Cell Death and Differentiation (2011), 1–13
& 2011 Macmillan Publishers Limited All rights reserved 1350-9047/11
www.nature.com/cdd
Review
The executioners sing a new song: killer caspases
activate microglia
JL Venero1, MA Burguillos2,3, P Brundin3 and B Joseph*,2
Activation of microglia and inflammation-mediated neurotoxicity are suggested to have key roles in the pathogenesis of several
neurodegenerative disorders. We recently published an article in Nature revealing an unexpected role for executioner caspases
in the microglia activation process. We showed that caspases 8 and 3/7, commonly known to have executioner roles for
apoptosis, can promote microglia activation in the absence of death. We found these caspases to be activated in microglia of PD
and AD subjects. Inhibition of this signaling pathway hindered microglia activation and importantly reduced neurotoxicity in cell
and animal models of disease. Here we review evidence suggesting that microglia can have a key role in the pathology of
neurodegenerative disorders. We discuss possible underlying mechanisms regulating their activation and neurotoxic effect.
We focus on the provocative hypothesis that caspase inhibition can be neuroprotective by targeting the microglia rather than the
neurons themselves.
Cell Death and Differentiation advance online publication, 12 August 2011; doi:10.1038/cdd.2011.107
Microglia, the Guardians of the Brain
The brain is partially isolated from the bloodstream by the
blood–brain barrier, which prevents infections from affecting
several vital brain functions. For this reason, brain and spinal
cord were considered ‘immune-privileged’ organs in that they
are separated from the rest of the body.
Several different types of neuroglia participate in multiple
CNS activities, from structural and metabolic functions to
information processing. They also seem to be involved in the
progression of neurodegenerative diseases. Neuroglial cells
are divided into different subtypes, which include astrocytes,
oligodendrocytes, NG2 glial cells and microglia.
Microglia are CNS macrophages first described by
Rio-Hortega, a disciple of Ramón y Cajal, in 1919.1 Microglia
populate the mammalian CNS in early embryonic development. By adulthood, microglia are found in all regions of the
brain and spinal cord, and comprise 10–15% of all CNS cells.
Microglia are the immune cells of the CNS – the guardians of
1
the brain – constantly scavenging for damaged neurons,
plaques and infectious agents by using their multiple surface
receptors. In addition to being sensitive to changes in their
environment, each microglial cell also regularly physically
surveys its domain. While moving through its precinct, if the
microglial cell finds any invading viruses, bacteria, damaged
cells, apoptotic cells, neural tangles, DNA fragments or
plaques it will undergo activation and phagocytize the material
or cell. Thereby microglial cells also act as ‘housekeepers’
that clean up the brain. Thus, microglial cells fulfill an
astonishing variety of tasks within the CNS (Box 1, FACTS).
Originally, the non-activated microglia were designated as
‘resting’, but nowadays the term ‘surveying’ is preferred,
reflecting the more dynamic nature of these cells. When
microglia are ‘activated’ they take on an amoeboid shape and
increase their gene expression, leading to the production of
numerous potentially neurotoxic mediators. During recent
years, researchers have tended to differentiate between
Facultad de Farmacia, Departamento de Bioquı́mica y Biologı́a Molecular, Universidad de Sevilla, and Instituto de Biomedicina de Sevilla (IBiS), Sevilla, Spain;
Department of Oncology-Pathology, Cancer Centrum Karolinska, Karolinska Institutet, Stockholm SE-171 76, Sweden and 3Neuronal Survival Unit, Department of
Experimental Medical Science, Wallenberg Neuroscience Center, Lund 221 84, Sweden
*Corresponding author: B Joseph, Department of Oncology-Pathology, Cancer Centrum Karolinska, Karolinska Institutet, Stockholm SE-171 76, Sweden.
Tel: þ 46 8 517 738 26; Fax: þ 46 8 33 90 31; E-mail: bertrand.joseph@ki.se
Keywords: microglia; caspases; neurodegenerative disorders
Abbreviations: Ab, b-amyloid; AD, Alzheimer’s disease; AIM2, absent in melanoma-2; ALS, amyotrophic lateral sclerosis; AMPA receptor, a-amino-3-hydroxy-5methyl-4-isoxazolepropionic acid receptor; AP-1, activator protein-1; APP, amyloid-b precursor protein; CoREST, REST co-repressor; COX, cyclooxygenase; DA,
dopaminergic; DAMP, danger-associated molecular pattern; fmk, fluoromethyl ketone; GSK3b, glycogen synthase kinase-3b; HD, Huntington’s disease; HSP, heatshock proteins; IL, interleukin; IL-1R, IL-1 receptor; IFN-g, interferon-g; IFNGR, IFN-g receptor; IKKe, IkB kinase-e; iNOS, inducible nitric oxide synthase; IRAK,
interleukin receptor-associated kinase; IRF, interferon-regulatory factor; Jak, Janus kinase; LPS, lipopolysaccharide; LTA, lipoteichoic acid; LRR, leucine-rich repeat;
MAPK, mitogen-activated protein kinase; MHC, major histocompatibility complex; MMP, matrix metalloproteinase; MPTP, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine;
MS, multiple sclerosis; MyD88, myeloid differentiation factor-88; NEMO, NF-kB essential modulator; NF-kB, nuclear factor k-light-chain enhancer of activated B cells;
NLRP, NLR family pyrin domain containing; NO, nitric oxide; O
2 , superoxide anion; Nurr-1, nuclear receptor related-1 protein; PAMP, pathogen-associated molecular
pattern; PD, Parkinson’s disease; PKCd, protein kinase-Cd; Q-VD-OPh, quinolyl-valyl-O-methylaspartyl-[-2,6-difluorophenoxy]-methylketone; RAGEs, receptors like
advanced glycation end products; ROS, reactive oxygen species; SN, substantia nigra; Socs, suppressors of cytokine signaling proteins; STAT, signal transducer and
activator of transcription; TANK, TRAF family member-associated NFKB activator; TBK1, TANK-binding kinase-1; TIR, Toll–interleukin-1 receptor; TLR, Toll-like
receptor; TNF-a, tumor necrosis factor-a; TRAF6, TNF receptor-associated factor-6; TIRAP, TIR domain-containing adaptor protein; TRAM, TRIF-related adapter
molecule; TRIF, TIR-domain-containing, adapter-inducing interferon-b; Tyk2, tyrosine kinase-2
2
Received 26.4.11; revised 09.6.11; accepted 21.6.11; Edited by G Melino
Killer caspases activate microglia
JL Venero et al
2
Box 1
FACTS:
K
Under physiological conditions, microglia cells serve as surveyors of the integrity and function of the brain. However, their function changes
under pathological conditions, during which microglia can have either a supportive or a detrimental role for neuronal survival.
K
Upon activation, microglia cells release proinflammatory factors (that is, cytokines), reactive oxygen/nitrogen species and proteases that
eventually provoke neuronal demise and death.
K
Several signaling pathways lead to microglial activation. Depending on the nature of the proinflammatory stimulus they can include TLRs,
MyD88, JAK/STAT and NF-kB.
K
Recent studies indicate that killer caspases can take on non-apoptotic roles. For example, caspases 8 and 3/7, commonly known as
executioners of apoptosis, can promote microglia activation in the absence of cell death.
K
Caspases 8 and 3/7 are activated in microglia in the substantia nigra in Parkinson’s disease subjects and the frontal cortex of Alzheimer’s
disease subjects.
K
Caspase inhibitors exert neuroprotective effects in several animal models of neurodegenerative disorders, which are characterized by
reactive microgliosis.
Table 1 Innate and adaptive immune responses in chronic neurodegenerative diseases
Innate immune response
Parkinson’s
disease
K
K
K
K
K
Alzheimer’s
disease
K
K
K
K
K
K
Amyotrophic
lateral
sclerosis
K
K
K
Multiple
sclerosis
K
K
K
Adaptive immune response
Microgliosis
Activation of cleaved caspase-8 and caspase-3
in reactive microglia
Release of matrix metalloproteinase-3, a-synuclein
and neuromelanin
Increased proinflammatory cytokines and
ROS-generating enzymes: TNF-a, IL-1b, IL-2, 4, 6, iNOS,
COX-2, NADPH oxidase
Upregulation of CD14, TLR2, TLR5
References
4,5,8,10,13,14,
18,21,103–105
K
Cytotoxic T-lymphocytes in the
parkinsonian brain
Suspected FasL-dependent mechanism
to trigger cell death of dopaminergic
neurons in the substantia nigra
Microgliosis
Activation of cleaved caspase-8 and caspase-3
in reactive microglia
Release of proinflammatory toxic Ab peptides
Increased proinflammatory cytokines and
ROS-generating enzymes: IL-1b, IL-6, iNOS, IL-18,
prostanglandins, COX-2, TNF-a
Increased complement components, chemokines
and TGF-b
Upregulation of CD14, TLR2 and TLR4 microglial
receptors
K
Uncertain
8,13–18,104,
106–108
Microgliosis
Increased proinflammatory cytokines IL-6, IL-1b
and TNF-a elevated in the cerebrospinal fluid
and/or spinal cord
Upregulation of COX-2 in spinal cord
K
T-lymphocytes at sites of lesion
109–112
Microgliosis
Presence of macrophages digesting myelin
at the lesion edge
Immunoglobulin and complement deposits found
in 50% of cases
K
Lymphocytes and activated myeloid
cells at sites of lesion
113–115
K
Abbreviations: Ab, b-amyloid; COX, cyclooxygenase; IL, interleukin; iNOS, inducible nitric oxide synthase; ROS, radical oxygen species; TGF, transforming growth
factor; TLR, toll-like receptor; TNF, tumor necrosis factor.
acute inflammation and chronic inflammation, and their
effects on neurons. It is widely accepted that sustained
inflammation is deleterious to neurons, a typical feature of
chronic neurodegenerative diseases including Parkinson’s
disease (PD) and Alzheimer’s disease (AD).
Microglia Turn Bad: Role for Microglia Activation
in Neurodegenerative Diseases
Activated microglia have key roles in neuroinflammation, and
depending on the nature of the initial stimulus their actions
may be either beneficial or detrimental to neuronal function. In
Cell Death and Differentiation
chronic neurodegenerative diseases, microglia ‘turn bad’ and
remain activated for an extended period during which the
production of mediators is sustained longer than usual. This
increase in mediators contributes to neuronal death. Activated
microglia have been increasingly linked to various neurodegenerative diseases (Table 1). We recently described their
involvement in the two most common forms of neurodegenerative disease, PD and AD.
Parkinson’s disease. Classical reports state that PD is
characterized by a clinical syndrome of hypokinesia, rigidity
Killer caspases activate microglia
JL Venero et al
3
and tremor. Most probably the motor dysfunction can be
largely attributed to the progressive degeneration of
dopaminergic (DA) neurons in the substantia nigra (SN).2
Relatively rare familial forms of PD are associated with
mutations in genes encoding several proteins, including
a-synuclein, parkin and ubiquitin C-terminal hydrolase-L1.3
However, the etiology of idiopathic PD, which accounts for
more than 90% of PD, is still not understood.2 The first
indications for a role of inflammation in the pathogenesis
of PD came from studies demonstrating the presence of
cytotoxic T-lymphocytes and reactive microglia in the
parkinsonian brain.4,5 Studies on brains from humans4 who
had self-administered 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and monkeys6 receiving the same toxin,
revealed a striking microglial activation along with significant
accumulation of extraneuronal neuromelanin, which can
activate microglia. These findings established that an acute
neurotoxic insult to the nigrostriatal system can result in longterm inflammatory response involving a reactive microgliosis
and possibly leading to a self-perpetuating process of
neurodegeneration.4 Several lines of evidence suggest
that reactive microgliosis is deleterious to the DA neurons
in the SN. Thus, a single intra-nigral injection of
lipopolysaccharide (LPS), the main component of the cell
walls of Gram-negative bacteria and a potent inductor of
inflammation, induces an acute inflammatory response with a
strong macrophage/microglial reaction. This leads to the
progressive death of DA neurons in the SN.7 Matrix
metalloproteinase-3 (MMP3), a-synuclein and neuromelanin
are released by damaged DA neurons, leading to the
production of reactive oxygen species (ROS) by overactivated microglia.8 Mice lacking the protein parkin, which
is encoded by a gene associated with autosomal recessive
young onset PD, show more severe nigral cell loss following
systemic LPS treatment than controls, thus providing a link
between a certain inherited form of PD with neuroinflammation.9 Regardless of the nature of the proinflammatory
stimulus
(proinflammatory
toxin,
neuronal
death,
extracellular neuromelanin or a-synuclein), the microglial
response to neuron damage can lead to further loss of
neurons over time. The presence of T-lymphocytes in the
midbrain of PD patients supports a role of peripheral
inflammation in the etiopathology of PD.5 Other studies
have demonstrated that (i) T-lymphocytes mediate nigral DA
neurotoxicity through an FasL-dependent mechanism10 and
(ii) peripheral inflammation induced by a single systemic LPS
injection triggers persistent microglia activation in different
brain areas, including the SN, and that DA neurons
progressively die later.11
Alzheimer’s disease. The primary clinical presentation of
AD is progressive cognitive decline, with memory loss being
an early sign.12 The AD brain shows prominent atrophic
changes in the hippocampal, frontal, parietal and temporal
areas as a consequence of extensive synaptic degeneration and
neuronal death.12 Furthermore, the AD brain is characterized by
extracellular accumulation of b-amyloid (Ab) aggregates
(amyloid plaques) and the appearance of intracellular
aggregates of tau, which are called neurofibrillary tangles.12
Altered cholinergic and glutamatergic neurotransmission,
apoptosis, oxidative stress, disrupted calcium homeostasis
and neuroinflammation have all been implicated in AD pathogenesis.12 We will highlight findings of neuroinflammation. In the
AD brain, microglia, astrocytes and neurons release various
neuroinflammatory mediators, including complement activators
and inhibitors, chemokines, cytokines and ROS.13–15
Historically, inflammation was thought to be secondary to
degeneration. However, compelling evidence suggests that
inflammatory mediators might have a significant role in the
pathogenesis of the disease. For example, Ab metabolism and
over-activation of microglia are thought to interact and trigger a
vicious cycle leading to neuronal death. Thus, toxic Ab peptides
are generated by the sequential action of two proteases denoted
as b-secretase and g-secretase, which cleave the amyloid
precursor protein. Ab is toxic to neurons, which in turn activates
microglia when they degenerate.15 The activated microglia, in
turn, are deleterious to neurons.8 In addition, toxic Ab peptides
show proinflammatory properties.16 The most prominent
activation of microglia in AD brain takes place around Ab
deposits.17 The stimulated microglia release a wide variety of
proinflammatory mediators, including cytokines, complement
factors, free radicals, nitric oxide (NO), chemokines (interleukin-18
(IL-18)) and prostanglandins, all of which potentially
contribute to further neuronal dysfunction and death.13,18
Activated microglia might also stimulate astrocytes that
enhance the inflammatory response. These cellular
interactions fuel a self-propelling cycle that might drive
progressive neurodegeneration in AD.15
Health cost of neurodegenerative disorders associated
with a microglia activation component. Neurodegenerative disorders have a tragic impact for those afflicted and their
families. In addition, their socioeconomic impact is
enormous, when both direct and indirect costs are
considered (Table 2). For both AD and PD the number of
affected people is predicted to increase dramatically because
of the increasing average age of the population. In 2010, 35.6
million people in the world had dementia (around 60% of
Table 2 Health cost of some neurodegenerative disorders
Year/area
Dementia (60% related to Alzheimer’s disease)
Multiple sclerosis
Amyotrophic lateral sclerosis
Parkinson’s disease
K
K
K
K
2010/worldwide
2005/Europe
2010/Germany
2007/USA
Health costs
K
K
K
K
604 billion US $
12.5 billion h
36 380 h/patient
10.8 billion US $
References
Annual World Alzheimer Report116
117
118
119
Direct medical costs as well as the indirect costs (i.e. lost working days for the patients and caregivers) are depicted.
Cell Death and Differentiation
Killer caspases activate microglia
JL Venero et al
4
Figure 1 Overview of the TLR-dependent and JAK/STAT-dependent signaling pathways in microglia. TLRs 1, 2, 5, 6, 7, 8 and 9 trigger the classical myeloid differentiation
primary response gene (88) (MyD88)-dependent signaling pathway. TLR3 triggers an alternative MyD88-independent, TIR-domain-containing, adapter-inducing IFN-b
(TRIF)-dependent pathway through the TRIF. TLR4 triggers both the MyD88-dependent pathway through TIR domain-containing adaptor protein (TIRAP)–MyD88 interaction
and the MyD88-independent pathway through TRIF-related adapter molecule (TRAM)–TRIF interaction. The MyD88-dependent pathway results in the activation of NF-kB,
mitogen-activated protein kinase (MAPK) or IRF7 downstream signaling pathways (the latest upon TLR9 activation) through the IL receptor-associated kinase (IRAK)
complex (which includes four subunits: two kinases, IRAK-1 and 4, and two non-catalytic units, IRAK-2 and M) and the TNF receptor-associated factor-6 (TRAF6). The MyD88independent pathway results in the activation of IRF3 through the use of two kinases (IkB kinase-e (IKKe) and TANK-binding kinase-1 (TBK1)). The TRAF family
member-associated NF-kB activator (TANK) interacts with NF-kB essential modulator (NEMO), TBK1 and IKKb, and may therefore bridge the MyD88-dependent and
MyD88-independent pathways. The ligation of the IFN-g receptor (IFNGR) with IFN-g leads to the activation of Jak1and Jak2. STAT1 is phosphorylated by Jak kinases
and translocates to the nucleus. Socs counteract STAT1 activation
which will be due to AD). It is estimated that this number will
increase to 65.7 million by 2030 and 115.4 million by 2050.
Microglia Plead Guilty to Neuron Death: Triggers of
Microglia-Mediated Neurotoxicity
Microglia are guilty of causing neuron death. The underlying
molecular mechanisms of neurotoxicity are clearly multiple
(Figure 1). In the present section we review each one of them
briefly.
Proinflammatory cytokines. Immunomodulators referred
to as cytokines are particularly important in the genesis of
inflammation. Proinflammatory, immunomodulatory and antiinflammatory cytokines include IL-1b, tumor necrosis factor-a
(TNF-a), IL-6, IL-8, IL-12, IL-15 and IL-10.19 The main
proinflammatory cytokines are IL-1b and TNF-a. Infusions of
LPS into the CNS activate Toll-like receptors (TLRs) on glial
cells and induce the activation of microglia.20 Through this
process, LPS stimulates the expression of multiple cytokines,
including IL-1, IL-6 and IL-12; cyclooxygenase-2 (COX-2)
Cell Death and Differentiation
and TNF-a.21 Release of these cytokines creates an
environment, which promotes neuronal cell death.7
Cytokines including IL-1b, interferon-g (IFN-g) and TNF-a
are elevated in postmortem PD brains.22 A recent metaanalysis study reported significantly higher concentrations of
the proinflammatory cytokines IL-6, TNF-a, IL-1b, IL-12 and
IL-18 in the peripheral blood of AD subjects as compared
with controls.23 Therefore, these molecules might be actively
involved in disease progression in both PD and AD.
Oxidative stress. Oxidative stress generated by reactive
microglia is supposed to be the most critical factor in inducing
the death of neuronal populations.13 It is highly relevant for
the pathogenesis of PD as nigral dopamine neurons are
highly vulnerable to oxidative stress.2 Activated microglia
upregulate different enzymes involved in the inflammatory
processes mediated by oxidative stress, including inducible
NO synthase (iNOS), NADPH oxidase, COX-2 and
myeloperoxidase.8 Direct evidence supporting a neurotoxic
role of microglial NADPH activity and NOS-derived NO
is supported by studies using intra-parenchymal LPS
Killer caspases activate microglia
JL Venero et al
5
injections. Thus, NADPH-deficient mice24 or pharmacological
inhibition of NOS confers protection in the LPS model.25
Proteases. MMPs are primarily produced and secreted
by inflammatory cells. In particular MMP9 is known to
significantly digest the basement membrane and negatively
impact on barrier integrity.26 MMP9 exerts also direct
neurotoxicity.26 MMPs are thought to have deleterious roles
in different brain diseases, including stroke, multiple sclerosis
(MS), infection, AD and PD.26
Environmental factors. Different environmental factors,
including toxins, and pollutants, may influence inflammatory
responses that contribute to the etiology of neurodegenerative
diseases. Several lines of evidence indicate that environment is
a source for compounds that are both directly toxic to neurons
and deleterious through direct stimulation of microglia
(reviewed by Glass et al.14).
Neuronal damage. Microglial activation in response to
neurodegeneration or neuron injury was initially perceived
as a transient event.27 However, increasing evidences
indicate that microglia can become chronically activated in
response to dying/damaged neurons, causing a selfpropelling cycle of neuron death, which is a proposed
mechanism of chronic neuronal loss across diverse
neurodegenerative diseases.6,13 Thus, damaged DA
neurons release several factors that seem to activate
microglia, such as MMP3, a-synuclein, neuromelanin and
m-calpain.8
Other factors. Traumatic brain injury activates both
microglia and astrocytes, and could potentially induce selfsustaining inflammatory responses in the brain.34
Accumulating evidence implicates traumatic brain injury as
a possible factor predisposing to AD.34 Activation of microglia
cells upon systemic infection might also be involved in the
early stages of AD pathogenesis.35 Recently, a strong
correlation between type-2 diabetes and AD was identified,
with hyper-insulinemia increasing the risk of AD.14 Different
mechanisms may explain this correlation. Type-2 diabetes is
mostly associated with obesity, which courses with a lowgrade but chronic form of inflammation in adipose tissue, liver
and other organs. Recent evidence suggests that systemic
inflammation contributes to the exacerbation of acute
symptoms of chronic neurodegenerative diseases, including
AD and PD, and may accelerate disease progression,35 a
plausible rationale to explain the correlation between type-2
diabetes and AD.
Once triggered by the factors described above, several
intracellular signaling pathways contribute to microglial
activation and can be said to sign the ‘contract’ for the killers
(Figure 2).
Protein aggregates. Protein aggregates have been found
to induce neuroinflammation in AD and PD. In AD, in vitro
experimental evidence indicates that protofibrils and
oligomers of Ab1–40 and Ab1–42 activate microglia by
binding to different receptors such as advanced glycation
end products (RAGEs),28 CD14, TLR2 and TLR4,29 and
through heterodimer formation of TLR4 and TLR6, a process
dependent on the binding of Ab to the CD36 scavenger
receptor.30 Further evidence demonstrating a link between
Ab and neuroinflammation comes from studies performed in
transgenic animals showing that cerebral amyloid deposition
is increased under inflammatory conditions.15 As mentioned
above, a-synuclein appears to have a pivotal role in the
pathogenesis of PD. For example, a-synuclein is the major
protein component of Lewy bodies.31 Early observations
showing that a-synuclein in the SN are often surrounded by
activated microglia suggested that the aggregates activated
microglia.5 In a transgenic mouse model expressing mutant
human a-synuclein, proinflammatory molecules are
increased and microglia are activated before SN neurons
begin to die.32 Interestingly neuroinflammation and protein
aggregation may be part of the same self-perpetuating
vicious cycle, because intra-nigral injections of LPS induce
the formation of intra-neuronal a-synuclein aggregates in
mice overexpressing a-synuclein.33 Aggregated a-synuclein
activated microglia, leading to enhanced DA neurotoxicity.
Microglial phagocytosis of a-synuclein and activation of
NADPH oxidase appeared to be pivotal to the process.
Pattern-recognition receptors. The first paper that
implicated TLRs in host immunity was by Lemaitre et al. in
1996. They observed that Toll-deficient Drosophila are
vulnerable to certain fungal infections.36 TLRs are a family
of pattern-recognition receptors in the innate immune
system. They are transmembrane proteins, each with
an ectodomain with leucine-rich repeats (LRRs) and a
cytoplasmic Toll–IL-1 receptor (TIR) motif homologous
to that of IL-1 receptor (IL-1R).37 There are 10 functional
TLRs (from TLR1 to TLR10) identified in humans and 11
(from TLR1 to TLR7, TLR9 and from TLR11 to TLR13) in
mice.37 Microglia express TLRs 1–9.38 TLRs can bind to
highly conserved structural motifs, so called pathogenassociated molecular patterns (PAMPs),39 as well as to
several host-derived ligands, for instance, heat-shock
proteins (HSP), mRNA, high-mobility group box-1 protein,
surfactant proteins A and D, hyaluronan and fibrinogen).39 All
these host-derived ligands can be released during tissue
damage in the CNS and they are collectively named dangerassociated molecular patterns or DAMPS. Thereby tissue
damage due to CNS disease can boost the inflammatory
response by activating TLRs, and consequently might further
promote cell death. Upon ligand binding, TLRs dimerize and
undergo conformational changes that induce a complex
cascade of intracellular signaling events, ultimately resulting
in the activation of the transcription factors NF-kB (nuclear
factor k-light-chain enhancer of activated B cells), activator
protein-1 (AP-1), and IFN-regulatory factor-3 (IRF3) and
Contract the Killers: Possible Underlying Mechanisms
for Microglia Activation
Microglia are activated in response to neuronal damage,
protein aggregate formation and several environmental
stimuli/toxins.
Cell Death and Differentiation
Killer caspases activate microglia
JL Venero et al
6
Figure 2 Mechanisms of cell death in AD and PD involving inflammation. A degenerating neuron shows key neuropathological features of PD (left side of the neuron) and
AD (right side of the neuron). Unfolded a-synuclein and proteasomal dysfunction induce the formation of oligomers of a-synuclein, the main component of Lewy bodies, which
are the most distinctive histopathological feature of PD. In AD, sequential action of b-secretase and g-secretase gives rise to the formation of toxic Ab (mainly Ab140 and
Ab142) from the amyloid-b precursor protein (APP). These peptides are proinflammatory and their extraneuronal accumulation form amyloid plaques, a typical
histopathological feature of AD. The presence of oxidative stress in PD and other factors in AD commit neurons to die. Degenerating DA neurons in PD release different
proinflammatory factors, including a-synuclein and neuromelanin. All these factors are recognized by different pattern-recognition receptors, including TLRs 2 and 4, CD14,
CD36 and RAGE. Binding to these receptors induces the activation of transcription factors such as NF-kB and AP-1 (not shown) leading to microglia activation. Nurr-1 is a
repressor of NF-kB, whereas active caspase-3 is an activator. Activation of microglia release different proinflammatory cytokines and activate ROS-producing enzymes such
as iNOS, NADPH oxidase and myeloperoxidase (not shown). NADPH oxidase is a major source of extracellular ROS in response to diverse stimuli. It is a membrane-bound
enzyme that catalyzes the production of superoxide anion (O
2 ) from oxygen and it is strongly induced in response to different proinflammatory stimuli. O2 easily reacts with
NO (mainly derived for upregulation of iNOS by reactive microglia) to produce peroxynitrite, the most reactive free radical, thus inducing nitrosative stress. Peroxynitrite has
the potential to both initiate and sustain an autotoxic loop considered as a neuronal damaging mechanism in neurodegenerative diseases. Over-activation of microglia is
deleterious to neurons, thus enhancing neuronal cell death, which in turn release more proinflammatory factors thus establishing a self-perpetuating process
of neuroinflammation and neurodegeneration
IRF7. These factors, in turn, regulate the expression of a
wide array of genes involved in inflammatory responses.
MyD88-dependent and -independent pathways. Activation of most TLRs mostly results in the recruitment of the
adaptor protein myeloid differentiation factor-88 (MyD88)
(see Figure 2 for further details), finally leading to the nuclear
translocation of NF-kB.39 In addition, there is an MyD88independent pathway.39
The Jak–STAT signaling pathway. Type-I and II cytokine
receptors are a conserved family of B40 members that
includes the receptors for ILs, IFNs and hormones.40 These
receptors are associated with a cytoplasmic kinase. These
cytoplasmic kinases comprise the four members of the Janus
kinase (Jak) family: Jak1, Jak2 and tyrosine kinase-2 (Tyk2)
transduce signaling from several receptors, whereas Jak3
does so only for one receptor, the common g-chain, or gc.
Cell Death and Differentiation
Upon cytokine binding, the receptor-associated Jaks are
activated and in turn phosphorylate the tyrosine residues in
the receptor cytoplasmic domain. This event provides a
docking site for proteins with Src homology-2 domains, one
important class of which is the signal transducer and
activator of transcription (Stat) family of transcription
factors. With seven members in all (Stat1, Stat2, Stat3,
Stat4, Stat5a, Stat5b and Stat6), these DNA-binding proteins
provide a rapid membrane-to-nucleus pathway for regulation
of gene expression.41 After cytokine stimulation, a family of
cytokine-induced inhibitors termed suppressors of cytokine
signaling (Socs proteins) is rapidly induced. The predominant
function of the Socs proteins is to block the generation of
the Stat signal from a cytokine receptor.42 In canonical
IFN-g–Jak–STAT1 signaling (reviewed by Stark43) the
binding of IFN-g to its receptor leads to the activation of
receptor-associated Jak1 and Jak2, and the phosphorylation
of a receptor tyrosine residue working as a docking site for
Killer caspases activate microglia
JL Venero et al
7
STAT1. STAT1 is then activated by phosphorylation and
translocates to the nucleus where it stimulates transcription
of STAT1 target genes.
Another mechanism for IFN-g to activate macrophages is by
enhancing macrophage responsiveness to other inflammatory stimuli, such as TLR ligands and TNF. This mechanism is
known by the name ‘priming’. By this ‘priming’ of IFN-g,
the TLR-dependant expression of several proinflammatory
cytokines and chemokines is greatly augmented.44 IFN-g
suppresses IL-10 production by increasing the activity of
glycogen synthase kinase-3b (GSK3b), a serine/threonine
kinase that inhibits the function of AP-1 and CREB, two
transcription factors critical for IL-10 expression.
NF-jB: Stress sensor/inflammatory trigger. The NF-kB
signaling pathway has a major role in the development,
maintenance and progression of most chronic diseases.
NF-kB controls the expression of genes involved in immune
inflammatory
responses,
acute-phase
inflammatory
responses, oxidative stress responses, cell adhesion,
differentiation and apoptosis.45 Ranjan Sen and David
Baltimore first identified NF-kB in 1986. They found that it
was bound to an enhancer element of the immunoglobulin
k-light chain gene in the nucleus of B cells.46 It is conserved
during evolution and is ubiquitous in nature, that is, present in
all cell type. It belongs to the family of Rel proteins that
includes c-Rel, RelA (p65), RelB, NF-kB1 and NF-kB2, all of
which can form hetero- or homodimers.46 NF-kB proteins are
present in the cytoplasm in an inactive form together with
inhibitory IkB (IkBa, IkBb and IkBe).46 NF-kB signaling is
divided (depending on which proteins are involved) in the
canonical (classical) pathway initiated by NF-kB1 and a noncanonical (alternative) pathway initiated by NF-kB2.
Whereas the classical pathway depends on the IKK complex
consisting of IKKa, IKKb, IKKg and the inhibitory subunit IkBs, the
alternative pathway depends on IKKa homodimers and the
NF-kB-inducing kinase (NIK).47 Once in the nucleus, activated
NF-kB undergoes a series of post-translational modifications,
including phosphorylation, acetylation and methylation. These
modifications regulate both the strength and the duration of
NF-kB activity.48 Activated NF-kB binds to specific DNA
sequences in target genes, which are designated as kB
elements, and regulates the transcription of over 500 genes
involved, in, for example, immunoregulation.
Astrocytes as Mediators of Inflammation
Astrocytes, the most abundant cells in the brain, besides
having functions in metabolic support, blood–brain barrier
integrity, K þ buffering and regulation of synaptic levels of
glutamate, have the ability to work as immunocompetent cells
in the CNS.49 Thus, astrocytes express major histocompatibility complex-II (MHC-II) and present antigen to T-cell
lines.50 In addition, astrocytes express different patternrecognition receptors, including TLRs, and produce a wide
array of chemokines and cytokines that act as immune
mediators.50 More important, selective inactivation of NF-kB
in astrocytes reduced inflammation and cell death in different
animal models of neurodegeneration, including spinal cord
injury,51 and in an animal model of MS.52 Keeping this view, it
has been reported that a-synuclein proteins, whose accumulation is a typical hallmark of PD, released from neuronal cells
are readily endocytosed by astrocytes and trigger immune
responses.53 When the internalized a-synuclein accumulates
in astrocytes, the cells produce glial inclusions and inflammatory responses.53 In AD, astroglial activation is primarily
triggered by amyloid deposits in the extracellular space and
AGE-modified proteins thus inducing the production of a
variety of proinflammatory factors to be released from
astrocytes.49 Several cytokines, including IL-1b and IL-6,
have been implicated in the induction and modulation of
reactive astrogliosis and pathological inflammatory
responses,50 a demonstration that a crosstalk between
microglia and astrocytes activation seems to contribute to
the inflammatory responses in the brain. Gage et al.54 have
demonstrated in primary cell cultures that LPS-induced
expression of factors such as IL-1b and TNF-a by microglia
results in a paracrine activation of astrocytes, with the
subsequent production of toxic mediators by astrocytes,
including NO and ROS. These factors are suggested to act
additively or synergistically with neurotoxic factors produced
by microglia. The detailed molecular mechanisms involved in
this crosstalk are emerging and some of the first important
signaling pathways (e.g. nuclear receptor related-1 protein-1
(Nurr1)/REST co-repressor (CoREST54)) were described
recently. Consequently, under pathological conditions, such
as those seen in chronic inflammation and neurodegeneration, it has been proposed that astroglia switch from metabolic
support cells to immunological cells capable of inducing
inflammation through the production of a variety of proinflammatory factors.49
Caspases, the Usual Suspects: Emerging Non-Apoptotic
Role for Killer Caspases
Proinflammatory caspase-1 and the inflammasomes.
Caspases, a family of cysteine proteases, were discovered
almost two decades ago. In 1992, two laboratories
simultaneously identified IL-1b-converting enzyme (ICE),
which is involved in the generation of active IL-1b. It was
later referred to as caspase-1 being the first mammalian
caspase characterized.55,56 Caspase-1 together with
caspase-11 and caspase-12 in mouse. and caspase-4 and
caspase-5 in human, comprises the proinflammatory
caspases.57 Caspase-1 catalytic activity is tightly regulated
by signal-dependent auto-activation within multi-protein
complexes called ‘inflammasomes’ that mediate caspase-1
autocatalytic activation and the subsequent cleavage of the
inactive precursors of IL-1b and IL-18 into bioactive
cytokines.58 For a review of the different inflammasomes,
which are, NLR family pyrin domain containing-1 (NLRP1),
NLRP3, IPAF and AIM2 (absent in melanoma-2)
inflammasomes, see reference Schroder and Tschopp.58
Importantly, strong associations between dysregulated
inflammasome activity, by extension caspase-1 activity,
and human inflammatory diseases highlight the importance
of this signaling pathway in tailoring the innate immune
response. Indeed, the significance of the inflammasome for
the initiation of the inflammatory response during systemic
Cell Death and Differentiation
Killer caspases activate microglia
JL Venero et al
8
diseases has already been shown, and members of the
inflammasome complex were found to be induced in acute
brain injury. However, the specific pathophysiological role of
the inflammasome in neurodegenerative disorders still
remains to be clarified (for a review see reference
Trendelenburg59). Interestingly, recently it was found that
Ab fibrils can activate NALP3 inflammasomes through the
lysosomal damage in mouse microglia60 and proposed to
contribute to AD pathology.61
Apoptotic caspases executioners of cell death. To date,
the mammalian genome encodes 14 caspases, seven of
which function in programmed cell death/apoptosis and will
henceforth be termed apoptotic caspases, or simply killer
caspases. During apoptosis, activated killer caspases cleave
selected target proteins to execute cell death.62 The
apoptotic caspases comprise two distinct classes: The
upstream initiator caspases, which include caspases 2, 8, 9
and 10, and the downstream effecter caspases, which
include caspases-3, 6 and 7. Effecter caspases cleave a
wide range of distinct proteins substrates in different cellular
compartments and are responsible for many of the changes
typical of apoptosis. They are expressed as zymogens, and
are activated when cleaved by the initiator caspases.
Initiators caspases are also expressed as zymogens but
are activated by recruitment into large multi-protein
complexes. Well-defined apoptotic caspase activating
complexes include the apoptosome (activating caspase-9),
the piddosome (activating caspase-2) and the death-inducing
signaling complex (DISC, activating caspases 8 and 10).63
Two major caspase-dependent pathways of apoptosis
signaling have been described, which trigger cell death
either by activation of death receptors at the cell surface (i.e.
extrinsic pathway) or through the disruption of the
outer mitochondrial membrane barrier function, with
the simultaneous release of proapoptotic molecules from
the mitochondria into the cytosol (i.e., intrinsic pathway).64
Non-apoptotic roles for killer caspases. Even if killer
caspases can been seen as ‘the usual suspect’ in the death
of cells, the opinion that the so-called ‘apoptotic caspases’
are more than just killers is supported by several recent
studies (Table 3). Miossec et al.,65 as early as 1997, reported
that T-lymphocytes upon stimulation acquire caspase-3
activation without evidence of ongoing apoptosis. Shortly
thereafter, inhibition of caspase activation was reported to
prevent T-cell proliferation.66 Finally, in vivo observations
corroborated these initial findings by convincingly proving
evidence that the initiator caspase-8 is indispensable for
T-cell activation and proliferation.67 Admittedly, anucleate
cell types, namely keratinocytes,68 lens fiber cells69 and
erythroid cells,70 are known to require effecter caspases
such as caspase-3 for the removal of their nucleus, as part of
their terminal differentiation. Likewise, nucleate cells,
including skeletal muscle cells,71 osteoblasts,72 B cells73
and myelomonocytic cells,74 are known to employ the
effecter caspase 3 or 6 for their differentiation.
Furthermore, initiator caspases such as caspase-8 and
caspase-9 are not always involved in cell death signaling.
For example, activation of the apical caspase-9, during
myoblastic differentiation, in turn leads to caspase-3 activation,
which contributes to myotube formation.75 Substantial evidence
has been built up regarding the non-apoptotic functions of
caspase-8, involving embryonic development;74 NF-kB
activation;76 cell migration;77 proliferation of T cells, B cells,
natural killer cells76 and hematopoietic progenitor cells;78 and
myeloid or lymphoid differentiation patterns.74
In the brain, which is the focus of this review, activation of
killer caspases can occur in various cell types as part of
multiple non-apoptotic, essential cell functions. Active
caspase-3 has been found in the non-apoptotic, proliferating
and differentiating neuronal cells of the forebrain ventricular
zone and the external granular layer of the developing
cerebellar cortex.79 In neurons, caspases are present in the
dendrites, axons and pre- and postsynaptic terminals, and
there is evidence that caspases can be activated in dendrites,
synaptosomes and growth cones, without inducing cell death.
Several studies have implicated apoptotic caspases in the
regulation of synaptic plasticity. Long-term synaptic depression and AMPA (a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor internalization in mouse
hippocampal neurons requires caspase-9 and caspase-3/7
Table 3 Non-apoptotic roles of apoptotic caspases
Caspases
Non-apoptotic function
References
Caspase-3
K
K
K
K
K
Upon stimulation, T-lymphocytes activate caspase-3 without evidence of apoptosis
Removal of nucleus in keratinocytes, lens fiber cells and erythroid cells
Differentiation of skeletal muscle cells, osteoblasts, B cells, myelomonocytic cells and Bergmann glia cells
Long-term synaptic depression and AMPA receptor internalization in hippocampal neuron
Cytoskeletal remodeling associated with astrogliosis
65
68–70
71–75,82
80,81
83
Caspase-8
K
K
K
K
K
K
K
Embryonic development
T-cell activation
Proliferation of T cells, B cells, natural killer and hemopoietic progenitor cells
NF-kB activation
Cell migration
Differentiation of myeloid and lymphoid cells
Microglia activation
74
67,120
67,76,78,120
76
77
74
21
Caspase-9
K
K
Differentiation of skeletal muscle cells
Long-term synaptic depression and AMPA receptor internalization in hippocampal neuron
75
80
Abbreviations: AMPA receptor, a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor; NF-kB, nuclear factor k-light-chain enhancer of activated B cells.
Cell Death and Differentiation
Killer caspases activate microglia
JL Venero et al
9
Figure 3 Apoptotic caspases control microglia activation. Activation of TLRs with lipoteichoic acid (LTA, TLR2 agonist), PamC3sk4 (synthetic lipopeptide TLR1/2 agonist)
or LPS (TLR4 agonist) leads to the orderly activation of caspase-8 and caspase-3. Caspase-3 activates the NF-kB pathways through processing and activation of PKCd.
Nuclear accumulation of NF-kB leads to the transcriptional activation of inflammatory gene expression
activity.80 Caspase-3 cleaves and activates calcineurin, which
in turn triggers the dephosphorylation and removal of the
GluR1 subunit of the AMPA-type receptor from postsynaptic
sites. This caspase-3-dependent mechanism is suggested
to promote early synaptic dysfunction in a mouse model of
AD at the onset of memory decline.81 Activation of caspase-3
in the postnatal cerebellum has also been linked to Bergmann
glia differentiation.82 In astroglia, activation of this enzyme,
in response to excitotoxic brain damage, is suggested to
have role in the cytoskeletal remodeling associated with
astrogliosis.83
License to Kill: Caspases Control Microglia Activation
We recently described an unexpected novel function for
caspases in the control of microglia activation and thereby
neurotoxicity21 (Figure 3). We showed that orderly activation
of caspase-8 and caspase-3 regulates microglia activation
through a protein kinase-Cd (PKCd)-dependent pathway.
Activation of microglia and inflammation-mediated neurotoxicity are believed to have important roles in the pathogenesis of
several neurodegenerative disorders. In our study, we found
that stimulation of microglia with various inflammogens
activates caspase-8 and caspase-3/7 in absence of cell death
in vitro and in vivo. Knockdown or chemical inhibition of these
caspases hindered microglia activation and importantly
reduced neurotoxicity. We also observed that these caspases
are activated in microglia in the SN in PD subjects and in the
frontal cortex of AD subjects. This novel view of the molecular
mechanisms underlying the activation of microglia opens up
avenues for new pharmacological interventions aimed at
mitigating brain inflammation and subsequent cell death.
Such novel strategies might lead to new treatments for
neurological disorders where neuroinflammation has a role,
such AD, PD, amyotrophic lateral sclerosis (ALS) and MS.
Under Arrest: Microglial Caspases are Potential Targets
for Future Neuroprotective Strategies
Neuronal cell death is a major feature of many chronic
neurodegenerative diseases, such as AD, PD, ALS and
Huntington’s disease (HD), as well as acute damage following
Cell Death and Differentiation
Killer caspases activate microglia
JL Venero et al
10
Box 2
OPEN QUESTIONS:
K It is unclear whether inhibition of caspase activation specifically in
microglia contributes to the neuroprotective effects of caspase
inhibitors. Could caspase inhibition be neuroprotective by
targeting the microglia rather than the neurons themselves?
K
It is also unclear whether the newly described caspasedependent signaling pathway controlling microglia activation
could also contribute to the beneficial effects of activated
microglia.
K
Could the selective inhibition of certain caspases in active
microglia be a possible therapy to decrease the demise under
several neuroinflammatory conditions such as Parkinson’s and
Alzheimer’s diseases?
stroke, spinal cord injury and brain trauma. While effective
neuroprotective therapies are still lacking, the most common
therapeutic strategies for those brain disorders aim at
rescuing the neurons. As caspase activation is considered a
feature of apoptosis, synthetic caspase inhibitors have been
developed both as research tools, and with the hope that they
may eventually be used to prevent cell death in the clinic.
Caspase inhibitors have been reported to successfully exert
neuroprotective effects in a number of animal models for
neurodegenerative disorders, including AD,84 HD,85 PD,86
ALS,87 as well as acute neurologic diseases, including
ischemia, stroke or traumatic injury.88,89 However, one should
also keep in mind these neurodegenerative diseases are also
characterized by reactive microgliosis,8,90 and that our recent
study now links apoptotic caspases to the control of microglia
activation.21 Presently, it is unclear whether inhibition of
caspase activation specifically in microglia contributes to the
neuroprotective effects of caspase inhibitors (Box 2, OPEN
QUESTIONS). Therapies that will effectively target microglia
activation and thereby their neurotoxicity could significantly
improve the therapeutic outcome of these brain diseases. In
conclusion, we propose that interfering with caspases that
contribute to the activation of microglial can reduce neuronal
cells loss.
Current
caspase
inhibitors,
opportunities
and
limitations. The next question is which caspase inhibitors
are available and might be suitable to target and control
microglia activation? We recently reported that intra-nigral
administration of fluoromethyl ketone (fmk)-derivative
tetrapeptide caspase inhibitors – DEVD-fmk targeting
caspase-3/7 or IETD-fmk targeting caspase-8 – hindered
in vivo microglia activation.21 The required administration
mode for these caspase inhibitors certainly limits their use in
a clinical setting. Furthermore, fmk metabolizes in the liver to
form fluoroacetate, an extremely toxic poison.91
Quinolyl-valyl-O-methylaspartyl-[-2,6-difluorophenoxy]methylketone (Q-VD-OPh) is a newer, third-generation broadspectrum caspase inhibitor that has potential as a therapeutic
compound.92 Most importantly, Q-VD-OPh crosses the
blood–brain barrier and appears to be far superior to Z-VAD
in many respects, including greater potency, selectivity,
stability and cell permeability.92 After acute treatment of mice
with Q-VD-OPh, all organs were normal suggesting a lack of
toxicity.92 Evidence for neuroprotection included studies
demonstrating efficacy of Q-VD-OPh in animal models of
Cell Death and Differentiation
AD, PD, HD, spinal cord injury and stroke.84,88,93,94 It remains
to be established whether Q-VD-OPh affects microglia
activation in those disease models and contributes to the
neuroprotective effect in that manner.
Finally, minocycline is particularly interesting in the context
of this review. It is a second-generation tetracycline that
effectively crosses the blood–brain barrier and shows
remarkably broad neuroprotective properties. Indeed, neuroprotection by minocycline has been observed in various
animal models of PD, HD, ALS, MS, cerebral ischemia, spinal
cord injury and traumatic brain injury.85,95,96 The precise
mechanism of minocycline’s neuroprotective effects remains
unclear. It is known, however, that minocycline is a potent
inhibitor of caspase-related apoptotic pathways.95 Minocycline has been shown to block the activation and proliferation
of microglia in an animal model of global brain ischemia, PD
and AD.90,95 Minocycline not only prevents the activation of
microglia, but also the upregulation of caspase-1 and the
formation of mature IL-1b; the activation of NADPH-oxidase
and iNOS; and the release of NO metabolites, all of which are
key microglial-derived cytotoxic mediators.90 Thus, minocycline is a potent inhibitor of both caspase activation and
microglial activation. However, the link between both awaits
further studies. Minocycline shows high bioavailability after
oral administration, crosses the blood–brain barrier and has a
proven track record for safety in a number of clinical trials
involving humans. It is being evaluated in clinical trials in
patients with HD, PD, MS and ALS, and for spinal cord injury.
A phase-III trial involving minocycline for treatment of ALS
was completed recently. Unfortunately, the results of the trial
were negative as the patients on minocycline declined more
quickly than those on placebo.97 These results are disappointing and certainly will probably have negative implications for
potential future clinical trials with minocycline.98 In addition,
a phase-II trial for PD indicated a decreased tolerability
of minocycline in the treatment group.99 At last, the results of
a futility phase-II study on HD with minocycline indicated that
further trials with minocycline in HD are not warranted.100
Perspectives
We have summarized the key aspects of neuroinflammation
and its obvious involvement in the etiopathogenesis of
different neurodegenerative diseases. The discovery that
apoptotic/killer caspases regulate microglia activation and
neurotoxicity will open up avenues for new therapeutic
strategies, aimed at mitigating neuroinflammation in neurodegeneration. Brain inflammation is typically divided into three
phases (acute, chronic and resolution), in which the microglia
are largely in three different morphologies (surveying,
activated and amoeboid/phagocytic). Considering the presumed dual role of microglia (both beneficial and detrimental),
it will be necessary to fully characterize first the role of
apoptotic caspases in each activation phase of microglia.
Different animal models of neurodegeneration (acute versus
chronic inflammation) should help to shed further light on
this issue.
We speculate that inhibition of apoptotic/killer caspases
might be a viable approach to therapeutic intervention. As
mentioned earlier, however, an increasing number of studies
Killer caspases activate microglia
JL Venero et al
11
demonstrate that these proteases have important roles in a
multitude of physiological conditions. Thus, inhibiting
caspases might conceivably inhibit functions that are essential for normal neuronal activity and thereby would preclude
the use of such an approach. The disappointing results of
minocycline in clinical trials of different neurodegenerative
diseases might be taken to suggest that it is important to
develop inhibitors, which are selective for microglial caspases. Thus a major challenge will be to develop tools
allowing the inhibition of specific caspases in selective cell
types. Different strategies might be useful for this purpose: (i)
A first approach may be suppression of caspase by targeted
delivery of small interfering RNA specifically to macrophage/
microglial cells;101 and (ii) the development of specific nontoxic nanocarriers able to cross the blood–brain barrier and to
safety release caspase inhibitors in those areas showing high
microgliosis.102 These are great technical challenges, but the
new insights into the non-lethal roles of caspases in microglia
certainly support that it could be worthwhile and indicate that
the idea of inhibiting caspases as a therapy for neurodegenerative diseases is anything but dead.
Conflict of Interest
The authors declare no conflict of interest.
Acknowledgements. This work has been supported by grants from
Spanish Ministerio de Ciencia y Tecnologı́a (SAF2009-13778) and Proyecto de
Excelencia from Junta de Andalucia (CTS-6494); the Swedish Research Council;
the Parkinson Foundation of Sweden and the Swedish Cancer Society. MAB and
PB are members of the Strong Research Environment Multipark (Multidisciplinary
Research in Parkinson’s Disease at Lund University).
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