorate brain edema formation following brain insults.

Targeting endothelin receptors A and B attenuates the inflammatory response and improves locomotor function following spinal cord injury in mice

After spinal cord injury (SCI), the disruption of blood-spinal cord barrier by activation of the endothelin (ET) system is a critical event leading to leukocyte infiltration, inflammatory response and oxidative stress, contributing to neurological disability. In the present study, we showed that blockade of ET receptor A (ETAR) and/or ET receptor B (ETBR) prevented early inflammatory responses directly via the inhibition of neutrophil and monocyte diapedesis and inflammatory mediator production following traumatic SCI in mice. Long-term neurological improvement, based on a series of tests of locomotor performance, occurred only in the spinal cord‑injured mice following blockade of ETAR and ETBR. We also examined the post‑traumatic changes of the micro-environment within the injured spinal cord of mice following blockade of ET receptors. Oxidative stress reflects an imbalance between malondialdehyde and superoxide dismutase in spinal cord‑injured mice treated with vehicle, whereas blockade of ETAR and ETBR reversed the oxidation state imbalance. In addition, hemeoxygenase-1, a protective protease involved in early SCI, was increased in spinal cord‑injured mice following the blockade of ETAR and ETBR, or only ETBR. Matrix metalloproteinase-9, a tissue-destructive protease involved in early damage, was decreased in the injured spinal cord of mice following blockade of ETAR, ETBR or a combination thereof. The findings of the present study therefore suggested an association between ETAR and ETBR in regulating early pathogenesis of SCI and determining the outcomes of long‑term neurological recovery.

The neglected co-star in the dementia drama: the putative roles of astrocytes in the pathogeneses of major neurocognitive disorders

Alzheimer's disease (AD) and vascular dementia are the major causes of cognitive disorders worldwide. They are characterized by cognitive impairments along with neuropsychiatric symptoms, and that their pathogeneses show overlapping multifactorial mechanisms. Although AD has long been considered the most common cause of dementia, individuals afflicted with AD commonly exhibit cerebral vascular abnormalities. The concept of mixed dementia has emerged to more clearly identify patients with neurodegenerative phenomena exhibiting both AD and cerebral vascular pathologies-vascular damage along with β-amyloid (Aβ)-associated neurotoxicity and τ-hyperphosphorylation. Cognitive impairment has long been commonly explained through a 'neuro-centric' perspective, but emerging evidence has shed light over the important roles that neurovascular unit dysfunction could have in neuronal death. Moreover, accumulating data have been demonstrating astrocytes being the essential cell type in maintaining proper central nervous system functioning. In relation to dementia, the roles of astrocytes in Aβ deposition and clearance are unclear. This article emphasizes the multiple events triggered by ischemia and the cytotoxicity exerted by Aβ either alone or in association with endothelin-1 and receptor for advanced glycation end products, thereby leading to neurodegeneration in an 'astroglio-centric' perspective.

Smooth muscle specific disruption of the endothelin-A receptor in mice reduces arterial pressure, and vascular reactivity and affects vascular development

AIMS

The role of vascular smooth muscle endothelin A receptors (ETA) in development and normal physiology remains incompletely understood. To address this, mice were generated with smooth muscle-specific knockout (KO) of ETA.

MAIN METHODS

Mice were homozygous for loxP-flanked exons 6-8 of the EDNRA gene (floxed) or were also hemizygous for a transgene expressing Cre recombinase under control of the smooth muscle-specific SM22 promoter (KO mice).

KEY FINDINGS

Genotyping at 17 days postnatal yielded a 10:1 ratio of floxed:KO mice. Smooth muscle actin staining of embryos at day E10.5 revealed increased tortuosity in dorsal aortae while E12.5 embryos had mandibular, vascular and thymic abnormalities. Mice surviving to weaning developed and bred normally. ETA KO mice aged 2-3 months manifested EDNRA gene recombination in all organs tested. Aortas from KO mice had a >90% reduction in ETA mRNA content, but no differences in ET-1 or ETB mRNA levels. Addition of 0.01-100 nM ET-1 to isolated femoral arteries from floxed, but not KO, mice dose-dependently decreased vessel diameter (up to 80% reduction in the presence of ETB blockade). Intravenous infusion of ET-1 into floxed, but not KO, mice increased mean arterial pressure (MAP) (by ~10 mm Hg). Telemetric analysis revealed decreased MAP in KO mice (reduced by ~7-10 mm Hg) when fed a high salt diet.

SIGNIFICANCE

Smooth muscle ETA is important for normal vascular, mandibular and thymic development and is involved in the maintenance of arterial pressure under physiological conditions.

Endothelin-1 stimulates cyclin D1 expression in rat cultured astrocytes via activation of Sp1

Endothelins (ETs), a family of vasoconstrictor peptides, are up-regulated in several pathological conditions in the brain, and induce astrocytic proliferation. We previously observed that ET-1 increased the expression of cyclin D1 protein. Thus, we confirmed the intracellular up-regulation of cyclin D1 by ET-1 in rat cultured astrocytes. Real-time PCR analysis indicated that ET-1 (100 nM) and Ala(1,3,11,15)-ET-1 (100 nM), a selective agonist of the ETB receptor, induced a time-dependent and transient increase in cyclin D1 mRNA. The effect of ET-1 was diminished by an ETB antagonist (1 μM BQ788) or inhibitors of Sp1 (500 nM mithramycin), ERK (50 μM PD98059), p38 (20 μM SB203580) and JNK (1 μM SP600125), but not inhibitors of NF-κB (10 μM SN50 and 100 μM pyrrolidine dithiocarbamate). The binding assay for Sp1 indicated that ET-1 increased the binding activity of Sp1 to consensus sequences, and two oligonucleotides of the cyclin D1 promoter including the Sp1-binding sites diminished the effect of ET-1. Western blot analysis showed that ET-1 induced time-dependent and transient phosphorylation of Sp1 on Thr453 and Thr739 via the ETB receptor. ET-1-induced phosphorylation of Sp1 was attenuated by PD98059 and SP600125. Additionally, ET-1 increased the incorporation of bromodeoxyuridine (BrdU) in cultured astrocytes and the number of BrdU-positive cells decreased in the presence of PD98059, SP600125 and mithramycin. These results suggest that ET-1 increases the expression of cyclin D1 via activation of Sp1 and induces astrocytic proliferation.

HIF-1 and c-Src mediate increased glucose uptake induced by endothelin-1 and connexin43 in astrocytes

In previous work we showed that endothelin-1 (ET-1) increases the rate of glucose uptake in astrocytes, an important aspect of brain function since glucose taken up by astrocytes is used to supply the neurons with metabolic substrates. In the present work we sought to identify the signalling pathway responsible for this process in primary culture of rat astrocytes. Our results show that ET-1 promoted an increase in the transcription factor hypoxia-inducible factor-1α (HIF-1α) in astrocytes, as shown in other cell types. Furthermore, HIF-1α-siRNA experiments revealed that HIF-1α participates in the effects of ET-1 on glucose uptake and on the expression of GLUT-1, GLUT-3, type I and type II hexokinase. We previously reported that these effects of ET-1 are mediated by connexin43 (Cx43), the major gap junction protein in astrocytes. Indeed, our results show that silencing Cx43 increased HIF-1α and reduced the effect of ET-1 on HIF-1α, indicating that the effect of ET-1 on HIF-1α is mediated by Cx43. The activity of oncogenes such as c-Src can up-regulate HIF-1α. Since Cx43 interacts with c-Src, we investigated the participation of c-Src in this pathway. Interestingly, both the treatment with ET-1 and with Cx43-siRNA increased c-Src activity. In addition, when c-Src activity was inhibited neither ET-1 nor silencing Cx43 were able to up-regulate HIF-1α. In conclusion, our results suggest that ET-1 by down-regulating Cx43 activates c-Src, which in turn increases HIF-1α leading to the up-regulation of the machinery required to take up glucose in astrocytes. Cx43 expression can be reduced in response not only to ET-1 but also to various physiological and pathological stimuli. This study contributes to the identification of the signalling pathway evoked after Cx43 down-regulation that results in increased glucose uptake in astrocytes. Interestingly, this is the first evidence linking Cx43 to HIF-1, which is a master regulator of glucose metabolism.

Endothelin-1 is increased in cerebrospinal fluid and associated with unfavorable outcomes in children after severe traumatic brain injury

Severe pediatric traumatic brain injury (TBI) is associated with unfavorable outcomes secondary to injury from activation of the inflammatory cascade, the release of excitotoxic neurotransmitters, and changes in the reactivity of cerebral vessels, causing ischemia. Hypoperfusion of injured brain tissues after TBI is also associated with unfavorable outcomes. Therapeutic hypothermia is an investigational treatment strategy for use in patients with severe TBI that has shown differential effects on various cerebrospinal fluid (CSF) mediators in pediatric patients. Endothelin-1 (ET-1) is a powerful vasoconstrictor that exerts its effects on the cerebrovascular endothelium for sustained periods after TBI. The purpose of this study was to determine if CSF concentrations of ET-1 are increased after severe TBI in children, and if they are associated with demographics and outcomes that are affected by therapeutic hypothermia. This was an ancillary study to a prospective, randomized-controlled trial of early hypothermia in a tertiary care pediatric intensive care unit. Children (n = 34, age 3 months-15 years) suffering from severe TBI were randomized to hypothermia (n = 19) and normothermia (n = 15) as part of the efficacy study. Children undergoing diagnostic lumbar puncture (n = 11) to rule out infection were used as controls. Patients received either mild to moderate hypothermia (32-33°C) or normothermia as part of their treatment protocol. CSF was serially collected during the first 5 days after TBI. ET-1 concentrations were quantitated in patient and control CSF samples by a validated ELISA in duplicate with a limit of quantification of 0.195 pg/mL. CSF ET-1 concentrations were increased by two- to threefold in children after TBI compared to controls, and the increase was sustained for up to 5 days post-TBI. This relationship was not affected by hypothermia, and there were no differences in ET-1 response between children with inflicted and accidental TBI. Group-based trajectory analysis revealed two distinct groups with similar ET-1 levels over time. Univariate analysis showed a significant association between ET-1 levels and Glasgow Outcome Scale (GOS) scores, for which higher ET-1 levels over time were associated with unfavorable outcomes. ET-1 is increased in children with severe TBI and is associated with unfavorable outcomes. This increase in ET-1 may mediate the hypoperfusion or cerebrovascular dysfunction accompanying severe TBI in children. Importantly, hypothermia does not affect the brain's ET-1 response as measured in the CSF.

Role of endothelin receptors in the control of central nervous system parasitism in Trypanosoma cruzi infection in rats

Endothelin has been implicated in the pathogenesis of experimental and human Chagas disease. In the present study, we investigated whether the treatment with bosentan, an antagonist of both ET(A)/ET(B) endothelin receptors, modified parasite load and inflammation in the central nervous system (CNS) of Trypanosomacruzi-infected rats. The cerebellum was the most affected region in the CNS with marked parasitism and inflammation. Treatment with bosentan enhanced parasitemia and CNS parasitism, but control of infection was eventually attained. There was also an increase in the levels of the cytokines TNF-alpha, IL-10, IFN-gamma, CCL2/MCP-1, CCL3/MIP-1alpha and CCL5/RANTES in the brain of infected animals at days 9, 13 and 18 after infection. Overall, bosentan has some effects on the expression of certain cytokines and this may be related to the initial enhanced parasite load. Altogether, our data suggest that endothelin action via ET(A) and ET(B) receptors may play a role in the initial resistance of the CNS to T. cruzi infection in rats.

Jagged1 regulates the activation of astrocytes via modulation of NFkappaB and JAK/STAT/SOCS pathways

The Notch pathway is implicated in many aspects of the central nervous system (CNS) development and functions. Recently, we and others identified the Notch pathway to be involved in inflammatory events of the CNS. To understand the implication of this pathway on astrocytes, we have studied the Jagged-Notch-Hes pathway under inflammatory conditions. LPS exposure induced an upregulation of Jagged1 expression on cultured astrocytes. To address the role of Jagged1 in the modulation of inflammation, we used a siRNA mediated silencing of Jagged1 (siRNA J1). Jagged1 inhibition induced important variations on the Notch pathway components like Hes1, Hes5, Notch3, and RBP-Jkappa. siRNA J1 repressed the mRNA expression of genes known as hallmarks of the gliosis like GFAP and endothelin(B) receptor. On activated astrocytes, the inhibition of Jagged1 had antiinflammatory effects and resulted in a decrease of LPS-induced proinflammatory cytokines (IL1beta, IL1alpha, and TNFalpha) as well as the iNOS expression. The inhibition of Jagged1 induced a modulation of the JAK/STAT/SOCS signaling pathway. Most interestingly, the siRNA J1 decreased the LPS-induced translocation of NFkappaB p65 and this could be correlated to the phosphorylation of IkappaBalpha. These results suggest that during inflammatory and gliotic events of the CNS, Jagged1/Notch signaling sustains the inflammation mainly through NFkappaB and in part through JAK/STAT/SOCS signaling pathways.

Sustained sensorimotor impairments after endothelin-1 induced focal cerebral ischemia (stroke) in aged rats

Despite recent advances, stroke remains a leading cause of neurological disability with the vast majority of victims being the elderly, who exhibit more severe neurological deficits and a reduced capacity to recover from these disabilities in comparison to young stroke survivors. The objective of the present study was to develop a model of focal ischemic stroke in aged rats using endothelin-1 (ET-1) to produce low mortality rates as well as reliable, robust sensorimotor deficits that resemble functional impairments associated with stroke in humans. Here, we studied the functional and histological outcome following unilateral ET-1 infusions into the sensorimotor cortex of aged rats (20-23 months old). This procedure resulted in low mortality rates (13.3%) and no loss in body weight one week following surgery. Functional assessment was performed using a number of reliable behavioural tests: staircase test (fine motor function), horizontal ladder (skilled locomotion), bilateral tactile stimulation test (somatosensory function) and cylinder test (postural weight support). Following ET-1 induced stroke, all tests demonstrated large and sustained sensorimotor deficits in both forelimb and hindlimb function that failed to improve over the 28-day testing period. In addition, histological assessment revealed a substantial loss of retrogradely labelled corticospinal neurons in the ipsilesional hemisphere following stroke. Our results establish a model for the use of aged rats in future preclinical studies, which will enhance assessment of the long-term benefit of potential neural repair and regenerative strategies.

Connexin43 is involved in the effect of endothelin-1 on astrocyte proliferation and glucose uptake

In previous studies, we showed that endothelin-1 increased astrocyte proliferation and glucose uptake. These effects were similar to those observed with other gap junction inhibitors, such as carbenoxolone (CBX). Because 24-h treatment with endothelin-1 or CBX downregulates the expression of connexin43, the main protein forming astrocytic gap junctions, which can also be involved in proliferation, in this study, we addressed the possible role of connexin43 in the effects of endothelin-1. To do so, connexin43 was silenced in astrocytes by siRNA. The knock down of connexin43 increased the rate of glucose uptake, characterized by the upregulation of GLUT-1 and type I hexokinase. Neither endothelin-1 nor CBX were able to further increase the rate of glucose uptake in connexin43-silenced astrocytes. In agreement, no effects of endothelin-1 and CBX on GLUT-1 and type I hexokinase were observed in connexin-43 silenced astrocytes or in astrocytes from connexin43 knock-out (KO) mice. Our previous studies suggested a close relationship between glucose uptake and astrocyte proliferation. Consistent with this, connexin43-silenced astrocytes exhibited an increase in Ki-67, a marker of proliferation. The effects of ET-1 on retinoblastoma phosphorylation on Ser780 and on the upregulation of cyclins D1 and D3 were affected by the levels of connexin43. In conclusion, our results indicate that connexin43 participates in the effects of endothelin-1 on glucose uptake and proliferation in astrocytes. Interestingly, although the rate of growth in connexin43 KO astrocytes has been reported to be reduced, we observed that an acute reduction in connexin43 by siRNA increased proliferation and glucose uptake.

Regulation of glial inflammatory mediators synthesis: possible role of endothelins

Endothelins are well known as modulators of inflammation in the periphery, but little is known about their possible role in brain inflammation. Stimulation of astrocyte prostaglandin, an inflammatory mediator, synthesis was shown so far only by endothelin 3 (ET-3). By contrast, several studies showed no change or slight decrease of basal nitric oxide synthesis after treatment of astrocytes with endothelin 1 (ET-1) and ET-3. However, a significant increase in astrocytic and microglial nitric oxide synthase (NOS) was observed after exposure to ET-1 and ET-3 in a model of forebrain ischaemia. Here we demonstrate that all three endothelins (ET-1, ET-2, ET-3) significantly enhanced the synthesis of prostaglandin E(2) and nitric oxide in glial cells. Each of the selective antagonists for ETA and ETB receptors (BQ123 and BQ788 respectively), significantly inhibited endothelins-induced production of both nitric oxide and prostaglandin E(2). These results suggest a regulatory mechanism of endothelins, interacting with both endothelin receptors, on glial inflammation. Therefore, inhibition of endothelin receptors may have a therapeutic potential in pathological conditions of the brain, when an uncontrolled inflammatory response is involved.

Phosphorylation in the C-terminal domain of Aquaporin-4 is required for Golgi transition in primary cultured astrocytes

Aquaporin-4 (AQP4) is expressed in the perivascular and subpial astrocytes end-feet in mammalian brain, and plays a critical component of an integrated water and potassium homeostasis. Here we examine whether AQP4 is phosphorylated in primary cultured mouse astrocytes. Astrocytes were metabolically labeled with [(32)P]phosphoric acid, then AQP4 was immunoprecipitated with anti-AQP4 antibody. We observed that AQP4 was constitutively phosphorylated, which is reduced by treatment with protein kinase CK2 inhibitors. To elucidate the phosphorylation of AQP4 by CK2, myc-tagged wild-type or mutant AQP4 was transiently transfected in primary cultured astrocytes. Substitution of Ala residues for four putative CK2 phosphorylation sites in the C terminus abolished the phosphorylation of AQP4. Immunofluorescent microscopy revealed that the quadruple mutant was localized in the Golgi apparatus. These observations indicate that the C-terminal domain of AQP4 is constitutively phosphorylated at least in part by protein kinase CK2 and it is required for Golgi transition.

Deficiency in endothelin receptor B reduces proliferation of neuronal progenitors and increases apoptosis in postnatal rat cerebellum

Endothelins regulate cellular functions in the mammalian brain through the endothelin receptors A and B (EDNRA and EDNRB). In this study, we investigated the role of EDNRB on cell proliferation in the cerebellum by using the spotting lethal (sl) rat, which carries a naturally occurring deletion in the EDNRB gene. Proliferating cells in the three genotypes, wild-type (+/+), heterozygous (+/sl) and homozygous mutant (sl/sl) rats were labelled by intraperitoneal injection of 5-bromo-2'-deoxyuridine (BrdU) at postnatal day 2. The density of BrdU-positive cells (per mm(2)) in the external germinal layer of sl/sl rats (Mean +/- SEM, 977 +/- 388) was significantly reduced compared to +/+ (4915 +/- 631) and +/sl (2304 +/- 557) rats. Subsequently, we examined the effects of EDNRB mutation on neural apoptosis by terminal deoxynucleotidyltransferase-mediated dUTP nick end-labelling assay. This showed that the density of apoptotic cells in the cerebella of sl/sl rats (9.3 +/- 0.5/mm(2)) was significantly more increased than +/+ rats (4 +/- 0.7). The expression of brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) were measured with standard ELISA, but were unchanged in all genotypes. These results suggest that ENDRB mediates neural proliferation and have anti-apoptotic effects in the cerebellum of the postnatal rat, and that these effects are independent of changes in the expression of BDNF and GDNF. Our findings will lead to better understanding of the morphological changes in the cerebellum of Hirschsprung's disease patients with congenital EDNRB mutation.

Role of endothelins as mediators of injury-induced alterations of glial glutamate turnover

Astroglia terminate glutamatergic neurotransmission and prevent excitotoxic extracellular glutamate concentration by clearing synaptically released glutamate through the high-affinity, sodium-dependent glutamate transporters GLT-1 and GLAST. Many brain injures are associated with the disturbed expression of glial glutamate transporters and a subsequent increase of extracellular glutamate to neurotoxic levels. We have now followed up initial hints pointing to endothelins, a family of injury-regulated peptides, as mediators of this injury-induced loss of glial glutamate transporter expression. We observed that, in line with such a role, endothelins not only act as potent inhibitors of basal and exogenously (dbcAMP)-induced expression of GLT-1 in cortical astrocytes as shown before, but likewise inhibit expression of GLT-1 or GLAST in astrocytes cultured from the diencephalon, mesencephalon, cerebellum, and spinal cord. We further demonstrate that endothelins equally inhibit GLT-1 expression in cortical slice cultures, a culture system closely resembling the in vivo situation. Although brain injuries are usually associated with an increase in the expression of the glutamate-converting enzyme glutamine synthetase, cultured cortical astrocytes maintained with endothelins showed an almost complete loss of glutamine synthetase. Interestingly, the inhibitory effects of endothelins on the expression of glutamine synthetase, but not of glutamate transporters, was overridden by high extracellular glutamate, indicating that the primarily inhibitory action of endothelins on the various components of glial glutamate turnover dissociates in the injured brain.

Acute astrocyte activation in brain detected by MRI: new insights into T(1) hypointensity

Increases in the T(1) of brain tissue, which give rise to dark or hypointense areas on T(1)-weighted images using magnetic resonance imaging (MRI), are common to a number of neuropathologies including multiple sclerosis (MS) and ischaemia. However, the biologic significance of T(1) increases remains unclear. Using a multiparametric MRI approach and well-defined experimental models, we have experimentally induced increases in tissue T(1) to determine the underlying cellular basis of such changes. We have shown that a rapid acute increase in T(1) relaxation in the brain occurs in experimental models of both low-flow ischaemia induced by intrastriatal injection of endothelin-1 (ET-1), and excitotoxicity induced by intrastriatal injection of N-methyl-D-aspartate (NMDA). However, there appears to be no consistent correlation between increases in T(1) relaxation and changes in other MRI parameters (apparent diffusion coefficient, T(2) relaxation, or magnetisation transfer ratio of tissue water). Immunohistochemically, one common morphologic feature shared by the ET-1 and NMDA models is acute astrocyte activation, which was detectable within 2 h of intracerebral ET-1 injection. Pretreatment with an inhibitor of astrocyte activation, arundic acid, significantly reduced the spatial extent of the T(1) signal change induced by intrastriatal ET-1 injection. These findings suggest that an increase in T(1) relaxation may identify the acute development of reactive astrocytes within a central nervous system lesion. Early changes in T(1) may, therefore, provide insight into acute and reversible injury processes in neurologic patients, such as those observed before contrast enhancement in MS.

Involvement of p38alpha in kainate-induced seizure and neuronal cell damage

We investigated how p38alpha mitogen-activated protein kinase (p38) is related to kainate-induced epilepsy and neuronal damages, by using the mice with a single copy disruption of the p38 alpha gene (p38alpha(+/-)). Mortality rate and seizure score of p38alpha(+/-) mice administered with kainate were significantly reduced compared with the case of wild-type (WT) mice. This was clearly supported by the electroencephalography data in which kainate-induced seizure duration and frequency in the brain of p38alpha(+/-) mice were significantly suppressed compared to those of WT mice. As a consequence of seizure, kainate induced delayed neuronal damages in parallel with astrocytic growth in the hippocampus and ectopic innervation of the mossy fibers into the stratum oriens in the CA3 region of hippocampus in WT mice, whose changes were moderate in p38alpha(+/-) mice. Likewise, kainate-induced phosphorylation of calcium/calmodulin-dependent kinase II in the hippocampus of p38alpha (+/-) mice was significantly decreased compared to that of WT mice. These results suggest that p38alpha signaling pathway plays an important role in epileptic seizure and excitotoxicity.

Microfluorimetry defines early axonal damage in a rat model of optic neuritis: a novel method targeting early CNS autoimmunity

Autoimmune optic neuritis is a common early manifestation of multiple sclerosis (MS), yet early therapeutic interventions for MS often have high ocular toxicity associated with increased risks for glaucoma, cataract, or retinopathy. This need to discover better early treatment options prompted our development of a sensitive and reliable means to quantify the broad range of pathologies that potentially develop very early in autoimmune optic neuritis. Tissue microfluorimetry was used to measure seven established markers for human MS pathology in normal and autoimmune optic nerves 13 days after antigen exposure, in a Brown Norway rat model of myelin oligodendrocyte glycoprotein (MOG) peptide (35-55)-induced autoimmune optic neuritis. Optic neuritis rats demonstrated early and significant pathologic changes in five established indices for neuroinflammation, immune infiltration, and demyelination that accurately modeled pathologies characteristic of MS. Two indices of MS-like axon damage advanced significantly within 13 days of antigen exposure. Fluorimetrically measured immunoreactivity (-ir) was significantly decreased for paranodin (PN, the requisite axonal paranodal junction protein) and significantly increased for amyloid precursor protein (APP), indicating loss of paranodal junctions and impaired fast axonal transport, respectively. Measurements showing decreased PN-ir with increased APP-ir quantitatively defined a pattern of early axonal damage in autoimmune optic neuritis.

CNS injury, glial scars, and inflammation: Inhibitory extracellular matrices and regeneration failure

Spinal cord and brain injuries lead to complex cellular and molecular interactions within the central nervous system in an attempt to repair the initial tissue damage. Many studies have illustrated the importance of the glial cell response to injury, and the influences of inflammation and wound healing processes on the overall morbidity and permanent disability that result. The abortive attempts of neuronal regeneration after spinal cord injury are influenced by inflammatory cell activation, reactive astrogliosis and the production of both growth promoting and inhibitory extracellular molecules. Despite the historical perspective that the glial scar was a mechanical barrier to regeneration, inhibitory molecules in the forming scar and methods to overcome them have suggested molecular modification strategies to allow neuronal growth and functional regeneration. Unlike myelin associated inhibitory molecules, which remain at largely static levels before and after central nervous system trauma, inhibitory extracellular matrix molecules are dramatically upregulated during the inflammatory stages after injury providing a window of opportunity for the delivery of candidate therapeutic interventions. While high dose methylprednisolone steroid therapy alone has not proved to be the solution to this difficult clinical problem, other strategies for modulating inflammation and changing the make up of inhibitory molecules in the extracellular matrix are providing robust evidence that rehabilitation after spinal cord and brain injury has the potential to significantly change the outcome for what was once thought to be permanent disability.

Gap junctional control of glial glutamate transporter expression

The uptake of glutamate into astroglia is the predominant mechanism to terminate glutamatergic neurotransmission and to prevent neurotoxic extracellular glutamate concentrations. Here, we show that uncoupling cultured cortical astrocytes with the gap junction blocker, propofol, or the Cx43 mimetic peptide, Gap27, inhibits the expression of GLT-1, the major glutamate transporter subtype in the cortex. The dependence of GLT-1 expression on gap junctions was further confirmed by the use of astrocytes in which either the expression of Cx43, the major astrocytic gap junction protein, was inhibited by RNA interference or which were derived from animals carrying an astrocyte-specific deletion of the Cx43 gene. In both cases, reduced astrocytic coupling was associated with a pronounced decline in GLT-1 expression. Finally, a luciferase reporter gene assay demonstrated that blockade of gap junctions/connexins suppressed transcriptional activity of GLT-1 promoter. These observations unravel a previously unrecognized role of gap junctions in the control of glial glutamate transport.

Neurotransmitter receptor expression and activity during neuronal differentiation of embryonal carcinoma and stem cells: from basic research towards clinical applications

Embryonal carcinoma and embryonic stem cells have served as models to understand basic aspects of neuronal differentiation and are promising candidates for regenerative medicine. Besides being well characterized regarding the capability of embryonal carcinoma and embryonic stem cells to be precursors of different tissues, the molecular mechanisms controlling neuronal differentiation are hardly understood. Neuropeptide and neurotransmitter receptors are expressed at early stages of differentiation prior to synaptogenesis, triggering transient changes in calcium concentration and inducing neurone-specific gene expression. In vitro neuronal differentiation of embryonal carcinoma and embryonic stem cells closely resembles early neuronal development in vivo. Murine P19 EC cells are a well-characterized model for in vitro differentiation, which upon treatment with retinoic acid differentiate into neurones. Expression and activity of various receptor proteins is regulated during their differentiation. Stimulation of kinin-B2, endothelin-B, muscarinic acetylcholine, and N-methyl-D-aspartate receptors results in transient increases of intracellular free calcium concentration [Ca(2+)](i) in P19 cells undergoing neuronal differentiation, whereas embryonal cells do not respond or show a smaller change in [Ca(2+)](i) than differentiating cells. Receptor inhibition, as studied with the example of the kinin-B2 receptor, aborts neuronal maturation of P19 cells, demonstrating the crucial importance of B2 receptors during the differentiation process. Future success in obtaining desired neuronal phenotypes from pluripotent cells in vitro may offer new therapeutic perspectives for curing genetic and acquired dysfunctions of the developing and adult nervous system.

Mitogen-activated protein kinases, Erk and p38, phosphorylate and regulate Foxo1

The members of the transcription factor Foxo family regulate the expression of genes concerned with the stress response, cell cycle and gluconeogenesis. Foxo1 (FKHR) contains 15 consensus phosphorylation sites for the mitogen-activated protein kinase (MAPK) family. Therefore, we hypothesized that MAPKs could directly regulate the transcriptional activity of Foxo1 via phosphorylation. In vitro kinase assay showed that Foxo1 was phosphorylated by extracellular signal-regulated kinase (Erk) and p38 MAPK (p38) but not by c-jun NH2-terminal kinase (JNK). In NIH3T3 cells, epidermal growth factor or anisomycin increased phosphorylation of exogenous Foxo1, which was significantly inhibited by pretreatment with an MEK 1 inhibitor, PD98059, or a p38 inhibitor, SB203580. Two-dimensional phosphopeptide mapping using mutation of phosphorylation sites for MAPK revealed that the nine serine residues in Foxo1 are specifically phosphorylated by Erk and that five of the nine residues are phosphorylated by p38 in vivo. Moreover, we also found that Foxo1 interacts with Ets-1 and functions as a coactivator for Ets-1 on the fetal liver kinase (Flk)-1 promoter in bovine carotid artery endothelial cells. Mutation of the nine phosphorylation sites for Erk in Foxo1 was shown to lead to less binding and synergistic activity for Ets-1 on the Flk-1 promoter when compared with wild-type Foxo1. These results suggest that Foxo1 is specifically phosphorylated by Erk and p38, and that this phosphorylation regulates the function of Foxo1 as a coactivator for Ets-1.

Kinetics of endothelin-induced inhibition and glucose permeability of astrocyte gap junctions

Gap junctions contribute to important functions of communicating glial cells in brain physiology and pathology. Endothelins (ETs), a vasoactive family of peptides present in the brain, have been described as potent inhibitors of astrocyte gap junctional communication. Through dye-coupling studies we demonstrate here that this inhibition occurs rapidly and then successively reverses and returns to control levels after 90 min of continuous ET1 or ET3 exposure. In addition, long-term exposure of cells to ET3, which acts mainly on ETB receptors, also desensitized the acute action of ET1, which was previously shown to act through either ETA or ETB receptor sites, or both. The gap junction blocker carbenoxolone did not show any time-dependent desensitization and was fully effective also in cultures treated with ETs for prolonged times. The ETs inhibitory effects were partially prevented when blocking pertussis toxin-sensitive G-proteins, chelating intracellular Ca2+, or omitting extracellular Ca2+. We further show that ETs modulate gap junction-mediated transfer of 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-Y1)amino]-2-deoxyglucose (2-NBDG), a fluorescent glucose molecule, indicating a role of astrocyte gap junction coupling in metabolic trafficking and suggesting the importance of these peptides in the control of intercellular diffusion of energetic compounds. These findings might have particular relevance in early tissue reactions after various cerebral injuries, which commonly involve increased cerebral ET levels.

Endothelin-1 and endothelin receptors in light-induced retinal degeneration

We have studied the distribution of endothelinergic molecules: prepro-endothelin-1 (PPET-1), endothelin-1 (ET-1), and receptors A and B (ET-A) and (ET-B) in the retina of mice. The localization of these molecules in normal mice was compared to their localization in retinas from animals submitted to continuous illumination during 1, 6, 9 or 18 days. We also evaluated the distribution of smooth muscle actin (SMA) and glial markers, glial fibrillary acidic protein (GFAP) and glutamine synthase (GS). PPET-1 immunoreactivity mainly appeared in retinal pigment epithelium (RPE) and cells of the ganglion cell layer (GCL), whereas ET-1 immunoreactivity was present in the RPE, outer plexiform layer (OPL) and astrocytes. Astrocytes exhibited the strongest immunostaining in the retina. ET-A immunoreactivity was observed in endothelium, RPE, OPL and cells of the GCL. By contrast, ET-B immunoreactivity could be detected in endothelial cells, horizontal cells and astrocytes. Astrocytes of the optic nerve also exhibited ET-1, ET-A, and ET-B immunoreactivities. After light-induced degeneration, there was an increase of RPE immunostaining. Degeneration of photoreceptors was accompanied by disappearance of immunoreactivity in the OPL. However, ET-A immunoreactivity appeared in the amacrine sublayer of the INL. There was an enormous increase in astrocytes and its cell processes. The increase of astrocytic immunoreactivities for ET-1 and ET-B was confirmed by quantitative image analysis. Growth of astrocytic cell processes was most marked around retinal blood vessels. Our findings indicate that there are at least three endothelinergic pathways in the normal retina: (1) between the RPE and choriocapillaris, (2) at the OPL, and (3) between blood vessels, astrocytes and cells of the GCL. After light-induced degeneration of photoreceptors, endothelinergic molecules were overexpressed at the RPE and astrocytes, but mostly disappeared from the OPL.

Neurochondrin negatively regulates CaMKII phosphorylation, and nervous system-specific gene disruption results in epileptic seizure

Neurochondrin is a novel cytoplasmic protein and possibly involved in neurite outgrowth, chondrocyte differentiation, and bone metabolism. Our previous trial in disclosing its role by the loss of function in mice failed because of the lethality in utero. In this study, we eliminated the neurochondrin gene expression preferentially in the nervous system by the conditional knockout strategy. Our results showed that neurochondrin is a negative regulator of Ca(2+)/calmodulin-dependent protein kinase II phosphorylation and essential for the spatial learning process but not for the differentiation or neurite outgrowth of the neuron. In addition, the nervous system-specific homozygous gene disruption resulted in epileptic seizure.

Pertussis toxin-sensitive modulation of glutamate transport by endothelin-1 type A receptors in glioma cells

Endothelin-1 (ET-1) is a 21 amino acids peptide that exerts several biological activities through interaction with specific G-protein coupled receptors. Increased ET-1 expression is frequently associated with pathological situations involving alterations in glutamate levels. In the present study, a brief exposure to ET-1 was found to increase aspartate uptake in C6 glioma cells, which endogenously express the neuronal glutamate transporter EAAC1 (pEC50 of 9.89). The stimulatory effect of ET-1 mediated by ETA receptors corresponds to a 62% increase in the Vmax with no modification of the affinity for the substrate. While protein kinase C activity is known to participate in the regulation of EAAC1, the effect of ET-1 on the glutamate uptake was found to be independent of this kinase activation. In contrast, the inactivation of Go/i type G-protein dependent signaling with pertussis toxin was found to impair ET-1-mediated regulation of EAAC1. An examination of the cell surface expression of EAAC1 by protein biotinylation studies or by confocal analysis of immuno-fluorescence staining demonstrated that ET-1 stimulates EAAC1 translocation to the cell surface. Hence, the disruption of the cytoskeleton with cytochalasin D prevented ET-1-stimulated aspartate uptake. Together, the data presented in the current study suggest that ET-1 participates in the acute regulation of glutamate transport in glioma cells. Considering the documented role of glutamate excitotoxicity in the development of brain tumors, endothelinergic system constitutes a putative target for the pharmacological control of glutamate transmission at the vicinity of glioma cells.

Endothelin-mediated induction of heme oxygenase-1 in the spinal cord is attenuated in transgenic mice overexpressing superoxide dismutase

Spinal cord blood flow and the induction of heme oxygenase-1 (HO-1), an indicator of oxidative stress, were studied in the spinal cords of adult wild-type and transgenic mice overexpressing the antioxidant copper, zinc superoxide dismutase (CuZn SOD) after intrathecal administration of the potent vasoactive peptide endothelin-1 (ET-1). Gelfoam, saturated with ET-1 (40, 80, or 400 micromol/L), was positioned in the intrathecal space at the midthoracic level in anesthetized animals. Blood flow was continuously monitored by laser Doppler for 10 min after the intrathecal application of ET-1. There was a significant reduction in spinal cord blood flow to approximately 40% of control values by 10 min after the intrathecal application of the peptide in both wild-type and transgenic mice. Moreover, SB209670, a nonselective endothelin receptor antagonist, blocked this reduction in flow. Each animal was euthanized 24 h after the intrathecal administration of ET-1, and the spinal cord was prepared for quantitative immunocytochemistry. HO-1 was primarily induced in astrocytes near the dorsal surface of the spinal cord in wild-type mice. This induction was attenuated in both wild-type, treated with SB209670, and untreated transgenic mice. Together, these findings suggest that ET-1 mediates oxidative stress in the spinal cord through the modulation of spinal cord blood flow.

Chronic endothelin exposure inhibits connexin43 expression in cultured cortical astroglia

Severe brain lesions are accompanied by sustained increases in endothelin (ET) levels, which in turn profoundly affect brain microcirculation and neural cell function. A known response of astrocytes to acute increases in ET levels is the rapid and transient closure of gap junctions and the subsequent decrease of gap junction-mediated intercellular communication (GJIC). Because evidence exists that the loss of GJIC alters astrocytic gene expression, we analyzed the effects of chronic ET exposure on astrocytic gap junction coupling. We found that within 24 hr, cultured cortical astrocytes respond to low nanomolar concentrations (2-10 nM) of either ET-1 or ET-3 with a robust inhibition of connexin (Cx)43 expression, the major junctional protein in astrocytes, and a subsequent decline of GIJC. We further observed that in the continuous presence of ETs, Cx43 expression remained inhibited for at least 7 days. In addition, a similar decrease of Cx43 expression occurred in cultured spinal cord astrocytes maintained with ET-1 for 3 days. Applying ETs in combination with the highly selective ETA and ETB receptor antagonists, BQ123 and BQ788, respectively, revealed that the inhibitory influences on astrocytic Cx43 expression depend on activation of ETB receptors. We suggest that the observed ET-dependent inhibition of Cx43 expression and the resulting decline of GJIC might represent a major pathway by which ETs regulate astrocytic gene expression in the injured brain.

Endothelin-1 modulates anterograde fast axonal transport in the central nervous system

Anterograde fast axonal transport (FAxT) maintains synaptic function and provides materials necessary for neuronal survival. Localized changes in FAxT are associated with a variety of central nervous system (CNS) neuropathies, where they may contribute to inappropriate remodeling, a process more appropriately involved in synaptic plasticity and development. In some cases, developmental remodeling is regulated by localized secretion of endothelins (ETs), neuroinflammatory peptides that are also pathologically elevated in cases of neurologic disease, CNS injury, or ischemia. To investigate the potential role of ETs in these processes, we decided to test whether locally elevated endothelin-1 (ET-1) modulates FAxT in adult CNS tissues. We used the established in vivo rat optic nerve model and a novel ex vivo rat hippocampal slice model to test this hypothesis. In vivo, exogenously elevated vitreal ET-1 significantly affected protein composition of FAxT-cargos as well as the abundance and peak delivery times for metabolically-labeled proteins that were transported into the optic nerve. Proteins with molecular weights of 139, 118, 89, 80, 64, 59, 51, 45, 42, 37, and 25 kDa were evaluated at injection-sacrifice intervals (ISIs) of 24, 28, 32, and 36 hr. In acute hippocampal slices maintained on nonvascular supplies of glucose and oxygen, ET-1 significantly decreased the distance traveled along the Schaffer collateral tract by nonmetabolically-labeled lipid rafts at 5 and 10 min after pulse-labeling. In both models, ET-1 significantly affected transport or targeted delivery of FaxT-cargos, suggesting that ET-1 has the potential to modulate FAxT in adult CNS tissues.

Mechanosensation and endothelin in astrocytes--hypothetical roles in CNS pathophysiology

Endothelin (ET) is a potent autocrine mitogen produced by reactive and neoplastic astrocytes. ET has been implicated in the induction of astrocyte proliferation and other transformations engendered by brain pathology, and in promoting the malignant behavior of astrocytomas. Reactive astrocytes containing ET are found in the periphery/penumbra of a wide array of CNS pathologies. Virtually all brain pathology deforms the surrounding parenchyma, either by direct mass effect or edema. Mechanical stress is a well established stimulus for ET production and release by other cell types, but has not been well studied in the brain. However, numerous studies have illustrated that astrocytes can sense mechanical stress and translate it into chemical messages. Furthermore, the ubiquitous reticular meshwork formed by interconnected astrocytes provides an ideal morphology for sensing and responding to mechanical disturbances. We have recently demonstrated stretch-induced ET production by astrocytes in vitro. Inspired by this finding, the purpose of this article is to review the literature on (1) astrocyte mechanosensation, and (2) the endothelin system in astrocytes, and to consider the hypothesis that mechanical induction of the ET system may influence astrocyte functioning in CNS pathophysiology. We conclude by discussing evidence supporting future investigations to determine whether specific inhibition of stretch-activated ion channels may represent a novel strategy for treating or preventing CNS disturbances, as well as the relevance to astrocyte-derived tumors.

Endothelins negatively regulate glial glutamate transporter expression

Glutamate is the main excitatory neurotransmitter in the mammalian central nervous system which at high extracellular levels leads to neuronal over-stimulation and subsequent excitotoxic neuronal cell death. Both the termination of glutamatergic neurotransmission and the prevention of neurotoxic extracellular glutamate concentrations are predominantly achieved by the uptake of extracellular glutamate into astroglia through the high-affinity glutamate transporters, excitatory amino acid transporter-2/glutamate transporter-1 (EAAT-2/GLT-1) and EAAT-1/glutamate aspartate transporter (GLAST). Although several injury-induced growth factors such as epidermal growth factor (EGF) and transforming growth factor alpha (TGFalpha) potently stimulate the expression of glutamate transporters in cultured astroglia, GLT-1 and/or GLAST expression temporarily decreases during acute brain injuries eventually contributing to secondary neuronal cell death. We now demonstrate that the stimulatory influences of these injury-regulated growth factors are overridden by endothelins (ETs), a family of peptides also upregulated in the injured brain. Exposure of cultured cortical astroglia to ET-1, ET-2, and ET-3 resulted in a major loss of basal glutamate transporter expression after 72 hours and the complete prevention of the known stimulatory influences of dibutyryl cyclic (dbc)AMP, pituitary adenylate cyclase-activating polypeptide (PACAP), EGF, and TGFalpha on both GLT-1 and GLAST expression. With all ET isoforms, the inhibitory effects were detectable with similar low nanomolar concentrations and persisted in endothelin B-receptor deficient astroglia, suggesting that the inhibitory action is equally induced by endothelin A and B receptors. In astroglial cultures maintained with endothelins alone or in combination with PACAP, the inhibitory action was remarkably long-lasting and was still detectable after 7 days. In apparent contrast, glutamate transporter expression partially recovered between days 5 and 7 in cultures maintained with a combination of ETs and the injury-regulated growth factors EGF or TGFalpha. These findings point to ETs as major mediators of injury-dependent down-regulation of glial glutamate transporters and subsequent glutamate-induced brain damage.

Endothelin-1 stimulates the translocation and upregulation of both glucose transporter and hexokinase in astrocytes: relationship with gap junctional communication

We have previously shown that endothelin-1 increases glucose uptake in astrocytes. In the present work we investigate the mechanism through which endothelin-1 (ET-1) increases glucose uptake. Our results show that ET-1 activates a short-term and a long-term mechanism. Thus, ET-1 induced a rapid change in the localization of both GLUT-1 and type I hexokinase. These changes are probably aimed at rapidly increasing the entry and phosphorylation of glucose. In addition, ET-1 upregulated GLUT-1 and type I hexokinase and induced the expression of isoforms not normally expressed in astrocytes, such as GLUT-3 and type II hexokinase. These changes provide astrocytes with the machinery required to sustain a high rate of glucose uptake for a longer period of time. Our previous work had suggested that the effect of ET-1 on glucose uptake was associated with the inhibition of gap junctions. In this work, we compare the effect of ET-1 with that of carbenoxolone, a classical inhibitor of gap junction communication. Carbenoxolone increased glucose uptake to the same extent as ET-1 following the same mechanisms. Thus, carbenoxolone induced a rapid change in the localization of both GLUT-1 and type I hexokinase, upregulated GLUT-1 and type I hexokinase and induced the expression of GLUT-3 and type II hexokinase. When the inhibition of gap junction was prevented by tolbutamide, neither ET-1 nor carbenoxolone were able to increase the levels of GLUT-1, GLUT-3, type I hexokinase or type II hexokinase, indicating that these events are closely related to gap junctions.

Continuous measurements of cerebral tissue oxygen pressure during hyperbaric oxygenation—HBO effects on brain edema and necrosis after severe brain trauma in rabbits

INTRODUCTION

Severe brain injury is one of the most frequent causes of severe disability in the young. In acute management of brain trauma, new approaches based on experimental animal investigations should be sought.

METHODS

Twenty male, juvenile Chinchilla-Bastard rabbits received standardized cold-injury-induced-brain-trauma (CIBT). A metal probe (temperature -196 degrees C) was applied epidurally over 10 s. The hyperbaric oxygenation (HBO) group (n=10) underwent 90-min HBO sessions with 100% oxygen at 2.5 atmospheres absolute (1 h, 24+/-2 h, 48+/-2 h after CIBT). Cerebral tissue pO2-measurements were performed 60 min after CIBT, during the three HBO sessions and on day 4. The control group (n=10) underwent no treatment. Animals were sacrificed on day 4, and brains were analyzed histologically.

RESULTS

In the HBO group, pO2 measurements showed a significant increase in pO2 between day 1 and day 4, whereas no significant changes were observed in the control group. During the first HBO session, mean pO2 was 169 mm Hg, during the second 305 mm Hg and during the third 420 mm Hg. The mean area of necrosis was 16.2 mm2 in the HBO group, in the control group 19.9 mm2. The areas of brain edema were significantly smaller in the HBO group. Mortality in the HBO group was 0%, in the control group 20%.

CONCLUSION

HBO appears to be beneficial as an adjunct treatment of severe head trauma. To find optimal treatment protocols, further clinical studies must be developed.

PKC Signaling Mediates Global Enhancement of Excitatory Synaptogenesis in Neurons Triggered by Local Contact with Astrocytes

Here we provide evidence that astrocytes affect neuronal synaptogenesis by the process of adhesion. Local contact with astrocytes via integrin receptors elicited protein kinase C (PKC) activation in individual dissociated neurons cultured in astrocyte-conditioned medium. This activation, initially focal, soon spread throughout the entire neuron. We then demonstrated pharmacologically that the arachidonic acid cascade, triggered by the integrin reception, is responsible for the global activation of PKC. Local astrocytic contact also facilitated excitatory synaptogenesis throughout the neuron, a process which could be blocked by inhibitors of both integrins and PKC. Thus, propagation of PKC signaling represents an underlying mechanism for global neuronal maturation following local astrocyte adhesion.

Endothelin receptor expression in the normal and injured spinal cord: potential involvement in injury-induced ischemia and gliosis

The endothelins (ETs) are a family of peptides that exert their biological effects via two distinct receptors, the endothelin A receptor (ET(A)R) and the endothelin B receptor (ET(B)R). To more clearly define the potential actions of ETs following spinal cord injury, we used immunohistochemistry and confocal microscopy to examine the protein expression of ET(A)R and ET(B)R in the normal and injured rat spinal cord. In the normal spinal cord, ET(A)R immunoreactivity (IR) is expressed by vascular smooth muscle cells and a subpopulation of primary afferent nerve fibers. ET(B)R-IR is expressed primarily by radial glia, a small population of gray and white matter astrocytes, ependymal cells, vascular endothelial cells, and to a lesser extent in smooth muscle cells. Fourteen days following compression injury to the spinal cord, there was a significant upregulation in both the immunoexpression and number of astrocytes expressing the ET(B)R in both gray and white matter and a near disappearance of ET(B)R-IR in ependymal cells and ET(A)R-IR in primary afferent fibers. Conversely, the vascular expression of ET(A)R and ET(B)R did not appear to change. As spinal cord injury has been shown to induce an immediate increase in plasma ET levels and a sustained increase in tissue ET levels, ETs would be expected to induce an initial marked vasoconstriction via activation of vascular ET(A)R/ET(B)R and then days later a glial hypertrophy via activation of the ET(B)R expressed by astrocytes. Strategies aimed at blocking vascular ET(A)R/ET(B)R and astrocyte ET(B)Rs following spinal cord injury may reduce the resulting ischemia and astrogliosis and in doing so increase neuronal survival, regeneration, and function.

Sources of endothelin-1 in hippocampus and cortex following traumatic brain injury

Endothelin 1 (ET-1) exerts normally a powerful vasoconstrictor role in the control of the brain microcirculation. In altered states, such as following traumatic brain injury (TBI), it may contribute to the development of ischemia and/or secondary cell injury. Because little is known of ET-1's cellular compartmentalization and its association to vulnerable neurons after TBI, we assessed its expression (both mRNA and protein) in cerebral cortex and hippocampus using correlative in situ hybridization and immunocytochemical techniques.Sprague-Dawley male rats were killed at 4, 24 or 48 h after TBI (450 g from 2 m, Marmarou's model). Semiquantitative analysis of our in situ hybridization results indicated a 2.5- and a 2.0-fold increase in ET-1 mRNA content in the hippocampus and cortex respectively which persisted up to 48 h post TBI. At 4 and 24 h after TBI enzyme-linked immunosorbent assay showed a tendency for increased ET-1 synthesis. In animals subjected to TBI, qualitative immunocytochemical analysis revealed a shift in ET-1 expression from astrocytes (in control animals) to endothelial cells, macrophages and neurons. Astrocytes and macrophages were identified unequivocally by using double immunofluorescence revealing ET-1 and glial fibrillary acidic protein or ED-1, respectively, the markers being specific for these cellular types. While this redistribution was most prominent at 4 and 24 h post TBI, at 48 h the endothelial cells remained strongly ET-1 immunopositive. The results suggest that cellular types which in the intact animal synthesize little or no ET-1 provide novel sources of the peptide after TBI. These sources may contribute to the sustained cerebrovascular hypoperfusion observed post TBI.

Endothelin-1 stimulated capacitative Ca2+ entry through ET(A) receptors of a rat brain-derived type-1 astrocyte cell line, IA-1g1

The present study demonstrated that endotheline-1 (ET-1) stimulated a biphasic (transient and sustained) increase in [Ca(2+)](i) and signaling was blocked by BQ123 and inhibited by BQ788. RT-PCR analysis revealed that ET(A) was expressed more than ET(B) mRNA-suggesting that ET(A) is the major receptor. Simply reintroducing Ca(2+) in the buffer stimulated a sustained increase in [Ca(2+)](i) and the effect was inhibited by U73122, thapsigargin (TG), miconazole and SKF96365. When measured in Ca(2+)-free buffer, the ET-1-stimulated Ca(2+) transient decreased by 73% and the reintroduction of Ca(2+) induced a large sustained increase in [Ca(2+)](i). These effects were not affected by nifedipine, but were inhibited by miconazole and SKF96365-indicating that the sustained increase in [Ca(2+)](i) mediated by ET-1 was mostly due to capacitative Ca(2+) entry (CCE). The ET-1-induced CCE was inhibited by phorbol ester (PMA) but was enhanced by GF109203X; it was also enhanced by 8-bromo-cyclic AMP (8-Br-cAMP) but was inhibited by H89. Thus, protein kinase C (PKC) negatively regulated and cAMP-dependent protein kinase (PKA) positively regulated the ET-1-mediated CCE in these cells.

Vasoactive intestinal contractor/endothelin-2 gene expression in the murine central nervous system

Vasoactive intestinal contractor (VIC) is a member of the endothelin (ET) family. We have investigated the regional distribution of VIC/ET-2 and of ET-1 gene expression in the adult murine brain and pituitary gland. We used real-time quantitative reverse transcription-linked polymerase chain reaction. VIC/ET-2 gene expression was observed at high levels in the pituitary gland and medulla oblongata in both the mouse and rat. Moderate to low levels of expression were observed in other brain regions. On the contrary, ET-1 gene expression was quite low in the pituitary gland in comparison with the levels observed in the cerebral cortex, striatum, and midbrain. Cold injury to the mouse cerebral cortex caused a significant decrease in VIC/ET-2 gene expression in this structure, whilst expression of the ET-1 gene was increased. These results suggest that VIC/ET-2 may have certain physiological roles that differ from those of ET-1 in the brain and pituitary gland.

Endothelin B receptors are expressed by astrocytes and regulate astrocyte hypertrophy in the normal and injured CNS

The ability of mammalian central nervous system (CNS) neurons to survive and/or regenerate following injury is influenced by surrounding glial cells. To identify the factors that control glial cell function following CNS injury, we have focused on the endothelin B receptor (ET(B)R), which we show is expressed by the majority of astrocytes that are immunoreactive for glial acid fibrillary protein (GFAP) in both the normal and crushed rabbit optic nerve. Optic nerve crush induces a marked increase in ET(B)R and GFAP immunoreactivity (IR) without inducing a significant increase in the number of GFAP-IR astrocytes, suggesting that the crush-induced astrogliosis is due primarily to astrocyte hypertrophy. To define the role that endothelins play in driving this astrogliosis, artificial cerebrospinal fluid (CSF), ET-1 (an ET(A)R and ET(B)R agonist), or Bosentan (a mixed ET(A)R and ET(B)R antagonist) were infused via osmotic minipumps into noninjured and crushed optic nerves for 14 days. Infusion of ET-1 induced a hypertrophy of ET(B)R/GFAP-IR astrocytes in the normal optic nerve, with no additional hypertrophy in the crushed nerve, whereas infusion of Bosentan induced a significant decrease in the hypertrophy of ET(B)R/GFAP-IR astrocytes in the crushed but not in the normal optic nerve. These data suggest that pharmacological blockade of astrocyte ET(B)R receptors following CNS injury modulates glial scar formation and may provide a more permissive substrate for neuronal survival and regeneration.

Endothelin downregulates the glutamate transporter GLAST in cAMP-differentiated astrocytes in vitro

Endothelin (ET) is a putative pathogenetic mediator associated with brain trauma and ischemia. Because a link between neuronal damage after these injuries and glial Na(+)-dependent L-glutamate transporter activity has been suggested, we investigated the effect of ET on the glutamate clearance ability of astrocytes. Dibutyryl cyclic adenosine monophosphate (dBcAMP), which is widely used to induce differentiation of cultured astrocytes, markedly increased [(3)H]glutamate transport activity in a concentration- and time-dependent manner. In the presence of ET, however, dBcAMP decreased the glutamate uptake. This effect was efficiently prevented by an antagonist of ET(B) receptor, but not of ET(A) receptor. ET per se was virtually ineffective. Eadie-Hofstee analysis demonstrated that dBcAMP increased the V(max) value of glutamate uptake activity by 43.4% in the absence of ET, but decreased it by 41.4% in the presence of ET, without apparent changes in the K(m) value. Accordingly, Western blot analysis indicated that the change in transport activity correlated closely with that in expression level of the glial glutamate transporter GLAST. These results may represent the mechanisms by which ET aggravates trauma- and ischemia-elicited neuronal damage.