Regulation of microglial activation in stroke.

TLDR: Various regulators of microglia/macrophage activation and the interaction betweenmicroglia and neurons in the context of ischemic stroke are discussed.

Abstract: When ischemic stroke occurs, oxygen and energy depletion triggers a cascade of events, including inflammatory responses, glutamate excitotoxicity, oxidative stress, and apoptosis that result in a profound brain injury. The inflammatory response contributes to secondary neuronal damage, which exerts a substantial impact on both acute ischemic injury and the chronic recovery of the brain function. Microglia are the resident immune cells in the brain that constantly monitor brain microenvironment under normal conditions. Once ischemia occurs, microglia are activated to produce both detrimental and neuroprotective mediators, and the balance of the two counteracting mediators determines the fate of injured neurons. The activation of microglia is defined as either classic (M1) or alternative (M2): M1 microglia secrete pro-inflammatory cytokines (TNFα, IL-23, IL-1β, IL-12, etc) and exacerbate neuronal injury, whereas the M2 phenotype promotes anti-inflammatory responses that are reparative. It has important translational value to regulate M1/M2 microglial activation to minimize the detrimental effects and/or maximize the protective role. Here, we discuss various regulators of microglia/macrophage activation and the interaction between microglia and neurons in the context of ischemic stroke.

Full text

Acta Pharmacologica Sinica
(2017) 38: 445–458
© 2017 CPS and SIMM All rights reserved 1671-4083/17
www.nature.com/aps
Introduction
Ischemic stroke is a leading cause of morbidity and mortality
worldwide. Ischemic brain injury is initiated by energy deple-
tion and massive cytoplasmic Na
+
/Ca
2+
accumulation. As
a result, a cascade of molecular events occurs, including the
production of reactive oxygen species (ROS), the activation of
Caspases/Calpain, the inhibition of protein synthesis, mito-
chondrial dysfunction, cerebral edema formation and cellular
DNA fragmentation, which together lead to primary ischemic
damage. Almost immediately after the onset of brain isch-
emia, microglia are activated, and cytokines are extensively
produced, which leads to the migration of leukocytes toward
the injured brain. The normally immune-privileged brain
environment is consequently exposed to systemic responses
that further exacerbate the immune responses and cause sec-
ondary neuronal damage
[1]
. The activated microglia are highly
plastic cells and are divided into classic (M1) and alternative
(M2) activation phenotypes
[2, 3]
, and the polarization of the cells
is dependent on the different stages of disease development.
Emerging studies have focused on the regulatory mechanisms
that underlie the activation of microglia and have aimed to
suppress the M1 phenotype and to promote the M2 phenotype
to provide neuroprotection. Different molecular pathways
and mediators that regulate microglia/macrophage activation
after cerebral ischemic injury are reviewed below.
Resting microglia
Microglia, the resident immunocytes in the brain
[4]
, have a
myeloid origin
[5, 6]
. These cells constantly survey their environ-
ment with highly motile processes and are thought to be the
immediate sensors of the brain pathology
[7]
. Resting microglia
not only constantly extend and retract their thin ramied pro-
cesses to inspect the microenvironment
[8, 9]
but also remodel
neural circuits by forming synaptic communications with
adjacent neurons in healthy brains
[10, 11]
.
During the embryonic stage, there is a major wave of migra-
tion of primitive myeloid progenitors that enter into the
brain and later develop into resident microglia. Microglia
are considered to be long-lived cells that can replenish them-
selves in the brain
[12]
; however, the topic of microglial renewal
and proliferation remains controversial
[13]
. There are other
macrophage-related populations present in the leptomeninges
and in the choroids plexus; the functions of these populations
are relatively poorly known, but they were proposed to have
monitoring and scavenging functions, given their locations
[14]
.
Regulation of microglial activation in stroke
Shou-cai ZHAO
1
, Ling-song MA
1
, Zhao-hu CHU
1
, Heng XU
1
, Wen-qian WU
1
, Fudong LIU
2,
*
1
Department of Neurology, Wannan Medical College Yijishan Hospital, Wuhu 241001, China;
2
Department of Neurology, University of
Texas Health Science Center at Houston McGovern Medical School, Houston, TX 77030, USA
Abstract
When ischemic stroke occurs, oxygen and energy depletion triggers a cascade of events, including inammatory responses, glutamate
excitotoxicity, oxidative stress, and apoptosis that result in a profound brain injury. The inammatory response contributes to secondary
neuronal damage, which exerts a substantial impact on both acute ischemic injury and the chronic recovery of the brain function.
Microglia are the resident immune cells in the brain that constantly monitor brain microenvironment under normal conditions.
Once ischemia occurs, microglia are activated to produce both detrimental and neuroprotective mediators, and the balance of the
two counteracting mediators determines the fate of injured neurons. The activation of microglia is dened as either classic (M1) or
alternative (M2): M1 microglia secrete pro-inammatory cytokines (TNFα, IL-23, IL-1β, IL-12, etc) and exacerbate neuronal injury,
whereas the M2 phenotype promotes anti-inammatory responses that are reparative. It has important translational value to regulate
M1/M2 microglial activation to minimize the detrimental effects and/or maximize the protective role. Here, we discuss various
regulators of microglia/macrophage activation and the interaction between microglia and neurons in the context of ischemic stroke.
Keywords:
cerebral ischemia; microglia; macrophage; neuroinammation; cytokines; microglia/neuron interaction; brain
Acta Pharmacologica Sinica (2017) 38: 445–458; doi: 10.1038/aps.2016.162; published online 6 Mar 2017
*
To whom correspondence should be addressed.
E-mail Fudong.Liu@uth.tmc.edu
Received 2016-08-22 Accepted 2016-11-06
Review

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Activation of microglia/macrophages
Classic (M1) or alternative (M2) activation has been mostly
reported for macrophage responses during peripheral inam-
mation, and, recently, microglia were found to have a similar
activation process upon ischemic insult
[15]
. M1 macrophages
are involved in the acute pro-inammatory response and pos-
sess antigen-presenting capacity. Several signaling pathways
have been proposed to contribute to M1 macrophage polariza-
tion. Interferon γ (IFNγ) secreted by T helper (Th) 1 cells is
instrumental in inducing the M1 phenotype of macrophages
[16]
.
Through Janus kinase (JAK)1/JAK2 signaling, IFNγ activates
signal transducer and activator of transcription 1 (STAT1)
factor and increases the production of pro-inammatory cyto-
kines, such as tumor necrosis factor α (TNFα), interleukin
(IL)-23, IL-1β, IL-12, chemotactic factors, reactive oxygen spe-
cies, and nitrogen monoxide (NO). Another pathway for the
induction of M1 activation is triggered by lipopolysaccharide
(LPS) or damage-associated molecular pattern (DAMP) stimu-
lation through Toll-like receptor 4 (TLR4)
[17]
, followed by the
formation of an “activation complex” that includes myeloid
differentiation factor 88 (Myd88), nuclear factor κB (NFκB),
p65, p38, and interferon regulatory factor 3 (IRF3)
[17, 18]
. The
complex in turn regulates the secretion of inammatory medi-
ators [inducible nitric oxide synthase (iNOS), CD16, CD32, etc]
and cell surface markers [histocompatibility complex (MHC-
II), CD86, etc] from the polarized cell
[17]
. The polarization of
the M1 phenotype is characterized by the expression of IL-
12
high
, IL-23
high
and IL-10
low[18]
. The alternative activation of
macrophages (M2) is usually induced by IL-4 or IL-10 and
IL-13 and is generally characterized by IL-12
low
, IL-23
low
, and
IL-10
high
. To activate macrophages toward M2, IL-4 or IL-13
combines with IL-4Rα or IL-13Rα1, respectively, to activate
transcription factors such as STAT6, peroxisome-proliferator-
activated receptor γ (PPARγ), Jumonji domain-containing pro-
tein 3
(
Jmjd3), and IRF4. As a result, cytokines such as IL-10,
transforming growth factor β (TGF β), IL-1 receptor agonist,
CD302, CD163, and other inflammatory mediators [platelet-
derived growth factor (PDGF), Fibronectin 1 and Arginase 1
(Arg1)] are released
[18]
.
In ischemic stroke, sterile inflammation is induced by
DAMPs released from injured brain tissue. Endogenous
microglia and recruited macrophages are activated and polar-
ize to the M2 phenotype in the acute stage and then gradually
switch toward the M1 phenotype, which particularly occurs
in peri-infarct regions adjacent to ischemic neurons
[15]
. Ex
vivo studies have found that when M1 microglia were added
to a cell culture, oxygen glucose deprivation (OGD)-induced
neuronal death was exacerbated; by contrast, M2 microglia
protected neurons against OGD
[15]
. It has been demonstrated
that microglial polarization also involves NFκB and STAT1
pathway activation, which is seen in inamed macrophages
[19]
.
Similar to macrophage activation, the production of pro-
inflammatory cytokines is essential for microglia to polarize
toward the M1 phenotype
[20]
. It has also been reported that
during neuroinflammation, microglia produce IL-10 and
TGFβ
[21, 22]
, which leads to anti-inammatory signaling (M2a)
and wound healing (M2c)
[22]
. During disease progression, the
M1 phenotype can switch to M2 or vice versa depending on
inammatory signals
[23]
. In addition, microglia can self-control
their polarization through autocrine and paracrine means, and
this response is protective but is downregulated once the dam-
age or pathogen has been addressed. If the self-regulation is
interrupted, unregulated, long-term, or chronic inammation
occurs that exacerbates tissue damage
[24]
.
Regulation of microglia/macrophage activation
A plethora of pathways and mediators have been documented
to regulate macrophage activation, and many of these path-
ways and mediators are shared by microglial activation. Most
of these signaling pathways overlap with each other to vary-
ing degrees, and do not appear to work independently but
rather function synergistically to cause an inammatory mael-
strom. Limited studies are available in the literature regarding
the regulatory pathways that underlie microglial activation in
the context of stroke; therefore, relevant data from other neu-
rodegenerative diseases are also reviewed here.
Toll-like receptors (TLRs)
TLRs are transmembrane proteins that are characterized as
pattern-recognition receptors and are the key factors in initiat-
ing inammatory responses. TLRs are expressed on various
cells, including macrophages, microglia
[25, 26]
, astrocytes
[26–28]
,
Schwann cells
[29]
, and neurons
[30]
. In the central nervous
system (CNS), both pathogen-associated molecular patterns
(PAMPs) and DAMPs can activate TLRs to initiate a signaling
cascade of immune responses that are regulated by microglia
and astrocytes
[25, 26, 31]
. PAMPs include bacterial DNA and LPS,
and DAMPs are known as endogenous ligands that are gener-
ated by sterile tissue injury. The cascade mobilizes Toll/IL-1R
(TIR) domain-containing adaptor proteins, such as MyD88,
NFκB-inducing kinase (NIK), and IκB kinase (IKK), to activate
NFκB; subsequently, the activated NFκB translocates into the
nucleus to induce the production of pro-inflammatory cyto-
kines such as TNFα, IL-1, and IL-12
[32–34]
. During spinal cord
injury, TLR2 mediates Nox2 gene expression and activates
microglia via the NFκB and p38 pathway
[35]
. It was found
that TLR2 activation led to the expression of IL-23 and IL-17
on microglia, which exacerbated inammatory responses and
neuronal damage
[36]
. TLR2 activation causes either M1 or M2
microglial polarization depending on the type of stimulation
and the progression of neuroinammation. After the admin-
istration of Pam2CSK4, a specific TLR2 agonist, microglia
robustly increased and were polarized to the M2 phenotype to
reduce the secondary degeneration of myelinated bers in the
CNS
[37]
. TLR3 was found to play vital roles in the JEV-induced
microglial response
[38]
. The TLR3-mediated activation of
human microglia can change the prole of immune responses
by transmitting Th1 polarizing signals to CD4 T cells. The
response leads to IFNγ secretion, Th1 polarization, and the
expression of major histocompatibility complex, costimula-
tory molecules (CD80, CD40, and CD86), and INFα and IL-23
in microglia
[39]
. TLR4 is mostly expressed by microglia in

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the brain
[40]
; however, both TLR4 and TLR2 expression were
increased in cerebral cortical neurons after ischemia/reperfu-
sion injury
[30]
. A shortage of TLR2 or TLR4 reduces infarct
size following focal cerebral ischemic injury
[41–43]
. The increase
in TLR4 expression leads to the activation of the NFκB path-
way, the release of the pro-inammatory cytokines TNFα and
IL-6, and the exacerbation of neuronal damage and apoptosis.
TLR4 activation also causes the elevated expression of p38,
c-Jun N-terminal kinase (JNK) and extracellular regulated pro-
tein kinase (ERK)1/2 and iNOS in the ischemic brain, which
are responsible for M2 microglial activation
[44]
.
Histamine and substance P
Histamine and substance P are important neuroinammatory
mediators in the CNS that can trigger microglia to become
activated and to release pro-inflammatory factors (TNFα
and IL-6). The underlying mechanism was proposed to be
histamine- or substance P-induced mitochondrial membrane
depolarization
[45]
. Antagonists of histamine or substance P
receptors can partially abolish the effect of microglial activa-
tion. However, controversy exists because another study
reported that in cultured microglia from adult mice, IL-4, an
anti-inflammatory mediator, increased the sizes of the hista-
mine-, substance P-, and somatostatin-sensitive cell popula-
tions
[46]
, which suggests that histamine and substance P are
also involved in anti-inammatory responses. Taken together,
the role of histamine and substance P in microglial activation
is still elusive.
Programmed cell death protein 1 (PD1)
PD1 is a transmembrane protein that belongs to the CD28
superfamily. As an inhibitory receptor, PD1 binds to its ligands
PDL1 and PDL2 to induce inhibitory signals to maintain the
balance between T-cell activation, tolerance, and immune-
mediated tissue insult
[47, 48]
. PD1 also suppresses innate and
adaptive immune responses
[49]
. The PD1/PDL1 signaling
pathway plays an important role in the polarization of mac-
rophage activation during spinal cord injury (SCI)
[50]
. At the
onset of SCI, both PD1 and PDL1 expression on microphage/
microglia are up-regulated; as SCI progresses, increased PDL1
recruits PD1 from other macrophage/microglia at the site of
SCI, providing a negative feedback signal that contributes to
an M2 phenotype switch
[50]
. Other studies have also shown
that up-regulated PD1 can suppress M1 polarization and
promote M2 polarization by increasing STAT6 phosphoryla-
tion
[50]
. By contrast, down-regulated PD1 signaling may initi-
ate the polarization of macrophages/microglia toward an M1
phenotype by increasing the phosphorylation of STAT1 and
NFκB
[50]
.
Secreted protein lipocalin 2 (LCN2)
LCN2 is a member of the lipocalin family that was initially
found in neutrophil granules
[51]
. LCN2 possesses multiple
functions to regulate cell death/survival
[52]
, cell migration/
invasion
[53]
, cell differentiation
[51]
and iron delivery
[54]
. A vari-
ety of inflammatory conditions can induce the expression of
LCN2 in macrophages
[55]
. In a mouse model of LPS-induced
neuroinflammation, the expression of LCN2 was notably
increased in microglia
[56]
. The secreted LCN2 from microg-
lia in turn stimulated and amplified the M1 polarization of
microglia in an autocrine manner. As a result, pro-inamma-
tory gene expression was increased, including the expression
of IL-12, IL-23, iNOS, TNFα, and the chemokine fractalkine
(CXCL)10. LCN2 was found to inhibit the phosphorylation
of STAT6 and to lead to the suppression of M2 signaling in
IL-4-stimulated microglia
[56]
. It has been suggested that LCN2
is an M1-amplifier in microglia and can cause skewed M1
polarization.
Mitogen-activated protein kinase (MAPK) and AMP-activated
protein kinase (AMPK)
The MAPK signaling pathway plays a vital role in microglial
activation. Through this signaling pathway, 4-oxobutyric acid
(DCPIB), a potent volume-regulated anion channel (VRAC)
inhibitor, suppressed the release of glutamate and the excit-
atory stimulation of neurotoxicity, and it inhibited M1 microg-
lial activation during ischemic stroke injury
[57]
. Upstream
of MAPK activation, the AMPK and Calcium-Calmodulin-
Dependent Protein Kinase Kinase (CaMKK) β-dependent sig-
naling pathway also participates in the activation of microglia.
This mechanism was identified through experiments show-
ing that microglia treated with hydrogen sulfide (H2S) can
polarize to the M2 phenotype”
[58]
. Another AMPK activator,
metformin, also induced the activation of microglia/macro-
phages to switch toward the M2 phenotype and signicantly
increased angiogenesis and neurogenesis in the ischemic brain
following middle cerebral artery occlusion (MCAO)
[58]
.
MicroRNAs (miRs)
MicroRNAs (miRs) are a family of small (approximately 22
nucleotides) noncoding RNAs. A total of 30%–90% of human
genes are regulated by miRs that modulate cell growth, activa-
tion, and differentiation
[59]
. miR124 is expressed in M2-pheno-
type macrophages, and the over-expression of miR124 down-
regulates the expression of M1 genes (MHC II, CD86) and
up-regulates M2 markers such as resistin-like α (Fizz1) and
Arg1. Exposure to IL-4 and IL-13 increased the expression of
miR124 in macrophages
[59]
. miR424 also plays an important
role in neuroprotection after stroke. In a cerebral ischemia/
reperfusion study, the over-expression of miR424 reduced
infarction volume and brain edema and decreased neuronal
apoptosis and microglia polarization by inhibiting ionized
calcium binding adaptor molecule-1 (Iba-1) expression and
activity after stroke. In vitro, miR424 mimics caused G
1
phase
cell-cycle arrest and inhibited BV2 microglia activity. miR424
suppresses microglial activation by inhibiting key transla-
tional activators of G
1
/S
[60]
. Other investigators found that
miR200b was also expressed in microglia, and the expression
was down-regulated after traumatic brain injury; knockdown
of miR200b in microglia increased JNK activity, pro-inamma-
tory cytokine secretion, iNOS synthesis and NO production,
which resulted in increased neuronal apoptosis. Conversely,

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the over-expression of miR200b in microglia suppressed JNK
activity, iNOS synthesis, NO production and the migratory
potential of activated microglia. miR200b regulates microglial
inammatory processes and neuronal survival by modulating
the c-Jun/ MAPK pathway
[61]
.
Notch signaling
Experimental stroke studies found that the activation of Notch
induces the M1 polarization of microglia, suppresses M2 acti-
vation, and worsens ischemic brain damage
[62]
. Moreover,
the blockade of Notch signaling during microglial polariza-
tion could be “memorized” by microglia, which suggests that
Notch signaling contributes to post-ischemic inflammation
by directly modulating the microglial innate response
[63]
. The
effects of Notch signaling on microglial activation may be
pleiotropic; Notch-induced microglial activation also contrib-
utes to cell apoptosis in the ischemic brain tissue
[64]
.
IRFs
Emerging data suggest that members of the family of inter-
feron regulatory factors (IRFs) mediate macrophage polariza-
tion. In mammals, the IRF family consists of nine members:
IRF1, IRF2, IRF3, IRF4, IRF5, IRF6, IRF7, IRF8/ICSBP, and
IRF9
[65]
. The variable domains at the C-terminus determine
the functional specicity of each IRF (Table 1).
IRF1
In the peripheral immune system, IRF1 is weakly expressed
in resting macrophages; however, the expression of IRF1 is
up-regulated in M1 polarized macrophages that are activated
by IFNγ-producing NKT cells or Th1 cells. By interacting
with MyD88, IRF1 migrates into the nucleus and triggers
the expression of TLR-mediated genes (IFNβ, iNOS and
IL-12p35)
[66]
. Moreover, IRF1 prevents other transcription
factors (eg, IFNγ) from binding to IL-4 and inhibits M2 macro-
phage activation, which leads to M1 microglial polarization in
the ischemic brain
[67, 68]
. The expression of the IRF1 gene was
markedly increased within 12 h of MCAO in mice and reached
a peak at d 4 of ischemia
[69]
. IRF1 expression was also present
in intravascular neutrophils that inltrated the ischemia tissue
after MCAO and in neurons at the outer border of the ischemic
territory
[70]
. In an ischemic stroke study with an IRF1
–/–
mouse
model, the infarct volume and neurological decit scores were
reduced compared to the WT cohort, indicating that IRF1 con-
tributes substantially to cerebral ischemic damage
[69]
.
IRF2
IRF2 is a transcriptional repressor with the capacity to inhibit
IFNγ expression and IRF1-dependent macrophage activa-
tion
[71, 72]
. IRF2 also suppresses LPS-induced TNFα expres-
sion and augments IL-1, IL-6, IL-12, and IFNγ secretion
[73]
.
Sumoylation increases IRF2’s ability to inhibit IRF1 tran-
scriptional activity
[74]
. The expression of TLR3, TLR4, and
TLR5 was regulated by IRF2 signaling after LPS stimulation
of murine macrophages
[75, 76]
. Thus far, there are limited data
available about the role of IRF2 in microglial activation after
Table 1.
The roles of IRFs in macrophage/microglia polarization.
Primary expression cell Induction Pathways involved Roles in regulating marophages/microgla References
IRF1
IRF2
IRF3
IRF4
IRF5
IRF7
IRF8
Microglia/
macrophage
Any cells
Ependymal cells and
choroid plexus
Bone marrow-derived
macrophages/
microglia
B cells, dendritic
cells, macrophages/
microglia
Microglia/
macrophages
Microglia
IFN-γ
IFN-γ and virus
Molecules
mediated by dsRNA
and dsDNA
Different mitogenic
stimuli
Type I IFN and viral
infection
Toll-like receptor 4
signaling
Type I IFN
Interact with MyD88.
Compete with IRF1 for DNA binding
sites.
Be the crucial transcription factor in
non-MyD88 pathway.
Through IL-4 signaling; mediated by the
transcription factor signal transducer
and activator of Stat6.
Decrease TLR3-, TLR4-, and TLR9-
dependent induction of TNFα and I IFN.
Suppress the activation of STAT1.
Activate gene expression that
transforms microglia into a reactive
phenotype.
Regulate TLR-mediated expression of pro-
inammatory genes.
Activator of H4 gene, VCAM-1, and TLR9
gene; augment LPS induced IL-1, IL-6,
IL-12, and IFN-γ secretion; repress the
transcriptional activation of the IFN-b gene.
Alter the microglial activation phenotype
from M1 to M2; transactivate the IFN-b,
CXCL10, CCL5, ISG56, IFIT1 , arginase II and
RIG-Ilike eceptors gene.
Control M2 polarization; regulate MHC-II,
Ciita, Cyp1b1, and Il1rn genes.
Regulate host immunity against extracellular
pathogens, DNA damage-induced apoptosis,
death receptor signaling, and classic
macrophage polarization.
Involved in demyelination.
Increase levels of Iba1, CD206, CD45,
CD11b and F4/80 and F4/80; decrease
levels of the chemokine receptors CCR2,
CCR5 and CX3CR1.
[66–70]
[71–76]
[77, 78]
[79–82]
[83, 84]
[85]
[86–91]

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cerebral ischemia.
IRF3
IRF3 is a transcription factor that can be induced by TLR3 or
TLR4 activation. A variety of cells express the IRF3 protein,
and it was reported that IRF3 could change the microglial
phenotype from M1 to M2. In an in vitro study with a glial-
neuronal co-culture system, transduction with adenovirus-
mediated IRF3 suppressed pro-inammatory gene expression,
including the expression of IL-1, TNFα, IL-6, and CXCL1 in
microglia. Moreover, IRF3 induced the expression of many
anti-inflammatory genes, including IL-1 receptor antagonist,
IL-10 and IFNβ
[77]
. It was found that IRF3 facilitated M2 polar-
ization of microglia and protected neurons from cytokine-
induced insult
[77]
. There was also evidence to indicate that
IRF3 suppressed neuroinflammation by regulating miR155,
which is highly expressed in lesions of multiple sclerosis and
is involved in the induction of pro-inflammatory cytokine
genes
[78]
.
IRF4
IRF4 is expressed in macrophages in various organs and tis-
sues
[79]
. During peripheral immune responses, IRF4 expression
is strongly induced in bone marrow-derived macrophages
via IL-4 signaling. IRF4 not only regulates the expression of
several secondary DNA binding proteins (eg, MHC II) that
promote M1 macrophage phenotype
[80]
but also increases the
expression of M2 phenotype marker genes such as Arg1, Ym1,
and Fizz1. IRF-4
–/–
macrophages produce pro-inflammatory
cytokines, including IL-1β and TNFα, to enhance M1 polariza-
tion
[81]
. In ischemic brains, the expression of IRF4 increases in
neurons and microglia in the penumbra after the onset of isch-
emic stroke in an attempt to rescue neurons from ischemia/
reperfusion-induced cell injury
[82]
. Neuron-specic IRF4 trans-
genic (IRF4-TG) mice exhibited reduced infarct volumes after
MCAO, which was reversed in the ischemic brains of IRF4
knockout mice
[82]
.
IRF5
IRF5 contributes to pro-inammatory responses
[83]
. Through
TLR signaling, IRF5 binds MyD88 to form homodimers, acti-
vates type I interferon, and up-regulates the expression of
M1-phenotype related cytokines, including TNFα, IL-6, and
IL-12. MyD88 is the key adaptor protein that mediates the
regulatory effect of IRF5 on M1 activation; however, IRF4
competes with IRF5 for binding to MyD88 and inhibits the
transmission of TLR outside-in signaling to NFκB and to other
pro-inflammatory transcription factors
[66]
. The competition
between IRF5 and IRF4 for MyD88 binding causes IRF4 to sup-
press M1 macrophage polarization
[84]
. The balance between
IRF4 and IRF5 might be a critical determinant in the modula-
tion of macrophage polarization.
IRF7
IRF7 also participates in the regulation of microglial polariza-
tion
[85]
. One study showed that IRF7 expression increased
when the phenotype of microglia switched from M2 to M1
after LPS treatment in vitro
[85]
. After the knockdown of IRF7
with siRNA, the phosphorylation of STAT1 was reduced,
and the expression of M1 markers (CD86, iNOS) was sup-
pressed
[85]
.
IRF8
IRF8 is an interferon consensus sequence-binding protein and
a key IRF for the phenotypic switch of microglial activation
in the brain
[86]
. IRF8 is constitutively expressed in neurons
and microglia
[87, 88]
. The expression of IRF8 was suppressed
at the onset of stroke and was further decreased to 26% of the
baseline at 72 h of ischemia
[88]
. However, IRF8 expression was
increased in microglia after peripheral nerve injury
[89]
. IRF8
promoted the M1 activation of microglia/macrophages and
exacerbated neuroinammation
[90]
. In an EAE disease model,
IRF8 activated microglia and induced the production of pro-
inammatory cytokines IL-12 and -23
[91]
.
The transcription factor p53
During HIV-associated neurocognitive disorders (HAND)-
induced neuroinammation, p53 was highly expressed and in
turn increased the expression of genes associated with classical
macrophage activation, leading to the secretion of pro-inam-
matory cytokines in microglia
[92]
. The phagocytic activity of
microglia and the expression of markers for alternative activa-
tion were increased in the ischemic brains of p53
–/–
mice
[92]
. In
a study of Alzheimer’s disease, the inhibition of p53 also led
to a decrease in microglial apoptosis and prevented microglial
neurotoxicity
[93]
. By contrast, the up-regulation of p53 expres-
sion in ischemic brains increased microglial apoptosis and BBB
damage, and worsened brain damage
[94]
.
Jumonji domain containing protein 3 (Jmjd3)
Jmjd3 is indispensable for the expression of the M2 pheno-
type during macrophage/microglia polarization
[95, 96]
. Jmjd3
can induce microglial polarization toward the M2 phenotype,
which ameliorates the inflammatory pathological changes in
Parkinson’s disease (PD)
[95]
. In the aforementioned study, the
suppression of Jmjd3 inhibited M2 microglia polarization and
simultaneously enhanced M1 activation and pro-inammatory
responses, resulting in extensive neuronal loss in the substan-
tia nigra area
[95]
. Currently, the role of Jmjd3 in microglial acti-
vation has only been reported in neurodegenerative diseases;
very little data are available from ischemic brains.
Peroxisome-proliferator-activated receptor (PPARγ) pathway
PPARγ aids in the phenotype switch of peripheral macro-
phages and modulates the secretion of many cytokines
[97]
.
PPARγ activation inhibits inammation and may have neuro-
protective effects. In
a mouse model of PD that was induced
by treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyr-
idine-probenecid (MPTPp), PPARγ was found to play a piv-
otal role in microglial activation
[98]
. After chronic treatment
with MPTPp, the expression of pro-inflammatory cytokines
(TNFα and IL-1β) gradually increased, whereas the levels