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Aging Cell (2004), pp169–176
Doi: 10.1111/j.1474-9728.2004.00101.x
REVIEW
Blackwell Publishing, Ltd.
How chronic inflammation can affect the brain and
support the development of Alzheimer’s disease in
old age: the role of microglia and astrocytes
Imrich Blasko,1 Michaela Stampfer-Kountchev,2
Peter Robatscher,2 Robert Veerhuis,3
Piet Eikelenboom3 and Beatrix Grubeck-Loebenstein2
1
Department of Psychiatry, University Hospital of Innsbruck,
Innsbruck, Austria
2
Institute for Biomedical Aging Research of the Austrian Academy
of Sciences, Innsbruck, Austria
3
Department of Psychiatry and Pathology, Research Institute of
Neurosciences, Vrije Universiteit Amsterdam, The Netherlands
Summary
A huge amount of evidence has implicated amyloid beta
β) peptides and other derivatives of the amyloid pre(Aβ
βAPP) as central to the pathogenesis of
cursor protein (β
Alzheimer’s disease (AD). It is also widely recognized that
age is the most important risk factor for AD and that the
innate immune system plays a role in the development
of neurodegeneration. Little is known, however, about the
molecular mechanisms that underlie age-related changes
of innate immunity and how they affect brain pathology.
Aging is characteristically accompanied by a shift within
innate immunity towards a pro-inflammatory status.
Pro-inflammatory mediators such as tumour necrosis
α or interleukin-1β
β can then in combination with
factor-α
interferon-γγ be toxic on neurons and affect the metabolism
of βAPP such that increased concentrations of amyloidogenic
peptides are produced by neuronal cells as well as by astrocytes. A disturbed balance between the production and
β can trigger chronic inflammatory
the degradation of Aβ
processes in microglial cells and astrocytes and thus
initiate a vicious circle. This leads to a perpetuation of
the disease.
Key words: aging; Alzheimer’s disease; amyloid beta; astrocytes; innate immune system; microglial cells.
Correspondence
Beatrix Grubeck-Loebenstein, MD, Institute for Biomedical Aging Research,
Austrian Academy of Sciences, Rennweg 10, A-6020 Innsbruck, Austria.
Tel.: +43 512583919 –0; fax: +43 512583919–8;
e-mail: [email protected]
Accepted for publication 25 May 2004
© Blackwell Publishing Ltd/Anatomical Society of Great Britain and Ireland 2004
Current state of knowledge on amyloid
precursor protein, its metabolism and
development of Alzheimer’s disease
Alzheimer’s disease (AD) is the most common dementia form of
old age. Currently, approximately 13 million people world-wide
suffer from this disorder. AD is the most comprehensively studied
neurodegenerative disease and in the last 15 years it has become
increasingly clear that the degeneration of neurons is connected
with a dysregulation in the metabolism of beta amyloid precursor
protein (βAPP) with a consequent transient overproduction or
decreased degradation of beta amyloid (Aβ) in the brain.
The pathological hallmarks of AD include profound neuronal
loss in the hippocampus, entorhinal and temporoparietal cortex,
the presence of intraneuronal neurofibrillary tangles (NFTs),
astrogliosis and deposition of Aβ. NFTs are composed of abnormally
folded and hyperphosphorylated Tau, a protein involved in microtubule formation. The presence of neurofibrillary pathology
(NFT and /or neuritic plaques) is mandatory for the diagnosis
of AD (Braak et al., 1993; Jellinger, 1998). Although the pathophysiological association of Aβ and Tau is not completely understood, it seems that in the pathogenesis of AD the dysregulation
in the βAPP metabolism precedes the Tau hyperphosphorylation.
Genetic studies support this evidence. Mutations in three genes,
βAPP, Presenilin 1 (PS1) and Presenilin 2 (PS2), all lead to increased
production of Aβ42. A dysregulation of the βAPP metabolism
leading to overproduction of Aβ is currently regarded as crucial
for the development of AD (Selkoe, 2000). βAPP is a transmembrane glycoprotein belonging to the super-family of amyloid
precursor-like proteins, the function of which is (a) adhesion to
extracellular matrix proteins and preservation of synaptic plasticity (Coulson et al., 2000), (b) mediation of intracellular sorting
mechanisms as a receptor molecule (Kamal et al., 2001) and
(c) degradation to cytoplasmic cleavage fragments that might
modulate gene expression (Ebinu & Yankner, 2002). βAPP is
cleaved by two major metabolic pathways. Secretory βAPP (βAPPs)
is cleaved from βAPP by a protease referred to as α-secretase.
It participates in the formation of synapses and in the integrity
of memory (Sisodia & Gallagher, 1998). Intact Aβ is cleaved from
βAPP by the sequential actions of two proteases referred to as
β- and γ-secretase ( Haass, 2004). Aβ occurs as a 40 and 42 amino
acid peptide ( Aβ40, Aβ42). Very recent observations suggest
that Aβ may play a role in normal synaptic physiology as well
as in pathological processes leading to AD, as Aβ depresses
synaptic transmission and the activity of neurons in a negative
169
170 Chronic inflammation and Alzheimer’s disease, I. Blasko et al.
feedback loop (Kamenetz et al., 2003). It is now well understood that an early and invariant step in the pathogenesis of
AD is the overproduction and/or decreased degradation of Aβ,
which leads to the oligomerization of Aβ in protofibrils and
oligomers, which are regarded as pathogenetic hallmark of AD.
The development of neuritic plaques precedes the deposition
of amyloid in non-fibrillar deposits of Ab42 in the neurophil
(diffuse plaques). Although the density of NFTs correlates better
with dementia than the frequency of Aβ plaques (Braak et al.,
1993; Jellinger, 1998), recent studies suggest that accumulation
of Aβ in the brain might precede the development of both
neurofibrillary tangles and cognitive impairment (Naslund et al.,
2000). Aggregated Aβ is toxic in vitro and in vivo and causes DNA
damage and apoptosis (Paradis et al., 1996). Similar to other systemic amyloidoses, masses of Aβ in the extracellular space might
also inflict physical or mechanical stress to passing axons and cause
diffuse axonal injury leading to neuronal death (Vickers et al., 2000).
Decreasing production and accelerating degradation of Aβ peptides seem crucial to avoid or postpone clinically overt AD.
Although the relatively rare familial early onset form of AD
is associated with a strongly increased Aβ generation, defective
A β clearance combined with an increased production is
important in late-onset AD (LOAD, age > 70 years). Approximately
90% of all cases are of the LOAD type (Selkoe, 2001). The
concept of a defective Aβ clearance in LOAD is also supported
by genetic evidence (Myers et al., 2000).
How innate immunity can support the
development of AD: a general overview
Aging of adaptive immunity has been profoundly studied in recent
years (Grubeck-Loebenstein & Wick, 2002). Considerably less
information is available on how aging affects the innate immune
system. However, there are clear indications that age-related
changes within the innate immune system may influence the
development of age-related neurodegenerative disorders such
as AD and vice versa that age-related degenerative changes can
modulate innate immunity.
Aβ and other proteins found in the senile plaques of AD
patients are frequently potent activators of the innate immune
response. This is of interest because chronic stimulation of the
innate immune system may lead to disadvantages outweighing
the advantages initiated by the efforts of an organism to degrade
toxic metabolites. Chronically activated microglia and astrocytes
can kill adjacent neurons by the release of highly toxic products
such as reactive oxygen intermediates, nitric oxide, inflammatory
cytokines, proteolytic enzymes, complement factors or excitatory
amino acids (for a review see Rogers et al., 2002). Activation of the
complement system leads to the production of complement activation fragments, including anaphylatoxins and opsonins (Bradt
et al., 1998). The continuous presence of complement activators
such as Aβ might vice versa stimulate the chronic state of complement activation and chronic inflammation in the AD brain.
Cytokines such as tumour necrosis factor-α (TNFα), interleukin1β (IL-1β) and IL-6 can be directly cytotoxic, when chronically
produced at high concentrations (Jeohn et al., 1998). They can
then also stimulate the synthesis of βAPP (Goldgaber et al., 1989)
or may, in combinations such as that of TNFα or IL-1β with
interferon-γ (IFN-γ), stimulate the production of Aβ peptides
(Blasko et al., 1999). Inflammatory cytokines additionally decrease
the secretion of the neuroprotective APPsα (Blasko et al., 1999).
Interestingly, βAPP expression is regulated by the same cytokinesensitive transcription factors that are involved in the expression of most acute phase proteins (Ge & Lahiri, 2002). Several of
these proteins such as α1-antichymotrypsin, apolipoprotein E,
serum amyloid A, transthyretin, certain proteoglycans and complement factors bind to Aβ and are regarded as pathological
chaperones promoting the fibrillar conformation of Aβ (Castillo
et al., 1996). The oligomer and fibrillar conformation of Aβ is
important for its neurotoxicity as well as its inflammation-inducing
properties. It is of interest that glial products such as IL-1β or
TNFα can induce the synthesis of most of these Aβ-binding proteins in astrocytes, microglia or neurons (Lieb et al., 1996).
Therefore, the chronic release of pro-inflammatory cytokines in
the brain is likely to maintain a chronic acute phase protein
secretion favouring the formation of Aβ fibrils.
The observation that inflammation is integrally involved
in the progression of dementia might have clinical implications.
The inducible form of cyclooxygenase, COX-2, is elevated in
the brain of early AD, and IL-6 and transforming growth factorβ1 (TGF-β1) were found to be elevated in severely demented AD
patients (Luterman et al., 2000; Ho et al., 2001; Pasinetti, 2001).
Therefore, future anti-inflammatory therapeutic strategies will
possibly have to depend on the clinical stage of the disease.
Inflammation in the AD brain is thus most likely the result of
amyloid overproduction, but there is abundant evidence that
it can be an additional cause of neurodegeneration and could
ultimately be as important as the conditions that gave rise to it.
The specific role of microglia
Microglial cells are the most important cells of the innate
immune system in the brain. They play the role of cerebral macrophages and recruit and stimulate astrocytes. There is considerable debate as to whether activated microglia are beneficial
or harmful. This may, however, depend upon the degree of activation. In recent years it has become obvious that microglial cells
can be activated by factors such as brain trauma, ischaemia or
neurodegeneration. Activated microglia express markers, such
as MHC class I and class II molecules, together with integrins
and Fc receptors (Eikelenboom et al., 1994; McGeer & McGeer,
1995). In the AD brain, activation of microglia by Aβ is associated with chemotactic responses to it, consistent with the extensive clustering of activated microglia at sites of Aβ deposition
( Terry & Wisniewski, 1975). Opsonization of Aβ oligomers and
fibrils with carrier proteins in serum facilitate its endocytosis
and degradation (Ard et al., 1996). Upon activation with Aβ and
the above-mentioned acute phase proteins, microglial cells produce the pro-inflammatory cytokines IL-1, IL-6 and TNFα, the
chemokines IL-8, macrophage inflammatory protein-1 (MIP-1) and
© Blackwell Publishing Ltd/Anatomical Society of Great Britain and Ireland 2004
Chronic inflammation and Alzheimer’s disease, I. Blasko et al. 171
monocyte chemoattractant peptide-1 (MCP-1), and the growth
factor macrophage-colony stimulating factor (M-CSF) (Lue
et al., 2001). The mRNAs for virtually all these proteins, as well
as for complement cascade proteins C1qB, C3 and C4, the IL1 receptor and receptor antagonist, and TGFβ have been
observed in AD microglia (Walker et al., 1995; Shen et al., 1997;
Strohmeyer et al., 2000). In peripheral monocytes a similar proinflammatory response has been demonstrated following stimulation with Aβ (Akiyama et al., 2000; Fiala et al., 1998).
How old age and cellular senescence change the response
of microglia to activation-inducing stimuli such as Aβ is not yet
known. The first morphological studies on the effect of healthy
aging on microglial cells have recently appeared. Microglial cells
from cognitively normal elderly donors exhibited several abnormalities within their cytoplasmic structure, including deramification, spheroid formation, gnarling and fragmentation of
processes (Streit et al., 2004). These changes were judged to
be different from morphological alterations that typically occur
during microglial activation in neurodegenerative disorders.
They were referred to as microglial dystrophy. Age-related
microglial senescence may still easily lead to functional defects,
which may then trigger an intracerebral pro-inflammatory
response that would support the development of neurodegenerative disorders such as AD in old age.
The specific role of astrocytes
General information on the role of astrocytes
Astrocytes are the most frequent cells of the brain. In the last
decade the view of neurobiologists on astrocytes has shifted
from the concept of a mainly supporting cell type to a multifunctional housekeeping cell that allows neurons to become
progressively specialized for the tasks of information processing.
Astrocytes not only organize the structural architecture of the
brain but also organize its communication pathways and plasticity. Neurons co-cultured with astrocytes develop approximately seven-fold more synapses, and have a seven-fold
increase in synaptic efficacy, compared with neurons raised in
the absence of astrocytes (Pfrieger & Barres, 1997). Astrocytes
cannot only modulate synaptic strength and activity, but in neurogenic regions of the adult human brain they positively and
negatively regulate neurogenesis (Goldman, 2003). Thus a
series of recent studies has clearly demonstrated that reciprocal
paracrine interactions between astrocytes, endothelial cells and
ependymal cells can regulate neurogenesis and gliogenesis from
resident precursor cells (Song et al., 2002). At the same time it
has become obvious that astrocytes can play an important role
in inflammatory processes (Wyss-Coray et al., 2003).
The participation of astrocytes in the
neurodegeneration of AD: potential beneficial effects
In the last decade considerable attention has been paid to
understanding the role of astrocytes in neurodegenerative
© Blackwell Publishing Ltd/Anatomical Society of Great Britain and Ireland 2004
disorders like AD. In contrast to microglia, astrocytes are able
to remove and to degrade Aβ without mediators or stimuli such
as opsonins or cytokines (Bard et al., 2000; Wyss-Coray et al.,
2003). Following activation, astrocytes can release cytokines
and growth factors similar to those produced by microglia
(McGeer & McGeer, 1995), but can also produce trophic substances such as nerve growth factor (NGF) (Aguado et al.,
1998), the neurotrophic signalling molecule S100β (Mrak &
Griffin, 2001), the brain-derived neurotrophic factor (BDNF)
(Hock et al., 2000), neurotrophin 3 (NT-3) (Blondel et al., 2000)
and neurotrophin 4/5 (NT-4/5), which have all been shown to
have a trophic effect on neurons. Astrocytes regulate extracellular levels of excitatory amino acids such as glutamate and
contribute to homeostasis within the central nervous system.
Neurotrophic factors, transporter molecules and enzymes
involved in the metabolism of excitatory amino acids or in the
antioxidant pathway may help to protect neurons and other
brain cells from damage by controlling the production of potentially toxic substances. The high flexibility of astrocytes to
express different molecules in dependence of age or in dependence of pathological stimuli is documented by the example of
experimental AD. Studies on transgenic animals overexpressing
human Aβ peptides show that in young animals astrocytes produce small amounts of the Aβ-degrading enzyme neprilysine.
However, as the animals get older and the amounts of Aβpeptides increase, the expression of neprilysine is stimulated
(Blondel et al., 2000; Apelt et al., 2003).
The influence of aging on the ability to degrade Aβ-peptides
is of interest in this context. In contrast to neonatal mouse and
rat astrocytes (Shaffer et al., 1995), adult mouse astrocytes efficiently clear exogenously added and surface-bound Aβ and are
capable of removing Aβ deposited in brain slices from mice overexpressing human βAPP (Wyss-Coray et al., 2003). Astrocytosis
is a typical morphological feature of the AD brain and represents
either proliferation of astrocytes in an effort to replace dying
neurons or a reaction to degrade the increasing amounts of
toxic Aβ peptides. The chemokine MCP-1 is believed to play an
important role in astrocytosis, as its levels were found to be
increased after brain injury in contrast to the levels of the proinflammatory cytokines TNFα, IL-6 and IL-1β (Little et al., 2002).
In the context of aging it is also of interest that adult mouse
astrocytes do not respond to stimulation with Aβ by increasing
their release of MCP-1 in the same way as young control cells
( Wyss-Coray et al., 2003).
The participation of astrocytes in the
neurodegeneration of AD
How Aβ affects astrocytes to produce inflammatory products
Because astrocytes greatly outnumber microglia in the brain
(Savchenko et al., 2000), these cells could have a more critical
role in the development of AD pathology than previously
thought. Depending on their degree of aggregation and deposition status, Aβ peptides can activate astrocytes. Thus, reactive
IL-1β and IL-6 positive astrocytes were found in close proximity
172 Chronic inflammation and Alzheimer’s disease, I. Blasko et al.
Table 1 The in vitro production of TGF-β1 (ng mL−1) by human astrocytes is
−1
suppressed following incubation with TNFα (1000 U mL for 24 h). Post
mortem astrocytes removed from the grey matter were used. They were
purified and cultured, as previously described (De Groot et al., 1997; Veerhuis
et al., 1998)
Astrocyte
line no.
Brain
bank no.
TGF-β1
TNFα−
TGF-β1
TNFα+
Suppression
1
2
3
99–138
99–139
00 – 010
86.8
118.7
334.9
51.8
56.7
164.7
40%
52%
51%
to both fibrillary and diffuse Aβ deposits detectable at very
early stages of plaque development in APP SW mice, whereas
activated microglia appeared in and around fibrillary Aβ plaques
only (Benzing et al., 1999). Reactive astrocytes can produce
TGF-β1, TGF-β3 and IL-10 in AD-transgenic mice (Apelt &
Schliebs, 2001), but have also been shown to secrete proinflammatory mediators such as MCP-1, RANTES, TNF-α and
IL-1 in response to stimulation with Aβ42 (Smits et al., 2002).
Using primary human astrocytes isolated post mortem from
AD patients, we recently demonstrated a high constitutive production of TGFβ-1. However, this potentially antiinflammatory mechanism could be inhibited by the addition of
TNFα (Table 1). This might indicate that pro-inflammatory
factors can down-regulate anti-inflammatory responses and
neutralize protective mechanisms. A vicious circle could thus
be initiated.
How inflammatory factors might affect the processing of βAPP
in astrocytes
Based on the evidence that inflammatory proteins were found
in brains of AD patients, several groups were stimulated to
investigate the influence of cytokines on the processing of βAPP
by astrocytes. βAPP holoprotein synthesis by astrocytes can be
stimulated by cytokines such as IL-1β, TNFα or TGF-β1 (Amara
et al., 1999; Rogers et al., 1999) through regulation of gene
transcription at the promoter level (Ge & Lahiri, 2002). Combinations of cytokines such as IL-1β or TNFα and IFN-γ are able
to affect the metabolism of βAPP and to stimulate the production of Aβ40 and Aβ42 peptides (Blasko et al., 1999; Sastre
et al., 2003). The mechanism by which cytokines stimulate Aβ
production is complex, but it seems likely that the maturation
(i.e. proper glycosylation) of the βAPP holoprotein is disturbed
(Blasko et al., 1999, 2000). At the same time, β-secretase activity
is stimulated (Sastre et al., 2003).
Can astrocyte senescence affect the response to inflammation?
There is presently no definitive answer to the question of why
aging is still the biggest risk factor for AD. It seems likely that
LOAD is orchestrated by a whole variety of different factors,
which are each subtle in influence, but highly detrimental in
their combination. LOAD is, for instance, not solely induced by
the overproduction of toxic metabolites but may coincide with
Table 2 Clinical and pathological characteristics of AD patients, from which
brain specimens were obtained for isolation of astrocytes. The Braak score
represents histopathological classification of AD, in which stages 5–6
represent the destruction of virtually all isocortical association areas (Braak &
Braak, 1991). The global deterioration scale (GDS) represents seven stages of
cognitive, functional and behavioural impairment in AD (Reisberg et al.,
1982). Apolipoprotein E (apoE) ε4 allele is a genetic risk factor for late-onset
familial and sporadic AD. The presence of at least one ε4 allele increases the
risk of AD 3–5 times in comparison with non-Apo ε4 carriers
Line
no.
Brain
bank no.
1
2
3
4
5
6
7
98–170
99– 068
99–138
99–139
00– 010
00– 045
00– 076
Sex
Age
(years)
Braak
score
(1–6)
GDS
stage
(1–7)
ApoE-ε4
allele
f
f
f
m
f
f
f
79
86
76
83
77
92
88
6
4
6
4
5
5
4
7
2
7
5
7
7
7
4/4
3/3
4/2
4/3
4/4
4/3
3/3
the decreased capacity of aged glial cells to degrade and /or
store toxic products. Little is known about age-related changes
in astrocytes (Flanary & Streit, 2004). As astrocytes can be easily
passaged in cell culture (Evans et al., 2003), we studied the replicative senescence of primary human astrocytes (Rozovsky
et al., 1998; Xie et al., 2003).
We measured the proliferative capacity and populationdoublings of post mortem isolated astrocytes obtained from
seven donors with AD. The cells were isolated from white matter
obtained either from corona radiata or parietal white matter as
well as from cortical grey brain matter from the frontal cortex.
The patients’ characteristics are listed in Table 2. The cells were
obtained from The Netherlands brain bank. Their isolation and
culture as well as their phenotype have recently been described
(De Groot et al., 1997; Veerhuis et al., 1998; Blasko et al.,
2000). In analogy to the well-known ‘in vitro senescence’
models on fibroblasts and endothelial cells ( Wagner et al., 2001;
Unterluggauer et al., 2003), astrocytes from AD patients proliferated rigorously in culture and underwent 6 – 8 population
doublings (PDs) before proliferation decreased and astrocytes
stopped dividing. Growth arrest was reached after 9 –10 PDs.
Astrocytes are therefore referred to as ‘late passage astrocytes’
after 8 PDs. Throughout the duration of the cultures, astrocytes
constantly expressed the glial fibrillary acidic protein (GFAP)
monitored by immunoblotting and immunofluorescence (data
not shown). The quantification of GFAP cellular content by
loading the same amount of cellular protein on gels revealed
that late passage astrocytes had a tendency (nonsignificant) to
express more GFAP in comparison with early passage astrocytes
(Fig. 1A; (Nichols et al., 1993, 1995). In agreement with this
observation, a higher GFAP in vivo content was found in aged
rats and mice in comparison with their young littermates (O’Callaghan & Miller, 1991; Yoshida et al., 1996; Kyrkanides et al.,
2001). Additionally, late passage astrocytes had a significantly
increased expression of senescence markers such as p16, p21
© Blackwell Publishing Ltd/Anatomical Society of Great Britain and Ireland 2004
Chronic inflammation and Alzheimer’s disease, I. Blasko et al. 173
Fig. 1 (A) Expression pattern of the senescence markers p16, p21, cyclin D1 and the differentiation marker GFAP. Each blot shown in the upper part of the
figure represents one characteristic Western blot experiment. The bars represent mean ± SEM (n = 3 in each group). The cells from early (PD 3) and late passage
(PD 12) are compared. Marker expression levels in PD 3 were considered as 100%. PD 12 results are presented as percentage change compared with early
passage cells. The levels of significance between both groups are indicated (Student’s t-test). (B) Comparison of the numbers of PDs of astrocytes from grey
or white matter after different periods of in vitro culture. The astrocytes from the grey matter proliferated better than their white counterparts (mean ± SEM,
n = 7 in each group, *P < 0.05 white vs. grey matter, Student’s t-test). (C) In vitro passaging of primary human astrocytes isolated post mortem from AD
patients leads to the occurrence of apoptosis after a certain number of PDs. Comparison of apoptosis of astrocytes from white and grey matter. Apoptosis
was assessed by propidium iodide staining of apoptotic nuclei. Cells were lysed by 0.1% Triton-X solution, stained with propidium iodide and FACS flowcytometry analysis was performed (Blasko et al., 1997). Apoptotic nuclei have characteristically lower fluorescence intensity than diploid cells. (D) Post-mitotic
grey matter astrocytes after 12 in vitro PDs retain their capacity to produce Aβ peptides following cytokine stimulation. The production of Aβ42 is compared
in early and late passage astrocytes. The production of Aβ peptides was induced by stimulating the cells with a combination of cytokines (IFN-γ, 500 U mL−1
and TNFα, 1000 U mL−1). The figure represents an immunoprecipitation experiment using the W0-2 antibody (Ida et al., 1996) following metabolic
labelling of the cells with S35. The figure represents one of five identical experiments. Similar experiments were performed also on white matter astrocytes
(data not shown).
and cyclin D1 (Fig. 1A; Stein et al., 1999; Morisaki et al., 1999;
Wainwright et al., 2001).
The grey matter astrocytes proliferated better than their white
matter counterparts. The number of PDs reached by grey matter
astrocytes in different culture periods was always higher than
by white matter astrocytes (Fig. 1B). Whether this increased
growth rate depends on in vivo degenerative or inflammatory
processes in AD patients is not yet known. To define whether
astrocytes died of apoptosis after having reached the end of
their replicative lifespan, the number of apoptotic nuclei was
analysed at the beginning and the end of the in vitro culture.
Figure 1(C) demonstrates that in contrast to fibroblasts (Wagner
et al., 2001), but in accordance with endothelial cells (Unterluggauer et al., 2003), astrocytes are prone to undergo apoptosis after they have lost their capacity to replicate.
© Blackwell Publishing Ltd/Anatomical Society of Great Britain and Ireland 2004
To analyse whether cellular senescence changes the capacity
of astrocytes to respond to cytokines and to produce Aβ we
stimulated early as well as late passage astrocytes with TNFα
and IFN-γ. Late passage astrocytes were still capable of producing Aβ peptides following stimulation, demonstrating that
stimulation of Aβ production by inflammatory products does
not decrease with aging (Fig. 1D).
Perspectives and conclusions: evidence that
anti-inflammatory agents might protect
against neurodegeneration in AD
Based on the epidemiological evidence that nonsteroidal antiinflammatory drugs (NSAIDs) decrease the risk of AD (Broe
et al., 2000; in t’Veld et al., 2001), many anti-inflammatory
174 Chronic inflammation and Alzheimer’s disease, I. Blasko et al.
drugs have been investigated for their potential influence on
βAPP processing and the production of Aβ peptides. The common feature of most NSAIDs is their capacity to inhibit COX
and consequently to decrease the production of prostaglandins
(PGs), in particular PG-E2. Ibuprofen, flurbiprofen, indometacine and sulindac sulphide are, however, able to decrease Aβ42
production by influencing γ-secretase cleavage without affecting COX activity ( Weggen et al., 2001; Eriksen et al., 2003). Ibuprofen, as a commonly over-the-counter used NSAID, also
decreases cytokine-stimulated Aβ production in human neuronal cells and astrocytes (Blasko et al., 2001). It also reduces neuritic plaque pathology and inflammation in a mouse model of
AD (Lim et al., 2000). Recently, ibuprofen at lower concentrations than used in previous studies has been shown to reduce
Aβ40 and Aβ42 by decreasing COX-mediated PG-E2 production
(Qin et al., 2003). The binding of ibuprofen to the peroxisome
proliferator-activated receptor-γ (PPARγ) receptor, which is
known to mediate anti-inflammatory activities, is thought to be
responsible for this effect (Sastre et al., 2003).
In summary, senescence of the innate immune system can
be associated with a pro-inflammatory status of glial cells. This
shifted background reactivity of cerebral defence responses in
old age may precipitate the perpetual progression of AD. The
overproduction or decreased degradation of potentially toxic
peptides such as Aβ may just be initial steps in a long cascade
of detrimental changes. A better understanding of how innate
immunity and chronic inflammation affect neurodegeneration
will help to develop new diagnostic and therapeutic concepts.
Acknowledgments
We thank the brain bank at The Netherlands Institute for Brain
Research (coordinator R. Ravid) for providing brain specimens.
This work was funded by the European Community (Project No.
QLRT-1999-02004 ‘MANAD’) and by the Austrian Science Fund
(Project No. P15347).
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