Enhanced expression of the blood-brain barrier GLUT1 glucose transporter gene by brain-derived factors

An In Vitro Model of the Blood-Brain Barrier for the Investigation and Isolation of the Key Drivers of Barriergenesis

The blood-brain barrier (BBB) tightly regulates substance transport between the bloodstream and the brain. Models for the study of the physiological processes affecting the BBB, as well as predicting the permeability of therapeutic substances for neurological and neurovascular pathologies, are highly desirable. Existing models, such as Transwell utilizing-models, do not mimic the extracellular environment of the BBB with their stiff, semipermeable, non-biodegradable membranes. To help overcome this, we engineered electrospun membranes from poly L-lactic acid in combination with a nanometric coating of poly(ethyl acrylate) (PEA) that drives fibrillogenesis of fibronectin, facilitating the synergistic presentation of both growth factors and integrin binding sites. Compared to commercial semi-porous membranes, these membranes significantly improve the expression of BBB-related proteins in brain endothelial cells. PEA-coated membranes in combination with different growth factors and extracellular protein coatings reveal nerve growth factor (NGF) and fibroblast growth factor (FGF-2) caused formation of better barriers in vitro. This BBB model offers a robust platform for studying key biochemical factors influencing barrier formation that marries the simplicity of the Transwell model with the highly tunable electrospun PEA-fibronectin membranes. This enables the generation of high-throughput drug permeability models without the need of complicated co-culture conditions.

Does synaptic hypometabolism or synaptic dysfunction, originate cognitive loss? Analysis of the evidence

Elderly persons with currently normal cognition who have cerebral hypometabolism as shown by low uptake of 18fluorine-fluorodeoxyglucose (18F-FDG), are at risk of future loss of cognition and, thus, of future Alzheimer's dementia (AD). Reduction of either 18F-FDG or cognition is assumed to reflect synaptic dysfunction, since synapses account for the majority of glucose use by the brain and cognition depends upon accurate synaptic function. The chronology of the connection between reduced cerebral synaptic function and hypometabolism is, therefore, a critical question, because if synaptic dysfunction came first, then correcting the hypometabolism would likely not benefit synaptic function; but if hypometabolism came first, then correcting the hypometabolism probably would benefit synaptic function. That correction might prevent initiation of the cognitive loss that eventuates in AD and, thereby, would benefit the vast numbers of persons in their eighth to tenth decades of life who are at risk for AD. Among the many citations reviewed in this presentation, seven show hypometabolism that precedes synaptic dysfunction, and two show the reverse. Thus the preponderance of evidence, 78%, suggests that the initiating event is synaptic hypometabolism and that it is 3.5-fold less likely that synaptic dysfunction is the initiator. In addition, it is inherently unlikely that synaptic dysfunction causes hypometabolism. This conclusion could be tested by a clinical trial whose primary objective would be to assess the benefit to cognition of improving synaptic metabolism in patients who are at risk for cognitive loss.

Caspase-1 has a critical role in blood-brain barrier injury and its inhibition contributes to multifaceted repair

Background

Excessive inflammation might activate and injure the blood-brain barrier (BBB), a common feature of many central nervous system (CNS) disorders. We previously developed an in vitro BBB injury model in which the organophosphate paraoxon (PX) affects the BBB endothelium by attenuating junctional protein expression leading to weakened barrier integrity. The objective of this study was to investigate the inflammatory cellular response at the BBB to elucidate critical pathways that might lead to effective treatment in CNS pathologies in which the BBB is compromised. We hypothesized that caspase-1, a core component of the inflammasome complex, might have important role in BBB function since accumulating evidence indicates its involvement in brain inflammation and pathophysiology.

Methods

An in vitro human BBB model was employed to investigate BBB functions related to inflammation, primarily adhesion and transmigration of peripheral blood mononuclear cells (PBMCs). Caspase-1 pathway was studied by measurements of its activation state and its role in PBMCs adhesion, transmigration, and BBB permeability were investigated using the specific caspase-1 inhibitor, VX-765. Expression level of adhesion and junctional molecules and the secretion of pro-inflammatory cytokines were measured in vitro and in vivo at the BBB endothelium after exposure to PX. The potential repair effect of blocking caspase-1 and downstream molecules was evaluated by immunocytochemistry, ELISA, and Nanostring technology.

Results

PX affected the BBB in vitro by elevating the expression of the adhesion molecules E-selectin and ICAM-1 leading to increased adhesion of PBMCs to endothelial monolayer, followed by elevated transendothelial-migration which was ICAM-1 and LFA-1 dependent. Blocking caspase-8 and 9 rescued the viability of the endothelial cells but not the elevated transmigration of PBMCs. Inhibition of caspase-1, on the other hand, robustly restored all of barrier insults tested including PBMCs adhesion and transmigration, permeability, and VE-cadherin protein levels. The in vitro inflammatory response induced by PX and the role of caspase-1 in BBB injury were corroborated in vivo in isolated blood vessels from hippocampi of mice exposed to PX and treated with VX-765.

Conclusions

These results shed light on the important role of caspase-1 in BBB insult in general and specifically in the inflamed endothelium, and suggest therapeutic potential for various CNS disorders, by targeting caspase-1 in the injured BBB.

Identification of a combination of transcription factors that synergistically increases endothelial cell barrier resistance

Endothelial cells (ECs) display remarkable plasticity during development before becoming quiescent and functionally mature. EC maturation is directed by several known transcription factors (TFs), but the specific set of TFs responsible for promoting high-resistance barriers, such as the blood-brain barrier (BBB), have not yet been fully defined. Using expression mRNA data from published studies on ex vivo ECs from the central nervous system (CNS), we predicted TFs that induce high-resistance barrier properties of ECs as in the BBB. We used our previously established method to  generate ECs from human pluripotent stem cells (hPSCs), and then we overexpressed the candidate TFs in hPSC-ECs and measured barrier resistance and integrity using electric cell-substrate impedance sensing, trans-endothelial electrical resistance and FITC-dextran permeability assays. SOX18 and TAL1 were the strongest EC barrier-inducing TFs, upregulating Wnt-related signaling and EC junctional gene expression, respectively, and downregulating EC proliferation-related genes. These TFs were combined with SOX7 and ETS1 that together effectively induced EC barrier resistance, decreased paracellular transport and increased protein expression of tight junctions and induce mRNA expression of several genes involved in the formation of EC barrier and transport. Our data shows identification of a transcriptional network that controls barrier resistance in ECs. Collectively this data may lead to novel approaches for generation of in vitro models of the BBB.

Blood-Brain Barrier Cellular Responses Toward Organophosphates: Natural Compensatory Processes and Exogenous Interventions to Rescue Barrier Properties

Organophosphorus compounds (OPs) are highly toxic chemicals widely used as pesticides (e.g., paraoxon (PX)- the active metabolite of the insecticide parathion) and as chemical warfare nerve agents. Blood-brain barrier (BBB) leakage has been shown in rodents exposed to PX, which is an organophosphate oxon. In this study, we investigated the cellular mechanisms involved in BBB reaction after acute exposure to PX in an established in vitro BBB system made of stem-cell derived, human brain-like endothelial cells (BLECs) together with brain pericytes that closely mimic the in vivo BBB. Our results show that PX directly affects the BBB in vitro both at toxic and non-toxic concentrations by attenuating tight junctional (TJ) protein expression and that only above a certain threshold the paracellular barrier integrity is compromised. Below this threshold, BLECs exhibit a morphological coping mechanism in which they enlarge their cell area thus preventing the formation of meaningful intercellular gaps and maintaining barrier integrity. Importantly, we demonstrate that reversal of the apoptotic cell death induced by PX, by a pan-caspase-inhibitor ZVAD-FMK (ZVAD) can reduce PX-induced cell death and elevate cell area but do not prevent the induced BBB permeability, implying that TJ complex functionality is hindered. This is corroborated by formation of ROS at all toxic concentrations of PX and which are even higher with ZVAD. We suggest that while lower levels of ROS can induce compensating mechanisms, higher PX-induced oxidative stress levels interfere with barrier integrity.

Angiopoietin/Tie2 Axis Regulates the Age-at-Injury Cerebrovascular Response to Traumatic Brain Injury

Although age-at-injury influences chronic recovery from traumatic brain injury (TBI), the differential effects of age on early outcome remain understudied. Using a male murine model of moderate contusion injury, we investigated the underlying mechanism(s) regulating the distinct response between juvenile and adult TBI. We demonstrate similar biomechanical and physical properties of naive juvenile and adult brains. However, following controlled cortical impact (CCI), juvenile mice displayed reduced cortical lesion formation, cell death, and behavioral deficits at 4 and 14 d. Analysis of high-resolution laser Doppler imaging showed a similar loss of cerebral blood flow (CBF) in the ipsilateral cortex at 3 and 24 h post-CCI, whereas juvenile mice showed enhanced subsequent restoration at 2-4 d compared with adults. These findings correlated with reduced blood-brain barrier (BBB) disruption and increased perilesional vessel density. To address whether an age-dependent endothelial cell (EC) response affects vessel stability and tissue outcome, we magnetically isolated CD31⁺ ECs from sham and injured cortices and evaluated mRNA expression. Interestingly, we found increased transcripts for BBB stability-related genes and reduced expression of BBB-disrupting genes in juveniles compared with adults. These differences were concomitant with significant changes in miRNA-21-5p and miR-148a levels. Accompanying these findings was robust GFAP immunoreactivity, which was not resolved by day 35. Importantly, pharmacological inhibition of EC-specific Tie2 signaling abolished the juvenile protective effects. These findings shed new mechanistic light on the divergent effects that age plays on acute TBI outcome that are both spatial and temporal dependent.SIGNIFICANCE STATEMENT Although a clear "window of susceptibility" exists in the developing brain that could deter typical developmental trajectories if exposed to trauma, a number of preclinical models have demonstrated evidence of early recovery in younger patients. Our findings further demonstrate acute neuroprotection and improved restoration of cerebral blood flow in juvenile mice subjected to cortical contusion injury compared with adults. We also demonstrate a novel role for endothelial cell-specific Tie2 signaling in this age-related response, which is known to promote barrier stability, is heightened in the injured juvenile vasculature, and may be exploited for therapeutic interventions across the age spectrum following traumatic brain injury.

Age-dependent metabolic dysregulation in cancer and Alzheimer's disease

Age is the main risk factor for cancer and neurodegeneration; two radically divergent diseases. Yet selective pressure to meet cellular metabolic needs may provide a common mechanism linking these two disorders. The exclusive use of glycolysis, despite the presence of oxygen, is commonly referred to as aerobic glycolysis and is the primary metabolic pathway of cancer cells. Recent evidence suggests that aerobic glycolysis is also a key regulator of synaptic plasticity in the brain that may positively influence cognition. Elevated aerobic glycolysis is a contributing factor to the development of cancer as increased glycolytic flux plays an important role in the biosynthesis of macromolecules and promotes proliferation. In contrast, decreased aerobic glycolysis in the brain occurs with age and could lead to a loss of cell survival mechanisms that counter pathogenic processes underlying neurodegeneration. In this review we discuss the recent findings from epidemiological studies demonstrating an inverse comorbidity of cancer and Alzheimer's disease. We summarize evidence linking the two diseases through changes in metabolism over the course of normal aging. We discuss the key steps and regulatory mechanisms of aerobic glycolysis and mitochondrial oxidative phosphorylation which could be exploited for the development of novel therapies. In addition, we outline the regulation of aerobic glycolysis at the transcriptional level by HIF-1α and Pin1 and their roles in cancer and neurodegeneration. Finally, we provide a possible explanation for metabolic dysregulation that occurs with age, and how it may be a contributing factor to age-related diseases. Determining how metabolism becomes dysregulated over time could lead to the development of effective interventions for ensuring metabolic homeostasis and healthy aging.

A genomic comparison of in vivo and in vitro brain microvascular endothelial cells

The blood-brain barrier (BBB) is composed of uniquely differentiated brain microvascular endothelial cells (BMEC). Often, it is of interest to replicate these attributes in the form of an in vitro model, and such models are widely used in the research community. However, the BMEC used to create in vitro BBB models de-differentiate in culture and lose many specialized characteristics. These changes are poorly understood at a molecular level, and little is known regarding the consequences of removing BMEC from their local in vivo microenvironment. To address these issues, suppression subtractive hybridization (SSH) was used to identify 25 gene transcripts that were differentially expressed between in vivo and in vitro BMEC. Genes affected included those involved in angiogenesis, transport and neurogenesis, and real-time quantitative polymerase chain reaction (qPCR) verified transcripts were primarily and significantly downregulated. Since this quantitative gene panel represented those BMEC characteristics lost upon culture, we used it to assess how culture manipulation, specifically BMEC purification and barrier induction by hydrocortisone, influenced the quality of in vitro models. Puromycin purification of BMEC elicited minimal differences compared with untreated BMEC, as assessed by qPCR. In contrast, qPCR-based gene panel analysis after induction with hydrocortisone indicated a modest shift of 10 of the 23 genes toward a more 'in vivo-like' gene expression profile, which correlated with improved barrier phenotype. Genomic analysis of BMEC de-differentiation in culture has thus yielded a functionally diverse set of genes useful for comparing the in vitro and in vivo BBB.

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

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

Manganese neurotoxicity: a focus on the neonate

Manganese (Mn) is an essential trace metal found in all tissues, and it is required for normal amino acid, lipid, protein, and carbohydrate metabolism. While Mn deficiency is extremely rare in humans, toxicity due to overexposure of Mn is more prevalent. The brain appears to be especially vulnerable. Mn neurotoxicity is most commonly associated with occupational exposure to aerosols or dusts that contain extremely high levels (>1-5 mg Mn/m(3)) of Mn, consumption of contaminated well water, or parenteral nutrition therapy in patients with liver disease or immature hepatic functioning such as the neonate. This review will focus primarily on the neurotoxicity of Mn in the neonate. We will discuss putative transporters of the metal in the neonatal brain and then focus on the implications of high Mn exposure to the neonate focusing on typical exposure modes (e.g., dietary and parenteral). Although Mn exposure via parenteral nutrition is uncommon in adults, in premature infants, it is more prevalent, so this mode of exposure becomes salient in this population. We will briefly review some of the mechanisms of Mn neurotoxicity and conclude with a discussion of ripe areas for research in this underreported area of neurotoxicity.

Drug delivery across the blood–brain barrier: why is it difficult? how to measure and improve it?

The development of drugs that act in the CNS has been significantly impeded by the difficulty of delivering them across the blood-brain barrier (BBB). This article aims to provide the reader with a critical overview of important issues in the discovery and development of drugs that need to enter the brain to elicit pharmacological activity, focusing particularly on i) the role of drug transporters in brain permeation and how to manipulate them to enhance drug brain bioavailability; ii) the successful application, limitations and challenges of commonly used in vitro and in vivo methodologies for measuring drug transport across the BBB, and iii) a discussion of recently developed strategies (e.g., modulation of efflux transporters by chemical inhibitors and the employment of delivery vectors taking advantage of native transport systems at the BBB) for facilitating drug penetration into the brain.

Manganese transport by rat brain endothelial (RBE4) cell-based transwell model in the presence of astrocyte conditioned media

Manganese (Mn), an essential nutrient, is neurotoxic at high levels and has been associated with the development of a parkinsonian syndrome termed manganism. Currently, the mechanisms responsible for transporting Mn across the blood-brain barrier (BBB) are unknown. By using rat brain endothelial 4 (RBE4) cell monolayers cultured in astrocyte-conditioned media (ACM), we examine the effects of temperature, energy, proton (pH), iron (Fe), and sodium (Na(+)) dependence on Mn transport. Our results suggest that Mn transport is temperature, energy, and pH dependent, but not Fe or Na(+) dependent. These data suggest that Mn transport across the BBB is an active process, but they also demonstrate that the presence of ACM in endothelial cell cultures decreases the permeability of these cells to Mn, reinforcing the use of ACM or astrocyte cocultures in studies examining metal transport across the BBB.

Glucose transport to the brain: a systems model

Glucose transport to the brain involves sophisticated interactions of solutes, transporters, enzymes, and cell signaling processes, within an intricate spatial architecture. The dynamics of the transport are influenced by the adaptive nature of the blood-brain barrier (BBB), the semi-impermeable membranes of brain capillaries. As both the gate and the gatekeeper between blood-borne nutrients and brain tissue, the BBB helps govern brain homeostasis. Glucose in the blood must cross the BBB's luminal and abluminal membranes to reach neural tissue. A robust representation of the glucose transport mechanism can highlight a target for brain therapeutic intervention, help characterize mechanisms behind several disease phenotypes, or suggest a new delivery route for drugs. The challenge for researchers is understanding the relationships between influential physiological variables in vivo, and using that knowledge to predict how alterations or interventions affect glucose transport. This paper reviews factors influencing glucose transport and approaches to representing blood-to-brain glucose transport including in vitro, in vivo, and kinetic models. Applications for different models are highlighted, while their limitations in answering arising questions about the human in vivo BBB lead to a discussion of an alternate approach. A developing complex systems simulation is introduced, initiating a single platform to represent the dynamics of glucose transport across the adapting human blood-brain barrier.

Developmental aspects of blood-brain barrier (BBB) and rat brain endothelial (RBE4) cells as in vitro model for studies on chlorpyrifos transport

The mammalian central nervous system (CNS) is characterized by the blood-brain barrier (BBB), a restrictive barrier endowed with the maintenance of homeostatic control of an optimal milieu within the brain. Whereas in tissues other than the CNS, concentrations of various metabolites (amino acids, K+) can undergo frequent fluctuations, the CNS must keep rigorous control over the extracellular cerebral fluid composition, preventing the mirroring of transient fluctuations in blood, because abrupt changes in these metabolites can translate to aberrant CNS function. The BBB is a specialized structure accomplished by individual endothelial cells that are continuously linked by tight junctions. This brief review will address pertinent issues to development of the BBB. Particular emphasis will be directed at the role of astrocytes in the induction and maintenance of the restrictive properties of this barrier, and the utility of in vitro culture models in surveying transport kinetics, exemplified by recent studies with the pesticide, chlorpyrifos.

Progress and limitations in the use of in vitro cell cultures to serve as a permeability screen for the blood-brain barrier

A relatively simple, widely applicable, and robust in vitro method of predicting blood-brain barrier (BBB) permeability to central nervous system-acting drugs is an increasing need. A cell-based model offers the potential to account for transcellular and paracellular drug diffusional processes, metabolism, and active transport processes, as well as nondefined interactions between a drug and cellular material that may impact upon a membrane's overall permeability profile. Any in vitro BBB cell model to be utilized for the transendothelial BBB permeability screening of potential central nervous system drugs must display reproducible solute permeability, and a number of other general criteria including: a restrictive paracellular barrier; a physiologically realistic cell architecture; the functional expression of key transporter mechanisms; and allow ease of culture to meet the technical and time constraints of a screening program. This article reviews the range of in vitro cell-based BBB models available, including the primary/low passage bovine and porcine brain endothelial cultures as well as the spectrum of immortalized brain endothelial cell lines that have been established. The article further discusses the benefits and limitations of exploiting such systems as in vitro BBB permeability screens.

Factor(s) released by glucose-deprived astrocytes enhance glucose transporter expression and activity in rat brain endothelial cells

Glucose transporter (GLUT) expression and regulation were studied in rat brain endothelial cells in primary culture (RBEC) and in immortalised RBE4 cells. Immunoblotting analysis showed a low expression of the endothelium-specific GLUT1 in RBEC and RBE4 cells compared to isolated brain capillaries. RBEC and RBE4 cells also expressed the GLUT3 isoform, whereas it was not present in isolated brain capillaries. No change in GLUT expression was observed in endothelial cells treated with astrocyte-conditioned medium. However, treatment with conditioned medium obtained from glucose-deprived astrocytes increased endothelial GLUT1 expression and glucose uptake. These results suggest that astrocytes submitted to hypoglycaemic conditions may release factor(s) that increase glucose uptake through the blood-brain barrier.

Overproduction of IFN-gamma and TNF-alpha from natural killer (NK) cells is associated with abnormal NK reactivity and cognitive derangement in Alzheimer's disease

Alterations of natural killer (NK) function can be involved in the neuroimmune mechanism of neurodegeneration in dementia of the Alzheimer's type (DAT). NK cell cytotoxicity (NKCC) and the generation and release of IFN-gamma and TNF-alpha (spontaneous and modulated by IL-2) from pure NK cells (CD 16+, CD 56+, CD 3-) were studied together with circulating IFN-gamma and TNF-alpha levels and cognitive function in 22 old patients with DAT and 15 healthy old subjects. Higher (p < 0.001) IL-2 modulated NKCC (with IL-2 50 U/mL and 100 U/mL) was demonstrated in DAT patients (+35% and +99% from baseline) than in healthy subjects (+6% and +76% from baseline). Increased spontaneous and IL-2-induced release of IFN-gamma and TNF-alpha from NK cells were found in DAT patients compared to healthy subjects (p < 0.001), whereas no difference of serum IFN-gamma and TNF-alpha was demonstrated between DAT and control groups. Significant negative correlations among the spontaneous release of IFN-gamma and TNF-alpha from NK and the decrease of the score of cognitive function (MMSE) were found in patients with DAT. In conclusion, alterations of NKCC control and NK-derived cytokine release in DAT could be involved in the neuroinflammatory mechanism related to the progression of neurodegeneration and dementia.

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.

GLUT1: a newly discovered immunohistochemical marker for juvenile hemangiomas

Juvenile hemangiomas are common, benign vascular tumors of infancy. These lesions enlarge rapidly through cellular hyperplasia during the first year of life and then involute over several years. Distinctive histopathologic features of hemangiomas diminish during this evolution, and differentiation from vascular malformations becomes increasingly difficult. This distinction has important therapeutic implications, as juvenile hemangiomas differ from malformations in natural history and in potential for recurrence. We report here that high endothelial immunoreactivity for the erythrocyte-type glucose transporter protein GLUT1 is a specific feature of juvenile hemangiomas during all phases of these lesions. In a retrospective study, we found intense endothelial GLUT1 immunoreactivity, involving more than 50% of lesional microvessels, in 97% (139 of 143) of juvenile hemangiomas from patients aged 1 month to 11 years. No endothelial GLUT1 immunoreactivity was found in any of 66 vascular malformations (17 arteriovenous, 33 venous, 11 lymphatic, and 5 port-wine) from patients aged 5 days to 75 years, or in any of 20 pyogenic granulomas or 7 granulation tissue specimens. Abundant Ki-67 positivity in these latter lesions established that GLUT1 expression does not simply reflect mitotically active endothelium. Focal GLUT1 immunoreactivity was found in 3 of 12 angiosarcomas, but not in any of 5 hemangioendotheliomas (epithelioid or infantile kaposiform). These findings establish GLUT1 immunoreactivity as a highly selective and diagnostically useful marker for juvenile hemangiomas. Because high levels of endothelial GLUT1 expression in normal tissue are restricted to microvessels with blood-tissue barrier function, these findings also have implications for the molecular and developmental pathogenic mechanisms of juvenile hemangiomas.

In vivo upregulation of the blood-brain barrier GLUT1 glucose transporter by brain-derived peptides

Glucose is the critical metabolic fluid for the brain, and the transport of this nutrient from blood to brain is limited by the blood-brain barrier (BBB) GLUT1 glucose transporter. The expression of the BBB-GLUT1 gene is augmented in brain endothelial cultured cells incubated with brain-derived trophic factors and the brain-derived peptide preparation Cerebrolysin (C1, EBEWE, Austria). The aim of the present investigation was to determine if C1 induces similar changes in the expression of the BBB-GLUT1 gene following its administration to rats in vivo. The BBB glucose transporter activity was investigated with the intracarotid artery perfusion technique using [3H]diazepam as cerebral blood flow marker. The acute or chronic administration of C1 markedly increased the brain permeability surface area of D-[14C]glucose compared to controls (D-[14C]glucose/[3H]diazepam ratio, 1.6- to 1.9-fold increase in frontal cortex, P < 0.05). Increased activity of the BBB glucose transporter was correlated with a significant rise in the abundance of the BBB-GLUT1 protein measured by both Western blot analysis and immunocytochemistry, and with a decrease in the transcript levels of this transporter. Data presented here demonstrate that the in vivo administration of Cl increases the transport of glucose from blood to brain via BBB-GLUT1 gene expression.

Glucose transport in brain and retina: implications in the management and complications of diabetes

Neural tissue is entirely dependent on glucose for normal metabolic activity. Since glucose stores in the brain and retina are negligible compared to glucose demand, metabolism in these tissues is dependent upon adequate glucose delivery from the systemic circulation. In the brain, the critical interface for glucose transport is at the brain capillary endothelial cells which comprise the blood-brain barrier (BBB). In the retina, transport occurs across the retinal capillary endothelial cells of the inner blood-retinal barrier (BRB) and the retinal pigment epithelium of the outer BRB. Because glucose transport across these barriers is mediated exclusively by the sodium-independent glucose transporter GLUT1, changes in endothelial glucose transport and GLUT1 abundance in the barriers of the brain and retina may have profound consequences on glucose delivery to these tissues and major implications in the development of two major diabetic complications, namely insulin-induced hypoglycemia and diabetic retinopathy. This review discusses the regulation of brain and retinal glucose transport and glucose transporter expression and considers the role of changes in glucose transporter expression in the development of two of the most devastating complications of long-standing diabetes mellitus and its management.

The pecten oculi of the chicken: a model system for vascular differentiation and barrier maturation

The pecten oculi is a convolute of blood vessels in the vitreous body of the avian eye. This structure is well known for more than a century, but its functions are still a matter of controversies. One of these functions must be the formation of a blood-retina barrier because there is no diffusion barrier for blood-borne compounds available between the pecten and the retina. Surprisingly, the blood-retina barrier characteristics of this organ have not been studied so far, although the pecten oculi may constitute a fascinating model of vascular differentiation and barrier maturation: Pectinate endothelial cells grow by angiogenesis from the ophthalmotemporal artery into the pecten primordium and consecutively gain barrier properties. The pectinate pigmented cells arise during development from retinal pigment epithelial cells and subsequently lose barrier properties. These inverse transdifferentiation processes may be triggered by the peculiar microenvironment in the vitreous body. In addition, the question is discussed whether the avascularity of the avian retina may be due to the specific metabolic activity of the pecten.

Hippocampal damage induced by carbon monoxide poisoning and spreading depression is alleviated by chronic treatment with brain derived polypeptides

A model of acute carbon monoxide poisoning combined with spreading depression (SD) induced metabolic stress was used to examine the protective effects of cerebrolysin (CL) on the development of electrophysiological, behavioral and morphological signs of hypoxic damage. Capillary electrodes were implanted into the neocortex and hippocampus of anesthetized rats which were then exposed for 90 min to breathing of 0.8% to 0.5% CO, while 3 to 4 waves of cortical and hippocampal SD were elicited by microinjections of 5% KCl. Duration of SD-provoked depolarization of cerebral cortex and hippocampus was noted. Nine and 18 to 19 days later propagation of SD waves was recorded with the same electrodes and decrease of their amplitude was used as an index of brain damage which was significant in the hippocampus but not in the cortex. CL-treatment (2.5 ml/kg per day) started after CO administration and continued for 14 days significantly improved hippocampal recovery manifested by increased amplitude of SD waves. Behavioral tests performed 10 and 20 days after CO poisoning in the Morris water maze revealed better performance (escape latency 7 s) in the CL-treated than in untreated animals (14 s). Morphological analysis showed marked damage in the hippocampus consonant with electrophysiological and behavioral findings in the same animals. No apparent histological damage was found in rats exposed to CO inhalation alone without the additional SD-provoked depolarization. It is concluded that chronic CL-treatment enhances recovery of hippocampal tissue after hypoxic damage of intermediate severity.

Molecular regulation of the blood-brain barrier GLUT1 glucose transporter by brain-derived factors

Glucose is the crucial metabolic fluid for the brain, and the transport of this nutrient from blood to brain is limited by the blood-brain barrier (BBB) GLUT1 glucose transporter. The activity of this transporter is altered in different pathophysiological conditions including Alzheimer's disease. The expression of the BBB-GLUT1 gene is directed by brain trophic factors, and the brain-derived peptide preparation Cerebrolysin (Cl, EBEWE, Austria), used in the treatment of Alzheimer's disease, increases the BBB-GLUT1 mRNA stability and the expression of the BBB-GLUT1 gene. In the present investigation, Cl markedly increased (p < 0.001) the expression of a BBB-GLUT1 reporter gene, named clone 753, that contains an important regulatory cis-acting element involved in the stabilization of this transcript in brain endothelial cultured cells (ECL). In experiments with a reporter gene lacking this regulatory element, Cl produced only a minimal fraction of the effect observed with clone 753. UV-cross linking/PAGE experiments showed that the GLUT1 transcript reacts with ECL cytosolic proteins to form a RNA/protein complex of approximately 80 kDa. The abundance of this cis/trans acting complex was found to be increased in Cl-treated cells. Overall, data presented here demonstrate that i) Cl increases the expression of a BBB-GLUT1-luciferase reporter gene containing a region of the 3'-untranslated region of BBB-GLUT1 mRNA with important regulatory cis-acting elements involved in the stabilization of this transcript, and ii) the increased expression of this BBB-GLUT1 reporter gene was associated with augmented abundance of a transacting factor that binds to the cis-acting element described in (i), suggesting that this association may be involved in the stabilization of GLUT1 mRNA induced by Cl.

The effect of MK-801 and of brain-derived polypeptides on the development of ischemic lesion induced by photothrombotic occlusion of the distal middle cerebral artery in rats

The effect of neuroprotective drugs on the early and late electrophysiological manifestations of photothrombotic occlusion of distal branches of middle cerebral artery was studied in rats treated with MK-801 and Cerebrolysin (CL). DC potentials were recorded from the irradiated cortex (ischemic core), from the adjacent penumbra zone and from remote intact cortex. Irradiation elicited after a few minutes of spontaneous spreading depression (SD) waves followed during 10-15 min by focal ischemic depolarization (FID) developing in the irradiated cortex and spreading into the perifocal areas. While the core FID amplitude reached about 30 mV and decayed during subsequent 2 h to 10-13 mV, FID in the penumbra zone was broken by periods of partial repolarization and returned during 30-90 min almost to baseline. At the same time, generation of spontaneous SD waves almost stopped. MK-801 (0.5 mg/kg, i.p., 45 min after ischemia) blocked SD waves, but did not shorten penumbra FID, the decay of which was slowed down to the rate found in the ischemic core. CL treatment (2.5 ml/kg, i.p. , 1 h after ischemia) did not influence FID in the acute phase of the experiment, but its 10-day administration facilitated post-ischemic recovery indicated by higher amplitude of evoked SD waves penetrating into the former penumbra zone. Morphological examination showed that the volume of total and partial necrosis was increased in the MK-801 group and marginally reduced in the CL group. It is suggested that the absence of the SD-induced hyperperfusion episodes in MK-801-treated rats may accelerate perifocal thrombotization in this model of focal ischemia.

Alkaline phosphatase expression in cultured endothelial cells of aorta and brain microvessels: induction by interleukin-6-type cytokines and suppression by transforming growth factor betas

Alkaline phosphatase (ALP) activity is markedly high in endothelial cells of the blood-brain barrier (BBB) type but absent from or low in those of the non-BBB type. Interleukin 6 (IL-6) has been identified as a glial cell line-derived factor that induces high ALP activity in cultured aortic endothelial cells. In the present study, we examined the effect of IL-6-type cytokines and transforming growth factor betas (TGF-betas) on ALP expression in cultures of calf pulmonary aortic endothelial (CPAE) cells and porcine brain microvascular endothelial (PBME) cells. Leukemia inhibitory factor, ciliary neurotrophic factor, and oncostatin M, which are known as IL-6-type cytokines, induced high ALP expression in the CPAE cells but not in the PBME cells. ALP levels in these cells were markedly suppressed by culture with TGF-betas. However, in cultured PBME cells, IL-6 and a derivative of cyclic adenosine monophosphate significantly increased ALP activity. Our findings raise the posibility that local concentrations of IL-6, IL-6-type cytokines, and TGF-betas affect the ALP levels in the endothelial cells of aorta and brain microvessels under normal development and also under inflammatory conditions.

Scavenger receptors are present on rabbit aortic endothelial cells in vivo

Endothelial cells metabolize modified LDL, but attempts to detect scavenger receptors in this cell type in vitro have been unsuccessful. To determine whether scavenger receptors are present on endothelial cells in vivo, species-specific reagents were developed to detect rabbit scavenger receptor protein. Antiserum against the rabbit scavenger receptor was generated with the use of synthetic peptides of two distinct regions: residues 3 to 21 in the cytoplasmic tail and residues 282 to 304 in the collagen-like region. Reactivity of antiserum against the synthetic peptides was confirmed with an enzyme-linked immunosorbent assay. Positive reactivity was also observed against fragments of scavenger receptor protein expressed in bacteria. Antiserum to both regions reacted with liver membrane proteins of sizes consistent with the scavenger receptor, as confirmed by Western blotting under reduced and nonreduced conditions. Immunocytochemical examination of rabbit aortic tissue by use of antiserum to both regions of scavenger receptor protein produced striking and identical patterns of staining of aortic endothelium. Immunostaining was abolished for both antisera by preadsorption with the specific peptide region used as immunogen. In contrast, incubation of scavenger receptor antiserum with a peptide of a region of the rabbit LDL receptor failed to influence immunoreactivity against endothelium. These data demonstrate the presence of scavenger receptors in rabbit endothelium in vivo, which may have fundamental implications for lipoprotein metabolism by the arterial wall.

Transport of glucose across the blood-tissue barriers

In specialized parts of the body, free exchange of substances between blood and tissue cells is hindered by the presence of a barrier cell layer(s). Specialized milieu of the compartments provided by these "blood-tissue barriers" seems to be important for specific functions of the tissue cells guarded by the barriers. In blood-tissue barriers, such as the blood-brain barrier, blood-cerebrospinal fluid barrier, blood-nerve barrier, blood-retinal barrier, blood-aqueous barrier, blood-perilymph barrier, and placental barrier, endothelial or epithelial cells sealed by tight junctions, or a syncytial cell layer(s), serve as a structural basis of the barrier. A selective transport system localized in the cells of the barrier provides substances needed by the cells inside the barrier. GLUT1, an isoform of facilitated-diffusion glucose transporters, is abundant in cells of the barrier. GLUT1 is concentrated at the critical plasma membranes of cells of the barriers and thereby constitutes the major machinery for the transport of glucose across these barriers where transport occurs by a transcellular mechanism. In the barrier composed of double-epithelial layers, such as the epithelium of the ciliary body in the case of the blood-aqueous barrier, gap junctions appear to play an important role in addition to GLUT1 for the transfer of glucose across the barrier.

Brain-derived peptides increase the expression of a blood-brain barrier GLUT1 glucose transporter reporter gene

The brain-derived peptide preparation Cerebrolysin (C1; EBEWE, Austria) increases the stability of blood-brain barrier (BBB)-GLUT1 transcript. To determine if the increase in BBB-GLUT1 mRNA stability is associated with an augmentation of gene expression, the present investigation studied the effect of C1 on the expression of a BBB-GLUT1-luciferase reporter gene in brain endothelial cultured (ECL) cells. Dose response studies showed that C1 markedly increased the expression of luciferase when the BBB-GLUT1-reporter gene was used. On the contrary, C1 produced no changes in the expression pattern of the control reporter gene, which lacks the GLUT1 regulatory sequence. Desensitization of the protein kinase C (PKC) receptor with the phorbol ester TPA, or inhibition with either 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7) or staurosporine, had no effect on the increased levels of luciferase induced by C1. Transfection efficiency was determined by measuring intracellular levels of the expression vector using a quantitative polymerase chain reaction (PCR) assay. The data presented here demonstrate that C1 increases BBB-GLUT1 gene expression in ECL cells through a mechanism that appears to be independent of activation of PKC.

Brain-derived peptides regulate the steady state levels and increase stability of the blood-brain barrier GLUT1 glucose transporter mRNA

The blood-brain barrier (BBB) GLUT1 glucose transporter gene expression is known to be regulated by putative brain trophic factors. Therefore, the present study investigated the effect of a brain-derived peptide rich preparation containing a neurotrophic factor-like action [Cerebrolysin (Cl), EBEWE, Austria]. In cultures of brain capillary endothelial cells, Cl induced a transient increase in the abundance of BBB-GLUT1 relative to actin measured by reverse transcription-polymerase chain reaction during the first 2 h of incubation, whereas a significant reduction in the GLUT1 transcript was observed at 20 and 48 h. In addition, Cl abolished the fall in GLUT1 levels induced by actinomycin D. The present data suggest that brain-derived factors in Cl are able to modulate the expression of the BBB-GLUT1 gene increasing the BBB-GLUT1 transcript stability.