Development of an in vitro blood–brain barrier model based on immortalized porcine brain microvascular endothelial cells

Progresses in the establishment, evaluation, and application of in vitro blood-brain barrier models

The blood-brain barrier (BBB) is a barrier between the circulatory system and the central nervous system (CNS), contributing to CNS protection and maintaining the brain homeostasis. Establishment of in vitro BBB models that are closer to the microenvironment of the human brain is helpful for evaluating the potential and efficiency of a drug penetrating BBB and thus the clinical application value of the drug. The in vitro BBB models not only provide great convenience for screening new drugs that can access to CNS but also help people to have a deeper study on the mechanism of substances entering and leaving the brain, which makes people have greater opportunities in the treatment of CNS diseases. Up to now, although much effort has been paid to the researches on the in vitro BBB models and many progresses have been achieved, no unified method has been described for establishing a BBB model and there is much work to do and many challenges to be faced with in the future. This review summarizes the research progresses in the establishment, evaluation, and application of in vitro BBB models.

Species-specific models in toxicology: in vitro epithelial barriers

Species-specific in vitro epithelial barriers represent interesting predictive tools for risk assessment evaluation in toxicological studies. Moreover, these models could be applied either as stand-alone methods for the study of absorption, bioavailability, excretion, transport, effects of xenobiotics, or through an Integrated Testing Strategy. The aim of this review is to give a comprehensive overview of in vitro species-specific epithelial barrier models from bovine, dog and swine. Bovine mammary epithelial barrier as a fundamental instrument for the evaluation of the toxicant excretion, the blood brain barrier as a useful first approach in toxicological and pharmacological studies, the porcine intestinal barrier, the canine skin barrier, and finally the pulmonary barrier from bovine and swine species are described in this review.

Phytochemical mediated-modulation of the expression and transporter function of breast cancer resistance protein at the blood-brain barrier: An in-vitro study

Clinical translation of BCRP inhibitors have failed due to neurotoxicity and novel approaches are required to identify suitable modulators of BCRP to enhance CNS drug delivery. In this study we examine 18 compounds, primarily phytochemicals, as potential novel modulators of AhR-mediated regulation of BCRP expression and function in immortalised and primary porcine brain microvascular endothelial cells as a mechanism to enhance CNS drug delivery. The majority of modulators possessed a cellular viability IC₅₀ >100µm in both cell systems. BCRP activity, when exposed to modulators for 1h, was diminished for most modulators through significant increases in H33342 accumulation at <10µm with 2,6,4-trimethoflavone increasing H33342 intracellular accumulation by 3.7-6.6 fold over 1-100µm. Western blotting and qPCR identified two inducers of BCRP (quercetin and naringin) and two down-regulators (17-β-estradiol and curcumin) with associated changes in BCRP efflux transport function further confirmed in both cell lines. siRNA downregulation of AhR resulted in a 1.75±0.08 fold change in BCRP expression, confirming the role of AhR in the regulation of BCRP. These findings establish the regulatory role AhR of in controlling BCRP expression at the BBB and confirm quercetin, naringin, 17-β-estradiol, and curcumin as novel inducers and down-regulators of BCRP gene, protein expression and functional transporter activity and hence potential novel target sites and candidates for enhancing CNS drug delivery.

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.

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.

The value of in vitro models of the blood-brain barrier and their uses

A range of in vitro BBB models are available, that can simplify the complexities associated with the in vivo study of the BBB. However, the adoption of these models, especially for studying the pathology of the BBB, is still poor, despite their ability to complement and reduce animal experiments

A modular approach to create a neurovascular unit-on-a-chip

In this work, we describe the fabrication and working of a modular microsystem that recapitulates the functions of the "Neurovascular Unit". The microdevice comprised a vertical stack of a poly(dimethylsiloxane) (PDMS) neural parenchymal chamber separated by a vascular channel via a microporous polycarbonate (PC) membrane. The neural chamber housed a mixture of neurons (~4%), astrocytes (~95%), and microglia (~1%). The vascular channel was lined with a layer of rat brain microvascular endothelial cell line (RBE4). Cellular components in the neural chamber and vascular channel showed viability (>90%). The neural cells fired inhibitory as well as excitatory potentials following 10 days of culture. The endothelial cells showed diluted-acetylated low density lipoprotein (dil-a-LDL) uptake, expressed von Willebrand factor (vWF) and zonula occludens (ZO-1) tight junctions, and showed decreased Alexafluor™-conjugated dextran leakage across their barriers significantly compared with controls (p < 0.05). When the vascular layer was stimulated with TNF-α for 6 h, about 75% of resident microglia and astrocytes on the neural side were activated significantly (p < 0.05 compared to controls) recapitulating tissue-mimetic responses resembling neuroinflammation. The impact of this microsystem lies in the fact that this biomimetic neurovascular platform might not only be harnessed for obtaining mechanistic insights for neurodegenerative disorders, but could also serve as a potential screening tool for central nervous system (CNS) therapeutics in toxicology and neuroinfectious diseases.

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.

Drug penetration across the blood-brain barrier: an overview

The brain is one of the most protected organs in the body. There are two key barriers that control the access of endogenous substances and xenobiotics (drugs or toxins) to the CNS. These physiological structures are the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier. The BBB represents the main determinant of the effective delivery of drugs to the CNS. Good access through the BBB is essential if the target site is located within the CNS or, in contrast, can be a disadvantage if adverse reactions occur at central level. The development of new drugs targeted to the CNS requires a better knowledge of the factors affecting BBB permeation as well as in vitro and in silico predictive tools to optimize screening, and to reduce the attrition rate at later stages of drug development. This review discusses the particular characteristics of the biology and physiology of the BBB with respect to the permeation and distribution of drugs into the brain. The factors affecting rate, extent and distribution into the brain are discussed and a brief description of the in silico, in vitro, in situ and in vivo methods used to measure BBB transport are presented. Finally, the lastest proposals and strategies to enhance transport across the BBB of new CNS drugs are summarized.

Experimental models for assaying microvascular endothelial cell pathophysiology in stroke

It is important to understand the molecular mechanisms underlying neuron death following stroke in order to develop effective neuroprotective strategies. Since studies on human stroke are extremely limited due to the difficulty in collecting post-mortem tissue at different time points after the onset of stroke, brain ischaemia research focuses on information derived from in-vitro models of neuronal death through ischaemic injury [1]. This review aims to provide an update on the different in-vitro stroke models with brain microvascular endothelial cells that are currently being used. These models provide a physiologically relevant tool to screen potential neuroprotective drugs in stroke and to study the molecular mechanisms involved in brain ischaemia.

Blood-brain barrier cell line PBMEC/C1-2 possesses functionally active P-glycoprotein

The blood-brain barrier (BBB) maintains the homeostasis between the central nervous system and the blood circulation. One of the main efflux transporter proteins at the BBB is P-glycoprotein (P-gP) also known as ABCB1 or MDR1. Due to the important role of P-gP for the transport barrier function of the BBB, the presence and functionality of P-gP was investigated in porcine cell line PBMEC/C1-2. Presence of P-gP was confirmed on the protein level by western blotting and immunofluorescence microscopy as well as on the mRNA level by qPCR. Functional assessment was accomplished by an established 96-well uptake assay using Rhodamine 123 and Doxorubicin as P-gP substrates and Verapamil as moderate P-gP inhibitor. In this regard, fluorescence microscopy confirmed a significant higher uptake of Rhodamine 123 into PBMEC/C1-2 cells when preincubated with Verapamil. Finally, knock-down of P-gP by antisense oligonucleotides revealed an increase of Rhodamine 123 uptake indicating decreased P-gP functionality. In summary, the presence and functionality of P-gP in the immortalised cell line PBMEC/C1-2 was proven with several techniques and assays. Thus, this cell line could be used for P-gP studies in the context of BBB relevant issues.

Validation of in vitro cell culture models of the blood-brain barrier: tightness characterization of two promising cell lines

In the course of the validation of blood-brain barrier in vitro models the aim of this work was to characterize two promising continuous cell lines with regard to their tightness properties. PBMEC/C1-2 and ECV304 cells were cultured in several media with different compositions on Transwell inserts. Inducibility and functionality of tightness were investigated by transendothelial electrical resistance (TEER) and by transport studies with transcellular marker diazepam, glycine antagonist Bu101 and paracellular marker APTS-dextran. Inducibility, expression and localization of tight junctional proteins were assessed by western blotting and immunofluorescence microscopy. Presence of factors derived from glioma cell line C6 resulted in increased TEER in both cell lines. Comparison to APTS-dextran data across Caco-2 layers emphasized that correlations between permeability of the paracellular marker and TEER are dependent on each investigated cell line and the corresponding growth medium. Presence and inducibility of tight junctional proteins ZO-1 and Occludin were proven for ECV304 layers. Cell line ECV304 seemed to be suitable for TEER dependent transport studies, whereas PBMEC/C1-2 showed higher potential for P-gP studies.

Side by side comparison between dynamic versus static models of blood–brain barrier in vitro: A permeability study

Endothelial cells in vivo are continuously exposed to shear stress, a tangential force generated by the flow of blood across their apical surfaces that affects endothelial cell structure and function. By contrast, the Transwell apparatus cannot reproduce the presence of intraluminal blood flow that is essential for the formation and differentiation of the BBB. In contrast, the dynamic in vitro model of the BBB (DIV-BBB) mimics both functionally and anatomically the brain microvasculature, creating quasi-physiological conditions for co-culturing human and non-human endothelial cells and astrocytes in a capillary-like structure. We used intraluminal bovine aortic endothelial cells (BAEC) co-cultured with extraluminal glial cells (C6) to obtain elevated trans-endothelial electrical resistance (TEER) and selective permeability to sucrose and phenytoin. The experiments were performed in parallel using Transwell systems DIV-BBB models and data were then cross compared. By contrast with Transwell, C6 and BAEC co-cultured in the DIV-BBB demonstrated predominantly aerobic metabolism evidenced by a robust increase in glucose consumption that was paralleled by a similar change in lactate production. BAEC exposed to glia under dynamic conditions grow in a monolayer fashion and developed a more stringent barrier as demonstrated by high TEER values and a selective permeability to [14C] phenytoin and the well-known paracellular marker [3H] sucrose. In conclusion, these data demonstrate that the exposure to intraluminal flow plays an essential role in promoting endothelial cell differentiation and increasing BBB tightness, thus making the use of the DIV-BBB well suited for pharmacological studies.

A novel tool to characterize paracellular transport: the APTS-dextran ladder

PURPOSE

The aim of this work was to develop an easy, manageable, and precise analytic tool to describe the tightness of cell layers by a molecular weight ladder.

METHODS

Dextrans were labeled by reductive amination with fluorescent 8-aminopyrene-1,3,6-trisulfonate (APTS). This mixture, including the internal standard diazepam, was used for transport studies in Transwell models using Caco-2, ECV304, and PBMEC/C1-2 cell lines. Samples were analyzed by fluorimetry, capillary electrophoresis, and reverse-phase high-performance liquid chromatography.

RESULTS

Following this approach, a logarithm correlation of R2 = 0.8958 between transepithelial electrical resistance (TEER) and APTS-dextran permeability was shown. In addition, a TEER-dependent permeability pattern could be observed including each single fraction from free APTS, APTS-glucose up to APTS-dextran consisting of 35 glucose units. The TEER-independent permeability coefficients of diazepam and confocal laser scanning microscopy images confirmed the paracellular transport of APTS-dextran.

CONCLUSIONS

All in all, the developed APTS-dextran ladder is a useful tool to characterize cell layer tightness and especially to describe paracellular transport ways and the extent of leakiness of cell layers (for blood-brain barrier or intestinal studies) over time--applying a wide array from smaller to larger molecules at the same time to refine TEER, sucrose, or Evans blue measurements.

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.

A novel flow based hollow-fiber blood-brain barrier in vitro model with immortalised cell line PBMEC/C1-2

A flow based hollow-fiber in vitro model of the blood-brain barrier (BBB) was established. The immortalised porcine brain microvascular endothelial cell line PBMEC/C1-2 was cultured in a pulsatile hollow-fiber cartridge system (Cellmax Quad). The usability of PBMEC/C1-2 in the flow based hollow-fiber model was increased from three days in the originally used Transwell model up to four months due to the application of shear stress and co-culturing with glioma cell line C6. It was shown that the tightness of PBMEC/C1-2 layers was enhanced significantly in astrocyte conditioned medium (ACM) and in co-culture. The morphology of PBMEC/C1-2 and C6 was visualised by environmental scanning electron microscopy (ESEM). Permeation studies were accomplished with a set of benzodiazepines. The raw data were processed with three different calculation models and the results were compared with permeability coefficients obtained with an established Transwell model. In summary a flow based hollow-fiber BBB in vitro model was developed, which can be used to perform experiments with physiological (e.g., regulation of BBB permeability), pharmacological (e.g., pharmacokinetics and dynamics) and pathophysiological (e.g., effects of diseases on BBB permeability and vice versa) objectives.

APTS-labeled dextran ladder: A novel tool to characterize cell layer tightness

The aim of this work was the development of an easy manageable analytic system for describing tightness of cell layers in a molecular size dependent manner, which is more precise than currently used ones. Dextrans were labeled by reductive amination with fluorescent 1-aminopyrene-3,6,8-trisulfonate (APTS). This mixture, including internal standard diazepam, was used for transport studies, which were accomplished with an established transwell blood-brain barrier model culturing an immortalized porcine brain microvascular endothelial cell line (PBMEC/C1-2). Samples were analyzed by fluorescence measurements, capillary electrophoresis and RP-LC. Following this approach, a permeability pattern could be achieved including each single fraction from APTS, APTS-glucose to APTS-dextran consisting of 31 glucose units. Permeability coefficients were calculated and ranged from 16.38+/-3.79 microm/min for APTS to 6.07+/-1.23 microm/min for the APTS-dextran with 31 glucose units (diazepam: 67.97+/-7.32 microm/min). All in all, the developed APTS-dextran ladder is an useful tool to characterize cell layer tightness--especially to describe paracellular transport ways and leakiness status of the blood-brain barrier over time--applying a wide range from smaller to larger molecules at the same time in order to refine, e.g. TEER, sucrose or Evans blue measurements.

Prediction of blood-brain barrier permeation in drug discovery from in vivo, in vitro and in silico models

The properties of the blood-brain barrier (BBB) are important considerations when designing drugs to target or avoid the brain. In vivo models provide some of the most reliable measurements of BBB drug permeability, whereas several new cell-based in vitro technologies give valuable mechanistic insights. Higher throughout methods (cell- and non-cell-based; in silico modelling) can help in prediction of permeation, but a combination of techniques is still required to cover the range of BBB drug entry and efflux mechanisms.: