Involvement of heterotrimeric G proteins in phagocytosis and recycling from the phagosomal compartment

Canonical and noncanonical g-protein signaling helps coordinate actin dynamics to promote macrophage phagocytosis of zymosan

Both chemotaxis and phagocytosis depend upon actin-driven cell protrusions and cell membrane remodeling. While chemoattractant receptors rely upon canonical G-protein signaling to activate downstream effectors, whether such signaling pathways affect phagocytosis is contentious. Here, we report that Gαi nucleotide exchange and signaling helps macrophages coordinate the recognition, capture, and engulfment of zymosan bioparticles. We show that zymosan exposure recruits F-actin, Gαi proteins, and Elmo1 to phagocytic cups and early phagosomes. Zymosan triggered an increase in intracellular Ca(2+) that was partially sensitive to Gαi nucleotide exchange inhibition and expression of GTP-bound Gαi recruited Elmo1 to the plasma membrane. Reducing GDP-Gαi nucleotide exchange, decreasing Gαi expression, pharmacologically interrupting Gβγ signaling, or reducing Elmo1 expression all impaired phagocytosis, while favoring the duration that Gαi remained GTP bound promoted it. Our studies demonstrate that targeting heterotrimeric G-protein signaling offers opportunities to enhance or retard macrophage engulfment of phagocytic targets such as zymosan.

Proteomics fingerprinting of phagosome maturation and evidence for the role of a Galpha during uptake

Phagocytosis, whether of food particles in protozoa or bacteria and cell remnants in the metazoan immune system, is a conserved process. The particles are taken up into phagosomes, which then undergo complex remodeling of their components, called maturation. By using two-dimensional gel electrophoresis and mass spectrometry combined with genomic data, we identified 179 phagosomal proteins in the amoeba Dictyostelium, including components of signal transduction, membrane traffic, and the cytoskeleton. By carrying out this proteomics analysis over the course of maturation, we obtained time profiles for 1,388 spots and thus generated a dynamic record of phagosomal protein composition. Clustering of the time profiles revealed five clusters and 24 functional groups that were mapped onto a flow chart of maturation. Two heterotrimeric G protein subunits, Galpha4 and Gbeta, appeared at the earliest times. We showed that mutations in the genes encoding these two proteins produce a phagocytic uptake defect in Dictyostelium. This analysis of phagosome protein dynamics provides a reference point for future genetic and functional investigations.

Reconstitution of recycling from the phagosomal compartment in streptolysin O-permeabilized macrophages: role of Rab11

By phagocytosis, macrophages engulf large particles, microorganisms and senescent cells in vesicles called phagosomes. Many internalized proteins rapidly shuttle back to the plasma membrane following phagosome biogenesis. Here, we report a new approach to the study of recycling from the phagosomal compartment: streptolysin O- (SLO) permeabilized macrophages. In this semi-intact cell system, energy and cytosol are required to efficiently reconstitute recycling transport. Addition of GDPbetaS strongly inhibits this transport step, suggesting that a GTP-binding protein modulates the dynamics of cargo exit from the phagosomal compartment. GTPases of the Rab family control vesicular trafficking, and Rab11 is involved in transferrin receptor recycling. To unravel the role of Rab11 in the phagocytic pathway, we added recombinant proteins to SLO-permeabilized macrophages. Rab11:S25N, a negative mutant, strongly diminishes the release of recycled proteins from phagosomes. In contrast, wild type Rab11 and its positive mutant (Rab11:Q70L) favor this vesicular transport event. Using biochemical and morphological assays, we confirm that overexpression of Rab11:S25N substantially decreases recycling from phagosomes in intact cells. These findings show the requirement of a functional Rab11 for the retrieval to the plasma membrane of phagosomal content. SLO-permeabilized macrophages likely constitute a useful tool to identify new molecules involved in regulating transport along the phagocytic pathway.

Calnexin, calreticulin and cytoskeleton-associated proteins modulate uptake and growth of Legionella pneumophila in Dictyostelium discoideum

The haploid amoebaDictyostelium discoideumis a versatile host system for studying cellular aspects ofLegionellapathogenicity. Previous studies have shown that the internalization ofL. pneumophilaleads to an endoplasmic reticulum (ER)-derived organelle that supports intracellular replication of the bacteria. In this study a roadmap of host-cell factors involved in this process was developed. Phagocytosis assays with specific cellular inhibitors and the effects of well defined host-cell mutants revealed that cytoplasmic calcium levels, cytoskeleton-associated proteins and the calcium-binding proteins of the ER, calreticulin and calnexin, specifically influence the uptake and intracellular growth ofL. pneumophila. Confocal microscopic time series with green fluorescent protein (GFP)-tagged calnexin and calreticulin demonstrated the accumulation of both proteins in the phagocytic cup ofL. pneumophila-infected host cells. In contrast to the control experiment withEscherichia coli-containing phagosomes, both proteins decorated the replicative vacuole ofL. pneumophiladuring the entire growth phase of the bacteria. The cumulative effects of cytosolic calcium levels, the spatial distribution of calnexin and calreticulin, and the defective invasion and replication ofL. pneumophilain calnexin- and calreticulin-minus cells suggest that these factors are part of a regulatory system that leads to the specific vacuole ofL. pneumophila.

Rab Coupling Protein Associates with Phagosomes and Regulates Recycling from the Phagosomal Compartment

The Rab coupling protein (RCP) is a recently identified novel protein that belongs to the Rab11-FIP family. RCP interacts specifically with Rab4 and Rab11, small guanosine-5'-triphosphatases that function as regulators along the endosomal recycling pathway. We used fluorescence confocal microscopy and biochemical approaches to evaluate the participation of RCP during particle uptake and phagosome maturation. In macrophages, RCP is predominantly membrane-bound and displays a punctuate vesicular pattern throughout the cytoplasm. RCP is mainly associated with transferrin-containing structures and Rab11-labeled endosomes. Overexpression of H13, the carboxyl-terminal region of RCP that contains the Rab binding domain, results in an abnormal endosomal compartment. Interestingly, we found that RCP is associated as discrete patches or protein domains to early phagosomal membranes. In macrophages, overexpression of full-length RCP stimulates recycling from the phagosomal compartment, whereas overexpression of H13 diminishes this vesicular transport step. It is likely that acting as an intermediate between Rab4 and Rab11, RCP regulates membrane flux along the phagocytic pathway via recycling events.

On phagosome individuality and membrane signalling networks

Cells such as macrophages take up pathogens into specialized membrane organelles (phagosomes) that fuse with other organelles, including lysosomes, in a process termed maturation. The fully matured phagolysosome is a low-pH, hydrolase-rich killing device that some pathogens can bypass. One might expect that phagosomes containing a given type of particle that entered cells simultaneously via the same receptor would behave the same, at least in a single cell. Surprisingly, however, recent data show that phagosomes formed via the same receptors can find themselves in different chemical states even within the same macrophage. Here, I argue that each phagosome is an individual entity whose behaviour depends on a finite number of stable equilibrium states in its membrane signalling networks.

Conserved features of endocytosis in Dictyostelium

Endocytosis in protozoa is often regarded as largely different from the pathways operating in mammalian cells. Experiments in the amoeba Dictyostelium, one of the genetically tractable single-celled organisms, have allowed us to manipulate the flow through endocytic compartments and to study the dynamic distribution of molecules by means of green fluorescent protein fusions. This review attempts to compile the molecular data available from Dictyostelium and assign them to specific steps of internalization by phagocytosis or macropinocytosis and to subsequent stages of the endocytic pathway. Parallels to phagocytes of the mammalian immune system are emphasized. The major distinctive feature between mammalian phagocytes and free-living cells is the need for osmoregulation. Therefore Dictyostelium cells possess a contractile vacuole that has occasionally obscured analysis of endocytosis but is now found to be entirely separate from endocytic organelles. In conclusion, the potential of Dictyostelium amoebas to provide a model system of mammalian phagocytes is ever increasing.