Budding of Ebola Virus Particles Requires the Rab11-Dependent Endocytic Recycling Pathway

MyosinVb tail inhibits transport of Marburg virus glycoprotein GP to VP40-enriched sites at the plasma membrane

Marburg virus (MARV) is the causative agent of severe fever with case fatality rates between 25 and 90 %. The glycoprotein GP is the only surface protein of MARV responsible for receptor recognition and fusion. Therefore, proper intracellular transport of GP to the plasma membrane and incorporation into virus particles is essential for the viral infection cycle. However, neither the exact post-Golgi trafficking route nor the host factors are known that support the transport of GP to the cell surface. Using quantitative confocal microscopy and correlative light and electron microscopy (CLEM), we show here that GP colocalized in both transiently transfected and MARV-infected cells with a dominant negative (DN) tail mutant of myosin Vb (MyoVbT), which inhibits trafficking through recycling endosomes. Overexpression of MyoVbT resulted in an aberrant distribution of GP that accumulated in or near perinuclear MyoVbT-containing structures. Simultaneously, we observed significantly reduced GP levels at the plasma membrane and especially at the viral budding sites characterized by clusters of the viral matrix protein VP40. Further, incorporation of GP into VP40-induced filamentous virus-like particles was impaired by MyoVbT. Overall, our results show that intracellular transport of MARV GP is disrupted by a DN mutant of the recycling endosome-associated motor protein MyoVb. These results might indicate a possible role for the endosomal recycling system in MARV GP trafficking to VP40-enriched budding-sites at the plasma membrane.

Marburg virus exploits the Rab11-mediated endocytic pathway in viral-particle production

Filoviruses produce viral particles with characteristic filamentous morphology. The major viral matrix protein, VP40, is trafficked to the plasma membrane and promotes viral particle formation and subsequent viral egress. In the present study, we assessed the role of the small GTPase Rab11-mediated endocytic pathway in Marburg virus (MARV) particle formation and budding. Although Rab11 was predominantly localized in the perinuclear region, it exhibited a more diffuse distribution in the cytoplasm of cells transiently expressing MARV VP40. Rab11 was incorporated into MARV-like particles. Expression of the dominant-negative form of Rab11 and knockdown of Rab11 decreased the amount of VP40 fractions in the cell periphery. Moreover, downregulation of Rab11 moderately reduced the release of MARV-like particles and authentic MARV. We further demonstrated that VP40 induces the distribution of the microtubule network toward the cell periphery, which was partly associated with Rab11. Depolymerization of microtubules reduced the accumulation of VP40 in the cell periphery along with viral particle formation. VP40 physically interacted with α-tubulin, a major component of microtubules, but not with Rab11. Taken together, these results suggested that VP40 partly interacts with microtubules and facilitates their distribution toward the cell periphery, leading to the trafficking of transiently tethering Rab11-positive vesicles toward the cell surface. As we previously demonstrated the role of Rab11 in the formation of Ebola virus particles, the results here suggest that filoviruses in general exploit the vesicle-trafficking machinery for proper virus-particle formation and subsequent egress. These pathways may be a potential target for the development of pan-filovirus therapeutics.IMPORTANCEFiloviruses, including Marburg and Ebola viruses, produce distinct filamentous viral particles. Although it is well known that the major viral matrix protein of these viruses, VP40, is trafficked to the cell surface and promotes viral particle production, details regarding the associated molecular mechanisms remain unclear. To address this knowledge gap, we investigated the role of the small GTPase Rab11-mediated endocytic pathway in this process. Our findings revealed that Marburg virus exploits the Rab11-mediated vesicle-trafficking pathway for the release of virus-like particles and authentic virions in a microtubule network-dependent manner. Previous findings demonstrated that Rab11 is also involved in Ebola virus-particle production. Taken together, these data suggest that filoviruses, in general, may hijack the microtubule-dependent vesicle-trafficking machinery for productive replication. Therefore, this pathway presents as a potential target for the development of pan-filovirus therapeutics.

Enterovirus-A71 exploits RAB11 to recruit chaperones for virus morphogenesis

BACKGROUND

Enterovirus 71 (EV-A71) causes Hand, Foot and Mouth Disease (HFMD) in children and has been associated with neurological complications. The molecular mechanisms involved in EV-A71 pathogenesis have remained elusive.

METHODS

A siRNA screen in EV-A71 infected-motor neurons was performed targeting 112 genes involved in intracellular membrane trafficking, followed by validation of the top four hits using deconvoluted siRNA. Downstream approaches including viral entry by-pass, intracellular viral genome quantification by qPCR, Western blot analyses, and Luciferase reporter assays allowed determine the stage of the infection cycle the top candidate, RAB11A was involved in. Proximity ligation assay, co-immunoprecipitation and multiplex confocal imaging were employed to study interactions between viral components and RAB11A. Dominant negative and constitutively active RAB11A constructs were used to determine the importance of the protein's GTPase activity during EV-A71 infection. Mass spectrometry and protein interaction analyses were employed for the identification of RAB11A's host interacting partners during infection.

RESULTS

Small GTPase RAB11A was identified as a novel pro-viral host factor during EV-A71 infection. RAB11A and RAB11B isoforms were interchangeably exploited by strains from major EV-A71 genogroups and by Coxsackievirus A16, another major causative agent of HFMD. We showed that RAB11A was not involved in viral entry, IRES-mediated protein translation, viral genome replication, and virus exit. RAB11A co-localized with replication organelles where it interacted with structural and non-structural viral components. Over-expression of dominant negative (S25N; GDP-bound) and constitutively active (Q70L; GTP-bound) RAB11A mutants had no effect on EV-A71 infection outcome, ruling out RAB11A's involvement in intracellular trafficking of viral or host components. Instead, decreased ratio of intracellular mature viral particles to viral RNA copies and increased VP0:VP2 ratio in siRAB11-treated cells supported a role in provirion maturation hallmarked by VP0 cleavage into VP2 and VP4. Finally, chaperones, not trafficking and transporter proteins, were found to be RAB11A's top interacting partners during EV-A71 infection. Among which, CCT8 subunit from the chaperone complex TRiC/CCT was further validated and shown to interact with viral structural proteins specifically, representing yet another novel pro-viral host factor during EV-A71 infection.

CONCLUSIONS

This study describes a novel, unconventional role for RAB11A during viral infection where it participates in the complex process of virus morphogenesis by recruiting essential chaperone proteins.

Coronavirus M Protein Trafficking in Epithelial Cells Utilizes a Myosin Vb Splice Variant and Rab10

The membrane (M) glycoprotein of coronaviruses (CoVs) serves as the nidus for virion assembly. Using a yeast two-hybrid screen, we identified the interaction of the cytosolic tail of Murine Hepatitis Virus (MHV-CoV) M protein with Myosin Vb (MYO5B), specifically with the alternative splice variant of cellular MYO5B including exon D (MYO5B+D), which mediates interaction with Rab10. When co-expressed in human lung epithelial A549 and canine kidney epithelial MDCK cells, MYO5B+D co-localized with the MHV-CoV M protein, as well as with the M proteins from Porcine Epidemic Diarrhea Virus (PEDV-CoV), Middle East Respiratory Syndrome (MERS-CoV) and Severe Acute Respiratory Syndrome 2 (SARS-CoV-2). Co-expressed M proteins and MYO5B+D co-localized with endogenous Rab10 and Rab11a. We identified point mutations in MHV-CoV M that blocked the interaction with MYO5B+D in yeast 2-hybrid assays. One of these point mutations (E121K) was previously shown to block MHV-CoV virion assembly and its interaction with MYO5B+D. The E to K mutation at homologous positions in PEDV-CoV, MERS-CoV and SARS-CoV-2 M proteins also blocked colocalization with MYO5B+D. The knockdown of Rab10 blocked the co-localization of M proteins with MYO5B+D and was rescued by re-expression of CFP-Rab10. Our results suggest that CoV M proteins traffic through Rab10-containing systems, in association with MYO5B+D.

Ebola Virus Activates IRE1α-Dependent XBP1u Splicing

Ebola (EBOV) and Marburg virus (MARV) are highly pathogenic filoviruses that influence cellular signaling according to their own needs. MARV has been shown to regulate the IRE1α-dependent unfolded protein response (UPR) to ensure optimal virus replication. It was not known whether EBOV affects this signaling cascade, which can be beneficial or detrimental for viruses. Activation of IRE1α leads to the expression of the transcription factor XBP1s, which binds to cis-acting UPR elements (UPRE), resulting in the expression of genes aimed at restoring homeostasis in the endoplasmic reticulum. We observed that EBOV infection, in contrast to MARV infection, led to UPR activation by IRE1α-dependent but not ATF6-dependent signaling. We showed an activation of IRE1α, XBP1s and UPRE target genes upon EBOV infection. ATF6, another UPRE transcription factor, was not activated. UPRE activation was mainly attributed to the EBOV nucleoprotein NP and the soluble glycoprotein sGP. Finally, activation of UPR by thapsigargin, a potent ER-stress inducer, in parallel to infection as well as knock-out of XBP1 had no effect on EBOV growth, while MARV proliferation was affected by thapsigargin-dependent UPR activation. Taken together EBOV and MARV differ in their strategy of balancing IRE1α-dependent signaling for their own needs.

Small GTPase-a Key Role in Host Cell for Coronavirus Infection and a Potential Target for Coronavirus Vaccine Adjuvant Discovery

Small GTPases are signaling molecules in regulating key cellular processes (e.g., cell differentiation, proliferation, and motility) as well as subcellular events (e.g., vesicle trafficking), making them key participants, especially in a great array of coronavirus infection processes. In this review, we discuss the role of small GTPases in the coronavirus life cycle, especially pre-entry, endocytosis, intracellular traffic, replication, and egress from the host cell. Furthermore, we also suggest the molecules that have potent adjuvant activity by targeting small GTPases. These studies provide deep insights and references to understand the pathogenesis of coronavirus as well as to propose the potential of small GTPases as targets for adjuvant development.

Respiratory syncytial virus ribonucleoproteins hijack microtubule Rab11 dependent transport for intracellular trafficking

Respiratory syncytial virus (RSV) is the primary cause of severe respiratory infection in infants worldwide. Replication of RSV genomic RNA occurs in cytoplasmic inclusions generating viral ribonucleoprotein complexes (vRNPs). vRNPs then reach assembly and budding sites at the plasma membrane. However, mechanisms ensuring vRNPs transportation are unknown. We generated a recombinant RSV harboring fluorescent RNPs allowing us to visualize moving vRNPs in living infected cells and developed an automated imaging pipeline to characterize the movements of vRNPs at a high throughput. Automatic tracking of vRNPs revealed that around 10% of the RNPs exhibit fast and directed motion compatible with transport along the microtubules. Visualization of vRNPs moving along labeled microtubules and restriction of their movements by microtubule depolymerization further support microtubules involvement in vRNPs trafficking. Approximately 30% of vRNPs colocalize with Rab11a protein, a marker of the endosome recycling (ER) pathway and we observed vRNPs and Rab11-labeled vesicles moving together. Transient inhibition of Rab11a expression significantly reduces vRNPs movements demonstrating Rab11 involvement in RNPs trafficking. Finally, Rab11a is specifically immunoprecipitated with vRNPs in infected cells suggesting an interaction between Rab11 and the vRNPs. Altogether, our results strongly suggest that RSV RNPs move on microtubules by hijacking the ER pathway.

Evidence for the role of Rab11-positive recycling endosomes as intermediates in coronavirus egress from epithelial cells

After their assembly by budding into the lumen of the intermediate compartment (IC) at the endoplasmic reticulum (ER)–Golgi interface, coronaviruses (CoVs) are released from their host cells following a pathway that remains poorly understood. The traditional view that CoV exit occurs via the constitutive secretory route has recently been questioned by studies suggesting that this process involves unconventional secretion. Here, using the avian infectious bronchitis virus (IBV) as a well-established model virus, we have applied confocal microscopy to investigate the pathway of CoV egress from epithelial Vero cells. We report a novel effect of IBV infection on cellular endomembranes, namely, the compaction of the pericentrosomal endocytic recycling compartment (ERC) defined by the GTPase Rab11, which coincides with the previously described Golgi fragmentation, as well as virus release. Despite Golgi disassembly, the IC elements containing the major IBV membrane protein (M)—which mostly associates with newly formed virus particles—maintain their close spatial connection with the Rab11-positive endocytic recycling system. Moreover, partial colocalization of the M protein with Rab11 was observed, whereas M displayed negligible overlap with LAMP-1, indicating that IBV egress does not occur via late endosomes or lysosomes. Synchronization of virus release using temperature-shift protocols was accompanied by increased colocalization of M and Rab11 in vesicular and vacuolar structures in the pericentrosomal region and at the cell periphery, most likely representing IBV-containing transport carriers. In conclusion, these results add CoVs to the growing list of viruses exploiting the endocytic recycling apparatus defined by Rab11 for their assembly and/or release.

Negative-sense RNA viruses: An underexplored platform for examining virus-host lipid interactions

Viruses are pathogenic agents that can infect all varieties of organisms, including plants, animals, and humans. These microscopic particles are genetically simple as they encode a limited number of proteins that undertake a wide range of functions. While structurally distinct, viruses often share common characteristics that have evolved to aid in their infectious life cycles. A commonly underappreciated characteristic of many deadly viruses is a lipid envelope that surrounds their protein and genetic contents. Notably, the lipid envelope is formed from the host cell the virus infects. Lipid-enveloped viruses comprise a diverse range of pathogenic viruses, which often lead to high fatality rates and many lack effective therapeutics and/or vaccines. This perspective primarily focuses on the negative-sense RNA viruses from the order Mononegavirales, which obtain their lipid envelope from the host plasma membrane. Specifically, the perspective highlights the common themes of host cell lipid and membrane biology necessary for virus replication, assembly, and budding.

The Endosomal Recycling Pathway-At the Crossroads of the Cell

The endosomal recycling pathway lies at the heart of the membrane trafficking machinery in the cell. It plays a central role in determining the composition of the plasma membrane and is thus critical for normal cellular homeostasis. However, defective endosomal recycling has been linked to a wide range of diseases, including cancer and some of the most common neurological disorders. It is also frequently subverted by many diverse human pathogens in order to successfully infect cells. Despite its importance, endosomal recycling remains relatively understudied in comparison to the endocytic and secretory transport pathways. A greater understanding of the molecular mechanisms that support transport through the endosomal recycling pathway will provide deeper insights into the pathophysiology of disease and will likely identify new approaches for their detection and treatment. This review will provide an overview of the normal physiological role of the endosomal recycling pathway, describe the consequences when it malfunctions, and discuss potential strategies for modulating its activity.

[Molecular mechanisms of highly pathogenic viruses' replication and their applications for a novel drug discovery]

Productive (lytic) replication of DNA viruses elicits host cell DNA damage responses, which cause both beneficial and detrimental effects on viral replication. Viruses utilize them and selectively cancel the 'noisy' downstream signaling pathways, leading to maintain high S-phase CDK activities required for viral replication. To achieve this fine tuning of cellular environment, herpesviruses encode many (>70) genes in their genome, which are expressed in a strictly regulated temporal cascade (immediate-early, early, and late). Here, I introduce and discuss how Epstein-Barr virus, an oncogenic herpesvirus, hijacks the cellular environment and adapt it for the progeny production.

Ebola Virus Uptake into Polarized Cells from the Apical Surface

Ebola virus (EBOV) causes severe hemorrhagic fever with high mortality rates. EBOV can infect many types of cells. During severe EBOV infection, polarized epithelial and endothelial cells are damaged, which promotes vascular instability and dysregulation. However, the mechanism causing these symptoms is largely unknown. Here, we studied virus infection in polarized Vero C1008 cells grown on semipermeable Transwell by using EGFP-labeled Ebola virus-like particles (VLPs). Our results showed that Ebola VLPs preferred to enter polarized Vero cells from the apical cell surface. Furthermore, we showed that the EBOV receptors TIM-1 and Axl were distributed apically, which could be responsible for mediating efficient apical viral entry. Macropinocytosis and intracellular receptor Niemann–Pick type C1 (NPC1) had no polarized distribution, although they played roles in virus entry. This study provides a new view of EBOV uptake and cell polarization, which facilitates a further understanding of EBOV infection and pathogenesis.