1、Crosstalk between autophagy and inflammatory signaling pathways:balancing host defence and homeostasisKen Cadwell1,21Kimmel Center for Biology and Medicine at the Skirball Institute,New York University School of Medicine,New York,NY 10016,USA2Departments of Microbiology and Medicine,New York Univers
2、ity School of Medicine,New York,NY 10016,USAAbstractAutophagy has broad functions in immunity ranging from cell autonomous defense to coordination of complex multicellular responses.The successful resolution of infection and avoidance of autoimmunity necessitates efficient and timely communication b
3、etween autophagy and pathways that sense the immune environment.In this article,progress in elucidating mechanisms of crosstalk between autophagy and inflammatory signaling cascades will be reviewed.The recent literature indicates that a variety of immune mediators induce or repress autophagy.It is
4、also becoming increasing clear that immune signaling cascades are subject to regulation by autophagy,and that a return to homeostasis following a robust immune response is critically dependent on this pathway.Importantly,examples of non-canonical forms of autophagy in mediating immunity are pervasiv
5、e.Improved mechanistic understanding of the autophagy machinery offers hope for treating infectious and inflammatory diseases.Macroautophagy,more commonly referred to simply as autophagy,is a fundamental cellular process in eukaryotes that is essential for responding and adapting to changes in the e
6、nvironment.In its most common form,autophagy involves sequestration of cytosolic material within a double membrane-bound vesicle termed the autophagosome,and the subsequent fusion of the autophagosome with endolysosomal vesicles leads to degradation and recycling of sequestered substrates.A critical
7、 function of autophagy is to breakdown macromolecules such as proteins to provide amino acids and other factors necessary to generate energy and synthesize new proteins.The ability to capture large material distinguishes this pathway from proteasomal degradation,making autophagy critical for maintai
8、ning cellular homeostasis in a variety of settings.Many of the substrates found within autophagosomes are those that threaten cell viability,such as damaged organelles,protein aggregates,and intracellular pathogens.Although autophagosomes are detected under steady-state conditions,their generation i
9、s increased substantially in response to stressors,of which nutrient deprivation is the best Correspondence to:Ken.Cadwellmed.nyu.edu.Competing interest statementThe author declares no competing interests.HHS Public AccessAuthor manuscriptNat Rev Immunol.Author manuscript;available in PMC 2017 Novem
10、ber 01.Published in final edited form as:Nat Rev Immunol.2016 November;16(11):661675.doi:10.1038/nri.2016.100.Author ManuscriptAuthor ManuscriptAuthor ManuscriptAuthor Manuscriptcharacterized.Depletion of amino acids or growth factors induces autophagy by inhibiting mechanistic target of rapamycin c
11、omplex 1(mTORC1),a master regulator of metabolism that inhibits the ULK1 complex through phosphorylation under nutrient replete conditions(Box 1).Once freed from inhibition,the ULK1 complex phosphorylates beclin-1 to activate the class III PI3K(PI3KC3)complex1,which creates PI3P-rich subdomains at r
12、egions associated with the endoplasmic reticulum(ER),such as the ER-Golgi intermediate compartment or ER-mitochondria contact sites2,3.PI3P binding proteins then recruit the ATG16L1 complex to this autophagosome precursor site(isolation membrane)4.The ATG16L1 complex anchors the ubiquitin-like molec
13、ule LC3 to the lipid bilayer by conjugating LC3 to phosphatidylethanolamine(PE)(Box 1).Next,LC3-PE mediates membrane tethering and fusion to extend the isolation membrane through recruiting lipids,which are likely derived from multiple membrane sources including endosomal vesicles harboring the tran
14、smembrane protein ATG9L1.The growing isolation membrane eventually fuses with itself to form the enclosed double-membrane structure,and class C VPS and SNARE-like proteins mediate fusion with endo-lysosomal vesicles leading to the degradation and recycling of the contents.Box 1Autophagosome biogenes
15、is and autophagy related(ATG)proteinsThe generation of the autophagosome is mediated by the sequential activities of three key protein complexes:the ULK1 complex(ULK1,FIP200,ATG13,and ATG101),the class III phosphatidylinositol 3-OH kinase(PI3KC3)complex(Beclin-1,VPS34,VPS15,and ATG14L),and the ATG16
16、L1 complex(ATG16L1,ATG5,and ATG12).Inhibition of mTORC1,such as through nutrient deprivation,allows the ULK1 complex to activate the PI3KC3 complex(see the figure).The ATG16L1 complex is recruited by the PI3P-binding protein WIPI2 to the autophagosome precursor structure(isolation membrane)when the
17、PI3KC3 complex creates PI3P-rich regions on the surface of the ER-Golgi intermediate compartment or other membranes.The ATG16L1 complex is generated through an ubiquitin-like(UBL)conjugation reaction in which the UBL molecule ATG12 is conjugated to ATG5 by the sequential action of ATG7 and ATG10.ATG
18、16L1 non-covalently binds the ATG5-ATG12 conjugate to form a multimeric complex.In parallel,another UBL molecule,LC3,is processed by the ATG4 protease and subjected to a second UBL conjugation reaction involving ATG7 and ATG3.The ATG16L1 complex conjugates LC3 and phosphatidylethanolamine(PE)to comp
19、lete the reaction.LC3-PE mediates membrane tethering and fusion to extend the isolation membrane through recruiting membranes from multiple sources,eventually leading to self-fusion and the formation of the autophagosome.Subsequent fusion with the lysosome,mediated by class C VPS and SNARE-like prot
20、eins,leads to the degradation of the contents.CadwellPage 2Nat Rev Immunol.Author manuscript;available in PMC 2017 November 01.Author ManuscriptAuthor ManuscriptAuthor ManuscriptAuthor ManuscriptIn this article,the function and regulation of autophagy in the context of inflammatory signals will be r
21、eviewed.It is now appreciated that autophagy function is critical to both cell autonomous defense and multicellular immunity alike,and autophagy dysfunction appears to be a recurring theme in inflammatory disease.First,the general function of autophagy in immunity will be summarized.The focus of the
22、 second section will be on specific modes of intersection between autophagy and immune pathways,especially signaling downstream of innate immune sensors.The third section will provide examples of how these mechanisms contribute to multicellular immune responses,with an emphasis on adaptive immunity
23、and inflammatory disease.Finally,the barriers for targeting the autophagy pathway to treat disease will be discussed.The role of autophagy in immunityAutophagy can eliminate an infectious threat by promoting xenophagy,whereby intracellular pathogens such as viruses,bacteria,and protozoans are trappe
24、d within an autophagosome and targeted to the lysosome for destruction.Xenophagy depends on adaptor molecules that crosslink pathogens with the autophagy machinery.For instance,sequestosome 1(SQSTM1;also known as p62),nuclear domain 10 protein NDP52,optineurin(OPTN),and TAX1-binding protein 1(TAX1BP
25、1)bind LC3 along with host molecules associated with damaged Salmonella-containing vacuoles,including ubiquitin and galectin-85.These binding events allow the isolation membrane to grow around Salmonella resulting in its sequestration within the autophagosome.One of the most compelling evidence that
26、 autophagy is an important cell-intrinsic defense mechanism is that many invasive pathogens encode virulence factors that counteract the pathway,as exemplified by the LC3-deconjugating enzyme RavZ secreted by Legionella pneumophila6.Also,autophagy enhances cell-to-cell spread of enteroviruses by pro
27、viding a membrane coat during egress7.Some pathogens are neutralized by non-canonical functions of the autophagy machinery,hereon referred to as non-canonical autophagy(Box 2).For instance,LC3-associated phagocytosis(LAP)mediates phagosome maturation,and requires many of the same proteins as convent
28、ional autophagy but occurs independently of autophagosome formation8.Another form of non-canonical autophagy induced by IFN restricts norovirus and CadwellPage 3Nat Rev Immunol.Author manuscript;available in PMC 2017 November 01.Author ManuscriptAuthor ManuscriptAuthor ManuscriptAuthor ManuscriptTox
29、oplasma gondii replication in macrophages in a lysosome-independent manner911.In the case of T.gondii infection,the ATG16L1 complex and LC3 disrupt the parasitophorous vacuole by recruiting p47 immunity-related GTPases(IRGs)and p65 guanylate binding proteins(GBPs)9,1215.Although it is not always cle
30、ar whether the classical or an unconventional form of autophagy is involved in host defense,autophagy proteins remain strong candidates as drug targets in infectious disease.Inducing autophagy with a cell permeable beclin-1 peptide protects against multiple pathogens in vitro and in vivo,including c
31、hikungunya and West Nile viruses16.Box 2Non-canonical autophagy and immunitySeveral processes have been described in which subsets of the canonical autophagy pathway(see box figure,part a)contribute to immune responses,frequently independent of autophagosome formation.LC3-associated phagocytosis(LAP
32、)mediates the maturation of phagosomes containing TLR ligands independently of the ULK1 complex,but requires the association of rubicon with the PI3KC3 complex8(see box figure,part b).Brucella abortus subverts the ULK1 and PI3KC3 complexes,but not the ATG16L1 complex or LC3,to generate pathogen-cont
33、aining vacuoles which can either be single or double membrane structures160(see box figure,part c).The recruitment of IFN-effector molecules(IRGs and GBPs)to the parasitophorous vacuole during Toxoplasma gondii infection is dependent on the the ATG16L1 complex but not autophagosome formation or lyso
34、somes11(see box figure,part d).A similar process is involved in inhibition of norovirus replication by IFN-10.ATG5 prevents neutrophil-mediated lung damage during Mycobacterium tuberculosis infection independently of the other ATG16L1 complex members161(see box figure,part e).It is unclear whether t
35、his function of ATG5 represents a type of non-canonical autophagy or a new function of ATG5.There are several other examples not depicted.Coronavirus replication occurs on ER-derived membranes decorated by the unconjugated form of LC3,and does not require the rest of the autophagy machinery162.ATG9L
36、1 represses DNA stimulation of STING independently of the LC3 conjugation machinery.LC3 can be recruited to double-membrane structures containing internalized Salmonella in the absence of ATG9L1 and subunits of the ULK1 and PI3KC3 complexes163.The inhibitory activity of ATG16L1 on NOD1 and NOD2 sign
37、aling can occur independently of the autophagy function of this protein164.CD4 T cell recognition of DCs harboring T.gondii was shown to depend on ATG5 and not ATG7165.Thus,the immune function of conserved autophagy proteins often reflects a non-canonical form of autophagy.CadwellPage 4Nat Rev Immun
38、ol.Author manuscript;available in PMC 2017 November 01.Author ManuscriptAuthor ManuscriptAuthor ManuscriptAuthor ManuscriptAutophagy also reduces damage inflicted by pathogens.During Sindbis virus infection,autophagy protects against neuronal cell death without affecting the degree of viral replicat
39、ion17.Similarly,in the absence of ATG16L1,the beneficial intestinal virus murine norovirus(MNV)18 triggers intestinal pathologies in mice through the cytokines TNF and IFN19.Viral replication is unaltered in these mice19,consistent with a role for autophagy in suppressing the negative impact of infl
40、ammatory cytokines produced in responses to an enteric virus.Furthermore,TFEB(HLH-30),a master transcriptional regulator of autophagy and lysosomal genes,protects Caenorhabditis elegans from Staphylococcus aureus infection by promoting a cytoprotective transcriptional response20.Autophagy is also es
41、sential for the survival of mice with S.aureus-mediated bacteremia and pneumonia.Instead of reducing bacterial burden in these animals,autophagy promotes the viability of endothelial cells in the presence of a pore-forming toxin produced by widely circulating methicillin resistant S.aureus(MRSA)stra
42、ins21.The vulnerability of autophagy-deficient cells reflects an increase in the toxin receptor ADAM10 at the protein level21,indicating that the protein turnover function of autophagy can indirectly contribute to infectious disease susceptibility.Therapeutically increasing autophagy also improves s
43、urvival in a mouse model of sepsis through reducing inflammation in the lung22.Thus,in addition to resistance mechanisms aimed at reducing the number of infectious agents,autophagy facilitates tolerance mechanisms that reduce the adverse effect of an infection23.A related function of autophagy is to
44、 inhibit the production of soluble inflammatory mediators,which can come at the cost of diminishing host defense.Deficiency in autophagy proteins protects against intestinal infection by Citrobacter rodentium,improves survival during lung infection by influenza virus,and prevents reactivation of mur
45、ine herpesvirus 68(MHV68)from latency2426.The enhanced host defense in these models are associated with large scale increases in cytokine levels and/or inflammatory gene expression.Although autophagy inhibition confers a short-term benefit during infection by these specific pathogens,the long-term c
46、onsequence of this heightened immunity may be chronic inflammatory disease.Additionally,as the above examples with S.aureus,Sindbis virus,and a large number of other studies illustrate,autophagy usually benefits the host during a life-threatening infection.The ultimate function of autophagy in immun
47、ity may be to mediate a CadwellPage 5Nat Rev Immunol.Author manuscript;available in PMC 2017 November 01.Author ManuscriptAuthor ManuscriptAuthor ManuscriptAuthor Manuscriptbalanced immune response whereby the greatest number of different types of infectious threats are neutralized with as little da
48、mage to the host as possible,thereby staving off long term disease.Crosstalk between autophagy and immune signaling cascadesAmino acid starvation during bacterial infection can induce autophagy27,raising the possibility that sensing changes in nutrient availability is an ancient mechanism to turn on
49、 autophagy in response to infectious threats.As the immune system evolved,autophagy and related pathways have been integrated into the complex signaling networks that coordinate multicellular defense strategies.An array of pathogen and damage associated molecular patterns(PAMPs and DAMPs)induce or i
50、nhibit autophagy in specific contexts.Also,autophagy can feedback on the process of PAMP and DAMP recognition to serve a regulatory role of preventing an over exuberant response.This creates a set of feedback loops between autophagy and immune signaling pathways that mediate host defense while limit