Electrical resistance across the blood-brain barrier in anaesthetized rats: a developmental study

The role of mutations associated with familial neurodegenerative disorders on blood-brain barrier function in an iPSC model

Background

Blood–brain barrier dysfunction is associated with many late-stage neurodegenerative diseases. An emerging question is whether the mutations associated with neurodegenerative diseases can independently lead to blood–brain barrier (BBB) dysfunction. Studies from patient-derived induced pluripotent stem cells suggest that mutations associated with neurodegenerative disease are non-cell autonomous, resulting in gain of toxic function in derived neurons and astrocytes. Here we assess whether selected mutations associated with neurodegenerative diseases can contribute to impairment of the blood–brain barrier.

Methods

We assessed barrier function of confluent monolayers of human brain microvascular endothelial cells (hBMECs) derived from induced pluripotent stem cells (iPSC) from three healthy individuals and eight individuals with neurodegenerative disease. We systematically assessed protein and gene expression of BBB biomarkers, transendothelial resistance (TEER), permeability of Lucifer yellow, permeability of d-glucose, permeability of rhodamine 123, the efflux ratio of rhodamine 123, and P-gp inhibition using Tariquidar for confluent monolayers of human brain microvascular endothelial cell (hBMECs).

Results

We provide evidence supporting the hypothesis that mutations associated with neurodegenerative disease can independently cause BBB dysfunction. These functional changes are not catastrophic since barrier breakdown would result in BBB impairment during development. Synergistic interactions between non-cell autonomous cerebrovascular dysfunction and the effects of gain-of-toxic function in neurons (e.g. toxic oligomers) are likely to increase disease burden through a positive feedback mechanism.

Conclusions

These results suggest that the accumulation of defects in brain microvascular endothelial cells may ultimately lead to impairment of the BBB. Small changes in barrier function over time could lead to accumulated defects that result in positive feedback to unrelated central nervous system diseases.

Electronic supplementary material

The online version of this article (10.1186/s12987-019-0139-4) contains supplementary material, which is available to authorized users.

Role of iPSC-derived pericytes on barrier function of iPSC-derived brain microvascular endothelial cells in 2D and 3D

Background

Pericytes of the blood–brain barrier (BBB) are embedded within basement membrane between brain microvascular endothelial cells (BMECs) and astrocyte end-feet. Despite the direct cell–cell contact observed in vivo, most in vitro BBB models introduce an artificial membrane that separates pericytes from BMECs. In this study, we investigated the effects of pericytes on BMEC barrier function across a range of in vitro platforms with varied spatial orientations and levels of cell–cell contact.

Methods

We differentiated RFP-pericytes and GFP-BMECs from hiPSCs and monitored transendothelial electrical resistance (TEER) across BMECs on transwell inserts while pericytes were either directly co-cultured on the membrane, indirectly co-cultured in the basolateral chamber, or embedded in a collagen I gel formed on the transwell membrane. We then incorporated pericytes into a tissue-engineered microvessel model of the BBB and measured pericyte motility and microvessel permeability.

Results

We found that BMEC monolayers did not require co-culture with pericytes to achieve physiological TEER values (> 1500 Ω cm²). However, under stressed conditions where TEER values for BMEC monolayers were reduced, indirectly co-cultured hiPSC-derived pericytes restored optimal TEER. Conversely, directly co-cultured pericytes resulted in a decrease in TEER by interfering with BMEC monolayer continuity. In the microvessel model, we observed direct pericyte-BMEC contact, abluminal pericyte localization, and physiologically-low Lucifer yellow permeability comparable to that of BMEC microvessels. In addition, pericyte motility decreased during the first 48 h of co-culture, suggesting progression towards pericyte stabilization.

Conclusions

We demonstrated that monocultured BMECs do not require co-culture to achieve physiological TEER, but that suboptimal TEER in stressed monolayers can be increased through co-culture with hiPSC-derived pericytes or conditioned media. We also developed the first BBB microvessel model using exclusively hiPSC-derived BMECs and pericytes, which could be used to examine vascular dysfunction in the human CNS.

Electronic supplementary material

The online version of this article (10.1186/s12987-019-0136-7) contains supplementary material, which is available to authorized users.

Parametric investigation of static and dynamic cell culture conditions and their impact on hCMEC/D3 barrier properties

In brain research, the hCMEC/D3 cell line is widely used for the establishment of a human in vitro blood-brain barrier (BBB) model. However, its barrier integrity seems to be insufficient for drug permeability studies, represented by rather low transendothelial electrical resistance (TEER) and high permeability of small molecules. Therefore, this study covers a parametric investigation of static and dynamic cell culture conditions to improve barrier functionality of hCMEC/D3. The effect of basal media was investigated by analyzing changes in proliferation rate, barrier integrity and gene expression of cellular junction proteins. The cells were able to grow in different cell culture media, including serum-free media. However, none of these media enhanced strongly the growth rate or barrier integrity compared to the microvascular endothelial cell growth medium-2 (EGM™-2 MV). Furthermore, hCMEC/D3 cells did not respond positively regarding TEER to any tested parameter neither supplements, coating materials nor co-cultures with the human immortalized astrocyte cell line SVGmm. Furthermore, the impact of dynamic conditions was examined by using the Dynamic Micro Tissue Engineering System (DynaMiTES). Cultivation conditions were successfully adapted to the DynaMiTES design and no negative effect was detected by analyzing cell viability and cell count, albeit TEER remained also unchanged. Consequently, the hCMEC/D3 model has considerable limitations and further improvements or alternative cell lines are required.

Advances in Microfluidic Blood-Brain Barrier (BBB) Models

Therapeutic options for neurological disorders currently remain limited. The intrinsic complexity of the brain architecture prevents potential therapeutics from reaching their cerebral target, thus limiting their efficacy. Recent advances in microfluidic technology and organ-on-chip systems have enabled the development of a new generation of in vitro platforms that can recapitulate complex in vivo microenvironments and physiological responses. In this context, microfluidic-based in vitro models of the blood-brain barrier (BBB) are of particular interest as they provide an innovative approach for conducting research related to the brain, including modeling of neurodegenerative diseases and high-throughput drug screening. Here, we present the most recent advances in BBB-on-chip devices and examine validation steps that will strengthen their future applications.

PDGF-B Is Required for Development of the Glymphatic System

The glymphatic system is a highly polarized cerebrospinal fluid (CSF) transport system that facilitates the clearance of neurotoxic molecules through a brain-wide network of perivascular pathways. Herein we have mapped the development of the glymphatic system in mice. Perivascular CSF transport first emerges in hippocampus in newborn mice, and a mature glymphatic system is established in the cortex at 2 weeks of age. Formation of astrocytic endfeet and polarized expression of aquaporin 4 (AQP4) consistently coincided with the appearance of perivascular CSF transport. Deficiency of platelet-derived growth factor B (PDGF-B) function in the PDGF retention motif knockout mouse line Pdgfbret/ret suppressed the development of the glymphatic system, whose functions remained suppressed in adulthood compared with wild-type mice. These experiments map the natural development of the glymphatic system in mice and define a critical role of PDGF-B in the development of perivascular CSF transport.

Evaluating the involvement of cerebral microvascular endothelial Na+/K+-ATPase and Na+-K+-2Cl- co-transporter in electrolyte fluxes in an in vitro blood-brain barrier model of dehydration

The blood-brain barrier (BBB) is involved in brain water and salt homeostasis. Blood osmolarity increases during dehydration and water is osmotically extracted from the brain. The loss of water is less than expected from pure osmotic forces, due to brain electrolyte accumulation. Although the underlying molecular mechanisms are unresolved, the current model suggests the luminally expressed Na⁺-K⁺-2Cl^- co-transporter 1 (NKCC1) as a key component, while the role of the Na⁺/K⁺-ATPase remains uninvestigated. To test the involvement of these proteins in brain electrolyte flux under mimicked dehydration, we employed a tight in vitro co-culture BBB model with primary cultures of brain endothelial cells and astrocytes. The Na⁺/K⁺-ATPase and the NKCC1 were both functionally dominant in the abluminal membrane. Exposure of the in vitro BBB model to conditions mimicking systemic dehydration, i.e. hyperosmotic conditions, vasopressin, or increased [K⁺]_o illustrated that NKCC1 activity was unaffected by exposure to vasopressin and to hyperosmotic conditions. Hyperosmotic conditions and increased K⁺ concentrations enhanced the Na⁺/K⁺-ATPase activity, here determined to consist of the α1 β1 and α1 β3 isozymes. Abluminally expressed endothelial Na⁺/K⁺-ATPase, and not NKCC1, may therefore counteract osmotic brain water loss during systemic dehydration by promoting brain Na⁺ accumulation.

Claudin-5: gatekeeper of neurological function

Tight junction proteins of the blood–brain barrier are vital for maintaining integrity of endothelial cells lining brain blood vessels. The presence of these protein complexes in the space between endothelial cells creates a dynamic, highly regulated and restrictive microenvironment that is vital for neural homeostasis. By limiting paracellular diffusion of material between blood and brain, tight junction proteins provide a protective barrier preventing the passage of unwanted and potentially damaging material. Simultaneously, this protective barrier hinders the therapeutic effectiveness of central nervous system acting drugs with over 95% of small molecule therapeutics unable to bypass the blood–brain barrier. At the blood–brain barrier, claudin-5 is the most enriched tight junction protein and its dysfunction has been implicated in neurodegenerative disorders such as Alzheimer’s disease, neuroinflammatory disorders such as multiple sclerosis as well as psychiatric disorders including depression and schizophrenia. By regulating levels of claudin-5, it is possible to abrogate disease symptoms in many of these disorders. This review will give an overview of the blood–brain barrier and the role of tight junction complexes in maintaining blood–brain barrier integrity before focusing on the role of claudin-5 and its regulation in homeostatic and pathological conditions. We will also summarise therapeutic strategies to restore integrity of cerebral vessels by targeting tight junction protein complexes.

Comparison between Polybutylcyanoacrylate Nanoparticles with Either Surface-Adsorbed or Encapsulated Brain-Derived Neurotrophic Factor on the Neural Differentiation of iPSCs

The brain-derived neurotrophic factor (BDNF) is vital in the neural differentiation of neural stem/progenitor cells, and together may have therapeutic potential for neural regeneration. In this study, a multiplexed polybutylcyanoacrylate nanoparticle (PBCA NP) delivery platform was constructed, incorporating either surface-adsorbed or encapsulated BDNF for the induction of neural differentiation in induced pleuripotent stem cells (iPSCs), where tween 80 (T80) and superparamagnetic iron oxide (SPIO) were added for central nervous system (CNS) targeting and magnetic resonance (MR) image tracking, respectively. Both methods by which the BDNF was carried resulted in loading efficiencies greater than 95%. The nanoparticle-mediated delivery of BDNF resulted in neural differentiation of iPSCs detected on immunofluorescence staining as early as 7 days, with enhanced differentiation efficiency by 1.3-fold compared to the control on flow cytometry; the delivery system of surface-adsorbed BDNF gave rise to cells that had the best neural development than the encapsulated formulation. T80-coating disrupted the in vitro blood–brain barrier model with a corresponding 1.5- to two-fold increase in permeability. SPIO-loaded PBCA NPs exhibited a concentration-dependent, rapid decay in signal intensity on the phantom MR experiment. This study demonstrates the versatility of the PBCA NP, and the surface-adsorption of BDNF is the preferred method of delivery for the differentiation of iPSCs.

Methods to Quantify Nanomaterial Association with, and Distribution Across, the Blood-Brain Barrier In Vivo

The role and functional anatomy of the blood-brain barrier (BBB) is summarized to enable the investigator to appropriately address evaluation of nanomaterial interaction with, and distribution across, it into brain tissue (parenchyma). Transport mechanisms across the BBB are presented, in relation to nanomaterial physicochemical properties. Measures and test substances to assess BBB integrity/disruption/permeation are introduced, along with how they are used to interpret the results obtained with the presented methods. Experimental pitfalls and misinterpretation of results of studies of brain nanomaterial uptake are briefly summarized, that can be avoided with the methods presented in this chapter. Two methods are presented. The in situ brain perfusion technique is used to determine rate and extent of nanomaterial distribution into the brain. The capillary depletion method separates brain parenchymal tissue from the endothelial cells that contribute to the BBB. It is used to verify nanomaterial brain tissue entry. These methods are best used together, the latter refining the results obtained with the former. Details of the materials and equipment needed to conduct these methods, and description of the procedures and data interpretation, are provided.

Benchmarking in vitro tissue-engineered blood-brain barrier models

The blood–brain barrier (BBB) plays a key role in regulating transport into and out of the brain. With increasing interest in the role of the BBB in health and disease, there have been significant advances in the development of in vitro models. The value of these models to the research community is critically dependent on recapitulating characteristics of the BBB in humans or animal models. However, benchmarking in vitro models is surprisingly difficult since much of our knowledge of the structure and function of the BBB comes from in vitro studies. Here we describe a set of parameters that we consider a starting point for benchmarking and validation. These parameters are associated with structure (ultrastructure, wall shear stress, geometry), microenvironment (basement membrane and extracellular matrix), barrier function (transendothelial electrical resistance, permeability, efflux transport), cell function (expression of BBB markers, turnover), and co-culture with other cell types (astrocytes and pericytes). In suggesting benchmarks, we rely primarily on imaging or direct measurements in humans and animal models.

Human iPSC-derived blood-brain barrier microvessels: validation of barrier function and endothelial cell behavior

Microvessels of the blood-brain barrier (BBB) regulate transport into the brain. The highly specialized brain microvascular endothelial cells, a major component of the BBB, express tight junctions and efflux transporters which regulate paracellular and transcellular permeability. However, most existing models of BBB microvessels fail to exhibit physiological barrier function. Here, using (iPSC)-derived human brain microvascular endothelial cells (dhBMECs) within templated type I collagen channels we mimic the cylindrical geometry, cell-extracellular matrix interactions, and shear flow typical of human brain post-capillary venules. We characterize the structure and barrier function in comparison to non-brain-specific microvessels, and show that dhBMEC microvessels recapitulate physiologically low solute permeability and quiescent endothelial cell behavior. Transcellular permeability is increased two-fold using a clinically relevant dose of a p-glycoprotein inhibitor tariquidar, while paracellular permeability is increased using a bolus dose of hyperosmolar agent mannitol. Lastly, we show that our human BBB microvessels are responsive to inflammatory cytokines via upregulation of surface adhesion molecules and increased leukocyte adhesion, but no changes in permeability. Human iPSC-derived blood-brain barrier microvessels support quantitative analysis of barrier function and endothelial cell dynamics in quiescence and in response to biologically- and clinically-relevant perturbations.

Efflux Pump Substrates Shuttled to Cytosolic or Vesicular Compartments Exhibit Different Permeability in a Quantitative Human Blood-Brain Barrier Model

Representative in vitro blood-brain barrier (BBB) models can support the development of strategies to efficiently deliver therapeutic drugs to the brain by aiding the characterization of their internalization, trafficking, and subsequent transport across the BBB. A collagen type I (COL1) hydrogel-based in vitro BBB model was developed to enable the simultaneous characterization of drug transport and intracellular processing using confocal microscopy, in a way that traditional insert-based in vitro BBB models cannot. Human induced pluripotent stem cells (hiPSCs) were differentiated into cells that exhibited a BBB-like phenotype on COL1 hydrogels, which included the expression of key BBB-specific proteins and low permeability of representative small and large molecule therapeutics. Furthermore, the BBB phenotype observed on the COL1 hydrogel was similar to that previously reported on porous inserts. The intracellular visualization of two small molecule efflux pump substrates within the hiPSC-derived BBB-like cells demonstrated a difference in cytosolic and vesicular accumulation, which complemented permeability measurements demonstrating a difference in transport rate. The easy-to-construct COL1-based hiPSC-derived BBB model presented here is the first in vitro two-dimensional BBB experimental system that enables the simultaneous quantification of cellular permeability and visualization of intracellular processes by utilizing confocal microscopy, which can provide insights regarding the relationship between transport and intracellular trafficking of therapeutic drugs.

Using superparamagnetic iron oxide nanoparticles to enhance bioavailability of quercetin in the intact rat brain

Background

Quercetin (QT) as a bioactive flavonoid has a potential therapeutic activity for numerous neuronal injuries and neurodegenerative diseases. However, the low absorption rate of QT, especially through the blood-brain barrier, restricts its bioactivity in the body. The current research took the advantage of superparamagnetic iron oxide nanoparticles (SPIONs) to enhance the bioavailability of quercetin.

Methods

Quercetin conjugated with SPIONs was prepared by means of nanoprecipitation method and was characterized by X-ray diffractometer, scanning electron microscope, and Fourier transformed infrared spectrometer analyses. Wistar male rats were orally fed by gavage with QT and QT-SPION at 50 and 100 mg/kg daily doses for 7 days. Using high-performance liquid chromatography (HPLC) method, biodistribution of QT was evaluated in plasma and brain tissue.

Results

The outcomes of this research revealed a higher concentration in the plasma and brain of the rats fed with QT-SPION in comparison to free QT.

Conclusion

The results of this study confirm that SPION as a targeted drug delivery system enhances the bioavailability of quercetin in the brain about ten folds higher than free quercetin and could be used for the treatment of neurodegenerative disorders.

Engineering and monitoring cellular barrier models

Epithelia and endothelia delineate tissue compartments and control their environments by regulating the passage of ions and solutes. This barrier function is essential for the development and maintenance of multicellular organisms, and its dysfunction is associated with numerous human diseases. Recent advances in biomaterials and microfabrication technologies have evolved in vitro approaches for modelling biological barriers. Current microphysiological systems have become more efficient and reliable in mimicking the cell microenvironment. Additionally, methods for the quantification of barrier permeability have long provided significant insight into their underlying mechanisms. In this review, we outline the current techniques to quantify the barrier function of engineered tissues, and we also give an overview of recent microphysiological systems of biological barriers that emulate the microenvironment and microarchitecture of native tissues.

Investigation on the effect of nanoparticle size on the blood-brain tumour barrier permeability by in situ perfusion via internal carotid artery in mice

The blood-brain barrier (BBB) is a limiting factor in nanoparticle drug delivery to the brain, and various attempts have been made to overcome it for efficient drug delivery. Nowadays, it was considered as further issue for brain-drug delivery that the nanoparticle delivered to brain through the BBB reach cancer cells in tumour tissue. In this study, we investigated the effect of nanoparticle size on blood-brain tumour barrier (BBTB) permeation of fluorescence-labelled gold nanoparticles (AuNPs) in a mouse model of orthotopic glioblastoma multiforme (GBM), established by intracranial implantation of luciferase-expressing human glioblastoma U87MG cells. AuNPs sized 10, 50, and 100 nm were perfused into the GBM mice via internal carotid artery (ICA) for 5 min. Immediately after perfusion, the brains were fixed and prepared for LSCM observation. The AuNPs distribution in the normal and tumorous brain tissues was analysed qualitatively and quantitatively. Higher distribution of AuNPs was observed in the tumorous tissue than in the normal tissue. Furthermore, the smallest nanoparticle, 10 nm AuNPs, was widely distributed in the brain tumour tissue, whereas the 50 and 100 nm AuNPs were located near the blood vessels. Therefore, nanoparticle size affected the permeation of nanoparticles from the blood into brain tumour tissue.

A differentiating neural stem cell-derived astrocytic population mitigates the inflammatory effects of TNF-α and IL-6 in an iPSC-based blood-brain barrier model

Inflammation can be a risk factor for neurodegenerative diseases such as Alzheimer's disease (AD) and may also contribute to the progression of AD. Here, we sought to understand how inflammation affects the properties of the brain microvascular endothelial cells (BMECs) that compose the blood-brain barrier (BBB), which is impaired in AD. A fully human in vitro BBB model with brain microvascular endothelial cells derived from induced pluripotent stem cells and differentiating neural stem cell (NSC)-derived astrocytic cells was used to investigate the effects of neuroinflammation on barrier function. The cytokines TNF-α and IL-6 directly cause BBB dysfunction measured by a decrease in transendothelial electrical resistance, an increase in sodium fluorescein permeability, and a decrease in cell polarity, providing a link between neuroinflammation and specific aspects of BBB breakdown. An NSC-derived astrocytic cell population was added to the model and secreted cytokines and chemokines were quantified in monoculture and coculture both in the presence and absence of TNF-α and IL-6. Increased concentrations of pro-inflammatory cytokines known to be secreted by astrocytes or endothelial cells such as MCP-1, IL-8, IP-10, MIP-1β, IL-1 β, MIG, and RANTES peaked in inflammatory conditions when NSC-astrocytic cells were present. Despite the presence of several pro-inflammatory cytokines, the NSC-derived astrocytic cells mitigated the effects of inflammation measured by a restoration of transendothelial electrical resistance and IgG permeability. These results also suggest a breakdown in transcellular transport that precedes any increase in paracellular permeability in neuroinflammation. This model has the potential to resolve questions about neurodegenerative disease progression and delivery of therapeutics to the brain.

Blood-brain barrier on a chip

The blood-brain barrier (BBB) plays a vital role in the maintenance of brain homeostasis. It strictly restricts the passage of molecules from the brain vasculature into the brain via its high transendothelial electrical resistance and low paracellular and transcellular permeability. Specialized brain endothelial cells, astrocytes, pericytes, neurons, and microglia contribute synergistically to the functional properties of the BBB. Because of its complexity and relative inaccessibility, BBB research is fraught with difficulties. Most studies rely on animal or cell culture models, which are not able to fully recapitulate the properties of the human BBB. The recent development of three-dimensional (3D) microfluidic models of the BBB could address this issue. This chapter aims to provide an overview of the recent advances in modeling the BBB on microdevices, and illustrate important considerations for the design of such models. In addition, protocols for the fabrication of a 3D BBB microfluidic chip and BBB assessment experiments, including immunocytochemistry for analyzing cell morphology and protein marker expression, permeability assay, and calcium imaging for studying neuronal function as a measure of BBB integrity, are presented here. It is envisioned that continued advancements in microtechnology can lead to the creation of realistic in vivo-like BBB-on-chip models.

Barriers-on-chips: Measurement of barrier function of tissues in organs-on-chips

Disruption of tissue barriers formed by cells is an integral part of the pathophysiology of many diseases. Therefore, a thorough understanding of tissue barrier function is essential when studying the causes and mechanisms of disease as well as when developing novel treatments. In vitro methods play an integral role in understanding tissue barrier function, and several techniques have been developed in order to evaluate barrier integrity of cultured cell layers, from microscopy imaging of cell-cell adhesion proteins to measuring ionic currents, to flux of water or transport of molecules across cellular barriers. Unfortunately, many of the current in vitro methods suffer from not fully recapitulating the microenvironment of tissues and organs. Recently, organ-on-chip devices have emerged to overcome this challenge. Organs-on-chips are microfluidic cell culture devices with continuously perfused microchannels inhabited by living cells. Freedom of changing the design of device architecture offers the opportunity of recapitulating the in vivo physiological environment while measuring barrier function. Assessment of barriers in organs-on-chips can be challenging as they may require dedicated setups and have smaller volumes that are more sensitive to environmental conditions. But they do provide the option of continuous, non-invasive sensing of barrier quality, which enables better investigation of important aspects of pathophysiology, biological processes, and development of therapies that target barrier tissues. Here, we discuss several techniques to assess barrier function of tissues in organs-on-chips, highlighting advantages and technical challenges.

Blood-brain barrier dysfunction in ischemic stroke: targeting tight junctions and transporters for vascular protection

The blood-brain barrier (BBB) is a physical and biochemical barrier that precisely controls cerebral homeostasis. It also plays a central role in the regulation of blood-to-brain flux of endogenous and exogenous xenobiotics and associated metabolites. This is accomplished by molecular characteristics of brain microvessel endothelial cells such as tight junction protein complexes and functional expression of influx and efflux transporters. One of the pathophysiological features of ischemic stroke is disruption of the BBB, which significantly contributes to development of brain injury and subsequent neurological impairment. Biochemical characteristics of BBB damage include decreased expression and altered organization of tight junction constituent proteins as well as modulation of functional expression of endogenous BBB transporters. Therefore, there is a critical need for development of novel therapeutic strategies that can protect against BBB dysfunction (i.e., vascular protection) in the setting of ischemic stroke. Such strategies include targeting tight junctions to ensure that they maintain their correct structure or targeting transporters to control flux of physiological substrates for protection of endothelial homeostasis. In this review, we will describe the pathophysiological mechanisms in cerebral microvascular endothelial cells that lead to BBB dysfunction following onset of stroke. Additionally, we will utilize this state-of-the-art knowledge to provide insights on novel pharmacological strategies that can be developed to confer BBB protection in the setting of ischemic stroke.

Direct current stimulation of endothelial monolayers induces a transient and reversible increase in transport due to the electroosmotic effect

We investigated the effects of direct current stimulation (DCS) on fluid and solute transport across endothelial cell (EC) monolayers in vitro. Our motivation was transcranial direct current stimulation (tDCS) that has been investigated for treatment of neuropsychiatric disorders, to enhance neurorehabilitation, and to change cognition in healthy subjects. The mechanisms underlying this diversity of applications remain under investigation. To address the possible role of blood-brain barrier (BBB) changes during tDCS, we applied direct current to cultured EC monolayers in a specially designed chamber that generated spatially uniform direct current. DCS induced fluid and solute movement across EC layers that persisted only for the duration of the stimulation suggesting an electroosmosis mechanism. The direction of induced transport reversed with DCS polarity - a hallmark of the electroosmotic effect. The magnitude of DCS-induced flow was linearly correlated to the magnitude of the applied current. A mathematical model based on a two-pore description of the endothelial transport barrier and a Helmholtz model of the electrical double layer describes the experimental data accurately and predicts enhanced significance of this mechanism in less permeable monolayers. This study demonstrates that DCS transiently alters the transport function of the BBB suggesting a new adjunct mechanism of tDCS.

Sex Steroids, Adult Neurogenesis, and Inflammation in CNS Homeostasis, Degeneration, and Repair

Sex steroidal hormones coordinate the development and maintenance of tissue architecture in many organs, including the central nervous systems (CNS). Within the CNS, sex steroids regulate the morphology, physiology, and behavior of a wide variety of neural cells including, but not limited to, neurons, glia, endothelial cells, and immune cells. Sex steroids spatially and temporally control distinct molecular networks, that, in turn modulate neural activity, synaptic plasticity, growth factor expression and function, nutrient exchange, cellular proliferation, and apoptosis. Over the last several decades, it has become increasingly evident that sex steroids, often in conjunction with neuroinflammation, have profound impact on the occurrence and severity of neuropsychiatric and neurodegenerative disorders. Here, I review the foundational discoveries that established the regulatory role of sex steroids in the CNS and highlight recent advances toward elucidating the complex interaction between sex steroids, neuroinflammation, and CNS regeneration through adult neurogenesis. The majority of recent work has focused on neuroinflammatory responses following acute physical damage, chronic degeneration, or pharmacological insult. Few studies directly assess the role of immune cells in regulating adult neurogenesis under healthy, homeostatic conditions. As such, I also introduce tractable, non-traditional models for examining the role of neuroimmune cells in natural neuronal turnover, seasonal plasticity of neural circuits, and extreme CNS regeneration.

Neuroimmune Axes of the Blood-Brain Barriers and Blood-Brain Interfaces: Bases for Physiological Regulation, Disease States, and Pharmacological Interventions

Central nervous system (CNS) barriers predominantly mediate the immune-privileged status of the brain, and are also important regulators of neuroimmune communication. It is increasingly appreciated that communication between the brain and immune system contributes to physiologic processes, adaptive responses, and disease states. In this review, we discuss the highly specialized features of brain barriers that regulate neuroimmune communication in health and disease. In section I, we discuss the concept of immune privilege, provide working definitions of brain barriers, and outline the historical work that contributed to the understanding of CNS barrier functions. In section II, we discuss the unique anatomic, cellular, and molecular characteristics of the vascular blood-brain barrier (BBB), blood-cerebrospinal fluid barrier, and tanycytic barriers that confer their functions as neuroimmune interfaces. In section III, we consider BBB-mediated neuroimmune functions and interactions categorized as five neuroimmune axes: disruption, responses to immune stimuli, uptake and transport of immunoactive substances, immune cell trafficking, and secretions of immunoactive substances. In section IV, we discuss neuroimmune functions of CNS barriers in physiologic and disease states, as well as pharmacological interventions for CNS diseases. Throughout this review, we highlight many recent advances that have contributed to the modern understanding of CNS barriers and their interface functions.

The Dynamics of Impaired Blood-Brain Barrier Restoration in a Rat Model of Co-morbid Injury

Defect in brain microperfusion is increasingly recognized as an antecedent event to Alzheimer's disease (AD) and ischemia. Nevertheless, studies on the role of impaired microperfusion as a pathological trigger to neuroinflammation, Aβ deposition as well as blood-brain barrier (BBB) disruption, and the etiological link between AD and ischemia are lacking. In this study, we employ in vivo sequential magnetic resonance imaging (MRI) and computed tomography (CT) imaging in a co-morbid rat model of β-amyloid toxicity (Aβ) and ischemia (ET1) with subsequent histopathology of striatal lesion core and penumbra at 1, 7, and 28 days post injury. Within 24 h, cerebral injury resulted in increased BBB permeability due to the dissolution of β-dystroglycan (β-DG) and basement membrane laminin by active matrix metalloproteinase9 (MMP9). As a result, net flow of circulating IgG down a hydrostatic gradient into the parenchyma led to vasogenic edema and impaired perfusion, thus increasing the apparent hyperintensity in true fast imaging with steady-state free precession (true FISP) imaging and acute hypoperfusion in CT. This was followed by a slow recruitment of reactive astroglia to the affected brain and depolarization of aquaporin4 (AQP4) expression resulting in cytotoxic edema-in an attempt to resolve vasogenic edema. On d28, functional BBB was restored in ET1 rats as observed by astrocytic MMP9 release, β-DG stabilization, and new vessel formation. This was confirmed by reduced hyperintensity on true FISP imaging and normalized cerebral blood flow in CT. While, Aβ toxicity alone was not detrimental enough, Aβ+ET1 rats showed delayed differential expression of MMP9, late recruitment of astroglial cells, protracted loss of AQP4 depolarization, and thus delayed BBB restoration and cerebral perfusion.

A role for sex and a common HFE gene variant in brain iron uptake

HFE (high iron) is an essential protein for regulating iron transport into cells. Mutations of the HFE gene result in loss of this regulation causing accumulation of iron within the cell. The mutated protein has been found increasingly in numerous neurodegenerative disorders in which increased levels of iron in the brain are reported. Additionally, evidence that these mutations are associated with elevated brain iron challenges the paradigm that the brain is protected by the blood-brain barrier. While much has been studied regarding the role of HFE in cellular iron uptake, it has remained unclear what role the protein plays in the transport of iron into the brain. We investigated regulation of iron transport into the brain using a mouse model with a mutation in the HFE gene. We demonstrated that the rate of radiolabeled iron (59Fe) uptake was similar between the two genotypes despite higher brain iron concentrations in the mutant. However, there were significant differences in iron uptake between males and females regardless of genotype. These data indicate that brain iron status is consistently maintained and tightly regulated at the level of the blood-brain barrier.

Modeling Neurovascular Disorders and Therapeutic Outcomes with Human-Induced Pluripotent Stem Cells

The neurovascular unit (NVU) is composed of neurons, astrocytes, pericytes, and endothelial cells that form the blood-brain barrier (BBB). The NVU regulates material exchange between the bloodstream and the brain parenchyma, and its dysfunction is a primary or secondary cause of many cerebrovascular and neurodegenerative disorders. As such, there are substantial research thrusts in academia and industry toward building NVU models that mimic endogenous organization and function, which could be used to better understand disease mechanisms and assess drug efficacy. Human pluripotent stem cells, which can self-renew indefinitely and differentiate to almost any cell type in the body, are attractive for these models because they can provide a limitless source of individual cells from the NVU. In addition, human-induced pluripotent stem cells (iPSCs) offer the opportunity to build NVU models with an explicit genetic background and in the context of disease susceptibility. Herein, we review how iPSCs are being used to model neurovascular and neurodegenerative diseases, with particular focus on contributions of the BBB, and discuss existing technologies and emerging opportunities to merge these iPSC progenies with biomaterials platforms to create complex NVU systems that recreate the in vivo microenvironment.

Effect of Centruroides antivenom on reversal of methamphetamine-induced hyperkinesis and hyperthermia in rats

Context

Methamphetamine (MA) toxicity is a major health concern causing agitation, hyperkinesia, hyperthermia, and even death, affecting 24.7 million people worldwide. It has been observed that MA generates movement disorders in children similar to that of scorpion envenomation. Four cases have been reported where MA intoxication in children were both subjectively and objectively improved as indicated by the reversal of nystagmus and movement disorders following administration of Centruroides antivenom (AV) therapy.

Objective

Here, we aimed to demonstrate the reversal of MA induced movement disorders and hyperthermia by scorpion AV equine immune F(ab')2 in rats.

Materials and Methods

Baseline core temperature and locomotor activity in adult male Sprague-Dawley rats (200-220 g) were evaluated prior to acute administration of AV (20 mg/kg, intraperitoneally, i.p.) + MA (10 mg/kg, i.p.) or control. Core body temperature was reassessed 10, 50, and 80 min post injection while locomotor activity was reassessed 20-35 and 60-75 min post injection.

Results

At 20-35 min, Saline + MA and BSA + MA groups showed a significant increase in the number of fine events compared to their respective control groups Saline + Saline and BSA + Saline, which indicates an increase in paw movements of animals in situ (p = 0.008, p = 0.006, respectively). In contrast, AV + MA demonstrated a non-significant increase in fine activity compared to the control group AV + Saline). At 60-75 min, the AV + MA treatment group were less likely to engage in locomotor activity indicated by the significant decrease in exploratory events compared to BSA + MA control group (p = 0.041). No significant percent change in core body temperature was observed in the AV + MA treatment group compared to the control groups, AV + Saline and BSA + MA.

Discussion

Here, we provide evidence for some aspects of MA-induced hyperkinesia but not hyperthermia reversed by scorpion AV. Further preclinical studies involving adolescent rodents may be necessary to completely mimic the reversal of MA toxicity seen in children in the clinic.

Transport Across the Blood-Brain Barrier

The blood-brain barrier (BBB) is a dynamic barrier essential for maintaining the microenvironment of the brain. Although the special anatomical features of the BBB determine its protective role for the central nervous system (CNS) from blood-borne neurotoxins, however, the BBB extremely limits the therapeutic efficacy of drugs into the CNS, which greatly hinders the treatment of major brain diseases. This chapter summarized the unique structures of the BBB; described a variety of in vivo and in vitro experimental methods for determining the transport properties of the BBB and the permeability of the BBB to water, ions, and solutes including nutrients, therapeutic agents, and drug carriers; and presented recently developed mathematical models which quantitatively correlate the anatomical structures of the BBB with its barrier functions. Recent findings for modulation of the BBB permeability by chemical and physical stimuli were described. Finally, drug delivery strategies through specific trans-BBB routes were discussed.

Engineering the human blood-brain barrier in vitro

The blood-brain barrier (BBB) is the interface between the vasculature and the brain, regulating molecular and cellular transport into the brain. Endothelial cells (ECs) that form the capillary walls constitute the physical barrier but are dependent on interactions with other cell types. In vitro models are widely used in BBB research for mechanistic studies and drug screening. Current models have both biological and technical limitations. Here we review recent advances in stem cell engineering that have been utilized to create innovative platforms to replicate key features of the BBB. The development of human in vitro models is envisioned to enable new mechanistic investigations of BBB transport in central nervous system diseases.

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

OBJECTIVES

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

METHODS

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

KEY FINDINGS

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

CONCLUSIONS

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

Effect of shear stress on iPSC-derived human brain microvascular endothelial cells (dhBMECs)

BACKGROUND

The endothelial cells that form the lumen of capillaries and microvessels are an important component of the blood-brain barrier. Cell phenotype is regulated by transducing a range of biomechanical and biochemical signals in the local microenvironment. Here we report on the role of shear stress in modulating the morphology, motility, proliferation, apoptosis, and protein and gene expression, of confluent monolayers of human brain microvascular endothelial cells derived from induced pluripotent stem cells.

METHODS

To assess the response of derived human brain microvascular endothelial cells (dhBMECs) to shear stress, confluent monolayers were formed in a microfluidic device. Monolayers were subjected to a shear stress of 4 or 12 dyne cm^-2 for 40 h. Static conditions were used as the control. Live cell imaging was used to assess cell morphology, cell speed, persistence, and the rates of proliferation and apoptosis as a function of time. In addition, immunofluorescence imaging and protein and gene expression analysis of key markers of the blood-brain barrier were performed.

RESULTS

Human brain microvascular endothelial cells exhibit a unique phenotype in response to shear stress compared to static conditions: (1) they do not elongate and align, (2) the rates of proliferation and apoptosis decrease significantly, (3) the mean displacement of individual cells within the monolayer over time is significantly decreased, (4) there is no cytoskeletal reorganization or formation of stress fibers within the cell, and (5) there is no change in expression levels of key blood-brain barrier markers.

CONCLUSIONS

The characteristic response of dhBMECs to shear stress is significantly different from human and animal-derived endothelial cells from other tissues, suggesting that this unique phenotype that may be important in maintenance of the blood-brain barrier. The implications of this work are that: (1) in confluent monolayers of dhBMECs, tight junctions are formed under static conditions, (2) the formation of tight junctions decreases cell motility and prevents any morphological transitions, (3) flow serves to increase the contact area between cells, resulting in very low cell displacement in the monolayer, (4) since tight junctions are already formed under static conditions, increasing the contact area between cells does not cause upregulation in protein and gene expression of BBB markers, and (5) the increase in contact area induced by flow makes barrier function more robust.

A Three-Dimensional Cell Culture System To Model RNA Virus Infections at the Blood-Brain Barrier

Neurotropic viral infections are significant sources of global morbidity and mortality. The blood-brain barrier (BBB) is composed in part of a layer of microvascular endothelial cells and functions to restrict viral access to the brain. In vitro models that recapitulate many of the properties of the human BBB endothelium are lacking, particularly with respect to the unique cellular and immunological mechanisms by which these cells restrict viral infections of the brain. Here, we developed a three-dimensional cell culture model that recapitulates many of the morphological and functional properties of the BBB microvasculature and apply this model to the study of RNA virus infections. The model we describe can therefore be used to study a variety of aspects of BBB physiology, including the mechanisms by which viruses might access the CNS, and could be used for the development and screening of antiviral therapeutics to limit this important step in viral pathogenesis.

The blood-brain barrier (BBB) comprises the foremost protective barrier in the brain and is composed in part of a layer of microvascular endothelial cells that line the capillaries surrounding the brain. Here, we describe a human three-dimensional (3-D) cell-based model of the BBB microvascular endothelium that recapitulates properties of these cells in vivo, including physiologically relevant transcriptional profiles, the capacity to induce potent antimicrobial innate immune signaling, and the ability to resist infection by diverse RNA viruses, including members of the enterovirus (coxsackievirus B, echovirus 11, enterovirus 71, poliovirus) and flavivirus (dengue virus, Zika virus [ZIKV]) families. We show that disruption of apical tight junctions by proinflammatory cytokine tumor necrosis factor alpha (TNF-α) sensitizes 3-D-cultured BBB cells to ZIKV infection and that 3-D derived BBB cells can be used to model the transmigration of ZIKV-infected monocytes across the endothelial barrier to access underlying astrocytes. Taken together, our findings show that human BBB microvascular endothelial cells cultured in 3-D can be used to model the mechanisms by which RNA viruses access the central nervous system (CNS), which could be used for the development and screening of therapeutics to limit this event.

IMPORTANCE Neurotropic viral infections are significant sources of global morbidity and mortality. The blood-brain barrier (BBB) is composed in part of a layer of microvascular endothelial cells and functions to restrict viral access to the brain. In vitro models that recapitulate many of the properties of the human BBB endothelium are lacking, particularly with respect to the unique cellular and immunological mechanisms by which these cells restrict viral infections of the brain. Here, we developed a three-dimensional cell culture model that recapitulates many of the morphological and functional properties of the BBB microvasculature and apply this model to the study of RNA virus infections. The model we describe can therefore be used to study a variety of aspects of BBB physiology, including the mechanisms by which viruses might access the CNS, and could be used for the development and screening of antiviral therapeutics to limit this important step in viral pathogenesis.

Abcb1a (P-glycoprotein) limits brain exposure of the anticancer drug candidate seliciclib in vivo in adult mice

Seliciclib displayed limited brain exposure in vivo in adult rats with mature blood-brain barrier (BBB). Selicilib was shown to be a specific substrate of human ABCB1 in vitro. To demonstrate that ABCB1/Abcb1 can limit brain exposure in vivo in mice we are showing that seliciclib is a substrate of mouse Abcb1a, the murine ABCB1 ortholog expressed in the BBB as LLC-PK-Abcb1a cells displayed an efflux ratio (ER) of 15.31±3.54 versus an ER of 1.44±0.10 in LLC-PK1-mock cells. Additionally, in the presence of LY335979, an ABCB1/Abcb1a specific inhibitor, the observed ER for seliciclib in the LLC-PK1-mMdr1a cells decreased to 1.05±0.25. To demonstrate in vivo relevance of seliciclib transport by Abcb1a mouse brain microdialysis experiments were carried out that showed that the AUC_brain/AUC_blood ratio of 0.143 in anesthetized mice increased about two-fold to 0.279 in the presence of PSC833 another ABCB1/Abcb1a specific inhibitor. PSC833 also increased the brain exposure (AUC_brain) of seliciclib close to 2-fold (136 vs 242) in awake mice. In sum, Abcb1a significantly decreases seliciclib permeability in vitro and is partly responsible for limited brain exposure of seliciclib in vivo in mice.

Sepsis-Associated Encephalopathy: The Blood-Brain Barrier and the Sphingolipid Rheostat

Sepsis is not only a significant cause of mortality worldwide but has particularly devastating effects on the central nervous system of survivors. It is therefore crucial to understand the molecular structure, physiology, and events involved in the pathogenesis of sepsis-associated encephalopathy, so that potential therapeutic advances can be achieved. A key determinant to the development of this type of encephalopathy is morphological and functional modification of the blood–brain barrier (BBB), whose function is to protect the CNS from pathogens and toxic threats. Key mediators of pathologic sequelae of sepsis in the brain include cytokines, including TNF-α, and sphingolipids, which are biologically active components of cellular membranes that possess diverse functions. Emerging data demonstrated an essential role for sphingolipids in the pulmonary vascular endothelium. This raises the question of whether endothelial stability in other organs systems such as the CNS may also be mediated by sphingolipids and their receptors. In this review, we will model the structure and vulnerability of the BBB and hypothesize mechanisms for therapeutic stabilization and repair following a confrontation with sepsis-induced inflammation.

Physical insights into the blood-brain barrier translocation mechanisms

The number of individuals suffering from diseases of the central nervous system (CNS) is growing with an aging population. While candidate drugs for many of these diseases are available, most of these pharmaceutical agents cannot reach the brain rendering most of the drug therapies that target the CNS inefficient. The reason is the blood-brain barrier (BBB), a complex and dynamic interface that controls the influx and efflux of substances through a number of different translocation mechanisms. Here, we present these mechanisms providing, also, the necessary background related to the morphology and various characteristics of the BBB. Moreover, we discuss various numerical and simulation approaches used to study the BBB, and possible future directions based on multi-scale methods. We anticipate that this review will motivate multi-disciplinary research on the BBB aiming at the design of effective drug therapies.

Organoid and Organ-On-A-Chip Systems: New Paradigms for Modeling Neurological and Gastrointestinal Disease

PURPOSE OF REVIEW

The modeling of biological processes in vitro provides an important tool to better understand mechanisms of development and disease, allowing for the rapid testing of therapeutics. However, a critical constraint in traditional monolayer culture systems is the absence of the multicellularity, spatial organization, and overall microenvironment present in vivo. This limitation has resulted in numerous therapeutics showing efficacy in vitro, but failing in patient trials. In this review, we discuss several organoid and "organ-on-a-chip" systems with particular regard to the modeling of neurological diseases and gastrointestinal disorders.

RECENT FINDINGS

Recently, the in vitro generation of multicellular organ-like structures, coined organoids, has allowed the modeling of human development, tissue architecture, and disease with human-specific pathophysiology. Additionally, microfluidic "organ-on-a-chip" technologies add another level of physiological mimicry by allowing biological mediums to be shuttled through 3D cultures.

SUMMARY

Organoids and organ-chips are rapidly evolving in vitro platforms which hold great promise for the modeling of development and disease.

Guidance on the risk assessment of substances present in food intended for infants below 16 weeks of age

Following a request from the European Commission to EFSA, the EFSA Scientific Committee (SC) prepared a guidance for the risk assessment of substances present in food intended for infants below 16 weeks of age. In its approach to develop this guidance, the EFSA SC took into account, among others, (i) an exposure assessment based on infant formula as the only source of nutrition; (ii) knowledge of organ development in human infants, including the development of the gut, metabolic and excretory capacities, the brain and brain barriers, the immune system, the endocrine and reproductive systems; (iii) the overall toxicological profile of the substance identified through the standard toxicological tests, including critical effects; (iv) the relevance for the human infant of the neonatal experimental animal models used. The EFSA SC notes that during the period from birth up to 16 weeks, infants are expected to be exclusively fed on breast milk and/or infant formula. The EFSA SC views this period as the time where health-based guidance values for the general population do not apply without further considerations. High infant formula consumption per body weight is derived from 95th percentile consumption. The first weeks of life is the time of the highest relative consumption on a body weight basis. Therefore, when performing an exposure assessment, the EFSA SC proposes to use the high consumption value of 260 mL/kg bw per day. A decision tree approach is proposed that enables a risk assessment of substances present in food intended for infants below 16 weeks of age. The additional information needed when testing substances present in food for infants below 16 weeks of age and the approach to be taken for the risk assessment are on a case-by-case basis, depending on whether the substance is added intentionally to food and is systemically available.

Functional Expression of P-glycoprotein and Organic Anion Transporting Polypeptides at the Blood-Brain Barrier: Understanding Transport Mechanisms for Improved CNS Drug Delivery?

Drug delivery to the central nervous system (CNS) is greatly limited by the blood-brain barrier (BBB). Physical and biochemical properties of the BBB have rendered treatment of CNS diseases, including those with a hypoxia/reoxygenation (H/R) component, extremely difficult. Targeting endogenous BBB transporters from the ATP-binding cassette (ABC) superfamily (i.e., P-glycoprotein (P-gp)) or from the solute carrier (SLC) family (i.e., organic anion transporting polypeptides (OATPs in humans; Oatps in rodents)) has been suggested as a strategy that can improve delivery of drugs to the brain. With respect to P-gp, direct pharmacological inhibition using small molecules or selective regulation by targeting intracellular signaling pathways has been explored. These approaches have been largely unsuccessful due to toxicity issues and unpredictable pharmacokinetics. Therefore, our laboratory has proposed that optimization of CNS drug delivery, particularly for treatment of diseases with an H/R component, can be achieved by targeting Oatp isoforms at the BBB. As the major drug transporting Oatp isoform, Oatp1a4 has demonstrated blood-to-brain transport of substrate drugs with neuroprotective properties. Furthermore, our laboratory has shown that targeting Oatp1a4 regulation (i.e., TGF-β signaling mediated via the ALK-1 and ALK-5 transmembrane receptors) represents an opportunity to control Oatp1a4 functional expression for the purpose of delivering therapeutics to the CNS. In this review, we will discuss limitations of targeting P-gp-mediated transport activity and the advantages of targeting Oatp-mediated transport. Through this discussion, we will also provide critical information on novel approaches to improve CNS drug delivery by targeting endogenous uptake transporters expressed at the BBB.

Accelerated differentiation of human induced pluripotent stem cells to blood-brain barrier endothelial cells

BACKGROUND

Due to their ability to limitlessly proliferate and specialize into almost any cell type, human induced pluripotent stem cells (iPSCs) offer an unprecedented opportunity to generate human brain microvascular endothelial cells (BMECs), which compose the blood-brain barrier (BBB), for research purposes. Unfortunately, the time, expense, and expertise required to differentiate iPSCs to purified BMECs precludes their widespread use. Here, we report the use of a defined medium that accelerates the differentiation of iPSCs to BMECs while achieving comparable performance to BMECs produced by established methods.

METHODS

Induced pluripotent stem cells were seeded at defined densities and differentiated to BMECs using defined medium termed E6. Resultant purified BMEC phenotypes were assessed through trans-endothelial electrical resistance (TEER), fluorescein permeability, and P-glycoprotein and MRP family efflux transporter activity. Expression of endothelial markers and their signature tight junction proteins were confirmed using immunocytochemistry. The influence of co-culture with astrocytes and pericytes on purified BMECs was assessed via TEER measurements. The robustness of the differentiation method was confirmed across independent iPSC lines.

RESULTS

The use of E6 medium, coupled with updated culture methods, reduced the differentiation time of iPSCs to BMECs from thirteen to 8 days. E6-derived BMECs expressed GLUT-1, claudin-5, occludin, PECAM-1, and VE-cadherin and consistently achieved TEER values exceeding 2500 Ω × cm² across multiple iPSC lines, with a maximum TEER value of 4678 ± 49 Ω × cm² and fluorescein permeability below 1.95 × 10^-7 cm/s. E6-derived BMECs maintained TEER above 1000 Ω × cm² for a minimum of 8 days and showed no statistical difference in efflux transporter activity compared to BMECs differentiated by conventional means. The method was also found to support long-term stability of BMECs harboring biallelic PARK2 mutations associated with Parkinson's Disease. Finally, BMECs differentiated using E6 medium responded to inductive cues from astrocytes and pericytes and achieved a maximum TEER value of 6635 ± 315 Ω × cm², which to our knowledge is the highest reported in vitro TEER value to date.

CONCLUSIONS

Given the accelerated differentiation, equivalent performance, and reduced cost to produce BMECs, our updated methods should make iPSC-derived in vitro BBB models more accessible for a wide variety of applications.

Accelerated differentiation of human induced pluripotent stem cells to blood-brain barrier endothelial cells

Background

Due to their ability to limitlessly proliferate and specialize into almost any cell type, human induced pluripotent stem cells (iPSCs) offer an unprecedented opportunity to generate human brain microvascular endothelial cells (BMECs), which compose the blood–brain barrier (BBB), for research purposes. Unfortunately, the time, expense, and expertise required to differentiate iPSCs to purified BMECs precludes their widespread use. Here, we report the use of a defined medium that accelerates the differentiation of iPSCs to BMECs while achieving comparable performance to BMECs produced by established methods.

Methods

Induced pluripotent stem cells were seeded at defined densities and differentiated to BMECs using defined medium termed E6. Resultant purified BMEC phenotypes were assessed through trans-endothelial electrical resistance (TEER), fluorescein permeability, and P-glycoprotein and MRP family efflux transporter activity. Expression of endothelial markers and their signature tight junction proteins were confirmed using immunocytochemistry. The influence of co-culture with astrocytes and pericytes on purified BMECs was assessed via TEER measurements. The robustness of the differentiation method was confirmed across independent iPSC lines.

Results

The use of E6 medium, coupled with updated culture methods, reduced the differentiation time of iPSCs to BMECs from thirteen to 8 days. E6-derived BMECs expressed GLUT-1, claudin-5, occludin, PECAM-1, and VE-cadherin and consistently achieved TEER values exceeding 2500 Ω × cm² across multiple iPSC lines, with a maximum TEER value of 4678 ± 49 Ω × cm² and fluorescein permeability below 1.95 × 10⁻⁷ cm/s. E6-derived BMECs maintained TEER above 1000 Ω × cm² for a minimum of 8 days and showed no statistical difference in efflux transporter activity compared to BMECs differentiated by conventional means. The method was also found to support long-term stability of BMECs harboring biallelic PARK2 mutations associated with Parkinson’s Disease. Finally, BMECs differentiated using E6 medium responded to inductive cues from astrocytes and pericytes and achieved a maximum TEER value of 6635 ± 315 Ω × cm², which to our knowledge is the highest reported in vitro TEER value to date.

Conclusions

Given the accelerated differentiation, equivalent performance, and reduced cost to produce BMECs, our updated methods should make iPSC-derived in vitro BBB models more accessible for a wide variety of applications.

Electronic supplementary material

The online version of this article (doi:10.1186/s12987-017-0059-0) contains supplementary material, which is available to authorized users.

PPAR-α, a lipid-sensing transcription factor, regulates blood-brain barrier efflux transporter expression

Lipid sensor peroxisome proliferator-activated receptor alpha (PPAR- α) is the master regulator of lipid metabolism. Dietary release of endogenous free fatty acids, fibrates, and certain persistent environmental pollutants, e.g. perfluoroalkyl fire-fighting foam components, are peroxisome proliferator-activated receptor alpha ligands. Here, we define a role for peroxisome proliferator-activated receptor alpha in regulating the expression of three ATP-driven drug efflux transporters at the rat and mouse blood-brain barriers: P-glycoprotein (Abcb1), breast cancer resistance protein (Bcrp/Abcg2), and multidrug resistance-associated protein 2 (Mrp2/Abcc2). Exposing isolated rat brain capillaries to linoleic acid, clofibrate, or PKAs increased the transport activity and protein expression of the three ABC transporters. These effects were blocked by the PPAR- α antagonist, GW6471. Dosing rats with 20 mg/kg or 200 mg/kg of clofibrate decreased the brain accumulation of the P-glycoprotein substrate, verapamil, by 50% (in situ brain perfusion; effects blocked by GW6471) and increased P-glycoprotein expression and activity in capillaries ex vivo. Fasting C57Bl/6 wild-type mice for 24 h increased both serum lipids and brain capillary P-glycoprotein transport activity. Fasting did not alter P-glycoprotein activity in PPAR- α knockout mice. These results indicate that hyperlipidemia, lipid-lowering fibrates and exposure to certain fire-fighting foam components activate blood-brain barrier peroxisome proliferator-activated receptor alpha, increase drug efflux transporter expression and reduce drug delivery to the brain.

Effects of diabetic retinopathy on the barrier functions of the retinal pigment epithelium

Diabetic retinopathy is a debilitating microvascular complication of diabetes mellitus. A rich literature describes the breakdown of retinal endothelial cells and the inner blood-retinal barrier, but the effects of diabetes on the retinal pigment epithelium (RPE) has received much less attention. RPE lies between the choroid and neurosensory retina to form the outer blood-retinal barrier. RPE's specialized and dynamic barrier functions are crucial for maintaining retinal health. RPE barrier functions include a collection of interrelated structures and activities that regulate the transepithelial movement of solutes, including: diffusion through the paracellular spaces, facilitated diffusion through the cells, active transport, receptor-mediated and bulk phase transcytosis, and metabolic processing of solutes in transit. In the later stages of diabetic retinopathy, the tight junctions that regulate the paracellular space begin to disassemble, but there are earlier effects on the other aspects of RPE barrier function, particularly active transport and metabolic processing. With advanced understanding of RPE-specific barrier functions, and more in vivo-like culture models, the time is ripe for revisiting experiments in the literature to resolve controversies and extend our understanding of how diabetes affects the outer blood-retinal barrier.

Hypoxic Stress and Inflammatory Pain Disrupt Blood-Brain Barrier Tight Junctions: Implications for Drug Delivery to the Central Nervous System

A functional blood-brain barrier (BBB) is necessary to maintain central nervous system (CNS) homeostasis. Many diseases affecting the CNS, however, alter the functional integrity of the BBB. It has been shown that various diseases and physiological stressors can impact the BBB's ability to selectively restrict passage of substances from the blood to the brain. Modifications of the BBB's permeability properties can potentially contribute to the pathophysiology of CNS diseases and result in altered brain delivery of therapeutic agents. Hypoxia and/or inflammation are central components of a number of diseases affecting the CNS. A number of studies indicate hypoxia or inflammatory pain increase BBB paracellular permeability, induce changes in the expression and/or localization of tight junction proteins, and affect CNS drug uptake. In this review, we look at what is currently known with regard to BBB disruption following a hypoxic or inflammatory insult in vivo. Potential mechanisms involved in altering tight junction components at the BBB are also discussed. A more detailed understanding of the mediators involved in changing BBB functional integrity in response to hypoxia or inflammatory pain could potentially lead to new treatments for CNS diseases with hypoxic or inflammatory components. Additionally, greater insight into the mechanisms involved in TJ rearrangement at the BBB may lead to novel strategies to pharmacologically increase delivery of drugs to the CNS.

Establishment of a Human Blood-Brain Barrier Co-culture Model Mimicking the Neurovascular Unit Using Induced Pluri- and Multipotent Stem Cells

In vitro models of the human blood-brain barrier (BBB) are highly desirable for drug development. This study aims to analyze a set of ten different BBB culture models based on primary cells, human induced pluripotent stem cells (hiPSCs), and multipotent fetal neural stem cells (fNSCs). We systematically investigated the impact of astrocytes, pericytes, and NSCs on hiPSC-derived BBB endothelial cell function and gene expression. The quadruple culture models, based on these four cell types, achieved BBB characteristics including transendothelial electrical resistance (TEER) up to 2,500 Ω cm² and distinct upregulation of typical BBB genes. A complex in vivo-like tight junction (TJ) network was detected by freeze-fracture and transmission electron microscopy. Treatment with claudin-specific TJ modulators caused TEER decrease, confirming the relevant role of claudin subtypes for paracellular tightness. Drug permeability tests with reference substances were performed and confirmed the suitability of the models for drug transport studies.

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Establishment of a standardized human BBB co-culture model based on hiPSCs and fNSCs

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Reflection of physiological BBB integrity and expression of relevant transporters/TJs

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Confirmation of TJ network functionality by claudin-specific TJ modulators

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Validation of physiological transcellular model tightness by permeability studies

Evaluation of [14C] and [13C]Sucrose as Blood-Brain Barrier Permeability Markers

Nonspecific quantitation of [¹⁴C]sucrose in blood and brain has been routinely used as a quantitative measure of the in vivo blood-brain barrier (BBB) integrity. However, the reported apparent brain uptake clearance (K_in) of the marker varies widely (∼100-fold). We investigated the accuracy of the use of the marker in comparison with a stable isotope of sucrose ([¹³C]sucrose) measured by a specific liquid chromatography-tandem mass spectrometry method. Rats received single doses of each marker, and the K_in values were determined. Surprisingly, the K_in value of [¹³C]sucrose was 6- to 7-fold lower than that of [¹⁴C]sucrose. Chromatographic fractionation after in vivo administration of [¹⁴C]sucrose indicated that the majority of the brain content of radioactivity belonged to compounds other than the intact [¹⁴C]sucrose. However, mechanistic studies failed to reveal any substantial metabolism of the marker. The octanol:water partition coefficient of [¹⁴C]sucrose was >2-fold higher than that of [¹³C]sucrose, indicating the presence of lipid-soluble impurities in the [¹⁴C]sucrose solution. Our data indicate that [¹⁴C]sucrose overestimates the true BBB permeability to sucrose. We suggest that specific quantitation of the stable isotope (¹³C) of sucrose is a more accurate alternative to the current widespread use of the radioactive sucrose as a BBB marker.