Distribution of anionic sites and glycoconjugates on the endothelial surfaces of the developing blood-brain barrier

Development of the blood-brain barrier within the paraventricular nucleus of the hypothalamus: influence of fetal glucocorticoid excess

The blood–brain barrier (BBB) is a critical contributor to brain function. To understand its development and potential function in different brain regions, the postnatal (P) BBB was investigated in the mouse cortex (CTX), lateral hypothalamus, and paraventricular nucleus of the hypothalamus (PVN). Brains were examined on postnatal days (P)12, P22 and P52 for BBB competency and for pericytes as key cellular components of the BBB demarcated by immunoreactive desmin. Glucocorticoid influences (excess dexamethasone; dex) during prenatal development were also assessed for their impact on the blood vessels within these regions postnatally. At P12, there was significantly more extravascular leakage of a low molecular weight dye (fluorescein isothiocyanate) in the CTX than within hypothalamic regions. For pericytes, there were low levels of desmin immunoreactivity at P12 that increased with age for all regions. There was more desmin immunoreactivity present in the PVN at each age examined. Fetal dex exposure resulted in decreased blood vessel density within the PVN at P20. In the CTX, dex exposure increased BBB competency, in contrast to the PVN where there was a decrease in BBB competency and increased pericyte presence. Overall, unique alterations in the functioning of the BBB within the PVN may provide a novel mechanism for fetal antecedent programming that may influence adult disorders.

Targeted delivery of antibody-based therapeutic and imaging agents to CNS tumors: crossing the blood-brain barrier divide

INTRODUCTION

Brain tumors are inherently difficult to treat in large part due to the cellular blood-brain barriers (BBBs) that limit the delivery of therapeutics to the tumor tissue from the systemic circulation. Virtually no large molecules, including antibody-based proteins, can penetrate the BBB. With antibodies fast becoming attractive ligands for highly specific molecular targeting to tumor antigens, a variety of methods are being investigated to enhance the access of these agents to intracranial tumors for imaging or therapeutic applications.

AREAS COVERED

This review describes the characteristics of the BBB and the vasculature in brain tumors, described as the blood-brain tumor barrier (BBTB). Antibodies targeted to molecular markers of central nervous system (CNS) tumors will be highlighted, and current strategies for enhancing the delivery of antibodies across these cellular barriers into the brain parenchyma to the tumor will be discussed. Noninvasive imaging approaches to assess BBB/BBTB permeability and/or antibody targeting will be presented as a means of guiding the optimal delivery of targeted agents to brain tumors.

EXPERT OPINION

Preclinical and clinical studies highlight the potential of several approaches in increasing brain tumor delivery across the BBB divide. However, each carries its own risks and challenges. There is tremendous potential in using neuroimaging strategies to assist in understanding and defining the challenges to translating and optimizing molecularly targeted antibody delivery to CNS tumors to improve clinical outcomes.

Maternal exposure to mercury chloride during pregnancy and lactation affects the immunity and social behavior of offspring

Developmental HgCl2 exposures of F1 offspring (H-2(q/s)) from unsociable SJL/J (H-2(s)) dams with high susceptibility to Hg-induced autoimmunity (SFvF1) and from highly sociable FVB/NJ (FVB; H-2(q)) dams with lower susceptibility to Hg-induced autoimmunity (FvSF1) were investigated. Hg exposure increased the serum IgG levels of all offspring at postnatal day 21 (pnd21) and of SJL/J dams but not of FVB dams. Serum IgG anti-brain antibody (Ab) levels of pnd21 SFvF1 offspring and SJL dams were higher than those of the FvSF1 offspring and FVB dams, but Hg only increased the titers of the FVB dams and their offspring. Hg significantly elevated the presence of IgG in all brain regions of the pnd21 SFvF1 offspring, and the SFvF1 offspring had greater amounts of IgG in the brain than the FvSF1 offspring, which had Hg-induced increases in only two brain regions. Cytokine levels were elevated in the brain regions of Hg-treated pnd21 SFvF1 but not of FvSF1 offspring, and SFvF1 females had more brain regions expressing cytokines than the males. At pnd70, the serum IgG, serum antibrain Abs, amounts of brain IgG, and brain cytokine levels of all of the Hg-treated offspring were equivalent to those of their appropriate controls, suggesting that developmental Hg exposure did not induce chronic immunological effects. However, the social behaviors of Hg-exposed SFvF1 offspring at pnd70 were significantly impaired, and SFvF1 females displayed greater decline in social behaviors than males, suggesting that the higher neuroinflammation of SFvF1 females earlier in life is associated with the altered behavior. Thus, developmental Hg exposure induces long-lasting effects on social behavior of offspring, which is dependent on sex and genetics and the induction of neuroinflammation.

ABC transporters and drug efflux at the blood-brain barrier

The blood-brain barrier (BBB) is a dynamic physical and biological barrier between blood circulation and the central nervous system (CNS). This unique feature of the BBB lies in the structure of the neurovascular unit and its cerebral micro-vascular endothelial cells. The BBB restricts the passage of blood-borne drugs, neurotoxic substances and peripheral immune cells from entering the brain, while selectively facilitating the transport of nutrients across the BBB into the brain. Thus, the integrity and proper function of the BBB is crucial to homeostasis and physiological function of the CNS. A number of transport and carrier systems are expressed and polarized on the luminal or abluminal surface of the BBB to realize these discrete functions. Among these systems, ABC transporters play a critical role in keeping drugs and neurotoxic substances from entering the brain and in transporting toxic metabolites out of the brain. A number of studies have demonstrated that ABCB1 and ABCG2 are critical to drug efflux at the BBB and that ABCC1 is essential for the blood-cerebral spinal fluid (CSF) barrier. The presence of these efflux ABC transporters also creates a major obstacle for drug delivery into the brain. We have comprehensively reviewed the literature on ABC transporters and drug efflux at the BBB. Understanding the molecular mechanisms of these transporters is important in the development of new drugs and new strategies for drug delivery into the brain.

Characterization of two blood-brain barrier mimicking cell lines: distribution of lectin-binding sites and perspectives for drug delivery

In the present study plant lectins with distinct sugar specificities were applied to two blood-brain barrier (BBB) mimicking cell lines, namely human ECV304 and porcine brain microvascular endothelial cells PBMEC/C1-2 in order to elucidate their glycosylation pattern and to evaluate the lectin-cell interaction for lectin-mediated targeting. The bioadhesive properties of fluorescein-labeled lectins were investigated with monolayers as well as single cells using fluorimetry and flow cytometry, followed by confirmation of the specificity of binding. For PBMEC/C1-2 layers highest binding capacity was found for wheat germ agglutinin (WGA), followed by Dolichus biflorus agglutinin (DBA) whereas single cell experiments revealed a predominance of DBA only. Analyzing ECV304 monolayers and single cells, WGA yielded the strongest interaction without any changes during cultivation. The binding capacities of the other lectins increased significantly during differentiation. As similar results to primary cells and brain sections were observed, both cell lines seem to be suitable as models for lectin-interaction studies. Thus, an additional focus was set on the mechanisms involved in uptake and intracellular fate of selected lectins. Cytoinvasion studies were performed with WGA for human ECV304 cells and WGA as well as DBA for PBMEC/C1-2 cells. For both lectins, the association rate to the cells was dependent on temperature which indicated cellular uptake.

MRI of the basement membrane using charged nanoparticles as contrast agents

The integrity of the basement membrane is essential for tissue cellular growth and is often altered in disease. In this work a method for noninvasively detecting the structural integrity of the basement membrane, based on the delivery of cationic iron-oxide nanoparticles, was developed. Cationic particles accumulate due to the highly negative charge of proteoglycans in the basement membrane. The kidney was used to test this technique because of its highly fenestrated endothelia and well-established disease models to manipulate the basement membrane charge barrier. After systemic injection of cationic or native ferritin (CF or NF) in rats, ex vivo and in vivo MRI showed selective accumulation of CF, but not NF, causing a 60% reduction in signal intensity in cortex at the location of individual glomeruli. Immunofluorescence and electron microscopy demonstrated that this CF accumulation was localized to the glomerular basement membrane (GBM). In a model of GBM breakdown during focal and segmental glomerulosclerosis, MRI showed reduced single glomerular accumulation of CF, but a diffuse accumulation of CF in the kidney tubules caused by leakage of CF through the glomerulus. Cationic contrast agents can be used to target the basement membrane and detect the breakdown of the basement membrane in disease.

Griffonia simplicifolia isolectin B4 identifies a specific subpopulation of angiogenic blood vessels following contusive spinal cord injury in the adult mouse

After traumatic spinal cord injury (SCI), disruption and plasticity of the microvasculature within injured spinal tissue contribute to the pathological cascades associated with the evolution of both primary and secondary injury. Conversely, preserved vascular function most likely results in tissue sparing and subsequent functional recovery. It has been difficult to identify subclasses of damaged or regenerating blood vessels at the cellular level. Here, adult mice received a single intravenous injection of the Griffonia simplicifolia isolectin B4 (IB4) at 1-28 days following a moderate thoracic (T9) contusion. Vascular binding of IB4 was maximally observed 7 days following injury, a time associated with multiple pathologic aspects of the intrinsic adaptive angiogenesis, with numbers of IB4 vascular profiles decreasing by 21 days postinjury. Quantitative assessment of IB4 binding shows that it occurs within the evolving lesion epicenter, with affected vessels expressing a temporally specific dysfunctional tight junctional phenotype as assessed by occludin, claudin-5, and ZO-1 immunoreactivities. Taken together, these results demonstrate that intravascular lectin delivery following SCI is a useful approach not only for observing the functional status of neovascular formation but also for definitively identifying specific subpopulations of reactive spinal microvascular elements.

Development of the blood-brain barrier: a historical point of view

Although there has been considerable controversy since the observation by Ehrlich more than 100 years ago that the brain did not take up dyes from the vascular system, the concept of an endothelial blood-brain barrier (BBB) was confirmed by the unequivocal demonstration that the passage of molecules from blood to brain and vice versa was prevented by endothelial tight junctions (TJs). There are three major functions implicated in the term "BBB": protection of the brain from the blood milieu, selective transport, and metabolism or modification of blood- or brain-borne substances. The BBB phenotype develops under the influence of associated brain cells, especially astrocytic glia, and consists of complex TJs and a number of specific transport and enzyme systems that regulate molecular traffic across the endothelial cells. The development of the BBB is a complex process that leads to endothelial cells with unique permeability characteristics due to high electrical resistance and the expression of specific transporters and metabolic pathways. This review article summarizes the historical background underlying our current knowledge of the cellular and molecular mechanisms involved in the development and maintenance of the BBB.

Effects of aging and HIF-1α deficiency on permeability of hippocampal vessels

We examined age-related changes in the blood-brain barrier (BBB) of neural cell-specific hypoxia inducible factor-1alpha (HIF-1alpha) deficient mice, which showed hydrocephalus with neuronal cell loss, to investigate an effect of neural cell-specific HIF-1alpha deficiency or hydrocephalus on vascular function. Vascular permeability of horseradish peroxidase (HRP) and binding of cationized ferritin (CF) particles to the endothelial cell luminal surface, as a marker of glycocalyx, were investigated. The thickness of CF-labeled glycocalyx was significantly decreased in the cortex in mutant mice compared with that of control mice, although it was not paralleled by increased vascular permeability. In addition, strong staining for HRP was seen around vessels located along the hippocampal fissure in 24-month-old mutant mice. The reaction product of HRP appeared in an increasing number of the endothelial cell abluminal vesicles and within the thickened basal lamina of arterioles in the hippocampus, showing increased vascular permeability. There were no leaky vessels in 10-week-old mutant mice or 10-week-old and 24-month-old control mice. These findings suggest the necessity of two factors, aging and hydrocephalus, for BBB dysfunction in HIF-1alpha deficient mice.

Oxidative damage in cerebral vessels of diabetic db/db mice

BACKGROUND

Oxidative stress in diabetes mellitus has recently received increasing attention as it has been proven to be associated with the development of diabetic vascular complications. Our aim was to examine whether microvascular changes, including oxidative damage, were induced in the brains of diabetic animals.

METHODS

The expression of 8-hydroxy-2'-deoxyguanosine (8-OHdG), a marker of oxidative DNA damage, the binding of cationized ferritin, a marker for evaluating endothelial glycocalyx, to the endothelial cells of capillaries and vascular permeability of intravenously injected horseradish peroxidase were examined in the cortices of 12- and 20-week-old db/db and db/+m mice.

RESULTS

Immunostaining for 8-OHdG was clearly seen in the vessels of the cortex of 20-week-old db/db mice, but was hardly seen in those of mice in the other groups. The immunopositive area of 8-OHdG was significantly increased in the cortex of 20-week-old db/db mice compared with that of 20-week-old db/+m mice. No extravasated leakage of horseradish peroxidase was seen in any groups of mice, while the numbers of cationized ferritin particles binding to the endothelial cells was significantly decreased in 12- and 20-week-old db/db mice compared with that of db/+m mice at the same age, respectively.

CONCLUSION

These findings suggest that changes in endothelial glycocalyx are induced in db/db mice and, in addition, the long-term diabetic condition of these mice induces oxidative DNA damage to the cerebral vessels.

Highly differential expression of SN1, a bidirectional glutamine transporter, in astroglia and endothelium in the developing rat brain

The transmitters glutamate and GABA also subserve trophic action and are required for normal development of the brain. They are formed from glutamine, which may be synthesized in glia or extracted from the blood. In the adult, the glutamine transporter SN1 is expressed in the astroglia. SN1 works in both directions, depending on the concentration gradients of its substrates and cotransported ions, and is thought to regulate extracellular glutamine and to supply the neurons with the transmitter precursor. In this article, we have quantified the expression and studied the localization of SN1 at different developmental stages. SN1 is expressed in astroglia throughout the CNS from embryonic stages through adulthood. No indication of SN1 staining in neuronal elements has been obtained at any stage. Quantitative immunoblotting of whole brain extracts demonstrates increasing expression of SN1 from P0, reaching a peak at P14, twice the adult level. A moderate and slower rise and fall of the expression levels of SN1 occurs in the cerebellum. Strong transient SN1-like staining is also found in Bergmann glia and vascular endothelium in the first postnatal weeks. Strong intracellular staining in the same time period suggests a high rate of SN1 synthesis in the early postnatal period. This coincides with the increasing levels of glutamate and GABA in the CNS and with the time course of synaptogenesis. This study suggests that the expression of SN1 is highly regulated, correlating with the demand for glutamine during the critical period of development.

Histochemical characterization of primary capillary endothelial cells from porcine brains using monoclonal antibodies and fluorescein isothiocyanate-labelled lectins: implications for drug delivery

Primary endothelial cells isolated from cerebral microvessels by combined mechanical and enzymatic treatment from porcine brains were characterized with regard to identity, purity and membrane surface characteristics. Cells were grown in culture to adherent monolayers and characterized morphologically and histochemically by their binding for fluorescently-labelled lectins and monoclonal antibodies detected by indirect immunofluorescence. The binding patterns of the cells were compared with the affinity of frozen tissue sections of porcine brain cortex for the markers. Endothelial cells in culture were characterized by the binding of von Willebrand factor, vimentin and fibronectin antibodies. They failed to react with anti-glial fibrillary acid protein, anti-galactocerebroside C and anti-neurofilament 160 antibodies characteristic for astrocytes, oligodendrocytes and neurons, respectively. Cell cultures were stained by the lectins, wheat germ agglutinin, horse gram agglutinin and soybean agglutinin, demonstrating the presence of N-acetylglucosamine and N-acetylgalactosamine residues on membrane surface. Binding sites for concanavalin A and peanut agglutinin characteristic for mannose and galactose could not be detected. Cell age and differentiation had no effect on lectin and antibody staining. Cell cultures gave staining results similar to those of microvessels in frozen tissue sections. The results of morphology, antibody and lectin staining pattern indicate that our in vitro endothelial cell culture model retained many histological characteristics observed for capillary microvessels in vivo and appears to be suitable for studying uptake and targeting properties of drug carrier systems with regard to the blood-brain barrier.

The blood-nerve barrier: enzymes, transporters and receptors--a comparison with the blood-brain barrier

The blood-brain barrier (BBB) has been much more extensively investigated than the blood-nerve barrier (BNB). Nevertheless it is clear that there are both similarities and differences in the molecular and morphophysiological characteristics of the two barrier systems. A number of enzymes, transporters and receptors have been investigated at both the BNB and BBB, as well as in the perineurium of peripheral nerves, which is also a metabolically active diffusion barrier. While there have been few systematic comparisons of the distribution of these molecules in both the BNB and BBB, it is apparent from the data available, reviewed in this article, that their distribution also supports the concept of the BNB and BBB having some features in common but also showing distinct identities. These similarities and differences cannot simply be accounted for by the presence of the inductive influences of astrocytes at the BBB and absence at the BNB. Whether the Schwann cell also has the capacity to induce some BNB properties remains to be determined.

Developmental pattern of reactive oxygen species generation and antioxidative defense machinery in rat cerebral microvessels

The developmental profile of certain enzymatic antioxidants as well as the generation of reactive oxygen species was studied in the rat cerebral microvessels during first three weeks of life and the levels were compared to those present in adults. The data showed a higher generation of superoxide anion (+67%) and H2O2 (+200%) at postnatal day (PND) 21. Superoxide anion production was significantly lower (-24%) at PND 14 and almost comparable to adult values at PND 7. The activity of superoxide dismutase increased with development and attained an adult level at PND 21. Catalase was higher in neonates with a maximum activity at PND 7 and 14 (+68, 69%). The measurement of microvessel glutathione and glutathione-related antioxidant enzymes showed that glutathione level was higher at PND 7, which declined to an adult level at PND 14. Se-dependent GPx showed a marked increase between PND 14 and 21, however, it declined in adults. The activity of Se-independent glutathione peroxidase was very low in cerebral microvessels. Glutathione reductase activity in 7-day-old, that was comparable to adult level, declined at PND 14 and 21. The level of glutathione S-transferase was higher (+43%) at PND 21. The activity of microvessel marker enzyme gamma-glutatmyl transpeptidase increased with age, whereas, alkaline phosphatase showed a slight increase up to PND 14 and thereafter it declined. Lipid peroxidation was found to be significantly lower (-18%) at PND 21 as compared to adults. It may be concluded that developing cerebral microvessels contain high levels of several antioxidant enzymes that are more or equal to those present in adult brain microvessels.

Development of blood-labyrinth barrier in the semicircular canal ampulla of the rat

Cationic polyethyleneimine (PEI) administered intravenously was transported to anionic sites on the capillary and subepithelial basal laminae (BL) in the vestibular labyrinth. Therefore, changes in the PEI distribution on the BL reflect changes in the transport system in the vestibular labyrinth. A 0.1% PEI solution was administered intravenously (7.5 ml/kg) to developing (1, 4, 7, 14 days after birth) and adult rats in order to investigate the development of the macromolecular transport in the ampulla of the semicircular canal as a function of age. After 1 h, the bony labyrinth was removed and embedded in Epoxy resin. Ultrathin sections of the ampulla were then examined with a transmission electron microscope. In the subepithelial BL in the dark cell area and capillary BL in the crista ampullaris, the PEI distribution in both 1- and 4-day-old rats was markedly increased compared to that in either 7-, 14-day or adult rats. In the sensory cells in 1-, 4-day or 7-day-old rats, PEI density and area was significantly greater than in the adult rats. These findings suggest that the macromolecular transport system in the developing rat ampulla becomes mature by 14 days after birth and that the maturation of its transport system in the ampulla is strongly associated with that in the stria vascularis.

Development of the blood-labyrinth barrier in the rat

Systemically administered cationic polyethyleneimine (PEI) passes through the capillary endothelial cell and attaches to anionic sites on the capillary basal lamina (BL). Thus, the distribution of PEI on the BL reflects the changes in the endothelial cell transport system. A 0.1% PEI solution was administered by intravenous injection (7.5 ml/kg) to developing (4, 7, 11, 14, 21 days after birth) and adult rats to evaluate the development of endothelial cell transport in the cochlear capillary as related to age. One hour later, the bony labyrinth was removed and embedded in Epoxy resin. Ultrathin sections of the cochlear lateral wall were then viewed with a transmission electron microscope. The distribution of PEI in the capillary BL in the stria vascularis of the 4-, 7- and 11-day-old rats was significantly greater compared to the adult rats. The distribution of PEI in the capillary BL in the spiral ligament of the 4- and 7-day-old rats was also significantly greater compared to the adult rats. These findings suggest that the endothelial cell transport is more robust in the developing rat cochlea and that the blood-labyrinth barrier becomes mature by 14 days after birth in rats.

Ontogenetic differences in convulsive action and cerebral uptake of uremic guanidino compounds in juvenile mice

Guanidinosuccinate (GSA) and methylguanidine (MG) are endogenous, convulsant guanidino compounds which have been shown to be greatly increased in uremic patients. In the present study, we have investigated the age-related differences in convulsive action and cerebral uptake of these compounds in juvenile mice of 7, 14 and 21 days old. An age-dependent decrease was apparent in the severity of the GSA- and MG-induced convulsions and toxicity. Mean latency for the appearance of clonic convulsions increased with increasing age. Two hours following the i.p. injection of GSA or MG in a dose of 250 mg/kg, the resulting brain concentration decreased with increasing age of the animals. This effect was more pronounced in the case of MG. Neither for GSA, nor for MG was this age-dependent effect apparent after 30 min. GSA and MG serum as well as brain concentrations were lower in 21-day-old mice than in 7-day-old ones. However, the brain/serum concentration ratios of GSA and of MG were significantly lower in 21-day-old mice than in 7-day-old ones, indicating that at least part of the difference in brain level can be explained by higher permeability of the immature blood-brain barrier to these uremic guanidino compounds. In addition, brain/serum ratios of GSA in mice of 7 days old and in mice of 21 days old were significantly lower than the ratios of MG in these age groups, indicative of lower overall blood-brain barrier permeability to GSA than to MG.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorptive-mediated endocytosis of an adrenocorticotropic hormone (ACTH) analogue, ebiratide, into the blood-brain barrier: studies with monolayers of primary cultured bovine brain capillary endothelial cells

The internalization of a neuromodulatory adrenocorticotropic hormone (ACTH) analogue, [125I]ebiratide (H-Met(O2)-Glu[125I]His-Phe-D-Lys-Phe-NH(CH2)2NH2), was examined in cultured monolayers of bovine brain capillary endothelial cells (BCEC). HPLC analysis of the incubation solution showed that [125I]ebiratide was not metabolized during the incubation with BCEC. The acid-resistant binding of [125I]ebiratide to BCEC increased with time for 120 min and showed a significant dependence on temperature and medium osmolarity. Pretreatment of BCEC with dansylcadaverine or phenylarsine oxide, endocytosis inhibitors, and 2,4-dinitrophenol, a metabolic inhibitor, decreased significantly the acid-resistant binding of [125I]ebiratide. The acid-resistant binding of [125I]ebiratide was saturable in the presence of unlabeled ebiratide (100 nM-1 mM). The maximal internalization capacity (Bmax) at 30 min was 7.96 +/- 3.27 pmol/mg of protein with a half-saturation constant (Kd) of 15.9 +/- 6.4 microM. The acid-resistant binding was inhibited by basic peptides such as poly-L-lysine, protamine, histone, and ACTH but was not inhibited by poly-L-glutamic acid, insulin, or transferrin. These results confirmed that ebiratide is transported through the blood-brain barrier via an absorptive-mediated endocytosis.

Molecular anatomy of the blood-brain barrier as defined by immunocytochemistry

This review outlines the recent developments and improvements of our knowledge concerning the molecular composition of the BBB as revealed by immunocytochemistry. Data have been accumulated which show that the BBB exhibits a specific collection of structural and metabolic properties which are also found in tight transporting epithelia. This conclusion is substantiated by (i) the implementation of antibodies which recognize proteins of non-BBB origin, to show that these biochemical markers and the functions that they represent are localized in the BBB endothelium; and (ii) the characterization of target molecules to which polyclonal or monoclonal antibodies which have been generated to epitopes of the BBB endothelium or brain homogenates. According to these data the protein assemblies comprising the phenotypical appearance of the BBB can therefore be defined by the particular selection as well as topological expression of common epithelial antigens, rather than the expression of BBB-unique molecular species. In this respect the immunocytochemical data corroborate the physiological assumption that the BBB possesses the character of a specific polarized epithelium. Attention is also given to the description of developmental expression of BBB-related immunomarkers. By collecting the data from different sources we introduce a classification of the BBB marker proteins according to their developmental appearance. Three groups of proteins are classified with respect to their sequential expression around the time of BBB closure: Phase E (early) markers which appear before BBB closure, phase I (intermediate) markers which are expressed at the time of BBB tightening, and phase L (late) markers which are detectable after the closure of the BBB. Such a scheme may to be useful in better defining the maturation process of BBB, which apparently is not a momentary event in brain development, but rather consists of a temporally sequenced process of hierarchically structured gene expression which finally define the molecular properties of the BBB. This process continues even after parturition, especially with regard to the achievement of immunological properties of the mature BBB. By examining the developmental spatio-temporal expression of different BBB markers we conclude that the mechanisms governing the pattern of BBB maturation are not limited to the interactions occurring between glial and endothelial cells. We therefore suggest a heuristic model in a triangular interrelationship that includes differentiation effects of neurons on glia and of glia cells on the BBB endothelium.(ABSTRACT TRUNCATED AT 400 WORDS)

Structural and functional aspects of the interaction of inflammatory cells with the blood-brain barrier in experimental brain inflammation

Interaction between various subclasses of inflammatory cells (ICs) and endothelial cells (ECs) lining selective blood vessels of the mammalian blood-brain barrier (BBB) is an initial, important event during inflammatory conditions of the central nervous system (CNS). In this review, we will present a brief ultracytochemical and immunocytochemical assessment of our perspective on this intimate cellular interaction which has been described recently in conditions that involve immunological alterations of the BBB. We will discuss some morphological aspects of what is currently known about acute and chronic inflammatory BBB disorders that are involved in inflammatory processes. We will focus, in particular, on experimental allergic encephalomyelitis (EAE), an animal model for human multiple sclerosis (MS). Many of the past and more recent concepts found in the literature concerning IC attachment and diapedesis, as well as our own experimental efforts over more than two decades will be presented.

Blood-nerve barrier: distribution of anionic sites on the endothelial plasma membrane and basal lamina

The distribution of anionic sites on the cell membranes and basal laminae of vascular endothelial cells in the rat sciatic nerve was investigated using cationic ferritin (CF) and cationic colloidal gold (CCG). Nerves fixed by perfusion followed by immersion were chopped into 400 microns thick slices and incubated in CF or embedded in LR White resin for staining with CCG. Using electron microscopy, the distribution of these tracers was investigated. The results indicated that microdomains of various charge densities exist. Diaphragms of caveolae and transendothelial channels, and luminal endothelial processes are highly anionic, the basal laminae of endothelial cells and pericytes and luminal membranes are medium and abluminal membranes least anionic. Inter-endothelial tight junctions were unlabelled and not penetrated by CF. These structures are thought to represent charge and size filters that control permeability of the vasa nervorum. The distribution of these charge-size filters is discussed in terms of the blood-nerve barrier, a physiological property present in the endo- but absent in the peri- and epineurial vessels.

Sequential appearance of anionic domains in the developing blood-brain barrier

The distribution of anionic sites in the walls of mouse brain micro-blood vessels (MBVs) during development and maturation of the blood-brain barrier (BBB) was studied by electron microscopy. Cationic colloidal gold (CCG) and Lowicryl K4M-embedded brain samples obtained from mouse fetuses (13th and 19th days) and from 1-, 5-, 12- and 24-day-old and adult mice were used. The labeling of anionic sites with CCG was more intense on the abluminal than on the luminal front of the endothelial cells (ECs) in fetuses and in newborn mice. Only a few anionic sites appear on the luminal front of the ECs of proliferating blood vessels invading the neural tissue in 13-day-old fetuses. They become slightly, although steadily, more abundant during further stages of development, and their number rapidly increases between the 12th and 24th day of life at which time they attain the density typical for mature animals. The maturation of the basement membrane (BM), which occurs during the myelinization period (12th-24th day of life), also coincides with an increasing concentration of anionic sites. These observations suggest that the gradual appearance of anionic sites on both fronts of the endothelium, as well as in the developing and maturing BM, represents one of the mechanisms responsible for differentiation of cerebral microvasculature into BBB-type MBVs.

Blood-brain barrier transport of cationized immunoglobulin G: enhanced delivery compared to native protein

IgG molecules are potential neuropharmaceuticals that may be used for therapeutic or diagnostic purposes. However, IgG molecules are excluded from entering brain, owing to a lack of transport of these plasma proteins through the brain capillary wall, or blood-brain barrier (BBB). The possibility of enhanced IgG delivery through the BBB by cationization of the proteins was explored in the present studies. Native bovine IgG molecules were cationized by covalent coupling of hexamethylenediamine and the isoelectric point was raised to greater than 10.7 based on isoelectric focusing studies. Native and cationized IgG molecules were radiolabeled with 125I and chloramine T. Cationized IgG, but not native IgG, was rapidly taken up by isolated bovine brain microvessels, which were used as an in vitro model system of the BBB. Cationized IgG binding was time and temperature dependent and was saturated by increasing concentrations of unlabeled cationized IgG (dissociation constant of the high-affinity binding site, 0.90 +/- 0.37 microM; Bmax, 1.4 +/- 0.4 nmol per mg of protein). In vivo studies documented enhanced brain uptake of 125I-labeled cationized IgG relative to [3H]albumin, and complete transcytosis of the 125I-labeled cationized IgG molecule through the BBB and into brain parenchyma was demonstrated by thaw-mount autoradiography of frozen sections of rat brain obtained after carotid arterial infusions of 125I-labeled cationized IgG. These studies demonstrate that cationization of IgG molecules greatly facilitates the transport of these plasma proteins through the BBB in vivo, and this process may provide a new strategy for IgG delivery through the BBB.

Role of molecular charge in disruption of the blood-brain barrier during acute hypertension

Acute hypertension disrupts the blood-brain barrier and may neutralize the negative charge on cerebral endothelium. The goal of this study was to determine the effects of molecular charge on permeability of the blood-brain barrier during acute hypertension. Intravital fluorescent microscopy and fluorescein-labeled dextrans were used to evaluate disruption of the blood-brain barrier during acute hypertension in rats. Disruption of the blood-brain barrier was quantitated by calculating clearance of neutral dextran and of anionic dextran sulfate in two groups of rats. Pressure in pial venules, which are the primary site of disruption of the blood-brain barrier during acute hypertension, was measured using a servo-null device. When systemic arterial pressure was increased from 87 +/- 5 (mean +/- SEM) to 188 +/- 5 mm Hg, clearance of neutral dextran increased from 0.04 +/- 0.01 to 4.38 +/- 0.72 ml/sec x 10(-6). When systemic arterial pressure was increased from 91 +/- 4 to 181 +/- 3 mm Hg, clearance of anionic dextran sulfate increased from 0.02 +/- 0.01 to only 0.70 +/- 0.23 ml/sec x 10(-6). Increases in pial venular pressure were similar in the two groups. Thus, similar increases in systemic arterial pressure and pial venular pressure during acute hypertension produce less disruption of the blood-brain barrier to anionic dextran sulfate than neutral dextran. The findings suggest that 1) the net negative charge of cerebral vessels may be preserved during acute hypertension, and 2) molecular charge is an important determinant of the severity of disruption of the blood-brain barrier during acute hypertension.

Sites of egress of inflammatory cells and horseradish peroxidase transport across the blood-brain barrier in a murine model of chronic relapsing experimental allergic encephalomyelitis

Results are reported of experiments designed to focus at attachment sites of inflammatory cells (ICs) on the luminal surface of brain endothelial cells (ECs) and on the mechanisms of horseradish peroxidase (HRP) transport across the altered blood-brain barrier (BBB) in a murine model of chronic relapsing experimental allergic encephalomyelitis. Cationized ferritin (CF) served as a marker for evaluating the electrostatic nature of brain microblood vessels (MBVs) on the plasma membranes of ICs or normal mouse peripheral white blood cells and erythrocytes. SJL/J mice demonstrating clinical illness were given HRP or CF, in vivo or in situ, respectively. Light microscopy and conventional transmission electron microscopy of cerebellum or thoracic and lumbar spinal cord regions demonstrated HRP leakage most pronounced in MBVs with perivascular infiltrates. HRP traversed across the ECs via numerous vesicles and tubular profiles located mostly in the parajunctional regions, while EC junctions appeared closed. Scanning electron microscopy demonstrated that IC attachment was primarily at parajunctional sites on the EC surface. We also observed increased microvillar projections extending from the EC surface into the lumen. CF demonstrated a patchy decoration on both the luminal EC surface and IC membranes but did not label uncoated invaginating membrane pits or tubular structures. Our data indicate that the points of attachment of the ICs on the EC surface may reflect specific receptor sites where the ICs eventually gain entrance into CNS across the BBB during brain inflammation.

Ultrastructural studies of glycoconjugates in brain micro-blood vessels and amyloid plaques of scrapie-infected mice

Lectin or glycoprotein-gold complexes and samples of scrapie-infected mouse brain embedded in Lowicryl K4M were used for ultrastructural localization of glycoconjugates. The lectins tested recognize the following residues: beta-D-galactosyl [RCA, Ricinus communis agglutinin (aggl.) 120], N-acetyl and N-glycolyl neuraminic acid (LFA, Limax flavus aggl.), N-acetyl-D-glucosaminyl and sialyl (WGA, Wheat germ aggl.), N-acetyl-D-galactosaminyl (HPA, Helix pomatia aggl., and DBA, Dolichos biflorus aggl.), alpha-D-mannosyl/alpha-D-glucosyl (Con A, Concanavalin A), alpha-D-galactosyl and alpha-D-galactopyranoside (BSA, Bandeirea simplicifolia aggl., izolectin B4). Labeling of the majority of micro-blood vessels (MBVs) located outside the plaque area and in the remaining cerebral cortex was similar to that which has been previously observed in non-infected animals. Some MBVs, however, located inside the plaque area and surrounded directly by amyloid fibers showed attenuation of the endothelium, the surface of which was scarcely and irregularly decorated with RCA, LFA, WGA and Con A. These abnormalities in the composition of glycoconjugates can be associated with previously noted increased permeability of some MBVs in the brains of scrapie-infected mice. Some vessels in the plaque area were encapsulated by perivascular deposits of homogeneous or flocculogranular material containing several glycoconjugates. A very intimate structural relation between reactive (microglial-like) cells and amyloid fibers suggests the participation of these cells in elaboration of plaque material. Labeling of the cell surface and adjacent amyloid fibers with the same lectins (RCA, WGA, DBA, Con A) suggests the possibility that the glycosylation of these fibers occurs extracellularly. Only WGA and DBA were occasionally labeling some Golgi elements of the reactive cells.

Current aspects of the development of the blood-brain barrier

Results of earlier studies clearly indicated that, during development, a number of enzymes are sequentially expressed in the brain endothelial cells correlating in time with the maturation of brain tissue. More recent data suggested that differentiation of endothelium in the intraparenchymal cerebral microvessels into one with blood-brain barrier characteristics seems to be induced by astrocytes at a specific time of embryonic development. Details of the above-mentioned and other important aspects of the development of the blood-brain barrier will be discussed in the present mini review.