Principles of polyoma- and papillomavirus uncoating

Evidence of the Mechanism by Which Polyomaviruses Exploit the Extracellular Vesicle Delivery System during Infection

Increasing evidence suggests that human viruses can hijack extracellular vesicles (EVs) to deliver proteins, mRNAs, microRNAs (miRNAs) and whole viral particles during viral persistence in the host. Human polyomavirus (PyV) miRNAs, which downregulate large T-antigen expression and target host factors, help the virus escape immune elimination and may have roles in the success of viral persistence/replication and the development of diseases. In this context, several investigations have detected PyV miRNAs in EVs obtained from cell culture supernatants after viral infection, demonstrating the ability of these vesicles to deliver miRNAs to uninfected cells, potentially counteracting new viral infection. Additionally, PyV miRNAs have been identified in EVs derived from the biological fluids of clinical samples obtained from patients with or at risk of severe PyV-associated diseases and from asymptomatic control healthy subjects. Interestingly, PyV miRNAs were found to be circulating in blood, urine, cerebrospinal fluid, and saliva samples from patients despite their PyV DNA status. Recently, the association between EVs and PyV viral particles was reported, demonstrating the ability of PyV viral particles to enter the cell without natural receptor-mediated entry and evade antibody-mediated neutralization or to be neutralized at a step different from that of the neutralization of naked whole viral particles. All these data point toward a potential role of the association between PyVs with EVs in viral persistence, suggesting that further work to define the implication of this interaction in viral reactivation is warranted.

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.

Extracellular Conformational Changes in the Capsid of Human Papillomaviruses Contribute to Asynchronous Uptake into Host Cells

Human papillomavirus 16 (HPV16) is the leading cause of cervical cancer. For initial infection, HPV16 utilizes a novel endocytic pathway for host cell entry. Unique among viruses, uptake occurs asynchronously over a protracted period of time, with half-times between 9 and 12 h. To trigger endocytic uptake, the virus particles need to undergo a series of structural modifications after initial binding to heparan sulfate proteoglycans (HSPGs). These changes involve proteolytic cleavage of the major capsid protein L1 by kallikrein-8 (KLK8), exposure of the N terminus of the minor capsid protein L2 by cyclophilins, and cleavage of this N terminus by furin. Overall, the structural changes are thought to facilitate the engagement of an elusive secondary receptor for internalization. Here, we addressed whether structural changes are the rate-limiting steps during infectious internalization of HPV16 by using structurally primed HPV16 particles. Our findings indicate that the structural modifications mediated by cyclophilins and furin, which lead to exposure and cleavage, respectively, of the L2 N terminus contribute to the slow and asynchronous internalization kinetics, whereas conformational changes elicited by HSPG binding and KLK8 cleavage did not. However, these structural modifications accounted for only 30 to 50% of the delay in internalization. Therefore, we propose that limited internalization receptor availability for engagement of HPV16 causes slow and asynchronous internalization in addition to rate-limiting structural changes in the viral capsid.IMPORTANCE HPVs are the main cause of anogenital cancers. Their unique biology is linked to the differentiation program of skin or mucosa. Here, we analyzed another unique aspect of HPV infections using the prototype HPV16. After initial cell binding, HPVs display an unusually protracted residence time on the plasma membrane prior to asynchronous uptake. As viruses typically do not expose themselves to host immune sensing, we analyzed the underlying reasons for this unusual behavior. This study provides evidence that both extracellular structural modifications and possibly a limited availability of the internalization receptor contribute to the slow internalization process of the virus. These findings indicate that perhaps a unique niche for initial infection that could allow for rapid infection exists. In addition, our results may help to develop novel, preventive antiviral measures.

Translocation of the papillomavirus L2/vDNA complex across the limiting membrane requires the onset of mitosis

The human papillomavirus type 16 (HPV16) L2 protein acts as a chaperone to ensure that the viral genome (vDNA) traffics from endosomes to the trans-Golgi network (TGN) and eventually the nucleus, where HPV replication occurs. En route to the nucleus, the L2/vDNA complex must translocate across limiting intracellular membranes. The details of this critical process remain poorly characterized. We have developed a system based on subcellular compartmentalization of the enzyme BirA and its cognate substrate to detect membrane translocation of L2-BirA from incoming virions. We find that L2 translocation requires transport to the TGN and is strictly dependent on entry into mitosis, coinciding with mitotic entry in synchronized cells. Cell cycle arrest causes retention of L2/vDNA at the TGN; only release and progression past G2/M enables translocation across the limiting membrane and subsequent infection. Microscopy of EdU-labeled vDNA reveals a rapid and dramatic shift in vDNA localization during early mitosis. At late G2/early prophase vDNA egresses from the TGN to a pericentriolar location, accumulating there through prometaphase where it begins to associate with condensed chromosomes. By metaphase and throughout anaphase the vDNA is seen bound to the mitotic chromosomes, ensuring distribution into both daughter nuclei. Mutations in a newly defined chromatin binding region of L2 potently blocked translocation, suggesting that translocation is dependent on chromatin binding during prometaphase. This represents the first time a virus has been shown to functionally couple the penetration of limiting membranes to cellular mitosis, explaining in part the tropism of HPV for mitotic basal keratinocytes.

Principles of Virus Uncoating: Cues and the Snooker Ball

Viruses are spherical or complex shaped carriers of proteins, nucleic acids and sometimes lipids and sugars. They are metastable and poised for structural changes. These features allow viruses to communicate with host cells during entry, and to release the viral genome, a process known as uncoating. Studies have shown that hundreds of host factors directly or indirectly support this process. The cell provides molecules that promote stepwise virus uncoating, and direct the virus to the site of replication. It acts akin to a snooker player who delivers accurate and timely shots (cues) to the ball (virus) to score. The viruses, on the other hand, trick (snooker) the host, hijack its homeostasis systems, and dampen innate immune responses directed against danger signals. In this review, we discuss how cellular cues, facilitators, and built‐in viral mechanisms promote uncoating. Cues come from receptors, enzymes and chemicals that act directly on the virus particle to alter its structure, trafficking and infectivity. Facilitators are defined as host factors that are involved in processes which indirectly enhance entry or uncoating. Unraveling the mechanisms of virus uncoating will continue to enhance understanding of cell functions, and help counteracting infections with chemicals and vaccines.

Kallikrein-8 Proteolytically Processes Human Papillomaviruses in the Extracellular Space To Facilitate Entry into Host Cells

The entry of human papillomaviruses into host cells is a complex process. It involves conformational changes at the cell surface, receptor switching, internalization by a novel endocytic mechanism, uncoating in endosomes, trafficking of a subviral complex to the Golgi complex, and nuclear entry during mitosis. Here, we addressed how the stabilizing contacts in the capsid of human papillomavirus 16 (HPV16) may be reversed to allow uncoating of the viral genome. Using biochemical and cell-biological analyses, we determined that the major capsid protein L1 underwent proteolytic cleavage during entry. In addition to a dispensable cathepsin-mediated proteolysis that occurred likely after removal of capsomers from the subviral complex in endosomes, at least two further proteolytic cleavages of L1 were observed, one of which was independent of the low-pH environment of endosomes. This cleavage occurred extracellularly. Further analysis showed that the responsible protease was the secreted trypsin-like serine protease kallikrein-8 (KLK8) involved in epidermal homeostasis and wound healing. Required for infection, the cleavage was facilitated by prior interaction of viral particles with heparan sulfate proteoglycans. KLK8-mediated cleavage was crucial for further conformational changes exposing an important epitope of the minor capsid protein L2. Occurring independently of cyclophilins and of furin that mediate L2 exposure, KLK8-mediated cleavage of L1 likely facilitated access to L2, located in the capsid lumen, and potentially uncoating. Since HPV6 and HPV18 also required KLK8 for entry, we propose that the KLK8-dependent entry step is conserved.

IMPORTANCE

Our analysis of the proteolytic processing of incoming HPV16, an etiological agent of cervical cancer, demonstrated that the capsid is cleaved extracellularly by a serine protease active during wound healing and that this cleavage was crucial for infection. The cleavage of L1 is one of at least four structural alterations that prime the virus extracellularly for receptor switching, internalization, and possibly uncoating. This step was also important for HPV6 and HPV18, which may suggest that it is conserved among the papillomaviruses. This study advances the understanding of how HPV16 initially infects cells, strengthens the notion that wounding facilitates infection of epidermal tissue, and may help the development of antiviral measures.

DNA virus uncoating

Virus genomes are condensed and packaged inside stable proteinaceous capsids that serve to protect them during transit from one cell or host organism, to the next. During virus entry, capsid shells are primed and disassembled in a complex, tightly-regulated, multi-step process termed uncoating. Here we compare the uncoating-programs of DNA viruses of the pox-, herpes-, adeno-, polyoma-, and papillomavirus families. Highlighting the chemical and mechanical cues virus capsids respond to, we review the conformational changes that occur during stepwise disassembly of virus capsids and how these culminate in the release of viral genomes at the right time and cellular location to assure successful replication.

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.

An L2 SUMO interacting motif is important for PML localization and infection of human papillomavirus type 16

Human papillomaviruses (HPV) induce warts and cancers on skin and mucosa. The HPV16 capsid is composed of the proteins L1 and L2. After cell entry and virus disassembly, the L2 protein accompanies the viral DNA to promyelocytic leukaemia nuclear bodies (PML-NBs) within the host nuclei enabling viral transcription and replication. Multiple components of PML-NBs are regulated by small ubiquitin-like modifiers (SUMOs) either based on covalent SUMO modification (SUMOylation), or based on non-covalent SUMO interaction via SUMO interacting motifs (SIMs). We show here that the HPV16 L2 comprises at least one SIM, which is crucial for the L2 interaction with SUMO2 in immunoprecipitation and colocalization with SUMO2 in PML-NBs. Biophysical analysis confirmed a direct L2 interaction with SUMO substantiated by identification of potential L2-SUMO interaction structures in molecular dynamics simulations. Mutation of the SIM resulted in absence of the L2-DNA complex at PML-NB and in a loss of infectivity of mutant HPV16 pseudoviruses. In contrast, we found that L2 SUMOylation has no effect on L2 localization in PML-NBs and SUMO interaction. Our data suggest that the L2 SIM is important for L2 interaction with SUMO and/or SUMOylated proteins, which is indispensable for the delivery of viral DNA to PML-NBs and efficient HPV infection.

The tetraspanin CD151 in papillomavirus infection

Human papillomaviruses (HPV) are non-enveloped DNA tumor viruses that infect skin and mucosa. The most oncogenic subtype, HPV16, causes various types of cancer, including cervical, anal, and head and neck cancers. During the multistep process of infection, numerous host proteins are required for the delivery of virus genetic information into the nucleus of target cells. Over the last two decades, many host-cell proteins such as heparan sulfate proteoglycans, integrins, growth factor receptors, actin and the tetraspanin CD151 have been described to be involved in the process of infectious entry of HPV16. Tetraspanins have the ability to organize membrane microdomains and to directly influence the function of associated molecules, including binding of receptors to their ligands, receptor oligomerization and signal transduction. Here, we summarize the current knowledge on CD151, and CD151-associated partners during HPV infection and discuss the underlying mechanisms.

Concepts of papillomavirus entry into host cells

Papillomaviruses enter basal cells of stratified epithelia. Assembly of new virions occurs in infected cells during terminal differentiation. This unique biology is reflected in the mechanism of entry. Extracellularly, the interaction of nonenveloped capsids with several host cell proteins, after binding, results in discrete conformational changes. Asynchronous internalization occurs over several hours by an endocytic mechanism related to, but distinct from macropinocytosis. Intracellular trafficking leads virions through the endosomal system, and from late endosomes to the trans-Golgi-network, before nuclear delivery. Here, we discuss the existing data with the aim to synthesize an integrated model of the stepwise process of entry, thereby highlighting key open questions. Additionally, we relate data from experiments with cultured cells to in vivo results.

Tetraspanin CD151 mediates papillomavirus type 16 endocytosis

Human papillomavirus type 16 (HPV16) is the primary etiologic agent for cervical cancer. The infectious entry of HPV16 into cells occurs via a so-far poorly characterized clathrin- and caveolin-independent endocytic pathway, which involves tetraspanin proteins and actin. In this study, we investigated the specific role of the tetraspanin CD151 in the early steps of HPV16 infection. We show that surface-bound HPV16 moves together with CD151 within the plane of the membrane before they cointernalize into endosomes. Depletion of endogenous CD151 did not affect binding of viral particles to cells but resulted in reduction of HPV16 endocytosis. HPV16 uptake is dependent on the C-terminal cytoplasmic region of CD151 but does not require its tyrosine-based sorting motif. Reexpression of the wild-type CD151 but not mutants affecting integrin functions restored virus internalization in CD151-depleted cells. Accordingly, short interfering RNA (siRNA) gene knockdown experiments confirmed that CD151-associated integrins (i.e., α3β1 and α6β1/4) are involved in HPV16 infection. Furthermore, palmitoylation-deficient CD151 did not support HPV16 cell entry. These data show that complex formation of CD151 with laminin-binding integrins and integration of the complex into tetraspanin-enriched microdomains are critical for HPV16 endocytosis.

Host-cell factors involved in papillomavirus entry

Papillomaviruses infect skin and mucosa where they induce warts and cancers. For entry to occur, they sequentially engage numerous host proteins, allowing them to deliver their genetic information into target cells. This multistep process starts with initial binding via its L1 major capsid protein, followed by structural changes of the capsid on the cell surface, engagement of different receptors, and endocytosis. The post-entry phase includes capsid disassembly, endosomal escape of a complex of the minor capsid protein L2 and the viral genome, its transport into the nucleus, and accumulation at nuclear substructures. This review summarizes the current knowledge of the papillomavirus entry pathway and the role of cellular proteins involved in this course of events.