PerspectiveHost-Pathogen Interactions

TBK1 Mediates Crosstalk Between the Innate Immune Response and Autophagy

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Sci. Signal.  23 Aug 2011:
Vol. 4, Issue 187, pp. pe39
DOI: 10.1126/scisignal.2002355

Abstract

The autophagic pathway participates in many physiological and pathophysiological processes. Autophagy plays an important role, as part of the innate immune response, in the first line of defense against intruding pathogens. Recognition of pathogens by the autophagic machinery is mainly mediated by autophagic adaptors, proteins that simultaneously interact with specific cargos and components of the autophagic machinery. However, the exact mechanisms and signaling pathways regulating this step are largely unknown. TANK-binding kinase 1 (TBK1) has been implicated recently in the autophagic clearance of the bacterium Salmonella enterica. After its activation by the invading bacteria, TBK1 directly phosphorylated the autophagic adaptor optineurin (OPTN). This modification led to enhanced interaction of OPTN with the family of mammalian Atg8 proteins, which are ubiquitin-like and essential for autophagy. Such interaction allows the autophagic machinery to be recruited to the intracellular loci of the bacteria, resulting in elimination of the bacteria by lysosomes. This study provides an example by which the innate immune response directly regulates cargo recruitment into autophagosomes.

Catabolic processes are fundamental for cell homeostasis. Autophagy plays a crucial role in these processes because it mediates the degradation of a large repertoire of proteins and organelles, providing a major recycling and energy-supply pathway (1). During this process, cytosolic components are enwrapped by double-membrane vesicles, the autophagosomes, and targeted for lysosomal degradation. New lines of evidence, however, point to specificity in the recruitment of cargo molecules into autophagosomes (1). This selectivity implicates the autophagic pathway not only as a bulk catabolic route but also in regulation of different cellular processes, including cell-autonomous defense against invasion of pathogens. Numerous autophagic receptors were reported to control the delivery of specific cargoes to the lysosomes through autophagy (1, 2). A substantial contribution was the identification of proteins such as Nbr1 and Nix as autophagy receptors, achieved by using the yeast two-hybrid system (3, 4). Wild et al. characterized a autophagic adaptor, optineurin (OPTN), as a key component of pathogen-induced autophagy (5). OPTN interacts with the family of Atg8 proteins, which are essential for autophagy, through an Atg8 family–interacting motif (AIM), a feature common to many autophagic adaptors. Atg8 proteins are conjugated to phosphatidylethanolamine (PE) on the autophagic membrane, a process essential both for autophagosome formation and for directing components to the nascent autophagosomal membrane (1). Concomitantly with its interaction with Atg8s, OPTN binds ubiquitin chains, often decorating molecules destined for autophagic degradation (5). These features allow OPTN to recognize ubiquitinated pathogens in the cytosol and recruit the autophagic machinery to their loci. Intriguingly, Wild et al. showed that this process was regulated by the activation of TANK-binding kinase 1 (TBK1), which binds and phosphorylates OPTN on Ser177, leading to enhanced binding to Atg8 proteins (Fig. 1). Apparently, OPTN acts together with the autophagic adapter p62, although on a different subdomain of the pathogen, in the recruitment of Salmonella into autophagosomes (5).

Fig. 1

Recognition of invasive Salmonella by the autophagic machinery. (A) Cytosolic salmonella undergoes ubiquitination (Ub). (B) The autophagic adaptors p62 and NDP52 bind to ubiquitin chains. (C) NDP52, located at the bacterial loci, enables recruitment of TBK1 into a complex with OPTN. (D) The autophagic adaptors bind to LC3 and consequently target the bacteria to the forming autophagosomal membrane. Phosphorylation (P) of OPTN by TBK1 promotes its interaction with LC3.

CREDIT: B. STRAUCH/SCIENCE SIGNALING

Autophagic adaptors were first identified in the yeast system (610). Atg19, a soluble protein, targets aminopeptidase and mannosidase from the cytosol to the vacuole through the autophagy-related cytoplasm-to-vacuole targeting pathway (6). In mammals, a few autophagic adaptors have been identified as mediators between the autophagic machinery and various cellular elements destined for lysosomal degradation (1, 2, 11). Although these adaptors are not essential for the autophagic process, they determine the content of autophagosomes. p62, the first mammalian autophagic adaptor identified (12), is apparently not committed to any particular cargo molecule but rather participates in many autophagic processes, including degradation of protein aggregates, selective clearance of cytosolic proteins such as Nrf2, mitophagy, and pathogen removal (4, 1214). The adaptors NDP52 and Tecpr1 also target bacterial pathogens for selective autophagy, but their participation in additional selective autophagic pathways is not established (15, 16). Whereas both p62 and NDP52 interact with LC3 (a mammalian Atg8 protein), Tecpr1 binds with Atg5 and WIPI-2, acting upstream in the autophagic pathway (15) (Fig. 1). p62 and NDP52 serve as links between the autophagic machinery and pathogens by binding to ubiquitin chains on the invasive bacteria (14, 16). In addition to ubiquitin, the diacylglycerol signaling cascade was reported to be an alternative pathway that controlled autophagic clearance of bacteria (17). It thus seems that host cells use numerous adaptors and signaling pathways to ensure the autophagic clearance of bacteria.

Autophagy serves as a defensive pathway against a broad range of pathogens. This is achieved either by direct elimination of the pathogen or indirectly through the production and delivery of foreign antigens to major histocompatibility complex class II antigen-presenting molecules (18). Accumulating evidence indicates that the latter process is mediated by sensors of the innate immune response. These include Toll-like receptors (TLRs), Nods, and Nod-like receptors, which are pattern-recognition receptors for pathogens (1921). After activation by bacterial lipopolysacharide (LPS), the phagosomal membrane proteins TLR and Fcγ stimulate the NADPH (reduced form of nicotinamide adenine dinucleotide phosphate) oxidase NOX2 to locally produce reactive oxygen species and, as a consequence, trigger autophagic induction (22). Furthermore, a few downstream effectors of TLR4 (a specific LPS receptor)—such as TRIF, RIP1, and p38 mitogen-activated protein kinase—participate in bacteria-dependent induction of autophagy (23). Nod2 and Nod1, the cytosolic sensors of the pathogen muramyl dipeptide, activate autophagy through their interaction with the autophagic factor Atg16L. This interaction facilitates the formation of autophagosomes at the bacterial entry site, providing a direct link between autophagic machinery and the innate immune system (19).

TBK1, a ubiquitously distributed inhibitor of nuclear factor κB kinase (IKK)–related enzyme, participates in the innate immune response downstream of TLRs (24). TBK1 participates in two signaling pathways, one leading to increased transcription of type 1 interferon genes and the other to nuclear translocation of the transcription factor nuclear factor κB (NF-κB). NF-κB is regulated by the “canonical” complex, composed of two IKK proteins and a regulatory subunit called NEMO, and a “noncanonical,” NEMO-independent complex regulated by TBK1. Induction of the canonical NF-κB cascade is dependent on the interaction of NEMO with ubiquitin chains (24). The link between TBK1 activation by the innate immune response and autophagic elimination of bacteria was demonstrated by Thurston et al. (16). According to that report, the autophagic adaptor NDP52 is recruited to ubiquitin-modified Salmonella and consequently targets TBK1 to the bacterial loci. OPTN constitutively interacts with TBK1, suggesting that these two factors arrive together at the ubiquitinated cytosolic bacteria (Fig. 1) (5, 25). These findings may indicate that NDP52 and OPTN function together in mediating Salmonella degradation, with NDP52 acting first. The study by Wild et al. supports this hypothesis (5). For example, simultaneous depletion of NDP52 and OPTN promotes Salmonella replication in a similar manner to depletion of each factor separately. Moreover, NDP52 and OPTN are detectable on the same subdomains on the bacterial surface, domains from which p62 is excluded (5). Thus, it is possible that NDP52 functions upstream of OPTN and locally activates TBK1 to enable recruitment of LC3 by OPTN. The possibility that TBK1 activates NDP52 by phosphorylation has yet to be tested.

OPTN has been identified as a causative gene in several diseases and disorders, such as glaucoma, Paget’s disease, and lateral sclerosis (2628). It is not clear, however, whether these pathologies are the result of defective clearance by the autophagic system. Mutations in two different autophagic adaptors, p62 and OPTN, have been implicated in Paget’s disease (26), implying that defective autophagic clearance may occur in this disorder. Moreover, the existence of such mutants raises the hypothesis that OPTN has a broader specificity toward autophagic cargoes other than pathogens. Another OPTN mutant (Glu487→Gly487) is associated with some forms of lateral sclerosis (27), where it was detected in protein inclusions associated with superoxide dismutase 1. Accumulation of these cytoplasmic inclusions may be a consequence of defective targeting of the autophagic machinery. Indeed, other autophagic receptors, such as p62 and Nbr1, accumulate in protein aggregates upon inhibition of autophagy (3, 29).

The above findings represent part of the growing body of evidence pointing to a major role for autophagy as a protective mechanism, induced in response to pathogen invasion and acting together with the innate immune system to fight intrusions. Although the invasive bacteria are finally eliminated by the autophagic system, the process is initiated by receptors of the innate immune system (18). The report by Wild et al. links the innate immune signaling pathway to the recruitment of pathogens into autophagosomes, a process mediated by TBK1 (5). Future studies will be needed to identify other factors that regulate the crosstalk between the immune system and selective autophagy. It is also important to determine whether TBK1 participates in other types of selective autophagy. In starvation-induced autophagy, the TBK-related kinase IKK acts independently of NF-κB activation (30). This may imply a broader role for IKK kinases in autophagy. Another crucial issue concerning the regulation of selective autophagy is the exact role of the ubiquitin system. For example, characterization of the E3 ubiquitin ligases that act in response to intrusion of different pathogens will provide yet another layer of regulation. A candidate ligase is TRAF6 [tumor necrosis factor receptor–associated factor 6], which previously has been shown to act in the autophagic induction downstream of TLR4. TRAF6 mediates ubiquitination of the autophagic factor beclin-1, which consequently dissociates from B cell lymphoma 2 and induces autophagy (31). Thus, the regulation of cargo recruitment into autophagosomes by the innate immune system is a multilayered process, the characterization of which will provide new and exciting insights into the interaction between autophagy and the immune system.

References and Notes

  1. Funding: Z.E. is the incumbent of the Harold Korda Chair of Biology and is supported in part by the Legacy Heritage Fund and by the Louis Brause Philanthropic Fund.
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