The phagosome proteome: insight into phagosome functions.

Phagosomes are key organelles for the innate ability of macrophages to participate in tissue remodeling, clear apoptotic cells, and restrict the spread of intracellular pathogens. To understand the functions of phagosomes, we initiated the systematic identification of their proteins. Using a proteomic approach, we identified >140 proteins associated with latex bead-containing phagosomes. Among these were hydrolases, proton pump ATPase subunits, and proteins of the fusion machinery, validating our approach. A series of unexpected proteins not previously described along the endocytic/phagocytic pathways were also identified, including the apoptotic proteins galectin3, Alix, and TRAIL, the anti-apoptotic protein 14-3-3, the lipid raft-enriched flotillin-1, the anti-microbial molecule lactadherin, and the small GTPase rab14. In addition, 24 spots from which the peptide masses could not be matched to entries in any database potentially represent new phagosomal proteins. The elaboration of a two-dimensional gel database of >160 identified spots allowed us to analyze how phagosome composition is modulated during phagolysosome biogenesis. Remarkably, during this process, hydrolases are not delivered in bulk to phagosomes, but are instead acquired sequentially. The systematic characterization of phagosome proteins provided new insights into phagosome functions and the protein or groups of proteins involved in and regulating these functions.

Leishmania promastigotes require lipophosphoglycan to actively modulate the fusion properties of phagosomes at an early step of phagocytosis.

The lipophosphoglycan (LPG) of Leishmania promastigotes plays key roles in parasite survival in both insect and mammalian hosts. Evidence suggests that LPG decreases phagosome fusion properties at the onset of infection in macrophages. The mechanisms of action of this molecule are, however, poorly understood. In the present study, we used a panoply of Leishmania mutants displaying modified LPG structures to determine more precisely how LPG modulates phagosome-endosome fusion. Using an in vivo fusion assay measuring, at the electron microscope, the transfer of solute materials from endosomes to phagosomes, we provided further evidence that the repeating Gal(beta1,4)Man(alpha1-PO4) units of LPG are responsible for the alteration in phagosome fusion. The inhibitory effect of LPG on phagosome fusion was shown to be more potent towards late endocytic organelles and lysosomes than early endosomes, explaining how Leishmania promastigotes can avoid degradation in hydrolase-enriched compartments. The involvement of other repeating unit-containing molecules, including the secreted acid phosphatase, in the inhibition process was ruled out, as an LPG-defective mutant (Ipg1-) which secretes repeating unit-containing glycoconjugates was present in highly fusogenic phagosomes. In L. major, oligosaccharide side-chains of LPG did not contribute to the inhibition process, as Spock, an L. major mutant lacking LPG side-chains, blocked fusion to the same extent as wild-type parasites. Finally, dead parasites internalized from the culture medium were not as efficient as live parasites in altering phagosome-endosome fusion, despite the presence of LPG. However, the killing of parasites with vital dyes after their sequestration in phagosomes had no effect on the fusion properties of this organelle. Collectively, these results suggest that living promastigotes displaying full-length cell surface LPG can actively influence macrophages at an early stage of phagocytosis to generate phagosomes with poor fusogenic properties.

Impaired recruitment of the small GTPase rab7 correlates with the inhibition of phagosome maturation by Leishmania donovani promastigotes.

We have shown recently that one of the survival strategies used by Leishmania donovani promastigotes during the establishment of infection in macrophages consists in inhibiting phagosome-endosome fusion. This inhibition requires the expression of lipophosphoglycan (LPG), the predominant surface glycoconjugate of promastigotes, as parasites expressing truncated forms of LPG reside in phagosomes that fuse extensively with endocytic organelles. In the present study, we developed a single-organelle fluorescence analysis approach to study and analyse the intracellular trafficking of 'fusogenic' and 'low-fusogenic' phagosomes induced by an LPG repeating unit-defective mutant (Ipg2 KO) or by wild-type L. donovani promastigotes respectively. The results obtained indicate that phagosomes containing mutant parasites fuse extensively with endocytic organelles and transform into phagolysosomes by losing the early endosome markers EEA1 and transferrin receptor, and acquiring the late endocytic and lysosomal markers rab7 and LAMP1. In contrast, a majority of 'low-fusogenic' phagosomes containing wild-type L. donovani promastigotes do not acquire rab7, wheres they acquire LAMP1 with slower kinetics. These results suggest that L. donovani parasites use LPG to restrict phagosome-endosome fusion at the onset of infection in order to prevent phagosome maturation. This is likely to permit the transformation of hydrolase-sensitive promastigotes into hydrolase-resistant amastigotes within a hospitable vacuole not displaying the harsh environment of phagolysosomes.

Flotillin-1-enriched lipid raft domains accumulate on maturing phagosomes.

Flotillin-1 was recently shown to be enriched on detergent-resistant domains of the plasma membrane called lipid rafts. These rafts, enriched in sphingolipids and cholesterol, sequester certain proteins while excluding others. Lipid rafts have been implicated in numerous cellular processes including signal transduction, membrane trafficking, and molecular sorting. In this study, we demonstrate both morphologically and biochemically that lipid rafts are present on phagosomes. These structures are enriched in flotillin-1 and devoid of the main phagosomes membrane protein lysosomal-associated membrane protein (LAMP1). The flotillin-1 present on phagosomes does not originate from the plasma membrane during phagocytosis but accumulates gradually on maturing phagosomes. Treatment with bafilomycin A1, a compound that inhibits the proton pump ATPase and prevents the fusion of phagosomes with late endocytic organelles, prevents the acquisition of flotillin-1 by phagosomes, indicating that this protein might be recruited on phagosomes from endosomal organelles. A proteomic characterization of the lipid rafts of phagosomes indicates that actin, the alpha- and beta-subunits of heterotrimeric G proteins, as well as subunits of the proton pump V-ATPase are among the constituents of these domains. Remarkably, the intracellular parasite Leishmania donovani can actively inhibit the acquisition of flotillin-1-enriched lipid rafts by phagosomes and the maturation of these organelles. These results indicate that specialized functions required for phagolysosome biogenesis may occur at focal points on the phagosome membrane, and therefore represent a potential target of intracellular pathogens.