6-Hydroxydopamine-induced alterations in blood-brain barrier permeability.

Vascular inflammation is well known for its ability to compromise the function of the blood--brain barrier (BBB). Whether inflammation on the parenchymal side of the barrier, such as that associated with Parkinson's-like dopamine (DA) neuron lesions, similarly disrupts BBB function, is unknown. We assessed BBB integrity by examining the leakage of FITC-labeled albumin or horseradish peroxidase from the vasculature into parenchyma in animals exposed to the DA neurotoxin 6-hydroxydopamine (6OHDA). Unilateral injections of 6OHDA into the striatum or the medial forebrain bundle produced increased leakage in the ipsilateral substantia nigra and striatum 10 and 34 days following 6OHDA. Microglia were markedly activated and DA neurons were reduced by the lesions. The areas of BBB leakage were associated with increased expression of P-glycoprotein and beta 3-integrin expression suggesting, respectively, a compensatory response to inflammation and possible angiogenesis. Behavioural studies revealed that domperidone, a DA antagonist that normally does not cross the BBB, attenuated apomorphine-induced stereotypic behaviour in animals with 6OHDA lesions. This suggests that drugs which normally have no effect in brain can enter following Parkinson-like lesions. These data suggest that the events associated with DA neuron loss compromise BBB function.

Fas activation reduces neutrophil adhesion to endothelial cells.

Polymorphonuclear neutrophils (PMN) express apoptotic markers and lose effector functions including adhesion, chemotaxis, and phagocytosis when cultured overnight. Although the loss of function correlates with apoptosis, it is not clear if functions are lost before an early marker of apoptosis, the display of phosphatidylserine (PS), targets PMN for removal by phagocytic cells. To address this question, freshly isolated PMN were treated with Fas-activating antibodies to induce apoptosis rapidly. Early markers of apoptosis and PMA-stimulated adhesion to endothelial cells were measured. After 1 h of Fas exposure, only 16% PMN had externalized PS. In contrast, Fas activation reduced PMA-stimulated adhesion between 68 and 27% depending on PMA concentration. The loss of adhesion was accompanied by a reduction in beta2 integrin expression and receptor clustering. These results indicate that the Fas-induced loss of adhesion may precede PS externalization and could limit participation in the inflammatory response before PS externalization targets PMN for removal.

Oriented endocytic recycling of alpha5beta1 in motile neutrophils.

During cell migration, integrin attachments to the substratum provide the means to generate the traction and force necessary to achieve locomotion. Once the cell has moved over these attachments, however, it is equally important that integrins detach from the substratum. The fate of integrins after detachment may include release from the cell, lateral diffusion across the cell surface, or endocytosis and redelivery to the cell surface. Polymorphonuclear neutrophils (PMNs) become stuck on the extracellular matrix proteins fibronectin and vitronectin when their intracellular free calcium concentration ([Ca(++)]i) is buffered. Taking advantage of this feature of PMN migration, we investigated the fate of integrins to differentiate among various models of migration. We demonstrate that alpha5beta1, one of the fibronectin-binding integrins, is responsible for immobilization of [Ca(++)](i)-buffered PMNs on fibronectin. We find that alpha5 and beta1 are in endocytic vesicles in PMNs and that alpha5 colocalizes with a marker for an endocytic recycling compartment. When [Ca(++)](i) is buffered, alpha5 and beta1 become concentrated in clusters in the rear of the adherent cells, suggesting that [Ca(++)](i) transients are required for alpha5beta1 detachment from the substratum. Inhibition of alpha5beta1 detachment by buffering [Ca(++)](i) results in the depletion of alpha5 from both endocytic vesicles and the recycling compartment, providing compelling evidence that integrins are normally recycled by way of endocytosis and intracellular trafficking during cell migration. This model is further refined by our demonstration that the endocytic recycling compartment reorients to retain its localization just behind the leading lamella as PMNs migrate, indicating that membrane recycling during neutrophil migration has directionality. (Blood. 2000;95:2471-2480)

Fibronectin enhances the migration rate of human neutrophils in vitro.

Efficient polymorphonuclear neutrophil (PMN) migration depends on specific interactions between PMNs, endothelial cells, and extracellular matrix (ECM) proteins. We investigated the relationship between PMN migration and the ECM molecule fibronectin (FN). We used an in vitro migration assay system to show that human PMNs migrated across an FN-coated filter barrier toward a formyl-Met-Leu-Phe (fMLP) chemoattractant gradient in greater numbers than across (uncoated) bare fitters. In 1 h of fMLP stimulation, 69 +/- 6% of the PMNs had migrated across the FN-coated filters, whereas 46 +/- 5% of PMNs migrated across bare filters. This effect was specific to FN; coating the filters with the ECM protein vitronectin did not enhance migration. Monoclonal antibodies against FN or against the alpha5 or beta1 integrin subunits of the FN receptor inhibited the enhanced PMN migration response across FN-coated filters. These findings indicate that the extracellular matrix protein FN enhances PMN migration and that this response is mediated by the alpha5beta1 FN receptor.

Intracellular calcium and calcineurin regulate neutrophil motility on vitronectin through a receptor identified by antibodies to integrins alphav and beta3.

Buffering of intracellular calcium ([Ca2+]i) or inhibition of the calcium/calmodulin-dependent phosphatase, calcineurin, results in neutrophils being unable to detach from vitronectin with a consequent loss of motility. Treatment of [Ca2+]i-buffered or calcineurin-inhibited neutrophils with monoclonal antibodies (MoAbs) to beta3 or alphav beta3 integrins allowed neutrophils to detach and restored motility. Quantitative immunofluorescence and flow cytometry showed that MoAbs specific for beta3, alphav, or alphav beta3 integrins bind to neutrophils. Immunolocalization studies using antibodies to the highly conserved cytoplasmic domains of alphav and beta3 also identified the receptor on neutrophils. Whereas antibodies to alphav, alphav beta3, and beta3 recognized the receptor in intact cells, only the beta3 MoAb immunoprecipitated the receptor from a neutrophil cell lysate. The alpha subunit co-immunoprecipitated by the beta3 antibody reacted with an antibody to alphav by Western blot. Peptide maps of V8 protease digests showed a strong similarity in alpha and beta chains precipitated by antibodies to beta3 from neutrophils and endothelial cells. These results indicate that [Ca2+]i and calcineurin regulate neutrophil motility on vitronectin through an alphav beta3-like receptor. Although we cannot rule out the possibility that neutrophils have an isoform of alphav, such an isoform would have to be similar enough to react with alphav- and alphav beta3-specific MoAbs in intact cells.

Regulation of neutrophil motility and adhesion by intracellular calcium transients.

Neutrophils are guided to the sites of infection or inflammation by gradients of chemoattractants. Chemoattractants stimulate rapid and repeated changes in neutrophil intracellular calcium, [Ca2+]i, which correlate with cell spreading, pseudopod extension, motility, change of direction and phagocytosis. However, blocking the [Ca2+]i transients has little effect on cell spreading, polarization or pseudopod extension. Thus, either the [Ca2+]i transients are not required for cell spreading, polarization or pseudopod extension or other redundant mechanisms are present that allow the cells to perform these functions in vitro. In contrast, cell motility is [Ca2+]i dependent when the cells are examined on physiological substrates such as fibronectin or vitronectin. Calcium-buffered cells appear to make repeated attempts to move but are unable to detach from a fibronectin or vitronectin substrate. Motility can be restored to [Ca2+]i buffered cells by blocking substrate attachment with RGD peptides or by using a less adherent substrate such as albumin. A similar inhibition of motility on vitronectin could be induced by inhibitors of the calcium/calmodulin-dependent phosphatase, calcineurin. Thus, the periodic increases in [Ca2+]i apparently activate the phosphatase calcineurin to initiate a cycle of detachment from the vitronectin substratum. These data suggest that the [Ca2+]i transients regulate motility by coordinating a series of substrate-specific attachment/detachment events.

Inhibition of neutrophil chemokinesis on vitronectin by inhibitors of calcineurin.

Migration of human polymorphonuclear neutrophils on vitronectin is dependent on repeated transient increases in the concentration of intracellular free calcium ([Ca2+]i). A specific peptide inhibitor of the Ca(2+)-calmodulin-dependent phosphatase calcineurin was introduced into the cytoplasm of neutrophils. The peptide inhibited neutrophil migration on vitronectin by interfering with the release of the cells from sites of attachment. A similar reduction in motility on vitronectin occurred when cells were treated with the immunosuppressant FK506, which also inhibits calcineurin when bound to its binding protein, FKBP. These results indicate that a rise in [Ca2+]i reduces integrin-mediated adhesion to vitronectin by a mechanism that requires calcineurin activity.

WS-1 human fibroblasts contain distinct calcium and protein kinase C-mediated pathways for activation of Na+/H+ exchange: contrasting effects of thrombin and PMA.

PMA and thrombin were examined for their ability to activate Na+/H+ exchange in growth-arrested WS-1 human fibroblasts. PMA or thrombin caused a cytoplasmic alkalinization that required extracellular sodium and was sensitive to 1 mM amiloride, suggesting that the rise in pH was mediated by the Na+/H+ exchanger. However, PMA and thrombin activated Na+/H+ exchange by distinctly different mechanisms. The rate of cytoplasmic alkalinization caused by 30 nM PMA was slower than 10 nM thrombin. The PMA-induced pH change was sensitive to the protein kinase inhibitors staurosporine (50 nM) and H-7 (100 microM). No increase in intracellular calcium was observed after PMA treatment and the cytoplasmic alkalinization caused by PMA was not sensitive to the drug TMB8 (200 microM) or the intracellular calcium-chelator BAPTA. In contrast, the thrombin-induced rise in cytoplasmic pH was insensitive to 50 nM staurosporine and only partially reduced with 100 microM H-7. The thrombin-induced activation of Na+/H+ exchange was inhibited by 200 microM TMB8 or pretreatment with BAPTA. PMA caused translocation of PKC activity from a cytoplasmic to membrane fraction whereas thrombin did not. Pretreatment with 50 nM staurosporine significantly reduced measurable PKC activity with or without PMA treatment. PMA and thrombin were also examined for their ability to induce DNA synthesis in growth-arrested WS-1 human fibroblasts. Unlike thrombin, PMA did not stimulate [3H]-thymidine incorporation in cells serum-deprived for 48 hours. In addition, PMA inhibited thrombin-induced DNA synthesis when added at the same time or as late as 10 hours after thrombin addition. Therefore, thrombin and PMA activate Na+/H+ exchange by distinct pathways, but only the thrombin-induced pathway correlates with a mitogenic response.

Attachment to fibronectin or vitronectin makes human neutrophil migration sensitive to alterations in cytosolic free calcium concentration.

Transient increases in cytosolic free calcium concentration, [Ca2+]i, appear to be required for the migration of human neutrophils on poly-D-lysine-coated glass in the presence of dilute serum (Marks, P. W., and F. R. Maxfield. 1990. J. Cell Biol. 110:43-52). In contrast, no requirement for [Ca2+]i transients exists when neutrophils migrate on albumin-coated glass in the absence of serum. To determine the mechanism that necessitates [Ca2+]i transients on poly-D-lysine in the presence of serum, migration was examined on substrates consisting of purified adhesive glycoproteins. In the absence of external Ca2+, a treatment which causes the cessation of [Ca2+]i transients, migration on fibronectin (fn) and vitronectin (vn) was significantly inhibited. Migration was also inhibited in Ca2(+)-buffered cells on these substrates, indicating that this effect was the result of an alteration of [Ca2+]i. In the absence of external Ca2+, the inhibition of migration on fn or vn was more pronounced when soluble fn or vn was added to cells migrating on these substrates. This effect of soluble adhesive glycoprotein was specific: in the absence of external Ca2+, soluble fn did not affect the migration of cells on vn, and soluble vn did not affect the migration on fn. No additional inhibition of migration was observed in Ca2(+)-buffered cells with the addition of soluble adhesive glycoprotein. These data indicate that [Ca2+]i transients are involved in continued migration of human neutrophils on fn or vn, proteins which are part of the extracellular matrix that neutrophils encounter in vivo.

Thrombin induces a calcium transient that mediates an activation of the Na+/H+ exchanger in human fibroblasts.

The calcium dependence of growth factor-induced cytoplasmic alkalinization was determined in serum-deprived human fibroblasts (WS-1 cells). Intracellular pH (pHi) and intracellular calcium (Ca2+i) were measured using the fluorescent dyes 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein and fura2, respectively. Thrombin (10 nM) induced an alkalinization (0.18 +/- 0.01 pH units, n = 23) that was Na+-dependent and amiloride-sensitive, suggesting that the alkalinization was mediated by the Na+/H+ exchanger. Thrombin treatment caused a transient increase in Ca2+i (325 +/- 39 nM, n = 12) that preceded the observed increase in pHi. The increases in Ca2+i and pHi were dependent on the concentration of thrombin. The thrombin-induced increase in Ca2+i occurred in the absence of external calcium indicating that thrombin released calcium from internal stores. Inhibition of the thrombin-induced increase in Ca2+i with 8-diethylaminooctyl 3,4,5-trimethoxybenzoate hydrochloride or bis-(o-aminophenoxy)ethane-N,N,N',N'- tetraacetic acid also inhibited the thrombin-stimulated increase in pHi. The calcium ionophore ionomycin was used to increase Ca2+i independent of growth factor stimulation. When Ca2+i was elevated with ionomycin, a concomitant increase in pHi was observed. The increase in pHi due to ionomycin was dependent on Na+ and sensitive to amiloride. The removal of external Ca2+i inhibited the ionomycin-induced elevation of both Ca2+i and pHi. The ionomycin-induced increases in Ca2+i and pHi were not inhibited by 8-diethylaminooctyl 3,4,5-trimethoxy-benzoate hydrochloride. The results suggest that thrombin treatment can activate the Na+/H+ exchanger, and this activation is mediated by an increase in Ca2+i.

Differential response of normal human fibroblasts to bombesin versus thrombin.

Normal human diploid fibroblasts (WS-1 cells) were growth-arrested under serum-free conditions for 48 hr. The addition of fetal bovine serum (10% final concentration) to these cells stimulated [3H]-thymidine incorporation into DNA and phosphoinositide breakdown over nine-fold. Thrombin, at concentrations above 0.1 unit/ml (u/ml), was also effective at stimulating DNA synthesis and phosphoinositide breakdown as well as causing a rise in intracellular pH. In contrast, the peptide bombesin (concentrations ranging from 1 nM to 100 nM) stimulated phosphoinositide breakdown but did not enhance DNA synthesis or cause an increase in cytoplasmic pH. The time course of accumulation of inositol phosphates differed in response to these agents. The thrombin effect peaked rapidly and leveled off after 5 min while the bombesin effect showed a constant increase for 30 min. Serum showed an intermediate response. The different rates of inositol phosphate accumulation observed with the two growth factors is viewed as representing a difference in the mechanism of phosphoinositide turnover. The relationship between the difference in phosphoinositide turnover and the initiation of DNA synthesis is also discussed.