Inhibitors of COP-mediated transport and cholera toxin action inhibit simian virus 40 infection

Antibacterial carbonic anhydrase inhibitors targeting Vibrio cholerae enzymes

INTRODUCTION

Cholera is a bacterial diarrheal disease caused by pathogen bacteria Vibrio cholerae, which produces the cholera toxin (CT). In addition to improving water sanitation, oral cholera vaccines have been developed to control infection. Besides, rehydration and antibiotic therapy are complementary treatment strategies for cholera. ToxT regulatory protein activates transcription of CT gene, which is enhanced by bicarbonate (HCO3-).

AREAS COVERED

This review delves into the genomic blueprint of V. cholerae, which encodes for α-, β-, and γ- carbonic anhydrases (CAs). We explore how the CAs contribute to the pathogenicity of V. cholerae and discuss the potential of CA inhibitors in mitigating the disease's impact.

EXPERT OPINION

CA inhibitors can reduce the virulence of bacteria and control cholera. Here, we reviewed all reported CA inhibitors, noting that α-CA from V. cholerae (VchCAα) was the most effective inhibited enzyme compared to the β- and γ-CA families (VchCAβ and VchCAγ). Among the CA inhibitors, acyl selenobenzenesulfonamidenamides and simple/heteroaromatic sulfonamides were the best VchCA inhibitors in the nM range. It was noted that some antibacterial compounds show good inhibitory effects on all three bacterial CAs. CA inhibitors belonging to other classes may be synthesized and tested on VchCAs to harness cholera.

The manipulation of cell signaling and host cell biology by cholera toxin

Vibrio cholerae colonizes the small intestine and releases cholera toxin into the extracellular space. The toxin binds to the apical surface of the epithelium, is internalized into the host endomembrane system, and escapes into the cytosol where it activates the stimulatory alpha subunit of the heterotrimeric G protein by ADP-ribosylation. This initiates a cAMP-dependent signaling pathway that stimulates chloride efflux into the gut, with diarrhea resulting from the accompanying osmotic movement of water into the intestinal lumen. G protein signaling is not the only host system manipulated by cholera toxin, however. Other cellular mechanisms and signaling pathways active in the intoxication process include endocytosis through lipid rafts, retrograde transport to the endoplasmic reticulum, the endoplasmic reticulum-associated degradation system for protein delivery to the cytosol, the unfolded protein response, and G protein de-activation through degradation or the function of ADP-ribosyl hydrolases. Although toxin-induced chloride efflux is thought to be an irreversible event, alterations to these processes could facilitate cellular recovery from intoxication. This review will highlight how cholera toxin exploits signaling pathways and other cell biology events to elicit a diarrheal response from the host.

GRK2 mediates β-arrestin interactions with 5-HT2 receptors for JC polyomavirus endocytosis

JC polyomavirus (JCPyV) infects the majority of the population, establishing a lifelong, asymptomatic infection in the kidney of healthy individuals. People that become severely immunocompromised may experience JCPyV reactivation, which can cause progressive multifocal leukoencephalopathy (PML), a neurodegenerative disease. Due to a lack of therapeutic options, PML results in fatality or significant debilitation among affected individuals. Cellular internalization of JCPyV is mediated by serotonin 5-hydroxytryptamine subfamily 2 receptors (5-HT₂Rs) via clathrin-mediated endocytosis. The JCPyV entry process requires the clathrin-scaffolding proteins β-arrestin, adaptor protein 2 (AP2), and dynamin. Further, a β-arrestin interacting domain, the Ala-Ser-Lys (ASK) motif, within the C-terminus of 5-HT_2AR is important for JCPyV internalization and infection. Interestingly, 5-HT₂R subtypes A, B, and C equally support JCPyV entry and infection, and all subtypes contain an ASK motif, suggesting a conserved mechanism for viral entry. However, the role of the 5-HT₂R ASK motifs and the activation of β-arrestin-associated proteins during internalization has not been fully elucidated. Through mutagenesis, the ASK motifs within 5-HT_2BR and 5-HT_2CR were identified as critical for JCPyV internalization and infectivity. Further, utilizing biochemical pulldown techniques, mutagenesis of the ASK motifs in 5-HT_2BR and 5-HT_2CR resulted in reduced β-arrestin binding. Utilizing small-molecule chemical inhibitors and RNA interference, G-protein receptor kinase 2 (GRK2) was determined to be required for JCPyV internalization and infection by mediating interactions between β-arrestin and the ASK motif of 5-HT₂Rs. These findings demonstrate that GRK2 and β-arrestin interactions with 5-HT₂Rs are critical for JCPyV entry by clathrin-mediated endocytosis and resultant infection.IMPORTANCE As intracellular parasites, viruses require a host cell to replicate and cause disease. Therefore, virus-host interactions contribute to viral pathogenesis. JC polyomavirus (JCPyV) infects most of the population, establishing a lifelong asymptomatic infection within the kidney. Under conditions of severe immunosuppression JCPyV may spread to the central nervous system, causing the fatal demyelinating disease progressive multifocal leukoencephalopathy (PML). Individuals living with HIV or undergoing immunomodulatory therapies are at risk for developing PML. The mechanisms of how JCPyV uses specific receptors on the surface of host cells to initiate internalization and infection is a poorly understood process. We have further identified cellular proteins involved in JCPyV internalization and infection and elucidated their specific interactions that are responsible for activation of receptors. Collectively, these findings illuminate how viruses usurp cellular receptors during infection, contributing to current development efforts for therapeutic options for the treatment or prevention of PML.

Sending mixed signals: polyomavirus entry and trafficking

Polyomaviruses are mostly non-pathogenic, yet some can cause human disease especially under conditions of immunosuppression, including JC, BK, and Merkel cell polyomaviruses. Direct interactions between viruses and the host early during infection dictate the outcome of disease, many of which remain enigmatic. However, significant work in recent years has contributed to our understanding of how this virus family establishes an infection, largely due to advances made for animal polyomaviruses murine and SV40. Here we summarize the major findings that have contributed to our understanding of polyomavirus entry, trafficking, disassembly, signaling, and immune evasion during the infectious process and highlight major unknowns in these processes that are open areas of study.

Taking the Scenic Route: Polyomaviruses Utilize Multiple Pathways to Reach the Same Destination

Members of the Polyomaviridae family differ in their host range, pathogenesis, and disease severity. To date, some of the most studied polyomaviruses include human JC, BK, and Merkel cell polyomavirus and non-human subspecies murine and simian virus 40 (SV40) polyomavirus. Although dichotomies in host range and pathogenesis exist, overlapping features of the infectious cycle illuminate the similarities within this virus family. Of particular interest to human health, JC, BK, and Merkel cell polyomavirus have all been linked to critical, often fatal, illnesses, emphasizing the importance of understanding the underlying viral infections that result in the onset of these diseases. As there are significant overlaps in the capacity of polyomaviruses to cause disease in their respective hosts, recent advancements in characterizing the infectious life cycle of non-human murine and SV40 polyomaviruses are key to understanding diseases caused by their human counterparts. This review focuses on the molecular mechanisms by which different polyomaviruses hijack cellular processes to attach to host cells, internalize, traffic within the cytoplasm, and disassemble within the endoplasmic reticulum (ER), prior to delivery to the nucleus for viral replication. Unraveling the fundamental processes that facilitate polyomavirus infection provides deeper insight into the conserved mechanisms of the infectious process shared within this virus family, while also highlighting critical unique viral features.

Mycobacterium tuberculosis and myeloid-derived suppressor cells: Insights into caveolin rich lipid rafts

Mycobacterium tuberculosis (M.tb) is likely the most successful human pathogen, capable of evading protective host immune responses and driving metabolic changes to support its own survival and growth. Ineffective innate and adaptive immune responses inhibit effective clearance of the bacteria from the human host, resulting in the progression to active TB disease. Many regulatory mechanisms exist to prevent immunopathology, however, chronic infections result in the overproduction of regulatory myeloid cells, like myeloid-derived suppressor cells (MDSC), which actively suppress protective host T lymphocyte responses among other immunosuppressive mechanisms. The mechanisms of M.tb internalization by MDSC and the involvement of host-derived lipid acquisition, have not been fully elucidated. Targeted research aimed at investigating MDSC impact on phagocytic control of M.tb, would be advantageous to our collective anti-TB arsenal. In this review we propose a mechanism by which M.tb may be internalized by MDSC and survive via the manipulation of host-derived lipid sources.

Choosing the right path: membrane trafficking and infectious entry of small DNA tumor viruses

To infect mammalian cells, all infectious viruses must cross a common set of biophysical membrane barriers to gain access to the cell. The virus capsid proteins attach to a host cell, become endocytosed, and traffic the viral genome to sites of replication. To do this they must interact with the membrane-confined organelles that control endocytosis, endosomal sorting, processing, and degradation of biological molecules. In this review, we highlight some recent advances in our understanding of the mechanisms that small non-enveloped DNA tumor viruses, such as Human Papillomavirus (HPV) and Polyomaviruses (PyV) employ to attain infectious entry. These viruses exploit different pathways to mediate entry, uncoating and subsequent transport to the nucleus via the Trans Golgi Network (TGN) or the Endoplasmic Reticulum (ER). Understanding how the viral capsid proteins interact with cellular membranous organelles sheds light on the novel ways by which viruses can hi-jack endocytic transport pathways and provides unique insights into how the highly complex machinery controlling cargo fate determination is regulated within the cell.

JC Polyomavirus Entry by Clathrin-Mediated Endocytosis Is Driven by β-Arrestin

JC polyomavirus (JCPyV) establishes a persistent, lifelong, asymptomatic infection within the kidney of the majority of the human population. Under conditions of severe immunosuppression or immune modulation, JCPyV can reactivate in the central nervous system (CNS) and cause progressive multifocal leukoencephalopathy (PML), a fatal demyelinating disease. Initiation of infection is mediated through viral attachment to α2,6-sialic acid-containing lactoseries tetrasaccharide c (LSTc) on the surface of host cells. JCPyV internalization is dependent on serotonin 5-hydroxytryptamine subfamily 2 receptors (5-HT₂Rs), and entry is thought to occur by clathrin-mediated endocytosis (CME). However, the JCPyV entry process and the cellular factors involved in viral internalization remain poorly understood. Treatment of cells with small-molecule chemical inhibitors and RNA interference of 5-HT₂R endocytic machinery, including β-arrestin, clathrin, AP2, and dynamin, significantly reduced JCPyV infection. However, infectivity of the polyomavirus simian virus 40 (SV40) was not affected by CME-specific treatments. Inhibition of clathrin or β-arrestin specifically reduced JCPyV internalization but did not affect viral attachment. Furthermore, mutagenesis of a β-arrestin binding domain (Ala-Ser-Lys) within the intracellular C terminus of 5-HT_2AR severely diminished internalization and infection, suggesting that β-arrestin interactions with 5-HT_2AR are critical for JCPyV infection and entry. These conclusions illuminate key host factors that regulate clathrin-mediated endocytosis of JCPyV, which is necessary for viral internalization and productive infection.IMPORTANCE Viruses usurp cellular factors to invade host cells. Activation and utilization of these proteins upon initiation of viral infection are therefore required for productive infection and resultant viral disease. The majority of healthy individuals are asymptomatically infected by JC polyomavirus (JCPyV), but if the host immune system is compromised, JCPyV can cause progressive multifocal leukoencephalopathy (PML), a rare, fatal, demyelinating disease. Individuals infected with HIV or taking prolonged immunomodulatory therapies have a heightened risk for developing PML. The cellular proteins and pathways utilized by JCPyV to mediate viral entry are poorly understood. Our findings further characterize how JCPyV utilizes the clathrin-mediated endocytosis pathway to invade host cells. We have identified specific components of this pathway that are necessary for the viral entry process and infection. Collectively, the conclusions increase our understanding of JCPyV infection and pathogenesis and may contribute to the future development of novel therapeutic strategies for PML.

Viral journeys on the intracellular highways

Viruses are obligate intracellular pathogens that are dependent on cellular machineries for their replication. Recent technological breakthroughs have facilitated reliable identification of host factors required for viral infections and better characterization of the virus-host interplay. While these studies have revealed cellular machineries that are uniquely required by individual viruses, accumulating data also indicate the presence of broadly required mechanisms. Among these overlapping cellular functions are components of intracellular membrane trafficking pathways. Here, we review recent discoveries focused on how viruses exploit intracellular membrane trafficking pathways to promote various stages of their life cycle, with an emphasis on cellular factors that are usurped by a broad range of viruses. We describe broadly required components of the endocytic and secretory pathways, the Endosomal Sorting Complexes Required for Transport pathway, and the autophagy pathway. Identification of such overlapping host functions offers new opportunities to develop broad-spectrum host-targeted antiviral strategies.

JC Polyomavirus Attachment and Entry: Potential Sites for PML Therapeutics

PURPOSE OF REVIEW

JC polyomavirus (JCPyV) is a significant human pathogen that causes an asymptomatic infection in the kidney in the majority of the population. In immunosuppressed individuals, the virus can become reactivated and spread to the brain, causing the fatal, demyelinating disease progressive multifocal leukoencephalopathy (PML). There are currently limited treatment options for this fatal disease. Attachment to receptors and entry into host cells are the initiating events in JCPyV infection and therefore an attractive target for therapeutics to prevent or treat PML. This review provides the current understanding of JCPyV attachment and entry events and the potential therapeutics to target these areas.

RECENT FINDINGS

JCPyV attachment and entry to host cells is mediated by α2,6-linked lactoseries tetrasaccharide c (LSTc) and 5-hydroxytryptamine receptors (5-HT₂Rs), respectively, and subsequent trafficking to the endoplasmic reticulum is required for infection. Recently, vaccines, monoclonal antibodies, and small molecules have shown promise as anti-viral and PML therapies.

SUMMARY

This review summarizes our current understanding of JCPyV attachment, entry, and trafficking and the development of potential PML therapeutics that inhibit these critical steps in JCPyV infection.

CSFV proliferation is associated with GBF1 and Rab2

The Golgi apparatus and its resident proteins are utilized and regulated by viruses to facilitate their proliferation. In this study, we investigated Classical swine fever virus (CSFV) proliferation when the function of the Golgi was disturbed. Golgi function was disturbed using chemical inhibitors, namely, brefeldin A (BFA) and golgicide A (GCA), and RNA interfering targets, such as the Golgi-specific BFA-resistance guanine nucleotide exchange factor 1 (GBF1) and Rab2 GTPases. CSFV proliferation was significantly inhibited during RNA replication and viral particle generation after BFA and GCA treatment. CSFV multiplication dynamics were retarded in cells transfected with GBF1 and Rab2 shRNA. Furthermore, CSFV proliferation was promoted by GBF1 and Rab2 overexpression using a lentiviral system. Hence, Golgi function is important for CSFV multiplication, and GBF1 and Rab2 participate in CSFV proliferation. Further studies must investigate Golgi-resident proteins to elucidate the mechanism underlying CSFV replication.

Nonenveloped capsid: mechanisms of viral entry

ABSTRACT 

Viruses are a diverse class of nanoparticles. However, they have evolved a few common mechanisms that enable successful infection of their host cells. The first stage of this process involves entry into the cell. For enveloped viruses this process has been well characterized. For nonenveloped viruses, the focus of this review, the entry mechanisms are less well understood. For these viruses, a typical pathway involves receptor attachment followed by internalization into cellular vesicles and subsequent viral escape to the cytosol and transport to the site of genome replication. Significantly, these viruses have evolved numerous mechanisms to fulfill this seemingly simple infection scheme. We focus on the latest observations for several families of nonenveloped viruses and highlight specific members for eukaryotic families: Adenoviridae, Papillomaviridae, Parvoviridae, Picornaviridae, Polyomaviridae and Reoviridae; and prokaryotic families: Microviridae, Myoviridae, Podoviridae and Siphoviridae.

ADP-ribosylation factor 6 acts as an allosteric activator for the folded but not disordered cholera toxin A1 polypeptide

The catalytic A1 subunit of cholera toxin (CTA1) has a disordered structure at 37°C. An interaction with host factors must therefore place CTA1 in a folded conformation for the modification of its Gsα target which resides in a lipid raft environment. Host ADP-ribosylation factors (ARFs) act as in vitro allosteric activators of CTA1, but the molecular events of this process are not fully characterized. Isotope-edited Fourier transform infrared spectroscopy monitored ARF6-induced structural changes to CTA1, which were correlated to changes in CTA1 activity. We found ARF6 prevents the thermal disordering of structured CTA1 and stimulates the activity of stabilized CTA1 over a range of temperatures. Yet ARF6 alone did not promote the refolding of disordered CTA1 to an active state. Instead, lipid rafts shifted disordered CTA1 to a folded conformation with a basal level of activity that could be further stimulated by ARF6. Thus, ARF alone is unable to activate disordered CTA1 at physiological temperature: additional host factors such as lipid rafts place CTA1 in the folded conformation required for its ARF-mediated activation. Interaction with ARF is required for in vivo toxin activity, as enzymatically active CTA1 mutants that cannot be further stimulated by ARF6 fail to intoxicate cultured cells.

Coat as a dagger: the use of capsid proteins to perforate membranes during non-enveloped DNA viruses trafficking

To get access to the replication site, small non-enveloped DNA viruses have to cross the cell membrane using a limited number of capsid proteins, which also protect the viral genome in the extracellular environment. Most of DNA viruses have to reach the nucleus to replicate. The capsid proteins involved in transmembrane penetration are exposed or released during endosomal trafficking of the virus. Subsequently, the conserved domains of capsid proteins interact with cellular membranes and ensure their efficient permeabilization. This review summarizes our current knowledge concerning the role of capsid proteins of small non-enveloped DNA viruses in intracellular membrane perturbation in the early stages of infection.

A cytosolic chaperone complexes with dynamic membrane J-proteins and mobilizes a nonenveloped virus out of the endoplasmic reticulum

Nonenveloped viruses undergo conformational changes that enable them to bind to, disrupt, and penetrate a biological membrane leading to successful infection. We assessed whether cytosolic factors play any role in the endoplasmic reticulum (ER) membrane penetration of the nonenveloped SV40. We find the cytosolic SGTA-Hsc70 complex interacts with the ER transmembrane J-proteins DnaJB14 (B14) and DnaJB12 (B12), two cellular factors previously implicated in SV40 infection. SGTA binds directly to SV40 and completes ER membrane penetration. During ER-to-cytosol transport of SV40, SGTA disengages from B14 and B12. Concomitant with this, SV40 triggers B14 and B12 to reorganize into discrete foci within the ER membrane. B14 must retain its ability to form foci and interact with SGTA-Hsc70 to promote SV40 infection. Our results identify a novel role for a cytosolic chaperone in the membrane penetration of a nonenveloped virus and raise the possibility that the SV40-induced foci represent cytosol entry sites.

A retrograde trafficking inhibitor of ricin and Shiga-like toxins inhibits infection of cells by human and monkey polyomaviruses

Polyomaviruses are ubiquitous pathogens that cause severe disease in immunocompromised individuals. JC polyomavirus (JCPyV) is the causative agent of the fatal demyelinating disease progressive multifocal leukoencephalopathy (PML), whereas BK polyomavirus (BKPyV) causes polyomavirus-induced nephropathy and hemorrhagic cystitis. Vaccines or antiviral therapies targeting these viruses do not exist, and treatments focus on reducing the underlying causes of immunosuppression. We demonstrate that retro-2(cycl), an inhibitor of ricin and Shiga-like toxins (SLTs), inhibits infection by JCPyV, BKPyV, and simian virus 40. Retro-2(cycl) inhibits retrograde transport of polyomaviruses to the endoplasmic reticulum, a step necessary for productive infection. Retro-2(cycl) likely inhibits polyomaviruses in a way similar to its ricin and SLT inhibition, suggesting an overlap in the cellular host factors used by bacterial toxins and polyomaviruses. This work establishes retro-2(cycl) as a potential antiviral therapy that broadly inhibits polyomaviruses and possibly other pathogens that use retrograde trafficking.

IMPORTANCE

The human polyomaviruses JC polyomavirus (JCPyV) and BK polyomavirus (BKPyV) cause rare but severe diseases in individuals with reduced immune function. During immunosuppression, JCPyV disseminates from the kidney to the central nervous system and destroys oligodendrocytes, resulting in the fatal disease progressive multifocal leukoencephalopathy. Kidney transplant recipients are at increased risk of BKPyV-induced nephropathy, which results in kidney necrosis and loss of the transplanted organ. There are currently no effective therapies for JCPyV and BKPyV. We show that a small molecule named retro-2(cycl) protects cells from infection with JCPyV and BKPyV by inhibiting intracellular viral transport. Retro-2(cycl) treatment reduces viral spreading in already established infections and may therefore be able to control infection in affected patients. Further optimization of retro-2(cycl) may result in the development of an effective antiviral therapy directed toward pathogens that use retrograde trafficking to infect their hosts.

Grape extracts inhibit multiple events in the cell biology of cholera intoxication

Vibrio cholerae produces cholera toxin (CT), an AB₅ protein toxin that is primarily responsible for the profuse watery diarrhea of cholera. CT is secreted into the extracellular milieu, but the toxin attacks its Gsα target within the cytosol of a host cell. Thus, CT must cross a cellular membrane barrier in order to function. This event only occurs after the toxin travels by retrograde vesicular transport from the cell surface to the endoplasmic reticulum (ER). The catalytic A1 polypeptide then dissociates from the rest of the toxin and assumes an unfolded conformation that facilitates its transfer to the cytosol by a process involving the quality control system of ER-associated degradation. Productive intoxication is blocked by alterations to the vesicular transport of CT and/or the ER-to-cytosol translocation of CTA1. Various plant compounds have been reported to inhibit the cytopathic activity of CT, so in this work we evaluated the potential anti-CT properties of grape extract. Two grape extracts currently sold as nutritional supplements inhibited CT and Escherichia coli heat-labile toxin activity against cultured cells and intestinal loops. CT intoxication was blocked even when the extracts were added an hour after the initial toxin exposure. A specific subset of host-toxin interactions involving both the catalytic CTA1 subunit and the cell-binding CTB pentamer were affected. The extracts blocked toxin binding to the cell surface, prevented unfolding of the isolated CTA1 subunit, inhibited CTA1 translocation to the cytosol, and disrupted the catalytic activity of CTA1. Grape extract could thus potentially serve as a novel therapeutic to prevent or possibly treat cholera.

The structure of the caspase recruitment domain of BinCARD reveals that all three cysteines can be oxidized

The caspase recruitment domain (CARD) is present in death-domain superfamily proteins involved in inflammation and apoptosis. BinCARD is named for its ability to interact with Bcl10 and inhibit downstream signalling. Human BinCARD is expressed as two isoforms that encode the same N-terminal CARD region but which differ considerably in their C-termini. Both isoforms are expressed in immune cells, although BinCARD-2 is much more highly expressed. Crystals of the CARD fold common to both had low symmetry (space group P1). Molecular replacement was unsuccessful in this low-symmetry space group and, as the construct contains no methionines, first one and then two residues were engineered to methionine for MAD phasing. The double-methionine variant was produced as a selenomethionine derivative, which was crystallized and the structure was solved using data measured at two wavelengths. The crystal structures of the native and selenomethionine double mutant were refined to high resolution (1.58 and 1.40 Å resolution, respectively), revealing the presence of a cis-peptide bond between Tyr39 and Pro40. Unexpectedly, the native crystal structure revealed that all three cysteines were oxidized. The mitochondrial localization of BinCARD-2 and the susceptibility of its CARD region to redox modification points to the intriguing possibility of a redox-regulatory role.

The VP1 subunit of JC polyomavirus recapitulates early events in viral trafficking and is a novel tool to study polyomavirus entry

JC polyomavirus (JCV) is an important human pathogen that causes the fatal demyelinating disease progressive multifocal leukoencephalopathy (PML). In this study we further delineate the early events of JCV entry in human glial cells and demonstrate that a pentameric subunit of the viral capsid is able to recapitulate early events in viral trafficking. We show that JCV traffics to the endoplasmic reticulum (ER) by 6h post infection, and that VP1 pentamers arrive at the ER with similar kinetics. Further, this JCV localization to the ER is critical for infection, as treatment of cells with agents that prevent ER trafficking, ER function, or ER quality control reduce JCV infectivity. These pentamers represent a new tool to study polyomavirus entry, and will be particularly useful in studying recently identified polyomaviruses that are difficult to propagate.

Caveolae and the regulation of endocytosis

Although clathrin-mediated endocytosis constitutes the main pathway for internalization of extracellular ligands and plasma membrane components it has generally been accepted that other uptake mechanisms-caveolae-mediated and noncaveolar raft-dependent endocytosis-also exist. During the last 20 years many papers have been published about caveolar endocytosis. These studies have fundamentally changed our view about the endocytotic role of caveolae. Views that caveolae are permanently static structures1 have been extensively considered and rejected. Although the initial steps leading to the pinching off of caveolae from the plasma membrane have been studied in details, there are still contradictory data about the intracellular trafficking of caveolae. It is still not entirely clear whether caveolar endocytosis represents an uptake pathway with distinct cellular compartments to avoid lysosomal degradation or ligands taken up by caveolae can also be targeted to late endosomes/lysosomes.In this chapter, we summarize the data available about caveolar endocytosis focusing on the intracellular route of caveolae and we provide data supporting that caveolar endocytosis can join the classical endocytotic pathway.

Role of Coatomer Protein I in Virus Replication

Coatomer protein I (COPI) is well known as the protein coat surrounding vesicles involved in returning endoplasmic reticulum (ER)-resident proteins to the ER. COPI coats are also found in vesicles involved in other trafficking processes including endocytosis, autophagy and anterograde transport in the secretory pathway. In view of the diverse functions of COPI proteins, it is expected that they will affect virus replication, and many reports of such COPI involvement have now appeared. The experimental approaches most often employ specific siRNA to deplete COPI subunits or brefeldin A to block COPI activation. Here we briefly describe the results obtained with viruses in which COPI is found to have a role in replication. The results demonstrate that COPI affects viruses quite differently with effects observed in processes such as entry, RNA replication, and intracellular transport of viral proteins.

Cell culture-adapted IBDV uses endocytosis for entry in DF-1 chicken embryonic fibroblasts

Although membrane perforation was suggested as the means of penetration mediated by IBDV, the cellular mechanism being hijacked to facilitate its entry is largely unknown. In this study, the entry pathway of cell culture adapted IBDV (caIBDV) was characterized in DF-1 chicken embryonic fibroblasts. We observed that the entry of caIBDV was inhibited by bafilomycin A1 and CaEGTA which interfere with the function of vacuolar H(+)-ATPase (V-ATPase) and retain endosomal Ca(2+). This result suggests that the intact caIBDV particle was transported to the V-ATPase positive vesicles for uncoating and implicates an essential role of endocytosis during the viral entry. The IBDV-mediated endocytosis was demonstrated to be clathrin-independent. Instead, the entry of caIBDV in DF-1 was reduced under the inhibitions or depletions of lipid raft, c-Src tyrosine kinase, dynamin and actin polymerization. In summary, this study confirmed the role of endocytosis in caIBDV entry and characterized the route of its endocytosis.

BAP31 and BiP are essential for dislocation of SV40 from the endoplasmic reticulum to the cytosol

How non-enveloped viruses overcome host cell membranes is poorly understood. Here, we show that after endocytosis and transport to the endoplasmic reticulum (ER), but before crossing the ER membrane to the cytosol, incoming simian virus 40 particles are structurally remodelled leading to exposure of the amino-terminal sequence of the minor viral protein VP2. These hydrophobic sequences anchor the virus to membranes. A negatively charged residue, Glu 17, in the α-helical, membrane-embedded peptide is essential for infection, most likely by introducing an 'irregularity' recognized by the ER-associated degradation (ERAD) system for membrane proteins. Using a siRNA-mediated screen, the lumenal chaperone BiP and the ER-membrane protein BAP31 (both involved in ERAD) were identified as being essential for infection. They co-localized with the virus in discrete foci and promoted its ER-to-cytosol dislocation. Virus-like particles devoid of VP2 failed to cross the membrane. The results demonstrated that ERAD-factors assist virus transport across the ER membrane.

Lipid-mediated endocytosis

Receptor-mediated endocytosis is used by a number of viruses and toxins to gain entry into cells. Some have evolved to use specific lipids in the plasma membrane as their receptors. They include bacterial toxins such as Shiga and Cholera toxin and viruses such as mouse polyoma virus and simian virus 40. Through multivalent binding to glycosphingolipids, they induce lipid clustering and changes in membrane properties. Internalization occurs by unusual endocytic mechanisms involving lipid rafts, induction of membrane curvature, trans-bilayer coupling, and activation of signaling pathways. Once delivered to early endosomes, they follow diverse intracellular routes to the lumen of the ER, from which they penetrate into the cytosol. The role of the lipid receptors is central in these well-studied processes.

Plasmalemmal vesicle associated protein (PV1) modulates SV40 virus infectivity in CV-1 cells

Plasmalemmal vesicle associated protein (Plvap/PV1) is a structural protein required for the formation of the stomatal diaphragms of caveolae. Caveolae are plasma membrane invaginations that were implicated in SV40 virus entry in primate cells. Here we show that de novo Plvap/PV1 expression in CV-1 green monkey epithelial cells significantly reduces the ability of SV40 virus to establish productive infection, when cells are incubated with low concentrations of the virus. However, in presence of high viral titers PV1 has no effect on SV40 virus infectivity. Mechanistically, PV1 expression does not reduce the cell surface expression of known SV40 receptors such as GM1 ganglioside and MHC class I proteins. Furthermore, PV1 does not reduce the binding of virus-like particles made by SV40 VP1 protein to the CV-1 cell surface and does not impact their internalization when cells are incubated with either high or low VLP concentrations. These results suggest that PV1 protein is able to block SV40 infectivity at low but not at high viral concentration either by interfering with the infective internalization pathway at the cell surface or at a post internalization step.

The SV40 late protein VP4 is a viroporin that forms pores to disrupt membranes for viral release

Nonenveloped viruses are generally released by the timely lysis of the host cell by a poorly understood process. For the nonenveloped virus SV40, virions assemble in the nucleus and then must be released from the host cell without being encapsulated by cellular membranes. This process appears to involve the well-controlled insertion of viral proteins into host cellular membranes rendering them permeable to large molecules. VP4 is a newly identified SV40 gene product that is expressed at late times during the viral life cycle that corresponds to the time of cell lysis. To investigate the role of this late expressed protein in viral release, water-soluble VP4 was expressed and purified as a GST fusion protein from bacteria. Purified VP4 was found to efficiently bind biological membranes and support their disruption. VP4 perforated membranes by directly interacting with the membrane bilayer as demonstrated by flotation assays and the release of fluorescent markers encapsulated into large unilamellar vesicles or liposomes. The central hydrophobic domain of VP4 was essential for membrane binding and disruption. VP4 displayed a preference for membranes comprised of lipids that replicated the composition of the plasma membranes over that of nuclear membranes. Phosphatidylethanolamine, a lipid found at high levels in bacterial membranes, was inhibitory against the membrane perforation activity of VP4. The disruption of membranes by VP4 involved the formation of pores of ∼3 nm inner diameter in mammalian cells including permissive SV40 host cells. Altogether, these results support a central role of VP4 acting as a viroporin in the perforation of cellular membranes to trigger SV40 viral release.

Viruses exploit host cells for their propagation. Once an adequate number of viral particles have been assembled within the cell through the aid of cellular machinery of the host cell, the virus must be released from the cell for the virus to spread. For nonenveloped viruses or viruses that are solely encapsulated by a protein shell, this step most commonly involves the perforation of cellular membranes resulting in the lysis or death of the host cell. The mechanism for how this key terminal step in the viral life cycle is performed is poorly understood. We demonstrated that for the model nonenveloped virus SV40, the newly discovered virally encoded protein, termed VP4, perforates membranes by forming pores with a diameter of ∼3 nm in host cell membranes. While these pores are not of a sufficient size to provide a conduit that permits the movement of the virus through the membrane, they support membrane destabilization that leads to the disintegration of the membrane of the host cell and viral release.

Role of endosomes in simian virus 40 entry and infection

After binding to its cell surface receptor ganglioside GM1, simian virus 40 (SV40) is endocytosed by lipid raft-mediated endocytosis and slowly transported to the endoplasmic reticulum, where partial uncoating occurs. We analyzed the intracellular pathway taken by the virus in HeLa and CV-1 cells by using a targeted small interfering RNA (siRNA) silencing screen, electron microscopy, and live-cell imaging as well as by testing a variety of cellular inhibitors and other perturbants. We found that the virus entered early endosomes, late endosomes, and probably endolysosomes before reaching the endoplasmic reticulum and that this pathway was part of the infectious route. The virus was especially sensitive to a variety of perturbations that inhibited endosome acidification and maturation. Contrary to our previous models, which postulated the passage of the virus through caveolin-rich organelles that we called caveosomes, we conclude that SV40 depends on the classical endocytic pathway for infectious entry.

Cholera toxin: an intracellular journey into the cytosol by way of the endoplasmic reticulum

Cholera toxin (CT), an AB(5)-subunit toxin, enters host cells by binding the ganglioside GM1 at the plasma membrane (PM) and travels retrograde through the trans-Golgi Network into the endoplasmic reticulum (ER). In the ER, a portion of CT, the enzymatic A1-chain, is unfolded by protein disulfide isomerase and retro-translocated to the cytosol by hijacking components of the ER associated degradation pathway for misfolded proteins. After crossing the ER membrane, the A1-chain refolds in the cytosol and escapes rapid degradation by the proteasome to induce disease by ADP-ribosylating the large G-protein Gs and activating adenylyl cyclase. Here, we review the mechanisms of toxin trafficking by GM1 and retro-translocation of the A1-chain to the cytosol.

Ocadaic acid retains caveolae in multicaveolar clusters

Caveola-mediated endocytosis exists parallel to other forms of endocytosis. Being ligand-triggered, caveolar endocytosis provides a more selective and highly regulated way for uptake of specified substances. Internalized caveolae accumulate in intermediate organelles called caveosomes. It is still debated whether caveosomes are independent organelles or the downstream caveosomes interact with the classical endocytotic compartments. In our work caveola internalization was stimulated with a serine/threonine phosphatase (PP1 and PP2A) inhibitor (ocadaic acid-OA). To find out whether caveolar clusters are really independent organelles or they are still connected to the cell surface we used an electron dense surface marker, ruthenium red (Ru red). Since we were especially interested in the fate of caveolar clusters, the cells were treated with OA for longer time. Stimulating caveola-mediated endocytosis, OA treatment resulted in a significant increase in the number of caveolar cluster. Most of these clusters were found Ru red positive indicating that they were still conneted to the cell surface. Our double labeling experiments on ultrathin frozen sections clearly showed that in OA-treated cells caveolae are not transported to late endosomes instead they are accumulted in large multicaveolar clusters. We think that PP2A can be one of the key components to regulate the fusion of various endocytotic compartments and /or the trafficking along the microtubules.

Cellular entry of polyomaviruses

Polyomaviruses (Pys) are nonenveloped DNA tumor viruses that include the murine polyomavirus (mPy), simian virus 40 (SV40), and the human BK, JC, KI, WU, and Merkel Cell viruses. To cause infection, Pys must enter host cells and navigate through various intracellular compartments, where they undergo sequential conformational changes enabling them to uncoat and deliver the DNA genome into the nucleus. The ensuing transcription and replication of the genome leads to lytic infection or cell transformation. In recent years, a more coherent understanding of how Pys are transported from the plasma membrane to the nucleus is starting to emerge. This review will focus on the decisive steps of Py entry, including engagement of the host cell receptor, targeting to the endoplasmic reticulum (ER), penetration across the ER membrane, nuclear entry, and genome release. Strikingly, a number of these steps resemble the intoxication pathway of the AB(5) bacterial toxins. Thus, as Pys and bacterial toxins hijack similar cellular machineries during infection, a general principle appears to guide their entry into host cells.

GM1 structure determines SV40-induced membrane invagination and infection

Incoming simian virus 40 (SV40) particles enter tight-fitting plasma membrane invaginations after binding to the carbohydrate moiety of GM1 gangliosides in the host cell plasma membrane through pentameric VP1 capsid proteins. This is followed by activation of cellular signalling pathways, endocytic internalization and transport of the virus via the endoplasmic reticulum to the nucleus. Here we show that the association of SV40 (as well as isolated pentameric VP1) with GM1 is itself sufficient to induce dramatic membrane curvature that leads to the formation of deep invaginations and tubules not only in the plasma membrane of cells, but also in giant unilamellar vesicles (GUVs). Unlike native GM1 molecules with long acyl chains, GM1 molecular species with short hydrocarbon chains failed to support such invagination, and endocytosis and infection did not occur. To conceptualize the experimental data, a physical model was derived based on energetic considerations. Taken together, our analysis indicates that SV40, other polyoma viruses and some bacterial toxins (Shiga and cholera) use glycosphingolipids and a common pentameric protein scaffold to induce plasma membrane curvature, thus directly promoting their endocytic uptake into cells.

Human papillomavirus type 16 infection of human keratinocytes requires clathrin and caveolin-1 and is brefeldin a sensitive

Human papillomavirus type 16 (HPV16) has been identified as being the most common etiological agent leading to cervical cancer. Despite having a clear understanding of the role of HPV16 in oncogenesis, details of how HPV16 traffics during infection are poorly understood. HPV16 has been determined to enter via clathrin-mediated endocytosis, but the subsequent steps of HPV16 infection remain unclear. There is emerging evidence that several viruses take advantage of cross talk between routes of endocytosis. Specifically, JCV and bovine papillomavirus type 1 have been shown to enter cells by clathrin-dependent endocytosis and then require caveolin-1-mediated trafficking for infection. In this paper, we show that HPV16 is dependent on caveolin-1 after clathrin-mediated endocytosis. We provide evidence for the first time that HPV16 infection is dependent on trafficking to the endoplasmic reticulum (ER). This novel trafficking may explain the requirement for the caveolar pathway in HPV16 infection because clathrin-mediated endocytosis typically does not lead to the ER. Our data indicate that the infectious route for HPV16 following clathrin-mediated entry is caveolin-1 and COPI dependent. An understanding of the steps involved in HPV16 sorting and trafficking opens up the possibility of developing novel approaches to interfere with HPV16 infection and reduce the burden of papillomavirus diseases including cervical cancer.

Encapsulated cargo internalized by fusogenic liposomes partially overlaps the endoplasmic reticulum

Few endocytosed ligands, including bacterial toxins and simian virus 40 (SV40) have been shown to reach the endoplasmic reticulum (ER) in mammalian cells. Using calcein and fluorescently labelled lactoferrin encapsulated in fusogenic liposomes we found that the cargo uses a microtubule-based pathway with ER delivery. Endocytic uptake of the lipid vesicles was cholesterol dependent in all cell lines tested, including the caveolin-1-deficient human hepatoma 7 cell line. The ligand was transported in non-caveosome organelles requiring acidic pH for maturation, but able to escape the lysosomal route. These organelles were not recycling endosomes either, as shown by the lack of co-localization with recycling transferrin. Co-localization with the ER-tracker, orange fluorescent protein with KDEL signal retention and cholera toxin in live microscopy revealed an ER distribution of the fluorescent ligand. Brefeldin A, which prevents Golgi-dependent retrograde trafficking, does not disrupt the cargo delivery to the ER. This new endocytic pathway making use of acidic endosome-like organelles is an alternative to the reported SV40 caveolae pathways. Exploiting a cellular route linking the cell surface to the ER, fusogenic liposomes may become efficient drug delivery vehicles for ER stress and diseases.

Endocytosis via caveolae: alternative pathway with distinct cellular compartments to avoid lysosomal degradation?

Endocytosis – the uptake of extracellular ligands, soluble molecules, protein and lipids from the extracellular surface – is a vital process, comprising multiple mechanisms, including phagocytosis, macropinocytosis, clathrin-dependent and clathrin-independent uptake such as caveolae-mediated and non-caveolar raft-dependent endocytosis. The best-studied endocytotic pathway for internalizing both bulk membrane and specific proteins is the clathrin-mediated endocytosis. Although many papers were published about the caveolar endocytosis, it is still not known whether it represents an alternative pathway with distinct cellular compartments to avoid lysosomal degradation or ligands taken up by caveolae can also be targeted to late endosomes/lysosomes. In this paper, we summarize data available about caveolar endocytosis. We are especially focussing on the intracellular route of caveolae and providing data supporting that caveolar endocytosis can join to the classical endocytotic pathway.

Structure, attachment and entry of polyoma- and papillomaviruses

Polyoma- (PY) and Papillomavirus (PV) virions have remarkable structural equivalence although no discernable sequence similarities among the capsid proteins can be detected. Their similarities include the overall surface organization, the presence of 72 capsomeres composed of five molecules of the major capsid proteins, VP1 and L1, respectively, the structure of the core segment of capsomeres with classical antiparallel "jelly roll" beta strands as the major feature, and the linkage of neighboring capsomeres by invading C-terminal arms. Differences include the size of surface exposed loops that contain the dominant neutralizing epitopes, the details of the intercapsomeric interactions, and the presence of 2 or 1 minor capsid proteins, respectively. These differences may affect the dramatic differences observed in receptor binding and internalization pathways utilized by these viruses, but as detailed later even structural differences cannot completely explain receptor and pathway usage. In recent years, technical advances aiding the study of entry processes have allowed the identification of novel endocytic compartments and an appreciation of the links between endocytic pathways that were previously thought to be completely separable. This review is intended to highlight recent advances in our understanding of virus receptor interactions and their consequences for endocytosis and intracellular trafficking.

A dynamic view of hepatitis C virus replication complexes

Hepatitis C virus (HCV) replicates its genome in a membrane-associated replication complex (RC). Specific membrane alterations, designated membranous webs, represent predominant sites of HCV RNA replication. The principles governing HCV RC and membranous web formation are poorly understood. Here, we used replicons harboring a green fluorescent protein (GFP) insertion in nonstructural protein 5A (NS5A) to study HCV RCs in live cells. Two distinct patterns of NS5A-GFP were observed. (i) Large structures, representing membranous webs, showed restricted motility, were stable over many hours, were partitioned among daughter cells during cell division, and displayed a static internal architecture without detectable exchange of NS5A-GFP. (ii) In contrast, small structures, presumably representing small RCs, showed fast, saltatory movements over long distances. Both populations were associated with endoplasmic reticulum (ER) tubules, but only small RCs showed ER-independent, microtubule (MT)-dependent transport. We suggest that this MT-dependent transport sustains two distinct RC populations, which are both required during the HCV life cycle.

High-throughput in planta expression screening identifies an ADP-ribosylation factor (ARF1) involved in non-host resistance and R gene-mediated resistance

To identify positive regulators of cell death in plants, we performed a high-throughput screening, employing potato virus X-based overexpression in planta of a cDNA library derived from paraquat-treated Nicotiana benthamiana leaves. The screening of 30,000 cDNA clones enabled the identification of an ADP-ribosylation factor 1 (ARF1) that induces cell death when overexpressed in N. benthamiana. Overexpression of the guanosine diphosphate (GDP)-locked mutant of ARF1 did not trigger cell death, suggesting that ARF1 guanosine triphosphatase (GTPase) activity is necessary for the observed cell death-inducing activity. The ARF1 transcript level increased strongly following treatment with Phytophthora infestans elicitor INF1, as well as inoculation with a non-host pathogen Pseudomonas cichorii in N. benthamiana. In addition, ARF1 was induced in the interaction between the N gene and tobacco mosaic virus (TMV) in Nicotiana tabacum. By contrast, inoculation with the virulent pathogen Pseudomonas syringae pv. tabaci did not affect ARF1 expression in N. benthamiana. Virus-induced gene silencing of ARF1 in N. benthamiana resulted in a stunted phenotype, and severely hampered non-host resistance towards P. cichorii. In addition, ARF1 silencing partially compromised resistance towards TMV in N. benthamiana containing the N resistance gene. By contrast, and in accordance with the ARF1 gene expression profile, silencing of ARF1 transcription did not alter the susceptibility of N. benthamiana towards the pathogen P. syringae pv. tabaci. These results strongly implicate ARF1 in the non-host resistance to bacteria and N gene-mediated resistance in N. benthamiana.

Regulation of virulence in Vibrio cholerae: the ToxR regulon

Vibrio cholerae is a gram-negative bacterium that is the causative agent of cholera. This disease consists of enormous fluid loss through stools, which can be fatal. Cholera epidemics appear in explosive outbreaks that have occurred repeatedly throughout history. The virulence factors toxin coregulated pilus (TCP) and cholera toxin (CT) are essential for colonization of the host and enterotoxicity, respectively. These virulence factors are under the control of ToxT, an AraC/XylS family protein that activates transcription of the genes encoding TCP and CT. ToxT is under the control of a virulence regulatory cascade known as the ToxR regulon, which responds to environmental stimuli to ensure maximal virulence-factor induction within the human intestine. An understanding of this intricate signaling pathway is essential for the development of methods to treat and prevent this devastating disease.

ERM is expressed by alveolar epithelial cells in adult mouse lung and regulates caveolin-1 transcription in mouse lung epithelial cell lines

We previously identified an Ets cis-element in the mouse caveolin-1 promoter that is selectively activated in lung epithelial (E10), but not lung endothelial murine lung endothelial cell line (MFLM-4), cell lines and therefore appears important for differential, cell-specific caveolin-1 transcription. In the present study, we demonstrate that immunostaining of adult mouse lung detects the ETS protein Ets-related molecule (ERM PEA3) in distal lung epithelium in alveolar type I and II cells, but not in bronchial epithelium or lung endothelial cells. We tested ERM and polyomavirus enhancer activator 3 (PEA3) for their ability to increase endogenous caveolin-1 transcripts and to activate caveolin-1 promoter fragments containing the -865 Ets cis-element. Chromatin immunoprecipitation (ChIP) assays show that both ERM and PEA3 bind to the caveolin-1 promoter in murine E10, but not MFLM-4, cells. Normalized luciferase activities show that only ERM activates the caveolin-1 promoter in E10 cells, but neither protein enhances promoter activity in MFLM-4 cells. Mutation of the Ets site blocks ERM-mediated promoter activation in E10 cells. Furthermore, overexpression of ERM increases the cellular content of caveolin-1 mRNA and protein, in E10, but not MFLM-4, cells. The effects of PEA3 on the cellular content of endogenous caveolin-1 expression are variable. These results demonstrate that ERM is involved in caveolin-1 regulation in a murine lung epithelial, but not lung endothelial cell line. We conclude that transcriptional regulation of caveolin-1 differs markedly between lung epithelial and endothelial cell lines, perhaps explaining why the onset of caveolin-1 expression differs in epithelial and endothelial cells during lung development.

Intracellular trafficking pathway of BK Virus in human renal proximal tubular epithelial cells

Intracellular trafficking of BK Virus (BKV) in human renal proximal tubular epithelial cells (HRPTEC) is critical for BKV nephritis. However, the major trafficking components utilized by BKV remain unknown. Coincubation of HRPTEC with BKV and microtubule disrupting agents prevented BKV infection as detected by immunofluorescence and western blot analysis with antibodies which recognize BKV large T antigen. However, inhibition of a dynein, cellular motor protein, did not interfere with BKV infection in HRPTEC. A colocalization study of BKV with the markers of the endoplasmic reticulum (ER) and the Golgi apparatus (GA), indicated that BKV reached the ER from 6 to 10 h, while bypassing the GA or passing through the GA too transiently to be detected. This study contributes to the understanding of mechanisms of intracellular trafficking used by BKV in the infection of HRPTEC.

N-glycolyl GM1 ganglioside as a receptor for simian virus 40

Carbohydrate microarrays have emerged as powerful tools in analyses of microbe-host interactions. Using a microarray with 190 sequence-defined oligosaccharides in the form of natural glycolipids and neoglycolipids representative of diverse mammalian glycans, we examined interactions of simian virus 40 (SV40) with potential carbohydrate receptors. While the results confirmed the high specificity of SV40 for the ganglioside GM1, they also revealed that N-glycolyl GM1 ganglioside [GM1(Gc)], which is characteristic of simian species and many other nonhuman mammals, is a better ligand than the N-acetyl analog [GM1(Ac)] found in mammals, including humans. After supplementing glycolipid-deficient GM95 cells with GM1(Ac) and GM1(Gc) gangliosides and the corresponding neoglycolipids with phosphatidylethanolamine lipid groups, it was found that GM1(Gc) analogs conferred better virus binding and infectivity. Moreover, we visualized the interaction of NeuGc with VP1 protein of SV40 by molecular modeling and identified a conformation for GM1(Gc) ganglioside in complex with the virus VP1 pentamer that is compatible with its presentation as a membrane receptor. Our results open the way not only to detailed studies of SV40 infection in relation to receptor expression in host cells but also to the monitoring of changes that may occur with time in receptor usage by the virus.

"Alternative" endocytic mechanisms exploited by pathogens: new avenues for therapeutic delivery?

Some pathogens utilize unique routes to enter cells that may evade the intracellular barriers encountered by the typical clathrin-mediated endocytic pathway. Retrograde transport and caveolar uptake are among the better characterized pathways, as alternatives to clathrin-mediated endocytosis, that are known to facilitate entry of pathogens and potential delivery agents. Recent characterization of the trafficking mechanisms of prion proteins and certain bacteria may present new paradigms for strategizing improvements in therapeutic spread and retention of therapy. This review will provide an overview of such endocytic pathways, and discuss current and future possibilities in using these routes as a means to improve therapeutic delivery.

Evidence that mono-ADP-ribosylation of CtBP1/BARS regulates lipid storage

Mono-ADP-ribosylation is emerging as an important posttranslational modification that modulates a variety of cell signaling pathways. Here, we present evidence that mono-ADP-ribosylation of the transcriptional corepressor C terminal binding protein, brefeldin A (BFA)-induced ADP-ribosylated substrate (CtBP1/BARS) regulates neutral lipid storage in droplets that are surrounded by a monolayer of phospholipid and associated proteins. CtBP1/BARS is an NAD-binding protein that becomes ribosylated when cells are exposed to BFA. Both endogenous lipid droplets and droplets enlarged by oleate treatment are lost after 12-h exposure to BFA. Lipid loss requires new protein synthesis, and it is blocked by multiple ribosylation inhibitors, but it is not stimulated by disruption of the Golgi apparatus or the endoplasmic reticulum unfolded protein response. Small interfering RNA knockdown of CtBP1/BARS mimics the effect of BFA, and mouse embryonic fibroblasts derived from embryos that are deficient in CtBP1/BARS seem to be defective in lipid accumulation. We conclude that mono-ADP-ribosylation of CtBP1/BARS inactivates its repressor function, which leads to the activation of genes that regulate neutral lipid storage.

Minor capsid proteins of simian virus 40 are dispensable for nucleocapsid assembly and cell entry but are required for nuclear entry of the viral genome

We investigated the roles of simian virus 40 capsid proteins in the viral life cycle by analyzing point mutants in Vp1 and Vp2/3, as well as a deletion mutant lacking the Vp2/3 coding sequence. The Vp1 mutants (V243E and L245E) and the Vp2/3 mutants (F157E-I158E and P164R-G165E-G166R) were previously shown to be defective in Vp1-Vp2/3 interaction and to be noninfectious or poorly infectious, respectively. Here, we show that all these point mutants form stable particles following DNA transfection into cells. The Vp2/3-mutant particles contained very low levels of Vp2/3, whereas the Vp1 mutant particles contained no detectable Vp2/3. As expected, the deletion mutant also formed particles that were noninfectious. We further characterized the two Vp1 point mutants and the deletion mutant. All three mutant particles comprised Vp1 and histone-associated viral DNA, and all were able to enter cells. However, the mutant complexes failed to associate with host importins (owing to the loss of the Vp2/3 nuclear localization signal), and the mutant viral DNAs prematurely dissociated from the Vp1s, suggesting that the nucleocapsids did not enter the nucleus. Consistently, all three mutant particles failed to express large T antigen. Together, our results demonstrate unequivocally that Vp2/3 is dispensable for the formation of nucleocapsids. Further, the nucleocapsids' ability to enter cells implies that Vp1 contains the major determinants for cell attachment and entry. We propose that the major role of Vp2/3 in infectivity is to mediate the nuclear entry of viral DNA.

SV40 VP2 and VP3 Insertion into ER Membranes Is Controlled by the Capsid Protein VP1: Implications for DNA Translocation out of the ER

Nonenveloped viruses such as Simian Virus 40 (SV40) exploit established cellular pathways for internalization and transport to their site of penetration. By analyzing mutant SV40 genomes that do not express VP2 or VP3, we found that these structural proteins perform essential functions that are regulated by VP1. VP2 significantly enhanced SV40 particle association with the host cell, while VP3 functioned downstream. VP2 and VP3 both integrated posttranslationally into the endoplasmic reticulum (ER) membrane. Association with VP1 pentamers prevented their ER membrane integration, indicating that VP1 controls the function of VP2 and VP3 by directing their localization between the particle and the ER membrane. These findings suggest a model in which VP2 aids in cell binding. After capsid disassembly within the ER lumen, VP3, and perhaps VP2, oligomerizes and integrates into the ER membrane, potentially creating a viroporin that aids in viral DNA transport out of the ER.

Rafting with cholera toxin: endocytosis and trafficking from plasma membrane to ER

Cholera toxin (CT), and members of the AB(5) family of toxins enter host cells and hijack the cell's endogenous pathways to induce toxicity. CT binds to a lipid receptor on the plasma membrane (PM), ganglioside GM1, which has the ability to associate with lipid rafts. The toxin can then enter the cell by various modes of receptor-mediated endocytosis and traffic in a retrograde manner from the PM to the Golgi and the endoplasmic reticulum (ER). Once in the ER, a portion of the toxin is unfolded and retro-translocated to the cytosol so as to induce disease. GM1 is the vehicle that carries CT from PM to ER. Thus, the toxin pathway from PM to ER is a lipid-based sorting pathway, which is potentially meditated by the determinants of the GM1 ganglioside structure itself.

Molecular dissection of nuclear entry-competent SV40 during infection

To establish viral infection, SV40 must expose nuclear localization signals (NLSs) that are internal in the virion architecture in order to enter the nucleus via interaction with the host's nuclear import machinery, which includes importin alpha and importin beta. The time course for SV40 association with the importins in infected cells was examined. The viral DNA associated with importin alpha by 1.5h post infection, before associating with the importin beta nuclear import receptor, by 3h post infection. Only a small fraction of cell-internalized SV40 that contained viral DNA was bound by the two importins. This fraction, termed "nuclear entry-competent SV40," was slightly smaller than the virion but, importantly, was larger than the viral chromatin and contained both Vp1 and Vp3. Furthermore, the internalized viral DNA in either anti-importin or anti-Vp3 immune complexes was sensitive to DNase I, whereas the viral DNA in mature virions was resistant. All these results suggest that once SV40 enters the cytoplasm, it undergoes an architectural modification that exposes the virion's NLSs for nuclear entry.

Caveolin is necessary for Wnt-3a-dependent internalization of LRP6 and accumulation of beta-catenin

beta-catenin-mediated Wnt signaling is critical in animal development and tumor progression. The single-span transmembrane Wnt receptor, low-density lipoprotein receptor-related protein 6 (LRP6), interacts with Axin to promote the Wnt-dependent accumulation of beta-catenin. However, the molecular mechanism of receptor internalization and its impact on signaling are unclear. Here, we present evidence that LRP6 is internalized with caveolin and that the components of this endocytic pathway are required not only for Wnt-3a-induced internalization of LRP6 but also for accumulation of beta-catenin. Overall, our data suggest that Wnt-3a triggers the interaction of LRP6 with caveolin and promotes recruitment of Axin to LRP6 phosphorylated by glycogen synthase kinase-3beta and that caveolin thereby inhibits the binding of beta-catenin to Axin. Thus, caveolin plays critical roles in inducing the internalization of LRP6 and activating the Wnt/beta-catenin pathway. We also discuss the idea that distinct endocytic pathways correlate with the specificity of Wnt signaling events.