Neutrophil haptotaxis induced by the lectin KM+

KM+ is a D(+)mannose binding lectin from Artocarpus integrifolia that induces neutrophil migration in vitro and in vivo.This attractant activity was shown to be caused by haptotaxis rather than chemotaxis. The inhibition by D(+)mannose of the neutrophil attraction exerted by KM+, both in vitro and in vivo, supports the idea that haptotaxis is triggered in vivo by the sugar binding sites interacting with glycoconjugates located on the neutrophil surface and in the extracellular matrix. In the present study an in vivo haptotaxis assay was performed by intradermally (i.d.) injecting 125I-KM+ (200 ng), which led to a selective staining of loose connective tissue and vascular endothelium. The radiolabelled area exhibited a maximum increase (five-fold) in neutrophil infiltration 3 h after injection, relative to i.d. 200 ng 125I-BSA. We characterized the ex vivo binding of KM+ to tissue elements by immunohistochemistry, using paraformaldehyde-fixed, paraffin-embedded, untreated rat skin. Bound KM+ was detected with an affinity-purified rabbit IgG anti-KM+ and visualized with an alkaline phosphatase based system. KM+ binding to connective tissue and vascular endothelium was inhibited by preincubating KM+ with 0.4 m MD(+)mannose and was potentiated by heparan sulfate (100 g ml–1). An in vitro assay carried out in a Boyden microchamber showed that heparan sulfate potentiated the attractant effect of 10 g KM+ by 34%. The present data suggest that KM+ induces neutrophil migration in vivo by haptotaxis and that the haptotactic gradient could be provided by the interaction of the KM+ carbohydrate recognition site(s) with mannose-containing glycoconjugate(s) in vascular endothelium and connective tissue. Heparan sulfate would act as an accessory molecule, enhancing the KM+ tissue binding and potentiating the induced neutrophil haptotaxis.     

Lectin KM+-induced neutrophil haptotaxis involves binding to laminin

The lectin KM+ from Artocarpus integrifolia, also known as artocarpin, induces neutrophil migration by haptotaxis. The interactions of KM+ with both the extracellular matrix (ECM) and neutrophils depend on the lectin ability to recognize mannose-containing glycans. Here, we report the binding of KM+ to laminin and demonstrate that this interaction potentiates the KM+-induced neutrophil migration. Labeling of lung tissue by KM+ located its ligands on the endothelial cells, in the basement membrane, in the alveolus, and in the interstitial connective tissue. Such labeling was inhibited by 400 mM D-mannose, 10 mM Manalpha1-3[Manalpha1-6]Man or 10 microM peroxidase (a glycoprotein-containing mannosyl heptasaccharide). Laminin is a tissue ligand for KM+, since both KM+ and anti-laminin antibodies not only reacted with the same high molecular mass components of a lung extract, but also determined colocalized labeling in basement membranes of the lung tissue. The relevance of the KM+-laminin interaction to the KM+ property of inducing neutrophil migration was evaluated. The inability of low concentrations of soluble KM+ to induce human neutrophil migration was reversed by coating the microchamber filter with laminin. So, the interaction of KM+ with laminin promotes the formation of a substrate-bound KM+ gradient that is able to induce neutrophil haptotaxis.     

Mast-cell degranulation induced by physical stimuli involves the activation of transient-receptor-potential channel TRPV2

A characteristic of mast cells is the degranulation in response to various stimuli. Here we have investigated the effects of various physical stimuli in the human mast-cell line HMC-1. We have shown that HMC-1 express the transient receptor potential channels TRPV1, TRPV2 and TRPV4. In the whole-cell patch-clamp configuration, increasing mechanical stress applied to the mast cell by hydrostatic pressure (-30 to -90 cm H(2)O applied via the patch pipette) induced a current that could be inhibited by 10 microM of ruthenium red. This current was also inhibited by 20 microM SKF96365, an inhibitor that is among TRPV channels specific for the TRPV2. A characteristic of TRPV2 is its activation by high noxious temperature; temperatures exceeding 50 °C induced a similar ruthenium-red-sensitive current. As another physical stimulus, we applied laser light of 640 nm. Here we have shown for the first time that the application of light (at 48 mW for 20 min) induced an SKF96365-sensitive current. All three physical stimuli that led to activation of SKF96365-sensitive current also induced pronounced degranulation in the mast cells, which could be blocked by ruthenium red or SKF96365. The results suggest that TRPV2 is activated by the three different types of physical stimuli. Activation of TRPV2 allows Ca(2+) ions to enter the cell, which in turn will induce degranulation. We, therefore, suggest that TRPV2 plays a key role in mast-cell degranulation in response to mechanical, heat and red laser-light stimulation.     

Specific monocyte adhesion to endothelial cells induced by oxidized phospholipids involves activation of cPLA2 and lipoxygenase

Oxidized phospholipids stimulate endothelial cells to bind monocytes, but not neutrophils, an initiating event in atherogenesis. Here, we investigate intracellular signaling events induced by oxidized phospholipids in human umbilical vein endothelial cells (HUVECs) that lead to specific monocyte adhesion. In a static adhesion assay, oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine and one of its components, 1-palmitoyl-2-oxovaleroyl-sn-glycero-3-phosphorylcholine, stimulated HUVECs to bind U937 cells and human peripheral blood monocytes but not HL-60 cells or blood neutrophils. Monocyte adhesion was dependent on protein kinases A and C, extracellular signal-regulated kinase 1/2, p38 mitogen activated protein kinases (MAPKs), and cytosolic phospholipase A(2) (cPLA(2)). Inhibition of 12-lipoxygenase (12-LOX), but not cyclooxygenases, blocked monocyte adhesion, and addition of 12-hydroxyeicosatetraenoic acid (12-HETE) mimicked the effects of oxidized phospholipids. Peroxisome proliferator-activated receptor alpha (PPARalpha) was excluded as a possible target for 12-HETE, because monocyte adhesion was still induced in endothelial cells from PPARalpha null mice. Together, our results suggest that oxidized phospholipids stimulate HUVECs to specifically bind monocytes involving MAPK pathways, which lead to the activation of cPLA(2) and 12-LOX. Further analysis of signaling pathways induced by oxidized phospholipids that lead to specific monocyte adhesion should ultimately lead to the development of novel therapeutic approaches against chronic inflammatory diseases.     

The novel lectin KM+ detects a specific subset of mannosyl-glycoconjugates in the rat cerebellum

KM+ is a D(+)mannose-specific lectin with a carbohydrate structure-affinity relationship different from those of most mannose-binding lectins. KM+ elicits carbohydrate-dependent biological effects in several mammalian cell types, but it has not yet been employed as a probe for the detection of its specific ligands. We show here for the first time the screening and partial identification of cerebellar mannosyl-glycoconjugates recognized by KM+, by means of lectin-histochemistry and lectin-blotting. Biotinylated KM+ stained most cellular structures in the adult rat cerebellum, particularly Purkinje cells bodies and the surface of granule cells, but not cellular processes. Capillaries in the choroid plexus were also strongly decorated, while blood vessels in the cerebellar parenchyma remained unstained. D(+)mannose, but not D(+)galactose, abolished the staining of all cerebellar structures. Higher inhibitory potencies were found for mannosyl-glycans such as mannotriose (man-alpha1,3-[man-alpha1,6]-man) and the biantennary heptasaccharide carried by the enzyme horseradish peroxidase. After separation of cerebellar proteins by SDS-PAGE, KM+ recognized three major unidentified mannosyl-glycoproteins of 132, 83 and 49 kDa. KM+ also detected high-Mw bands corresponding to the light and heavy chains of Type-I laminin, but not a 160-kDa cleavage product of laminin. We conclude that KM+ binds preferentially to a specific subset of mannose-containing glycoproteins in cerebellar tissue, thus being much more restricted than other mannose-specific lectins. KM+ can be used as a novel probe to screen the central nervous system for this specific subset of complex mannosyl-glycoconjugates.     

The novel lectin KM+ detects a specific subset of mannosyl-glycoconjugates in the rat cerebellum

KM+ is a D(+)mannose-specific lectin with a carbohydrate structure-affinity relationship different from those of most mannose-binding lectins. KM+ elicits carbohydrate-dependent biological effects in several mammalian cell types, but it has not yet been employed as a probe for the detection of its specific ligands. We show here for the first time the screening and partial identification of cerebellar mannosyl-glycoconjugates recognized by KM+, by means of lectin-histochemistry and lectin-blotting. Biotinylated KM+ stained most cellular structures in the adult rat cerebellum, particularly Purkinje cells bodies and the surface of granule cells, but not cellular processes. Capillaries in the choroid plexus were also strongly decorated, while blood vessels in the cerebellar parenchyma remained unstained. D(+)mannose, but not D(+)galactose, abolished the staining of all cerebellar structures. Higher inhibitory potencies were found for mannosyl-glycans such as mannotriose (man-α1,3-[man-α1,6]-man) and the biantennary heptasaccharide carried by the enzyme horseradish peroxidase. After separation of cerebellar proteins by SDS-PAGE, KM+ recognized three major unidentified mannosyl-glycoproteins of 132, 83 and 49 kDa. KM+ also detected high-Mw bands corresponding to the light and heavy chains of Type-I laminin, but not a 160-kDa cleavage product of laminin. We conclude that KM+ binds preferentially to a specific subset of mannose-containing glycoproteins in cerebellar tissue, thus being much more restricted than other mannose-specific lectins. KM+ can be used as a novel probe to screen the central nervous system for this specific subset of complex mannosyl-glycoconjugates. Published in 2004.     

Calpain activation by cooperative Ca2+ binding at two non-EF-hand sites

The active site residues in calpain are mis-aligned in the apo, Ca(2+)-free form. Alignment for catalysis requires binding of Ca2+ to two non-EF-hand sites, one in each of the core domains I and II. Using domain swap constructs between the protease cores of the mu and m isoforms (which have different Ca2+ requirements) and structural and biochemical characterization of site-directed mutants, we have deduced the order of Ca2+ binding and the basis of the cooperativity between the two sites. Ca2+ binds first to the partially preformed site in domain I. Knockout of this site through D106A substitution eliminates binding to this domain as shown by the crystal structure of D106A muI-II. However, at elevated Ca2+ concentrations this mutant still forms the double salt bridge that links the two Ca2+ sites and becomes nearly as active as muI-II. Elimination of the bridge in E333A muI-II has a more drastic effect on enzyme action, especially at low Ca2+ concentrations. Domain II Ca2+ binding appears essential, because Ca(2+)-coordinating side-chain mutants E302R and D333A have severely impaired muI-II activation and activity. The introduction of mutations into the whole heterodimeric enzyme that eliminate the salt bridge or Ca2+ binding to domain II produce similar phenotypes, suggesting that the protease core Ca2+ switch is crucial and cannot be overridden by Ca2+ binding to other domains.     

Structural and thermodynamic studies of KM+, a d-mannose binding lectin from Artocarpus integrifolia seeds

The KM+ lectin exhibits a novel and unusual circular dichroism (CD) spectrum that could be explained by a high proline content that would be inducing deformation of the beta-structure and/or unusual turns. KM+ was shown to be a very rigid lectin, which was very stable under a broad variety of conditions (urea, guanidine, hydrolysis, pH, etc.). Only incubation for 60 min at 333-338 K and extreme basic pH were able to induce conformational changes which could be observed by CD and fluorescence measurements. Data from CD are typical for protein denaturing associated with changes in the overall secondary structure. Data from high-performance size exclusion chromatography (SEC) showed that the denatured forms produced at pH 12.0 are eluted in clusters that co-elute with the native forms. A significant contribution from the tyrosines to the fluorescence emission upon denaturation was observed above 328 K. In fact at 328 K some broadening of the emission spectrum takes place followed by the appearance of a shoulder (approx. 305 nm) at 333 K and above. The sensitivity of tryptophan fluorescence to the addition of sugar suggests a close proximity of the tryptophan residues to the sugar binding site, K(a)=(2.9+/-0.6)x10(3) M(-1). The fraction of chromophore accessible to the quencher obtained is f(a)=0.43+/-0.08, suggesting that approximately 50% of the tryptophan residues are not accessible to quenching by d-mannose. KM+ thermal denaturation was found to be irreversible and was analyzed using a two-state model (N-->D). The results obtained for the activation energy and transition temperature from the equilibrium CD studies were: activation energy, E(a)=134+/-11 kJ/mol and transition temperature, T(m)=339+/-1 K, and from the fluorescence data: E(a)=179+/-18 kJ/mol and T(m)=337+/-1 K. Kinetic studies gave the following values: E(a)=108+/-18 kJ/mol and E(a)=167+/-12 kJ/mol for CD and fluorescence data, respectively.     

Corepressor binding to progesterone and glucocorticoid receptors involves the activation function-1 domain and is inhibited by molybdate

Corepressors are known to interact via their receptor interaction domains (RIDs) with the ligand binding domain in the carboxyl terminal half of steroid/nuclear receptors. We now report that a portion of the activation function-1 domain of glucocorticoid receptors (GRs) and progesterone receptors (PRs), which is the major transactivation sequence, is necessary but not sufficient for corepressor [nuclear receptor corepressor (NCoR) and silencing mediator of retinoid and thyroid hormone receptor (SMRT)] RID binding to GRs and PRs in both mammalian two-hybrid and coimmunoprecipitation assays. Importantly, these two receptor sequences are functionally interchangeable in the context of GR for transactivation, corepressor binding, and corepressor modulatory activity assays. This suggests that corepressors may act in part by physically blocking portions of receptor activation function-1 domains. However, differences exist in corepressor binding to GRs and PRs. The C-terminal domain of PRs has a higher affinity for corepressor than that of GRs. The ability of some segments of the coactivator TIF2 to competitively inhibit corepressor binding to receptors is different for GRs and PRs. With each receptor, the cell-free binding of corepressors to ligand-free receptor is prevented by sodium molybdate, which is a well-known inhibitor of receptor activation to the DNA-binding state. This suggests that receptor activation precedes binding to corepressors. Collectively, these results indicate that corepressor binding to GRs and PRs involve both N- and C-terminal sequences of activated receptors but differ in ways that may contribute to the unique biological responses of each receptor in intact cells.     

Kinetic analysis of cooperative interactions induced by Mn2+ binding to the chloroplast H(+)-ATPase

The kinetics of Mn2+ binding to three cooperatively interacting sites in chloroplast H(+)-ATPase (CF1) were measured by EPR following rapid mixing of the enzyme with MnCl2 with a time resolution of 8 ms. Mixing of the enzyme-bound Mn2+ with MgCl2 gave a measure of the rate of exchange. The data could be best fitted to a kinetic model assuming three sequential, positively cooperative binding sites. (1) In the latent CF1, the binding to all three sites had a similar on-rate constants of (1.1 +/- 0.04) X 10(4) M-1s-1. (2) Site segregation was found in the release of ions with off-rate constants of 0.69 +/- 0.04 s-1 for the first two and 0.055 +/- 0.003 s-1 for the third. (3) Addition of one ADP per CF1 caused a decrease in the off-rate constants to 0.31 +/- 0.02 and 0.033 +/- 0.008 s-1 for the first two and the third sites, respectively. (4) Heat activation of CF1 increased the on-rate constant to (4.2 +/- 0.92) X 10(4) M-1s-1 and the off-rate constants of the first two and the third site to 1.34 +/- 0.08 and 0.16 +/- 0.07 s-1, respectively. (5) The calculated thermodynamic dissociation constants were similar to those previously obtained from equilibrium binding studies. These findings were correlated to the rate constants obtained from studies of the catalysis and regulation of the H(+)-ATPase. The data support the suggestion that regulation induces sequential progress of catalysis through the three active sites of the enzyme.     

Neutrophil activation detected by monoclonal antibodies

Monoclonal antibodies have been produced against three neutrophil-associated membrane proteins (p 90, p 170, and p 70) expressed at different maturation stages of the cells. The reactivity of the antibodies against p 90 (B13.9) and p 170 (CLB-gran 10), as measured by quantitative flow cytofluorometry, increased after stimulation of the neutrophils by the calcium ionophore A23187, by phorbol myristate acetate, or by the chemoattractant formylmethionyl-leucyl-phenylalanine in combination with cytochalasin B. This increase is regulated independently of the simultaneously increased expression of the C3bi receptor, because neutrophils of a patient deficient for the C3bi receptor showed a normal increase in membrane expression of p 90 and p 170. Neutrophil cytoplasts were not inducible to increased membrane expression, suggesting that the cytoplasts lack the internal pool of these proteins. The reactivity of the antibody against p 70 (CLB-gram 5) was not affected by activation. The antibodies B13.9 and CLB-gran 10 may be useful to detect neutrophil activation.     

The conformation of the activation peptide of protein C is influenced by Ca2+ and Na+ binding

Previous studies have suggested that the conformation of the activation peptide of protein C is influenced by the binding of Ca(2+). To provide direct evidence for the linkage between Ca(2+) binding and the conformation of the activation peptide, we have constructed a protein C mutant in the gamma-carboxyglutamic acid-domainless form in which the P1 Arg(169) of the activation peptide is replaced with the fluorescence reporter Trp. Upon binding of Ca(2+), the intrinsic fluorescence of the mutant decreases approximately 30%, as opposed to only 5% for the wild-type, indicating that Trp(169) is directly influenced by the divalent cation. The K(d) of Ca(2+) binding for the mutant protein C was impaired approximately 4-fold compared with wild-type. Interestingly, the conformation of the activation peptide was also found to be sensitive to the binding of Na(+), and the affinity for Na(+) binding increased approximately 5-fold in the presence of Ca(2+). These findings suggest that Ca(2+) changes the conformation of the activation peptide of protein C and that protein C is also capable of binding Na(+), although with a weaker affinity compared with the mature protease. The mutant protein C can no longer be activated by thrombin but remarkably it can be activated efficiently by chymotrypsin and by the thrombin mutant D189S. Activation of the mutant protein C by chymotrypsin proceeds at a rate comparable to the activation of wild-type protein C by the thrombin-thrombomodulin complex.     

Neutrophil degranulation induced by Haemophilus ducreyi

Haemophilus ducreyi is the causative bacterium of genital ulcers, which are collectively known as chancroid. Little is known about the cytotoxicity of H. ducreyi. The virulent strains are relatively resistant to phagocytosis and apoptosis by neutrophils. Therefore, experiments were designed to examine whether neutrophil degranulation caused by H. ducrey would provide insights into the virulence mechanisms through which cellular damage is affected by the organism. Clinical isolates of eight strains of H. ducreyi and the culture strain type CIP542 (Collection Institute Pasteur) were incubated with neutrophils harvested from human donor blood. The release by the organism of lysosomal enzymes from intracellular granules of neutrophils was indicative of degranulation. The results showed that H. ducreyi triggered the release of lysosomal enzymes from human neutrophils, and that the magnitude of the release was dependent both on the ratio of bacteria to neutrophils and the duration of incubation. In vitro experiments involving HeLa cells were designed to determine the manner in which H. ducreyi initiated the process of degranulation. The morphological changes associated with degranulation were visualized by confocal and transmission electron microscopy. This is the first report that describes degranulation of neutrophils induced by H. ducreyi which causes chancroid infection.     

Apoptosis induction by epigallocatechin gallate involves its binding to Fas

Epigallocatechin gallate (EGCG) is known to induce apoptosis in various types of tumor cells, but the precise mechanism by which EGCG induces apoptosis remains to be elucidated. The Fas-Fas ligand system is one of the major pathways operating in the apoptotic cascade. The aim of this study was to examine the possibility that EGCG-binding to Fas triggers the Fas-mediated apoptosis. The EGCG treatment of human monocytic leukemia U937 cells resulted in elevation of caspase 8 activity and fragmentation of caspase 8. The DNA ladder formation caused by the EGCG treatment was inhibited by the caspase 8 inhibitor. These findings suggested the involvement of the Fas-mediated cascade in the EGCG-induced apoptosis in U937 cells. Affinity chromatography revealed the binding between EGCG and Fas. Thus, the results suggest that EGCG-binding to Fas, presumably on the cell surface, triggers the Fas-mediated apoptosis in U937 cells.     

Neutrophil activation by sea hydrobiont biopolymers]

Biopolymers of sea hydrobionts such as mytilan, alpha-1,4;1,6-D-glycan isolated from the muntle of the mussel Crenomytilus grayanus; translam, beta-1,3;1,6-D-glucan isolated from the seaweed Laminaria cichorioides; fucoidan, a sulfated polysccharide isolated from the algae Fucus evanescens; zosterin, a pectin isolated from sea grass of the family Zosteraceae were comparatively studied. The mechanisms of the phagocyte activation were investigated and the dose-dependent ability of the biopolymers to increase in vitro adhesion of the intact cells and to restore the neutrophil functions at cyclophosphamide-induced immunodepression was detected. The neutrophil activation by mytilan, zosterin and fucoidan linked with the adhesion potentiation was shown to be associated with their ability to increase the number of the adhesion receptors and in particular CD116b on the cell surface. The lower potential of the neutrophils preincubated in vitro with high doses of translam beta-glucan could be due to blockade of the beta-glucan receptors participating in the complex multicomponent adhesion process. The use of the biopolymers of the sea hydrobionts of the glycobiological nature for modulation of the immunity processes provided rather convenient in vivo management of intracellular processes through direct and competing carbohydrate specific interactions of the modifiers with the membrane receptors and formation of active and inactive lectin-glycoligand and carbohydrate-carbohydrate complexes.     

Neutrophil tolerance to oxidative stress induced by hypoxia/reoxygenation

Repetitive episodes of hypoxia/reoxygenation induce cellular adaptations resulting in a tolerance process against oxidative stress. We studied the effects of chronic episodes of hypoxia/reoxygenation on neutrophil antioxidant defenses, neutrophil oxidative capability, and oxidative damage induced in neutrophils and plasma. Seven professional apnea divers participated in the study. Blood samples were taken under basal conditions, after a diving apnea session, and under basal conditions after five consecutive days of diving apnea sessions (basal post-diving). Chronic episodes of hypoxia/reoxygenation increased malondialdehyde (MDA), carbonyl derivates and creatine kinase (CPK) in plasma. Neutrophil catalase (CAT) levels were higher in basal post-diving. Neutrophil oxidative burst was maintained after diving, although the maximum response was delayed in basal post-diving. Neutrophil thioredoxin reductase (TR) activity increased in basal post-diving, and glutathione reductase (GR) activity was maintained. Chronic, repetitive episodes of diving apnea induce neutrophil adaptations in order to delay the oxidative burst response and to facilitate protein reduction. Diving apnea could be a good model to study tolerance to the oxidative stress generated by hypoxia/reoxygenation.     

Plasma membrane depolarization induced by abscisic acid in Arabidopsis suspension cells involves reduction of proton pumping in addition to anion channel activation, which are both Ca2+ dependent

In Arabidopsis suspension cells a rapid plasma membrane depolarization is triggered by abscisic acid (ABA). Activation of anion channels was shown to be a component leading to this ABA-induced plasma membrane depolarization. Using experiments employing combined voltage clamping, continuous measurement of extracellular pH, we examined whether plasma membrane H(+)-ATPases could also be involved in the depolarization. We found that ABA causes simultaneously cell depolarization and medium alkalinization, the second effect being abolished when ABA is added in the presence of H+ pump inhibitors. Inhibition of the proton pump by ABA is thus a second component leading to the plasma membrane depolarization. The ABA-induced depolarization is therefore the result of two different processes: activation of anion channels and inhibition of H(+)-ATPases. These two processes are independent because impairing one did not suppress the depolarization. Both processes are however dependent on the [Ca2+]cyt increase induced by ABA since increase in [Ca(2+)](cyt) enhanced anion channels and impaired H(+)-ATPases.     

Depolarization-induced Ca2+ release in ischemic spinal cord white matter involves L-type Ca2+ channel activation of ryanodine receptors

The mechanisms of Ca(2+) release from intracellular stores in CNS white matter remain undefined. In rat dorsal columns, electrophysiological recordings showed that in vitro ischemia caused severe injury, which persisted after removal of extracellular Ca(2+); Ca(2+) imaging confirmed that an axoplasmic Ca(2+) rise persisted in Ca(2+)-free perfusate. However, depletion of Ca(2+) stores or reduction of ischemic depolarization (low Na(+), TTX) were protective, but only in Ca(2+)-free bath. Ryanodine or blockers of L-type Ca(2+) channel voltage sensors (nimodipine, diltiazem, but not Cd(2+)) were also protective in zero Ca(2+), but their effects were not additive with ryanodine. Immunoprecipitation revealed an association between L-type Ca(2+) channels and RyRs, and immunohistochemistry confirmed colocalization of Ca(2+) channels and RyR clusters on axons. Similar to "excitation-contraction coupling" in skeletal muscle, these results indicate a functional coupling whereby depolarization sensed by L-type Ca(2+) channels activates RyRs, thus releasing damaging amounts of Ca(2+) under pathological conditions in white matter.