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Whereas the marking of phagocytic targets by opsonins has long been recognized to engage specific receptor-mediated signaling pathways, the contribution of environmental signals to the alteration of these pathways is less well understood. Environmental regulation is composed of both positive and negative signals that act to tune the phagocytic response to an appropriate threshold. This homeostatic control limits the release of self-damaging products generated by the phagocytic process to the site of infection or inflammation. Thus, unactivated circulating phagocytes are capable of only minimal ingestion, and they develop their full phagocytic potential after exposure to additional signals, such as bacterial peptides, fragments of complement, clotting proteins, arachidonate metabolites, and cytokines that predominate at sites of infection and inflammation.

One important function of these positive signals is to overcome negative signals that act as constitutive brakes on ingestion. Negative signals are important for raising the threshold of phagocyte activation, terminating overwhelming inflammatory responses, and discriminating between an appropriate target and a host cell that are marked for uptake. Both positive and negative signals must be coordinated with the signals provided by the primary opsonin receptor "eat me" to provide fine control of the phagocytic response.


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  • Phagocytosis.

Thus, the overall milieu in which a phagocyte exists directly affects its phagocytic potential. Positive Regulation.

A major mechanism to enhance the rate and extent of phagocytosis by neutrophils and monocytes is adhesion to extracellular matrix proteins. Teleologically, phagocytes contact extracellular matrix in tissues only after exodus from the blood, and this acts to inform these cells that they have migrated out of the vasculature and are present at a site of infection or tissue injury in which their full phagocytic potential is required.

Demonstrated initially for fibronectin-induced enhancement of both Fc g R- and complement-mediated ingestion by monocytes , this mechanism been extended to include neutrophils, multiple opsonins, and additional adhesive proteins, including entactin, laminin, collagen, fibrinogen, and vitronectin.

The Interaction Between Brucella and the Host Cell in Phagocytosis

The signal for enhanced phagocytosis by many of these adhesive proteins is mediated by a short peptide sequence, Arg-Gly-Asp, contained within these molecules. Although this peptide is a recognition motif for multiple members of the integrin receptor family, a v b 3 integrin expressed by both monocytes and neutrophils plays an essential role in adhesive protein-amplified phagocytic responses , Phagocyte activation by this receptor has been studied in depth; it involves the physical association of a v b 3 with an immunoglobulin superfamily member, CD47 integrin-associated protein That these two proteins function as a signal transduction unit for enhancement of phagocytosis was confirmed by the failure of neutrophils from CDdeficient mice to enhance IgG-dependent ingestion when stimulated with an Arg-Gly-Asp peptide mimic The early History of Phagocytosis T.

Phagocytosis by Nonprofessional Phagocytes D. Williams-Herman and Z.

Introduction

Section II Receptors. Platt, R. Haworth, R. Mannose Receptor and Phagocytosis I. Fraser, R.

Colorimetric Phagocytosis Assays

Alan and B. Integrin Receptors of Phagocytes S. Blystone and E. Fc Receptor-Mediated Phagocytosis S. Section III Signaling. Heterogeneity in Macrophage Phagocytosis A. Aderem and D. Section IV The Pathway. Pathways through the Macrophage Vacuolar Compartment J. Alvarez-Dominguez, L. Mayorga, and P. Hackam, O. Rotstein, and S. The Phagocyte Actin Cytoskeleton H.


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Sun, K. Lin, M. Yamamoto, and H. Section V Responses.

5.2 Pathogen Evolution: Evasion of Host Defenses

Gruenheid, E. Skamene, and P. Under this model, the organism that acquired the mitochondrial endosymbiont was an archaeon without a cell wall that morphologically might resemble the extant Thermoplasma and possessed an actin-like-protein based cytoskeleton in the form of filament networks.

Introduction

One of such occasions would eventually lead to the mitochondrial endosymbiosis. Conceptually, this process can be regarded as the simplest, primordial form of phagocytosis. Indeed, it has been shown that some bacteria enter the host eukaryotic cell by inducing protrusions of the host plasma membrane [ ]. This model, essentially, takes the middle ground between the "primitive phagocyte" and "fateful encounter" scenarios of endosymbiosis: it is proposed that the archaeal host had no full-fledged phagocytosis but did possess a primitive mechanism for the engulfment of other prokaryotic cells.

Such an ability would substantially increase the frequency of engulfment and the likelihood that a stable enosymbiosis would be established. Thus, this model is generally compatible with the hypotheses which portray the protoeukaryote as a predator or scavenger that fed on other prokaryotes [ 18 , , ]; however, compared, say, to modern amoebas, this would be a rather ineffective predator.

Clearly, this scenario is also compatible with the earlier "you are what you eat" idea of Doolittle according to which the protoeukaryote continuously acquired diverse genes from bacteria engulfed as food or transient endosymbionts [ 19 ]. The proposed endosymbiotic scenario of eukaryogenesis and subsequent origin of phagocytosis. The evolutionary tree of archaea is shown as a multifurcation of 5 major branches: Crenarchaeota, Euryarchaeota, Korarchaeota, Thaumarchaeota, and the hypothetical Archaeal Ancestor of Eukaryotes which is depicted as an irregular shape to emphasize the likely absence of a rigid cell wall.

The primary radiation of eukaryotes is shown as a multifurcation of 5 supergroups: Unikonts, Chromalveolata, Excavates, Rhizaria, and Planta. At least, three of the supergroups evolved full-fledged phagocytosis Ph. A specific relationship between eukaryotes and Thermoplasma has been suggested on the basis of the results of supertree analysis [ ] but a recent comprehensive phylogenetic study suggests that the archaeal "parent" of eukaryotes, most likely, belonged to a deep branch of archaea that lies outside the currently known archaeal diversity [ 60 ].

The observed distribution of actin homologs is compatible with this view in that the eukaryotic-type actin-like proteins are not seen in Euryarchaeota but are present in two other distinct, major branches of archaea, namely, a subset of Crenarchaeota and Korarchaeota. Thus, we hypothesize that the host cell that engulfed the future mitochondrion was a mesophilic archaeon that belonged to a still unknown, deep archaeal branch and possessed an actin-like protein capable of forming networks of filaments. The alternative assignment of the archaeal root of eukaryotes to Crenarchaeota eocytes that is suggested by some phylogenetic analyses [ — ] would imply that the actin-like proteins emerged within this archaeal phylum and that the archaeal parent of eukaryotes was a mesophilic crenarchaeon As seen with modern mesophilic archaea, such as Methanosarcina [ , ], an archaeon with such a life style regardless of its exact phylogenetic position would likely acquire a variety of bacterial genes via HGT, even prior to the endosymbiosis, a process that might account for the diversity of genes of apparent bacterial origin seen in eukaryotes [ 2 , ].

With the input of the horizontally transferred bacterial genes and, particularly, the endosymbiont genome, Rho-GTPases and some of the actin-interacting proteins such as ARPC1 and coronin containing the WD40 domain or profiling containing the Rossmann-type domain that could be a highly diverged derivative of PAS or GAF were recruited as regulators of actin assembly. Conceivably, endosymbiosis would put a high premium on the evolution of the cytoskeleton that is intimately involved, among other processes, in mitochondrial dynamics [ , ]. The modern-type phagocytosis as well as cell adhesion could evolve only after the endocytic system because endomembrane delivery is required for the formation of the phagocytic cup [ 25 ].

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Furthermore, the origin of phagocytosis must have succeeded the evolution of lysosomes given that the phagosome-lysosome fusion is a crucial step of phagocytosis in all modern phagocytic eukaryotes [ 27 ]. In the course of the subsequent evolution, full-fledged phagocytosis seems to have evolved independently in, at least, three of the five supergroups of eukaryotes [ , ], namely, Unikonts metazoa and amoebozoa , Chromalveolata ciliates , and Excavates trypanosomes. The protein composition of phagosomes is highly variable among eukaryotes, suggesting that the modern-type, advanced phagocytosis evolved independently and relatively late in the course of evolution of several major eukaryotic lineages.

However, actin, actin-related proteins, and the core set of proteins that are involved in actin filament remodeling as well as some of the key regulatory proteins are conserved across eukaryotes. All these proteins are required for a variety of cytoskeleton-dependent process not for phagocytosis specifically. Among the key proteins involved in phagocytosis, only the actin family and the regulatory GTPases of the Ras-family have well-conserved prokaryotic orthologs. Phylogenetic analysis and structural comparison of eukaryotic actin family proteins with archaeal actin-like proteins suggest that the ancestral actin-like proteins could have been capable of the formation of branched filament structures and networks.

This capacity would allow the hypothetical archaeal host of the mitochondrion to form protrusions resembling modern eukaryotic lamellipodia or filopodia and facilitating engulfment of other prokaryotes. Such engulfment of bacteria would be decisive for the acquisition of the mitochondrial endosymbiont.

Phagocytosis (article) | Cells | Khan Academy

The Ras-family GTPases appear to be of bacterial origin and might have been recruited for the regulation of actin filament remodeling from the endosymbiont or even earlier, via horizontal gene transfer from bacteria. Thus, under the proposed model, a primitive process of particle engulfment by actin-encoding archaea might antedate eukaryogenesis whereas the full-fledged phagocytosis was a late development that occurred independently in several major branches of eukaryotes.

The Entamoeba histolytica phagoproteome analyzed in this study consisted of non-redundant proteins. The first set contained proteins from the phagosome of the wild-type and myosin IB-overproducing strains of E. Proteins found only in the mutant phagosomes were omitted. The second set included proteins that were identified in latex-bead containing phagosomes of three E.