Age-associated changes in the blood-brain barrier: comparative studies in human and mouse

Aims

While vascular pathology is a common feature of a range of neurodegenerative diseases, we hypothesized that vascular changes occur in association with normal ageing. Therefore, we aimed to characterize age‐associated changes in the blood–brain barrier (BBB) in human and mouse cohorts.

Methods

Immunohistochemistry and Evans blue assays were used to characterize BBB dysfunction (tight junction protein expression and serum plasma protein accumulation), vascular pathology (pericyte loss and vascular density) and glial pathology (astrocyte and microglial density) in ageing neurological control human prefrontal cortex (a total of 23 cases from 5 age groups representing the spectrum of young adult to old age: 20–30 years, 31–45 years, 46–60 years, 61–75 years and 75+) and C57BL/6 mice (3 months, 12 months, 18 months and 24 months, n = 5/6 per group).

Results

Quantification of the tight junction protein ZO‐1 within the cortex and cerebellum of the mouse cohort showed a significant trend to both increased number (cortex P < 0.001, cerebellum P < 0.001) and length (cortex P < 0.001, cerebellum P < 0.001) of junctional breaks associated with increasing age. GFAP expression significantly correlated with ageing in the mice (P = 0.037). In the human cohort, assessment of human protein accumulation (albumin, fibrinogen and human IgG) demonstrated cells morphologically resembling clasmatodendritic astrocytes, indicative of BBB dysfunction. Semiquantitative assessment of astrogliosis in the cortex expression revealed an association with age (P = 0.003), while no age‐associated changes in microglial pathology, microvascular density or pericyte coverage were detected.

Conclusions

This study demonstrates BBB dysfunction in normal brain ageing, both in human and mouse cohorts.

Zinc-imidazolate polymers (ZIPs) as a potential carrier to brain capillary endothelial cells

Herein, we report the synthesis and characterization of nanospheres of a biodegradable zinc-imidazolate polymers (ZIPs) as a proof-of-concept delivery vehicle into human brain endothelial cells, the main component of the blood-brain barrier (BBB). The ZIP particles can readily encapsulate functional molecules such as fluorophores and inorganic nanoparticles at the point of synthesis producing stable colloidal dispersions. Our results show that these biodegradable particles are not cytotoxic, and are able to penetrate and release cargo species to human brain capillary endothelial cells in vitro thus exhibiting significant potential as a novel platform for brain targeting treatments.

Protective effect of the multitarget compound DPH-4 on human SSAO/VAP-1-expressing hCMEC/D3 cells under oxygen-glucose deprivation conditions: an in vitro experimental model of cerebral ischaemia

BACKGROUND AND PURPOSE

Stroke and Alzheimer's disease (AD) are related pathologies in which the cerebrovascular system is involved. Plasma levels of semicarbazide-sensitive amine oxidase/vascular adhesion protein 1 (SSAO/VAP-1, also known as Primary Amine Oxidase -PrAO) are increased in both stroke and AD patients and contribute to the vascular damage. During inflammation, its enzymatic activity mediates leukocyte recruitment to the injured tissue, inducing damage in the blood-brain barrier (BBB) and neuronal tissue. We hypothesized that by altering cerebrovascular function, SSAO/VAP-1 might play a role in the stroke-AD transition. Therefore, we evaluated the protective effect of the novel multitarget-directed ligand DPH-4, initially designed for AD therapy, on the BBB.

EXPERIMENTAL APPROACH

A human microvascular brain endothelial cell line expressing human SSAO/VAP-1 was generated, as the expression of SSAO/VAP-1 is lost in cultured cells. To simulate ischaemic damage, these cells were subjected to oxygen and glucose deprivation (OGD) and re-oxygenation conditions. The protective role of DPH-4 was then evaluated in the presence of methylamine, an SSAO substrate, and/or β-amyloid (Aβ).

KEY RESULTS

Under our conditions, DPH-4 protected brain endothelial cells from OGD and re-oxygenation-induced damage, and also decreased SSAO-dependent leukocyte adhesion. DPH-4 was also effective at preventing the damage induced by OGD and re-oxygenation in the presence of Aβ as a model of AD pathology.

CONCLUSIONS AND IMPLICATIONS

From these results, we concluded that the multitarget compound DPH-4 might be of therapeutic benefit to delay the onset and/or progression of the neurological pathologies associated with stroke and AD, which appear to be linked.

Characterizing the pathotype of neonatal meningitis causing Escherichia coli (NMEC)

Background

Neonatal meningitis-causing Escherichia coli (NMEC) is the predominant Gram-negative bacterial pathogen associated with meningitis in newborn infants. High levels of heterogeneity and diversity have been observed in the repertoire of virulence traits and other characteristics among strains of NMEC making it difficult to define the NMEC pathotype. The objective of the present study was to identify genotypic and phenotypic characteristics of NMEC that can be used to distinguish them from commensal E. coli.

Methods

A total of 53 isolates of NMEC obtained from neonates with meningitis and 48 isolates of fecal E. coli obtained from healthy individuals (HFEC) were comparatively evaluated using five phenotypic (serotyping, serum bactericidal assay, biofilm assay, antimicorbial susceptibility testing, and in vitro cell invasion assay) and three genotypic (phylogrouping, virulence genotyping, and pulsed-field gel electrophoresis) methods.

Results

A majority (67.92 %) of NMEC belonged to B2 phylogenetic group whereas 59 % of HFEC belonged to groups A and D. Serotyping revealed that the most common O and H types present in NMEC tested were O1 (15 %), O8 (11.3 %), O18 (13.2 %), and H7 (25.3 %). In contrast, none of the HFEC tested belonged to O1 or O18 serogroups. The most common serogroup identified in HFEC was O8 (6.25 %). The virulence genotyping reflected that more than 70 % of NMEC carried kpsII, K1, neuC, iucC, sitA, and vat genes with only less than 27 % of HFEC possessing these genes. All NMEC and 79 % of HFEC tested were able to invade human cerebral microvascular endothelial cells. No statistically significant difference was observed in the serum resistance phenotype between NMEC and HFEC. The NMEC strains demonstrated a greater ability to form biofilms in Luria Bertani broth medium than did HFEC (79.2 % vs 39.9 %).

Conclusion

The results of our study demonstrated that virulence genotyping and phylogrouping may assist in defining the potential NMEC pathotype.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0547-9) contains supplementary material, which is available to authorized users.

BBB on chip: microfluidic platform to mechanically and biochemically modulate blood-brain barrier function

The blood-brain barrier (BBB) is a unique feature of the human body, preserving brain homeostasis and preventing toxic substances to enter the brain. However, in various neurodegenerative diseases, the function of the BBB is disturbed. Mechanisms of the breakdown of the BBB are incompletely understood and therefore a realistic model of the BBB is essential. We present here the smallest model of the BBB yet, using a microfluidic chip, and the immortalized human brain endothelial cell line hCMEC/D3. Barrier function is modulated both mechanically, by exposure to fluid shear stress, and biochemically, by stimulation with tumor necrosis factor alpha (TNF-α), in one single device. The device has integrated electrodes to analyze barrier tightness by measuring the transendothelial electrical resistance (TEER). We demonstrate that hCMEC/D3 cells could be cultured in the microfluidic device up to 7 days, and that these cultures showed comparable TEER values with the well-established Transwell assay, with an average (± SEM) of 36.9 Ω.cm(2) (± 0.9 Ω.cm(2)) and 28.2 Ω.cm(2) (± 1.3 Ω.cm(2)) respectively. Moreover, hCMEC/D3 cells on chip expressed the tight junction protein Zonula Occludens-1 (ZO-1) at day 4. Furthermore, shear stress positively influenced barrier tightness and increased TEER values with a factor 3, up to 120 Ω.cm(2). Subsequent addition of TNF-α decreased the TEER with a factor of 10, down to 12 Ω.cm(2). This realistic microfluidic platform of the BBB is very well suited to study barrier function in detail and evaluate drug passage to finally gain more insight into the treatment of neurodegenerative diseases.

CCL2 binding is CCR2 independent in primary adult human astrocytes

Chemokines are low relative molecular mass proteins, which have chemoattractant actions on many cell types. The chemokine, CCL2, has been shown to play a major role in the recruitment of monocytes in central nervous system (CNS) lesions in multiple sclerosis (MS). Since resident astrocytes constitute a major source of chemokine synthesis including CCL2, we were interested to assess the regulation of CCL2 by astrocytes. We showed that CCL2 bound to the cell surface of astrocytes and binding was not modulated by inflammatory conditions. However, CCR2 protein was not detected nor was activation of the classical CCR2 downstream signaling pathways. Recent studies have shown that non-signaling decoy chemokine receptors bind and modulate the expression of chemokines at site of inflammation. Here, we show that the D6 chemokine decoy receptor is constitutively expressed by primary human adult astrocytes at both mRNA and protein level. In addition, CCL3, which binds to D6, but not CCL19, which does not bind to D6, displaced CCL2 binding to astrocytes; indicating that CCL2 may bind to this cell type via the D6 receptor. Our results suggest that CCL2 binding to primary adult human astrocytes is CCR2-independent and is likely to be mediated via the D6 decoy chemokine receptor. Therefore we propose that astrocytes are implicated in both the establishment of chemokine gradients for the migration of leukocytes into and within the CNS and in the regulation of CCL2 levels at inflammatory sites in the CNS.

Molecular cytogenetic characterization of the human cerebral microvessel endothelial cell line hCMEC/D3

The immortalized human cerebral microvessel endothelial cell line hCMEC/D3 has been repeatedly used as a model of human blood-brain barrier (BBB). hCMEC/D3 cells between passage 25 and 35 are most often applied in research, remained phenotypically nontransformed, and cells maintained many characteristics of human brain endothelial cells. Also hCMEC/D3 was thought to have conserved a normal diploid karyotype over all these passages. Here we characterized the cell line using high-resolution multicolor fluorescence in situ hybridization (FISH) approaches and revealed a complex karyotype in the 30th passage. Clonal cryptic unbalanced structural rearrangements and numerical aberrations were discovered and described as follows: 45 approximately 48,XX, -X,del(5)(q11)[2],del(9)(q11)[3],+9[3],del(11)(q13 approximately 14)[2], der(14)t(14;21)(q32.33;q22.3)[28],der(15)t(9;15)(p11;p11)[13], dup(15)(p11q11)[5],der(21)t(17;21)(p12;q22)[9],-22[6][cp28]. In summary, a complex karyotype with clonal unbalanced chromosomal rearrangements is present in hCMEC/D3. Thus, we solicit to include molecular cytogenetics in the testing of all cell lines prior to application of their use in complex studies.

Differential activation of mitochondrial apoptotic pathways by vasculotropic amyloid-beta variants in cells composing the cerebral vessel walls

Cerebral amyloid angiopathy (CAA) is an age-associated condition and a common finding in Alzheimer's disease in which amyloid-beta (Abeta) vascular deposits are featured in >80% of the cases. Familial Abeta variants bearing substitutions at positions 21-23 are primarily associated with CAA, although they manifest with strikingly different clinical phenotypes: cerebral hemorrhage or dementia. The recently reported Piedmont L34V Abeta mutant, located outside the hot spot 21-23, shows a similar hemorrhagic phenotype, albeit less aggressive than the widely studied Dutch E22Q variant. We monitored the apoptotic events occurring after stimulation of human brain microvascular endothelial and smooth muscle cells with nonfibrillar structures of both variants and wild-type Abeta40. Induction of analogous caspase-mediated mitochondrial pathways was elicited by all peptides, although within different time frames and intensity. Activated pathways were susceptible to pharmacological modulation either through direct inhibition of mitochondrial cytochrome c release or by the action of pan- and pathway-specific caspase inhibitors, giving a clear indication of the independent or synergistic engagement of both extrinsic and intrinsic mechanisms. Structural analyses of the Abeta peptides showed that apoptosis preceded fibril formation, correlating with the presence of oligomers and/or protofibrils. The data support the notion that rare genetic mutations constitute unique paradigms to understand the molecular pathogenesis of CAA.

Amyloid-beta-induced occludin down-regulation and increased permeability in human brain endothelial cells is mediated by MAPK activation

Vascular dysfunction is emerging as a key pathological hallmark in Alzheimer’s disease (AD). A leaky blood–brain barrier (BBB) has been described in AD patient tissue and in vivo AD mouse models. Brain endothelial cells (BECs) are linked together by tight junctional (TJ) proteins, which are a key determinant in restricting the permeability of the BBB. The amyloid β (Aβ) peptides of 1–40 and 1–42 amino acids are believed to be pivotal in AD pathogenesis. We therefore decided to investigate the effect of Aβ 1–40, the Aβ variant found at the highest concentration in human plasma, on the permeability of an immortalized human BEC line, hCMEC/D3. Aβ 1–40 induced a marked increase in hCMEC/D3 cell permeability to the paracellular tracer 70 kD FITC-dextran when compared with cells incubated with the scrambled Aβ 1–40 peptide. Increased permeability was associated with a specific decrease, both at the protein and mRNA level, in the TJ protein occludin, whereas claudin-5 and ZO-1 were unaffected. JNK and p38MAPK inhibition prevented both Aβ 1–40-mediated down-regulation of occludin and the increase in paracellular permeability in hCMEC/D3 cells. Our findings suggest that the JNK and p38MAPK pathways might represent attractive therapeutic targets for preventing BBB dysfunction in AD.

Blood-brain barrier-specific properties of a human adult brain endothelial cell line

Establishment of a human model of the blood-brain barrier has proven to be a difficult goal. To accomplish this, normal human brain endothelial cells were transduced by lentiviral vectors incorporating human telomerase or SV40 T antigen. Among the many stable immortalized clones obtained by sequential limiting dilution cloning of the transduced cells, one was selected for expression of normal endothelial markers, including CD31, VE cadherin, and von Willebrand factor. This cell line, termed hCMEC/D3, showed a stable normal karyotype, maintained contact-inhibited monolayers in tissue culture, exhibited robust proliferation in response to endothelial growth factors, and formed capillary tubes in matrix but no colonies in soft agar. hCMEC/D3 cells expressed telomerase and grew indefinitely without phenotypic dedifferentiation. These cells expressed chemokine receptors, up-regulated adhesion molecules in response to inflammatory cytokines, and demonstrated blood-brain barrier characteristics, including tight junctional proteins and the capacity to actively exclude drugs. hCMEC/D3 are excellent candidates for studies of blood-brain barrier function, the responses of brain endothelium to inflammatory and infectious stimuli, and the interaction of brain endothelium with lymphocytes or tumor cells. Thus, hCMEC/D3 represents the first stable, fully characterized, well-differentiated human brain endothelial cell line and should serve as a widely usable research tool.

Cross-Linking of Brain Endothelial Intercellular Adhesion Molecule (ICAM)-1 Induces Association of ICAM-1 With Detergent-Insoluble Cytoskeletal Fraction

Abstract —Intercellular adhesion molecule (ICAM)-1 plays a vital role in the process of leukocyte transmigration through endothelial cell (EC) barriers and has been shown to mediate signal transduction events in ECs induced either by its cross-linking or by the binding of T lymphocytes. Immunoblotting of ICAM-1 of Triton X-100 detergent fractions demonstrated that the majority of ICAM-1 was contained within the detergent-soluble fraction (noncytoskeletal associated) under basal conditions. After cross-linking of endothelial ICAM-1 with monoclonal antibody or coculture with T lymphocytes, EC ICAM-1 was observed to partition with a Triton X-100–insoluble (cytoskeletal associated) fraction in a dose- and time-dependent manner. Redistribution of ICAM-1 was specific, inasmuch as no association with the Triton X-100–insoluble fraction was observed after cross-linking of vascular cell adhesion molecule-1, nor did cross-linking of ICAM-1 result in a redistribution of the platelet and EC adhesion molecule. ICAM-1 association with the endothelial cytoskeleton after cross-linking was unaffected after treatment of the cells with cytochalasin D, C3-transferase, removal of extracellular calcium ions, or chelation of intracellular calcium ions. These data show that ICAM-1 colocalizes with the endothelial cytoskeleton and associates with a detergent-insoluble fraction after cross-linking.

Chemokines control fat accumulation and leptin secretion by cultured human adipocytes

In addition to their role in inflammation, cytokines like TNFalpha have been reported to regulate the adipose tissue function suggesting a role for these soluble mediators in metabolism. However, it is not known whether adipocytes have the capacity to secrete chemokines, a group of low molecular weight inflammatory mediators that control leukocyte migration into tissues. Here we show that primary cultures of human preadipocytes constitutively produce three chemokines, interleukin-8 (IL-8), macrophage inflammatory protein-1alpha (MIP-1alpha) and monocyte chemotactic protein-1 (MCP-1), while their level of expression is low in mature adipocytes. Upon TNFalpha treatment, the expression of all the three chemokines is upregulated in adipocytes differentiated in vitro. In addition, we describe the presence of seven different chemokine receptors, mainly in mature adipocytes, both in vitro and in human fat tissue sections. Prolonged stimulation of cultured human adipocytes with exogenous chemokines leads to a decrease in lipid content in association with the downregulation of PPARgamma mRNA expression. Moreover, chemokines positively control the secretion of leptin, a hormone that regulates appetite, by a post-transcriptional mechanism. These findings reveal a new role for chemokines in the regulation of adipose tissue and suggest a novel therapeutic basis for the treatment of obesity, diabetes and cachexia.

Annexin 1 binds to U937 monocytic cells and inhibits their adhesion to microvascular endothelium: involvement of the alpha 4 beta 1 integrin

Annexin 1 (ANX1), a calcium-binding protein, participates in the regulation of early inflammatory responses. Whereas some of its effects depend on intracellular interactions, a growing number of observations indicate that ANX1 may also act via autocrine/paracrine functions following externalization to the outer side of the plasma membrane. We studied the effects of ANX1 on leukocyte adhesion to endothelial cells using as a model system the monocytic cell line U937 and human bone marrow microvascular endothelial cells. Exogenous rANX1, as well as endogenous ANX1 externalized by U937 differentiated in vitro, inhibited monocyte firm adhesion to vascular endothelium. Both binding of ANX1 to U937 cells and ANX1-mediated inhibition of cell adhesion involved the short N-terminal domain of the ANX1 molecule. Under experimental conditions in which ANX1 inhibited U937 adhesion to human bone marrow microvascular endothelial cells, this protein specifically colocalized with the alpha 4 integrin, and a direct interaction between ANX1 and the alpha 4 integrin could be documented by immunoprecipitation experiments. Moreover, ANX1 competed with the endothelial integrin counterreceptor, VCAM-1, for binding to alpha 4 integrin. These results indicate that ANX1 plays an important physiological role in modulating monocyte firm adhesion to the endothelium.

Interactions between brain endothelial cells and human T-cell leukemia virus type 1-infected lymphocytes: mechanisms of viral entry into the central nervous system

Human T-cell leukemia virus type 1 (HTLV-1) is associated with a variety of clinical manifestations, including tropical spastic paraparesis or HTLV-1-associated myelopathy (TSP/HAM). Viral detection in the central nervous system (CNS) of TSP/HAM patients demonstrates the ability of HTLV-1 to cross the blood-brain barrier (BBB). To investigate viral entry into the CNS, rat brain capillary endothelial cells were exposed to human lymphocytes chronically infected by HTLV-1 (MT2), to lymphocytes isolated from a seropositive patient, or to a control lymphoblastoid cell line (CEM). An enhanced adhesion to and migration through brain endothelial cells in vitro was observed with HTLV-1-infected lymphocytes. HTLV-1-infected lymphocytes also induced a twofold increase in the paracellular permeability of the endothelial monolayer. These effects were associated with an increased production of tumor necrosis factor alpha by HTLV-1-infected lymphocytes in the presence of brain endothelial cells. Ultrastructural analysis showed that contact between endothelial cells and HTLV-1-infected lymphocytes resulted in a massive and rapid budding of virions from lymphocytes, followed by their internalization into vesicles by brain endothelial cells and apparent release onto the basolateral side, suggesting that viral particles may cross the BBB using the transcytotic pathway. Our study also demonstrates that cell-cell fusion occurs between HTLV-1-infected lymphocytes and brain endothelial cells, with the latter being susceptible to transient HTLV-1 infection. These aspects may help us to understand the pathogenic mechanisms associated with neurological diseases induced by HTLV-1 infection.

Dexamethasone regulation of P-glycoprotein activity in an immortalized rat brain endothelial cell line, GPNT

The blood-brain barrier (BBB) plays an important role in controlling the passage of molecules from the blood to the extracellular fluid environment of the brain. The multidrug efflux pump P-glycoprotein (P-gp) is highly expressed in the luminal membrane of brain capillary endothelial cells, thus forming a functional barrier to lipid-soluble drugs, notably, antitumor agents. It is of interest to develop an in vitro BBB model that stably expresses P-gp to investigate the mechanisms of regulation in expression and activity. The rat brain endothelial cell line, GPNT, was derived from a previously characterized rat brain endothelial cell line. A strong expression of P-gp was found in GPNT monocultures, whereas the multidrug resistance-associated pump Mrp1 was not expressed. The transendothelial permeability coefficient of the P-gp substrate vincristine across GPNT monolayers was close to the permeability coefficient of bovine brain endothelial cells cocultured with astrocytes, a previously documented in vitro BBB model. Furthermore, the P-gp blocker cyclosporin A induced a large increase in apical to basal permeability of vincristine. Thus, P-gp is highly functional in GPNT cells. A 1-h treatment of GPNT cells with dexamethasone resulted in decreased uptake of vincristine without any increase in P-gp expression. This effect could be mimicked by protein kinase C (PKC) activation and prevented by PKC inhibition, strongly suggesting that activation of P-gp function may involve a PKC-dependent pathway. These results document the GPNT cell line as a valuable in vitro model for studying drug transport and P-gp function at the BBB and suggest that activation of P-gp activity at the BBB might be considered in chemotherapeutic treatment of cancer patients.

Transendothelial permeability changes induced by free radicals in an in vitro model of the blood-brain barrier

In the present study, we investigated the changes in blood-brain barrier (BBB) permeability following brain endothelial cell exposure to different xenobiotics able to promote free radical generation during their metabolism. Our in vitro BBB model consisted of confluent monolayers of immortalized rat brain capillary endothelial cells (RBE4) grown on collagen-coated filters in the presence of C6 glioma cells grown in the lower compartment. We have recently shown that a range of xenobiotics, including menadione, nitrofurazone, and methylviologen (paraquat) may undergo monoelectronic redox cycling in isolated brain capillaries, giving rise to reactive oxygen species. In this study, addition of 100 microM menadione to the culture medium for 30 min significantly increased the permeability of endothelial cell monolayers to radiolabeled sucrose. The effect on endothelial permeability induced by menadione was dose-dependent and reversible. These permeability changes preceded the onset of cell death, as assessed by the Trypan blue exclusion method. Pre-incubation with superoxide dismutase and catalase blocked changes in sucrose permeability to control levels in a dose-dependent manner, suggesting the involvement of reactive oxygen species in menadione-induced BBB opening.

Toxic effects of beta-amyloid(25-35) on immortalised rat brain endothelial cell: protection by carnosine, homocarnosine and beta-alanine

The effect of a truncated form of the neurotoxin beta-amyloid peptide (A beta25-35) on rat brain vascular endothelial cells (RBE4 cells) was studied in cell culture. Toxic effects of the peptide were seen at 200 microg/ml A beta using a mitochondrial dehydrogenase activity (MTT) reduction assay, lactate dehydrogenase release and glucose consumption. Cell damage could be prevented completely at 200 microg/ml A beta and partially at 300 microg/ml A beta, by the dipeptide carnosine. Carnosine is a naturally occurring dipeptide found at high levels in brain tissue and innervated muscle of mammals including humans. Agents which share properties similar to carnosine, such as beta-alanine, homocarnosine, the anti-glycating agent aminoguanidine, and the antioxidant superoxide dismutase (SOD), also partially rescued cells, although not as effectively as carnosine. We postulate that the mechanism of carnosine protection lies in its anti-glycating and antioxidant activities, both of which are implicated in neuronal and endothelial cell damage during Alzheimer's disease. Carnosine may therefore be a useful therapeutic agent.

The HIV-1 nef protein inhibits extracellular signal-regulated kinase-dependent DNA synthesis in a human astrocytic cell line

The role of nonproductive infection of astrocytes by human immunodeficiency virus type 1 (HIV-1), characterized by the overexpression of nef, in brain disease progression is largely unknown. We investigated the consequences of stable expression of nef from the HIV-1 strain LAI in the human astrocytic cell line U373. DNA synthesis induced by endothelin-1 (ET-1) was largely decreased by nef. Stable expression of nef did not affect the ET-1-induced tyrosine phosphorylation of focal adhesion kinase, an adhesion-dependent pathway known to participate in DNA synthesis in astrocytes. Conversely, the activation of extracellular signal-regulated kinase (ERK) by ET-1 was largely inhibited in cells stably or transiently expressing nef. A similar inhibitory action of nef on ERK activation was observed after direct stimulation of G proteins. Furthermore, the inhibitory action of nef did not require protein kinase C (PKC) and affected mainly the PKC-independent pathway of ERK activation. Following chemokine receptor CXCR4-mediated infection of U373 cells stably expressing CXCR4 with the T-tropic HIV-1 strain m7-NDK, ET-1-induced activation of ERK was also inhibited. Altogether, these results indicate that intracellular signaling pathways associated with the growth factor activity of ET-1 are impaired in nef-expressing and HIV-1-infected astrocytes, suggesting that infection of astrocytes may play a significant role in the neuropathogenesis of HIV-1 encephalopathy.

Acute energy deprivation syndromes: investigation of m-dinitrobenzene and alpha-chlorohydrin toxicity on immortalized rat brain microvessel endothelial cells

Acute energy deprivation syndromes share a common pattern of CNS pathology resulting in symmetrical spongiform brain stem lesions in rodents. However, some toxicants are proposed to act on astrocytes alone (alpha-chlorohydrin) whilst others are associated with petechial haemorrhages and blood-brain barrier (BBB) breakdown (m-dinitrobenzene). In this study, we investigated the toxicity of alpha-chlorohydrin (alpha-CH) and m-dinitrobenzene (m-DNB) in an in vitro BBB model, the rat brain capillary endothelial cell line RBE4. Cytotoxicity was observed after treatment of RBE4 cells with both toxicants, as manifested by a decrease in protein content and MTT reduction (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) over control values at concentrations > or = 1 mM for m-DNB and > or = 20 mM for alpha-CH. m-DNB caused a dose-dependent increase in glucose consumption and lactate production in RBE4 cells, while alpha-CH had no effect on these parameters. The distribution of F-actin at the cell margin observed in control cultures was changed to a diffuse pattern over the cell cytoplasm after treatment with both toxins at subcytotoxic concentrations. However, a reduction in F-actin content was only observed at concentrations > or = 1 mM for m-DNB and > or = 20 mM for alpha-CH. The permeability of RBE4 cell monolayers cultured on filters above primary rat astrocytes was measured using 14C-sucrose (M.Wt.=342) and FITC-dextran (M.Wt.=4000). m-DNB (0.5 mM) increased the permeability of RBE4 cell monolayers to both tracers, while alpha-CH (30 mM) had no effect. The results from this study indicate that m-DNB may have direct toxic effects on brain endothelial cells which lead to loss of barrier function. Whilst alpha-CH caused some toxic effects in RBE4 cells, it did not alter endothelial cell monolayer permeability.

F-actin cytoskeleton and sucrose permeability of immortalised rat brain microvascular endothelial cell monolayers: effects of cyclic AMP and astrocytic factors

The immortalised RBE4 cell line, derived from rat brain capillary endothelial cells, preserves many features of the in vivo brain endothelium, and hence is of interest as a potential in vitro model of the blood-brain barrier (BBB). This study reports the effects of elevated intracellular cAMP and factors released by astrocytes on the F-actin cytoskeleton and paracellular sucrose permeability of monolayers of RBE4 cells. RBE4 cells grown in control medium showed a marked increase in the F-actin staining at the cytoplasmic margin at confluence, which was not significantly enhanced by elevation of intracellular cAMP and/or addition of astrocyte-conditioned medium (ACM). The formation of the marginal band of F-actin was accompanied by an increase in the F-actin content of the RBE4 cells up to confluence, and a decline in F-actin content thereafter. Elevation of intracellular cAMP or co-culture above astrocytes significantly decreased the paracellular sucrose permeability of confluent RBE4 cell monolayers grown on collagen filters (P < 0.01 and P < 0.001, respectively). Co-culture above astrocytes together with elevated cAMP also produced a significant decrease in the sucrose permeability of the monolayer (P < 0.01) but this was no greater than with astrocytes alone. These findings show that the RBE4 cell line may serve as a useful in vitro model for the study of brain endothelial cell physiology and agents which alter the permeability of the BBB.

Growth factor activity of endothelin-1 in primary astrocytes mediated by adhesion-dependent and -independent pathways

Endothelin-1 (ET-1) has been shown to induce DNA synthesis in primary astrocytes by stimulating the extracellular signal-regulated kinase (ERK) pathway. To clarify the mechanisms responsible for the anchorage-dependent growth of astrocytes, the relationships between cell adhesion and ERK activation were investigated. Here it is reported that ET-1 promotes the formation of stress fibers and focal adhesions and the tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin, as well as Src activation and association of phosphorylated FAK with Grb2. Pretreatment of astrocytes with cytochalasin D or C3-transferase, which inhibits actin polymerization or Rho activity, respectively, prevented the activation/phosphorylation of Src, FAK, and paxillin after ET-1 stimulation; by contrast, the ERK pathway was not significantly affected. This differential activation of FAK/Src and ERK pathways was also observed with astrocytes 10 and 60 min after replating on poly-L-ornithine-precoated dishes. Collectively, these findings indicate that activation of FAK and Src is dependent on actin cytoskeleton integrity, Rho activation, and adhesion to extracellular matrix, whereas ERK activation is independent of these intracellular events and seems to correlate with activation of the newly identified protein tyrosine kinase PYK2. Induction of DNA synthesis by ET-1, however, was reduced dramatically in astrocytes pretreated with either cytochalasin D or C3-transferase. This study provides a demonstration of Rho- and adhesion-dependent activation of FAK/Src, which collaborates with adhesion-independent activation of PYK2/ERK for DNA synthesis in ET-1-stimulated astrocytes.

Metabolic and permeability changes caused by thiamine deficiency in immortalized rat brain microvessel endothelial cells

The possible involvement of blood-brain barrier (BBB) breakdown in the pathogenesis of thiamine deficiency encephalopathy was investigated in RBE4 cells, an immortalized rat brain endothelial cell line. The effects of thiamine deficiency produced by addition of pyrithiamine and by reduction of thiamine in the culture medium, on the metabolism and permeability of the RBE4 monolayer was examined. Pyrithiamine treatment in low thiamine medium (M199) for 7 days caused cytotoxic effects on RBE4 cells at all concentrations (10-50 microg/ml). Pyrithiamine caused a concentration- and time-dependent decrease in MTT reduction and a significant increase in glucose consumption and lactate production compared to controls. Pyrithiamine treatment for 3 days caused a significant decrease in MTT reduction at 50 microg/ml only. In contrast, increased glucose consumption and lactate production by the RBE4 cells was observed after treatment for 3 days with concentrations of 25 microg/ml pyrithiamine and above. The permeability of RBE4 cell monolayers to [14C]sucrose (Mw 342), but not FITC-dextran (Mw 4000) was significantly increased by treatment with pyrithiamine concentrations of 25 microg/ml and above for 3 days. These effects were not accompanied by detectable changes in F-actin distribution or content, although F-actin content was significantly reduced by 7 days exposure to pyrithiamine. These results suggest that metabolic and permeability changes in thiamine-deficient RBE4 cells may be important early events in thiamine-deficiency encephalopathy. The relative role of the BBB in the pathogenesis of thiamine deficiency is discussed.

Effects of energy deprivation induced by fluorocitrate in immortalised rat brain microvessel endothelial cells

The effects of the mitochondrial aconitase inhibitor, fluorocitrate on the immortalised rat brain endothelial cell line (RBE4) were investigated. Treatment with different concentrations of fluorocitrate (0-1 mM) for 24 h induced a significant, concentration-dependent decrease in the MTT reduction (an index of mitochondrial function), intracellular ATP content, glucose consumption and lactate production by RBE4 cell monolayers but did not alter the glucose to lactate ratio at concentrations lower than 0.5 mM. At all concentrations, fluorocitrate induced a significant decrease in the protein content per well. Fluorocitrate treatment of confluent RBE4 cells induced a marked redistribution of the F-actin cytoskeleton from a characteristic marginal band to a more diffuse cytosolic pattern. This redistribution of the cytoskeleton coincided with a reduction in the total cellular F-actin content of the RBE4 cells at fluorocitrate concentrations greater than 0.5 mM. Treatment of confluent RBE4 cells with fluorocitrate had no significant effect on RBE4 cell monolayer permeability measured by FITC-dextran or [14C]sucrose. These results show that whilst energy deprivation following fluorocitrate treatment induces significant changes in the RBE4 cell F-actin cytoskeleton and cellular metabolism, it does not have any significant effect on endothelial cell monolayer permeability. These results demonstrate that profound toxic effects on endothelial cell structure and metabolism are not necessarily accompanied by changes in endothelial cell monolayer permeability.

An in vitro study of m-dinitrobenzene toxicity on the cellular components of the blood-brain barrier, astrocytes and endothelial cells

We have studied the effects of an industrial compound, m-dinitrobenzene (DNB), on cultured astrocytes and brain capillary endothelial cells in an in vitro blood-brain barrier (BBB) model. In single cultures, the threshold DNB concentration that induced cell death, as assessed by morphology and lactose dehydrogenase leakage into the culture medium, was 1 mM for both cell types after 1 day of incubation. In cocultures, astrocytes showed a dose-response curve similar to that obtained in single cultures, while endothelial cells showed an increased sensitivity to DNB cytotoxicity. DNB induced a dose-dependent increase in glucose consumption and lactate production in both cell types in single cultures, although astrocytes appeared to be more sensitive than endothelial cells. The role of oxidative stress was also studied using the reduction of nitroblue tetrazolium as an index of generation of active oxygen species. A dose-dependent increase was observed for both cell types in single cultures, although this could not be prevented by the addition of superoxide dismutase to the culture medium. Addition of 0.3 mM carmustine to the culture medium increased the cytotoxicity of 0.5 mM DNB in both astrocytes and endothelial cells, indicating a role of oxidative stress in DNB-induced damage. Desferrioxamine (20 mm) completely protected endothelial cells from damage by 1 mM DNB, suggesting that hydroxyl radicals mediated at least part of the DNB neurotoxicity. However, astrocytic damage by 2 mM DNB was only partially prevented by desferrioxamine. We conclude that our in vitro model is suitable for studying interactions between astrocytes and endothelial cells in toxicological situations.

Transporting therapeutics across the blood-brain barrier

In 1996, we are half-way through the Decade of the Brain, yet we still have few effective treatments for major disorders of the central nervous system. These include affective disorders, epilepsy, neurodegenerative disorders, brain tumours, infections and HIV encephalopathy; sufferers far outnumber the morbidity of cancer or heart disease. Increased understanding of the pharmacology of the brain and its blood supply, and methods for rational drug design, are leading to potential new drug therapies based on highly specific actions on particular target sites, such as neurotransmitter receptors and uptake systems. These methods are capable of reducing the side effects that are common with more general treatments. However, all these treatments and potential treatments meet a formidable obstacle--the blood-brain barrier. In this article, we review the properties of this barrier that complicate drug delivery to the brain, and some of the most hopeful strategies for overcoming or bypassing the barrier in humans.

Early metabolic changes during m-Dinitrobenzene neurotoxicity and the possible role of oxidative stress

m-Dinitrobenzene (m-DNB) is an industrial chemical causing gliovascular lesions in the brain stem similar to those produced by nitroimidazoles and by thiamine deficiency. To identify early preneuropathic indices of toxicity we examined the action of m-DNB on glycolysis and on measures of oxidative stress in the brain both in vivo and in vitro. Significant increases in local cerebral glucose utilization were seen in 14 of 30 brain regions prior to development of lesions. Rat brain astrocyte cultures also showed increases in both glucose consumption and lactic acid formation in the first 24 h following exposure to 0.5 mM m-DNB and prior to the development of cytotoxicity. The concentration of reduced glutathione in these cultures was decreased to about half of control values over a 2-h incubation period, indicating an early disturbance of redox balance. The rate of reduction of nitroblue tetrazolium increased eightfold during a 1-h incubation period, suggesting a free radical-mediated process. Superoxide dismutase partially prevented this increase, although other protective agents failed to do so possibly due to lack of cellular penetration. These observations show that m-DNB neurotoxicity involves early metabolic stimulation and redox disruption that may be causally associated with the production of free radicals.

Astrocyte-endothelial interaction: physiology and pathology

The blood-brain barrier of higher vertebrates is formed by the layer of endothelial cells lining the brain microvessels. The close anatomical association between endothelial cells and perivascular astrocytic end feet suggests cooperation between these cell types in forming and maintaining the blood-brain barrier. This review considers evidence from grafting experiments, developmental studies and culture models of the brain endothelium, concerning the inductive influences acting on the endothelium, and from endothelial cells acting on perivascular astrocytes. Examples from pathology and neurotoxicology which may involve breakdown of induction are also considered.