Endothelins increase tyrosine phosphorylation of astrocytic focal adhesion kinase and paxillin accompanied by their association with cytoskeletal components

Inhibitory input directs astrocyte morphogenesis through glial GABABR

Communication between neurons and glia has an important role in establishing and maintaining higher-order brain function1. Astrocytes are endowed with complex morphologies, placing their peripheral processes in close proximity to neuronal synapses and directly contributing to their regulation of brain circuits2-4. Recent studies have shown that excitatory neuronal activity promotes oligodendrocyte differentiation5-7; whether inhibitory neurotransmission regulates astrocyte morphogenesis during development is unclear. Here we show that inhibitory neuron activity is necessary and sufficient for astrocyte morphogenesis. We found that input from inhibitory neurons functions through astrocytic GABAB receptor (GABABR) and that its deletion in astrocytes results in a loss of morphological complexity across a host of brain regions and disruption of circuit function. Expression of GABABR in developing astrocytes is regulated in a region-specific manner by SOX9 or NFIA and deletion of these transcription factors results in region-specific defects in astrocyte morphogenesis, which is conferred by interactions with transcription factors exhibiting region-restricted patterns of expression. Together, our studies identify input from inhibitory neurons and astrocytic GABABR as universal regulators of morphogenesis, while further revealing a combinatorial code of region-specific transcriptional dependencies for astrocyte development that is intertwined with activity-dependent processes.

Inhibitory input directs astrocyte morphogenesis through glial GABA B R

Communication between neurons and glia plays an important role in establishing and maintaining higher order brain function. Astrocytes are endowed with complex morphologies which places their peripheral processes in close proximity to neuronal synapses and directly contributes to their regulation of brain circuits. Recent studies have shown that excitatory neuronal activity promotes oligodendrocyte differentiation; whether inhibitory neurotransmission regulates astrocyte morphogenesis during development is unknown. Here we show that inhibitory neuron activity is necessary and sufficient for astrocyte morphogenesis. We found that input from inhibitory neurons functions through astrocytic GABA B R and that its deletion in astrocytes results in a loss of morphological complexity across a host of brain regions and disruption of circuit function. Expression of GABA B R in developing astrocytes is regulated in a region-specific manner by SOX9 or NFIA and deletion of these transcription factors results in region-specific defects in astrocyte morphogenesis, which is conferred by interactions with transcription factors exhibiting region-restricted patterns of expression. Together our studies identify input from inhibitory neurons and astrocytic GABA B R as universal regulators of morphogenesis, while further revealing a combinatorial code of region-specific transcriptional dependencies for astrocyte development that is intertwined with activity-dependent processes.

News from the endothelin-3/EDNRB signaling pathway: Role during enteric nervous system development and involvement in neural crest-associated disorders

The endothelin system is a vertebrate-specific innovation with important roles in regulating the cardiovascular system and renal and pulmonary processes, as well as the development of the vertebrate-specific neural crest cell population and its derivatives. This system is comprised of three structurally similar 21-amino acid peptides that bind and activate two G-protein coupled receptors. In 1994, knockouts of the Edn3 and Ednrb genes revealed their crucial function during development of the enteric nervous system and melanocytes, two neural-crest derivatives. Since then, human and mouse genetics, combined with cellular and developmental studies, have helped to unravel the role of this signaling pathway during development and adulthood. In this review, we will summarize the known functions of the EDN3/EDNRB pathway during neural crest development, with a specific focus on recent scientific advances, and the enteric nervous system in normal and pathological conditions.

Endothelin-3 stimulates cell adhesion and cooperates with β1-integrins during enteric nervous system ontogenesis

Endothelin-3 (EDN3) and β1-integrins are required for the colonization of the embryonic gut by enteric neural crest cells (ENCCs) to form the enteric nervous system (ENS). β1-integrin-null ENCCs exhibit migratory defects in a region of the gut enriched in EDN3 and in specific extracellular matrix (ECM) proteins. We investigated the putative role of EDN3 on ENCC adhesion properties and its functional interaction with β1-integrins during ENS development. We show that EDN3 stimulates ENCC adhesion to various ECM components in vitro. It induces rapid changes in ENCC shape and protrusion dynamics favouring sustained growth and stabilization of lamellipodia, a process coincident with the increase in the number of focal adhesions and activated β1-integrins. In vivo studies and ex-vivo live imaging revealed that double mutants for Itgb1 and Edn3 displayed a more severe enteric phenotype than either of the single mutants demonstrated by alteration of the ENS network due to severe migratory defects of mutant ENCCs taking place early during the ENS development. Altogether, our results highlight the interplay between the EDN3 and β1-integrin signalling pathways during ENS ontogenesis and the role of EDN3 in ENCC adhesion.

Distinct detergent-resistant membrane microdomains (lipid rafts) respectively harvest K(+) and water transport systems in brain astroglia

The detergent-resistant microdomains (DRM) of cell membranes scaffold different molecules and regulate cell functions by orchestrating various signaling pathways including G-proteins and tyrosine kinase. Here we report a novel role for DRM in astroglial cells. K(+)-buffering inwardly rectifying Kir4.1 channels and the water channel AQP4 are expressed in astrocytes and they may be functionally coupled to maintain ionic and osmotic homeostasis in the brain. Kir4.1 and AQP4 channel proteins were abundant in noncaveloar DRM in the brain and also in HEK293T cells when exogenously expressed. In HEK293T cells, depletion of membrane cholesterol by methyl-beta-cyclodextrin (MbetaCD) resulted in loss of Kir4.1 association with DRM and its channel activity but did not affect either the distribution or the function of AQP4. Immunolabeling showed that Kir4.1 and AQP4 were occasionally distributed in close proximity but in distinct compartments of the astroglial cell membrane. Astroglial noncaveolar DRM may therefore be made up of at least two distinct compartments, MbetaCD-sensitive and MbetaCD-resistant microdomains, which control localization and function of, respectively, Kir and AQP4 channels on the cell membrane.

Multiple signaling pathways involved in the effect of endothelin type B receptor in rat median eminence

We assessed the possible link between endothelin receptor mediated phosphoinositide breakdown and NO/cGMP signaling pathways in rat arcuate nucleus-median eminence fragments (AN-ME), brain structures known to contain a rich plexus of nitric oxide synthase (NOS)-containing neurons and fibers, together with densely arranged endothelin ETB-receptors-like immunoreactive fibres. Our data show that ET-1, ET-3 and the ETB-receptors agonist, IRL 1620, increased inositol monophosphate (InsP1) accumulation, NOS activity and cGMP formation, in a similar degree. The stimulatory effect of ETs on InsP1 accumulation and cGMP formation was inhibited by the phospholipase C (PLC) inhibitor, neomycin, and the absence of extracellular calcium, suggesting that calcium is involved in endothelin receptor-induced PLC activation. The L-arginine analog, L-NAME, inhibited ET-1 or IRL1620-stimulated cGMP formation. The ETA receptor antagonists BQ 123, did not alter, while the ETB receptor antagonists BQ788 inhibited ETs-induced increase in the PI metabolism, NOS activity and cGMP generation. Our data indicate that in AN-ME, ETB receptor signals through receptor-mediated calcium dependent-stimulation of phosphoinositide breakdown and activation of NOS/cGMP signaling pathway.

Establishment of Vero E6 cell clones persistently infected with severe acute respiratory syndrome coronavirus

Little information is available on persistent infection of severe acute respiratory syndrome (SARS) coronavirus (CoV). In this study, we established persistent infection of SARS-CoV in the Vero E6 cell line. Acute infection of Vero E6 with SARS-CoV produced a lytic infection with characteristic rounding cytopathic effects (CPE) and the production of a large number of infectious particles in the culture fluid within 3 days post-infection. Upon subsequent culturing of the remaining adherent cells, the cells gradually proliferated and recovered normal morphology similar to that of the parental cells, and continued to produce large numbers of infectious viral particles during the observation period of 5 months. Among a total of 87 cell clones obtained from the persistently infected Vero E6, only four cell clones (named #13, #18, #21, and #34) were positive for viral RNA. Clones #13, #18, and #34 shifted to viral RNA-negative during subsequent cultures, while #21 continuously produced infectious particles at a high rate. The SARS-CoV receptor, angiotensin-converting enzyme 2, was almost completely down regulated from the cell surface of persistently infected cells. Western blot analysis as well as electron microscopy indicated that the ratios of spike to nucleocapsid protein in clone #21 as well as its parental persistently infected cells were lower than that in the cells in the acute phase of infection. These Vero E6 cells persistently infected with SARS-CoV may be useful for clarifying the mechanism of the persistent infection and also for elucidating the possible pathophysiologic significance of such long-term maintenance of this virus.

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors signaling complexes in Bergmann glia

Glutamate, the major excitatory neurotransmitter, induces a wide array of signals from the membrane to the nucleus regulating gene expression. In Bergmann glia, Ca2+ -permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazole- propionic acid (AMPA) receptors are involved in the short- and long-term interactions between these cells and the neurons that they surround. After activation, AMPA receptors become tyrosine phosphorylated and by these means form multiprotein signaling complexes. To characterize these events, cultured chick Bergmann glia cells as well as chick cerebellar slices were exposed to glutamate, and, by using a combination of immunoprecipitation assays coupled to Western blot analysis, we identified several signaling proteins that become associated with these receptors. A dose- and time-dependent association among AMPA receptors, the focal adhesion kinase pp125FAK, the phosphatidylinositol-3 kinase and paxillin was found. These results extend the concept of the transducisome to AMPA receptors and provide a framework in which a plausible control of the cytoskeletal network by glutamate is taking place, most possibly through AMPA receptors.

Hypoxia augments TNF-alpha-mediated endothelin-1 release and cell proliferation in human optic nerve head astrocytes

The effect of hypoxia (24 h) on TNF-alpha-mediated release of endothelin-1 (ET-1) from human optic nerve head astrocytes (hONAs) and TNF-alpha- and ET-1-induced hONA proliferation was determined. ET-1 synthesis and release was quantitated using ELISA while TNF-alpha (10 nM)- and ET-1 (100 nM)-mediated hONA proliferation was assessed by CellTiter 96 aqueous one-solution cell proliferation assay, respectively. hONAs appeared to be more rounded with fewer processes following 24 h hypoxia compared to thodr seen in normoxia. Hypoxia enhanced TNF-alpha-mediated ET-1 synthesis and release (by 5-fold) and also significantly increased TNF-alpha- and ET-1-mediated hONA proliferation. PD142893 (1 microM), an ET(A/B) receptor antagonist, blocked ET-1-mediated hONA proliferation both under normoxia and hypoxia, while doing so only under normoxia following TNF-alpha treatment. Also, U0126 (10 microM; an upstream ERK1/2 inhibitor) completely blocked agonist-induced hONA proliferation in normoxia and partially blocked the same in hypoxia. These results demonstrate for the first time that hONAs secrete ET-1 and that TNF-alpha and hypoxia can regulate its levels. Moreover, hypoxia augments the proliferative responses of hONAs to TNF-alpha and ET-1. These agonist-mediated effects following hypoxia could contribute to astroglial activation as seen in glaucomatous optic nerve heads.

Increase by FK960, a novel cognitive enhancer, in glial cell line-derived neurotrophic factor production in cultured rat astrocytes

We examined the effect of N-(4-acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate (FK960), a novel anti-dementia drug, on neurotrophic factor production in cultured rat astrocytes. FK960 (100nM) increased mRNA and protein levels of glial cell line-derived neurotrophic factor (GDNF). FK960 did not affect mRNA levels of neurotrophic factors other than GDNF. The effect of FK960 was not affected by antagonists of dopamine and alpha7-nicotinic acetylcholine receptors. FK960 stimulated phosphorylation of mitogen-activated protein/extracellular signal-regulated kinase (ERK) without any effect on phosphoryolation of p38 and c-Jun N-terminal kinase. FK960 increased the levels of c-Fos and phosphorylation of cAMP responsive element binding protein (CREB). The effect of FK960 on c-Fos was inhibited by PD98059 (10microM), an ERK kinase inhibitor, and cycloheximide (1microg/ml), a transcription inhibitor, and the effect of FK960 on CREB phosphorylation was blocked by PD98059. The effect of FK960 on GDNF mRNA expression was attenuated by PD98059, curcumin (10microM), an activator protein-1 inhibitor, cycloheximide and actinomycin D (10microg/ml). These results suggest that FK960 stimulates GDNF production in c-Fos- and CREB-dependent mechanisms in cultured astrocytes and that ERK signal is responsible for both c-Fos expression and CREB phosphorylation in the cascades.

Focal adhesion kinase is required for endothelin-induced cell cycle progression of cultured astrocytes

When the brain is damaged, astrocytes often cause hyperplasia resulting in glial scar formation at the injured sites. Endothelins (ETs) have been shown to be involved in the pathophysiologic responses of astrocytes, including proliferation. In this study, we examined the mechanisms underlying the ET-induced astrocytic G1/S-phase cell cycle transition by focusing on focal adhesion kinase (FAK). A transient transfection with wild-type FAK was followed by an increase in bromodeoxyuridine (BrdU) incorporation into cultured rat astrocytes. The increases in BrdU incorporation induced by 100 nM ET-1 were not found in astrocytes transfected with dominant-negative FAK mutants (FRNK and dC14-FAK). The increases in BrdU incorporation induced by 10 nM phorbol 12-myristate 13-acetate (PMA) were not affected by the FAK mutants. Wild-type FAK did not induce stress fiber formation in cultured astrocytes. The dominant negative FAK mutant dC14-FAK did not prevent ET-induced astrocytic stress fiber formation. These results suggest that FAK mediated the astrocytic G1/S cell cycle transition induced by ET-1 downstream of the cytoskeletal actin reorganization.

Intracerebroventricular administration of an endothelin ETB receptor agonist increases expressions of GDNF and BDNF in rat brain

Endothelins (ETs) are suggested to be involved in functional alterations of astrocytes after brain injury, including proliferation, hypertrophy and production of neurotrophic factors. In this study, effects of Ala1,3,11,15-endothelin-1 (Ala1,3,11,15-ET-1), an ETB receptor selective agonist, on neurotrophic factor production were examined in rat brain. A continuous intracerebroventricular administration of Ala1,3,11,15-ET-1 (500 pmol/day for 7 days) increased the numbers of GFAP- and vimentin-positive astrocytes in the hippocampus, caudate putamen and cerebrum. Ala1,3,11,15-ET-1 did not induce neuronal degeneration and activation of microglia/macrophage in these brain regions. The intracerebroventricular administration of Ala1,3,11,15-ET-1 for 7 days caused two- to three-fold increases in glial cell line-derived neurotrophic factors (GDNF) mRNA in the hippocampus and cerebrum. The mRNA levels of brain-derived neurotrophic factors (BDNF) in caudate putamen were increased by Ala1,3,11,15-ET-1. Expressions of nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) mRNA in these regions were not largely affected by Ala1,3,11,15-ET-1, except cerebral NGF mRNA level was increased. The Ala1,3,11,15-ET-1-induced increases in GDNF and BDNF mRNA levels were accompanied by increases in immunoreactive GDNF and BDNF. Immunohistochemical observations showed that GFAP-positive astrocytes expressed GDNF and BDNF in the brain regions of Ala1,3,11,15-ET-1-infused rats. In cultured rat astrocytes, Ala1,3,11,15-ET-1 (100 nm) increased mRNA levels of GDNF and BDNF. These results suggest that activation of brain ETB receptors induced GDNF and BDNF expression in astrocytes.

Endothelin-1 stimulates glial cell line-derived neurotrophic factor expression in cultured rat astrocytes

Effects of endothelin-1 (ET-1) on glial cell line-derived neurotrophic factor (GDNF) production in cultured astrocytes were examined. Treatment of cultured astrocytes with ET-1 (100 nM) increased mRNA levels of GDNF in 1-6h. The effect of ET-1 was inhibited by BQ788, an ET(B) receptor antagonist, but not by FR139317, an ET(A) receptor antagonist. ET-1 stimulated release of GDNF into culture medium. Dexamethasone (1 microM) and pyrrolidine dithiocarbamate (PDTC, 100 microM), which inhibit activation of NFkappaB, prevented the increases in GDNF mRNA by H(2)O(2). In contrast, the effect of ET-1 was not affected by dexamethasone and PDTC. The increase of astrocytic GDNF mRNA by ET-1 was inhibited by BAPTA/AM (30 microM) and PD98059 (50 microM), but not by calphostin C, staurosporine, and cyclosporine A. These results suggest that ET-1 stimulated expression of astrocytic GDNF through ET(B) receptor-mediated increases in cytosolic Ca(2+) and ERK activation.

[Functional alterations of astroglia on brain pathologies and their intracellular mechanisms]

A phenotypic alteration of astroglia, "astroglial activation", is a common phenomenon observed on brain pathologies. The hypertrophy/hyperplasia of activated astroglia causes a glial scar, which prevents synaptic re-generation. In contrast, many neurotrophic substances are produced by the activated astroglia. Thus, the functional alteration of astroglia is important in tissue repair processes of the damaged CNS. Endothelins (ETs) are involved in the pathophysiological responses of the CNS. We found that injection of ETs into rat brain induced activated astroglia. A selective ETB-receptor antagonist attenuated the induction of activated astroglia. In cultured astroglia, ETs reproduce the functional alterations characterizing activated astroglia; i.e., increases in proliferation, morphological changes and stimulation of several gene transcriptions. ETs re-organized astroglial cytoskeletal actin through a small GTP-binding protein, rho, which may underlie the astroglial hypertrophy. Analysis of gene expression showed that transcriptions of neurotrophic factors (GDNF and BDNF) were stimulated by ETs. ETs stimulated astroglial proliferation by both adhesion-dependent and -independent mechanisms, where FAK and ERK plays key roles, respectively. These findings suggest important roles of ETs in the regulation of astroglial functions.

Glutamate activates PP125(FAK) through AMPA/kainate receptors in Bergmann glia

Glial glutamate receptors are likely to play a role in plasticity, learning, and memory and in a number of neuropathologies. An enhanced glutamate-dependent tyrosine phosphorylation has been detected in such processes. Using primary cultures of chick Bergmann glia cells and chick cerebellar slices, we addressed whether glial glutamate receptors can activate the nonreceptor tyrosine kinase pp125 focal adhesion kinase (pp125(FAK)). A dose- and time-dependent tyrosine phosphorylation of pp125(FAK) was found in both preparations upon glutamate treatment. This effect was mediated through alpha-amino-3-hydroxy-5-methyl-4-isoaxazolepropionate (AMPA)/kainate (KA) receptors, as shown by its inhibition by the specific antagonists 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7- sulfonamide (NBQX) and 6,7-dinitroquinoxaline-2,3-dione (DNQX) and the lack of effect of metabotropic agonists. FAK tyrosine phosphorylation was dependent on phosphatidylinositol 3-kinase activity. As expected, an increase in pp125(FAK) catalytic activity was found upon glutamate treatment. Immunprecipitation experiments demonstrated that FAK associates with ionotropic glutamate receptors. Taken together, these results suggest a role for glial glutamate receptors in cytoskeletal rearrengments and focal adhesion contact formation and provide new insight into the signaling transactions elicited by this neurotransmitter in glial cells.