Restricted accumulation of phosphatidylinositol 3-kinase products in a plasmalemmal subdomain during Fc gamma receptor-mediated phagocytosis

The efferocytosis dilemma: how neutrophil extracellular traps and PI3K/Rac1 complicate diabetic wound healing

AIMS/HYPOTHESIS

The resolution of apoptotic cells (ACs) is crucial for wound healing and tissue remodeling and is often impaired by persistent inflammation. This study aimed to elucidate the impact of neutrophil extracellular traps (NETs) on diabetic wound healing by targeting the phosphoinositide 3-kinase/Ras-related C3 botulinum toxin substrate 1 (PI3K/Rac1) signaling pathway, which is pivotal for macrophage efferocytosis.

METHODS

A streptozotocin-induced diabetic mouse model was used to assess the impact of NETs on efferocytosis in vivo. The effects of NETs on macrophage efferocytosis and wound healing were evaluated using specific inhibitors and agonists targeting the PI3K/Rac1 pathway. In vitro, macrophages from diabetic wounds or cell lines (Raw264.7) were treated with NETs and a panel of pharmacological agents of the PI3K/Rac1 pathway to evaluate macrophage efferocytosis.

RESULTS

NETs were found to inhibit macrophage efferocytosis, resulting in delayed clearance of ACs that accumulate within the wounds. Inhibition of NET formation in diabetic mice rescued impaired efferocytosis, accompanied by reactivation of PI3K and Rac1 in macrophages. Moreover, pharmacological agents targeting the PI3K/Rac1 pathway restored NETs-induced impairment in efferocytosis, leading to rapid wound healing. Raw264.7 cells exhibited elevated activation levels of PI3K and Rac1 when co-cultured with ACs in vitro. Nevertheless, this signaling activation was inhibited when cultured in a NETs-conditioned medium, leading to attenuated efferocytosis.

CONCLUSIONS/INTERPRETATION

Targeting NETs and the PI3K/Rac1 pathway emerges as a potential therapeutic strategy to enhance healing in diabetic wounds by promoting macrophage efferocytosis.

Podosome Nucleation Is Facilitated by Multivalent Interactions between Syk and ITAM-containing Membrane Complexes

Immune cells survey their microenvironment by forming dynamic cellular protrusions that enable chemotaxis, contacts with other cells, and phagocytosis. Podosomes are a unique type of protrusion structured by an adhesive ring of active integrins that surround an F-actin-rich core harboring degradative proteases. Although the features of podosomes, once-established, have been well defined, the steps that lead to podosome formation remain poorly understood by comparison. In this study, we report that spleen tyrosine kinase (Syk) is a critical regulator of podosome formation. Deletion of Syk or targeting its kinase activity eliminated the ability for murine macrophages to form podosomes. We found that the kinase activity of Syk was important for the phosphorylation of its substrates, HS1 and Pyk2, both of which regulate podosome formation. Additionally, before podosomes form, we report that the tandem Src homology 2 domains of Syk afforded multivalent clustering of ITAM-containing adaptors that associated with integrins to structure platforms that initiate podosomes. We therefore propose that Syk has a dual role in regulating podosomes: first, by facilitating the assembly of multivalent signaling hubs that nucleate their formation and second, by sustaining tyrosine kinase activity of the podosomes once they form against their substrates. In cells expressing recently identified gain-of-function variants of SYK, podosomes were dysregulated. These results implicate SYK in the (patho)physiological functions of podosomes in macrophages.

The LCLAT1/LYCAT acyltransferase is required for EGF-mediated phosphatidylinositol-3,4,5-trisphosphate generation and Akt signaling

Receptor tyrosine kinases such as EGF receptor (EGFR) stimulate phosphoinositide 3 kinases to convert phosphatidylinositol-4,5-bisphosophate [PtdIns(4,5)P2] into phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P3]. PtdIns(3,4,5)P3 then remodels actin and gene expression, and boosts cell survival and proliferation. PtdIns(3,4,5)P3 partly achieves these functions by triggering activation of the kinase Akt, which phosphorylates targets like Tsc2 and GSK3β. Consequently, unchecked upregulation of PtdIns(3,4,5)P3-Akt signaling promotes tumor progression. Interestingly, 50-70% of PtdIns and PtdInsPs have stearate and arachidonate at sn-1 and sn-2 positions of glycerol, respectively, forming a species known as 38:4-PtdIns/PtdInsPs. LCLAT1 and MBOAT7 acyltransferases partly enrich PtdIns in this acyl format. We previously showed that disruption of LCLAT1 lowered PtdIns(4,5)P2 levels and perturbed endocytosis and endocytic trafficking. However, the role of LCLAT1 in receptor tyrosine kinase and PtdIns(3,4,5)P3 signaling was not explored. Here, we show that LCLAT1 silencing in MDA-MB-231 and ARPE-19 cells abated the levels of PtdIns(3,4,5)P3 in response to EGF signaling. Importantly, LCLAT1-silenced cells were also impaired for EGF-driven and insulin-driven Akt activation and downstream signaling. Thus, our work provides first evidence that the LCLAT1 acyltransferase is required for receptor tyrosine kinase signaling.

Making cups and rings: the 'stalled-wave' model for macropinocytosis

Macropinocytosis is a broadly conserved endocytic process discovered nearly 100 years ago, yet still poorly understood. It is prominent in cancer cell feeding, immune surveillance, uptake of RNA vaccines and as an invasion route for pathogens. Macropinocytic cells extend large cups or flaps from their plasma membrane to engulf droplets of medium and trap them in micron-sized vesicles. Here they are digested and the products absorbed. A major problem - discussed here - is to understand how cups are shaped and closed. Recently, lattice light-sheet microscopy has given a detailed description of this process in Dictyostelium amoebae, leading to the 'stalled-wave' model for cup formation and closure. This is based on membrane domains of PIP3 and active Ras and Rac that occupy the inner face of macropinocytic cups and are readily visible with suitable reporters. These domains attract activators of dendritic actin polymerization to their periphery, creating a ring of protrusive F-actin around themselves, thus shaping the walls of the cup. As domains grow, they drive a wave of actin polymerization across the plasma membrane that expands the cup. When domains stall, continued actin polymerization under the membrane, combined with increasing membrane tension in the cup, drives closure at lip or base. Modelling supports the feasibility of this scheme. No specialist coat proteins or contractile activities are required to shape and close cups: rings of actin polymerization formed around PIP3 domains that expand and stall seem sufficient. This scheme may be widely applicable and begs many biochemical questions.

FcγRIIb Exacerbates LPS-Induced Neuroinflammation by Binding with the Bridging Protein DAP12 and Promoting the Activation of PI3K/AKT Signaling Pathway in Microglia

Introduction

This paper focuses on the expression and role of FcγRIIb in neuroinflammation, exploring the molecular mechanisms by which FcγRIIb interacts with the bridging protein DAP12 to regulate the PI3K-AKT signaling pathway that promote neuroinflammation and aggravate neuronal injury.

Methods

LPS-induced neuroinflammation models in vivo and in vitro were constructed to explore the role and mechanism of FcγRIIb in CNS inflammation. Subsequently, FcγRIIb was knocked down or overexpressed to observe the activation of BV2 cell and the effect on PI3K-AKT pathway. Then the PI3K-AKT pathway was blocked to observe its effect on cell activation and FcγRIIb expression. We analyzed the interaction between FcγRIIb and DAP12 by Immunoprecipitation technique. Then FcγRIIb was overexpressed while knocking down DAP12 to observe its effect on PI3K-AKT pathway. Finally, BV2 cell culture supernatant was co-cultured with neuronal cell HT22 to observe its effect on neuronal apoptosis and cell activity.

Results

In vivo and in vitro, we found that FcγRIIb expression was significantly increased and activated the PI3K-AKT pathway. Contrary to the results of overexpression of FcγRIIb, knockdown of FcγRIIb resulted in a significant low level of relevant inflammatory factors and suppressed the PI3K-AKT pathway. Furthermore, LPS stimulation induced an interaction between FcγRIIb and DAP12. Knockdown of DAP12 suppressed inflammation and activation of the PI3K-AKT pathway in BV2 cells, and meantime overexpression of FcγRIIb suppressed the level of FcγRIIb-induced AKT phosphorylation. Additionally, knockdown of FcγRIIb inhibited microglia activation, which induced neuronal apoptosis.

Discussion

Altogether, our experiments indicate that FcγRIIb interacts with DAP12 to promote microglia activation by activating the PI3K-AKT pathway while leading to neuronal apoptosis and exacerbating brain tissue injury, which may provide a new target for the treatment of inflammatory diseases in the central nervous system.

Phagocytosis

One hundred years have passed since the death of Élie Metchnikoff (1845-1916). He was the first to observe the uptake of particles by cells and realized the importance of this process, named phagocytosis, for the host response to injury and infection. He also was a strong advocate of the role of phagocytosis in cellular immunity, and with this, he gave us the basis for our modern understanding of inflammation and the innate immune response. Phagocytosis is an elegant but complex process for the ingestion and elimination of pathogens, but it is also important for the elimination of apoptotic cells and hence fundamental for tissue homeostasis. Phagocytosis can be divided into four main steps: (i) recognition of the target particle, (ii) signaling to activate the internalization machinery, (iii) phagosome formation, and (iv) phagolysosome maturation. In this chapter, we present a general view of our current knowledge on phagocytosis performed mainly by professional phagocytes through antibody and complement receptors and discuss aspects that remain incompletely understood.

Localisation of Intracellular Signals and Responses during Phagocytosis

Phagocytosis is one of the most polarised of all cellular activities. Both the stimulus (the target for phagocytosis) and the response (its internalisation) are focussed at just one part of the cell. At the locus, and this locus alone, pseudopodia form a phagocytic cup around the particle, the cytoskeleton is rearranged, the plasma membrane is reorganised, and a new internal organelle, the phagosome, is formed. The effect of signals from the stimulus must, thus, both be complex and yet be restricted in space and time to enable an effective focussed response. While many aspects of phagocytosis are being uncovered, the mechanism for the restriction of signalling or the effects of signalling remains obscure. In this review, the details of the problem of restricting chemical intracellular signalling are presented, with a focus on diffusion into the cytosol and of signalling lipids along the plasma membrane. The possible ways in which simple diffusion is overcome so that the restriction of signalling and effective phagocytosis can be achieved are discussed in the light of recent advances in imaging, biophysics, and cell biochemistry which together are providing new insights into this area.

Quantifying Phagocytosis by Immunofluorescence and Microscopy

Phagocytosis is carried out by cells such as macrophages of the immune system, whereby particulates like bacteria and apoptotic bodies are engulfed and sequestered within phagosomes for subsequent degradation. Hence, phagocytosis is important for infection resolution and tissue homeostasis. Aided by the innate and adaptive immune system, the activation of various phagocytic receptors triggers a cascade of downstream signaling mediators that drive actin and plasma membrane remodeling to entrap the bound particulate within the phagosome. Modulation of these molecular players can lead to distinct changes in the capacity and rates of phagocytosis. Here, we present a fluorescence microscopy-based technique to quantify phagocytosis using a macrophage-like cell line. We exemplify the technique through the phagocytosis of antibody-opsonized polystyrene beads and Escherichia coli. This method can be extended to other phagocytes and phagocytic particles.

Functional Role of AGAP2/PIKE-A in Fcγ Receptor-Mediated Phagocytosis

In phagocytes, cytoskeletal and membrane remodeling is finely regulated at the phagocytic cup. Various smaFll G proteins, including those of the Arf family, control these dynamic processes. Human neutrophils express AGAP2, an Arf GTPase activating protein (ArfGAP) that regulates endosomal trafficking and focal adhesion remodeling. We first examined the impact of AGAP2 on phagocytosis in CHO cells stably expressing the FcγRIIA receptor (CHO-IIA). In unstimulated CHO-IIA cells, AGAP2 only partially co-localized with cytoskeletal elements and intracellular compartments. In CHO-IIA cells, AGAP2 transiently accumulated at actin-rich phagocytic cups and increased Fcγ receptor-mediated phagocytosis. Enhanced phagocytosis was not dependent on the N-terminal GTP-binding protein-like (GLD) domain of AGAP2. AGAP2 deleted of its GTPase-activating protein (GAP) domain was not recruited to phagocytic cups and did not enhance the engulfment of IgG-opsonized beads. However, the GAP-deficient [R618K]AGAP2 transiently localized at the phagocytic cups and enhanced phagocytosis. In PLB-985 cells differentiated towards a neutrophil-like phenotype, silencing of AGAP2 reduced phagocytosis of opsonized zymosan. In human neutrophils, opsonized zymosan or monosodium urate crystals induced AGAP2 phosphorylation. The data indicate that particulate agonists induce AGAP2 phosphorylation in neutrophils. This study highlights the role of AGAP2 and its GAP domain but not GAP activity in FcγR-dependent uptake of opsonized particles.

Phosphoinositide species and filamentous actin formation mediate engulfment by senescent tumor cells

Cancer cells survive chemotherapy and cause lethal relapse by entering a senescent state that facilitates expression of many phagocytosis/macrophage-related genes that engender a novel cannibalism phenotype. We used biosensors and live-cell imaging to reveal the basic steps and mechanisms of engulfment by senescent human and mouse tumor cells. We show filamentous actin in predator cells was localized to the prey cell throughout the process of engulfment. Biosensors to various phosphoinositide (PI) species revealed increased concentration and distinct localization of predator PI(4) P and PI(4,5)P2 at the prey cell during early stages of engulfment, followed by a transient burst of PI(3) P before and following internalization. PIK3C2B, the kinase responsible for generating PI(3)P, was required for complete engulfment. Inhibition or knockdown of Clathrin, known to associate with PIK3C2B and PI(4,5)P2, severely impaired engulfment. In sum, our data reveal the most fundamental cellular processes of senescent cell engulfment, including the precise localizations and dynamics of actin and PI species throughout the entire process.

LEDGF is a new growth factor in fetal serum

Fetal serum supports the immortal growth of mammalian cell lines in culture while adult serum leads to the terminal differentiation and death of cells in culture. Many of the proteins in fetal serum that support the indefinite division and growth of cancerous cell lines remain obscure. The peptides and proteins of fetal versus adult serum were analyzed by liquid chromatography, nano electrospray ionization and tandem mass spectrometry (LC-ESI-MS/MS). Three batches of fetal serum contained the Alpha Fetoprotein marker while adult serum batches did not. Insulin (INS), and insulin-like growth factor (ILGF), fibroblast growth factor (FGF), epidermal growth factor (EGF) and platelet derived growth factor (PDGF) were increased in fetal serum. New fetal growth factors including MEGF, HDGFRP and PSIP1 and soluble growth receptors such as TNFR, EGFR, NTRK2 and THRA were discovered. Addition of insulin or the homeotic transcription factor PSIP1, also referred to as Lens Epithelium Derived Growth Factor (LEDGF), partially restored the rounded phenotype of rapidly dividing cells but was not as effective as fetal serum. Thus, a new growth factor in fetal serum, LEDGF/PSIP1, was directly observed by tandem mass spectrometry and confirmed by add back experiments to cell culture media alongside insulin.

PI3K Isoforms in Cell Signalling and Innate Immune Cell Responses

Phosphoinositide-3-kinases (PI3Ks) are enzymes involved in signalling and modification of the function of all mammalian cells. These enzymes phosphorylate the 3-hydroxyl group of the inositol ring of phosphatidylinositol, resulting in lipid products that act as second messengers responsible for coordinating many cellular functions, including activation, chemotaxis, proliferation and survival. The identification of the functions that are mediated by a specific PI3K isoform is complex and depends on the specific cell type and inflammatory context. In this chapter we will focus on the role of PI3K isoforms in the context of innate immunity, focusing on the mechanisms by which PI3K signalling regulates phagocytosis, the activation of immunoglobulin, chemokine and cytokines receptors, production of ROS and cell migration, and how PI3K signalling plays a central role in host defence against infections and tissue injury.

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.

Phagosome maturation in macrophages: Eat, digest, adapt, and repeat

Phagocytosis is a dynamic process that requires an intricate interplay between phagocytic receptors, membrane lipids, and numerous signalling proteins and their effectors, to coordinate the engulfment of a bound particle. These particles are diverse in their physico-chemical properties such as size and shape and include bacteria, fungi, apoptotic cells, living tumour cells, and abiotic particles. Once engulfed, these particles are enclosed within a phagosome, which undergoes a striking transformation referred to as phagosome maturation, which will ultimately lead to the processing and degradation of the enclosed particulate. In this review, we focus on recent advancements in phagosome maturation in macrophages, highlighting new discoveries and emerging themes. Such advancements include identification of new GTPases and their effectors and the intricate spatio-temporal dynamics of phosphoinositides in governing phagosome maturation. We then explore phagosome fission and recycling, the emerging role of membrane contact sites, and delve into mechanisms of phagosome resolution to recycle and reform lysosomes. We further illustrate how phagosome maturation is context-dependent, subject to the type of particle, phagocytic receptors, the phagocytes and their state of activation during phagocytosis. Lastly, we discuss how phagosomes serve as signalling platforms to help phagocytes adapt to their environmental conditions. Overall, this review aims to cover recent findings, identify emerging themes, and highlight current challenges and directions to improve our understanding of phagosome maturation in macrophages.

Molecular Mechanisms of Borrelia burgdorferi Phagocytosis and Intracellular Processing by Human Macrophages

Lyme disease is the most common vector-borne illness in North America and Europe. Its causative agents are spirochetes of the Borrelia burgdorferi sensu latu complex. Infection with borreliae can manifest in different tissues, most commonly in the skin and joints, but in severe cases also in the nervous systems and the heart. The immune response of the host is a crucial factor for preventing the development or progression of Lyme disease. Macrophages are part of the innate immune system and thus one of the first cells to encounter infecting borreliae. As professional phagocytes, they are capable of recognition, uptake, intracellular processing and final elimination of borreliae. This sequence of events involves the initial capture and internalization by actin-rich cellular protrusions, filopodia and coiling pseudopods. Uptake into phagosomes is followed by compaction of the elongated spirochetes and degradation in mature phagolysosomes. In this review, we discuss the current knowledge about the processes and molecular mechanisms involved in recognition, capturing, uptake and intracellular processing of Borrelia by human macrophages. Moreover, we highlight interactions between macrophages and other cells of the immune system during these processes and point out open questions in the intracellular processing of borreliae, which include potential escape strategies of Borrelia.

Phosphoinositides: Roles in the Development of Microglial-Mediated Neuroinflammation and Neurodegeneration

Microglia are increasingly recognized as vital players in the pathology of a variety of neurodegenerative conditions including Alzheimer’s (AD) and Parkinson’s (PD) disease. While microglia have a protective role in the brain, their dysfunction can lead to neuroinflammation and contributes to disease progression. Also, a growing body of literature highlights the seven phosphoinositides, or PIPs, as key players in the regulation of microglial-mediated neuroinflammation. These small signaling lipids are phosphorylated derivates of phosphatidylinositol, are enriched in the brain, and have well-established roles in both homeostasis and disease.Disrupted PIP levels and signaling has been detected in a variety of dementias. Moreover, many known AD disease modifiers identified via genetic studies are expressed in microglia and are involved in phospholipid metabolism. One of these, the enzyme PLCγ2 that hydrolyzes the PIP species PI(4,5)P₂, displays altered expression in AD and PD and is currently being investigated as a potential therapeutic target.Perhaps unsurprisingly, neurodegenerative conditions exhibiting PIP dyshomeostasis also tend to show alterations in aspects of microglial function regulated by these lipids. In particular, phosphoinositides regulate the activities of proteins and enzymes required for endocytosis, toll-like receptor signaling, purinergic signaling, chemotaxis, and migration, all of which are affected in a variety of neurodegenerative conditions. These functions are crucial to allow microglia to adequately survey the brain and respond appropriately to invading pathogens and other abnormalities, including misfolded proteins. AD and PD therapies are being developed to target many of the above pathways, and although not yet investigated, simultaneous PIP manipulation might enhance the beneficial effects observed. Currently, only limited therapeutics are available for dementia, and although these show some benefits for symptom severity and progression, they are far from curative. Given the importance of microglia and PIPs in dementia development, this review summarizes current research and asks whether we can exploit this information to design more targeted, or perhaps combined, dementia therapeutics. More work is needed to fully characterize the pathways discussed in this review, but given the strength of the current literature, insights in this area could be invaluable for the future of neurodegenerative disease research.

Better Together: Current Insights Into Phagosome-Lysosome Fusion

Following phagocytosis, the nascent phagosome undergoes maturation to become a phagolysosome with an acidic, hydrolytic, and often oxidative lumen that can efficiently kill and digest engulfed microbes, cells, and debris. The fusion of phagosomes with lysosomes is a principal driver of phagosomal maturation and is targeted by several adapted intracellular pathogens. Impairment of this process has significant consequences for microbial infection, tissue inflammation, the onset of adaptive immunity, and disease. Given the importance of phagosome-lysosome fusion to phagocyte function and the many virulence factors that target it, it is unsurprising that multiple molecular pathways have evolved to mediate this essential process. While the full range of these pathways has yet to be fully characterized, several pathways involving proteins such as members of the Rab GTPases, tethering factors and SNAREs have been identified. Here, we summarize the current state of knowledge to clarify the ambiguities in the field and construct a more comprehensive phagolysosome formation model. Lastly, we discuss how other cellular pathways help support phagolysosome biogenesis and, consequently, phagocyte function.

Monitoring Phosphoinositide Fluxes and Effectors During Leukocyte Chemotaxis and Phagocytosis

The dynamic re-organization of cellular membranes in response to extracellular stimuli is fundamental to the cell physiology of myeloid and lymphoid cells of the immune system. In addition to maintaining cellular homeostatic functions, remodeling of the plasmalemma and endomembranes endow leukocytes with the potential to relay extracellular signals across their biological membranes to promote rolling adhesion and diapedesis, migration into the tissue parenchyma, and to ingest foreign particles and effete cells. Phosphoinositides, signaling lipids that control the interface of biological membranes with the external environment, are pivotal to this wealth of functions. Here, we highlight the complex metabolic transitions that occur to phosphoinositides during several stages of the leukocyte lifecycle, namely diapedesis, migration, and phagocytosis. We describe classical and recently developed tools that have aided our understanding of these complex lipids. Finally, major downstream effectors of inositides are highlighted including the cytoskeleton, emphasizing the importance of these rare lipids in immunity and disease.

Sphingomyelin Biosynthesis Is Essential for Phagocytic Signaling during Mycobacterium tuberculosis Host Cell Entry

Mycobacterium tuberculosis (Mtb) invades alveolar macrophages through phagocytosis to establish infection and cause disease. The molecular mechanisms underlying Mtb entry are still poorly understood.

Phagocytosis by alveolar macrophages is the obligate first step in Mycobacterium tuberculosis (Mtb) infection, yet the mechanism underlying this process is incompletely understood. Here, we show that Mtb invasion relies on an intact sphingolipid biosynthetic pathway. Inhibition or knockout of early sphingolipid biosynthetic enzymes greatly reduces Mtb uptake across multiple phagocytic cell types without affecting other forms of endocytosis. While the phagocytic receptor dectin-1 undergoes normal clustering at the pathogen contact sites, sphingolipid biosynthetic mutant cells fail to segregate the regulatory phosphatase CD45 from the clustered receptors. Blocking sphingolipid production also impairs downstream activation of Rho GTPases, actin dynamics, and phosphoinositide turnover at the nascent phagocytic cup. Moreover, we found that production of sphingomyelin, not glycosphingolipids, is essential for Mtb uptake. Collectively, our data support a critical role of sphingomyelin biosynthesis in an early stage of Mtb infection and provide novel insights into the mechanism underlying phagocytic entry of this pathogen.

Pseudomonas aeruginosa Planktonic- and Biofilm-Conditioned Media Elicit Discrete Metabolic Responses in Human Macrophages

Macrophages (MΦs) are prevalent innate immune cells, present throughout human bodily tissues where they orchestrate innate and adaptive immune responses to maintain cellular homeostasis. MΦs have the capacity to display a wide array of functional phenotypes due to different microenvironmental cues, particularly soluble bacterial secretory products. Recent evidence has emerged demonstrating that metabolism supports MΦ function and plasticity, in addition to energy and biomolecular precursor production. In this study, 1D ¹H-NMR-based metabolomics was used to identify the metabolic pathways that are differentially altered following primary human monocyte-derived MΦ exposure to P. aeruginosa planktonic- and biofilm-conditioned media (PCM and BCM). Metabolic profiling of PCM- and BCM-exposed MΦs indicated a significant increase in glycolytic metabolism, purine biosynthesis, and inositol phosphate metabolism. In addition, these metabolic patterns suggested that BCM-exposed MΦs exhibit a hyperinflammatory metabolic profile with reduced glycerol metabolism and elevated catabolism of lactate and amino acids, relative to PCM-exposed MΦs. Altogether, our study reveals novel findings concerning the metabolic modulation of human MΦs after exposure to secretory microbial products and contributes additional knowledge to the field of immunometabolism in MΦs.

Coordinated Ras and Rac Activity Shapes Macropinocytic Cups and Enables Phagocytosis of Geometrically Diverse Bacteria

Engulfment of extracellular material by phagocytosis or macropinocytosis depends on the ability of cells to generate specialized cup-shaped protrusions. To effectively capture and internalize their targets, these cups are organized into a ring or ruffle of actin-driven protrusion encircling a non-protrusive interior domain. These functional domains depend on the combined activities of multiple Ras and Rho family small GTPases, but how their activities are integrated and differentially regulated over space and time is unknown. Here, we show that the amoeba Dictyostelium discoideum coordinates Ras and Rac activity using the multidomain protein RGBARG (RCC1, RhoGEF, BAR, and RasGAP-containing protein). We find RGBARG uses a tripartite mechanism of Ras, Rac, and phospholipid interactions to localize at the protruding edge and interface with the interior of both macropinocytic and phagocytic cups. There, we propose RGBARG shapes the protrusion by expanding Rac activation at the rim while suppressing expansion of the active Ras interior domain. Consequently, cells lacking RGBARG form enlarged, flat interior domains unable to generate large macropinosomes. During phagocytosis, we find that disruption of RGBARG causes a geometry-specific defect in engulfing rod-shaped bacteria and ellipsoidal beads. This demonstrates the importance of coordinating small GTPase activities during engulfment of more complex shapes and thus the full physiological range of microbes, and how this is achieved in a model professional phagocyte.

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We identify a new regulator that shapes macropinocytic and phagocytic cups

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Shaping protrusions into cups requires differential regulation of Ras and Rac

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Cups are organized by integrating interactions with phospholipids and multiple GTPases

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Defective cup formation causes a target shape-specific defect in phagocytosis

Phagocytosis: Our Current Understanding of a Universal Biological Process

Phagocytosis is a cellular process for ingesting and eliminating particles larger than 0.5 μm in diameter, including microorganisms, foreign substances, and apoptotic cells. Phagocytosis is found in many types of cells and it is, in consequence an essential process for tissue homeostasis. However, only specialized cells termed professional phagocytes accomplish phagocytosis with high efficiency. Macrophages, neutrophils, monocytes, dendritic cells, and osteoclasts are among these dedicated cells. These professional phagocytes express several phagocytic receptors that activate signaling pathways resulting in phagocytosis. The process of phagocytosis involves several phases: i) detection of the particle to be ingested, ii) activation of the internalization process, iii) formation of a specialized vacuole called phagosome, and iv) maturation of the phagosome to transform it into a phagolysosome. In this review, we present a general view of our current understanding on cells, phagocytic receptors and phases involved in phagocytosis.

Endocytosis and the internalization of pathogenic organisms: focus on phosphoinositides

Despite their comparatively low abundance in biological membranes, phosphoinositides are key to the regulation of a diverse array of signaling pathways and direct membrane traffic. The role of phosphoinositides in the initiation and progression of endocytic pathways has been studied in considerable depth. Recent advances have revealed that distinct phosphoinositide species feature prominently in clathrin-dependent and -independent endocytosis as well as in phagocytosis and macropinocytosis. Moreover, a variety of intracellular and cell-associated pathogens have developed strategies to commandeer host cell phosphoinositide metabolism to gain entry and/or metabolic advantage, thereby promoting their survival and proliferation. Here, we briefly survey the current knowledge on the involvement of phosphoinositides in endocytosis, phagocytosis, and macropinocytosis and highlight several examples of molecular mimicry employed by pathogens to either “hitch a ride” on endocytic pathways endogenous to the host or create an entry path of their own.

Phagolysosome resolution requires contacts with the endoplasmic reticulum and phosphatidylinositol-4-phosphate signalling

Phosphoinositides have a pivotal role in the maturation of nascent phagosomes into microbicidal phagolysosomes. Following degradation of their contents, mature phagolysosomes undergo resolution, a process that remains largely uninvestigated. Here we studied the role of phosphoinositides in phagolysosome resolution. Phosphatidylinositol-4-phosphate (PtdIns(4)P), which is abundant in maturing phagolysosomes, was depleted as they tubulated and resorbed. Depletion was caused, in part, by transfer of phagolysosomal PtdIns(4)P to the endoplasmic reticulum, a process mediated by oxysterol-binding protein-related protein 1L (ORP1L), a RAB7 effector. ORP1L formed discrete tethers between the phagolysosome and the endoplasmic reticulum, resulting in distinct regions with alternating PtdIns(4)P depletion and enrichment. Tubules emerged from PtdIns(4)P-rich regions, where ADP-ribosylation factor-like protein 8B (ARL8B) and SifA- and kinesin-interacting protein/pleckstrin homology domain-containing family M member 2 (SKIP/PLEKHM2) accumulated. SKIP binds preferentially to monophosphorylated phosphoinositides, of which PtdIns(4)P is most abundant in phagolysosomes, contributing to their tubulation. Accordingly, premature hydrolysis of PtdIns(4)P impaired SKIP recruitment and phagosome resolution. Thus, resolution involves phosphoinositides and tethering of phagolysosomes to the endoplasmic reticulum.

Interaction of microtubules and actin during the post-fusion phase of exocytosis

Exocytosis is the intracellular trafficking step where a secretory vesicle fuses with the plasma membrane to release vesicle content. Actin and microtubules both play a role in exocytosis; however, their interplay is not understood. Here we study the interaction of actin and microtubules during exocytosis in lung alveolar type II (ATII) cells that secrete surfactant from large secretory vesicles. Surfactant extrusion is facilitated by an actin coat that forms on the vesicle shortly after fusion pore opening. Actin coat compression allows hydrophobic surfactant to be released from the vesicle. We show that microtubules are localized close to actin coats and stay close to the coats during their compression. Inhibition of microtubule polymerization by colchicine and nocodazole affected the kinetics of actin coat formation and the extent of actin polymerisation on fused vesicles. In addition, microtubule and actin cross-linking protein IQGAP1 localized to fused secretory vesicles and IQGAP1 silencing influenced actin polymerisation after vesicle fusion. This study demonstrates that microtubules can influence actin coat formation and actin polymerization on secretory vesicles during exocytosis.

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.

Tied up: Does altering phosphoinositide-mediated membrane trafficking influence neurodegenerative disease phenotypes?

Phosphoinositides are a class of membrane lipids that are found on several intracellular compartments and play diverse roles inside cells, such as vesicle formation, protein trafficking, endocytosis etc. Intracellular distribution and levels of phosphoinositides are regulated by enzymes that generate and breakdown these lipids as well as other proteins that associate with phosphoinositides. These events lead to differing levels of specific phosphoinositides on different intracellular compartments. At these intracellular locations, phosphoinositides and their associated proteins, such as Rab GTPases, dynamin and BAR domain-containing proteins, regulate a variety of membrane trafficking pathways. Neurodegenerative phenotypes in disorders such as Parkinson's disease (PD) can arise as a consequence of altered or hampered intracellular trafficking. Altered trafficking can cause proteins such as α-synuclein to aggregate intracellularly. Several trafficking pathways are regulated bymaster regulators such as LRRK2,which is known to regulate the activity of phosphoinositide effector proteins. Perturbing either the levels of phosphoinositides or their interactions with different proteins disrupts intracellular trafficking pathways, contributing to phenotypes often observed in disorders such as Alzheimer's or PDs. Thus, studying phosphoinositide regulation and its role in trafficking can give us a deeper understanding of the contribution of disrupted trafficking to neurodegenerative phenotypes.

7-Ketocholesterol impairs phagocytosis and efferocytosis via dysregulation of phosphatidylinositol 4,5-bisphosphate

The plasma membrane is inhomogeneously organized containing both highly ordered and disordered nanodomains. 7-Ketocholesterol (7KC), an oxysterol formed from the nonenzymatic oxidation of cholesterol, is a potent disruptor of membrane order. Importantly, 7KC is a component of oxidized low-density lipoprotein and accumulates in macrophage and foam cells found in arterial plaques. Using a murine macrophage cell line, J774, we report that both IgG-mediated and phosphatidylserine-mediated phagocytic pathways are inhibited by the accumulation of 7KC. Examination of the well-studied Fcγ receptor pathway revealed that the cell surface receptor abundance and ligand binding are unaltered while downstream signaling and activation of spleen tyrosine kinase is not affected. However, while the recruitment of phospholipase Cγ1 was unaffected its apparent enzymatic activity was compromised resulting in sustained phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂ ] levels and polymerized actin at the base of the phagocytic cup. Additionally, we found that 7KC prevented the activation of PLCβ downstream of the P2Y₆ G-protein coupled receptor and that 7KC impaired PLCγ activity in response to a direct elevation of cytosolic calcium induced by ionomycin. Finally, we demonstrate that 7KC partly attenuates the activity of rapamycin recruitable constitutively active PLCβ3. Together, our results demonstrate that the accumulation of 7KC impairs macrophage function by altering PtdIns(4,5)P₂ catabolism and, thus, impairing actin depolymerization required for the completion of phagocytosis.

mTORC2 Activity Disrupts Lysosome Acidification in Systemic Lupus Erythematosus by Impairing Caspase-1 Cleavage of Rab39a

Lysosomes maintain immune homeostasis through the degradation of phagocytosed apoptotic debris; however, the signaling events regulating lysosomal maturation remain undefined. In this study, we show that lysosome acidification, key to the maturation process, relies on mTOR complex 2 (mTORC2), activation of caspase-1, and cleavage of Rab39a. Mechanistically, the localization of cofilin to the phagosome recruits caspase-11, which results in the localized activation of caspase-1. Caspase-1 subsequently cleaves Rab39a on the phagosomal membrane, promoting lysosome acidification. Although caspase-1 is critical for lysosome acidification, its activation is independent of inflammasomes and cell death mediated by apoptosis-associated speck-like protein containing a caspase recruitment domain, revealing a role beyond pyroptosis. In lupus-prone murine macrophages, chronic mTORC2 activity decouples the signaling pathway, leaving Rab39a intact. As a result, the lysosome does not acidify, and degradation is impaired, thereby heightening the burden of immune complexes that activate FcγRI and sustain mTORC2 activity. This feedforward loop promotes chronic immune activation, leading to multiple lupus-associated pathologies. In summary, these findings identify the key molecules in a previously unappreciated signaling pathway that promote lysosome acidification. It also shows that this pathway is disrupted in systemic lupus erythematosus.

Integrin-based diffusion barrier separates membrane domains enabling the formation of microbiostatic frustrated phagosomes

Candida albicans hyphae can reach enormous lengths, precluding their internalization by phagocytes. Nevertheless, macrophages engulf a portion of the hypha, generating incompletely sealed tubular phagosomes. These frustrated phagosomes are stabilized by a thick cuff of F-actin that polymerizes in response to non-canonical activation of integrins by fungal glycan. Despite their continuity, the surface and invaginating phagosomal membranes retain a strikingly distinct lipid composition. PtdIns(4,5)P₂ is present at the plasmalemma but is not detectable in the phagosomal membrane, while PtdIns(3)P and PtdIns(3,4,5)P₃ co-exist in the phagosomes yet are absent from the surface membrane. Moreover, endo-lysosomal proteins are present only in the phagosomal membrane. Fluorescence recovery after photobleaching revealed the presence of a diffusion barrier that maintains the identity of the open tubular phagosome separate from the plasmalemma. Formation of this barrier depends on Syk, Pyk2/Fak and formin-dependent actin assembly. Antimicrobial mechanisms can thereby be deployed, limiting the growth of the hyphae.

Differential ability of proinflammatory and anti-inflammatory macrophages to perform macropinocytosis

Macropinocytosis mediates the uptake of antigens and of nutrients that dictate the regulation of cell growth by mechanistic target of rapamycin complex 1 (mTORC1). Because these functions differ in proinflammatory and anti-inflammatory macrophages, we compared the macropinocytic ability of two extreme polarization states. We found that anti-inflammatory macrophages perform vigorous macropinocytosis constitutively, while proinflammatory cells are virtually inactive. The total cellular content of Rho-family GTPases was higher in anti-inflammatory cells, but this disparity failed to account for the differential macropinocytic activity. Instead, reduced activity of Rac/RhoG was responsible for the deficient macropinocytosis of proinflammatory macrophages, as suggested by the stimulatory effects of heterologously expressed guanine nucleotide-exchange factors or of constitutively active (but not wild-type) forms of these GTPases. Similarly, differences in the activation state of phosphatidylinositol 3-kinase (PtdIns3K) correlated with the macropinocytic activity of pro- and anti-inflammatory macrophages. Differences in PtdIns3K and Rho-GTPase activity were attributable to the activity of calcium-sensing receptors (CaSRs), which appear to be functional only in anti-inflammatory cells. However, agonists of PtdIns3K, including cytokines, chemokines, and LPS, induced macropinocytosis in proinflammatory cells. Our findings revealed a striking difference in the macropinocytic ability of pro- and anti-inflammatory macrophages that correlates with their antigen-presenting and metabolic activity.

Vibrio vulnificus MARTX cytotoxin causes inactivation of phagocytosis-related signaling molecules in macrophages

Background

Vibrio vulnificus is a marine bacterial species that causes opportunistic infections manifested by serious skin lesions and fulminant septicemia in humans. We have previously shown that the multifunctional autoprocessing repeats in toxin (MARTX_Vv1) of a biotype 1 V. vulnificus strain promotes survival of this organism in the host by preventing it from engulfment by the phagocytes. The purpose of this study was to further explore how MARTX_Vv1 inhibits phagocytosis of this microorganism by the macrophage.

Methods

We compared between a wild-type V. vulnificus strain and its MARTX_Vv1-deficient mutant for a variety of phagocytosis-related responses, including morphological change and activation of signaling molecules, they induced in the macrophage. We also characterized a set of MARTX_Vv1 domain-deletion mutants to define the regions associated with antiphagocytosis activity.

Results

The RAW 264.7 cells and mouse peritoneal exudate macrophages underwent cell rounding accompanied by F-actin disorganization in the presence of MARTX_Vv1. In addition, phosphorylation of some F-actin rearrangement-associated signaling molecules, including Lyn, Fgr and Hck of the Src family kinases (SFKs), focal adhesion kinase (FAK), proline-rich tyrosine kinase 2 (Pyk2), phosphoinositide 3-kinase (PI3K) and Akt, but not p38, was decreased. By using specific inhibitors, we found that these kinases were all involved in the phagocytosis of MARTX_Vv1-deficient mutant in an order of SFKs-FAK/Pyk2-PI3K-Akt. Deletion of the effector domains in the central region of MARTX_Vv1 could lead to reduced cytotoxicity, depending on the region and size of deletion, but did not affect the antiphagocytosis activity and ability to cause rounding of macrophage. Reduced phosphorylation of Akt was closely associated with inhibition of phagocytosis by the wild-type strain and MARTX_Vv1 domain-deletion mutants, and expression of the constitutively active Akt, myr-Akt, enhanced the engulfment of these strains by macrophage.

Conclusions

MARTX_Vv1 could inactivate the SFKs-FAK/Pyk2-PI3K-Akt signaling pathway in the macrophages. This might lead to impaired phagocytosis of the V. vulnificus-infected macrophage. The majority of the central region of MARTX_Vv1 is not associated with the antiphagocytosis activity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-017-0368-2) contains supplementary material, which is available to authorized users.

The life cycle of phagosomes: formation, maturation, and resolution

Phagocytosis, the regulated uptake of large particles (>0.5 μm in diameter), is essential for tissue homeostasis and is also an early, critical component of the innate immune response. Phagocytosis can be conceptually divided into three stages: phagosome, formation, maturation, and resolution. Each of these involves multiple reactions that require exquisite spatial and temporal orchestration. The molecular events underlying these stages are being unraveled and the current state of knowledge is briefly summarized in this article.

Molecular Mechanisms of Phagosome Formation

Phagocytosis refers to the active process that allows cells to take up large particulate material upon binding to surface receptors. The discovery of phagocytosis in 1883 by Elie Metchnikoff, leading to the concept that specialized cells are implicated in the defense against microbes, was one of the starting points of the field of immunology. After more than a century of research, phagocytosis is now appreciated to be a widely used process that enables the cellular uptake of a remarkable variety of particles, including bacteria, fungi, parasites, viruses, dead cells, and assorted debris and solid materials. Uptake of foreign particles is performed almost exclusively by specialized myeloid cells, commonly termed "professional phagocytes": neutrophils, monocytes, macrophages, and dendritic cells. Phagocytosis of microbes not only stops or at least restricts the spread of infection but also plays an important role in regulating the innate and adaptive immune responses. Activation of the myeloid cells upon phagocytosis leads to the secretion of cytokines and chemokines that convey signals to a variety of immune cells. Moreover, foreign antigens generated by the degradation of microbes following phagocytosis are loaded onto the major histocompatibility complex for presentation to specific T lymphocytes. However, phagocytosis is not restricted to professional myeloid phagocytes; an expanding diversity of cell types appear capable of engulfing apoptotic bodies and debris, playing a critical role in tissue remodeling and in the clearance of billions of effete cells every day.

Phagocytosis: A Fundamental Process in Immunity

One hundred years have passed since the death of Élie Metchnikoff (1845-1916). He was the first to observe the uptake of particles by cells and realized the importance of this process for the host response to injury and infection. He also was a strong advocate of the role of phagocytosis in cellular immunity, and with this he gave us the basis for our modern understanding of inflammation and the innate and acquired immune responses. Phagocytosis is an elegant but complex process for the ingestion and elimination of pathogens, but it is also important for the elimination of apoptotic cells and hence fundamental for tissue homeostasis. Phagocytosis can be divided into four main steps: (i) recognition of the target particle, (ii) signaling to activate the internalization machinery, (iii) phagosome formation, and (iv) phagolysosome maturation. In recent years, the use of new tools of molecular biology and microscopy has provided new insights into the cellular mechanisms of phagocytosis. In this review, we present a general view of our current knowledge on phagocytosis. We emphasize novel molecular findings, particularly on phagosome formation and maturation, and discuss aspects that remain incompletely understood.

Inhibitory receptor FcγRIIb mediates the effects of IgG on a phagosome acidification and a sequential dephosphorylation system comprising SHIPs and Inpp4a

The relative abundance of phosphoinositide (PI) species on the phagosome membrane fluctuates over the course of phagocytosis. PtdIns(3,4,5)P3 and PtdIns(3,4)P2 rapidly increase in the forming of the phagocytic cup, following which they disappear after sealing of the cup. In the present study, we monitored the clearance of these PI species using the enhanced green fluorescent protein-fused pleckstrin homology domain of Akt, a fluorescence probe that binds both PtdIns(3,4,5)P3 and PtdIns(3,4)P2 in Raw 264.7 macrophages. The clearance of PIs was much faster when the phagocytosed particles were coated with IgG. The effect of IgG was not observed in the macrophages deficient in FcγRIIb, an inhibitory IgG receptor. To identify the lipid phosphatases responsible for the FcγRIIb-accelerated PI clearance, we prepared a panel of lipid phosphatase-deficient cells. The lack of a PI 5-phosphatase Src homology 2 domain-containing inositol-5-phosphatase (SHIP)1 or SHIP2 impaired the FcγRIIb-accelerated clearance of PIs. The lack of a PI 4-phosphatase Inpp4a also impaired the accelerated PIs clearance. In the FcγRIIb- and Inpp4a-deficient cells, acidification of the formed phagosome was slowed. These results suggested that FcγRIIb drives the sequential dephosphorylation system comprising SHIPs and Inpp4a, and accelerates phagosome acidification.

Purified Streptococcus pneumoniae Endopeptidase O (PepO) Enhances Particle Uptake by Macrophages in a Toll-Like Receptor 2- and miR-155-Dependent Manner

Insights into the host-microbial virulence factor interaction, especially the immune signaling mechanisms, could provide novel prevention and treatment options for pneumococcal diseases. Streptococcus pneumoniae endopeptidase O (PepO) is a newly discovered and ubiquitously expressed pneumococcal virulence protein. A PepO-mutant strain showed impaired adherence to and invasion of host cells compared with the isogenic wild-type strain. It is still unknown whether PepO is involved in the host defense response to pneumococcal infection. Here, we demonstrated that PepO could enhance phagocytosis of Streptococcus pneumoniae and Staphylococcus aureus by peritoneal exudate macrophages (PEMs). Further studies showed that PepO stimulation upregulated the expression of microRNA-155 (miR-155) in PEMs in a time- and dose-dependent manner. PepO-induced enhanced phagocytosis was decreased in cells transfected with an inhibitor of miR-155, while it was increased in cells transfected with a mimic of miR-155. We also revealed that PepO-induced upregulation of miR-155 in PEMs was mediated by Toll-like receptor 2 (TLR2)-NF-κB signaling and that the increased expression of miR-155 downregulated expression of SHIP1. Taken together, these results indicate that PepO induces upregulation of miR-155 in PEMs, contributing to enhanced phagocytosis and host defense response to pneumococci and Staphylococcus aureus.

Quantifying Phagocytosis by Immunofluorescence and Microscopy

Phagocytosis is the actin-driven internalization of solid particles, utilized by phagocytic immune cells to sequester potentially infectious microorganisms. Aided by the innate and adaptive immune system, the activation of various phagocytic receptors triggers a cascade of downstream signaling mediators that drive actin and plasma membrane remodeling. Modulation of these molecular players can lead to distinct changes in the capacity and rates of phagocytosis. Here, we present a fluorescence microscopy based technique to quantify phagocytosis using a macrophage-like cell line. We exemplify the technique through the phagocytosis of antibody-opsonized polystyrene beads. This method can be extended to other phagocytes and phagocytic particles.

Phagocytosis: Hungry, Hungry Cells

Phagocytosis is the cellular internalization and sequestration of particulate matter into a `phagosome, which then matures into a phagolysosome. The phagolysosome then offers a specialized acidic and hydrolytic milieu that ultimately degrades the engulfed particle. In multicellular organisms, phagocytosis and phagosome maturation play two key physiological roles. First, phagocytic cells have an important function in tissue remodeling and homeostasis by eliminating apoptotic bodies, senescent cells and cell fragments. Second, phagocytosis is a critical weapon of the immune system, whereby cells like macrophages and neutrophils hunt and engulf a variety of pathogens and foreign particles. Not surprisingly, pathogens have evolved mechanisms to either block or alter phagocytosis and phagosome maturation, ultimately usurping the cellular machinery for their own survival. Here, we review past and recent discoveries that highlight how phagocytes recognize target particles, key signals that emanate after phagocyte-particle engagement, and how these signals help modulate actin-dependent remodeling of the plasma membrane that culminates in the release of the phagosome. We then explore processes related to early and late stages of phagosome maturation, which requires fusion with endosomes and lysosomes. We end this review by acknowledging that little is known about phagosome fission and even less is known about how phagosomes are resolved after particle digestion.

Quantitative Live-Cell Fluorescence Microscopy During Phagocytosis

Phagocytosis is a receptor-mediated process whereby professional phagocytes internalize invading pathogens and apoptotic bodies into an intracellular vacuole or phagosome, leading to their degradation. During the formation and maturation of the phagosome, several lipids undergo changes and effector proteins are recruited on the nascent phagosome in a concerted manner. These highly localized, dynamic, and transient processes can only be studied by methods capable of high spatial and temporal resolution. The use of genetically encoded chimeric constructs coupled with fluorescence confocal microscopy enables the continuous, noninvasive analysis of the distribution and metabolism of lipids and effector proteins during phagocytosis. Here, we describe a method where the mouse macrophage cell line, RAW 264.7, and primary macrophages are transiently transfected with fluorescent chimeric probes to analyze and quantify phagocytosis of immunoglobulin-opsonized particles, using confocal microscopy.

Particles from the Echinococcus granulosus laminated layer inhibit IL-4 and growth factor-driven Akt phosphorylation and proliferative responses in macrophages

Proliferation of macrophages is a hallmark of inflammation in many type 2 settings including helminth infections. The cellular expansion is driven by the type 2 cytokine interleukin-4 (IL-4), as well as by M-CSF, which also controls homeostatic levels of tissue resident macrophages. Cystic echinococcosis, caused by the tissue-dwelling larval stage of the cestode Echinococcus granulosus, is characterised by normally subdued local inflammation. Infiltrating host cells make contact only with the acellular protective coat of the parasite, called laminated layer, particles of which can be ingested by phagocytic cells. Here we report that a particulate preparation from this layer (pLL) strongly inhibits the proliferation of macrophages in response to IL-4 or M-CSF. In addition, pLL also inhibits IL-4-driven up-regulation of Relm-α, without similarly affecting Chitinase-like 3 (Chil3/Ym1). IL-4-driven cell proliferation and up-regulation of Relm-α are both known to depend on the phosphatidylinositol (PI3K)/Akt pathway, which is dispensable for induction of Chil3/Ym1. Exposure to pLL in vitro inhibited Akt activation in response to proliferative stimuli, providing a potential mechanism for its activities. Our results suggest that the E. granulosus laminated layer exerts some of its anti-inflammatory properties through inhibition of PI3K/Akt activation and consequent limitation of macrophage proliferation.

A plasma membrane template for macropinocytic cups

Macropinocytosis is a fundamental mechanism that allows cells to take up extracellular liquid into large vesicles. It critically depends on the formation of a ring of protrusive actin beneath the plasma membrane, which develops into the macropinocytic cup. We show that macropinocytic cups in Dictyostelium are organised around coincident intense patches of PIP3, active Ras and active Rac. These signalling patches are invariably associated with a ring of active SCAR/WAVE at their periphery, as are all examined structures based on PIP3 patches, including phagocytic cups and basal waves. Patch formation does not depend on the enclosing F-actin ring, and patches become enlarged when the RasGAP NF1 is mutated, showing that Ras plays an instructive role. New macropinocytic cups predominantly form by splitting from existing ones. We propose that cup-shaped plasma membrane structures form from self-organizing patches of active Ras/PIP3, which recruit a ring of actin nucleators to their periphery.

Multiphasic dynamics of phosphatidylinositol 4-phosphate during phagocytosis

Phosphatidylinositol 4-phosphate undergoes striking multiphasic changes during phagosome formation and maturation. The molecular mechanisms underlying these changes and the role of phosphatidylinositol 4-phosphate in phagolysosome formation are investigated.

We analyzed the distribution, fate, and functional role of phosphatidylinositol 4-phosphate (PtdIns4P) during phagosome formation and maturation. To this end, we used genetically encoded probes consisting of the PtdIns4P-binding domain of the bacterial effector SidM. PtdIns4P was found to undergo complex, multiphasic changes during phagocytosis. The phosphoinositide, which is present in the plasmalemma before engagement of the target particle, is transiently enriched in the phagosomal cup. Soon after the phagosome seals, PtdIns4P levels drop precipitously due to the hydrolytic activity of Sac2 and phospholipase C, becoming undetectable for ∼10 min. PtdIns4P disappearance coincides with the emergence of phagosomal PtdIns3P. Conversely, the disappearance of PtdIns3P that signals the transition from early to late phagosomes is accompanied by resurgence of PtdIns4P, which is associated with the recruitment of phosphatidylinositol 4-kinase 2A. The reacquisition of PtdIns4P can be prevented by silencing expression of the kinase and can be counteracted by recruitment of a 4-phosphatase with a heterodimerization system. Using these approaches, we found that the secondary accumulation of PtdIns4P is required for proper phagosomal acidification. Defective acidification may be caused by impaired recruitment of Rab7 effectors, including RILP, which were shown earlier to displace phagosomes toward perinuclear lysosomes. Our results show multimodal dynamics of PtdIns4P during phagocytosis and suggest that the phosphoinositide plays important roles during the maturation of the phagosome.

Phosphoinositides in Control of Membrane Dynamics

Most functions of eukaryotic cells are controlled by cellular membranes, which are not static entities but undergo frequent budding, fission, fusion, and sculpting reactions collectively referred to as membrane dynamics. Consequently, regulation of membrane dynamics is crucial for cellular functions. A key mechanism in such regulation is the reversible recruitment of cytosolic proteins or protein complexes to specific membranes at specific time points. To a large extent this recruitment is orchestrated by phosphorylated derivatives of the membrane lipid phosphatidylinositol, known as phosphoinositides. The seven phosphoinositides found in nature localize to distinct membrane domains and recruit distinct effectors, thereby contributing strongly to the maintenance of membrane identity. Many of the phosphoinositide effectors are proteins that control membrane dynamics, and in this review we discuss the functions of phosphoinositides in membrane dynamics during exocytosis, endocytosis, autophagy, cell division, cell migration, and epithelial cell polarity, with emphasis on protein effectors that are recruited by specific phosphoinositides during these processes.

A Journey of Cytolethal Distending Toxins through Cell Membranes

The multifunctional role of lipids as structural components of membranes, signaling molecules, and metabolic substrates makes them an ideal partner for pathogens to hijack host cell processes for their own survival. The properties and composition of unique membrane micro-domains such as membrane rafts make these regions a natural target for pathogens as it affords them an opportunity to hijack cell signaling and intracellular trafficking pathways. Cytolethal distending toxins (Cdts), members of the AB2 family of toxins are comprised of three subunits, the active, CdtB unit, and the binding, CdtA-CdtC unit. Cdts are cyclomodulins leading to cell cycle arrest and apoptosis in a wide variety of cell types. Cdts from several species share a requirement for membrane rafts, and often cholesterol specifically for cell binding and CdtB mediated cytotoxicity. In this review we focus on how host–cell membrane bilayer organization contributes to the cell surface association, internalization, and action of bacteria derived cytolethal distending toxins (Cdts), with an emphasis on Aggregatibacter actinomycetemcomitans Cdt.