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The Journal of Neuroscience, February 1, 2003, 23(3):955
Formation of Complement Membrane Attack Complex in Mammalian
Cerebral Cortex Evokes Seizures and Neurodegeneration
Zhi-Qi
Xiong1,
Weihua
Qian1,
Katsuaki
Suzuki1, and
James O.
McNamara1, 2, 3, 4
Departments of 1 Neurobiology, 2 Medicine
(Neurology), and 3 Pharmacology and Molecular Cancer
Biology, Duke University Medical Center, Durham, North Carolina 27710, and 4 Durham Veterans Affairs Medical Center, Durham, North
Carolina 27710
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ABSTRACT |
The complement system consists of >30 proteins that interact in a
carefully regulated manner to destroy invading bacteria and prevent the
deposition of immune complexes in normal tissue. This complex system
can be activated by diverse mechanisms proceeding through distinct
pathways, yet all converge on a final common pathway in which five
proteins assemble into a multimolecular complex, the membrane attack
complex (MAC). The MAC inserts into cell membranes to form a functional
pore, resulting in ion flux and ultimately osmotic lysis.
Immunohistochemical evidence of the MAC decorating neurons in cortical
gray matter has been identified in multiple CNS diseases, yet the
deleterious consequences, if any, of MAC deposition in the cortex of
mammalian brain in vivo are unknown. Here we demonstrate
that the sequential infusion of individual proteins of the membrane
attack pathway (C5b6, C7, C8, and C9) into the hippocampus of awake,
freely moving rats induced both behavioral and electrographic seizures
as well as cytotoxicity. The onset of seizures occurred during or
shortly after the infusion of C8/C9. Neither seizures nor cytotoxicity resulted from the simultaneous infusion of all five proteins premixed in vitro. The requirement for the
sequential infusion of all five proteins together with the temporal
relationship of seizure onset to infusions of C8/C9 implies that the
MAC was formed in vivo and triggered both seizures and
cytotoxicity. Deposition of the complement MAC in cortical gray matter
may contribute to epileptic seizures and cell death in diverse diseases
of the human brain.
Key words:
complement; membrane attack complex; C5b9; seizure; neurodegeneration; hippocampus; Fluoro-Jade B
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INTRODUCTION |
Complement is an important member of
the innate immune system and also one of the major effector mechanisms
of the humoral component of the adaptive immune system (Walport, 2001 ).
Although diverse mechanisms can activate complement, each activation
pathway culminates in the formation of C5b, the first component of the membrane attack pathway. Once formed, C5b binds to C6 to produce a
stable and soluble complex, C5b6. Next, C7 binds C5b6 to form C5b7, which can attach to the surface of cell membranes without disturbing membrane integrity. The binding of C8 to the membrane-bound C5b7 forms C5b8, which becomes more deeply incorporated in the membrane
and causes the cell to become slightly leaky. The C5b8 complex in turn
forms a receptor for C9 molecules. The binding of the initial C9
molecule to C5b8 transforms the C9 molecule from a globular,
hydrophilic structure to an elongated, amphipathic structure, which
inserts into and through the membrane; these conformational changes in
C9 expose binding sites for additional C9 to bind, unfold, and insert
into the membrane. Addition of as many as 18 copies of C9 to the C5b8
complex forms the membrane attack complex (MAC), resulting in ion flux
and ultimately lysis of target cells (Morgan, 1999).
Although its normal function is critical to the health of the organism,
inappropriate activation of complement can itself damage tissue of
multiple organs and contribute to disease in humans (Morgan, 1994;
Matis and Rollins, 1995; Morgan, 1995; Asghar and Pasch, 2000). With
respect to diseases of the CNS, inappropriate activation of
complement appears to contribute to demyelination in multiple
sclerosis. Evidence of inappropriate activation of complement in
multiple sclerosis includes MAC immunoreactivity in myelin (Compston et
al., 1989) together with MAC immunoreactive membranes isolated from the
spinal fluid of patients with multiple sclerosis (Scolding et
al., 1989). Moreover, transgenic overexpression of a complement
inhibitory protein eliminates demyelination in an animal model of
multiple sclerosis (Davoust et al., 1999), underscoring the deleterious
effects of complement activation in this disease. A wealth of
immunohistochemical evidence has demonstrated inappropriate activation
of complement, including the formation of MAC, on neurons in cortical
gray matter in diverse neurodegenerative diseases, including
Alzheimer's (McGeer et al., 1989; Itagaki et al., 1994; Webster et
al., 1997; Yasojima et al., 1999), Huntington's (Singhrao et al.,
1999), and Pick's (Yasuhara et al., 1994) diseases, as well as in the
putative autoimmune disease Rasmussen's encephalitis (Whitney et al.,
1999). The evidence of MAC formation on cortical neurons in these
diverse diseases notwithstanding, whether the deposition of MAC in
cortical gray matter in the mammalian brain has deleterious
consequences is uncertain. The determination that the formation of MAC
in muscle cell membranes results in depolarization (Jackson et al.,
1981) together with the cytotoxic effects of MAC in cortical neurons in vitro (Whitney and McNamara, 2000) led us to hypothesize
that MAC deposition on neurons could trigger seizures and
neurodegeneration. We tested this hypothesis by sequential infusion of
individual proteins of the membrane attack pathway into the rat
hippocampus to form MAC in vivo.
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Materials and Methods |
Electrode implantation and intrahippocampal
cannulation. Male Sprague Dawley rats (250-300 gm) were
anesthetized with sodium pentobarbital (60 mg/kg) and placed in a
stereotaxic frame. Teflon-coated stainless-steel bipolar electrodes
were implanted into the left dorsal hippocampus (the bregma was used as
the reference; coordinates:  3.5 mm anteroposterior, +2.5 mm lateral,
3.2 mm ventral to the dura) (Paxinos and Watson, 1982). The tip of a
guide cannula (26 gauge; Plastics One, Roanoke, VA) was placed into the
contralateral (right) dorsal hippocampus (3.5 mm anteroposterior,
+2.5 mm lateral, and 2.5 mm ventral to the dura). The cannula and
electrode were secured firmly to the skull with dental acrylic and a
ground wire was attached to an anchor screw (see Fig.
1A). After surgery, a removable stylet was placed in
the guide cannula and the animals were allowed to recover for 4 d.
The EEG is recorded from the hippocampus contralateral to the injected
hippocampus, because the rich commissural connections facilitate
seizure propagation between the two hippocampi, thereby simplifying the
detection of electrographic seizure; also any damage resulting from the placement of the recording electrode will not confound the assessment of damage caused by the infusion of complement.
Intrahippocampal infusion and EEG recording. On the day of
infusion, the stylet was removed and an infusion cannula (28 gauge) was
inserted through the guide cannula to a depth of 3.5 mm below the dura.
Solutions were infused into the hippocampus of awake, freely moving
rats at a rate of 0.20 µl/min, and EEG and behavior were continuously
monitored. The intensity of behavioral seizures was scored according to
a modification of Racine (1972), as follows: class 1, facial clonus and
chewing; class 2, head nodding; class 3, unilateral forelimb clonus;
class 4, rearing with bilateral forelimb clonus; class 5, rearing and
falling (loss of postural control); class 6, running; class 7, running,
jumping, and tonic-clonic seizure.
Perfusion. Animals were killed at varying intervals after
infusion by deep anesthesia (pentobarbital, 60 mg/kg, i.p.) followed by
perfusion with 100 ml of cold saline and subsequently with 250 ml of
0.1 M sodium phosphate-buffered (pH 7.4)
paraformaldehyde (4%). After additional fixation overnight in buffered
paraformaldehyde, the brains were cryopreserved for 48 hr in 20%
sucrose in phosphate buffer and frozen in isopentene chilled in a dry
ice-methanol bath. Coronal sections (40 µm) were collected in 24 well culture plates containing PBS or mounted onto
gelatin-coated slides and stored at 4°C until use.
MAC immunohistochemistry. Floating sections were washed in
PBS, incubated for 1 hr in blocking buffer (PBS containing 0.1% Triton
X-100 and 2% bovine serum albumin), and then incubated with rabbit
anti-human C5b9 in blocking buffer (final concentration, 1 µg/ml;
Calbiochem, La Jolla, CA) for 1 hr at room temperature. After three
washes, sections were incubated for 1 hr in biotinylated goat
anti-rabbit IgG (1.5 µg/ml) (Jackson ImmunoResearch Laboratories, West Grove, PA) diluted in blocking buffer (PBS containing 0.1% Triton
X-100 and 5% goat serum). To visualize biotinylated secondary antibodies, cultures were washed three times in PBS, followed by a 30 min incubation with streptavidin-linked rhodamine (final concentration
4 µg/ml; Molecular Probes, Eugene, OR) in PBS. After three washes,
the sections were dried, mounted, and viewed with a microscope equipped
for epifluorescence.
Nissl staining. Sections were mounted from 0.1 M sodium phosphate buffer, pH 7.4, onto
gelatin-coated slides and dried at room temperature. After rinsing with
tap water, sections were stained for 5 min in 0.5% toluidine blue.
After graded alcohol washes, the inserts were briefly dipped in 95%
ethanol containing 1% glacial acetic acid. Sections were then
rehydrated and mounted.
Fluoro-Jade B staining. Degenerating neurons were detected
with Fluoro-Jade B as described by Schmued and Hopkins (2000). Briefly,
sections were mounted from 0.1 M sodium phosphate
buffer, pH 7.4, onto gelatin-coated slides, dried, and immersed
in 100% ethanol for 3 min, followed by 70% ethanol for 5 min and then distilled water for 2 min; they were then incubated in
0.06% potassium permanganate for 15 min. After rinsing with distilled
water, the slides were incubated in 0.001% Fluoro-Jade B (Histo-Chem,
Inc., Jefferson, AR) in 0.1% acetic acid for 25 min, rinsed in water three times, dried at room temperature, dehydrated in xylene, and
coverslipped with DPX (Electron Microscopy Sciences, Fort Washington, PA).
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Results |
To induce the formation of MAC in mammalian cerebral cortex,
purified complement proteins (Advanced Research Technologies, San
Diego, CA) of the membrane attack pathway were infused sequentially (Fig. 1B) through a
guide cannula in the right hippocampus of awake freely moving rats
while continuously monitoring behavior; the EEG was recorded from the
contralateral hippocampus (Fig. 1A). A representative
experiment (Fig. 1B,C) revealed a normal EEG before
the infusion (baseline) and during and after the infusion (3 nmol) of
C5b6, after which the injection cannula was withdrawn and replaced with
another injection cannula through which C7 (4 nmol) was infused. A
normal EEG was also recorded during and after the infusion of C7. At
this point, the injection cannula was again withdrawn and replaced with
a third injection cannula through which a solution containing C8 (5 nmol) and C9 (14 nmol) was infused. Six electrographic seizures were
observed at the times denoted by asterisks in Figure
1B. The first seizure occurred 5 min after initiating
the infusion of C8/C9 and lasted 280 sec (data not shown). The sixth
seizure (Fig. 1C) occurred 20 min after terminating the
infusion of C8/9 and lasted for 160 sec, during which the animal
exhibited clonic contractions of the forelimbs typical of class 3 and
class 4 seizures. No additional seizures were detected despite
continuous monitoring for at least 1 hr and a brief monitoring session
(30 min) the next day. In these initial experiments, seizures (three to
six per animal) were observed in each of three animals infused with
these doses, yet no seizures were observed in three control animals in
which artificial CSF (ACSF) was substituted for C5b6, C7, or
C8/C9. Likewise no seizures were observed in either of two
additional controls infused with "preformed C5b9," demonstrating
that simple infusion of five proteins is not sufficient to induce
seizures; that is, all five proteins were mixed in vitro, resulting in the formation of the C5b9 complex in solution, after which
the complexes would not be expected to insert into a cell membrane
(Morgan, 1999). The temporal relationship of seizure onset with the
introduction of C8/9, together with the fact that seizures were evoked
by C8/9 only in animals with previous infusion of C5b6 and C7, implies
that the MAC was formed in vivo and triggered the
seizures.
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Figure 1.
Formation of MAC in rat brain evokes seizures.
A, Placements of recording electrode and infusion
cannula inserted through the guide cannula. B, Timeline
of experiments in which complement proteins are infused into the right
hippocampus of awake, freely moving rats (top), and the
occurrence of seizures detected in EEG recordings from a representative
animal (bottom; asterisks indicate
seizure events). C, EEG recordings between the tips of
bipolar electrodes in hippocampus (H-H) or from
one of these electrodes referenced to a skull screw
(H-G) from a representative animal in which the infusion
of C5b9 induced seizures. This is the sixth seizure in this
animal.
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The fact that assembly of the MAC can be limited by the amount of C5b
generated by the early activation pathway of the complement cascade
suggested a strategy with which to establish a dose-response curve.
Varying amounts of C5b6 were infused, followed by a fixed, and
presumably molar, excess amount of C7 and C8/C9, which should thereby
vary the density of MAC formed in the tissue. The doses were plotted as
amounts (in nanomoles) of C5b6 infused, and the responses were plotted
as the percentage of animals exhibiting seizures (Fig.
2A), behavioral
intensity, or the number or duration of seizures (Fig.
2B-D). The highest dose (3 nmol of C5b6) evoked seizures in 10 of 11 rats, resulting in an average of five seizures per
animal with an average behavioral intensity of class 4. The effects
were dose dependent, as evidenced by fewer seizures of shorter duration
and diminished behavioral intensity with the doses of 1.5 and 0.75 nmol
of C5b6.
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Figure 2.
Concentration dependence of effects of C5b6 in the
presence of excess C7/C8/C9 on seizure parameters. A,
Effects of varying amounts of C5b6 followed by C7, C8, and C9 on the
percentage of animals exhibiting seizures. The number of animals
receiving each amount of C5b6 is shown above the data points. The
results presented in this figure were obtained from animals described
in Results as well as from additional animals studied to confirm and
extend the findings of the initial experiments. B-D,
Results of experiments presented in A are presented as
seizure severity (B), the number of seizures per
animal (C), and the total duration of seizure
activity per animal during the recording period
(D). Error bars indicate SEM.
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To determine whether MAC formation also caused cytotoxicity, animals
from the experiments described in the preceding paragraph were killed
2 d after infusion for histological study. In contrast to the
normal hippocampal architecture of the recording (noninfused) side
(Fig. 3A,B, left),
extensive destruction of each of the principal populations of
hippocampal neurons is evident in Nissl stains of the hippocampus
infused with 3 nmol of C5b6 followed by C7/C8/C9 (Fig. 3A,B,
right top). Fluoro-Jade B staining of nearby sections revealed degenerating neurons in the infused but not the contralateral hippocampus (Fig. 3A,B, compare bottom
right and bottom left). Infusion of a lower dose
of C5b6 (0.75 nmol) produced less cell loss, as evidenced by a
representative animal in which destruction was confined to the dentate
gyrus, hilus, and CA4 area near the site of the infusion (Fig.
3C,D). The destruction evident on Nissl and Fluoro-Jade B
staining was not simply a consequence of the infusion, because controls
in which ACSF was substituted for C5b6, C7, or C8/C9 exhibited
negligible cell death (data not shown). Likewise, small inflammatory
cell infiltrates but minimal or no cytotoxicity were evident in animals
infused with preformed C5b9, as evidenced by the representative animal
shown in Figure 3E,F. The dose dependence of the
cytotoxicity was also evident in the semiquantitative measures of
destruction of the three principal neuronal populations of hippocampus
(Fig. 4).
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Figure 3.
Formation of MAC in rat brain induces
neurodegeneration. A, Nissl (top) and
Fluoro-Jade B (FJB; bottom) staining of
rat brain sections after the sequential infusion of C5b6 (3 nmol), C7
(4 nmol), and C8 (5 nmol)/C9 (14 nmol) into the right hippocampus of
the rat. There is massive cell death on the injected side but not on
the recording side. B, High-power photomicrograph of the
areas designated by boxes in A disclosed
the loss of CA1 pyramidal neurons in Nissl staining (top
right) and the presence of Fluoro-Jade B labeling of
degenerating cell bodies and proximal dendrites of CA1 pyramidal
neurons (bottom right). C, Nissl
(top) and Fluoro-Jade B (bottom) staining
of rat hippocampus after the sequential infusion of lower amounts (0.75 nmol) of C5b6 in the presence of excess C7/C8/C9. Cell loss and
Fluoro-Jade-B-positive cells were confined to the injected area,
including the granule cell layer, hilar neurons, and CA4 pyramidal
cells. D, High-power photomicrograph of the boxed
area of C disclosed the presence of Fluoro-jade
B labeling of degenerating cell bodies and proximal dendrites of CA4
pyramidal neurons. E, Nissl (top) and
Fluoro-Jade B (bottom) staining of rat hippocampus after
the infusion of preformed 3 nmol C5b9. There is no obvious cell loss,
but there are minimal inflammatory cell infiltrates in the injected
area evident on Nissl staining. F, High-power
photomicrograph of the boxed area of E.
DG, Dentate gyrus.
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Figure 4.
Concentration dependence of effects of C5b6 in the
presence of excess C7/C8/C9 on neuronal cell death. The intensity of
cell death was graded on a semiquantitative scale ranging from 0 to 5. The numbers of animals included in these histological analyses are
specified above the CA3 columns and represent a subset of
the animals presented in Figure 2; the remaining animals were used for
different histological studies, the results of which are not presented
here. Note that cell death was evident in the hippocampus of all
animals exhibiting seizures; however, some cell death was also detected
in the injected hippocampus after the infusion of complement in which
seizures were not detected. DG, Dentate
gyrus.
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To further assess MAC formation in vivo, immunohistochemical
experiments were performed. In these experiments, five additional animals underwent the sequential infusion of C5b6 (3 nmol), followed by
C7 and finally C8/C9, using the highest dose shown in Figure 2A-D; in contrast to previous experiments, these
animals were killed 1 hr after the infusion. The interval of just 1 hr
between the infusion and death was selected because the occurrence of seizures during and shortly after the infusion of C8/C9 implied that
the MAC was formed during or shortly after the infusion and thus should
be detectable at this time. Seizures were observed in each of the five
animals infused. In the animal presented in Figure
5A,B, intense MAC
immunoreactivity was evident in the hippocampus sequentially infused
with C5b6, C7, and C8/C9 (right); in contrast, no MAC
immunoreactivity was detected in the contralateral hippocampus from
which the electrographic seizure activity was recorded (Fig. 5A,B, left). The immunoreactivity was widespread
within, but did not extend beyond, the infused hippocampus (Fig.
5A,B, right). Elimination of the
immunoreactivity by preincubating the primary antibody with soluble
C5b9 (Fig. 5C) reinforces the specificity of the antibody.
The occurrence of seizures temporally related to the infusion of C8/C9
implies that the MAC was formed in all five animals, yet MAC
immunoreactivity was detected in only three of the five animals
infused. The most likely explanation for the absence of detectable MAC
immunoreactivity in the remaining two animals is that the epitope was
destroyed by proteolytic enzymes during the hour after the infusion;
the fact that no additional seizures occurred later than 30 min after
terminating the infusion of C8/C9 suggests that functional MACs are
rapidly eliminated and thus could have escaped detection by
immunohistochemistry. Other potential explanations include
inaccessibility of the epitope to the antibody, as found by Morgan et
al. (1987), or the presence of MAC in a section of hippocampus not
selected for immunohistochemical study.
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Figure 5.
MAC immunohistochemistry after intrahippocampal
infusion of complement proteins. A, MAC immunoreactivity
in the hippocampus of an animal killed 1 hr after the sequential
infusion of C5b6 (3 nmol), C7 (4 nmol), and C8 (5 nmol)/C9 (14 nmol)
into the right hippocampus (right). Note the absence of
detectable MAC immunoreactivity in the uninjected hippocampus
(left) used for recording of the same animal.
B, High-power photomicrograph of the boxed
areas in A. C, The MAC
immunoreactivity was eliminated when the anti-MAC antibodies were
preincubated with soluble C5b9 in sections adjacent to those
portrayed in A, demonstrating the antibody
specificity.
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Discussion |
The principal findings emerging from these studies include the
following. Sequential infusion of the five proteins of the membrane
attack pathway into the hippocampus of awake, freely moving rats
induced both behavioral and electrographic seizures as well as
cytotoxicity. The onset of seizures occurred during or shortly after
the infusion of C8/C9. In contrast, the simultaneous infusion of all
five proteins premixed in vitro evoked neither seizures nor
cytotoxicity. Likewise, the sequential infusion of just three or four
of the five proteins of the membrane attack pathway evoked neither
seizures nor cytotoxicity. The requirement for the sequential infusion
of all five proteins together with the temporal relationship of seizure
onset to infusions of C8/C9 implies that the MAC was formed in the
infused hippocampus in vivo and triggered both seizures and
cytotoxicity. The presence of MAC immunoreactivity in a subset of
animals undergoing infusion reinforces this conclusion.
Studies of MAC effects in both excitable and nonexcitable cells
in vitro provide a clue to the mechanism by which MAC
deposition on neurons in vivo might trigger seizures. The
fully assembled MAC contains one molecule each of C5b, C6, C7, and C8
and multiple C9 molecules; insertion of MACs into cell membranes
creates transmembrane channels. Single-channel analyses of MACs formed
in planar lipid bilayers by the sequential application of purified
human complement proteins, C5b6, C7, C8, and C9 (Benz et al., 1986)
revealed striking variation of the single-channel conductances, the
largest of which were 3.1 nS. The channels exhibited linear
current-voltage relationships and, interestingly, a threefold to
fourfold greater permeability for cations compared with anions. Indeed,
MAC formation in erythrocyte membranes leads to the influx of
Ca2+ and Na+
and the efflux of K+ (Halperin et al.,
1993); the net effect is a dose-dependent depolarization of membrane
potential before hemolysis (Wiedmer and Sims, 1985). MAC deposition on
the membrane of cardiac myocytes induced increases in intracellular
Ca2+ and contractility (Berger et al.,
1993). Thus, it seems plausible that the deposition of MACs on the
membranes of a small subset of hippocampal neurons may trigger similar
cation fluxes and depolarization; the resulting high-frequency firing
of this subset might recruit otherwise unaffected but synaptically
coupled neurons into a larger neuronal population firing synchronously,
thus mediating the electrographic and behavioral seizures observed in
this study. Similar mechanisms might underlie the medically refractory
seizures commonly observed in Rasmussen's encephalitis, in which MAC
deposition on cortical neurons has been identified
immunohistochemically (Whitney et al., 1999).
Mechanisms similar to those by which MAC deposition evokes seizures
likely underlie the cytotoxic consequences of MACs in the present
study. Although the deposition of one or a few MACs might evoke cation
flux and depolarization, triggering high-frequency firing, the
diffusion of ions down their electrochemical gradients through large
numbers of MACs would be expected to induce cell swelling and osmotic
lysis, the hallmarks of death of cortical neurons undergoing lethal
attack by complement in vitro (Whitney and McNamara, 2000).
Importantly, analyses of diverse nucleated cell types have elucidated
an elaborate set of defense mechanisms aimed at fending off MAC
deposition on cell membranes. An early defense is mediated by CD59, a
cell-surface complement regulatory protein, which binds nascent C5b9
complexes and prevents the incorporation of additional C9 molecules and
formation of a full MAC (Morgan, 1999). If this defense is overwhelmed
and MACs are deposited in the cell membrane, the cell eliminates the
membrane fragments containing the MACs by both endocytosis and
exocytosis (Morgan et al., 1987; Scolding et al., 1989; Stein
and Luzio, 1991), simultaneously increasing ion pumping to restore
ionic homeostasis. In the present experiments, the rapid infusion of
high concentrations of proteins of the membrane attack pathway
presumably overwhelmed the defense mechanisms and effected rapid
necrotic death of neurons, as evidenced both by Nissl and Fluoro-Jade
B staining.
Although the present studies elucidate some deleterious consequences of
MAC deposition in cortical gray matter in vivo, the complement cascade almost certainly has both adaptive and maladaptive consequences in a number of pathological settings. One common disease
exemplifying this idea is Alzheimer's disease, in which the
accumulation of extracellular A peptide contributes to the pathogenesis (Hardy and Selkoe, 2002). Aggregated A peptide can activate both the classical and alternative complement cascades in vitro by binding C1q and C3b, respectively (Jiang et al.,
1994; Bradt et al., 1998); this is likely to be the trigger of
sequential activation of these pathways, resulting in the formation of
C5b, the initial component of the membrane attack pathway, thereby leading to the deposition of the MAC evident in Alzheimer's disease brains (McGeer et al., 1989; Itagaki et al., 1994; Webster et al.,
1997; Yasojima et al., 1999). Interestingly, the binding of A
peptide by C3b likely promotes its phagocytosis and removal, because
the transgenic overexpression of an inhibitor of C3b formation increases A peptide accumulation in mice expressing the human amyloid precursor protein (Wyss-Coray et al., 2002); importantly, the
increased A peptide deposition is accompanied by enhanced neurodegeneration in these mice (Wyss-Coray et al., 2002). These adaptive consequences notwithstanding, the triggering of the early activation pathway of complement by A peptide leaves the
complement cascade poised to transition to the membrane attack pathway
and deposition of the MAC. Notably, the transition to the membrane attack pathway is not an inevitable consequence of triggering the early
activation pathway; rather, a diversity of complement regulatory
proteins expressed on neurons and other cell types inhibits diverse
steps in the complement cascade, seeking to limit the deleterious
consequences of complement activation on host cells (Liszewski et al.,
1996). Immunohistochemical evidence of MAC deposition on neurons in
Alzheimer's brains implies that the endogenous defenses are not
completely effective. Interestingly, supervening sublethal excitotoxic
insults, such as might occur with transient ischemia or seizures,
compromise the endogenous defenses and promote the transition from the
early activation pathway to MAC deposition on cortical neurons in
vitro (Xiong and McNamara, 2002). The devastating consequences of
MAC deposition in cortical gray matter demonstrated here underscore the
need to identify small molecule inhibitors selectively targeting the membrane attack pathway, which should reduce or even eliminate the
maladaptive consequences of the complement cascade in Alzheimer's disease.
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FOOTNOTES |
Received Sept. 20, 2002; revised Nov. 4, 2002; accepted Nov. 8, 2002.
This work was supported by National Institute of Neurological Disorders
and Stroke Grant NS3038319. We thank Dr. Steve Danzer for his comments
on this manuscript.
Correspondence should be addressed to Dr. James O. McNamara, Department
of Neurobiology, Box 3676, Duke University Medical Center, Durham, NC
27710. E-mail: jmc{at}neuro.duke.edu.
K. Suzuki's present address: Department of Psychiatry and Neurology,
Hamamatsu University School of Medicine, Hamamatsu, Shizuoka 431-3192, Japan.
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