Signaling and membrane dynamics during phagocytosis: many roads lead to the phagos(R)ome

Quantitative and dynamic assessment of the contribution of the ER to phagosome formation

Phagosomes were traditionally thought to originate from an invagination and scission of the plasma membrane to form a distinct intracellular vacuole. An alternative model implicating the endoplasmic reticulum (ER) as a major component of nascent and maturing phagosomes was recently proposed (Gagnon et al., 2002). To reconcile these seemingly disparate hypotheses, we used a combination of biochemical, fluorescence imaging, and electron microscopy techniques to quantitatively and dynamically assess the contribution of the plasmalemma and of the ER to phagosome formation and maturation. We could not verify even a transient physical continuity between the ER and the plasma membrane, nor were we able to detect a significant contribution of the ER to forming or maturing phagosomes in either macrophages or dendritic cells. Instead, our data indicate that the plasma membrane is the main constituent of nascent and newly formed phagosomes, which are progressively remodeled by fusion with endosomal and eventually lysosomal compartments as phagosomes mature into acidic, degradative organelles.

A Role for Mammalian Diaphanous-Related Formins in Complement Receptor (CR3)-Mediated Phagocytosis in Macrophages

Macrophages, dendritic cells, and neutrophils use phagocytosis to capture and clear off invading pathogens. The process is triggered by the interaction of ligands on the pathogens' surface with specific phagocytic receptors, including immunoglobulin (FcR) and complement C3bi (CR3) receptors (integrin alpha(M)beta2, Mac1) . Localized actin-filament assembly that acts as the driving force for particle engulfment is controlled by Rho-family small GTPases . RhoA regulates CR3-mediated phagocytosis through a mechanism that is still unclear . Mammalian Diaphanous-related (mDia) formins participate in the generation of a diverse set of actin-remodeling events downstream of RhoA , and mDia1 is recruited around fibronectin-coated beads in a RhoA-dependent manner in fibroblasts . Here, we set out to examine whether mDia proteins are involved in CR3-mediated phagocytosis in macrophages. We show that the RhoA effector mDia1 is recruited early during CR3-mediated phagocytosis and colocalizes with polymerized actin in the phagocytic cup. Interfering with mDia activity inhibits CR3-mediated phagocytosis while having no effect on FcR-mediated phagocytosis. These results indicate a new function for mDia proteins in the regulation of actin polymerization during CR3-mediated phagocytosis.

Phagocytosis, endosomal/lysosomal system and other cellularaspects of macrophage activation by Canova medication

Canova is a homeopathic medication with immunomodulatory properties, recommended for diseases where the immune system is depressed. Our research aims to study the activation of mice peritoneal macrophages when submitted to in vivo and in vitro Canova treatment. Morphological parameters and acid phosphatase activity were analyzed using light and transmission electron microscopy. Differential interference contrast microscopy, including serial time acquisition in living cells, was also performed. The results demonstrated a greater spreading ability in Canova treated macrophages, a higher phagocytic activity of non-infective microorganisms (Saccharomyces cerevisiae and Tripanosoma cruzi epimastigotes) and a tendency to lower the phagocytic activity of the infective microorganisms T. cruzi trypomastigotes and Leishmania amazonensis, when compared with control cells. Acid phosphatase activity was analyzed and showed that Canova treatment stimulates an increase of the endosomal/lysosomal system. Treated macrophages that do or do not interact with yeast present a higher number of acid phosphatase marked vesicles compared to control cells. In contrast, the activity of tartrate resistant acid phosphatase (TRAP), is lower in Canova treated macrophages. The net results demonstrate that Canova medication is an effective stimulator of macrophage activity.

A ubiquitin-based assay for the cytosolic uptake of protein transduction domains

Protein transduction domains (PTDs) are promising tools for transducing presynthesized polypeptides across the plasma membrane. However, the development and optimization of PTDs are hampered by many technical problems and artifacts resulting notably from the tight binding of PTDs to the cell surface and the difficulty in discriminating, through imagery analyses, truly cytosolic from cytoplasmic vesicular compartments. To circumvent these problems, we have developed an unambiguous enzymatic assay of the cytosolic uptake of PTD-driven proteins, based on the processing by ubiquitin-specific C-terminal proteases (DUBs). This method, coupled with fluorometry and fluorescence microscopy, shows that the TAT PTD derived from human immunodeficiency virus type 1 is rapidly taken up by cells but fails to reach their cytosol, except when dendritic cells, which are known to take up circulating antigens for cross-presentation, are used. In addition to its usefulness in assessing cytosolic uptake, DUB processing of PTD-linked proteins can ensure the intracellular release of cargo proteins, which might prove helpful for MHC-I-based vaccination or intracellular delivery of biologically active polypeptides.

ARF6 GTPase controls bacterial invasion by actin remodelling

The obligate intracellular bacterium Chlamydia penetrates the host epithelial cell by inducing cytoskeleton and membrane rearrangements reminiscent of phagocytosis. Here we report that Chlamydia induces a sharp and transient activation of the endogenous small GTP-binding protein ARF6, which is required for efficient uptake. We also show that a downstream effector of ARF6, phosphatidylinositol 4-phosphate 5-kinase and its product, phosphatidylinositol 4,5-bisphosphate were instrumental for bacterial entry. By contrast, ARF6 activation of phospholipase D was not required for Chlamydia uptake. ARF6 activation was necessary for extensive actin reorganization at the invasion sites. Remarkably, these signalling players gathered with F-actin in a highly organized three-dimensional concentric calyx-like protrusion around invasive bacteria. These results indicate that ARF6, which controls membrane delivery during phagocytosis of red blood cells in macrophages, has a different role in the entry of this small bacterium, controlling cytoskeletal reorganization.

Differential use of endoplasmic reticulum membrane for phagocytosis in J774 macrophages

Sustained phagocytosis requires the continuous replacement of cell-surface membrane from intracellular sources. Depending on the nature of the engulfed particles, a variety of endocytic compartments have been demonstrated to contribute membranes needed for the formation of phagosomes. It has recently been reported that the endoplasmic reticulum (ER) can also fuse with the plasma membrane during phagocytosis [Gagnon, E., Duclos, S., Rondeau, C., Chevet, E., Cameron, P. H., Steele-Mortimer, O., Paiement, J., Bergeron, J. J. & Desjardins, M. (2002) Cell 110, 119-131]. However, there is currently no known mechanistic basis for this fusion process to occur. Here we report that direct ER-plasma membrane fusion during phagocytosis requires the ER resident soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein ERS24/Sec22b and that J774-macrophages react toward the challenge of large (3.0-microm) but not small (0.8-microm) particles by triggering this fusion mechanism, allowing them to access the most abundant endogenous membrane source in the cell, the ER.

Pathways for antigen cross presentation

Dendritic cells (DCs) have the unique ability to capture cellular tissue antigens, and to present them on MHC class I molecules to antigen-specific CD8(+) T lymphocytes after migration to the draining lymph nodes. This process, called "cross presentation" can lead either to the tolerization or activation of antigen-specific CD8(+) T cells. Antigen capture is believed to occur by phagocytosis of antigen-bearing dead cells. Recent studies suggest that the antigen transferred from the phagocytosed cell to the DC during cross presentation is a proteasome substrate, rather than a proteasomal degradation product. In most cases, the formation of the peptide-MHC class I complexes in DCs requires the export of protein antigens from phagosomes to the cytosol, where they undergo proteasomal degradation. The resulting peptides are then translocated by TAP to the lumen of a cross presentation-loading compartment, for association to MHC class I under the control of chaperones and oxido-reductases. This loading compartment may be either the endoplasmic reticulum (ER) or a mix phagosome-ER compartment. MHC class I egress from the loading compartment to cell surface remains to be analyzed.

TI-VAMP/VAMP7 is required for optimal phagocytosis of opsonised particles in macrophages

Phagocytosis relies on extension of plasmalemmal pseudopods generated by focal actin polymerisation and delivery of membranes from intracellular pools. Here we show that compartments of the late endocytic pathway, bearing the tetanus neurotoxin-insensitive vesicle-associated membrane protein (TI-VAMP/VAMP7), are recruited upon particle binding and undergo exocytosis before phagosome sealing in macrophages during Fc receptor (FcR)-mediated phagocytosis. Expression of the dominant-negative amino-terminal domain of TI-VAMP or depletion of TI-VAMP with small interfering RNAs inhibited phagocytosis mediated by Fc or complement receptors. In addition, inhibition of TI-VAMP activity led to a reduced exocytosis of late endocytic vesicles and this resulted in an early blockade of pseudopod extension, as observed by scanning electron microscopy. Therefore, TI-VAMP defines a new pathway of membrane delivery required for optimal FcR-mediated phagocytosis.