Modulation of the effects of extracellular ATP on [Ca2+]i in rat brain microvacular endothelial cells

Neuroprotective effects of tongluojiunao in neurons exposed to oxygen and glucose deprivation

ETHNOPHARMACOLOGICAL RELEVANCE

Tongluojiunao (TLJN) is an herb extract that mainly contains ginsenoside Rg1 and geniposide, which are clinically used for treating ischemic damages in the brain.

AIM OF THE STUDY

In the stroke, cerebral ischemia followed by oxygen reperfusion induced apoptosis in hippocampal neurons, while extension of axons and dendrites in neurons may compensate for and repair damages of neuronal network in the hypoxia brain. In this study, we investigated whether TLJN can protect neurons against damages by ischemia in brain vasculature.

MATERIALS AND METHODS

We measured cell viability and lactate dehydrogenase (LDH) release from primary culture of rat hippocampal neurons before and after the neurons were deprived of oxygen and glucose (OGD). In addition, the effects were evaluated with cell viability and neurite outgrowth before or after OGD.

RESULTS

We found that TLJN could play a neuroprotective role to cultured primary rat hippocampal neurons under both normal and oxygen/glucose-deprivation (OGD) conditions. TLJN could protect both cultured primary rat hippocampal neurons and brain microvascular endothelial cells (BMECs) from cell death under both normal and oxygen/glucose-deprivation (OGD) conditions. Moreover, under the same conditions, BMECs-conditioned media pretreated by TNJN could also promote neuron viability and neurite outgrowth, indicating that TLJN stimulated BMECs to secret some neuroprotective/neurotrophic factors.

CONCLUSION

These findings suggest that TLJN has a marked neuroprotective and neurotrophic roles by either direct or indirect operation, and provide insight into the mechanism of clinical efficacy of this drug against stroke.

Role of nitric oxide on purinergic signalling in the cochlea

In the inner ear, there is considerable evidence that extracellular adenosine 5'-triphosphate (ATP) plays an important role in auditory neurotransmission as a neurotransmitter or a neuromodulator, although the potential role of adenosine signalling in the modulation of auditory neurotransmission has also been reported. The activation of ligand-gated ionotropic P2X receptors and G protein-coupled metabotropic P2Y receptors has been reported to induce an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) in inner hair cells (IHCs), outer hair cells (OHCs), spiral ganglion neurons (SGNs), and supporting cells in the cochlea. ATP may participate in auditory neurotransmission by modulating [Ca(2+)](i) in the cochlear cells. Recent studies showed that extracellular ATP induced nitric oxide (NO) production in IHCs, OHCs, and SGNs, which affects the ATP-induced Ca(2+) response via the NO-cGMP-PKG pathway in those cells by a feedback mechanism. A cross-talk between NO and ATP may therefore exist in the auditory signal transduction. In the present article, I review the role of NO on the ATP-induced Ca(2+) signalling in IHCs and OHCs. I also consider the possible role of NO in the ATP-induced Ca(2+) signalling in SGNs and supporting cells.

Effects of external ATP on Ca(2+) signalling in endothelial cells isolated from mouse islets

External ATP is believed to initiate and propagate Ca(2+) signals co-ordinating the insulin release pulses within and among the different islets in the pancreas. The possibility that islet endothelial cells participate in this process was evaluated by comparing the effects on [Ca(2+)](i) of purinoceptor activation in these cells with those in beta-cells. beta-Cell-rich pancreatic islets were isolated from ob/ob mice and dispersed into single cells/aggregates. After culture with or without endothelial cell growth supplement (ECGS), the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) was measured with ratiometric fura-2 technique. Presence of ECGS or prolongation of culture (>5 days) resulted in proliferation of endothelial cells and altered their phenotype from rounded to elongated. Endothelial cells, preliminarily identified by attachment of Dynabeads coated with the Bandeiraea simplicifolia 1 lectin (BS-1), responded in a similar way as those stained with CD31 antibodies after measurements of [Ca(2+)](i). Spontaneous transients and oscillations of [Ca(2+)](i )were seen in beta-cells, but not in endothelial cells exposed to 20 mM glucose. Addition of ATP (10 microM) resulted in pronounced and more extended rise of [Ca(2+)](i) in endothelial cells than in beta-cells. The endothelial cells differed from the beta-cells by also responding with a rise of [Ca(2+)](i) to 10 microM UTP, but not to equimolar ADP and acetylcholine. The results support the idea of mutual interactions between islet endothelium and beta-cells based on ATP-induced Ca(2+) signals. It is suggested that the endothelial cells have a tonic inhibitory action on beta-cell P2 purinoceptors resulting in impaired synchronization of the insulin release pulses.

Probable role of spinal purinoceptors in the analgesic effect of Trigonella foenum (TFG) leaves extract

In our previous work, we demonstrated that Trigonella foenum (TFG) leaves extract can exert analgesic effects in both formalin (F.T.) and tail flick (T.F.) tests. Spinal serotonergic system, but not endogenous opioid system, was involved in TFG induced analgesia (in the second phase of formalin test). Some reports concern the similarity between NSAIDs and TFG extract in many pharmacological effects or the interaction between NSAIDs and purinergic system; so the present study was designed to investigate the relationship between TFG extract and purinergic system or the inhibition of cyclo-oxygenase (COX). We examined the effect of TFG extract on: (1) the response of rabbit platelets to ADP induced aggregation, (2) the contraction of mouse vas deferens induced by alpha,beta-Me-ATP (a P(2) receptor agonist; this receptor mediates the rapid phase of ADP- and ATP-evoked influx of Ca(2+) through a non-specific cation channel in platelets), (3) alpha,beta-Me-ATP induced hyperalgesia in tail flick test in male rats and (4) the specific inhibition of COX-1 and COX-2. Our results showed that TFG extract (0.5, 1, 1.5, 3 mg/ml) inhibited ADP (10(-5) mol) induced platelet aggregation (IC(50)=1.28 mg/ml). alpha,beta-Me-ATP (30 microM) induced isometric contraction in vas deferens while suramin (a P(2) receptor antagonist, 50, 150, 300 microM) or TFG extract (0.5, 1, 2, 3 mg/ml) inhibited this effect significantly (IC(50) were 91.07 microM and 1.57 mg/ml, respectively). Moreover, alpha,beta-Me-ATP (3 microg/rat, i.t.) induced hyperalgesia in tail flick test, but it was prevented by co-injection of alpha,beta-Me-ATP with suramin (120 microg/rat, i.t.) or TFG extract (1mg/rat, i.t.). Effective concentrations of TFG extract in the above mentioned experiments did not inhibit COX enzymes in EIA tests. In conclusion, these results indicate that the blocking of spinal purinoceptors may contribute in the analgesic effect of TFG leaves extract.

Acute effects of glucocorticoids on ATP-induced Ca2+ mobilization and nitric oxide production in cochlear spiral ganglion neurons

Rapid, non-genomic effects of glucocorticoids on extracellular adenosine 5'-triphosphate (ATP)-induced intracellular Ca(2+) concentration ([Ca(2+)](i)) changes and nitric oxide (NO) production were investigated in type I spiral ganglion neurons (SGNs) of the guinea-pig cochlea using the Ca(2+)-sensitive dye fura-2 and the NO-sensitive dye 4,5-diaminofluorescein (DAF-2). Pretreatment of SGNs with 1 microM dexamethasone for 10 min, a synthetic glucocorticoid hormone, enhanced the ATP-induced [Ca(2+)](i) increase in SGNs. RU 38486, a competitive glucocorticoid receptor antagonist eliminated the effects of dexamethasone on the ATP-induced [Ca(2+)](i) increase in SGNs. These acute effects of dexamethasone were dependent on the presence of extracellular Ca(2+), thereby suggesting that dexamethasone may rapidly enhance the Ca(2+) influx through the activation of ionotropic P2X receptors which may interact with glucocorticoid-mediated membrane receptors. Extracellular ATP increased the intensity of DAF-2 fluorescence, indicating NO production in SGNs. The ATP-induced NO production was mainly due to the Ca(2+) influx through the activation of P2 receptors. S-nitroso-N-acetylpenicillamine, a NO donor, enhanced the ATP-induced [Ca(2+)](i) increase in SGNs while L-N(G)-nitroarginine methyl ester (L-NAME), a NO synthesis inhibitor, inhibited it. Dexamethasone enhanced the ATP-induced NO production in SGNs. The augmentation of dexamethasone on ATP-induced NO production was abolished in the presence of l-NAME. It is concluded that the ATP-induced [Ca(2+)](i) increase induces NO production which enhances a [Ca(2+)](i) increase in SGNs by a positive-feedback mechanism. Dexamethasone enhances the ATP-induced [Ca(2+)](i) increase in SGNs which results in the augmentation of NO production. The present study suggests that NO may play an important role in auditory signal transduction. Our results also indicate that glucocorticoids may rapidly affect auditory neurotransmission due to a novel non-genomic mechanism.

Dynamics of CNS Barriers: Evolution, Differentiation, and Modulation

(1) Three main barrier layers at the interface between blood and tissue protect the central nervous system (CNS): the endothelium of brain capillaries, and the epithelia of the choroid plexus (CP) and the arachnoid. The classical work on these barriers in situ until the 1970s laid the foundations for modern understanding. Techniques for brain endothelial cell isolation and culture pioneered by Ferenc Joó in the 1970s opened up new fields of examination, enabling study of mechanisms at the cellular and molecular level. (2) Astrocytic glial cells are closely associated with the brain endothelial barrier. During evolution the barrier appears to have shifted from the glial to the endothelial layer, in parallel with the increasing importance of the microvasculature and its regulation. Vestiges of the barrier potential of glia remain in the modern mammalian CNS. (3) Evolutionary evidence suggests that the advantage derived from ionic homeostasis around central synapses was the major selective pressure leading to refinement of CNS barrier systems. This is one element of the modern 'multitasking' barrier function. (4) While epithelia are constitutively able to form barriers at appropriate interfaces, the 'default' condition for endothelia is more leaky; inductive influences from associated cells especially astrocytes are important in generating the full blood-brain barrier (BBB) phenotype in brain capillaries. The underlying mechanisms are being elucidated at the molecular and genomics level. (5) The barrier layers of the nervous system can be modulated by a number of receptor-mediated processes, involving several signal transduction pathways, both calcium dependent and independent. Some agents acting as 'inducers' in the long term can act as 'modulators' in the short-term, with some overlap of signaling pathways. Modulating agents may be derived both from the blood and from cells associated with cerebral vessels. Less is known about the modulation of the CP. (6) The challenge for the next era of CNS barrier studies will be to apply new knowledge from proteomics and genomics to understanding the in vivo condition in physiology and pathology.

Nitric oxide mediates hypoxia-induced changes in paracellular permeability of cerebral microvasculature

Ischemic stroke from a reduction in blood flow to the brain microvasculature results in a subsequent decreased delivery of oxygen (i.e., hypoxia) and vital nutrients to endothelial, neuronal, and glial cells. Hypoxia associated with stroke has been shown to increase paracellular permeability of the blood-brain barrier, leading to the release of cellular mediators and brain tissue injury. Whereas reperfusion does not occur in all ischemic strokes, increased permeability has been seen in posthypoxic reoxygenation. Currently, it is unknown whether these deleterious effects result from cellular mechanisms stimulated by decreased oxygen during stroke or posthypoxic reoxygenation stress. This study used primary bovine brain microvessel endothelial cells (BBMECs) to examine the involvement of nitric oxide (NO) as a mediator in hypoxia-induced permeability changes. Hypoxia-induced increased transport of [14C]sucrose across BBMEC monolayers compared with normoxia was attenuated by either posthypoxic reoxygenation or inhibition of NO synthase (NOS). The hypoxia-induced permeability effect was further reduced when NOS inhibition was combined with posthypoxic reoxygenation. Additionally, a significant increase in total NO was seen in BBMECs after hypoxic exposure. This correlation was supported by the increased [14C]sucrose permeability observed when BBMECs were exposed to the NO donor diethylenetriaamine NONOate. Western blot analyses of NOS isoforms showed a significant increase in the inducible isoform after hypoxic exposure with a subsequent reduction in expression on reoxygenation. Results from this study suggest that hypoxia-induced blood-brain barrier breakdown can be diminished by inhibition of NO synthesis, decreased concentration of NO metabolites, and/or reoxygenation.

Microvascular basal lamina antigen loss after traumatic brain injury in the rat

The loss of microvascular basal lamina antigen is known to be a consequence of cerebral ischemia, but little information is available on its role in traumatic brain injury (TBI). The aim of our study was (1) to test the hypothesis that there is damage to the basal lamina of brain microvasculature after TBI, (2) to localize microvascular damage, and (3) to compare this loss with that in ischemia. Rats (n=14) were either sham operated (n=5) or subjected to fluid percussion injury (n=9; TBI=1.5 atm) and killed after 0 (n=5, sham), 12 (n=4), or 24 h (n=5). Collagen-type-IV immunoreactivity and a digital image-processing system were used to localize and quantify the number of stained vascular elements and the total collagen stained area. Western blot was used to compare collagen-type-IV content on the traumatic and nontraumatic brain side. The cortex of animals subjected to TBI and killed after 24 h showed a reduction in the area of stained collagen amounting to 19+/-4% (p<0.009) and a reduction in the total number of microvessels identified by collagen stain (29+/-6%; p<0.02). The Western blot revealed a 31+/-6% (p<0.03) reduction of collagen, compared to the mirror cortical area after 24 h. No significant reduction was found in the group that survived 12 h or in basal ganglia in both groups. TBI causes microvascular basal lamina damage. Whereas TBI affected only cortical areas, cerebral ischemia also induced microvascular basal lamina damage in the basal ganglia. After 24 h, the extent of severe basal lamina damage due to TBI was less severe than in ischemia.

Nitric oxide inhibits adenosine 5'-triphosphate-induced Ca2+ response in inner hair cells of the guinea pig cochlea

To investigate the interaction between Ca(2+) and nitric oxide (NO) in inner hair cells of the guinea pig cochlea (IHCs), the extracellular adenosine 5'-triphosphate (ATP)-induced NO production and the effects of NO on ATP-induced increase of intracellular Ca(2+) concentrations ([Ca(2+)](i)) were investigated in IHCs using the NO-sensitive dye DAF-2 and the Ca(2+)-sensitive dye Fura-2. Extracellular ATP induced an increase in DAF-2 fluorescence, which thus indicates NO production in IHCs. The ATP-induced NO production was mainly due to Ca(2+) influx through the activation of P2 receptor. L-N(G)-nitroarginine methyl ester, a NO synthesis inhibitor, enhanced the ATP-induced [Ca(2+)](i) increase in IHCs while S-nitroso-N-acetylpenicillamine, a NO donor, inhibited it. We conclude that NO inhibits the ATP-induced [Ca(2+)](i) increase in IHCs by a negative-feedback mechanism.

Modulation of insulin transport in rat brain microvessel endothelial cells by an ecto-phosphatase activity

The physiological function of alkaline phosphatase (ALP) remains controversial. It was recently suggested that this membrane-bound enzyme has a role in the modulation of transmembranar transport systems into hepatocytes and Caco-2 cells. ALP activity expressed on the apical surface of blood-brain barrier cells, and its relationship with (125)I-insulin internalization were investigated under physiological conditions using p-nitrophenylphosphate (p-NPP) as substrate. For this, an immortalized cell line of rat capillary cerebral endothelial cells (RBE4 cells) was used. ALP activity and (125)I-insulin internalization were evaluated in these cells. The results showed that RBE4 cells expressed ALP, characterized by an ecto-oriented active site which was functional at physiological pH. Orthovanadate (100 microM), an inhibitor of phosphatase activities, decreased both RBE4-ALP activity and (125)I-insulin internalization. In the presence of L-arginine (1 mM) or adenosine (100 microM) RBE4-ALP activity and (125)I-insulin, internalization were significantly reduced. However, D-arginine (1 mM) had no significant effect. Additionally, RBE4-ALP activity and (125)I-insulin internalization significantly increased in the presence of the bioflavonoid kaempferol (100 microM), of the phorbol ester PMA (80 nM), IBMX (1 mM), progesterone (200 microM and 100 microM), beta-estradiol (100 microM), iron (100 microM) or in the presence of all-trans retinoic acid (RA) (10 microM). The ALP inhibitor levamisole (500 microM) was able to reduce (125)I-insulin internalization to 69.1 +/- 7.1% of control. Our data showed a positive correlation between ecto-ALP activity and (125)I-insulin incorporation (r = 0.82; P < 0.0001) in cultured rat brain endothelial cells, suggesting that insulin entry into the blood-brain barrier may be modulated through ALP.

*NO and oxyradical metabolism in new cell lines of rat brain capillary endothelial cells forming the blood-brain barrier

To investigate the relevance of *NO and oxyradicals in the blood-brain barrier (BBB), differentiated and well-proliferating brain capillary endothelial cells (BCEC) are required. Therefore, rat BCEC (rBCEC) were transfected with immortalizing genes. The resulting lines exhibited endothelial characteristics (factor VIII, angiotensin-converting enzyme, high prostacyclin/thromboxane release rates) and BBB markers (gamma-glutamyl transpeptidase, alkaline phosphatase). The control line rBCEC2 (mock transfected) revealed fibroblastoid morphology, less factor VIII, reduced gamma-glutamyl transpeptidase, weak radical defence, low prostanoid metabolism, and limited proliferation. Lines transfected with immortalizing genes (especially rBCEC4, polyoma virus large T antigen) conserved primary properties: epitheloid morphology, subcultivation with high proliferation rate under pure culture conditions, and powerful defence against reactive oxygen species (Mn-, Cu/Zn-superoxide dismutase, catalase, glutathione peroxidase, glutathione) effectively controlling radical metabolism. Only 100 microM H2O2 overcame this defence and stimulated the formation of eicosanoids similarly as in primary cells. Some BBB markers were expressed to a lower degree; however, cocultivation with astrocytes intensified these markers (e.g., alkaline phosphatase) and paraendothelial tightness, indicating induction of BBB properties. Inducible NO synthase was induced by a cytokine plus lipopolysaccharide mixture in all lines and primary cells, resulting in *NO release. Comparing the cell lines obtained, rBCEC4 are stable immortalized and reveal the best conservation of properties from primary cells, including enzymes producing or decomposing reactive species. These cells can be subcultivated in large amounts and, hence, they are suitable to study the role of radical metabolism in the BBB and in the cerebral microvasculature.

Regulatory pathways and uptake of L-DOPA by capillary cerebral endothelial cells, astrocytes, and neuronal cells

We examined the nature and regulation of the inward L-3,4-dihydroxyphenylalanine (L-DOPA) transporter in rat capillary cerebral endothelial (RBE4) cells, type 1 astrocytes (DI TNC1), and Neuro-2a neuroblastoma cells. In all three cell types, the inward transfer of L-DOPA was largely promoted through the 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid-sensitive and sodium-independent L-type amino acid transporter. Only in DI TNC1 cells was the effect of maneuvers that increase intracellular cAMP levels accompanied by increases in L-DOPA uptake. Also, only in DI TNC1 cells was the effect of the guanylyl cyclase inhibitor LY-83583 accompanied by a 65% increase in L-DOPA accumulation, whereas the nitric oxide donor sodium nitroprusside produced a 25% decrease in L-DOPA accumulation. In all three cell types, the Ca2+/calmodulin inhibitors calmidazolium and trifluoperazine inhibited L-DOPA uptake in a noncompetitive manner. Thapsigargin (1 and 3 microM) and A-23187 (1 and 3 microM) failed to alter L-DOPA accumulation in RBE4 and Neuro-2a cells but markedly increased L-DOPA uptake in DI TNC1 cells. We concluded that L-DOPA in RBE4, DI TNC1, and Neuro-2a cells is transported through the L-type amino acid transporter and appears to be under the control of Ca2+/calmodulin-mediated pathways. Astrocytes, however, are endowed with other processes that appear to regulate the accumulation of L-DOPA, responding positively to increases in intracellular Ca2+ and cAMP and to decreases in cGMP.

The pharmacology of nucleotide receptors on primary rat brain endothelial cells grown on a biological extracellular matrix: effects on intracellular calcium concentration

1. Brain capillary endothelial cells express a variety of nucleotide receptors, but differences have been reported between culture models. This study reports examination of nucleotide receptors on primary cultured rat brain capillary endothelial cells (RBCEC) grown on a biological extracellular matrix (ECM) to produce a more differentiated phenotype. 2. Fura-2 fluorescence ratio imaging was used to monitor intracellular free calcium concentration [Ca(2+)](i). ATP, UTP, and 2-methylthioATP (2-MeSATP) increased [Ca(2+)](i) to similar levels, while 2-MeSADP, ADP and adenosine gave smaller responses. 3. Removal of extracellular calcium caused no significant change in the [Ca(2+)](i) response to 2-MeSATP, evidence that the response was mediated by a metabotropic (P2Y) receptor. 4. All cells tested responded to ATP, UTP, 2-MeSATP and ADP, while 63% responded to adenosine and 50% to 2-MeSADP. No cells responded to alpha, beta-methyleneATP. Cells grown on rat tail collagen instead of ECM gave smaller and less uniform [Ca(2+)](i) responses, suggesting that the differentiating effect of the ECM contributed to a more uniform receptor profile. 5. The [Ca(2+)](i) response to the P2Y(1)-selective agonist 2-MeSADP was abolished in the presence of the subtype-selective antagonist adenosine 3'-phosphate 5'-phosphosulphate (PAPS). 6. The P2Y(2) antagonist suramin completely blocked the response to ATP and inhibited the response to UTP by 66%. 7. The A(1) subtype-selective adenosine receptor agonist N(6)-Cyclopentyladenosine (CPA) gave a small but characteristic [Ca(2+)](i) response, while A(2A) and A(2B) subtype-selective agonists failed to generate [Ca(2+)](i) changes. 8. The results are consistent with the presence on RBCEC of a P2Y(2)-like receptor coupled to phospholipase C, and a P2Y(1)-like receptor mobilizing intracellular Ca(2+). The role of multiple nucleotide receptors in the function of the brain endothelium is discussed.

L-DOPA transport properties in an immortalised cell line of rat capillary cerebral endothelial cells, RBE 4

The present study aimed to determine the kinetics of L-3, 4-dihydroxyphenylalanine (L-DOPA) uptake in an immortalised cell line of rat capillary cerebral endothelial cells (clones RBE 4 and RBE 4B), to define the type of inhibition produced by L-5-hydroxytryptophan (L-5-HTP), 2-aminobicyclo(2,2,1)-heptane-2-carboxylic acid (BHC) and N-(methylamino)-isobutyric acid (MeAlB) and its sodium dependence. Non-linear analysis of the saturation curves for L-DOPA and L-5-HTP revealed in RBE 4 cells Km values (in microM) of 72 and 102 and in RBE 4B cells Km values (in microM) of 60 and 118, respectively. IC50 values for L-5-HTP (RBE 4, 1026 microM; RBE 4B, 831 microM) obtained in the presence of a nearly saturating (250 microM) concentration of L-DOPA were almost 5-fold those obtained when non-saturating (25 microM) concentrations of L-DOPA were used. IC50 values for BHC obtained in the presence of a nearly saturating (250 microM) concentration of L-DOPA were also 6- to 5-fold those obtained when non-saturating (25 microM) concentrations of L-DOPA were used. MeAlB (up to 2.5 mM) was found not to interfere with the uptake of L-DOPA. In RBE 4 cells, Vmax values for L-DOPA uptake were identical in the absence and the presence of 150 microM L-5-HTP or 150 microM BHC, but Km values (microM) were significantly greater (P<0.05) when L-DOPA uptake was studied in the presence of L-5-HTP or BHC. Similar findings were observed when RBE 4B cells were used. Uptake of (250 microM) L-DOPA in the absence of sodium in the incubation medium was similar to that observed in the presence of increasing concentrations of sodium (20 to 140 mM). It is concluded that RBE 4 and RBE 4B cells are endowed with the L-type amino acid transporter through which L-DOPA and L-5-HTP can be taken up, and suggested that this immortalised cell line of rat capillary cerebral endothelium might constitute an interesting in vitro model for the study of BBB mechanisms, namely those concerning solute and nutrient transfer across the brain capillary endothelium.