Distinct roles of class I and class III phosphatidylinositol 3-kinases in phagosome formation and maturation

Roles of PI3K in neutrophil function

Neutrophils are terminally differentiated cells that play a vital role in host defense. It has recently become evident that phospholipid regulation plays an import role in many neutrophil functions. We review the regulation of neutrophil functions such as chemotaxis, superoxide production, and phagocytosis by phosphatidylinositol-3,4,5-trisphosphate (PIP3), which is generated in neutrophils by PI3K(gamma). Several lines of evidence are presented demonstrating the importance of this kinase in regulating chemotaxis, in particular the directionality of chemotactic migration. Evidence suggesting that this kinase is important for phagocytosis, especially during engulfment and the internalization of large particles, is also reviewed. Finally, it is suggested that PI3K is important for superoxide production and neutrophil priming. The common link between these seemingly diverse functions is that PI3K(gamma), via its phospholipid products, appears to be providing spatial-temporal cues for the binding of actin-organizing proteins.

Protein targeting to endosomes and phagosomes via FYVE and PX domains

Phosphatidylinositol 3-phosphate (PI3P) is generated on early endosomal and phagosomal membranes by PI 3-kinases. This lipid serves important regulatory functions in phagocytosis, endocytic traffic, receptor signalling and microbial killing through the recruitment and activation of a number of effector proteins. Almost all of these effectors contain FYVE or PX domains, functional protein modules which are conserved from yeast to mammals. Structural information is available regarding the binding of FYVE and PX domains to PI3P. The two domains are highly different, but they have in common that clusters of basic residues mediate ligand binding through interactions with the phosphate groups of PI3P. Most proteins that contain FYVE or PX domains serve as regulators of endocytic membrane trafficking, whereas others function as regulators of phagosome maturation, signal transduction, microbial killing and other cellular activities of relevance for the immune system.

Stable gene silencing in human monocytic cell lines using lentiviral-delivered small interference RNA. Silencing of the p110alpha isoform of phosphoinositide 3-kinase reveals differential regulation of adherence induced by 1alpha,25-dihydroxycholecalciferol and bacterial lipopolysaccharide

Studying mononuclear phagocyte cell biology through genetic manipulation by non-viral transfection methods has been challenging due to the dual problems of low transfection efficiency and the difficulty in obtaining stable transfection. To overcome this problem, we developed a system for mediating RNA interference in monocytic cells. The p110alpha isoform of phosphoinositide 3-kinases (PI3Ks) was silenced using a lentiviral vector expressing short hairpin RNA (shRNA). This resulted in the generation of stable THP-1 and U-937 monocytic cell lines deficient in p110alpha. Notably, p110alpha was silenced without affecting levels of either the other class I(A) PI3K catalytic subunits p110beta and p110delta, or the p85alpha regulatory subunit. The role of p110alpha in mediating cell adherence was examined. Monocyte adherence induced in response to either lipopolysaccharide (LPS) or 1alpha,25-dihydroxycholecalciferol (D(3)) was blocked by the PI3K inhibitor LY294002. However, although adherence induced in response to D(3) was sensitive to silencing of p110alpha, LPS-induced adherence was not. Expression of the monocyte differentiation marker CD11b was also induced by D(3) in a PI3K-dependent manner and gene silencing using shRNA showed that p110alpha was also required for this effect. Taken together, these findings demonstrate that LPS and D(3) use distinct isoforms of class I(A) PI3K to induce functional responses and that lentiviral-mediated delivery of shRNA is a powerful approach to study monocyte biology.

Vacuolar H+-ATPase binding to microfilaments: regulation in response to phosphatidylinositol 3-kinase activity and detailed characterization of the actin-binding site in subunit B

Vacuolar H(+)-ATPase (V-ATPase) binds microfilaments, and that interaction may be mediated by an actin binding domain in subunit B of the enzyme. To test for possible physiologic functions of the actin binding activity of V-ATPase, early responses of resorbing osteoclasts to inhibition of phosphatidylinositol 3-kinase activity by wortmannin and LY294002 were examined. Rapid co-localization between V-ATPase and F-actin was demonstrated by immunocytochemistry, and corresponding association between V-ATPase and F-actin in immunoprecipitations and pelleting assays was detected. This response was reversed as osteoclasts recovered resorptive activity after inhibitors were removed. By expressing and characterizing fusion proteins containing segments of the actin-binding amino-terminal regions of the B subunits of V-ATPase, we mapped the actin-binding site to a 44-amino acid domain. An 11-amino acid segment with a sequence similar to the actin-binding site of human profilin I was detected within this region. 13-Mers containing these profilin-like segments bound actin in fluorescent anisotropy studies and competed with profilin for binding to actin. Using site-directed mutagenesis, the 11-amino acid profilin-like actin-binding motifs (amino acids 49-59 of B1 and 55-65 of B2) were replaced with an 11-amino acid spacer with a sequence based on the homologous sequence from subunit B of Pyrococcus horikoshii, an organism that lacks an actin cytoskeleton. These substitutions eliminated the actin-binding activity of the B subunit fusion proteins. In summary, binding between V-ATPase and F-actin in osteoclasts occurs in response to blocking phosphatidylinositol 3-kinase activity. This response was fully reversible. The actin binding activities of the B subunits of V-ATPase required 11-amino acid actin-binding motifs that are similar in sequence to the actin-binding site of mammalian profilin I.

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.

Involvement of the AP-1 adaptor complex in early steps of phagocytosis and macropinocytosis

The best described function of the adaptor complex-1 (AP-1) is to participate in the budding of clathrin-coated vesicles from the trans-Golgi network and endosomes. Here, we show that AP-1 is also localized to phagocytic cups in murine macrophages as well as in Dictyostelium amoebae. AP-1 is recruited to phagosomal membranes at this early stage of phagosome formation and rapidly dissociates from maturing phagosomes. To establish the role of AP-1 in phagocytosis, we made used of Dictyostelium mutant cells (apm1(-) cells) disrupted for AP-1 medium chain. In this mutant, phagocytosis drops by 60%, indicating that AP-1 is necessary for efficient phagocytosis. Furthermore, phagocytosis in apm1(-) cells is more affected for large rather than small particles, and cells exhibiting incomplete engulfment are then often observed. This suggests that AP-1 could participate in the extension of the phagocytic cup. Interestingly, macropinocytosis, a process dedicated to fluid-phase endocytosis and related to phagocytosis, is also impaired in apm1(-) cells. In summary, our data suggest a new role of AP-1 at an early stage of phagosome and macropinosome formation.

The regulation of phagosome maturation in Dictyostelium

Macropinocytosis (fluid uptake) and phagocytosis (particle uptake) are processes that result in the formation of intracellular membrane enclosed vacuoles termed macropinosomes and phagosomes, respectively. Macropinosomes and phagosomes are modified by fission and fusion reactions with the endo-lysosomal pathway that eventually transform these vacuoles into a lysosomal environment. Many human bacterial pathogens, including species of Mycobacteria, Legionella, and Chlamydia, are thought to survive by disrupting the normal membrane trafficking events that usually result in the formation of phago-lysosomes and death of the microorganism. In addition, a number of important pathogens facilitate homotypic phagosome fusion in order to generate an intracellular environment conducive for survival. A greater understanding of the regulation of phagosomal maturation and fusion will be critical in designing new therapies to treat infections caused by intracellular pathogens. The genetically tractable phagocyte, D. discoideum, has proven extremely useful in dissecting the signaling pathways regulating macropinocytosis, phagocytosis, phagosomal maturation and phagosome-phagosome fusion. A body of knowledge has accumulated and demonstrates important roles for Rab GTPases, the cytoskeleton, phosphoinositide metabolism and pH regulation in regulating phagosome maturation. This review will summarize the current state of knowledge.

Topographical expression of class IA and class II phosphoinositide 3-kinase enzymes in normal human tissues is consistent with a role in differentiation

Background

Growth factor, cytokine and chemokine-induced activation of PI3K enzymes constitutes the start of a complex signalling cascade, which ultimately mediates cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. The PI3K enzyme family is divided into 3 classes; class I (subdivided into IA and IB), class II (PI3K-C2α, PI3K-C2β and PI3K-C2γ) and class III PI3K. Expression of these enzymes in human tissue has not been clearly defined.

Methods

In this study, we analysed the immunohistochemical topographical expression profile of class IA (anti-p85 adaptor) and class II PI3K (PI3K-C2α and PI3K-C2β) enzymes in 104 formalin-fixed, paraffin embedded normal adult human (age 33–71 years, median 44 years) tissue specimens including those from the gastrointestinal, genitourinary, hepatobiliary, endocrine, integument and lymphoid systems. Antibody specificity was verified by Western blotting of cell lysates and peptide blocking studies. Immunohistochemistry intensity was scored from undetectable to strong.

Results

PI3K enzymes were expressed in selected cell populations of epithelial or mesenchymal origin. Columnar epithelium and transitional epithelia were reactive but mucous secreting and stratified squamous epithelia were not. Mesenchymal elements (smooth muscle and endothelial cells) and glomerular epithelium were only expressed PI3K-C2α while ganglion cells expressed p85 and PI3K-C2β. All three enzymes were detected in macrophages, which served as an internal positive control. None of the three PI3K isozymes was detected in the stem cell/progenitor compartments or in B lymphocyte aggregates.

Conclusions

Taken together, these data suggest that PI3K enzyme distribution is not ubiquitous but expressed selectively in fully differentiated, non-proliferating cells. Identification of the normal in vivo expression pattern of class IA and class II PI3K paves the way for further analyses which will clarify the role played by these enzymes in inflammatory, neoplastic and other human disease conditions.

Class I phosphoinositide 3-kinase p110beta is required for apoptotic cell and Fcgamma receptor-mediated phagocytosis by macrophages

Phosphoinositide 3-kinases (PI3Ks) play an important role in a variety of cellular functions, including phagocytosis. PI3Ks are activated during phagocytosis induced by several receptors and have been shown to be required for phagocytosis through the use of inhibitors such as wortmannin and LY294002. Mammalian cells have multiple isoforms of PI3K, and the role of the individual isoforms during phagocytosis has not been addressed. The class I PI3Ks consist of a catalytic p110 isoform associated with a regulatory subunit. Mammals have three genes for the class IA p110 subunits encoding p110alpha, p110beta, and p110delta and one gene for the class IB p110 subunit encoding p110gamma. Here we report a specific recruitment of p110beta and p110delta (but not p110alpha) isoforms to the nascent phagosome during apoptotic cell phagocytosis by fibroblasts. By microinjecting inhibitory antibodies specific to class IA p110 subunits, we have shown that p110beta is the major isoform required for apoptotic cell and Fcgamma receptor-mediated phagocytosis by primary mouse macrophages. Macrophages from mice expressing a catalytically inactive form of p110delta showed no defect in the phagocytosis of apoptotic cells and IgG-opsonized particles, confirming the lack of a major role for p110delta in this process. Similarly, p110gamma-deficient macrophages phagocytosed apoptotic cells normally. Our findings demonstrate that p110beta is the major class I catalytic isoform required for apoptotic cell and Fcgamma receptor-mediated phagocytosis by primary macrophages.

Phagosomes fuse with late endosomes and/or lysosomes by extension of membrane protrusions along microtubules: role of Rab7 and RILP

Nascent phagosomes must undergo a series of fusion and fission reactions to acquire the microbicidal properties required for the innate immune response. Here we demonstrate that this maturation process involves the GTPase Rab7. Rab7 recruitment to phagosomes was found to precede and to be essential for their fusion with late endosomes and/or lysosomes. Active Rab7 on the phagosomal membrane associates with the effector protein RILP (Rab7-interacting lysosomal protein), which in turn bridges phagosomes with dynein-dynactin, a microtubule-associated motor complex. The motors not only displace phagosomes in the centripetal direction but, strikingly, promote the extension of phagosomal tubules toward late endocytic compartments. Fusion of tubules with these organelles was documented by fluorescence and electron microscopy. Tubule extension and fusion with late endosomes and/or lysosomes were prevented by expression of a truncated form of RILP lacking the dynein-dynactin-recruiting domain. We conclude that full maturation of phagosomes requires the retrograde emission of tubular extensions, which are generated by activation of Rab7, recruitment of RILP, and consequent association of phagosomes with microtubule-associated motors.

Novel PI 3-kinase-dependent mechanisms of trypanosome invasion and vacuole maturation

Mammalian cell invasion by the protozoan parasite, Trypanosoma cruzi, is facilitated by the activation of host cell phosphatidylinositol 3 (PI 3)-kinases. We demonstrate that the well-characterized Ca2+-regulated lysosome-mediated parasite entry pathway is abolished by wortmannin pretreatment. In addition, we have characterized a novel route of T. cruzi invasion unexpectedly revealed in the course of this study. For over a decade, targeted exocytosis of lysosomes at the host cell plasma membrane was considered as the primary mechanism for T. cruzi entry into non-professional phagocytic cells. We now provide evidence that a significant fraction (50% or greater) of invading T. cruzi trypomastigotes exploit an alternate actin-independent entry pathway that involves formation of a tightly associated host cell plasma membrane-derived vacuole enriched in the lipid products of class I PI 3-kinases, PtdInsP3/PtdIns(3,4)P2. Initially devoid of lysosomal markers, the resultant parasite-containing vacuoles gradually acquire lysosome associated membrane protein 1 (lamp-1) and fluid phase endocytic tracer from the lysosomal compartment. In striking contrast to latex bead phagosomes, few T. cruzi vacuoles associate with the early endosomal marker, EEA1 and the 'maturation' process becomes refractory to PI 3-kinase inhibition immediately following parasite internalization. Jointly, these data provide a new paradigm for T. cruzi invasion of non-professional phagocytic cells and reveal a novel vacuole maturation process that appears to bypass the requirement for EEA1.

Tuberculosis toxin blocking phagosome maturation inhibits a novel Ca2+/calmodulin-PI3K hVPS34 cascade

The capacity of Mycobacterium tuberculosis to infect latently over one billion people and cause two million fatalities annually rests with its ability to block phagosomal maturation into the phagolysosome in infected macrophages. Here we describe how M. tuberculosis toxin lipoarabinomannan (LAM) causes phagosome maturation arrest, interfering with a new pathway connecting intracellular signaling and membrane trafficking. LAM from virulent M. tuberculosis, but not from avirulent mycobacteria, blocked cytosolic Ca2+ increase. Ca2+ and calmodulin were required for a newly uncovered Ca2+/calmodulin phosphatidylinositol (PI)3 kinase hVPS34 cascade, essential for production of PI 3 phosphate (PI3P) on liposomes in vitro and on phagosomes in vivo. The interference of the trafficking toxin LAM with the calmodulin-dependent production of PI3P described here ensures long-term M. tuberculosis residence in vacuoles sequestered away from the bactericidal and antigen-processing organelles in infected macrophages.

Insulin induces phosphatidylinositol-3-phosphate formation through TC10 activation

Phosphatidylinositol-3-phosphate (PtdIns-3-P) is considered as a lipid constitutively present on endosomes; it does not seem to have a dynamic role in signalling. In contrast, phosphatidylinositol-3,4,5-trisphosphate (PtdIns-3,4,5-P(3)) plays a crucial role in different signalling pathways including translocation of the glucose transporter protein GLUT4 to the plasma membrane upon insulin receptor activation. GLUT4 translocation requires activation of two distinct pathways involving phosphatidylinositol 3-kinase (PI 3-K) and the small GTP-binding protein TC10, respectively. The contribution of each pathway remains to be elucidated. Here we show that insulin specifically induces the formation of PtdIns-3-P in insulin- responsive cells. The insulin-mediated formation of PtdIns-3-P occurs through the activation of TC10 at the lipid rafts subdomain of the plasma membrane. Exogenous PtdIns-3-P induces the plasma membrane translocation of both overexpressed and endogenous GLUT4. These data indicate that PtdIns-3-P is specifically produced downstream from insulin-mediated activation of TC10 to promote the plasma membrane translocation of GLUT4. These results give a new insight into the intracellular role of PtdIns-3-P and shed light on some aspects of insulin signalling so far not completely understood.

Critical role for scaffolding adapter Gab2 in Fc gamma R-mediated phagocytosis

Grb2-associated binder 2 (Gab2), a member of the Dos/Gab subfamily scaffolding molecules, plays important roles in regulating the growth, differentiation, and function of many hematopoietic cell types. In this paper, we reveal a novel function of Gab2 in Fcgamma receptor (FcgammaR)-initiated phagocytosis in macrophages. Upon FcgammaR activation, Gab2 becomes tyrosyl phosphorylated and associated with p85, the regulatory subunit of phosphoinositide 3-kinase (PI3K), and the protein-tyrosine phosphatidylinositol Shp-2. FcgammaR-mediated phagocytosis is severely impaired in bone marrow-derived macrophages from Gab2-/- mice. The defect in phagocytosis correlates with decreased FcgammaR-evoked activation of Akt, a downstream target of PI3K. Using confocal fluorescence microscopy, we find that Gab2 is recruited to the nascent phagosome, where de novo PI3K lipid production occurs. Gab2 recruitment requires the pleckstrin homology domain of Gab2 and is sensitive to treatment with the PI3K inhibitor wortmannin. The Grb2 binding site on Gab2 also plays an auxiliary role in recruitment to the phagosome. Because PI3K activity is required for FcgammaR-mediated phagocytosis, our results indicate that Gab2 acts as a key component of FcgammaR-mediated phagocytosis, most likely by amplifying PI3K signaling in the nascent phagosome.

Dynamics of phosphoinositides in membrane retrieval and insertion

The phosphoinositides PtdIns(4,5)P2 and PtdIns(3,4,5)P3 are concentrated in plasma membranes of eukaryotic cells, and excluded from endosomes, whereas PtdIns(3)P is formed in these latter intracellular membranes and is apparently excluded from the plasma membrane. The logic of this asymmetric disposition is now revealed by the nature of the effector proteins that selectively bind these lipids through specific modules and by the processes that they catalyze. PtdIns(3,4,5)P3 has a role in directing exocytosis, in addition to many other signaling events, whereas PtdIns(4,5)P2 directs endocytosis through its ability to anchor several coat proteins to the plasma membrane. Remarkably, the elimination of PtdIns(4,5)P2 from forming endosomes may be required for membrane fission to occur. Thus membrane insertion and retrieval can be regulated by plasma membrane concentrations of PtdIns(3,4,5)P3 and PtdIns(4,5)P2, whereas PtdIns(3)P directs the downstream trafficking and recycling of intracellular membranes through its attraction of proteins that catalyze these processes. The phosphoinositides thereby control many cell features that depend upon protein sorting, including the composition of the plasma membrane itself, which in turn determines the cell's responses to its environment.

Phagosome maturation: a few bugs in the system

Cells of the innate immune system ingest and destroy invading microorganisms by initially engulfing them into a specialized vacuole, known as the phagosome. The membrane of the forming phagosome is similar to the plasmalemma and its contents resemble the extracellular milieu. As such, the nascent phagosome is not competent to kill and eliminate the ingested microorganisms. However, shortly after sealing, the phagosome undergoes a series of rapid and extensive changes in its composition, the result of a sophisticated sequence of membrane fusion and fission reactions. Understanding the molecular basis of these events is of particular importance, since they are often the target of disruption by intracellular parasites such as Mycobacterium, Salmonella and Legionella. The objective of this review is to summarize the current knowledge of the molecular mechanisms underlying phagosomal maturation and its subversion by parasitic microorganisms.

Role of lipid-mediated signal transduction in bacterial internalization

Receptor-mediated phagocytosis normally represents an important first line of immune defence. Invading microbes are internalized into phagosomes and are typically killed by exposure to a battery of microbicidal agents. To some intracellular pathogens, however, receptor-mediated phagocytosis represents an opportunity to access a protected niche within the host cell. Another type of intracellular pathogen, including Salmonella enterica serovar Typhimurium and Shigella flexneri, invade host cells in a more direct manner. These pathogens deliver effectors into the host cell via a type III secretion apparatus, initiating a ruffling response that leads to their uptake into intracellular vacuoles. Recent studies have demonstrated the importance of lipid signal transduction events in the uptake of pathogenic bacteria by both receptor-mediated phagocytosis and type III secretion-mediated invasion. In this review we highlight some of these discoveries, with a focus on phospholipid-dependent signalling events.

Dynamin regulates focal exocytosis in phagocytosing macrophages

Phagocytosis in macrophages is thought to involve insertion of cytoplasmic vesicles at sites of membrane expansion before particle ingestion ("focal" exocytosis). Capacitance (Cm) measurements of cell surface area were biphasic, with an initial rise indicative of exocytosis followed by a fall upon phagocytosis. Unlike other types of regulated exocytosis, the Cm rise was insensitive to intracellular Ca2+, but was inhibited by guanosine 5'-O-(2-thio)diphosphate. Particle uptake, but not Cm rise, was affected by phosphatidylinositol 3-kinase inhibitors. Inhibition of actin polymerization eliminated the Cm rise, suggesting possible coordination between actin polymerization and focal exocytosis. Introduction of anti-pan-dynamin IgG blocked Cm changes, suggesting that dynamin controls focal exocytosis and thereby phagocytosis. Similarly, recombinant glutathione S-transferase*amphiphysin-SH3 domain, but not a mutated form that cannot bind to dynamin, inhibited both focal exocytosis and phagocytosis. Immunochemical analysis of endogenous dynamin distribution in macrophages revealed a substantial particulate pool, some of which localized to a presumptive endosomal compartment. Expression of enhanced green fluorescent protein*dynamin-2 showed a motile dynamin pool, a fraction of which migrated toward and within the phagosomal cup. These results suggest that dynamin is involved in the production and/or movement of vesicles from an intracellular organelle to the cell surface to support membrane expansion around the engulfed particle.

Mycobacterium tuberculosis glycosylated phosphatidylinositol causes phagosome maturation arrest

The tubercle bacillus parasitizes macrophages by inhibiting phagosome maturation into the phagolysosome. This phenomenon underlies the tuberculosis pandemic involving 2 billion people. We report here how Mycobacterium tuberculosis causes phagosome maturation arrest. A glycosylated M. tuberculosis phosphatidylinositol [mannose-capped lipoarabinomannan (ManLAM)] interfered with the phagosomal acquisition of the lysosomal cargo and syntaxin 6 from the trans-Golgi network. ManLAM specifically inhibited the pathway dependent on phosphatidylinositol 3-kinase activity and phosphatidylinositol 3-phosphate-binding effectors. These findings identify ManLAM as the M. tuberculosis product responsible for the inhibition of phagosomal maturation.

Phagocytosis and the microtubule cytoskeleton

Phagocytosis is a critical host defense mechanism used by macrophages and neutrophils to clear invading pathogens. The complex sequence of events resulting in internalization and degradation of the pathogens is a coordinated process involving lipids, signaling proteins, and the cytoskeleton. Here, we examine the role of the microtubule cytoskeleton in supporting both the engulfment of pathogens and their elimination within phagolysosomes.

Phosphatidylinositol 3-kinases in carcinoembryonic antigen-related cellular adhesion molecule-mediated internalization of Neisseria gonorrhoeae

Neisseria gonorrhoeae can be internalized by mammalian cells through interactions between bacterial opacity-associated (Opa) adhesins and members of the human carcinoembryonic antigen-related cellular adhesion molecule (CEACAM) family. We examined the role of phosphatidylinositol 3-kinases (PI3Ks) in gonococcal invasion of epithelial cell lines expressing either CEACAM1 or CEACAM3. CEACAM3-mediated internalization, but not that mediated by CEACAM1, was accompanied by localized and transient accumulation of the class I PI3K product phosphatidylinositol 3,4,5-trisphosphate at sites of bacterial engulfment. Inhibition of phosphatidylinositol 3-kinases reduced CEACAM3-mediated uptake but, paradoxically, led to an increase in intracellular survival of bacteria internalized via either CEACAM1 or CEACAM3, suggesting additional roles for PI3K products. Consistent with this finding, the class III PI3K product phosphatidylinositol 3-phosphate accumulated and persisted in the membrane of gonococcal phagosomes after internalization. Inhibition of PI3K blocked phagosomal acquisition of the late endosomal marker lysosome-associated membrane protein 2 and reduced phagosomal acidification. Inhibiting phagosomal acidification with concanamycin A also increased survival of intracellular gonococci. These results suggest two modes of action of phosphatidylinositol 3-kinases during internalization of gonococci: synthesis of phosphatidylinositol 3,4,5-trisphosphate is important for CEACAM3-mediated uptake, while phosphatidylinositol 3-phosphate is needed for phagosomal maturation and acidification, which are required for optimal bacterial killing.

Distinct phosphoinositide 3-kinases mediate mast cell degranulation in response to G-protein-coupled versus FcepsilonRI receptors

Phosphoinositide (PI) 3-kinases are critical regulators of mast cell degranulation. The Class IA PI 3-kinases p85/p110beta and p85/p110delta but not p85/p110alpha are required for antigen-mediated calcium flux in RBL-2H3 cells (Smith, A. J., Surviladze, Z., Gaudet, E. A., Backer, J. M., Mitchell, C. A., and Wilson, B. S. et al., (2001) J. Biol. Chem. 276, 17213-17220). We now examine the role of Class IA PI 3-kinases isoforms in degranulation itself, using a single-cell degranulation assay that measures the binding of fluorescently tagged annexin V to phosphatidylserine in the outer leaflet of the plasma membrane of degranulated mast cells. Consistent with previous data, antibodies against p110delta and p110beta blocked FcepsilonR1-mediated degranulation in response to FcepsilonRI ligation. However, antigen-stimulated degranulation was also inhibited by antibodies against p110alpha, despite the fact that these antibodies have no effect on antigen-induced calcium flux. These data suggest that p110alpha mediates a calcium-independent signal during degranulation. In contrast, only p110beta was required for enhancement of antigen-stimulated degranulation by adenosine, which augments mast cell-mediated airway inflammation in asthma. Finally, we examined carbachol-stimulated degranulation in RBL2H3 cells stably expressing the M1 muscarinic receptor (RBL-2H3-M1 cells). Surprisingly, carbachol-stimulated degranulation was blocked by antibody-mediated inhibition of the Class III PI 3-kinase hVPS34 or by titration of its product with FYVE domains. Antibodies against Class IA PI 3-kinases had no effect. These data demonstrate: (a) a calcium-independent role for p110alpha in antigen-stimulated degranulation; (b) a requirement for p110beta in adenosine receptor signaling; and (c) a requirement for hVPS34 during M1 muscarinic receptor signaling. Elucidation of the intersections between these distinct pathways will lead to new insights into mast cell degranulation.

ER-mediated phagocytosis: a new membrane for new functions

Genomics and other high-throughput approaches, such as proteomics, are changing the way we study complex biological systems. In the past few years, these approaches have contributed markedly to improving our understanding of phagocytosis. Indeed, the ability to study biological systems by monitoring hundreds of proteins provides a level of resolution that is not attainable by the usual 'one protein at a time' approach. In this article, I discuss how proteomic approaches have revealed surprising findings that enable us to revisit established concepts, such as the origin of the phagosome membrane, and to propose new models of cell organization and the link between innate and adaptive immunity.

The role of PI3K in immune cells

Members of the phosphoinositide-3 kinase (PI3K) family control several cellular responses including cell growth, survival, cytoskeletal remodeling and the trafficking of intracellular organelles in many different types of cell. In particular PI3K has important functions in the immune system. It has been difficult to evaluate the roles of distinct PI3Ks in cellular immune responses because no PI3K inhibitors are specific for individual family members and because most stimuli activate several PI3K enzymes. The development of gene-targeted mice now enables us to examine the physiological functions of individual PI3K enzymes in the immune system in vivo.

Modulation of Rab5 and Rab7 recruitment to phagosomes by phosphatidylinositol 3-kinase

Phagosomal biogenesis is central for microbial killing and antigen presentation by leukocytes. However, the molecular mechanisms governing phagosome maturation are poorly understood. We analyzed the role and site of action of phosphatidylinositol 3-kinases (PI3K) and of Rab GTPases in maturation using both professional and engineered phagocytes. Rab5, which is recruited rapidly and transiently to the phagosome, was found to be essential for the recruitment of Rab7 and for progression to phagolysosomes. Similarly, functional PI3K is required for successful maturation. Remarkably, inhibition of PI3K did not preclude Rab5 recruitment to phagosomes but instead enhanced and prolonged it. Moreover, in the presence of PI3K inhibitors Rab5 was found to be active, as deduced from measurements of early endosome antigen 1 binding and by photobleaching recovery determinations. Though their ability to fuse with late endosomes and lysosomes was virtually eliminated by wortmannin, phagosomes nevertheless recruited a sizable amount of Rab7. Moreover, Rab7 recruited to phagosomes in the presence of PI3K antagonists retained the ability to bind its effector, Rab7-interacting lysosomal protein, suggesting that it is functionally active. These findings imply that (i) dissociation of Rab5 from phagosomes requires products of PI3K, (ii) PI3K-dependent effectors of Rab5 are not essential for the recruitment of Rab7 by phagosomes, and (iii) recruitment and activation of Rab7 are insufficient to induce fusion of phagosomes with late endosomes and lysosomes. Accordingly, transfection of constitutively active Rab7 did not bypass the block of phagolysosome formation exerted by wortmannin. We propose that Rab5 activates both PI3K-dependent and PI3K-independent effectors that act in parallel to promote phagosome maturation.

Loss of phosphatidylinositol 3-phosphate binding by the C-terminal Tiam-1 pleckstrin homology domain prevents in vivo Rac1 activation without affecting membrane targeting

Dbl family guanine nucleotide exchange factors (GEFs) for Rho family small GTPases invariably contain a pleckstrin homology (PH) domain that immediately follows their Dbl homology (DH) domain. Although the DH domain is responsible for GEF activity, the role of the PH domain is less clear. We previously reported that PH domains from several Dbl family members bind phosphoinositides with very low affinity (K(d) values in the 10 microM range). This suggests that, unlike several other PH domains, those from Dbl proteins will not function as independent membrane-targeting modules. To determine the functional relevance of low affinity phosphoinositide binding, we mutated the corresponding PH domain from Tiam-1 to abolish its weak, specific binding to phosphatidylinositol 3-phosphate. We first confirmed in vitro that phosphoinositide binding by the isolated DH/PH domain was impaired by the mutations but that intrinsic GEF activity was unaffected. We then introduced the PH domain mutations into full-length Tiam-1 and found that its ability to activate Rac1 or serum response factor in vivo was abolished. Immunofluorescence studies showed that membrane targeting of Tiam-1 was essentially unaffected by mutations in the C-terminal PH domain. Our studies therefore indicate that low affinity phosphatidylinositol 3-phosphate binding by the C-terminal PH domain may be critical for in vivo regulation and activity of Tiam-1 but that the PH domain exerts its regulatory effects without altering membrane targeting. We suggest instead that ligand binding to the PH domain induces conformational and/or orientational changes at the membrane surface that are required for maximum exchange activity of its adjacent DH domain.

Phosphoinositide-3-kinase-independent contractile activities associated with Fcgamma-receptor-mediated phagocytosis and macropinocytosis in macrophages

Previous studies have shown that Fcgamma receptor (FcR)-mediated phagocytosis and macropinocytosis in macrophages consist of two dissociable activities: a phosphoinositide 3-kinase (PI3K)-independent extension of phagocytic cups and a PI3K-dependent contractile mechanism that closes phagosomes and ruffles into intracellular organelles. Here, we identify an additional contractile activity that persists in the presence of the PI3K inhibitor wortmannin. ML-7, an inhibitor of myosin-light-chain kinase (MLCK), inhibited FcR-mediated phagocytosis, macropinocytosis and cell movements associated with ruffling. Scanning electron microscopy demonstrated a striking difference in morphology between phagocytic cups in the different inhibitors: whereas phagocytic cups of control cells and wortmannin-treated cells conformed closely to particles and appeared to have constricted them, the phagocytic cups in cells treated with ML-7 were more open. Video microscopy of macrophages expressing green-fluorescent-protein (GFP)-actin fusions revealed that bound IgG-opsonized erythrocytes were squeezed during phagosome formation and closure. In ML-7, GFP-actin-rich protrusions extended outward but failed to squeeze particles. Moreover, in contrast to the effects of PI3K inhibitors, ML-7 markedly reduced ruffle movement, and perturbed circular ruffle formation. These PI3K-independent myosin-II-based contractile activities that squeeze phagocytic cups and curve ruffles therefore represent a third component activity of the actin cytoskeleton during phagocytosis and macropinocytosis.

A novel assay system implicates PtdIns(3,4)P(2), PtdIns(3)P, and PKC delta in intracellular production of reactive oxygen species by the NADPH oxidase

Activated neutrophils assemble an NADPH oxidase enzyme complex to produce superoxide for microbial killing. Much of the initial oxidase assembly occurs on intracellular granules, followed by movement of the oxidase to phagolysosomes and the plasma membrane. We have developed a novel assay system using Streptolysin-O permeabilized neutrophils that recapitulates the initial intracellular activation process while maintaining the ultrastructural features of this granulocytic cell type. Using this system, we biochemically dissect molecular events and signaling pathways involved in NADPH oxidase assembly and demonstrate specific roles for PKC delta, PI(3,4)P(2)/PI(3,4,5)P(3), and PI(3)P in the PMA-dependent intracellular activation process. This system should be of great utility for the study of cell signaling events that regulate the intracellular production of reactive oxygen species by neutrophils.

Inhibition of autophagy in mitotic animal cells

In nutrient-deprived cells autophagy recycles cytoplasmic constituents by engulfing and degrading them in membrane-bound autophagic vacuoles. The regulation of autophagic vacuole formation is poorly understood, but here we show this process is under strict cell-cycle control in cultured animal cells. We found strong inhibition of autophagic vacuole accumulation in nocodazole-arrested pseudo-prometaphase cells, and also in metaphase and anaphase cells generated on release from the nocodazole arrest. Autophagic vacuoles reappeared after closure of the nuclear envelope in telophase/G1. Treatment with phosphoinositide 3(PI3)-kinase inhibitors wortmannin, LY294002 and 3-methyladenine (known to inhibit the autophagic response in interphase cells) rescued autophagy in mitotic cells without inducing reassembly of vesiculated ER and Golgi compartments. The autophagy induced in mitotic cells was inhibited by amino acids, and the resulting autophagosomes contained proteins LC3 and Lamp1, known to be associated with autophagosomes in interphase cells. The mitotic inhibition of autophagy was not relieved by rapamycin treatment or in PDK1-/- embryonic stem cells, by microinjection of inhibitory antibodies against the class III PI3 kinase VPS34, or in cell lines lacking the p85 regulatory subunits of class IA PI3 kinases. Our results show that autophagy is under strict mitotic control and indicate a novel role for phosphoinositide 3-kinases or other wortmannin/LY294002-sensitive kinases in mitotic membrane traffic regulation.

Phagosome maturation: aging gracefully

Foreign particles and apoptotic bodies are eliminated from the body by phagocytic leucocytes. The initial stage of the elimination process is the internalization of the particles into a plasma membrane-derived vacuole known as the phagosome. Such nascent phagosomes, however, lack the ability to kill pathogens or to degrade the ingested targets. These properties are acquired during the course of phagosomal maturation, a complex sequence of reactions that result in drastic remodelling of the phagosomal membrane and contents. The determinants and consequences of the fusion and fission reactions that underlie phagosomal maturation are the topic of this review.

Transforming growth factor beta activates Smad2 in the absence of receptor endocytosis

Like many other cell surface receptors, transforming growth factor beta (TGF-beta) receptors are internalized upon ligand stimulation. Given that the signaling-facilitating molecules Smad anchor for receptor activation (SARA) and Hrs are mainly localized in early endosomes, it was unclear whether receptor internalization is required for Smad2 activation. Using reversible biotin labeling, we directly monitored internalization of the TGF-beta type I receptor. Our data indicate that TGF-beta type I receptor is endocytosed via a clathrin-dependent mechanism and is effectively blocked by depletion of intracellular potassium or by expression of a mutant dynamin (K44A). However, blockage of receptor endocytosis by these two means has no effect on TGF-beta-mediated Smad2 activation. Furthermore, TGF-beta-induced Smad2 activation was unaffected by inhibition of hVPS34 activity with wortmannin or inhibitory anti-hVPS34 antibodies. Finally, we demonstrated that Smad2 interacted with cell surface receptors and that a SARA binding-deficient Smad2 mutant was phosphorylated by the receptors. Thus, our findings suggest that receptor endocytosis is dispersible for TGF-beta-mediated activation of Smad2 and that this activation can be mediated by both SARA-dependent and -independent mechanisms.

Organelle identity and the targeting of peripheral membrane proteins

Within the secretory pathway, most proteins involved in vesicle formation, motor recruitment and vesicle tethering are not integral membrane proteins but, rather, peripheral membrane proteins recruited to the relevant organelles from the cytosol. From recent studies on diverse organelles, it appears that such recruitment is usually mediated by binding to a labile determinant, such as an activated G protein or a short-lived lipid species, whose distribution is restricted to a single organelle. This suggests that these determinants are what specify organelle identity, and raises interesting questions about how they are generated in an organelle-specific fashion.

Potential role of phosphatidylinositol 3 kinase, rather than DNA-dependent protein kinase, in CpG DNA-induced immune activation

Unmethylated CpG motifs present in bacterial DNA stimulate a strong innate immune response. There is evidence that DNA-dependent protein kinase (DNA-PK) mediates CpG signaling. Specifically, wortmannin (an inhibitor of phosphatidylinositol 3 kinase [PI3]-kinases including DNA-PK) interferes with CpG-dependent cell activation, and DNA-PK knockout (KO) mice fail to respond to CpG stimulation. Current studies establish that wortmannin actually inhibits the uptake and colocalization of CpG DNA with toll-like receptor (TLR)-9 in endocytic vesicles, thereby preventing CpG-induced activation of the NF-kappaB signaling cascade. We find that DNA-PK is not involved in this process, since three strains of DNA-PK KO mice responded normally to CpG DNA. These results support a model in which CpG signaling is mediated through TLR-9 but not DNA-PK, and suggest that wortmannin-sensitive member(s) of the PI3-kinase family play a critical role in shuttling CpG DNA to TLR-9.

How to eat something bigger than your head

Macrophage phagocytosis, the engulfment of large particles, is important for host defense against infection and for the removal of dead cells. These phagocytes can internalize particles larger than themselves, obtaining membrane from internal pools including recycling endosomes. Desjardins and coworkers report in this issue of Cell that the endoplasmic reticulum is a major reservoir of membrane for phagocytosis, an observation that may shed light on a number of conundrums in immunology.

Endoplasmic reticulum-mediated phagocytosis is a mechanism of entry into macrophages

Phagocytosis is a key aspect of our innate ability to fight infectious diseases. In this study, we have found that fusion of the endoplasmic reticulum (ER) with the macrophage plasmalemma, underneath phagocytic cups, is a source of membrane for phagosome formation in macrophages. Successive waves of ER become associated with maturing phagosomes during phagolysosome biogenesis. Thus, the ER appears to possess unexpectedly pluripotent fusion properties. ER-mediated phagocytosis is regulated in part by phosphatidylinositol 3-kinase and used for the internalization of inert particles and intracellular pathogens, regardless of their final trafficking in the host. In neutrophils, where pathogens are rapidly killed, the ER is not used as a major source of membrane for phagocytosis. We propose that intracellular pathogens have evolved to adapt and exploit ER-mediated phagocytosis to avoid destruction in host cells.

Phagocytosis of microbes: complexity in action

The phagocytic response of innate immune cells such as macrophages is defined by the activation of complex signaling networks that are stimulated by microbial contact. Many individual proteins have been demonstrated to participate in phagocytosis, and the application of high-throughput tools has indicated that many more remain to be described. In this review, we examine this complexity and describe how during recognition, multiple receptors are simultaneously engaged to mediate internalization, activate microbial killing, and induce the production of inflammatory cytokines and chemokines. Many signaling molecules perform multiple functions during phagocytosis, and these molecules are likely to be key regulators of the process. Indeed, pathogenic microorganisms target many of these molecules in their attempts to evade destruction.

Role of 3-phosphoinositides in the maturation of Salmonella-containing vacuoles within host cells

Salmonella typhimurium invades mammalian cells and replicates within a vacuole that protects it from the host's microbicidal weapons. The Salmonella-containing vacuole (SCV) undergoes a remodelling akin to that of the host cell's endocytic pathway, but SCV progression is arrested prior to fusion with lysosomes. We studied the role of phosphatidylinositol 3-kinase (PI3-K) in SCV maturation within HeLa cells. Phosphatidylinositol 3-phosphate (PI3P), monitored in situ using fluorescent conjugates of FYVE or PX domains, was found to accumulate transiently on the SCV. Wortmannin prevented PI3P accumulation and the recruitment of EEA1 but did not affect the association of Rab5 with the SCV. Importantly, inhibition of PI3-K also impaired fusion of the SCV with vesicles containing LAMP-1. Rab7, which is thought to be required for association of LAMP-1 with the SCV, still associated with SCV in wortmannin-treated cells. We have therefore concluded that a 3-phosphoinositide-dependent step exists following recruitment of Rab7 to the SCV. The data also imply that 3-phosphoinositide-dependent effectors of Rab5 are not an absolute requirement for recruitment of Rab7. Despite failure to acquire LAMP-1, the SCV persists and allows effective replication of Salmonella within wortmannin-treated host cells. These findings imply that PI3-K is involved in the development of the SCV but is not essential for intracellular survival and proliferation of Salmonella.

Roles of PI3Ks in leukocyte chemotaxis and phagocytosis

Through selective disruption of phosphoinositide 3-kinase (PI3K) activity and the use of green fluorescent protein tagged derivatives of domains capable of specifically binding the lipid products of PI3Ks in vivo, it has been shown that this family of signalling enzymes have vital and distinct roles in chemotaxis, phagocytosis and phagosome maturation in leukocytes.

The PX domain: a new phosphoinositide-binding module

The PX domain, which until recently was an orphan domain, has emerged as the latest member of the phosphoinositide-binding module superfamily. Structural studies have revealed that it has a novel fold and identified key residues that interact with the bound phosphoinositide, enabling some prediction of phosphoinositide-binding specificity. Specificity for PtdIns(3)P appears to be the most common, and several proteins containing PX domains localise to PtdIns(3)P-rich endosomal and vacuolar structures through their PX domains: these include the yeast t-SNARE Vam7p, mammalian sorting nexins (involved in membrane trafficking events) and the Ser/Thr kinase CISK, which is implicated in cell survival. Additionally,phosphoinositide binding to the PX domains of p40phox and p47phox appears to play a critical role in the active assembly of the neutrophil oxidase complex.

Modular phosphoinositide-binding domains--their role in signalling and membrane trafficking

The membrane phospholipid phosphatidylinositol is the precursor of a family of lipid second-messengers, known as phosphoinositides, which differ in the phosphorylation status of their inositol group. A major advance in understanding phosphoinositide signalling has been the identification of a number of highly conserved modular protein domains whose function appears to be to bind various phosphoinositides. Such 'cut and paste' modules are found in a diverse array of multidomain proteins and recruit their host protein to specific regions in cells via interactions with phosphoinositides. Here, with particular reference to proteins involved in membrane traffic pathways, we discuss recent advances in our understanding of phosphoinositide-binding domains.

Phosphoinositides and phagocytosis

Phosphoinositide 3 kinases (PI3Ks)*Abbreviation used in this paper: PI3K, phosphoinositide 3 kinase. are known as regulators of phagocytosis. Recent results demonstrate that class I and III PI3Ks act consecutively in phagosome formation and maturation, and that their respective products, phosphatidylinositol 3,4,5-trisphosphate (PI[3,4,5]P(3)) and phosphatidylinositol 3-phosphate (PI[3]P), accumulate transiently at different stages. Phagosomes containing Mycobacterium tuberculosis do not acquire the PI(3)P-binding protein EEA1, which is required for phagosome maturation. This suggests a possible mechanism of how this microorganism evades degradation in phagolysosomes.