γ-Secretase promotes membrane insertion of the human papillomavirus L2 capsid protein during virus infection

The BICD2 dynein cargo adaptor binds to the HPV16 L2 capsid protein and promotes HPV infection

During entry, human papillomavirus (HPV) traffics from the endosome to the trans Golgi network (TGN) and Golgi and then the nucleus to cause infection. Although dynein is thought to play a role in HPV infection, how this host motor recruits the virus to support infection and which entry step(s) requires dynein are unclear. Here we show that the dynein cargo adaptor BICD2 binds to the HPV L2 capsid protein during entry, recruiting HPV to dynein for transport of the virus along the endosome-TGN/Golgi axis to promote infection. In the absence of BICD2 function, HPV accumulates in the endosome and TGN and infection is inhibited. Cell-based and in vitro binding studies identified a short segment near the C-terminus of L2 that can directly interact with BICD2. Our results reveal the molecular basis by which the dynein motor captures HPV to promote infection and identify this virus as a novel cargo of the BICD2 dynein adaptor.

A Peptide Derived from Sorting Nexin 1 Inhibits HPV16 Entry, Retrograde Trafficking, and L2 Membrane Spanning

High risk human papillomavirus (HPV) infection is responsible for 99% of cervical cancers and 5% of all human cancers worldwide. HPV infection requires the viral genome (vDNA) to gain access to nuclei of basal keratinocytes of epithelium. After virion endocytosis, the minor capsid protein L2 dictates the subcellular retrograde trafficking and nuclear localization of the vDNA during mitosis. Prior work identified a cell-permeable peptide termed SNX1.3, derived from the BAR domain of sorting nexin 1 (SNX1), that potently blocks the retrograde and nuclear trafficking of EGFR in triple negative breast cancer cells. Given the importance of EGFR and retrograde trafficking pathways in HPV16 infection, we set forth to study the effects of SNX1.3 within this context. SNX1.3 inhibited HPV16 infection by both delaying virion endocytosis, as well as potently blocking virion retrograde trafficking and Golgi localization. SNX1.3 had no effect on cell proliferation, nor did it affect post-Golgi trafficking of HPV16. Looking more directly at L2 function, SNX1.3 was found to impair membrane spanning of the minor capsid protein. Future work will focus on mechanistic studies of SNX1.3 inhibition, and the role of EGFR signaling and SNX1- mediated endosomal tubulation, cargo sorting, and retrograde trafficking in HPV infection.

HPV-associated oropharyngeal cancer: in search of surrogate biomarkers for early lesions

The incidence of oropharyngeal cancer (OPSCC) has escalated in the past few decades; this has largely been triggered by high-risk human papillomavirus (HPV). Early cancer screening is needed for timely clinical intervention and may reduce mortality and morbidity, but the lack of knowledge about premalignant lesions for OPSCC poses a significant challenge to early detection. Biomarkers that identify individuals at high risk for OPSCC may act as surrogate markers for precancer but these are limited as only a few studies decipher the multistep progression from HPV infection to OPSCC development. Here, we summarize the current literature describing the multistep progression from oral HPV infection, persistence, and tumor development in the oropharynx. We also examine key challenges that hinder the identification of premalignant lesions in the oropharynx and discuss potential biomarkers for oropharyngeal precancer. Finally, we evaluate novel strategies to improve investigations of the biological process that drives oral HPV persistence and OPSCC, highlighting new developments in the establishment of a genetic progression model for HPV + OPSCC and in vivo models that mimic HPV + OPSCC pathogenesis.

γ-Secretase inhibitor alleviates lipopolysaccharide-induced myocardial injury through regulating JAK2/STAT3 signaling

BACKGROUND

Septic myocardial injury is one of the most life-threatening organ dysfunction. The γ-secretase-based approaches have been developed as potential strategies for diverse diseases management. Unfortunately, the role of γ-secretase inhibitor in septic myocardial injury is unclarified. The present study aims to investigate the effect of γ-secretase inhibitor in septic myocardial injury and reveal its mechanism.

METHODS

The mouse model of septic myocardial injury was established by intraperitoneally injection of lipopolysaccharide (LPS), and γ-secretase inhibitor MW167 was applied in this model. RNA-sequencing analysis and further bioinformatics analyses were used to screen differential expressed genes (DEGs) and potentially enriched pathways between LPS and LPS + MW167 mice. Pathological examination was performed using haematoxylin and eosin (HE) staining. SD-1029 was used to block JAK2/STAT3 signaling in H9C2 cells and western blot analysis quantified JAK2/STAT3-related proteins.

RESULTS

LPS induced myocardial injury accompanied with significant inflammatory infiltration and more apoptotic cells. Transcriptome sequencing screened 36 DEGs and bioinformatics identified JAK2/STAT3 signaling pathway was significantly enriched. Further in vitro experiments showed that γ-secretase inhibitor MW167 activated JAK2/STAT3 pathway. Additionally, MW167 restored cell viability, decreased myocardial injury markers including cardiac troponin I (cTnI) and brain natriuretic peptide (BNP), inhibited pro-inflammatory cytokines such as interleukin (IL)-1β and tumor necrosis factor-α (TNF-α) and reduced nitric oxide (NO), cyclooxygenase-2 (COX2) and inducible nitric oxide synthase (iNOS) release. Application of SD-1029 reversely deteriorated LPS-induced myocardial injury and inflammatory response in γ-secretase inhibitor-treated myocardial cells.

CONCLUSION

The results demonstrate that γ-secretase inhibitor alleviates septic myocardial injury via activating JAK2/STAT3 signaling, and provide novel therapeutic direction for septic myocardial injury.

Elevated A-to-I RNA editing in COVID-19 infected individuals

Given the current status of coronavirus disease 2019 (COVID-19) as a global pandemic, it is of high priority to gain a deeper understanding of the disease's development and how the virus impacts its host. Adenosine (A)-to-Inosine (I) RNA editing is a post-transcriptional modification, catalyzed by the ADAR family of enzymes, that can be considered part of the inherent cellular defense mechanism as it affects the innate immune response in a complex manner. It was previously reported that various viruses could interact with the host's ADAR enzymes, resulting in epigenetic changes both to the virus and the host. Here, we analyze RNA-seq of nasopharyngeal swab specimens as well as whole-blood samples of COVID-19 infected individuals and show a significant elevation in the global RNA editing activity in COVID-19 compared to healthy controls. We also detect specific coding sites that exhibit higher editing activity. We further show that the increment in editing activity during the disease is temporary and returns to baseline shortly after the symptomatic period. These significant epigenetic changes may contribute to the immune system response and affect adverse outcomes seen in post-viral cases.

Engineering Proteins for Cell Entry

Proteins are essential for life, as they participate in all vital processes in the body. In the past decade, delivery of active proteins to specific cells and organs has attracted increasing interest. However, most proteins cannot enter the cytoplasm due to the cell membrane acting as a natural barrier. To overcome this challenge, various proteins have been engineered to acquire cell-penetrating capacity by mimicking or modifying natural shuttling proteins. In this review, we provide an overview of the different types of engineered cell-penetrating proteins such as cell-penetrating peptides, supercharged proteins, receptor-binding proteins, and bacterial toxins. We also discuss some strategies for improving endosomal escape such as pore formation, the proton sponge effect, and hijacking intracellular trafficking pathways. Finally, we introduce some novel methods and technologies for designing and detecting engineered cell-penetrating proteins.

Noncanonical Rab9a action supports retromer-mediated endosomal exit of human papillomavirus during virus entry

Rab GTPases play key roles in controlling intracellular vesicular transport. GTP-bound Rab proteins support vesicle trafficking. Here, we report that, unlike cellular protein cargos, retromer-mediated delivery of human papillomaviruses (HPV) into the retrograde transport pathway during virus entry is inhibited by Rab9a in its GTP-bound form. Knockdown of Rab9a inhibits HPV entry by modulating the HPV-retromer interaction and impairing retromer-mediated endosome-to-Golgi transport of the incoming virus, resulting in the accumulation of HPV in the endosome. Rab9a is in proximity to HPV as early as 3.5 h post-infection, prior to the Rab7-HPV interaction, and HPV displays increased association with retromer in Rab9a knockdown cells, even in the presence of dominant negative Rab7. Thus, Rab9a can regulate HPV-retromer association independently of Rab7. Surprisingly, excess GTP-Rab9a impairs HPV entry, whereas excess GDP-Rab9a reduces association between L2 and Rab9a and stimulates entry. These findings reveal that HPV and cellular proteins utilize the Rab9a host trafficking machinery in distinct ways during intracellular trafficking.

Folliculin Prevents Lysosomal Degradation of Human Papillomavirus To Support Infectious Cell Entry

Human papillomavirus (HPV) infects epithelial basal cells in the mucosa and either proliferates with the differentiation of the basal cells or persists in them. Multiple host factors are required to support the HPV life cycle; however, the molecular mechanisms involved in cell entry are not yet fully understood. In this study, we performed a genome-wide clustered regularly interspaced short palindromic repeat (CRISPR)-CRISPR-associated protein 9 (Cas9) knockout (KO) screen in HeLa cells and identified folliculin (FLCN), a GTPase-activating protein for Rag GTPases, as an important host factor for HPV infection. The introduction of single guide RNAs for the FLCN gene into HeLa, HaCaT, and ectocervical Ect1 cells reduced infection by HPV18 pseudovirions (18PsVs) and 16PsVs. FLCN KO HeLa cells also exhibited strong resistance to infection with 18PsVs and 16PsVs; nevertheless, they remained highly susceptible to infections with vesicular stomatitis virus glycoprotein-pseudotyped lentivirus and adeno-associated virus. Immunofluorescence microscopy revealed that the numbers of virions binding to the cell surface were slightly increased in FLCN KO cells. However, virion internalization analysis showed that the internalized virions were rapidly degraded in FLCN KO cells. This degradation was blocked by treatment with the lysosome inhibitor bafilomycin A1. Furthermore, the virion degradation phenotype was also observed in Ras-related GTP-binding protein C (RagC) KO cells. These results suggest that FLCN prevents the lysosomal degradation of incoming HPV virions by enhancing lysosomal RagC activity. IMPORTANCE Cell entry by human papillomavirus (HPV) involves a cellular retrograde transport pathway from the endosome to the trans-Golgi network/Golgi apparatus. However, the mechanism by which this viral trafficking is safeguarded is poorly understood. This is the first study showing that the GTPase-activating protein folliculin (FLCN) protects incoming HPV virions from lysosomal degradation and supports infectious cell entry by activating the Rag GTPases, presumably through the suppression of excessive lysosomal biosynthesis. These findings provide new insights into the effects of small GTPase activity regulation on HPV cell entry and enhance our understanding of the HPV degradation pathway.

Noncanonical Rab9a action supports endosomal exit of human papillomavirus during virus entry

Rab GTPases play key roles in controlling intracellular vesicular transport. GTP-bound Rab proteins support vesicle trafficking. Here, we report that, unlike cellular protein cargos, the delivery of human papillomaviruses (HPV) into the retrograde transport pathway during virus entry is inhibited by Rab9a in its GTP-bound form. Knockdown of Rab9a hampers HPV entry by regulating the HPV-retromer interaction and impairing retromer-mediated endosome-to-Golgi transport of the incoming virus, resulting in the accumulation of HPV in the endosome. Rab9a is in proximity to HPV as early as 3.5 h post-infection, prior to the Rab7-HPV interaction. HPV displays increased association with retromer in Rab9a knockdown cells, even in the presence of dominant negative Rab7. Thus, Rab9a can regulate HPV-retromer association independently of Rab7. Surprisingly, excess GTP-Rab9a impairs HPV entry, whereas excess GDP-Rab9a stimulates entry. These findings reveal that HPV employs a trafficking mechanism distinct from that used by cellular proteins.

Master mitotic kinases regulate viral genome delivery during papillomavirus cell entry

Mitosis induces cellular rearrangements like spindle formation, Golgi fragmentation, and nuclear envelope breakdown. Similar to certain retroviruses, nuclear delivery during entry of human papillomavirus (HPV) genomes is facilitated by mitosis, during which minor capsid protein L2 tethers viral DNA to mitotic chromosomes. However, the mechanism of viral genome delivery and tethering to condensed chromosomes is barely understood. It is unclear, which cellular proteins facilitate this process or how this process is regulated. This work identifies crucial phosphorylations on HPV minor capsid protein L2 occurring at mitosis onset. L2's chromosome binding region (CBR) is sequentially phosphorylated by the master mitotic kinases CDK1 and PLK1. L2 phosphorylation, thus, regulates timely delivery of HPV vDNA to mitotic chromatin during mitosis. In summary, our work demonstrates a crucial role of mitotic kinases for nuclear delivery of viral DNA and provides important insights into the molecular mechanism of pathogen import into the nucleus during mitosis.

HPV is a cargo for the COPI sorting complex during virus entry

During entry, human papillomavirus (HPV) traffics from the cell surface to the endosome and then to the trans-Golgi network (TGN) and Golgi apparatus. HPV must transit across the TGN/Golgi and exit these compartments to reach the nucleus to cause infection, although how these steps are accomplished is unclear. Combining cellular fractionation, unbiased proteomics, and gene knockdown strategies, we identified the coat protein complex I (COPI), a highly conserved protein complex that facilitates retrograde trafficking of cellular cargos, as a host factor required for HPV infection. Upon TGN/Golgi arrival, the cytoplasmic segment of HPV L2 binds directly to COPI. COPI depletion causes the accumulation of HPV in the TGN/Golgi, resembling the fate of a COPI binding-defective L2 mutant. We propose that the L2-COPI interaction drives HPV trafficking through the TGN and Golgi stacks during virus entry. This shows that an incoming virus is a cargo of the COPI complex.

Non-enveloped virus membrane penetration: New advances leading to new insights

Host cell membranes pose a particular challenge for non-enveloped viruses. Whereas enveloped viruses enter cells by fusing their lipid envelopes with the cellular membrane, non-enveloped viruses generally must (1) enter cells via endocytosis, then (2) penetrate the cellular endomembrane to reach the cytosol. Only then can the viruses begin to replicate (or transit to the nucleus to replicate). Although membrane penetration of non-enveloped viruses is a crucial entry step, many of the precise molecular details of this process remain unclear. Recent findings have begun to untangle the various mechanisms by which non-enveloped viral proteins disrupt and penetrate cellular endomembranes. Specifically, high-resolution microscopy studies have revealed precise conformational changes in viral proteins that enable penetration, while biochemical studies have identified key host proteins that promote viral penetration and transport. This brief article summarizes new discoveries in the membrane penetration process for three of the most intensely studied families of non-enveloped viruses: reoviruses, papillomaviruses, and polyomaviruses.

Epidemiology and Molecular Biology of HPV Variants in Cervical Cancer: The State of the Art in Mexico

Cervical cancer (CC) continues to be a major public health problem in Mexico, ranking second among cancers in women. A persistent infection with human papillomaviruses (HPV) is the main risk factor for CC development. In addition, a significant fraction of other cancers including those of the anus, oropharynx, and penis are also related to HPV infection. In CC, HPV-16 is the most prevalent high-risk HPV type, followed by HPV-18, both being responsible for 70% of cases. HPV intratype variant lineages differ in nucleotide sequences by 1–10%, while sublineages differ by 0.5–1%. Several studies have postulated that the nucleotide changes that occur between HPV intratype variants are reflected in functional differences and in pathogenicity. Moreover, it has been demonstrated that HPV-16 and -18 intratype variants differentially affect molecular processes in infected cells, changing their biological behavior that finally impacts in the clinical outcome of patients. Mexico has participated in providing knowledge on the geographical distribution of intratype variants of the most prevalent HPVs in premalignant lesions of the cervix and cervical cancer, as well as in other HPV-related tumors. In addition, functional studies have been carried out to assess the cellular effects of intratype variations in HPV proteins. This review addresses the state of the art on the epidemiology of HPV-16 and HPV-18 intratype variants in the Mexican population, as well as their association with persistence, precancer and cervical cancer, and functional aspects related to their biological behavior.

Peroxisomal Membrane Protein PMP34 Is Involved in the Human Papillomavirus Infection Pathway

Infection with high-risk human papillomavirus (HPV) types is linked to the onset of several cancers. The mechanism of HPV infection, however, has not yet been fully elucidated. Here, using the newly developed HPV infectious pseudovirion (HPV PsV) and a genome-wide clustered regularly interspaced short palindromic repeat (CRISPR) screening system, we established an experimental system and searched for genes involved in HPV infection. The HPV PsV has the truncated herpes simplex virus thymidine kinase (dTK) to kill PsV-infected cells when combined with ganciclovir. The five rounds of selection of 293FT cells by infection with HPV PsVs identified two candidate genes involved in the HPV infection pathway. The validation experiments showed that SLC25A17, which encodes the peroxisomal membrane protein PMP34, was involved in the HPV infection pathway. The gRNAs against SLC25A17 attenuated the efficiency of HPV PsV infection in 293FT and HeLa cells. Although further experiments are required to determine whether PMP34 acts as the HPV infection pathway, these results indicate that our screening system is useful for identification of the genes involved in the HPV infection pathway.

Human Papillomavirus L2 Capsid Protein Stabilizes γ-Secretase during Viral Infection

Intracellular trafficking of human papillomavirus (HPV) during virus entry requires γ-secretase, a cellular protease consisting of a complex of four cellular transmembrane (TM) proteins. γ-secretase typically cleaves substrate proteins but it plays a non-canonical role during HPV entry. γ-secretase binds to the HPV minor capsid protein L2 and facilitates its insertion into the endosomal membrane. After insertion, L2 protrudes into the cytoplasm, which allows HPV to bind other cellular factors required for proper virus trafficking into the retrograde transport pathway. Here, we further characterize the interaction between γ-secretase and HPV L2. We show that γ-secretase is required for cytoplasmic protrusion of L2 and that L2 associates strongly with the PS1 catalytic subunit of γ-secretase and stabilizes the γ-secretase complex. Mutational studies revealed that a putative TM domain in HPV16 L2 cannot be replaced by a foreign TM domain, that infectivity of HPV TM mutants is tightly correlated with γ-secretase binding and stabilization, and that the L2 TM domain is required for protrusion of the L2 protein into the cytoplasm. These results provide new insight into the interaction between γ-secretase and L2 and highlight the importance of the native HPV L2 TM domain for proper virus trafficking during entry.

HPV16 Entry into Epithelial Cells: Running a Gauntlet

During initial infection, human papillomaviruses (HPV) take an unusual trafficking pathway through their host cell. It begins with a long period on the cell surface, during which the capsid is primed and a virus entry platform is formed. A specific type of clathrin-independent endocytosis and subsequent retrograde trafficking to the trans-Golgi network follow this. Cellular reorganization processes, which take place during mitosis, enable further virus transport and the establishment of infection while evading intrinsic cellular immune defenses. First, the fragmentation of the Golgi allows the release of membrane-encased virions, which are partially protected from cytoplasmic restriction factors. Second, the nuclear envelope breakdown opens the gate for these virus–vesicles to the cell nucleus. Third, the dis- and re-assembly of the PML nuclear bodies leads to the formation of modified virus-associated PML subnuclear structures, enabling viral transcription and replication. While remnants of the major capsid protein L1 and the viral DNA remain in a transport vesicle, the viral capsid protein L2 plays a crucial role during virus entry, as it adopts a membrane-spanning conformation for interaction with various cellular proteins to establish a successful infection. In this review, we follow the oncogenic HPV type 16 during its long journey into the nucleus, and contrast pro- and antiviral processes.

Recent Advances in Our Understanding of the Infectious Entry Pathway of Human Papillomavirus Type 16

Papillomaviruses are a diverse viral species, but several types such as HPV16 are given special attention due to their contribution towards the pathogenesis of several major cancers. In this review, we will summarize how the knowledge of HPV16 entry has expanded since the last comprehensive HPV16 entry review our lab published in 2017.

The long and winding road: human papillomavirus entry and subcellular trafficking

Human papillomaviruses (HPVs) infect and replicate in differentiating mucosal and cutaneous epithelium. Most HPV infections are asymptomatic or cause transient benign neoplasia. However, persistent infections by oncogenic HPV types can progress to cancer. During infectious entry into host keratinocytes, HPV particles interact with many host proteins, beginning with major capsid protein L1 binding to cellular heparan sulfate and a series of enzymatic capsid modifications that promote infectious cellular entry. After utilizing the endosomal pathway to uncoat the viral genome (vDNA), the minor capsid protein L2/vDNA complex is retrograde trafficked to the Golgi, and thereafter, to the nucleus where viral transcription initiates. Post-Golgi trafficking is dependent on mitosis, with L2-dependent tethering of vDNA to mitotic chromosomes before accumulation at nuclear substructures in G1. This review summarizes the current knowledge of the HPV entry pathway, the role of cellular proteins in this process, and notes many gaps in our understanding.

Retromer stabilizes transient membrane insertion of L2 capsid protein during retrograde entry of human papillomavirus

Retromer, a cellular protein trafficking complex, sorts human papillomaviruses (HPVs) into the retrograde pathway for transport of HPV to the nucleus during virus entry. Here, we conducted a protein modulation screen to isolate four artificial transmembrane proteins called traptamers that inhibit different steps of HPV entry. By analyzing cells expressing pairs of traptamers, we ordered the trafficking steps during entry into a coherent pathway. One traptamer stimulates ubiquitination of the L2 capsid protein or associated proteins and diverts incoming virus to the lysosome, whereas the others act downstream by preventing sequential passage of the virus through retrograde compartments. Complex genetic interactions between traptamers revealed that a cell-penetrating peptide (CPP) on L2 mediates transient insertion of L2 into the endosome membrane, which is stabilized by retromer-L2 binding. These results define the retrograde entry route taken by HPV and show that retromer can play a role in CPP-mediated membrane insertion.

How DNA and RNA Viruses Exploit Host Chaperones to Promote Infection

To initiate infection, a virus enters a host cell typically via receptor-dependent endocytosis. It then penetrates a subcellular membrane, reaching a destination that supports transcription, translation, and replication of the viral genome. These steps lead to assembly and morphogenesis of the new viral progeny. The mature virus finally exits the host cell to begin the next infection cycle. Strikingly, viruses hijack host molecular chaperones to accomplish these distinct entry steps. Here we highlight how DNA viruses, including polyomavirus and the human papillomavirus, exploit soluble and membrane-associated chaperones to enter a cell, penetrating and escaping an intracellular membrane en route for infection. We also describe the mechanism by which RNA viruses—including flavivirus and coronavirus—co-opt cytosolic and organelle-selective chaperones to promote viral endocytosis, protein biosynthesis, replication, and assembly. These examples underscore the importance of host chaperones during virus infection, potentially revealing novel antiviral strategies to combat virus-induced diseases.

A Ran-binding protein facilitates nuclear import of human papillomavirus type 16

Human papillomaviruses (HPVs) utilize an atypical mode of nuclear import during cell entry. Residing in the Golgi apparatus until mitosis onset, a subviral complex composed of the minor capsid protein L2 and viral DNA (L2/vDNA) is imported into the nucleus after nuclear envelope breakdown by associating with mitotic chromatin. In this complex, L2 plays a crucial role in the interactions with cellular factors that enable delivery and ultimately tethering of the viral genome to mitotic chromatin. To date, the cellular proteins facilitating these steps remain unknown. Here, we addressed which cellular proteins may be required for this process. Using label-free mass spectrometry, biochemical assays, microscopy, and functional virological assays, we discovered that L2 engages a hitherto unknown protein complex of Ran-binding protein 10 (RanBP10), karyopherin alpha2 (KPNA2), and dynein light chain DYNLT3 to facilitate transport towards mitotic chromatin. Thus, our study not only identifies novel cellular interactors and mechanism that facilitate a poorly understood step in HPV entry, but also a novel cellular transport complex.

Human papillomaviruses (HPVs) cause proliferative lesions such as benign warts or malignant invasive cancers. Like other DNA viruses, HPV has to deliver its genome to the nucleus for viral genome transcription and replication. After initial attachment, HPVs are endocytosed to be eventually directed to the trans-Golgi-network (TGN) by intracellular trafficking, where they reside until cell division. Mitosis onset enables access of the virus to cellular chromatin after nuclear envelope breakdown. Tethering of the virus to mitotic chromatin ensures nuclear delivery upon reformation of the nuclear envelope after mitosis. Our previous work showed that the minor capsid protein L2 facilitates nuclear delivery. However, the detailed mechanism, namely, how HPV trafficks from cytosol to the nuclear space, is barely understood. Here, we identified for the first time cellular proteins that interacted with L2 for nuclear import. Mechanistically, the proteins formed a hitherto unknown cellular transport complex that interacted with L2 to direct the virus to mitotic chromosomes by microtubular transport. Our findings provided not only evidence for a transport mechanism of a poorly understood step of HPV entry, but also discovered a novel cellular transport complex.

Human Papillomavirus infection requires the CCT Chaperonin Complex

Human papillomavirus (HPV) infection is a multi-step process that implies complex interactions of the viral particles with cellular proteins. The HPV capsid includes the two structural proteins L1 and L2, that play crucial roles on infectious viral entry. L2 is particularly relevant for the intracellular trafficking of the viral DNA towards the nucleus. Here, using proteomic studies we identified CCT proteins as novel interaction partners of HPV-16 L2. The CCT multimeric complex is an essential chaperonin which interacts with a large number of protein targets. We analysed the binding of different components of the CCT complex to L2. We confirmed the interaction of this structural viral protein with the CCT subunit 3 (CCT3) and we found that this interaction requires the N-terminal region of L2. Defects in HPV-16 pseudoviral particle (PsVs) infection were revealed by siRNA-mediated knockdown of some CCT subunits. While a substantial drop in the viral infection was associated with the ablation of CCT component 2, even more pronounced effects on infectivity were observed upon depletion of CCT component 3. Using confocal immunofluorescence assays, CCT3 co-localised with HPV PsVs at early times after infection, with L2 being required for this to occur. Further analysis showed the colocalization of several other subunits of CCT with the PsVs. Moreover, we observed a defect in capsid uncoating and a change in PsVs intracellular normal processing when ablating CCT3. Taken together, these studies demonstrate the importance of CCT chaperonin during HPV infectious entry.ImportanceSeveral of the mechanisms that function during the infection of target cells by HPV particles have been previously described. However, many aspects of this process remain unknown. In particular, the role of cellular proteins functioning as molecular chaperones during HPV infections has been only partially investigated. To the best of our knowledge, we describe here for the first time, a requirement of the CCT chaperonin for HPV infection. The role of this cellular complex seems to be determined by the binding of its component 3 to the viral structural protein L2. However, CCT's effect on HPV infection most probably comprises the whole chaperonin complex. Altogether, these studies define an important role for the CCT chaperonin in the processing and intracellular trafficking of HPV particles and in subsequent viral infectious entry.

Advances in Targeting HPV Infection as Potential Alternative Prophylactic Means

Infection by oncogenic human papillomavirus (HPV) is the primary cause of cervical cancer and other anogenital cancers. The majority of cervical cancer cases occur in low- and middle- income countries (LMIC). Concurrent infection with Human Immunodeficiency Virus (HIV) further increases the risk of HPV infection and exacerbates disease onset and progression. Highly effective prophylactic vaccines do exist to combat HPV infection with the most common oncogenic types, but the accessibility to these in LMIC is severely limited due to cost, difficulties in accessing the target population, cultural issues, and maintenance of a cold chain. Alternative preventive measures against HPV infection that are more accessible and affordable are therefore also needed to control cervical cancer risk. There are several efforts in identifying such alternative prophylactics which target key molecules involved in early HPV infection events. This review summarizes the current knowledge of the initial steps in HPV infection, from host cell-surface engagement to cellular trafficking of the viral genome before arrival in the nucleus. The key molecules that can be potentially targeted are highlighted, and a discussion on their applicability as alternative preventive means against HPV infection, with a focus on LMIC, is presented.

Vesicular trafficking permits evasion of cGAS/STING surveillance during initial human papillomavirus infection

Oncogenic human papillomaviruses (HPVs) replicate in differentiating epithelium, causing 5% of cancers worldwide. Like most other DNA viruses, HPV infection initiates after trafficking viral genome (vDNA) to host cell nuclei. Cells possess innate surveillance pathways to detect microbial components or physiological stresses often associated with microbial infections. One of these pathways, cGAS/STING, induces IRF3-dependent antiviral interferon (IFN) responses upon detection of cytosolic DNA. Virion-associated vDNA can activate cGAS/STING during initial viral entry and uncoating/trafficking, and thus cGAS/STING is an obstacle to many DNA viruses. HPV has a unique vesicular trafficking pathway compared to many other DNA viruses. As the capsid uncoats within acidic endosomal compartments, minor capsid protein L2 protrudes across vesicular membranes to facilitate transport of vDNA to the Golgi. L2/vDNA resides within the Golgi lumen until G2/M, whereupon vesicular L2/vDNA traffics along spindle microtubules, tethering to chromosomes to access daughter cell nuclei. L2/vDNA-containing vesicles likely remain intact until G1, following nuclear envelope reformation. We hypothesize that this unique vesicular trafficking protects HPV from cGAS/STING surveillance. Here, we investigate cGAS/STING responses to HPV infection. DNA transfection resulted in acute cGAS/STING activation and downstream IFN responses. In contrast, HPV infection elicited minimal cGAS/STING and IFN responses. To determine the role of vesicular trafficking in cGAS/STING evasion, we forced premature viral penetration of vesicular membranes with membrane-perturbing cationic lipids. Such treatment renders a non-infectious trafficking-defective mutant HPV infectious, yet susceptible to cGAS/STING detection. Overall, HPV evades cGAS/STING by its unique subcellular trafficking, a property that may contribute to establishment of infection.

p120 catenin recruits HPV to γ-secretase to promote virus infection

During internalization and trafficking, human papillomavirus (HPV) moves from the cell surface to the endosome where the transmembrane protease γ-secretase promotes insertion of the viral L2 capsid protein into the endosome membrane. Protrusion of L2 through the endosome membrane into the cytosol allows the recruitment of cytosolic host factors that target the virus to the Golgi en route for productive infection. How endosome-localized HPV is delivered to γ-secretase, a decisive infection step, is unclear. Here we demonstrate that cytosolic p120 catenin, likely via an unidentified transmembrane protein, interacts with HPV at early time-points during viral internalization and trafficking. In the endosome, p120 is not required for low pH-dependent disassembly of the HPV L1 capsid protein from the incoming virion. Rather, p120 is required for HPV to interact with γ-secretase-an interaction that ensures the virus is transported along a productive route. Our findings clarify an enigmatic HPV infection step and provide critical insights into HPV infection that may lead to new therapeutic strategies against HPV-induced diseases.

Gamma secretase inhibition impairs HCMV replication by reduction of immediate early gene expression at the transcriptional level

Due to diverse pathogenic potentials, there is a growing need for anti-HCMV agents. In this study, we show that treatment with DAPT, a γ-secretase inhibitor (GSI), impairs HCMV replication as assessed by a progeny assay based on immunostaining. This effect is not limited to DAPT because other GSIs with different structures and distinct mechanisms of action also exhibit a similar level of inhibitory effects on HCMV viral production, indicating that γ-secretase activity is required for efficient HCMV replication. Western blot and qPCR analyses reveal that DAPT does not interfere with the viral entry process, but reduces expression of the immediate early protein IE1 at the transcriptional level. Furthermore, we exclude the possible involvement of Notch signaling pathway during HCMV replication by showing that expression of the dominant-negative form of MAML1, which disrupts the transactivational ability of Notch intracellular domain (NICD), does not reduce viral particle formation, and that NICD cannot rescue the DAPT-treated outcomes. Taken together, these findings indicate that γ-secretase activity plays an important role in a key step of the HCMV life cycle and γ-secretase inhibition could potentially be used as a novel preventive and therapeutic strategy against HCMV infection and HCMV-related diseases.

Papillomaviruses Go Retro

Human papillomaviruses are important pathogens responsible for approximately 5% of cancer as well as other important human diseases, but many aspects of the papillomavirus life cycle are poorly understood. To undergo genome replication, HPV DNA must traffic from the cell surface to the nucleus. Recent findings have revolutionized our understanding of HPV entry, showing that it requires numerous cellular proteins and proceeds via a series of intracellular membrane-bound vesicles that comprise the retrograde transport pathway. This paper reviews the evidence supporting this unique entry mechanism with a focus on the crucial step by which the incoming virus particle is transferred from the endosome into the retrograde pathway. This new understanding provides novel insights into basic cellular biology and suggests novel rational approaches to inhibit HPV infection.

The Role of Promyelocytic Leukemia Nuclear Bodies During HPV Infection

Promyelocytic leukemia (PML) nuclear bodies (NBs) are highly dynamic subnuclear structures. Their name giving major component, PML protein, is essential for their formation. PML is present in many different isoforms due to differential splicing, which seem to contribute differently to PML NBs function. Sp100 and DAXX are also permanently residing in these structures. PML NBs disassemble in mitosis to form large cytoplasmic aggregates and reassemble after completion of cell division. Posttranslational modifications such as SUMOylation play important roles for protein association with PML NBs. In addition to the factors permanently associated with PML NBs, a large number of proteins may transiently reside in PML NBs dependent on cell stage, type, and condition. PML NBs have been indirectly implicated in a large number of cellular processes including apoptosis, transcriptional regulation, DNA repair and replication. They are considered hot spots for posttranslational modifications and may serve as readily accessible protein depots. However, a precise function has been difficult to assign. Many DNA viruses target PML NBs after entry often resulting in reorganization of these subnuclear structures. Antiviral activity has been assigned to PML NBs partially based on the observation that PML protein is an interferon stimulated gene. In contrast, human papillomavirus (HPV) infection requires the presence of PML protein suggesting that PML NBs may be essential to establish infection. This review will summarize and discuss recent advances in our understanding of the role of PML NBs and individual protein components in the establishment of HPV infection.

Glutathione contributes to efficient post-Golgi trafficking of incoming HPV16 genome

Human papillomavirus (HPV) is the most common sexually transmitted pathogen in the United States, causing 99% of cervical cancers and 5% of all human cancers worldwide. HPV infection requires transport of the viral genome (vDNA) into the nucleus of basal keratinocytes. During this process, minor capsid protein L2 facilitates subcellular retrograde trafficking of the vDNA from endosomes to the Golgi, and accumulation at host chromosomes during mitosis for nuclear retention and localization during interphase. Here we investigated the relationship between cellular glutathione (GSH) and HPV16 infection. siRNA knockdown of GSH biosynthetic enzymes results in a partial decrease of HPV16 infection. Likewise, infection of HPV16 in GSH depleted keratinocytes is inefficient, an effect that was not seen with adenoviral vectors. Analysis of trafficking revealed no defects in cellular binding, entry, furin cleavage of L2, or retrograde trafficking of HPV16, but GSH depletion hindered post-Golgi trafficking and translocation, decreasing nuclear accumulation of vDNA. Although precise mechanisms have yet to be defined, this work suggests that GSH is required for a specific post-Golgi trafficking step in HPV16 infection.

The Known and Potential Intersections of Rab-GTPases in Human Papillomavirus Infections

Papillomaviruses (PVs) were the first viruses recognized to cause tumors and cancers in mammalian hosts by Shope, nearly a century ago (Shope and Hurst, 1933). Over 40 years ago, zur Hausen (1976) first proposed that human papillomaviruses (HPVs) played a role in cervical cancer; in 2008, he shared the Nobel Prize in Medicine for his abundant contributions demonstrating the etiology of HPVs in genital cancers. Despite effective vaccines and screening, HPV infection and morbidity remain a significant worldwide burden, with HPV infections and HPV-related cancers expected increase through 2040. Although HPVs have long-recognized roles in tumorigenesis and cancers, our understanding of the molecular mechanisms by which these viruses interact with cells and usurp cellular processes to initiate infections and produce progeny virions is limited. This is due to longstanding challenges in both obtaining well-characterized infectious virus stocks and modeling tissue-based infection and the replicative cycles in vitro. In the last 20 years, the development of methods to produce virus-like particles (VLPs) and pseudovirions (PsV) along with more physiologically relevant cell- and tissue-based models has facilitated progress in this area. However, many questions regarding HPV infection remain difficult to address experimentally and are, thus, unanswered. Although an obligatory cellular uptake receptor has yet to be identified for any PV species, Rab-GTPases contribute to HPV uptake and transport of viral genomes toward the nucleus. Here, we provide a general overview of the current HPV infection paradigm, the epithelial differentiation-dependent HPV replicative cycle, and review the specifics of how HPVs usurp Rab-related functions during infectious entry. We also suggest other potential interactions based on how HPVs alter cellular activities to complete their replicative-cycle in differentiating epithelium. Understanding how HPVs interface with Rab functions during their complex replicative cycle may provide insight for the development of therapeutic interventions, as current viral counter-measures are solely prophylactic and therapies for HPV-positive individuals remain archaic and limited.

Efficient Inhibition of Human Papillomavirus Infection by L2 Minor Capsid-Derived Lipopeptide

The amino (N)-terminal region of human papillomavirus (HPV) minor capsid protein (L2) is a highly conserved region which is essential for establishing viral infection. Despite its importance in viral infectivity, the role of the HPV N-terminal domain has yet to be fully characterized. Using fine mapping analysis, we identified a 36-amino-acid (aa) peptide sequence of the L2 N terminus, termed L2N, that is critical for HPV infection. Ectopic expression of L2N with the transmembrane sequence on the target cell surface conferred resistance to HPV infection. Additionally, L2N peptide with chemical or enzymatic lipidation at the carboxyl (C) terminus efficiently abrogated HPV infection in target cells. Among the synthetic L2N lipopeptides, a stearoylated lipopeptide spanning aa 13 to 46 (13-46st) exhibited the most potent anti-HPV activity, with a half-maximal inhibitory concentration (IC50) of ∼200 pM. Furthermore, we demonstrated that the 13-46st lipopeptide inhibited HPV entry by blocking trans-Golgi network retrograde trafficking of virion particles, leading to rapid degradation. Fundamentally, the inhibitory effect of L2N lipopeptides appeared to be evolutionarily conserved, as they showed cross-type inhibition among various papillomaviruses. In conclusion, our findings provide new insights into the critical role of the L2N sequence in the HPV entry mechanism and identify the therapeutic potential of L2N lipopeptide as an effective anti-HPV agent.IMPORTANCE HPV is a human oncogenic virus that causes a major public health problem worldwide, which is responsible for approximately 5% of total human cancers and almost all cases of cervical cancers. HPV capsid consists of two structure proteins, the major capsid L1 protein and the minor capsid L2 protein. While L2 plays critical roles during the viral life cycle, the molecular mechanism in viral entry remains elusive. Here, we performed fine mapping of the L2 N-terminal region and defined a short 36-amino-acid peptide, called L2N, which is critical for HPV infection. Specifically, L2N peptide with carboxyl-terminal lipidation acted as a potent and cross-type HPV inhibitor. Taken together, data from our study highlight the essential role of the L2N sequence at the early step of HPV entry and suggests the L2N lipopeptide as a new strategy to broadly prevent HPV infection.

Human Papillomavirus 16 Capsids Mediate Nuclear Entry during Infection

Infectious human papillomavirus 16 (HPV16) L1/L2 pseudovirions were found to remain largely intact during vesicular transport to the nucleus. By electron microscopy, capsids with a diameter of 50 nm were clearly visible within small vesicles attached to mitotic chromosomes and to a lesser extent within interphase nuclei, implying nuclear disassembly. By confocal analysis, it was determined that nuclear entry of assembled L1 is dependent upon the presence of the minor capsid protein, L2, but independent of encapsidated DNA. We also demonstrate that L1 nuclear localization and mitotic chromosome association can occur in vivo in the murine cervicovaginal challenge model of HPV16 infection. These findings challenge the prevailing concepts of PV uncoating and disassembly. More generally, they document that a largely intact viral capsid can enter the nucleus within a transport vesicle, establishing a novel mechanism by which a virus accesses the nuclear cellular machinery.IMPORTANCE Papillomaviruses (PVs) comprise a large family of nonenveloped DNA viruses that include HPV16, among other oncogenic types, the causative agents of cervical cancer. Delivery of the viral DNA into the host cell nucleus is necessary for establishment of infection. This was thought to occur via a subviral complex following uncoating of the larger viral capsid. In this study, we demonstrate that little disassembly of the PV capsid occurs prior to nuclear delivery. These surprising data reveal a previously unrecognized viral strategy to access the nuclear replication machinery. Understanding viral entry mechanisms not only increases our appreciation of basic cell biological pathways but also may lead to more effective antiviral interventions.

How non-enveloped viruses hijack host machineries to cause infection

Viruses must navigate the complex endomembranous network of the host cell to cause infection. In the case of a non-enveloped virus that lacks a surrounding lipid bilayer, endocytic uptake from the plasma membrane is not sufficient to cause infection. Instead, the virus must travel within organelle membranes to reach a specific cellular destination that supports exposure or arrival of the virus to the cytosol. This is achieved by viral penetration across a host endomembrane, ultimately enabling entry of the virus into the nucleus to initiate infection. In this review, we discuss the entry mechanisms of three distinct non-enveloped DNA viruses-adenovirus (AdV), human papillomavirus (HPV), and polyomavirus (PyV)-highlighting how each exploit different intracellular transport machineries and membrane penetration apparatus associated with the endosome, Golgi, and endoplasmic reticulum (ER) membrane systems to infect a host cell. These processes not only illuminate a highly-coordinated interplay between non-enveloped viruses and their host, but may provide new strategies to combat non-enveloped virus-induced diseases.

Phosphorylation of Human Papillomavirus Type 16 L2 Contributes to Efficient Virus Infectious Entry

The human papillomavirus (HPV) capsid comprises two viral proteins, L1 and L2, with the L2 component being essential to ensure efficient endocytic transport of incoming viral genomes. Several studies have previously reported that L1 and L2 are posttranslationally modified, but it is uncertain whether these modifications affect HPV infectious entry. Using a proteomic screen, we identified a highly conserved phospho-acceptor site on the HPV-16 and bovine papillomavirus 1 (BPV-1) L2 proteins. The phospho-modification of L2 and its presence in HPV pseudovirions (PsVs) were confirmed using anti-phospho-L2-specific antibodies. Mutation of the phospho-acceptor sites of both HPV-16 and BPV-1 L2 resulted in the production of infectious virus particles, with no differences in efficiencies of packaging the reporter DNA. However, these mutated PsVs showed marked defects in infectious entry. Further analysis revealed a defect in uncoating, characterized by a delay in the exposure of a conformational epitope on L1 that indicates capsid uncoating. This uncoating defect was accompanied by a delay in the proteolysis of both L1 and L2 in mutated HPV-16 PsVs. Taken together, these studies indicate that phosphorylation of L2 during virus assembly plays an important role in optimal uncoating of virions during infection, suggesting that phosphorylation of the viral capsid proteins contributes to infectious entry.IMPORTANCE The papillomavirus L2 capsid protein plays an essential role in infectious entry, where it directs the successful trafficking of incoming viral genomes to the nucleus. However, nothing is known about how potential posttranslational modifications may affect different aspects of capsid assembly or infectious entry. In this study, we report the first phospho-specific modification of the BPV-1 and HPV-16 L2 capsid proteins. The phospho-acceptor site is very highly conserved across multiple papillomavirus types, indicating a highly conserved function within the L2 protein and the viral capsid. We show that this modification plays an essential role in infectious entry, where it modulates susceptibility of the incoming virus to capsid disassembly. These studies therefore define a completely new means of regulating the papillomavirus L2 proteins, a regulation that optimizes endocytic processing and subsequent completion of the infectious entry pathway.

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.

Refolding and in vitro characterization of human papillomavirus 16 minor capsid protein L2

The minor capsid protein L2 of papillomaviruses exhibits multiple functions during viral entry including membrane interaction. Information on the protein is scarce, because of its high tendency of aggregation. We determined suitable conditions to produce a functional human papillomavirus (HPV) 16 L2 protein and thereby provide the opportunity for extensive in vitro analysis with respect to structural and biochemical information on L2 proteins and mechanistic details in viral entry. We produced the L2 protein of high-risk HPV 16 in Escherichia coli as inclusion bodies and purified the protein under denaturing conditions. A successive buffer screen resulted in suitable conditions for the biophysical characterization of 16L2. Analytical ultracentrifugation of the refolded protein showed a homogenous monomeric species. Furthermore, refolded 16L2 shows secondary structure elements. The N-terminal region including the proposed transmembrane region of 16L2 shows alpha-helical characteristics. However, overall 16L2 appears largely unstructured. Refolded 16L2 is capable of binding to DNA indicating that the putative DNA-binding regions are accessible in refolded 16L2. Further the refolded protein interacts with liposomal membranes presumably via the proposed transmembrane region at neutral pH without structural changes. This indicates that 16L2 can initially interact with membranes via pre-existing structural features.

PML nuclear body-residing proteins sequentially associate with HPV genome after infectious nuclear delivery

Subnuclear promyelocytic leukemia (PML) nuclear bodies (NBs) are targeted by many DNA viruses after nuclear delivery. PML protein is essential for formation of PML NBs. Sp100 and Small Ubiquitin-Like Modifier (SUMO) are also permanently residing within PML NBs. Often, large DNA viruses disassemble and reorganize PML NBs to counteract their intrinsic antiviral activity and support establishment of infection. However, human papillomavirus (HPV) requires PML protein to retain incoming viral DNA in the nucleus for subsequent efficient transcription. In contrast, Sp100 was identified as a restriction factor for HPV. These findings suggested that PML NBs are important regulators of early stages of the HPV life cycle. Nuclear delivery of incoming HPV DNA requires mitosis. Viral particles are retained within membrane-bound transport vesicles throughout mitosis. The viral genome is released from transport vesicles by an unknown mechanism several hours after nuclear envelope reformation. The minor capsid protein L2 mediates intracellular transport by becoming transmembranous in the endocytic compartment. Herein, we tested our hypothesis that PML protein is recruited to incoming viral genome prior to egress from transport vesicles. High-resolution microscopy revealed that PML protein, SUMO-1, and Sp100 are recruited to incoming viral genomes, rather than viral genomes being targeted to preformed PML NBs. Differential immunofluorescent staining suggested that PML protein and SUMO-1 associated with transport vesicles containing viral particles prior to egress, implying that recruitment is likely mediated by L2 protein. In contrast, Sp100 recruitment to HPV-harboring PML NBs occurred after release of viral genomes from transport vesicles. The delayed recruitment of Sp100 is specific for HPV-associated PML NBs. These data suggest that the virus continuously resides within a protective environment until the transport vesicle breaks down in late G1 phase and imply that HPV might modulate PML NB assembly to achieve establishment of infection and the shift to viral maintenance.

Cell-Penetrating Peptide Mediates Intracellular Membrane Passage of Human Papillomavirus L2 Protein to Trigger Retrograde Trafficking

Cell-penetrating peptides (CPPs) are short protein segments that can transport cargos into cells. Although CPPs are widely studied as potential drug delivery tools, their role in normal cell physiology is poorly understood. Early during infection, the L2 capsid protein of human papillomaviruses binds retromer, a cytoplasmic trafficking factor required for delivery of the incoming non-enveloped virus into the retrograde transport pathway. Here, we show that the C terminus of HPV L2 proteins contains a conserved cationic CPP that drives passage of a segment of the L2 protein through the endosomal membrane into the cytoplasm, where it binds retromer, thereby sorting the virus into the retrograde pathway for transport to the trans-Golgi network. These experiments define the cell-autonomous biological role of a CPP in its natural context and reveal how a luminal viral protein engages an essential cytoplasmic entry factor.