Polarized distribution of viral envelope proteins in the plasma membrane of infected epithelial cells

Elucidation of host-virus surfaceome interactions using spatial proteotyping

The cellular surfaceome and its residing extracellularly exposed proteins are involved in a multitude of molecular signaling processes across the viral infection cycle. Successful viral propagation, including viral entry, immune evasion, virion release and viral spread rely on dynamic molecular interactions with the surfaceome. Decoding of these viral-host surfaceome interactions using advanced technologies enabled the discovery of fundamental new functional insights into cellular and viral biology. In this review, we highlight recently developed experimental strategies, with a focus on spatial proteotyping technologies, aiding in the rational design of theranostic strategies to combat viral infections.

Multifaceted Functions of Host Cell Caveolae/Caveolin-1 in Virus Infections

Virus infection has drawn extensive attention since it causes serious or even deadly diseases, consequently inducing a series of social and public health problems. Caveolin-1 is the most important structural protein of caveolae, a membrane invagination widely known for its role in endocytosis and subsequent cytoplasmic transportation. Caveolae/caveolin-1 is tightly associated with a wide range of biological processes, including cholesterol homeostasis, cell mechano-sensing, tumorigenesis, and signal transduction. Intriguingly, the versatile roles of caveolae/caveolin-1 in virus infections have increasingly been appreciated. Over the past few decades, more and more viruses have been identified to invade host cells via caveolae-mediated endocytosis, although other known pathways have been explored. The subsequent post-entry events, including trafficking, replication, assembly, and egress of a large number of viruses, are caveolae/caveolin-1-dependent. Deprivation of caveolae/caveolin-1 by drug application or gene editing leads to abnormalities in viral uptake, viral protein expression, or virion release, whereas the underlying mechanisms remain elusive and must be explored holistically to provide potential novel antiviral targets and strategies. This review recapitulates our current knowledge on how caveolae/caveolin-1 functions in every step of the viral infection cycle and various relevant signaling pathways, hoping to provide a new perspective for future viral cell biology research.

Apical Trafficking Pathways of Influenza A Virus HA and NA via Rab17- and Rab23-Positive Compartments

The envelope proteins of influenza A virus, hemagglutinin (HA) and neuraminidase (NA), play critical roles in viral entry to host cells and release from the cells, respectively. After protein synthesis, they are transported from the trans-Golgi network (TGN) to the apical plasma membrane (PM) and assembled into virus particles. However, the post-TGN transport pathways of HA and NA have not been clarified. Temporal study by confocal microscopy revealed that HA and NA colocalized soon after their synthesis, and relocated together from the TGN to the upper side of the cell. Using the Rab family protein, we investigated the post-TGN transport pathways of HA and NA. HA partially colocalized with AcGFP-Rab15, Rab17, and Rab23, but rarely with AcGFP-Rab11. When analyzed in cells stably expressing AcGFP-Rab, HA/NA colocalized with Rab15 and Rab17, markers of apical sorting and recycling endosomes, and later colocalized with Rab23, which distributes to the apical PM and endocytic vesicles. Overexpression of the dominant-negative (DN) mutants of Rab15 and Rab17, but not Rab23, significantly delayed HA transport to the PM. However, Rab23DN impaired cell surface expression of HA. Live-cell imaging revealed that NA moved rapidly with Rab17 but not with Rab15. NA also moved with Rab23 in the cytoplasm, but this motion was confined at the upper side of the cell. A fraction of HA was localized to Rab17 and Rab23 double-positive vesicles in the cytoplasm. Coimmunoprecipitation indicated that HA was associated with Rab17 and Rab23 in lipid raft fractions. When cholesterol was depleted by methyl-β-cyclodextrin treatment, the motion of NA and Rab17 signals ceased. These results suggest that HA and NA are incorporated into lipid raft microdomains and are cotransported to the PM by Rab17-positive and followed by Rab23-positive vesicles.

Friend or Foe: The Role of the Cytoskeleton in Influenza A Virus Assembly

Influenza A Virus (IAV) is a respiratory virus that causes seasonal outbreaks annually and pandemics occasionally. The main targets of the virus are epithelial cells in the respiratory tract. Like many other viruses, IAV employs the host cell’s machinery to enter cells, synthesize new genomes and viral proteins, and assemble new virus particles. The cytoskeletal system is a major cellular machinery, which IAV exploits for its entry to and exit from the cell. However, in some cases, the cytoskeleton has a negative impact on efficient IAV growth. In this review, we highlight the role of cytoskeletal elements in cellular processes that are utilized by IAV in the host cell. We further provide an in-depth summary of the current literature on the roles the cytoskeleton plays in regulating specific steps during the assembly of progeny IAV particles.

Directional Fluid Transport across Organ-Blood Barriers: Physiology and Cell Biology

Directional fluid flow is an essential process for embryo development as well as for organ and organism homeostasis. Here, we review the diverse structure of various organ-blood barriers, the driving forces, transporters, and polarity mechanisms that regulate fluid transport across them, focusing on kidney-, eye-, and brain-blood barriers. We end by discussing how cross talk between barrier epithelial and endothelial cells, perivascular cells, and basement membrane signaling contribute to generate and maintain organ-blood barriers.

Interplay between influenza A virus and host factors: targets for antiviral intervention

Influenza A viruses (IAVs) pose a major public health threat worldwide. Recent experience with the 2013 H7N9 outbreak in China and the 2009 "swine flu" pandemic have shown that antiviral vaccines and drugs fall short of controlling the spread of disease in a timely and effective manner. Major problems include rapid emergence of drug-resistant influenza virus strains and the slow process of vaccine production. With the threat of a highly pathogenic H5N1 bird-flu pandemic looming large, it is crucial to develop novel ways of combating influenza A viruses. Targeting the host factors critical for influenza A virus replication has shown promise as a strategy to develop novel antiviral molecules with broad-spectrum protection. In this review, we summarize the role of currently identified host factors that play a critical role in the influenza A virus life cycle and discuss the most promising candidates for anti-influenza therapeutics.

Co-assembly of viral envelope glycoproteins regulates their polarized sorting in neurons

Newly synthesized envelope glycoproteins of neuroinvasive viruses can be sorted in a polarized manner to the somatodendritic and/or axonal domains of neurons. Although critical for transneuronal spread of viruses, the molecular determinants and interregulation of this process are largely unknown. We studied the polarized sorting of the attachment (NiV-G) and fusion (NiV-F) glycoproteins of Nipah virus (NiV), a paramyxovirus that causes fatal human encephalitis, in rat hippocampal neurons. When expressed individually, NiV-G exhibited a non-polarized distribution, whereas NiV-F was specifically sorted to the somatodendritic domain. Polarized sorting of NiV-F was dependent on interaction of tyrosine-based signals in its cytosolic tail with the clathrin adaptor complex AP-1. Co-expression of NiV-G with NiV-F abolished somatodendritic sorting of NiV-F due to incorporation of NiV-G•NiV-F complexes into axonal transport carriers. We propose that faster biosynthetic transport of unassembled NiV-F allows for its proteolytic activation in the somatodendritic domain prior to association with NiV-G and axonal delivery of NiV-G•NiV-F complexes. Our study reveals how interactions of viral glycoproteins with the host's transport machinery and between themselves regulate their polarized sorting in neurons.

Neurons are highly polarized cells exhibiting somatodendritic and axonal domains with distinct protein and lipid compositions. Some enveloped viruses target neurons by binding of the viral envelope glycoproteins to neuronal surface receptors. The ensuing fusion of the viral and neuronal membranes delivers the genetic material of the virus into the neurons. During viral replication in neurons, newly synthesized envelope glycoproteins are sorted to the somatodendritic and/or axonal domains. Although critical for viral propagation, the mechanisms responsible for this sorting are largely unknown. We studied the neuronal sorting of the attachment (NiV-G) and fusion (NiV-F) glycoproteins of Nipah virus, a pathogen that causes fatal human encephalitis. When analyzed individually, NiV-G was delivered to both the axonal and somatodendritic domains. In contrast, NiV-F was exclusively targeted to the somatodendritic domain by virtue of interaction of specific signals in this protein with AP-1, a component of the neuronal protein transport machinery. Assembly with NiV-G, however, abolished somatodendritic sorting of NiV-F due to incorporation of complexes into axon-bound vesicles. Thus, coordinated interactions of viral glycoproteins with the host's sorting machinery and between themselves allow temporal and spatial regulation of their distribution in neurons. We propose that this coordination facilitates viral spread among neurons.

Palmitoylation is required for intracellular trafficking of influenza B virus NB protein and efficient influenza B virus growth in vitro

All influenza viruses bud and egress from lipid rafts within the apical plasma membrane of infected epithelial cells. As a result, all components of progeny virions must be transported to these lipid rafts for assembly and budding. Although the mechanism of transport for other influenza proteins has been elucidated, influenza B virus (IBV) glycoprotein NB subcellular localization and transport are not understood completely. To address the aforementioned properties of NB, a series of trafficking experiments were conducted. Here, we showed that NB co-localized with markers specific for the endoplasmic reticulum (ER) and Golgi region. The data from chemical treatment of NB-expressing cells by Brefeldin A, a fungal antibiotic and a known chemical inhibitor of the protein secretory pathway, further confirmed that NB is transported through the ER-Golgi pathway as it restricted NB localization to the perinuclear region. Using NB deletion mutants, the hydrophobic transmembrane domain was identified as being required for NB transport to the plasma membrane. Furthermore, palmitoylation was also required for transport of NB to the plasma membrane. Systematic mutation of cysteines to serines in NB demonstrated that cysteine 49, likely in a palmitoylated form, is also required for transport to the plasma membrane. Surprisingly, further analysis demonstrated that in vitro replication of NBC49S mutant virus was delayed relative to the parental IBV. The results demonstrated that NB is the third influenza virus protein to have been shown to be palmitoylated and together these findings may aid in future studies aimed at elucidating the function of NB.

Transsynaptic tracing with vesicular stomatitis virus reveals novel retinal circuitry

The use of neurotropic viruses as transsynaptic tracers was first described in the 1960s, but only recently have such viruses gained popularity as a method for labeling neural circuits. The development of retrograde monosynaptic tracing vectors has enabled visualization of the presynaptic sources onto defined sets of postsynaptic neurons. Here, we describe the first application of a novel viral tracer, based on vesicular stomatitis virus (VSV), which directs retrograde transsynaptic viral spread between defined cell types. We use this virus in the mouse retina to show connectivity between starburst amacrine cells (SACs) and their known synaptic partners, direction-selective retinal ganglion cells, as well as to discover previously unknown connectivity between SACs and other retinal ganglion cell types. These novel connections were confirmed using physiological recordings. VSV transsynaptic tracing enables cell type-specific dissection of neural circuitry and can reveal synaptic relationships among neurons that are otherwise obscured due to the complexity and density of neuropil.

Nipah virus entry and egress from polarized epithelial cells

Highly pathogenic Nipah virus (NiV) infections are transmitted via airway secretions and urine, commonly via the respiratory route. Epithelial surfaces represent important replication sites in both primary and systemic infection phases. NiV entry and spread from polarized epithelial cells therefore determine virus entry and dissemination within a new host and influence virus shedding via mucosal surfaces in the respiratory and urinary tract. To date, there is no knowledge regarding the entry and exit sites of NiV in polarized epithelial cells. In this report, we show for the first time that NiV can infect polarized kidney epithelial cells (MDCK) from both cell surfaces, while virus release is primarily restricted to the apical plasma membrane. Substantial amounts of basolateral infectivity were detected only after infection with high virus doses, at time points when the integrity of the cell monolayer was largely disrupted as a result of cell-to-cell fusion. Confocal immunofluorescence analyses of envelope protein distribution at early and late infection stages suggested that apical virus budding is determined by the polarized sorting of the NiV matrix protein, M. Studies with stably M-expressing and with monensin-treated cells furthermore demonstrated that M protein transport is independent from the glycoproteins, implying that the M protein possesses an intrinsic apical targeting signal.

A 14-3-3γ dimer-based scaffold bridges CtBP1-S/BARS to PI(4)KIIIβ to regulate post-Golgi carrier formation

Large pleiomorphic carriers leave the Golgi complex for the plasma membrane by en bloc extrusion of specialized tubular domains, which then undergo fission. Several components of the underlying molecular machinery have been identified, including those involved in the budding/initiation of tubular carrier precursors (for example, the phosphoinositide kinase PI(4)KIIIβ, the GTPase ARF, and FAPP2), and in the fission of these precursors (for example, PKD, CtBP1-S/BARS). However, how these proteins interact to bring about carrier formation is poorly understood. Here, we describe a protein complex that mediates carrier formation and contains budding and fission molecules, as well as other molecules, such as the adaptor protein 14-3-3γ. Specifically, we show that 14-3-3γ dimers bridge CtBP1-S/BARS with PI(4)KIIIβ, and that the resulting complex is stabilized by phosphorylation by PKD and PAK. Disrupting the association of these proteins inhibits the fission of elongating carrier precursors, indicating that this complex couples the carrier budding and fission processes.

High throughput method to quantify anterior-posterior polarity of T-cells and epithelial cells

The virologic synapse (VS), which is formed between a virus-infected and uninfected cell, plays a central role in the transmission of certain viruses, such as HIV and HTLV-1. During VS formation, HTLV-1-infected T-cells polarize cellular and viral proteins toward the uninfected T-cell. This polarization resembles anterior-posterior cell polarity induced by immunological synapse (IS) formation, which is more extensively characterized than VS formation and occurs when a T-cell interacts with an antigen-presenting cell. One measure of cell polarity induced by both IS or VS formation is the repositioning of the microtubule organizing center (MTOC) relative to the contact point with the interacting cell. Here we describe an automated, high throughput system to score repositioning of the MTOC and thereby cell polarity establishment. The method rapidly and accurately calculates the angle between the MTOC and the IS for thousands of cells. We also show that the system can be adapted to score anterior-posterior polarity establishment of epithelial cells. This general approach represents a significant advancement over manual cell polarity scoring, which is subject to experimenter bias and requires more time and effort to evaluate large numbers of cells.

Remodeling epithelial cell organization: transitions between front-rear and apical-basal polarity

Polarized epithelial cells have a distinctive apical-basal axis of polarity for vectorial transport of ions and solutes across the epithelium. In contrast, migratory mesenchymal cells have a front-rear axis of polarity. During development, mesenchymal cells convert to epithelia by coalescing into aggregates that undergo epithelial differentiation. Signaling networks and protein complexes comprising Rho family GTPases, polarity complexes (Crumbs, PAR, and Scribble), and their downstream effectors, including the cytoskeleton and the endocytic and exocytic vesicle trafficking pathways, together regulate the distributions of plasma membrane and cytoskeletal proteins between front-rear and apical-basal polarity. The challenge is to understand how these regulators and effectors are adapted to regulate symmetry breaking processes that generate cell polarities that are specialized for different cellular activities and functions.

Apical trafficking in epithelial cells: signals, clusters and motors

In the early days of epithelial cell biology, researchers working with kidney and/or intestinal epithelial cell lines and with hepatocytes described the biosynthetic and recycling routes followed by apical and basolateral plasma membrane (PM) proteins. They identified the trans-Golgi network and recycling endosomes as the compartments that carried out apical-basolateral sorting. They described complex apical sorting signals that promoted association with lipid rafts, and simpler basolateral sorting signals resembling clathrin-coated-pit endocytic motifs. They also noticed that different epithelial cell types routed their apical PM proteins very differently, using either a vectorial (direct) route or a transcytotic (indirect) route. Although these original observations have generally held up, recent studies have revealed interesting complexities in the routes taken by apically destined proteins and have extended our understanding of the machinery required to sustain these elaborate sorting pathways. Here, we critically review the current status of apical trafficking mechanisms and discuss a model in which clustering is required to recruit apical trafficking machineries. Uncovering the mechanisms responsible for polarized trafficking and their epithelial-specific variations will help understand how epithelial functional diversity is generated and the pathogenesis of many human diseases.

Clathrin and AP1B: key roles in basolateral trafficking through trans-endosomal routes

Research following introduction of the MDCK model system to study epithelial polarity (1978) led to an initial paradigm that posited independent roles of the trans Golgi network (TGN) and recycling endosomes (RE) in the generation of, respectively, biosynthetic and recycling routes of plasma membrane (PM) proteins to apical and basolateral PM domains. This model dominated the field for 20 years. However, studies over the past decade and the discovery of the involvement of clathrin and clathrin adaptors in protein trafficking to the basolateral PM has led to a new paradigm. TGN and RE are now believed to cooperate closely in both biosynthetic and recycling trafficking routes. Here, we critically review these recent advances and the questions that remain unanswered.

Phosphoinositide signaling pathways: promising role as builders of epithelial cell polarity

Polarity is a prerequisite for proper development and function of epithelia in metazoa. The major feature of polarized epithelial cells is the presence of specialized domains with asymmetric distribution of macromolecular contents including proteins and lipids. The apical domain is involved in exchange with the organ lumen, and the basolateral membrane maintains contact with neighboring cells and the underlying extracellular matrix. The two domains are separated by tight junctions, which act as a diffusion barrier to prevent free mixing of domain-specific proteins and lipids. Extensive studies have shed light on the numerous protein families involved in cell polarization. However, many questions still remain regarding the molecular mechanisms of polarity regulation and in particular very little is known about the role of lipids in building polarity. In this chapter, essential determinants of epithelial polarity will be reviewed with a particular focus on metabolism and function of phosphoinositides.

Human respiratory syncytial virus glycoproteins are not required for apical targeting and release from polarized epithelial cells

Human respiratory syncytial virus (HRSV) is released from the apical membrane of polarized epithelial cells. However, little is known about the processes of assembly and release of HRSV and which viral gene products are involved in the directional maturation of the virus. Based on previous studies showing that the fusion (F) glycoprotein contained an intrinsic apical sorting signal and that N- and O-linked glycans can act as apical targeting signals, we investigated whether the glycoproteins of HRSV were involved in its directional targeting and release. We generated recombinant viruses with each of the three glycoprotein genes deleted individually or in groups. Each deleted gene was replaced with a reporter gene to maintain wild-type levels of gene expression. The effects of deleting the glycoprotein genes on apical maturation and on targeting of individual proteins in polarized epithelial cells were examined by using biological, biochemical, and microscopic assays. The results of these studies showed that the HRSV glycoproteins are not required for apical maturation or release of the virus. Further, deletion of one or more of the glycoprotein genes did not affect the intracellular targeting of the remaining viral glycoproteins or the nucleocapsid protein to the apical membrane.

Biogenesis of epithelial cell plasma membranes

Polarized monolayers of cultured epithelial cells, such as the kidney-derived MDCK cell line, when infected with enveloped viruses, provide a convenient model system for study of the intracellular routes followed by newly synthesized glycoproteins to reach specific domains of the plasma membrane. The polarized nature of the monolayers is reflected in the asymmetric assembly of enveloped viruses, some of which, such as influenza and simian virus 5 (SV5), bud from the apical surfaces of the cells, while others, such as vesicular stomatitis virus (VSV), emerge from the basolateral surfaces. MDCK cells can sustain double infection with viruses of different budding polarity, and within such cells the envelope glycoproteins of the two viruses are synthesized simultaneously and assembled into virions at different sites. Immunoelectron microscopic observations of doubly infected cells show that glycoproteins of influenza and VSV traverse the same Golgi apparatus. This indicates that critical sorting steps must take place during or after passage of the glycoproteins through the organelle. Following passage through the Golgi, the HA glycoprotein accumulates almost exclusively at the apical surface, where the influenza virions assemble. Significant amounts of the G protein, however, are detected on both plasma membranes in singly and doubly infected cells, although VSV virion assembly is limited to basolateral domains. These observations indicate that the site of VSV budding is not exclusively determined by the presence of G polypeptides on a given cell-surface domain. It is possible that other cellular or viral components are responsible for the selection of the appropriate budding domain or that the G protein found on the apical surface must be transferred to the basolateral domain before it becomes competent for assembly.

Chapter 6 Protein Sorting in the Secretory Pathway

This chapter focuses on protein sorting in the secretory pathway. From primary and secondary biosynthetic sites in the cytosol and mitochondrial matrix, respectively, proteins and lipids are distributed to more than 30 final destinations in membranes or membrane-bound spaces, where they carry out their programmed function. Molecular sorting is defined, in its most general sense, as the sum of the mechanisms that determine the distribution of a given molecule from its site of synthesis to its site of function in the cell. The final site of residence of a protein in a eukaryotic cell is determined by a combination of various factors, acting in concert: (1) site of synthesis, (2) sorting signals or zip codes, (3) signal recognition or decoding mechanisms, (4) cotranslational or posttranslational mechanisms for translocation across membranes, (5) specific fusion-fission interactions between intracellular vesicular compartments, and (6) restrictions to the lateral mobility in the plane of the bilayer. Improvements in cell fractionation, protein separation, and immune precipitation procedures in the past decade have made them possible. Very little is known about the mechanisms that mediate the localization and concentration of specific proteins and lipids within organelles. Various experimental model systems have become available for their study. The advent of recombinant DNA technology has shortened the time needed for obtaining the primary structure of proteins to a few months.

AP1B sorts basolateral proteins in recycling and biosynthetic routes of MDCK cells

The epithelial-specific adaptor AP1B sorts basolateral proteins, but the trafficking routes where it performs its sorting role remain controversial. Here, we used an RNAi approach to knock down the medium subunit of AP1B (mu1B) in the prototype epithelial cell line Madin-Darby canine kidney (MDCK). Mu1B-knocked down MDCK cells displayed loss of polarity of several endogenous and exogenous basolateral markers transduced via adenovirus vectors, but exhibited normal polarity of apical markers. We chose two well characterized basolateral protein markers, the transferrin receptor (TfR) and the vesicular stomatitis virus G protein, to study the sorting role of AP1B. A surface-capture assay introduced here showed that mu1B-knocked down MDCK cells plated on filters at confluency and cultured for 4.5 d, sorted TfR correctly in the biosynthetic route but incorrectly in the recycling route. In contrast, these same cells missorted vesicular stomatitis virus G apically in the biosynthetic route. Strikingly, recently confluent MDCK cells (1-3 d) displayed AP1B-dependence in the biosynthetic route of TfR, which decreased with additional days in culture. Sucrose density gradient analysis detected AP1B predominantly in TfR-rich endosomal fractions in MDCK cells confluent for 1 and 4 d. Our results are consistent with the following model: AP1B sorts basolateral proteins in both biosynthetic and recycling routes of MDCK cells, as a result of its predominant functional localization in recycling endosomes, which constitute a post-Golgi station in the biosynthetic route of some plasma membrane proteins. TfR utilizes a direct route from Golgi to basolateral membrane that is established as the epithelial monolayer matures.

Bidirectional virus secretion and nonciliated cell tropism following Andes virus infection of primary airway epithelial cell cultures

Hantavirus pulmonary syndrome (HPS) is an acute disease resulting from infection with any one of a number of New World hantaviruses. HPS has a mortality rate of 40% and, unlike many other severe respiratory diseases, often occurs in young, healthy adults. Infection is usually initiated after inhalation of rodent excreta containing virus particles, but human-to-human transmission has been documented. Postmortem tissue samples show high levels of viral antigen within the respiratory endothelium, but it is not clear how the virus can traverse the respiratory epithelium in order to initiate infection in the microvasculature. We have utilized Andes virus infection of primary, differentiated airway epithelial cells to investigate the ability of the virus to interact with and cross the respiratory epithelium. Andes virus infects the Clara and goblet cell populations but not the ciliated cells, and this infection pattern corresponds to the expression of beta(3) integrin, the viral receptor. The virus can infect via the apical or basolateral membrane, and progeny virus particles are secreted bidirectionally. There is no obvious cytopathology associated with infection, and beta(3) integrins do not appear to be critical for respiratory epithelial cell monolayer integrity. Our data suggest that hantavirus infection of the respiratory epithelium may play an important role in the early or prodrome phase of disease as well as serving as a source of virus involved in transmission.

IN AWE OF SUBCELLULAR COMPLEXITY: 50 Years of Trespassing Boundaries Within the Cell

In this review I describe the several stages of my research career, all of which were driven by a desire to understand the basic mechanisms responsible for the complex and beautiful organization of the eukaryotic cell. I was originally trained as an electron microscopist in Argentina, and my first major contribution was the introduction of glutaraldehyde as a fixative that preserved the fine structure of cells, which opened the way for cytochemical studies at the EM level. My subsequent work on membrane-bound ribosomes illuminated the process of cotranslational translocation of polypeptides across the ER membrane and led to the formulation, with Gunter Blobel, of the signal hypothesis. My later studies with many talented colleagues contributed to an understanding of ER structure and function and aspects of the mechanisms that generate and maintain the polarity of epithelial cells. For this work my laboratory introduced the now widely adopted Madin-Darby canine kidney (MDCK) cell line, and demonstrated the polarized budding of envelope viruses from those cells, providing a powerful new system that further advanced the field of protein traffic.

Trafficking of viral membrane proteins

Many viruses express membrane proteins. For enveloped viruses in particular, membrane proteins are frequently structural components of the virus that mediate the essential tasks of receptor recognition and membrane fusion. The functional activities of these proteins require that they are sorted correctly in infected cells. These sorting events often depend on the ability of the virus to mimic cellular protein trafficking signals and to interact with the cellular trafficking machinery. Importantly, loss or modification of these signals can influence virus infectivity and pathogenesis.

A novel type of detergent-resistant membranes may contribute to an early protein sorting event in epithelial cells

One sorting mechanism of apical and basolateral proteins in epithelial cells is based on their solubility profiles with Triton X-100. Nevertheless, apical proteins themselves are also segregated beyond the trans-Golgi network by virtue of their association or nonassociation with cholesterol/sphingolipid-rich microdomains (Jacob, R., and Naim, H. Y. (2001) Curr. Biol. 11, 1444-1450). Therefore, extractability with Triton X-100 does not constitute an absolute criterion of protein sorting. Here, we investigate the solubility patterns of apical and basolateral proteins with other detergents and demonstrate that the mild detergent Tween 20 is adequate to discriminate between apical and basolateral proteins during early stages in their biosynthesis. Although the mannose-rich forms of the apical proteins sucrase-isomaltase, lactase-phlorizin hydrolase, aminopeptidase N, and dipeptidylpeptidase IV reveal similar solubility profiles comprising soluble and nonsoluble fractions, the basolateral proteins, vesicular stomatitis virus G protein, major histocompatibility complex class I, and CD46 are entirely soluble with this detergent. The insoluble Tween 20 membranes are enriched in phosphatidylinositol and phosphatidylglycerol compatible with their synthesis in the endoplasmic reticulum and the existence of a novel class of detergent-resistant membranes. The association of the mannose-rich biosynthetic forms of the apical proteins, sucraseisomaltase, lactase-phlorizin hydrolase, aminopeptidase N, and dipeptidylpeptidase IV with the Tween 20-resistant membranes suggests an early polarized sorting mechanism prior to maturation in the Golgi apparatus.

CtBP3/BARS drives membrane fission in dynamin-independent transport pathways

Membrane fission is a fundamental step in membrane transport. So far, the only fission protein machinery that has been implicated in in vivo transport involves dynamin, and functions in several, but not all, transport pathways. Thus, other fission machineries may exist. Here, we report that carboxy-terminal binding protein 3/brefeldin A-ribosylated substrate (CtBP3/BARS) controls fission in basolateral transport from the Golgi to the plasma membrane and in fluid-phase endocytosis, whereas dynamin is not involved in these steps. Conversely, CtBP3/BARS protein is inactive in apical transport to the plasma membrane and in receptor-mediated endocytosis, both steps being controlled by dynamin. This indicates that CtBP3/BARS controls membrane fission in endocytic and exocytic transport pathways, distinct from those that require dynamin.

Lipid raft-dependent targeting of the influenza A virus nucleoprotein to the apical plasma membrane

Influenza virus acquires a lipid raft-containing envelope by budding from the apical surface of epithelial cells. Polarised budding involves specific sorting of the viral membrane proteins, but little is known about trafficking of the internal virion components. We show that during the later stages of virus infection, influenza nucleoprotein (NP) and polymerase (the protein components of genomic ribonucleoproteins) localised to apical but not lateral or basolateral membranes, even in cell types where haemagglutinin was found on all external membranes. Other cytosolic components of the virion either distributed throughout the cytoplasm (NEP/NS2) or did not localise solely to the apical plasma membrane in all cell types (M1). NP localised specifically to the apical surface even when expressed alone, indicating intrinsic targeting. A similar proportion of NP associated with membrane fractions in flotation assays from virus-infected and plasmid-transfected cells. Detergent-resistant flotation at 4 degrees C suggested that these membranes were lipid raft microdomains. Confirming this, cholesterol depletion rendered NP detergent-soluble and furthermore, resulted in its partial redistribution throughout the cell. We conclude that NP is independently targeted to the apical plasma membrane through a mechanism involving lipid rafts and propose that this helps determine the polarity of influenza virus budding.

Organization of vesicular trafficking in epithelia

Experiments using mammalian epithelial cell lines have elucidated biosynthetic and recycling pathways for apical and basolateral plasma-membrane proteins, and have identified components that guide apical and basolateral proteins along these pathways. These components include apical and basolateral sorting signals, adaptors for basolateral signals, and docking and fusion proteins for vesicular trafficking. Recent live-cell-imaging studies provide a real-time view of sorting processes in epithelial cells, including key roles for actin, microtubules and motors in the organization of post-Golgi trafficking.

Rhabdovirus assembly and budding

Rhabdoviruses are a diverse, widely-distributed group of enveloped viruses that assemble and bud from the plasma membrane of host cells. Recent advances in the identification of domains on both the envelope glycoprotein and the matrix protein of rhabdoviruses that contribute to virus assembly and release have allowed us to refine current models of rhabdovirus budding and to describe in better detail the interplay between both viral and cellular components involved in the budding process. In this review we discuss the steps involved in rhabdovirus assembly beginning with genome encapsidation and the association of nucleocapsid-matrix protein pre-assembly complexes with the inner leaflet of the plasma membrane, how condensation of these complexes may occur, how microdomains containing the envelope glycoprotein facilitate bud site formation, and how multiple forms of the matrix protein may participate in virion extrusion and release.

Molecular mechanisms of membrane polarity in renal epithelial cells

Exciting discoveries in the last decade have cast light onto the fundamental mechanisms that underlie polarized trafficking in epithelial cells. It is now clear that epithelial cell membrane asymmetry is achieved by a combination of intracellular sorting operations, vectorial delivery mechanisms and plasmalemma-specific fusion and retention processes. Several well-defined signals that specify polarized segregation, sorting, or retention processes have, now, been described in a number of proteins. The intracellular machineries that decode and act on these signals are beginning to be described. In addition, the nature of the molecules that associate with intracellular trafficking vesicles to coordinate polarized delivery, tethering, docking, and fusion are also becoming understood. Combined with direct visualization of polarized sorting processes with new technologies in live-cell fluorescent microscopy, new and surprising insights into these once-elusive trafficking processes are emerging. Here we provide a review of these recent advances within an historically relevant context.

Cell Adhesion, Polarity, and Epithelia in the Dawn of Metazoans

Transporting epithelia posed formidable conundrums right from the moment that Du Bois Raymond discovered their asymmetric behavior, a century and a half ago. It took a century and a half to start unraveling the mechanisms of occluding junctions and polarity, but we now face another puzzle: lest its cells died in minutes, the first high metazoa (i.e., higher than a sponge) needed a transporting epithelium, but a transporting epithelium is an incredibly improbable combination of occluding junctions and cell polarity. How could these coincide in the same individual organism and within minutes? We review occluding junctions (tight and septate) as well as the polarized distribution of Na+-K+-ATPase both at the molecular and the cell level. Junctions and polarity depend on hosts of molecular species and cellular processes, which are briefly reviewed whenever they are suspected to have played a role in the dawn of epithelia and metazoan. We come to the conclusion that most of the molecules needed were already present in early protozoan and discuss a few plausible alternatives to solve the riddle described above.

Transcytosis: crossing cellular barriers

Transcytosis, the vesicular transport of macromolecules from one side of a cell to the other, is a strategy used by multicellular organisms to selectively move material between two environments without altering the unique compositions of those environments. In this review, we summarize our knowledge of the different cell types using transcytosis in vivo, the variety of cargo moved, and the diverse pathways for delivering that cargo. We evaluate in vitro models that are currently being used to study transcytosis. Caveolae-mediated transcytosis by endothelial cells that line the microvasculature and carry circulating plasma proteins to the interstitium is explained in more detail, as is clathrin-mediated transcytosis of IgA by epithelial cells of the digestive tract. The molecular basis of vesicle traffic is discussed, with emphasis on the gaps and uncertainties in our understanding of the molecules and mechanisms that regulate transcytosis. In our view there is still much to be learned about this fundamental process.

Membrane and cytoskeleton dynamics during axonal elongation and stabilization

Proper nervous activities are gradually developing events. Reflecting this, embryonic neurons start differentiation by sprouting multiple extensions, neurites, which do not bear clear axonal or dendritic structural and molecular characteristics. Later in development one of these multiple neurites elongates further, generating a morphologically polarized neuron with a single long axon and many short dendrites. Still, despite such morphological differences these processes can switch destiny, further reflecting their immaturity. Final and irreversible axonal and dendritic commitment occurs after both axons and dendrites have elongated considerably. Recent evidence suggests that the transition from axonal immaturity to maturity reflects changes in the mechanisms used by neurons to control the precise membrane and cytoskeleton polarization. This chapter provides an overview of how these mechanisms contribute to the formation of an axon.

Infection of polarized epithelial cells with flavivirus West Nile: polarized entry and egress of virus occur through the apical surface

Both polarized epithelial Vero (C1008) and non-polarized Vero (control) cells were grown on permeable cell culture inserts and infected either apically or basolaterally with West Nile (WN) or Kunjin (KUN) virus. KUN virus (closely related to WN virus) was used as a comparison. Using indirect immunofluorescence and plaque assays of productive virus titres, entry of WN and KUN viruses was confined to the apical surface of polarized epithelial cells. For the first time, these results provided evidence on the distribution of flavivirus-specific receptor(s) in polarized epithelial cells; that is to say that receptor expression was shown to be predominant at the apical surface. In addition, the release of these viruses from polarized Vero C1008 epithelial cells was also examined. Egress of WN virus strain Sarafend (S) was observed to occur predominantly at the apical surface of Vero C1008 cells. In contrast, the release of KUN virus was bi-directional from polarized Vero C1008 cells. Furthermore, disruption of the cellular microtubule network was shown to inhibit the apical release of WN (S) virus but had no effect on the release of KUN virus. Hence, the difference in the release of these closely related viruses suggested the involvement of a microtubule-dependent, polarized sorting mechanism for WN virus proteins but not for KUN virus proteins in polarized epithelial cells.

Apical budding of a recombinant influenza A virus expressing a hemagglutinin protein with a basolateral localization signal

Influenza virions bud preferentially from the apical plasma membrane of infected epithelial cells, by enveloping viral nucleocapsids located in the cytosol with its viral integral membrane proteins, i.e., hemagglutinin (HA), neuraminidase (NA), and M2 proteins, located at the plasma membrane. Because individually expressed HA, NA, and M2 proteins are targeted to the apical surface of the cell, guided by apical sorting signals in their transmembrane or cytoplasmic domains, it has been proposed that the polarized budding of influenza virions depends on the interaction of nucleocapsids and matrix proteins with the cytoplasmic domains of HA, NA, and/or M2 proteins. Since HA is the major protein component of the viral envelope, its polarized surface delivery may be a major force that drives polarized viral budding. We investigated this hypothesis by infecting MDCK cells with a transfectant influenza virus carrying a mutant form of HA (C560Y) with a basolateral sorting signal in its cytoplasmic domain. C560Y HA was expressed nonpolarly on the surface of infected MDCK cells. Interestingly, viral budding remained apical in C560Y virus-infected cells, and so did the location of NP and M1 proteins at late times of infection. These results are consistent with a model in which apical viral budding is a shared function of various viral components rather than a role of the major viral envelope glycoprotein HA.

Mutually exclusive apicobasolateral sorting of two oligodendroglial membrane proteins, proteolipid protein and myelin/oligodendrocyte glycoprotein, in Madin-Darby canine kidney cells

Oligodendrocytes elaborate an extensive membrane network that ensheathes CNS axons in multilamellar wrappings. A compaction process excludes much of the cytoplasm in mature myelin membranes, giving rise to distinct lipid/protein compositions in two membrane compartments (compact myelin and membranes of the cell body and processes). Insofar as oligodendrocytes arise from neuroepithelial progenitors, it seems likely that some elements are shared for protein targeting by these two cell types. We hypothesized that certain membrane proteins targeting different oligodendroglial membrane compartments would preferentially sort to opposite domains when transfected into Madin-Darby canine kidney (MDCK) epithelial cells. Myelin/oligodendrocyte glycoprotein (MOG) is found in uncompacted membrane (cell body, processes), and it sorts exclusively to MDCK basolateral membrane. Proteolipid protein (PLP) is found in compact myelin, and it sorts exclusively to MDCK apical membrane. Myelin-associated glycoprotein (MAG) is primarily in the periaxonal inner loop of myelin; however, it fails to target preferentially within MDCK cells. This inability of MAG to sort within MDCK cells suggests a lack of required oligodendroglial-specific targeting components. In contrast, the sorting machinery in both oligodendrocytes and MDCK cells recognizes targeting signals for MOG and PLP, and we propose that these oligodendroglial membrane proteins delineate cognate basolateral and apical domains, respectively.

Synapsin I is expressed in epithelial cells: localization to a unique trans-Golgi compartment

Synapsin I is abundant in neural tissues. Its phosphorylation is thought to regulate synaptic vesicle exocytosis in the pre-synaptic terminal by mediating vesicle tethering to the cytoskeleton. Using anti-synapsin antibodies, we detected an 85 kDa protein in liver cells and identified it as synapsin I. Like brain synapsin I, non-neuronal synapsin I is phosphorylated in vitro by protein kinase A and yields identical 32P-peptide maps after limited proteolysis. We also detected synapsin I mRNA in liver by northern blot analysis. These results indicate that the expression of synapsin I is more widespread than previously thought. Immunofluorescence analysis of several non-neuronal cell lines localizes synapsin I to a vesicular compartment adjacent to trans-elements of the Golgi complex, which is also labeled with antibodies against myosin II; no sub-plasma membrane synapsin I is evident. We conclude that synapsin I is present in epithelial cells and is associated with a trans-Golgi network-derived compartment; this localization suggests that it plays a role in modulating post-TGN trafficking pathways.

Transport of viral proteins to the apical membranes and interaction of matrix protein with glycoproteins in the assembly of influenza viruses

Influenza virus assembly and morphogenesis require transport of viral components to the assembly site at the apical plasma membrane of polarized epithelial cells and interaction among the viral components. In this report we have discussed the apical determinants present in the transmembrane domain (TMD) of influenza virus hemagglutinin (HA) and neuraminidase (NA), and the interaction of M1 with influenza virus HA and NA. Earlier studies have shown that the NA and HA TMDs possess determinant(s) for apical sorting and raft-association (Kundu et al., 1996. J. Virol 70, 6508-6515; Lin et al., 1998. J. Cell Biol. 142, 51-57). Analysis of chimeric constructs between NA and TR (human transferring receptor) TMDs and the mutations in the NA and HA TMD sequences showed that the COOH terminus of the NA TMD and NH(2) terminus of the HA TMD encompassing the exoplasmic leaflet of the lipid bilayers were significantly involved in lipid raft-association and that apical determinants were not discrete sequences but rather dispersed within the TMD of HA and NA. These analyses also showed that although both signals for apical sorting and raft-association resided in the NA TMD, they were not identical and varied independently. Interactions of M1 protein with HA or NA, the influenza virus envelope glycoproteins, were investigated by TX-100 detergent treatment of membrane fractions and floatation in sucrose gradients. Results from these analyses showed that the interaction of M1 with mature HA and NA, which associated with the detergent-resistant lipid rafts caused an increased detergent-resistance of the membrane-bound M1 and that M1 interacted with HA and NA both in influenza virus-infected cells as well as in recombinant vaccinia virus-infected cells coexpressing M1 with HA and/or NA. Furthermore, both the cytoplasmic tail and the TMD of HA caused an increased detergent-resistance of the membrane-bound M1 supporting their interaction with M1. Immunofluorescence analysis by confocal microscopy also showed colocalization supporting the interaction of M1 with HA and NA at the cell surface and during exocytic transport both in influenza virus-infected cells as well as in coexpressing cells.

The highly conserved C-terminal dileucine motif in the cytosolic domain of the human immunodeficiency virus type 1 envelope glycoprotein is critical for its association with the AP-1 clathrin adaptor [correction of adapter]

Short amino acid sequences in the cytosolic domains of transmembrane proteins are recognized by specialized adaptor [corrected] proteins which are part of coated vesicles utilized to transport membrane proteins between the trans-Golgi network (TGN) and the plasma membrane (forward and backward). Previously, we and others reported that the membrane-proximal tyrosine residues Y712 (human immunodeficiency virus [HIV]) and Y721 (simian immunodeficiency virus [SIV]) in the envelope glycoprotein (Env) of the primate lentiviruses are crucial for the association of Env with clathrin-associated adaptor [corrected] complex AP-2. The same tyrosine-based endocytosis motifs in the cytosolic domains (EnvCD) of transmembrane gp41 of HIV type 1 (HIV-1) and SIV, respectively, were also shown to modulate the interaction with TGN- and endosome-based clathrin-associated complex AP-1. Our findings suggested that EnvCD binding to AP-1, unlike the association of EnvCD with AP-2, is dependent largely on residues other than Y712 and Y721. Here, we tested if motifs downstream of Y712 affect HIV-1 EnvCD-AP-1 binding and Env trafficking. Mutational analysis revealed that the C-terminal leucine-based motif in Env was crucial for the recruitment of AP-1 in vitro and in Env-expressing cells. In addition to affecting Env-AP-1 association, mutations at the C terminus of Env also altered the subcellular localization of Env, suggesting that proper post-Golgi routing of Env depends on its recruitment of AP-1. Finally, the C-terminal dileucine was shown to assist the membrane-proximal Y712 motif in restricting the cell surface expression of Env.

E3-13.7 integral membrane proteins encoded by human adenoviruses alter epidermal growth factor receptor trafficking by interacting directly with receptors in early endosomes

Animal cell viruses provide valuable model systems for studying many normal cellular processes, including membrane protein sorting. The focus of this study is an integral membrane protein encoded by the E3 transcription region of human adenoviruses called E3-13.7, which diverts recycling EGF receptors to lysosomes without increasing the rate of receptor internalization or intrinsic receptor tyrosine kinase activity. Although E3-13.7 can be found on the plasma membrane when it is overexpressed, its effect on EGF receptor trafficking suggests that the plasma membrane is not its primary site of action. Using cell fractionation and immunocytochemical experimental approaches, we now report that the viral protein is located predominantly in early endosomes and limiting membranes of endosome-to-lysosome transport intermediates called multivesicular endosomes. We also demonstrate that E3-13.7 physically associates with EGF receptors undergoing E3-13.7-mediated down-regulation in early endosomes. Receptor-viral protein complexes then dissociate, and EGF receptors proceed to lysosomes, where they are degraded, while E3-13.7 is retained in endosomes. We conclude that E3-13.7 is a resident early endocytic protein independent of EGF receptor expression, because it has identical intracellular localization in mouse cells lacking endogenous receptors and cells expressing a human cytomegalovirus-driven receptor cDNA. Finally, we demonstrate that EGF receptor residues 675-697 are required for E3-13.7-mediated down-regulation. Interestingly, this sequence includes a known EGF receptor leucine-based lysosomal sorting signal used during ligand-induced trafficking, which is also conserved in the viral protein. E3-13.7, therefore, provides a novel model system for determining the molecular basis of selective membrane protein transport in the endocytic pathway. Our studies also suggest new paradigms for understanding EGF receptor sorting in endosomes and adenovirus pathogenesis.

Evidence against the acidification hypothesis in cystic fibrosis

The pleiotropic effects of cystic fibrosis (CF) result from the mislocalization or inactivity of an apical membrane chloride channel, the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR may also modulate intracellular chloride conductances and thus affect organelle pH. To test the role of CFTR in organelle pH regulation, we developed a model system to selectively perturb the pH of a subset of acidified compartments in polarized cells and determined the effects on various protein trafficking steps. We then tested whether these effects were observed in cells lacking wild-type CFTR and whether reintroduction of CFTR affected trafficking in these cells. Our model system involves adenovirus-mediated expression of the influenza virus M2 protein, an acid-activated ion channel. M2 expression selectively slows traffic through the trans-Golgi network (TGN) and apical endocytic compartments in polarized Madin-Darby canine kidney (MDCK) cells. Expression of M2 or treatment with other pH perturbants also slowed protein traffic in the CF cell line CFPAC, suggesting that the TGN in this cell line is normally acidified. Expression of functional CFTR had no effect on traffic and failed to rescue the effect of M2. Our results argue against a role for CFTR in the regulation of organelle pH and protein trafficking in epithelial cells.

Patterned entry and egress by Epstein-Barr virus in polarized CR2-positive epithelial cells

In polarized epithelium direction of viral entry and release correlates with proclivity of a virus to establish local versus systemic infection. The Epstein-Barr virus (EBV), whose principal tissue reservoir is B lymphocytes, also has disease manifestations in epithelium, suggesting intertissue spread potentially influenced by epithelial cell polarity. We stably transfected the B lymphocyte EBV receptor (CR2/CD21) into Madin-Darby canine kidney (MDCK) epithelial cells used extensively to study effects of cell polarity on infection by both DNA and RNA viruses. CR2/CD21 was detected on both apical and basolateral surfaces of polarized MDCK cells, with predominant expression basolaterally. However, infectivity was up to four-fold greater apically, suggesting that endogenous cell surface molecules, sorted asymmetrically onto polarized plasma membranes, may be involved in EBV entry into MDCK cells. EBV gp350/220, a replicative cycle glycoprotein added to the virus envelope on egress through the cell membrane, was immunolocalized by confocal microscopy to basolateral cell surfaces only. Apical entry of EBV with subsequent basolateral release of newly replicated virus favors systemic infection by viral dissemination to underlying lymphocytic aggregations. Under conditions of long-term culture, latent EBV was not stably maintained in these cells, suggesting that the epithelial phase of acute EBV infection may be transient.

Pathogenicity of Sendai viruses adapted into polarized MDCK cells

Apically and basally released Sendai viruses (SeV) were obtained after infection of polarized Madin-Darby canine kidney (MDCK) cells grown on permeable membrane culture inserts. After 20 passages of adaptation in MDCK cells, we compared their in vivo and in vitro pathogenicity with the parental Mol-strain of SeV. These viruses had comparable in vitro pathogenicity, but the in vivo pathogenicities were varied. The apically released MDCK-adapted virus showed comparable pathogenicity with the parental virus, in contrast with the basally released MDCK-adapted virus, which showed in vivo attenuation.

MAL, an integral element of the apical sorting machinery, is an itinerant protein that cycles between the trans-Golgi network and the plasma membrane

The MAL proteolipid is a nonglycosylated integral membrane protein found in glycolipid-enriched membrane microdomains. In polarized epithelial Madin-Darby canine kidney cells, MAL is necessary for normal apical transport and accurate sorting of the influenza virus hemagglutinin. MAL is thus part of the integral machinery for glycolipid-enriched membrane-mediated apical transport. At steady state, MAL is predominantly located in perinuclear vesicles that probably arise from the trans-Golgi network (TGN). To act on membrane traffic and to prevent their accumulation in the target compartment, integral membrane elements of the protein-sorting machinery should be itinerant proteins that cycle between the donor and target compartments. To establish whether MAL is an itinerant protein, we engineered the last extracellular loop of MAL by insertion of sequences containing the FLAG epitope or with sequences containing residues that became O-glycosylated within the cells or that displayed biotinylatable groups. The ectopic expression of these modified MAL proteins allowed us to investigate the surface expression of MAL and its movement through different compartments after internalization with the use of a combination of assays, including surface biotinylation, surface binding of anti-FLAG antibodies, neuraminidase sensitivity, and drug treatments. Immunofluorescence and flow cytometric analyses indicated that, in addition to its Golgi localization, MAL was also expressed on the cell surface, from which it was rapidly internalized. This retrieval implies transport through the endosomal pathway and requires endosomal acidification, because it can be inhibited by drugs such as chloroquine, monensin, and NH(4)Cl. Resialylation experiments of surface MAL treated with neuraminidase indicated that approximately 30% of the internalized MAL molecules were delivered to the TGN, probably to start a new cycle of cargo transport. Together, these observations suggest that, as predicted for integral membrane members of the late protein transport machinery, MAL is an itinerant protein cycling between the TGN and the plasma membrane.

Polarity of osteoblasts and osteoblast-like UMR-108 cells

Enveloped viruses, such as vesicular stomatitis virus (VSV) and Influenza virus, have been widely used in studying epithelial cell polarity. Viral particles of VSV-infected epithelial cells bud from the basolateral membrane, which is in contact with the internal milieu and the blood supply. Influenza-infected cells bud viral particles from the apical surface facing the external milieu. This feature can be utilized in labeling polarized membrane domains. We studied the polarity of mesenchymal osteoblasts using osteosarcoma cell line UMR-108 and endosteal osteoblasts in situ in bone tissue cultures. Immunofluorescence confocal microscopy revealed that the VSV glycoprotein (VSV G) was targeted to the culture medium-facing surface. In endosteal osteoblasts, VSV G protein was found in the surface facing bone marrow and circulation. On the contrary, Influenza virus hemagglutinin (HA) was localized to the bone substrate-facing surface of the UMR-108 cells. Electron microscopy showed that in the cases where the cells were growing as a single layer, VSV particles were budding from the culture medium-facing surface, whereas Influenza viruses budded from the bone substrate-facing surface. When the cells overlapped, this polarity was lost. Cell surface biotinylation revealed that 55% of VSV G protein was biotinylated, whereas Influenza virus HA was only 22% biotinylated. These findings suggest that osteoblasts are polarized at some point of their life cycle. The bone-attaching plasma membrane of osteoblasts is apical, and the circulation or bone marrow-facing plasma membrane is basolateral in nature.

Interaction of viral infections with muscarinic receptors

Both in humans and in experimental animals, much of the airway hyperresponsiveness that accompanies viral infections is the result of increased reflex bronchoconstriction. The M3 muscarinic receptors on the airway smooth muscle function normally during viral infections so that the direct effects of acetylcholine on the smooth muscle are not altered. In contrast, the M2 muscarinic receptors on the vagal nerve endings, which normally inhibit acetycholine release, are markedly dysfunctional during viral infections. This leads to substantial increases in acetylcholine release and potentiated reflex bronchoconstriction. Multiple mechanisms account for virus-induced M2 receptor dysfunction. Viral neuraminidase may deglycosylate the M2 receptor, decreasing acetylcholine affinity. Furthermore, both viruses and interferon-gamma decrease M2 receptor gene expression. Finally, in atopic hosts, viral infection causes M2 receptor dysfunction by activating eosinophils, causing them to release major basic protein which binds to the M2 receptor, functioning as an endogenous antagonist.