Morphology of blood-brain interfaces

The blood-brain barrier choline transporter as a brain drug delivery vector

Choline is a ubiquitous molecule, found throughout almost every tissue in the body. Given it is a charged cation, nearly every cellular membrane has a transport mechanism to meet the intracellular and membrane need for choline. The blood-brain barrier is no exception in that a carrier-mediated transport mechanism is present to deliver choline from plasma to brain. The carrier consists of an anionic binding area that attracts positively charged quaternary ammonium groups or simple cations. Recent reports have shown this vector to be efficacious in delivering quaternary ammonium analogs of nicotine to brain. Future work is being completed to determine if other cationic or positively charged therapeutics can be effectively delivered to brain via this carrier.

Relation of beta-casomorphin to apnea in sudden infant death syndrome

Sudden infant death syndrome (SIDS) is the most common cause of death in infants and its pathogenesis is complex and multifactorial. The aim of this review is to summarize recent novel findings regarding the possible association of beta-casomorphin (beta-CM) to apnea in SIDS, which has not been widely appreciated by pediatricians and scientists. beta-CM is an exogenous bioactive peptide derived from casein, a major protein in milk and milk products, which has opioid activity. Mechanistically, circulation of this peptide into the infant's immature central nervous system might inhibit the respiratory center in the brainstem leading to apnea and death. This paper will review the possible relationship between beta-CM and SIDS in the context of passage of beta-CM through the gastrointestinal tract and the blood-brain barrier (BBB), permeability of the BBB to peptides in infants, and characterization of the casomorphin system in the brain.

The blood-brain barrier and its role in immune privilege in the central nervous system

The blood-brain barrier (BBB) provides both anatomical and physiological protection for the central nervous system (CNS), strictly regulating the entry of many substances and blood borne cells into the nervous tissue. Increased understanding of how the unique microenvironment in the CNS influences the BBB is crucial for developing novel therapeutic approaches to CNS diseases. In this review, we discuss those characteristics of the BBB that play an important role in maintaining immune privilege in the CNS, as well as factors that regulate immune cell invasion through the BBB and thereby modulate immune responses in the nervous tissue. In general, immune cell invasion across the BBB is highly restricted and carefully regulated. A florid invasion of activated white blood cells can create a predominantly proinflammatory local environment in the CNS, leading to immune-mediated diseases of the nervous tissue. Recent developments in cellular and molecular biological methods have allowed closer analysis of BBB function, and led to an improved understanding of the active role of the BBB in immune-mediated diseases of the CNS.

Corpus luteum derived copper, zinc-superoxide dismutase serves as a luteinizing hormone-release inhibiting factor in sheep

In the present study, we report the purification and characterization of a polypeptide from the sheep corpus luteum of pregnancy with a potent luteinizing hormone-release inhibiting factor (LH-RIF) bioactivity that stained as a single band in SDS-PAGE with an apparent molecular mass of 16000 Da. The amino acid sequences obtained after sequence analysis of peptides derived from the trypsin digestion of LH-RIF were subjected to a protein data bank search and were found to be identical with regions of sheep copper, zinc-superoxide dismutase (Cu,Zn-SOD). The measured mass of LH-RIF (15604.2+/-1.9 Da) was found to be similar to the theoretical mass of sheep Cu,Zn-SOD (15603.5 Da), with a disulfide bond and N acetylated alanine at the N-terminus. The inhibitory action of Cu,Zn-SOD on pulsatile LH secretion would suggest that this antioxidant may play an important role, either independently or in concert with some neurotransmitters, in the neuroendocrine regulation of sheep female reproductive function.

Drug and gene delivery to the brain: the vascular route

Brain drug development of either small molecule or large molecule (recombinant proteins, gene medicines) neurotherapeutics has been limited, owing to the restrictive transport properties of the brain microvasculature, which forms the blood-brain barrier (BBB) in vivo. Widespread drug delivery to the brain, while not feasible via craniotomy and intracerebral injection, is possible if the drug is delivered to brain via the transvascular route through the BBB. Novel brain drug delivery and drug targeting strategies can be developed from an understanding of the molecular and cellular biology of the brain microvascular and BBB transport processes.

Rat blood-brain barrier genomics. II

The tissue-specific gene expression at the brain microvasculature, which forms the blood-brain barrier (BBB) can be elucidated with a brain vascular genomics program, which starts with the isolation of gene products derived from purified brain microvessels. Genes commonly expressed in peripheral organs are subtracted with the suppression subtractive hybridization method using driver cDNA produced from a pool of rat liver/kidney-derived poly A+RNA. From a screen of 480 clones in the subtracted tester cDNA library, 156 clones were sequenced. The clones fell into 3 groups: known genes (51%), rat expressed sequence tags (31%), and novel rat genes not found in databases (18%). The known genes could be categorized into families of common function including vascular remodeling, signal transduction, transcription factors, biologic transport, vascular amyloid, hemostasis, myelin, lipids, secretion, cytoskeleton, and junctional complexes. Brain vascular genomics, or BBB genomics, allows for an accelerated discovery of the gene families that are differentially expressed at the microvasculature in brain.

Peptide delivery to the brain via adsorptive-mediated endocytosis: advances with SynB vectors

Biological membranes normally restrict the passage of hydrophilic molecules. This impairs the use of a wide variety of drugs for biomedical applications. To overcome this problem, researchers have developed strategies that involve conjugating the molecule of interest to one of a number of peptide entities that are efficiently transported across the cell membranes. In the past decade, a number of different peptide families with the ability to cross the cell membranes have been identified. Certain of these families enter the cells by a receptor-independent mechanism, are short (10-27 amino acid residues), and can deliver successfully various cargoes across the cell membrane into the cytoplasm or nucleus. Surprisingly, some of these vectors, the SynB vectors, have also shown the ability to deliver hydrophilic molecules across the blood-brain barrier, one of the major obstacles to the development of drugs to combat diseases affecting the CNS.

Drug delivery to brain via the blood-brain barrier

1. Many neurodegenerative diseases, cancer and infections of the brain become more prevalent as populations become older. Despite major advances in neuroscience, the blood-brain barrier (BBB) ensures that many potential therapeutic cannot reach the central nervous system (CNS). The BBB is formed by the complex tight junctions between the endothelial cells of the brain capillaries and their low endocytic activity. This results in the capillary wall that behaves as a continuous lipid bilayer and prevents the passage of polar and lipid-insoluble substances. It is, therefore, the major obstacle to drugs that may combat diseases affecting the CNS. 2. Several strategies for delivering drugs to the CNS have been developed. These enhance the capacity of therapeutic molecules to cross the BBB by modifying the drug itself, or by coupling it to a vector for receptor-mediated or adsorption-mediated transcytosis. 3. The current challenge is to develop drug-delivery systems that ensure that drugs cross the BBB in a safe and effective manner. This review focuses on the strategies developed to enhance drug delivery across the BBB.

Drug and gene targeting to the brain with molecular Trojan horses

Getting drugs and genes into the brain is a tall order. This is because the presence of the blood-brain barrier prevents many molecules from crossing into the brain. Overcoming this problem will have a profound effect on the treatment of many neurological disorders, allowing larger water-soluble molecules to pass into the brain. Transport vectors, such as endogenous peptides, modified proteins or peptidomimetic monoclonal antibodies, are one way of tricking the brain into allowing these molecules to pass. This article will review such molecular Trojan Horses, and the progress that has been made in the delivery of drugs and genes to the brain.

Intrathecal chemotherapy with antineoplastic agents in children

Intrathecal chemotherapy with antineoplastic agents is mainly utilised in children with leukaemia and lymphoma, and in selected brain tumours. In these diseases, intrathecal use is restricted to methotrexate (MTX), cytosine arabinoside (Ara-C) and corticosteroids. A number of other agents are, at the present time, under evaluation. Intrathecal MTX administered sequentially with systemic high dose MTX infusion prolongs therapeutic cerebral spinal fluid (CSF) levels of the drug. Prolonged therapeutic CSF levels can also be achieved by giving repeated small intrathecal doses of MTX over an extended period in selected patients, with an implanted Ommaya reservoir. In the CSF, the metabolic inactivation of Ara-C is significantly lower than in plasma with a CSF clearance similar to the rate of CSF bulk flow. A slow-release formulation of Ara-C may be given intrathecally, resulting in a prolonged cytotoxic concentration in the CSF. CNS relapse and neurotoxicity in patients with acute lymphoblastic leukaemia, especially younger children, may be reduced by using age-related dosing of intrathecal MTX and Ara-C. Hydrocortisone is used in combination with MTX and Ara-C for so-called 'triple intrathecal chemotherapy' in the treatment of meningeal leukaemia. Intrathecal thiotepa does not appear to be advantageous over systemic administration in patients with brain and meningeal leukaemia. Monoclonal antibodies, reactive with tumour-associated antigens, can be used as delivery systems for chemotherapeutic agents and radionuclides. However, the development of this new approach is currently under evaluation in larger clinical studies. Neurological adverse effects may be expected with intrathecal chemotherapy and are increased by high dose systemic therapy, concomitant cranial radiotherapy or meningeal infiltration by neoplastic cells. Inadvertant intrathecal administration of antineoplastic agents that are indicated for systemic administration only, is dangerous and may result in a fatal outcome.

Vector-mediated drug delivery to the brain

As a consequence of the growing ageing population, many neurodegenerative diseases, cancer and infections of the brain will become more prevalent. Despite major advances in neuroscience, many potential therapeutic agents are denied access to the central nervous system (CNS) because of the existence of the blood-brain barrier (BBB). This barrier is formed by the endothelial cells of the brain capillaries and its primary characteristic is the impermeability of the capillary wall due to the presence of complex tight junctions and a low endocytic activity. The BBB behaves as a continuous lipid bilayer and prevents the passage of polar and lipid-insoluble substances. The BBB is, therefore, the major obstacle to drugs that are potentially useful for combating diseases affecting the CNS. Extensive efforts have been made to develop CNS drug delivery strategies in order to enhance delivery of therapeutic molecules across the BBB. The current challenge is to develop drug-delivery strategies that will allow the passage of therapeutic drugs through the BBB in a safe and effective manner. This review focuses specifically on the strategies developed to enhance drug delivery across the BBB with an emphasis on the vector-mediated strategy.

Amplification of blood-brain barrier GLUT1 glucose transporter gene expression by brain-derived peptides

Glucose is a critical nutrient for the brain, and the transport of this hexose from blood to brain is mediated by the blood-brain barrier (BBB) GLUT1 glucose transporter. The expression of the BBB-GLUT1 gene is compromised in different pathological conditions and it is modulated by brain trophic factors. The brain-derived peptide preparation Cerebrolysin (Cl, EBEWE, Austria) increases the expression of the BBB-GLUT1 via mRNA stabilization. In order to gain more insights into the mechanism of BBB-GLUT1 gene regulation, the present investigation studied the effect of Cl on the expression of both the GLUT1 protein and GLUT1 reporter genes in brain endothelial cultured cells (ECL). Cl markedly increased the expression of reporter genes containing GLUT1 translational control elements and cis-acting elements involved in the stabilization of the GLUT1 mRNA transcript in a dose dependent manner. Cl produced only marginal effects on the reporter gene control lacking the GLUT1 regulatory elements. In parallel experiments, Cl markedly increased the uptake of 3H-2-deoxy-D-glucose and the levels of the GLUT1 protein measured by ELISA. Data presented here demonstrate: (i) that Cl increases the expression of BBB-GLUT1 reporter genes containing regulatory cis-elements involved in the stabilization and translation of the GLUT1 transcript; (ii) that the effect on both regulatory elements cooperates to increase gene expression; and (iii) that the increased levels of the BBB-GLUT1 reporter genes in Cl-treated ECL cells are associated with an increase in the glucose uptake and in the expression of the GLUT1 protein.

Cloning and expression in Pichia pastoris of a genetically engineered single chain antibody against the rat transferrin receptor

The present investigation describes the construction of a genetically engineered single chain antibody (scFv) against the rat transferrin receptor (OX26), and demonstrates that this scFv antibody can be fully processed and expressed as a soluble secreted molecule in the methylotrophic yeast Pichia pastoris. Restriction endonuclease sites located at both 5'- and 3'-flanking regions of OX26 coding region in the prokaryote pOPE-OX26 vector were engineered to incorporate yeast compatible restriction endonuclease sites (i.e. EcoRI and SmaI or AvrII). The modified OX26 cDNA was subcloned into the Pichia expression vectors pPIC9 and pHIL-S1. An OX26 scFv high producer clone [GS115 His+ Mut+ (pPIC-OX26 SacI)] was isolated and used for large-scale production and characterization. Because the engineered scFv contains both a c-myc tag and a (His)5 tail, the OX26 scFv was purified to homogeneity by immobilized metal affinity chromatography. The identity of the OX26 scFv was confirmed by Western blot analyses with both anti c-myc and anti poly-His antibodies. Minor immunoreactive bands corresponding to hyperglycosylated and partially processed alpha-factor leader prosequence were also detected in the purified OX26 scFv, and these contaminants were markedly reduced when the expression of the OX26 scFv was performed in minimal methanol medium buffered with phosphate at pH = 7. The present investigation suggests that this expression system may be useful for the production of anti-receptor single chain antibodies that can be used as brain drug delivery vectors.

Inhibitory effect of somatostatin on neutral amino acid transport in isolated brain microvessels

In the presence of somatostatin-14 or some of its receptorial agonists, the uptake of large neutral amino acids by isolated brain microvessels was found to be inhibited up to 50%, no other transport system being affected. Although the luminal and abluminal sides of brain endothelial cells are both capable of taking up large neutral amino acids, only uptake from the abluminal side appears to be inhibited by somatostatin. The involvement of a type-2 somatostatin receptor was suggested by assays with a series of receptor-specific somatostatin agonists, and was confirmed by the release of inhibition caused by a specific type-2 receptor antagonist. A type-2-specific mRNA was indeed shown to be present in both bovine brain microvessels ex vivo and primary cultures of endothelial cells from rat brain microvessels.

Delivery of peptides and proteins through the blood-brain barrier

Peptide and protein therapeutics are generally excluded from transport from blood to brain, owing to the negligible permeability of these drugs to the brain capillary endothelial wall, which makes up the blood-brain barrier (BBB) in vivo. However, peptides or protein therapeutics may be delivered to the brain with the use of the chimeric peptide strategy for peptide drug delivery. Chimeric peptides are formed when a non-transportable peptide therapeutic is coupled to a BBB drug transport vector. Transport vectors are proteins such as cationized albumin, or the OX26 monoclonal antibody to the transferrin receptor; these proteins undergo absorptive-mediated and receptor-mediated transcytosis through the BBB, respectively. In addition to vector development, another important element of the chimeric peptide strategy is the design of strategies for coupling drugs to the vector that give high efficiency coupling and result in the liberation of biologically active peptides following cleavage of the bond linking the therapeutic and the transport vector. The avidin/biotin system has been recently shown to be advantageous in fulfilling these criteria for successful linker strategies. The use of the OX26 monoclonal antibody, the use of the avidin/biotin system as a linker strategy, and the design of a vasoactive intestinal peptide (VIP) analogue that is suitable for monobiotinylation and retention of biologic activity following cleavage, allowed for the recent demonstration of in vivo pharmacologic effects in brain following the systemic administration of relatively low doses (12 microg/kg) of neuropeptide.

Targeting drugs to the brain by redox chemical delivery systems

Chemical delivery systems (CDSs) based on the redox conversion of a lipophilic dihydropyridine to an ionic, lipid-insoluble pyridinium salt have been developed to improve the access of therapeutic agents to the central nervous system. A dihydropyridinium-type CDS or a redox analog of the drug is sufficiently lipophilic to enter the brain by passive transport, then undergoes an enzymatic oxidation to an ionic pyridinium compound, which promotes retention in the CNS. At the same time, peripheral elimination of the entity is accelerated due to facile conversion of the CDS in the body. This review discusses chemical, physicochemical, biochemical, and biological aspects in relation to the principles and practical implementation of the redox brain-targeting approach to various classes of drugs. Representative examples to the brain-enhanced delivery of neurotransmitters, steroids, anticonvulsants, antibiotics, antiviral, anticancer and antidementia agents, and neuropeptides and their analogs are presented in detail. In vivo and in vitro studies and preliminary clinical data of several novel derivatives have been promising, which could lead to a practical use of the redox CDSs after proper pharmaceutical development. The investigations accentuate the need for considering physicochemical, metabolic, and pharmacokinetic properties in designing of carrier systems that are able to target drugs into the central nervous system.

Selective Lutheran glycoprotein gene expression at the blood-brain barrier in normal brain and in human brain tumors

The Lutheran (LU) glycoprotein was shown to be a specific marker of brain capillary endothelium, which forms the blood-brain barrier (BBB) in vivo. A 1.5 kb partial cDNA encoding the bovine LU was isolated from a bovine brain capillary cDNA library. Sequence analysis showed that the bovine and human LU had a 75% and 79% identity in the amino acid and nucleotide sequences, respectively. Northern blot analysis demonstrated a very high level of gene expression of the LU transcript in freshly isolated bovine brain capillaries, but no measurable LU mRNA in whole bovine brain. The high level of LU gene expression was maintained when bovine brain capillary endothelium was grown in tissue culture. Because many BBB specific genes are downregulated in tissue culture and in brain tumors, the expression of the LU mRNA and immunoactive LU protein was investigated in primary and metastatic human brain tumors. Immunocytochemistry of fresh frozen human brain and human brain tumors showed abundant immunostaining of brain capillary endothelium. Northern blot analysis showed the presence of LU transcripts in a panel of primary and metastatic human brain tumors. These studies demonstrated that the LU glycoprotein was a novel new marker of the BBB, and unlike other BBB specific genes, there was a persistent gene expression of the LU glycoprotein both in brain capillary endothelial cells grown in culture and in the endothelium of capillaries perfusing human brain cancer.

Brain drug delivery technologies: novel approaches for transporting therapeutics

The blood-brain barrier (BBB) denies many therapeutic agents access to brain tumours and other diseases of the central nervous system (CNS). Despite remarkable advances in our understanding of the mechanisms involved in the development of the brain diseases and the actions of neuroactive agents, drug delivery to the brain remains a challenge. For more than 20 years, extensive efforts have been made to enhance delivery of therapeutic molecules across vascular barriers of the CNS. The current challenge is to develop drug-delivery strategies that will allow the passage of drug molecules through the BBB in a safe and effective manner, and this review will provide an insight into some of the strategies developed to enhance drug delivery across the BBB.

Glutamic acid, twenty years later

This review examines progress in understanding the physiologic functions of glutamic acid in the body since the first symposium on glutamic acid physiology and biochemistry was held at the Mario Negri Institute in Milan in 1978. The topics reviewed, although not exhaustive, include the metabolism of glutamic acid, umami taste, the role of glutamic acid as a neurotransmitter, glutamate safety and the development of new drugs resulting from the knowledge of the neurodegeneration induced by high doses of glutamic acid.

Determinants of passive drug entry into the central nervous system

1. The blood-brain barriers restrict the passive diffusion of many drugs into the brain and constitute a significant obstacle in the pharmacological treatment of central nervous system diseases and disorders. The degree of restriction they impose is variable, with some lipid-insoluble drugs effectively excluded from the brain, while many lipid-soluble drugs do not appear to be subject to any restriction. 2. The ease with which any particular drug diffuses across the blood-brain barrier is determined largely by the number and strength of intermolecular forces "holding" it to surrounding water molecules. By quantifying the molecular features that contribute to these forces, it is possible to predict the in vivo blood-brain barrier permeability of a drug from its molecular structure. Dipolarity, polarizability, and hydrogen bonding ability are factors that appear to reduce permeability, whereas molecular volume (size) and molar refraction are associated with increased permeability. 3. Increasing the passive entry of "restricted" drugs into the central nervous system can be achieved by disrupting the blood-brain barrier (increased paracellular diffusion) or by modifying the structure of "restricted" drugs to temporarily or permanently increase their lipid solubility (increased transcellular permeability). 4. Competitive inhibition of outwardly directed active efflux mechanisms (P-glycoprotein and MRP, the multidrug resistance-related protein) can also significantly increase the accumulation of certain drugs within the central nervous system.

Genetically engineered brain drug delivery vectors: cloning, expression and in vivo application of an anti-transferrin receptor single chain antibody-streptavidin fusion gene and protein

A single chain Fv antibody-streptavidin fusion protein was expressed and purified from bacterial inclusion bodies following cloning of the genes encoding the variable region of the heavy chain and light chain of the murine OX26 monoclonal antibody to the rat transferrin receptor. The latter undergoes receptor mediated transcytosis through the brain capillary endothelial wall in vivo, which makes up the blood-brain barrier (BBB); therefore, the OX26 monoclonal antibody and its single chain Fv analog may act as brain drug delivery vectors in vivo. Attachment of biotinylated drugs to the antibody vector is facilitated by production of the streptavidin fusion protein. The bi-functionality of the OX26 single chain Fv antibody-streptavidin fusion protein was retained, as the product both bound biotin and the rat transferrin receptor in vitro and in vivo, based on pharmacokinetic and brain uptake analyses in anesthetized rats. The attachment of biotin-polyethyleneglycol-fluorescein to the OX26 single chain Fv antibody-streptavidin fusion protein resulted in illumination of isolated rat brain capillaries in confocal fluorescent microscopy. In conclusion, these studies demonstrate that genetically engineered single chain Fv antibody-streptavidin fusion proteins may be used for non-invasive neurotherapeutic delivery to the brain using endogenous BBB transport systems such as the transferrin receptor.

Monoclonal antibodies against vascular endothelial antigens expressed in normal human brain

A panel of monoclonal antibodies reactive with human-brain vessels was raised by immunizing BALB/c mice with homogenate of whole human brain, obtained from temporal lobectomies. Hybridoma supernates were screened by immunohistochemical methods on frozen sections of human brain, liver and spleen and 16 clones were isolated. The pattern of immunoreactivity varied with respect to the type of brain blood vessels predominantly labelled and to tissue specificity. Some antibodies cross-reacted with cow or squirrel monkey forebrain microvessels with an intensity equal to that shown by human brain. The immunoreactivity patterns reflected antigenic heterogeneity among different subsets of vascular endothelial cells in human brain.

Drug delivery of antisense molecules to the brain for treatment of Alzheimer's disease and cerebral AIDS

Antisense oligonucleotides (ODNs) and peptide nucleic acids (PNAs) are potential therapeutics for eradication of malignancies, viral infections, and other pathologies. However, ODNs and PNAs in general are unable to cross cellular membranes and blood-tissue barriers, such as the blood-brain barrier (BBB), which is only permeable to lipophilic molecules of molecular weight <600 Da. Cellular delivery systems based on conjugates of streptavidin (SA) and the OX26 monoclonal antibody directed to the transferrin receptor may be employed as a universal carrier for the transport of mono-biotinylated peptides, ODNs, or PNAs. 3'-Biotinylation of phosphodiester (PO)-ODN produces complete protection of ODN against serum and cellular 3'-exonucleases, facilitating the conjugation to avidin-based delivery systems and maintaining the activation of RNase H. These delivery systems markedly increased the cellular uptake and antisense efficacy of 3'-biotinylated ODNs in models of Alzheimer's disease and HIV-AIDS. In vivo brain delivery studies demonstrated that 3'-protected PO-ODNs and PO-phosphorothioate(PS)-ODN hybrids containing a single PO linkage are subjected to endonuclease degradation in vivo. On the contrary PS-ODNs, which were also protected at 3'-terminus by biotinylation, are metabolically stable in vivo and resistant to exo/endonuclease degradation. However, because of the strong binding of these oligomers to plasma protein, PS-ODNs are poorly transported into the brain through the BBB by the OX26-SA delivery vector following intravenous administration. PNAs are also resistant to exo/endonuclease and protease degradation, and these molecules biotinylated at the amino terminal group were transported into the brain by the OX26-SA delivery system with brain uptake levels comparable to that of morphine. Using the rev gene of HIV as a model target, RNase protection assays and cell-free translation arrest showed that the PNA-OX26-SA conjugate maintained active recognition and inactivation of target mRNA, respectively. The overall experimental evidence suggests that PNA-OX26-SA conjugates represent optimal antisense molecules for drug delivery to the brain.

Brain drug delivery system bearing dopamine hydrochloride for effective management of parkinsonism

Dopamine hydrochloride bearing positively charged small liposomes was prepared by sonicating the multilamellar vesicles. These vesicles were characterized for their physical attributes (shape, size, charge, drug entrapment efficiency, and drug leakage). The drug release kinetics from the liposomes were also studied and found to be Fickian-type diffusion. In vivo performance of the drug-entrapped liposomes was assessed by periodic measurement of drug- (chlorpromazine) induced catatonia in Sprague-Dowley rats. These results were compared with the plain dopamine HCl and levodopa preparations as well with the marketed formulation of levodopa containing carbidopa (Syndopa). These studies revealed that the dopamine can be effectively delivered to the brain by incorporating it into liposomes, and its degradation in circulation can also be protected. The results of liposomal formulation were found to be superior compared to plain levodopa as well as Syndopa.

Mechanisms of neuronal cell death in Wernicke's encephalopathy

Wernicke's Encephalopathy (WE) is a serious neurological disorder resulting from thiamine deficiency, encountered in chronic alcoholics and in patients with grossly impaired nutritional status. Neuropathologic studies as well as Magnetic Resonance Imaging reveal selective diencephalic and brainstem lesions in patients with WE. The last decade has witnessed major advances in the understanding of pathophysiologic mechanisms linking thiamine deficiency to the selective brain lesions characteristic of WE. Activities of the thiamine-dependent enzyme alpha-ketoglutarate dehydrogenase, a rate-limiting tricarboxylic acid cycle enzyme are significantly reduced in autopsied brain tissue from patients with WE and from rats treated with the central thiamine antagonist, pyrithiamine. In the animal studies, evidence suggests that such enzyme deficits result in focal lactic acidosis, cerebral energy impairment and depolarization resulting from increased release of glutamate in vulnerable brain structures. It has been proposed that this depolarization may result in N-Methyl-D-Aspartate receptor-mediated excitotoxicity as well as increased expression of immediate early genes such as c-fos and c-jun resulting in apoptotic cell death. Other mechanisms involved in thiamine deficiency-induced cell loss may involve free radicals and alterations of the blood-brain barrier. Additional studies are still required to identify the site of the initial cellular insult and to explain the predilection of diencephalic and brainstem structures due to thiamine deficiency.

Blood-brain barrier carrier-mediated transport and brain metabolism of amino acids

The transport of neutral amino acids through the brain capillary endothelial wall, which makes up the blood-brain barrier (BBB) in vivo, is an important control point for the overall regulation of cerebral metabolism, including protein synthesis and neurotransmitter production. The Michaelis-Menten kinetics of BBB amino acid transport have been investigated in vivo with the brain uptake index (BUI) technique, and in vitro with the isolated human brain capillary preparation. The only amino acid that is albumin-bound is tryptophan, and the majority of albumin-bound tryptophan in the plasma is available for transport through the BBB via an enhanced dissociation mechanism that operates at the surface of the brain capillary endothelium. The availability in brain of amino acids is predicted from the BBB Km values to be sharply influenced by supra-physiological concentrations of phenyalanine in the 200-500 microM range. Moreover, the measurement of cerebral protein synthesis with an internal carotid artery perfusion technique and HPLC-based measurements of aminoacyl-transfer RNA specific activities shows an inverse relationship between cerebral protein synthesis and plasma phenyalanine concentrations in the 200-500 microM range. These findings indicate the neurotoxicity of hyperphenylalninemia is not restricted to the phenylketonuria range of approximately 2000 microM, but is exerted in the supra-physiological range of 200-500 microM.

Vitamin C crosses the blood-brain barrier in the oxidized form through the glucose transporters

Vitamin C concentrations in the brain exceed those in blood by 10-fold. In both tissues, the vitamin is present primarily in the reduced form, ascorbic acid. We identified the chemical form of vitamin C that readily crosses the blood-brain barrier, and the mechanism of this process. Ascorbic acid was not able to cross the blood-brain barrier in our studies. In contrast, the oxidized form of vitamin C, dehydroascorbic acid (oxidized ascorbic acid), readily entered the brain and was retained in the brain tissue in the form of ascorbic acid. Transport of dehydroascorbic acid into the brain was inhibited by d-glucose, but not by l-glucose. The facilitative glucose transporter, GLUT1, is expressed on endothelial cells at the blood-brain barrier, and is responsible for glucose entry into the brain. This study provides evidence showing that GLUT1 also transports dehydroascorbic acid into the brain. The findings define the transport of dehydroascorbic acid by GLUT1 as a mechanism by which the brain acquires vitamin C, and point to the oxidation of ascorbic acid as a potentially important regulatory step in accumulation of the vitamin by the brain. These results have implications for increasing antioxidant potential in the central nervous system.

The effect of beta-turn structure on the permeation of peptides across monolayers of bovine brain microvessel endothelial cells

PURPOSE

To investigate the effects of the beta-turn structure of a peptide on its permeation via the paracellular and transcellular routes across cultured bovine brain microvessel endothelial cell (BBMEC) monolayers, an in vitro model of the blood-brain barrier (BBB).

METHODS

The effective permeability coefficients (Peff) of the model peptides were determined across BBMEC monolayers. The dimensions of the aqueous pores in the tight junctions (TJs) of the BBMEC monolayers were determined using a series of hydrophilic permeants. This value and the molecular radius of each peptide were used to calculate the theoretical paracellular (PP*) and transcellular (PT*) permeability coefficients for each peptide.

RESULTS

A comparison of the theoretical PP* values with the observed Peff values was made for a series of model peptides. For the most hydrophobic peptides (Ac-PheProXaaIle-NH2 and Ac-PheProXaaIleVal-NH2; Xaa = Gly, Ile), it was concluded that the Gly-containing peptide of each pair more readily permeates BBMEC monolayers via the transcellular pathway than the Ile-containing analog. In addition, the Gly-containing peptides, which exhibit more beta-turn structure, were shown to be more lipophilic than the Ile-containing peptides as estimated by the log of their 1-octanol:HBSS partition coefficients (log Po/w). However, the three hydrophilic peptide pairs (Ac-TyrProXaaAspVal-NH2, Ac-TyrProXaaAsnVal-NH2, and Ac-TyrProXaaIleVal-NH2; Xaa = Gly, Ile) were found to permeate BBMEC monolayers predominantly via the paracellular pathway. No differences were observed in the Peff values of the hydrophilic peptides having higher beta-turn structures as compared to the peptides lacking these structural features. In addition, the Ile-containing peptides exhibited significantly higher log Po/w values than the Gly-containing hydrophilic peptides.

CONCLUSIONS

Hydrophobic peptides that exhibit significant beta-turn structure in solution are more lipophilic as measured by log Po/w, and more readily permeate BBMEC monolayers via the transcellular route than hydrophobic peptides that lack this type of solution structure. Similar secondary structural features in hydrophilic peptides do not appear to sufficiently alter the physicochemical properties of the peptides so as to alter their paracellular flux through BBMEC monolayers.

Deltorphin transport across the blood-brain barrier

In vivo antinociception studies demonstrate that deltorphins are opioid peptides with an unusually high blood-brain barrier penetration rate. In vitro, isolated bovine brain microvessels can take up deltorphins through a saturable nonconcentrative permeation system, which is apparently distinct from previously described systems involved in the transport of neutral amino acids or of enkephalins. Removing Na+ ions from the incubation medium decreases the carrier affinity for deltorphins (-25%), but does not affect the Vmax value of the transport. The nonselective opiate antagonist naloxone inhibits deltorphin uptake by brain microvessels, but neither the selective delta-opioid antagonist naltrindole nor a number of opioid peptides with different affinities for delta- or mu-opioid receptors compete with deltorphins for the transport. Binding studies demonstrate that mu-, delta-, and kappa-opioid receptors are undetectable in the microvessel preparation. Preloading of the microvessels with L-glutamine results in a transient stimulation of deltorphin uptake. Glutamine-accelerated deltorphin uptake correlates to the rate of glutamine efflux from the microvessels and is abolished by naloxone.

Bifunctional fusion between nerve growth factor and a transferrin receptor antibody

The cDNAs encoding the variable regions of the heavy and light chains of a murine antibody specific for the human TfR were cloned and a human chimera (gamma1, kappa) was produced. A gene fusion was created by joining the 3' end of the coding region of the human nerve growth factor (NGF) precursor to the 5' end of the heavy chain variable region of the chimeric antibody. When expressed with the unmodified light chain in mammalian cells, the protein fusion is properly processed, assembled, and secreted. Subsequent purification and characterization established the uncompromised bifunctional activities of the protein, relative to the unmodified components, as demonstrated by its ability to both bind to the human TfR and induce neurite outgrowth in primary sympathetic or spinal ganglia and in trkA-transfected pheochromocytoma cells. The ability to generate biologically active NGF fused to a TfR targeting antibody, which was previously shown to cross the blood-brain barrier, may offer a novel way to deliver NGF and other neurotrophic factors to the central nervous system.

Palmitoylation of brain capillary proteins

Palmitoylation is a reversible posttranslational modification which is involved in the regulation of several membrane proteins such as beta 2-adrenergic receptor, p21ras and trimeric G-protein alpha-subunits. This covalent modification could be involved in the regulation of the numerous membrane proteins present in the blood-brain barrier capillaries. The palmitoylation activity present in brain capillaries was characterized using [3H]palmitate labeling followed by chloroform methanol precipitation. Palmitate solubilizing agents such as detergents and bovine serum albumin (BSA), were used for optimizing activity. Some palmitoylated substrates were identified using [3H]palmitate labeling followed by immunoprecipitation with specific antibodies. Two optimal palmitate solubilization conditions were found, one involves cell permeabilization (Triton X-100) and the other represents a more physiological condition where membrane integrity is conserved (BSA). Sensitivity to the cysteine modifier N-ethylmaleimide and to hydrolysis, using hydroxylamine or alkaline methanolysis, indicated that palmitic acid was bound to the proteins by a thioester bond. Maximal palmitate incorporation was reached after 30 or 60 min of incubation in the presence of Triton or BSA, respectively. Depalmitoylation was observed in the presence of BSA, but not with detergents. The palmitoylation reaction was optimal at pH 8 or 9 in the presence of Triton or BSA, respectively, but palmitoylated substrates were detectable over a wide range of pH values. In the presence of Triton X-100, the addition of ATP, CoA and Mg2+ to the incubation medium increased palmitoylation by up to 80-fold. Two palmitoylated substrates were identified, a 42 kDa G-protein alpha subunit and p21ras. The study shows that the utilization of palmitate solubilizing agents is essential to measure in vitro palmitoylation in brain capillaries. Several palmitoylated proteins are present in the blood-brain barrier including five major substrates of 12, 21, 35, 42 and 55 kDa. It is suggested that palmitoylation could play a crucial role in the regulation of brain capillary function, since the two substrates identified in this study are known to be involved in signal transduction, vesicular transport and cell differentiation.

Excitability of neurons in the ventromedial nucleus in rat hypothalamic slices: modulation by amino acids at cerebrospinal fluid levels

Free amino acid composition of cerebrospinal fluid (CSF) in the rat was measured to examine if electrical activity of neurons in the ventromedial nucleus of the hypothalamus (VMH) is modulated by amino acids at CSF levels. In CSF collected through a cannula in the cerebromedullary cistern of freely moving rats, the total concentration of amino acids was 979 microM, while that in plasma was 3.50 mM. Glutamine was the major (60%) component and each of other amino acids was less than 8.0% (lysine) of total. Among neutral amino acids, concentrations of serine, threonine and alanine were much higher than those of others. Although concentrations of basic amino acids such as lysine and arginine were also relatively high, those of acidic amino acids such as glutamate and aspartate were very low. Spontaneous firing rates of 73% of the neurons in the VMH in brain slices in vitro were facilitated by changing the perfusion medium from artificial CSF (ACSF) without amino acids to one containing each amino acid close to CSF levels (AA-ACSF). Glutamine (600 microM) alone similarly facilitated the activity in 64% of VMH neurons, though the facilitatory effect of amino acids was blocked by simultaneous application of neither AP-5 nor CNQX, antagonists for NMDA or non-NMDA receptors, respectively, to AA-ACSF at 100 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Human insulin receptor monoclonal antibody undergoes high affinity binding to human brain capillaries in vitro and rapid transcytosis through the blood-brain barrier in vivo in the primate

PURPOSE

The ability of monoclonal antibodies against the human insulin receptor to undergo transcytosis through the blood-brain barrier (BBB) was examined in the present studies.

METHODS

Two murine monoclonal antibodies (MAb83-7 and MAb83-14) which bind different epitopes within the alpha-subunit of the human insulin receptor were examined using isolated human brain capillaries, frozen sections of primate brain, and in vivo pharmacokinetic studies in anesthetized Rhesus monkeys.

RESULTS

Both antibodies strongly illuminated capillary endothelium in immunocytochemical analysis of frozen sections of brain from Rhesus monkey but not squirrel monkey. Both monoclonal antibodies, in the iodinated forms, bound to human brain microvessels, although the binding and endocytosis of MAb83-14 was approximately 10-fold greater than MAb83-7. The active binding of MAb83-14 to the human insulin receptor was paralleled by a very high rate of transport of this antibody through the BBB in vivo in two anesthetized Rhesus monkeys. The BBB permeability-surface area (PS) product in neocortical gray matter was 5.4 +/- 0.6 microL/min/g, which is severalfold greater than previous estimates of the PS product for receptor-specific monoclonal antibody transport through the BBB. The brain delivery of MAb83-14 to the Rhesus monkey brain was high and 3.8 +/- 0.4% of the injected dose was delivered to 100 g of brain at 3 hours after a single intravenous injection. In contrast, there was no brain uptake of the mouse IgG2a isotype control antibody.

CONCLUSIONS

These studies demonstrate an unexpected high degree of transcytosis of a monoclonal antibody through the primate BBB in vivo.

Palmitoylation of the glucose transporter in blood-brain barrier capillaries

Palmitoylation of GLUT1 was investigated in brain capillaries. The glucose transporter was shown to be palmitoylated using [3H]palmitate labeling and immunoprecipitation. The labeling was sensitive to methanolic KOH or hydroxylamine hydrolysis, indicating the presence of an ester or thioester bond. The released fatty acid was analyzed by reverse-phase HPLC and was identified as [3H]palmitate. Specificity of the immunoprecipitation was assessed by competitive inhibition of anti-GLUT1 binding with a synthetic C-terminal peptide against which the antibody was raised. In vivo studies were performed using capillaries isolated from control rats, streptozotocin-induced diabetic rats and diet-induced hyperglycemic rats. Glycemia was increased 2- and 5-fold in the hyperglycemic and diabetic groups, respectively. GLUT1 expression was evaluated in the three groups by Western blot analysis. A 36% decrease in GLUT1 expression was observed in the diabetic group, while there was no significant variation in GLUT1 expression in the hyperglycemic group. Palmitoylation of GLUT1 was increased in both diet-induced hyperglycemic and diabetic groups. These results suggest that palmitoylation may be involved in the regulation of glucose transport activity in hyperglycemia.

P-glycoprotein of blood brain barrier: cross-reactivity of Mab C219 with a 190 kDa protein in bovine and rat isolated brain capillaries

P-glycoprotein (P-gp), an active efflux pump of antitumor drugs, is strongly expressed in endothelial cells of the blood brain barrier (BBB). Two proteins (155 and 190 kDa) were detected by Western blot analysis of beef and rat capillaries with the monoclonal antibody (MAb) C219. In order to characterize the nature of these proteins, their profile of solubilization by different detergents was established and compared with that of P-gp from the CHRC5 tumoral cell line. The 155 kDa protein (p155) of capillaries and the P-gp of CHRC5 cells were well solubilized by deoxycholate and Elugent, whereas the 190 kDa kDa protein (p190) was only solubilized by sodium dodecylsulfate (SDS). Both proteins have different patterns of extraction by Triton X-114, p155 partitioning as a membrane protein, while p190 was insoluble. Deglycosylation of capillary proteins resulted in a 27-28 kDa decrease in the apparent molecular weight of p155, similar to that observed for the P-gp of CHRC5 cells, but a decrease of only 7-8 for p190. Only p155 was immunoprecipitated by MAb C219. These results suggest that only p155 is the P-gp in BBB and that MAb C219 cross-reacts with a 190 kDa MDR-unrelated glycosylated protein. Consequently, the use of this antibody, which is frequently used to detect P-gp in tumors, could be a pitfall of immunohistochemistry screening for cancer tissues and lead to false positive in the diagnosis of MDR.

Animal models for studying transport across the blood-brain barrier

There are many reasons for wishing to determine the rate of uptake of a drug from blood into brain parenchyma. However, when faced with doing so for the first time, choosing a method can be a formidable task. There are at least 7 methods from which to choose: indicator dilution, brain uptake index, microdialysis, external registration, PET scanning, in situ perfusion, and compartmental modeling. Each method has advantages and disadvantages. Some methods require very little equipment while others require equipment that can cost millions of dollars. Some methods require very little technical experience whereas others require complex surgical manipulation. The mathematics alone for the various methods range from simple algebra to complex integral calculus and differential equations. Like most things in science, as the complexity of the technique increases, so does the quantity of information it provides. This review is meant to serve as a starting point for the researcher who wishes to study transport and uptake across the blood-brain barrier in animal models. An overview of the mathematical theory, as well as an introduction to the techniques, is presented.

In vivo evidence for ATP-dependent and P-glycoprotein-mediated transport of cyclosporin A at the blood-brain barrier

To evaluate the significance of P-glycoprotein (P-gp)-mediated active efflux on the blood-brain barrier (BBB) permeability of cyclosporin A (CsA) in vivo, we investigated the effects of ATP depletion in the brain and of a multidrug-resistant (MDR) reversing agent on the transport of CsA across the BBB. Using transient brain ischemia obtained by 4-vessel occlusion of vertebral and common carotid arteries in rats to deplete ATP content in the brain, the estimated permeability surface area product (PS) value of [3H]CsA was increased 2.7-fold compared with that in normal rats, whereas the PS value of [14C]sucrose was not altered. Additionally, when quinidine hydrochloride (QND) was infused into the brain through a microdialysis probe implanted in the rat hippocampus, the extravascular extraction of CsA was increased to approximately 2.5-fold of the control, whereas no difference in the extravascular extraction between control and normal rats having no implanted dialysis probe was observed. Furthermore, the efflux rate from brain to blood of CsA was decreased remarkably to 5% of control at steady-state by co-administration of CsA with QND directly into the brain through the dialysis probe. The ATP-dependent and QND-sensitive efflux of CsA from the brain strongly indicates that P-gp in the brain capillary endothelial cells functions as an efflux pump under the physiological state, and that P-gp-mediated efflux of CsA is a major mechanism of the restricted transfer from blood into the brain.

Intracarotid infusion of RMP-7, a bradykinin analog: a method for selective drug delivery to brain tumors

The bradykinin analog, RMP-7, was investigated for its ability to selectively increase uptake of molecular tracers in RG2 glial tumors. When infused in low doses (0.1 microgram/kg/min) through the intracarotid artery ipsilateral to RG2 gliomas in rats, RMP-7 significantly increased the permeability of tumor capillaries to methotrexate and to four other tracers of varying molecular weights, compared to intracarotid infusion of vehicle alone. Tracers used to examine permeability included radiolabeled alpha-aminoisobutyric acid (M(r) 103 D), sucrose (M(r) 342 D), methotrexate (M(r) 454.5 D), inulin (M(r) 5000 D), and dextran (M(r) 70,000 D). Permeability was expressed as the unidirectional transfer constant, Ki (microliters/gm/min). The permeability (Ki) of tumors in the RMP-7 group compared to the vehicle control group was as follows: alpha-aminoisobutyric acid, 35.3 +/- 9.11 versus 12.7 +/- 4.56 (p < 0.001); sucrose, 16.5 +/- 3.83 versus 9.28 +/- 3.12 (p < 0.05); methotrexate, 26.3 +/- 10.3 versus 8.98 +/- 6.78 (p < 0.005); inulin, 13.5 +/- 3.23 versus 6.55 +/- 4.32 (p < 0.005); dextran, 15.2 +/- 3.42 versus 1.47 +/- 1.24 (p < 0.001). The permeability of RG2 gliomas to high-molecular-weight dextran (70,000 D) was 10.3-fold higher in the RMP-7 group than in the vehicle control group. Intracarotid infusion of RMP-7 did not significantly increase the blood volume in tumor or brain tissue. The permeability of normal brain capillaries was unaffected by intracarotid infusion of 0.1 microgram/kg/min RMP-7 relative to that achieved in tumor. These data support the idea that intracarotid infusion of RMP-7 will be a useful technique for selective delivery of antitumor compounds to brain tumors.

Differential expression of alpha-actin mRNA and immunoreactive protein in brain microvascular pericytes and smooth muscle cells

Hypertension has been linked to opening of the blood-brain barrier and may be related to the expression of the smooth muscle alpha-actin gene in contractile cells at the brain microvasculature. However, the cellular origin (i.e., endothelial cells, pericytes, smooth muscle cells) of the alpha-actin mRNA in the brain microvasculature is not clearly identified. Therefore, we investigated the abundance of actin mRNA by Northern blot analysis in isolated brain microvessels and in brain microvascular endothelial or pericytes in tissue culture. All samples showed the characteristic 2.1 kb transcript corresponding to cytoplasmic beta and gamma isoform mRNA. The 1.7 kb transcript corresponding to smooth muscle alpha-actin was detected in freshly isolated bovine brain microvessels, in primary cultures of brain microvascular pericytes, or endothelial cells; the latter cultures contain both endothelial cells and pericytes. The alpha-actin mRNA was absent in a cloned bovine brain endothelial cell line. The relative abundance of the alpha/(beta + gamma) actin transcript ratio was: cultured pericytes > freshly isolated microvessels > endothelial primary. The cellular distribution of the smooth muscle alpha-actin immunoreactive protein was studied by immunocytochemistry in cytospun/methanol-fixed isolated bovine brain microvessels with a monoclonal antibody directed to the amino-terminal decapeptide of the smooth muscle alpha-actin isoform. This antibody reacted strongly with precapillary arterioles of isolated microvessels, whereas no immunostaining was observed in either capillary endothelial cells or in pericytes. In conclusion, the alpha-actin mRNA is expressed in brain microvascular pericytes in tissue culture, but the immunoreactive alpha-actin protein is not expressed in brain microvascular pericytes in vivo. These data suggest that either 1) alpha-actin gene expression is induced in capillary pericytes in tissue culture or 2) alpha-actin mRNA in brain capillary pericytes in vivo is subject to translational repression resulting in no detectable alpha-actin protein under normal conditions.

Fluid-phase endocytosis of horseradish peroxidase by cerebral endothelial cells in primary culture: characterization and kinetic analysis

Endocytosis of horseradish peroxidase (HRP) was studied in primary cultures of cerebral endothelial cells prepared from 2-week-old rats. These cultures were considered "endothelial-like" on the basis of their ability to internalize acetylated low density lipoprotein. Cellular localization of HRP protein was examined at the light and ultrastructural levels and endocytosis of the protein was evaluated by a colorimetric assay. HRP was localized in discrete cytoplasmic granules by light microscopy. At the ultrastructural level these granules corresponded to pleomorphic membrane-bound structures that were present throughout the cytoplasm. The amount of internalized HRP was directly related to the concentration of the protein in the medium, was not saturable at high concentrations of HRP, and increased with time. Endocytosis proceeded at 37 degrees C, but was abolished at 4 degrees C. In pulse-chase experiments, the quantity of internalized protein in the cells did not significantly change during the 2 hr chase period. Taken together, these findings suggest that internalization of HRP occurs by fluid-phase endocytosis, a non-receptor-mediated process, and that the protein is stable within an intracellular compartment for at least several hours.

New approaches to drug delivery through the blood-brain barrier

The development of recombinant proteins, monoclonal antibodies, or antisense oligonucleotides as pharmaceuticals for the brain will require the parallel development of practical strategies for delivery of these pharmaceuticals in vivo through the endothelial wall of capillaries in the brain, the blood-brain barrier. The brain and spinal cord constitute the only organ to be perfused by capillaries having such a barrier, which excludes the uptake into the brain of circulating molecules that do not have access to several specialized transport systems within the barrier. The current challenge for biotechnology is to develop effective drug-delivery strategies to the brain in parallel with the ongoing drug-discovery programs for this organ.

Hydrogen bonding potential as a determinant of the in vitro and in situ blood-brain barrier permeability of peptides

With the exception of various central nervous system (CNS)-required nutrients for which specific, saturable transport systems exist, the passage of most water-soluble solutes through the blood-brain barrier (BBB) is believed to depend largely on the lipid solubility of the solutes. Most peptides, therefore, do not enter the CNS because of their hydrophilic character. Recently, utilizing homologous series of model peptides and Caco-2 cell monolayers as a model of the intestinal mucosa, it was concluded that the principal determinant of peptide transport across the intestinal cellular membrane is the energy required to desolvate the polar amide bonds in the peptide (P. S. Burton et al., adv. Drug Deliv. Rev. 7:365, 1991). To determine whether this correlation can be extended to the BBB, the permeabilities of the same peptides were determined using an in vitro as well as an in situ BBB model. The peptides, blocked on the N- and C-terminal ends, consisted of D-phenylalanine (F) residues: AcFNH2, AcF2NH2, AcF3NH2, AcF2(NMeF)NH2, AcF(NMeF)2NH2, Ac(NMeF)3NH2, and Ac(NMeF)3NHMe. A good correlation among the permeabilities of these model peptides across the bovine brain microvessel endothelial cell (BBMEC) monolayers, an in vitro model of the BBB, and their permeabilities across the BBB in situ was observed (r = 0.928, P < 0.05). The permeabilities of these peptides did not correlate with the octanol-buffer partition coefficients of the peptides (r = 0.389 in vitro and r = 0.155 in situ; P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of the tight junction-associated protein ZO-1 in circumventricular organs of the CNS

The immunofluorescent distribution of ZO-1, a tight junction-associated protein, was studied in murine circumventricular organs. These regions generally express a less restrictive blood-brain barrier than is found in other areas of the CNS. In the remaining brain parenchyma, where a characteristic blood-brain barrier exists, ZO-1 was localized in discrete, continuous lines along blood vessels, presumably in association with endothelial cell tight junctions. The ependymal cells in the ventricular walls displayed a more punctate pattern of ZO-1 distribution, indicative of discontinuous tight junctions. In two of the circumventricular organs examined, the median eminence and the subfornical organ, many capillaries lacked detectable ZO-1 immunoreactivity while the apical aspects of the specialized ependymal cells (tanycytes) revealed an unbroken ZO-1 distribution. Scant labelling of ZO-1 in blood vessels was found in the area postrema, and only weak and discontinuous ZO-1 labelling was present in the ventricular wall. Capillaries of the organum vasculosum laminae terminalis expressed ZO-1 immunoreactivity which was comparable to the pattern observed in CNS regions with typical blood-brain barrier. The subcommissural organ, known to contain a blood-brain barrier, also displayed continuous ZO-1 staining in blood vessels. Unbroken ZO-1 distribution was observed in the specialized ependymal cells adjacent to both the organum vasculosum laminae terminalis and subcommissural organ. These immunocytochemical data demonstrate a distribution of ZO-1 in CNS parenchyma outside the circumventricular organs that is consistent with an organization of tight junctions which prevent free paracellular exchange of substances between blood and neuropil but which allow for continuity between CSF and the neuronal environment. The ZO-1 staining pattern in blood vessels and ventricular walls of the circumventricular organs is heterogeneous despite the prevalent absence of a functional blood-brain barrier.

Neovascularization and the appearance of morphological characteristics of the blood-brain barrier in the embryonic mouse central nervous system

Vascularization and expression of blood-brain barrier (bbb)-associated morphological characteristics during the embryonic development of the mouse central nervous system (CNS) was studied by ultrastructural analysis. At embryonic day 9 (E9) capillaries were only found in the perineural mesenchymal tissue. These capillaries showed fenestrations, and pericyte like cells (PC) were found joined to the vessel walls. Around E10 endothelial cells (EC) together with PC started invading the intraneural section. At this stage, the immigrating endothelial cells lost their fenestrations and exhibited numerous, partly extended junctional complexes, which appeared 'tight' in some places. A first intraneural anastomotic plexus was observed at E10, as evidenced by the presence of blood cells in all capillary lumens. While the number of junctional complexes remained constant in intraneural capillaries, the frequency of pinocytotic vesicles decreased significantly from E10 to E17. These findings indicate that from the first day of intraneural vascularization onwards, the morphological properties of the bbb are present in the early embryonic mouse cerebral cortex.

An electron microscopic immunogold analysis of developmental up-regulation of the blood-brain barrier GLUT1 glucose transporter

Electron microscopy was used to quantitate blood-brain barrier (BBB) glucose transporters in newborn, 14-day-old suckling, 28-day-old weanling, and adult rabbits. A rabbit polyclonal antiserum to a synthetic peptide encoding the 13 C-terminal amino acids of the human erythrocyte glucose transporter (GLUT1) was labeled with 10-nm gold particle-secondary antibody conjugates and localized immunoreactive GLUT1 molecules in rabbit brain capillary endothelia. Three distinct populations of brain capillary profiles were identified in newborn rabbits: prepatent capillary buds, partially patent capillaries with highly amplified luminal membranes, and patent capillaries. Immunogold analyses indicated that the GLUT1 transporter abundance positively correlated with capillary developmental status. The mean number of gold particles per capillary profile increased at each developmental age examined, suggesting that developmental up-regulation of the BBB glucose transporter occurred in rabbits. GLUT1 immunoreactivity was three- to fourfold greater on the abluminal than luminal capillary membranes among all ages examined. Changes in the proportions of GLUT1 transporter were also seen, and possible reasons for the postnatal decrease in the percentage of cytoplasmic GLUT1 transporter are discussed. The numbers of cytoplasmic and membrane-associated immunogold particles increased with age. We conclude that regulatory modulations of BB glucose transport may be characterized by increases in BBB glucose transporter density with age and state of development. In addition, modulation of glucose transporter activity may be reflected by minor postnatal shifts of GLUT1 from cytoplasmic to membrane compartments, which can be demonstrated with quantitative immunogold electron microscopy.