The dynamic phagosomal proteome and the contribution of the endoplasmic reticulum Lindsay D. Rogers and Leonard J. Foster* Centre for Proteomics, Department of Biochemistry and Molecular Biology, University of British Columbia, 301-2185 East Mall, Vancouver, BC, Canada V6T 1Z4 Edited by Emil R. Unanue, Washington University School of Medicine, St. Louis, MO, and approved October 4, 2007 (received for review June 23, 2007) Macrophages use phagocytosis to control the spread of pathogens LTQ-Orbitrap to measure the dynamics of the maturing phago- in the body, to clear apoptotic cells, and to aid in tissue remodeling. somal proteome with unparalleled accuracy. The phagosomal membrane is traditionally thought to originate IgG was chosen as an opsonin rather than whole serum (6) to from the plasmalemma and then go through a series of maturation reduce potentially confounding effects of phagocytosis through steps involving sequential fusion with endosomal compartments, different receptor systems. IgG-opsonized latex beads were leading to the formation of a phagolysosome. A recent model allowed to internalize for 10 min, and then latex bead-containing suggests that the endoplasmic reticulum (ER) is involved in the vacuoles (LBVs) were harvested at seven different time points maturation as well. Here we use stable isotope labeling and (Fig. 1). We focused on events within2hofphagocytosis because multiple quantitative proteomic approaches to follow the dynamic most internalized objects are dead or destroyed by this time and composition of the maturing phagosome in RAW 264.7 macro- there is no apparent physiological relevance to allowing vacuoles phage cells to a greater depth and higher temporal resolution than containing inert latex to mature for longer periods. From at least was previously possible. Analysis of the results suggests that the three biological replicates of each time point we identified 505 traditional model of a linear sequence of fusion events with proteins associated with LBVs, 382 of which could be reliably different compartments is more complex or variable than previ- quantified across various time points [supporting information ously thought. By concomitantly measuring the degree to which (SI) Table 1]. Based on current models of phagosome maturation each component is enriched on phagosomes, our data argue that (4), we expected to find a large influx of EE markers, followed the amount of ER involved in phagocytosis is much less than by LE markers and ending with a bolus of LS proteins. Certain markers of these compartments did peak at the expected time predicted by the model of ER-mediated phagocytosis. points, but endosomal proteins and the vesicle trafficking ma- chinery as a whole did not arrive in three discrete packages, as innate immunity ͉ organelle ͉ phagocytosis ͉ stable isotope labeling ͉ predicted. latex bead vacuoles Biochemical enrichment of an organelle is never perfect, because complete separation from all other organelles is essen- hagocytosis is the process by which cells engulf particles. tially never achieved. Given what is known about phagosome PPrimitive eukaryotes use phagocytosis primarily to obtain development, one can easily imagine a situation where the actin nutrients (1), whereas in more complex organisms it serves mesh surrounding LBVs at certain stages could entrap pieces of additional functions such as the clearance of apoptotic cells and other organelles, leading to their apparent time-dependent various pathogens (2). Internalized objects are contained in a copurification with LBVs. In our experimental approach, one membrane-bound vacuole called the phagosome, and current would expect an unchanging SILAC profile over time if proteins models have the phagosome traversing a complex maturation were enriched at the same time as LBVs but independent of the process involving sequential fusion with early endosomes (EE), LBV maturation itself. However, such is not the case for most late endosomes (LE), and ultimately with lysosomes (LS) (3, 4). markers of other organelles that we have measured here (SI In this model, full maturation is characterized by luminal acid- Table 2). One possible explanation for this could be that some ification and acquisition of hydrolytic enzymes, which serve to organelles get trapped to varying degrees with the LBVs (e.g., degrade or kill the cargo, typically microbes, within 2–4 h after caught in the surrounding actin mesh) at certain time points. An internalization (5). Previous studies have aimed to characterize alternative explanation could be that the cell lysis procedure the phagosome proteomes in cell lines from mouse and fruit fly, disrupts LBVs themselves, leading to some nonspecific associ- as well as from Dictyostelium discoideum, Entamoeba histolytica, ation of cytoplasmic proteins with the beads. Proteins from other and Tetrahymena thermophila (6–10). Of particular note, the organelles should generally not affect profiles of LBV proteins, studies of Desjardins and colleagues (6) in mouse macrophages however, except where a protein is shared between LBVs and the led them and others to propose a role for the endoplasmic copurifying organelle. reticulum (ER) in phagocytosis (11, 12), although this model has been challenged recently (13). Here we apply stable isotope Multiple Visits to the LBV. One by one the movement of most known endosomal markers has been followed on maturing labeling by amino acids in cell culture (SILAC) to develop a phagosomes using Western blotting (14). Our data set contains comprehensive, quantitative model of phagosome maturation that allows us to test several hypotheses suggested by current models of the process. Author contributions: L.J.F. designed research; L.J.F. performed research; L.J.F. contributed new reagents/analytic tools; L.D.R. and L.J.F. analyzed data; and L.D.R. and L.J.F. wrote the Results and Discussion paper. Maturation into a phagolysosome is absolutely crucial for even- The authors declare no conflict of interest. tual destruction of phagocytosed objects, whereas the success of This article is a PNAS Direct Submission. pathogens such as Salmonella enterica and Mycobacterium tuber- Freely available online through the PNAS open access option. culosis depends largely on their ability to avoid phagolysosomal *To whom correspondence should be addressed. E-mail: [email protected]. killing. To gain a better understanding into the process of This article contains supporting information online at www.pnas.org/cgi/content/full/ phagosome maturation we used latex beads to model phagocy- 0705801104/DC1. tosis in RAW 264.7 mouse macrophage cells and SILAC with an © 2007 by The National Academy of Sciences of the USA 18520–18525 ͉ PNAS ͉ November 20, 2007 ͉ vol. 104 ͉ no. 47 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0705801104 Downloaded by guest on September 28, 2021 Plasma a Membrane Y Y Y Y Y Y Y Y Internalization Maturation 01020 30 45 60 90 120 Expt 1 Arg6/Lys4 Arg10/Lys8 Arg0/Lys0 Expt 2 Arg0/Lys0 Arg4/Lys6 Arg10/Lys8 Expt 3 Arg0/Lys0 Arg6/Lys4 Arg10/Lys8 b 130000 65000 0 130000 521 522 523 524 519 520 65000 0 130000 519 520 521 522 523 524 Intensity 65000 0 c 130000 519 520 521 522 523 524 65000 0 Fig. 1. Application of SILAC to phagosome maturation. (a) IgG-opsonized, 0.8-␮m latex beads were added to RAW 264.7 cells (T ϭ 0) and allowed to phagocytose for 10 min, at which point external beads were washed away and LBVs were allowed to mature for the times indicated by dashed gray lines. Because SILAC is limited to a maximum of three conditions per experiment, one time point in each experiment was used to scale the other time points across experiments as described in Materials and Methods. For instance, the 90-min time point was used to scale between experiments 2 and 3 in this example. (b) Measured spectra for the [M ϩ 2H]2ϩ ion of ATIGADFLTK from Rab7 covering all seven time points in three experiments are shown. The abscissa scale for each peak cluster is m/z, but note that the three clusters in each row are not ordered by m/z but rather by the time point that they represent. The ordinate axes on each have been corrected by the isotope enrichment factor measured for each experiment. (c) The overall profile that would be calculated for ATIGADFLTK based on only the three experiments shown. In reality the profile for a protein was the averaged, normalized intensity at each time point for each peptide identified from that protein. the majority of proteins these early studies focused on, and the brane proteins (LAMPs) and cathepsins peak at 90 or 120 min profiles measured by SILAC largely agree with those reported (Fig. 2). However, we also observed that many proteins, includ- previously. For instance, EE antigen 1 is most abundant at 30 ing LAMP-1, heterotrimeric G proteins from the plasma mem- min, Niemann–Pick C1 and charged multivesicular body protein brane, and some cathepsins, display a biphasic profile with a 6 (CMVB6) peak around 60 min, and the LS-associated mem- smaller peak earlier in the time course. Although not explicitly CELL BIOLOGY 0.3 CathB CathS CathZ 0 LAMP2 CathA LAMP1 -0.3 NPC1 (Relative abundance) EEA1 10 10 20 30 45 60 90 120 CMVB6 Log Gβ -0.6 - CathA 1 Goα2 10 20 30 45 60 90 120 Time (min) Fig. 2. Endosomal proteins repeatedly move on and off LBVs. LBV profiles of several endosomal and plasma membrane proteins identified in this study are shown. Cath, cathepsins; NPC1, Niemann–Pick C1 protein; CMVB6, charged multivesicular body protein 6. Shown are average profiles from at least three replicates of each time point. (Inset) Western blot of CathA across the time points indicated. Rogers and Foster PNAS ͉ November 20, 2007 ͉ vol. 104 ͉ no. 47 ͉ 18521 Downloaded by guest on September 28, 2021 0.4 - VAMP4 SNAP23 10 20 30 45 60 90 120 Sxn7 VAMP4 0 Vti1b NSF (Relative abundance) 10 20 30 45 60 90 120 10 SNAP29 NSF - Log VAMP8 -0.4 10 20 30 45 60 90 120 Time (min) Fig.