An in vitro blood-brain barrier model: cocultures between endothelial cells and organotypic brain slice cultures

Effect of Parenchymal Arachnoid on Brain Fluid Transport

Introduction

The pia-arachnoid is a critical component of cerebrospinal fluid removal. It covers and invaginates into the brain parenchyma, and physiologic failure results in hydrocephalus and cerebral edema. The purpose of this study was to characterize the role of arachnoid within brain parenchyma and determine if water flux and solute transport are affected by these intra-parenchymal cells.

Methods

An immortalized arachnoid rat cell line was used to seed 300-μm organotypic rat brain slices of 4-week-old rats. Fluid and tracer transport analyses were conducted following a 7-10 day intraparenchymal growth period. The development of an arachnoid brain slice model was characterized using diffusion chamber experiments to calculate permeability, diffusion coefficient, and flux.

Results

Labeled rat arachnoid cells readily penetrated organotypic cultures for up to 10 days. A significant reduction of dye and water flux across arachnoid-impregnated brain slices was observed after 3 hours in the diffusion chamber. Permeability decreased in whole brain slices containing arachnoid cells compared to slices without arachnoid cells. In comparison, a significant reduction of dextran across all slices occurred when molecular weights increased from 40 to 70 kDa.

Conclusion

Tracer and small molecule studies show that arachnoid cells' presence significantly impacts water's movement through brain parenchyma. Size differential experiments indicate that the permeability of solute changed substantially between 40 and 70 kDa, an essential marker of blood-CSF barrier definition. We have developed an arachnoid organotypic model that reveals their ability to alter permeability and transport.

Recent advances in cellular models for discovering prion disease therapeutics

INTRODUCTION

Prion diseases are a group of rare and lethal rapidly progressive neurodegenerative diseases arising due to conversion of the physiological cellular prion protein into its pathological counterparts, denoted as "prions". These agents are resistant to inactivation by standard decontamination procedures and can be transmitted between individuals, consequently driving the irreversible brain damage typical of the diseases.

AREAS COVERED

Since its infancy, prion research has mainly depended on animal models for untangling the pathogenesis of the disease as well as for the drug development studies. With the advent of prion-infected cell lines, relevant animal models have been complemented by a variety of cell-based models presenting a much faster, ethically acceptable alternative.

EXPERT OPINION

To date, there are still either no effective prophylactic regimens or therapies for human prion diseases. Therefore, there is an urgent need for more relevant cellular models that best approximate in vivo models. Each cellular model presented and discussed in detail in this review has its own benefits and limitations. Once embarking in a drug screening campaign for the identification of molecules that could interfere with prion conversion and replication, one should carefully consider the ideal cellular model.

Study of BBB Dysregulation in Neuropathogenicity Using Integrative Human Model of Blood-Brain Barrier

The blood-brain barrier (BBB) is a cellular and physical barrier with a crucial role in homeostasis of the brain extracellular environment. It controls the imports of nutrients to the brain and exports toxins and pathogens. Dysregulation of the blood-brain barrier increases permeability and contributes to pathologies, including Alzheimer's disease, epilepsy, and ischemia. It remains unclear how a dysregulated BBB contributes to these different syndromes. Initial studies on the role of the BBB in neurological disorders and also techniques to permit the entry of therapeutic molecules were made in animals. This review examines progress in the use of human models of the BBB, more relevant to human neurological disorders. In recent years, the functionality and complexity of in vitro BBB models have increased. Initial efforts consisted of static transwell cultures of brain endothelial cells. Human cell models based on microfluidics or organoids derived from human-derived induced pluripotent stem cells have become more realistic and perform better. We consider the architecture of different model generations as well as the cell types used in their fabrication. Finally, we discuss optimal models to study neurodegenerative diseases, brain glioma, epilepsies, transmigration of peripheral immune cells, and brain entry of neurotrophic viruses and metastatic cancer cells.

The neurovascular unit in leukodystrophies: towards solving the puzzle

The neurovascular unit (NVU) is a highly organized multicellular system localized in the brain, formed by neuronal, glial (astrocytes, oligodendrocytes, and microglia) and vascular (endothelial cells and pericytes) cells. The blood-brain barrier, a complex and dynamic endothelial cell barrier in the brain microvasculature that separates the blood from the brain parenchyma, is a component of the NVU. In a variety of neurological disorders, including Alzheimer's disease, multiple sclerosis, and stroke, dysfunctions of the NVU occurs. There is, however, a lack of knowledge regarding the NVU function in leukodystrophies, which are rare monogenic disorders that primarily affect the white matter. Since leukodystrophies are rare diseases, human brain tissue availability is scarce and representative animal models that significantly recapitulate the disease are difficult to develop. The introduction of human induced pluripotent stem cells (hiPSC) now makes it possible to surpass these limitations while maintaining the ability to work in a biologically relevant human context and safeguarding the genetic background of the patient. This review aims to provide further insights into the NVU functioning in leukodystrophies, with a special focus on iPSC-derived models that can be used to dissect neurovascular pathophysiology in these diseases.

Neuroprotection Mediated by Human Blood Plasma in Mouse Hippocampal Slice Cultures and in Oxidatively Stressed Human Neurons

Neuroprotection from oxidative stress is critical during neuronal development and maintenance but also plays a major role in the pathogenesis and potential treatment of various neurological disorders and neurodegenerative diseases. Emerging evidence in the murine system suggests neuroprotective effects of blood plasma on the aged or diseased brain. However, little is known about plasma-mediated effects on human neurons. In the present study, we demonstrate the neuroprotective effect mediated by human plasma and the most abundant plasma–protein human serum albumin against oxidative stress in glutamatergic neurons differentiated from human neural crest-derived inferior turbinate stem cells. We observed a strong neuroprotective effect of human plasma and human serum albumin against oxidative stress-induced neuronal death on the single cell level, similar to the one mediated by tumor necrosis factor alpha. Moreover, we detected neuroprotection of plasma and human serum albumin against kainic acid-induced excitatory stress in ex vivo cultured mouse hippocampal tissue slices. The present study provides deeper insights into plasma-mediated neuroprotection ultimately resulting in the development of novel therapies for a variety of neurological and, in particular, neurodegenerative diseases.

Organotypic Brain Slices of ADULT Transgenic Mice: A Tool to Study Alzheimer's Disease

Transgenic mice have been extensively used to study the Alzheimer pathology. In order to reduce, refine and replace (3Rs) the number of animals, ex vivo cultures are used and optimized. Organotypic brain slices are the most potent ex vivo slice culture models, keeping the 3-dimensional structure of the brain and being closest to the in vivo situation. Organotypic brain slice cultures have been used for many decades but were mainly prepared from postnatal (day 8-10) old rats or mice. More recent work (including our lab) now aims to culture organotypic brain slices from adult mice including transgenic mice. Especially in Alzheimer´s disease research, brain slices from adult transgenic mice will be useful to study beta-amyloid plaques, tau pathology and glial activation. This review will summarize the studies using organotypic brain slice cultures from adult mice to mimic Alzheimer's disease and will highlight advantages and also pitfalls using this technique.

Organotypic Brain Slice Cultures: An Efficient and Reliable Method for Neurotoxicological Screening and Mechanistic Studies

This paper reviews the current state of the use of organotypic brain slice cultures for neurotoxicological and neuropharmacological screening and mechanistic studies, as exemplified by excitotoxin application. At present, no in vitro systems have been approved by the regulatory authorities for neurotoxicity testing. For the evaluation of the slice culture method, organotypic hippocampal slice cultures were exposed to toxic doses of the excitotoxins, glutamate, N-methyl-D-aspartate (NMDA), kainic acid and 2-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), and the glial toxin, DL-alpha-aminoadipic acid (DLAAA). Neuronal cell death was quantified by propidium iodide (PI) uptake, and visualised by Fluoro-Jade (FJ) staining. General cell death was monitored by lactate dehydrogenase (LDH) release into the culture medium. EC50 values for the different compounds, based on PI uptake after exposure for 48 hours in entire cultures, were: glutamate, 3.5 mM; DL-AAA, 2.3 mM; kainic acid, 13 microM; NMDA, 11 microM; and AMPA, 3.7 microM. In the slice cultures, the hippocampal subfields displayed the same differences in vulnerability as those observed in vivo. When subfield analysis was performed on the cultures, the CA1 subfield was most susceptible to glutamate, NMDA and AMPA, while CA3 was most susceptible to kainic acid. The amount of LDH release for DL-AAA was about four times that of L-glutamate, in accordance with the additional toxic effect on glial cells, which was also found by confocal microscopy to stain for FJ. In conclusion, it was found that organotypic brain slice culture, combined with standardised protocols and quantifiable markers, such as PI and FJ staining, is a relevant and feasible in vitro system for neurotoxicity testing. Considering the amount and quality of the available published data, it is recommended that the brain slice culture method could be subjected to pre-validation and formal validation for inclusion in a tiered in vitro neurotoxicity testing scheme to supplement and replace conventional animal tests.

The cerebral endothelial cell as a key regulator of inflammatory processes in sterile inflammation

Cerebral endothelial cells accomplish numerous tasks connected to the maintenance of homeostasis of the central nervous system. They create a barrier between the central nervous system and peripheral blood and regulate mechanotransduction, vascular permeability, rheology, thrombogenesis, and leukocyte adhesion. In pathophysiological conditions (e.g., stroke or ischemia-reperfusion injury) the endothelial functions are impaired, leading to increased vascular permeability, vascular inflammation, leukocyte-endothelium interactions, and transendothelial migration, driving CNS inflammation and neuronal destruction. This review describes the current knowledge on the regulatory roles of endothelial cells in neuroinflammatory processes.

Isolation, cultivation, and characterization of primary bovine cochlear pericytes: A new in vitro model of stria vascularis

The study of strial pericytes has gained great interest as they are pivotal for the physiology of stria vascularis. To provide an easily accessible in vitro model, here we described a growth medium-based approach to obtain and cultivate primary bovine cochlear pericytes (BCP) from the stria vascularis of explanted bovine cochleae. We obtained high-quality pericytes in 8-10 days with a > 90% purity after the second passage. Immunocytochemical analysis showed a homogeneous population of cells expressing typical pericyte markers, such as neural/glial antigen 2 (NG2), platelet-derived growth factor receptorβ (PDGFRβ), α-smooth muscle actin (α-SMA), and negative for the endothelial marker von Willebrand factor. When challenged with tumor necrosis factor or lipopolysaccharide, BCP changed their shape, similarly to human retinal pericytes (HRPC). The sensitivity of BCP to ototoxic drugs was evaluated by challenging with cisplatin or gentamicin for 48 hr. Compared to human retinal endothelial cells and HRPC, cell viability of BCP was significantly lower ( p < 0.05) after the treatment with gentamicin or cisplatin. These data indicate that our protocol provides a simple and reliable method to obtain highly pure strial BCP. Furthermore, BCP are suitable to assess the safety profile of molecules which supposedly exert ototoxic activity, and may represent a valid alternative to in vivo tests.

Brain vascular heterogeneity: implications for disease pathogenesis and design of in vitro blood-brain barrier models

The vertebrate blood–brain barrier (BBB) is composed of cerebral microvascular endothelial cells (CEC). The BBB acts as a semi-permeable cellular interface that tightly regulates bidirectional molecular transport between blood and the brain parenchyma in order to maintain cerebral homeostasis. The CEC phenotype is regulated by a variety of factors, including cells in its immediate environment and within functional neurovascular units. The cellular composition of the brain parenchyma surrounding the CEC varies between different brain regions; this difference is clearly visible in grey versus white matter. In this review, we discuss evidence for the existence of brain vascular heterogeneity, focusing on differences between the vessels of the grey and white matter. The region-specific differences in the vasculature of the brain are reflective of specific functions of those particular brain areas. This BBB-endothelial heterogeneity may have implications for the course of pathogenesis of cerebrovascular diseases and neurological disorders involving vascular activation and dysfunction. This heterogeneity should be taken into account when developing BBB-neuro-disease models representative of specific brain areas.

Organotypic brain explant culture as a drug evaluation system for malignant brain tumors

Therapeutic options for malignant brain tumors are limited, with new drugs being continuously evaluated. Organotypic brain slice culture has been adopted for neuroscience studies as a system that preserves brain architecture, cellular function, and the vascular network. However, the suitability of brain explants for anticancer drug evaluation has been unclear. We here adopted a mouse model of malignant glioma based on expression of H-RasV12 in Ink4a/Arf-/- neural stem/progenitor cells to establish tumor-bearing brain explants from adult mice. We treated the slices with cisplatin, temozolomide, paclitaxel, or tranilast and investigated the minimal assays required to assess drug effects. Serial fluorescence-based tumor imaging was sufficient for evaluation of cisplatin, a drug with a pronounced cytotoxic action, whereas immunostaining of cleaved caspase 3 (a marker of apoptosis) and of Ki67 (a marker of cell proliferation) was necessary for the assessment of temozolomide action and immunostaining for phosphorylated histone H3 (a marker of mitosis) allowed visualization of paclitaxel-specific effects. Staining for cleaved caspase 3 was also informative in the assessment of drug toxicity for normal brain tissue. Incubation of explants with fluorescently labeled antibodies to CD31 allowed real-time imaging of the microvascular network and complemented time-lapse imaging of tumor cell invasion into surrounding tissue. Our results suggest that a combination of fluorescence imaging and immunohistological staining allows a unified assessment of the effects of various classes of drug on the survival, proliferation, and invasion of glioma cells, and that organotypic brain slice culture is therefore a useful tool for evaluation of antiglioma drugs.

Culture media-based selection of endothelial cells, pericytes, and perivascular-resident macrophage-like melanocytes from the young mouse vestibular system

The vestibular blood-labyrinth barrier (BLB) is comprised of perivascular-resident macrophage-like melanocytes (PVM/Ms) and pericytes (PCs), in addition to endothelial cells (ECs) and basement membrane (BM), and bears strong resemblance to the cochlear BLB in the stria vascularis. Over the past few decades, in vitro cell-based models have been widely used in blood-brain barrier (BBB) and blood-retina barrier (BRB) research, and have proved to be powerful tools for studying cell-cell interactions in their respective organs. Study of both the vestibular and strial BLB has been limited by the unavailability of primary culture cells from these barriers. To better understand how barrier component cells interact in the vestibular system to control BLB function, we developed a novel culture medium-based method for obtaining EC, PC, and PVM/M primary cells from tiny explants of the semicircular canal, sacculus, utriculus, and ampullae tissue of young mouse ears at post-natal age 8-12 d. Each phenotype is grown in a specific culture medium which selectively supports the phenotype in a mixed population of vestibular cell types. The unwanted phenotypes do not survive passaging. The protocol does not require additional equipment or special enzyme treatment. The harvesting process takes less than 2 h. Primary cell types are generated within 7-10 d. The primary culture ECs, PCs, and PVM/M shave consistent phenotypes more than 90% pure after two passages (∼ 3 weeks). The highly purified primary cell lines can be used for studying cell-cell interactions, barrier permeability, and angiogenesis.

Pathophysiology of the cochlear intrastrial fluid-blood barrier (review)

The blood-labyrinth barrier (BLB) in the stria vascularis is a highly specialized capillary network that controls exchanges between blood and the intrastitial space in the cochlea. The barrier shields the inner ear from blood-born toxic substances and selectively passes ions, fluids, and nutrients to the cochlea, playing an essential role in the maintenance of cochlear homeostasis. Anatomically, the BLB is comprised of endothelial cells (ECs) in the strial microvasculature, elaborated tight and adherens junctions, pericytes (PCs), basement membrane (BM), and perivascular resident macrophage-like melanocytes (PVM/Ms), which together form a complex "cochlear-vascular unit" in the stria vascularis. Physical interactions between the ECs, PCs, and PVM/Ms, as well as signaling between the cells, is critical for controlling vascular permeability and providing a proper environment for hearing function. Breakdown of normal interactions between components of the BLB is seen in a wide range of pathological conditions, including genetic defects and conditions engendered by inflammation, loud sound trauma, and ageing. In this review, we will discuss prevailing views of the structure and function of the strial cochlear-vascular unit (also referred to as the "intrastrial fluid-blood barrier"). We will also discuss the disrupted homeostasis seen in a variety of hearing disorders. Therapeutic targeting of the strial barrier may offer opportunities for improvement of hearing health and amelioration of auditory disorders. This article is part of a Special Issue entitled <Annual Reviews 2016>.

Three-Dimensional Blood-Brain Barrier Model for in vitro Studies of Neurovascular Pathology

Blood–brain barrier (BBB) pathology leads to neurovascular disorders and is an important target for therapies. However, the study of BBB pathology is difficult in the absence of models that are simple and relevant. In vivo animal models are highly relevant, however they are hampered by complex, multi-cellular interactions that are difficult to decouple. In vitro models of BBB are simpler, however they have limited functionality and relevance to disease processes. To address these limitations, we developed a 3-dimensional (3D) model of BBB on a microfluidic platform. We verified the tightness of the BBB by showing its ability to reduce the leakage of dyes and to block the transmigration of immune cells towards chemoattractants. Moreover, we verified the localization at endothelial cell boundaries of ZO-1 and VE-Cadherin, two components of tight and adherens junctions. To validate the functionality of the BBB model, we probed its disruption by neuro-inflammation mediators and ischemic conditions and measured the protective function of antioxidant and ROCK-inhibitor treatments. Overall, our 3D BBB model provides a robust platform, adequate for detailed functional studies of BBB and for the screening of BBB-targeting drugs in neurological diseases.

Organotypic brain slice cultures as a model to study angiogenesis of brain vessels

Brain vessels are the most important structures in the brain to deliver energy and substrates to neurons. Brain vessels are composed of a complex interaction between endothelial cells, pericytes, and astrocytes, controlling the entry of substrates into the brain. Damage of brain vessels and vascular impairment are general pathologies observed in different neurodegenerative disorders including e.g., Alzheimer's disease. In order to study remodeling of brain vessels, simple 3-dimensional in vitro systems need to be developed. Organotypic brain slices of mice provide a potent tool to explore angiogenic effects of brain vessels in a complex 3-dimensional structure. Here we show that organotypic brain slices can be cultured from 110 μm thick sections of postnatal and adult mice brains. The vessels are immunohistochemically stained for laminin and collagen IV. Co-stainings are an appropriate method to visualize interaction of brain endothelial cells with pericytes and astrocytes in these vessels. Different exogenous stimuli such as fibroblast growth factor-2 or vascular endothelial growth factor induce angiogenesis or re-growth, respectively. Hyperthermia or acidosis reduces the vessel density in organotypic slices. In conclusion, organotypic brain slices exhibit a strong vascular network which can be used to study remodeling and angiogenesis of brain vessels in a 3-dimensional in vitro system.

Organotypic brain slice cultures: A review

In vitro cell cultures are an important tool for obtaining insights into cellular processes in an isolated system and a supplement to in vivo animal experiments. While primary dissociated cultures permit a single homogeneous cell population to be studied, there is a clear need to explore the function of brain cells in a three-dimensional system where the main architecture of the cells is preserved. Thus, organotypic brain slice cultures have proven to be very useful in investigating cellular and molecular processes of the brain in vitro. This review summarizes (1) the historical development of organotypic brain slices focusing on the membrane technology, (2) methodological aspects regarding culturing procedures, age of donors or media, (3) whether the cholinergic neurons serve as a model of neurodegeneration in Alzheimer’s disease, (4) or the nigrostriatal dopaminergic neurons as a model of Parkinson’s disease and (5) how the vascular network can be studied, especially with regard to a synthetic blood–brain barrier. This review will also highlight some limits of the model and give an outlook on future applications.

Dopamine increases CD14+CD16+ monocyte migration and adhesion in the context of substance abuse and HIV neuropathogenesis

Drug abuse is a major comorbidity of HIV infection and cognitive disorders are often more severe in the drug abusing HIV infected population. CD14⁺CD16⁺ monocytes, a mature subpopulation of peripheral blood monocytes, are key mediators of HIV neuropathogenesis. Infected CD14⁺CD16⁺ monocyte transmigration across the blood brain barrier mediates HIV entry into the brain and establishes a viral reservoir within the CNS. Despite successful antiretroviral therapy, continued influx of CD14⁺CD16⁺ monocytes, both infected and uninfected, contributes to chronic neuroinflammation and the development of HIV associated neurocognitive disorders (HAND). Drug abuse increases extracellular dopamine in the CNS. Once in the brain, CD14⁺CD16⁺ monocytes can be exposed to extracellular dopamine due to drug abuse. The direct effects of dopamine on CD14⁺CD16⁺ monocytes and their contribution to HIV neuropathogenesis are not known. In this study, we showed that CD14⁺CD16⁺ monocytes express mRNA for all five dopamine receptors by qRT-PCR and D1R, D5R and D4R surface protein by flow cytometry. Dopamine and the D1-like dopamine receptor agonist, SKF38393, increased CD14⁺CD16⁺ monocyte migration that was characterized as chemokinesis. To determine whether dopamine affected cell motility and adhesion, live cell imaging was used to monitor the accumulation of CD14⁺CD16⁺ monocytes on the surface of a tissue culture dish. Dopamine increased the number and the rate at which CD14⁺CD16⁺ monocytes in suspension settled to the dish surface. In a spreading assay, dopamine increased the area of CD14⁺CD16⁺ monocytes during the early stages of cell adhesion. In addition, adhesion assays showed that the overall total number of adherent CD14⁺CD16⁺ monocytes increased in the presence of dopamine. These data suggest that elevated extracellular dopamine in the CNS of HIV infected drug abusers contributes to HIV neuropathogenesis by increasing the accumulation of CD14⁺CD16⁺ monocytes in dopamine rich brain regions.

A neurovascular blood-brain barrier in vitro model

The cerebral microvasculature possesses certain cellular features that constitute the blood-brain barrier (BBB) (Abbott et al., Neurobiol Dis 37:13-25, 2010). This dynamic barrier separates the brain parenchyma from peripheral blood flow and is of tremendous clinical importance: for example, BBB breakdown as in stroke is associated with the development of brain edema (Rosenberg and Yang, Neurosurg Focus 22:E4, 2007), inflammation (Kuhlmann et al., Neurosci Lett 449:168-172, 2009; Coisne and Engelhardt, Antioxid Redox Signal 15:1285-1303, 2011), and increased mortality. In vivo, the BBB consists of brain endothelial cells (BEC) that are embedded within a precisely regulated environment containing astrocytes, pericytes, smooth muscle cells, and glial cells. These cells experience modulation by various pathways of intercellular communication and by pathophysiological processes, e.g., through neurovascular coupling (Attwell et al., Nature 468:232-243, 2010), cortical spreading depression (Gursoy-Ozdemir et al., J Clin Invest 113:1447-1455, 2004), or formation of oxidative stress (Yemisci et al., Nat Med 15:1031-1037, 2009). Hence, this interdependent assembly of cells is referred to as the neurovascular unit (NVU) (Zlokovic, Nat Med 16:1370-1371, 2010; Zlokovic, Neuron 57:178-201, 2008). Experimental approaches to investigate the BBB in vitro are highly desirable to study the cerebral endothelium in health and disease. However, due to the complex interactions taking place within the NVU in vivo, it is difficult to mimic this interplay in vitro.Here, we describe a murine blood-brain barrier coculture model consisting of cortical organotypic slice cultures and brain endothelial cells that includes most of the cellular components of the NVU including neurons, astrocytes, and brain endothelial cells. This model allows the experimental analysis of several crucial BBB parameters such as transendothelial electrical resistance or tight junction protein localization by immunohistochemistry and live cell imaging of reactive oxygen species.

Isolation and culture of endothelial cells, pericytes and perivascular resident macrophage-like melanocytes from the young mouse ear

This protocol describes a growth medium-based approach for obtaining cochlear endothelial cells (ECs), pericytes (PCs) and perivascular resident macrophage-like melanocytes (PVM/Ms) from the stria vascularis of mice aged between P10 and P15 (P, postnatal day). The procedure does not involve mechanical or enzymatic digestion of the sample tissue. Explants of stria vascularis, 'mini-chips', are selectively cultured in growth medium, and primary cell lines are obtained in 7-10 d. The method is simple and reliable, and it provides high-quality ECs, PVM/Ms and PCs with a purity >90% after two passages. This protocol is suitable for producing primary culture cells from organs and tissues of small volume and high anatomical complexity, such as the inner ear capillaries. The highly purified primary cell lines enable cell culture-based in vitro modeling of cell-cell interactions, barrier control function and drug action.

A simple method for measuring organotypic tissue slice culture thickness

This paper presents a simple method to measure tissue slice thicknesses using an ohmmeter. The circuit described here is composed of a metal probe, an ohmmeter, a counter electrode, culture medium or physiological buffer, and tissue slice. The probe and the electrode are on opposite interfaces of an organotypic hippocampal slice culture. The circuit closes when the metal probe makes contact with the surface of the tissue slice. The probe position is recorded and compared to its position when it makes contact with the insert membrane on which the tissue grows, thus yielding a thickness measurement. The method does not reduce the viability of slice cultures. Thicknesses of the slice cultures were measured under a number of culturing protocols. An initial drop in thickness occurred between 0 and 4 days in culture. Thicknesses are rather constant thereafter. The type of culture medium and the initial thickness of the tissue explant influence the thickness. Slice thicknesses were compared to a known technique by using optical measurements of slice cross-sections to obtain thicknesses. In contrast to this known technique, the proposed method does not sacrifice the slice culture for measurement purposes. The proposed measurement technique described is straightforward and rapid, about 1 min per culture.

Slice cultures as a model to study neurovascular coupling and blood brain barrier in vitro

Proper neuronal functioning depends on a strictly regulated interstitial environment and tight coupling of neuronal and metabolic activity involving adequate vascular responses. These functions take place at the blood brain barrier (BBB) composed of endothelial cells, basal lamina covered with pericytes, and the endfeet of perivascular astrocytes. In conventional in vitro models of the BBB, some of these components are missing. Here we describe a new model system for studying BBB and neurovascular coupling by using confocal microscopy and fluorescence staining protocols in organotypic hippocampal slice cultures. An elaborated network of vessels is retained in culture in spite of the absence of blood flow. Application of calcein-AM either from the interstitial or from the luminal side resulted in different staining patterns indicating the maintenance of a barrier. By contrast, the ethidium derivative MitoSox penetrated perivascular basal lamina and revealed free radical formation in contractile cells embracing the vessels, likely pericytes.

Nanomedicine Faces Barriers

Targeted nanoparticles have the potential to improve drug delivery efficiencies by more than two orders of magnitude, from the ~ 0.1% which is common today. Most pharmacologically agents on the market today are small drug molecules, which diffuse across the body’s blood-tissue barriers and distribute not only into the lesion, but into almost all organs. Drug actions in the non-lesion organs are an inescapable part of the drug delivery principle, causing “side-effects” which limit the maximally tolerable doses and result in inadequate therapy of many lesions. Nanoparticles only cross barriers by design, so side-effects are not built into their mode of operation. Delivery rates of almost 90% have been reported. This review examines the significance of these statements and checks how far they need qualification. What type of targeting is required? Is a single targeting sufficient? What new types of clinical challenge, such as immunogenicity, might attend the use of targeted nanoparticles?

Studying the neurovascular unit: an improved blood-brain barrier model

The blood-brain barrier (BBB) closely interacts with the neuronal parenchyma in vivo. To replicate this interdependence in vitro, we established a murine coculture model composed of brain endothelial cell (BEC) monolayers with cortical organotypic slice cultures. The morphology of cell types, expression of tight junctions, formation of reactive oxygen species, caspase-3 activity in BECs, and alterations of electrical resistance under physiologic and pathophysiological conditions were investigated. This new BBB model allows the application of techniques such as laser scanning confocal microscopy, immunohistochemistry, fluorescent live cell imaging, and electrical cell substrate impedance sensing in real time for studying the dynamics of BBB function under defined conditions.

Integration and application of vitrified collagen in multilayered microfluidic devices for corneal microtissue culture

This paper describes the fabrication and application of microfluidic devices containing collagen vitrigel (CV) used as both a functional and sacrificial cell growth substrate for the development of corneal microtissue patches. Within the device, vacuum fixation of the CV in a dehydrated state enables quick integration with standard multilayer soft lithographic techniques, while on-chip rehydration results in a gel-like collagen substrate for microfluidic cell culture. Fluidic connectivity to both the apical and basal side of the CV permits bilayered culture of epithelium and supporting stromal cell layers. In addition, microfluidic introduction of a collagenase etching media enables sacrificial degradation of the supporting CV membrane for development of barrier tissue constructs containing minimal synthetic substrate. The utility of this platform was evaluated by miniaturizing the standard transepithelial permeability (TEP) assay in order to measure the integrity of an array of corneal tissue micropatches.

TAT-mediated intracellular protein delivery to primary brain cells is dependent on glycosaminoglycan expression

Although some studies have shown that the cell penetrating peptide (CPP) TAT can enter a variety of cell lines with high efficiency, others have observed little or no transduction in vivo or in vitro under conditions mimicking the in vivo environment. The mechanisms underlying TAT-mediated transduction have been investigated in cell lines, but not in primary brain cells. In this study we demonstrate that transduction of a green fluorescent protein (GFP)-TAT fusion protein is dependent on glycosaminoglycan (GAG) expression in both the PC12 cell line and primary astrocytes. GFP-TAT transduced PC12 cells and did so with even higher efficiency following NGF differentiation. In cultures of primary brain cells, TAT significantly enhanced GFP delivery into astrocytes grown under different conditions: (1) monocultures grown in serum-containing medium; (2) monocultures grown in serum-free medium; (3) cocultures with neurons in serum-free medium. The efficiency of GFP-TAT transduction was significantly higher in the monocultures than in the cocultures. The GFP-TAT construct did not significantly enter neurons. Experimental modulation of GAG content correlated with alterations in TAT transduction in PC12 cells and astrocyte monocultures grown in the presence of serum. In addition, this correlation was predictive of TAT-mediated transduction in astrocyte monocultures grown in serum free medium and in coculture. We conclude that culture conditions affect cellular GAG expression, which in turn dictates TAT-mediated transduction efficiency, extending previous results from cell lines to primary cells. These results highlight the cell-type and phenotype-dependence of TAT-mediated transduction, and underscore the necessity of controlling the phenotype of the target cell in future protein engineering efforts aimed at creating more efficacious CPPs.

Organotypic entorhino-hippocampal slice cultures--a tool to study the molecular and cellular regulation of axonal regeneration and collateral sprouting in vitro

Organotypic slice cultures of the brain are widely used as a tool to study fundamental questions in neuroscience. In this chapter, we focus on a protocol based on organotypic slice cultures of mouse entorhinal cortex and hippocampus that can be employed to study axonal regeneration and collateral sprouting in the central nervous system in vitro. Using pharmacological as well as genetic approaches, axonal regeneration and sprouting can be influenced, and some of the molecular and cellular mechanisms involved in these processes can be identified. The protocol describes in detail (1) the generation of organotypic entorhino-hippocampal slice cultures, (2) the conditions needed for the analysis of axonal regeneration and collateral sprouting, respectively, (3) the lesioning technique, (4) tracing techniques to visualize regenerating entorhinal axons, and (5) an immunohistochemical technique to visualize sprouting fibers.

Neuron-glial interactions in blood-brain barrier formation

The blood brain barrier (BBB) evolved to preserve the microenvironment of the highly excitable neuronal cells to allow for action potential generation and propagation. Intricate molecular interactions between two main cell types, the neurons and the glial cells, form the underlying basis of the critical functioning of the nervous system across species. In invertebrates, interactions between neurons and glial cells are central in establishing a functional BBB. However, in vertebrates, the BBB formation and function is coordinated by interactions between neurons, glial cells, and endothelial cells. Here we review the neuron-glial interaction-based blood barriers in invertebrates and vertebrates and provide an evolutionary perspective as to how a glial-barrier system in invertebrates evolved into an endothelial barrier system. We also summarize the clinical relevance of the BBB as this protective barrier becomes disadvantageous in the pharmacological treatment of various neurological disorders.

Early excitability changes in a novel acute model of transient focal ischemia and reperfusion in the in vitro isolated guinea pig brain

The study of the early events that characterize cerebral ischemia is limited in available experimental models. The study of neurophysiological network changes that occur in brain tissue during the early minutes that follow focal ischemia induction is restricted in the in vivo condition. Very simplified systems, such as in vitro brain slices and in isolated neurons, have been utilized for this type of studies. We describe here a new model of transient focal ischemia and reperfusion developed in the isolated guinea pig brain, maintained in vitro by arterial perfusion with a complex saline solution without blood cells. In this preparation, that combines the advantage of an in vitro preparation with the functional preservation of both vascular and neuronal compartments, the arteries of the Willis circle are directly accessible by visual control. To induce transitory focal ischemia, one medial cerebral artery (MCA) was transiently tied for 30 min, while brain activity was recorded with multiple electrodes positioned in brain areas within and outside MCA territory. Anoxic depression in ischemic areas propagated to the surrounding tissue and was associated with the abolition of evoked responses due to both functional impairment of afferent olfactory input and tissue depression. Recovery of evoked responses was obtained after MCA reperfusion. The spatial distribution of hypoxic depressions was characterized and was correlated with the extension of brain damage, defined by immunohistochemical analysis with antibodies against microtubule-associated protein (MAP-2). We propose that the present model can be utilized to analyze brain activity changes that occur in early stages of focal brain ischemia and reperfusion.

Astrocyte-endothelial interactions at the blood-brain barrier

The blood-brain barrier, which is formed by the endothelial cells that line cerebral microvessels, has an important role in maintaining a precisely regulated microenvironment for reliable neuronal signalling. At present, there is great interest in the association of brain microvessels, astrocytes and neurons to form functional 'neurovascular units', and recent studies have highlighted the importance of brain endothelial cells in this modular organization. Here, we explore specific interactions between the brain endothelium, astrocytes and neurons that may regulate blood-brain barrier function. An understanding of how these interactions are disturbed in pathological conditions could lead to the development of new protective and restorative therapies.

Activation of cerebral endothelium is required for mononuclear cell recruitment in a novel in vitro model of brain inflammation

Brain inflammation is a common event in the pathogenesis of several neurological diseases. It is unknown whether leukocyte/endothelium interactions are sufficient to promote homing of blood-borne cells into the brain compartment. The role of mononuclear cells and endothelium was analyzed in a new experimental model, the isolated guinea-pig brain maintained in vitro by arterial perfusion. This preparation allows one to investigate early steps of brain inflammation that are impracticable in vivo. We demonstrate by confocal microscopy analysis that in vitro co-perfusion of pro-inflammatory agents and pre-activated fluorescent mononuclear cells induced endothelial expression of selectins and intracellular adhesion molecule-1 in correspondence of arrested mononuclear cells, and correlates with a moderate increase in blood-brain barrier permeability. Separate perfusion of pro-inflammatory agents and mononuclear cells induced neither mononuclear cell adhesion nor adhesion molecule expression. We demonstrate that co-activation of mononuclear cells and cerebral endothelium is an essential requirement for cell arrest and adhesion in the early stages of experimental cerebral inflammation.

Brain activation of monocyte lineage cells: brain-derived soluble factors differentially regulate BV2 microglia and peripheral macrophage immune functions

Brain injury and subsequent neurodegeneration are often associated with infiltrating leukocytes and the activation of microglia as well as other infiltrating cells. However, the characteristics of activation are poorly understood. The objective of this study was to further the understanding of brain regulation of microglial activation. We used an organotypic coculture paradigm to assess how brain-derived soluble factors modulate microglia and peripheral macrophage activation through microscopy and flow cytometry techniques. In the presence of brain-derived soluble factors, the BV2 microglia cell line increased MHC II and phagocytic receptor (Fcgamma II/III) expression. The increased expression correlated with a functional increase in phagocytic activity, but did not correlate with an increase in allostimulation ability. Furthermore, this interaction was selective to an interaction between brain-derived soluble factor(s) and BV2 microglia, since it was not observed in the ANA1 macrophage cell line or in primary peritoneal macrophages. The results indicated that brain-derived soluble factor(s) modulate microglial activation in a manner that is distinct from the effects on peripheral macrophages. Moreover, our results suggest that inflammatory events associated with some types of brain injury may be induced by the brain without dependence on infiltrating peripheral macrophages or T lymphocytes.

Dynamics of CNS barriers: evolution, differentiation, and modulation

(1) Three main barrier layers at the interface between blood and tissue protect the central nervous system (CNS): the endothelium of brain capillaries, and the epithelia of the choroid plexus (CP) and the arachnoid. The classical work on these barriers in situ until the 1970s laid the foundations for modern understanding. Techniques for brain endothelial cell isolation and culture pioneered by Ferenc Joó in the 1970s opened up new fields of examination, enabling study of mechanisms at the cellular and molecular level. (2) Astrocytic glial cells are closely associated with the brain endothelial barrier. During evolution the barrier appears to have shifted from the glial to the endothelial layer, in parallel with the increasing importance of the microvasculature and its regulation. Vestiges of the barrier potential of glia remain in the modern mammalian CNS. (3) Evolutionary evidence suggests that the advantage derived from ionic homeostasis around central synapses was the major selective pressure leading to refinement of CNS barrier systems. This is one element of the modern 'multitasking' barrier function. (4) While epithelia are constitutively able to form barriers at appropriate interfaces, the 'default' condition for endothelia is more leaky; inductive influences from associated cells especially astrocytes are important in generating the full blood-brain barrier (BBB) phenotype in brain capillaries. The underlying mechanisms are being elucidated at the molecular and genomics level. (5) The barrier layers of the nervous system can be modulated by a number of receptor-mediated processes, involving several signal transduction pathways, both calcium dependent and independent. Some agents acting as 'inducers' in the long term can act as 'modulators' in the short-term, with some overlap of signaling pathways. Modulating agents may be derived both from the blood and from cells associated with cerebral vessels. Less is known about the modulation of the CP. (6) The challenge for the next era of CNS barrier studies will be to apply new knowledge from proteomics and genomics to understanding the in vivo condition in physiology and pathology.

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.

In vitro model of the outer blood–retina barrier

The outer blood-retina barrier (BRB) is formed by the retinal pigment epithelium (rpe) and functions similarly to the blood-brain barrier (BBB). In contrast to the BBB, which is composed of a myriad of capillaries, the rpe can in principle be prepared as an intact planar tissue sheet without disruption of its barrier and carrier functions. Both a rapid and gentle procedure to isolate porcine rpe and a method to implement the harvested rpe in drug penetration testing are presented. Enucleated eyes were flat-mounted and the RPE/choroid tissue sheets with or without the retina were isolated. Fluorescence microscopy based on double-labeling with propidium iodide/calcein and scanning electron microscopy revealed well-preserved cell and tissue architecture. For drug evaluation, specimens were immobilized as the interface between test compartments in a dual-chamber device. Ten different test agents were added to one chamber at defined concentrations. After an incubation time of 30 min at 37 degrees C permeated drug levels in both compartments were quantified by HPLC-tandem mass spectrometry or HPLC with fluorescence detection. Sodium fluorescein used as a barrier marker indicated that the rpe model had excellent seal integrity. The use of a representative subset of pharmaceuticals with known BBB permeability characteristics demonstrated that the rpe model had a large permeability dynamic range (factor >350). These findings showed that the model represents a valuable tool for the investigation of the blood barrier penetration of test compounds.

Chemokine-dependent mechanisms of leukocyte trafficking across a model of the blood-brain barrier

Leukocyte transmigration across the blood-brain barrier (BBB) is a multistep process that can be mediated by chemokines. These low-molecular-weight chemoattractant proteins are secreted by cells within the central nervous system (CNS) in response to injury or on activation. Leukocytes transmigrate toward this chemokine gradient, crossing the BBB and gaining access to the CNS parenchyma. Depending on the chemokine, the nature of the insult, and the type of cell that transmigrates, the BBB integrity may be disrupted, leading to its increased permeability. Both the inflammation resulting from leukocyte transmigration and BBB perturbations contribute to CNS pathology. The mechanisms that mediate leukocyte transmigration and BBB disruption, as well as tissue culture models that are used to study leukocyte trafficking, are the focus of this review.

Blood-brain barrier preservation in the in vitro isolated guinea pig brain preparation

The morphofunctional preservation of the blood-brain barrier (BBB) was evaluated in the isolated guinea pig brain maintained in vitro by arterial perfusion. Electron microscopy evaluation after 5 hr in vitro demonstrated that cerebral capillaries and BBB specializations in this preparation retain features compatible with structural integrity. BBB-impermeable and -permeable atropine derivatives arterially perfused to antagonize carbachol-induced fast oscillatory activity confirmed the functional preservation of the BBB in vitro. To study BBB function further, changes in extracellular K+ concentration during arterial perfusion of a high-K+ solution were measured with K+-sensitive electrodes positioned in the cortex and, as control, at the brain venous outlet, where the solution perfused through the brain arterial system was collected. After 5 hr in vitro, the [K+](o) values measured during high-K+ perfusion in the piriform and entorhinal cortices were 5.02 +/- 0.17 mM (mean +/- SE) and 5.2 +/- 0.21 mM, respectively (n = 6). Coperfusion of the high-K+ solution with the Na+/K+ pump blocker ouabain (10 microM; n = 4) induced consistently spreading depression preceded by a rise in [K+](o). Finally, sporadic, isolated spots of extravasation of the fluorescent marker fluorescein isothiocyanate (FITC)-dextran preferentially circumscribed to deep cortical layers was observed in brains perfused with FITC-dextran after 5 hr in vitro. The study demonstrates that the in vitro isolated guinea pig brain is viable for studying cerebrovascular interactions and BBB permeability of compounds active in the central nervous system.

Characteristics of endothelial cells derived from the blood-brain barrier and of astrocytes in culture

In this study, cultures of astrocytes and capillary endothelial cells from the blood-brain barrier (BBB) of the postnatal (P1) mouse cerebral cortex were analyzed with the aim of acquiring information on the distinguishing characteristics of each cell type. For isolation and purification of astrocyte cells, the methods of McCarthy and DeVellis [J. Cell Biol. 85 (1980) 890] were employed. The methods of Chen et al. [Lab. Invest. 78 (1998) 353], Duport et al. [Proc. Natl. Acad. Sci. USA 95 (1998) 1840], Rubin et al. [J Cell Biol. 115 (1991) 1725] and Tontsch and Bauer [Microvasc. Res. 37 (1989) 148] were utilized for culturing of cells from the BBB. A simple protocol was also created for isolating and purifying brain endothelial cells with 10 mM sodium cyanide. The vascular system of the cerebral cortex is derived from the leptomeningeal blood vessels [Qin and Sato, Dev. Dyn. 202 (1995) 172; Risau et al., EMBO J. 5 (1986) 3179]. With this in mind, cultures of the P1 mouse meninges were used as a comparative cell type in order to differentiate between BBB cells and astrocytes. In this regard, the expression of a number of markers were correlated, and an antibody double labeling technique was employed. The staining of these markers was then compared to cells cultured from leptomeninges and to two other types of endothelial cells, human umbilical vein and bovine aortic. Reverse transcription-polymerase chain reaction (RT-PCR) was performed on total RNA isolated from adult mouse brain, cells cultured from P1 mouse cortex or meninges, bovine aortic endothelial cells and human umbilical vein endothelial cells (HUV-EC) to detect the expression of glial fibrillary acidic protein (GFAP), Von Willebrand factor (factor VIII-related antigen) and fibronectin. These analyses revealed the presence of GFAP mRNA in the cultures of cortical and leptomeningeal cells and of protein in all cell types; Von Willebrand factor mRNA was detectable in HUV-EC cells but undetectable in cortical, leptomeningeal and bovine aortic endothelial cells. Fibronectin mRNA and protein were present in all of the cell types. Given the results of our investigations we conclude that in culture, astrocytes are actually brain endothelial cells.

Metabolic activity of cultured rat brainstem, hippocampal and spinal cord slices

The use of cultured brain slices has become an accepted technique for the ex vivo analysis of neural mechanisms, yet the viability of this preparation is not routinely measured. The tetrazolium dye 3-(4, 5-dimethlythiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) is reduced by active mitochondria to an insoluble purple precipitate which accumulates within living cells and is easily visualized with bright field or phase contrast microscopy. In this study, the MTT assay was used to assess the viability of cultured brainstem, hippocampal and spinal cord slices (150-300 micrometer) from 0 to 22 day-old neonatal rats at post-explant time points ranging from 2 to 29 days. After 2 weeks, 180-300 micrometer cultured slices from 4-13 day old rats remained 90-100% viable. Those from 0-1 day old rats had similar viability but displayed peripheral tissue outgrowth. Slices from older 18-22 day rats were no longer viable after 10-14 days. After 4 weeks, the thicker (300 micrometer) slices of hippocampus and spinal cord retained 75-89% viability, in contrast to the 50-74% viability of the brainstem. Thinner brainstem and hippocampal slices (150-220 micrometer) slices were less than 50% viable at 4 weeks. Morphologic characteristics of the brain regions gradually degenerated over the 4-week culture period. Slice viability was markedly influenced by tissue thickness, donor age and brain region. Use of the MTT assay provides an inexpensive and expeditious means to assess a significant functional parameter of regional slice viability under variable conditions and enhances the feasibility of this preparation for functional studies, such as those concerned with genetic and protein expression within circumscribed areas of the brain.

Isolation and culture of human neuromicrovascular endothelial cells for the study of angiogenesis in vitro

Neovascularization in the adult central nervous system occurs as a response to several pathophysiological conditions such as ischemia, wound repair, or neoplasia. Endothelial cells from different blood vessel types, different organs, and different species are heterogeneous; therefore, the appropriate cell type should be used to study specific aspects of vascular pathology. We have developed a method to isolate human cerebral microvascular endothelial cells (CMECs) from small, freshly obtained specimens of normal brain adherent to human arteriovenous malformations (AVMs). The isolation procedure involves enzymatic digestions and gradient centrifugations, yielding over 95% pure primary cultures. Alternative isolation methods using magnetic beads, panning, or cloning were not superior with regard to cell purity or yield. CMECs were identified by their immunoreactivity for vWF, CD34, EN4, binding of Ulex europeus lectin, and uptake of DiI-Ac-LDL. They displayed ultrastructural features characteristic of blood-brain barrier endothelial cells and expressed GLUT-1. CMECs were subcultured; however, prolonged culture led to reduced culture purity. Vascular endothelial growth factor, basic fibroblast growth factor and hepatocyte growth factor/scatter factor stimulated the directional motility of CMECs, with dose-response profiles similar to human umbilical vein endothelial cells (HUVECs). In contrast, to stimulate proliferation, lower concentrations of growth factors tended to be necessary for CMECs than for the large vessel endothelial cells. CMECs formed capillary tube-like structures in an in vitro angiogenesis assay using matrigel. This study expands the spectrum of available tissue sources for the isolation of human neuromicrovascular endothelial cells, which are essential for the in vitro study of blood-brain barrier function and cerebral angiogenesis.

Assessment of pharmacodynamic and pharmacokinetic characteristics of drugs using microdialysis sampling and capillary electrophoresis

Microdialysis sampling combined with capillary electrophoresis is emerging as a new approach in drug studies. It allows the continuous monitoring, in vivo or in vitro, of changes in free endogenous compounds as well as in drug substances, following the administration of pharmacological agents. The low volume requirement of capillary electrophoresis for injection allows the collection of dialysates during short sampling times, leading to a precise temporal description of drug-induced biochemical changes or pharmacokinetics. Various protocols can be used for analyzing endogenous compounds and drug substances in microdialysis samples. Capillary electrophoresis with laser-induced fluorescence detection often affords the high sensitivity level which is needed in most studies. Furthermore, the direct on-line coupling of microdialysis, derivatization of samples, and electrophoretic analysis now brings a separation-based biosensor, allowing a real-time description of chemical events with a high molecular specificity. Microdialysis sampling combined with capillary electrophoresis has recently been used to assess pharmacodynamic and pharmacokinetic characteristics of various drugs in animal studies; it may also represent a new approach in clinical pharmacology in the near future.