A porcine astrocyte/endothelial cell co-culture model of the blood–brain barrier

Porcine Astrocytes and Their Relevance for Translational Neurotrauma Research

Astrocytes are essential to virtually all brain processes, from ion homeostasis to neurovascular coupling to metabolism, and even play an active role in signaling and plasticity. Astrocytic dysfunction can be devastating to neighboring neurons made inherently vulnerable by their polarized, excitable membranes. Therefore, correcting astrocyte dysfunction is an attractive therapeutic target to enhance neuroprotection and recovery following acquired brain injury. However, the translation of such therapeutic strategies is hindered by a knowledge base dependent almost entirely on rodent data. To facilitate additional astrocytic research in the translatable pig model, we present a review of astrocyte findings from pig studies of health and disease. We hope that this review can serve as a road map for intrepid pig researchers interested in studying astrocyte biology.

Recent advancements on in vitro blood-brain barrier model: A reliable and efficient screening approach for preclinical and clinical investigation

INTRODUCTION

The efficiency of brain therapeutics is greatly hindered by the blood-brain barrier (BBB). BBB's protective function, selective permeability, and dynamic functionality maintain the harmony between the brain and peripheral region. Thus, the design of any novel drug carrier system requires the complete study and investigation of BBB permeability, efflux transport, and the effect of associated cellular and non-vascular unit trafficking on BBB penetrability. The in vitro BBB models offer a most promising, and reliable mode of initial investigation of BBB permeability and associated factors as strong evidence for further preclinical and clinical investigation.

AREA COVERED

This review work covers the structure and functions of BBB components and different types of in vitro BBB models along with factors affecting BBB model development and model selection criteria.

EXPERT OPINION

In vivo models assume to reciprocate the physiological environment to the maximum extent. However, the interspecies variability, NVUs trafficking, dynamic behavior of BBB, etc., lead to non-reproducible results. The in vitro models are comparatively less complex, and flexible, as per the study design, could generate substantial evidence and help identify suitable in vivo animal model selection.

Current Status of In vitro Models of the Blood-Brain Barrier

Disorders of the brain are the most debilitating situation affected globally with increased mortality rates every year, while brain physiology and cumbersome drug development processes exacerbate this. Although blood-brain barrier (BBB) and its components are important for brain protection, their complexity creates major obstacles for brain drug delivery and the BBB is the primary cause of treatment failure leading to disease progression. Therefore, developing an ideal platform which can predict the behavior of a drug delivery system in the brain at the early development phase is extremely crucial. In this direction, since the last two decades, numerous in vitro BBB models have been developed and investigated by researchers to understand the barrier properties and how closely the in vitro models mimic with in vivo BBB. In-vitro BBB models are mainly culture of endothelial cells or their coculture with other perivascular cells either in two or three-dimensional platforms. In this article, we have briefly summarized the fundamentals of BBB and outlined different types of in vitro BBB models with their pros and cons. Based on the available reports, no model seems to be robust that can truly mimic the entire properties of the in vivo BBB microvasculature. However, human stem cells, coculture and three-dimensional models were found to mimic the complexity of the barrier integrity not completely but more precisely than other in vitro models. More studies aiming towards combining them together would be needed to develop an ideal in vitro model that can overcome the existing limitations and unravel the mysterious BBB.

Synthesis and Evaluations of Novel Apocynin Derivatives as Anti-Glioma Agents

Apocynin (4-hydroxy-3-methoxyacetophenone) is a natural polyphenolic compound with multiple biological activities. In the present study, a series of apocynin derivatives were designed and synthesized. The in silico ADMET prediction, blood–brain barrier (BBB) penetration assay, anti-NADPH oxidase activity, reactive oxygen species (ROS) levels, and anti-glioma effects of these apocynin derivatives were evaluated. The anti-glioma mechanisms of candidate compounds were studied by flow cytometer and Western blot. The results showed that D31 exhibited higher BBB penetration, increased ROS generations and significant anti-glioma effects both in vitro and in vivo. Further studies showed that D31 inhibited the activations of NF-κB pathway. Overall, our data demonstrated that D31 inhibited growth and induced apoptosis of glioma, which might be caused by ROS-related NF-κB activation. The current study suggested that D31 could be further explored for its potential use in anti-glioma therapy.

Improved In Vitro Model for Intranasal Mucosal Drug Delivery: Primary Olfactory and Respiratory Epithelial Cells Compared with the Permanent Nasal Cell Line RPMI 2650

Background: The epithelial layer of the nasal mucosa is the first barrier for drug permeation during intranasal drug delivery. With increasing interest for intranasal pathways, adequate in vitro models are required. Here, porcine olfactory (OEPC) and respiratory (REPC) primary cells were characterised against the nasal tumour cell line RPMI 2650. Methods: Culture conditions for primary cells from porcine nasal mucosa were optimized and the cells characterised via light microscope, RT-PCR and immunofluorescence. Epithelial barrier function was analysed via transepithelial electrical resistance (TEER), and FITC-dextran was used as model substance for transepithelial permeation. Beating cilia necessary for mucociliary clearance were studied by immunoreactivity against acetylated tubulin. Results: OEPC and REPC barrier models differ in TEER, transepithelial permeation and MUC5AC levels. In contrast, RPMI 2650 displayed lower levels of MUC5AC, cilia markers and TEER, and higher FITC-dextran flux rates. Conclusion: To screen pharmaceutical formulations for intranasal delivery in vitro, translational mucosal models are needed. Here, a novel and comprehensive characterisation of OEPC and REPC against RPMI 2650 is presented. The established primary models display an appropriate model for nasal mucosa with secreted MUC5AC, beating cilia and a functional epithelial barrier, which is suitable for long-term evaluation of sustained release dosage forms.

In vitro models and systems for evaluating the dynamics of drug delivery to the healthy and diseased brain

The blood-brain barrier (BBB) plays a crucial role in maintaining brain homeostasis and transport of drugs to the brain. The conventional animal and Transwell BBB models along with emerging microfluidic-based BBB-on-chip systems have provided fundamental functionalities of the BBB and facilitated the testing of drug delivery to the brain tissue. However, developing biomimetic and predictive BBB models capable of reasonably mimicking essential characteristics of the BBB functions is still a challenge. In addition, detailed analysis of the dynamics of drug delivery to the healthy or diseased brain requires not only biomimetic BBB tissue models but also new systems capable of monitoring the BBB microenvironment and dynamics of barrier function and delivery mechanisms. This review provides a comprehensive overview of recent advances in microengineering of BBB models with different functional complexity and mimicking capability of healthy and diseased states. It also discusses new technologies that can make the next generation of biomimetic human BBBs containing integrated biosensors for real-time monitoring the tissue microenvironment and barrier function and correlating it with the dynamics of drug delivery. Such integrated system addresses important brain drug delivery questions related to the treatment of brain diseases. We further discuss how the combination of in vitro BBB systems, computational models and nanotechnology supports for characterization of the dynamics of drug delivery to the brain.

Development of a direct contact astrocyte-human cerebral microvessel endothelial cells blood-brain barrier coculture model

OBJECTIVES

In conventional in-vitro blood-brain barrier (BBB) models, primary and immortalized brain microvessel endothelial cell (BMEC) lines are often cultured in a monolayer or indirect coculture or triculture configurations with astrocytes or pericytes, for screening permeation of therapeutic or potentially neurotoxic compounds. In each of these cases, the physiological relevancy associated with the direct contact between the BMECs, pericytes and astrocytes that form the BBB and resulting synergistic interactions are lost. We look to overcome this limitation with a direct contact coculture model.

METHODS

We established and optimized a direct interaction coculture system where primary human astrocytes are cultured on the apical surface of a Transwell® filter support and then human cerebral microvessel endothelial cells (hCMEC/D3) seeded directly on the astrocyte lawn.

KEY FINDINGS

The studies suggest the direct coculture model may provide a more restrictive and physiologically relevant model through a significant reduction in paracellular transport of model compounds in comparison with monoculture and indirect coculture. In comparison with existing methods, the indirect coculture and monoculture models utilized may limit cell-cell signaling between human astrocytes and BMECs that are possible with direct configurations.

CONCLUSIONS

Paracellular permeability reductions with the direct coculture system may enhance therapeutic agent and potential neurotoxicant screening for BBB permeability better than the currently available monoculture and indirect coculture in-vitro models.

In vitro screening of nanomedicines through the blood brain barrier: A critical review

The blood-brain barrier accounts for the high attrition rate of the treatments of most brain disorders, which therefore remain one of the greatest health-care challenges of the twenty first century. Against this background of hindrance to brain delivery, nanomedicine takes advantage of the assembly at the nanoscale of available biomaterials to provide a delivery platform with potential to raising brain levels of either imaging or therapeutic agents. Nevertheless, to prevent later failure due to ineffective drug levels at the target site, researchers have been endeavoring to develop a battery of in vitro screening procedures that can predict earlier in the drug discovery process the ability of these cutting-edge drug delivery platforms to cross the blood-brain barrier for biomedical purposes. This review provides an in-depth analysis of the currently available in vitro blood-brain barrier models (both cell-based and non-cell-based) with the focus on their suitability for understanding the biological brain distribution of forthcoming nanomedicines. The relationship between experimental factors and underlying physiological assumptions that would ultimately lead to a more predictive capacity of their in vivo performance, and those methods already assayed for the evaluation of the brain distribution of nanomedicines are comprehensively discussed.

High-throughput platforms for the screening of new therapeutic targets for neurodegenerative diseases

Despite the recent progress in the understanding of neurodegenerative disorders, a lack of solid fundamental knowledge on the etiology of many of the major neurodegenerative diseases has made it difficult to obtain effective therapies to treat these conditions. Scientists have been looking to carry out more-human-relevant studies, with strong statistical power, to overcome the limitations of preclinical animal models that have contributed to the failure of numerous therapeutics in clinical trials. Here, we identify currently existing platforms to mimic central nervous system tissues, healthy and diseased, mainly focusing on cell-based platforms and discussing their strengths and limitations in the context of the high-throughput screening of new therapeutic targets and drugs.

In Vitro Blood-Brain Barrier Models-An Overview of Established Models and New Microfluidic Approaches

The societal need for new central nervous system (CNS) medicines is substantial, because of the global increase in life expectancy and the accompanying increase in age-related CNS diseases. Low blood-brain barrier (BBB) permeability has been one of the major causes of failure for new CNS drug candidates. There has therefore been a great interest in cell models, which mimic BBB permeation properties. In this review, we present an overview of the performance of monocultured, cocultured, and triple-cultured primary cells and immortalized cell lines, including key parameters such as transendothelial electrical resistance values, permeabilities of paracellular flux markers, and expression of BBB-specific marker proteins. Microfluidic systems are gaining ground as a new automated technical platform for cell culture and systematic analysis. The performance of these systems was compared with current state-of-the-art models and it was noted that, although they show great promise, these systems have not yet reached beyond the proof-of-concept stage. In general, it was found that there were large variations in experimental protocols, BBB phenotype markers, and paracellular flux markers used. It is the author's opinion that the field may benefit greatly from developing standardized methodologies and initiating collaborative efforts on optimizing culture protocols.

Superparamagnetic plasmonic nanoshells for improved imaging, separation and seeding of co-cultured cells

Multifunctional superparamagnetic nanoshells were applied for improved 2D and 3D two-photon luminescence imaging, separation and seeding of co-cultured cells.

Blood-brain barrier models and their relevance for a successful development of CNS drug delivery systems: a review

During the research and development of new drugs directed at the central nervous system, there is a considerable attrition rate caused by their hampered access to the brain by the blood-brain barrier. Throughout the years, several in vitro models have been developed in an attempt to mimic critical functionalities of the blood-brain barrier and reliably predict the permeability of drug candidates. However, the current challenge lies in developing a model that retains fundamental blood-brain barrier characteristics and simultaneously remains compatible with the high throughput demands of pharmaceutical industries. This review firstly describes the roles of all elements of the neurovascular unit and their influence on drug brain penetration. In vitro models, including non-cell based and cell-based models, and in vivo models are herein presented, with a particular emphasis on their methodological aspects. Lastly, their contribution to the improvement of brain drug delivery strategies and drug transport across the blood-brain barrier is also discussed.

Transport of active flavonoids, based on cytotoxicity and lipophilicity: an evaluation using the blood-brain barrier cell and Caco-2 cell models

This in vitro study aims to evaluate and compare transmembrane transport of eight cardio-cerebrovascular protection flavonoids including puerarin, rutin, hesperidin, quercetin, genistein, kaempferol, apigenin and isoliquiritigenin via the rat blood-brain barrier cell and Caco-2 cell monolayer models, based on the data of cytotoxicity and lipophilicity. The cytotoxicity of the flavonoids to rat brain microvessel endothelial cell was determined by the MTT assay. The apparent permeability coefficients (Papp) of the flavonoids were calculated from the unilateral transport assays in Transwell system with simultaneous determination using a high performance liquid chromatography. The results showed that the cytotoxicity and oil-water partition coefficient of the flavonoids modified by the number and position of the glycoside and hydroxyl group were the key determinant for the transmembrane transport. The Papp values of the flavonoids reduced adversely when the numbers of glycoside and hydroxyl groups of the flavonoids increased accordingly. The tested flavonoids exhibited time-dependent Papp values in these models. The efflux mechanism related with P-glycoprotein also existed with the polar flavonoids; verapamil could enhance the permeation of rutin and quercetin via inhibition of P-glycoprotein. We propose that genistein and isoliquiritigenin with the permeation priority in vitro Caco-2 and BBB cell model could be better as the drug candidates for cardio-cerebral vascular protection. These findings provided important information for establishing the transport relationship for the flavonoid compounds and evaluating the potential oral bioavailability and brain distribution of the flavonoids.

In vitro and in vivo study of dolichyl phosphate on the efflux activity of P-glycoprotein at the blood-brain barrier

It has been commonly recognized that accumulated amyloid-β (Aβ) in the brain plays a crucial role in the pathogenesis of Alzheimer's disease (AD). Since the deficiency of the P-glycoprotein (P-gp) at the blood-brain barrier (BBB) in AD may aggravate Aβ deposition and the P-gp reversal agents display lower selectivity of the action, to selectively restore activity of the efflux pump is eagerly required. This study was designed to investigate the influence of dolichyl-phosphate (dolichyl-P) on the P-gp at the BBB. The results revealed that treatment with dolichyl-P increased transendothelial transfer of Rhodamine123 (Rh123) and Aβ42 from the apical compartment to the basolateral compartment but reduced that from the basolateral compartment to the apical compartment in the co-culture of rat brain microvessel endothelial cells (rBMECs) and astrocytes, down regulated P-gp expression in rBMECs and significantly elevated content of Rh123 in rat cortex and hippocampus tissues. The present results implied that accumulated dolichyl-P in the brain may exert an important role in the depression of the P-gp at the BBB, which may suggest valuable clues to promote function of the pump at the BBB in AD.

Modulation of P-glycoprotein in rat brain microvessel endothelial cells under oxygen glucose deprivation

Objectives

To investigate modulation of P-glycoprotein (P-gp) in rat brain microvessel endothelial cells (rBMECs) under oxygen glucose deprivation (OGD).

Methods

The coculture of rBMECs and astrocytes was established to investigate the time course of P-gp, tumour necrosis factor-α (TNF-α), endothelin-1 (ET-1), nitric oxide synthase (NOS) and protein kinase C (PKC) expression in the rBMECs as well as rhodamine 123 (Rh123) transendothelial transfer under OGD using Western blot and HPLC, respectively. The influence of pharmacological tools including H398, JKC-301, RES-701-1, L-NMMA, BIM and SN50 on the P-gp expression as well as Rh123 transendothelial transfer was evaluated at 3 h time point of OGD.

Key findings

Elevated P-gp, TNF-α, ET-1, NOS and PKC expression in the rBMECs, as well as increased P-gp efflux activity were observed after 2 h or more time of OGD. Incubation of H398 and other pharmacological tools downregulated P-gp expression and functional activity in the rBMECs at 3 h time point of OGD.

Conclusions

This report suggested that TNF-α, ET-1, NOS and PKC may mediate upregulation of P-gp in the rBMECs under OGD, which may be worthy of being referenced for the investigation of P-gp at the blood–brain barrier in the early period of stroke.

An immortalised astrocyte cell line maintains the in vivo phenotype of a primary porcine in vitro blood-brain barrier model

Whilst it is well documented that all components of the neurovascular unit contribute to the restrictive nature of the blood-brain barrier (BBB), astrocytes have been identified as the cellular component most likely to play an essential role in maintaining the barrier properties. The aim of this study was to examine the impact of the rat astrocyte cell line, CTX-TNA2, on the structural and functional characteristics of an in vitro BBB and determine the capacity of this astrocyte cell line to maintain the BBB phenotype. Co-culture of the CTX-TNA2 cells with primary porcine brain endothelial cells produced an in vitro BBB model which retains key features of the in vivo BBB. High transendothelial electrical resistances, comparable to those reported in vivo, were obtained. Ultrastructural analysis revealed distinct intercellular tight junction protein complexes and immunocytochemistry confirmed expression of the tight junction proteins ZO-1 and occludin. Western blotting and fluorescent tracer assays confirmed expression and functional activity of P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2) efflux transporters. Studies employing Alexa-fluor 555-conjugated human transferrin revealed temperature-sensitive internalisation indicating the BBB model retains functional receptor-mediated transferrin uptake. The findings of this study indicate that a robust BBB model has been produced and this is the first report of the inductive capacity of the CTX-TNA2 cell line. Since this in vitro BBB model possesses many key characteristics of the BBB in vivo it has the potential to be a valuable tool for the study of biochemical and physiological processes associated with the BBB.

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.

Influence of P-glycoprotein on brucine transport at the in vitro blood-brain barrier

Brucine is a central agonist that can pass through the blood-brain barrier (BBB). The goal of this study is to examine whether brucine is one of the substrates of the drug transporter P-glycoprotein (P-gp) and to examine the effects of P-gp on the brucine transport at the in vitro BBB model. The P-gp ATPase assay was utilized to investigate the in vitro affinity of P-gp to brucine. Results suggested that K(m) of brucine (11.4 μmol/l) was smaller than the positive control, verapamil (16.4 μmol/l). In this study, we developed an in vitro BBB model, comprising a co-culture of primary rat brain microvessel endothelial cells and astrocytes for the transport study. The validated model was correct and available. Transendothelial electrical resistance reached (283.78 ± 18.85) Ω cm(2). The model displayed limited permeability to fluorescein sodium and [(125)I]albumin, with the apparent permeability coefficient Papp of (10.36 ± 0.86) × 10(-6) cm/s and (6.00 ± 0.78) × 10(-6)cm/s, respectively. The quantity of the bidirectional transport of brucine was determined by ultra-performance liquid chromatography-tandem mass spectrometry. In the absence of verapamil, the transport of brucine from basolateral compartment to apical compartment (BL-AP) was higher than from AP to BL at low, middle, and high concentrations (P<0.05). The excretion rate was 1.32, 1.56, and 1.54, respectively. However, following exposure to verapamil, the excretion rate at three different concentrations was decreased (P<0.05). All the results suggest that P-gp prevented brucine from passing through the in vitro BBB model.

Isolation and cultivation of porcine astrocytes

A procedure for the isolation and cultivation of astrocytes from swine is described. More specifically, the donor animals are adolescent minipigs about 3 months in age and 10 kg in weight. About 20 g of cerebral tissue can be isolated from the piglet, yielding enough astrocytes of homogeneous genetics for experimentation after only one passage in culture. The astrocyte isolation procedure includes mechanical and enzymatic digestion of the brain tissue followed by separation of the brain fragments, based on size and density. Astrocytes are further purified from any residual nonastrocytes by differential attachment during the first passage. The resulting culture is purely astrocytes (>98%) based upon their appearance in phase-contrast microscopy and their uniform expression of glial fibrillary acidic protein. More importantly for our purposes, the astrocytes greatly enhanced the functionality of our in vitro blood-brain barrier model when cocultured with porcine brain capillary endothelial cells.

Barrier functionality of porcine and bovine brain capillary endothelial cells

INTRODUCTION

To date, isolated cell based blood-brain barrier (BBB) models have been widely used for brain drug delivery and targeting, due to their relatively proper bioelectrical and permeability properties. However, primary cultures of brain capillary endothelial cells (BCECs) isolated from different species vary in terms of bioelectrical and permeability properties.

METHODS

To pursue this, in the current investigation, primary porcine and bovine BCECs (PBCECs and BBCECs, respectively) were isolated and used as an in vitro BBB model. The bioelectrical and permeability properties were assessed in BCECs co-cultured with C6 cells with/without hydrocortisone (550 nM). The bioelectrical properties were further validated by means of the permeability coefficients of transcellular and paracellular markers.

RESULTS

The primary PBCECs displayed significantly higher trans-endothelial electrical resistance (~900 Ω.cm(2)) than BBCECs (~700 Ω.cm(2)) - both cocultured with C6 cells in presence of hydrocortisone. Permeability coefficients of propranolol/diazepam and mannitol/sucrose in PBCECs were ~21 and ~2 (×10(-6) cm.sec(-1)), where these values for BBCECs were ~25 and ~5 (×10(-6) cm.sec(-1)).

CONCLUSION

Upon our bioelectrical and permeability findings, both models display discriminative barrier functionality but porcine BCECs seem to provide a better platform than bovine BCECs for drug screening and brain targeting.

Development of a three-dimensional, all-human in vitro model of the blood-brain barrier using mono-, co-, and tri-cultivation Transwell models

In vitro models of the blood-brain barrier (B-BB) generally utilise murine or porcine brain endothelium and rat astrocytes which are commonly grown in foetal calf serum supplemented conditions which modulate cell growth rates. Consequently, results gained from these experimental models can be difficult to extrapolate to the human in vivo situation since they are not of human origin. The proposed in vitro Transwell model of the B-BB is a multi-culture human cell system. It requires reconstruction of the human derived B-BB components in vitro (cerebral microvascular endothelial cells, astrocytes, and brain vascular pericytes) in a three-dimensional (3D) configuration based on Transwell filters. Different cell permutations (mono-, co-, and tri-cultivation) were investigated to find the most effective model in terms of tight junction resistance of the human cerebral microvascular endothelial cells. The B-BB model permutations comprised of human astrocytes (CC-2565 and SC-1810), human brain vascular pericytes (HBVP), and human cerebral microvascular endothelial cells (hCMEC/D3), under human serum supplementation. The models were assessed by trans-endothelial electrical resistance (TEER) measurements using an epithelial voltohmmeter, to validate the tight junction formation between hCMEC/D3 cells. Mono-, co-, and tri-cultivation Transwell models constructed with human brain-derived cells under human serum supplementation demonstrated that co-cultivation of astrocytes with endothelial cells produced the most successful model, as determined by TEER. Pericytes on the other hand improved tight junction formation when co-cultured with endothelial cells but did not improve the model to such an extent when grown in tri-cultivation with astrocytes.

Effect of human astrocytes on the characteristics of human brain-microvascular endothelial cells in the blood-brain barrier

A blood-brain barrier (BBB) model in vitro was established by cultivating human brain-microvascular endothelial cells (HBMECs) with the regulation of human astrocytes (HAs) (HBMEC/HA). Astrocyte-conditioned medium (ACM) was employed to constitute a confluent monolayer of HBMECs without directly conjugated HAs. HBMECs exhibited an orientated multiplication on the supporting membrane; while HAs grew in an overlapping fashion. In addition, HBMECs could propagate over the membrane pore, and the end-feet of HAs extended into the membrane pore to improve the integral feature of the BBB. HBMEC/HA demonstrated a high transendothelial electrical resistance (TEER) about 230 Ω cm² and low permeability of propidium iodide (PI) about 4 × 10⁻⁶ cm/s. The order in TEER was HBMEC/HA>HBMECs with 100% ACM>HBMECs with 50% ACM > HBMECs. The reverse order was valid for the permeability of PI and uptake of calcein-AM by HBMECs. The tranwell culture of HBMECs and HAs displays appropriate characteristics of the BBB and can be applied to estimate the delivery efficiency of therapeutic chemicals for the brain-related disease.

Cultured adult porcine astrocytes and microglia express functional interferon-γ receptors and exhibit toxicity towards SH-SY5Y cells

In vitro cultures of various glial cell types are common systems used to model neuroinflammatory processes associated with age-dependent human neurodegenerative diseases. Even though most researchers choose to use neonatal rodent brain tissues as the source of glial cells, there are significant variations in glial cell functions that are species and age dependent. It has been established that human and swine immune systems have a number of similarities, which suggests that cultured porcine microglia and astrocytes may be good surrogates for human glial cell types. Here we describe a method that could be used to prepare more than 90% pure microglia and astrocyte cultures derived from adult porcine tissues. We demonstrate that both microglia and astrocytes derived from adult porcine brains express functional interferon-γ receptors (IFN-γ-R) and CD14. They become toxic towards SH-SY5Y neuroblastoma cells when exposed to proinflammatory mediators. Upon such stimulation with lipopolysaccharide (LPS) and interferon-γ (IFN-γ), adult porcine microglia, but not astrocytes, secrete tumor necrosis factor-α (TNF-α) while both cell types do not secrete detectable levels of nitric oxide (NO). Comparison of our experimental data with previously published studies indicates that adult porcine glial cultures have certain functional characteristics that make them similar to human glial cells. Therefore adult porcine glial cells may be a useful model system for studies of human diseases associated with adulthood and advanced age. Adult porcine tissues are relatively easy to obtain in most countries and could be used as a reliable and inexpensive source of cultured cells.

In vitro and in vivo investigation of glucose-mediated brain-targeting liposomes

New glycosyl derivative of cholesterol was synthesized as a material for preparing novel liposome to overcome the ineffective delivery of normal drug formulations to brain by targeting the (glucose transporters) GLUTs on the BBB. Coumarin-6 was used as fluorescent probe. The results have shown that the cytotoxicity for the brain capillary endothelial cells (BCECs) of the glucose-mediated brain targeting liposome containing coumarin-6 was less than that of conventional liposome. The BBB model in vitro was established by coculturing of BCECs and astrocytes (ACs) of rat to test the transendothelial ability crossing the BBB. The transendothelial ability was confirmed strengthen alone with the amount of the new glycosyl derivative of cholesterol used in liposome. After i.v. administration of LIP, control liposome (CLP), and GLP-4, the AUC(0-t) of coumarin-6 for GLP-4 was 2.85 times higher than that of LIP, and 3.33 times higher than that of CLP. The C(max) of CLP-4 was 1.43 times higher than that of LIP, and 3.10 times higher than that of CLP. Both pharmacokinetics and distribution in mice were also investigated to show that this novel brain targeting drug delivery system was promising.

Looking at the blood-brain barrier: molecular anatomy and possible investigation approaches

The blood-brain barrier (BBB) is a dynamic and complex interface between blood and the central nervous system that strictly controls the exchanges between the blood and brain compartments, therefore playing a key role in brain homeostasis and providing protection against many toxic compounds and pathogens. In this review, the unique properties of brain microvascular endothelial cells and intercellular junctions are examined. The specific interactions between endothelial cells and basement membrane as well as neighboring perivascular pericytes, glial cells and neurons, which altogether constitute the neurovascular unit and play an essential role in both health and function of the central nervous system, are also explored. Some relevant pathways across the endothelium, as well as mechanisms involved in the regulation of BBB permeability, and the emerging role of the BBB as a signaling interface are addressed as well. Furthermore, we summarize some of the experimental approaches that can be used to monitor BBB properties and function in a variety of conditions and have allowed recent advances in BBB knowledge. Elucidation of the molecular anatomy and dynamics of the BBB is an essential step for the development of new strategies directed to maintain or restore BBB integrity and barrier function and ultimately preserve the delicate interstitial brain environment.

Closing the gap between the in-vivo and in-vitro blood-brain barrier tightness

Numerous in-vitro models of the blood-brain barrier (BBB) have been developed in the hope to mimic as closely as possible the in-vivo BBB characteristics. Most models however display BBB tightness properties still very remote from those found in-vivo. We describe here the properties of an in-vitro BBB model in three configurations: primary porcine brain endothelial cells (PBEC) grown in a monoculture, or as a co-culture in close proximity to rat glial cells (contact), or with the latter at distance (non-contact). The BBB tightness as reflected by measurements of the permeability (Pe) to sucrose and of the transendothelial electrical resistance (TEER) showed that only the contact co-culture closely mimic the in-vivo BBB (Pe=0.1910(-6)+/-0.01 cm/s and TEER up to 1650 Omegacm2). While no changes in the expression pattern of three of the major tight junction proteins, claudin-5, occludin and ZO-1, were observed using immunohistochemistry, western blot analysis showed that the expression levels of claudin-5 and occludin increase when PBEC are cultured in contact with glial cells. In addition, we found, in the contact co-culture model, a reduced sensitivity of the TEER to vinblastine, a P-glycoprotein (Pgp) substrate that disrupts the cell cytoskeleton, indicating an improved functionality of the Pgp transporters in this configuration. We conclude that the close proximity of astrocytes is crucial to the development of a tight BBB.

Biomedical Technologies for in vitro Screening and Controlled Delivery of Neuroactive Compounds

Cell culture models can provide information pertaining to the effective dose, toxiciology, and kinetics, for a variety of neuroactive compounds. However, many in vitro models fail to adequately predict how such compounds will perform in a living organism. At the systems level, interactions between organs can dramatically affect the properties of a compound by alteration of its biological activity or by elimination of it from the body. At the tissue level, interaction between cell types can alter the transport properties of a particular compound, or can buffer its effects on target cells by uptake, processing, or changes in chemical signaling between cells. In any given tissue, cells exist in a three-dimensional environment bounded on all sides by other cells and components of the extracellular matrix, providing kinetics that are dramatically different from the kinetics in traditional two-dimensional cell culture systems. Cell culture analogs are currently being developed to better model the complex transport and processing that occur prior to drug uptake in the CNS, and to predict blood-brain barrier permeability. These approaches utilize microfluidics, hydrogel matrices, and a variety of cell types (including lung epithelial cells, hepatocytes, adipocytes, glial cells, and neurons) to more accurately model drug transport and biological activity. Similar strategies are also being used to control both the spatial and temporal release of therapeutic compounds for targeted treatment of CNS disease.

Regulation of bovine brain microvascular endothelial tight junction assembly and barrier function by laminar shear stress

Blood-brain barrier (BBB) controls paracellular solute diffusion into the brain microenvironment and is maintained primarily by tight junctions between adjacent microvascular endothelial cells. Studies implicate blood flow-associated shear stress as a pathophysiological mediator of BBB function, although detailed biochemical data are scarce. We hypothesize that shear stress upregulates BBB function via direct modulation of expression and properties of pivotal tight-junction proteins occludin and zonula occludens-1 (ZO-1). Bovine brain microvascular endothelial cells (BBMvECs) were exposed to either steady or pulsatile shear stress (10 and 14 dyn/cm2, respectively) for 24 h. Sheared BBMvECs were monitored for occludin-ZO-1 expression, association, and subcellular localization, and transendothelial permeability of BBMvECs to FITC-dextran and14[C]sucrose was assessed. Actin reorganization and BBMvEC realignment were observed following steady shear stress for 24 h. Substantial increases in occludin mRNA and protein expression (2.73 ± 0.26- and 1.83 ± 0.03-fold) and in occludin-ZO-1 association (2.12 ± 0.15-fold) were also observed. Steady shear stress also induced clear relocalization of both proteins to the cell-cell border in parallel with reduced transendothelial permeability to FITC-dextran (but not sucrose). Following pulsatile shear stress, increased protein levels for both occludin and ZO-1 (2.15 ± 0.02- and 1.67 ± 0.21-fold) and increased occludin-ZO-1 association (2.91 ± 0.14-fold) were observed in parallel with a reduction in transendothelial permeability to14[C]sucrose. Shear stress upregulates BBMvEC barrier function at the molecular level via modulation of expression, association, and localization of occludin and ZO-1. The pulsatile shear model appeared to give the most profound biochemical responses.

Strengthening tight junctions of retinal microvascular endothelial cells by pericytes under normoxia and hypoxia involving angiopoietin-1 signal way

PURPOSE

To determine the effects of pericytes and angiopoietin-1 on the expression of occludin and zonula occludens-1 (ZO-1) in retinal endothelial cells (ECs) under both normoxic and hypoxic conditions.

METHODS

Rat primary retinal microvascular ECs were cultured under normoxia or hypoxia in either absence or presence of pericytes conditioned medium (PCM). PCM was pretreated with or without angiopoietin-1 neutralizing antibody. Immuofluorescent staining, Western blot and RT-PCR were used to detect the alterations of occludin and ZO-1 expression.

RESULTS

Under normoxia, PCM strengthened occludin and ZO-1 immunofluorescent staining at cytomembrane as well as increased their expression at both protein and mRNA level. When pretreated with angiopoietin-1 neutralizing antibody, occludin upregulation induced by PCM was significantly blocked at protein level (62%) and mRNA level (34%). Under hypoxia, the continuity of occludin and ZO-1 staining at cell boundaries was disrupted consistent with a decrease of their protein level by 31 and 27%, respectively. Also occludin and ZO-1 mRNA level decreased by 46 and 57%, respectively. PCM was observed to partially increase expression of occludin at protein and mRNA level. Angiopoietin-1 antibody slightly inhibited (16%) PCM induced occludin mRNA increase under hypoxia.

CONCLUSION

Pericytes improved the integrity of endothelial barrier through inducing occludin and ZO-1 expression at protein and mRNA level under normoxia. Under hypoxia, pericytes could partially reverse occludin decrease. These protecting effects of pericytes on endothelial barrier were at least in part mediated by angiopoietin-1.

Porcine brain microvessel endothelial cells as an in vitro model to predict in vivo blood-brain barrier permeability

The objective of the study was to establish primary cultured porcine brain microvessel endothelial cells (PBMECs) as an in vitro model to predict the blood-brain barrier (BBB) permeability in vivo. The intercellular tight junction formation of PBMECs was examined by electron microscopy and measured by transendothelial electrical resistance (TEER). The mRNA expression of several BBB transporters in PBMECs was determined by reverse transcriptionpolymerase chain reaction analysis. The in vitro permeability of 16 structurally diverse compounds, representing a range of passive diffusion and transporter-mediated mechanisms of brain penetration, was determined in PBMECs. Except for the perfusion flow rate marker diazepam, the BBB permeability of these compounds was determined either in our laboratory or as reported in literature using in situ brain perfusion technique in rats. Results in the present study showed that PBMECs had a high endothelium homogeneity, an mRNA expression of several BBB transporters, and high TEER values. Culturing with rat astrocyte-conditioned medium increased the TEER of PBMECs, but had no effect on the permeability of sucrose, a paracellular diffusion marker. The PBMEC permeability of lipophilic compounds measured under stirred conditions was greatly increased compared with that measured under unstirred conditions. The PBMEC permeability of the 15 test compounds, determined under the optimized study conditions, correlated with the in situ BBB permeability with an r2 of 0.60. Removal of the three system L substrates increased the r2 to 0.89. In conclusion, the present PBMEC model may be used to predict or rank the in vivo BBB permeability of new chemical entities in a drug discovery setting.

Development of an in vitro blood-brain barrier model to study the effects of endosulfan on the permeability of tight junctions and a comparative study of the cytotoxic effects of endosulfan on rat and human glial and neuronal cell cultures

Endosulfan, an organochlorine (OC) insecticide that belongs to the cyclodiene group, is one of the most commonly used pesticides to control pests in vegetables, cotton, and fruits. Porcine brain microvascular endothelial cells were used to develop a model to study the effects of endosulfan on the permeability of tight junctions in the blood-brain barrier (BBB). BBB permeability, measured as transendothelial electrical resistance, decreased in a dose- and time-dependent manner when treated with alpha-endosulfan, beta-endosulfan, or endosulfan sulfate. Cytotoxicity testing revealed that the three endosulfans did not cause cell death at concentrations of 10 microM and below. The ratio of the average permeability of the filter-grown endothelial cell monolayer to 14C-endosulfan (Pe) going from the outer to the inner compartments with that going from the inner to the outer compartments was approximately 1:1.2-2.1 after exposure to concentrations of 0.01-10 microM. alpha-Endosulfan, beta-endosulfan, and endosulfan sulfate had cytotoxic effects on rat glial (C6) and neuronal (PC12) cell cultures as well as on human glial (CCF-STTG1) and neuronal (NT2) cell cultures. The effects of alpha-endosulfan were highly selective, with a wide range of LC50 values found in the different cultures, ranging from 11.2 microM for CCF-STTG1 cells to 48.0 microM for PC12 cells. In contrast, selective neurotoxicity was not so manifest in glial and neuronal cell cultures after exposure to endosulfan sulfate, as LC50 values were in the range of 10.4-21.6 microM. CCF-STTG1 cells were more sensitive to alpha-endosulfan and endosulfan sulfate, whereas NT2 cells were more sensitive to beta-endosulfan.

Growth and characterisation of a cell culture model of the feline blood–brain barrier

An in vitro model of the feline blood-brain barrier was developed using primary cultures of brain capillary endothelial cells derived from adult cats. They were grown in the presence of astrocytes obtained from newborn kittens. Feline endothelial cell cultures were characterised by uptake of DiI-acetylated low-density lipoprotein (DiI-Ac-LDL) and expression of von Willebrand factor. Astrocytes were characterised based on their expression of glial fibrillary acidic protein (GFAP). Electron microscopy revealed junctional specialisation between endothelial cells. Occludin and ZO-1 expression by the endothelial cell cultures was detected by Western blot analysis. Barrier function of co-cultured endothelial cells and astrocytes was confirmed by a transendothelial electrical resistance (TEER) value of 30-35 Omegacm2 and apparent permeability coefficients (Papp) for FD-40 (FITC-dextran, 40 kDa) of 4x10(-6) cm/s and for FD-4 (4kDa) of 1.92x10(-5) cm/s. In endothelial cell monolayers grown with astrocyte-conditioned medium, the TEER value was lower (20-25 Omegacm2), and Papp of FD-40 and FD-4 was higher at 6.27x10(-6) and 3.96x10(-5) cm/s, respectively. This model should have useful applications in the examination of events occurring at the BBB early in FIV infection, and may provide knowledge applicable to HIV infection.

Evaluation of Drug Penetration into the Brain: A Double Study by in Vivo Imaging with Positron Emission Tomography and Using an in Vitro Model of the Human Blood-Brain Barrier

The blood-brain barrier (BBB) permeabilities of 11 compounds were measured both in vitro with a newly developed coculture-based model of human BBB and in vivo with positron emission tomography (PET). The 11 compounds were fluoropyridinyl derivatives labeled with the positron-emitter fluorine-18, [(18)F]F-A-85380 [2-[(18)F]fluoro-3-[2(S)-2 azetidinylmethoxy]pyridine], and 10 selected N-substituted-azetidinyl and pyrrolidinyl closely related [(18)F]fluoropyridinyl derivatives (including [N'-aromatic/aliphatic]-thioureas, -ureas, and -amides). The in vitro BBB model, a new coculture system of primary human brain endothelial cells and astrocytes, was used to measure the permeability coefficient for each compound. Dynamic PET studies were performed in rats with the same compounds, and a two-compartment model analysis was used to calculate their in vivo permeability coefficients. The 11 derivatives differed in their degree of BBB passage and transport mechanism. The analysis of PET data showed a significant cerebral uptake for six derivatives, for which the in vitro evaluation indicated active influx or free diffusion. Five derivatives displayed low in vivo cerebral uptake, in agreement with the observation of an in vitro active efflux. Overall, there was a remarkable correlation between the in vitro and in vivo permeability coefficients (r = 0.99). This double study proves a close correlationship between the assessment of the BBB passage in vitro and in vivo. The in vitro model of human BBB offers the possibility of subtle discrimination of various BBB permeability degrees and transport mechanisms. Conversely, small animal PET imaging appears suitable to screen directly in vivo brain targeting of drugs or radiopharmaceutical candidates.

The critical component to establish in vitro BBB model: Pericyte

The blood-brain barrier (BBB), a highly regulated membranous barrier of brain capillaries, consists of an intricate network of tight junctions (TJs) that segregate the central nervous system (CNS) from systemic blood circulation and maintain a delicate homeostasis of the CNS environment. While endothelial cells (ECs) of brain capillaries are clearly the principal cellular element of BBB, the formation and regulation of intact BBB structure appear to require the interactions of endothelial cells with other cellular components. Astrocytes, one of the major non-neural cells in the brain, associate closely and interact with capillary endothelial cells during the angiogenesis and BBB development. Current in vitro cellular models for the study of BBB functions often incorporate astrocytes with endothelial cells. However, another foremost cell type, CNS pericyte, which intimately embraces brain capillary endothelium, attracts relatively little attention for its role in developing the in vitro BBB system. This review will analyze the critical functions of pericytes in angiogenesis in various systems and discuss the relevance of these functions in mediating the development, maintenance, and regulation of BBB. The author will also discuss the functional role of actin in both ECs and pericytes, and further elaborate the molecular mechanisms of BBB permeability regulation that involves the transduction pathway-mediated actin remodeling process. Finally, the rationale of incorporating pericytes for establishing a better in vitro BBB model will be emphasized.

Dynamics of sulfur amino acids in mammalian brain: assessment of the astrocytic-neuronal cysteine interaction by a mathematical hybrid model

A mathematically hybrid model was used to analyze three mechanisms by which cysteine could be produced in the brain to be used as preferential substrate in the synthesis of neuronal glutathione. In that way, the fluxes of sulfur-compounds at the brain-blood barrier were integrated with their transport in astrocytes and neurons, and with their metabolism in astrocytes. We concluded that cysteine, in contrast with its precursor cystine, would not be taken up from the blood at the blood-brain barrier, but instead it must be lost continuously from astrocytes. Cysteine efflux is produced because the uptake of cystine in astrocytes is much greater than their cysteine demand to synthesize glutathione, hypotaurine and taurine. Once in the interstitial parenchyma, cysteine would be taken for the neurons, as backwardly by the endothelial cells. Remarkably, a close sulfur-macro balance can be maintained only if the surplus of the produced cysteine is transferred from the endothelial cells to the blood together with significant amounts of other sulfur-compounds, probably taurine and hypotaurine. In addition, the results obtained shown that alternative mechanisms of cysteine generation (i.e., nonenzymatic-thiol-disulfide exchange reaction, enzymatic cleavage of the glutathione effluxed from astrocytes) are not quantitatively significant under physiological conditions, in situ.