Rab GTPases regulating phagosome maturation are differentially recruited to mycobacterial phagosomes

Mycobacterium tuberculosis (M. tb) is an intracellular pathogen that can replicate within infected macrophages. The ability of M. tb to arrest phagosome maturation is believed to facilitate its intracellular multiplication. Rab GTPases regulate membrane trafficking, but details of how Rab GTPases regulate phagosome maturation and how M. tb modulates their localization during inhibiting phagolysosome biogenesis remain elusive. We compared the localization of 42 distinct Rab GTPases to phagosomes containing either Staphylococcus aureus or M. tb. The phagosomes containing S. aureus were associated with 22 Rab GTPases, but only 5 of these showed similar localization kinetics as the phagosomes containing M. tb. The Rab GTPases responsible for phagosome maturation, phagosomal acidification and recruitment of cathepsin D were examined in macrophages expressing the dominant-negative form of each Rab GTPase. LysoTracker staining and immunofluorescence microscopy revealed that Rab7, Rab20 and Rab39 regulated phagosomal acidification and Rab7, Rab20, Rab22b, Rab32, Rab34, Rab38 and Rab43 controlled the recruitment of cathepsin D to the phagosome. These results suggest that phagosome maturation is achieved by a series of interactions between Rab GTPases and phagosomes and that differential recruitment of these Rab GTPases, except for Rab22b and Rab43, to M. tb-containing phagosomes is involved in arresting phagosome maturation and inhibiting phagolysosome biogenesis.     

Induction of p38 mitogen-activated protein kinase reduces early endosome autoantigen 1 (EEA1) recruitment to phagosomal membranes

Mycobacterium tuberculosis survives in the infected host by parasitizing macrophages in which the bacillus resides in a specialized phagosome sequestered from the phagolysosomal degradative pathway. Here we report a role of the stress-induced p38 mitogen-activated protein kinase (p38 MAPK) in the component of M. tuberculosis phagosome maturation arrest that has been linked previously to the reduced recruitment of the endosomal and phagosomal membrane-tethering molecule called early endosome autoantigen 1 (EEA1; Fratti, R. A., Backer, J. M., Gruenberg, J., Corvera, S., and Deretic, V. (2001) J. Cell Biol. 154, 631-644). A pharmacological inhibition of M. tuberculosis var. bovis Bacillus Calmette-Guérin-induced p38 MAPK activity caused a marked increase in EEA1 colocalization with mycobacterial phagosomes. Consistent with the increase in EEA1 association and its role in phagosomal maturation, the pharmacological block of p38 activity caused phagosomal acidification and enrichment of the late endocytic markers lysobisphosphatidic acid and CD63 (lysosomal integral membrane protein 1) on mycobacterial phagosomes. A negative regulatory role of p38 MAPK activation in phagosome maturation was further demonstrated by converse experiments with latex bead phagosomes. Artificial activation of p38 MAPK caused a decrease in EEA1 colocalization with model latex bead phagosomes, which normally acquire EEA1 and subsequently mature into the phagolysosome. These findings show that p38 MAPK activity contributes to the arrest of M. tuberculosis phagosome maturation and demonstrate a negative regulatory role of p38 in phagolysosome 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.     

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.     

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 glycolipid 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-gamma 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.     

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.     

Autophagy is a defense mechanism inhibiting BCG and Mycobacterium tuberculosis survival in infected macrophages

Mycobacterium tuberculosis is an intracellular pathogen persisting within phagosomes through interference with phagolysosome biogenesis. Here we show that stimulation of autophagic pathways in macrophages causes mycobacterial phagosomes to mature into phagolysosomes. Physiological induction of autophagy or its pharmacological stimulation by rapamycin resulted in mycobacterial phagosome colocalization with the autophagy effector LC3, an elongation factor in autophagosome formation. Autophagy stimulation increased phagosomal colocalization with Beclin-1, a subunit of the phosphatidylinositol 3-kinase hVPS34, necessary for autophagy and a target for mycobacterial phagosome maturation arrest. Induction of autophagy suppressed intracellular survival of mycobacteria. IFN-gamma induced autophagy in macrophages, and so did transfection with LRG-47, an effector of IFN-gamma required for antimycobacterial action. These findings demonstrate that autophagic pathways can overcome the trafficking block imposed by M. tuberculosis. Autophagy, which is a hormonally, developmentally, and, as shown here, immunologically regulated process, represents an underappreciated innate defense mechanism for control of intracellular pathogens.     

A tale of two lipids: Mycobacterium tuberculosis phagosome maturation arrest

Mycobacterium tuberculosis persistence in human populations relies on its ability to inhibit phagosomal maturation. M. tuberculosis resides in a pathogen-friendly phagosome escaping lysosomal bactericidal mechanisms and efficient antigen presentation in the host phagocytic cell. M. tuberculosis phagosome maturation arrest includes the action of mycobacterial lipid products, which mimic mammalian phosphatidylinositols, targeting host cell membrane trafficking processes. These products interfere with membrane trafficking and organelle biogenesis processes initiated by Ca(2+) fluxes, and ending with host cell Rab GTP-binding proteins and their effectors. The block includes phosphatidylinositol 3-kinase and membrane tethering molecules that prepare phagosomes for fusion with other organelles. Understanding these processes could provide new targets for pharmacological intervention in tuberculosis.     

Sphingosine kinase 1 (SK1) is recruited to nascent phagosomes in human macrophages: inhibition of SK1 translocation by Mycobacterium tuberculosis

Mycobacterium tuberculosis (M.tb) is a leading cause of global infectious mortality. The pathogenesis of tuberculosis involves inhibition of phagosome maturation, leading to survival of M.tb within human macrophages. A key determinant is M.tb-induced inhibition of macrophage sphingosine kinase (SK) activity, which normally induces Ca2+ signaling and phagosome maturation. Our objective was to determine the spatial localization of SK during phagocytosis and its inhibition by M.tb. Stimulation of SK activity by killed M.tb, live Staphylococcus aureus, or latex beads was associated with translocation of cytosolic SK1 to the phagosome membrane. In contrast, SK1 did not associate with phagosomes containing live M.tb. To characterize the mechanism of phagosomal translocation, live cell confocal microscopy was used to compare the localization of wild-type SK1, catalytically inactive SK1G82D, and a phosphorylation-defective mutant that does not undergo plasma membrane translocation (SK1S225A). The magnitude and kinetics of translocation of SK1G82D and SK1S225A to latex bead phagosomes were indistinguishable from those of wild-type SK1, indicating that novel determinants regulate the association of SK1 with nascent phagosomes. These data are consistent with a model in which M.tb inhibits both the activation and phagosomal translocation of SK1 to block the localized Ca2+ transients required for phagosome maturation.     

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.     

Cutting edge: Mycobacterium tuberculosis blocks Ca2+ signaling and phagosome maturation in human macrophages via specific inhibition of sphingosine kinase

One-third of the world's population is infected with Mycobacterium tuberculosis (Mtb), and three million people die of tuberculosis each year. Following its ingestion by macrophages (MPs), Mtb inhibits the maturation of its phagosome, preventing progression to a bactericidal phagolysosome. Phagocytosis of Mtb is uncoupled from the elevation in MP cytosolic Ca(2+) that normally accompanies microbial ingestion, resulting in inhibition of phagosome-lysosome fusion and increased intracellular viability. This study demonstrates that the mechanism responsible for this failure of Ca(2+)-dependent phagosome maturation involves mycobacterial inhibition of MP sphingosine kinase. Thus, inhibition of sphingosine kinase directly contributes to survival of Mtb within human MPs and represents a novel molecular mechanism of pathogenesis.     

Mycobacterium tuberculosis-specific phagosome proteome and underlying signaling pathways

The phagosome is very important to host immunity and tissue homeostasis maintenance. The destiny of the phagosome is closely associated with the outcome of the pathogen within. Most pathogens are successfully delivered to the lysosome and destroyed via the fusion of the phagosome with the lysosome. Mycobacterium tuberculosis has evolved multiple tactics to deflect the normal fusion process, such as delaying the phagosome maturation and acidification, thereby evading the immune recognition and subsequent elimination. Identification of the specific constituents of M. tuberculosis phagosome and the underlying signaling pathways are pivotal to define the key molecular features of this process and better targets to control this recalcitrant pathogen. Proteomic profiling is a comprehensive approach to define the protein inventory. In this review, currently available mycobacteria-containing phagosome proteome data were compiled. Ten putative evolutionarily conserved phagosome proteins were summarized. Unique proteins of the M. tuberculosis-containing phagosome proteome were compiled via comparison with other phagosomes, especially the inert latex bead phagosome. Signaling events associated with these unique proteins, such as Rab GTPase and PI3P, were also found and discussed. The data will facilitate better characterization of the M. tuberculosis specific phagosome constituents and involved signaling, and host-derived targets for better tuberculosis control.     

Phagosome dynamics and function

Phagocytosis of microorganisms and other particles is mediated most efficiently by receptors such as Fc-receptors (FcR) and complement-receptors (C3R). Interaction between these receptors and ligands on the particle results in signal transduction events that lead to actin polymerisation and phagosome formation. The phagosome then undergoes a maturation process whereby it transforms into a phagolysosome. Phagosome maturation depends on interactions (fusion events) with early and late endosomes as well as with lysosomes. The fusion processes are regulated by small GTP-binding proteins and other proteins that are also involved in fusion processes in the endocytic pathway. Although most phagocytosed microorganisms are killed in the lysosome, some pathogens have developed survival strategies and are able to live in the harsh conditions in the phagolysosome or interfere with the maturation process and thereby evade destruction by acid hydrolases.     

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.     

Selected lipids activate phagosome actin assembly and maturation resulting in killing of pathogenic mycobacteria

Pathogenic mycobacteria such as Mycobacterium tuberculosis and Mycobacterium avium facilitate disease by surviving intracellularly within a potentially hostile environment: the macrophage phagosome. They inhibit phagosome maturation processes, including fusion with lysosomes, acidification and, as shown here, membrane actin assembly. An in vitro assay developed for latex bead phagosomes (LBPs) provided insights into membrane signalling events that regulate phagosome actin assembly, a process linked to membrane fusion. Different lipids were found to stimulate or inhibit actin assembly by LBPs and mycobacterial phagosomes in vitro. In addition, selected lipids activated actin assembly and phagosome maturation in infected macrophages, resulting in a significant killing of M. tuberculosis and M. avium. In contrast, the polyunsaturated sigma-3 lipids behaved differently and stimulated pathogen growth. Thus, lipids can be involved in both stimulatory and inhibitory signalling networks in the phagosomal membrane.     

Mycobacterium tuberculosis reprograms waves of phosphatidylinositol 3-phosphate on phagosomal organelles

The potent human pathogen Mycobacterium tuberculosis persists in macrophages within a specialized, immature phagosome by interfering with the pathway of phagolysosome biogenesis. The molecular mechanisms underlying this process remain to be fully elucidated. Here, using four-dimensional microscopy, we detected on model phagosomes, which normally mature into phagolysosomes, the existence of cyclical waves of phosphatidylinositol 3-phosphate (PI3P), a membrane trafficking regulatory lipid essential for phagosomal acquisition of lysosomal characteristics. We show that mycobacteria interfere with the dynamics of PI3P on phagosomal organelles by altering the timing and characteristics of the PI3P waves on phagosomes. The default program of cyclical PI3P waves on model phagosomes is composed of an initial stage (phase I), represented by a strong PI3P burst occurring only upon the completion of phagosome formation, and a subsequent stage (phase II) of recurring PI3P waves on maturing phagosomes with the average periodicity of 20 min. Mycobacteria alter this program in two ways: (i) by inducing, in a cholesterol-dependent fashion, a neophase I* of premature PI3P production, coinciding with the process of mycobacterial entry into the macrophage, and (ii) by inhibiting the calmodulin-dependent phase II responsible for the acquisition of lysosomal characteristics. We conclude that the default pathway of phagosomal maturation into the phagolysosome includes temporally organized cyclical waves of PI3P on phagosomal membranes and that this process is targeted for reprogramming by mycobacteria as they prevent phagolysosome formation.     

Kinetics of phosphatidylinositol-3-phosphate acquisition differ between IgG bead-containing phagosomes and Mycobacterium tuberculosis-containing phagosomes

A key aspect of Mycobacterium tuberculosis pathogenesis is the ability of the bacteria to survive within the host macrophage. A phagosome containing an IgG-coated bead matures into a lysosomal compartment as evidenced by a decrease in pH and an increased acquisition of hydrolytic enzymes. In contrast, when M. tuberculosis is phagocytosed, the maturation of the bacteria-containing phagosome is arrested, and the bacterium resides within a vacuole that retains characteristics of early endosomal compartments. M. tuberculosis-containing phagosomes are delayed in the recruitment of the early endosome autoantigen EEA1. Acquisition of EEA1 is dependent on the presence of phosphatidylinositol-3-phosphate (PI-3-P) generated by the kinase Vps34. We tested the hypothesis that delayed recruitment of EEA1 was due to altered kinetics of PI-3-P accumulation at the phagosomal membrane. Biochemical analysis of the phosphatidylinositol phosphates on M. tuberculosis-containing phagosomes revealed that PI-3-P acquisition was markedly retarded and reduced in comparison to IgG bead-containing phagosomes. Given the role these lipids play in the regulation of phagosome maturation these findings have implications with respect to the mechanisms behind the arrest of phagosome maturation.     

The Leishmania donovani Lipophosphoglycan Excludes the Vesicular Proton-ATPase from Phagosomes by Impairing the Recruitment of Synaptotagmin V

We recently showed that the exocytosis regulator Synaptotagmin (Syt) V is recruited to the nascent phagosome and remains associated throughout the maturation process. In this study, we investigated the possibility that Syt V plays a role in regulating interactions between the phagosome and the endocytic organelles. Silencing of Syt V by RNA interference revealed that Syt V contributes to phagolysosome biogenesis by regulating the acquisition of cathepsin D and the vesicular proton-ATPase. In contrast, recruitment of cathepsin B, the early endosomal marker EEA1 and the lysosomal marker LAMP1 to phagosomes was normal in the absence of Syt V. As Leishmania donovani promastigotes inhibit phagosome maturation, we investigated their potential impact on the phagosomal association of Syt V. This inhibition of phagolysosome biogenesis is mediated by the virulence glycolipid lipophosphoglycan, a polymer of the repeating Galβ1,4Manα1-PO₄ units attached to the promastigote surface via an unusual glycosylphosphatidylinositol anchor. Our results showed that insertion of lipophosphoglycan into ganglioside GM1-containing microdomains excluded or caused dissociation of Syt V from phagosome membranes. As a consequence, L. donovani promatigotes established infection in a phagosome from which the vesicular proton-ATPase was excluded and which failed to acidify. Collectively, these results reveal a novel function for Syt V in phagolysosome biogenesis and provide novel insight into the mechanism of vesicular proton-ATPase recruitment to maturing phagosomes. We also provide novel findings into the mechanism of Leishmania pathogenesis, whereby targeting of Syt V is part of the strategy used by L. donovani promastigotes to prevent phagosome acidification.     

Modulation of cellular phosphatidylinositol 3-phosphate levels in primary macrophages affects heat-killed but not viable Mycobacterium avium's transport through the phagosome maturation process

Most disease causing mycobacteria are intramacrophage pathogens which replicate within nonacidified phagosomes that can interact with the early endosomal network but fail to mature to a phagolysosome. The mycobacterial phagosome retain some proteins required for fusion with endocytic vesicles including Rab5 but lack others such as early endosomal autoantigen 1 (EEA1). As the membrane lipid phosphatidylinositol 3-phosphate (PtdIns-3-P) is required for EEA1 membrane association and phagosome maturation, it may be a potential target of pathogenic mycobacteria. To test this hypothesis, macrophage cellular levels of PtdIns-3-P were altered by retroviral introduction of the type III Phosphoinositide 3-Kinase (VPS34) and the PtdIns-3-P phosphatase myotubularin 1 (MTM1). By utilizing the PtdIns-3-P-specific probes FYVE and PX coupled to EGFP (EGFP-2-FYVE and EGFP-PX, respectively), the expression of PtdIns-3-P on the mycobacterial phagosome was addressed. All phagosomes containing viable Mycobacterium avium stained positive for EGFP-2-FYVE and EGFP-PX despite obvious differences in PtdIns-3-P concentrations in cells expressing MTM1 or VPS34. Altering PtdIns-3-P cellular concentrations did not affect trafficking of live bacilli. However, a significant increase in the transport of killed bacilli to a late endosomal/lysosomal compartment was observed in VPS34-compared to MTM1-transduced macrophages. Therefore, although overexpression of PdtIns-3-P in macrophages can facilitate phagosome maturation, its effect on phagosomes containing viable M. avium was negligible.     

Biogenesis of phagolysosomes: the 'kiss and run' hypothesis

Particles such as microorganisms that are taken up by the cell into phagosomes are usually ultimately degraded in phagolysosomes. However, despite its importance, phagolysosome biogenesis is poorly understood. This article presents a model for phagosome maturation into phagolysosomes that involves multiple transient fusion-fission interactions of phagosomes with endocytic organelles via a fusion-pore-like structure. This dynamic process may be modulated by the sequential appearance and disappearance of key phagosome proteins.