nick end labeling) assay], was cholesterol dependent (fig. S9, D and E), and was rescued by
caspase-1 inhibitors, pointing to an inflammasome-mediated pyroptotic cell death pathway (Fig. 4G
and fig. S10). Thus, cholesterol derived from
myelin debris can activate the NLRP3 inflammasome in macrophages when toxic levels build up
intracellularly in the absence of sufficient lipoprotein carriers.
Because jamming the lysosomal system with
myelin debris resulted in cholesterol crystallization and inflammasome activation, we asked
whether this pathway was responsible for
limiting regeneration in old mice. To examine
whether increased inflammasome activation
contributed to the poor recovery of old mice, we
analyzed spinal cord lesions of aged WT and
NLRP3-deficient mice. As in G W3965-treated
old animals (Fig. 4, I and J), we found significantly improved remyelination in aged NLRP3-
deficient mice as compared with aged WT mice
at 21 dpi. Thus, inflammasome activation, possibly downstream of cholesterol accumulation,
drives a maladaptive immune response that
hampers inflammation resolution and repair
in aged mice.
Self-resolving inflammation is essential for a
proper restorative process after tissue damage,
whereas uncontrolled inflammation can leave
lasting marks that permanently alter tissue
homeostasis (16). We made the surprising discov-
ery that the self-limiting inflammatory response,
which is necessary to initiate a regenerative pro-
cess, is maladaptive in the CNS of aged mice. It
appears that the inability of aged phagocytes to
clear the enormous amounts of cholesterol that
are released from myelin after myelin breakdown
in demyelinating diseases results in a phase tran-
sition of free cholesterol into crystals, inducing
lysosomal rupture and inflammasome stimula-
tion, consistent with the beneficial effects of
nuclear receptor agonists in remyelination (17, 18).
The unexpected link between lipid metabolisms
and tissue regeneration provides opportunities
for the development of regenerative medicines
for remyelination and for improving functional
recovery after CNS injury (19).
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We thank L. Vaculčiaková for help with the calculations of
three-dimensional reconstructions and A. Kerksiek for
technical assistance. This work was supported by a European
Research Council (ERC) Consolidator Grant (M.S.) and
grants from the German Research Foundation (DFG)
(SI 746/9-1,10-1, SPP1757, TRR128, and TRR43), the Klaus
Tschira Stiftung, the Adelson Foundation, Excellence Cluster
for Systems Neurology (SyNergy), and Excellence Cluster
for Nanoscale Microscopy and Molecular Physiology of the
Brain (CNMPB). W.M. is supported by an ERC grant to
K.-A. Nave. M.B.-Q. is supported by a Boehringer Ingelheim
stipend. All data are presented in the main text and
Materials and Methods
Figs. S1 to S10
10 April 2017; resubmitted 5 November 2017
Accepted 11 December 2017
Published online 4 January 2018
688 9 FEBRUARY 2018 • VOL 359 ISSUE 6376 sciencemag.org SCIENCE
Fig. 4. Defective myelin debris clearance activates the NLRP3 inflammasome. (A) Immunoblot of cytosol and membrane fractions of primary
BMDMs 12 hours after treatment with myelin debris for cathepsin B
and g-tubulin. (B) Cathepsin B activity assay of the cytosolic fraction of
control macrophages 12 hours after treatment with myelin debris in the
presence or absence of the cathepsin B inhibitor CA074me (10 mM). AMC,
7-amino-4-methylcoumarin. (C and D) Immunoblot and quantification of the
active subunit of caspase 1 (p20) after myelin debris treatment of WT or
NLRP3−/− BMDMs. The intensity of the p20 band was normalized to
g-tubulin. (E) Caspase 1 activity in lysates from lysolecithin lesions of WT
and APOE−/− mice at 21 dpi. (F) Enzyme-linked immunosorbent assay for
IL-1b release in WT BMDMs, after treatment with myelin debris with or without
YVAD. (G) Quantification of the percentage of dead cells [propidium iodide–
positive (PI+)] after myelin debris treatment (12 hours) in the presence or
absence of a caspase-1 inhibitor (YVAD). (H) Quantification of PI+ cells after
treatment of WT and NLRP3−/− BMDMs with myelin debris for 12 hours.
(I) Methylenblue-azur II staining of remyelinating lesions in the spinal cord of
3M, 12M, GW3965-treated 12M, and 12M NLRP3−/− mice and (J) relative
quantification of myelinated fibers. All data are mean ± SEM (error bars);
*P < 0.05, **P < 0.01, ***P < 0.001 by one-way ANOVA test, with Tukey’s
multiple comparison test and two-tailed Student’s t test (E). Scale bar
in (I), 25 mm.