Trends in Cell Biology
Volume 30, Issue 2, February 2020, Pages 157-167
Journal home page for Trends in Cell Biology

Review
Squeezing in a Meal: Myosin Functions in Phagocytosis

https://doi.org/10.1016/j.tcb.2019.11.002Get rights and content

Highlights

  • Phagocytosis in both mammalian and nonmammalian cells requires F-actin and involves multiple myosin isoforms.

  • Specific steps in phagocytosis rely on the activity of specific myosin isoforms.

  • Phagocytes encounter targets with varying physical characteristics, which may affect the activity and distribution of the involved myosins.

  • The physiological setting of a phagocyte affects its actomyosin cytoskeleton and in turn its phagocytic behavior.

Phagocytosis is a receptor-mediated, actin-dependent process of internalization of large extracellular particles, such as pathogens or apoptotic cells. Engulfment of phagocytic targets requires the activity of myosins, actin-dependent molecular motors, which perform a variety of functions at distinct steps during phagocytosis. By applying force to actin filaments, the plasma membrane, and intracellular proteins and organelles, myosins can generate contractility, directly regulate actin assembly to ensure proper phagocytic internalization, and translocate phagosomes or other cargo to appropriate cellular locations. Recent studies using engineered microenvironments and phagocytic targets have demonstrated how altering the actomyosin cytoskeleton affects phagocytic behavior. Here, we discuss how studies using genetic and biochemical manipulation of myosins, force measurement techniques, and live-cell imaging have advanced our understanding of how specific myosins function at individual steps of phagocytosis.

Introduction

Phagocytosis is an ancient biological process, utilized initially for nutrient uptake and now a critical activity for immune defense [1]. Professional phagocytes, including neutrophils, dendritic cells, and macrophages, regularly rid the body of pathogens, apoptotic cells, and cellular debris. These phagocytic targets are recognized by specific cell-surface receptors, which relay distinct downstream signals through protein and lipid kinases and phosphatases, small GTPases, and other signaling proteins [2]. Targets coated by plasma- or cell-derived components (opsonins) are recognized by opsonic receptors, including Fc receptors (FcRs) that bind the conserved domain of immunoglobulins and complement receptors (CRs), which respond to targets coated in the complement derivative iC3b [1]. Non-opsonic receptors (receptors binding to specific motifs on phagocytic targets) include Dectin-1 receptors that recognize fungal β-glucan and scavenger receptors that interact with both apoptotic and microbial ligands [1,3]. For internalization, phagocytic receptors can work in concert, with others serving to modulate the phagocytic response [4]. Despite the diversity of phagocytic receptors recognizing various targets, the process of target internalization always requires the actin cytoskeleton [5, 6, 7, 8, 9].

Phagocytosis by FcRs is classically divided into multiple steps. First, phagocytes search for targets using membrane protrusions, such as filopodia or ruffles, generated by actin polymerization [10]. This active process of probing the cell’s microenvironment can be stimulated, or primed, by soluble molecules such as TLR ligands and growth factors, which increase actin-dependent ruffling [11]. Once contact is made, phagocytic receptors cluster resulting in a more stable adhesion [12]. Downstream signaling from engaged receptors leads to robust actin polymerization that deforms the plasma membrane, causing the cell to encircle the target in a structure known as the phagocytic cup [13]. Closure of the phagocytic cup produces a sealed membrane-bound organelle called the phagosome, which is trafficked along microtubules further into the cell. Fusion of the phagosome with endosomal vesicles and later lysosomes is required for pathogen degradation and antigen presentation [1,14].

Myosins, actin-dependent molecular motors (Box 1), work to apply direct force on actin filaments, but their mechanical role in phagocytosis is not fully understood. This is due partially to the multitude of myosin isoforms that are expressed in phagocytes (Figure 1), as well as to the relatively fast and complex multistep nature of phagocytosis. Myosins can regulate actin assembly, actin filament crosslinking and rearrangement, actin-dependent membrane deformation, and protein localization. In this review, we examine our current understanding of the role of myosins in the various steps of phagocytosis, giving special attention to the mechanochemical features of each myosin and how they function in the overall biomechanics of the process. Furthermore, we discuss how the use of force measurements and engineered phagocytic targets and microenvironments has offered new insights into myosin activity during this process.

Section snippets

Target Capture Using Myosin-X-Guided Filopodia

Phagocytes capture their targets through actin-based membrane ruffles or filopodia, which can act as cellular tentacles, extending outward to bind a target and retracting back to pull the target to the cell surface [10,15]. Myosin-X is ubiquitously expressed among all immune cells and is well known for its striking localization at filopodial tips [16,17]. It is a double-headed processive motor with a tail that includes a myosin tail homology 4 domain and band 4.1/ezrin/radixin/moesin domain

Assembling the Phagocytic Cup: Roles for Myosin-I and Myosin-IX

Once a phagocytic target contacts the surface of the cell, receptor clustering and downstream signaling initiate the formation of the phagocytic cup that surrounds the target. The assembly of this actin-based structure is likely to involve at least two myosin classes: myosin-I and myosin-IX (Figure 2).

Myosin-Is are small, monomeric motors that represent one of the largest groups in the myosin superfamily. Myosin-Is contain a single heavy chain with a tail domain divided into three conserved

Closing the Phagocytic Cup and Myosin-II-Generated Contractility

A role for myosin-II in phagocytosis has been suspected since its detection at sites of ingestion by early immunofluorescence studies of macrophages [55]. Mammalian cells contain three non-muscle myosin-II isoforms: non-muscle myosin-2A (NM2A), NM2B, and NM2C, encoded by the MYH9, MYH10, and MYH14 genes in humans. NM2A is the dominant myosin-II isoform expressed in immune cells [32,56]. Like skeletal muscle myosin-II, this myosin functions as a heterohexamer comprising a dimer of heavy chains

Transporting the Phagosome with Myosin-V

Closure of the phagocytic cup creates a membrane-bound organelle known as the phagosome, which is shuttled further into the cell for processing and degradation [1,14]. Retrograde transport of the phagosome is known to occur along microtubule tracks [75], but it also involves myosin-V, a large, two-headed myosin motor. In macrophages from dilute-lethal (Myo5a-null) mice, following internalization, phagosomes move toward the cell center twice as fast as in wild-type cells, rapidly accumulating in

Concluding Remarks

The majority of myosin phagocytosis studies to date have been performed on cells in vitro. However, phagocytes reside in tissues with distinct physical characteristics and encounter phagocytic targets with various geometrical, mechanical, and chemical properties and the process of phagocytosis is affected by both cell environment and target properties. Since the actin cytoskeleton is influenced by substrate stiffness and geometry, myosin activity during phagocytosis is also likely to be

Acknowledgments

This work was supported by the AHA (18PRE34070066) grant to S.R.B., the Italian Association for Cancer Research (AIRC) Investigator Grant (IG) 20716 to N.C.G., and the National Institute of Diabetes and Digestive and Kidney Diseases of the NIH under Award R01DK083345 to M.K. The authors are grateful to Allyson Porter for help in preparing the illustrations and to Dr Joseph W. Sanger and anonymous reviewers for the helpful suggestions.

References (112)

  • P. de Lanerolle

    Myosin light chain phosphorylation does not increase during yeast phagocytosis by macrophages

    J. Biol. Chem.

    (1993)
  • S. Yamauchi

    Myosin II-dependent exclusion of CD45 from the site of Fcγ receptor activation during phagocytosis

    FEBS Lett.

    (2012)
  • J.D. Rotty

    Arp2/3 complex is required for macrophage integrin functions but is dispensable for FcR phagocytosis and in vivo motility

    Dev. Cell

    (2017)
  • D.T. Kovari

    Frustrated phagocytic spreading of J774A-1 macrophages ends in myosin II-dependent contraction

    Biophys. J.

    (2016)
  • I.M. Olazabal

    Rho-kinase and myosin-II control phagocytic cup formation during CR, but not FcγR, phagocytosis

    Curr. Biol.

    (2002)
  • M.S. Shutova et al.

    Mammalian nonmuscle myosin II comes in three flavors

    Biochem. Biophys. Res. Commun.

    (2018)
  • M.V. Baranov

    SWAP70 organizes the actin cytoskeleton and is essential for phagocytosis

    Cell Rep.

    (2016)
  • M.L. Markwardt

    A genetically encoded biosensor strategy for quantifying non-muscle myosin II phosphorylation dynamics in living cells and organisms

    Cell Rep.

    (2018)
  • X.D. Li

    Activation of myosin Va function by melanophilin, a specific docking partner of myosin Va

    J. Biol. Chem.

    (2005)
  • Y. Tabata et al.

    Effect of the size and surface charge of polymer microspheres on their phagocytosis by macrophage

    Biomaterials

    (1988)
  • G. Sharma

    Polymer particle shape independently influences binding and internalization by macrophages

    J. Control. Release

    (2010)
  • D. Paul

    Phagocytosis dynamics depends on target shape

    Biophys. J.

    (2013)
  • N.G. Sosale

    Cell rigidity and shape override CD47’s “self”-signaling in phagocytosis by hyperactivating myosin-II

    Blood

    (2015)
  • M.J. Greenberg

    A perspective on the role of myosins as mechanosensors

    Biophys. J.

    (2016)
  • E. Groves

    Molecular mechanisms of phagocytic uptake in mammalian cells

    Cell. Mol. Life Sci.

    (2008)
  • H.S. Goodridge

    Mechanisms of Fc receptor and Dectin-1 activation for phagocytosis

    Traffic

    (2012)
  • S.A. Freeman et al.

    Phagocytosis: receptors, signal integration, and the cytoskeleton

    Immunol. Rev.

    (2014)
  • S.L. Newman

    Differential requirements for cellular cytoskeleton in human macrophage complement receptor- and Fc receptor-mediated phagocytosis

    J. Immunol.

    (1991)
  • S.G. Axline et al.

    Inhibition of phagocytosis and plasma membrane mobility of the cultivated macrophage by cytochalasin B. Role of subplasmalemmal microfilaments

    J. Cell Biol.

    (1974)
  • R.C. May et al.

    Phagocytosis and the actin cytoskeleton

    J. Cell Sci.

    (2001)
  • R.S. Flannagan

    Dynamic macrophage “probing” is required for the efficient capture of phagocytic targets

    J. Cell Biol.

    (2010)
  • P.C. Patel et al.

    Membrane ruffles capture C3bi-opsonized particles in activated macrophages

    Mol. Biol. Cell

    (2008)
  • A. Sobota

    Binding of IgG-opsonized particles to Fc gamma R is an active stage of phagocytosis that involves receptor clustering and phosphorylation

    J. Immunol.

    (2005)
  • D.M. Underhill et al.

    Information processing during phagocytosis

    Nat. Rev. Immunol.

    (2012)
  • L. Vonna

    Micromechanics of filopodia mediated capture of pathogens by macrophages

    Eur. Biophys. J.

    (2007)
  • J.S. Berg et al.

    Myosin-X is an unconventional myosin that undergoes intrafilopodial motility

    Nat. Cell Biol.

    (2002)
  • S. Nagy

    A myosin motor that selects bundled actin for motility

    Proc. Natl Acad. Sci. U. S. A.

    (2008)
  • D. Cox

    Myosin X is a downstream effector of PI(3)K during phagocytosis

    Nat. Cell Biol.

    (2002)
  • A.C. Bachg

    Phenotypic analysis of Myo10 knockout (Myo10tm2/tm2) mice lacking full-length (motorized) but not brain-specific headless myosin X

    Sci. Rep.

    (2019)
  • S.V. Kim et al.

    Myosin I: from yeast to human

    Cell. Mol. Life Sci.

    (2008)
  • B.B. McIntosh et al.

    Myosin-I molecular motors at a glance

    J. Cell Sci.

    (2016)
  • E.M. Ostap

    Dynamic localization of myosin-I to endocytic structures in Acanthamoeba

    Cell Motil. Cytoskeleton

    (2003)
  • H. Voigt

    Myosin IB from Entamoeba histolytica is involved in phagocytosis of human erythrocytes

    J. Cell Sci.

    (1999)
  • G. Jung et al.

    Generation and characterization of Dictyostelium cells deficient in a myosin I heavy chain isoform

    J. Cell Biol.

    (1990)
  • G. Jung

    Dictyostelium mutants lacking multiple classic myosin I isoforms reveal combinations of shared and distinct functions

    J. Cell Biol.

    (1996)
  • U. Durrwang

    Dictyostelium myosin-IE is a fast molecular motor involved in phagocytosis

    J. Cell Sci.

    (2006)
  • Y. Fukui

    Myosin I is located at the leading edges of locomoting Dictyostelium amoebae

    Nature

    (1989)
  • C.L. Chen

    Myosin I links PIP3 signaling to remodeling of the actin cytoskeleton in chemotaxis

    Sci. Signal.

    (2012)
  • E.C. Schwarz

    Dictyostelium myosin IK is involved in the maintenance of cortical tension and affects motility and phagocytosis

    J. Cell Sci.

    (2000)
  • J.L. Maravillas-Montero et al.

    The myosin family: unconventional roles of actin-dependent molecular motors in immune cells

    J. Leukoc. Biol.

    (2012)
  • Cited by (0)

    View full text