Induction of the blood/brain-barrier-associated enzyme alkaline phosphatase in endothelial cells from cerebral capillaries is mediated via cAMP

Tissue-Nonspecific Alkaline Phosphatase in Central Nervous System Health and Disease: A Focus on Brain Microvascular Endothelial Cells

Tissue-nonspecific alkaline phosphatase (TNAP) is an ectoenzyme bound to the plasma membranes of numerous cells via a glycosylphosphatidylinositol (GPI) moiety. TNAP's function is well-recognized from earlier studies establishing its important role in bone mineralization. TNAP is also highly expressed in cerebral microvessels; however, its function in brain cerebral microvessels is poorly understood. In recent years, few studies have begun to delineate a role for TNAP in brain microvascular endothelial cells (BMECs)-a key component of cerebral microvessels. This review summarizes important information on the role of BMEC TNAP, and its implication in health and disease. Furthermore, we discuss current models and tools that may assist researchers in elucidating the function of TNAP in BMECs.

Culture-induced changes in mRNA expression levels of efflux and SLC-transporters in brain endothelial cells

Background

The complexity of the neurovascular unit (NVU) poses a challenge in the investigations of drug transport across the blood–brain barrier (BBB) and the function of the brain capillary endothelium. Several in vitro models of the brain capillary endothelium have been developed. In vitro culture of primary endothelial cells has, however, been reported to alter the expression levels of various brain endothelial proteins. Only a limited number of studies have addressed this in detail. The aim of the present study was to investigate mRNA levels of selected BBB transporters and markers in in vitro models of the BBB based on bovine primary endothelial cells and compare these to the levels estimated in freshly isolated bovine brain capillaries.

Methods

Brain capillaries were isolated from bovine cerebral cortex grey matter. Capillaries were seeded in culture flasks and endothelial cells were obtained using a brief trypsinization. They were seeded onto permeable supports and cultured in mono-, non-contact- or contact co-culture with/without primary rat astrocytes. mRNA-expression levels of the selected BBB markers and transporters were evaluated using qPCR and monolayer integrity of resulting monolayers was evaluated by measuring the transendothelial electrical resistance (TEER).

Results

The capillary mRNA transcript profile indicated low expression of ABCC1 and CLDN1. The mRNA expression levels of TPA, OCLN, ABCB1, SLC2A1, SLC16A1 and SLC7A5 were significantly decreased in all culture configurations compared to freshly isolated bovine brain capillaries. ALP, VWF, ABCC1 and ABCC4 were upregulated during culture, while the mRNA expression levels of F11R, TJP1, CLDN5, CLDN1 and ABCG2 were found to be unaltered. The mRNA expression levels of VWF, ALP, ABCB1 and ABCC1 were affected by the presence of rat astrocytes.

Conclusion

The endothelial mRNA transcript profile in bovine capillaries obtained in this study correlated nicely with profiles reported in mice and humans. Cultured endothelial cells drastically downregulated the mRNA expression of the investigated SLC transporters but maintained expression of efflux transporter and junctional protein mRNA, implying that the bovine in vitro BBB models may serve well to investigate basic barrier biology and in vivo permeation of passively permeating compounds and efflux transporter substrates but may be less well suited for investigations of SLC-mediated transport.

The Evolving Concept of the Blood Brain Barrier (BBB): From a Single Static Barrier to a Heterogeneous and Dynamic Relay Center

The blood–brain barrier (BBB) helps maintain a tightly regulated microenvironment for optimal central nervous system (CNS) homeostasis and facilitates communications with the peripheral circulation. The brain endothelial cells, lining the brain’s vasculature, maintain close interactions with surrounding brain cells, e.g., astrocytes, pericytes and perivascular macrophages. This function facilitates critical intercellular crosstalk, giving rise to the concept of the neurovascular unit (NVU). The steady and appropriate communication between all components of the NVU is essential for normal CNS homeostasis and function, and dysregulation of one of its constituents can result in disease. Among the different brain regions, and along the vascular tree, the cellular composition of the NVU varies. Therefore, differential cues from the immediate vascular environment can affect BBB phenotype. To support the fluctuating metabolic and functional needs of the underlying neuropil, a specialized vascular heterogeneity is required. This is achieved by variances in barrier function, expression of transporters, receptors, and adhesion molecules. This mini-review will take you on a journey through evolving concepts surrounding the BBB, the NVU and beyond. Exploring classical experiments leading to new approaches will allow us to understand that the BBB is not merely a static separation between the brain and periphery but a closely regulated and interactive entity. We will discuss shifting paradigms, and ultimately aim to address the importance of BBB endothelial heterogeneity with regard to the function of the BBB within the NVU, and touch on its implications for different neuropathologies.

Monocultures of primary porcine brain capillary endothelial cells: Still a functional in vitro model for the blood-brain-barrier

The main obstacle for the treatment of brain diseases is the restriction of the passage of pharmaceuticals across the blood-brain barrier. Endothelial cells line up the cerebral micro vessels and prevent the uncontrolled transfer of polar substances by intercellular tight junctions. In addition to this physical barrier, active transporters of the multi-drug-resistance prevent the passage of hydrophobic substances by refluxing them back to the blood stream. This paper reviews the development and selected applications of an in vitro porcine brain derived primary cell culture system established in the authors lab that closely resembles the BBB in vivo and could thus be used to study beyond other applications drug delivery to the brain. An essential technique to control the intactness or destruction of the barrier, the impedance spectroscopy, will be introduced. It will be shown that nanoparticles can cross the blood brain barrier by two mechanisms: opening the tight junctions and thus allowing parallel import of substances into the brain as well as receptor mediated endocytosis using brain specific target molecules. However cytotoxic effects have to be considered as well which beside standard cytotoxicity assays could be also determined by impedance technology. Moreover it will be shown that enzymes e.g. for enzyme replacement therapy could be transferred across the barrier by proper tuning or chemical modification of the enzyme. Since this review is based on a conference presentation it will mainly focus on applications of the monoculture system developed in the authors lab which under given culture conditions is useful due to its easy availability, robustness, good reproducibility and also due to its simplicity. Improvements of this model made by other groups will be acknowledged but not discussed here in detail.

Association of serum gamma-glutamyltransferase and C-reactive proteins with neuromyelitis optica and multiple sclerosis

BACKGROUND

Many studies have demonstrated that serum gamma glutamyltransferase (GGT) within normal range might be an early marker of oxidative stress. However the role of GGT in neuromyelitis optica (NMO) and multiple sclerosis (MS) is unknown.

METHODS

We assessed the correlations among GGT and C-reactive protein (CRP) levels, as well as clinical characteristics of NMO and MS. Serum GGT and CRP levels were measured in 106 NMO patients, 87 MS patients, 79 patients with non-inflammatory neurological diseases (Parkinson disease) and 80 healthy controls (HC). Clinical parameters, blood-brain barrier (BBB) index and Delpech index of MS and NMO were also investigated.

RESULTS

We found that NMO patients had higher serum GGT and CRP levels within their normal ranges compared to MS, PD, healthy controls. NMO patients exhibited significantly higher EDSS scores than MS patients. The BBB index in NMO patients was significantly higher than that in MS patients. Significant correlations existed between serum GGT and CRP levels and EDSS scores, BBB index in NMO and MS patients.

CONCLUSION

Elevated GGT and CRP levels within their normal ranges in NMO and MS may be associated with inflammatory response, oxidative stress and BBB disturbance in the diseases. Further study into the underlying pathophysiology of this relationship is warranted.

Tissue Non-specific Alkaline Phosphatase (TNAP) in Vessels of the Brain

The microvessels of the brain represent around 3-4 % of the brain compartment but constitute the most important length (400 miles) and surface of exchange (20 m(2)) between the blood and the parenchyma of brain. Under influence of surrounding tissues, the brain microvessel endothelium expresses a specific phenotype that regulates and restricts the entry of compounds and cells from blood to brain, and defined the so-called blood-brain barrier (BBB). Evidences that alkaline phosphatase (AP) is a characteristic feature of the BBB phenotype that allows differentiating capillary endothelial cells from brain to those of the periphery have rapidly emerge. Thenceforth, AP has been rapidly used as a biomarker of the blood-brain barrier phenotype. In fact, brain capillary endothelial cells (BCECs) express exclusively tissue non-specific alkaline phosphatase (TNAP). There are several lines of evidence in favour of an important role for TNAP in brain function. TNAP is thought to be responsible for the control of transport of some compounds across the plasma membrane of the BCECs. Here, we report that levamisole-mediated inhibition of TNAP provokes an increase of the permeability to Lucifer Yellow of the endothelial monolayer. Moreover, we illustrate the disruption of the cytoskeleton organization. Interestingly, all observed effects were reversible 24 h after levamisole removal and correlated with the return of a full activity of the TNAP. This reversible effect remains to be studied in details to evaluate the potentiality of a levamisole treatment to enhance the entry of drugs in the brain parenchyma.

Modelling the endothelial blood-CNS barriers: a method for the production of robust in vitro models of the rat blood-brain barrier and blood-spinal cord barrier

Background

Modelling the blood-CNS barriers of the brain and spinal cord in vitro continues to provide a considerable challenge for research studying the passage of large and small molecules in and out of the central nervous system, both within the context of basic biology and for pharmaceutical drug discovery. Although there has been considerable success over the previous two decades in establishing useful in vitro primary endothelial cell cultures from the blood-CNS barriers, no model fully mimics the high electrical resistance, low paracellular permeability and selective influx/efflux characteristics of the in vivo situation. Furthermore, such primary-derived cultures are typically labour-intensive and generate low yields of cells, limiting scope for experimental work. We thus aimed to establish protocols for the high yield isolation and culture of endothelial cells from both rat brain and spinal cord. Our aim was to optimise in vitro conditions for inducing phenotypic characteristics in these cells that were reminiscent of the in vivo situation, such that they developed into tight endothelial barriers suitable for performing investigative biology and permeability studies.

Methods

Brain and spinal cord tissue was taken from the same rats and used to specifically isolate endothelial cells to reconstitute as in vitro blood-CNS barrier models. Isolated endothelial cells were cultured to expand the cellular yield and then passaged onto cell culture inserts for further investigation. Cell culture conditions were optimised using commercially available reagents and the resulting barrier-forming endothelial monolayers were characterised by functional permeability experiments and in vitro phenotyping by immunocytochemistry and western blotting.

Results

Using a combination of modified handling techniques and cell culture conditions, we have established and optimised a protocol for the in vitro culture of brain and, for the first time in rat, spinal cord endothelial cells. High yields of both CNS endothelial cell types can be obtained, and these can be passaged onto large numbers of cell culture inserts for in vitro permeability studies. The passaged brain and spinal cord endothelial cells are pure and express endothelial markers, tight junction proteins and intracellular transport machinery. Further, both models exhibit tight, functional barrier characteristics that are discriminating against large and small molecules in permeability assays and show functional expression of the pharmaceutically important P-gp efflux transporter.

Conclusions

Our techniques allow the provision of high yields of robust sister cultures of endothelial cells that accurately model the blood-CNS barriers in vitro. These models are ideally suited for use in studying the biology of the blood-brain barrier and blood-spinal cord barrier in vitro and for pre-clinical drug discovery.

TNAP and EHD1 are over-expressed in bovine brain capillary endothelial cells after the re-induction of blood-brain barrier properties

Although the physiological properties of the blood-brain barrier (BBB) are relatively well known, the phenotype of the component brain capillary endothelial cells (BCECs) has yet to be described in detail. Likewise, the molecular mechanisms that govern the establishment and maintenance of the BBB are largely unknown. Proteomics can be used to assess quantitative changes in protein levels and identify proteins involved in the molecular pathways responsible for cellular differentiation. Using the well-established in vitro BBB model developed in our laboratory, we performed a differential nano-LC MALDI-TOF/TOF-MS study of Triton X-100-soluble protein species from bovine BCECs displaying either limited BBB functions or BBB functions re-induced by glial cells. Due to the heterogeneity of the crude extract, we increased identification yields by applying a repeatable, reproducible fractionation process based on the proteins' relative hydrophobicity. We present proteomic and biochemical evidence to show that tissue non-specific alkaline phosphatase (TNAP) and Eps15 homology domain-containing protein 1(EDH1) are over-expressed by bovine BCECs after the re-induction of BBB properties. We discuss the impact of these findings on current knowledge of endothelial and BBB permeability.

Regulation of Mct1 by cAMP-dependent internalization in rat brain endothelial cells

In the cerebrovascular endothelium, monocarboxylic acid transporter 1 (Mct1) controls blood-brain transport of short chain monocarboxylic and keto acids, including pyruvate and lactate, to support brain energy metabolism. Mct1 function is acutely decreased in rat brain cerebrovascular endothelial cells by β-adrenergic signaling through cyclic adenosine monophosphate (cAMP); however, the mechanism for this acute reduction in transport capacity is unknown. In this report, we demonstrate that cAMP induces the dephosphorylation and internalization of Mct1 from the plasma membrane into caveolae and early endosomes in the RBE4 rat brain cerebrovascular endothelial cell line. Additionally, we provide evidence that Mct1 constitutively cycles through clathrin vesicles and recycling endosomes in a pathway that is not dependent upon cAMP signaling in these cells. Our results are important because they show for the first time the regulated and unregulated vesicular trafficking of Mct1 in cerebrovascular endothelial cells; processes which have significance for better understanding normal brain energy metabolism, and the etiology and potential therapeutic approaches to treating brain diseases, such as stroke, in which lactic acidosis is a key component.

Establishment of a simplified in vitro porcine blood-brain barrier model with high transendothelial electrical resistance

Good in vitro blood-brain barrier (BBB) models that mimic the in vivo BBB phenotype are essential for studies on BBB functionality and for initial screening in drug discovery programmes, as many potential therapeutic drug candidates have poor BBB permeation. Difficulties associated with the availability of human brain tissue, coupled with the time and cost associated with using animals for this kind of research have led to the development of non-human cell culture models. However, most BBB models display a low transendothelial electrical resistance (TEER), which is a measure of the tightness of the BBB. To address these issues we have established and optimised a robust, simple to use in vitro BBB model using porcine brain endothelial cells (PBECs). The PBEC model gives high TEER without the need for co-culture with astrocytes (up to 1300 O cm(2) with a mean TEER of ~800 O cm(2)) with well organised tight junctions as shown by immunostaining for occludin and claudin-5. Functional assays confirmed the presence of high levels of alkaline phosphatase (ALP), and presence of the efflux transporter, P-glycoprotein (P-gp, ABCB1). Presence of the breast cancer resistance protein (BCRP, ABCG2) was confirmed by TaqMan real-time RT-PCR assay. Real-time RT-PCR assays for BCRP, occludin and claudin-5 demonstrated no significant differences between batches of PBECs, and also between primary and passage 1 PBECs. A permeability screen of 10 compounds demonstrated the usefulness of the model as a tool for drug permeability studies. Qualitative and quantitative results from this study confirm that this in vitro porcine BBB model is reliable and robust; it is also simpler to generate than most other BBB models. This article is part of a Special Issue entitled Electrical Synapses.

Expression of Tight Junction and Drug Efflux Transporter Proteins in an in vitro Model of Human Blood-Brain Barrier

Interendothelial cell tight junctions (TJs) proteins contribute to maintain the structural and functional integrity of the blood-brain barrier (BBB) and several efflux transporters regulate transport of compounds across BBB. A unique double compartment-model of the BBB, consisting of cerebral endothelial cells isolated from cryopreserved human glial tumors, alone and in the presence of human astroglial cells derived from the same tissue preparation was established. Endothelial cell viability and transendothelial electrical resistance (TEER) were measured in this model and three representative TJ proteins - occludin (OCLN), zonula occludens-1 (ZO-1) and claudin-5 (CLN-5) - as well as several drug efflux transporters - P-glycoprotein (P-gp), multidrug resistance protein-1 and 2 (MRP-1 and MRP-2), organic anion-transporting polypeptide-1 and 3 (oatp1 and oatp3) were analyzed at both the protein and gene transcript level. Functional activity of P-gp and MRP-1 was also assessed. Endothelial cell viability as well as TEER significantly increased in the presence of glial cells. A significant increase of expression of OCLN, ZO-1, and CLN-5 proteins as well as of several drug transporter proteins except oatp3 and MRP-1, was also found in the presence of glial cells. All the gene transcripts protein analyzed were found to be significantly increased in the presence of glial cells. A suitable functional activity of P-gp and MRP-1 was also found. These results demonstrate that this brain endothelium culture system mimics a physiologically relevant situation and may therefore provide a new tool for studying the effects of biological fluids such as serum and cerebrospinal fluid from patients with neurological disorders underlying a BBB alteration in disease pathogenesis.

The impact of glia-derived extracellular matrices on the barrier function of cerebral endothelial cells: an in vitro study

The blood-brain barrier (BBB) is composed of the cerebral microvascular endothelium, which, together with astrocytes, pericytes, and the extracellular matrix (ECM), contributes to a "neurovascular unit". It was our objective to clarify the impact of endogenous extracellular matrices on the barrier function of BBB microvascular endothelial cells cultured in vitro. The study was performed in two consecutive steps: (i) The ECM-donating cells (astrocytes, pericytes, endothelial cells) were grown to confluence and then removed from the growth substrate by a protocol that leaves the ECM behind. (ii) Suspensions of cerebral endothelial cells were seeded on the endogenous matrices and barrier formation was followed with time. In order to quantify the tightness of the cell junctions, all experiments were performed on planar gold-film electrodes that can be used to read the electrical resistance of the cell layers as a direct measure for endothelial barrier function (electric cell-substrate impedance sensing, ECIS). We observed that endogenously isolated ECM from both, astrocytes and pericytes, improved the tightness of cerebral endothelial cells significantly compared to ECM that was derived from the endothelial cells themselves as a control. Moreover, when cerebral endothelial cells were grown on extracellular matrices produced by non-brain endothelial cells (aorta), the electrical resistances were markedly reduced. Our observations indicate that glia-derived ECM - as an essential part of the BBB - is required to ensure proper barrier formation of cerebral endothelial cells.

Tight junction protein expression and barrier properties of immortalized mouse brain microvessel endothelial cells

Understanding the molecular and biochemical mechanisms regulating the blood-brain barrier is aided by in vitro model systems. Many studies have used primary cultures of brain microvessel endothelial cells for this purpose. However, primary cultures limit the generation of material for molecular and biochemical assays since cells grow slowly, are prone to contamination by other neurovascular unit cells, and lose blood-brain barrier characteristics when passaged. To address these issues, immortalized cell lines have been generated. In these studies, we assessed the suitability of the immortalized mouse brain endothelial cell line, bEnd3, as a blood-brain barrier model. RT-PCR and immunofluorescence indicated expression of multiple tight junction proteins. bEnd3 cells formed barriers to radiolabeled sucrose, and responded like primary cultures to disrupting stimuli. Exposing cells to serum-free media on their basolateral side significantly decreased paracellular permeability; astrocyte-conditioned media did not enhance barrier properties. The serum-free media-induced decrease in permeability was correlated with an increase in claudin-5 and zonula occludens-1 immunofluorescence at cell-cell contracts. We conclude that bEnd3 cells are an attractive candidate as a model of the blood-brain barrier due to their rapid growth, maintenance of blood-brain barrier characteristics over repeated passages, formation of functional barriers and amenability to numerous molecular interventions.

Lipid polarity in brain capillary endothelial cells

Brain capillary endothelial cells (BCEC) represent an epithelial like cell type with continuous tight junctions and polar distributed proteins. In this paper we investigated whether cultured BCEC show a polar distribution of membrane lipids as this was demonstrated for many epithelial cell types. Therefore we applied a high yield membrane fractionation method to isolate pure fractions of the apical and the basolateral plasma membrane (PM) domains. Using a set of methods for lipid analysis we were able to determine the total lipid composition of the whole cells and the PM fractions. Both membrane domains showed a unique lipid composition with clear differences to each other and to the whole cell composition. Three lipid species were polar distributed between the two PM domains. Phosphatidylcholine was enriched in the apical membrane whereas sphingomyelin and glucosylceramide were enriched in the basolateral membrane. The possible function of this lipid polarity for the blood-brain barrier mechanism is the generation of a suitable lipid environment for polar distributed membrane proteins and the generation of two PM domains with different biophysical properties and permeabilities.

Characterisation of the brain multidrug resistance protein (BMDP/ABCG2/BCRP) expressed at the blood-brain barrier

The blood-brain barrier (BBB) plays the predominant role in controlling the passage of endogenous and xenobiotic substances between the circulating blood and the extracellular fluid environment of the brain. The transfer of compounds is strictly regulated by brain capillary endothelial cells (BCEC), which are interconnected to each other by well developed tight junctions, without fenestrations. Although hydrophobic molecules such as nicotine and ethanol readily cross the BBB by diffusion, the brain microvasculature shows a highly restrictive permeability to hydrophobic antitumor agents. So far, this multidrug resistance has been almost exclusively attributed to the most prominent member of the ATP-binding cassette (ABC) transporter family, P-glycoprotein located in the luminal membrane of brain capillary endothelial cells and to a minor extent to the multidrug resistance-associated proteins (MRPs). The brain multidrug resistance protein (BMDP) has recently been discovered at the porcine BBB and was shown to be highly homologous to the human breast cancer resistance protein (BCRP/ABCG2). Here, we demonstrate by northern blot and RT-PCR analysis that BMDP mRNA is more highly expressed in the capillary endothelial cells compared to other cell types of the brain. Immunocytochemistry of porcine BCEC showed a clear plasma membrane localisation of BMDP. Analysis of the total mRNA pool revealed that BMDP is more strongly expressed than P-glycoprotein and MRP1. Consistently, first transport studies indicate that active exclusion of the chemotherapeutic drug daunorubicin from the central nervous system is mediated mainly by this new transporter compared to P-glycoprotein or MRP1. Thus, we hypothesise that BMDP might play an important role in the exclusion of xenobiotics from the porcine brain.

Neural induction of the blood-brain barrier: still an enigma

1. The study of the blood-brain barrier and its various realms offers a myriad of opportunities for scientific exploration. This review focuses on two of these areas in particular: the induction of the blood-brain barrier and the molecular mechanisms underlying this developmental process. 2. The creation of the blood-brain barrier is considered a specific step in the differentiation of cerebral capillary endothelial cells, resulting in a number of biochemical and functional alterations. Although the specific endothelial properties which maintain the homeostasis in the central nervous system necessary for neuronal function have been well described, the inductive mechanisms which trigger blood-brain barrier establishment in capillary endothelial cells are unknown. 3. The timetable of blood-brain barrier formation is still a matter of debate, caused largely by the use of varying experimental systems and by the general difficulty of quantitatively measuring the degree of blood-brain barrier "tightness." However, there is a general consensus that a gradual formation of the blood-brain barrier starts shortly after intraneural neovascularization and that the neural microenvironment (neurons and/or astrocytes) plays a key role in inducing blood-brain barrier function in capillary endothelial cells. This view stems from numerous in vitro experiments using mostly cocultures of capillary endothelial cells and astrocytes and assays for easily measurable blood-brain barrier markers. In vivo, there are great difficulties in proving the inductive influence of the neuronal environment. Also dealt with in this article are brain tumors, the least understood in vivo systems, and the induction or noninduction of barrier function in the newly established tumor vascularization. 4. Finally, this review tries to elucidate the question concerning the nature of the inductive signal eliciting blood-brain barrier formation in the cerebral microvasculature.

An improved low-permeability in vitro-model of the blood-brain barrier: transport studies on retinoids, sucrose, haloperidol, caffeine and mannitol

Primary cultures of porcine brain capillary endothelial cells grown on collagen coated polycarbonate membranes were used to build up an in vitro-model for the blood-brain barrier. Improved cultivation techniques allowed cell-storage and experiments under serum-free conditions. We employed this model to perform permeability studies in vitro with the radioactively labelled marker substances sucrose, retinoic acid, retinol, haloperidol, caffeine, and mannitol. Permeability values obtained with this blood-brain barrier model (1. 0x10-6 cm/s for sucrose, 6.2x10-6 cm/s for retinoic acid, 4.8x10-6 cm/s for retinol, 49.5x10-6 cm/s for haloperidol, 62.4x10-6 cm/s for caffeine, and 1.8x10-6 cm/s for mannitol) show a good correlation to data which are already known from in vivo-experiments. As judged by the sucrose permeability our blood-brain barrier model is less permeable than numerous other models published so far. Therefore it represents a powerful tool for in vitro-prediction of blood-brain barrier permeability of drugs and offers the possibility to scan a large quantity of drugs for their potential to enter the brain.

Alkaline phosphatase expression in cultured endothelial cells of aorta and brain microvessels: induction by interleukin-6-type cytokines and suppression by transforming growth factor betas

Alkaline phosphatase (ALP) activity is markedly high in endothelial cells of the blood-brain barrier (BBB) type but absent from or low in those of the non-BBB type. Interleukin 6 (IL-6) has been identified as a glial cell line-derived factor that induces high ALP activity in cultured aortic endothelial cells. In the present study, we examined the effect of IL-6-type cytokines and transforming growth factor betas (TGF-betas) on ALP expression in cultures of calf pulmonary aortic endothelial (CPAE) cells and porcine brain microvascular endothelial (PBME) cells. Leukemia inhibitory factor, ciliary neurotrophic factor, and oncostatin M, which are known as IL-6-type cytokines, induced high ALP expression in the CPAE cells but not in the PBME cells. ALP levels in these cells were markedly suppressed by culture with TGF-betas. However, in cultured PBME cells, IL-6 and a derivative of cyclic adenosine monophosphate significantly increased ALP activity. Our findings raise the posibility that local concentrations of IL-6, IL-6-type cytokines, and TGF-betas affect the ALP levels in the endothelial cells of aorta and brain microvessels under normal development and also under inflammatory conditions.

Endothelial cell surface alkaline phosphatase activity is induced by IL-6 released during wound repair

Phosphatase activity on endothelial cell surfaces is responsible, in part, for the conversion of adenosine nucleotides to adenosine, a potent vasodilator and anti-inflammatory mediator that can protect tissues from the ischemic damage that results from injury. To evaluate whether phosphatases are actively induced by a soluble factor released following injury, the effect of tissue fluids collected from porcine or human skin wounds was tested on primary cultures of endothelial cells. Phosphatase activity increased approximately 50-fold following 48-h culture in the presence of wound fluid. Inductive activity was present only in fluids collected during the inflammatory phase of wound repair. The phosphatase activity metabolized adenosine monophosphate to free phosphate and was the liver/bone/kidney alkaline phosphatase isoenzyme: activity was temperature- and levamisole-sensitive, 1-phenylalanine-resistant, and linked to the cell surface via phospholipid, and migrated at a size identical to this isozyme. interleukin-6 was identified as the phosphatase-inducing factor in wound fluid and the related cytokines, leukaemia inhibiting factor, and oncostatin M, caused a similar degree of alkaline phosphatase induction. Therefore, following injury, accumulation of interleukin-6 can lead to production by alkaline phosphatase of adenosine and subsequent protection from ischemic injury.

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.