Differentiating embryonic neural progenitor cells induce blood-brain barrier properties

Her4.3+ radial glial cells maintain the brain vascular network through activation of Wnt signaling

During vascular development, radial glial cells (RGCs) regulate vascular patterning in the trunk and contribute to the early differentiation of the blood-brain barrier. Ablation of RGCs results in excessive sprouting vessels or the absence of bilateral vertebral arteries. However, interactions of RGCs with later brain vascular networks after pattern formation remain unknown. Here, we generated a her4.3 transgenic line to label RGCs and applied the MTZ/NTR system to ablate her4.3+ RGCs. The ablation of her4.3+ RGCs led to the collapse of the cerebral vascular network, disruption of the blood-brain barrier, and downregulation of Wnt signaling. The inhibition of Wnt signaling resulted in the collapse of cerebral vasculature, similar to that caused by her4.3+ RGC ablation. The defects in the maintenance of brain vasculature resulting from the absence of her4.3+ RGCs were partially rescued by the activation of Wnt signaling or overexpression of Wnt7aa or Wnt7bb. Together, our study suggests that her4.3+ radial glial cells maintain the cerebral vascular network through Wnt signaling.

Involvement of Astrocytes in the Formation, Maintenance, and Function of the Blood-Brain Barrier

The blood-brain barrier (BBB) is a fundamental structure that protects the composition of the brain by determining which ions, metabolites, and nutrients are allowed to enter the brain from the blood or to leave it towards the circulation. The BBB is structurally composed of a layer of brain capillary endothelial cells (BCECs) bound to each other through tight junctions (TJs). However, its development as well as maintenance and properties are controlled by the other brain cells that contact the BCECs: pericytes, glial cells, and even neurons themselves. Astrocytes seem, in particular, to have a very important role in determining and controlling most properties of the BBB. Here, we will focus on these latter cells, since the comprehension of their roles in brain physiology has been continuously expanding, even including the ability to participate in neurotransmission and in complex functions such as learning and memory. Accordingly, pathological conditions that alter astrocytic functions can alter the BBB's integrity, thus compromising many brain activities. In this review, we will also refer to different kinds of in vitro BBB models used to study the BBB's properties, evidencing its modifications under pathological conditions.

Applications and Considerations for Microfluidic Systems To Model the Blood-Brain Barrier

In a myriad of developmental and degenerative brain diseases, characteristic pathological biomarkers are often associated with cerebral blood flow and vasculature. However, the relationship between vascular dysfunction and markers of brain disease is not well-defined. Additionally, it is difficult to deliver effective therapeutics to the brain due to the highly regulated blood-brain barrier (BBB) at the microvasculature interface of the brain. This Review first covers the need for modeling the BBB and the challenges of modeling the BBB. In vitro models of the BBB enable the study of the relationship between vascular dysfunction, BBB function, and disease progression and can serve as a platform to screen therapeutics. In particular, microfluidic-based in vitro BBB models are useful for studying brain vasculature as they support cell culture within the presence of continuous perfusion, which mirrors the in vivo flow and associated stress conditions in the brain. Early microfluidic models of the BBB created the most simplistic models possible that still displayed some functional aspects of the in vivo BBB. Therefore, this Review also discusses the emerging unique ways in which microfluidics in tandem with recent advancements in cell culture, biomaterials, and in vitro modeling can be used to develop more complex and physiologically relevant models of the BBB. Finally, we discuss the current and future state-of-the-art application of microfluidic BBB models for drug development and disease modeling, and the ongoing areas of needed innovation in this field.

Advances in BBB on Chip and Application for Studying Reversible Opening of Blood-Brain Barrier by Sonoporation

The complex structure of the blood-brain barrier (BBB), which blocks nearly all large biomolecules, hinders drug delivery to the brain and drug assessment, thus decelerating drug development. Conventional in vitro models of BBB cannot mimic some crucial features of BBB in vivo including a shear stress environment and the interaction between different types of cells. There is a great demand for a new in vitro platform of BBB that can be used for drug delivery studies. Compared with in vivo models, an in vitro platform has the merits of low cost, shorter test period, and simplicity of operation. Microfluidic technology and microfabrication are good tools in rebuilding the BBB in vitro. During the past decade, great efforts have been made to improve BBB penetration for drug delivery using biochemical or physical stimuli. In particular, compared with other drug delivery strategies, sonoporation is more attractive due to its minimized systemic exposure, high efficiency, controllability, and reversible manner. BBB on chips (BOC) holds great promise when combined with sonoporation. More details and mechanisms such as trans-endothelial electrical resistance (TEER) measurements and dynamic opening of tight junctions can be figured out when using sonoporation stimulating BOC, which will be of great benefit for drug development. Herein, we discuss the recent advances in BOC and sonoporation for BBB disruption with this in vitro platform.

Interactions between Major Bioactive Polyphenols of Sugarcane Top: Effects on Human Neural Stem Cell Differentiation and Astrocytic Maturation

Sugarcane (Saccharum officinarum L.) is a tropical plant grown for sugar production. We recently showed that sugarcane top (ST) ameliorates cognitive decline in a mouse model of accelerated aging via promoting neuronal differentiation and neuronal energy metabolism and extending the length of the astrocytic process in vitro. Since the crude extract consists of multicomponent mixtures, it is crucial to identify bioactive compounds of interest and the affected molecular targets. In the present study, we investigated the bioactivities of major polyphenols of ST, namely 3-O-caffeoylquinic acid (3CQA), 5-O-caffeoylquinic acid (5CQA), 3-O-feruloylquinic acid (3FQA), and Isoorientin (ISO), in human fetal neural stem cells (hNSCs)- an in vitro model system for studying neural development. We found that multiple polyphenols of ST contributed synergistically to stimulate neuronal differentiation of hNSCs and induce mitochondrial activity in immature astrocytes. Mono-CQAs (3CQA and 5CQA) regulated the expression of cyclins related to G1 cell cycle arrest, whereas ISO regulated basic helix-loop-helix transcription factors related to cell fate determination. Additionally, mono-CQAs activated p38 and ISO inactivated GSK3β. In hNSC-derived immature astrocytes, the compounds upregulated mRNA expression of PGC-1α, a master regulator of astrocytic mitochondrial biogenesis. Altogether, our findings suggest that synergistic interactions between major polyphenols of ST contribute to its potential for neuronal differentiation and astrocytic maturation.

In Vitro Models of the Human Blood-Brain Barrier Utilising Human Induced Pluripotent Stem Cells: Opportunities and Challenges

The blood-brain barrier (BBB) is a component of the neurovascular unit formed by specialized brain microvascular endothelial cells surrounded by astrocytes end-feet processes, pericytes, and a basement membrane. The BBB plays an important role in the maintenance of brain homeostasis and has seen a growing involvement in the pathophysiology of various neurological diseases. On the other hand, the presence of such a barrier remains an important challenge for drug delivery to treat such illnesses.Since the pioneering work describing the isolation and cultivation of primary brain microvascular cells about 50 years ago until now, the development of an in vitro model of the BBB that is scalable, capable to form tight monolayers, and predictive of drug permeability in vivo remained extremely challenging.The recent description of the use of induced pluripotent stem cells (iPSCs) as a modeling tool for neurological diseases raised momentum into the use of such cells to develop new in vitro models of the BBB. This chapter will provide an exhaustive description of the use of iPSCs as a source of cells for modeling the BBB in vitro, describe the advantages and limitations of such model, as well as describe their prospective use for disease modeling and drug permeability screening platforms.

Biology and Models of the Blood-Brain Barrier

The blood-brain barrier (BBB) is one of the most selective endothelial barriers. An understanding of its cellular, morphological, and biological properties in health and disease is necessary to develop therapeutics that can be transported from blood to brain. In vivo models have provided some insight into these features and transport mechanisms adopted at the brain, yet they have failed as a robust platform for the translation of results into clinical outcomes. In this article, we provide a general overview of major BBB features and describe various models that have been designed to replicate this barrier and neurological pathologies linked with the BBB. We propose several key parameters and design characteristics that can be employed to engineer physiologically relevant models of the blood-brain interface and highlight the need for a consensus in the measurement of fundamental properties of this barrier.

Review of Design Considerations for Brain-on-a-Chip Models

In recent years, the need for sophisticated human in vitro models for integrative biology has motivated the development of organ-on-a-chip platforms. Organ-on-a-chip devices are engineered to mimic the mechanical, biochemical and physiological properties of human organs; however, there are many important considerations when selecting or designing an appropriate device for investigating a specific scientific question. Building microfluidic Brain-on-a-Chip (BoC) models from the ground-up will allow for research questions to be answered more thoroughly in the brain research field, but the design of these devices requires several choices to be made throughout the design development phase. These considerations include the cell types, extracellular matrix (ECM) material(s), and perfusion/flow considerations. Choices made early in the design cycle will dictate the limitations of the device and influence the end-point results such as the permeability of the endothelial cell monolayer, and the expression of cell type-specific markers. To better understand why the engineering aspects of a microfluidic BoC need to be influenced by the desired biological environment, recent progress in microfluidic BoC technology is compared. This review focuses on perfusable blood-brain barrier (BBB) and neurovascular unit (NVU) models with discussions about the chip architecture, the ECM used, and how they relate to the in vivo human brain. With increased knowledge on how to make informed choices when selecting or designing BoC models, the scientific community will benefit from shorter development phases and platforms curated for their application.

Bioengineering the neurovascular niche to study the interaction of neural stem cells and endothelial cells

The ability of mammalian neural stem cells (NSCs) to self-renew and differentiate throughout adulthood has made them ideal to study neurogenesis and attractive candidates for neurodegenerative disease therapies. In the adult mammalian brain, NSCs are maintained in the neurovascular niche (NVN) where they are found near the specialized blood vessels, suggesting that brain endothelial cells (BECs) are prominent orchestrators of NSC fate. However, most of the current knowledge of the mammalian NVN has been deduced from nonhuman studies. To circumvent the challenges of in vivo studies, in vitro models have been developed to better understand the reciprocal cellular mechanisms of human NSCs and BECs. This review will cover the current understanding of mammalian NVN biology, the effects of endothelial cell-derived signals on NSC fate, and the in vitro models developed to study the interactions between NSCs and BECs.

Estimating Brain Permeability Using In Vitro Blood-Brain Barrier Models

The blood-brain barrier (BBB) is a vital biological interface that regulates transfer of different molecules between blood and brain and, therefore, maintains the homeostatic environment of the CNS. In order to perform high-throughput screening of therapeutics in drug discovery, specific properties of the BBB are investigated within in vitro BBB platforms. In this chapter, we detail the process and steps for the iPSC to BMEC and astrocyte differentiation as well as TEER and permeability measurement in Transwell platform of in vitro BBB model. Also, advanced microfluidic iPSCs-derived BMECs on chip and permeability measurement within this model have been elucidated.

Astrocyte-derived Wnt growth factors are required for endothelial blood-brain barrier maintenance

Maintenance of the endothelial blood-brain-barrier (BBB) through Wnt/β-catenin signalling is essential for neuronal function. The cells however, providing Wnt growth factors at the adult neurovascular unit (NVU) are poorly explored. Here we show by conditionally knocking out the evenness interrupted (Evi) gene in astrocytes (Evi^ΔAC) that astrocytic Wnt release is crucial for BBB and NVU integrity. Evi^ΔAC mice developed brain oedema and increased vascular tracer leakage. While brain vascularization and endothelial junctions were not altered in 10 and 40 week-old mice, endothelial caveolin(Cav)-1-mediated vesicle formation was increased in vivo and in vitro. Moreover, astrocytic end-feet were swollen, and aquaporin-4 distribution was disturbed, coinciding with decreased astrocytic Wnt activity. Vascular permeability correlated with increased neuronal activation by c-fos staining, indicative of altered neuronal function. Astrocyte-derived Wnts thus serve to maintain Wnt/β-catenin activity in endothelia and in astrocytes, thereby controlling Cav-1 expression, vesicular abundance, and end-feet integrity at the NVU.

Drug Development for Central Nervous System Diseases Using In vitro Blood-brain Barrier Models and Drug Repositioning

An important goal of biomedical research is to translate basic research findings into practical clinical implementation. Despite the advances in the technology used in drug discovery, the development of drugs for central nervous system diseases remains challenging. The failure rate for new drugs targeting important central nervous system diseases is high compared to most other areas of drug discovery. The main reason for the failure is the poor penetration efficacy across the blood-brain barrier. The blood-brain barrier represents the bottleneck in central nervous system drug development and is the most important factor limiting the future growth of neurotherapeutics. Meanwhile, drug repositioning has been becoming increasingly popular and it seems a promising field in central nervous system drug development. In vitro blood-brain barrier models with high predictability are expected for drug development and drug repositioning. In this review, the recent progress of in vitro BBB models and the drug repositioning for central nervous system diseases will be discussed.

A 3D Biohybrid Real-Scale Model of the Brain Cancer Microenvironment for Advanced In Vitro Testing

The modeling of the pathological microenvironment of the central nervous system (CNS) represents a disrupting approach for drug screening for advanced therapies against tumors and neuronal disorders. The in vitro investigations of the crossing and diffusion of drugs through the blood-brain barrier (BBB) are still not completely reliable, due to technological limits in the replication of 3D microstructures that can faithfully mimic the in vivo scenario. Here, an innovative 1:1 scale 3D-printed realistic biohybrid model of the brain tumor microenvironment, with both luminal and parenchyma compartments, is presented. The dynamically controllable microfluidic device, fabricated through two-photon lithography, enables the triple co-culture of hCMEC/D3 cells, forming the internal biohybrid endothelium of the capillaries, of astrocytes, and of magnetically-driven spheroids of U87 glioblastoma cells. Tumor spheroids are obtained from culturing glioblas-toma cells inside 3D microcages loaded with superparamagnetic iron oxide nanoparticles (SPIONs). The system proves to be capable in hindering dextran diffusion through the bioinspired BBB, while allowing chemotherapy-loaded nanocarriers to cross it. The proper formation of the selective barrier and the good performance of the anti-tumor treatment demonstrate that the proposed device can be successfully exploited as a realistic in vitro model for high-throughput drug screening in CNS diseases.

Neuronal Activity Regulates Blood-Brain Barrier Efflux Transport through Endothelial Circadian Genes

The blood vessels in the central nervous system (CNS) have a series of unique properties, termed the blood-brain barrier (BBB), which stringently regulate the entry of molecules into the brain, thus maintaining proper brain homeostasis. We sought to understand whether neuronal activity could regulate BBB properties. Using both chemogenetics and a volitional behavior paradigm, we identified a core set of brain endothelial genes whose expression is regulated by neuronal activity. In particular, neuronal activity regulates BBB efflux transporter expression and function, which is critical for excluding many small lipophilic molecules from the brain parenchyma. Furthermore, we found that neuronal activity regulates the expression of circadian clock genes within brain endothelial cells, which in turn mediate the activity-dependent control of BBB efflux transport. These results have important clinical implications for CNS drug delivery and clearance of CNS waste products, including Aβ, and for understanding how neuronal activity can modulate diurnal processes.

Profiling the mouse brain endothelial transcriptome in health and disease models reveals a core blood-brain barrier dysfunction module

Blood vessels in the CNS form a specialized and critical structure, the blood-brain barrier (BBB). We present a resource to understand the molecular mechanisms that regulate BBB function in health and dysfunction during disease. Using endothelial cell enrichment and RNA sequencing, we analyzed the gene expression of endothelial cells in mice, comparing brain endothelial cells with peripheral endothelial cells. We also assessed the regulation of CNS endothelial gene expression in models of stroke, multiple sclerosis, traumatic brain injury and seizure, each having profound BBB disruption. We found that although each is caused by a distinct trigger, they exhibit strikingly similar endothelial gene expression changes during BBB disruption, comprising a core BBB dysfunction module that shifts the CNS endothelial cells into a peripheral endothelial cell-like state. The identification of a common pathway for BBB dysfunction suggests that targeting therapeutic agents to limit it may be effective across multiple neurological disorders.

An isogenic neurovascular unit model comprised of human induced pluripotent stem cell-derived brain microvascular endothelial cells, pericytes, astrocytes, and neurons

Background

Brain microvascular endothelial cells (BMECs) astrocytes, neurons, and pericytes form the neurovascular unit (NVU). Interactions with NVU cells endow BMECs with extremely tight barriers via the expression of tight junction proteins, a host of active efflux and nutrient transporters, and reduced transcellular transport. To recreate the BMEC-enhancing functions of NVU cells, we combined BMECs, astrocytes, neurons, and brain pericyte-like cells.

Methods

BMECs, neurons, astrocytes, and brain like pericytes were differentiated from human induced pluripotent stem cells (iPSCs) and placed in a Transwell-type NVU model. BMECs were placed in co-culture with neurons, astrocytes, and/or pericytes alone or in varying combinations and critical barrier properties were monitored.

Results

Co-culture with pericytes followed by a mixture of neurons and astrocytes (1:3) induced the greatest barrier tightening in BMECs, supported by a significant increase in junctional localization of occludin. BMECs also expressed active P-glycoprotein (PGP) efflux transporters under baseline BMEC monoculture conditions and continued to express baseline active PGP efflux transporters regardless of co-culture conditions. Finally, brain-like pericyte co-culture significantly reduced the rate of non-specific transcytosis across BMECs.

Conclusions

Importantly, each cell type in the NVU model was differentiated from the same donor iPSC source, yielding an isogenic model that could prove enabling for enhanced personalized modeling of the NVU in human health and disease.

Cell and Gene Therapies for Mucopolysaccharidoses: Base Editing and Therapeutic Delivery to the CNS

Although individually uncommon, rare diseases collectively account for a considerable proportion of disease impact worldwide. A group of rare genetic diseases called the mucopolysaccharidoses (MPSs) are characterized by accumulation of partially degraded glycosaminoglycans cellularly. MPS results in varied systemic symptoms and in some forms of the disease, neurodegeneration. Lack of treatment options for MPS with neurological involvement necessitates new avenues of therapeutic investigation. Cell and gene therapies provide putative alternatives and when coupled with genome editing technologies may provide long term or curative treatment. Clustered regularly interspaced short palindromic repeats (CRISPR)-based genome editing technology and, more recently, advances in genome editing research, have allowed for the addition of base editors to the repertoire of CRISPR-based editing tools. The latest versions of base editors are highly efficient on-targeting deoxyribonucleic acid (DNA) editors. Here, we describe a number of putative guide ribonucleic acid (RNA) designs for precision correction of known causative mutations for 10 of the MPSs. In this review, we discuss advances in base editing technologies and current techniques for delivery of cell and gene therapies to the site of global degeneration in patients with severe neurological forms of MPS, the central nervous system, including ultrasound-mediated blood-brain barrier disruption.

A Novel Transwell Blood Brain Barrier Model Using Primary Human Cells

Structural alterations and breakdown of the blood brain barrier (BBB) is often a primary or secondary consequence of disease, resulting in brain oedema and the transport of unwanted substances into the brain. It is critical that effective in vitro models are developed to model the in vivo environment to aid in clinically relevant research, especially regarding drug screening and permeability studies. Our novel model uses only primary human cells and includes four of the key cells of the BBB: astrocytes, pericytes, brain microvascular endothelial cells (HBMEC) and neurons. We show that using a larger membrane pore size (3.0 μM) there is an improved connection between the endothelial cells, astrocytes and pericytes. Compared to a two and three cell model, we show that when neurons are added to HBMECs, astrocytes and pericytes, BBB integrity was more sensitive to oxygen-glucose deprivation evidenced by increased permeability and markers of cell damage. Our data also show that a four cell model responds faster to the barrier tightening effects of glucocorticoid dexamethasone, when compared to a two cell and three cell model. These data highlight the important role that neurons play in response to ischaemia, particularly how they contribute to BBB maintenance and breakdown. We consider that this model is more representative of the interactions at the neurovascular unit than other transwell models and is a useful method to study BBB physiology.

Studying Human Neurological Disorders Using Induced Pluripotent Stem Cells: From 2D Monolayer to 3D Organoid and Blood Brain Barrier Models

Neurological disorders have emerged as a predominant healthcare concern in recent years due to their severe consequences on quality of life and prevalence throughout the world. Understanding the underlying mechanisms of these diseases and the interactions between different brain cell types is essential for the development of new therapeutics. Induced pluripotent stem cells (iPSCs) are invaluable tools for neurological disease modeling, as they have unlimited self-renewal and differentiation capacity. Mounting evidence shows: (i) various brain cells can be generated from iPSCs in two-dimensional (2D) monolayer cultures; and (ii) further advances in 3D culture systems have led to the differentiation of iPSCs into organoids with multiple brain cell types and specific brain regions. These 3D organoids have gained widespread attention as in vitro tools to recapitulate complex features of the brain, and (iii) complex interactions between iPSC-derived brain cell types can recapitulate physiological and pathological conditions of blood-brain barrier (BBB). As iPSCs can be generated from diverse patient populations, researchers have effectively applied 2D, 3D, and BBB models to recapitulate genetically complex neurological disorders and reveal novel insights into molecular and genetic mechanisms of neurological disorders. In this review, we describe recent progress in the generation of 2D, 3D, and BBB models from iPSCs and further discuss their limitations, advantages, and future ventures. This review also covers the current status of applications of 2D, 3D, and BBB models in drug screening, precision medicine, and modeling a wide range of neurological diseases (e.g., neurodegenerative diseases, neurodevelopmental disorders, brain injury, and neuropsychiatric disorders). © 2019 American Physiological Society. Compr Physiol 9:565-611, 2019.

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.

Protective effects of primary neural stem cell treatment in ischemic stroke models

Strokes are a major cause of neurological disability. Stem cell replacement therapy is a potential novel strategy of treating patients that have experienced strokes. The present study examined the protective role of neural stem cell (NSC) administration in oxygen-glucose deprivation (OGD) injury and ischemic stroke animal models. Primary cultured embryonic NSCs and brain microvascular endothelial cells were indirectly co-cultured for in vitro testing. A rat model of embolic middle cerebral artery occlusion (MCAO) was used to assess the morphological and functional changes that occur following treatment with NSCs. The role of the phosphoinositide 3-kinase/protein kinase b/glycogen synthase kinase 3β (PI3K/Akt/GSK-3β) signaling pathway in the neuroprotective effects of NSC treatment was also determined. It was demonstrated in vivo and in vitro that NSC administration may attenuate the brain injury caused by stroke. Furthermore, the results suggest that activation of PI3k/Akt/GSK-3β signaling pathway serves a role in attenuating OGD injury. Inflammation, synaptic remodeling and autophagy may be improved following NSC treatment and behavioral testing suggests that treatment with NSCs improves functional recovery in rats following MCAO.

Neurovascular Communication during CNS Development

A precise communication between the nervous and the vascular systems is crucial for proper formation and function of the central nervous system (CNS). Interestingly, this communication does not only occur by neural cells regulating the growth and properties of the vasculature, but new studies show that blood vessels actively control different neurodevelopmental processes. Here, we review the current knowledge on how neurons in particular influence growing blood vessels during CNS development and on how vessels participate in shaping the neural compartment. We also review the identified molecular mechanisms of this bidirectional communication.

Appearance of β-dystroglycan precedes the formation of glio-vascular end-feet in developing rat brain

Dystroglycan has an important role in binding of perivascular glial end-feet tothe basal lamina. Its β-subunit is localized in the glial end-feet. The investigation period lasted from E(embryonic day)12 to E20. Laminin and β-dystroglycan were detected by immunohistochemistry, the glial localization of the latter one was supported  by electron microscopy. The immatureglial structures were visualized by the immunostaining of nestin. The β-dystroglycan immunoreactivity appeared at E16 following the laminin of basal lamina but preceding the perivascular processes of radial glia (E18) and astrocyte-like cells (E20). It occurred in cell bodies which attached to the vessels directly but not with vascular processes and end-feet. The presence of β-dystroglycan in such immature cells may promote their differentiation to perivascular astrocytes and influence the formation of the glio-vascular processes.

Blood-brain barrier-supported neurogenesis in healthy and diseased brain

Adult neurogenesis is one of the most important mechanisms contributing to brain development, learning, and memory. Alterations in neurogenesis underlie a wide spectrum of brain diseases. Neurogenesis takes place in highly specialized neurogenic niches. The concept of neurogenic niches is becoming widely accepted due to growing evidence of the important role of the microenvironment established in the close vicinity to stem cells in order to provide adequate control of cell proliferation, differentiation, and apoptosis. Neurogenic niches represent the platform for tight integration of neurogenesis and angiogenesis supported by specific properties of cerebral microvessel endothelial cells contributing to establishment of partially compromised blood-brain barrier (BBB) for the adjustment of local conditions to the current metabolic needs of stem and progenitor cells. Here, we review up-to-date data on microvascular dynamics in activity-dependent neurogenesis, specific properties of BBB in neurogenic niches, endothelial-driven mechanisms of clonogenic activity, and future perspectives for reconstructing the neurogenic niches in vitro.

Modeling the blood-brain barrier: Beyond the endothelial cells

The blood-brain barrier (BBB) regulates the transport of ions, nutrients, and metabolites to help maintain proper brain function. This restrictive interface formed by brain microvascular endothelial cells excludes the majority of small and large molecule drugs from entering the brain, and blood-brain barrier dysfunction is a signature of many neurological diseases. Thus, in vitro models of the BBB based on brain endothelial cells have been developed to facilitate screening drugs for BBB permeability. However, while brain endothelial cells form the main interface, they work in concert with other brain-resident cells such as neural progenitor cells, pericytes, astrocytes, and neurons to form the neurovascular unit (NVU). Importantly, non-endothelial cells of the NVU play key roles in eliciting BBB phenotypes and in regulating the dynamic responses of the BBB to brain activity and disease. As a result, emerging in vitro BBB models have incorporated these NVU cell types in addition to endothelial cells. These multicellular BBB or NVU models have found increasing application not only in drug screening, but also in studying complex cellular and molecular mechanisms underlying BBB biology and disease.

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

The blood-brain barrier (BBB) is composed of specialized endothelial cells that are critical to neurological health. A key tool for understanding human BBB development and its role in neurological disease is a reliable and scalable source of functional brain microvascular endothelial cells (BMECs). Human pluripotent stem cells (hPSCs) can theoretically generate unlimited quantities of any cell lineage in vitro, including BMECs, for disease modeling, drug screening, and cell-based therapies. We demonstrate a facile, chemically defined method to differentiate hPSCs to BMECs in a developmentally relevant progression via small-molecule activation of key signaling pathways. hPSCs are first induced to mesoderm commitment by activating canonical Wnt signaling. Next, these mesoderm precursors progress to endothelial progenitors, and treatment with retinoic acid leads to acquisition of BBB-specific markers and phenotypes. hPSC-derived BMECs generated via this protocol exhibit endothelial properties, including tube formation and low-density lipoprotein uptake, as well as efflux transporter activities characteristic of BMECs. Notably, these cells exhibit high transendothelial electrical resistance above 3000 ohm·cm2. These hPSC-derived BMECs serve as a robust human in vitro BBB model that can be used to study brain disease and inform therapeutic development.

Neuroinflammation and Infection: Molecular Mechanisms Associated with Dysfunction of Neurovascular Unit

Neuroinflammation is a complex inflammatory process in the central nervous system, which is sought to play an important defensive role against various pathogens, toxins or factors that induce neurodegeneration. The onset of neurodegenerative diseases and various microbial infections are counted as stimuli that can challenge the host immune system and trigger the development of neuroinflammation. The homeostatic nature of neuroinflammation is essential to maintain the neuroplasticity. Neuroinflammation is regulated by the activity of neuronal, glial, and endothelial cells within the neurovascular unit, which serves as a “platform” for the coordinated action of pro- and anti-inflammatory mechanisms. Production of inflammatory mediators (cytokines, chemokines, reactive oxygen species) by brain resident cells or cells migrating from the peripheral blood, results in the impairment of blood-brain barrier integrity, thereby further affecting the course of local inflammation. In this review, we analyzed the most recent data on the central nervous system inflammation and focused on major mechanisms of neurovascular unit dysfunction caused by neuroinflammation and infections.

Paving the Way Toward Complex Blood-Brain Barrier Models Using Pluripotent Stem Cells

A tissue with great need to be modeled in vitro is the blood-brain barrier (BBB). The BBB is a tight barrier that covers all blood vessels in the brain and separates the brain microenvironment from the blood system. It consists of three cell types [neurovascular unit (NVU)] that contribute to the unique tightness and selective permeability of the BBB and has been shown to be disrupted in many diseases and brain disorders, such as vascular dementia, stroke, multiple sclerosis, and Alzheimer's disease. Given the progress that pluripotent stem cells (PSCs) have made in the past two decades, it is now possible to produce many cell types from the BBB and even partially recapitulate this complex tissue in vitro. In this review, we summarize the most recent developments in PSC differentiation and modeling of the BBB. We also suggest how patient-specific human-induced PSCs could be used to model BBB dysfunction in the future. Lastly, we provide perspectives on how to improve production of the BBB in vitro, for example by improving pericyte differentiation protocols and by better modeling the NVU in the dish.

An isogenic blood-brain barrier model comprising brain endothelial cells, astrocytes, and neurons derived from human induced pluripotent stem cells

The blood-brain barrier (BBB) is critical in maintaining a physical and metabolic barrier between the blood and the brain. The BBB consists of brain microvascular endothelial cells (BMECs) that line the brain vasculature and combine with astrocytes, neurons and pericytes to form the neurovascular unit. We hypothesized that astrocytes and neurons generated from human-induced pluripotent stem cells (iPSCs) could induce BBB phenotypes in iPSC-derived BMECs, creating a robust multicellular human BBB model. To this end, iPSCs were used to form neural progenitor-like EZ-spheres, which were in turn differentiated to neurons and astrocytes, enabling facile neural cell generation. The iPSC-derived astrocytes and neurons induced barrier tightening in primary rat BMECs indicating their BBB inductive capacity. When co-cultured with human iPSC-derived BMECs, the iPSC-derived neurons and astrocytes significantly elevated trans-endothelial electrical resistance, reduced passive permeability, and improved tight junction continuity in the BMEC cell population, while p-glycoprotein efflux transporter activity was unchanged. A physiologically relevant neural cell mixture of one neuron: three astrocytes yielded optimal BMEC induction properties. Finally, an isogenic multicellular BBB model was successfully demonstrated employing BMECs, astrocytes, and neurons from the same donor iPSC source. It is anticipated that such an isogenic facsimile of the human BBB could have applications in furthering understanding the cellular interplay of the neurovascular unit in both healthy and diseased humans. Read the Editorial Highlight for this article on page 843.

A 3D neurovascular microfluidic model consisting of neurons, astrocytes and cerebral endothelial cells as a blood-brain barrier

The neurovascular unit is a complex, interdependent system composed of neurons and neural supporting cells, such as astrocytes, as well as cells that comprise the vascular system including endothelial cells, pericytes, and smooth muscle cells. Each cell type in the neurovascular unit plays an essential role, either in transmitting and processing neural signals or in maintaining the appropriate microenvironmental conditions for healthy neural function. In vitro neurovascular models can be useful for understanding the different roles and functions of the cells composing the neurovascular unit, as well as for assessing the effects on neural function of therapeutic compounds after crossing the endothelial barrier. Here, we report a novel three-dimensional neurovascular microfluidic model consisting of primary rat astrocytes and neurons together with human cerebral microvascular endothelial cells. These three cell types in our neurovascular chip (NVC) show distinct cell type-specific morphological characteristics and functional properties. In particular, morphological and functional analysis of neurons enables quantitative assessment of neuronal responses, while human cerebral endothelial cells form monolayers with size-selective permeability similar to existing in vitro blood-brain barrier (BBB) models.

New experimental models of the blood-brain barrier for CNS drug discovery

INTRODUCTION

The blood-brain barrier (BBB) is a dynamic biological interface which actively controls the passage of substances between the blood and the central nervous system (CNS). From a biological and functional standpoint, the BBB plays a crucial role in maintaining brain homeostasis inasmuch that deterioration of BBB functions are prodromal to many CNS disorders. Conversely, the BBB hinders the delivery of drugs targeting the brain to treat a variety of neurological diseases. Area covered: This article reviews recent technological improvements and innovation in the field of BBB modeling including static and dynamic cell-based platforms, microfluidic systems and the use of stem cells and 3D printing technologies. Additionally, the authors laid out a roadmap for the integration of microfluidics and stem cell biology as a holistic approach for the development of novel in vitro BBB platforms. Expert opinion: Development of effective CNS drugs has been hindered by the lack of reliable strategies to mimic the BBB and cerebrovascular impairments in vitro. Technological advancements in BBB modeling have fostered the development of highly integrative and quasi- physiological in vitro platforms to support the process of drug discovery. These advanced in vitro tools are likely to further current understanding of the cerebrovascular modulatory mechanisms.

Post-ischemic salubrinal treatment results in a neuroprotective role in global cerebral ischemia

This study describes the neuroprotective effect of treatment with salubrinal 1 and 24 h following 15 min of ischemia in a two-vessel occlusion model of global cerebral ischemia. The purpose of this study was to determine if salubrinal, an enhancer of the unfolded protein response, reduces the neural damage modulating the inflammatory response. The study was performed in CA1 and CA3 hippocampal areas as well as in the cerebral cortex whose different vulnerability to ischemic damage is widely described. Characterization of proteins was made by western blot, immunofluorescence, and ELISA, whereas mRNA levels were measured by Quantitative PCR. The salubrinal treatment decreased the cell demise in CA1 at 7 days as well as the levels of matrix metalloprotease 9 (MMP-9) in CA1 and cerebral cortex at 48 h and ICAM-1 and VCAM-1 cell adhesion molecules. However, increases in tumor necrosis factor α and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) inflammatory markers were observed at 24 h. Glial fibrillary acidic protein levels were not modified by salubrinal treatment in CA1 and cerebral cortex. We describe a neuroprotective effect of the post-ischemic treatment with salubrinal, measured as a decrease both in CA1 cell demise and in the blood-brain barrier impairment. We hypothesize that the ability of salubrinal to counteract the CA1 cell demise is because of a reduced ability of this structure to elicit unfolded protein response which would account for its greater ischemic vulnerability. Data of both treated and non-treated animals suggest that the neurovascular unit present a structure-dependent response to ischemia and a different course time for CA1/cerebral cortex compared with CA3. Finally, our study reveals a high responsiveness of endothelial cells to salubrinal in contrast to the limited responsiveness of astrocytes. The alleviation of ER stress by enhancing UPR with salubrinal treatment reduces the ischemic damage. This effect varies across the different neurovascular unit cell types. The salubrinal neuroprotective effect on CA1 supports differences in neurovascular unit for different brain regions and involves the inflammatory response and its time course. Thus, UPR modulation could be a therapeutic target in cerebral ischemia.

Identification and dynamic regulation of tight junction protein expression in human neural stem cells

Recent reports indicate that neural stem cells (NSCs) exist in a cluster-like formation in close proximity to cerebral microvessels. Similar appearing clusters can be seen ex vivo in NSC cultures termed neurospheres. It is known that this neurosphere configuration is important for preserving stemness and a proliferative state. How NSCs form neurospheres or neuroclusters remains largely undetermined. In this study, we show that primary human NSCs express the tight junction proteins (TJPs): zonula occludens-1 (ZO-1), occludin, claudin-1, -3, -5, and -12. The relative mRNA expression was measured by quantitative polymerase chain reaction, and protein expression was confirmed by flow cytometry and immunofluorescence microscopy. Our results show that downregulation of TJPs occurs as neuronal differentiation is induced, suggesting that control of TJPs may be tied to the neuronal differentiation program. Importantly, upon specific knockdown of the accessory TJP, ZO-1, undifferentiated NSCs showed decreased levels of key stem cell markers. Taken together, our results indicate that TJPs possibly aid in maintaining the intercellular configuration of NSCs and that reduction in TJP expression consequently affects the stemness status.

Concise review: tissue-specific microvascular endothelial cells derived from human pluripotent stem cells

Accumulating evidence suggests that endothelial cells (ECs) display significant heterogeneity across tissue types, playing an important role in tissue regeneration and homeostasis. Recent work demonstrating the derivation of tissue-specific microvascular endothelial cells (TS-MVECs) from human pluripotent stem cells (hPSCs) has ignited the potential to generate tissue-specific models which may be applied to regenerative medicine and in vitro modeling applications. Here, we review techniques by which hPSC-derived TS-MVECs have been made to date and discuss how current hPSC-EC differentiation protocols may be directed toward tissue-specific fates. We begin by discussing the nature of EC tissue specificity in vivo and review general hPSC-EC differentiation protocols generated over the last decade. Finally, we describe how specificity can be integrated into hPSC-EC protocols to generate hPSC-derived TS-MVECs in vitro, including EC and parenchymal cell coculture, directed differentiation, and direct reprogramming strategies.

Glycolysis-mediated control of blood-brain barrier development and function

The blood-brain barrier (BBB) consists of differentiated cells integrating in one ensemble to control transport processes between the central nervous system (CNS) and peripheral blood. Molecular organization of BBB affects the extracellular content and cell metabolism in the CNS. Developmental aspects of BBB attract much attention in recent years, and barriergenesis is currently recognized as a very important and complex mechanism of CNS development and maturation. Metabolic control of angiogenesis/barriergenesis may be provided by glucose utilization within the neurovascular unit (NVU). The role of glycolysis in the brain has been reconsidered recently, and it is recognized now not only as a process active in hypoxic conditions, but also as a mechanism affecting signal transduction, synaptic activity, and brain development. There is growing evidence that glycolysis-derived metabolites, particularly, lactate, affect barriergenesis and functioning of BBB. In the brain, lactate produced in astrocytes or endothelial cells can be transported to the extracellular space via monocarboxylate transporters (MCTs), and may act on the adjoining cells via specific lactate receptors. Astrocytes are one of the major sources of lactate production in the brain and significantly contribute to the regulation of BBB development and functioning. Active glycolysis in astrocytes is required for effective support of neuronal activity and angiogenesis, while endothelial cells regulate bioavailability of lactate for brain cells adjusting its bidirectional transport through the BBB. In this article, we review the current knowledge with regard to energy production in endothelial and astroglial cells within the NVU. In addition, we describe lactate-driven mechanisms and action of alternative products of glucose metabolism affecting BBB structural and functional integrity in developing and mature brain.

Three-dimensional culture conditions differentially affect astrocyte modulation of brain endothelial barrier function in response to transforming growth factor β1

Blood-brain barrier (BBB) function is regulated by dynamic interactions among cell types within the neurovascular unit, including astrocytes and endothelial cells. Co-culture models of the BBB typically involve astrocytes seeded on two-dimensional (2D) surfaces, which recent studies indicate cause astrocytes to express a phenotype similar to that of reactive astrocytes in situ. We hypothesized that the culture conditions of astrocytes would differentially affect their ability to modulate BBB function in vitro. Brain endothelial cells were grown alone or in co-culture with astrocytes. Astrocytes were grown either as conventional (2D) monolayers, or in a collagen-based gel which allows them to grow in a three-dimensional (3D) construct. Astrocytes were viable in 3D conditions, and displayed a marked reduction in their expression of glial fibrillary acidic protein (GFAP), suggesting reduced activation. Stimulation of astrocytes with transforming growth factor (TGF)β1 decreased transendothelial electrical resistance (TEER) and reduced expression of claudin-5 in co-cultures, whereas treatment of endothelial cells in the absence of astrocytes was without effect. The effect of TGFβ1 on TEER was significantly more pronounced in endothelial cells cultured with 3D astrocytes compared to 2D astrocytes. These results demonstrate that astrocyte culture conditions differentially affect their ability to modulate brain endothelial barrier function, and suggest a direct relationship between reactive gliosis and BBB permeability. Moreover, these studies demonstrate the potential importance of physiologically relevant culture conditions to in vitro modeling of disease processes that affect the neurovascular unit.

Measuring direct current trans-epithelial electrical resistance in organ-on-a-chip microsystems

Trans-epithelial electrical resistance (TEER) measurements are widely used as real-time, non-destructive, and label-free measurements of epithelial and endothelial barrier function. TEER measurements are ideal for characterizing tissue barrier function in organs-on-chip studies for drug testing and investigation of human disease models; however, published reports using this technique have reported highly conflicting results even with identical cell lines and experimental setups. The differences are even more dramatic when comparing measurements in conventional Transwell systems with those obtained in microfluidic systems. Our goal in this work was therefore to enhance the fidelity of TEER measurements in microfluidic organs-on-chips, specifically using direct current (DC) measurements of TEER, as this is the most widely used method reported in the literature. Here we present a mathematical model that accounts for differences measured in TEER between microfluidic chips and Transwell systems, which arise from differences in device geometry. The model is validated by comparing TEER measurements obtained in a microfluidic gut-on-a-chip device versus in a Transwell culture system. Moreover, we show that even small gaps in cell coverage (e.g., 0.4%) are sufficient to cause a significant (~80%) drop in TEER. Importantly, these findings demonstrate that TEER measurements obtained in microfluidic systems, such as organs-on-chips, require special consideration, specifically when results are to be compared with measurements obtained from Transwell systems.

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

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

The molecular, cellular, and morphological components of blood-brain barrier development during embryogenesis

The blood brain barrier (BBB) is a hallmark of blood vessels in the brain and functions to protect the brain from unwanted blood born materials, support the unique metabolic needs of the brain, and define a stable environment crucial for brain homeostasis. The temporal profile of BBB development was long debated until recent studies produced convincing evidence demonstrating that the BBB is established and functional during embryogenesis. Here we review research focused on the molecular, cellular and morphological characteristics of BBB development. Our review discusses the precise temporal profile of BBB formation, the development of endothelial cell ultrastructure and the molecular components that provide sealing and transporting properties, the molecular pathways involved in the induction of BBB specific endothelial cell differentiation, the signaling pathways driving developmental angiogenesis versus barrier-genesis, and finally the contribution of other cell types to BBB formation. We examine aspects of BBB development that are still unresolved while highlighting research tools that could provide new insight to answer these open questions.

"You Shall Not Pass"-tight junctions of the blood brain barrier

The structure and function of the barrier layers restricting the free diffusion of substances between the central nervous system (brain and spinal cord) and the systemic circulation is of great medical interest as various pathological conditions often lead to their impairment. Excessive leakage of blood-borne molecules into the parenchyma and the concomitant fluctuations in the microenvironment following a transient breakdown of the blood-brain barrier (BBB) during ischemic/hypoxic conditions or because of an autoimmune disease are detrimental to the physiological functioning of nervous tissue. On the other hand, the treatment of neurological disorders is often hampered as only minimal amounts of therapeutic agents are able to penetrate a fully functional BBB or blood cerebrospinal fluid barrier. An in-depth understanding of the molecular machinery governing the establishment and maintenance of these barriers is necessary to develop rational strategies allowing a controlled delivery of appropriate drugs to the CNS. At the basis of such tissue barriers are intimate cell-cell contacts (zonulae occludentes, tight junctions) which are present in all polarized epithelia and endothelia. By creating a paracellular diffusion constraint TJs enable the vectorial transport across cell monolayers. More recent findings indicate that functional barriers are already established during development, protecting the fetal brain. As an understanding of the biogenesis of TJs might reveal the underlying mechanisms of barrier formation during ontogenic development numerous in vitro systems have been developed to study the assembly and disassembly of TJs. In addition, monitoring the stage-specific expression of TJ-associated proteins during development has brought much insight into the “developmental tightening” of tissue barriers. Over the last two decades a detailed molecular map of transmembrane and cytoplasmic TJ-proteins has been identified. These proteins not only form a cell-cell adhesion structure, but integrate various signaling pathways, thereby directly or indirectly impacting upon processes such as cell-cell adhesion, cytoskeletal rearrangement, and transcriptional control. This review will provide a brief overview on the establishment of the BBB during embryonic development in mammals and a detailed description of the ultrastructure, biogenesis, and molecular composition of epithelial and endothelial TJs will be given.

Neural Regulation of CNS Angiogenesis During Development

Vertebrates have evolved a powerful vascular system that involves close interactions between blood vessels and target tissues. Vascular biology had been mostly focused on the study of blood vessels for decades, which has generated large bodies of knowledge on vascular cell development, function and pathology. We argue that the prime time has arrived for vascular research on vessel-tissue interactions, especially target tissue regulation of vessel development. The central nervous system (CNS) requires a highly efficient vascular system for oxygen and nutrient transport as well as waste disposal. Therefore, neurovascular interaction is an excellent entry point to understanding target tissue regulation of blood vessel development. In this review, we summarize signaling pathways that transmit information from neural cells to blood vessels during development and the mechanisms by which they regulate each step of CNS angiogenesis. We also review important mechanisms of neural regulation of blood-brain barrier establishment and maturation, highlighting different functions of neural progenitor cells and pericytes. Finally, we evaluate potential contribution of malfunctioning neurovascular signaling to the development of brain vascular diseases and discuss how neurovascular interactions could be involved in brain tumor angiogenesis.

In vitro models of the blood-brain barrier for the study of drug delivery to the brain

The most important obstacle to the drug delivery into the brain is the presence of the blood-brain barrier, which limits the traffic of substances between the blood and the nervous tissue. Therefore, adequate in vitro models need to be developed in order to characterize the penetration properties of drug candidates into the central nervous system. This review article summarizes the presently used and the most promising in vitro BBB models based on the culture of brain endothelial cells. Robust models can be obtained using primary porcine brain endothelial cells and rodent coculture models, which have low paracellular permeability and express functional efflux transporters, showing good correlation of drug penetration data with in vivo results. Models mimicking the in vivo anatomophysiological complexity of the BBB are also available, including triple coculture (culture of brain endothelial cells in the presence of pericytes and astrocytes), dynamic, and microfluidic models; however, these are not suitable for rapid, high throughput studies. Potent human cell lines would be needed for easily available and reproducible models which avoid interspecies differences.

A novel blood-brain barrier co-culture system for drug targeting of Alzheimer's disease: establishment by using acitretin as a model drug

In the pathogenesis of Alzheimer’s disease (AD) the homeostasis of amyloid precursor protein (APP) processing in the brain is impaired. The expression of the competing proteases ADAM10 (a disintegrin and metalloproteinase 10) and BACE-1 (beta site APP cleaving enzyme 1) is shifted in favor of the A-beta generating enzyme BACE-1. Acitretin–a synthetic retinoid–e.g., has been shown to increase ADAM10 gene expression, resulting in a decreased level of A-beta peptides within the brain of AD model mice and thus is of possible value for AD therapy. A striking challenge in evaluating novel therapeutically applicable drugs is the analysis of their potential to overcome the blood-brain barrier (BBB) for central nervous system targeting. In this study, we established a novel cell-based bio-assay model to test ADAM10-inducing drugs for their ability to cross the BBB. We therefore used primary porcine brain endothelial cells (PBECs) and human neuroblastoma cells (SH-SY5Y) transfected with an ADAM10-promoter luciferase reporter vector in an indirect co-culture system. Acitretin served as a model substance that crosses the BBB and induces ADAM10 expression. We ensured that ADAM10-dependent constitutive APP metabolism in the neuronal cells was unaffected under co-cultivation conditions. Barrier properties established by PBECs were augmented by co-cultivation with SH-SY5Y cells and they remained stable during the treatment with acitretin as demonstrated by electrical resistance measurement and permeability-coefficient determination. As a consequence of transcellular acitretin transport measured by HPLC, the activity of the ADAM10-promoter reporter gene was significantly increased in co-cultured neuronal cells as compared to vehicle-treated controls. In the present study, we provide a new bio-assay system relevant for the study of drug targeting of AD. This bio-assay can easily be adapted to analyze other Alzheimer- or CNS disease-relevant targets in neuronal cells, as their therapeutical potential also depends on the ability to penetrate the BBB.

Novel insights into the development and maintenance of the blood-brain barrier

The blood-brain barrier (BBB) is essential for maintaining homeostasis within the central nervous system (CNS) and is a prerequisite for proper neuronal function. The BBB is localized to microvascular endothelial cells that strictly control the passage of metabolites into and out of the CNS. Complex and continuous tight junctions and lack of fenestrae combined with low pinocytotic activity make the BBB endothelium a tight barrier for water soluble moleucles. In combination with its expression of specific enzymes and transport molecules, the BBB endothelium is unique and distinguishable from all other endothelial cells in the body. During embryonic development, the CNS is vascularized by angiogenic sprouting from vascular networks originating outside of the CNS in a precise spatio-temporal manner. The particular barrier characteristics of BBB endothelial cells are induced during CNS angiogenesis by cross-talk with cellular and acellular elements within the developing CNS. In this review, we summarize the currently known cellular and molecular mechanisms mediating brain angiogenesis and introduce more recently discovered CNS-specific pathways (Wnt/β-catenin, Norrin/Frizzled4 and hedgehog) and molecules (GPR124) that are crucial in BBB differentiation and maturation. Finally, based on observations that BBB dysfunction is associated with many human diseases such as multiple sclerosis, stroke and brain tumors, we discuss recent insights into the molecular mechanisms involved in maintaining barrier characteristics in the mature BBB endothelium.

A retinoic acid-enhanced, multicellular human blood-brain barrier model derived from stem cell sources

Blood-brain barrier (BBB) models are often used to investigate BBB function and screen brain-penetrating therapeutics, but it has been difficult to construct a human model that possesses an optimal BBB phenotype and is readily scalable. To address this challenge, we developed a human in vitro BBB model comprising brain microvascular endothelial cells (BMECs), pericytes, astrocytes and neurons derived from renewable cell sources. First, retinoic acid (RA) was used to substantially enhance BBB phenotypes in human pluripotent stem cell (hPSC)-derived BMECs, particularly through adherens junction, tight junction, and multidrug resistance protein regulation. RA-treated hPSC-derived BMECs were subsequently co-cultured with primary human brain pericytes and human astrocytes and neurons derived from human neural progenitor cells (NPCs) to yield a fully human BBB model that possessed significant tightness as measured by transendothelial electrical resistance (~5,000 Ωxcm(2)). Overall, this scalable human BBB model may enable a wide range of neuroscience studies.