Frustrated Phagocytic Spreading of J774A-1 Macrophages Ends in Myosin II-Dependent Contraction

Plasma membrane abundance dictates phagocytic capacity and functional cross-talk in myeloid cells

Professional phagocytes like neutrophils and macrophages tightly control what they consume, how much they consume, and when they move after cargo uptake. We show that plasma membrane abundance is a key arbiter of these cellular behaviors. Neutrophils and macrophages lacking the G protein subunit Gβ4 exhibited profound plasma membrane expansion, accompanied by marked reduction in plasma membrane tension. These biophysical changes promoted the phagocytosis of bacteria, fungus, apoptotic corpses, and cancer cells. We also found that Gβ4-deficient neutrophils are defective in the normal inhibition of migration following cargo uptake. Sphingolipid synthesis played a central role in these phenotypes by driving plasma membrane accumulation in cells lacking Gβ4. In Gβ4 knockout mice, neutrophils not only exhibited enhanced phagocytosis of inhaled fungal conidia in the lung but also increased trafficking of engulfed pathogens to other organs. Together, these results reveal an unexpected, biophysical control mechanism central to myeloid functional decision-making.

High-viscosity driven modulation of biomechanical properties of human mesenchymal stem cells promotes osteogenic lineage

Biomechanical cues could effectively govern cell gene expression to direct the differentiation of specific stem cell lineage. Recently, the medium viscosity has emerged as a significant mechanical stimulator that regulates the cellular mechanical properties and various physiological functions. However, whether the medium viscosity can regulate the mechanical properties of human mesenchymal stem cells (hMSCs) to effectively trigger osteogenic differentiation remains uncertain. The mechanism by which cells sense and respond to changes in medium viscosity, and regulate cell mechanical properties to promote osteogenic lineage, remains elusive. In this study, we demonstrated that hMSCs, cultured in a high-viscosity medium, exhibited larger cell spreading area and higher intracellular tension, correlated with elevated formation of actin stress fibers and focal adhesion maturation. Furthermore, these changes observed in hMSCs were associated with activation of TRPV4 (transient receptor potential vanilloid sub-type 4) channels on the cell membrane. This feedback loop among TRPV4 activation, cell spreading and intracellular tension results in calcium influx, which subsequently promotes the nuclear localization of NFATc1 (nuclear factor of activated T cells 1). Concomitantly, the elevated intracellular tension induced nuclear deformation and promoted the nuclear localization of YAP (YES-associated protein). The concurrent activation of NFATc1 and YAP significantly enhanced alkaline phosphatase (ALP) for pre-osteogenic activity. Taken together, these findings provide a more comprehensive view of how viscosity-induced alterations in biomechanical properties of MSCs impact the expression of osteogenesis-related genes, and ultimately promote osteogenic lineage.

Ninein promotes F-actin cup formation and inward phagosome movement during phagocytosis in macrophages

Phagocytosis by macrophages is a highly polarized process to destroy large target cells. Binding to particles induces extensive cortical actin-generated forces that drive the formation of elaborate pseudopods around the target particle. Postinternalization, the resultant phagosome is driven toward the cell interior on microtubules (MTs) by cytoplasmic dynein. However, it is unclear whether dynein and cargo-adaptors contribute to the earlier steps of particle internalization and phagosome formation. Here we reveal that ninein, a MT minus-end-associated protein that localizes to the centrosome, is also present at the phagocytic cup in macrophages. Ninein depletion impairs particle internalization by delaying the early F-actin recruitment to sites of particle engagement and cup formation, with no impact on F-actin dynamics beyond this initial step. Ninein forms membrane-bound clusters on phagocytic cups that do not nucleate acentrosomal MTs but instead mediate the assembly of dynein-dynactin complex at active phagocytic membranes. Both ninein depletion and pharmacological inhibition of dynein activity reduced inward displacement of bound particles into macrophages. We found that ninein and dynein motor activity were required for timely retrograde movement of phagosomes and for phagolysosome formation. Taken together, these data show that ninein, alone and with dynein, play significant roles during phagocytosis.

PCSK9 activation promotes early atherosclerosis in a vascular microphysiological system

Atherosclerosis is a primary precursor of cardiovascular disease (CVD), the leading cause of death worldwide. While proprotein convertase subtilisin/kexin 9 (PCSK9) contributes to CVD by degrading low-density lipoprotein receptors (LDLR) and altering lipid metabolism, PCSK9 also influences vascular inflammation, further promoting atherosclerosis. Here, we utilized a vascular microphysiological system to test the effect of PCSK9 activation or repression on the initiation of atherosclerosis and to screen the efficacy of a small molecule PCSK9 inhibitor. We have generated PCSK9 over-expressed (P+) or repressed (P-) human induced pluripotent stem cells (iPSCs) and further differentiated them to smooth muscle cells (viSMCs) or endothelial cells (viECs). Tissue-engineered blood vessels (TEBVs) made from P+ viSMCs and viECs resulted in increased monocyte adhesion compared to the wild type (WT) or P- equivalents when treated with enzyme-modified LDL (eLDL) and TNF-α. We also found significant viEC dysfunction, such as increased secretion of VCAM-1, TNF-α, and IL-6, in P+ viECs treated with eLDL and TNF-α. A small molecule compound, NYX-1492, that was originally designed to block PCSK9 binding with the LDLR was tested in TEBVs to determine its effect on lowering PCSK9-induced inflammation. The compound reduced monocyte adhesion in P+ TEBVs with evidence of lowering secretion of VCAM-1 and TNF-α. These results suggest that PCSK9 inhibition may decrease vascular inflammation in addition to lowering plasma LDL levels, enhancing its anti-atherosclerotic effects, particularly in patients with elevated chronic inflammation.

Plasma membrane abundance dictates phagocytic capacity and functional crosstalk in myeloid cells

Professional phagocytes like neutrophils and macrophages tightly control what they eat, how much they eat, and when they move after eating. We show that plasma membrane abundance is a key arbiter of these cellular behaviors. Neutrophils and macrophages lacking the G-protein subunit Gb4 exhibit profound plasma membrane expansion due to enhanced production of sphingolipids. This increased membrane allocation dramatically enhances phagocytosis of bacteria, fungus, apoptotic corpses, and cancer cells. Gb4 deficient neutrophils are also defective in the normal inhibition of migration following cargo uptake. In Gb4 knockout mice, myeloid cells exhibit enhanced phagocytosis of inhaled fungal conidia in the lung but also increased trafficking of engulfed pathogens to other organs. These results reveal an unexpected, biophysical control mechanism lying at the heart of myeloid functional decision-making.

Coupled mechanical mapping and interference contrast microscopy reveal viscoelastic and adhesion hallmarks of monocyte differentiation into macrophages

Monocytes activated by pro-inflammatory signals adhere to the vascular endothelium and migrate from the bloodstream to the tissue ultimately differentiating into macrophages. Cell mechanics and adhesion play a crucial role in macrophage functions during this inflammatory process. However, how monocytes change their adhesion and mechanical properties upon differentiation into macrophages is still not well understood. In this work, we used various tools to quantify the morphology, adhesion, and viscoelasticity of monocytes and differentiatted macrophages. Combination of atomic force microscopy (AFM) high resolution viscoelastic mapping with interference contrast microscopy (ICM) at the single-cell level revealed viscoelasticity and adhesion hallmarks during monocyte differentiation into macrophages. Quantitative holographic tomography imaging revealed a dramatic increase in cell volume and surface area during monocyte differentiation and the emergence of round and spread macrophage subpopulations. AFM viscoelastic mapping showed important stiffening (increase of the apparent Young's modulus, E₀) and solidification (decrease of cell fluidity, β) on differentiated cells that correlated with increased adhesion area. These changes were enhanced in macrophages with a spread phenotype. Remarkably, when adhesion was perturbed, differentiated macrophages remained stiffer and more solid-like than monocytes, suggesting a permanent reorganization of the cytoskeleton. We speculate that the stiffer and more solid-like microvilli and lamellipodia might help macrophages to minimize energy dissipation during mechanosensitive activities. Thus, our results revealed viscoelastic and adhesion hallmarks of monocyte differentiation that may be important for biological function.

Molecular Force Imaging Reveals That Integrin-Dependent Mechanical Checkpoint Regulates Fcγ-Receptor-Mediated Phagocytosis in Macrophages

Macrophages are a type of immune cell that helps eliminate pathogens and diseased cells. Recent research has shown that macrophages can sense mechanical cues from potential targets to perform effective phagocytosis, but the mechanisms behind it remain unclear. In this study, we used DNA-based tension probes to study the role of integrin-mediated forces in FcγR-mediated phagocytosis. The results showed that when the phagocytic receptor FcγR is activated, the force-bearing integrins create a "mechanical barrier" that physically excludes the phosphatase CD45 and facilitates phagocytosis. However, if the integrin-mediated forces are physically restricted at lower levels or if the macrophage is on a soft matrix, CD45 exclusion is significantly reduced. Moreover, CD47-SIRPα "don't eat me" signaling can reduce CD45 segregation by inhibiting the mechanical stability of the integrin barrier. These findings demonstrate how macrophages use molecular forces to identify physical properties and combine them with biochemical signals from phagocytic receptors to guide phagocytosis.

Analysis of Efferocytic Receptor Dynamics and Synapse Formation in a Frustrated Efferocytosis Model

Efferocytes express multiple receptors that mediate the recognition and engulfment of apoptotic cells through a process known as efferocytosis. Ligation of these receptors induces the formation of a structured efferocytic synapse that mediates the engulfment of the apoptotic cell by the efferocyte. The lateral diffusion of these receptors allows for clustering-mediated receptor activation and is central for the formation of the efferocytic synapse. This chapter describes a single particle tracking protocol to analyze the diffusion of efferocytic receptors within a frustrated efferocytosis model. This enables high-resolution tracking of efferocytic receptors throughout synapse formation, allowing the user to simultaneously quantify synapse formation and the dynamics of receptor diffusion as the efferocytic synapse evolves.

Mechanisms of frustrated phagocytic spreading of human neutrophils on antibody-coated surfaces

Complex motions of immune cells are an integral part of diapedesis, chemotaxis, phagocytosis, and other vital processes. To better understand how immune cells execute such motions, we present a detailed analysis of phagocytic spreading of human neutrophils on flat surfaces functionalized with different densities of immunoglobulin G (IgG) antibodies. We visualize the cell-substrate contact region at high resolution and without labels using reflection interference contrast microscopy and quantify how the area, shape, and position of the contact region evolves over time. We find that the likelihood of the cell commitment to spreading strongly depends on the surface density of IgG, but the rate at which the substrate-contact area of spreading cells increases does not. Validated by a theoretical companion study, our results resolve controversial notions about the mechanisms controlling cell spreading, establishing that active forces generated by the cytoskeleton rather than cell-substrate adhesion primarily drive cellular protrusion. Adhesion, on the other hand, aids phagocytic spreading by regulating the cell commitment to spreading, the maximum cell-substrate contact area, and the directional movement of the contact region.

Spatiotemporal control of actomyosin contractility by MRCKβ signaling drives phagocytosis

Phagocytosis requires actin dynamics, but whether actomyosin contractility plays a role in this morphodynamic process is unclear. Here, we show that in the retinal pigment epithelium (RPE), particle binding to Mer Tyrosine Kinase (MerTK), a widely expressed phagocytic receptor, stimulates phosphorylation of the Cdc42 GEF Dbl3, triggering activation of MRCKβ/myosin-II and its coeffector N-WASP, membrane deformation, and cup formation. Continued MRCKβ/myosin-II activity then drives recruitment of a mechanosensing bridge, enabling cytoskeletal force transmission, cup closure, and particle internalization. In vivo, MRCKβ is essential for RPE phagocytosis and retinal integrity. MerTK-independent activation of MRCKβ signaling by a phosphomimetic Dbl3 mutant rescues phagocytosis in retinitis pigmentosa RPE cells lacking functional MerTK. MRCKβ is also required for efficient particle translocation from the cortex into the cell body in Fc receptor–mediated phagocytosis. Thus, conserved MRCKβ signaling at the cortex controls spatiotemporal regulation of actomyosin contractility to guide distinct phases of phagocytosis in the RPE and represents the principle phagocytic effector pathway downstream of MerTK.

Engineering physical microenvironments to study innate immune cell biophysics

Innate immunity forms the core of the human body's defense system against infection, injury, and foreign objects. It aims to maintain homeostasis by promoting inflammation and then initiating tissue repair, but it can also lead to disease when dysregulated. Although innate immune cells respond to their physical microenvironment and carry out intrinsically mechanical actions such as migration and phagocytosis, we still do not have a complete biophysical description of innate immunity. Here, we review how engineering tools can be used to study innate immune cell biophysics. We first provide an overview of innate immunity from a biophysical perspective, review the biophysical factors that affect the innate immune system, and then explore innate immune cell biophysics in the context of migration, phagocytosis, and phenotype polarization. Throughout the review, we highlight how physical microenvironments can be designed to probe the innate immune system, discuss how biophysical insight gained from these studies can be used to generate a more comprehensive description of innate immunity, and briefly comment on how this insight could be used to develop mechanical immune biomarkers and immunomodulatory therapies.

Integrative experimental/computational approach establishes active cellular protrusion as the primary driving force of phagocytic spreading by immune cells

The dynamic interplay between cell adhesion and protrusion is a critical determinant of many forms of cell motility. When modeling cell spreading on adhesive surfaces, traditional mathematical treatments often consider passive cell adhesion as the primary, if not exclusive, mechanistic driving force of this cellular motion. To better assess the contribution of active cytoskeletal protrusion to immune-cell spreading during phagocytosis, we here develop a computational framework that allows us to optionally investigate both purely adhesive spreading ("Brownian zipper hypothesis") as well as protrusion-dominated spreading ("protrusive zipper hypothesis"). We model the cell as an axisymmetric body of highly viscous fluid surrounded by a cortex with uniform surface tension and incorporate as potential driving forces of cell spreading an attractive stress due to receptor-ligand binding and an outward normal stress representing cytoskeletal protrusion, both acting on the cell boundary. We leverage various model predictions against the results of a directly related experimental companion study of human neutrophil phagocytic spreading on substrates coated with different densities of antibodies. We find that the concept of adhesion-driven spreading is incompatible with experimental results such as the independence of the cell-spreading speed on the density of immobilized antibodies. In contrast, the protrusive zipper model agrees well with experimental findings and, when adapted to simulate cell spreading on discrete adhesion sites, it also reproduces the observed positive correlation between antibody density and maximum cell-substrate contact area. Together, our integrative experimental/computational approach shows that phagocytic spreading is driven by cellular protrusion, and that the extent of spreading is limited by the density of adhesion sites.

Building the phagocytic cup on an actin scaffold

Cells ingest large particles, such as bacteria, viruses, or apoptotic cells, via the process of phagocytosis, which involves formation of an actin-rich structure known as the phagocytic cup. Phagocytic cup assembly and closure results from a concerted action of phagocytic receptors, regulators of actin polymerization, and myosin motors. Recent studies using advanced imaging approaches and biophysical techniques have revealed new information regarding phagocytic cup architecture, regulation of actin assembly, and the distribution, direction, and magnitude of the forces produced by the cytoskeletal elements that form the cup. These findings provide insights into the mechanisms leading to the assembly, expansion, and closure of phagocytic cups. The new data show that engulfment and internalization of phagocytic targets rely on several distinct yet complementary mechanisms that support the robust uptake of foreign objects and may be precisely tailored to the demands of specific phagocytic pathways.

Distinct timing of neutrophil spreading and stiffening during phagocytosis

Phagocytic cells form the first line of defense in an organism, engulfing microbial pathogens. Phagocytosis involves cell mechanical changes that are not yet well understood. Understanding these mechanical modifications promises to shed light on the immune processes that trigger pathological complications. Previous studies showed that phagocytes undergo a sequence of spreading events around their target followed by an increase in cell tension. Seemingly in contradiction, other studies observed an increase in cell tension concomitant with membrane expansion. Even though phagocytes are viscoelastic, few studies have quantified viscous changes during phagocytosis. It is also unclear whether cell lines behave mechanically similarly to primary neutrophils. We addressed the question of simultaneous versus sequential spreading and mechanical changes during phagocytosis by using immunoglobulin-G-coated 8- and 20-μm-diameter beads as targets. We used a micropipette-based single-cell rheometer to monitor viscoelastic properties during phagocytosis by both neutrophil-like PLB cells and primary human neutrophils. We show that the faster expansion of PLB cells on larger beads is a geometrical effect reflecting a constant advancing speed of the phagocytic cup. Cells become stiffer on 20- than on 8-μm beads, and the relative timing of spreading and stiffening of PLB cells depends on target size: on larger beads, stiffening starts before maximal spreading area is reached but ends after reaching maximal area. On smaller beads, the stiffness begins to increase after cells have engulfed the bead. Similar to PLB cells, primary cells become stiffer on larger beads but start spreading and stiffen faster, and the stiffening begins before the end of spreading on both bead sizes. Our results show that mechanical changes in phagocytes are not a direct consequence of cell spreading and that models of phagocytosis should be amended to account for causes of cell stiffening other than membrane expansion.

Snail Augments Nuclear Deformability to Promote Lymph Node Metastasis of Head and Neck Squamous Cell Carcinoma

Up to 50% of head and neck squamous cell carcinoma (HNSCC) patients have lymph node (LN) metastasis, resulting in poor survival rate. Numerous studies have supported the notion that the alterations of gene expression and mechanical properties of cancer cells play an important role in cancer metastasis. However, which genes and how they regulate the biomechanical properties of HNSCC cells to promote LN metastasis remains elusive. In this study, we used an LN-metastatic mouse model in vivo to generate an LN-metastatic head and neck squamous cell carcinoma cell line and compared the differences in the biomolecular and biomechanical properties of LN-metastatic and non-metastatic cells. Our results showed that LN-metastatic cells had a higher level of Snail expression compared to non-LN-metastatic cells. The higher Snail expression promoted the cellular invasion capability in confined environments, mainly by increasing the longitudinal strain of the cell nuclei, which could be attributed to the stronger cell traction force and softer nuclear stiffness. These two biomechanical changes were correlated, respectively, to a larger amount of focal adhesion and less amount of nuclear lamins. Taken together, our works revealed not only the biomechanical profiles of LN-metastatic cells but also the corresponding biomolecular expressions to pinpoint the key process in LN metastasis.

Phagocytic 'teeth' and myosin-II 'jaw' power target constriction during phagocytosis

Phagocytosis requires rapid actin reorganization and spatially controlled force generation to ingest targets ranging from pathogens to apoptotic cells. How actomyosin activity directs membrane extensions to engulf such diverse targets remains unclear. Here, we combine lattice light-sheet microscopy (LLSM) with microparticle traction force microscopy (MP-TFM) to quantify actin dynamics and subcellular forces during macrophage phagocytosis. We show that spatially localized forces leading to target constriction are prominent during phagocytosis of antibody-opsonized targets. This constriction is largely driven by Arp2/3-mediated assembly of discrete actin protrusions containing myosin 1e and 1f ('teeth') that appear to be interconnected in a ring-like organization. Contractile myosin-II activity contributes to late-stage phagocytic force generation and progression, supporting a specific role in phagocytic cup closure. Observations of partial target eating attempts and sudden target release via a popping mechanism suggest that constriction may be critical for resolving complex in vivo target encounters. Overall, our findings present a phagocytic cup shaping mechanism that is distinct from cytoskeletal remodeling in 2D cell motility and may contribute to mechanosensing and phagocytic plasticity.

TBC1D10C is a cytoskeletal functional linker that modulates cell spreading and phagocytosis in macrophages

Cell spreading and phagocytosis are notably regulated by small GTPases and GAP proteins. TBC1D10C is a dual inhibitory protein with GAP activity. In immune cells, TBC1D10C is one of the elements regulating lymphocyte activation. However, its specific role in macrophages remains unknown. Here, we show that TBC1D10C engages in functions dependent on the cytoskeleton and plasma membrane reorganization. Using ex vivo and in vitro assays, we found that elimination and overexpression of TBC1D10C modified the cytoskeletal architecture of macrophages by decreasing and increasing the spreading ability of these cells, respectively. In addition, TBC1D10C overexpression contributed to higher phagocytic activity against Burkholderia cenocepacia and to increased cell membrane tension. Furthermore, by performing in vitro and in silico analyses, we identified 27 TBC1D10C-interacting proteins, some of which were functionally classified as protein complexes involved in cytoskeletal dynamics. Interestingly, we identified one unreported TBC1D10C-intrinsically disordered region (IDR) with biological potential at the cytoskeleton level. Our results demonstrate that TBC1D10C shapes macrophage activity by inducing reorganization of the cytoskeleton-plasma membrane in cell spreading and phagocytosis. We anticipate our results will be the basis for further studies focused on TBC1D10C. For example, the specific molecular mechanism in Burkholderia cenocepacia phagocytosis and functional analysis of TBC1D10C-IDR are needed to further understand its role in health and disease.

A mechanical perspective on phagocytic cup formation

Phagocytosis is a widespread and evolutionarily conserved process with diverse biological functions, ranging from engulfment of invading microbes during infection to clearance of apoptotic debris in tissue homeostasis. Along with differences in biochemical composition, phagocytic targets greatly differ in physical attributes, such as size, shape, and rigidity, which are now recognized as important regulators of this process. Force exertion at the cell-target interface and cellular mechanical changes during phagocytosis are emerging as crucial factors underlying sensing of such target properties. With technological developments, mechanical aspects of phagocytosis are increasingly accessible experimentally, revealing remarkable organizational complexity of force exertion. An increasingly high-resolution picture is emerging of how target physical cues and cellular mechanical properties jointly govern important steps throughout phagocytic engulfment.

Physical Constraints and Forces Involved in Phagocytosis

Phagocytosis is a specialized process that enables cellular ingestion and clearance of microbes, dead cells and tissue debris that are too large for other endocytic routes. As such, it is an essential component of tissue homeostasis and the innate immune response, and also provides a link to the adaptive immune response. However, ingestion of large particulate materials represents a monumental task for phagocytic cells. It requires profound reorganization of the cell morphology around the target in a controlled manner, which is limited by biophysical constraints. Experimental and theoretical studies have identified critical aspects associated with the interconnected biophysical properties of the receptors, the membrane, and the actin cytoskeleton that can determine the success of large particle internalization. In this review, we will discuss the major physical constraints involved in the formation of a phagosome. Focusing on two of the most-studied types of phagocytic receptors, the Fcγ receptors and the complement receptor 3 (αMβ2 integrin), we will describe the complex molecular mechanisms employed by phagocytes to overcome these physical constraints.

Fibroblast rejuvenation by mechanical reprogramming and redifferentiation

The mechanical reprogramming of fibroblasts, followed by their redifferentiation into rejuvenated fibroblasts in an optimized 3D collagen matrix, made these cells more contractile and more efficient at synthesizing matrix components including laminin, fibronectin, and collagen-IV. Moreover, the rejuvenated fibroblasts obtained through this approach exhibited a decrease in DNA damage. The rejuvenated fibroblasts derived from this method precisely align into tissue architectures, suggesting its potential application as clinical implants in tissue engineering and regenerative medicine.

Over the course of the aging process, fibroblasts lose contractility, leading to reduced connective-tissue stiffness. A promising therapeutic avenue for functional rejuvenation of connective tissue is reprogrammed fibroblast replacement, although major hurdles still remain. Toward this, we recently demonstrated that the laterally confined growth of fibroblasts on micropatterned substrates induces stem-cell-like spheroids. In this study, we embedded these partially reprogrammed spheroids in collagen-I matrices of varying densities, mimicking different three-dimensional (3D) tissue constraints. In response to such matrix constraints, these spheroids regained their fibroblastic properties and sprouted to form 3D connective-tissue networks. Interestingly, we found that these differentiated fibroblasts exhibit reduced DNA damage, enhanced cytoskeletal gene expression, and actomyosin contractility. In addition, the rejuvenated fibroblasts show increased matrix protein (fibronectin and laminin) deposition and collagen remodeling compared to the parental fibroblast tissue network. Furthermore, we show that the partially reprogrammed cells have comparatively open chromatin compaction states and may be more poised to redifferentiate into contractile fibroblasts in 3D-collagen matrix. Collectively, our results highlight efficient fibroblast rejuvenation through laterally confined reprogramming, which has important implications in regenerative medicine.

Phagocytosis: Phenotypically Simple Yet a Mechanistically Complex Process

Phagocytosis is a pivotal immunological process, and its discovery by Elia Metchnikoff in 1882 was a step toward the establishment of the innate immune system as a separate branch of immunology. Elia Metchnikoff received the Nobel Prize in physiology and medicine for this discovery in 1908. Since its discovery almost 140 years before, phagocytosis remains the hot topic of research in immunology. The phagocytosis research has seen a great advancement since its first discovery. Functionally, phagocytosis is a simple immunological process required to engulf and remove pathogens, dead cells and tumor cells to maintain the immune homeostasis. However, mechanistically, it is a very complex process involving different mechanisms, induced and regulated by several pattern recognition receptors, soluble pattern recognition molecules, scavenger receptors (SRs) and opsonins. These mechanisms involve the formation of phagosomes, their maturation into phagolysosomes causing pathogen destruction or antigen synthesis to present them to major histocompatibility complex molecules for activating an adaptive immune response. Any defect in this mechanism may predispose the host to certain infections and inflammatory diseases (autoinflammatory and autoimmune diseases) along with immunodeficiency. The article is designed to discuss its mechanistic complexity at each level, varying from phagocytosis induction to phagolysosome resolution.

Microparticle traction force microscopy reveals subcellular force exertion patterns in immune cell-target interactions

Force exertion is an integral part of cellular behavior. Traction force microscopy (TFM) has been instrumental for studying such forces, providing spatial force measurements at subcellular resolution. However, the applications of classical TFM are restricted by the typical planar geometry. Here, we develop a particle-based force sensing strategy for studying cellular interactions. We establish a straightforward batch approach for synthesizing uniform, deformable and tuneable hydrogel particles, which can also be easily derivatized. The 3D shape of such particles can be resolved with superresolution (<50 nm) accuracy using conventional confocal microscopy. We introduce a reference-free computational method allowing inference of traction forces with high sensitivity directly from the particle shape. We illustrate the potential of this approach by revealing subcellular force patterns throughout phagocytic engulfment and force dynamics in the cytotoxic T-cell immunological synapse. This strategy can readily be adapted for studying cellular forces in a wide range of applications.

Physical Constraints and Forces Involved in Phagocytosis

Phagocytosis is a specialized process that enables cellular ingestion and clearance of microbes, dead cells and tissue debris that are too large for other endocytic routes. As such, it is an essential component of tissue homeostasis and the innate immune response, and also provides a link to the adaptive immune response. However, ingestion of large particulate materials represents a monumental task for phagocytic cells. It requires profound reorganization of the cell morphology around the target in a controlled manner, which is limited by biophysical constraints. Experimental and theoretical studies have identified critical aspects associated with the interconnected biophysical properties of the receptors, the membrane, and the actin cytoskeleton that can determine the success of large particle internalization. In this review, we will discuss the major physical constraints involved in the formation of a phagosome. Focusing on two of the most-studied types of phagocytic receptors, the Fcγ receptors and the complement receptor 3 (αMβ2 integrin), we will describe the complex molecular mechanisms employed by phagocytes to overcome these physical constraints.

Force Measurement of Living Professional Phagocytes of the Immune System

In higher organisms, the professional phagocytes of the immune system (dendritic cells, neutrophils, monocytes, and macrophages) are responsible for pathogen clearance, the development of immune responses via cytokine secretion and presentation of antigens derived from internalized material, and the normal turnover and remodelling of tissues and disposal of dead cells. These functions rely on the ability of phagocytes to migrate and adhere to sites of infection, dynamically probe their environments to make contact with phagocytic targets, and perform phagocytosis, a mechanism of internalization of large particles, microorganisms, and cellular debris for intracellular degradation. The cell-generated forces that are necessary for the professional phagocytes to act in their roles as ‘first responders’ of the immune system have been the subject of mechanical studies in recent years. Methods of force measurement such as atomic force microscopy, traction force microscopy, micropipette aspiration, magnetic and optical tweezers, and exciting new variants of these have accompanied classical biological methods to perform mechanical investigations of these highly dynamic immune cells.

Squeezing in a Meal: Myosin Functions in Phagocytosis

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.

Actin Waves and Dynamic Patterning of the Plasma Membrane

Plasma membrane and underlying actin network are connected to a functional unit that by non-linear interactions is capable of forming patterns. For instance, in cell motility and chemotaxis, cells polarize to form a protruding front and a retracting tail. Here we address dynamic patterns that are formed on a planar substrate surface and are therefore easily accessible to optical recording. In these patterns two distinct areas of the membrane and actin cortex are interconverted at the site of circular actin waves. The inner territory circumscribed by a wave is distinguished from the external area by a high PIP3 content and high Ras activity. In contrast, the external area is occupied with the PIP3-degrading phosphatase PTEN. In the underlying cortex, these areas differ in the proteins associated with the actin network. Actin waves can be formed at zones of increasing as well as decreasing Ras activity. Both types of waves are headed by myosin IB. When waves collide, they usually extinguish each other, and their decay is accompanied by the accumulation of coronin. No membrane patterns have been observed after efficient depolymerization of actin, suggesting that residual actin filaments are necessary for the pattern generating system to work. Where appropriate, we relate the experimental data obtained with Dictyostelium to human normal and malignant cell behavior, in particular to the role of Ras-GAP as an enhancer of macropinocytosis, to mutations in the tumor suppressor PTEN, to frustrated phagocytosis, and to the role of coronin in immune cells and neurons.

Membrane-cytoskeletal crosstalk mediated by myosin-I regulates adhesion turnover during phagocytosis

Phagocytosis of invading pathogens or cellular debris requires a dramatic change in cell shape driven by actin polymerization. For antibody-covered targets, phagocytosis is thought to proceed through the sequential engagement of Fc-receptors on the phagocyte with antibodies on the target surface, leading to the extension and closure of the phagocytic cup around the target. We find that two actin-dependent molecular motors, class 1 myosins myosin 1e and myosin 1f, are specifically localized to Fc-receptor adhesions and required for efficient phagocytosis of antibody-opsonized targets. Using primary macrophages lacking both myosin 1e and myosin 1f, we find that without the actin-membrane linkage mediated by these myosins, the organization of individual adhesions is compromised, leading to excessive actin polymerization, slower adhesion turnover, and deficient phagocytic internalization. This work identifies a role for class 1 myosins in coordinated adhesion turnover during phagocytosis and supports a mechanism involving membrane-cytoskeletal crosstalk for phagocytic cup closure.

The phagocytic state of brain myeloid cells after ischemia revealed by superresolution structured illumination microscopy

BACKGROUND

Phagocytosis is a key function of myeloid cells and is highly involved in brain ischemic injury. It has been scarcely studied in vivo, thus preventing a deep knowledge of the processes occurring in the ischemic environment. Structured illumination microscopy (SIM) is a superresolution technique which helps study phagocytosis, a process involving the recruitment of vesicles sized below the resolution limits of standard confocal microscopy.

METHODS

Mice underwent permanent occlusion of the middle cerebral artery and were sacrificed at 48 h or 7 days after insult. Immunofluorescence for CD11b, myeloid cell membrane marker, and CD68, lysosomal marker was done in the ischemic area. Images were acquired using a SIM system and verified with SIM check. Lysosomal distribution was measured in the ischemic area by the gray level co-occurrence matrix (GLCM). SIM dataset was compared with transmission electron microscopy images of macrophages in the ischemic tissue at the same time points. Cultured microglia were stimulated with LPS to uptake 100 nm fluorescent beads and imaged by time-lapse SIM. GLCM was used to analyze bead distribution over the cytoplasm.

RESULTS

SIM images reached a resolution of 130 nm and passed the quality control diagnose, ruling out possible artifacts. After ischemia, GLCM applied to the CD68 images showed that myeloid cells at 48 h had higher angular second moment (ASM), inverse difference moment (IDM), and lower entropy than myeloid cells at 7 days indicating higher lysosomal clustering at 48 h. At this time point, lysosomal clustering was proximal (< 700 nm) to the cell membrane indicating active target internalization, while at 7 days, it was perinuclear, consistent with final stages of phagocytosis or autophagy. Electron microscopy images indicated a similar pattern of lysosomal distribution thus validating the SIM dataset. GLCM on time-lapse SIM from phagocytic microglia cultures revealed a temporal decrease in ASM and IDM and increase in entropy, as beads were uptaken, indicating that GLCM informs on the progression of phagocytosis.

CONCLUSIONS

GLCM analysis on SIM dataset quantitatively described different phases of macrophage phagocytic behavior revealing the dynamics of lysosomal movements in the ischemic brain indicating initial active internalization vs. final digestion/autophagy.

Mechanistic Understanding of Single-Cell Behavior is Essential for Transformative Advances in Biomedicine

Most current efforts to advance medical technology proceed along one of two tracks. The first is dedicated to the improvement of clinical tasks through the incremental refinement of medical instruments. The second comprises engineering endeavors to support basic science studies that often only remotely relate to human medicine. Here we survey emerging research approaches that aim to populate the sprawling frontier between these tracks. We focus on interdisciplinary single-live-cell techniques that have overcome limitations of traditional biological methods to successfully address vital questions about medically relevant cellular behavior. Most of the presented case studies are based on the controlled manipulation of nonadherent human immune cells using one or more micropipettes. The included studies have (i) examined one-on-one encounters of immune cells with real or model pathogens, (ii) assessed the physiological role of the expandable surface area of immune cells, and (iii) started to dissect the spatiotemporal organization of signaling processes within these cells. The unique aptitude of such single-live-cell studies to fill conspicuous gaps in our quantitative understanding of medically relevant cause-effect relationships provides a sound basis for new insights that will inform and drive future biomedical innovation.

Label-free and live cell imaging by interferometric scattering microscopy

Despite recent remarkable advances in microscopic techniques, it still remains very challenging to directly observe the complex structure of cytoplasmic organelles in live cells without a fluorescent label.

Despite recent remarkable advances in microscopic techniques, it still remains very challenging to directly observe the complex structure of cytoplasmic organelles in live cells without a fluorescent label. Here we report label-free and live-cell imaging of mammalian cell, Escherischia coli, and yeast, using interferometric scattering microscopy, which reveals the underlying structures of a variety of cytoplasmic organelles as well as the underside structure of the cells. The contact areas of the cells attached onto a glass substrate, e.g., focal adhesions and filopodia, are clearly discernible. We also found a variety of fringe-like features in the cytoplasmic area, which may reflect the folded structures of cytoplasmic organelles. We thus anticipate that the label-free interferometric scattering microscopy can be used as a powerful tool to shed interferometric light on in vivo structures and dynamics of various intracellular phenomena.

How cells engulf: a review of theoretical approaches to phagocytosis

Phagocytosis is a fascinating process whereby a cell surrounds and engulfs particles such as bacteria and dead cells. This is crucial both for single-cell organisms (as a way of acquiring nutrients) and as part of the immune system (to destroy foreign invaders). This whole process is hugely complex and involves multiple coordinated events such as membrane remodelling, receptor motion, cytoskeleton reorganisation and intracellular signalling. Because of this, phagocytosis is an excellent system for theoretical study, benefiting from biophysical approaches combined with mathematical modelling. Here, we review these theoretical approaches and discuss the recent mathematical and computational models, including models based on receptors, models focusing on the forces involved, and models employing energetic considerations. Along the way, we highlight a beautiful connection to the physics of phase transitions, consider the role of stochasticity, and examine links between phagocytosis and other types of endocytosis. We cover the recently discovered multistage nature of phagocytosis, showing that the size of the phagocytic cup grows in distinct stages, with an initial slow stage followed by a much quicker second stage starting around half engulfment. We also address the issue of target shape dependence, which is relevant to both pathogen infection and drug delivery, covering both one-dimensional and two-dimensional results. Throughout, we pay particular attention to recent experimental techniques that continue to inform the theoretical studies and provide a means to test model predictions. Finally, we discuss population models, connections to other biological processes, and how physics and modelling will continue to play a key role in future work in this area.

Cdc42 regulates the cellular localization of Cdc42ep1 in controlling neural crest cell migration

The member of Rho family of small GTPases Cdc42 plays important and conserved roles in cell polarity and motility. The Cdc42ep family proteins have been identified to bind to Cdc42, yet how they interact with Cdc42 to regulate cell migration remains to be elucidated. In this study, we focus on Cdc42ep1, which is expressed predominantly in the highly migratory neural crest cells in frog embryos. Through morpholino-mediated knockdown, we show that Cdc42ep1 is required for the migration of cranial neural crest cells. Loss of Cdc42ep1 leads to rounder cell shapes and the formation of membrane blebs, consistent with the observed disruption in actin organization and focal adhesion alignment. As a result, Cdc42ep1 is critical for neural crest cells to apply traction forces at the correct place to migrate efficiently. We further show that Cdc42ep1 is localized to two areas in neural crest cells: in membrane protrusions together with Cdc42 and in perinuclear patches where Cdc42 is absent. Cdc42 directly interacts with Cdc42ep1 (through the CRIB domain) and changes in Cdc42 level shift the distribution of Cdc42ep1 between these two subcellular locations, controlling the formation of membrane protrusions and directionality of migration as a consequence. These results suggest that Cdc42ep1 elaborates Cdc42 activity in neural crest cells to promote their efficient migration.