Trends in Cell Biology
ReviewSqueezing in a Meal: Myosin Functions in 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.
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