Enzyme cytochemistry of blood-brain barrier (BBB) disturbances

Tissue Non-specific Alkaline Phosphatase (TNAP) in Vessels of the Brain

The microvessels of the brain represent around 3-4 % of the brain compartment but constitute the most important length (400 miles) and surface of exchange (20 m(2)) between the blood and the parenchyma of brain. Under influence of surrounding tissues, the brain microvessel endothelium expresses a specific phenotype that regulates and restricts the entry of compounds and cells from blood to brain, and defined the so-called blood-brain barrier (BBB). Evidences that alkaline phosphatase (AP) is a characteristic feature of the BBB phenotype that allows differentiating capillary endothelial cells from brain to those of the periphery have rapidly emerge. Thenceforth, AP has been rapidly used as a biomarker of the blood-brain barrier phenotype. In fact, brain capillary endothelial cells (BCECs) express exclusively tissue non-specific alkaline phosphatase (TNAP). There are several lines of evidence in favour of an important role for TNAP in brain function. TNAP is thought to be responsible for the control of transport of some compounds across the plasma membrane of the BCECs. Here, we report that levamisole-mediated inhibition of TNAP provokes an increase of the permeability to Lucifer Yellow of the endothelial monolayer. Moreover, we illustrate the disruption of the cytoskeleton organization. Interestingly, all observed effects were reversible 24 h after levamisole removal and correlated with the return of a full activity of the TNAP. This reversible effect remains to be studied in details to evaluate the potentiality of a levamisole treatment to enhance the entry of drugs in the brain parenchyma.

An alkaline phosphatase transport mechanism in the pathogenesis of Alzheimer's disease and neurodegeneration

Systemic inflammation is associated with loss of blood-brain barrier integrity and neuroinflammation that lead to the exacerbation of neurodegenerative diseases. It is also associated specifically with the characteristic amyloid-β and tau pathologies of Alzheimer's disease. We have previously proposed an immunosurveillance mechanism for epithelial barriers involving negative feedback-regulated alkaline phosphatase transcytosis as an acute phase anti-inflammatory response that hangs in the balance between the resolution and the progression of inflammation. We now extend this model to endothelial barriers, particularly the blood-brain barrier, and present a literature-supported mechanistic explanation for Alzheimer's disease pathology with this system at its foundation. In this mechanism, a switch in the role of alkaline phosphatase from its baseline duties to a stopgap anti-inflammatory function results in the loss of alkaline phosphatase from cell membranes into circulation, thereby decreasing blood-brain barrier integrity and functionality. This occurs with impairment of both amyloid-β efflux and tau dephosphorylating activity in the brain as alkaline phosphatase is replenished at the barrier by receptor-mediated transport. We suggest systemic alkaline phosphatase administration as a potential therapy for the resolution of inflammation and the prevention of Alzheimer's disease pathology as well as that of other inflammation-related neurodegenerative diseases.

Mediators of cerebral edema

The blood-brain barrier (BBB) which is located in the continuous endothelial lining of cerebral blood vessels rigidly controls exchange of water soluble compounds under physiological conditions. Under pathological conditions such as trauma or ischemia, BBB permeability may increase thus allowing plasma constituents to escape into brain tissue. This "opening" of the BBB may, at least in part, be mediated by massive release of autacoids resulting in vasogenic brain edema. Five criteria have to be fulfilled by an individual autacoid to be considered a mediator candidate of cerebral edema: i) a permeability-enhancing action under physiological conditions, ii) a vasodilatory action, iii) the ability to induce vasogenic brain edema, iv) an increase of concentration in the tissue or interstitial fluid under pathological conditions, and v) a decrease of brain edema by specific interference with the release or action of a given autacoid. Among the mediator candidates considered, bradykinin is the only one to meet all criteria. Histamine, arachidonic acid and free radicals including nitric oxide may also be considered mediators of brain edema, but for each of these compounds evidence is less clear than for bradykinin. Although the concept of mediators inducing brain edema is well established by experimental studies, only a bradykinin receptor antagonist has so far gained entrance into clinical evaluation.

Permeability function related to cerebral microvessel enzymes during ageing in rats

Cerebral microvessels from rats were prepared and characterized by their enrichment of specific markers, namely alkaline phosphatase (AP) and tau-glutamyl transpeptidase (tau-GT). Further, it was observed that AP and tau-GT registered marked increase in aged rats. On the contrary, lactate dehydrogenase (LDH) activity decreased with the increasing age. Monoamine oxidase A activity in the microvessels decreased with age whereas MAO-B moved in the reverse direction. No noticeable change was seen in acetyl-cholinesterase activity with increasing age of rats.

Changes of the Na/K ATPase activity in the cerebral cortical microvessels of rat after single intraperitoneal administration of mercuric chloride: histochemical demonstration with light and electron microscopy

Since inorganic mercury salts only poorly penetrate the cerebral microvascular endothelial cells comprising the blood-brain barrier (BBB), their neurotoxicity may be predicted to result from interference with BBB transport enzymes. In the present study, we tested the effect of mercuric chloride (HgCl2) on Na+/K+ ATPase activity, a key enzyme involved in the ion transport in and out of the brain. Routine histochemical staining in conjunction with light and electron microscopy was used to evaluate the changes in the Na+/K+ ATPase activity in cerebral cortical microvessels of rats who received a single intraperitoneal injection of 6 mg/kg HgCl2. At 1 h after HgCl2 administration, light microscopy revealed uniform reduction of the Na+/K+ ATPase reaction in all cortical layers. Electron microscopy confirmed the enzyme reaction to be very weak to completely absent in both the luminal and abluminal endothelial cell membranes, and the luminal plasmalemma showed invaginations and pinocytic vesicles indicative of changes in its transport functions. The enzyme inhibition coincided with, and was likely to contribute to, profound perivascular swelling, involving mainly the astrocytic endfeet. The enzyme activity showed a partial recovery 18 h after HgCl2 treatment, mainly in cortical layers II and III. After 5 days, the recovery of the enzyme activity appeared complete as observed by light and electron microscopy. The recovery of the microvascular Na+/K+ ATPase coincided with the appearance of a strongly positive Na+/K+ ATPase reaction in the adjacent astrocytic processes and with the diminution of perivascular swelling.

The blood-brain barrier: cellular basis

Perfusion experiments with horseradish peroxidase have established that the morphological substrate of the blood-brain barrier is represented by microvascular endothelial cells. They are characterized by complexly arranged tight junctions and a very low rate of transcytotic vesicular transport. They express transport enzymes, carrier systems and brain endothelial cell-specific molecules of unknown function not expressed by any other endothelial cell population. These blood-brain barrier properties are not intrinsic to these cells but are inducible by the surrounding brain tissue. Type I astrocytes injected into the anterior eye chamber of the rat or onto the chick chorioallantoic membrane are able to induce a host-derived angiogenesis and some blood-brain barrier properties in endothelial cells of non-neural origin. Recently we have shown that this cellular interaction is due to the secretion of a soluble astrocyte derived factor(s). Astrocytes are also implicated in the maintenance, functional regulation and the repair of the blood-brain barrier. Complex interactions between other constituents of the microenvironment surrounding the endothelial cells, such as the basement membrane, pericytes, nerve endings, microglial cells and the extracellular fluid, take place and are required for the proper functioning of the blood-brain barrier, which in addition is regionally different as reflected by endothelial cell heterogeneity.

A cytochemical study of cerebrovascular lesions in mice infected with Plasmodium berghei

Mice with a Plasmodium berghei infection exhibit morphological and cytochemical changes in the blood-brain barrier. Changes in activity and localization of alkaline phosphatase and adenosine triphosphatase, enzymes with important functions in the maintenance of the blood-brain barrier, were observed. Changes in activity and localization of those enzymes in and near the endothelial cells of the microvasculature, concomitant with an increase in pinocytotic activity, and formation of irregular cytoplasmic extensions in these cells, as well as loosening of the basal lamina are indicative of a functional deterioration of the blood-brain barrier in the course of infection.

Quantitative detection of blood-brain barrier-associated enzymes in cultured endothelial cells of porcine brain microvessels

The present study deals with a rapid and convenient assay for blood-brain barrier (BBB)-associated enzymes, gamma-glutamyl transpeptidase (gamma-GTP) and alkaline phosphatase (ALP), in cultured endothelial cells and other cells. These enzyme activities in cultured cells could be efficiently measured by direct incubation of each substrate in the culture plates without pretreatment of the cells. This new direct in situ-in plate assay was more rapid and convenient than conventional ex-plate assays, and these assays gave similar values for specific enzyme activities. gamma-GTP and ALP activities could be detected by this in situ method in primary-cultured endothelial cells of porcine brain microvessels, but their levels were lower than those before culture. The degree of loss due to culture differed between gamma-GTP and ALP; a relatively large amount of ALP remained but the gamma-GTP level decreased greatly. In this direct in situ-in plate assay, cultured porcine aortic endothelial cells exhibited negligibly small activities for both enzymes, whereas cultured astroglial cells of neonatal porcine brain showed moderate gamma-GTP activity and a trace of ALP activity. This direct in situ-in plate assay can be used for microculture and automatic measurement and offers a convenient means for studying the possible regulatory mechanisms of the expression of the BBB-associated enzymes.

Pharmacological properties of voltage-dependent calcium channels in functional microvessels isolated from rat brain

Voltage-operated calcium channels were studied in rat intracerebral microvessels. The contractile reactivity to KCl-depolarization was assessed by the measurement of internal diameter of superfused microvessels. Dihydropyridine receptor sites associated with calcium channels were identified and characterized using 3H(+)PN 200-110 [isopropyl-4-(2,1,3-benzodiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5- -methoxycarbonyl-pyridine-3-carboxylate]. Depolarization induced by high-KCl solution produced a marked reduction of the internal diameter of cerebral microvessels which was associated with the appearance of rhythmic activity. The vessel contraction was reversible and abolished by nimodipine. Binding studies with 3H(+)PN 200-110 revealed the existence of a single class of specific, stereoselective and voltage-dependent binding sites which bound (+)PN 200-110 with a KD of 88 +/- 6.6 pmol l-1 at 37 degrees C in microvessels incubated in NaCl medium. When microvessels were incubated in KCl-medium, the apparent KD value was reduced to 35 +/- 2 pmol l-1. Bmax was not significantly changed. The effect of KCl was not related to concomitant changes in the Na concentration. The potency of various dihydropyridine derivatives in inhibiting 3H(+)PN 200-110 binding was in agreement with their pharmacological potency in smooth muscle preparations. The effect of PN 200-110 and of nimodipine was stereoselective. Ki values of PN 200-110 and of nimodipine were increased in depolarized preparations, while nifedipine's potency was unchanged. Verapamil was only a partial inhibitor of 3H(+)PN 200-110 binding. The effect of diltiazem was stereoselective: the (+)-cis isomer enhanced the binding and the (-)-cis isomer of diltiazem poorly inhibited the binding of PN 200-110. Results showed that isolated cerebral microvessels possess functional voltage-operated calcium channels, which contain potential-modulated receptors for dihydropyridine calcium entry blockers with characteristics similar to those described in other tissues.

Ultrastructural and ultracytochemical studies on the blood-brain barrier in chronic relapsing experimental allergic encephalomyelitis

We induced chronic relapsing experimental allergic encephalomyelitis (EAE), and studied the ultrastructural and ultracytochemical changes of the blood-brain barrier (BBB) in the demyelinating lesions of various stages of EAE. In the chronic, inactive stage with gliosis and perivascular fibrosis, the basal lamina (BL) of the perivascular processes of astrocytes was formed only partially, and neural parenchyma was not fully separated from the perivascular mesenchymal tissues by the BL of astrocytic processes. Vascular permeability of the BBB was studied using exogenous horseradish peroxidase (HRP) as the tracer: HRP extravasation was marked during the stages of both active myelin breakdown and removal of debris, and was recognized even at the inactive stage, although the degree was reduced to a very low level. The functions of the endothelia, assessed by ouabain-sensitive, K+-dependent p-nitrophenylphosphatase activity, were impaired as EAE progressed. The decrease in HRP leakage at the inactive stage suggests the endothelial impairment of active transport of metabolites including HRP. Along with the development of inflammatory demyelination in EAE, the BBB in affected areas became more and more altered, and gradual morphological and functional impairment of the BBB developed.

Quantitative enzyme histochemistry of rat foetal brain and trigeminal ganglion

The increasing concern and the efforts in determining neurological effects in offsprings resulting from maternal exposure to xenobiotics are faced with several difficulties in monitoring damage to the central nervous system. In this paper, the efficiency of several enzyme histochemical reactions for analysing the forebrain and the trigeminal ganglia of rat foetuses are reported. Brains of 20-day-old Sprague-Dawley rat foetuses were frozen and analysed for 18 enzymes that had previously been used to monitor initial injury caused by toxic compounds in liver and other organs. Eight enzymes appeared suitable as histochemical markers for the functional integrity of different areas in brain and ganglia of rats exposed to xenobiotics. They were lactate, malate, glycerophosphate (NAD-linked), succinate, aldehyde and glucose 6-phosphate dehydrogenases, alpha-glycerophosphate-menadione oxidoreductase and cytochrome c oxidase. The activities of the enzymes were determined by microphotometry and the arrangement of absorbances of the enzyme final reaction products into appropriate analytical tables is proposed as an efficient procedure for data analysis.

Differential localization of alkaline phosphatase in barrier tissues of the frog and rat nervous systems: a cytochemical and biochemical study

We investigated the localization of alkaline phosphatase (AP) in the peripheral and central nervous systems of the frog (Rana pipiens) and rat. In the frog sciatic nerve, AP reaction product was seen as a precipitate within caveolae and vesicular profiles of perineurial cells, and frequently filled the extracellular space. In the rat peripheral nerve, AP reaction product appeared as small tufts on the cell surfaces and within vesicular profiles of endoneurial blood vessels. AP reaction product was not detected in the rat perineurium or in endoneurial blood vessels of the frog. In the frog central nervous system, AP reaction product was detected in the arachnoid membrane adjacent to the subarachnoid space, but not in the cerebral or pial vessels, whereas in the rat it was detected in the outer arachnoid membrane and in the cerebral and pial blood vessels. Biochemical analysis indicated a sevenfold higher AP activity in the frog perineurium over the endoneurium, whereas in the rat, threefold more activity was measured in the endoneurium over the perineurium. Levamisole, an AP inhibitor, decreased the enzyme activity by 95% in rat tissues, and by 70% in frog tissues and in plasma from both animals. Similar decrements were observed cytochemically. This study suggests that: (1) the distribution of AP varies between species, but that it is always present in at least one component of the blood-brain and blood-nerve barriers, (2) because barrier tissues of the nervous system have enzymatic activity, they may biochemically modify the adjacent environment, (3) vesicular profiles and caveolae in the blood vessels and perineurium may function as microenvironments for enzymatic activity, and (4) in the rat and frog, different isozymes of AP may be present.

Ultracytochemical localization of alkaline phosphatase activity in endothelial cells in chronic relapsing experimental allergic encephalomyelitis

To investigate the functions of endothelial cells (ECs) in chronic relapsing experimental allergic encephalomyelitis (EAE), we examined ECs ultracytochemically in various stages of EAE, in conjunction with the localization of alkaline phosphatase (AP) activity. We also studied the relation between the specific localization of AP activity and pathological features at each stage. Chronic relapsing EAE was induced in strain-13 guinea pigs by inoculation with homologous myelin. Controls were inoculated with complete Freund's adjuvant. The controls showed AP activity on the luminal and abluminal surfaces of the plasmalemma, and in pinocytic vesicles and vesicular pits. The localization of AP activity in the preclinical stage of EAE was similar to that in control animals. The initial inflammatory and actively demyelinating stage with perivascular cuffs of mononuclear cells showed AP-positive reactions on the abluminal surface of the plasmalemma, and in vesicles and pits, but not on the luminal surface in many ECs. In a later stage showing relatively old plaques with perivascular accumulation of debris-containing macrophages, AP activity continued to show localization similar to that seen in the initial stage, except for the presence of AP activity on some segments of the abluminal plasmalemma. Inactive lesions with marked perivascular fibrosis showed no AP reaction products. AP activity in unaffected areas showed the same localization as that in control animals throughout the various clinical stages of EAE. These findings suggest that AP activity decreased as the inflammatory demyelination in EAE progressed. The gradual disappearance of AP activity suggests development of functional impairment of ECs.

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

The distribution of anionic sites detected in vitro with cationized ferritin and lectin-binding sites on the endothelial cell (EC) surface of brain micro-blood vessels was studied by electron microscopy. Gold-labeled lectins and glycoproteins and Lowicryl K4M-embedded brain samples obtained from mouse embryos (19th day), and from 1-, 5-, 12-, 24- and 48-day-old and adult mice were used. It was shown that the functional maturation of the blood-brain barrier (BBB) occurring in the mouse after birth between the 12th and 24th day of life is accompanied by a disappearance of vesicular transport in capillaries and by the formation of a uniform, thin, negatively charged layer on the surface of the EC. Concomitantly the binding of lectins specific for beta-D-galactosyl (RCA) and sialyl (LFA and WGA) residues become progressively more intense and uniform on both luminal and abluminal fronts of the EC. The concentration of HPA-binding sites on the abluminal side of the EC and in the basement membrane increases. Similarly the binding of Con A becomes more intense on abluminal than on luminal front of the EC. These observations suggest that extensive remodeling of anionic sites and surface glycoprotein layer and also the elaboration of ECs polarity occur during BBB maturation.

Glial cells influence membrane-associated enzyme activity at the blood-brain barrier

Glial cells have been shown to influence several cerebral endothelial cell properties in vitro. A situation similar to the endothelial-astrocyte relationship existing at the blood-brain barrier (BBB) can be produced by growing cultured cerebral endothelium on one side of a filter and glial cells on the other in an enclosed double chamber. In this setting membrane-associated reaction product on the cerebral endothelial cell for both Na+,K+-ATPase and non-specific alkaline phosphatase was markedly increased when the endothelial cells were co-cultured with glial cells. In addition, the distribution of reaction product on the cerebral endothelial cell membrane was similar to that reported in vivo. These observations support a glial influence on enzyme activity at the BBB.

Changes in the distribution of endothelial surface glycoconjugates associated with altered permeability of brain micro-blood vessels

Lectin-binding sites located on the endothelial cell (EC) surfaces in unaltered, leaking and resorbing micro-blood vessels (MBVs) in cryo-injured cat brain were studied. Lectin or glycoprotein-gold complexes and brain samples embedded in hydrophilic resin Lowicryl K4M were used. The lectins tested recognize the following residues: beta-D-galactosyl (Ricinus communis agglutinin 120, RCA and peanut agglutinin, PNA), sialyl (Limax flavus agglutinin), N-acetyl-D-galactosaminyl (Helix pomatia agglutinin and soybean agglutinin, SBA), alpha-D-glucosyl and alpha-D-mannosyl (concanavalin A). The luminal front was labeled with SBA, and both fronts of the EC were labeled with PNA only after neuraminidase digestion. The most abundant and regularly distributed on both fronts of the EC were beta-D-galactosyl residues (RCA). These residues were also most affected in altered MBVs. The labeling of sialic acid residues was less pronounced on both sides of the EC. Following alteration of the function of the blood-brain barrier by cold-lesion injury, in leaking MBVs which represent increased luminal transport, we observed a conspicuous diminution of the labeling of the luminal surface of the EC with some lectins. On the other hand, in resorbing blood vessels located in the area of edema, where a presumably reverse (abluminal) transport occurs, major changes in the distribution of lectin-binding sites occurred on the abluminal front of the EC and in the basement membrane. The results reported here indicate that luminal and abluminal fronts of the EC change their properties in various functional conditions of MBVs, and that these changes can also be a reflection of functional polarity of brain endothelium.

Ultrastructural observations on the transvascular route of protein removal in vasogenic brain edema

Micro-blood vessels (MBVs), located in the area of edema, were studied in cat brain at various time intervals (1 h, 24 h, 7 days) after cold-lesion injury. Both cold-injured and adjacent gyri were examined for blood-brain barrier (BBB) permeability to i. v. injected horseradish peroxidase (HRP) with circulation times of 40 min and 24 h. Evans blue (EB) was used as a tracer for gross evaluation of the extension of brain edema. Localization of alkaline phosphatase (AP) and binding of cationized ferritin (CF), considered as a marker of anionic sites, were also studied ultrastructurally. Twenty-four hours after cold injury, the extravasated edema fluid, outlined by EB tracer, was observed to be spreading through the white matter (WM) into the adjacent gyrus. At this time, numerous, larger than capillary MBVs, presumably arterioles and venules located in the edematous WM, showed accumulations of HRP injected at the time of the operation, in the basement membrane, in abluminal pits, and in numerous pinocytotic vesicles and vacuoles of endothelial cells (ECs). The animals killed after 24 h with 40 min HRP circulation showed extravasation of HRP tracer in a zone underlying the necrotic cold injury lesion. On the other hand, there was no evidence of an abnormal HRP leakage in the further removed areas of edema in the WM, particularly in the adjacent gyrus.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural studies of concanavalin A receptors and 5'-nucleotidase localization in normal and injured mouse cerebral microvasculature

Plant lectin concanavalin A conjugated with ferritin (Con A-F) injected i.v. was used for the detection of the specific monosaccharide residues (alpha-D-mannosyl and alpha-D-glucosyl) on the luminal surface of endothelial cells (ECs) in brain micro-blood vessels (MBVs). Both normal mice and animals with mechanically damaged blood-brain barrier (BBB) were used in this study. In addition, the activity of 5'-nucleotidase (5'N), the putative receptor for Con A, was studied cytochemically. Various methodologic experiments indicated that the reaction product formed on the luminal plasmalemma of ECs after incubation of samples in the cytochemical medium for the detection of 5'N activity results from the action of unspecific phosphatase hydrolyzing both specific and nonspecific substrates. The abluminal side of the wall of MBVs seems to be a major location of 5'N activity. Thus, no correlation between cytochemically demonstrable 5'N activity and Con A receptor sites on the luminal surface of ECs was noted. After damage of the BBB, extensive internalization of the luminal plasmalemma forming the limiting membranes of pinocytotic vesicles, vacuoles, and endothelial channel-like structures was observed. This process was represented by a relatively rapid translocation of Con A receptors from luminal surface into the interior of the ECs and to the abluminal side of the vessel wall.

Ultracytochemical studies of vesicular and canalicular transport structures in the injured mammalian blood-brain barrier

An ultracytochemical investigation was performed to study the origin of pinocytic vesicles and canalicular structures within endothelial cells (EC) of the injured mammalian blood-brain barrier (BBB). To accomplish this goal, two electron-dense tracers, native ferritin (NF) and horseradish peroxidase (HRP), were used in conjunction with the detection of alkaline phosphatase (AP) activity, a known marker of EC plasmalemma of brain micro-blood vessels. Brain ECs from (1) mice subjected to crude leptomeningeal damage for 1, 2, or 3 days and (2) cats subjected to cold lesion injury for 1, 4, or 24 h were evaluated for tracer transport and AP activity. Fine structural analysis of leaking segments of micro-blood vessels from damaged cerebral cortex or basal ganglia demonstrated pinocytic vesicles, deep invaginations of the luminal plasmalemma and elongated, tubular profiles, all containing tracer. Because we observed in ECs from both experimental models of brain injury a positive reaction for AP activity in the luminal plasmalemma, in its deep invaginations, in delimiting membranes of pinocytic vesicles, and in tubulo-canalicular structures, we conclude that all types of transport structures derive from the same 100A thick exoplasmic plasmalemmal membranes. Further, besides the pinocytic vesicular transport system (PTS), the canalicular transport system (CTS) appears to serve as an additional important mechanism for macromolecular transport across the damaged mammalian BBB.