Differentiating embryonic neural progenitor cells induce blood-brain barrier properties

Purification and culture of central nervous system endothelial cells

Blood vessels are critical for delivering oxygen and nutrients to all tissues in the body. This is especially important in the central nervous system, which is extremely sensitive to hypoxia and ischemia. Blood vessels are made of two main cell types: endothelial cells and mural cells. Endothelial cells form the walls of the blood vessels that generate a lumen through which blood flows. Mural cells are support cells thought to be involved in vessel contractility, vascular remodeling, and regulation of endothelial permeability. On large vessels, including arteries and veins, mural cells are termed vascular smooth muscle cells. On the small vessels of the capillary bed, they are called pericytes. Here, we provide a brief introduction to the methods for purification of endothelial cells, including an immunopanning method that we developed for isolating these cells from the rodent brain and optic nerve.

'Sealing off the CNS': cellular and molecular regulation of blood-brain barriergenesis

From their initial ingression into the neural tube to the established, adult vascular plexus, blood vessels within the CNS are truly unique. Covered by a virtually continuous layer of perivascular cells and astrocytic endfeet and connected by specialized cell-cell junctional contacts, mature CNS blood vessels simultaneously provide nutritive blood flow and protect the neural milieu from potentially disruptive or harmful molecules and cells flowing through the vessel lumen. In this review we will discuss how the CNS vasculature acquires blood-brain barrier (BBB) properties with a specific focus on recent work identifying the cell types and molecular pathways that orchestrate barriergenesis.

Modelling the endothelial blood-CNS barriers: a method for the production of robust in vitro models of the rat blood-brain barrier and blood-spinal cord barrier

Background

Modelling the blood-CNS barriers of the brain and spinal cord in vitro continues to provide a considerable challenge for research studying the passage of large and small molecules in and out of the central nervous system, both within the context of basic biology and for pharmaceutical drug discovery. Although there has been considerable success over the previous two decades in establishing useful in vitro primary endothelial cell cultures from the blood-CNS barriers, no model fully mimics the high electrical resistance, low paracellular permeability and selective influx/efflux characteristics of the in vivo situation. Furthermore, such primary-derived cultures are typically labour-intensive and generate low yields of cells, limiting scope for experimental work. We thus aimed to establish protocols for the high yield isolation and culture of endothelial cells from both rat brain and spinal cord. Our aim was to optimise in vitro conditions for inducing phenotypic characteristics in these cells that were reminiscent of the in vivo situation, such that they developed into tight endothelial barriers suitable for performing investigative biology and permeability studies.

Methods

Brain and spinal cord tissue was taken from the same rats and used to specifically isolate endothelial cells to reconstitute as in vitro blood-CNS barrier models. Isolated endothelial cells were cultured to expand the cellular yield and then passaged onto cell culture inserts for further investigation. Cell culture conditions were optimised using commercially available reagents and the resulting barrier-forming endothelial monolayers were characterised by functional permeability experiments and in vitro phenotyping by immunocytochemistry and western blotting.

Results

Using a combination of modified handling techniques and cell culture conditions, we have established and optimised a protocol for the in vitro culture of brain and, for the first time in rat, spinal cord endothelial cells. High yields of both CNS endothelial cell types can be obtained, and these can be passaged onto large numbers of cell culture inserts for in vitro permeability studies. The passaged brain and spinal cord endothelial cells are pure and express endothelial markers, tight junction proteins and intracellular transport machinery. Further, both models exhibit tight, functional barrier characteristics that are discriminating against large and small molecules in permeability assays and show functional expression of the pharmaceutically important P-gp efflux transporter.

Conclusions

Our techniques allow the provision of high yields of robust sister cultures of endothelial cells that accurately model the blood-CNS barriers in vitro. These models are ideally suited for use in studying the biology of the blood-brain barrier and blood-spinal cord barrier in vitro and for pre-clinical drug discovery.

Derivation of blood-brain barrier endothelial cells from human pluripotent stem cells

The blood-brain barrier (BBB) is crucial to the health of the brain and is often compromised in neurological disease. Moreover, because of its barrier properties, this endothelial interface restricts uptake of neurotherapeutics. Thus, a renewable source of human BBB endothelium could spur brain research and pharmaceutical development. Here we show that endothelial cells derived from human pluripotent stem cells (hPSCs) acquire BBB properties when co-differentiated with neural cells that provide relevant cues, including those involved in Wnt/β-catenin signaling. The resulting endothelial cells have many BBB attributes, including well-organized tight junctions, appropriate expression of nutrient transporters and polarized efflux transporter activity. Notably, they respond to astrocytes, acquiring substantial barrier properties as measured by transendothelial electrical resistance (1,450 ± 140 Ω cm2), and they possess molecular permeability that correlates well with in vivo rodent blood-brain transfer coefficients.

Modeling the blood-brain barrier using stem cell sources

The blood–brain barrier (BBB) is a selective endothelial interface that controls trafficking between the bloodstream and brain interstitial space. During development, the BBB arises as a result of complex multicellular interactions between immature endothelial cells and neural progenitors, neurons, radial glia, and pericytes. As the brain develops, astrocytes and pericytes further contribute to BBB induction and maintenance of the BBB phenotype. Because BBB development, maintenance, and disease states are difficult and time-consuming to study in vivo, researchers often utilize in vitro models for simplified analyses and higher throughput. The in vitro format also provides a platform for screening brain-penetrating therapeutics. However, BBB models derived from adult tissue, especially human sources, have been hampered by limited cell availability and model fidelity. Furthermore, BBB endothelium is very difficult if not impossible to isolate from embryonic animal or human brain, restricting capabilities to model BBB development in vitro. In an effort to address some of these shortcomings, advances in stem cell research have recently been leveraged for improving our understanding of BBB development and function. Stem cells, which are defined by their capacity to expand by self-renewal, can be coaxed to form various somatic cell types and could in principle be very attractive for BBB modeling applications. In this review, we will describe how neural progenitor cells (NPCs), the in vitro precursors to neurons, astrocytes, and oligodendrocytes, can be used to study BBB induction. Next, we will detail how these same NPCs can be differentiated to more mature populations of neurons and astrocytes and profile their use in co-culture modeling of the adult BBB. Finally, we will describe our recent efforts in differentiating human pluripotent stem cells (hPSCs) to endothelial cells with robust BBB characteristics and detail how these cells could ultimately be used to study BBB development and maintenance, to model neurological disease, and to screen neuropharmaceuticals.

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

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

(-)-Epigallocatechin gallate inhibits endotoxin-induced expression of inflammatory cytokines in human cerebral microvascular endothelial cells

Background

(−)-Epigallocatechin gallate (EGCG) is a major polyphenol component of green tea that has antioxidant activities. Lipopolysaccharide (LPS) induces inflammatory cytokine production and impairs blood–brain barrier (BBB) integrity. We examined the effect of EGCG on LPS-induced expression of the inflammatory cytokines in human cerebral microvascular endothelial cells (hCMECs) and BBB permeability.

Methods

The expression of TNF-α, IL-1β and monocyte chemotactic protein-1 (MCP-1/CCL2) was determined by quantitative real time PCR (qRT-PCR) and ELISA. Intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule (VCAM) in hCMECs were examined by qRT-PCR and Western blotting. Monocytes that adhered to LPS-stimulated endothelial cells were measured by monocyte adhesion assay. Tight junctional factors were detected by qRT-PCR (Claudin 5 and Occludin) and immunofluorescence staining (Claudin 5 and ZO-1). The permeability of the hCMEC monolayer was determined by fluorescence spectrophotometry of transmembrane fluorescin and transendothelial electrical resistance (TEER). NF-kB activation was measured by luciferase assay.

Results

EGCG significantly suppressed the LPS-induced expression of IL-1β and TNF-α in hCMECs. EGCG also inhibited the expression of MCP-1/CCL2, VCAM-1 and ICAM-1. Functional analysis showed that EGCG induced the expression of tight junction proteins (Occludin and Claudin-5) in hCMECs. Investigation of the mechanism showed that EGCG had the ability to inhibit LPS-mediated NF-κB activation. In addition, 67-kD laminin receptor was involved in the anti-inflammatory effect of EGCG.

Conclusions

Our results demonstrated that LPS induced inflammatory cytokine production in hCMECs, which could be attenuated by EGCG. These data indicate that EGCG has a therapeutic potential for endotoxin-mediated endothelial inflammation.

Differentiation state determines neural effects on microvascular endothelial cells

Growing evidence indicates that nerves and capillaries interact paracrinely in uninjured skin and cutaneous wounds. Although mature neurons are the predominant neural cell in the skin, neural progenitor cells have also been detected in uninjured adult skin. The aim of this study was to characterize differential paracrine effects of neural progenitor cells and mature sensory neurons on dermal microvascular endothelial cells. Our results suggest that neural progenitor cells and mature sensory neurons have unique secretory profiles and distinct effects on dermal microvascular endothelial cell proliferation, migration, and nitric oxide production. Neural progenitor cells and dorsal root ganglion neurons secrete different proteins related to angiogenesis. Specific to neural progenitor cells were dipeptidyl peptidase-4, IGFBP-2, pentraxin-3, serpin f1, TIMP-1, TIMP-4 and VEGF. In contrast, endostatin, FGF-1, MCP-1 and thrombospondin-2 were specific to dorsal root ganglion neurons. Microvascular endothelial cell proliferation was inhibited by dorsal root ganglion neurons but unaffected by neural progenitor cells. In contrast, microvascular endothelial cell migration in a scratch wound assay was inhibited by neural progenitor cells and unaffected by dorsal root ganglion neurons. In addition, nitric oxide production by microvascular endothelial cells was increased by dorsal root ganglion neurons but unaffected by neural progenitor cells.

Characterization of a microfluidic in vitro model of the blood-brain barrier (μBBB)

The blood-brain barrier (BBB), a unique selective barrier for the central nervous system (CNS), hinders the passage of most compounds to the CNS, complicating drug development. Innovative in vitro models of the BBB can provide useful insights into its role in CNS disease progression and drug delivery. Static transwell models lack fluidic shear stress, while the conventional dynamic in vitro BBB lacks a thin dual cell layer interface. To address both limitations, we developed a microfluidic blood-brain barrier (μBBB) which closely mimics the in vivo BBB with a dynamic environment and a comparatively thin culture membrane (10 μm). To test validity of the fabricated BBB model, μBBBs were cultured with b.End3 endothelial cells, both with and without co-cultured C8-D1A astrocytes, and their key properties were tested with optical imaging, trans-endothelial electrical resistance (TEER), and permeability assays. The resultant imaging of ZO-1 revealed clearly expressed tight junctions in b.End3 cells, Live/Dead assays indicated high cell viability, and astrocytic morphology of C8-D1A cells were confirmed by ESEM and GFAP immunostains. By day 3 of endothelial culture, TEER levels typically exceeded 250 Ω cm(2) in μBBB co-cultures, and 25 Ω cm(2) for transwell co-cultures. Instantaneous transient drop in TEER in response to histamine exposure was observed in real-time, followed by recovery, implying stability of the fabricated μBBB model. Resultant permeability coefficients were comparable to previous BBB models, and were significantly increased at higher pH (>10). These results demonstrate that the developed μBBB system is a valid model for some studies of BBB function and drug delivery.

Cell transplantation: stem cells and precursor cells

Stem cells have been used to approach four different therapeutic repair strategies in spinal cord injury (SCI): (1) replacement of lost neurons, (2) replacement of oligodendrocytes to promote remyelination of demyelinated and/or regenerated axons, (3) providing a permissive substrate for axonal regeneration to overcome the intrinsic inhibition of surface molecules, and (4) engendering host repair. The first two strategies involve cell-specific differentiation of engrafted neural cells and the latter two may involve grafted neural or non-neural cells. The preclinical data for all of these approaches is at times contradictory and there is no consensus as to what type of stem cell is optimal to facilitate repair in specific injuries. Remyelination has been the most successful stem cell replacement strategy. Partial lineage restriction and pharmacological and/or genetic manipulation to express additional trophic support or restrict responses to host signals appears necessary for optimal neuronal and oligodendrocytic differentiation. However, these modifications will make their clinical application exceedingly difficult. Effects of grafted stem cells on abrogating host immune responses and engendering intrinsic repair is also a mechanism through which stem cells are likely therapeutically beneficial. While clinical trials with stem cell grafting into the injured spinal cord are ongoing, preclinical studies have yet to define mechanisms of action that can be definitively translated to those clinical approaches.

Optimization of endothelial cell growth in a murine in vitro blood-brain barrier model

In vitro cell culture models of the blood-brain barrier (BBB) are important tools used to study cellular physiology and brain disease therapeutics. Although the number of model configurations is expanding across neuroscience laboratories, it is not clear that any have been effectively optimized. A sequential screening study to identify optimal primary mouse endothelial cell parameter set points, grown alone and in combination with common model enhancements, including co-culturing with primary mouse or rat astrocytes and addition of biochemical agents in the media, was performed. A range of endothelial cell-seeding densities (1-8 × 10(5) cells/cm(2) ) and astrocyte-seeding densities (2-8 × 10(4) cells/cm(2) ) were studied over seven days in the system, and three distinct media-feeding strategies were compared to optimize biochemical agent exposure time. Implementation of all optimal set points increased transendothelial electrical resistance by over 200% compared to an initial model and established a suitable in vitro model for brain disease application studies. These results demonstrate the importance of optimizing cell culture growth, which is the most important parameter in creating an in vitro BBB model as it directly relates the model to the in vivo arrangement.

A novel co-culture model of the blood-retinal barrier based on primary retinal endothelial cells, pericytes and astrocytes

Loss of blood-retinal barrier (BRB) properties is an important feature in the pathology of diabetic macular edema (DME), but cellular mechanisms underlying BRB dysfunction are poorly understood. Therefore, we developed and characterized a novel in vitro BRB model, based on primary bovine retinal endothelial cells (BRECs). These cells were shown to maintain specific in vivo BRB properties by expressing high levels of the endothelial junction proteins occludin, claudin-5, VE-cadherin and ZO-1 at cell borders, and the specific pumps glucose transporter-1 (GLUT1) and efflux transporter P-glycoprotein (MDR1). To investigate the influence of pericytes and astrocytes on BRB maintenance in vitro, we compared five different co-culture BRB models, based on BRECs, bovine retinal pericytes (BRPCs) and rat glial cells. Co-cultures of BRECs with BRPCs and glial cells showed the highest trans-endothelial resistance (TEER) as well as decreased permeability of tracers after vascular endothelial growth factor (VEGF) stimulation, suggesting a major role for these cell types in maintaining barrier properties. To mimic the in vivo situation of DME, we stimulated BRECs with VEGF, which downregulated MDR1 and GLUT1 mRNA levels, transiently reduced expression levels of endothelial junctional proteins and altered their organization, increased the number of intercellular gaps in BRECs monolayers and influence the permeability of the model to differently-sized molecular tracers. Moreover, as has been shown in vivo, expression of plasmalemma vesicle-associated protein (PLVAP) was increased in endothelial cells in the presence of VEGF. This in vitro model is the first co-culture model of the BRB that mimicks in vivo VEGF-dependent changes occurring in DME.

Blood-brain barrier modeling with co-cultured neural progenitor cell-derived astrocytes and neurons

In vitro blood-brain barrier (BBB) models often consist of brain microvascular endothelial cells (BMECs) that are co-cultured with other cells of the neurovascular unit, such as astrocytes and neurons, to enhance BBB properties. Obtaining primary astrocytes and neurons for co-culture models can be laborious, while yield and heterogeneity of primary isolations can also be limiting. Neural progenitor cells (NPCs), because of their self-renewal capacity and ability to reproducibly differentiate into tunable mixtures of neurons and astrocytes, represent a facile, readily scalable alternative. To this end, differentiated rat NPCs were co-cultured with rat BMECs and shown to induce BBB properties such as elevated trans-endothelial electrical resistance, improved tight junction continuity, polarized p-glycoprotein efflux, and low passive permeability at levels indistinguishable from those induced by primary rat astrocyte co-culture. An NPC differentiation time of 12 days, with the presence of 10% fetal bovine serum, was found to be crucial for generating NPC-derived progeny capable of inducing the optimal response. This approach could also be extended to human NPC-derived astrocytes and neurons which similarly regulated BBB induction. The distribution of rat or human NPC-derived progeny under these conditions was found to be a roughly 3 : 1 mixture of astrocytes to neurons with varying degrees of cellular maturity. BMEC gene expression analysis was conducted using a BBB gene panel, and it was determined that 23 of 26 genes were similarly regulated by either differentiated rat NPC or rat astrocyte co-culture while three genes were differentially altered by the rat NPC-derived progeny. Taken together, these results demonstrate that NPCs are an attractive alternative to primary neural cells for use in BBB co-culture models.

The effect of astrocytes on the induction of barrier properties in aortic endothelial cells

Construction of in vitro models of the blood-brain barrier (BBB) using primary brain microvascular endothelial cells (BMEC) is time intensive and not high throughput, in part due to a lack of culture purity, low yields, and cellular dedifferentiation after the first passage. This problem has created interest in the substitution of BMEC with immortalized brain endothelial cells (EC), or peripheral EC such as bovine aortic EC (BAEC). Many BBB models have focused on further inducing the brain and peripheral ECs by incorporating astrocyte back-to-back or nonback-to-back cocultures. However, previous studies demonstrating induction effects of astrocytes on BAEC in back-to-back cocultures failed to recognize the extensive barrier properties of astrocytes alone, which can have a significant effect on interpreting the results. This manuscript reports the establishment of back-to-back and nonback-to-back cocultures between astrocytes and BAEC or BMEC (as a control) with primary focus on the properties of astrocytes alone and with a linear contrast statistical methodology to interpret the results. Transendothelial electrical resistance and permeability studies revealed that astrocytes can significantly increase the barrier tightening of BMEC by 167%, while having no effect on BAEC. Immunocytochemical studies also revealed the reorganization of BMEC occludin junctions in the presence of astrocytes, while indicating the absence of this junctional protein in BAEC. In contrast to a previous report, here the linear contrast statistical analysis revealed that observed decreases in permeability of BAEC in back-to-back cocultures is due to the addition of astrocytes' properties in series and not due to induction.

Murine in vitro model of the blood-brain barrier for evaluating drug transport

In vitro blood-brain barrier (BBB) models help predict brain uptake of potential central nervous system drug candidates. Current in vitro models are composed of brain microvascular endothelial cells (BMEC) that are isolated from rat, bovine, or porcine. However, most in vivo studies on drug transport through the BBB are performed in small laboratory animals, specially mouse and thus murine in vitro BBB models serve as better surrogates to correlate with these studies. Here we describe the functional characterization of a reproducible in vitro model composed of murine BMEC co-cultured with rat primary astrocytes in the presence of biochemical inducing agents. The co-cultures presented high TEER and low sodium fluorescein permeability. Expression of specific BBB tight junction proteins (occludin, claudin-5, ZO-1) and the functionality of transporters (Pgp, GLUT1) were detected by immunocytochemistry and Western blotting. These results indicated a 2.5-fold increase in the expression levels of these proteins in the presence of astrocytes. In addition, a high correlation coefficient (0.98) was obtained between the permeability of a series of hydrophobic and hydrophilic drugs and their corresponding in vivo values. These results together establish the utility of this murine model for future drug transport, pathological, and pharmacological characterizations of the BBB.

The mouse blood-brain barrier transcriptome: a new resource for understanding the development and function of brain endothelial cells

The blood-brain barrier (BBB) maintains brain homeostasis and limits the entry of toxins and pathogens into the brain. Despite its importance, little is known about the molecular mechanisms regulating the development and function of this crucial barrier. In this study we have developed methods to highly purify and gene profile endothelial cells from different tissues, and by comparing the transcriptional profile of brain endothelial cells with those purified from the liver and lung, we have generated a comprehensive resource of transcripts that are enriched in the BBB forming endothelial cells of the brain. Through this comparison we have identified novel tight junction proteins, transporters, metabolic enzymes, signaling components, and unknown transcripts whose expression is enriched in central nervous system (CNS) endothelial cells. This analysis has identified that RXRalpha signaling cascade is specifically enriched at the BBB, implicating this pathway in regulating this vital barrier. This dataset provides a resource for understanding CNS endothelial cells and their interaction with neural and hematogenous cells.

Pericytes are required for blood-brain barrier integrity during embryogenesis

Vascular endothelial cells in the central nervous system (CNS) form a barrier that restricts the movement of molecules and ions between the blood and the brain. This blood-brain barrier (BBB) is crucial to ensure proper neuronal function and protect the CNS from injury and disease. Transplantation studies have demonstrated that the BBB is not intrinsic to the endothelial cells, but is induced by interactions with the neural cells. Owing to the close spatial relationship between astrocytes and endothelial cells, it has been hypothesized that astrocytes induce this critical barrier postnatally, but the timing of BBB formation has been controversial. Here we demonstrate that the barrier is formed during embryogenesis as endothelial cells invade the CNS and pericytes are recruited to the nascent vessels, over a week before astrocyte generation. Analysing mice with null and hypomorphic alleles of Pdgfrb, which have defects in pericyte generation, we demonstrate that pericytes are necessary for the formation of the BBB, and that absolute pericyte coverage determines relative vascular permeability. We demonstrate that pericytes regulate functional aspects of the BBB, including the formation of tight junctions and vesicle trafficking in CNS endothelial cells. Pericytes do not induce BBB-specific gene expression in CNS endothelial cells, but inhibit the expression of molecules that increase vascular permeability and CNS immune cell infiltration. These data indicate that pericyte-endothelial cell interactions are critical to regulate the BBB during development, and disruption of these interactions may lead to BBB dysfunction and neuroinflammation during CNS injury and disease.

Three-Dimensional Scaffolds as a Model System for Neural and Endothelial ‘In Vitro’ Culture

Biomaterials based on the hydrophobic homopolymer poly(ethyl acrylate), PEA, and its copolymers with hydroxyethyl acrylate, p(EA-co-HEA) and methacrylic acid, p(EA-co-MAAc) were prepared as polymeric scaffolds with interconnected pores of 90 microns and tested in vitro as culture substrates and compared for their impact on the differentiation of neural stem cells (NSC) obtained from the subventricular zone (SVZ) of postnatal rats and human endothelial cells (HUVEC). Immunocytochemical staining assay for specific markers show that p(EA-co-MAAc) scaffolds were suitable substrates to promote cell attachment and differentiation of adult NSC and HUVEC cells.

Astrocytes: biology and pathology

Astrocytes are specialized glial cells that outnumber neurons by over fivefold. They contiguously tile the entire central nervous system (CNS) and exert many essential complex functions in the healthy CNS. Astrocytes respond to all forms of CNS insults through a process referred to as reactive astrogliosis, which has become a pathological hallmark of CNS structural lesions. Substantial progress has been made recently in determining functions and mechanisms of reactive astrogliosis and in identifying roles of astrocytes in CNS disorders and pathologies. A vast molecular arsenal at the disposal of reactive astrocytes is being defined. Transgenic mouse models are dissecting specific aspects of reactive astrocytosis and glial scar formation in vivo. Astrocyte involvement in specific clinicopathological entities is being defined. It is now clear that reactive astrogliosis is not a simple all-or-none phenomenon but is a finely gradated continuum of changes that occur in context-dependent manners regulated by specific signaling events. These changes range from reversible alterations in gene expression and cell hypertrophy with preservation of cellular domains and tissue structure, to long-lasting scar formation with rearrangement of tissue structure. Increasing evidence points towards the potential of reactive astrogliosis to play either primary or contributing roles in CNS disorders via loss of normal astrocyte functions or gain of abnormal effects. This article reviews (1) astrocyte functions in healthy CNS, (2) mechanisms and functions of reactive astrogliosis and glial scar formation, and (3) ways in which reactive astrocytes may cause or contribute to specific CNS disorders and lesions.

The gut immune barrier and the blood-brain barrier: are they so different?

In order to protect itself from a diverse set of environmental pathogens and toxins, the body has developed a number of barrier mechanisms to limit the entry of potential hazards. Here, we compare two such barriers: the gut immune barrier, which is the primary barrier against pathogens and toxins ingested in food, and the blood-brain barrier, which protects the central nervous system from pathogens and toxins in the blood. Although each barrier provides defense in very different environments, there are many similarities in their mechanisms of action. In both cases, there is a physical barrier formed by a cellular layer that tightly regulates the movement of ions, molecules, and cells between two tissue spaces. These barrier cells interact with different cell types, which dynamically regulate their function, and with a different array of immune cells that survey the physical barrier and provide innate and adaptive immunity.

Intact and injured endothelial cells differentially modulate postnatal murine forebrain neural stem cells

Neural stem cells (NSCs) persist in the forebrain subventricular zone (SVZ) within a niche containing endothelial cells. Evidence suggests that endothelial cells stimulate NSC expansion and neurogenesis. Experimental stroke increases neurogenesis and angiogenesis, but how endothelial cells influence stroke-induced neurogenesis is unknown. We hypothesized intact or oxygen-glucose deprived (OGD) endothelial cells secrete factors that enhance neurogenesis. We co-cultured mouse SVZ neurospheres (NS) with endothelial cells, or differentiated NS in endothelial cell-conditioned medium (ECCM). NS also were expanded in ECCM from OGD-exposed (OGD-ECCM) endothelial cells to assess injury effects. ECCM significantly increased NS production. NS co-cultured with endothelial cells or ECCM generated more immature-appearing neurons and oligodendrocytes, and astrocytes with radial glial-like/reactive morphology than controls. OGD-ECCM stimulated neuroblast migration and yielded neurons with longer processes and more branching. These data indicate that intact and injured endothelial cells exert differing effects on NSCs, and suggest targets for stimulating regeneration after brain insults.

Dynamic distribution and stem cell characteristics of Sox1-expressing cells in the cerebellar cortex

Bergmann glia cells are a discrete radial glia population surrounding Purkinje cells in the cerebellar cortex. Although Bergmann glia are essential for the development and correct arborization of Purkinje cells, little is known about the regulation of this cell population after the developmental phase. In an effort to characterize this population at the molecular level, we have analyzed marker expression and established that adult Bergmann glia express Sox1, Sox2 and Sox9, a feature otherwise associated with neural stem cells (NSCs). In the present study, we have further analyzed the developmental pattern of Sox1-expressing cells in the developing cerebellum. We report that before becoming restricted to the Purkinje cell layer, Sox1-positive cells are present throughout the immature tissue, and that these cells show characteristics of Bergmann glia progenitors. Our study shows that these progenitors express Sox1, Sox2 and Sox9, a signature maintained throughout cerebellar maturation into adulthood. When isolated in culture, the Sox1-expressing cerebellar population exhibited neurosphere-forming ability, NSC-marker characteristics, and demonstrated multipotency at the clonal level. Our results show that the Bergmann glia population expresses Sox1 during cerebellar development, and that these cells can be isolated and show stem cell characteristics in vitro, suggesting that they could hold a broader potential than previously thought.

Acquired immunodeficiency syndrome and the blood-brain barrier

The blood-brain barrier (BBB) plays a critical role in normal physiology of the central nervous system by regulating what reaches the brain from the periphery. The BBB also plays a major role in neurologic disease including neuropathologic sequelae associated with infection by human immunodeficiency virus (HIV) in humans and the closely related simian immunodeficiency virus (SIV) in macaques. In this review, we provide an overview of the function, structure, and components of the BBB, followed by a more detailed discussion of the subcellular structures and regulation of the tight junction. We then discuss the ways in which HIV/SIV affects the BBB, largely through infection of monocytes/macrophages, and how infected macrophages crossing the BBB ultimately results in breakdown of the barrier.

Cell therapy for CNS trauma

Cell therapy plays an important role in multidisciplinary management of the two major forms of central nervous system (CNS) injury, traumatic brain injury and spinal cord injury, which are caused by external physical trauma. Cell therapy for CNS disorders involves the use of cells of neural or non-neural origin to replace, repair, or enhance the function of the damaged nervous system and is usually achieved by transplantation of the cells, which are isolated and may be modified, e.g., by genetic engineering, when it may be referred to as gene therapy. Because the adult brain cells have a limited capacity to migrate to and regenerate at sites of injury, the use of embryonic stem cells that can be differentiated into various cell types as well as the use of neural stem cells has been explored. Preclinical studies and clinical trials are reviewed. Advantages as well as limitations are discussed. Cell therapy is promising for the treatment of CNS injury because it targets multiple mechanisms in a sustained manner. It can provide repair and regeneration of damaged tissues as well as prolonged release of neuroprotective and other therapeutic substances.

Wnt/beta-catenin signaling is required for CNS, but not non-CNS, angiogenesis

Despite the importance of CNS blood vessels, the molecular mechanisms that regulate CNS angiogenesis and blood-brain barrier (BBB) formation are largely unknown. Here we analyze the role of Wnt/beta-catenin signaling in regulating the formation of CNS blood vessels. First, through the analysis of TOP-Gal Wnt reporter mice, we identify that canonical Wnt/beta-catenin signaling is specifically activated in CNS, but not non-CNS, blood vessels during development. This activation correlates with the expression of different Wnt ligands by neural progenitor cells in distinct locations throughout the CNS, including Wnt7a and Wnt7b in ventral regions and Wnt1, Wnt3, Wnt3a, and Wnt4 in dorsal regions. Blockade of Wnt/beta-catenin signaling in vivo specifically disrupts CNS, but not non-CNS, angiogenesis. These defects include reduction in vessel number, loss of capillary beds, and the formation of hemorrhagic vascular malformations that remain adherent to the meninges. Furthermore, we demonstrate that Wnt/beta-catenin signaling regulates the expression of the BBB-specific glucose transporter glut-1. Taken together these experiments reveal an essential role for Wnt/beta-catenin signaling in driving CNS-specific angiogenesis and provide molecular evidence that angiogenesis and BBB formation are in part linked.

Modelling of the blood–brain barrier in drug discovery and development

The market for neuropharmaceuticals is potentially one of the largest sectors of the global pharmaceutical market owing to the increase in average life expectancy and the fact that many neurological disorders have been largely refractory to pharmacotherapy. The brain is a delicate organ that can efficiently protect itself from harmful compounds and precisely regulate its microenvironment. Unfortunately, the same mechanisms can also prove to be formidable hurdles in drug development. An improved understanding of the regulatory interfaces that exist between blood and brain may provide novel and more effective strategies to treat neurological disorders.