Shaping cups into phagosomes and macropinosomes

Search for inhibitors of endocytosis: Intended specificity and unintended consequences

We discuss here the variety of approaches that have been taken to inhibit different forms of endocytosis. Typically, both non-specific and specific chemical inhibitors of endocytosis are tried in order to “classify” entry of a new plasma membrane protein into one of the various types of endocytosis. This classification can be confirmed through genetic approaches of protein depletion or overexpression of mutants of known endocytosis machinery components. Although some new compounds have been designed to be selective in biochemical assays, we caution investigators to be alert to the unintended consequences that sometimes arise when these compounds are applied to intact cells.

Delivery of CSF-1R to the lumen of macropinosomes promotes its destruction in macrophages

Activation of the macrophage colony stimulating factor-1 receptor (CSF-1R) by CSF-1 stimulates pronounced macropinocytosis and drives proliferation of macrophages. Although the role of macropinocytosis in CSF-1R signaling remains unknown, we show here that, despite internalizing large quantities of plasma membrane, macropinosomes contribute little to the internalization of the CSF-1-CSF-1R complex. Rather, internalization of the CSF-1R in small endocytic vesicles that are sensitive to clathrin disruption, outcompetes macropinosomes for CSF-1R endocytosis. Following internalization, small vesicles carrying the CSF-1R underwent homotypic fusion and then trafficked to newly formed macropinosomes bearing Rab5. As these macropinosomes matured, acquiring Rab7, the CSF-1R was transported into their lumen and degraded. Inhibition of macropinocytosis delayed receptor degradation despite no disruption to CSF-1R endocytosis. These data indicate that CSF-1-stimulated macropinosomes are sites of multivesicular body formation and accelerate CSF-1R degradation. Furthermore, we demonstrate that macropinocytosis and cell growth have a matching dose dependence on CSF-1, suggesting that macropinosomes might be a central mechanism coupling CSF-1R signaling and macrophage growth.

A novel alpha kinase EhAK1 phosphorylates actin and regulates phagocytosis in Entamoeba histolytica

Phagocytosis plays a key role in nutrient uptake and virulence of the protist parasite Entamoeba histolytica. Phagosomes have been characterized by proteomics, and their maturation in the cells has been studied. However, there is so far not much understanding about initiation of phagocytosis and formation of phagosomes at the molecular level. Our group has been studying initiation of phagocytosis and formation of phagosomes in E. histolytica, and have described some of the molecules that play key roles in the process. Here we show the involvement of EhAK1, an alpha kinase and a SH3 domain containing protein in the pathway that leads to formation of phagosomes using red blood cell as ligand particle. A number of approaches, such as proteomics, biochemical, confocal imaging using specific antibodies or GFP tagged molecules, expression down regulation by antisense RNA, over expression of wild type and mutant proteins, were used to understand the role of EhAK1 in phagocytosis. EhAK1 was found in the phagocytic cups during the progression of cups, until closure of phagosomes, but not in the phagosomes themselves. It is recruited to the phagosomes through interaction with the calcium binding protein EhCaBP1. A reduction in phagocytosis was observed when EhAK1 was down regulated by antisense RNA, or by over expression of the kinase dead mutant. G-actin was identified as one of the major substrates of EhAK1. Phosphorylated actin preferentially accumulated at the phagocytic cups and over expression of a phosphorylation defective actin led to defects in phagocytosis. In conclusion, we describe an important component of the pathway that is initiated on attachment of red blood cells to E. histolytica cells. The main function of EhAK1 is to couple signalling events initiated after accumulation of EhC2PK to actin dynamics.

Striking the Right Balance Determines TB or Not TB

Mycobacterium tuberculosis continues to be one of the most successful pathogens on earth. Upon inhalation of M. tuberculosis by a healthy individual, the host immune system will attempt to eliminate these pathogens using a combination of immune defense strategies. These include the recruitment of macrophages and other phagocytes to the site of infection, production of cytokines that enhance the microbicidal capacity of the macrophages, as well as the activation of distinct subsets of leukocytes that work in concert to fight the infection. However, being as successful as it is, M. tuberculosis has evolved numerous strategies to subvert host immunity at virtual every level. As a consequence, one third of the world inhabitants carry M. tuberculosis, and tuberculosis continuous to cause disease in more than 8 million people with deadly consequences in well over 1 million patients each year. In this review, we discuss several of the strategies that M. tuberculosis employs to circumvent host immunity, as well as describe some of the mechanisms that the host uses to counter such subversive strategies. As for many other infectious diseases, the ultimate outcome is usually defined by the relative strength of the virulence strategies employed by the tubercle bacillus versus the arsenal of immune defense mechanisms of the infected host.

The mechanism of phagocytosis: two stages of engulfment

Despite being of vital importance to the immune system, the mechanism by which cells engulf relatively large solid particles during phagocytosis is still poorly understood. From movies of neutrophil phagocytosis of polystyrene beads, we measure the fractional engulfment as a function of time and demonstrate that phagocytosis occurs in two distinct stages. During the first stage, engulfment is relatively slow and progressively slows down as phagocytosis proceeds. However, at approximately half-engulfment, the rate of engulfment increases dramatically, with complete engulfment attained soon afterwards. By studying simple mathematical models of phagocytosis, we suggest that the first stage is due to a passive mechanism, determined by receptor diffusion and capture, whereas the second stage is more actively controlled, perhaps with receptors being driven toward the site of engulfment. We then consider a more advanced model that includes signaling and captures both stages of engulfment. This model predicts that there is an optimum ligand density for quick engulfment. Further, we show how this model explains why nonspherical particles engulf quickest when presented tip-first. Our findings suggest that active regulation may be a later evolutionary innovation, allowing fast and robust engulfment even for large particles.

Down-regulation of desmosomes in cultured cells: the roles of PKC, microtubules and lysosomal/proteasomal degradation

Desmosomes are intercellular adhesive junctions of major importance for tissue integrity. To allow cell motility and migration they are down-regulated in epidermal wound healing. Electron microscopy indicates that whole desmosomes are internalised by cells in tissues, but the mechanism of down-regulation is unclear. In this paper we provide an overview of the internalisation of half-desmosomes by cultured cells induced by calcium chelation. Our results show that: (i) half desmosome internalisation is dependent on conventional PKC isoforms; (ii) microtubules transport internalised half desmosomes to the region of the centrosome by a kinesin-dependent mechanism; (iii) desmosomal proteins remain colocalised after internalisation and are not recycled to the cell surface; (iv) internalised desmosomes are degraded by the combined action of lysosomes and proteasomes. We also confirm that half desmosome internalisation is dependent upon the actin cytoskeleton. These results suggest that half desmosomes are not disassembled and recycled during or after internalisation but instead are transported to the centrosomal region where they are degraded. These findings may have significance for the down-regulation of desmosomes in wounds.

Small GTPases and phosphoinositides in the regulatory mechanisms of macropinosome formation and maturation

Macropinosome formation requires the sequential activation of numerous signaling pathways that coordinate the actin-driven formation of plasma membrane protrusions (ruffles) and circular ruffles (macropinocytic cups), followed by the closure of these macropinocytic cups into macropinosomes. In the process of macropinosome formation, localized productions of phosphoinositides such as PI(4,5)P₂ and PI(3,4,5)P₃ spatiotemporally orchestrate actin polymerization and rearrangement through recruiting and activating a variety of actin-associated proteins. In addition, the sequential activation of small GTPases, which are known to be master regulators of the actin cytoskeleton, plays a pivotal role in parallel with phosphoinositides. To complete macropinosome formation, phosphoinositide breakdown and Rho GTPase deactivation must occur in appropriate timings. After the nascent macropinosomes are formed, phosphoinositides and several Rab GTPases control macropinosome maturation by regulating vesicle trafficking and membrane fusion. In this review, we summarize recent advances in our understanding of the critical functions of phosphoinositide metabolism and small GTPases in association with their downstream effectors in macropinocytosis.

Phosphatidylserine receptors: enhancers of enveloped virus entry and infection

A variety of both RNA and DNA viruses envelop their capsids in a lipid bilayer. One of the more recently appreciated benefits this envelope is incorporation of phosphatidylserine (PtdSer). Surface exposure of PtdSer disguises viruses as apoptotic bodies; tricking cells into engulfing virions. This mechanism is termed apoptotic mimicry. Several PtdSer receptors have been identified to enhance virus entry and we have termed this group of proteins PtdSer-mediated virus entry enhancing receptors or PVEERs. These receptors enhance entry of a range of enveloped viruses. Internalization of virions by PVEERs provides a broad mechanism of entry with little investment by the virus itself. PVEERs may allow some viruses to attach to cells, thereby making viral glycoprotein/cellular receptor interactions more probable. Alternatively, other viruses may rely entirely on PVEERs for internalization into endosomes. This review provides an overview of PtdSer receptors that serve as PVEERs and the biology behind virion/PVEER interaction.

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Phosphatidylserine (PtdSer) receptors can mediate entry of enveloped viruses.

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PtdSer is present on the outer leaflet of the virion envelope.

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PtdSer receptors are expressed on a variety of primary cells and cell lines.

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Characteristics of PtdSer receptors that mediate virus entry are defined.

Entry of Bombyx mori nucleopolyhedrovirus into BmN cells by cholesterol-dependent macropinocytic endocytosis

Bombyx mori nucleopolyhedrovirus (BmNPV) is a serious viral pathogen of silkworm, and no drug or specific protection against BmNPV infection is available at present time. Although functions of most BmNPV genes were depicted in recent years, knowledge on the mechanism of BmNPV entry into insect cells is still limited. Here BmNPV cell entry mechanism is investigated by different endocytic inhibitor application and subcellular analysis. Results indicated that BmNPV enters BmN cells by clathrin-independent macropinocytic endocytosis, which is mediated by cholesterol in a dose-dependent manner, and cholesterol replenishment rescued the BmNPV infection partially.

Phosphoinositides in phagocytosis and macropinocytosis

Professional phagocytes provide immunoprotection and aid in the maintenance of tissue homeostasis. They perform these tasks by recognizing, engulfing and eliminating pathogens and endogenous cell debris. Here, we examine the paramount role played by phosphoinositides in phagocytosis and macropinocytosis, two major endocytic routes that mediate the uptake of particulate and fluid matter, respectively. We analyze accumulating literature describing the molecular mechanisms whereby phosphoinositides translate environmental cues into the complex, sophisticated responses that underlie the phagocytic and macropinocytic responses. In addition, we exemplify virulence strategies involving modulation of host cell phosphoinositide signaling that are employed by bacteria to undermine immunity. This article is part of a Special Issue entitled Phosphoinositides.

Herpes simplex virus internalization into epithelial cells requires Na+/H+ exchangers and p21-activated kinases but neither clathrin- nor caveolin-mediated endocytosis

Herpes simplex virus 1 (HSV-1) is an alphaherpesvirus that has been reported to infect some epithelial cell types by fusion at the plasma membrane but others by endocytosis. To determine the molecular mechanisms of productive HSV-1 cell entry, we perturbed key endocytosis host factors using specific inhibitors, RNA interference (RNAi), or overexpression of dominant negative proteins and investigated their effects on HSV-1 infection in the permissive epithelial cell lines Vero, HeLa, HEp-2, and PtK2. HSV-1 internalization required neither endosomal acidification nor clathrin- or caveolin-mediated endocytosis. In contrast, HSV-1 gene expression and internalization were significantly reduced after treatment with 5-(N-ethyl-N-isopropyl)amiloride (EIPA). EIPA blocks the activity of Na(+)/H(+) exchangers, which are plasma membrane proteins implicated in all forms of macropinocytosis. HSV-1 internalization furthermore required the function of p21-activated kinases that contribute to macropinosome formation. However, in contrast to some forms of macropinocytosis, HSV-1 did not enlist the activities of protein kinase C (PKC), tyrosine kinases, C-terminal binding protein 1, or dynamin to activate its internalization. These data suggest that HSV-1 depends on Na(+)/H(+) exchangers and p21-activated kinases either for macropinocytosis or for local actin rearrangements required for fusion at the plasma membrane or subsequent passage through the actin cortex underneath the plasma membrane.

IMPORTANCE

After initial replication in epithelial cells, herpes simplex viruses (HSVs) establish latent infections in neurons innervating these regions. Upon primary infection and reactivation from latency, HSVs cause many human skin and neurological diseases, particularly in immunocompromised hosts, despite the availability of effective antiviral drugs. Many viruses use macropinocytosis for virus internalization, and many host factors mediating this entry route have been identified, although the specific perturbation profiles vary for different host and viral cargo. In addition to an established entry pathway via acidic endosomes, we show here that HSV-1 internalization depended on sodium-proton exchangers at the plasma membrane and p21-activated kinases. These results suggest that HSV-1 requires a reorganization of the cortical actin cytoskeleton, either for productive cell entry via pH-independent fusion from macropinosomes or for fusion at the plasma membrane, and subsequent cytosolic passage to microtubules that mediate capsid transport to the nucleus for genome uncoating and replication.

Phosphoinositides and engulfment

Cellular engulfment of particles, cells or solutes displaces large domains of plasma membrane into intracellular membranous vacuoles. This transfer of membrane is accompanied by major transitions of the phosphoinositide (PI) species that comprise the cytoplasmic face of membrane bilayers. Mapping of membrane PIs during engulfment reveals distinct patterns of protein and PI distributions associated with each stage of engulfment, which correspond with activities that regulate the actin cytoskeleton, membrane movements and vesicle secretion. Experimental manipulation of PI chemistry during engulfment indicates that PIs integrate organelle identity and orient signal transduction cascades within confined subdomains of membrane. These pathways are exploited by microbial pathogens to direct or redirect the engulfment process.

Coronaviruses induce entry-independent, continuous macropinocytosis

Macropinocytosis is exploited by many pathogens for entry into cells. Coronaviruses (CoVs) such as severe acute respiratory syndrome (SARS) CoV and Middle East respiratory syndrome CoV are important human pathogens; however, macropinocytosis during CoV infection has not been investigated. We demonstrate that the CoVs SARS CoV and murine hepatitis virus (MHV) induce macropinocytosis, which occurs late during infection, is continuous, and is not associated with virus entry. MHV-induced macropinocytosis results in vesicle internalization, as well as extended filopodia capable of fusing with distant cells. MHV-induced macropinocytosis requires fusogenic spike protein on the cell surface and is dependent on epidermal growth factor receptor activation. Inhibition of macropinocytosis reduces supernatant viral titers and syncytia but not intracellular virus titers. These results indicate that macropinocytosis likely facilitates CoV infection through enhanced cell-to-cell spreading. Our studies are the first to demonstrate virus use of macropinocytosis for a role other than entry and suggest a much broader potential exploitation of macropinocytosis in virus replication and host interactions. Importance: Coronaviruses (CoVs), including severe acute respiratory syndrome (SARS) CoV and Middle East respiratory syndrome CoV, are critical emerging human pathogens. Macropinocytosis is induced by many pathogens to enter host cells, but other functions for macropinocytosis in virus replication are unknown. In this work, we show that CoVs induce a macropinocytosis late in infection that is continuous, independent from cell entry, and associated with increased virus titers and cell fusion. Murine hepatitis virus macropinocytosis requires a fusogenic virus spike protein and signals through the epidermal growth factor receptor and the classical macropinocytosis pathway. These studies demonstrate CoV induction of macropinocytosis for a purpose other than entry and indicate that viruses likely exploit macropinocytosis at multiple steps in replication and pathogenesis.

Routes and mechanisms of extracellular vesicle uptake

Extracellular vesicles (EVs) are small vesicles released by donor cells that can be taken up by recipient cells. Despite their discovery decades ago, it has only recently become apparent that EVs play an important role in cell-to-cell communication. EVs can carry a range of nucleic acids and proteins which can have a significant impact on the phenotype of the recipient. For this phenotypic effect to occur, EVs need to fuse with target cell membranes, either directly with the plasma membrane or with the endosomal membrane after endocytic uptake. EVs are of therapeutic interest because they are deregulated in diseases such as cancer and they could be harnessed to deliver drugs to target cells. It is therefore important to understand the molecular mechanisms by which EVs are taken up into cells. This comprehensive review summarizes current knowledge of EV uptake mechanisms. Cells appear to take up EVs by a variety of endocytic pathways, including clathrin-dependent endocytosis, and clathrin-independent pathways such as caveolin-mediated uptake, macropinocytosis, phagocytosis, and lipid raft–mediated internalization. Indeed, it seems likely that a heterogeneous population of EVs may gain entry into a cell via more than one route. The uptake mechanism used by a given EV may depend on proteins and glycoproteins found on the surface of both the vesicle and the target cell. Further research is needed to understand the precise rules that underpin EV entry into cells.

Extending single-molecule microscopy using optical Fourier processing

This article surveys the recent application of optical Fourier processing to the long-established but still expanding field of single-molecule imaging and microscopy. A variety of single-molecule studies can benefit from the additional image information that can be obtained by modulating the Fourier, or pupil, plane of a widefield microscope. After briefly reviewing several current applications, we present a comprehensive and computationally efficient theoretical model for simulating single-molecule fluorescence as it propagates through an imaging system. Furthermore, we describe how phase/amplitude-modulating optics inserted in the imaging pathway may be modeled, especially at the Fourier plane. Finally, we discuss selected recent applications of Fourier processing methods to measure the orientation, depth, and rotational mobility of single fluorescent molecules.

Rab8a interacts directly with PI3Kγ to modulate TLR4-driven PI3K and mTOR signalling

Toll-like receptor 4 (TLR4) is activated by bacterial lipopolysaccharide (LPS) to mount innate immune responses. The TLR4-induced release of pro- and anti-inflammatory cytokines generates robust inflammatory responses, which must then be restrained to avoid disease. New mechanisms for the critical regulation of TLR-induced cytokine responses are still emerging. Here we find TLR4 complexes localized in LPS-induced dorsal ruffles on the surface of macrophages. We discover that the small GTPase Rab8a is enriched in these ruffles and recruits phosphatidylinositol 3-kinase (PI3Kγ) as an effector by interacting directly through its Ras-binding domain. Rab8a and PI3Kγ function to regulate Akt signalling generated by surface TLR4. Rab8a and PI3Kγ do not affect TLR4 endocytosis, but instead regulate mammalian target of rapamycin signalling as a mechanism for biasing the cytokine profile to constrain inflammation in innate immunity.

Rho GTPases, phosphoinositides, and actin: a tripartite framework for efficient vesicular trafficking

Rho GTPases are well known regulators of the actin cytoskeleton that act by binding and activating actin nucleators. They are therefore involved in many actin-based processes, including cell migration, cell polarity, and membrane trafficking. With the identification of phosphoinositide kinases and phosphatases as potential binding partners or effectors, Rho GTPases also appear to participate in the regulation of phosphoinositide metabolism. Since both actin dynamics and phosphoinositide turnover affect the efficiency and the fidelity of vesicle transport between cell compartments, Rho GTPases have emerged as critical players in membrane trafficking. Rho GTPase activity, actin remodeling, and phosphoinositide metabolism need to be coordinated in both space and time to ensure the progression of vesicles along membrane trafficking pathways. Although most molecular pathways are still unclear, in this review, we will highlight recent advances made in our understanding of how Rho-dependent signaling pathways organize actin dynamics and phosphoinositides and how phosphoinositides potentially provide negative feedback to Rho GTPases during endocytosis, exocytosis and membrane exchange between intracellular compartments.

Overexpression of atypical protein kinase C in HeLa cells facilitates macropinocytosis via Src activation

Atypical protein kinase C (aPKC) is the first recognized kinase oncogene. However, the specific contribution of aPKC to cancer progression is unclear. The pseudosubstrate domain of aPKC is different from the other PKC family members, and therefore a synthetic peptide corresponding to the aPKC pseudosubstrate (aPKC-PS) sequence, which specifically blocks aPKC kinase activity, is a valuable tool to assess the role of aPKC in various cellular processes. Here, we learned that HeLa cells incubated with membrane permeable aPKC-PS peptide displayed dilated heterogeneous vesicles labeled with peptide that were subsequently identified as macropinosomes. A quantitative membrane binding assay revealed that aPKC-PS peptide stimulated aPKC recruitment to membranes and activated Src. Similarly, aPKC overexpression in transfected HeLa cells activated Src and induced macropinosome formation. Src-aPKC interaction was essential; substitution of the proline residues in aPKC that associate with the Src-SH3 binding domain rendered the mutant kinase unable to induce macropinocytosis in transfected cells. We propose that aPKC overexpression is a contributing factor to cell transformation by interacting with and consequently promoting Src activation and constitutive macropinocytosis, which increases uptake of extracellular factors, required for altered cell growth and accelerated cell migration.

Arl13b and the non-muscle myosin heavy chain IIA are required for circular dorsal ruffle formation and cell migration

The Arf-like protein Arl13b has been implicated in ciliogenesis and Sonic hedgehog signaling. Furthermore, we have previously shown that it regulates endocytic recycling traffic and interacts with actin. Herein, we report that the non-muscle myosin heavy chain IIA, also known as Myh9, is an Arl13b effector. Moreover, we found that both proteins localized to circular dorsal ruffles (CDRs) induced by platelet-derived growth factor stimulation and are required for their formation. CDRs are ring-shaped actin-dependent structures formed on the dorsal cell surface and are involved in diverse processes, such as macropinocytosis, integrin recycling, internalization of receptor tyrosine kinases and cell migration. We found that Arl13b or Myh9 silencing impaired cell migration, suggesting that Arl13b is required for this function through the interaction with Myh9. Moreover, Arl13b silencing impaired neural crest cell migration in zebrafish embryos. Furthermore, we showed that Arl13b is required for the formation of CDRs in migrating cells. Thus, our results indicate a new role for Arl13b in actin cytoskeleton remodeling through the interaction with Myh9, by driving the formation of CDRs necessary for cell migration.

Biokinetics of nanoparticles and susceptibility to particulate exposure in a murine model of cystic fibrosis

BACKGROUND

Persons with cystic fibrosis (CF) are at-risk for health effects from ambient air pollution but little is known about the interaction of nanoparticles (NP) with CF lungs. Here we study the distribution of inhaled NP in a murine CF model and aim to reveal mechanisms contributing to adverse effects of inhaled particles in susceptible populations.

METHODS

Chloride channel defective CftrTgH (neoim) Hgu mice were used to analyze lung function, lung distribution and whole body biokinetics of inhaled NP, and inflammatory responses after intratracheal administration of NP. Distribution of 20-nm titanium dioxide NP in lungs was assessed on ultrathin sections immediately and 24 h after a one-hour NP inhalation. NP biokinetics was deduced from total and regional lung deposition and from whole body translocation of inhaled 30-nm iridium NP within 24 h after aerosol inhalation. Inflammatory responses were assessed within 7 days after carbon NP instillation.

RESULTS

Cftr mutant females had moderately reduced lung compliance and slightly increased airway resistance compared to wild type mice. We found no genotype dependent differences in total, regional and head deposition or in secondary-organ translocation of inhaled iridium NP. Titanium dioxide inhalation resulted in higher NP uptake by alveolar epithelial cells in Cftr mutants. Instillation of carbon NP induced a comparable acute and transient inflammatory response in both genotypes. The twofold increase of bronchoalveolar lavage (BAL) neutrophils in Cftr mutant compared to wild type mice at day 3 but not at days 1 and 7, indicated an impaired capacity in inflammation resolution in Cftr mutants. Concomitant to the delayed decline of neutrophils, BAL granulocyte-colony stimulating factor was augmented in Cftr mutant mice. Anti-inflammatory 15-hydroxyeicosatetraenoic acid was generally significantly lower in BAL of Cftr mutant than in wild type mice.

CONCLUSIONS

Despite lacking alterations in lung deposition and biokinetics of inhaled NP, and absence of significant differences in lung function, higher uptake of NP by alveolar epithelial cells and prolonged, acute inflammatory responses to NP exposure indicate a moderately increased susceptibility of lungs to adverse effects of inhaled NP in Cftr mutant mice and provides potential mechanisms for the increased susceptibility of CF patients to air pollution.

The noodle defense

Macrophages ingest and kill microbes by phagocytosis and delivery to lysosomes. In this issue, Prashar et al. (2013, J. Cell Biol.http://dx.doi.org/10.1083/jcb.201304095) demonstrate that the elongated morphology of filamentous bacteria does not prevent ingestion by macrophages or the fusion of lysosomes, but creates a chimeric, unclosed phagolysosomal compartment whose leakiness blunts the toxicity of lysosomal enzymes, thereby increasing bacterial survival.

LRRK2 and neuroinflammation: partners in crime in Parkinson's disease?

It is now well established that chronic inflammation is a prominent feature of several neurodegenerative disorders including Parkinson’s disease (PD). Growing evidence indicates that neuroinflammation can contribute greatly to dopaminergic neuron degeneration and progression of the disease. Recent literature highlights that leucine-rich repeat kinase 2 (LRRK2), a kinase mutated in both autosomal-dominantly inherited and sporadic PD cases, modulates inflammation in response to different pathological stimuli. In this review, we outline the state of the art of LRRK2 functions in microglia cells and in neuroinflammation. Furthermore, we discuss the potential role of LRRK2 in cytoskeleton remodeling and vesicle trafficking in microglia cells under physiological and pathological conditions. We also hypothesize that LRRK2 mutations might sensitize microglia cells toward a pro-inflammatory state, which in turn results in exacerbated inflammation with consequent neurodegeneration.

RETRACTED: Vulnerability of glioblastoma cells to catastrophic vacuolization and death induced by a small molecule

Glioblastoma multiforme (GBM) is the most aggressive form of brain cancer with marginal life expectancy. Based on the assumption that GBM cells gain functions not necessarily involved in the cancerous process, patient-derived glioblastoma cells (GCs) were screened to identify cellular processes amenable for development of targeted treatments. The quinine-derivative NSC13316 reliably and selectively compromised viability. Synthetic chemical expansion reveals delicate structure-activity relationship and analogs with increased potency, termed Vacquinols. Vacquinols stimulate death by membrane ruffling, cell rounding, massive macropinocytic vacuole accumulation, ATP depletion, and cytoplasmic membrane rupture of GCs. The MAP kinase MKK4, identified by a shRNA screen, represents a critical signaling node. Vacquinol-1 displays excellent in vivo pharmacokinetics and brain exposure, attenuates disease progression, and prolongs survival in a GBM animal model. These results identify a vulnerability to massive vacuolization that can be targeted by small molecules and point to the possible exploitation of this process in the design of anticancer therapies.

Molecular interplays involved in the cellular uptake of octaarginine on cell surfaces and the importance of syndecan-4 cytoplasmic V domain for the activation of protein kinase Cα

Arginine-rich cell-penetrating peptides (CPPs) are promising carriers for the intracellular delivery of various bioactive molecules. However, many ambiguities remain about the molecular interplays on cell surfaces that ultimately lead to endocytic uptake of CPPs. By treatment of cells with octaarginine (R8), enhanced clustering of syndecan-4 on plasma membranes and binding of protein kinase Cα (PKCα) to the cytoplasmic domain of syndecan-4 were observed; these events potentially lead to the macropinocytic uptake of R8. The cytoplasmic V domain of syndecan-4 made a significant contribution to the cellular uptake of R8, whereas the cytoplasmic C1 and C2 domains were not involved in the process.

Sequential breakdown of 3-phosphorylated phosphoinositides is essential for the completion of macropinocytosis

Macropinocytosis is a highly conserved endocytic process by which extracellular fluid and solutes are internalized into cells. Macropinocytosis starts with the formation of membrane ruffles at the plasma membrane and ends with their closure. The transient and sequential emergence of phosphoinositides PI(3,4,5)P3 and PI(3,4)P2 in the membrane ruffles is essential for macropinocytosis. By making use of information in the Caenorhabditis elegans mutants defective in fluid-phase endocytosis, we found that mammalian phosphoinositide phosphatase MTMR6 that dephosphorylates PI(3)P to PI, and its binding partner MTMR9, are required for macropinocytosis. INPP4B, which dephosphorylates PI(3,4)P2 to PI(3)P, was also found to be essential for macropinocytosis. These phosphatases operate after the formation of membrane ruffles to complete macropinocytosis. Finally, we showed that KCa3.1, a Ca(2+)-activated K(+) channel that is activated by PI(3)P, is required for macropinocytosis. We propose that the sequential breakdown of PI(3,4,5)P3 → PI(3,4)P2 → PI(3)P → PI controls macropinocytosis through specific effectors of the intermediate phosphoinositides.

Cryptococcus neoformans is internalized by receptor-mediated or 'triggered' phagocytosis, dependent on actin recruitment

Cryptococcosis by the encapsulated yeast Cryptococcus neoformans affects mostly immunocompromised individuals and is a frequent neurological complication in AIDS patients. Recent studies support the idea that intracellular survival of Cryptococcus yeast cells is important for the pathogenesis of cryptococcosis. However, the initial steps of Cryptococcus internalization by host cells remain poorly understood. Here, we investigate the mechanism of Cryptococcus neoformans phagocytosis by peritoneal macrophages using confocal and electron microscopy techniques, as well as flow cytometry quantification, evaluating the importance of fungal capsule production and of host cell cytoskeletal elements for fungal phagocytosis. Electron microscopy analyses revealed that capsular and acapsular strains of C. neoformans are internalized by macrophages via both 'zipper' (receptor-mediated) and 'trigger' (membrane ruffle-dependent) phagocytosis mechanisms. Actin filaments surrounded phagosomes of capsular and acapsular yeasts, and the actin depolymerizing drugs cytochalasin D and latrunculin B inhibited yeast internalization and actin recruitment to the phagosome area. In contrast, nocodazole and paclitaxel, inhibitors of microtubule dynamics decreased internalization but did not prevent actin recruitment to the site of phagocytosis. Our results show that different uptake mechanisms, dependent on both actin and tubulin dynamics occur during yeast internalization by macrophages, and that capsule production does not affect the mode of Cryptococcus uptake by host cells.

PIP₃-dependent macropinocytosis is incompatible with chemotaxis

In eukaryotic chemotaxis, the mechanisms connecting external signals to the motile apparatus remain unclear. The role of the lipid phosphatidylinositol 3,4,5-trisphosphate (PIP₃) has been particularly controversial. PIP₃ has many cellular roles, notably in growth control and macropinocytosis as well as cell motility. Here we show that PIP₃ is not only unnecessary for Dictyostelium discoideum to migrate toward folate, but actively inhibits chemotaxis. We find that macropinosomes, but not pseudopods, in growing cells are dependent on PIP₃. PIP₃ patches in these cells show no directional bias, and overall only PIP₃-free pseudopods orient up-gradient. The pseudopod driver suppressor of cAR mutations (SCAR)/WASP and verprolin homologue (WAVE) is not recruited to the center of PIP₃ patches, just the edges, where it causes macropinosome formation. Wild-type cells, unlike the widely used axenic mutants, show little macropinocytosis and few large PIP₃ patches, but migrate more efficiently toward folate. Tellingly, folate chemotaxis in axenic cells is rescued by knocking out phosphatidylinositide 3-kinases (PI 3-kinases). Thus PIP₃ promotes macropinocytosis and interferes with pseudopod orientation during chemotaxis of growing cells.

Dissecting the roles of Rac1 activation and deactivation in macropinocytosis using microscopic photo-manipulation

Macropinocytosis, a fluid-phase endocytosis, is a crucial pathway for antigen uptake and presentation in macrophages. We attempted to characterise the activation and deactivation of a small GTPase molecular switch, Rac1, in macropinocytosis using microscopic photo-manipulation. Expression of genetically encoded photoactivatable-Rac1 (PA-Rac1) in RAW264 macrophages enabled the local, reversible control of macropinocytosis using blue laser irradiation. Marked membrane ruffling and unclosed pre-macropinosomes were observed in the irradiated region of macrophages under the persistent activation of PA-Rac1. Although phosphatidylinositol 4,5-bisphosphate and actin were also localised to this region, the recruitment of maturating endosome markers, such as phosphatidylinositol 3-phosphate and Rab21, was restricted until PA-Rac1 deactivation. After deactivating PA-Rac1 by ceasing irradiation, membrane ruffling immediately receded, and the macropinosomes acquired maturation markers. These data suggest that activation of Rac1 is sufficient to induce membrane ruffling and macropinocytic cup formation, but subsequent deactivation of Rac1 is required for macropinosome closure and further maturation.

A combination of SILAC and nucleotide acyl phosphate labelling reveals unexpected targets of the Rsk inhibitor BI-D1870

Protein kinase inhibitors frequently have interesting effects that cannot be fully ascribed to the intended target kinase(s) but identifying additional targets that might explain the effects is not straightforward. By comparing two different inhibitors of the Rsk (p90 ribosomal S6 kinase) kinases, we found that the increasingly used compound BI-D1870 had biological effects in murine DCs (dendritic cells) that could not be solely ascribed to Rsk or other documented targets. We assessed the ability of BI-D1870 and a second Rsk inhibitor, BIX 02565 to protect enzyme active sites from reaction with biotinylated nucleotide acyl phosphates. Using SILAC (stable isotope labelling by amino acids in cell culture)-labelled DC lysates as a source of enzyme targets, we identify several kinases that interact with BI-D1870 but not with BIX 02565. We confirmed that these kinases, including Slk, Lok and Mst1, are inhibited by BI-D1870 but to a much lesser extent by BIX 02565 and that phosphorylation of some of their substrates is blocked by BI-D1870 in living cells. Our results suggest that the BI-D1870 inhibitor should be used with caution. The SILAC-based methodology we used should be useful for further comparative unbiased profiling of the target spectrum of kinase inhibitors with interesting biological effects under conditions that closely mimic those found in cells.

Effect of sauchinone, a lignan from Saururus chinensis, on bacterial phagocytosis by macrophages

AMP-activated protein kinase (AMPK) plays an important role in inflammation in various cells and increases the phagocytic ability of macrophages. In this study, we found that sauchinone increased the phosphorylation of AMPK and acetyl-CoA carboxylase (ACC), a downstream target of AMPK, in mouse peritoneal macrophages. Sauchinone increased macrophage phagocytosis of fluorescent Escherichia coli, which was blocked by compound C, an AMPK inhibitor. Sauchinone also increased the phosphorylation of p38 mitogen activated protein kinase (MAPK) in cultured macrophages in a concentration-dependent fashion, which was not blocked by compound C. However, the increase of sauchinone-induced phagocytosis was prevented by SB203580. An inhibitor of the upstream kinase TGF-beta-activated kinase (TAK1), (5z)-7-oxozeaenol, abolished the phosphorylation of ACC and p38 MAPK. Systemic administration of sauchinone to mice led to increased phosphorylation of AMPK and p38 MAPK in the lung, and enhanced phagocytosis of fluorescent E. coli in bronchoalveolar lavage fluid as compared with control mice. These results suggest sauchinone to be a useful adjunctive treatment for bacterial infection.

Ca²⁺-sensor proteins in the autophagic and endocytic traffic

Autophagy and endocytosis are two evolutionarily conserved catabolic processes that comprise vesicle trafficking events for the clearance of the sequestered intracellular and extracellular cargo. Both start differently but end in the same compartment, the lysosome. Mounting evidences from the last years have established the involvement of proteins sensitive to intracellular Ca²⁺ in the control of the early autophagic steps and in the traffic of autophagic, endocytic and lysosomal vesicles. However, this knowledge is based on dispersed outcomes that do not set up a consensus model of the Ca²⁺-dependent control of autophagy and endocytosis. Here, we will provide a critical synopsis of insights from the last decade on the involvement of Ca²⁺-sensor proteins in the activation of autophagy and in fusion events of endocytic vesicles, autophagosomes and lysosomes.

Inflammatory stimuli reprogram macrophage phagocytosis to macropinocytosis for the rapid elimination of pathogens

Following an infectious challenge, macrophages have to be activated in order to allow efficient clearance of infectious pathogens, but how macrophage activation is coupled to increased clearance remains largely unknown. We here describe that inflammatory stimuli induced the reprogramming of the macrophage endocytic machinery from receptor-mediated phagocytosis to macropinocytosis, allowing the rapid transfer of internalized cargo to lysosomes in a receptor-independent manner. Reprogramming occurred through protein kinase C-mediated phosphorylation of the macrophage protein coronin 1, thereby activating phosphoinositol (PI)-3-kinase activity necessary for macropinocytic uptake. Expression of a phosphomimetic form of coronin 1 was sufficient to induce PI3-kinase activation and macropinocytosis even in the absence of inflammatory stimuli. Together these results suggest a hitherto unknown mechanism to regulate the internalization and degradation of infectious material during inflammation.

The main cells that are involved in cleaning up microbial pathogens are macrophages. Upon an infection, macrophages are being recruited to the site of infection by a number of different stimuli. In addition, during an infection, macrophages are also activated by cytokines such as interferon-γ and tumor necrosis factor-α that is released from other immune cells. Such macrophage activation is important to achieve a rapid and efficient clearance of microbial pathogens. In this study, we found that macrophage activation induces uptake through macropinocytosis rather than receptor-mediate phagocytosis. As a consequence, microbial material as well as particles can be internalized far more efficiently; In addition, the internalized cargo is rapidly destroyed within lysosomes. We also provide the mechanisms for the switch from phagocytosis to macropinocytosis, which turned out to be the cytokine-induced phosphorylation of the host protein coronin 1. Phosphorylated coronin 1 activated the lipid kinase phosphoinositide 3-kinase, which is known to be responsible for the entry of cargo through macropinocytosis. Together these results provide evidence for a hitherto unrecognized mechanisms to regulate the internalization and degradation of infectious material during an infection.

Quantitative assessment of neutrophil phagocytosis using flow cytometry

Neutrophils have an incredible ability to find and eradicate intruders such as bacteria and fungi. They do this largely through the process of phagocytosis, where the target is internalized into a phagosome, and eventually destroyed by the hostile phagosomal environment. It is important to study phagocytosis in order to understand how neutrophils interact with various pathogens and how they respond to different stimuli. Here, I describe a method to study neutrophil phagocytosis of bacteria using flow cytometry. The bacteria are fluorescently labeled before being introduced to neutrophils. After phagocytosis, both any remaining extracellular bacteria and neutrophils are labeled using one-step staining before three-color analysis. To assess phagocytosis, first the average time it takes for the neutrophils to internalize all bound bacteria is determined. Experiments are then performed using that time point while varying the bacteria-to-neutrophil ratio for full control of the analysis. Due to the ease with which multiple samples can be analyzed, and the quantitative nature of flow cytometry, this approach is both reproducible and sensitive.

The Annexin A2/p11 complex is required for efficient invasion of Salmonella Typhimurium in epithelial cells

The facultative intracellular pathogen, Salmonella enterica, triggers its own uptake into non-phagocytic epithelial cells. Invasion is dependent on a type 3 secretion system (T3SS), which delivers a cohort of effector proteins across the plasma membrane where they induce dynamic actin-driven ruffling of the membrane and ultimately, internalization of the bacteria into a modified phagosome. In eukaryotic cells, the calcium- and phospholipid-binding protein Annexin A2 (AnxA2) functions as a platform for actin remodelling in the vicinity of dynamic cellular membranes. AnxA2 is mostly found in a stable heterotetramer, with p11, which can interact with other proteins such as the giant phosphoprotein AHNAK. We show here that AnxA2, p11 and AHNAK are required for T3SS-mediated Salmonella invasion of cultured epithelial cells and that the T3SS effector SopB is required for recruitment of AnxA2 and AHNAK to Salmonella invasion sites. Altogether this work shows that, in addition to targeting Rho-family GTPases, Salmonella can intersect the host cell actin pathway via AnxA2.

Assessment of phagocytic activity of cultured macrophages using fluorescence microscopy and flow cytometry

Phagocytosis is the process by which phagocytes, including macrophages, neutrophils and monocytes, engulf and kill invading pathogens, remove foreign particles, and clear cell debris. Phagocytes and their ability to phagocytose are an important part of the innate immune system and are critical for homeostasis of the host. Impairment in phagocytosis has been associated with numerous diseases and disorders. Different cytokines have been shown to affect the phagocytic process. Cytokines including TNFα, IL-1β, GM-CSF, and TGF-β1 were found to promote phagocytosis, whereas high mobility group box-1 (HMGB1) inhibited the phagocytic function of macrophages. Here, we describe two commonly used methods to assess the phagocytic function of cultured macrophages, which can easily be applied to other phagocytes. Each method is based on the extent of engulfment of FITC-labeled latex minibeads by macrophages under different conditions. Phagocytic activity can be assessed either by counting individual cells using a fluorescence microscope or measuring fluorescence intensity using a flow cytometer.

[Phagocytosis and cytokinesis: highlights on common themes and differences]

Eukaryotic cells 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 are described, and questions that remain open in these dynamic areas of research are also highlighted.

Recombinant lentogenic Newcastle disease virus expressing Ebola virus GP infects cells independently of exogenous trypsin and uses macropinocytosis as the major pathway for cell entry

BACKGROUND

Using reverse genetics, we generated a recombinant low-pathogenic LaSota strain Newcastle disease virus (NDV) expressing the glycoprotein (GP) of Ebola virus (EBOV), designated rLa-EBOVGP, and evaluated its biological characteristic in vivo and in vitro.

RESULTS

The introduction and expression of the EBOV GP gene did not increase the virulence of the NDV vector in poultry or mice. EBOV GP was incorporated into the particle of the vector virus and the recombinant virus rLa-EBOVGP infected cells and spread within them independently of exogenous trypsin. rLa-EBOVGP is more resistant to NDV antiserum than the vector NDV and is moderately sensitive to EBOV GP antiserum. More importantly, infection with rLa-EBOVGP was markedly inhibited by IPA3, indicating that rLa-EBOVGP uses macropinocytosis as the major internalization pathway for cell entry.

CONCLUSIONS

The results demonstrate that EBOV GP in recombinant NDV particles functions independently to mediate the viral infection of the host cells and alters the cell-entry pathway.

Properties and prospects of adjuvants in influenza vaccination - messy precipitates or blessed opportunities?

Influenza is a major challenge to healthcare systems world-wide. While prophylactic vaccination is largely efficient, long-lasting immunity has not been achieved in immunized populations, at least in part due to the challenges arising from the antigen variation between strains of influenza A virus as a consequence of genetic drift and shift. From progress in our understanding of the immune system, the mode-of-action of vaccines can be divided into the stimulation of the adaptive system through inclusion of appropriate vaccine antigens and of the innate immune system by the addition of adjuvant to the vaccine formulation. A shared property of many vaccine adjuvants is found in their nature of water-insoluble precipitates, for instance the particulate material made from aluminum salts. Previously, it was thought that embedding of vaccine antigens in these materials provided a "depot" of antigens enabling a long exposure of the immune system to the antigen. However, more recent work points to a role of particulate adjuvants in stimulating cellular parts of the innate immune system. Here, we briefly outline the infectious medicine and immune biology of influenza virus infection and procedures to provide sufficient and stably available amounts of vaccine antigen. This is followed by presentation of the many roles of adjuvants, which involve humoral factors of innate immunity, notably complement. In a perspective of the ultrastructural properties of these humoral factors, it becomes possible to rationalize why these insoluble precipitates or emulsions are such a provocation of the immune system. We propose that the biophysics of particulate material may hold opportunities that could aid the development of more efficient influenza vaccines.

Properties and prospects of adjuvants in influenza vaccination - messy precipitates or blessed opportunities?

Influenza is a major challenge to healthcare systems world-wide. While prophylactic vaccination is largely efficient, long-lasting immunity has not been achieved in immunized populations, at least in part due to the challenges arising from the antigen variation between strains of influenza A virus as a consequence of genetic drift and shift. From progress in our understanding of the immune system, the mode-of-action of vaccines can be divided into the stimulation of the adaptive system through inclusion of appropriate vaccine antigens and of the innate immune system by the addition of adjuvant to the vaccine formulation. A shared property of many vaccine adjuvants is found in their nature of water-insoluble precipitates, for instance the particulate material made from aluminum salts. Previously, it was thought that embedding of vaccine antigens in these materials provided a "depot" of antigens enabling a long exposure of the immune system to the antigen. However, more recent work points to a role of particulate adjuvants in stimulating cellular parts of the innate immune system. Here, we briefly outline the infectious medicine and immune biology of influenza virus infection and procedures to provide sufficient and stably available amounts of vaccine antigen. This is followed by presentation of the many roles of adjuvants, which involve humoral factors of innate immunity, notably complement. In a perspective of the ultrastructural properties of these humoral factors, it becomes possible to rationalize why these insoluble precipitates or emulsions are such a provocation of the immune system. We propose that the biophysics of particulate material may hold opportunities that could aid the development of more efficient influenza vaccines.

Exploiting endocytosis for nanomedicines

In this article, we briefly review the endocytic pathways used by cells, pointing out their defining characteristics and highlighting physical limitations that may direct the internalization of nanoparticles to a subset of these pathways. A more detailed description of these pathways is presented in the literature. We then focus on the endocytosis of nanomedicines and present how various nanomaterial parameters impact these endocytic processes. This topic is an area of active research, motivated by the recognition that an improved understanding of how nanomaterials interact at the molecular, cellular, and whole-organism level will lead to the design of better nanomedicines in the future. Next, we briefly review some of the important nanomedicines already on the market or in clinical development that serve to exemplify how endocytosis can be exploited for medical benefit. Finally, we present some key unanswered questions and remaining challenges to be addressed by the field.

Exploring the contribution of mycobacteria characteristics in their interaction with human macrophages

Tuberculosis (TB) is a major health problem. The emergence of multidrug resistant (MDR) Mycobacterium tuberculosis (Mtb) isolates confounds treatment strategies. In Portugal, cases of MDR-TB are reported annually with an increased incidence noted in Lisbon. The majority of these MDR-TB cases are due to closely related mycobacteria known collectively as the Lisboa family and Q1 cluster. Genetic determinants linked to drug resistance have been exhaustively studied resulting in the identification of family and cluster specific mutations. Nevertheless, little is known about other factors involved in development of mycobacteria drug resistance. Here, we complement genetic analysis with the study of morphological and structural features of the Lisboa family and Q1 cluster isolates by using scanning and transmission electron microscopy. This analysis allowed the identification of structural differences, such as cell envelope thickness, between Mtb clinical isolates that are correlated with antibiotic resistance. The infection of human monocyte derived macrophages allowed us to document the relative selective advantage of the Lisboa family isolates over other circulating Mtb isolates.

P2Y4 receptor-mediated pinocytosis contributes to amyloid beta-induced self-uptake by microglia

Brain disturbances, like injuries or aberrant protein deposits, evoke nucleotide release or leakage from cells, leading to microglial chemotaxis and ingestion. Recent studies have identified P2Y12 purinergic receptors as triggers for microglial chemotaxis and P2Y6 receptors as mediators for phagocytosis. However, pinocytosis, known as the internalization of fluid-phase materials, has received much less attention. We found that ATP efficiently triggered pinocytosis in microglia. Pharmacological analysis and knockdown experiments demonstrated the involvement of P2Y4 receptors and the phosphatidylinositol 3-kinase/Akt cascade in the nucleotide-induced pinocytosis. Further evidence indicated that soluble amyloid beta peptide 1-42 induced self-uptake in microglia through pinocytosis, a process involving activation of P2Y4 receptors by autocrine ATP signaling. Our results demonstrate a previously unknown function of ATP as a "drink me" signal for microglia and P2Y4 receptors as a potential therapeutic target for the treatment of Alzheimer's disease.

High-throughput quantification of early stages of phagocytosis

Phagocytosis—the engulfment of cells and foreign bodies—is an important cellular process in innate immunity, development, and disease. Quantification of various stages of phagocytosis, especially in a rapid screening fashion, is an invaluable tool for elucidating protein function during this process. However, current methods for assessing phagocytosis are largely limited to flow cytometry and manual image-based assays, providing limited information. Here, we present an image-based, semi-automated phagocytosis assay to rapidly quantitate three distinct stages during the early engulfment of opsonized beads. Captured images are analyzed using the image-processing software ImageJ and quantified using a macro. Modifications to this method allowed quantification of phagocytosis only in fluorescently labeled transfected cells. Additionally, the time course of bead internalization could be measured using this approach. The assay could discriminate perturbations to stages of phagocytosis induced by known pharmacological inhibitors of filamentous actin and phosphoinositol-3-kinase. Our methodology offers the ability to automatically categorize large amounts of image data into the three early stages of phagocytosis within minutes, clearly demonstrating its potential value in investigating aberrant phagocytosis when manipulating proteins of interest in drug screens and disease.

Ca²⁺ and calpain control membrane expansion during the rapid cell spreading of neutrophils

Following adherence of neutrophils to the endothelium, neutrophils undergo a major morphological change that is a necessary prelude to their extravasation. We show here that this shape change is triggered by an elevation of cytosolic inositol (1,4,5)-trisphosphate (IP3), to provoke physiological Ca(2+) influx through a store-operated mechanism. This transition from a spherical to 'flattened' neutrophil morphology is rapid (∼100 seconds) and is accompanied by an apparent rapid expansion of the area of the plasma membrane. However, no new membrane is added into the plasma membrane. Pharmacological inhibition of calpain-activation, which is triggered by Ca(2+) influx during neutrophil spreading, prevents normal cell flattening. In calpain-suppressed cells, an aberrant form of cell spreading can occur where an uncoordinated and localised expansion of the plasma membrane is evident. These data show that rapid neutrophil spreading is triggered by Ca(2+) influx, which causes activation of calpain and release of furled plasma membrane to allow its apparent 'expansion'.

Photoreceptor phagocytosis is mediated by phosphoinositide signaling

Circadian oscillations in peripheral tissues, such as the retinal compartment of the eye, are critical to anticipating changing metabolic demands. Circadian shedding of retinal photoreceptor cell discs with subsequent phagocytosis by the neighboring retinal pigmented epithelium (RPE) is essential for removal of toxic metabolites and lifelong survival of these postmitotic neurons. Defects in photoreceptor phagocytosis can lead to severe retinal pathology, but the biochemical mechanisms remain poorly defined. By first documenting a 2.8-fold burst of photoreceptor phagocytosis events in the mouse eye in the morning compared with the afternoon by serial block face imaging, we established time points to assess transcriptional readouts by RNA sequencing (RNA-Seq). We identified 365 oscillating protein-coding transcripts that implicated the phosphoinositide lipid signaling network mediating the discrete steps of photoreceptor phagocytosis. Moreover, examination of overlapping cistromic sites by core clock transcription factors and promoter elements of these effector genes provided a functional basis for the circadian cycling of these transcripts. RNA-Seq also revealed oscillating expression of 16 long intergenic noncoding RNAs and key histone modifying enzymes critical for circadian gene expression. Our phenotypic and genotypic characterization reveals a complex global landscape of overlapping and temporally controlled networks driving the essential circadian process in the eye.

The non-canonical roles of clathrin and actin in pathogen internalization, egress and spread

The role of clathrin in pathogen entry has received much attention and has highlighted the adaptability of clathrin during internalization. Recent studies have now uncovered additional roles for clathrin and have put the spotlight on its role in pathogen spread. Here, we discuss the manipulation of clathrin by pathogens, with specific attention to the processes that occur at the plasma membrane. In the majority of cases, both clathrin and the actin cytoskeleton are hijacked, so we also examine the interplay between these two systems and their role during pathogen internalization, egress and spread.