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Vol. 49, Issue 2, 143-156, June 1997
Division of Pharmacology (H.E.d.V., A.G.d.B., D.D.B.) and Division of Biopharmaceutics (H.E.d.V., J.K., T.J.C.V.B.), Leiden/Amsterdam Center for Drug Research, University of Leiden, The Netherlands
I. The Blood-Brain Barrier
A. Morphology and Function
B. Mechanisms of Transport of Compounds Across the Blood-Brain Barrier
II. Pathophysiology of the Blood-Brain Barrier in Neurological Diseases
A. Inflammatory Mediators during Neurological Diseases
1. Cytokines.
2. Eicosanoids.
3. Free radicals.
4. Adhesion molecules.
B. Neurological Diseases Affecting the Blood-Brain Barrier
1. Multiple sclerosis and experimental allergic encephalomyelitis.
2. Bacterial meningitis.
3. Ischemia.
4. Brain edema.
5. Alzheimer's disease.
6. Acquired immune deficiency syndrome dementia complex.
III. Final Considerations
References
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I. The Blood-Brain Barrier |
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The existence of the blood-brain barrier (BBB)b
was revealed through studies by Ehrlich (1885)
in the late
19th century, describing that brain tissue remained
unstained after injection of a vital dye into the systemic blood
circulation of rats. In contrast, the brain tissue was stained after
direct injection of trypan blue into the brain ventricular system,
indicating the existence of some kind of barrier at the site of the
brain microvessels (Goldmann, 1909). At first it was generally believed
that the BBB was formed by the glial sheets covering the brain
capillaries (Dempsey and Wislocki, 1955). Administration of the
electron-dense marker horseradish peroxidase (HRP), however, revealed
that the anatomical localization of the BBB could in fact be detected
at the level of the cerebral endothelial cells (CEC). No HRP was found
in the extracellular space surrounding the brain capillaries after
intravenous injection. When administered directly into the brain
ventricular system, HRP passed astrocytic end processes readily and was
retained at the cerebral endothelial plasma membrane (Reese and
Karnovsky, 1967; Brightman and Reese, 1969).
The homeostasis of the central nervous system (CNS) environment is maintained by the BBB, which separates the brain from the systemic blood circulation. The cerebral capillaries are organized such that the brain is protected from blood-borne compounds, since a strict homeostasis of the neuronal environment and an intact barrier are essential for optimal brain functioning. However, during various neurological diseases the permeability of the BBB may be changed. This review will discuss the role of the BBB and especially of the CEC in various neuroinflammatory diseases.
A. Morphology and Function
The BBB is formed by a complex cellular system of endothelial
cells, astroglia, pericytes, perivascular macrophages, and a basal
lamina (Bradbury, 1985). Astrocytes project their end feet tightly to
the CEC, influencing and conserving the barrier function of these
cells. CEC are embedded in the basal lamina together with pericytes and
perivascular macrophages (Graeber et al., 1989). Pericytes are
characterized as contractile cells that surround the brain capillaries
with long processes, and are believed to play a role in controlling the
growth of endothelial cells. Due to their close contact with the
endothelial cells, pericytes may influence the integrity of the
capillaries and conserve the barrier function. Pericytes additionally
limit the transport by the ability to phagocytose compounds which have
crossed the endothelial barrier, as observed in a healthy BBB
(Broadwell and Salcman, 1981). Finally, the lumen of the cerebral
capillaries is covered by CEC in which the functional and morphological
basis of the BBB resides (fig. 1).
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CEC exhibit various functional and morphological differences in
comparison with endothelial cells derived from peripheral organs. CEC
possess narrow intercellular tight junctional structures. The tight
junctions are composed of a complex of belt-like zonula occludens,
which is localized close to the lumen of the capillary (Heimark, 1993).
Electrical resistance in vivo across the barrier can increase to
approximately 1200 ohm·cm2 due to these intercellular
structures (Butt et al., 1990). These tight junctions hinder
paracellular transport of hydrophilic compounds across the cerebral
endothelium. The absence of fenestrae in the endothelial plasma
membrane and the presence of high densities of mitochondria in the
cytosol are other prominent morphological features of the CEC. In
addition, pinocytotic vesicular activity seems to be virtually absent
at the endothelial plasma membrane, which implies that fluid phase
uptake is limited (Cervos-Navarro et al., 1988).
Recently, P-glycoprotein (Pgp) expression, which appears to be
associated with multidrug resistance (MDR) in numerous tumors, has also
been detected at the site of the BBB. The discovery of its presence on
the BBB has much contributed to the understanding of the penetration of
various drugs into the brain (Cordon-Cardo et al., 1989; Thiebaut et
al., 1987). Pgp is a 170-kDa glycoprotein and belongs to the
superfamily of the ATP-binding-cassette transporters (Higgins et al.,
1986). The system is comprised of two almost identical halves within a
total of 12 membrane spanning domains and two ATP-binding sites. In
humans the MDR1 and the MDR2 genes have been
identified, which encode for the two different isotypes of Pgp
(Gottesman, 1993). The MDR1-Pgp is mainly found in epithelial tissues of the intestine, kidney, pancreas, adrenal gland, and in the
endothelium of various tissues like the endocervix, glumeruli, intestine, ovarian cortex, prostate, spleen, testes, and the BBB (Hegmann et al., 1992; Cordon-Cardo et al., 1989). In rodents there are
three genes encoding for the mdr1a-, the mdr1b-, and the
mdr2-Pgp (Schinkel et al., 1995a). The mdr1a- and the
mdr1b- gene products fulfill the same function as the
MDR1 gene product in humans (Borst and Schinkel, 1996).
MDR2 and mdr2, however, do not seem to play an important role
in the transport of drugs. It is abundantly expressed in the liver and
it may function in the transport of phospholipids across the canicular
membranes in hepatocytes into the bile (Smit et al., 1993).
MDR1- and the mdr1a-Pgp are located at the luminal side of
the cerebral endothelium and function as an efflux pump for several drugs. In particular cytostatic drugs such as anthracyclines, taxanes,
epipodophyllotoxins, and vinca alkaloids (Gottesman, 1993) are
transported out of the cells. In addition, noncytotoxic drugs such as
ivermectin, digoxin, cyclosporin A, dexamethasone (Schinkel et al.,
1995a; Mayer et al., 1996), also other drugs such as domperidone,
ondansetron, and loperamide (Schinkel et al., 1995b) are effluxed by
this system. MDR1- and mdr1a-Pgp-mediated transport can be
inhibited by so-called reversal agents such as verapamil (particularly
R-verapamil), cyclosporin A, SDZ PCS 833, but also by small peptides
(Gottesman, 1993; Ford, 1996). In vivo, microdialysis studies showed
considerably higher rhodamine concentrations in the brain of
mdr1a-deficient mice in comparison to wild type mice.
Normally mdr1b-Pgp is not detectable in vivo at the level of
the BBB (Schinkel et al., 1995a). However, in BBB cell cultures, the
expression of the mdr1b-Pgp has been demonstrated,
indicating that changed circumstances induced by culture conditions may
induce its expression (Barrand et al., 1995).
Certain enzymes which reside selectively in the CEC constitute a
metabolic barrier, which also contributes to the protective function of
the BBB. For instance, enzymes like monoamine oxidase A and B, catechol
O-methyltransferase, or pseudocholinesterase are involved in
the degradation of neurotransmitters present in the CNS. In addition,
blood-borne compounds that have entered the CEC can be degraded by
enzymatic activity (Maxwell et al., 1987; Baranczyk-Kuzma et al., 1986;
Kalaria and Harik, 1987; Betz and Goldstein, 1984).
In more than 99% of the brain capillaries, a BBB is present, but in
some areas of the brain a blood-cerebrospinal fluid (CSF) barrier can
be found instead. This barrier is present in the circumventricular organs such as the median eminence, pituitary, choroid plexus, subfornical organ, organum vasculosum of the lamina terminalis and the
area postrema (Hashimoto, 1992). The blood-CSF barrier is not as strict
as the BBB but it also prevents the entrance of blood-borne compounds
into the brain. Since the surface of the BBB is about 5000-fold larger
than that of the blood-CSF barrier, the main route of entry for
compounds from plasma into the brain is via the brain capillaries
(Pardridge, 1986).
B. Mechanisms of Transport of Compounds Across the Blood-Brain Barrier
The presence of the BBB has major implications for the passage of relatively large and hydrophilic compounds into the brain. As a result the entry of certain endogenous compounds such as nutrients is restricted. Essential nutrients are transported into the brain by means of (selective) carrier mechanisms. Several transport systems have been characterized varying from passive transport (such as diffusion) to active and energy requiring processes.
The diffusion of compounds across the plasma membranes of the
endothelial cells of the BBB is dependent on the physicochemical properties of these compounds, such as lipid solubility, molecular weight, electrical charge, or extent of ionization. Rapoport et al.
(1979) described the correlation between diffusion across the BBB and
lipid solubility of compounds. Lipid soluble substances penetrated the
cerebral endothelial plasma membranes readily and also equilibrated
easily between blood and brain tissue (Bradbury, 1985). In vitro
studies revealed also close correlation of the lipid solubility of
compounds and their BBB permeability (Van Bree et al., 1988). In
contrast to these observations, compounds which are a substrate for Pgp
are less efficiently transported across the BBB as would be expected on
the basis of their lipophilicity as discussed before (section A).
Specific carrier systems mediating active transport of certain
compounds into the brain have been identified. A selective stereospecific glucose carrier system (GLUT-1 within the
sodium-independent glucose transporter supergene family) has been
characterized transporting 2-deoxyglucose,
3-O-methylglucose, mannose, galactose, and glucose with a
high capacity. GLUT-1 (55 kDa) was expressed asymmetrically both at the
abluminal and luminal membrane of the CEC (Farrell and Pardridge,
1991). Three high affinity amino acid carrier systems have been
described for large neutral amino acids (LNAA system), for basic amino
acids and for acidic amino acids, respectively. Furthermore, carrier
systems for purine, nucleoside, thiamine, monocarboxylic acid, and
thyroid hormones have been identified (Pardridge, 1986; Spector, 1990).
Thus, a functional barrier is of great importance for the maintenance
of a constant environment of the CNS and for its optimal functioning.
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II. Pathophysiology of the Blood-Brain Barrier in Neurological Diseases |
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Under healthy conditions, the BBB not only regulates the entry of
drugs or endogenous compounds into the brain, but also cellular infiltration is lower compared to peripheral organs. The normal endothelial cell layer provides a thromboresistant surface that prevents platelet and leukocyte adhesion and activation of any coagulation system. The highly specialized CEC form a tight barrier which isolates the brain from immune surveillance, and allow only a few
mononuclear cells (such as activated T-cells) to migrate into the CNS.
The low expression of major histocompatibility complex antigens, the
low number of antigen-presenting cells in the healthy CNS, and the fact
that the CNS is not properly drained by a fully developed lymphatic
vasculature, make the brain an "immunosecluded" site (Hafler and
Weiner, 1987; Wekerle et al., 1986).
However, when inflammation occurs, an extensive leukocyte migration
into the brain takes place, for instance during multiple sclerosis (MS)
or encephalitis (Andersson et al., 1992; Lassmann et al., 1991). The
migration of mononuclear cells into the CNS is often accompanied by an
increased flux of serum proteins which are transferred to the CSF.
Besides the CEC, other cell types such as microglial cells and
perivascular macrophages may eventually be involved in the neuroimmune
response.
The barrier function of the BBB can change dramatically during various
diseases of the CNS i.e., during hypertension or seizures, or during
cerebral inflammation such as MS or cerebral infections. Enhanced BBB
permeability is considered to be the result of either opening of tight
junctions or of enhanced pinocytotic activity and the formation of
transendothelial channels (Juhler et al., 1985)
The BBB itself may play an active role in the mediation of the neuroimmune response either by production of inflammatory mediators or by the expression of adhesion molecules. Various BBB-related factors involved in the development of CNS inflammatory diseases will be discussed in the following sections.
A. Inflammatory Mediators during Neurological Diseases
1. Cytokines.
An early step in inflammation is the secretion
of various mediators. Cytokines such as tumor necrosis factor (TNF),
interleukin-1 (IL-1), and interleukin-6 (IL-6) are of crucial
importance in the development of the inflammatory response. Cells in
the CNS that can produce cytokines upon activation include macrophages, microglial cells, astrocytes, and CEC. The cytokines TNF, IL-1
, and
IL-6 are predominantly present in the CNS after injury or inflammation.
These cytokines play a major role in mediating the pathogenesis of a
fever response (Hashimoto et al., 1991), in the host defense response
(Beutler and Cerami, 1988), activation of the
hypothalamus-pituitary-adrenal axis, and they may trigger the release
of other cytokines in the CNS.
). Administration of TNF-
to an in vitro model for the BBB resulted in enhanced transport of
inulin and sucrose, which was accompanied by the reorganization of
actin filaments (Deli et al., 1996). Intracisternal administration of
TNF in newborn piglets resulted in a constriction of the cerebral
arteries and a dose- and time-dependent increase in the brain uptake of
marker compound (Megyeri et al., 1992). Moreover, TNF administration
(intracerebroventricular) to rats resulted in an increased number of
white blood cells in the CSF and enhanced levels of radiolabeled
albumin, indicating BBB disruption (Kim et al., 1992).
, the predominant form of IL-1 in CNS tissue, can be present not
only after local synthesis by astrocytes or microglia but also after
transport from the peripheral blood into the brain tissue (Banks et
al., 1991). Production of IL-1 by CEC in a damaged BBB has also been
reported (Plata-Salamán, 1991) and the permeability of the BBB is
affected by IL-1. Intracisternal administration of IL-1 to rats
revealed an increase of BBB permeability to radiolabeled albumin with a
peak concentration in the CSF after 3 h of inoculation. The
alteration in BBB permeability was dose-dependent and could be
inhibited by pretreatment of animals with antibodies directed against
IL-1 (Quagliarello et al., 1991). Receptors for IL-1 were
detected in the cerebral vasculature (Ericsson et al., 1995) and in
vitro on rat brain endothelial cells, which appear to be functionally
coupled to the release of IL-6 and eicosanoids (Van Dam et al., 1996).
Hence, the production of cytokines by cells of the BBB, such as
microglial cells, astrocytes, and endothelial cells contribute to the
total inflammatory response of the CNS after injury or infection and
also affect the function of the BBB.
2. Eicosanoids.
The derivatives of arachidonic acid (AA),
called eicosanoids, and their metabolism play an important role in the
mediation of the inflammatory response and the pathogenesis of fever.
AA is released from phospholipids present in cell membranes by the activation of phospholipase A2. AA can be converted by two
different enzymes. Through the cyclo-oxygenase pathway, AA is
metabolized into prostaglandins (PGs) such as PGD2,
PGE2, PGF1
(prostacyclin; PGI2) and thromboxane A2. Through the
lipoxygenase pathway, AA is converted initially into the mono- or
dihydroxyeicosatetraenoic acids and leukotrienes, lipoxins, and the
peptidoleukotrienes (Shimizu and Wolfe, 1990) (fig. 2).
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) and the hydroxyeicosatetranoic acids can act as chemotactics on
leukocytes (Piper and Samhoun, 1987). Predominantly PGE2
and PGI2 are involved at the site of the inflammation and both are secreted by inflamed tissue and by vascular endothelium (Moncada et al., 1985).
During brain injury, eicosanoids play an important role in the
pathogenesis of CNS inflammatory diseases. In ischemic human brain, for
instance, high levels of PGE2, PGI2, and
PGF2 have been detected. Increased levels of
PGs were also found in the CSF of patients suffering from suspected
intracranial disease (Saeed Abdel-Halim et al., 1980). Augmented levels
of PGs could either be produced in the CNS itself or by entry into the
CNS, as was shown for PGE2 (Dascombe and Milton, 1979). One
possible site of PG synthesis is the CEC, since these cells have been
shown to produce PGE2 and PGI2 when exposed to
traces of AA (Moore et al., 1988; De Vries et al., 1995).
Synthesis and secretion of eicosanoids in the brain during cerebral
inflammatory diseases denotes the importance of these mediators,
besides cytokines, in modulation of the function of the BBB in such
diseases. In addition, the cerebral endothelium itself may contribute
to the total pool of PGs produced in the CNS after injury.
3. Free radicals.
Upon activation, cells of the immune system
can produce a range of free radicals, such as reactive oxygen species
(ROS) or nitric oxide (NO), which can contribute to tissue damage. Free radicals are defined as ions with an electron that possess unusual chemical reactivity, including an ability to alter and to fragment membrane lipids (Fishman and Chan, 1980). In healthy conditions, the
constantly produced oxygen-derived free radicals are scavenged by
endogenous antioxidants such as, e.g. superoxide dismutase and
glutathione peroxidase. During pathological conditions, such as
ischemia and inflammation, however, this defense mechanism is perturbed
and results in the overproduction of oxygen-derived free radicals (Chan
et al., 1991).
; Kim and Kim,
1991).
Another free radical that can damage the BBB is NO. NO is synthesized
in the presence of nitric oxide synthetase (NOS) out of the guanidine
group of L-arginine. Two isoforms of NOS are known, the
constitutive form (cNOS) and its inducible (iNOS) form, which can be
induced by various cytokines. Endothelial cells of the BBB are known to
possess the inducible form of NOS. Inflammatory mediators released in
the CNS during viral or bacterial infections are able to induce iNOS,
present in endothelial cells, astrocytes and brain macrophages
(Feinstein et al., 1994; Morin and Stanboli, 1994). Studies on BBB
opening during infections revealed the involvement of NO in this
process. Increased permeability of the BBB, after endotoxin
administration to rats, could be blocked in the presence of a inhibitor
of NOS, N-nitro-l-arginine methyl ester. On the other hand the effect could be potentiated after administration of
L-arginine a substrate for NOS (Shukla et al., 1995). In
addition to other mediators influencing the permeability of the BBB,
ROS, and NO generated by BBB endothelial cells, can play an important role in the response to injury or inflammation in the CNS.
4. Adhesion molecules.
Within minutes after the release of
inflammatory mediators such as cytokines or eicosanoids, neutrophils
arrive at the site of inflammation followed by the migration of
antigen-specific B- and T-lymphocytes and monocytes into the inflamed
site (Osborn, 1990). Three families of homologous adhesion molecules
are responsible for the adhesion and migration of leukocytes into an
inflamed site: the immunoglobulin (Ig) superfamily, the integrins, and the selectins (Osborn, 1990; Springer, 1990). The immunoglobulin superfamily comprises a large group of molecules, characterized by the
presence of one or more Ig homology units (Springer, 1990). On
endothelial cells this group is represented by intercellular adhesion
molecule-1 (ICAM-1), intercellular adhesion molecule-2 (ICAM-2), and
vascular cell adhesion molecule-1 (VCAM-1). These molecules recognize
their leukocytic ligand and permit adhesion and migration of these
cells out of the bloodstream (Osborn, 1990; Springer, 1990) (Table
1).
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(IFN-), TNF-, and IL-1 (Fabry et al., 1992). Migration of lymphocytes across monolayers of CEC could be observed after treatment of the CEC either with IFN- or TNF-. The
migration of lymphocytes could subsequently be suppressed by antibodies directed against lymphocyte functional antigen-1 (LFA-1), indicating the relevance of the LFA-1/ICAM-1 pathway (Male et al., 1992). In
addition, the involvement of very late antigen-4 (VLA-4)/VCAM-1 pathway
in the adhesion of lymphocytes to CEC in vitro stimulated with LPS,
IL-1, and IL-6 was also demonstrated (De Vries et al., 1994). Other
studies illustrated that the barrier function of the cerebral
endothelium is decreased during an inflammatory challenge and after the
adhesion of lymphocytes (Huynh and Dorovini-Zis, 1993).
The promotion of leukocyte adhesion to the cerebral endothelium may be
mediated through the LFA-1/ICAM-1 as well as the VLA-4/VCAM-1 pathway
during cerebral inflammatory diseases. These adhesion molecules are of
importance for enabling the migration of lymphocytes across the BBB to
battle the inflammatory process inside the brain tissue after injury.
Inflammatory mediators play an active part in the mediation of the
immune response during MS or in its corresponding animal model
experimental allergic encephalomyelitis (Sharief et al., 1993a, b).
IFN-, for instance, has been identified at the site of the lesions
in the CNS tissue. It is suggested to be involved in the lesion growth
during MS and may exert local immunosuppressive effects. Also TNF has
been identified at the site of the lesions, and is likely to mediate
cytotoxic effects on the oligodendrocytes which produce myelin (Sharief
et al., 1993a, b). Elevated levels of cytokines like IL-1, IL-6, and
TNF are observed in the CSF of MS patients (Sharief et al., 1993a, b;
Gallo et al., 1989; Shaw et al., 1995) and also in CNS tissue of mice
suffering from experimental allergic encephalomyelitis (Gijbels et al.,
1990)
CEC are involved in the encephalitic processes during experimental
allergic encephalomyelitis. They attract autoantigen-specific T-cells
migrating into the brain (Raine, 1990). During experimental allergic
encephalomyelitis, the expression of the adhesion molecule ICAM-1 was
6-fold higher on the CEC, perivascular cells, and some of the
astrocytes than in controls. During remission, the level of ICAM-1
expression was similar to that before the onset of the clinical signs
(Cannella et al., 1990). Wilcox et al. (1990) also reported an
up-regulation of ICAM-1 on cerebral endothelia of the guinea pig
suffering from acute experimental allergic encephalomyelitis. Furthermore, traces of soluble ICAM-1 in the CSF of patients with active MS were found, which correlated with BBB damage (Gijbels et al.,
1990; Hartung et al., 1993). It was suggested that in addition to
ICAM-1 other adhesion molecules are involved in these adhesion
processes. The high expression of VCAM-1 on microvessels isolated from
CNS tissue from MS patients was shown recently in addition to the
presence of ICAM-1 (Washington et al., 1994).
During experimental allergic encephalomyelitis the integrity of the BBB
itself is reduced (Hawkins et al., 1991). Transport studies using
radiolabeled mannitol as a marker for the transport of compounds across
the BBB revealed a 2-fold increase of transport at day 14 post
inoculation, when the clinical signs of experimental allergic
encephalomyelitis were most abundant (Daniel et al., 1983). Using
several markers for the transport of compounds across the BBB, Juhler
(1988) demonstrated enhanced transport of either radiolabeled sodium,
chloride, sucrose, or inulin in all regions. Changes in BBB
permeability preceded the occurrence of the clinical signs (Claudio et
al., 1989). Transport of radiolabeled albumin and IgG was enhanced in
the caudal region and increased transport occurred in the more cranial
regions when the clinical signs became more severe. It was demonstrated
that the BBB disruptions were near regions which are called "leaky
areas," such as the entry zone of the trigeminal nerve and the spinal
roots, area postrema and choroid plexus (Claudio et al., 1989).
Changes in BBB permeability and the loss of BBB function may either be
generated by the formation of pores in the CEC, or by an increase in
the number of multivesicular bodies and transcytotic vesicles in the
CEC. Claudio et al. (1989) demonstrated enhanced vesicular transport
and a decrease in the number of mitochondria in the CEC during
experimental allergic encephalomyelitis. Disruption of tight junctions
may also occur, possibly induced by fibrinolysis mediated by
endothelial derived plasminogen activator and by production of
fibrin-fibrinogen products (Koh et al., 1992).
The current opinion on MS is that the BBB together with active
inflammatory processes at the lesion sites, leading to enhanced BBB
permeability, plays an important role in the development and progression of the disease (Moor et al., 1994; Poser, 1993).
Long-standing MS lesions contain a permanent damaged BBB, detected by
serum proteins leakage (Claudio et al., 1996). The continuous exposure of demyelinated areas to blood-borne compounds may have a negative influence on the oligodendrocyte regeneration and thus prevent remyelination of these areas (Kwon and Prineas, 1994). The opening of
the BBB itself may also reinforce the progression of the disease.
2. Bacterial meningitis.
A disease of bacterial origin can
also affect the integrity of the BBB. Certain bacteria can cross the
BBB and penetrate the CNS tissue and the CSF where they multiply and
cause bacterial meningitis. The three most frequent causative bacterial
species are Hemophilus influenza, Necisseria
meningitidis, also called meningococcus, and Streptococcus
pneumonia (Tuomanen et al., 1985).
).
In animals with experimental meningitis, it was shown that the bacteria
multiply in the CSF, and then at sufficiently high concentration, fever
and inflammation develop, accompanied by the release of cytokines,
predominantly IL- and TNF- (Jacobs and Tabor, 1990). There upon,
an increase in the number of white blood cells migrating through the
BBB into the CNS tissue was observed. These events may lead to
degeneration of neurons and permanent damage of the visual and hearing
system, even after treatment with penicillin (Tuomanen, 1993).
During bacterial meningitis the permeability of the BBB may be altered
(Tuomanen et al., 1985). Morphological changes, observed in animals
after intracisternal administration of bacterial endotoxin, LPS, as a
model for bacterial meningitis, revealed an enhanced pinocytotic
activity in the CEC. A progressive opening of tight junctions was
observed from 4 to 18 h after inoculation, which resulted in
enhanced transport of radiolabeled albumin into the CSF. Time-dependent
effect of LPS on BBB permeability was postulated to be partly mediated
by cytokines, released after intracerebroventricular inoculation of LPS
(Wispelwey et al., 1988).
3. Ischemia.
During and after stroke or cerebral injury,
ischemic processes will occur in the brain due to insufficient blood
circulation. Stroke is a form of cardiovascular disease that affects
the cerebral arteries. Cerebral thrombosis and cerebral embolism are
the most common types of stroke, caused by clots that plug an artery.
Cerebral and subarachnoid hemorrhage can also appear, caused by
ruptured arteries. Stroke may damage the neurons and lead to the loss
of speech, memory, or motility, and may change behavior dramatically. During ischemia an overproduction of free radicals, such as superoxide and NO has been observed, functioning as mediators in the ischemic process. The release of these factors in the CNS can lead to cellular injury of glia or neurons by membrane disruption and increasing regional cerebral blood flow (Chan et al., 1984; Koide et al., 1986;
Pfister et al., 1990; Ikeda et al., 1989; Chan et al., 1991).
) and HRP (Pluta et al., 1994; Tanno et al.,
1992). Ischemic sites remain permeable to HRP up to 72 h after
induction of the injury.
The opening of the BBB itself will probably not contribute to the
progressive damage observed after the ischemic event, although injury
to the cerebral microvasculature may lead to the formation of brain
edema (Preston et al., 1993). However, increased infiltration of
monocytes and macrophages into damaged regions can lead to perivascular
inflammatory reactions, in which the BBB may play a role by production
of chemotactic substances. The increase in BBB permeability after an
ischemic event could be significantly inhibited by superoxide dismutase
and catalase, acting as scavengers for superoxide. These observations
indicate that superoxide, released in the CNS after an ischemic event,
may influence the BBB permeability (Nelson, 1992). In addition, the BBB
can contribute to ischemic damage by the production of NO and the
up-regulation of adhesion molecules responsible for platelet
aggregation as observed in the ischemic site (Said et al., 1993).
4. Brain edema.
Brain edema can be classified into two
different types on the basis of morphological characteristics: (1)
Vasogenic or "wet" edema, the result of an increased BBB
permeability, and (2) cytotoxic or "dry" edema, the result of the
actual swelling of the cells of the brain parenchyma (Klatzo, 1967).
Vasogenic edema is the type of edema most often present in the brain
after injury, induced by ischemic stroke, brain tumors or inflammatory
lesions. The BBB expresses morphological changes during the onset of
vasogenic brain edema, such as the opening of tight junctions and a
damaged endothelial cell membrane, followed by migration of leukocytes into the CNS (Klatzo, 1987).
). Histamine may also
contribute to the change of pinocytotic activity in the CEC. Joó
(1990) assumed that after brain damage excessive amounts of histamine
are released from damaged histaminergic nerves and mast cells. Released
histamine may activate the histamine (H2) receptor on the
CEC, which is coupled to adenylate cyclase. The rise in intracellular
cyclic AMP may result in an increase in the number of pinocytotic
vesicles. In addition to cyclic AMP, other second messengers such as
cyclic GMP and AA may be involved in augmented pinocytotic activity
(Joó, 1990; Joó and Klatzo, 1989; Wahl et al., 1988;
Michiels et al., 1993). Other mediators in the formation of vasogenic
edema may be leukotrienes (Baba et al., 1992) and NO (Nagafuji et al.,
1992). The synthesis of secondary messengers by the CEC may contribute
to the local changes of the BBB permeability as observed in vasogenic
brain edema (Joó, 1990).
Cytotoxic brain edema is the most prominent clinical disorder after
ischemic processes in the CNS and is characterized by an increase in
the water content of the cells of the CNS, which may be caused by a
disturbance in the transport systems for potassium and sodium rather
than from changes in the permeability of the BBB.
5. Alzheimer's disease.
Alzheimer's disease is a chronic
neurodegenerative condition that affects approximately 10% of the
individuals in age over 65 years. Five percent of this group of
patients suffers from severe dementia. Memory decline as well as the
inability to absorb new information are the most prominent clinical
signs. Imaging techniques revealed disturbances in cerebral blood flow
and glucose metabolism. Until now a deficiency in the neurotransmitter
acetylcholine has been hypothesized to be involved in Alzheimer's
disease, since cortical acetylcholine synthesis is markedly diminished
in Alzheimer's patients. Other neurotransmitters which may be involved
in the course of the disease are dopamine, -aminobutyric acid,
vasoactive intestinal peptide, and glutamate (Struble et al., 1985).
-amyloid protein, derived from a larger precursor protein. This amyloid peptide induces
the release of the so-called -peptide or the A4 peptide, which
becomes stacked into a -pleated sheet structure with a high degree
of intermolecular hydrogen bonding. Usually, the amyloid peptide
precursor (A4P) with an apparent molecular mass of 112 kDa is found in
the brain (Masters et al., 1985). During Alzheimer's disease, the A4P
is processed abnormally into a 43-amino acid amyloiditic peptide that
arises from the near C terminus of the A4P (Marotta et al., 1992).
Pathology of Alzheimer brains reveals granulovacuolar degeneration of
neurons within the region of the hippocampus. Furthermore,
neurofibrillary tangles are detected which are intraneuronal
accumulations of dense non-membrane bound fibrillary material forming
paired helices. Other lesions are composed of amyloid, predominantly
associated with microglial cells (Cras et al., 1990; Marotta et al.,
1992).
Inflammatory reactions, surrounding the cerebral microvasculature, are
often observed during Alzheimer's disease. The number of perivascular
macrophages increases and hypertrophy of astrocytes and microglia is
observed in brain sections of the Alzheimer's disease dementia
patients. IL-1 is found in the CNS of patients suffering from
Alzheimer's disease, and its synthesis is postulated as an early event
in the onset of Alzheimer's disease. IL-1 affects the synthesis of
-amyloid precursor protein and its subsequent deposition of
-amyloid (Glenner and Wong, 1984). ICAM-1 accumulates in the brain
of Alzheimer's disease patients in the microvascular endothelial cells
as well as in the senile plaque (Verbeek et al., 1994), indicating that
infiltration of lymphocytes into the brain tissue is involved in
inflammatory reactions occurring during Alzheimer's disease.
Brain capillaries themselves are also affected in the course of
Alzheimer's disease. Evident is the cerebral microvascular angiopathy
located at the level of the brain capillaries (Vinters et al., 1990).
Vascular amyloid surrounds the microvessels of the Alzheimer's disease
brain on the antiluminal surface of the vessels (Vinters et al., 1994).
Brain capillaries may degenerate by the deposition of amyloid at the
basement membrane. The thickened capillaries reduce vessel elasticity,
which may disturb the cerebral blood flow, which eventually influences
the transport of compounds into the brain (Vinters, et al., 1994).
Albumin concentrations in the CSF of Alzheimer patients are enhanced
significantly at the early onset of Alzheimer's disease and result
from an increase in BBB permeability (Mecocci et al., 1991).
Two-dimensional gel electrophoresis showed the presence of a high
molecular weight protein, haptoglobulin, molecular mass of 13.5 kDa) in
the CSF of Alzheimer's disease patients. The presence of this protein
was suggested to be the result of an increased penetration of this
protein across an impaired BBB (Mattila et al., 1994). Alzheimer's
disease patients have a diminished density in mitochondria and features
of interendothelial junctions in the CEC, suggesting leakiness of the
vessels (Stewart, et al., 1992). The diminished cerebral metabolic rate
observed during Alzheimer's disease can be partly explained by a
decreased density of glucose transporters (GLUT-1) in the brain
capillaries (Harik, 1992). Changes in the intracellular signal
transduction in the cerebral microvessels of Alzheimer's disease
patients, in comparison with age-matched controls, have also been
observed. Protein kinase C activity is strongly diminished in the
brain microvessels, which lead to altered protein phosphorylation.
Protein kinase C activity during Alzheimer's disease is of importance
in the inhibition of the processing of -amyloid precursor protein
into soluble A protein. The intracellular signaling in the
cerebrovasculature may be one of the targets in Alzheimer's disease,
and the balance of various second messengers may be disturbed (Grammas
et al., 1995).
Perivascular inflammatory reactions during Alzheimer's disease are
likely to cause additional changes in the function of the BBB. Recent
studies showed that the BBB is indeed disrupted in brains of
Alzheimer's disease patients. In addition, cultures of endothelial
cells derived from brain capillaries of Alzheimer's disease patients
revealed production of the -amyloid precursor protein. In addition,
apolipoprotein E4 which is considered as a risk factor for the
development of Alzheimer's disease, was also produced by these cells
(Wells et al., 1995). Changes in the perivascular environment were also
found, like increased collagen content of the cerebral microvessels
(Kalaria and Pax, 1995; Kalaria, 1992). Confirmation of these
observations were made by Claudio (1996), who detected in addition to
an increased collagen content in the vascular basement membranes also
focal necrotic changes in endothelial cells. Furthermore, abnormalities
in the microvasculature include profound irregularities in the course
of the vessels and the basement membrane, and changes in specific
receptors or proteins associated with the cerebral endothelium were
reported (Kalaria, 1992).
These studies provide evidence that the function of the BBB is impaired
in Alzheimer's disease patients. The disruption of the BBB can lead to
an entry of neurotoxic environmental factors into the brain or
circulating amyloid. One hypothesis is that BBB opening is the initial
insult that may cause Alzheimer's disease, although more research
needs to be conducted to confirm this hypothesis. Until then, it can be
concluded that opening of the BBB is a characteristic feature of
Alzheimer's disease.
6. Acquired immune deficiency syndrome dementia complex.
Acquired immune deficiency syndrome (AIDS) severely affects the CNS.
About 60% of the patients develop AIDS dementia complex and suffer
from neurological dysfunctions (Gulevitch and Wiley, 1991). Neuronal
disorders related to the AIDS dementia complex include opportunistic
brain infections due to immunodeficiency, neoplasms of the CNS and
meningeal infections. Multinucleated cell encephalitis is marked by the
presence of infiltrates, consisting of macrophages and multinucleated
cells with some lymphocytes and microglia. Cellular infiltrates are
predominantly found in the white matter, and infiltration is
accompanied by demyelination by macrophages, and finally vacuolization
will occur at these sites. This process is most noticeable in the white
matter of the thoracic spinal cord with predominant involvement of the
posterior and lateral columns (Navia et al., 1986).
in the CSF of AIDS dementia complex patients are elevated. The
cytokines may be derived from activated mononuclear cells (Liuzzi et
al., 1992). In tissue samples and CSF of patients suffering from AIDS
dementia complex, increased levels of PGE2 and thromboxane
B2 were detected (Griffin et al., 1994). Presence of
adhesion molecules was also assessed by the detection of traces of
soluble VCAM-1 in CSF of animals infected by simian immunodeficiency virus, serving as an animal model for AIDS (Sasseville et al., 1992).
Furthermore, Nottet et al. (1996) suggested that soluble factors
derived from HIV-infected monocytes are able to induce the expression
of the adhesion molecules E-selectin and VCAM on brain microvascular
endothelial cells.
The function of the BBB is dramatically changed during the onset of
AIDS dementia complex. Lenhardt et al. (1989) examined the role of
humorally mediated damage within the CNS of HIV+ patients,
and observed perivascular inflammation and immunoreactive fibrinogen
and immunoglobulin in the cortex and basal ganglia, probably caused by
leakage of the BBB. Rhodes (1991) detected leakage of serum proteins
across the BBB into the brains of AIDS patients. Local production of
cytokines by BBB endothelial cells may cause a disruption of the BBB,
leading to increased permeability (Moses and Nelson, 1994).
Enhanced permeability will not only lead to changes in transport of
compounds across the BBB during AIDS dementia complex but also
facilitate the entry of HIV-1 infected monocytes into the CNS tissue
(Hurwitz et al., 1994). Furthermore, due to the enhanced expression of
adhesion molecules on the endothelial cells surface, increased
infiltration of macrophages or monocytes infected with HIV-1 will be
able to enter the CNS which will lead to additional infection of the
CNS tissue. The exact role of the BBB in the course of AIDS dementia
complex is not yet known, although perivascular inflammation may play a
key role in the development and onset of this complex.
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III. Final Considerations |
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Several pathological conditions of the brain are associated with structural and functional abnormalities of the cerebral microvasculature. In most diseases described an inflammatory process involves cerebral microvasculature, and severe CNS inflammatory diseases affect BBB permeability and its structure. Until now, most attention has been paid to the inflammatory activities of the glial cells during such diseases and the secretion of inflammatory mediators by these cells.
The role of the endothelial cells of the BBB in these processes can, however, not be ignored. Enhanced expression of adhesion molecules on the CEC in vivo during a pathological state facilitates the entry of leukocytes into the cerebral tissue. The possibility that the endothelial cells may secrete inflammatory mediators may indicate an important "gate" function for these cells between the immune system and the brain. In that respect, the BBB endothelial cells may and will contribute both to the onset and the progression of the disease. For instance, the production of enhanced levels of IL-6 and PGE2 by CEC, suggests that these mediators can be involved in the transmission of the inflammatory signal to other cell types in the brain, such as astrocytes, microglia, pericytes, and perivascular macrophages. The CEC may be the connection of the CNS and the peripheral immune system resulting in the neuro-immune response (fig. 3).
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The presence of cytokines in the CSF and the brain has also been described for diseases like Parkinson and schizophrenia. It may be that in the case of these disorders, the production of cytokines influences the permeability of the BBB. Future research may elucidate the specific role of the BBB and in particular that of the CEC in those types of cerebral pathology which are not of an inflammatory origin.
A more passive role for the endothelial cells in the increased transport of compounds across the BBB, either by the opening of tight junctions, or by an increased vesicular transport, may also be of importance for the progression of the disease. Changes of the capillaries may impair nutrition of the parenchyma. The effect of disease on the functioning of the BBB will secondarily affect the cerebral blood flow and the vascular tone in the brain, which also influences transport across the BBB.
The presence of various antigen presenting cells in the CNS after
infection of disease may lead to novel therapeutic strategies to fight
neuroinflammatory diseases. The use of antibodies directed against
adhesion molecules expressed on the brain endothelial cells, could be a
possible mechanism to block the infiltration of blood-borne macrophages
or encephalitic T-cells into the CNS. Thus, inhibiting the inflammatory
reactions in the cerebrovasculature. For instance, antibodies directed
against the very late antigen-4 could efficiently block the clinical
signs in the experimental allergic encephalomyeliis model (Yednock et
al., 1992). Recently, the Food and Drug Administration has approved the
use of IFN- as a therapeutic agent for the treatment of ambulant
relapsing-remitting MS patients. The mechanism of action may be the
influence on macrophage actions, like the expression of glucocorticoid
receptors, and down-regulation of cytokine production.
In conclusion, in neuroinflammatory diseases changes at the BBB, like the production of cytokines, free radicals or eicosanoids, and the expression of adhesion molecules at the cell surface, may contribute to the onset and progression of various neuroinflammatory diseases. The suppression of the inflammatory events at the site of the BBB should be further explored as a therapeutic strategy against neuroinflammatory diseases.
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Footnotes |
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a Address correspondence to: H. E. de Vries, Division of Biopharmaceutics, Wassenaarseweg 72, P.O. Box 9503, 2300 RA Leiden, The Netherlands.
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Abbreviations |
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BBB, blood-brain barrier; HRP, horseradish peroxidase; CEC, cerebral endothelial cells; CNS, central nervous system; Pgp, P-glycoprotein; MDR, multidrug resistance; CSF, cerebrospinal fluid; MS, multiple sclerosis; TNF, tumor necrosis factor; IL, interleukin; AA, arachidonic acid; PG, prostaglandin; ROS, reactive oxygen species; NO, nitric oxide; NOS, NO synthetase; cNOS, constitutive NOS; iNOS, inducible NOS; Ig, immunoglobulin; ICAM, intercellular adhesion molecule; VCAM, vascular cell adhesion molecule; LPS, lipopolysaccharide; IFN, interferon; A4P, amyloid peptide precursor; AIDS, acquired immune deficiency syndrome; HIV, human immunodeficiency virus.
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References |
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