Science - 16.08.2019

phagocytosed DNA is recognized by TLR9 ( 53 – 55 ).
Viral double-stranded RNA (dsRNA) is recognized
by the relatives RIG I and MDA5 ( 56 )andbythe
oligoadenylate synthetase (OAS)/ribonuclease L
(RNase L) antiviral pathway ( 57 ). Interestingly,
cGAS that binds dsDNA is evolutionarily related
to OAS that binds dsRNA ( 58 ). They both synthe-
size 2′ 5 ′–linked nucleotides upon double-stranded
polynucleotide binding to activate downstream
effectors: STING to induce interferon-b(IFNb)
and RNase L to degrade RNA, respectively. Fur-
thermore, the type III CRISPR-Cas system shares
overlapping mechanisms. In bacteria, upon the
CRISPR interference complex binding to invader
RNA, the Cas10 subunit links ATP into a 3′-5′–
linked cyclic hexaadenylate that in turn activates
the RNase activity of Csm6. Thus, some nucleic
acid–sensing pathways of mammalian innate im-
munity appear to have evolved from primordial
bacterial defense mechanisms ( 59 , 60 ).
Not surprisingly, some antibacterial innate
immune receptors can cross-react with molecules
that mitochondria share with bacteria, called
damage-associated molecular patterns (DAMPs).
Mitochondrial DAMPs are associated with poor
prognosis in cardiovascular disease ( 61 ), acute
respiratory distress syndrome ( 62 ), and several
other disorders ( 63 ). The innate immune recep-
tors are known for several mitochondrial DAMPs,
suggesting pathways that may be targeted to ame-
liorate disease.
Bacterial protein translation depends on
N-formylmethonine as a start codon, distinct from
cytosolic eukaryotic translation but shared with
the mitochondrial matrix translation machinery.
Two G protein–coupled receptors, FPR1 and FPR2,
reside on the plasma membrane of neutrophils
and macrophages facing the extracellular space
that bind and signal the presence ofN-formyl
peptides, inducing chemotaxis of the white blood
cells toward theN-formyl peptide source—either
bacterial or mitochondrial ( 64 ). This innate im-
mune pathway would detect mitochondrial

N-formyl peptides after cell and tissue damage that

release the DAMP into the extracellular space

( 65 ). Cardiolipin, a dimeric phospholipid found

in bacteria and in the mitochondrial inner mem-

brane, is also a potential DAMP linked to inflam-

mation. NLRP3 appears to directly interact with

cardiolipin, and inflammasome activation has

been correlated with cardiolipin levels in macro-

phages ( 66 ).

Among several stress signals, NLRP3 also rec-

ognizes mitochondrial damage that stems from

reactive oxygen radicals (ROS) during Ox/Phos;

although the mechanism of how ROS can activate

NLRP3 remains unclear, it may involve changes

in cytosolic potassium levels ( 67 ). Upon activation,

NLRP3 forms a heptameric ring that binds ASC

and procaspase 1, cleaving and activating caspase

1 based on proximity, which in turn processes the

proforms of cytokines interleukin-1b(IL-1b)and

IL-18to initiate inflammation (Fig. 4A). When

mitochondria are damaged, they are normally elim-

inated by the phosphatase and tensin homolog

(PTEN)–induced putative kinase protein 1 (PINK1)/

Parkin mitophagy pathway in macrophages (Fig.

2B). Loss of Parkin or p62, the ubiquitin-binding

autophagy receptor, in macrophages causes an

accumulation of damaged mitochondria and ex-

cessive activation of the NLRP3 inflammasome

( 68 ). Oxidized mtDNA in the cytosol binds and

activates the NLRP3 ( 69 ) but not the AIM2 in-

flammasome ( 68 ), adding to the broad molecular

diversity of NLRP3 activators ( 70 ). Mitochondrial

ROS is thought to lead to the oxidization of the

mtDNA. In contrast to the NLRP3 inflamma-

some activation in macrophages, mtDNA activates

the cGAS/STING pathway of IFNbinduction in

other cell types. When the mtDNA nucleoid is

disrupted, mtDNA is released to the cytosol, where

it activates cGAS and STING-induced inflamma-

tion ( 71 ). Preventing mitochondrial fusion in muscle

tissue activates TLR9 and nuclear factorkB(NF-kB)

expression, apparently from release of mtDNA and

activation of macrophages cell-nonautonomously

( 72 ). InC. elegans, the mitochondrial proteotoxic

stress sensor ATFS-1 induces innate immune gene

expression ( 73 ). In some of the above scenarios,

mtDNA is thought to actively participate in mam-

malian innate immunity and host defense and not

to simply represent deleterious off-target inflam-

matory response to be avoided. Such models are

hard to test in vivo because mtDNA cannot be

removed from most cells in mammals except in

cultured cell lines.

In early stages of apoptosis, if caspases are

blocked (Fig. 4), mtDNA can be released from

mitochondria when the proapoptotic Bcl-2 family

members Bax and Bak permeabilize the outer

membrane. First the inner membrane herniates

through Bax pores in the outer membrane and

then ruptures to release cytosolic mtDNA. This

mtDNA activates cGAS and STING induction of

IFNbexpression ( 74 , 75 ). Normally, apoptosis is

immunologically silent as caspases eliminate the

cGAS induction of interferon. However, low-level

activation of Bax and Bak without caspase acti-

vation can induce cytokine expression and bacte-

rial resistance ( 76 ).

Mitochondrial dsRNA also activates IFNb

production if its catabolism within mitochondria

is inhibited ( 77 ). Mitochondrial dsRNA appears to

escape mitochondria by way of Bax and Bak pores

and then binds MDA5, which activates mitochon-

drial antiviral signaling (MAVS) and subsequent

IFNbproduction. Interestingly, MAVS has a

C-terminal hydrophobic tail that localizes it to the

mitochondrial outer membrane, and without mito-

chondrial localization, MAVS fails to signal cytokine

production ( 78 ).However,theroleofmitochon-

drial localization of MAVS remains unclear.

Control of mitochondrial DAMPs

One pathway that removes mitochondrial DAMPs

involves mitophagy and lysosomal degradation.

Mouse cardiomyocytes that lack lysosomal deoxy-

ribonuclease II fail to degrade mitochondria,

leading to extracellular mtDNA activating TLR9

Youle,Science 365 , eaaw9855 (2019) 16 August 2019 4of7

Apoptosis

Inflammation

Interferon β STING activation

mtDNA

Caspase 3 activation

IL-1β activation Caspase 1 activation

Caspase 9 activation

APAF-1

apoptosome

Bax and Bak

ABMitochondrial damage induced inflammation Mitochondrial targeted to induce apoptosis

Cyclic GAMP

cGAS activation

NLRP3

inflammasome

Lysis

Fig. 4. mtDNA activation of inflammation has similarities to apoptosis.(A) When mitochondria are sufficiently damaged to break the membranes,
mtDNA leaks into the cytoplasm, where it can activate the NLRP3 inflammasome in macrophages and cGAS in other cell types. This leads to IL-1b
and IFN-bcytokine activation, respectively, and inflammation. (B) The apoptosome, which resembles the inflammasome, induces apoptosis and a form
of innate immunity. When the outer mitochondrial membrane is permeabilized by BCL-2 family proteins (blue circles), cytochrome c (red circles) is
released, which activates the apoptosome. The apoptosome, a relative of the inflammasome, activates Caspase 9 to induce apoptotic cell death. This
pathway is another innate defense mechanism to prevent the spread of viruses in mammalian tissues.

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