Luminal localization of blood-brain barrier sodium, potassium adenosine triphosphatase is dependent on fixation

Evaluating the involvement of cerebral microvascular endothelial Na+/K+-ATPase and Na+-K+-2Cl- co-transporter in electrolyte fluxes in an in vitro blood-brain barrier model of dehydration

The blood-brain barrier (BBB) is involved in brain water and salt homeostasis. Blood osmolarity increases during dehydration and water is osmotically extracted from the brain. The loss of water is less than expected from pure osmotic forces, due to brain electrolyte accumulation. Although the underlying molecular mechanisms are unresolved, the current model suggests the luminally expressed Na⁺-K⁺-2Cl^- co-transporter 1 (NKCC1) as a key component, while the role of the Na⁺/K⁺-ATPase remains uninvestigated. To test the involvement of these proteins in brain electrolyte flux under mimicked dehydration, we employed a tight in vitro co-culture BBB model with primary cultures of brain endothelial cells and astrocytes. The Na⁺/K⁺-ATPase and the NKCC1 were both functionally dominant in the abluminal membrane. Exposure of the in vitro BBB model to conditions mimicking systemic dehydration, i.e. hyperosmotic conditions, vasopressin, or increased [K⁺]_o illustrated that NKCC1 activity was unaffected by exposure to vasopressin and to hyperosmotic conditions. Hyperosmotic conditions and increased K⁺ concentrations enhanced the Na⁺/K⁺-ATPase activity, here determined to consist of the α1 β1 and α1 β3 isozymes. Abluminally expressed endothelial Na⁺/K⁺-ATPase, and not NKCC1, may therefore counteract osmotic brain water loss during systemic dehydration by promoting brain Na⁺ accumulation.

Fluid and ion transfer across the blood-brain and blood-cerebrospinal fluid barriers; a comparative account of mechanisms and roles

The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na+-pumps. K+ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood-brain barrier lining brain microvasculature, allows passage of O2, CO2, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood-brain barrier Na+ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood-brain barrier is the most important interface for maintaining interstitial fluid (ISF) K+ concentration within tight limits. This is most likely because Na+-pumps vary the rate at which K+ is transported out of ISF in response to small changes in K+ concentration. There is also evidence for functional regulation of K+ transporters with chronic changes in plasma concentration. The blood-brain barrier is also important in regulating HCO3- and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood-brain barrier HCO3- transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO3- concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pHISF and pHplasma are similar. The key similarities and differences of the two interfaces are summarized.

Apicobasal polarity of brain endothelial cells

Normal brain homeostasis depends on the integrity of the blood-brain barrier that controls the access of nutrients, humoral factors, and immune cells to the CNS. The blood-brain barrier is composed mainly of brain endothelial cells. Forming the interface between two compartments, they are highly polarized. Apical/luminal and basolateral/abluminal membranes differ in their lipid and (glyco-)protein composition, allowing brain endothelial cells to secrete or transport soluble factors in a polarized manner and to maintain blood flow. Here, we summarize the basic concepts of apicobasal cell polarity in brain endothelial cells. To address potential molecular mechanisms underlying apicobasal polarity in brain endothelial cells, we draw on investigations in epithelial cells and discuss how polarity may go awry in neurological diseases.

Endothelial cell heterogeneity of blood-brain barrier gene expression along the cerebral microvasculature

The blood-brain barrier (BBB) refers to the network of microvessels that selectively restricts the passage of substances between the circulation and the central nervous system (CNS). This microvascular network is comprised of arterioles, capillaries and venules, yet the respective contribution of each of these to the BBB awaits clarification. In this regard, it has been postulated that brain microvascular endothelial cells (BMEC) from these different tributaries might exhibit considerable heterogeneity in form and function, with such diversity underlying unique roles in physiological and pathophysiological processes. Means to begin exploring such endothelial differences in situ, free from caveats associated with cell isolation and culturing procedures, are crucial to comprehending the nature and treatment of CNS diseases with vascular involvement. Here, the recently validated approach of immuno-laser capture microdissection (immuno-LCM) coupled to quantitative real-time PCR (qRT-PCR) was used to analyze gene expression patterns of BMEC retrieved in situ from either capillaries or venules. From profiling 87 genes known to play a role in BBB function and/or be enriched in isolated brain microvessels, results imply that most BBB properties reside in both segments, but that capillaries preferentially express some genes related to solute transport, while venules tend toward higher expression of an assortment of genes involved in inflammatory-related tasks. Fuller appreciation of such heterogeneity will be critical for efficient therapeutic targeting of the endothelium and the management of CNS disease.

Blood-brain barrier Ca2+-ATPase cytochemistry: incubation media and fixation methods for differentiating Ca2+-specific ATPase from ecto-ATPase

Ca2+-ATPase cytochemistry frequently uses the incubation medium of Ando et al. that was introduced in 1981. Some studies, however, have suggested that this medium localizes ecto-ATPase in addition to Ca2+-ATPase and that Ca2+-ATPase is sensitive to fixation. Strong activity of the enzyme on the luminal surface of the blood-brain barrier (BBB) also is considered indicative of immature or pathological microvessels. We address here five questions. 1) Is the incubation medium of Ando et al. specific for BBB Ca2+-ATPase or does it also localize ecto-ATPase? 2) How are the two enzymes distributed in the BBB? 3) How would data interpretation be prone to error if the cytochemical study does not use controls identifying ecto-ATPase? 4) Does the amount of reaction product of both enzymes vary significantly when the cortical tissue is exposed to different fixatives? 5) Does the presence of Ca2+-ATPase on the luminal membrane of the BBB necessarily indicate immature or abnormal brain endothelial cells? Adult male Sprague-Dawley rats were perfused with one of two different fixatives and vibratome slices of the brain cortex were incubated in the medium of Ando et al. The controls used were those demonstrating the ecto-ATPase and those that do not. The results indicate that the incubation medium is not specific for Ca2+-ATPase, because it also localizes the ecto-ATPase. Ca2+-ATPase appears to be localized primarily on the luminal surface of the BBB, while ecto-ATPase is localized on both the luminal and abluminal surfaces. The portion of the reaction product contributed by Ca2+-ATPase would not have been identified if the controls uniquely identifying the ecto-ATPase had not been used. The amount of reaction product formed by Ca2+-ATPase is strongly dependent on the type of fixative used. The strong localization of Ca2+-ATPase on the luminal surface of the BBB is not only normal, but also better accounts for the physiological homeostasis of Ca2+ across the blood-brain interface and should not be interpreted as indicative of immature or pathological microvessels.

Polarized P-glycoprotein expression by the immortalised human brain endothelial cell line, hCMEC/D3, restricts apical-to-basolateral permeability to rhodamine 123

P-glycoprotein (P-gp) expression at the blood-brain barrier prevents unwanted blood-borne toxins and signalling molecules from entering the brain. Primary and immortalised human brain endothelial cells (BECs) represent two suitable options for studying P-gp function in vitro. The limited supply of primary human BECs and their instability over passage number make this choice unattractive for medium/high throughput studies. The aim of this study was to further characterise the expression of P-gp by an immortalised human BEC line, hCMEC/D3, in order to evaluate their use as an in vitro human blood-brain barrier model. P-gp expression was stable over a high passage number (up to passage 38) and was polarised on the apical plasma membrane, consistent with human BECs in vivo. In addition, hCMEC/D3 cell P-gp expression was comparable, albeit slightly lower to that observed in primary isolated human BECs although P-gp function was similar in both cell lines. The P-gp inhibitors tariquidar and vinblastine prevented the efflux of rhodamine 123 (rh123) from hCMEC/D3 cells, indicative of functional P-gp expression. hCMEC/D3 cells also displayed polarised P-gp transport, since both tariquidar and vinblasine selectively increased the apical-to-basolateral permeability of hCMEC/D3 cells to rh123. The results presented here demonstrate that hCMEC/D3 cells are a suitable model to investigate substrate specificity of P-gp in BECs of human origin.

Normal endothelium

In recent decades, it has become evident that the endothelium is by no means a passive inner lining of blood vessels. This 'organ' with a large surface (approximately 350 m2) and a comparatively small total mass (approximately 110 g) is actively involved in vital functions of the cardiovascular system, including regulation of perfusion, fluid and solute exchange, haemostasis and coagulation, inflammatory responses, vasculogenesis and angiogenesis. The present chapter focusses on two central aspects of endothelial structure and function: (1) the heterogeneity in endothelial properties between species, organs, vessel classes and even within individual vessels and (2) the composition and role of the molecular layer on the luminal surface of endothelial cells. The endothelial lining of blood vessels in different organs differs with respect to morphology and permeability and is classified as 'continuous', 'fenestrated' or 'discontinuous'. Furthermore, the mediator release, antigen presentation or stress responses of endothelial cells vary between species, different organs and vessel classes. Finally there are relevant differences even between adjacent endothelial cells, with some cells exhibiting specific functional properties, e.g. as pacemaker cells for intercellular calcium signals. Organ-specific structural and functional properties of the endothelium are marked in the vascular beds of the lung and the brain. Pulmonary endothelium exhibits a high constitutive expression of adhesion molecules which may contribute to the margination of the large intravascular pool of leucocytes in the lung. Furthermore, the pulmonary microcirculation is less permeable to protein and water flux as compared to large pulmonary vessels. Endothelial cells of the blood-brain barrier exhibit a specialised phenotype with no fenestrations, extensive tight junctions and sparse pinocytotic vesicular transport. This barrier allows a strict control of exchange of solutes and circulating cells between the plasma and the interstitial space. It was observed that average haematocrit levels in muscle capillaries are much lower as compared to systemic haematocrit, and that flow resistance of microvascular beds is higher than expected from in vitro studies of blood rheology. This evidence stimulated the concept of a substantial layer on the luminal endothelial surface (endothelial surface layer, ESL) with a thickness in the range of 0.5-1 microm. In comparison, the typical thickness of the glycocalyx directly anchored in the endothelial plasma membrane, as seen in electron micrographs, amounts to only about 50-100 microm. Therefore it is assumed that additional components, e.g. adsorbed plasma proteins or hyaluronan, are essential in constituting the ESL. Functional consequences of the ESL presence are not yet sufficiently understood and acknowledged. However, it is evident that the thick endothelial surface layer significantly impacts haemodynamic conditions, mechanical stresses acting on red cells in microvessels, oxygen transport, vascular control, coagulation, inflammation and atherosclerosis.

A quantitative overview of glucose dynamics in the gliovascular unit

While glucose is constantly being "pulled" into the brain by hexokinase, its flux across the blood brain barrier (BBB) is allowed by facilitative carriers of the GLUT family. Starting from the microscopic properties of GLUT carriers, and within the constraints imposed by the available experimental data, chiefly NMR spectroscopy, we have generated a numerical model that reveals several hidden features of glucose transport and metabolism in the brain. The half-saturation constant of glucose uptake into the brain (K(t)) is close to 8 mM. GLUT carriers at the BBB are symmetric, show accelerated-exchange, and a K(m) of zero-trans flux (K(zt)) close to 5 mM, determining a ratio of 3.6 between maximum transport rate and net glucose flux (T(max)/CMR(glc)). In spite of the low transporter occupancy, the model shows that for a stimulated hexokinase to pull more glucose into the brain, the number or activity of GLUT carriers must also increase, particularly at the BBB. The endothelium is therefore predicted to be a key modulated element for the fast control of energy metabolism. In addition, the simulations help to explain why mild hypoglycemia may be asymptomatic and reveal that [glucose](brain) (as measured by NMR) should be much more sensitive than glucose flux (as measured by PET) as an indicator of GLUT1 deficiency. In summary, available data from various sources has been integrated in a predictive model based on the microscopic properties of GLUT carriers.

Localization of brain endothelial luminal and abluminal transporters with immunogold electron microscopy

Immunogold electron microscopy has identified a variety of blood-brain barrier (BBB) proteins with transporter and regulatory functions. For example, isoforms of the glucose transporter, protein kinase C (PKC), and caveolin-1 are BBB specific. Isoform 1 of the facilitative glucose transporter family (GLUT1) is expressed solely in endothelial (and pericyte) domains, and approximately 75% of the protein is membrane-localized in humans. Evidence is presented for a water cotransport function of BBB GLUT1. A shift in transporter polarity characterized by increased luminal membrane GLUT1 is seen when BBB glucose transport is upregulated; but a greater abluminal membrane density is seen in the human BBB when GLUT1 is downregulated. PKC colocalizes with GLUT1 within these endothelial domains during up- and downregulation, suggesting that a PKC-mediated mechanism regulates human BBB glucose transporter expression. Occludin and claudin-5 (like other tight-junctional proteins) exhibit a restricted distribution, and are expressed solely within interendothelial clefts of the BBB. GFAP (glial fibrillary acidic protein) is uniformly expressed throughout the foot-processes and the entire astrocyte. But the microvascular-facing membranes of the glial processes that contact the basal laminae are also polarized, and their transporters may also redistribute within the astrocyte. Monocarboxylic acid transporter and water channel (Aquaporin-4) expression are enriched at the glial foot-process, and both undergo physiological modulation. We suggest that as transcytosis and efflux mechanisms generate interest as potential neurotherapeutic targets, electron microscopic confirmation of their site-specific expression patterns will continue to support the CNS drug discovery process.

Electron microscopical enzyme histochemistry on unfixed tissues and cells. Bridging the gap between LM and EM enzyme histochemistry

In principle, enzyme histochemistry should be performed on unfixed tissues and cells to avoid inhibition of enzyme activity by chemical fixation. For EM enzyme histochemistry, unfixed tissue specimens include fresh tissue blocks, non-frozen tissue chopper sections, cryostat sections and cell preparations. Studies on localization of enzyme activity at the ultrastructural level in unfixed specimens, be it fresh or frozen, are reviewed here. Preservation of ultrastructural morphology is discussed with special attention to the effects of freezing. It is concluded that unfixed cryostat sections are the best alternative for EM histochemistry of tissues, when interposing a semipermeable membrane in between cryostat section and gelled incubation medium. It is an adequate method to preserve structural integrity of unfixed tissue on the one hand and to avoid inactivation of the enzyme by chemical fixation on the other. For EM cytochemistry on individual cells, a better preservation of ultrastructure may be obtained because freezing can be avoided, but mild pretreatment with a fixative or detergent may be necessary to permeabilize cellular membranes for demonstration of intracellular enzyme activity.

Effect of reduced flow on blood-brain barrier transport systems

Pathological states (i.e. stroke, cardiac arrest) can lead to reduced blood flow to the brain potentially altering blood-brain barrier (BBB) permeability and regulatory transport functions. BBB disruption leads to increased cerebrovascular permeability, an important factor in the development of ischemic brain injury and edema formation. In this study, reduced flow was investigated to determine the effects on cerebral blood flow (CBF), pressure, basal BBB permeability, and transport of insulin and K+ across the BBB. Anesthetized adult female Sprague-Dawley rats were measured at normal flow (3.1 ml min(-1)), half flow (1.5 ml min(-1)), and quarter flow (0.75 ml min(-1)), using bilateral in situ brain perfusion for 20 min followed by capillary depletion analysis. Reduction in perfusion flow rates demonstrated a modest reduction in CBF (1.27-1.56 ml min(-1) g(-1)), a decrease in pressure, and no significant effect on basal BBB permeability indicating that autoregulation remained functional. In contrast, there was a concomittant decrease in BBB transport of both insulin and K+ with reduced flow. At half and quarter flow, insulin transport was significantly reduced (R(Br)%=17.2 and R(Br)%=16.2, respectively) from control (R(Br)%=30.4). Additionally, a significant reduction in [86Rb+] was observed at quarter flow (R(Br)%=2.5) as compared to control (R(Br)%=4.8) suggesting an alteration in ion homeostasis as a result of low flow. This investigation suggests that although autoregulation maintains CBF, BBB transport mechanisms were significantly compromised in states of reduced flow. These flow alterations may have a significant impact on brain homeostasis in pathological states.