Ontogenic expression of the erythroid-type glucose transporter (Glut 1) in the telencephalon of the mouse: correlation to the tightening of the blood-brain barrier

IL-1β disrupts the initiation of blood-brain barrier development by inhibiting endothelial Wnt/β-catenin signaling

During neuroinflammation, the proinflammatory cytokine interleukin-1β (IL-1β) impacts blood-brain barrier (BBB) function by disrupting brain endothelial tight junctions, promoting vascular permeability, and increasing transmigration of immune cells. Here, we examined the effects of Il-1β on the in vivo initiation of BBB development. We generated doxycycline-inducible transgenic zebrafish to secrete Il-1β in the CNS. To validate the utility of our model, we showed Il-1β dose-dependent mortality, recruitment of neutrophils, and expansion of microglia. Using live imaging, we discovered that Il-1β causes a significant reduction in CNS angiogenesis and barriergenesis. To demonstrate specificity, we rescued the Il-1β induced phenotypes by targeting the zebrafish il1r1 gene using CRISPR-Cas9. Mechanistically, we determined that Il-1β disrupts the initiation of BBB development by decreasing Wnt/β-catenin transcriptional activation in brain endothelial cells. Given that several neurodevelopmental disorders are associated with inflammation, our findings support further investigation into the connections between proinflammatory cytokines, neuroinflammation, and neurovascular development.

IL-1β disrupts blood-brain barrier development by inhibiting endothelial Wnt/β-catenin signaling

During neuroinflammation, the proinflammatory cytokine Interleukin-1β (IL-1β) impacts blood-brain barrier (BBB) function by disrupting brain endothelial tight junctions, promoting vascular permeability, and increasing transmigration of immune cells. Here, we examined the effects of Il-1β on the in vivo development of the BBB. We generated a doxycycline-inducible transgenic zebrafish model that drives secretion of Il-1β in the CNS. To validate the utility of our model, we showed Il-1β dose-dependent mortality, recruitment of neutrophils, and expansion of microglia. Using live imaging, we discovered that Il-1β causes a significant reduction in CNS angiogenesis and barriergenesis. To demonstrate specificity, we rescued the Il-1β induced phenotypes by targeting the zebrafish il1r1 gene using CRISPR/Cas9. Mechanistically, we determined that Il-1β disrupts BBB development by decreasing Wnt/β-catenin transcriptional activation in brain endothelial cells. Given that several neurodevelopmental disorders are associated with inflammation, our findings support further investigation into the connections between proinflammatory cytokines, neuroinflammation, and neurovascular development.

Blood-brain borders: a proposal to address limitations of historical blood-brain barrier terminology

Many neuroscientists use the term Blood-Brain Barrier (BBB) to emphasize restrictiveness, often equating or reducing the notion of BBB properties to tight junction molecules physically sealing cerebral endothelial cells, rather than pointing out the complexity of this biological interface with respect to its selectivity and variety of exchange between the general blood circulation and the central nervous tissue. Several authors in the field find it unfortunate that the exquisitely dynamic interfaces between blood and brain continue to be viewed primarily as obstructive barriers to transport. Although the term blood-brain interface is an excellent descriptor that does not convey the idea of a barrier, it is important and preferable for the spreading of an idea beyond specialist communities to try to appeal to well-chosen metaphors. Recent evidence reviewed here indicates that blood-brain interfaces are more than selective semi-permeable membranes in that they display many dynamic processes and complex mechanisms for communication. They are thus more like 'geopolitical borders'. Furthermore, some authors working on blood-brain interface-relevant issues have started to use the word border, for example in border-associated macrophages. Therefore, we suggest adopting the term Blood-Brain Border to better communicate the flexibility of and movement across blood-brain interfaces.

Brain endothelial cells acquire blood-brain barrier properties in the absence of Vegf-dependent CNS angiogenesis

During neurovascular development, brain endothelial cells (BECs) respond to secreted signals from the neuroectoderm that regulate CNS angiogenesis, the formation of new blood vessels in the brain, and barriergenesis, the acquisition of blood-brain barrier (BBB) properties. Wnt/β-catenin signaling and Vegf signaling are both required for CNS angiogenesis; however, the relationship between these pathways is not understood. Furthermore, while Wnt/β-catenin signaling is essential for barriergenesis, the role of Vegf signaling in this vital process remains unknown. Here, we provide the first direct evidence, to our knowledge, that Vegf signaling is not required for barriergenesis and that activation of Wnt/β-catenin in BECs is independent of Vegf signaling during neurovascular development. Using double transgenic glut1b:mCherry and plvap:EGFP zebrafish (Danio rerio) to visualize the developing brain vasculature, we performed a forward genetic screen and identified a new mutant allele of kdrl, an ortholog of mammalian Vegfr2. The kdrl mutant lacks CNS angiogenesis but, unlike the Wnt/β-catenin pathway mutant gpr124, acquires BBB properties in BECs. To examine Wnt/β-catenin pathway activation in BECs, we chemically inhibited Vegf signaling and found robust expression of the Wnt/β-catenin transcriptional reporter line 7xtcf-Xla.Siam:EGFP. Taken together, our results establish that Vegf signaling is essential for CNS angiogenesis but is not required for Wnt/β-catenin-dependent barriergenesis. Given the clinical significance of either inhibiting pathological angiogenesis or stimulating neovascularization, our study provides valuable new insights that are critical for the development of effective therapies that target the vasculature in neurological disorders.

Developmental regulation of barrier- and non-barrier blood vessels in the CNS

The blood-brain barrier (BBB) is essential for creating and maintaining tissue homeostasis in the central nervous system (CNS), which is key for proper neuronal function. In most vertebrates, the BBB is localized to microvascular endothelial cells that acquire barrier properties during angiogenesis of the neuroectoderm. Complex and continuous tight junctions, and the lack of fenestrae combined with low pinocytotic activity render the BBB endothelium a tight barrier for water-soluble molecules that may only enter the CNS via specific transporters. The differentiation of these unique endothelial properties during embryonic development is initiated by endothelial-specific flavours of the Wnt/β-catenin pathway in a precise spatiotemporal manner. In this review, we summarize the currently known cellular (neural precursor and endothelial cells) and molecular (VEGF and Wnt/β-catenin) mechanisms mediating brain angiogenesis and barrier formation. Moreover, we introduce more recently discovered crosstalk with cellular and acellular elements within the developing CNS such as the extracellular matrix. We discuss recent insights into the downstream molecular mechanisms of Wnt/β-catenin in particular, the recently identified target genes like Foxf2, Foxl2, Foxq1, Lef1, Ppard, Zfp551, Zic3, Sox17, Apcdd1 and Fgfbp1 that are involved in refining and maintaining barrier characteristics in the mature BBB endothelium. Additionally, we elute to recent insight into barrier heterogeneity and differential endothelial barrier properties within the CNS, focussing on the circumventricular organs as well as on the neurogenic niches in the subventricular zone and the hippocampus. Finally, open questions and future BBB research directions are highlighted in the context of taking benefit from understanding BBB development for strategies to modulate BBB function under pathological conditions.

Immunological defense of CNS barriers against infections

Neuroanatomical barriers with physical, chemical, and immunological properties play an essential role in preventing the spread of peripheral infections into the CNS. A failure to contain pathogens within these barriers can result in very serious CNS diseases. CNS barriers are inhabited by an elaborate conglomerate of innate and adaptive immune cells that are highly responsive to environmental challenges. The CNS and its barriers can also be protected by memory T and B cells elicited by prior infection or vaccination. Here, we discuss the different CNS barriers from a developmental, anatomical, and immunological standpoint and summarize our current understanding of how memory cells protect the CNS compartment. We then discuss a contemporary challenge to CNS-barrier system (SARS-CoV-2 infection) and highlight approaches to promote immunological protection of the CNS via vaccination.

Form and Function of the Vertebrate and Invertebrate Blood-Brain Barriers

The need to protect neural tissue from toxins or other substances is as old as neural tissue itself. Early recognition of this need has led to more than a century of investigation of the blood-brain barrier (BBB). Many aspects of this important neuroprotective barrier have now been well established, including its cellular architecture and barrier and transport functions. Unsurprisingly, most research has had a human orientation, using mammalian and other animal models to develop translational research findings. However, cell layers forming a barrier between vascular spaces and neural tissues are found broadly throughout the invertebrates as well as in all vertebrates. Unfortunately, previous scenarios for the evolution of the BBB typically adopt a classic, now discredited 'scala naturae' approach, which inaccurately describes a putative evolutionary progression of the mammalian BBB from simple invertebrates to mammals. In fact, BBB-like structures have evolved independently numerous times, complicating simplistic views of the evolution of the BBB as a linear process. Here, we review BBBs in their various forms in both invertebrates and vertebrates, with an emphasis on the function, evolution, and conditional relevance of popular animal models such as the fruit fly and the zebrafish to mammalian BBB research.

Medications for pregnant women: A balancing act between the interests of the mother and of the fetus

Drug entry into the adult brain is controlled by efflux mechanisms situated in various brain barrier interfaces. The effectiveness of these protective mechanisms in the embryo, fetus and newborn brain is less clear. The longstanding belief that "the" blood-brain barrier is absent or immature in the fetus and newborn has led to many misleading statements with potential clinical implications. Here we review the properties of brain barrier mechanisms in the context of drug entry into the developing brain and discuss the limited number of studies published on the subject. We noticed that most of available literature suffers from some experimental limitations, notably that drug levels in fetal blood and cerebrospinal fluid have not been measured. This means that the relative contribution to the overall brain protection provided by individual barriers such as the placenta (which contains similar efflux mechanisms) and the brain barriers cannot be separately ascertained. Finally, we propose that systematic studies in appropriate animal models of drug entry into the brain at different stages of development would provide a rational basis for use of medications in pregnancy and in newborns, especially prematurely born, where protection usually provided by the placenta is no longer present.

Development of the brain vasculature and the blood-brain barrier in zebrafish

To ensure tissue homeostasis the brain needs to be protected from blood-derived fluctuations or pathogens that could affect its function. Therefore, the brain capillaries develop tissue-specific properties to form a selective blood-brain barrier (BBB), allowing the passage of essential molecules to the brain and blocking the penetration of potentially harmful compounds or cells. Previous studies reported the presence of this barrier in zebrafish. The intrinsic features of the zebrafish embryos and larvae in combination with optical techniques, make them suitable for the study of barrier establishment and maturation. In this review, we discuss the most recent contributions to the development and formation of a functional zebrafish BBB. Moreover, we compare the molecular and cellular characteristic of the zebrafish and the mammalian BBB.

Physiology and molecular biology of barrier mechanisms in the fetal and neonatal brain

Properties of the local internal environment of the adult brain are tightly controlled providing a stable milieu essential for its normal function. The mechanisms involved in this complex control are structural, molecular and physiological (influx and efflux transporters) frequently referred to as the 'blood-brain barrier'. These mechanisms include regulation of ion levels in brain interstitial fluid essential for normal neuronal function, supply of nutrients, removal of metabolic products, and prevention of entry or elimination of toxic agents. A key feature is cerebrospinal fluid secretion and turnover. This is much less during development, allowing greater accumulation of permeating molecules. The overall effect of these mechanisms is to tightly control the exchange of molecules into and out of the brain. This review presents experimental evidence currently available on the status of these mechanisms in developing brain. It has been frequently stated for over nearly a century that the blood-brain barrier is not present or at least is functionally deficient in the embryo, fetus and newborn. We suggest the alternative hypothesis that the barrier mechanisms in developing brain are likely to be appropriately matched to each stage of its development. The contributions of different barrier mechanisms, such as changes in constituents of cerebrospinal fluid in relation to specific features of brain development, for example neurogenesis, are only beginning to be studied. The evidence on this previously neglected aspect of brain barrier function is outlined. We also suggest future directions this field could follow with special emphasis on potential applications in a clinical setting.

Parvins Are Required for Endothelial Cell-Cell Junctions and Cell Polarity During Embryonic Blood Vessel Formation

OBJECTIVE

During vascular development, integrin-mediated signaling regulates the formation and stabilization of cell-cell junctions, which are required for endothelial cell (EC) apical-basal polarity and proper deposition of the vascular basement membrane. Parvins are actin-binding proteins that facilitate the interaction of integrins with the actin cytoskeleton. The endothelium expresses 2 parvin isoforms: α-pv (α-parvin) and β-pv (β-parvin). Recently, we have shown that α-pv is critical for vessel growth and vessel stability at late embryonic developmental stages. The role of parvins during early embryonic development is unknown.

APPROACH AND RESULTS

To investigate the role of endothelial parvins in the developing vasculature, we generated mice with ECs lacking both parvin isoforms by deleting α-pv in ECs in global β-pv^-/- mice (α-pv^ΔEC;β-pv^-/- mice). Here, we show that α-pv^ΔEC;β-pv^-/- mice die around embryonic day 11.5 and exhibit hemorrhages, immature capillary beds, and severe vascular defects in the central nervous system, including reduced vessel branching, increased vessel diameter, and balloon-like hemorrhagic clusters of ECs. Vessels in α-pv^ΔEC;β-pv^-/- embryos display disorganized cell-cell junctions, impaired endothelial apical-basal polarity, and discontinuous basement membranes. These vascular defects are accompanied by defective pericyte-vessel interaction.

CONCLUSIONS

Our results show that parvins are critical for the organization of endothelial cell-cell junctions, the establishment of endothelial apical-basal polarity, and the integrity of the basement membrane.

Blood-brain barrier and foetal-onset hydrocephalus, with a view on potential novel treatments beyond managing CSF flow

Despite decades of research, no compelling non-surgical therapies have been developed for foetal hydrocephalus. So far, most efforts have pointed to repairing disturbances in the cerebrospinal fluid (CSF) flow and to avoid further brain damage. There are no reports trying to prevent or diminish abnormalities in brain development which are inseparably associated with hydrocephalus. A key problem in the treatment of hydrocephalus is the blood–brain barrier that restricts the access to the brain for therapeutic compounds or systemically grafted cells. Recent investigations have started to open an avenue for the development of a cell therapy for foetal-onset hydrocephalus. Potential cells to be used for brain grafting include: (1) pluripotential neural stem cells; (2) mesenchymal stem cells; (3) genetically-engineered stem cells; (4) choroid plexus cells and (5) subcommissural organ cells. Expected outcomes are a proper microenvironment for the embryonic neurogenic niche and, consequent normal brain development.

Development and Function of the Blood-Brain Barrier in the Context of Metabolic Control

Under physiological conditions, the brain consumes over 20% of the whole body energy supply. The blood-brain barrier (BBB) allows dynamic interactions between blood capillaries and the neuronal network in order to provide an adequate control of molecules that are transported in and out of the brain. Alterations in the BBB structure and function affecting brain accessibility to nutrients and exit of toxins are found in a number of diseases, which in turn may disturb brain function and nutrient signaling. In this review we explore the major advances obtained in the understanding of the BBB development and how its structure impacts on function. Furthermore, we focus on the particularities of the barrier permeability in the hypothalamus, its role in metabolic control and the potential impact of hypothalamic BBB abnormities in metabolic related diseases.

CNS angiogenesis and barriergenesis occur simultaneously

The blood-brain barrier (BBB) plays a vital role in the central nervous system (CNS). A comprehensive understanding of BBB development has been hampered by difficulties in observing the differentiation of brain endothelial cells (BECs) in real-time. Here, we generated two transgenic zebrafish line, Tg(glut1b:mCherry) and Tg(plvap:EGFP), to serve as in vivo reporters of BBB development. We showed that barriergenesis (i.e. the induction of BEC differentiation) occurs immediately as endothelial tips cells migrate into the brain parenchyma. Using the Tg(glut1b:mCherry) transgenic line, we performed a genetic screen and identified a zebrafish mutant with a nonsense mutation in gpr124, a gene known to play a role in CNS angiogenesis and BBB development. We also showed that our transgenic plvap:EGFP line, a reporter of immature brain endothelium, is initially expressed in newly formed brain endothelial cells, but subsides during BBB maturation. Our results demonstrate the ability to visualize the in vivo differentiation of brain endothelial cells into the BBB phenotype and establish that CNS angiogenesis and barriergenesis occur simultaneously.

Neuronal and vascular interactions

The brain, which represents 2% of body mass but consumes 20% of body energy at rest, has a limited capacity to store energy and is therefore highly dependent on oxygen and glucose supply from the blood stream. Normal functioning of neural circuits thus relies on adequate matching between metabolic needs and blood supply. Moreover, not only does the brain need to be densely vascularized, it also requires a tightly controlled environment free of toxins and pathogens to provide the proper chemical composition for synaptic transmission and neuronal function. In this review, we focus on three major factors that ensure optimal brain perfusion and function: the patterning of vascular networks to efficiently deliver blood and nutrients, the function of the blood-brain barrier to maintain brain homeostasis, and the regulation of cerebral blood flow to adequately couple energy supply to neural function.

GPR124 functions as a WNT7-specific coactivator of canonical β-catenin signaling

G protein-coupled receptor 124 (GPR124) is an orphan receptor in the adhesion family of GPCRs, and previous global or endothelial-specific disruption of Gpr124 in mice led to defective CNS angiogenesis and blood-brain barriergenesis. Similar developmental defects were observed following dual deletion of Wnt7a/Wnt7b or deletion of β-catenin in endothelial cells, suggesting a possible relationship between GPR124 and canonical WNT signaling. Here, we show using in vitro reporter assays, mutation analysis, and genetic interaction studies in vivo that GPR124 functions as a WNT7A/WNT7B-specific costimulator of β-catenin signaling in brain endothelium. WNT7-stimulated β-catenin signaling was dependent upon GPR124's intracellular PDZ binding motif and a set of leucine-rich repeats in its extracellular domain. This study reveals a vital role for GPR124 in potentiation of WNT7-induced canonical β-catenin signaling with important implications for understanding and manipulating CNS-specific angiogenesis and blood-brain barrier-genesis.

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.

The rights and wrongs of blood-brain barrier permeability studies: a walk through 100 years of history

Careful examination of relevant literature shows that many of the most cherished concepts of the blood-brain barrier are incorrect. These include an almost mythological belief in its immaturity that is unfortunately often equated with absence or at least leakiness in the embryo and fetus. The original concept of a blood-brain barrier is often attributed to Ehrlich; however, he did not accept that permeability of cerebral vessels was different from other organs. Goldmann is often credited with the first experiments showing dye (trypan blue) exclusion from the brain when injected systemically, but not when injected directly into it. Rarely cited are earlier experiments of Bouffard and of Franke who showed methylene blue and trypan red stained all tissues except the brain. The term “blood-brain barrier” “Blut-Hirnschranke” is often attributed to Lewandowsky, but it does not appear in his papers. The first person to use this term seems to be Stern in the early 1920s. Studies in embryos by Stern and colleagues, Weed and Wislocki showed results similar to those in adult animals. These were well-conducted experiments made a century ago, thus the persistence of a belief in barrier immaturity is puzzling. As discussed in this review, evidence for this belief, is of poor experimental quality, often misinterpreted and often not properly cited. The functional state of blood-brain barrier mechanisms in the fetus is an important biological phenomenon with implications for normal brain development. It is also important for clinicians to have proper evidence on which to advise pregnant women who may need to take medications for serious medical conditions. Beliefs in immaturity of the blood-brain barrier have held the field back for decades. Their history illustrates the importance of taking account of all the evidence and assessing its quality, rather than selecting papers that supports a preconceived notion or intuitive belief. This review attempts to right the wrongs. Based on careful translation of original papers, some published a century ago, as well as providing discussion of studies claiming to show barrier immaturity, we hope that readers will have evidence on which to base their own conclusions.

"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.

Onset of aquaporin-4 expression in the developing mouse brain

The main water channel in the brain, aquaporin-4 (AQP4) is involved in maintaining homeostasis and water exchange in the brain. In adult mammalian brains, it is expressed in astrocytes, mainly, and in high densities in the membranes of perivascular and subpial endfeet. Here, we addressed the question how this polarized expression is established during development. We used immunocytochemistry against AQP4, zonula occludens protein-1, glial fibrillary acidic protein, and β-dystroglycan to follow astrocyte development in E15 to P3 NMRI mouse brains, and expression of AQP4. In addition we used freeze-fracture electron microscopy to detect AQP4 in the form of orthogonal arrays of particles (OAPs) on the ultrastructural level. We analyzed ventral, lateral, and dorsal regions in forebrain sections and found AQP4 immunoreactivity to emerge at E16 ventrally before lateral (E17) and dorsal (E18) areas. AQP4 staining was spread over cell processes including radial glial cells in developing cortical areas and became restricted to astroglial endfeet at P1-P3. This was confirmed by double labeling with GFAP. In freeze-fracture replicas OAPs were found with a slight time delay but with a similar ventral to dorsal gradient. Thus, AQP4 is expressed in the embryonic mouse brain starting at E16, earlier than previously reported. However a polarized expression necessary for homeostatic function and water balance emerges at later stages around and after birth.

Mfsd2a is critical for the formation and function of the blood-brain barrier

The central nervous system (CNS) requires a tightly controlled environment free of toxins and pathogens to provide the proper chemical composition for neural function. This environment is maintained by the 'blood-brain barrier' (BBB), which is composed of blood vessels whose endothelial cells display specialized tight junctions and extremely low rates of transcellular vesicular transport (transcytosis). In concert with pericytes and astrocytes, this unique brain endothelial physiological barrier seals the CNS and controls substance influx and efflux. Although BBB breakdown has recently been associated with initiation and perpetuation of various neurological disorders, an intact BBB is a major obstacle for drug delivery to the CNS. A limited understanding of the molecular mechanisms that control BBB formation has hindered our ability to manipulate the BBB in disease and therapy. Here we identify mechanisms governing the establishment of a functional BBB. First, using a novel tracer-injection method for embryos, we demonstrate spatiotemporal developmental profiles of BBB functionality and find that the mouse BBB becomes functional at embryonic day 15.5 (E15.5). We then screen for BBB-specific genes expressed during BBB formation, and find that major facilitator super family domain containing 2a (Mfsd2a) is selectively expressed in BBB-containing blood vessels in the CNS. Genetic ablation of Mfsd2a results in a leaky BBB from embryonic stages through to adulthood, but the normal patterning of vascular networks is maintained. Electron microscopy examination reveals a dramatic increase in CNS-endothelial-cell vesicular transcytosis in Mfsd2a(-/-) mice, without obvious tight-junction defects. Finally we show that Mfsd2a endothelial expression is regulated by pericytes to facilitate BBB integrity. These findings identify Mfsd2a as a key regulator of BBB function that may act by suppressing transcytosis in CNS endothelial cells. Furthermore, our findings may aid in efforts to develop therapeutic approaches for CNS drug delivery.

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.

Barrier mechanisms in the developing brain

The adult brain functions within a well-controlled stable environment, the properties of which are determined by cellular exchange mechanisms superimposed on the diffusion restraint provided by tight junctions at interfaces between blood, brain and cerebrospinal fluid (CSF). These interfaces are referred to as “the” blood–brain barrier. It is widely believed that in embryos and newborns, this barrier is immature or “leaky,” rendering the developing brain more vulnerable to drugs or toxins entering the fetal circulation from the mother. New evidence shows that many adult mechanisms, including functionally effective tight junctions are present in embryonic brain and some transporters are more active during development than in the adult. Additionally, some mechanisms present in embryos are not present in adults, e.g., specific transport of plasma proteins across the blood–CSF barrier and embryo-specific intercellular junctions between neuroependymal cells lining the ventricles. However developing cerebral vessels appear to be more fragile than in the adult. Together these properties may render developing brains more vulnerable to drugs, toxins, and pathological conditions, contributing to cerebral damage and later neurological disorders. In addition, after birth loss of protection by efflux transporters in placenta may also render the neonatal brain more vulnerable than in the fetus.

Barriers in the developing brain and Neurotoxicology

The brain develops and grows within a well-controlled internal environment that is provided by cellular exchange mechanisms in the interfaces between blood, cerebrospinal fluid and brain. These are generally referred to by the term "brain barriers": blood-brain barrier across the cerebral endothelial cells and blood-CSF barrier across the choroid plexus epithelial cells. An essential component of barrier mechanisms is the presence of tight junctions between the endothelial and epithelial cells of these interfaces. This review outlines historical evidence for the presence of effective barrier mechanisms in the embryo and newborn and provides an up to date description of recent morphological, biochemical and molecular data for the functional effectiveness of these barriers. Intercellular tight junctions between cerebral endothelial cells and between choroid plexus epithelial cells are functionally effective as soon as they differentiate. Many of the influx and efflux mechanisms are not only present from early in development, but the genes for some are expressed at much higher levels in the embryo than in the adult and there is physiological evidence that these transport systems are functionally more active in the developing brain. This substantial body of evidence supporting the concept of well developed barrier mechanisms in the developing brain is contrasted with the widespread belief amongst neurotoxicologists that "the" blood-brain barrier is immature or even absent in the embryo and newborn. A proper understanding of the functional capacity of the barrier mechanisms to restrict the entry of harmful substances or administered therapeutics into the developing brain is critical. This knowledge would assist the clinical management of pregnant mothers and newborn infants and development of protocols for evaluation of risks of drugs used in pregnancy and the neonatal period prior to their introduction into clinical practice.

Oxidative stress and blood-brain barrier dysfunction under particular consideration of matrix metalloproteinases

A cell's "redox" (oxidation and reduction) state is determined by the sum of all redox processes yielding reactive oxygen species (ROS), reactive nitrogen species (RNS), and other reactive intermediates. Low amounts of ROS/RNS are generated by different mechanisms in every cell and are important regulatory mediators in many signaling processes (redox signaling). When the physiological balance between the generation and elimination of ROS/RNS is disrupted, oxidative/nitrosative stress with persistent oxidative damage of the organism occurs. Oxidative stress has been suggested to act as initiator and/or mediator of many human diseases. The cerebral vasculature is particularly susceptible to oxidative stress, which is critical since cerebral endothelial cells play a major role in the creation and maintenance of the blood-brain barrier (BBB). This article will only contain a focused introduction on the biochemical background of redox signaling, since this has been reported already in a series of excellent recent reviews. The goal of this work is to increase the understanding of basic mechanisms underlying ROS/RNS-induced BBB disruption, with a focus on the role of matrix metalloproteinases, which, after all, appear to be a key mediator in the initiation and progression of BBB damage elicited by oxidative stress.

Integration of tight junctions and claudins with the barrier functions of the retinal pigment epithelium

The retinal pigment epithelium (RPE) forms the outer blood-retinal barrier by regulating the movement of solutes between the fenestrated capillaries of the choroid and the photoreceptor layer of the retina. Blood-tissue barriers use various mechanisms to accomplish their tasks including membrane pumps, transporters, and channels, transcytosis, metabolic alteration of solutes in transit, and passive but selective diffusion. The last category includes tight junctions, which regulate transepithelial diffusion through the spaces between neighboring cells of the monolayer. Tight junctions are extraordinarily complex structures that are dynamically regulated. Claudins are a family of tight junctional proteins that lend tissue specificity and selectivity to tight junctions. This review discusses how the claudins and tight junctions of the RPE differ from other epithelia and how its functions are modulated by the neural retina. Studies of RPE-retinal interactions during development lend insight into this modulation. Notably, the characteristics of RPE junctions, such as claudin composition, vary among species, which suggests the physiology of the outer retina may also vary. Comparative studies of barrier functions among species should deepen our understanding of how homeostasis is maintained in the outer retina. Stem cells provide a way to extend these studies of RPE-retinal interactions to human RPE.

Croix B. GPR124, an orphan G protein-coupled receptor, is required for CNS-specific vascularization and establishment of the blood-brain barrier

Every organ in the body requires blood vessels for efficient delivery of oxygen and nutrients, but independent vascular beds are highly specialized to meet the individual needs of specific organs. The vasculature of the brain is tightly sealed, with blood-brain barrier (BBB) properties developing coincident with neural vascularization. G protein-coupled receptor 124 (GPR124) (tumor endothelial marker 5, TEM5), an orphan member of the adhesion family of G protein-coupled receptors, was previously identified on the basis of its overexpression in tumor vasculature. Here, we show that global deletion or endothelial-specific deletion of GPR124 in mice results in embryonic lethality associated with abnormal angiogenesis of the forebrain and spinal cord. Expression of GPR124 was found to be required for invasion and migration of blood vessels into neuroepithelium, establishment of BBB properties, and expansion of the cerebral cortex. Thus, GPR124 is an important regulator of neurovasculature development and a potential drug target for cerebrovascular diseases.

Angiogenic potential of microvessel fragments is independent of the tissue of origin and can be influenced by the cellular composition of the implants

We have demonstrated that MFs isolated from adipose retain angiogenic potential in vitro and form a mature, perfused network when implanted. However, adipose-derived microvessels are rich in provascularizing cells that could uniquely drive neovascularization in adipose-derived MFs implants.

OBJECTIVE

Investigate the ability of MFs from a different vascular bed to recapitulate adipose-derived microvessel angiogenesis and network formation and analyze adipose-derived vessel plasticity by assessing whether vessel function could be modulated by astrocyte-like cells.

METHODS

MFs were isolated by limited collagenase digestion from rodent brain or adipose and assembled into 3D collagen gels in the presence or absence of GRPs. The resulting neovasculatures that formed following implantation were assessed by measuring 3D vascularity and vessel permeability to small and large molecular tracers.

RESULTS

Similar to adipose-derived MFs, brain-derived MFs can sprout and form a perfused neovascular network when implanted. Furthermore, when co-implanted in the constructs, GRPs caused adipose-derived vessels to express the brain endothelial marker glucose transporter-1 and to significantly reduce microvessel permeability.

CONCLUSION

Neovascularization involving isolated microvessel elements is independent of the tissue origin and degree of vessel specialization. In addition, adipose-derived vessels have the ability to respond to environmental signals and change vessel characteristics.

Glucose transporter distribution in the vessels of the central nervous system of the axolotl Ambystoma mexicanum (Urodela: Ambystomatidae)

The GLUT-1 isoform of the glucose transporter is commonly considered a reliable molecular marker of blood-brain barrier endothelia in the neural vasculature organized in a three-dimensional network of single vessels. The central nervous system of the axolotl Ambystoma mexicanum is characterized by a vascular architecture that contains both single and paired vessels. The presence and distribution of the GLUT-1 transporter are studied in this urodele using both immunoperoxidase histochemistry and immunogold technique. Light microscopy reveals immunopositivity in both parenchymal and meningeal vessels. The transverse-sectioned pairs of vessels do not show the same size. Furthermore, in the same pair, the two elements often differ in diameter. The main regions of the central nervous system show a different percentage of the paired structures. Only immunogold cytochemistry reveals different staining intensity in the two adjoined elements of a vascular pair. Colloidal gold particles show an asymmetric distribution in the endothelia of both single and paired vessels. These particles are more numerous on the abluminal surface than on the luminal one. The particle density is calculated in both vascular types. The different values could indicate functional differences between single and paired vessels and between the two adjoined elements of a pair, regarding glucose transport.

Glucose transporter isoform-3 mutations cause early pregnancy loss and fetal growth restriction

Glucose transporter isoform-3 (GLUT3) is the trophoblastic facilitative glucose transporter. To investigate the role of this isoform in embryonic development, we created a novel GLUT3-null mouse and observed arrested early embryonic development and loss at neurulation stage when both alleles were mutated. This loss occurred despite the presence of other related isoforms, particularly GLUT1. In contrast, when a single allele was mutated, despite increased embryonic cell apoptosis, adaptive changes in the subcellular localization of GLUT3 and GLUT1 in the preimplantation embryo led to postimplantation survival. This survival was compromised by decreased GLUT3-mediated transplacental glucose transport, causing late-gestation fetal growth restriction. This yielded young male and female adults demonstrating catch-up growth, with normal basal glucose, insulin, insulin-like growth factor-I and IGF-binding protein-3 concentrations, fat and lean mass, and glucose and insulin tolerance. We conclude that GLUT3 mutations cause a gene dose-dependent early pregnancy loss or late-gestation fetal growth restriction despite the presence of embryonic and placental GLUT1 and a compensatory increase in system A amino acid placental transport. This critical life-sustaining functional role for GLUT3 in embryonic development provides the basis for investigating the existence of human GLUT3 mutations with similar consequences during early pregnancy.

Development of the retinal vasculature

Blood vessels that supply the inner portion of the retina are extensively reorganized during development. The vessel regression, sprouting angiogenesis, vascular remodelling and vessel differentiation events involved critically depend on cell-cell signalling between different cellular components such as neurons, glia, endothelial cells, pericytes and immune cells. Studies in mice using transgenic and gene deletion approaches have started to unravel the genetic basis of some of these signalling pathways and have lead to a much improved understanding of the molecular mechanisms controlling retinal blood vessel behaviour both during development and under pathological conditions. Such insight will provide the basis of future therapeutic approaches aimed at manipulating retinal blood vessels.

Differentiating embryonic neural progenitor cells induce blood-brain barrier properties

The blood-brain barrier (BBB) is a multifunctional endothelial interface separating the bloodstream from the brain interior. Although the mature BBB is well characterized, the embryonic development of this complex system remains poorly understood. Embryonic neural progenitor cells (NPC) are a potential inductive cell type populating the developing brain, and their ability to influence BBB properties was therefore examined. When puromycin-purified brain microvascular endothelial cells (BMEC) were co-cultured with embryonic NPC in a two-compartment Transwell system, the BMEC exhibited enhanced barrier properties in the form of increased transendothelial electrical resistance (TEER) and decreased permeability to the small molecule tracer, sodium fluorescein. These changes required the presence of NPC in the early stages of differentiation and were accompanied by alterations in the fidelity of BMEC tight junctions as indicated by occludin, claudin 5, and zonula occluden-1 redistribution at cell-cell borders. In contrast to the findings with NPC, post-natal astrocytes elicited a delayed, but longer duration response in BMEC TEER. BMEC co-culture also suppressed neuronal differentiation of NPC indicating a reciprocal signaling between the two cell populations. This study demonstrates that NPC-BMEC interactions are prevalent and for the first time demonstrates that NPC are capable of inducing BBB properties.

Glucose transporter (GLUT-1) distribution in the brain vessels of the adult Italian wall lizard, Podarcis sicula

The GLUT-1 isoform of the glucose transporter is commonly accepted as a reliable molecular marker of blood-brain barrier endothelia in neural vasculature organized in a three-dimensional network of single vessels. The brain of the lizard Podarcis sicula is characterized by a vascular architecture based on a pattern of paired vessels. The presence and distribution of GLUT-1 were studied in adult lizards using both light and transmission electron microscopic techniques. Immunoperoxidase histochemistry was applied to sections from paraffin-embedded brain using gold-conjugated secondary antibodies to localize this antigen on ultrathin sections. The transverse sectioned pairs of vessels did not show the same size and, in particular, the two elements of the same pair often differed in their diameters. Light microscopy revealed immunopositivity in both parenchymal and meningeal vessels. In each transverse-sectioned vascular pair, one element was intensely labelled, and the adjacent one showed only slight or negligible reaction. Colloidal gold particles were restricted to endothelial cells, showing an asymmetric labelling pattern, which was always characterized by markedly higher density of immunolabelling of the abluminal rather than the luminal plasmalemma. Moreover, in every vascular pair, one profile had lower amounts of scantier labelling by gold particles than the adjacent element. This pattern indicates functional differences between the adjacent vascular limbs regarding glucose transport.

Functional effectiveness of the blood-brain barrier to small water-soluble molecules in developing and adult opossum (Monodelphis domestica)

We have evaluated a small water-soluble molecule, biotin ethylenediamine (BED, 286 Da), as a permeability tracer across the blood-brain barrier. This molecule was found to have suitable characteristics in that it is stable in plasma, has low plasma protein binding, and appears to behave in a similar manner across brain barriers as established by permeability markers such as sucrose. BED, together with a 3000-Da biotin-dextran (BDA3000), was used to investigate the effectiveness of tight junctions in cortical vessels during development and adulthood of a marsupial opossum (Monodelphis domestica). Marsupial species are born at an early stage of brain development when cortical vessels are just beginning to appear. The tracers were administered systemically to opossums at various ages and localized in brains with light and electron microscopy. In adults, the tight junctions restricted the movement of both tracers. In neonates, as soon as vessels grow into the neocortex, their tight junctions are functionally restrictive, a finding supported by the presence of claudin-5 in endothelial cells. However, both tracers are also found within brain extracellular space soon after intraperitoneal administration. The main route of entry for the tracers into immature neocortex appears to be via the cerebrospinal fluid over the outer (subarachnoid) and inner (ventricular) surfaces of the brain. These experiments demonstrate that the previously described higher permeability of barriers to small molecules in the developing brain does not seem to be due to leakiness of cerebral endothelial tight junctions, but to a route of entry probably via the choroid plexuses and cerebrospinal fluid.

Distinct spatio-temporal expression of ABCA and ABCG transporters in the developing and adult mouse brain

Using in situ hybridization for the mouse brain, we analyzed developmental changes in gene expression for the ATP-binding cassette (ABC) transporter subfamilies ABCA1-4 and 7, and ABCG1, 2, 4, 5 and 8. In the embryonic brains, ABCA1 and A7 were highly expressed in the ventricular (or germinal) zone, whereas ABCA2, A3 and G4 were enriched in the mantle (or differentiating) zone. At the postnatal stages, ABCA1 was detected in both the gray and white matter and in the choroid plexus. On the other hand, ABCA2, A3 and A7 were distributed in the gray matter. In addition, marked up-regulation of ABCA2 occurred in the white matter at 14 days-of-age when various myelin protein genes are known to be up-regulated. In marked contrast, ABCA4 was selective to the choroid plexus throughout development. ABCG1 was expressed in both the gray and white matters, whereas ABCG4 was confined to the gray matter. ABCG2 was diffusely and weakly detected throughout the brain at all stages examined. Immunohistochemistry of ABCG2 showed its preferential expression on the luminal membrane of brain capillaries. Expression signals for ABCG5 and G8 were barely detected at any stages. The distinct spatio-temporal expressions of individual ABCA and G transporters may reflect their distinct cellular expressions in the developing and adult brains, presumably, to regulate and maintain lipid homeostasis in the brain.

In Search of the Astrocytic Factor(s) Modulating Blood–Brain Barrier Functions in Brain Capillary Endothelial Cells In Vitro

(1) The blood-brain barrier (BBB) is formed by brain capillary endothelial cells (ECs). There are various cell types, in particular astrocytes, but also pericytes and neurons, located in close vicinity to the capillary ECs which may influence formation and function of the BBB. Based on this consideration, this paper discusses various aspects of the influence of the surrounding cells on brain capillary ECs with special focus on the role of astrocytes. (2) Based on the morphology of the BBB, important aspects of brain EC functions are summarized, such as transport functions and maintenance of low paracellular permeability. Moreover, various facets are discussed with respect to the influence of astrocytes, pericytes, microglia, and neurons on the BBB. Data on the role of glial cells in the ontogenesis of the BBB are presented subsequently. The knowledge on this subject is far from being complete, however, these data imply that the neural/neuronal environment rather than glial cells may be of importance in the maturation of the barrier. (3) The role of glial cells in the induction and maintenance of the BBB is discussed under physiological as well as pathological conditions. Although the literature presents manifold evidence for a great variety of effects induced by astroglia, there are also many controversies, which may result from different cellular models and experimental conditions used in the respective studies. Numerous factors secreted by astrocytes have been shown to induce a BBB phenotype. On the molecular level, increased expression of barrier-relevant proteins (e.g., tight junction proteins) is documented in the presence of astrocyte-derived factors, and many studies demonstrate the improvement of physiological parameters, such as increased transendothelial resistance and decreased paracellular permeability, in different in vitro models of the BBB. Moreover, one has to take into account that the interaction of brain ECs and astrocytes is bi-directional, and that the other cell types surrounding the brain microvasculature also contribute to BBB function or dysfunction, respectively. (4) In conclusion, it is expected that the present and future research focused on molecular mechanisms and signaling pathways will produce new and exciting insights into the complex network of BBB regulation: the cornerstone is laid.

Neurobarrier coupling in the brain: a partner of neurovascular and neurometabolic coupling?

Neurovascular and neurometabolic coupling help the brain to maintain an appropriate energy flow to the neural tissue under conditions of increased neuronal activity. Both coupling phenomena provide us, in addition, with two macroscopically measurable parameters, blood flow and intermediate metabolite fluxes, that are used to dynamically image the functioning brain. The main energy substrate for the brain is glucose, which is metabolized by glycolysis and oxidative breakdown in both astrocytes and neurons. Neuronal activation triggers increased glucose consumption and glucose demand, with new glucose being brought in by stimulated blood flow and glucose transport over the blood-brain barrier. Glucose is shuttled over the barrier by the GLUT-1 transporter, which, like all transporter proteins, has a ceiling above which no further stimulation of the transport is possible. Blood-brain barrier glucose transport is generally accepted as a nonrate-limiting step but to prevent it from becoming rate-limiting under conditions of neuronal activation, it might be necessary for the transport parameters to be adapted to the increased glucose demand. It is proposed that the blood-brain barrier glucose transport parameters are dynamically adapted to the increased glucose needs of the neural tissue after activation according to a neurobarrier coupling scheme. This review presents neurobarrier coupling within the current knowledge on neurovascular and neurometabolic coupling, and considers arguments and evidence in support of this hypothesis.

Hypoxia and hypoxia-inducible factor-1 target genes in central nervous system radiation injury: a role for vascular endothelial growth factor

PURPOSE

Microvascular permeability changes and loss of blood-brain barrier integrity are important features of central nervous system (CNS) radiation injury. Expression of vascular endothelial growth factor (VEGF), an important determinant of microvascular permeability, was examined to assess its role in CNS radiation damage. Because hypoxia mediates VEGF up-regulation through hypoxia-inducible factor-1alpha (HIF1alpha) induction, we studied the relationships of hypoxia, HIF1alpha expression, and expression of VEGF in this damage pathway.

EXPERIMENTAL DESIGN

Expression of HIF1alpha, VEGF, and another hypoxia-responsive gene, glucose transporter-1, was assessed in the irradiated rat spinal cord using immunohistochemistry and in situ hybridization. Hypoxic areas were identified using the nitroimidazole 2-(2-nitro-1H-imidazole-L-yl)-N-(2,2,3,3,3,-pentafluoropropyl) acetamide. To determine the causal importance of VEGF expression in radiation myelopathy, we studied the response of transgenic mice with greater (VEGF-A(hi/+)), reduced (VEGF-A(lo/+)), and wild-type VEGF activity to thoracolumbar irradiation.

RESULTS

In rat spinal cord, the number of cells expressing HIF1alpha and VEGF increased rapidly from 16 to 20 weeks after radiation, before white matter necrosis and forelimb paralysis. A steep dose response was observed in expression of HIF1alpha and VEGF. HIF1alpha and VEGF expressing cells were identified as astrocytes. Hypoxia was present in regions where up-regulation of VEGF and glucose transporter-1 and increased permeability was observed. VEGF-A(lo/+) mice had a longer latency to development of hindlimb weakness and paralysis compared with wild-type or VEGF-A(hi/+) mice.

CONCLUSIONS

VEGF expression appears to play an important role in CNS radiation injury. This focuses attention on VEGF and other genes induced in response to hypoxia as targets for therapy to reduce or prevent CNS radiation damage.

Immunolocalization of tight junction proteins in the adult and developing human brain

The formation of endothelial tight junctions (TJs) is crucial in blood-brain barrier (BBB) differentiation, and the expression and targeting of TJ-associated proteins mark the beginning of BBB functions. Using confocal microscopy, this study analyzed endothelial TJs in adult human cerebral cortex and the fetal telencephalon and leptomeninges in order to compare the localization of two TJ-associated transmembrane proteins, occludin and claudin-5. In the arterioles and microvessels of adult brain, occludin and claudin-5 form continuous bands of endothelial immunoreactivity. During fetal development, occludin and claudin-5 immunoreactivity is first detected as a diffuse labeling of endothelial cytoplasm. Later, at 14 weeks, the immunosignal for both proteins shifts from the cytoplasm to the interface of adjacent endothelial cells, forming a linear, widely discontinuous pattern of immunoreactivity that achieves an adult-like appearance within a few weeks. These results demonstrate that occludin and claudin-5 expression is an early event in human brain development, followed shortly by assembly of both proteins at the junctional areas. This incremental process suggests more rapid establishment of the human BBB, consistent with its specific function of creating a suitable environment for neuron differentiation and neurite outgrowth during neocortical histogenesis.

Immunolocalization of tight junction proteins in the adult and developing human brain

The formation of endothelial tight junctions (TJs) is crucial in blood-brain barrier (BBB) differentiation, and the expression and targeting of TJ-associated proteins mark the beginning of BBB functions. Using confocal microscopy, this study analyzed endothelial TJs in adult human cerebral cortex and the fetal telencephalon and leptomeninges in order to compare the localization of two TJ-associated transmembrane proteins, occludin and claudin-5. In the arterioles and microvessels of adult brain, occludin and claudin-5 form continuous bands of endothelial immunoreactivity. During fetal development, occludin and claudin-5 immunoreactivity is first detected as a diffuse labeling of endothelial cytoplasm. Later, at 14 weeks, the immunosignal for both proteins shifts from the cytoplasm to the interface of adjacent endothelial cells, forming a linear, widely discontinuous pattern of immunoreactivity that achieves an adult-like appearance within a few weeks. These results demonstrate that occludin and claudin-5 expression is an early event in human brain development, followed shortly by assembly of both proteins at the junctional areas. This incremental process suggests more rapid establishment of the human BBB, consistent with its specific function of creating a suitable environment for neuron differentiation and neurite outgrowth during neocortical histogenesis.

Collateral growth and angiogenesis around cortical stroke

BACKGROUND AND PURPOSE

We tested the hypothesis that there are significant long-term local vascular changes after ministroke that could form a basis for functional recovery.

METHODS

A 6- to 8-mm cranial window was opened over the barrel cortex, which was identified by an intrinsic optical signal during mechanical stimulation of the whiskers in anesthetized female Wistar rats. Branches of the middle cerebral artery (MCA) to this region were ligated. Fluorescein isothiocyanate (FITC) transits were recorded by videomicroscopy in each rat just before, immediately after, and 30 days after ligation. Changes in surface vessels and parenchymal perfusion were measured. In similarly prepared rats, angiogenesis was identified by 5-bromo-2-deoxyuridine labeling and immunohistochemistry for the integrin family member alpha(v)beta(3).

RESULTS

The intrinsic optical signal disappeared immediately after MCA ligations. FITC injection just after ligation demonstrated 3 concentric regions: 1 region of unchanged perfusion, surrounding 1 region of reduced perfusion (the ischemic border) surrounding a central core with little observable perfusion. At 30 days, the following had taken place: (1) diameters and lengths of surface collaterals in the ischemic border had grown significantly, but no new surface vessels were detected, (2) FITC entered occluded MCA segments, (3) arteriocapillary latencies in the ischemic border were shortened compared with latencies just after ligation, and (4) small infarcts were virtually identical to the poorly perfused core. Angiogenesis was confined to the ischemic border.

CONCLUSIONS

Arteriolar collateral growth and new capillaries support restored perfusion in the ischemic border after ministroke and could support long-term functional recovery.

A new gene trap construct enriching for insertion events near the 5' end of genes

The gene trap approach is based on the integration of a gene trap vector into the genome. This can be done either by electroporation of a plasmid construct or by infection with a viral vector. Commonly used viral gene trap vectors have been shown to select for integrations near the 5' end of genes. To date, no plasmid vector with a similar tendency has been reported. In this paper we describe a new plasmid vector, pKC199beta geo. This vector contained a short splice acceptor fragment from the Hoxc9 gene, a full length lacZ gene, including an ATG, and a reduced activity, mutant neomycin phosphotransferase gene as a selectable marker. This vector enriched the population of trapped genes in our gene trap screen for insertion events in the 5' end of genes. In the two cases examined the beta-galactosidase activity pattern accurately reflected the endogenous promotor activity.

Immunogold cytochemistry of the blood-brain barrier glucose transporter GLUT1 and endogenous albumin in the developing human brain

The blood-brain barrier (BBB) glucose transporter, GLUT1, was detected by immunogold electron microscopy on the microvascular compartment of the human foetus telencephalon at the 12th and 18th weeks of gestation. By computerized morphometry, the cellular and subcellular localization of the immunosignal for GLUT1 was quantitatively evaluated. The study showed that the glucose transporter is strongly expressed by endothelial cells while a very low signal is detected on vascular pericytes. The GLUT1 antigenic sites are preferentially associated to the ablumenal and junctional plasma membranes of the endothelial cells and tend to increase significantly with age. A parallel study carried out by the endogenous serum protein albumin demonstrated that already at the 12th week the endothelial routes are hindered to the protein as happens at the blood-endothelium interface of mature brain. The results demonstrate that in the human foetus the brain microvessels express BBB-specific functional activities early.

Regulation of glucose transporters during development of the retinal pigment epithelium

The retinal pigment epithelium (RPE) separates the outer retina from its blood supply. To satisfy the retina's large requirement for glucose, the RPE expresses high levels of glucose transporters. In most rat cells, the transporter GLUT3 provides a basal level of transport, but the expression of GLUT1 can be regulated. The opposite is true in chicken (P. Wagstaff, H.Y. Kang, D. Mylott, P.J. Robbins, M.K. White, Characterization of the avian GLUT1 glucose transporter: differential regulation of GLUT1 and GLUT3 in chicken embryo fibroblasts, Mol. Biol. Cell 6 (1995) 1575-1589). We examined chick RPE to determine which isoform is regulated during development, and if the neural retina regulates GLUT expression. By RT-PCR, RPE expressed GLUT1 and GLUT3, but not GLUT2. Only the level of GLUT1 increased between E5 and E18. A corresponding increase in GLUT1 protein was observed by immunoblotting. Most of the increase occurred between E14 and E18, which corresponds to the late stage of tight junction development. A culture model of development was used to examine the intermediate phase, which extends from E7 to E14. While medium conditioned by the neural retina decreased paracellular diffusion across the tight junctions, it increased diffusion through the glucose transporters. Unlike mammals, chick upregulates different isoforms in quiescent RPE and proliferating fibroblasts. Further, the upregulation of glucose transport is coordinated with the development of tight junctions in the blood-retinal barrier.

Neural induction of the blood-brain barrier: still an enigma

1. The study of the blood-brain barrier and its various realms offers a myriad of opportunities for scientific exploration. This review focuses on two of these areas in particular: the induction of the blood-brain barrier and the molecular mechanisms underlying this developmental process. 2. The creation of the blood-brain barrier is considered a specific step in the differentiation of cerebral capillary endothelial cells, resulting in a number of biochemical and functional alterations. Although the specific endothelial properties which maintain the homeostasis in the central nervous system necessary for neuronal function have been well described, the inductive mechanisms which trigger blood-brain barrier establishment in capillary endothelial cells are unknown. 3. The timetable of blood-brain barrier formation is still a matter of debate, caused largely by the use of varying experimental systems and by the general difficulty of quantitatively measuring the degree of blood-brain barrier "tightness." However, there is a general consensus that a gradual formation of the blood-brain barrier starts shortly after intraneural neovascularization and that the neural microenvironment (neurons and/or astrocytes) plays a key role in inducing blood-brain barrier function in capillary endothelial cells. This view stems from numerous in vitro experiments using mostly cocultures of capillary endothelial cells and astrocytes and assays for easily measurable blood-brain barrier markers. In vivo, there are great difficulties in proving the inductive influence of the neuronal environment. Also dealt with in this article are brain tumors, the least understood in vivo systems, and the induction or noninduction of barrier function in the newly established tumor vascularization. 4. Finally, this review tries to elucidate the question concerning the nature of the inductive signal eliciting blood-brain barrier formation in the cerebral microvasculature.

The nature and composition of the internal environment of the developing brain

1. The fetal brain develops within its own environment, which is protected from free exchange of most molecules among its extracellular fluid, blood plasma, and cerebrospinal fluid (CSF) by a set of mechanisms described collectively as "brain barriers." 2. There are high concentrations of proteins in fetal CSF, which are due not to immaturity of the blood-CSF barrier (tight junctions between the epithelial cells of the choroid plexus), but to a specialized transcellular mechanism that specifically transfers some proteins across choroid plexus epithelial cells in the immature brain. 3. The proteins in CSF are excluded from the extracellular fluid of the immature brain by the presence of barriers at the CSF-brain interfaces on the inner and outer surfaces. These barriers are not present in the adult. 4. Some plasma proteins are present within the cells of the developing brain. Their presence may be explained by a combination of specific uptake from the CSF and synthesis in situ. 5. Information about the composition of the CSF (electrolytes as well as proteins) in the developing brain is of importance for the culture conditions used for experiments with fetal brain tissue in vitro, as neurons in the developing brain are exposed to relatively high concentrations of proteins only when they have cell surface membrane contact with CSF. 6. The developmental importance of high protein concentrations in CSF of the immature brain is not understood but may be involved in providing the physical force (colloid osmotic pressure) for expansion of the cerebral ventricles during brain development, as well as possibly having nutritive and specific cell development functions.