A Clathrin, Caveolae, and Dynamin-independent Endocytic Pathway Requiring Free Membrane Cholesterol Drives HIV-1 Internalization and Infection in Polarized Trophoblastic Cells

Impact of Caveolin-Mediated Endocytosis on the Trafficking of HIV within the Colonic Barrier

In the United States, most new cases of human immunodeficiency virus (HIV) belong to the at-risk group of gay and bisexual men. Developing therapies to reverse viral latency and prevent spread is paramount for the HIV cure agenda. In gay and bisexual men, a major, yet poorly characterized, route of HIV entry is via transport across the colonic epithelial barrier. While colonic tears and paracellular transport contribute to infection, we hypothesize that HIV entry through the colonic mucosa proceeds via a process known as transcytosis, involving (i) virion binding to the apical surface of the colonic epithelium, (ii) viral endocytosis, (iii) transport of virions across the cell, and (iv) HIV release from the basolateral membrane. Using Caco-2 colonic epithelial cells plated as a polarized monolayer in transwells, we characterized the mechanism of HIV transport. After exposing the monolayer to HIV apically, reverse transcription quantitative PCR (RT-qPCR) of the viral genome present in the basolateral chamber revealed that transport is dose dependent, cooperative, and inefficient, with released virus first detectable at 12 h. Inefficiency may be associated with >50% decline in detectable intracellular virus that correlates temporally with increased association of the virion with lysosomal-associated membrane protein 1 (LAMP-1+) endosomes. Microscopy revealed green fluorescent protein (GFP)-labeled HIV within the confines of the epithelial monolayer, with no virus detectable between cells, suggesting that viral transport is transcellular. Treatment of the monolayer with endocytosis inhibitors, cholesterol reducing agents, and small interfering RNA (siRNA) to caveolin showed that viral endocytosis is mediated by caveolin-coated endosomes contained in lipid rafts. These results indicate that HIV transport across the intestinal epithelial barrier via transcytosis is a viable mechanism for viral spread and a potential therapeutic target.

IMPORTANCE Despite the success of combination antiretroviral therapy in suppressing HIV replication and the emergence and effectiveness of PrEP-based prevention strategies, in 2018, 37,968 people in the United States received a new HIV diagnosis, accompanied by 15,820 deaths. While the annual number of new diagnoses decreased 7% from 2014 to 2018, 14% of people with HIV did not know they were infected. Gay and bisexual men accounted for 69% of all HIV diagnoses and 83% of diagnoses among males. Due to the scope of the HIV epidemic, determining and understanding precise routes of infection and the mechanisms of viral spread are paramount to ending the epidemic. Since transcellular transport of HIV across an intact colonic epithelial barrier is poorly understood, our overall goal is to characterize the molecular events involved in HIV transcytosis across the intestinal epithelial cell.

Membrane Rafts: Portals for Viral Entry

Membrane rafts are dynamic, small (10-200 nm) domains enriched with cholesterol and sphingolipids that compartmentalize cellular processes. Rafts participate in roles essential to the lifecycle of different viral families including virus entry, assembly and/or budding events. Rafts seem to participate in virus attachment and recruitment to the cell surface, as well as the endocytic and non-endocytic mechanisms some viruses use to enter host cells. In this review, we will introduce the specific role of rafts in viral entry and define cellular factors implied in the choice of one entry pathway over the others. Finally, we will summarize the most relevant information about raft participation in the entry process of enveloped and non-enveloped viruses.

Transcriptional response of vaginal epithelial cells to medroxyprogesterone acetate treatment results in decreased barrier integrity

Medroxyprogesterone acetate (MPA) is a frequently used hormonal contraceptive that has been shown to significantly increase HIV-1 susceptibility by approximately 40 %. However, the underlying mechanism by which this occurs remains unknown. Here, we examined the biological response to MPA by vaginal epithelial cells, the first cells to encounter HIV-1 during sexual transmission, in order to understand the potential mechanism(s) of MPA-mediated increase of HIV-1 infection. Using microarray analysis and in vitro assays, we characterized the response of vaginal epithelial cells, grown in biologically relevant air-liquid interface (ALI) cultures, to physiological levels of female sex hormones, estradiol (E2), progesterone (P4), or MPA. Transcriptional profiling of E2, P4 or MPA-treated vaginal epithelial cells indicated unique transcriptional profiles associated with each hormone. MPA treatment increased transcripts of genes related to cholesterol/sterol synthesis and decreased transcripts related to cell division and cell-cell adhesion, results not seen with E2 or P4 treatments. MPA treatment also resulted in unique gene expression indicative of decreased barrier integrity. Functional assays confirmed that MPA, but not E2 or P4 treatments, resulted in increased epithelial barrier permeability and inhibited cell cycle progression. The effects of MPA on vaginal epithelial cells seen in this study may help explain the increase of HIV-1 infection in women who use MPA as a hormonal contraceptive.

Potential for Virus Endogenization in Humans through Testicular Germ Cell Infection: the Case of HIV

Viruses have colonized the germ line of our ancestors on several occasions during evolution, leading to the integration in the human genome of viral sequences from over 30 retroviral groups and a few nonretroviruses. Among the recently emerged viruses infecting humans, several target the testis (e.g., human immunodeficiency virus [HIV], Zika virus, and Ebola virus). Here, we aimed to investigate whether human testicular germ cells (TGCs) can support integration by HIV, a contemporary retrovirus that started to spread in the human population during the last century. We report that albeit alternative receptors enabled HIV-1 binding to TGCs, HIV virions failed to infect TGCs in vitro Nevertheless, exposure of TGCs to infected lymphocytes, naturally present in the testis from HIV+ men, led to HIV-1 entry, integration, and early protein expression. Similarly, cell-associated infection or bypassing viral entry led to HIV-1 integration in a spermatogonial cell line. Using DNAscope, HIV-1 and simian immunodeficiency virus (SIV) DNA were detected within a few TGCs in the testis from one infected patient, one rhesus macaque, and one African green monkey in vivo Molecular landscape analysis revealed that early TGCs were enriched in HIV early cofactors up to integration and had overall low antiviral defenses compared with testicular macrophages and Sertoli cells. In conclusion, our study reveals that TGCs can support the entry and integration of HIV upon cell-associated infection. This could represent a way for this contemporary virus to integrate into our germ line and become endogenous in the future, as happened during human evolution for a number of viruses.IMPORTANCE Viruses have colonized the host germ line on many occasions during evolution to eventually become endogenous. Here, we aimed at investigating whether human testicular germ cells (TGCs) can support such viral invasion by studying HIV interactions with TGCs in vitro Our results indicate that isolated primary TGCs express alternative HIV-1 receptors, allowing virion binding but not entry. However, HIV-1 entered and integrated into TGCs upon cell-associated infection and produced low levels of viral proteins. In vivo, HIV-1 and SIV DNA was detected in a few TGCs. Molecular landscape analysis showed that TGCs have overall weak antiviral defenses. Altogether, our results indicate that human TGCs can support HIV-1 early replication, including integration, suggesting potential for endogenization in future generations.

HIV internalization into oral and genital epithelial cells by endocytosis and macropinocytosis leads to viral sequestration in the vesicles

Recently, we showed that HIV-1 is sequestered, i.e., trapped, in the intracellular vesicles of oral and genital epithelial cells. Here, we investigated the mechanisms of HIV-1 sequestration in vesicles of polarized tonsil, foreskin and cervical epithelial cells. HIV-1 internalization into epithelial cells is initiated by multiple entry pathways, including clathrin-, caveolin/lipid raft-associated endocytosis and macropinocytosis. Inhibition of HIV-1 attachment to galactosylceramide and heparan sulfate proteoglycans, and virus endocytosis and macropinocytosis reduced HIV-1 sequestration by 30-40%. T-cell immunoglobulin and mucin domain 1 (TIM-1) were expressed on the apical surface of polarized tonsil, cervical and foreskin epithelial cells. However, TIM-1-associated HIV-1 macropinocytosis and sequestration were detected mostly in tonsil epithelial cells. Sequestered HIV-1 was resistant to trypsin, pronase, and soluble CD4, indicating that the sequestered virus was intracellular. Inhibition of HIV-1 intraepithelial sequestration and elimination of vesicles containing virus in the mucosal epithelium may help in the prevention of HIV-1 mucosal transmission.

Medroxyprogesterone acetate-treated human, primary endometrial epithelial cells reveal unique gene expression signature linked to innate immunity and HIV-1 susceptibility

PROBLEM

Medroxyprogesterone acetate (MPA), a progestin-based hormonal contraceptive designed to mimic progesterone, has been linked to increased human immunodeficiency virus (HIV-1) susceptibility. Genital epithelial cells (GECs) form the mucosal lining of the female genital tract (FGT) and provide the first line of protection against HIV-1. The impact of endogenous sex hormones or MPA on the gene expression profile of GECs has not been comprehensively documented.

METHOD OF STUDY

Using microarray analysis, we characterized the transcriptional profile of primary endometrial epithelial cells grown in physiological levels of E2, P4, and MPA.

RESULTS

Each hormone treatment altered the gene expression profile of GECs in a unique manner. Interestingly, although MPA is a progestogen, the gene expression profile induced by it was distinct from P4. MPA increased gene expression of genes related to inflammation and cholesterol synthesis linked to innate immunity and HIV-1 susceptibility.

CONCLUSION

The analysis of gene expression profiles provides insights into the effects of sex hormones and MPA on GECs and allows us to posit possible mechanisms of the MPA-mediated increase in HIV-1 acquisition.

RNA-Associated Early-Stage Antiviral Factor Is a Major Component of Lv2 Restriction

Human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) replication in human cells is restricted at early postentry steps by host inhibitory factors. We previously described and characterized an early-phase restriction of HIV-1 and -2 replication in human cell lines, primary macrophages, and peripheral blood mononuclear cells. The restriction was termed lentiviral restriction 2 (Lv2). The viral determinants of Lv2 susceptibility mapped to the HIV-2 envelope (Env) and capsid (CA). We subsequently reported a whole-genome small interfering RNA screening for factors involved in HIV that identified RNA-associated early-stage antiviral factor (REAF). Using HIV-2 chimeras of susceptible and nonsusceptible viruses, we show here that REAF is a major component of the previously described Lv2 restriction. Further studies of the viral CA demonstrate that the CA mutation I73V (previously called I207V), a potent determinant for HIV-2, is a weak determinant of susceptibility for HIV-1. More potent CA determinants for HIV-1 REAF restriction were identified at P38A, N74D, G89V, and G94D. These results firmly establish that in HIV-1, CA is a strong determinant of susceptibility to Lv2/REAF. Similar to HIV-2, HIV-1 Env can rescue sensitive CAs from restriction. We conclude that REAF is a major component of the previously described Lv2 restriction.IMPORTANCE Measures taken by the host cell to combat infection drive the evolution of pathogens to counteract or sidestep them. The study of such virus-host conflicts can point to possible weaknesses in the arsenal of viruses and may lead to the rational design of antiviral agents. Here we describe our discovery that the host restriction factor REAF fulfills the same criteria previously used to describe lentiviral restriction (Lv2). We show that, like the HIV-2 CA, the CA of HIV-1 is a strong determinant of Lv2/REAF susceptibility. We illustrate how HIV counteracts Lv2/REAF by using an envelope with alternative routes of entry into cells.

The role of trophoblast cell receptor expression in HIV-1 passage across the placenta in pre-eclampsia: an observational study

OBJECTIVE

To compare expression of markers of HIV and associated receptors (p24, CD4, CCR5 and ICAM-2) in placentae and umbilical cords of HIV-associated and pre-eclamptic pregnancies to elucidate any association between these conditions in mother-to-child transmission.

DESIGN

Cross-sectional immunohistochemical analysis of target receptor expression.

SETTING

Laboratory-based study of primigravidae attending a district hospital in South Africa.

POPULATION OR SAMPLE

Retrospectively collected placental tissue (stratified into four groups according to HIV status of normotensive and pre-eclamptic participants (n = 20/group).

METHOD

Immunohistochemistry utilising CD4 (1:1), p24 (1:10), CCR5 (1:80) and ICAM-2 (1:100) antibodies was performed using light microscopy for image acquisition and analysis.

MAIN OUTCOME MEASURES

Evaluate the expression of receptors on syncytiotrophoblast involved in in utero transmission of HIV.

RESULTS

Syncytiotrophoblast was immunopositive for CD4 and CCR5 antibody with greater expression of CCR5 in HIV-positive versus HIV-negative groups (F_1,159  = 6.979, P = 0.009) and normotensive versus pre-eclamptic groups (F_1,159  = 8.803, P = 0.003). p24 was present in both placentae and umbilical cords of babies that were HIV-negative at 6 weeks. ICAM-2 immunostaining was observed in the syncytiotrophoblast across study groups and was significantly higher in the HIV-negative pre-eclamptic group (χ² (3)  = 45.3; P < 0.001).

CONCLUSION

Concurrent CD4 and CCR5 receptor expression demonstrates possible in utero viral entry routes across the placental barrier. ICAM-2 expression may influence HIV passage across the placenta or restoration of risk of pre-eclampsia in HAART-treated mothers. HIV was found in fetal circulation regardless of antiretroviral treatment. Further confirmatory ultrastructural and molecular work is warranted.

TWEETABLE ABSTRACT

CD4, CCR5 and ICAM-2 on syncytiotrophoblast may facilitate HIV infection of passage across the placenta.

Cholesterol regulates multiple forms of vesicle endocytosis at a mammalian central synapse

Endocytosis in synapses sustains neurotransmission by recycling vesicle membrane and maintaining the homeostasis of synaptic membrane. A role of membrane cholesterol in synaptic endocytosis remains controversial because of conflicting observations, technical limitations in previous studies, and potential interference from non-specific effects after cholesterol manipulation. Furthermore, it remains unclear whether cholesterol participates in distinct forms of endocytosis that function under different activity levels. In this study, applying the whole-cell membrane capacitance measurement to monitor endocytosis in real time at the rat calyx of Held terminals, we found that disrupting cholesterol with dialysis of cholesterol oxidase or methyl-β-cyclodextrin impaired three different forms of endocytosis, including slow endocytosis, rapid endocytosis, and endocytosis of the retrievable membrane that exists at the surface before stimulation. The effects were observed when disruption of cholesterol was mild enough not to change Ca(2+) channel current or vesicle exocytosis, indicative of stringent cholesterol requirement in synaptic endocytosis. Extracting cholesterol with high concentrations of methyl-β-cyclodextrin reduced exocytosis, mainly by decreasing the readily releasable pool and the vesicle replenishment after readily releasable pool depletion. Our study suggests that cholesterol is an important, universal regulator in multiple forms of vesicle endocytosis at mammalian central synapses.

Distinct requirements for HIV-cell fusion and HIV-mediated cell-cell fusion

Whether HIV-1 enters cells by fusing with the plasma membrane or with endosomes is a subject of active debate. The ability of HIV-1 to mediate fusion between adjacent cells, a process referred to as "fusion-from-without" (FFWO), shows that this virus can fuse with the plasma membrane. To compare FFWO occurring at the cell surface with HIV-cell fusion through a conventional entry route, we designed an experimental approach that enabled the measurements of both processes in the same sample. The following key differences were observed. First, a very small fraction of viruses fusing with target cells participated in FFWO. Second, whereas HIV-1 fusion with adherent cells was insensitive to actin inhibitors, post-CD4/coreceptor binding steps during FFWO were abrogated. A partial dependence of HIV-cell fusion on actin remodeling was observed in CD4(+) T cells, but this effect appeared to be due to the actin dependence of virus uptake. Third, deletion of the cytoplasmic tail of HIV-1 gp41 dramatically enhanced the ability of the virus to promote FFWO, while having a modest effect on virus-cell fusion. Distinct efficiencies and actin dependences of FFWO versus HIV-cell fusion are consistent with the notion that, except for a minor fraction of particles that mediate fusion between the plasma membranes of adjacent cells, HIV-1 enters through an endocytic pathway. We surmise, however, that cell-cell contacts enabling HIV-1 fusion with the plasma membrane could be favored at the sites of high density of target cells, such as lymph nodes.

Differentially expressed microRNAs and affected signaling pathways in placentae of transgenic cloned cattle

Placental deficiencies are related to the developmental abnormalities of transgenic cattle produced by somatic cell nuclear transfer, but the concrete molecular mechanism is not very clear. Studies have shown that placental development can be regulated by microRNAs (miRNAs) in normal pregnancy. Thus, this study screened differentially expressed miRNAs by the next-generation sequencing technology to reveal the relationship between miRNAs expression and aberrant development of placentae produced by the transgenic-clone technology. Expressions of miRNAs and mRNAs in different placentae were compared, the placentae derived from one natural pregnancy counterpart (PNC), one natural pregnancy of a cloned offspring as a mother (PCM), and two transgenic (human beta-defensin-3) cloned pregnancy: one offspring was alive after birth (POL) and the other offspring was dead in 2 days after birth (POD). Further, signaling pathway analysis was conducted. The results indicated that 694 miRNAs were differentially expressed in four placental samples, such as miR-210, miR-155, miR-21, miR-128, miR-183, and miR-145. Signaling pathway analysis revealed that compared with PNC, significantly upregulated pathways in POL, POD, and PCM mainly included focal adhesion, extracellular matrix-receptor interaction, pathways in cancer, regulation of actin cytoskeleton, endosytosis, and adherens junction, and significantly downregulated pathways mainly included malaria, nucleotide binding oligomerization domain-like receptor signaling, cytokine-cytokine receptor interaction, Jak-STAT signaling pathway. In conclusion, this study confirmed alterations of the expression profile of miRNAs and signaling pathways in placentae from transgenic (hBD-3) cloned cattle (PTCC), which could lead to the morphologic and histologic deficiencies of PTCC. This information would be useful for the relative research in future.

Sterol dynamics during endocytic trafficking in Arabidopsis

Sterols are lipids found in membranes of eukaryotic cells. Functions of sterols have been demonstrated for various cellular processes including endocytic trafficking in animal, fungal, and plant cells. The ability to visualize sterols at the subcellular level is crucial to understand sterol distribution and function during endocytic trafficking. In plant cells, the polyene antibiotic filipin is the most extensively used tool for the specific detection of fluorescently labeled 3-β-hydroxysterols in situ. Filipin can to some extent be used to track sterol internalization in live cells, but this application is limited, due to the inhibitory effects filipin exerts on sterol-dependent endocytosis. Nevertheless, filipin-sterol labeling can be performed on aldehyde-fixed cells which allows for sterol detection in endocytic compartments. This approach can combine studies correlating sterol distribution with experimental manipulations of endocytic trafficking pathways. Here, we describe step-by-step protocols and troubleshooting for procedures on live and fixed cells to visualize sterols during endocytic trafficking. We also provide a detailed discussion of advantages and limitations of both methods. Moreover, we illustrate the use of the endocytic recycling inhibitor brefeldin A and a genetically modified version of one of its target molecules for studying endocytic sterol trafficking.

Massive endocytosis triggered by surface membrane palmitoylation under mitochondrial control in BHK fibroblasts

Large Ca transients cause massive endocytosis (MEND) in BHK fibroblasts by nonclassical mechanisms. We present evidence that MEND depends on mitochondrial permeability transition pore (PTP) openings, followed by coenzyme A (CoA) release, acyl CoA synthesis, and membrane protein palmitoylation. MEND is blocked by inhibiting mitochondrial Ca uptake or PTP openings, depleting fatty acids, blocking acyl CoA synthesis, metabolizing CoA, or inhibiting palmitoylation. It is triggered by depolarizing mitochondria or promoting PTP openings. After mitochondrial MEND blockade, MEND is restored by cytoplasmic acyl CoA or CoA. MEND is blocked by siRNA knockdown of the plasmalemmal acyl transferase, DHHC5. When acyl CoA is abundant, transient H2O2 oxidative stress or PKC activation initiates MEND, but the immediate presence of H2O2 prevents MEND. The PTP inhibitor, NIM811, significantly increases plasmalemma in normally growing cells. Thus, the MEND pathway may contribute to constitutive as well as pathological plasmalemma turnover in dependence on mitochondrial stress signaling. DOI: http://dx.doi.org/10.7554/eLife.01293.001.

HIV entry: a game of hide-and-fuse?

Human Immunodeficiency Virus (HIV) initiates infection by fusing its envelope membrane with the cell membrane through a process which is triggered through interactions with the cellular receptor and coreceptor. Although the mechanism of HIV fusion has been extensively studied, the point of its entry into cells remains controversial. HIV has long been thought to fuse directly with the cell plasma membrane. However, several lines of evidence suggest that endocytic entry of HIV can lead to infection and, moreover, that endocytosis could be the predominant HIV entry pathway into different cell types. This review discusses recent findings pertinent to HIV entry routes and novel approaches to pinpoint the sites of virus entry.

IL-10 produced by trophoblast cells inhibits phagosome maturation leading to profound intracellular proliferation of Salmonella enterica Typhimurium

INTRODUCTION

Salmonella enterica Typhimurium (ST) is a phagosomal pathogen that can infect placental trophoblast cells leading to abortion and severe maternal illness. It is unclear how the trophoblast cells promote profound bacterial proliferation.

METHODS

The mechanism of internalization, intracellular growth and phagosomal biogenesis in ST-infected human epithelial (HeLa), macrophage (THP-1) and trophoblast-derived cell lines (JEG-3, BeWo and HTR-8) was studied. Specific inhibitors were used to block bacterial internalization. Phagosomal maturation was determined by confocal microscopy, Western-blotting and release of lysosomal β-galactosidase by infected cells. Bacterial colony forming units were determined by plating infected cell lysates on agar plates.

RESULTS

ST proliferated minimally in macrophages but replicated profoundly within trophoblast cells. The ST-ΔinvA (a mutant of Salmonella pathogenicity island-1 gene effector proteins) was unable to infect epithelial cells, but was internalized by scavenger receptors on trophoblasts and macrophages. However, ST was contrastingly localized in early (Rab5⁺) or late (LAMP1⁺) phagosomes within trophoblast cells and macrophages respectively. Furthermore trophoblast cells (unlike macrophages) did not exhibit phagoso-lysosomal fusion. ST-infected macrophages produced IL-6 whereas trophoblast cells produced IL-10. Neutralizing IL-10 in JEG-3 cells accelerated phagolysomal fusion and reduced proliferation of ST. Placental bacterial burden was curtailed in vivo in anti-IL-10 antibody treated and IL-10-deficient mice.

DISCUSSION

Macrophages phagocytose but curtail intracellular replication of ST in late phagosomes. In contrast, phagocytosis by trophoblast cells results in an inappropriate cytokine response and proliferation of ST in early phagosomes.

CONCLUSION

IL-10 production by trophoblast cells that delays phagosomal maturation may facilitate proliferation of pathogens in placental cells.

Lipid raft- and SRC family kinase-dependent entry of coxsackievirus B into human placental trophoblasts

Maternal-fetal transmission of group B coxsackieviruses (CVB) during pregnancy has been associated with a number of diverse pathological outcomes, including hydrops fetalis, fetal myocarditis, meningoencephalitis, neurodevelopmental delays, congenital skin lesions, miscarriage, and/or stillbirth. Throughout pregnancy, the placenta forms a critical antimicrobial protective barrier at the maternal-fetal interface. Despite the severity of diseases accompanying fetal CVB infections, little is known regarding the strategies used by CVB to gain entry into placental trophoblasts. Here we used both a trophoblast cell line and primary human trophoblasts to demonstrate the mechanism by which CVB gains entry into polarized placental trophoblasts. Our studies revealed that the kinetics of CVB entry into placental trophoblasts are similar to those previously described for polarized intestinal epithelial cells. Likewise, CVB entry into placental trophoblasts requires decay-accelerating factor (DAF) binding and involves relocalization of the virus from the apical surface to intercellular tight junctions. In contrast, we have identified a divergent mechanism for CVB entry into polarized trophoblasts that is clathrin, caveolin-1, and dynamin II independent but requires intact lipid rafts. In addition, we found that members of the Src family of tyrosine kinases were required for CVB entry. Our studies highlight the complexity of viral entry into human placental trophoblasts and may serve as a model for mechanisms used by diverse pathogens to penetrate the placental barrier.

Retrovirus entry by endocytosis and cathepsin proteases

Retroviruses include infectious agents inducing severe diseases in humans and animals. In addition, retroviruses are widely used as tools to transfer genes of interest to target cells. Understanding the entry mechanism of retroviruses contributes to developments of novel therapeutic approaches against retrovirus-induced diseases and efficient exploitation of retroviral vectors. Entry of enveloped viruses into host cell cytoplasm is achieved by fusion between the viral envelope and host cell membranes at either the cell surface or intracellular vesicles. Many animal retroviruses enter host cells through endosomes and require endosome acidification. Ecotropic murine leukemia virus entry requires cathepsin proteases activated by the endosome acidification. CD4-dependent human immunodeficiency virus (HIV) infection is thought to occur via endosomes, but endosome acidification is not necessary for the entry whereas entry of CD4-independent HIVs, which are thought to be prototypes of CD4-dependent viruses, is low pH dependent. There are several controversial results on the retroviral entry pathways. Because endocytosis and endosome acidification are complicatedly controlled by cellular mechanisms, the retrovirus entry pathways may be different in different cell lines.

Differential HIV-1 endocytosis and susceptibility to virus infection in human macrophages correlate with cell activation status

HIV-1 is an enveloped virus that enters target cells by fusion either directly at the plasma membrane or at the endosomal membrane. The latter mechanism follows a rapid engulfment of HIV-1 after its receptor engagement at the cell surface, and its scale depends on cellular endocytosis/degradation rates and virus fusion kinetics. HIV-1 has recently been shown to exploit a novel Pak1-dependent macropinocytosis mechanism as a way to productively infect macrophages. However, macrophages are highly heterogeneous cells that can adapt functionally to their changing environment, and their endosomal/lysosomal pathway is highly regulated upon cell activation. These changes might impact the ability of HIV-1 to exploit endocytosis as a way to productively infect macrophages. In this study, we compared HIV-1 endocytosis/degradation rates in nonactivated, M1-activated, and M2a-activated monocyte-derived macrophages (MDMs). We found that the rate of HIV-1 endocytosis was increased in M1-activated but decreased in M2a-activated MDMs. However, both M1 and M2a activations of MDMs led specifically to a greater clathrin-mediated endocytosis of HIV-1, which was independent of CD4 and CCR5 binding. Furthermore, clathrin-mediated endocytosis is unlikely to result in productive HIV-1 infection, given that it leads to increased viral degradation. Therefore, we suggest that viral fusion following endocytosis is restricted in activated macrophages.

Endocytosis at the nanoscale

Endocytosis is a fundamental process in which eukaryotic cells internalise molecules and macromolecules via deformation of the membrane and generation of membrane-bound carriers. Functional aspects are not only limited to uptake of nutrients, but also play a primary role in evolutionary conserved processes such as the regulation of plasma membrane protein activity (i.e. signal-transducing receptors, small-molecule transporters and ion channels), cell motility and mitosis. The macromolecular nature of the material transported by endocytosis makes this route one of the most important targets for nanomedicine. Indeed, many nanoparticle formulations have been customised to enter cells through endocytosis and deliver the cargo within the cell. In this critical review, we present an overview of the biology of endocytosis and discuss its implications in cell internalisation of nanoparticles. We discuss how nanoparticle size, shape and surface chemistry can control this process effectively. Finally, we discuss different drug delivery strategies on how to evade lysosomal degradation to promote effective release of the cargo (376 references).

Barriers of the Human Organism and Their Achilles’ Heels

The human body is covered by barriers separating it from the external and internal surroundings. The “milieu enterieur” has to be stabilised in spite of the variable external and internal conditions of toxic, osmotic, microbial and climatic environmental circumstances. This first line of barriers is composed of skin and mucous membranes of complicated structures.

A second line of barrier system is present in our organisms. Certain organs have to be separated from the immune system and other parts of the body because of evolutionary reasons (eye-bulb and testicles) because of unique proteins “unknown” for the acquired immune system. The blood-brain barrier (BBB) is providing enhanced safety circumstances for the central nervous system. The second line of barriers is represented by the special properties of the capillary endothelial system.

The maternal-fetal barrier is the most complex. At the maternal fetal interface two individuals of two different haplotypes has to be live 9 months separated by a very complicated dynamic barrier.

The placenta is the organ, which is separating the maternal and fetal tissues. Similar to others the bidirectional transport of gasses, metabolites, cells, proteins, regulatory substances, are transported by active or passive transcellular and intercellular mechanisms.

The fetal immune system develops immunotolerance to all maternal cells and antigens transferred transplacentally. The problem is to mitigate the maternal immune system to tolerate the paternal haplotype of the fetus. In the case of normal pregnancy a complex series of physiological modifications can solve the problem without harmful consequences to the mother and fetus.

The outermost contact cells of trophoblasts express instead of HLA-class Ia and class II antigens non-variable HLA-C, HLA-E, HLA-F and HLA-G antigens. The first consequence of this is reduction of the activity of maternal natural killer cells and maternal dendritic cells;

Progesteron, micro-RNA and mediators influence the development of T effector-cells. The production of soluble HLA-G(5 and 6) and IL-10 supports the differentiation of Th-2 CD4⁺ helper cells, reducing the ability of maternal cells to kill fetal cells.

Series of receptors and costimulators are expressed by the different lines of semi-allogenic trophoblast cells to bind HLA-G and mitigate maternal immune response;

The maternal immunotolerance is further facilitated by the activation of CD4⁺CD25^brightFoxp3⁺ regulatory T (T_REG) cells.

Infections have to be prevented during pregnancy. The cells of placenta express 10 Toll-like receptors a group of pattern recognition receptors responsible for innate immunity. The interferon level is also higher in the placental tissues than in the somatic fetal or maternal cells.

The complement system is also adapted to the requirements of the pregnancy and fetal damage is inhibited by the production of “assymmetric IgG antibodies” under hormonal and placental-regulation. These modifications prevent the activation of complement, cytotoxic activity, opsonising ability, antigen clearance and precipitating activity of the molecules.

The Achilles’ heels of the different barriers are regularly found by virus infections. Lamina cribrosa of the blood-brain barrier, optical nerve of the eyes, etc. the risk factors of the maternal-fetal barrier has been summarised in Table 1.1.

Francisella tularensis uses cholesterol and clathrin-based endocytic mechanisms to invade hepatocytes

Francisella tularensis are highly infectious microbes that cause the disease tularemia. Although much of the bacterial burden is carried in non-phagocytic cells, the strategies these pathogens use to invade these cells remains elusive. To examine these mechanisms we developed two in vitro Francisella-based infection models that recapitulate the non-phagocytic cell infections seen in livers of infected mice. Using these models we found that Francisella novicida exploit clathrin and cholesterol dependent mechanisms to gain entry into hepatocytes. We also found that the clathrin accessory proteins AP-2 and Eps15 co-localized with invading Francisella novicida as well as the Francisella Live Vaccine Strain (LVS) during hepatocyte infections. Interestingly, caveolin, a protein involved in the invasion of Francisella in phagocytic cells, was not required for non-phagocytic cell infections. These results demonstrate a novel endocytic mechanism adopted by Francisella and highlight the divergence in strategies these pathogens utilize between non-phagocytic and phagocytic cell invasion.

Clathrin-mediated endocytic proteins are upregulated in the cortex of the Tg2576 mouse model of Alzheimer's disease-like amyloid pathology

Amyloid-β (Aβ) is cleaved from amyloid precursor protein (APP) predominantly after APP has trafficked through the secretory pathway and then become re-internalised by endocytosis. Clathrin-mediated and, more recently, clathrin-independent endocytosis have both been implicated in this process. Furthermore, endocytic abnormalities have been identified in cases of Alzheimer's disease (AD), however, the relevance of these changes to the aetiology of the disease remains unclear. We therefore examined the expression of proteins related to these endocytic processes in the cortex of Tg2576 mice that overexpress the Swedish mutation in APP, and consequently overexpress Aβ, to determine if there were any changes in their associated pathways. We identified significant increases in the levels of clathrin, dynamin and PICALM, all proteins intimately involved with the clathrin-mediated endocytic pathway, in the transgenic animals. However, levels of proteins associated with flotillin or caveolin-mediated endocytic pathways remained unchanged. These results emphasise the importance of clathrin-mediated endocytosis in the aetiology of AD and reinforce the results of the recent GWAS studies that identified genes for clathrin-mediated endocytosis as susceptibility genes for AD. Such studies in transgenic mice will allow us to learn more about the role of clathrin-mediated endocytosis in AD.

Engineered lentiviral vectors pseudotyped with a CD4 receptor and a fusogenic protein can target cells expressing HIV-1 envelope proteins

Lentiviral vectors (LVs) derived from human immunodeficiency virus type 1 (HIV-1) are promising vehicles for gene delivery because they not only efficiently transduce both dividing and non-dividing cells, but also maintain long-term transgene expression. Development of an LV system capable of transducing cells in a cell type-specific manner can be beneficial for certain applications that rely on targeted gene delivery. Previously it was shown that an inverse fusion strategy that incorporated an HIV-1 receptor (CD4) and its co-receptor (CXCR4 or CCR5) onto vector surfaces could confer to LVs the ability to selectively deliver genes to HIV-1 envelope-expressing cells. To build upon this work, we aim to improve its relatively low transduction efficiency and circumvent its inability to target multiple tropisms of HIV-1 by a single vector. We investigated a method to create LVs co-enveloped with the HIV-1 cellular receptor CD4 and a fusogenic protein derived from the Sindbis virus glycoprotein and tested its efficiency to selectively deliver genes into cells expressing HIV-1 envelope proteins. The engineered LV system yields a higher level of transduction efficiency and a broader tropism towards cells displaying the HIV-1 envelope protein (Env) than the previously developed system. Furthermore, we demonstrated in vitro that this engineered LV can preferentially deliver suicide gene therapy to HIV-1 envelope-expressing cells. We conclude that it is potentially feasible to target LVs towards HIV-1-infected cells by functional co-incorporation of the CD4 and fusogenic proteins, and provide preliminary evidence for further investigation on a potential alternative treatment for eradicating HIV-1-infected cells that produce drug-resistant viruses after highly active antiretroviral therapy (HAART).

Viral infection: Moving through complex and dynamic cell-membrane structures

Viruses have developed different survival strategies in host cells by crossing cell-membrane compartments, during different steps of their viral life cycle. In fact, the non-regenerative viral membrane of enveloped viruses needs to encounter the dynamic cell-host membrane, during early steps of the infection process, in which both membranes fuse, either at cell-surface or in an endocytic compartment, to promote viral entry and infection. Once inside the cell, many viruses accomplish their replication process through exploiting or modulating membrane traffic, and generating specialized compartments to assure viral replication, viral budding and spreading, which also serve to evade the immune responses against the pathogen. In this review, we have attempted to present some data that highlight the importance of membrane dynamics during viral entry and replicative processes, in order to understand how viruses use and move through different complex and dynamic cell-membrane structures and how they use them to persist.

Involvement of caveolin-1 in the Jak-Stat signaling pathway and infectious spleen and kidney necrosis virus infection in mandarin fish (Siniperca chuatsi)

Caveolae, the major source of caveolin-1 protein, are specialized invaginated microdomains of the plasma membrane that act as organizing centers for signaling molecules in the immune system. In the present study, we report the cloning and characterization of caveolin-1 (mCav-1) from mandarin fish (Siniperca chuatsi) and study on the roles of mCav-1 in the fish Jak–Stat signaling pathway and in virus infection. The cDNA sequence of mCav-1 was 707 bp in size, encoding a protein of 181 amino acids, which was different from the mammalian protein (178 amino acids). The deduced amino acid sequence of mCav-1 shared similar architecture with vertebrate caveolin-1 proteins, but mCav-1 lacked a phosphorylation site (y14). The major subcellular location of mCav-1 was in the caveolae, where the protein appeared to have major functions. Real-time PCR revealed that the expression of the mandarin fish Mx, IRF-1, SOCS1, and SOCS3 genes involved in the poly(I:C)-induced Jak–Stat signaling pathway was impaired by the mCav-1 scaffolding domain peptide (mSDP). In mandarin fish fry (MFF-1) cells, the protein levels of mCav-1 were markedly up-regulated at 12 and 24 h post-infection with ISKNV (infectious spleen and kidney necrosis virus). In addition, ISKNV entry into MFF-1 cells was significantly inhibited by mSDP, and the inhibition was dose-dependent. Thus, ISKNV infection was apparently associated with mCav-1 protein and may utilize the caveolae-related endocytosis pathway. The findings reported here further our understanding of the function of caveolin-1 in the complex signal transduction network in fish immune systems and in the cellular entry mechanism of iridoviruses.

Entry of tiger frog virus (an Iridovirus) into HepG2 cells via a pH-dependent, atypical, caveola-mediated endocytosis pathway

Tiger frog virus (TFV), in the genus Ranavirus of the family Iridoviridae, causes high mortality of cultured tiger frog tadpoles in China. To explore the cellular entry mechanism of TFV, HepG2 cells were treated with drugs that inhibit the main endocytic pathways. We observed that TFV entry was inhibited by NH(4)Cl, chloroquine, and bafilomycin, which can all elevate the pH of acidic organelles. In contrast, TFV entry was not influenced by chlorpromazine or overexpression of a dominant-negative form of Esp15, which inhibit the assembly of clathrin-coated pits. These results suggested that TFV entry was not associated with clathrin-mediated endocytosis, but was related to the pH of acidic organelles. Subsequently, we found that endocytosis of TFV was dependent on membrane cholesterol and was inhibited by the caveolin-1 scaffolding domain peptide. Dynamin and actin were also required for TFV entry. In addition, TFV virions colocalized with the cholera toxin subunit B, indicating that TFV enters as caveola-internalized cargo into the Golgi complex. Taken together, our results demonstrated that TFV entry occurs by caveola-mediated endocytosis with a pH-dependent step. This atypical caveola-mediated endocytosis is different from the clathrin-mediated endocytosis of frog virus 3 (FV3) by BHK cells, which has been recognized as a model for iridoviruses. Thus, our work may help further the understanding of the initial steps of iridovirus infection in lower vertebrates.

CD4-independent human immunodeficiency virus infection involves participation of endocytosis and cathepsin B

During a comparison of the infectivity of mNDK, a CD4-independent human immunodeficiency virus type 1 (HIV-1) strain, to various cell lines, we found that HeLa cells were much less susceptible than 293T and TE671 cells. Hybridoma cells between HeLa and 293T cells were as susceptible as 293T cells, suggesting that cellular factors enhance the mNDK infection in 293T cells. By screening a cDNA expression library in HeLa cells, cystatin C was isolated as an enhancer of the mNDK infection. Because cathepsin B protease, a natural ligand of cystatin C, was upregulated in HeLa cells, we speculated that the high levels of cathepsin B activities were inhibitory to the CD4-independent infection and that cystatin C enhanced the infection by impairing the excessive cathepsin B activity. Consistent with this idea, pretreatment of HeLa cells with 125 µM of CA-074Me, a cathepsin B inhibitor, resulted in an 8-fold enhancement of the mNDK infectivity. Because cathepsin B is activated by low pH in acidic endosomes, we further examined the potential roles of endosomes in the CD4-independent infection. Suppression of endosome acidification or endocytosis by inhibitors or by an Eps15 dominant negative mutant reduced the infectivity of mNDK in which CD4-dependent infections were not significantly impaired. Taken together, these results suggest that endocytosis, endosomal acidification, and cathepsin B activity are involved in the CD4-independent entry of HIV-1.

HIV-1 requires Arf6-mediated membrane dynamics to efficiently enter and infect T lymphocytes

As the initial barrier to viral entry, the plasma membrane along with the membrane trafficking machinery and cytoskeleton are of fundamental importance in the viral cycle. However, little is known about the contribution of plasma membrane dynamics during early human immunodeficiency virus type 1 (HIV-1) infection. Considering that ADP ribosylation factor 6 (Arf6) regulates cellular invasion via several microorganisms by coordinating membrane trafficking, our aim was to study the function of Arf6-mediated membrane dynamics on HIV-1 entry and infection of T lymphocytes. We observed that an alteration of the Arf6-guanosine 5'-diphosphate/guanosine 5'-triphosphate (GTP/GDP) cycle, by GDP-bound or GTP-bound inactive mutants or by specific Arf6 silencing, inhibited HIV-1 envelope-induced membrane fusion, entry, and infection of T lymphocytes and permissive cells, regardless of viral tropism. Furthermore, cell-to-cell HIV-1 transmission of primary human CD4(+) T lymphocytes was inhibited by Arf6 knockdown. Total internal reflection fluorescence microscopy showed that Arf6 mutants provoked the accumulation of phosphatidylinositol-(4,5)-biphosphate-associated structures on the plasma membrane of permissive cells, without affecting CD4-viral attachment but impeding CD4-dependent HIV-1 entry. Arf6 silencing or its mutants did not affect fusion, entry, and infection of vesicular stomatitis virus G-pseudotyped viruses or ligand-induced CXCR4 or CCR5 endocytosis, both clathrin-dependent processes. Therefore we propose that efficient early HIV-1 infection of CD4(+) T lymphocytes requires Arf6-coordinated plasma membrane dynamics that promote viral fusion and entry.

PI3KC2α, a class II PI3K, is required for dynamin-independent internalization pathways

Increasing evidence indicates that cellular uptake of several molecules can occur independently of functional dynamin, but the molecular players that regulate dynamin-independent endocytosis and the subsequent trafficking steps are still largely unknown. A survival-based short-hairpin (sh) RNA screen using a cell line expressing a diphtheria toxin receptor (DTR, officially known as HBEGF) anchored to GPI (DTR–GPI), which internalizes diphtheria toxin (DT, officially known as DTX) in a dynamin-independent manner, identified PI3KC2α, a class II phosphoinositide 3-kinase (PI3K), as a specific regulator of dynamin-independent DT internalization. We found that the internalization of several proteins that enter the cell through dynamin-independent pathways led to a relocalization of PI3KC2α to cargo-positive vesicles. Furthermore, downregulation of PI3KC2α impaired internalization of CD59 as well as fluid-phase endocytosis. Our data suggest a general role for PI3KC2α in regulating physiologically relevant dynamin-independent internalization pathways by recruiting early endosome antigen 1 (EEA1) to vesicular compartments, a step required for the intracellular trafficking of vesicles generated by dynamin-independent endocytic pathways.

Non-productive HIV-1 infection of human glomerular and urinary podocytes

Podocyte damage induced by HIV-1 is critical to the pathogenesis of HIV-1 associated nephropathy (HIVAN) and is believed to result from productive replication of the virus. Here we demonstrate that HIV-1 readily enters human podocytes by a dynamin-mediated endocytosis but does not establish productive infection. We provide evidence suggesting that viral nucleic acids and proteins detected in podocytes are delivered by viral particles internalized by the cells. Endocytosed HIV-1 is only transiently harbored by podocytes and is subsequently released to the extracellular milieu as fully infectious virus. Similarly, primary podocytes established from normal human urine do not support productive infection by HIV-1 but sustain replication of VSV-G pseudotyped virus that bypasses HIV-1 entry receptors. Moreover, transfected podocytes expressing CD4 and CXCR4 receptors support productive replication of HIV-1. This further confirms that lack of HIV-1 entry receptors is the major barrier preventing productive infection of podocytes in vitro.

DC-specific ICAM-3-grabbing nonintegrin mediates internalization of HIV-1 into human podocytes

Human immunodeficiency virus (HIV)-1 has been demonstrated to contribute to the pathogenesis of HIV-associated nephropathy. In renal biopsy studies, podocytes have been reported to be infected by HIV-1. However, the mechanism involved in HIV-1 internalization into podocytes is not clear. In the present study, we evaluated the occurrence of HIV-1 internalization into conditionally immortalized human podocytes and the mechanism involved. Human podocytes rapidly internalized R5 and X4 HIV-1 primary strains via an endocytosis-dependent pathway, without establishing a productive infection. The HIV-1 internalization was dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN) receptor mediated. The role of DC-SIGN was confirmed by using specific blocking antibodies and transfection with small interfering (si) RNA/DC-SIGN. Since podocyte HIV-1 trafficking was not altered by pH-modulating agents, it appeared that HIV-1 routing occurred through nonacid vesicular compartments. Interestingly, transfection of podocytes with neither siRNA/caveolin-1 nor siRNA/clathrin heavy chain inhibited podocyte viral accumulation. Thus it appears that clathrin-coated vesicles and caveosomes may not be contributing to HIV-1-associated membrane traffic.

Low cholesterol? Don't brag yet ... hypocholesterolemia blunts HAART effectiveness: a longitudinal study

BACKGROUND

In vitro studies suggest that reducing cholesterol inhibits HIV replication. However, this effect may not hold in vivo, where other factors, such as cholesterol's immunomodulatory properties, may interact.

METHODS

Fasting blood samples were obtained on 165 people living with HIV at baseline and after 24 weeks on highly active antiretroviral therapy (HAART). Participants were classified as hypocholesterolemic (HypoCHL; <150 mg/dl) or non-HypoCHL (>150 mg/dl) and were compared on viro-immune outcomes.

RESULTS

At baseline, participants with HypoCHL (40%) exhibited lower CD4 (197 +/- 181 vs. 295 +/- 191 cells/mm3, p = 0.02) and CD8 (823 +/- 448 vs. 1194 +/- 598 cells/mm3, p = 0.001) counts and were more likely to have detectable viral loads (OR = 3.5, p = 0.01) than non-HypoCHL controls. After HAART, participants with HypoCHL were twice as likely to experience a virological failure >400 copies (95% CI 1-2.6, p = 0.05) and to exhibit <200 CD4 (95% CI 1.03-2.9, p = 0.04) compared with non-HypoCHL. Low thymic output was related to poorer CD4 cell response in HypoCHL subjects. Analyses suggest a dose-response relationship with every increase of 50 mg/dl in cholesterol related to a parallel rise of 50 CD4 cells.

CONCLUSIONS

The study implicates, for the first time, HypoCHL with impaired HAART effectiveness, including limited CD4 repletion by the thymus and suboptimal viral clearance.

A clathrin independent macropinocytosis-like entry mechanism used by bluetongue virus-1 during infection of BHK cells

Acid dependent infection of Hela and Vero cells by BTV-10 occurs from within early-endosomes following virus uptake by clathrin-mediated endocytosis (Forzan et al., 2007: J Virol 81: 4819–4827). Here we report that BTV-1 infection of BHK cells is also dependent on a low endosomal pH; however, virus entry and infection were not inhibited by dominant-negative mutants of Eps15, AP180 or the ‘aa’ splice variant of dynamin-2, which were shown to inhibit clathrin-mediated endocytosis. In addition, infection was not inhibited by depletion of cellular cholesterol, which suggests that virus entry is not mediated by a lipid-raft dependent process such as caveolae-mediated endocytosis. Although virus entry and infection were not inhibited by the dominant-negative dynamin-2 mutant, entry was inhibited by the general dynamin inhibitor, dynasore, indicating that virus entry is dynamin dependent. During entry, BTV-1 co-localised with LAMP-1 but not with transferrin, suggesting that virus is delivered to late-endosomal compartments without first passing through early-endosomes. BTV-1 entry and infection were inhibited by EIPA and cytochalasin-D, known macropinocytosis inhibitors, and during entry virus co-localised with dextran, a known marker for macropinocytosis/fluid-phase uptake. Our results extend earlier observations with BTV-10, and show that BTV-1 can infect BHK cells via an entry mechanism that is clathrin and cholesterol-independent, but requires dynamin, and shares certain characteristics in common with macropinocytosis.

Celiac anti-tissue transglutaminase antibodies interfere with the uptake of alpha gliadin peptide 31-43 but not of peptide 57-68 by epithelial cells

Celiac disease is characterized by the secretion of IgA-class autoantibodies that target tissue transglutaminase (tTG). It is now recognized that anti-tTG antibodies are functional and not mere bystanders in the pathogenesis of celiac disease. Here we report that interaction between anti-tTG antibodies and extracellular membrane-bound tTG inhibits peptide 31-43 (but not peptide 57-68) uptake by cells, thereby impairing the ability of p31-43 to drive Caco-2 cells into S-phase. This effect did not involve tTG catalytic activity. Because anti-tTG antibodies interfered with epidermal growth factor endocytosis, we assume that they exert their effect by reducing peptide 31-43 endocytosis. Our results suggest that cell-surface tTG plays a hitherto unknown role in the regulation of gliadin peptide uptake and endocytosis.

Endocytosis of murine norovirus 1 into murine macrophages is dependent on dynamin II and cholesterol

Although noroviruses cause the vast majority of nonbacterial gastroenteritis in humans, little is known about their life cycle, including viral entry. Murine norovirus (MNV) is the only norovirus to date that efficiently infects cells in culture. To elucidate the productive route of infection for MNV-1 into murine macrophages, we used a neutral red (NR) infectious center assay and pharmacological inhibitors in combination with dominant-negative (DN) and small interfering RNA (siRNA) constructs to show that clathrin- and caveolin-mediated endocytosis did not play a role in entry. In addition, we showed that phagocytosis or macropinocytosis, flotillin-1, and GRAF1 are not required for the major route of MNV-1 uptake. However, MNV-1 genome release occurred within 1 h, and endocytosis was significantly inhibited by the cholesterol-sequestering drugs nystatin and methyl-beta-cyclodextrin, the dynamin-specific inhibitor dynasore, and the dominant-negative dynamin II mutant K44A. Therefore, we conclude that the productive route of MNV-1 entry into murine macrophages is rapid and requires host cholesterol and dynamin II.

Heterogeneous pathways of maternal-fetal transmission of human viruses (review)

Several viruses can pass the maternal-fetal barrier, and cause diseases of the fetus or the newborn. Recently, however, it became obvious, that viruses may invade fetal cells and organs through different routes without acute consequences. Spermatozoa, seminal fluid and lymphocytes in the sperm may transfer viruses into the human zygotes. Viruses were shown to be integrated into human chromosomes and transferred into fetal tissues. The regular maternal-fetal transport of maternal cells has also been discovered. This transport might implicate that lymphotropic viruses can be released into the fetal organs following cellular invasion. It has been shown that many viruses may replicate in human trophoblasts and syncytiotrophoblast cells thus passing the barrier of the maternal-fetal interface. The transport of viral immunocomplexes had also been suggested, and the possibility has been put forward that even anti-idiotypes mimicking viral epitopes might be transferred by natural mechanisms into the fetal plasma, in spite of the selective mechanisms of apical to basolateral transcytosis in syncytiotrophoblast and basolateral to apical transcytosis in fetal capillary endothelium. The mechanisms of maternal-fetal transcytosis seem to be different of those observed in differentiated cells and tissue cultures. Membrane fusion and lipid rafts of high cholesterol content are probably the main requirements of fetal transcytosis. The long term presence of viruses in fetal tissues and their interactions with the fetal immune system might result in post partum consequences as far as increased risk of the development of malignancies and chronic pathologic conditions are discussed.

Murine norovirus-1 cell entry is mediated through a non-clathrin-, non-caveolae-, dynamin- and cholesterol-dependent pathway

For many viruses, endocytosis and exposure to the low pH within acidic endosomes is essential for infection. It has previously been reported that feline calicivirus uses clathrin-mediated endocytosis for entry into mammalian cells. Here, we report that infection of RAW264.7 macrophages by the closely related murine norovirus-1 (MNV-1) does not require the clathrin pathway, as infection was not inhibited by expression of dominant-negative Eps15 or by knockdown of the adaptin-2 complex. Further, infection was not inhibited by reagents that raise endosomal pH. RAW264.7 macrophages were shown not to express caveolin, and flotillin depletion did not inhibit infection, suggesting that caveolae and the flotillin pathway are not required for cell entry. However, MNV-1 infection was inhibited by methyl-beta-cyclodextrin and the dynamin inhibitor, dynasore. Addition of these drugs to the cells after a period of virus internalization did not inhibit infection, suggesting the involvement of cholesterol-sensitive lipid rafts and dynamin in the entry mechanism. Macropinocytosis (MPC) was shown to be active in RAW264.7 macrophages (as indicated by uptake of dextran) and could be blocked by 5-(N-ethyl-N-isopropyl) amiloride (EIPA), which is reported to inhibit this pathway. However, infection was enhanced in the presence of EIPA. Similarly, actin disruption, which also inhibits MPC, resulted in enhanced infection. These results suggest that MPC could contribute to virus degradation or that inhibition of MPC could lead to the upregulation of other endocytic pathways of virus uptake.

Ca2+ regulation of dynamin-independent endocytosis in cortical astrocytes

Astrocytes release ATP and glutamate through vesicular exocytosis to mediate neuron-glial interactions. In contrast to exocytosis, the endocytic pathways in astroglial cells are poorly understood. Here, we identify a constitutive endocytic pathway in cultured astrocytes that is dependent on neither clathrin nor dynamin. This dynamin-independent endocytic pathway is regulated by Rab5, an early endosome protein. The endocytosed vesicles show fast transition from early endosomes to late endosomes and lysosomes within a few minutes. Interestingly, this clathrin- and dynamin-independent endocytosis in astrocytes is potently regulated by intracellular Ca(2+). ATP and glutamate greatly enhance the dynamin-independent endocytosis through elevating the intracellular Ca(2+). In addition, amyloid-beta peptide (A beta) also enhances the dynamin-independent endocytosis by inducing Ca(2+) transients in astrocytes. These results demonstrate a novel endocytic pathway in glial cells that is dynamin independent but tightly regulated by intracellular Ca(2+). The regulation by ATP, glutamate, and A beta suggests an important role of the dynamin-independent endocytosis in both physiological and pathological conditions.

Alternative infectious entry pathways for dengue virus serotypes into mammalian cells

The entry of two dengue virus (DENV) serotypes into Vero cells was analysed using biochemical inhibitors, dominant negative mutants of cellular proteins involved in endocytic pathways, fluorescence microscopy and infectivity determinations. By treatment with dansylcadaverine and chlorpromazine and overexpression of a dominant negative form of the Eps15 protein, a clathrin‐mediated endocytosis for productive DENV‐1 internalization into Vero cells was demonstrated whereas the infectious entry of DENV‐2 in the same cell system was independent of clathrin. Treatment with the inhibitors nystatin and methyl‐β‐cyclodextrin, as well as transfection of Vero cells with dominant negative caveolin‐1, had no effect on DENV‐2 virus infection. It was also shown, by using the K44A mutant and the inhibitor dynasore, that dynamin was required for DENV‐2 entry. Consequently, the infectious entry of DENV‐2 into Vero cells occurs by a non‐classical endocytic pathway independent of clathrin, caveolae and lipid rafts, but dependent on dynamin. By contrast, DENV‐2 entry into A549 cells was clathrin‐dependent, as previously reported in HeLa, C6/36 and BS‐C‐1 cells. Our results conclusively show, for the first time, a differential mode of infective entry for DENV‐1 and DENV‐2 into a common host cell, Vero cells, as well as alternative entry pathways for a given serotype, DENV‐2, into different types of cells.

Proteasome-independent degradation of HIV-1 in naturally non-permissive human placental trophoblast cells

BACKGROUND

The human placenta-derived cell line BeWo has been demonstrated to be restrictive to cell-free HIV-1 infection. BeWo cells are however permissive to infection by VSV-G pseudotyped HIV-1, which enters cells by a receptor-independent mechanism, and to infection by HIV-1 via a cell-to-cell route.

RESULTS

Here we analysed viral entry in wild type BeWo (CCR5+, CXCR4+) and BeWo-CD4+ (CD4+, CCR5+, CXCR4+) cells. We report that HIV-1 internalisation is not restricted in either cell line. Levels of internalised p24 antigen between VSV-G HIV-1 pseudotypes and R5 or X4 virions were comparable. We next analysed the fate of internalised virions; X4 and R5 HIV-1 virions were less stable over time in BeWo cells than VSV-G HIV-1 pseudotypes. We then investigated the role of the proteasome in restricting cell-free HIV-1 infection in BeWo cells using proteasome inhibitors. We observed an increase in the levels of VSV-G pseudotyped HIV-1 infection in proteasome-inhibitor treated cells, but the infection by R5-Env or X4-Env pseudotyped virions remains restricted.

CONCLUSION

Collectively these results suggest that cell-free HIV-1 infection encounters a surface block leading to a non-productive entry route, which either actively targets incoming virions for non-proteasomal degradation, and impedes their release into the cytoplasm, or causes the inactivation of mechanisms essential for viral replication.

Microglia mediate the clearance of soluble Abeta through fluid phase macropinocytosis

Alzheimer's disease is characterized by the progressive deposition of beta-amyloid (Abeta) within the brain parenchyma and its subsequent accumulation into senile plaques. Pathogenesis of the disease is associated with perturbations in Abeta homeostasis and the inefficient clearance of these soluble and insoluble peptides from the brain. Microglia have been reported to mediate the clearance of fibrillar Abeta (fAbeta) through receptor-mediated phagocytosis; however, their participation in clearance of soluble Abeta peptides (sAbeta) is largely unknown. We report that microglia internalize sAbeta from the extracellular milieu through a nonsaturable, fluid phase macropinocytic mechanism that is distinct from phagocytosis and receptor-mediated endocytosis both in vitro and in vivo. The uptake of sAbeta is dependent on both actin and tubulin dynamics and does not involve clathrin assembly, coated vesicles or membrane cholesterol. Upon internalization, fluorescently labeled sAbeta colocalizes to pinocytic vesicles. Microglia rapidly traffic these soluble peptides into late endolysosomal compartments where they are subject to degradation. Additionally, we demonstrate that the uptake of sAbeta and fAbeta occurs largely through distinct mechanisms and upon internalization are segregated into separate subcellular vesicular compartments. Significantly, we found that upon proteolytic degradation of fluorescently labeled sAbeta, the fluorescent chromophore is retained by the microglial cell. These studies identify an important mechanism through which microglial cells participate in the maintenance of Abeta homeostasis, through their capacity to constitutively clear sAbeta peptides from the brain.

A human polymorphism affects NEDD4L subcellular targeting by leading to two isoforms that contain or lack a C2 domain

Background

Ubiquitination serves multiple cellular functions, including proteasomal degradation and the control of stability, function, and intracellular localization of a wide variety of proteins. NEDD4L is a member of the HECT class of E3 ubiquitin ligases. A defining feature of NEDD4L protein isoforms is the presence or absence of an amino-terminal C2 domain, a class of subcellular, calcium-dependent targeting domains. We previously identified a common variant in human NEDD4L that generates isoforms that contain or lack a C2 domain.

Results

To address the potential functional significance of the NEDD4L common variant on NEDD4L subcellular localization, NEDD4L isoforms that either contained or lacked a C2 domain were tagged with enhanced green fluorescent protein, transfected into Xenopus laevis kidney epithelial cells, and imaged by performing confocal microscopy on live cells. We report that the presence or absence of this C2 domain exerts differential effects on the subcellular distribution of NEDD4L, the ability of C2 containing and lacking NEDD4L isoforms to mobilize in response to a calcium stimulus, and the intracellular transport of subunits of the NEDD4L substrate, ENaC. Furthermore, the ability of the C2-containing isoform to influence β-ENaC mobilization from intracellular pools involves the NEDD4L active site for ubiquitination. We propose a model to account for the potential impact of this common genetic variant on protein function at the cellular level.

Conclusion

NEDD4L isoforms that contain or lack a C2 domain target different intracellular locations. Additionally, whereas the C2-containing NEDD4L isoform is capable of shuttling between the plasma membrane and intracellular compartments in response to calcium stimulus the C2-lacking isoform can not. The C2-containing isoform differentially affects the mobilization of ENaC subunits from intracellular pools and this trafficking step requires NEDD4L ubiquitin ligase activity. This observation suggests a new mechanism for the requirement for the PY motif in cAMP-mediated exocytosis of ENaC. We have elucidated how a common genetic variant can underlie significant functional diversity in NEDD4L at the cellular level. We propose a model that describes how that functional variation may influence blood pressure. Moreover, our observations regarding differential function of the NEDD4L isoforms may impact other aspects of physiology that involve this ubiquitin ligase.

Defining macropinocytosis

Macropinocytosis represents a distinct pathway of endocytosis in mammalian cells. This actin-driven endocytic process is not directly co-ordinated by the presence of cargo but can be induced upon activation of growth factor signalling pathways. The capacity to dissect the contribution of macropinocytosis to cellular processes has been hampered by a lack of unique molecular markers and defining features. While aspects of macropinosome formation and maturation are common to those shared by the other endocytic pathways, a number of key differences have recently begun to emerge and will be discussed in this study. It is now well established that macropinocytosis significantly contributes to antigen presentation by the immune system and is exploited by a range of pathogens for cellular invasion and avoidance of immune surveillance.

Porcine circovirus 2 infection of epithelial cells is clathrin-, caveolae- and dynamin-independent, actin and Rho-GTPase-mediated, and enhanced by cholesterol depletion

Epithelial cells are the major in vivo target cells for porcine circovirus type 2 (PCV2). Although these cells are used for most studies of PCV2 gene expression and, little is known on PCV2 entry, attachment and internalization, in epithelial cells. PCV2 attachment to epithelial cells occurred rapidly and in a time-dependent manner. In contrast to attachment, internalization was slow. Immunofluorescent stainings revealed that during internalization, PCV2 co-localized with clathrin, but not caveolin. Blocking clathrin-mediated endocytosis increased instead of decreased the number of PCV2-infected cells by threefold, suggesting that it does not represent the main internalization pathway leading to a full replication. Further analysis with different inhibitors revealed that also macropinocytosis, dynamin-dependent internalization and membrane cholesterol play no role in PCV2 entry that leads to infection. Inhibition of small GTPases with Clostridium difficile toxin B reduced the number of PCV2-infected PK-15, SK and STs to 63+/-25%, 47+/-21% and 14+/-6%, respectively. Finally, inhibiting actin polymerization also blocked PCV2 infection, showing the need for actin during PCV2 infection. Together, these data indicate that a dynamin- and cholesterol-independent, but actin- and small GTPase-dependent pathway, allows PCV2 internalization in epithelial cells that leads to infection and that clathrin-mediated PCV2 internalization in epithelial cells is not followed by a full replication.