Distinct pharmacological properties of ET-1 and ET-3 on astroglial gap junctions and Ca(2+) signaling

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.

Physiology of Astroglia

Astrocytes are principal cells responsible for maintaining the brain homeostasis. Additionally, these glial cells are also involved in homocellular (astrocyte-astrocyte) and heterocellular (astrocyte-other cell types) signalling and metabolism. These astroglial functions require an expression of the assortment of molecules, be that transporters or pumps, to maintain ion concentration gradients across the plasmalemma and the membrane of the endoplasmic reticulum. Astrocytes sense and balance their neurochemical environment via variety of transmitter receptors and transporters. As they are electrically non-excitable, astrocytes display intracellular calcium and sodium fluctuations, which are not only used for operative signalling but can also affect metabolism. In this chapter we discuss the molecules that achieve ionic gradients and underlie astrocyte signalling.

Increased Levels of Endothelin-1 in Cerebrospinal Fluid Are a Marker of Poor Visual Recovery after Optic Neuritis in Multiple Sclerosis Patients

Background

Multiple sclerosis (MS), a chronic inflammatory and degenerative disease of the central nervous system, typically features immune-mediated focal demyelination and secondary axonal degeneration. Cerebral hypoperfusion of the normal-appearing white matter (NAWM) has been reported in MS patients and may be mediated by elevated levels of endothelin-1 (ET-1), a most potent vasoconstrictive peptide released from reactive astrocytes in MS focal lesions. Optic neuritis (ON) is one of the most frequent manifestations of MS and also shows peripapillary vascular hypoperfusion in combination with disc swelling.

Aims

We aimed to compare serum and cerebrospinal fluid (CSF) levels of ET-1 as a potential prognostic marker of MS-ON in two groups of patients differing for severity of MS-ON clinical presentation.

Materials and Methods

A cross-sectional study to compare serum and CSF levels of ET-1 between patients with clinically aggressive MS-ON (A-MS-ON) and nonaggressive MS-ON (NA-MS-ON) according to conventional ophthalmological criteria, including optical coherence tomography. CSF and serum concentrations of ET-1 were measured using a commercially available ELISA method.

Results

Sixteen patients consecutively referred to the Units of Neurology for visual disturbances attributable to MS were recruited, 11 (69%) patients with A-MS-ON and 5 (31%) with NA-MS-ON. Median CSF ET-1 levels and CSF/serum ET-1 quotient were significantly higher in patients with A-MS-ON (0.30 vs. 0.56 ng/ml) as compared to NA-MS-ON (0.16 vs. 0.16).

Conclusions

Severity and failure in the recovery from ON in MS patients may depend from vascular hypoperfusion of the optic nerve induced by high intrathecally produced ET-1, a potential prognostic marker of ON recovery in MS. The detection of CSF ET-1 levels may allow identifying groups of ON patients potentially benefitting from treatment with ET-1 antagonists (e.g., bosentan).

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Location and Number of Astrocytes Determine Dopaminergic Neuron Survival and Function Under 6-OHDA Stress Mediated Through Differential BDNF Release

While astrocytes throughout the CNS share many common traits, they exhibit significant differences in function and number among brain regions. The aim of the present study is to assess the effect of region-specificity and number of astrocytes on the survival of dopaminergic neurons under stress, and to understand the possible mechanism by which these astrocytes extend neuroprotection to dopaminergic neurons. Purified astrocytes obtained from forebrain, midbrain, and hindbrain region were characterized through FACS and immunofluorescence. Co-culture experiments (using trans-wells) were then performed to measure the effect of region-specificities and numbers of astrocytes on primary midbrain culture under 6-OHDA stress. Cell survival augmented with an increase in astrocyte seeding number and total cell survival was comparable among the different region-specific astrocytes for all numbers. However, striking differences were observed in dopaminergic neuronal (TH) cell survival in the presence of midbrain astrocytes in comparison to forebrain and hindbrain astrocytes at all seeding numbers. At 75 μM 6-OHDA insult, while cell survival was comparable in purified astrocytes from the different brain regions, a distinct increase in BDNF secretion (significantly higher than its constitutive release) was noted for midbrain astrocytes compared to forebrain and hindbrain astrocytes. The TH immunopositive population decreased when TrkB inhibitor was added to the co-culture under 6-OHDA toxicity, suggesting that BDNF released by co-cultured astrocytes plays a key role in the survival of dopaminergic neurons. This BDNF release decreased in presence of NO inhibitor and increased in the presence of NO donor (DETA/NO). We conclude that the BDNF released from astrocytes under 6-OHDA toxicity is mediated through NO release through both autocrine and paracrine signaling, and this BDNF release is primarily responsible for the differential effect of region-specific astrocytes on TH neuron survival under these conditions.

Contribution of Astroglial Cx43 Hemichannels to the Modulation of Glutamatergic Currents by D-Serine in the Mouse Prefrontal Cortex

Astrocytes interact dynamically with neurons by modifying synaptic activity and plasticity. This interplay occurs through a process named gliotransmission, meaning that neuroactive molecules are released by astrocytes. Acting as a gliotransmitter, D-serine, a co-agonist of the NMDA receptor at the glycine-binding site, can be released by astrocytes in a calcium [Ca²⁺]_i-dependent manner. A typical feature of astrocytes is their high expression level of connexin43 (Cx43), a protein forming gap junction channels and hemichannels associated with dynamic neuroglial interactions. Pharmacological and genetic inhibition of Cx43 hemichannel activity reduced the amplitude of NMDA EPSCs in mouse layer 5 prefrontal cortex pyramidal neurons without affecting AMPA EPSC currents. This reduction of NMDA EPSCs was rescued by addition of D-serine in the extracellular medium. LTP of NMDA and AMPA EPSCs after high-frequency stimulation was reduced by prior inhibition of Cx43 hemichannel activity. Inactivation of D-serine synthesis within the astroglial network resulted in the reduction of NMDA EPSCs, which was rescued by adding extracellular D-serine. We showed that the activity of Cx43 hemichannels recorded in cultured astrocytes was [Ca²⁺]_I dependent. Accordingly, in acute cortical slices, clamping [Ca²⁺]_i at a low level in astroglial network resulted in an inhibition of NMDA EPSC potentiation that was rescued by adding extracellular D-serine. This work demonstrates that astroglial Cx43 hemichannel activity is associated with D-serine release. This process, occurring by direct permeation of D-serine through hemichannels or indirectly by Ca²⁺ entry and activation of other [Ca²⁺]_i-dependent mechanisms results in the modulation of synaptic activity and plasticity.SIGNIFICANCE STATEMENT We recorded neuronal glutamatergic (NMDA and AMPA) responses in prefrontal cortex (PFC) neurons and used pharmacological and genetic interventions to block connexin-mediated hemichannel activity specifically in a glial cell population. For the first time in astrocytes, we demonstrated that hemichannel activity depends on the intracellular calcium concentration and is associated with D-serine release. Blocking hemichannel activity reduced the LTP of these excitatory synaptic currents triggered by high-frequency stimulation. These observations may be particularly relevant in the PFC, where D-serine and its converting enzyme are highly expressed.

The effects of endothelin-1 on satellite glial cells in peripheral ganglia

Endothelins (ET) are a family of highly active neuropeptides with manifold influences via ET receptors (ETR) in both the peripheral and central nervous systems. We have shown previously that satellite glial cells (SGCs) in mouse trigeminal ganglia (TG) are extremely sensitive to ET-1 in evoking [Ca²⁺]_in increase, apparently via ET_BR activation, but there is no functional information on ETR in SGCs of other peripheral ganglia. Here we tested the effects of ET-1 on SGCs in nodose ganglia (NG), which is sensory, and superior cervical ganglia (Sup-CG), which is part of the sympathetic nervous system, and further investigated the influence of ET-1 on SGCs in TG. Using calcium imaging we found that SGCs in intact, freshly isolated NG and Sup-CG are highly sensitive to ET-1, with threshold concentration at 0.1nM. Our results showed that [Ca²⁺]_in elevation in response to ET-1 was partially due to Ca²⁺ influx from the extracellular space and partially to Ca²⁺ release from intracellular stores. Using receptor selective ETR agonists and antagonists, we found that the responses were mediated by mixed ET_AR/ET_BR in SGCs of NG and predominantly by ET_BR in SGCs of Sup-CG. By employing intracellular dye injection we examined coupling among SGCs around different neurons in the presence of 5nM ET-1 and observed coupling inhibition in all the three ganglion types. In summary, our work showed that SGCs in mouse sensory and sympathetic ganglia are highly sensitive to ET-1 and that this peptide markedly reduces SGCs coupling. We conclude that ET-1, which may participate in neuron-glia communications, has similar functions in wide range of peripheral ganglia.

Glutamate mediated astrocytic filtering of neuronal activity

Neuron-astrocyte communication is an important regulatory mechanism in various brain functions but its complexity and role are yet to be fully understood. In particular, the temporal pattern of astrocyte response to neuronal firing has not been fully characterized. Here, we used neuron-astrocyte cultures on multi-electrode arrays coupled to Ca2+ imaging and explored the range of neuronal stimulation frequencies while keeping constant the amount of stimulation. Our results reveal that astrocytes specifically respond to the frequency of neuronal stimulation by intracellular Ca2+ transients, with a clear onset of astrocytic activation at neuron firing rates around 3-5 Hz. The cell-to-cell heterogeneity of the astrocyte Ca2+ response was however large and increasing with stimulation frequency. Astrocytic activation by neurons was abolished with antagonists of type I metabotropic glutamate receptor, validating the glutamate-dependence of this neuron-to-astrocyte pathway. Using a realistic biophysical model of glutamate-based intracellular calcium signaling in astrocytes, we suggest that the stepwise response is due to the supralinear dynamics of intracellular IP3 and that the heterogeneity of the responses may be due to the heterogeneity of the astrocyte-to-astrocyte couplings via gap junction channels. Therefore our results present astrocyte intracellular Ca2+ activity as a nonlinear integrator of glutamate-dependent neuronal activity.

Interplay between endothelin and erythropoietin in astroglia: the role in protection against hypoxia

We show that, under in vitro conditions, the vulnerability of astroglia to hypoxia is reflected by alterations in endothelin (ET)-1 release and capacity of erythropoietin (EPO) to regulate ET-1 levels. Exposure of cells to 24 h hypoxia did not induce changes in ET-1 release, while 48–72 h hypoxia resulted in increase of ET-1 release from astrocytes that could be abolished by EPO. The endothelin receptor type A (ETA) antagonist BQ123 increased extracellular levels of ET-1 in human fetal astroglial cell line (SV-FHAS). The survival and proliferation of rat primary astrocytes, neural precursors, and neurons upon hypoxic conditions were increased upon administration of BQ123. Hypoxic injury and aging affected the interaction between the EPO and ET systems. Under hypoxia EPO decreased ET-1 release from astrocytes, while ETA receptor blockade enhanced the expression of EPO mRNA and EPO receptor in culture-aged rat astroglia. The blockade of ETA receptor can increase the availability of ET-1 to the ETB receptor and can potentiate the neuroprotective effects of EPO. Thus, the new therapeutic use of combined administration of EPO and ETA receptor antagonists during hypoxia-associated neurodegenerative disorders of the central nervous system (CNS) can be suggested.

Gap junction channels and hemichannels in the CNS: regulation by signaling molecules

Coordinated interaction among cells is critical to develop the extremely complex and dynamic tasks performed by the central nervous system (CNS). Cell synchronization is in part mediated by connexins and pannexins; two different protein families that form gap junction channels and hemichannels. Whereas gap junction channels connect the cytoplasm of contacting cells and coordinate electric and metabolic activities, hemichannels communicate intra- and extra-cellular compartments and serve as diffusional pathways for ions and small molecules. Cells in the CNS depend on paracrine/autocrine communication via several extracellular signaling molecules, such as, cytokines, growth factors, transmitters and free radical species to sense changes in microenvironment as well as to adapt to them. These signaling molecules modulate crucial processes of the CNS, including, cellular migration and differentiation, synaptic transmission and plasticity, glial activation, cell viability and microvascular blood flow. Gap junction channels and hemichannels are affected by different signaling transduction pathways triggered by these paracrine/autocrine signaling molecules. Most of the modulatory effects induced by these signaling molecules are specific to the cell type and the connexin and pannexin subtype expressed in different brain areas. In this review, we summarized and discussed most of the relevant and recently published information on the effects of signaling molecules on connexin or pannexin based channels and their possible relevance in CNS physiology and pathology. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Endothelin B receptor agonist, IRL-1620, provides long-term neuroprotection in cerebral ischemia in rats

We have earlier shown that stimulation of endothelin B receptors by IRL-1620 provides significant neuroprotection at 24h following cerebral ischemia. However, the effect of IRL-1620 is not known in the subacute phase of cerebral ischemia, where development of cerebral edema further contributes towards brain damage. This study was designed to determine the effect of IRL-1620 on neurological functions, infarct volume, oxidative stress, and endothelin receptors following permanent middle cerebral artery occlusion for 7 days. Rats received three intravenous injections of either vehicle or IRL-1620 [Suc-[Glu9,Ala11,15]-Endothelin-1(8-12)] at 2, 4, and 6h post occlusion. Treatment with IRL-1620 reduced infarct volume (54.06 ± 14.12 mm(3) vs. 177.06 ± 13.21 mm(3)), prevented cerebral edema and significantly improved all neurological and motor function parameters when compared to the vehicle-treated group. Vehicle-treated middle cerebral artery occluded rats demonstrated high levels of malondialdehyde and low levels of reduced glutathione and superoxide dismutase; these effects were reversed in IRL-1620 treated rats. No change in expression of endothelin A receptor was observed 7 days after induction of cerebral ischemia in vehicle or IRL-1620 treated rats. Rats receiving IRL-1620 demonstrated an upregulation of endothelin B receptor only in the infarcted hemisphere 7 days following occlusion. All effects of IRL-1620 were blocked by endothelin B receptor antagonist, BQ788. Results of the present study demonstrate that IRL-1620, administered on day 1, provides significant neuroprotection till 7 days after the induction of cerebral ischemia in rats. Selective endothelin B receptor activation may prove to be a novel therapeutic target in the treatment of cerebral ischemia.

Endothelin-mediated calcium responses in supraoptic nucleus astrocytes influence magnocellular neurosecretory firing activity

In addition to their peripheral vasoactive effects, accumulating evidence supports an important role for endothelins (ETs) in the regulation of the hypothalamic magnocellular neurosecretory system, which produces and releases the neurohormones vasopressin (VP) and oxytocin (OT). Still, the precise cellular substrates, loci and mechanisms underlying the actions of ETs on the magnocellular system are poorly understood. In the present study, we combined patch-clamp electrophysiology, confocal Ca(2+) imaging and immunohistochemistry to study the actions of ETs on supraoptic nucleus (SON) magnocellular neurosecretory neurones and astrocytes. Our studies show that ET-1 evoked rises in [Ca(2+) ](i) levels in SON astrocytes (but not neurones), an effect largely mediated by the activation of ET(B) receptors and mobilisation of thapsigargin-sensitive Ca(2+) stores. The presence of ET(B) receptors in SON astrocytes was also verified immunohistochemically. ET(B) receptor activation either increased (75%) or decreased (25%) SON firing activity, both in VP and putative OT neurones, and these effects were prevented when slices were preincubated in glutamate receptor blockers or nitric oxide synthase blockers, respectively. Moreover, ET(B) -mediated effects in SON neurones were also prevented by a gliotoxin compound, and when changes in [Ca(2+) ](i) were prevented with bath-applied BAPTA-AM or thapsigargin. Conversely, intracellular Ca(2+) chelation in the recorded SON neurones failed to block ET(B) -mediated effects. In summary, our results indicate that ET(B) receptor activation in SON astrocytes induces the mobilisation of [Ca(2+) ](i) , likely resulting in the activation of glutamate and nitric oxide signalling pathways, evoking in turn excitatory and inhibitory SON neuronal responses, respectively. Taken together, our study supports an important role for astrocytes in mediating the actions of ETs on the magnocellular neurosecretory system.

6-Mercaptopurine reverses experimental vasospasm and alleviates the production of endothelins in NO-independent mechanism-a laboratory study

INTRODUCTION

Increased endothelin-1 (ET-1) production and diminished nitric oxide synthase (NOS) bioavailability has been observed in aneurysmal subarachnoid hemorrhage (SAH). The authors previously found that 6-mercaptopurine (6-mp) is effective in preventing and reversing arterial narrowing in a rodent SAH model. This present study is of interest to examine the effect of 6-mp on ET-1/endothelial nitric oxide synthase (eNOS) in this animal model.

METHODS

A rodent double hemorrhage SAH model was employed. Animals were randomly assigned to six groups (sham, SAH only, vehicle, 0.5, 1.0 and 2 mg kg(-1) day(-1) 6-mp treatment). Monoclonal CD45 immunostaining was utilized to evaluate monocytes and microglia. The level of pro-inflammatory cytokines, such as IL-1, IL-6 and TNF-α(RT-PCR), and ET-1 (ELISA) was measured. The basilar arteries (BAs) were harvested and sliced, and their cross-sectional areas were determined. Radiolabeled NOS assay kit was applied to detect eNOS.

RESULTS

Morphologically, convolution of internal elastic lamina, endothelial cells distortion, and necrotic smooth muscle were prevalently present in the basilar artery of SAH groups, which was absent in the 1 and 2 mg kg(-1) day(-1) 6-mp plus SAH group or the healthy controls. Significant vasospasm was noted in the vehicle group (lumen patency, 54.6%, p ≤ 0.01 compared with the sham group), but it was less prominent in the 2 mg kg(-1) day(-1) 6-mp treatment group (lumen patency, 87.6%, p < 0.05). In addition, administration with 2 mg kg(-1) day(-1) 6-mp reduced cytokine levels by 11%, 47%, and 34% for IL-1, IL-6, and TNF-α, respectively, and increased ET-1 levels were found in all the animals subject to SAH (SAH only, SAH plus vehicle, SAH plus 0.5 and 1.0 mg kg(-1) day(-1) 6-mp) except in the 2 mg kg(-1) day(-1) 6-mp SAH group, when compared with the healthy controls (no SAH). Meanwhile, treatment with 6-mp did not induce the levels of expressed eNOS in BAs in the 6-mp groups (0.5, 1.0, and 2 mg kg(-1) day(-1) 6-mp plus SAH) when compared with that in the SAH groups (p > 0.1).

CONCLUSION

In summary, treatment with 6-mp decreased the release of pro-inflammatory cytokines and diminished experimental vasospasm. This study offered first evidence that 6-mp dose-dependently reduces the level of ET-1 in a NO-independent mechanism, which corresponds to its antivasospastic effect in the condition of chronic vasospasm.

Opposing actions of endothelin-1 on glutamatergic transmission onto vasopressin and oxytocin neurons in the supraoptic nucleus

Endothelin (ET-1) given centrally has many reported actions on hormonal and autonomic outputs from the CNS. However, it is unclear whether these effects are due to local ischemia via its vasoconstrictor properties or to a direct neuromodulatory action. ET-1 stimulates the release of oxytocin (OT) and vasopressin (VP) from supraoptic magnocellular (MNCs) neurons in vivo; therefore, we asked whether ET-1 modulates the excitatory inputs onto MNCs that are critical in sculpting the activity of these neurons. To investigate whether ET-1 modulates excitatory synaptic transmission, we obtained whole-cell recordings and analyzed quantal glutamate release onto MNCs in the supraoptic nucleus (SON). Neurons identified as VP-containing neurosecretory cells displayed a decrease in quantal frequency in response to ET-1 (10-100 pm). This decrease was mediated by ET(A) receptor activation and production of a retrograde messenger that targets presynaptic cannabinoid-1 receptors. In contrast, neurons identified as OT-containing MNCs displayed a transient increase in quantal glutamate release in response to ET-1 application via ET(B) receptor activation. Application of TTX to block action potential-dependent glutamate release inhibited the excitatory action of ET-1 in OT neurons. There were no changes in quantal amplitude in either MNC type, suggesting that the effects of ET-1 were via presynaptic mechanisms. A gliotransmitter does not appear to be involved as ET-1 failed to elevate astrocytic calcium in the SON. Our results demonstrate that ET-1 differentially modulates glutamate release onto VP- versus OT-containing MNCs, thus implicating it in the selective regulation of neuroendocrine output from the SON.

Differential influence of non-synaptic mechanisms in two in vitro models of epileptic field bursts

Non-synaptic interactions are known to promote epileptiform activity through mechanisms that have primarily been studied in one particular in vitro model (low Ca(2+) model). Here we characterize another non-synaptic model, where ictal-like field bursts are induced in the CA1 area of rat hippocampal slices by exposure to Cs(+) (4-5mM) together with blockers of fast chemical synaptic transmission, and compare it with the low Ca(2+) model. The Cs-induced field bursts were blocked by 1 microM tetrodotoxin, but persisted in the presence of 200 microM Cd(2+) or 300 microM Ni(2+). Hyperosmotic condition (addition of 30 mM sucrose), reduced burst amplitude, but, unlike field bursts induced by 0mM Ca(2+)/5.25 mM K(+), sucrose had no effect on frequency or duration. Intracellular alkalinization-acidification sequence induced by NH(4)Cl potentiated and blocked, respectively, the field bursts. Octanol (100-250 microM) blocked all activity in most experiments. A quantitative comparison of three gap junction antagonists (carbenoxolone (100 microM), quinidine (100-250 microM), and endothelin-3 (1-2 microM)) indicated that gap junction communication is implicated in both models. However, endothelin-3 had selective effect on the low Ca(2+)-induced field burst. The data suggest that extracellular space-dependent processes, including field effects, significantly contribute to ongoing field burst activity, whereas initiation of a field burst can occur with or without the aid of such interactions, depending on the level of neuronal excitability. Gap junctions seem to have a general role in initiating field bursts. However, the contribution to this effect from neuronal versus glial connexin types differs in the two epileptic models studied.

Differential endothelin receptor expression and function in rat myometrial cells and leiomyoma ELT3 cells

Uterine leiomyoma are the most common benign tumors of the myometrium. We previously identified endothelin (ET)-1 as a proliferative and antiapoptotic factor in Eker rat-derived leiomyoma (ELT3) cells. A major role of ETB receptor in the prosurvival effect was revealed. Here we investigated, in ELT3 and myometrial cells, the respective contribution of ETA and ETB in the proliferative effect of ET-1. In myometrial cells, binding experiments show that ETA is almost exclusively expressed and stimulates phospholipase C (PLC) activity and ERK1/2 phosphorylation and proliferation. In ELT3 cells, ETB is expressed at about the same level as ETA, and the two receptors are differently coupled to Gi protein. The ETB agonist, sarafotoxin S6c, stimulates PLC activity 60% less than ET-1 but is as potent as ET-1 to increase ERK1/2 phosphorylation and induce proliferation. However, the ability of ETA to activate ERK1/2 is observed after ETB desensitization. Although ETA and ETB antagonists partially reduce ET-1 stimulated PLC activity, they are without effect on ET-1-induced ERK1/2 phosphorylation and proliferation. Only the simultaneous use of ETA and ETB antagonists reduces ET-1-triggered ERK1/2 activation. These unconventional properties of ETRs may reveal the existence of functional ETA-ETB heterodimers. Finally, treatment of ELT3 cells with ETB but not ETA-directed small interfering RNA reduces the proliferative effect of ET-1. All the data obtained in ELT3 cells strengthen the relation between ETB overexpression, which decreases the ETA to ETB ratio, and the ability of leiomyoma cells to highly proliferate and resist apoptosis.

Electrical coupling between hippocampal astrocytes in rat brain slices

Gap junctions in astrocytes play a crucial role in intercellular communication by supporting both biochemical and electrical coupling between adjacent cells. Despite the critical role of electrical coupling in the network organization of these glial cells, the electrophysiological properties of gap junctions have been characterized in cultures while no direct evidence has been sought in situ. In the present study, gap-junctional currents were investigated using simultaneous dual whole-cell patch-clamp recordings between astrocytes from rat hippocampal slices. Bidirectional electrotonic coupling was observed in 82% of the cell pairs with an average coupling coefficient of 5.1%. Double patch-clamp analysis indicated that junctional currents were independent of the transjunctional voltage over a range from -100 to +110 mV. Interestingly, astrocytic electrical coupling displayed weak low-pass filtering properties compared to neuronal electrical synapses. Finally, during uncoupling processes triggered by either the gap-junction inhibitor carbenoxolone or endothelin-1, an increase in the input resistance in the injected cell paralleled the decrease in the coupling coefficient. Altogether, these results demonstrate that hippocampal astrocytes are electrically coupled through gap-junction channels characterized by properties that are distinct from those of electrical synapses between neurons. In addition, gap-junctional communication is efficiently regulated by endogenous compounds. This is taken to represent a mode of communication that may have important implications for the functional role of astrocyte networks in situ.

Protective role of reactive astrocytes in brain ischemia

Reactive astrocytes are thought to protect the penumbra during brain ischemia, but direct evidence has been lacking due to the absence of suitable experimental models. Previously, we generated mice deficient in two intermediate filament (IF) proteins, glial fibrillary acidic protein (GFAP) and vimentin, whose upregulation is the hallmark of reactive astrocytes. GFAP(-/-)Vim(-/-) mice exhibit attenuated posttraumatic reactive gliosis, improved integration of neural grafts, and posttraumatic regeneration. Seven days after middle cerebral artery (MCA) transection, infarct volume was 210 to 350% higher in GFAP(-/-)Vim(-/-) than in wild-type (WT) mice; GFAP(-/-), Vim(-/-) and WT mice had the same infarct volume. Endothelin B receptor (ET(B)R) immunoreactivity was strong on cultured astrocytes and reactive astrocytes around infarct in WT mice but undetectable in GFAP(-/-)Vim(-/-) astrocytes. In WT astrocytes, ET(B)R colocalized extensively with bundles of IFs. GFAP(-/-)Vim(-/-) astrocytes showed attenuated endothelin-3-induced blockage of gap junctions. Total and glutamate transporter-1 (GLT-1)-mediated glutamate transport was lower in GFAP(-/-)Vim(-/-) than in WT mice. DNA array analysis and quantitative real-time PCR showed downregulation of plasminogen activator inhibitor-1 (PAI-1), an inhibitor of tissue plasminogen activator. Thus, reactive astrocytes have a protective role in brain ischemia, and the absence of astrocyte IFs is linked to changes in glutamate transport, ET(B)R-mediated control of gap junctions, and PAI-1 expression.

Na(+)-Ca(2+) cosignaling in the stimulation of the glucose transporter GLUT1 in cultured astrocytes

Glutamate triggers an acute stimulation of the glucose transporter GLUT1 in cultured astrocytes, a phenomenon thought to facilitate energy delivery to active areas in the brain. Here we have explored the cell signaling mechanisms involved in this response. Half-stimulation of GLUT1 occurred at low micromolar glutamate, thus within the physiological range estimated in brain interstitium. The effect was mimicked by D-aspartate and inhibited by L-threo-beta-benzyloxyaspartate or Na(+) replacement with NMDG(+), showing the participation of the Na(+)-glutamate co-transporter. AMPA and the mGLURI agonist DHPG had no effect. The stimulation of GLUT1 was fully inhibited by ouabain, but independent activation of the Na(+)/K(+) ATPase pump with gramicidin did not affect glucose transport. Simultaneous with the Na(+) rise, glutamate and D-aspartate triggered a Ca(2+)signal, whose inhibition with BAPTA prevented the stimulation of GLUT1. However, an isolated Ca(2+) signal, triggered with endothelin 1, ATP or DHPG, did not affect glucose transport. The stimulation of GLUT1 could finally be mimicked by simultaneous induction of Na(+) and Ca(2+) signals. The requirement for both cations in the stimulation of the astrocytic glucose transporter, may help to explain how glucose metabolism in the brain is strongly activated by glutamate, but not by GABA or by inter-astrocytic signaling.

Endothelin-1 (ET-1) causes death of retinal neurons through activation of nitric oxide synthase (NOS) and production of superoxide anion

Endothelin-1 (ET-1) is the most potent and long-acting vasoconstricting peptide presently known. In addition to its vascular effects, endothelin signaling pathway exists in the central nervous system (CNS), which is deeply related to neuronal degeneration. In the present study, we evaluated the effect of ET-1 on death of retinal neurons consisting mainly of amacrine cells, and its interaction with nitric oxide synthase (NOS) and superoxide production. Cultured retinal neurons from fetal rats were exposed to various doses of ET-1 (0.1, 1.0, 10 and 100nM). Neuronal toxicity of ET-1 was assessed by trypan blue exclusion, Hoechst 33,258 staining and TUNEL assay at different times. Intracellular levels of nitric oxide (NO), superoxide and peroxynitrite were determined semiquantitatively by DAF2-DA, hydroethidine and dihydrorhodamine-123, respectively. The effects of adding SOD (100U/ml) and L-NAME with ET-1 on these changes were evaluated. In addition, the receptor mechanisms involved in these reactions were determined by BQ-123 and BQ-788, receptor antagonists for ET A and ET B receptors, respectively. Exposure of cultured retinal neurons to ET-1 reduced the percentage of living cells in a dose- and time-dependent way, and the percentage of living cells was significantly increased by addition of SOD and L-NAME. Fluorometric analyses revealed that ET-1 increased the intracellular NO level in a dose- and time-dependent manner. The intracellular superoxide and peroxynitrite levels were also significantly increased 24h after incubation with 100nM of ET-1, and this elevation was suppressed by SOD and L-NAME. These ET-1-induced alterations were significantly suppressed when both BQ-123 and BQ-788 were added simultaneously with ET-1 to the medium. These results indicate that the neuronal death caused by ET-1 is most likely mediated by the activation of NOS in association with the formation of superoxides and peroxynitrite.

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.

Subepithelial fibroblasts in intestinal villi: roles in intercellular communication

Ingestion of food and water induces chemical and mechanical signals that trigger peristaltic reflexes in the gut. Intestinal villi are motile, equipped with chemosensors and mechanosensors, and transduce signaling to sensory neurons, but the exact mechanisms have not yet been elucidated. Subepithelial fibroblasts located under the villous epithelium form contractile cellular networks via gap junctions. The networks ensheathe lamina propria and are in close contact with epithelium, neural and capillary networks, smooth muscles, and immune cells. Unique characteristics of subepithelial fibroblasts have been revealed by primary cultures isolated from rat duodenal villi. They include rapid reversal changes in cell shape by cAMP reagents and endothelins, cell shape-dependent mechanosensitivity that induces ATP release as a paracrine mediator, contractile ability, and expression of various receptors for vasoactive and neuroactive substances. Herein, we review these characteristics that play a key role in the villi. They serve as a barrier/sieve, flexible mechanical frame, mechanosensor, and signal transduction machinery in the intestinal villi, which are regulated locally and dynamically by rapid cell shape conversion.

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.

Anti-inflammatory effect of augmented nitric oxide production in chronic lung infection

Chronic infection of the lungs with Pseudomonas aeruginosa complicates many long-term lung diseases including cystic fibrosis, bronchiectasis, chronic obstructive lung disease, and mechanical ventilation. In acute inflammatory lung diseases, increased nitric oxide synthase (NOS-2) expression leads to excess nitric oxide (NO) production, resulting in the production of reactive nitrogen intermediates, which contribute to tissue damage. In contrast, the contribution of NO to pulmonary damage in chronic Pseudomonas infection of the lung has not been directly examined and is unclear. Although NOS-2 expression is increased in this condition, NO production is not abnormally elevated. It was hypothesized that chronic infection of the airways does not cause increased NO production but, in contrast, leads to inappropriately low NO concentrations that are pro-inflammatory. A rodent model of chronic airway infection was used to examine the effects on lung damage of augmenting or inhibiting NO production after airway infection with P. aeruginosa was well established. Three days post-infection, L-arginine, which augments NO production, or L-NAME, an inhibitor of NO production, was administered in drinking water. Lung damage was assessed 12 days later. L-arginine treatment reduced tissue damage, inhibited neutrophil recruitment, and reduced the pro-inflammatory cytokine interleukin (IL)-1beta. Treatment with L-NAME caused loss of alveolar walls, greater vascular damage, and increased levels of the pro-inflammatory cytokine IL-6. Thus, in chronic airway infection, inhibition of NO production worsened lung damage, whereas augmenting NO ameliorated this damage. This is the first demonstration that augmenting endogenous NO production in chronic infective lung disease caused by P. aeruginosa is anti-inflammatory. Given that infection with this organism complicates many chronic lung diseases, most notoriously cystic fibrosis, these findings have important clinical implications.

Exploiting the vitamin B12 pathway to enhance oral drug delivery via polymeric micelles

Vitamin B12 (VB12)-modified dextran-g-polyethyleneoxide cetyl ether (DEX-g-PEO-C16) was synthesized by linking VB12 residues to a DEX-g-PEO-C16 copolymer via a 2,2'-(ethylenedioxy)bis(ethylamine) spacer. The level of VB12 substitution on the DEX-g-PEO-C16 copolymer reached 1.68% (w/w). In aqueous solution, DEX-based copolymers form micelles that can entrap within their hydrophobic core up to 8.5% w/w of cyclosporin A (CsA), a poorly water soluble immunosuppressant. The permeability of Caco-2 cell membranes to CsA incorporated in VB12 modified and unmodified polymeric micelles was monitored in the presence and absence of intrinsic factor (IF). The apical (AP) to basolateral (BL) permeation of CsA through Caco-2 cell monolayers after 24 h of transport was significantly higher (1.8 and 2.3 times in absence and presence of IF, respectively) in the case of CsA loaded in VB12-modified polymeric micelles, compared to CsA in unmodified micelles. The results point to possible improvement in the application of polysaccharide-based polymeric micelles as targeted polymeric drug carriers for the oral delivery of poorly water soluble drugs.

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.

Gap junction blockage limits intercellular spreading of astrocytic apoptosis induced by metabolic depression

Astrocytes are highly coupled by gap junction channels, which allow transfer of intracellular signalling molecules and metabolites between connected cells. Astrocytic gap junctions remain open during ischemic conditions as previously demonstrated in vitro and in situ. In this study, we investigated the effect of gap junction blockage on iodoacetate-induced ATP depression and cell death progression in astrocytes in primary rat hippocampal cultures. We demonstrated that blockage of gap junctions during iodoacetate-induced inhibition of the glycolysis induced an earlier onset of the ATP depression. Moreover, initiation of apoptotic processes, demonstrated by binding of Annexin V, was critically dependent on the ATP levels. The apoptotic event was also shown to spread and involve neighbouring cells, a process that was inhibited by blockage of gap junction communication. Chelating intracellular calcium using BAPTA-AM decelerated the iodoacetate-induced ATP depression. The chelation also decelerated the spreading of apoptotic processes. Inhibition of caspases did not alter the expansion of cell groups being Annexin V positive. However, the proportion of Annexin V positive cells also being propidium iodide positive was increased after caspase inhibition. The results show that inhibition of gap junctions during cellular metabolic depression interferes with the metabolic status and cell death progression in astrocytes.

Endothelin-1 overexpression leads to further water accumulation and brain edema after middle cerebral artery occlusion via aquaporin 4 expression in astrocytic end-feet

Stroke patients have increased levels of endothelin-1 (ET-1), a strong vasoconstrictor, in their plasma or cerebrospinal fluid. Previously, we showed high level of ET-1 mRNA expression in astrocytes after hypoxia/ischemia. It is unclear whether the contribution of ET-1 induction in astrocytes is protective or destructive in cerebral ischemia. Here, we generated a transgenic mouse model that overexpress ET-1 in astrocytes (GET-1) using the glial fibrillary acidic protein promoter to examine the role of astrocytic ET-1 in ischemic stroke by challenging these mice with transient middle cerebral artery occlusion (MCAO). Under normal condition, GET-1 mice showed no abnormality in brain morphology, cerebrovasculature, absolute cerebral blood flow, blood-brain barrier (BBB) integrity, and mean arterial blood pressure. Yet, GET-1 mice subjected to transient MCAO showed more severe neurologic deficits and increased infarct, which were partially normalized by administration of ABT-627 (ET(A) antagonist) 5 mins after MCAO. In addition, GET-1 brains exhibited more Evans blue extravasation and showed decreased endothelial occludin expression after MCAO, correlating with higher brain water content and increased cerebral edema. Aquaporin 4 expression was also more pronounced in astrocytic end-feet on blood vessels in GET-1 ipsilateral brains. Our current data suggest that astrocytic ET-1 has deleterious effects on water homeostasis, cerebral edema and BBB integrity, which contribute to more severe ischemic brain injury.

Inhibition of Rho-kinase attenuates hypoxia-induced angiogenesis in the pulmonary circulation

Pulmonary hypertension (PH) is a common complication of chronic hypoxic lung diseases, which increase morbidity and mortality. Hypoxic PH has previously been attributed to structural changes in the pulmonary vasculature including narrowing of the vascular lumen and loss of vessels, which produce a fixed increase in resistance. Using quantitative stereology, we now show that chronic hypoxia caused PH and remodeling of the blood vessel walls in rats but that this remodeling did not lead to structural narrowing of the vascular lumen. Sustained inhibition of the RhoA/Rho-kinase pathway throughout the period of hypoxic exposure attenuated PH and prevented remodeling in intra-acinar vessels without enlarging the structurally determined lumen diameter. In chronically hypoxic lungs, acute Rho kinase inhibition markedly decreased PVR but did not alter the alveolar to arterial oxygen gap. In addition to increased vascular resistance, chronic hypoxia induced Rho kinase-dependent capillary angiogenesis. Thus, hypoxic PH was not caused by fixed structural changes in the vasculature but by sustained vasoconstriction, which was largely Rho kinase dependent. Importantly, this vasoconstriction had no role in ventilation-perfusion matching and optimization of gas exchange. Rho kinase also mediated hypoxia-induced capillary angiogenesis, a previously unrecognized but potentially important adaptive response.

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.

Localization of the endothelin system in human diffuse astrocytomas

BACKGROUND

Endothelin-1 (ET-1), a vasoconstrictor and mitogen, has recently been implicated in the pathogenesis of human glioblastoma, neuroblastoma, and meningioma. ET-1, formed by proteolysis of the propeptide big ET-1 by endothelin-converting enzyme-1 (ECE-1), mediates its cellular actions through ETA and ETB receptors. Because only immunoreactive ET-1 has been observed within human astrocytic tumor cells, the authors investigated the localization of the entire ET-1 system (ET-1 mRNA, ET-1, ECE-1, ETA and ETB receptors) in surgical samples of human diffuse astrocytomas WHO Grade II (n = 6).

METHODS

ET-1 mRNA expression was elucidated by in situ reverse transcriptase polymerase chain reaction (RT-PCR) using synthetic primers. Polyclonal antibodies were used to localize ET-1, ECE-1, ETA and ETB receptors by immunocytochemistry.

RESULTS

All ET components were detected in the six tumor samples. Intense (3+) cytoplasmic ET-1 mRNA labeling was observed in more than 75% of cells in all 6 astrocytomas. Up to 75% of tumor cells displayed intense ET-1 and ECE-1 immunolabeling distributed throughout their cytoplasm. Immunoreactive ETA and ETB receptors, observed in 25% to 75% of astrocytic tumor cells, were of moderate intensity. In addition, all components of the ET system were seen within endothelial cells of tumor blood vessels.

CONCLUSIONS

The presence of ET-1 mRNA, ECE-1, and ET-1 within tumor astrocytes suggests local ET synthesis and processing. The mitogenic and antiapoptotic properties of ET-1, as well as the vasodilatory signaling of ETB receptors, may promote tumorigenesis.

Ethanol acutely decreases astroglial gap junction permeability in primary cultures from defined brain regions

The acute effect of hyperosmotic ethanol on gap junction permeability was examined in astroglial cells in primary culture from five different brain regions. Gap junction permeability was analyzed by measuring dye spreading from cell to cell with the low molecular weight dye Lucifer Yellow. Ethanol concentrations 25-300 mM significantly decreased dye spreading in cultures from the cerebral cortex in a dose-dependent but time-independent manner for up to 60 min. Besides cerebral cortex, exposure to 150 mM ethanol decreased dye spreading in astroglial cultures from the hippocampus and from the brain stem, while cultures from the olfactory bulb and from the hypothalamus were not significantly affected. The ethanol-induced decrease in dye spreading in cultures from the cerebral cortex was not mediated through changes in cell volume, osmolarity, protein kinase C (PKC) phosphorylation, intracellular pH, or intracellular calcium concentration ([Ca(2+)](i)). The decrease in dye spreading was abolished upon incubation in sodium-reduced buffer, and after blockage of the Na(+)/K(+)/2Cl(-) cotransporter with furosemide. The results presented here indicate that ethanol-mediated decrease in dye spreading is directly or indirectly dependent on sodium.

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.

Characteristics of cultured subepithelial fibroblasts in the rat small intestine. II. Localization and functional analysis of endothelin receptors and cell-shape-independent gap junction permeability

Subepithelial fibroblasts form a cellular network with gap junctions under the epithelium of the gastrointestinal tract. Previously, we have reported their unique characteristics, such as reversible rapid cell-shape changes from a flat to a stellate configuration induced by dBcAMP and endothelins (ETs), and Ca2+ responses to, for example, ETs, ATP, and substance-P. We have now investigated the subtypes of ET receptors both in the rat small intestine and in primary cultured subepithelial fibroblasts isolated from rat duodenal villi. Their properties were compared between wild-type and endothelin-B-receptor-mutant sl/sl rats. Light- and electron-microscopic immunohistochemistry showed intense ETA immunoreactivity in the subepithelial fibroblasts from the small intestine and colon of both wild-type and sl/sl rats. In culture, immunocytochemistry, reverse transcription/polymerase chain reaction analysis, Ca2+ response measurements, and cell-shape change analysis indicated functional ETA and ETB receptors in the wild-type cells, but only ETA in the sl/sl cells. However, wild-type cells were more sensitive to ET-1 than to ET-3 by about one order of magnitude. ETA seemed to be dominant both in vivo and in vitro. The relationship between cell-shape change and gap junction permeability was examined by fluorescence recovery after photobleaching; the gap junctions were usually open but were blocked by carbenoxolone. Permeability did not change significantly with cell-shape change. This network of differentiated subepithelial fibroblasts may maintain intercellular communication via gap junctions to transduce signals evoked in the local network to the whole network. The cell-shape change of the cells through ETA activation may play an important role as a barrier and for intercellular signaling in the intestinal villi.

ATP-induced inhibition of gap junctional communication is enhanced by interleukin-1 beta treatment in cultured astrocytes

Nucleotides are signaling molecules involved in variety of interactions between neurons, between glial cells as well as between neurons and glial cells. In addition, ATP and other nucleotides are massively released following brain insults, including inflammation, and may thereby be involved in mechanisms of cerebral injury. Recent concepts have shown that in astrocytes intercellular communication through gap junctions may play an important role in neuroprotection. Therefore, we have studied the effects of nucleotides on gap junction communication in astrocytes. Based on measurement of intercellular dye coupling and recording of junctional currents, the present study shows that ATP (10-100 microM) induces a rapid and a concentration-dependent inhibition of gap junction communication in cultured cortical astrocytes from newborn mice. Effects of agonists and antagonists of purinergic receptors indicate that the inhibition of gap junctional communication by ATP mainly involves the stimulation of metabotropic purinergic 1 (P2Y(1)) receptors. Pretreatment with the pro-inflammatory cytokine interleukin-1beta (10 ng/ml, 24 h), which has no effect by itself on gap junctional communication, increases the inhibitory effect of ATP and astrocytes become sensitive to uridine 5'-triphosphate (UTP). As indicated by the enhanced expression of P2Y(2) receptor mRNA, P2Y(2) receptors are responsible for the increased responses evoked by ATP and UTP in interleukin-1beta-treated cells. In addition, the effect of endothelin-1, a well-known inhibitor of gap junctional communication in astrocytes was also exacerbated following interleukin-1beta treatment. We conclude that ATP decreases intercellular communication through gap junctions in astrocytes and that the increased sensitivity of gap junction channels to nucleotides and endothelin-1 is a characteristic feature of astrocytes exposed to pro-inflammatory treatments.

Ligand-dependent Differences in the Internalization of Endothelin A and Endothelin B Receptor Heterodimers

Endothelin-1 (ET-1) is a potent vasoactive peptide that acts on endothelin A (ET(A)) and endothelin B (ET(B)) receptors. Although both receptor subtypes are co-expressed in numerous cells, little is known about their ability to form heterodimers. Here we show that both receptors were co-immunoprecipitated with an ET(B)-specific antibody using extracts from HEK293 cells stably co-expressing a fusion protein consisting of a myc-tagged ET(A) receptor and CFP (ET(A)myc.CFP) and a fusion protein consisting of an ET(B) receptor and YFP (ET(B).YFP). Co-immunoprecipitation was also observed with extracts from HEK293 cells transiently co-expressing FLAG-tagged ET(B) and myc-tagged ET(A) receptors, thereby excluding that heterodimerization is mediated by the CFP/YFP moieties. Heterodimerization was further confirmed in fluorescence resonance energy transfer (FRET) analysis of HEK293 cells transiently co-expressing ET(A)myc.CFP and ET(B).YFP receptors. FRET efficiencies were between 12 and 18% in untreated and antagonist- or ET-1-treated cells, indicating constitutive heterodimerization. Prolonged stimulation (30 min) with the ET(B) receptor-selective agonist BQ3020 decreased FRET efficiency by 50%. This decrease was not observed when internalization was inhibited by co-expression of dominant-negative K44A.dynamin I or incubation with 450 mm sucrose. Enzyme-linked immunosorbent assay and laser scanning microscopy of cell clones stably co-expressing ET(A)myc.CFP/ET(B)flag.YFP receptors revealed a slower sequestration of the ET(B)flag.YFP receptors upon stimulation with ET-1 than with BQ3020. No difference in ET-1 or BQ3020-mediated sequestration was observed with cell clones expressing ET(B)flag.YFP receptors alone. The data suggest that ET(A) and ET(B) receptors form constitutive heterodimers, which show a slower sequestration upon stimulation with ET-1 than with BQ3020. Heterodimer dissociation along the endocytic pathway only occurs upon ET(B)-selective stimulation.

p38/SAPK2 controls gap junction closure in astrocytes

Astrocyte gap junction communication (GJC) is thought to contribute to death signal propagation following central nervous system injury, noteworthy in some ischemia/anoxia models. The inhibition of p38/stress-activated protein kinase 2 (p38/SAPK2) by a pyrimidyl imidazole derivative has been reported to reduce the extent of the lesion area after cerebral ischemia. Therefore, interleukin-1beta (IL-1beta), which contributes to stroke-induced brain injury and activates p38/SAPK2, and hyperosmolarity induced by sorbitol, a potent stimulus of p38/SAPK2 in non-neuronal cells, were used to investigate a possible involvement of p38/SAPK2 in GJC modulation in mouse cultured astrocytes. Both stimuli inhibited dye coupling within minutes. The IL-1beta effect was transient, while that of sorbitol lasted up to 90 min. Both stimuli induced a rapid p38/SAPK2 activation, the kinetic of which matched that of induction of dye coupling inhibition. Immunocytochemical studies showed that IL-1beta and sorbitol induced a p38/SAPK2 translocation from the nucleus to the cytoplasm. The pharmacological agent SB203580 specifically blocked p38/SAPK2 activation, cytoplasmic translocation and reversed the IL-1beta and sorbitol-induced inhibition of GJC. Further characterization of the p38/SAPK2 mode of action on GJC, performed with sorbitol, revealed an increased phosphorylation of protein kinase C (PKC) substrates abolished by both PKC inhibitors and SB203580. Expression and serine phosphorylation of connexin 43, the main component of astrocyte gap junctions, were unchanged, suggesting the existence of additional intracellular signaling mechanisms modulating the channel gating. Altogether, these results demonstrate that p38/SAPK2 is a central mediator of IL-1beta and sorbitol inhibitory actions on GJC and establish PKC among the distal effectors of p38/SAPK2.

Endothelins regulate astrocyte gap junctions in rat hippocampal slices

Gap junctional communication (GJC) is a typical feature of astrocytes proposed to contribute to the role played by these glial cells in brain physiology and pathology. In acutely isolated hippocampal slices from rat (P11-P19), intercellular diffusion of biocytin through gap junction channels was shown to occur between hundreds of cells immuno-positive for astrocytic markers studied in the CA1/CA2 region. Single-cell RT-PCR demonstrated astrocytic mRNA expression of several connexin (Cx) subtypes, the molecular constituent of gap junction channels, whereas immunoblotting confirmed that Cx43 and Cx30 are the main gap junction proteins in hippocampal astrocytes. In the brain, astrocytes represent a major target for endothelins (Ets), a vasoactive family of peptides. Our results demonstrate that Ets decrease the expression of phosphorylated Cx43 forms and are potent inhibitors of GJC. The Et-induced effects were investigated using specific Et receptor agonists and antagonists, including Bosentan (Tracleer trade mark ), an EtA/B receptor antagonist, and using hippocampal slices and cultures from EtB-receptor-deficient rats. Interestingly, the pharmacological profile of Ets effects did not follow the classical profile established in cardiovascular systems. The present study therefore identifies Ets as potent endogenous inhibitory regulators of astrocyte networks. As such, the action of these peptides on astrocyte GJC might be involved in the contribution of astrocytes to neuroprotective processes and have a therapeutic potential in neuropathological situations.

ET-1 induces cortical spreading depression via activation of the ETA receptor/phospholipase C pathway in vivo

Recently, it has been shown that brain topical superfusion of endothelin (ET)-1 at concentrations around 100 nM induces repetitive cortical spreading depressions (CSDs) in vivo. It has remained unclear whether this effect of ET-1 is related to a primary neuronal/astroglial effect, such as an increase in neuronal excitability or induction of interastroglial calcium waves, or a penumbra-like condition after vasoconstriction. In vitro, ET-1 regulates interastroglial communication via combined activation of ET(A) and ET(B) receptors, whereas it induces vasoconstriction via single activation of ET(A) receptors. We have determined the ET receptor profile and intracellular signaling pathway of ET-1-induced CSDs in vivo. In contrast to the ET(B) receptor antagonist BQ-788 and concentration dependently, the ET(A) receptor antagonist BQ-123 completely blocked the occurrence of ET-1-induced CSDs. The ET(B) receptor antagonist did not increase the efficacy of the ET(A) receptor antagonist. Direct stimulation of ET(B) receptors with the selective ET(B) agonist BQ-3020 did not trigger CSDs. The phospholipase C (PLC) antagonist U-73122 inhibited CSD occurrence in contrast to the protein kinase C inhibitor Gö-6983. Our findings indicate that ET-1 induces CSDs through ET(A) receptor and PLC activation. We conclude that the induction of interastroglial calcium waves is unlikely the primary cause of ET-1-induced CSDs. On the basis of the receptor profile, likely primary targets of ET-1 mediating CSD are either neurons or vascular smooth muscle cells.

Endothelin-1 decreases gap junctional intercellular communication by inducing phosphorylation of connexin 43 in human ovarian carcinoma cells

Endothelin-1 (ET-1) is overexpressed in ovarian carcinoma and acts as an autocrine factor selectively through the ETA receptor (ETAR) to promote tumor cell proliferation, survival, neovascularization, and invasiveness. Loss of gap junctional intercellular communication (GJIC) is critical for tumor progression by allowing the cells to escape growth control. Exposure of HEY and OVCA 433 ovarian carcinoma cell lines to ET-1 led to a 50-75% inhibition in intercellular communication and to a decrease in the connexin 43 (Cx43)-based gap junction plaques. To investigate the phosphorylation state of Cx43, ovarian carcinoma cell lysates were immunoprecipitated and transient tyrosine phosphorylation of Cx43 was detected in ET-1-treated cells. BQ 123, a selective ETAR antagonist, blocked the ET-1-induced Cx43 phosphorylation and cellular uncoupling. Gap junction closure was prevented by tyrphostin 25 and by the selective c-Src inhibitor, PP2. Furthermore, the increased Cx43 tyrosine phosphorylation was correlated with ET-1-induced increase of c-Src activity, and PP2 suppressed the ET-1-induced Cx43 tyrosine phosphorylation, indicating that inhibition of Cx43-based GJIC is mainly mediated by the Src tyrosine kinase pathway. In vivo, the inhibition of human ovarian tumor growth in nude mice induced by the potent ETAR antagonist, ABT-627, was associated with a reduction of Cx43 phosphorylation. These findings indicate that the signaling mechanisms involved in GJIC disruption on ovarian carcinoma cells depend on ETAR activation, which leads to the Cx43 tyrosine phosphorylation mediated by c-Src, suggesting that ETAR blockade may contribute to the control of ovarian carcinoma growth and progression also by preventing the loss of GJIC.

Effect of endothelin-1 on astrocytic protein content

The astrocytic endothelin (ET) receptors, ET(A) and ET(B), modulate calcium signaling and the astrocytic gap junctional network. The nonselective ET receptor ligand ET-1 inhibits gap junction permeability, an effect that can be blocked by tolbutamide. This mechanism may play a role in pathophysiological conditions such as ischemic stroke, characterized by elevated tissue ET-1 levels and hypertrophic-appearing reactive astrocytes. Therefore, the effect of ET-1 on cellular protein content was investigated in confluent once-passaged rat astrocyte cultures under serum-free conditions, by the Lowry method. Gap junction permeability was determined by the dye transfer technique. ET-1 prevented the decrease in astrocytic protein content observed in controls. The effect of ET-1 on cellular protein content was most pronounced in cultures seeded at high density, but it was attenuated in ET(B)-deficient (sl/sl) astrocytes. This effect could be blocked by the nonselective ET antagonist LU 302872 (10 micro M), as well as by the protein synthesis inhibitor cycloheximide (10 micro M). This increase in astrocytic protein content was inhibited by the ATP-sensitive K(+) channel blocker tolbutamide, which also antagonized the ET-1-induced reduction of gap junction permeability and reversed the morphological changes observed in astrocytes upon ET-1 treatment. Cytosine arabinoside (10 micro M), a DNA synthesis blocker, inhibited the ET-1-induced BrdU uptake without affecting the ET-1-induced increase in astrocytic protein content. To conclude, ET-1 induces an increase in astrocytic protein content as well as changes in astrocyte morphology in vitro. This hypertrophic response involves uncoupling of the astrocytic gap junctional network and is not dependent on DNA synthesis.

Glial neuronal signaling in the central nervous system

Glial cells are known to interact extensively with neuronal elements in the brain, influencing their activity. Astrocytes associated with synapses integrate neuronal inputs and release transmitters that modulate synaptic sensitivity. Glial cells participate in formation and rebuilding of synapses and play a prominent role in protection and repair of nervous tissue after damage. For glial cells to take an active part in plastic alterations under physiological conditions and pathological disturbances, extensive specific signaling, both within single cells and between cells, is required. In recent years, intensive research has led to our first insight into this signaling. We know there are active connections between astrocytes in the form of networks promoting Ca2+ and ATP signaling; we also know there is intense signaling between astrocytes, microglia, oligodendrocytes, and neurons, with an array of molecules acting as signaling substances. The cells must be functionally integrated to facilitate the enormous dynamics of and capacity for reconstruction within the nervous system. In this paper, we summarize some basic data on glial neuronal signaling to provide insight into synaptic modulation and reconstruction in physiology and protection and repair after damage.

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.

Type 2 nitric oxide synthase and protein nitration in chronic lung infection

Inflammation in the lung can lead to increased expression of inducible nitric oxide synthase (iNOS) and enhanced NO production. It has been postulated that the resultant highly reactive NO metabolites may have an important role in host defence, although they might also contribute to tissue damage. However, in a number of inflammatory lung diseases, including bronchiectasis, iNOS expression is increased but no elevation of airway NO can be detected. A potential explanation for this finding is that NO is rapidly scavenged by reaction with superoxide radicals, forming peroxynitrite, which is preferentially metabolized via nitration and nitrosation reactions. To test this hypothesis, anaesthetized, specific pathogen-free rats were inoculated with Pseudomonas aeruginosa incorporated into agar beads (chronically infected group) or sterile agar beads (control group). Ten to 15 days later, the lungs were isolated and fixed. Pseudomonas organisms were isolated from the lungs of the chronically infected group. These lungs showed extensive inflammatory cell infiltration and tissue damage, which were not observed in control lungs. Expression of iNOS was increased in the chronically infected group when compared with the control group. However, the mean number of cells staining for nitrotyrosine in the chronically infected group was not significantly different from that in the controls, nor was there an excess of nitrotyrosine, nitrate, nitrite or nitrosothiol concentrations in the infected lungs. Thus, no evidence was found of increased NO metabolites in chronically infected lungs, including products of the peroxynitrite pathway. These findings suggest that chronic infection does not cause increased iNOS activity in the lung, despite increased expression of iNOS.