Use of a three-dimensional in vitro model of the rat blood-brain barrier to assay nucleoside efflux from brain

Inosine pretreatment of pregnant rats ameliorates maternal inflammation-mediated hypomyelination in pups via microglia polarization switch

Periventricular leukomalacia (PVL) is a neurological condition observed in premature infants, characterized by hypomyelination and activation of microglia. Maternal inflammation-induced brain injury in offspring significantly contributes to the development of PVL. Currently, there are no clinical pharmaceutical interventions available for pregnant women to prevent maternal inflammation-mediated brain injury in their offspring. Inosine has been shown to modulate the immune response in diverse stressful circumstances, such as injury, ischemia, and inflammation. The aim of this investigation was to examine the potential prophylactic impact of inosine on offspring PVL induced by maternal inflammation. This was accomplished by administering a 1 mg/ml inosine solution (40 ml daily) to pregnant Sprague-Dawley (SD) rats for 16 consecutive days prior to their intraperitoneal injection of lipopolysaccharide (350 µg/kg, once a day, for two days). The results showed that maternal inosine pretreatment significantly reversed the reduction in MBP and CNPase (myelin-related markers), CC-1 and Olig2 (oligodendrocyte-related markers) in their PVL pups (P7), suggesting that inosine administration during pregnancy could improve hypomyelination and enhance the differentiation of oligodendrocyte precursor cells (OPCs) in their PVL pups. Furthermore, the protective mechanism of inosine against PVL is closely associated with the activation and polarization of microglia. This is evidenced by a notable reduction in the quantity of IBA 1-positive microglia, a decrease in the level of CD86 (a marker for M1 microglia), an increase in the level of Arg 1 (a marker for M2 microglia), as well as a decrease in the level of pro-inflammatory factors TNF-α, IL-1β, and IL-6, and an increase in the level of anti-inflammatory factors IL-4 and IL-10 in the brain of PVL pups following maternal inosine pretreatment. Taken together, inosine pretreatment of pregnant rats can improve hypomyelination in their PVL offspring by triggering the M1/M2 switch of microglia.

Challenges and prospects in geriatric epilepsy treatment: the role of the blood-brain barrier in pharmacotherapy and drug delivery

The blood-brain barrier (BBB) is pivotal in maintaining neuronal physiology within the brain. This review delves into the alterations of the BBB specifically in the context of geriatric epilepsy. We examine how age-related changes in the BBB contribute to the pathogenesis of epilepsy in the elderly and present significant challenges in pharmacotherapy. Subsequently, we evaluate recent advancements in drug delivery methods targeting the BBB, as well as alternative approaches that could bypass the BBB's restrictive nature. We particularly highlight the use of neurotropic viruses and various synthetic nanoparticles that have been investigated for delivering a range of antiepileptic drugs. Additionally, the advantage and limitation of these diverse delivery methods are discussed. Finally, we analyze the potential efficacy of different drug delivery approaches in the treatment of geriatric epilepsy, aiming to provide insights into more effective management of this condition in the elderly population.

Experimental Models to Study the Functions of the Blood-Brain Barrier

The purpose of this paper was to discuss the achievements of in vitro modeling in terms of the blood-brain barrier [BBB] and to create a clear overview of this research area, which is useful in research planning. The text was divided into three main parts. The first part describes the BBB as a functional structure, its constitution, cellular and noncellular components, mechanisms of functioning and importance for the central nervous system, in terms of both protection and nourishment. The second part is an overview of parameters important in terms of establishing and maintaining a barrier phenotype that allows for formulating criteria of evaluation of the BBB in vitro models. The third and last part discusses certain techniques for developing the BBB in vitro models. It describes subsequent research approaches and models, as they underwent change alongside technological advancement. On the one hand, we discuss possibilities and limitations of different research approaches: primary cultures vs. cell lines and monocultures vs. multicultures. On the other hand, we review advantages and disadvantages of specific models, such as models-on-a-chip, 3D models or microfluidic models. We not only attempt to state the usefulness of specific models in different kinds of research on the BBB but also emphasize the significance of this area of research for advancement of neuroscience and the pharmaceutical industry.

On the quest of reliable 3D dynamic in vitro blood-brain barrier models using polymer hollow fiber membranes: Pitfalls, progress, and future perspectives

With the increasing concern of neurodegenerative diseases, the development of new therapies and effective pharmaceuticals targeted to central nervous system (CNS) illnesses is crucial for ensuring social and economic sustainability in an ageing world. Unfortunately, many promising treatments at the initial stages of the pharmaceutical development process, that is at the in vitro screening stages, do not finally show the expected results at the clinical level due to their inability to cross the human blood-brain barrier (BBB), highlighting the inefficiency of in vitro BBB models to recapitulate the real functionality of the human BBB. In the last decades research has focused on the development of in vitro BBB models from basic 2D monolayer cultures to 3D cell co-cultures employing different system configurations. Particularly, the use of polymeric hollow fiber membranes (HFs) as scaffolds plays a key role in perfusing 3D dynamic in vitro BBB (DIV-BBB) models. Their incorporation into a perfusion bioreactor system may potentially enhance the vascularization and oxygenation of 3D cell cultures improving cell communication and the exchange of nutrients and metabolites through the microporous membranes. The quest for developing a benchmark 3D dynamic in vitro blood brain barrier model requires the critical assessment of the different aspects that limits the technology. This article will focus on identifying the advantages and main limitations of the HFs in terms of polymer materials, microscopic porous morphology, and other practical issues that play an important role to adequately mimic the physiological environment and recapitulate BBB architecture. Based on this study, we consider that future strategic advances of this technology to become fully implemented as a gold standard DIV-BBB model will require the exploration of novel polymers and/or composite materials, and the optimization of the morphology of the membranes towards thinner HFs (<50 μm) with higher porosities and surface pore sizes of 1–2 µm to facilitate the intercommunication via regulatory factors between the cell co-culture models of the BBB.

A Novel Dynamic Human In Vitro Model for Studying the Blood-Brain Barrier

Constructing a reliable in vitro blood-brain barrier (BBB) model using human primary cells has been considered a major challenge during the past decades. These systems could provide valuable information regarding the effect of therapeutic compounds on different BBB cell types (endothelial cells, astrocytes, pericytes) and their ability to cross the barrier in order to reach the brain. Several attempts have been made to develop in vitro BBB models, but these studies mainly used rat, bovine, and porcine cells rather than human primary cells. Genetically modified cell lines have also been used, but they do not appear to maintain physiological properties of the BBB. Here, we describe a detailed protocol for co-culturing and maintaining human brain primary endothelial cells, pericytes, and astrocytes under flow to create an in vitro human BBB model, which can be used for toxicity testing and for studying cross-interaction among different cell types involved in the BBB formation.

Hollow Fiber and Nanofiber Membranes in Bioartificial Liver and Neuronal Tissue Engineering

To date, the creation of biomimetic devices for the regeneration and repair of injured or diseased tissues and organs remains a crucial challenge in tissue engineering. Membrane technology offers advanced approaches to realize multifunctional tools with permissive environments well-controlled at molecular level for the development of functional tissues and organs. Membranes in fiber configuration with precisely controlled, tunable topography, and physical, biochemical, and mechanical cues, can direct and control the function of different kinds of cells toward the recovery from disorders and injuries. At the same time, fiber tools also provide the potential to model diseases in vitro for investigating specific biological phenomena as well as for drug testing. The purpose of this review is to present an overview of the literature concerning the development of hollow fibers and electrospun fiber membranes used in bioartificial organs, tissue engineered constructs, and in vitro bioreactors. With the aim to highlight the main biomedical applications of fiber-based systems, the first part reviews the fibers for bioartificial liver and liver tissue engineering with special attention to their multifunctional role in the long-term maintenance of specific liver functions and in driving hepatocyte differentiation. The second part reports the fiber-based systems used for neuronal tissue applications including advanced approaches for the creation of novel nerve conduits and in vitro models of brain tissue. Besides presenting recent advances and achievements, this work also delineates existing limitations and highlights emerging possibilities and future prospects in this field.

Hollow Fiber Membranes of PCL and PCL/Graphene as Scaffolds with Potential to Develop In Vitro Blood-Brain Barrier Models

There is a huge interest in developing novel hollow fiber (HF) membranes able to modulate neural differentiation to produce in vitro blood-brain barrier (BBB) models for biomedical and pharmaceutical research, due to the low cell-inductive properties of the polymer HFs used in current BBB models. In this work, poly(ε-caprolactone) (PCL) and composite PCL/graphene (PCL/G) HF membranes were prepared by phase inversion and were characterized in terms of mechanical, electrical, morphological, chemical, and mass transport properties. The presence of graphene in PCL/G membranes enlarged the pore size and the water flux and presented significantly higher electrical conductivity than PCL HFs. A biocompatibility assay showed that PCL/G HFs significantly increased C6 cells adhesion and differentiation towards astrocytes, which may be attributed to their higher electrical conductivity in comparison to PCL HFs. On the other hand, PCL/G membranes produced a cytotoxic effect on the endothelial cell line HUVEC presumably related with a higher production of intracellular reactive oxygen species induced by the nanomaterial in this particular cell line. These results prove the potential of PCL HF membranes to grow endothelial cells and PCL/G HF membranes to differentiate astrocytes, the two characteristic cell types that could develop in vitro BBB models in future 3D co-culture systems.

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.

Hollow fiber bioreactor technology for tissue engineering applications

Hollow fiber bioreactors are the focus of scientific research aiming to mimic physiological vascular networks and engineer organs and tissues in vitro. The reason for this lies in the interesting features of this bioreactor type, including excellent mass transport properties. Indeed, hollow fiber bioreactors allow limitations to be overcome in nutrient transport by diffusion, which is often an obstacle to engineer sizable constructs in vitro. This work reviews the existing literature relevant to hollow fiber bioreactors in organ and tissue engineering applications. To this purpose, we first classify the hollow fiber bioreactors into 2 categories: cylindrical and rectangular. For each category, we summarize their main applications both at the tissue and at the organ level, focusing on experimental models and computational studies as predictive tools for designing innovative, dynamic culture systems. Finally, we discuss future perspectives on hollow fiber bioreactors as in vitro models for tissue and organ engineering applications.

From Single Cells to Engineered and Explanted Tissues: New Perspectives in Bacterial Infection Biology

Cell culture techniques are essential for studying host-pathogen interactions. In addition to the broad range of single cell type-based two-dimensional cell culture models, an enormous amount of coculture systems, combining two or more different cell types, has been developed. These systems enable microscopic visualization and molecular analyses of bacterial adherence and internalization mechanisms and also provide a suitable setup for various biochemical, immunological, and pharmacological applications. The implementation of natural or synthetical scaffolds elevated the model complexity to the level of three-dimensional cell culture. Additionally, several transwell-based cell culture techniques are applied to study bacterial interaction with physiological tissue barriers. For keeping highly differentiated phenotype of eukaryotic cells in ex vivo culture conditions, different kinds of microgravity-simulating rotary-wall vessel systems are employed. Furthermore, the implementation of microfluidic pumps enables constant nutrient and gas exchange during cell cultivation and allows the investigation of long-term infection processes. The highest level of cell culture complexity is reached by engineered and explanted tissues which currently pave the way for a more comprehensive view on microbial pathogenicity mechanisms.

Development of a physiologically-based pharmacokinetic model of the rat central nervous system

Central nervous system (CNS) drug disposition is dictated by a drug's physicochemical properties and its ability to permeate physiological barriers. The blood-brain barrier (BBB), blood-cerebrospinal fluid barrier and centrally located drug transporter proteins influence drug disposition within the central nervous system. Attainment of adequate brain-to-plasma and cerebrospinal fluid-to-plasma partitioning is important in determining the efficacy of centrally acting therapeutics. We have developed a physiologically-based pharmacokinetic model of the rat CNS which incorporates brain interstitial fluid (ISF), choroidal epithelial and total cerebrospinal fluid (CSF) compartments and accurately predicts CNS pharmacokinetics. The model yielded reasonable predictions of unbound brain-to-plasma partition ratio (Kpuu,brain) and CSF:plasma ratio (CSF:Plasmau) using a series of in vitro permeability and unbound fraction parameters. When using in vitro permeability data obtained from L-mdr1a cells to estimate rat in vivo permeability, the model successfully predicted, to within 4-fold, Kpuu,brain and CSF:Plasmau for 81.5% of compounds simulated. The model presented allows for simultaneous simulation and analysis of both brain biophase and CSF to accurately predict CNS pharmacokinetics from preclinical drug parameters routinely available during discovery and development pathways.

Inflammatory cell trafficking across the blood-brain barrier: chemokine regulation and in vitro models

The blood-brain barrier (BBB) is the brain-specific capillary barrier that is critical for preventing toxic substances from entering the central nervous system (CNS). In contrast to vessels of peripheral organs, the BBB limits the exchange of inflammatory cells and mediators under physiological and pathological conditions. Clarifying these limitations and the role of chemokines in regulating the BBB would provide new insights into neuroprotective strategies in neuroinflammatory diseases. Because there is a paucity of in vitro BBB models, however, some mechanistic aspects of transmigration across the BBB still remain largely unknown. In this review, we summarize current knowledge of BBB cellular components, the multistep process of inflammatory cells crossing the BBB, functions of CNS-derived chemokines, and in vitro BBB models for transmigration, with a particular focus on new and recent findings.

Are you in or out? Leukocyte, ion, and neurotransmitter permeability across the epileptic blood-brain barrier

The credo that epileptic seizures can be initiated only by "epileptic" neurons has been recently challenged. The recognition of key astrocytic-neuronal communication, and the close interaction and crosstalk between astrocytes and brain endothelial cells, has shifted attention to the blood-brain barrier (BBB) and the "neurovascular unit." Therefore, the pursuit of mechanisms of seizure generation and epileptogenesis now includes investigations of cerebral blood flow and permeability of cerebral microvessels. For example, leukocyte adhesion molecules at the BBB have been proposed to play a role as an initiating factor for pilocarpine-induced status epilepticus, and a viral infection model with a strong BBB etiology has been used to study epileptogenesis. Finally, the fact that in nonepileptic subjects seizures can be triggered by BBB disruption, together with the antiseizure effects obtained by administration of potent antiinflammatory "BBB repair" drugs, has increased the interest in neuroinflammation; both circulating leukocytes and resident microglia have been studied in this context. The dual scope of this review is the following: (1) outline the proposed role of BBB damage and immune cell activation in seizure disorders; and (2) explain how increased cerebrovascular permeability causes neuronal misfiring. The temporal sequence linking seizures to peripheral inflammation and BBB dysfunction remains to be clarified. For example, it is still debated whether seizures cause systemic inflammation or vice versa. The topographic localization of fundamental triggers of epileptic seizures also remains controversial: Are immunologic mechanisms required for seizure generation brain-specific or is systemic activation of immunity sufficient to alter neuronal excitability? Finally, the causative role of "BBB leakage" remains a largely unresolved issue.

An optimized in vitro blood-brain barrier model reveals bidirectional transmigration of African trypanosome strains

The transmigration of African trypanosomes across the human blood-brain barrier (BBB) is the critical step during the course of human African trypanosomiasis. The parasites Trypanosoma brucei gambiense and T. b. rhodesiense are transmitted to humans during the bite of tsetse flies. Trypanosomes multiply within the bloodstream and finally invade the central nervous system (CNS), which leads to the death of untreated patients. This project focused on the mechanisms of trypanosomal traversal across the BBB. In order to establish a suitable in vitro BBB model for parasite transmigration, different human cell lines were used, including ECV304, HBMEC and HUVEC, as well as C6 rat astrocytes. Validation of the BBB models with Escherichia coli HB101 and E. coli K1 revealed that a combination of ECV304 cells seeded on Matrigel as a semi-synthetic basement membrane and C6 astrocytes resulted in an optimal BBB model system. The BBB model showed selective permeability for the pathogenic E. coli K1 strain, and African trypanosomes were able to traverse the optimized ECV304-C6 BBB efficiently. Furthermore, coincubation indicated that paracellular macrophage transmigration does not facilitate trypanosomal BBB traversal. An inverse assembly of the BBB model demonstrated that trypanosomes were also able to transmigrate the optimized ECV304-C6 BBB backwards, indicating the relevance of the CNS as a possible reservoir of a relapsing parasitaemia.

A dynamic in vitro BBB model for the study of immune cell trafficking into the central nervous system

Although there is significant evidence correlating overreacting or perhaps misguided immune cells and the blood-brain barrier (BBB) with the pathogenesis of neuroinflammatory diseases, the mechanisms by which they enter the brain are largely unknown. For this purpose, we revised our humanized dynamic in vitro BBB model (DIV-BBBr) to incorporate modified hollow fibers that now feature transmural microholes (2 to 4 μm Ø) allowing for the transendothelial trafficking of immune cells. As with the original model, this new DIV-BBBr reproduces most of the physiological characteristics of the BBB in vivo. Measurements of transendothelial electrical resistance (TEER), sucrose permeability, and BBB integrity during reversible osmotic disruption with mannitol (1.6 mol/L) showed that the microholes do not hamper the formation of a tight functional barrier. The in vivo rank permeability order of sucrose, phenytoin, and diazepam was successfully reproduced in vitro. Flow cessation followed by reperfusion (Fc/Rp) in the presence of circulating monocytes caused a biphasic BBB opening paralleled by a significant increase of proinflammatory cytokines and activated matrix metalloproteinases. We also observed abluminal extravasation of monocytes but only when the BBB was breached. In conclusion, the DIV-BBBr represents the most realistic in vitro system to study the immune cell trafficking across the BBB.

Matrix metalloproteinase-7 facilitates immune access to the CNS in experimental autoimmune encephalomyelitis

BACKGROUND

Metalloproteinase inhibitors can protect mice against experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis (MS). Matrix metalloproteinase-9 (MMP-9) has been implicated, but it is not clear if other MMPs are also involved, including matrilysin/MMP-7 - an enzyme capable of cleaving proteins that are essential for blood brain barrier integrity and immune suppression.

RESULTS

Here we report that MMP-7-deficient (mmp7-/-) mice on the C57Bl/6 background are resistant to EAE induced by myelin oligodendrocyte glycoprotein (MOG). Brain sections from MOG-primed mmp7-/-mice did not show signs of immune cell infiltration of the CNS, but MOG-primed wild-type mice showed extensive vascular cuffing and mononuclear cell infiltration 15 days after vaccination. At the peak of EAE wild-type mice had MMP-7 immuno-reactive cells in vascular cuffs that also expressed the macrophage markers Iba-1 and Gr-1, as well as tomato lectin. MOG-specific proliferation of splenocytes, lymphocytes, CD4+ and CD8+ cells were reduced in cells isolated from MOG-primed mmp7-/- mice, compared with MOG-primed wild-type mice. However, the adoptive transfer of splenocytes and lymphocytes from MOG-primed mmp7-/- mice induced EAE in naïve wild-type recipients, but not naïve mmp7-/- recipients. Finally, we found that recombinant MMP-7 increased permeability between endothelial cells in an in vitro blood-brain barrier model.

CONCLUSION

Our findings suggest that MMP-7 may facilitate immune cell access or re-stimulation in perivascular areas, which are critical events in EAE and multiple sclerosis, and provide a new therapeutic target to treat this disorder.

Immortalized human brain endothelial cells and flow-based vascular modeling: a marriage of convenience for rational neurovascular studies

In evaluating drugs that enter or are excluded from the brain, novel pharmaceutical strategies are needed. For this reason, we have developed a humanized Dynamic In vitro Blood-Brain Barrier model (hDIV-BBB) based on a novel human brain vascular endothelial cell line (HCMEC/D3), which closely mimics the BBB in vivo. In this system, HCMEC/D3 was grown in the lumen of hollow microporous fibers and exposed to a physiological pulsatile flow. Comparison with well-established humanized DIV-BBB models (based on human brain and non-brain vascular endothelial cells co-cultured with abluminal astrocytes) demonstrated that HCMEC/D3 cells cultured under flow conditions maintain in vitro physiological permeability barrier properties of the BBB in situ even in the absence of abluminal astrocytes. Measurements of glucose metabolism demonstrated that HCMEC/D3 cells retain an aerobic metabolic pathway. Permeability to sucrose and two relevant central nervous system drugs showed that the HCMEC/D3 cells grown under dynamic conditions closely mimic the physiological permeability properties of the BBB in situ (slope=0.93). Osmotic disruption of the BBB was also successfully achieved. Peak BBB opening in the DIV-BBB lasted from 20 to 30 mins and was completely reversible. Furthermore, the sequence of flow cessation/reperfusion in the presence of leukocytes led to BBB failure as demonstrated by a biphasic decrease in transendothelial electrical resistance. Additionally, BBB failure was paralleled by the intraluminal release of proinflammatory factors (interleukin-6 and interleukin-1beta) and matrix metalloproteinase-9 (MMP-9). Pretreatment with ibuprofen (0.125 mmol/L) prevented BBB failure by decreasing the inflammatory response after flow cessation/reperfusion.

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.

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.

Functional and molecular characterization of adenosine transport at the rat inner blood–retinal barrier

The purpose of the present study was to characterize the adenosine transport system(s) at the inner blood-retinal barrier (inner BRB). A conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2), used as an in vitro model of the inner BRB, expresses equilibrative nucleoside transporter 1 (ENT1), ENT2, concentrative nucleoside transporter 2 (CNT2), and CNT3 mRNAs. TR-iBRB2 cells exhibited an Na+-independent and concentration-dependent [3H]adenosine uptake with a Michaelis-Menten constant of 28.5 microM and a maximum uptake rate of 814 pmol/(min mg protein). [3H]Adenosine uptake by TR-iBRB2 cells was strongly inhibited by 2 mM adenosine, inosine, uridine, and thymidine. On the other hand, this process was not inhibited by 100 nM nitrobenzylmercaptopurine riboside and dipyridamole. These uptake studies suggest that ENT2 is involved in [3H]adenosine uptake by TR-iBRB2 cells. Quantitative real-time PCR revealed that the expression of ENT2 mRNA is 5.5-fold greater than that of ENT1 mRNA. An in vivo study suggested that [3H]adenosine is transported from the blood to the retina and significantly inhibited by adenosine and thymidine. The results of this study show that ENT2 most likely mediates adenosine transport at the inner BRB and is expected to play an important role in regulating the adenosine concentration in the retina.

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.

Permeability studies on in vitro blood-brain barrier models: physiology, pathology, and pharmacology

(1) The specifically regulated restrictive permeability barrier to cells and molecules is the most important feature of the blood-brain barrier (BBB). The aim of this review was to summarize permeability data obtained on in vitro BBB models by measurement of transendothelial electrical resistance and by calculation of permeability coefficients for paracellular or transendothelial tracers. (2) Results from primary cultures of cerebral microvascular endothelial cells or immortalized cell lines from bovine, human, porcine, and rodent origin are presented. Effects of coculture with astroglia, neurons, mesenchymal cells, blood cells, and conditioned media, as well as physiological influence of serum components, hormones, growth factors, lipids, and lipoproteins on the barrier function are discussed. (3) BBB permeability results gained on in vitro models of pathological conditions including hypoxia and reoxygenation, neurodegenerative diseases, or bacterial and viral infections have been reviewed. Effects of cytokines, vasoactive mediators, and other pathogenic factors on barrier integrity are also detailed. (4) Pharmacological treatments modulating intracellular cyclic nucleotide or calcium levels, and activity of protein kinases, protein tyrosine phosphatases, phospholipases, cyclooxygenases, or lipoxygenases able to change BBB integrity are outlined. Barrier regulation by drugs involved in the metabolism of nitric oxide and reactive oxygen species, as well as influence of miscellaneous treatments are also listed and evaluated. (5) Though recent advances resulted in development of improved in vitro BBB model systems to investigate disease modeling, drug screening, and testing vectors targeting the brain, there is a need for checking validity of permeability models and cautious interpretation of data.

Pericyte abundance affects sucrose permeability in cultures of rat brain microvascular endothelial cells

The blood-brain barrier is a physical and metabolic barrier that restricts diffusion of blood-borne substances into brain. In vitro models of the blood-brain barrier are used to characterize this structure, examine mechanisms of damage and repair and measure permeability of test substances. The core component of in vitro models of the blood-brain barrier is brain microvascular endothelial cells. We cultured rat brain microvascular endothelial cells (RBMEC) from isolated rat cortex microvessels. After 2-14 days in vitro (DIV), immunohistochemistry of these cells showed strong labeling for zona occludens 1 (ZO-1), a tight junction protein expressed in endothelial cells. Pericytes were also present in these cultures, as determined by expression of alpha-actin. The present study was performed to test different cell isolation methods and to compare the resulting cell cultures for abundance of pericytes and for blood-brain barrier function, as assessed by 14C-sucrose flux. Two purification strategies were used. First, microvessels were preabsorbed onto uncoated plastic for 4 h, then unattached microvessels were transferred to coated culture ware. Second, microvessels were incubated with an antibody to platelet-endothelial cell adhesion molecule 1 (PECAM-1; CD31) precoupled to magnetic beads, and a magnetic separation procedure was performed. Our results indicate that immunopurification, but not preadsorption, was an effective method to purify microvessels and reduce pericyte abundance in the resulting cultures. This purification significantly reduced 14C-sucrose fluxes across cell monolayers. These data indicate that pericytes can interfere with the development of blood-brain barrier properties in in vitro models that utilize primary cultures of RBMECs.

The Effect of Intravenous Infusion of Adenosine on Electrically Evoked Hyperalgesia in a Healthy Volunteer Model of Central Sensitization

Human pain models invoking central sensitization, one of the key mechanisms of chronic pain, may be useful for characterizing new analgesics. A new model of electrical hyperalgesia can detect the efficacy of several analgesic mechanisms. Because IV adenosine can alleviate neuropathic pain, we investigated its effect on experimental sensitization. This was a double-blinded, randomized, two-period crossover study in 20 healthy volunteers. Current pulses (0.5 ms; 1 Hz) were applied intracutaneously to achieve pain rating of approximately 5 on a 0-10 numeric rating scale. Pain, areas of pinprick hyperalgesia, and tactile allodynia were assessed during the 2.5-h stimulation period. Adenosine (50 microg. kg(-1). min(-1)) and placebo were infused IV over 60 min. Additional testing was performed 24 h after each treatment. Adenosine reduced the area of pinprick hyperalgesia during the infusion compared with placebo; there was no significant effect on tactile allodynia or pain rating. The effect on hyperalgesia developed over 15 min and was significant (P < or = 0.05) for the rest of the infusion period. There was no difference between treatments at 24 h. Thus, in accordance with reports on neuropathic pain, adenosine reduced central sensitization in the human model of electrical hyperalgesia. However, adenosine did not have the long-term effects seen in patients. The model can investigate mechanisms of drugs for the treatment of chronic pain.

Culture of murine brain microvascular endothelial cells that maintain expression and cytoskeletal association of tight junction-associated proteins

A readily obtainable in vitro paradigm of the blood-brain barrier (BBB) would offer considerable benefits. Toward this end, in this study, we describe a novel method for purifying murine brain microvascular endothelial cells (BMEC) for culture. The method uses limited collagenase-dispase digestion of enriched brain microvessels, followed by immunoisolation of digested, microvascular fragments by magnetic beads coated with antibody to platelet-endothelial cell adhesion molecule-1. When plated onto collagen IV-coated surfaces, these fragments elaborated confluent monolayers of BMEC that expressed, as judged by immunocytochemistry, the adherens junction-associated proteins, VE-cadherin and beta-catenin, as well as the tight junction (TJ)-associated proteins, claudin-5, occludin, and zonula occludin-1 (ZO-1), in concentrated fashion along intercellular borders. In contrast, cultures of an immortalized and transformed line of murine brain capillary-derived endothelial cells, bEND.3, displayed diffuse cytoplasmic localization of occludin and ZO-1. This difference in occludin and ZO-1 staining between the two endothelial cell types was also reflected in the extent of association of these proteins with the detergent-resistant cytoskeletal framework (CSK). Although both occludin and ZO-1 largely partitioned with the CSK fraction in BMEC, they were found predominantly in the soluble fraction of bEND.3 cells, and claudin-5 was found associated equally with both fractions in BMEC and bEND.3 cells. Moreover, detergent-extracted cultures of the BMEC retained pronounced immunostaining of occludin and ZO-1, but not claudin-5, along intercellular borders. Because both occludin and ZO-1 are thought to be functionally coupled to the detergent-resistant CSK and high expression of TJs is considered a seminal characteristic of the BBB, these results impart that this method of purifying murine BMEC provides a suitable platform to investigate BBB properties in vitro.