Selective translocation of annexins during intracellular redistribution of Chlamydia trachomatis in HeLa and McCoy cells

Streptococcus pneumoniae inhibits purinergic signaling and promotes purinergic receptor P2Y2 internalization in alveolar epithelial cells

Bacterial pneumonia is a global health challenge that causes up to 2 million deaths each year. Purinergic signaling plays a pivotal role in healthy alveolar epithelium. Here, we used fluorophore-based analysis and live-cell calcium imaging to address the question of whether the bacterial pathogen Streptococcus pneumoniae directly interferes with purinergic signaling in alveolar epithelial cells. Disturbed purinergic signaling might result in pathophysiologic changes like edema formation and atelectasis, which are commonly seen in bacterial pneumonia. Purine receptors are mainly activated by ATP, mediating a cytosolic calcium response. We found that this purinergic receptor P2Y₂-mediated response is suppressed in the presence of S. pneumoniae in A549 and isolated primary alveolar cells in a temperature-dependent manner. Downstream inositol 3-phosphate (IP₃) signaling appeared to be unaffected, as calcium signaling via protease-activated receptor 2 remained unaltered. S. pneumoniae-induced suppression of the P2Y₂-mediated calcium response depended on the P2Y₂ phosphorylation sites Ser-243, Thr-344, and Ser-356, which are involved in receptor desensitization and internalization. Spinning-disk live-cell imaging revealed that S. pneumoniae induces P2Y₂ translocation into the cytosol. In conclusion, our results show that S. pneumoniae directly inhibits purinergic signaling by inducing P2Y₂ phosphorylation and internalization, resulting in the suppression of the calcium response of alveolar epithelial cells to ATP, thereby affecting cellular integrity and function.

Impact of calcium signaling during infection of Neisseria meningitidis to human brain microvascular endothelial cells

The pili and outer membrane proteins of Neisseria meningitidis (meningococci) facilitate bacterial adhesion and invasion into host cells. In this context expression of meningococcal PilC1 protein has been reported to play a crucial role. Intracellular calcium mobilization has been implicated as an important signaling event during internalization of several bacterial pathogens. Here we employed time lapse calcium-imaging and demonstrated that PilC1 of meningococci triggered a significant increase in cytoplasmic calcium in human brain microvascular endothelial cells, whereas PilC1-deficient meningococci could not initiate this signaling process. The increase in cytosolic calcium in response to PilC1-expressing meningococci was due to efflux of calcium from host intracellular stores as demonstrated by using 2-APB, which inhibits the release of calcium from the endoplasmic reticulum. Moreover, pre-treatment of host cells with U73122 (phospholipase C inhibitor) abolished the cytosolic calcium increase caused by PilC1-expressing meningococci demonstrating that active phospholipase C (PLC) is required to induce calcium transients in host cells. Furthermore, the role of cytosolic calcium on meningococcal adherence and internalization was documented by gentamicin protection assay and double immunofluorescence (DIF) staining. Results indicated that chelation of intracellular calcium by using BAPTA-AM significantly impaired PilC1-mediated meningococcal adherence to and invasion into host endothelial cells. However, buffering of extracellular calcium by BAPTA or EGTA demonstrated no significant effect on meningococcal adherence to and invasion into host cells. Taken together, these results indicate that meningococci induce calcium release from intracellular stores of host endothelial cells via PilC1 and cytoplasmic calcium concentrations play a critical role during PilC1 mediated meningococcal adherence to and subsequent invasion into host endothelial cells.

Streptococcus pneumoniae infection of host epithelial cells via polymeric immunoglobulin receptor transiently induces calcium release from intracellular stores

The pneumococcal surface protein C (PspC) is a major adhesin of Streptococcus pneumoniae (pneumococci) that interacts in a human-specific manner with the ectodomain of the human polymeric immunoglobulin receptor (pIgR) produced by respiratory epithelial cells. This interaction promotes bacterial colonization and bacterial internalization by initiating host signal transduction cascades. Here, we examined alterations of intracellular calcium ([Ca(2+)](i)) levels in epithelial cells during host cell infections with pneumococci via the PspC-hpIgR mechanism. The release of [Ca(2+)](i) from intracellular stores in host cells was significantly increased by wild-type pneumococci but not by PspC-deficient pneumococci. The increase in [Ca(2+)](i) was dependent on phospholipase C as pretreatment of cells with a phospholipase C-specific inhibitor U73122 abolished the increase in [Ca(2+)](i). In addition, we demonstrated the effect of [Ca(2+)](i) on pneumococcal internalization by epithelial cells. Uptake of pneumococci was significantly increased after pretreatment of epithelial cells with the cell-permeable calcium chelator 1,2-bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid-tetraacetoxymethyl ester or use of EGTA as an extracellular Ca(2+)-chelating agent. In contrast, thapsigargin, an inhibitor of endoplasmic reticulum Ca(2+)ATPase, which increases [Ca(2+)](i) in a sustained fashion, significantly reduced pIgR-mediated pneumococcal invasion. Importantly, pneumococcal adherence to pIgR-expressing cells was not altered in the presence of inhibitors as demonstrated by immunofluorescence microscopy. In conclusion, these results demonstrate that pneumococcal infections induce mobilization of [Ca(2+)](i) from intracellular stores. This may constitute a defense response of host cells as the experimental reduction of intracellular calcium levels facilitates pneumococcal internalization by pIgR-expressing cells, whereas elevated calcium levels diminished bacterial internalization by host epithelial cells.

Mechanisms of Chlamydia trachomatis entry into nonphagocytic cells

The mechanisms of entry for the obligate intracellular bacterium C. trachomatis were examined by functional disruption of proteins essential for various modes of entry. RNA interference was used to disrupt proteins with established roles in clathrin-mediated endocytosis (clathrin heavy chain, dynamin-2, heat shock 70-kDa protein 8, Arp2, cortactin, and calmodulin), caveola-mediated endocytosis (caveolin-1, dynamin-2, Arp2, NSF, and annexin II), phagocytosis (RhoA, dynamin-2, Rac1, and Arp2), and macropinocytosis (Pak1, Rac1, and Arp2). Comparative quantitative PCR analysis was performed on small interfering RNA-transfected HeLa cells to accurately determine the extent of C. trachomatis entry after these treatments. Key structural and regulatory factors associated with clathrin-mediated endocytosis were found to be involved in Chlamydia entry, whereas those for caveola-mediated endocytosis, phagocytosis, and macropinocytosis were not. Thus, clathrin and its coordinate accessory factors were required for entry of C. trachomatis, although additional, uncharacterized mechanisms are also utilized.

Ca2+ release from host intracellular stores and related signal transduction during Campylobacter jejuni 81-176 internalization into human intestinal cells

Campylobacter jejuni is the leading bacterial cause of human diarrhoeal disease in many parts of the world, including the USA. The ability of C. jejuni to invade the host intestinal epithelium is an important determinant of virulence. A common theme among pathogenic invasive micro-organisms is their ability to usurp the eukaryotic cell-signalling systems both to allow for invasion and to trigger disease pathogenesis. Ca(2+) is very important in a great variety of eukaryotic cell-signalling processes (e.g. calmodulin-activated enzymes, nuclear transcriptional upregulation, and cytoskeletal rearrangements). This study analyses the effects of Ca(2+) availability on invasion of human INT407 intestinal epithelial cells by C. jejuni strain 81-176. The ability of C. jejuni to invade INT407 cells was not blocked by chelation of any remaining extracellular Ca(2+) from host cells incubated in Ca(2+)-free, serum-free media. In contrast, C. jejuni invasion was markedly reduced either by chelating host intracellular Ca(2+) with 1,2-bis-(2-)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA, AM) or by blocking the release of Ca(2+) from intracellular stores with dantrolene or U73122. Moreover, Bay K8644, a plasma-membrane Ca(2+)-channel agonist, was observed to stimulate C. jejuni invasion, presumably by increasing host intracellular free Ca(2+) levels. Measurement of host-cell cytosolic Ca(2+) via spectrofluorimetry and fluorescence microscopy revealed an increase in Ca(2+) from 10 min post-infection. Monolayer pretreatment with either a calmodulin antagonist or a specific inhibitor of protein kinase C was found to cause a marked reduction in C. jejuni invasion, suggesting roles for these Ca(2+)-activated modulators in signal-transduction events involved in C. jejuni invasion. These results demonstrate that C. jejuni induces the mobilization of Ca(2+) from host intracellular stores, which is an essential step in the invasion of intestinal cells by this pathogen.

The chlamydial inclusion: escape from the endocytic pathway

Chlamydiae, bacterial obligate intracellular pathogens, are the etiologic agents of several human diseases. A large part of the chlamydial intracellular survival strategy involves the formation of a unique organelle called the inclusion that provides a protected site within which they replicate. The chlamydial inclusion is effectively isolated from endocytic pathways but is fusogenic with a subset of exocytic vesicles that deliver sphingomyelin from the Golgi apparatus to the plasma membrane. A combination of host and parasite functions contribute to the biogenesis of this compartment. Establishment of the mature inclusion is accompanied by the insertion of multiple chlamydial proteins, suggesting that chlamydiae actively modify the inclusion to define its interactions with the eukaryotic host cell. Despite being sequestered within a membrane-bound vacuole, chlamydiae clearly communicate with and manipulate the host cell from within this privileged intracellular niche.

Chlamydia trachomatis disrupts N-cadherin-dependent cell-cell junctions and sequesters beta-catenin in human cervical epithelial cells

The cadherin/catenin complex serves as an important structural component of adherens junctions in epithelial cells. Under certain conditions, beta-catenin can be released from this complex and interact with transcription factors in the nucleus to stimulate the expression of genes that regulate apoptosis and cell cycle control. While studying the effects of the bacterial pathogen Chlamydia trachomatis on human cervical epithelial cells in culture, we observed that C. trachomatis caused the epithelial cells to separate from each other without detaching from their growing surface. The objective of the present study was to determine if this effect might involve the disruption of the cadherin/catenin complex. Primary cultures of human cervical epithelial cells or HeLa cells were infected with C. trachomatis serovar E. Cadherin-like immunoreactive materials and beta-catenin were visualized by immunofluorescence. Preliminary studies showed that N-cadherin was the primary cadherin expressed in both the primary cultures and the HeLa cells. In noninfected cells, N-cadherin and beta-catenin were colocalized at the intercellular junctional complexes. By contrast, the infected cells showed a marked loss of both N-cadherin and beta-catenin labeling from the junctional complexes and the concomitant appearance of intense beta-catenin labeling associated with the chlamydial inclusion. The results of Western blot analyses of extracts of C. trachomatis showed no evidence of cross-reactivity with the beta-catenin antibody. These results indicate that C. trachomatis causes the breakdown of the N-cadherin/beta-catenin complex and that the organism can sequester beta-catenin within the chlamydial inclusion. This could represent an important mechanism by which C. trachomatis alters epithelial cell function.

Chlamydia trachomatis-induced apoptosis occurs in uninfected McCoy cells late in the developmental cycle and is regulated by the intracellular redox state

Infections with the obligate intracellular bacterium Chlamydia trachomatis are characterized by avoidance of fusion between chlamydia-containing endosomes and lysosomes, bacterial persistence and development of post-infectious sequelae. In this report we show that C. trachomatis induces apoptosis in McCoy and HeLa cells. Apoptosis was monitored by three different techniques; enzyme-linked immunoassay (EIA) of fragmented nucleosomes, terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) and flow cytometry of propidium iodide-stained cells. Apoptosis occurred in uninfected cells, was induced late in the chlamydial developmental cycle, beyond 24 h post-infection and was dependent on bacterial protein synthesis. Apoptosis was not significantly increased in infected, inclusion-containing cells. Treatment of cells with the antioxidants ascorbic acid (10 microM) and alpha-tocopherol (10 microM) reduced the degree of apoptosis. These results suggest that host cells infected with C. trachomatis generate proapoptotic stimuli that induce apoptosis in uninfected, neighbouring cells and that the redox state of the cell is a regulator in chlamydia-induced apoptosis.

Intracellular survival by Chlamydia

Chlamydiae are obligate intracellular bacterial pathogens whose entry into mucosal epithelial cells is required for intracellular survival and subsequent growth. After a seemingly stealthy entry, chlamydiae quickly modify their vacuole (i) for exit from the endosomal pathway to the exocytic pathway and (ii) to permit fusion with intercepted endoplasmic reticulum- and Golgi-derived vesicles carrying glycerophospholipids and sphingolipids for chlamydiae-containing vacuole membrane expansion. Chlamydiae possess novel hollow proteinaceous structures, termed projections, which they use to pierce the inclusion membrane, possibly to acquire from the epithelial cytoplasm nutrients they cannot synthesize; whether or not these truncated flagellar-like structures serve a dual exchange function for secretion of molecules to programme host cell signalling is unknown. Despite the accumulation of some 500-1000 progeny in the enormously enlarged inclusion, host cell function is surprisingly little disrupted, and progeny escape can be unobtrusive. This elegant adaptive pathogen strategy, which leads to silent, chronic human infection, is fascinating from a cellular microbiology perspective.

Annexin V binds to the actin-based cytoskeleton at the plasma membrane of activated platelets

Immunocytochemical studies demonstrate that annexin V relocates to the plasma membranes of intact stimulated blood platelets. Anti-annexin V antibodies label the cytoplasmic side of the substrate-adherent plasma membrane of mechanically unroofed, glass-activated platelets and colocalize with actin. In addition, crosslinking experiments using detergent-solubilized membranes of activated platelets have identified an 85-kDa complex containing annexin V. The 85-kDa complex is also recognized by antibodies against actin, suggesting that annexin V interacts with actin. In addition, annexin V was found to associate with filamentous actin in the presence of millimolar Ca(2+). Annexin V was also shown by immunofluorescence microscopy to be associated with platelet cytoskeletons, colocalizing with actin in the presence of micromolar Ca(2+). These findings provide the first evidence for annexin V binding to the plasma membrane and to the actin-based cytoskeleton in activated platelets and indicate that annexin V may function in both cytoskeletal and membrane domains.

Human granulocytic ehrlichiosis agent and Ehrlichia chaffeensis reside in different cytoplasmic compartments in HL-60 cells

The human granulocytic ehrlichiosis (HGE) agent resides and multiplies exclusively in cytoplasmic vacuoles of granulocytes. Double immunofluorescence labeling was used to characterize the nature of the HGE agent replicative inclusions and to compare them with inclusions containing the human monocytic ehrlichia, Ehrlichia chaffeensis, in HL-60 cells. Although both Ehrlichia spp. can coinfect HL-60 cells, they resided in separate inclusions. Inclusions of both Ehrlichia spp. were not labeled with either anti-lysosome-associated membrane protein 1 or anti-CD63. Accumulation of myeloperoxidase-positive granules were seen around HGE agent inclusions but not around E. chaffeensis inclusions. 3-(2, 4-Dinitroanilino)-3'-amino-N-methyldipropylamine and acridine orange were not localized to either inclusion type. Vacuolar-type H+-ATPase was not colocalized with HGE agent inclusions but was weakly colocalized with E. chaffeensis inclusions. E. chaffeensis inclusions were labeled with the transferrin receptor, early endosomal antigen 1, and rab5, but HGE agent inclusions were not. Some HGE agent and E. chaffeensis inclusions colocalized with major histocompatibility complex class I and II antigens. These two inclusions were not labeled for annexins I, II, IV, and VI; alpha-adaptin; clathrin heavy chain; or beta-coatomer protein. Vesicle-associated membrane protein 2 colocalized to both inclusions. The cation-independent mannose 6-phosphate receptor was not colocalized with either inclusion type. Endogenously synthesized sphingomyelin, from C6-NBD-ceramide, was not incorporated into either inclusion type. Brefeldin A did not affect the growth of either Ehrlichia sp. in HL-60 cells. These results suggest that the HGE agent resides in inclusions which are neither early nor late endosomes and does not fuse with lysosomes or Golgi-derived vesicles, while E. chaffeensis resides in an early endosomal compartment which accumulates the transferrin receptor.

Localization of intracellular Ca2+ stores in HeLa cells during infection with Chlamydia trachomatis

Chlamydia trachomatis elementary bodies (EBs) enter epithelial cells within membrane-bound endosomes that aggregate with each other in a calcium-regulated process, but avoid fusion with lysosomes. Annexin III but not I translocates to chlamydial aggregates and inclusions. In this study, we localize the intracellular Ca2+ stores during the course of infection by analyzing the distribution of three intracellular Ca2+ store proteins: calreticulin, type-1 inositol-1,4, 5-trisphosphate receptor (IP3-R), and Sarcoplasmic/Endoplasmic Reticulum Ca2+ ATPase type 2 (SERCA2) in HeLa cells infected with C. trachomatis serovar L2. In uninfected cells, immunofluorescence staining of the proteins showed a fine granular distributed pattern for all three proteins. After infection with C. trachomatis, calreticulin was found at the periphery of chlamydial aggregates and inclusions from 3 to 48 hours post-infection. In infected cells, SERCA2 was intimately associated with chlamydial inclusions after 3 and 24 hours, but not after 48 hours. Moreover, IP3-R was translocated to and colocalized with EB aggregates and chlamydial inclusions and had a distribution very similar to that of SERCA 2. After 24 hours incubation with chlamydiae, there was a local accumulation of [Ca2+]i (105+/-17 nM) in the proximity of chlamydial inclusions, compared to 50+/-13 nM in other parts of the cell cytoplasm. In the absence of extracellular Ca2+, this local accumulation of Ca2+ increased to 295+/-50 nM after adding 50 microM ATP, and to a similar extent after adding 100 nM thapsigargin (Tg). These data indicate that during infection of HeLa cells with chlamydiae, intracellular Ca2+ stores are redistributed, causing local accumulation of Ca2+ in the vicinity of chlamydial inclusions. These changes may trigger the association of certain proteins such as annexins with chlamydia-containing vesicles, and thereby regulation of membrane-membrane interaction during endosome aggregation and inclusion formation.

Fusion of Chlamydia trachomatis-containing inclusions is inhibited at low temperatures and requires bacterial protein synthesis

The human pathogen Chlamydia trachomatis is an obligate intracellular bacterium with a unique developmental cycle. Within the host cell cytoplasm, it resides within a membrane-bound compartment, the inclusion. A distinguishing characteristic of the C. trachomatis life cycle is the fusion of the chlamydia-containing inclusions with each other in the host cell cytoplasm. We report that fusion of inclusions does not occur at 32 degreesC in multiple mammalian cell lines and with three different serovars of C. trachomatis. The inhibition of fusion was inclusion specific; the fusion with sphingolipid-containing secretory vesicles and the interaction with early endosomes were unaffected by incubation at 32 degreesC. The inhibition of fusion of the inclusions was not primarily the result of delayed maturation of the inclusion, as infectious progeny was produced in host cells incubated at 32 degreesC, and the unfused inclusions remained competent to fuse up to 48 h postinfection. The ability to reverse the inhibition of fusion by shifting the infected cells from 32 to 37 degreesC allowed the measurement of the rate and the time of fusion of the inclusions after entry of the bacteria. Most significantly, we demonstrate that fusion of inclusions with each other requires bacterial protein synthesis and that the required bacterial protein(s) is present, but inactive or not secreted, at 32 degreesC.

Reprogramming the phagocytic pathway--intracellular pathogens and their vacuoles (review)

Phagocytic immune cells (particularly macrophages and neutrophils) take up and digest particles that have invaded our bodies. In doing so, they represent a very early line of defence against a microbial attack. During uptake, the particles are wrapped by a portion of the phagocyte's plasma membrane, and a new endocytic compartment, the phagosome, is formed. The typical fate of a phagosome is its fusion with lysosomes to yield a phagolysosome in which the particle is digested. Recent data show that some 'intracellular microorganisms' that can cause severe illnesses (tuberculosis, leprosy, legionnaire's disease and others) manage to reprogramme the host phagocytes not to deliver them to the lysosomal compartment. This probably results in increased survival of the pathogens. The analysis of the composition of such 'novel' compartments and research on the molecular mechanisms underlying the microbial interference with host cell functions are likely to yield important insights into: (1) which endocytic/phagocytic compartments phagocytes employ to handle ingested material in general; (2) how some pathogenic microorganisms can reprogramme the phagocytic pathway; and possibly (3) how infections caused by these microorganisms can be treated more effectively. Here, some studies are presented analysing which compartments intracellular pathogens inhabit and how microbes might be able to reprogramme their host cells.

Preparation and characterization of an endogenously fluorescent annexin for detection of apoptotic cells

Annexin proteins specifically bind anionic phospholipids such as phosphatidylserine, which are normally confined to the cytoplasmic leaflet of cellular membranes. During programmed cell death, or apoptosis, this phospholipid asymmetry is lost, and anionic phospholipids are exposed on the extracellular leaflet of the plasma membrane where they are accessible to exogenously added, labeled annexins. Chemically [e.g., fluoroscein isothiocyanate (FITC)]-modified annexin V has been widely used to detect and enumerate apoptotic cells by flow cytometry. We prepared chimeric proteins containing green fluorescent protein (GFP) fused to annexin V. A chimera containing GFP fused to the C-terminus of annexin V was soluble and fluorescent, but was unable to bind phospholipids. In contrast, a chimera containing GFP fused to the N-terminus of annexin V specifically bound apoptotic cells. GFP-annexin V represents a sensitive and facile alternative to FITC-annexin V for studies of apoptosis.

Roles of calcium and annexins in phagocytosis and elimination of an attenuated strain of Mycobacterium tuberculosis in human neutrophils

The phagocytic function of neutrophils is a crucial element in the host defence against invading microorganisms. We investigated phagocytosis and intracellular killing of an attenuated strain of Mycobacterium tuberculosis(H37Ra) by human neutrophils focusing on the role of the cytosolic free calcium concentration [Ca2+]i and certain cytosolic calcium-dependent membrane-binding proteins annexins. Phagocytic uptake did not trigger a calcium rise and occurred independently of different calcium conditions, and in a serum-dependent manner. Changes in the viability of H37Ra were determined by agar plate colony count and a radiometric assay. Neutrophils showed a capacity to kill ingested mycobacteria and this occurred without a rise in [Ca2+]i. The ability to kill H37Ra decreased in the absence of extracellular calcium and when intra-extracellular calcium was reduced. Immunofluorescence staining revealed that during phagocytosis of H37Ra, annexins III, IV and VI translocated from cytoplasm to the proximity of the H37Ra-containing phagosomes, whereas the localization of annexin I and V remained unchanged. The translocation of annexin IV occurred even when Ca2+-depleted neutrophils ingested H37Ra in the absence of extracellular calcium. We concluded that neutrophil-mediated killing of mycobacteria is a Ca2+-dependent process. The fact that the association of certain annexins to the membrane vesicle containing H37Ra differ from other phagosomes suggests a selective regulatory mechanism during phagocytosis of mycobacteria by neutrophils.

Mouse annexin III cDNA, genetic mapping and evolution

Mouse annexin III cDNA was characterized from I.M.A.G.E. Consortium (LLNL) expressed sequence tag clones by molecular sequencing, chromosomal mapping and systematic analysis. cDNA sequences extended the known 5' and 3' untranslated regions and confirmed the location of intron 7 with respect to the human gene. The Anx3 locus mapped to the middle of mouse chromosome 5 between Areg and Fgf5. Protein-coding regions were compared with homologous annexins to establish subfamily identity, structural conservation and divergence pattern. Annexin III exhibited low functional constraint against structural change and weak phylogenetic association with known annexins. The rapid, constant divergence of human and rodent annexins III from each other and from other annexin subfamilies was used to estimate gene separation times. Phylogenetic, phenetic and structural data suggested a possible direct or indirect separation of annexin III from XI approximately 317 million years ago.

Role of annexins in endocytosis of antigens in immature human dendritic cells

We have evaluated the uptake of a soluble protein antigen, denitrophenylated human serum albumin (DNP-HSA), and two different intracellular bacteria; Chlamydia trachomatis serovar L2 and Mycobacterium tuberculosis strain H37Ra, by immature human dendritic cells. These were generated by culturing progenitor cells from blood in the presence of cytokines (granulocyte-macrophage colony-stimulating factor and interleukin-4). Dendritic cells play a crucial part in antigen presentation for the induction of T-cell-dependent immune responses in various tissues. Recently, macropinocytic and phagocytic activity has been shown for immature dendritic cells of mouse, rat and human origin. In the present study, macropinocytosis characterized the uptake of the soluble protein-antigen DNP-HSA, whereas the C. trachomatis were ingested via receptor-mediated endocytosis in coated pits, and opsonized M. tuberculosis via phagocytosis. To follow the intracellular routes of the antigens, their positions were compared with the localization of annexins, a family of Ca(2+)-and phospholipid-binding proteins, involved in membrane fusion, aggregation and transport of different vesicles. To elucidate further the intracellular pathway of the antigens, two other proteins, lysosome-associated membrane protein-1 (LAMP-1) and cathepsin D, were labelled. They are known to colocalize with major histocompatibility complex class II compartments in the immature dendritic cells. We observed a distinct translocation of annexin V to DNP-HSA containing endosomes, and annexin III to vesicles with C. trachomatis. Furthermore, annexin III, IV and V redistributed to phagosomes with M. tuberculosis. Both LAMP-1 and cathepsin D colocalized with DNP-HSA endosomes, and with phagosomes with M. tuberculosis. Thus, immature human dendritic cells have the capacity to phagocytose. Moreover, the handling of these antigens by dendritic cells may represent three distinct intracellular pathways, albeit some properties and compartments are shared.

Safe haven: the cell biology of nonfusogenic pathogen vacuoles

Our understanding of both membrane traffic in mammalian cells and the cell biology of infection with intracellular pathogens has increased dramatically in recent years. In this review, we discuss the cell biology of the host-microbe interaction for four intracellular pathogens: Chlamydia spp., Legionella pneumophila, Mycobacterium spp., and the protozoan parasite Toxoplasma gondii. All of these organisms reside in vacuoles inside cells that have restricted fusion with host organelles of the endocytic cascade. Despite this restricted fusion, the vacuoles surrounding each pathogen display novel interactions with other host cell organelles. In addition to the effect of infection on host membrane traffic, we focus on these novel interactions and relate them where possible to nutrient acquisition by the intracellular organisms.

Localization of five annexins in J774 macrophages and on isolated phagosomes

Annexins are a family of structurally related proteins which bind phospholipids in a calcium-dependent manner. Although the precise functions of annexins are unknown, there is an accumulating set of data arguing for a role for some of them in vesicular transport and, specifically, in membrane-membrane or membrane-cytoskeletal interactions during these processes. Here we describe our qualitative and quantitative analysis of the localization of annexins I-V in J774 macrophages that had internalized latex beads, both with and without IgG opsonization. Our results show that whereas all these annexins are present on both the plasma membrane and on phagosomes, the localization on other organelles differs. Annexins I, II, III and V were detected on early endosomes, while only annexin V was seen on late endocytic organelles and mitochondria. Annexins I and II distributed along the plasma membrane non-uniformly and co-localized with F-actin at the sites of membrane protrusions. We also investigated by western blot analysis the association of annexins with purified phagosomes isolated at different time-points after latex bead internalization. While the amounts of annexins I, II, III and V associated with phagosomes were similar at all times after their formation, the level of annexin IV was significantly higher on older phagosomes. Whereas annexins I, II, IV and V could be removed from phagosome membranes with a Ca2+ chelator they remained membrane bound under low calcium conditions. In contrast, annexin III was removed under these conditions and needed a relatively high Ca2+ concentration to remain phagosome bound. Because of their purity and ease of preparation we suggest that phagosomes are a powerful system to study the potential role of annexins in membrane traffic.

Characterization of the Chlamydia trachomatis vacuole and its interaction with the host endocytic pathway in HeLa cells

Chlamydia trachomatis, an obligate intracellular parasite and a major human pathogen, invades eukaryotic host cells and replicates within a membrane-bound compartment (termed the vacuole or inclusion) in the cytoplasm of the host cell. In this report, we describe in detail the characteristics of the vacuole throughout the chlamydial life cycle in terms of the endocytic pathway, as determined by epifluorescent and confocal immunofluorescence microscopy. By indirect immunofluorescence, the transferrin receptor (TfR), a component of early endosomes, and the cation-independent mannose-6-phosphate receptor (CI-M6PR), a component of late endosomes, were found in close association with the chlamydial vacuole as early as 4 h postinfection (hpi) and as late as 20 hpi. Fluorescein isothiocyanate (FITC)-labeled Tf was also found to colocalize with the vacuole at 4, 12, and 20 hpi, indicating that exogenously added ligands can be transported to the region of the vacuole. Antibodies to several different lysosomal proteins failed to label the chlamydial vacuole at any time point during the life cycle. Indirect immunofluorescence of cells infected with chlamydiae stained with an antibody to the trans-Golgi network (TGN) protein TGN38 demonstrated that in infected cells, the integrity and structure of the TGN was altered. The rates of Tf recycling in infected and uninfected cells were compared by fluorescence microscopy and quantitated with 125I-Tf. While the rate of FITC-Tf recycling from endocytic compartments in chlamydia-infected cells did not appear different from that of uninfected cells, a small pool of FITC-Tf that had accumulated adjacent to the chlamydial vacuole recycled at a slower rate. Quantitation of Tf recycling with 125I-Tf showed that Tf was recycled more slowly in infected cells than in uninfected cells. The altered distribution of several endocytic pathway markers and the slowed Tf recycling are consistent with the hypothesis that the chlamydial vacuole interacts with the endocytic pathway of the host. These results furthermore suggest that the chlamydial vacuole does not correspond to a canonical endocytic compartment but that it is a unique and dynamic organelle that shares several characteristics with recycling endosomes of the host cell. Interactions with the early and/or late endosomal compartments, in addition to the Golgi apparatus, may provide a source of membrane or nutrients for the replicating organisms.

Annexins II, IV, V and VI relocate in response to rises in intracellular calcium in human foreskin fibroblasts

Annexins are a family of proteins implicated in a number of cellular processes involving calcium. We studied annexins I, II, IV, V and VI and found that they are all present in human foreskin fibroblasts and, from immunocytochemical studies, have distinct locations in the cell. Only annexin IV and annexin V have unstructured cytoplasmic staining patterns consistent with predominantly cytosolic locations. Annexin VI partially colocalizes with the endoplasmic reticulum. In contrast, annexins I and II are both associated with the plasma membrane with annexin II having a very homogeneous staining compared with the punctate pattern observed for annexin I. Annexins I, IV and V are all present in the nucleus at higher concentrations than in the cytoplasm. Treatment of cells with the calcium ionophore A23187 to raise intracellular calcium, results in relocations of annexin II, IV, V and VI. Intranuclear annexins IV and V relocate to the nuclear membrane whereas the cytosolic pools of these annexins relocate to the plasma membrane. Annexin II relocates to granular structures at the plasma membrane whereas annexin VI relocates to a more homogeneous distribution on the plasma membrane. These results are consistent with an important role for annexins in mediating the calcium signal at the plasma membrane and within the nuclei of fibroblasts.

Mobilization of annexin V during the uptake of DNP-albumin by human dendritic cells

Dendritic cells play a crucial role in antigen presentation in various tissues. The endocytic capacity of these cells has been regarded as minimal, but recent work on dendritic cells from mouse spleen has disclosed that the fluid-phase traffic through late endosomes is as active in dendritic cells as in other antigen-presenting cell types. We show that cultured human dendritic cells express the annexins I, III, IV, V and VI, as detected by immunofluorescence staining. The annexins are cytosolic Ca(2+)-dependent proteins with the ability to promote vesicle aggregation and membrane fusion through their capacity to bind to membrane phospholipids. Annexin I and VI appeared to outline the cytoskeleton and the plasma membrane in cultured human dendritic cells. Studies using confocal laser scanning microscopy showed that during the endocytosis of fluorescent dinitrophenyl-conjugated albumin by dendritic cells, there was a redistribution of annexin V which was found to colocalize with vesicles containing dinitrophenyl-FITC-conjugated albumin.

The varied lifestyles of intracellular pathogens within eukaryotic vacuolar compartments

Many bacterial pathogens and eukaryotic parasites can enter mammalian cells and live intracellularly inside membrane-bound vacuoles. The intravacuolar lifestyle of these pathogens plays a key role in pathogenesis. Understanding the molecular basis of the development of these specialized intracellular compartments is critical to understanding how these organisms cause disease.

Chlamydial envelope components and pathogen-host cell interactions

Few bacterial pathogens are as widespread in nature or as capable of eliciting such a diversity of disease syndromes as are the chlamydiae. As obligate intracellular organisms, they pose a special research challenge in defining the molecular components and mechanisms for productive growth within host cells and the overall progress of infection throughout host tissue. Although a comprehensive view of chlamydial envelope composition and respective functions in pathogenesis is far from complete, ongoing investigations continue to expose new and intriguing avenues for exploration.

Cytoskeletal requirements in Chlamydia trachomatis infection of host cells

Infection of genital epithelial cells by the closely related sexually transmitted pathogens Chlamydia trachomatis serovars E and L2 results in different clinical disease manifestations. Following entry into target host cells, individual vesicles containing chlamydiae fuse with one another to form one large inclusion. At the cellular level, the only obvious difference between these serovars is the time until inclusion maturation, which is 48 h for the invasive serovar L2 and 72 h for serovar E. To begin to define the intracellular events of these pathogens, the effect of cytoskeletal disruption on early endosome fusion and inclusion development in epithelial (HEC-1B) and fibroblast (McCoy) cells was analyzed by fluorescence microscopy. Disruption of microfilaments with cytochalasin D markedly reduced serovar E, but not serovar L2, infection of both cell lines. Conversely, microfilament as well as microtubule disruption, with colchicine or nocodazole, had no effect on serovar E inclusion development but resulted in the formation of multiple serovar L2 inclusions per cell during early and mid-development. Later in serovar L2 inclusion development (> 36 h postinfection), vesicles containing chlamydiae fused to form one large inclusion in the absence of an intact cytoskeleton. These results imply that (i) C. trachomatis serovar E may utilize a different pathway for uptake and development from serovar L2; (ii) these differences are consistent in both epithelial cells and fibroblasts; and (iii) the cytoskeleton plays a unique role in the infection of host cells by these two genital pathogens.

Chlamydia trachomatis serovar L2 induces protein tyrosine phosphorylation during uptake by HeLa cells

Chlamydia trachomatis L2 is an obligate intracellular microorganism with a unique biphasic life cycle. The extracellular form, the elementary body (EB), is infectious but metabolically inactive. Attachment of EBs to host cells is medicated by a heparan sulfate-like glycosaminoglycan. Following attachment, the EB is internalized within a membrane-bound vesicle, and during the first 8 h of infection the vesicles are transported to a perinuclear location where they aggregate and fuse. By use of a monoclonal antibody against phosphotyrosine, we showed that three classes of proteins are tyrosine phosphorylated: a triple band of 68, 66, and 64 kDa, a 97-kDa band, and a 140-kDa band. The phosphorylation could be detected by immunoblotting from 15 min after infection of HeLa cells. We followed the movement of the EBs and the tyrosine phosphorylation of proteins by double-labelling immunofluorescence microscopy with the same monoclonal anti-phosphotyrosine antibody and a polyclonal antibody against the C. trachomatis L2 outer membrane complex. During the first 8 h of infection, the phosphorylation colocalized with EBs. Sixteen hours after infection, EBs have reorganized to the replicating reticulate bodies, forming an inclusion. At this time, phosphorylation was seen as dotted spots in the periphery of the inclusion.