410 | Nature | Vol 584 | 20 August 2020
Article
Extracellular proteostasis prevents
aggregation during pathogenic attack
Ivan Gallotta^1 , Aneet Sandhu1,2, Maximilian Peters^3 , Martin Haslbeck^4 , Raimund Jung^1 ,
Sinem Agilkaya^1 , Jane L. Blersch1,2, Christian Rödelsperger^5 , Waltraud Röseler^5 ,
Chaolie Huang^1 , Ralf J. Sommer^5 & Della C. David1,6 ✉In metazoans, the secreted proteome participates in intercellular signalling and
innate immunity, and builds the extracellular matrix scaffold around cells. Compared
with the relatively constant intracellular environment, conditions for proteins in the
extracellular space are harsher, and low concentrations of ATP prevent the activity of
intracellular components of the protein quality-control machinery. Until now, only a
few bona fide extracellular chaperones and proteases have been shown to limit the
aggregation of extracellular proteins^1 –^5. Here we performed a systematic analysis of
the extracellular proteostasis network in Caenorhabditis elegans with an RNA
interference screen that targets genes that encode the secreted proteome. We
discovered 57 regulators of extracellular protein aggregation, including several
proteins related to innate immunity. Because intracellular proteostasis is upregulated
in response to pathogens^6 –^9 , we investigated whether pathogens also stimulate
extracellular proteostasis. Using a pore-forming toxin to mimic a pathogenic attack,
we found that C. elegans responded by increasing the expression of components of
extracellular proteostasis and by limiting aggregation of extracellular proteins. The
activation of extracellular proteostasis was dependent on stress-activated MAP kinase
signalling. Notably, the overexpression of components of extracellular proteostasis
delayed ageing and rendered worms resistant to intoxication. We propose that
enhanced extracellular proteostasis contributes to systemic host defence by
maintaining a functional secreted proteome and avoiding proteotoxicity.Extracellular pathological deposits are associated with a variety of
diseases such as Alzheimer’s disease, spongiform encephalopathies,
cardiac amyloidosis and type II diabetes. A better understanding of
the regulation of protein homeostasis (proteostasis) in the extracel-
lular space could ultimately expand treatment options. The cellular
proteostasis network comprises more than 2,000 factors^10 , yet these
factors are largely inactive outside the cell. In support of an active
extracellular proteostasis network, a growing number of extracellular
chaperones^1 –^3 have been identified to act as suppressors of aggrega-
tion by binding to misfolded proteins or oligomers and promoting
their removal through receptor-mediated endocytosis. One of the
main obstacles delaying the exploration of extracellular proteostasis
in vivo is the lack of an amenable model for a comprehensive study.
Here, we took advantage of the simplicity of C. elegans, in which the
extracellular fluid in the body cavity (pseudocoelom) bathes all inter-
nal organs and provides a medium to exchange intercellular signals
and distribute nutrients^11. Six scavenger cells (coelomocytes) ensure
turnover of extracellular components by non-specific endocytosis^12.
Except for this basic control by the coelomocytes, to our knowledge,
nothing is known about an extracellular quality-control system for
damaged proteins.
To discover extracellular regulators (ECRs), we constructed a C. elegans
model to follow protein aggregation in the extracellular space. A
previous proteomic analysis of the C. elegans aggregating proteome
identified several secreted proteins highly prone to aggregate with
age^13. One of these extracellular proteins was lipid-binding protein 2
(LBP-2). Of note, mutations linked to Charcot–Marie–Tooth in the clos-
est orthologue of LBP-2 in humans, fatty acid-binding protein myelin
P2 (E value = 9.5 × 10−9), increase its aggregation propensity^14. LBP-2
labelled with the fluorescent protein tagRFP is secreted by body-wall
muscles and is diffusely localized in the pseudocoelom in young animals
and taken up by the coelomocytes (Fig. 1a–c, Extended Data Fig. 1a).
Toxin-mediated ablation of coelomocytes causes LBP-2 to accumulate
with a similar distribution to GFP secreted from the body-wall mus-
cles^12 (Extended Data Fig. 1b, c). As animals aged and in young animals
lacking coelomocytes, we observed the formation of LBP-2 puncta
localized in the same space as secreted GFP outside of the body-wall
muscles or neurons, consistent with its aggregation in the pseudo-
coelom (Fig. 1d–i, Extended Data Fig. 1d–f, Supplementary Video 1).
Increased formation of LBP-2 puncta correlated with an increase in
detergent-insoluble LBP-2, a hallmark of age-dependent protein aggre-
gation and disease-associated protein aggregation (Fig. 1j). By contrast,https://doi.org/10.1038/s41586-020-2461-z
Received: 18 October 2018
Accepted: 16 April 2020
Published online: 8 July 2020
Check for updates(^1) German Center for Neurodegenerative Diseases (DZNE), Tübingen, Germany. (^2) Graduate Training Centre of Neuroscience, International Max Planck Research School, University of Tübingen,
Tübingen, Germany.^3 Department of Medical Neurobiology, Hebrew University of Jerusalem, Jerusalem, Israel.^4 Department of Chemistry, Technical University of Munich, Garching, Germany.
(^5) Max Planck Institute for Developmental Biology, Department for Integrative Evolutionary Biology, Tübingen, Germany. (^6) Interfaculty Institute of Biochemistry, University of Tübingen, Tübingen,
Germany. ✉e-mail: [email protected]