George B. Kyei

Toll-like receptors control autophagy.

Autophagy is a newly recognized innate defense mechanism, acting as a cell‐autonomous system for elimination of intracellular pathogens. The signals and signalling pathways inducing autophagy in response to pathogen invasion are presently not known. Here we show that autophagy is controlled by recognizing conserved pathogen‐associated molecular patterns (PAMPs). We screened a PAMP library for effects on autophagy in RAW 264.7 macrophages and found that several prototype Toll‐like receptor (TLR) ligands induced autophagy. Single‐stranded RNA and TLR7 generated the most potent effects. Induction of autophagy via TLR7 depended on MyD88 expression. Stimulation of autophagy with TLR7 ligands was functional in eliminating intracellular microbes, even when the target pathogen was normally not associated with TLR7 signalling. These findings link two innate immunity defense systems, TLR signalling and autophagy, provide a potential molecular mechanism for induction of autophagy in response to pathogen invasion, and show that the newly recognized ability of TLR ligands to stimulate autophagy can be used to treat intracellular pathogens.

Autophagy pathway intersects with HIV-1 biosynthesis and regulates viral yields in macrophages

1. 1. Kyei, 2. et al . 2009. J. Cell Biol. doi:[10.1083/jcb.200903070][1] [1]: /lookup/doi/10.1083/jcb.200903070

Mycobacterium tuberculosis inhibition of phagolysosome biogenesis and autophagy as a host defence mechanism

A marquee feature of the powerful human pathogen Mycobacterium tuberculosis is its macrophage parasitism. The intracellular survival of this microorganism rests upon its ability to arrest phagolysosome biogenesis, avoid direct cidal mechanisms in macrophages, and block efficient antigen processing and presentation. Mycobacteria prevent Rab conversion on their phagosomes and elaborate glycolypid and protein trafficking toxins that interfere with Rab effectors and regulation of specific organellar biogenesis in mammalian cells. One of the major Rab effectors affected in this process is the type III phosphatidylinositol 3‐kinase hVPS34 and its enzymatic product phosphatidylinositol 3‐phosphate (PI3P), a regulatory lipid earmarking organellar membranes for specific trafficking events. PI3P is also critical for the process of autophagy, recently recognized as an effector of innate and adaptive immunity. Induction of autophagy by physiological, pharmacological or immunological signals, including the major antituberculosis Th1 cytokine IFN‐γ and its downstream effector p47 GTPase LRG‐47, can overcome mycobacterial phagosome maturation block and inhibit intracellular M. tuberculosis survival. This review summarizes the findings centred around the PI3P‐nexus where the mycobacterial phagosome maturation block and execution stages of autophagy intersect.

Delivery of Cytosolic Components by Autophagic Adaptor Protein p62 Endows Autophagosomes with Unique Antimicrobial Properties

Autophagy allows cells to self-digest portions of their own cytoplasm for a multitude of physiological purposes, including innate and adaptive immunity functions. In one of its innate immunity manifestations, autophagy, is known to contribute to the killing of intracellular microbes, including Mycobacterium tuberculosis, although the molecular mechanisms have been unclear. Here, we delineated sequential steps of the autophagic pathway necessary to control intracellular M. tuberculosis and found that in addition to autophagy initiation and maturation, an accessory autophagy-targeting molecule p62 (A170 or SQSTM1) was required for mycobactericidal activity. The p62 adaptor protein delivered specific ribosomal and bulk ubiquitinated cytosolic proteins to autolysosomes where they were proteolytically converted into products capable of killing M. tuberculosis. Thus, p62 brings cytosolic proteins to autolysosomes where they are processed from innocuous precursors into neo-antimicrobial peptides, explaining in part the unique bactericidal properties of autophagic organelles.

Rab14 is critical for maintenance of Mycobacterium tuberculosis phagosome maturation arrest

Mycobacterium tuberculosis arrests phagosomal maturation in infected macrophage, and, apart from health significance, provides a superb model system to dissect the phagolysosomal biogenesis pathway. Here, we demonstrate a critical role for the small GTPase Rab14 in maintaining mycobacterial phagosome maturation block. Four‐dimensional microscopy showed that phagosomes containing live mycobacteria accumulated Rab14 following phagocytosis. The recruitment of Rab14 had strong functional consequence, as a knockdown of endogenous Rab14 by siRNA or overexpression of Rab14 dominant‐negative mutants (Rab14S25N and Rab14N125I) released the maturation block and allowed phagosomes harboring live mycobacteria to progress into phagolysosomes. Conversely, overexpression of the wild‐type Rab14 and the constitutively active mutant Rab14Q70L prevented phagosomes with dead mycobacteria from undergoing default maturation into phagolysosomal organelles. Mechanistic studies demonstrated a role for Rab14 in stimulating organellar fusion between phagosomes and early endosomes but not with late endosomes. Rab14 enables mycobacterial phagosomes to maintain early endosomal characteristics and avoid late endosomal/lysosomal degradative components.

Mechanism of Inducible Nitric Oxide Synthase Exclusion from Mycobacterial Phagosomes

Mycobacterium tuberculosis is sensitive to nitric oxide generated by inducible nitric oxide synthase (iNOS). Consequently, to ensure its survival in macrophages, M. tuberculosis inhibits iNOS recruitment to its phagosome by an unknown mechanism. Here we report the mechanism underlying this process, whereby mycobacteria affect the scaffolding protein EBP50, which normally binds to iNOS and links it to the actin cytoskeleton. Phagosomes harboring live mycobacteria showed reduced capacity to retain EBP50, consistent with lower iNOS recruitment. EBP50 was found on purified phagosomes, and its expression increased upon macrophage activation, paralleling expression changes seen with iNOS. Overexpression of EBP50 increased while EBP50 knockdown decreased iNOS recruitment to phagosomes. Knockdown of EBP50 enhanced mycobacterial survival in activated macrophages. We tested another actin organizer, coronin-1, implicated in mycobacterium-macrophage interaction for contribution to iNOS exclusion. A knockdown of coronin-1 resulted in increased iNOS recruitment to model latex bead phagosomes but did not increase iNOS recruitment to phagosomes with live mycobacteria and did not affect mycobacterial survival. Our findings are consistent with a model for the block in iNOS association with mycobacterial phagosomes as a mechanism dependent primarly on reduced EBP50 recruitment.

Higher order Rab programming in phagolysosome biogenesis

Phagosomes offer kinetically and morphologically tractable organelles to dissect the control of phagolysosome biogenesis by Rab GTPases. Model phagosomes harboring latex beads undergo a coordinated Rab5–Rab7 exchange, which is akin to the process of endosomal Rab conversion, the control mechanisms of which are unknown. In the process of blocking phagosomal maturation, the intracellular pathogen Mycobacterium tuberculosis prevents Rab7 acquisition, thus, providing a naturally occurring tool to study Rab conversion. We show that M. tuberculosis inhibition of Rab7 acquisition and arrest of phagosomal maturation depends on Rab22a. Four-dimensional microscopy revealed that phagosomes harboring live mycobacteria recruited and retained increasing amounts of Rab22a. Rab22a knockdown in macrophages via siRNA enhanced the maturation of phagosomes with live mycobacteria. Conversely, overexpression of the GTP-locked mutant Rab22aQ64L prevented maturation of phagosomes containing heat-killed mycobacteria, which normally progress into phagolysosomes. Moreover, Rab22a knockdown led to Rab7 acquisition by phagosomes harboring live mycobacteria. Our findings show that Rab22a defines the critical checkpoint for Rab7 conversion on phagosomes, allowing or disallowing organellar transition into a late endosomal compartment. M. tuberculosis parasitizes this process by actively recruiting and maintaining Rab22a on its phagosome, thus, preventing Rab7 acquisition and blocking phagolysosomal biogenesis.

Autophagy in Immune Defense Against Mycobacterium tuberculosis

Autophagy is a newly recognized innate and adaptive immunity defense against intracellular pathogens, in keeping with its role as a cytoplasmic maintenance pathway. Induction of autophagy by physiological, pharmacological or immunological means can eliminate intracellular Mycobacterium tuberculosis, providing one of the first examples of the immunological role of autophagy. Under normal circumstances, M. tuberculosis survives in macrophages by inhibiting phagolysosome biogenesis. Induction of autophagy overcomes the mycobacterial phagosome maturation block, and delivers the tubercle bacilli to degradative, compartments, where they are eliminated.

Endosomal membrane traffic: convergence point targeted by Mycobacterium tuberculosis and HIV

Inhibition of phagolysosome biogenesis in infected macrophages is a classical pathogenesis determinant of Mycobacterium tuberculosis. In this review we primarily cover the cellular mechanisms of M. tuberculosis phagosome maturation arrest. A detailed picture is beginning to emerge, involving regulators of membrane trafficking in mammalian cells and phagosomal interactions with endosomal organelles and the trans‐Golgi network. We also present a hypothesis that overlaps may exist between the mycobacterial interference with the host cell membrane trafficking processes and the targeting of the late endosomal sorting machinery by HIV during viral budding in macrophages. We propose that interference with the endosomal sorting machinery contributes to the synergism between the two significant human diseases – AIDS and tuberculosis.

Phosphoinositides in phagolysosome and autophagosome biogenesis

Interconversions of phosphoinositides play a pivotal role during phagocytosis and at the subsequent stages of phagosomal maturation into the phagolysosome. Several model systems have been used to study the role of phosphoinositides in phagosomal membrane remodelling. These include phagosomes formed by inanimate objects such as latex beads, or pathogenic bacteria, e.g. Mycobacterium tuberculosis. The latter category provides naturally occurring tools to dissect membrane trafficking processes governing phagolysosome biogenesis. M. tuberculosis persists in infected macrophages by blocking Rab conversion and affecting Rab effectors. One of the major Rab effectors involved in this process is the type III phosphatidylinositol 3-kinase hVPS34. The lipid kinase hVPS34 and its enzymatic product PtdIns3P are critical for the default pathway of phagosomal maturation into phagolysosomes. Mycobacteria block PtdIns3P production and thus arrest phagosomal maturation. PtdIns3P is also critical for the process of autophagy, recently recognized as an effector of innate immunity defenses. Induction of autophagy by pharmacological, physiological, or immunological means, overcomes mycobacterial phagosome maturation block in a PtdIns3P generation dependent manner and eliminates intracellular M. tuberculosis. PtdIns3P and PtdIns3P-dependent processes represent an important cellular nexus where fundamental trafficking processes, disease causing host-pathogen interactions, and innate and adaptive immunity defense mechanisms meet.