Membrane orientation of the human papillomavirus type 16 E5 oncoprotein

Functions of Viroporins in the Viral Life Cycle and Their Regulation of Host Cell Responses

Viroporins are virally encoded transmembrane proteins that are essential for viral pathogenicity and can participate in various stages of the viral life cycle, thereby promoting viral proliferation. Viroporins have multifaceted effects on host cell biological functions, including altering cell membrane permeability, triggering inflammasome formation, inducing apoptosis and autophagy, and evading immune responses, thereby ensuring that the virus completes its life cycle. Viroporins are also virulence factors, and their complete or partial deletion often reduces virion release and reduces viral pathogenicity, highlighting the important role of these proteins in the viral life cycle. Thus, viroporins represent a common drug-protein target for inhibiting drugs and the development of antiviral therapies. This article reviews current studies on the functions of viroporins in the viral life cycle and their regulation of host cell responses, with the aim of improving the understanding of this growing family of viral proteins.

Unravelling the Immunomodulatory Effects of Viral Ion Channels, towards the Treatment of Disease

The current COVID-19 pandemic has highlighted the need for the research community to develop a better understanding of viruses, in particular their modes of infection and replicative lifecycles, to aid in the development of novel vaccines and much needed anti-viral therapeutics. Several viruses express proteins capable of forming pores in host cellular membranes, termed "Viroporins". They are a family of small hydrophobic proteins, with at least one amphipathic domain, which characteristically form oligomeric structures with central hydrophilic domains. Consequently, they can facilitate the transport of ions through the hydrophilic core. Viroporins localise to host membranes such as the endoplasmic reticulum and regulate ion homeostasis creating a favourable environment for viral infection. Viroporins also contribute to viral immune evasion via several mechanisms. Given that viroporins are often essential for virion assembly and egress, and as their structural features tend to be evolutionarily conserved, they are attractive targets for anti-viral therapeutics. This review discusses the current knowledge of several viroporins, namely Influenza A virus (IAV) M2, Human Immunodeficiency Virus (HIV)-1 Viral protein U (Vpu), Hepatitis C Virus (HCV) p7, Human Papillomavirus (HPV)-16 E5, Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) Open Reading Frame (ORF)3a and Polyomavirus agnoprotein. We highlight the intricate but broad immunomodulatory effects of these viroporins and discuss the current antiviral therapies that target them; continually highlighting the need for future investigations to focus on novel therapeutics in the treatment of existing and future emergent viruses.

Cellular Functions of HPV16 E5 Oncoprotein during Oncogenic Transformation

The human papillomavirus (HPV) is recognized as the main etiologic agent associated with cervical cancer. HPVs are epitheliotropic, and the ones that infect the mucous membranes are classified into low-risk (LR) and high-risk (HR) types. LR-HPVs produce benign lesions, whereas HR-HPVs produce lesions that may progress to cancer. HR-HPV types 16 and 18 are the most frequently found in cervical cancer worldwide. E6 and E7 are the major HPV oncogenic proteins, and they have been profusely studied. Moreover, it has been shown that the HPV16 E5 (16E5) oncoprotein generates transformation, although the molecular mechanisms through which it carries out its activity have not been well defined. In contrast to E6 and E7, the E5 open reading frame is lost during the integration of the episomal HPV DNA into the cellular genome. This suggests that E5 acts at the early stages of the transformation process. In this review, we focused on the biochemical characteristics and functions of the HPV E5 oncoprotein, mainly on its association with growth factor receptors and other cellular proteins. Knowledge of the HPV E5 biology is important to understand the role of this oncoprotein in maintaining the viral cycle through the modulation of proliferation, differentiation, and apoptosis, as well as the alteration of other processes, such as survival, adhesion, migration, and invasion during early carcinogenesis. Finally, we summarized recent research that uses the E5 oncoprotein as a therapeutic target, promising a novel approach to the treatment of cervical cancer in its early stages.

The scaffold protein IQGAP1 is crucial for extravasation and metastasis

IQGAP1 is a scaffold protein involved in a range of cellular activities, including migration, invasion, adhesion and proliferation. It is also oncogenic in a variety of cancers, promoting primary tumor growth and invasiveness. However, the role of IQGAP1 in tumor progression and metastasis remains unclear. In this study, we use both knockdown and knockout of IQGAP1 to investigate its role in the metastatic cascade of both melanoma and breast cancer cells in vivo. We find that reduction of IQGAP1 expression decreases the formation of both spontaneous and experimental metastases, without limiting primary or metastatic tumor growth. Furthermore, IQGAP1 knockout significantly inhibits extravasation of tumor cells from circulation, possibly involving invadopodial function. By expressing mutant forms of IQGAP1 in a knockout context, we also determine that IQGAP1’s pro-metastatic functions are dependent on multiple domains and functions. These data demonstrate that IQGAP1 is crucial for metastasis in vivo through regulation of extravasation and suggest that it may represent a valid therapeutic target for inhibiting metastasis.

Murine neuroblastoma cell lines developed by conditional reprogramming preserve heterogeneous phenotypes observed in vivo

Neuroblastoma (NB) is a pediatric tumor of the peripheral nervous system. Treatment of the disease represents an unsolved clinical problem, as survival of patients with aggressive form of NB remains below 50%. Despite recent identification of numerous potential therapeutic targets, clinical trials validating them are challenging due to the rarity of the disease and its high patient-to-patient heterogeneity. Hence, there is a need for the accurate preclinical models that would allow testing novel therapeutic approaches and prioritizing the clinical studies, preferentially in personalized way. Here, we propose using conditional reprogramming (CR) technology for rapid development of primary NB cell cultures that could become a new model for such tests. This newly established method allowed for indefinite propagation of normal and tumor cells of epithelial origin in an undifferentiated state by their culture in the presence of Rho-associated kinase (ROCK) inhibitor, Y-27632, and irradiated mouse feeder cells. Using a modification of this approach, we isolated cell lines from tumors arising in the TH-MYCN murine transgenic model of NB (CR-NB). The cells were positive for neuronal markers, including Phox2B and peripherin and consisted of two distinct populations: mesenchymal and adrenergic expressing corresponding markers of their specific lineage. This heterogeneity of the CR-NB cells mimicked the different tumor cell phenotypes in TH-MYCN tumor tissues. The CR-NB cells preserved anchorage-independent growth capability and were successfully passaged, frozen and biobanked. Further studies are required to determine the utility of this method for isolation of human NB cultures, which can become a novel model for basic, translational, and clinical research, including individualized drug testing.

Characteristics and Functions of the Yip1 Domain Family (YIPF), Multi-Span Transmembrane Proteins Mainly Localized to the Golgi Apparatus

Yip1 domain family (YIPF) proteins are multi-span, transmembrane proteins mainly localized in the Golgi apparatus. YIPF proteins have been found in virtually all eukaryotes, suggesting that they have essential function(s). Saccharomyces cerevisiae contains four YIPFs: Yip1p, Yif1p, Yip4p, and Yip5p. Early analyses in S. cerevisiae indicated that Yip1p and Yif1p bind to each other and play a role in budding of transport vesicles and/or fusion of vesicles to target membranes. However, the molecular basis of their functions remains unclear. Analysis of YIPF proteins in mammalian cells has yielded significant clues about the function of these proteins. Human cells have nine family members that appear to have overlapping functions. These YIPF proteins are divided into two sub-families: YIPFα/Yip1p and YIPFβ/Yif1p. A YIPFα molecule forms a complex with a specific partner YIPFβ molecule. In the most broadly hypothesized scenario, a basic tetramer complex is formed from two molecules of each partner YIPF protein, and this tetramer forms a higher order oligomer. Three distinct YIPF protein complexes are formed from pairs of YIPFα and YIPFβ proteins. These are differently localized in either the early, middle, or late compartments of the Golgi apparatus and are recycled between adjacent compartments. Because a YIPF protein is predicted to have five transmembrane segments, a YIPF tetramer complex is predicted to have 20 transmembrane segments. This high number of transmembrane segments suggests that YIPF complexes function as channels, transporters, or transmembrane receptors. Here, the evidence from functional studies of YIPF proteins obtained during the last two decades is summarized and discussed.

Beyond Channel Activity: Protein-Protein Interactions Involving Viroporins

Viroporins are short polypeptides encoded by viruses. These small membrane proteins assemble into oligomers that can permeabilize cellular lipid bilayers, disrupting the physiology of the host to the advantage of the virus. Consequently, efforts during the last few decades have been focused towards the discovery of viroporin channel inhibitors, but in general these have not been successful to produce licensed drugs. Viroporins are also involved in viral pathogenesis by engaging in critical interactions with viral proteins, or disrupting normal host cellular pathways through coordinated interactions with host proteins. These protein-protein interactions (PPIs) may become alternative attractive drug targets for the development of antivirals. In this sense, while thus far most antiviral molecules have targeted viral proteins, focus is moving towards targeting host proteins that are essential for virus replication. In principle, this largely would overcome the problem of resistance, with the possibility of using repositioned existing drugs. The precise role of these PPIs, their strain- and host- specificities, and the structural determination of the complexes involved, are areas that will keep the fields of virology and structural biology occupied for years to come. In the present review, we provide an update of the efforts in the characterization of the main PPIs for most viroporins, as well as the role of viroporins in these PPIs interactions.

Infectious Bursal Disease Virus Hijacks Endosomal Membranes as the Scaffolding Structure for Viral Replication

Birnaviruses are unconventional members of the group of double-stranded RNA (dsRNA) viruses that are characterized by the lack of a transcriptionally active inner core. Instead, the birnaviral particles organize their genome in ribonucleoprotein complexes (RNPs) composed by dsRNA segments, the dsRNA-binding VP3 protein, and the virally encoded RNA-dependent RNA polymerase (RdRp). This and other structural features suggest that birnaviruses may follow a completely different replication program from that followed by members of the Reoviridae family, supporting the hypothesis that birnaviruses are the evolutionary link between single-stranded positive RNA (+ssRNA) and dsRNA viruses. Here we demonstrate that infectious bursal disease virus (IBDV), a prototypical member of the Birnaviridae family, hijacks endosomal membranes of infected cells through the interaction of a viral protein, VP3, with the phospholipids on the cytosolic leaflet of these compartments for replication. Employing a mutagenesis approach, we demonstrated that VP3 domain PATCH 2 (P2) mediates the association of VP3 with the endosomal membranes. To determine the role of VP3 P2 in the context of the virus replication cycle, we used avian cells stably overexpressing VP3 P2 for IBDV infection. Importantly, the intra- and extracellular virus yields, as well as the intracellular levels of VP2 viral capsid protein, were significantly diminished in cells stably overexpressing VP3 P2. Together, our results indicate that the association of VP3 with endosomes has a relevant role in the IBDV replication cycle. This report provides direct experimental evidence for membranous compartments such as endosomes being required by a dsRNA virus for its replication. The results also support the previously proposed role of birnaviruses as an evolutionary link between +ssRNA and dsRNA viruses.IMPORTANCE Infectious bursal disease (IBD; also called Gumboro disease) is an acute, highly contagious immunosuppressive disease that affects young chickens and spreads worldwide. The etiological agent of IBD is infectious bursal disease virus (IBDV). This virus destroys the central immune organ (bursa of Fabricius), resulting in immunosuppression and reduced responses of chickens to vaccines, which increase their susceptibility to other pathogens. IBDV is a member of Birnaviridae family, which comprises unconventional members of dsRNA viruses, whose replication strategy has been scarcely studied. In this report we show that IBDV hijacks the endosomes of the infected cells for establishing viral replication complexes via the association of the ribonucleoprotein complex component VP3 with the phospholipids in the cytosolic leaflet of endosomal membranes. We show that this interaction is mediated by the VP3 PATCH 2 domain and demonstrate its relevant role in the context of viral infection.

Conditional cell reprogramming involves non-canonical β-catenin activation and mTOR-mediated inactivation of Akt

The combination of irradiated fibroblast feeder cells and Rho kinase inhibitor, Y-267362, converts primary epithelial cells growing in vitro into an undifferentiated adult stem cell-like state that is characterized by long-term proliferation. This cell culture method also maintains the proliferation of adult epithelial stem cells from various tissues. Both primary and adult stem cells retain their tissue-specific differentiation potential upon removal of the culture conditions. Due to the ability to modulate the proliferation and differentiation of the cells, this method is referred to as conditional reprogramming and it is increasingly being used in studies of tumor heterogeneity, personalized medicine and regenerative medicine. However, little is known about the biology of these conditionally reprogrammed (CR) cells. Previously we showed that β-catenin activation, a hallmark of stem cells in vivo, occurs in CR human ectocervical cells (HECs). Here we show that β-catenin-dependent transcription is necessary for the induction of epithelial stem cell markers, and that β-catenin is activated via a non-canonical pathway that is independent of Wnt and Akt/GSK-3. Active Akt actually decreases due to increased mTOR signaling, with a consequent increase in dephosphorylated, active GSK-3. Despite the increase in active GSK-3, β-catenin associates with protein phosphatase 2A (PP2A) and is activated. Inhibition of PP2A catalytic activity reduces both the level of active β-catenin and the acute induction of stem cell markers, suggesting an important role for PP2A in the activation of β-catenin. Moreover, we demonstrate similar results using human prostate and breast cells, indicating that these changes are not restricted to ectocervical epithelial cells and may represent a more fundamental property of conditional reprogramming.

Glycosylated Triterpenoids as Endosomal Escape Enhancers in Targeted Tumor Therapies

Protein-based targeted toxins play an increasingly important role in targeted tumor therapies. In spite of their high intrinsic toxicity, their efficacy in animal models is low. A major reason for this is the limited entry of the toxin into the cytosol of the target cell, which is required to mediate the fatal effect. Target receptor bound and internalized toxins are mostly either recycled back to the cell surface or lysosomally degraded. This might explain why no antibody-targeted protein toxin has been approved for tumor therapeutic applications by the authorities to date although more than 500 targeted toxins have been developed within the last decades. To overcome the problem of insufficient endosomal escape, a number of strategies that make use of diverse chemicals, cell-penetrating or fusogenic peptides, and light-induced techniques were designed to weaken the membrane integrity of endosomes. This review focuses on glycosylated triterpenoids as endosomal escape enhancers and throws light on their structure, the mechanism of action, and on their efficacy in cell culture and animal models. Obstacles, challenges, opportunities, and future prospects are discussed.

Papillomaviruses: a systematic review

In the last decades, a group of viruses has received great attention due to its relationship with cancer development and its wide distribution throughout the vertebrates: the papillomaviruses. In this article, we aim to review some of the most relevant reports concerning the use of bovines as an experimental model for studies related to papillomaviruses. Moreover, the obtained data contributes to the development of strategies against the clinical consequences of bovine papillomaviruses (BPV) that have led to drastic hazards to the herds. To overcome the problem, the vaccines that we have been developing involve recombinant DNA technology, aiming at prophylactic and therapeutic procedures. It is important to point out that these strategies can be used as models for innovative procedures against HPV, as this virus is the main causal agent of cervical cancer, the second most fatal cancer in women.

Weak Selectivity Predicted for Modeled Bundles of Viral Channel-Forming Protein E5 of Human Papillomavirus-16

Protein E5 is a polytopic 83 amino acid membrane protein with three transmembrane domains (TMDs), encoded by high-risk human papillomavirus-16 (HPV-16). HPV-16 is found to be the causative agent for cervical cancer. Protein E5, among other proteins (e.g., E6, E7), is expressed at an "early" (E) stage when the cell turns malignant. It has been experimentally found that E5 forms hexameric assemblies, which show the characteristics of the class of so-called channel-forming proteins by rendering lipid membranes permeable to ions and small molecules. Protein E5 is used to achieve structural models of the protein in assembled bundles using a force field-based docking approach. Extended molecular dynamics simulations of selected bundles in fully hydrated lipid bilayers suggest the second TMD to be pore-lining, allowing for water columns to exist within the lumen of the pore. Full correlation analysis indicates asymmetric dynamics within the monomers of the bundle. Potential of mean force calculations of a snapshot structure of the putative open pore of the protein bundle propose low selectivity.

Emerging From the Unknown: Structural and Functional Features of Agnoprotein of Polyomaviruses

Agnoprotein is an important regulatory protein of polyomaviruses, including JCV, BKV, and SV40. In the absence of its expression, these viruses are unable to sustain their productive life cycle. It is a highly basic phosphoprotein that localizes mostly to the perinuclear area of infected cells, although a small amount of the protein is also found in nucleus. Much has been learned about the structure and function of this important regulatory protein in recent years. It forms highly stable dimers/oligomers in vitro and in vivo through its Leu/Ile/Phe-rich domain. Structural NMR studies revealed that this domain adopts an alpha-helix conformation and plays a critical role in the stability of the protein. It associates with cellular proteins, including YB-1, p53, Ku70, FEZ1, HP1α, PP2A, AP-3, PCNA, and α-SNAP; and viral proteins, including small t antigen, large T antigen, HIV-1 Tat, and JCV VP1; and significantly contributes the viral transcription and replication. This review summarizes the recent advances in the structural and functional properties of this important regulatory protein. J. Cell. Physiol. 231: 2115-2127, 2016. © 2016 Wiley Periodicals, Inc.

Emerging Roles of Viroporins Encoded by DNA Viruses: Novel Targets for Antivirals?

Studies have highlighted the essential nature of a group of small, highly hydrophobic, membrane embedded, channel-forming proteins in the life cycles of a growing number of RNA viruses. These viroporins mediate the flow of ions and a range of solutes across cellular membranes and are necessary for manipulating a myriad of host processes. As such they contribute to all stages of the virus life cycle. Recent discoveries have identified proteins encoded by the small DNA tumor viruses that display a number of viroporin like properties. This review article summarizes the recent developments in our understanding of these novel viroporins; describes their roles in the virus life cycles and in pathogenesis and speculates on their potential as targets for anti-viral therapeutic intervention.

Human papillomavirus molecular biology and disease association

Human papillomaviruses (HPVs) have evolved over millions of years to propagate themselves in a range of different animal species including humans. Viruses that have co‐evolved slowly in this way typically cause chronic inapparent infections, with virion production in the absence of apparent disease. This is the case for many Beta and Gamma HPV types. The Alpha papillomavirus types have however evolved immunoevasion strategies that allow them to cause persistent visible papillomas. These viruses activate the cell cycle as the infected epithelial cell differentiates in order to create a replication competent environment that allows viral genome amplification and packaging into infectious particles. This is mediated by the viral E6, E7, and E5 proteins. High‐risk E6 and E7 proteins differ from their low‐risk counterparts however in being able to drive cell cycle entry in the upper epithelial layers and also to stimulate cell proliferation in the basal and parabasal layers. Deregulated expression of these cell cycle regulators underlies neoplasia and the eventual progression to cancer in individuals who cannot resolve high‐risk HPV infection. Most work to date has focused on the study of high‐risk HPV types such as HPV 16 and 18, which has led to an understanding of the molecular pathways subverted by these viruses. Such approaches will lead to the development of better strategies for disease treatment, including targeted antivirals and immunotherapeutics. Priorities are now focused toward understanding HPV neoplasias at sites other than the cervix (e.g. tonsils, other transformation zones) and toward understanding the mechanisms by which low‐risk HPV types can sometimes give rise to papillomatosis and under certain situations even cancers. Copyright © 2015 John Wiley & Sons, Ltd.

Topography of the Human Papillomavirus Minor Capsid Protein L2 during Vesicular Trafficking of Infectious Entry

The human papillomavirus (HPV) capsid is composed of the major capsid protein L1 and the minor capsid protein L2. During entry, the HPV capsid undergoes numerous conformational changes that result in endosomal uptake and subsequent trafficking of the L2 protein in complex with the viral DNA to the trans-Golgi network. To facilitate this transport, the L2 protein harbors a number of putative motifs that, if capable of direct interaction, would interact with cytosolic host cell factors. These data imply that a portion of L2 becomes cytosolic during infection. Using a low concentration of digitonin to selectively permeabilize the plasma membrane of infected cells, we mapped the topography of the L2 protein during infection. We observed that epitopes within amino acid residues 64 to 81 and 163 to 170 and a C-terminal tag of HPV16 L2 are exposed on the cytosolic side of intracellular membranes, whereas an epitope within residues 20 to 38, which are upstream of a putative transmembrane region, is luminal. Corroborating these findings, we also found that L2 protein is sensitive to trypsin digestion during infection. These data demonstrate that the majority of the L2 protein becomes accessible on the cytosolic side of intracellular membranes in order to interact with cytosolic factors to facilitate vesicular trafficking.

IMPORTANCE

In order to complete infectious entry, nonenveloped viruses have to pass cellular membranes. This is often achieved through the viral capsid protein associating with or integrating into intracellular membrane. Here, we determine the topography of HPV L2 protein in the endocytic vesicular compartment, suggesting that L2 becomes a transmembrane protein with a short luminal portion and with the majority facing the cytosolic side for interaction with host cell transport factors.

YIP1 family member 4 (YIPF4) is a novel cellular binding partner of the papillomavirus E5 proteins

E5 proteins are amongst the least understood of the Human Papillomavirus (HPV) encoded gene products. They are small, membrane-integrated proteins known to modulate a number of critical host pathways associated with pathogenesis including growth factor receptor signaling and immune evasion. Their role in the virus life cycle is less clear, indicating a role in the productive stages of the life cycle. However, a mechanism for this is currently lacking. Here we describe the identification of a novel binding partner of E5, YIPF4 using yeast two-hybrid analysis. YIPF4 is also a poorly characterized membrane spanning protein. Mutagenesis studies implicated the transmembrane regions of each protein as important for their interaction. Binding to YIPF4 was found for all E5 proteins tested suggesting that this interaction may mediate a conserved E5 function. In normal human keratinocytes YIPF4 expression was down-regulated upon differentiation and this reduction was partially rescued in cells harbouring HPV. Despite the conserved nature of the interaction with E5, siRNA mediated depletion of YIPF4 failed to impede two well-characterized functions of E5, namely EGFR trafficking or HLA class I presentation. Continued studies of YIPF4 are warranted to determine its role in the PV life cycle.

Quantification of Cytosolic vs. Vacuolar Salmonella in Primary Macrophages by Differential Permeabilization

Intracellular bacterial pathogens can replicate in the cytosol or in specialized pathogen-containing vacuoles (PCVs). To reach the cytosol, bacteria like Shigella flexneri and Francisella novicida need to induce the rupture of the phagosome. In contrast, Salmonella typhimurium replicates in a vacuolar compartment, known as Salmonella-containing vacuole (SCV). However certain mutants of Salmonella fail to maintain SCV integrity and are thus released into the cytosol. The percentage of cytosolic vs. vacuolar bacteria on the level of single bacteria can be measured by differential permeabilization, also known as phagosome-protection assay. The approach makes use of the property of detergent digitonin to selectively bind cholesterol. Since the plasma membrane contains more cholesterol than other cellular membranes, digitonin can be used to selectively permeabilize the plasma membrane while leaving intracellular membranes intact. In brief, following infection with the pathogen expressing a fluorescent marker protein (e.g. mCherry among others), the plasma membrane of host cells is permeabilized with a short incubation in digitonin containing buffer. Cells are then washed and incubated with a primary antibody (coupled to a fluorophore of choice) directed against the bacterium of choice (e.g. anti-Salmonella-FITC) and washed again. If unmarked bacteria are used, an additional step can be done, in which all membranes are permeabilized and all bacteria stained with a corresponding antibody. Following the staining, the percentage of vacuolar and cytosolic bacteria can be quantified by FACS or microscopy by counting single or double-positive events. Here we provide experimental details for use of this technique with the bacterium Salmonella typhimurium. The advantage of this assay is that, in contrast to other assay, it provides a quantification on the level of single bacteria, and if analyzed by microscopy provides the exact number of cytosolic and vacuolar bacteria in a given cell.

Viroporins: structure, function and potential as antiviral targets

The channel-forming activity of a family of small, hydrophobic integral membrane proteins termed 'viroporins' is essential to the life cycles of an increasingly diverse range of RNA and DNA viruses, generating significant interest in targeting these proteins for antiviral development. Viroporins vary greatly in terms of their atomic structure and can perform multiple functions during the virus life cycle, including those distinct from their role as oligomeric membrane channels. Recent progress has seen an explosion in both the identification and understanding of many such proteins encoded by highly significant pathogens, yet the prototypic M2 proton channel of influenza A virus remains the only example of a viroporin with provenance as an antiviral drug target. This review attempts to summarize our current understanding of the channel-forming functions for key members of this growing family, including recent progress in structural studies and drug discovery research, as well as novel insights into the life cycles of many viruses revealed by a requirement for viroporin activity. Ultimately, given the successes of drugs targeting ion channels in other areas of medicine, unlocking the therapeutic potential of viroporins represents a valuable goal for many of the most significant viral challenges to human and animal health.

Exon 4-encoded sequence is a major determinant of cytotoxicity of apolipoprotein L1

The apolipoprotein L1 (APOL1) gene (APOL1) product is toxic to kidney cells, and its G1 and G2 alleles are strongly associated with increased risk for kidney disease progression in African Americans. Variable penetrance of the G1 and G2 risk alleles highlights the significance of additional factors that trigger or modify the progression of disease. In this regard, the effect of alternative splicing in the absence or presence of G1 or G2 alleles is unknown. In this study we investigated whether alternative splicing of non-G1, non-G2 APOL1 (APOL1 G0) affects its biological activity. Among seven APOL1 exons, exons 2 and 4 are differentially expressed in major transcripts. We found that, in contrast to APOL1 splice variants B3 or C, variants A and B1 demonstrate strong toxicity in human embryonic kidney (HEK293T) cells. Subsequently, we established that exon 4 is a major determinant of toxicity of variants A and B1 and that extracellular release of these variants is dispensable for their cytotoxicity. Although only variants A and B1 induced nuclear translocation of transcription factor EB (TFEB), a master regulator of lysosomal biogenesis and autophagy, exon 4-positive and -negative APOL1 variants stimulated perinuclear accumulation of unprocessed autophagosomes. Knockdown of endogenous TFEB did not attenuate APOL1 cytotoxicity, indicating that nuclear translocation of TFEB is dispensable for APOL1 toxicity. Our findings that a human podocyte cell line expresses exon 4-positive and -negative APOL1 transcripts suggest that these variants may play a differential role in podocyte pathology. In summary, we have identified exon 4 as a major determinant of APOL1 G0 cytotoxicity.

Determination of the topology of endoplasmic reticulum membrane proteins using redox-sensitive green-fluorescence protein fusions

Membrane proteins of the endoplasmic reticulum (ER) are involved in a wide array of essential cellular functions. Identification of the topology of membrane proteins can provide significant insight into their mechanisms of action and biological roles. This is particularly important for membrane enzymes, since their topology determines the subcellular site where a biochemical reaction takes place and the dependence on luminal or cytosolic co-factor pools and substrates. The methods currently available for the determination of topology of proteins are rather laborious and require post-lysis or post-fixation manipulation of cells. In this work, we have developed a simple method for defining intracellular localization and topology of ER membrane proteins in living cells, based on the fusion of the respective protein with redox-sensitive green-fluorescent protein (roGFP). We validated the method and demonstrated that roGFP fusion proteins constitute a reliable tool for the study of ER membrane protein topology, using as control microsomal 11β-hydroxysteroid dehydrogenase (11β-HSD) proteins whose topology has been resolved, and comparing with an independent approach. We then implemented this method to determine the membrane topology of six microsomal members of the 17β-hydroxysteroid dehydrogenase (17β-HSD) family. The results revealed a luminal orientation of the catalytic site for three enzymes, i.e. 17β-HSD6, 7 and 12. Knowledge of the intracellular location of the catalytic site of these enzymes will enable future studies on their biological functions and on the role of the luminal co-factor pool.

Viral channel proteins in intracellular protein-protein communication: Vpu of HIV-1, E5 of HPV16 and p7 of HCV

Viral channel forming proteins are known for their capability to make the lipid membrane of the host cell and its subcellular compartments permeable to ions and small compounds. There is increasing evidence that some of the representatives of this class of proteins are also strongly interacting with host proteins and the effectiveness of this interaction seems to be high. Interaction of viral channel proteins with host factors has been proposed by bioinformatics approaches and has also been identified experimentally. An overview of the interactions with host proteins is given for Vpu from HIV-1, E5 from HPV-16 and p7 from HCV. This article is part of a Special Issue entitled: Viral Membrane Proteins - Channels for Cellular Networking.

The E5 proteins

The E5 proteins are short transmembrane proteins encoded by many animal and human papillomaviruses. These proteins display transforming activity in cultured cells and animals, and they presumably also play a role in the productive virus life cycle. The E5 proteins are thought to act by modulating the activity of cellular proteins. Here, we describe the biological activities of the best-studied E5 proteins and discuss the evidence implicating specific protein targets and pathways in mediating these activities. The primary target of the 44-amino acid BPV1 E5 protein is the PDGF β receptor, whereas the EGF receptor appears to be an important target of the 83-amino acid HPV16 E5 protein. Both E5 proteins also bind to the vacuolar ATPase and affect MHC class I expression and cell-cell communication. Continued studies of the E5 proteins will elucidate important aspects of transmembrane protein-protein interactions, cellular signal transduction, cell biology, virus replication, and tumorigenesis.

The biology and life-cycle of human papillomaviruses

Human papillomaviruses (HPVs) comprise a diverse group, and have different epithelial tropisms and life-cycle strategies. Many HPVs are classified as low-risk, as they are only very rarely associated with neoplasia or cancer in the general population. These HPVs typically cause inapparent/inconspicuous infections, or benign papillomas, which can persist for months or years, but which are eventually resolved by the host's immune system. Low-risk HPVs are difficult to manage in immunosuppressed people and in individuals with genetic predispositions, and can give rise to papillomatosis, and in rare instances, to cancer. The high-risk HPV types are, by contrast, a cause of several important human cancers, including almost all cases of cervical cancer, a large proportion of other anogenital cancers and a growing number of head and neck tumours. The high-risk HPV types constitute a subset of the genus Alphapapillomavirus that are prevalent in the general population, and in most individuals cause only inconspicuous oral and genital lesions. Cancer progression is associated with persistent high-risk HPV infection and with deregulated viral gene expression, which leads to excessive cell proliferation, deficient DNA repair, and the accumulation of genetic damage in the infected cell. Although their life-cycle organisation is broadly similar to that of the low-risk HPV types, the two groups differ significantly in their capacity to drive cell cycle entry and cell proliferation in the basal/parabasal cell layers. This is thought to be linked, at least in part, to different abilities of the high- and low-risk E6 proteins to modulate the activity of p53 and PDZ-domain proteins, and the differential ability of the E7 proteins to target the several different members of the retinoblastoma protein family. This article forms part of a special supplement entitled "Comprehensive Control of HPV Infections and Related Diseases" Vaccine Volume 30, Supplement 5, 2012.

Conditionally reprogrammed cells represent a stem-like state of adult epithelial cells

The combination of irradiated fibroblast feeder cells and Rho kinase inhibitor, Y-27632, conditionally induces an indefinite proliferative state in primary mammalian epithelial cells. These conditionally reprogrammed cells (CRCs) are karyotype-stable and nontumorigenic. Because self-renewal is a recognized property of stem cells, we investigated whether Y-27632 and feeder cells induced a stem-like phenotype. We found that CRCs share characteristics of adult stem cells and exhibit up-regulated expression of α6 and β1 integrins, ΔNp63α, CD44, and telomerase reverse transcriptase, as well as decreased Notch signaling and an increased level of nuclear β-catenin. The induction of CRCs is rapid (occurs within 2 d) and results from reprogramming of the entire cell population rather than the selection of a minor subpopulation. CRCs do not overexpress the transcription factor sets characteristic of embryonic or induced pluripotent stem cells (e.g., Sox2, Oct4, Nanog, or Klf4). The induction of CRCs is also reversible, and removal of Y-27632 and feeders allows the cells to differentiate normally. Thus, when CRCs from ectocervical epithelium or tracheal epithelium are placed in an air-liquid interface culture system, the cervical cells form a well differentiated stratified squamous epithelium, whereas the tracheal cells form a ciliated airway epithelium. We discuss the diagnostic and therapeutic opportunities afforded by a method that can generate adult stem-like cells in vitro without genetic manipulation.

Human papillomavirus 16/18 E5 promotes cervical cancer cell proliferation, migration and invasion in vitro and accelerates tumor growth in vivo

High-risk human papillomaviruses (HR-HPVs) are consistently associated with human cervical cancer Additionally, the early oncoproteins of HPVs E5, E6 and E7 are known to contribute to tumor progression. The role of E5 is still nebulous. In this study, we aimed to explore the mechanism of E5 action during the human cervical carcinogenesis process. We created four cell models overexpressing HPV16 or HPV18 E5 (HPV16/18 E5) and investigated their ability to proliferate, along with their metastatic characteristics such as migration and invasion. The expression of HPV16/18 E5 protein in various cell lines was analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR). In addition, we compared the levels of phosphorylated paxillin as well as E-cadherin in cell models and controls by western blot analysis. Finally, we assessed the tumor growth rate of human cervical cancer cells overexpressing HPV16/18 E5 in vivo. We discovered that the expression of HPV16/18 E5 consistently increased the malignant potential of various human cervical cancer cells compared with the primary counterparts. We demonstrated the involvement of HPV16/18 E5 in proliferation, migration, invasion and regulation of the actin cytoskeleton in human cervical cancer cells. In particular we discovered that HPV16/18 E5 overexpression in human cervical cancer cells correlated with higher levels of paxillin proteins phosphorylated on tyrosine residues and with the downregulation of E-cadherin. Importantly, injection of HPV16/18 E5-overexpressing human cervical cancer cells into mice increased both HPV-and non-HPV-derived tumor growth. Collectively, our data indicate that HPV16/18 E5 influences progression of the human cervical cancer malignant phenotype. This study provides new insights into HPV16/18 E5 as a possible agent that may have an impact on the therapeutic strategies targeting human cervical cancer.

Cancer associated human papillomaviruses

A small group of human papillomaviruses (HPVs) cause almost all cervical carcinoma and a significant percentage of other anogenital tract and oral carcinoma. Another group of HPVs causes non-melanoma skin cancers in genetically predisposed or immune suppressed patients upon UV exposure. HPV genome replication requires the host cell's DNA synthesis machinery and HPVs encode proteins that maintain differentiated epithelial cells in a replication competent state. The resulting rewiring of cellular signal transduction circuits triggers several innate cellular tumor suppressor responses that HPVs need to inactivate in order to establish persistent and/or productive infections. This review emphasizes this interplay between virus and the infected host cells and points out biological similarities and differences between different groups of HPVs.

High-risk human papillomavirus E5 oncoprotein displays channel-forming activity sensitive to small-molecule inhibitors

High-risk human papillomavirus type 16 (HPV16) is the primary causative agent of cervical cancer and therefore is responsible for significant morbidity and mortality worldwide. Cellular transformation is mediated directly by the expression of viral oncogenes, the least characterized of which, E5, subverts cellular proliferation and immune recognition processes. Despite a growing catalogue of E5-specific host interactions, little is understood regarding the molecular basis of its function. Here we describe a novel function for HPV16 E5 as an oligomeric channel-forming protein, placing it within the virus-encoded "viroporin" family. The development of a novel recombinant E5 expression system showed that E5 formed oligomeric assemblies of a defined luminal diameter and stoichiometry in membranous environments and that such channels mediated fluorescent dye release from liposomes. Hexameric E5 channel stoichiometry was suggested by native PAGE studies. In lieu of high-resolution structural information, established de novo molecular modeling and design methods permitted the development of the first specific small-molecule E5 inhibitor, capable of both abrogating channel activity in vitro and reducing E5-mediated effects on cell signaling pathways. The identification of channel activity should enhance the future understanding of the physiological function of E5 and could represent an important target for antiviral intervention.

Identification of high-risk human papillomavirus (hrHPV)-associated genes in early stage cervical squamous cell carcinomas

This retrospective study investigated gene expression in tumour samples from 38 patients with early stage human papillomavirus (HPV)-associated cervical squamous cell carcinoma (CSCC). The patients were divided into two groups based on the presence of viral markers of HPV16 or HPV18 infection. Gene expression profiles of tumour samples and the corresponding normal cervical epithelium were analysed using cDNA microarrays. Several genes showed differential expression between the two groups of HPV-infected CSCC patients, although seven genes showed similar changes in both groups. The four genes encoding cyclin-dependent kinase inhibitor 2A, matrix metallopeptidase 9, laminin γ-1, and epidermal growth factor receptor were up-regulated, and the three genes encoding transforming growth factor β receptor 1, interleukin-1α and insulin-like growth factor-binding protein 6 were down-regulated, in both HPV16(+) and HPV18(+) CSCC. These proteins are involved in cell proliferation, cell structure and cell attachment, so their expression might be involved in the mechanism of HPV-induced carcino genesis. A clearer understanding of HPV type-specific gene expression might aid diagnosis and treatment.

Papillomavirus E5: the smallest oncoprotein with many functions

Papillomaviruses (PVs) are established agents of human and animal cancers. They infect cutaneous and mucous epithelia. High Risk (HR) Human PVs (HPVs) are consistently associated with cancer of the uterine cervix, but are also involved in the etiopathogenesis of other cancer types. The early oncoproteins of PVs: E5, E6 and E7 are known to contribute to tumour progression. While the oncogenic activities of E6 and E7 are well characterised, the role of E5 is still rather nebulous. The widespread causal association of PVs with cancer makes their study worthwhile not only in humans but also in animal model systems. The Bovine PV (BPV) system has been the most useful animal model in understanding the oncogenic potential of PVs due to the pivotal role of its E5 oncoprotein in cell transformation. This review will highlight the differences between HPV-16 E5 (16E5) and E5 from other PVs, primarily from BPV. It will discuss the targeting of E5 as a possible therapeutic agent.

The human papillomavirus type 16 E5 oncoprotein translocates calpactin I to the perinuclear region

The human papillomavirus type 16 (HPV-16) E5 oncoprotein is embedded in membranes of the endoplasmic reticulum and nuclear envelope with its C terminus exposed to the cytoplasm. Among other activities, E5 cooperates with the HPV E6 oncoprotein to induce koilocytosis in human cervical cells and keratinocytes in vitro. The effect of E5 on infected cells may rely on its interactions with various cellular proteins. In this study we identify calpactin I, a heterotetrameric, Ca(2+)- and phospholipid-binding protein complex that regulates membrane fusion, as a new cellular target for E5. Both the annexin A2 and p11 subunits of calpactin I coimmunoprecipitate with E5 in COS cells and in human epithelial cell lines, and an intact E5 C terminus is required for binding. Moreover, E5-expressing cells exhibit a perinuclear redistribution of annexin A2 and p11 and show increased fusion of perinuclear membrane vesicles. The C terminus of E5 is required for both the perinuclear redistribution of calpactin I and increased formation of perinuclear vacuoles. These results support the hypothesis that the E5-induced relocalization of calpactin I to the perinuclear region promotes perinuclear membrane fusion, which may underlie the development of koilocytotic vacuoles.

Molecular activities, biosynthesis and evolution of triterpenoid saponins

Saponins are bioactive compounds generally considered to be produced by plants to counteract pathogens and herbivores. Besides their role in plant defense, saponins are of growing interest for drug research as they are active constituents of several folk medicines and provide valuable pharmacological properties. Accordingly, much effort has been put into unraveling the modes of action of saponins, as well as in exploration of their potential for industrial processes and pharmacology. However, the exploitation of saponins for bioengineering crop plants with improved resistances against pests as well as circumvention of laborious and uneconomical extraction procedures for industrial production from plants is hampered by the lack of knowledge and availability of genes in saponin biosynthesis. Although the ability to produce saponins is rather widespread among plants, a complete synthetic pathway has not been elucidated in any single species. Current conceptions consider saponins to be derived from intermediates of the phytosterol pathway, and predominantly enzymes belonging to the multigene families of oxidosqualene cyclases (OSCs), cytochromes P450 (P450s) and family 1 UDP-glycosyltransferases (UGTs) are thought to be involved in their biosynthesis. Formation of unique structural features involves additional biosynthetical enzymes of diverse phylogenetic background. As an example of this, a serine carboxypeptidase-like acyltransferase (SCPL) was recently found to be involved in synthesis of triterpenoid saponins in oats. However, the total number of identified genes in saponin biosynthesis remains low as the complexity and diversity of these multigene families impede gene discovery based on sequence analysis and phylogeny. This review summarizes current knowledge of triterpenoid saponin biosynthesis in plants, molecular activities, evolutionary aspects and perspectives for further gene discovery.

The human papillomavirus type 16 E5 oncoprotein inhibits epidermal growth factor trafficking independently of endosome acidification

The human papillomavirus type 16 E5 oncoprotein (16E5) enhances acute, ligand-dependent activation of the epidermal growth factor receptor (EGFR) and concomitantly alkalinizes endosomes, presumably by binding to the 16-kDa "c" subunit of the V-ATPase proton pump (16K) and inhibiting V-ATPase function. However, the relationship between 16K binding, endosome alkalinization, and altered EGFR signaling remains unclear. Using an antibody that we generated against 16K, we found that 16E5 associated with only a small fraction of endogenous 16K in keratinocytes, suggesting that it was unlikely that E5 could significantly affect V-ATPase function by direct inhibition. Nevertheless, E5 inhibited the acidification of endosomes, as determined by a new assay using a biologically active, pH-sensitive fluorescent EGF conjugate. Since we also found that 16E5 did not alter cell surface EGF binding, the number of EGFRs on the cell surface, or the endocytosis of prebound EGF, we postulated that it might be blocking the fusion of early endosomes with acidified vesicles. Our studies with pH-sensitive and -insensitive fluorescent EGF conjugates and fluorescent dextran confirmed that E5 prevented endosome maturation (acidification and enlargement) by inhibiting endosome fusion. The E5-dependent defect in vesicle fusion was not due to detectable disruption of actin, tubulin, vimentin, or cytokeratin filaments, suggesting that membrane fusion was being directly affected rather than vesicle transport. Perhaps most importantly, while bafilomycin A(1) (like E5) binds to 16K and inhibits endosome acidification, it did not mimic the ability of E5 to inhibit endosome enlargement or the trafficking of EGF. Thus, 16E5 alters EGF endocytic trafficking via a pH-independent inhibition of vesicle fusion.

The natural history of human papillomavirus infections of the mucosal epithelia

Human papillomaviruses (HPVs), members of a very large family of small DNA viruses, cause both benign papillomas and malignant tumors. While most research on these viruses over the past 30 years has focused on their oncogenic properties in the genital tract, they also play an important role in diseases of the upper aerodigestive tract. Rapidly accelerating advances in knowledge have increased our understanding of the biology of these viruses and this knowledge, in turn, is being applied to new approaches to prevent, diagnose, and treat HPV-induced diseases. In this introductory article, we provide an overview of the structure and life cycle of the mucosal HPVs and their interactions with their target tissues and cells. Finally, we provide our thoughts about treatments for HPV-induced diseases, present and future.