Membrane capacitance changes associated with particle uptake during phagocytosis in macrophages

Membrane backtracking at the maximum capacity of nondigestible antigen phagocytosis in macrophages

The zipper model has been dominantly used to describe the driving mechanism of the engulfment process and its specific identification of antigens during phagocytosis in macrophages. However, the abilities and limitations of the zipper model, capturing the process as an irreversible reaction, have not been examined yet under the critical conditions of engulfment capacity. Here, we demonstrated the phagocytic behavior of macrophages after reaching the maximum engulfment capacity by tracking the progression of their membrane extension during engulfment using IgG-coated nondigestible polystyrene beads and glass microneedles. The results showed that, after macrophages reached their maximum engulfment capacity, they induced membrane backtracking (the reverse phenomenon of engulfment) in both polystyrene beads and glass microneedles, regardless of the difference in the shape of these antigens. We evaluated the correlation of engulfment in simultaneous stimulations of two IgG-coated microneedles and found that each microneedle was regurgitated by the macrophage independently of the advancement or backtracking of membranes on the other microneedle. Moreover, assessing the total engulfment capacity determined by the maximum amount the macrophage was capable of engulfing when imposing each antigen geometry showed that the capacity increased as the attached antigen areas increased. These results indicate that the mechanism of engulfment should imply the following: 1) macrophages have a backtracking function to recover their phagocytic activity after reaching maximal engulfment limit, 2) both phagocytosis and backtracking are local phenomena of the macrophage membrane that operates independently, and 3) the maximum engulfment capacity is determined not only by mere local cell membrane capacity but also by the whole-cell volume increase during simultaneous phagocytosis of multiple antigens by the single macrophages. Thus, the phagocytic activity may entail a hidden backtracking function, adding to the conventionally known irreversible zipper-like ligand-receptor binding mechanism during membrane progression to recover the macrophages that are saturated from engulfing targets beyond their capacity.

Dielectrophoretic characterization of macrophage phenotypes

Different macrophage phenotypes play important roles in diverse biological processes and diseases. In this study, we have characterized the dielectrophoretic responses of human monocytes and macrophage phenotypes: nonactivated (M0), pro-inflammatory (M1), and pro-healing (M2a). Dielectrophoretic responses of cells change as a function of frequency of the applied electric field. We measured the crossover frequency at which cells transition from negative to positive dielectrophoresis (DEP) or vice versa using interdigitated electrodes. For these characterization experiments, we also developed a new low-conductivity media formulation that retained 100% of the initial viability for 1 h. Human THP1 monocytes showed a distinguishable DEP response from mature macrophages. M1 macrophages also showed a distinct DEP response compared to M0 and M2a macrophages. No clear distinction could be drawn between M0 and M2a. The median values of the crossover frequencies of monocytes, M0, M1, and M2a were 38, 21, 11, and 23 kHz, respectively. Membrane capacitances of these cells were calculated consequently, and the values were 0.0111, 0.0128, 0.0244, and 0.0117 F/m² for monocytes, M0, M1, and M2a, respectively. These results show how bioelectric properties are influenced by changes in macrophage phenotype.

PKC-ε regulates vesicle delivery and focal exocytosis for efficient IgG-mediated phagocytosis

Protein kinase C (PKC)-ε is required for membrane addition during IgG-mediated phagocytosis, but its role in this process is ill defined. Here, we performed high-resolution imaging, which reveals that PKC-ε exits the Golgi and enters phagosomes on vesicles that then fuse. TNF and PKC-ε colocalize at the Golgi and on vesicles that enter the phagosome. Loss of PKC-ε and TNF delivery upon nocodazole treatment confirmed vesicular transport on microtubules. That TNF+ vesicles were not delivered in macrophages from PKC-ε null mice, or upon dissociation of the Golgi-associated pool of PKC-ε, implies that Golgi-tethered PKC-ε is a driver of Golgi-to-phagosome trafficking. Finally, we established that the regulatory domain of PKC-ε is sufficient for delivery of TNF+ vesicles to the phagosome. These studies reveal a novel role for PKC-ε in focal exocytosis - its regulatory domain drives Golgi-derived vesicles to the phagosome, whereas catalytic activity is required for their fusion. This is one of the first examples of a PKC requirement for vesicular trafficking and describes a novel function for a PKC regulatory domain. This article has an associated First Person interview with the first author of the paper.

The cytoskeleton in phagocytosis and macropinocytosis

Cells of the innate immune system, notably macrophages, neutrophils and dendritic cells, perform essential antimicrobial and homeostatic functions. These functions rely on the dynamic surveillance of the environment supported by the formation of elaborate membrane protrusions. Such protrusions - pseudopodia, lamellipodia and filopodia - facilitate the sampling of the surrounding fluid by macropinocytosis, as well as the engulfment of particulates by phagocytosis. Both processes entail extreme plasma membrane deformations that require the coordinated rearrangement of cytoskeletal polymers, which exert protrusive force and drive membrane coalescence and scission. The resulting vacuolar compartments undergo pronounced remodeling and ultimate resolution by mechanisms that also involve the cytoskeleton. Here, we describe the regulation and functions of cytoskeletal assembly and remodeling during macropinocytosis and phagocytosis.

Examining the Underappreciated Role of S-Acylated Proteins as Critical Regulators of Phagocytosis and Phagosome Maturation in Macrophages

Phagocytosis is a receptor-mediated process used by cells to engulf a wide variety of particulates, including microorganisms and apoptotic cells. Many of the proteins involved in this highly orchestrated process are post-translationally modified with lipids as a means of regulating signal transduction, membrane remodeling, phagosome maturation and other immunomodulatory functions of phagocytes. S-acylation, generally referred to as S-palmitoylation, is the post-translational attachment of fatty acids to a cysteine residue exposed topologically to the cytosol. This modification is reversible due to the intrinsically labile thioester bond between the lipid and sulfur atom of cysteine, and thus lends itself to a variety of regulatory scenarios. Here we present an overview of a growing number of S-acylated proteins known to regulate phagocytosis and phagosome biology in macrophages.

Origins of eukaryotic excitability

All living cells interact dynamically with a constantly changing world. Eukaryotes, in particular, evolved radically new ways to sense and react to their environment. These advances enabled new and more complex forms of cellular behaviour in eukaryotes, including directional movement, active feeding, mating, and responses to predation. But what are the key events and innovations during eukaryogenesis that made all of this possible? Here we describe the ancestral repertoire of eukaryotic excitability and discuss five major cellular innovations that enabled its evolutionary origin. The innovations include a vastly expanded repertoire of ion channels, the emergence of cilia and pseudopodia, endomembranes as intracellular capacitors, a flexible plasma membrane and the relocation of chemiosmotic ATP synthesis to mitochondria, which liberated the plasma membrane for more complex electrical signalling involved in sensing and reacting. We conjecture that together with an increase in cell size, these new forms of excitability greatly amplified the degrees of freedom associated with cellular responses, allowing eukaryotes to vastly outperform prokaryotes in terms of both speed and accuracy. This comprehensive new perspective on the evolution of excitability enriches our view of eukaryogenesis and emphasizes behaviour and sensing as major contributors to the success of eukaryotes. This article is part of the theme issue 'Basal cognition: conceptual tools and the view from the single cell'.

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.

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.

A Multiparametric and High-Throughput Assay to Quantify the Influence of Target Size on Phagocytosis

Phagocytosis by macrophages represents a fundamental process essential for both immunity and tissue homeostasis. It consists in the uptake of pathogenic or cellular targets larger than 0.5 μm. For the biggest particles, the phagocytic process involves a massive reorganization of membrane and actin cytoskeleton as well as an important intracellular deformation all in a matter of minutes. The study of the role of the size of objects in their phagocytosis has led to contradictory results in the last decades. We designed a method using confocal microscopy, automated image analysis, and databases for fast quantitative analysis of phagocytosis assays. It yields comprehensive data on the cells and targets geometric and fluorescence intensity parameters, automatically discriminates internalized from external targets, and stores the relationship between a cell and the targets it has engulfed. We used two types of targets (solid polystyrene beads and liquid lipid droplets) to investigate the influence of size on the phagocytic uptake of macrophages. The method made it possible not only to perform phagocytic assays with functionalized droplets and beads of different sizes but to use polydisperse particles to further our understanding of the role of size in phagocytosis. The use of monodisperse and polydisperse objects shows that whereas smaller monodisperse objects are internalized in greater numbers, objects of different sizes presented simultaneously are internalized without preferred size. The total surface engulfed by the cell is thus the main factor limiting the uptake of particles, regardless of their nature or size. A meta-analysis of the literature reveals that this dependence in surface is consistently conserved throughout cell types, targets' nature, or activated receptors.

The role of relative membrane capacitance and time delay in cerebellar Purkinje cells

The membrane capacitance of a neuron can influence the synaptic efficacy and the speed of electrical signal propagation. Exploring the role of membrane capacitance will help facilitate a deeper understanding of the electrical properties of neurons. Thus, in this paper, we investigated the neuronal firing behaviors of a two-compartment model in Purkinje cells. We evaluated the influence of membrane capacitance under two different circumstances: in the absence of time delay and in the presence of time delay. Firstly, we separately studied the influence of somatic membrane capacitance Cs and dendritic membrane capacitance Cd on neuronal firing patterns. Through numerical simulation, we observed that they had two different types of period-adding scenarios, i.e. with and without chaotic bursting. Secondly, our results indicated that when the time delay was included in the model, periodic motions were more inclined to be destroyed, while at the same time, corresponding new chaotic motions were induced. These findings suggested that membrane capacitance and time delay play a pivotal functional role in modulating dynamical firing properties of neurons, especially aspects which lead to behaviors which result in changes to bursting patterns.

Membrane dynamics and organelle biogenesis-lipid pipelines and vesicular carriers

Discoveries spanning several decades have pointed to vital membrane lipid trafficking pathways involving both vesicular and non-vesicular carriers. But the relative contributions for distinct membrane delivery pathways in cell growth and organelle biogenesis continue to be a puzzle. This is because lipids flow from many sources and across many paths via transport vesicles, non-vesicular transfer proteins, and dynamic interactions between organelles at membrane contact sites. This forum presents our latest understanding, appreciation, and queries regarding the lipid transport mechanisms necessary to drive membrane expansion during organelle biogenesis and cell growth.

Maxed out macs: physiologic cell clearance as a function of macrophage phagocytic capacity

The phagocytic clearance of host cells is important for eliminating dying cells and for the therapeutic clearance of antibody-targeted cells. As ubiquitous, motile and highly phagocytic immune cells, macrophages are principal players in the phagocytic removal of host cells throughout the body. In recent years, great strides have been made in identifying the molecular mechanisms that control the recognition and phagocytosis of cells by macrophages. However, much less is known about the physical and metabolic constraints that govern the amount of cellular material macrophages can ingest and how these limitations affect the overall efficiency of host cell clearance in health and disease. In this review we will discuss, in the contexts of apoptotic cells and antibody-targeted malignant cells, how physical and metabolic factors associated with the internalization of host cells are relayed to the phagocytic machinery and how these signals can impact the overall efficiency of cell clearance. We also discuss how this information can be leveraged to increase cell clearance for beneficial therapeutic outcomes.

Ca2+-dependent Focal Exocytosis of Golgi-derived Vesicles Helps Phagocytic Uptake in Macrophages

The role of Golgi apparatus during phagocytic uptake by macrophages has been ruled out in the past. Notably, all such reports were limited to Fcγ receptor-mediated phagocytosis. Here, we unravel a highly devolved mechanism for recruitment of Golgi-derived secretory vesicles during phagosome biogenesis, which was important for uptake of most cargos, except the IgG-coated ones. We report recruitment of mannosidase-II-positive Golgi-derived vesicles during uptake of diverse targets, including latex beads, Escherichia coli, Salmonella typhimurium, and Mycobacterium tuberculosis in human and mouse macrophages. The recruitment of mannosidase-II vesicles was an early event mediated by focal exocytosis and coincided with the recruitment of transferrin receptor, VAMP3, and dynamin-2. Brefeldin A treatment inhibited mannosidase-II recruitment and phagocytic uptake of serum-coated or -uncoated latex beads and E. coli However, consistent with previous studies, brefeldin A treatment did not affect uptake of IgG-coated latex beads. Mechanistically, recruitment of mannosidase-II vesicles during phagocytic uptake required Ca²⁺ from both extra- and intracellular sources apart from PI3K, microtubules, and dynamin-2. Extracellular Ca²⁺ via voltage-gated Ca²⁺ channels established a Ca²⁺-dependent local phosphatidylinositol 1,4,5-trisphosphate gradient, which guides the focal movement of Golgi-derived vesicles to the site of uptake. We confirmed Golgi-derived vesicles recruited during phagocytosis were secretory vesicles as their recruitment was sensitive to depletion of VAMP2 or NCS1, whereas recruitment of the recycling endosome marker VAMP3 was unaffected. Depletion of both VAMP2 and NCS1 individually resulted in the reduced uptake by macrophages. Together, the study provides a previously unprecedented role of Golgi-derived secretory vesicles in phagocytic uptake, the key innate defense function.

Modeling sleep apnea severity using bioimpedance measurements

Obstructive sleep apnea (OSA) is a common disorder in adults characterized by repetitive collapse of the pharynx. OSA prevalence increases in fluid retaining patients such as those with heart or renal failure, and worsens with overnight fluid accumulation in the neck. The objective of this study was to develop a new method of measuring changes in intracellular water (ICW) in the neck, and investigate metrics that represent total neck impedance and their relationship to sleep apnea severity. In 18 non-obese men, neck fluid volume (NFV) was measured before and after sleep using bioelectrical impedance at 50 kHz. For each participant, resistance and reactance was extracted from the impedance measurements. A model was developed to estimate the cell membrane capacitance which could represent changes in intracellular fluid in the neck. OSA severity was assessed using polysomnography to estimate the apnea-hypopnea index (AHI) as well as the obstructive AHI (OAHI). Our results showed a strong correlation between the changes in NFV from before to after sleep with the changes in cell membrane capacitance from before to after sleep, indicating an increase in ICW in the neck during sleep. Using linear stepwise regression we were also able to develop models to accurately predict AHI and OAHI using baseline anthropometric and bioimpedance measurements. These promising results demonstrate that non-invasive measurements of bioimpedance can be used to develop a novel biomarker to model sleep apnea severity, and assess patients at high risk of OSA.

Inhalable Particles for "Pincer Therapeutics" Targeting Nitazoxanide as Bactericidal and Host-Directed Agent to Macrophages in a Mouse Model of Tuberculosis

Nitazoxanide (NTZ) has moderate mycobactericidal activity and is also an inducer of autophagy in mammalian cells. High-payload (40-50% w/w) inhalable particles containing NTZ alone or in combination with antituberculosis (TB) agents isoniazid (INH) and rifabutin (RFB) were prepared with high incorporation efficiency of 92%. In vitro drug release was corrected for drug degradation during the course of study and revealed first-order controlled release. Particles were efficiently taken up in vitro by macrophages and maintained intracellular drug concentrations at one order of magnitude higher than NTZ in solution for 6 h. Dose-dependent killing of Mtb and restoration of lung and spleen architecture were observed in experimentally infected mice treated with inhalations containing NTZ. Adjunct NTZ with INH and RFB cleared culturable bacteria from the lung and spleen and markedly healed tissue architecture. NTZ can be used in combination with INH-RFB to kill the pathogen and heal the host.

Comparative Anatomy of Phagocytic and Immunological Synapses

The generation of phagocytic cups and immunological synapses are crucial events of the innate and adaptive immune responses, respectively. They are triggered by distinct immune receptors and performed by different cell types. However, growing experimental evidence shows that a very close series of molecular and cellular events control these two processes. Thus, the tight and dynamic interplay between receptor signaling, actin and microtubule cytoskeleton, and targeted vesicle traffic are all critical features to build functional phagosomes and immunological synapses. Interestingly, both phagocytic cups and immunological synapses display particular spatial and temporal patterns of receptors and signaling molecules, leading to the notion of “phagocytic synapse.” Here, we discuss both types of structures, their organization, and the mechanisms by which they are generated and regulated.

Insight into the efficient transfection activity of a designed low aggregated magnetic polyethyleneimine/DNA complex in serum-containing medium and the application in vivo

In vitro fate of designed low aggregated magnetic polyethyleneimine/DNA (MPD-cc) complexes and in vivo study via systemic administration.

Lysosomal exocytosis and lipid storage disorders

Lysosomes are acidic compartments in mammalian cells that are primarily responsible for the breakdown of endocytic and autophagic substrates such as membranes, proteins, and lipids into their basic building blocks. Lysosomal storage diseases (LSDs) are a group of metabolic disorders caused by genetic mutations in lysosomal hydrolases required for catabolic degradation, mutations in lysosomal membrane proteins important for catabolite export or membrane trafficking, or mutations in nonlysosomal proteins indirectly affecting these lysosomal functions. A hallmark feature of LSDs is the primary and secondary excessive accumulation of undigested lipids in the lysosome, which causes lysosomal dysfunction and cell death, and subsequently pathological symptoms in various tissues and organs. There are more than 60 types of LSDs, but an effective therapeutic strategy is still lacking for most of them. Several recent in vitro and in vivo studies suggest that induction of lysosomal exocytosis could effectively reduce the accumulation of the storage materials. Meanwhile, the molecular machinery and regulatory mechanisms for lysosomal exocytosis are beginning to be revealed. In this paper, we first discuss these recent developments with the focus on the functional interactions between lipid storage and lysosomal exocytosis. We then discuss whether lysosomal exocytosis can be manipulated to correct lysosomal and cellular dysfunction caused by excessive lipid storage, providing a potentially general therapeutic approach for LSDs.

The cell surface proteome of Entamoeba histolytica

Surface molecules are of major importance for host-parasite interactions. During Entamoeba histolytica infections, these interactions are predicted to be of prime importance for tissue invasion, induction of colitis and liver abscess formation. To date, however, little is known about the molecules involved in these processes, with only about 20 proteins or protein families found exposed on the E. histolytica surface. We have therefore analyzed the complete surface proteome of E. histolytica. Using cell surface biotinylation and mass spectrometry, 693 putative surface-associated proteins were identified. In silico analysis predicted that ∼26% of these proteins are membrane-associated, as they contain transmembrane domains and/or signal sequences, as well as sites of palmitoylation, myristoylation, or prenylation. An additional 25% of the identified proteins likely represent nonclassical secreted proteins. Surprisingly, no membrane-association sites could be predicted for the remaining 49% of the identified proteins. To verify surface localization, 23 proteins were randomly selected and analyzed by immunofluorescence microscopy. Of these 23 proteins, 20 (87%) showed definite surface localization. These findings indicate that a far greater number of E. histolytica proteins than previously supposed are surface-associated, a phenomenon that may be based on the high membrane turnover of E. histolytica.

A TRP channel in the lysosome regulates large particle phagocytosis via focal exocytosis

Phagocytosis of large extracellular particles such as apoptotic bodies requires delivery of the intracellular endosomal and lysosomal membranes to form plasmalemmal pseudopods. Here, we identified mucolipin TRP channel 1 (TRPML1) as the key lysosomal Ca2+ channel regulating focal exocytosis and phagosome biogenesis. Both particle ingestion and lysosomal exocytosis are inhibited by synthetic TRPML1 blockers and are defective in macrophages isolated from TRPML1 knockout mice. Furthermore, TRPML1 overexpression and TRPML1 agonists facilitate both lysosomal exocytosis and particle uptake. Using time-lapse confocal imaging and direct patch clamping of phagosomal membranes, we found that particle binding induces lysosomal PI(3,5)P2 elevation to trigger TRPML1-mediated lysosomal Ca2+ release specifically at the site of uptake, rapidly delivering TRPML1-resident lysosomal membranes to nascent phagosomes via lysosomal exocytosis. Thus phagocytic ingestion of large particles activates a phosphoinositide- and Ca2+-dependent exocytosis pathway to provide membranes necessary for pseudopod extension, leading to clearance of senescent and apoptotic cells in vivo.

Plasma membrane tension orchestrates membrane trafficking, cytoskeletal remodeling, and biochemical signaling during phagocytosis

Phagocytes clear the body of undesirable particles such as infectious agents and debris. To extend pseudopods over the surface of targeted particles during engulfment, cells must change shape through extensive membrane and cytoskeleton remodeling. We observed that pseudopod extension occurred in two phases. In the first phase, pseudopods extended rapidly, with actin polymerization pushing the plasma membrane forward. The second phase occurred once the membrane area from preexisting reservoirs was depleted, leading to increased membrane tension. Increased tension directly altered the small Rho GTPase Rac1, 3'-phosphoinositide, and cytoskeletal organization. Furthermore, it activated exocytosis of vesicles containing GPI-anchored proteins, increasing membrane area and phagocytosis efficiency for large particles. We thus propose that, during phagocytosis, membrane remodeling, cytoskeletal organization, and biochemical signaling are orchestrated by the mechanical signal of membrane tension. These results put a simple mechanical signal at the heart of understanding immunological responses.

Cdc42 interacts with the exocyst complex to promote phagocytosis

The process of phagocytosis in multicellular organisms is required for homeostasis, clearance of foreign particles, and establishment of long-term immunity, yet the molecular determinants of uptake are not well characterized. Cdc42, a Rho guanosine triphosphatase, is thought to orchestrate critical actin remodeling events needed for internalization. In this paper, we show that Cdc42 controls exocytic events during phagosome formation. Cdc42 inactivation led to a selective defect in large particle phagocytosis as well as a general decrease in the rate of membrane flow to the cell surface. Supporting the connection between Cdc42 and exocytic function, we found that the overproduction of a regulator of exocytosis, Rab11, rescued the large particle uptake defect in the absence of Cdc42. Additionally, we demonstrated a temporal interaction between Cdc42 and the exocyst complex during large particle uptake. Furthermore, disruption of exocyst function through Exo70 depletion led to a defect in large particle internalization, thereby establishing a functional role for the exocyst complex during phagocytosis.

Phagocytosis and cytokinesis: do cells use common tools to cut and to eat? Highlights on common themes and differences

Eukaryotic cells with specialized functions often use and adapt common molecular machineries. Recent findings have highlighted that actin polymerization, contractile activity and membrane remodelling with exocytosis of internal compartments are required both for successful phagocytosis, the internalization of particulate material and for cytokinesis, the last step of cell division. Phagocytosis is induced by the triggering of specific cell surface receptors, which leads to membrane deformation, pseudopod extension and contraction to engulf particles. Cytokinesis relies on intense contractile activity and eventually leads to the physical scission of sister cells. In this review, shared features of signalling, cytoskeletal reorganization and vesicular trafficking used in both phagocytosis and cytokinesis will be described, but non-common mechanisms and questions that remain open in these dynamic areas of research are also highlighted.

A novel function for SNAP29 (synaptosomal-associated protein of 29 kDa) in mast cell phagocytosis

Mast cells play a critical role in the innate immune response to bacterial infection. They internalize and kill a variety of bacteria and process antigen for presentation to T cells via MHC molecules. Although mast cell phagocytosis appears to play a significant role during bacterial infection, little is known about the proteins involved in its regulation. In this study, we demonstrate that the SNARE protein SNAP29 is involved in mast cell phagocytosis. SNAP29 is localized in the endocytic pathway and is transiently recruited to Escherichia coli (E. coli)-containing phagosomes. Interestingly, overexpression of SNAP29 significantly increases the internalization and killing of E. coli, while it does not affect mast cell exocytosis of inflammatory mediators. To our knowledge, these data are the first to demonstrate a novel function of SNAP29 in mast cell phagocytosis and have implications in protection against bacterial infection.

Magnetoresistive-based real-time cell phagocytosis monitoring

The uptake of large particles by cells (phagocytosis) is an important factor in cell biology and also plays a major role in biomedical applications. So far, most methods for determining the phagocytic properties rely on cell-culture incubation and end-point detection schemes. Here, we present a lab-on-a-chip system for real-time monitoring of magnetic particle uptake by human fibroblast (NHDF) cells. It is based on recording the time evolution of the average position and distribution of magnetic particles during phagocytosis by giant-magnetoresistive (GMR) type sensors. We employ particles with a mean diameter of 1.2 μm and characterize their phagocytosis-relevant properties. Our experiments at physiological conditions reveal a cellular uptake rate of 45 particles per hour and show that phagocytosis reaches saturation after an average uptake time of 27.7h. Moreover, reference phagocytosis experiments at 4°C are carried out to mimic environmental or disease related inhibition of the phagocytic behavior, and our measurements clearly show that we are able to distinguish between cell-membrane adherent and phagocytosed magnetic particles. Besides the demonstrated real-time monitoring of phagocytosis mechanisms, additional nano-biointerface studies can be realized, including on-chip cell adhesion/spreading as well as cell migration, attachment and detachment dynamics. This versatility shows the potential of our approach for providing a multifunctional platform for on-chip cell analysis.

Quantitative proteomics reveals that only a subset of the endoplasmic reticulum contributes to the phagosome

Phagosomes, by killing and degrading pathogens for antigen presentation, are organelles implicated in key aspects of innate and adaptive immunity. Although it has been well established that phagosomes consist of membranes from the plasma membrane, endosomes, and lysosomes, the notion that the endoplasmic reticulum (ER) membrane could play an important role in the formation of the phagosome is debated. However, a method to accurately estimate the contribution of potential source organelles and contaminants to the phagosome proteome has been lacking. Herein, we have developed a proteomic approach for objectively quantifying the contribution of various organelles to the early and late phagosomes by comparing these fractions to their total membrane and postnuclear supernatant of origin in the J774A.1 murine macrophage cell line. Using quantitative label-free mass spectrometry, the abundance of peptides corresponding to hundreds of proteins was estimated and attributed to one of five organelles (e.g. plasma membrane, endosomes/lysosomes, ER, Golgi, and mitochondria). These data in combination with a stable isotope labeling in cell culture method designed to detect potential contaminant sources revealed that the ER is part of the phagosomal membrane and contributes ≈ 20% of the early phagosome proteome. In addition, only a subset of ER proteins is recruited to the phagosome, suggesting that a specific subdomain(s) of the ER might be involved in phagocytosis. Western blotting and immunofluorescence substantially validated this conclusion; we were able to demonstrate that the fraction of the ER in which the ER marker GFP-KDEL accumulates is excluded from the phagosomes, whereas that containing the mVenus-Syntaxin 18 is recruited. These results highlight promising new avenues for the description of the pathogenic mechanisms used by Leishmania, Brucella, and Legionella spp., which thrive in ER-rich phagosomes.

Linking exocytosis and endocytosis during phagocytosis

Phagocytosis is used by macrophages, dendritic cells and neutrophils to capture and destroy pathogens and particulate antigens. Although localized assembly of actin filaments is the driving force for particle internalization, exocytosis of intracellular compartments, and in particular endocytic compartments, has been shown recently to be required for the early steps of phagosome formation. Here we report on the different compartments undergoing exocytosis during phagocytosis, with a special focus on late endosomes. We then compare this process with secretion from lysosomes or lysosome-related organelles in specialized cells. Finally, we discuss how some of the molecular mechanisms responsible for lysosome-related organelle secretion could also be implicated in phagosome formation.

Defective macrophage function in aquaporin-3 deficiency

Macrophages play an essential role in innate immunity. We found that mouse resident peritoneal macrophages (mRPMs) express the aquaglyceroporin aquaporin-3 (AQP3) in a plasma membrane pattern. AQP3-deficient (AQP3(-/-)) mice showed significantly greater mortality than wild-type (AQP3(+/+)) mice in a model of bacterial peritonitis. To establish the cellular mechanism of the peritonitis phenotype, measurements were made of mRPM phagocytosis, migration, and water/glycerol permeability. We found significantly impaired engulfment of Escherichia coli and chicken erythrocytes in AQP3(-/-) vs. AQP3(+/+) mRPMs, as well as impaired migration of AQP3(-/-) mRPMs in response to a chemotactic stimulus. In AQP3(+/+) mRPMs, AQP3 was polarized to pseudopodia at the leading edge during migration and around the phagocytic cup during engulfment. Water and glycerol permeabilities in mRPMs from AQP3(-/-) mice were reduced compared to mRPMs from AQP3(+/+) mice. Cellular glycerol and ATP content were remarkably lower in AQP3(-/-) vs. AQP3(+/+) mRPMs, and glycerol supplementation partially rescued the reduced ATP content and impaired function of AQP3(-/-) mRPMs. These data implicate AQP3 as a novel determinant in macrophage immune function by a cellular mechanism involving facilitated water and glycerol transport, and consequent phagocytic and migration activity. This is the first study demonstrating involvement of an aquaporin in innate immunity. Our results suggest AQP3 as a novel therapeutic target in modulating the immune response in various infectious and inflammatory conditions.

The zipper mechanism in phagocytosis: energetic requirements and variability in phagocytic cup shape

BACKGROUND

Phagocytosis is the fundamental cellular process by which eukaryotic cells bind and engulf particles by their cell membrane. Particle engulfment involves particle recognition by cell-surface receptors, signaling and remodeling of the actin cytoskeleton to guide the membrane around the particle in a zipper-like fashion. Despite the signaling complexity, phagocytosis also depends strongly on biophysical parameters, such as particle shape, and the need for actin-driven force generation remains poorly understood.

RESULTS

Here, we propose a novel, three-dimensional and stochastic biophysical model of phagocytosis, and study the engulfment of particles of various sizes and shapes, including spiral and rod-shaped particles reminiscent of bacteria. Highly curved shapes are not taken up, in line with recent experimental results. Furthermore, we surprisingly find that even without actin-driven force generation, engulfment proceeds in a large regime of parameter values, albeit more slowly and with highly variable phagocytic cups. We experimentally confirm these predictions using fibroblasts, transfected with immunoreceptor FcγRIIa for engulfment of immunoglobulin G-opsonized particles. Specifically, we compare the wild-type receptor with a mutant receptor, unable to signal to the actin cytoskeleton. Based on the reconstruction of phagocytic cups from imaging data, we indeed show that cells are able to engulf small particles even without support from biological actin-driven processes.

CONCLUSIONS

This suggests that biochemical pathways render the evolutionary ancient process of phagocytic highly robust, allowing cells to engulf even very large particles. The particle-shape dependence of phagocytosis makes a systematic investigation of host-pathogen interactions and an efficient design of a vehicle for drug delivery possible.

Ral isoforms are implicated in Fc gamma R-mediated phagocytosis: activation of phospholipase D by RalA

Phagocytosis is an essential element of the immune response permitting the elimination of pathogens, cellular debris, apoptotic cells, and tumor cells. Recently, both phospholipase D (PLD) isoforms, PLD1 and PLD2, were shown to be necessary for efficient FcgammaR-mediated phagocytosis. In this study, we investigated the role of a potential PLD regulator, the Ral GTPases RalA and RalB, in murine RAW 264.7 macrophages. Both Ral isoforms are expressed in macrophages and are transiently activated following FcgammaR stimulation. When Ral expression levels were varied using Ral mutants or interference RNA, phagocytosis assays revealed that Ral isoforms have antagonistic effects; RalA is a positive modulator, whereas RalB plays a negative role. We then focused on RalA and its possible relationship with PLD. The increase in PLD activity that occurs when phagocytosis is stimulated was inhibited in cells with reduced RalA protein, but it was unaffected by reduced levels of RalB. Furthermore, in macrophages transfected with dsRed-RalA and GFP-PLD1 or GFP-PLD2, RalA colocalized with PLD1 and PLD2 at the phagocytic cup during phagosome formation. Additional results obtained from immunoprecipitation of PLD from macrophages transfected with myc-RalA and hemagglutinin-tagged PLD1 or PLD2 indicated an enhanced interaction of RalA with both PLD isoforms during phagocytic stimulation. The increase in RalA and PLD1 interaction was transient and correlated with the time course of RalA activation. These findings reveal a novel pathway involving RalA and PLD in the regulation of FcgammaR-mediated phagocytosis.

Antimicrobial mechanisms of phagocytes and bacterial evasion strategies

Professional phagocytes have a vast and sophisticated arsenal of microbicidal features. They are capable of ingesting and destroying invading organisms, and can present microbial antigens on their surface, eliciting acquired immune responses. To survive this hostile response, certain bacterial species have developed evasive strategies that often involve the secretion of effectors to co-opt the cellular machinery of the host. In this Review, we present an overview of the antimicrobial defences of the host cell, with emphasis on macrophages, for which phagocytosis has been studied most extensively. In addition, using Mycobacterium tuberculosis, Listeria monocytogenes, Legionella pneumophila and Coxiella burnetii as examples, we describe some of the evasive strategies used by bacteria.

Particles induce apical plasma membrane enlargement in epithelial lung cell line depending on particle surface area dose

Background

Airborne particles entering the respiratory tract may interact with the apical plasma membrane (APM) of epithelial cells and enter them. Differences in the entering mechanisms of fine (between 0.1 μm and 2.5 μm) and ultrafine ( ≤ 0.1 μm) particles may be associated with different effects on the APM. Therefore, we studied particle-induced changes in APM surface area in relation to applied and intracellular particle size, surface and number.

Methods

Human pulmonary epithelial cells (A549 cell line) were incubated with various concentrations of different sized fluorescent polystyrene spheres without surface charge (∅ fine – 1.062 μm, ultrafine – 0.041 μm) by submersed exposure for 24 h. APM surface area of A549 cells was estimated by design-based stereology and transmission electron microscopy. Intracellular particles were visualized and quantified by confocal laser scanning microscopy.

Results

Particle exposure induced an increase in APM surface area compared to negative control (p < 0.01) at the same surface area concentration of fine and ultrafine particles a finding not observed at low particle concentrations. Ultrafine particle entering was less pronounced than fine particle entering into epithelial cells, however, at the same particle surface area dose, the number of intracellular ultrafine particles was higher than that of fine particles. The number of intracellular particles showed a stronger increase for fine than for ultrafine particles at rising particle concentrations.

Conclusion

This study demonstrates a particle-induced enlargement of the APM surface area of a pulmonary epithelial cell line, depending on particle surface area dose. Particle uptake by epithelial cells does not seem to be responsible for this effect. We propose that direct interactions between particle surface area and cell membrane cause the enlargement of the APM.

Facilitated monocyte-macrophage uptake and tissue distribution of superparmagnetic iron-oxide nanoparticles

Background

We posit that the same mononuclear phagocytes (MP) that serve as target cells and vehicles for a host of microbial infections can be used to improve diagnostics and drug delivery. We also theorize that physical and biological processes such as particle shape, size, coating and opsonization that affect MP clearance of debris and microbes can be harnessed to facilitate uptake of nanoparticles (NP) and tissue delivery.

Methods

Monocytes and monocyte-derived macrophages (MDM) were used as vehicles of superparamagnetic iron oxide (SPIO) NP and immunoglobulin (IgG) or albumin coated SPIO for studies of uptake and distribution. IgG coated SPIO was synthesized by covalent linkage and uptake into monocytes and MDM investigated related to size, time, temperature, concentration, and coatings. SPIO and IgG SPIO were infused intravenously into naïve mice. T₂ measures using magnetic resonance imaging (MRI) were used to monitor tissue distribution in animals.

Results

Oxidation of dextran on the SPIO surface generated reactive aldehyde groups and permitted covalent linkage to amino groups of murine and human IgG and F(ab')₂ fragments and for Alexa Fluor® 488 hydroxylamine to form a Schiff base. This labile intermediate was immediately reduced with sodium cyanoborohydride in order to stabilize the NP conjugate. Optical density measurements of the oxidized IgG, F(ab')₂, and/or Alexa Fluor® 488 SPIO demonstrated ∼50% coupling yield. IgG-SPIO was found stable at 4°C for a period of 1 month during which size and polydispersity index varied little from 175 nm and 200 nm, respectively. In vitro, NP accumulated readily within monocyte and MDM cytoplasm after IgG-SPIO exposure; whereas, the uptake of native SPIO in monocytes and MDM was 10-fold less. No changes in cell viability were noted for the SPIO-containing monocytes and MDM. Cell morphology was not changed as observed by transmission electron microscopy. Compared to unconjugated SPIO, intravenous injection of IgG-SPIO afforded enhanced and sustained lymphoid tissue distribution over 24 hours as demonstrated by MRI.

Conclusions

Facilitated uptake of coated SPIO in monocytes and MDM was achieved. Uptake was linked to particle size and was time and concentration dependent. The ability of SPIO to be rapidly taken up and distributed into lymphoid tissues also demonstrates feasibility of macrophage-targeted nanoformulations for diagnostic and drug therapy.

Differential role of the Ca(2+) sensor synaptotagmin VII in macrophages and dendritic cells

Synaptotagmin VII (Syt VII) is a Ca(2+) sensing molecule that regulates lysosomal exocytosis in several cell types. In macrophages (MØ), Syt VII is required for efficient uptake of large particle loads, by promoting the delivery of lysosomal membrane to phagocytic cups. Here we compare the phagocytic capacity of bone marrow-derived MØs and dendritic cells (DC), and show that the requirement for Syt VII correlates with the unique ability of MØs for continuous phagocytosis. In contrast to MØs, Syt VII(+/+) and Syt VII(-/-) immature DCs show similar levels of initial phagocytosis, followed by a marked decrease in particle uptake. [Ca(2+)](i) chelation and PI-3 kinase inhibition reduce particle uptake by MØs, but are markedly less inhibitory in DCs. Thus, immature DCs appear to lack the Syt VII, Ca(2+) and PI-3 kinase-dependent forms of phagocytosis that are present in MØs. Interestingly, expression of Syt VII is up-regulated during LPS-induced DC maturation, a stimulus that also induces Syt VII translocation from intracellular compartments to the plasma membrane. Syt VII(-/-) DCs show a delayed translocation of MHC class II to the cell surface during maturation, consistent with the possibility that Syt VII facilitates exocytosis and/or surface retention of molecules critical for antigen presentation.

Cell (A549)-particle (Jasada Bhasma) interactions using Raman spectroscopy

Current methods for the evaluation of cell interactions with particles are nonspecific, slow, and invasive to the cells. Raman spectroscopy is a noninvasive technique, and is used in the present study to investigate particle-cell interactions. The main focus of the present study is to employ Raman spectroscopy for investigating the interaction of human lung adenocarcinoma cell line (A549) with the particulate system Jasada Bhasma, a traditional Indian medicine. Jasada Bhasma is a unique preparation of zinc and is traditionally used for the treatment of various diseases like diabetes, age-related eye diseases, and as a health promotional tonic. The Raman spectral analysis is executed by identifying the difference in intracellular DNA/RNA, and proteins and lipids concentration between particles--treated and untreated cells. Comparison between Bhasma-treated and -untreated cells indicates that vibrational peaks corresponding to the DNA/RNA molecule show a significant increase in cells treated with the Jasada Bhasma. Apart from the DNA molecule, several other vibrational peaks related to the protein molecules also show a significant increase in A549 cells after treatment with Bhasma. These results indicate that Bhasma treatment of A549 possibly delays DNA degradation and enables retention of higher amount of protein molecules in the cells.

Contributions of proteomics to understanding phagosome maturation

In metazoans macrophage cells use phagocytosis, the process of engulfing large particles, to control the spread of pathogens in the body, to clear dead or dying cells, and to aid in tissue remodelling, while the same process is also used by unicellular eukaryotes to ingest food. Phagocytosing cells essentially swallow the particles, trapping them in vacuoles called phagosomes that go through a series of maturation steps, culminating in the destruction of the internalized cargo. Because of their central role in innate immunity and their relatively simple structure (one membrane bilayer surrounding a single particle), phagosomes have been a popular subject for organelle proteomics studies. Qualitative proteomic technologies are now very sensitive so hundreds of different proteins have been identified in phagosomes from several species, revealing new properties of these intriguing compartments. More recently, quantitative proteomic approaches have also been applied, shedding new light on the dynamics and composition of maturing phagosomes. In this review we summarize the studies that have applied proteomic technologies to phagosomes and how they have changed our understanding of phagosome biology.

Fusion, fission, and secretion during phagocytosis

Phagocytosis is essential for the elimination of pathogens and for clearance of apoptotic bodies. The ingestion process entails extensive remodeling of the cellular membranes, particularly when large and/or multiple particles are engulfed. The membrane fusion and fission events that accompany phagocytosis are described. The coordinated sequence of membrane trafficking events required for phagocytosis involves multiple organelles and also serves other cellular functions, such as cytokine secretion.

Possible roles of the endocytic cycle in cell motility

Starving, highly motile Dictyostelium cells maintain an active endocytic cycle, taking up their surface about every 11 minutes. Cell motility depends on a functional NSF (N-ethylmaleimide sensitive factor) protein--also essential for endocytosis and membrane trafficking generally--and we, therefore, investigated possible ways in which the endocytic cycle might be required for cell movement. First, NSF, and presumably membrane trafficking, are not required for the initial polarization of the leading edge in a cyclic-AMP gradient. Second, we can detect no evidence for membrane flow from the leading edge, as photobleached or photoactivated marks in the plasma membrane move forward roughly in step with the leading edge, rather than backwards from it. Third, we find that the surface area of a cell--measured from confocal reconstructions--constantly fluctuates during movement as it projects pseudopodia and otherwise changes shape; increases of 20-30% can often occur over a few minutes. These fluctuations cannot be explained by reciprocal changes in filopodial surface area and they substantially exceed the 2-3% by which membranes can stretch. We propose that the endocytic cycle has a key function in motility by allowing adjustment of cell surface area to match changes in shape and that, without this function, movement is severely impaired.

The dynamic phagosomal proteome and the contribution of the endoplasmic reticulum

Macrophages use phagocytosis to control the spread of pathogens in the body, to clear apoptotic cells, and to aid in tissue remodeling. The phagosomal membrane is traditionally thought to originate from the plasmalemma and then go through a series of maturation steps involving sequential fusion with endosomal compartments, leading to the formation of a phagolysosome. A recent model suggests that the endoplasmic reticulum (ER) is involved in the maturation as well. Here we use stable isotope labeling and multiple quantitative proteomic approaches to follow the dynamic composition of the maturing phagosome in RAW 264.7 macrophage cells to a greater depth and higher temporal resolution than was previously possible. Analysis of the results suggests that the traditional model of a linear sequence of fusion events with different compartments is more complex or variable than previously thought. By concomitantly measuring the degree to which each component is enriched on phagosomes, our data argue that the amount of ER involved in phagocytosis is much less than predicted by the model of ER-mediated phagocytosis.

Regulation of vacuolar pH and its modulation by some microbial species

To survive within the host, pathogens such as Mycobacterium tuberculosis and Helicobacter pylori need to evade the immune response and find a protected niche where they are not exposed to microbicidal effectors. The pH of the microenvironment surrounding the pathogen plays a critical role in dictating the organism's fate. Specifically, the acidic pH of the endocytic organelles and phagosomes not only can affect bacterial growth directly but also promotes a variety of host microbicidal responses. The development of mechanisms to avoid or resist the acidic environment generated by host cells is therefore crucial to the survival of many pathogens. Here we review the processes that underlie the generation of organellar acidification and discuss strategies employed by pathogens to circumvent it, using M. tuberculosis and H. pylori as examples.

Quantitative analysis of membrane remodeling at the phagocytic cup

Nascent phagosomes, which are derived from the plasma membrane, acquire microbicidal properties through multiple fusion and fission events collectively known as maturation. Here we show that remodeling of the phagosomal membrane is apparent even before sealing, particularly when large particles are ingested. Fluorescent probes targeted to the plasma membrane are cleared from the region lining the particle before engulfment is completed. Extensive clearance was noted for components of the inner as well as outer monolayer of the plasmalemma. Segregation of lipid microdomains was ruled out as the mechanism underlying membrane remodeling, because markers residing in rafts and those that are excluded were similarly depleted. Selective endocytosis was also ruled out. Instead, several lines of evidence indicate that endomembranes inserted by exocytosis at sites of ingestion displace the original membrane constituents from the base of the phagosomal cup. The Fcgamma receptors that trigger phagocytosis remain associated with their ligands. By contrast, Src-family kinases that are the immediate effectors of receptor activation are flushed away from the cup by the incoming membranes. Together with the depletion of phosphoinositides required for signal transduction, the disengagement of receptors from their effectors by bulk membrane remodeling provides a novel means to terminate receptor signaling.

Ca2+ and synaptotagmin VII-dependent delivery of lysosomal membrane to nascent phagosomes

Synaptotagmin (Syt) VII is a ubiquitously expressed member of the Syt family of Ca2+ sensors. It is present on lysosomes in several cell types, where it regulates Ca2+-dependent exocytosis. Because [Ca2+]i and exocytosis have been associated with phagocytosis, we investigated the phagocytic ability of macrophages from Syt VII-/- mice. Syt VII-/- macrophages phagocytose normally at low particle/cell ratios but show a progressive inhibition in particle uptake under high load conditions. Complementation with Syt VII rescues this phenotype, but only when functional Ca2+-binding sites are retained. Reinforcing a role for Syt VII in Ca2+-dependent phagocytosis, particle uptake in Syt VII-/- macrophages is significantly less dependent on [Ca2+]i. Syt VII is concentrated on peripheral domains of lysosomal compartments, from where it is recruited to nascent phagosomes. Syt VII recruitment is rapidly followed by the delivery of Lamp1 to phagosomes, a process that is inhibited in Syt VII-/- macrophages. Thus, Syt VII regulates the Ca2+-dependent mobilization of lysosomes as a supplemental source of membrane during phagocytosis.