Science - 16.08.2019

dislodge at least some tail-anchored proteins,
such as Pex15, from the outer mitochondrial
membrane, allowing protein retargeting to their
proper location in the peroxisomal membrane
( 33 , 34 ). Clogging of mitochondrial import chan-
nels induces the heat shock factor Hsf1, which in-
creases Rpn4, a transcription factor that increases
proteasome subunit expression ( 35 ). Consistent
with ( 32 ), Pdr3, which induces transcription of
CIS1, is another downstream target of Rpn4. Hsf1
also induces a number of cytosolic chaperones

and represses cytosolic protein translation. In-

terestingly, clogging mitochondrial import re-

presses nuclear encoded TCA cycle and Ox/

Phos gene transcription. Ox/Phos protein re-

pression is mediated through inhibition of the

HAP transcription complex. How HAP is inhi-

bited, which is suggested to be posttranslational,

remains unclear. As for mtDNA-encoded pro-

teins, mitochondrial translation machinery and

translation itself (but not transcription) is markedly

down-regulated after clogging ( 35 ). This would

help balance mtDNA-encoded protein produc-

tion with the loss of import of nuclear encoded

proteins, which is consistent with other studies

that use Lon inhibition in mammalian cells ( 22 ),

and during mitochondrial biogenesis in yeast

( 13 ). Removal of proteins stuck in the TOM chan-

nel uses the AAA ATPase Cdc48 and an adaptor

Ubx2 that is constitutively bound to the TOM

complex. This work also implicates ubiquitination

of stuck substrates, although the ubiquitin ligase

that mediates ubiquitination remains unknown

( 36 ). If mitochondrial proteins are stalled during

translation, Vms1 prevents protein aggregation

that would occur in the matrix compartment

downstream of ribosome quality control mod-

ifications ( 37 ). In mammalian cells, a protein

family with ubiquitin-like domains and ubiquitin-

binding domains, called ubiquilins, bind to

tail-anchored outer mitochondrial membrane

proteins and redirect at least matrix proteins

unable to enter the mitochondria for proteosomal

degradation ( 38 ).

Thus, numerous processes maintain endosym-

biont proteostasis, including (i) increasing trans-

cription of chaperones; (ii) inhibition of mtDNA

gene translation; (iii) inhibition of nuclear en-

coded mitochondrial gene transcription; (iv) prote-

olysis of mislocalized and misfolded mitochondrial

proteins in several compartments, including the

cytosol and mitochondrial subcompartments; (v)

mitochondrial-derived vesicles; and (vi) autoph-

agy, both piecemeal and likely wholesale. When

these processes fail, mitochondria can lose meta-

bolic capacity, leading to numerous diseases

commonly linked to the nervous system ( 39 ).

Additionally, a failure of quality control leading

to mitochondrial breakdown may also activate

inflammation, with severe consequences.

Mitophagy at the mtDNA level

Mitochondria accumulate mutations during aging

and are purified during maternal oocyte develop-

ment, starting each generation of mammals with

a homogenous (homoplastic) and fully functional

mitochondrial genome. InDrosophila, deleterious

mtDNA mutations are sensed based on mito-

chondrial function, and impaired mitochondria

areeliminatedthroughmitophagy( 9 ). InC. elegans

and in mammals, paternal mtDNA in sperm is

eliminated upon fertilization, at least in part

through autophagy of the paternal mitochon-

dria ( 40 – 42 ).

It is not clear whether quality control exists

at the level of mtDNA in somatic cells. Neither

mouse liver hepatocytes nor human colonic epi-

thelial cells display purifying selection against

the accumulation of deleterious mtDNA muta-

tions ( 7 , 43 ). However, inherited mtDNA muta-

tions, in contrast to sporadic mutations, can be

selected against in proliferating tissues ( 44 , 45 ).

Whether this selection occurs intracellularly at

the mtDNA level or through the death of stem

cells with high mutation loads is not yet clear. In

Drosophila, there is also evidence against purify-

ing selection in somatic tissues ( 46 ). Studies of

cultured mammalian cell lines that contain a

mixture of wild-type and mutant mtDNA, called

cybrids, indicate that mitophagy has the poten-

tial to select against certain deleterious mtDNA

mutations but not others ( 47 , 48 ). Because

mitochondria continuously fuse and divide in many

tissues and cultured cells ( 49 ), mixing their con-

tents, physical links between a deleterious gene

product and the mtDNA encoding the mutant

gene appear to be tenuous. Thus, wild-type and

mutant mtDNA, when both are present in hetero-

plasmic cells, would be challenging to distinguish

on the basis of their gene products that become

mixed throughout the mitochondriome. Further-

more,morerapidlydiffusiblemitochondrialcom-

ponents such as tRNA would be harder to physically

link to mutant mtDNA nucleoids than to large

membrane-spanning electron transport complexes,

and thus more challenging to select against under

heteroplasmic conditions. Consistently, mutations

in tRNA genes are substantially overrepresented

among heteroplasmic mtDNA mutation disease

patients relative to protein-coding mutations. Not

only may mutant DNA be selected against, mito-

chondrial proteins in yeast and human cells can

be segregated and, in human cells, destroyed,

cleansing the mitochondriome of debris ( 18 , 50 ).

This may be related to processes in bacteria in

which daughter cells differentially inherit dam-

aged proteins to avoid a posttranscriptional form

of meltdown with the gradual accumulation of

protein aggregates over generations ( 51 ).

Mitochondria share PAMPs with bacteria

Another challenge for mitochondria is to main-

tain sequestration of their contents from cytosolic

and extracellular innate immune sensors. During

metazoan evolution, immune pathways arose in

invertebrates that innately recognize virus and

bacterial PAMPs. These pathways involve Toll-like

receptors (TLRs), nucleotide-binding oligomeriza-

tion domain-containing protein (NOD) receptors,

and certain G protein–coupled receptors that re-

cognize bacterial components, such as the bacte-

rial cell wall lipopolysaccharide. Activation of innate

immune pathways by PAMPs triggers intracellular

signaling pathways to induce chemotaxis of mono-

cytes, secretion of cytokines, and type 1 interferons,

which transmit the danger signal to neighboring

cells. Pathways downstream of innate immune

signaling further link inflammation to adaptive

immune responses in vertebrates ( 52 ).

Viral double-stranded DNA (dsDNA) in the

cytosol is recognized by the cyclic guanosine

monophosphate–adenosine monophosphate

(cGAMP) synthase (cGAS)–stimulator of inferon

genes (STING) pathway and the absent in melanoma

2 (AIM2) inflammasome, whereas extracellular or

Youle,Science 365 , eaaw9855 (2019) 16 August 2019 3of7

Healthy mitochondria degrade ATFS-1 and PINK1

Damaged or depolarized mitochondria

stabilize ATFS-1 and PINK1

Nucleus

Parkin

PINK1

PINK1

MTS

MTS

NLS

AT FS -1

NLS

AT FS -1 Nuclear

envelope

Nuclear

pore

Transcription

Fig. 3. Mitochondrial protein import regu-
lates two quality control processes.The
transcription factor ATFS-1 and the ubiquitin
kinase PINK1 are regulated through mitochon-
drial fidelity. (Top) Healthy mitochondria
degrade ATFS-1 and PINK1. When mitochondria
are damaged and fail to import ATFS-1 and
PINK1, their degradation is prevented. ATFS-
1 then traffics to the nucleus and induces the
transcription of mitochondrial chaperones and
proteases. PINK1 accumulates bound to the
outer mitochondrial membrane TOM complex,
where it phosphorylates ubiquitin already
attached to outer mitochondrial membrane
proteins. (Bottom) These phospho-ubiquitin
chains recruit Parkin from the cytosol and
activate Parkin E3 ubiquitin ligase activity.
Parkin ubiquitinates more mitochondrial pro-
teins for PINK1 to phosphorylate, forming an
amplification loop. These ubiquitin chains recruit
autophagy receptors that induce autophagoso-
mal engulfment selective for the damaged
mitochondria.

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